Resilient marker compounds

Abstract

The present invention relates to a compound which is an acridan of formula (I) wherein X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl; R1 is unsubstituted or substituted C1-20 alkyl, optionally wherein R1 is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; and R2, R3, R4, R5, R6, R7, R8 and R9 are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and – C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl; wherein R10, R11 and R12 are each independently selected from H or unsubstituted or substituted C1-6 alkyl.

Description

RESILIENT MARKER COMPOUNDSFIELD OF THE INVENTIONThe invention relates to compounds, which are useful for marking materials. The invention also relates to processes for producing the compounds. The invention also relates to methods of marking a product using the compounds, the products marked with the compound and methods of detecting a marker compound in a marked product.BACKGROUND TO THE INVENTIONThe secure marking of commercial products or personal items for the purpose of identification can provide a variety of benefits including (a) proof of authenticity (b) brand protection (c) quantifying levels of adulteration in premium products (d) in-process or retrospective testing of specified blending activities and (e) taxation status and / or provide evidence of sustainability. A wide range of products can be marked using the invention described herein including petroleum and fuel products, grease, plastics, fabrics, chemical products, packaging materials, manufactured goods, clothing, inks, vegetable oils, food and beverages, tobacco, drugs including cannabis and related products, perfumes, graphic art products, timber or plants and seeds.The marking of fuels by currently utilized means typically involves (relatively) large concentrations (from 2.5ppm to 50ppm) of markers. This can involve coloured dyes which are subsequently quantified using colourimetry. The drawbacks of dye-markers include: (i) colouration of the fuel which makes it obvious that a marker is present; (ii) given large concentrations of dye it is relatively easy to adulterate a premium fuel with low grade material and then add sufficient dye to imitate the premium (marked) fuel. Other currently available techniques utilize similarly high concentrations and can involve radioactive or toxic materials. Therefore, there is a need for a different type of marking technology which can address the above issues.Acridinium esters have previously been proposed (EP0637743A1) as candidates for fuel marking and can be detected via chemiluminescence at concentrations in the region of a few parts per billion. However, it is clear from this prior art that the solubility of acridinium esters in fuels was conditional on the use of polar solvents (eg acetonitrile) prior to dilution into a fuel. In addition, EP0637743A1 claims that acridinium esters are stable to fuel additives but does not provide experimental evidence for this claim.A further challenge is presented by the fact that the composition of contemporary fuels often includes high levels of alcohols (e.g. ethanol in E10 Petrolat 10%). Indeed, new fuels including pure methanol are being developed as net carbon neutral replacements for petroleum fuels. Therefore, more resilient chemiluminescent compounds are required to address these above demands.In summary, there is no current marking technology that can meet all of the following requirements: (a) no colouration of marked fuel, (b) very low (ppb) concentrations of marker toprotect fuel performance, (c) low toxicity dosing solutions, (d) dosing solutions which have no impact upon critical fuel properties, (e) low volume (economical) dosing solutions, (f) markers with excellent (intrinsic) solubility in fuels, (g) robust chemical stability in the presence of fuel additives including amines, (h) cost effective and portable technology suitable for detecting ppb levels of marker using an in-field test, (i) a detection method which does not generate significant levels of ‘background noise’ associated with an unmarked-fuel and (j) applicable to a wide range of fuel types and grades.Prior art methods for the quantification of chemiluminescent compounds rely upon photomultiplier tube (PMT) technology and usually involve aqueous type media. Although costly, such instrumentation provides an extremely sensitive means of detection. Typically, PMT technology is used in a stable laboratory environment. Accordingly, it is also an objective of the present inventors to develop an economical and rugged technique for use in the field (eg. on board ships or fuel distribution depots). Therefore, instrumentation based upon more robust and economical solid-state detectors (eg G& H-ITL Lumini) rather than PMT technology is preferred. Most fuels are nonpolar and display poor miscibility in aqueous media. It would therefore be attractive if the chemistry of the detection method can operate in an environment which is (i) low in water content and (ii) in which the marked fuel is readily soluble.Chemiluminescent compounds have been widely employed in the analytical detection and quantification of inorganic and organic species as well as the detection and quantification of biomolecules. Most of these methods employ luminol or acridinium esters which are popular components in commercially available testing kits. On the other hand, one commercially available testing kit (Lumi-Phos HRP) uses an acridan ester which can be used to detect the enzyme Horse Radish Peroxidase (HRP) (see J. Org. Chem

[1998] , 930 and EP0778946 Bl). This prior art teaches that the acridan phenoxy ester is first oxidised to an acridinium ester (by HRP) which can then be detected by chemiluminescence using hydrogen peroxide.On the other hand, work published by McCapra (Pure & Applied Chemistry

[1970] , 611) describes the chemiluminescence of a series of Acridan Esters which were unsubstituted on the acridan nucleus but included substituents on the phenoxy group. McCapra claimed that these compounds would undergo chemiluminescence when reacted with potassium phthalimide in dimethylformamide (a polar aprotic solvent) providing chemiluminescent quantum yields in the range of 10-5to 0.10. Quantum yields at the upper end of this range were observed when using phenoxy groups containing strongly electronegative functionalities.Using the G& H ITL Lumini instrument, the present inventors have evaluated the method reported by McCapra (see above) involving chemiluminescence of acridan esters triggered by potassium phthalimide in dimethylformamide solvent. When injecting the potassium phthalimide reagent into a toluene solution of either compound 3 or 33 (at lOppb), the present inventors observed very poor chemiluminescence. At best, a Peak height of 2,489 and a Peak Area of 1,758 RLU / s was obtained using a Lumini detector scanning over a 10 second period.Compound 3Therefore, there remains a need to provide a system which can provide reliable, strong luminescent signals via the means of a field instrument.SUMMARY OF THE INVENTIONThe present invention provides a series of novel marker compounds which are designed to meet the requirements for secure marking and overcome the difficulties associated with existing technologies outlined above, including the demands presented by different categories of fuels including diesels, gasolines (petrols), alcohols as well as reactive environments including fuel additive packages as well as in all the other applications identified above. In particular, the present inventors have developed marker technologies based upon acridan esters as well as novel methods for the detection of acridan esters.It is a finding of the inventors that the acridan ester compounds of the present invention can be directly dissolved in low polarity liquids (e.g. fuels, alkylbenzenes or alkanes) without resorting to polar aprotic carrier solvents or surfactants. In some embodiments the compounds are soluble in pure alkanes, for instance in n-hexane. The inherent solubility of these acridan ester compounds provides stable solutions at ppb concentrations without the limitations of a micelle formulation. Another finding relates to excellent chemical stability of these compounds when dissolved in compositions comprising mixtures of amines and alcohols which are representative of many fuel-additive packages.Accordingly, the present invention provides a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted Ci -e alkyl.Typically, when X is phenyl X is substituted with at least one group other than halo.Often when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(0)0CH3.It is a finding of the present inventors that the Acridan ester compounds (comprising a substituted or unsubstituted nucleus) can be detected and quantified using a Lumini instrument together with a basic reagent system comprising an alkali metal alkoxide (preferably a hindered potassium alkoxide) in non-aqueous solvents (preferably tert- Amyl alcohol). This type of base and solvent combination produces a high intensity signal such that a lOppb concentration of acridan marker (in toluene) can produce an emission Peak height >5 million RLU and Peak Area of > 0.5 million RLU / s). After introducing a phase transfer catalyst (which is often a quaternary ammonium salt such as cetyl trimethylammonium chloride (CTAC)) to the alkali metal alkoxide reagent system, significant improvements to the peak shape (shorter flash time) are achieved. This effect is useful for the measurement of marked diesel fuels which, in the absence of a catalyst such as CTAC, slow down the kinetics of the chemiluminescence. For example, without adding the quaternary ammonium salt, the peak shape becomes extended and less symmetrical. In addition, it has been found that the addition of acetonitrile to the marked fuel sample, prior to injection of reagent (alkali metal alkoxide, and optionally, a phase transfer catalyst and non-aqueous solvent), further enhances the intensity and sharpness of the chemiluminescence. In summary, this novel method of detection enables use of a solid-state detector for the quantification of a wide variety of acridan esters in the application of fuel markers.According, the present invention also provides a method of producing light emission from a marked product, which method comprises:(i) contacting a sample comprising a marked composition;wherein the marked composition comprises (a) a product and (b) a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12and unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl,with an alkali metal alkoxide.The inventors of the present method also investigated different categories of chemiluminescent esters beyond Acridan Esters. Phenolic esters of (simple) luciferase analogues were evaluated based upon the findings of Prescher (Chem. Bio. Chem.

[2016] , 564) and Kato (Photochemical and Photobiological Sciences

[2014] , 1640). Also, phenolic esters of xanthene-9-carboxylic acid were evaluated. For example, compound 69 was synthesized according to the method of Yan (iScience

[2019] , 13, 478) and subsequently tested using the above reagent system and the Lumini detector.Compound 69A 20ppb solution of 69 in dimethylformamide produced a relatively low level of chemiluminescence with peak areas in the region of 86,000 RLU / s when using the Lumini detector.It is also a finding of the present inventors that acridan esters with defined structural features are required to satisfy all the demands required in commercial fuel marking applications. In general, acridan esters of alkyl substituted phenols provide the most useful properties, namely (i) soluble in Diesel and Petrol (at least 0.1 %wv) (ii) peak shapes are near symmetrical and complete within lOsecs (iii) good stability in amine / alcohol mixtures and (iv) detectable at 20ppb with a Lumini peak area >300,000 RLU / s. Acridan esters of alkyl substituted phenols may also be detectable at lOppb with a Lumini peal area >300,000 RLU / s. When these phenolic esters are substituted with electron withdrawing groups (eg. Cl or CN substituted), we found that such compounds usually provide fast kinetics and one of these compounds (15) provides adequate solubility in diesel as well as adequate stability in amines. It is also a finding of the present inventors that certain acridan esters prepared from long chain or polyalkyl substituted phenols, for example 4-poly(isobutyl)-phenol, provide improved solubility in diesel or simple hydrocarbons such as n-hexane.In the prior art (H. Akhaven-Tafti J. Org. Chem, (1998), 63, 930 and EP0778946 (2002)), it has been claimed that acridan esters with electron withdrawing groups on the phenoxy ester, produce greater light intensity in the order F> Cl> H> OMe with 2,6-difluorophenol representing one of the preferred compounds. It should be noted however, that this prior art involved oxidation (using HRP and hydrogen peroxide) of the acridan to an acridinium intermediate prior to triggering of chemiluminescence using hydrogen peroxide. Referring to this prior art, it can be concluded that the observed trend regarding electron withdrawing groups is related to the acridinium intermediate and not the acridan ester (precursor).The detection method described in the present invention does not use an oxidising reagent nor attempt to form an acridinium type intermediate. Contrary to the above prior art, we have found that acridan ester of 2,6-difluorophenol produces a relatively low intensity of light (120,000RLU / s at lOppb) when compared to acridan esters of alkyl substituted phenols (mentioned above). In addition to acridan esters of alkyl phenols, we have also found that acridan esters of certain haloalkanes provide useful properties in this application. The acridan esters of l,l,l,3,3,3-hexafluoro-2-methyl-2-propanol and 2,2,2-trichloroethanol provide fast kinetics and good solubility. However, from these two compounds, only the 2,2,2-trichloroethanol ester has adequate stability in alcohol / amine mixtures.The reagent system as described herein (i.e. a system comprising an alkali metal alkoxide, and optionally a phase transfer catalyst) can sometimes be affected by the level of background noise resulting from unmarked fuels and other components used in the detection method (eg. solvents). For the sake of comparison, treatment of various unmarked diesels using the potassium tert-butoxide reagent system can give peak areas ranging from 20,000 to 500,000 RLU / s. These values tend to vary depending upon factors including the grade of fuel, its age and the storage conditions (including temperature and level of exposure to air). The present inventors have found that the addition of tertiary amines such as triethylamine (pKa 10.75) to the reagent system is an effective way to limit the level of background noise associated with fuels or solvents. Unfortunately, however, using excessive quantities of triethylamine can also limit the intensity of chemiluminescence generated by the acridan ester.It is also a finding of the present invention that some acridan esters will undergo chemiluminescence in the presence of very strong organic bases which are often referred to as superbases (book authored by T. Ishikawa ‘Superbases for Organic Synthesis’

