WO1994026729A1

WO1994026729A1 - Tribochromic compounds and their applications

Info

Publication number: WO1994026729A1
Application number: PCT/GB1994/001031
Authority: WO
Inventors: Harry George Heller; Abdullah Mohamed Asiri
Original assignee: University College Cardiff Consultants Limited
Priority date: 1993-05-14
Filing date: 1994-05-13
Publication date: 1994-11-24
Also published as: GB9309992D0; AU6654894A

Abstract

Tribochromic compounds are disclosed which are furan derivatives of general formula (I), wherein B represents an adamantylidene or a cycloalkylidene group, or the group (a) where R?1 and R2¿ independently represent an alkyl group or a cycloalkyl group, and Ar?1 and Ar2¿ independently represent an aryl or substituted aryl group. Tribochromic compounds in accordance with the invention are useful for producing security documents, e.g. cheques, financial documents, certificates and tickets, in which a colour change caused by rubbing or pressure indicates authenticity or cancellation of the document.

Description

Tribochromic Compounds and their Applications Field of the Invention

The present invention relates to crystalline materials which undergo a major colour change (which may or may not be reversible) when the crystal is broken or ground. Background of the Invention

Thermochromism can be defined as the phenomenon of a material undergoing a reversible colour change either on heating or on cooling.

Photochromism can be defined as the phenomenon of a material undergoing a reversible colour change on exposure to activating radiation, the reverse reaction occurring thermally on removal of the activating radiation or by radiation at an appropriate different wavelength.

Triboluminescence is the phenomenon of emission of light when a crystal is broken or ground. The effect has been explained in terms of excitation of the molecule by an electrical discharge between the surfaces of the fractured crystal and subsequent fluorescence, e.g. crystals of N-acetylanthranilic acid show this phenomenon.

Piezochromism is the phenomenon of a crystalline material undergoing a major colour change by mechanically grinding or pressing the crystals of the compound for a prolonged period. The induced colour reverts to the original colour when the compound is kept in the dark or treated with an organic solvent.

For example, the pale yellow crystals of 2,2' ,4,4' ,5,5'-hexaphenylbisimidazole (1,R=H) (see Reaction Scheme A) are converted into a reddish-purple solid on grinding for 5 minutes or by pressing at 30k/cm² for 5 minutes at room temperature. The reddish-purple solid reverts back to the original pale yellow when kept at 50°C in the dark or when subjected to liquid or the vapour of ether, ethanol, acetone or benzene. Derivatives (l,R=Me or Cl) (Figure 1, Reaction Scheme A) are piezochromic also undergoing a colour change to pale bluish purple (T. Hayashi and K. Maeda, Bull, Chem. Soc. Japan, 1965 38 685) .

Tribochromism is used herein to define the phenomenon of a crystalline material undergoing a major colour change (e.g. from bright yellow to deep red) by a fracture of the crystals, including dissolution in a solvent or melting. Due to the severe molecular overcrowding in these compounds, the compounds are not able to adopt a planar configuration. Crystals separate in the bright yellow folded configuration which, on fracture of the crystals, are converted into the intense dark red twisted configuration (established by X-ray crystallographic analysis). Typically, when yellow tribochromic crystals are dissolved in common organic solvents, the solutions are deep red.

The tribochromic compounds may be thermally and photochemically stable under ambient temperature and light conditions. For the purposes of this application, a tribochromic compound is defined as one which has a coloured crystalline form and when the crystals are broken mechanically or physically (including dissolution in a solvent or melting), the compound exhibits a colour change. Tribochromic crystals of this invention are distinguished from piezochromic crystals in the following respects:-

1. Tribochromic crystals change colour instantaneously on fracturing whereas piezochromic crystals require intense pressure or prolonged grinding to produce a colour change.

2. The colour change of tribochromic crystals effected by fracturing is essentially permanent, whereas piezochromic crystals revert to their original colour in the dark or when contacted with a solvent or solvent vapour. Summary of the Invention

According to one aspect of the invention there is provided a furan derivative of the general formula:-

wherein B represents an adamantylidene or cycloalkylidene

group, or -

wherein R¹ and R² independently represent an alkyl group or a cycloalkyl group, and r and Ar² independently represent an aryl group, including a substituted aryl group. Preferably, Ar^- and Ar² independently represent phenyl or 4-substituted phenyl where the substituent in the 4-position is alkyl (e.g. methyl, ethyl, propyl, cyclopropyl, butyl or pentyl), fluoro, chloro or bromo.

