GB1590955A

GB1590955A - Spectral sensitization of photographic silver halides and dyes therefor

Info

Publication number: GB1590955A
Application number: GB4727676A
Authority: GB
Original assignee: Kodak Ltd
Current assignee: Kodak Ltd
Priority date: 1976-11-12
Filing date: 1976-11-12
Publication date: 1981-06-10

Description

(54) SPECTRAL SENSITIZATION OF PHOTOGRAPHIC SILVER HALIDES AND NOVEL DYES THEREFOR (71) We, KODAK LIMITED, a Company registered under the Law of England, of Kodak House, Station Road, Hemel Hempstead, Hertfordshire, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the spectral sensitization of photographic silver halides.

Photographic layers containing sensitive silver halides are commonly spectrally sensitized using cyanine or merocyanine dyes. We have found that certain azo dyes may be used for this purpose.

According to the present invention, there is provided a photographic silver halide spectrally sensitized with an azo dye of the formula: A-N=N-B (1) wherein A and B are moieties which complete a chromophoric system which includes the azo group, one or each of A and B comprising a heterocyclic nucleus including a nitrogen atom which carries an acidic hydrogen atom which enables the dye to form a substantially water-insoluble silver salt.

Preferred salt-forming heterocyclic nuclei which may constitute or form part of the moieties A and B are: triazole, pyrazolotriazole, tetrazole, hydroxytetraazaindene, indazole, imidazole, benzimidazole, imidazotetrazole, benzotriazole, pyridopyrazine.

In addition to the essential silver salt-forming heterocyclic nucleus or nuclei, either or both of the moieties A and B may comprise non-silver salt-forming aromatic and/or heterocyclic nuclei. Preferred such aromatic nuclei are: phenyl, p-methoxyphenyl, 3-amido-4-hydroxyphenyl, p-dimethylaminophenyl, 2-hydroxynaphth- l-yl, l-hydroxy-4-methoxynaphth-2-yl, and p-benzamido Examples of suitable, non-silver salt-forming, heterocyclic nuclei are 3 isopropyl, 4-isopropyl, 2-isoimidazole, 2H-pyrrole, tetrazine, triazine and pyridine nuclei.

Preferably A or each of A and B has a 5 to 12 membered heterocylic ring and B, if an aryl group, has 6 to 14 carbon atoms.

The moiety A may comprise more than one nucleus and these may be linked directly or through a second diazo group giving a bis azo dye. In the latter case A may be represented as: A 1-N=N-A2- wherein each of A' and A2 is an aromatic or heterocyclic nucleus.

Any nuclei present in the moieties A and B.may include substituents such as halogen atoms, and alkyl, alkoxyl, alkoxycarbonyl, alkylsulphonyl, amino, amido, hydroxyl, nitro and aryl (especially phenyl or naphthyl) groups.

Azo dyes particularly useful for the invention are the dyes represented by the following formula:

wherein A is as defined for Formula I, D is a heterocyclic nucleus containing a 5- or 6-membered heterocyclic ring which includes a nitrogen atom which carries an acidic hydrogen atom which enables the dye to form a substantially water-insoluble silver salt. R indicates the possible presence of one or more simple substituents, such as halogen atoms, and alkyl, hydroxyl, amino, amido, nitro, alkylsulphonyl and alkoxyl groups, and of atoms which complete a fused-on aromatic ring, x is 0 or 1, and E is a group of formula -CONR1-, -NR1CO-, -CONR1CH2-, -NR1CONR1-,

-SO2NR1- or -NR1SO2- wherein R is as already defined and R1 is hydrogen, an alkyl or substituted alkyl group, or an aryl or substituted aryl group, any substituent forming part of R1 being a simple substituent such as one of those listed above for R.

The azo dyes of Formula 1 are useful for sensitizing any sensitive silver halide photographic composition, such as a gelatino-silver halide emulsion. The presence of the acidic hydrogen atom in the heterocyclic ring enables the dye to react with the silver halide and so become strongly adsorbed thereto. The dye is therefore not readily displaced from the silver halide by other substances, and unsensitization is not likely to occur. For this reason a photographic material of the invention may contain any of a wide variety of constituents.

