Dye-Borate complexes

Abstract: Compounds are described of the formula (I): D+ is a cationic dye moiety. R¹ is a branched chain alkyl group. R², R³ and R⁴ are independently selected from alkyl, aryl, aralkyl, alkaryl, alkenyl, alkynyl, alicyclic, heterocyclic, and allyl groups.

Description: The present invention relates to a novel class of cationic dye-borate anion complexes. U.S. Patents 4,772,541 and 4,772,530 to The Mead Corporation disclose a novel class of photoinitiators which are compounds consisting of an ionic photoreducible or photooxidizable dye and a counter ion which absorb light and generate a free radical. The preferred photoinitiators are complexes of a cationic dye and a borate anion. While these photoinitiators are suitable for many applications there is a desire to design photoinitiators which provide higher film speeds. The present invention provides a novel class of photoinitiators which can provide relatively higher film speeds. These photoinitiators are represented by the formula (I) where D⁺ is a cationic dye, R¹ is a branched chain alkyl group and R², R³ and R⁴ are independently selected from the alkyl, aryl, alkaryl, allyl, aralkyl, alkenyl, alkynyl, alicyclic and saturated or unsaturated heterocyclic groups. We have found that only a small amount of n-alkyl borate radicals in cyanine borate initiated polymerization of acrylates escape the solvent cage to initiate polymerization. Recombination of the borate and cyanine radicals may be a significant factor in the loss of productive radicals. With the present complexes a more sterically hindered radical is provided through a branched chain alkyl group. This radical is less likely to recombine with the cyanine radical before diffusing from the solvent cage. Consequently, these cyanine borate photoinitiators should provide media with greater photographic speed than previous initiators which use n -alkylborate. Preferably the initiators are represented by the formula (II) where y¹ and y² are the same or different and represent an oxygen atom, a sulfur atom, a selenium atom, a vinyl group, >C(CH₃)₂, or N-alkyl and may be the same or different; n is 0, 1, 2, or 3; z is 1 to 4; R¹, R², R³ and R⁴ are defined as in formula (I); R⁵ and R⁶ are alkyl groups and preferably an alkyl group having at least 4 and preferably at least 7 carbon atoms; X is selected based on the desired absorption characteristics of the dye and may be selected from a large number of substituents in a manner known in the art. X may be a hydrogen atom; an electron donating group such as amino, dimethylamino, hydroxy, methoxy, t-butyl, and methyl; or X may be an electron withdrawing group such as phenyl, fluoro, chloro, bromo, iodo, -COOH, -COOR where R is a lower alkyl group, methycarbonyl, trifluoromethyl, cyano, methylsulfonyl, and nitro. The invention also extends to a photohardenable composition comprising a photoinitiator of formula (I) and a polymerizable compound; and to a photosensitive material comprising a support having a layer of the composition coated thereon. Methods which can be usefully employed in making examples of our present photoinitiators, photohardenable compositions, and photosensitive materials are described in U.S. Patents 4,772,530 and 4,772,541. In our present photoinitiators the borate anion includes a branched chain alkyl group and preferably a secondary alkyl group containing 3 to 10 carbon atoms. Still more preferably R², R³ and R⁴ represent aryl groups or alkaryl groups such as a phenyl group, an anisyl group, napthyl, etc. Useful dyes form photoreducible but dark stable complexes with borate anions and can be cationic methine, polymethine, triarylmethane, indoline, thiazine, xanthene, oxazine and acridine dyes. More specifically, the dyes may be cationic cyanine, carbocyanine, hemicyanine, rhodamine and azomethine dyes. The dyes should not contain groups which would neutralize or desensitize the complex or render the complex poorly dark stable. With respect to the borate anion, useful examples of branched chain alkyl groups are more particularly secondary alkyl groups such as sec-butyl, neopentyl, neophyl, 3-methyl-2 butene, etc. These alkyl groups may be substituted, for example, by one or more halogen, cyano, acyloxy, acyl, alkoxy or hydroxy groups. Representative examples of aryl groups represented by R²-R⁴ include phenyl, naphthyl and substituted aryl groups such as anisyl and alkaryl groups such as methylphenyl, dimethylphenyl, etc. Representative examples of aralkyl groups represented by R²-R⁴ groups include benzyl and naphthylmethyl. Representative alicyclic groups include cyclobutyl, cyclopentyl, and cyclohexyl groups. Examples of an alkynyl group are propynyl and ethynyl, and examples of alkenyl groups include a vinyl group. Examples of allyl groups include 3,3-diphenyl-2-propenyl, 3,3-dimethyl-2-propenyl, etc. As a general rule, useful cationic dye-borate anion complexes must be identified empirically, however, potentially useful cationic dye and borate anion combinations can be identified by reference