GB1603663A - Polymeric compounds for use as sensitizers in photoconductive compositions - Google Patents

Description

(54) POLYMERIC COMPOUNDS FOR USE AS SENSITIZERS IN PHOTOCONDUCTIVE COMPOSITIONS (71) We, EASTMAN KODAK COMPANY, a Company organized under the Laws of the State of New Jersey, United States of America of 343 State Street, Rochester, New York 14650, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to polymeric compounds useful as sensitizers in photoconductive compositions and to multiactive photo conductive insulating elements containing these compounds.

Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Patent Nos.

2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a photoconductive insulating 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.

Various types of photoconductive insulating elements are known for use in electrophotographic imaging processes. In many conventional elements, the active components of the photoconductive insulating composition are contained in a single layer composition. This composition is typically placed in electrical contact affixed, with a conductive support during the electrophotographic imaging process.

Among the many different kinds of photoconductive compositions which may be employed in typical single active layer photoconductive elements are inorganic photoconductive materials such as vacuum evaporated selenium, particulate zinc oxide dispersed in a polymeric binder and homogeneous organic photoconductive compositions composed of an organic photoconductor solubilized in a polymeric binder.

The use of two or more active layers in a photoconductive element has been discussed in the patent literature. Such multi-active-layer photoconductive elements are sometimes referred to hereinafter simply as "multi-active" photoconductive elements. A partial listing of representative patents discussing or at least alluding to "multi-active" photoconductive elements includes U.S. Patents Nos. 3,165,405, 3,041,166, 3,394,001, 3,679,405 and 3,725,058; Canadian Patent 930,591; Canadian Patents Nos. 932,197-199 and British Patents Nos. 1,343,671 and 1,337,228.

Although there has been a fairly extensive description of specific types of multi-active photoconductive elements in the literature, various shortcomings still exist in these elements so that there is a need to investigate alternative kinds of multi-active elements. For example, the multi-active elements described in the aforementioned U.S. Patent No. 3,165,405 suffer from the disadvantages of generally low speed and difficult to clean zinc oxide materials in both active layers of the element. Other multi-active elements such as those described in Canadian Patents 930,591 and 932,199 appear to be primarily designed for use in a positve charging mode of operation and therefore may not generally be suitable for use in an electrophotographic process in which a negative charging mode is employed.

French Application No. 2,295,461 discloses a multiactive photoconductive insulating element having at least two layers comprising an inorganic photoconductor-containing layer in electrical contact with an aggregate photoconductive layer.

Belgian Patent No. 836,892 discloses a multi-active photoconductive insulating element having at least two layers comprising an aggregate or charge generation layer in electrical contact with an organic photoconductor-containing or chargetransport layer. The aggregate photoconductive layer used in both previously mentioned patents includes a continuous electrically insulating polymer phase having dispersed therein a finely divided, particulate co-crystalline complex containing at least one pyrylium-type dye salt and at least one polymer having an alkylidene diarylene group in a recurring unit.

The aggregate layer used in both French Application No. 2,295,461 and Belgian Patent No. 836,892 are of the type described in U.S. Patent No. 3,615,414.

Typically, it has its principle absorption band for radiation in the visible region of the spectrum within the range of from about 520 nm to about 700 nm. Within this range the aggregate layer provides an exceptional level of sensitivity. However, below 520 nm, especially in the region of 460 nm, the aggregate layer exhibits low absorption thereby lowering the overall efficiency of such multi-active elements for white light exposure as well as decreasing the ability of such elements to discriminate red copy from a white background. Clearly, there exists a need in the art for multi-active photoconductive elements comprising an aggregate photoconductive layer which has greater sensitivity in the region of the visible spectrum below about 520 nm especially in the blue region of the spectrum around 460 nm.

We have discovered a new class of polymeric compounds which when incorporated into the aggregate photoconductive layer of a multi-active photoconductive element, results in an increase in the electrophotographic sensitivity of the element.

In accordance with the invention there is provided a polymeric compound having the formula:

wherein R1 and R3, which may be the same or different, represent a substituted or unsubstituted alkyl group having from 1 to 18 carbon atoms or a substituted or unsubstituted aryl group; R2 and R4, which may be the same or different, represent an alkylene group having from 2 to 10 carbon atoms or a substituted or unsubstituted arylene group: R5 and R which may be the same or different, represent hydrogen or an electron withdrawing group selected from --CN, --CF,, -NO2, -CO2R8 and -SO2F wherein R8 is an alkyl group having from 1 to 12 carbon atoms; R, represents an oxy, imino, thio, oxycarbonyl, iminocarbonyl, carbonyldioxy, ureylene, carbonyloxycarbonyl, sulphonyl, iminosulphonyl or iminocarbonyloxy group; Ar is an unsubstituted or a substituted arylene group wherein the substituent is selected from -CN, -CO2R9, -OR9, -CF3, -NO2, -Cl, -SR9, and -R9 wherein R9 is an alkyl group having from 1 to 12 carbon atoms; each of a, b and c independently is an integer of from 1 to 25; d is0 or 1; and n is an integer of from 2 to 150.

In the groups -C*C- and -C*C- R5 R6 of Formula I, R5 and R6 can be bonded to either carbon atom or the respective group, and a hydrogen atom is bonded to the other carbon atom in the group.

Preferably, Ar is a phenylene, naphthylene or anthrylene group.

Preferably, each of a, b and c independently is an integer of from 1 to 10.

In accordance with another aspect of the invention there is provided a multiactive photoconductive insulating element having at least two layers including an aggregate photoconductive layer in electrical contact with a photoconductorcontaining layer, wherein: (a) the photoconductor-containing layer, contains either an inorganic or organic photoconductor; and (b) the aggregate photoconductive layer comprises (i) a continuous, electrically insulating polymer phase, (ii) a discontinuous phase dispersed in the continuous phase and comprising a finely-divided, particulate co-crystalline complex of at least one polymer having an alkylidene diarylene group in a recurring unit and at least one pyrylium-type dye salt, and (iii) a polymeric compound of the invention.

The invention also provides a method of photographic reproduction which comprises charging a photoconductive element of the invention, imagewise exposing the element to activating radiation to form a charge pattern, and applying a toner to the charge pattern to form a negative or positive image having optical density.

