Heat development image forming process thermally decoloring image recording process and process for decoloring cyanine dye

A heat developable light-sensitive material comprises a support, a light-sensitive layer and a non-light-sensitive layer. The light-sensitive layer contains silver halide and a reducing agent. The non-light-sensitive layer contains a cyanine dye represented by the formula (I) or a salt thereof and a base precursor: ##STR1## in which R.sup.1 is hydrogen, an aliphatic group, an aromatic group, --NR.sup.21 R.sup.24, --OR.sup.21 or --SR.sup.21, each of R.sup.21 and R.sup.24 independently is hydrogen, an aliphatic group or an aromatic group, or R.sup.21 and R.sup.24 are combined to form a nitrogen-containing heterocyclic ring; R.sup.2 is hydrogen, an aliphatic group or an aromatic group; R.sup.3 is an aliphatic group; L.sup.1 is a methine chain consisting of an odd number of methines; and each of Z.sup.1 and Z.sup.2 independently is an atomic group forming a five-membered or six-membered nitrogen-containing heterocyclic ring. A heat development image forming process, a thermal image recording material, a thermally decoloring image recording process and a process for decoloring a cyanine dye are also disclosed.

FIELD OF THE INVENTION
 The present invention relates to a heat developable light-sensitive
 material, a heat development image forming process, a thermal image
 recording material, a thermally decoloring image recording process and a
 process for decoloring cyanine dye.
 BACKGROUND OF THE INVENTION
 A heat developable light-sensitive material (or a photothermographic
 material) has already been proposed, and is described in U.S. Pat. Nos.
 3,152,904, 3,457,075, and B. Shely "Thermally Processed Silver Systems"
 (Imaging Processes and Materials, Neblette eighth edition, edited by
 Sturge, V. Walworth and A. Shepp, page 2, 1996).
 The heat developable light-sensitive material generally has a
 light-sensitive layer, which contains a catalytically active amount of a
 photo catalyst (e.g., silver halide), a reducing agent, a reducible silver
 salt (e.g., organic silver salt) and a color toning agent dispersed in a
 binder matrix. The color toning agent has a function of controlling color
 tone of silver. A heat development image forming process comprises steps
 of imagewise exposing to light the heat developable light-sensitive
 material, and then heating the light-sensitive material at an elevated
 temperature (not lower than 80.degree. C.) to cause an oxidation-reduction
 reaction between the silver halide or the reducible silver salt (which
 functions as an oxidizing agent) and the reducing agent. Thus a black
 silver image is formed. The oxidation-reduction reaction is accelerated by
 a catalytic function of a silver halide latent image formed at the
 exposing step. Accordingly, the black silver image is formed within the
 exposed area.
 The heat development does not require processing solutions of a wet
 development. The heat development can easily and rapidly be conducted,
 compared with the wet development. However, the wet development is still a
 major photographic technology, while the heat development is minor. The
 heat development has unsolved problems, while the wet development does not
 have the problems.
 A photographic material usually contains a dye, such as a filter dye, an
 antihalation dye or an antiirradiation dye. The dye functions at the
 exposing step. If the dye remains in the photographic material after image
 formation, a formed image would be colored with the dye. Therefore, the
 dye should be removed from a photographic material at a developing step.
 At the wet development, the dye can easily be removed from a photographic
 material by using processing solutions. On the other hand, it is very
 difficult (substantially impossible) to remove the dye from a photographic
 material at the heat development.
 A simple, easy and rapid development has been desired in the field of
 recent photography, especially in the field of recent clinical or printing
 photography. The improvement of the conventional wet development, however,
 has nearly reached its limits. Therefore, much attention has been paid
 again to a heat development image forming process in the field of clinical
 or printing photography.
 Since it is very difficult to remove a dye at the heat development, it has
 been proposed to decolor the dye at the heat development. For example,
 U.S. Pat. No. 5,135,842 discloses a method of decoloring a polymethine dye
 of a specific structure by heating a photographic material. U.S. Pat. Nos.
 5,314,795, 5,324,627 and 5,384,237 disclose a method of decoloring a
 polymethine dye by heating a photographic material in the presence of a
 carbanion forming agent (nucleophilic agent).
 SUMMARY OF THE INVENTION
 The known process of decoloring a dye by heat has some problems. For
 example, some dyes are not sufficiently decolored at heat development.
 Other dyes are decolored while storing a heat developable light-sensitive
 material because the dyes are not stable. Further, some known dyes are
 decolored to form decomposition products that have light absorption.
 Therefore, a formed image (particularly highlighted area) is colored with
 the decomposition products. Furthermore, some decolored dyes are colored
 again after the heat development (particularly in the presence of an
 acid). Moreover, a process of decoloring a dye with another compound such
 as a nucleophilic agent is influenced with a (stoichiometrical or
 dimensional) relation between the dye and the agent. Accordingly, the
 decoloring reaction between the dye and the agent is relatively slow.
 An object of the present invention is to provide a heat developable
 light-sensitive material containing a dye that is free from the
 above-mentioned problems.
 Another object of the invention is to provide a heat development image
 forming method that can form a clear image in which the dye is completely
 decolored.
 A further object of the invention is to provide a new thermal image
 recording material.
 A furthermore object of the invention is to provide a thermally decoloring
 image forming process that forms a decolored image in a simple manner.
 A still further object of the invention is to provide a process of
 decoloring a dye that is stable at room temperature by a substantially
 irreversible quick reaction.
 The present invention provides a heat developable light-sensitive material
 comprising a support, a light-sensitive layer and a non-light-sensitive
 layer, said light-sensitive layer containing silver halide and a reducing
 agent, and said non-light-sensitive layer containing a cyanine dye
 represented by the formula (I) or a salt thereof and a base precursor:
 ##STR2##
 in which R.sup.1 is hydrogen, an aliphatic group, an aromatic group,
 --NR.sup.21 R.sup.24, --OR.sup.21 or --SR.sup.21, each of R.sup.21 and
 R.sup.24 independently is hydrogen, an aliphatic group or an aromatic
 group, or R.sup.21 and R.sup.24 are combined to form a nitrogen-containing
 heterocyclic ring; R.sup.2 is hydrogen, an aliphatic group or an aromatic
 group; R.sup.3 is an aliphatic group; L.sup.1 is a methine chain
 consisting of an odd number of methines; and each of Z.sup.1 and Z.sup.2
 independently is an atomic group forming a five-membered or six-membered
 nitrogen-containing heterocyclic ring, which may be condensed with an
 aromatic ring.
 The invention also provides a heat development image forming process
 comprising steps of:
 imagewise exposing to light a heat developable light-sensitive material
 comprising a support, a light-sensitive layer and a non-light-sensitive
 layer, said light-sensitive layer containing silver halide and a reducing
 agent, and said non-light-sensitive layer containing a cyanine dye
 represented by the formula (I) or a salt thereof and a base precursor: and
 then heating the heat developable light-sensitive material at 80 to
 200.degree. C. to form a base from the base precursor whereby the cyanine
 dye is decolored and to develop the silver halide.
 The invention further provides a thermal image recording material
 comprising a support and an image recording layer, said image recording
 layer containing a cyanine dye represented by the formula (I) or a salt
 thereof and a base precursor.
 The invention furthermore provides a thermally decoloring image recording
 process comprising imagewise heating a thermal image recording material at
 80 to 200.degree. C., said image recording material comprising a support
 and an image recording layer, said image recording layer containing a
 cyanine dye represented by the formula (I) or a salt thereof and a base
 precursor to form a base from the base precursor whereby the cyanine dye
 is decolored.
 The invention still further provides a process for decoloring a cyanine dye
 comprising heating a cyanine dye represented by the formula (II) or a salt
 thereof at 80 to 200.degree. C. in the presence of a base:
 ##STR3##
 in which X.sup.21 is --NR.sup.24 --, --O-- or --S--; each of R.sup.21 and
 R.sup.24 independently is hydrogen, an aliphatic group or an aromatic
 group, or R.sup.21 and R.sup.24 are combined to form a nitrogen-containing
 heterocyclic ring; R.sup.22 is hydrogen, an aliphatic group or an aromatic
 group; R.sup.23 is an aliphatic group; L.sup.21 is a methine chain
 consisting of an odd number of methines; and each of Z.sup.21 and Z.sup.22
 independently is an atomic group forming a five-membered or six-membered
 nitrogen-containing heterocyclic ring, which may be condensed with an
 aromatic ring.
 The present inventors have found that the cyanine dye represented by the
 formula (I) is advantageously added to a non-light-sensitive layer of a
 heat developable light-sensitive material. The cyanine dye represented by
 the formula (I) is quickly decolored by a substantially irreversible
 reaction at heat development in an image forming method. According to
 study of the present inventors, a substantially colorless compound is
 formed from the cyanine dye represented by the formula (I) by an
 intramolecular ring forming reaction when the dye is heated in the
 presence of a base (under a basic condition). The reaction rapidly
 proceeds without influence caused by another agent because the decoloring
 reaction is an intramolecular reaction. Further, the decoloring reaction
 is a ring forming reaction that forms a five-membered or seven-membered
 ring condensed with the basic nucleus (onium form) of the cyanine dye. The
 formed compound is substantially colorless and relatively stable.
 Accordingly, the decoloring reaction is substantially irreversible. For
 the reasons mentioned above, the heat developable light-sensitive material
 of the present invention can form a clear image in which the dye is
 completely decolored.
 Further, a thermal image recording material can be prepared by using the
 cyanine dye represented by the formula (I). A thermally decolored image
 can be easily formed by a simple step of imagewise heating the thermal
 image recording material.
 Furthermore, the cyanine dye represented by the formula (II) is a stable
 compound at room temperature. According to the process of decoloring a
 dye, the stable dye can be decolored by a substantially irreversible quick
 reaction.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention uses a cyanine dye represented by the formula (I) or
 a salt thereof.
