Persistent photoconductive compositions
United States Patent 11 1 Williams et al.  
 [73] Assignee: Xerox Corporation, Stamford,  
 Conn.  
 [22] Filed: Dec. 18, 1972 21 Appl. No.: 316,152  
  52 u.s.c1 96/l.6; 96/1 R; 96/l.5 51 1111.01 ..G03g 5/00 [58] Field of Search 96/15, 1.6, 1 R  
 [56] References Cited UNITED STATES PATENTS 11/1972 Yamaguchi ct al 96/l.5 5/1973 Gosselink et al. 6/1973 Contois et al 96/15 Apr. 22, 1975 [57] ABSTRACT Photoeonductive compositions comprising an organic photoconductive material, an activator capable of forming a charge transfer complex with the photoconductive material, and a protonic acid. Imaging members provided with an imaging layer prepared from the above composition are highly light sensitive. requiring only brief exposure times, and exhibit a photoinduced state of elevated conductivity which persists long after exposure to light is terminated. These compositions can be returned to their relatively insulative state by merely subjecting the imaging layer to heat in the dark, thereby erasing this photoinduced image pattern of elevated conductivity.  
 19 Claims, No Drawings PERSISTENT PHOTOCONDUCTIVE COMPOSITIONS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a photoconductive composition. an imaging method. an imaging member and a method of elevating the level of conductivity of a photoconductive material. More specifically. the compositions of this invention exhibit a photoinduced state of elevated conductivity which persists long after exposure to light is terminated. This characteristic. hereinafter also referred to as persistent conductivity. enables the utilization of such materials in imaging systems wherein the conductivity of the photoconductive imaging layer must persist for extended periods of time after imaging of said layer. Such materials are also useful in cyclic imaging systems. since the elevated state of conductivity which persists in these selectively illuminated areas can be readily thermally erased and the imaging layer thus restored to its previous uniformly insulative state.  
 2. Description of the Prior Art The formation and development ofimages on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes. more commonly known as xerography. involves forming a latent electrostatic image on an imaging surface of an imaging member by first uniformly electrostatically charging this imaging surface and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging surface are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed. the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material. known in the art as toner.&#34; This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus render visible the latent image.  
  The developed image can then be read or permanently affixed to the photoconductor where the imaging surface is not to be reused. This latter practice is usually followed with respect to the binder type photoconductive films (e.g. ZnO) where the photoconductive imaging layer is also an integral part of the finished copy.  
  ln so-called plain paper&#34; copying systems. the latent image can be developed on a reusable photoconductive surface or transferred to another surface. such as a sheet of paper. and thereafter developed. When the latent image is developed on a reusable photoconductive surface. it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films. and solvent or thermal fusion of the toner particles to the supportive substrate.  
  in the above plain paper copying system. the materials used in the photoconductive layer should preferably be capable of rapid switching from insulative to conductive to insulative state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulative state prior to the succeeding charging sequence will result in an increase in the dark decay rate of the photoconductor. This phenomenon, commonly referred to in the art as fatigue. has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene. sulfur. selenium and mixtures thereof (US. Pat. No. 2.297.691); selenium being preferred because of its superior photosensitivity.  
  Many materials which persist in their conductivity after illumination can also be satisfactorily used in electrophotography by simple revision of the imaging sequence. In such a revised imaging sequence. the uncharged imaging layer is initially exposed to a light and shadow image and thus rendered persistently conductive in imagewise configuration in these light struck areas. After exposure, the imaged layer is electrostatically charged in the dark whereby an electrostatic charge pattern is formed on the non-conductive areas. This charge pattern can then be developed directly or transferred to another surface for development. Development can be performed by any of the standard tech niques available to the art. Subsequent to transfer of the latent image from the imaging surface, the imaging layer is uniformly illuminated to dissipate any residual charge pattern and then restored to its former insulative state by heating in the dark for a brief interval. The above imaging system is more comprehensively described in US. Pat. No. 3.545.969. which is hereby incorporated by way of reference.  