The multi-active photoconductive element of the invention may be employed as the image-forming member in a variety of electrophotographic processes, including transfer electrophotographic processes, employing a reusable photoconductive element; non-transfer electrophotographic processes wherein a final visible image is formed on a non-reusable photoconductive element; and the so-called TESI processes (i.e., Transfer of ElectroStatic Images) such as described by R. M. Schaffert in the book entitled Electrophotography, at pp. 87-96, The Focal Press, New York (1965). For convenience and purposes of illustration, the multi-active photoconductive element of the invention will be described herein with reference to its use in conventional electrophotographic processes in which an electrostatic charge image is formed at or near the surface of the photoconductive element by employing the now well-known steps of (a) applying a uniform electrostatic charge to the top surface of the photoconductive insulating element in the absence of activating radiation while the bottom surface of the element is maintained at a suitable reference potential, thereby creating an electric field through the photoconductive element and (b) imagewise exposing the photoconductive element to activating radiation. However, it will be appreciated by those familiar with the art that the multi-active element of the invention may also be advantageously employed in a wide variety of other. known electrophotographic processes. For a greater understanding of multi-active photoconductive elements comprising aggregate photoconductive layers the reader is directed to French Application No. 2,295,461 and Belgian Patent No.

836,892.

The term "activating radiation" as used in the present specification is defined as electromagnetic radiation which is capable of generating electron-hole pairs in the aggregate photoconductive layer and the inorganic photoconductor containing layer upon exposure thereof. Thus, for an example, when the aggregate photoconductive layer is exposed to activating radiation, charge carriers, i.e.

electron-hole pairs, are photogenerated therein.

The multi-active photoconductive element of the present invention may be employed in electrophotographic processes using either positive or negative charging of the photoconductive element. Typically, when the multi-active photoconductive element is employed in an electrophotographic process, the element is affixed, either permanently or temporarily, on a conductive support. In such case, by appropriate selection of the photoconductor material included in the photoconductor containing layer, the multi-active element is capable of providing useful electrostatic charge images when used in either a positive or negative charge mode, regardless of whether the aggregate photoconductive layer or the photoconductor containing layer is located adjacent the conductive support.

Typically, in formula I representing polymeric compounds of the invention, R1 and R3 may represent any of the following alkyl or aryl groups, and R2 and R4 may represent the equivalent alkylene or arylene groups.

1. an alkyl group having one to 18 carbon atoms e.g., methyl, ethyl, propyl, butyl, isobutyl, octyl, and dodecyl, including a substituted alkyl group having one to 18 carbon atoms such as a. alkoxyalkyl e.g., ethoxypropyl, methoxybutyl and propoxymethyl, b. aryloxyalkyl e.g., phenoxyethyl, naphthoxymethyl and phenoxypentyl, c. aminoalkyl, e.g., aminobutyl, aminoethyl and aminopropyl, d. hydroxyalkyl e.g., hydroxypropyl and hydroxyoctyl, e. aralkyl e.g., benzyl and phenethyl, f. alkylaminoalkyl e.g., methylaminopropyl, methylaminoethyl and also including dialkylaminoalkyl e.g., diethylaminoethyl, dimethylaminopropyl and propylaminooctyl, g. arylaminoalkyl, e.g., phenylaminoalkyl, diphenylaminoalkyl, N-phenyl-N ethylaminopentyl, N-phenyl-N-ethylaminohexyl and naphthylaminomethyl, h. nitroalkyl, e.g., nitrobutyl, nitroethyl and nitropentyl, i. cyanoalkyl, e.g., cyanopropyl, cyanobutyl and cyanoethyl, j. haloalkyl, e.g., chloromethyl, bromopentyl and chlorooctyl, k. alkyl substituted with an acyl group having the formula

wherein R10 is hydroxy, hydrogen, aryl, e.g., phenyl and naphthyl; lower alkyl having one to eight carbon atoms, e.g., methyl, ethyl and propyl; amino including substituted amino, e.g., diloweralkylamino; lower alkoxy having one to about eight carbon atoms e.g., butoxy and methoxy; and aryloxy, e.g., phenoxy and naphthoxy; 1. alkyl acetates e.g., methyl acetate and ethyl acetate, 2. an aryl group, e.g., phenyl, naphthyl, anthryl and fluorenyl, including a substituted aryl group such as a. alkoxyaryl, e.g., ethoxyphenyl, methoxyphenyl and propoxynaphthyl, b. aryloxyaryl, e.g., phenoxyphenyl, naphthoxyphenyl and phenoxynaphthyl, c. aminoaryl, e.g., aminophenyl, aminonaphthyl and aminoanthryl, d. hydroxyaryl, e.g., hydroxyphenyl, hydroxynaphthyl and hydroxyanthryl, e. btphenylyl, f. alkylaminoaryl, e.g., methylaminophenyl, methaylaminonaphthyl and also including dialkylaminoaryl, e.g., diethylaminophenyl, and dipropylaminophenyl, g. arylaminoaryl, e.g., phenylaminophenyl, diphenylaminophenyl, N-phenyl N-ethylaminophenyl and naphthylaminophenyl, h. nitroaryl e.g., nitrophenyl, nitronaphthyl and nitroanthryl, i. cyanoaryl, e.g., cyanophenyl, cyanonaphthyl and cyanoanthryl, j. haloaryl, e.g., chlorophenyl, bromophenyl and chloronaphthyl, k. aryl substituted with an acryl group having the formula

wherein R10 is hydroxy, hydrogen, aryl, e.g., phenyl and naphthyl; amino including substituted amino, e.g., diloweralkylamino, lower alkoxy having one to eight carbon atoms, e.g., butoxy and methoxy; aryloxy, e.g., phenoxy and naphthoxy; lower alkyl having one to eight carbon atoms, e.g., methyl, ethyl, propyl and butyl, 1. alkaryl, e.g., tolyl, ethylphenyl, propyl and naphthyl.

Minimum and maximum values for n are determined by solubility factors.

When the molecular weight is too low, i.e., a value of n below 2, the sensitizer will tend to diffuse into adjacent layers resulting in a decrease in the efficiency of front exposures. When the molecular weight of the polymeric sensitizer is too high, then it will be difficult to incorporate the sensitizer in sufficient amounts into the aggregate photoconductive layer.

While some of the compounds of the present invention possess some photoconductive properties, the use in multiactive elements in which the aggregate particles are absent results in exceptionally reduced blue sensitivity. It appears that such compounds interact in some manner with the aggregate particle and the photoconductors from the adjacent layer to produce multi-active elements having increased sensitivity.

Typical compounds included within Formula I are set out in Table I.

