CA1040917A - Spectrally sensitized direct positive emulsion layers - Google Patents

Abstract

A B S T R A C T

Photographic direct positive silver halide emulsions are spec-trally sensitized with a cyanine dye having a thiazole ring which is con-nected in 5-position through a methine chain to a 5- or 6-membered hetero-cyclic ring and which thiazole ring carries in 2-position a -SR9 or -R10R11-group as specified hereinafter.

Description

1~4~17 The invention relates to photographic direct positive sil-ver halide emulsions which are sensitized with special cyanine dyesO
Numerous sensitizing dyes are known for the usual negative silver halide emulsions, e.g. mono- or trimethine cyanines, merocyanines or rhodacyaninesO
The known sensitizing dyes, however, cannot usually be used for sensitizing direct positive emùlsions, i.e. emulsions which are processed to produce positive images after the usual exposure and development, because they generally cause flatte-ning of the ~~-value or gradationO On the other hand, compounds have already been described which are only suitable as sensi-tizing dyes for direct positive emulsionsO If used in negative emulsions, these compounds cause fogglng and their sensitizing effect is unsatisfactory. Indocyanines are examples of sensi--tizing dyes which have been used for direct positive emulsions.
Although these have a quite advantageous effect on the sensi- -tivity of the emulsions to the red region of the spectrum, their - -sensitizing effect is still not completely satisfactory. This also applies to the direct positive indole sensitizing dyes for the green region o* the spectrum and to bis-thiazol and bis-selen-azolyl compounds which are known as sensitizers for direct posi-tive emul~ions. -- ~ ~ ......
It i8 among the ob~ects of the present invention to provide sensitizing dye~ for direct positive silver halide emulsion~
which havc a ~ufficient~y intense sensltizing effect and do not adversely affec~ the gradation.
. ' , ".

. ~ ' ,.

. . ' ~4~3917 We now have found that cyanine dyes which have a thiazole nucleus whose carbon atom in the 5 position is connected via a methine chain to a second nucleus of the type usually present in known cyanine dyes, are highly effective cyanine dyes, suitable for the preparation of direct posotive photographic silver halide emùlsionsO The new cyanine dyes are particularly suitable as so-called electron acceptors and spectral sensi-tizers for developable fogged silver halide emulsions. They produce both excellent general speed and a selective sensitivity to light from the green and red regions of the visible spectrum.
Images obtained with such silver halide emulsions are except-ionally clear and sharp and have excellent contrast~ m e second nucleus which is connected to the thiazole nucleus by a methine group may be a 5- or 6-membered nitrogen containing nucleus of the type normally found in known cyanine dyes.
Particular utility is exhibited by cyanine dyes character-ized by one of the following formulae I or II:

:
- .
I R1 _ N = (CH - CH=)nC - CH = CH ~ ~ R3 Anion 0 R N ~
R6 ' R8 R ~ C - (CH = CH~ 6 R4 ~ Anion ~
.. . .

1~J4~917 wherein Rl represents ~1) a saturated or unsaturated aliphatic hydrocarbon group which preferably contains up to 6 carbon atoms and may be substituted, e.g.
with phenyl, hydroxyl, halogen, carboxyl, sulfo, carbonamido, carbalkoxy, sulfato or thiosulfato, sulfonamido or phosphato, (2) cycloalkyl such as cyclohexyl, or (3) aryl, particularly phenyl;
R2, R4, R8 stand for (1) hydrogen, (2) saturated or unsaturated aliphstic groups preferably containing up to 6 carbon atoms, for example methyl or ethyl, or t3) aryl, for example phenyl which may be substituted, for example with alkyl, or alkoxy;
R3 represents -SR or -NR OR l;
R5, R6, R represent (1) hydrogen, (2) saturated or unsaturated aliphatic groups preferably containing up to 3 carbon atoms, for example methyl or ethyl, t3) aryl, for example phenyl, or t4) carbalkoxy such as carbethoxy, and R5 and R may also together represent the ring members required for completing -a condensed benzene or naphthalene ring; ~ --R9, R10, Rll represent (1) saturated or unsaturated aliphatic groups prefer-ably containing up to 6 carbon atoms, for example methyl or ethyl, or (2) aryl, for example phenyl; R10 and Rll may also together represent the ring ; ---members required for completing a heterocyclic ring, e.g. for completing a pyrrolidine, piperidine, morpholine or thiomorpholine, indoline or tetra- -hydroquinoline ring;
n = 0 or 1, -m ~ 0 or 1, ~
' ' F
.

