US3560208A - Cyanine dye containing a pyrrole nucleus used as a sensitizer for organic photoconductors - Google Patents

Abstract

ORGANIC PHOTOCONDUCTORS ARE SPECTRALLY SENSITIZED WITH A CYANINE DYE WHICH FEATURES A PYRROLE NUCLEUS LINKED BY THE 2-CARBON ATOM THEREOF TO THE METHINE CHAIN OF THE DYE.

Description

United States Patent 3,560,208 CYANINE DYE CONTAINING A PYRROLE NUCLEUS USED AS A SENSITIZER FOR ORGANIC PHOTOCONDUCTORS Arthur Fumia, Jr., and Donald W. Heseltine, Rochester,

N.Y., assiguors to Eastman Kodak Company, Rochester, N.Y.

No Drawing. Filed Feb. 15, 1968, Ser. No. 705,619

Int. Cl. G03g 51/06 U.S. Cl. 961.6 Claims ABSTRACT OF THE DISCLOSURE Organic photoconductors are spectrally sensitized with a cyanine dye which features a pyrrole nucleus linked by the 2-carbon atom thereof to the methine chain of the dye.

This invention relates to electrophotography, and more particularly to materials and elements useful in the electrophotographic process.

Elements useful in the electrophotographic process com monly comprise an electrically conductive support bearing a stratum including a photoconductive insulating layer which has a resistivity substantially greater in the dark than in light actinic thereto. Such elements can be used in electrophotographic processes, for example, by first adapting the element in the dark to obtain a uniformly high resistivity in the photoconductive insulating layer, and electrostatically charging the element in the dark to obtain a relatively high potential which may be either negative or positive in polarity. The element can then be exposed to a light pattern which lowers the resistivity and thereby the charge density of the illuminated area imagewise in proportion to the intensity of illumination incident upon each point of the illuminated areas. A latent electrostatic image is obtained. Visible images can be formed from the latent electrostatic image in any convenient manner, such as by dusting with a finely divided, fusible pigment the particles of which bear an electrostatic charge opposite that remaining on the surface of the photoconductive insulating layer. Thereafter, the pigment particles can be fused to the surface to provide a permanent image.

Various photoconductive substances have been employed in photographic elements and processes of the type described above. Typical inorganic photoconductive materials include selenium and zinc oxide. Such inorganic photoconductive materials have inherent disadvantages, such as an inability to be readily adapted to reflex copying systems, or to produce images on transparent supports except by indirect means. Organic photoconductors avoid such disadvantages, but, generally have relatively poor sensitivity to visible radiation. It has been proopsed to increase the spectral sensitivity of organic photoconductors with certain cyanine or merocyanine dyes, for example, such as listed in Table D hereinafter. The spectral sensitivity imparted by such dyes has been very weak. It therefore appears highly desirable to provide effective spectral sensitizers for organic photoconductors.

One object of this invention is to provide novel sensitized organic photoconductors.

Another object of this invention is to provide novel spectrally sensitized organic photoconductor materials.

Still another object of this invention is to provide novel compositions of matter comprising organic photoconductors and certain spectral sensitizers.

A further object of this invention is to provide novel compositions of matter comprising organic photoconductor, binder and certain spectral sensitizers for the organic photoconductor.

Still another object of this invention is to provide a "ice novel electrophotographic material including a conductive support having coated thereon an insulating layer containing spectrally sensitized organic photoconductor.

A further object of this invention is to provide methods for spectrally sensitizing organic photoconductors.

Still other objects of the invention will be apparent from the following disclosure and the appended claims.

In accordance with one embodiment of this invention, novel compositions of matter are provided comprising organic photoconductors spectrally sensitized with the dyes defined more fully below. These compositions can be incorporated in a suitable binder and coated on a conductive support for use in electrophotography.

In another embodiment of this invention, compositions of matter are provided comprising organic photoconductors spectrally sensitized with the dyes described below, dispersed in an insulating binder. These compositions of matter can be coated on a conductive support and used in electrophotographic processes.

In still another embodiment of this invention, electrophotographic materials are provided comprising a conductive support having coated thereon a layer comprising an insulating binder, an organic photoconductor and a spectral sensitizing quantity of a dye defined more fully below.

In another embodiment of this invention, a method is provided for spectrally sensitizing organic photoconductors which comprises mixing a dye of the type described below with an organic photoconductor, in a concentration sufficient to effectively spectrally sensitize the organic photoconductor. Preferably, the dye and organic photoconductor are mixed in a suitable solvent.

The spectral sensitizing dyes which are employed in this invention are certain cyanine dyes containing certain pyrrole nuclei which, when incorporated in a test negative gelatin silver bromoiodide emulsion consisting of 99.35 mole percent bromide and .65 mole percent iodide, at a concentration of 0.2 millimole of dye per mole of silver halide, desensitize the emulsion more than 0.4 log E when the test emulsion is coated on a support, exposed through a step Wedge in a sensitometer (to obtain D to light having a wavelength of 365 nm., processed for three minutes at 20 C. in Kodak Developer D-19, and is fixed, washed and dried. As used herein and in the appended claims, the test negative silver bromoiodide emulsions are prepared as follows:

In a container with temperature control is put a solution with the following composition:

Potassium bromidel g. Potassium iodide5 g. Gelatin65 g. Water1700 cc.

And in another container is put a filtered solution consisting of:

Silver nitrate200 g. Water2000 c.c.

Solution A is kept at a temperature of 54 C. during precipitation and ripening, while solution B is put in a separating funnel at a temperature of 54 C. The silver nitrate solution runs from the separating funnel through a calibrated nozzle into the container, the contents of which are kept in constant motion during precipitation and ripening, and later during finishing, by a mechanical stirrer. The precipitation is conducted over a period of 10 minutes.

The developer employed in the test referred to above is Kodak developer D-l9 which has the following composition:

N-methyl-p-aminophenol sulfate2.0 g. Sodium sulfite, desiccated90.0 g. Hydroquinone8.0 g.

Sodium carbonate, monohydrated52.5 g. Potassium bromide5.0 g.