TABLE I Compound:

TABLE I (Con't.)

Compound: rr \0 -Ni\0 > HC=CZ C =CH -Ju 0/ l C:.

4. :.l (clIH2)2 (ooh2)2 O - C = /70(OH2 16 0 14 /00 --14 -o =C- =CH cr ON -. ON 5. $D:i (OH2)2 (OH2)2 0-0=0 /C-OtCH2 7 I C '22 1 56 NG0)-Y-0(G0?-HO ;H31 v-HC =CH 0-0 (e I 10 (cm2)2 ------O- C=O 141 TABLE I (Con't.)

Compound: ON I . ,O=OH ~~ 7 X -HC=C tJ 7 ON (OH2 - 10 CN 0 34 0-0=0 /OMe H3C-e - HC = C H - 0O ~~~ 8 NI̸ MeO = OHl\G/oHO=OHo(0D?,\oyofL)oOH3 8.

olD! Me0 (or2)2 (cm2)2 0-0=0 //O0(OH210 0 38 9 X C 42 CN 9-CH3 ; 5 ON ON !D! 5 (0' H2)2 O-C=O O(C H 2l)i 0 44 H3C- -HC=CH--HC=CH- CH3 10. 0 C' \.

(CH2)2 (cm2)2 ----O-C=O //O0(OH210 0 21 TABLE I (Con't.)

Compound: II. N/G)0No(G/\oH0=Oe(G/\oO -HC -f iB c =c H-i\2 -N--\ ON CN CN 0 0 11 ---- o -( H2)2 II 0-(0H2)2 (CH2)2-OC(CH2)8 C 5 S 0-0.- 00 |C2H5- I -(\OX -HC=C~L -C=CH--\ N-C2H5 ON ON 0',".

12 N CN CN *\ i (OH2)2 (OH2)2 C=O C-O(CH2t 10 0 10 0-00 0 0O 00 5 q\7N07HO=O0 13. ON ON 5 (C H2)2 (C H2)2 0-0=0 - C =O SOC - O(C H2 .V 2 1 0 21 0H30\ /\ GN .'," 0OH3 tC' 7\\ '," '," ON tt 14. L .~~~ % ~!N/ \007N1 I( )I I (ooh2)2 (eH2)2 0-0=0 0=O-0-(0H2 10 26 TABLE I (Con't) Compound:

The polymeric compounds of the present invention may be prepared by generally known methods such as disclosed in Fieser and Fieser, Advanced Organic Chemistry (Reinhold Publishing Co., New York, N.Y. 1961), H. O. House, Modern Synthetic Reactions (2nd Ed. Benjamine Publishing Co., New York, N.Y. 1972) and numerous review articles such as J. Boutagy and R. Thomas, Chemical Review, 74, 89 (1974). For illustration purposes, Compound 1, Table I, is prepared as follows: Preparation of 4 - Formyl - 4' - (p - methoxycarbonylethyl) triphenylamine.

To a solution of 42 g (0.126 mole) of 4 - methoxycarbonylethyl)triphenylamine in 125 ml of dimethylformamide (DMF) heated to about 75"C under nitrogen was added dropwise 22.2 g (0.145 mole) of phosphoryl chloride. The rate of addition was controlled so that the temperature of the reaction did not exceed 85"C. Heating and stirring were continued for an additional six hours. The reaction mixture was allowed to cool and poured into I litre of saturated sodium acetate solution. The aqueous solution was extracted with several portions of benzene. The organic extracts were dried, filtered and the solvent removed. The tan oil was chromatographed on a column of silica gel.

Preparation of Dimethyl Ester:

A mixture of 10.2 g (0.028 mole) of 4- formyl- 4' - methoxycarbonylethyl)triphenylamitie, 2.1 g (0.0135 mole) benzene diacetonitrile, 40 ml dimethylformamide, 250 ml methanol and 0.350 g of sodium methoxide was heated under nitrogen at 500C for 48 hours. The reaction mixture was allowed to cool and filtered. The orange cake was recrystallized twice from ethyl acetatemethanol, mp 167-1690C.

Preparation of Polyester:

The compounds thus prepared are, in general, useful in any multi-active photoconductive insulating element in which an aggregate photoconductive layer is used.

The multi-active photoconductive elements of the present invention are formed, according to one mode of operation, by coating the aggregate photoconductive layer onto a suitable support and then overcoating the aggregate photoconductive layer with the photoconductor-containing layer. In another mode of operation, the photoconductor-containing layer may be coated onto a suitable support and then overcoated with the aggregate photoconductive layer. Optionally, protective overcoats, interlayers and subbing layers may be used.

When the multi-active photoconductive element of this invention comprises an inorganic photoconductor containing layer in electrical contact with an aggregate photoconductive layer both layers may generate charge carriers, i.e., holes, or electrons, and inject them into the other layer, which, in turn, can transport these injected charge carriers. That is the aggregate photoconductive layer can transport charge carriers, for example, electrons injected into it from a selenium-containing or zinc oxide-containing inorganic photoconductive layer; and the aggregate photoconductive layer can, in turn, generate its own charge carriers and inject them into the selenium-containing or zinc oxide-containing inorganic photoconductive layer. Some inorganic photoconductive materials inject charge carriers into the aggregate photoconductive layer or they accept and transport charge carriers generated from within the aggregage photoconductive composition less efficiently than selenium-containing or zinc oxide-containing layers.

The term "inorganic photoconductor" as used herein is defined as any inorganic photoconductive element or compound, including inorganic polymers, consisting solely of inorganic molecules. A partial list of particularly useful photoconductors useful in the invention includes selenium containing or zinc-oxide containing inorganic photoconductive materials, the various structural forms of selenium such as metallic selenium and amorphous selenium, cadmium selenide and arsenic triselenide.

The inorganic photoconductor-containing layer used in the present invention may be composed solely of an inorganic photoconductor, such as a vacuum evaporated selenium layer (with or without various known sensitizer(s) or dopant(s) for the selenium layer), or it may be composed of a mixture of inorganic photoconductors in an electrically insulating material. The total amount of inorganic photoconductor employed together with an electrically insulating binder material, when one is used, may vary considerably. Typically, the amount of inorganic photoconductor(s) used in admixture with an electrically insulating binder varies within the range of from 5 to 99 percent by weight, preferably from 50 to 90 weight percent, based on the total weight of the inorganic photoconductorcontaining layer.