1~4~917 Anion ~ represents any anion, eOg. a halide such as chloride, bromide or iodide, perchlorate, sulfate, methylsulfate, p-toluenesulfonate, acetate or oxalate; the anion is absent in cases where R1 or R4 contains an acid group in the anionic form so that a betaine is present;
Z represents a radical required for completing a hetero-cyclic group containing a 5- or 6-membered heterocyclic ring; the heterocyclic group may contain a condensed benzene or naphthalene ring a~d other substituents;
the heterocyclic groups are those commonly used in cyanine chemistry, for example those based on thiazole, (eOg. thiazole, 4-methylthiazole, 5-methylthiazole, 4,5-di-methylthiazole, 4-phenylthiazole, 5-phenylthiazolejor 4,5-di-phenylthiazole), benzothiazole, (eOgObenzothiazole, 4-chloro-benzothiazole,`~5-chlorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 6-bromobenzothiazole, 5-iodobenzothia-zole, 6-iodobenzothiazole, 4-methylbenzothiazole, 5-methyl-benzothiazole, 6-methylbenzothiazole, 5,6-dimethylbenzothia-zole, 4-phenylbenzothiazole, 5-phenylbenzothiazole, 6-phenyl-benzothiazole, 5-hydroxybenzothiazole, 6-hydroxybenzothiazole, 5-ethoxybenzothiazole, 6-ethoxybenzothiazole, 5,6--dimethoxy-benzothiazole, 5,6-methylene-dihydroxy-benzothiazole, 5-di-ethylaminobenzothiazole, 6-diethylaminobenzothiazole, 6-cyano-benzothiazole, 5-carboxybenzothiazole, 5-sulfobenzothiazole, tetrahydrobenzothiazole or 7-oxotetrahydrobenzothiazole), naphthothiazole (e.g. naphto~ 1j2-d_7thiazole, naphtho~ 2,1-d 7 thiazole, 7-methoxynaphtho~ 2,1-d 7thiazole or 8-methoxynaphtho ! 1,2-d 7thiazole), selenazole (eOg. 4-methylselenazole or 4-phenyl-sel~nazole), benzoselenazole (e.g. benzoselenazole, ~4~)9~7 5-chlorobenzoselenazole, 5,6-dlmethyl-benzoselenazole, 5-hydroxy-benzoselenazole, 5-methoxybenzoselenazole or tetrahydrobenzo-selenezole), naphthoselenazole (eO~0 naphtho~ 1,2-d_7selenazole or naphtho-~ 2,1-d 7selenazole), oxazole (eOg. oxazole, 4-meth-oxyoxazole, 4-phenyloxazole or 4,5-diphenyloxazole), benzoxazole (benzoxezole, 5-ohlorobenzoxazole, 6-chlorobenzoxazole, 5,6-dl-methylbenzox~zole, 5-phenylbenzoxazole, 5-hydroxybenzoxazole, 5-methoxybenzoxazole, 5-phenylbenzoxazolè, 5-hydroxybenzoxazole, 5-methoxybenzoxazole, 5-ethoxybenzoxazoie, 6-dlalkylamlnobenz-oxazole, 5-oarboxybenzoxazole, 5-~ul~obenzoxazole, sulfonamldo-benzoxazole, or 5-carboxyvinylbenzoxHzole), naphthoxazole (e.g.
nephthol 1,2-d_70xazole, naphtho~ 2,1-d_70xazole or naphtho ~ 2,3-d_70xazolo), imid~zole (o.g, 1-methyllmldazole, 1-ethyl-4-phenylimld~zole or 1-butyl-4,5-dlmethyllmldazole), benzlmld-1.5 azole (e.g, 1-methylb0nzlmldazole, 1-butyl-4-methylbenzlmld-azole, 1-ethyl-5,6-dlchlorobenzlmldazole or 1-ethyl-5-trl~luoro-mothylbonz~mldazole), n~phthlmldazole (o.g. 1-methylnaphtho-1~2-~_71mldazolc or 1-ethylnaphtho/~2,3-d_7imidazolo), 3,3-dlalkylindolonlne (~.g. 3,3-dlmethyllndolenlne, 3,3,5-trl-methylindolenlne or 3,3-dlmethyl-5-methoxylndolenlne),

