US3684502A

US3684502A - Photoconductive co-crystalline complex of pyrylium dye and polymer used in electrophotography

Info

Publication number: US3684502A
Application number: US90778A
Authority: US
Inventors: Eugene P Gramza; David D Schreiber
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1970-11-18
Filing date: 1970-11-18
Publication date: 1972-08-15
Anticipated expiration: 1989-08-15
Also published as: DE2155905A1; GB1373638A; FR2114838A5; JPS5612867B1; DE2155905B2; CA968942A; AU456072B2; BE775523A; DE2155905C3; AU3581271A; IT940689B

Abstract

A METHOD IS PROVIDED FOR FORMING A CO-CRYSTALLINE COMPLEX OF PYRYLIUM DYE AND POLYMER WHICH INVOLVES DISSOLVING THE COMPONENTS IN A SOLVENT SYSTEM IN WHICH THE SOLUBILITIES OF THE COMPONENTS ARE SUB-STANTIALLY EQUAL. A NONPOLAR PRECIPITATING LIQUID IN WHICH THE COMPONENTS ARE INSOLUBLE IS COMBINED WITH THE SOLTUION ABOVE TO PRODUCE PRECIPITATION OF THE COMPLEX WHICH IS THEN REMOVED FROM THE VARIOUS LIQUIDS. THE RESULTANT COMPLEX IS PHOTOCONDUCTIVE AND CAN BE USED TO SENSITIZE OTHER PHOTOCONDUCTORS. ELECTROPHOTOGRAPHIC ELEMENTS ARE PREPARED BY DISPERSING THE SO-FORMED COMPLEX IN A POLYMERIC VEHICLE AND COATING THE DISPERSION ON A SUPPORT.

Description

United States Patent 3,684,502 PHOTOCONDUCTIVE CO-CRYSTALLINE COM- PLEX 0F PYRYLIUM DYE AND POLYMER USED IN ELECTROPHOTOGRAPHY Eugene P. Gramza and David D. Schreiber, both of Kodak Park, 1669 Lake Ave., Rochester, N.Y. 14615 No Drawing. Filed Nov. 18, 1970, Ser. No. 90,778

Int. Cl. G03g 5/04 U.S. Cl. 961.6 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electrophotography and to photoconductive elements and structures useful in electrophotography. In addition, this invention relates to methods for preparing electrophotographic elements.

Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature.

Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well known in the art, can then be employed to produce a permanent record of the image.

One type of photoconductive insulating structure or element particularly useful in electrophotography utilizes a composition containing a photoconductive insulating material dispersed in a resinous material. A unitary electrophotographic element is generally produced in a multilayer type of structure by coating a layer of the photoconductive composition onto a film support previously overcoated with a layer of conducting material or the photoconductive composition may be coated directly onto a conducting support of metal or other suitable conducting material. Such photoconductive compositions have shown improved speed and/or spectral response, as well as other desired electrophotographic characteristics when one or more photosensitizing materials or addenda are incorporated into the photoconductive composition. Typical addenda of this latter type are disclosed in US. Pat. Nos. 3,250,615, issued May 10, 1966, by Van Allan; 3,141,770, issued July 21, 1964, by Davis et al.; and 2,987,395, issued June 6, 1961, by Jarvis. Generally, photosensitizing addenda to photoconductive compositions are incorporated to etfect a change in the sensitivity or speed of a particular photoconductor system and/or a change in its spectral response characteristics. Such addenda can enhance the sensitivity of an element to radiation at a particular wavelength or to a broad range of wavelengths Where desired. The mechanism of such sensitization is presently not fully understood. The phenomenon, however, is extremely useful. The importance of such effects is evidenced by the extensive search currently conducted by workers in the art for compositions and compounds which are capable of photosensitiz- 3,684,502 Patented Aug. 15, 1972 c CC ing photoconductive compositions in the manner described.

Usually, the desirability of a change in electrophotographic properties is dictated by the end use contemplated for the photoconductive element. For example, in document copying applications, the spectral electrophotographic response of the photoconductor should be capable of reproducing the Wide range of colors which are normally encountered in such use. If the response of the photocnductor falls short of these design criteria, it is highly desirable if the spectral response of the composition can be altered by the addition of photosensitizing addenda to the composition. Likewise, various applications specifically require other characteristics such as the ability of the element to accept a high surface potential, and exhibit a low dark decay of electrical charge. It is also desirable for the photoconductive element to exhibit high speed as measured in an electrical speed or characteristic curve, a low residual potential after exposure and resistance to fatigue. Sensitization of many photoconductive compositions by the addition of certain dyes selected from the large number of dyes presently known has hitherto been widely used to provide for the desired flexibility in the design of photoconductive elements in particular photoconductor-containing systems. Conventional dye addenda to photoconductor compositions have generally shown only a limited capability for over-all improvement in the totality of electrophotographic properties which cooperate to produce a useful electrophotographic element or structure. The art is still searching for improvements in shoulder and toe speeds, improved solid area reproduction characteristics, rapid recovery and useful electrophotographic speed from either positive or negative electrostatic charging.

A high speed heterogeneous or aggregate photoconductive system was developed by William A. Light which overcomes many of the problems of the prior art. This aggreate composition is the subject matter of copending application Ser. No. 804,266, filed Mar. 4, 1969, now U.S. Pat. No. 3,615,414, and entitled Novel Photoconductive Compositions and Elements. The addenda disclosed therein are responsible for the exhibition of desirable electrophotographic properties in photoconductive elements prepared therewith. However, in accordance with the procedures described therein, the preparation of electrophotographic elements uses a solvent treatment step subsequent to the coating step. In an effort to avoid this secondary treatment step, a novel method of preparation of photoconductive compositions of the type described by Light is disclosed in copending Eugene P. Gramza Application Ser. No. 821,513, filed May 2, 1969, now U.S. Pat. No. 3,615,415, and entitled Method for the Preparation of Photoconductive Compositions. This latter method involves the high speed shearing of the photoconductive composition prior to coating and thus eliminates subsequent solvent treatment. However, it is often desirable to have photoconductive compositions of even higher speeds than those obtainable with the above compositions. Thus, copending Edward J. Seus application Ser. No. 764,302, filed Oct. 1, 1968, now U.S. Pat. No. 3,591,374, and entitled High Speed Electrophotographic Elements and Method for Preparation Thereof discloses a technique for substantially increasing the speed of the above compositions. This technique involves forming electrophotographic layers by the above techniques'and then overcoating such layers with a solution of suitable dye. This latter procedure uses a secondary coating step. Accordingly, there is a need situ in a binder composition which may or may not be that desired for the ultimate use of the electrophotographic element thus formed. Accordingly, there is a need for a method of forming isolated aggregate compositions which can be dispersed as addenda in a variety of photoconductive compositions.