Water to makel.0 liter As indicated above, the cyanine dyes employed in this invention desensitize conventional negative silver halide emulsions. Such emulsions are inherently sensitive to blue radiation. The present dyes reduce that sensitivity. In addition, these dyes fail to provide practical spectral sensitization from such emulsions. Therefore, it was quite unexpected to find that they spectrally sensitized organic photoconductors.

Another characteristic of the cyanine dyes of this invention is that they are substantially non-photoconductive. The term substantially non-photoconductive as used herein means that no image is formed when a solution of 0.002 g. of the dye and 0.5 g. of polyester binder (described in Examples 1 to below) are dissolved in 5.0 ml. of methylene chloride, and is coated and tested (in the absence of any photoconductor) as described in Examples 1 to 5 below.

The cyanine dyes of this invention increase the speed of organic photoconductors by extending or increasing the response of the photoconductor to visible radiation (i.e., radiation in the range of about 400 nm. to 700 nm.) In the concentrations used, the dyes herein appear to function as spectral sensitizers when employed with eflicient U organic photoconductors. When the organic photoconductor used is poor or inefiicient, the dyes seem to function as speed increasing compounds as well as spectral sensitizers.

The cyanine dyes that are useful in practicing the invention include those comprising first and second 5- or 6-membered nitrogen containing heterocyclic nuclei joined together by a methine linkage; said first nucleus being a pyrrole nucleus joined at the Z-carbon atom thereof to said linkage; and said second nucleus being of the type used in cyanine dyes, preferably an electron accepting nucleus, joined at a carbon atom thereof to said linkage, to complete said cyanine dyes.

The preferred cyanine dyes that are useful herein include those represented by the following general formula:

( I R3 C-C-R4 wherein n represents a positive integer of from 1 to 2, L represents a methine linkage, e.g., -CH=,

-C(C H etc., R etc.; R represents an alkyl group, including substituted alkyl (preferably a lower alkyl containing from 1 to 4 carbon atoms), e.g., methyl, ethyl, propyl, isopropyl, butyl, hexyl, cyclohexyl, decyl, dodecyl, etc., and substituted alkyl groups (preferably a substituted lower alkyl containing from 1 to 4 carbon atoms), such as a hydroxylalkyl group, e.g., ,B-hydroxyethyl, whydroxybutyl, etc., ran alkoxyalkyl group, e.g., ,B-methoxyethyl, w-butoxybutyl, etc., a carboxylalkyl group, e.g., pcarboxyethyl, w-carboxybutyl, etc., a sulfoalkyl group, e.g., ,B-sulfoethyl, w-sulfobutyl, etc., a sulfatoalkyl group, e.g., ,B-sulfatoethyl, w-sulfatobutyl, etc., an acyloxyalkyl group, e.g., fi-acetoxyethyl, 'y-acetoxypropyl, w-butyryloxybutyl, etc., an alkoxycarbonylalkyl group, e.g., ,B-methoxycarbonylethyl, w-ethoxycarbonylbutyl, etc, or an aralkyl group, e.g., benzyl phenethyl, etc.; an alkenyl group, e.g., allyl, l-propenyl, Z-butenyl, etc., or an aryl group, e.g., phenyl, tolyl, naphthyl, methoxyphenyl, chlorophenyl, etc.; R R R and R each represents a hydrogen atom, an alkyl group (preferably a lower alkyl containing from 1 to 4 carbon atoms), e.g., methyl, ethyl, propyl,

