Electrophotographic sensitive material

Abstract: An electrophotographic sensitive material is provided which has a photosensitive layer formed on an electrocon­ ductive substrate, the photosensitive layer containing a pyrrolopyrrole type compound represented by the following general formula (1) and a benzidine derivative represented by the following general formula (2) : In the general formulas (1) and (2), R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, ℓ, m, n, o, p and q have the same meanings as defined in the text of the specification.

Description: This invention relates to an electrophotographic sensitive material useful advantageously in image-forming apparatus such as copying machines. In recent years, as electrophotographic materials for use in such image-forming apparatus as copying machines, sensitive materials of the kind permitting a wide freedom in design of functions have been proposed. Particularly, electrophotographic materials comprising a photosensitive layer of two-function type containing an electrical charge generating material capable of generating electric charges upon exposure to light and an electrical charge transferring material capable of transferring generated electrical charges provided in one single or in two separate layers have been proposed. For example, electrophotographic materials comprising a single-layer type photosensitive layer containing an electrical charge generating material, an electrical charge transferring material, and a binding resin, and electrophotographic materials comprising a laminate type photosensitive layer formed by superposition of an electrical charge generating layer containing an electrical charge generating material on an electrical charge transferring layer containing an electrical charge transferring material and a binding resin have been proposed. In the formation of a copied image by use of an electrophotographic material, the Carlson process is widely utilized. The Carlson process basically comprises a charging step for uniformly charging a sensitive material by corona discharge, an exposing step for exposing the charged sensitive material through a given original image to light thereby forming on the sensitive material an electrostatic latent image in conformity to the original image, a developing step for developing the electrostatic latent image with a developer containing a toner thereby forming a toner image, a transferring step for causing the toner image to be transferred onto a substrate such as paper, a fixing step for fixing the toner image transferred on the substrate, and a cleaning step for removing the toner remaining on the sensitive material after the transferring step. For producing an image of high quality in the Carlson process, the electrophotographic material is required to excel in the charging and photosensitivity properties and, at the same time, to have a low residual potential after the exposure to light. The electrophotographic properties of the photosensitive material of the separate function type mentioned above are dependent to a large extent on the combination of an electric charge generating material and an electric charge transferring material. For example, an electrophotographic sensitive material comprising a photosensitive layer using a pyrrolopyrrole type compound as an electrical charge generating material (US-A-4 632 893, JP-A-162 555/1986) in combination with a hydrazone type compound such as N-ethyl-3-carbazolylaldehyde-N,N-diphenyl hydrazone, the drift mobility of which depends heavily upon the intensity of the electric field, has a high residual potential and insufficient sensitivity. The hydrazone type compound has no sufficient stability to resist light because it is liable to be isomerized and dimerized on light exposure. The sensitive material, therefore, has the disadvantage that it suffers from a gradual decrease in sensitivity and a gradual increase in its residual potential through repeated printing cycles. Electrophotographic materials comprising pyrrolopyrrole type compound as charge generating material have been known from EP-A-187 620. The electrical charge transferring materials are disclosed in this document only in a very general manner as aromatic compounds preferably containing nitrogen, such as those described in DE-A-3447685. The only example therefor given in this document is a hydrazone of the formula Some of the benzidine derivatives of the electrical charge transferring material (II) as used according to the present invention have been known from DE-A-36 38 418 wherein electrophotographic materials are disclosed which comprise electrical charge generating material being an aromatic diphenylaminohydrazone compound. Further, a photosensitive material which uses a phthalocyanine type compound as an electrical charge generating material in combination with a styryl triphenylamine type compound represented by 4-styryl-4′-methoxy-triphenylamine, 4-(4-methylstyryl)-4′-methyl-triphenylamine or 4-(3,5-dimethylstyryl)-4′-methyl-triphenylamine as an electrical charge transferring material has been proposed (JP-A-115 167/1987). Electrophotographic materials comprising a photosensitive layer containing a styryl triphenylamine type compound generally excel in electrical properties and sensitivity properties as compared with electrophotographic materials containing other electrical charge transferring materials. The styryl triphenylamine type compounds, however, exhibit no sufficient compatibility with binding resins, possess only a small electron donor capacity, and are deficient in the electric charge transferring properties. Electrophotographic materials produced by using the styryl triphenylamine type compounds, therefore, have the disadvantage that the charging properties and the sensitivity are in sufficient, and the residual potential is unduly high. It is the object of the present invention to provide an electrophotographic material which is excellent in its light stability and its charging and photosensitive