A partial listing of representative materials which may be employed as binders in the inorganic photoconductor-containing layer are film-forming polymeric materials having a fairly high dielectric strength and good electrically insulating properties. Such binders include styrene-butadiene copolymers; polyvinyl toluenestyrene copolymers; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins vinylidene chloride-vinyl chloride copolymers; poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); nitrated polystyrene; polymethylstyrene, isobutylene polymers; polyesters, such as poly[ethylenecoalkylenebis(alkyleneoxyaryl) phenylenedicarboxylatel: phenolformaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly[ethylene - co - isopropylidene - 2,2 bis(ethyleneoxyphenylene)terephthalate]; copolymers of vinyl haloarylates and vinyl acetate such as poly(vinyl - m - bromobenzoate - co - vinyl acetate); and chlorinated poly(olefins) such as chlorinated poly(ethylene). Other types of binders which may be used in the inorganic photoconductor-containing layers include such materials as paraffin and mineral waxes, as well as combinations of binder materials.

The inorganic photoconductor-containing layer may also contain, if necessary or desirable depending on the particular inorganic photoconductor(s) selected and the specific spectral and electrical speed response desired, an effective amount of one or more sensitizers or dopants, such as thiapyrylium dye salts and selenapyrylium dye salts disclosed in U.S. Patent No. 3,250,615; fluorenes, such as 7,12 - dioxo - 13 - dibenzo(a,h)fluorene, 5,10 - dioxo - 4a,l 1 diazobenzo(b)fluorene and 3,13 - dioxo - 7 - oxadibenzo(b,g)fluorene; aromatic nitro compounds such as those described in U.S. Patent No. 2,610,120; anthrones such as those disclosed in U.S. Patent No. 2,670,284; quinones, such as those disclosed in U.S. Patent No. 2,670,286; benzophenones such as those disclosed in U.S. Patent No. 2,670,287; thiazoles, such as those disclosed in U.S. Patent No.

2,732,301; mineral acids; carboxylic acids; such as maleic acid, dichloroacetic acid, trichloroacetic acid, and salicyclic acid; sulphonic acids; phosphoric acids; and various dyes, such as cyanine (including carbocyanine), merocyanine, diarylmethane, thiazine, azine, oxazine, xanthene, phthalein, acridine, azo and anthraquinone dyes, and mixtures thereof.

Where a sensitizing compound is employed in the inorganic photoconductorcontaining layer, it is the normal practice, when the inorganic photoconductorcontaining layer is applied as a liquid coating dope, to mix a suitable amount of the sensitizing compound with the coating composition so that, after thorough mixing, the sensitizing compound is uniformly distributed in the coated layer. In general, useful results can be obtained where an appropriate sensitizer is added in a concentration range of from 0.001 to 30 percent by weight based on the dry weight of the inorganic photoconductor-containing layer. Normally, when used, a sensitizer is added to the layer in an amount by weight of from 0.005 to 10.0 percent by weight of the layer.

The inqrganic photoconductor-containing layer may also contain other addenda such as leveling agents, surfactants and plasticizers to enhance or improve various physical properties of the layer.

Liquid coating vehicles useful for coating inorganic photoconductorcontaining layers (which include a binder) onto a suitable substrate can include a wide variety of aqueous and organic vehicles. Typical organic coating vehicles include: 1) Aromatic hydrocarbons such as benzene and naphthalene, including substituted aromatic hydrocarbons such as toluene, xylene and mesitylene; 2) Ketones such as acetone and 2-butanone; 3) Halogenated aliphatic hydrocarbons such as methylene chloride, chloroform and ethylene chloride; and 4) Ethers including cyclic ethers such as tetrahydrofuran and ethylether.

When the multi-active photoconductive element of the present invention comprises an organic photoconductor-containing layer in electrical contact with an aggregate photoconductive layer, the organic photoconductor-containing layer functions as a charge transport layer and the aggregate photoconductive layer functions as a charge generation layer. The term "organic", as used herein, refers to both organic and metallo-organic materials.

The organic photoconductor-containing layer contains as the active chargetransport material one or more organic photoconductors capable of accepting and transporting charge carriers generated by the aggregate photoconductive layer.

Useful organic photoconductors can generally be divided into two classes depending upon the electronic charge-transport properties of the material. That is most charge-transport materials generally will preferentially accept and transport either positive charges, i.e. holes (p-type charge transport materials), or negative charges, i.e. electrons (n-type charge transport materials), generated by the chargegeneration layer. Of course, there are materials (amphoteric) which will accept and transport either positive charges or negative charges.

The capability of a given organic photoconductor to accept and transport charge carriers generated by the aggregate photoconductive layer can be conveniently determined by coating a layer of the particular organic photoconductor under consideration for use as a charge-transport material (e.g. a 5 to 10 micron thick layer containing about 30 weight percent or more of the organic photoconductive material together with up to about 70 weight percent of a binder, if one is used), on the surface of an aggregate photoconductive layer (e.g., a 0.5 to 2 micron aggregate photoconductive layer) which is, in turn, coated on a conducting substrate. The resultant unitary element may th triarylamines in which at least one of the aryl radicals is substituted by an active hydrogen-containing group as described in U.S. Patent No. 3,658,520 and tritolylamine.

3. polyarylalkane materials of the type described in U.S. Patents Nos.

3,274,000, 3,542,547, 3,542,544 and 3,615,402. Preferred polyarylalkane photoconductors are represented by the formula:

wherein D and G, which may be the same or different, represent aryl groups and J and E, which may be the same or different, represent a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and G containing an amino substituent.

An especially useful polyarylalkane photoconductor which may be employed as the charge transport material is a polyarylalkane having the formula noted above wherein J and E represent a hydrogen atom, an aryl group or an alkyl group and D and G represent substituted aryl groups having as a substituent thereof a group represented by the formula:

wherein R represents an unsubstituted aryl group such as phenyl or an alkyl substituted aryl group such as a tolyl group. Additional information concerning certain of these latter polyarylalkane photoconductors may be found in Belgian Patent No. 836,891.

4. strong Lewis base materials such as various aromatic including aromatically unsaturated heterocyclic-containing materials which are free to strong electron withdrawing groups. A partial listing of such aromatic Lewis base materials includes tetraphenylpyrene, I-methylpyrene, perylene, chrysene, anthracene, tetraphene, 2-phenyl naphthalene, azapyrene, fluorene, fluorenone, l-ethylpyrene, acetyl pyrene, 2,3-benzochrysene, 3,4-benzopyrene, 1 4-bromopyrene, phenylindole, polyvinyl carbazole, polyvinyl pyrene, polyvinyl tetracene, polyvinyl perylene, and polyvinyl tetraphene.

