USE IN IMAGING SYSTEMS

BACKGROUND

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

[0002] The present disclosure relates to benzofluoran-based 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 benzofluoran- based 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 benzofluoran-based 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 molecule can turn transparent again 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 invention will be described. These elements are listed with specific embodiments, however, it should be understood that 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 invention 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 application should be considered disclosed by the description of the present application unless the context indicates otherwise.

[0011] The present disclosure provides a benzofluoran-based compound represented by formula la or lb

wherein

R1 to R6 can be the same or different, and are each independently selected from hydrogen, halogen, haloalkyl (preferably CF3), linear or branched alkyl (preferably methyl), cycloalkyl, alkenyl, alkynyl, alkoxy, cycloalkoxy, benzyl, aryl or heteroaryl;

R1’ to R6’ can be the same or different, and are each independently selected from hydrogen, halogen, haloalkyl (preferably CF3), linear or branched alkyl (preferably methyl), cycloalkyl, alkenyl, alkynyl, alkoxy, cycloalkoxy, benzyl, aryl or heteroaryl;

R7 to R9 can be the same or different, and are each independently selected from hydrogen, or alkyl (preferably methyl or ethyl);

R7’ to R9’ can be the same or different, and are each independently selected from hydrogen, or alkyl (preferably methyl or ethyl);

R10 to R13 can be the same or different, and are each independently selected hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), carboxyl, alkoxy, aryl or heteroaryl;

R10’ to R13’ can be the same or different, and are each independently selected hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), carboxyl, alkoxy, aryl or heteroaryl; R14 to R17 can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), CN, alkoxy, cycloalkoxy, cycloalkyl, amine, benzyl, aryl or heteroaryl;

R14’ to R17’ can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), CN, alkoxy, cycloalkoxy, cycloalkyl, amine, benzyl, aryl or heteroaryl;

R18 to R21 can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), aryl or heteroaryl;

R18’ to R21’ can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), aryl or heteroaryl;

X is selected from O, S, NRc, CRD, SIRE;

Rc, RD and RE are each independently selected from alkyl, cycloalkyl, alkenyl, alkynyl, amino, benzyl, heteroaryl or aryl.

[0012] 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;

(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).

[0013] The present disclosure provides the use of a compound according to the present disclosure in a thermo-imaging system or for full color imaging.

[0014] The present disclosure provides the use of a composition according to the present disclosure in a thermo-imaging system or for full color imaging.

[0015] 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; (iii) optionally, a heat source.

[0016] 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.

[0017] 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 naphthol and methane sulfonic acid in solvent to obtain 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3' -one;

(3) dispersing 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3' -one in solvent and adding pyridine and trifluoromethanesulfonic anhydride to obtain 3'-oxo-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-9-yl trifluoromethanesulfonate; and

(4) mixing 3'-oxo-3'H-spiro[benzo[a]xanthene-12, l'-isobenzofuran]-9-yl trifluoromethane- sulfonate with trimethyl indoline to obtain 9-(3,3,5-trimethylindolin-l-yl)-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3'-one.

[0018] 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

[0019] 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: [0020] Figure 1 shows a a general synthetic route for the preparation of the compounds of the present disclosure, namely benzo [a] fluoran and benzo[Z>] fluoran structures.

[0021] Figure 2 shows the synthetic route used for the preparation of a specific cyan benzo[a]fluoran, compound 7 in the figure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] As discussed above, the present disclosure provides a benzofluoran-based compound represented by formula la or lb wherein

R1 to R6 can be the same or different, and are each independently selected from hydrogen, halogen, haloalkyl (preferably CF3), linear or branched alkyl (preferably methyl), cycloalkyl, alkenyl, alkynyl, alkoxy, cycloalkoxy, benzyl, aryl or heteroaryl;

R1’ to R6’ can be the same or different, and are each independently selected from hydrogen, halogen, haloalkyl (preferably CF3), linear or branched alkyl (preferably methyl), cycloalkyl, alkenyl, alkynyl, alkoxy, cycloalkoxy, benzyl, aryl or heteroaryl;

R7 to R9 can be the same or different, and are each independently selected from hydrogen, or alkyl (preferably methyl or ethyl);

R7’ to R9’ can be the same or different, and are each independently selected from hydrogen, or alkyl (preferably methyl or ethyl); R10 to R13 can be the same or different, and are each independently selected hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), carboxyl, alkoxy, aryl or heteroaryl;