2-pyrldlno (e,g. pyrldlne, 3-methylpyrldine, 4-methylpyrldlne, 5-methylpyrldlne, 6-methylpyridlne, 3,4-dimethylpyridine,

3,5-dlmothylpyrldine, 3,6-dlmethylpyridlne, 4,5-dimethylpyridlne,

4,6-dlmothylpyrldine, 4-ohloropyrldine, 5-chloropyridlne, 6-chloropyrldlne, 3-hydroxypyrldlne, 4-hydroxypyridlne,

5-hydroxypyridlne, 3-phenylpyrldlne, 4-phenylpyridine or

6-phonylpyridine), 4-pyrldlne (e.g. 2-methylpyrldlne, 3-methyl- :.-.
pyrldlno, 2,3-dlmethylpyrldlne, 2,5-dimethylpyrldine, 2,6-dl-mothylpyrltlno, 2-¢hloropyrldine, 3-ohloropyrldine, 2-hydroxy- ....
A-a 854 - 6 -~5~41~9~7 pyridine, or 3-hydroxypyridine), 2-quinoline (eOg. quinoline, 3-methylquinoline, 5-methylquinoline, 7-methylquinoline, 8-methyl-quinoline, 6-chloroquinoline, 8-chloroquinoline, 6-methoxyquino-line, 6-ethoxyquinoline, 6-hydroxyquinoline, 8-hydroxyquinoline or 5-oxo-5,6,7,8-tetrahydroquinoline), 4-quinoline (eOgO quino-line, 6-methoxyq~inoline, 7-methylquinoline or 8-methylquino-line), isoquinoline (eOg. isoquinoline or 3,4-dihydroisoquino-line), thiazoline (e.g. thiazoline or 4-methylthiazoline), and those based on pyrroline, tetrahydropyridine, thiadiazole, oxadiazole, pyrimidine, triazine or benzothiazine and pyrimi-done or thiopyrimidoneO The aryl groups and heterocyclic groups may carry any other additional substituents, eOgO alkyI, . preferably with up to 3 carbon atoms such as methyl or ethyl, h~logen such as chlorine or bromine, hydroxyl, alkoxy preferab-ly with up to 3 carbon atoms such as methoxy or ethoxy, hydroxy-alkyl, alkylthio, aryl such as phenyl, aralkyl such as benzyl, amino, substituted amino or nitroO
The following are examples of suitable compounds. The absorption maxima are measured in methanolic solution unless otherwise indicatedO In the following table, DMS0 represents dimethylsulfoxide.

-- : -No. Dye ~: ~ Absorp- Sensiti-tion zation maximum maximum (nm) (nm) ~ ~ N ~ ; 5~3 A-G 854 . - 7 -~()4(~917 No. Dye Absorp- Sensiti-tion zation maximum maximum (nm) (nm) 2 02N ~ ~ -CH = CH- ~ b~ SCH 471 550 3 ~ ~ -CH = OH- ~ S~ SCH

4 H3C~N~ 478 540 S~ CH = CH~ SCH

` C104 ~

H3C.~N 466 540 CH ~ SCH3~DMS0) :~

, 6 02~ G~ ~ 476 550 A-G 854. - 8 -~ . .

~409~7 NoO . Dye Absorp- Sensiti-tion zation maximum maximum (nm) (nm) H C

7 3 \N~ 483 555 i N I-CH = CH ~ (DMS0) (brelt) CH3~ ~ ~ N~ 3 H3C~ CH 548 580 3 ~ ~ ~CH3 (D~30) 3 H3C ~ ~ ~

.
CH
~P~ e===CH S / 3 ~ N ~ 539 610 N' ~ ~ N ~ Aceto~) (breit) C~

, H
3C~ ~ CH3 ` 542 585 o~ CH~C~ -A-G 854 - C104 ~

_ 9 _ .. .

