US3503740A - Photoconductive elements containing organic photoconductors and sensitizers - Google Patents

Description

United States Patent ABSTRACT OF THE DISCLOSURE Photoconductive compositions and elements containing organic photoconductors and salts of certain substituted cycloheptenyl compounds are described. The salts are used a as sensitizers for the organic photoconductors.

This invention relates to electrophotography, and in particular to novel sensitized photoconductive compositions and elements having coated thereon such compositions.

The process of xerography, as disclosed by Carlson in US. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident actinic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of the surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or in the absence of charge pattern as desired. The deposited marking material may then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it may similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and a developed there.

Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder have found wide application in present-day document copying applications.

Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result a very large number of organic compounds are known to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Optically clear organic photoconductor-containing elements having desirable electrophotographic properties can be especially useful in electrophotography. Such electrophotographic elements may be exposed through -a trans parent base if desired, thereby providing unusual flexibility in equipment design. Such compositions, when coated as a film or layer on a suitable support also yield an element 3,503,740 Patented Mar. 31, 1970 which is reusable; that is, it can be used to form subsequent images after residual toner from prior images has been removed by transfer and/or cleaning.

Although some of the organic photoconductors comprising the materials described are inherently light sensitive, their degree of sensitivity is usually low and in the short wave length portion of the spectrum so that it is common practice to add materials to increase the speed and to shift the sensitivity toward the longer wave length portion of the visible spectrum. Increasing the speed and shifting the sensitivity of such systems into the visible regions of the spectrum has several advantages: it makes available inexpensive and convenient light sources such as incandescent lamps; it reduces exposure time; it makes possible the recording of a wide range of colors in proper tonal relationship, and allows projection printing through various optical systems. By increasing the speed through the use of sensitizers, photoconductors which would otherwise have been unsatisfactory are useful in processes where high speeds are required such as document copying. However, while some of the sensitizers have appreciably increased the speed of a photoconductive system, they have been unstable when exposed to moisture for a period of time.

It is, therefore, an object of this invention to provide a novel class of sensitizers for use in combination with certain photoconductors so that improved electrophotographic speeds are obtained.

Another object of this invention is to provide novel sensitized photoconductive elements.

It is also an object to provide novel sensitized photoconductive compositions which can be positively and negatively charged.

It is a further object of this invention to provide novel photoconductive compositions sensitized with a class of sensitizers which are stable when exposed to moisture for an extended period of time.

These and other objects of this invention are accomplished with photoconductive compositions containing a photoconductor and a sensitizer which is a salt of a cycloheptenyl compound. The term cycloheptenyl compound is meant to include derivatives of cycloheptane, cycloheptene, cycloheptadiene and cycloheptatriene.

The cycloheptenyl salts of this invention can be sub stituted in the nucleus, if desired, with a wide variety of substituents such as:

(a) An aryl radical including substituted and unsubstituted aryl radicals such as phenyl and naphthyl, and particularly phenyl radicals containing amino groups such as alkylamino groups, as well as aryl radicals containing a fused heterocyclic ring,

(b) A heterocyclic radical including substituted or unsubstituted heterocyclic radicals, and

(c) A hydrogen atom.

The salt is preferably a perchlorate or fiuoroborate but any anion capable of forming a salt with the cycloheptenyl radical is suitable, e.g., sulfate, chloride, bromide, iodide,

etc.

When the cycloheptenyl salts of this invention are used in conjunction with composition containing conventional photoconductors, substantial increases are noted in the electrical speeds of the composition. Also, these sensitizers are markedly stable to moisture when so exposed for a period of time.

The preferred cycloheptenyl salt sensitizers of this irivention are represented by the following structures:

. 3 where E, G and D can be either (a) A phenyl radical,

(b) A naphthyl radical,

(c) A heterocyclic radical having to 6 atoms in the heterocyclic nucleus and at least one hetero nitrogen atom, or

(d) A hydrogen atom;

(R and R (R and R (R and R and (R and R are together the necessary atoms to complete a benzene ring fused to the cycloheptenyl nucleus; X is an anion such as a perchlorate, fluoroborate, sulfate, chloride, bromide or iodide.

