Patent Publication Number: US-5298390-A

Title: Speed enhancers and stabilizers for photothermography

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to photothermographic materials and in particular to speed enhancing and post-processing stabilization of photothermographic silver-containing materials. 
     2. Background of the Art 
     Silver halide containing photothermographic imaging materials processed with heat, and without liquid development have been known in the art for many years. These materials, also known as dry silver materials, generally comprise a support having thereon a photographic light-sensitive silver halide, a light-insensitive organic silver salt, and a reducing agent for the organic silver salt. 
     The light-sensitive silver halide is in catalytic proximity to the light-insensitive organic silver salt so that the latent image, formed by irradiation of the silver halide, serves as a catalyst nucleus for the oxidation-reduction reaction of the organic silver salt with the reducing agent when the emulsion is heated above about 80° C. Such media are described, for example, in U.S. Pat. Nos. 3,457,075; 3,839,049; and 4,260,677. The silver halide may also be generated in the media by a preheating step in which halide ion is released to form silver halide. 
     A variety of ingredients may be added to these basic components to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732 and 4,021,249. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are known and described in U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure, March 1989, item 29963. 
     A common problem that exists with photothermographic systems is post-processing instability of the image. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver during room light handling or exposure to heat or humidity. Thus, there exists a need for stabilization of the unreacted silver halide. The addition of separate post-processing image stabilizers has been used to impart post-processing stability. Most often these are sulfur containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure, June 1978, item 17029. U.S. Pat. Nos. 4,245,033; 4,837,141; and 4,451,561 describe sulfur compounds that are development restrainers for photothermographic systems. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles such as 3-amino-5-benzothio-1,2,4-triazole as post-processing stabilizers are described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure, May 1978, items 16977 and 16979. U.S. Pat. No. 5,158,866 describes the use of omega-substituted 2-propionamidoacetyl or 3-propionamidopropionyl stabilizer precursors as post-processing stabilizers in photothermographic emulsions. U.S. Pat. No. 5,175,081 describes the use of certain azlactones as stabilizers. 
     Problems arising from the addition of stabilizers may include thermal fogging during processing and losses in photographic speed, maximum density or contrast at effective stabilizer concentrations. 
     Stabilizer precursors are materials which have blocking or modifying groups that are usually cleaved during processing with heat and/or alkali. The cleaving provides the primary active stabilizer which can combine with the photoactive silver halide in the unexposed and undeveloped areas of the photographic material. For example, in the presence of a stabilizer precursor in which a sulfur atom is unblocked upon processing, the resulting silver mercaptide will be more stable than the silver halide to light, atmospheric, and ambient conditions. 
     Various blocking techniques have been utilized in developing the stabilizer precursors. U.S. Pat. No. 3,615,617 describes acyl blocked photographically useful stabilizers. U.S. Pat. Nos. 3,674,478 and 3,993,661 describe hydroxyarylmethyl blocking groups. Benzylthio releasing groups are described in U.S. Pat. No. 3,698,898. Thiocarbonate blocking groups are described in U.S. Pat. No. 3,791,830, and thioether blocking groups in U.S. Pat. Nos. 4,335,200; 4,416,977; and 4,420,554. Photographically useful stabilizers which are blocked as urea or thiourea derivatives are described in U.S. Pat. No. 4,310,612. Blocked imidomethyl derivatives are described in U.S. Pat. No. 4,350,752, and imide or thioimide derivatives are described in U.S. Pat. No. 4,888,268. Removal of all of these aforementioned blocking groups from the photographically useful stabilizers is accomplished by an increase of pH during alkaline processing conditions of the exposed imaging material. 
     Thermally sensitive blocking groups are also known. These blocking groups are removed by heating the imaging material during processing. Photographically useful stabilizers blocked as thermally sensitive carbamate derivatives are described in U.S. Pat. Nos. 3,844,797 and 4,144,072. These carbamate derivatives presumably regenerate the photographic stabilizer through loss of an isocyanate. Hydroxymethyl blocked photographic reagents which are unblocked through loss of formaldehyde during heating are described in U.S. Pat. No. 4,510,236. Development inhibitor releasing couplers releasing tetrazolyolthio moieties are described in U.S. Pat. No. 3,700,457. Substituted benzylthio releasing groups are described in U.S. Pat. No. 4,678,735. U.S. Pat. Nos. 4,351,896 and 4,404,390 utilize carboxybenzylthio blocking groups for mesoionic 1,2,4-triazolium-3-thiolate stabilizers. Photographic stabilizers that are blocked by a Michael-type addition to the carbon-carbon double bond of either acrylonitrile or alkyl acrylates are described in U.S. Pat. Nos. 4,009,029 and 4,511,644, respectively. Heating of these blocked derivatives causes unblocking by a retro-Michael reaction. 
     Various disadvantages attend these different blocking techniques. Highly basic solutions necessary to cause deblocking of the alkali sensitive blocked derivatives are corrosive and irritating to the skin. With photographic stabilizers that are blocked with a heat removable group, it is often found that the liberated reagent or by-product can react with other components of the imaging construction and cause adverse effects. Also, inadequate or premature release of the stabilizing moiety during heat processing may occur. 
     There has been a continued need for improved post-processing stabilizers or stabilizer precursors that do not fog or desensitize photothermographic materials, and for stabilizer precursors that release the stabilizing moiety at the appropriate time and do not have any detrimental effects on the photosensitive material or user of said material. 
     Photolytically active stabilizer precursors for photothermographic silver imaging compositions which apparently release bromine atoms are described in U.S. Pat. No. 4,459,350. 
     U.S. Pat. No. 4,207,108 describes the use of thione compounds as a photographic speed enhancing additive, U.S. Pat. No. 4,873,184 describes the use of metal chelating agents to enhance speed in silver halide systems, and U.S. Pat. No. 4,264,725 describes the use of benzyl alcohol and 2-phenoxyethanol as speed enhancing solvents for photothermographic materials. 
     Stabilizer precursors of this type can be added to photothermographic formulations without the necessity of rebalancing the formulation to compensate for effects on sensitometry, as is often the case with other stabilizers in the art. 
     SUMMARY OF THE INVENTION 
     In one aspect this invention relates to photothermographic articles comprising a photothermographic composition coated on a substrate wherein the photothermographic composition comprises a photographic silver salt, an organic silver salt, and a reducing agent for the organic silver salt, and a stabilizer having a central nucleus of the formula: 
     