R10’ to R13’ can be the same or different, and are each independently selected hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), carboxyl, alkoxy, aryl or heteroaryl;

R14 to R17 can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), CN, alkoxy, cycloalkoxy, cycloalkyl, amine, benzyl, aryl or heteroaryl;

R14’ to R17’ can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), CN, alkoxy, cycloalkoxy, cycloalkyl, amine, benzyl, aryl or heteroaryl;

R18 to R21 can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), aryl or heteroaryl;

R18’ to R21’ can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), aryl or heteroaryl;

X is selected from O, S, NRc, CRD, SIRE;

Rc, RD and RE are each independently selected from alkyl, cycloalkyl, alkenyl, alkynyl, amino, benzyl, heteroaryl or aryl.

[0023] The benzofluoran-based compounds are found in two isomeric forms: benzo[a]fluoran, as shown in formula la, and benzo [b]fluoran, as shown in formula lb.

[0024] In one embodiment, aryl or heteroaryl are selected from:

wherein

RA and RB can be the same or different, and are each independently selected from hydrogen, alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), alkoxy, cycloalkoxy, alkenyl, alkynyl, amino, benzyl, aryl or heteroaryl, and n is 1 to 5.

[0025] In one embodiment aryl or heteroaryl of R1 to R6, or Rr to R6', and/or R14 to R17 or R14’ to R17’ are selected from wherein RA and RB can be the same or different, and are each independently selected from hydrogen, alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), alkoxy, cycloalkoxy, alkenyl, alkynyl, amino, benzyl, aryl or heteroaryl, and n is 1 to 5.

[0026] In one embodiment, the benzofluoran-based compound is represented by formula la, wherein R1 to R21 are H.

[0027] In one embodiment, the benzofluoran-based compound is represented by formula la, whereinR18 and R19 are alkyl, preferably methyl.

[0028] In one embodiment, the benzofluoran-based compound is represented by formula la, wherein Ri6 is selected from alkyl (preferably methyl), aryl, heteroaryl, cycloalkyl, with RA being alkyl (preferably methyl), or haloalkyl (preferably -CF3), and n is

1 to 3, and with RA being alkyl (preferably methyl) and n is 1 to 3.

[0029] In one embodiment, the benzofluoran-based compound is represented by formula la, wherein R3 is with RA being haloalkyl (preferably CF3) or alkoxy (preferably -

OCH3) and n is 1 to 3.

[0030] In one embodiment, the benzofluoran-based compound is represented by formula la, wherein R10 to R13 or Rn and R12 are the same and are halogen, preferably F.

[0031] In one embodiment, wherein the benzofluoran-based compound is represented by formula la, the benzofluoran-based compound is represented by any of the following structures

[0032] In one embodiment, the benzofluoran-based compound is represented by formula lb, wherein R1 to R21’ are H.

[0033] In one embodiment, the benzofluoran-based compound is represented by formula lb, wherein R18’ and R19’ are alkyl, preferably methyl.

[0034] In one embodiment, the benzofluoran-based compound is represented by formula lb, wherein R16’ is selected from alkyl (preferably methyl), aryl, heteroaryl, cycloalkyl, with RA being alkyl (preferably methyl), or haloalkyl (preferably -CF3) and n is

1 to 3, and with RA being alkyl (preferably methyl) and n is 1 to 3.

[0035] In one embodiment, the benzofluoran-based compound is represented by formula lb, wherein R5’ is selected from cycloalkyl or with RA being alkyl (preferably methyl), or haloalkyl (preferably -CF3) and n is 1 to 3.

[0036] In one embodiment, the benzofluoran-based compound is represented by formula lb, wherein R10’ to R13’ or R11 and R12’ are the same and are halogen, preferably F.

[0037] In one embodiment, wherein the benzofluoran-based compound is represented by formula lb, the benzofluoran-based compound is represented by any of the following structures

[0038] A benzofluoran-based compound according to the present disclosure preferably absorbs in the absorption range between about 560 nm to 700 nm, the cyan absorption range.

[0039] A benzofluoran-based compound according to the present disclosure preferably shows a lower absorption coefficient in the range between about 400 nm to 560nm.

[0040] A benzofluoran-based compound according to the present disclosure preferably shows an extinction coefficient of > 10 ⁴ Lmol ^-1 cm ^-1 , when in colored form.

