Heat-developable light-sensitive material
FIELD OF THE INVENTION 
     The present invention relates to a heat-developable light-sensitive material containing a base precursor. 
     BACKGROUND OF THE INVENTION 
     Into heat-developable light-sensitive materials, bases or base precursors are frequently incorporated for the purpose of accelerating development upon heating. In view of preservability of the light-sensitive materials, it is preferred to employ base precursors which release basic substances upon heat decomposition. 
     Examples of typical base precursors are described in British Pat. No. 998,949. A preferred base precursor is a salt of a carboxylic acid and an organic base, and examples of the suitable carboxylic acids include trichloroacetic acid and trifluoroacetic acid and examples of the suitable bases including guanidine, piperidine, morpholine, p-toluidine and 2-picoline, etc. 
     Guanidine trichloroacetate as described in U.S. Pat. No. 3,220,846 is particularly preferred. Further, aldonic amides as described in Japanese Patent Application (OPI) No. 22625/75 (the term &#34;OPI&#34; as used herein means a &#34;published unexamined Japanese patent application&#34;) are preferably used because they decompose at a high temperature to form bases. 
     When these known base precursors are employed, however, the following problems are often encountered. Specifically, a relatively long time is required for obtaining images, and a high level of fog is formed. Further, they have a disadvantage in that they are susceptible to air and humidity, and tend to decompose; as a result, photographic properties of the light-sensitive materials undergo a change and preservability of the light-sensitive materials seriously degrade. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a heat developable light-sensitive material containing a base precursor, which can form an image having a high density in a short time and have good preservability. 
     These and other objects of the present invention will become more apparent from the following detailed description and examples. 
     These objects of the present invention are accomplished with a heat developable light-sensitive material containing a compound represented by formula (I) or (II) ##STR3## wherein R 1  and R 2  each represents a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an aryl group, a heterocyclic group, a carboxyl group or a salt thereof, a halogen atom, a cyano group, an alkylsulfonyl or arylsulfonyl group, a sulfamoyl group, a carbamoyl group, an alkoxycarbonyl or aryloxycarbonyl group, a di- or mono-alkylphosphoryl or arylphosphoryl group, a di- or mono-alkylphosphinyl or arylphosphinyl group, an alkylsulfinyl or arylsulfinyl group, an acyl group, an amino group, an acylamino group, an acyloxy group or a group represented by X; X represents a decarboxylation accelerating group; or any two of R 1 , R 2 , and X may combine to form a ring; and the groups represented by R 1 , R 2  and X may further be substituted with a substituent; B represents an organic base; and n represents 1 when B represents a monoacid base or 2 when B represents a diacid base, provided that when ##STR4## is a free dibasic acid, n is 1/2 of the above described numbers, respectively. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The heat developable light-sensitive material of the present invention is characterized by containing the compound represented by formula (I) or (II) as described above as a base precursor. 
     The base precursor represented by formula (I) or (II) according to the present invention is described in detail in the following. 
     In formula (I) or (II), R 1  and R 2  each represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, a carboxyl group or a salt thereof, a halogen atom, a cyano group, a substituted or unsubstituted alkylsulfonyl or arylsulfonyl group, a substituted or unsubstituted alkylsulfamoyl or arylsulfamoyl group, a substituted or unsubstituted alkylcarbamoyl or arylcarbamoyl group, a substituted or unsubstituted alkoxycarbonyl or aryloxycarbonyl group, a substituted or unsubstituted di- or mono-alkylphosphoryl or arylphosphoryl group, a substituted or unsubstituted di- or mono-alkylphosphinyl or arylphosphinyl group, a substituted or unsubstituted alkylsulfinyl or arylsulfinyl group, a substituted or unsubstituted acyl group, a substituted or unsubstituted acylamino group, an amino group, a substituted or unsubstituted di- or mono-alkylamino or arylamino group, a substituted or unsubstituted acyloxy group or a group represented by X. 
     For R 1 , a substituted or unsubstituted alkyl group having from 1 to 8 carbon atoms, for example, a butyl group, a hexyl group, an octyl group, etc.; a substituted or unsubstituted aryl group having from 6 to 12 carbon atoms, for example, a phenyl group, a tolyl group, a p-chlorophenyl group, a p-methoxyphenyl group, etc.; a substituted or unsubstituted aralkyl group having from 7 to 14 carbon atoms, for example, a benzyl group, a β-phenethyl group, an α-methylbenzyl group, etc.; an alkenyl group having from 2 to 8 carbon atoms, for example, a crotyl group, a styryl group, etc.; an alkynyl group having from 2 to 8 carbon atoms, for example, an ethynyl group, a phenylethynyl group, etc.; a cyano group; a substituted carbamoyl group, for example, a dimethylcarbamoyl group, a diethylcarbamoyl group, a morpholinocarbamoyl group, etc.; a substituted or unsubstituted amino group, for example, a diethylamino group, a piperidino group, an N-methylanilino group, etc.; a carboxyl group; and, as a group represented by X, an alkylthio group, an alkoxy group, an aryloxy group, an acylamino group and a nitrogen-containing heterocyclic group, etc. are preferred. 
     Of these groups, a phenyl group, a tolyl group and a p-methoxyphenyl group, etc., are particularly preferred. 
     On the other hand, for R 2 , a hydrogen atom, the above described substituted or unsubstituted alkyl group having from 1 to 8 carbon atoms; the above described substituted or unsubstituted aryl group having from 6 to 12 carbon atoms; a cyano group; a halogen atom, for example, a chlorine atom, a bromine atom, an iodine atom, etc.; a substituted or unsubstituted alkylsulfonyl or arylsulfonyl group, for example, a methanesulfonyl group, a benzenesulfonyl group, a toluenesulfonyl group, etc.; a substituted sulfamoyl group, for example, a dimethylsulfamoyl group, a diethylsulfamoyl group, a morpholinosulfonyl group, etc.; the above described substituted carbamoyl group; the above described dialkylamino group; the above described diarylamino group; and, as a group represented by X, an alkylthio group; an arylthio group; an alkoxy group; an aryloxy group; an acylamino group; and a nitrogen-containing heterocyclic group; etc., are preferred. 
     Of these groups, a hydrogen atom, a methyl group, an ethyl group, a phenyl group, and a p-chlorophenyl group, etc., are particularly preferred. 
     Examples of other substituents which may be present in the groups represented by R 1  or R 2  include a halogen atom, an alkoxy group, an alkyl group, a hydroxyl group, a cyano group, an acyl group, an acylamino group, an acyloxy group, a nitro group and a dialkylamino group, etc. 
     The decarboxylation accelerating group represented by X is a group capable of accelerating a decarboxylation reaction from the CO 2  H group by heating. 
     As the group capable of accelerating decarboxylation, a leaving group is usually used. The leaving group is a group capable of being released at the same time with the decarboxylation by heating. 
     For X, a group derived from a conjugate acid (HX) having pKa of 20 or less, and particularly pKa from 2 to 18 is preferred. Further, a group having -7 or more of an L-value of Swain which is an indicator of releasability as described in E. R. Thornton, Solvolysis Mechanism, page 165, Ronald Press Co., New York (1964) is preferred. 
     Specific examples of preferred leaving groups include, an alkoxy group or an aryloxy group, for example, a phenoxy group, a 2,4-dichlorophenoxy groups, a p-cyanophenoxy group, etc., an acylamino group, for example, a trifluoroacetylamino group, a heptafluorobutyroylamino group, etc., a sulfonylamino group, for example, a methanesulfonylamino group, a benzenesulfonylamino group, a p-chlorophenylsulfonylamino group, etc., an imido group, for example, a succinimido group, a hydantoinyl group, a phthalimido group, etc., an acyloxy group, for example, an acetoxy group, a benzoyloxy group, a p-chlorobenzoyloxy group, etc., a halogen atom, for example, a chlorine atom, a bromine atom, an iodine atom, etc., an alkylthio group or an arylthio group, for example, a phenylthio group, a 1-naphthylthio group, a 2-naphthylthio group, etc., an alkoxycarbonyloxy group or an aryloxycarbonyloxy group, for example, an ethoxycarbonyloxy group, a phenoxycarbonyloxy group, etc., a dialkylcarbamoyloxy group, for example, a diethylcarbamoyloxy group, a morpholinocarbonyloxy group, etc., and a nitrogen-containing heterocyclic group, for example, a 1-pyrazolyl group, a 1-imidazolyl group, a 1-benzimidazolyl group, a 1-benzotriazolyl group, etc. 
