Unsymmetrical perylene dimers

Abstract: Photoconductive imaging members comprised of unsymmetrical dimeric perylene as a charge generator, wherein said perylene is of the following formula wherein R is hydrogen, alkyl, cycloalkyl, substituted alkyl, aryl, substituted aryl, aralkyl or substituted aralkyl group, and X-Y is an unsymmetrical bridging moiety of alkylene, substituted alkylene, arylene, substituted arylene, aralkylene or substituted aralkylene.

Description: The present invention is directed generally to dimeric perylene pigments suitable for use in photoconductive imaging members. Generally, layered photoresponsive imaging members are described in a number of U.S. patents, such as U.S. Patent 4,265,900, wherein there is illustrated an imaging member comprised of a photogenerating layer, and an aryl amine hole transport layer. Examples of photogenerating layer components include trigonal selenium, metal phthalocyanines, vanadyl phthalocyanines, and metal free phthalocyanines. The selection of selected perylene pigments as photoconductive substances is also known. There is thus described in EP-A-0040402, the use of N,N'-disubstituted perylene-3,4,9,10-tetracarboxyldiimide pigments as photoconductive substances. There are also disclosed in U.S. Patent 3,871,882 photoconductive substances comprised of specific perylene-3,4,9,10-tetracarboxylic acid derivative dyestuffs. In U.S. Patent 4,587,189 there is illustrated a layered imaging member with a nonhalogenated perylene pigment photogenerating component. While prior art perylene-based photoreceptors, or photoconductive imaging members may exhibit desirable xerographic electrical characteristics, most of the bisimides are red to brown in color, and possess, it is believed, relatively poor spectral response, particularly to the 600 to 700 nanometers region of the spectrum. The majority of the bis(imidazo) pigments, especially those with a purple to violet color, have poor spectral response in the blue (400 to 450 nanometers) region of the spectrum. Ideally, a photoconductive pigment used for light lens imaging, particularly for color photocopying, should have a uniform spectral response, that is, be panchromatic throughout the visible spectrum from 400 to 700 nanometers. A need, therefore, exists for imaging members containing photoconductive components with improved xerographic electrical performance including higher charge acceptance, lower dark decay, increased charge generation efficiency and charge injection into the transporting layer, tailored PIDC curve shapes to enable a variety of reprographic applications, reduced residual charge and/or reduced erase energy, improved long term cycling performance, and less variability in performance with environmental changes in temperature and relative humidity. There is also a need for imaging members with photoconductive components comprised of certain photogenerating pigments with enhanced dispersability in polymers and solvents. There is also a need for photogenerating pigments which permit the preparation of coating dispersions, particularly in dipcoating operations, which are colloidally stable and wherein settlement is avoided or minimized, for example little settling for a period of from 20 to 30 days in the absence of stirring. Further, there is a need for photoconductive materials with enhanced dispersability in polymers and solvents that enable low cost coating processes in the manufacture of photoconductive imaging members. Additionally, there is a need for photoconductive materials that enable imaging members with enhanced photosensitivity in the red region of the light spectrum, enabling the resulting imaging members thereof to be selected for imaging by red diode and gas lasers. Furthermore, there is a need for photogenerator pigments with spectral response in the green and blue regions of the spectrum to enable imaging by newly emerging blue and green electronic imaging light sources. A need also exists for improved panchromatic pigments with broad spectral response from 400 to 700 nanometers for color copying using light-lens processes. These and other needs may be accomplished, it is believed, in embodiments of the present invention. It is an object of the present invention to provide unsymmetrical perylene bisimide dimers and imaging members thereof with many of the advantages illustrated herein. According to one aspect of the present invention there are provided unsymmetrical perylene dimers of the formula wherein R is hydrogen, alkyl, cycloalkyl, substituted alkyl, aryl, substituted aryl, aralkyl, substituted aralkyl, and X-Y represents an unsymmetrical bridging moiety such as alkylene, substituted alkylene, arylene, substituted arylene, aralkylene or substituted aralkylene. Alkyl includes linear and branched components with, for example, from 1 to 25, and preferably from 1 to 10 carbon atoms, such as methyl, ethyl, propyl, butyl, pentyl, heptyl, octyl, and decyl. Cycloalkyl includes homologous rings from, for example, cyclopropane to cyclododecane. Substituted alkyl groups contain substituents such as hydroxy, alkoxy, carboxy, cyano, and dialkylamino. Aryl includes components with, for example, from 6 to 24 carbon atoms such as phenyl, naphthyl, biphenyl, and terphenyl. Substituted aryl groups contain, for example, one to five substituents such as alkyl like methyl, or tertiary-butyl, halogen (fluoro, chloro, bromo, and iodo), hydroxy, alkoxy like methoxy, nitro, cyano and dimethylamino. Aralkyl includes components with from 7 to 24 carbon atoms such as benzyl, phenethyl, and fluorenyl. Substituted aralkyl groups can contain the same substituents as the aforementioned aryl groups, and more specifically, for example, methyl, tertiary-butyl, halogen, hydroxy, methoxy, nitro and dialkylamino. Embodiments of the present invention include a process for the preparation of the unsymmetncal perylene bisimide dimers in high yield and high purity, which process comprises the reaction of preferably at least two parts of a perylene monoimido anhydride of the following Formula 2, with an unsymmetrical diamine in a high boiling solvent, such as N-methylpyrrolidine, and washing the resultant product with hot solvents to remove residual starting components and other byproducts. wherein R represents the groups or substituents described in Formula 1. The unsymmetrical perylene dimers illustrated herein can be selected as a photoactive