Compounds with thermally activated delayed fluorescence

Abstract

The present invention relates to polycyclic compounds comprising electron-accepting pyrazine cores and one or more dendron-like donor groups. Such compounds have been shown to have properties advantageous for use in electroluminescent devices such as TADF. Thus, the invention also relates to uses of such compounds in electroluminescent devices, electroluminescent devices comprising such compounds, and methods of manufacturing such electroluminescent devices.

Description

M&C PE962338GB 1 COMPOUNDS FIELD OF THE INVENTION The present invention relates to polycyclic compounds comprising electron-accepting pyrazine cores and one or more dendron-like donor groups. Such compounds have been shown to have properties advantageous for use in electroluminescent devices. Thus, the invention also relates to uses of such compounds in electroluminescent devices, electroluminescent devices comprising such compounds, and methods of manufacturing such electroluminescent devices. BACKGROUND OF THE INVENTION Organic Light Emitting Diode (OLED) displays are used extensively in small displays such as smart phones, watches and automotive displays owing to their excellent colour reproduction, high contrast ratio and low power consumption relative to alternative technologies, such as liquid crystal displays (LCDs). Vacuum deposition (VD) technologies are commonly used to manufacture display and lighting products based on OLEDs. However, the cost to manufacture large-area OLED displays using this technique is extremely high and OLED displays are only available in very expensive, premium displays. In addition, VD fabrication techniques are energy intensive, wasteful of materials and scale poorly to large display areas in comparison to other display technologies like LCDs. Solution-processing (SP) technology such as ink-jet printing provides a lower-cost alternative that uses less energy and materials to fabricate OLED displays and easily scales to large area panels. However, despite the lower production cost, the adoption of SP-OLEDs has been slow owing to poor display quality, with SP-OLEDs exhibiting poorer colour reproduction, brightness, and efficiency in comparison to VD-OLEDs. Recent improvements in the materials available for use in SP-OLEDs have improved display performance to the point where SP is becoming suitable for commercial fabrication. Like VD-OLEDS, current SP-OLEDs generally use emitters that are based on iridium, which is scarce and expensive. Replacing such emitters with sustainable materials is essential for cost-effective manufacturing of large area OLED displays. SP-OLEDs must 55402883-3M&C PE962338GB 2 address two technical challenges to become a viable alternative. Firstly, the performance of SP devices must improve to compete with VD-OLEDs. Secondly, the emitter materials must be switched to more sustainable sources. Dendrimers are branched macromolecules in which branched dendrons are attached to a core structure. Owing to their excellent film-forming ability, high solubility and well- defined structure, dendrimer-like compounds are more promising for SP (such as ink-jet printing and roll-to-roll processing) than small molecules, which often crystallize during operation. The first OLED comprising dendrimer-like compounds as light-emitting materials was reported by Wang et al. in Adv. Mater.1996, 8, 237-241. These dendrimers contained a highly fluorescent 9,10-bis(phenylethynyl)-anthracene core and phenylacetylene as surrounding dendrons for electron capture, as well as tertiary butyl groups as peripheral groups for enhanced solubility. Most solution-processable fluorescent OLEDs exhibit low efficiencies. In order to improve device performance, phosphorescent dendrimers have been developed. Lupton et al., in Control of Electrophosphorescence in Conjugated Dendrimer Light ^Emitting Diodes . Adv. Funct. Mater.2001, 11, 287-294, reported the first phosphorescent dendrimer based on a platinum porphyrin core unit, which demonstrates the concept of a triplet harvesting dopant in OLEDs. A series of phosphorescent dendrimers based on iridium cores were then developed including those Phosphorescent Dendrimer for Light-Emitting Diodes . Adv. Mater.2002, 14, 975-979 High-triplet-energy dendrons: enhancing the luminescence of deep blue phosphorescent iridium(III) complexes . J. Am. Chem. Soc. 2009, 131, 16681- 16688. Ding et al. Highly Efficient Green ^Emitting Phosphorescent Iridium Dendrimers Based on Carbazole Dendrons . Adv. Funct. Mater.2006, 16, 575-581, reported a series of green-emitting carbazole conjugated dendrimers containing iridium(III) complexes. A maximum external quantum efficiency (EQE) of 10.3% and current efficiency (CE) of ved for a non-doped green OLED. Red-emitting triphenylamine dendrimers containing iridium(III) complexes were reported by Zhou et al., in Triphenylamine-Dendronized Pure Red Iridium Phosphors with Superior OLED 55402883-3M&C PE962338GB 3 Efficiency / Color Purity Trade-Offs . Angew. Chem. Int. Ed.2007, 46, 1149-1151. The maximum EQE was reported to be 7.4%. The first successful demonstration of sky-blue-emitting alkynylgold(III) complexes featuring dendritic alkynyl ligands with carbazole moieties as dendrons and peripheral groups Sky-Blue-Emitting Dendritic Alkynylgold(III) Complexes for Solution-Processable Organic Light-Emitting Devices . J. Am. Chem. Soc.2017, 139(30), 10539-10550. High-performance solution processable OLEDs with maximum current efficiency of up to 23.7 cd A1and EQE of up to 6.9% are described. In contrast to phosphorescent dendrimer-like compounds, which are based on the noble metals- iridium(III), platinum(II), ruthenium(II) and gold(III), thermally activated delayed fluorescence (TADF) dendrimers do not comprise such metals. Dendrimers based on a TADF mechanism have performance advantages over conventional fluorescent dendrimers, since TADF dendrimers can harvest both singlet and triplet excitons for light emission, thus 100% internal quantum efficiency (IQE) at maximum could in theory be achieved. TADF dendrimers are gradually supplanting phosphorescent dendrimers and boosting the commercialization of inkjet printing OLEDs. Albrecht et al. Processable Thermally Activated Delayed ^Fluorescence Materials . Angew. Chem. Int. Ed.2015, 54 (19), 5677-5682, proposed the first TADF dendrimer- like compounds by bonding tri-substituted dendronized carbazole donor groups onto triazine acceptor core units. A further improvement of this type of dendrimer-like compound was also reported by modifying the peripheral groups (see Albrecht et al. in A dendrimer emitter doped in a dendrimer host: efficient thermally activated delayed fluorescence OLEDs with fully-solution processed organic-layers . Mater. Chem. Front. 2018, 2 (6), 1097-1103). However, the performance lags behind that of OLEDs based on small molecules, which have already reached more than 30% maximum EQE. The development of yellow to red TADF emitters is hampered by an intrinsically small energy gap between excited electronic states and the ground state, and thus a higher probability of non-radiative transitions. Non-radiative decay from TADF emitters may be reduced through the use of rigid polyaromatic acceptors. However, - stacking typically reduces solubility of such emitters. 55402883-3M&C PE962338GB 4 Sun et al. CBP derivatives dendronized self-host TADF dendrimer: achieving efficient non-doped near-infrared organic light-emitting diodes . Dyes and Pigments, 147, 436- 443 Near-infrared thermally activated delayed fluorescent dendrimers for the efficient non-doped solution-processed organic light-emitting diodes . Organic Electronics, 48, 389-396, reported the first red TADF dendrimer-like compounds based on encapsulating known red TADF core molecules with dendronized hosts. Spin-coated OLEDs employing these compounds as emitters exhibited maximum EQE of 0.064% to 0.62%, with peak emissions ranging from 698 nm to 728 nm Orange-red and saturated red thermally activated delayed fluorescent dendrimers for non-doped solution-processed OLEDs . Dyes and Pigments, 203, 110385, developed two red TADF dendrimer-like compounds. SP-OLED devices based on these achieved maximum EQEs ranging from 0.17% to 1.82%, with emission maxima at 600 nm to 649 nm. In CN 106967060 A, fluorescent materials for OLEDs are described. The materials are of a specific structure that does not allow dendron-like donors to be bonded to adjacent positions of an acceptor core. Color tuning of dibenzo[a,c]phenazine-2,7-dicarbonitrile-derived thermally activated delayed fluorescence emitters from yellow to deep-red J. Mater. Chem. C, 2020, 8, 7059, report the direct attachment of three different electron-donating groups, namely 3,6-di-tert-butylcarbazole, 9,9-dimethyl-9,10-dihydroacridine, and N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-diamine, to a dibenzo[a,c]phenazine- 2,7-dicarbonitrile acceptor unit to produce yellow to red TADF emitters. Approaching Nearly 40% External Quantum Efficiency in Organic Light Emitting Diodes Utilizing a Green Thermally Activated Delayed Fluorescence Emitter with an Extended Linear Donor Acceptor Donor Structure 2103293 and Solution-processable orange-red thermally activated delayed fluorescence emitters with 3,6-disubstituted carbazole for highly efficient OLEDs with low efficiency roll-off J. Mater. Chem. C, 2022, 10, 2034-2041 report TADF emitters comprising either pyrazino- or dicyanopyrazino-phenanthrene acceptor cores. 55402883-3M&C PE962338GB 5 Dibenzodipyridophenazines with Dendritic Electron Donors Exhibiting Deep-Red Emission and Thermally Activated Delayed Fluorescence Chem., 2023, 88, 7, 4224-4233, describe three luminescent donor-acceptor compounds, each comprising a dibenzodipyridophenazinyl, and different carbazole-based donors. The compounds are reported to emit visible light ranging in colour from yellow to deep- red. There is a need for alternative compounds capable of TADF, preferably with efficient emission (e.g. high EQE), particularly in the yellow to red part of the spectrum, and with solubility in solvents commonly employed in SP technology, such as toluene, chlorobenzene, chloroform and tetrahydrofuran. SUMMARY OF THE INVENTION The present invention is based on the finding that polycyclic compounds comprising at least four fused aromatic rings including electron accepting pyrazine cores, and one or more specific dendron-like donor groups bonded either directly to the fused aromatic rings or through benzene or 5- or 6-membered heteroaromatic rings, have properties advantageous for use in electroluminescent devices, including TADF, efficient emission (e.g. high EQE), particularly in the yellow to red part of the spectrum and / or solubility in solvents commonly employed in SP technology. According to the molecular design employed, the strong electron-withdrawing ability of the pyrazine cores and the strong donating abilities of the specific donor groups leads to emission of light of longer wavelengths, particularly yellow to red light, under photo / electrical excitation. In addition, the dendron-like groups and their specific positioning on the compounds of the invention influences intermolecular interactions between two or more compounds of the invention, owing to steric effects. This controls molecular aggregation and the likelihood of emission quenching and / or exciton annihilation. Viewed from a first aspect, therefore, the present invention provides a compound of formula (I), (II) or (III): 55402883-3M&C PE962338GB 6wherein: each A is independently benzene or a 5- or 6-membered heteroaromatic ring; n is independently 0 to 2; each X and Z is independently CH or N; and one of G1to G6is or more than one of G1to G6are independently selected from formulae (IVa) to (IVe):55402883-3M&C PE962338GB 7wherein: X1, X , X4and X are independently N or CR1and X2, X , X3and X are independently N or CR2; with the proviso that no more than one of X1, X2, X3and X4is N and no more than one of X , X , X and X is N; each R1is independently selected from H, halo, ester, nitro, cyano, SF5, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, phenoxy, 5- or 6-membered heteroaryl, 5- or 6-membered heteroaryloxy, (C1-6alkyl)3silyl and (C1-6alkyl)3silyloxy; each R2is independently selected from H, halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, phenoxy, 5- or 6-membered heteroaryl, 5- or 6-membered heteroaryloxy, (C1-6alkyl)3silyl, (C1-6alkyl)3silyloxy, B(C1-6alkyl-substituted phenyl)2and any one of formulae (Va) to (Vf); and those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf):, wherein: each A2 is independently selected from benzene, napthalene and a 5- or 6- membered heteroaromatic ring, each of which is optionally substituted with one or more groups selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl 55402883-3M&C PE962338GB 8 C1-6alkoxy, phenyl, C1-6alkyl-substituted phenyl, C1-6haloalkyl-substituted phenyl, silyloxy and (C1-6alkyl)3silyl; X2is selected from C(R3)2, N, S, O and Si(R3)2, wherein each R3is independently selected from: C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, benzene, napthalene and 5- or 6-membered heteroaromatic ring, wherein the benzene naphthalene and 5- or 6-membered heteroaromatic ring are each optionally substituted with one or more selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl C1-6alkoxy, (C1-6alkyl)3silyl and (C1-6alkyl)3silyloxy; or the two R3groups combine together with the C or Si atom to which they are bonded such that C(R3)2or Si(R3)2forms a 5- or 6-membered ring, optionally fused with one or two benzene rings; and the dashed line of formula (Vb) is an optionally present bond; and when only one of G1to G6is selected from formulae (IVa) to (IVe), at least one of those of G1to G6not of formulae (IVa) to (IVe) is selected from formulae (Va) to (Vf). The compounds of the invention comprising only one dendron-like group selected from formulae (IVa) to (IVe), also comprise at least one donor group selected from formulae (Va) to (Vf). This means that the compounds of the invention comprise at least two donor groups, beneficial for yellow to red emission under photo / electrical excitation. As described above, the compounds of the invention have properties advantageous for use in electroluminescent devices, including TADF, efficient emission (e.g. high EQE), particularly in the yellow to red part of the spectrum and / or solubility in solvents commonly employed in SP technology. Viewed from a second aspect, therefore, the present invention provides an electroluminescent device, such as an OLED, comprising a compound of the first aspect. Also, viewed from a third aspect, the invention provides for the use of a compound of the first aspect in an electroluminescent device, such as an OLED. The compounds of the invention have been found to be surprisingly effective emitters in OLEDs in the absence of additional host materials. Thus, host materials need not be present in OLEDs of the invention, i.e. the OLEDs may be non-doped. 