[2009] J Wiley). Superbases based upon Amidine or Guanidine structures are especially attractive when considering cost, stability and tolerance of moisture.Accordingly, the present invention also provides a method of producing light emission from a marked product, which method comprises:(i) contacting a sample comprising a marked composition;wherein the marked composition comprises (a) a product and (b) a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl,with an organic superbase which is an amidine or guanidine compound.The invention also provides a method of detecting a marker compound in a product, which method comprises:(i) a method of the invention for producing light emission from a marked product; and(ii) detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition.Example of amidine and guanidine compounds include l,8-diazabicyclo[5.4.0]undec-7-ene (DBU), l,5-diazabicyclo[4.3.0]non-5-ene (DBN), l,5-diazabicyclo[4.4.0]dec-6-ene (DBD), 7-methyl-l,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) and l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). In general, superbases in this list have pKa values >23. The preferred reagent, TBD, has the highest pKa (ca. 26) and provides a faster chemiluminescence profde than the other superbases. Pure TBD is a solid that can be used as a solution in an organic solvent such as diethylene glycol dimethyl ether (DEGDME) or DMSO and therefore enables addition via injection to trigger chemiluminescence. Pure TBD can also advantageously be used in solution in a mixture of organic solvents such as in diethylene glycol dimethyl ether (DEGDME) and DMSO. In addition, it has been found that the addition of acetonitrile to the marked fuel sample (prior to injection of TBD) further enhances the intensity and sharpness of the chemiluminescence event. Organic superbases are highly effective reagents for the chemiluminescence of acridan esters bearing electron donating groups on the acridan nucleus (for example -OR or -NRR’).Compound 3 Compound 48 On the other hand, acridan esters without electron donating groups on the nucleus (including those compounds referred to by McCapra) may not undergo any significant chemiluminescence in the presence of the Amidine or Guanidine superbases. Instead, chemiluminescence of unsubstituted acridan esters is possible using much stronger superbases including those selected from phosphazenes, guanidinophosphazenes and proazaphosphatranes. However, these stronger superbases are highly sensitive to moisture, need to be handled in an inert atmosphere, and are more suited to a laboratory environment rather than an ‘in field’ test setting.It is also a finding of the present invention that the combination of organic superbases (eg. TBD) with tertiary amines (eg. triethylamine) is a highly effective way to produce chemiluminescence of certain acridan esters whilst (at the same time) reducing background noise due to fuels (esp. diesel) and / or solvents (eg. acetonitrile). For example, treatment of various unmarked diesels with 10% triethylamine in acetonitrile followed by TBD (as a diethyleneglycol dimethylether solution) will tend to give peak areas ranging from 20,000 to 25,000 RLU / s. After marking various diesels with acridan esters (possessing electron donating substituents) at lOppb, addition of (i) triethylamine / acetonitrile followed by (ii) TBD / DEGDME provides peak areas in the region of 0.4 million RLU / s. These results represent a signal to noise ratio of 19: 1. In general, the issue of signal: noise is easier to manage when marking a Petrol, rather than a Diesel type fuel. For example, applying the same approach to samples of marked (lOppb) and unmarked Petrol provides a signal to noise ratio of 35:1. Alternatively, when using a 10% solution of TBD in dimethyl sulfoxide (DMSO) a signal: noise of >200 can be achieved even without using triethylamine in acetonitrile (see Table 1 and Figures 1 and 2). Similarly, by using a solution of TBD in dimethyl sulfoxide (DMSO) or in a mixture of DMSO and DEGDME, a signal: noise of >160 can be achieved even without using triethylamine in acetonitrile (see Table 1). Similar improvements in Diesel are achieved when using TBD in a mixture of DMSO and DEGDME, the signal: noise becomes >29 and the magnitude of the RLU / s area counts are further improved. Mixtures with a DMSO to DEGDME ratio equal to or less than 1:2 do not freeze at 3°C. Therefore, the practical issues associated with DMSO freezing at ambient temperatures (ca. 18°C) can be avoided. Also, reagents based upon TBD in a mixture of DMSO and DEGDMEavoid the need for a second reagent system (e.g. triethylamine in acetonitrile). Furthermore, the reagents based upon DMSO / DEGDME mixtures offer stable area counts (RLU / s) across a wise temperature range. For example, we have evaluated these reagents at +6°C, +16°C and +22°C and observe good repeatability (see Table 2) within 1%.Table 1Table 2These improvements in signal: noise ratio, extend the applicability of the technology for wider ranges and grades of fuel and other commercial products. In addition, these improvementssupport marking of fuels using even lower concentrations (<10ppb) of acridan ester and consequently offer a reduced cost of implementation.The present invention also provides a marked product which comprises (a) a product and (b) a marker compound which is a compound as defined herein.It is also a finding of the present invention that combinations of several different acridan esters in different quantities can be used to provide an authentic and bespoke marking system. This type of ‘combination marking’ provides a method to (for example) mark a specific batch of fuel (or product) that differentiates one batch from another batch. To realize this approach, the present invention describes a series of novel acridan esters which each undergo chemiluminescence to emit a different wavelength of light. For example, compound 3 emits light at 425nm and compound 48 emits light at 460nm. Further compounds (eg. 68) are disclosed which emit light in the region of 480 to 800nm. Moreover, within the mixture of different acridan esters, it is also possible to vary the quantity of each individual acridan ester. Using a band pass filter in the detection system it is possible to selectively quantify a specific acridan ester and to exclude the detection of wavelengths from the other acridan esters. Repeating the chemiluminescence detection with a different band pass filter allows quantification of the next acridan ester in the mixture and so on. The commercial availability of different band pass filters affords exclusivity of detection within + / - 15nm of the following wavelengths, 420, 440, 460, 480, 500, 520, 540, 560, 580, 600, 620, 685, 700, 720, 750, 780, 800 and 850 nm and so forth.Accordingly, the present invention also provides a marked product of the invention as described herein, i.e. a marked product which comprises (a) a product and (b) a marker compound which is a compound as defined herein, wherein the marked product further comprises (c) an additional marker compound which is an acridan of formula (I)X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substitutedaryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl,wherein the marker compound (b), and the additional marker compound (c) which is an acridan of formula (I), emit light at different wavelengths.The invention also provides a method of recovering a marker compound from a marked product, the method comprising:(i) taking a sample from a marked product of the invention as defined herein, and optionally:(ii) extracting a marker compound from the sample, wherein the marker compound is a compound of formula (I) as defined herein.The present invention also relates to a method of marking a product, which method comprises treating a product with a compound as described herein.The present invention also relates to a method of detecting a marker compound in a product, which method comprises:(i) contacting a sample comprising a marked composition;wherein the marked composition comprises (a) a product and (b) a compound as described hereinwith one or more reagents for causing the marker compound to undergo a chemiluminescent reaction to emit light; and(ii) detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition.The present invention also provides a formulation for use in marking a product wherein said formulation comprises a marker compound as defined herein, and wherein said formulation is a grease or resin.The invention also provides the use of a formulation of the invention for marking a product, wherein said use comprises coating said product in said formulation.The invention also provides an intermediate (suitable for use in producing a compound of the invention), wherein said intermediate comprises a cation of formula (III)and an anion, Yn-, wherein n is an integer of 1 to 4,X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted Ci -e alkylprovided that;(a) when X is phenyl, X is substituted with at least one group other than halo;(b) when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and-C(O)OCH3; and(c) the compound is other than:The present invention also provides a process for producing a compound of Formula (I) as described herein, wherein said process comprises treating with a reducing agent an intermediate which comprises a cation of formula (III):and an anion, Yn-, wherein n is an integer of 1 to 4,X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl.Typically when X is phenyl, X is substituted with at least one group other than halo.Often when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(0)0CH3.The invention also provides a precursor (suitable for use in producing a compound according to the invention), which precursor is a compound of formula (IV)X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted Ci- 20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkylprovided that;(a) when X is phenyl, X is substituted with at least one group other than halo;(b) when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH methyl, -OCH3 and-C(O)OCH3; and(c) said compound is other thanandA method of marking a product which method comprises treating said product with a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted Ci -e alkyl; andan antioxidant.A method of marking a product which method comprises treating said product with a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted Ci -e alkyl; anda UV stabiliser or absorber.A marked product which comprises:(a) a product;(b) a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted Ci -e alkyl; and(a) an antioxidant; and / ora UV stabiliser or absorber.BRIEF DESCRIPTION OF THE FIGURESFigure 1 shows the chemiluminescence of compound 48 (at 10ppb) in E5 Petrol using TBD in DMSO, with time in 0.1 second (x-axis), and relative light units (y-axis).Figure 2 shows the background chemiluminescence of E5 petrol, with time in 0.1 second (x-axis), and relative light units (y-axis).Figure 3 shows the linearity of detection of compound 3 in E5 petrol, with concentration in parts per billion (x-axis) and relative light units (y-axis).Figure 4 shows the linearity of detection of compound 3 in B7 diesel, with concentration in parts per billion (x-axis) and relative light units (y-axis).Figure 5 shows the linearity of detection of compound 48 in B7 diesel, with concentration in parts per billion (x-axis) and relative light units (y-axis).Figure 6 shows the linearity of detection of compound 48 in E5 petrol, with concentration in parts per billion (x-axis) and relative light units (y-axis).Figure 7 shows the stability of compound 48 (at a concentration of 20 ppb) stored B7 diesel at a temperature of 40°C and evaluated over a 3 month period, with time lapsed in days (x-axis) and relative light units (y-axis).Figure 8 shows the stability of compound 48 (at a concentration of 20 ppb) stored E5 petrol at a temperature of 40°C and evaluated over a 3 month period, with time lapsed in days (x-axis) and relative light units (y-axis).Figure 9 shows the linearity of detection of compound 51 in ethanol, with concentration in parts per billion (x-axis) and relative light units (y-axis).Figure 10 shows the linearity of detection of compound 51 in methanol, with concentration in parts per billion (x-axis) and relative light units (y-axis).DETAILED DESCRIPTION OF THE INVENTIONDefinitionsThe term “hydrocarbon”, as used herein, takes its normal meaning. Thus, unless it is explicitly said to be a “substituted hydrocarbon”, a hydrocarbon is a compound which consists only of carbon and hydrogen. For the avoidance of doubt, hydrocarbons include straight chained and branched, saturated and unsaturated aliphatic hydrocarbon compounds, including alkanes, alkenes, and alkynes, as well as saturated and unsaturated cyclic aliphatic hydrocarbon compounds, including cycloalkanes, cycloalkenes and cycloalkynes. Hydrocarbons also include aromatic hydrocarbons, i.e. hydrocarbons comprising one or more aromatic rings. The aromatic rings may be monocyclic or polycyclic.Aliphatic hydrocarbons which are substituted with one or more aromatic hydrocarbons, and aromatic hydrocarbons which are substituted with one or more aliphatic hydrocarbons, are also of course encompassed by the term “hydrocarbon” (such compounds consisting only of carbon and hydrogen) as are straight-chained or branched aliphatic hydrocarbons that are substituted with one or more cyclic aliphatic hydrocarbons, and cyclic aliphatic hydrocarbons that are substituted with one or more straight-chained or branched aliphatic hydrocarbons.A “substituted hydrocarbon” is a hydrocarbon as defined above which bears one or more nonhydrocarbon substituents. The one or more non -hydrocarbon substituents may be selected from cyano, amino, nitro, C1-10 alkylamino, di(C1-10)alkylamino, arylamino, diarylamino, aryl(C1-10)alkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, C1-10alkoxy, aryloxy, halo(C1-10)alkyl, sulfonic acid, thiol, C1-10 alkylthio, arylthio, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid, phosphonate ester and SO,. Typically, the one or more non-hydrocarbon substituents are selected from cyano, amino, nitro, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, sulfonic acid, thiol, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid, phosphonate ester and SO3. When a hydrocarbon is substituted, it may for instance bear from 1 to 20non-hydrocarbon substituents, for example 1, 2, 3 or 4 non-hydrocarbon substituents. For instance, a substituted hydrocarbon may have 1, 2 or 3 non-hydrocarbon substituents, or for example 1 or 2 non-hydrocarbon substituents.A “Cn-m hydrocarbon”, where n and m are integers, is a hydrocarbon, as defined above, having from n to m carbon atoms. For instance, a C1-150 hydrocarbon is a hydrocarbon as defined above which has from 1 to 150 carbon atoms, a C5-150 hydrocarbon is a hydrocarbon as defined above which has from 5 to 150 carbon atoms, and a C10-150 hydrocarbon is a hydrocarbon as defined above which has from 10 to 150 carbon atoms. A C1-100 hydrocarbon is a hydrocarbon as defined above which has from 1 to 100 carbon atoms, a C5-100 hydrocarbon is a hydrocarbon as defined above which has from 5 to 100 carbon atoms, and a C10-100 hydrocarbon is a hydrocarbon as defined above which has from 10 to 100 carbon atoms. A C5-50 hydrocarbon is a hydrocarbon as defined above which has from 5 to 50 carbon atoms, and a C10-50 hydrocarbon is a hydrocarbon as defined above which has from 10 to 50 carbon atoms. A C1-10 hydrocarbon is a hydrocarbon as defined above which has from 1 to 10 carbon atoms, and a C1.4 hydrocarbon is a hydrocarbon as defined above which has from 1 to 4 carbon atoms.The term “alkane”, as used herein, refers to a linear or branched chain saturated hydrocarbon compound. A “Cn-malkane” refers to an alkane having from n to m carbon atoms. Thus, for instance, an alkane may be a C1-20 alkane, i.e. an alkane having from 1 to 20 carbon atoms, or for instance a Ci-10 alkane, i.e. an alkane having from 1 to 10 carbon atoms. It may for instance be a Ci-s alkane, a C1-6 alkane, a C1-5 alkane, or a C1.4 alkane, or for instance a C2-20 alkane, a C2-10 alkane, a C2-8 alkane, a C2-6 alkane, a C2-5 alkane, a C2-4 alkane, or a C2-3 alkane. It is often a C1.4 alkane, C1-3 alkane or C2-3 alkane in the present invention. Examples of smaller alkanes, e.g. of a CMO alkane, are for instance, methane, ethane, propane, butane, isobutane, pentane, isopentane, hexane, methylpentane, dimethylbutane, heptane, methylhexane, dimethylpentane, octane, methylheptane, dimethylhexane, trimethylpentane, nonane, decane. The term “n-alkane” as used herein, refers to a straight chain alkane. The term “i-alkane” as used herein, refers to a branched chain alkane. Alkanes such as dimethylbutane may be one or more of the possible isomers of this compound. Thus, dimethylbutane includes 2,3-dimethybutane and 2,2-dimethylbutane. This also applies for all hydrocarbon compounds referred to herein including cycloalkane, alkene, cycloalkene.The term “cycloalkane”, as used herein, refers to a saturated cyclic aliphatic hydrocarbon compound. A “Cn-mcycloalkane” refers to a cycloalkane having from n to m carbon atoms. A cycloalkane may for instance be a C3-20 cycloalkane, a C3-10 cycloalkane, a C3-8 cycloalkane, or a C3-4 cycloalkane. Examples of a C3-8 cycloalkane include cyclopropane, cyclobutane, cyclopentane, cyclohexane, methylcyclopentane, cycloheptane, methylcyclohexane, dimethylcyclopentane and cyclooctane. The terms “cycloalkane” and “naphthene” may be used interchangeably.The term “alkene”, as used herein, refers to a linear or branched chain hydrocarbon compound containing one or more double bonds. A “Cn-malkene” refers to an alkene having from nto m carbon atoms. Thus, for instance, an alkene may be a C2-20 alkene, i.e. an alkene having from 2 to 20 carbon atoms, or for instance a C2-10 alkane, i.e. an alkane having from 2 to 10 carbon atoms. It may for instance be a C2-8 alkane, a C2-6 alkane, a C2-5 alkane, a C2-4 alkane, or a C2-3 alkane. It is often a C2-4 alkene or a C2-3 alkene in the present invention. Examples of smaller alkenes, e.g. of C2-12 alkenes are ethene (i.e. ethylene), propene (i.e. propylene), butene, pentene, methylbutene, hexene, methylpentene, dimethylbutene, heptene, methylhexene, dimethylpentene, octene, methylheptene, nonene, decene, undecene and dodecene. Alkenes typically comprise one or two double bonds. The terms “alkene” and “olefin” may be used interchangeably. The one or more double bonds may be at any position in the hydrocarbon chain. The alkenes may be cis- or trans-alkenes (or as defined using E- and Z- nomenclature). An alkene comprising a terminal double bond may be referred to as an “alk-l-ene” (e.g. hex-l-ene), a “terminal alkene” (or a “terminal olefin”), or an “alpha-alkene” (or an “alpha-olefin”). The term “alkene”, as used herein also often includes cycloalkenes.The term “cycloalkene”, as used herein, refers to partially unsaturated cyclic hydrocarbon compound. A “Cn-mcycloalkene” refers to a cycloalkene having from n to m carbon atoms. A cycloalkene may for instance be a C3-20 cycloalkene a C3-10 cycloalkene, a C3-8 cycloalkene or a C3-4 cycloalkene. Examples of a C3-8 cycloalkene include cyclopropene, cyclobutene, cyclopentene, cyclohexene, cyclohexa-l,3-diene, methylcyclopentene, cycloheptene, methylcyclohexene, dimethylcyclopentene and cyclooctene. A cycloalkene may comprise one or two double bonds. The term “aromatic compound”, “aromatic hydrocarbon” or “aromatic hydrocarbon compound”, as used herein, refers to a hydrocarbon compound comprising one or more aromatic rings. The aromatic rings may be monocyclic or polycyclic. Typically, an aromatic compound comprises a benzene ring. An aromatic compound may for instance be a Ce-i4 aromatic compound, a Ce-i2 aromatic compound or a Ce-io aromatic compound. Examples of Ce-i4 aromatic compounds are benzene, toluene, xylene, ethylbenzene, methylethylbenzene, diethylbenzene, naphthalene, methylnaphthalene, ethylnaphthalene and anthracene. The terms “aromatic compounds”, “aromatics” and “arenes” may be used interchangeably.The term “alkyl”, as used herein, refers to a linear or branched chain saturated hydrocarbon radical. A “Cn-malkyl” refers to an alkyl having from n to m carbon atoms. Thus, an alkyl group may be a Ci-400 alkyl group, a Ci-360 alkyl group, a Ci-iso alkyl group, a C1-50 alkyl group, a Ce-4oo alkyl group, a Ce-360 alkyl group, a Ce-iso alkyl group, a Ce-so alkyl group, a C1-20 alkyl group, a CM S alkyl group, a Ci-14 alkyl group, a C1-10alkyl group, a C1-6 alkyl group or a C1.4 alkyl group. Examples of a Ci-10 alkyl group are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl.Examples of C1-6 alkyl groups are methyl, ethyl, propyl, butyl, pentyl or hexyl. Examples of C1.4 alkyl groups are methyl, ethyl, i-propyl, n-propyl, t-butyl, s-butyl or n-butyl. If the term “alkyl” is used without a prefix specifying the number of carbons anywhere herein, it has from 1 to 6 carbons.An alkyl as defined herein - for instance a Cn-4oo alkyl, a Cn-3eo alkyl, a Cn-iso alkyl or a Cn-50 alkyl - may be a polymer, i.e. the alkyl may have at least 3 repeating units. Such an alkyl may, for example, comprise the following formula QI:wherein RQ1is, for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2- CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100. The number of repeating units, t, may for instance be an integer from 3 to 90. Typically, such an alkyl comprises the repeating units of formula QI and a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2 or -C(CH3)3 group (typically a terminal -CH3 group or -C(CH3)3 group). In some instances, the repeating units of formula QI are bonded to a -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2- group (in other words the alkyl may further comprise such a -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2- group) at the end of the formula QI that is opposite to the terminal group. Such an alkyl group which is a polymer may comprise, or may be, poly(isobutylene).The term “alkenyl”, as used herein, refers to a linear or branched chain hydrocarbon radical containing one or more double bonds. A “Cn-malkenyl” refers to an alkenyl having from n to m carbon atoms. Thus, an alkenyl group may be a C2-400 alkenyl group, a C2-360 alkenyl group, a C2-180 alkenyl group, a C2-50 alkenyl group, a C6-400alkenyl group, a Ce-36o alkenyl group, a Ce-iso alkenyl group, a Ce-so alkenyl group, a C2-20 alkenyl group, a C2-18 alkenyl group, a C2-14 alkenyl group, a C2-10 alkenyl group, a C2-6 alkenyl group or a C2-4 alkenyl group. Examples of a C2-10 alkenyl group are ethenyl (vinyl), propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl.Examples of C2-6 alkenyl groups are ethenyl, propenyl, butenyl, pentenyl or hexenyl. Examples of C2-4 alkenyl groups are ethenyl, i-propenyl, n-propenyl, s-butenyl or n-butenyl. Alkenyl groups typically comprise one or two double bonds.An alkenyl as defined herein - for instance a Cn-4oo alkenyl, a Cn-3eo alkenyl, a Cn-iso alkenyl or a Cn-50 alkenyl - may comprise a polymer, i.e. the alkenyl may have at least 3 repeating units.Such an alkenyl may, for instance, comprise at least one unsaturated unit of formula -CH=CH-, -CH=C(CH3)-CH2-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2- (for instance, an unsaturated unit of formula -CH=C(CH3)-CH2-). In addition to the at least one unsaturated unit, such an alkenyl may, for instance, comprise one or more saturated alkylene units, for instance one or more saturated units of formula -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-.Such an alkenyl may, for example, comprise an unsaturated unit of formula -CH=CH-, -CH=C(CH3)-CH2-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2- (for instance, an unsaturated unit of formula -CH=C(CH3)-CH2-) which is in turn bonded to a polymer of formula QI:wherein RQ1is, for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100. The polymer of formula (QI) is typically in turn bonded to a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2 or -C(CH3)3group (typically a terminal -CH3group or -C(CH3)3group).Alternatively, such an alkenyl may, for example, comprise a polymer of the following formula QII:wherein the number of repeating units, t, is an integer from 3 to 100 and wherein each RQ2is an unsaturated repeating unit, for instance, -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2-, or wherein each RQ2is independently selected from an unsaturated repeating unit (for instance, -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2-) and a saturated repeating unit (for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-), provided that at least one RQ2is an unsaturated repeating unit. The number of repeating units, t, may for instance be an integer from 3 to 90. Typically such an alkenyl comprises the repeating units of formula QII and a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2 or -C(CH3)3group (typically a terminal -CH3group or -C(CH3)3group).The term “alkynyl”, as used herein, refers to a linear or branched chain hydrocarbon radical containing one or more triple bonds. A “Cn-malkynyl” refers to an alkynyl having from n to m carbon atoms. Thus, an alkynyl group may be a C2-18 alkynyl group, a C2-14 alkynyl group, a C2-10 alkynyl group, a C2-6 alkynyl group or a C2-4 alkynyl group. Examples of a C2-10 alkynyl group are ethynyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, octynyl, nonynyl or decynyl. Examples of C1-6 alkynyl groups are ethynyl, propynyl, butynyl, pentynyl or hexynyl. Alkynyl groups typically comprise one or two triple bonds.A C3-20 heterocyclyl group is a monovalent moiety obtained by removing a hydrogen atom from a ring atom of a heterocyclic compound, which moiety has from 3 to 20 ring atoms (unless otherwise specified), of which from 1 to 10 are ring heteroatoms. A “Cn-mheterocyclyl” refers to a heterocyclyl having from n to m ring atoms. Preferably, each ring has from 3 to 7 ring atoms (i.e. it is a C3-7 heterocyclyl), of which from 1 to 4 are ring heteroatoms.Examples of 5- and 6- membered saturated heterocyclyl groups include piperazine, piperidine, morpholine, 1,3-oxazinane, pyrrolidine, imidazolidine, and oxazolidine, including quatemised derivatives thereof, as defined herein. Examples of 5- and 6- membered partially saturated heterocyclyl groups include tetrahydropyrazine, tetrahydropyridine, dihydro- 1,4-oxazine, tetrahydropyrimidine, dihydro-1, 3-oxazine, dihydropyrrole, dihydroimidazole and dihydrooxazole, including quatemised derivatives thereof, as defined herein.Examples of 9- and 10- membered fused heterobicyclyl groups include 9-membered fused heterobicyclic groups such as indoline, 2,3-dihydrobenzofuran, 2,3 -dihydrobenzo [b]thiophene, 2,3-dihydro-lH-benzo[d]imidazole, 2,3-dihydrobenzo[d]oxazole, 2,3-dihydrobenzo[d]thiazole, benzo[d][l,3]dioxole, 4,5,6,7-tetrahydrothiazolo[5,4-c]pyridine and 4,5,6,7-tetrahydrothiazolo[4,5-c]pyridine, including quatemised derivatives thereof, as defined herein; and 10-membered heterobicyclyl groups such as 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroisoquinoline, chromane, isochromane, thiochromane, isothiochromane, 1,2,3,4-tetrahydroquinoxaline, 1, 2,3,4-tetrahydroquinazoline, l,4-dihydro-2H-benzo[d][l,3]oxazine, 3,4-dihydro-2H-benzo[b][l,4]oxazine, 3,4-dihydro-2H-benzo[b] [l,4]thiazine, l,4-dihydro-2H-benzo[d] [l,3]thiazine, 4H-benzo[d][l,3]dioxine and 2,3-dihydrobenzo[b][l,4]dioxine, including quatemised derivatives thereof. Preferably, the fused heterobicyclyl group comprises 1, 2 or 3, preferably 1 or 2 nitrogen atoms.For the avoidance of doubt, references to a heterocyclyl group also include fused polycyclic ring systems, including for instance fused bicyclic systems in which a heterocyclic group is fused to an aryl group. When the heterocyclyl group is such a fused heterocyclyl group, preferred examples are fused ring systems wherein a 5- to 6-membered heterocyclyl group is fused to a phenyl group. References to a heterocyclyl group also include spiro ring systems, for example 7-membered heterocyclic groups e.g. 2,6-diazaspiro[3.3]heptane.The term “aryl”, as used herein, refers to a monocyclic, bicyclic or polycyclic aromatic ring which contains up to 14 carbon atoms, typically from 6 to 10 carbon atoms, in the ring portion.Examples include phenyl, naphthyl, indenyl and indanyl groups. The term “aryl group”, as used herein, includes heteroaryl groups. The term “heteroaryl”, as used herein, refers to monocyclic or bicyclic heteroaromatic rings which typically contains from six to ten atoms in the ring portion including one or more heteroatoms. A heteroaryl group is generally a 5- or 6-membered ring, containing at least one heteroatom selected from O, S, N, P, Se and Si. It may contain, for example, one, two or three heteroatoms. Examples of heteroaryl groups include pyridyl, pyrazinyl,pyrimidinyl, pyridazinyl, furanyl, thienyl, pyrazolidinyl, pyrrolyl, oxazolyl, oxadiazolyl, isoxazolyl, thiadiazolyl, thiazolyl, isothiazolyl, imidazolyl, pyrazolyl, quinolyl and isoquinolyl.The terms “hydrocarbylene”, “alkylene”, “cycloalkylene”, “alkenylene”, “alkynylene”, and “arylene”, as used herein, refer to bivalent groups obtained by removing a hydrogen atom from a hydrocarbyl, alkyl, cycloalkyl, alkenyl, alkynyl, or aryl group, respectively. Such bidentate groups may be substituted or unsubstituted. An alkylene group may be a C1-20 alkylene group, a CMS alkylene group, a Ci-14 alkylene group, a Ci-w alkylene group, a C1-6 alkylene group or a C1.4 alkylene group. Examples of C1-6 alkylene groups are methylene, ethylene, propylene, butylene, pentylene and hexylene. A cycloalkylene group may be a C3-10 cycloalkylene group, a C3-8 cycloalkylene group or a C3-6 cycloalkylene group. Examples of C3-6 cycloalkylene groups include cyclopentylene and cyclohexylene. An alkenylene group may be a C2-18 alkenylene group, a C2-14 alkenylene group, a C2-10 alkenylene group, a C2-6 alkenylene group or a C2-4 alkenylene group. Examples of a C2-4 alkenylene group include ethenylene (vinylene), propenylene and butenylene. An alkynylene group may be a C2-18 alkynylene group, a C2-14 alkynylene group, a C2-10 alkynylene group, a C2-6 alkynylene group or a C2-4 alkynylene group. Examples of a C2-4 alkynylene group include ethynylene and propynylene. Examples of arylene groups include phenylene and encompass heteroarylene groups such as, for instance, a diradical derived from thiophene, a diradical derived from chromane, and a diradical derived from chromanol. For alkylene, cycloalkylene, alkenylene, alkynylene, and arylene, these groups may be bonded to other groups at any two positions on the group. Thus, propylene includes -CH2CH2CH2- and -CFECF^CFE)-, and phenylene includes ortho-, meta- and paraphenylene.The term “substituted”, as used herein, in the context of substituted organic compounds and groups, refers to an organic compound or group, e.g. an alkane, an alkyl group, an alkylene group, an alkyl group, an alkoxy group or an aryloxy group, which bears one or more substituents selected from an alkyl group as defined herein (for instance Ci-400 alkyl, or C O alkyl), an alkenyl group as defined herein (for instance C2-400 alkenyl, or C2-10 alkenyl), C3-10 cycloalkyl, C3-7 heterocyclyl, aryl, heteroaryl, cyano, amino, nitro, C1-10alkylamino, di(Ci-io)alkylamino, arylamino, diarylamino, aryl(Ci-io)alkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, C1-10alkoxy, aryloxy, halo(Ci-io)alkyl, sulfonic acid, thiol, C O alkylthio, arylthio, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid, phosphonate ester and SO,. Typically, the one or more substituents are selected from cyano, amino, nitro, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, sulfonic acid, thiol, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid, phosphonate ester and SO,. When a compound or group is substituted, it typically bears 1, 2, 3 or 4 substituents. For instance, a substituted compound or group may have 1, 2 or 3 substituents, or for example 1 or 2 substituents, for instance 1 substituent.However, when a group is halo -substituted, for instance fluoro-substituted, the group may bear 1, 2, 3 or 4 halo substituents, or it may bear more than four halo substituents. In fact, the groupmay be perhalo-substituted, i.e. all hydrogen atoms of the group may be replaced by halogen atoms. The group may for instance be perfluoro-substituted, i.e. perfluorinated, i.e. all hydrogen atoms of the group may be replaced by fluorine atoms. Accordingly, the term “substituted”, as used herein, in the context of substituted organic groups, for instance in the context of substituted hydrocarbyl groups, substituted alkyl groups, substituted cycloalkyl groups, substituted alkenyl groups, substituted alkynyl groups, substituted aryl groups, substituted hydrocarbylene groups, substituted alkylene groups, substituted cycloalkylene groups, substituted alkenylene groups, substituted alkynylene groups, and substituted arylene (including substituted heteroarylene) groups, encompasses the perhalo-substituted groups, in particular the perfluoro-substituted groups. Thus, for example, the term “substituted Cn-malkyl” as used herein encompasses Cn-mperfluoroalkyl, the term “substituted Cn-malkylene” as used herein encompasses Cn-mperfluoroalkylene, the term “substituted Cn-mhydrocarbyl” as used herein encompasses Cn-mperfluorohydrocarbyl and the term “substituted Cn-mhydrocarbylene” as used herein encompasses Cn-mperfluorohydrocarbylene, the term “substituted Cn-malkoxy” as used herein encompasses Cn-mperfluoroalkoxy, and so-on.As used herein the term oxo represents a group of formula: =0As used herein the term acyl represents a group of formula: -C(=0)R, wherein R is an acyl substituent, for example, a substituted or unsubstituted C1-20 alkyl group, a substituted or unsubstituted C3-20 heterocyclyl group, or a substituted or unsubstituted aryl group. Examples of acyl groups include, but are not limited to, -C(=0)CH3 (acetyl), -C(=O)CH2CH3 (propionyl), -C(=O)C(CH3)3 (t-butyryl), and -C(=O)Ph (benzoyl, phenone).As used herein the term ester (or carboxylate, carboxylic acid ester or oxycarbonyl) represents a group of formula: -C(=0)0R, wherein R is an ester substituent, for example, a substituted or unsubstituted C1-20 alkyl group, a substituted or unsubstituted C3-20 heterocyclyl group, or a substituted or unsubstituted aryl group (typically a phenyl group). Examples of ester groups include, but are not limited to, -C(=0)0CH3, -C(=O)OCH2CH3, -C(=O)OC(CH3)3, and -C(=O)OPh.As used herein the term acyloxy (or reverse ester) represents a group of formula: -0C(=0)R, wherein R is an acyloxy substituent, for example, substituted or unsubstituted C1-20 alkyl group, a substituted or unsubstituted C3-20 heterocyclyl group, or a substituted or unsubstituted aryl group, typically a C1-6 alkyl group. Examples of acyloxy groups include, but are not limited to, -0C(=0)CH3 (acetoxy), -OC(=O)CH2CH3, -OC(=O)C(CH3)3, -OC(=O)Ph, and -OC(=O)CH2Ph.As used herein the term phosphonic acid represents a group of the formula: -P(=0)(0H)2. As would be understood by the skilled person, a phosphonic acid group can exist in protonated and deprotonated forms (i.e. -P(=0)(0H)2, -P(=0)(0 )2 and -P(=0)(0H)(0 )) all of which are within the scope of the term “phosphonic acid”.As used herein the term phosphonic acid salt represents a group which is a salt of a phosphonic acid group. For example a phosphonic acid salt may be a group of the formula -P(=O)(OH)(O X+) wherein X is a monovalent cation. X+may be an alkali metal cation. X+may be Na+or K+, for example.As used herein the term phosphonate ester represents a group of one of the formulae:-P(=O)(OR)2 and -P(=O)(OR)O wherein each Ris independently a phosphonate ester substituent, for example, -H, substituted or unsubstituted C1-20 alkyl, substituted or unsubstituted C3-20 heterocyclyl, C3-20 heterocyclyl substituted with a further C3-20 heterocyclyl, substituted or unsubstituted C1-20 alkylene -C3-20 heterocyclyl, substituted or unsubstituted C3-25 cycloalkyl, substituted or unsubstituted C1-20 alkylene-C3-25 cycloalkyl, aryl, substituted or unsubstituted C1-20 alkylene-aryl. Examples of phosphonate ester groups include, but are not limitedto, -P(=O)(OCH3)2, -P(=O)(OCH2CH3)2, -P(=O)(O-t-Bu)2, and -P(=O)(OPh)2,As used herein the term phosphoric acid represents a group of the formula:-OP(=O)(OH)2.As used herein the term phosphate ester represents a group of one of the formulae:-0P(=0)(0R)2 and -OP(=O)(OR)O wherein each R is independently a phosphate ester substituent, for example, -H, substituted or unsubstituted C1-20 alkyl, substituted or unsubstituted C3-20 heterocyclyl, C3-20 heterocyclyl substituted with a further C3-20 heterocyclyl, substituted or unsubstituted C1.20alkylene-C3.20 heterocyclyl, substituted or unsubstituted C3-25 cycloalkyl, substituted or unsubstituted C1.20alkylene-C3.25 cycloalkyl, aryl, substituted or unsubstituted C1-20 alkylene-aryl. Examples of phosphate ester groups include, but are not limitedto, -OP(=O)(OCH3)2, -OP(=O)(OCH2CH3)2, -OP(=O)(O-t-Bu)2, and -OP(=O)(OPh)2.As used herein the term amino represents a group of formula -NH2. The term C1-C10 alkylamino represents a group of formula -NHR' wherein R' is a C O alkyl group, preferably a C1-6 alkyl group, as defined previously. The term di(Ci-io)alkylamino represents a group offormula -NR'R” wherein R' and R” are the same or different and represent C1-10alkyl groups, preferably C1-6alkyl groups, as defined previously. The term arylamino represents a group of formula -NHR' wherein R' is an aryl group, preferably a phenyl group, as defined previously. The term diarylamino represents a group of formula -NR'R' ' wherein R' and R' ' are the same or different and represent aryl groups, preferably phenyl groups, as defined previously. The term arylalkylamino represents a group of formula -NR'R' ' wherein R' is a C O alkyl group, preferably a C1-6 alkyl group, and R” is an aryl group, preferably a phenyl group.As used herein the term amido represents a group of formula: -C(=O)NR R ”, wherein R and R are independently amino substituents, as defined for di(Ci-io)alkylamino groups. Examples of amido groups include, but are not limitedto, -C(=O)NH2, -C(=O)NHCH3, -C(=O)N(CH3)2, -C(=O)NHCH2CH3, and -C(=O)N(CH2CH3)2, as well as amido groups in which R and R ”, together with the nitrogen atom to which they are attached, form a heterocyclic structure as in, for example, piperidinocarbonyl, morpholinocarbonyl, thiomorpholinocarbonyl, and piperazinocarbonyl.As used herein the term acylamido represents a group of formula: -NR1C(=O)R2, wherein R1is an amide substituent, for example, hydrogen, a Ci-2oalkyl group, a C3-20 heterocyclyl group, an aryl group, preferably hydrogen or a C1-20 alkyl group, and R2is an acyl substituent, for example, a C1-20 alkyl group, a C3-20 heterocyclyl group, or an aryl group, preferably hydrogen or a C1-20 alkyl group. Examples of acylamide groups include, but are not limited to, -NHC(=0)CH3, -NHC(=O)CH2CH3, -NHC(=O)Ph, -NHC(=O)CI5H3I and -NHC(=O)C9HI9. Thus, a substituted C120 alkyl group may comprise an acylamido substituent defined by the formula -NHC(=0)-CI-2O alkyl, such as -NHC(=O)CI5H3I or -NHC(=O)C9HI9. R1and R2may together form a cyclic structure, as in, for example, succinimidyl, maleimidyl, and phthalimidyl:succinimidyl maleimidyl phthalimidylA Ci-10 alkylthio group is a said C1-10 alkyl group, preferably a C1-6 alkyl group, attached to a thio group. An arylthio group