 ##STR4##
 In the formula (I), R.sup.1 is hydrogen, an aliphatic group, an aromatic
 group, --NR.sup.21 R.sup.24, --OR.sup.21 or --SR.sup.21. Each of R.sup.21
 and R.sup.24 independently is hydrogen, an aliphatic group or an aromatic
 group, or R.sup.21 and R.sup.24 are combined to form a nitrogen-containing
 heterocyclic ring. R.sup.1 preferably is --NR.sup.21 R.sup.24, --OR.sup.21
 or --SR.sup.21, as is defined in the formula (II). The details of
 --NR.sup.21 R.sup.24, --OR.sup.21 and --SR.sup.21 are described about the
 formula (II).
 In the present specification, the aliphatic group means an alkyl group, a
 substituted alkyl group, an alkenyl group, a substituted alkenyl group, an
 alkynyl group, a substituted alkynyl group, an aralkyl group and a
 substituted aralkyl group. The alkyl group, the substituted alkyl group,
 the alkenyl group, the substituted alkenyl group, the aralkyl group and
 the substituted aralkyl group are preferred, and the alkyl group, the
 substituted alkyl group, the aralkyl group and the substituted aralkyl
 group are more preferred. The aliphatic group preferably has a chain
 structure rather than a cyclic structure. The aliphatic group of the chain
 structure may be branched.
 The alkyl group preferably has 1 to 30 carbon atoms, more preferably has 1
 to 20 carbon atoms, further preferably has 1 to 15 carbon atoms, and most
 preferably has 1 to 12 carbon atoms. The alkyl moiety of the substituted
 alkyl group is the same as the above-described alkyl group.
 The alkenyl group and the alkynyl group preferably has 2 to 30 carbon
 atoms, more preferably has 2 to 20 carbon atoms, further preferably has 2
 to 15 carbon atoms, and most preferably has 2 to 12 carbon atoms. The
 alkenyl moiety of the substituted alkenyl group and the alkynyl moiety of
 the substituted alkynyl group are the same as the above-described alkenyl
 group and alkynyl group respectively.
 The aralkyl group preferably has 7 to 35 carbon atoms, more preferably has
 7 to 25 carbon atoms, further preferably has 7 to 20 carbon atoms, and
 most preferably has 7 to 15 carbon atoms. The aralkyl moiety of the
 substituted aralkyl group is the same as the above-described aralkyl
 group.
 Examples of the substituent groups of the aliphatic groups (the substituted
 alkyl group, the substituted alkenyl group, the substituted alkynyl group
 and the substituted aralkyl group) include a halogen atom (fluorine,
 chlorine, bromine), hydroxyl, nitro, carboxyl, sulfo, an acyl group, an
 alkoxy group, an alkoxycarbonyl group, an alkylthio group, an
 alkylthiocarbonyl group, an aryloxy group, an aryloxycarbonyl group and a
 carbamoyl group. Carboxyl and sulfo can be in the form of a salt. The
 cation forming a salt with carboxyl or sulfo preferably is an alkali metal
 ion (e.g., sodium ion, potassium ion).
 In the present specification, the aromatic group means an aryl group and a
 substituted aryl group.
 The aryl group preferably has 6 to 30 carbon atom, more preferably has 6 to
 20 carbon atoms, further preferably has 6 to 15 carbon atoms, and most
 preferably has 6 to 12 carbon atoms. The aryl moiety of the substituted
 aryl group is the same as the above-described aryl group.
 Examples of the substituent groups of the aromatic group (the substituted
 aryl group) include a halogen atom (fluorine, chlorine, bromine),
 hydroxyl, nitro, carboxyl, sulfo, an alkyl group, an acyl group, an alkoxy
 group, an alkoxycarbonyl group, an alkylthio group, an alkylthiocarbonyl
 group, an aryloxy group, an aryloxycarbonyl group and a carbamoyl group.
 Carboxyl and sulfo can be in the form of a salt. The cation forming a salt
 with carboxyl or sulfo preferably is an alkali metal ion (e.g., sodium
 ion, potassium ion).
 In the formula (I), R.sup.2 is hydrogen, an aliphatic group or an aromatic
 group. The aliphatic group and the aromatic group are defined above.
 R.sup.2 preferably is hydrogen or an aliphatic group, more preferably is
 hydrogen or an alkyl group, further preferably is hydrogen or an alkyl
 group having 1 to 15 carbon atoms, and most preferably is hydrogen.
 In the formula (I), R.sup.3 is an aliphatic group. The aliphatic group is
 defined above. R.sup.3 preferably is a substituted alkyl group. In view of
 synthesis of the compound, R.sup.3 preferably is a substituted alkyl group
 having the same meanings as --CHR.sup.2 --CO--R.sup.1.
 In the formula (I), L.sup.1 is a methine chain consisting of an odd number
 of methines. The number of the methines preferably is 3, 5, 7 or 9, more
 preferably is 3, 5 or 7, further preferably is 5 or 7, and most preferably
 is 5.
 The methine may have a substituent group. Examples of the substituent
 groups include a halogen atom, an aliphatic group, an aromatic group,
 --NR.sup.5 R.sup.6, --OR.sup.5 and --SR.sup.5. Each of R.sup.5 and R.sup.6
 independently is hydrogen, an aliphatic group or an aromatic group. The
 aliphatic group and the aromatic group are defined above. The substituent
 groups of the methine can be combined to form an unsaturated aliphatic
 ring or an unsaturated heterocyclic ring. The unsaturated aliphatic ring
 is preferred to the unsaturated heterocyclic ring. The formed ring
 preferably is a five-membered or six-membered ring. Cyclohexene ring is
 particularly preferred. The methine chain preferably is not substituted,
 or forms cyclohexene ring by combining substituent groups.
 In the formula (I), each of Z.sup.1 and Z.sup.2 independently is an atomic
 group forming a five-membered or six-membered nitrogen-containing
 heterocyclic ring. Examples of the nitrogen-containing heterocyclic rings
 include oxazole ring, thiazole ring, selenazole ring, pyrroline ring,
 imidazole ring and pyridine ring. A six-membered ring is preferred to a
 five-membered ring. The nitrogen-containing heterocyclic ring may be
 condensed with an aromatic ring (benzene ring, naphthalene ring). The
 nitrogen-containing heterocyclic ring and the condensed ring may have a
 substituent group. Examples of the substituent groups include a halogen
 atom (fluorine, chlorine, bromine), hydroxyl, nitro, carboxyl, sulfo and
 an alkyl group. Carboxyl and sulfo can be in the form of a salt. The
 cation forming a salt with carboxyl or sulfo preferably is an alkali metal
 ion (e.g., sodium ion, potassium ion).
 The cyanine dye represented by the formula (I) is preferably used in the
 form of a salt, which consists of the dye and an anion. In the case that
 the cyanine dye represented by the formula (I) has an anionic group such
 as carboxyl and sulfo, the dye can form an intramolecular salt. In the
 other cases, the cyanine dye preferably forms a salt with an anion other
 than its molecule. The anion preferably is monovalent or divalent, and
 more preferably is monovalent. Examples of the anions include halide ion
 (Cl, Br, I), p-toluenesulfonate ion, ethylsulfate ion,
 1,5-disulfonaphthalene dianion, PF.sub.6, BF.sub.4 and ClO.sub.4.
 A preferred cyanine dye is represented by the formula (Ia).
 ##STR5##
 In the formula (Ia), R.sup.11 is hydrogen, an aliphatic group, an aromatic
 group, --NR.sup.31 R.sup.34, --OR.sup.31 or --SR.sup.31. Each of R.sup.31
 and R.sup.34 independently is hydrogen, an aliphatic group or an aromatic
 group, or R.sup.31 and R.sup.34 are combined to form a nitrogen-containing
 heterocyclic ring. R.sup.11 preferably is --NR.sup.31 R.sup.34,
 --OR.sup.31 or --SR.sup.31, as is defined in the formula (IIa). The
 details of --NR.sup.31 R.sup.34, --OR.sup.31 and --SR.sup.31 are described
 about the formula (IIa).
 In the formula (Ia), R.sup.12 is hydrogen, an aliphatic group or an
 aromatic group. R.sup.12 preferably is hydrogen or an aliphatic group,
 more preferably is hydrogen or an alkyl group, further preferably is
 hydrogen or an alkyl group having 1 to 15 carbon atoms, and most
 preferably is hydrogen.
 In the formula (Ia), R.sup.13 is an aliphatic group. R.sup.13 preferably is
 a substituted alkyl group. In view of synthesis of the compound, R.sup.13
 preferably is a substituted alkyl group having the same meanings as
 --CHR.sup.12 --CO--R.sup.11.
 In the formula (Ia), L.sup.11 is a methine chain consisting of an odd
 number of methines. The number of the methines preferably is 3, 5, 7 or 9,
 more preferably is 3, 5 or 7, further preferably is 5 or 7, and most
 preferably is 5.
 The methine may have a substituent group. Examples of the substituent
 groups include a halogen atom, an aliphatic group, an aromatic group,
 --NR.sup.15 R.sup.16, --OR.sup.15 and --SR.sup.15. Each of R.sup.15 and
 R.sup.16 independently is hydrogen, an aliphatic group or an aromatic
 group. The aliphatic group and the aromatic group are defined above. The
 substituent groups of the methine can be combined to form an unsaturated
 aliphatic ring or an unsaturated heterocyclic ring. The unsaturated
 aliphatic ring is preferred to the unsaturated heterocyclic ring. The
 formed ring preferably is a five-membered or six-membered ring.
 Cycloheptene ring and cyclohexene ring are particularly preferred. The
 methine chain preferably is not substituted, or forms cycloheptene ring or
 cyclohexene ring by combining substituent groups.
 In the formula (Ia), each of y.sup.11 and y.sup.12 independently is
 --CR.sup.14 R.sup.15 --, --NR.sup.14 --, --O--, --S--or --Se--. Each of
 R.sup.14 and R.sup.15 independently is hydrogen or an aliphatic group or
 R.sup.14 and R.sup.15 are combined to form an aliphatic ring. The
 aliphatic group preferably is an alkyl group or a substituted alkyl group.