component in photoconductive imaging members used in electrophotographic printing, organic solar cells and, because of the asymmetry induced by the bridging moiety, they are expected to possess non-linear optical properties. Moreover, in embodiments the unsymmetrical dimers can be selected as a colorant in polymeric composite materials such as plastic objects, xerographic toners, and the like. The perylenes of Formula 1 can be selected, it is believed, as a component for solid state devices such as in solar cells, chemical sensors, electroluminescent devices and non-linear optical devices. They can also be used as dispersed colorants for coloration of, for example, plastics. Important embodiments of the present invention include photoconductive imaging members comprised of a supporting substrate, a photogenerating layer comprised of the perylene dimer pigments illustrated herein of Formula 1 and a charge transport layer. Furthermore, the perylene dimer pigments are highly colored and can be prepared with a variety of hues such as orange, red, magenta, maroon, brown, black, greenish black, and the like depending, for example, on the R and X-Y substituents. With the present invention in embodiments, photoconductive imaging members with the perylene dimer pigments obtained by coupling two perylene monomers together via an unsymmetrical bridging group (X-Y in Formula 1) enable a number of advantages with respect, for example, to photoconductive imaging members with monomeric perylene pigments or with symmetrical dimeric perylene pigments. In embodiments, the present invention is directed to photogenerating pigments comprised of unsymmetrical perylene bisimide dimers. Embodiments of the present invention are directed to an imaging member comprised of a supporting substrate, a photogenerating layer comprised of an unsymmetrical perylene dimer of Formula 1 and, more specifically, wherein where R = n-propyl, n-butyl, isobutyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, n-hexyl, n-heptyl and n-octyl and X-Y = 2-methyl-1,4-tetramethylene, 2-methyl-1,5-pentamethylene, toluene-a-4-diyl, ethylbenzene-b-4-diyl, diphenyl ether-3,4'-diyl, and a charge, especially hole, transport layer. More specifically, the unsymmetrical perylene dimers of the present invention are comprised of two different perylene bisimide molecules having the same terminal substituent (R group in Formula 1) wherein the two perylene moieties are distinguishable by being bonded to different ends of the unsymmetrical -X-Y-bridging group. The perylenes of the present invention can be characterized as having no center of symmetry. Imaging members with the photogenerating pigments of the present invention are sensitive to wavelengths of from about 400 to about 650 nanometers, that is in the visible region of the light spectrum. In embodiments thereof, the imaging members of the present invention generally possess broad spectral response to white light or, specifically to red, green and blue light emitting diodes and stable electrical properties over long cycling times. Many of the unsymmetrical perylene bisimide dimers of the present invention when selected as photogenerator pigments. exhibit excellent charge acceptance of about 800 volts surface potential in a layered device, dark decay of less than about 50 volts per second, for example 35 to 45, photosensitivities ranging from E 1/2 of 4x10 -7 to 2x10 -6 J/cm 2 (4 to 20 ergs/centimeter), excellent dispersibility and low solubility in typical coating compositions, such as solutions of certain polymers in organic solvents, such as methylene chloride, selected for the preparation of layered photoresponsive imaging members. Embodiments of the present invention relate to the provision of layered imaging members comprised of a supporting substrate, a photogenerating layer comprised of photogenerating pigments comprised of unsymmetrical perylene bisimide dimers, such as those of Formula 1, and more specifically, wherein R is hydrogen, alkyl, such as methyl, ethyl, n-propyl, n-butyl, isobutyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, n-hexyl, phenyl, benzyl, phenethyl and the like, and X-Y is an unsymmetrical bridging group such as alkylene, arylene, and aralkylene. Unsymmetrical alkylenes, representative formulas of which are illustrated hereinafter, include 1,2-propylene, 1-methyl-1,3-propylene. 1-ethyl-1,3-propylene, 1-methyl-1,4-tetramethylene, 2-methyl-1,4-tetramethylene, 1-methyl-1,5-pentamethylene, 2-methyl-1,5-pentamethylene and higher unsymmetric alkylene groups with up to 20 carbon atoms. Unsymmetric substituted alkylenes include, for example, 3-hydroxy-1,2-propylene, 2-hydroxy-1,4-tetramethylene, 2-methoxy1,4-tetramethylene, 2-carboxy-1,4-tetramethylene and 2-dimethylamino-1,4-tetramethylene. Arylene refers, for example, to unsymmetrically substituted bridging groups such as 2,4-, 2,3'-, 2,4'-, and 3,4'-biphenylene, and 1,3-, 1,6- and 1,7-naphthylene. Substituted arylenes refers, for example, to groups such as 2-chloro-1,4-phenylene, 2-methyl-4,4'-biphenylene, N-phenylbenzamide-3,4'-diyl, diphenylsulfone-3,4'-diyl and diphenylether-3,4'-diyl. Aralkylene includes benzyl-, phenethyl-, phenylpropyl- and fluorenyl- groups in which one perylene bisimide moiety is bonded to the alkyl group and the second is bonded to the 2-, 3- or 4- position of the aromatic ring. Substituted aralkylene refers, for example, to groups of the aforementioned class in which substituents such as methyl, tertiary-butyl, halogen of, for example, fluoro, chloro, bromo, and iodo, hydroxy, methoxy, nitro, cyano and dimethylamino are attached to the aromatic ring. In embodiments, the imaging members of the present invention are comprised of, preferably in the order indicated, a conductive substrate, a photogenerating layer comprising unsymmetrical perylene bisimide dimer pigments dispersed in a resinous binder composition, and a charge transport layer, which comprises charge transporting molecules dispersed in an inactive resinous binder composition. In embodiments, the photoconductive imaging member comprises a conductive substrate, a hole transport layer comprising a hole transport composition, such as an aryl amine, dispersed in an inactive resinous binder composition, and as a top layer a photogenerating layer comprised of unsymmetrical perylene bisimide dimer pigments optionally dispersed in a resinous binder composition; or a conductive