     Further, leaving groups of 2-equivalent couplers useful in a conventional photographic system are preferred examples for X. Examples of these releasing groups include a 3,5-dimethyl-1-pyrazolyl group, a benzylethoxyhydantoinyl group, a dodecylsuccinimido group and a 1-pyridinium group, etc. 
     In such cases, X is released at the same time with decarboxylation by heating and form HX (or X - ). When the HX thus formed has a photographic effect, the base precursors containing such a group are particularly preferred in accordance with the present invention. 
     Preferred examples of HX include sulfonamides, imides, and nitrogen-containing heterocyclic compounds. 
     On the other hand, when X is not released but only accelerates the decarboxylation reaction, the effectiveness of the base precursor according to the present invention is not adversely affected at all. 
     Such a decarboxylation accelerating group is generally a group derived from a conjugate acid having pKa of 15 or more. Specific examples of these groups include an ethoxy group, a tert-butoxy group, an acetylamino group, and a benzoylamino group, etc. 
     Moreover, the group represented by X may further have a substituent. Examples of such substituents include an alkyl group, an alkoxy group, a hydroxyl group, a cyano group, an acyloxy group, an acyl group, an acylamino group, a halogen atom, a sulfonyl group, and a nitro group, etc. 
     Furthermore, R 1  and X or R 2  and X in formula (I), or R 1  and X in formula (II) may connect to each other to form a ring. 
     Preferred examples of the compounds having such a ring are set forth in the following: ##STR5## 
     It is not preferred that R 1  and R 2  are connected. 
     Of the organic base represented by B, those having pKa of 7 or more and having 12 or less carbon atoms are desirable, and those having pKa of 10 or more and a boiling point of 150° C. or higher and being low volatile are preferred. Examples of particularly preferred organic bases include acyclic guanidines, cyclic guanidines, acyclic amidines, cyclic amidines, tetraalkyl ammonium hydroxides, etc. 
     Specific examples of preferred organic bases for B include the followings: 
     Piperidine, piperazine, ethylenediamine, N,N&#39;-dimethylethylenediamine, acetoamidine, diazabicyclononene, diazabicycloundecene, tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, ##STR6## 
     n represents 1 when B represents a monoacid base or 2 when B represents a diacid base, provided that when ##STR7## is a free dibasic acid, n is 1/2 of the above described numbers, respectively. 
     Specific examples of the base precursors which can be preferably used in the present invention are set forth below, but the present invention should not be construed as being limited thereto. ##STR8## 
     The base precursor according to the present invention can be synthesized by various methods. More specifically, it can be easily synthesized by: (A) addition of a nucleophilic reagent to the β-position of an acetylene carboxylic acid ester derivative; (B) addition release of a nucleophilic reagent to the β-position of a β-haloacrylic acid ester derivative; or (C) reaction of an α,β-dihalopropionic acid derivative with a nucleophilic reagent. 
     The base precursors obtained by the above described methods are generally cis-trans isomers, that is, those represented by mixtures of the above described formulae (I) and (II), respectively. 
    
    
     In the following, a method for synthesis of the base precursor according to the present invention is described with reference to the specific example. 
     SYNTHESIS EXAMPLE 
     Synthesis of Base Precursor (1) 
     Phenylpropiolic acid was synthesized according to the method as described in J. Chem. Soc., Vol. 83, page 1154. To a mixture of 42 g of phenylpropiolic acid and 50 ml of methylene chloride was added 42 ml of thionyl chloride, and the mixture was mildly refluxed by heating on a water bath. After the completion of the generation of gas, the solvent and excessive thionyl chloride were distilled off under reduced pressure. The pale yellow liquid thus obtained was gradually added to 300 ml of methanol under cooling with ice. After being allowing to stand overnight, the solvent was distilled off under a reduced pressure, and the residual liquid was washed with water and dried to obtain 44.5 g of almost pure methyl phenylpropiolate. 
     A mixture of 28 g of methyl phenylpropiolate thus obtained, 35.7 g of imidazole and 60 ml of dry acetonitrile was refluxed by heating for 3 hours. After cooling, acetonitrile was distilled off under a reduced pressure and to the residue were added 300 ml of water and 200 ml of ethyl acetate. The mixture was thoroughly shaken, and the organic layer was separated, washed with water, and dried. The solvent was distilled off under reduced pressure, and the residue thus obtained was purified by silica gel chromatography to obtain 34.2 g of the adduct (a mixture of cis and trans forms) as a colorless viscous liquid. 
     The adduct thus obtained was dissolved in 50 ml of methanol and the resulting solution was gradually added to 500 ml of a 1N aqueous solution of sodium hydroxide under cooling with ice. The mixture was stirred at 5° to 10° C. until it became a uniform solution. Then, the solution was further stirred for 1 hour at 5° to 10° C. and neutralized with cool diluted hydrochloric acid while cooling with ice to pH of 5 to 6. The pale yellow oil isolated was gradually crystallized. The crystals thus obtained were separated by filtration, washed with water, and air-dried to obtain 24 g of β-(1-imidazolyl)cinnamic acid (a mixture of cis and trans forms). 
     21.4 g of β-(1-imidazolyl)cinnamic acid thus obtained was dissolved in 100 ml of methanol and to the solution was added, little by little, a solution containing 9.0 g of guanidine carbonate dissolved in 50 ml of water. After stirring the mixture for 1 hour at room temperature, the solvent was distilled off under reduced pressure at 50° C. or less, to obtain 27.4 g of Base Precursor (1) as a colorless viscous liquid. 
     Other base precursors are synthesized in an analogous manner to that described above. 
     The base precursor according to the present invention is especially effective when it is employed together with a spectrally sensitized light-sensitive silver halide emulsion, and in particular the degree of increase in image density is particularly large in this case. 
     The spectral sensitization of silver halide emulsions can be performed using methine dyes or other dyes. Suitable dyes which can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxonol dyes. Of these dyes, cyanine dyes, merocyanine dyes and complex merocyanine dyes are particularly useful. Any conventionally utilized nucleus for cyanine dyes, such as a basic heterocyclic nucleus, is applicable to dyes useful in the present invention, including a pyrroline nucleus, an oxazoline nucleus, a thiazoline nucleus, a pyrrole nucleus, an oxazole nucleus, a thiazole nucleus, a selenazole nucleus, an imidazole nucleus, a tetrazole nucleus or a pyridine nucleus. Furthermore, nuclei formed by condensing alicyclic hydrocarbon rings with these nuclei and nuclei formed by condensing aromatic hydrocarbon rings with these nuclei, e.g., an indolenine nucleus, a benzindolenine nucleus, an indole nucleus, a benzoxazole nucleus, a naphthoxazole nucleus, a benzothiazole nucleus, a naphthothiazole nucleus, a benzoselenazole nucleus, a benzimidazole nucleus or a quinoline nucleus may also be used. The carbon atoms of these nuclei may be substituted. 
     In merocyanine dyes and complex merocyanine dyes, nuclei having a ketomethylene structure can include 5- or 6-membered heterocyclic nuclei such as a pyrazolin-5-one nucleus, a thiohydantoin nucleus, a 2-thiooxazolidin-2,4-dione nucleus, a thiazolidin-2,4-dione nucleus, a rhodanine nucleus or a thiobarbituric acid nucleus. 
     These sensitizing dyes can be employed individually, and can also be employed in combination thereof. A combination of sensitizing dyes is often used, particularly for the purpose of supersensitization. 