55402883-3M&C PE962338GB 9 Alternatively, the compounds of the invention may be part of a composition further comprising host materials. Accordingly, viewed from a further aspect, there is provided a composition comprising a compound of the invention and a host material, such as the host materials described in Chem. Phys. Rev. 2, 031304 (2021); doi: 10.1063 / 5.0049513. The host material may be one or more selected from 1,3- bis(carbazol-9-yl)benzene (mCP), 4,4 -bis(9-carbazolyl)biphenyl (CBP), bis[2- (diphenylphosphino)phenyl]ether oxide (DPEPO), 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (POT2T), 9-(4-tert- butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), and 2,8-bis(diphenyl- phosphoryl)-dibenzo[b,d]thiophene (PPT). In addition, viewed from a fourth aspect, the invention provides a method of manufacturing an electroluminescent device comprising contacting a compound of the first aspect with one or more components of an electroluminescent device. Since the compounds are soluble in solvents commonly employed in SP technology, the compound may be solution-processed, such as ink-jet printed or roll-to-roll processed. BRIEF DESCRIPTION OF THE FIGURES Fig. 1. The photoluminescence spectrum of 3,6-bis(3,3'',6,6''-tetra-tert-butyl-9'H- [9,3':6',9''-tercarbazol]-9'-yl)tetrabenzo[a,c,h,j]phenazine in toluene. Fig. 2. The photoluminescence spectrum of 12,15-bis(3,3'',6,6''-tetra-tert-butyl-9'H- [9,3':6',9''-tercarbazol]-9'-yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine in toluene. Fig. 3. The photoluminescence spectrum of 10,10'-(11,12-bis(3,3'',6,6''-tetra-tert-butyl- 9'H-[9,3':6',9''-tercarbazol]-9'-yl)dibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) in toluene. Fig. 4. The photoluminescence spectrum of 2,3-bis(4-(3,3'',6,6''-tetra-tert-butyl-9'H- [9,3':6',9''-tercarbazol]-9'-yl)phenyl)pyrazino[2,3-f][1,10]phenanthroline in toluene. Fig.5. (a) Frontier molecular orbitals (isovalue: 0.02) and (b) vertical excitation energy levels of (12,15-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'- 55402883-3M&C PE962338GB 10 yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-] phenazine (2GCzBPPZ) and 11,12-bis(3,3'',6,6''- tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'-yl)dipyrido[3,2-a:2',3'-c]phenazine (2GCzBPN), (c) Natural transition orbitals (unoccupied (hole, darker grey shading towards the top of each schematic) & occupied (electron, lighter grey shading towards the bottom of each schematic), (isovalue: 0.02) of S1 and T1 calculated at the optimized S0geometry in the gas phase at the PBE0 / 6-31G(d,p) level. (d) comparison of optimized structures at S0(dotted lines) and S1(solid lines) state. Fig.6. (a) Cyclic and differential pulse voltammograms measured in degassed DCM with 0.1 M [nBu4N]PF6as the supporting electrolyte and Fc / Fc+as the internal reference (0.46 V vs SCE). Scan rate = 100 mV s1. (b) UV-vis absorption and steady-state photoluminescence (PL) spectra of 2GCzBPPZ and 2GCzBPN recorded in toluene at room temperature (exc= 340 nm). (c) PL solvatochromism study of 2GCzBPPZ and 2GCzBPN (exc= 343 nm). (d) Prompt fluorescence and phosphorescence spectra of 2GCzBPPZ and 2GCzBPN 2-MeTHF at 77 K (exc= 360 nm, prompt fluorescence and phosphorescence spectra were acquired across a 1-100 ns and 1 10 ms time range, respectively). Fig. 7. (a) Concentration-dependent fluorescence spectra for 2GCzBPPZ in n-hexane solution ( ^exc= 340 nm); (b) Ratiometric plot of I475 / I575(solid dots) and I575 / I475(hollow dots) vs increasing concentration of 2GCzBPPZ; (c) Concentration-dependent emission mapping; (d) Concentration-dependent absorption spectra for 2GCzBPPZ in n-hexane solution. Fig. 8. (a) PL spectra under degassed and aerated conditions at the concentration of 2.0 ^10^5M in n-hexane ( ^exc = 340 nm); (b) PL decay of the emission ^em= 475 nm (top) and ^em = 575 nm (bottom) under degassed and aerated n-hexane ( ^exc = 375 nm); (c) Normalized fluorescence spectra of increasing doping concentrations (from 0.1 wt% to 50 wt%) for 2GCzBPPZ in PMMA ( ^exc = 340 nm); (d) the corresponding PL decays of the doped films in PMMA ( ^exc = 340 nm). Fig. 9. (a) Temperature-dependent emission spectra of 2GCzBPPZ in n-hexane at a concentration of 1.6 ^10^5M ( ^exc = 360 nm); (b) CIE plot of the emission spectra in (a) of 2GCzBPPZ; (d) Ratiometric plot of I575 / I475 vs temperature upon decreasing the temperature below RT; (e) Ratiometric plot of I475 / I575vs temperature upon increasing the 55402883-3M&C PE962338GB 11 temperature from RT (right); (f) Schematic representation of the thermal response and two-state equilibration model describing the observed abnormal temperature-responsive dual emission phenomenon of 2GCzBPPZ. Fig.10. Prompt fluorescence (1-100 ns) and phosphorescence spectra (1-10 ms) in 10 wt% doped in mCP at 77 K of (a) 2GCzBPPZ and (b) 2GCzBPN (exc= 345 nm). (c) Steady-state PL spectra of 10 wt% doped films of 2GCzBPPZ and 2GCzBPN in mCP film at room temperature (exc=345 nm); (d) PL decay of 10 wt% doped films of 2GCzBPPZ and 2GCzBPN in mCP at 298 K (exc=375 nm); Temperature-dependent PL decay of 10 wt% doped films of (e) 2GCzBPPZ and (f) 2GCzBPN in mCP (exc= 375 nm). Fig.11. (a) Energy level diagram and structure of materials used in the devices; (b) EL spectra, (c) Current density and luminance versus voltage characteristics, (d) External quantum efficiency versus luminance curves for the devices. DETAILED DESCRIPTION OF THE INVENTION As described above, the inventors have found that polycyclic compounds comprising at least four fused aromatic rings including electron accepting pyrazine cores, and one or more specific dendron-like donor groups bonded either directly to the fused aromatic rings or through benzene or 5- or 6-membered heteroaromatic rings, have properties advantageous for use in electroluminescent devices, including TADF, efficient emission (e.g. high EQE), particularly in the yellow to red part of the spectrum, and / or solubility in solvents commonly employed in SP technology. The dendron-like groups and their specific positioning on the compounds of the invention influences intermolecular interactions between two or more compounds of the invention, owing to steric effects. This controls the likelihood of molecular aggregation and thus the likelihood of emission quenching and / or exciton annihilation. In the discussion that follows, reference is made to a number of terms, which have the meanings provided below, unless a context indicates to the contrary. The nomenclature used herein for defining compounds, in particular the compounds according to the invention, is in general based on the rules of the IUPAC organisation for chemical (Gold with 55402883-3M&C PE962338GB 12 a definition provided herein, the definition herein is to prevail. Furthermore, if a compound structure is in conflict with the name provided for the structure, the structure is to prevail. imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step or group of elements, integers or steps. umber or value, is used to refer to values that lie within ± 5% of the value specified. For example, where a racemate is defined as comprising a pair of enantiomers in a ratio of about 1:1, ratios of 0.95:1.05 to 1.05: 0.95 are included. arenes, by replacement of one or more methine ( C=) and / or vinylene ( CH=CH ) groups by trivalent or divalent heteroatoms, respectively, in such a way as to maintain -electron system characteristic of aromatic systems and a number of out-of- -electrons corresponding to the Hückel rule. The Hückel rule is often used in the art to assess aromatic character; monocyclic planar (or almost planar) systems of trigonally (or so -electrons (where n is a non-negative integer) will exhibit aromatic character. Heteroaromatic rings may be 5- or 6-membered. Examples of 5- or 6-membered aromatic rings include pyridine, thiophene, pyrrole, furan, imidazole, oxazole, thiazole, pyrazole, isoxazole and isothiazole. Heteroaryl groups are univalent groups derived from heteroaromatic rings by removal of a hydrogen atom from any carbon atom. Halo refers to a halogen radical. Typically, halo refers to any selected from fluoro, bromo, chloro and iodo. In some cases, halo refers to fluoro. 55402883-3M&C PE962338GB 13 sulfonic acids by the removal of a hydroxy group. The sulfonyl has a general formula of -SO2R, where R may be an organyl group such as an optionally substituted phenyl, optionally substituted naphthalenyl or an optionally substituted C1-6alkyl group (such as an optionally substituted methyl or ethyl). The optional substituents of the phenyl or napthalenyl may be any one or more selected from C1-6alkyl (such as methyl), C1-6haloalkyl (such as trifluoromethyl), halo (such as bromo or fluoro), nitro and diC1-6alkylamino (such as dimethylamino). The optional substituents of the C1-6alkyl may be any one or more selected from halo (such as fluoro), nitro and diC1-6alkylamino (such as dimethylamino). stituent comprising carbon) with one free valence at a carbon atom. Typically, the organic substituent comprises only non-metallic elements. Non-metallic elements include reactive non-metals and metalloids. and refers herein to compounds comprising a -C=O moiety. Used herein, the term carbonyl refers to a moiety of general formula 2 and independently selected from organyl groups such as C1-6alkyl, C1-6haloalkyl, C1-6alkylol, phenyl and C1-6alkylphenyl. alkanes by removal of a hydrogen atom from any carb is intended to define acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2 1-6alkyl groups. In some cases, alkyl groups are C1-4alkyl groups. C1-4alkyl refers to any selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl and tert-butyl. groups by replacement of one or more hydrogen atoms from one or more carbon atoms with a halo group. Haloalkyl groups may comprise one or more different types of halo. For example, one or more independently selected from fluoro, chloro, bromo and iodo. In some cases, the haloalkyl is a fluoroalkyl. 55402883-3M&C PE962338GB 14 groups derived from alkanes by the replacement of a hydrogen atom with a hydroxy group. Often, alkoxy groups are C1-6alkoxy groups or C1-4alkoxy groups. heteroaryloxy compounds by removal of a hydrogen atom from the OH group, wherein the term define compounds derived from heteroaromatic rings by the replacement of a hydrogen atom with a hydroxy group. oxy univalent groups derived from silyl ethers by removal of the alkyl group bonded to the oxygen atom of the silyl ether. Silyl ethers comprise silicon atoms bonded to an alkoxy group. The silyloxy has a general formula of OSiR3, where R may be an organyl group such as an optionally substituted alkyl or an optionally substituted phenyl. Silyloxy groups may be (C1-6alkyl)3silyloxy groups, i.e. -OSi(C1- 6alkyl)3 groups. hydrocarbylboranes by removal of a hydrocarbyl group. The hydrocarbylboranyl has a general formula of BR2, where each R may be independently selected from alkyl- substituted phenyl (such as C1-6alkyl-substituted phenyl), phenyl and alkyl. In some cases, the hydrocarbylboranyl is -B(C1-6alkyl-substituted phenyl)2. For the avoidance of doubt, a wavy line in a chemical structure bisects the bond linking the moiety shown to the rest of the compound. molecular formulae and sequence of bonded atoms, but which differ in the arrangement of their atoms in space. images of each other and non-superimposable, i.e. cannot be brought into coincidence by translation and rigid rotation transformations. Enantiomers are chiral molecules, i.e. are distinguishable from their mirror image. 55402883-3M&C PE962338GB 15 a substantially equimolar mixture of a pair of enantiomers, which typically comprises a pair of enantiomers in a ratio of about 1:1. stereoisomers that are not related as mirror images. as a compound or salt of the compound, and a solvent. If the solvent is water, the solvate may be termed a hydrate, for example a mono-hydrate, di-hydrate, tri-hydrate etc., depending on the number of water molecules present per molecule of substrate. element, in which the nucleus necessarily has the same atomic number but has a different mass number owing to it possessing a different number of neutrons. As described above, the compounds of the invention are of formula (I), (II) or (III):. Formulae (I) and (II) comprise A rings, which are each independently benzene or a 5- or 6-membered heteroaromatic ring. The 5- or 6-membered heteroaromatic ring comprises one or more heteroatoms, typically one or two heteroatoms. The heteroatoms may be one or more selected from nitrogen, oxygen and sulfur atoms. In some embodiments, the 5- or 6-membered heteroaromatic ring comprises one heteroatom selected from nitrogen, oxygen and sulfur. In some embodiments, each A is independently selected from benzene, pyridine, thiophene and furan, such as benzene and pyridine. In some embodiments, each A is the same, for example each A is benzene or each A is pyridine. 55402883-3M&C PE962338GB 16 Each A ring is optionally substituted with up to two G5or G6groups, i.e. each A ring is optionally substituted up to two times with dendron-like groups independently selected from those of formulae (IVa) to (IVe), halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf). In some embodiments, n is independently 0 or 1, i.e. each A ring is unsubstituted or substituted once with a dendron-like group selected from those of formulae (IVa) to (IVe), halo, cyano, SF5, C1-6haloalkyl and those of formulae (Va) to (Vf). X of the compounds is independently CH or N. Z of compounds of formulae (III) is independently CH or N, such as CH. At least one of G1to G6is or more than one of G1to G6are independently selected from the dendron-like groups of formulae (IVa) to (IVe):55402883-3M&C PE962338GB 17. With respect to formula (IVa), no more than one of X1, X2, X3and X4is N and no more than one of X , X , X and X is N. Those of X1, X , X4and X that are not N are CR1and those of X2, X , X3and X that are not N are CR2. In some embodiments, X2and / or X is N and in other embodiments, X3and / or X3is N. In some embodiments, X1, X , X4and X are CR1and X2, X , X3and X are CR2. Each R1of formulae (IVa) to (IVe) is independently selected from H, halo, nitro, cyano, SF5, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, phenoxy, 5- or 6- membered heteroaryl, 5- or 6-membered heteroaryloxy, and (C1-6alkyl)3silyl. In some embodiments, each R1is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, and C1-6alkoxy. For example, each R1may be independently selected from H, and halo, such as H. Each R2is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, phenoxy, 5- or 6-membered heteroaryl, 5- or 6- membered heteroaryloxy, (C1-6alkyl)3silyl, B(C1-6alkyl-substituted phenyl)2 and any one of formulae (Va) to (Vf) (described in more detail below). In some embodiments, each R2is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C2-6alkyl, C1- 6haloalkyl, C2-6alkoxy, phenyl, phenoxy, 5- or 6-membered heteroaryl, 5- or 6-membered 55402883-3M&C PE962338GB 18 heteroaryloxy, (C1-6alkyl)3silyl, B(C1-6alkyl-substituted phenyl)2 and any one of formulae (Va) to (Vf). In some embodiments, each R2is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy and B(C1-6alkyl- substituted phenyl)2, such as B(2,4,6-trimethylphenyl)2. In some embodiments, each R2is independently selected from H, halo, C1-6alkyl, and C1-6haloalkyl, such as H and C1-6alkyl. In some embodiments, each R2is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C2-6alkyl, C1-6haloalkyl, C2-6alkoxy and B(C1-6alkyl-substituted phenyl)2, such as B(2,4,6-trimethylphenyl)2. In some embodiments, each R2is independently selected from H, halo, C2-6alkyl, and C1-6haloalkyl, such as H and C2-6alkyl. In some embodiments, each R2is the same. In