 The aliphatic ring preferably is a saturated aliphatic ring, more
 preferably is five-membered ring (cyclopentane ring), six-membered ring
 (cyclohexane ring) or seven-membered ring (cycloheptane ring), and most
 preferably is cyclohexane ring.
 In the formula (Ia), the benzene rings of Z.sup.11 and Z.sup.12 may be
 condensed with another benzene ring. The benzene rings of Z.sup.11,
 Z.sup.12 and the condensed ring may have a substituent group. Examples of
 the substituent groups include a halogen atom (fluorine, chlorine,
 bromine), hydroxyl, nitro, carboxyl, sulfo and an alkyl group. Carboxyl
 and sulfo can be in the form of a salt. The cation forming a salt with
 carboxyl or sulfo preferably is an alkali metal ion (e.g., sodium ion,
 potassium ion).
 The cyanine dye represented by the formula (Ia) is preferably used in the
 form of a salt, which consists of the dye and an anion. The salt is
 described about the formula (I).
 A more preferred cyanine dye is represented by the formula (Ib).
 ##STR6##
 In the formula (Ib), the two groups of R.sup.41 are identical. R.sup.41 is
 hydrogen, an aliphatic group, an aromatic group, --NR.sup.51 R.sup.52,
 --OR.sup.51 or --SR.sup.51. Each of R.sup.51 and R.sup.52 independently is
 hydrogen, an aliphatic group or an aromatic group, or R.sup.51 and
 R.sup.52 are combined to form a nitrogen-containing heterocyclic ring.
 R.sup.41 preferably is --NR.sup.51 R.sup.52, --OR.sup.51 or --SR.sup.51,
 as is defined in the formula (IIa). The details of --NR.sup.51 R.sup.52,
 --OR.sup.51 and --SR.sup.51 are described about the formula (IIb).
 The cyanine dye represented by the formula (Ib) is preferably used in the
 form of a salt, which consists of the dye and an anion. The salt is
 described about the formula (I).
 In the formula (I), R.sup.1 preferably is --NR.sup.21 R.sup.24, --OR.sup.21
 or --SR.sup.21. Where R.sup.1 is hydrogen, an aliphatic group or an
 aromatic group, the cyanine dye is quickly decolored by a base at an
 elevated temperature. However, the dye having hydrogen, an aliphatic group
 or an aromatic group as R.sup.1 is sometimes decolored while storing it
 because the dye is relatively labile. The stability of the dye is improved
 where R.sup.1 is --NR.sup.21 R.sup.24, --OR.sup.21 or --SR.sup.21. The
 stable cyanine dye is represented by the formula (II).
 ##STR7##
 In the formula (II), X.sup.21 is --NR.sup.24 --, --O-- or --S--. Each of
 R.sup.21 and R.sup.24 independently is hydrogen, an aliphatic group or an
 aromatic group, or R.sup.21 and R.sup.24 are combined to form a
 nitrogen-containing heterocyclic ring. R.sup.21 preferably is an aliphatic
 group or an aromatic group, and more preferably is an alkyl group, a
 substituted alkyl group, an aralkyl group, a substituted aralkyl group, an
 aryl group or a substituted aryl group. R.sup.24 preferably is hydrogen or
 an aliphatic group, and more preferably is hydrogen, an alkyl group or a
 substituted alkyl group. The nitrogen-containing heterocyclic ring formed
 by combining R.sup.21 and R.sup.24 preferably is a five-membered ring or a
 six-membered ring. The nitrogen-containing heterocyclic ring may contain a
 hetero atom (e.g., oxygen, sulfur) in addition to nitrogen.
 In the formula (II), R.sup.22 is hydrogen, an aliphatic group or an
 aromatic group. R.sup.22 preferably is hydrogen or an aliphatic group,
 more preferably is hydrogen or an alkyl group, further preferably is
 hydrogen or an alkyl group having 1 to 15 carbon atoms, and most
 preferably is hydrogen.
 In the formula (II), R.sup.23 is an aliphatic group. R.sup.23 preferably is
 a substituted alkyl group. In view of synthesis of the compound, R.sup.23
 preferably is a substituted alkyl group having the same meanings as
 --CHR.sup.22 --CO--R.sup.21.
 In the formula (II), L.sup.21 is a methine chain consisting of an odd
 number of methines. The number of the methines preferably is 3, 5, 7 or 9,
 more preferably is 3, 5 or 7, further preferably is 5 or 7, and most
 preferably is 5.
 The methine may have a substituent group. Examples of the substituent
 groups include a halogen atom, an aliphatic group, an aromatic group,
 --NR.sup.25 R.sup.26, --OR.sup.25 and --SR.sup.25. Each of R.sup.25 and
 R.sup.26 independently is hydrogen, an aliphatic group or an aromatic
 group. The aliphatic group and the aromatic group are defined above. The
 substituent groups of the methine can be combined to form an unsaturated
 aliphatic ring or an unsaturated heterocyclic ring. The unsaturated
 aliphatic ring is preferred to the unsaturated heterocyclic ring. The
 formed ring preferably is a five-membered or six-membered ring.
 Cycloheptene ring is particularly preferred. The methine chain preferably
 is not substituted, or forms cyclohexene ring by combining substituent
 groups.
 In the formula (II), each of Z.sup.21 and Z.sup.22 independently is an
 atomic group forming a five-membered or six-membered nitrogen-containing
 heterocyclic ring. Examples of the nitrogen-containing heterocyclic rings
 include oxazole ring, thiazole ring, selenazole ring, pyrroline ring,
 imidazole ring and pyridine ring. A six-membered ring is preferred to a
 five-membered ring. The nitrogen-containing heterocyclic ring may be
 condensed with an aromatic ring. The nitrogen-containing heterocyclic ring
 and the condensed ring may have a substituent group. Examples of the
 substituent groups include a halogen atom, hydroxyl, nitro, carboxyl,
 sulfo and an alkyl group. Carboxyl and sulfo can be in the form of a salt.
 The cation forming a salt with carboxyl or sulfo preferably is an alkali
 metal ion.
 The cyanine dye represented by the formula (II) is preferably used in the
 form of a salt, which consists of the dye and an anion. The salt is
 described about the formula (I).
 In the formula (Ia), R.sup.11 preferably is --NR.sup.31 R.sup.34,
 --OR.sup.31 or --SR.sup.31. The preferred cyanine dye is represented by
 the formula (IIa).
 ##STR8##
 In the formula (IIa), X.sup.31 is --NR.sup.34 --, --O-- or --S--. Each of
 R.sup.31 and R.sup.34 independently is hydrogen, an aliphatic group or an
 aromatic group, or R.sup.31 and R.sup.34 are combined to form a
 nitrogen-containing heterocyclic ring. R.sup.31 preferably is an aliphatic
 group or an aromatic group, and more preferably is an alkyl group, a
 substituted alkyl group, an aralkyl group, a substituted aralkyl group, an
 aryl group or a substituted aryl group. R.sup.34 preferably is hydrogen or
 an aliphatic group, and more preferably is hydrogen, an alkyl group or a
 substituted alkyl group. The nitrogen-containing heterocyclic ring formed
 by combining R.sup.31 and R.sup.34 preferably is a five-membered ring or a
 six-membered ring. The nitrogen-containing heterocyclic ring may contain a
 hetero atom in addition to nitrogen.
 In the formula (IIa), R.sup.32 is hydrogen, an aliphatic group or an
 aromatic group. R.sup.32 preferably is hydrogen or an aliphatic group,
 more preferably is hydrogen or an alkyl group, further preferably is
 hydrogen or an alkyl group having 1 to 15 carbon atoms, and most
 preferably is hydrogen.
 In the formula (IIa), R.sup.33 is an aliphatic group. R.sup.33 preferably
 is a substituted alkyl group. In view of synthesis of the compound,
 R.sup.33 preferably is a substituted alkyl group having the same meanings
 as --CHR.sup.32 --CO--R.sup.31.
 In the formula (IIa), L.sup.31 is a methine chain consisting of an odd
 number of methines. The number of the methines preferably is 3, 5, 7 or 9,
 more preferably is 3, 5 or 7, further preferably is 5 or 7, and most
 preferably is 5.
 The methine may have a substituent group. Examples of the substituent
 groups include a halogen atom, an aliphatic group, an aromatic group,
 --NR.sup.35 R.sup.36, --OR.sup.35 and --SR.sup.35. Each of R.sup.35 and
 R.sup.36 independently is hydrogen, an aliphatic group or an aromatic
 group. The aliphatic group and the aromatic group are defined above. The
 substituent groups of the methine can be combined to form an unsaturated
 aliphatic ring or an unsaturated heterocyclic ring. The unsaturated
 aliphatic ring is preferred to the unsaturated heterocyclic ring. The
 formed ring preferably is a five-membered or six-membered ring.
 Cycloheptene ring and cyclohexene ring are particularly preferred. The
 methine chain preferably is not substituted, or forms cycloheptene ring or
 cyclohexene ring by combining substituent groups.
 In the formula (IIa), Y.sup.31 and Y.sup.32 independently is --CR.sup.37
 R.sup.38 --, --NR.sup.37 --, --O--, --S-- or --Se--. Each of R.sup.37 and
 R.sup.38 independently is hydrogen or an aliphatic group or R.sup.37 and
 R.sup.38 are combined to form an aliphatic ring. The aliphatic group
 preferably is an alkyl group or a substituted alkyl group. The aliphatic
 ring preferably is a saturated aliphatic ring, more preferably is
 cyclopentane ring, cyclohexane ring or cycloheptane ring, and most
 preferably is cyclohexane ring.
 In the formula (IIa), the benzene rings of Z.sup.31 and Z.sup.32 may be
 condensed with another benzene ring. The benzene rings of Z.sup.11,
 Z.sup.12 and the condensed ring may have a substituent group. Examples of
 the substituent groups include a halogen atom, hydroxyl, nitro, carboxyl,
 sulfo and an alkyl group. Carboxyl and sulfo can be in the form of a salt.
