Electrophotographic Material Containing Sensitizers

This invention relates to novel light-sensitive layers and more particularly to electrophotographic light-sensitive polymer layers.

Various light-sensitive layers are well known in the electrophotographic art for making copies of documents, drawings, transparencies, etc. These layers contain organic photoconductive compounds, such as poly-N-vinylcarbazole, brominated poly-N-vinylcarbazoles, polyacenaphthylene, etc. These layers are non-conductors of electricity before exposure and become electrical conductors upon exposure.

It is necessary for the electrophotographic art that the photoconductive material has a high photoconductivity in the long wavelength region of the visible spectrum. Such a photoconductive material makes it possible for the electrophotograph art to employ inexpensive and convenient light sources such as incandescent lamps in a reduced exposure time and to reproduce colored pictures. The photoconductive materials mentioned above, however, have usually a low photoconductivity and spectrum characteristics sensitive to the short wavelength region of the exposure light and do not satisfy entirely the above requirement.

It is known that an addition of a so-called sensitizer can improve photoconductivity and spectrum characteristics of the above photoconductive compounds. Conventional sensitizers are dyestuff compounds, such as triarylmethane dyes, xanthene dyes, triazine dyes or acridine dyes, but the conventional sensitizers are not entirely satisfactory to improve the photoconductivity and the spectrum characteristics of the available photoconductive compounds.

In addition to the high photoconductivity and spectrum characteristics sensitive to visible light, the electrophotographic art requires a high electric resistance of photoconductive materials in the dark. Conventional sensitizers are apt to increase the dark conductivity whereas they promote the photoconductivity. A high dark conductivity of photoconductive materials is not desirable because it results in a loss of the applied electrostatic charge in the dark.

An object of the invention is to provide electrophotographic materials having a high photoconductivity and spectrum characteristics sensitive to a long wavelength region of the visible spectrum.

Another object of the invention is to provide electrophotographic materials having a high electric resistance in the dark.

These and other objects are accomplished by adding a new sensitizer shown hereinafter to a photoconductive polymeric compound such as poly-N-vinylcarbazole, brominated poly-N-vinylcarbazole or polyacenaphthylene, as a light-sensitive film-forming composition.

The invention is further explained in the following description with reference to the accompanying drawings wherein:

FIG. 1 represents the wedge spectrogram for an electrophotographic material coated with an unsensitized solution of 10 weight percent of brominated poly-N-vinylcarbazole in chlorobenzene.

FIG. 2 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-(.alpha.-phenyl-p-methoxystyryl)-4-[(2'-phenyl-4'-benzopyranylidene)-met hyl]benzopyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

FIG. 3 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-styryl-3-phenyl-4-[(2'-styryl-4'-benzopyranylidene)methyl]-6-methylbenzo pyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

FIG. 4 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-p-methoxystyryl-4-[2'-.alpha.-phenyl-p-methoxystyryl-4'-benzopyranyliden e)methyl]benzopyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

FIG. 5 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-p-methoxystyryl-4-[(2'-phenyl-4'-benzothiopyranylidene)-methyl]benzopyry lium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

FIG. 6 represents the wedge spectrogram for an electrophotograph material coated with a solution containing 10 weight percent of brominated poly-N-vinyl carbazole sensitized with 0.04 weight percent of 2-[(2'-phenyl-4'-benzopyranylidene)methyl]3-phenylbenzopyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four parts of chlorobenzene and one weight part of dichloroethane.

FIG. 7 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-[(4'-benzothiopyranylidene)methyl]-3-phenylbenzopyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

FIG. 8 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-[(2'-.alpha.-phenyl-p-methoxystyryl-4'-benzopyranylidene)methyl]-3-pheny lbenzopyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

FIG. 9 represents the wedge spectrogram for an electrophotographic material coated with a solution containing 10 weight percent of brominated poly-N-vinylcarbazole sensitized with 0.04 weight percent of 2-[(2'-phenyl-4'-benzothiopyranylidene)methyl]-3-phenyl-benzopyrylium perchlorate, the brominated poly-N-vinylcarbazole being dissolved in a solvent of four weight parts of chlorobenzene and one weight part of dichloroethane.