particular embodiments, each R2is independently selected from H, halo, C1-6alkyl, and C1-6haloalkyl, such as H, halo, C2-6alkyl and C2-6haloalkyl or H, halo, C3-6alkyl and C3-6haloalkyl. In particular embodiments, each R2is independently selected from H, branched C3-6alkyl and branched C3-6haloalkyl, such as H, branched C3-5alkyl and branched C3-5haloalkyl. In some embodiments, each R2of formula (IVa) and (IVb) is independently selected from H, halo, C1-6alkyl, and C1-6haloalkyl, such as H, halo, C2-6alkyl and C2-6haloalkyl or H, halo, C3-6alkyl and C3-6haloalkyl. In particular embodiments, each R2of formula (IVa) and (IVb) is independently selected from H, branched C3-6alkyl and branched C3-6haloalkyl, such as H, branched C3-5alkyl and branched C3-5haloalkyl. In some embodiments, formulae (IVa) to (IVe) are of formulae (VIIa) to (VIIe):55402883-3M&C PE962338GB 19. In some embodiments, each R2of formula (VIIa) is independently selected from C1-6alkyl, and C1-6haloalkyl, such as C2-6alkyl and C2-6haloalkyl or C3-6alkyl and C3-6haloalkyl. In particular embodiments, each R2of formula (VIIa) is independently selected from branched C3-6alkyl and branched C3-6haloalkyl, such as branched C3-5alkyl and branched C3-5haloalkyl. In some embodiments, each R2of formulae (VIIa) to (VIIe) is independently selected from C1-6alkyl, and C1-6haloalkyl, such as C2-6alkyl and C2-6haloalkyl or C3-6alkyl and C3-6haloalkyl. In particular embodiments, each R2of formula (VIIa) to (VIIe) is independently selected from branched C3-6alkyl and branched C3-6haloalkyl, such as branched C3-5alkyl and branched C3-5haloalkyl. Those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf):. In some embodiments, none of G1to G6are cyano, i.e. G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, SF5, C1-6haloalkyl and any one of formulae 55402883-3M&C PE962338GB 20 (Va) to (Vf). In more specific embodiments, G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, C1-6haloalkyl and any one of formulae (Va) to (Vf). Each A2 of formulae (Va) to (Vf) is independently selected from benzene, napthalene and a 5- or 6-membered heteroaromatic ring, each of which is optionally substituted with one or more groups selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, C1-6alkyl-substituted phenyl, C1-6haloalkyl-substituted phenyl, silyloxy and (C1-6alkyl)3silyl. The 5- or 6-membered heteroaromatic ring of A2 comprises one or more heteroatoms, typically one or two heteroatoms. The heteroatoms may be one or more selected from nitrogen, oxygen and sulfur atoms. In some embodiments, the 5- or 6-membered heteroaromatic ring comprises one heteroatom selected from nitrogen, oxygen and sulfur, such as nitrogen. In some embodiments, the 5- or 6-membered heteroaromatic ring is a 6-membered heteroaromatic ring, for example a 6-membered heteroaromatic ring comprising one or more nitrogen atoms. In some embodiments, each A2 is independently selected from benzene, naphthalene and pyridine, each of which is optionally substituted. In some embodiments, each A2 is the same, for example each A2 is benzene, each A2 is naphthalene, or each A2 is pyridine. In some embodiments, the optional substituents of A2 are independently selected from halo, C1-6alkyl, C1-6haloalkyl, phenyl, C1-6alkyl-substituted phenyl, and C1-6haloalkyl- substituted phenyl. For example, the optional substituents of A2 may be halo, C1-6alkyl, C1-6haloalkyl, and phenyl. X2of formula (Va) is selected from C(R3)2, N, S, O and Si(R3)2. Each R3is independently selected from C1-6alkyl, C1-6haloalkyl, C1-6alkoxy and optionally substituted benzene, napthalene and 5- or 6-membered heteroaromatic ring, each of which is optionally substituted with one or more selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl C1-6alkoxy, and (C1-6alkyl)3silyl; or the two R3groups combine together with the C or Si atom to which they are bonded such that C(R3)2 or Si(R3)2 forms a 5- or 6-membered ring, optionally fused with one or two benzene rings. 55402883-3M&C PE962338GB 21 In some embodiments, X2is selected from C(CH3)2, C(C6H5)2, O, S, Si(CH3)2 and Si(C6H5)2 or C(R3)2 or Si(R3)2, where the two R3groups combine together with the C or Si atom to which they are bonded such that C(R3)2 or Si(R3)2 forms a 5- or 6-membered ring (such as a 5-membered ring, optionally fused with one or two benzene rings. The two R3groups may combine together with the C or Si atom to which they are bonded such that C(R3)2or Si(R3)2forms a 5-membered ring, which is fused with two benzene rings. The dashed line of formula (Vb) is an optionally present bond. In some embodiments, formulae (Va) to (Vf) are of formulae (VIa) to (VIu):55402883-3M&C PE962338GB 22, wherein X3is selected from C(CH3)2, C(C6H5)2, O, S, Si(CH3)2and Si(C6H5)2and the dashed line in formula (VIe) is an optionally present bond. In more particular embodiments, those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and any one of formulae (VIa) to (VIu). In even more particular embodiments, G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and formula (Va). In yet more particular embodiments, G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and any one of formulae (VIa), (VId), (VIs), (VIt), and (VIu). When only one of G1to G6is selected from formulae (IVa) to (IVe), at least one of those of G1to G6not of formulae (IVa) to (IVe) is selected from formulae (Va) to (Vf). As described above, this means that when compounds of the invention comprise only one dendron-like group selected from formulae (IVa) to (IVe), they also comprise at least one donor group selected from those of formulae (Va) to (Vf). This means that the compounds of the invention comprise at least two donor groups, which is beneficial for yellow to red emission under photo / electrical excitation. In some embodiments, the compounds of the invention comprise at least two dendron- like groups selected from formulae (IVa) to (IVe), i.e. two or more of G1to G6are selected from formulae (IVa) to (IVe). In these embodiments, the two or more dendron-like groups selected from formulae (IVa) to (IVe) may be separated from each other by at least three carbon atoms. In such embodiments, at least one of G1and G2, at least one of G3and G4, and at least one of G5and G6of formula (III) is not selected from formulae (IVa) to (IVe). In some embodiments, at least one of G1and G2is H and at least one of G3and G4is H. In such embodiments, the compounds of formulae (I) and (II) comprise two to six dendron-like groups and the compounds of formula (III) comprise two to four dendron- like groups. 55402883-3M&C PE962338GB 23 In specific embodiments, both G5and G6of formula (III) are not H, for example G5and G6may each be independently selected from the groups depicted in formulae (IVa) to (IVe). As described above, the specific positioning of the dendron-like groups on the compounds of the invention influences intermolecular interactions between two or more compounds of the invention owing to steric effects. Where there are two dendron-like groups bonded at G5and G6, the probability of non-radiative decay may be reduced. In some embodiments, either one of G1to G4is or two or more of G1to G4are dendron- like donors and G5and G6are not dendron-like donors; or one of G5and G6is or two or more of G5and G6are dendron-like donors and G1to G4are not dendron-like donors. Typically, one of G5and G6is or two or more of G5and G6are dendron-like donors and G1to G4are not dendron-like donors. In other words, one of G5and G6is or two or more of G5and G6are dendron-like groups selected from formulae (IVa) to (IVe), and G1to G4are not of formulae (IVa) to (IVe). In some embodiments, two or more of G5and G6are dendron-like groups selected from formulae (IVa) to (IVe), and G1to G4are not of formulae (IVa) to (IVe). In these embodiments, the two or more dendron-like groups selected from formulae (IVa) to (IVe) may be separated from each other by at least three carbon atoms. The inventors have found that such compounds are able to form aggregates (see example section), which are surprisingly efficient emitters and are capable of emitting light of longer wavelength at lower temperatures and of shorter wavelength at higher temperatures (thereby finding use as a temperature sensor). Aggregate formation is dependent on concentration and temperature, with the compounds exhibiting dual emission owing to the presence of a combination of monomers and aggregates emitting light at different wavelengths. Accordingly, viewed from a further aspect, the invention provides a temperature sensor comprising a compound of the invention of formula (I) or formula (II) (e.g. formula (I)), wherein two or more of G5and G6are dendron-like groups selected from formulae (IVa) to (IVe) and are separated from each other by at least three carbon atoms, and G1to G4are not of formulae (IVa) to (IVe). The temperature sensor may further comprise a non- polar solvent that may be a liquid or a wax. As is known in the art, waxes are essentially a functionally defined class of substances, which comprise thermoplastic water-repellent lipid substances having low softening temperatures (e.g. about 80 °C), formed from long-chain fatty acids and alcohols and secreted by animals or which form a protective outer layer on plants; and various mineral and synthetic organic compounds, generally 55402883-3M&C PE962338GB 24 hydrocarbons, having similar properties to naturally occurring lipid waxes. It is to be understood that long-chain fatty acid soaps, in which the acidic hydrogen atom of the long chain fatty acid has been replaced by an alkali metal ion, such as Li+ , Na+ , and K+ , typically Na+ , and long chain fatty acid esters, preferably mono-, di-, and tri-(long chain fatty acid) glycerol esters are to be considered waxes. Many naturally occurring and synthetic waxes comprise mixtures of compounds and so, therefore, may the coating material of the coated particles described herein, although the coated particles' coatings may comprise a single type of compound. The non-polar solvent may have a dielectric constant at 20 °C of about 2.4, such as 2.3. In particular embodiments, the non-polar solvent is hexane or paraffin wax. The paraffin wax may be any of those described in EP 0040091 A1 (Unilever plc & Unilever N.V.). Paraffin waxes are widely available commercially from, for example, Merck, of Darmstadt (Germany) and Boler, of Wayne, Pennsylvania (USA). Petroleum (paraff in) waxes of the microcrystalline type, melting at various temperatures, may be employed. Suitable micro-crystalline waxes include Shell micro-crystalline wax - HMP, and -W4, and micro-crystalline waxes sold by Witco, and many other suppliers. Other suitable waxes include Fischer-Tropsch and oxidised Fischer-Tropsch waxes, ozokerite, ceresin, montan wax, beeswax, candelilla wax (melting point between 68-70 °C), and carnauba wax (melting point between 80-88 °C), and spermacetti, and other ester waxes. Also provided is use of a compound of the invention of formula (I) or formula (II) (e.g. formula (I)) in or as a temperature sensor, wherein two or more of G5and G6are dendron- like groups selected from formulae (IVa) to (IVe) and are separated from each other by at least three carbon atoms, and G1to G4are not of formulae (IVa) to (IVe). Also provided is a method of manufacturing a temperature sensor comprising contacting a compound of the invention of formula (I) or formula (II) (e.g. formula (I)), with a non- polar solvent, wherein two or more of G5and G6are dendron-like groups selected from formulae (IVa) to (IVe) and are separated from each other by at least three carbon atoms, and G1to G4are not of formulae (IVa) to (IVe). In some embodiments, each G1to G6of formula (III) is not cyano, i.e. one of G1to G6is or more than one of G1to G6are independently selected from formulae (IVa) to (IVe), and those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, 55402883-3M&C PE962338GB 25 halo, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf). In some embodiments, one of G1to G6of formula (III) is or more than one of G1to G6of formula (III) are independently selected from formulae (IVa) to (IVe), and those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, C1-6haloalkyl and any one of formulae (Va) to (Vf). In some embodiments, at least one of X of formula (III) or at least one of Z of formula (III) is N, for example at least one X of formula (III) is N. In some embodiments, each X of formula (III) is N. In some embodiments, at least one of G1to G6is or at least two of G1to G6are of formula (IVa). Those of G1to G6not of formula (IVa) may be independently selected from H and formula (Va). In particular embodiments, the at least one of G1to G6is or more than one of G1to G6are independently selected from the dendron-like groups of formula (IVa). In other words the dendron-like groups are of formula (IVa). In some embodiments, G5and G6of formula (I) and formula (II) are not of formulae (IVa) to (IVe), ie., they may be independently selected from halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf). In some embodiments, the compounds are of formulae (I) or formula (II), or of formula (I) or formula (III). In more specific embodiments, the compounds are of formula (I). In particular embodiments, the compounds of formulae (I), (II) and (III) are of formula (VIII-i), (VIII-ii) or (VIII-iii), respectively:, 55402883-3M&C PE962338GB 26 wherein X and G1to G6are as defined above, and X5is CH or N. (VIII-i) may be of formulae (VIII-ia) or (VIII-ib):, such as of formula (VIII-ic) or (VIII-id):. In specific embodiments, (VIII-i) is of formula (VII-ie) or (VIII-if):. In some cases, G2and G4of (VIII-ie) and (VIII-if) are not of formulae (IVa) to (IVe), ie., they may be independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf). 