 The cation forming a salt with carboxyl or sulfo preferably is an alkali
 metal ion.
 The cyanine dye represented by the formula (IIa) is preferably used in the
 form of a salt, which consists of the dye and an anion. The salt is
 described about the formula (I).
 In the formula (IIb), R.sup.41 preferably is --NR.sup.51 R.sup.52,
 --OR.sup.51 or --SR.sup.51. The most preferred cyanine dye is represented
 by the formula (IIb).
 ##STR9##
 In the formula (IIb), the two groups of X.sup.51 are identical. The two
 groups of R.sup.51 are also identical. X.sup.51 is --NR.sup.52 --, --O--
 or --S--. Each of R.sup.51 and R.sup.52 independently is hydrogen, an
 aliphatic group or an aromatic group, or R.sup.51 and R.sup.52 are
 combined to form a nitrogen-containing heterocyclic ring. R.sup.51
 preferably is an aliphatic group or an aromatic group, and more preferably
 is an alkyl group, a substituted alkyl group, an aralkyl group, a
 substituted aralkyl group, an aryl group or a substituted aryl group.
 R.sup.52 preferably is hydrogen or an aliphatic group, and more preferably
 is hydrogen, an alkyl group or a substituted alkyl group. The
 nitrogen-containing heterocyclic ring formed by combining R.sup.51 and
 R.sup.52 preferably is a five-membered ring or a six-membered ring. The
 nitrogen-containing heterocyclic ring may contain a hetero atom in
 addition to nitrogen.
 The cyanine dye represented by the formula (IIb) is preferably used in the
 form of a salt, which consists of the dye and an anion. The salt is
 described about the formula (I).
 Examples of the cyanine dyes represented by the formula (Ib) are shown
 below. The anion (X) and R.sup.41 of the formula (Ib) are shown in the
 examples.
 ##STR10##
 ##STR11##
 ##STR12##
 ##STR13##
 ##STR14##
 ##STR15##
 The other cyanine dyes represented by the formula (I) are shown below.
 ##STR16##
 ##STR17##
 ##STR18##
 SYNTHESIS EXAMPLE 1
 Synthesis of Cyanine Dye (1)
 With 30 ml of ethanol, 33.4 g of ethyl bromoacetate and 15.9 g of
 2,3,3-trimethylindolenine were mixed. The mixture was refluxed for 5 hours
 while heating. After the reaction was completed, 50 ml of acetone and 500
 ml of ethyl acetate were added to the mixture. Precipitated quaternary
 salt was filtered off. The yield of the quaternary salt was 25.4 g. The
 melting point was higher than 250.degree. C.
 With 19.0 g of acetic anhydride, 16.3 g of the quaternary salt, 4.9 g of
 tetramethoxypropane, 75 g of N-methylpyrrolidone and 2.85 g of acetic acid
 were mixed. The mixture was heated at 50.degree. C. for 3 hours. After the
 reaction was completed, 50 ml of water was added to the mixture.
 Precipitated crystals were filtered off, and recrystallized with a mixture
 of methanol, isopropanol and ethyl acetate. The yield was 13.1 g, the
 melting point was higher than 250.degree. C., .lambda.max was 637.5 nm,
 and .epsilon. was 2.16.times.10.sup.5 (methanol).
 SYNTHESIS EXAMPLE 2
 Synthesis of Cyanine Dye (3)
 With 57 ml of acetic acid, 30.8 g of di(n-butyl)iodoacetamide and 15.9 g of
 2,3,3-trimethylindolenine were mixed. The mixture was heated at
 100.degree. C. for 10 hours. After the reaction was completed, 11.1 g of
 3-anilino-N-phenyl-2-propenylideneimine, 8.1 ml of pyridine, 9.4 ml of
 acetic anhydride and 30 ml of dimethylformaldehyde were added to the
 mixture. The resulting mixture was stirred at room temperature for 1 hour.
 The product was purified by a flash column chromatography. The yield was
 14.4 g, the melting point was higher than 250.degree. C., .lambda.max was
 639.5 nm, and .epsilon. was 2.15.times.10.sup.5 (methanol).
 Other cyanine dyes can be synthesized in a manner similar to the synthesis
 examples. The similar synthesis methods are described in Japanese Patent
 Provisional Publication Nos. 61(1986)-123454 and 7(1995)-333784.
 The cyanine dye represented by the formula (I) or a salt thereof can be
 decolored by heating in the presence of a base. The present inventors have
 found that an active methylene group of the cyanine dye represented by the
 formula (I) is deprotonated in the presence of a base to form a
 nucleophilic spices, which attacks the methine chain to form a
 substantially colorless intramolecular cyclic compound. The base in the
 decoloring reaction should have a basicity of deprotoning the active
 methylene group of the cyanine dye. The ring formed by the decoloring
 reaction is considered to be a five-membered or seven-membered ring.
 The formed substantially colorless compound is stable, and does not return
 to the cyanine dye. Accordingly, the present invention is free from the
 problem of color reversion.
 The decoloring reaction can be conducted according to a solvent system or a
 non-solvent system. The solvent system is preferably conducted by heating
 a solution of a cyanine dye and a base (or base precursor). The solvent is
 a liquid at a heating temperature (described below), which dissolves the
 cyanine dye and the base (or base precursor). Examples of the solvents
 include dimethyl sulfoxide and dimethylacetamide. The non-solvent (dry)
 system is preferably conducted by heating a sheet on which a cyanine dye
 and a base (or base precursor) are coated (such as an image recording
 material or a light-sensitive material). The non-solvent system, namely
 the image recording material or the light-sensitive material is described
 below.
 The heating temperature at the decoloring reaction is preferably in the
 range of 40 to 200.degree. C., more preferably in the range of 80 to
 150.degree. C., further preferably in the range of 100 to 130.degree. C.,
 and most preferably in the range of 115 to 125.degree. C. The heating time
 is preferably in the range of 5 to 120 seconds, more preferably in the
 range of 10 to 60 seconds, further preferably in the range of 12 to 30
 seconds, and most preferably in the range of 15 to 25 seconds.
 A heat developable light-sensitive material (described below) is heated for
 heat development. A base is preferably formed by heating a base precursor
 of a thermal decomposition type. In the case of using the heat developable
 light-sensitive material or the base precursor, the heating temperature
 and the heating time is determined by considering the temperature and time
 for the heat development or the thermal decomposition as well as the
 above-described decoloring reaction.
 The decoloring reaction can use a base in a broad sense. The bases in the
 broad sense include a nucleophilic agent (Lewis base) as well as a base in
 a narrow sense. The cyanine dye would be decolored in the presence of a
 base even at room temperature. Accordingly, the base is preferably
 separated from the cyanine dye when the base and the dye are stored, and
 the cyanine dye is preferably contacted with the base when they are heated
 (when the dye should be decolored). The base can be separated from the
 cyanine dye by using chemical means or physical means.
 The physical separating means include use of microcapsules, addition of a
 base to hot-melt particles and addition of a base to a layer separated
 from a layer containing a cyanine dye. The microcapsules can be ruptured
 by pressure or heat. Microcapsules ruptured by heat (described in Hiroyuki
 Moriga, Introduction of Chemistry of Specific Paper (written in Japanese,
 1975) or Japanese Patent Provisional Publication No. 1(1989)-150575) are
 advantageously used because the decoloring reaction is conducted at an
 elevated temperature. One of the base and cyanine dye is contained in the
 microcapsules for separation. In the case that the shell of the
 microcapsule is opaque, the base is preferably contained in the
 microcapsules. The base or the cyanine dye (preferably the base) can be
 contained in hot-melt particles for separation. The hot-melt particles are
 formed of a substance that can be melt by heat such as wax. The melting
 point of the substance is arrange between the room temperature and the
 above-described heating temperature. In an image recording material or a
 light-sensitive material, a base can be contained in a layer separated
 from a layer containing a cyanine dye. A barrier layer containing a
 hot-melt substance is preferably provided between the layer containing the
 base and the layer containing the cyanine dye.
 The chemical separating means are preferred to the physical separating
 means. A base precursor is a representative chemical separating means.
 Various base precursors have been proposed. A precursor of forming (or
 releasing) a base at an elevated temperature is advantageously used
 because the decoloring reaction is also conducted at an elevated
 temperature. A base precursor of a thermal decomposition type is
 preferred. The base precursor of the thermal decomposition type more
 preferably consists of a salt of a base with a carboxylic acid
 (decarboxylation type). When the base precursor of the decarboxylation
 type is heated, carboxyl of the carboxylic acid is decarboxylated to
 release a base. The acid preferably has carboxyl that can easily be
 decarboxylated. In this regard, a sulfonylacetic acid and a propionic acid
 are preferred. The sulfonylacetic acid and the propionic acid preferably
 has an aromatic group (an aryl group or an unsaturated heterocyclic
 group), which has a function of accelerating decarboxylation reaction. A
 base precursor of a sulfonylacetic salt is described in Japanese Patent
 Provisional Publication No. 59(1984)-168441. A base precursor of a
 propionic salt is described in Japanese Patent Provisional Publication No.
 59(1984)-180537.
 The base precursor of the decarboxylation type preferably contains an
 organic base as a base component. The organic base preferably is an
 amidine, a guanidine or their derivatives. The organic base also
 preferably is a diacidic, triacidic or tetraacidic base, more preferably
 is a diacidic base, and most preferably is a diacidic base of an amidine
 or guanidine derivative.
 A precursor of a diacidic, triacidic or tetraacidic base of an amidine
 derivative is described in Japanese Patent Publication No. 7(1995)-59545.
 A precursor of a diacidic, triacidic or tetraacidic base of a guanidine
 derivative is described in Japanese Patent Publication No. 8(1996)-10321.