substrate, a hole blocking metal oxide layer, an optional adhesive layer, a photogenerating layer comprised of the unsymmetrical perylene bisimide dimer pigment of the present invention, optionally dispersed in a resinous binder composition, and an aryl amine hole transport layer comprising aryl amine hole transport molecules optionally dispersed in a resinous binder. Specific examples of unsymmetrical perylene dimer pigments of the present invention and encompassed by the Formula 1 illustrated herein include those wherein R is hydrogen, methyl, ethyl, n-propyl, isopropyl, 3-methoxypropyl, 3-hydroxypropyl, cyclopropyl, cyclopropylmethyl, n-butyl, isobutyl, secbutyl, cyclobutyl, n-pentyl, 2-pentyl, 3-pentyl, 2-(3-methyl)butyl, 2-methylbutyl, 3-methylbutyl, neopentyl, cyclopentyl, n-hexyl, 2-ethylhexyl, cyclohexyl, n-heptyl, cycloheptyl, n-octyl, cyclooctyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, cyclododecyl, phenyl, benzyl, phenethyl and substituted phenyl, benzyl and phenethyl radicals in which the aromatic nng contains from 1 to 5 substituents inclusive of fluorine, chlorine, bromine, iodine, methyl, hydroxymethyl, trifluoromethyl, tertiary-butyl, tertiary-butoxy, methoxy, trifluoromethoxy, nitro, cyano, dimethylamino, diethylamino, and the like and X-Y represents an unsymmetrical bridging group. Specific examples of photogenerating unsymmetric perylene bisimide dimers of the present invention include those encompassed by Formula 1 wherein R is hydrogen, methyl, ethyl, n-propyl, allyl, 3-methoxypropyl, n-butyl, isobutyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, neopentyl, n-hexyl, n-heptyl, n-octyl, phenyl, benzyl, 3-chlorobenzyl and phenethyl, and X-Y is propane-1,2-diyl, butane-1,2-diyl, butane-1,3-diyl, 2-methylbutane-1,4-diyl, pentane-1,3-diyl, pentane-1,4-diyl, 2-methylpentane-1,5-diyl, toluene-a,4-diyl, ethylbenzene-b,4-diyl and diphenyl ether-3',4'-diyl. The substrate can be formulated entirely of an electrically conductive material, or it can be comprised of an insulating material having an electrically conductive surface. The substrate can be of an effective thickness, generally up to 2.54 mm (100 mils), and preferably from 0.025 to 1.27 mm (1 to 50 mils), although the thickness can be outside of this range. Thus, this layer may be of substantial thickness, for example over 2.54 mm (100 mils), or of minimal thickness. In a preferred embodiment, the thickness of this layer is from 0.025 to 0.25 mm (3 mils to 10 mils). The substrate can be opaque or substantially transparent and can comprise numerous suitable materials having the desired mechanical properties. The entire substrate can comprise the same material as that in the electrically conductive surface, or the electrically conductive surface can merely be a coating on the substrate. Typical electrically conductive materials include copper, brass, nickel, zinc, chromium, stainless steel, conductive plastics and rubbers, aluminum, semitransparent aluminum, steel, cadmium, titanium, silver, gold, paper rendered conductive by the inclusion of a suitable material therein or through conditioning in a humid atmosphere to ensure the presence of sufficient water content to render the material conductive, indium, tin, metal oxides, including tin oxide and indium tin oxide. Generally, the conductive layer ranges in thickness of from 5 nm to many centimeters, although the thickness can be outside of this range. When a flexible electrophotographic imaging member is desired, the thickness typically is from 10 nm to 75 nm. Typical substrate materials include insulating nonconducting materials such as various resins known for this purpose including polycarbonates, polyamides, polyurethanes, paper, glass, plastic, polyesters such as MYLAR® (available from E.I. OuPont) or MELINEX 447® (available from ICI Americas, Inc.). In addition, the substrate can comprise a metallized plastic, such as titanized or aluminized MYLAR® , wherein the metallized surface is in contact with the photogenerating layer or any other layer situated between the substrate and the photogenerating layer. In embodiments, intermediate adhesive layers between the substrate and subsequently applied layers may be desirable to improve adhesion. When such adhesive layers are utilized, they preferably have a dry thickness of from 0.1 micrometer to 5 micrometers, although the thickness can be outside of this range. Typical adhesive layers include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrrolidone, polycarbonate, polyurethane, polymethylmethacrylate, and the like as well as mixtures thereof. Since the surface of the substrate can be a metal oxide layer or an adhesive layer, the expression substrate is intended to also include a metal oxide layer with or without an adhesive layer on a metal oxide layer. Moreover other known layers may be selected for the photoconductive imaging members of the present invention, such as polymer protective overcoats. The photogenerating layer is of an effective thickness, for example, of from 0.05 micrometer to 10 micrometers or more, and in embodiments has a thickness of from 0.1 micrometer to 3 micrometers. The thickness of this layer can be dependent primanly upon the concentration of photogenerating material in the layer, which may generally vary from 5 to 100 percent The 100 percent value generally occurs when the photogenerating layer is prepared by vacuum evaporation of the pigment. When the photogenerating material is present in a binder material, the binder contains, for example, from 25 to 95 percent by weight of the photogenerating material, and preferably contains 60 to 80 percent by weight of the photogenerating material. Typical transport layers are described, for example, in U.S. Patents, 4,265,990; 4,609,605; 4,297,424, 4,921,773, 4,306,008; 4,304,829; 4,233,384; 4,115,116; 4,299,897; 4,081,274, and 5,139,910. Typical diamine hole transport molecules include N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(4-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(2-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(3-ethylphenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(4-ethylphenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(4-n-butylphenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(3-chlorophenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(4-chlorophenyl)-(1,1'-biphenyl)-4,4'-diamine, N,N'-diphenyl-N,N'-bis(phenylmethyl)-(1,1'-biphenyl)-4,4'-diamine, N,N,N',N'-tetraphenyl-[2,2'-dimethyl-1,1'-biphenyl]-4,4'-diamine, N,N,N',N'-tetra-(4-methylphenyl)-[2,2'-dimethyl-1,1'-biphenyl]-4,4'-diamine, N,N'-diphenyl-N,N'-bis(4-methylphenyl)-[2,2'-dimethyl-1,1'-biphenyl]-4,4'-diamine, N,N'-diphenyl-N,N'-bis(2-methylphenyl)-[2,2'-dimethyl-1,1'-biphenyl]-4,4'-diamine, N,N'-diphenyl-N,N'-bis(3-methylphenyt)-[2,2'-dimethyl-1,1'-biphenyl]-4,4'-diamine, and