CA1137348A - Dimers derived from unsymmetrical 2,4,5- triphenylimidazole compounds as photoinitiators - Google Patents

Abstract

Title Dimers Derived From Unsymmetrical 2,4,5-Triphenylimidazole Compounds As Photoinitiators Abstract of the Invention Photoimaging compositions comprising (A) 2,4,5-triphenylimidazolyl dimer having selected substituents on the 2,4 and 5 phenyl rings and an extinction coefficient determined in methylene chloride at 10-5 to 10-3 mol/liter at 350 nm of at least 4000 liters/mol-cm and at 400 nm of at least 250 liters/mol-cm; and at least one of (B1) leuco dye or (B2) addition polymerizable ethylenically unsaturated monomeric compound. The new imaging compositions are useful in preparing dual response photoimaging products such as proofing papers, printout paper, overlay films and photopolymerizable elements. Improved imaging speed is achieved at equal concentration levels when compared with conventional 2,4,5-triphenylimidazolyl dimers.

Description

Title Dimers Derived From Unsymmetrical

2,4,5-Triphenylimidazole Compounds as Photoinitiators Description 5Technical Field This invention relates to photoimaging compositions. More particularly, it relates to photoimaging compositions containing selectively substituted 2,4,5-triphenylimidazolyl dimers.
10Background Art Photoimaging compositions utilizing hexaarylbiimidazole compounds in conjunction with either a leuco dye or an ethylenically unsaturated monomer compound, as well as other additives, are known. Such compositions are sensitive to radiation in the shorter wavelength range of the ultraviolet spectrum. For many reasons it is desirable that the spectral sensitivity of these compositions be extended into the longer wavelength range of the spectrum or the reactivity of these compositions be increased so that they can respond most efficiently to less costly lower energy light exposure sources.
To accomplish this many additives have been used in conjunction with hexaarylbiimidazoles, e.g., aromatic hydrocarbons, coumarins, carbocyanine dyes, hydroxy-phthalein dyes, acridine dyes, aminophenylketones, etc. The use of additives, while effective, often is not desired because not only does the presence of the specific additive increase the cost of the formulation, but it requires experimental work of skilled chemists to find additives of appropriate solubility characteristics and chemical properties to permit existence of the additive in the photoimaging composition, to develop the optimum formulations, and to maintain appropriate and ef~ective quality control over the chosen additives. The known additives have S been found to be effective in the formulations typical of hexaaryl~iimidazole-containing photoimaging compositions. After a period of time, however, it has been found that there may be a tendency for crystallization to occur causing an undesired decrease in photosensitivity and a discontinuity in photo-response in the area~ where the crystals form.
It is desirable that the above disadvantages be overcome and that formulations be prepared containing hexaphenylbiimidazole compounds sensitive to longer wavelengtshs of the spectrum and having increased radical reactivity.
Disclosure of the Invention In accordance with this invention there is provided a photoimaging composition comprising an admixture of ~ A) an 2,4,5-triphenylimidazolyl dimer of the formula:

L~2 wherein Rl is 2-bromo, 2-chloro, 2-fluoro, 2-alkyl of 1 to 4 carbon atoms and 2,4-dichioro;
R2 is 2-bromo, 2-chloro, 2-fluoro, 4-chioro, 2-alkyl of 1 to 4 carbon atoms, 2-cyano, and 2-al~oxy wherein the alkyl radical is of 1 to 4 carbon atoms;
and R3 is 3,4-dimethoxy, 3,4-diethoxy, 2,3-dimethoxy, 2,4,6-trimethoxy, 4-alkoxy wherein the 1 alkyl radical is of 1 to 4 carbon atoms and

3,4-methylenedioxy; the imidazolyl dimer having an extinction coefficient determined in methylene chioride at 10 5 to 10 3 mol/litPr at 350 nm of at least 4000 liters/mol-cm and at 400 nm of at least 250 liters/mol-cm; and at least one compound taken from the group consisting of (Bl) a ieuco dye that is oxidizable to dye by the imidazolyl radicals; and (B2) an addition polymerizable ethylenically unsaturated monomeric compound.
The photoimaging compositions of the invention comprise the specific 2,4,5-triphenylimidazolyl dimers and either a dye in its leuco form, or, in the event that the composition is photopolymerizable, a compound having ethylenic un~aturation. Both the leuco dye and ethylenically unsaturated compound can be present in the composition as noted below. The photoimaging composition containing the specific 2,4,5-triphenylimidazolyl dimer ana leuco dye is stabilized to prevent color build-up in the nonimage areas. The following processes have oeen found to be effective to achieve such stabilization: treatment with solution containing a free radical trap, e.g., hydro~uinone, phenidone, etc.; inclusion in the coating of precursors of hydroquinone which lead to its ~eneration by heat, e.g., dihydropyran adduct of ditertiarybutylhydroquinone; inclusion of quinones (photoactivatible oxidants) and hydrogen donor compounds (reductant components) which may be employed to generate hydroquinones by light exposure, preferably at wavelength distinct from the colorforming exposure; and photopolymerizable compounds which act as plasticizers to promote color formation until polymerized when they limit diffusion of color forming species and prevent formation of image color. It is desirable to add to the photoimaging composition a film-forming polymeric binder. Suitable inert solvents are generally present in preparing the formulations and plasticizers are commonly used therein. Additional components which can be present include:
anti-blocking agents, dyes, and white and colored pigments which do not act as sensitizers, etc.
In the photopolymerizable composition containing the specific 2,4,5-triphenylimidazolyl dimer and addition polymerizable ethylenically unsaturated compound there can be present a free radical producing, electron donor agent hydrogen donor (hydrogen donor), e.g., organic amines, mercaptans, certain halogen-containing compounds, active methylene compounds, etc. Optional components hat can be present in the photopolymerizable composition are: film-forming polymeric binders,

- 4 -1~37348 inert solvents, plasticizers, chain transfer agents, energy transfer dyes, oxygen scavengers, ultraviolet absorbers, etc. After imagewise exposure an image pattern can be observed in the photopolymerizable composition (in layer form) by toning with a colored toner or by solvent wasAout.
The specific 2,4,5-triphenylimidazolyl aimers useful in the photoimagin~ compositions are defined above. The dimers have what is classified as 10 n intra-radical asymmetry". The selected triphenylimidazole precursor to the dimer is asymmetrical about a plane perpendicular to the plane of the triphenylimidazole moiecule and bisecting the N-C-N bond angle of the formula set forth above.
This type of asymmetry is dependent on the substitution in the phenyl rings at tAe 4- and ;-positions on the imidazole ring. It is determined by the choice of the benzil precursor to the triphenylimidazole. Preferred compounds used in the compositions of this invention have been found to give rise to free radicals which, in the absence of leuco dyes, possess extended free radical life, i.e., show reduced tendency to recombine to give corresponding dimers. As dimerization removes reactive species, i.e., imidazolyl radicals, from the photoimaging compositions, reduced tendency to dimerize enhances imagins speed and performance.
Examples 1 to 10 beiow iliustrate the synthesis of the asymmetrical hexaphenyl~iimidazoles o~ the invention wherein tl~e selected benzoin ana benzaidehyde are refluxed in Methanol in the pressnce of copper acetate an~ ammonia, an adaptation of the procedure of Wiedenhagen et al., Ber. 70,570 (1937).

- 5 -~3~348 Another preparation procedure involves heating a benzil and a benzaldehyde at 180 to 190C in formamide solution as disclosed in Belgian Patent 589,417.
The specific triphenylimidazolyl dimers are present in 0.1 to 10.0 percent by weight of solids in the,photoimaging compos-tions and 0.1 to 15.4% by weight of solids in the photopolymerizable composition.
The leuco form of the dye which comprises one component of a photoimaging composition of the present invention is the reduced form of the dye having one or two hydrogen atoms, the removal of which together with an additional electron in certain cases produces the dye. Such dyes have been described, for example, in U.S. Patent 3,445,234, column 2, line 49 to column 8, line 55.
The following classes are included:
(a) aminotriarylmethanes (b) aminoxanthenes (c) aminothioxafithenes (d) amino-9,10-dihydroacridines (e) aminophenoxazines (f) aminophenothiazines (g) aminodihydrophenazines (h) aminodiphenylmethanes (i) leuco indamines (j) aminohydrocinnamic acids (cyanoethanes, leuco methines) (k) hydrazines (1) leuco indigoid dyes (m) amino-2,3 dihydroanthraquinones (n) tetrahalo-p,p'-biphenols (o) 2(p-hydroxyphenyl)-4,5-diphenylimidazoles (p) phenethylanilines 37~3~8 Of these leuco forms, (a) through (i) form the dye by losing one hydrogen atom, w;nile the leuco forms (j) through (p) lose two hydrogen atoms to produce the parent aye. Aminotriarylmethanes are preferred. A
general preferred aminotriarylmethane class is that of the acid salts of aminotriarylmethanes wherein at least two of the aryl groups are phenyl groups having (a) an ~lR2N-substituent in the position para to the bond to the methane carbon atom wherein Rl and R2 are each groups selected from hydrogen, Cl to C10 alkyl, 2-hydroxyethyl, 2-cyano-ethyl, or benzyl and (b) a group ortho to the methane carbon atom which is selected from lower alkyi (C is 1 to 4), l; lower alkoxy (C is 1 to 4), fluorine, chlorine or bromine; and the third aryl group may be the same as or aifferent from the first two, and when dif~erent is selected from (a) Phenyl which can be substituted with lower aiky~, lower alkoxy, chloro, diphenylamino, cyano, nitro, hydroxy, fluoro or bromo;
(b) Naphthyl which can be substituted with amino, di-lower alkylamino, alkylamino;
(c) Pyridyl which can be substituted with alkyl;
(d) Quinolyl;
(e) Indolinylidene which can be substituted with alkyl.
Preferably, Rl and R2 are hydrogen or alkyl of 1-4 carbon atoms. Leuco dye is present in 0.1 to 5.0 percent by weight of solids in the photoimaging composition.
With tAe leuco form of dyes which have amino or substituted amino groups witnin the dye structure and which are characterized as cationic dyes, an ~ 7 ~