It is, therefore, an object of this invention to provide the art of electrography with a novel method of preparing photoconductive compositions.

It is an additional object of this invention to provide novel isolated dye-polymer aggregates which can subsequently be dispersed in a polymeric binder to form a photoconductive composition and a novel method for their preparation.

It is another object to provide a novel process for forming electrophotographic elements.

These and other objects and advantages will become apparent from the following description of the invention.

It has been discovered that when the components of the compositions described by Light are manipulated in a certain prescribed manner, aggregates are formed which can be isolated and then be dispersed in any desired binder system. This technique allows extended latitude in the use of these aggregates in that it is no longer necessary to form the aggregates in situ in a particular binder system. This technique thus provides greater freedom in selecting a binder system desired and provides a simplified means of forming electrophotographic elements without any required secondary treatment. This method involves dissolving the polymer and dye components of the aggregate in a halogenated solvent system in which the solubilities of the two ingredients are substantially equal. A precipitating liquid is then added in which the aggregate is insoluble. The addition of the precipitating liquid causes the aggregate compositions to precipitate. The solvent and precipitating liquid are separated from the precipitate which is then dried.

The aggregate crystals are now ready to be dispersed in a suitable binder system which can contain any desired photoconductor. Aggregate crystals are added to a solution of binder and photoconductor in a solvent which has substantially no solvent action on the aggregate. The composition is mixed by any suitable means and coated using standard coating techniques. After drying, the resultant layer comprises a multiphase composition, the heterogeneous nature of which is generally apparent when viewed under 2500 magnification, although such compositions may appear to be substantially optically clear to the naked eye in the absence of magnification. There can, of course, be a macroscopic heterogeneity. Suitably, the dye-containing aggregate in the discontinuous phase is predominantly in the size range of from about 0.01 to microns. However, it should be noted that when the aggregate crystals prepared according to the invention are use to sensitize a particulate photoconductor, such as zinc oxide, another discontinuous phase will be present which may not fall within this size range.

In general, the heterogeneous compositions formed, as described above, are multiphase organic solids containing dye and polymeric vehicle. The polymeric vehicle forms an amorphous matrix or continuous phase which contains a discrete discontinuous phase as distinguished from a solution. The discontinuous phase is comprised of aggregate crystals prepared by the instant precipitation technique and are comprised of a co-crystalline complex of dye and polymer. When aggregate crystals prepared as described herein are used in conjunction with an organic photoconductor, the resultant photoconductive composition generally contains only two phases as the photoconductor usually forms a solid solution with the continuous phase of polymeric vehicle. On the other hand, when a particulate photoconductor, such as as zinc oxide, is used three phases may be present. In such a case, there would be a continuous polymeric phase, a discontinuous phase containing aggregate as discussed above and another discontinuous phase comprised of the particulate photoconductor. Of course, such multiphase compositions may also contain additional discontinuous phases of trapped impurities, etc. Another feature characteristic of the heterogeneous compositions formed as described above is that the wavelength of the radiation absorption maximum characteristic of such compositions is substantially shifted from the wavelength of the radiation absorption maximum of a substantially homogeneous solid solution formed of similar constituents. Such an absorption maximum shift in the formation of multiphase heterogeneous systems for the present invention is generally of the magnitude of at least about 10 nm. If mixtures of dyes are used, one dye may cause an absorption maximum shift to a long wavelength and another dye cause an absorption maximum to a shorter wavelength.

Sensitizing dyes and electrically insulating polymeric materials are used in forming the aggregate compositions. Typically, pyrylium dyes, including pyrylium, thiapyrylium and selenapyrylium dye salts are useful in forming such. compositions. Such dyes include those which can be represented by the following general formula:

Raxi Re wherein R, R, R, R and R can each represent (a) a hydrogen atom; (b) an alkyl group typically having from 1 to 15 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, amyl, isoamyl, hexyl, octyl, nonyl, dodecyl, etc., (c) alkoxy groups like methoxy, ethoxy, propoxy, butoxy, amyloxy, hexoxy, octoxy, and the like; and (d) aryl groups including substituted aryl groups such as phenyl, 4-diphenyl, alkylphenyls as 4- ethylphenyl, 4-propylphenyl, and the like, alkoxyphenyls as 4-ethoxyphenyl, 4-methoxyphenyl, 4-amyloxyphenyl, 2- hexoxyphenyl, 2-methoxyphenyl, 3,4-dimethoxyphenyl, and the like, fi-hydroxy alkoxyphenyls as Z-hydroxyethoxyphenyl, 3-hydroxyethoxyphenyl, and the like, 4- hydroxyphenyl, halophenyls as 2,4-dichlorophenyl, 3,4-dibrornophenyl, 4-chlorophenyl, 3,4-dichlorophenyl, and the like, azidophenyl, nitrophenyl, aminophenyls as 4-diethylaminophenyl, 4-dimethylaminophenyl and the like, naphthyl; and vinyl substituted aryl groups such as styryl, methoxystyryl, diethoxystyryl, dimethylaminostyryl, lbutyl-4-p-dimethylaminophenyl-l,3-butadienyl, 8-ethyl-4- dimethylaminostyryl, and the like; and where X is a sulfur, oxygen or selenium atom, and Z- is an anionic function, including such anions as perchlorate, fluoroborate, iodide, chloride, bromide, sulfate, periodate, p-toluenesulfonate, and the like. In addition, the pair R and R as well as the pair R and R can together he the necessary atoms to complete an aryl ring fused to the pyrylium nucleus. Typical members of such pyrylium dyes are listed in Table 1.

atoms necessary to form a cyclic hydrocarbon radical including cycloalkanes such as cyclohexyl and polycycloalkanes such as norbornyl, the total number of carbon atoms in R and R being up to 19;

R and R7 can each be hydrogen, an alkyl radical of from 1 to 5 carbon atoms or a halogen such as chloro, bromo, iodo, etc. and

R is a divalent radical selected from the following:

0 ll ll ll -c-o-, o-o-cm Preferred polymers useful in the present method of forming aggregate crystals are hydrophobic carbonate polymers comprised of the following recurring unit:

Each R is a phenylene radical including halo-substituted phenylene radicals and alkyl-substituted phenylene radicals; and R and R are as described above. Such compositions are disclosed, for example, in U.S. Pat. Nos. 3,028,365 by Schnell et al., issued Apr. 3, 1962 and 3,317,466 by Caldwell et al., issued May 2, 1967. Preferably polycarbonates containing an alkylidene diarylene moiety in the recurring unit such as those prepared with Bisphenol A and including polymeric products of ester exchange between diphenylcarbonate and 2,2-bis(4-hydroxyphenyl)propane are useful in the practice of this invention. Such compositions are disclosed in the following U.S. patents: 2,999,750 by Miller et al., issued Sept. 12, 1961; 3,038,874 by Laakso et 211., issued June 12, 1962; 3,038,879 by Laakso et a1., issued June 12, 1962; 3,038,880 by Laakso et al., issued June 12, 1962; 3,106,544 by Laakso et 211., issued Oct. 8, 1963; 3,106,545 by Laakso et al., issued Oct. 8, 196-3; and 3,106,546 by Laakso et al., issued Oct. 8, 1963. A wide range of filmforming polycarbonate resins are useful, with completely satisfactory results being obtained when using commercial polymeric materials which are characterized by an inherent viscosity of about 0.5 to about 1.8.

The following polymers are included among the materials useful in the practice of this invention:

dimethylcarb onate) Poly (3, 3-ethylenedioxyphenylene thiocarbonate). P013161, 4)-isopropylidenediphenylene carbonnte-cotereph t a ate 4 Poly (4, 4-isopropylidencdiphenylene carbonate).

Poly(4=, -t-isopropyledc ne diphenylene thioearbonate) Pcly(2, 2-butancbis-4-phenylene carbonate). Poly (4. 4-lsopropylidcnediphenylene carbonate-blocketltylene oxide) 8 Poly (4, t J-isopropylidenediphenylene carbonate-blocktetramethylenoxide) 9 Polyl l, l-isopropylidenebis(Z-methylphenylene)-carbonate 10 Poly (4, 4-is0propylidene diphenylene-co-l, l-phenylene carbonate 11 Poly (4, at-isopropylidenediphenylene-co-l, 3-phenylene carbonate).

12 Poly (4, 4J-isopropylidene dlpheuylene-co-4, 4-diphenylene carbonate).

13 Poly(4, a-isopropylidenedlphenylene-co-e, 4'-oxydtphenylene carbon l4 Poly(4, 4' 15opropylidenediphenylene-c0-4, 4-carbonyldiphenylene carbonate).

15 Poly (4, 4-isopropylldenediphenylene-co-, 4-ethylene diphenylene carbonate).

16. Polyl l, 4-methylenebis (Z-methylphenylene) carbonate].

17 Polyll, 1-](p-bromophenylethane)bis(4-phenylcne)carbonate 18 Polyl l,a-iscpropylidenediphenylene-co-sul[onylbis-(4- phenylene)carbonate].

l9 Polyfl, 1-cyclohcxanebis( rphenylenekarbonatel.

20 loly[4, l-isopropylidcne bis(Z-chloropheuylcne)-carbonate].

- PolylZ, 2-(3-methylbutane)bls-4-phenylene carbonate].

olyl2, 2-(3, 3dimethylbutane) bisA-phenylene carbonate]. Poly {1, 1-[1(1-naphthyl)]bis-4-phenylene carbonate 2 27 Poly[2, 2 (4-methylpentane)bis-4-phenylene carbonate}.

2S. Pol y [4, 4 -(2-n0rbornylidene) diphenylene carbonate].

29 Polyi t, 4-(hexahydro'4, 7-methanolndan-5-ylldene)- diphenylene carbonate].

30 Poly(4, 4-isopropylidenediphenylene carbonate-blockoxytetramethylane).

The aggregate crystals formed according to the present invention can readily be used for enhancing the sensitivity and extending the spectral range of sensitivity of a variety of organic photoconductors and inorganic photoconductors including both nand p-type photoconductors. A typical example of an inorganic photoconductor would be zinc oxide. The present invention can be used in connection with many organic, including organo-metallic, photoconducting materials which have little or substantially no persistence of photoconductivity. Representative organometallic compounds are the organic derivatives of Group Illa, Na, and Va metals such as those having at least one amino-aryl group attached to the metal atom. Exemplary organo-metallic compounds are the triphenyl-p-dialkylaminophenyl derivatives of silicon, germanium, tin and lead, the tri-p-dialkylaminophenyl derivatives of arsenic, antimony, phosphorus, bismuth, boron, aluminum, gallium, thallium and indium. Useful photoconductors of this type are described in copending Goldman and Johnson U.S. patent application Ser. No. 650,664, filed July 3, 1967 and Johnson application Ser. No. 755,711, filed Aug. 27, 1968 now U.S. Pat. No. 3,607,257.

An especially useful class of organic photoconductors is referred to herein as organic amine photoconductors. Such organic photocondnctors have as a common structural feature at least one amino group. Useful organic photoconductors which can be spectrally sensitized in accordance with this invention include, therefore, arylamine compounds comprising (1) diarylamines such as diphenylamine, dinaphthylamine, N,N' diphenylbenzidine, N- phenyl 1 naphthylamine, N phenyl 2 naphthylamine, N,N' diphenyl p phenylenediamine, 2 carboxy 5 chloro 4 methoxydiphenylamine, p anilinophenol, N,N' di 2 naphthyl p phenylenediamine, those described in Fox U.S. Pat. 3,240,597, issued Mar. 15, 1966, and the like, and (2) triarylarnines including (a) nonpolymeric triarylamines, such as triphenylamine, N,N,N',N' tetraphenyl m phenylenediamine, 4-acetyltriphenylamine, 4 hexanoyltriphenylamine, 4 lauroyltriphenylamine, 4 hexyltriphenylamine, 4 dodecyltriphenylamine, 4,4 bis(diphenylarnino)benzil, 4,4 bis (diphenylamino)bcnzophenone and the like, and (b) polymeric triarylamines such as poly[N,4" (N,N,N- triphenylbenzidine)], polyadipyltriphenylamine, polysebacyltriphenylamine, polydecamethylenetriphcnylamine, poly N (4 vinylphenyl)diphenylamine, poly N- (vinylphenyl) 0:,0c' dinaphthylamine and the like. Other useful amine-type photoconductors are disclosed in U.S. Pat. 3,180,730, issued Apr. 27, 1965.