         A--(CH.sub.2).sub.n --X--(CH.sub.2).sub.n --A 
    
     A represents any monovalent group for which the corresponding compound AH functions as a post-processing stabilizer having from 1 to 50 carbon atoms, and 
     X represents --S--, --SO-- or --SO 2  --. The A groups may of course independently bear substituents that are photographically inert or physically useful (e.g., solubilizing, ballasting, etc.) and may be independently represented by a group R, for example selected from hydrogen, alkyl, alkoxycarbonyl, alkenyl, aryl, hydroxy, mercapto, amino, amido, thioamido, carbamoyl, thiocarbamoyl, cyano, nitro, sulfo, carboxyl, fluoro, formyl, sulfoxyl, sulfonyl, hydrodithio, ammonio, phosphonio, silyl, and silyloxy groups having up to 18 carbon atoms, and wherein any two or three R groups such as R 1 , R 2 , and R 3  may together form a fused ring structure with any central benzene ring, and 
     n is 0 or 1-10 and may be equal on both sides of the molecule or assymmetrical. 
     The preferred blocked derivatives of heterocyclic compounds that stabilize silver images according to the present invention are those where n is 1 or 2 and are symmetrical compounds. They typically comprise from about 0.01 wt % to 10 wt % of the dry photothermographic composition. They may be incorporated directly into the silver containing layer or into an adjacent layer. The stabilizers of the invention are especially useful in articles and compositions for the preparation of photothermographic color and black-and-white images. 
     The stabilizers of the present invention stabilize silver halide and/or minimize untimely leuco oxidation for improved post-processing stabilization without desensitization or fogging during heat processing. 
     The stabilizers of this invention are believed to be deblocked to release the parent stabilizer by the action of heat and therefore offer advantages over unprotected stabilizers and stabilizers released by other mechanisms by being inert and inactive during the processing step, and being resistant to thermal release during shelf aging. They are only released when they are needed. They are useful in a wide range of photothermographic media and processing conditions, since they do not appear to have specific requirements for release that attend most other masking groups. 
     Whether or not specifically describing substituents, substitution is anticipated on the compounds of the present invention. Where the term &#34;group&#34; or &#34;nucleus&#34; is used, these terms include the use of additional substituents beyond the literal definition of alkyl or the nucleus. For example, alkyl group includes ether groups (e.g., CH 3  --CH 2  --CH 2  --O--CH 2  --), haloalkyls, nitroalkyls, carboxyalkyls, hydroxyalkyls, sulfoalkyls, etc., while the term &#34;alkyl&#34; or &#34;alkyl radical&#34; includes only hydrocarbons. Substituents which react with active ingredients, such as very strongly electrophilic or oxidizing substituents, would of course be excluded as not being inert or harmless. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Photothermographic articles of the present invention comprise a photothermographic composition coated on a substrate wherein the photothermographic construction comprises a photographic silver salt, an organic silver salt, a reducing agent for the organic silver salt, and a stabilizer having the formula: 
     
         A--(CH.sub.2).sub.n --X--(CH.sub.2).sub.n --A 
    
     
         wherein: 
    