     Useful sensitizing dyes include those described in German Pat. No. 929,080, U.S. Pat. Nos. 2,493,748, 2,503,776, 2,519,001, 2,912,329, 3,656,959, 3,672,897, 3,694,217, 4,025,349 and 4,046,572, British Pat. No. 1,242,588, and Japanese Patent Publication Nos. 14030/69 and 24844/77. 
     A suitable amount of the sensitizing dye to be used is from about 0.001 g to 20 g, and preferably from about 0.01 g to 2 g, per 100 g of silver contained in the emulsion. 
     The amount of base precursor according to the present invention used can vary in a broad range. It is suitably used in an amount of about 50% by weight or less, and more preferably in a range from about 0.01% by weight to 40% by weight, based on the total weight of dried coatings of the light-sensitive material. 
     Any unit and layer structure can be applied to the light-sensitive material of the present invention. The base precursor may be incorporated into any one of various layers of the light-sensitive material. When a light-sensitive emulsion layer and a layer containing a dye-providing substance are separately constructed, the base precursor may be incorporated into any of these layers. Further, it can be incorporated into an inter layer or a protective layer. 
     Moreover, two or more base precursors according to the present invention can be used. 
     In the present invention, silver halide as a light-sensitive substance is preferably employed. 
     The silver halide used in the present invention can be any conventional light-sensitive silver halide, including silver chloride, silver chlorobromide, silver chloroiodide, silver bromide, silver iodobromide, silver chloroiodobromide and silver iodide. 
     Any conventional process for preparing those silver halides can be used, such as a typical method of preparing silver iodobromide by first adding a silver nitrate solution to a potassium bromide solution to form silver bromide grains and then adding potassium iodide to the mixture. 
     Two or more silver halides in which the grain size and/or halogen composition are different from each other may be used in combination. 
     The average grain size of the silver halide used in the present invention is preferably from about 0.001 μm to 10 μm and more preferably from about 0.001 μm to 5 μm. 
     The silver halide used in the present invention may be unsensitized or chemically sensitized with a conventional chemical sensitizing agent such as compounds of sulfur, selenium or tellurium, or compounds of gold, platinum, palladium, rhodium or iridium, a reducing agent such as tin halide, or a combination thereof. The details of suitable sensitization methods are described in T. H. James, The Theory of the Photographic Process, pages 149 to 169, (4th ed., 1977). 
     A suitable coating amount of the light-sensitive silver halide according to the present invention is from about 1 mg to 10 g/m 2  calculated as silver. 
     In a particularly preferred embodiment of the heat developable light-sensitive material according to the present invention, an organic silver salt oxidizing agent is used together with silver halide. The organic silver salt oxidizing agent is a silver salt which forms a silver image by reacting with the hereinafter described reductive dye-providing substance or with optional reducing agents present together with the image forming substances, when it is heated to a temperature of above about 80° C., and, preferably, above about 100° C. in the presence of exposed silver halide. Combined use of such an organic silver salt oxidizing agent, the light-sensitive material which provides higher color density can be obtained. 
     The silver halide used in conjunction with an organic silver salt oxidizing agent does not necessarily contain pure silver iodide crystal in the case of using the silver halide alone. Any silver halide which is known in the art can be used. 
     Examples of such organic silver salt oxidizing agents include those described in U.S. Pat. No. 4,500,626, and specifically include the following. 
     A silver salt of an organic compound having a carboxyl group can be used, including a silver salt of an aliphatic carboxylic acid and a silver salt of an aromatic carboxylic acid. 
     In addition, a silver salt of a compound containing a mercapto group or a thione group or a derivative thereof can be used. 
     Further, a silver salt of a compound containing an imino group can be used, including a silver salt of benzotriazole and a derivative thereof described in Japanese Patent Publication No. 30270/69 and U.S. Pat. No. 3,635,719, e.g., a silver salt of benzotriazole, a silver salt of an alkyl-substituted benzotriazole such as a silver salt of methylbenzotriazole, a silver salt of a halogen-substituted benzotriazole such as a silver salt of 5-chlorobenzotriazole, a silver salt of carboimidobenzotriazole such as a silver salt of butylcarboimidobenzotriazole, a silver salt of 1,2,4-triazole or 1-H-tetrazole as described in U.S. Pat. No. 4,220,709, a silver salt of carbazole, a silver salt of saccharin, a silver salt of imidazole and an imidazole derivative. 
     Moreover, a silver salt as described in Research Disclosure, Vol. 170, No. 17029 and an organic metal salt such as copper stearate, can be used as organic metal salt oxidizing agents in the present invention. 
     Methods of preparing these silver halide and organic silver salt oxidizing agents and methods of blending them are described in Research Disclosure, No. 17029, Japanese Patent Application (OPI) Nos. 42529/76, 13224/74 and 17216/75, and U.S. Pat. Nos. 3,700,458 and 4,076,539. 
     A suitable coating amount of the light-sensitive silver halide and the organic silver salt oxidizing agent employed in the present invention is a total of from about 50 mg/m 2  to 10 g/m 2  calculated as silver. 
     In the present invention, a wide variety of image forming substances can be used in addition to silver as an image forming substance. 
     For instance, couplers capable of forming color images by bonding to oxidation products of developing agents employed in conventional liquid development processing can be used in the present invention, with specific examples including magenta couplers such as 5-pyrazolone couplers, pyrazolobenzimidazole couplers, cyanoacetylcumarone couplers and open chain acylacetonitrile couplers, yellow couplers such as acylacetamide couplers (e.g., benzoylacetanilides and pivaloylacetanilides), and cyan couplers such as naphthol couplers and phenol couplers. 
     It is generally desirable that these couplers should be rendered non-diffusible by a hydrophobic group (i.e., a &#34;ballast group&#34;) in their molecules, or that the couplers should be polymeric couplers. The couplers may be either 4-equivalent or 2-equivalent with respect to silver ion. Further, these couplers may be colored couplers having a color correction effect, or couplers capable of releasing development restrainers upon development (i.e., &#34;DIR couplers&#34;). 
     In addition, dyes which can produce positive color images using light-sensitive silver dye bleach processes, for example, dyes described in Research Disclosure, pages 30-32, RD-14433 (April, 1876); Research Disclosure, pages 14-15, RD-15227 (December, 1976), and U.S. Pat. No. 4,235,957, and leuco dyes described in U.S. Pat. Nos. 3,985,565 and 4,022,617, can be used. 
     Also, dyes into which nitrogen-containing heterocyclic groups are introduced described in Research Disclosure, pages 54-58, RD-16966 (May, 1978) can be used. 
     Moreover, dye-providing substances described in European Pat. Nos. 67,455 and 79,056, West German Pat. No. 3,217,853, which release mobile dyes by a coupling reaction with reducing agents oxidized by a redox reaction with silver halide or organic silver salt oxidizing agents under high temperatures, and dye-providing substances described in European Pat. Nos. 66,282 and 76,492, West German Pat. No. 3,215,485, and Japanese Patent Application (OPI) Nos. 154445/84 and 152440/84, which undergo a redox reaction with silver halide or organic silver salt oxidizing agents under high temperatures, and release mobile dyes as a result of this reaction, can be used. 
     Dye-providing substances which can be used in the above-described processes are preferably represented by the following formula (C I): 
     
         (Dye-X.sub.0).sub.q -Y                                     (C I) 
    
     wherein Dye represents a dye which becomes mobile when it is released from the molecule of the compound represented by the formula (C I); X 0  represents a simple bond or a connecting group; q represents 1; and Y represents a group which releases Dye in correspondence or counter-correspondence to light-sensitive silver salts having a latent image distributed imagewise, the diffusibility of a dye released being different from that of the compound represented by (Dye-X 0 ) q  -Y. 
     The dye represented by Dye is preferably a dye having a hydrophilic group. Examples of the dye which can be used include azo dyes, azomethine dyes, anthraquinone dyes, naphthoquinone dyes, styryl dyes, nitro dyes, quinoline dyes, carbonyl dyes and phthalocyanine dyes. These dyes can also be used in a color-shifted form having temporarily shorter wavelengths, the original color of which is recoverable in development processing. 