  Depending upon the level of such persistent conductivity. the imaging layer can be used for short term image storage similar to standard photographic films. Inorganic phosphors. such as zinc cadmium sulfide. have reportedly been used in such an imaging mode: however. due to only short lived persistence. have not received broad commercial acceptance in electrophotography. Other disadvantages frequently encountered in the use of such materials is their relative slow exposure speed and nonerasable photoinduced conductivity making them thus unsuitable for a rapid cyclic imaging process.  
 A number of organic photoconductive materials having persistent photoconductivity have also been disclosed in the patent literature (US. Pat. No. 3.1 H.022); however, these materials reportedly suffer many of the same inadequacies encountered in the use of the previously discussed inorganic compositions. A relatively recent reference (US. Pat. No. 3512966) reportedly discloses thermally erasable persistently conductive organic compositions suitable for use in electrophotography. This composition is prepared from a dispersion of a polymer. such as poly-N- vinylcarbazole. a dye and an activator selected from a group consisting essentially of specific carboxylic acids; carboxylic acid anhydrides; nitrophenols; and nitroanilines.  
  Although the above organic composition purportedly resolves many of the deficiencies heretofore present in the materials previously discussed. it still does not possess the speed and level of persistent conductivity requisite for use in an electrophotographic device where the recorded image is to be stored for periods of up to 24 hours prior to development.  
  It is. therefore. an object of this invention to remove this as well as other related deficiencies in the prior art.  
  A more specific object of this invention is to provide a novel photoconductive composition.  
  Still another of the objects of this invention is to provide a photoconductive composition capable of retention of a recorded image for extended periods of time.  
  A further object of this invention is to provide a photoconductive composition having both the sensitivity and image retention capacity to be suitable for use in an electrophotographic recording device.  
  A still further object of this invention includes the use of this photoconductive composition in an imaging method and a method of elevating the level of conductivity in organic photoconductive compositions.  
 SUMMARY OF THE INVENTION The foregoing and related objects are accomplished by providing a photoconductive composition suitable for use in electrophotographic imaging which comprises an organic photoconductive material. an activator capable of formation of a charge transfer complex with said organic photoconductive material and a protonic acid. The acid component of the composition must be present in sufficient concentration relative to the activator to enhance the stability of a complex formed during illumination between the anion radical form of the activator and a proton. The stability of this protonated activator radical is believed determinative of the degree and duration of the photoinduced state of elevated conductivity of the photoconductive composition which persists subsequent to selective illumination. This photoconductive composition can be used as either a rapidly switching or persistent imaging layer depending upon the degree of exposure given to it and the acidity of the environment of the charge transfer complex. The preferred compositions of this invention have a strong tendency toward this elevated state of conductivity and can be restored to a relatively insulative condition by subjecting the imaged composition to heat, thereby erasing this conductive image pattern.  
  The invention also embraces electrostatographic imaging methods employing the above photoconductive composition. an imaging member comprising an imaging layer of the above composition, and a method of enhancing the tendency of complexing species within such photoconductive compositions toward this elevated state of photoinduced conductivity.  
 DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS Organic photoconductive electron donor materials which can be used in preparation of the photoconductive compositions of the present invention include what can be termed small molecule photoconductors dispersed in an inert cohesive matrix and any of a number of the polymeric photoconductive materials.  
  These so-called small molecule photoconductive materials include the following: oxadiazoles; e.g., 2,5- bis[4-diethylaminophenyl]-l,3.4-oxadiazole. 2,5-bis- [4&#39;-(n-propylamino)&#39;2&#39;-chlorophenyl-( l l 3.4- oxadiazole. 2,5-bis-[4-N-ethyl-N-n-propylaminophenyl-( l l ,3,4-oxadiazole. 2.5-bis-[4-dimethylaminophenyll-l,3,4-oxadiazole; triazoles, e.g., l-methyl-2,5- bis-[4-diethylaminophenyl]- l ,3,4-triazole; imidazoles. e.g., 2-( 4&#39;-dimethylaminophenyl )-6-methoxybenzimidazole; oxazoles. e.g. 2-(4&#39;-chlorophenyl)- phenanthreno-(9-l0:4 5)-oxazole; thiazoles, e.g., 2- (4&#39;-diethylaminophenyl)-benzthiazole; thiophenes. e.g.  