55402883-3M&C PE962338GB 27 In some embodiments, (VIII-ii) is of formula (VIII-iia):, wherein X, G1, G3, G5and G6are as defined above. In some cases, G1and G3of (VIII- iia) are not of formulae (IVa) to (IVe), ie., they may be independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf). In some embodiments, (VIII-iii) is of formula (VIII-iiia) or (VIII-iiib):. In some cases, G1and G3of (VIII-iiia) are not of formulae (IVa) to (IVe), ie., they may be independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf). In particular embodiments, the compounds are of formula (IX-ia), (IX-ib), (IX-iia), (IX-iiia) or (IX-iiib): 55402883-3M&C PE962338GB 2855402883-3M&C PE962338GB 2955402883-3M&C PE962338GB 30. The compounds of the invention may exist in different stereoisomeric forms. All stereoisomeric forms and mixtures thereof, including enantiomers and racemic mixtures, are included within the scope of the invention. Such stereoisomeric forms include enantiomers and diastereoisomers. Individual stereoisomers of compounds of the invention, i.e., associated with, for example mixed with, less than 5%, preferably less than 2% and in particular less than 1% of the other stereoisomer, are included. Mixtures of stereoisomers in any proportion, for example a racemic mixture comprising substantially equal amounts of two enantiomers are also included within the invention. Also included are solvates and isotopically-labelled compounds of the invention. Isotopically-labelled compounds are identical to those recited herein, but for the fact that one or more atoms are replaced by an atom having an atomic mass or mass number different from the atomic mass or mass number predominantly found in nature. Examples of isotopes that can be incorporated into compounds of the invention include isotopes of hydrogen, carbon, nitrogen, oxygen, sulfur, fluorine and chlorine, such as2H (i.e. deuterium),3H,13C,14C,15N,18O,17O,35S,18F, and36Cl, respectively. A protium atom (H) is a hydrogen atom with zero neutrons. A deuterium atom (D or2H) is a hydrogen atom with one neutron. Naturally occurring hydrogen contains about 0.02 molar per cent deuterium and 99.98% protium. Physical chemical properties between 55402883-3M&C PE962338GB 31 protium and deuterium are small but measurable. Deuterium is slightly less lipophilic than protium, has a smaller molar volume and carbon-deuterium bonds are shorter than carbon-protium bonds. Deuterium keeps the 3D surface, shape and steric flexibility of a molecule unaltered compared to H. A ratio of deuterium:protium in a compound greater than 1:99 is considered to be greater than that found naturally in hydrogen. In particular ratio of deuterium:protium at this position is greater than the natural isotopic abundance of deuterium, i.e. the percentage of deuterium found at his position of the compounds of the invention is greater than its natural isotopic abundance in hydrogen, which is about 0.02 mol%. All amorphous and crystalline forms of the compounds of the invention are included. The compounds of the invention have properties advantageous for use in electroluminescent devices. Thus, the second aspect of the invention provides an electroluminescent device, such as an OLED, comprising a compound of the first aspect and the third aspect of the invention provides invention provides for the use of a compound of the first aspect in an electroluminescent device, such as an OLED. In addition, the fourth aspect provides a method of manufacturing an electroluminescent device comprising contacting a compound of the first aspect with one or more components of an electroluminescent device. An electroluminescent device is a device capable of emitting light on application of a voltage to the device. The skilled person is well aware of such devices, and examples may be found in (Geffroy Organic light ^emitting diode (OLED) technology: materials, devices and display technologies . Polymer international, 2006, 55(6): 572- 582). OLEDs are an example of an electroluminescent device, and comprise an anode, a cathode and layers including emissive electroluminescent layers and conductive layers. In OLEDs, the emissive electroluminescent layer comprises an organic compound, such as a compound of the invention, which emits light when an electric current is applied. The electroluminescent device of the second, and third and fourth aspects may comprise the compound of the first aspect as an emitter material or as a host material. For example, the compound may be a TADF emitter of light of a wavelength of about 570 to about 780 nm. 55402883-3M&C PE962338GB 32 The compounds of the invention have been found to be surprisingly effective emitters in electroluminescent devices in the absence of additional host materials. Thus, host materials need not be present in the electroluminescent device of the second, third and fourth aspects of the invention, i.e. the electroluminescent devices may be non-doped. Since the compounds of the invention are soluble in solvents commonly employed in SP technology, the electroluminescent device of the second, third and fourth aspects may comprise compounds of the invention that have been solution-processed, such as ink- jet printed or roll-to-roll processed (see Jaeheung et al., Organic Electronics 19 (2015): 147-156 and Yang, et al., Journal of Materials Science: Materials in Electronics 11 (2000): 89-96 for reviews of these processes). Ink-jet printing involves depositing inks (which are solutions of organic functional materials) onto substrates in a patterned manner. Roll-to-roll processing is a continuous method where materials are deposited, patterned, or treated as they move between two rollers. This technology is particularly used for fabrication on flexible substrates like plastics. Vacuum deposition fabrication techniques are energy-intensive, wasteful of materials and scale poorly to large display areas. As a result, fabrication is costly in comparison to other display technologies like LCDs for displays. In contrast, solution-processing fabrication provides a lower-cost alternative that uses less energy and materials to fabricate OLED displays and easily scales to large-area panels. For the avoidance of doubt, any embodiments described in connection with the first aspect may apply mutatis mutandis to the second to fourth aspects. For example, the electroluminescent device of the second to fourth aspects may comprise a compound of formula (I), (II) or (III), wherein those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and any one of formulae (VIa), (VId), (VIs), (VIt), and (VIu); formulae (IVa) to (IVe) are of formulae (VIIa) to (VIIe); and / or each A is independently selected from benzene, pyridine, thiophene and furan. Each and every patent and non-patent reference referred to herein is hereby incorporated by reference in its entirety, as if the entire contents of each reference were set forth herein in their entirety. 55402883-3M&C PE962338GB 33 The invention may be further understood with reference to the following non- limiting clauses: Clause 1. A compound of formula (I), (II) or (III):wherein: each A is independently benzene or a 5- or 6-membered heteroaromatic ring; n is independently 0 to 2; each X and Z is independently CH or N; and one of G1to G6is or more than one of G1to G6are independently selected from formulae (IVa) to (IVe):55402883-3M&C PE962338GB 34wherein: X1, X , X4and X are independently N or CR1and X2, X , X3and X are independently N or CR2; with the proviso that no more than one of X1, X2, X3and X4is N and no more than one of X , X , X and X is N; each R1is independently selected from H, halo, nitro, cyano, SF5, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, phenoxy, 5- or 6-membered heteroaryl, 5- or 6-membered heteroaryloxy, (C1-6alkyl)3silyl and (C1-6alkyl)3silyloxy; each R2is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy, phenyl, phenoxy, 5- or 6-membered heteroaryl, 5- or 6-membered heteroaryloxy, (C1-6alkyl)3silyl, (C1-6alkyl)3silyloxy, B(C1-6alkyl-substituted phenyl)2and any one of formulae (Va) to (Vf); and those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf):, wherein: each A2 is independently selected from benzene, napthalene and a 5- or 6- membered heteroaromatic ring, each of which is optionally substituted with one or more groups selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl 55402883-3M&C PE962338GB 35 C1-6alkoxy, phenyl, C1-6alkyl-substituted phenyl, C1-6haloalkyl-substituted phenyl, silyloxy and (C1-6alkyl)3silyl; X2is selected from C(R3)2, N, S, O and Si(R3)2, wherein each R3is independently selected from: C1-6alkyl, C1-6haloalkyl, C1-6alkoxy and optionally substituted benzene, napthalene and 5- or 6-membered heteroaromatic ring, each of which is optionally substituted with one or more selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl C1-6alkoxy, (C1-6alkyl)3silyl and (C1-6alkyl)3silyloxy; or the two R3groups combine together with the C or Si atom to which they are bonded such that C(R3)2or Si(R3)2forms a 5- or 6-membered ring, optionally fused with one or two benzene rings; and the dashed line of formula (Vb) is an optionally present bond; and when only one of G1to G6is selected from formulae (IVa) to (IVe), at least one of those of G1to G6not of formulae (IVa) to (IVe) is selected from formulae (Va) to (Vf). Clause 2. The compound of clause 1, wherein those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and formula (Va). Clause 3. The compound of clause 1 or clause 2, wherein A2 is an optionally substituted benzene, napthalene or 6-membered heteroaromatic ring. Clause 4. The compound of clause 1 or clause 2, wherein A2 is an optionally substituted benzene, naphthalene or pyridine ring. Clause 5. The compound of clause 1, wherein those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and any one of formulae (VIa) to (VIu):55402883-3M&C PE962338GB 36, wherein: X3is selected from C(CH3)2, C(C6H5)2, O, S, Si(CH3)2 and Si(C6H5)2; and the dashed line in formula (VIe) is an optionally present bond. Clause 6. The compound of clause 5, wherein those of G1to G6not of formulae (IVa) to (IVe) are independently selected from H, halo, cyano and any one of formulae (VIa), (VId), (VIs), (VIt), and (VIu). Clause 7. The compound of any one preceding clause, wherein each R1is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1- 6haloalkyl, and C1-6alkoxy. 55402883-3M&C PE962338GB 37 Clause 8. The compound of clause 7, wherein each R1is independently selected from H and halo, such as H. Clause 9. The compound of any one preceding clause, wherein each R2is independently selected from H, halo, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl, C1-6alkoxy and B(2,4,6-trimethylphenyl)2. Clause 10. The compound of clause 9, wherein each R2is independently selected from H, halo, C1-6alkyl, and C1-6haloalkyl. Clause 11. The compound of clause 9, where each R2is independently selected from H and C1-6alkyl. Clause 12. The compound of any one preceding clause, wherein formula (IVa) is of. Clause 13. The compound of any one preceding clause, wherein formulae (IVa) to (IVe) are of formulae (VIIa) to (VIIe):55402883-3M&C PE962338GB 38. Clause 14. The compound of clause 13, wherein each R2is the same. Clause 15. The compound of any one preceding clause, wherein the 5- or 6- membered heteroaromatic ring of A comprises one or more heteroatoms selected from N, O and S. Clause 16. The compound of clause 15, wherein each A is independently selected from benzene, pyridine, thiophene and furan. Clause 17. The compound of clause 15, wherein each A is independently selected from benzene and pyridine. Clause 18. The compound of any one preceding clause, wherein each A is the same. Clause 19. The compound of any one preceding clause, wherein each Z is CH. Clause 20. The compound of any one preceding clause, wherein n is independently 0 or 1. 55402883-3M&C PE962338GB 39 Clause 21. The compound of any one preceding clause, wherein at least one of G1and G2is H and at least one of G3and G4is H. Clause 22. The compound of any one preceding clause, wherein G5and G6of formula (III) are not H. Clause 23. The compound of any one preceding clause, which is of formula (I) or (III). Clause 24. The compound of any one preceding clause, wherein at least one of G1to G6is or at least two of G1to G6are of formula (IVa). Clause 25. The compound of clause 24, wherein those of G1to G6not of formula (IVa) are independently selected from H and formula (Va). Clause 26. The compound of clause 1, which is of formula (IX-ia), (IX-ib), (IX-iia), (IX- iiia) or (IX-iiib):55402883-3M&C PE962338GB 4055402883-3M&C PE962338GB 41. Clause 27. An electroluminescent device, such as an organic light-emitting diode, comprising a compound of any one preceding clause. 55402883-3M&C PE962338GB 42 Clause 28. Use of a compound of any one of clauses 1 to 26 in an electroluminescent device, such as an organic light-emitting diode. Clause 29. The use of clause 28, wherein the compound is used as an emitter material or as a host material. Clause 30. The use of clause 28 or clause 29, wherein the compound is used as an emitter of thermally activated delayed fluorescence. Clause 31. The use of any one of clauses 28 to 30, wherein the compound is solution- processed, such as ink-jet printed or roll-to-roll processed. Clause 32. A method of manufacturing an electroluminescent device comprising contacting a compound of any one of clauses 1 to 26 with one or more components of an electroluminescent device. 55402883-3M&C PE962338GB 43 EXPERIMENTAL General Synthesis The skilled person is aware of many different synthetic routes that are appropriate to synthesise the compounds of the invention. Examples are described below. The compounds of the invention may be synthesised by reaction of suitable diketones with diamines to form the pyrazine cores, as shown in Scheme 1.Scheme 1: Reaction of suitable diketone and diamine to form compound of the invention The dendron-like ligands of formulae (IVa) to (IVe) and the donor ligands of formulae (Va) to (Vf) may be introduced to halogenated precursors of the compounds of the 55402883-3M&C PE962338GB 44 invention, by reaction of suitable halogenated precursors with the desired amine ligand to form the compound, as shown in Scheme 2.Scheme 2: Reaction of suitable halogenated precursor and desired amine to form compound of the invention 55402883-3M&C PE962338GB 45 These reactions are typically carried out in 1-butanol or other alcohols with relatively high boiling points, i.e. boiling points of Materials All commercially available chemicals and reagent grade solvents were used as received. Air-sensitive reactions were performed under a nitrogen atmosphere using Schlenk techniques. Flash column chromatography was carried out using silica gel (Silia-P from Silicycle, 60 Å, 40-63 µm). Analytical thin-layer-chromatography (TLC) was performed with silica plates with aluminium backings (250 µm with F-254 indicator). TLC visualization was accomplished by 254 / 365 nm UV lamp.1H and13C NMR spectra were recorded on a Bruker Advance spectrometer (400 or 500 MHz for1H, 101 or 126 MHz for13ultiplet. Deuterated chloroform (CDCl3) and THF-d8 were used as the solvents of record.1H NMR and13C NMR spectra were referenced to the solvent peak. Steady-state emissions were recorded at 298 K using an Edinburgh Instruments FS5 fluorimeter, where the samples were excited at 343 nm using a Xenon lamp, Synthesis 3,6-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'- yl)tetrabenzo[a,c,h,j]phenazine (IX-ia), also referred to as 2GCzBPPZ 55402883-3M&C PE962338GB 46In a 100 ml Schlenk flask, a mixture of 3,6-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''- tercarbazol]-9'-yl)phenanthrene-9,10-dione (330 mg, 0.2 mmol, 1.0 equiv.) and phenanthrene-9,10-diamine (42 mg, 0.2 mmol, 1 equiv.) in 1-butanol (20 ml) was stirred at 110 °C for 24 h. After cooling to room temperature, the reaction was extracted with chloroform and washed with water (3 × 30 mL). The organic phase was dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using 1:20 ethyl acetate / hexane as eluent and then further purification was performed with preparative GPC column using THF as eluent to afford a red solid (150 mg). Yield: 41%.1H NMR (500 MHz, Chloroform-d) 8.99 (d, J = 7.1 Hz, 1H), 8.99 8.92 (m, 1H), 8.85 (d, J = 2.0 Hz, 1H), 8.23 (d, J = 2.3 Hz, 2H), 8.08 (d, J = 2.2 Hz, 2H), 7.97 (dd, J = 4.4, 2.2 Hz, 2H), 7.94 (dd, J = 7.3, 2.0 Hz, 1H), 7.91 (d, J = 2.1 Hz, 1H), 7.76 7.65 (m, 5H), 7.61 7.54 (m, 4H), 7.47 7.40 (m, 3H), 7.22 (dd, J = 7.8, 2.0 Hz, 2H), 1.37 (s, 72H). 