Fogged direct positive silver halide emulsions containing photographic sensitizing dyes derived from substituted salicylaldehydes

Abstract

Cyanine and merocyanine photographic sensitizing dyes derived from substituted salicylaldehydes and light sensitive silver halide emulsions containing such dyes are disclosed. Also disclosed are certain reversible spiropyranmerocyanine dye systems, the stabilization of a dye in either the spiropyran form or the merocyanine form and observations of these photochromic materials when coated in polymeric matrices.

Description

This invention relates to photographic silver halide emulsions containing sensitizing dyes derived from substituted salicylaldehydes.

In accordance with this invention there are provided photographic silver halide emulsions containing a sensitizing dye represented by the following general formulas: ##SPC1##

And ##SPC2##

Wherein a is 0 or 1; R preferably represents a lower alkyl group having 1 to 8 carbon atoms such as methyl, ethyl, propyl, butyl, etc., and including substituted lower alkyl groups such as a carboxyalkyl group (e.g., carboxymethyl, carboxyethyl, carboxypropyl, carboxybutyl, etc.), an alkoxycarbonyl alkyl group (e.g., methoxycarbonylmethyl, ethoxycarbonylethyl, propoxycarbonylethyl, propoxycarbonylbutyl, etc.), a sulfoalkyl group (e.g., sulfomethyl, sulfoethyl, sulfopropyl, sulfobutyl, etc.), a hydroxyalkyl group (e.g., hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, etc.), an alkoxyalkyl group (e.g., methoxyethyl, methoxypropyl, ethoxyethyl, ethoxypropyl, etc.), a sulfoalkoxyalkyl group, as well as an aryl group such as phenyl, 4-methylphenyl, 4-chlorophenyl, etc.; R.sub.1 and R.sub.2 may be the same or different groups and each represents hydrogen, a nitro group, a halo group such as chloro, bromo, fluoro, or iodo, a lower alkoxy group such as methoxy, ethoxy, propoxy, etc., or a lower alkyl group such as previously defined for R provided that at least one of the groups R.sub.1 and R.sub.2 must be a nitro group or a halo group; and Z represents the nonmetallic atoms necessary to complete a heterocyclic nucleus containing from 5 to 6 atoms in the heterocyclic ring, such as a heterocyclic nucleus of the thiazole series (e.g., thiazole, 4-methylthiazole, 5-methylthiazole, 4-phenylthiazole, 5-phenylthiazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thiazole, etc.), those of the benzothiazloe series (e.g., benzothiazole, 4-chlorobenzothiazole, 5-chlorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, 5-methylbenzothiazole, 6-methylbenzothiazole, 5-bromobenzothiazole, 6-bromobenzothiazole, 4-phenylbenzothiazole, 5-phenylbenzothiazole, 4-methoxybenzothiazole, 5-methoxybenzothiazole, 6-methoxybenzothiazole, 5-iodobenzothiazole, 6-iodobenzothiazole, 4-ethoxybenzothiazole, 5-ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6-dimethoxybenzothiazole, 5,6-dihydroxymethylenebenzothiazole, 5-hydroxybenzothiazole, etc.), those of the naphthothiazole series (e.g., naphtho[2,1-d]thiazole, naphtho[1,2-d]thiazol, 5-methoxynaphtho[1,2-d]thiazole, 5-ethoxynaphtho[1,2-d]thiazole, 7-methoxynaphtho[1,2-d]thiazole, 8-methoxynaphtho[1,2-d]thiazole, etc.), those of the thieno[1,2-a]benzothiazole series (e.g., 5-methoxythieno[1,2-a]benzothiazole etc.), those of the oxazole series (e.g., 4-methyloxazole, 5-methyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, 5-phenyloxazole, etc.), those of the benzoxazole series (e.g., benzoxazole, 5-chlorobenzoxazole, 5-phenylbenzoxazole, 5-methylbenzoxazole, 6-methylbenzoxazole, 5,6-dimethylbenzoxazole, 4,6-dimethylbenzoxazole, 5-methoxybenzoxazole, 5-ethoxybenzoxazole, 6-chlorobenzoxazole, 5-hydroxybenzoxazole, 6-hydroxybenzoxazole, etc.), those of the naphthoxazole series (e.g., naphth[2,1-d]oxazole, naphth[1,2-d]-oxazole, etc.), those of the selenazole series (e.g., 4-methylselenazole, 4-phenylselenazole, etc.), those of the benzoselenazole series (e.g., benzoselenazole, 5-chlorobenzoselenazole, 5-methoxybenzoselenazole, 5-hydroxybenzoselenazole, tetrahydrobenzoselenazole, etc.), those of the naphthoselenazole series (e.g., naphtho[2,1-d]selenazole, naphtho[1,2-d]selenazole, etc.), those of the thiazoline series (e.g., thiazoline, 4-methylthiazoline, etc.), those of the 2-quinoline series (e.g., quinoline, 3-methylquinoline, 5-methylquinoline, 7-methylquinoline, 8-methylquinoline, 6-chloroquinoline, 8-chloroquinoline, 6-methoxyquinoline, 6-ethoxyquinoline, 6-hydroxyquinoline, 8-hydroxyquinoline, etc.), those of the 4-quinoline series (e.g., quinoline, 6-methoxyquinoline, 7-methylquinoline, 8-methylquinoline, etc.), those of the 1-isoquinoline series (e.g., isoquinoline, 3,4-dihydroisoquinoline, etc.) those of the 3,3-dialkylindolenine series (e.g., 3,3-dimethylindolenine, 3,3,5-trimethylindolenine, 3,3,7-trimethylindolenine, etc.), those of the 2-pyridine series (e.g., pyridine, 3-methylpyridine, 4-methylpyridine, 5-methylpyridine, 6-methylpyridine, 3,4-dimethylpyridine, 3,5-dimethylpyridine, 3,6-dimethylpyridine, 4,5-dimethylpyridine, 4,6-dimethylpyridine, 4-chloropyridine, 5-chloropyridine, 6-chloropyridine, 3-hydroxypyridine, 4-hydroxypyridine, 5-hydroxypyridine, 6-hydroxypyridine, 3-phenylpyridine, 4-phenylpyridine, 6-phenylpyridine, etc.), those of the 4 pyridine series (e.g., 2-methylpyridine, 3-methylpyridine, 2-chloropyridine, 3-chloropyridine, 2,3-dimethylpyridine, 2,5-dimethylpyridine, 2,6-dimethylpyridine, 2-hydroxypyridine, 3-hydroxypyridine, etc.), those of the imidazole series (e.g., imidazole, 4-methylimidazole, 5-ethylimidazole, 4-chloroimidazole, 4,5-dichloroimidazole, 4-methoxyimidazole, 5-phenylimidazole, etc.), those of the benzimidazole series (e.g., benzimidazole, 4-methylbenzimidazole, 5-methylbenzimidazole, 6-methylbenzimidazole, 5,6-dichlorobenzimidazole, 5-chlorobenzimidazole, 5-phenylbenzimidazole, 6-phenylbenzimidazole, etc.), those of the naphthimidazole series (e.g., naphth[1,2-d]imidazole, naphth[2,1-d]imidazole, etc.), those of the lepidine series (e.g., lepidine, 7-chlorolepidine, 7-methyllepidine, etc.).