The concentration of a particular dye of Formula 1 to be used for sensitizing a photographic silver halide is best determined by preliminary testing. The concentration will often be found to be in the range 0.01 to 2.0 grams of dye per mole of silver halide. Taking into account the approximate range of 250 to 500 for the molecular weight of the dyes, this range is equivalent to a range of 0.02 to 8 millimoles of dye per mole of silver halide. The dye may be incorporated in a photographic emulsion or dispersion in any convenient manner. It is preferably added as a solution in an organic solvent such as methanol, ethanol, acetone or N,N-dimethylformamide.

Azo dyes of the Formula 1 are particularly suitable for sensitizing photothermographic layers which contain, in addition to a silver halide, an oxidation-reduction image-forming composition comprising a substantially lightinsensitive metal salt-commonly a silver salt of a long chain fatty acid (e.g. silver behenateHand a reducing agent therefor. Photothermographic layers of this kind may be prepared as described in, for instance, United Kingdon Patent Specifications 1,110,046, 1,342,522 and 1,346,252.

The silver halide of a photothermographic material may be prepared either ex situ, preferably in a non-aqueous medium as described in "Research Disclosure" September 1974, Item 12537, or in situ by reaction between a silver salt and an added halide, as described in United Kingdon Patent Specification 1,110,146. The silver halide may (as in a conventional gelatin-containing emulsion) be chemically sensitized with reducing agents; with sulphur, selenium or tellurium compounds; with noble metal, especially gold, compounds; or with combinations of these sensitizers. The silver halide may be protected against the production of fog and loss of sensitivity with antifoggants and stabilizers.

A photothermographic material of the invention may contain a silver salt of a long chain fatty acid (i.e. having at least 10 and preferably having from 12 to 22 carbon atoms e.g. behenic acid) and a reducing agent therefor (preferably a substituted phenol or naphthol; and especially a bis p-naphthol or a sulphonamidophenolHsee "Product Licensing Index" May 1971 Items 8526 and 8527, and "Research Disclosure" January 1973 Item 10513), may contain a development modifier (sometimes termed an activator toning agent-see United Kingdom, Patent Specifications 1,339,767, 1,342,522 and 1,342,523), an image stabilizer precursor (see United Kingdom Patent Specification 1,346,252), a colourless onium halide (see United Kingdom Patent Specification 1,342,525), or a polar organic solvent (see United Kingdom Patent Specification 1,342,524).

A photothermographic material of the invention may be designed for producing a dye image. For instance, a 2,6-dichloro or 2,6-dibromo-4benzenesulphonamidophenol reducing agent may be used with a four-equivalent dye-forming coupler as described in U.S. Patent 4,021,240; or a phenolic leuco dye reducing agent may be used as described in U.K. Patent Application 29481/75 (Serial No. 1,512,046) and U.S. Patent 3,985,565. When a coupler is used, it is typically present at a concentration of from 0.25 to 4.0 moles per mole of the reducing agent.

Useful silver salts of long-chain fatty acids are, for example, silver behenate, silver stearate, silver oleate, silver laurate, silver hydroxystearate, silver caprate, silver myristate and silver palmitate. Other reducible silver salts which may be included in materials of the present invention silver benzoate, silver phthalate, silver acetate, silver acid phthalate, silver phthalazinone, silver benzotriazole and silver saccharin. Mixtures of silver salt oxidizing agents can be used.

Minor proportions of reducible salts of other metals can be used with the silver salts, if desired,-such as zinc oxide, gold stearate, mercury behenate and gold behenate.

It is preferred to have a long-chain fatty acid in the photothermographic material. For example, when silver behenate is the long-chain fatty acid silver salt, it is typically desirable to have some behenic acid present to provide an improved image. A typical concentration of fatty acid is from 0.1 mole to 2.0 moles of the fatty acid per mole of the fatty acid silver salt or salts.

The preferred coverages for the main constituents in a spectrally sensitized photothermographic material of the invention are as follows: millimoles/metre2 a) organic silver salt 0.2 to 20 b) organic reducing agent 0.15 to 40 c) photographic silver halide 0.0075 to 20 d) azo dye of Formula 1 0.0002 to 0.25 e) binder 0.5-10 The preferred molar quanitites of (b) and (c) and of certain other possible addenda for such a material, relative to the molar quantity of the organic silver salt (a) are as follows: a) organic silver salt 1 b) organic reducing agent 0.2 to 2.0 c) photographic silver halide 0.01 to 2.0 f) development modifier 0.001 to 1.1 g) image stabilizer precursor 0.002 to 0.1 The amount of azo dye (d) to be used is best decided with reference to the halide concentration as described above. Typical suitable coverages lie in the range 0.1 to 50 mg per square metre.