 2.3 ,S-triphenylthiophene; triazines, e.g. 3-( 4 aminophenyl)-5.6-dipyridyl-(2&#39;) l,2,4-triazine, 3-(4&#39;- dimethylaminophenyl )-5 ,6-di( 4&#39;-phenoxyphenyl l 2,4-triazine; hydrazones, e.g. 4- dimethylaminobenzaldehyde isonicotinic acid hydrazone; styryl compounds. e.g. 2-(4- dimethylaminostyryl)-6-methyl-4-pyridone, 2-(4&#39;- dimethylaminostyryl)-5-(or 6)-amino-benzimidazole, bis (4-dimethylamin0styryl) ketone; azomethines, e.g. 4-dimethylaminobenzylidene-B-naphthylamine; acylhydrazones, e.g. 4-dimethylaminobenzylidenebenzhydrazine. 4-dimethylaminobenzylidene-4- hydroxybenzoic hydrazide. 4-dimethylaminobenzylidene-2 -aminobenzoic hydrazide, 4- dimethylaminobenzylidene-4-methoxybenzoic hydrazide. 4-dimethylaminobenzylidene-iso-nicotinic hydrazide, 4-dimethyIaminobenzylidene-2-methylbenzoic hydrazide; pyrazolines. e.g. l,3,S-triphenylpyrazoline,  
 l.3:diphenyl-S-[4-methoxy-phenyl]-pyrazoline. 1,3- diphenyl-5 4&#39;-dimethylaminophenyl ]pyrazoline; 1.5- diphenyl-3-styrylpyrazoline; l-phenyl-3 4 dimethylaminostyryl ]5-[ 4-dimethylaminophenyl pyrazoline; imidazolones. e.g. 4-[4&#39;-dimethylaminophenyl]-5-phenylimidazolone. 4-furfuryl-5- phenylimidazolone; imidazolethiones, e.g. 4-[4- dimethylaminophenyl]-5-phenylimidazolethione. 3,4,- 5-tetraphenylimidazolethione; l,3,5-triphenyl-4-[4&#39;- dimethylaminophenyl limidazolethione; [.3 .4- triphenyl-S-furfurylimidazolethione; benzimidazoles, e.g. 2-[4&#39;-dimethylaminophenyl]-benzimidazole. lmethyl-2-l4&#39;-dimethylaminophenyll-benzimidazole. I-phenyI-Z-l 4-dimethylaminophenyl]-benzinidazole: benzoxazoles. e.g. 2-[4&#39;-dimethylaminophenyl]- benzoxazole; and benzothiazoles. e.g. 2-[4&#39;- dimethylaminophenyl]-benzothiazole.  
  Materials which can be effectively used to provide the inert cohesive matrix for dispersion of the above small molecule photoconductors are polymers having fairly high dielectric strength and which are good electrically insulating film forming vehicles. Typical of such inert polymer matrices are: styrenebutadiene copolymers; silicone resins, styrene-alkyd resins; soyaalkyd resins; polyvinyl chloride; polyvinylidene chloride; vinylidene chloride-acrylonitrile copolymers; polyvinyl acetate; vinyl acetate-vinyl chloride copolymers; polyvinyl acetals. such as polyvinyl formal; polyacrylic and methacrylic esters, such as polymethyl methacrylate. poly-n-butyl methacrylate, polyisobutyl methacrylate: polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as polyethylene-alkaryloxyalkylene terephthalate; phenolformaldehyde resins; ketone resins; polyamide; and polycarbonates. Methods of making resins of this type have been described in the prior art. for example, styrenealkyd resins can be prepared according to the method described in US. Pat. ,Nos. 2,361,019 and 2,258,423.  