5. other useful p-type charge-transport materials which may be employed in the present invention are any of the p-type organic photoconductors, including metallo-organo materials, known to be useful in electrophotographic processes, such as any of the organic photoconductive materials described in Research Disclosure, Vol. 109, May 1973, pages 6167, paragraph IV (A) (2) through (13) which are p-type photoconductors.

Representative of typical n-type charge-transport materials which are believed to be useful are strong Lewis acids such as organic, including metallo-organic, materials containing one or more aromatic, including aromatically unsaturated heterocyclic, materials bearing an electron withdrawing substituent. These materials are considered useful because of their characteristic electron accepting capability. Typical electron withdrawing substituents include cyano and nitro groups; sulphonate groups; halogens such as chlorine, bromine, and iodine; ketone groups; ester groups: acid anhydride groups; and other acid groups such as carboxyl and quinone groups. A partial listing of such representative n-type aromatic Lewis acid materials having electron withdrawing substituents include phthalic anhydride, tetrachlorophthalic anhydride, benzil, metallitic anhydride, Stricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene, 2,4dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4,6-trinitroanisol, trichlorotrinitrobenzene, trinitro-O-toluene, 4,6-dichloro- 1 ,3-dinitrobenzene, 4,6dibromo-1,3-dinitrobenzene, P-dinitrobenzene, chloranil, bromanil, 2,4,7-trinitro9-fluorenone, 2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridene, tetracyanopyrene, dinitroanthraquinone, and mixtures thereof.

Other useful n-type charge-transport materials which may be employed in the present invention are conventional n-type organic photoconductors, for example, complexes of 2,4,6-trinitro-9-fluorenone and poly(vinyl carbazole). Still other ntype organic, including metallo-organo, photoconductive materials useful as n-type charge-transport materials in the present invention are any of the organic photoconductive materials known to be useful in electrophotographic processes such as any of the materials described in Research Disclosure, Vol. 109, May 1973, pages 61-67, paragraph IV (A) (2) through (13) which are n-type photoconductors.

The organic photoconductor-containing or charge-transport layer may consist entirely of the organic photoconductors described hereinabove, or, as is more usually the case, the organic photoconductor-containing layer may contain a mixture of the organic photoconductors in a suitable film-forming polymeric binder material. The binder material may, if it is an electrically insulating material, help to provide the charge-transport layer with electrical insulating organic photoconductor-containing characteristics, and it also serves as a film-forming material useful in (a) coating the organic photoconductor containing layer, (b) adhering the organic photoconductor-containing layer to an adjacent substrate, and (c) providing a smooth, easy to clean, and wear resistant surface. Of course, in instances where the organic photoconductor may be conveniently applied without a separate binder, for example, where the organic photoconductor-containing material is itself a polymeric material, such as a polymeric arylamine or poly(vinyl carbazole), there may be no need to use a separate polymeric binder. However, even in many of these cases, the use of a polymeric binder may enhance desirable physical properties such as adhesion and resistance to cracking.

Where a polymeric binder material is employed in the organic photoconductor-containing layer, the optimum ratio of charge-transport material to binder material may vary widely depending on the particular polymer binder(s) and particular organic photoconductor(s) used. In general, it has been found that when a binder material is used, useful results are obtained when the amount of active organic photoconductor contained within the organic photoconductorcontaining layer varies within the range of from 5 to 90 weight percent based on the dry weight of the charge-transport layer.

A partial listing of representative materials which may be employed as binders in the organic photoconductor-containing layer as film-forming polymeric materials having a fairly high dielectric strength and good electrically isulating properties. Such binders are listed hereinbefore as binders for the inorganic photoconductor-containing layer.

In general, it has been found that polymers containing aromatic or heterocyclic groups are most effective as the binder materials for use in the organic photoconductor-containing layer because these polymers, by virtue of their heterocyclic or aromatic groups, tend to provide little or no interference with the transport of charge carriers through the layer. Heterocyclic or aromatic-containing polymers which are especially useful in p-type organic photoconductor-containing layers include styrene-containing polymers, bisphenol-A polycarbonate polymers, phenol-formaldehyde resins, polyesters such as poly[ethylene - co isopropylidene - 2,2 - bis(ethyleneoxyphenylene)lterephthalate, and copolymers of vinyl haloarylates and vinylacetate such as poly(vinyl - m - bromobenzoate - co vinyl acetate).

The organic photoconductor-containing layer may also contain other addenda such as leveling agents, surfactants, plasticizers to enhance or improve various physical properties of the charge-transport layer. In addition, various addenda to modify the electrophotographic response of the element may be incorporated in the organic photoconductor layer. For example; various contrast control materials, such as certain hole-trapping agents and certain easily oxidized dyes may be incorporated in the charge-transport layer. Various such contrast control materials are described in Research Disclosure, Volume 122, June, 1974, p. 33 in an article entitled "Additives for Contrast Control in Organic Photoconductor Compositions and Elements".

The thickness of the organic photoconductor-containing layer may vary. It is especially advantageous to use an organic photoconductor-containing layer which is thickner than that of the aggregate photoconductive layer, with best results generally being obtained when the organic photoconductor layer is from 5 to 200 times, and particularly from 10 to 40 times, as thick as the aggregate photoconductive layer. A useful thickness for the aggregate photoconductive layer is within the range of from 0.1 to 15 microns dry thickness, particularly from 0.5 to 2 microns. However, good results can also obtained using an organic photoconductor-containing layer which is thinner than the aggregate photoconductive-layer.

The organic photoconductor-containing layers described herein are typically applied to the desired substrate by coating on the substrate a liquid dispersion or solution containing the organic photoconductor-containing layer components.

Typically, the liquid coating vehicle used is an organic vehicle. Typical organic coating vehicles are listed hereinbefore as coating vehicles for the inorganic photoconductor-containing layer.