isopropyl, butyl, decyl, dodecyl, etc., or an aryl group, e.g., phenyl, tolyl, naphthyl, methoxyphenyl, chlorophenyl, nitrophenyl, etc.; X represents an acid anion, e.g., chloride, bromide, iodide, perchlorate, sulfamate, thiocyanate, p-toluenesulfonate, methyl, sulfate, etc.; and Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing from 5 to 6 atoms in the heterocyclic ring, which nucleus may contain a second hetero atom such as oxygen, sulfur, selenium or nitrogen, i.e., a nucleus of the type used in the production of cyanine dyes, preferably an electron-accepting nucleus, such as the following representative nuclei: a thiazole nucleus, e.g., thiazole, 4-methylthiazole, 4-phenylthiazole, S-methylthiazole, 5-phenylthiazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thiazole, benzothiazole, -4 chlorobenzothiazole, 4 or 5 nitrobenzothiazole, 5 chlorobe'nzothiazole, 6 chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, S-methylbenzothiazole, 6 methylbenzothiazole, 6 nitrobenzothiazole, S-bromobenzothiazole, 6-bromobenzothiazole, 5- chloro 6 nitrobenzothiazole, 4 phenylbenzothiazole, 4- methoxybenzothiazole, S-methoxybenzothiazole, 6-methoxybenzothiazole, 5 iodobenzothiazole, 6 iodobenzothiazole, 4 ethoxybenzothiazole, 5 ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6-dimethoxybenzothiazole, 5,6- dioxymethylenebenzothiazole, S-hydroxybenzothiazole, 6- hydroxybenzothiazole, naphtho[2,l-d]thiazole, naphtho- [l,2-d]thiazole, naphtho[2,3-d]thiazole, 5-methoxynaphtho[2,3-d]thiazole, 5 ethoxynaphtho[l,2-d]thiazole, 8- methoxynaphtho[2,1-d]thiazole, 7 methoxynaphtho[2, 1-d]thiazole, 4 methoxythianaphtheno-7',6,4,S-thiazole, nitro group substituted naphthothiazoles, etc.; an oxazole nucleus, e.g., 4-methyloxazole, 4-nitrooxazole, S-methyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyl0xaz0le, S-phenyloxazole, benzoxazole, S-chlorobenzoxazole, S-methylbenzoxazole, S-phenylbenzoxazole, 5- or 6-nitrobenzoxazole, 5-chloro-6-nitrobenzoxazole, 6-methylbenzoxazole, 5,6-dimethylbenzoxazole, 4,6-dimethylbenzoxazole, S-methoxybenzoxazole, 5- ethoxybenzoxazole, S-chlorobenzoxazole, 6-methoxybenzoxazole, S-hydroxybenzoxazole, 6-hydroxybenzoxazole, group substituted naphthoxazoles, etc.; a selenazole nucleus, e.g., 4-methylselenazole, 4-nitroselenazole, 4-phenylselenazole, benzoselenazole, S-chlorobenzoselenazole, 5- methoxybenzoselenazole, 5-hydroxybenzoselenazole, 5- or 6-nitrobenzoselenazole, 5 chloro-6-nitrobenzoselenazole, tetrahydrobenzoselenazole, a-naphthoselenazole, fi-naphthoselenazole, nitro group substituted naphthoselenazoles, etc.; a thiazoline nucleus, e.g., thiazoline, 4-methylthiazoline, 4-nitrothiazoline, etc.; a pyridine nucleus, e.g., 2- pyridine, 5-methyl-2-pyridine, 4-pyridine, 3-methyl-4-pyridine, nitro group substituted pyridines, etc.; a quinoline nucleus, e.g., 2-quinoline, 3-methyl-2-quinoline, S-ethyl- Z-quinoline, 6chloro-2-quinoline, 6-nitro-2-quinoline, 8- chloro-Z-quinoline, 6-rnethoxy-2-quinoline, 8 ethoxy-Z- quinoline, 8-hydroXy-2-quinoline, 4-quinoline, 6-methoxy- 4-quinoline, 6-nitro-4-quinoline, 7-methyl-4-quinoline, 8- chloro-4-quinoline, l-isoquinoline, 6-nitro-l-isoquinoline, 3,4-dihydro-l-isoquinoline, 3-isoquinoline, etc.; a 3,3-dialkylindolenine nucleus, preferably having a nitro or cyano substituent, e.g., 3,3-dimethyl-5-or 6-nitroind0lenine, 3,3-dimethyl-5- or 6-cyanoindolenine, etc.; and, an imidazole nucleus e.g., imidazole, l-alkylimidazole, 1-alkyl-4 phenylimidazole, 1 alkyl-4,S-dimethylimidazole, benzimidazole, l-alkylbenzimidazole, l-alkyl 5 nitrobenzimidazole, 1-aryl-5,6-dichlorobenzimidazole, 1-alkyl-1H- naphthimidazole, 1 aryl-3H-naphthimidazole, l-alkyl-S- methoxy 1H naphthirnidazole, or, an imidazo[4,5-b] quinoxaline nucleus, e.g., 1,3-dialkylimidazo[4,5-b1quinoxaline such as 1,3-diethylimidazo[4,5-b1quinoxaline, 6- chloro-1,3-diethylimidazo[4,5-b]quinoxaline, etc., 1,3-dialkenylimidazo[4,5-b]quinoxaline such as 1,3-diallylimidazo[4,5-b]quinoxaline, 6-chloro 1,3 diallylimidazo[4, S-bjlquinoxaline, etc., 1,3 diarylimidazo[4,5-b1quinoxaline such as 1,3 -diphenylimidazo[4,5-b]quinoxaline, 6-

chloro-l,3-diphenylimidazo[4,5-d]quinoxaline, etc.; a 3,3- dialkyl-3H pyrrolo[2,3-b]pyridine nucleus e.g., 3,3-dimethyl-3H-pyrrolo[2,3-b]pyridine, 3,3 diethyl-3H-pyrrolo[2,3-b]pyridine, etc.; a thiazolo[4,5-b]quinoline nucleus; and the like. Nuclei wherein Z is above Formula I completes an electron-accepting nucleus, e.g., an imidazo [4,5-b]quinoxaline nucleus, or a nitro group substituted thiazole, oxazole, selenazole, thiazoline, pyridine, quinoline, indole, or imidazole nucleus; or a 3,3-dialkyl-3H- pyrrolo[2,3-b]pyridine or thiazolo[4,5-b]quinline nucleus; and the like; provide particularly efficacious spectral sensitizing dyes for the photoconductor compositions and elements of this invention. Dyes containing such nuclei are the preferred sensitizing dye species herein.

As used herein electron-accepting nucleus refers to those nuclei which, when converted to a symmetrical carbocyanine dye and added to gelatin silver chlorobromide emulsion containing 40 mole percent chloride and 60 mole percent bromide, at a concentration of from 0.01 to 0.2 gram dye per mole of silver, cause by electron trapping at least about an 80 percent loss in the blue speed of the emulsion when sensitometrically exposed and developed three minutes in Kodak developer Dl9 at room temperature. .Advantageously, the electron-accepting nuclei are those which, when converted to a symmetrical carbocyanine dye and tested as just described, essentially completely desensitize the test emulsion to blue radiation (i.e., cause more than about 90 to 95% loss of speed of blue radiation.)

The cyanine dyes defined by Formula I above are conveniently prepared, for example, by reacting at from 15 C. to refluxing temperatures a mixture of 1) a heterocyclic compound of the formula:

wherein R R R R are as previously defined, in approximately equimolar proportions, in the presence or absence, as desired, of a condensing agent such as a trialkylamine, e.g., triethylamine, etc., piperidine, N-methylpiperidine, etc., in an inert solvent medium such as an alkanol, e.g., ethanol, or acetic anhydride. Chain-substituted dyes are prepared when R represents ethyl, benzyl etc. The crude dyes are then separated from the reaction mixtures and purified by one or more recrystallizations from appropriate solvents such as methanol, mixtures of dimethylacetamide and methanol, and the like.

The intermediate compounds of Formula III above are conveniently prepared, for example, by reacting a mixture of approximately equimolar amounts of (1) a compound of the formula:

wherein R R R and R are as previously defined, and (2) an N,N-dimethylformamide/phosphoryl chloride complex, in an inert solvent medium such as dichloroethane. Preferably (2) is agitated with the dichloroethane and the pyrrole compound (1) is slowly added thereto in the form of a solution thereof in dichloroethane, at the temperature of an ice bath. The mixture is then heated to reflux for a short period, cooled, and an aqueous solution of sodium acetate is then added. After further refluxing, the crude product is separated by appropriate decantation and extraction methods and recrystallized from a suitable solvent such as ligroin.

Further details for the preparation of the dyes herein can be had by reference to our copending application Ser. No. 673,001, filed Oct. 5, 1967.

