Amide-substituted fluoran compounds as yellow thermochromic DYES and their use in imaging systems

Abstract

Disclosed is an amide-substituted fluoran compound represented by formula (la) wherein R1 to R6 can be the same or different, and are each independently selected from the group consisting of hydrogen, halogen, linear or branched alkyl, cycloalkyl, alkoxy, benzyl, aryl and heteroaryl; R7 to R10 can be the same or different, and are each independently selected from the group consisting of hydrogen, linear or branched alkyl, halogen, carboxyl, alkoxy, aryl or heteroaryl, nitro, amide, and amine; R11 and R12 can be the same or different, and are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, cycloalkoxy, carboxyl and benzyl, or are linked via cycloalkyl; and X is either OH or SH. Furthermore a composition, use of a compound or a composition, a thermo-imaging system, a method for thermo-imaging or full color imaging and a method for synthesizing the compound disclosed herein are described.

Description

AMIDE-SUBSTITUTED FLUORAN COMPOUNDS AS YELLOW THERMOCHROMIC DYES AND THEIR USE IN IMAGING SYSTEMSBACKGROUND

[0001] The field of the DISCLOSURE lies in active materials in imaging systems.

[0002] The present disclosure relates to amide-substituted fluoran active materials and their use in thermo-imaging and full color imaging and methods for their synthesis.

[0003] The present disclosure also relates to compositions comprising the amide-substituted fluoran active materials according to the present disclosure.

[0004] Moreover, the present disclosure relates to a thermo -imaging system comprising a thermochromic layer comprising at least one amide-substituted fluoran compound according to the present disclosure and to a method for thermo -imaging or full color imaging.DESCRIPTION OF THE RELATED ART

[0005] The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.

[0006] Leuco dyes are materials that undergo controlled color changes resulting in a shift from a colorless state (absorbing below 350 nm) to an intense color in the visible (380 nm - 700 nm) or even in near infrared region (700 nm - 1200 nm) depending on its molecular structure. The change from colorless to colored state is triggered by an acidic environment, modifying its molecular structure, increasing its conjugation length, generating the absorption in longer wavelengths. This process is reversible, and the system can turn transparent again (no absorption in the visible range of the electromagnetic spectrum) by adding a base into the mixture, which makes the materials quite interesting for different types of application, for instance, textile, thermo -printing, bio-labelling, etc.

[0007] The coloration and discoloration process can also be controlled by heat, the process is then called thermochromism, commonly used in thermal printing technology. The dyes in this case are called thermochromic dyes and, therefore, a thermochromic dye is a compound that changes color when accessible to heat.

[0008] The development of new dyes to be used in thermochromic systems has been intense in the last few decades, leading to the development of many materials and new applications, such as the thermal rewritable recording technology that uses leuco dyes based in a black and white color system. However, for reaching high quality for a printed photographic image, meaning a full-color image with high quality and resolution, materials are required that absorb in very specific areas of the visible region.

[0009] The design and development of new dyes for thermochromic systems presenting high thermal stability and light resistance, as well as to obtain full-color images with high quality and resolution, having negligible or very low color quality variation after thermal treatment used during process preparation, is a challenge. One possible solution is to continue with the development of new dyes with improved properties.SUMMARY

[0010] In the following, the elements of the disclosure will be described. These elements are listed with specific embodiments; however, they may be combined in any manner and in any number to create additional embodiments. The variously described examples and preferred embodiments should not be construed to limit the present disclosure to only the explicitly described embodiments. This description should be understood to support and encompass embodiments which combine two or more of the explicitly described embodiments or which combine the one or more of the explicitly described embodiments with any number of the disclosed and / or preferred elements. Furthermore, any permutations and combinations of all described elements in this disclosure should be considered disclosed by the description of the present disclosure unless the context indicates otherwise.

[0011] The present disclosure provides an amide-substituted fluoran compound represented by formula lawhereinR1 to R6 are the same or different, and are each independently selected from the group consisting of hydrogen, halogen, linear or branched alkyl, cycloalkyl, alkoxy, benzyl, aryl and heteroaryl;R7 to R10 are the same or different, and are each independently selected from the group consisting of hydrogen, linear or branched alkyl, halogen, carboxyl, alkoxy, aryl or heteroaryl, nitro, amide, and amine;R11 and R12 are the same or different, and are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, cycloalkoxy, carboxyl, benzyl, or are linked via cycloalkyl, lactams; andX is either OH or SH.

[0012] The present disclosure provides a composition comprising:(a) at least one compound according to the present disclosure;(b) a color developer;(c) a polymer matrix; and(d) optionally, a photo-thermal conversion agent (PC A).

[0013] The present disclosure provides the use of a compound according to the present disclosure or of a composition according to the present disclosure in an imaging system.

[0014] The present disclosure provides a thermo-imaging system comprising(i) a thermochromic layer comprising at least one compound according to the present disclosure, or a composition according to the present disclosure;(ii) a printable substrate or surface to which the thermochromic layer (i) is applied; and(iii) optionally, a heat source.

[0015] The present disclosure provides a method for thermo-imaging or full color imaging, comprising the steps:(a) providing a printable substrate or surface;(b) applying at least one compound according to the present disclosure, or a composition according to the present disclosure;(c) inducing the transition from the colorless state of the compound to a color state by applying heat and producing a compound-developer complex, and(d) keeping the color state of the compound by rapid cooling to room temperature.