The aggregate photoconductive layer used in the present invention comprises an aggregage composition as described in U.S. Patent 3,615,414. These aggregate compositions have a multiphase structure comprising (a) a discontinuous phase of at least one particulate co-crystalline compound or complex of a pyrylium-type dye salt and an electrically insulating, film-forming polymeric material containing an alkylidene diarylene group as a recurring unit and (b) a continuous phase comprising an electrically insulating film-forming polymeric material. Optionally, one or more charge-transport material(s) may also be incorporated in this multiphase structure. Of course, these multi-phase compositions may also contain other addenda such as leveling agents, surfactants, plasticizers and contrast control materials to enhance or improve various physical properties or electrophotographic response characteristics of the charge-generation layer.

The aggregate composition may be prepared by several techniques, such as, for example, the so-called "dye first" technique described in U.S. Patent No.

3,615,396. Alternatively, these compositions may be prepared by the so-called "shearing" method described in U.S. Patent No. 3,615,415. Still another method of preparation involves preforming the finely-divided aggregate particles such as is described in U.S. Patent No. 3,732,180 and simply storing these preformed aggregate particles until it is desired to prepare the charge-generating layer. At this time, the preformed aggregate particles may be dispersed in an appropriate coating vehicle together with the desired film-forming polymeric material and coated on a suitable substrate to form the resultant aggregate photoconductive layer.

In any case, by whatever method prepared; the aggregate photoconductive layer containing compounds according to Formula I, exhibits a separately indentifiable multi-phase structure. The aggregate nature of this multi-phase structure. The aggregate nature of this multi-phase layer is generally apparent when viewed under at least 2500x magnification, although such layers may appear to be substantially optically clear to the naked eye in the absence of magnification.

There can, of course, by microscope heterogeneity. Suitably, the co-crystalline complex particles present in the continuous phase of the aggregate photoconductive layer are finely-divided, that is, typically predominantly in the size range of from 0.01 to 25 microns.

The terms "co-crystalline complex" or "co-crystalline compound" are used interchangeably herein and have reference to a co-crystalline compound which contains dye and polymer molecules co-crystallized in a single crystalline structure to form a regulator array of molecules in a three-dimensional pattern. It is this particulate co-crystalline material dispersed in the continuous polymer phase of the aggregate photoconductive layer which, upon being exposed to activating radiation in the presence of an electric field, generates and/or transports electron-hole pairs in the multi-active photoconductive elements of the present invention.

Another feature characteristic of conventional heterogeneous or aggregate compositions such as those described in U.S. Patents 3,615,414 and 3,732.180, 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 dye-polymer solid solution formed of similar constituents. The new absorption maximum characteristic of the aggregate composition is not necessarily an overall maximum for the system as this will depend on the relative amount of dye in the aggregate. The shift in absorption maximum which occurs due to the formation of the co-crystalline complex in conventional aggregate compositions is generally of the magnitude of at least about 10 nanometers.

The pyrylium-type dye salts useful in preparing the co-crystalline complex contained in the aggregate photoconductive layer of the present invention includes pyrylium, bispyrylium, thiapyrylium, and selenapyrylium dye salts; and also salts of pyrylium compounds containing condensed ring systems such as salts of benzopyrylium and naphthopyrylium dyes are useful in forming such compositions.

Typical pyrylium-type dye salts from these classes which are useful in forming these co-crystalline complexes are disclosed in Light, U.S. Patent 3,615,414 noted above.

The co-crystalline complex contained in the aggregate photoconductive layer used in the present invention may include any of a variety of film-forming polymeric materials which are electrically insulating and have an alkylidene diarylene group in a recurring unit such as those disclosed in U.S. Patent 3,615,414.

The amount of the above-described pyrylium type dye salt used in forming the aggregate photoconductive layer may vary. Useful results are obtained by employing the described pyrylium-type dye salts in amounts of from 0.001 to 50 percent based on the dry weight of the aggregate photoconductive-layer.

The amount of dialkylidene diarylene group-containing polymer used in the aggregate photoconductive layer of the multi-active elements of the invention may vary. Typically, the aggregate photoconductive layer contains an amount of this polymer within the range of from 20 to 98 weight percent based on the dry weight of the aggregate photoconductive layer, although larger or smaller amounts may also be used.

The amount of compound represented by Formula I included in the aggregate photoconductive layer may vary widely. In general amounts from 0.1% by weight to 50% by weight of the dried layer is effective although amounts outside of this range will work. However, amounts of 5% by weight to 20% by weight are preferred.

Optionally, one or more organic photoconductors may be incorporated into the aggregate photoconductive composition. Organic photoconductors including metallo-organic, materials which can be solubilized in the continuous phase of the aggregate photoconductive composition may be used.

If an organic photoconductor is incorporated in the aggregate photoconductive layer of the multi-active element of the invention as is described above, the particular material selected should be electronically compatible with the organic photoconductor used in the organic photoconductive layer. That is, if an ntype organic photoconductor is used in the organic photoconductor-containing layer, then an n-type organic photoconductor should be incorporated in the aggregate photoconductive composition. Similarly, if a p-type organic photoconductor is used in the organic photoconductor-containing layer, then a ptype organic photoconductor should be incorporated in the aggregate photoconductive layer of the element.

The multi-active elements of the invention may be affixed, if desired, to a variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminium-paper laminates; metal foils such as aluminium foil and zinc foil; metal plates, such as aluminium copper, zinc, brass and galvanized plates; vapour deposited metal layers such as silver, nickel and aluminium coated on paper or conventional photographic film bases such as cellulose acetate and polystyrene. Such conducting materials as nickel may be vacuum deposited on transparent film supports in sufficiently thin layers to allow electrophotographic elements prepared therewith to be exposed from either side of such elements. An especially useful conducting support may 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 U.S. Patent No. 3,245,833. Other useful conducting layers include compositions consisting essentially of an intimate mixture of at least one protective inorganic oxide and from 30 to 70 percent by weight of at least one conducting metal, e.g., a vacuum-deposited cermet conducting layer as described in U.S.

Patent No. 3,880,657. Similarly, a suitable conducting coating may 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. Patents Nos. 3,007,901 and 3,262,807.

The multi-layer photoconductive elements of the invention may be affixed, if desired, directly to a conducting substrate. In some cases, it may be desirable to use one or more intermediate subbing layers between the conducting substrate to improve adhesion to the conducting substrate and/or to act as an electrical barrier layer between the multi-active element and the conducting substrate as described in U.S. Patent No. 2,940,348. Such subbing layers, if used, typically have a dry thickness in the range of from 0.1 to 5 microns. Typical subbing layer materials which may be used include film-forming polymers such as cellulose nitrate, polyesters, copolymers or poly(vinyl pyrrolidone) and vinylacetate, and various vinylidene chloride-containing polymers including two, three or four component polymers prepared from a polymerizable blend of monomers or prepolymers containing at least 60 percent by weight of vinylidene chloride. A partial list of representative vinylidene chloride-containing polymers includes vinylidene chloride-methyl methacrylate-itaconic acid terpolymers such as disclosed in U.S.