N,N'-diphenyl-N,N'-bis(3-methylphenyl)-pyrenyl-1,6-diamine. In embodiments of the present invention, the preferred hole transport layer, since it enables excellent effective transport of charges, is comprised of aryldiamine components as represented, or essentially represented, by the general formula of, for example, the U.S. patents indicated herein, such as 4,265,990, wherein X, Y and Z are selected from the group consisting of hydrogen, an alkyl group with, for example, from 1 to 25 carbon atoms and a halogen, preferably chlorine, and at least one of X, Y and Z is independently an alkyl group or chlorine. When Y and Z are hydrogen, the compound may be N,N'-diphenyl-N,N'-bis(alkylphenyl)-(1,1'-biphenyl)-4,4'-diamine wherein alkyl is, for example, methyl, ethyl, propyl, n-butyl, or the compound may be N,N'-diphenyl-N,N'-bis(chlorophenyl)-(1,1'-biphenyl)-4,4'-diamine. The charge transport component is present in the charge transport layer in an effective amount, generally from 5 to 90 percent by weight, preferably from 20 to 75 percent by weight, and more preferably from 30 to 60 percent by weight, although the amount can be outside of this range. Examples of the highly insulating and transparent resinous components or inactive binder resinous material for the transport layer include binders such as those described in U.S. Patent 3,121,006. Specific examples of suitable organic resinous materials include polycarbonates, acrylate polymers, vinyl polymers, cellulose polymers, polyesters, polysiloxanes, polyamides, polyurethanes, polystyrenes, and epoxies as well as block, random or alternating copolymers thereof. Preferred electrically inactive binder materials are polycarbonate resins having a molecular weight of from 20,000 to 100,000 with a molecular weight in the range of from 50,000 to 100,000 being particularly preferred. Generally, the resinous binder contains from 5 to 90 percent by weight of the active material corresponding to the foregoing formula, and preferably from 20 percent to 75 percent of this material. Similar binder materials may be selected for the photogenerating layer, including polyesters, polyvinyl butyrals, polyvinylcarbazole, polycarbonates, polyvinyl formals, poly(vinylacetals) and those illustrated in U.S. Patent 3,121,006. The photoconductive imaging member may optionally contain a charge blocking layer situated between the conductive substrate and the photogenerating layer. This layer may comprise metal oxides, such as aluminum oxide and the like, or materials such as silanes and nylons. The primary purpose of this layer is to prevent charge injection from the substrate during and after charging. This layer is of a thickness of less than 5 nm to 10 micrometers, preferably being no more than 2 micrometers. In addition, the photoconductive imaging member may also optionally contain an adhesive interface layer situated between the hole blocking layer and the photogenerating layer. This layer may comprise a polymeric material such as polyester, polyvinyl butyral, polyvinyl pyrrolidone and the like. Typically, this layer is of a thickness of less than about 0.6 micrometer. The unsymmetrical dimers of the present invention can be readily prepared by reaction, or condensation of 2 to 5 equivalents of a perylene monoimide-monoanhydride as illustrated in Formula 2, with one equivalent of an unsymmetrical diamine such as 1,2-diaminopropane, 2-methyl-1,5-diaminopentane, 4-aminobenzylamine, 4-aminophenethylamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminobenzanilide or 3,4'-diaminodiphenylsulfone in an organic solvent, such as chloronaphthalene, trichlorobenzene, decalin, tetralin, aniline, dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone and the like with the optional use of appropriate catalysts, such as zinc acetate or zinc iodide, in an amount equivalent to 1 to 50 mole percent of the perylene. The concentration of reactants in the solvent can range from 50 weight percent combined diamine and anhydride, and 50 percent solvent to 2 percent diamine and anhydride, and 98 percent solvent with a preferred range being from 5 percent diamine and anhydride and 95 percent solvent to 20 percent diamine and anhydride and 80 percent solvent The reactants are stirred in the solvent and heated to a temperature of from 100°C to 300°C, and preferably from 150°C to 205°C for a period of from 10 minutes to 8 hours depending on the rate of the reaction. The mixture is subsequently cooled to a temperature of between 25°C to 175°C, and the solid pigment perylene product is separated from the mother liquors by filtration through, for example, a fine porosity sintered glass filter funnel or a glass fiber filter. The pigment product is then subjected to a number of washing steps using hot and cold solvents such as dimethyl formamide, methanol, water and alcohols. Optionally, the pigment may be washed with dilute hot or cold aqueous base solution such as 5 percent of sodium hydroxide or potassium carbonate which serves to remove by conversion to a water soluble salt any residual starting anhydride and other acidic contaminants. Also, optionally the unsymmetrical dimeric perylene pigment product may also be washed with dilute acid such as 2 percent aqueous hydrochloric acid which serves to remove residual metal salts, such as for example zinc acetate which can be optionally used as a reaction catalyst. Finally, the pigment is dried either at ambient temperature or at temperatures up to 200°C at atmospheric pressure or under vacuum. The yield of product referred to as "as-synthesized pigment", ranges from 50 percent to nearly 100 percent. Also optionally, in situations where the hot, for example 60 to 150°C, solvent, for example DMF. fails to completely remove all the excess starting monoanhydride from the dimer the product can be dispersed in dilute, for example 1 to 5 percent of aqueous potassium hydroxide for period of time of from 1 hour to 24 hours, and preferably from 7 to 20 hours, at