     A represents any monovalent group for which the corresponding compound AH functions as a post-processing stabilizer having from 1 to 50 carbon atoms. 
     X represents --S--, --SO--, or --SO 2  --, 
     n is 0 or 1-10. Preferably n is 1 or 2. 
     In photothermographic articles of the present invention the layer(s) that contain the photographic silver salt are referred to herein as emulsion layer(s). According to the present invention the blocked stabilizer is added either to one or more emulsion layers or to a layer or layers adjacent to one or more emulsion layers. Layers that are adjacent to emulsion layers may be for example, primer layers, image-receiving layers, interlayers, opacifying layers, antihalation layers, barrier layers, auxiliary layers, etc. 
     The bridging group acts as a blocking group to block the activity of the primary stabilizer AH. If AH is left unblocked and added to the photothermographic emulsion at the same molar equivalent concentration as the blocked compound, AH desensitizes or fogs the emulsion. Deblocking to release the active stabilizer occurs after exposure and development at elevated temperatures. Thus, the blocked stabilizers of the present invention overcome the problems of desensitization and fogging that occur when the stabilizers are used in their unblocked form. 
     A is preferably attached to the blocking group through a nitrogen atom. Post-processing stabilizing groups for stabilizing silver ion AH usually have a heteroatom such as nitrogen available for complexing silver ion. The compounds are usually ring structures with the heteroatom within the ring or external to the ring. These compounds are well known to one ordinarily skilled in the photographic art. Non-limiting examples of AH include nitrogen containing heterocycles, substituted or unsubstituted, including but not limited to, imidazoles such as benzimidazole and benzimidazole derivatives; triazoles such as benzotriazole, 1,2,4-triazole, 3-amino-1,2,4-triazole, and 2-thioalkyl-5-phenyl-1,2,4-triazoles; tetrazoles such as 5-amino tetrazole and phenylmercaptotetrazole; triazines such as mercaptotetrahydrotriazine; piperidones; tetraazaindans; 8-azaguanine; thymine; thiazolines such as 2-amino-2-thiazoline, indazoles; hypoxanthines; pyrazolidinones 2H-pyridooxazin-3(4H)-one and other nitrogen containing heterocycles; or any such compound that stabilizes the emulsion layer, and particularly those that have deleterious effects on the initial sensitometry or excessive fog if used unblocked. 
     Many of such compounds are summarized in Research Disclosure, March 1989, item 29963. AH may also be a compound which stabilizes a leuco dye, usually a reducing agent which has an active hydrogen. An example of a useful reducing agent is 1-phenyl-3-pyrazolidinone (described in U.S. Pat. No. 4,423,139 for stabilizing leuco dyes). Masking of such reducing agents during the processing step is usually necessary since they may act as developers or development accelerators to cause unacceptable fogging. 
     In another preferred embodiment of the invention, AH is a post-processing stabilizer identified to be most advantageous for a given photothermographic construction; for instance, 1-phenyl-3-pyrazolidinone, benzotriazole, or 3-(n-hexylthio)-5-phenyl-1,2,4-triazole. 
     Non-limiting, representative examples of A portions of post-processing stabilizers AH are shown below. ##STR1## 
     Photothermographic articles of the invention may contain other post-processing stabilizers or stabilizer precursors in combination with the compounds of the invention, as well as other additives in combination with the compound of the invention such as shelf-life stabilizers, toners, development accelerators and other image modifying agents. 
     The amounts of the above described stabilizer ingredients that are added to the emulsion layer according to the present invention may be varied depending upon the particular compound used and upon the type of emulsion layer (i.e., black and white or color). However, the ingredients are preferably added in an amount of 0.01 to 100 mol, per mol of silver halide and more preferably from 0.1 to 50 mol per mol of silver halide in the emulsion layer. 
     The photothermographic dry silver emulsions of this invention may be constructed of one or more layers on a substrate. Single layer constructions must contain the silver source material, the silver halide, the developer and binder as well as optional additional materials such as toners, coating aids, and other adjuvants. Two-layer constructions must contain the silver source and silver halide in one emulsion layer (usually the layer adjacent to the substrate) and some of the other ingredients in the second layer or both layers, although two layer constructions comprising a single emulsion layer containing all the ingredients and a protective topcoat are envisioned. Multicolor photothermographic dry silver constructions may contain sets of these bilayers for each color, or they may contain all ingredients within a single layer as described in U.S. Pat. No. 4,708,928. In the case of multilayer multicolor photothermographic articles the various emulsion layers are generally maintained distinct from each other by the use of functional or non-functional barrier layers between the various photosensitive layers as described in U.S. Pat. No. 4,460,681. 
     While not necessary for practice of the present invention, it may be advantageous to add mercury (II) salts to the emulsion layer(s) as an antifoggant. Preferred mercury (II) salts for this purpose are mercuric acetate and mercuric bromide. 
     The light sensitive silver halide used in the present invention may typically be employed in a range of 0.75 to 25 mol percent and, preferably, from 2 to 20 mol percent of organic silver salt. 
     The silver halide may be any photosensitive silver halide such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, etc. The silver halide may be in any form which is photosensitive including, but not limited to cubic, orthorhombic, tabular, tetrahedral, etc., and may have epitaxial growth of crystals thereon. 
     The silver halide used in the present invention may be employed without modification. However, it may be chemically sensitized with a chemical sensitizing agent such as a compound containing sulfur, selenium or tellurium etc., or a compound containing gold, platinum, palladium, rhodium or iridium, etc., a reducing agent such as a tin halide, etc., or a combination thereof. The details of these procedures are described in T. N. James The Theory of the Photographic Process, Fourth Edition, Chapter 5, pages 149 to 169. 
     The silver halide may be added to the emulsion layer in any fashion which places it in catalytic proximity to the silver source. Silver halide and the organic silver salt which are separately formed or &#34;preformed&#34; in a binder can be mixed prior to use to prepare a coating solution, but it is also effective to blend both of them in a ball mill for a long period of time. Further, it is effective to use a process which comprises adding a halogen-containing compound in the organic silver salt prepared to partially convert the silver of the organic silver salt to silver halide. 
     Methods of preparing these silver halide and organic silver salts and manners of blending them are known in the art and described in Research Disclosure, June 1978, item 17029, and U.S. Pat. No. 3,700,458. 
     The organic silver salt may be any organic material which contains a reducible source of silver ions. Silver salts of organic acids, particularly long chain (10 to 30 preferably 15 to 28 carbon atoms) fatty carboxylic acids are preferred. Complexes of organic or inorganic silver salts wherein the ligand has a gross stability constant between 4.0 and 10.0 are also desirable. The silver source material should preferably constitute from about 5 to 30 percent by weight of the imaging layer. 
     The use of preformed silver halide emulsions of this invention can be unwashed or washed to remove soluble salts. In the latter case the soluble salts can be removed by chill-setting and leaching or the emulsion can be coagulation washed, e.g., by the procedures described in U.S. Pat. Nos. 2,618,556; 2,614,928; 2,565,418; 3,241,969; and 2,489,341. The silver halide grains may have any crystalline habit including, but not limited to cubic, tetrahedral, orthorhombic, tabular, laminar, platelet, etc. 
     The light-sensitive silver halides may be advantageously spectrally sensitized with various known dyes including cyanine, merocyanine, styryl, hemicyanine, oxonol, hemioxonol and xanthene dyes. Useful cyanine dyes include those having a basic nucleus, such as a thiazoline nucleus, an oxazoline nucleus, a pyrroline nucleus, a pyridine nucleus, an oxazole nucleus, a thiazole nucleus, a selenazole nucleus and an imidazole nucleus. Useful merocyanine dyes which are preferred include those having not only the above described basic nuclei but also acid nuclei, such as a thiohydantoin nucleus, a rhodanine nucleus, an oxazolidinedione nucleus, a thiazolidinedione nucleus, a barbituric acid nucleus, a thiazolinone nucleus, a malononitrile nucleus and a pyrazolone nucleus. In the above described cyanine and merocyanine dyes, those having imino groups or carboxyl groups are particularly effective. Practically, the sensitizing dyes to be used in the present invention may be properly selected from known dyes such as those described in U.S. Pat. Nos. 3,761,279, 3,719,495, and 3,877,943, British Pat Nos. 1,466,201, 1,469,117 and 1,422,057, and can be located in the vicinity of the photocatalyst according to known methods. Spectral sensitizing dyes may be typically used in amounts of about 10 -4  mol to about 1 mol per 1 mol of silver halide. 
     The organic silver salt which can be used in the present invention is a silver salt which is comparatively stable to light, but forms a silver image when heated to 80° C. or higher in the presence of an exposed photocatalyst (such as photographic silver halide) and a reducing agent. 
     Preferred organic silver salts include silver salts of organic compounds having a carboxy group. Non-limiting examples thereof include silver salts of an aliphatic carboxylic acid and a silver salt of an aromatic carboxylic acid. Preferred examples of the silver salts of aliphatic carboxylic acids include silver behenate, silver stearate, silver oleate, silver laurate, silver caproate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartrate, silver linoleate, silver butyrate and silver camphorate, mixtures thereof, etc. Silver salts with a halogen atom or a hydroxyl on the aliphatic carboxylic acid can also be effectively used. Preferred examples of the silver salts of aromatic carboxylic acids and other carboxyl group-containing compounds include silver benzoate, a silver substituted benzoate such as silver 3,5-dihydroxybenzoate, silver o-methylbenzoate, silver m-methylbenzoate, silver p-methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silver p-phenylbenzoate, etc., silver gallate, silver tannate, silver phthalate, silver terephthalate, silver salicylate, silver phenylacetate, silver pyromellitate, a silver salt of 3-carboxymethyl-4-methyl-4-thiazoline-2-thione or the like as described in U.S. Pat. No. 3,785,830, and silver salt of an aliphatic carboxylic acid containing a thioether group as described in U.S. Pat. No. 3,330,663, etc. 
     Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also be used. Preferred examples of these compounds include a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole, a silver salt of 2-mercaptobenzimidazole, a silver salt of 2-mercapto-5-aminothiadiazole, a silver salt of 2-(ethylglycolamido)benzothiazole, a silver salt of thioglycolic acid such as a silver salt of an S-alkyl thioglycolic acid (wherein the alkyl group has from 12 to 22 carbon atoms), a silver salt of a dithiocarboxylic acid such as a silver salt of dithioacetic acid, a silver salt of a thioamide, a silver salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine, a silver salt of mercaptotriazine, a silver salt of 2-mercaptobenzoxazole, a silver salt as described in U.S. Pat. No. 4,123,274, for example, a silver salt of 1,2,4-mercaptothiazole derivative such as a silver salt of 3-amino-5-benzylthio-1,2,4-thiazole, a silver salt of thione compound such as a silver salt of 3-(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione as disclosed in U.S. Pat. No. 3,301,678. 
     Furthermore, a silver salt of a compound containing an imino group may be used. Preferred examples of these compounds include silver salts of benzothiazole and derivatives thereof, for example, silver salts of benzothiazoles such as silver methylbenzotriazolate, etc., silver salt of halogen-substituted benzotriazoles, such as silver 5-chlorobenzotriazolate, etc., silver salts of carboimidobenzotriazole, etc., silver salt of 1,2,4-triazoles or 1H-tetrazoles as described in U.S Pat. No. 4,220,709, silver salts of imidazoles and imidazole derivatives, and the like. Various silver acetylide compounds can also be used, for example, as described in U.S. Pat. Nos. 4,761,361 and 4,775,613. 
     It is also found convenient to use silver half soaps, of which an equimolar blend of silver behenate and behenic acid, prepared by precipitation from aqueous solution of the sodium salt of commercial behenic acid and analyzing about 14.5 percent silver, represents a preferred example. Transparent sheet materials made on transparent film backing require a transparent coating and for this purpose the silver behenate full soap, containing not more than about four or five percent of free behenic acid and analyzing about 25.2 percent silver may be used. 
     The method used for making silver soap dispersions is well known in the art and is disclosed in Research Disclosure, April 1983, item 22812, Research Disclosure, October 1983, item 23419 and U.S. Pat. No. 3,985,565. 
     The reducing agent for the organic silver salt may be any material, preferably organic material, that can reduce silver ion to metallic silver. Conventional photographic developers such as phenidone, hydroquinones, and catechol are useful but hindered phenol reducing agents are preferred. The reducing agent should be present as 1 to 10 percent by weight of the imaging layer. In multilayer constructions, if the reducing agent is added to a layer other than an emulsion layer, slightly higher proportions, of from about 2 to 15 percent tend to be more desirable. 