to the Weller equation (Rehm, D. and Weller, A., Isr. J. Chem. (1970), 8, 259-271), which can be simplified as follows. ΔG=E ox -E red -E hv (Eq. 3) where ΔG is the change in the Gibbs free energy, E ox is the oxidation potential of the borate anion, E red is the reduction potential of the cationic dye, and E hv is the energy of light used to excite the dye. Useful complexes will have a negative-free energy change. Similarly, the difference between the reduction potential of the dye and the oxidation potential of the borate must be negative for the complex to be dark stable, i.e., E ox -E red >O. As indicated, Eq. 3 is a simplification and it does not absolutely predict whether a particular complex will provide good results or not. There are a number of other factors which will influence this determination. One such factor is the effect of the monomer on the complex. It is also known that if the Weller equation produces too negative a value, deviations from the equation are possible. Furthermore, the Weller equation only predicts electron transfer, it does not predict whether a particular dye complex is an efficient initiator of polymerization. The equation is a useful first approximation. Specific examples of cationic dye-borate anion complexes useful in the present invention are shown in the examples with their λ max. The most typical example of a free radical addition polymerizable or crosslinkable compound useful in practice of present invention is an ethylenically unsaturated compound and, more specifically, a polyethylenically unsaturated compound. These compounds include both monomers having one or more ethylenically unsaturated groups, such as vinyl or allyl groups, and polymers having terminal or pendant ethylenic unsaturation. Such compounds are well known in the art and include acrylic and methacrylic esters of polyhydric alcohols such as trimethylolpropane, pentaerythritol, and the like; and acrylate or methacrylate terminated epoxy resins, acrylate or methacrylate terminated polyesters, etc. Representative examples include ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylol-propane triacrylate (TMPTA), pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hydroxpentacrylate (DPHPA), hexanediol-1,6-dimethacrylate, and diethyleneglycol dimethacrylate. The cationic dye-borate anion complex is usually used in an amount up to about 1% by weight based on the weight of the photopolymerizable or crosslinkable species in the photohardenable composition. More typically, the cationic dye-borate anion complex is used in an amount of about 0.2% to 0.5% by weight. While the cationic dye-borate anion complex can be used alone as the initiator, film speeds tend to be quite low and oxygen inhibition is observed. We have found that it is preferable to use the complex in combination with an autoxidizer. An autoxidizer is a compound which is capable of consuming oxygen in a free radical chain process. Representative examples of N,N-dialkylanilines useful in practice of the present invention as autoxidisers are 4-cyano-N, N-dimethylaniline, 4-acetyl-N,N-dimethylaniline, 4-bromo-N, N-dimethylaniline, ethyl 4-(N,N-dimethylamino) benzoate, 3-chloro-N,N-dimethylaniline, 4-chloro-N,N-dimethylaniline, 3-ethoxy-N,N-dimethylaniline, 4-fluoro-N,N-dimethylaniline, 4-methyl-N,N-dimethylthioanicidine, 4-amino-N,N-dimethylaniline, 3-hydroxy-N,N-dimethylaniline, N,N,N′,N′-tetramethyl-1,4-dianiline, 4-acetamido-N,N-dimethylaniline, etc. Preferred N,N-dialkylanilines are 2,6-diisopropyl-N,N-dimethylaniline, 2,6-diethyl-N,N-dimethylaniline, N,N,2,4,6-pentamethylaniline (PMA) and p-t-butyl-N,N-dimethylaniline. The autoxidizers are preferably used in concentrations of about 4-5% by weight. Thiols and disulfides also appear to enhance film speed although the mechanism is not clear. Particularly preferred thiols are selected from mercaptobenzoxazoles, mercaptotetrazines, mercaptotriazines and mercaptobenzothiazoles. Representative examples include 2-mercaptobenzothiazole, 6-ethoxy-2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 4-methyl-4H-1,2,4-triazole-3-thiol, 2-mercapto-1-methylimidazole, 2-mercapto-5-methylthio-1,3,4-thiadiazole, 5- n -butylthio-2-mercapto-1,3,4-thiadiazole, 4-methoxybenzenethiol, 1-phenyl-1H-tetrazole-5-thiol, 4-phenyl-4H-1,2,4-triazole-3-thiol, 2-mercaptobenzimidazole, pentaerythritol tetrakis (mercaptoacetate), pentaerythritol tetrakis (3-mercaptoproprionate), trimethylolpropane tris(mercaptoacetate), 4-acetamidothiophenol, mercaptosuccinic acid, dodecanethiol, 2-mercaptopyridine, 4-mercaptopyridine, 2-mercapto-3H-quinazoline, and 2-mercaptothiazoline. Disulfides as described in our European Patent Application No: 90302540.1 (Publication No: EP-A-0 387 087) are also useful. Our photohardenable compositions may be used in a wide variety of applications. They may be used to prepare photocurable resists, photoadhesives, photocurable coatings, photocurable inks, etc. They may also be microencapsulated and used in the full colour panchromatic imaging system described in