Useful photoconductive substances capable of being sensitized in accordance with this invention are disclosed in Fox U.S. Pat. 3,265,496, issued Aug. 9, 1966, and include those represented by the following general forwherein T represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear (cg, phenyl, naphthyl, biphenyl, binaphthyl, etc.), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy, etc.), or a nitro group; M represents a mononuclear or polynuclear monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), or a substituted monovalent aromatic radical wherein said substituent can comprise a member, such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogen atom, a halogen atom, or an aromatic amino group, such as MNH-; b represents an integer from 1 to about 12; and R represents a hydrogen atom, a mononuclear or polynuclear aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), a substituted aromatic radical wherein said substituent comprises an alkyl group, an alkoxy group, an acyl group, or a nitro group, or a poly(4'-vinyl henyl) group which is bonded to the nitrogen atom by a carbon atom of the phenyl group.

Polyarylalkane photoconductors are particularly useful in producing the present invention. Such photoconductors are described in US. Pat. 3,274,000 by Noe et al., issued Sept. 20, 1966, French Pat. 1,383,461 and in copending application of Seus and Goldman, titled Photoconductive Elements Containing Organic Photoconductors, Ser. No. 627,857, filed Apr. 3, 1967, now US. Pat. No. 3,542,544. These photoconductors include leuco bases of diaryl or triaryl methane dye salts, 1,1,l-triarylalkanes wherein the alkane moiety has at least two carbon atoms and tetraarylmethanes, there being substituted an amine group on at least one of the aryl groups attached to the alkane and methane moieties of the latter two classes of photoconductors which are non-leuco base materials.

Preferred polyarylalkane photoconductors can be represented by the formula:

wherein each of D, E and G is an aryl group and J is a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and G containing an amino substituent. The aryl groups attached to the central carbon atom are preferabl; phenyl groups, although naphthyl groups can also be used. Such aryl groups can contain such substituents as alkyl and alkoxy typically having 1 to 8 carbon atoms, hydroxy, halogen, etc., in the ortho, meta or para positions, ortho-substituted phenyl being preferred. The aryl groups can also be joined together or cyclized to form a fluorene moiety, for example. The amino substituent can be represented by the formula:

wherein each L can be an alkyl group typically having 1 to 8 carbon atoms, a hydrogen atom, an aryl group, or together the necessary atoms to form a heterocyclic amino group typically having to 6 atoms in the ring such as morpholino, piperidino, tetrahydropyrrole, etc. At least one of D, E, and G is preferably p-dialkylaminophenyl group. When I is an alkyl group, such an alkyl group more generally has 1 to 7 carbon atoms.

Representative useful polyarylalkane photoconductors include the compounds listed in Table 3.

TABLE 3 Compound Number Name of Compound 1 4,4-benzylidene bis(N,N-diethyl-rn-toluidlne). 5 2 4,4-diamlno4-dimethylamino-2',2-dimethyltriphenylme ane. 3 4 ,4 -bls (diethylamlno) -2,6-diehloro-2 ,2" dimethyltrlphenylmethane. 4 4',4-bis(diethylamlno)-2,2-dlmethyldiphenylnaphthylmethane. 5 2,2"-ifimethyl-4,4,4"-trls(dimethylamlno)triphenylme ane. 1O 6 4,4-bis(diethylamtno)-4-dimethylamino-2,2-dimethyltrlphenylmethane. 7 4 ,4 -bis (diethylamino) -2-ehloro-2 ,2 -dlmethyl-4- dimethylaminotrlphenylmethane. 8 4' ,4 -bis(diethylamino) -4-dlmethylamlno-2,2 ,2

trimethyltriphenylmethane. 9 4,4-bis(dimethylamino)-?rchloro-2',2-dlmethyltriphenylmethane.

l0 4,4=-bis(dimethylamino)-2,2-dlmethyl-4-methoxytriphenylmethane. 11 Bis(4-diethylamino)-1,1,1-triphenylethane. 12 Bis(4-diethylamino)tetraphenylmethane. 13 4'A gs(benzylethylamino)-2,2"-dtmethyltriphenylme ane. 2O 4,4-bls(diethylamlno)-2,2-diethoxytrlphenylmethane 15 4-bis(dimethylamlno)-1,l,1-triphenylethane.

1-(4-N,N-dimethylaminophenyl)-1,14iipheny1etl1ane. 4dimethylaminotetraphenylmethane. 18 4-diethylaminotetraphenylmethane.

Another class of photoconductors useful in this invention are the 4-diarylamino-substituted chalcones. Typical compounds of this type are low molecular weight nonpolymeric ketones having the general formula:

wherein R and R are each phenyl radicals including substituted phenyl radicals and particularly when R is a phenyl radical having the formula:

where R, and R are each aryl radicals, aliphatic residues of l to 12 carbon atoms such as alkyl radicals preferably having 1 to 4 carbon atoms or hydrogen. Particularly advantageous results are obtained when R is a phenyl radical including substituted phenyl radicals and where R is diphenylaminophenyl, dimethylaminophenyl or phenyl.

Other photoconductors which can be used with the present aggregate compositions include rhodamine B, malachite green, crystal violet, phenosafranine, cadmium sulfide, cadmium selenide, parachloronil, benzil, trinitrofluorenone, tetranitrofluorenone, etc.

In preparing photoconductive compositions in accordance with this invention, useful results are obtained when an organic, including organo-metallic, photoconductor is present in an amount equal to at least about /z% by weight of the total solids added to the coating solvent. The upper limit of the amount of photoconductor pres ent can be varied widely with up to 99% by weight of total solids being useful. A preferred weight range for the photoconductor is from about 10 to about weight percent. Of course, if it is desired to use the present aggregate compositions alone as the photoconductive substance, then no photoconductor would be added. In addition to the photoconductors described above, polymeric photoconductors, e.g., poly(N-vinylcarbazole), halogenated poly(N-vinylcarbazole) can also be used if desired.

According to the process of the invention, a pyrylium dye as hereinbefore defined is dissolved together with a hydrophobic polymer in an organic solvent system which contains a halogenated hydrocarbon. Suitable halogenated hydrocarbons can be selected from any solvent that has substantial solvent action on both the dye and the polymer. Particularly useful solvents include, for example, chlorinated lower alkanes having from 1 to about 4 carbon atoms, e.g., dichloromethane, chloroform, carbon tetrachloride, 1,1 dichloroethane, 1,2 dichloroethane, 1,1,2-trichloroethane, etc., and chlorinated aromatic hydrocarbons, e.g., chlorobenzene, bromobenzene, 1,2-dichlorobenzene, etc. The conditions by which solution of the dye is elfected are not particularly critical, providing that temperature is not a factor in adjusting the relative solubility of the dye and polymer in the succeeding precipitation step. Recognizing this factor, the temperature can be any convenient temperature below the boiling point of the solvent and below the decomposition temperature of any of the components. Similarly, sufficient agitation may be employed to promote solution in a reasonable period of time.