properties. This object is achieved according to claim 1. The dependent claims relate to prefered embodiments. The electrophotographic material of the present invention comprises an electroconductive substrate and a photosensitive layer formed on the substrate and comprising (I) an electrical charge generating material which is a pyrrolopyrrole type compound of the general formula (1), wherein are: R¹, R² independently an aryl group, optionally containing a substituent, an aralkyl group, optionally containing a substituent, or a heterocyclic group, and R³, R⁴ independenlty an hydrogen atom, an alkyl or aryl group, optionally containing a substituent, and (II) an electrical charge transferring material which is an aromatic, nitrogen containing compound; it is characterized in that the electrical charge transferring material (II) is a benzidine derivative of the general formula (2), wherein are: R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰ independently a hydrogen atom, a lower alkyl, lower alkoxy group or a halogen atom, l, m, n and o independently 1, 2 or 3, and p and q independently 1 or 2. The electroconductive substrate may be in the form of a sheet or in the form of a drum. As regards the material of the electroconductive substrate, the substrate itself may be made of a material possessing electroconductivity, or of a material not possessing electroconductivity which is provided with a surface having electroconductivity. The electroconductive substrate is desired to exhibit high mechanical strength at the time of its use. Various materials possessing electroconductivity are available for the production of the electroconductive substrate meeting the description given above. Concrete examples therefor are simple metals such as aluminium, copper, tin, platinum, gold, silver, vanadium, molybdenum, chromium, cadmium, titanium, nickel, palladium, indium, stainless steel, and brass, plastic materials having these metals vacuum deposited or superposed thereon, and glass sheets coated with aluminium iodide, tin oxide, or indium oxide. From these materials which are available for the electroconductive substrate, aluminium is used desirably for the purpose of preventing occurrence of black spots and pinholes in a copied image and, at the same time, enhancing the tightness of adhesion between the photosensitive layer and the substrate. The aluminium which has undergone electrolysis in an oxalic acid solution and which consequently has no crystal particles of aluminium retained on the surface thereof is particularly prefered for this purpose. The aluminium which, due to the anodization, has been provided with an oxide coating of 5 to 12 µm in thickness and not more than 1.5 µm in surface roughness is used most advantageously for this purpose. As concrete examples of the aryl group of R¹ and R² in the general formula 1 of the pyrrolopyrrole type compound to be contained in the photosensitive layer, phenyl, naphthyl, anthryl, phenanthryl, fluorenyl and 1-pyrenyl may be cited. Among the aryl groups mentioned above, the phenyl group and the naphthyl group are particularly desirable. The phenyl group is most desirable. As concrete examples of the aralkyl group, there may be cited benzyl, phenylethyl and naphthylmethyl. The substituents in the aryl group or the aralkyl group may be selected from halogen atoms, lower alkyl groups containing a halogen atom, a cyano group, alkyl groups, alkoxy groups, and dialkylamino group, for example. The halogen atoms include fluorine, chlorine, bromine, and iodine. Among the halogen atoms mentioned above, chlorine and bromine are prefered. As concrete examples of the alkyl group containing a halogen atom, there may be cited chloromethyl, dichloromethyl, trichloromethyl, 2-chloroethyl, 2,2-dichloroethyl, 2,2,2-trichloroethyl and trifluoromethyl. As concrete examples of the alkyl group, there may be cited such alkyl groups which have 1 to 18 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl and stearyl. Among the alkyl groups mentioned above, linear or branched alkyl groups having 1 to 12 carbon atoms are desirable, linear or branched lower alkyl groups having 1 to 6 carbon atoms are more desirable, and linear or branched lower alkyl groups having 1 to 4 carbon atoms are most desirable. As concrete examples of the alkoxy group, there may be cited methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, tert-butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, and stearyloxy. Among the alkoxy groups mentioned above, linear or branched alkoxy groups having 1 to 12 carbon atoms are desirable, linear or branched lower alkoxy groups having 1 to 6 carbon atoms are more desirable, and linear or branched lower alkoxy groups having 1 to 4 carbon atoms are most desirable. As concrete examples of the dialkylamino group, there may be cited such dialkylamino groups as dimethylamino, diethylamino, methylethylamino, dipropylamino, diisopropylamino, dibutylamino, diisobutylamino, di-tert-butylamino, dipentylamino and dihexylamino which have an alkyl moiety of 1 to 6 carbon atoms. As concrete examples of the heterocyclic group, there may be cited thienyl, thianthrenyl, furyl, pyranyl, isobenzofuranyl, chromenyl, xanthenyl, phenoxazinyl, pyrrolyl, imidazolyl, pyrazolyl, isothiazolyl, isooxazolyl, indolysinyl, isoindolyl, indolyl, indazolyl, purinyl, pyridyl, pyrazinyl, pyrimidyl, pyridazinyl, quinolidinyl, isoquinolyl, quinolyl, phthalazinyl, naphthylidinyl, quinoxalinyl, quinazolinyl, cinnolinyl, pteridinyl, carbazolyl, carbonylyl, phenanthridinyl, acridinyl, perimidinyl, phenanthrolinyl, phenazinyl, phenarsazinyl, phenothiazinyl, furazanyl, phenoxazinyl, isochromanyl, chromanyl, pyrrolidinyl, pyrrolinyl, imidazolidinyl, imidazolinyl, pyrazolidinyl, pyrazolinyl, piperidyl, piperidino, piperazinyl, indolinyl, isoindolinyl, quinuclidinyl, morpholinyl, and morpholino, and the condensed heterocyclic groups of the condensed heterocyclic type compounds obtained by ortho-condensation or peri-condensation of compounds containing the heterocyclic