A spectrally sensitized photothermographic material of the invention can contain various synthetic polymeric binders alone or in combination in the layer or the layers present. The preferred binders are hydrophobic, but hydrophilic materials can be used. Suitable binders include such substances as cellulose derivatives and polyvinyl compounds examples being, polyvinyl butyral, cellulose acetate butyrate, polymethylmethacrylate, ethyl cellulose, polystyrene, polyvinyl chloride), chlorinated rubber, polyisobutylene, butadiene-styrene copolymers, vinyl chloride-vinyl acetate copolmers, copolymers of vinyl acetate, vinyl chloride and maleic acid and poly(vinyl alcohol); hydroxypropylcellulose; sodium ethyl cellulose sulphate and polyacrylamide.

A spectrally sensitized photothermographic composition of the invention may be coated on any support which can withstand the necessary processing temperature, usually in the range 100 to 2500 C. The components of the photothermographic composition may be disposed in a single layer on the support or may be in adjacent layers provided that these are such as will allow the image-forming reactions to occur at the processing temperature. Preferably, the photothermographic composition is overcoated with a polymer layer, such as an acrylamide layer.

The dyes of Formula 2 wherein x is 0 may be prepared by diazotizing a compound of the formula

and reacting the diazonium salt obtained with a coupler of formula AH.

The dyes of the Formula 2 wherein x is 1 may be prepared either by forming an azo dye of the formula:

wherein A and R are as defined for Formula 2 and reacting this dye with a heterocyclic compound of the formula DR3-where D is as defined for Formula 2 and R2 and R3 are substituents which react to form the desired link E, or by diazotizing a compound of the formula

and reacting the diazonium salt obtained with a coupler of formula AH. Very suitable groups for R2 and R3 are -COCl and -NH2 groups which react to give a -CONH- linkage.

The silver salts of dyes of Formula (1) may be prepared by reacting a solution of the dye with a solution of a silver salt. A preferred procedure is as follows. A solution of the dye (0.006 mole) is prepared in a mixture of dimethylformamide (100 ml) and aqueous ammonia (20 ml, S.G. 0.88). To this solution is added, with stirring, a solution of silver nitrate (0.006 mole) in 3 ml of aqueous ammonia (S.G. 0.88) diluted with 4 ml of water. Water (100 ml) is added to the mixture and the dye silver salt is filtered off, washed successively with 1% aqueous ammonia, water and methanol. If the silver salt is of very small particle size, it can be collected using a centrifuge instead of a filter.

The preparation of dyes used in the Examples, and of silver salts of certain of these dyes, is described below. Compounds A to D are dyes which are not capable of forming substantially water-insoluble silver salts and are used in the Examples to provide comparative data.

Preparation of Compounds Compound I 2-B enzotriazol-5-ylazo-4-methoxynaphth- 1 -ol

5-Aminobenzotriazole (4.3 g) was dissolved in a mixture of concentrated hydrocholoric acid (24 ml) and water (100 ml) and diazotised at 0 C with a solution of sodium nitrite (2.2 g) in water (15 ml). The resulting diazonium solution was added at OOC to 4-methoxynaphth - 1 - ol (5.3 g) dissolved in 3N sodium hydroxide (90 ml) containing sodium acetate (20 g). The resulting mixture was stirred for one hour at OOC and then acidified with hydrochloric acid. The dye was filtered off and crystallised from a pyridine/water mixture of give pure material (7.4 g), m.p. 254, Amax (methanol)=516 nm.

Compound II 1 -(Benzotriazol-5-vlazo)naphth-2-ol

5-Aminobenzotriazole (10 g) was diazotized as described for the preparation of Compound I and the resulting diazonium solution was added at 0 C to naphth - 2 ol (10.75 g) dissolved in 3N sodium hydroxide (185 ml) containing sodium acetate (100 g). The resulting mixture was stirred for 90 minutes and acidified with hydrochloric acid. The dye was filtered off and recrystallised from a pyridine/water mixture to give pure material (18.6 g), m.p. 250 , Amax (methanol)=475 nm.