The alkyl groups or moieties referred to herein typically have 1 to 20 carbon atoms, lower alkyl radicals or moieties have 1 to 8 carbon atoms and aryl groups or moieties typically have 6 to 20 carbon atoms.

The dye compounds defined by Formulas I and II are prepared by heating a mixture of a substituted salicylaldehyde having the formula ##SPC3##

wherein R.sub.1 and R.sub.2 are as previously defined with a nitrogen-containing heterocyclic compound containing a reactive methyl group or a methylene group represented by the general formulas ##EQU1## wherein R, Z and a are as previously defined and A represents an anionic group such as chloride, bromide, iodide, p-toluenesulfonate, thiocyanate, perchlorate, acetate, methylsulfate, ethylsulfate, etc. The reaction is preferably conducted in an inert solvent such as tetrahydrofuran, pyridine, quinoline, isoquinoline, ethanol, propanol, butanol, etc., and in the presence of a basic condensing agent such as the trialkyl amines (e.g., triethylamine, tri-n-propylamine, tri-n-butylamine etc.), piperidine, N-methylpiperidine, N-ethylpiperidine, N, N-dimethylaniline, etc.

The following examples disclose the preparation of some of the dyes of Formulas I and II:

EXAMPLE 1

1',3'-Diethyl-2',3'-dihydro-6-nitrospiro(2H)-1-benzopyran-2,2'-1H-imidazo[4 ,5-b]quinoxaline ##SPC4##

A solution of 1,3-diethyl-2-methylimidazo[4,5-b]quinoxolium p-toluenesulfonate (4.12 g., .01 mole), 5-nitrosalicylaldehyde (1.67 g., .01 mole) and piperidine (1.0 g.) in ethanol (30 ml.) is heated to reflux for 10 minutes. During this period a dye crystallizes as green crystals and is collected upon cooling. The yield of dye after washing with ethanol and ether and drying is 2.4 g. (62% yield). The dye is refluxed in ethanol (130 ml.) containing 3 drops of triethylamine for 24 hours. The volume is then reduced to 40 ml. After cooling, 1.75 g. (45% yield) of a pale green solid is obtained, having a melting point of 215.degree.-217.degree.C. Photocolorization occurs when a solution of this compound is irradiated with UV light.

Analysis: Calculated: C, 64.8%; H, 49.0%; N 18.0%. Found: C, 64.7%; H, 5.1%; N 18.2%.

EXAMPLE 2

6-[(1-Methylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cyclo hexadienone ##SPC5##

A mixture of 1,2-dimethylnaphtho[1,2-d]thiazolium p-toluenesulfonate (3.85 g., 0.01 mole), 5-nitrosalicylaldehyde (1.67 g., 0.01 mole) and piperidine (0.87 g., 0.01 mole) is refluxed for 10 minutes in ethanol (25 ml.). The dye precipitates upon cooling and is collected and washed with ethanol to give 3.35 g. of product. A sample of 1.5g. of this crude product is digested in dimethylformamide containing a small amount of ethanol. The dye is collected and dried. The yield of dye is 1.4 g. (85% yield) having a melting point of 243.degree.-245.degree.C. The dye can be reversibly bleached with visible light.

Analysis: Calculated: C, 65.9%; H, 4.2%; N, 7.7%. Found: C, 66.2%; H, 4.2%; N, 7.7%.

EXAMPLE 3

6-[(1-Ethylnaphtho[1,2,-d]thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cyclo hexadienone ##SPC6##

A solution of 1-ethyl-2-methylnaphtho[1,2-d]thiazolium p-toluenesulfonate (3.99 g., 0.01 mole) and 5-nitrosalicylaldehyde (1.67 g., 0.01 mole) in ethanol (50 ml.) is prepared and filtered at room temperature. Piperidine (1.0 g.) is added and the reaction mixture is heated for 10 minutes. A precipitate is formed and collected upon cooling and then washed with ethanol. The yield of dye is 2.83 g. (75% yield) having a melting point of 230.degree.C.

Analysis: Calculated: C, 67.0%; A, 4.3%; N 7.4%. Found: C, 66.9%; H, 4.4%; N 6.9%.

EXAMPLE 4

6[(3-Ethyl-2-benzoselenazolinylidene)ethylidene]4-nitro-2,4-cyclohexadienon e ##SPC7##

A solution of 3-ethyl-2-methylbenzoselenazolium p-toluenesulfonate (3.49 g., 0.01 mole) and 5-nitrosalicyladehyde (1.67 g., 0.01 mole) is prepared in ethanol (45 ml.) and filtered at room temperature. Piperidine (0.87 g., 0.01 mole) is added and the reaction mixture is heated for 10 minutes. A solid precipitates and is collected and washed with ethanol. The yield of dye after drying is 2.78 g. (75% yield) having a melting point of 239.degree.C. The dye can be reversibly bleached with visible light.

Analysis: Calculated: C 54.7%; H 3.8%; N 7.5% Found: C 54.4%; H 3.8%; N 7.3%

EXAMPLE 5

6-[(1-Ethylnaphtho[2,3-d]thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cycloh exadienone ##SPC8##

A solution of 1-ethyl-2-methylnaphtho[2,3-d]thiazolium p-toluenesulfonate (4.0 g., 0.01 mole) and 5-nitrosalicyladehyde (1.67 g., 0.01 mole) in ethanol (100 ml.) is prepared and filtered at room temperature. Piperidine (0.87 g., 0.01 mole) is added to the solution and the reaction mixture is refluxed for 15 minutes. After cooling, the dye is collected and washed with ethanol. This provides 2.5 g. of product. This material is dissolved in m-cresol with slight warming and filtered hot. Boiling methanol is then added until the dye begins to crystallize. After cooling, the dye is collected, washed and dried. The yield of dye is 0.9 g. (24% yield) having a melting point of 207.degree.-209.degree.C.