U.S. Patent 4,772,541. A preferred method for microencapsulating the composition is described in our European Patent Application No: 88311479.5 (Publication No: EP-A-0319337). A preferred developer material is described in our European Patent Application No: 87307985.9 (Publication No: EP-A-0260129). Neopentylborates can be made by the reaction of neopentylmagnesium chloride with triphenylboron. sec -Butylborate can be made by the reaction of sec -butyllithium with triphenylboron. The resulting borate is metastisized with an appropriate dye. The invention is illustrated in more detail in the examples below using the following compounds. Example 1 To compare the photographic characteristics of examples of dye complexes in accordance with the present invention and previous dye complexes, the dye complexes of compounds 1 to 5, each being in accordance with the present invention, and the comparative complexes of compounds A to C were encapsulated with photopolymerizable compositions. The encapsulated compositions were coated on a sheet of ALPET in an amount of 6 g/cm² using a slot head coater and dried. The coated compositions were exposed through a 30 step wedge. The results are shown in Table 1. The data clearly shows an increase in speed of two steps in shoulder speed and one step in toe speed for magenta media using neopentylborate photoinitiator. These results also show a potential increase in the shoulder speed of cyan media utilizing neopentylborate of nearly three stops and an increase in the toe of one step over current n-butylborate based media. Thus, sterically hindered borates do tend to lead to faster photoinitiators. Example 2 illustrates an example of a photosensitive material in accordance with the present invention. Example 2 A. The following oily internal phase compositions were prepared and preheated to 60°C. B. The continuous phase for emulsification was prepared as follows and preheated to 60°C: 430g H₂O 8.0g Versa TL - dissolved for 15 minutes or more 12.65g Pectin - dry blended with sodium bicarbonate and dissolved into water phase 0.24g NaHCO₃ - dry blended with pectin and added concurrently pH - adjusted by dropwise addition of acid and/or to pH 6.0 C. External continuous phase is brought to 3000 rpm. D. The internal phase material was added into a mixing continuous phase. E. 3000 rpm mixing maintained for 15 minutes after oil phase addition. F. 246g melamine-formaldehyde precondensate is added to emulsion with mixing at 1500 rpm. G. pH is adjusted to pH 6.0. H. Reaction mixture is covered and held at 70°C for 1 hour while mixing. I. 46.2g Urea @ 50% solution is added to reaction mixture and the reaction is allowed to continue at 70°C for another 40 min. (This step scavenges unreacted formaldehyde.) J. The pH is adjusted to about 9.5 with NaOH (20% solution), covered and allowed to cool while mixing gently. The process is repeated for each internal phase composition to prepare three batches of microcapsules. Once prepared, the microcapsules are blended in an amount which provides good colour balance, coated on a sheet of aluminized polyethylene terephthalate, and dried to provide a visible light sensitive, panchromatic photosensitive material.

Claims: A compound of the formula (I) where D⁺ is a cationic dye moiety; characterised in that R¹ is a branched chain alkyl group, and R², R³ and R⁴ are the same or different and selected from alkyl, aryl, aralkyl, alkaryl, alkenyl, alkynyl, alicyclic, heterocyclic, and allyl groups. A compound according to Claim 1, further characterised in that R¹ is a branched chain secondary alkyl group and R², R³ and R⁴ each represent an aryl group, preferably phenyl. A compound according to Claims 1 or 2, further characterised in that R₁ is sec-butyl or neopentyl. A compound according to any of Claims 1, 2 or 3, further characterised in that said cationic dye moiety is represented by the formula: where y¹ and y² are the same or different and represent an oxygen atom, a sulfur atom, a selenium atom, a vinyl group, >C(CH₃)₂, or N-R⁷; n is 0, 1, 2, or 3; z is 1 to 4; R⁷ is a short chain alkyl group; R⁵ and R⁶ represent alkyl groups and X represents a hydrogen atom, an electron withdrawing group or an electron donating group. A compound according to Claim 4, further characterised in that said cationic dye is selected from cationic cyanine, carbocyanine and hemicyanine dyes. A photohardenable composition characterised in comprising a free radical polymerizable compound and a compound according to any preceding claim. A photohardenable composition according to Claim 6, further characterised in being microencapsulated. A photosensitive material characterised in comprising a support having a layer of a photohardenable composition according to Claims 6 or 7 on the surface thereof.

Inventors: GOTTSCHALK PETER, SHANKLIN MICHAEL SAMUEL, GOTTSCHALK, PETER, SHANKLIN, MICHAEL SAMUEL
Applicants: MEAD CORP, THE MEAD CORPORATION
Application year: 1990
Main classification: C09B69
IPC codes: C09B69, G03F7, C07F5, C09B21, C09B17, G03C1, C09B23, C09B11