[0016] The present disclosure provides a method for synthesizing a compound according to the present disclosure, comprising the steps of(1) treating fluorescein with a base, preferably NaOH, to obtain 2-(2,4-dihydroxybenzoyl) benzoic acid;(2) mixing 2-(2,4-dihydroxybenzoyl) benzoic acid with 3 -bromophenol and methane sulfonic acid in solvent to obtain 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]- 3 -one;(3) dispersing 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]-3-one inO^NH2solvent and mixing with acetamide I to obtain N-(3'-hydroxy-3-oxo-3H- spiro[isobenzofuran-l,9'-xanthen]-6'-yl)acetamide:

[0017] The foregoing paragraphs have been provided by way of general introduction and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

[0019] Figure 1 shows a general structure of the amide-substitute fluoran compound.

[0020] Figure 2 shows the suggested synthetic route for the preparation of the compounds of the present disclosure, namely amide-substitute fluoran structures used for the preparation of a yellow dye, compound MC in the figure. Intermediate (A) can be obtained by different methods found in the literature.

[0021] Figure 3 shows the suggested synthetic route for the preparation of the intermediate (A) for the synthesis of amide-substitute fluoran structures

[0022] Figure 4 shows the suggested synthetic route for the preparation of amide-substitute fluoran structure used for the preparation of a yellow dye, compound MC in the figure.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] As discussed above, the present disclosure provides an amide-substituted fluoran compound represented by formula la:

[0024] whereinR1 to R6 are the same or different, and are each independently selected from the group consisting of hydrogen, halogen, linear or branched alkyl, cycloalkyl, alkoxy, benzyl, aryl and heteroaryl;R7 to R10 are the same or different and are each independently selected from the group consisting of hydrogen, linear or branched alkyl, halogen, carboxyl, alkoxy, aryl or heteroaryl, nitro, amide, and amine.Rn and R12 are the same or different, and are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, cycloalkoxy, carboxyl, benzyl, or are linked via cycloalkyl; andX is either OH or SH.

[0025] In embodiments, wherein Rn and R12 are linked via cycloalkyl, Rn and R12 may be a lactam.

[0026] The alkyl in formula (la) may be a C1-C10 alkyl, preferably a C4-C8 alkyl.

[0027] The cycloalkyl in formula (la) may be a C3-C8 cycloalkyl, preferably a C4-C5 cycloalkyl.

[0028] The alkoxy in formula (la) may be a Ci-Cs alkoxy, preferably a C4-C6 alkoxy.

[0029] The carboxyl in formula (la) may be a C1-C10 carboxyl, preferably a C4-C6 carboxyl, more preferably a C2-C3 carboxyl.

[0030] In one embodiment, in the amide-substituted fluoran compound according to the present disclosure, Ri to Re can be the same or different, and are each independently selected from the group consisting of hydrogen, halogen, linear or branched C1-C6 alkyl, C3-C6 cycloalkyl, C1-C6 alkoxy, benzyl, aryl and hctcroaryl; and R7 to Rio can be the same or different, and are each independently selected from the group consisting of hydrogen, linear or branched C1-C4 alkyl, halogen, C1-C6 carboxyl, C1-C4 alkoxy, aryl or heteroaryl, nitro, amide, and amine.

[0031] In one embodiment, wherein the amide-substituted fluoran compound is represented by formula la, the amide-substituted fluoran compound is represented by any of the following structures:N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)acetamide(C22H15NO5)2,2,2-trifluoro-N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'- yl)acetamide (C22H12F3NO5)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)hexanamide (C26H23NO5)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)benzamide (C27H17NO5)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-4- methoxybenzamide (C28H19NO6)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-N-phenethyl acetamide (C30H23NO5)

[0032] An amide-substituted fluoran compound according to the present disclosure preferably absorbs in the absorption range between about 380 nm to 520 nm.

[0033] An amide-substituted fluoran compound according to the present disclosure preferably shows a lower absorption coefficient in the range between about 520 nm to 700nm.

[0034] An amide-substituted fluoran compound according to the present disclosure preferably shows a correspondingly high extinction coefficient when in colored form.

[0035] An amide-substituted fluoran compound according to the present disclosure preferably shows an extinction coefficient of > 104Lmol^cm1, when in colored form.

[0036] The compounds of the present disclosure preferably exhibit high thermal and photo resistance, wherein the retention of optical density is preferably kept above 80 % of the initial value under storage tests and light endurance tests. One example of storage tests is to expose samples at about 60 °C and 15-90 % RH for several hours. One example of light endurance tests is to expose samples at about 60 °C for several hours under light irradiation using, for instance, a 60 W / m2Xe weather meter.

[0037] The compounds of the present disclosure preferably exhibit degradation temperatures above 250°C, as determined by thermalgravimetric analysis.

[0038] An amide-substituted fluoran compound according to the present disclosure is preferably used as color former in thermal-imaging systems or methods for producing full color image.

[0039] An amide-substituted fluoran compound according to the present disclosure is preferably a thermochromic compound.

[0040] An amide-substituted fluoran compound according to the present disclosure preferably undergoes a heat-induced transition from a colorless state (with absorption below about 350 nm) to a color state in the visible range (from about 380 nm to 520 nm), which is preferably reversible.

[0041] As discussed above, the present disclosure provides a composition comprising:(a) at least one compound according to the present disclosure;(b) a color developer; and(c) a polymer matrix; and(d) optionally, a photo-thermal conversion agent (PC A).

[0042] In one embodiment, the composition is a mixture.

[0043] In one embodiment, the at least one compound (a), compound (b) and, optionally, compound (d) are dispersed in the polymer matrix (c). For example, the compound(s) of the present disclosure are dispersed in a polymer matrix containing a mixture of the compound(s), developer, and in some cases a photo-thermal conversion agent (PCA) that is used to convert NIR light into heat. This composition or mixture can also be called leuco media.

[0044] In one embodiment, the color developer (b) is a compound that assists a colorless dye to develop color.

[0045] In one embodiment, the color developer (b) is a compound comprising, preferably in its main skeleton structure, one or more of the following functional groups: phenol, carboxylic acid, amide, amine, urea, or urethane derivatives.