[0041] A benzofluoran-based compound according to the present disclosure preferably shows a correspondingly high extinction coefficient when in colored form. [0042] 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 thermochromic samples at about 60 °C and 15-90 % RH for several hours. One example of light endurance tests is to expose thermochromic samples at about 60 °C and 60 % RH for several hours under light irradiation using, for instance, a 60 W/m ² Xe weather meter.

[0043] The compounds of the present disclosure preferably exhibit degradation temperatures above 250°C, as determined by thermalgravimetric analysis.

[0044] A benzofluoran-based compound according to the present disclosure is preferably used as color former in thermal-imaging systems or methods for producing full color image.

[0045] A benzofluoran-based compound according to the present disclosure is preferably a thermochromic compound.

[0046] A benzofluoran-based 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 560 nm to 700 nm), which is preferably reversible.

[0047] As discussed above, the present disclosure provides a composition comprising

(a) at least one compound according to any one of claims 1 to 5;

(b) a color developer; and

(c) a polymer matrix;

(d) optionally, a photo-thermal conversion agent (PCA).

[0048] In one embodiment, the composition is a mixture.

[0049] 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. [0050] In one embodiment, the color developer (b) is a compound that assists a colorless dye to develop color.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] In one embodiment, the polymer matrix (c) is selected from the group of polyvinylchlorides, polyester, amorphous copolyesters, polyurethane, polyamide, polyimide, polycarbonate, polymetrachylic ester, polyvinyl alcohol, cellulose, cellulose nanofiber, liquid silicone rubbers or combinations thereof.

[0055] In one embodiment, the PCA (d) is a near-infrared (NIR) absorbing dye, that absorbs near infrared light, then producing heat.

[0056] In one embodiment, 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).

[0057] In one embodiment, the PCA (d) is 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.

[0058] As discussed above, the present disclosure provides the use of a compound according the present disclosure in a thermo-imaging system.

[0059] As discussed above, the present disclosure provides the use of a compound according the present disclosure for full color imaging.

[0060] In one embodiment, the compound is used as cyan dye or color former in a thermoimaging system.

[0061] As discussed above, the present disclosure provides the use of a composition according to the present disclosure in a thermo-imaging system [0062] As discussed above, the present disclosure provides the use of a composition according to the present disclosure for full color imaging.

[0063] In one embodiment, the use of the compound or composition comprises inducing the transition from the colorless state of the compound to a color state by applying heat.

[0064] In one embodiment, the heat is applied by direct heating.

[0065] Direct heating can be, for example, applied via a printhead of a thermal printer that preferably comprises one or more arrays of small heaters.

[0066] In one embodiment, the heat is applied by NIR laser irradiation in the range from about 700 nm to 1,500 nm (NIR region), when PCA is present.

[0067] In one embodiment, the use comprises 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.

[0068] In one embodiment, the thermo-imaging system is a reversible thermo-imaging system.

[0069] In one embodiment, the use comprises 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. The decomposition temperature depends on the compound and the developer used, or the composition used, and, thus, needs to be determined for each compound and developer / composition.

[0070] 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.

(iii) optionally, a heat source.

[0071] In one embodiment, the substrate (ii) is a textile, membrane or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, polymethracrylic ester, cellulose, or cellulose nanofiber. [0072] In one embodiment, the heat source (iii) is a printhead of a thermal printer that preferably comprises one or more arrays of small heaters, when, preferably, heat is to be applied directly. In one embodiment, the heat source (iii) is a NIR laser, when a PCA is present.

[0073] In one embodiment, the thermochromic layer (i) is used as cyan thermochromic layer in a multilayer thermochromic stack (iv). Said multilayer thermochromic stack (iv) comprises Cyan, Magenta and Yellow thermochromic layers for producing full color images.

[0074] In said multilayer thermochromic stack (iv), each thermochromic layer is separated or isolated from each other by heat isolation layers between them and any other protective layers, and the substrate (ii),

[0075] 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).

[0076] In one embodiment, the thermo-imaging system further comprises

(vi) an UV protective layer on top of the thermochromic stack (iv).

[0077] In one embodiment, the thermo-imaging system is a reversible thermo-imaging system. Therefore, an image can be written only once, or it can be rewritable, when the process is repeated.

[0078] 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. [0079] In one embodiment, the printable substrate or surface provided in step (a) is a textile, membrane or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, polymethracrylic ester, cellulose, or cellulose nanofiber.