 Typical polymeric photoconductive materials suitable for use in preparation of such photoconductive compositions include: poly-N-acrylylphenothiazine, poly-N-(B-acrylyloxyethyl )-phenothiazine. poly-N-( 2- acrylyloxy propyI)-phenothiazine, polyallylcarbazole, poIy-N-( 2-acrylyloxy-Z-methyI-N-ethyl) carbazole. poly-N-( 2-p-vinylbenzoyl-ethyl )-carbazole, poly-N- propenylcarbazole. poly-N-vinyl-carbazole, poly-N-2- meth-acrylyloxypropyl carbazole, poly-N-acrylylcarbazole. poly-(N-ethyl-3-vinylcarbazole), poly-4- vinyl&#39;p-(-N-carba2yl)-toluene, poly (vinylanisal acetophenone), poly(vinylpyrene) and polyidenes.  
  lf desired, the monomers of the polymeric photoconductors can be copolymerized with each other or with other monomers, such as vinyl acetate. methylacrylate. vinylcinnamate, polystyrene. Z-vinylpyridine.  
  The photoresponsiveness of the above photoconductive materials are enhanced with respect to speed and spectral response by the addition thereto of any of a number of standard activators (electron acceptors) and, optionally, any one of a number of dyestuff sensitizersv The quantity of activator in the photoconductive compositions will vary depending upon the level of enhancement of conductivity desired and the effect such inclusions have on the physical properties of the composition. Generally, the amount of activator present in the photoconductive composition will range from about 0.1 to 50.0 weight percent based upon the weight of the photoconductive material, with l-6 weight percent ordinarily being preferred. The quantity of dyestuff sensitizer that can be optionally added to the com position is similarly limited. Representative of activators which can be added to these compositions include nitrobenzene, m-dinitrobenzene; o-dinitrobenzene; pdinitrobenzene; l-nitro-napthalene; Z-nitronapthalene; 2,5-dinitrophenapthrenequinone; 2,7-dinitrophenapthrenequinone; 3,o-dinitrophenapthrenequinone; 2,4 dinitrofluorene-A -malononitrile; 2,5 dinitrofluorene-A -rnalononitrile; 2,6 dinitrofluorene- A -malononitrile: 2,7 dinitrofluorene-N malononitrile; 3,6 dinitrofluorene-A -malononitrile; 2,4,7 trinitrofluorene-A -malononitrile; 2,4,5] tetronitrofluorene-N&#39; -malononitrile; 2,4- dinitrofluorenone; 2,5-dinitrofluorenone; 2,6- dinitrofluorenone; 2,7-dinitrofluorenone; and 2,4,7-trinitro-9fluorenone. Especially preferred activators of the type described above are the nitroaromatics. Examples of dyestuff sensitizers suitable for incorporation in the photoconductive compositions of this invention are the triarylmethane dyestuffs such as Malachite Green, Brilliant Green, Victoria lBlue B. Methyl Violet, rystal Violet, Acid Violet 6B; xanthene dyestuffs, namely rhodamines, such as Rhodamine B, Rhodamine 6G. Rhodamine G Extra, and Fast Acid Eosin G, as also phthaleins such as Eosin S, Eosin A, Erythrosin, Phloxin, Rose Bengal, and Fluorescein; thiazine dyestuffs such as Methylene Blue; acridine dyestuffs such as Acridine Yellow, Acridine Orange and Trypafiavine; and cyanine dyestuffs such as Pinacyanol, Cryptocyanine and fyanine.  
  The protonic acids which can be used in extending the conductivity of the photoconductive compositions of this invention can be any proton donor having an aqueous dissociation constant of 10&#34; and preferably greater. The upper concentration of acid relative to the photoconductive material is only limited by the solubility of such material in the photoconductive composition. Good results have been obtained utilizing as little as about 0.004 weight percent acid based upon the combined weight of the essential components of the photoconductive composition. In the preferred embodiments of this invention the acid concentration will range from about 0.1-4 weight percent.  
  Typical of the acids which can be used in the photoconductive compositions of this invention are: napthalinesulfonic acid, benzosulfonic acid, oaminobenzosulfonic acid, p-arninobenzosulfonic acid, m-arninobenzosulfonic acid, iodoacetic acid, bromoacetic acid, dichloroacetic acid, trichloroacetic acid, dichloroacetylacetic acid, dimethylmalonic acid, dinicotinic acid, fluorobenzoic acid, o-hydroxybenzoic acid, lutidinic acid, maleic acid, malonic acid, oxalic acid. quinolinic acid ct-tartaric acid, phosphoric acid and sulfurous acid.  