 The diacidic base of the amidine or guanidine derivative comprises (A) two
 amidine or guanidine moieties, (B) substituent groups of the amidine or
 guanidine moieties and (C) a divalent linking group combining the two
 amidine or guanidine moieties. Examples of the substituent groups of (B)
 include an alkyl group (including a cycloalkyl group), an alkenyl group,
 an alkynyl group, an aralkyl group and a heterocyclic group. Two or more
 substituent groups can be combined to form a nitrogen-containing
 heterocyclic group. The linking group of (C) preferably is an alkylene
 group or phenylene.
 Examples of the diacidic base precursors of the amidine or guanidine
 derivatives are shown below.
 ##STR19##
 ##STR20##
 ##STR21##
 The amount (mol) of the base precursor is preferably 1 to 100 times, and
 more preferably 3 to 30 times of the amount (mol) of the cyanine dye.
 The cyanine dye can be used in various technical fields to make an
 advantage of the above-described decoloring reaction. For example, a
 solution of a cyanine dye and a base precursor can be used as an ink,
 which can be decolored by heat. Further, the solution can be coated on a
 transparent support to form a color sheet (filter), which can be decolored
 by heat.
 The cyanine dye and the base precursor can be used in a thermal image
 recording material, which comprises a support (preferably transparent
 support) and an image recording layer. The image recording-layer contains
 the cyanine dye and the base precursor. The image recording layer can be
 formed by coating a solution or dispersion of the dye and the base
 precursor on the support. The cyanine dye is preferably in the form of
 solid particles, which are dispersed in the image recording layer. The
 cyanine dye in the form of the solid particles can be formed by using a
 dispersion of the dye. The base precursor is also preferably in the form
 of solid particles. The image recording layer preferably further contains
 a binder. The binder preferably is a hydrophilic polymer (e.g., polyvinyl
 alcohol, gelatin).
 The thermal image recording material is imagewise heated to form a
 decolored image within the heated area. The material can easily be
 imagewise heated by using a thermal head, which is attached to a facsimile
 machine or a thermal printer. The heating temperature is preferably in the
 range of 80 to 200.degree. C., and more preferably in the range of 100 to
 200.degree. C.
 The cyanine dye can be advantageously used in a heat developable
 light-sensitive material. The cyanine dye and the base precursor is added
 to a non-light-sensitive layer of the light-sensitive material. The
 non-light-sensitive layer containing the cyanine dye can function as a
 filter layer or an antihalation layer. The heat developable
 light-sensitive material usually comprises a non-light-sensitive layer as
 well as a light-sensitive layer. In view of arrangement, the
 non-light-sensitive layer can be classified into (1) an overcoating layer
 provided on a light-sensitive layer, (2) an intermediate layer provided
 between light-sensitive layers, (3) an undercoating layer provided between
 a support and a light-sensitive layer and (4) a backing layer provided on
 a support (the side on which a light-sensitive layer is not provided). The
 filter layer is provided on the light-sensitive material as (1) an
 overcoating layer or (2) an intermediate layer. The antihalation layer is
 provided as (3) an undercoating layer or (4) a backing layer.
 The cyanine dye and the base precursor is preferably contained in the same
 non-light-sensitive layer. The cyanine dye and the base precursor can be
 contained in separated but adjacent two layers respectively. A barrier
 layer can be further provided between two layers. In the present
 specification, the expression "layer contains a cyanine dye and a base
 precursor" includes the case that two layers contains the cyanine dye and
 the base precursor separately.
 The cyanine dye can be contained in the non-light-sensitive layer by adding
 a solution, emulsion, solid particle dispersion or polymer impregnant of
 the dye to a coating solution of the layer. Further, the dye can be added
 to the non-light-sensitive layer by using a polymer mordant.
 A latex (described in U.S. Pat. No. 4,199,363, German Patent Publication
 Nos. 2,541,230, 2,541,274, European Patent Publication No. 029,104 and
 Japanese Patent Publication No. 53(1978)-41091) can be used for the
 polymer impregnant. An emulsion of a dye can be prepared by adding the
 cyanine dye to a polymer solution and emulsifying the polymer solution, as
 is described in International Patent Publication No. 88/00723.
 The amount of the cyanine dye depends on use of the dye. The amount of the
 dye is usually so adjusted that the optical density (absorbance) at a
 desired wavelength is higher than 0.1. The optical density is preferably
 in the range of 0.2 to 2. The amount of the dye for the abovementioned
 optical density is usually in the range of 0.001 to 1 g per m.sup.2. After
 the cyanine dye is decolored according to the present invention, the
 optical density can be reduced to not higher than 0.1.
 Two or more cyanine dyes can be used in combination. Two or more base
 precursors can also be used in combination.
 The heat developable light-sensitive material is described below in more
 detail.
 The heat developable light-sensitive material preferably is a monosheet
 type, which means that an image can directly be formed on a heat
 developable light-sensitive material without using another sheet such as
 an image-receiving material.
 The heat developable light-sensitive material has a light-sensitive layer
 containing silver halide (an catalytically active amount of photo
 catalyst) and a reducing agent. The light-sensitive layer preferably
 further contains a binder (usually a synthetic polymer), an organic silver
 salt (reducible silver source), a hydrazine compound (ultra-hard gradation
 agent) and a color toning agent (controlling color tone of silver). Two or
 more light-sensitive layers may be provided in the light-sensitive
 material. For example, a high sensitive layer and a low sensitive layer
 can be provided in the heat developable light-sensitive material to
 control gradation. The high- sensitive layer and the low sensitive layer
 may be arranged in any order. For example, the low sensitive layer may be
 arranged on the lower side (support side), or the high sensitive layer may
 be arranged on the lower side.
 The heat developable light-sensitive material further has a
 non-light-sensitive layer containing the cyanine dye and the base
 precursor, as is described above. The light-sensitive material can
 furthermore has another non-light-sensitive layer such as a surface
 protective layer.
 Examples of the support of the heat developable light-sensitive material
 include a paper, a paper coated with polyethylene, a paper coated with
 polypropylene, a parchment, a cloth, a sheet or a thin film of metal
 (e.g., aluminum, copper, magnesium, zinc), a glass board, a glass board
 coated with metal (e.g., chromium alloy, steel, silver, gold, platinum)
 and a plastic film. Examples of the plastics used for the support include
 polyalkyl methacrylate (e.g., polymethyl methacrylate), polyester (e.g.,
 polyethylene terephthalate), polyvinyl acetal, polyamide (e.g., nylon) and
 cellulose ester (e.g., cellulose nitrate, cellulose acetate, cellulose
 acetate propionate, cellulose acetate butyrate).
 The support may be coated with a polymer. Examples of the polymers include
 polyvinylidene chloride, an acrylic acid polymer (e.g., polyacrylonitrile,
 polymethyl acrylate), a polymer of an unsaturated dicarboxylic acid (e.g.,
 itaconic acid), carboxymethyl cellulose and polyacrylamide. A copolymer
 can also be used. In place of coating a polymer on the support, an
 undercoating layer containing a polymer can be provided on the support.
 Silver bromide, silver iodide, silver chloride, silver chlorobromide,
 silver iodobromide or silver chloroiodobromide can be used as silver
 halide. Silver halide preferably contains silver iodide.
 The silver halide is used in an amount of preferably 0.03 to 0.6 g per
 m.sup.2, more preferably 0.05 to 0.4 g per m.sup.2, and most preferably
 0.1 to 0.4 g per m.sup.2.
 The silver halide is generally prepared in the form of a silver halide
 emulsion by a reaction of silver nitrate with a soluble halogen salt. The
 silver halide may be prepared by causing silver soap to react with a
 halogen ion and thereby subject the soap moiety of the silver soap to
 halogen conversion. A halogen ion may be added during the formation of the
 silver soap.
 As the reducing agent, phenidone, hydroquinone, catechol or hindered phenol
 is preferable. The reducing agent is described in U.S. Pat. Nos.
 3,770,448, 3,773,512, 3,593,863, 4,460,681, and Research Disclosure Nos.
 17029 and 29963.
 Examples of the reducing agents include aminohydroxycycloalkenone compounds
 (e.g., 2-hydroxy-piperidine-2-cyclohexenone), N-hydroxyurea derivatives
 (e.g., N-p-methylphenyl-N-hydroxyurea), hydrazones of aldehyde or ketone
 (e.g., anthracenealdehyde phenylhydrazone), phosphamide phenols,
 phosphamide anilines, polyhydroxybenzenes (e.g., hydroquinone,
 t-butyl-hydroquinone, isopropyl-hydroquinone,
 2,5-dihydroxy-phenylmethylsulfone), sulfohydroxamic acids (e.g.,
 benzenesulfohydroxamic acid), sulfonamideanilines (e.g.,
 4-(N-methanesulfonamide)aniline), 2-tetrazolylthiohydroquinones (e.g.,
 2-methyl-5-(1-phenyl-5-tetrazolylthio)hydroquinone),
 tetrahydroquinoxalines (e.g., 1,2,3,4-tetrahydroquninoxaline.),
 amidoxines, combinations of azines (e.g., aliphatic carboxylic acid
 arylhydrazides) and ascorbic acid, combination of polyhydroxybenzene and
 hydroxyamine, reductone, hydrazine, hydroxamic acids, combinations of
 azines and sulfonamidophenols, .alpha.-cyanophenylacetic acid derivative,
 combination of bis-.beta.-naphthol and 1,3-dihydroxybenzene derivative,
 5-pyrazolones, sulfonamidophenols, 2-phenylindane-1,3-dione, chroman,
 1,4-dihydropyridines (e.g.,
 2,6-dimethoxy-3,5-dicarboethoxy-1,4-dihydropyridine), bisphenols (e.g.,
 bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane,
 bis(6-(hydroxy-m-tri)mesitol, 2,2-bis(4-hydroxy-3-methylphenyl)propane,
 4,4-ethylidene-bis(2-t-butyl-6-methyl)phenol), ultraviolet-sensitive
 ascorbic acid derivative and 3-pyrazolidone.
 An ester of aminoreductone which functions as a precursor of a reducing
 agent (e.g., piperidinohexose reductone monoacetate) can be used as the
 reducing agent.
 A particularly preferred reducing agent is a hindered phenol.