Patent No. 3,143,421. Various vinylidene chloride containing hydrosol tetrapolymers which may be used include tetrapolymers of vinylidene chloride, methyl acrylate, acrylonitrile, and acrylic acid such as disclosed in U.S. Patent No.

3,640,708. A partial listing of other useful vinylidene chloride-containing copolymers includes poly(vinylidene chloride-methyl acrylate), poly(vinylidene chloride-methacrylonitrile), poly(vinylidene chloride-acrylonitrile), and poly(vinylidene chloride-acrylonitrile-methyl acrylate). Other useful subbing materials include the so-called tergels which are described in U.S. Patent No.

3,501,301.

One especially useful subbing layer which may be employed in the multi-active element of the invention is a hydrophobic film-forming polymer or copolymer free from any acid-containing group, such as a carboxyl group, prepared from a blend of monomers or prepolymers, each of said monomers or prepolymers containing one or more polymerizable ethylenically unsaturated groups. A partial listing of such useful materials includes many of the above-mentioned copolymers, and, in addition, the following polymers: copolymers of polyvinyl-pyrrolidone and vinyl acetate; and poly(vinylidene chloride-methyl methacrylate).

Optional overcoat layers may be used in the present invention, if desired. For example, to improve surface hardness and resistance to abrasion, the surface layer of the multi-active element of the invention may be coated with one or more electrically insulating, organic polymer coatings or electrically insulating, inorganic coatings. A number of such coatings are well known in the art and accordingly extended discussion thereof is unnecessary. Typical useful such overcoats are described, for example, in Research Disclosure, "Electrophotographic Elements, Materials, and Processes", Volume 109, page 63, Paragraph V, May, 1973.

The following examples are presented to further illustrate and clarify the invention: General Method for Preparation of Multi-active Elements Base Aggregate Photoconductive Layer a. Bisphenol-A-polycarbonate High Molecular Weight 3.26 g b. 4 - (4 - dimethylaminophenyl) - 2,6 diphenylthiapyrylium hexafluorophosphate 1.59 g c. Dichloromethane 171.6 g d. 1,1 ,2Trichloroethane 73.5 g e. Polymeric compound from Table I 0.84 g The base aggregate layer was formulated and coated according to procedures substantially similar to those disclosed in U.S. Patent No. 3,706,554.

Carrier Transport Layer a. Bisphenol-A-polycarbonate High Molecular Weight 8.6 g b. Bisphenol-A-polycarbonate (Lexan 145, General Electric Co.) ("Lexan" is a Registered Trade Mark) 77.8 g c. Tri-p-tolyamine 38.2 g d. 1,1 - Bis(di - p - tolylaminophenyl) cyclohexane 19.4 g e. Chloroform 1056.0 g This formulation was prepared by first dissolving the two binder polymers in chloroform over a 2 hour period with a speed controlled, heavy duty, mechanical stirrer. The photoconductors were then added and stirred into solution for 30 minutes. The final solution was filtered and coated in two passes of 0.7 and 0.9 g/ft2 onto the aggregate photoconductive layers to yield the completed multi-active photoconductive element.

Examples 1-1 2: Twelve different multi-active photoconductive elements and a control element were prepared according to the procedure described immediately above. Each element contained a different polymeric sensitizer selected from Table I. Table II indicates the sensitivity of each element relative to the control element which included tri-p-tolylamine instead of a compound from Table I.

TABLE II Relative Compound from Sensitivity* Element Table I -460 nm Control ** 1.

1 1 4.2 2 2 7.4 3 3 4.5 4 4 4.5 5 5 3.9 6 6 5.5 7 7 4.1 8 8 6.7 9 9 7.3 10 10 7.4 11 11 5.4 12 12 4.9 * Relative sensitivity represents the reciprocal of the relative energy required to discharge the multi-active photoconductive element from -500 volts -100 volts residual potential as compared to the control element which is arbitrarily assigned a relative sensitivity value of 1.0. The listed values are for front exposures to 460 nm. light energy.

**Contains tri-p-tolylamine.

The relative sensitivity measurements reported in this and the following examples are relative reciprocal electrical sensitivity measurements. The relative reciprocal electrical sensitivity measures the speed of a given photoconductive element relative to other elements typically within the same test group of elements.

The relative reciprocal sensitivity values are not absolute sensitivity values.

However, relative reciprocal sensitivity values are related to absolute sensitivity values. The relative reciprocal electrical sensitivity is a dimensionless number and is obtained simply by arbitrarily assigning a value, Ro, to one particular absolute reciprocal sensitivity of one particular photoconductive control element. The relative reciprocal sensitivity Rn, of any other photoconductive element, n, relative to this value, Ro, may then be calculated as follows: Rn=(A,)(Ro/Ao) wherein An is the absolute reciprocal electrical sensitivity (in cm2/ergs.) of n, Ro is the sensitivity value arbitrarily assigned to the control element, and Ao is the absolute reciprocal electrical sensitivity (measured in cm2/ergs.) of the control element.

This data demonstrates that the polymeric sensitizers of the present invention result in a substantial increase in sensitivity of a multi-active photoconductive element compared to the control. Indeed, in some cases, the improvement was almost ten fold.

Example 13: Two separate elements were prepared according to the general procedure.

Element I contained an aggregate photoconductive layer containing Compound I from Table I. In the second element tri-p-tolylamine was used for a control: Electrophotographic measurements showed that the sensitivity of element 1 was almost 10 times that of the control upon both front and rear exposure in the blue region of the spectrum. Moreover, element 1 demonstrated an enhanced sensitivity throughout a much greater area of the blue region than did the control and thus provided a more panchromatic element.

Example 14: To demonstrate and synergistic increase in sensitivity of certain multi-active photoconductive elements of the invention, three separate elements were prepared according to the general procedure except for the differences indicated in the following Table III. The relative sensitivity of each of the elements were determined at 460 nm using front exposure.

TABLE III Blue Response of Various Multi-Active Photoconductors Relative Sensitivity* (=460 nm) Element Description Front Exposure 1 Prepared according to general procedure using Compound 1 from Table I 4.6 2 Prepared according to general procedure and coated without a compound from Table I 1.0 3 Prepared according to general procedure except formulated and coated without aggregate (included Compound I from Table I) .04 *Relative Sensitivity measurement carried out as in Table II.