The organic solvent system in which the dye and polymer are dissolved may contain additional solvents other than one chosen from those mentioned above. The reason for this will become evident upon consideration of the succeeding steps in the process of the invention.

After the dye and polymer are dissolved in the halogenated hydrocarbon-containing solvent system, a precipitating liquid is slowly combined with the solvent system to cause the dissolved components of the solution to precipitate in a predetermined manner. This precipitating liquid is characterized in that it is a non-halogenated, non-polar liquid in which neither the dye nor the polymer is soluble. It may be a liquid of low dielectric constant, for example, below about 5.0. Suitable liquids include, for example, alkanes having from about 6 to about 12 carbon atoms and which may be straight or branched chain, e.g., hexane, octane, decane, dodecane, 2,2,4-trimethylpentane, etc., ligroin, and similar liquids and mixtures thereof.

The conditions of precipitation are such that a cocrystalline complex of dye and polymer is formed in the presence of the organic solvent system and the precipitating liquid. In order to ensure that a co-crystalline complex of the dye and polymer is formed and not merely a different crystalline form of dye or a precipitated form of polymer alone, the conditions of precipitation are carefully controlled. The dye and polymer are initially dissolved in a good mutual solvent, such as dichloromethane. A precipitating liquid such as hexane, for example, is then added to the solution. T his precipitating liquid is a non-solvent for both the polymer and the dye. The precipitate is examined to determine whether it is dye or polymer alone. This can be done visually or by conventional analytical methods, and forms no part of the present invention. If it is determined that the dye has precipitated, for example, there may be added to the solution a solvent, e.g., toluene, which is a better solvent for the dye than for the polymer. As an alternative, the amount of dye in the solution initially may be reduced with respect to the amount of polymer. As another alternative, the temperature of the solution may be changed in such a direction that the solubility of the dye increases at a greater rate than that of the polymer. Conversely, if it is determined that the polymer has precipitated, the concentration of polymer may be reduced, a preferential solvent for the polymer may be added, or the temperature may be changed in such a direction as to increase the solubility of the polymer with respect to that of the dye. The preferred method is to adjust the relative concentrations of dye and polymer so that they precipitate at about the same time and rate, in the form of the co-crystalline complex. As a rough guide to the relative amounts of dye and polymer to be used initially, the solubility of each can be determined initially, and the concentration of each to be used selected to be in approximately direct proportion to the solubility. For example, if the dye is less soluble, its concentration is reduced in the solvent system. When it has been determined that the precipitate is the co-crystalline complex, the precipitating liquid is added in sufiicient quantity to completely precipitate the product. As previously indicated, the product can be identified as being the feature material of the aforementioned Light 12 application by virtue of the presence of a spectral absorption shift from that of the absorption spectrum of the dye alone. The shift appears as a change in the position of the spectral absorption maximum from that of the dye by an amount of at least about 10 nm.

It has been mentioned that the precipitating liquid is slowly combined with the solvent system to precipitate the desired co-crystalline complex. A preferred method of combining the two liquid media is to add one of the liquids dropwise to the other, at a rate of about one to three drops per second. It is particularly preferred to dropwise add the solvent system containing dye and polymer to the precipitating liquid, although the reverse will yield the same product, although at a much slower rate.

The ratio of dye to polymer in the halogenated hydrocarbon-containing solvent system can be varied over a limited range and still obtain reliable precipitation of the co-crystalline complex upon being combined with the precipitating liquid. Ratios of dye to polymer in the general range of from about 1:10 to about 2:1 and preferably 1:7 to 1:1 generally yield reliable precipitation of the complex. Similarly, the ratio of precipitating liquid to solvent can be varied over a relatively wide range, with values in the range of from about 50:1 to about 1:1 being useful with 20:1 to about 2.5:1 being preferred.

After the formation of the feature material, it is separated from the liquid medium in which it is formed. Separation can be effected by any known means of separating a liquid from a suspended solid, such as by filtration, centrifuging, decanting, and the like. A preferred method is to pour the contents of the vessel into a Biichner funnel fitted with a piece of filter paper. After removal of the liquid by, for example, application of a mild vacuum, the crystals are found to be retained by the paper. The residual liquid is then removed from the crystals by drying the filter paper and its contents at a suitable temperature of from about 40 C. to about C. for a period of time ranging from about a few minutes to an hour, depending on the temperature employed.

After the crystals have been separated from the solvent system and precipitating liquid, they are ready to be dispersed in a suitable polymeric vehicle or binder system, as indicated previously. The binder is dissolved in a suitable solvent, and the feature material or aggregate crystals dispersed therein by any convenient technique such as low or medium speed mixers of the types well known for the purpose. If necessary or desirable, one or more photoconductors and any other desired addenda can also be added to the solution of vehicle containing dispersed aggregate crystals. It should be noted, however, that the aggregate crystals retain their crystalline form when so dispersed, and are not re-dissolved.

Solvents of choice for preparing the photoconductive compositions and elements to be coated therefrom in accordance with this invention can include a number of organic solvents for the polymeric vehicle such as alkanes having from about 5 to about 12 carbon atoms including cycloalkanes such as pentane, cyclohexane; iso-octane, nonane, decane, dodecane, etc.; aromatic hydrocarbons including lower al-kyl-substituted such as benzene, toluene, xylene, ethylbenzene, propylbenzene, etc.; ketones such as dialkyl ketones having 1 to about 3 carbon atoms in the alkyl moiety such as dimethylketone, methylethylketone, etc.; or mixtures of solvents. It is necessary, of course, that the solvent have substantially no solvent effect on the aggregate crystalline phase once it has formed.