12,15-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'- yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine (IX-ib) 55402883-3M&C PE962338GB 47In a 100 ml Schlenk flask, a mixture of 3,6-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''- tercarbazol]-9'-yl)phenanthrene-9,10-dione (330 mg, 0.2 mmol, 1.0 equiv.) and 1,10- phenanthroline-5,6-diamine (42 mg, 0.2 mmol, 1 equiv.) in 1-butanol (20 ml) was stirred at 110 °C for 24 h. After cooling to room temperature, the reaction was extracted with chloroform and washed with water (3 × 30 mL). The organic phase was dried over Na2SO4and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using 1:20 ethyl acetate / hexane as eluent and then further purification was performed with preparative GPC column using THF as eluent to afford a red solid (110 mg). Yield: 30 %.1H NMR (500 MHz, Chloroform-d) 9.05 (dd, J = 8.5, 2.0 Hz, 1H), 9.02 8.96 (m, 2H), 8.85 (d, J = 2.0 Hz, 1H), 8.23 (d, J = 2.2 Hz, 2H), 8.08 (d, J = 2.2 Hz, 2H), 7.97 (dd, J = 4.3, 2.2 Hz, 2H), 7.94 (dd, J = 7.3, 2.0 Hz, 1H), 7.91 (d, J = 2.2 Hz, 1H), 7.75 7.61 (m, 5H), 7.59 (d, J = 7.7 Hz, 2H), 7.47 7.40 (m, 3H), 7.25 7.20 (m, 1H), 1.37 (s, 72H). 2,3-bis(4-(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'- yl)phenyl)pyrazino[2,3-f][1,10]phenanthroline (IX-iia) 55402883-3M&C PE962338GB 48In a 100 ml Schlenk flask, a mixture of 1,2-bis(4-(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''- tercarbazol]-9'-yl)phenyl)ethane-1,2-dione (330 mg, 0.2 mmol, 1.0 equiv.) and 1,10- phenanthroline-5,6-diamine (42 mg, 0.2 mmol, 1 equiv.) in 1-butanol (20 ml) was stirred at 110 °C for 24 h. After cooling to room temperature, the reaction was extracted with chloroform and washed with water (3 × 30 mL). The organic phase was dried over Na2SO4and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using 1:20 ethyl acetate / hexane as eluent and then further purification was performed with preparative GPC column using THF as eluent to afford a red solid (139 mg). Yield: 38 %.1H NMR (500 MHz, Chloroform-d) 8.98 (dd, J = 4.2, 2.0 Hz, 1H), 8.82 (dd, J = 8.5, 2.0 Hz, 1H), 8.05 (d, J = 2.2 Hz, 2H), 7.98 7.92 (m, 2H), 7.90 (dd, J = 9.4, 2.1 Hz, 3H), 7.84 (d, J = 1.9 Hz, 1H), 7.79 7.73 (m, 3H), 7.69 7.62 (m, 2H), 7.60 (dd, J = 6.6, 2.1 Hz, 1H), 7.52 (d, J = 7.7 Hz, 2H), 7.46 (dd, J = 6.4, 2.1 Hz, 1H), 7.35 (dd, J = 7.8, 2.1 Hz, 2H), 7.28 7.20 (m, 4H). 55402883-3M&C PE962338GB 49 10,10'-(11,12-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'- yl)dibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (IX-iiia)Under nitrogen, a mixture of 3,3'',6,6''-tetrakis(tert-butyl-9'H-9,3':6',9''-tercarbazole) (289 mg, 0.4 mmol, 2.0 equiv.), 10,10'-(11,12-difluorodibenzo[a,c]phenazine-3,6-diyl)bis(10H- phenoxazine) (136 mg, 0.2 mmol, 1 equiv.) and caesium carbonate (391 mg, 1.2 mmol, 6 equiv.) in dry DMSO (10 ml) was stirred at 140 °C for 24 h. After cooling to room temperature, the reaction was extracted with chloroform and washed with water (3 × 30 mL). The organic phase was dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using 1:10 dichloromethane / hexane as eluent and then further purification was performed with preparative GPC column using THF as eluent to afford a red solid (210 mg). Yield: 50%.1H NMR (500 MHz, Chloroform-d) 8.54 (d, J = 8.0 Hz, 0H), 8.24 (dd, J = 16.6, 2.1 Hz, 1H), 8.08 (d, J = 2.2 Hz, 1H), 7.92 7.87 (m, 1H), 7.75 7.65 (m, 2H), 7.59 (d, J = 7.7 Hz, 1H), 7.52 7.43 (m, 1H), 7.46 7.39 (m, 1H), 7.28 7.21 (m, 2H), 7.24 7.17 (m, 2H), 6.84 6.78 (m, 1H), 1.37 (s, 72H). 55402883-3M&C PE962338GB 50 [11,12-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'-yl)dipyrido[3,2- a:2',3'-c]phenazine (2GCzBPN)4,5-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'-yl)benzene-1,2-diamine (0.98 g, 0.63 mmol, 1.0 equiv.) and 1,10-phenanthroline-5,6-dione (0.13 g, 0.63 mmol, 1.0 equiv.) were added to 40 mL of 1-butanol and then heated to at 110 °C for 24 h. After cooling to room temperature, the reaction was extracted with chloroform and washed with water (3 × 30 mL). The organic phase was dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using 1:20 ethyl acetate / hexane as eluent and then further purification was performed with a preparative GPC column using THF as eluent to afford a yellow solid (0.83 g). Yield: 76%.1H NMR (500 MHz, CDCl3) 9.87 (dd, J = 8.1, 1.6 Hz, 2H), 9.51 9.41 (m, 2H), 9.23 (s, 2H), 8.11 (s, 8H), 8.05 (d, J = 2.0 Hz, 4H), 7.98 (dd, J = 8.1, 4.4 Hz, 2H), 7.49 (d, J = 8.6 Hz, 4H), 7.33 (dd, J = 8.7, 2.0 Hz, 4H), 7.09 (s, 16H), 1.37 (s, 72H). 55402883-3M&C PE962338GB 51 Results Summary of photophysical properties of VIIIa, VIIIb, VIIIc and VIIId. Table 1nm / % eV eV eV eV eV IX-ia 576 78 (70) 2.58 / 2.33 0.25 -5.38 -2.79 2.59 IX-ib 650 57 (45) 2.38 / 2.21 0.17 -5.30 -2.94 2.36 IX-iia 618 71 (60) 2.45 / 2.29 0.16 -5.44 -2.90 2.54 IX-iiia 720 22 (18) 2.05 / 2.03 0.02 -5.02 -3.00 2.02aexc = 340 nm);bObtained from the onset of the prompt fluorescence (time window: 1 ns 100 ns) and phosphorescence spectra (time window: 1 ms 10 ms) measured in 2-MeTHF glass at 77 K, exc = 343 nm.cIn DCM with 0.1 M [nBu4N]PF6 as the supporting electrolyte and Fc / Fc+as the internal reference (0.46 V vs. SCE). The HOMO and LUMO energies were determined using EHOMO / LUMO=-(Eox / Ered+4.8) eV where Eoxand Eredare anodic and cathodic peak potentials versus Fc / Fc+, respectively, obtained from the DPV. ^E= ^EHOMO-ELUMO ^ Compounds IX-ib and IX-iia exhibit red-shifted emissions compared to compound IX-ia due to enhanced electron-withdrawing capability of their acceptors, stemming from the addition of nitrogen atoms. Among these four compounds, compound IX-iiia emits the deepest red emission ( ^PL= 720 nm). This is attributed to its multi-donor substitution and the integration of the strong donor phenoxazine. The S1 energies are 2.58, 2.38, 2.45 and 2.05 eV, while the T1 energies are 2.33, 2.21, 2.29 and 2.03 eV, for compound IX- ia, IX-ib, IX-iia and IX-iiia, respectively, corresponding to ^ESTvalues of 0.25 eV, 0.17 eV, 0.16 eV and 0.02 eV. Compound IX-iiia possess the smallest ^ESTof 0.02 eV, 55402883-3M&C PE962338GB 52 suggesting that it has the fastest reverse intersystem crossing rate constant (kRISC) and superior TADF efficiency. Comparison of the properties of 2GCzBPPZ (IX-ia) and 2GCzBPN (IX-iiib) Compound 2GCzBPPZ possesses a complex concentration- and temperature- dependent dual emission where the longer wavelength emission band shows TADF. Through a combination of detailed photophysical studies and theoretical calculations, the dual emission is attributed to contributions from both monomolecular and aggregates species. Due to the more twisted geometry and the presence of bulky substituents, 2GCzBPN exhibits TADF and aggregates formation is suppressed. The distinct photophysical properties of 2GCzBPPZ motivated the inventors to use it as a temperature sensor. 2GCzBPPZ exhibits excellent temperature sensitivity across a very wide temperature range ( ^70 ^C to 70 °C), corresponding to a large colour change from yellow at ^70 °C through to white at room temperature, to sky blue at 70 °C, which is one of the best TADF-based temperature sensors based on its large dynamic spectral range and associated wide temperature detection range emanating from a single material. Finally, the two compounds were employed as emitters in solution-processed OLEDs. Theoretical Calculations The ground-state (S0) geometries of 2GCzBPPZ and 2GCzBPN were optimized using density functional theory (DFT) at the PBE0 / 6-31G(d,p) level of theory in the gas phase starting from a geometry generated using Chem3D (Adamo and Barone, J. Chem. Phys. 1999, 110, 6158; Petersson, Tensfeldt, and Montgomery, J. Chem. Phys.1991, 94, 6091; Kerwin, J. Am. Chem. Soc.2010, 132, 2466). At the optimized S0 geometries, due to the larger steric hindrance caused by two of the dendron-like donors substituted on neighboring positions of the BPN acceptor, 2GCzBPN possesses a much more twisted geometry than that of 2GCzBPPZ, where the torsion angles between donor and acceptor were on average about 67 ^ and 43 ^, respectively. The frontier molecular orbital (FMO) of 2GCzBPPZ and 2GCzBPN are shown in Fig.5a. As expected, the highest occupied molecular orbitals (HOMOs) are localized on the donors while lowest unoccupied molecular orbitals (LUMOs) are localized on the acceptor group. Notably, the acceptor moiety of 2GCzBPPZ (LUMO = ^2.19 eV) is a weaker electron-acceptor than that of 2GCzBPN (LUMO = ^2.38 eV). 55402883-3M&C PE962338GB 53 Thus, the HOMO-LUMO gap, ^EHOMO-LUMO, decreases from 2.87 eV for 2GCzBPPZ to 2.36 eV for 2GCzBPN. The excited-state properties were calculated using time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation (TDA-DFT) based on the optimized S0geometries (Fig. 5b and 5c, the excited-state properties were also calculated based on optimized S1geometries; Hirata and Head-Gordon, Chem. Phys. Lett.1999, 314, 291; and Grimme, Chem. Phys. Lett.1996, 259, 128). The S1energies are 2.54 eV for 2GCzBPPZ and 1.95 eV for 2GCzBPN, while the T1 energies are 2.43 eV to 1.91 eV, respectively, corresponding to ESTvalues of 0.11 eV for 2GCzBPPZ and only 0.04 eV for 2GCzBPN, due to its more twisted conformation. Fig.5c displays the natural transition orbitals (NTOs) for S1and T1. For both compounds, the hole and electron densities are clearly separated for S1, indicating charge-transfer (CT) transitions from the ground state. The T1 state of 2GCzBPN is also CT in nature while that of 2GCzBPPZ is better described as one having a mixed locally excited (LE) and CT character. Reflecting the somewhat stronger overlap between the HOMO and LUMO electron densities, the calculated oscillator strength, f, for the S0-S1transition in 2GCzBPPZ (f = 0.13) is larger than in 2GCzBPN (f = 0.06). To evaluate the geometric rigidity of the compounds, the inventors calculated the root-mean-square displacement (RMSD) value (Fig.5d) using the VMD program between S0and S1geometries each optimized at the PBE0 / 6-31G(d,p) level (Humphrey, Dalke, Schulten, J. Mol. Graph. 1996, 14, 33). 2GCzBPN shows a relatively smaller geometry relaxation (RMSD = 0.22 Å) compared to 2GCzBPPZ (RMSD = 0.41 Å), suggesting that 2GCzBPN possesses a more rigid geometry, so nonradiative decay should be relatively attenuated for this compound compared to 2GCzBPPZ. Electrochemistry Next, the energies of the frontier molecular orbitals (FMOs) were inferred from the electrochemical behavior of 2GCzBPPZ and 2GCzBPN by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in degassed dichloromethane (DCM) with tetra-n- butylammonium hexafluorophosphate ([nBu4N]PF6) as the supporting electrolyte (Fig. 6a). The reduction potential (Eredfor 2GCzBPPZ 2GCzBPN, respectively. The inferred LUMO energies of 2GCzBPPZ and 2GCzBPN, respectively, are consistent with the trend of the energies from the theoretical calculation (Fig.5a). 55402883-3M&C PE962338GB 54 2GCzBPPZ and 2GCzBPN both have two resolvable quasi-reversible oxidation waves with Eoxof 1.00 V and 1.13 V for 2GCzBPPZ, 0.92 V and 1.07 V for 2GCzBPN, which correspond to the oxidation of the inner carbazole and the peripheral tert-butylcarbazole, respectively (Duda et al., J. Phys. Chem. B.2022, 126, 552; and Sun et al., Adv. Mater. 2022, 2110344). Due to the presence of two adjacent donors, 2GCzBPN has a catholically shifted first oxidation potential (Eox= 0.92 V) than that of 2GCzBPPZ (Eox= 1.00 V), HOMO LUMO gaps are 2.36 and 2.01 eV for 2GCzBPPZ and 2GCzBPN, respectively, which mirror the trend in the DFT calculated values of 2.87 and 2.36 eV (Fig.5a). Photophysical Properties The UV-Vis absorption spectra of 2GCzBPPZ and 2GCzBPN in dilute toluene are shown in Fig.6b, and the photophysical properties are summarized in Table 2. Both compounds exhibit similar strong double hump absorption with peaks at 337 and 346 nm, which can be attributed to locally excited (LE) transitions of the 2GCz dendron-like donors based on a comparison with literature data of 2GCz (Sun et al., 2022, supra; Connelly and Geiger, Chem. Rev.1996, 96, 877; Sun et al., Adv. Sci.2022, 2201470). The absorption band at around 400 nm for both compounds is assigned to an LE transition of the acceptor moieties as these align with the absorption of BPN (Furue et al., Adv. Opt. Mater. 2018, 6, 1).2GCzBPPZ possesses a stronger ICT absorption band peaking at 432 nm ( ^ = 19×103M-1cm-1) than that of 2GCzBPN (475 nm, ^ = 10×103M-1cm-1), which is consistent with the calculated higher f = 0.13 for 2GCzBPPZ (S0 1) than f = 0.06 for 2GCzBPN (S0 1). The unstructured and broad PL spectra in toluene for both compounds indicate excited states with strong ICT character, with peak maxima,PL, at 534 nm and 624 nm for 2GCzBPPZ and 2GCzBPN, respectively Fig.7a). A strong positive solvatochromism is observed for both compounds (Fig. 6c), which is consistent with the CT nature of the emissive excited state. Furthermore, 2GCzBPPZ exhibits a more significant positive solvatochromism with a ^f = 6194 cm^1than that of 2GCzBPN ( ^f = 4563 cm^1), corresponding to the larger transition dipole moment in the excited state, which is corroborated by DFT calculations, for 2GCzBPPZ (3.63 D) than 2GCzBPN (2.83 D). The optical bandgaps, Eg, calculated from the intersection of the normalized absorption and emission spectra are 2.62 eV and 2.26 eV for 2GCzBPPZ and 2GCzBPN, respectively. 55402883-3M&C PE962338GB 55 Prompt fluorescence and phosphorescence spectra in 2-MeTHF glass at 77 K were used to extract the S1and T1energies from their respective onsets Fig.5d, Table 2). The S1energies of 2GCzBPPZ and 2GCzBPN are 2.63 and 2.35 eV, and the T1 energies are 2.46 ESTvalues for 2GCzBPPZ and 2GCzBPN are 0.17 eV and 0.14 eV, respectively, which align with the TDA-DFT calculated results (Fig.5b). The structured character of the phosphorescence spectrum for both emitters implies a T1state with LE character, corroborated by the TDA-DFT calculations to mixed3CT / 3LE state (LE localized on the acceptor, especially for BPPZ, Fig.5c). Time-resolved PL (TRPL) decays of 2GCzBPPZ and 2GCzBPN in degassed toluene measured using time-correlated single-photon counting show biexponential kinetics with a prompt fluorescence lifetimp, of 24.3 ns and a delayed fluorescence d, of 7.5 2GCzBPN. However, the emission of 2GCzBPPZ decays P of 22.3 ns, and no delayed emission is observed in solution. In both compounds, the emission intensity in the toluene solutions was enhanced upon oxygen removal, demonstrating that oxygen is quenching accessible triplet states in both compounds. Table 2. Photophysical properties of 2GCzBPPZ and 2GCzBPN.346(45), 2.63 / 57 2GCzBPPZ 400(19), 534 0.17 531 18.0 73.1 2.46 (45) 432(19) 348(51), 2.35 / 71 2GCzBPN 397(15), 624 0.14 601 34.1 2.9 2.21 (60) 475(10) S1 / T1e / eV ESTe / eV HOMOf / eV LUMOf / eV ^Ef / eV 2.71 / 2GCzBPPZ 0.26 -5.30 -2.94 2.36 2.50 55402883-3M&C PE962338GB 56 2.38 / 2GCzBPN 0.12 -5.22 -3.21 2.01 2.26aIn PhMe at 298 K (exc= 340 nm).bObtained from the onset of the prompt fluorescence (time window: 1 ns 100 ns) and phosphorescence spectra (time window: 1 ms 10 ms) measured in 2-MeTHF glass at 77 K,exc= 343 nm.cThin films of 10 wt% emitters doped in mCP were prepared by spin-coating, andPLvalues were determined using an integrating sphere (exc= 345 nm). Values quoted are under N2. Values in parentheses are in air.dAverage lifetime ( ^^ ^ ^^ ^ ^^^ ^ ^ ^ ^ ^^^^, where ^^is the pre-exponential for lifetime ^^). Prompt and delayed emissions were measured by TCSPC and MCS, respectively ( exc = 379 nm).eS1 was obtained from the onset of the prompt emission (time window: 1 100 ns) measured in doped film at 77 K and T1was obtained from the onset of the phosphorescence spectrum (time window: 1 10 ms) measured in doped film at 77 K.fIn DCM with 0.1 M [nBu4N]PF6 as the supporting electrolyte and Fc / Fc+as the internal reference (0.46 V vs. SCE).