Analysis: Calculated: C, 67.0%; H, 4.3%. Found: C, 67.0%; H, 4.6%.

EXAMPLE 6

6-Nitro-2',3'-dihydro-1',3',3'-trimethylspiro(2H)-1-benzopyran-2,2'(1H)pyrr olo[2,3-b]pyridine ##SPC9##

A solution of 2-methylene-1,3,3-trimethyl-3H-pyrrolo[2,3-b]pyridine (3.2 g., 0.02 mole) and 5-nitrosalicylaldehyde (3.34 g., 0.02 mole) is refluxed for 1 hour in ethanol (50 ml.) containing a drop of piperidine. Upon cooling, a solid crystallizes from the reaction mixture. The solid is recrystallized from ethanol-methylcyclohexane to obtain 1.7 g. (26% yield) of a colorless solid having a melting point of 163.degree.-164.degree.C. Photocoloration is observed when solutions of this compound are irradiated with UV light.

EXAMPLE 7

6-[(1,3-Diethyl-2-benzimidazolinylidene)ethylidene]4-nitro-2,4-cyclohexadie none ##SPC10##

A solution of 1,3-diethyl-2-methylbenzimidazolium iodide (3.16 g., 0.01 mole), 5-nitrosalicylaldehyde (1.67 g., 0.01 mole) and piperidine (0.87 g.) is refluxed in ethanol (15 ml.) for one hour. Upon cooling, the dye precipitates and is collected, washed with ethanol and dried. This provides 2.86 g. of dye. The material is recrystallized once from dimethylformamide to obtain 1.0 g. of dye having a melting point of 271.degree.C. (dec.). Color changes are observed when solutions of this dye are irradiated.

Analysis: Calculated: C, 67.6%; H, 5.7%; N 12.5%. Found: C, 67.5%; H, 6.0%; N 12.4%.

EXAMPLE 8

6-[(3-Ethyl-2-benzoxazolinylidene)ethylidene]-4-nitro-2,4-cyclohexadieneone ##SPC11##

A mixture of 3-ethyl-2-methylbenzoxazolium iodide (2.89 g., 0.01 mole), 5-nitrosalicylaldehyde (1.67 g., 0.01 mole) and piperidine (1.0 g.) is refluxed in tetrahydrofuran (75 ml.) for 20 minutes. The dye precipitates and after cooling is collected, washed with tetrahydrofuran and dried. This provides 2.4 g. of dye. This material is digested in water, collected and dried. This solid is then recrystallized from m-cresol-methanol to obtain 1.87 g. (60% yield) of dye having a melting point of 228.degree.-230.degree.C. Solutions of the dye can be reversibly photobleached with visible light.

EXAMPLE 9

6-[(1-Ethyl-4(1H)-quinolylidene)ethylidene]-4-nitro2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 3 in an 88% yield. The product has a melting point of 260.degree.-262.degree.C.

Analysis: Calculated: C, 71.2%; H, 5.0%. Found: C, 71.0%; H, 5.1%.

EXAMPLE 10

6-[(3-Ethylnaphtho[2,1-d]thiazolin-2-ylidine)ethylidene]-4-nitro-2,4-cycloh exadienone

This dye is prepared by the procedure described in Example 5 in a 75% yield. The product has a melting point of 239.degree.-240.degree.C. and is photoactive.

EXAMPLE 11

6-[(5,6-Dichloro-1,3-diethyl-2-benzimidazolinylidene)ethylidene]-4-nitro-2, 4-cyclohexadienone

This dye is prepared by the procedure described in Example 3 in an 86% yield. The product has a melting point of 292.degree.-294.degree.C. (dec.) and is photoactive.

Analysis: Calculated: C, 58.8%; H, 4.4%. Found: C, 58.6%; H, 4.4%.

EXAMPLE 12

6-[(3,4-Diphenyl-4-thiazolin-2-ylidene)ethylidene]4-nitro-2,4-cyclohexadien one

This dye is prepared by the procedure described in Example 7 except that the product is recrystallized from ethanol. The product is obtained in a 19% yield, has a melting point of 192.degree.-197.degree.C. and is photoactive.

EXAMPLE 13

6-[(1-Methyl-2(1H)-quinolylidene)ethylidene]-4nitro-2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 3 in an 82% yield. The product has a melting point of 250.degree.-253.degree.C.

Analysis: Calculated: C, 70.6%; H, 4.6%. Found: C, 70.5%; H, 4.6%.

EXAMPLE 14

6-[(3-Ethyl-2-benzothiazolinylidene)ethylidene]-4nitro-2,4-cyclohexadienone

This dye is prepared by the procedure desribed in Example 3 in a 72.5% yield. The product has a melting point of 235.degree.-240.degree.C. and is photoactive.

Analysis: Calculated: C, 62.6%; H, 4.3%. Found: C, 62.7%; H, 4.4%.

EXAMPLE 15

6-[(1,3-Diethyl-1H-imidazo-[4,5-b]quinoxalin-2(3H)ylidene)ethylidene]-2-met hoxy-4-nitro-2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 5 in a 25% yield. The product has a melting point of 241.degree.C. and is photoactive.

Analysis: Calculated: C, 63.0%; H, 5.0%. Found: C, 62.6%; H, 5.2%.

EXAMPLE 16

6-[(3-Ethyl-2-benzothiazolinylidene)ethylidene]-2-methoxy-4-nitro-2,4-cyclo hexadienone

This dye is prepared by the procedure described in Example 3 in a 50% yield. The product has a melting point of 242.degree.-244.degree.C.

Analysis: Calculated: C, 60.7%; H, 4.5%. Found: C, 61.0%; H, 4.7%.

EXAMPLE 17

6-[(1-Methylnaphtho[1,2-d]-thiazolin-2-ylidene)ethylidene]-2-nitro-2,4-cycl ohexadienone

This dye is prepared by the procedure described in Example 3 in an 85% yield. The product has a melting point of 202.degree.-206.degree.C. and is photoactive.