is an aryl group, preferably a phenyl group, attached to a thio group.A C1-20 alkoxy group is a said substituted or unsubstituted C1-20 alkyl group attached to an oxygen atom. A C1-6 alkoxy group is a said substituted or unsubstituted C1-6 alkyl group attached to an oxygen atom. A C1.4 alkoxy group is a substituted or unsubstituted C1.4 alkyl group attached to an oxygen atom. A substituted C1-20 alkoxy group includes a C 1.20 perfluoroalkoxy group. A C1-20 perfluoroalkoxy group is a C1-20 perfluoroalkyl group attached to an oxygen atom. An example of a C1-20 perfluoroalkoxy group is a tert-nonafluorobutyloxy group, -OC(CF3)3.An aryloxy group is a substituted or unsubstituted aryl group, as defined herein, attached to an oxygen atom. It may for instance be unsubstituted or substituted phenoxy. An example of an aryloxy group is -OPh (phenoxy).Unless otherwise specified, included in the above are the well known ionic, salt, solvate, and protected forms of these substituents. For example, a reference to carboxylic acid or carboxyl group (-COOH) also includes the anionic (carboxylate) form (-COO ), a salt or solvate thereof, as well as conventional protected forms. Similarly, a reference to an amino group includes the protonated form (-N HR1R2). a salt or solvate of the amino group, for example, a hydrochloride salt, as well as conventional protected forms of an amino group. Similarly, a reference to a hydroxyl group also includes the anionic form (-0 ), a salt or solvate thereof, as well as conventional protected forms.Standard ambient temperature and pressure, abbreviated herein to SATP, refers to a temperature of 298.15 K (25 °C) and a pressure of 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm).Parts per NotationAll quantities specified herein using the parts-per notation, for instance parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt), are mass fractions unless otherwise specified. The terms parts per million by mass, parts per billion by mass and parts per trillion by mass may be used to signify that the fraction is a mass fraction, instead of the terms parts per million, parts per billion and parts per trillion, respectively. Similarly, the notations ppmw, ppbw and pptw may be used instead of ppm, ppb and ppt, respectively, to signify that the fraction is a mass fraction. For the avoidance of doubt, one part per million (ppm) denotes one part per 1,000,000 (106) parts. One part per billion (ppb) denotes one part per 1,000,000,000 (109) parts. One part per trillion (ppt) denotes one part per 1,000,000,000,000 (1012) parts. Each part is a unit of mass unless otherwise specified. ChemiluminescenceChemiluminescence is the emission of light as a result of a chemical reaction. The marker compounds employed in the present invention are chemiluminescent precursor compounds, also referred to herein as chemiluminescent precursors. A chemiluminescent precursor compound (or “chemiluminescent precursor”) is a compound capable of emitting light when it undergoes a chemical reaction. Typically, a chemiluminescent precursor reacts to form a product in an electronically excited state, and the product emits electromagnetic radiation (light) upon returning to its ground state. The light emitted is often in the visible spectrum, although it can, in principle, be in the ultraviolet, visible or infrared region of the electromagnetic spectrum.Generally, the light produced upon reaction of a chemiluminescent precursor can either occur over a very short time period (for instance less than 5 seconds) or over extended periods (e.g. minutes). Chemiluminescent precursors that emit light over short periods - which may be referred to as “flashers” - are preferred for use as the marker compound in the present invention.Compounds of the inventionIn one aspect the invention relates to a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted Ci -e alkyl.Typically when X is phenyl X is substituted with at least one group other than halo.Often when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(O)OCH3.Often (a) when X is phenyl X is substituted with at least one group other than halo; and / or (b) when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(O)OCH3.Therefore, often when X is phenyl, X is substituted with at least one group selected from unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Often when X is phenyl, X is substituted with at least one group selected from unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Often, when R1is methyl, X is aryl, and R2to R9are all H:(a) X has at least three substituents;(b) X has at least two different substituents;(c) X has at least two substituents selected from halo, substituted C1-400alkyl, unsubstituted C2-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) X has at least one substituent selected from halo, substituted Ci-400 alkyl, unsubstituted C2-400 alkyl; unsubstituted or substituted C2-400 alkenyl, CN, C(O)ORX, C(O)NRy2, and ORZ, wherein Rxis H,unsubstituted or substituted C3-10 heterocyclyl, substituted Ci-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Often, when R1is methyl, X is aryl, and R2to R9are all H:(a) X has at least three substituents;(b) X has at least two different substituents;(c) X has at least two substituents selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) X has at least one substituent selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl; unsubstituted or substituted C2-10 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Often when X is phenyl, X is substituted with at least one group selected from unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andwhen R1is methyl, X is aryl, and R2to R9are all H:(a) X has at least three substituents;(b) X has at least two different substituents;(c) X has at least two substituents selected from halo, substituted C1-400alkyl, unsubstituted C2-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) X has at least one substituent selected from halo, substituted Ci-400 alkyl, unsubstitutedC2-400 alkyl; unsubstituted or substituted C2-400 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl andunsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted Ci-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Often when X is phenyl, X is substituted with at least one group selected from unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andwhen R1is methyl, X is aryl, and R2to R9are all H:(a) X has at least three substituents;(b) X has at least two different substituents;(c) X has at least two substituents selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) X has at least one substituent selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl; unsubstituted or substituted C2-10 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically, X is substituted with at least one group selected from unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl.Typically, X is substituted with at least one group selected from unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl.Typically, X has at least three substituents.Typically, X has at least two different substituents.Typically, X has at least two substituents selected from halo, substituted Ci-400 alkyl, unsubstituted C2-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl.Typically, X has at least two substituents selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl. Typically, X has at least one substituent selected from halo, substituted C1-400alkyl, unsubstituted C2-400 alkyl; unsubstituted or substituted C2-400 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl; Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and Rzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.X is typically substituted aryl or substituted C2-6 alkyl. Often X is a substituted 6-membered aryl (e.g. phenyl) or substituted C2-6 alkyl.Typically, X has at least one substituent selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl; unsubstituted or substituted C2-10 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl; Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and Rzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.X is typically substituted aryl or substituted C2-6 alkyl.Often X is a substituted 6-membered aryl (e.g. phenyl) or substituted C2-6 alkyl.The X group in the acridans of formula (I) - particularly when X is aryl, for instance phenyl -may be substituted with a longer-chain alkyl or alkenyl group, which may for instance be a fatty moiety (such as dodecyl or octadecyl) or a polymeric alkyl or alkenyl moiety.Thus, X may be aryl, for instance phenyl, substituted with at least one, for instance one, two or three groups independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl. Thus, X may be aryl, for instance phenyl, substituted with at least one, for instance one, two, three, four or five, and preferably one, two or three, groups independently selected from halo, unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl, provided that X is substituted with at least one, for instance one, two or three groups selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl.X may be aryl, for instance phenyl, substituted with at least one group, for instance one, two or three groups, or for instance one or two groups, independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl. Optionally, X may be further substituted with at least one group, for instance one, two, three or four groups, for instance one, two orthree groups, independently selected from halo, unsubstituted or substituted C1-3 alkyl, unsubstituted or substituted C2-5 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl.When X is substituted with at least one, for instance one, two or three groups independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, said unsubstituted or substituted C4-400 alkyl may be unsubstituted or substituted C4-30 alkyl. Often, it is unsubstituted or substituted C5-18 alkyl, C8-13alkyl or C10-12 alkyl, for instance unsubstituted C5-18 alkyl, C8-13alkyl or C10-12 alkyl. Preferably, it is unsubstituted or substituted C12 alkyl, for instance, unsubstituted C12 alkyl. Alternatively, it is unsubstituted or substituted C10-30 alkyl, C15-25 alkyl or C17-19 alkyl, for instance unsubstituted or substituted C10-30 alkyl, C15-25 alkyl or C17-19 alkyl. Preferably, it is unsubstituted or substituted C18alkyl, for instance, unsubstituted C18alkyl. Alternatively, it is unsubstituted or substituted C40-400 alkyl, C48-250 alkyl, or C60-200alkyl, for instance unsubstituted C40-400 alkyl, C48-250 alkyl, or C60-200alkyl. The C40-400 alkyl typically comprises at least three repeat units, for instance at least three repeat units of formula -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, wherein the number of repeat units may be from 10 to 100, for instance from 10 to 90, or from 10 to 80, and is often from 10 to 50 such as from 20 to 50, from 30 to 50, from 35 to 45, most preferably around 39; or from 10 to 20, such as from 12 to 18, most preferably around 16. Often the number of repeat units is from 10 to 80, and is often from 10 to 50, such as 20 to 50, from 30 to 48, from 34 to 44, most preferably around 39; or from 11 to 21, such as from 13 to 19, most preferably around 16. The C40-400 alkyl may also comprise a terminal -CH3 or -C(CH₃)₃ group, and may also be attached to the aryl, for instance phenyl, via a group such as CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-.Often, when X is substituted with at least one, for instance one, two or three groups independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, said unsubstituted or substituted C4-400 alkyl may be a C6-400alkyl comprising the following formula QI:wherein RQ1is, for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100. Typically the C6-400alkyl comprises said repeating units of formula QI and a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2 or -C(CH3)3 group (typically a terminal -CH3 group or -C(CH3)3 group). In some instances, the C6-400alkyl further comprises a -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2- group, which is bonded to the end of the formula QI that is opposite to the terminal group.The number of repeating units, t, is typically an integer from 10 to 50, such as from 20 to 50, from 30 to 50, from 35 to 45, most preferably around 39; or from 10 to 20, such as from 12 to 18, most preferably around 16. Often the number of repeating units (t) is from 10 to 80, and is often from 10 to 50, such as 20 to 50, from 30 to 48, from 34 to 44, most preferably around 39; or from 11 to 21, such as from 13 to 19, most preferably around 16. Most preferably each RQ1is isobutylene, such that X is substituted with poly(isobutylene). Thus, typically the C6-400alkyl group comprises, or is,poly(isobutylene).When X is substituted with at least one, for instance one, two or three groups independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, said C6-400alkenyl may be a C6-400alkenyl comprising an unsaturated unit of formula -CH=CH-, -CH=C(CH3)-CH2-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, - CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2- (for instance, an unsaturated unit of formula -CH=C(CH3)-CH2-) which is in turn bonded to a polymer of formula QI:wherein RQ1is, for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100. The polymer of formula (QI) is typically in turn bonded to a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2or -C(CH3)3group (typically a terminal -CH3group or -C(CH3)3group).Alternatively, said C6-400alkenyl may, for example, comprise a polymer of the following formulawherein the number of repeating units, t, is an integer from 3 to 100 and wherein each RQ2is an unsaturated repeating unit, for instance, -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2-, or wherein each RQ2is independently selected from an unsaturated repeating unit (for instance, -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, - C(CH3)=C(CH3)- or -CH2-CH=CH-CH2-) and a saturated repeating unit (for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-), provided that at least one RQ2is an unsaturated repeating unit. The number of repeating units, t, may for instance be an integer from 3 to 90. Typically, such an alkenyl comprises the repeating units of formula QII and aterminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH₃, -CH(CH3)2 or -C(CH3)3group (typically a terminal -CH3group or -C(CH3)3group).Often, X is aryl, for instance phenyl, substituted with at least one, for instance one, two, three, four or five, and preferably one, two or three, groups independently selected from halo, unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, provided that X is substituted with at least one, for instance one, two or three groups selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl (which may be as further defined above).Often, X is substituted with at least one, for instance one, two or three groups selected from unsubstituted or substituted C8-400alkyl and unsubstituted or substituted C8-400alkenyl (which may be as further defined above) and X is optionally further substituted with at least one, for instance one, two, three, or four, and preferably one or two groups independently selected from halo (for instance, chloro) and unsubstituted or substituted C1-7 alkyl, preferably unsubstituted C1-7 alkyl.Often, X is substituted with at least one, for instance one, two or three groups selected from unsubstituted or substituted C8-400alkyl and unsubstituted or substituted C8-400alkenyl (which may be as further defined above) and X is optionally further substituted with at least one, for instance one, two, three, or four, and preferably one or two groups independently selected from halo (for instance, chloro) and unsubstituted C1.4 alkyl, for instance, methyl or isopropyl.Alternatively, X is substituted with at least one, for instance one, two or three groups selected from unsubstituted or substituted C8-400alkyl and unsubstituted or substituted C8-400alkenyl (which may be as further defined above) and X is not further substituted (i.e. it is otherwise unsubstituted).The present inventors have found that compounds of the present invention can be directly dissolved into low polarity liquids (e.g. fuels, alkylbenzenes or alkanes), and that carrier solvents are not essential to mark a product with such a compound. The solubility of these compounds provide stable solutions at ppb concentrations, and is not subject to the limitations of micelle formulations (i.e. collapse and potential precipitation when diluted below critical micelle concentration). The compounds of the invention also display excellent chemical stability in the presence of alcohols and amines. This is especially useful, as amines and alcohols are typical components of fuel additives.Sometimes X is substituted 6-membered aryl, such that the compound is an acridan of formula (la):R1to R9may be as defined herein for formula (I).For compounds of Formula (la), Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl.Typically, this is provided that at least one of Ra, Rb, Rc, Rdand Reis other than H and other than halo.Often this is provided that when R1is methyl and R2to R9are all H, at least three of Ra, Rb, Rc, Rdand Reare other than H, at least two of Ra, Rb, Rc, Rdand Reare other than H and are different, at least two of Ra, Rb, Rc, Rdand Reare other than H and methyl, and / or at least one of Ra, Rb, Rc, Rdand Reis other than H, -OH, methyl, -OCH3and -C(O)OCH3.Typically, this is provided that (a) at least one of Ra, Rb, Rc, Rdand Reis other than H and other than halo; and(b) when R1is methyl and R2to R9are all H, at least three of Ra, Rb, Rc, Rdand Reare other than H, at least two of Ra, Rb, Rc, Rdand Reare other than H and are different, at least two of Ra, Rb, Rc, Rdand Reare other than H and methyl, and / or at least one of Ra, Rb, Rc, Rdand Reis other than H, -OH, methyl, -OCH3 and -C(0)0CH3.Typically, for compounds of Formula (la), Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, this is provided that at least one of Ra, Rb, Rc, Rdand Reis other than H and other than halo.Often this is provided that when R1is methyl and R2to R9are all H, at least three of Ra, Rb, Rc, Rdand Reare other than H, at least two of Ra, Rb, Rc, Rdand Reare other than H and are different, at least two of Ra, Rb, Rc, Rdand Reare other than H and methyl, and / or at least one of Ra, Rb, Rc, Rdand Reis other than H, -OH, methyl, -OCH3and -C(O)OCH3.Typically, this is provided that (a) at least one of Ra, Rb, Rc, Rdand Reis other than H and other than halo; and(b) when R1is methyl and R2to R9are all H, at least three of Ra, Rb, Rc, Rdand Reare other than H, at least two of Ra, Rb, Rc, Rdand Reare other than H and are different, at least two of Ra, Rb, Rc, Rdand Reare other than H and methyl, and / or at least one of Ra, Rb, Rc, Rdand Reis other than H, -OH, methyl, -OCH3 and -C(0)0CH3.Therefore, often at least one of Ra, Rb, Rc, Rdand Reis selected from unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl.Often at least one of Ra, Rb, Rc, Rdand Reis selected from unsubstituted or substituted C1-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, when R1is methyl and R2to R9are all H(a) at least three of Ra, Rb, Rc, Rdand Reare selected from halo, unsubstituted or substituted Ci- 400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;(b) at least two of Ra, Rb, Rc, Rdand Reare different groups selected from halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3- 10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;(c) at least two of Ra, Rb, Rc, Rdand Reare selected from halo, substituted Ci-400 alkyl;unsubstituted or substituted C2-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) at least one of Ra, Rb, Rc, Rdand Reare selected from halo, substituted Ci- 400 alkyl, unsubstituted C2-400 alkyl; unsubstituted or substituted C2-400 alkenyl, CN, C(O)ORX, C(O)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyland unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted Ci-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically, when R1is methyl and R2to R9are all H(a) at least three of Ra, Rb, Rc, Rdand Reare selected from halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;(b) at least two of Ra, Rb, Rc, Rdand Reare different groups selected from halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;(c) at least two of Ra, Rb, Rc, Rdand Reare selected from halo, substituted C1-10alkyl;unsubstituted or substituted C2-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) at least one of Ra, Rb, Rc, Rdand Reare selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl; unsubstituted or substituted C2-10 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically, at least three of Ra, Rb, Rc, Rdand Reare selected from halo, unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, at least three of Ra, Rb, Rc, Rdand Reare selected from halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, at least two of Ra, Rb, Rc, Rdand Reare different groups selected from halo, unsubstituted or substituted Ci-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR,C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl.Typically, at least two of Ra, Rb, Rc, Rdand Reare different groups selected from halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, at least two of Ra, Rb, Rc, Rdand Reare selected from halo, substituted Ci-400 alkyl; unsubstituted or substituted C2-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, at least two of Ra, Rb, Rc, Rdand Reare selected from halo, substituted C1-10alkyl; unsubstituted or substituted C2-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, at least one of Ra, Rb, Rc, Rdand Reare selected from halo, substituted Ci-400 alkyl, unsubstituted C2-400 alkyl; unsubstituted or substituted C2-400 alkenyl, CN, C(O)ORX, C(O)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl; Ryis H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and Rzis unsubstituted or substituted C3-10heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically, at least one of Ra, Rb, Rc, Rdand Reare selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl; unsubstituted or substituted C2-10 alkenyl, CN, C(O)ORX, C(O)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl; Ryis H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and Rzis unsubstituted or substituted C3-10heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically, for compounds of Formula (la) at least one of Ra, Rb, Rc, Rdand Reis selected from unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andwhen R1is methyl and R2to R9are all H(a) at least three of Ra, Rb, Rc, Rdand Reare selected from halo, unsubstituted or substituted Ci- 400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, whereinR is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl;(b) at least two of Ra, Rb, Rc, Rdand Reare different groups selected from halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3- 10 heterocyclyl, unsubstituted or substituted C alkyl and unsubstituted or substituted aryl;(c) at least two of Ra, Rb, Rc, Rdand Reare selected from halo, substituted Ci-400 alkyl;unsubstituted or substituted C2-400 alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl; and / or(d) at least one of Ra, Rb, Rc, Rdand Reare selected from halo, substituted Ci -400 alkyl, unsubstituted C2-400 alkyl; unsubstituted or substituted C2-400 alkenyl, CN, C(O)ORX, C(0)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically, for compounds of Formula (la) at least one of Ra, Rb, Rc, Rdand Reis selected from unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andwhen R1is methyl and R2to R9are all H(a) at least three of Ra, Rb, Rc, Rdand Reare selected from halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;(b) at least two of Ra, Rb, Rc, Rdand Reare different groups selected from halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;(c) at least two of Ra, Rb, Rc, Rdand Reare selected from halo, substituted C1-10alkyl;unsubstituted or substituted C2-10 alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and / or(d) at least one of Ra, Rb, Rc, Rdand Reare selected from halo, substituted C1-10alkyl, unsubstituted C2-10 alkyl; unsubstituted or substituted C2-10 alkenyl, CN, C(O)ORX, C(O)NRy2, and ORZ, wherein Rxis H, unsubstituted or substituted C3-10 heterocyclyl, substituted C alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl;Ryis H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; andRzis unsubstituted or substituted C3-10 heterocyclyl, substituted C1-6 alkyl, unsubstituted or substituted C2-6 alkyl and unsubstituted or substituted aryl.Typically:(a) X is aryl and X has at least three substituents (optionally wherein the acridan is an acridan of formula la as described herein and at least three of Ra, Rb, Rc, Rdand Reare other than H);(b) X is aryl and X has at least two different substituents (optionally wherein the acridan is an acridan of formula la as described herein and at least two of Ra, Rb, Rc, Rdand Reare other than H and are different from each other);(c) X is aryl and X has at least two substituents other than methyl (optionally wherein the acridan is an acridan of formula la as described herein and at least two of Ra, Rb, Rc, Rdand Reare other than H and other than methyl); and / or(d) X is aryl and X has at least one substituent other than -OH, methyl, -OCH3 and -C(0)0CH3 (optionally wherein the acridan is an acridan of formula la as described herein and at least one of Ra, Rb, Rc, Rdand Reis other than H, -OH, methyl, -OCH3 and -C(0)0CH3);optionally wherein:(e) each of R2, R3, R4, R5, R6, R7, R8and R9are H, or(f) each of R2, R3, R4, R5, R6, R7, R8and R9are H and R1is methyl.Often at least one of Ra, Rb, Rc, Rdand Reis other than H. Typically one, two or three of Ra, Rb, Rc, Rdand Reis other than H. Often two or three of Ra, Rb, Rc, Rdand Reare other than H.Therefore one, two or three of Ra, Rb, Rc, Rdand Remay be independently selected from halo, unsubstituted or substituted C1-400alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl.Often at least one of Ra, Rb, Rc, Rdand Reis other than H. Typically one, two or three of Ra, Rb, Rc, Rdand Reis other than H. Often two or three of Ra, Rb, Rc, Rdand Reare other than H.Therefore one, two or three of Ra, Rb, Rc, Rdand Remay be independently selected from halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl.When one of Ra, Rb, Rc, Rdand Reis other than H, this could be any of Ra, Rb, Rc, Rdand Re. For example Racould be other than H and each of Rb, Rc, Rdand Reis H; Rbis other than H, and each of Ra, Rc, Rdand Reis H; Rcis H and each of Ra, Rb, Rdand Reis H; Rdis H and each of Ra, Rb, Rc, and Reis H; or Reis H is each of Ra, Rb, Rc, and Rdis H.When two of Ra, Rb, Rc, Rdand Reis other than H, this can be any two of Ra, Rb, Rc, Rdand Re. Often Raand Reare other than H, and Rb, Rcand Rdare H.When three of Ra, Rb, Rc, Rdand Reis other than H, this can be any three of Ra, Rb, Rc, Rdand Re. Often, Ra, Rcand Reare other than H, and Rband Rdare H; or Ra, Rdand Reare other than H, and Rband Rcare H.Often one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from halo, unsubstituted or substituted C1-400alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6alkyl; and preferably the others of Ra, Rb, Rc, Rdand Reare all H.Often one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6alkyl; and preferably the others of Ra, Rb, Rc, Rdand Reare all H Typically, when Ra, Rb, Rc, Rdand Reare other than H, they are independently selected from halo, unsubstituted C1-400alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted Ci -e alkyl.Therefore, often one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from halo, unsubstituted C1-400alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted C1-6 alkyl; and the others of Ra, Rb, Rc, Rdand Reare all H.Typically, when Ra, Rb, Rc, Rdand Reare other than H, they are independently selected from halo, unsubstituted C1.4 alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted C i-6 alkyl.Therefore often one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from halo, unsubstituted C1.4 alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted C1-6 alkyl; and the others of Ra, Rb, Rc, Rdand Reare all H.Often one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from chloro, methyl, isopropyl, CN, C(O)OMe, and OMe; and the others of Ra, Rb, Rc, Rdand Reare all H.Ramay be H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically, Rais unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, or H. Any one of the aforementioned C1-400alkyl or C2-400 alkenyl groups may be a C4-400 alkyl or a C6-400alkenyl group which may be as further defined anywhere herein.Rbmay be H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically, Rbis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, or H. Any one of theaforementioned C1-400alkyl or C2-400 alkenyl groups may be a C4-400 alkyl or a C6-400alkenyl group which may be as further defined anywhere herein.Rcmay be H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically, Rcis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, halo or H. Any one of the aforementioned Ci-400 alkyl or C2-400 alkenyl groups may be a C4-400 alkyl or a C6-400alkenyl group which may be as further defined anywhere herein.Rdmay be H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically, Rdis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, or H. Any one of the aforementioned C1-400alkyl or C2-400 alkenyl groups may be a C4-400 alkyl or a C6-400alkenyl group which may be as further defined anywhere herein.Remay be H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically, Reis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, or H. Any one of the aforementioned C1-400alkyl or C2-400 alkenyl groups may be a C4-400 alkyl or a C6-400alkenyl group which may be as further defined anywhere herein.Often, Rais unsubstituted C1-400alkyl, unsubstituted C2-400 alkenyl, or H; Rbis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, or H; Rcis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, halo or H; Rdis unsubstituted Ci-400 alkyl, unsubstituted C2-400 alkenyl, or H; and Reis unsubstituted C1-400alkyl, unsubstituted C2-400 alkenyl, or H. Any one of the aforementioned C1-400alkyl or C2-400 alkenyl groups may be a C4-400 alkyl or a C6-400alkenyl group which may be as further defined anywhere herein.Ramay be H, halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically Rais unsubstituted C1-6 alkyl, or OR, wherein R is selected from H and unsubstituted C1-6 alkyl. Ramay be methyl, isopropyl, or OMe.Rbmay be H, halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically Rbis H.Rcmay be H, halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically Rcis H, CN, halo, unsubstituted C1-6 alkyl, or C(O)OR, wherein R is selected from H and unsubstituted C1-6 alkyl. Rcmay be H, methyl, chloro, C(O)OMe, or CN.Rdmay be H, halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted C1-6 alkyl. Typically Rdis H or C1-6 unsubstituted alkyl. Typically Rdis H or methyl.Remay be H, halo, unsubstituted or substituted C1-10alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted or substituted Ci-6 alkyl. Typically Reis halo, unsubstituted Ci-6 alkyl, or OR, wherein R is selected from H and unsubstituted Ci-6 alkyl. Remay be chloro, methyl, isopropyl, or OMe.Often, Rais unsubstituted Ci-6 alkyl, or OR, wherein R is selected from H and unsubstituted Ci-6 alkyl; Rbis H; Rcis H, CN, halo, unsubstituted Ci-6 alkyl, or C(O)OR, wherein R is selected from H and unsubstituted Ci-6 alkyl; Rdis H or Ci-6 unsubstituted alkyl; and Reis halo, unsubstituted Ci-6 alkyl, or OR, wherein R is selected from H and unsubstituted Ci-6 alkyl.Often Rais methyl, isopropyl, or OMe; Rbis H; Rcis H, methyl, chloro, C(O)OMe, or CN; Rdis H or methyl; Reis chloro, methyl, isopropyl, or OMe.Alternatively, X may be a C2-20 alkyl which is unsubstituted or substituted with from 1 to 9 groups selected from halo, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, X is a C2-6 alkyl which is unsubstituted or substituted with from 1 to 9 halo groups. In particular X may be a C2-4 alkyl substituted with from 1 to 6 chloro or fluoro groups. X may be trichloroethyl, or hexafluorobutyl.Often R1is unsubstituted C1-6 alkyl. Typically R1is methyl.Each of R2, R3, R4, R5, R6, R7, R8and R9may be H.Often one, two, or more than two of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, NRnR12, unsubstituted or substituted C1-20 alkyl, and -C=C-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently is H or unsubstituted C1-6 alkyl;wherein R10, R11and R12are each independently is H or unsubstituted C1-6 alkyl;and each of the others of R2, R3, R4, R5, R6, R7, R8and R9are HOften one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from halo; unsubstituted C1-20 alkoxy;-N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6alkyl.Often one of R2, R3, R4, R5, R6, R7, R8and R9is other than H, and the remainder of R2, R3, R4, R5, R6, R7, R8and R9are H. Any one of R2, R3, R4, R5, R6, R7, R8and R9may be other than H. Often R3or R8is other than H.Often two of R2, R3, R4, R5, R6, R7, R8and R9are other than H, and the remainder of R2, R3, R4, R5, R6, R7, R8and R9are H. Any two of R2, R3, R4, R5, R6, R7, R8and R9may be other than H. Typically, R3and R8are other than H.Often one, two, or more than two of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from bromo, -OMe, branched C5-C10 alkyl, -N(CH3)C(O)H, -NHMe;and a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me. Typically two of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from bromo, -OMe, branched C5-C10 alkyl, -N(CH3)C(O)H, -NHMe;and a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me.Often one, two, or more than two of R2, R3, R4, R5, R6, R7, R8and R9are OMe or NR11R12, when R11and R12are selected from H or unsubstituted C1-6alkyl (typically methyl). Often two of R2, R3, R4, R5, R6, R7, R8and R9are OMe or NR11R12, when R11and R12are selected from H or unsubstituted C1-6 alkyl (typically methyl). Often R3and R8are OMe or -NHCH3, and R2, R4, R5, R6, R7and R9are H.Often one, two, or more than two of R2, R3, R4, R5, R6, R7, R8and R9are OMe. Often R3and R8are OMe, and R2, R4, R5, R6, R7and R9are H.When two of R2, R3, R4, R5, R6, R7, R8and R9are other than H, they may be the same or different. Typically, they are the same.R2may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl. Typically R2is selected from H, halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NRnR12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6 alkyl.Often R2is H.R3may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl; wherein R10, R11and R12are each independently is H or unsubstituted C1-6 alkyl. Typically R3is halo, unsubstituted C1-20 alkoxy, -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6 alkyl. Often R3is selected from H, OMe, Br, C10 branched alkoxy, C5 branched alkoxy, -N(CH3)C(0)H, -NHMe or a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me.R4may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl. Typically R4is selected from H, halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6alkyl.Often R4is H.R5may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl. Typically R5is selected from H, halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6 alkyl.Often R5is H.R6may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl. Typically R6is selected from H, halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6 alkyl.Often R6is H.R7may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl. Typically R7is selected from H, halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6 alkyl.Often R7is H.R8may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl; wherein R10, R11and R12are each independently is H or unsubstituted C1-6 alkyl. Typically R8is halo, unsubstituted C1-20 alkoxy, -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6alkyl. Often R8is selected from H, OMe, Br, Cio branched alkoxy, C5 branched alkoxy, -N(CH3)C(O)H, -NHMe or a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me.R9may be selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl. Typically R9is selected from H, halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NRnR12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl;and a group of formula (II):wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6alkyl.Often R9is H.Often R2is H; R3is selected from H, OMe, Br, Cio branched alkoxy, C5 branched alkoxy, -N(CH3)C(O)H, -NHMe or a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me; R4is H; R5is H; R6is H, R7is H; R8is H, Br, OMe, Cio branched alkoxy, C5 branched alkoxy, -N(CH3)C(O)H, -NHMe or a group of formulawherein R14is selected from -NMe2, -OH and -OC(O)Me; and R9is H.Often X is substituted a 6-membered aryl, such that the compound is an acridan of formula (la):R1to R9may be as defined herein for formula (I).Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl. Any of Ra, Rb, Rc, Rdand Remay be as further defined anywhere herein.Often one, two, or three of R2, R3, R4, R5, R6, R7, R8and R9are C1-20 alkoxy. Preferably, two of R2, R3, R4, R5, R6, R7, R8and R9are C1-20 alkoxy. Typically, each of the others of R2, R3, R4, R5, R6, R7, R8and R9are H. Typically, the C1-20 alkoxy is C1-10alkoxy, preferably C1-5 alkoxy. For instance, the C1-20 alkoxy may be OMe, OEt or OPr, preferably OMe.Therefore, typically, one, two, or three of R2, R3, R4, R5, R6, R7, R8and R9are OMe. Often, two of R2, R3, R4, R5, R6, R7, R8and R9are OMe.Often R3or R8is other than H. Typically, R3and R8are other than H. Often, R3or R8are Ci-20 alkoxy. Often, R3and R8are C1-20 alkoxy. Typically, R3or R8are OMe. Often, R3and R8are OMe. Often, R3and R8are OMe and R2, R4, R5, R6, R7and R9are H.Often, at least one, for instance one, two or three, or one or two, of Ra, Rb, Rc, Rdand Reare independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl. The others of Ra, Rb, Rc, Rdand Reare generally each independently selected from H, halo, unsubstituted or substituted C1-3 alkyl, unsubstituted or substituted C2-5 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl.Typically, the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-3 alkyl, and unsubstituted or substituted C2-5 alkenyl.More typically, the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, and unsubstituted or substituted C1-3 alkyl.Often, the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, and unsubstituted C1-3 alkyl.Usually, the others are each independently selected from H, chloro, methyl and isopropyl. When at least one, for instance one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, said unsubstituted or substituted C4-400 alkyl may be unsubstituted or substituted C4-30 alkyl, for instance unsubstituted or substituted C8-30alkyl, or for instance unsubstituted or substituted C8-20alkyl. Often, it is unsubstituted or substituted C5-18 alkyl, C8-13alkyl or C10-12 alkyl, for instance unsubstituted C5-18 alkyl, C8-13alkyl or C10-12 alkyl. Preferably, it is unsubstituted or substituted C12 alkyl, for instance, unsubstituted C12 alkyl. Alternatively, it is unsubstituted or substituted C10-30 alkyl, C15-25 alkyl or C17-19 alkyl, for instance unsubstituted or substituted C10-30 alkyl, C15-25 alkyl or C17-19 alkyl. Preferably, it is unsubstituted or substituted C18alkyl, for instance, unsubstituted C18alkyl.Alternatively, it is unsubstituted or substituted C40-400 alkyl, C48-250 alkyl, or C60-200alkyl, for instance unsubstituted C40-400 alkyl, C48-250 alkyl, or C60-200alkyl. The C40-400 alkyl typically comprises at least three repeat units, for instance at least three repeat units of formula -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, wherein the number of repeat units may be from 10 to 100, for instance from 10 to 90, or from 10 to 80, and is often from 10 to 50 such as from 20 to 50, from 30 to 50, from 35 to 45, most preferably around 39; or from 10 to 20, such as from 12 to 18, most preferably around 16. Often the number of repeat units is from 10 to 80, and is often from 10 to 50, such as 20 to 50, from 30 to 48, from 34 to 44, most preferably around 39; or from 11 to 21, such as from 13 to 19, most preferably around 16. The C40-400 alkyl may also comprise a terminal -CH3 or -C(CH3)3 group, and may also be attached to the phenyl via a group such as CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-.Often, when at least one, for instance one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, said unsubstituted or substituted C4-400 alkyl may be a C6-400alkyl comprising the following formula QI:wherein RQ1is, for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100. Typically the C6-400alkyl comprises said repeating units of formula QI and a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2 or -C(CH3)3 group (typically a terminal -CH3 group or -C(CH3)3 group). In some instances, the C6-400alkyl furthercomprises a -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2- group, which is bonded to the end of the formula QI that is opposite to the terminal group.The number of repeating units, t, is typically an integer from 10 to 50, such as from 20 to 50, from 30 to 50, from 35 to 45, most preferably around 39; or from 10 to 20, such as from 12 to 18, most preferably around 16. Often the number of repeating units (t) is from 10 to 80, and is often from 10 to 50, such as 20 to 50, from 30 to 48, from 34 to 44, most preferably around 39; or from 11 to 21, such as from 13 to 19, most preferably around 16. Most preferably each RQ1is isobutylene, such that at least one, for instance one, two or three of Ra, Rb, Rc, Rdand Reare poly(isobutylene). Thus, typically the C6-400alkyl group comprises, or is,poly(isobutylene).When at least one, for instance one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, said C6-400alkenyl may be a C6-400alkenyl comprising an unsaturated unit of formula -CH=CH-, -CH=C(CH3)-CH2-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, - CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2- (for instance, an unsaturated unit of formula -CH=C(CH3)-CH2-) which is in turn bonded to a polymer of formula QI:wherein RQ1is, for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100. The polymer of formula (QI) is typically in turn bonded to a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CH3, -CH(CH3)2or -C(CH3)3group (typically a terminal -CH3group or -C(CH3)3group).Alternatively, said C6-400alkenyl may, for example, comprise a polymer of the following formulawherein the number of repeating units, t, is an integer from 3 to 100 and wherein each RQ2is an unsaturated repeating unit, for instance, -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2-, or wherein each RQ2is independently selected from an unsaturated repeating unit (for instance, -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2-) and a saturated repeating unit (for instance, -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-), provided that at least one RQ2is an unsaturated repeating unit. The number of repeating units, t, may for instance bean integer from 3 to 90. Typically such an alkenyl comprises the repeating units of formula QII and a terminal alkyl group having from 1 to 4 carbon atoms, for instance a terminal -CHs, -CH(CH3)2 or -CfC h) group (typically a terminal -CH3 group or -C(CH3)3 group).Therefore, often, at least one, for instance one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, which may be as further defined anywhere above, and typically the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-3 alkyl, unsubstituted or substituted C2-5 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.Typically, the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-3 alkyl, and unsubstituted or substituted C2-5 alkenyl.More typically, the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, and unsubstituted or substituted C1-3 alkyl.Often, the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, and unsubstituted C1-3 alkyl.Usually, the others are each independently selected from H, chloro, methyl and isopropyl. Often, Rais H; Rbis H; Rcis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); Rdis H; and Reis H. Often Rcis unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 72. Often Rcis unsubstituted C40-400 alkyl. Often the acridan of formula (la) is compound 99.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); Rdis H; and Reis unsubstituted C1-3 alkyl. Often Rcis unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 76, 80 or 88 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis H; Rdis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); and Reis unsubstituted C1-3 alkyl. Often Rdis unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 76, 80 or 88 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C1-3 alkyl; Rdis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); and Reis H. Often Rdis unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 84 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C1-3 alkyl; Rdis H; and Reis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted Ce-4ooalkenyl (which may be as further defined anywhere herein). Often Reis unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 84 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis halo; Rdis H; and Reis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein). Often Reis unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 92 herein.Often, Rais unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); Rbis H; Rcis halo; Rdis H; and Reis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein). Often Raand Reare unsubstituted C8-20alkyl. Often the acridan of formula (la) is compound 96 herein.Often X is substituted a 6-membered aryl, such that the compound is an acridan of formula (la):Rais unsubstituted C1-6 alkyl, or OR, wherein R is selected from H and unsubstituted C1-6 alkyl; Rbis H; Rcis H, CN, halo, unsubstituted C1-6 alkyl, or -C(O)OR, wherein R is selected from H and unsubstituted C1-6 alkyl; Rdis H or C1-6 unsubstituted alkyl; and Reis halo, unsubstituted C1-6 alkyl, or OR, wherein R is selected from H and unsubstituted C1-6 alkyl; orX is C1-6 alkyl which is unsubstituted or substituted with from 1 to 9 halo groups; andR2is H; R3is selected from H, OMe, Br, C10 branched alkoxy, C5 branched alkoxy, N(CH3)C(0)H, NHMe or a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me; R4is H; R5is H; R6is H, R7is H; R8is H, OMe, Cio branched alkoxy, C5 branched alkoxy, N(CH3)CH(O), NHMe or a group of formula (II):wherein R14is selected from -NMe2, -OH and -OC(O)Me; and R9is H.Often, when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than Ci alkyl and / or X has at least one substituent other than Ci alkyl, Ci alkoxy or Ci ester.Therefore, when the compound is an acridan of Formula (la)often it is the case that either:at least three of Ra, Rb, Rc, Rdand Reis other than H;at least two of Ra, Rb, Rc, Rdand Reare other than H, and are different from each other; or at least two of Ra, Rb, Rc, Rdand Reare other than H, -OH, methyl, -OCH3, or -C(O)OCH3.In addition, often, when X is aryl, X is substituted with at least one group other than halo. This means that typically, when the compound is an acridan of Formula (la)at least one of Ra, Rb, Rc, Rdand Reis other than H, and other than halo.Therefore when the compound is an acridan of Formula (la)often it is the case that either:at least three of Ra, Rb, Rc, Rdand Reis other than H;at least two of Ra, Rb, Rc, Rdand Reare other than H, and are different from each other; or at least two of Ra, Rb, Rc, Rdand Reare other than H, -OH, methyl, -OCH3, or -C(O)OCH3; and at least one of Ra, Rb, Rc, Rdand Reis other than H, and other than halo.A compound of the invention, accordingly, may be selected from:wherein t may be as defined herein and is typically from 10 to 80, and is often from 10 to 50, for instance from 20 to 50, from 30 to 48 or from 34 to 44, most preferably around 39.By [R1+ R2= CnH2n+2] herein is meant the moietybonded to the phenyl ring is an unsubstituted Cn+1alkyl group that may be attached to the phenyl ring by a single covalent bond to any one of the n+1 carbon atoms in said Cn+1alkyl group. For instance, by [R1+ R2= C11H24] herein R1+ R2is meant the moietybonded to the phenyl ring is a dodecyl group that may be attached to the phenyl ring by a single covalent bond to any one of the 12 carbon atoms in said dodecyl group. Thus, when [R1+ R2= C11H24], R1may be H and R2may be C11H23; or R1may be CH3and R2may be C10H21; or R1may be C2H5and R2may be C9H19; or R1may be C3H7and R2may be C8H17; or R1may be C4H9and R2may be C7H15; or R1may be C5H11and R2may be C6H13; or R1may be C6H13and R2may be C5H11; or R1may be C7H15and R2may be C4H9; or R1may be C8H17and R2may be C3H7; or R1may be C9H19and R2may be C2H5; or R1may be C10H21and R2may be CH3; or R1may be C11H23and R2may be H.Likewise, by [R1+ R2= C17H36], herein is meant the moietybonded to the phenyl ring is an octadecyl group that may be attached to the phenyl ring by a single covalent bond to any one of the 18 carbon atoms in said octadecyl group. Thus, when [R1+ R2= C17H36], R1may be H and R2may be C17H35; or R1may be CH3and R2may be C16H33; or R1may be C2H5and R2may be C15H31; or R1may be C3H7and R2may be C14H29; or R1may be C4H9and R2may be C13H27; or R1may be C5H11and R2may be C12H25; or R1may be C6H13and R2may be C11H23; or R1may be C7H15and R2may be C10H21; or R1may be C8H17and R2may be C9H19; or R1may be C9H19and R2may be C8H17; or R1may be C10H21and R2may be C7H15; or R1may be C11H23and R2may be C6H13; or R1may be C12H25and R2may be C5H11; or R1may be C13H27and R2may be C4H9; or R1may be C14H29and R2may be C3H7; or R1may be C15H31and R2may be C2H5; or R1may be C16H33and R2may be CH3; or R1may be C17H35and R2may be H.Where a moietyshown herein as being bonded to a phenyl ring in between two carbon atoms of the phenyl ring, this means that the moiety can be bonded either to one or the other of the two carbon atoms in question.A compound of the invention may also be selected from:The compound of the invention may be selected from:The present inventors have found that some compounds of the invention have especially high solubility in pure alkanes, for instance in pure n-hexane. Such compounds are of formula (la) as defined above and may be as further defined in the following paragraphs.Often, Rais H; Rbis H; Rcis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); Rdis H; and Reis H. Often Rcis unsubstituted C8-20alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often the acridan of formula (la) is compound 72 Often Rcis unsubstituted C40-400 alkyl. Often R1is methyl. Often the acridan of formula (la) is compound 99.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C15-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C15-400 alkenyl (which may be as further defined anywhere herein); Rdis H; and Reis unsubstituted C1-3 alkyl. Often Rcis unsubstituted C16-20 alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 80 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis H; Rdis unsubstituted C15-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C15-400 alkenyl (which may be as furtherdefined anywhere herein); and Reis unsubstituted C1-3 alkyl. Often Rdis unsubstituted C16-20 alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 80 herein.Often, Rais unsubstituted C2-3 alkyl; Rbis H; Rcis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); Rdis H; and Reis unsubstituted C2-3 alkyl. Often Rcis unsubstituted C8-20alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 88 herein.Often, Rais unsubstituted C2-3 alkyl; Rbis H; Rcis H; Rdis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); and Reis unsubstituted C2-3 alkyl. Often Rdis unsubstituted C8-20alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 88 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C1-3 alkyl; Rdis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); and Reis H. Often Rdis unsubstituted C8-20alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 84 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C1-3 alkyl; Rdis H; and Reis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein). Often Reis unsubstituted C8-20alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 84 herein.Often, Rais unsubstituted C1-3 alkyl; Rbis H; Rcis halo; Rdis H; and Reis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein). Often Reis unsubstituted C8-20alkyl. Often, R3and R8are Ci-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 92 herein.Often, Rais unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein); Rbis H; Rcis halo; Rdis H; and Reis unsubstituted C4-400 alkyl (which may be as further defined anywhere herein) or unsubstituted C6-400alkenyl (which may be as further defined anywhere herein). Often Raand Reare unsubstituted C8-20alkyl. Often, R3and R8are C1-20 alkoxy, for instance OMe and R2, R4, R5, R6, R7and R9are H. Often R1is methyl. Often the acridan of formula (la) is compound 96 herein.For example, such compounds may be selected from:wherein t may be as defined herein and is typically from 10 to 80, and is often from 10 to 50, for instance from 20 to 50, from 30 to 48 or from 34 to 44, most preferably around 39.The present inventors have found that some compounds of the invention have especially high solubility in fuels. In particular, compounds of formula (I) wherein one, two, or more than two of R2, R3, R4, R5, R6, R7, R8and R9are unsubstituted C5-10 alkoxy display excellent solubility. Typically for such compounds one of R2, R3, R4, R5, R6, R7, R8and R9is an unsubstituted C5-10 alkoxy, one of R2, R3, R4, R5, R6, R7, R8and R9is OMe, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H; or two of R2, R3, R4, R5, R6, R7, R8and R9are unsubstituted C5-10 alkoxy, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H. Often the substituents which are other than H are R3and R8.For such compounds, typically the compounds are compounds of Formula (la), and at least two of Ra, Rb, Rc, Rdand Reare other than H. Typically two of Ra, Rb, Rc, Rdand Reare unsubstituted C1-4 alkyl, such as methyl or isopropyl. Typically the groups other than H are Raand Re.For example, such compounds may be selected from:The present inventors have found that some compounds of the invention are useful as they emit light at a longer wavelength (e.g. from around 540 nm to around 840 nm) than other acridans of the invention. In particular, these may be compounds of formula (I), wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is -N(R10)C(O)H, -NR11R12, or -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl, wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl, and wherein the others of R2, R3, R4, R5, R6, R7, R8and R9are H.For such compounds, typically the compounds are compounds of Formula (la), and at least two of Ra, Rb, Rc, Rdand Reare other than H. Typically two of Ra, Rb, Rc, Rdand Reare unsubstituted C1-4 alkyl, such as methyl. Typically the groups other than H are Raand Re.For example, such compounds may be selected from:Synthesis of AcridansThe present invention also relates to an intermediate for use in producing a compound of Formula (I), as described herein, wherein said intermediate comprises a cation of Formula (III)and an anion, Yn-, wherein n is an integer of 1 to 4,X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl.Typically, when X is phenyl, X is substituted with at least one group other than halo.Often, when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and-C(O)OCH3.X, R1, R2, R3R4, R5, R6, R7, R8and R9may be as further defined anywhere herein (i.e. for Formula (I)).Typically, the intermediate is other than:The invention further provides an intermediate wherein the cation of Formula (III) is a cation of Formula (Illa):wherein Ra, Rb, Rc, Rd, Re, R1, R2, R3, R4, R5, R6, R7, R8and R9are as defined anywhere herein for Formula (I) and Formula (la).Often Ra, Rb, Rc, Rd and Re are each independently selected from H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;at least one of Ra, Rb, Rc, Rdand Reis selected from halo, CN, OR wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, and substituted or unsubstituted C3 alkyl; orat least one of R2, R3, R4, Rs, Rd, R7, Rs and Rg is halo.Often Ra, Rb, Rc, Rd and Re are each independently selected from H, halo, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted C2-10 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl and unsubstituted or substituted aryl;at least one of Ra, Rb, Rc, Rdand Reis selected from halo, CN, OR wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, and substituted or unsubstituted C3 alkyl; orat least one of R2, R3, R4, Rs, Rd, R7, Rs and Rg is halo.For Formula (III) and Formula (Illa), n is typically 1 or 2, and often n is 1.Typically, Y is a sulfonate anion. Often Y is an aryl sulfonate, or a substituted or unsubstituted C1-10alkyl sulfonate, wherein substituted alkyl sulfonate is substituted with one or more groups selected from halo, CN, and OH. Often Y is phenyl sulfonate, toluene sulfonate or trifluoromethyl sulfonate.The intermediate is typically selected from:The intermediate may also be selected from:wherein t may be as defined herein and is typically from 10 to 80, and is often from 10 to 50, for instance from 20 to 50, from 30 to 48 or from 34 to 44, most preferably around 39.Often, the intermediate is selected from:The present invention also provides a method of producing a compound (i.e. an acridan of Formula (I)) as described herein. Accordingly, the invention provides a process for producing acompound as described herein, wherein said process comprises treating with a reducing agent an intermediate which comprises a cation of formula (III):X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl.Typically when X is phenyl, X is substituted with at least one group other than halo.Often when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and-C(O)OCH3.Any reducing agent may be appropriately used. Typically the reducing agent is a borohydride, typically sodium borohydride or sodium triacetoxyborohydride. Often the reducing agent is sodium borohydride or sodium triacetoxyborohydride.Said treatment may be treatment in a solvent, for example in a polar, aprotic solvent, such as acetonitrile. Said solvent may also be dichloromethane (DCM). DCM is typically used when thereducing agent is sodium triacetoxyborohydride. Use of DCM can simplify the work-up and purification of the product. In particular as it is non-miscible with washes such as aqueous carbonate washes, and has a low boiling point, it can be easy to concentrate the product.Said treatment may also occur in the present of acid, for example in the presence of acetic acid. The present invention also relates to a precursor for use in producing a compound of the invention (i.e. an acridan of Formula (I)), wherein said precursor is a compound of formula (IV)X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted Ci- 20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl.Often when X is phenyl, X is substituted with at least one group other than halo.Typically when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and-C(O)OCH3X, R1, R2, R3R4, R5, R6, R7, R8and R9may be as further defined anywhere herein (i.e. for Formula (I)).The compound is typically other than:andTypically, the compound is a compound of Formula (IVa):wherein Ra, Rb, Rc, Rd, Re, R1, R2, R3, R4, R5, R6, R7, R8and R9are as defined anywhere herein for Formula (I) and Formula (la).Often, the precursor is a compound selected from:The precursors may also be a compound selected from:[ R1+ R2= C11H24]wherein t may be as defined herein and is typically from 10 to 80, and is often from 10 to 50, for instance from 20 to 50, from 30 to 48 or from 34 to 44, most preferably around 39.Typically, the precursor is a compound selected from:The invention therefore also provides a process of producing a compound of the invention (i.e. an acridan of Formula (I) or Formula (la)) wherein said process comprises the treatment with a reducingagent of an intermediate which comprises a cation of formula (III) and an anion, Yn-, as described above, and said process further comprising an initial step of treating a compound of formula (IV) with an alkylating reagent comprising R1and Y, to form said intermediate which comprises said cation of formula (III) and said anion, Yn-, wherein R1is as defined for the cation of formula (III) and wherein Y is a charge-neutral moiety that results in the formation of said anion Yn-, wherein n is said integer of 1 to 4, and wherein the formula (IV) is:wherein X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl.Typically, when X is phenyl, X is substituted with at least one group other than halo.Often when X is aryl, and R2to R9are all H, X is substituted with at least two different substituents, X has at least three substituents, and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(O)OCH3As stated above, the alkylating reagent comprises Ri and Y, wherein Ri is as defined for the cation of formula (III), and Y is a charge -neutral moiety that results in the formation of the anion Yn-, wherein n is said integer of 1 to 4. Often, n is 1 or 2.For example, the alkylating reagent may be a compound of formula (Ri)p(Y)q, p is typically 1 to 8, preferably 1 to 4, more preferably 1 to 2, even more preferably 1; and q is typically 1 to 2, preferably 1. Typically p=nq, wherein n is as described for Yn-above, i.e. p is the product of n and q,such that, for example, when n is 1 and q is 1, p is also 1. In some preferred embodiments, p, q, and n are all 1, such that (Ri)p(Y)qis RiYTypically, Ri is an alkyl group, typically unsubstituted Ci-6 alky, and R’Y is typically an alkylating agent. RiY, may therefore be an unsubstituted Ci-6 alkyl halide, unsubstituted Ci-6 alkyl-OSOsMe, unsubstituted Ci-6 alkyl-OsSCHs, unsubstituted Ci-6 alkyl-OsSCR and unsubstituted Ci-6 alkyl-OsSAr. Ar in this context is typically toluene or phenyl. Typically R1is methyl, and RiY is a methylating agent. When the alkylation step is a methylation step, the methylation step can be performed with a wide variety of methylation reagents. For example, RiY, may be a methyl halide, MeOSOsMe, MeOsSCHs, MeOsSCFs and MeOsSAr. Typically R’Y is MeOsSCFs, or MeOsSAr, wherein Ar is toluene or phenyl. The use of methyl arylsulfonates for example methyl toluenesulfonate, is especially convenient for large scale synthesis providing a less hazardous and more cost-effective process than traditional options such as methyl triflate.Said treatment typically occurs at standard atmospheric pressure and temperature as defined herein. Said treatment typically occurs in a solvent, often a polar solvent, such as dichloromethane. Such conditions may be used when the treatment is with MeO3SCF3.Said treatment typically occurs at an elevated temperature, such as 80 to 120 °C, such as around 100 °C. This is typical for treatment with methyl toluenesulfonate. Such treatment may comprise a further step of granulation in a solvent (e.g. in ethyl acetate).The process for producing a compound according to formula (I) may also involve a step of substituting substituents on the tricyclic nucleus (e.g. substitution of one or more R1, R2, R3, R4, R5, R6, R7, R8or R9group). This step may comprise treatment of a compound of formula (I) wherein one or more of R1, R2, R3, R4, R5, R6, R7, R8or R9is a leaving group (e.g. a halo group) with suitable reagents for a substitution.For example, such treatment may be treatment with tritolylphosphine, a catalyst (e.g. a palladium (0) catalyst, such as palladium bis dibenzylideneactone, and 4 -acetoxy styrene. Such a reaction may take place in a solvent, such as a mixture of triethylamine and acetonitrile.The process may further comprise a step of ester hydrolysis to convert a group of formula (II):wherein R14is -OC(O)R16, wherein R16is H or unsubstituted Ci-6 alkyl; to a group offormula (II):wherein R14is -OR15, wherein R15is H or unsubstituted C1-6 alkyl.Said ester hydrolysis may occur in a solution of tetrahydrofuran (THF) and ammonia.Marked productThe invention also comprises a marked product which comprises (a) a product and (b) a marker compound which is a compound of the invention of Formula (I) as defined anywhere herein.The “product” (as referred to in the context of the marked product or marked composition of the invention, or in the context of the method of the invention of detecting a marker compound, or in the context of any of the other methods of the invention, or in the context of the compositions of the invention) may be a liquid, a solid or a gas. This means that the product may be one which is in the liquid state at standard ambient temperature and pressure (SATP), or it may be a product which is in the solid state at SATP. Alternatively, it may be a product which is a gas at SATP, for instance a C1-4 alkane or C2-4 alkene. Standard ambient temperature and pressure, abbreviated herein to SATP, refers to a temperature of 298.15 K (25 °C) and a pressure of 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm). Typically, the product is a liquid. In other words, the product is typically one which is in the liquid state at SATP.The term “product”, as used herein, refers to a material or continuous phase which makes up the bulk of the marked composition. Although the marked composition may optionally comprise water - for example in a relatively small amount - the product, which makes up the bulk of the marked composition, is typically not water or an aqueous solution; it is typically a non-aqueous product.The product may be, for example, a non-aqueous liquid, i.e. a liquid other than water and other than an aqueous solution. The product may, for example, be a hydrocarbon fuel, an oil (such as a fuel oil, crude oil, lubricating oil, or a vegetable oil), a fuel additive, or an organic solvent.The product may alternatively be a solid. When the non-aqueous product is a solid it may, for example, be a wax, a plastic, a polymer, wood, a solid foodstuff, a naturally-occuring organic solid material, for instance solid biomass, vegetable material, animal fat or vegetable fat.The product may alternatively be a gas, for instance a C1-4 alkane or C2-4 alkene, such as for instance methane, ethane, ethylene, propane or butane, or a mixture of C1-4 hydrocarbons.Usually, the marked product comprises less than 10% by weight of water, and more typically less than 5% by weight of water, for instance less than 4% by weight of water, for example less than 3% by weight of water, optionally less than 2% by weight of water. Often, the marked composition comprises less than 1% by weight of water, and more typically less than 0.7% by weight of water, for instance less than 0.5% by weight of water, for example less than 0.1% by weight of water. The marked product may not comprise water.The product employed in the methods and compositions of the invention, including the above defined method of detecting a marker compound, may be hydrophobic. It is often immiscible with water.The product may comprise a petroleum product, a fuel (including a solid fuel), a plastic, a fabric, a chemical product, a packaging material (e.g. a packaging material for pharmaceutical products), a food, a beverage, a herbal medicine or related product, a perfume, a graphic art material, a plant, a seed, a fuel additive, a lubricating oil, a crude oil, a vegetable oil or fat, an animal oil or fat, algae oil, a fatty acid methyl ester or biodiesel, solid fuel, wood, vegetable product or finished goods in general.The product may comprise a petroleum product, a fuel (including a solid fuel), a plastic, a fabric, a chemical product, a packaging material (e.g. a packaging material for pharmaceutical products), a food, a beverage, a herbal medicine or related product, a perfume, a graphic art material, a plant, a seed, a fuel additive, a lubricating oil, a crude oil, a vegetable oil or fat, an animal oil or fat, algae oil, and a fatty acid methyl ester or biodiesel.As an example, the product to be marked may comprise of crude oil or a petroleum product, a fuel, fuel additive, lubricating oil, a vegetable oil or fat, animal oil or fat, algae oil, a fatty acid methyl ester or biodiesel. The product may comprise a petroleum product, a fuel, fuel additive, lubricating oil, crude oil, a vegetable oil or fat, animal oil or fat, algae oil, a fatty acid methyl ester or biodiesel.The fuel may comprise a hydrocarbon fuel or ethanol or methanol. Further examples of petroleum and fuel products include hydrocarbons, ethanol, methanol, fuel additives, lubricating oil, crude oil, a vegetable oil, animal oil, algae oil, a fatty acid methyl ester or biodiesel, synthetic fuels (including a synthetic fuel obtainable by methanol-to-gasoline technology (MTG), a synthetic fuel obtainable by coal-to-liquid technology (CTL), a synthetic fuel obtainable by gas-to-liquid technology (GTL), a synthetic fuel obtainable by biomass-to-liquid technology (BTL), a synthetic fuel obtainable by coal -biomass to liquid technology (CBTL) and hydrotreated renewal jet fuel (HRJ)).The fuel may be a synthetic fuel. The synthetic fuel may be selected from a synthetic fuel obtainable by methanol-to-gasoline technology (MTG), a synthetic fuel obtainable by coal-to-liquid technology (CTL), a synthetic fuel obtainable by gas-to-liquid technology (GTL), a synthetic fuelobtainable by biomass-to-liquid technology (BTL), a synthetic fuel obtainable by coal-biomass to liquid technology (CBTL) and hydrotreated renewal jet fuel (HRJ)The product may be a plastic. Further examples of plastics include recovered or waste plastic materials. The recovered or waste plastic materials may require authenticity (or sustainability) of origin prior to recycling and processing.The product may be a plant. Further examples of plants include trees and timber products. Such products may require authenticity of sustainable production and protection from forgery.The product may be a fabric. Further examples of fabric products include raw materials such as cotton which are required to meet the demands of a sustainable source of origin. Alternatively, fabrics used in branded garments which require proof of authenticity. Accordingly, the product may be a fabric, wherein the fabric is a raw material, such as cotton, or wherein the fabric is a fabric used in a branded garment.The product may be a herbal medicine or related product. Further examples of herbal medicines and related products include approved sources of tobacco, vapes or legalised cannabis which must satisfy requirements of quality and safety. Accordingly, the product may be a herbal medicine or related product, wherein the herbal medicine or related product is tobacco, a vape or legalised cannabis.The product may be a food or beverage product. Further examples of food and beverage products include sustainable sources of vegetable matter such as palm oil and rape seed oil as well as branded alcoholic goods. Accordingly, the product may be a food or a beverage, a vegetable, vegetable matter, palm oil, rape seed oil, or a branded alcoholic product.A marked product of the invention may further comprise an additional marker compound which is an acridan of formula (I)X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl,wherein the at least one marker compound (i.e. acridan of formula (I)) of the marked product, and the additional marker compound which is an acridan of formula (I) emit light at different wavelengths.The present invention also comprises a marked product which comprises:(a) a product;(b) a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl; and(c) an antioxidant; and / ora UV stabiliser or absorber.Often the marked product comprises a product, a compound which is an acridan of formula (I) and an antioxidant. Often the marked product comprises a product, a compound which is an acridan of formula (I) and a UV stabiliser or absorber.The antioxidant is typically ascorbic acid or ascorbyl palmitate. The UV stabiliser or absorber is typically a hydrophenyl benzotriazole or a substituted triazine.The marker might be applied to a product using a formulation that forms a physical or chemical bond with a commercial product. Such formulations include a grease like material, a glue, a setting resin or a formulation which is applied for the purpose of drying a commercial product.Accordingly, the invention also provides a formulation for use in marking a product, wherein said formulation comprises a marker compound which is an acridan of Formula (I) as described anywhere herein, and wherein said formulation is a grease or a resin. Said formulation may comprise at least two compounds which are acridans of Formula (I) as defined herein, wherein said compounds emit light at different wavelengths.The formulation may comprise, for example, at least one marker compound which is an acridan of Formula (I), and an adhesive (typically a spreadable adhesive).The invention also provides use of such a formulation for marking a product.Hydrocarbon-based productsThe product is often hydrocarbon-based. The product may therefore comprise an unsubstituted or substituted hydrocarbon. The unsubstituted or substituted hydrocarbon may have a particular number of carbon atoms as defined further herein for substituted or unsubstituted hydrocarbons. Thus, the product may for instance comprise an unsubstituted or substituted C1-150 hydrocarbon, or for instance an unsubstituted or substituted C1-100 hydrocarbon, an unsubstituted or substituted C1-50 hydrocarbon, an unsubstituted or substituted C1-30 hydrocarbon, or an unsubstituted or substituted C1-20 hydrocarbon. This includes natural gas (methane), propane and butane, in addition to higher hydrocarbons which are in the liquid or solid state at SATP. In other embodiments, the product may comprise a C5-150 hydrocarbon, or for instance an unsubstituted or substituted C5-100 hydrocarbon, an unsubstituted or substituted C5-50 hydrocarbon, an unsubstituted or substituted C5-30 hydrocarbon, or an unsubstituted or substituted C5-20 hydrocarbon.The product may comprise a single unsubstituted or substituted C1-150 hydrocarbon (an example of which would be marking pure ethanol, e.g. for duty purposes). For instance, the product may comprise a single substituted or unsubstituted Ci-150 hydrocarbon, for example a single substituted or unsubstituted C1-50 hydrocarbon, or for instance a single substituted or unsubstituted C1-30hydrocarbon, or a single substituted or unsubstituted C1-20 hydrocarbon. The product may for instance comprise a single unsubstituted or substituted C5-150 hydrocarbon, or for instance a single unsubstituted or substituted C5-100 hydrocarbon, a single unsubstituted or substituted C5-50 hydrocarbon, a single unsubstituted or substituted C5-30 hydrocarbon, or a single unsubstituted or substituted C5-20 hydrocarbon. The product may for instance comprise a single unsubstituted or substituted C10-150 hydrocarbon, or for instance a single unsubstituted or substituted C10-100 hydrocarbon, a single unsubstituted or substituted C10-50 hydrocarbon, a single unsubstituted or substituted C10-30 hydrocarbon, or a single unsubstituted or substituted C10-20 hydrocarbon.Often, however, the product comprises a mixture of two or more different substituted or unsubstituted C1-150 hydrocarbons. For instance, the product may comprise a mixture of two or more different substituted or unsubstituted C1-150 hydrocarbons, for example a mixture of two or more different substituted or unsubstituted C1-50 hydrocarbons, or for instance a mixture of two or more different substituted or unsubstituted C1-30 hydrocarbons, or a mixture of two or more different substituted or unsubstituted C1-20 hydrocarbons. The product may for instance comprise a mixture of two or more different unsubstituted or substituted C5-150 hydrocarbons, or for instance a mixture of two or more different unsubstituted or substituted C5-100 hydrocarbons, a mixture of two or more different unsubstituted or substituted C5-50 hydrocarbons, a mixture of two or more different unsubstituted or substituted C5-30 hydrocarbons, or a mixture of two or more different unsubstituted or substituted C5-20 hydrocarbons. The product may for instance comprise a mixture of two or more different unsubstituted or substituted C10-150 hydrocarbons, or for instance a mixture of two or more different unsubstituted or substituted C10-100 hydrocarbons, a mixture of two or more different unsubstituted or substituted C10-50 hydrocarbons, a mixture of two or more different unsubstituted or substituted C10-30 hydrocarbons, or a mixture of two or more different unsubstituted or substituted C10-20 hydrocarbons.The product is typically a liquid comprising an unsubstituted or substituted hydrocarbon, wherein the unsubstituted or substituted hydrocarbon may be as further defined hereinbefore. The product is often a liquid comprising a mixture of two or more different unsubstituted or substituted hydrocarbons, wherein the mixture of two or more different unsubstituted or substituted hydrocarbons may be as further defined hereinbefore.The product employed in the methods and compositions of the invention often comprises a hydrocarbon (i.e. an unsubstituted hydrocarbon). The hydrocarbon may have a particular number of carbon atoms as defined further herein for hydrocarbons. The product may for instance comprise a C1-150 hydrocarbon, a C5-150 hydrocarbon, or for instance a C1-100 hydrocarbon, a C1-50 hydrocarbon, a C1-30 hydrocarbon, or a C1-20 hydrocarbon, or for instance a C5-100 hydrocarbon, a C5-50 hydrocarbon, a C5-30 hydrocarbon, or a C5-20 hydrocarbon.The C1-150 hydrocarbon, for instance the C1-100, C1-50, C1-30, or C1-20 hydrocarbon, or the the C5-100, C5-50, C5-30, or C5-20 hydrocarbon, may be an alkane, a cycloalkane, an alkene, a cycloalkene, or an aromatic compound. Thus, the hydrocarbon may be a C1-50 alkane, or for instance a C1-30 alkane or a C1-20 alkane. It may be a C5-50 alkane, or for instance a C5-30 alkane or a C5-20 alkane. Alternatively, the hydrocarbon may be a C3-30 cycloalkane, or for example a C3-20 cycloalkane, a C5-30 cycloalkane, or for example a C5-20 cycloalkane. The hydrocarbon may for instance be a C2-50 alkene, a C5-50 alkene, or for instance a C2-30 alkene, a C5-30 alkene, C1-20 alkene or a C5-20 alkene. Alternatively, the hydrocarbon may be a C3-30 cycloalkene, or for example a C5-30 cycloalkene, or a C5-20 cycloalkene. The hydrocarbon may for instance be an aromatic hydrocarbon, for instance a C5-16 aromatic hydrocarbon, for instance benzene, toluene, diethylbenzene, naphthalene, methylnaphthalene, ethylnaphthalene, anthracene, methylanthracene or ethylanthracene.The product employed in the methods and compositions of the invention may comprise a single hydrocarbon but it often comprises a mixture of two or more different hydrocarbons (i.e. two or more different unsubstituted hydrocarbons). For instance, the product may comprise a mixture of two or more different Ci-150 hydrocarbons, such as a mixture of two or more different C2-150 hydrocarbons, or for instance a mixture of two or more different C5-150 