  Both the essential and optional ingredients used in preparation of the herein disclosed photoconductive compositions are presently commercially available or can be prepared by wellknown chemical synthesis.  
  The photoconductive compositions of this invention can be prepared by dispersal of the above ingredients in their appropriate proportion in a suitable dispersal medium, forming a film of the dispersal on a conductive substrate and thereafter evaporation of the dispersant. The liquid dispersal can be applied to the conductive substrate by any of a number of standard coating techniques. Film thickness is controlled&#39;by either adjustment of the viscosity of the dispersal or by mechanical means or both. The films thus produced form a substantially uniform, continuous and adherent coating on the conductive substrate. Ordinarily, an average film thickness of about 5 to about 50 microns will provide the conductive substrate with any imaging layer of the requisite insulative and photodischarge characteristics to be suitable for imaging in either xerographic or persistent imaging modes.  
  Liquid dispersal media suitable for use in preparation of coatings of these photoconductive compositions include benzene; toluene; acetone; Z-butanone; Chlorinated hydrocarbons, e.g., methylene chloride. ethylene ethers, e.g, tetrahydrofuran, and mixtures thereof.  
  The substrate material bearing the above photoconductive film can be virtually almost any conductive. self-supporting material. Examples of such supporting materials include conductive paper; metals, e.g., copper, aluminum, Zinc. tin. iron and lead; polyethylene terephthalate having a thin overcoating of aluminum and copper; and NESA glass. Under certain conditions, injection of carriers from the substrate into the overlying film will occur. This can be prevented by the interfacing of an insulative barrier layer between the photoconductive film and the substrate. The resistivity of this interfacial barrier should be about 1 to l0 megohms per square. Materials which are suitable in providing such a charge injection barrier include any of the traditionally used metal oxides and insulating polymeric resins.  
  Once the organic photoconductive composition is operatively associated with a conductive substrate, the resultant imaging member is ready for use in an electrostatographic imaging system. This imaging member can be used in the traditional xerographic mode of imaging where the imaging layer is charged and then exposed or a persistent imaging mode where exposure preceeds charging. in both types of imaging situations, the light struck areas of the imaging layer undergo photoinduced elevation of the level of conductivity in the illuminated areas, this conductivity persisting for an extended period subsequent to illumination. Where the imaging member is to be reused relatively soon after transfer of the latent or developed image from its imaging layer, the conductive image pattern in this layer must be erased, that is, the conductive areas restored to their relatively insulative state, prior to re-exposure to another light pattern. Erasure can be accomplished by first uniformly illuminating this imaging layer thus rendering it conductive in the previously nonilluminated areas and then heating said layer in the dark to a temperature in the range of about 50 to about 150C for an interval sufficient to restore this layer to its former insulative state. The mechanics for achieving such erasure can comprise contacting the surface of the conductive imaging layer with a heated plate for the requisite interval or passing a heated roller over the surface of the imaging layer at a constant linear velocity: both plate and roller being within the prescribed temperature range.  
  The photoinduced processes associated with the light induced thermally erasable conductivity of the persistent photoconductive compositions prepared from the above ingredients were analyzed by standard electron spin resonance (esr) techniques. A photoinduced thermally erasable esr signal was observed in these materials. The intensity of this esr signal closely parallels the level of electrical conductivity of these photoconductive materials and varies with the temperature. duration and intensity of the light source. This esr signal is believed attributable to the reversible formation of a protonated anion radical of the activator and the electrical conductivity to the simultaneous formation of the mobile positive charge on the cation radical of the photoconductive material.  
  The Examples which follow further define. describe and illustrate preparation of a representative number of specific photoconductive compositions having the hereinbefore described physical properties. lmaging techniques and apparatus employed in such Examples, where not explicitly set forth, are presumed to be standard or as hereinbefore described. Example Vlll has been included to provide a basis for comparison of a dyestuff sensitized photoconductive composition of a type previously disclosed in the literature with those compositions embraced within the scope of this invention.  