groups mentioned above with an aryl compound. As concrete examples of the alkyl group of R³ and R⁴ in the general formula 1 of the pyrrolopyrrole type compound to be contained in the photosensitive layer, there may be cited the lower alkyl groups cited above with respect to the substituents R¹ and R², those lower alkyl groups having 1 to 6 carbon atoms, and preferably 1 to 4 carbon atoms. The aryl groups containing a substituent are preferably substituted phenyl groups. The substituent is preferably selected from halogen atoms, lower alkyl groups containing a halogen atom, alkyl groups, alkoxy groups, alkylthio groups, and nitro groups. As concrete examples of the halogen atoms, the lower alkyl groups containing a halogen atom, the alkyl groups, and the alkoxy groups, there may be cited the same substituents as cited above with respect to R¹ and R². As concrete examples of the alkylthio group, there may be cited methylthio, ethylthio, propylthio isopropylthio, butylthio, isobutylthio, tert-butylthio, pentylthio, hexylthio, heptylthio, octylthio, nonylthio, decylthio, undecylthio, dodecylthio and stearylthio. Among the alkylthio groups mentioned above, linear or branched alkylthio groups having 1 to 12 carbon atoms are desirable, linear or branched lower alkylthio groups having 1 to 6 carbon atoms are more desirable, and linear or branched lower alkylthio groups having 1 to 4 carbon atoms are most desirable. It is especially prefered that the substituents R³ and R⁴ are both a hydrogen atom. Prefered examples of the pyrrolopyrrole type compounds as described above are 1,4-dithioketo-3,6-diphenylpyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(4-tolyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-ethylphenyl)-pyrrolo[3,4-c]pyrrole, 1-4-dithioketo-3,6-bis(4-propylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-isopropylphenyl)-pyrrolo[3,4-c]-pyrrole, 1,4-dithioketo-3,6-bis(4-butylphenyl)-(pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-isobutylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-tert-butylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-pentylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-hexylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(3,5-dimethylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(3,4,5-trimethylphenyl)-pyrrolo[3,4-c]-pyrrole, 1,4-dithioketo-3,6-bis(4-methoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-ethoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-propoxy-phenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-isopropoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-butoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-isobutoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-tert-butoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-pentyloxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-hexyloxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(3,5-dimethoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(3,4,5-trimethoxyphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-dibenzylpyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-dinaphthylpyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-cyanophenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(2-bromophenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-trifluoromethylphenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-dimethylaminophenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(4-diethylaminophenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-diphenylpyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-ditolylpyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-ethylphenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-isopropylphenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-tert-butylphenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(3,4,5-trimethoxyphenyl)-pyrrolo[3,4-c]-pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-methoxyphenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-ethoxyphenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-isopropoxyphenyl)-pyrrolo[3,4-c]-pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(4-tert-butoxyphenyl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-bis(3,4,5-trimethoxyphenyl)-pyrrolo[3,4-c]-pyrrole, 1,4-dithioketo-3,6-di-(pyrrolol-3-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-oxazol-4-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-thiazol-4-yl)pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-diimidazolylpyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(imidazo-2-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(imidazol-4-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(pyrid-4-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(pyrimidin-2-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-dipiperidinopyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(piperidin-4-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-dimorpholinopyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(quinol-2-yl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-bis(3-benzo[b]thiophenyl)-pyrrolo[3,4-c]pyrrole, 1,4-dithioketo-3,6-di-(quinol-2-yl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-di-( imidazol-4-yl)-pyrrolo[3,4-c]pyrrole, N,N′-dimethyl-1,4-dithioketo-3,6-dimorpholinopyrrolo[3,4-c]pyrrole, and N,N′-dimethyl-1,4-dithioketo-3,6-di-(pyrid-4-yl)-pyrrolo[3,4-c]pyrrole. The pyrrolopyrrole type compounds represented by the aforementioned general formula 1 are used either alone or in the form of a mixture of two or more compounds. Optionally, the pyrrolopyrrole type compounds of formula 1 may be used in combination with a different electrical charge generating material in such a ratio that the photosensitivity, for example, is not impaired. As concrete examples of the electrical charge generating material usable in this case, there may be cited selenium, selenium-tellurium, amorphous silicon, pyrylium salts, azo type compounds, azido type compounds, phthalocyanine type compounds, anthanthrone