[0046] In one embodiment, the color developer (b) is phase separated from the at least one compound (a) in the polymer matrix (c) before color development.

[0047] In one embodiment, the polymer matrix (c) is a material wherein the at least one compound (a), compound (b) and, optionally, compound (d) are easily dispersed.

[0048] In one embodiment, the polymer matrix (c) is selected from the group consisting of polyvinylchlorides, polyester, amorphous co-polyesters, polyurethane, polyamide, polyimide,polycarbonate, poly methacrylic ester, polyvinyl alcohol, cellulose, cellulose nanofiber, liquid silicone rubbers or combinations thereof.

[0049] In one embodiment, the PCA (d) is a near-infrared (NIR) absorbing dye, that absorbs near infrared light, then producing heat.

[0050] The PCA (d) may be a near-infrared (NIR) absorbing dye having an absorption peak from 700 nm to 1500 nm and having low absorption in the visible region (380 nm to 700 nm).

[0051] The PCA (d) may be selected from the group consisting of naphthalocyanines and their metallonaphthalocyanine derivatives, phthalocyanines and their metallophthalocyanine derivatives, porphyrins and their metalloporphyrins derivatives, cyanines, squarylium dyes, croconaine dyes, and a thiolate complex.

[0052] Disclosed is also the use of a compound according to the present disclosure or of a composition according to the present disclosure in an imaging system.

[0053] Further disclosed is the use of a compound according to the present disclosure in a thermo-imaging system.

[0054] Further disclosed is the use of a compound according to the present disclosure for full color imaging.

[0055] The compound may be used as yellow dye or color former in a thermo-imaging system.

[0056] Disclosed is also the use of a composition according to the present disclosure in a thermo-imaging system.

[0057] Further disclosed is the use of a composition according to the present disclosure for full color imaging.

[0058] The use of the compound or composition may comprise inducing the transition from the colorless state of the compound to a color state by applying heat.

[0059] The heat may be applied by direct heating.

[0060] Direct heating can be, for example, applied via a printhead of a thermal printer that preferably comprises one or more arrays of small heaters.

[0061] The heat may be applied by NIR laser irradiation in the range from about 700 nm to 1500 nm (NIR region), when PCA is present.

[0062] The use may comprise a rapid cooling to room temperature after the heat-induced transition from the colorless state to a color state, for keeping the color state of the compound.

[0063] The thermo-imaging system may be a reversible thermo-imaging system.

[0064] The use may comprise changing back the color state of the compound to the colorless state by heating to a temperature about 10 % above the decomposition temperature of thecompound-developer complex, and then slow cooling to room temperature. The decomposition temperature may depend on the compound and the developer used, or the composition used, and, thus, needs to be determined for each compound and developer / composition.

[0065] As discussed above, the present disclosure provides a thermo-imaging system comprising:(i) a thermochromic layer comprising at least one compound according to the present disclosure, or a composition according to the present disclosure;(ii) a printable substrate or surface to which the thermochromic layer (i) is applied; and(iii) optionally, a heat source.

[0066] In one embodiment, the substrate (ii) is a textile, membrane, glass, or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, polymethracrylic ester, cellulose, or cellulose nanofiber.

[0067] Exemplary substrates are PET, glass, and Nylon 6,6.

[0068] The heat source (iii) may be a printhead of a thermal printer that preferably comprises one or more arrays of small heaters, when, preferably, heat is to be applied directly. The heat source (iii) may be a NIR laser, when a PCA is present.

[0069] The thermochromic layer (i) may be used as Yellow thermochromic layer in a multilayer thermochromic stack (iv). Said multilayer thermochromic stack (iv) may comprise Cyan, Magenta and Yellow thermochromic layers for producing full color images.

[0070] In said multilayer thermochromic stack (iv), each thermochromic layer may be separated or isolated from each other by heat isolation layers between them and any other protective layers, and the substrate (ii).

[0071] In one embodiment, the thermo-imaging system further comprises:(v) oxygen barrier layers that are present as top and bottom layer on the thermochromic stack (iv).

[0072] In one embodiment, the thermo-imaging system further comprises:(vi) an UV protective layer on top of the thermochromic stack (iv).

[0073] In one embodiment, the thermo-imaging system is a reversible thermo-imaging system. Therefore, an image cam be written only once, or it can be rewritable, when the process is repeated.

[0074] As discussed above, the present disclosure provides a method for thermo-imaging or full color imaging, comprising the steps:(a) providing a printable substrate or surface;(b) applying at least one compound according to the present disclosure, or a composition according to the present disclosure;(c) inducing the transition from the colorless state of the compound to a color state by applying heat and producing a compound-developer complex,(d) keeping the color state of the compound by rapid cooling to room temperature.

[0075] In one embodiment, the printable substrate or surface provided in step (a) is a textile, membrane, glass, or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, cellulose, paper, or cellulose nanofiber.

[0076] Exemplary printable substrates or surfaces are glass, cellulose, or PET.

[0077] In one embodiment, in step (c) the heat is applied by direct heating. The heat can be directly applied via a printhead of a thermal printer that preferably comprises one or more arrays of small heaters.

[0078] In step (c) the heat may be applied by laser irradiation in the range from about 700 nm to 1500 nm (NIR region), when PCA is present.

[0079] In one embodiment, the method further comprises the step:(e) changing back the color state of the compound to the colorless state.

[0080] The color state of the compound can be changed back to the colorless state by heating to a temperature which is about 10 % above the decomposition temperature of the compound- developer complex, and then slow cooling to room temperature (step e). The decomposition temperature may depend on the compound and the developer used, or the composition used, and, thus, needs to be determined for each compound and developer / composition.

[0081] In one embodiment, steps (c) to (e) are repeated. In this embodiment, the method or system is called “rewritable”.