Typical cycloheptenyl compounds which belong to the herein'described general class of compounds include the following:

(1) trop ylium perchlonate,

(2) (4-dimethylaminophenyl)tropylium fluoroborate,

(3) 5-(4-dimethylaminophenyl)-5H-dibenzo[a,d]

tropylium fiuoroborate,

(4) 5-(Z-nrethyl-4-diethylaminophenyl)-5H-dibenzo [a,d]tropylium fluoroborate,

(5) 5-(4-dimethylaminol-naphthyD-SH-dibenzoa,d]

tropylium fluoroborate,

(6) 5-(9-julolidyl)-5H-dibenzo[a,d]tropyliu'm fiuoroborate,

(7) 5H-dibenzo[a,d] tropylium fiuoroborate, and

(8) (5H-dibenzo[a,d]cycloheptenyl)-1,2,3-triazolyl fluoroborate.

Electrophotographic elements of the invention can be prepared with any photoconductive compound and the sensitizers of this invention in the usual manner, i.e., by blending a dispersion or solution of the photoconductive compound together with a binder, when necessary or desirable, and coating or forming a self-supporting layer with the photoconductive composition. Generally, a suitable amount of the sensitizing compound is mixed with the photoconductive coating composition so that, after thorough mixing, the sensitizing compound is uniformly distributed throughout the desired layer of the coated element. The amount of sensitizer that can be added to a photoconductor-incorporating layer to give effective increases in speed can vary widely. The optimum concentration in any given case will vary with the specific photoconductor and sensitizing compound used. In general, sub stantial speed gains can be obtained where an appropriate sensitizer is added in a concentration range from about 0.0001 to about 30 percent by weight based on the weight of the film-forming coating composition. Normally, a sensitizer is added to the coating composition in an amount from about 0.005 to about 5.0 percentby weight of the total coating composition.

The sensitizers of this invention are effective for enhancing the electrophotosensitivity of a wide variety of photoconductors. The preferred photoconductors are those organic compounds which exhibit an electrophotosensitivity to light and are capable of forming transparent elements. An especially useful class of organic photoconductors is referred to herein as organicamine photoconductors. Such organic photoconductors 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;

4,4 benzylidene bis N,N diethyl m toluidine), those described in Fox US. Patent 3,240,597 issued Mar. 15, 1966, and the like, and (2) triarylamines 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(diphenylamino)benzyl, 4,4- bis(diphenylamino)benzophenone, and the like, and (b) polymeric triarylamines such as poly[N,4"-(-N,N',N-triphenylbenzidinefl; polyadipyltriphenylamine, polysebacyltriphenylamine; polydecamethylenetriphenylamine; poly- N (4 vinylphenyl)diphenylamine, poly N (vinylphenyl) 00,12 dinaphthylamine and the like. Other useful amine-type photoconductors are disclosed in US. Patent 3,180,730 issued Apr. 27, 1965.

Useful photoconductive substances capable of being spectrally sensitized in accordance with this invention are disclosed in Fox U.S. Patent 3,265,496 issued Aug. 9, 1966, and include those represented by the following general formula:

GI:NA] Q, A

wherein A represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, binaphthyl, etc.), or a substituted divalent aromatic radical of these typeswherein 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; A 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., rnethoxy, propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogen atom, a halogen atom or an aromatic amino group, such as ANH; b represents an integer from 1 to about 12, and G 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'-vinylphenyl) 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. Patent 3,274,000; French Patent 1,383,461 and in copending application of Seus and Goldman titled Photoconductive Elements Containing Organic Photoconductors, filed Apr. 3, 1967. These photoconductors include leuco bases of diaryl or triaryl methane dye salts, 1,1,1-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 polyaryl alkane photoconductors can be rep resented by the formula:

wherein each of D, E and G is an aryl group and I 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 preferably 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 to gether the necessary atoms to form a heterocyclic amino group typically having 5 to 6 atoms in the ring such as morpholino, pyridyl, pyrryl, 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 below:

TABLE A Compound No.2

(1) 4,4 benzylidine bis(N,N diethylm-toluidine) (2) 4',4" diamino 4 dimethylamino- 2,2"-dimethyltriphenylmethane.

-(3) 4',4" bis(diethylamino) 2,6 dichloro 2',2" dimethyltriphenylmethane.