Where R¹ and/or R² represent alkyl or a cycloalkyl group, these are preferably lower alkyl, e.g. methyl, ethyl, propyl, butyl, pentyl or cyclopropyl.

Substituents have a marked influence on the nature of the crystals as to whether they are yellow and tribochromic or dark red and non-tribochromic. Not all compounds having the above general formula are tribochromic. The phenomenon of tribochromism is related to the structure of the compound and to its crystal structure. Practical Applications of tribochromic compounds

The application of tribochromic compounds depend on the change in colour which is induced by fracturing the crystals.

According to a second aspect of the present invention, there is provided a tribochromic ink which comprises a dispersion or suspension of crystals of an organic tribochromic compound in a liquid medium.

A further aspect of the invention includes a tribochromic device in which crystals of an organic tribochromic compound are impregnated into, incorporated in or coated onto a support, said crystals being activated to undergo a colour change by application of pressure, friction, abrasion, heat or light.

Thus, the crystals of the tribochromic compound can be impregnated into the material directly (e.g. paper) or films or plates may be prepared by coating a support with a dispersion or emulsion containing a suspension of tribochromic material in its crystalline state (e.g. as a lacquer or paint or ink, using a vehicle in which the crystals are insoluble) . Alternatively, the crystals can be suspended in soft but rigid plastic matrix so that the crystalline nature of the tribochromic material is not affected and that on application of pressure or by friction the crystals can still be fractured to produce the colour change. Tribochromic coatings can be covered and protected by film (such as coated cellophane). The tribochromic marks have a dual function of authentication and cancellation. They can be used to establish whether an item (e.g. cheque, ticket, or any article containing a tribochromic print or paint mark) is genuine on the basis that no other types of compounds are known which can produce a similar colour change on rubbing, scraping, pressure or friction or any process resulting in crystal fracture. Instead of using mechanical means, the media containing the crystals could be warmed to cause the colour change since this could allow the tribochromic compound to dissolve in the medium in which it is suspended. While the colour of the tribochromic compound after crystal fracture may fade with time, the process is generally slow and conditions can be set so that the initial colour and appearance can not be fully restored. Once the tribochromic crystals have been fractured, the coloured mark can act as a cancellation mark. Thus, by fracturing the crystals in the mark, this can produce a date, or symbol which denotes cancellation and, if there are a number of marks, then they can be cancelled on separate occasions such as on a season ticket or tickets for major sporting functions, concerts, lottery tickets etc. Alternatively, the tribochromic mark can be printed in the form of a symbol which changes colour when the whole mark is subjected to a process which fractures or disrupts the crystals. It can be used on a ticket on shop items in which the mark is switched in colour to indicate that the shop item has been purchased and that the item has not been purchased, tampered with and returned to the shelf. The tribochromic material may be contained within icrocapsules to produce a coating on a material. The tribochromic sheets can be written upon by application of pressure sufficient to fracture the tribochromic crystals.

The invention therefore includes a method of marking an article or substrate with a mark or device which changes colour on rubbing, pressure or friction which comprises of printing or marking onto the article or substrate a tribochromic ink or paint or a label, or a document or card which incorporates a tribochromic mark or device, or a substrate printed thereon having a clear superficial protective layer over the said mark or device which gives protection for the tribochromic crystals against degradation by atmospheric moisture and oxygen.

Writing instruments containing tribochromic inks can be produced for security documents, which when subjected to the heat and light of a photocopier, would change colour not by pressure but photochemically and by dissolving in the ink vehicle, indicating that a photocopy had been made of the original. A further method of using the tribochromic compounds of this invention in security documents is to print an ink containing a tribochromic compound onto the paper or to incorporate such a compound in the surface of the paper during the manufacture Scratching, heating or exposing the surface to light will indicate whether the document is genuine (as indicated by the appropriate colour change) or a copy or a forgery produced by printing onto a substitute paper.