room temperature, 25°C to 90°C, which treatment converts the monoimide to a watersoluble, deep purple-colored dipotassium carboxylate salt, followed by filtration and washing the solid with water until the filtrate becomes colorless. The residual starting anhydride in the product can be detected by known spectroscopic methods such as FT-IR and NMR, or by a color spot test in which the product is stirred in dilute, for example about 2 percent of aqueous potassium hydroxide solution with the presence of monoanhydride being indicated by the development of a deep reddish purple color characteristic of the dipotassium salt of the monoimide. The present invention also encompasses a method of generating images with the photoconductive imaging members disclosed herein. The method comprises the steps of generating an electrostatic latent image on a photoconductive imaging member of the present invention, developing the latent image with a toner comprised of resin, pigment like carbon black, and a charge additive, and transferring the developed electrostatic image to a substrate. Optionally, the transferred image can be permanently affixed to the substrate. Development of the image may be achieved by a number of methods, such as cascade, touchdown, powder cloud, magnetic brush, and the like. Transfer of the developed image to a substrate, such as paper, may be by any method, including those making use of a corotron or a biased roll. The fixing step may be performed by means of any suitable method, such as flash fusing, heat fusing, pressure fusing, and vapor fusing. Any substrate selected for xerographic copiers and printers, including digital copiers, may be used as a substrate, such as paper, and transparency material. Specific embodiments of the invention will now be described in detail. These Examples are intended to be illustrative, and the invention is not limited to the materials, conditions, or process parameters set forth in these embodiments. All parts and percentages are by weight unless otherwise indicated. SYNTHESIS EXAMPLES The starting monoanhydride monoimides in the following Examples were prepared by the methods described in U.S. Patent 4,501,906, or by minor adaptations of the processes described therein. The structures of the product dimers described below were mainly established by 1 H and 13 C nuclear magnetic resonance spectrometry in trifluoroacetic acid-containing solvent mixtures. Visible absorption spectra in trifluoroacetic acid-methylene chloride solution were also measured for each product. The bisimide dimers evidence absorbance maxima at 500 and 540 nanometers which is diagnostic for the perylene bisimide chromophore in this solvent system. Trivial names, based on the substituent groups and referring to the perylene bisimide moiety as the imidoperyleneimido group have been used. To avoid or minimize confusion and ambiguity, the compounds are also described in relation to the structures shown in Formula 1. The synthesis Examples that follow are representative of the general synthesis and purification processes selected. SYNTHESIS EXAMPLE 1 Preparation of 1,5-Bis(n-butylimidoperyleneimido)-2-methylpentane (Formula 1, R and R = n-butyl, X-Y = 2-methyl-1,5-pentamethylene): A suspension of mono-n-butylimidoperylene monoanhydride (Formula 2, R = n-butyl; 2.46 grams, 0.0055 mole) in 100 milliliters of N-methylpyrollidinone (NMP) was treated with 0.2905 gram (0.338 milliliter, 0.00250 mole) of 1,5-diamino-2-methylpentane (Dytek A). The resulting mixture was then stirred and was heated to reflux (202°C) for 2.1/2 hours. The resultant thick dark brown reaction mixture was cooled to 150°C then was filtered through a 9 centimeter glass fiber filter, Whatman Grade 934AH, which had been preheated by pouring 100 milliliters of boiling dimethylformamide (DMF) solvent (boiling point 150°C) through it. The resulting solid product was washed in the funnel with 3 X 75 milliliters portions of boiling DMF. The final wash filtrate was a faint pink color. The solid was washed with 25 milliliters of cold DMF then with 2 X 25 milliliters of methanol and was dried at 60°C to provide 2.25 grams (92 percent yield) of dark chocolate brown solid product. A spot test for the presence of starting monoanhydride which was accomplished by stirring 50 milligrams of the above product pigment in 2 milliliters of 2 percent aqueous potassium hydroxide solution for 4 hours at room temperature, 25°C, was negative, there being no sign of the deep red-purple color characteristic of the monoimide dicarboxylate salt. SYNTHESIS EXAMPLE 2 Preparation of 1,5-Bis(n-pentylimidoperyleneimido)-2-methylpentane (Formula 1, R and R = n-pentyl, X-Y = 2-methyl-1,5-pentamethylene): A mixture of 2.54 grams (0.0055 mole) of mono-n-pentylimidoperylene monoanhydride (Formula 2, R = n-pentyl) and Dytek A diamine (0.338 milliliter, 0.00250 mole) in 100 milliliters of NMP was stirred and heated at reflux (202°C) for 2.74 hours, then was cooled to 150°C. The solid product resulting was hot filtered and washed with boiling DMF, cold DMF and methanol as in the above Example 1. Drying at 60°C for 16 hours provided 2.20 grams (88 percent yield) of a brownish red solid product. A spot test for the presence of starting monoimide was negative. SYNTHESIS EXAMPLE 3 Preparation of Bis-1,3-(n-propylimidoperyleneimido)-1-ethylpropane (Formula 1, R and R = n-propyl, X-Y = 1-ethyl-1,3-propylene): A suspension of 4.76 grams (0.011 mole) of monopropylimidoperylene monoanhydride (Formula 2, R = n-propyl) in 300 milliliters of NMP was treated with 0.511 gram ( 0.598 milliliters, 0.005 mole) of 1,3-diaminopentane (Dytek EP). The resulting mixture was stirred at room temperature, 25°C throughout, for 15 minutes, then was heated at reflux for 2. 