Compound III 2-(Indazol-5-ylazo)-4-methoxynaphth- 1 -ol

5-Aminoindazole was diazotised and coupled with 4-methoxynaphth-1-ol by a method similar to that used in preparing Compound I. The product was crystallized from aqueous pyridine, m.p. 2400 decomp., Amax (methanol)=523 nm.

Compound IV 2-(B enzimidazol-6-ylazo)-4-methoxynaphth- 1 -ol

6-Aminobenzimidazole was diazotised and coupled with 4-methoxynaphth-lol by a method similar to that used in preparing Compound I. The product was crystallized from aqueous pyridine, m.p. 2350 decomp., Amax (methanol)=525 nm.

Compound A 4-Methoxy-2-phenylazonaphth-l-ol

Aniline was diazotized and coupled with 4-methoxynaphth-l-ol by a method similar to that used in preparing Compound I. The product was crystallised from aqueous pyridine m.p. 1370, Amax (methanol)=514 nm.

Compound B 2-Hydroxy-5-(4-methoxyphenylazo)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride (1 g) was carefully warmed with 0.880 ammonium hydroxide (10 ml) for 10 min. The mixture was then acidified with glacial acetic acid and the amide removed by filtration. The product crystallised from ethanol (0.60 g), 65 /", m.p. 2313 , Amax (methanol)=352 nm.

Compound C 4-(4-N,N-Dimethylaminophenylazo)benzoic acid

4-Aminobenzoic acid (13.7 g) was diazotised in 1.5 M hydrochloric acid (200 ml) and coupled with dimethylaniline (17.8 ml) in the presence of sodium acetate (21 g). Yield 26.8 g, m.p. 245 , Amax (methanol)=433 nm.

Compound V N-B enzotriazol-5'-yl.4-(4-dimethylaminophenylazo)benzamide

A mixture of 5 - aminobenzotriazole (0.47 g), 4 - (N,N dimethylamino)phenylazobenzoyl chloride, hydrochloride (1.13 g) and N,Ndimethylaniline (0.84 g) in dry tetrahydrofuran was stirred overnight and the resulting precipitate was filtered off and recrystallised from a dimethylformamide/water mixture to give pure dye, 0.73 g, m.p. 246 , Amax (methanol)=435 nm.

Compound VI 4-Methoxy-2-(4-triazol-5'-ylphenylazo)naphth- 1 -ol

5-(4-Aminophenyl)triazole was diazotised and coupled with 4methoxynaphth - 1 - ol using a method similar to that described for the preparation of Compound I. The product was crystallised from aqueous pyridine, m.p. 30340,;lmax (methanol)=510 nm.

Compound VII N - (Benzotriaol - 5 - yl) - 2 - hydroxy - 5 - (4 - methoxyphenylazo)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride (1.46 g) in dry tetrahydrofuran (10 ml) was added dropwise with stirring to a solution of 5-aminobenzotriazole (0.67 g) in dry tetrahydrofuran (15 ml) and dimethylaniline (0.61 g). After 2-3 hours the mixture was poured into water and the solid removed by filtration. The product crystallised from aqueous pyridine as yellow plates of the monohydrate (1.5 g, 8lav0), m.p. 2756 , Amax (methanol)=352 nm.

Compound VIII 2-Hydroxy-N-(indazol-5-yl)-5-(4-methoxyphenylazo)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride and 5-aminoindazole were reacted under the conditions described for the preparation of Compound VII to form 2 hydroxy - N - (indazol - 5 - yl) - 5 - (4 - methoxyphenylazo)benzamide. The product crystallised from aqueous pyridine, m.p. 2767 , Amax (methanol)=348 nm.

Compound IX 2 - Hydroxy - N - (indazol - 6 - yl) - 5 - (4 - methoxyphenylazo)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride and 6-aminoindazole were reacted together by a method similar to that described for the preparation of Compound VII. The product crystallised from aqueous pyridine, m.p. 2790, Amax (methanol)=350 nm.

Compound X N - (Benzimidazol - 6 - yl) - 2 - hydroxy - 5 - (4 - methoxyphenylazo)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride and 6-aminobenzimidazole were reacted together by a method similar to that described for the preparation of Compound VII. The product crystallised from aqueous pyridine, m.p. 2740, Amax (methanol)=350 nm.