The preparation of tribochromic compounds in accordance with the invention is illustrated in the following Examples. The preparation of compounds in accordance with the invention is illustrated in the Reaction Scheme B shown in Figure 1. The formulae of the compounds prepared in the following Examples are given in the attached Figures 2 to 4. The numbering of the Figures corresponds with that of the Examples. For example, Formula (3) shows the structure of the compound of Example 3. Method of Preparation Example 1

5-Dicyanomethylene-4-diphenylmethylene-3-isopropyl- idene-tetrahydrofuran-2-one (1,X=C(CN)₂)

Diethylamine (0.53g, 2 mole equivalents) was added dropwise to a stirred solution of diphenylmethylene(iso- propylidene)succinic anhydride (1,X=0) (1.1 g, 1 mole equivalents) and malononitrile (0.24g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 12 hours. On addition of ether (25 ml), the colourless salt (2) (1.51 g, 95% yield) separated, was filtered off, and treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (2 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether, chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as elutant, and recrystallised from the solvent mixture, giving tribochromic yellow crystals of compound (1,X=C(CN)₂ (0.42 g 32% yield), m.p. 182-183° (deep red melt) . Example 2

3-Adamantylidene-5-dicyanomethylene-4-diphenylmethyl- ene tetrahydrofuran-2-one (2,X=C(CN)₂)

Diethylamine (0.19g, 2 mole equivalents) was added dropwise to a stirred solution of adamantylidene(diphenyl- methylene)succinic anhydride (2,X=0)(0.51 g, 1 mole equivalents) and malononitrile (0.85 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for 24 hours. On addition of ether (25 ml), the colourless salt (2) (0.77 g, 98% yield) separated, was filtered off, and treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (2 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether, chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C), as elutant, and recrystallised from the same solvent mixture, giving tribochromic yellow crystals of compound (2,X=C(CN)₂) (0.32 g, 56% yield), m.p. 213-215° (deep red melt). Example 3

5-Dicyanomethylene-3-dicyclopropylmethylene)-4- diphenylmethylene-tetrahydrofuran-2-one (3,X=C(CN)₂) Diethylamine (0.53 g, 2 mole equivalents) was added dropwise to a stirred solution of dicyclopropylmethylene (diphenylmethylene)succinic anhydride (3,X=0)(1.26 g, 1 mole equivalents) and malononitrile (0.24 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 12 hours. On addition of ether (25 ml), the colourless salt (2) (1.30 g, 64% yield) separated, which was filtered off, and treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (2 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C, as elutant, and recrystallised from the same solvent mixture, giving tribochromic yellow crystals of the dicyanomethylene compound (3,X=C(CN)₂) (0.30 g, 21% yield), m.p. 188-189° (deep red melt). Example 4

Z-Butanylidene-5-dicyanomethylene-4-diphenylm thyl- ene-tetrahydrofuran-2-one (4,X=C(CN)₂)

Diethylamine (0.69 g, 2 mole equivalents) was added dropwise to a stirred solution of Z-2-butanylidene- (diphenylmethylene)succinic anhydride (4,X=0), 1.5 g, 1 mole equivalents) and malononitrile (0.31 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 12 hours. On addition of ether (25 ml), the colourless salt (2) (2g, 80% yield) separated, which was filtered off, and treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (2 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether, chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C, as elutant, and recrystallised from the same solvent mixture, giving tribochromic yellow crystals of compound (4,X=C(CN)₂ (0.69 g, 40% yield), m.p. 160-162° (deep red melt) . Example 5

E-3-(l-Cyclopropylethylidene)-5-dicyanomethylene-4- diphenylmethylene-tetrahydrofuran-2-one (5,X=C(CN)₂) (0.85g, 50% yield)

Diethylamine (0.66 g, 2 mole equivalents) was added dropwise to a stirred solution of 1-eyelopropylethylidene- (diphenylmethylene)succinic anhydride (5,X=0) (1.5 g, 1 mole equivalents) and malononitrile (0.31 g, 1 mole equivalent) in dry tetrahydrofuran (15 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 18 hours. On addition of ether (25 ml), the colourless salt () (2.1 g, 85% yield) separated, which was filtered off, and treated with acetyl chloride (20 ml) in dichloromethane (10 ml). The solution was stirred (18 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether, chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C), as elutant, and recrystallised from the same solvent mixture, giving tribochromic yellow crystals of compound (5,X=C(CN)₂ (0.73g, 43% yield), m.p. 193-195° (deep red melt) . Example 6

5-Dicyanomethylene-3-diphenylmethylene-4-(2-heptanyl- idene)-tetrahydrofuran-2-one(1,X=C(CN)₂)