hydrocarbons. For instance, the product may comprise a mixture of two or more different C1-50 hydrocarbons, or for instance a mixture of two or more different C1-30 hydrocarbons, or a mixture of two or more different C1-20 hydrocarbons. The product may for instance comprise a mixture of two or more different C2-150 hydrocarbons, or for instance a mixture of two or more different C2-100 hydrocarbons, a mixture of two or more different C2-50 hydrocarbons, a mixture of two or more different C2-30 hydrocarbons, or a mixture of two or more different C2-20, C5-20 or C10-20 hydrocarbons. For example, the product may comprise a mixture of two or more different C5-50 hydrocarbons, or for instance a mixture of two or more different C5-30 hydrocarbons, or a mixture of two or more different C5-20 hydrocarbons. The product may for instance comprise a mixture of two or more different C10-150 hydrocarbons, or for instance a mixture of two or more different C10-100 hydrocarbons, a mixture of two or more different C10-50 hydrocarbons, a mixture of two or more different C 10-30 hydrocarbons, or a mixture of two or more different C 10-20 hydrocarbons.The mixture of two or more different hydrocarbons typically comprises two or more different hydrocarbons selected from alkanes, cycloalkanes, alkenes, cycloalkenes, and aromatic compounds. The mixture may for instance comprise two or more different hydrocarbons independently selected from C1-50 alkanes (for instance from C1-30 alkanes or from C1-20 alkanes); C3-30 cycloalkanes (for instance from C3-20 cycloalkanes or from C3-10 cycloalkanes); C2-50 alkenes (for instance from C2-30 alkenes or from C2-20 alkenes); C3-30 cycloalkenes (for instance from C3-20 cycloalkenes or from C3-10 cycloalkenes); C6-16 aromatic hydrocarbons (for instance benzene, toluene, xylene, ethylbenzene, methylethylbenzene, diethylbenzene, naphthalene, methylnaphthalene, ethylnaphthalene, anthracene,methylanthracene or ethylanthracene). The mixture may for instance comprise two or more different hydrocarbons independently selected from C5-50 alkanes (for instance from C5-30 alkanes or from C5-20 alkanes); C4-30 cycloalkanes (for instance from C5-20 cycloalkanes or from C5-10 cycloalkanes); C2-50 alkenes (for instance from C5-30 alkenes or from C5-20 alkenes); C4-30 cycloalkenes (for instance from C5-20 cycloalkenes or from C5-10 cycloalkenes); Ce-ie aromatic hydrocarbons (for instance benzene, toluene, xylene, ethylbenzene, methylethylbenzene, diethylbenzene, naphthalene, methylnaphthalene, ethylnaphthalene, anthracene, methylanthracene or ethylanthracene). The mixture may for instance comprise two or more different hydrocarbons independently selected from C5-30 alkanes, C4-20 cycloalkanes, C2-30 alkenes, C4-20 cycloalkenes, and Ce-ie aromatic hydrocarbons. For example, the mixture may comprise two or more different hydrocarbons independently selected from C5-20 alkanes, C4-10 cycloalkanes, C2-20 alkenes, C4-10 cycloalkenes and C6-16 aromatic hydrocarbons (for instance benzene, toluene, xylene, ethylbenzene, methylethylbenzene, diethylbenzene, naphthalene, methylnaphthalene, ethylnaphthalene, anthracene, methylanthracene or ethylanthracene).The mixture of two or more different hydrocarbons may comprise gasoline range (C5-12) hydrocarbons, or diesel range (C8-24) hydrocarbons, or both. It may additionally or alternatively comprise one or more solid hydrocarbons, including paraffin waxes and hydrocarbon polymers, such as for instance, polyolefins, including for example polyethylene and polypropylene. The hydrocarbon polymers, may, for instance, be polyolefins, including for example polyethylene, polypropylene, polymethylpentene, polybutene- 1, and including for instance polyolefin elastomers, such as, for example, polyisobutylene, ethylene propylene rubber, and ethylene propylene diene monomer (M-class) rubber. The hydrocarbon polymers may, for example, be hydrocarbon polymers which have a molecular weight of at least 1,800 Da. The mixture of two or more different hydrocarbons may additionally or alternatively comprise one or more hydrocarbons which are gaseous at SATP, including for example C1.4 hydrocarbons such as methane (e.g. as part of natural gas), ethane, ethylene, propane or butane.Often, when the product comprises a mixture of two or more different hydrocarbons, at least one of those hydrocarbons is a C8-150 hydrocarbon, for instance a C8-100, C8-50, C8-30, or C8-20 hydrocarbon. The C8-150 hydrocarbon (or for instance the C8-100, C8-50, C8-30, or C8-20 hydrocarbon) may be as further defined hereinbefore. For instance, when the non-aqueous product comprises a mixture of two or more different hydrocarbons, at least one of those hydrocarbons may be a C9-150 hydrocarbon, for instance a C9-100, C9-50, C9-30, or C9-20 hydrocarbon. The C9-150 hydrocarbon (or for instance the C9-100, C9-50, C9-30, or C9-20 hydrocarbon) may be as further defined hereinbefore.Often, when the product comprises a mixture of two or more different hydrocarbons, at least one of those hydrocarbons is a C10-150 hydrocarbon, for instance a C10-100, C10 -50, C10-30, or C10-20hydrocarbon. The C10-150 hydrocarbon (or for instance the C10-100, C10-50, C10-30, or C10-20 hydrocarbon) may be as further defined hereinbefore.As mentioned above, the product employed in the composition, uses, methods and processes of the invention is typically a liquid. The product is typically a liquid comprising a hydrocarbon, i.e. an unsubstituted hydrocarbon, wherein the (unsubstituted) hydrocarbon may be as further defined hereinbefore. The product is often a liquid comprising a mixture of two or more different hydrocarbons, i.e. two or more different unsubstituted hydrocarbons, wherein the mixture of the two or more different (unsubstituted) hydrocarbons may be as further defined hereinbefore.Petroleum productsThe product (which is typically a non-aqueous product) may for instance comprise, or be, a petroleum product, such as crude oil or a fuel.The product may for instance comprise, or be, crude oil.The product may comprise, or be, a fuel, for instance a hydrocarbon fuel.Often, the product is a fuel.The fuel may be a distillate fuel, for instance gasoil, diesel, gasoline, or liquified petroleum gas. The fuel may for instance be gasoline (petrol). Gasoline typically comprises a mixture of C5-12 hydrocarbons.The fuel may be liquified petroleum gas.Alternatively, the fuel may be diesel or gas oil (red diesel). Diesel and gas oil both typically comprise a mixture of Cs-25 hydrocarbons. They may for instance comprise a mixture of C10-15 hydrocarbons.Alternatively, the fuel may be a fuel oil. The term “fuel oil” is used herein to refer to fuels such as heavy fuel oil (HFO), intermediate fuel oils (IFO), and marine diesel oil (MDO), all of which contain at least in part the residue of distillation of crude oil. Marine diesel oil (MDO), for instance, is a blend of gasoil (a distillate fuel) and heavy fuel oil (a residual fuel). The fact that fuel oils contain residual fuels differentiates them from distillate fuels such diesel, gasoil, gasoline and liquified petroleum gas.Alternatively, the fuel may comprise methanol. Methanol can be desirable as a fuel, as it can be synthesised using CO2 and H2, meaning the process of synthesising and burning methanol fuel can be net carbon neutral. Methanol can also be synthesised used CO and H2, which is a less “green” process. The marker compounds of the present invention could be utilised for marking of methanol fuel to provide evidence that the fuel comes from a source that means it is carbon neutral.Alternatively, the fuel may comprise ethanol.The product may for example comprise, or be, marine diesel, also known as marine diesel oil (MDO). Marine diesel typically comprises a mixture of C9.20 hydrocarbons. More typically, it comprises a mixture of C10-20 hydrocarbons. Thus, in one embodiment the product comprises a mixture of two or more, for instance a mixture of five or more, or a mixture of ten or more, different Ci 0-20 hydrocarbons.The product may comprise, or be, intermediate fuel oil (IFO). Intermediate fuel oil typically comprises a mixture of C10-70 hydrocarbons, for instance a mixture of C12-70 hydrocarbons. Thus, in one embodiment the non-aqueous product comprises a mixture of two or more, for instance a mixture of five or more, or a mixture of ten or more, different C10-70 hydrocarbons. The product may for instance comprise a mixture of two or more, for instance a mixture of five or more, or a mixture of ten or more, different Ci 2-70 hydrocarbons.The product may comprise, or be, heavy fuel oil (HFO). Heavy fuel oil typically comprises a mixture of C20-70 hydrocarbons. Thus, in one embodiment the product comprises a mixture of two or more, for instance a mixture of five or more, or a mixture of ten or more, different C20-70 hydrocarbons.The product may comprise, or be, marine lubricant oil, for instance marine diesel engine lubricating oil.The product may comprise, or be, crude oil.Vegetable, animal and algae oils and biodieselThe product may comprise, or be, a vegetable oil, an algae oil (i.e. an oil produced by microalgae), an animal oil or animal fat.Thus, the product may comprise a mixture of oxygen-containing organic compounds selected from oxygen containing organic compounds found in vegetable oils, animal oils and fats and / or algae oils.Often, the product comprises, or is, a vegetable oil. Any vegetable oil may be the nonaqueous product; vegetable oils are well known and include, but are not limited to, oil from soybean, rapeseed, jatropha, mahua, mustard, flax, sunflower, safflower, palm, hemp, cottonseed, field pennycress, wheatgerm, olive, com, coconut, Millettia pinnata and Pongamia pinnata. The vegetable oil may for instance be palm oil, rapeseed oil, jatropha oil, soybean oil, Pongamia oil or canola oil. Palm oil or soybean oil may for instance be employed.The product may however comprise, or be, a naturally-occuring oil other than a vegetable oil, for instance an animal oil or fat, or an algae oil. Many of these are known in the art. The animal fat or oil may for example be tallow, which consists mainly of triglycerides whose major constituents are derived from stearic and oleic acids. The algae oil may for instance be oil produced by any of the following types of microalgae: Botryococcus braunii, Chlorella, Dunaliella tertiolecta, Gracilaria, Pleurochrysis carterae (also called CCMP647), Sargassum, Ankistrodesmus, Chlorella protothecoides, Cyclotella, Dunaliella tertiolecta, Hantzschia, Nannochloris, Nannochloropsis, Nitzschia, Phaeodactylum tricornutum, Scenedesmus, Stichococcus, Tetraselmis suecica, Thalassiosira pseudonana, Crypthecodinium cohnii, Neochloris oleoahundans, Schiochytrium.Additionally or alternatively, the product may comprise, or be, biodiesel. Thus, the nonaqueous product may comprise an alkyl ester, typically a Ci-6 alkyl ester, of a fatty acid, for instance a methyl, ethyl or propyl ester of a fatty acid. Such alkyl esters are the main constituents of biodiesel.As mentioned above, naturally occurring oils and biodiesel comprise oxygen-containing organic compounds, for instance fatty acid esters and fatty acids. The product may therefore comprise an organic compound comprising at least one oxygen atom, or two or more different said organic compounds. The organic compound(s) comprising at least one oxygen atom are typically selected from fatty acids and fatty acid esters. Such a fatty acid ester may for instance be a triglyceride, or an alkyl ester of a fatty acid. Similarly, the product may comprise a mixture of compounds selected from fatty acid esters and fatty acids. It may for instance comprise a mixture of more than one fatty acid ester, or a mixture of more than one fatty acid, or for instance a mixture comprising more than one fatty acid ester and more than one fatty acid. The fatty acid ester or esters may for example be selected from triglycerides, i.e. esters derived from glycerol and three fatty acids, and alkyl esters of fatty acids, for instance Ci-6 alkyl esters of fatty acids.Triglycerides are the main constituents of vegetable oil (in which case the triglycerides are typically more unsaturated) animal fats (in which case the triglycerides are typically more saturated), and algae oil. Such vegetable, animal and algae oils and fats typically contain complex mixtures of triglycerides.Thus, the product may therefore comprise a triglyceride, e.g. a triglyceride of formula (a) below. Also, the non-aqueous product may comprise at least two, for instance at least three, or at least four, different triglycerides, which may be selected from triglycerides of formula (a) below. The product may further comprise a (free) fatty acid, or for instance two or more different free fatty acids, each of which may be of formula (b) below.Additionally or alternatively, the product may comprise an alkyl ester of a fatty acid, for instance a Ci-6 alkyl ester such as a methyl, ethyl or propyl ester of a fatty acid. Such alkyl esters areIllthe main constituents of biodiesel. The non-aqueous matrix may therefore comprise an ester of formula (c) below.Alternatively, the product may comprise a free fatty acid, e.g. of formula (b) below.Likewise, the product may comprise at least one fatty acid, and may for instance comprise two or more fatty acids, which may be of formula (b) below.Thus, the product may comprise at least one compound, but more typically a mixture of compounds, selected from:a triglyceride of formula (a):whereinR1, R2and R3are the same or different and are independently selected from unsubstituted or substituted C1-40 alkyl (for instance unsubstituted or substituted C4-30 alkyl), unsubstituted or substituted C2-40 alkenyl (for instance unsubstituted or substituted C4-30 alkenyl) and unsubstituted or substituted C2-40 alkynyl (for instance unsubstituted or substituted C4-30 alkynyl);an acid of formula (b):whereinR4is unsubstituted or substituted C1-40 alky l(for instance unsubstituted or substituted C4-30 alkyl), unsubstituted or substituted C2-40 alkenyl (for instance unsubstituted or substituted C4-30 alkenyl), or unsubstituted or substituted C2-40 alkynyl (for instance unsubstituted or substituted C4-30 alkynyl); andan ester of formula (c):whereinR5is unsubstituted or substituted C1-10 alkyl; andR6is unsubstituted or substituted C1-40 alkyl (for instance unsubstituted or substituted C4-30 alkyl), unsubstituted or substituted C2-40 alkenyl (for instance unsubstituted or substituted C4-30 alkenyl), or unsubstituted or substituted C2-40 alkynyl (for instance unsubstituted or substituted C4-30 alkynyl).Usually, R1, R2and R3in the triglyceride of formula (a) are independently selected from unsubstituted or substituted C4-30 alkyl and unsubstituted or substituted C4-30 alkenyl. For instance, R1, R2and R3may be independently selected from unsubstituted or substituted C7-25 alkyl and unsubstituted or substituted C7-25 alkenyl. More typically, R1, R2and R3are independently selected from unsubstituted C7-25 alkyl and unsubstituted C7-25 alkenyl. In some embodiments, for instance, R1, R2and R3are independently selected from unsubstituted C7-21 alkyl and unsubstituted C7-21 alkenyl, or, for instance, from unsubstituted C10-20 alkyl and unsubstituted C10-20 alkenyl. Often, for example, R1, R2and R3are independently selected from unsubstituted C13-18 alkyl and unsubstituted C13-18 alkenyl.In the acid of formula (b), R4is usually unsubstituted or substituted C4-30 alkyl or unsubstituted or substituted C4-30 alkenyl. It may for instance be unsubstituted or substituted C7-25 alkyl or unsubstituted or substituted C7-25 alkenyl. More typically, however, R4is unsubstituted C7-25 alkyl or unsubstituted C7-25 alkenyl. In some embodiments, for instance, R4is unsubstituted C7-21 alkyl or unsubstituted C7-21 alkenyl, or, for instance, unsubstituted C10-20 alkyl or unsubstituted C10-20 alkenyl. Often, for example, R4is unsubstituted C13-18 alkyl or unsubstituted C13-18 alkenyl.In the ester of formula (c), R5is usually unsubstituted or substituted C1-6 alkyl, for instance unsubstituted or substituted C1-4 alkyl. Typically, R5is unsubstituted C1-6 alkyl. R5may for instance be unsubstituted C1-4 alkyl. R6in the ester of formula (III) is often unsubstituted or substituted C4-30 alkyl or unsubstituted or substituted C4-30 alkenyl. It may for instance be unsubstituted or substituted C7-25 alkyl or unsubstituted or substituted C7-25 alkenyl. More typically, R6is unsubstituted C7-25 alkyl or unsubstituted C7-25 alkenyl. In some embodiments, for instance, R6is unsubstituted C7-21 alkyl or unsubstituted C7-21 alkenyl, or, for instance, unsubstituted C10-20 alkyl or unsubstituted C10-20 alkenyl. Often, for example, R6is unsubstituted C13-18 alkyl or unsubstituted C13-18 alkenyl.The marker compound may be dissolved or suspended in the product, or it may be dispersed in or on the product. Typically, the product is a liquid and the marker compound is dissolved, suspended or dispersed in the product. Usually, the marker compound is dissolved in the product. In other cases,however, the product may be a solid and the marker compound may for instance be dispersed on a surface of the solid, dispersed under a surface of the solid, or dispersed throughout the solid. Usually, however, the product is a liquid and the marker compound is dissolved or suspended in the product, and more typically dissolved.Other product materialsThe product may for instance comprise, or be, a fuel additive. The product may comprise, or be a lubricating oil.The product employed in the methods and compositions of the invention, including the above defined method of detecting a marker compound, may be a solid.The product employed in the methods and compositions of the invention, including the above defined method of detecting a marker compound, may, for instance, comprise, or consist of, a wax, a plastic, a polymer, wood, a solid foodstuff, or a naturally -occurring organic solid material, for instance solid biomass or vegetable material.Concentration of the marker compoundAs mentioned above, it is a finding of the invention that the marker compound (a chemiluminescent precursor) may successfully be introduced into a product to be marked at very low concentrations and still be detected afterwards from a sample of the material.Thus, typically, the amount of the marker compound in the marked product is less than 100 parts per billion (ppb) by mass. More typically, the amount of the marker compound in the marked product is less than 50 ppb by mass, or less than 20 ppb by mass.Preferably, however, the amount of the marker compound in the marked product is equal to or less than 15 parts per billion by mass, for instance equal to or less than 10 parts per billion by mass. The amount of the marker compound in the marked product may for instance be equal to or less than 5 ppb by mass, for instance less than 2 ppb by mass.In some embodiments, the amount of the marker compound in the marked composition may be equal to or less than 1 ppb by mass.In some embodiments, however, the marker compound can be detected at even lower concentrations, for instance at concentrations of one twentieth of the above. The amount of the marker compound in the marked composition may for instance be equal to or less than 0.1 parts per billion by mass, for instance equal to or less than 0.05 parts per billion by mass, or equal to or less than 0.02 parts per billion by mass. The amount of the marker compound may for instance be equal to or less than 0.01 ppb by mass, for instance less than 0.005 ppb by mass. In some embodiments, the marker compound can be detected at concentrations in the region of parts per trillion (ppt), for example less than 100 ppt, less than 50 ppt, or in some embodiments less than 10 ppt.As the skilled person would appreciate, the marker compound need not be used at low concentrations. It may for instance be used at relatively high concentrations for example of up to 1.0 % by mass, or even for instance up to 5% by mass. Thus, the amount of the marker compound in the marked composition may be less than or equal to 5% by mass, for instance less than or equal to 1% by mass, for example less than or equal to 0.1 % by mass. The amount may be less than or equal to 500 parts per million (ppm) by mass, for instance less than or equal to 100 ppm by mass, or for example, less than or equal to 50 ppm by mass. The amount of the marker compound in the marked composition may be less than or equal to 20 ppm by mass, or less than or equal to 10 ppm by mass for instance less than or equal to 1 ppm by mass.The concentration of the compound of the invention in a marked product, is the same as the concentration described in a marked composition as described above. In particular, the concentration of the compound of formula (I) in a product is often from around 1 ppb to 100 ppm.The concentration of the compound of the invention in a marked product may be 1 ppm to 100 ppm. If the compound of the invention is compound of Formula (I) wherein two or more of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from unsubstituted or substituted C1-20 alkoxy, and -NR11R12, wherein R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl; and the concentration of the compound in a marked product is from 1 ppm to 100 ppm, then a method of detecting the compound of the invention using an organic superbase (TBD), may comprise an initial step of diluting the marked product. If the compound of the invention is compound of Formula (I) wherein fewer than two of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from unsubstituted or substituted C1-20 alkoxy, and -NR11R12, wherein R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl; and the concentration of the compound in a marked product is from 1 ppm to 100 ppm, then a method of detecting the compound of the invention using an organic superbase, may comprise directly contacting the product with an organic superbase (e.g. TBD).Often, in a method of detecting a marker compound in a product, the method comprises a step of diluting the sample. This step is typical when the concentration of the compound of the invention in the product results in an excessively strong emission such that the detector becomes saturated.Additional components in the marked product or compositionThe marked product or composition may further comprise an additive.The additive may be a fuel additive. Thus, the product may be crude oil or a fuel, for instance a hydrocarbon fuel, such as a distillate fuel or a fuel oil, or for example biodiesel, and the additive may be a fuel additive.Often, the product is a hydrocarbon fuel, for instance a distillate fuel or a fuel oil which may be as further defined herein, and the additive is a fuel additive.The marker compound may have been present in the additive at a known concentration before it was added to the product fuel. Thus, quantitative detection of the marker compound in the final marked composition can not only flag the presence of the additive in the first place, but it can reveal whether or not the additive was added at the correct concentration or not.Accordingly, in the method of detecting the marker compound in the product, the presence of the marker compound in the product may signify the presence of the additive in the product, and the method may comprise detecting the light emitted from the sample and thereby determining the presence of the marker compound and the additive in the product.As mentioned above, often, the amount of the additive in the marked product relative to the amount of the marker compound in the marked composition is known. Thus, the method may further comprise: measuring the amount of light emitted from the sample; determining the amount of the marker compound in the marked product based on the amount of light emitted; and calculating the amount of the additive in the marked composition based on the amount of the marker compound in the marked composition.Determining the amount of marker compound in the marked product typically comprises determining the mass fraction or concentration of the marker compound in the marked composition. Typically, calculating the amount of the additive in the marked product comprises calculating the mass fraction or concentration of the additive in the marked product (based on the mass fraction or concentration of the marker compound in the marked composition).Typically, the amount of the marker compound in the marked product is equal to or less than 15 parts per billion by mass, for instance equal to or less than 10 parts per billion by mass. The amount may for instance be equal to or less than 5 ppb by mass, for instance less than 2 ppb by mass. The amount is often, for instance, equal to or less than 1 ppb by mass, equal to or less than 0.5 ppb by mass, or equal to or less than 0.2 ppb by mass. The amount of the marker compound in the marked composition may for instance be equal to or less than 0.1 parts per billion by mass, for instance equal to or less than 0.05 parts per billion by mass, or equal to or less than 0.02 parts per billion by mass. The amount of the marker compound may for instance be equal to or less than 0.01 ppb by mass, for instance less than 0.005 ppb by mass. The amount of the marker compound may for instance be equal to or less than 100 ppt, for instance less than 50 ppt, for instance less than 10 ppt.Method of marking a productThe present invention also provides a method of marking a product, which method comprises treating said product with a compounds of the invention, wherein said compound is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl.For said compound, typically when X is phenyl, X is substituted with at least one group other than halo.Often, for said compound, when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl and / or X has at least one substituent other than -OH, methyl, -OCH3 or -C(0)0CH3.The compound may be a compound according to Formula (I) (or Formula (la)) as further defined anywhere herein.Said treatment with a marker compound may comprise directly adding the marker compound to a product or composition, with no further excipients.Said treatment with a marker compound may comprise treatment with a formulation as described herein. For example, the treatment may comprise spreading the formulation across the surface of a product.Said treatment with a marker compound as defined above, may comprise contacting a product with a carrier composition as defined herein, wherein the carrier composition comprises the marker compound and a carrier component. Often the step of treating the product comprises mixing the carrier composition with the product, or wetting the surface of the product with the carrier composition. As the skilled person would appreciate, mixing may be more appropriate when marking a liquid product, and wetting of a surface may be more appropriate when marking a solid product (such as an organic solid, a polymer, a packaging material, solid fuel, wood, mineral ore, metal, other mineral product or finished goods).The carrier composition may comprise a marker compound and a carrier solvent, wherein the marker compound is a chemiluminescent precursor (i.e. a compound of Formula (I) or Formula (la)) which is soluble in the carrier solvent.The carrier solvent may be one solvent or a mixture of two or more solvents. Often the carrier solvent comprises ethyl benzoate, and / or a mixture of aromatic hydrocarbons Typically the carrier solvent is ethyl benzoate. Ethyl benzoate has the advantages of being relatively inexpensive as a solvent and having low toxicity.Often the carrier solvent comprises one or more aromatic esters, for instance methyl benzoate, ethyl benzoate, phenyl acetate and phenyl benzoate.Compounds of the invention (markers) have very high solubility in aromatic esters including methyl benzoate, ethyl benzoate, phenyl acetate and phenyl benzoate. These solvents support high concentrations of marker for subsequent dilution into a product (e.g. fuel).If necessary for the application, some of the markers are soluble in the presence of DMSO, dimethyl acetamide and dimethylformamide. Often the carrier solvent comprises DMSO. Also, some of the markers are soluble in polar solvents such as oligomers of ethylene glycol and their ethers.Accordingly, the carrier solvent often comprises an aromatic ester, for instance methyl benzoate, ethyl benzoate, phenyl acetate or phenyl benzoate; DMSO; dimethyl acetamide and dimethylformamide; oligomers of ethylene glycol; or ethers of oligomers of ethylene glycol.The present invention also provides a concentrated solution of a marked compound, comprising a marker compound of the invention and a carrier solvent. The carrier solvent often comprises ethyl benzoate, and / or a mixture of aromatic hydrocarbons. The carrier solvent is often ethyl benzoate.Accordingly, the present invention also provides a marked product comprising (a) a product (typically a fuel) and (b) a concentrated solution of the invention.The carrier solvent is typically miscible with a non-aqueous liquid. Typically the nonaqueous liquid petroleum product, a fuel, fuel additive, lubricating oil, crude oil, a vegetable oil, animal oil, algae oil, a fatty acid methyl ester or biodiesel. Often the fuel comprises a hydrocarbon fuel or ethanol or methanol.The product may be as defined anywhere herein. The carrier solvent and the marker compound may also be as further defined anywhere herein, as may the carrier composition.The product is often a liquid, and, in embodiments where the product is a liquid the carrier solvent is generally miscible with the product. Thus the product may be a liquid at SATP. It is often crude oil, a fuel oil or a distillate fuel, or for instance a vegetable oil or biodiesel.Typically, the amount of the carrier composition contacted with the product is less than or equal to 1% by volume, based on the total volume of the carrier composition and the non-aqueous product. It is often for instance less than or equal to 0.2 % by volume, or less than or equal to 0.1% by volume. In some embodiments, the amount of the carrier composition contacted with the product is less than or equal to 0.01% by volume. The amount of the carrier composition contacted with the product may for instance be less than or equal to one part per hundred thousand (0.001%) by mass, or for example, less than or equal to one part per million (1 ppm) by mass, based on the total mass of the carrier composition and the non-aqueous product.Typically, contacting the product with the carrier composition comprises introducing the carrier composition into the product, for instance by mixing the product with the carrier composition, optionally by high shear mixing. High shear mixing is particularly useful with more viscous matrices such as crude oil or heavy fuel oils.The product often comprises a petroleum product, a fuel, crude oil, a vegetable oil, animal oil, algae oil or biodiesel and the marker compound is often (i) a compound of the invention of formula (I), which compound is as defined herein.The product may for instance comprise a fuel (e.g. a distillate fuel or a fuel oil) or crude oil, and the carrier composition may for instance comprise:a first solvent which comprises an ester such as ethyl benzoate or phenyl acetate; and / or a second solvent which comprises a mixture of aromatic hydrocarbons;and a compound of the invention.Typically, the product comprises a fuel (e.g. a distillate fuel or a fuel oil) or crude oil, and the carrier composition may for instance comprise:a first solvent which comprises an ester ethyl benzoate;and / or a second solvent which comprises a mixture of aromatic hydrocarbons;and a compound of the invention.In some embodiments, contacting the carrier composition with a product comprises contacting simultaneously, sequentially, or separately with more than one carrier composition. Forexample, the product could be contacted with one carrier composition comprising a compound of the invention as defined herein, and, simultaneously, sequentially, or separately, with a second compound of the invention as defined herein. Typically said first and second compound emit light at different wavelengths.Typically, the amount of the marker compound is less than 100 parts per billion (ppb) by mass based on the total mass of the carrier composition and the product. More typically, the amount of the marker compound is less than 50 ppb by mass, or less than 20 ppb by mass. Preferably, however, the amount of the marker compound is equal to or less than 15 parts per billion by mass based on the total mass of the carrier composition and the non-aqueous product, for instance equal to or less than 10 parts per billion by mass. The amount of the marker compound may for instance be equal to or less than 5 ppb by mass, for instance less than 2 ppb by mass. The amount of the marker compound may for instance be equal to or less than 1 ppb by mass, or for instance equal to or less than 0.5 ppb by mass, or equal to or less than 0.2 ppb by mass. In some embodiments, the amount of marker compound is less that 100 parts per trillion (ppt) by mass based on the total mass of the carrier composition and the product. For example, the amount of marker compound may be less than 50 ppt, or less than 20 ppt.Typically, in the method of marking a product of the invention, the carrier composition further comprises a tracer compound, which may be as further defined anywhere herein. The tracer compound typically comprises a nucleic acid, and the nucleic acid is preferably an oligonucleotide. Alternatively, the tracer compound may be a deuterated hydrocarbon, a rare earth element, a compound comprising a rare earth element, a radioactive compound, or a biomolecule, for instance a biomolecule capable of binding to a second molecule (e.g. an antibody capable of binding to an antigen, or an antigen capable of binding to an antibody).Typically, however, the tracer compound comprises an oligonucleotide.Usually, the amount of the tracer compound is less than or equal to 10 parts per million by mass, optionally less than or equal to one part per million by mass, optionally less than or equal to one part per billion by mass, optionally less than or equal to one part per trillion by mass, based on the total mass of the carrier composition and the non-aqueous product.Typically, in this embodiment, the marker compound signifies the presence of the tracer compound. Furthermore, the identity of the tracer compound typically signifies information about the non-aqueous product, for instance information as further described and defined hereinbefore.The method of marking a product may further comprise adding an additive to the product. The additive may be a fuel additive. For instance, the product may be a hydrocarbon fuel, crude oil or biodiesel, typically a fuel or crude oil, and the additive may be a fuel additive. Often, the product is a hydrocarbon fuel and the additive is a fuel additive.Typically, the amount of the additive added to the marked composition relative to the amount of said marker compound is known, so that the amount of the additive in the product may be calculated based on the amount of the marker compound.The carrier composition may be mixed with the additive prior to the step of contacting the product with the carrier composition. Thus, the method may comprise mixing the carrier composition with the additive, and then contacting the product with the carrier composition and the additive. Typically, the amount of the additive relative to the amount of said marker compound is known, so that the amount of the additive in the product may be calculated based on the amount of the marker compound, e.g. as determined by the method of the invention as defined herein for detecting a marker compound in a product.Alternatively the method of marking a product may comprise simply adding at least one compound of the invention to a product (such as dissolving at least one compound of the invention in a product, e.g. a fuel).The invention further provides a method of marking a product which method comprises treating said product with a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl; andan antioxidant.Said method may be used in conjunction with any other method of the invention.Furthermore, the compound which is an acridan of formula (I) may be as further defined anywhere herein.The antioxidant is typically selected from ascorbic acid and ascorbyl palmitate. The addition of an antioxidant to a composition comprising an acridan of formula (I) can enhance the oxidation stability of the compounds of formula (I), without impacting their chemiluminescence.The present invention also provides a method of marking a product which method comprises treating said product with a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl; anda UV stabiliser or absorber.Said method may be used in conjunction with any other method of the invention. In particular, said method may comprise an additional step of treating with an antioxidant, as described above. Furthermore, the compound which is an acridan of formula (I) may be as further defined anywhere herein.The UV stabiliser or absorber is typically selected from hydrophenyl benzotriazole and a substituted triazine. The addition of a UV stabiliser or absorber to a composition comprising an acridan of formula (I) can enhance the photostability of the compounds of formula (I), without impacting their chemiluminescence.Detecting the markerThe invention further comprises a method of detecting a marker compound in a product, which method comprises:(i) contacting a sample comprising a marked composition,wherein the marked composition comprises (a) a product and (b) a marker compound which is a compound as described anywhere herein (i.e. an acridan of Formula (I)),with one or more reagents for causing the marker compound to undergo a chemiluminescent reaction to emit light; and(ii) detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition.The method of detecting a marker compound may also be a method of quantifying a marker compound. Said method comprises:(i) contacting a sample comprising a marked composition,wherein the marked composition comprises (a) a product and (b) a marker compound which is a compound as described anywhere herein (i.e. an acridan of Formula (I)),with one or more reagents for causing the marker compound to undergo a chemiluminescent reaction to emit light;(ii) detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition;(iii) quantifying light emitted from the sample (for example in terms of relative light units) and thereby determining the quantity of the marker compound in the marked composition.Said quantifying method can be especially useful to detect the purity of a marked composition (or, for example, if it has been diluted). For such a quantifying method it can be that the marked composition has a known (or supposed) concentration of marker compound with a known (or supposed) quantity of light emitted. The quantity of light emitted from the sample can then be compared to the expected quantity to assess if the quantity of marker compound in the marked composition is as expected. Typically, the amount of light emitted by the marker compound will vary in a linear relationship with the concentration of the marker in a product (for example, see figures 5 and 6).Typically, in the method of the invention, detecting light emitted from the sample comprises detecting light emitted from the sample over a period of x seconds beginning once the samplecomprising the marked