     A wide range of reducing agents has been disclosed in dry silver systems including amidoximes such as phenylamidoxime, 2-thienylamidoxime and p-phenoxyphenylamidoxime, azines (e.g., 4-hydroxy-3,5-dimethoxybenzaldehydeazine); a combination of aliphatic carboxylic acid aryl hydrazides and ascorbic acid, such as 2,2&#39;-bis(hydroxymethyl)propionyl-β-phenylhydrazide in combination with ascorbic acid; a combination of polyhydroxybenzene and hydroxylamine, a reductone and/or a hydrazine (e.g., a combination of hydroquinone and bis(ethoxyethyl)hydroxylamine, piperidinohexose reductone or formyl-4-methylphenylhydrazine); hydroxamic acids such as phenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, and β-alaninehydroxamic acid; a combination of azines and sulfonamidophenols, (e.g., phenothiazine and 2,6-dichloro-4-benzenesulfonamidophenol); α-cyanophenylacetic acid derivatives such as ethyl-α-cyano-2-methylphenylacetate, ethyl α-cyanophenylacetate; bis-β-naphthols as illustrated by 2,2&#39;-dihydroxy-1,1&#39;-binaphthyl, 6,6&#39;-dibromo-2,2&#39;-dihydroxy-1,1&#39;-binaphthyl, and bis(2-hydroxy-1-naphthyl)methane; a combination of bis-β-naphthol and a 1,3-dihydroxybenzene derivative, (e.g., 2,4-dihydroxybenzophenone or 2,4-dihydroxyacetophenone); 5-pyrazolones such as 3-methyl-1-phenyl-5-pyrazolone; reductones as illustrated by dimethylaminohexose reductone, anhydrodihydroaminohexose reductone, and anhydrodihydropiperidonehexose reductone; sulfonamidophenol reducing agents such as 2,6-dichloro-4-benzenesulfonamidophenol, and p-benzenesulfonamidophenol;  2-phenylindane-1,3-dione and the like; chromans such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman; 1,4-dihydropyridines such as 2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridine; bisphenols (e.g., bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2-t-butyl-6-methylphenol), and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane); ascorbic acid derivatives (e.g., 1-ascorbyl palmitate, ascorbyl stearate); and aldehydes and ketones, such as benzil and biacetyl; 3-pyrazolidones and certain indane-1,3-diones. 
     In addition to the aformementioned ingredients it may be advantageous to include additives known as &#34;toners&#34; that improve the image. Toner materials may be present, for example, in amounts from 0.1 to 10 percent by weight of all silver bearing components. Toners are well known materials in the photothermographic art as shown in U.S. Pat. Nos. 3,080,254; 3,847,612 and 4,123,282. 
     Examples of toners include phthalimide and N-hydroxyphthalimide; cyclic imides such as succinimide, pyrazoline-5-ones, and quinazolinone, 3-phenyl-2-pyrazoline-5-one, 1-phenylurazole, quinazoline, and 2,4-thiazolidinedione; naphthalimides (e.g., N-hydroxy-1,8-naphthalimide); cobalt complexes (e.g., cobaltic hexammine trifluoroacetate); mercaptans as illustrated by 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole and 2,5-dimercapto-1,3,4-thiadiazole; N-(aminomethyl)aryldicarboximides, (e.g., (N,N-dimethylaminomethyl)phthalimide, and N,N-(dimethylaminomethyl)naphthalene-2,3-dicarboximide); and a combination of blocked pyrazoles, isothiuronium derivatives and certain photobleaching agents (e.g., a combination of N,N&#39;-hexamethylene bis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-diazaoctane)bis(isothiuronium trifluoroacetate) and 2-(tribromomethylsulfonyl)benzothiazole); and merocyanine dyes such as 3-ethyl-5[(3-ethyl-2-benzothiazolinylidene)-1-methylethylidene]-2-thio-2,4-oxazolidinedione; phthalazinone and phthalazinone derivatives or metal salts or these derivatives such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone, and 2,3-dihydro-1,4-phthalazinedione; a combination of phthalazinone plus phthalic acid derivatives (e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride); quinazolinediones, benzoxazine or naphthoxazine derivatives; rhodium complexes functioning not only as tone modifiers, but also as sources of halide ion for silver halide formation in situ, such as ammonium hexachlororhodate (III), rhodium bromide, rhodium nitrate and potassium hexachlororhodate (III); inorganic peroxides and persulfates (e.g., ammonium peroxydisulfate and hydrogen peroxide); benzoxazine-2,4-diones such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione, and 6-nitro-1,3-benzoxazine-2,4-dione; pyrimidines and asymmetric triazines (e.g., 2,4-dihydroxypyrimidine, 2-hydroxy-4-aminopyrimidine), azauracils, and tetrazapentalene derivatives (e.g, 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetrazapentalene, and 1,4-di(o-chlorophenyl)-3,6-dimercapto-1H,4H-2,3 a,5,6a-tetrazapentalene). 
     A number of methods are known in the art for obtaining color images with dry silver systems including: a combination of silver benzotriazole, well known magenta, yellow and cyan dye-forming couplers, aminophenol developing agents, a base release agent such as guanidinium trichloroacetate and silver bromide in poly(vinyl butyral) as described in U.S. Pat. Nos. 4,847,188 and 5,064,742; preformed dye release systems such as those described in U.S. Pat. No. 4,678,739; a combination of silver bromoiodide, sulfonamidophenol reducing agent, silver behenate, poly(vinyl butyral), an amine such as n-octadecylamine and 2-equivalent or 4-equivalent cyan, magenta or yellow dye-forming couplers; leuco dye bases which oxidize to form a dye image (e.g., Malachite Green, Crystal Violet and para-rosaniline); a combination of in situ silver halide, silver behenate, 3-methyl-1-phenylpyrazolone and N,N&#39;-dimethyl-p-phenylenediamine hydrochloride; incorporating phenolic leuco dye reducing agents such as 2(3,5-di-(t-butyl)-4-hydroxyphenyl)-4,5-diphenylimidazole, and bis(3,5-di-(t-butyl)-4-hydroxyphenyl)phenylmethane, incorporating azomethine dyes or azo dye reducing agents; silver dye bleach processes (for example, an element comprising silver behenate, behenic acid, poly(vinyl butyral), poly(vinyl butyral) peptized silver bromoiodide emulsion, 2,6-dichloro-4-benzenesulfonamidophenol, 1,8-(3,6-diazaoctane)bis(isothiuronium-p-toluenesulfonate) and an azo dye can be exposed and heat processed to obtain a negative silver image with a uniform distribution of dye, and then laminated to an acid activator sheet comprising polyacrylic acid, thiourea and p-toluenesulfonic acid and heated to obtain well defined positive dye images); and amines such as aminoacetanilide (yellow dye-forming), 3,3&#39;-dimethoxybenzidine (blue dye-forming) or sulfanilide (magenta dye forming) that react with the oxidized form of incorporated reducing agents such as 2,6-dichloro-4-benzenesulfonamidophenol to form dye images. Neutral dye images can be obtained by the addition of amines such as behenylamine and p-anisidine. 
     Leuco dye oxidation in such silver halide systems for color formation is disclosed in U.S. Pat. Nos. 4,021,240; 4,374,821; 4,460,681; and 4,883,747. 
     Representative classes of leuco dyes that are suitable for use in the present invention include, but are not limited to, bisphenol and bisnaphthol leuco dyes, phenolic leuco dyes, indoaniline leuco dyes, imidazole leuco dyes, azine leuco dyes, oxazine leuco dyes, diazine leuco dyes, and thiazine leuco dyes. Preferred classes of dyes are described in U S. Pat. Nos. 4,460,681 and 4,594,307. 
     One class of leuco dyes useful in this invention are those derived from imidazole dyes. Imidazole leuco dyes are described in U.S. Pat. No. 3,985,565. 
     Another class of leuco dyes useful in this invention are those derived from so-called &#34;chromogenic dyes.&#34; These dyes are prepared by oxidative coupling of a p-phenylenediamine with a phenolic or anilinic compound. Leuco dyes of this class are described in U.S. Pat. No. 4,594,307. Leuco chromogenic dyes having short chain carbamoyl protecting groups are described in assignee&#39;s copending application U.S. Ser. No. 07/939,093, incorporated herein by reference. 
     A third class of dyes useful in this invention are &#34;aldazine&#34; and &#34;ketazine&#34; dyes. Dyes of this type are described in U.S. Pat. Nos. 4,587,211 and 4,795,697. 
     Another preferred class of leuco dyes are reduced forms of dyes having a diazine, oxazine, or thiazine nucleus. Leuco dyes of this type can be prepared by reduction and acylation of the color-bearing dye form. Methods of preparing leuco dyes of this type are described in Japanese Patent No. 52-89131 and U.S. Pat. Nos. 2,784,186; 4,439,280; 4,563,415; 4,570,171; 4,622,395; and 4,647,525, all of which are incorporated herein by reference. 
     Another class of dye releasing materials that form a dye upon oxidation are known as preformed-dye-release (PDR) or redox-dye-release (RDR) materials. In these materials the reducing agent for the organic silver compound releases a pre-formed dye upon oxidation. Examples of these materials are disclosed in Swain, U.S. Pat. No. 4,981,775, incorporated herein by reference. 
     The optical leuco dyes of this invention, can be prepared as described in H. A. Lubs The Chemistry of Synthetic Dyes and Pigments; Hafner; New York, N.Y.; 1955 Chapter 5; in H. Zollinger Color Chemistry: Synthesis, Properties and Applications of Organic Dyes and Pigments; VCH; New York, N.Y.; pp. 67-73, 1987, and in U.S. Pat. No. 5,149,807; and EPO Laid Open Application No. 0,244,399. 
     Silver halide emulsions containing the stabilizers of this invention can be protected further against the additional production of fog and can be stabilized against loss of sensitivity during shelf storage. Suitable antifoggants, stabilizers, and stabilizer precursors which can be used alone or in combination, include thiazolium salts as described in U.S. Pat. Nos. 2,131,038 and 2,694,716; azaindenes as described in U.S. Pat. Nos. 2,886,437 and 2,444,605; mercury salts as described in U.S. Pat. No. 2,728,663; urazoles as described in U.S. Pat. No. 3,287,135; sulfocatechols as described in U.S. Pat. No. 3,235,652; oximes as described in British Patent No. 623,448; nitrones; nitroindazoles; polyvalent metal salts as described in U.S. Pat. No. 2,839,405; thiouronium salts as described in U.S. Pat. No. 3,220,839; and palladium, platinum and gold salts described in U.S. Pat. Nos. 2,566,263 and 2,597,915; halogen-substituted organic compounds as described in U.S. Pat. Nos. 4,108,665 and 4,442,202; triazines as described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and 4,459,350; and phosphorous compounds as described in U.S. Pat. No. 4,411,985. 
     Stabilized emulsions of the invention can contain plasticizers and lubricants such as polyalcohols (e.g., glycerin and diols of the type described in U.S. Pat. No. 2,960,404); fatty acids or esters such as those described in U.S. Pat. No. 2,588,765 and U.S. Pat. No. 3,121,060; and silicone resins such as those described in British Patent No. 955,061. 
     The photothermographic elements of the present invention may include image dye stabilizers. Such image dye stabilizers are illustrated by British Patent No. 1,326,889; U.S. Pat. Nos. 3,432,300; 3,698,909; 3,574,627; 3,573,050; 3,764,337 and 4,042,394. 
     Photothermographic elements containing emulsion layers stabilized according to the present invention can be used in photographic elements which contain light absorbing materials and filter dyes such as those described in U.S. Pat. Nos. 3,253,921; 2,274,782; 2,527,583; and 2,956,879. If desired, the dyes can be mordanted, for example, as described in U.S. Pat. No. 3,282,699. 
     Photothermographic elements containing emulsion layers stabilized as described herein can contain matting agents such as starch, titanium dioxide, zinc oxide, silica, polymeric beads including beads of the type described in U.S. Pat. No. 2,992,101 and U.S. Pat. No. 2,701,245. 
     Emulsions stabilized in accordance with this invention can be used in photothermographic elements which contain antistatic or conducting layers, such as layers that comprise soluble salts (e.g., chlorides, nitrates, etc.), evaporated metal layers, ionic polymers such as those described in U.S. Pat. Nos. 2,861,056 and 3,206,312 or insoluble inorganic salts such as those described in U.S. Pat. No. 3,428,451. 
     The binder may be selected from any of the well-known natural or synthetic resins such as gelatin, polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, polyolefins, polyesters, polystyrene, polyacrylonitrile, polycarbonates, and the like. Copolymers and terpolymers are of course included in these definitions. The preferred photothermographic silver containing polymers are polyvinyl butyral, butyl ethyl cellulose, methacrylate copolymers, maleic anhydride ester copolymers, polystyrene, and butadiene-styrene copolymers. 
     Optionally, these polymers may be used in combinations of two or more thereof. Such a polymer is used in an amount sufficient to carry the components dispersed therein, that is, within the effective range of the action as the binder. The effective range can be appropriately determined by one skilled in the art. As a guide in the case of carrying at least an organic silver salt, it can be said that a preferable ratio of the binder to the organic silver salt ranges from 15:1 to 1:2, and particularly from 8:1 to 1:1. 
     Photothermographic emulsions containing a stabilizer according to the present invention may be coated on a wide variety of supports. Typical supports include polyester film, subbed polyester film, poly(ethylene terephthalate) film, cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polycarbonate film and related or resinous materials, as well as glass, paper metal and the like. Typically, a flexible support is employed, especially a paper support, which may be partially acetylated or coated with baryta and/or an α-olefin polymer, particularly a polymer of an α-olefin containing 2 to 10 carbon atoms such as polyethylene, polypropylene, ethylene-butene copolymers and the like. Substrates may be transparent or opaque. 
     Substrates with a backside resistive heating layer may be used in color photothermographic imaging systems such as shown in U.S. Pat. Nos. 4,460,681 and 4,374,921. 
     Photothermographic emulsions of this invention can be coated by various coating procedures including dip coating, air knife coating, curtain coating, or extrusion coating using hoppers of the type described in U.S. Pat. No. 2,681,294. If desired, two or more layers may be coated simultaneously by the procedures described in U.S. Pat. No. 2,761,791 and British Patent No. 837,095. 
     Additional layers may be incorporated into photothermographic articles of the present invention such as dye receptive layers for receiving a mobile dye image, an opacifying layer when reflection prints are desired, a protective topcoat layer and a primer layer as is known in the photothermographic art. Additionally, it may be desirable in some instances to coat different emulsion layers on both sides of a transparent substrate, especially when it is desirable to isolate the imaging chemistries of the different emulsion layers. These compounds of the invention find utility in systems free of mercury and systems spectrally sensitized to the infrared. 
     The present invention will be illustrated in detail in the following examples, but the embodiment of the present invention is not limited thereto. 
    