Diethylamine (1.83g, 2 mole equivalents) was added dropwise to a stirred solution of diphenylmethylene(2- heptanylidene)succinic anhydride (6,X=0) (2.0 g, 1 mole equivalents) and malononitrile (1.83 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 48 hours. Ether (25 ml) and acetyl chloride (10 ml) in dichloromethane (10 ml) were added. The solution was stirred (6 hours) below 10°C and the solvent removed under reduced pressure. The residue was chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as elutant, and crystallised from the a 1:4 mixture of dichloromethane and petroleum 60-80°C giving tribochromic yellow crystals of compound (6,X=C(CN)₂ (0.050 g. 3% yield), m.p. 143-145°C (deep red melt) .

Example 7

E-5-Dicyanomethylene-3-dicyclopropyimethyiene-4-[4-methylphenyl- (phenyl)methylene]tetrahydroturan-2-one (7, X=C(CN)2) Diethylamine (0.59 g, 2 mole equivalents) was added dropwise to a stirred solution of a 1 :1 E/Z mixture of dicyclopropylmethylene[(4-phenyl- (pheπyl)]methylene)succinic anhydride (7, X=0) (1.5 g, 1 mole equivalent) and malononitile (0.26 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 20 hours. On addition of ether (20 ml), no salt separated. Solvent was removed and the residue which was treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (2 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as elutant and recrystallised from the same solvent mixture, giving tribochromic yellow crystals of compound (7, C=C(CN)₂), (0.52 g, 31% yield), m.p. 183-185° (deep red melt).

Example 8

5-Dicyanomethylene-3-dicyclopropylmethylene- 4-[di(4-fluorophenyl)methylene]tetrahydrofuran-2-one (8 ,X=C(CN)₂) Diethylamine (0.60 g, 2 mole equivalents) was added dropwise to a stirred solution of dicyclopropylmethylene[di(4-fluorophenyi)methylene]succinic anhydride (8, X=0) (1.5 g, 1 mole equivalent) and malononitile (0.25 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 18 hours. On addition of ether (20 ml), no salt separated. Solvent was removed and the residue which was treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (2 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as elutant and recrystallised from the same solvent mixture, giving tribochromic yellow crystals of compound (8, C=C(CN)₂), (1.0 g, 59% yield), m.p. 203-205° (deep red melt). Example 9

5-Dicyanomethylene-4-[di(4-chlorophenyl)methylene- 3-dicyclopropylmethylene -tetrahydrofuran-2-one (9, X=C(CN) ) Diethylamine (0.41 g, 2 mole equivalents) was added dropwise to a stirred solution of dicyclopropylmethylene[di(4-chlorophenyl)methylene]succinic anhydride (9, X=0) (1.5 g, 1 mole equivalent) and malononitile (0.19 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 3 hours. On addition of ether (20 ml), no salt separated. Solvent was removed and the residue which was treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (3 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using a 1 :4 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as- elutant and recrystallised from the same solvent mixture, giving tribochromic golden crystals of compound (9, C=C(CN)2), (0.56 g, 42% yield), m.p. 195-197° (deep red melt)

Example 10

E-3-Adamantylidene-5-dicyanomethylene-4-[4-methylphenyl (phenyl)methylene]- tetrahydrofuran-2-one (10, X=C(CN)₂) Diethylamine (0.36 g, 2 mole equivalents) was added dropwise to a stirred solution of a 1 :1 E/Z mixture of adamantylidene[4- phenyl(phenyl)methyϊene]succinic anhydride (10, X=0) (1.0 g, 1 mole equivalent) and malononitile (0.16 g, 1 mole equivalent) in dry tetrahydrofuran (15 ml) at -30°C.When the addition was complete, the temperature was allowed to rise to room temperature and the reaction mixture was stirred for a further 22 hours. On addition of ether (20 ml), no salt separated. Solvent was removed and the residue which was treated with acetyl chloride (15 ml) in dichloromethane (10 ml). The solution was stirred (6 hours) at room temperature and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using a 3:7 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as elutant and recrystallised from the same solvent mixture, giving tribochromic orange crystals of the E-isomer (10, C=C(CN)₂), (0.10 g, 9% yield), m.p. 222-224° (deep red melt). Example 11

3-Adamantylidene-5-dicyanomethylene-4-[di(4-chlorophenyl)methylene]- tetrahydrofuran-2-one (11 , X=C(CN)₂)

Diethylamine (0.1 g, 2 mole equivalents) was added dropwise to a stirred solution of adamantylidene[di(4-chlorophenyl)methylene]succinic anhydride (11 , X=0) (0.3 g, 1 mole equivalent) and malononitile (0.04 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 18 hours. On addition of ether (15 ml), no salt separated. Solvent was removed and the residue which was treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (3 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using petroleum (b.p. 60-80°C) as elutant, giving tribochromic colourless crystals of compound (11 , C=C(CN)₂), (0.16 g, 48% yield), m.p. 245-247° (deep red melt (after recrystallisation from petrol and ether).