     More specifically, dyes described in European Pat. No. 76,492 can be utilized. 
     Examples of the connecting group represented by X 0  include --N-- (wherein R represents a hydrogen atom, an alkyl group, or a substituted alkyl group), --SO 2  --, --CO--, an alkylene group, a substituted alkylene group, a phenylene group, a substituted phenylene group, a naphthylene group, a substituted naphthylene group, --O--, --SO--, or a group containing two or more of the foregoing groups in combination. 
     In the following, preferred embodiments of Y in formula (C I) are described in greater detail. 
     In one embodiment, Y is selected so that the compound represented by formula (C I) is a non-diffusible image forming compound which is oxidized as a result of development, thereby undergoing self-cleavage and releasing a diffusible dye. 
     An example of Y which is effective for compounds of this type is a N-substituted sulfamoyl group. For example, a group represented by formula (C II) is illustrated for Y. ##STR9## wherein 
     β represents non-metallic atoms necessary for forming a benzene ring, which may optionally be condensed with a carbon ring or a hetero ring to form, for example, a naphthalene ring, a quinoline ring, a 5,6,7,8-tetrahydronaphthalene ring, a chroman ring or the like. 
     α represents a group of --OG 11  or --NHG 12  (wherein G 11  represents hydrogen or a group which forms a hydroxyl group upon being hydrolyzed, and G 12  represents hydrogen, an alkyl group containing 1 to 22 carbon atoms or a hydrolyzable group), 
     Ball represents a ballast group, and 
     b represents an integer of 0, 1 or 2. 
     Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 33826/73 and 50736/78. 
     Other examples of Y suited for this type of compound are those represented by formula (C III): ##STR10## wherein Ball, α, and b are the same as defined with (C II), β&#39; represents atoms necessary for forming a carbon ring, e.g., a benzene ring which may optionally be condensed with another carbon ring or a hetero ring to form a naphthalene ring, quinoline ring, 5,6,7,8-tetrahydronaphthalene ring, chroman ring or the like. Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 113624/76, 12642/81, 16131/81, 16130/81, 4043/82 and 650/82, and U.S. Pat. No. 4,053,312. 
     Further examples of Y suited for this type of compound are those represented by formula (C IV): ##STR11## wherein Ball, α, and b are the same as defined with formula (C II), and β&#34; represents atoms necessary for forming a hetero ring such as a pyrazole ring, a pyridine ring or the like, said hetero ring being optionally bound to a carbon ring or a hetero ring. Specific examples of this type of Y are described in Japanese Patent Application (OPI) No. 104343/76. 
     Still further examples of Y suited for this type of compound are those represented by formula (C V): ##STR12## wherein γ preferably represents a hydrogen, a substituted or unsubstituted alkyl, aryl or heterocyclic group, or --CO--G 21  ; G 21  represents --OG 22 , --S--G 22  or ##STR13## (wherein G 22  represents a hydrogen, an alkyl group, a cycloalkyl group or an aryl group, G 23  is the same group as defined for said G 22 , or G 23  represents an acyl group derived from an aliphatic or aromatic carboxylic acid or sulfonic acid, and G 24  represents hydrogen atom or an unsubstituted or substituted alkyl group); and δ represents a residue necessary for completing a condensed benzene ring. 
     Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 104343/76, 46730/78, 130122/79 and 85055/82. 
     Still further examples of Y suited for this type of compound are those represented by formula (C VI): ##STR14## wherein Ball is the same as defined with formula (CII); ε represents an oxygen atom or ═NG 32  (wherein G 32  represents a hydroxyl group or an optionally substituted amino group) (examples of H 2  N--G 32  to be used for forming the group of ═NG 32  including hydroxylamine, hydrazines, semicarbazides, thiosemicarbazides, etc.); β&#34;&#39; represents a saturated or unsaturated nonaromatic 5-, 6- or 7-membered hydrocarbon ring; and G 31  represents a hydrogen or a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom, etc.). 
     Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 3819/78 and 48534/79. 
     Other examples of Y of this type of compound are described in Japanese Patent Publication Nos. 32129/73, 39165/73, Japanese Patent Application (OPI) No. 64436/74, U.S. Pat. No. 3,443,934, etc. 
     Still further examples of Y are those represented by formula (C VII): ##STR15## wherein α represents --OR 41  or --NHR 42  ; R 41  represents hydrogen or a hydrolyzable component; R 42  represents hydrogen, an alkyl group containing 1 to 50 carbon atoms or a hydrolyzable group; A 41  represents atoms necessary for forming an aromatic ring; Ball represents an organic immobile group existing on the aromatic ring, with Ball&#39;s being the same or different from each other; m represents an integer of 1 or 2; X represents a divalent organic group having 1 to 8 atoms, with the nucleophilic group (Nu) and an electrophilic center (asterisked carbon atom) formed by oxidation forming a 5- to 12-membered ring; Nu represents a nucleophilic group; n represents an integer of 1 or 2; and α may be the same as defined with the above-described formula (C II). Specific examples of this type of Y are described in Japanese Patent Application (OPI) No. 20735/82. 
     As still further type of examples represented by the formula of (C I), there are dye providing non-diffusible substances which release a diffusible dye in the presence of a base as a result of self cyclization or the like but which, when reacted with an oxidation product of a developing agent, substantially never release the dye. 
     Examples of Y effective for this type of compound are those which are represented by formula (CVIII): ##STR16## wherein 
     α&#39; represents an oxidizable nucleophilic group (e.g., a hydroxyl group, a primary or secondary amino group, a hydroxyamino group, a sulfonamido group or the like), or a precursor thereof; 
     α&#34; represents a dialkylamino group or an optional group defined for α&#39;; 
     G 51  represents an alkylene group having 1 to 3 carbon atoms; 
     a represents 0 or 1; 
     G 52  represents a substituted or unsubstituted alkyl group having 1 to 40 carbon atoms or a substituted or unsubstituted aryl group having 6 to 40 carbon atoms; 
     G 53  represents an electrophilic group such as --CO--, or --CS--; 
     G 54  represents an oxygen atom, a sulfur atom, a selenium atom, a nitrogen atom or the like, when G 54  represents a nitrogen atom, it has hydrogen or may be substituted by an alkyl or substituted alkyl group having 1  to 10 carbon atoms or an aromatic residue having 6 to 20 carbon atoms; and 
     G 55 , G 56  and G 57  each represents hydrogen, a halogen atom, a carbonyl group, a sulfamoyl group, a sulfonamido group, an alkyloxy group having 1 to 40 carbon atoms or an optional group defined for G 52 , G 55  and G 56  may form a 5- to 7-membered ring, and G 56  may represent ##STR17## with the proviso that at least one of G 52 , G 55 , G 56  and G 57  represents a ballast group. Specific examples of this type of Y are described in Japanese Patent Application (OPI) No. 63618/76. 
     Further examples of Y suited for this type of compound are those which are represented by formulae (C IX) and (C X): ##STR18## wherein Nu 61  and Nu 62 , which may be the same or different, each represents a nucleophilic group or a precursor thereof; Z 61  represents a divalent atom group which is electrically negative with respect to the carbon atom substituted by R 64  and R 65  ; R 61 , R 62 , and R 63  each represents hydrogen, a halogen atom, an alkyl group, an alkoxy group or an acylamino group or, when located at adjacent positions on the ring, R 61  and R 62  may form a condensed ring together with the rest of the molecule, or R 62  and R 63  may form a condensed ring together with the rest of the molecule; R 64  and R 65 , which may be the same or different, each represents hydrogen, a hydrocarbon group or a substituted hydrocarbon group; with at least one of the substituents, R 61 , R 62 , R 63 , R 64  and R 65  having a ballast group, Ball, of an enough size so as to render the above-described compounds immobile. Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 69033/78 and 130927/79. 