    
     EXAMPLES 
     The following procedure is representitive of the method for preparation of the stabilizers of the present invention. Compounds 1 and 3 were prepared as described in J. Amer. Chem. Soc. 1952, 74, 3868 and as follows. 
     Preparation of 1-Hydroxymethylbenzotriazole: A mixture of benzotriazole (60 g, 0.5 mole), formalin (40  ml, 40% by volume), acetic acid (50 ml) and water (100 ml) (gave a white color precipitate after few minutes) was allowed to stand for over two hours at room temperature. The product which had precipitated was filtered off, and dried and recrystallized from 1200 ml of hot (80°-85° C.) not boiling water to give 68 g of (90% yield) the desired product; mp 148° C. (lit. 148°-151° C.). 
     Preparation of 1-Chloromethylbenzotriazole: To 1-hydroxymethylbenzotriazole (59.6 g), prepared above, kept at ice-bath temp., 175 ml of thionyl chloride was added dropwise as long as a vigorous reaction continued. The reminder was added more rapidly. The mixture was then stirred and refluxed for 90 minutes. Excess thionyl chloride was removed by distillation, the last traces were removed by heating for a short time with 200 ml of methanol. After cooling and collecting on a funnel, the product weighted 45 g. (67%); melting point: 136° C; (lit. 136°-138° C.). 
     Preparation of Bis(benzotriazole-1-yl-methyl)sulfide: 1-(Chloromethyl)benzotriazole (20.4 g), prepared above, and sodium sulfide monohydrate (14.4 g) in anhydrous ethanol (75 ml) were stirred overnight (20 hours) at room temperature. Water (240 ml) was added to the reaction mixture. The precipitate was filtered, washed with water to remove inorganic salts and dried. Crystallization from ethanol gave 15.6 g (88%) of desired product (Compound 1); m.p. 179°-181° C. (lit. 182°-184° C.). Thin Layer Chromotography in (CHCl 3  /Ethyl acetate) showed only one spot. 
     Preparation of Bis(benzotriazole-1-yl-methyl)sulfone: To the suspension of bis(benzotriazole-1-yl-methyl)sulfide (2.96 g)in CH 2  Cl 2  was added m-chloroperoxybenzoic acid (85%) (5.4 g) portion wise with cooling the reaction mixture on ice-water bath. The reaction mixture was stirred at room temperature for 4 hours, and the next portion of m-CPBA (2.2 g) was added and the reaction mixture was stirred at rom temperature for 2 days. The solvent was evaporated, the residue was treated with 80 ml of water, and the reaction mixture was then extracted with CH 2  Cl 2 . After drying the organic phase over anhydrous MgSO 4 , the solvent was evaporated to give 3.3 g of crude sulfone (100%). Crystallization from EtOH-AcOH gave 2.7 g (81%) of colorless microcrystals (Compound 5). m.p. 208° C. ##STR2## 
     EXAMPLE 1 
     A dispersion of silver behenate half soap was made at 10% solids in toluene and acetone (90:10, w:w) by homogenization. To 223.3 g of the silver half soap dispersion was added 0.34 g of polyvinylbutyral. After 15 minutes of mixing, 7.6 ml of a 0.963 g/19.0 g mercuric acetate in methanol solution and 21.2 ml of a 1.0 g/49.0 g calcium bromide in ethanol solution were added. Then an additional 14.5 ml of 1.45 g/48.5 g calcium bromide in ethanol was added 60 minutes later. After 60 minutes of mixing, 41.2 g of polyvinylbutryal was added. 
     To 29.3 g of the prepared silver premix described above was added 1.47 ml of the sensitizing dye A (0.021 g/50 ml methanol) shown below. 
     After 30 minutes, a magenta color-forming leuco dye solution was added as shown below: 
     