`` `` 1~37348 amine salt-forming mineral acid, organic acid, or an acid from a compound supplying acid is employed. The ~mount of acid usually varies from 0.33 mol to 1 mol per mol of amino nitrogen in the dye. The preferred quantity of acid is about 0.5 to 0.9 mol per mol of amino nitrogen. Representative acids which form the required amine salts are hyarochloric, hydrobromic, sulfuric, phospnoric, acetic, oxalic, p-toluenesulfonic, trichloroacetic, trifluoroacetic an~ perfluoroheptanoic acid. Other acids such as acids in the "Lewis" sense or acid sources which may be employed in the presence of water or moisture include zinc chloride, zinc bromide, and ferric chloride. Representative leuco dye salt~ include tris- (4-diethylamino-o-tolyl) methane zinc chloride, tris- (4-diethylamino-o-tolyl) methane oxalate, tris-(4-diethylamino-o-tolyl) methane p-toluene-sulfonate and the like.
The redox couple useful in the photoimag1ng composition is described in U.S. Patent 3,658,543, column 9, lines 1 to 46.
Preferred oxidants include ~,10-phenantnrenequinone alone or in admixture with 1,6- and 1,8-pyrenequinone which absorb ~rincipally in the 430 to 550 nm region. The reductant component of the redox couple is 100 to 10 percent of an acyl ester of triethanolamine of the formula:
' O
N(CH2c~2Oc-R)3 where R is alkyl of i to 4 carbon atoms, and 0 to 30 percent of a Cl to C4 alkyl ester of _ ~ _ nitrilotriacetic aci~ or of 3,3',3"-nitrilo-tripropionic acid. Triethanolamine triacetate and dibenzylethanolamine acetate are preferred reductant components. The molar ratios of oxidants to biimidazole used ranges from 0.01:1 to 2:1, preferably 0.2:1 to 0.6:1. The molar ratios of reductant to biimidazole used ranges from about 1:1 to about 90:1, preferably 10:1 to 20:1.
Optionally, other additives can be present in the photoimaging composition. Polymeric binders can be adde~ to thicken the formulations or adhere them to substrates. The binders can also serve as a matrix for the color-forming composition.
Light-transparent and film-forming polymers are preferred. Examples are ethyl cellulose, polyvinyl alcohol, polyvinyl chloride, polystyrene, polyvinyl acetate, poly-(methyl, propyl or butyl methacrylate), cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, cnlorinated rubber, copolymers of the above vinyl monomers, etc.
The binder can be present in an amount from about 0.5 part to about 200 parts by weight per part of combined weight of the hexaphenyl~iimidazole and 2~ leuco dye. Generally 5 to 20 parts by weight are used.
The binder composition can also contain inert infusible fillers such as titanium dioxide, organophilic colloidal silica, bentonite, powdered glass, micron-sized alumina and mica in minor, noninterfering amounts. Formulations containing micron-sized silicas, as, for example, the "Syloid"
silica gels, sold by W. R. Grace & Co., are particularly useful ror proviaing a "tooth" for ~f~ade _ g _ - ~ 1137348 pencil or ink receptivity and elim~nating blocking tendencies.
Witn some polymers, it is desirable to add a -plasticizer, e.g., solid or liquid, to give flexibility to the film or coating. Suitable plasticizers are disclosed ~n U.S. Patent 3,658,543, coiumn 10, lines 20 to 73.
A preferred liquid plasticizer is NonyLphenoxypoly(ethyleneoxy)-ethanol. A preferred solid plasticizer is N-ethyl-p-toluenesulfonamide.
The plasticizers can be used in concentration ranging from 1:2~ to 5:3, preferably 1:5 to 1:2, based on the weight of polymeric binder used.
In preparing the formuiation generally inert solvents are employed which are volatiie at ordinary ~ressures. Examples include alcohols and ether alcohols such as methanol, ethanol, l-propanol, 2-propanol, butanol, and ethyiene glycoi; esters such as methyl acetate and ethyl acetate; aromatics such as benzene, o-d~chlorobenzene and toluene; ketones such as acetone, metnyl ethyl ketone and 3-pentanone;
alip~atic halocarbons such as methylene chloride, chloroform, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane and 1,1,2-trichloroethylene;
miscellaneous solvents such as dimethylsulfoxide, pyridine, tetrahydrofuran, dioxane, dicyanocyclobutane and l-methyl-2-oxo-hexamethyleneimine; and mixtures of these solvents in various proportions as may be required to attain solutions. It is often beneficial to leave a small residue of solvent in the dried composition so that the desired degree of imaging can be obtained upon subse~uent irra~ation.