Included among the dyes of Formula I above are the following typical dye compounds. The method for preparing Dye No. I is included in Table A below to illustrate in general how the dyes herein are prepared.

TABLE A Dye No. Compound toluenesulfonate [4,5-b1quinoxalinium perchlorate.

NorE.3-ethyl-2-methyl-6-nitrobenzothiazoliurn p-toluenesulfonate (7.9 g., .02 mole), pyrrole-zearboxaldehyde (1.9 g., .02 mole) and piperidine (3 drops) are dissolved in ethanol (25 ml.) and heated at reflux with stirring, for 5 minutes. After chilling, the solid is collected on a filter, rinsed with ethanol and dried. The yield of crude dye is 5.9 g. (78%) and, after one recrystallization from methanol, the yield of purified 3. dye is5 g. (46%), M.P. 254-255 0., decomposes.

Other useful related dyes embraced by Formula I above include, for example, 3-ethyl-2-[2-(l-methyl-Z-pyrrolyl) vinyl]-6-nitrobenzoxazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 3-ethyl-2-[2-(l-methyl-Z-pyrrolyl)vinyl]-6-nitrobenzoselenazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluene-sulfonate, etc. salts), the dye 3 ethyl-6-nitro-2-[2-(2-phenyl-2-pyrrolyl)vinyl]benzoxazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 3-ethyl-6- nitro 2 [2 (1-phenyl-2-pyrrolyl)vinyl]benzoselenazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salt), 3-phenyl-6-nitro-2-[2-(1- phenyl-2-pyrrolyl)vinyl]benzothiazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 1,3-diallyl-2-[2-(2-phenyl-2pyrrolyl)- vinyl]imidazo[4,5-b]quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 1,3-diphenyl-2-[2-(1-phenyl-2-pyrrolyl) vinyl]imidazo[4,5-b]quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 2-[2-(1,5-diphenyl-2-pyrrolyl)vinyl]-1,3- diallylimidazo[4,'5-b]quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 2-[2-(1,5-diphenyl-2-pyrrolyl)vinyl]-1,3- diphenylimidazo[4,5-b]quinoxalinium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), and the like dyes.

Still other highly useful cyanine dyes of the invention wherein Z of Formula I above completes a sensitizing nucleus includes, for example, the dye 3-ethyl-2-[2-(1- methyl-2-pyrrolyl)vinyl]benzothiazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 3-ethyl-2-[2-(l-methyl-Z-pyrrolyl) vinyl]benzoxazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts), the dye 3-ethyl-2- 2-( I-methyl-Z-pyrrolyl vinyl] benzoselenazolium salt (e.g., the chloride, bromide, iodide, perchlorate, p-toluenesulfonate, etc. salts) and the like dyes.

Dyes such as illustrated above can be used alone, or a combination of one or more of the above described dyes can be used to impart the desired spectral sensitivity. All of them are spectral sensitizers for organic photoconductors. Suitable organic photoconductors which are effectively spectrally sensitized by such dyes include both monomeric and polymeric organic photoconductors. The invention is particularly useful in increasing the speed of organic photoconductors which are substantially insensitive, or which have low sensitivity (e.g., a speed less than 25 but generally less than 10 when tested as described in Examples 1 to 5 below) to radiation of 400 to 700 nrn.

An especially useful class of organic photoconductors is referred to herein as organic amine photoconductors. Such organic photoconductors have as a common structural feature at least one amino group. Useful organic photoconductors which can be spectrally sensitized in accordance with this invention include, therefore, arylamine compounds comprising (1) diarylamines such as diphenylamine, dinaphthylamine, N,N'-diphenylbenzidine, N phenyl-l-naphthylamine; N-phenyl-Z-naphthylamine; N,N diphenyl-p-phenylenediamine; 2-carboxy-5-chloro- 4'-rnethoxydiphenylamine; p-anilinophenol; N,N-di-2- naphthyl-p-phenylene diamine; 4,4'-benzylidene-bis(N,N- diethyl-m-toluidine), those described in Fox U.S. Pat. 3,240,597 issued Mar. 15, 1966, and the like, and (2) triarylamines including (a) nonpolymeric triarylamines, such as triphenylamine, N,N,N,N'-tetraphenyl-m-phenylenediamine; 4-acetyltriphenylamine, 4-hexanoyltriphenylamine; 4-lauroyltriphenylamine; 4-hexyltriphenylamine, 4-dodecyltriphenylamine, 4,4'-bis diphenylamino -benzil, 4,4'-bis(diphenylamino)-benzophenone, and the like, and (b) polymeric triarylamines such as poly[N,4"-(N,N,N- triphenylbenzidine)]; polyadipyltriphenylamine, polysebacyltriphenylamine; polydecamethylenetriphenylamine; poly N (4-vinylphenyl)-diphenylamine, poly-N-(vinylphenyl)-a,a-dinaphthylamine and the like. Other useful amine-type photoconductors are disclosed in U.S. Pat. 3,180,730, issued Apr. 27, 1965.

Other very useful photoconductive substances capable of being spectrally sensitized in accordance with this invention are disclosed in Fox U.S. Pat. 3,265,496 issued Aug. 9, 1966, and include those represented by the following general formula:

[NAlbQ wherein A represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear (e.g., phenylene, naphthylene, biphenylene, naphthylene, etc.), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy, etc.), or a nitro group; A represents a mononuclear or polynuclear monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), or a substituted monovalent aromatic radical wherein said substituent can comprise a member, such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogen atom, a halogen atom or an aromatic amino group, such as A'NH; b represents an integer from 1 to about 12, and G represents a hydrogen atom, a mononuclear or polynuclear aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), a substituted aromatic radical wherein said substituent comprises an alkyl group, an alkoxy group, an acyl group, or a nitro group, or a poly(4-vinylphenyl) group which is bonded to the nitrogen atom by a carbon atom of the phenyl group. Certain nitrogen heterocyclic compounds are also useful photoconductors in the invention such as, for example, 1,3,5- triphenyl-Z-pyrazoline, 2,3,4,5-tetraphenylpyrrole, etc.