[0066] The HOMO and LUMO energies were determined using EHOMO / LUMO=- (Eox / Ered+4.8) eV where Eoxand Eredare anodic and cathodic peak potentials versus Fc / Fc+, respectively, obtained from the DPV.

[0066] g^E= HOMO-ELUMO Aggregate modulation D -conjugation inherent in both acceptors, there is likely to be . To explore and modulate the emergence of emission from the 2GCzBPPZ and 2GCzBPN aggregates, we measured the PL spectra of different concentrations of 2GCzBPPZ and 2GCzBPN in n- hexane solution (Fig.7). As shown in Fig.7a, there are two emission peaks observed for 2GCzBPPZ where the high-energy emission band at 475 nm converts to the low-energy emission band at 575 nm as a function of increasing concentration of the emitter, indicating that a new species, either aggregate or excimer, is responsible for the low- energy emission; however, there is negligible change in the emission spectra of 2GCzBPN with increasing concentration. The corresponding emission color of 2GCzBPPZ evolves from sky-blue to white and then to yellow. The corresponding ratios of the intensity of the emission at 475 to 575 nm (I475 / I575) and (I575 / I475) upon increasing concentration are shown in Fig. 7b. An isosbestic point was identified at a concentration of 1.6 ^10^5M where the emission intensity of the monomer 55402883-3M&C PE962338GB 57 ( ^PL= 475 nm) is equal to that of the aggregates ( ^PL= 575 nm). Specifically, at low concentrations of 2GCzBPPZ (<1.6 ^10^5M), the system is dominated by emission from the monomer as molecules are on average not sufficiently close to each other to form aggregates to any major extent (Fig.7c). As the concentration increases (>1.6 ^10^5M), the probability of intermolecular interactions increases, and the population of aggregates begins to grow (Fig. 7c). Fig. 7d reveals that with increasing concentration, the absorption maximum associated with the ICT band of 2GCzBPPZ monomer at ~432 nm gradually decreases along with the emergence of a red-shifted absorption tail from 452 nm to 550 nm, indicating the formation of aggregates through strong intermolecular interactions in the ground state (Förster, Angew. Chemie Int. Ed. English 1969, 8, 333). Furthermore, the absorption of 2GCzBPPZ in different solvents at the same concentration also demonstrates that the formation of aggregates occurs in n-hexane but not in higher polarity solvents like toluene, diethyl ether, or dichloromethane. This is likely due to strong solute-solvent interactions that competitively act to suppress the intermolecular interactions between 2GCzBPPZ molecules responsible for aggregate formation (Chen, Fimmel, and Würthner, Org. Biomol. Chem.2012, 10, 5845). Fig.8a shows that white emission can be obtained in 2 ^ 10^5M solutions of 2GCzBPPZ in n-hexane as a result of contributions from two distinct emission bands at lPL of 475 and 575 nm. Both emission bands are quenched upon exposure to oxygen, indicating that triplet states are accessible in both emission processes. TRPL studies in n-hexane under degassed and aerated conditions reveal that the emission band at 475 nm decays with a similar lifetime,p, of around 5.7 ns (Fig.8b, top), which may reflect the quenching of this emission due to fast Förster resonance energy transfer from the monomers to aggregates. Under degassed conditions, the emission band at 575 nm decays with pdof 7.5 Fig.8b, bottom), the delayed emission was largely quenched after exposure to air, which could be explained as the quenching of the aggregates-induced TADF in solution (Yang, Fang and Li, Aggregate 2020, 1, 6). The inventors next investigated the photophysical properties of 2GCzBPPZ as doped films in polymethyl methacrylate (PMMA) (Fig.8c). For the 0.1 wt% doped film in PMMA, 2GCzBPPZ exhibits a weak green emission with the main peak at 515 nm and shoulder peak at 580 nm. With increasing doping concentration, the intensity of the high-energy emission band decreases while the low-energy emission band becomes dominant, 55402883-3M&C PE962338GB 58 reflecting a similar behavior to that observed in n-hexane solution. The TRPL decays reveal biexponential decay kinetics with associated tp and td for both emission bands in 0.1 wt% doped film in PMMA. The delayed lifetime of the low-energy emission band decreased from 2.12 ms to 150.5 ^s with increasing doping concertation from 0.1 wt% to 50 wt% (Fig.8d). While the corresponding prompt lifetime slightly increased from 18.7 ns to 25.9 ns with increasing doping concertation. Colorimetric temperature sensing Intrigued by the unusual dual-emissive nature of 2GCzBPPZ, the inventors sought to explore in greater detail the photophysical properties and temperature-dependence of the emission in n-hexane (Fig. 9). At room temperature, the 1.6 ^10^5M solution of 2GCzBPPZ is dual-emissive and the sample appears to emit white light (Fig.9a), where there is approximately equal contributions from the emission from the monomer (475 nm) and aggregates (575 nm). Upon decreasing the temperature towards the solvent freezing point, the low-energy emission band increases in intensity dramatically while the high- energy emission band is completely quenched. The corresponding ratio of the intensity of the emission at 575 to 475 nm (I575 / I475) exponentially decreases with increasing temperature from ^70 ^C to room temperature (r2= 0.998) (Fig. 9d). Such an exponential relationship could result from a combination of various factors at low temperature, like high viscosity and low solubility that will both affect the population of the aggregates (Peng et al., J. Phys. Chem. C 2020, 124, 22684). On the other hand, increasing the temperature beyond room temperature reveals a complementary effect where the high-energy emission band becomes more intense and the low-energy emission band all but disappears. In this temperature regime there is a linear relationship between I475 / I575 versus T (r2= 0.988 over a temperature region of 25 to 70 °C), corresponding to a ratiometric increase of 6.6% ± 0.2% K1(Fig.9e). Overall, 2GCzBPPZ features excellent temperature sensitivity across a broad range of ^70 ^C to 70 °C, manifested in distinct colorimetric readout from yellow at ^70 °C to white at RT and finally to sky blue at 70 °C, corresponding to CIE coordinates of (0.50, 0.49) at ^70 °C that shift to CIE coordinates of (0.23, 0.32) associated with blue emission (Fig. 9b). The broad range of temperature detection coupled with the significant color change exhibited by 2GCzBPPZ make it a promising temperature sensor, whose properties are much superior to previously reported organic fluorescent temperature sensors (Sun et al., Biotechnol. Bioeng. 2023, 120, 7; Mazza and Raymo, J. Mater. 55402883-3M&C PE962338GB 59 Chem. C 2019, 7, 5333; Vetrone et al., ACS Nano 2010, 4, 3254). Generally, most organic fluorescent temperature sensors rely only on changes in emission intensity with negligible color change and have a narrow temperature detection range, usually of between RT to ~70 °C. We have interpreted the origin of the wide dynamic spectral range of our optical temperature sensor to result from a temperature-dependent equilibrium between monomeric and aggregate species. Illustrated in Fig. 9f, upon increasing the temperature, the p-stacking interactions necessary for aggregate formation are disrupted and the monomer population increases, reflected in the emergence of blue emission. As the temperature is decreased, the mobility of the molecules in solution is reduced, which can increase the probability of intermolecular interactions and promote the formation of aggregates as the molecules have less energy to overcome the repulsive forces between them and the attractive forces become more significant. Note that FRET occurs from monomer to the aggregates and so as the population of aggregates increases, so does the FRET rate, leading to an accelerated quenching of the high-energy emission of the monomeric species. Recognizing the potential of this compound to act as a temperature sensor, we translated its properties into the solid state by embedding the compound into Paraffin. When photoexcited at 360 nm the solid Paraffin emits in the yellow at room temperature. As the temperature increases from 20 towards 80 °C, paraffin starts to melt at around 50 °C and the emission gradually blue-shifts from yellow to green. When the temperature increases beyond 160 °C, the liquid paraffin emits in the blue, emulating the emission observed in n-hexane beyond 60 °C. This distinctive performance in paraffin makes it ideal as a spatio-temperature probe. The 2GCzBPPZ-embedded paraffin was melted and poured into a test tube (length:160 mm, diameter: 16 mm) and then allowed to solidify upon cooling. At room temperature, the entire test tube with the solid paraffin shows yellow emission. As the top part of the temperature is selectively heated and the local temperature increases, this part of the paraffin sample exhibits a noticeable color change, while the bottom section of the paraffin maintains its yellow emission as this part is still at room temperature. The inventors have correlated the optical response of the 2GCzBPPZ-embedded paraffin with an external temperature probe to demonstrate that it can act as an accurate temperature sensor. 55402883-3M&C PE962338GB 60 In an attempt to use a non-polar solid host with a higher melting temperature, the inventors also incorporated 2GCzBPPZ into 4,4 -bis(9-carbazolyl)-biphenyl (CBP). However, despite having a similar polarity to paraffin, dual emission was not observed in this host. Although the doped film of 2GCzBPPZ in PMMA exhibits concentration- dependent dual emission, there was no temperature-dependent emission observed, which implies that in this host it is not possible to change significantly intramolecular distances as a function of temperature. The fact that the optical thermometer phenomenon works in paraffin but not the other host may be due to the paraffin having a greater thermal expansion coefficient compared to the other host compound (Ruan et al., Tribol. Int. 2022, 165, 107303). 2GCzBPPZ thus shows unrivaled temperature sensitivity and with a broad temperature-dependent spectral response compared to previously reported organic temperature sensors. TADF properties in the solid state To assess the emission properties of 2GCzBPPZ and 2GCzBPN in the solid state, their photophysical properties were investigated in an OLED-relevant host 1,3-bis(N- carbazolyl)benzene (mCP) as this host matrix has a sufficiently high triplet energy (T1 = 2.91 eV)