Compound XI 2- Hydroxy - 5 - (4 - methoxyphenylazo)- N- (4 - triazol- 5' ylphenyl)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride and 5 - (4 - aminophenyl)triazole were reacted together as described for the preparation of Compound VII. The product crystallised from aqueous pyridine, m.p. 2680, Amax (methanol)=352 nm.

Compound D N - [3 - (1 - Acetyl - 6 - methyl - 1H - pyrazolo[3,2 - d - S - triazol - 3 - yl) - 4 methoxyphenyll - 2 - hydroxy - 5 - (4 - methoxyphenylazo)benzamide

5-(4-Methoxyphenylazo)salicoyl chloride and 1 - acetyl - 3 - (5 - amino - 2 methoxyphenyl) - 6 - methyl - lH - pyrazolo[3,2 - c] - s - triazole were reacted together as described for the preparation of Compound VII. The product crystallised as the monohydrate from aqueous pyridine, m.p. 2823 , Amax (methanol)=350 nm.

Compound XII N - 14 - Methoxy - 3 - (6 - methyl - 1H - pyrazolo[3.2 - c] - S - triazol - 3 - yl)phenyl - 2 - hydroxy - 5 - (4 - methoxyphenylazo)benzamide

N - [3 - (1 - Acetyl - 6 - methyl - 1H - pyrazolo[3,2 - c] - S - triazol - 3 yl) - 4 - methoxyphenyl] - 2 - hydroxy - 5 - (4 - methoxyphenylazo)benzamide (1.4 g) was dissolved in methanol (100 ml) and a solution of sodium hydroxide (1.0 g) in water (30 ml) added. After stirring for 15 minutes the solution was acidified with glacial acetic acid when a gelatinous precipitate of the product appeared. This was removed by filtration, washed with water and crystallised from aqueous pyridine (850 mg; 66%), m.p. 295--60 Amax (methanol)=350 nm.

Compound XIII N - (Benzotriazol - 5 - yl) - 4 -(5 - hydroxy - 3 - methyl - 1 - phenylpyrazol - 4 - ylazo)benzamide

A mixture of 5-aminobenzotriazole (0.78 g), 4 - (5 - hydroxy - 3 - methyl 1 - phenylpyrazol - 4 - ylazo)benzoyl chloride (2.0 g) and N,N-dimethylaniline (0.71 g) in dry tetrahydrofuran (15 ml) was stirred overnight and water (50 ml) was added. The precipitate was filtered and crystallised from aqueous pyridine (1.9 g), m.p. 2850 decomp., Amax (methanol)=396 nm.

Compound XIV N - (Benzotriazol - 5 - yl) - 1 - hydroxy - 4 - (2 - methylsulphonyl - 4 nitrophenylazo)naphth - 2 - amide

2 - Methylsulphonyl - 4 - nitroaniline (0.66 g) was diazotized in a mixture of 3:1 v/v glacial acetic acid, propionic acid (40 ml) by adding sodium nitrite (0.21 g) in concentrated sulphuric acid (2.5 ml) dropwise at OOC. The resulting diazonium solution was added to N - (benzotriazol - 5 - yl)naphth - 2 - amide (1.0 g) dissolved in 3:1 v/v glacial acetic acid, propionic acid (100 ml) containing ammonium acetate (16.2 g) and the mixture was stirred at room temperature for one hour. The mixture was poured.onto ice and the product was filtered and crystallised from aqueous pyridine. Yield 1.16 g m.p. 2600 decomp., Amax (methanol)=577 nm.

Compound XV 2 - Hydroxy - N - (4 - hydroxy - 6 - methyl - 1,3,3a,7 - tetraazainden - 5 - yl) 5 - (4- methoxyphenylazo)benzamide

5 - Amino - 4 - hydroxy - 6 - methyltetraazaindene (0.5 g) was dissolved in dry methylformamide (10 ml) and dimethylaniline (0.37 g) added. 5 - (4 Methoxyphenylazo)salicyloyl chloride (0.88 g) was added portionwise at room temperature with stirring. After 12 hours the mixture was poured into iced dilute hydrochloric acid and the crude product collected, washed with water and dried.