Polyarylalkane photoconductors are particularly useful in producing the present invention. Such photoconductors are described in U.S. Pat. 3,274,000; French Pat. 1,383,461 and in a copending application of Seus et a1. Ser. No. 624,233, Photoconductive Elements Containing Organic Photoconductors filed Mar. 20, 1967. These photoconductors include leucobases of diaryl or triaryl methane dye salts, 1,1,1-triarylalkanes wherein the alkane moiety has at least two carbon atoms and tetraarylmethanes, there being substituted an amine group of at least one of the aryl groups attached to the alkane and methane moieties of the latter two classes of photoconductors which are non-leuco base materials.

Preferred polyaryl alkane photoconductors can be represented by the formula:

wherein each of D, E and G is an aryl group and J is a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and G containing an amino substituent. The aryl groups attached to the central carbon atom are preferably phenyl groups, although naphthyl groups can also be used. Such aryl groups can contain such substituents as alkyl and alkoxy typically having 1 to 8 carbon atoms, hydroxy, halogen etc. in the ortho, meta or para positions, ortho-substituted phenyl being preferred. The aryl groups can also be joined together or cyclized to form a fluorene moiety, for example. The amino substituent can be represented by the formula Rs N wherein each R can be an alkyl group typically having 1 to 8 carbon atoms, a hydrogen atom, an aryl group, or together the necessary atoms to form a heterocyclic amino group typically having 5 to 6 atoms in the ring such as morpholino, pyridyl, pyrryl, etc. At least one of D, E and G is preferably p-dialkylaminophenyl group. When I is an alkyl group, such an alkyl group more generally has 1 to 7 carbon atoms.

Representative useful polyarylalkane photoconductors include the compounds listed below:

TABLE B Compound No.

(1) 4,4-bis-(diethylamlno)-2,2-dimethyltriphenylmethane.

(2) 4' ,4 -diamino4-dimethylamino-2 ,2 -dirnethy1tri-.

phenylmethane.

(3) 4',4 -bis (diethylamino) -2,6-dichloro-2 ,2 -dimethyltn'phenylmethane.

(4) 4 ,4 -bis (diethylamino) -2,2 -dimethyldiphenylnaphthylmethane.

(5) 2,2"-dimethyl-4,4,4-tris(d1methylammo) trlphenylmethane.

(6) 4 ,4"-bis (diethylamlno) -4-dimethylamim-2 ,2

dimethyltriphenylmethane.

(7) 4 ,4 -bis (dietllylamino) -2chlor0-2' ,2 -dimethy1-4- dimethylaminotriphenylmethane.

(8) 4'-4"-bls (diethylamino)-4-dimethylam1no-2,2,2"-

trimethyltriphenylmethane.

(9) 4,4 -bis (dimethylamino) -2-chloro-2 ,2 -dim ethyltriphenylmethane.

(10) 4,4-bis (dimethylamino) -2 ,2 -dimethyl-4-methoxytriphenylrnethane.

(11). 4,4-bis(benzylethylamino) -2,2"dimethyltrlphenylmethane.

(12). 4,4'-bis(diethylamino) -2,2-diethoxytriphenylmethane.

(13)- 4,4 -bis(d.imethylamino) -1,1,1-tr1pheny lethane.

(14) 1-(4:119,N-dimethylaminophenyl)-1,1-d1pl1enyle ane.

(15) 4-dimethylaminotetraphenylmethane.

(16) 4-diethylaminotetraphenylmethane.

As described herein a wide variety of photoconductor compounds can be spectrally sensitized with the dyes referred to above. Some organic photoconductors will, of course, be preferred to others; but in general useful results may be obtained from substantially all of the pressently known organic photoconductors.

The following Table C comprises a partial listing of US. patents describing such organic photoconductors and compositions which can be used in place of those more particularly described herein.

TABLE 0 Inventor Issued Patent No Noe et a1 Feb. 25, 1964 3, 122, 435 S115 et a1... Mar. 31, 1964. 3, 127, 266 pr. 21, 1964 3,130,046 Apr. 28, 1964 3, 131, 060 June 30, 1964 3, 139, 338 June 30, 1964 3, 139, 339 July 14, 1964 3,140,936 July 21, 1964..-. 3,141,770 Sept. 15, 19 3, 148, 982 Nov. 3, 1964 3, 155. 503 0V. 24, 19("4.... 3, 158,475 Dec. 15, 1964. 3, 161, 505 Dec. 29, 1964. 3,163,530 Dec. 29, 1964. 3, 163, 531 Dec. 29, 1964- 3, 163, 532 Feb. 9, 1965. 3,169,060 Mar. 23, 1965 3, 174. 854 Apr. 27, 1965. 3,180, 729 Apr. 27, 1965. 3, 180, 730 June 15, 1965... 3, 189, 447 Sept. 14, 1965.... 3, 206, 306 July 21, 1964.-.. 3, 141, 770

June 5, 1962...- 3, 037, 8 June 26, 1962.-. 3, 041,165 Nov. 27, 1962.-. 3, 066, 023 Bethe. Jan. 8, 1963... 3, 072, 479 Klupfel et al... July 9, 1963.-. a, 047, 095 Neugebauer et al Nov. 26, 1963-.. 3,112,197 Cassiers et al. Dec. 3, 1963. 3,113,022 Sch1esi ger Dec. 17, 1963." 3,114,633 Kosche et al-.. Aug. 9, 1966..-- 3, 265, 497 Noe et al Sept. 20, 1966 3, 214, 000

The spectrally sensitized organic photoconductor compositions of this invention can, in certain arrangements, be employed in electrophotographic elements in the absence of binder. For example, the photoconductor itself is sometimes capable of film formation, and therefore requires no separate binder. An example of such filmforming photoconductor is poly(vinylcarbazole). However, the more common arrangement is to provide a binder for the spectrally sensitized organic photoconductive materials. Any suitable binder material can be utilized for the spectrally sensitized organic photoconductors of the invention. Such binders should possess high dielectric strength, and have good insulating properties (at least in the absence of actinic radiation) as well as good film forming properties. Preferred binder materials are polymers such as polystyrene, poly(methylstyrene), styrenebutadiene polymers, poly(vinyl chloride), poly(vinylidene chloride), poly(vinyl acetate), vinyl acetate-vinyl chloride polymers, poly(vinyl acetals), polyacrylic and methacrylic acid esters, polyesters such as poly(ethylene alkaryloxy-alkylene terephthalates), phenol-formaldehyde resins, polyamides, polycarbonates and the like.