113~348 Useful optional antiblocking agents present to prevent the coatings from adhering to one another include n CF3 (CF2CF2) 17CH2CH2-0-C (CH2) 16CE~3 and other known agents.
In the photopolymerizable embodiment of this invention, in addition to the hexaphenylbiimidazole compounds described above, at least one addition polymerizable ethylenically unsaturated compound having at least one polymerizable ethylenic group is present. Such compounds are capable of forming a high polymer by free-radical initiated, chain-propagating, addition polymerization.
Preferably, the monomeric compound has at least two terminal ethylenically unsaturated groups, e.g., 2 to 4 groups. The monomeric compounds are nongaseous, i.e., at 2nc and atmospheric pressure, have a normal boiling point about 100~ and a plasticizing action on any thermoplastic polymeric binder that may be present.
Ethylenically unsaturated monomeric compounds useful in this invention include monomeric compounds or polymers wherein the ethylenic unsaturation is present as an extralinear substituent attached to the polymer backbone. Useful monomeric compounds are: alkylene or a polyalkylene glycol diacrylate prepared from an alkylene glycol of 2 to 15 carbons or a polyalkylene ether glycol of 1 to 10 ether linkages; unsaturated esters of alcohols, preferably polyols and particularly such esters of the alphamethylene carboxylic acids, e.g., ethylene glycol diacrylate, diethylene glycol diacrylate, glycerol diacrylate, glycerol triacrylate, ethylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate, 1,4-benzenediol dimethacrylate, pentaerythritol tetramethacrylate, 1,3-propanediol diacrylate, 1,5-pentanediol dimethacrylate, pentaerythritol triacrylate; the bis-acrylates and methacrylates of polyethylene glycols of molecular weight 200-500, etc.;
unsaturated amides, particularly those of the alpha-methylene carboxylic acids, and especially those of alpha-omega-diamines and oxygen-interrupted omega-diamines, such as methylene bis-acrylamide, methylene bis-methacrylamide, ethylene bis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylene triamine trismethacrylamide, bis(gamma-methacrylamidopropoxy) ethane, beta methacrylamidoethyl methacrylate, N-(betahydroxyethyl)-beta-(methacrylamido) ethyl acrylate and N,N-bis (beta-methacryloxyethyl) acrylamide; vinyl esters such as divinyl succinate, divinyl adipate, divinyl phthalate, divinyl terephthalate; divinyl benzene-1,4-disulfonate, and divinyl butane-1,4-disulfonate, styrene and derivatives thereof and unsaturated aldehyde, such as sorbaldehyde (hexadienal).
Useful polymers having ethylenically unsaturated groups attached thereto are: the polymerizable, ethylenically unsaturated polymers of U.S. Patent 3,043,805 and U.S. Patent 2,929,710, Ii37348 e.g., polyvinyl acetate/acrylate, cellulose acetate/acrylate, cellulose, acetate/methacrylate, N-acrylyloxymetnyl polyamide, etc.; polyoxyethylated trimethylol propane triacrylate, polytetramethylene glycol diacrylate, etc., disclosed in U.S. Patent 3,418,295.
Suitable thermal polymerization inhibitors that can be used in photopolymerizable compositions include p-methoxyphenol, hyaroquinone, and al~yl- and aryl-substituted hydroquinones and quinones, tert-butyl catechol, ~yrogallol, copper resinate, naphthylamines, beta-naphthol, cuprous chloride, 2,6-di-tert-butyl-p-cresol, phenothiazine, pyridine, nitrobenzene and dinitrobenzene. Other useful inhibitors, include p-toluquinone and chloranil, and thiazine dyes, e.g., Thionine Blue G (C.I. Basic Blue 25), and Toluidine Blue O (C.I. Basic Blue 17).
In certain embodiments of the invention containing certain dye photoinitiators, however, no thermal inhibitor is required since these initiators have a dual function and in the dark serve as thermal inhibitors.
Free radical producing, electron donor agents lhydrogen donor) and active methylene compounds that can be present in the photopolymeriza~le photoimagins composition are described in U.S. Patent 3,479,185, column 2, line ~0 to column 3, line 3. The electron donor agent has a reactive atom, usually hydrogen, which is removable and in the presence of the radi~al of the substituted 2,4,5-triphenyl-imidazolyl dimer yields a radical which reacts with the monomeric compound to initiate growth of polymer chains.
Examples of preferred electron or hydrogen donor compounds include compounds that form a stable composition with the hexaphenylbiimidazole compound in the dark. The agent can be an amine, e.g., a tertiary amine. The amine-substituted leuco dyes are useful, especially those having at least one dialkylamino group. Also, any leuco triphenylamine dye or various salts of the dye, e.g., the HCl salt of the leuco blue dye can be used. Illustrations of suitable dyes include tris-(4-N,N-diethyl-amino-o-tolyl)-methane trihydrochloride, bis(4-N,N-diethylamino-o-tolyl)triphenylmethane, bis(4-N~N-diethylamino-o-tolyl) methylene-dioxyphenylmethane, leuco neutral shade dye, i.e., bis(4-N,N-diethylamino-o-tolyl)-benzyl thiophenylmethane, Leuco Malachite Green (C.I. Basic Green 4), leuco forms of Crystal Violet, Brilliant Green (C.I. Basic Green 1), Victorial Green 3B (C.I.
Basic Green 4), Acid Green GG (C.I. Acid Green 3), Methyl Violet (C.I. Basic Violet 1), Rosaniline (C.I.
Basic Violet 14), etc. The salt forms, e.g., HCl salt, salts with Lewis acid, sulfuric acid salts, p-toluene sulfonic acid salts, etc., of the leuco dye is preferred for use.
Additional suitable, electron donor agents which can be used singly or in combination include aniline, N-methylaniline, N,N-diethylaniline, N,N-diethylcresidine, triethanolamine, ascorbic acid, 2-allylthiourea, sarcosin, N,N-diethylglycine, trihexylamine, diethylcyclohexylamine, ~,N,N',N'-tetramethylethylenediamine, diethyl-aminoethanol, ethylaminoethanol, N,N,N',N'-ethylenediaminotetracetic acid, N-methylpyrrolidone, N,N,N',N",Nn-pentamethyldiethylenetriamine, N,N-diethylxylidene, N,N'-dimethyl-1,4-piperazine, N-~-hydroxyethylpiperidine, N-ethylmorpholine, and related amino compounds. While the tertiary amines and especially the aromatic tertiary amines having at least one CH2 group adjacent to the nitrogen atoms are preferred, a combination of two radical generating agents such as a tertiary amine, e.g., N,N-dimethylaniline, and a secondary amine, e.g., N-phenylglycine, appear especially useful.
In a photoimaging composition containing the hexaphenylbiimidazole, monomeric compound and electron donor agent, the light-sensitivity, speed, or degree of polymerization is dependent on the concentration of the hexaphenylbiimidazole and electron donor agent. Useful compositions may be limited in part by the solubilities of the components. It was discovered that the speed increased up to a certain concentration of hexaphenylbiimidazole and electron donor agent, and an increase of the concentration past that level did not produce any increase in speed and in some instances the speed dropped. When a leuco dye was used as the electron donor agent, a mole ratio of leuco dye to the hexaphenylbiimidazole of 1.0 to 1.4 gave the best results as to photospeed and stability.
As indicated above, optionally, ~ut preferably, a polymeric binder can be present in the photopolymerizable photoimaging system. Suitable binders include: the polymerized methylmethacrylate resins including copolymers thereof, polyvinyl acetals such as polyvinyl butyral and polyvinyl formal, vinylidene chloride copolymers (e.g., vinylidene chloride/acrylonitrile, vinylidene chloride/methacrylate and vinylidene chloride/vinylacetate copolymers), synthetic rubbers (e.g, butadiene/acrylonitrile copolymers and chloro-2-butadine-1,3 polymers~ cellulose esters (e.g, cellulose acetate, cellulose acetate succinate and cellulose acetate butyrate), polyvinyl esters (e.g, polyvinyl acetate/acrylate, polyvinyl acetate/methacrylate and polyvinyl acetate), polyvinyl chloride and copolymers (e.g, polyvinyl chloride/acetate), polyurethanes, polystyrene and the polymeric binders described in U.S. Patent 3,418,295. The monomeric compound and polymeric binder are present in the photoimaging composition in from 3 to 97 to 97 to 3 parts by weight, ~ respectively. 0.001 to 2.0 parts by weight per 100 parts by weight of monomer and binder of a thermal addition polymerization inhibitor preferably is present.
Solvents, plasticizers, e.g., 10 to 50 percent by weight based on the weight of monomer, antiblocking agents, ultraviolet absorbers as described in the Examples, e.g., Example 18, can be present in the photopolymerizable photoimaging composition. In addition, oxygen scavengers, e.g., 2-allyl thiourea, dimethylsulfoxide, stannous chloride, N-phenylglycine, etc., can be present. The oxygen scavenger appears to eliminate or reduce the induction period usually found in a photopolymerization reaction, possibly by consumption of oxygen in the layer prior to exposure.
Another additive that can be present in the pho.opolymerizable composition is a chain transfer agent, in an amount of from 0.01 to 0.1 mol/mol electron donor agent such as a leuco dye, e.g., N-phenylglycine, 1,1-dimethyl-3,5-diketocyclohexane, or organic thiols, e.~., 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, pentaerythritol tetrakis (mercaptoacetatP), 4-acetamidothiophenol, mercaptosuccinic acid, dodecanethiol, ~eta-mercaptoethanol, beta-mercaptoethanol, or other organic thiol.
Still another additive is an energy-transfer dye of the type disclosed in U.S. Patent 3,479,185, column 5, lines 57 to 74.
Generally such energy-transfer dyes are present in 0.5 to 3.0% by weight based on the weight of monomer - or binder component, if present.
For imaging uses, the compositions of tnis invention may be coated upon or impregnated in substrates following known techniques. Substrates include materials commonly used in the graphic arts and in decorative applications such as paper ranging from tissue paper to heavy cardboard, films of plastics and polymeric materials such as regenerated cellulose, cellulose acetate, cellulose nitrate, polyethylene terephthalate, vinyl polymers and copolymers, polyethylene, polyvinyl-acetate, polymethyl methacrylate, polyvinylchloride; textile .., ~137348 fabrics; glass, wood and metals. The composition, usually as a solution in a carrier solvent described above, may be sprayed, brushed, applied by a roller or an immersion coater, flowed over the surface, picked up by i.~mersion or spread by other means, and the solvent evaporated.
Any convenient source providing radiation of wavelengths in the range of 200 nm to 420 nm can be used to activate the photoimaging composition for triphenylimidazolyl radical formation, image formation, and photopolymerization initiation. The radiation may be natural or artificial, monochromatic or polychromatic, incoherent or coherent, and should be sufficiently intense to activate a substantial proportion of the photoinitiator.
Conventional light sources include fluorescent lamps, mercury, metal additive and arc lamps. Coherent light sources ~re the pulsed nitrogen-, xenon, argon ion- and ionized neon-lasers whose emissions fall within or overlap the ultraviolet or visible absorption bands of the photoinitiator. Ultraviolet and near-visible radiation-emitting cathode ray tubes widely useful in printout systems for writing on photosensitive materials are also useful with the subject compositions.
Images may be formed by writing with a beam of the activating light or by exposing to such light a selected area behind a negative, stencil, or other relatively opaque pattern. The negative may be silver on cellulose acetate or polyester film or one in which its opacity results from aggregations of li37~348 areas having different refractive indices. Image formation may also be effected in conventional diazo printing apparatus, graphic arts expo~ure or electronic flash equipment an~ by projection as ~escribed in U.S. Patent 3,661,461. The light exposure time may vary from a fraction of a second to several minutes, depending upon the intensity and spectral energy distribution of the light, its distance from the composition, the nature and amount of the composition available, and the intensity of color in the image desired.
Best Mode for Carr~ Out the Invention The best mode is illustrated in Example 1 wherein the hexaarylbiimidazole compound is 2,2',4,4'-tetrakis(o-chlorophenyl)-5,5'-~is(m,p-dimethoxyphenyl)-biimidazole.
Industrial Applicability The hexaphenylbiimidazoles are useful as photoinitiators in various photoimaging formulations which can be used in the manufacture o~ dual response photoimaging products, where controlled sequential exposure with ultraviolet and visible light may yield negative or positive images, e.g., Dylux~ proofing papers, printout paper, e.g., for the Du Pont "aca"
automatic clinical analyzer; garment pattern papers, overlay films, heatfix type papers and films; and in photopolymerizable layers and elements useful for printing purposes as well as a variety of copying, i.e., office copying, recording, decorative, and photoresist applications. The formulations containing the hexaarylbiimidazoles have improved ab~orption in the imaging areas of 345 to 400 nm and form imidazolyl radicals with increased radical life ..

which permits their more effective utilization at lower concentration ieveis. The biack formuiation may be imageà with a variety of exposure sources, e.g., black light blue, black light, xenon, mercury vapor, etc. without use of a fiiter due to extended absorption. The sta~ility of the formulations is improve~. It is possible to fix the formulations faster because a lower amount of hexaphenyl-biimidazole compound is present and must subsequently be destroyed during fixing. The imaging speed is also improved at equal concentration levels with current photoinitiators. Cost of the formulations is reduced because of tbe lower cost of ingredients for manufacture of t'he biimidazole as well as the fact tAat lower concentrations can be used.
Examples The following examples illustrate the invention wherein tne percentages are by weight.
ExamPles l to 10 Synthesis of HexaphenYlbiimidazoles The asymmetrical hexap~enylbiimidazoles of this invention are prepared according to the following procedures.
A. Benzoin Condensation TO a one-liter flask equipped with a stirrer, reflux condenser and nitrogen inlet tube is charged:
Ingredient Amount (q) 3,4-~imethoxybenzalaehyde 200 (1.20 mols) 2-Chlorobenzaldehyde 174 (l.24 mols) Methanol 225 ml Water 65 ml Potassium ~yanide 10 The system is purged with nitrogen, and the reaction mixture is refluxeà for 45 minutes. The flask is cooled in an ice batn to induce crystallization which is foliowed by an aaditional 4.25 hours refluxing and cooling to room temperature with stirring. The precipitated benzoin is filtered off, is washed with a i75 ml methanol/25 mi water mixture, followed by a 1000 ml of water wash and is then recrystallized trom ethanol. Table 1 summarizes the resuits of the benzoin preparation. Except for the indicated variations in the molar scale of the reactions, tne procedure described above was followed with appropriate modification of the amounts of reactants.
Table 1 Benzoins R-CO-CH(O~
Reaction Scale ~1.P. Yield R aldehyde Rl A~dehyde (Molar) (C) (~) 2~3,4-dimethoxy 2-chloro 1.2 116-118 69 3,4-dimethoxy 2-bromo 0.2 122-124 58 3,4-dimethoxy 2-methyl 1.0 118-19 231 4-ethoxy 2-chloro 1.2 123-5 74 3,4-dimethoxy 4-chloro 1.2 100-2 212 253,4-dimethox~ 2-fluoro 0.9 124-6 52 3,4-methyl-eneoxy 2-chloro 1.2 115-6 63 i Oil from reaction; crystals obtained from 3~ ether extraction.
2 Oil from reaction; crystals obtained from ethanol/petroleum ether.