 The light-sensitive layer and the non-light-sensitive layer preferably
 contain a binder. As the binder, a colorless, transparent or translucent
 polymer is generally employed. A natural polymer or a semisynthetic
 polymer (e.g., gelatin, gum arabic, hydroxyethyl cellulose, cellulose
 ester, casein, starch) is employable, but a synthetic polymer is
 preferable to the natural or semisynthetic polymer in consideration of
 heat resistance. Though the cellulose ester (e.g., acetate, cellulose
 acetate butyrate) is a semisynthetic polymer, it is preferably used as a
 binder of the heat developable light-sensitive material because it is
 relatively resistant to heat.
 Examples of the synthetic polymers include polyvinyl alcohol, polyvinyl
 pyrrolidone, polyacrylic acid, polymethyl methacrylate, polyvinyl
 chloride, polymethacrylic acid, styrene/maleic anhydride copolymer,
 styrene/acrylonitrile copolymer, styrene/butadiene copolymer, polyvinyl
 acetal (e.g., polyvinyl formal, polyvinyl butyral), polyester,
 polyurethane, phenoxy resin, polyvinylidene chloride, polyepoxide,
 polycarbonate, polyvinyl acetate and polyamide. A hydrophobic polymer is
 preferable to a hydrophilic polymer. Of these, therefore, preferable are
 styrene/acrylonitrile copolymer, styrene/butadiene copolymer, polyvinyl
 acetal, polyester, polyurethane, cellulose acetate butyrate, polyacrylic
 acid, polymethyl methacrylate, polyvinyl chloride and polyurethane. More
 preferable are styrene/butadiene copolymer and polyvinyl acetal.
 The binder is used by dissolving or emulsifying it in a solvent (water or
 organic solvent) of a coating solution for forming the light-sensitive
 layer or the non-light-sensitive layer. When the binder is emulsified in
 the coating solution, an emulsion of the binder may be mixed with the
 coating solution.
 The amount of the binder in the layer containing the dye is preferably
 adjusted so that the coating weight of the dye is 0.1 to 60 wt % of the
 binder. The coating weight of the dye is more preferably 0.2 to 30 wt % of
 the binder, most preferably 0.5 to 10 wt % of the binder.
 The light-sensitive layer or the non-light-sensitive layer preferably
 further contains an organic silver salt. An organic acid for forming the
 silver salt is preferably a long-chain fatty acid. The fatty acid
 preferably has 10 to 30 carbon atoms, and more preferably has 15 to 25
 carbon atoms. A complex of the organic silver salt is also available. The
 ligand of the complex preferably has a total stability constant against
 the silver ion in the range of 4.0 to 10.0. The organic silver salt is
 described in Research Disclosure Nos. 17029 and 29963.
 Examples of the organic silver salts include a silver salt of a fatty acid
 (e.g., gallic acid, oxalic acid, behenic acid, stearic acid, palmitic
 acid, lauric acid), a silver salt of carboxyalkylthiourea (e.g.,
 l-(3-carboxypropyl)thiourea, 1-(3-carboxypropyl)-3,3-dimethylthiourea), a
 silver complex of a polymer reaction product of aldehyde (e.g.,
 formaldehyde, acetaldehyde, butylaldehyde) and a hydroxy-substituted
 aromatic carboxylic acid, a silver salt of an aromatic carboxylic acid
 (e.g., salicylic acid, benzoic acid, 3,5-dihydroxybenzoic acid,
 5,5-thiodisalicyclic acid), a silver salt or a silver complex of thioene
 (e.g., 3-(2-carboxyethyl)-4-hydroxymethyl-4-thiazoline-2-thioene,
 3-carboxymethyl-4-thiazoline-2-thioene), a silver salt or a silver complex
 of nitrogen acid (e.g., imidazole, pyrazole, urazole, 1,2,4-thiazole,
 1H-tetrazole, 3-amino-5-benzylthio-1,2,4-triazole, benzotriazole), a
 silver salt of saccharin, a silver salt of 5-chlorosalicylaldoxime, and a
 silver salt of mercaptide. Most preferable is the silver behenate. The
 organic acid silver salt is used in an amount of preferably not more than
 3 g/m.sup.2, more preferably not more than 2 g/m.sup.2, in terms of
 silver.
 The light-sensitive layer or the non-light-sensitive layer preferably
 further contains an ultra-hard gradation agent. When the heat developable
 light-sensitive material is used in the field of printing photography,
 reproduction of continuous gradation dot image or line image is important.
 By the use of the ultra-hard gradation agent, the reproducibility of the
 dot image or the line image can be improved. As the ultra-hard gradation
 agent, a hydrazine compound, a quaternary ammonium compound or an
 acrylonitrile compound (described in U.S. Pat. No. 5,545,515) is employed.
 The hydrazine compound is particularly preferable as the ultra-hard
 gradation agent.
 The hydrazine compound includes hydrazine (H.sub.2 N-NH.sub.2) and a
 compound wherein at least one hydrogen of said hydrazine is substituted.
 As for the substituent group, its aliphatic group, aromatic group or
 heterocyclic group is directly attached to the nitrogen atom of the
 hydrazine, or its aliphatic group, aromatic group or heterocyclic group is
 attached to the hydrazine through a connecting group. Examples of the
 connecting groups include --CO--, --CS--, --SO.sub.2 --, --P(.dbd.O)R-- (R
 is an aliphatic group, an aromatic group or a heterocyclic group), --CNH--
 and combinations thereof.
 The hydrazine compound is described in U.S. Pat. Nos. 5,464,738, 5,496,695,
 5,512,411, 5,536,622, Japanese Patent Publication Nos. 6(1994)-77138,
 6(1994)-93082, and Japanese Patent Provisional Publication Nos.
 6(1994)-230497, 6(1994)-289520, 6(1994)-313951, 7(1995)-5610,
 7(1995)-77783 and 7(1995)-104426.
 The hydrazine compound can be added to a coating solution for forming the
 light-sensitive layer by dissolving it in an appropriate organic solvent.
 Examples of the organic solvents include alcohol (e.g., methanol, ethanol,
 propanol, fluorinated alcohol), ketone (e.g., acetone, methyl ethyl
 ketone), dimethylformamide, dimethyl sulfoxide and methyl cellosolve. A
 solution obtained by dissolving the hydrazine compound in an oily
 (co)solvent may be emulsified in the coating solution. Examples of the
 oily (co)solvents include dibutyl phthalate, tricresyl phosphate, glycerol
 triacetate, diethyl phthalate, ethyl acetate and cyclohexanone. A solid
 dispersion of the hydrazine compound may be added to the coating solution.
 The hydrazine compound can be dispersed using a known dispersing machine
 such as a ball mill, a colloid mill, a Mantongoring, a microfluidizer or
 an ultrasonic dispersing machine.
 The ultra-hard gradation agent is used in an amount of preferably
 1.times.10.sup.-6 to 1.times.10.sup.-2 mol, more preferably
 1.times.10.sup.-5 to 5.times.10.sup.-3 mol, most preferably
 2.times.10.sup.-5 to 5.times.10.sup.-3 mol, based on 1 mol of the silver
 halide.
 In addition to the ultra-hard gradation agent, a hard gradation accelerator
 may be used. Examples of the hard gradation accelerators include an amine
 compound (described in U.S. Pat. No. 5,545,505), a hydroxamic acid
 (described in U.S. Pat. No. 5,545,507), acrylonitriles (described in U.S.
 Pat. No. 5,545,507) and a hydrazine compound (described in U.S. Pat. No.
 5,558,983).
 The light-sensitive layer or the non-light-sensitive layer preferably
 further contains a color toning agent. The color toning agent is described
 in Research Disclosure No. 17029.
 Examples of the color toning agents include imides, (e.g., phthalimide),
 cyclic imides (e.g., succinimide), pyrazoline-5-ones (e.g.,
 3-phenyl-2-pyrazoline-5-one, 1-phenylurazole), quinazolinones (e.g.,
 quinazoline, 2,4-thiazolidinedione), naphthalimides (e.g.,
 N-hydroxy-1,8-naphthalimide), cobalt complex (e.g., hexamine cobalt
 trifluoroacetate), mercaptans (e.g., 3-mercapto-1,2,4-triazole),
 N-(aminomethyl)aryldicarboxyimides (e.g.,
 N-(dimethylaminomethyl)phthalimide), blocked pyrazoles (e.g.,
 N,N'-hexamethylene-1-carbamoyl-3,5-dimethylpyrazole), combination of
 isothiuronium derivative (e.g., 1,8-(3,6-dioxaoctane)-bis(isothiuronium
 trifluoroacetate) and a photo bleaching agent (e.g.,
 2-(tribromomethylsulfonyl)benzothiazole), merocyanine dye (e.g.,
 3-ethyl-5-((3-ethyl-2-benzothiazolinylidene)-1-methylethylidene)-2-thio-2,
 4-oxazolidinedione), a phthalazinone compound and a metallic salt thereof
 (e.g., phthalazinone, 4-(l-naphthyl)phthalazinone, 6-chlorophthalazinone,
 5,7-dimethyloxyphthalazinone, 2,3-dihydro-1,4-phthalazinone,
 8-methylphthalazinone), combination of a phthalazinone compound and
 sulfinic acid derivative (e.g., sodium benzenesulfinate), combination of a
 phthalazinone compound and sulfonic acid derivative (e.g., sodium
 p-toluenesulfonate), combination of phthalazine and phthalic acid,
 combination of phthalazine or phthalazine adduct and dicarboxylic acid
 (preferably o-phenylene acid) or anhydride thereof (e.g., maleic
 anhydride, phthalic acid, 2,3-naphthalenedicarboxylic acid, phthalic
 anhydride, 4-methylphthalic acid, 4-nitrophthalic acid,
 tetrachlorophthalic anhydride), quinazolinediones, benzoxazine,
 naphthoxazine derivative, benzoxazine-2,4-diones (e.g.,
 1,3-benzoxazine-2,4-dione), pyrimidines, asymmetric triazines (e.g.,
 2,4-dihydroxypyrimidine), tetrazapentalene derivative (e.g.,
 3,6-dimercapto-1,4-diphenyl-1H, 4H-2,3a, 5, 6a-tetrazapentalene), and
 phthalazine. Phthalazine is particularly preferred.