type compounds, perylene type compounds, indigo type compounds, triphenylmethane type compounds, threne type compounds, toluidine type compounds, pyrazoline type compounds, and quinacridone type compounds. The electrical charge generating materials mentioned above are used either singly or in the form of a mixture of two or more compounds. As concrete examples of the lower alkyl group, the lower alkoxy group, and the halogen atom of the substituents R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ in the general formula 2 representing the benzidine derivative to be contained in the photosensitive layer, there may be cited those substituents cited above with respect to the substituents R¹ and R². Among the substituents R⁵,R⁶, R⁷, R⁸, R⁹ and R¹⁰ mentioned above, hydrogen, alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, and halogen atoms are desirable. The substituents R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ may be attached to suitable positions in a benzene ring or a biphenyl backbone. Among the benzidine derivatives of formula 2 shown below including the position indications, the compounds comprised in the following Table 1 may be cited as desirable examples. The benzidine derivatives of formula 2 are used either singly or in the form of a mixture of two or more compounds. The benzidine derivatives of formula 2 are excellent in light stability and do not undergo such reactions as isomerization upon exposure to light. The benzidine derivatives possess a high degree of drift mobility and have a small dependency of the drift mobility upon the intensity of an electrical field. An electrophotographic material of high sensitivity and low residual potential is obtained by producing a photosensitive layer using a benzidine derivative represented by the aforementioned general formula 2 in combination with a pyrrolopyrrole type compound represented by the aforementioned general formula 1. This electrophotographic material produces images of high quality free from fogging. The compounds represented by the aforementioned general formula 2 can be produced according to various methods. They may be produced, for example, by causing a compound represented by the following general formula 3 to react with compounds represented by the following general formulae 4 to 7 simultaneously or sequentially: (wherein R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, ℓ, m, n, o, p, and q have the same meanings as defined above, and X stands for a halogen atoms such as iodine). The reaction of the compound represented by the aforementioned general formula 3 with the compounds represented by the aforementioned general formulae 4 to 7 is generally carried out in an organic solvent. Any of the organic solvents available may be used for this reaction on the sole condition that the solvent to be used does not adversely affect the solution. As concrete examples of the organic solvent, there may be cited nitrobenzene, dichlorobenzene, quinoline, N,N-dimethylformamide, N-methylpyrrolidone, and dimethylsulfoxide. The reaction is generally carried out at a temperature in the range of 150 to 250°C in the presence of a metal or metal oxide catalyst such as copper powder, copper oxide, or a copper halide or a basic catalyst such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogen carbonate, or potassium hydrogen carbonate. Of the benzidine derivatives of formula 2, those which have the substituents R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ attached at controlled positions can be produced, for example, by causing a compound represented by the following general formula 8 to react with compounds of formulae 4 and 6, thereby producing a compound represented by the general formula 9, then deacylating the compound of formula 9 by means of hydrolysis thereby producing a compound represented by the general formula 10, and further causing the compound of formula 10 to react with compounds of formulae 5 and 7, to obtain a derivative of formula 2a : (wherein R¹¹ and R¹² each stand for a lower alkyl group, and R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, ℓ, m, n, o, p, q and X have the same meanings as defined above). The reaction of the compound represented by the aforementioned general formula 8 with the compounds represented by the aforementioned general formulae 4 and 6 can be carried out in the same manner as the reaction of the compound represented by the aforementioned general formula 3 with the compounds represented by the aforementioned general formulae 4 to 7. The deacylation of the compound of formula 9 can be carried out by the conventional method in the presence of a basic catalyst. The reaction of the compound of formula 10 with the compounds of formulae 5 and 7 can be carried out in the same manner as the reaction of the compound of formula 3 with the compounds of formulae 4 to 7. Of the benzidine derivatives represented by the aforementioned general formula 2, those compounds whose substituents R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ are invariable halogen atoms may be produced by causing a compound represented of formula 10 to react with compounds of formulae 5 and 7 and subsequently halogenating the resulting reaction product. After the reaction is completed, the reaction mixture is concentrated. Optionally, the concentrated reaction mixture may be further separated and purified by conventional means such as recrystallization, solvent extraction, and column chromatography. An electrophotographic material with high sensitivity and low residual potential is obtained by preparing a photosensitive layer using a benzidine derivative of formula 2 in combination with a pyrrolopyrrole type compound of formula 1. These reactants, when necessary, may be used further in combination with other electrical charge transferring materials in such a ratio that the charging property and the photosensitivity are not impaired. As concrete examples of the other electrical charge transferring material usable herein, there may be cited