NO129424B - - Google Patents

Description

Direct-positive photographic silver halide emulsion.

The present invention relates to a direct-positive

photographic silver halide emulsion.

Direct-positive materials have been prepared by adding a compound of a metal more electropositive than silver, as well as an electroacceptor, to a reducing agent for silver halide emulsions. One of the biggest advantages of such direct-positive emulsions is that high-light areas in images produced with these materials are essentially free of blurring. However, known film materials of this type, such as e.g. described in British Patent No. 723,019, required exposure to light in the wavelength range ^000 - 4800 Å. In other words, they have been substantially insensitive to longer wavelength irradiation, such as e.g. green and red. A new class of dyes has now been found which are effective electron acceptors or electron scavengers and are also effective spectral sensitizers for these emulsions. This dye class differs from the dyes used in British patent no. 723,019 in that they provide better spectral sensitization. This enables the use of only one compound which provides both the desired spectral sensitization and electron capture.

According to the present invention, it is provided

a direct-positive silver halide photographic emulsion obscured with a reducing agent and a compound of a metal selected from gold, rhodium, platinum, palladium and iridium, wherein on the surface of the silver halide grains is adsorbed an electron acceptor, characterized in that the electron acceptor has an anodic polarographic half-wave potential and a cathodic polarographic half-wave potential, which potentials together give a positive sum, and that the acceptor sensitizes the emulsion to irradiation with a wavelength above 480 m^u so that the ratio between relative minus blue velocity and relative blue velocity is greater than 7.

The relative blue speed and the relative minus blue speed of the emulsion are measured by applying the emulsion to a support exposing the coating on an Eastman Ib sensitometer to a tungsten source through "Wratten 35 + 38A" filters (blue exposure) and a "Wratten 16" filter (minus blue exposure), develop the exposed coatings for 6 minutes at 20°C in a "Kodak D-19" developer, fix, wash and dry and then read the results.

The spectrally sensitizing electron acceptors used can be dimethine and trimethine cyanine dyes containing an aromatic indole core substituted in the 2-position and bound to the methine chain through the core's 3-carbon atom. Furthermore, these dyes also advantageously contain a desensitizing core in addition to the indole core. By the term "desensitizing core" used is meant such cores which, when converted to a symmetrical carbocyanine dye and added to a gelatin-silver chlorobromide emulsion containing 40 mol-% chloride and 60 mol-% bromide in a concentration of between 0.01 and 0.2 g dye per moles of silver, by electron capture causes at least 80% loss in blue speed, when the emulsion is sensitometrically exposed and developed for 3 minutes in "Kodak Developer D-19" at 20°C. This developer has the following composition:

The desensitizing cores are preferably such that, when converted to symmetrical carbocyanine dyes and subjected to samples as described, substantially completely desensitize the sample emulsion to blue light. When the term substantially complete desensitization is used, this means core which, when tested as above, results in a loss of at least 90%, and preferably more than 95%, "loss of blue light sensitivity".

The spectrally sensitizing electron acceptors used according to the invention can have the following general formula:

where L denotes a methine chain containing 2-3 carbon atoms,

D denotes an aromatic indole nucleus substituted in the 2-position, and bound to the methine chain through the 3-carbon atom of the indole nucleus, and B denotes a heterocyclic nucleus which, when L denotes a methine chain of 2 carbon atoms, stands for a desensitizing nucleus forming an unsymmetrical dimethine- cyanine dye, and when L denotes a methine chain of 3 carbon atoms, B denotes an aromatic indole nucleus substituted in the 2-position and bound to the methine chain through the 3-carbon atom of the indole nucleus. A particularly useful desensitizing core, when L denotes a methine chain containing 2 carbon atoms, is an imidazo-(4,5-b)-conoxaline core bonded to the methine chain through the 2 carbon atom of the core.