 The light-sensitive layer or the non-light-sensitive layer (preferably
 non-light-sensitive layer) can contain an antifogging agent. A
 mercury-free antifogging agent (described in U.S. Pat. Nos. 3,874,946,
 4,546,075, 4,452,885, 4,756,999, 5,028,523, British Patent Nos.
 92221383.4, 9300147.7, 9311790.1, Japanese Patent Provisional Publication
 No. 59(1984)-57234) is preferred to a mercury antifogging agent (described
 in U.S. Pat. No. 3,589,903).
 A heterocyclic compound having a methyl group substituted with halogen (F,
 Cl, Br or I) is preferably used as the antifogging agent.
 In the use of the silver halide, the silver halide is generally subjected
 to spectral sensitization. In the present invention, the silver halide is
 preferably spectrally sensitized in the near infrared region. The spectral
 sensitizing dye is described in Japanese Patent Provisional Publication
 Nos. 60(1985)-140336, 63(1988)-159841, 63(1988)-231437, 63(1988)-259651,
 63(1988)-304242, 63(1988)-15245, and U.S. Pat. Nos. 4,639,414, 4,740455,
 4,741,966, 4,751,175 and 4,835,096.
 Into the heat developable light-sensitive material, various additives such
 as a surface active agent, an antioxidant, a stabilizer, a plasticizer, an
 ultraviolet light absorber and a coating aid may be incorporated. The
 additives are added to either the light-sensitive layer or the
 non-light-sensitive layer.
 The heat developable light-sensitive material is preferably imagewise
 exposed to a near infrared light. The present invention is particularly
 effective for the exposure to the near infrared light (particularly near
 infrared laser). The wavelength of the near infrared light is in the range
 of preferably 700 to 1,100 nm, more preferably 750 to 860 nm, most
 preferably 780 to 830 nm. Examples of the near infrared light sources
 employable in the invention include a xenon flash lamp, various laser
 sources and light emitting diode.
 After the imagewise exposure, the heat developable light-sensitive material
 is heated to perform development. Through the heat development, a black
 silver image is formed. The heating temperature is in the range of
 preferably 80 to 250.degree. C., more preferably 100 to 200.degree. C. The
 heating time is in the range of usually 1 second to 2 minutes.
 EXAMPLE 1
 Decoloring Reaction of Cyanine Dye
 In 10 ml of dimethyl sulfoxide, 0.73 g of the cyanine dye (47) was
 dissolved. To the solution, 0.7 ml of triethylamine was added. The mixture
 was heated at 120.degree. C. for 1 minute. Immediately after heating the
 mixture, the blue color of the solution disappeared, and the color of the
 solution was turned to pale yellow. The solution was left to cool it, and
 precipitated white crystals were filtered off. The obtained crystals were
 made of a strongly hydrophobic and neutral compound. The compound was
 subjected to a mass spectral analysis. As a result, the molecular weight
 of the compound was 626, which means that the compound is formed by
 removing the counter anion and one proton atom from the cyanine dye (47).
 Further, the results of .sup.1 H-NMR spectral analysis confirmed that the
 compound was decolored by a ring forming reaction.
 The experiment was repeated except that 1,8-diazabicyclo[5,4,0]-7-undecene,
 guanidine or sodium hydroxide was used in place of triethylamine (base).
 The results of the experiments also confirmed that the dye was decolored
 by a ring forming reaction.
 Further, the cyanine dye was dissolved in dimethyl sulfoxide-d.sub.6
 (substituted with heavy hydrogen) without use of triethylamine. The
 solution was heated at 160.degree. C. for 2 hours. The solution was
 subjected to .sup.1 H-NMR spectral analysis. As a result, no reactions
 were confirmed. The result confirmed that the cyanine dye was very stable
 without a base.
 The experiments were further repeated, except that the cyanine dye (1),
 (48), (49), (2) or (4) was used in place of the cyanine dye (47). The
 results of the experiments also confirmed that the dye was decolored by a
 ring forming reaction.
 EXAMPLE 2
 Decoloring Reactions of Various Dyes
 To a dimethylacetamide solution (1.times.10.sup.-5 mol per dm.sup.3) of the
 dye set forth in Table 1, the base precursor (1.times.10.sup.-4 mol per
 dm.sup.3) was added. The mixture was heated at 110.degree. C. for 30
 seconds. The absorbance at the primary absorption band (.lambda.max) was
 measured to determine the remaining ratio of the dye. The results are set
 forth in Table 1.
 TABLE 1
 Dye .lambda.max Remaining
 ratio
 (1) 645.8 nm 0.2%
 (47) 682.4 nm 0.0%
 (48) 550.8 nm 0.2%
 (2) 647.2 nm 0.3%
 (3) 647.2 nm 1.1%
 (5) 647.0 nm 1.5%
 (6) 645.5 nm 0.0%
 (61) 566.0 nm 0.2%
 Comparative dye 1 644.0 nm 24.3%
 Comparative dye 2 679.4 nm 98.6%
 Comparative dye 3 646.2 nm 38.7%
 (Comparative dye 1)
 ##STR22##
 (Comparative dye 2)
 ##STR23##
 (Comparative dye 3)
 ##STR24##
 EXAMPLE 3
 Preparation of Solid Particle Dispersion of Base Precursor
 In a dispersing container of 300 ml, 52.5 g of 3 wt. % aqueous solution of
 polyvinyl alcohol, 52.5 g of 3 wt. % aqueous solution of carboxymethyl
 cellulose, 40 g of the base precursor (BP-41) and 150 ml of glass beads
 (diameter: 0.5 to 0.75 mm) were placed. The mixture was stirred at 3,000
 rpm for 30 minutes in a Dynomill dispersing device. The dispersion was
 adjusted to pH 6.5 by using 2N sulfuric acid to obtain a solid particle
 dispersion of the base precursor (BP-41). The average particle size was
 about 1 .mu.m.
 Preparation of Particle Dispersion of Dye
 In 30 g of ethyl acetate, 2.1 g of the cyanine dye (3) was dissolved.
 Independently, 31 g of 20 wt. % aqueous solution of polyvinyl alcohol and
 21 g of water were mixed with 10 g of 5 wt. % aqueous solution of sodium
 dodecylbenzenesulfonate. The mixture was place in a homogenizer cup of 200
 ml. The solution of the dye was added to the mixture. The resulting
 mixture was stirred at 10,000 rpm for 5 minutes to obtain an emulsion of
 the dye. The emulsion was stirred at 50.degree. C. for 2 hours. After
 ethyl acetate was removed from the emulsion, water (the amount of
 evaporated water) was added to the emulsion to obtain a particle
 dispersion of the cyanine dye (3). The average particle size was about 0.4
 .mu.m.
 Preparation of Thermal Image Recording Material
 To 5.1 g of the particle dispersion of the cyanine dye, 0.5 g of water and
 20 wt. % aqueous solution of polyvinyl alcohol were mixed. To the mixture,
 2 g of the solid particle dispersion of the base precursor was added to
 prepare a coating solution of an image recording layer. The coating
 solution was coated on a gelatin undercoating layer of a polyethylene film
 support (thickness: 100 .mu.m), and dried. The coating amount was 10.5 g
 per m.sup.2.
 With 4.8 g of water, 4 g of 10 wt. % aqueous solution of polyvinyl alcohol,
 1 g of 2 wt. % aqueous solution of poly(n=1)ethylene glycol dodecyl ether
 (surface active agent) and 0.5 g of 20 wt. % aqueous dispersion of zinc
 stearate (average particle size: 0.2 .mu.m) were mixed to prepare a
 coating solution of a protective layer. The coating solution was coated on
 the image recording layer, and dried. The coating amount was 17.5 g per
 m.sup.2.
 Thus, a thermal image recording material was prepared.
 Thermal Image Formation
 The thermal image recording material was imagewise heated by using a
 thermal imager (FTI210, Fuji Photo Film Co., Ltd.) at 8 gradation steps to
 form a negative image in which an area of high energy was decolored.
 Independently, the thermal image recording material was stored at
 40.degree. C. and at the relative humidity of 80% for 3 days. As a result,
 the image recording layer was not decolored. The stored thermal image
 recording material was imagewise heated in the same manner as is described
 above. As a result, a similar clear negative image was formed.
 EXAMPLE 3
 Preparation of Solid Particle Dispersion of Base Precursor
 With 43.5 g of water, 5.12 g of the base precursor (BP-7) and 1.02 g of
 polyvinyl alcohol were mixed. The mixture was stirred in a sand mill
 (1/16G sand grinder mill, Aimex Co., Ltd.) to prepare a solid particle
 dispersion of the base precursor (BP-7).
 Preparation of Emulsion of Dye
 In 35 g of ethyl acetate, 1.2 g of the cyanine dye (3) was dissolved to
 form an organic phase. The organic phase was mixed with 84 g of 6 wt. %
 aqueous solution of polyvinyl alcohol. The mixture was emulsified at room
 temperature to prepare an emulsion of the cyanine dye (3). The average
 particle size was 1.2 .mu.m.
 Preparation of Coatina Solution of Antihalation Layer
 With 28 g of 4 wt. % aqueous solution of polyvinyl alcohol, 4 g of the
 solid particle dispersion of the base precursor and 4 g of the emulsion of
 the dye were mixed. The mixture was stirred to prepare a coating solution
 of an antihalation layer.
 Formation of Antihalation Layer
 A moistureproofing undercoating layer containing vinylidene chloride was
 formed on one side of a polyethylene terephthalate film support
 (thickness; 175 .mu.m). A gelatin undercoating layer was formed on the
 other side of the support. The coating solution of the antihalation layer
 was coated on the moistureproofing undercoating layer, and dried to form
 an antihalation layer (dry coating amount: 2 g per m.sup.2).