[0080] 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.

[0081] In one embodiment, in step (c) the heat is applied by laser irradiation in the range from about 700 nm to 1,500 nm (NIR region), when PCA is present;

[0082] In one embodiment, the method further comprises the step

(e) changing back the color state of the compound to the colorless state.

[0083] 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 compounddeveloper complex, and then slow cooling to room temperature. The decomposition temperature depends on the compound and the developer used, or the composition used, and, thus, needs to be determined for each compound and developer / composition.

[0084] In one embodiment, steps (c) to (e) are repeated. In this embodiment, the method or system is called “rewritable”.

[0085] The image can be written only once, or it can be rewritable, when the process is repeated (many times).

[0086] As discussed above, the present disclosure provides a method for synthesizing a compound of the present disclosure.

[0087] 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 naphthol and methane sulfonic acid in solvent to obtain 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3' -one;

(3) dispersing 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3' -one in solvent and adding pyridine and trifluoromethanesulfonic anhydride to obtain 3'-oxo-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-9-yl trifluoromethanesulfonate; (4) mixing 3'-oxo-3'H-spiro[benzo[a]xanthene-12, l'-isobenzofuran]-9-yl trifluoromethane- sulfonate with trimethyl indoline to obtain 9-(3,3,5-trimethylindolin-l-yl)-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3'-one.

[0088] Note that the present technology can also be configured as described below.

(1) An benzofluoran-based compound represented by formula la or lb wherein

R1 to R6 can be the same or different, and are each independently selected from hydrogen, halogen, haloalkyl (preferably CF3), linear or branched alkyl (preferably methyl), cycloalkyl, alkenyl, alkynyl, alkoxy, cycloalkoxy, benzyl, aryl or heteroaryl;

R1’ to R6’ can be the same or different, and are each independently selected from hydrogen, halogen, haloalkyl (preferably CF3), linear or branched alkyl (preferably methyl), cycloalkyl, alkenyl, alkynyl, alkoxy, cycloalkoxy, benzyl, aryl or heteroaryl;

R7 to R9 can be the same or different, and are each independently selected from hydrogen, or alkyl (preferably methyl or ethyl);

R7’ to R9’ can be the same or different, and are each independently selected from hydrogen, or alkyl (preferably methyl or ethyl); R10 to R13 can be the same or different, and are each independently selected hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), carboxyl, alkoxy, aryl or heteroaryl;

R10’ to R13’ can be the same or different, and are each independently selected hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), carboxyl, alkoxy, aryl or heteroaryl;

R14 to R17 can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), CN, alkoxy, cycloalkoxy, cycloalkyl, amine, benzyl, aryl or heteroaryl;

R14’ to R17’ can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), CN, alkoxy, cycloalkoxy, cycloalkyl, amine, benzyl, aryl or heteroaryl;

R18 to R21 can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), aryl or heteroaryl;

R18’ to R21’ can be the same or different, and are each independently selected from hydrogen, linear or branched alkyl (preferably methyl), aryl or heteroaryl;

X is selected from O, S, NRc, CRD, SIRE;

Rc, RD and RE are each independently selected from alkyl, cycloalkyl, alkenyl, alkynyl, amino, benzyl, aryl or heteroaryl.

(2) The compound according to (1), wherein aryl or heteroaryl, preferably aryl or heteroaryl of R1 to R6 or R1 to R6' and/or R14 to R17 or R14 to R17’ are selected from

wherein

RA and RB can be the same or different, and are each independently selected from hydrogen, alkyl (preferably methyl), halogen, haloalkyl (preferably CF3), alkoxy, cycloalkoxy, alkenyl, alkynyl, amino, benzyl, aryl or heteroaryl, and n is 1 to 5.