composition has been contacted with said one or more reagents, wherein x is at least 1.Similarly, when the method involves measuring the amount of light emitted from the sample (e.g. for quantitative detection of the marker), said measuring typically comprises measuring the amount of light emitted from the sample over a period of x seconds beginning once the sample comprising the marked composition has been contacted with said one or more reagents, wherein x is at least 1.Accordingly, when two acridan compounds are involved, said detecting or measuring typically begins after addition of the first developer solution has been added and immediately prior to addition of the second developer solution has been added. (A developer solution may be as defined anywhere herein.)Thus, when the one or more reagents for causing the marker compound to undergo a chemiluminescent reaction are present in a first developer solution and a second developer solution, said detecting light emitted from the sample typically comprises detecting light emitted from the sample over a period of x seconds starting from contacting the marked composition with the second developer solution, wherein x is at least 1. Similarly, when the one or more reagents for causing the marker compound to undergo a chemiluminescent reaction are present in a first developer solution and a second developer solution, said measuring the amount of light emitted from the sample typically comprises measuring the amount of light emitted from the sample over a period of x seconds starting from contacting the marked composition with the second developer solution, wherein x is at least 1.The period of x seconds is often long enough to measure all of the light, or at least 90% of the light, emitted by the chemiluminescent reaction of the marker compound. Typically, x is at least 2 seconds, and more typically at least 3 seconds, for instance at least 6 seconds. Thus, x may for instance be from 2 to 20 seconds, for instance from 4 to 15 seconds. Often, x is 8 seconds. Thus, usually, x is at least 2 and preferably at least 6. x is often from 6 to 10, for instance from 7 to 9, e.g. about 8.Often, when the developer solution comprises TBD in DMSO, x is typically 2 or 3. Often, when the developer solution comprises TBD in DEGDME, x is typically 7 to 9, often 8.Often, the light emitted has a wavelength of from 350 nm to 900 nm. The light emitted is typically visible light. The light emitted typically has a wavelength of from 390 nm to 840 nm.Different compounds of the invention may emit light at different wavelengths. Therefore the present method of detecting a marker compound in a product may comprise a method wherein the step of detecting light emitted from the sample comprises detecting light of a particular wavelength or detecting light having particular emission kinetics, and thereby identifying the marker compound as having a particular acridan of formula (I).Typically, compounds wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is unsubstituted C1-20 alkyl; and wherein the others of R2, R3, R4, R5, R6, R7, R8and R9are H;or wherein each of R2, R3, R4, R5, R6, R7, R8and R9are H, will emit light with a wavelength of from around 350 nm to 500 nm, typically 390 nm to 480 nm.Typically, compounds wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is unsubstituted or substituted C1-20 alkoxy, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H, will emit light with a wavelength of from around 400 nm to 550 nm, typically 410 nm to 520 nm.Typically, compounds wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H, will emit light with a wavelength of from around 520 nm to 860 nm, typically 540 nm to 840 nm. Often such compounds emit light at a wavelength of from 650 nm to 750 nm, typically around 700 nm.Typically, compounds wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is -NR11R12, unsubstituted or substituted C1-20 alkyl,wherein R11and R12are each independently is H or unsubstituted C1-6 alkyl, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H, will emit light with a wavelength of from around 500 nm to 700 nm. Often such compounds emit light at a wavelength of from 550 nm to 650 nm, typically around 600 nm.Sometimes, in the methods of the invention, the marked composition comprises at least two different acridans of formula (I), wherein the at least two different acridans emit light at at least two different wavelengths.Therefore, for example, the marked composition may comprise at least one acridan which emits light at a wavelength of from 350 nm to 500 nm, and at least one acridan which emits light at a wavelength of from 500 nm to 850 nm. Alternatively, the marked composition may comprises at least one acridan which emits light at a wavelength of from 350 nm to 600 nm, and at least one acridan which emits light at a wavelength of from 600 nm to 850 nm.The marked composition may therefore comprises at least two different acridans selected from:a) a compound which is an acridan of formula (I) wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is unsubstituted C1-20 alkyl; and wherein the others of R2, R3, R4, R5, R6, R7, R8and R9are H;or wherein each of R2, R3, R4, R5, R6, R7, R8and R9are H;b) a compound which is an acridan of formula (I) wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is unsubstituted or substituted C1-20 alkoxy, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H;c) a compound which is an acridan of formula (I) wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl, and the others of R2, R3, R4, R5, R6, R7, R8and R9are Hd) a compound which is an acridan of formula (I) wherein one, two or more than two of R2, R3, R4, R5, R6, R7, R8and R9is -NR11R12, unsubstituted or substituted C1-20 alkyl,wherein R11and R12are each independently is H or unsubstituted C1-6alkyl, and the others of R2, R3, R4, R5, R6, R7, R8and R9are H.Typically, in the method of the invention, the steps of: (i) contacting the sample comprising the marked composition or product with the one or more reagents for causing the marker compound to undergo a chemiluminescent reaction, and (ii) detecting light emitted from the sample (and optionally measuring the amount of light emitted by the sample), are carried out in a luminometer.As would be appreciated by the skilled person, a luminometer is an instrument that measures weak emissions of visible light coming from a sample, by means of a photomultiplier tube. Luminometers are very sensitive devices that are used to measure small amounts of light. They usually only require a small sample, e.g. only a few microliters. A wide range of commercially available luminometers may be employed in the present invention, for instance the G& H-ITL Lumini.The method of the invention may further comprise introducing the sample comprising the marked composition or product and the reagents for causing the marker compound to undergo the chemiluminescent reaction into the luminometer. Typically, introducing the reagents for causing the marker compound to undergo the chemiluminescent reaction into the luminometer comprises introducing one or more developer solutions comprising the reagents into the luminometer prior to introducing the sample comprising the marked composition. Introducing the one or more developer solutions may comprise introducing the first developer solution as defined herein and the second developer solution as defined herein into the luminometer.The luminometer is preferably a portable luminometer. Employing a portable luminometer advantageously allows the presence of the marker compound to be quantitatively detected in a sample in the field.The luminometer may be a solid state luminometer. The luminometer may be a Hygiena™, Ensure™, Odyssey™, Horiba™ or Lu-mini ™. It is typically a Lu-mini™.The method of the invention of detecting a marker compound in a product may further comprise producing the marked composition. The marked composition may be produced by the method of the invention of marking a product, as defined herein.In some embodiments, the product subjected to the detection method of the invention comprises a petroleum product, a fuel, crude oil, lubricating oil, a vegetable oil or fat, animal oil or fat, algae oil, a fuel additive, a fatty acid methyl ester or biodiesel, optionally wherein the fuel comprises a hydrocarbon fuel, ethanol or methanol.The present invention also provides a method of producing light emission from a marked product, which method comprises:(i) contacting a sample comprising a marked composition;wherein the marked composition comprises (a) a product and (b) a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12and unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl,with an alkali metal alkoxide.The compound used in said method may be a compound of Formula (I) as further described anywhere herein.Advantageously, the present inventors have found that using compounds of the invention with an alkali metal oxide allows detection of the compound of the invention using solid state detection instruments. Typically, these instruments are more portable and less expensive that other detection technologies (e.g. PMT), and so this detection method can diversify and increase accessibility of marking technology.The alkali metal alkoxide is a compound of the formula MOR17, wherein M is an alkali metal and R17is an unsubstituted C1-10alkyl. M is often selected from lithium, sodium or potassium, often sodium or potassium. Typically M is potassium.The alkali metal alkoxide may be a lithium, sodium or potassium alkoxide. Typically the alkali metal alkoxide is a potassium alkoxide. The potassium alkoxide is often a compound of the formula KOR17. R17is an unsubstituted C1-10alkyl. Typically R17is a C2-6 alkyl. Typically R17is abutyl group. Therefore typically the potassium alkoxide is potassium butoxide, often potassium tert-butoxide.Typically, the amount of alkali metal alkoxide present in the mixture comprising the sample and the alkali metal alkoxide (and other optional components, e.g. phase transfer catalyst) is less than or equal to 10% by weight / volume (w / v). It is preferably from 0.1% by w / v to 5% by w / v. Often, for example, the amount of alkali metal alkoxide present in the mixture comprising the sample and the alkali metal alkoxide (and other optional components, e.g. phase transfer catalyst) is from 0.1% to 1% by w / v, for instance about 0.5% by w / v.The method, as described herein, may further comprise contacting the marked compound with a phase transfer catalyst. This may happen simultaneously, sequentially or subsequently with the contacting of the marked compound with the alkali metal alkoxide. Adding a phase transfer catalyst to the sample can improve the kinetics of the reaction.A phase transfer catalyst is a reagent which facilitates migration of a species from a polar phase to a non-polar phase, or a non-polar to a polar phase.Typically the phase transfer catalyst as referred to herein is a quaternary ammonium salt. Often the phase transfer catalyst is a halogenated quaternary ammonium salt. For example, the phase transfer catalyst may be tricaprylmethyl ammonium chloride, cetyl trimethyl ammonium bromide, or cetyl trimethyl ammonium chloride (CTAC). The phase transfer catalyst is often cetyl trimethyl ammonium chloride. Addition of a catalyst such as CTAC significantly improves peak shape (shorter flash time), and prevents the peak from being extended and unsymmetrical which then becomes more difficult to quantify.Typically, the amount of phase transfer catalyst present in the mixture comprising the sample, the alkali metal alkoxide and the phase transfer catalyst is less than or equal to 10% by w / v. It is preferably from 0.09% by w / v to 4.5% by w / v. Often, for example, the amount of CTAC present the mixture comprising the sample, the alkali metal alkoxide and the phase transfer catalyst is from 0.09% to 0.9% by w / v, for instance about 0.45% by w / v.The method, as described herein, may further comprise contacting the sample comprising the marked compound with acetonitrile. Acetonitrile is typically added to a marked product or composition prior to addition of the alkali metal alkoxide, as such the method of the invention may comprise a first step of adding acetonitrile to a sample of a marked composition or product, and a second step of adding an alkali metal alkoxide (and optionally a phase transfer catalyst). Addition of acetonitrile further enhances the intensity and sharpness of chemiluminescence of a sample.The present method also has the advantage that it does not require oxidation with an oxidation reagent such as hydrogen peroxide, which leads to variability associated with the additional chemical reactionOften, for such a method of the invention, one of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from unsubstituted or substituted C1-20 alkoxy, halo, -NR10CH(O); -NR11R12wherein R10, R11and R12are each independently H or unsubstituted or substituted C1-6alkyl; unsubstituted or substituted C1-20 alkyl; and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl; and the others of R2, R3, R4, R5, R6, R7, R8and R9are HAlternatively at least one of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from halo, -NR10CH(O); wherein R10is H or unsubstituted or substituted C1-6alkyl; unsubstituted or substituted C1-20 alkyl; and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl; and the others of R2, R3, R4, R5, R6, R7, R8and R9are H.Alternatively, each of R2, R3, R4, R5, R6, R7, R8and R9are H.Often, for such methods, the alkali metal alkoxide is in a solvent. This is typically a nonaqueous solvent. Suitable solvents include dimethylformamide, toluene, tetrahydrofuran or tertiary alcohols including tert-butanol or tert-pentanol (referred to as tert-amyl alcohol). Often the solvent is a tertiary alcohol. Often the solvent is tert-amyl alcohol.Often, the methods as described herein (method using alkali metal alkoxides) comprise a step of contacting the sample with acetonitrile.Often the step of contacting the sample with acetonitrile preceeds a step of contacting the sample with an alkali metal alkoxide (e.g. potassium tert-butoxide), a solvent, and optionally a phase transfer catalyst.The method of the invention may therefore include a first step of contacting the sample with acetonitrile, and a second step of contacting the sample with an alkali metal alkoxide, a solvent, and optionally a phase transfer catalyst. Typically the alkali metal alkoxide is potassium tert-butoxide. Often, the solvent is tert-amyl alcohol. Typically the phase transfer catalyst is CTAC. Often, the phase transfer catalyst, alkali metal alkoxide and solvent are pre-mixed (and optionally sonicated), such that they can be contacted with the sample in a single step.Therefore the method of the invention may include a first step of contacting the sample with acetonitrile, and a second step of contacting the sample with potassium tert-butoxide in tert-amyl alcohol, optionally wherein the second step is a step of contacting the sample with potassium tert-butoxide and CTAC in tert-amyl alcohol.Often reagent systems can be affected by background noise resulting from unmarked fuels and other components in a sample. The present inventors have found that the addition of a tertiary amine (e.g. triethylamine) can reduce background noise. Therefore, the present invention provides a method wherein the method further comprises contacting the sample with triethylamine.It has been found that an excessive quantity of tertiary amine (e.g. triethylamine) can reduce the chemiluminescence of a sample. Therefore, typically, the amount of triethylamine relative to the total amount of sample, alkali metal alkoxide, solvent and tertiary amine (e.g. triethylamine) (and optionally phase transfer catalyst) is less than or equal to 25% by w / v. It is preferably from 0.1% by w / v to 15% by w / v. Often, for example, the amount of tertiary amine (e.g. triethylamine) relative to the total amount of sample, alkali metal alkoxide, solvent and tertiary amine (e.g. triethylamine) (and optionally phase transfer catalyst) is from 0.5% to 12% by w / v, for instance about 10% by w / v.The present invention also relates to a method of producing light emission from a marked product, which method comprises contacting a sample comprising a marked composition;wherein the marked composition comprises (a) a product and (b) a compound which is an acridan of formula (I)whereinX is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; andR2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl,with an organic superbase.The compound used in said method may be a compound of Formula (I) as further described anywhere herein.The present method also has the advantage that it does not require oxidation with an oxidation reagent such as hydrogen peroxide, which leads to variability associated with the additional chemical reaction.Often reagent systems can be affected by background noise resulting from unmarked fuels and other components in a sample. The present inventors have found that the addition of a tertiary amine (e.g. triethylamine) can reduce background noise. Therefore, the present invention provides a method wherein the method further comprises contacting the sample with triethylamine.An organic superbase, as used herein, is a charge-neutral compound with a basicity which is greater than that of a proton sponge (pKBH+= 18.6 in MeCN).Typically an organic superbase is an amidine, guanidine or phosphazene compound. Often the organic superbase is an amidine or guanidine compound.An amidine compound is defined here in as a compound including an amidine functional group, i.e. a group of structure:wherein each R may be the same or different, and wherein two R groups may join together to create cyclic structures. A guanidine compound is defined herein as a compound including a guanidine functional group, i.e. a group of structure:wherein each R may be the same or different, and wherein two R groups may join together to create cyclic structures.Often the amidine or guanidine compound is selected from l,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5 -diazabicyclo [4.3.0]non-5-ene (DBN), l,5-diazabicyclo[4.4.0]dec-6-ene (DBD), 7-methyl-l,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) and l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).Typically the amidine or guanidine compound is TBD. TBD provides an especially quick chemiluminescence profile.Typically, the amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is less than or equal to 25% by weight / volume (w / v). It is preferably from 0.1% by w / v to 20% by w / v. Often, for example, the amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is from 0.5% to 3% by w / v, for instance about 2.5% by w / v. This may be the concentration when the organic superbase is dissolved e.g. in DEGDME. Often, the amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is from 8% to 12% by w / v, for instance about 10% by w / v. This may be the concentration when the organic superbase is dissolved e.g. in DMSO.Typically, the amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is less than or equal to 25% by weight / volume (w / v). Often, the organic superbase is dissolved in a mixture of DEGDME and DMSO. This is advantageous because it negates the need for the tertiary amine. Additionally, it has been found that the solvent which is a mixture of DMSO and DEGDME performs well over a wider temperature range with low variability in the luminescence across different temperatures. Often the volume ratio of the DEGDME and the DMSO is from 6: 1 to 1:1, for instance from 5: 1 to 3:2. The amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is often from 0.1% by w / v to 20% by w / v. Often, for example, the amount is from 0.5% to 5% by w / v, for instance about 2.5% by w / v or 3.4% by w / v. Alternatively, the amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is from 5% to 15%, for instance 8% to 12% by w / v, for example about 10% by w / v.Organic superbases are highly effective reagents in particular for the chemiluminescence of esters bearing electron donating groups (e.g. alkoxy or amine groups). Often, therefore, two or more of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from unsubstituted or substituted C1-20 alkoxy, and -NR11R12, wherein R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl; and wherein the others of R2, R3, R4, R5, R6, R7, R8and R9are H.Typically, two or more of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from unsubstituted or substituted C1-20 alkoxy, and -NR11R12, wherein R11and R12are each independently selected from H or unsubstituted C1-4 alkyl; and wherein the others of R2, R3, R4, R5, R6, R7, R8and R9are H.Often two of R2, R3, R4, R5, R6, R7, R8and R9are -OCH3 or -NHCH3 and the others of R2, R3, R4, R5, R6, R7, R8and R9are H. Typically, in such cases, R3and R8are -OCH3 or -NHCH3.Often, two of R2, R3, R4, R5, R6, R7, R8and R9are -OCH3 and the others of R2, R3, R4, R5, R6, R7, R8and R9are H. Typically, in such cases, R3and R8are -OCH3.Organic superbases may also be used for chemiluminescence of esters bearing electron withdrawing groups (e.g. halo groups). Therefore, when one, two or more than two R2, R3, R4, R5, R6, R7, R8and R9are halo groups, the organic superbase may be a phosphazene, guanidinophosphazene or proazaphosphatrane.Often the superbase is in a solvent. Typically in a non-aqueous solvent. Often the solvent is a polar aprotic solvent. The solvent may be acetonitrile, dimethylsulfoxide (DMSO), N-methyl pyrrolidone, N, N-dimethyl acetamide, dimethyl phthalate, diethylene glycol diethyl ether (DGDEE) or diethylene glycol dimethyl ether (DEGDME). The solvent is typically dimethyl sulfoxide (DMSO) or diethylene glycol dimethyl ether (DEGDME). Often, the solvent comprises both DMSO and DEGDME. Often the volume ratio of the DEGDME and the DMSO is from 6: 1 to 1:1, for instance from 5: 1 to 3:2, for instance about 4: 1 or about 2:1.It is also advantageous to add a tertiary amine such as triethylamine which decreases the level of background noise associated with the product or other solvents employed in the method.Triethylamine can also be added together with the subsequently, sequentially or simultaneously with the organic superbase and optional solvent. The combination of an organic super base and a tertiary amine (e.g. triethylamine) is a highly effective method to produce chemiluminescence of acridans of the invention with reduced background noise.As shown in Table 1 of the specification, this effect is not observed for known acridans in the prior art (such as compound 3).Accordingly, the method may further comprise contacting said sample comprising the marked composition with a tertiary amine. Said tertiary amine may be triethylamine.Typically, the amount of tertiary amine (e.g. triethylamine) relative to the total amount of sample, organic superbase, solvent and tertiary amine (e.g. triethylamine) is less than or equal to 25% by w / v. It is preferably from 0.1% by w / v to 15% by w / v. Often, for example, the amount of tertiary amine (e.g. triethylamine) relative to the total amount of sample, organic superbase, solvent and tertiary amine (e.g. triethylamine) is from 0.5% to 12% by w / v, for instance about 10% by w / v.Preferably, the superbase is in a solvent which is a mixture of DMSO and DEGDME. This is advantageous because it negates the need for the tertiary amine. Additionally, it has been found that the solvent which is a mixture of DMSO and DEGDME performs well over a wider temperature range with low variability in the luminescence across different temperatures. Often the volume ratio of the DEGDME and the DMSO is from 6: 1 to 1:1, for instance from 5: 1 to 3:2.Often, for any method described herein (i.e. methods using alkali metal alkoxides and methods using an organic superbase), the method further comprises contacting a compound of the invention with acetonitrile. This may be subsequently, sequentially or simultaneously with contact with an alkali metal alkoxide or an organic superbase.Acetonitrile is typically added to a marked product or composition prior to addition of the superbase, such the method of the invention may comprise a first step of adding acetonitrile to a sample of a marked composition or product, and a second step of adding a superbase. Addition of acetonitrile further enhances the intensity and sharpness of chemiluminescence of a sample.Often, the methods as described herein (method using alkali metal alkoxides and methods using an organic superbase, but typically methods using an organic superbase) comprise a step of contacting the sample with acetonitrile and contacting the sample with a tertiary amine (e.g. triethylamine). These steps often happen simultaneously. Indeed, the step may be a single step of contacting the sample with a mixture of acetonitrile and a tertiary amine.Often the step of contacting the sample with tertiary amine and acetonitrile precedes a step of contacting the sample with an organic superbase (e.g. TBD) and a solvent.The method of the invention may therefore include a first step of contacting the sample with acetonitrile and a tertiary amine, and a second step of contacting the sample with an organic superbase and a solvent. Often the tertiary amine is triethylamine. Typically the organic superbase is TBD. Often, the solvent is diethylene glycol dimethyl ether (DEGDME).Therefore the method of the invention may include a first step of contacting the sample with acetonitrile and triethylamine, and a second step of contacting the sample with TBD in diethylene glycol dimethyl ether.Such a method may typically be used when the marked composition comprises a product which is diesel.Often the method of producing light emission from a marked product comprises contacting a sample comprising a marked composition with an organic superbase in a solvent, wherein the organic superbase is typically TBD, and the solvent is typically DMSO. Such a method is often employed when the marked composition comprises a product which is petrol.Alternatively, the methods described herein do not include a step (for instance said first step) of contacting the sample with a tertiary amine, for instance contacting the sample with acetonitrile and a tertiary amine. Typically the organic superbase is TBD. Often, the solvent is a mixture of DMSO and DEGDME. Often the volume ratio of the DEGDME and the DMSO is from 6: 1 to 1:1, for instance from 5: 1 to 3:2, for instance about 4: 1 or about 2:1. Often in such embodiments, the method does not comprise contacting the sample comprising the marked composition with triethylamine.Typically, in such embodiments, the method does not comprise contacting the sample comprising the marked composition with any tertiary amine.It may be that the method of the invention comprises contacting a compound of the invention with both an alkali metal alkoxide and an organic superbase, as described anywhere herein.Typically, the amount of marker compound in sample used in the methods herein, is the same as the amount of the marker compound described for marked compositions here. Although it may decrease if the marker composition or marked product is diluted (e.g. by a solvent).In some methods of the invention (such as methods using organic superbases), the amount of the compound of the invention in a marked composition may be from 1 ppm to 1000 ppm or from 1 ppm to 100 ppm. The amount of a compound in a sample may be the same as the concentration of the compound in a marked composition, or maybe less if the sample comprises a further component, e.g. a solvent or carrier composition. Typically the concentration of a compound of the invention in a sample is the same as the concentration of a compound in a marked composition. Therefore, in some methods of the invention (such as methods using organic superbases), the amount of the compound of the invention in a sample may be from 1 ppm to 1000 ppm or from 1 ppm to 100 ppm.If the concentration of the compound in the marked composition is from around 100 ppm to 1000 ppm, the marked composition may be diluted prior detection.The methods described above (i.e. methods using alkali metal alkoxides and methods using an organic superbase) may comprise an additional step of detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition. (If the marked composition comprises more than one marker compound as described herein, the presence of each of these may be determined.)The step of detecting light may be carried out using any suitable means and any suitable instrument. For example, the step of detecting light may comprise detecting light using PMT technology or using solid state detector (e.g. a portable solid state detector). While PMT technology is not as portable as many solid state detectors, it could allow for detection of compounds of the invention at even lower concentrations (e.g. ppt). For such low concentrations, the marked product may be other than a marked fuel. Often the step of detecting light is carried out using a solid state detector. The solid state detector is often an easily transportable solid state detector, i.e. a field solid state detector. The solid state detector may be a Hygiena™, Ensure™, Odyssey™, Horiba™ or Lu-mini ™. It is typically a Lu-mini™.In the method of the invention, the step of detecting light emitted from the sample may comprise detecting light of a particular wavelength or detecting light having particular emission kinetic-profile, and thereby identifying the marker compound as having a particular acridan of Formula (I).Often, the marked composition comprises at least two different acridans of Formula (I). Typically two or more different acridans emit light at two or more different wavelengths.For methods as described herein, the product may comprise a petroleum product, a fuel, crude oil, lubricating oil, a vegetable oil or fat, animal oil or fat, algae oil, a fuel additive, a fatty acid methyl ester or biodiesel. Often the fuel comprises a hydrocarbon fuel, ethanol or methanol.In the method described herein, the marked composition may further comprise an additive. Typically the amount of the additive in the marked composition relative to the amount of the marker compound in the marked composition is known. Therefore, the method may further comprise measuring the amount of light emitted from the sample, determining the amount of the marker compound in the marked composition based on the amount of light emitted, and calculating the amount of the additive in the marked composition based on the amount of the marker compound in the marked composition.Quantitative detectionFor method described herein the method may further comprise measuring the amount of light emitted from a sample (typically in relative light units per second (RLU / s)), and determining the amount of the marker compound in the marked composition based on the amount of light emitted.The concentration (or mass fraction) of the marker compound in a sample may reliably be determined by measuring the amount of light emitted from the sample after initiating the marker compound’s chemiluminescent reaction. By determining the marker’s concentration in the bulk through quantitative detection, useful information may be derived. For instance, a lower-than-expected concentration may reveal that a marked material has been diluted or mixed with a different material, either deliberately or inadvertently. Alternatively, if the marker compound was initially present in an additive (such as a fuel additive) at a known concentration, quantitative detection of the marker in a bulk fuel would reveal whether or not the additive is present at the correct concentration in the fuel.Accordingly, the method of detecting a marker compound in a product may further comprise measuring the amount of the light emitted from the sample; and determining the amount of the marker compound in the marked composition or marked product based on the amount of light emitted.The amount of light emitted from a sample may be measured by a luminometer, and it is typically reported by the luminometer as the average relative light units emitted per second (RLU / s), over the measurement period. The measurement period is typically less than 20 seconds beginning with initiation of the chemiluminescent reaction of the marker compound. More typically, it is less than 10 seconds beginning with initiation of the chemiluminescent reaction of the marker compound. Knowing the relationship between the amount of light emitted and the amount (e.g. the concentration or the mass fraction) of the marker compound in a given material, the amount of the marker compound present may be determined from the amount of light emitted.Thus, typically, determining the amount of the marker compound in the marked composition comprises determining the mass fraction or concentration of the marker compound in the marked composition, based on the amount of light emitted. Typically, the amount is equal to or less than 20 parts per billion by mass, for instance equal to or less than 10 parts per billion by mass. The amount may for instance be equal to or less than 5 ppb by mass, for instance less than 2 ppb by mass.The method of detecting the marker compound in the product may further comprise: determining whether the amount of the marker compound in the marked composition or product is the same as, more than, or less than, a reference amount. An amount less than the reference amount may indicate that the marked composition has been contaminated or diluted. Often, both the amount and the reference amount are mass fractions or concentrations. They are usually mass fractions, for instance expressed in ppb by mass.Preparing the sample comprising the marked composition for exposure to the one or more reagentsIn the method of the invention of detecting a marker compound in a marked product, the sample comprising the marked composition may comprise (i) the marked product itself and (ii) an organic solvent.Thus, prior to contacting the marked product with the one or more reagents for causing the marker compound to undergo a chemiluminescent reaction, the marked product may be added to an organic solvent.Accordingly, the method of the invention of detecting a marker compound in a product may further comprise: producing the sample comprising the marked product by adding the marked composition to the organic solvent.Typically the organic solvent comprises acetonitrile. Typically, the organic solvent is a mixture of acetonitrile and triethylamine. Typically, the amount of Triethylamine present in the acetonitrile solution is less than or equal to 25% by volume. It is preferably from 0.1% by to 15% by volume. Often, for example, the amount of triethylamine present in the acetonitrile solution is from 0.5% to 12% by volume, for instance about 10% by volume.The addition of such an organic solvent has been found to facilitate the process of exposing the marked product to the reagents for causing the marker compound to undergo a chemiluminescent reaction, which typically involves transfer of the sample comprising the marked composition into a luminometer where it is then exposed to the reagents, typically by exposing the sample to one or more “developer” solutions which comprise the reagents. The addition of the organic solvent to the marked product to produce the same may usefully reduce the viscosity of the marked composition prior tocontacting it with the one or more developer solutions and / or usefully increase the compatibility of the sample with the one or more developer solutions.Furthermore, in embodiments where the product is solid, the organic solvent may usefully extract or disperse or dissolve the solid matrix and the marker compound therein, prior to exposing the sample to the one or more reagents for causing the marker compound to undergo a chemiluminescent reaction.When the sample comprising the marked composition comprises (i) the marked product and (ii) an organic solvent, the volume ratio of the marked product to the organic solvent is typically from 1:50 to 10: 1. It may for instance be from 1:50 to 5:1, or for instance from 1:40 to 4: 1. Often, the volume ratio of the marked composition to the organic solvent is from 1: 10 to 3: 1. It may for instance be about 2:1.In other embodiments, however, no organic solvent is employed, and the marked composition itself is exposed directly to the one or more reagents for causing the marker compound to undergo a chemiluminescent reaction. Thus, the sample comprising the marked product may consist of the marked product.Reagents for causing the marker compound to undergo a chemiluminescent reaction For marker compounds of the invention, chemiluminescence can be initiated using a strongly basic compound under substantially non-aqueous conditions and in the presence of a source of oxygen (for example air) to initiate chemiluminescence. Examples of suitable strong bases include (but are not limited to) alkali metal alkoxides (e.g. potassium tert-butoxide) or organic superbases (e.g. 