The photoconductive compositions of the invention can be coated on any of the electrically conductive supports conventionally used in electrophotographic processes, such as metal plates or foils, metal foils laminated to paper or plastic films, electrically conductive papers and films, papers and films coated with transparent electrically conductive resins and the like. Other useful conducting layers include thin layers of nickel coated by high vacuum deposition and cuprous iodide layers as described in US. Pat. 3,245,833. Transparent, translucent or opaque support material can be used. Exposure by reflex requires that the support transmit light while no such requirement is necessary for exposures by projection. Similarly transparent supports are desired if the reproduction is to be used for projection purposes; translucent supports are preferred for reflex prints; and opaque supports are adequate if the image is subsequently transferred by any means to another support, the reproduction is satisfactory as obtained, or the reproduction is to be used as a printing plate for preparing multiple copies of the original.

The quantity of the above-described dye required to spectrally sensitize an organic photoconductor varies with the results desired, the particular dye used, and the particular organic photoconductor used. Best results are obtained with about .01 to 10 parts by weight dye and about 1 to 75 parts by Weight of the organic photoconductor based on the photoconductive composition. Binder can be employed in such compositions, when desired, at preferred ranges of to 99 parts by weight. In addition, the composition can contain other sensitizers, either spectral sensitizers or speed increasing compounds, or both.

As used herein and in the appended claims, the terms insulating and electrically conductive have reference to materials the surface resistivities of which are greater than 10 ohms per square unit (e.g., per square foot) and less than 10 ohms per square unit (e.g., per square foot) respectively.

Coating thicknesses of the photoconductive compositions of the invention on a support can vary widely. As a general guide, a dry coating in the range from about 1 to 200 microns is useful for the invention. The preferred range of dry coating thickness is in the range from about 3 to microns.

To produce a reproduction of an image utilizing the electrophotographic elements of our invention, the photoconductive layer is preferably dark adapted, and then is charged either negatively or positively by means of, for example, a corona discharge device maintained at a potential of from 6000-7000 volts. The charged element is then exposed to light through a master, or by reflex in contact with a master, to obtain an electrostatic image corresponding to the master. This invisible image may then be rendered visible by being developed by contact with a developer including a carrier and toner. The carrier can be, for example, small glass or plastic balls, or iron power. The toner can be, for example, a pigmented thermoplastic resin having a grain size of from about 1-100 which may be fused to render the image permanent. Alternatively, the developer may contain a pigment or pigmented resin suspended in an insulating liquid which optionally may contain a resin in solution. If the polarity of the charge on the toner particles is opposite to that of the electrostatic latent image on the photoconductive element, a reproduction corresponding to the original is obtained. If, however, the polarity of the toner charge is the same as that of the electrostatic latent image, a reversal or negative of the original is obtained.

Although the development techniques described hereinabove produce a visible image directly on the electrophotographic element, it is also possible to transfer either the electrostatic latent image, or the developed image to a second support which may then be processed to obtain the final print. All of these development techniques are well known in the art and have been described in a number of U8. and foreign patents.

This invention is further illustrated by the following representative examples.

EXAMPLES 1 TO 5 These examples show the great increase in speed of organic photoconductors when the dyes employed in this invention are added thereto. This increase in speed is due to the spectral sensitivity imparted to the photoconductor by the dyes described herein. The examples also show that the maximum sensitivity peaks (Abs max.) occur in most cases at radiations in the region of the spectrum of from about 350 to 625 nm.

A series of solutions are prepared consisting of 5.0 ml. methylene chloride (solvent); 0 .15 g. 4,4-bis(diethylamino)-2,2' dimethyltriphenylmethane (organic photoconductor); 0.50 g. polyester composed of terephthalic acid and a glycol mixture comprising a 9:1 weight ratio of 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane and ethylene glycol (binder) and 0.0065 g. of the spectral sensitizing dye indicated by identifying number from above Table A. Each solution is coated on an aluminum surface maintained at 25 C., and dried. All operations are carried out in a darkened room. A sample of each coating is uniformly charged by means of a corona to a potential of about 600 volts and exposed through a transparent member bearing a pattern of varying optical density to a 3000 K. tungsten source. The resultant electrostatic image pattern is then rendered visible by cascading a developer composition comprising finely divided colored thermoplastic electrostatically responsive toner particles carried on glass beads over the surface of the element. The image is then developed by deposition of the toner in an imagewise manner on the element. (Other development techniques such as those described in US. 2,786,439; 2,786,440; 2,786,441; 2,811,465; 2,874,063; 2,984,163; 3,040,704; 3,117,884; re. 25,779; 2,297,691; 2,551,582; and in RCA Review, vol. 15 (1954) pages 469-484, can be used with similar results.) An image is formed on each sample, as indicated in Table I. Another sample of each coating is tested to determine its electrical speed and maximum sensitivity peak. This is accomplished by giving each element a positive or negative charge (as indicated in Table I) with a corona source until the surface potential, as measured by an electrometer probe, reaches 600 volts. It is then exposed to light from a 3000 K. tugnsten source of 20-foot candle illuminance at the exposure surface. The exposure is made through a stepped density gray scale. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potential, V0, to some lower potential, V, whose exact value depends on the actual amount of exposure in meter-candle-seconds received by the area. The results of these measurements are plotted on a graph of surface potential V vs. log exposure for each step. The actual speed of each element is expressed in terms of the reciprocal of the exposure required to reduce the surface potential by 100 volts. Hence, the speeds given in Table I are the numerical expression of 10 divided by the exposure in meter-candle-seconds required to reduce the 600 volts charged surface potential by 100- volts. The results are shown in Table I below.