.. .. ", .....

~C~4Q917 NoO Dye Absorp- Sensiti-tion zation maximum maximum (nm) (nm) 12 02N ~ N~ N / ~ 557 605 13 H3C`N ~ CH S f 3 556 600 CH3 ~ ~ -N

. . ........................................................ ..
` CH2-CH=CH
14 ~ ~ -CN ~ ~-N ~ 548 600 CH2-CH=CH2 H3C- ~ S03 CH

J

"
3 ':

16 ~ ~ -N ~ ~ 541 590 .
A-G 854 . J ~
- 10 - . .
....

1~4~7 ~ e Absorp- Sensiti-NOL ~ tion zation maximum. maximu~
(nm) (nm) CH
~CH ~ ~ ~ 493 535 N~ CH ~ ~ -N

J ~

H3C~N ~ /CH3 529 580 l9~ N~ -$ = CN- ~ -R ~

Cl04 ~

0zN ~ 'NC H ~ H C ~ N~ ~ :

H C

~:C104 ~ -,, ~, . . . . . . . . . . . .. .

~)4~917 No. Dye Absrop- Sensiti-tion zation maximum maximum (nm) (nm) fH2-CH=CH2 ~N ~ N ~ ~ ~ SCH3 468 530 CH2 -CH=CH2 H3C ~ 3 ;.
, :,,, CH2-CH=CH2 ; , N ~ N ~ ~ ~ N 605 CH2-CH=CH2 H3C ~ so~3 N ~~ ~ N / 485 530 CH3 ~3 ~ (Aceton) C104 :' ......

- 12 ~
,' ~" , ~ ` '. .:
, 1~409~7 NoO . Dye Absorp- Sensiti-tion zation maximum. maximum (nm) (nm) ;

31~ > -CH ~ ~-N ~ 476 S15 C2H5 J ~

32~ ~ -CH - CN- ~ S\ ~ ~ ( 449 ) 490 ~ .

33~ ~ - CH = CH - ~ ~ -N~CH 653 720 C ~ ~ C 04 02N ~ ' CN ~ ON 559 610 H~C CH3 ~D5So) 600 CH3 ~ ~ ~ - ~ CH3 C104 ~
. -. . .

1~)4~9~7 No, Dye Ab~orp- Senslti-tion zatlon maximum maxlmum (nm) (nm) O ~N~-CH ~ CH- ~ ~ N ~ 3 517 575 C104 ~

9 ~ X~ -CH OH ~ ~ ~ N~ 3 ~ 334 ) 590 - 0104-~

02N ~ ~ -ON ~ CH- ~ g~ -N/ ~ ~ 54 ~2~5 ';
.'.~

. .
;~ ' , ' ,' F ~
A-G 854 - 4!~- . -.. . .. . . .. ... . . . . .. ... .. .. .. .. . .. . . . . .. ... . ..

9~7 The new cyanine dyes of the above formula I can easily be obtained by reacting a heterocyclic compound of the following formula III:

III R1 _ N = (CH - CH =)nC - R' '~9 Anion wherein R , n, Z and anion have the meaning already indicated and R' represents a methyl group, with a thiazolealdehyde of the structural formula IV

OHC S
IV R2 ~ N

.

.
wherein R' and R~ have the meanings mentioned herein before~
The reaction is preferably carried out at temperatures of between 15C.and the reflux temperature of the mixture, preferably using equimolar or approximately equimolar ratios. The reaction may be carried out in the presence or absence of a condensing agent, for example a trialkylamine, in an inert solvent, for example an alkanol, e.gO ethanol, or acetic acid anhydrideO
The dyes of Formula II are obtained by condensation of a pyrrole or indole of Formula V:

A-G 854 ~

.