1/2 hours. The resultant dark reddish brown solution was cooled to room temperature and was filtered. The solid product was washed with DMF until the filtrate became a light orange color. The solid was washed with 2 X 20 milliliters of methanol and then was dried at 60°C to provide 2.6 grams of a brown solid product of bis-1,3-(n-propylimidoperyleneimido)-1-ethylpropane (Formula 1, R = n-propyl, X-Y = 1-ethyl-1,3-propylene). When a sample, 10 grams, of the above obtained product was stirred in a 2 percent aqueous solution of potassium hydroxide, a reddish purple color developed which is indicative of residual starting anhydride in the product. The product was stirred in 200 milliliters of water containing 4 grams of potassium hydroxide for 5 hours at 60°C, the resultant suspension was filtered, and the solid product was washed with 5 X 100 milliliters of water until the filtrate changed from reddish purple to colorless. The solid was then dried at 60°C to provide 1.3 gram (28 percent yield) of the above dark brown solid product. SYNTHESIS EXAMPLE 4 Preparation of a-4-Bis(n-butylimidoperyleneimido)methylbenzene (Formula 1, R and R = n-butyl, X-Y = a-4-tolyl): A mixture of n-butylimidoperylene monoanhydride (Formula 2, R = n-butyl; 2.46 grams, 0.0055 mole) and 4-aminobenzylamine (0.305 gram, 0.00250 mole) and zinc acetate dihydrate (0.44 gram, 0.0020 mole) in 100 milliliters of NMP was stirred at room temperature, 25°C, for 30 minutes. The resulting mixture was then heated at reflux for 45 minutes, was cooled to 155°C, and was then filtered through a preheated glass fiber filter. The solid product was washed with 4 X 30 milliliters portions of boiling DMF, then with 20 milliliters of cold DMF and 2 X 20 milliliters portions of methanol. Drying at 60°C provided 2.20 grams of a black solid product of a-4-bis(n-butylimidoperyleneimido)methylbenzene (Formula 1, R = n-butyl, X-Y = a-4-tolyl), (91 percent yield). A spot test for the presence of starting monoanhydride was negative. SYNTHESIS EXAMPLE 5 Preparation of b-4-Bis(n-pentylimidoperyleneimido)ethylbenzene (Formula 1, R and R = n-pentyl, X-Y = b-4-ethylbenzene): A mixture of n-pentylimidoperylene monoanhydride (Formula 2, R = n-pentyl; 2.54 grams. 0.0055 mole), b-(4-amino)phenethylamine (0.341 gram, 0.329 milliliter, 0.00250 mole) and zinc acetate dihydrate (0.55 gram, 0.0025 mole) in 100 milliliters of NMP was stirred at room temperature for 30 minutes, followed by heating at reflux for 1.75 hours. The resultant reddish brown suspension was cooled to 150°C, filtered and washed as in Example 4. Drying at 60°C provided 2.3 grams (90 percent) of the above reddish brown solid product of b-4-bis(n-pentylimidoperyleneimido)ethylbenzene (Formula 1, R = n-pentyl, X-Y = b-4-ethylbenzene). A spot test for starting monoanhydride reactant was negative. SYNTHESIS EXAMPLE 6 Preparation of 4,4'-Bis(n-propylimidoperyleneimido)benzanilide (Formula 1, R and R = n-propyl, X-Y = benzanilide-4,4'-diyl): Propylimidoperylene monoanhydride (4.76 grams, 0,011 mole), 4,4'-diaminobenzanilide (1.135 grams, 0.0050 mole) and zinc acetate dihydrate (1.1 grams, 0 0050 mole) were stirred and heated to reflux in 200 milliliters of NMP. After 2.33 hours, the mixture was cooled to 150°C and was filtered through an 11 centimeter glass fiber filter (Whatman Grade GF/F) which had been preheated with 50 milliliters of boiling OMF. The solid product was washed in the funnel with 5 X 50 milliliter portions of boiling DMF then with 50 milliliters of cold DMF, and 2 X 25 milliliters of water. The resulting wet cake was stirred in 300 milliliters of 3 percent aqueous hydrochloric acid for 2 hours at 60°C. The resultant dispersion was filtered and the solid product was washed with 4 X 100 milliliter portions of water, then was dried at 60°C to provide 3.9 grams (74 percent) of the above orange solid product A spot test indicated that there was no residual starting monoimide in this product. SYNTHESIS EXAMPLE 7 Preparation of 3,4'-Bis(n-pentylimidoperyleneimido)diphenyl ether (Formula 1, R and R = n-pentyl, X-Y = diphenyl ether-3,4"-diyl): A mixture of n-pentylimidoperylene monoanhydride (Formula 2, R = n-Pentyl; 5.07 grams, 0.011 mole), 3,4'-diaminophenyl ether (1.00 gram, 0.0050 mole) and zinc acetate dihydrate (1.1 grams, 0.005 mole) in 200 milliliters of NMP was stirred and heated to reflux under argon. After 3.33 hours at reflux the dark brown reaction mixture was cooled to 160°C, was filtered and washed with boiling DMF then wate,r and then treated with 3 percent aqueous hydrochloric acid in the same manner as Example 6. The product resulting above was dried at 60°C to provide 4.9 grams (90 percent) to provide as a dull red powder. COMPARATIVE SYNTHESIS EXAMPLE 1 A MONOMERIC PERYLENE BISIMIDE Preparation of N,N'-Bis(n-pentyl)perylene-3,4,9,10-tetracarboxylic diimide : A mixture of 3,4.9,10-perylenetetracarboxylic dianhydride (3.92 grams, 0.010 mole) and n-pentylamine (4.5 grams, 0.060 mole) in 200 milliliters of NMP was stirred and heated to reflux. After 1/2 hour at reflux the resulting dark brown solution was cooled to 150°C and the suspension was filtered through a preheated sintered glass funnel. The solid product resulting was washed in the funnel with 3 X 100 milliliters of boiling DMF then with 50 milliliters of cold DMF and 2 X 25 milliliters of methanol. The solid product was dried at 60°C to provide 2.8 grams (56 percent) of the above product compound as dark brown light fluffy crystals. COMPARATIVE SYNTHESIS EXAMPLE 2 A SYMMETRICAL PERYLENE BISIMIDE DIMER Preparation of 1,6-Bis(n-pentylimidoperyleneimido)hexane (Formula 1, R and R = n-pentyl, X-Y = 1,6-hexamethylene): Pentylimidoperylene monoanhydride (Formula 2, R = n-pentyl; 2.40 grams, 0.0050 mole) and 1,6-diaminohexane (0.232 grams, 0.0020 mole) were stirred in 100 milliliters of NMP for 1 hour at room temperature. The mixture was then heated to reflux (202°C) for 3 hours, was cooled to 150°C, and was then filtered through a preheated 9 centimeter glass fiber filter (Whatman Grade GF/F). The solid product was then washed with 3 X 100 milliliter portions of boiling DMF, then with 50 milliliters of cold DMF, followed by 2 X 25 milliliter portions of 5 percent aqueous potassium hydroxide and finally with 4 X 25 milliliters of water. The product was dried at 60°C to provide 2.0 grams (97 percent) of the above symmetrical dimer product as a dark brown solid. Preparation of Dispersions of Unsymmetrical Perylene Bisimide Dimers in Poly(vinyl acetate): To demonstrate the application of the invention unsymmetrical dimers as dispersed colorants, sample of pigments dispersed in poly(vinyl acetate) were prepared as follows: 0.2 gram of perylene pigment, 8 milliliters of a 1.5 percent W/W solution of poly(vinyl acetate) (M w = 45,000; Polysciences, Inc.) in dichloromethane and 70 grams of 3 mm (1/8 inch) diameter stainless balls were charged into a 30 milliliter glass jar. The jar was sealed and the mixture was milled on a roll mill for 3 to 5 days until the pigment particles were submicrometer, below 1 micrometer in diameter size, and