(3) The compound according to (1) or (2), which is represented by formula la, wherein

R1 to R21 are H; or

R18 and R19 are alkyl, preferably methyl; or

Ri6 is selected from alkyl (preferably methyl), aryl, heteroaryl, cycloalkyl, with RA being alkyl (preferably methyl), or haloalkyl (preferably -CF3), and n is 1 to 3, and with RA being alkyl (preferably methyl) and n is 1 to 3; or

R3 is with RA being haloalkyl (preferably CF3) or alkoxy (preferably -

OCH3) and n is 1 to 3; or

R10 to R13 or R11 and R12 are the same and are halogen, preferably F, which is preferably represented by any one of the structures

(4) The compound according to (1) or (2), which is represented by formula lb wherein

R1 to R21’ are H; or

R18’ and R19’ are alkyl, preferably methyl; or

R16’ is selected from alkyl (preferably methyl), aryl, heteroaryl, cycloalkyl, with RA being alkyl (preferably methyl), or haloalkyl (preferably -CF3) and n is 1 to 3, and with RA being alkyl (preferably methyl) and n is 1 to 3; or

R5’ is selected from cycloalkyl or with RA being alkyl (preferably methyl), or haloalkyl (preferably -CF3) and n is 1 to 3; or

R10’ to R13’ or R11 and R12’ are the same and are halogen, preferably F, which is preferably represented by any one of the structures

(5) The compound according to any one of (1) to (4), wherein the compound

- absorbs in the absorption range between about 560 nm to 700 nm;

- preferably shows a lower absorption coefficient in the range between about 400 to 560nm;

- shows an extinction coefficient of > 10 ⁴ Lmol ^-1 cm ^-1 , when in colored form;

- exhibits degradation temperatures above 250°C;

- is a thermochroic 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 560 nm to 700 nm), which is preferably reversible. (6) A composition comprising

(a) at least one compound according to any one of (1) to (5);

(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).

(7) The composition of (6), 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.

(8) The composition of (6) or (7), wherein the polymer matrix (c) is selected from the group of polyvinylchlorides, polyester, amorphous copolyesters, polyurethane, polyamide, polyimide, polycarbonate, polymetrachylic ester, polyvinyl alcohol, cellulose, cellulose nanofiber, liquid silicone rubbers or combinations thereof.

(9) The composition of any one of (6) to (8), 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.

(10) Use of a compound according to any one of (1) to (5) in a thermo-imaging system or for full color imaging, preferably as cyan dye or color former in a thermo-imaging system.

(11) Use of a composition according to any one of (6) to (9) in a thermo-imaging system or for full color imaging. (12) The use of (10) or (11), comprising inducing the transition from the colorless state of the compound to a color state by applying heat.

(13) The use of (12), 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.

(14) The use of claim (12) or (13), 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.

(15) The use of any one of (12) to (14), wherein the thermo-imaging system is a reversible thermo-imaging system.

(16) The use of (15), 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.

(17) A thermo-imaging system comprising

(i) a thermochromic layer comprising at least one compound according to any one of (1) to (5), or a composition according to any one of (6) to (9);

(ii) a printable substrate or surface to which the thermochromic layer (i) is applied, such as a textile, membrane or paper, which can be polyester, polyurethane, polyamide, polyimide, polycarbonate, polymethracrylic ester, cellulose, or cellulose nanofiber;

(iii) optionally, a heat source.

(18) The thermo-imaging system of (17), wherein the thermochromic layer (i) is used as cyan 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),

(19) The thermo-imaging system of (17) or (18), 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).

(20) The thermo-imaging system of (19), which is a reversible thermo-imaging system.

(21) 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 (5), or a composition according to any one of (6) to (9);

(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.

(22) The method of (21), 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;

(23) The method of (21) or (22), 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, polymethracrylic ester, cellulose, or cellulose nanofiber.

(24) The method of any one of (21) to (23), 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.

(25) The method of any one of (21) to (24), wherein steps (c) to (e) are repeated.

(26) A method for synthesizing a compound of any one of (1) to (5), 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 naphthol and methane sulfonic acid in solvent to obtain 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]- 3'-one; (3) dispersing 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3' -one in solvent and adding pyridine and trifluoromethanesulfonic anhydride to obtain 3'-oxo-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-9-yl trifluoromethanesulfonate; and

(4) mixing 3'-oxo-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-9-yl trifluoromethane-sulfonate with trimethyl indoline to obtain 9-(3,3,5-trimethylindolin-l-yl)- 3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3'-one.

[0089] The term “benzofluoran compound” or “benzofluoran-based compound”, as used herein, refers to a compound having a 12H-benzo[a or b]xanthene unit and an isobenzofuran- l(3H)-one unit. The benzofluoran-based compounds are found in two isomeric forms: benzo[a]fluoran, as shown in formula la, and benzo [b]fluoran, as shown in formula lb, both shown below.

[0090] The term “absorption in the cyan absorption range” or “dye exhibiting absorption in the cyan 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 560 nm to 700 nm.