     Further examples of Y suited for this type of compound are those which are represented by formula of (C XI): ##STR19## wherein 
     Ball and β&#39; are the same as defined for those in formula (CIII), and G 71  represents an alkyl group (including a substituted alkyl group). Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 111628/74 and 4819/77. 
     As different type of compound represented by formula (C I), there are illustrated dye providing non-diffusible substances which themselves do not release any dye but, upon reaction with a reducing agent, release a dye. with these compounds, compounds which mediate the redox reaction (called electron donors) are preferably used in combination. 
     Examples of Y effective for this type of compound are those represented by formula (CXII): ##STR20## wherein Ball and β&#39; are the same as defined for those in formula (C III), and G 71  represents an alkyl group (including a substituted alkyl group). Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 35533/78 and 110827/78. 
     Further examples of Y suited for this type of compound are those which are represented by formula (C XIII): ##STR21## wherein α&#39; ox  and α&#34; ox  represent groups capable of giving α&#39; and α&#34;, respectively, upon reduction, and α&#39;, α&#34;, G 51 , G 52 , G 53 , G 54 , G 55 , G 56 , G 57  and a are the same as defined with respect to formula (CVIII). Specific examples of Y described above are described in Japanese Patent Application (OPI) No. 110827/78, U.S. Pat. Nos. 4,356,249 and 4,358,525. 
     Further examples of Y suited for this type of compound are those which are represented by formula (CXIV A) and (CXIV B); ##STR22## wherein (Nu ox ) 1  and (Nu ox ) 2 , which may be the same or different, each represents an oxidized nucleophilic group, and other notations are the same as defined with respect to formulae (CIX) and (CX). Specific examples of this type of Y are described in Japanese Patent Application (OPI) Nos. 130927/79 and 164342/81. 
     The puplicly known documents having been referred to with respect to (CXII), (CXIII), (CXIV A) and (CXIV B) describe electron donors to be used in combination. 
     As still further different type of compound represented by formula (CI), there are illustrated LDA compounds (Linked Donor Acceptor Compounds). These compounds are dye providing non-diffusible substances which cause donor-acceptor reaction in the presence of a base to release a diffusible dye but, upon reaction with an oxidation product of a developing agent, they substantially do not release the dye any more. 
     Examples of Y effective for this type of compound are those represented by formula of (CXV) (specific examples thereof being described in Japanese Patent Application (OPI) No. 60289/83): ##STR23## wherein n, x, y, and z each represents 1 or 2; m represents an integer of 1 or more; D on  represents a group containing an electron donor or its precursor moiety; L 1  represents an organic group linking Nup to --El--Q or D on  ; Nup represents a precursor of a nucleophilic group; El represents an electrophilic center; Q represents a divalent group; Ball represents a ballast group; L 2  represents a linking group; and M 1  represents an optional substituent. 
     The ballast groups is an organic ballast group which can render the dye providing substance non-diffusible, and is preferably a group containing a C 8-32  hydrophobic group. Such organic ballast group is bound to the dye providing substance directly or through a linking group (e.g., an imino bond, an ether bond, a thioether bond, a carbonamido bond, a sulfonamido bond, a ureido bond, an ester bond, an imido bond, a carbamoyl bond, a sulfamoyl bond, etc., and combination thereof). 
     Two or more kinds of the dye providing substances can be employed together. In such a case two or more kinds of the dye providing substances may be used together in order to provide the same hue or in order to reproduce black color. 
     Specific examples of dye image forming substances which can be used in the present invention are described in the patents cited hereinbefore. Since length prevents illustrating all preferred examples thereof, only a portion thereof is described hereinafter. Specific examples of the dye providing substances represented by formula (CI) are set forth below. ##STR24## 
     The above described compounds are only given as examples and the present invention should not be construed as being limited thereto. 
     Many of the above-described substances form an imagewise distribution of mobile dyes corresponding to exposure in a light-sensitive material by heat development, and methods for transferring these image dyes into a dye fixing material (so-called diffusion transfer) to produce images are described in the above described patents and Japanese Patent Application (OPI) Nos. 168439/84 and 182447/84. 
     The dye providing substance used in the present invention can be introduced into a layer of the light-sensitive material by known methods such as the method as described in U.S. Pat. 2,322,027, such as by using an organic solvent having a high boiling point or an organic solvent having a low boiling point as described in the Examples. For example, the dye-providing substance is dispersed in a hydrophilic colloid after dissolved in an organic solvent having a high boiling point, for example, a phthalic acid alkyl ester (for example, dibutyl phthalate, dioctyl phthalate, etc.), a phosphoric acid ester (for example, diphenyl phosphate, triphenyl phosphate, tricresyl phosphate, dioctylbutyl phosphate, etc.), a citric acid ester (for example, tributyl acetylcitrate, etc.), a benzoic acid ester (for example, octyl benzoate, etc.), an alkylamide (for example, diethyl laurylamide, etc.), an aliphatic acid ester (for example, dibutoxyethyl succinate, dioctyl azelate, etc.), a trimesic acid ester (for example, tributyl trimesate, etc.), etc. or an organic solvent having a boiling point of about 30° C. to 160° C., for example, a lower alkyl acetate such as ethyl acetate, butyl acetate, etc., ethyl propionate, secondary butyl alcohol, methyl isobutyl ketone, β-ethoxyethyl acetate, methyl cellosolve acetate, cyclohexanone, etc. The above-described organic solvents having a high-boiling point and organic solvents having a low boiling point may be used as a mixture thereof. 
     Further, it is possible to use a dispersion method using a polymer as described in Japanese Patent Publication No. 39853/76 and Japanese Patent Application (OPI) No. 59943/76. Moreover, various surface active agents can be used when the dye providing substance is dispersed in a hydrophilic colloid. For this purpose, the surface active agents illustrated in other part of the specification can be used. 
     An amount of the organic solvents having a high-boiling point used in the present invention is 10 g or less, and preferably 5 g or less, per gram of the dye providing substance. 
     In the present invention, if necessary, a reducing agent may be used. 
     The reducing agents used in the present invention include the following compounds. 
     Hydroquinone compounds (for example, hydroquinone, 2,5-dichlorohydroquinone, 2-chlorohydroquinone, etc.), aminophenol compounds (for example, 4-aminophenol, N-methylaminophenol, 3-methyl-4-aminophenol, 3,5-dibromoaminophenol, etc.), catechol compounds (for example, catechol, 4-cyclohexylcatechol, 3-methoxycatechol, 4-(N-octadecylamino)-catechol, etc.), phenylenediamine compounds (for example, N,N-diethyl-p-phenylenediamine, 3-methyl-N,N-diethyl-p-phenylenediamine, 3-methoxy-N-ethyl-N-ethoxy-p-phenylenediamine, N,N,N&#39;,N&#39;-tetramethyl-p-phenylenediamine, etc.). 
     Various combinations of developing agents as described in U.S. Pat. No. 3,039,869 can also be used. 
     In the present invention, an amount of the reducing agent added is from 0.01 mol to 20 mols per mol of silver and more preferably from 0.1 mol to 10 mols per mol of silver. 
     In the present invention, various dye-releasing activators can be used. These dye-releasing activators of the present invention are compounds which are basic and are capable of accelerating development, or so-called nucleophilic compounds. Namely bases or base precursors are used. 
     Although base precursors of the the present invention can be used as a dye-releasing activator, other bases or base precursors can be also incorporated. 
     The dye-releasing activator can be incorporated in the light-sensitive material or a dye-fixing material. In the case that the dye-releasing activator is incorporated in the light-sensitive material, it is advantageous to use a base precursor. 
     In the present invention, various development stopping agents can be used in order to always obtain constant image regardless of any changes of processing temperature and processing time in heat development. 
     These development stopping agents mean compounds capable of neutralizing a base or capable of reacting with a base to reduce the basicity in layer immediately after completion of appropriate development. In the concrete, acid precursors releasing an acid by heating, or compounds drops the basicity by reacting with a base coexisting by heating are used as a development stopping agent. For example, the embodiments of the acid precursor include oxime esters as described in Japanese Patent Application Nos. 216928/83 and 48305/84, and compounds releasing an acid due to Lossen rearrangement as described in Japanese Patent Application No. 85834/84. The compounds reacting with a base by heating are described in Japanese Patent Application No. 85836/84. 