tetracyanoethylene, fluorenone type compounds such as 2,4,7-trinitro-9-fluorenone, nitrated compounds such as 2,4,8-trinitrothioxanthone and dinitroanthracene, succinic anhydride, maleic anhydride, dibromomaleic anhydride, oxadiazole type compounds such as 2,5-bis(4-dimethylaminophenyl)-1,3,4-oxadiazole, styryl type compounds such as 9-(4-diethylaminostyryl-anthracene, carbazole type compounds such as polyvinyl carbazole, pyrazoline type compounds such as 1-phenyl-3-(p-dimethylaminophenyl)-pyrazoline, amine derivatives such as 4,4′,4″-tris(4-diethylaminophenyl)-triphenylamine, conjugated diene compounds such as 1,1-diphenyl-4,4-bis(4-dimethylaminophenyl)-1,3-butadiene, hydrazone type compounds such as 4-(N,N-diethylamino)-benzaldehyde-N,N-diphenyl hydrazone, nitrogen-containing cyclic compounds such as indole type compounds, oxazole type compounds, isooxazole type compounds, thiazole type compounds, thiadiazole type compounds, imidazole type compounds, pyrazole type compounds, and triazole type compounds, and condensed polycyclic compounds. Among the photoconductive polymers cited above as electrical charge transferring materials, poly-N-vinyl carbazole, for example, may be used as a binding resin. The photosensitive layer may comprise various additives such as the conventional sensitizers represented by terphenyl, halonaphthoquinones, and acenaphthylene, quenchers represented by fluorene type compounds like 9-(N,N-diphenylhydrazino)-fluorene and 9-carbazolyliminofluorene, plasticizers, and deterioration inhibitors represented by antioxidants and ultraviolet absorbers. The photosensitive layer containing a pyrrolopyrrole type compound as electrical charge generating material of formula 1 and the benzidine derivative as electrical charge transferring material of formula 2 may be either a single layer containing the pyrrolopyrrole type compound of formula 1, the benzidine derivative of formula 2, and a binding resin, or a laminate type photosensitive layer composed of an electrical charge generating layer containing the pyrrolopyrrole type compound of formula 1 and an electrical charge transferring layer containing the benzidine derivative of formula 2 and a binding resin. The construction of the laminate type photosensitive layer is either such that the electrical charge transferring layer is superposed on the electrical charge generating layer or such that the electrical charge generating layer is superposed on the electrical charge transferring layer. A wide variety of binding resins are available for the use mentioned above. The binding resins useful herein include styrene type polymers, acryl type polymers, styrene-acryl type copolymers, olefin type polymers such as polyethylene, ethylene-vinyl acetate copolymers, chlorinated polyethylene, polypropylene, and ionomers, polyvinyl chloride, vinyl chloride-vinyl acetate copolymers, polyesters, alkyd resins, polyamides, polyurethanes, epoxy resins, polycarbonates, polyallylates, polysulfones, diallylphthalate resins, silicone resins, ketone resins, polyvinyl butyral resins, polyether resins, phenol resins, photosetting resins such as epoxy acrylates, and various polymers, for example. These binding resins may be used either singly or in the form of a mixture of two or more compounds. In the formation of the single layer type photosensitive layer, the mixing ratio of the pyrrolopyrrole type compounds of formula 1 and the benzidine derivative of formula 2 is not specifically restricted but may be suitably selected to achieve the desired properties of the electrophotographic material. The proportion of the pyrrolopyrrole type compound is desired to be in the range of 2 to 20 parts by weight, preferably 3 to 15 parts by weight, and that of the benzidine derivative in the range of 40 to 200 parts by weight, preferably 50 to 100 parts by weight, based on 100 parts by weight of the binding resin. If the amounts of the pyrrolopyrrole type compound and the benzidine derivative are less than the lower limits of the respective ranges mentioned above, the electrophotographic material has insufficient sensitivity and unduly high residual potential. If these amounts exceed the upper limits of the respective ranges, the electrophotographic material is deficient in wear resistance. The single layer type photosensitive layer may be formed in a suitable thickness. This thickness is desired to be in the range of 10 to 50 µm, preferably 15 to 25 µm. The electrical charge generating layer of the laminate type photosensitive layer may be formed of a film obtained by vacuum depositing or sputtering a pyrrolopyrrole type compound of formula 1. In the case of the electrical charge generating layer which is formed in combination with a binding resin, the mixing ratio of the pyrrolopyrrole type compound and the binding resin in the electrical charge generating layer may be suitably selected. Generally the proportion of the pyrrolopyrrole type compound is desired to be in the range of 5 to 500 parts by weight, preferably 10 to 250 parts by weight, based on 100 parts by weight of the binding resin. If the amount of the pyrrolopyrrole type compound is less than 5 parts by weight, there results the disadvantage that the electrical charge generating layer is deficient in its electrical charging capacity. If this amount exceeds 500 parts by weight, there arises the disadvantage that the electrical charge generating layer has inferior adhesion. The electrical charge generating layer may be formed in a suitable thickness. This thickness is desired to be approximately in the range of 0.01 to 3 µm, preferably 0.1 to 2 µm. In the formation of the electrical charge transferring layer, the mixing ratio of the binding resin and the benzidine derivative of formula 2 may be suitably selected. The proportion of the benzidine derivative is desired to be in the range of 10 to 500 parts by weight, preferably 25 to 200 parts by weight, based on 100 parts by weight of the binding resin. If the amount of the benzidine derivative is less than 10 parts by