~)46)917 V ~ (C=CH-)mH

wherein R4, R5, R6, R7, R8 and m ha~e the meanings already indicated with an aldehyde of formula IV with the addition of at lea~t 1 mol of an acid, e.g. acetic acidO The condensation may advan-tageously also be carried out in glacial acetic acid with the -addition of a condensing agent such as pho~phorus oxychloride.
The thiazole aldehydes of formula IV are easily obtained from the correspond1ng thlazole~ by the Vilsmeier reaction as described in German Patent SpecificationsNo. 1,137,024 and 1,147,584.
The preparation~ of dyes 1, 8 and 10 are described in detail below:
Dre 1 ~3 g ~ 2-methyl-3-ethyl-Oenzothiazolium-tosylate and 1.4 g of 2-methylmercapto-4-phenyl-thiazolealdehyde-5 in 20 ml of acetic acid anhydride are heated under reflux for -10 minute` The solution i8 cooled and the dye~ i8 ~precipitated , . ~ .
with potas~ium lodide ~olution, filtered under suotion and recry~tallized twice,~each time from 100 ml of methanol, with th~ addition o~ active oharcoal. 1.4 g of dye of ~elting polnt 228C (decompo~ition~ 18 obtained.
-, F' ,,~ ~ ~ , A~G 854 ~7-,' ..-1~)4~917 Dye 8 2.4 g of 1,3,6-trimethyl-2-oxo-pyrimidinium perchlorate and 203 g of 2-dimethylamino-4-phenyl-thiazole-aldehyde-5 in 20 ml of acetic acid anhydride are heated under reflux for 15 minutesO
The dye precipitates after heating for a short time. The mixture is cooled and the dye is filtered under suction and recrystall-ized from 250 ml of glacial acetic acidO
Yield = 4.2 g, m.pO: 277C (decomposition)O
Dye 10 2.0 g of 1-methyl-2-phenyl-indole and 3.0 g of 2-methyl-phenylamino-4-phenyl-thiazole-aldehyde-5 in 10 ml of glacial acetic acid with the addition of 2 ml of phosphorus oxychloride are heated under reflux for one hour. The dye solution is cooled and the dye is precipitated with sodium perchlorate solution, filtered under suction and recrystallized from 150 ml of alcoholO
Yield = 1.6 g, mOp. 160 ~ 162Co The dyes used according to the invention generally have no sensitizing effect in conventional negative emulsions but on the contrary reduce the overall sensitivity and reinforce the unwanted uniform grey fogO In direct positive emulsions, on the other hand, these dyes have a spectral sensitizing effect resulting in exceptionally high sensitivity and excellent steepness of the gradation.
According to a particularly preferred embodiment of the invention,-the new cyanine dyes are used for the preparation direct positive photographic silver halide emulsions, in partlcular developable fogged silver halide emulsionsO

F ,7 A-G 854 _ ~

lg~4~9~7 The emulsions may be fogged by the usual methods, for example by the action of light or so-called chemical fogging agents, for example stannous chloride, formaldehyde, thiourea dioxideO
Another advantageous method of fogging the emulsions consists in adding a reducing compound, for example thiourea dioxide, and the compound of a metal which is more electropositive than silver, for example a gold salt, e.g. potassium chloroaurate, as described in British Patent Specificàtion NoO 723,019.
Typical reducing compounds which are suitable for preparing :
such fogged silver halide emulsions are, for example stannous salts, such as stannous chloride, hydrazine, sulfur compounds such as thiourea dioxide, phosphonium salts, e O g O tetra-(hydroxymethyl)-phosphonium chloride. Typical compounds of metals which are more electropositive than silver are, for example compounds of gold, rhodium, platinum, palladium and iridium. It is preferred to use soluble salts of the said noble metals, e.gO potassium chloroauràte, auric chloride and (NH4)2-PdCl6.
The concentration of the reducing agents or the compounds of the metal which is more electropositive than silver, used for fogging can vary within wide limitsO In general, concen-trationA of .0005 to about .06 milli-equivalent~ of reducing agent and about .001 to about .01 milli-mols of the noble metal salt per mol silver halide have proved suffic~ent.