[0082] The image can be written only once, or it can be rewritable, when the process is repeated (many times).

[0083] As discussed above, the present disclosure provides a method for synthesizing a compound of the present disclosure.

[0084] The synthesis method comprises the steps of:(1) treating fluorescein with a base, preferably NaOH, to obtain 2-(2,4-dihydroxybenzoyl) benzoic acid;(2) mixing 2-(2,4-dihydroxybenzoyl) benzoic acid with a halogenated phenol, preferably 3- bromophenol, in methane sulfonic acid to obtain 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran- l,9'-xanthen]-3-one;(3) dispersing 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]-3-one in solvent O^NH2and mixing with acetamide I to obtain N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran- l,9'-xanthen]-6'-yl)acetamide:

[0085] In step (1) of the synthesis method, the base may be selected from NaOH or KOH. Preferably, the base may be NaOH.

[0086] In step (2) of the synthesis method, the halogenated phenol may also be selected from 3 -bromophenol, 3 -chlorophenol or 3 -iodophenol.

[0087] In step (3) of the synthesis method, the 3 ’-bromo-furan based material may also react with a secondary or tertiary amide or a lactam, in the presence of a solvent (preferably dioxane, tetrahydrofuran, or toluene), a palladium catalyst (preferably the ones used for C-N cross coupling, or Buchwald-Hartwig cross coupling reactions) and a base (preferably cesium carbonate)

[0088] The term “amide-substituted fluoran compound”, as used herein, refers to a compound having a 9 / Z-xanthcnc unit and an isobenzofuran- 1 (3 / / )-onc unit. The amide- substituted fluoran compounds are shown in formula la below.

[0089] The term “absorption in the yellow absorption range” or “dye exhibiting absorption in the yellow absorption range”, as used herein, is meant to refer to a compound / dye that is able to absorb light in only one or several parts of the entire range indicated or over the total range.It refers to an absorption in the range between about 380 nm to 520 nm.

[0090] The term “absorption in the NIR range” or “dye exhibiting absorption in the NIR range”, as used herein, is meant to refer to a compound / dye that can absorb light in wavelength ranges above 700 nm, preferably between about 700 nm to 1500 nm.

[0091] The present inventors have developed novel amide-substituted fluoran compounds and compositions comprising these compounds. They can be used in in thermo imaging and full color imaging systems. The novel amide-substituted fluoran compounds are highly versatile thermochromic material for imaging applications.

[0092] The requirements for the development of a thermochromic material for imaging applications can be summarized as follows:• The color former dyes should present an absorption spectrum in the visible range (380- 700 nm) or in near infrared region (700 nm to 1200 nm) when in colored state and should be transparent in the visible region when in colorless form.• The color former dyes should present high extinction coefficient, E > 104Lmof'cm- 1- and a correspondingly high absorption coefficient when in colored form.• The color former dyes should present high thermal and photo resistance (the retention of optical density should be kept above 80 % under storage tests and light endurance tests), and should present degradation temperatures above 250°C, as determined by thermalgravimetric analysis. o One example of storage tests is exposure of the thermochromic samples at about 60 °C and 15-90 % RH for several hours. o One example of light endurance tests is exposure of the thermochromic samples at about 60 °C for several hours under light irradiation using, for instance, a 60 W / m2Xe weather meter.• The color former dyes should be dispersed in a polymer matrix with a color developer agent, and optionally, with a photo-thermal conversion agent (PCA).• The color developer agent structures are compounds that assist a colorless dye to develop color, and it preferably contains one or more of the following functional groups in the main skeleton structure: a phenol, carboxylic acid, amide, amine, urea, or urethane derivative.• The PCA is preferably a near-infrared (NIR) absorbing dye, that absorbs near infrared light, having an absorption peak from 700 nm to 1500 nm and having low absorption in the visible region (380 nm to 700 nm), and it can be chosen from one of the following examples: a naphthalocyanine and their metallonaphthalocyanine derivatives, phthalocyanine and their metallophthalocyanine derivatives, porphyrins and theirmetalloporphyrins derivatives, cyanines, squarylium dyes, croconaine dyes, or a thiolate complex.• The transition from a decolored state to a colored state should occur when the polymer mixture is heated up, allowing color former dye and color developer to form a color former-color developer complex. This colored state can be kept by a rapid cooling of the polymer mixture to room temperature.• The heat can be achieved by direct heating, as for instance using a printhead of a thermal printer that comprehends one or more arrays of small heaters, or by NIR laser irradiation in the range from about 700 nm to 1500 nm (NIR region), when PCA is present.• The color former dye can change back to transparent (or leuco form) by heating the mixture to temperatures of ca. 10% above the color former-color developer decomposition temperature, and then slow cooling to room temperature.• The process of changing color from transparent to colored and colored to transparent is reversible and it can be repeated several times (rewritable).• The colorless state of these materials should not present color before conversion to the colored state, which plays a crucial role for printing applications to avoid so called background fogging.

[0093] The compounds, compositions, their imaging uses as well as the imaging systems and methods disclosed herein fulfill these requirements.

[0094] Fluoran dyes offer the possibility of easy modification of the molecular structure, which also enables relatively good control of specific color hues, processability (solubility) that can be tuned by attachment of bulky groups, etc. As fluoran-chemistry involves some already well-known techniques, synthesizing materials on several 100 mg up to multi gram (or even kg) scale in most cases can be accomplished within a reasonable time frame. Another important feature is the high molar extinction coefficient in the colored state, which is usually above 104Lmol ’cm1. Additionally, fluoran dyes provide good thermal stability up to 250°C (or above) and photo stability is known to be reasonable. The material can be used in solid state by dispersing it in a polymer matrix to be used in printing technology.