 Preparation of Silver Halide Emulsion
 In 700 ml of water, 22 g of phthalated gelatin and 30 mg of potassium
 bromide were dissolved. The solution was adjusted to pH 5.0 at 35.degree.
 C. for 5 minutes. To the solution, 159 ml of an aqueous solution
 containing 18.6 g of silver nitrate and 0.9 g of ammonium nitrate and an
 aqueous solution of potassium bromide and potassium iodide (molar ratio
 92:8) were added for 10 minutes according to a controlled double jet
 method while keeping pAg of 7.7. To the mixture, 476 ml of an aqueous
 solution containing 55.4 g of silver nitrate and 2 g of ammonium nitrate
 and an aqueous solution of dipotassium hexachloroiridate (10 .mu. mole per
 liter) and potassium bromide (1 mole per liter) were added for 30 minutes
 according to a controlled double jet method while keeping pAg of 7.7.
 Further, 1 g of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene was added to
 the mixture. The pH of the emulsion was lowered to cause sedimentation,
 and the emulsion was desalted. To the emulsion 0.1 g of phenoxyethanol was
 added. The emulsion was adjusted to pH 5.9 and pAg 8.2 to complete
 formation of silver iodobromide grains. The iodide content in the core was
 8 mol %, and the average iodide content in the whole grains was 2 mol %.
 The average grain size was 0.05 .mu.m, the distribution coefficient of the
 projected area was 8%, the ratio of (100) face was 88%, and the grain
 shape was cubic.
 The emulsion was heated to 60.degree. C. To the emulsion, 85 .mu. mole
 (based on 1 mole of silver) of sodium thiosulfate, 11 .mu. mole of
 2,3,4,5,6-heptafluorophenyldiphenyl phosphineselenide, 15 .mu. mole of the
 following tellurium compound, 3.5 .mu. mole of chloroauric acid and 270
 .mu. mole of thiocyanic acid was added. The emulsion was ripened for 120
 minutes, and quickly cooled to 30.degree. C. to obtain a silver halide
 emulsion.
 ##STR25##
 Preparation of Organic Silver Salt Emulsion
 To 850 ml of distilled water, 7 g of stearic acid, 4 g of arachidic acid
 and 36 g of behenic acid were added. While stirring the mixture vigorously
 at 90.degree. C., 187 ml of 1N aqueous solution of sodium hydroxide was
 added to the mixture. The resulting mixture was stirred for 60 minutes.
 After 60 ml of 1N nitric acid was added to the mixture, the resulting
 mixture was cooled to 50.degree. C. While stirring the mixture more
 vigorously, 0.62 g of N-bromosuccinimide was added to the mixture. After
 10 minutes, the silver halide emulsion (amount of silver halide: 6.2
 mmole) was added to the mixture. Further, 125 ml of an aqueous solution
 containing 21 g of silver nitrate was added to the mixture for 100
 seconds. The resulting mixture was stirred for 10 minutes. To the mixture,
 0.62 g of N-bromosuccinimide was added. The mixture was left for 10
 minutes. The solid contents were filtered off through vacuum filtration.
 The solid contents were washed with water until the conductivity of the
 filtrate water was 30 .mu.S per cm. To the obtained solid contents, 150 g
 of 0.6 wt. % butyl acetate solution of polyvinyl acetate was added. After
 the mixture was stirred, the mixture was left to cause separation between
 oily phase and aqueous phase. The aqueous phase containing salts was
 removed from the mixture to obtain the oily phase. To the oily phase, 80 g
 of 2.5 wt. % 2-butanone solution of polyvinyl butyral (Denka Butyral
 #3000-K, Denki Kagaku Kogyo K.K.) was added, and the mixture was stirred.
 To the resulting mixture, 0.1 mmole of pyridinium perbromide, 0.15 mole of
 calcium bromide dihydrate and 0.7 g of methanol were added. To the
 mixture, 200 g of 2-butanone and 59 g of polyvinyl butyral (BUTVAR-76,
 Monsanto Co.) were added. The mixture was stirred in a homogenizer to
 obtain an organic silver salt emulsion (average minor size of needle-like
 grains: 0.04 .mu.m, average major size: 1 .mu.m, distribution coefficient:
 30%).
 Preparation of Coating Solution of Light-sensitive Layer
 To the organic silver salt emulsion, the following components (first
 addition and second addition) were added while stirring to prepare a
 coating solution of a light-sensitive layer. The following amounts were
 based on 1 mole of silver.

Components of light-sensitive layer (second addition)
 5-Tribromomethylsuofonyl-2-methylthiazole 8 g
 2-Tribromomethylsulfonylbenzothiazole 6 g
 4,6-Ditrichloromethyl-2-phenyltriazine 5 g
 The following disulfide compound 2 g
 1,1-Bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane 155 g
 Fluorine-containing surface active agent (Megafax F-176P, 1.1 g
 Dainippon Ink & Chemicals Inc.)
 2-Butanone 590 g
 Methyl isobutyl ketone 10 g
 (Disulfide compound)
 ##STR27##
 Preparation of Coatina Solution of Emulsion Surface Protective Layer
 In 3070 g of 2-butanone and 30 g of ethyl acetate, 75 g of cellulose
 acetate butyrate (CAB-171-15S, Eastman Chemicals), 5.7 g of 2-methyl
 phthalate, 1.5 g of tetrachlorophthalic anhydride, 0.3 g of a
 fluorine-containing surface active agent (Megafax F-176P, Dainippon Ink &
 Chemicals Inc.), 2 g of spherical silica particles having the average
 particle size of 3 .mu.m (Sildex H31, Dokai Chemicals) and 6 g of
 polyisocyanate (Sumidur N3500, Sumitomo Bayer Urethane Co., Ltd.) were
 dissolved to prepare a coating solution of an emulsion surface protective
 layer.
 Preparation of Coating Solution of Backing Surface Protective Layer
 In 500 g of water, 10 g of gelatin, 0.6 g of polymethyl methacrylate
 particles (average particle size: 7 .mu.m), 0.4 g of sodium
 dodecylbenzenesulfonate and 0.9 g of a silicone compound (X-22-2809,
 Shinetsu Silicone Co., Ltd.) were dissolved to prepare a coating solution
 of a backing surface protective layer.
 Preparation of Heat Developable Light-sensitive Material 101
 On the emulsion side (on which the antihalation layer was not provided) of
 the support, the coating solution of the light-sensitive layer was coated
 (coated silver amount: 2.3 g per m.sup.2). On the antihalation layer, the
 coating solution of the backing surface protective layer was coated (dry
 thickness: 0.9 .mu.m). On the light-sensitive layer, the coating solution
 of the emulsion surface protective layer was coated (dry thickness: 2
 .mu.m) to prepare a heat developable light-sensitive material 101.
 Preparation of Heat Developable Light-Sensitive Materials 102 to 110
 Heat developable light-sensitive materials 102 to 109 were prepared in the
 same manner as in the preparation of the material 101, except that the
 cyanine dyes (52), (50), (6), (10), (33) and the comparative dyes 1 to 3
 were used respectively in place of the cyanine dye (3).
 Further, a heat developable light-sensitive material 110 was prepared in
 the same manner as in the preparation of the material 101, except that the
 cyanine dye (3) was not used.
 Evaluation of Photographic Characteristics
 The heat developable light-sensitive materials were exposed to light by
 using a semiconductor laser sensitometer. The light-sensitive material was
 heated (developed) at 120.degree. C. for 15 seconds. The obtained image
 was measured by using a densitometer. The results were evaluated based on
 he minimum density (Dmin) corresponding to fog and the sensitivity
 (reciprocal value of the ratio of the exposure for the density of Dmin
 plus 1.0). The results are set forth in Table 2. In Table 2, the
 sensitivity means a relative sensitivity wherein the sensitivity of the
 material is 100.
 Evaluation of Sharpness
 The heat developable light-sensitive materials were exposed to light by
 using a semiconductor laser sensitometer. The exposed area had a square
 shape of 1 cm.sup.2. The exposure (x) for the density of 2.5 and the
 exposure (y) for the density of 0.5 were determined. Further, rectangle
 areas (width: 100 .mu.m, length: 1 cm) neighboring each other along the
 length side of the light-sensitive material were exposed to light at the
 exposures (x) and (y) alternatively. The maximum density and the minimum
 density within the exposed area were measured by using a
 microdensitometer. The difference between the maximum density and the
 minimum density was divided by 2 to evaluate the sharpness. The results
 are set forth in Table 2.
 Evaluation of Storage Stability
 The heat developable light-sensitive materials 101 to 110 were stored at a
 high temperature (50.degree. C.) and a high humidity (relative humidity:
 80%) for 3 hours. The absorption at 650 nm was measured before and after
 storage. The remaining ratio of the dye was determined by the absorption
 before storage (Db) of the materials 101 to 109, the absortion after
 storage (Da) of the materials 101 to 109 and the absorption after storage
 (DO) of the material 110 according to the following formula. There was no
 change in absorption of the material 110 before and after storage.
EQU 100.times.(Da-D0)/(Db-D0)
 Where the above-defined value is large (near 100), the dye is excellent in
 the storage stability. The results are set forth in Table 2.
 TABLE 2
 Minimum Sensi- Sharp- Stabil-
 Material Dye density tivity ness ity
 101 (3) 0.15 100 0.98 95
 102 (52) 0.14 100 0.95 93
 103 (50) 0.13 95 0.92 82
 104 (6) 0.12 95 0.91 35
 105 (10) 0.13 100 0.98 100
 106 (33) 0.12 100 0.98 100
 107 CD1 0.27 95 0.97 98
 108 CD2 0.65 95 0.89 100
 109 CD3 0.34 100 0.97 97
 110 None 0.21 95 0.42 --
 (Remark)
 CD1: Comparative dye 1 used in Example 2
 CD2: Comparative dye 2 used in Example 2
 CD3: Comparative dye 3 used in Example 2