were finely dispersed. Colored films were prepared by coating a clear plastic sheet, such as MYLAR® polyester with the dispersion using a #8 wire-wound rod. The nominal film wet thickness was 20 micrometers, and the dried film was 1 micrometer. The films, comprised of 60 percent of well-dispersed pigment in PVA, had an optical density of 1 and exhibited a variety of colors. The large range of colors available by varying the R and X-Y groups of the unsymmetrical dimers of this invention is illustrated in Table 1, and which table provides the color of films prepared from five representative compounds. Colors of Raw Pigments And PVA-Dispersed Films of Some Unsymmetrical Perylene Bisimide Dimers R X-Y Color of Raw Pigment Color of PVA Dispersion n-propyl a-4-tolyl black brown n-propyl b-4-phenethyl red-brown red n-pentyl 2-methyl-1,5-pentamethylene brown red-brown n-butyl benzanilide-4,4'-diyl red-brown orange n-pentyl diphenyl ether-3,4'-diyl red magenta Xerographic Evaluation of Unsymmetrical Perylene Bisimide Dimers: Photoresponsive imaging members were fabricated with the unsymmetrical perylene dimer pigments obtained by Synthesis Examples 1, 3, 4, 5 and 7, respectively, and from the Comparative Synthesis Examples 1 and 2, representing the monomeric perylene bisimide and the symmetrical bisimide dimer corresponding to Synthesis Example 2. These photoresponsive, or photoconductive imaging members are generally known as dual layer photoreceptors containing a photogenerator layer, and thereover a charge transport layer. The photogenerator layer was prepared from a pigment dispersion as follows: 0.2 gram of the perylene dimer pigment was mixed with 0.05 gram of polyvinylcarbazole (PVK) polymer and 8.1 milliliters of methylene chloride in a 30 milliliter glass bottle containing 70 grams of 3 mm (1/8-inch) stainless steel balls. The bottle was placed on a roller mill and the dispersion was milled for 4 days. Using a film applicator of 0.038 mm (1 5 mil gap), the pigment dispersion was coated to form the photogenerator layer on a titanized MYLAR® substrate of 75 micrometers in thickness, which had a gamma amino propyl triethoxy silane layer, 0.1 micrometer in thickness, thereover, and E.I. DuPont 49,000 polyester adhesive thereon in a thickness of 0.1 micrometer. Thereafter, the photogenerator layer formed was dried in a forced air oven at 135°C for 20 minutes. Photogenerator layers for each device were each overcoated with an amine charge transport layer prepared as follows. A transport layer solution was made by mixing 8.3 grams of MAKROLON™, a polycarbonate resin, 4.4 grams of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine and 82.3 grams of methylene chloride. The solution was coated onto the above photogenerating layer using a film applicator of 0.25 mm (10 mil) gap. The resulting members were dried at 135°C in a forced air oven for 20 minutes. The final dried thickness of transport layer was 20 micrometers. The xerographic electrical properties of each imaging member were then determined by electrostatically charging its surface with a corona discharging device until the surface potential, as measured by a capacitively coupled probe attached to an electrometer, attained an initial value V o . After resting for 0.5 second in the dark, the charged member reached a surface potential of V ddp , dark development potential, and was then exposed to light from a filtered xenon lamp. A reduction in the surface potential to V bg , background potential due to photodischarge effect, was observed. The dark decay in volt/second was calculated as (V o -V ddp )/0.5. The lower the dark decay value, the superior is the ability of the member to retain its charge prior to exposure by light. Similarly, the tower the V ddp , the poorer is the charging behavior of the member. The percent photodischarge was calculated as 100 percent x (V ddp -V bg )/V ddp . The light energy used to photodischarge the imaging member during the exposure step was measured with a light meter. The photosensitivity of the imaging member can be described in terms of E 1/2 , amount of exposure energy in 10 -7 J/cm 2 (erg/cm 2 ) required to achieve 50 percent photodischarge from the dark development potential. The higher the photosensitivity, the smaller the E 1/2 value. High photosensitivity (lower E 1/2 value), lower dark decay and high charging are desired for the improved performance of xerographic imaging members. The following Table 2 summarizes the xerographic electrical results when the exposed light used was at a wavelength of 500 nanometers. Imaging Member No. Perylene Dimer Synthesis Example Dark Decay V/s E ½ 10 7 J/cm 2 (erg/cm 2) 1 1,5-Bis(n-butylimidoperyleneimido)-2-methylpentane 1 27.6 8.0 2 1,5-Bis(n-pentylimidoperyleneimido)-2-methylpentane 2 35.8 4.3 3 a-4-Bis(n-butylimidoperyleneimido) toluene 4 15.8 19 4 b-4-Bis(n-pentylimidoperyleneimido) ethylbenzene 5 31 14 5 3,4'-Bis(n-pentylimidoperyleneimido) diphenyl ether 7 34.6 7.6 6 N,N'-Bis(n-pentyl)perylene-3,4,9,10-tetracarboxylic acid dicarboximide Comparative Synthesis Example 1 1.2 6.7 7 1,6-Bis(n-pentylimidoperyleneimido) hexane Comparative Synthesis Example 2 49.4 8.9 All the imaging members with the invention unsymmetrical photogenerating pigments exhibited acceptable charge acceptance, and most showed low to moderate dark decay ranging from 20 to <50 volts per second, and photosensitivities ranging from excellent (E 1/2 of 4.3 x 10 -7 J.cm 2 (4.3 ergs/cm 2 )) to moderate (E 1/2 of 1.9 x 10 -6 J/cm 2 (19 ergs/cm 2 )) indicating that these unsymmetrical perylene dimers would be very useful for xerographic imaging applications. There does not appear to be an empirical or theoretical correlation between the chemical structures of the perylene pigments and their efficacies as photogenerator pigments for xerographic imaging applications. However, the much higher sensitivity (E 1/2 = 4.3 x 10 -7 J/cm 2 (4.3 ergs/cm 2 )) observed for the unsymmetrical dimer of Example 2, compared to the related n-pentyl monomeric perylene bisimide of Comparative Example 1 (E 1/2 6.7 x 10 -7 J/cm 2 (6.7 ergs/cm 2 )) and the related symmetrical dimer of Comparative Example 2 (E 1/2 = 8.9 x 10 -7 J/cm 2 (8.9 ergs/cm 2 )), which has a 6-carbon symmetrical bridging group instead of the unsymmetrical 6-carbon bridge of Example 2, indicates that the unsymmetrical dimers of this invention possess unique xerographic electrical properties.