         ______________________________________                                    
Component                 Amount                                          
______________________________________                                    
Leuco Dye B               0.61   g                                        
Phthalazinone             0.916  g                                        
Tetrahydrofuran           22.4   g                                        
Methyl Ethyl Ketone       33.6   g                                        
VAGH (Union Carbide)      2.2    g                                        
(vinyl acetate/vinyl chloride copolymer)                                  
Polyvinylbutyral          9.8    g                                        
______________________________________                                    
 
    
     Dye A is disclosed in U.S. Pat. No. 4,476,220 and has the formula shown below. 
     Dye B is a magenta leuco dye disclosed in U.S. Pat. No. 4,795,697 and has the formula shown below. ##STR3## 
     A topcoat solution was prepared consisting of 23% by weight polystyrene and 3.1% by weight Acryloid B-66™ in approximately 50:50 mixture of toluene and methyl ethyl ketone. Acryloid B-66™ is a poly(methyl methacrylate) available from Rohm and Haas. 
     To 10.0 g of the magenta silver coating solution was added 0.5 ml or 1.0 ml of compounds 1 or 5 at concentrations of 0.078 g/2 ml dimethylformamide and 0.088 g/2 ml of dimethylformamide, respectively; or 0.5 ml or 1.0 ml of benzotriazole (BZT) at a concentration of 0.16 g/5 ml of methanol. 
     The magenta silver layer and topcoat were each coated at a wet thickness of 2 mils and dried for 5 minutes at 82° C. The samples were exposed for 10 -3  seconds through a Wratten 58 filter and a 0-3 continuous wedge and developed by heating for 6 seconds (at approximately 138° C.). The samples were measured using a green filter of a computer densitometer. The initial sensitometric data are shown below: 
     
