Method for preparation of a photothermographic material with increased photosensitivity

1. A process for preparing a photothermographic material with increased photosensitivity, said photothermographic material comprising a support and a photo-addressable thermally developable element, exclusive of a compound R-S(M) n wherein R is an aliphatic hydrocarbon, aryl or heterocyclic group, M is a hydrogen atom, or cation, and letter n is a number determined so as to render the molecule neutral and also exclusive of a compound capable of releasing a mobile dye corresponding to or inversely corresponding to the reduction of silver halide to silver at elevated temperatures, said photo-addressable thermally developable element containing a light-insensitive organic silver salt, a photosensitive agent in catalytic association with said light-insensitive organic silver salt, a reducing agent for said light-insensitive organic silver salt in thermal working relationship therewith, a silver salt having a water-solubility greater than 0.1 g in 1 L water at 20° C. and a binder, comprising the steps of:

(i) increasing the photosensitivity of a photosensitive silver halide by chemical sensitisation with a chemical sensitising merocyanine dye containing a thione group in a non-oxidative aqueous medium, optionally in the presence of said light-insensitive organic silver salt and/or in the presence of said silver salt having a water-solubility greater than 0.1 g in 1 L water at 20° C., thereby preparing said photosensitive agent;

(ii) coating said support with one or more aqueous solutions or dispersions together containing said light-insensitive organic silver salt, said photosensitive agent, said reducing agent, said silver salt having a water-solubility greater than 0.1 g in 1 L water at 20° C. and said binder; and

(iii) drying said coating or coatings thereby producing said photo-addressable thermally developable element.

2. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye is capable of spectrally sensitizing said photosensitive silver halide.

3. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprises a rhodanine ring, a thiohydantoin ring, a thiobarbituric acid ring or a 2-thio-4-oxo-oxazolidine ring, optionally substituted with alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, substituted arylalkyl, heteroaryl, or substituted heteroaryl.

4. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 and A 2 are each independently selected from one of the following structures:

R selected from alkyl, substituted alkyl, arylalkyl, and substituted arylalkyl; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl; and substituted alkyl; R 4 to R 7 are each independently selected from alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, substituted arylalkyl, heteroaryl and substituted heteroaryl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0, 1, 2 or 3; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, and —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —Cl, —Br, —I, alkoxy, metboxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR17, —NH—SO 2 —NHR 17 , —CN, —CF 3 , —SO 2 —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 R 3 , —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl, and 2-thienyl.

5. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye or dyes selected from the group consisting of:

6. The process according to claim 1 , wherein said photosensitive silver halide is chemically sensitised with at least two of said chemical sensitising merocyanine dyes containing a thione group in a non-oxidative aqueous medium.

7. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein (I)=

wherein A 1 and A 2 are each independently selected from one of the following structures:

R is selected from alkyl, substituted alkyl, arylalkyl, and substituted arylalkyl; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl; and substituted alkyl; R 4 to R 7 are each independently selected from alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, substituted arylalkyl, heteroaryl, and substituted heteroaryl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 -COOH or salts; n, m, p and q are each independently 0 or 1; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, and —C(CH 3 ) 2 ; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; γ is selected from alkyl, —Cl, —Br, —l, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO-R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NH—SO 2 —NHR 17 R, —CN, —CF 3 , —SO 2 —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 R 3 , —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl and 2-thienyl.

8. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 and A 2 are each independently selected from one of the following structures:

R is selected from alkyl, substituted alkyl, arylalkyl, and substituted arylalkyl; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S——NR 1 —, and—CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl; and substituted alkyl; R 4 to R 7 are each independently selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 8 or salts, —(CH 2 ) s , —SO 2 —NH—CO—R 8 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 8 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 or salts; R 8 is selected from alkyl and substituted alkyl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0, 1, 2 or 3; r is 1, 2, 3, 4, 5 or 6; s is 2, 3 or 4; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, and —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocycli ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —Cl, —Br, —I, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NW—SO 2 —NHR 17 , —CN, —CF 3 , —SO 2 —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 R 3 , —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl, and 2-thienyl.

9. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 and A 2 are each independently selected from one of the following structures:

R is selected from alkyl, substituted alkyl, arylalkyl, and substituted arylalkyl; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl; and substituted alkyl; R 4 to R 7 are each independently selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 8 or salts, —(CH 2 ) s —SO 2 —NH—CO—R 8 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 8 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl and —CH 2 —CO—NH—SO 2 —CH 3 or salts; R 8 is selected from alkyl and substituted alkyl; R 9 to R16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0 or 1; r is 1, 2, 3, 4, 5 or 6; s is 2, 3 or 4; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, and —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —Cl, —Br, —I, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1—pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NH—SO 2 —NHR 17 , —CN, —CF 3 , —SO 2 —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 R 3 , —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl and 2-thienyl.

10. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 and A 2 are each independently selected from one of the following structures:

R is selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 3 or salts, —(CH 2 ) s —SO 2 —NH—CO—R 3 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 3 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 or salts; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; H 3 is selected from alkyl and substituted alkyl; R 4 to R 7 are each independently selected from alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, substituted arylalkyl, heteroaryl and substituted heteroaryl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, bentyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0, 1, 2 or 3; r is 1, 2, 3, 4, 5 or 6; s is 2, 3 or 4; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —CI, —Br, —I, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NH—SO 2 —NHR 17 , —CN, —CF 3 , —SO 2 —CF 3 , —SO—CH 3 , —SO 2 —NR 2 R 3 , —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl, and 2-thienyl.

11. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 and A 2 are each independently selected from one of the following structures:

R is selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 3 or salts, —(CH 2 ) s —SO 2 —NH—CO—R 3 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 3 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 or salts; Q 1 , Q 2 and Q 3 are independently selected from —O —, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl; and substituted alkyl; R 4 to R 7 are each independently selected from alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, substituted arylalkyl, heteroaryl, and substituted heteroaryl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0 or 1; r is 1, 2, 3, 4, 5 or 6; s is 2, 3 or 4; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, and —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —Cl, —Br, —I, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NH—SO 2 —NHR17, —CN, —CF 3 , —SO 2 —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 , R 3 —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl and 2-thienyl.

12. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 or and A 2 are each independently selected from one of the following structures:

R is selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 3 or salts, —(CH 2 ) s —SO 2 NH—CO—R 3 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 3 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 , or salts; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl and substitured alkyl; R 4 to R 7 are each independently selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 8 or salts, —(CH 2 ) s —SO 2 —NH—CO—R 8 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 8 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 or salts; R 8 is selected from alkyl and substituted alkyl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0, 1, 2 or 3; r is 1, 2, 3, 4, 5 or 6; s is 2, 3 or 4; x is 0 or 1; Z is selected from —O—, —S—, —NR— 1 —, —CH—═CH—,and —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —Cl, —Br, —I, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo—ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NH—SO 2 —NHR 17 , —CN, —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 R 3 , and —CO 2 —R 2 , —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-furyl, and 2-thienyl.

13. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprising formula (I) or (II) or (III) wherein

wherein A 1 and A 2 are each independently selected from one of the following structures:

R is selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 3 or salts, —(CH 2 ) s —SO 2 —NH—CO—R 3 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 —R 3 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3—sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 or salts; Q 1 , Q 2 and Q 3 are independently selected from —O—, —S—, —NR 1 —, and —CO—NR 2 —; R 1 is selected from alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and substituted heteroaryl; R 2 is selected from —H, alkyl, and substituted alkyl; R 3 is selected from alkyl, substituted alkyl; R 4 to R 7 are each independently selected from methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 8 or salts, —(CH 2 ) s —SO 2 —NH—CO—R 8 or salts, —(CH 2 ) s—SO 2 —NH—SO 2 —R 8 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, and —CH 2 —CO—NH—SO 2 —CH 3 or salts; R 8 is selected from alkyl and substituted alkyl; R 9 to R 16 are each independently selected from —H, methyl, ethyl, isopropyl, phenyl, substituted phenyl, benzyl, substituted benzyl, cyclopropyl, and —(CH 2 ) 2 —COOH or salts; n, m, p and q are each independently 0 or 1; r is 1, 2, 3, 4, 5 or 6; s is 2, 3 or 4; x is 0 or 1; Z is selected from —O—, —S—, —NR 1 —, —CH═CH—, —C(CH 3 ) 2 —; G represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; Y represents the atoms necessary to complete a carbocyclic ring or a heterocyclic ring; T is selected from alkyl, —Cl, —Br, —I, alkoxy, methoxy, ethoxy, hydroxy, —S—CH 3 , phenyl, substituted phenyl, annulated benzo-ring, 1-indolyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-thienyl, 3-thienyl, 2-furyl, 3-furyl, —NH—CO—R 17 , —CO—NHR 17 , —NH—CO—NHR 17 , —NH—SO 2 —NHR 17 , —CN, —CF 3 , —SO 2 —CF 3 , —SO 2 —CH 3 , —SO 2 —NR 2 R 3 , —CO 2 —R 2 , and —CO—NR 2 R 2 ; and R 17 is selected from an alkyl containing 1 to 6 carbon atoms, 2-fury, and 2-thienyl.

14. The process according to claim 1 , wherein said chemically sensitizing merocyanine dye comprises a rhodanine ring, a thiohydantoin ring, a thiobarbituric acid ring or a 2-thio- 4-oxo-oxazolidine ring, optionally substituted with methyl, ethyl, hydroxy ethyl, —(CH 2 ) r —COOH or salts, —(CH 2 ) s —SO 3 H or salts, —(CH 2 ) r —CO—NH—SO 2 —R 8 or salts, or salt, —(CH 2 ) s —SO 2 —NH—CO—R 8 or salts, —(CH 2 ) s —SO 2 —NH—SO 2 or salts, carboxy-methyl, carboxy-ethyl, 2-sulpho-ethyl, 3-sulpho-propyl, 4-sulpho-butyl, or —CH 2 —CO—NH—SO 2 —CH 3 wherein R 8 is selected from alkyl and substituted alkyl; r is 1, 2, 3, 4, 5 or 6; and s is 2, 3 or 4.

15. A photothermographic material with increased photosensitivity obtained by a process for preparing a photothermographic material with increased photosensitivity, said photothermographic material comprising a support and a photo-addressable thermally developable element, exclusive of a compound R—S(M) n wherein R is an aliphatic hydrocarbon, aryl or heterocyclic group, M is a hydrogen atom, or cation, and letter n is a number determined so as to render the molecule neutral and also exclusive of a compound capable of releasing a mobile dye corresponding to or inversely corresponding to the reduction of silver halide to silver at elevated temperatures, said photo-addressable thermally developable element containing a light-insensitive organic silver salt, a photosensitive agent in catalytic association with said light-insensitive organic silver salt, a reducing agent for said light-insensitive organic silver salt in thermal working relationship therewith, a silver salt having a water-solubility greater than 0.1 g in 1 L water at 20° C. and a binder, comprising the steps of:

(i) increasing the photosensitivity of a photosensitive silver halide by chemical sensitisation with a chemical sensitising merocyanine dye containing a thione group in a non-oxidative aqueous medium, optionally in the presence of said light-insensitive organic silver salt and/or in the presence of said silver salt having a water-solubility greater than 0.1 g in 1 L water at 20° C., thereby preparing said photosensitive agent;

(ii) coating said support with one or more aqueous solutions or dispersions together containing said light-insensitive organic silver salt, said photosensitive agent, said reducing agent, said silver salt having a water-solubility greater than 0.1 g in 1 L water at 20° C. and said binder; and

(iii) drying said coating or coatings thereby producing said photo-addressable thermally developable element.