[0095] The absorption, energy levels and the morphology of dye in a solid state mixture Eire tunable by the type of substituent R1-R12. This makes the fluoran based molecules very versatile structures to be used as yellow color form in a full-color image system.

[0096] The main advantages of the amide-substitute fluoran based molecules reported herein, for the application as active materials in full -color imaging system is:They exhibit good photo- and thermal resistancethey form Yellow dyes with good color qualityTuning of the absorbion maximum (optical band gap) and spectral shape over a broad range is possible.The possibility to adjust the absorption spectrum by moleculer structure modification High extinction coefficients ( E > 104LmoT'cnT1) are expected.

[0097] Note that the present technology can also be configured as described below.(1) An amide-substitute fluoran compound represented by formula lawhereinRi to Rfi are the same or different, and are each independently selected from the group consisting of hydrogen, halogen, linear or branched alkyl, cycloalkyl, alkoxy, benzyl, aryl or heteroaryl;R7 to Rio are the same or different and are each independently selected from the group consisting of hydrogen, linear or branched alkyl, halogen, carboxyl, alkoxy, aryl or heteroaryl, nitro, amide, and amine.Rn and R12 are the same or different, and are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, cycloalkoxy, carboxyl, benzyl, or are linked via cycloalkyl; andX is either OH or SH.

[0098] (2) The amide-substituted fluoran compound according to (1), whereinRi to Re Eire the seime or different, and are each independently selected from the group consisting of hydrogen, halogen, linesir or branched C1-C6 alkyl, C3-C6 cycloalkyl, C1-C6 alkoxy, benzyl, siryl and heteroaryl; andR7 to Rio are the same or different and are each independently selected from the group consisting of hydrogen, linear or branched C1-C4 alkyl, halogen, C1-C6 carboxyl, C1-C4 alkoxy, aryl or heteroaryl, nitro, amide, and amine.

[0099] (3) The amide-substituted fluoran compound according to (1) or (2), wherein the amide- substituted fluoran compound is represented by any of the following structures:

[0100] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)acetamide (C22H15NO5)

[0101] 2,2,2-trifluoro-N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'- yl)acetamide (C22H12F3NO5)

[0102] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)hexanamide(C26H23NO5)

[0103] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)benzamide(C27H17NO5)

[0104] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-4- methoxybenzamide (C28H19NO6)

[0105] l-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)piperidin-2-one(C25H19NO5)

[0106] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-N-phenethyl acetamide (C30H23NO5)

[0107] (4) A composition comprising:(a) at least one compound according to any one of (1) to (3);(b) a color developer;(c) a polymer matrix; and(d) optionally, a photo-thermal conversion agent (PC A).

[0108] (5) The composition according to (4), wherein the at least one compound (a), compound (b) and, optionally, compound (d) are dispersed in the polymer matrix (c).

[0109] (6) The composition according to (4) or (5), wherein the color developer (b) is a compound comprising, preferably in its main skeleton structure, one or more of the following functional groups: phenol, carboxylic acid, amide, amine, urea, or urethane derivatives.

[0110] (7) The composition according to any one of (4) to (6), wherein the polymer matrix (c) is selected from the group consisting of polyvinylchlorides, polyester, amorphous co- polyesters, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, polyvinyl alcohol, cellulose, cellulose nanofiber, liquid silicone rubbers and combinations thereof.

[0111] (8) The composition according to any one of (4) to (7), wherein the PCA (d) is a near-infrared (NIR) absorbing dye, that absorbs near infrared light, then producing heat.

[0112] (9) Use of a compound according to any one of (1) to (3) or of a composition according to any one of (4) to (8) in an imaging system.

[0113] (10) A thermo-imaging system comprising:(i) a thermochromic layer comprising at least one compound according to any one of (1) to (3), or a composition according to any one of (4) to (8).(ii) a printable substrate or surface to which the thermochromic layer (i) is applied; and(iii) optionally, a heat source.

[0114] (11) The thermo-imaging system according to (10), wherein the substrate (ii) is a textile, membrane, or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, polymethracrylic ester, cellulose, or cellulose nanofiber.

[0115] (12) The thermo-imaging system according to (10) or (11), further comprising:(v) oxygen barrier layers that are present as top and bottom layer on the thermochromic stack (iv).

[0116] (13) The thermo-imaging system according to any one of (10) to (12), further comprising:(vi) an UV protective layer on top of the thermochromic stack (iv).

[0117] (14) The thermo-imaging system according to any one of (10) to (13), wherein the thermo-imaging system is a reversible thermo-imaging system.

[0118] (15) A method for thermo-imaging or full color imaging, comprising the steps:(a) providing a printable substrate or surface;(b) applying at least one compound according to any one of (1) to (3), or a composition according to any one of (4) to (8);(c) inducing the transition from the colorless state of the compound to a color state by applying heat and producing a compound-developer complex,(d) keeping the color state of the compound by rapid cooling to room temperature.

[0119] (16) The method according to (15), wherein the printable substrate or surface provided in step (a) is a textile, membrane, glass, or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, cellulose, or cellulose nanofiber.

[0120] (17) The method according to (15) or (16), wherein in step (c) the heat is applied by direct heating.

[0121] (18) The method according to any one of (15) to (17), further comprising the step:(e) changing back the color state of the compound to the colorless state.

[0122] (19) The method according to one of ( 15) to ( 18), wherein steps (c) to (e) are repeated.