Claims: Unsymmetrical perylene dimer of the formula: wherein R is hydrogen, alkyl, cycloalkyl, substituted alkyl, aryl, substituted aryl, aralkyl or substituted aralkyl group, and X-Y is an unsymmetrical bridging moiety, alkylene, substituted alkylene, arylene, substituted arylene, aralkylene or substituted aralkylene. A dimer in accordance with claim 1 wherein R is hydrogen. A dimer in accordance with claim 1 wherein alkyl contains from 1 to 25 carbon atoms, aryl contains from 6 to 24 carbon atoms, and aralkyl contains from 7 to 30 carbon atoms. A dimer in accordance with claim 3 wherein R is methyl, ethyl, n-propyl, 3-methoxypropyl, n-butyl, isobutyl, n-pentyl, 2-pentyl, 3-pentyl, 2-methylbutyl, 3-methylbutyl, neopentyl, n-hexyl, n-heptyl, n-octyl, benzyl, or phenethyl. A dimer in accordance with claim 3 wherein alkyl is methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, 2-methylbutyl, 3-methylbutyl, n-pentyl, 2-pentyl, 3-pentyl, neopentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl or n-decyl; or wherein cycloalkyl is cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl or cyclododecyl, and wherein substituted alkyl is 3-hydroxypropyl, 2-methoxyethyl, 3-methoxypropyl, 3-ethoxypropyl, 4-methoxybutyl, 2-carboxyethyl, 3-carboxybutyl or 3-dimethylaminopropyl; or wherein aryl is phenyl, 2-, 3-, or 4-phenylphenyl, or 1- or 2-naphthyl; or wherein substituted aryl is 2-, 3-, or 4-hydroxyphenyl, 2-, 3-, or 4-methylphenyl, 2-, 3-, or 4-tertiary-butylphenyl, 2-, 3-, or 4-methoxyphenyl, 2-, 3-, or 4-halophenyl, 2-, 3-, or 4-nitrophenyl, 2-, 3-, or 4-cyanophenyl or 2-, 3-, or 4-dimethylaminophenyl; or wherein aralkyl is benzyl, phenethyl or 3-phenylpropyl; or wherein substituted aralkyl is 2-, 3-, or 4- hydroxybenzyl, 2-, 3-, or 4-methylbenzyl, 2-, 3-, or 4-tertiary-butylbenzyl, 2-, 3-, or 4-methoxybenzyl, 2-, 3-, or 4-halobenzyl, 2-, 3-, or 4-nitrobenzyl, 2-, 3-, or 4-cyanophenyl, 2-, 3-, or 4-dimethylaminobenzyl, 2-, 3-, or 4-hydroxyphenethyl, 2-, 3-, or 4-methylphenethyl, 2-, 3-, or 4- tertiary-butylphenethyl, 2-, 3-, or 4-methoxyphenethyl, 2-, 3-, 4-halophenethyl, 2-, 3-, or 4- nitrophenethyl, 2-, 3-, or 4-cyanophenethyl or 2-, 3-, or 4-dimethylaminophenethyl, and wherein halo is chloro, fluoro, iodo, or bromo. A dimer in accordance with any of claims 1 to 5 wherein alkylene contains from 3 to 20 carbon atoms and arylene contains from 6 to 24 carbon atoms. A dimer in accordance with any of claims 1 to 5 wherein alkylkene is 1,2 - propylene, butane-1,2-diyl, butane-1,3-diyl, pentane-1,3-diyl, pentane-1,4-diyl, 2-methylbutane-1,4-diyl, hexane-1,5-diyl, or 2-methylpentane-1,5-diyl; or wherein substituted alkylene is substituted 2-methoxybutane-1,4-diyl, 2-hydroxybutane-1,4-diyl or 2-dimethylaminobutane-1,4-diyl; or wherein arylene is biphenyl-2,3-diyl, biphenyl-2,4'-diyl, biphenyl-1,4-diyl, naphthalene-1,3-diyl, naphthalene-1,6-diyl or naphthalene-1,7-diyl, wherein substituted arylene is 2-fluoro-, 2-chloro-, 2-bromo-, 2-hydroxy-, 2-methyl-, 2-methoxy-, 2-dimethylamino-, 2-cyano-, or 2-nitro-phenyl, 2-fluoro-, 2-chloro-, 2-bromo-, 2-hydroxy-, 2-methyl-, 2-methoxy-, 2-dimethylamino-, 2-cyano-, or 2-nitro-biphenyl or 3-fluoro-, 3-chloro-. 3-bromo-, 3-hydroxy-, 3-methyl-, 3-methoxy-, 3-dimethylamino-, 3-cyano-, or 3-nitro-biphenyl, diphenyl ether-3,4'-diyl, diphenylsulfone-3,4'-diyl or benzanilide-4,4'-diyl; or wherein aralkylene is toluene-a,3-diyl, toluene-a,4-diyl, ethylbenzene-b,4-diyl, propylbenzene-g,4-diyl, fluorene-2,9-diyl or fluorene-3,9-diyl; or wherein substituted aralkylene is 2-fluoro-, 2-chloro-, 2-bromo-, 2-hydroxy-, 2-methyl-, 2-methoxy-, 2-dimethylamino-, 2-cyano- or 2-nitro-toluene-a,4-diyl, 2-fluoro-, 2-chloro-, 2-bromo-, 2-hydroxy-, 2-methyl-, 2-methoxy-, 2-dimethylamino-, 2-cyano- or 2-nitro-ethylbenzene-b,4-diyl, or 3-fluoro-, 3-chloro-, 3-bromo-, 3-hydroxy-, 3-methyl-, 3-methoxy-, 3-dimethylamino-, 3-cyano- or 3-nitro-ethylbenzene-b,4-diyl. A photoconductive imaging member comprised of an unsymmetrical perylene dimer in accordance with any of claims 1 to 7. A photoconductive imaging member in accordance with claim 8 comprised of a supporting substrate and a photogenerator layer comprised of an unsymmetrical perylene dimer. An imaging member in accordance with either of claims 8 or 9 wherein the unsymmetrical perylene dimer is dispersed in a resinous binder in an amount of from 5 percent to 95 percent by weight.

Inventors: ALLEN GEOFFREY C, DUFF JAMES M, HAMER GORDON K, HOR AH-MEE, ALLEN, GEOFFREY C., DUFF, JAMES M., HAMER, GORDON K., HOR, AH-MEE
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1998
Main classification: C09B5
IPC codes: G03G5, C09B3, C07D471, C07D519