Fogging can be also accomplished with the method of æilver halide dlgestion described by W00D in "J.phot.Science 1 (1963) page 163, at pAg valueA between 2 and 5 and pH values of a~out 6.5.
F 1~

1~4~917 Any type of direct positive photographic emulsions can be spectrally sensitized by the sensitizing dyes of the present inventionO Suitable direct positive emulsions include silver halide emulsions the grains of which have high internal sensi-tivity in particular silver halide emulsions containing internalelectron trapsO`Particular utility is exhibited by direct posi-tive silver halide grains comprising a central core of a sllver halide which contains centres which act as electron traps and an outer shell covering said core comprising a fogged silver halide that develops to silver without exposure. ~mulsions of that type are disclosed by Eo MOISAR and Fo WAGNER in "Berichte der Bunsengesellschaft f~r physikalische Chemie" 67 (1963), pages 365 - 359. We furthermore refer to British patent peci-fications 1,027,146 and 1,151,781 or to French patent specifi--cation 1,585,791. m e formations of the centres or specks in the inside of the grains particularly on the core of the composite grain are produced as known per se by chemical sensi-tizing the emulsions with compounds of noble metals in particu-lar gold or iridium salts are sulfur compounds such as thio-~ulfates. In particular u8eful is a treatment with noble metal salts and sulfur compoundsO
.
The spectral sensitizers of the present invention can also be applied to direct positive emulsions which are fogged on - the surface so that they can be developed to silver without exposure and which contain at the surface electron acceptors auch as desensitizing dyes. Suitable dyes for this purpose are well-known. Referen¢e i8 made eOg. to pinacryptol yellow or ~9 A-G 854 - 2~-i~4~9~7 nitro-substituted polymethine dyes. m e technic of desensiti-zation and the chemical structure of suitable desensitizing dyes are described e.gO by 0. RI~ST~R in "Mitteilungen aus den Forschungslaboratorien der Agfa", Volume 1 (1955), page 44 or 5 in the handbook '!Grundlagen der photographischen Prozesse mit Silberhalogeniden", Volume 3, page 10770 The direct positive sensitizing dyes of the present invention can be used in combination with further sensitizing dyes capable of sensitizing direct positive emulsions and desensitizing negative silver halide emulsions as described for example in US patent speci-fications 3,31~, 796 or 3, 505,070. Further suitable direct positive emulsion are described in German patent specifications 606,392 or 642,222 or in UK patent specifications 581,773 or 655,0090 The direct positive emulsions may also contain mercury salts and thallium salts as described in British patent speci-fication 1,203,744 or US patent specification 3,620,7500 The silver halide of the silver halide emulsions to which the new.cyanine dyes may be added may consist of one of the u~ual silver halides used for preparing photographic silver halide emulsions, i.eO for example silver bromide, silver iodide, silver chloride, silver chlorobromide, silver bromoiodide and silver chlorobromoiodide.
The new dyes may be incorporated with silver halLde emulsions at the usual concentrations, for example concentrations of about 50 to 2.000 mg, preferably about 400 to 800 mg/mol of sllyer halide.

o ~4~7 The ne~ cyanine dyes are preferably added to the washed, prepared silver halide emulsions and dispersed in them as uni-formly as possible. The incorporation of the dyes in the emulsions may be carried out by the usual known methods, for example the dyes may be added to the emulsions from solutions in suitable solvents. The solvent must, of course, be selected so that they have no adverse effect on the light-sensitive photographic material which is to be produced. Suitable solvents are, for example, methanol, isopropanol, acetone and water, either separately or mixed with each otherO
me binder of the silver halide emulsions to which the cyanine dyes according to the invention may be added may be any of the hydrophilic colloids commonly used for the prepa-ration of photographic silver halide emulsions, for example -naturally substances such as proteins, in particular gelatin, albumin, agar-agar, gum arabic or alginic acid or synthetic hydrophilic resins such as polyvinyl alcohol, polyvinyl pyrrolidone, cellulose ethers or partly hydrolyzed cellulose acetate.
- The emulsions containing one or more cyanine dyes accor-dlng to the invention may be applied to the usual opaque or transparent photographlc supports used for the preparation Or photographic materials, ~or example supports of glass, cellulose acetate, cellulose acetobutyrates or polyesters, e.g. polyethylene terephthalate, or supports of baryta-coated paper or paper coated with polyole~ines such as polyethylene. --F z~