weight, the electrical charge transferring layer is deficient in its electrical charge transferring capacity. If this amount exceeds 500 parts by weight, the electrical charge transferring layer has only poor mechanical strength. The electrical charge transferring layer may be formed in a suitable thickness. This thickness is desired to be approximately in the range of 2 to 100 µm, preferably 5 to 30 µm. The electrical charge generating layer may contain the aforementioned benzidine derivative as electrical charge transferring material in addition to the pyrrolopyrrole type compound as electrical charge generating material. In this case, the mixing ratio of the pyrrolopyrrole type compound, the benzidine derivative, and the binding resin may be suitably selected. This mixing ratio is desired to be similar to that of the pyrrolopyrrole type compound, the benzidine derivative and the binding resin in the aforementioned single layer type photosensitive layer. The electrical charge generating layer may be formed in a suitable thickness. Generally, this thickness is approximately in the range of 0.1 to 50 µm. The single layer type photosensitive layer can be formed by preparing a photosensitive layer coating liquid containing the pyrrolopyrrole type compound, the benzidine derivative, and the binding resin, applying this coating liquid to the electroconductive substrate, and drying or setting the applied layer of the coating liquid. The laminate type photosensitive layer can be formed by preparing an electrical charge generating layer coating liquid containing the pyrrolopyrrole type compound, the binding resin, etc. and an electrical charge transferring layer coating liquid containing the benzidine derivative, the binding resin, etc., applying the coating liquids sequentially to the electroconductive substrate, and drying or setting the applied layers of the coating liquids. In the preparation of the coating liquids mentioned above, various kinds of organic solvents may be selected to comply with the particular kind of binding resin used. The organic solvents useful herein include aliphatic hydrocarbons such as n-hexane, octane, and cyclohexane; aromatic hydrocarbons such as benzene, toluene, and xylene; halogenated hydrocarbons such as dichloromethane, dichloroethane, carbon tetrachloride, and chlorobenzene; ethers such as dimethyl ether, diethyl ether, tetrahydrofuran, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and diethylene glycol dimethyl ether; ketones such as acetone, methylethyl ketone and cyclohexanone; esters such as ethyl acetate and methyl acetate; dimethyl formamide; and dimethyl sulfoxide, for example. These organic solvents may be used either singly or in the form of one or more compounds. Further in the preparation of the coating liquids mentioned above, a surfactant, a leveling agent, etc. may be added for the purpose of enhancing the dispersibility and coatability. The coating liquids can be prepared by the conventional method using a mixing device such as, for example, a mixer, a ball mill, a paint shaker, a sand mill, an attriter, or an ultrasonic dispersing device. The electrophotographic material of the present invention can be obtained by sequentially applying the coating liquids to the electroconductive substrate and thereafter heating the applied layers of the coating liquid to eliminate the solvent. Optionally, for the purpose of enhancing a tigh adhesion between the electroconductive substrate and the photosensitive layer, an undercoat layer may be formed between the electroconductive substrate and the photosensitive layer. In this case, the undercoat layer is formed by applying to a given surface a solution containing a natural or synthetic macromolecular compound in an amount calculated to form a dry film of approximately 0.01 to 1 µm in thickness. For the purpose of enhancing the tight adhesion between the electroconductive substrate and the photosensitive layer, the electroconductive substrate may be treated with a surface treating agent such as, for example, a silane coupling agent or a titanium coupling agent. Then, for the purpose of protecting the photosensitive layer, a surface protecting layer may be formed on the photosensitive layer. The surface protecting layer is formed by preparing a mixed liquid consisting of various binding resins mentioned above or of a binding resin and additives such as a deterioration preventing agent, and applying this mixed liquid to a given surface in an amount calculated to produce a dry layer of 0.1 to 10 µm in thickness. Preferably, this thickness is approximately in the range of 0.2 to 5 µm. The electrophotographic material of the present invention excels in light stability and in sensitivity and has a high surface potential. The electrophotographic material of the present invention, therefore, can be used advantageously in copying machines, laser beam printers, etc. In the following, the present invention will be described more specifically with reference to working examples. Electrophotographic materials comprising a laminate type photosensitive layer were prepared as follows, using various pyrrolopyrrole type compounds and various benzidine derivatives shown in the foregoing table. All indicated parts are by weight. Pyrrolopyrrole type compounds The pyrrolopyrrole type compounds mentioned above are identified by the following symbols in Tables 2 to 4. A : 1,4-Dithioketo-3,6-diphenylpyrrolo[3,4-c]pyrrole B : 1,4-Dithioketo-3,6-di-(4-tolyl)-pyrrolo[3,4-c]pyrrole C : 1,4-Dithioketo-3,6-bis(4-methoxyphenyl)-pyrrolo[3,4-c]pyrrole D : 1,4-Diketo-3,6-diethylpyrrolo[3,4-c]pyrrole E : N,N′-Diethyl-1,4-dithioketo-3,6-di-tert-butyl-pyrrolo[3,4-c]pyrrole F : 1,4-Dithioketo-3,6-distearylpyrrolo[3,4-c]pyrrole G : N,N′-Dimethyl-1,4-dithioketo-3,6-dibenzyl-pyrrolo[3,4-c]pyrrole M : 1,4-Dithioketo-3,6-dinaphthylpyrrolo[3,4-c]pyrrole I : 1,4-Dithioketo-3,6-di-(pyrid-4-yl)-pyrrolo[3,4-c]pyrrole J : N,N′-Diethyl-1,4-dithioketo-3,6-di-(quinol-2-yl)-pyrrolo[3,4-c]pyrrole K : N,N′-Diethyl-1,4-dithioketo-3,6-bis(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole L : 1,4-Dithioketo-3,6-bis[4-(2,2,2-trifluoroethyl)-phenyl]-pyrrolo[3,4-c]pyrrole M : 1,4-Dithioketo-3,6-bis(4-diethylaminophenyl)-pyrrolo[3,4-c]pyrrole N : N,N′-Dimethyl-1,4-dithioketo-3,6-bis(4-hexyloxyphenyl)-pyrrolo[3,4-c]pyrrole C : 1,4-Dithioketo-3,6-bis(4-cyanophenyl)-pyrrolo[3,4-c]pyrrole P : 1,4-Dithioketo-3,6-bis(2-bromophenyl)-pyrrolo[3,4-c]pyrrole Q : N,N′-Diethyl-1,4-dithioketo-3,6-bis(4-dodecylphenyl)-pyrrolo[3,4-c]pyrrole Examples 1 to 22 : An electrical charge generating layer coating liquid consisting of 2 parts of a pyrrolo-pyrrole type compound as indicated above, 1 part of a vinyl chloride-vinyl acetate copolymer, and 10.7 parts of tetrahydrofuran was prepared, applied to an aluminum sheet, and heated at a temperature of 100°C for 30 min to produce an electrical charge generating layer of about 0.5 µm in thickness. Then, an electrical charge transferring layer was formed using a benzidine derivative identified by the compound No. in the preceding table as electrical charge transferring material. Specifically, an electrical charge transferring layer coating liquid was prepared by mixing and dissolving 8 parts of a compound indicated in Tables 2 to 4, 10 parts of a bisphenol Z type polycarbonate, and 90 parts of benzene. The coating liquid was applied to the aforementioned electrical charge generating layer and dried by heating to form an electrical charge transferring layer of about 25 µm in thickness. Thus, an electrophotographic material comprising a laminate type photosensitive layer was obtained. Comparative Example 1 : An electrophotographic material comprising a laminate type photosensitive layer was obtained by following the procedure of Example 1, except that N-ethyl-3-carbazolylaldehyde-N,N-diphenyl hydrazone was used in place of the benzidine derivative. Comparative Example 2 : An electrophotographic material comprising a laminate type photosensitive layer was obtained by following the procedure of Example 2, except that β-type metal-free phthalocyanine "Heliogen® Blue-7800") and 4-styryl-4′-methoxytriphenylamine were used in place of the pyrrolopyrrole type compound and the benzidine derivative. Comparative Example 3 : An electrophotographic material comprising a laminate type photosensitive layer was obtained by following the procedure of Example 3, except that β-type metal-free phthalocyanine BASF "Heliogen® Blue-7800") and 4-(3,5-dimethylstyryl)-4′-methyltriphenylamine were used in place of the pyrrolopyrrole type compound and the benzidine derivative. To test the charging property and the sensitivity, the electrophotographic materials obtained in Examples 1 to 22 and in Comparative Examples 1 to 3 were each negatively charged by exposure to a corona discharge generated at -6.0 kV in an electrostatic test copier. The initial surface potential, Vs.p. (V), of each electrophotographic material was measured and, at the same time, the surface of the electrophotographic material was exposed to the light from a tungsten lamp of 10 lux, with measuring the time required for the surface potential Vs.p. to decrease to 1/2 the initial value for calculating the half-life exposure, E 1/2 (µJ/cm²). The surface potential measured after lapse of 0.15 s following the exposure is indicated as residual potential, V r.p. (V). The results of the test of the electrophotographic materials of Examples 1 to 22 and Comparative Experiments 1 to 3 for charging property and sensitivity are shown in Tables 2 to 4. It is noted from Tables 2 to 4 that the electrophotographic sensitive materials of Comparative Examples 1 to 3 were invariably low in sensitivity and high in residual potential. In contrast, the electrophotographic materials of Examples 1 to 22 were invariably high in sensitivity and low in residual potential. The test has demonstrated that the electrophotographic material of Example 22 particularly excelled in charging property and sensitivity and, at the same time, possessed a very low residual potential. The high sensitivity of the electrophotographic material of Example 22 may be explained by the following reasons (1) to (3). (1) Since the ionization potential (IP) of compound No. 233 (4,4-bis[N-(2,4-dimethylphenyl-N-phenylamino]diphenyl) which is 5.43 eV is smaller that of compound A (1,4-dithioketo-3,6-diphenylpyrrolo[3,4-c]pyrrole) which is 5.46 eV, the injection of holes from compound A into compound No. 233 encounters no energy barrier. As the result, the injection of holes takes place efficiently. (2) Since the difference between the IP of compound A and that of compound No. 233 is as small as 0.03 eV, the possibility of compound A's IP surface level existing between the IP of compound A and that of compound No. 233 is very low even if compound A has an IP surface level (the IP level originating from the irregularity of the molecular configuration on the surface of compound A existing as microcrystals). As the result, holes are quickly injected from compound A into compound No. 233 without being trapped on their way. (3) Since the difference between the IP of compound A and that of compound No. 233 is very slight as mentioned above, the energy (Gibbs free energy difference, ΔG) radiated from compound A during the injection of holes from compound A into compound No. 233 is small. Otherwise, the surrounding binding resin, for example, generates electrical dipoles on exposure to the radiated energy. The electrical dipoles are oriented in the holes (cationic radicals) injected into compound No. 233 and thus help to stabilize the holes. Thus, the possiblity of the intermolecular transfer of holes in compound No. 233 being impeded by the stabilization of holes is extremely low. As described above, the electrophotographic material of the present invention has high sensitivity and low residual potential because the photosensitive layer thereof contains the specific combination of the pyrrolopyrrole type compound of formula (1) and the benzidine derivative of formula (2).