         ______________________________________                                    
            D.sub.min                                                     
                   D.sub.max                                              
                          Speed.sup.1                                     
                                    Contrast.sup.2                        
______________________________________                                    
Control (0.0 ml)                                                          
              0.09     1.86   2.30    1.95                                
0.5 ml BZT    0.07     *      *       *                                   
1.0 ml BZT    0.07     *      *       *                                   
0.5 ml Compound 1                                                         
              0.08     1.92   2.28    1.83                                
1.0 ml Compound 1                                                         
              0.10     1.84   2.32    1.63                                
0.5 ml Compound 5                                                         
              0.09     1.95   2.20    1.92                                
1.0 ml Compound 5                                                         
              0.11     2.06   2.14    2.00                                
______________________________________                                    
 *This means value was not obtainable (no image).                         
 .sup.1 Log Exposure corresponding to density of 0.6 above D.sub.min.     
 .sup.2 Average contrast measured by the slope of the line joining density
 points 0.6 and 1.2 above D.sub.min.                                      
 
    
     Post-processing stability was measured by exposing imaged samples to 1200 foot candles of illumination (daylight fluorescent bulbs) for 6 and 24 hours at 65% RH and 26.7° C. or 100 foot candles of illumination for 7 days at 73% RH and 70° F. The results are shown in the following Table. 
     
         ______________________________________                                    
                         100 Foot                                         
       1200 Foot Candles Candles                                          
       6 Hours   24 Hours    7 Davs                                       
Sample   ΔD.sub.min                                                 
                 ΔD.sub.max                                         
                         ΔD.sub.min                                 
                               ΔD.sub.max                           
                                     ΔD.sub.min                     
                                           ΔD.sub.max               
______________________________________                                    
Control  +0.21   -0.42   +0.30 -0.83 +0.26 -0.37                          
0.5 ml BZT                                                                
         +0.15   *       +0.18 *     +0.18 *                              
1.0 ml BZT                                                                
         +0.06   *       +0.09 *     +0.06 *                              
0.5 ml Cpd 1                                                              
         +0.19   -0.39   +0.27 -0.91 +0.22 -0.53                          
1.0 ml Cpd l                                                              
         +0.15   -0.45   +0.22 -0.97 +0.15 -0.57                          
0.5 ml Cpd 5                                                              
         +0.17   -0.37   +0.24 -0.98 +0.18 -0.50                          
1.0 ml Cpd 5                                                              
         +0.14   -0.40   +0.19 -0.94 +0.13 -0.51                          
______________________________________                                    
 
    
     At these concentrations of primary stabilizer BZT, D min  post-processing improvements were observed but with significant desensitization of the silver halide emulsion. With the use of the blocked dimeric benzotriazole Compound 1 and Compound 5 initial desensitization effects were minimized and, in fact, speed was enhanced 0.15 log E for compound Compound 5. 
     The comparison of BZT equivalents to the dimeric BZT blocked compounds assumed 2 moles of BZT are being released per mole of dimeric BZT. The post-processing results would then suggest partial release of BZT was occurring but with still significant D min  post-processing stabilization of 40-50%. 
     EXAMPLE 2 
     A dispersion of silver behenate half soap was made at 10% solids in a 50/50 ratio of toluene and 2-butanone by homogenization. To 153.5 g of this silver half soap dispersion was added 170.9 g 2-butanone, 76.7 g isopropyl alcohol and 0.6 g poly(vinyl butyral). After 15 minutes of mixing 1.2 ml of a pyridine solution (1 ml/9 ml 2-butanone) and 3.0 ml of a mercuric bromide (0.17 g/2.4 g ethanol) were added. Then 8.1 ml of calcium bromide (0.22 g/6.5 g ethanol) was added 30 minutes later. After two hours of mixing, 25.7 g of poly(vinylpyrrolidone) was added, and 32.1 g of poly(vinylbutyral) was added one hour later. 
     To 73.2 g the prepared silver premix described above was added 6.0 ml of sensitizing dye C (0.045 g/50 ml ethanol). Sensitizing dye C is described in U.S. Pat. No. 4,123,282 and has the formula: ##STR4## 
     After 20 minutes a yellow color-forming leuco dye solution was added as shown below. 
     
         ______________________________________                                    
Component       Amount                                                    
______________________________________                                    
Leuco dye D     0.90 g                                                    
Tribenzylamine  0.75 g                                                    
Phthalazinone   0.44 g                                                    
2-butanone      14.70 g                                                   
Ethanol         14.70 g                                                   
______________________________________                                    
 
    
     The leuco yellow dye, Dye D, is disclosed in U.S. Pat. No. 4,883,747 and has the following formula. ##STR5## 
     After sensitization with the dye and the addition of the leuco dye solution, compound 1 or 5 were added in the amounts of 0.035 ml or 1.0 ml at a concentration of 0.08 g/2 ml dimethylformamide and 0.088 g/2 ml dimethylformamide, respectively; or 0.35 ml or 1.0 ml of benzotriazole (BZT) at a concentration of 0.16 g/5 ml of methanol to 9.9 g aliquot of the yellow coating solution. The resulting solutions were coated onto a vesicular polyester base at a wet thickness of 3 mils (76.2 μm) and dried at 82° C. for 5 minutes. A topcoat solution was coated a wet thickness of 3 mils (76.2 μm) over the silver halide layer and dried at 82° C. for 5 minutes. The topcoat solution consisted of 7.3% poly(vinyl alcohol), 0.06% phthalazine, 0.0008% benzotriazole, 1.42% tetrachlorophthalic acid, and 0.35% sodium acetate. 
     The samples were exposed for 10 -3  seconds through a 47B Wratten filter and a 0 to 3 continuous wedge and developed by heating to approximately 138° C. for 6 seconds. The density of dye was measured using a blue filter of a computer densitometer. Post processing stability was measured as described previously. The initial sensitometric data are shown below. 
     
         ______________________________________                                    
Sample        D.sub.min                                                   
                       D.sub.max                                          
                              Speed.sup.1                                 
                                      Contrast.sup.2                      
______________________________________                                    
Control (0.0 ml)                                                          
              0.10     2.45   1.83    4.48                                
0.35 ml BZT   0.09     1.19   2.68    *                                   
1.00 ml BZT   0.09     *      *       *                                   
0.35 ml Compound 1                                                        
              0.10     2.49   1.80    5.96                                
1.00 ml Compound 1                                                        
              0.09     2.43   1.84    4.63                                
0.35 ml Compound 5                                                        
              0.09     2.52   1.65    4.88                                
1.00 ml Compound 5                                                        
              0.09     2.38   1.63    3.14                                
______________________________________                                    
 *This means value was not obtainable (no image).                         
 .sup.1 Log Exposure corresponding to density of 0.6 above D.sub.min.     
 .sup.2 Average contrast measured by the slope of the line joining density
 points 0.6 and 1.2 abovs D.sub.min.                                      
 
    
     Post-processing stability was measured by exposing imaged samples to 1200 foot candles of illumination (daylight fluorescent bulbs) for 6 and 24 hours at 65% RH and 26.7° C. or 100 foot candles of illumination for 7 days at 73% RH and 70° F. The results are shown in the following Table. 
     