Analysis: Calculated: C, 66.3%; H, 3.9%. Found: C, 66.0%; H, 4.0%.

EXAMPLE 18

6-[(3-Ethyl-2-benzothiazolinylidene)ethylidene]-2-nitro-2,4-cyclohexadienon e

This dye is prepared by the procedure described in Example 7 except that the product is recrystallized in ethanol. The product is obtained in a 16% yield, has a melting point of 191.degree.-193.degree.C. (dec.), and is photoactive.

Analysis: Calculated: C, 62.6%; H, 4.3%. Found: C, 62.6%; H, 4.3%.

EXAMPLE 19

3,5-Dibromo-6-[(3-ethyl-2-benzothiazolinylidene)ethylidene]-2,4-cyclohexadi enone

This dye is prepared by the procedure described in Example 3 in an 85% yield. The product has a melting point of 194.degree.-196.degree.C.

Analysis: Calculated: C, 48.2%; H, 3.1%. Found: C, 47.9%; H, 3.3%.

EXAMPLE 20

6-[(1,3-Diethyl-1H-imidazo[4,5-b]quinoxalin -2(3H)-ylidene)ethylidene]2,4-dinitro-2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 3 in a 56% yield. The product has a melting point of 274.degree.C. (dec.) and is photoactive.

Analysis: Calculated: C, 58.1%; H, 4.2%. Found: C, 57.9%; H, 4.5%.

EXAMPLE 21

6-[(1-Methylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-2,4-dinitro-2,4-c yclohexadienone

This dye is prepared by the procedure described in Example 5 in a 36% yield. The product has a melting point of 303.degree.C. (dec.) and is photoactive.

Analysis: Calculated: C, 59.0%; H, 3.2%. Found: C, 59.1%; H, 3.3%.

EXAMPLE 22

6-[(5,6-Dichloro-1,3-diethyl-2-benzimidazolinylidene)ethylidene]-2,4-dinitr o-2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 5 in a 6% yield. The product has a melting point of 325.degree.C. (dec.) and is photoactive.

Analysis: Calculated: C, 50.6%; H, 3.6%. Found: C, 50.9%; H, 3.7%.

EXAMPLE 23

4-Nitro-6-[(4,5,6,7-tetrachloro-1,3-diethyl-2-benzimidazolinylidene)ethylid ene]-2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 3 in a 50% yield. The product has a melting point of 250.degree.C. (dec.) and is photoactive.

Analysis: Calculated: C, 48.0%; H, 3.2%. Found: C, 48.2%; H, 3.5%.

EXAMPLE 24

6-[(3-Ethyl-4-phenyl-4-thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cyclohex adienone

This dye is prepared by the procedure described in Example 3 in a 90% yield. The product has a melting point of 221.degree.-222.degree.C. (dec.) and is photoactive.

Analysis: Calculated: C, 64.8%; H, 4.6%. Found: C, 64.7%; H, 4.5%.

EXAMPLE 25

6-[(1,2-Dihydropyrrolo[2,1-b]benzothiazol-3-yl)methylene]-4-nitro-2,4-cyclo hexadienone

This dye is prepared by the procedure described in Example 3 in a 65% yield. The product has a melting point of 254.degree.-255.degree.C.

Analysis: Calculated: C, 62.9%; H, 3.7%. Found: C, 62.6%; H, 3.7%.

EXAMPLE 26

6-[(2,3-Dihydro-1H-pyrido[2,1-b]benzothiazol-4-yl)methylene]-4-nitro-2,4-cy clohexadienone

This dye is prepared by the procedure described in Example 3 in a 54% yield and is digested in ethanol. The product has a melting point of 234.degree.-239.degree.C. (dec.).

EXAMPLE 27

6-[(1-Ethylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-2,4-dinitro-2,4-cy clohexadienone

This dye is prepared by the procedure described in Example 3 in a 39% yield. The product has a melting point of 308.degree.-309.degree.C. and is photoactive.

EXAMPLE 28

2,4-Dibromo-6-[(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-2,4-cy clohexadienone

This dye is prepared by the procedure described in Example 5 in a 75% yield. The product has a melting point of 190.degree.C. and is photoactive.

EXAMPLE 29

3,5-Dichloro-6-[(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-2,4-c yclohexadienone

This dye is prepared by the procedure described in Example 3 in a 53% yield. The product is recrystallized from a mixture of dimethyl formamide, methanol and hexane. The product has a melting point of 215.degree.-217.degree.C. and is photoactive.

EXAMPLE 30

3,5-Dichloro-6-[(3-ethylnaphtho[2,1-d]thiazolin-2-ylidene)ethylidene]-2,4-c yclohexadienone

This dye is prepared by the procedure described in Example 30 in a 50% yield. The product has a melting point of 217.degree.C. and is photoactive.

EXAMPLE 31

3,5-Dichloro-6-[(3-ethyl-2-benzothiazolinylidene)ethylidene]-2,4-cyclohexad ienone

This dye is prepared by the procedure described in Example 5 in a 58% yield. The product has a melting point of 142.degree.-148.degree.C. and is photoactive.

EXAMPLE 32

3,5-Dichloro-6-[(3-methyl-2-benzothiazolinylidene)ethylidene]-2,4-cyclohexa dienone

This dye is prepared by the procedure described in Example 11 in a 42% yield. The product has a melting point of 216.degree.-217.degree.C. and is photoactive.

EXAMPLE 33

6,8-Dibromo-1',3'-diethyl-2',3'-dihydrospiro[2H]-1-benzopyran-2,2'-(1H)-imi dazo[4,5-b]quinoxaline

This dye is prepared by the procedure described in Example 3 in a 36% yield. The product is recrystallized from methylene chloride, has a melting point of 136.degree.-137.degree.C. and is photoactive.

EXAMPLE 34

6-[(1-Ethylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-2-methoxy-5-nitro- 2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 3 in a 40% yield. The product is recrystallized from a mixture of dimethyl formamide and ethanol. The product has a melting point of 240.degree.-241.degree.C. and is photoactive.

EXAMPLE 35

6-[(3-Ethylnaphtho[2,1-d]thiazolin-2-ylidene)ethylidene]-2-methoxy-5-nitro- 2,4-cyclohexadienone

This dye is prepared by the procedure described in Example 11 in a 59% yield. The product has a melting point of 239.degree.C. and is photoactive.