[0076] to confine the excitons onto the emitter. As shown in Fig.9, as 10 wt% doped films in mCP, 2GCzBPNPLof 601 nm, which is red-shifted compared to 2GCzBPPZ,PLof 531 nm (Table 2). The corresponding PL) of 2GCzBPN and 2GCzBPPZ are 71 and 57%, respectively, which decrease to 60 and 45%, respectively, under air. The S1 / T1energy levels of 2GCzBPPZ (2.76 / 2.50 eV) and 2GCzBPN (2.38 / 2.26 eV) were inferred from the onsets of the prompt fluorescence and phosphorescence spectra, respectively, at 77 K (Fig. 10a and 10b). For 2GCzBPN, both the fluorescence and phosphorescence spectra are broad and structureless, indicating that both the S1and T1states have CT character. However, the phosphorescence spectrum of 2GCzBPPZ is structured, suggesting that the T1 state has LE character, while the S1 state is CT in EST values are 0.26 and 0.12 eV for 2GCzBPPZ and 2GCzBPN, respectively, which matched the trend observed in 2-MeTHF glass at 77 K (DEST of 0.17 eV for 2GCzBPPZ and 0.14 eV for 2GCzBPN). The room temperature emission from both compounds shows multiexponential decay kinetics, with average prompt fluorescence lifetimes,^p, of 18.0 and 34.1 ns, and average 55402883-3M&C PE962338GB 61 delayed emission lifetimes, ^d2GCzBPPZ and 2GCzBPN, respectively (Fig.10d). The corresponding rate constants of intersystem crossing (kISC) for both compounds in mCP films are 1.61 × 107s1and 0.45 × 107s1for 2GCzBPPZ, and 2GCzBPN, respectively, while the rate constants of reverse intersystem crossing (kRISC) for 2GCzBPN reached 4.1 × 105s1, a value 20 times faster than in 2GCzBPPZ of 1.93 × 104s1. The relative intensities of the delayed PL increased with increasing temperature from 100 K to 298 K, corroborating the TADF nature of the emission of both compounds in the mCP films (Fig.10e and 10f). OLED characterization The inventors next fabricated solution-processed (SP) OLEDs employing 10 wt% emitters doped in mCP films as the emissive layer (EML) using the following device stack: ITO (indium tin oxide) / poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (40 nm) / mCP (60%): oxadiazolyl]phenylene (OXD-7) (30%): 10 wt% emitters (20 nm) / ( - -(3-(pyridin-3-yl)phenyl)- -terphenyl]- - diyl)dipyridine) (TmPyPB) (45 nm) / LiF (1 nm) / Al (Fig.11a and 11b), where ITO and Al serve as the anode and cathode, respectively, and PEDOT:PSS and TmPyPB are the hole-transporting layer electron-transporting layer, respectively. As shown in Fig. 11b, the OLEDs with 2GCzBPPZ and 2GCzBPN exhibit green and orange emission with emission maxima, ^EL, of 552 and 608 nm and corresponding (0.39, 0.55) and (0.58, 0.40), respectively, which match the PL emission (Fig. 11c). The devices with 2GCzBPPZ showed lower turn-on voltages (Von) of 3.5 V and high brightness of up to 8000 cd m^2compared to Vonof 4.1 V and a maximum luminance of 1068 cd m^2for the devices with 2GCzBPN (Fig.11c and Table 3). At the same voltages, the current density (J) of the 2GCzBPN-based device is lower than that of the 2GCzBPPZ-based device, which we ascribe to the relatively lower electron mobility of 2GCzBPN in the mCP film. The devices with 2GCzBPPZ showed a higher maximum external quantum efficiency (EQEmax) of 15.0% at 581 cd m^2with negligible efficiency roll-off at 1000 cd m^2(EQE1000= 14.0%), while the devices with 2GCzBPN showed poorer performance, with EQEmax of 5.3% (Fig.11d) PL (Table 2) and assuming 25% outcoupling efficiency associated with an isotropic orientation of the transition dipole moment of the emitter, the EQEmax for devices with 2GCzBPPZ and 2GCzBPN were expected to be 55402883-3M&C PE962338GB 62 14.3% and 17.8%, respectively. This indicates that the device with 2GCzBPPZ has effectively unity exciton utilization efficiency, which can be attributed to the stabilizing effect of aggregation on excitons (Liu et al., Aggregate 2022, 3, DOI 10.1002 / agt2.164). Table 3. Electroluminescence data for the devices Emitter Vona / V nm cd m^2cd A-1lm W-1x,y 2GCzBPPZ 3.5 552 7207 51.7 28.8 15.0 / 10.2 / 14.0 0.39, 0.55 2GCzBPN 4.1 608 1068 10.2 5.1 5.3 / 5.1 / 3.0 0.58, 0.40 a The turn--2.bThe electroluminescence maximum recorded at 6 V.cEQEmax / EQE100 / EQE1000.dThe CIE coordinates recorded at 6 V Conclusions The inventors have designed two TADF dendrimers 2GCzBPPZ and 2GCzBPN by using different rigid and planar nitrogen-doped polycyclic aromatic hydrocarbons as the acceptors, combined with two second-generation tercarbazole donor dendrons. Owing to the adoption of a less twisted geometry and the use of a large ^-conjugation acceptor, 2GCzBPPZ shows an unusual white emission in n-hexane that results from dual emission from a combination of monomer and aggregates emitting, respectively, at 475 nm and 575 nm. The dual emission behavior of 2GCzBPPZ is quite sensitive to both the concentration and temperature. The dual emission behavior was exploited in a temperature sensor by embedding 2GCzBPPZ knowledge, 2GCzBPPZ shows the broadest spectra and temperature response of any organic temperature sensor. Due to its more twisted geometry, 2GCzBPN shows efficient TADF and there is neither significant aggregate formation nor dual emission. Finally, the two compounds were employed as emitters in solution-processed OLEDs. The SP- OLEDs with 2GCzBPPZ showed around a three-times higher EQEmaxof 15.0% at lELof 552 nm than the device with 2GCzBPN (EQEmax = 5.3% at lEL= 608 nm). 55402883-3