Preferred binders for use in preparing the photoconductive layers which can be formed in accordance with the method of this invention comprise polymers having fairly high dielectric strength which are good electrically insulating film-forming vehicles. Materials of this type comprise styrene-butadiene copolymers; polyvinyltoluenestyrene copolymers; silicone resins; styrene-alkyd resins; silicone alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chlorideacrylonitrile copolymer; vinylidene chloride-vinylchloride copolymers; poly(vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); polyacrylic and methacrylic esters, such as poly- (methylmethacrylate), poly(n-butylmethacrylate), poly- (isobutyl methacrylate), etc.; polystyrene, nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly(ethylene alkaryloxyalkylene terephthalate); cellulose esters; phenol-formaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly(ethyleneglycol-co-bishydroxyethoxyphenylpropane terephthalate); nuclear substituted polyvinyl haloacrylates; etc. 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. Pats. 2,361,019 by Gerhart, issued Oct. 24, 1944 and 2,258,423 by Rust, issued Oct. 7, 1941. Suitable resins of the type contemplated for use in the photoconductive layers of the invention are sold under such tradenames as Vitel PE-l, Cymac, Piccopale 100, Saran F-220 and Lexan 105 and 145. Other types of binders which can be used in the photoconductive layers of the invention include such materials as paraifin, mineral waxes, etc.

Suitable supporting materials for coating sensitiZer-containing photoconductive layers in accordance with the method of this invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminumpaper laminates; metal foils such as aluminum foil, zinc foil, etc; metal plates, such as aluminum, copper, zinc, brass and galvanized plates; vapor deposited metal layers such as silver, nickel, aluminum and the like coated on paper or conventional photographic film bases such as cellulose acetate, polystyrene, etc. Such conducting materials as nickel can be vacuum deposited on transparent film supports in sufiiciently thin layers to allow electrophotographic elements prepared therewith to be exposed from either side of such elements. An especially useful conducting support can be prepared by coating a support material such as poly(ethylene terephthalate) with a conducting layer containing a semiconductor dispersed in a resin or vacuum deposited on the support. Such conducting layers both with and without insulating barrier layers are described in US. Pat. 3,245,833 by Trevoy, issued Apr. 12, 1966. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of maleic anhydride and a vinyl acetate polymer. Such kinds of conducting layers and methods for their optimum preparation and use are disclosed in U.S. 3,007,901 by Minsk, issued Nov. 7, 1961 and 3,262,- 807 by Sterman et al., issued July 26, 1966.

Coating thickness of the photoconductive composition on the support can vary widely. Normally, a coating in the range of about 10 microns to about 300 microns before drying is useful for the practice of this invention. The preferred range of coating thickness is found to be in the range from about 50 microns to about 150 microns before drying, although useful results can be obtained outside of this range. The resultant dry thickness of the coating is preferably between about 2 microns and about 50 microns, although useful results can be obtained with a dry coating thickness between about 1 and about 200 IIJJCI'OBS.

After the photoconductive elements prepared according to the method of this invention have been dried, they can be employed in any of the well-known electrophotographic processes which require photoconductive layers. One such process is the xerographic process. In a process of this type, an electrophotographic element is held in the dark and given a blanket electrostatic charge by placing it under a corona discharge. This uniform charge is retained by the layer because of the substantial dark insulating property of the layer, i.e., the low conductivity of the layer in the dark. The electrostatic charge formed on the surface of the photoconductive layer is then selectively dissipated from the surface of the layer by imagewise exposure to light by means of a conventional exposure operation such as, for example, by a contact printing technique, or by lens projection of an image, and the like, to thereby form a latent electrostatic image in the photoconductive layer. Exposing the surface in this manner forms a pattern of electrostatic charge by virtue of the fact that light energy striking the photoconductor causes the electrostatic charge in the light struck areas to be conducted away from the surface in proportion to the intensity of the illumination in a particular area.

The charge pattern produced by exposure is then developed or transferred to another surface and developed there, i.e., either the charge or uncharged areas rendered visible, by treatment with a medium comprising electrostatically responsive particles having optical density. The developing electrostatically responsive particles can be in the form of a dust, i.e., powder, or a pigment in a resinous carrier, i.e., toner. A prefererd method of applying such toner to a latent electrostatic image for solid area development is by the use of a magnetic brush. Methods of forming and using a magnetic brush toner applicator are described in the following US. Pats: 2,786,439 by Young issued Mar. 26, 1957; 2,786,440 by Giaimo, issued Mar. 26, 1957; 2,786,441 by Young, issued Mar. 26, 1957; 2,874,063 by Greig, issued Feb. 17, 1959. Liquid development of the latent electrostatic image may also be used. In liquid development, the developing particles are carried as to the image-bearing surface in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature, for example US. Pat. 2,907,674 by Metcalfe et al., issued Oct. 6, 1959. In dry development processes, the most widely used method of obtaining a permanent record is achieved by selecting a developing particle which has as one of its components a low melting resin. Heating the powder image then causes the resin to melt or fuse into or on the element. The powder is, therefore, caused to adhere permanently to the surface of the photoconduct layer. In other cases a transfer of the electrostatic charge image formed on the photoconductive layer can be made to a second support such as paper which would then become the final print after development and fusing. Techniques of the type indicated are well known in the art and have been described in the literature such as in RCA Review vol. 15 (1954), pp. 469-484.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A 0.2 gram portion of the dye 4-(4-dimethylaminophenyl)-2,G-diphenylthiapyrylium perchlorate and a 0.2 gram portion of poly(4,4'-isopropylidenediphenylene carbonate) are dissolved with stirring in 10.0 grams of dichloromethane. To the solution of dye and polymer are added 5.0 grams of toluene, causing partial precipitation of the dye. Sufficient dichloromethane is added to redissolve the dye, or about 5 grams. To the solvent system comprising dichloromethane and toluene are added about 50 grams of n-hexane, with stirring. After a few minutes, a bluish-green crystalline material precipitates from the mixture of the solvent system and precipitating solvent. The crystals are separated from the solvent system by pouring the contents into a Buchner funnel containing filter paper and applying a mild vacuum to the funnel using an aspirator. The crystals thus separated are then dried by heating them to about C. for about 15 minutes. They are identified as being aggregate crystals by virtue of the fact that their spectral absorption maximum is at a wavelength of 685 nm., while that of the dye alone is 590 nm. Photoconductive compositions are then prepared by dissolving 0.58 gram of a modified poly(vinyl butyral) containing approximately '9 Weight percent of poly (vinyl alcohol) units (Butvar B-76, Monsanto Chem- 15 ical Co.) and 0.40 gram of 4,4'-benzylidenebis(N,N-diethyl-m-toluidine) in 9.0 grams of toluene, and adding to each of three such solutions a separate portion of aggregate crystals prepared as described above. The weight of crystals added to the first solution is 0.04 gram, to the second solution, 0.08 gram, and to the third solution, 0.12 gram. Each composition thus obtained is shaken in a metal container with small steel balls for about minutes to produce a uniform mixture. Each of the compositions is then coated at a wet thickness of 100 microns on a conductive support comprising poly(ethylene terephthalate) film support bearing a layer of cuprous iodide as in the aforementioned Trevoy U .8. Pat. 3,245,833, and dried. The resultant electrophotographic elements are then electrostatically charged under a negative corona source until the surface potential, as measured by an electrometer probe, reaches about 600 volts. The charged element is then exposed to a 3000 K. tungsten light source through a stepped density gray scale. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potential, V0, to some lower potential, V, whose exact value depends on the actual amount of exposure in meter-candleseconds received by the area. The results of these measurements are then plotted on a graph of surface potential V vs. log exposure for each step. The actual negative speed of the photoconductive composition can then be expressed in terms of the reciprocal of the exposure required to reduce the surface potential to any fixed arbitrarily selected value. Herein, unless otherwise stated, the actual negative speed is the numerical expression of 104 divided by the exposure in meter-candle-seconds required to reduce the 600 volt charged surface potential by 100 volts. The speeds of the elements thus obtained, in order of increasing concentration of co-crystalline complex crystals, are 1600, 3200 and 4000. As a control, a composition is prepared as first described above for preparing the co-crystalline complex but omitting the polycarbonate. The precipitate thus obtained is, therefore, only dye. Dye crystals are incorporated in a photoconductor-containing composition and coated to form electrophotographic elements as above. The speeds of the elements thus obtained are less than 20. It is thus seen that the crystals formed according to the process of this invention truly comprise a co-crystalline complex of dye and polymer of the type described by Light.