Claims: Electrophotographic material comprising an electroconductive substrate and a photosensitive layer formed on the substrate and comprising (I) an electrical charge generating material which is a pyrrolopyrrole type compound of the general formula (1), wherein are: R¹, R² independently an aryl group, optionally containing a substituent, an aralkyl group, optionally containing a substituent, or a heterocyclic group, and R³, R⁴ independently a hydrogen atom, an alkyl or aryl group, optionally containing a substituent, and (II) an electrical charge transferring material which is an aromatic, nitrogen containing compound, characterized in that the electrical charge transferring material (II) is a benzidine derivative of the general formula (2), wherein are: R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently a hydrogen atom, a lower alkyl group, a lower alkoxy group or a halogen atom, l, m, n and o independently 1, 2 or 3, and p and q independently 1 or 2. The electrophotographic material according to claim 1, characterized in that the photosensitive layer is a single layer which contains the pyrrolopyrrole type compound of formula 1 representing the electrical charge generating material (I) and the benzidine derivative of formula 2 representing the electrical charge transferring material (II) in a binding resin. The electrophotographic material according to claim 1, characterized in that the photosensitive layer is a laminate type layer composed of an electrical charge generating layer which contains the pyrrolopyrrole type compound of formula 1 representing the electrical charge generating material (I), and an electrical charge transferring layer which contains the benzidine derivative of formula 2 representing the electrical charge transferring material (II) in a binding resin, the electrical charge generating layer being provided upon or below the electrical charge transferring layer. The electrophotographic material according to one of claims 1 to 3, characterized by a pyrrolopyrrole type compound of formula 1 wherein R¹ and R² are an aryl or an aralkyl group containing a substituent selected from halogen, lower alkyl containing a halogen atom, a cyano alkyl, alkoxy, and dialkylamino group and/or wherein R³ and R⁴ are a hydrogen atom, a C₁₋₄-alkyl or phenyl group which may contain a substituent selected from a halogen atom, a lower alkyl group containing a halogen atom, an alkyl, alkoxy, alkylthio and nitro group. The electrophotographic material according to one of claims 1 to 4, characterized by a benzidine derivative of formula 2 wherein R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently are a hydrogen atom, a C₁₋₄-alkyl group, a C₁₋₄-alkoxy group or a halogen atom. The electrophotographic material according to one of claims 1 to 5, characterized by a pyrrolopyrrole type compound of formula 1 wherein R¹ and R² are a phenyl group, and a benzidine derivative of formula 2 wherein R³ and R⁴ are a hydrogen atom. The electrophotographic material according to one of claims 1 to 6, characterized by a pyrrolopyrrole type compound of formula 1 wherein R¹ and R² are a phenyl group, and R³ and R⁴ are a hydrogen atom, and/or a benzidine derivative of formula 2 wherein R⁵, R⁷, R⁹ and R¹⁰ are a hydrogen atom, and are 2,4-dimethylphenyl. The electrophotographic material according to one of claims 2 to 7, characterized in that the pyrrolopyrrole type compound of formula 1 is present in an amount of 2 to 20 parts by weight, and preferably of 3 to 15 parts by weight, and the benzidine derivative of formula 2 is present in an amount of 40 to 200 parts by weight, and preferably of 50 to 100 parts by weight, in 100 parts by weight of the binding resin. The electrophotographic material according to one of claims 3 to 7, characterized in that the electrical charge generating layer of the laminate consists of the pyrrolopyrrole type compound of formula 1 or contains the pyrrolopyrrole type compound of formula 1 in an amount of 5 to 500 parts by weight, and preferably of 10 to 250 parts by weight, in 100 parts of the binding resin, and the electrical charge transferring layer contains the benzidine derivative of formula 2 in an amount of 10 to 500 parts by weight, and preferably of 25 to 200 parts by weight, in 100 parts by weight of the binding resin. The electrophotographic material according to one of claims 1 to 9, characterized in that the photosensitive layer further comprises an electrical charge generating material different from that of formula 1 and/or an electrical charge transferring material different from that of formula 2.

Inventors: HANATANI YASUYUKI, NAKATANI KANAME, HANATANI, YASUYUKI, NAKATANI, KANAME
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1988
Main classification: G03G5
IPC codes: G03G5