     When base precursors are used, the most excellent effect of a development stopping agent is obtained. In that case, the molar ratio of a base precursor/an acid precursor is from 1/20 to 20/1, and preferably from 1/5 to 5/1. 
     The binder which can be used in the present invention can be employed alone or in combination thereof. A hydrophilic binder can be used as the binder according to the present invention. The typical hydrophilic binder is a transparent or translucent hydrophilic binder, examples of which include a natural substance, for example, protein such as gelatin, a gelatin derivative, etc., a polysaccharide such as starch, gum arabic, a cellulose derivatives, etc., and a synthetic polymer, for example, a water-soluble polyvinyl compound such as polyvinyl alcohol, polyvinyl pyrrolidone, acrylamide polymer, etc. Another example of the synthetic polymer compound is a dispersed vinyl compound in a latex form which is used for the purpose of increasing dimensional stability of a photographic material. 
     Further, in the present invention, it is possible to use a compound which activates development simultaneously while stabilizing the image. Particularly, it is preferred to use isothiuroniums including 2-hydroxyethylisothiuronium trichloroacetate as described in U.S. Pat. No. 3,301,678, bisisothiuroniums including 1,8-(3,6-dioxaoctane)-bis(isothiuronium trichloroacetate), etc., as described in U.S. Pat. No. 3,669,670, thiol compounds as described in West German Patent Application (OLS) No. 2,162,714, thiazolium compounds such as 2-amino-2-thazolium trichloroacetate, 2-amino-5-bromoethyl-2-thiazolium trichloroacetate, etc., as described in U.S. Pat. No. 4,012,260, compounds having α-sulfonylacetate as an acid part such as bis(2-amino-2-thiazolium)methylenebis(sulfonylacetate), 2-amino-2-thiazolium phenylsulfonylacetate, etc., as described in U.S. Pat. No. 4,060,420, and compounds having 2-carboxycarboxamide as an acid part as described in U.S. Pat. No. 4,088,496. 
     Furthermore, it is also preferred to use azolthioether and blocked-azoline thione compound as described in Belgian Pat. No. 768,071, 4-aryl-1-carbamyl-2-tetrazoline-5-thione compound as described in U.S. Pat. No. 3,893,859, and compounds as described in U.S. Pat. Nos. 3,839,041, 3,844,788 and 3,877,940. 
     The light-sensitive material of the present invention can contain a toning agent as occasion arises. Effective toning agents are 1,2,4-triazoles, 1H-tetrazoles, thiouracils, 1,3,4-thiadiazoles, and like compounds. Examples of preferred toning agents include 5-amino-1,3,4-thiadiazole-2-thiol, 3-mercapto-1,2,4-triazole, bis(dimethylcarbamyl)disulfide, 6-methylthiouracil, 1-phenyl-2-tetrazoline-5-thione, and the like. Particularly effective toning agents are compounds which can impart a black color tone to images. 
     The content of such a toning agent as described above, though depending upon the kind of a heat developable light-sensitive material used, processing conditions, desired images and various other factors, generally ranges from about 0.001 to 0.1 mol per mol of silver in the light-sensitive material. 
     The above-described various ingredients to constitute a heat developable light-sensitive material can be arranged in arbitrary positions, if desired. For instance, one or more of the ingredients can be incorporated in one or more of the constituent layers of a light-sensitive material, if desired. In some cases, it is desired that particular portions of reducing agent, image stabilizing agent and/or other additives should be distributed in a protective layer. As a result of the distribution in the above-described manner, migration of additives among constituent layers of a heat developable light-sensitive material can be reduced. Therefore, such distribution of additives is of advantage to some cases. 
     The heat developable light-sensitive materials of the present invention are effective in forming both negative or positive images. The negative or positive image can be formed depending mainly on the type of the light-sensitive silver halide. For instance, in order to produce direct positive images, internal image type silver halide emulsions described in U.S. Pat. Nos. 2,592,250, 3,206,313, 3,367,778 and 3,447,927, or mixtures of surface image type silver halide emulsions with internal image type silver halide emulsions as described in U.S. Pat. No. 2,996,382 can be used. 
     Various means of exposure can be used in the present invention. Latent images are obtained by imagewise exposure by radiant rays including visible rays. Generally, light sources used for conventional color prints can be used, examples of which include sun light, flash lamp, strobe light, tungsten lamps, mercury lamps, halogen lamps such as iodine lamps, xenon lamps, laser light sources, CRT light sources, plasma light source, fluorescent tubes and light-emitting diodes, etc. 
     In the present invention, after the heat-developable color photographic material is exposed to light, the resulting latent image can be developed by heating the whole material to a suitably elevated temperature. 
     As the heating means, a simple heat plate, iron, heat roller, heat generator utilizing carbon or titanium white, etc., or analogues thereof may be used. 
     A support used in the light-sensitive material and the dye fixing material employed, if desired, according to the present invention is that which can endure at the processing temperature. As an ordinary support, not only glass, paper, metal or analogues thereof may be used, but also an acetyl cellulose film, a cellulose ester film, a polyvinyl acetal film, a polystyrene film, a polycarbonate film, a polyethylene terephthalate film, and film related thereto or a plastic material may be used. Further, a paper support laminated with a polymer such as polyethylene, etc., can be used. The polyesters described in U.S. Pat. Nos. 3,634,089 and 3,725,070 are preferably used. 
     In the photographic light-sensitive material and the dye fixing material of the present invention, the photographic emulsion layer and other binder layers may contain inorganic or organic hardeners. It is possible to use chromium salts (chromium alum, chromium acetate, etc.), aldehydes (formaldehyde, glyoxal, glutaraldehyde, etc.), N-methylol compounds (dimethylolurea, methylol dimethylhydantoin, etc.), dioxane derivatives (2,3-dihydroxydioxane, etc.), active vinyl compounds (1,3,5-triacryloyl-hexahydro-s-triazine, 1,3-vinylsulfonyl-2-propanol, etc.), active halogen compounds (2,4-dichloro-6-hydroxy-s-triazine, etc.), mucohalogenic acids (mucochloric acid, mucophenoxychloric acid, etc.), etc. which are used alone or as a combination thereof. 
     The transfer of dyes from the light-sensitve layer to the dye-fixing layer can be carried out using a dye transfer assistant. 
     The dye transfer assistants suitably used in a process wherein it is supplied from the outside include water and an aqueous solution containing sodium hydroxide, potassium hydroxide or an inorganic alkali metal salt. Further, a solvent having a low boiling point such as methanol, N,N-dimethylformamide, acetone, diisobutyl ketone, etc., and a mixture of such a solvent having a low boiling point with water or an alkaline aqueous solution can be used. The dye transfer assistant may be used by wetting the image receiving layer with the transfer assistant. 
     When the dye transfer assistant is incorporated into the light-sensitive material or the dye-fixing material, it is not necessary to supply the transfer assistant from the outside. In this case, the above described dye transfer assistant may be incorporated into the material in the form of water of crystallization or microcapsules or as a precursor which releases a solvent at a high temperature. 
     More preferred process is a process wherein a hydrophilic thermal solvent which is solid at an ambient temperature and melts at a high temperature is incorporated into the light-sensitive material or the dye-fixing material. The hydrophilic thermal solvent can be incorporated either into any of the light-sensitive material and the dye-fixing material or into both of them. Although the solvent can be incorporated into any of the emulsion layer, the inter layer; the protective layer and the dye-fixing layer, it is preferred to incorporate it into the dye-fixing layer and/or adjacent layers thereto. 
     Examples of the hydrophilic thermal solvents include ureas, pyridines, amides, sulfonamides, imides, alcohols, oximes and other heterocyclic compounds. 