Referring to the above Table I, it will be seen that the control example containing the same photoconductor but no dye shows speeds of only 8 and 7 for the positively and negatively charged surfaces, respectively, whereas the corresponding values for those of the invention represented by Examples 1 to 5 are clearly of a different order of magnitude. For example, the highest speed is shown by Example 3 (Dye No. III) of 810 and 630 for the positively and negatively charged surfaces, respectively, with maximum sensitivity peak at 490 nm., thus indicating a speed increase over that of the control by a factor of about 100 for the positively charged and about 90 for the negatively charged. Also of great significance in the above results is the extension of the absolute sensitivity to the region of 540 nm. (Example 5). Even with the least speed shown for the compositions and element of the invention as illustrated by Example 4 (Dye No. IV) the improvement in speed is impressive in comparison with that of the control by factors of about 25 and 55 for the positively charged and negatively charged surfaces, respectively. Similar results are obtained when Dyes VIXII or the dyes listed after Table A above are substituted for Dyes IV of Examples 1-5.

Similar results to those shown in above Table I are obtained, when, for example, the organic photoconductor 4,4-bis(diethylamino) 2,2 dimethyltriphenylmethane is replaced with 0.15 g. of triphenylamine (using the p-toluenesulfonate salt of each dye), or 1,3,5 triphenyl 2- pyrazoline, or 2,3,4,5 tetraphenylpyrrole, or 4,4'-bis-diethylaminobenzophenone or when other dyes of the invention embraced by Formula I above are used. These results show that the dyes of this invention effectively spectrally sensitize a wide variety of organic photoconductors. The dyes of this invention are not in themselves photoconductive. Also, it should be noted that the above mentioned photoconductors when used alone have very low photoconductive speed to visible light. However, as shown by the tests, the combination of the dyes of the invention with the photoconductors of the invention provide compositions and elements of outstanding speed and excellent quality of image.

This invention is highly unexpected because dyes previously suggested for spectral sensitizers impart weak spectral sensitization to organic photoconductors. Typical dyes proposed by the prior art as spectral sensitizers, which produce weak spectral sensitization in these systems, are shown in Table D below.

TABLE D Dye identi- Name ficatlon A Pinacyenol.

Kryptocyanine. O Anhydro 3-ethy1-9-methy1-3-(3-suliobutyl)-thiacarbocyanine hydroxide. D 3,3-diethyl-9-methylthiacarbocyanine bromide. E 3-carboxymethyl-5-[(3-methyl-2-thiazolidinylidene)-1- methylethyhde e]rhodanine. F Anhydro-5,5-d1chloro-3,9-diethyl-3- (3-su1fobutyl) thiaearbocyanine hydroxide. l-ethyl-3;n1ethylthia-2-cyanine chloride. H 1,1-d1ethyl-2,2-cyanine chloride.

graphic silver halide emulsions.

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

We claim:

1. A composition of matter comprising an organic photoconductor spectrally sensitized with a cyanine dye comprising first and second and G-membered nitrogen containing heterocyclic nuclei joined by a dimethine linkage; the first of said nuclei being a pyrrole nucleus joined at the 2-carbon atom thereof to said dimethine linkage.

2. A composition as defined in claim 1 wherein said second nucleus of said dye is an electron-accepted nucleus.

3. A composition as defined in claim 1 wherein said second nucleus of said dye is substituted by a nitro group on a carbon atom thereof.

4. A composition as defined in claim 1 wherein said second nucleus of said dye is an imidazo [4,5-b1quinoxaline nucleus.

5. A composition as defined by claim 1 wherein said organic photoconductor is selected from the group consisting of: a triphenylamine; a 1,3,S-triaryl-Z-pyrazoline; a 4,4-bis-(dialkylamino)-2,2-dialkyltriarylamine; a 2,3, 4,5-tetraarylpyrrole; and a 4,4'-bis-dialkylaminobenz0- phenone.

6. A comopsition of matter comprising an organic photoconductor spectrally sensitized with a cyanine dye selected from those represented by the following formula:

n-l e wherein n represents a positive integer of from 1 to 2; L represents a methine linkage; R represents a member selected from the group consisting of an alkyl group, an alkenyl group and an aryl group; R R R and R each represents a member selected from the group consisting of a hydrogen atom, an alkyl group, and an aryl group; X represents an acid anion; and Z represents the nonmetallic atoms required to complete a 5- to G-membered nitrogen containing nucleus of the type used in the production of cyanine dyes.

7. A composition of matter as defined in claim 6 wherein said second nucleus of said dye is an electron-accepting nucleus.

8. A composition as defined in claim 6 wherein said Z of said dye represents the non-metallic atoms necessary to complete an electron accepting nucleus selected from the group consisting of: a nitrobenzothiazole nucleus; a nitrobenzoxazole nucleus; a nitrobenzoselenazole nucleus; an imidazo[4,5-b]quinoxaline nucleus; and a 3,3-dialkyl- BH-nitroindole nucleus.

9. A composition as defined by claim 6 wherein said organic photoconductor has the following formula:

wherein each of D, E and G is an aryl group and J is selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group, at least one of D, E and G containing an amino substituent from the group consisting of a secondary amino group and a tertiary amino group.

10. A composition as defined by claim 6 wherein said organic photoconductor is selected from the group consisting of: triphenylamine; 1,3,S-triphenyl-Z-pyrazoline; 4,4 bis(diethylamino)-2,2-dimethyltriphenylamine; 2,3, 4,5-tetraphenylpyrrole; and, 4,4'-bis-diethylaminobenzo phenone.

11. A composition as defined by claim 6 wherein said photoconductor comprises from 1 to 75 parts by Weight of said composition, said photoconductor being spectrally sensitized with from .01 to 10 parts by weight of said composition of said cyanine dye.

12. A composition as defined by claim 6 wherein said organic photoconductor and said dye are incorporated in an insulating binder.

13. A composition as defined by claim 6 wherein said organic photoconductor and said dye are dispersed in from 25 to 99 parts by weight of a polyester of terephthalic acid and a glycol mixture consisting of a 9:1 weight ratio of 2,2-bis-[4-(2-hydroxyethoxy)phenyl]- propane and ethylene glycol as insulating binder.