The material was extracted with a large volume of ether and finally crystallised in aqueous ethanol as the monohydrate. (0.8 g, 61%), m,p. 323324 decomp., Amax (methanol)=354 nm.

Compound XVI 2 - Hydroxy - 5 - (4 - methoxyphenylazo) - N - (lH - tetrazol - 5 - ylmethyl) benzamide

5 - (4 - Methoxyphenylazo)salicylyi chloride and 5 - amino - methyltetrazole were reacted together under the conditions described for the preparation of Compound VII to form Compound XV, m.p. 220225 decomp., after crystallisation from aqueous acetic acid, Amax (methanol)=350 nm.

Compound XVII 5 - [4 - (5 - Hydroxy - 3 - methylcarbamoyl - I - phenylpyrazol - 4 ylazo)benzene sulphonamido]benzotriazole

4 - (5 - Hydroxy - 3 - methylcarbamoyl - 1 - phenylpyrazol - 4ylazo)benzene sulphonyl chloride (2.0 g) and 5-aminobenzotriazole (0.64 g) were stirred at room temperature in dry tetrahydrofuran (10 ml) and a solution of pyridine (5 ml) in dry tetrahydrofuran (10 ml) was slowly added over 2 hours. The mixture was allowed to stir for 20 hours and acidified with 3N hydrochloric acid.

The resulting precipitate was filtered off, dried and purified by chromatography (using a 'Florisil' (trade mark) column and acetone as eluent). Yield 1.18 g, m.p.

1420C, Amax (methanol) 414 nm.

Compound XVIII N - (Benzotriazol - 5 - yl) - 4 - (1 - hydroxy - 4 - methoxynaphth - 2 ylazo)benzamide

A mixture of 4 - (I - hydroxy - 4 - methoxynaphth - 2 - ylazo)benzoyl chloride (3.0 g), 5-aminobenzotriazole (1.23 g) and N,N-dimethylaniline (1.11 g) in dry tetrahydrofuran (60 ml) was stirred overnight and water (100 ml) was then added. The resulting precipitate was filtered and crystallized from aqueous pyridine to give pure material (3.6 g), m.p. 21 5C, Amax (methanol) 514 nm.

Compound XIX N - (Benzotriazol - 5 - yl) - 2 - hydroxy 5 - [4 - (1 - hydroxy - 4 methoxynaphth - 2 - ylazo)phenylazo]benzamide

2 - Hydroxy - 5 - ( I - hydroxy - 4 - methoxynaphth - 2 - ylazophenyl - 4' azo)benzoyl chloride (1 g) was added portionwise at room temperature to a mixture of 5-aminobenzotriazole (0.27 g), dimethylaniline (0.25 g) and dry tetrahydrofuran (20 ml). After stirring overnight the mixture was thrown into dilute hydrochloric acid (100 ml) and the solid removed by filtration and washed with water. The product crystallized as the monohydrate from aqueous dimethylformamide. Yield 0.63 g (54%) m.p. 260C (dec).

Example 1 This example illustrates the method of the invention as applied to a silver behenate photothermographic material.

The following dispersion was ball-milled for 18 hours: 50 mol % silver behenate l 50 mol /n behenic acid 3.0 g polyvinyl butyral* 0.3 g dichloromethane 30 ml *the polyvinyl butyral used in this and the following Examples was 'Butvar' B76: 'Butvar' is a trademark of the Monsanto Company.

The following solution was made up: 2,6-dichloro-4-benzenesulphonamidophenol 0.70 g N-hydroxy-1,8-naphthalimide 0.018g polyvinyl butyral 4.0 g Methanol 10 ml dichloromethane 80 ml polydimethylsiloxane, 2% solution in toluene 0.4 ml 7 ml Portions of this solution were taken and varying quantites of sensitising dye (see table of results below) dissolved therein. Then was added 0.40 ml of a silver bromide dispersion prepared by mixing acetone solutions of silver trifluoroacetate and lithium bromide in the presence of polyvinyl butyral as a peptizer (see "Research Disclosure" September 1974, Item No. 12537). The silver bromide grains were approximately cubic in shape, having an average edge length of 80 nm.

The dispersion contained 0.37 mole of silver bromide and 33 grams of polyvinyl butyral per kilogram.