     Other compounds which can be used in the light-sensitive material of the present invention, for example, sulfamide derivatives, cationic compounds containing a pyridinium group, surface active agents having polyethylene oxide chains, antihalation and anti-irradiation dyes, hardeners, mordants and so on, are those described in U.S. Pat. Nos. 4,500,626, 4,478,927, 4,463,079, and Japanese Patent Application No. 28928/83 (corresponding to U.S. patent application Ser. No. 582,655 filed on Feb. 23, 1984) and U.S. Pat. No. 4,503,137. Methods for the exposure and so on cited in the above-described patents can be employed in the present invention also. 
     According to the present invention, an image having a high density can be obtained in a short time since the compound represented by formula (I) or (II) is incorporated into the heat developable light-sensitive material as a base precursor. Further, the heat developable light-sensitive material exhibits little change in photographic properties over time and has extremely good preservability. 
     The present invention will be explained in greater detail with reference to the following examples, but the present invention should not be construed as being limited thereto. 
     Unless otherwise specified all ratios, percents, etc., are by weight. 
     EXAMPLE 1 
     Method for Preparation of Silver Iodobromide Emulsion 
     40 g of gelatin and 26 g of potassium bromide (KBr) were dissolved in 3,000 ml of water and the solution was maintained at 50° C. with stirring. A solution of 34 g of silver nitrate dissolved in 200 ml of water and 200 ml of a solution of 0.02 g of Dye I described below dissolved in 300 ml of methanol were simultaneously added to the above prepared solution over a 10 minute period. Then, a solution of 3.3 g of potassium iodide (KI) dissolved in 100 ml of water was added over a 2 minute period. The thus prepared silver iodobromide emulsion was adjusted in pH and precipitated, and the excess salts were removed. It was then adjusted to a pH of 6.0, whereby 400 g of a silver iodobromide emulsion was obtained. 
     Dye I ##STR25## 
     Method for Preparation of Dispersion of Coupler in Gelatin 
     A mixture of 5 g of 2-dodecylcarbamoyl-1-naphthol, 0.5 g of sodium 2-ethylhexyl sulfosuccinate, 2.5 g of tricresyl phosphate (TCP) and 30 ml of ethyl acetate was dissolved. This solution was mixed with 100 g of a 10% aqueous solution of gelatin with stirring and the mixture was dispersed by means of a homogenizer at 10,000 rpm for 10 minutes. 
     A coating solution having the composition shown below was coated on a polyethylene terephthalate support at a wet thickness of 60 μm and dried to prepare a light-sensitive material. 
     (a) Silver iodobromide emulsion 10 g 
     (b) Dispersion of coupler in gelatin 3.5 g 
     (c) Base Precursor (1) according to the present invention 0.30 g 
     (d) Gelatin (10% aqueous solution) 5 g 
     (e) Solution of 0.2 g of 2,6-dichloro-p-aminophenol dissolved in 17 ml of water 
     The light-sensitive material thus-obtained was exposed imagewise at 2,000 lux for 5 seconds using a tungsten lamp and then uniformly heated for 20 seconds on a heat block which had been heated at 150° C., whereby a negative cyan color image was obtained. 
     The density of the cyan color image was measured using a Macbeth transmission densitometer (TD-504). The minimum density (D min ) was 0.20 and the maximum density (D max ) was 2.01. 
     From the result it will be understood that the compound according to the present invention provides a high density. 
     EXAMPLE 2 
     The same silver iodobromide emulsion as described in Example 1 and the following dispersion of dye-providing substance were employed. 
     Method for Preparation of Dispersion of Dye-Providing Substance 
     A mixture of 5 g of Dye Providing Substance CI-2 described above, 0.5 g of sodium 2-ethylhexyl sulfosuccinate, as a surface active agent, 5 g of tricresyl phosphate (TCP) and 30 ml ethyl acetate was dissolved by heating at about 60° C. to prepare a solution. This solution was mixed with 100 g of a 10% aqueous solution of gelatin with stirring and the mixture was dispersed by means of a homogenizer at 10,000 rpm for 10 minutes. 
     A method for preparation of a light-sensitive coating solution is described below. 
     Components 
     (a) Light-sensitive silver iodobromide emulsion (the same as prepared in Example 1) 25 g 
     (b) Dispersion of dye-providing substance CI-2 33 g 
     (c) 5 vol% Aqueous solution of a compound having the following formula 10 ml ##STR26## (d) 10% Aqueous solution of a compound having the following formula 4 ml 
     
         H.sub.2 NSO.sub.2 N(CH.sub.3).sub.2 
    
     (e) Base Precursor (1) according to the present invention 3.1 g 
     (f) Water 20 ml 
     The above components (a) to (f) were mixed and dissolved by heating and the mixture was coated on a polyethylene terephthalate film at a wet thickness of 30 μm and dried. This light-sensitive material is designated Sample A. 
     The light-sensitive material thus obtained was exposed imagewise at 2,000 lux for 10 seconds using a tungsten lamp and then uniformly heated for 20 seconds on a heat block which had been heated at 150° C. 
     For comparison, Samples B, C, and D were prepared an analogous procedure as described for Sample A, except that 1.8 g of guanidine trichloroacetate, 2.1 g of guanidine phenylsulfonyl acetate and 2.2 g of guanidine 3-sulfamoylphenylsulfonyl acetate were used in place of Base Precursor (1) according to the present invention as component (e), respectively. 
     A method for preparation of an image receiving material having an image receiving layer is described below. 
     10 g of poly(methyl acrylate-co-N,N,N-trimethyl-N-vinylbenzylammonium chloride) (molar ratio of methyl acrylate to vinylbenzylammonium chloride was 1/1) was dissolved in 200 ml of water and then uniformly mixed with 100 g of a 10% aqueous solution of lime-processed gelatin. The resulting mixture was uniformly coated at a wet thickness of 90 μm on a paper support laminated with polyethylene with titanium dioxide dispersed therein. The thus prepared material was dried and then used as an image receiving material. 
     The above described image receiving material was dipped in water and then superimposed on each of the above heated light-sensitive materials, i.e., Samples A, B, C, and D in such a manner that their coated layers were in contact with each other. 
     After heating for 6 seconds on a heat block maintained at 80° C., the image-receiving material was separated from the light-sensitive material, whereupon a negative magenta color image was obtained in the image-receiving material. 
     The maximum density (D max ) and the minimum density (D min ) of the negative color image were measured using a Macbeth reflection densitometer (RD-519). 
     Further, Samples A, B, C and D were preserved at 60° C. for 2 days and then subjected to the same procedure as described above to obtain negative magenta color images. The maximum density (D&#39; max ) and the minimum density (D&#39; min ) of these images were measured in the same manner as described above. 
     The results are shown in Table 1. 
     
                       TABLE 1                                                     
______________________________________                                    
Sample No.    D.sub.max                                                   
                      D.sub.min D&#39;.sub.max                                
                                      D&#39;.sub.min                          
______________________________________                                    
A (Present Invention)                                                     
              1.99    0.21      2.01  0.28                                
B (Comparison)                                                            
              2.14    0.58      Fog in whole area                         
C (Comparison)                                                            
              1.28    0.16      1.33  0.20                                
D (Comparison)                                                            
              1.45    0.15      1.49  0.27                                
______________________________________                                    
 
    
     From the results shown in Table 1, it is understood that the base precursor according to the present invention provides both a high maximum density and a low minimum density, and has good preservability. 
     EXAMPLE 3 
     The same procedures as described in Example 2 were repeated, except using the base precursors according to the present invention shown in Table 2. The results thus obtained are shown in Table 2. 
     
                       TABLE 2                                                     
______________________________________                                    
                Amount                                                    
Sample                                                                    
      Base      Added                                                     
No.   Precursor (g)      D.sub.max                                        
                               D.sub.min                                  
                                     D&#39;.sub.max                           
                                           D&#39;.sub.min                     
______________________________________                                    
E      (3)      3.2      2.18  0.22  2.25  0.26                           
F      (4)      3.4      2.25  0.26  2.20  0.29                           
G     (13)      3.3      2.09  0.20  2.08  0.27                           
H     (26)      3.4      2.20  0.21  2.24  0.29                           
______________________________________                                    
 
    
     From the results shown in Table 2, it is understood that the base precursors according to the present invention provide both a high maximum density and a low minimum density, and have good preservability. 