B. Benzil Synthesis To a 3-liter flask fitted with a stirrer, reflux condenser and thermometer, is added Mixture 1 which contains the following ingredients in the amounts indicated:
Mixture 1 Ingredient Amount (g) Cupric acetate 2.18 Water 121.0 Acetic acid 303.0 Ammonium nitrate 112.0 Mixture 1 is heated to about 95C to obtain a solution. Mixture 2 is prepared from the following ingredients in the amounts indicated:
Mixture 2 Ingredient Amount (g) 3,4-Dimethoxy-2'-chlorobenzoin 303.0 Acetic acid gog.o Mixture 2 is heated to 70-80C to maintain solution and is added portionwise to Mixture 1 in the flask as follows:
Portion 1: about 10% (~120 g) of Mixture 2 to start the reaction; nitrogen evolution is observed in 1 to 3 minutes.
Portion 2: About 40% (~450 g) of Mixture 2; the reaction temperature is maintained at 95 to 105C.
Portion 3: Balance of Mixture 2 (~600 g) 1 to 2 hours after addition of Portion 2; the temerature of the mixture is held at 105C for an additional hour.

Following an additional five-hour reflux period to complete the reaction, the mixture is allowed to cool overnight to room conditions. The mixture is chilled to 5 to 10C with stirring and is filtered twice, first with a water (80 g)/acetone (320 g) solution chilled to 5 to 10C, and then with 2000 ml of water. The filter cake is held for the preparation of the hexaphenylbiimidazoles.
Table 2 summarizes the results of benzil preparation.
Table 2 O O
n n Benzils R2-C-C-~3 R2 R3 M.P.(C) 3,4-dimethoxy 2-Cl 3,4-dimethoxy 2-Br 122-124 3,4-dimethoxy 2-methyl 124-126 4-ethoxy 2-Cl 90-91 3,4-dimethoxy 4-C1 135-136 3,4-dimethoxy 2-F 96-98 3,4-methylene-oxy 2-C1 91-92 C. Imidazole Synthesis To a 500 ml flask equipped with a stirrer, reflux condenser and nitrogen inlet tube is charged:
Ingredient Amount t~) 3enzil Compound (See Table 3 below) (0.133 mol) Benzaldehyde compound(See Table 3 below) (0.146 mol) Ammonium acetate 45 Glacial acetic acid 120 ml `` 1137348 The reaction mixture is swept with nitrogen and refluxed for 12 hours and allowed to cool to room temperature overnight. The reaction mixture is then poured into 2 liters of distilled water containing 7 g of potassium bisulfite to complex the unreacted aldehyde. A white solid precipitates which is filtered, washed with 2000 ml water, and dried.
N.M.R. analysis shows that the acetate salt of the imidazole formed. No attempt is made to prepare the free base because in the final step of the synthesis the oxidative dimerization is carried out in strong base and the acetate salt is converted to the base at that stage.
Table 3 15 Compound No. Benzil ComPound Amount (g) 1. 3,4-Dimethoxy-2'-chlorobenzil 40.6 2. 3,4-Dimethoxy-2' fluorobenzil 38.3 3. 4-Ethoxy-2'-chlorobenzil 38.3 4. 3,4-Dimethoxy-2'-chlorobenzil 40.6 5. 3,4-Methylenedi-oxy-2'-chlorobenzil 33.8

6. 3,4-Dimethoxy-4'-chlorobenzil 40.6

7. 3,4-Dimethoxy-2'-chlorobenzil 40.6

8. 3,4-Dimethoxy-2'-chlorobenzil 40.6

9. 3,4-Dimethoxy-2'-methylbenzil 37.8

10. 3,4-Dimethoxy-2'-bromobenzil 46.4 Table 4 Compound No. Benzaldehyde Compound Amount (g) 1. 2-Chlorobenzaldehyde 20.6 2. 2-Chlorobenzaldehyde 20.6 3. 2-Chlorobenzaldehyde 20.6 4. 2-Bromobenzaldehyde 27.0 5. 2-Chlorobenzaldehyde 20.6 6. 2-Chlorobenzaldehyde 20.6 7. 2-Methylbenzaldehyde 17.5 8. 2,4-Dichlorobenzaldehyde 25.4 9. 2-Chlorobenzaldehyde - 20.6 10. 2-Chlorobenzaldehyde 20.6 Table 5 summarizes the results of triphenylimidazole preparation.

li37348 Table 5 ~ ~
A

~ -H

D

D C

A A' B ~ D D' 1. Cl ~ OCX3 OCH3 Cl 2. Cl ~ O~H3 OCH3 F
3. Cl - - OEt Cl 204. Br ~ OCH3 OCH3 Cl 5. Cl ~ C~2 o Cl 6. Cl ~ OC~3 OCH3 - Cl 7- C~3 C~3 OCH3 Cl 8. C1 C1 OCH3 oC~3 Cl 259. Cl ~ O~H3 OCX3 ~3 10. Cl ~ OCH3 OC~3 Br *derived from piperonal The dimers of the triphenyiimidazolyl radicals derived from each of the specific triphenylimidazoles shown in Table 5 above are prepared by the following procedure. Into a 250 ml flask equipped with stirrer and condensor is placed one of the ten triphenylimidazoles in the amount indicated:

113734~

Ingredient Amount (g) Triphenylimidazole (0.0471 mol) 1. 20.0 2. 19.2 3- 19.2 4- 22.1 5. 19.2 6. 20.0 7. 19.0 8. 21.7 9. 19.0 10. 22.1 Methylene chloride 125.0 ml K3Fe(CN)6 20.7 Water 65.0 ml Sodium hydroxide (50%) 20.0 After refluxing for about 18 hours and cooling, 100 ml of water is added. The methylene chloride layer is separated and the aqueous phase is extracted with 50 ml methylene chloride. The combined methylene chloride layers are washed twice with 70 ml water and are dried over anhydrous sodium sulphate.
The methylene chloride is evaporated carefully to determine the yield and methylene chloride is added to make a 15% solution.

Table 6 Dimer Molar No. Con- Extinction 5 and ver- Coefficient* Density (Trans.) ~a) sion (~ (c) Speed Rank (%) 350 nm 400 nm Dmax Step Dmin 1. 88 5763 1451 0.73 0.59 0.30 2. 58 4633 506 0.68 0.56 0.26 10 ~ 3. 65 4672 253 0.65 0.45 0.22 4. 89 4941 588 0.57 0.41 0.19 5. 54 5848 1089 0.60 0.39 0.17 6. 85 4364 844 0.81 0.48 0.2 7. 96 5338 1128 0.46 0.41 0.19 8. 79 4598 1150 - 0.45 0.24 9. 88 5338 502 0~60 0.45 0.29 10. 70 4706 971 0.78 0.52 0.28 *Molar Extinction Coefficient is determined in methylene chloride at 10 5 to 10 3 mol/liter, the values being expressed in liters/mol-cm.
(a) Visual comparison of sensitometric strips, lowest number is fastest. Dimers derived from triphenylimidazoles Table 5.
(b) Extinction coefficient for Control 2, Example 11 is 410 and Control 1, Example 11 is 2550.
(c) Extinction coefficient for Controls 1 and 2, Example 11 are <200.
The above ten triphenylbiimidazolyl dimers are used as new photoinitiators in photosensitive compositions containing the following insredients in the amounts indicated:

1-13734~

A~ount Ingredient (weight %) Acetone (solvent) 72.000 Isopropanol (solvent) 8.000 Dodecylbenzenesulfonic acid (proton acid) 0.727 Tris(p-diethylamino-o-tolyl)methane (leuco cyan dye) 0.275 N-Ethyl-p-toluenesulfonamide (solid plasticizer) 2.890 o-Phenylphenol condensate with average of ~2.25 mols ethyleneoxide (plasticizer) 2.385 Triphenylimidazolyl dimer (photoinitiator)*
(one of compounds 1 to 10, Table 6)0.434 Mixture of ~83% 1,6- and ~17% 1,8-pyrenequinones (oxidant) 0.004 9,10-Phenanthrenequinone (oxidant)0.139 Triethanolamine triacetate (hydrogen donor) 2.013 n CF3 (CF~CF2) 17C~I2CH2-0-C- (CH2) 16CH3 (antl-blocking agent) 0.020 Silica gel of 9 ~ average particle diameter surface area of 675 meters squared per gram, bulk density ~465 kg/m3 (anti-blocking agent)0.606 Cellulose acetate butyrate (~27% butyryl) content, ~1 hydroxyl/2 anydroglucose units, and a viscosity of 56-131 poises by ASTM D-817-65-D (binder) 10.507 Total100.000 *added as a 15% solution by weight in methylene chloride (2.2 ml) The photosensitive compositions are coated with a 0.004 inch (~100 nm) doctor blade on Schweitzer 32-HG
paper and are dried at normal room conditions. The dry coatings are imaged by exposure through an image-bearin~ transparency for 30 seconds to 2.75 microwatts/cm2 of black light blue radiation and are deactivated by exposure to room light to remove yellow background and render coatings insensitive to ~ further ultraviolet irradiation. All the compositions provide good colored images with low background density.
Example 11 Three coating lacquers are prepared as follows:
15 Ingredient Amount (g) Acetone 184.0 Cellulose acetate butyrate ~27%
butyryl content hydroxyl/2 anhydroglucose units and viscosity is 7.5-15 poises by ASTM D-817-65-D 24.0 o-Phenylphenol condensed with average of ~2.25 mols ethylene oxide 16.0 f Tris-(2-methyl-4-diethylaminophenyl)-methane] 1.2 p-Toluenesulfonic acid 1.4 7-Diethylamino-4-methylcoumarin 9.4 To each of three 11.4 g aliquots of the above lacquers is added 1.6 x 10 4 mol of one of the following hexaphenylbiimidazole compounds:
1. 2,2'-bis(o-chlorophenyl)-4,4',S,5'-tetrakis-S (m-methoxyphenyl)-biimidazole (Control 1) 2. 2,2'-bis~o-chlorophenyl)-4,4',5,5'-tetraphenyl-biimidazole (Control 2) 3. 2,2,'4,4'-tetrakis(o-chlorophenyl)-S,5'-bis-(m,p-dimetho~yphenyl)-biimidazole.
Films are cast with a 040 coating bar on Schweitzer 32-~G paper and are dried at normal room conditions.
Portions of the films are exposed as indicated in Table 7. The reflectance densities are measured with a MacBeth Quantalo~ reflectance densitometer.

