3,488,705 United States Patent Office Patented Jan. 6, 1970 2 3,488,705 having desirable electrophotographic properties can be THERMALLY UNSTABLE ORGANIC ACD SALTS especially useful in elecetrophotography. Such electro OF TRARYLMETHANE DYES AS SENSTEZERS photographic elements may be exposed through a trans FOR ORGANIC PHOTOCONDUCTORS parent base if desired, thereby providing unusual flexibility Charles J. Fox and Arthur L. Johnson, Rochester, N.Y., in equipment design. Such compositions, when coated as assignors to Eastman Kodak Company, Rochester, a film or layer on a suitable support also yield an element N.Y., a corporation of New Jersey No Drawing. Continuation-in-part of application Ser. No. which is reusable; that is, it can be used to form subse 447,937, Mar. 16, 1965. This application Dec. 4, 1967, quent images after residual toner from prior images has Ser. No. 687,503 been removed by transfer and/or cleaning. nt. C. G03g 5/06, 9/00 0. Although some of the organic photoconductors com U.S. C. 96-1.6 32 Claims prising the materials described are inherently light sensi tive, their degree of sensitivity is usually low and in the short wavelength portion of the spectrum so that it is ABSTRACT OF THE DISCLOSURE common practice to add materials to increase the speed 15 and to shift the sensitivity toward the longer wavelength Organic acid salts of triarylmethane dyes are useful as portion of the visible spectrum. Increasing the speed and sensitizers in electrophotographic elements. They are shifting the sensitivity of such systems into the visible thermally unstable and thus readily bleachable. Accord regon of the spectrum has several advantages: it makes ingly, exposed and developed electrophotographic ele available inexpensive and convenient light sources such ments having colorless background areas are attainable. 20 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 This application is a continuation-in-part of Ser. No. various optical systems. By increasing the speed through 447,937, filed Mar. 16, 1965, now U.S. Patent 3,387,973, the use of sensitizers, photoconductors which would other which in turn is a division of Ser. No. 163,092, filed Dec. 25 wise have been unsatisfactory are useful in processes 29, 1961, now U.S. Patent 3,234,280. where high speeds are required such as document copy This invention relates to electrophotography, and in ing. However, while some of the sensitizers have appre particular to novel sensitized photoconductive composi ciably increased the speed of a photoconductive system, tions and elements having coated thereon such composi they have also produced a colored background in the tions. 30 finished print. This is particularly true of the triaryl The process of xerography, as disclosed by Carlson in methane dye Sensitizers such as crystal violet, rhodamine U.S. Patent No. 2,297,691, employs an electrophotographic B and hexaphenylpararosaniline. While these dyes are element comprising a support material bearing a coating generally considered to be effective for sensitizing photo of a normally insulating material whose electrical resist conducive compounds, they also impart color to the initial ance varies with the amount of incident actinic radiation photoconductive composition. The intense color of these it receives during an imagewise exposure. The element, dyes is maintained throughout the processing of the photo commonly termed a photoconductive element, is first conductive coatings and thus appears in the final product. given a uniform surface charge, generally in the dark It is, therefore, an object of this invention to provide after a suitable period of dark adaptation. It is then ex a novel class of triarylmethane dye sensitizers for use posed to a pattern of actinic radiation which has the effect 40 in combination with photoconducting compounds so that of differentially reducing the potential of the surface colorless final reproductions are obtainable. charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential Another object of this invention is to provide novel surface charge or electrostatic latent image remaining on Sensitized photoconductive elements. the electrophotographic element is then made visible by It is also an object to provide novel sensitized photo contacting the surface with a suitable electroscopic mark 45 conductive compositions which can be positively and ing material. Such marking material or toner, whether negatively charged. contained in an insulating liquid or on a dry carrier, can These and other objects of this invention are accom be deposited on the exposed surface in accordance with plished with photoconductive compositions containing a either the