         ______________________________________                                    
       1200 Foot Candles 100 Foot Candles                                 
       6 hrs.    24 hrs.     7 days                                       
Sample   ΔD.sub.min                                                 
                 ΔD.sub.max                                         
                         ΔD.sub.min                                 
                               ΔD.sub.max                           
                                     ΔD.sub.min                     
                                           ΔD.sub.max               
______________________________________                                    
Control  +0.41   -0.21   +0.53 -0.22 +0.47 -0.33                          
0.35 ml BZT                                                               
         +0.35   -0.10   *     *     +0.32 *                              
1.00 ml BZT                                                               
         +0.12   *       +0.15 *     +0.09 *                              
0.35 ml Cpd 1                                                             
         +0.42   -0.18   +0.52 -0.15 +0.45 -0.45                          
1.00 ml Cpd l                                                             
         +0.37   -0.21   +0.47 -0.25 +0.42 -0.35                          
0.35 ml Cpd 5                                                             
         +0.41   -0.19   +0.50 -0.17 +0.44 -0.31                          
1.00 ml Cpd 5                                                             
         +0.26   -0.21   +0.32 -0.24 +0.41 -0.34                          
______________________________________                                    
 
    
     ΔD min  represents the difference between the final minimum density minus the initial minimum density. Similarly, ΔD max  represents the difference between final maximum density minus initial maximum density. At these concentrations of primary stabilizer BZT, D min  post-processing improvements were observed but with significant desensitization of the emulsion. With the use of the dimeric benzotriazole compounds of this invention, Compounds 1 and 5, show no desensitization effects and speed was enhanced 0.20 log E for Compound 5. 
     The comparison of BZT equivalents to the dimeric BZT blocked compounds assumed 2 moles of BZT are being released per mole of dimeric BZT. The post processing results would suggest partial release of BZT was occurring but with 12-40% D min  post-processing stabilization. 
     EXAMPLE 3 
     Example 3 demonstrates the use of the stabilizers as speed enhancers of this invention in a black-and white photothermographic system. 
     A 13.6 wt % dispersion of silver behenate/behenic acid half soap was made in acetone by homogenization. To 201.5 g of this dispersion was added Butvar B-76™ (1.12 g) and the mixture was stirred 30 minutes more. Three 1.00 ml aliquots of a solution of 10.0 g zinc bromide in 100.0 ml methanol were added sequentially with stirring for 10 minutes after each addition. Toluene (66.66 g) was added and the mixture was stirred for an additional 15 minutes. A solution (2.40 ml) containing 4.00 g of pyridine in 100 ml methyl ether ketone was added with continued stirring for 15 minutes. The mixture was allowed to stand for 4 hours. 
     To the mixture was added Butvar B-76™ (31.75 g) and then stirred for 30 minutes followed by the addition of 2.73 ml of a solution of 1.33 g N-bromosuccinimide in 100 ml methanol. CAO-05™ (4.20 g, an antioxidant purchased from Rohm and Haas Co., Philadelphia, Pa.) was added with stirring for 5 minutes. Acryloid 21™ (27.22 g, also from Rohm and Haas) was added with stirring for 5 minutes. 
     The following steps were carried under green safe lights. 
     A 6.00 ml aliquot of a solution of 0.03 g of Sensitizing Dye-E, 25.00 ml methanol, and 75 ml toluene was added and the mixture was stirred for 5 minutes. The viscosity of the resultant solution should be between 180 and 220 centipoise. If greater than 220 centipoise, acetone should be added to bring the viscosity into the appropriate range. 
     Sensitizing Dye-E is disclosed in U.S. Pat. No. 3,719,495 and has the following formula: ##STR6## 
     The silver trip formulation was coated at 4.4 mils (112 μm) wet thickness (to give a dry coating weight of 1.25 g/ft 2 ) onto paper and dried at 180° F. (82.2° C.) for one minute. 
     A topcoat solution was coated onto the coated samples prepared above. A master batch of topcoat solution was prepared by mixing: 164.728 g acetone, 82.350 g 2-butanone, 33.300 g methanol, 13.500 g C.A. 398-6 (a cellulose acetate, Eastman Kodak), 1.542 g phthalazine, 1.068 g 4-methyl-phthalic acid, 0.636 g tetrachlorophthalic acid, and 0.800 g tetrachlorophthalic anhydride. Each stabilizer was added to 7.00 g of topcoat solution (master batch) before coating. The compounds tested in the dry silver paper topcoat formulation at concentration levels of 4.0, 0.8, and 0.2 mmoles/7.00 g of topcoat master batch. 
     The topcoat formulation was coated at 2.8 mils (71 μm) wet thickness, on top of the silver emulsion and dried for 3 minutes at 70° F. to provide a dry coating weight of 0.24 gm/ft 2 . 
     The coated paper was imaged by exposing with a photometric sensitometer (Eastman Kodak #101 tungsten light source). After exposure, the strips (25 cm×17.8 cm) were processed at 250° F. (121° C.) by heating for 6 seconds in a hot roll processor. The images obtained were evaluated by a computer densitometer. Sensitometeric results include D min , D max  and Speed. In these samples, the lower the speed number, the &#34;faster&#34; the paper. The post processing results, shown below. 
     
         ______________________________________                                    
Post-Exposure Results                                                     
Sample      Amount     D.sub.min                                          
                                D.sub.max                                 
                                     Speed.sup.1                          
______________________________________                                    
Control     (0.0 mmoles)                                                  
                       0.365    1.60 0.940                                
Compound 1  (4.0 mmoles)                                                  
                       0.304    1.70 0.755                                
            (0.8 mmoles)                                                  
                       0.229    1.60 0.782                                
            (0.2 mmoles)                                                  
                       0.180    1.70 0.806                                
Compound 2  (4.0 mmoles)                                                  
                       0.431    1.69 0.808                                
            (0.8 mmoles)                                                  
                       0.250    1.66 0.818                                
            (0.2 mmoles)                                                  
                       0.249    1.64 0.853                                
Compound 5  (4.0 mmoles)                                                  
                       0.174    1.68 0.802                                
            (0.8 mmoles)                                                  
                       0.222    1.68 0.810                                
            (0.2 mmoles)                                                  
                       0.229    1.60 0.868                                
Compound 6  (4.0 mmoles)                                                  
                       0.318    1.69 0.750                                
            (0.8 mmoles)                                                  
                       0.260    1.66 0.810                                
            (0.2 mmoles)                                                  
                       0.407    1.65 0.920                                
______________________________________                                    
 .sup.1 Log Exposure corresponding to density of 0.6 above D.sub.min.     
 
    
     The results shown, demonstrate that the compounds of this invention are as good or better than the control. The use of dimeric benzotriazole compounds (Compounds 1, 2, 5, and 6) show that speed was enhanced from a range of 0.190 to 0.020 Log E. D min  values were lowered as much as 0.191 for compound 5.