EXAMPLE 36

2-Methoxy-6-[(1-methyl-naphtho[1,2-d]thiazolin-2ylidene)]-4-nitro-2,4-cyclo hexadienone

This dye is prepared by the procedure described in Example 3 in a 97% yield. The product has a melting point of 251.degree.-254.degree.C.

Analysis: Calculated: C, 64.3%; H 4.1%. Found: C, 64.5%; H 4.3l%.

Depending upon the nature of the nitrogen-containing heterocyclic nucleus, i.e., the group ##EQU2## the dyes at ambient temperatures exist in either the merocyanine open colored form of Formula I or the closed colorless spiropyran form of Formula II. Regardless of which form they are in at ambient temperatures, they generally may be converted to the other form. Thus, these dyes exhibit extremely efficient photochromic behavior. By control of the dye structure and the exciting wavelength, the dyes may be used in either negative or positive silver halide photographic systems. The dyes of Formula I are particularly useful as spectral sensitizers for direct positive silver halide photographic systems and for organic photoconductors.

Photographic silver halide emulsions which can be spectrally sensitized by the dyes of this invention can comprise, for example, silver chloride, silver bromide, silver bromoiodide, silver chlorobromide, silver chloroiodide, silver chlorobromoiodide crystals or mixtures thereof. Suitable emulsions are described in Product Licensing Index, Volume 92, December 1971, Publication 9232, page 107, Paragraph I.

The silver halide emulsions can be unwashed or washed to remove soluble salts after precipitation of the silver halide as described in the aforementioned Product Licensing Index reference at page 107, Paragraph II.

The dyes of this invention are advantageously incorporated in the washed, finished emulsion and should be uniformly distributed throughout the emulsion. The dyes can be added from solutions in appropriate solvents which are compatible with the emulsion and which are substantially free from deleterious effects on the light sensitive materials.

The types of silver halide emulsions that can be sensitized with the new dyes of this invention include those prepared with hydrophilic colloids that are known to be satisfactory vehicles for dispersed silver halides as described in the aforementioned Product Licensing Index reference at Paragraph VIII.

The concentration of the new dyes in the emulsion can vary widely, e.g., from about 5 to 100 mg. per liter of flowable emulsion containing, for example, 50 g. of silver bromide per liter. The specific concentration will vary according to the type of light-sensitive material in the emulsion and the effects desired. The suitable and most economical concentration for a given emulsion will be apparent to those skilled in the art upon making the tests and observations customarily used in the art of emulsion making.

To prepare a gelatin-silver halide emulsion sensitized with one of the dyes of this invention, the following procedure is satisfactory. A quantity of the dye is dissolved in a suitable solvent and a volume of this solution containing from 5 to 100 mg. of dye is slowly added to one liter of a gelatin-silver halide emulsion. With most of the dyes, 10 to 20 mg. of dye per liter of emulsion suffices to produce the maximim sensitizing effect with the ordinary gelatin-silver bromide (including bromoiodide and chlorobromide) emulsions. With fine grain emulsions, which include most of the ordinarily employed gelatin-silver chloride emulsions, somewhat larger concentrations of dye may be necessary to obtain the optimum sensitizing effect. While this procedure has dealt with emulsions comprising gelatin, it will be appreciated that these remarks apply generally to any emulsion wherein all or part of the gelatin is substituted by another suitable hydrophilic colloid as mentioned above.

Photographic silver halide emulsions containing the sensitizing dyes of this invention can also contain other addenda such as chemical sensitizers, e.g., reducing agents; sulfur, selenium or tellurium compounds; gold, platinum or palladium compounds; or combinations of these. Procedures for chemically sensitizing silver halide emulsions are described in Sheppard et al. U.S. Pat. No. 1,623,499 issued Apr. 5, 1927; Waller et al. U.S. Pat. No. 2,399,083 issued Apr. 23, 1946; McVeigh U.S. Pat. No. 3,297,477 issued Jan. 10, 1967 and Dunn U.S. Pat. No. 3,297,446 issued Jan. 10, 1967. The emulsions can contain development modifiers that function as speed increasing compounds such as polyalkylene glycols, cationic surface active agents and thioethers or combinations of these as described in Piper U.S. Pat. No. 2,886,437 issued May 12, 1959; Dann et al U.S. Pat. No. 3,046,900 issued July 12, 1960 and Goffe U.S. Pat. No. 3,294,540 issued Dec. 27, 1966.

The silver halide emulsions can be protected against the production of fog and can be stabilized against loss of sensitivity during keeping. Useful antifoggants and stabilizers are listed in the aforementioned Product Licensing Index reference at Paragraph V.

The photographic silver halide emulsions of this invention can be coated on a wide variety of supports. Typical supports include cellulose nitrate film, cellulose acetate film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film and related films or resinous materials, as well as glass, paper, metal and the like. Typically, a flexible support is employed, especially a paper support, which can be partially acetylated or coated with baryta and/or an alpha-olefin polymer, particularly a polymer of an alpha-olefin containing 2 to 10 carbon atoms such as polyethylene, polypropylene, ethylenebutene copolymers and the like.

Photographic elements having silver halide emulsions coated thereon can contain developing agents such as those listed in the aforementioned Product Licensing Index reference at Paragraph VI. The emulsions layers can be hardened by various organic or inorganic hardeners, alone or in combination, such as those disclosed in the aforementioned Product Licensing Index reference at Paragraph VII.

The photographic layers of this invention may also include other addenda known to be beneficial in photographic emulsions such as those listed in the Product Licensing Index reference at Paragraphs IV, XI, XII and XIII.

EXAMPLE 37

The sensitizing effects of several of the dyes described above are illustrated in the following tabulation where the sensitizing range and sensitizing minimum values are shown for these dyes. The dyes are tested in a bromoiodide direct positive emulsion containing 2.5 mole percent iodide and having an average grain size of 0.2 micron as described in Berriman U.S. Pat. No. 3,367,778. The dyes, dissolved in suitable solvents are added to separate portions of the emulsion at the concentrations indicated. The emulsions are chemically fogged and are coated at a coverage of 100 mg. of silver per square foot on a cellulose acetate film support. A sample of each coating is exposed on an Eastman 1B Sensitometer, processed for 6 minutes in an Elon-hydroquinone developer, fixed, washed and dried. The photographic results from these tests are set forth in Table I.