1137~348 Table 7 Exposure Source Time(sec) Rerlectance Densities 1. None 0.08 0.04 0.09 2. Biack light blue (365 nm peak) 60 1.46 1.53 1.65 3. Vivita~ 292 Photoflash 6000R, no filter 0.001 1.52 1.52 1.55 4. Same as source 3 but use O-51 filter(a) 0.001 1.08 1.03 1.25 5. Same as source 3 but use 3-75 filter(b) 0.001 0.99 0.99 1.10 6. Same as source 3 but use 3-74 filter(C) 0.001 0.66 0.44 0.69 7. Same as source 3 but use 3-72 filter(d) 0.001 0.24 0.18 0.45 (a) less than 0.5% transmission at less than 334 nm;
greater than 65% transmission at greater than 405 nm (b) less than 0.5% transmission at less than 373 nm;
greater than 65% transmission at greater than 427 nm (c) less than 0.5% transmission at less than ~03 nm;
greater than 65~ transmission at greater than 436 nm 35 (d) less than 0.5% transmission at Less than 444 nm;
greater than 65% transmission at gr ater than 466 nm `` 1137348 This example illustrates that the hexaphenylbiimidazole compound of the invention present in coating 3 has greater absorption of actinic radiation at longer wavelengths as compared to the two control hexaphenylbiimidazole compounds present in coatings 1 and 2.
Examples 12 to 18 which follow illustrate various uses for the preferred hexaphenylbiimidazole compound, 2,2',4,4'-tetrakis(o-chlorophenyl)-5,5'-bis-(m,p-dimethoxyphenyl)-biimidazole.

- ~137348 Example 12 This example illustrates a magenta color-forming system.
A coating solution is prepared by mixing the following ingredients in the amounts indicated:
Ingredient Amount (g) ~ethylene chloride 295.0 Isopropanol 21.0 10 Cellulose acetate butyrate described in Example 11 42.2 Triethanolamine triacetate 0.01 l-Phenylpyrazolidine-3-one 0.106 15 p-Toluenesulfonic acid 0.347 N-Ethyl-p-toluenesulfonamiae 3.93 Nonylphenoxypoly(etheneoxy)-ethanol, nonionic surfactant and plasticizer 3.93 20 7-Diethylamino-4-methylcoumarin 1.0 Hexaphenylbiimidazole No. 3, Example 11 4.64 The solution is coated on 0.003 inch (0.076 mm) thick polyethylene terephthalate film with a 025 coating bar and is allowed to dry. A 10-second exposure to ultraviolet radiation from BLB lamps gives a magenta image reflectance density of 0.64 when read over a white paper surface. A 20-second exposure to visible light with SDZ fluorescent lamps, gives an optical density of 1.0~. The coating is deactivated by passing it through a solution containing 235 g water, 115 g isopropanol, 3.5 g sodium sulfite, 3.5 g acetic acid and 7.0 g l-phenylpyrazolidine-3-one.

Example 13 This example illustrates a negative-working garment pattern paper.
A coating solution is prepared by mixing the following ingredients in the amounts indicated:
Ingredient Amount (g) Methylene chloride 3a5.0 Isopropanol 70.0 10 Cellulose acetate butyrate as in Example 11 except that the viscosity is 56-131 poises by ASTM D-817-65-D 38.0 Nonylphenoxypoly(etheneoxy)-ethanol 28.0 15 p-Toluenesulfonic acid 2~74 Silica gel of 9.0 ~ average particle diameter, 675 m2/gm surface area, bulk density ~465 Kg/m3 2.90 Mixture of ~83% 1,6- and ~17% 1,8-pyrenequinones 0.047 Phenanthrenequinone 0.141 n CF3(cF2cF2)l7cH2cH~-o-c(cH2)l6cH3 P.30 7-Diethylamino-4-methylcoumarin 0.46 3,4-Dimethoxyphenyl-bis(4-diethylamine-2-methylphenyl) methane 0.38 Rhodamine 5 GLD (C . I. 45160) 0.18 Hexaphenylbiimidazole No. 3, Example 11 0.57 The solution is coated on Schweitzer SUb 14 paper with a 010 coating bar to give a coating weight of ~ 4.0 mg/dm2. After exposure to an ultraviolet radiation source for 60 seconds, an optical density of 0.7 is obtained.
Example 14 This example illustrates the preparation of proofpaper, A yields cyan and B yields black images.
A coating lacquer is prepared by mixing the following ingredients in the amounts indicated:
Coating A

Ingredient Amount (g) Methylene chloride 37.1 Isopropanol 4.1 Cellulose acetate butyrate described in Example 13 5.2 Pyrenequinone as described in Example 13 0.0031 Phenanthrequinone 0.050 n CF3(cF2cF2)l7cH2c~2-o-c(cH2)l6cH3 0 044 7-Diethylamino-4-methylcoumarin 0.078 Dodecylbenzenesulfonic acid 0.435 Triethanolamine triacetate 0.96 o-Phenylphenol condensed with average of ~2.25 mols ethylene oxide 1.0 Silica gel described in Example 13 0.35 Hexaphenylbiimidazole No. 3, Example 11 0.152 113;'348 The solution is coated on Schweitzer 32-HG paper with a 020 bar, and the dried coating is exposed for 60 seconds to ultraviolet radiation (BLB lamps) through a lithosraphic negative. An image density of 1.0 is read on a MacBeth Quantalog~Reflectance densitometer The coating can also be exposed in a positive manner, with the initial light source a bank of SDZ fluorescent lamps, through a lithographic negative, followed by a flooding exposure with BLB
lamps. An image with density of 1.0, and background of 0.1 is attained.
A similar coating solution as described above, but containing 0.18 g of the hexaphenylbiimidazole instead of 0.152 g, results in increasea contrast; 60 seconds exposure to ultraviolet radiation gives an optical density of 1.3, with background of 0.15, after 1 hour ambient light exposure.
The concentration of the hexaphenyl-2(! biimidazole is reduced further to 0.12 g in the above coating lacquer. A reduced optical density of 0.7 with background of 0.11 is obtained, after a 60-second exposure to ultraviolet radiation and a 1 hour ambient light exposure.
A coating lacquer in which 2,2'-bis-(o-chlorophenyl)-4,4',5,5'-tetrakis-(m-methoxyphenyl)-biimidazole is employed instead of the hexaphenylbiimidazole No. 3 as aescribed above, requires 0.36 g to achieve an optical density of 1.05, with background of 0.1 and 0.61 g for an image density of 1.1. At the higher concentration, it is not possible to obtain a low background in a positive exposure mode, as a color buildup of 0.41 is observed. A lower background is observed, however, when samples are exposed in ambient light for an hour.
A coating lacquer is prepared by mixing the following ingredients in the amounts indicated:
Coating B
Ingredient Amount (g) Methylene chloride 36.0 10 Isopropanol 4.0 Cellulose acetate butyrate described in Example 13 4.53 Phenanthrene~uinone 0.045 O
CF3(cF2cF2)l7c~2cH2-o-c(cH2)l6cH3 0.038 3-Methoxy-4-octamidophenyl-bis(4-diethylamino-2-methylphenyl) methane 0.088 Trans-3-hydroxy-2-(p-diethylamino-benzyl)indanone 0.038 20 N-ethyl-p-toluenesulfonamide 0.98 Dodecylbenzenesulfonic acid 0.493 Triethanolamine triacetate 1.66 o-Phenylphenol condensed with average of ~2.25 mols ethylene oxide 1.19 Silica gel described in Example 13 0.30 Hexaphenylbiimidazole No. 3, Example 11 0.30 The coating lacquer is coated on Schweitzer 32-HG
paper with a 032 wire-wound rod, and dried with hot air. Exposure to BLB fluorescent lamps in a contact printer, at an irradiance of 6 mw/cm2 through a silver negative generates black imaqes with a density of 1.22. Substantially no color develops when the coating is exposed to 80 foot candles (~860 meter candles) of cool white fluorescent light for 1 hour, but a whitening of the yellow areas is noticed. The Dmax is 2-1; the ODmin is 0.15.
Example 15 This example illustrates a fast photofix composition.
A coating solution is prepared by mixing the following ingredients in the amounts indicated:
Ingredient Amount (g) Methylene chloride 107.0 15 Isopropanol 6.8 Cellulose acetate butyrate described in Example 13 11.3 Pyrenequinone as described in Example 13 0.0113 20 Phenanthrenequinone 0.225 n CF3 (CF2CF2) 17CH2C~2-0-C (CH2) 15C~30 .101 7-Diethylamino-4-methylcoumarin 0.262 25 N-Ethyl-p-toluenesulfonamide 2.63 Dodecylbenzenesulfonic acid 3.375 Triethanolamine triacetate 4.123 o-Phenylphenol condensed with average of ~2.25 mols ethylene oxide 3.0 Silica gel described in Example 14 0.79 Hexaphenylbiimidazole No. 3, Example 11 0.64 3~

1~37348 The solution is applied onto Schweitzer 32-HG paper with a 010 coating bar, at a dry coa~ing weight of 82 mg/dm2. Exposure of the dried coating to ultraviolet radiation from a bank of BLB lamps, with an irradiance of 9 mw/cm2 for 20 seconds produces an image density of 0.9 (reflectance). A positive image results when the coating is exposed through a lithographic negative for 10 seconds in a bank of SDZ
fluorescent lamps, followed by a 5-second exposure to the BL3 lamps. An image density of 0.8, with a background of 0.1 is observed.
A slightly lower image density results when 0.51 g of the hexapnenylbiimidazole is used instead of 0.64 g; the image density for coatings of about 80 mg/dm2 is 0.7.
When 2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetrakis-(m-methoxyphenyl)-biimidazole (control)* is used as photoinitiator, 1.28 g is required to achieve a similar level of performance as is achieved with the lower levels of the hexaphenylbiimidazole described above.