(4) 4',4" bis(diethylamino) 2,2" dimethyldiphenylnaphthylmethane.

(5) 2,2" dimethyl 4,4,4 tris(dimethylamino triphenylmethane.

(6) 4',4" bis(diethylamino) 4 dimethylamino 2,2 dimethyltriphenylmethane.

(7) 4',4" bis (diethylamino) 2 chloro- 2',2" dimethyl 4 dimethylaminotriphenylmethane.

(8) 4',4 bis(diethylamino) 4 dimethylamino 2,2',2" trimethyltriphenylmethane.

(9) 4,4'-' bis(dimethylamino) 2 chlo- 4 I0 2,2" dimethyltriphenylmethane.

'(10) 4',4" bis(dimethylarnino) 2,2"-

, dimethyl 4 methoxytriphenylmethane.

(ll) Bis(4 diethylamino) 1,1,1 triphenylethane.

(12) Bis(4 diethylamino)tetraphenylmethane.

(13) 4,4 bis(benzylethylamino) 2',

2"-dimethyltriphenylmethane.

(14) 4,4" bis(diethylamino) 2,2" diethoxytriphenylmethane.

(15) 4,4 -bis(dimethylamino) 1,1,1 triphenylethane.

(l6) 1 (4 N,N dimethylaminophenyl) 1,1-diphenylethane. ('17) 4-dimethylaminotetraphenylmethane. (18) 4-diethylaminotetraphenylmethane.

Preferred binders for use in preparing the present photoconductive layers comprise polymers having fairly high dielectric strength which are good electrically insulating film-forming vehicles. Materials of this type comprise styrene-butadiene copolymers; silicone resins; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; "poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride. L acrylonitrile 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(ethylenealkaryloxyalkylene terephthalate); phenol formaldehyde resins; ketone resins; polyamide; polycarbonates; polythiocarbonates; poly (ethyleneglycolco bishydroxyethoxyphenyl propane terephthalate); etc. Methods of making resins of this type have been described in the prior art, for example, styrene-alkyd resins can be prepared according to the method described in US. Patents 2,361,019 and 2,258,423. Suitable resins of the type contemplated for use in the photoconductive layers of the invention are sold under such trade names as Vitel PE-101, Cymac, Piccopale 100, Saran F-220 and Lexan 105. Other types of binders which can be used in the photoconductive layers of the invention include such materials as paraffin, mineral Waxes, etc.

Solvents of choice for preparing coating compositions of the present invention can include a number of solvents such as benzene, toluene, acetone, Z-butanone, chlorinated hydrocarbons, e.g., methylene chloride, ethylene chloride, etc., ethers, e.g., tetrahydrofuran, or mixtures of these solvents, etc.

In preparing the coating composition useful results are obtained Where the photoconductor substance is present in an amount equal to at least about 1 weight percent of the coating composition. The upper limit in the amount of photoconductor substance present can be widely varied in accordance with usual practice. In those cases where a binder is employed, it is normally required that the photoconductor substance be present in an amount from about 1 weight percent of the coating composition to about 99 weight percent of the coating composition. A preferred weight range for the photoconductor substance in the coating composition is from about 10 weight percent to about 60 weight percent.

Coating thickness of the photoconductive composition on a support can vary widely. Normally, a coating in the range of about 0.001 inch to about 0.01 inch before drying is useful for the practice of this invention. The preferred range of coating thickness was found to be in the range from about 0.002 inch to about 0.006 inch before drying although useful results can be obtained outside of this range.

Suitable supporting materials for coating the photoconductive layers of the present invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminum-paper 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 or aluminum and the like. An especially useful conducting support can be prepared by coating a support material such as polyethylene terephthalate with a layer containing a semiconductor dispersed in a resin. Such conducting layers both with and without insulating barrier layers are described in US. Patent 3,245,833. 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 and 3,267,807.