charge pattern or the absence of charge pattern photoconductor and a sensitizer which is an organic acid as desired. The deposited marking material may then be 50 Salt of a triarylmethane dye. In accordance with this in either permanently fixed to the surface of the sensitive vention, it has been found that the organic acid salts of element by known means such as heat, pressure, solvent these dyes have a range of thermal instability. The ad vapor, or the like, or transferred to a second element to vantage of employing a thermally unstable sensitizing which it may similarly be fixed. Likewise, the electrostatic dye is that the intense color of the dye in the background latent image can be transferred to a second element and 55 areas of the image-bearing element may be eliminated developed there. simply by applying heat to the coating. The use of the Various photoconductive insulating materials have been organic acid salt does not alter the ability of the triaryl employed in the manufacture of electrophotographic ele methane to effectively sensitize the photoconductive com ments. For example, vapors of selenium and vapors of 60 position. While the chloride salts, the usual form in which selenium alloys deposited on a suitable support and par the dyes exist, are relatively stable, the corresponding ticles of photoconductive zinc oxide held in a resinous, film-forming binder have found wide application in pres organic acid salt derivatives of these dyes are thermally ent-day document copying applications. unstable. Thus, upon heating an element after an image Since the introduction of electrophotography, a great is obtained, the dye compound is decomposed and bleach many organic compounds have also been screened for 65 ing occurs. By this technique the background areas of their photoconductive properties. As a result, a very large the reproduction are rendered colorless. number of organic compounds are known to possess Some The anion of the triarylmethane organic acid salts of degree of photoconductivity. Many organic compounds this invention are derived from a wide variety of organic have revealed a useful level of photoconduction and have acids. The term organic acid salt includes mono-salt de been incorporated into photoconductive compositions. Op 70 rivatives as well as di-salt derivatives obtained from tically clear organic photoconductor-containing elements monocarboxylic and polycarboxylic acids. Typical organic 3,488,705 3 4. acid Salt derivatives which form the anion of the sensi di-(dichloroacetate) salt of poly-hexaphenylpararosaniline tizers of this invention include the following: di-(cyanoacetate) salt of poly-hexaphenylpararosaniline Alkanoate derivatives having one to ten carbon atoms The organic acid salts of the triarylmethane dyes of Such as formate, acetate, propionate and butyrate radicals this invention are obtained by converting the chloride salt, including substituted alkanoate derivatives such as halo 5 the usual form in which the dye exists, to the carbinol acetate, cyanoacetate and oxalate radicals; base and subsequently treating the carbinol base with the Aryloate derivatives such as benzoate and naphthoate desired acid. The di-salt is obtained by using twice the radicals and including substituted aryloate derivatives such stoichiometric amount of acid required. The conversion to as a salicylate radical; the carbinol base is accomplished by treating the chloride Alkenoate derivatives having one to ten carbon atoms O salt in solution with dilute alkali. such as acrylate, crotonate and vinylacetate radicals; and Electrophotographic elements of the invention can be Aryl and alkane sulfonate derivatives such as benzene prepared with any photoconductive compound and the Sulfonate and methylsulfonate radicals. sensitizers of this invention in the usual manner, i.e., by The cation of the sensitizing salts of this invention is blending a dispersion or solution of the photoconduc formed from any of the triarylmethane dyes which ex tive compound together with a binder, when necessary or hibit a sensitizing effect when used with a photoconductive desirable, and coating or forming a self-supporting layer composition. Such cation furnishing dyes include malachite with the photoconductive composition. Generally, a suit green; various dyes from the rosaniline series such as able amount of the sensitizing compound is mixed with the pararosaniline, hexaphenylpararosaniline, tri-para-tolyl photoconductive coating composition so that, after pararosaniline, methyl violet and crystal violet; various 20 thorough mixing, the sensitizing compound is uniformly phthaleins such as phenol phthalein; and Xanthenes such distributed