 EXAMPLE I A photoconductive composition of the present invention is prepared from poly (N-vinylcarbazole), odinitrobenzene and trichloroacetic acid in the following manner: Ten grams of poly (N-vinylcarbazole) (molecular weight-300,000) are reprecipitated twice from a mixture containing equal parts of tetrahydrofuran (THF) and methanol for removal of impurities and the recovered polymer solids are then dissolved in sufficient THF to form a solution containing 15 weight percent of the polymer. o-dinitrobenzene is similarly purified by recrystallization from methanol and water. The o-dinitrobenzene and trichloroacetic acid (anhydrous solid) are then added to the polymer solution in sufficient quantities such that the relative weight ratio of the three components in solution is approximately 24 parts polymer: parts activator: 0.5 parts acid. Once thoroughly mixed. the resulting solution is cast on an aluminum plate with the assistance of a doctor blade having a wet gap setting of 0.005 inches. The cured photoconductive film has an average thickness of about 10 microns. After the photoconductive composition is sufficiently cured, it is evaluated for use in a persistent imaging mode. This procedure involves initially heating the film to 100C for 10 seconds in the dark, selectively masking the photoconductive surface. exposing the unmasked areas of the photoconductive surface with a I watt high intensity lamp (GE photoglood Model BBA) from a distance of 12 centimeters for 5 minutes thus forming a persistently conductive image pattern within the photoconductive layer. A dielectric sheet is then placed over the masked photoconductive layer and corona charged to a positive potential of 1100 volts. The dielectric sheet is then grounded. peeled from the masked photoconductive surface and developed with Xerox 813 toner (a thermoplastic styrene/nbutyl methacrylate copolymer containing a carbon black pigment). Ten additional copies are prepared from this persistently conductive imaging member in the manner described above without any reexposure of the masked photoconductive surface. Copy quality with respect to image intensity and resolution remain substantially unchanged from first through eleventh copy. Elapsed time for preparation of from the first to the eleventh copy is approximately 15 minutes.  
 EXAMPLE II The procedure of Example I is repeated except that the dielectric sheet placed over the masked photoconductive layer is corona charged to a positive potential of 1450 volts. The dielectric sheet is peeled from the masked photoconductive surface and developed as described in Example 1. Copy quality with respect to image intensity and resolution remains substantially unchanged from first through eleventh copy. Elapsed time for preparation of from the first to the eleventh copy is approximately 15 minutes.  
 EXAMPLE Ill The procedure of Example I is repeated except that the dielectric sheet placed over the masked photoconductive layer is corona charged to a positive potential of 2000 volts. The dielectric sheet is peeled from the masked photoconductive surface and developed as described in Example I. Copy quality with respect to image intensity and resolution remains substantially unchanged from first through eleventh copy. Elapsed time for preparation of from the first to the eleventh copy is approximately 15 minutes.  
  Comparison of copy quality of Examples l-lIl indicates that image intensity varies directly with the potential generated by the corona discharge and, thus. the copies prepared in Example I&#34; proved to be superior.  
 EXAMPLE lV-Vl The procedure of Example I is repeated except for variation in the relative weight ratio of trichloroacetic acid to the polymer and activator.  
 Example No. Polymer: Activator: Acid IV 24 l ().()l  
 \&#39;l 24 l H) With respect to the powdered photoconductive samples. they are initially spread out on a thin layer on a glass plate. heated in the dark at 100C for I minute. allowed to cool to room temperature (-23C). and then illuminated with a 150 watt high intensity light source (GE Photoflood Model BBA) from a distance of l2 centimeters for a 5 minute period. Immediately subsequent to illumination. the powdered sample is packed into an esr tube having an outside diameter of 4 millimeters, the tube inserted into the dual cavity of a Varian X-band esr spectrometer operating at a modulation frequency of 6 KHz, and the sample esr spectrum recorded.  
  The following table shows signal intensity and the level of persistent conductivity for each of the three samples tested.  
 TABLE I Example Relative esr Level of Persistent No. Signal Intensity Conductivity in (ohmsf&#39; l\&#39; l 2.5 X 10&#34; It thus appears that both the intensity of the esr signal and level of the persistent conductivity are proportional to the square root of the acid concentration, and the level of persistent conductivity and esr signal intensity proportional to one another.  