[0091] 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 is able to absorb light in wavelength ranges above 700 nm, preferably between about 700 nm to 1500 nm. [0092] The present inventors have developed novel benzofluoran-based compound and compositions comprising these compounds. They can be used in in thermo imaging and full color imaging systems. The novel benzofluoran-based compounds are highly versatile thermochromic material for imaging applications.

[0093] 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 be transparent in the visible region when in colorless form.

• The color former dyes should present high extinction coefficient, e > 10 ⁴ Lmol ^-1 cm ^-1 - 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 and 60 % RH for several hours under light irradiation using, for instance, a 60 W/m ² Xe 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 derivatives.

• The PCA is a near-infrared (NIR) absorbing dye, that absorbs near infrared light, with it has absorption peak from 700 nm to 1500 nm and has 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 their metalloporphyrins 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 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 PC A 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.

[0094] The compounds, compositions, their imaging uses as well as the imaging systems and methods disclosed herein fulfill these requirements.

[0095] Benzofluoran 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 benzofluoran-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 10 ⁴ Lmol ^-1 cm ^-1 . Additionally, benzofluoran 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.

[0096] The absorption, energy levels and the morphology of dye in a solid state mixture are tunable by the type of substituent R1-R21. This makes the benzofluoran based molecules very versatile structures to be used as Cyan color form in a full-color image system.

[0097] The main advantages of the benzofuoran-based molecules reported herein, for the application as active materials in full-color imaging system is: They exhibit good photo- and thermal resistance

- they form Cyan dyes with good color quality

Tuning 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 ( ε > 10 ⁴ Lmol ^-1 cm ^-1 ) are expected.

EXAMPLES

EXAMPLE 1 :

[0098] In the scheme shown in Figure 1, the general synthetic route for the preparation of the benzofluoran-based compounds, the a and b isomers, according to the present disclosure is depicted.

EXAMPLE 2:

[0099] In the scheme shown in Figure 2, the synthetic route used for the preparation of a cyan benzo[a]fluoran (9-(3,3,5-trimethylindolin-l-yl)-3'H-spiro[benzo[a]xanthene- 12,l'- isobenzofuran]-3'-one, compound 7) is presented. The cyan benzofluoran dye is used as color former in the thermochromic layer.

[00100] The same synthetic route can be used for benzo[b]fluorans.

The steps 1-4 are described in detail below.

- Step 1:

[00101] 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.

[00102] The aqueous mother liquor, which had been made slightly alkaline, was extracted with EtOAc once (Fraction 1, F1), 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.

[00103] 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. Structure of the product (2) was confirmed by 1HNMR 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). 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%). [00104] Step 1 was adapted from ACS Omega 2017, 2, 1, 154-163 (doi . org/ 10.1021 / acsomega.6b00403 ) .

- Step 2:

[00105] A mixture of 2-(2,4-dihydroxybenzoyl)benzoic acid (2, 2.20 g, 8.52 mmol), 2- naphthol (3, 1.47 g; 10.22 mmol; 1.2 eq.), 25 mL methane sulfonic acid and 50 mL toluene was stirred at 65 °C for roughly one hour. Heating was discontinued and the mixture was stirred at room temperature overnight. Water and EtOAc were added to the mixture, which became warm upon addition. The mixture was stirred at room temperature for ca. 30 minutes and then extraction was carried out with several portions of EtOAc. Saturated brine was added from time to time to facilitate better phase separation. The combined organic layers were washed with saturated brine and then dried over sodium sulphate.

[00106] Additional to the extracted fraction, there was remaining solid from the reaction mixture which was barely soluble in EtOAc. This fraction was washed with EtOAc and water a few times and then dissolved in acetone. This fraction contained some product as well. Solvent was removed under reduced pressure to receive a nearly black residue. This fraction was purified together with the extracted fraction.

[00107] Roughly 1.5 g (4.09 mmol; 48 %) of pure material was isolated after column chromatography using CHCL / EtOAc mixture (99: 1 -> 9: 1 -> 1 : 1) for elution; purity was confirmed by TLC. For improving purity, the material can be crystallized from EtOAc with hexane, followed by further washing with hexane. [00108] The product (4), 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,T-isobenzofuran]-3'- one was confirmed by GC-MS(measured m/z = 366.1, expected m/z = 366.1).