The elements of the present invention can be employed in any of the well-known electrophotographic processes which require photoconductive layers. One such process is the aforementioned xerographic process. As explained previously, in a process of this type the electrophotographic element is given a blanket electrostatic charge by placing the same under a corona discharge which serves to give a uniform charge to the surface of the photoconductive layer. This charge is retained by the layer owing to the substantial 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 photo-conducting layer is then selectively dissipated from the surface of the layer by exposure to light through an 7 image-bearing transparency by a conventional exposure operation such as, for example, by contact-printing technique, or by lens projection of an image, etc., to form a latent image in the photoconducting layer. By exposure of the surface in this manner, a charged pattern is created by virtue of the fact that light causes the charge to be conducted away in proportion to the intensity of the illumination in a particular area. The charge pattern remaining after exposure is then developed, i.e., rendered visible, by treatment with a medium comprising electrostatically attractable particles having optical density. The

developing electrostatically attractable particles can be in the form of a dust, e.g., powder, a pigment in a resinous carrier, i.e., toner, or a liquid developer may be used in which the developing particles are carried in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature in such patents, for example, a U.S. 2,297,691, and in Australian Patent 212,- 315. In processes of electrophotographic reproduction such as in xerography, by selecting a developing particle which has as one of its components, a low-melting resin, it is possible to treat the developed photoconductive material with heat and cause the powder to adhere permanently to the surface of the photo-conductive layer. In other cases, a transfer of the image formed on the photoconductive layer can be made to a second support, which would then become the final print. Techniques of the type indicated are well known in the art and have been described in a number of US. and foreign patents, such as US. Patents 2,297,691 and 2,551,582, and in RCA Review, vol. 15 1964), pages 469-484.

The present invention is not limited to any particular mode of use of the new electrophotographic materials,

and the exposure technique, the charging method, the transfer (if any), the developing method, and the fixing method as well as the materials used in these methods can be selected and adapted to the requirements of any particular technique.

Electrophotographic materials according to the present invention can be applied to reproduction techniques wherein difierent kinds of radiations, i.e., electromagnetic radiations as well as nuclear radiations, can be used. For this reason, it is pointed out herein that although materials according to the invention are mainly intended for use in connection with methods comprising an exposure, the term electrophotography wherever appearing in the description and the claims, is to be interpreted broadly and understood to comprise both xerography and xeroradiography.

The invention is further illustrated by the following examples which include preferred embodiments thereof.

EXAMPLE 1 Preparation of tropylium perchlorate To a solution of 6.86 g. (0.02 m.) of triphenylmethyl perchlorate in about 150 ml. of acetonitrile is added 1.84 g. 0.02 m.) of cycloheptatriene. The solvent is removed and the residue triturated with ether. The solid is collected on a filter and recrystallized from acetonitrileethylacetate. The yield is 92% (3.5 g.) and the melting point is greater than 280 C.

EXAMPLE 2 Preparation of N,N-dimethylaminophenyl tropylium fluoroborate This compound is prepared by the method of Jutz and Voithenleitner, Ber. Dtsch. Chem. Ges. 97:29 (1964) p. 45

8 EXAMPLE 3 Preparation of 5-(4-dimethylaminophenyl)-5H I dibenzo [a,d] tropylium fluoroborate A solution of 0.1 mole of N,N-dimethylaniline in 50 ml. of 1,2-dimethoxyethane is added to a solution (0.048 mole) of 5,5 dichloro-SI-I-di-benzo[a,d]cycloheptene in 50 ml. of 1,2-dimethoxyethane. After one hour the solvent is removed and the residue dissolved in 200 ml. of 3 percent HCl. An excess of fluoroboric acid is added and the oil that separates is taken up in ml. of pro; pionic anhydride. The product is precipitated by the addition of ether and recrystallized from ethyl acetate-nitromethane. The yield is 8.0 g. (42 percent) of a solid which exhibits a melting point of 242-243 C.

EXAMPLE 4 Preparation of 5-(4-dimethylamino-1-naphthyl)-5H- dibenzo [a,d]tropylium fluoroborate The precedure of Example 3 is followed, with the exception that 0.1 mole of 4(N,N-dimethyl)-1-naphthalene was used in place of the N,N-dimethylaniline. The product is obtained in 44 percent yield and has a melting point of 283-285 C. after purification by recrystallization from acetonitrile.