 EXAMPLE V&#34; EXAMPLE Vlll In order to evaluate the effect that dye sensitization has on the electrical properties of the photoconductive composition of this invention, two polymeric solutions are prepared. one from the composition of Example I and a second from the composition of Example I modified by the addition of 1 weight percent Malachite Green oxalate dye. Each film is cast on a conductive glass substrate (NESA Glass. Corning Glass. Corning. N.Y.). After curing of each of the films. a gold electrode is evaporated on a portion of the surface of each film and connected by means of a gold wire, anchored by a silver paste, to the ammeter which in turn is connected to the conductive substrate.  
  The photoresponsiveness of the dye sensitized and unsensitized photoconductive films are evaluated under identical conditions. In the initial phase of this evaluation. the films are heated in the dark to C for 1 minute and then allowed to cool to room temperature (-23C). Each sample is then separately illuminated by a watt high intensity light source (GE Photoflood Model BBA). at a distance of 12 centimeters for 5 minutes. allowed to remain in a light tight enclosure for 5 minutes. and then restored to the level of conductivity formerly prevailing in the dark. This cycle of exposure. resting and erasure are repeated an additional 5 times. The conductivity in these films is continuously monitored before, during. and subsequent to each phase of this cycle.  
  The dye sensitized sample exhibits substantially enhanced conductivity in comparison to that of the unsensitized film during illumination. however. subsequent to illumination and after erasure the differences in the conductivity persisting in each of these films are functionally insignificant. Apparently. dye sensitization of the photoconductive composition of this invention does not result in appreciably more efficient utilization of incident light in the elevation of the level of conductivity of these materials; however. such dyestuff sensitizers do extend the range of spectral response of these photoconductive films thus accounting for greater response during illumination.  
 EXAMPLE IX A photoconductive composition is prepared accord ing to the procedures of Example Vlll from poly (N- vinylcarbazole). 2.4,7-trinitro-9-fluorenone (TNF). and trichloroacetic acid; in the relative weight ratio of 24 parts polymerzl part activator: 1 part acid.  
  The photoresponsiveness of this composition is evaluated in the manner described in Examples lV-Vl (esr spectra) and Example Vlll (magnitude of photoinduced conductivityy The intensity of the esr signal is larger by a factor of three than the signal generated by illumination of the composition of Example VI; however. the dark decay rate with respect to the level of conductivity of the composition containing TNF is greater by a factor of 2 to 3 than the composition containing o-dinitrobenzene. Thus it appears that although higher levels of conductivity are more easily generated in the presence of TNF. that compositions containing odinitrobenzene are superior in terms of photoinduced conductivity persisting subsequent to irradiation.  
 EXAMPLE X XIX The following compositions are prepared in accordance with the procedures of Example l. The relative weight ratio of ingredients in each composition is 24 parts polymerzl part activatorzl part acid.  
 carbazole) Continued Ex. No. Polymer Activator Acid XV poly(N-ethyl-3-vinyl- TNF maleic acid carbazole) XVI poly(vinylpyrene) o-dinitrobenzene trichloroacetic acid XVll poly(vinylpyrene) TNF trichloroacetic acid XVlll poly(vinylpyrenc) o-dinitrobenzcne maleic acid XlX poly(v1nylpyrene) TNF maleic acid All of the photoconductive films prepared from the above compositions are useful in both xerographic and persistent modes of imaging and upon selective illumination exhibit a photoinduced state of elevated conductivity in these light struck areas which persists long after illumination ceases.  
 What is claimed is:  
  1. An electrostatographic imaging method which comprises the steps of:  
 a. providing an imaging member having a photoconductive imaging layer comprising an organic photoconductive material, an activator capable of formation of a charge transfer complex with said photoconductive material. and a protonic acid. said acid being present in sufficient concentration in relation to the activator to enhance the stability of a complex formed during illumination between the anion radical form of the activator and a proton and thus extend the elevated level of conductivity of the photoconductive imaging layer subsequent to the illumination thereof;  
 b. illuminating the imaging layer in image configuration for an interval sufficient to generate a photoinduced state of elevated conductivity within said imaging surface in the light struck areas which persists subsequent to illumination;  
 c. forming a latent electrostatic image on the imaging layer by charging said layer in the dark;  
  2. The electrostatographic imaging method of claim 1, wherein the latent electrostatic image is rendered visible by development with colored electroscopic toner particles.  