The spectrally sensitizing electron acceptors may in particular have the following general formula:

where Å denotes an aryl radical, e.g. phenyl or naphthyl, optionally substituted with alkyl groups with 1-8 carbon atoms, alkoxy groups with 1-8 carbon atoms, halogen atoms, or aryl groups, such as phenyl, or denotes a heterocyclic aromatic group with 5 or 6 carbon atoms, where the heteroatom may be nitrogen, sulfur or oxygen , R ? and R^

both denote a hydrogen atom, a halogen atom, an alkyl radical with 1-8 carbon atoms, which can be substituted with e.g. hydroxyl, alkoxy with 1-8 carbon atoms, or where R_ and R-j together denote the necessary atoms for the completion of an aromatic ring with 6 carbon atoms, R^ denotes an alkyl radical with 1-8 carbon atoms, which can be substituted with e.g. a sulfo, sulfate or carboxy group, R^ and R^

both denote an alkyl group with 1-18 carbon atoms, such as e.g. may be substituted with a sulfo, sulfate, carboxy or hydroxy group, or alkenyl group, e.g. propenyl, allyl, butenyl; with alkenyl, e.g. propargyl, cycloalkyl such as e.g. cyclobutyl and cyclohexyl, dialkylaminoalkyl, which. e.g. dimethylaminoethyl, or denotes aryl groups, such as e.g. phenyl, p-tolyl, o-tolyl or 3,4-dichlorophenyl, Rg denotes the same as R~ or together with R^ denotes an alkylene group, such as e.g. trimethylene or dimethylene, R 7 denotes a hydrogen atom, a halogen atom or a nitro group, k denotes a number from 1-3 and X denotes an anion, preferably chloride, bromide, iodide, p-toluene sulphonate, thiocyanate, sulphonate, methyl sulphate, ethyl sulphate or perchlorate .

Alternatively, the spectrally sensitizing dyes can have the following formula:

where R-]_25R,,°S R]_ij denotes an alkyl group with 1-b carbon atoms or an aryl group, such as e.g. phenyl, X has the above meaning and Q denotes (1) -CH=Q1 where Q1 stands for the necessary atoms for the completion of a desensitizing nucleus during the formation of a trimethine cyanine dye, such as e.g. a 6-nitrobenzthiazole nucleus, a 5-nitroindolenine nucleus, an imidazo(4,5-b)-quinoxaline nucleus or a pyrrolo(2,3_b)-pyrido nucleus, or (2) the necessary atoms for the completion of a desensitizing nucleus to form a dimethine cyanine dye, e.g. a pyrazole core or an indole core substituted in the 2-position with an aromatic group and bonded to the methine chain through the core's 3-carbon atom.

Among dyes of formula Ia, suitable spectrally sensitizing electron acceptors can have the formula:

wherein X, A, R^, R2J R^ and Rg have the above meanings, A^, Kg, Kg, ti-^ Q and R-^ designate the same groups respectively as A,

R 1 , R 2 , R 1 and R 8 , and where Y denotes a hydrogen atom, an aryl group, such as e.g. phenyl, alkyl, preferably of 1-8 carbon atoms, alkoxy, containing 1-8 carbon atoms, substituted phenyl or heterocyclic-aromatic group, such as e.g. thiophene.

Symmetrical imidazo-(4,5~b)-quinoxaline-trimethinecyanine dyes, where each nucleus is bound through its 2-carbon atom to the methine chain, can be used to carry out the invention. Typical such dyes have the following general formula:

wherein kj X, R^, R^ and R^ have the above meanings, R^ and R-L2 are the same groups as R^ and R^, K-^g is the same as R^

and 1 denotes a number from 1-3-

Finally, the spectrally sensitizing electron acceptor can be a carbocyanine dye in which at least one nucleus, and preferably 2, contains desensitizing substituents, e.g. nitro groups.

Suitable dyes can be produced on any

preferably a practical way, such as reaction under reflux in a suitable solvent between a carboxyaldehyde derivative of a compound which provides a nucleus, and an alkyl-substituted quaternary salt which provides the other nucleus. Dyes of formula II can be prepared as described in US patent 2,903-694 and dyes of formula III as described

in Belgian patent 660,253- The aromatic indole dimethinecyanine dyes substituted in the 2-position, containing a desensitizing core according to the invention, are described in US patent 3•505,070, (US claim no. 533-459). The dyes XVI, XVII and XVIII can be prepared as described in US patent 2,903-694.

A further advantage of the dyes described in formula Ia, where D denotes either an aromatic benz(g^indole nucleus substituted in the 2-position or a 2-pyridyl nucleus, is that the dyes can be used in color coupler dispersions, while the known unsymmetrical indole dyes substituted in the 2 position cannot be used as sensitizers in the presence of coupling dispersions.

The best concentration for spectrally sensitizing electron acceptors in the blurred emulsions according to the invention is between 50 and 2000, preferably between 400 and 800, mg per moles of silver halide. However, these concentrations are given as a guide only, and are found in each individual case, depending on the compound used.