EXAMPLE 5 Preparation of 5-(Z-methyl-4-diethylaminophenyl)- SH-dibenzo [a,d]tropylium fluoroborate The procedure of Example 3 is followed, using as the starting material 0.1 mole of 3-methyl-N,N-(diethylaniline in place of the N,N-dimethylaniline. The product is obtained in 14 percent yield and has a melting point of 233-235 C. after recrystallization from acetonitrileethyl acetate mixture.

EXAMPLE 6 Preparation of 5-(9-julolidy1)-5H-dibenzo [a,d]tropylium fluoroborate The procedure of Example 3 is followed, using as the starting material 0.1 mole of julolidine in place of the N,N-dimethylaniline. The product is obtained in 24 percent yield and has a melting point of 214215 C. after recrystallization from acetonitrile.

EXAMPLE 7 Photoconductive compositions containing cycloheptenyl salt sensitizers of the type described herein are separately incorporated into a coating dope having the following composition:

Organic photoconductor-0.15 g.

Polymeric binder-0.50 g.

Sensitizer0.002 g.

Methylene chloride or tetrahydrofuran5 ml.

These compositions are then separately coated on a wet thickness of 0.004 inch on an aluminum surface maintained at 100 F. to provide the coatings described in Table 1 below. In a darkened room, the surface of each of the photoconductive layers so prepared is either positively or negatively charged to a potential of about 600 volts under a corona charger. The charged layer is exposed through a stepped density gray scale to the radiation from a 60-watt incandescent lamp at a distance of 100 cm. which provides an illumination-intensity of about 75 meter-candles. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potential, V to some lower potential, V, whose exact value depends upon the actual amount of exposure received by the area. The element is exposed through each step ofthe scale and the resultant surface potential measured. The results of these measurements are plotted on a graph of surface potential V versus log of the exposure for each step. The actual speed of each element is expressed in terms of the reciprocal of the exposure required to reduce the surface potential to any fixed arbitrarily assigned value. The results of these measurements are set forth in the following Table 1. The speed is the quotient of 10 divided by the exposure in meter-candle-seconds required to reduce the potential by 100 volts. The polymeric binder used in the coating compositions is a polyester of terephthalic acid and a mixture of ethylene glycol (1 part by weight) and 2,2-bis(4- hydroxy-ethoxyphenyl) propane (9 parts by weight). The photoconductors referred to in the table are as follows:

MTriphenylamine N1,3,S-triphenyl-Z-pyrazoline O4,4'-bis (diethylamino) -2,2-dimethyltriphenylmethane P2,3,4,5-tetraphenylpyrrole Q-4,4'-bis(diethylamino)benzophenone TABLE 1 Speed Photosensitizer conductor 5-(4-dimethylaminophenyl)-5H-dibenzo M 400 320 [a,d]tropylium fluoroborate. N 100 100 O 160 100 P 80 120 Q 120 63 5-(9-1 ulolidyl)-5H-d1benzo[a,d]tropylium M 160 100 fluoroborate. N 250 100 200 80 P 250 100 Q, 120 32 -(4-dimethylamino-l-naphthyl)-5H- M 320 50 dibenzo[a,d]tropylium fiuoroborate. N 160 25 O 63 16 P 120 Q 164 12 5-(2 methyl 4-diethylaminophenyl)5H- M 200 63 d1benzo[a,d]tropylium fluoroborate. N 320 80 O 200 50 P 200 50 Q, 160 4-(d1methylaminophenyl) -tropylium N fluoroborate 0 30 Tropylium perchlorate M None M 16 EXAMPLE 8 Coating compositions containing the sensitizers of this invention are prepared and coated in the manner described in Example 1. In a darkened room, the surface of each of the photoconductive layers so prepared is charged to a potential of about +600 volts under a corona charger. The layer is then covered with a transparent sheet hearing a pattern of opaque and light transmitting areas and exposed to the radiation from an incandescent lamp with an illumination intensity of about 75 meter-candles for 12 seconds. The resulting electrostatic latent image is developed in the usual manner by cascading over the surface of the layer a mixture of negatively charged black thermoplastic toner particles and glass beads. A good reproduction of the pattern results in each instance.