  3. The electrostatographic imaging method of claim 1. wherein the latent electrostatic image is transferred from the surface of the imaging layer to a transfer sheet and then developed.  
  4. The electrostatographic imaging method of claim 1, wherein the imaging layer is restored to its relatively insulative state by the additional steps of d. dissipating the latent electrostatic image by uniformly illuminating the imaging layer; and  
  e. subjecting the imaging layer to a temperature of about 50 to about l50C for an interval sufficient to restore it to its former insulative condition.  
  5. The electrostatographic imaging method of claim 4 wherein steps (a) through (e) are repeated in sequence at least one additional time.  
  6. The electrostatographic imaging method of claim 1, wherein the photoconductive imaging layer contains a l:l weight ratio of activator to protonic acid.  
  7. The electrostatographic imaging method of claim 1 wherein from photoconductive imaging layer contains structural units form the monomer, N-vinylcarbazole.  
  8. The electrostatographic imaging method of claim 1, wherein the activator is o-dinitrobenzene.  
  9. The electrostatographic imaging method of claim 1 wherein the activator is 2,4,7-trinitro-9-fluorenone.  
  10. An electrostatographic imaging method which comprises the steps of:  
 a. providing an imaging member having a photoconductive imaging layer comprising an organic photoconductive material. an activator capable of formation of a charge transfer complex with said photoconductive material and a protonic acid selected from the group consisting of napthalenesulfonic acid. benzosulfonic acid, o-aminobenzosulfonic acid. p-aminobenzosulfonic acid. m-aminobenzosulfonic acid. iodoacetic acid. bromoacetic acid. dichloroacetic acid, trichloroacetic acid. dichloroacetylacetic acid, dimethylmalonic acid. dinicotinic acid, fluorobenzoic acid. 0- hydroxybenzoic acid, lutidinic acid, maleic acid. malonic acid, oxalic acid. quinolinic acid a-tartaric acid. phosphoric acid and sulfurous acid, said acid being present in sufficient concentration in relation to the activator to enhance the stability of a complex formed during illumination between the anion radical form of the activator and a proton and thus extend the elevated level of conductivity of the photoconductive imaging layer in the illuminated areas subsequent to the illumination thereof;  
 b. illuminating the imaging layer in imagewise configuration for an interval sufficient to generate a photoinduced state of elevated conductivity within said imaging layer in the illuminated areas. the level of illumination being sufficient to cause said photoinduced state of elevated conductivity to persist subsequent to such illumination; and  
 c. forming a latent electrostatic image on the imaging layer by charging said layer in the dark.  
  11. The electrostatic imaging method of claim 10, wherein the photoconductive imaging layer contains from about 0.1 to about 4 weight percent protonjc acid.  
  12. The electrostatographic imaging method of claim 10, wherein the latent electrostatic image is rendered visible by development with colored electroscopic toner particles.  
  13. The electrostatographic imaging method of claim l0, wherein the latent electrostatic image is transferred from the surface of the imaging layer to a transfer sheet and then developed.  
  14. The electrostatographic imaging method of claim 10, wherein the imaging layer is restored to. its relatively insulative state by the additional steps of d. dissipating the latent electrostatic image by uniformly illuminating the imaging layer; and  
 e. subjecting the imaging layer to a temperature of about 50 to about l50C for an interval sufficient to restore it to its former insulating condition.  
  15. The electrostatographic imaging method of claim 14, wherein steps (a) through (e) are repeated in sequence at least one additional time.  
  16. The electrostatographic imaging method of claim 10, wherein the photoconductive imaging layer contains a 1:] weight ratio of activator to protonic acid.  
  17. The electrostatographic imaging method of claim 10, wherein the photoconductive imaging layer contains structural units from the monomer. N- vinylcarbazole.  
  18. The electrostatographic imaging method of claim 10, wherein the activator is o-dinitrobenzene.  
 19. The electrostatographic imaging method of claim 10, wherein the activator is 2,4.7-trinitro-9-fluorenone.