[0123] (20) A method for synthesizing a compound of any one of (1) to (3), the method comprises the steps of:(1) treating fluorescein with a base to obtain 2-(2,4-dihydroxybenzoyl) benzoic acid;(2) mixing 2-(2,4-dihydroxybenzoyl) benzoic acid with 3 -bromophenol and methane sulfonic acid in solvent to obtain 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]- 3 -one;(3) dispersing 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]-3-one in solvent o;_,NH2and mixing with acetamide I to obtain N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran- l,9'-xanthen]-6'-yl)acetamide:

[0124] (21) In one embodiment, wherein the amide-substituted fluoran compound is represented by formula la, the amide-substituted fluoran compound is represented by any of the following structures:

[0125] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)acetamide(C22H15NO5)

[0126] 2,2,2-trifhioro-N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'- yl)acetamide (C22H12F3NO5)

[0127] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)hexanamide(C26H23NO5)

[0128] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)benzamide(C27H17NO5)

[0129] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-4- methoxybenzamide (C28H19NO6)

[0130] l-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)piperidin-2-one(C25H19NO5)

[0131] N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-N-phenethyl acetamide (C30H23NO5)(22) The compound according to any one of (1) to (3), wherein the compound- absorbs in the absorption range between about 380 nm to 520 nm;- preferably shows a lower absorption coefficient in the range between about 520 nm to 700 nm;- shows an extinction coefficient of > 104Lmol^cm'1, when in colored form;- exhibits degradation temperatures above 250°C;- is a thermochromic compound;- undergoes a heat-induced transition from a colorless state (with absorption below about 350 nm) to a color state in the visible range (from about 380 nm to 520 nm), which is preferably reversible.(23) A composition comprising(a) at least one compound according to any one of (1) to (3);(b) a color developer; and(c) a polymer matrix;(d) optionally, a photo-thermal conversion agent (PCA), wherein, preferably, the at least one compound (a), compound (b) and, optionally, compound (d) are dispersed in the polymer matrix (c).(24) The composition of (23), wherein the color developer (b) is a compound comprising one or more of the following functional groups: phenol, carboxylic acid, amide, amine, urea, or urethane derivatives, and wherein, preferably, the color developer (b) is phase separated from the at least one compound (a) in the polymer matrix (c) before color development.(25) The composition of (23) or (24), wherein the polymer matrix (c) is selected from the group of polyvinylchlorides, polyester, amorphous co-polyesters, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, polyvinyl alcohol, cellulose, cellulose nanofiber, liquid silicone rubbers or combinations thereof.(26) The composition of anyone (23) to (25), wherein the PCA (d) is a near-infrared (NIR) absorbing dye having an absorption peak from 700 nm to 1500 nm and having low absorption in the visible region (380 nm to 700 nm), and is preferably selected from naphthalocyanines and their metallonaphthalocyanine derivatives, phthalocyanines and their metallophthalocyanine derivatives, porphyrins and their metalloporphyrins derivatives, cyanines, squarylium dyes, croconaine dyes, or a thiolate complex.(27) Use of a compound according to any one of (1) to (3) in a thermo-imaging system or for full color imaging, preferably as yellow dye, or color former in a thermo-imaging system.(28) Use of a composition according to any one (23) to (25) in a thermo-imaging system or for full color imaging.(29) The use of (27) or (28), comprising inducing the transition from the colorless state of the compound to a color state by applying heat.(30) The use of (29), wherein the heat is applied by direct heating or by laser irradiation in the range from about 700 nm to 1500 nm (NIR region), when PCA is present.(31) The use of (29) or (30), comprising a rapid cooling to room temperature after the heat- induced transition from the colorless state to a color state, for keeping the color state of the compound.(32) The use of any one of (29) to (31), wherein the thermo-imaging system is a reversible thermo-imaging system.(33) The use of claim (32), comprising changing back the color state of the compound to the colorless state by heating to a temperature about 10 % above the decomposition temperature of the compound-developer complex, and then slow cooling to room temperature(34) A thermo-imaging system comprising:(i) a thermochromic layer comprising at least one compound according to any one of (1) to (3), or a composition according to any one of (4) to (8);(ii) a printable substrate or surface to which the thermochromic layer (i) is applied, such as a textile, membrane, glass, or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, cellulose, or cellulose nanofiber.(iii) optionally, a heat source.(35) The thermo-imaging system of (34), wherein the thermochromic layer (i) is used as yellow thermochromic layer in a multilayer thermochromic stack (iv) comprising Cyan, Magenta and Yellow thermochromic layers, wherein each thermochromic layer is preferably separated or isolated from each other by heat isolation layers between them and any other protective layers, and the substrate (ii),(36) The thermo-imaging system of (34) or (35), further comprising:(v) oxygen barrier layers that are present as top and bottom layer on the thermochromic stack (iv); and / or(vi) an UV protective layer on top of the thermochromic stack (iv).(37) The thermo-imaging system of (36), which is a reversible thermo-imaging system.(a) A method for thermo-imaging or full color imaging, comprising the steps(b) providing a printable substrate or surface.(c) applying at least one compound according to any one of (1) to (3), or a composition according to any one of (4) to (8);(c) inducing the transition from the colorless state of the compound to a color state by applying heat and producing a compound-developer complex,(d) keeping the color state of the compound by rapid cooling to room temperature.(38) The method of (37), wherein in step (c) the heat is applied by direct heating or by laser irradiation in the range from about 700 nm to 1500 nm (NIR region), when PCA is present.(39) The method of (37) or (38), wherein the printable substrate or surface provided in step (a) is a textile, membrane, or paper, and is preferably selected from polyester, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, paper, or cellulose nanofiber.(40) The method of any one of (37) to (39), furthermore comprising the step:(e) changing back the color state of the compound to the colorless state, preferably by heating to a temperature which is about 10 % above the decomposition temperature of the compound-developer complex, and then slow cooling to room temperature.(41) The method of any one of (37) to (40), wherein steps (c) to (e) are repeated.(42) A method for synthesizing a compound of any one of (1) to (3), comprising the steps of:(1) treating fluorescein with a base, preferably NaOH, to obtain 2-(2,4-dihydroxybenzoyl) benzoic acid;(2) mixing 2-(2,4-dihydroxybenzoyl) benzoic acid with 3 -bromophenol and methane sulfonic acid in solvent to obtain 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]- 3 -one;(3) dispersing 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]-3-one in solvent and mixing with acetamideto obtain N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-1 ,9'-xanthen]-6'-yl)acetamide:EXAMPLESEXAMPLE 1:

[0132] Figure 2 shows a synthetic route for the preparation of compounds according to the present disclosure, namely amide-substitute fluoran structures used for the preparation of ayellow dye, compound MC. Intermediate (A) may be obtained by different methods found in the literature.