.
* On a molecular weight basis the hexaphenyl-biimidazole of this example is 2.2 times better than the control hexaphenylbiimidazole.

~5 Example 16 This example illustrates the preparation of a heat-fix formulation.
The following ingredients are mixed together in the amounts indicated:
Ingredient Amount (g) Acetone 250.0 Cellulose acetate butyrate described in Example 11 60.0 3,4-Dimethoxyphenyl-bis(4-diethylamino-2-methylphenyl)methane 2.48 p-Toluenesulfonic acid 2.06 Di-t-butyl hydroquinone adduct with dihydropyran, mol wt is 274 2.88 p-Cresol-ethylene oxide adduct, mol wt is ~170 18.0 Hexaphenylbiimidazole No. 3, Example 11 0.40 The formulation is coated with a 020 bar onto Schweitzer Sub 14 paper and dried with air. It is imaged for 20 seconds by exposure through a negative with BLB ultraviolet lamps to give an image density of 1.09. The coated paper is brought into contact with an aluminum block heated to 300F (149C) ~5 for 15 seconds. Subsequent exposure to cool white fluorescent lamps [680 ft. candles (~7320 meter candles) intensity] for 2.75 hours produces no further color, indicating that the photoinitiator is deactivated; background density is 0.05.

`` 1:137348 - ~2 -Example l7 This example illustrates a photopolymerizable composition containing a hexaphenylbiimidazole of the invention as a photoinitiator.
A prepolymer is prepared as follows: to 300 9 of an aliphatic aiisocyanate polymer, specific gravity 1.07, Hyiene~ ~, E. I. du Pont de Nemours and Company, Wilmington, DE stirred under nitrogen, is added 149 g of 2-hydroxyethylmethylmethacrylate and 0.001 g of phenothiazine. Their reaction mixture is heated to 55 & , followed by two hours at 60C.
The prepolym^r is pre~ent in the foliowing mixture:
Inqredient Amount (g) Prepolymer (binder) 10.0 ~exaphenylbiimidazole No. 3, Example 11 0.05 Michler's ketone ~initiator) 0.02 ~0 N-Phenylglycine (hydrogen donor) 0.04 Methylene chloride (solvent) 5.00 The mixture is stirred in the dark, and then is cast on a 0.003 inch (0.076 mm) thic~ polyethylene terephthalate film. After air-drying the coating is covered with a 0.001 inch t0.025 mm) thick polyethylene terephthalate film, and the element formed is exposed to RLB ultravioiet lamps for 60 seconds. A tack-free photopolymer is observed.
A similar coating is made on a glossy paper with a 040 coating bar. It is overlaid with thin polyethylene terephthalate film and i- exposed to give a tack-free coating after 60 seconds.

~137348 _ 43 --Example 18 A solution is prepared as follows:
Ingredient Amount (g) 1. Trimethylolpropane-trimethacrylate (monomer)16.796 2. Monolauryl ether of tetra(oxy-ethylene) glycol (plasticizer)5.599 3. Methylene chloride (solvent)441.600 4. (a) Copolymer of methylmethacrylate and ethylacrylate, mol wt 500,000 determined by gel permeation chromatography, Brookfield viscosity 10% in toluene, 25C, 60 rpm, #1 - spindle is 15 to 45 cps (binder) 25.529 (b) 2,2'-Dihydroxy-4-methoxy-bénzophenone (UV absorber) 0.045 (c) 2-Mercaptobenzothiazole (chain transfer agent) 0.448 5. (a) 2~(Stilbyl-4")-(naphtho-1',4,5)-1,2,3-triazole-2'-sulfonic acid phenyl ester (UV absorber)0.112 (b) 7-(4'-Chloro-6'-diethylamino-1',3',5'-triazine-4'-yl)amino-3-phenyl coumarin (UV absorber)0.546 To a 10 g (1.0 g solids) aliquot of the above solution is added 0.183 g of one of the hexaphenylbiimidazole compounds of Example 11, i.e., 1, 2 or 3. The solutions are coated using a 0.075 mm knife onto sub-coated polyethylene terephthalate film, 0.075 mm in thickness, are air dried for about 10 minutes, and are laminated with a polyethylene terephthalate cover film, 0.025 mm in thi~kness. The photopolymer films having a coating weight of about ` ~ 11373~8 46 mg/dm are each exposed for 2 seconds using the following sources:
(1) 200Q watt pulsed Xenon manufactured by nuArc Company, Chicago, Illinois [17 inches (43.18 cm) from vacuum frame].
(2) A bank of 10 black light blue fluorescent lamps manufactured by GTE Sylvania 18T8/BLB/40/180 maintained 2 inches (5.08 cm) from pressure held frame.
After exposure throuqh a ~ step wedge image the cover film is removed and colored toner is applied to each. The following results are obtained:
Exposure ~exaphenylbiimidazoleSource Photospeed*

* 3t~r (step wedge) step totally polymerized to step ~otally unpolymerized.
The samples (3) usinq a hexaphenylbiimidazole compound o the invention are on the average about 1.8 and 1.67 faster than the controls (1 and 2) using exposure source 1 and 2, respectively.

Example 19 This example illustrates a high speed imaging system.
A coating solution is prepared by mixing the following ingredients in the amounts indicated:
Ingredient Amount (g) Acetone 65.0 Di-i-propylamine l.Q
10 p-Formylphenoxyacetic acid 0.8 o-Formylphenoxyacetic acid 0.8 Nonylphenoxypoly(etheneoxy)-ethanol 1.0 15 Tricresyl phosphate 1.0 Leuco Crystal Violet (C.I. 42555) 0.4 Benzophenone 0.6 Ethylen~ediamine tetracetic acid 1.0 20 Cellulose acetate butyrate described in Example 13 6.0 l-Phenylpyrazolidine-3-one 0.048 Hexaphenylbiimidazole No. 3, Example 11 0.32 The solution is coated on Schweitzer 32-HG paper with a 0015 coating bar, and is exposed with BLB
ultraviolet lamps or 10 seconds to give a reflectance optical density of 1.25. A 60-second exposure gives an optical density of 1.47. A
30-second exposure to visible light (SD~Z lamps) gives an optical density of 1.36.
Examples 2Q to 22 Three coating solutions are prepared by mixing the following ingredients in the amounts indicated:

Amount (g) IngredientEx.20 Ex.21 Ex.22 Methylene chloride 432.0 530.0 514.0 Isopropanol 48.0 50.0 56.0 Dodecylbenzenesulfonic acid4.31 Tris(p-diethylamino-o-tolyl) methane 1.65 - 2.14 N-Ethyl-p-toluenesulfonamide 17.1 22.1 o-Phenylphenol condensate with average of ~2.25 mols ethyleneoxide(plasticizer)14.1 18.3 32.2 Mixture of ~83% 1,6- and 17%
1,8-pyrenequinones 0.0234 -9~lO-Phenanthrenequinone0.826 0.273 Triethanolamine triacetate11.95 15.5 Silica gel of 9 ~ average particle diameter surface area of 675 meters squared per gram, bulk density ~465 kg/m3 3.60 6.3 Cellulose acetate butyrate as described in Example 1362.4 80.0 n CF3(CF2CF2)17CH2c~2-O-cl H3C(H2C)160.12 - -p-Toluenesulfonic acid - 3.23 2.24 3-Methoxy-4-octamido-phenyl-bis(4-diethyl-amino-2-methylphenyl) methane - 2.01 Trans-3-hydroxy-2-(p-diethylaminobenzyl)-indanone - 0.90 Cellulose acetate butyrate described in Example 11 - - 92.0 ,. , 1137;1t48 Ali~uots, 15. 4 g for ~xample 20, 15.15 g for Example 21 and 13.2 g for E~ample 22, are ta~en and to each aliquot is added 1.2 x 10 4 mol of a hexaphenylbiimidazole set forth ~elow in Table 8.
After stirring for at least 30 minutes, the solutions are coated on Schweit2er 32-~G paper or 0.002 inch thick (0.05 mm) pol~ethylene terephthalate film with a 032 wire-wound rod. The coatings are air-dried overnight, and are exposed and Examples 20 and 21 formulations are fixed. Example 22 formulation, coating weights are the same as corresponding formulation of Example but are not photodeactivated.
In Table 8 the exposure sources are:
Examples 2C and 21 8LB fluorescent lamps (~LB max) through a step tablet for 90 seconds with an irradiance of about 8 mw/cm , followed by a 90-second photodeactivation exposure using GTE Sylvania lamps which peak at 440-450 nm. Positive images (Pos. Im.) are prepared by initially exposing through a litho negative image for 90 seconds with the GTE Sylvania lamps followed by a 10-second exposure to the 3LB
lamps after removing the negative.
Example 22 (a) BLB fluorescent lamps through a step tablet for 90 seconds with an irradiance of about 8 mw/cm2;
(b) SDZ fluorescent lamps, directly for 60 seconds with an irradiance of abou~ 3 mw/cm2;
(c) as in (b) but an interposed weatherable polyethylene terephthalate film is placed between the exposure source and the photosensitive coating.