[00109] Step 2 was adapted from Azizian et al., Chem. Comm., 2012, 48, 750-752 (Doi: 10.1039/clccl5854f).

- Step 3:

[00110] 9-hydroxy-3'H-spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3' -one (4, 410 mg; 1.12 mmol) was dispersed in 10 mL dry DCM and cooled to 0 °C. Under inert atmosphere, pyridine (354 mg; 347 pL; 4.48 mmol; 4 eq.) and trifluoromethanesulfonic anhydride (632 mg; 370 pL; 2.24 mmol; 2 eq.) were added slowly. The cooling bath was removed, and the mixture stirred at room temperature for 3-4 hours under inert atmosphere. With time, the mixture became clear and changed its color to orange. Water was added and it was further stirred for several minutes, then the mixture was extracted with DCM several times. The combined organic layers were washed with saturated copper sulfate solution and saturated brine, then dried over sodium sulfate. After removal of the drying agent, solvent was removed under reduced pressure. The crude product was dissolved in a small amount of DCM / pentane 2: 1 and then purified over silica gel column using the same solvent mixture for elution. About 490 mg of a colorless solid was isolated, which makes for a yield of 88 % (0.98 mmol). Structure was confirmed by GC-MS (measured m/z = 498.1, expected m/z = 498.0).

[00111] Step 3 was adapted from Peng and Yang, Org. Lett., 2010, 12, 3, 496-499 (DOI: 10.1021/ol902706b).

- Step 4:

[00112] A mixture of trimethyl indoline (6, 788 mg; 4.89 mmol; 1.4 eq.), 3'-oxo-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-9-yl trifluoromethanesulfonate (5, 1.74 g; 3.49 mmol), cesium carbonate (1.71 g; 5.24 mmol; 1.5 eq.), Pd(OAc)2 (78 mg; 349 pmol; 10 mol%) and BINAP (326 mg; 542 pmol; 15 mol%) in 80 mL anhydrous toluene was stirred at 100 °C under inert atmosphere overnight (ca. 19 hours). After cooling, water was added to the reaction mixture and stirred for a few hours. After that, extraction was carried out with toluene. The combined organic layers were washed with saturated brine, and then dried over sodium sulfate. Solvent was removed under reduced pressure. The brownish crude material was dissolved in ca. 50 mL of DCM and then applied onto silica gel column. Elution was started with DCM / pentane 2:1 then changed to 3 : 1 to pure DCM to DCM / MeOH 97:3. The fractions were analyzed by TLC and the ones containing the desired product were isolated and the solvent striped off, and a yellowish solid was received. Pentane was added to the material; the mixture was then sonicated for a short time. The solid was removed by filtration and washed with further small amounts of pentane. This resulted in a slightly yellowish filtrate and white solid in the filter paper. This procedure was carried out for three different fractions, which ended up in powders of slightly different color (two more white, the other one rather beige), but practically identical on TLC. In total, 1.05 g of material was isolated after drying under ambient conditions.

[00113] Structure and purity of compound 7 (9-(3,3,5-trimethylindolin-l-yl)-3'H- spiro[benzo[a]xanthene-12,l'-isobenzofuran]-3'-one, compound 7) were confirmed APCI- ASAP and 1H-NMR. APCLASAP (measured m/z = 510.7, expected m/z 509.6); IH-NMR in DCM-d (8.14-8.12 ppm, d, 1H; 7.97-7.94 ppm, d, 1H; 7.84-7.82 ppm, d, 1H; 7.66-7.58 ppm, p, 2H; 7.48-7.46 ppm, d, 1H; 7.37-7.30 ppm, t, 1H; 7.20-7.08 ppm, m, 4H; 7.03 ppm, d, 1H; 6.96 ppm, s, 1H; 6.93-6.90 ppm, d, 2H; 6.68-6.66 ppm, d, 1H; 3.70 ppm, t, 2H; 2.28ppm, t, 3H; 1.32ppm, s, 6h) .

[00114] Step 4 was adapted from Peng and Yang, Org. Lett., 2010, 12, 3, 496-499 (DOI: 10.1021/ol902706b) and Grimm and Lavis, Org. Lett., 2011, 13, 24, 6354-6357 (DOI: 10.1021/ol202618t).

[00115] Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. As will be understood by those skilled in the art, the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present disclosure is intended to be illustrative, but not limiting of the scope of the disclosure, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, define, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.