TABLE I ______________________________________ Dye Concentration (mg/mole of silver Sensitizing Sensitizing Example halide) Range (nm) Maximum (nm) ______________________________________ 1 400 to 590 550 2 200 to 580 550 3 200 to 550 630 4 200 to 580 550 5 400 to 600 550 7 200 to 470 -- 9 200 to 470 -- 10 400 to 560 530 11 200 to 470 -- 12 200 to 470 -- 14 400 to 575 535 ______________________________________

EXAMPLE 38

The dyes of Examples 1 to 7, 9 to 12 and 14 are tested in a silver bromoiodide emulsion containing 2.5 mole percent iodide and having an average particle size of 0.2 micron. The dyes, dissolved in suitable solvents, are added to separate portions of the emulsion at the concentrations indicated in Table II. The emulsions are digested at 40.degree.C. for 10 minutes and coated at a coverage of 100 mg. of silver per square foot on a cellulose aacetate film support. A sample of each coating is then exposed to a mercury vapor lamp for a 365 nm emission on a spectrophotometer to obtain the sensitizing range. The coatings are processed to 6 minutes in an Elon-hydroquinone developer, fixed, washed and dried. The photographic results from these tests are set forth in Table II.

TABLE II ______________________________________ Dye Concentration Sensi- (mg/mole of silver tizing Desensitization Example halide) Range (nm) in Log E Units ______________________________________ 1 100 to 484 -.27 2 100 to 480 -.40 3 200 to 470 -.75 4 100 to 465 -1.02 5 100 to 460 -1.37 6 100 to 485 -.01 7 100 to 500 +.07 sens. 9 100 to 485 -.28 10 100 to 485 -.30 11 100 to 510 -.03 12 100 to 490 -.05 14 100 to 485 -.35 ______________________________________

EXAMPLE 39

As previously noted, the merocyanine dyes of Formula I and the spiropyran compounds of Formula II exhibit photochromic behavior. To illustrate this effect, various tests are conducted on the compounds shown in Table III. The tests are carried out in test tubes at room temperature using a 200 watt Xenon flash lamp. In most cases, photobleaching is more efficient when an ultraviolet filter is placed between the lamp and the sample. Table III sets forth the photochromic properties of the dyes tested in various solvents. The notation "decolorized" indicates that the dye is photobleached. For most of the dyes, the values for .lambda..sub.max and E .times. 10.sup.4 are determined at the time the solution is prepared and again after 24 hours. In the last 4 columns of Table III, the numbers in parentheses are the values for E .times. 10.sup.4 and the numbers which are not in parentheses are the values of .lambda..sub.max. The notation "s" following a value for .lambda..sub.max indicates the presence of a shoulder on a larger absorption band.

In all cases the 24 hour reading represents the position of thermodynamic equilbrum. In many cases while the dye form was isolated the solution equilibra involves some spiropyran (colorless) form.

TABLE III __________________________________________________________________________ .lambda..sub.max (E .times. 10.sup.4) 10% M-Cresol- Photoactivity Pyridine 90% Ethyl Alcohol Dye of Example Number Methanol Pyridine Initial After 24 hrs. Initial After 24 hrs. __________________________________________________________________________ 1 color color spiropyran 525 (0.23) forms forms stable 2 decolor- decolor- 570 not soluble 510 (2.95) 510 (2.83) ized ized dye stable 3 decolor- decolor- 570 (5.13) 570 (4.95) 513 (3.08) 513 (2.47) ized ized dye stable dye stable 4 decolor- decolor- 570 (3.69) 570 (2.91) 522 (2.91) 522 (1.94) ized ized mostly dye some dye 14 decolor- decolor- 565 (4.24) 563 (1.45) 511 (3.08) 511 (3.02) ized ized mostly spiropyran mostly dye at equilibrum 5 decolor- decolor- 582 (2.84) 582 (0.08) 533 (3.69) 533 (3.09) ized ized all spiropyran all dye at equilibrum 10 decolor- decolor- 575 (3.44) 575 (3.10) 518 (2.03) 518 (1.85) ized ized mostly dye mostly dye 12 no decolor- 553 (3.40) 553 (3.29) 502 (1.54) 502 (1.37) change ized all dye all dye 9 no no 605 (3.84) 605 (3.71) 529 (2.30) 529 (2.24) change change all dye all dye 10 no no 541 (3.69) 541 (3.69) 480 (2.31) 480 (2.27) change change all dye all dye 13 no no 573 (4.75) 572 (2.46) 508 (2.88) 508 (2.77) change change some spiropyran all dye 12 no no 518 (3.46) 518 (3.14) 457 (2.08) 457 (1.84) change change all dye mostly dye 11 color color 496 (2.09) 510s(0.82) 426 (2.25) 415 (2.25) change change 445 (2.25) 438 (2.56) 409 (2.13) undergoes cis-trans isomerism 7 color color 441 (2.61) 438 (2.81) 427 (1.67) 417 (1.60) change change change in 395s(1.55) curve shape undergoes cis-trans isomerism 37 no no change change 592 (3.51) 592 (3.45) 541 (2.53) 541 all dye 15 color color 609 (0.14) 561 (2.98) 561 (0.84) generated generated all spiropyran all spiropyran 16 no no 589 (4.51) 589 (4.21) 541 (3.31) 541 change change all dye 18 decolor- ized 19 no no 605 (3.83) 605 (3.51) 552 (2.50) 552 change change mostly dye 20 color decolor- 544 (4.81) 544 (0.65) 492 (1.56) 492 (1.17) change ized mostly spiropyran mostly dye 21 not decolor- 535 (3.66) 535 (3.18) 491 (2.63) 491s(0.98) soluble ized mostly dye mostly spiropyran or decomposition __________________________________________________________________________

Dye Numbers 7 and 11 are not decolorized but definite changes in color are observed. Dye Number 15 gives an initially weak colored solution, the color being intensified by flashing in the absence of an ultraviolet filter. The failure of photobleaching to occur in Dye Numbers 15, 16 and 37 may be caused by a very rapid thermal return in these systems. The 2-methoxy group appears to be responsible for this effect. The most efficient compounds in the photobleaching reactions are those derived from 5-nitrosalicylaldehyde and which contain 5-member heterocyclic nuclei. The data in Table III indicates that the dye form is generally more stable when the heterocyclic nuclei become more basic.