This data showed that the sensitivity of the multi-active element comprising Compound 1 from Table I is more than the combined sensitivity of elements 2 and 3.

Attention is drawn to our co-pending Application No. 53258/77, Serial No.

1,599,166, which describes and claims a multi-active photoconductive insulating element having at least two layers comprising an aggregate photoconductive layer in electrical contact with a photoconductor-containing layer, wherein: (a) the photoconductor-containing layer contains either an inorganic or organic photoconductor; and (b) the aggregate photoconductive layer comprises (i) a continuous, electrically insulating polymer phase, (ii) a discontinuous phase dispersed in the continuous phase and comprising a finely-divided, particulate co-crystalline complex of at least one polymer having an alkylidene diarylene group in a recurring unit and at least one pyrylium-type dye salt and (iii) at least one compound having the structure: where A represents:

in which R1, R2, R3, and R4, which may be the same or different, represent a substituted or unsubstituted alkyl group having from I to 18 carbon atoms or a substituted or unsubstituted aryl group; R5 and R6, which may be the same or different, represent an electron withdrawing group, phenyl or substituted phenyl; R7 and R8, which may be the same or different represent an electron withdrawing group or hydrogen except that when Ar is unsubstituted phenylene or unsubstituted anthrylene, R7 and R8 must be other than hydrogen; R11 is an electron withdrawing group; Ar represents a substituted or unsubstituted arylene group.

Claims (25)

WHAT WE CLAIM IS:

1. A polymeric compound having the formula:

wherein R, and R3, which may be the same or different, represent a substituted or unsubstituted alkyl group having from 1 to 18 carbon atoms or a substituted or unsubstituted aryl group; R2 and R4, which may be the same or different, represent an alkylene group having from 2 to 10 carbon atoms or a substituted or unsubstituted arylene group; Rs and R6, which may be the same or different, represent hydrogen or an electron withdrawing group selected from --CN, CF3, -NO2, CO2R9 and -SO2F wherein R8 is an alkyl group having from 1 to 12 carbon atoms; R7 represents an oxy, imino, thio, oxycarbonyl, iminocarbonyl, carbonyldioxy, ureylene, carbonyloxy-carbonyl, sulphonyl, iminosulphonyl or iminocarbonyloxy group; Ar is an unsubstituted or a substituted arylene group wherein the substituent is selected from -CN, -CO2R9, -OR9, -CF3, -NO2, -Cl, -SR9, and -R9 wherein R9 is an alkyl group having from 1 to 12 carbon atoms; each of a, b and c independently is an integer of from 1 to 25; d is 0 or 1; and n is an integer of from 2 to 150.

2. A compound as claimed in Claim 1 wherein Ar is a phenylene, naphthylene or anthrylene group.

3. A compound as claimed in Claim 1 or Claim 2 wherein each of a, b and c independently is an integer of from 1 to 10.

4. A compound as claimed in Claim 1 having the formula:

5. A compound as claimed in Claim 1 substantially as hereinbefore described in any one of the Examples.

6. A multi-active photoconductive insulating element having at least two layers including an aggregate photoconductive layer in electrical contact with a photoconductor-containing layer, wherein: (a) the photoconductor-containing layer contains either an inorganic or organic photoconductor; and (b) the aggregate photoconductive layer comprises (i) a continuous electrically insulating polymer phase, (ii) a discontinuous phase dispersed in the continuous phase and comprising a finely-divided, particulate co-crystalline complex of at least one polymer having an alkylidene diarylene group in a recurring unit and at least one pyrylium-type dye salt and (iii) a polymeric compound as claimed in any one of the preceding claims.

7. An element as claimed in Claim 6 wherein the polymeric compound (b) (iii) is present in an amount of from 0.1 to 50 percent by weight of the aggregate photoconductive layer.

8. An element as claimed in Claim 7 wherein the polymeric compound is present in an amount of from 5 to 20 percent by weight of the aggregate photoconductive layer.

9. An element as claimed in any one of Claims 6 to 8 wherein the photoconductor-containing layer contains at least one inorganic photoconductive material.

10. An element as claimed in Claim 9 wherein the inorganic photoconductive material is a selenium- or a zinc oxide-containing material.

11. An element as claimed in any one of Claims 6 to 8 wherein the photoconductor-containing layer contains at least one organic photoconductive material.

12. An element as claimed in Claim 11 wherein the photoconductor-containing layer is from 5 to 200 times as thick as the aggregate photoconductive layer.

13. An element as claimed in Claim 12 wherein the photoconductor-containing layer is from 10 to 40 times as thick as the aggregate photoconductive layer.

14. An element as claimed in any one of Claims 11 to 13 wherein the aggregate photoconductive layer has a thickness of from 0.1 to 15 microns.

15. An element as claimed in claim 14 wherein the aggregate photoconductive layer has a thickness of from 0.5 to 2 microns.

16. An element as claimed in any one of Claims 6 to 15 wherein the pyrylium type dye salt is present in an amount of from 0.001 to 50 percent by weight based on the dry weight of the aggregate photoconductive layer.

17. An element as claimed in any one of Claims 6 to 16 wherein the polymer having an alkylidene diarylene group in a recurring unit is present in an amount of from 20 to 98 percent by weight based on the dry weight of the aggregate photoconductive layer.

18. An element as claimed in any one of Claims 6 to 17 affixed to an electrically conducting support.

19. An element as claimed in Claim 18 wherein the aggregate photoconductive layer is closer to the support than the photoconductor-containing layer.

20. An element as claimed in Claim 6 substantially as hereinbefore described in any one of the Examples.

21. A method of photographic reproduction which comprises charging a photoconductive element as claimed in any one of Claims 6 to 20, imagewise exposing the element to activating radiation to form a charge pattern, and applying a toner to the charge pattern to form a negative or positive image having optical density.

22. A method as claimed in Claim 20 wherein prior to the application of a toner, the charge pattern is transferred to a receiving sheet.

23. A method as claimed in Claim 20 wherein subsequent to the application of a toner, the image having optical density is transferred to a receiving sheet.

24. A method of photographic reproduction as claimed in Claim 21 substantially as herein described.

25. A photographic reproduction whenever produced by a method as claimed in any one of Claims 21 to 24.