14. A composition of matter comprising from 1 to 75 parts by weight of an organic photoconductor selected from the group consisting of: triphenylamine; 1,3,5-triphenyl-Z-pyrazoline; 4,4 bis-diethylamino-2,2-dimethyltriphenylmethane; 2,3,4,5-tetraphenylpyrr0le; 4,4-bis-diethylaminobenzophenone; said organic photoconductor being spectrally sensitized with from .01 to 10 parts by weight of a dye selected from the group consisting of 3 ethyl 6-nitro-2-[2-(2-pyrrolyl)vinyl]benzothiazolium salt; 1,3,3 trimethyl-S-nitro-2-[2-(2-pyrrolyl)vinyl]-3H- indolium salt; 3-ethyl-2-[2-(1methyl-2-pyrrolyl)vinyl]-6- nitrobenzothiazolium salt; l,3,3-trimethyl-2-[2-(l-methyl- 2-pyrrolyl)vinyl]-5-nitro-3H-indolium salt and 3-ethyl-2- [2 (5 -methyl-2-pyrrolyl)vinyl]-6-nitrobenzothiazolium salt.

15. A composition of matter as defined in claim 14 wherein said organic photoconductor and said dye are dispersed in from 25 to 99 parts by weight of a polyester of terephthalic acid and a glycol mixture consisting of a 9:1 weight ratio of 2,2-bis-[4-(2-hydroxyethoxy)-phenyl]-propane and ethylene glycol as insulating binder.

16. An electrophotographic element comprising a conductive support having thereon a layer comprising an organic photoconductor in an insulating binder, said organic photoconductor being spectrally sensitized with a cyanine dye comprising first and second 5- to 6-membered nitrogen containing heterocyclic nuclei joined together by a dimethine linkage; said first nucleus being a pyrrole nucleus joined at the Z-carbon atom thereof to said dimethine linkage.

17. An electrophotographic element as defined in claim 16 wherein said second nucleus of said dye is an electronaccepting nucleus.

18. An electrophotographic element as defined in claim 16 wherein said second nucleus of said dye is substituted by a nitro group on a carbon atom thereof.

19. An electrophotographic element as defined in claim 16 wherein said second nucleus of said dye is an imidazo- [4,5-b] quinoxaline nucleus.

20. An electrophotographic element as defined in claim 16 wherein said organic photoconductor is selected from the group consisting of: a triphenylamine; a 1,3,5-triaryl- 2-pyrazoline; a 4,4-bis-(dialkylamino)-2,2-dialkyltriarylamine; a 2,3,4,5-tetraarylpyrrole; and a 4,4-bis-dialkylaminobenzophenone.

21. An electrophotographic element comprising a conductive support having thereon a layer comprising an organic photoconductor spectrally sensitized with a dye selected from those represented by the following formula:

wherein n represents a positive integer of from 1 to 2; L represents a methine linkage; R represents a member selected from the group consisting of an alkyl group, an alkenyl group and an aryl group; R R R and R each represents a member selected from the group consisting of a hydrogen atom, an alkyl group, and an aryl group; X represents an acid anion; and Z represents the nonmetallic atoms required to complete a 5- to 6-membered nitrogen containing nucleus of the type used in the production of cyanine dyes.

22. An electrophotographic element as defined in claim 21 wherein said second nucleus of said dye is an electronaccepting nucleus.

23. An electrophotographic element as defined in claim 21 wherein said Z of said dye represents the non-metallic atoms necessary to complete an electron accepting nucleus selected from the group consisting of: a nitrobenzothiazole nucleus; a nitrobenzoxazole nucleus; a nitrobenzoselenazole nucleus; an imidazo[4,5-b]quinoxaline nucleus; and a 3,3-dialkyl-3H-nitroindole nucleus.

24. An electrophotographic element as defined in claim 21 wherein said organic photoconductor has the following formula:

wherein each of D, E and G is an aryl group and J is selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group, at least one of D, E and G containing an amino substituent selected from the group consisting of a secondary amino group and a tertiary amino group.

25. An electrophotographic element as defined in claim 21 wherein said organic photoconductor is selected from the group consisting of: triphenylamine; 1,3,5-triphenyl-2- pyrazoline; 4,4-bis-(diethylamino)-2,2-dimethyltriphenylamine; 2,3,4,5-tetraphenylpyrrole; and 4,4-bis-diethylaminobenzophenone.

26. An electrophotographic element as defined in claim 21 wherein said organic photoconductor comprises from 1 to 75 parts by weight of said composition, said photoconductor being spectrally sensitized with from .01 to parts by weight of said composition of said cyanine dye.

27. An electrophotographic element as defined in claim 21 wherein said organic photoconductor and said dye are incorporated in an insulating binder.

28. An electrophotographic element as defined in claim 21 wherein said organic photoconductor and said dye are dispersed in from 25 to 99 parts by weight of a polyester of terephthalic acid and a glycol mixture consisting of a 1,3 ,3-trimethyl-5-nitro-2- [2- (2-pyrrolyl) vinyl] -3H- indolium salt;

3-ethyl-2- [2-( l-methyl-2-pyrrolyl vinyl] -6-nitrobenzothiazolium salt;

1,3,3-trimethyl-2-[2-( l-methyl-Z-pyrrolyl) vinyl] -5- nitro-3H-indolium salt;

3-ethyl-6-nitro-2-[2-(2-pyrrolyl)vinyl]benzothiazolium salt and 3-ethyl-2-[Z-(S-methyl-Z-pyrrolyl) vinyl] -6-nitrobenzothiazolium salt.

30. An electrophotographic element as defined in claim 29 wherein said organic photoconductor and said dye are dispersed in from 25 to 99 parts by weight of a polyester of terephthalic acid and a glycol mixture consisting of a 9: 1 weight ratio of 2,2-bis-[4-(2-hydroxyethoxy)- phenyl]-propane and ethylene glycol as insulating binder.

References Cited UNITED STATES PATENTS 2,298,731 10/1942 Brooker et a1 260--240X 2,466,523 4/1949 White et al. 96102X 2,503,775 4/1950 Sprague 96106X 3,174,854 3/1965 Stumpf et al 961.6

FOREIGN PATENTS 964,873 7/1964 Great Brtiain 96-1.6

GEORGE F. LE'SMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 96106; 260240