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)), as described above. Either of these can be dissolved in a suitable solvent.The invention therefore also provides a developer solution comprising an organic superbase (as described herein) or an alkali metal alkoxide (as described herein). The developer solution may also comprise a solvent.When using a metal alkoxide type base (e.g. potassium tert-butoxide), suitable solvents include dimethylformamide, toluene, tetrahydrofuran or preferably tertiary alcohols including tertbutanol or tert-pentanol (referred to as tert-amyl alcohol). It is also advantageous to add a phase transfer catalyst such as Cetyl Trimethyl Ammonium Chloride (CTAC) which improves the kinetics of reaction.Typically, the amount of alkali metal alkoxide present in the one or more developer solutions is less than or equal to 10% by weight / volume (w / v). It is preferably from 0.1% by w / v to 5% by w / v.Often, for example, the amount of alkali metal alkoxide present in the one or more developer solutions is from 0.1% to 1% by w / v, for instance about 0.5% by w / v.Therefore, typically, the amount of potassium tert-butoxide present in the one or more developer solutions is less than or equal to 10% by weight / volume (w / v). It is preferably from 0.1% by w / v to 5% by w / v. Often, for example, the amount of potassium tert-butoxide present in the one or more developer solutions is from 0.1% to 1% by w / v, for instance about 0.5% by w / v.Typically, the amount of phase transfer catalyst present in the one or more developer solutions is less than or equal to 10% by w / v. It is preferably from 0.09% by w / v to 4.5% by w / v. Often, for example, the amount of phase transfer catalyst present in the one or more developer solutions is from 0.09% to 0.9% by w / v, for instance about 0.45% by w / v.Therefore, typically, the amount of CTAC present in the one or more developer solutions is less than or equal to 10% by w / v. It is preferably from 0.09% by w / v to 4.5% by w / v. Often, for example, the amount of CTAC present in the one or more developer solutions is from 0.09% to 0.9% by w / v, for instance about 0.45% by w / v.When using an organic superbase (eg. TBD), satisfactory results are obtained using acridan esters which possess suitable electron donating substituents in the nucleus (for example a nucleus comprising a 2,7-dialkoxyacridan ester). It is preferable to make a developer solution by dissolving the TBD in a suitable solvent including acetonitrile, dimethylsulfoxide (DMSO), N-methyl pyrrolidone, N, N-dimethyl acetamide, dimethyl phthalate or preferably diethylene glycol dimethyl ether (DEGDME). It is also advantageous to add a tertiary amine such as triethylamine which decreases the level of background noise associated with the product or other solvents employed in the method. A tertiary amine can also be added together with the superbase / solvent mixture or alternatively (as described above) can be combined with the marked product containing the acridan ester prior to addition of the anhydrous base. Alternatively, the developer solution can be made by dissolving the TBD in a suitable solvent. Often, the solvent comprises both DMSO and DEGDME. This is advantageous because it negates the need for the tertiary amine for the purposes of improving the signal to noise ratio. Additionally, it has been found that the solvent which is a mixture of DMSO and DEGDME performs well over a wider temperature range with low variability in the luminescence across different temperatures. Often the volume ratio of the DEGDME and the DMSO is from 6: 1 to 1: 1, for instance from 5: 1 to 3: 2, for instance about 4: 1 or about 2:1.Typically, the amount of organic superbase present in the (trigger) developer solution is less than or equal to 25% by weight / volume (w / v). It is preferably from 0.1% by w / v to 10% by w / v. Often, for example, the amount of organic superbase present in the one or more developer solutions is from 0.5% to 3% by w / v, for instance about 2.5% by w / v.Therefore, typically, the amount of TBD present in the (trigger) developer solution is less than or equal to 25% by weight / volume (w / v). It is preferably from 0.1% by w / v to 10% by w / v. Often, for example, the amount of TBD present in the one or more developer solutions is from 0.5% to 3% by w / v, for instance about 2.5% by w / v.The amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is often from 0.1% by w / v to 20% by w / v. Often, for example, the amount is from 0.5% to 5% by w / v, for instance about 2.5% by w / v or 3.4% by w / v. This may be the concentration when the organic superbase is dissolved e.g. in a mixture of DMSO and DEGDME. Alternatively, the amount of organic superbase in relation to the total amount of sample and organic superbase (and other optional components) is from 5% to 15%, for instance 8% to 12% by w / v, for example about 10% by w / v. This may be the concentration when the organic superbase is dissolved e.g. in a mixture of DMSO and DEGDME.Typically, the amount of tertiary amine (e.g. triethylamine) present in the one or more developer solutions is less than or equal to 25% by w / v. It is preferably from 0.1% by w / v to 15% by w / v. Often, for example, the amount of tertiary amine (e.g. triethylamine) present in the one or more developer solutions is from 0.5% to 12% by w / v, for instance about 10% by w / v.Recovering the marked productThe invention also provides a method of recovering a marker compound from a marked product, the method comprising:(i) taking a sample from a marked product or composition as defined herein, and optionally;(ii) extracting a marker compound from the sample, wherein the marker compound is a compound of formula (I) as defined herein.The sample can be taken from the marked product by any appropriate means, such as cutting, scraping, extracting, pipetting, pouring, or syringing.To extract the marker compound from the sample, any appropriate method can be used. This includes but is not limited to: drying, crystallisation, liquid-liquid separation, solvent extraction, chromatography (such as column chromatography), distillation, or filtration.Tracer compoundsA marked product as discussed above, including in the context of a marked product of the invention or in the context of a method of the invention, may further comprise a tracer compound, optionally wherein the tracer compound comprises a nucleic acid, a deuterated hydrocarbon, a radioactive compound, a compound comprising a rare earth element, an antibody or an antigen. In some embodiments, the tracer compound comprises a nucleic acid, optionally wherein the nucleic acid is an oligonucleotide.Further, in some embodiments, the presence of the marker compound signifies the presence of a tracer compound in the marked composition.Therefore, the present invention also describes a method of detecting a marker compound in a product, as described anywhere herein, wherein the method comprises detecting the light emitted from the sample and thereby determining the presence of the marker compound and the tracer compound in the marked composition.The marked product described herein may, in some embodiments, comprise more than one tracer compound, i.e. it may comprise at least two different tracer compounds, for example many different tracer compounds.The terms “tracer compound” and “tag”, as used herein, refer to a compound which can be used to mark a product or composition to enable information about the product or composition to be identified. For instance, a tracer compound may signify the origin of the product or composition, so that if the tracer compound is identified in a product or composition the product or composition can be traced back to a particular source. A tracer compound should preferably be invisible, environmentally safe, secure, robust, detectable, recoverable and decipherable (so that information encoded in the compound can be revealed).Typically, the amount of the tracer compound in the marked composition is less than or equal to 100 parts per million (ppm) by mass, and it is often less than 10 ppm by mass. It may for instance be less than or equal to one part per million by mass, or for instance less than or equal to one part per billion by mass. It is often, for instance less than or equal to one part per trillion by mass.As mentioned above, the marker compounds (chemiluminescent precursors) described herein may be used as tracer compounds in their own right. Often, however, the marker compounds described herein are used as a “flag”, to flag up the fact that a tracer compound, or “tag” (that contains further information about the product) is also present. This is referred to as “flag and tag” technology.Typically, therefore, the presence of the marker compound in the marked product signifies the presence of the tracer compound in the marked product, and the method of the invention comprises detecting the light emitted from the sample and thereby determining the presence of the marker compound and the tracer compound in the marked product.Tracer compounds are known and include nucleic acids, and in particular oligonucleotides. For example, nucleic acid tracers have been developed for a wide range of commercial tracing applications (WO 91 / 17265; WO 95 / 02702; WO 00 / 61799; WO 03 / 080931; EP1354097B). Other kinds of tracer compound include, but are not limited to, deuterated hydrocarbons, rare earth elements and compounds comprising them, radioactive materials, and biomolecules, for instance a biomoleculecapable of binding to a second molecule (e.g. an antibody capable of binding to an antigen, or an antigen capable of binding to an antibody).Preferably, the tracer compound comprises a nucleic acid, for instance an oligonucleotide. A nucleic acid, such as an oligonucleotide has the ability to provide a large number of unique, secure codes (“fingerprints”) that can readily be decoded by the producer of the tag and generally by them alone. The tracer compound may therefore have a unique signature which effectively adds a “fingerprint” to the product, which can encode useful information about the product such as for instance its source and / or, for instance, the date of application of the tracer compound to the product, or date the matrix was produced at or obtained from its source, or for example the name of a particular vehicle or vessel on which the product was stored or transported, or for instance a particular place at which the product was stored, and / or the date it was so-stored or transported on a particular vehicle or vessel. The tracer compound can then be recovered from the product at any point and decoded by the producer of the tracer compound to verify the source of the product and / or any other useful encoded information such as that mentioned above or elsewhere herein.If need be, an oligonucleotide tracer compound can be modified with a hydrophobic group, e.g. as described in WO 2018 / 142131, to ensure solubility or stability in a non-aqueous product. Additionally or alternatively, it can be encapsulated for a greater level of protection of the tag in case of exposure to harsh conditions. An oligonucleotide tracer is advantageous because it meets all the requirements for an invisible, environmentally safe, secure, robust, recoverable and decipherable tag, which could allows enforcement agencies and other stakeholders to verify the source of a nonaqueous product (for instance a fuel or crude oil) at any point in a supply chain.Typically, therefore, the tracer compound in the marked product comprises an oligonucleotide. Oligonucleotides may be synthesised using well-established methods, for example by solid-phase synthesis using the phosphoramidite method and by using phosphoramidite building blocks derived from protected 2'-deoxynucleosides (dA, dC, dG, and T), ribonucleosides (A, C, G, and U), or chemically modified nucleosides, e.g. LNA or BNA. Oligonucleotide synthesis is typically carried out automatically, using commercially available, computer-controlled oligonucleotide synthesizers.Synthetic oligonucleotides are themselves available commercially from oligonucleotide synthesis companies such as ATDBio Ltd. (www.atdbio.com).An oligonucleotide tracer compound can readily be identified by performing standard techniques on the tracer compound such as, for instance amplification (typically by PCR) followed by sequencing. In this way, information encoded in the tracer compound can be revealed.The oligonucleotide is generally deoxyribonucleic acid (DNA). Accordingly, the nucleotides present in the oligonucleotide are typically independently selected from: deoxyadenosinemonophosphate (dAMP), deoxyguanosine monophosphate (dGMP), deoxythymidine monophosphate (dTMP) and deoxy cytidine monophosphate (dCMP).The oligonucleotide is generally single-stranded. However, as will be appreciated by the skilled person, a single stranded oligonucleotide may have one or more primers or probes hybridised thereto. The length of the oligonucleotide in the tracer compound may be from 25 to 200 nucleotides, for instance from 30 to 150 nucleotides, or from 35 to 120 nucleotides. Often, however, it is from 40 to 110 nucleotides, for instance from 50 to 90 nucleotides, e.g. from 60 to 80 nucleotides.Generally, the oligonucleotide of the tracer compound comprises a unique sequence of nucleotides, which provides information about a product that is tagged with the tracer compound. Thus, typically the oligonucleotide comprises a sequence of nucleotides which represents information about the product.The unique sequence of nucleotides may for instance denote a particular source, so that a nonaqueous matrix which was tagged with the tracer compound can subsequently be identified as originating from that source. The source is typically for instance an approved source, a certified source, a sustainable source and / or a legal source.Accordingly, said oligonucleotide typically comprises a sequence of nucleotides, which identifies the source from which the product was obtained or at which it was produced. The matrix can then subsequently be identified as originating from that source. The oligonucleotide may additionally indicate, for instance, a particular batch from which the product was obtained, and / or the time of producing the batch from which the product was obtained. The unique sequence of nucleotides may for instance denote a particular location. A product which was tagged with the tracer compound can then subsequently be identified as having been present at that particular location. The unique sequence of nucleotides may for instance denote a particular location at a particular point in time. A product which was tagged with the tracer compound can then subsequently be identified as having been present at that particular location at the particular point in time. The unique sequence of nucleotides may for example denote a particular batch. A product which was tagged with the tracer compound can then subsequently be identified as being from that particular batch. The unique sequence of nucleotides may for instance denote a particular batch produced at a particular location. A product which was tagged with the tracer compound can then subsequently be identified as being from that particular batch produced at that particular location. The unique sequence of nucleotides may for instance denote a particular batch produced at a particular location at a particular point in time. A product tagged with the tracer compound can then subsequently be identified as having been from that particular batch which was produced at the particular location at the particular point in time. The unique sequence of nucleotides may for instance denote a particular vehicle or vessel on which the product was stored or transported, and it may optionally for instance denote a particular date ordates during which the product was stored or transported on the particular vehicle or vessel, for instance it may denote a specific voyage on a particular vehicle or vessel.The unique sequence of nucleotides in the oligonucleotide, which provides information about a non-aqueous product that is labelled (or is to be labelled) with the tracer compound, is typically in a central region of the oligonucleotide (as opposed to at either of the 5’ or 3’ ends of the oligonucleotide). This is because the oligonucleotide generally also comprises a sequence of nucleotides at or near the 5’ end, and a sequence of nucleotides at or near the 3’ end, which carry predetermined sequences that will recognise appropriate complementary primers for use in PCR amplification and in sequencing of the PCR amplified nucleic acid.The oligonucleotide may for instance be represented as follows:wherein:region AB comprises a predetermined sequence for recognising a complementary primer suitable for amplifying nucleic acid in the oligonucleotide;region BC comprises a sequence of nucleotides which represents information about the matrix; andregion CD comprises a predetermined sequence for recognising a complementary primer suitable for amplifying nucleic acid in the oligonucleotide.The regions AB and CD, respectively, typically comprise predetermined sequences that will recognise complementary primers for use in PCR amplification (e.g. qPCR amplification). Regions AB and CD optionally additionally comprise complementary primers for use in sequencing of the amplified nucleic acid.The regions AB and CD generally will not vary from product to product. Rather, they will generally be constant for all tracer compounds in a particular batch, or “library”, of tracer compounds to be employed in accordance with the present invention. Each of the regions AB and CD of the oligonucleotide typically has a length of from 8 to 50 nucleotides, for instance from 10 to 40 nucleotides, or for instance from 15 to 30 nucleotides.The region BC, on the other hand, comprises a unique sequence of nucleotides that represents information about the product that is labelled (or is to be labelled) with the tracer compound. This is the region that gives each tracer compound its unique, characteristic signal, and will vary as the information about the product to be labelled with the tracer compound varies. The uniqueness of thesequence of nucleotides that represents information about the product is generally known only to the individuals using the tracer, thereby guaranteeing security of the information.The region BC may for instance comprise a sequence of nucleotides which identifies the source from which the product was obtained. The region BC may for instance comprise a sequence of nucleotides which identifies product as: having been present at a particular location, having been present at a particular location at a particular point in time, being from a particular batch produced at a particular location, or being from a particular batch produced at a particular location at a particular point in time.The region BC of the oligonucleotide may have a length of, for instance, from 5 to 120 nucleotides. It may for instance have a length of from 10 to 80 nucleotides, or for example from 15 to 50 nucleotides, for instance from 20 to 40 nucleotides. Indeed, if the BC region is 10 nucleotides in length then with the four bases available for a DNA molecule, there will be 1.048 x 106unique molecules capable of being synthesised. If the BC region is 15 bases long, then 1.07 x 109unique molecules can be synthesised. If the BC region is 30 bases long, 1.15 x 1018unique molecules can be synthesised. Each of these unique molecules can potentially represent different information about a product to be labelled.Thus, in the method of detecting the marker compound in the product (as well as in the other methods of the invention and the compositions of the invention), the marked product may further comprise a tracer compound, or a plurality of different tracer compounds. The tracer compound is typically an oligonucleotide, which may be as further defined above, although any other kind of tracer compound may be employed, for instance a deuterated hydrocarbon, a radioactive compound, a compound comprising a rare earth element, or a biomolecule capable of binding to a second molecule (for instance an antibody capable of binding to an antigen, or an antigen capable of binding to an antibody).Often, the amount of the tracer compound in the marked composition is less than or equal to 10 parts per million by mass. It may for instance be less than or equal to one part per million by mass, and is often less than or equal to one part per billion by mass, for instance less than or equal to one part per trillion by mass.Typically, the presence of the marker compound in the marked composition signifies the presence of the tracer compound in the marked composition. (This is referred to as “flag” and “tag” technology where the marker compound is the flag, which flags up the presence of the tracer compound which is the tag.) The method of the invention may therefore comprise detecting the light emitted from the sample and thereby determining the presence of the marker compound and the tracer compound in the marked composition. Having determined the presence of the tracer compound, the method may further comprise recovering the tracer compound from a sample of the markedcomposition. The method may then further comprise analysing the tracer compound to determine the identity of the tracer compound; and using the identity of the tracer compound to identify information about the marked product.The tracer compound may comprise a nucleic acid - for instance an oligonucleotide which may be as further defined above - and analysing the tracer compound may comprise analysing the nucleic acid of the tracer compound to determine the identity of the tracer compound. The identity of the tracer compound can readily be determined by amplifying and sequencing the nucleic acid. Thus, for instance, the tracer compound may be an oligonucleotide, and the abovementioned step of analysing the tracer compound to determine the identity of the tracer compound may comprise: (a) amplifying the oligonucleotide; and (b) sequencing the amplified oligonucleotide. The oligonucleotide may be amplified by performing PCR (the polymerase chain reaction) on the tracer compound. Accordingly, step (a) may comprise performing PCR on the tracer compound. PCR and sequencing are both very well-known techniques. The PCR may for instance be qPCR (quantitative polymerase chain reaction), which is also well known. The sequencing in step (b) may be by any suitable method. Methods for sequencing oligonucleotides are well known in the art.The methods of the invention may therefore further comprise recovering a tracer compound from a sample of a marked composition.The methods may therefore also comprise analysing the tracer compound to determine the identity of the tracer compound; and using the identity of the tracer compound to identify information about the marked composition. Often in such methods the tracer compound comprises a nucleic acid and analysing the tracer compound comprises analysing the nucleic acid of the tracer compound to determine the identity of the tracer compound. Determining the identity of a tracer compound may be by amplifying and sequencing the nucleic acid.The step of using the identity of the tracer compound to identify information about the marked composition may comprise entering the sequence of the tracer compound into a database and retrieving the information about the marked composition that is associated with the sequence.The information retrieved may for instance be information such as the source from which the product was obtained or at which it was produced, a particular batch from which the product was obtained, a particular location at which the product was present, a particular vehicle or vessel on which the product was stored or transported, or a particular point in time at which the product was produced, obtained, stored, transported, or present at a particular location.EXAMPLESSynthesis of Acridan Esters.Example 1 - Preparation of 9-(2,6-dimethylphenoxycarbonyl)-10-methylacridinium benzenesulfonate (2).Compound 1 Compound- 2To a 50ml reaction flask was added 1.25g of 2,6-dimethylphenyl 9-acridinecarboxylate (1) (prepared from 9-Acridine carboxylic acid and 2,6-dimethylphenol using conditions reported in W02008 / 067055). To this was added 2ml of methyl benzenesulfonate and heated at 110°C for 4hrs. After cooling to ambient, 40ml of ethyl acetate was added and the mixture was granulated for Ihr. The product was collected by filtration and washed with 4ml of methyl tert-butyl ether. After drying at 40°C, 1.8g (94% yield) of 9-(2,6-dimethylphenoxycarbonyl)-10-methylacridinium benzenesulfonate (2) was obtained as a yellow solid.1H NMR (d6DMSO) delta: 2.45(6H); 5.0(3H); 7.1(3H); 7.4-7.7(5H); 8.2(2H); 8.55-8.65(4H); 9.0(2H).Example 2: Preparation of 9-(2,6-dimethylphenoxycarbonyl)-10-methylacridan (3).Compound 3To a 150ml reaction flask was added 1.8g of (2) followed by 100ml of acetonitrile and 20ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 65mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 25ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.7g (57%) of 9-(2,6-dimethylphenoxy carbonyl)-! 0-methylacridan (3)1H NMR (CDCl3) delta: 1.7(6H); 3.4(3H); 5.2(1H); 6.92(3H); 7.0(4H); 7.32(2H); 7.38(2H).TOF MS ES+ data for M+l: 344Chemiluminescence: 200uL (at lOppb) of (3) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.64million RLU / s.Example 3 - Preparation of 9-(2,6-diisopropylphenoxycarbonyl)-10-methylacridinium benzenesulfonate (5)Compound 4 Compound 5To a 50ml reaction flask was added 0.7g of 2,6-diisopropylphenyl 9-acridinecarboxylate (4) (prepared from 9-Acridine carboxylic acid and 2,6-diisopropylphenol using conditions reported in W02008 / 067055). To this was added 1.2ml of methyl benzene sulfonate and heated at 110°C for 4hrs. After cooling to ambient, 2ml toluene and 7.5ml methyl tert-butylether were added and the mixture was granulated for 16hr. The product was collected by filtration and washed with 4ml of methyl tertbutyl ether. After drying at 40°C, 0.91g (91% yield) of 9-(2,6-diisopropylphenoxycarbonyl)-10-methylacridinium benzene sulfonate (5) was obtained as a yellow solid.’HNMR (d6DMSO) delta: 1.3(12H); 3.2(2H); 5.0(3H); 7.3-7.4(5H); 7.5(3H): 8.2(2H); 8.55-8.65(4H); 9.0(2H).ExampleCompound 5 Compound 6To a 50ml reaction flask was added 0.85g of (5) followed by 20ml of acetonitrile and 10ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 25ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.5g (82%) of 9-(2,6-diisopropylphenoxycarbonyl)-10-methylacridan (6)1H NMR (CDCl3) delta: 0.8-1.0(12H); 2.4(2H); 3.4(3H); 5.2(1H); 6.98-7.02(6H); 7.1(1H); 7.34(2H); 7.39(2H).TOF MS ES+ data for M+l: 400Chemiluminescence: 200uL (at lOppb) of (6) in toluene triggered with (Method A) KOBu’ in tert Amyl alcohol reagent gave 0.3million RLU / s.Example 5 - Preparation of 9-(2,4,6-trimethylphenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (8)Compound 7 Compound 8The synthesis of compound 8 involves methylation of 7 and was reported by K. Kryminski, Spectrochimica Acta Part A 78 (2011) 401.Example 6: Preparation of9-(2,4,6-trimethylphenoxycarbonyl)-10-methylacridan (9).Compound 9To a 50ml reaction flask was added 0.74g of (8) followed by 20ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.52g (99%) of 9-(2,4,6-trimethylphenoxycarbonyl)-10-methylacridan (9)’HNMR CCDCh) delta: 1.7(6H); 2.2(3H); 3.3(3H); 5.2(1H); 6.72(2H); 6.99(4H); 7.31(2H); 7.38(2H). TOF MS ES+ data for M+1 : 358Chemiluminescence: 200uL (at lOppb) of (9) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.46million RLU / s.Example 7 - Preparation of9-(2,3,6-trimethylphenoxycarbonyl)-10-methylacridiniumtri fluoromethane sulfonate (11)Compound 10 Compound 11To a 50ml reaction flask was added 0.7g of 2,3,6-trimethylphenyl 9-acridinecarboxylate (10) (prepared from 9-Acridine carboxylic acid and 2,3,6-trimethylphenol using conditions reported in W02008 / 067055). To this was added 10ml dichloromethane and 0.7ml of methyl trifluoromethanesulfonate and stirred at ambient over 16hrs. The product was collected by filtration and washed with 4ml of methyl tert-butyl ether. After drying at 40°C, 0.88g (85% yield) of 9-(2,3,6-trimethylphenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (11) was obtained as a yellow solid.1H NMR (d6DMSO) delta: 2.25(3H); 2.3(3H); 2.4(3H); 5.0(3H); 7.2(2H); 8.2(2H); 8.5-8.6(4H); 9.0(2H).Example 8: Preparation of9-(2,3,6-trimethylphenoxycarbonyl)-10-methylacridan (12).C 3 Compound 12To a 50ml reaction flask was added 0.74g of (11) followed by 20ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.52g (99%) of 9-(2,3,6-trimethylphenoxycarbonyl)-10-methylacridan (12)’HNMR CCDCh) delta: 1.65(3H); 1.7(3H); 2.2(3H); 3.4(3H); 5.2(1H); 6.82(2H); 7.0(4H); 7.31(2H); 7.39(2H).TOF MS ES+ data for M+1 : 358Chemiluminescence: 200uL (at lOppb) of (12) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.73million RLU / s.Example 9 - Preparation of 9-(4-chloro-2,6-dimethylphenoxycarbonyl)-10-methylacridinium trifluoromethane sulfonate (14)Compound 13 Compound 14To a 50ml reaction flask was added 0.7g of 4-chloro-2,6-dimethylphenyl 9-acridinecarboxylate (13) (prepared from 9-Acridine carboxylic acid and 4-chloro-2,6-dimethylphenol using conditions reported in W02008 / 067055). To this was added 10ml dichloromethane and 0.8ml of methyl trifluoromethanesulfonate and stirred at ambient over 16hrs. The product was collected by filtration and washed with 4ml of methyl tert-butylether. After drying at 40°C, 1g (99% yield) of 9-(4-chloro-2,6-dimethylphenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (14) was obtained as a yellow solid.'HNMR (d6DMSO) delta: 2.45(6H); 5.0(3H); 7.5(2H); 8.25(2H); 8.5-8.6(4H); 9.0(2H).Example 10:CH3 Compound 15To a 50ml reaction flask was added 0.85g of (14) followed by 15ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in lOOmg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry wastreated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.55g (90%) of 9-(4-chloro-2,6-dimethylphenoxy carbonyl)-! 0-methylacridan (15)1H NMR (CDCl3) delta: 1.75(6H); 3.4(3H); 5.2(1H); 6.9(2H); 7.0(4H); 7.32(2H); 7.36(2H).TOF MS ES+ data for M+1 : 378Chemiluminescence: 200uL (at lOppb) of (15) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.81million RLU / s.Example 11 - Preparation of9-(2,6-dimethoxyphenoxycarbonyl)-10-methylacridiniumtri fluoromethane sulfonate (17).Compound- 16 Compound 17The synthesis of compound 17 involves methylation of 16 and was reported by K. Krzymiski, Acta Crystallogr Section E (2009), 65, 789.Example 12: Preparation of 9-(2,6-dimethoxyphenoxycarbonyl)-10-methylacridan (18).Compound 18To a 50ml reaction flask was added 0.64g of (17) followed by 25ml of acetonitrile and 10ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a gum. The gum was treated with 25ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.45g (99%) of 9-(2, 6-dimethoxyphenoxycarbonyl)-! 0-methylacridan (18)1H NMR (CDCl3) delta: 3.4(3H); 3.6(6H); 5.3(1H); 6.5(2H); 6.98(4H); 7.03(1H); 7.3(2H); 7.45(2H). TOF MS ES+ data for M+1 : 376Chemiluminescence: 200uL (at lOppb) of (18) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.21million RLU / s.Example 13 - Preparation of 9-(4-methoxycarbonyl-2,6-dimethoxyphenoxycarbonyl)-10-methylacridinium benzene sulfonate (20).Compound 19 Compound 20To a 50ml reaction flask was added 0.8g of 4-methoxy carbonyl -2, 6-dimethoxyphenyl 9-acridinecarboxylate (19) (prepared from 9-Acridine carboxylic acid and methyl syringate using conditions reported in W02008 / 067055). To this was added 1.3ml of methyl benzene sulfonate and stirred at 110C for 3hrs. The reaction was cooled to 50°C and diluted with 5ml of ethyl acetate and stirred for 30mins. The product was then collected by filtration and washed with 2ml of ethyl acetate. After drying at 40°C, 0.6g (53% yield) of 9-(4-methoxycarbonyl-2,6-dimethoxyphenoxycarbonyl)-10-methylacridinium benzene sulfonate (20) was obtained as a yellow solid.1H NMR (d6DMSO) delta: 3.9(3H); 4.1(6H); 4.9(3H); 7.3(3H); 7.5(2H); 7.6(2H); 8.3(2H); 8.5-8.6(4H); 8.9(2H).Example 14: Preparation of 9-(4-methoxycarbonyl-2,6-dimethoxyphenoxycarbonyl)-10-methylacridan (21).Compound 21To a 50ml reaction flask was added 0.64g of (20) followed by 25ml of acetonitrile and 10ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a gum. The gum was treated with 25ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated di chloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.45g (99%) of 9-(4-methoxycarbonyl-2,6-dimethoxyphenoxycarbonyl)-10-methylacridan (21)1H NMR (CDCl3) delta: 3.4(3H); 3.65(6H); 3.9(3H); 5.3(1H); 6.99(4H); 7.21(2H); 7.3(2H); 7.43(2H). TOF MS ES+ data for M+1 : 434Chemiluminescence: 200uL (at lOppb) of (21) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.14million RLU / s.Example 15 - Preparation of 9-(2,6-difluorophenoxycarbonyl)-10-methylacridinium trifluoromethane sulfonate (23).Compound 22 Compound 23The synthesis of compound 23 involves methylation of compound 22 and was reported by K. Smith Luminescence (2024) 39 (6) 4794.Example 16: Preparation of 9-(2,6-difluorophenoxycarbonyl)-10-methylacridan (24).Compound 24To a 50ml reaction flask was added 0.4g of (23) followed by 12ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.25g (90%) of 9-(2, 6- difluorophenoxycarbonyl)-! 0-methylacridan (24)’HNMRCCDCh ) delta: 3.4(3H); 5.3(1H); 6.86(2H); 7.0(2H); 7.03(2H); 7.07(lH); 7.33(2H);7.40(2H).TOF MS ES+ data for M+l: 352Chemiluminescence: 200uL (at lOppb) of (24) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.12million RLU / s.Example 17 - Preparation of 9-(2-methoxy-6-methylphenoxycarbonyl)-10-methylacridinium trifluoromethane sulfonate (26).Compound 25 Compound 26To a 50ml reaction flask was added 0.7g of 2-methoxycarbonyl-6-methylphenyl 9-acridinecarboxylate (25) (prepared from 9-Acridine carboxylic acid and methyl 2-methoxy-6-methylphenol using conditions reported in W02008 / 067055). To this was added 10ml dichloromethane, 0.7ml of methyl trifluoromethanesulfonate and stirred at ambient for 16hrs. The product was collected by filtration and washed with 4ml of methyl tert-butyl ether. After drying at 40°C, 0.93g (93% yield) of 9-(2-methoxy-6-methylphenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (26) was obtained as a yellow solid.1H NMR(d6DMSO) delta: 2.4(3H); 4.1(3H); 5.0(3H); 7.1(1H); 7.2(1H); 7.3(1H); 8.2(2H); 8.6(2H); 8.7(2H); 8.9(2H).Example 18: Preparation of 9-(2-methoxy-6-methylphenoxycarbonyl)-10-methylacridan (27).Compound 27To a 50ml reaction flask was added 0.45g of (26) followed by 12ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a gum. The gum was treated with 25ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give 0.36g crude solid. The solid was recrystallized from 1ml methanol, filtered and dried at 40°C to provide 0.15g (47%) of 9-(2-methoxy-6-methylphenoxycarbonyl)-10-methylacridan (27)1H NMR (CDCl3) delta: 1.8(3H); 3.4(3H); 3.6(3H); 5.25(1H); 6.7(2H); 7.0(5H); 7.3(2H); 7.42(2H). TOF MS ES+ data for M+1 : 360Chemiluminescence: 200uL (at lOppb) of (27) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.59million RLU / s.Example 19 - Preparation of 9-(4-cyano-2,6-dimethylphenoxycarbonyl)-10-methylacridinium trifluoromethane sulfonate (29)Compound 29To a 50ml reaction flask was added 1.0g of 4-cyano-2,6-dimethylphenyl 9-acridinecarboxylate (28) (prepared from 9-Acridine carboxylic acid and 4-cyano-2,6-dimethylphenol using conditions reported in W02008 / 067055). To this was added 10ml dichloromethane and 1.0ml of methyl trifluoromethanesulfonate and stirred at ambient over 16hrs. The product was collected by filtration and washed with 4ml of methyl tert-butylether. After drying at 40°C, 1g (68% yield) of 9-(4-cyano-2,6-dimethylphenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (29) was obtained as a yellow solid.1H NMR (d6DMSO) delta: 2.45(6H); 5.0(3H); 7.5(2H); 8.2(2H); 8.55(2H); 8.65(2H); 9.0(2H).Example 20: Preparation of 9-(4-cyano-2,6-dimethylphenoxycarbonyl)-10-methylacridan (30).Compound 30To a 50ml reaction flask was added 1.0g of (29) followed by 50ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in lOOmg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.7g (98%) of 9-(4-cyano-2, 6-dimethylphenoxy carbonyl)-! 0-methylacridan (30)’HNMR CCDCh ) delta: 1.75(6H); 3.4(3H); 5.2(1H); 7.01(4H); 7.23(2H); 7.33(2H); 7.35(2H).TOF MS ES+ data for M+1 : 369Chemiluminescence: 200uL (at lOppb) of (30) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.52million RLU / s.Example 21 - Preparation of 9-(phenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (32)Compound 31 Compound 32The synthesis of compound 32 involves methylation of 31 and was reported by K. Kryminski, Spectrochimica Acta Part A 78 (2011) 401.Example 22:C 3 Compound 33To a 50ml reaction flask was added 1.0g of (32) followed by 50ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in lOOmg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a thick slurry. The slurry was treated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was dried at 40°C to provide 0.62g (95%) of 9-(phenoxycarbonyl)-10- methylacridan (33)1HNMR (CDC13) delta: 3.4(3H); 5.2(1H); 6.93(2H); 7.01(4H); 7.15(1H); 7.27(2H); 7.32(2H);7.39(2H).TOF MS ES+ data: 316Chemiluminescence: 200uL (at lOppb) of (33) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.56million RLU / s.Example 23 - Preparation of 9-(2-chloro-6-methylphenoxycarbonyl)-10-methylacridinium trifluoromethane sulfonate (35).Compound 34 Compound 35To a 50ml reaction flask was added 0.7g of 2-chloro-6-methylphenyl 9-acridinecarboxylate (34) (prepared from 9-Acridine carboxylic acid and 2-chloro-6-methylphenol using conditions reported in W02008 / 067055). To this was added 10ml dichloromethane, 0.7ml of methyl trifluoromethanesulfonate and stirred at ambient for 16hrs. The product was collected by filtration and washed with 4ml of methyl tert-butyl ether. After drying at 40°C, 0.96g (93% yield) of 9-(2-chloro-6- methylphenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (35) was obtained as a yellow solid.'HNMR (d6DMSO) delta:2.4(3H); 5.0(3H); 7.4-7.7(3H); 8.2(2H); 8.6(2H); 8.7(2H); 9.0(2H).Example 24: ycarbonyl)-10-methylacridan (36).Compound 36To a 50ml reaction flask was added 0.9g of (35) followed by 12ml of acetonitrile and 6ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 50mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a gum. The gum was treated with 30ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated and dried at 40°C to provide 0.63g (98%) of 9-(2-chloro-6-methylphenoxycarbonyl)-10-methylacridan (36)’HNMR CCDCh ) delta: 1.8(3H); 3.4(3H); 5.3(1H); 7.0(6H); 7.15(1H); 7.32(2H); 7.42(2H).TOF MS ES+ data for M+l: 364Chemiluminescence: 200uL (at lOppb) of (35) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.80million RLU / s.Example 25 - Preparation of 9-(methoxycarbonyl)-10-methylacridinium benzenesulfonate (38).Compound 37 Compound 38To a 50ml reaction flask was added 1.6g of methyl-9-acridinecarboxylate (37) (prepared from 9-Acridine carboxylic acid and methanol using conditions reported in W02008 / 067055). To this was added 2.5ml of methyl benzene sulfonate and stirred at 110C for 4hrs. Next, the reaction was cooled to 40C, then diluted with 7ml methyl tert-butylether and stirred overnight. The product was collected by filtration and washed with 2ml of methyl tert-butyl ether. After drying at 40°C, 2.4g (87% yield) of 9-(methoxycarbonyl)-lO-methylacridinium benzenesulfonate (38) was obtained as a yellow solid.1HNMR (d6DMSO) delta: 4.3(3H); 4.9(3H); 7.3-7.6(5H); 8.2(2H); 8.45(2H); 8.5(2H); 8.9(2H). Example 26: Preparation of 9-(methoxycarbonyl)-10-methylacridan (39).Compound 39To a 50ml reaction flask was added 2g of (38) followed by 100ml of acetonitrile and 20ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 200mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a gum. The gum was treated with 50ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The solid was recrystallized from 1ml methanol mixed with 1ml water, filtered and dried at 40°C to give 1g (85%) of 9-(2-methoxycarbonyl-6-methylphenoxycarbonyl)-10-methylacridan (39)1HNMR (CDC13) delta: 3.4(3H); 3.6(3H); 4.9(1H); 6.97(4H); 7.27(2H); 7.29(2H).TOF MS ES+ data for M+l: 254Chemiluminescence: 200uL (at lOppb) of (39) in toluene triggered with (Method A) KOBu‘ in tert Amyl alcohol reagent gave 0.02million RLU / s.Example 27 - Preparation of 9-(2,2,2-trichloroethoxycarbonyl)-10-methylacridinium trifluoromethane sulfonate (41).Compound 40 Compound 41To a 50ml reaction flask was added 1.0g of (2,2,2-trichloroethyl)-9-acridinecarboxylate (40) (prepared from 9-Acridine carboxylic acid and 2,2,2-trichloroethanol using conditions reported in W02008 / 067055). To this was added 15ml of dichloromethane, 0.5ml methyl trifluoromethanesulfonate and then stirred at ambient overnight. Next, the reaction was concentrated to a solid using a rotor-evaporator then diluted with 7ml methyl tert-butylether and stirred for lOhrs. The product was collected by filtration and washed with 2ml of methyl tert-butyl ether. After drying at 40°C, 1.1g (75% yield) of 9-(2,2,2-trichloroethoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate (41) was obtained as a yellow solid.'HNMR (d6DMSO) delta:4.93(3H); 5.68(2H); 8.15(2H); 8.53(2H); 8.57(2H); 8.93(2H).Example 28:C 3 Compound 42To a 50ml reaction flask was added 2g of (41) followed by 100ml of acetonitrile and 20ml acetic acid at 10°C. To this yellow solution was added sodium borohydride (in 200mg portions) until the solution was decolourised. The reaction was stirred for Ihr and 1.5ml water added. After stirring for a further 30mins, the mixture was concentrated on a rotary evaporator to give a gum. The gum was treated with50ml dichloromethane and washed twice with saturated aqueous sodium carbonate then brine. The separated dichloromethane phase was dried over magnesium sulfate and concentrated to give a crude solid. The...