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

We claim:

1. An electrophotographic element comprising a support having coated thereon a photoconductive composition comprising an organic photoconductor dispersed in a resin binder and a sensitizer having a formula selected from the group consisting of:

(a) a phenyl radical,

(b) a naphthyl radical,

(c) a heterocyclic radical having 5 to 6 atoms in the heterocyclic nucleus, and including at least one hetero nitrogen atom, and v (d) a hydrogen atom;

(R and R (R and R (R and R and (R and R are together the necessary atoms to complete a benzene ring fused to the cycloheptenyl nucleus; and

X is an anion.

2. The electrophotographic element of claim 1 wherein the sensitizer is a perchlorate salt of the cycloheptenyl compound.

3. The electrophotographic element of claim 1 wherein the sensitizer is a fluoroborate salt of the cycloheptenyl compound.

4. A photoconductive composition comprising an organic photoconductor dispersed in a resin binder and a sensitizer selected from the group consisting of tropylium perchlorate, (4-dimethy1aminophenyl)-tropyliurn fluoro- 0 borate, 5-(4-dimethylaminophenyl) 5H dibenzo[a,d]

tropylium fluoroborate, 5-(2-methyl 4 diethylaminophenyD-SH-dibenzo [a,d]tropylium fiuoroborate, 5- (4-dimethylamino1-naphthyl) 5H dibenzo[a,d]tropylium fluoroborate and 5-(9-julolidyl) 5H dibenzo[a,d]tropylium fluoroborate.

5. An electrophotographic element comprising a support having coated thereon the photoconductive composition of claim 4.

6. The electrophotographic element of claim 5 wherein the photoconductor is selected from the group consisting of triphenylamine, 1,3,5-triphenyl 2 pyrazoline, bis(4- diethylamino)-l,l,1-triphenylethane, 4,4 bis(diethy1- amino)-2,2'-dimethyltriphenylmethane, bis(4 diethylamino)tetraphenylmethane, 2,3,4,5 tetraphenylpyrrole and 4,4'-bis(diethylamino)-benzophenone.

7. A photoconductive element for use in electrophotography comprising a support having coated thereon a photoconductive Composition comprising:

(a) about 10 to by weight based on said photoconductive composition of an organic photoconductor,

(b) a film-forming polymeric binder for the said photoconductor, and

(c) 0.005 to 5% of 5-(4-dimethylaminophenyl) -5H- dibenzo[a,d]tropylium fiuoroborate as a sensitizer for the said photoconductor.

8. A photoconductive element for use in electrophotography comprising a support having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductive composition of an organic photoconductor,

(b) a film-forming polymeric binder for the said photoconductor, and

(c) 0.005 to 5% of 5-(9-julolidyl)-5H-dibenzo[a,d] tropylium fluoroborate as a sensitizer for the said photoconductor.

9. A photoconductive element for use in electrophotography comprising a support having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductive composition of an organic photoconductor,

(b) a film-forming polymeric binder for the said photo conductor, and

(c) 0.005 to 5% of 5-(4-dimethylamino-1-naphthyl)- 5H-dibenzo[a,d]tropylium fluoroborate as a sensitizer for the said photoconductor.

10. A photoconductive element for use in electrophotography comprising a support having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductiv composition of an organic photoconductor,

11. n w o n ia mm ng po ymeriebinder for the said ,photoconductor; and

- (c) 0.005 to 5%vof5-(2-methyl-4-diethyleminophenyl)- 5H-dibenzo[a,'d]tropy1inm fluorobor'ate as a sensitizer for the said photoconductor. 1 11. 'A photoconductive element for use ine'lectrophotogfra'phy comprising a support having coated thereon a ph'oto'conductive' composition comprising: I (a) about 10 to 60% by weight based on'said photoconductive composition of an organic photocon- 'd'ucto'r, (b) a film-forming polymeric binder for the said photoconductor, and 1 12 (c) 0.005 to'5% of' tropylium' perchlorate 'as 'asensi tizer for the said photoconductor. t I References Cited" v NITE D STATE PATENTS 3,141,770 7/1964 Davis et'a1.v4 9641' .'3,143,544 h 8/1964 Van Dormail j. 260 24 O GEORGE F. LESME S, Primary Examiner 10 J'. c, COOPER, As sist ant Exeminer U.S. Cl. X.R. 96-1.6