According to the invention, any type of silver halide emulsion can be used. E.g. can emulsions with silver halide, e.g. silver chloride or silver bromide and mixed silver halides such as silver bromoiodide, silver chlorobromide or silver-chlorobromoiodide are used. Particularly suitable are silver-bromine-halide emulsions containing over 50 mol-% bromide. These emulsions are blurred with a combination of a reducing agent and a compound of a metal that is more electropositive than silver. Typical suitable reducing agents include divalent tin salts, such as stannous chloride, hydrazine, sulfur compounds, such as thiourea dioxide, phosphonium salts such as tetra(hydroxymethyl)phosphonium chloride and the like. Compounds of metals such as gold, rhodium, platinum, palladium and iridium are preferably used in the form of soluble salts, such as e.g. auric chloride and salts of the type (NH4)2PdCl6.

The spectrally sensitizing electron acceptors which give particularly good results when carrying out the invention can be characterized by their polarographic half-wave potentials, i.e. their redox potentials determined by polarography. Cathodic measurements can be made with a 1 x 10 ^-molar solution of the electron acceptor in a solvent, e.g. methanol,

containing 0.05 mol of lithium chloride using a dripping mercury electrode, the polarographic half-wave potential of the most positive cathodic wave being denoted E . Anodic measurements can be made with a 1 x 10 -molar aqueous solution, e.g. methanol solutions of the electron acceptor containing 0.05 mol sodium acetate and 0.005 mol acetic acid, and with a carbon paste or pyrolytic graphite electrode, where the voltammetric half-wave potential for the most negative anodic wave is denoted Ea. In each measurement, the reference electrode is an aqueous silver-silver chloride electrode at 20°C (saturated potassium chloride). Electrochemical measurements of this type are known in the art and are described in "New Instrumental Methods in Electrochemistry" by Delahay, Interscience Publischers, New York, New York, 1954; "Polarography", by Kolthoff and Lingane, 2nd ed., Interscience Publishers, New York, New York, 1952; Analytical Chemistry, 36 2426 (1964) by Elving, and Analytical Chemistry 30, 1576 (1958) by Adams. Compounds that can be used as spectrally sensitizing electron acceptors when carrying out the invention include organic compounds with an anodic polarographic half-wave potential (EQ) and a cathodic polarographic potential (E ) which together give a positive sum.

The best results are obtained by using 0.05 to 40 mg, preferably 0.05 to 5 mg, of reducing agent per moles of silver halide, and 0.5 to 15.0 mg, preferably 0.5 to 4.0 mg,

of metal compound (e.g. gold salt) per moles of silver halide. However, these concentrations can be varied over a considerable range while achieving the desired results.

The new emulsions according to the invention can be applied to any suitable photographic substrate, such as e.g. glass, film substrate such as cellulose acetate, cellulose acetate butyrate, polyester such as polyethylene terephthalate, paper, baryte-coated paper, polyolefin-coated paper, e.g. with polyethylene or polypropylene, which may be electron bombarded to promote emulsion adhesion.

The emulsions of the invention can be dispersed in any suitable photographic hydrophilic colloid, such as e.g. mentioned in US patent 3,039,873, column 13, and may contain all types of hardeners or plasticizers which are mentioned in columns 12 and 13 of said patent.

As used herein, "blurry" denotes emulsions containing silver halide grains which give a density of at least 0.5 when developed without exposure for 5 minutes at 20°C in developer DK-50, when the emulsion is applied in a silver amount of 0.054- 0.54 mg silver per cm.

The invention shall be illustrated by the following examples. Examples 1-4 relate to emulsions clouded with reducing agent and a compound of a metal that is more electropositive than silver, together with an electron acceptor known in the art. Examples 5-17 show the improved results obtained with the used spectrally sensitizing electron acceptors.

Examples 1-17.

A silver bromoiodide-gelatin emulsion is produced

(2.5 mol-% of the halide as iodide), with an average grain size of approx. 0.2 micron, by adding an aqueous solution of potassium bromide and potassium iodide, and an aqueous solution of silver nitrate, simultaneously to a rapidly stirred aqueous gelatin solution at 70°C during approx. 35 minutes. The emulsion is cooled, torn up and washed by leaching with cold water in a known manner. The emulsion is reduction- and gold-blurred by first adding a

0.75 mg thiourea dioxide per moles of silver and heat for 60 minutes at 65°C, and then add 7>5 mg of potassium chloraurate per moles of silver and heat for 60 minutes at 65°C. The blurred emulsion is divided into several portions, and 0.87 millimol per

moles of silver halide of each of the dyes listed in the table below are added to a portion of the emulsion.- Each portion is applied to a cellulose acetate substrate in an amount of 0.12 mg of silver and 0.43 mg of gelatin per cm. Each layer is exposed with a tungsten light source in an Eastman Ib sensitometer. The emulsions are given blue exposure through filters Wratten 35 + 38A, and a negative blue exposure through a filter Wratten 16. The exposed layers are developed for 6 minutes at room temperature in Kodak D-19~developer, fixed, washed and dried with the results below.