     EXAMPLE 4 
     In the following, an example using an organic silver salt oxidizing agent is illustrated. 
     Method for Preparation of Silver Benzotriazole Emulsion 
     28 g of gelatin and 13.2 g of benzotriazole were dissolved in 3,000 ml of water and the solution was maintained at 40° C. with stirring. A solution of 17 g of silver nitrate dissolved in 100 ml of water was added to the above prepared solution over a 2 minute period. The thus prepared silver benzotriazole emulsion was adjusted in pH and precipitated, and the excess salts were removed. It was then adjusted to a pH of 6.0, whereby 400 g of a silver benzotriazole emulsion was obtained. 
     Using the above described silver benzotriazole emulsion, the following light-sensitive material was prepared. 
     Components 
     (a) Silver iodobromide emulsion (same as described in Example 1) 20 g 
     (b) Silver benzotriazole emulsion 10 g 
     (c) Dispersion of dye providing substance (same as described in Example 2) 33 g 
     (d) 5% Aqueous solution of compound having the following formula: 10 ml ##STR27## (e) 10% Aqueous solution of compound having the following formula: 4 ml 
     
         H.sub.2 NSO.sub.2 N(CH.sub.3).sub.2 
    
     (f) Base Precursor (1) according to the present invention 3.3 g 
     (g) Dispersion of acid precursor in gelatin shown below 8 ml 
     (h) Water 12 ml 
     The dispersion of acid precursor in gelatin described in (g) above was prepared in the following manner. 
     10 g of the compound shown below was added to 100 g of a 1% aqueous solution of gelatin and the mixture was pulverized in a mill containing 100 g of glass beads with an average diameter of about 0.6 mm for 10 minutes. The glass beads were separated by filtration, and a dispersion of acid precursor in gelatin was obtained. 
     Acid Precursor ##STR28## 
     The above components (a) to (h) were mixed, followed by the same procedure as described in Example 2 to prepare a light-sensitive material and then the light-sensitive material was processed and evaluated in the same manner as described in Example 2. The results thus obtained are shown in Table 3. 
     
                       TABLE 3                                                     
______________________________________                                    
                      Maximum   Minimum                                   
Sample                Density   Density                                   
______________________________________                                    
A&#39;  containing Base Precursor (1)                                         
                          2.11      0.21                                  
    (Present Invention)                                                   
B&#39;  containing guanidine trichloroacetate                                 
                          2.33      0.61                                  
    (Comparison)                                                          
C&#39;  containing guanidine phenylsufonyl-                                   
                          1.47      0.19                                  
    acetate                                                               
    (Comparison)                                                          
______________________________________                                    
 
    
     From the results shown in Table 3, it is understood that the base precursor according to the present invention provides both a high maximum density and a low minimum density. 
     Further, Samples A&#39;, B&#39; and C&#39; were preserved at 60° C. for 2 days and then subjected to the same procedure as described above. The minimum densities and the maximum densities of Samples A&#39; and C&#39; were 0.28 and 2.14 and 0.20 and 1.52, respectively. Sample B&#39; exhibited fog in the whole area. From these results, it is apparent that the sample according to the present invention has good preservability. 
     EXAMPLE 5 
     Method for Preparation of Silver Benzotriazole Emulsion Containing Light-Sensitive Silver Bromide 
     10 g of gelatin and 6.5 g of benzotriazole were dissolved in 1,000 ml of water and the solution was maintained at 50° C. with stirring. A solution of 8.5 g of silver nitrate dissolved in 100 ml of water was added to the above prepared solution over a 2 minute period. Then, a solution of 1.2 g of potassium bromide dissolved in 50 ml of water was added over a 2 minute period. The thus prepared emulsion was adjusted in pH and precipitated, and the excess salts were removed. It was then adjusted to a pH of 6.0, whereby 200 g of a silver benzotriazole emulsion containing silver bromide was obtained. 
     Method for Preparation of Dispersion of Dye Providing Substance CI-16 in Gelatin 
     A mixture of 10 g of Dye Providing Substance CI-16 shown below, 0.5 g of sodium 2-ethylhexyl sulfosuccinate, as a surface active agent, 4 g of tricresyl phosphate (TCP) and 20 ml of cyclohexanone was dissolved by heating at about 60° C. to prepare a uniform solution. This solution was mixed with 100 g of a 10% aqueous solution of lime-processed gelatin with stirring and the mixture was dispersed by means of a homogenizer at 10,000 rpm for 10 minutes. 
     Dye Providing Substance CI-16 ##STR29## 
     A method for preparation of a light-sensitive material is described below. 
     (a) Silver benzotriazole emulsion containing light-sensitive silver bromide 10 g 
     (b) Dispersion of dye-providing substance 3.5 g 
     (c) Base Precursor (1) according to the present invention 0.32 g 
     (d) Gelatin (10% aqueous solution) 5 g 
     (e) Solution containing 200 mg of 2,6-dichloro-4-aminophenol dissolved in 4 ml of methanol 
     The above components (a) to (e) were mixed and dissolved by heating and the mixture was coated on a polyethylene terephthalate film having a thickness of 180 μm at a wet thickness of 30 μm, and dried. 
     The light-sensitive material thus obtained was exposed imagewise at 2,000 lux for 10 seconds using a tungsten lamp, and then uniformly heated for 20 seconds on a heat block which had been heated at 150° C. 
     The same procedure as described in Example 2 was conducted using the image receiving material as described in Example 2, and a negative magenta color image was obtained in the image-receiving material. 
     The density of the negative image was measured by means of a Macbeth reflection densitometer (RD-519). The maximum density was 2.05 and the minimum density was 0.21. These results demonstrate that the compound according to the present invention exhibits excellent effect. 
     EXAMPLE 6 
     Method for Preparation of Dispersion of Dye-Providing Substance CI-17 in Gelatin 
     A mixture of 5 g of Dye Providing Substance CI-17 shown below, 4 g of an electron donating substance shown below, 0.5 g of sodium 2-ethylhexyl sulfosuccinate, 10 g of tricresyl phosphate (TCP) and 20 ml of cyclohexanone was dissolved by heating at about 60° C. to prepare a solution. This solution was mixed with 100 g of a 10% aqueous solution of gelatin with stirring and the mixture was dispersed by means of a homogenizer at 10,000 rpm for 10 minutes. Dye Providing Substance CI-17: ##STR30## wherein R represents a group having the following formula: ##STR31## 
     Electron Donating Substance ##STR32## 
     A method for preparation of a light-sensitive material is described below. 
     Components 
     (a) Silver benzotriazole emulsion containing light-sensitive silver bromide (same as described in Example 5) 10 g 
     (b) Dispersion of dye-providing substance (described above) 3.5 g 
     (c) Base Precursor (1) according to the present invention 0.41 g 
     (d) 5% Aqueous solution of compound having the following formula: ##STR33## 
     The above components (a) to (d) and 4 ml of water were mixed and dissolved by heating and the mixture was coated on a polyethylene terephthalate film at a wet thickness of 30 μm and dried to prepare a light-sensitive material. 
     The light-sensitive material thus obtained was exposed imagewise at 2,000 lux for 10 seconds using a tungsten lamp and then uniformly heated for 40 seconds on a heat block which had been heated at 140° C. 
     The same image receiving material as described in Example 2 was dipped in water and then superimposed on the above heated light-sensitive material in such a manner that their coated layers were in contact with each other. 
     The positive magenta color image was obtained in the image receiving material. The density of the positive image was measured by means of a Macbeth reflection densitometer (RD-519). The maximum density and minimum density to green light were 2.08 and 0.28, respectively. 
     These results demonstrate the effectiveness of the base precursors according to the present invention. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.