The following examples illustrate the behavior of dyes in polymers of varying polarity and the effect of metal ions and metal chelates, particularly of the rare earth series, on stabilizing the dye in either the merocyanine or the spiropyran form:

EXAMPLE 40

This example employs Dye Number 1 of Table III which in most polymers and solutions exists as the closed (colorless) spiropyran and forms the intensely colored merocyanine (blue-purple) when exposed to light with a wavelength of less than 400 nm. The colored form is rapidly bleached by light of wavelength greater than 400 nm. A solution of the dye, dissolved in pyridine, is added to a 10% solution of a 50% butyl acrylate-50% styrene copolymer in ethyl acetate, so as to give a 10.sup.-.sup.5 M concentration of the dye in the polymer solution. The resulting highly colored solution fades to colorless shortly after mixing. The solution is hand-coated on a conductive polyethylene terephthalate film base at a knife setting of 0.008 inches under red safelights, then air dried. A portion of this coating is exposed to an ultraviolet lamp (140 watts) for 15 seconds through a resolving-power chart (etched chrome plated quartz glass). The exposed area of the coating becomes a deep purple, giving an excellent color negative. This negative image can be bleached immediately to give the colorless spiropyran by exposure to light from a photoflood lamp (150 watts) of a wavelength greater than 400 nm or by exposure for 15 minutes under conditions of normal lighting (about 100 ft. cdls.). The re-exposure of the resulting colorless film to ultraviolet light intensifies the previously unexposed areas with the background considerably less colored. This image appears as a positive and indicates some dye fatigue after the first exposure to ultraviolet light. This latent image positive can be repeatedly intensified after bleaching by exposure to ultraviolet light. When the colorless, previously unexposed, film is intensified in a non-imagewise manner by exposure to ultraviolet light and then imagewise exposed to light of wavelength greater than 400 nm, the resulting positive can be bleached with visible light, then re-intensified with ultra-violet light, and imagewise exposed with visible light without a trace of the first exposure (i.e., no latent image). This procedure may also be repeated many times. It is thus seen that fatigue occurs during exposure to ultraviolet light (but either not at all or to a much lesser extent) during exposure to visible light (less than 400 nm). The coating is also thermally sensitive, and by subjecting the coating to heat, the photochromic system will revert to the colorless spiropyran in seconds.

EXAMPLE 41

Dye Number 1 of Table III and Dye Number 23 having the formula ##SPC12##

are dissolved in pyridine and mixed with a solution of a polymer as described in Example 40. The mixture of Dye Number 1 in solution is initially colored and changes to colorless when coated or upon standing. Both the solution and coating of Dye Number 23 are pink. The coating containing Dye Number 23 slowly turns dark brown at which point the coating is relatively stable. Exposure of this coating to ultraviolet light gives little intensification and the coating does not bleach to light of wavelength greater than 400 n. If exposures are made shortly after coating, the image is stable for several days to light of wavelength greater than 400 nm, and is stable for several weeks if kept in the dark.

The following example illustrates the stabilization of a photochromic system in either its merocyanine form or the spiropyran form by forming an insoluble metal salt or complex of the available oxygen or sulfur on the nucleus of the merocyanine dye or by complexing the closed spiropyran.

EXAMPLE 42

A mixture of 3 .times. 10.sup.-.sup.5 mole of Dye Number 1 and 1 .times. 10.sup.-.sup.5 mole of a europium chelate salt having the formula ##SPC13##

in a butylacrylate-styrene polymer is dissolved in a mixture of pyridine and ethyl acetate. Before the addition of the europium chelate salt, the polymer solution containing Dye Number 1 is purple. The solution immediately turns magenta upon the addition of the europium chelate salt. The mixture turns colorless when coated on a substrate and then turns red after several days if kept in the dark. When this coating is exposed imagewise to an ultraviolet light source of 140 watts, the exposed area becomes magenta. This material is faster to form an image that the corresponding polymer solution in the absence of the europium chelate salt and is fairly stable to tungsten light. Under conditions of normal lighting, the dye-image fades within 2-3 hours. The coating containing the metal chelate retains its image for many weeks if kept in the dark whereas the same coating without the metal chelate fades in 15 minutes. The coating containing the metal chelate turns red upon heating and cannot be photobleached whereas the same coating which does not contain the metal chelate rapidly turns colorless when heated.

EXAMPLE 43

Example 42 is repeated substituting Dye Number 23 for Number 1. Separate coatings are made containing cadmium, cobalt, mercury, silver and zinc ions instead of the europium chelates used in Example 42. These coatings have an excellent image to background density after imagewise exposure to ultraviolet light. The dye image is stable up to one week when viewed periodically under tungsten light and the remainder of the time stored in a refrigerator in the dark. At equilibrium, the dye exists in the merocyanine form, and after a week's time becomes colored and blends with the image. At this point, the coatings are stable to tungsten, ultraviolet light and heat. The color of the coating containing the dye alone, i.e., containing no metal ions, is brown, the color of the coating containing cadmium ions is brown, the color of the coating containing cobalt ions is tan, the color of the coating containing silver ions is yellow and the color of the coating containing zinc ions is brown.

As used in the structural formulas herein, "C.sub.2 H.sub.5 " is an ethyl group and "C.sub.6 H.sub.5 " is a phenyl group.

The invention has been described in detail with reference to particular embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

Claims

I claim:

1. A fogged direct positive photographic silver halide emulsion containing at least one photographic silver halide sensitizing dye selected from those having the formulas: ##SPC14##

and ##SPC15##

wherein a is 0 or 1; Z represents a heterocyclic nucleus selected from the group consisting of thiazoles, benzothiazoles, naphthothiazoles, thieno-[1,2-a]benzothiazoles, oxazoles, benzoxazoles, naphthoxazoles, selenazoles, benzoselenazoles, naphthoselenazoles, thiazolines, 2-quinolines, 4-quinolines, 3,3-dialkylindolenines, 2-pyridines, 4-pyridines, imidazoles, benzimidazoles, naphthimidazoles, and lepidines; R represents a lower alkyl group or an aryl group; and R.sub.1 and R.sub.2 each represents hydrogen, a nitro group, a halo group, a lower alkoxy group or a lower alkyl group, provided that at least one of the groups R.sub.1 and R.sub.2 is a nitro group or a halo group.