[0133] Exemplary synthesis methods for Intermediate (A) are described in Kawai et al., Am. Chem. Soc. 2013, 135, 34, 12690-12696, Grimm et al., Org. Lett. 2011, 13, 24, 6354- 6357, Peng et al., Org. Lett. 2010, 12, 3, 496-499, Poronik et al., Chem. Soc. Rev. 2019,48, 5242-5265 and Woodroofe et al, Tetrahedron 2005, 61 (12), 3097-3105.

[0134] Figure 3 shows a synthetic route for the preparation of the intermediate (A) for the synthesis of amide-substitute fluoran structures. In step (1) NaOH is added to compound (1) to obtain compound (2). In step (2) halogenated compound (3) is added to obtain compound (A).

[0135] Exemplary synthesis methods for Intermediate (A) are described in Wang et al., ACS Omega 2017, 2, 1, 154-163 and Woodroofe et al, Tetrahedron 2005, 61 (12), 3097- 3105.EXAMPLE 2:

[0136] In the scheme shown in Figure 4, the synthetic route used for the preparation of the amide-substituted fluoran yellow dye N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'- xanthen]-6'-yl)acetamide (called MC), is presented. The yellow dye is used as color former in the thermochromic layer. The steps 1-3 are described in detail below.- Step 1 :

[0137] The reaction was carried out using fluorescein (1) (5.00 g; 15.05 mmol) in 50 mL of 12 M aqueous NaOH. The mixture was stirred overnight at 140 °C under nitrogen atmosphere. Next, the mixture was cooled to 0 °C and concentrated hydrochloric acid was added for acidification. During addition, formation of a whitish precipitate was observed. When the pH had reached a value of 2-3, the previously brown-orange color had disappeared and only the whitish precipitate was remaining.

[0138] The aqueous mother liquor, which had been made slightly alkaline, was extracted with EtOAc once (Fraction 1, Fl), then made (approximately) pH-neutral and then extracted further with EtOAc (Fraction 2, F2). Both fractions were analyzed by MS technique (APCI- ASAP). The Fl contained small amounts of the sodium salt of the product (m / z = 280), whereas the second fraction contained large amounts of the product itself (m / z = 258). Then, F2 was dried over sodium sulfate.

[0139] After removal of the drying agent by filtration, the solvent was stripped off, which left an orange-brown resinous residue in the flask. A small amount of water was added to the residue and a few milliliters of IM aq. HC1 were added. Initially, the resinous residue remained as an oily droplet in the aqueous surrounding, but which solidified after sonication for some time. The formed solid was isolated by filtration and washed with small amounts of water. After drying in air overnight, roughly 1 g of a brownish, crystalline solid was received. Further extraction was done, and further fractions of product were isolated. In total, 3.0 - 3.5 g of product (2) were isolated (77 - 90%).

[0140] Structure of the product (2) was confirmed by1H NMR in DMSO-d: 8.00-7.98 ppm, d, 1H; 7.73-7.69 ppm, t, 1H; 7.66-7.62 ppm, t, 1H; 7.43-7.41 ppm, d, 1H; 6.92-6.90 ppm, d, 1H; 6.32-6.32 ppm, d, 1H; 6.29-6.27 ppm, 2d, 1H);13C NMR in DMSO-d (ppm): 102.4, 108.2, 113.2, 127.3, 129.4, 129.6, 128.9, 132.2, 134.6, 139.9, 164.3, 164.9, 166.7, 200; MS (APCI-ASAP) m / z: 257.9, m / z: 258.05 (calc.)

[0141] Step 1 was adapted from ACS Omega 2017, 2, 1, 154-163 (doi . org / 10.1021 / acsomega.6b00403 ) .- Step 2:

[0142] A mixture of 2-(2,4-dihydroxybenzoyl)benzoic acid (2) (4.45 g, 17.25 mmol), 3- bromophenol (3) (3.8 g; 22.00 mmol; 1.28 eq.), 50 mL methanesulfonic acid and 100 mL toluene was stirred at 65 °C for roughly 1.5 hours. Heating was discontinued and the mixture was stirred at room temperature overnight. The reaction was poured into 200 mL of stirred ice water, stirred for 20 min, and then filtered. The resulting mixture was extracted with ethyl acetate (3x 200mL), the organic phase was dried over magnesium sulphate. Solvent was removed under reduced pressure and the resulting material purified over column chromatography (silica gel, and ethyl acetate as eluent) and ca. 2.3g of 3'-bromo-6'-hydroxy- 3H-spiro[isobenzofuran-l,9'-xanthen]-3-one as a yellow-beige color solid was received (ca. 33% yield); *H NMR (CDC13): d 8.05 (dd, 1H); 7.67 (p, 2H); 7.50 (d, 2H); 7.20 (dd, 2H);7.14 (d, 1H); 6.71 (d, 2H).13C NMR (CDC13): 169.4, 153.2, 151.5, 135.9, 130.7, 129.6, 127.9, 126.2, 125.9, 124.6, 124.1, 120.8, 118.3, 81.5.