~137348 The density readings are measured with a MacBeth Quantalo~ densitometer for a visual density reading (reflectance mode).
For the film coatings a white Morest~ card as used for pigmPnt drawdown is employed under the coated films. The steps column in Table 8 refers to the maximum number of steps totally polymerized.

Table 8 Blue (Ex. 20) Dimer Ctg. BLB
(See Wt. Max Pos. Im.
Tables Mols (m~/ Re- Dmax/Dmin S & 6) X103 dm ) flect Steps Reflect 1 0.119 157 1.32 21 1.53/0.12 2 0.120 156 1.21 22 4 0.118 145 1.35 21 1.46/0.18 0.120 184 1.32 21 6 0.118 16~ 1.35 22 7 0.120 185 1.20 20 1.25/0.12 8 0.120 151 1.41 23 1.28/0.13 9 0.119 169 1.24 22 0.100 146 1.21 20 1.24/0.1 25 Control 1 (Example

11) 0.118 164 0.88 19 Control 2 (Example 11) ~.118 210 0.99 15 ~.97/0.14 Table 8 (~ontinued) Black (Ex. 21) Dimer Ctg. BLB
(See Wt. Max Pos. Im.
5 Tables Mols (mg/ ~e- ~max/Dmin 5 & 6) X103 dm2) flect Steps Reflect 1 0.119 133 1.29 21 1.32/0.12 2 0.120 131 1.08 2i 1.17/0.11 4 0.118 141 1.34 25 1.30/0.16 0.120 133 1.32 22 1.29/0.14 6 0.118 149 1.12 22 7 0.120 136 1.21 20 i.23/0.11 8 0.120 185 1.08 20 9 O.llg 138 1.03 20 15 10 0.100 165 0.77 20 Control 1 (Example 11) 0.118 131 1.21 17 1.20/0.17 Control 2 (Example 20 11) 0.118 140 0.53 11 0.51/0.12 li37;~48 Table 8 (continued) Blue - No Fix (Ex. 22) BLB
Dimer (a) 5 (See Max SDZ SDZ
Tables Mols Re- (b) (c) 5 ~ 6) X103 flect Steps Reflect Reflect .

1 0.119 1.60 24 1.52 0.73 2 0.120 1.58 24 1.59 0.76 3 0.120 L.72 24 1.30 Q.53 4 0.118 1.50 25 1.46 0.72 0.120 1.70 25 1.60 0.71 6 0.118 1.49 23 1.35 0.58 7 0.120 1.48 24 1.44 0.57 8 0.120 1.51 23 1.31 0.77 9 0.119 1.50 23 1.35 0.47 0.100 1.44 25 1.40 0.66 Control 1 (Example 11) 0.118 1.72 24 1.13 0.33 Control 2 (Example 11) 0.118 1.65 20 1.19 0.41 Example 23 The following stock solution is prepared:
Ingredient Amount ~g) 2-~Stilbyl-4")-(naphtho-1',4,5)-1,2,3-triazole-2'-sulfonic acid Phenyl ester (UV a~sorber) 0.78 7-(4'-Chloro-6'-diethylamino-1',3',5'-triazine-4'-yl)amino-3-phenyl coumarin (W absorber) 3.88 Trimethylolpropanetrimethacrylate139.50 Polyoxyethylated trimethylol propane ~riacrylate, mol wt 1000-120031.00 Monolauryl ether of tetra(oxy-ethylene)glycol (plasticizer)31.00 Polymethylmethacrylate, Very high mol wt, Inherent Viscosity 1.37 determined in 0.25 g of CHC13, 25C using a No. 50 Cannon-Fenske Viscometer 127.10 Polyvinyl acetate, ~86 monomer units chain length 49.60 2,2'-Dihydroxy-4-methoxy-benzophenone 0.31 Hydroquinone 0.20 2-Mercaptobenzoxazole 2.79 Methylene chloride 3476.00 :

To 38.6 g aliquots of this solution are added 9.4 x 10 5 mol of each of the photoinitiators listed in Table 6 (Controls as solids, Dimer 1 as 31% solution in methyl ethyl ketone, the rest as 15% solutions in me~hylene chloride). The solutions are stirred and are coated on polyethylene terephthalate supports using a 0.003 inch (0.076 mm) knife to yield a coating 0.0003 inch (0.0076 mm) in thickness. After air drying for about 10 minutes, a po7ypropylene cover sheet 0.00075 inch (0.019 mm) in thickness is applied using a squeegee. The coated films are exposed simultaneously through a ~ step tablet to each o~ two radiation sources:
(1) 2000-watt pulsed xenon lamp in a lS nuArc~ flip-top-exposure frame, 20 seconds, 17 inches (43.18 cm) from the films in the pressure frame.
(2) Series of SDZ fluorescent lamps with an output of 450 ft.-candles (4840 meter-candles), Ç0 seconds, 2 inches (5.08 cm) from films in the pressure frame.
After exposure the cover sheets are removed and a colored toner is applied by dusting each exposed film. The following results are obtained, Table 9 illustrates radiztion source (1) and Table 10 radiation source (2).

` ~37348 Table 9 Dimer ~See Tables 5 and 6) Photospeed* Photospeed**
1 7 to 10 1.63 2 7 to 12 3 . O
3 5 to 9 1.13 4 7 to 1~ 1.63 7 to 11 1.79 6 7 to 11 1.79 7 7 to 9 1.50 8 8 to 11 2.05 9 6 to 10 1. 42 6 to 9 1.29 Control 1, Example 11 5 to 11 Control 2, Example 11 4 to 9 1.00 . _ * Step totally polymerized to step totally unpolymerized ** Speed factor relative to Control 2 which is 1.0; speed factor equals [lantilog [(step no.
experimental - step no. control 2) left value times log ( ~ )] ~ antilog [(step no.
experimental - step no. Control 2) right value x log ~ )]] divided by 2.

.! 53 -` 1137348 Table 10 Dimer (See Tables 5 and 6) Photospeed* Photospeed**
1 12 to 14 1.13 2 13 to 15 1.42 3 11 to 14 1.00 4 13 to 15 1.42

12 to 15 1.26 6 14 to 16 1.79 7 13 to 15 1.42 8 14 to 16 1.79 9 13 to 15 1.42 12 to 15 1.26 1 Control 1, Example 11 11 to 14 Control 2, Example 11 11 to 14 1.00 . _ * Step totally polymerized to step totally unpolymerized ** Speed factor relative to Control 2 which is 1.0 Speed factor is determined by rormula beneath Table 9.

`- 1137348 Example 24 The following stock solution is prepared:
Ingredient Amount (g) ~ichler's ketone 5.0 5 Trimethylolpropane triacrylate 145.0 Triethyleneglycol diacetate 20.0 Tri~resyl phosphate 20.0 Dioctylphthalate 20.0 Peroxide-initiated polymethyl-1~ methacrylate, high mol wt, inherent viscosity 1.37 determined in 0.25 g of CHC13, 25C using a No. 50 Cannon-Fenske Viscometer 20.0 Peroxide-initiated polymethyl-methacrylate, low mol wt, inherent viscosity 0.21 determined in 0.25 g of CHC13, 25C using a No. 50 Cannon-Fenske Viscometer 145.0 Victoria Pure Blue BO (CI 42595) 0.15 Benzotriazole 1.0 Tris-(p-diethylamino-o-tolyl)methane 1.5 20 Tris-(p-dimethylaminophenyl)-methane 0.5 Methylene chloride 2709.0 To 31.9 aliquots of this solution are added 3.03 x 10 4 mol of each of the photoinitiators listed in Table 6 (Controls as solids~ Dimer 1 as 31% solution in methyl ethyl ketone, the rest as 15% solutions in methylene chloride). The solutions are stirred and are coated on polyethylene terephthalate films using a 0.010 inch (0.25 mm) knife to yield a coating O.OOlS inch (0.038 mm) in thickness. After air drying for 30 minutes, the films are laminated onto ~137348 precleaned copper circuit boards at 40 psi (2.81 kg/sq cm) pressure and 220F (105C). The coated films are exposed for 30 units (about 90 seconds) through a 21 step ~ step tablet on a Berkey-Ascor unit (to a 2kw Addalux~ mercury photopolymer type lamp). After the exposure, the polyethylene terephthalate films are removed and the unexposed areas are washed out during a one-minute development cycle in a Riston~ processor usin~
chlorothene. The following results are obtained:

Table 11 Dimer (See Tables 5 and 6) Photospeed* Photospeed**
1 6 to 8 1.71 2 6 to 8 1.71 3 6 to 7 1.41 4 7 to 8 2.0 6 to 8 1.71 6 5 to 6 1.0 7 5 to 6.5 1.09 8 7 to 8 2.0 9 6 to 7 1.41 ` 6 to 8 1.71 Control 1, Example 11 3 to 5 Control 2, Example 11 5 to 6 1.00 . . . _ * Step totally polymerized to step totally unpolymerized.
** Speed factor relative to Control 2 which is 1Ø
Speed factor is determined bv formula beneath Table 9 except that log (3~ 2) is replaced by log 2) li37348 It is known that imiàazolyl radicals exhibit unique signals when examined in an esr spectrometer;
diminution of the signals indicates disappearance of the raaical to give the starting dimer. It is further known that many biimidazoie compounds on pnotolysis give rise to colored free radicais. In the absence of exciting radiation, tnese radicals recombine to give the starting dimer. This aimerization process may be speeded up by heat or increased plasticization of the medium. Techniques for measuring absorption spectra of the radicals or colorometric measurements of films or treated papers may thus be employed to make comparisons in the lifetime of imidazolyl radicals.
The following ~xamples illustrate the increasea radical lifetime of radicals formed from imidazolyl dimers of this invention.
ExamPle 25 Electron Spin Resonance (esr) Measurements (A) Solutions of 0.01 molarity in methylene chloride are made with Control 1 (Example 11) and hexaphen~Lbiimidazole No. 3, Example 11. These are inserted in 3.0 mm (ID) Pyrex~ tubes inside the cavity of a Bruker~ BR420 esr spectrometer, and are exposed to several flashes with an eiectronic flashgun (Exposure Source 3 of Table 7). The signal generated by the ra~ical is recorded over a period of time, and disappears completely after 200 seconds for the control. Signal strength of one-third the original is detected after more than 2000 seconds for hexaphenylbiimidazole NO. 3, Example 11, a lifetime of over 10 times greater in solution for the radicals derived from the hexaphenylbiimidazole of this invention.
(B) Films are prepared by coating a lacquer containing 0.96 g cellulose ace~ate butyrate described in the photosensitive compositions of Examples 1 to iO, 0.15 g hexaphenylbiimidazole, and 8.0 g methylene chioride onto a polyethylene terephthalate film using a 032 coating bar. The film weight is approximately 0.056 g/dm2. A film made with hexaphenylbiimiaazole No. 3, Example 11, is - inserted in a 4.0 mm (ID) Pyrex~ tube, and is exposed for several minutes with uitraviolet light. The sample is placed in the cavity of the above-described esr instrument, and the signal is recorded. After 66 hours the sample is scannea again. It retains 40% of the originai signal strength. A similarly exposed film made with Controi 1 (Example 11) shows a signal initially but shows no signal after 16 hours, indicating appreciably longer radical lifetime in films prepared containlng the hexaphenylbiimidazole of this invention.
The solution (A) containing hexaphenylbiimidazole No. 3, Example 11, shows a single esr absorption 7 gauss wide with G-factor of 2.0028. ~his signal is also observed in irradiated films (B) except that a slightly broader line width of 8 gauss is ~ound.
ExamPle 26 (A) StabllitY of Radicals aY Cptical Measurements Films are prepared as described in Example 25 (B) except that the below listed hexaphenylbiimidazole compounds are used. The films are scanned in a Cary Model 17 spectrophotometer from 700 nm to 300 nm. Samples are then irradiated wit~
ultraviolet light from B~B iamps for 110 seconds.
Exposed samples are tnen returned to the spectrophotometer for additional scanning over the same wavelength range. The scanning is repeated after 22 hours. Approximately the same length of time is employed in handling each sample. Scanning from 700 to 300 nm takes about 4 minutes.
A wavelength is selected at which there is a maximum difference between the exposed and unexposed film sample. The 22-hour old sample shows a decrease in optical density at that wavelength. The decrease in radical concentration is determined according to the formula:
Loss in radical ODIRR-OD22 concentration = ODIRR-ODUN
~R-) where ODIRR is the optical density of the film after irradiation, OD22 is the optical density of the film after 22 hours, and O~uN is the optical density of the unirradiated film.
Wavelength Loss Sample ~nm~ Formula (%) Control 1, 380 0.29 - 0.23 100 Example 11 0.~9 - 0.23 Control 2, 360 0.38 - 0.23 75 Example 11 0.38 - G.13 Compound 10, 460 0.215 - 0.12 63 Tables 5 and 6 0.215 - 0.065 Compouna 3, 400 0.75 - 0.44 57 Tables 5 and 6 0.75 - 0.21 Compound 1, 400 0.90 - 0.55 52 Tables ; and 6 0.90 - 0.23 113734~?

These data show that the compounds of the invention have longer radical life versus the controls. The differences between tne controls and hexaphenylbiimidazole compounds of this invention indicate that electronic effects, arising from substituents, as well as steric factors which may be present, as shown b~ 4-phenyl ortho-substituted compounds of this invention influence radical li~e.
(B) Colorometric Measurements Sampies of the above films are irradiated ror 9~ seconds with a light source containing BLB
lamps, with an irradiance of about 9mw/cm2.
Reflectance measurements are made on the films prior to exposure, immediately after exposure, 2 hours after exposure and 67 hours later. Reflectance readings are made with the film over a black tile, using a Photomatch~ 300 colorimeter; the readings are ~L (lighter/darker), aa (redder/greener) and ~b (yellower/bluer). The values for~ set forth in Table 12 below are derived from the square root of the sum of the squares ( ~L2 + ~a2 + ~b2) and give an approximation of the overall change of color of the film sample. It is observed that the control hexaphenylbiimidazoles form relatively little color after exposure which disappeared after 2 hours. A'l film samples of this invention retained color longer. This confirms that the hexaphenylbiimidazole compounds of this invention have longer radical life.

- il37348 Table 12 Chan~ein~E Values Film 5 Sample (see 2 Hrs. 67 Hrs.
Tables Before After After After 5 and 6) Exposure ExPosure ExPosure Exposure i 1.5 6.2 5.1 5.1 2 1.3 5.0 3.3 3.1 3 0.~ 5.0 2.3 1.8 4 1.4 6.1 4.4 4.4 1.6 5.7 3.7 2.8 6 1.7 3.1 2.6 2.6 7 1.8 5.2 3.3 2.8 8 1.6 5.3 4.3 4.3 9 1.5 5.1 3.5 3.5 1.3 6.0 4.7 4.7 Control 1, 1.1 1.9 1.1 --Example 11 Control 2, 1.5 2.3 1.0 --Example li (C) Radical Recombination By Heating A 15% solution of hexaphenylbiimidazole No.
3, Example 11, in methylene chloride is spotted on filter paper, and after color formation by ultraviole~ irradiation, a portion of the colored pattern is placed in an oven set at 85-90C.
Initially, before exposure, optical density is 0.38; after a 90 second exposure the optical density is 1.40.

li373~8 - ~2 -Room Temp. Oven ~eated Time (O.D.) (O.D.) 1 hour i.37 1.13 1.75 hours 1.34 1.06 2 hours 1.33 1.02 20 hours 1.20 0.64 44 hours 1.16 0.56 This shows that tAe ra~icals formed by the exposure to uitraviolet light can be made to recombine at eievated temperatures over an extended period.
Reexposure of the heated sample to uitraviolet light regeneratesthe color to an optical density of 1.22.

Claims (16)

63

1. Photoimaging composition comprising an admixture of (A) a 2,4,5-triphenylimidazolyl dimer of the formula wherein R1 is 2-bromo, 2-chloro, 2-fluoro, 2-alkyl of 1 to 4 carbon atoms and 2,4-dichloro;
R2 is 2-bromo, 2-chloro, 2-fluoro, 4-chloro, 2-alkyl of 1 to 4 carbon atoms, 2-cyano, and 2-alkoxy wherein the alkyl radical is of 1 to 4 carbon atoms;
and R3 is 3,4-dimethoxy, 3,4-diethoxy, 2,3-dimethoxy, 2,4,6-trimethoxy, 4-alkoxy wherein the alkyl radical is of 1 to 4 carbon atoms and 3,4-methylenedioxy; the imidazolyl dimer having an extinction coefficient determined in methylene chloride at 10-5 to 10-3 mol/liter at 350 nm of at least 4000 liters/mol-cm and at 400 nm of at least 250 liters/mol-cm; and at least one compound taken from group consisting of (B1) a leuco dye that is oxidizable to dye by the imidazolyl radicals; and (B2) an addition polymerizable ethylenically unsaturated monomeric compound.

2. A photoimaging composition according to Claim 1 having present at least one leuco dye which has one to two removable hydrogens, the removal of which forms a differently colored compound, with the proviso that when the leuco form has only one removable hydrogen and the resultant dye is cationic, there is also present a mineral acid, organic acid or acid-supplying compound which forms a salt with the leuco form of the dye.

3. A photoimaging composition according to Claim 2 wherein the leuco dye is the salt of an acid in leuco form of a triphenylmethane dye having, in at least two of the phenyl rings positioned para to the methane carbon atom, a substituent selected from the group consisting of amino, and C1 to C4 dialkyl amino groups, the acid being a mineral acid, an organic acid, or an acid-supplying compound.

4. A photoimaging composition according to Claim 2 wherein (C) a redox couple of a photoactivatable oxidant component and a reductant component is present in the admixture.

5. A photoimaging composition according to Claim 4 wherein the photoactivatable oxidant component is a polynuclear quinone absorbing principally in the 430 nm to 550 nm region.

6. A photoimaging composition according to Claim 4 wherein the reductant component is an acyl ester of triethanolamine of the formula where R is alkyl of 1 to 4 carbon atoms.

7. A photoimaging composition according to Claim 4 wherein (D) a film-forming polymeric binder is present.

8. A photoimaging composition according to Claim 7 wherein the polymeric binder is cellulose acetate butyrate.

9. A photoimaging composition according to Claim 1 wherein the 2,4,5-triarylimidazolyl compound is 2,2',4,4'-tetrakis(o-chloro-phenyl)-5,5'-bis-(m,p-dimethoxyphenyl)-biimidazole dimer.

10. A photopolymerizable composition according to Claim 1 having present at least one addition polymerizable ethylenically unsaturated compound (B2) having at least one polymerizable ethylenic group.

11. A photopolymerizable composition according to Claim 10 wherein (C) at least one agent selected from the group consisting of a free radical producing hydrogen donor agent and an active methylene compound is present.

12. A photopolymerizable composition according to Claim 11 wherein the agent is a leuco dye.

13. A photopolymerizable composition according to Claim 11 wherein (D) a film-forming polymeric binder is present.

14. A photopolymerizable composition according to Claim 13 wherein the polymeric binder is a methylmethacrylate containing polymer or copolymer.

15. A photopolymerizable composition according to Claim 13 wherein (E) a chain transfer agent is present.

16. A photopolymerizable composition according to Claim 15 wherein the chain transfer agent is 2-mercaptobenzothiazole.