2. A fogged direct positive photographic silver halide emulsion containing a photographic silver halide sensitizing dye of the formula: ##SPC16## wherein a is 0 or 1; Z represents a heterocyclic nucleus selected from the group consisting of thiazoles, benzothiazoles, naphthothiazoles, thieno-[1,2-a]benzothiazoles, oxazoles, benzoxazoles, naphthoxazoles, selenazoles, benzoselenazoles, naphthoselenazoles, thiazolines, 2-quinolines, 4-quinolines, 3,3-dialkylindolenines, 2-pyridines, 4-pyridines, imidazoles, benzimidazoles, naphthimidazoles, and lepidines; R represents a lower alkyl group or an aryl group; R.sub.1 and R.sub.2 each represents hydrogen, a nitro group, a halo group, a lower alkoxy group or a lower alkyl group, provided that at least one of the groups R.sub.1 and R.sub.2 is a nitro group or a halo group.

3. A fogged direct positive photographic silver halide emulsion containing a photographic silver halide sensitizing dye of the formula: ##SPC17##

wherein a is 0 or 1; Z represents a heterocyclic nucleus selected from the group consisting of thiazoles, benzothiazoles, oxazoles, benzoxazoles, naphthoxazoles, selenazoles, benzoselenazoles, naphthoselenazoles, thiazolines, 2-quinolines, 4-quinolines, 3,3-dialkylindolenines, 2-pyridines, 4-pyridines, imidazoles, benzimidazoles, naphthimidazoles, and lepidines; R represents a lower alkyl group or an aryl group; and R.sub.1 and R.sub.2 each represents hydrogen, a nitro group, a halo group, a lower alkoxy group or a lower alkyl group, provided that at least one of the groups, R.sub.1 and R.sub.2 is a nitro group or a halo group.

4. A photographic silver halide emulsion as defined in claim 1 wherein said sensitizing dye is 1',3'-diethyl-2',3'-dihydro-6-nitrospiro(2H)-1-benzopyran-2,2'-1H-imidazo[ 4,5-b]quinoxaline.

5. A photographic silver halide emulsion as defined in claim 1 wherein said sensitizing dye is 6-[(1-methylnaphtho-[1,2-d]thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cyc lohexadienone.

6. A photographic silver halide emulsion as defined in claim 1 wherein said sensitizing dye is 6[(3-ethylbenzoselenazolinylidene)ethylidene]-4-nitro-2,4-cyclohexadienone

7. A photographic silver halide emulsion as defined in claim 1 wherein said sensitizing dye is 6-[(1-ethylnaphtho-[2,3-d]thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cycl ohexadienone.

8. A photographic silver halide emulsion as defined in claim 1 wherein said sensitizing dye is 6-[(3-ethyl-2-benzothiazolinylidene)ethylidene]-4-nitro-2,4-cyclohexadieno ne.

9. A photographic silver halide emulsion as defined in claim 1 wherein said sensitizing dye is selected from the group consisting of

6-[(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)ethylidene]-4-nitro-2,4-cycloh exadienone;

6-nitro-2',3'-dihydro-1',3',3'-trimethylspiro(2H)-1-benzopyran2,2'-(1H)-pyr rolo[2,3-b]pyridine;

6-[(1,3-diethyl-2-benzimidazolinylidene)ethylidene]-4-nitro2,4-cyclohexadie none;

6-[(1-ethyl-4-(1H)-quinolylidene)ethylidene]-4-nitro-2,4-cyclohexadienone;

6-[(5,6-dichloro-1,3-diethyl-2-benzimidazolinylidene)ethylidene]-4-nitro-2, 4-cyclohexadienone; and

6-[(3,4-diphenyl-4-thiazolin-2-ylidene)ethylidene]-4-nitro2,4-cyclohexadien one.

10. A fogged direct positive photographic element comprising a support having thereon at least one layer of a photographic silver halide emulsion containing at least one photographic sensitizing dye selected from those having the formulas: ##SPC18##

and ##SPC19##

wherein a is 0 or 1; Z represents a heterocyclic nucleus selected from the group consisting of thiazoles, benzothiazoles, naphthothiazoles, thieno-[1,2-a]benzothiazoles, oxazoles, benzoxazoles, naphthoxazoles, selenazoles, benzoselenazoles, naphthoselenazoles, thiazolines, 2-quinolines, 4-quinolines, 3,3-dialkylindolenines, 2-pyridines, 4-pyridines, imidazoles, benzimidazoles, naphthimidazoles, and lepidines; R represents a lower alkyl group or an aryl group; and R.sub.1 and R.sub.2 each represents hydrogen, a nitro group, a halo group, a lower alkoxy group or a lower alkyl group, provided that at least one of the groups R.sub.1 and R.sub.2 is a nitro group or a halo group.

11. A fogged direct positive photographic element comprising a support having thereon at least one layer of a photographic silver halide emulsion containing at least one photographic sensitizing dye of the formula: ##SPC20## wherein a is 0 or 1; Z represents a heterocyclic nucleus selected from the group consisting of thiazoles, benzothiazoles, naphthothiazoles, thieno-[1,2-a] benzothiazoles, oxazoles, benzoxazoles, naphthoxazoles, selenazoles, benzoselenazoles, naphthoselenazoles, thiazolines, 2-quinolines, 4-quinolines, 3,3-dialkylindolenines, 2-pyridines, 4-pyridines, imidazoles, benzimidazoles, naphthimidazoles, and lepidines; R represents a lower alkyl group or an aryl group; and R.sub.1 and R.sub.2 is a nitro group or a halo group.

12. A fogged direct positive photographic element comprising a support having thereon at least one layer of a photographic silver halide emulsion containing at least one photographic sensitizing dye of the formula: ##SPC21## wherein a is 0 or 1; Z represents a heterocyclic nucleus; selected from the group consisting of thiazoles, benzothiazoles, naphthothiazoles, thieno-[1,2-a] benzothiazoles, oxazoles, benzoxazoles, naphthoxazoles, selenazoles, benzoselenazoles, naphthoselenazoles, thiazolines, 2-quinolines, 4-quinolines, 3,3-dialkylindolenines, 2-pyridines, 4-pyridines, imidazoles, benzimidazoles, naphthimidazoles, and lepidines; R represents a lower alkyl group or an aryl group; and R.sub.1 and R.sub.2 each represents hydrogen, a nitro group, a halo group, a lower alkoxy group or a lower alkyl group, provided that at least one of the groups R.sub.1 and R.sub.2 is a nitro group or a halo group.