In the above table it will be seen that the invention's spectrally sensitizing electron acceptors which (1) have an anodic polarographic potential (E Sl) and a cathodic polarographic potential (E ) measured as described, which together give a positive sum (determined as described below) and (2) which provides a spectral sensitization that is so great that the ratio between the relative sensitivity to min.us blue light and the relative sensitivity to blue light, measured as described, is greater than 7.0, provides significantly improved spectral sensitization compared to previously known dyes. Furthermore, this improvement has been achieved by the addition of just one substance, which acts both as a spectral sensitizer and an electron acceptor. The following dyes were used in the above examples:

If the blurring of the emulsion in the above examples is done with 0.2 mg of thiourea dioxide and 4.0 mg of potassium chloroaurate, similar results are obtained, the emulsions showing an increase in overall sensitivity.

Claims (6)

1. Direct-positive photographic silver halide emulsion blurred with a reducing agent and a compound of a metal selected from gold, rhodium, platinum, palladium and iridium, wherein an electron acceptor is adsorbed on the surface of the silver halide grains, characterized in that the electron acceptor has an anodic polarographic half-wave potential and a cathodic polarographic half-wave potential, which potentials together give a positive sum, and that the acceptor sensitizes the emulsion to irradiation with a wavelength above 480 m^ u so that the ratio between relative minus blue velocity and relative blue velocity is greater than 7. 2. Emulsion according to claim 1, characterized in that the electron acceptor spectrally sensitizes the emulsion so that the ratio between relative minus blue velocity and relative blue velocity is greater than 10. 3. Emulsion according to claim 1, characterized in that it contains 50-2000 mg per moles of silver halide of the electron acceptor. 4. Emulsion according to claims 1-3, characterized in that the electron acceptor has the general formula: where the A groups which may be the same or different are aryl radicals or aromatic heterocyclic radicals containing 5 or 6 atoms in the heterocyclic ring, the heteroatom being nitrogen, sulfur or oxygen, B.^ and R^ , which may be the same or different , each is hydrogen, halogen or alkyl radicals which may be substituted by hydroxyl or alkoxy, or R^ and R^ together comprise the atoms required to constitute an aromatic ring with 6 carbon atoms in the ring, the R^ groups which may be the same or different are alkyl radicals which may be substituted with a sulfo, sulfate or carboxy group, the Rg groups which may be the same or different are hydrogen, halogen or alkyl radicals which may be substituted with hydroxyl or alkoxy, or Rg together with R constitute alkylene radicals, Y is hydrogen or alkyl, alkoxy, aryl or an aromatic heterocyclic radical, and where X is an anion. 5" Emulsion according to claims 1-3" characterized in that the electron acceptor has the general formula: where Rjj and Rj-, which may be the same or different, where is an alkyl radical which may be substituted with a sulfo-, sulfate-elier carboxy group, or an alkenyl-, alkynyl-, cycloalkyl- or aryl radical, the R^ -groups which may be the same or different are hydrogen, halogen or nitro groups, k is 1, 2 or 3, and X is an anion. 6. Emulsion according to claims 1-3, characterized in that the electron acceptor is a dye with the general formula: where R-[_2> rtt° S Rl4 is each an alkyl or aryl radical, X is an anion, and Q is (1) -CH=Q^ which includes the atoms required to complete a desensitizing nucleus in a trimethine cyanine dye , and which constitute a 6-nitrobenzthiazole nucleus, a 5-nitroindolene nucleus, an imidazo-(4,5-b)-conoxaline nucleus or a pyrrolo(2,3~ b)-pyrido nucleus, or (2) the atoms required to completing a desensitizing nucleus in a dimethinecyanine dye, and which constitutes a pyrazole nucleus or an aromatic indole nucleus substituted in the 2-position with an aromatic group, which nucleus is bound by means of the 3-carbon atom to the methine chain.