Claims

1. CLAIMS1. A compound which is an acridan of formula (I)4. 6.wherein7.X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;8.R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; and9.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;10.wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl,11.provided that:12.(a) when X is phenyl, X is substituted with at least one group other than halo; and13.(b) when R1is methyl, X is aryl, and R2to R9are all H: X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(0)0CH3.

2. A compound according to claim 1, wherein X is aryl substituted with at least one group, for instance one or two groups, independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl, and optionally X is further substituted with at least one group, for instance one, two or three groups, independently selected from halo, unsubstituted or substituted C1-3 alkyl, unsubstituted or substituted C2-5 alkenyl, CN, C(O)OR, C(O)NR2, and OR,wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl.

3. A compound according to claim 1 or claim 2, wherein the acridan of formula (I) is an acridan of formula (la)17. 19.wherein Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl; and20.R1, R2, R3, R4, R5, R6, R7, R8and R9are as defined in claim 1,21.provided that:22.(a) at least one of Ra, Rb, Rc, Rdand Reis other than H and other than halo; and23.(b) when R1is methyl and R2to R9are all H: at least three of Ra, Rb, Rc, Rdand Reare other than H, at least two of Ra, Rb, Rc, Rdand Reare other than H and are different from each other, at least two of Ra, Rb, Rc, Rdand Reare other than H and other than methyl, and / or at least one of Ra, Rb, Rc, Rdand Reis other than H, -OH, methyl, -OCH3and -C(O)OCH3.

4. A compound according to claim 3, wherein one, two or three of Ra, Rb, Rc, Rdand Reare independently selected from halo, unsubstituted Ci-400 alkyl, CN, C(O)OR, and OR, wherein R is selected from H and unsubstituted C1-6 alkyl; and the others of Ra, Rb, Rc, Rdand Reare all H; and R1, R2, R3, R4, R5, R6, R7, R8and R9are as defined in claim 1.

5. A compound according to claim 1, wherein X is a C2-20 alkyl which is unsubstituted or substituted with from 1 to 9 groups selected from halo, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl, and25.R1, R2, R3, R4, R5, R6, R7, R8and R9are as defined in claim 1.

6. A compound according to any one of claims 1 to 5, wherein R1is unsubstituted C1-6alkyl, preferably wherein R1is methyl.

7. A compound according to any one of claims 1 to 6, wherein one, two, or more than two of R2, R3, R4, R5, R6, R7, R8and R9are independently selected from halo; unsubstituted C1-20 alkoxy; -N(R10)C(O)H; -NR11R12wherein R10, R11and R12are each H or unsubstituted C1-3 alkyl; and a group of formula (II):

29. 31.wherein R14is selected from -NR15R16, -OR15and -OC(O)R16, wherein R15and R16are each independently selected from H and unsubstituted C1-6alkyl.

8. A compound according to claim 3 or claim 4 wherein at least one of, for instance one or two of, Ra, Rb, Rc, Rdand Reare independently selected from unsubstituted or substituted C4-400 alkyl and unsubstituted or substituted C6-400alkenyl,33.and the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, unsubstituted or substituted C1-3 alkyl, unsubstituted or substituted C2-5 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl.

9. A compound according to claim 8 wherein at least one of, for instance one or two of, Ra, Rb, Rc, Rdand Reare independently selected from:35.unsubstituted or substituted C8-30 alkyl;36.unsubstituted or substituted C8-30 alkenyl;37.a Ce-400 alkyl comprising the following formula QI:

40.

41. wherein RQ1is -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100, optionally wherein formula (QI) is bonded to a terminal alkyl group having from 1 to 4 carbon atoms;42.• a C6-400alkenyl comprising an unsaturated unit of formula -CH=CH-, -CH=C(CH3)-CH2-, - CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, - C(CH3)=C(CH3)- or -CH2-CH=CH-CH2- which unsaturated unit is bonded to formula QI:

45. 47.wherein RQ1is -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2CH2- or -CH2-C(CH3)2-, and wherein the number of repeating units, t, is an integer from 3 to 100, optionally wherein formula (QI) is bonded to a terminal alkyl group having from 1 to 4 carbon atoms; and48.• a C6-400alkenyl comprising the following formula QII:

50. 52.wherein the number of repeating units, t, is an integer from 3 to 100 and wherein each RQ2is independently selected from an unsaturated repeating unit of formula -CH=CH-, -CH=CH-CH2-, -CH=C(CH3)-, -CH2-CH2-CH=CH-, -CH2-CH=C(CH3)-, -CH=CH-CH(CH3)-, -C(CH3)=C(CH3)- or -CH2-CH=CH-CH2- and a saturated repeating unit of formula -CH2-CH2-, -CH(CH3)-, -CH2-CH2-CH2-, -CH2-CH(CH3)-, -CH2-CH2-CH2-CH2- or -CH2-C(CH3)2-, provided that at least one RQ2is an unsaturated repeating unit, optionally wherein formula (QII) is bonded to a terminal alkyl group having from 1 to 4 carbon atoms;53.optionally wherein the others of Ra, Rb, Rc, Rdand Reare each independently selected from H, halo, and unsubstituted C1-3 alkyl.

10. A compound according to claim 8 or claim 9 wherein one, two, or three of R2, R3, R4, R5, R6, R7, R8and R9are C1-20 alkoxy, each of the others of R2, R3, R4, R5, R6, R7, R8and R9are H, and R1is unsubstituted C1-6alkyl.

11. A compound according to any one of claims 8 to 10 wherein R3is -OCH3, R8is -OCH3, R2, R4, R5, R6, R7and R9are H, and R1is -CH3.

12. A compound according to any one of claims 8 to 11 wherein:57.(a) Rais H; Rbis H; Rcis unsubstituted C8-20 alkyl; Rdis H; and Reis H;58.(b) Rais H; Rbis H; Rcis unsubstituted C40-400 alkyl; Rdis H; and Reis H; (c) Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C16-20 alkyl; Rdis H; and Reis unsubstituted C1-3 alkyl;59.(d) Rais unsubstituted C1-3 alkyl; Rbis H; Rcis H; Rdis unsubstituted C16-20 alkyl; and Reis unsubstituted C1-3 alkyl;60.(e) Rais unsubstituted C2-3 alkyl; Rbis H; Rcis unsubstituted C8-20alkyl; Rdis H; and Reis unsubstituted C2-3 alkyl;61.(f) Rais unsubstituted C2-3 alkyl; Rbis H; Rcis H; Rdis unsubstituted C8-20alkyl; and Reis unsubstituted C2-3 alkyl;62.(g) Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C1-3 alkyl; Rdis unsubstituted C8-20alkyl; and Reis H;63.(h) Rais unsubstituted C1-3 alkyl; Rbis H; Rcis unsubstituted C1-3 alkyl; Rdis H; and Reis unsubstituted C8-20alkyl;64.(i) Rais unsubstituted C1-3 alkyl; Rbis H; Rcis halo; Rdis H; and Reis unsubstituted C8-20alkyl; or65.(j) Rais unsubstituted C8-20alkyl; Rbis H; Rcis halo; Rdis H; and Reis unsubstituted C8-20alkyl.

13. A compound according to claim 1, wherein the acridan has any one of the following structures:

68.

70.

71.

14. A compound according to claim 7, wherein the acridan has any one of the following structures:

73.

74. 9SZ76.

78. 80.wherein t is from 10 to 80, optionally from 10 to 50, for instance from 20 to 50, from 30 to 48 or from 34 to 44, most preferably around 39,82.

84.

86.

88.

90.

15. A method of marking a product, which method comprises treating a product with a compound of any one of claims 1 to 14.

16. A method of detecting a marker compound in a product, which method comprises:94.(i) contacting a sample comprising a marked composition;95.wherein the marked composition comprises (a) a product and (b) a compound according to any one of claim 1 to 1496.with one or more reagents for causing the marker compound to undergo a chemiluminescent reaction to emit light; and97.(ii) detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition.

17. A method of producing light emission from a marked product, which method comprises contacting a sample comprising a marked composition,99.wherein the marked composition comprises (a) a product and (b) a compound which is an acridan of formula (I)101. 103.wherein104.X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl; R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6alkyl or unsubstituted or substituted aryl; and105.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12and unsubstituted or substituted C1-20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl,106.with an alkali metal alkoxide.

18. A method according to claim 17, wherein the alkali metal alkoxide is a compound of the formula MOR17, wherein M is an alkali metal and R17is an unsubstituted C1-10 alkyl, preferably wherein M is potassium, and preferably wherein the alkali metal alkoxide is potassium butoxide.

19. A method of producing light emission from a marked product, which method comprises contacting a sample comprising a marked composition;109.wherein the marked composition comprises (a) a product and (b) a compound which is an acridan of formula (I)111. 113.wherein114.X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;115.R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; and116.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl; wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl,117.with an organic superbase which is an amidine or guanidine compound.

20. A method according to claim 19, wherein the amidine or guanidine compound is selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,5-diazabicyclo[4.4.0]dec-6-ene (DBD), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), preferably wherein the amidine or guanidine derivative is TBD.

21. A method of detecting a marker compound in a product, which method comprises:120.(i) a method according to any one of claims 17 to 20; and121.(ii) detecting light emitted from the sample and thereby determining the presence of the marker compound in the marked composition.

22. A marked product which comprises (a) a product and (b) a marker compound which is a compound as defined in any one of claims 1 to 14.

23. A marked product according to claim 22 wherein the product comprises a petroleum product, a fuel, a plastic, a fabric, a chemical product, a packaging material (e.g. a packaging material for pharmaceutical products), a food, a beverage, a herbal medicine or related product, a perfume, a graphic art material, a plant, a seed, a fuel additive, a lubricating oil, a crude oil, a vegetable oil or fat, an animal oil or fat, algae oil, a fatty acid methyl ester or biodiesel, solid fuel, wood, vegetable product or finished goods in general.

24. A method of recovering a marker compound from a marked product, the method comprising: (i) taking a sample from a marked product as defined in claim 22 or claim 23, and optionally;125.(ii) extracting a marker compound from the sample, wherein the marker compound is a compound of formula (I) any of claims 1 to 14.

25. A method or marked product according to any one of claims 17 to 24 wherein the product further comprises a tracer compound, optionally wherein the tracer compound comprises a nucleic acid, a deuterated hydrocarbon, a radioactive compound, a compound comprising a rare earth element, an antibody or an antigen.

26. A formulation for use in marking product, wherein said formulation comprises a marker compound which is a compound as defined in any one of claims 1 to 14, and wherein said formulation is a grease or a resin.

27. An intermediate for use in producing a compound according to any one of claims 1 to 14, wherein said intermediate comprises a cation of formula (III)130. 132.X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;133.R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; and134.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;135.wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl136.provided that;137.(a) when X is phenyl, X is substituted with at least one group other than halo;138.(b) when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and139.-C(O)OCH3; and140.(c) the compound is other than:

141.

28. An intermediate according to claim 27, wherein the cation of Formula (III) is a cation of Formula (Illa):

145. 147.wherein Ra, Rb, Rc, Rd and Re are each independently selected from H, halo, unsubstituted or substituted C1-400alkyl, unsubstituted or substituted C2-400 alkenyl, CN, C(O)OR, C(O)NR2, and OR, wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, unsubstituted or substituted C1-6 alkyl and unsubstituted or substituted aryl;148.at least one of Ra, Rb, Rc, Rdand Reis selected from halo, CN, OR wherein R is H, unsubstituted or substituted C3-10 heterocyclyl, and substituted or unsubstituted C3 alkyl; or149.at least one of R2, R3, R4, Rs, Rb, R7, Rs and R< is halo.

29. A process for producing a compound of any one of claims 1 to 14, wherein said process comprises treating with a reducing agent an intermediate which comprises a cation of formula (III):

152. 154.X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;155.R1is unsubstituted or substituted C1-20 alkyl, optionally wherein R1is unsubstituted C1-20 alkyl or C1-20 alkyl substituted with a group of formula -C(=O)OR, wherein R is H, unsubstituted or substituted C3-20 heterocyclyl, unsubstituted or substituted C1-6 alkyl or unsubstituted or substituted aryl; and156.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;157.wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6 alkyl158.provided that:159.(a) when X is phenyl, X is substituted with at least one group other than halo;160.(b) when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and161.-C(O)OCH3.

30. A precursor for use in producing a compound according to any one of claims 1 to 14; wherein said precursor is a compound of formula (IV)164. 166.X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl;167.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted Ci- 20 alkyl, and -C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;168.wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl169.provided that;170.(a) when X is phenyl, X is substituted with at least one group other than halo;171.(b) when R1is methyl and X is aryl, and R2to R9are all H, X has at least three substituents, X has at least two different substituents, X has at least two substituents other than methyl, and / or X has at least one substituent other than -OH, methyl, -OCH3 and172.-C(O)OCH3; and173.(c) said compound is other than175. 177.or31. The process according to claim 29, wherein the process comprises an initial step of treating a compound of formula (IV) with an alkylating reagent comprising R1and Y, to form said intermediatewhich comprises said cation of formula (III) and said anion, Yn-, wherein R1is as defined for the cation of formula (III) and wherein Y is a charge -neutral moiety that results in the formation of said anion Yn-, wherein n is said integer of 1 to 4, wherein the formula (IV) is:

180. 182.wherein X is a group selected from unsubstituted or substituted aryl, and unsubstituted or substituted C2-20 alkyl; and183.R2, R3, R4, R5, R6, R7, R8and R9are independently selected from H, halo, unsubstituted or substituted C1-20 alkoxy, -N(R10)C(O)H, -NR11R12, unsubstituted or substituted C1-20 alkyl, and - C(H)=C(H)-Ar, wherein Ar is unsubstituted or substituted aryl;184.wherein R10, R11and R12are each independently selected from H or unsubstituted or substituted C1-6alkyl,185.provided that;186.(a) when X is phenyl, X is substituted with at least one group other than halo; and187.(b) when X is aryl, and R2to R9are all H, X is substituted with at least two different substituents, X has at least three substituents, and / or X has at least one substituent other than -OH, methyl, -OCH3 and -C(O)OCH3.

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