EXAMPLE 2 The crystals of co-crystalline complex formed as in Example 1 are incorporated in a photoconductive composition similar to that of Example 1 except that a polystyrene resin having a ditferent average molecular weight (Koppers 8X, Koppers Co., Inc.) is used as the binder for the photoconductive composition and cyclohexane as the coating solvent. The weight of aggregate or co-crystalline complex crystals used is 0.12 gram. Coating conditions are the same as in Example 1. The speed of the element thus produced is 800, while the speed of an element prepared from dye alone instead of co-crystalline complex is zero. This is because the dye alone does not sensitize the photoconductor in a polystyrene binder system.

EXAMPLE 3 The general procedure of Example 1 is followed in preparing co-crystalline complex crystals. In the present example, a 2.5 Wt./vol. percent solution of the dye in dichloromethane is prepared to which is added a 2.5 wt./ vol. percent solution of the resin, also in dichloromethane. The dye and resin used are those of Example 1. No further solvent is added to the dye-polymer solution. To the solution is then added dropwise an amount of ligroin having a boiling range of about 63 to 75 C. (Eastman Organic Chemicals P513), the amount being about 2.5 times the volume of solvent used to dissolve dye and polymer. Precipitation of co-crystalline complex occurs as in Example 1, after which the crystals are isolated and dried.

16 The crystals are then used to prepare three elements as in Example 1. Theseelements each give an electrophotographic response similar to that obtained in Example 1. The crystals are identified as the complex by the presence of the shift in the wavelength of maximum spectral absorption.

EXAMPLE 4 The procedure of Example 3 is followed using 2.0 wt./ col. percent solutions of dye and polymer, and 1,2-dichl0- roethane as the solvent. The precipitating liquid is n-hexane (Eastman Organic Chemicals P1135), the amount being about 2.4 times the volume of solvent used. Crystals similar to those in Example 3 are obtained which show the spectral absorption shift characteristic of the aggregate or co-crystalline complex and which confer similar high speeds to elements prepared therefrom.

EXAMPLE 5 The procedure of Example 3 is followed using twice the weight concentration of polymer in the initial solution. The percipitating liquid is the liquid of Example 4, the amount being 2.4 times the volume of solvent. Results simular to those in Examples 3 and 4 are obtained.

EXAMPLE 6 Using the dye and polymer of Example 1, a dye-polymer solution is prepared which contains 2.0 weight percent of both dye and polymer in dichloromethane. The order of addition of Example 1 is reversed, in that the dyepolymer solution is added dropwise to the precipitating liquid. The precipitating liquid is the ligroin of Example 3, its volume being three times that of initial dye-polymer solution. Crystals of aggregate begin to form immediately upon addition of the solution to the precipitating liquid. When the entire volume of initial solution has been added, the crystals are separated and dried as in the previous examples. The method of this example is preferred, as the complete precipitation requires a much shorter time. Identification of the crystals as the aggregate is carried out as before, as is the preparation of an electrophotographic element which, when tested, has similar high speeds.

EXAMPLE 7 A solution is prepared from the following ingredients:

4 (4 dimethylaminophenyl) 2,6 diphenyl thiapyrylium fluoroborate (Dye II) g 2.7 4-(4 dimethylaminophenyl) 2 (4-eth0xyphenyl)- 6-phenyl thiapyry-lium fluorobor-ate (Dye Ill) g 0.3 Lexan g 3.0 Methylene chloride ml 525 Toluene ml 375 gate crystals are then used to prepare the following compositions:

Amount Composi- Millition Ingredients liters Grams Chlorinated polyethylene (65.8% 01), Texas 1. 2

Eastman Co. 'IX199-14 (binder). A 4,4-diethylamino-2,2-dimethyltriphenyl 0. 8

"""" methane (photoconductor).

Aggregate crystals 0. 2 Toluene 12 Polyvinyl-vinylidene chloride, (Goodrich 1. 2

Rubber 00., Glenn 222}. B 4,4-diethylam.ino-2,2-dimethyltriphenyl 0. 8

""'"' methane (photoconductor).

Aggregate crystals 0. 2 Toluene 12 P01y(vinyl-m-bromobenzoate-co-viny1 1. 2

acetate. 0 4,4-diethylamlno-2,2-dimethyltriphenyl 0. 8

""""" methane (photoconductor).

Aggregate crystals 0. 2 Toluene 12 Porcelain balls A3") are added to each of the above compositions and each is shaken for two hours on a low amplitude, high frequency vibrating mixer. Each of the resulting formulations is coated at .004" wet thickness on a 0.4 CD. nickel coated substrate to form an electrophotographic element which is measured for its speed as described above. The speeds are shown below.