Example 12

3-Adamantylidene-5-dicyanomethylene-4-[di(4-fluorophenyl)methylene]- tetrahydrofuran-2-one (12, X=C(CN_/2).

Diethylamine (0.52 g, 2 mole equivalents) was added dropwise to a stirred solution of adamantylidene[di(4-fluorophenyl)methylene]succinic anhydride (12, X=0) (1.5 g, 1 mole equivalent) and malononitile (0.23 g, 1 mole equivalent) in dry tetrahydrofuran (10 ml) at 0°C. When the addition was complete, the reaction mixture was stirred for a further 12 hours. On addition of ether (20 ml), the salt separated in quantitative yield and was filtered off. The salt was treated with acetyl chloride (10 ml) in dichloromethane (10 ml). The solution was stirred (6 hours) below 10°C and the solvent removed under reduced pressure. The residue was washed with cold ether and chromatographed on a column of silica gel using a 1 :4 mixture of ethyl acetate and petroleum (b.p. 60-80°C) as elutant, giving tribochromic yellow crystals of compound (12, C=C(CN)2), (0.6 g, 59% yield), m.p. 235-237° (deep red melt ) from toluene. All compounds described in the Examples gave satisfactory combustion analyses.

Each of the compounds prepared in the above Examples were obtained as yellow crystals which could be transformed into a deep red coloured state by fracturing the crystals by rubbing them with a glass rod or by dissolving them in a solvent, such as petroleum. Formulae 7(a) and 7(b) show the structure of the compound of Example 7 in, respectively, the undisturbed state and after rubbing them with a glass rod.

Claims

CLAIMS : -

A furan derivative of the general formula:

wherein B represents an adamantylidene or a cycloalkylidene group, or the group

, wherein R1^x and R*9' independently represent an alkyl

group or a cycloalkyl group, and Ar-'- and Ar² independently represent an aryl or substituted aryl group.

2. A tribochromic compound having the general formula set forth in claim 1 wherein Ar¹ and Ar² represent phenyl or alkylphenyl groups.

3. A compound according to claim 2 wherein the alkyl phenyl group is substituted in the 4-position with an alkyl or halo group.

4. A tribochromic compound according to claim 2 or 3 wherein B is an adamantylidene group.

5. A tribochromic compound according to claim 2 or 3 wherein B represents a lower alkyl or cyclopropyl group.

6. A compound as claimed in any one of the preceding claims which forms yellow crystals which change to red when the crystals are caused to fracture.

7. A tribochromic ink which comprises a dispersion or suspension of crystals of an organic tribochromic compound in a liquid medium.

8. An ink as claimed in claim 7 wherein the tribochromic compound is a compound as claimed in any one of claims 1 to 6.

9. An ink as claimed in claim 7 or 8 in which the tribochromic compound dissolves in the liquid medium when heated or subjected to radiation.

10. A tribochromic device in which crystals of an organic tribochromic compound are impregnated into, incorporated in or coated onto a support, said crystals being activated to undergo a colour change by application of pressure, friction, abrasion, heat or light or by dissolution in a solvent.

11. A device as claimed in claim 10 wherein the crystals are encapsulated in rupturable microcapsules.

12. A device as claimed in claim 10 or 11 wherein the tribochromic compound forms a pattern in the device so that on activation it gives a substantially permanent marking.

13. A device as claimed in any one of claims 10 to 12 wherein the tribochromic compound is protected against degradation by atmospheric oxygen and moisture.

14. A device as claimed in any one of claims 10 to 13 wherein the tribochromic compound is a furan derivative having a dicyanomethylene substituent in the 5-position.

15. A device as claimed in any one of claims 10 to 13 wherein the tribochromic compound is a compound as claimed in any one of claims 1 to 6.