1~4~917 The photographic silver halide emulsions to which the new cyanine dyes are added, as well as the other layers of a photo-graphic material which is prepared using these emulsions may be hardened by the addition of the usual hardeners such as aldehyde hardeners, eOgO formaldehyde or mucochloric acid.
The good sensitizing effect is preserved even in the presence of water-soluble and emulsified color couplers. The emulsion may also contain the usual wetting agents, stabilizers, brightening agents and other additivesO
10 li xamPle 45 mg of Dye 3 in the form of a 1:1000 solution in methanol are added with stirring to 1 kg of a direct positive emulsion which has been fogged by chemical fogging with ammonia and excess silver nitrate and which contains 004 mol of silver halide per kilogram of casting solution with an iodine content of 205 mol percent, based on the amount of silver, and the mixture i8 left to digest for 10 minutesO 10 ml of a 4 % solution of saponin as wetting agent and 25 ml of a 5 % solution of mucochloric acid are then addedO

The emulsion is then cast on a cellulose acetate support in the usual mannerO When dry, the layer is exposed to white light in a sensltometer and developed for 5 minutes at 20C with a de~eloper of the following composition:

F ~ z~
A-G 854 _ 2, _ -p-methylaminophenol 305 g anhydrous sodium sulfite60.0 g hydroquinone 900 g anhydrous sodium carbonate40.0 g potassium bromide 3.5 g made up to 1 litre with waterO
The emulsion is then fixed in the usual mannerO A positive image of the step wedge with exceptional clear whites and excellent gradation is obtained. The spectral sensitization curve is shown in Figure 20 Figure 1 shows for comparison the sensitivity curve obtained without a sensitizer.
If Dye 3 is replaced by an equal quantity of Dye 11, the ~ensitiza~ion curve shown in Figure 3 is obtained, and if with ... .
an equal quantity of Dye 12, the sensitization curve shown in Figure 4 is obtained. Using the same photographic process, the 8ensitization curves shown in Figures 5, 6 and 7 are obtained using Dyes 13, 31 and 340 ,- , .:' .
., , " , - ' `.

, , , . , , , . , , , ,. ".. ,,; " , "; " , ... .

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Photographic material comprising at least one direct positive silver halide emulsion layer, containing a sensitizing cyanine dye which has one of the following formulae:

and wherein R1 represents a saturated or unsaturated aliphatic group having up to 6 carbon atoms, cycloalkyl or aryl;
R2, R4 and R8 represent hydrogen, a saturated or unsaturated aliphatic group having up to 6 carbon atoms or aryl;
R3 stands for -SR9 or -NR10R11;
R5, R6 and R7 represent hydrogen, a saturated or unsaturated aliphatic group having up to 3 carbon atoms, aryl or carbalkoxy; or R5 and R6 may together represent the ring members required for completing a condensed benzene or naphthalene ring;
R9, R10 and R11 stand for a saturated or unsaturated aliphatic group having up to 6 carbon atoms; or R10 and R11 may together represent the ring members required for completing a heterocyclic group containing a 5- or 6-membered heterocyclic ring;
n is 0 or 1, m is 0 or 1, Anion ? represents an anion but is absent when R1 or R4 contains an acid group in the anionic form so that a betaine is present;

Z stands for a radical required for completing a heterocyclic group contain-ing a 5- or 6-membered heterocyclic ring.

2. The material of claim 1, wherein Z represents the ring members required for completing one of the following rings: benzothiazole, pyrimi-done, thiopyrimidone, indolenine.

3. A photographic emulsion in accordance with claim 1 where the desensitizing heterocyclic ring is 4-chlorbenzothiazole.

4. A photographic emulsion in accordance with claim 1 wherein the desensitizing heterocyclic ring is 6-chlorbenzothiazole.

5. A photographic emulsion in accordance with claim 1 where the desensitizing heterocyclic ring is 5-nitrobenzothiazole.