SPEED Positive Negative omposition Shoulder Toe Shoulder Toe Similar results are also obtained using Dye I and 4-(4- dimethylaminophenyD-Z-(4-ethoxyphenyl) 6 phenyl thiapyrylium perchlorate (Dye IV).

EXAMPLE 8 Two stock solutions are prepared each containing: Dye H g 0.44 Dye III g 0.06 Lexan 145 g 3.2 Methylene chloride ..ml 350 Toluene ml 250 One solution is added dropwise to 3000 ml. of hexane and cooled to -80 C. by means of a Dry Ice-acetone bath. The other solution is added to 3000 ml. of hexane maintained at room temperature (-23 C.). The resulting aggregrate precipitates are collected by suction filtration. Upon examination of the two precipitates, it is observed that the one prepared at 80 C. has a much finer particle size than the one prepared at room temperature.

EXAMPLE 9 Four solutions are prepared from the ingredients listed below:

Solution No. 1 2 3 4 Lexan 145, g 3. 5 3. 5 1. 2 13. 3 Dye, H g 0.44 0.44 0.15 6.65 Dye g 0.06 0. 06 0. 02 Methylene chloride, ml 350 850 118 1, 750 Toluene, ml 250 250 83 1, 250

EXAMPLE 10 A mixture is prepared of 0.9 gram of polyvinylcarbazole, 0.2 gram of the aggregate crystals of Example 7 and 12 ml. of benzene by stirring the polyvinylcarbazole into solution, adding the aggregate crystals and agitating with a vibrating shaker for 2 hours using A;- inch porcelain balls. The formulation is then coated at 27 C. on a 0.4 0D. nickel-coated substrate at a .00 wet thickness. A control coating is also prepared which contains 1.8 grams of polyvinylcarbazole, 30 ml. of benzene, 0.2 g. of Lexan 145 and 0.18 g. of Dye II and 0.02 g. of Dye III. The Lexan and dye of this composition are simply dissolved together and are present as a homogeneous combination not in the form of a co-crystalline complex. This control coating is also used to form an electrophotographic element as above and the two elements are measured for their speed. The element containing the aggregate has a positive 100 volt shoulder speed of 700 and a toe speed of 100. The control element which contains no aggregate has a 100 volt shoulder speed more than 10 times slower than the aggregate-containing element. The 100 volt toe speed of the control is 0.

The term co-crystalline complex as used herein has reference to a crystalline compound which contains dye and polymer molecules co-crystallized in a single crystalline structure to form a regular array of the molecules in a three dimensional pattern.

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

We claim:

1. A method of preparing a heterogeneous photoconductive composition containing a dispersion of a cocrystalline complex of dye and polymer, said method comprising the steps of:

(a) dissolving a pyrylium dye and a carbonate polymer in an organic solvent system containing a halogenated hydrocarbon to form a solution thereof, said dye and carbonate polymer having substantially equal solubilities in the organic solvent system;

(b) slowly combining the solution with a precipitating liquid comprised of a non-halogenated, non-polar organic liquid in which the dye and carbonate polymer are insoluble, thereby precipitating a co-crystalline complex of dye and polymer;

(0) separating the complex from the solvent system and precipitating liquid; and

(d) dispersing the complex in a film-forming organic polymeric vehicle to form a heterogeneous photoconductive composition.

2. A method according to claim 1 comprising the additional step of dispersing an organic photoconductor in the organic polymeric vehicle.

3. A method according to claim 1 wherein the carbonate polymer contains the following recurring unit:

wherein each R is a phenylene radical and R and R when taken separately are selected from the group consisting of a hydrogen atom, an alkyl radical having from one to about 10 carbon atoms, and an aryl radical, and when taken together represent the number of carbon atoms necessary to complete a cyclic hydrocarbon radical, the total number of carbon atoms in R and R being up to 19.

4. The method as described in claim 1 wherein the halogenated hydrocarbon in said solvent system is selected from the group consisting of dic'hloromethane, 1,2-dichloroethane and 1,1,2-trichloroethane and said precipitating liquid is selected from hexane and octane.

'5. A method according to claim 1 wherein the dye is selected from the group consisting of pyrylium and thiapyrylium dye salts.

6. -A method according to claim 5 wherein the dye salt corresponds to the formula:

wherein each of R and R is a phenyl radical, R is an alkylamino-substituted phenyl radical having from 1 to about 6 carbon atoms in the alkyl moiety, X is selected from the group consisting of an oxygen atom and a sulfur atom, and Z- is an anionic function.

19 7. A method of preparing an electrophotographic element comprising the steps of:

(a) dissolving in an organic solvent system containing a halogenated hydrocarbon, a pyrylium dye salt corresponding to the formula:

R; i R2 wherein each of R and R is a phenyl radical, R is an alkylamino-substituted phenyl radical having from 1 to about 6 carbon atoms in the alkyl moiety, X is selected from the group consisting of an oxygen atom and a sulfur atom, and Z- is an anionic function,

and a carbonate polymer having the recurring unit:

wherein each R is a phenylene radical and R and R when taken separately are selected from the group consisting of a hydrogen atom, an alkyl radical having from one to about 10 carbon atoms, and an aryl radical, and when taken together, represent the number of carbon atoms necessary to complete a cyclic hydrocarbon radical, the total number of carbon atoms in R and R being up to 19';

(b) adding a precipitating liquid comprised of a nonhalogenated, non-polar organic liquid in which the dye and carbonate polymer are insoluble, said liquid having a dielectric constant less than about 5;

(c) precipitating a co-crystalline complex of pyrylium dye and carbonate polymer;

(d) separating the complex from the solvent system and precipitating liquid;

(e) dispersing the complex and an organic photocondoctor in a polymeric film-forming organic vehicle; and

(f) coating a thin film of the resultant dispersion on a conductive support.

8. The method as described in claim 7 wherein said dye salt is selected from the group consisting of salts of 4 (4-dimethylaminopheny-l)-2-(4-ethoxyphenyl)-6-phenylthiapyrylium, 4-(4-dirnethylaminophcnyD-Z,6-diphenylthiapyryliurn and 4-(4-dimethylaminophenyl)-2-(4-methoxy- 15 phenyl)-6-phenylthiapyryliurn.

9. The method as described in claim 7 wherein said polymer is poly(4,4'-isopropylidenediphenylene carbon- GEORGE F. LESMES, Primary Examiner I. R. MILLER, Assistant Examiner U.S. C1. X.R,

961; 252-50l; 260-342, 37 PC