Claims

M&C PE962338GB 63 CLAIMS:

1. A compound of formula (I), (II) or (III):wherein: each A is independently benzene or a 5- or 6-membered heteroaromatic ring; n is independently 0 to 2; each X and Z is independently CH or N; and one of G1to G6is or more than one of G1to G6are independently selected from formulae (VIIa) to (VIIe):55402883-3M&C PE962338GB 64wherein: each R2is independently selected from C2-6alkyl and C2-6haloalkyl, or C3-6alkyl and C3-6haloalkyl; and those of G1to G6not of formulae (VIIa) to (VIIe) are independently selected from H, halo, cyano, SF5, C1-6haloalkyl and any one of formulae (Va) to (Vf):, wherein: each A2 is independently selected from benzene, napthalene and a 5- or 6- membered heteroaromatic ring, each of which is optionally substituted with one or more groups selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl C1-6alkoxy, phenyl, C1-6alkyl-substituted phenyl, C1-6haloalkyl-substituted phenyl and silyl; X2is selected from C(R3)2, N, S, O and Si(R3)2, wherein each R3is independently selected from: C1-6alkyl, C1-6haloalkyl, C1-6alkoxy and optionally substituted benzene, napthalene and 5- or 6-membered heteroaromatic ring, each of which is optionally substituted with one or more selected from halo, ester, nitro, cyano, sulfonyl, carbonyl, C1-6alkyl, C1-6haloalkyl C1-6alkoxy, and (C1-6alkyl)3silyl; or 55402883-3M&C PE962338GB 65 the two R3groups combine together with the C or Si atom to which they are bonded such that C(R3)2 or Si(R3)2 forms a 5- or 6-membered ring, optionally fused with one or two benzene rings; and the dashed line of formula (Vb) is an optionally present bond; and when only one of G1to G6is selected from formulae (VIIa) to (VIIe), at least one of those of G1to G6not of formulae (VIIa) to (VIIe) is selected from formulae (Va) to (Vf).

2. The compound of claim 1, wherein those of G1to G6not of formulae (VIIa) to (VIIe) are independently selected from H, halo, cyano and formula (Va).

3. The compound of claim 1 or claim 2, wherein A2 is an optionally substituted benzene, napthalene or 6-membered heteroaromatic ring.

4. The compound of claim 1 or claim 2, wherein A2 is an optionally substituted benzene, naphthalene or pyridine ring.

5. The compound of claim 1, wherein those of G1to G6not of formulae (VIIa) to (VIIe) are independently selected from H, halo, cyano and any one of formulae (VIa) to (VIu):55402883-3M&C PE962338GB 66, wherein: X3is selected from C(CH3)2, C(C6H5)2, O, S, Si(CH3)2and Si(C6H5)2; and the dashed line in formula (VIe) is an optionally present bond.

6. The compound of claim 5, wherein those of G1to G6not of formulae (VIIa) to (VIIe) are independently selected from H, halo, cyano and any one of formulae (VIa), (VId), (VIs), (VIt), and (VIu).

7. The compound of any one preceding claim, wherein each R2is independently selected from C3-6alkyl and C3-6haloalkyl.

8. The compound of claim 7, wherein each R2is independently selected from branched C3-6alkyl and branched C3-6haloalkyl, such as branched C3-5alkyl and branched C3-5haloalkyl.

9. The compound of any one preceding claim, wherein each R2is the same.

10. The compound of any one preceding claim, wherein the 5- or 6-membered heteroaromatic ring of A comprises one or more heteroatoms selected from N, O and S. 55402883-3M&C PE962338GB 67 11. The compound of claim 10, wherein each A is independently selected from benzene, pyridine, thiophene and furan.

12. The compound of claim 10, wherein each A is independently selected from benzene and pyridine.

13. The compound of any one preceding claim, wherein each A is the same.

14. The compound of any one preceding claim, wherein each Z is CH.

15. The compound of any one preceding claim, wherein n is independently 0 or 1.

16. The compound of any one preceding claim, wherein at least one of G1and G2is H and at least one of G3and G4is H.

17. The compound of any one preceding claim, wherein G5and G6of formula (III) are not H.

18. The compound of any one preceding claim, wherein at least one of G1to G6is or at least two of G1to G6are of formula (VIIa).

19. The compound of claim 18, wherein those of G1to G6not of formula (VIIa) are independently selected from H and formula (Va).

20. An electroluminescent device, such as an organic light-emitting diode, comprising a compound of any one preceding claim.

21. Use of a compound of any one of claims 1 to 19 in an electroluminescent device, such as an organic light-emitting diode.

22. The use of claim 21, wherein the compound is used as an emitter material or as a host material.

23. The use of claim 21 or claim 22, wherein the compound is used as an emitter of light of a wavelength of about 570 to about 780 nm. 55402883-3M&C PE962338GB 68 24. The use of any one of claims 21 to 23, wherein the compound is solution- processed, such as ink-jet printed or roll-to-roll processed.

25. A method of manufacturing an electroluminescent device comprising contacting a compound of any one of claims 1 to 19 with one or more components of an electroluminescent device. 55402883-3

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