The mixture was allowed to stand for a few minutes, then was added 2.5 ml of the silver behenate dispersion described above. After mixing, the composition was coated at approximately 100 mVm2 on photographic paper base, and the coating dried.

Portions of the coatings made in this way were exposed on a wedge spectrograph, then developed by heating for about 5 seconds at 1200C. An image of that part of the visible spectrum to which the coating was sensitive resulted, and the results are tabulated below. Coatings without sensitising dye added showed an intrinsic sensitivity from 400 nm (the short wavelength limit of the spectrograph) to about 480 nm. Sensitivity beyond this indicates spectral sensitisation. Results are quoted for the optimum quantity of dye added.

Spectral Sensitization of Photothermographic Materials Long wavelength Quantity added Spectral limit of sensitiz Dye mg/m2 Sensitisation ation (nm) A 1.0 very weak 600 B 1.0and5.0 none C 1.0 weak 590 D 1.0 weak 590 I 1.0 very strong 670 II 1.0 moderate 600 III 1.0 strong 660 IV 0.1 moderate 650 V 1.0 strong 630 VI 0.3 weak 620 VII 3.0 moderate 590 VIII 10 moderate 600 IX 0.3 weak 610 X 10 strong 580 XI 3.0 strong 570 XII 3.0 moderate 580 XIII 0.5 weak 610 XIV 0.5 weak 700 XV 4.0 strong 570 A further coating was made, adding dye I at 1.0 mg/m2 but omitting the silver bromide emulsion. No sensitivity was shown by this coating, showing the effect of the dye is a spectral sensitisation involving silver bromide, and not due to the intrinsic light sensitivity of the dye silver salt generated in situ from dye and silver behenate.

The above results show that silver behenate-silver bromide systems may be spectrally sensitised by azo dyes. The inclusion of a heterocyclic 'nitrogen acid' in the dye molecule gives improved sensitisation compared with analogous dyes not having such a group.

Example 2 This illustrates the use according to the invention of an azo sensitizing dye in a photothermographic material based on the silver salt of the dye. The silver salt was prepared as described in our United Kingdom patent application 47277/76 (Serial No 1590956) and 'Research Disclosure', May 1978, item No. 16966.

The following dispersion was ball-milled for 18 hours: silver salt of dye Compound II 0.3 g polyvinyl butyral 0.3 g dichloromethane 30 ml The following solution was prepared: l-phenyl-3-pyrazolidone 0.40 g phthalazinone 0.40 g polyvinyl butyral 1.40 g triphenylphosphate 0.50 g methanol 5 ml dichloromethane 35 ml

Claims

**WARNING** start of CLMS field may overlap end of DESC **. A further coating was made, adding dye I at 1.0 mg/m2 but omitting the silver bromide emulsion. No sensitivity was shown by this coating, showing the effect of the dye is a spectral sensitisation involving silver bromide, and not due to the intrinsic light sensitivity of the dye silver salt generated in situ from dye and silver behenate. The above results show that silver behenate-silver bromide systems may be spectrally sensitised by azo dyes. The inclusion of a heterocyclic 'nitrogen acid' in the dye molecule gives improved sensitisation compared with analogous dyes not having such a group. Example 2 This illustrates the use according to the invention of an azo sensitizing dye in a photothermographic material based on the silver salt of the dye. The silver salt was prepared as described in our United Kingdom patent application 47277/76 (Serial No 1590956) and 'Research Disclosure', May 1978, item No. 16966. The following dispersion was ball-milled for 18 hours: silver salt of dye Compound II 0.3 g polyvinyl butyral 0.3 g dichloromethane 30 ml The following solution was prepared: l-phenyl-3-pyrazolidone 0.40 g phthalazinone 0.40 g polyvinyl butyral 1.40 g triphenylphosphate 0.50 g methanol 5 ml dichloromethane 35 ml polydimethylsiloxane, 2% in toluene 0.25 ml To 5 ml portion of this solution was added varying quantities of the dye Compound I. Then was added 0.5 ml of silver bromide dispersion, as described in Example 1. After a few minutes, 4 ml of the silver dye salt dispersion was added the mixture coated at approximately 100 mVm2 on film base, and the coating dried. Portions of the coatings were exposed to a wedge spectrograph and developed by heating for 7 seconds at 1400 C. The amounts of sensitising dye added were 0, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10 mg/m2. The coating containing no sensitising dye showed only the spectral sensitivity of silver bromide, with a long wavelength limit of about 460 nm. The coatings containing dye showed increasing spectral sensitisation up to 5.0 mg/m2 of dye; at 10 mg/m2 there was no further increase. At these levels, the long wavelength limit of sensitisation was about 620 nm. Example 3 A conventional aqueous silver bromide-gelatin emulsion was used in this example. It was prepared by normal double-jet controlled pAg techniques to give cubic grains of edge length about 0.2,us. The pH of the emulsion was 6.6, pAg was 8.0, and the dilution was 7 litres per mole silver bromide. The dye Compound I was added as a solution in ethanol/acetone, at the rate of 300 mg/mole silver bromide, to the molten emulsion at 400 C. The pH remained at 6.6, pAg increased to 8.1. A portion of this dyed emulsion was coated at about 100 ml/m2 on film base (Coating A). Silver nitrate solution was added to another portion of the dyed emulsion to bring the pAg to 4.0. The emulsion was left for five minutes, then the pAg restored to 8.0 by adding potassium bromide solution. The pH remained at 6.6 throughout. This emulsion was also coated on film base at about 100 mVm2 (Coating B). Portions of the dried coatings were exposed on a wedge spectrograph and developed in 'Kodak' D163 Developer (diluted 1+3 with water), then fixed in 'Kodak' Rapid Fixer and washed ('Kodak' is a trade mark). Coating A showed silver bromide sensitivity out to 480 nm and very weak dye sensitisation to 620 nm. Coating B showed silver halide sensitivity to 480 nm, and moderately strong dye sensitisation out to 660 nm, with peaks at 530 and 580 nm. WHAT WE CLAIM IS:

1. A photographic silver halide.spectrally sensitized with an azo dye of the formula: A-N=N-B (I)

wherein A and B are moieties which complete a chromophoric system which includes the azo group, one or each of A and B comprising a heterocyclic silver salt-forming nucleus including a nitrogen atom which carries an acidic hydrogen atom which enables the dye to form a substantially water-insoluble silver salt.

2. A photographic silver halide according to Claim 1 wherein each heterocyclic silver salt-forming nucleus is a triazole, tetrazole, indazole, benzimidazole, benzotriazole, pyrazolotriazole or hydroxytetraazaindene nucleus.

3. A photographic silver halide according to Claim 1 or 2 wherein the azo dye is a bis azo dye, the moiety A being of the formula A1-N=N-A2- wherein each of A' and A2 is an aromatic or heterocyclic nucleus.

4. A photographic silver halide according to any of the preceding claims wherein the azo dye is of the formula:

wherein A is as defined for Formula 1, D is a heterocyclic silver salt-forming nucleus containing a 5- or 6-membered heterocyclic ring which includes a nitrogen atom which carries an acidic hydrogen atom which enables the dye to form a substantially water-insoluble silver salt, R indicates the possible presence of one or more simple substituents and of atoms which complete a fused-on aromatic ring, x is 0 or 1 and E is a group of formula -CONR1-, -NR1.CO-, -CONR1CH2-, --NR'CONR''-,

-SO2NR1- or -NR1SO2- wherein R is as already defined and RX is hydrogen, an alkyl or substituted alkyl group, or an aryl or substituted aryl group.

5. A photographic silver halide according to Claim 1 wherein the azo dye is one of the Dyes I to XIX specified herein.

6. A photographic silver halide according to any of the preceding claims sensitized with 0.02 to 8 millimoles of the azo dye per mole of the silver halide.

7. A sensitive photothermographic material which comprises a support and, incorporated in a layer or layers coated on one side thereof, (a) a spectrally sensitized photographic silver halide according to any of the preceding claims, and an oxidation-reduction image-forming system comprising (b) a substantially light- insensitive reducible metal salt and (c) a reducing agent therefor.

8. A photothermographic material according to Claim 7 wherein the metal salt (b) is a silver saft of a long chain fatty acid having at least 10 carbon atoms.

9. A photothermographic material according to Claim 8 wherein the reducing agent (c) is a substituted phenol or naphthol.

10. A sensitive photographic material which comprises a support bearing a layer of a gelatino-silver halide emulsion, the grains of which comprise a spectrally sensitized silver halide according to any of Claims 1 to 6.