[0143] Step 2 was adapted from Woodroofe et al., Tetrahedron, 2005, 61, 3097-3105 (https: / / doi.Org / 10.1016 / j.tet.2005.01.024).- Step 3:

[0144] A vial was charged with 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]- 3-one (1.58 g, 4.0 mmol), acetamide (1.085 g, 9.6 mmol), Pd2dba3 (366 mg, 0.4 mmol), Xantphos (694 mg, 1.2 mmol), and CS2CO3 (3.258 g, 10 mmol). The vial was sealed, dried under vacuum for 2h and flushed with nitrogen. Dry dioxane (60 mL) was added, and the reaction was flushed with nitrogen. The reaction was stirred at 100 °C for 16 h.

[0145] It was then cooled to room temperature and filtered. The collected residue was dissolved in MeOH, filtered through celite with MeOH, and evaporated. The residue was purified by silica gel chromatography (DCM / MeOH = 100 / 0 -> 95 / 5) to afford 860 mg (ca. 58% yield) of MC as a pale-yellow powder.

Claims

CLAIMS1. An amide-substituted fluoran compound represented by formula lawhereinRi to Rfi are the same or different, and are each independently selected from the group consisting of hydrogen, halogen, linear or branched alkyl, cycloalkyl, alkoxy, benzyl, aryl and heteroaryl;R7 to Rio are the same or different, and are each independently selected from the group consisting of hydrogen, linear or branched alkyl, halogen, carboxyl, alkoxy, aryl or heteroaryl, nitro, amide, and amine;Rn and R12 are the same or different, and are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, cycloalkoxy, carboxyl and benzyl, or are linked via cycloalkyl; andX is either OH or SH.

2. The amide-substituted fluoran compound according to claim 1, whereinRi to Re Eire the seime or different, and are each independently selected from the group consisting of hydrogen, halogen, linesir or branched Ci-Cg alkyl, C3-C6 cycloalkyl, C1-C6 alkoxy, benzyl, siryl and heteroaryl; andR7 to Rio £tre the SEime or different Eind are each independently selected from the group consisting of hydrogen, linesir or branched C1-C4 alkyl, halogen, C1-C6 carboxyl, C1-C4 alkoxy, aryl or heteroaryl, nitro, amide, and amine.

3. The amide-substituted fluoran compound according to claim 1, wherein the amide- substituted fluoran compound is represented by any of the following structures:N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)acetamide(C22H15NO5)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)hexanamide (C26H23NO5)- N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)benzamide (C27H17NO5)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-4- methoxybenzamide (C28H19NO6)l-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)piperidin-2-one(C25H19NO5)N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-l,9'-xanthen]-6'-yl)-N-phenethyl acetamide (C30H23NO5)4. A composition comprising:(a) at least one compound according to claim 1 ;(b) a color developer;(c) a polymer matrix; and(d) optionally, a photo-thermal conversion agent (PC A).

5. The composition according to claim 4, wherein the at least one compound (a), compound (b) and, optionally, compound (d) are dispersed in the polymer matrix (c).

6. The composition according to claim 4, wherein the color developer (b) is a compound comprising one or more of the following functional groups: phenol, carboxylic acid, amide, amine, urea, or urethane derivatives.

7. The composition according to claim 4, wherein the polymer matrix (c) is selected from the group consisting of polyvinylchlorides, polyester, amorphous co-polyesters, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, polyvinyl alcohol, cellulose, cellulose nanofiber, liquid silicone rubbers and combinations thereof.

8. The composition according to claim 4, wherein the PCA (d) is a near-infrared (NIR) absorbing dye, that absorbs near infrared light, then producing heat.

9. Use of a compound according to claim 1 or of a composition according to claim 4 in an imaging system.

10. A thermo-imaging system comprising:(i) a thermochromic layer comprising at least one compound according to claim 1, or a composition according to claim 4;(ii) a printable substrate or surface to which the thermochromic layer (i) is applied; and(iii) optionally, a heat source.

11. The thermo-imaging system according to claim 10, wherein the substrate (ii) is a textile, membrane, glass, or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, polymethacrylic ester, cellulose, or cellulose nanofiber.

12. The thermo-imaging system according to claim 10, further comprising:(v) oxygen barrier layers that are present as top and bottom layer on the thermochromic stack (iv).

13. The thermo-imaging system according to claim 10, further comprising:(vi) an UV protective layer on top of the thermochromic stack (iv).

14. The thermo-imaging system according to claim 10, wherein the thermo-imaging system is a reversible thermo -imaging system.

15. A method for thermo-imaging or full color imaging, comprising the steps:(a) providing a printable substrate or surface;(b) applying at least one compound according to claim 1, or a composition according to claim 4;(c) inducing the transition from the colorless state of the compound to a color state by applying heat and producing a compound-developer complex, and(d) keeping the color state of the compound by rapid cooling to room temperature.

16. The method according to claim 15, wherein the printable substrate or surface provided in step (a) is a textile, membrane, glass or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, poly methacrylic ester, cellulose, or cellulose nanofiber.

17. The method according to claim 15, wherein in step (c) the heat is applied by direct heating.

18. The method according to claim 15, further comprising the step:(e) changing back the color state of the compound to the colorless state.

19. The method according to claim 15, wherein steps (c) to (e) are repeated.

20. A method for synthesizing a compound of claim 1, the method comprises the steps of:(1) treating fluorescein with a base to obtain 2-(2,4-dihydroxybenzoyl) benzoic acid;(2) mixing 2-(2,4-dihydroxybenzoyl) benzoic acid with 3 -bromophenol and methane sulfonic acid in solvent to obtain 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]- 3 -one;(3) dispersing 3'-bromo-6'-hydroxy-3H-spiro[isobenzofuran-l,9'-xanthen]-3-one in solventO^NH2and mixing with acetamide I to obtain N-(3'-hydroxy-3-oxo-3H-spiro[isobenzofuran-1 ,9'-xanthen]-6'-yl)acetamide:

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