The new sensitizer, according to the present invention has the following formula: ##SPC4##

where B represents a radical selected from the group consisting of, ##SPC5## ##SPC6##

R.sub.1 is hydrogen, phenyl or an ethenyl or an ethenyl radical selected from the group consisting of styryl,p-methoxystyryl, 3,4-dimethoxystyryl, .alpha.-phenylstyryl, .alpha.-phenyl-p-methoxystyryl, .beta.-furyl(.alpha.')ethenyl and .alpha.-phenyl-.beta.-furyl(.alpha.')-ethenyl;

R.sub.2 is hydrogen or phenyl;

R.sub.3 is hydrogen, methyl or phenyl;

R.sub.4 is hydrogen or phenyl;

R.sub.5 is hydrogen, phenyl or an ethenyl radical selected from the group consisting of styryl, p-methoxystyryl, 3,4-dimethoxystyryl, .alpha.-phenyl-p-methoxystyryl, .beta.-furyl(.alpha.')ethenyl and .alpha.-phenyl-.beta.-furyl(.alpha.')ethenyl;

R.sub.6 is hydrogen or phenyl;

R.sub.7 is hydrogen, methyl or phenyl;

R.sub.8 is hydrogen or phenyl;

R.sub.9 is hydrogen or p-methoxyphenyl;

the anion is anionic function selected from the group consisting of perchlorate, fluoroborate, chloroferrate, chlorozincate and nitrate; and

X is oxygen or sulfur atom.

The sensitizers having the formula above mentioned are prepared by the following chemical equations A or B: ##SPC7##

In the case of Equation A, 4-[(4'-benzopyranylidene)-methyl]benzopyrylium derivative (III) is obtained by an addition reaction of 4-methoxybenzopyrylium o-nitrobenzene sulfonate (I) with a benzopyrylium salt (II) in an acetic anhydride solution. The benzopyrylium salt has an active methyl or methylene radical.

In the case of Equation B, 2-[(4'-benzopyranylidene)-methyl]benzopyrylium derivative (V) is obtained by an addition reaction of 4-methoxybenzopyrylium o-nitrobenzene sulfonate (I) with a benzopyrylium salt (IV) in an acetic anhydride solution. The benzopyrylium salt has an active methyl radical.

In both cases, methyl 2,4-dinitrobenzene sulfonate, dimethyl sulfate or methyl iodide can be used instead of methyl o-nitrobenzene sulfonate.

Representative examples of the benzopyranylidenemethylbenzopyrylium salts and benzothiopyranylidene-methylbenzopyrylium salts according to the invention are listed in Table I.

The following description will explain a practical method for making benzopyranylidene-methylbenzopyrylium salts or benzothiopyranylidene-methylbenzopyrylium salts with reference to exemplary compounds. The details of the preparation of other compounds will be apparent to the skilled in the art from the preceding disclosure and the following illustrative examples of preparation methods of various compounds according to the invention:

(a) 2-phenyl-4-[(2'-phenyl-4'-benzopyranylidene)-benzyl]benzopyrylium perchlorate (compund number 1) ##SPC8##

5.5 Grams of 2-phenyl-4-methoxybenzopyrylium o-nitrobenzene sulfonate, which is obtained from flavone and methyl o-nitrobenzenesulfonate, and 5 grams of 2-phenyl-4-benzylbenzopyrylium perchlorate are dissolved in 150 milliliters of acetic anhydride to a solution. The solution is then refluxed for 15 minutes and poured into 900 milliliters of 10 weight percent perchloric acid. The solution is filtered to obtain a precipitate. The precipitate is dried and dissolved again in dichloroethan to a solution. For purification the solution is poured into ether. Precipitated crystals are filtered off, washed with ether and dried to obtain 4 grams (theoretical yield 53%) of green crystals having a melting point of 135.degree. to 139.degree. C.

(b) 2-(.alpha.-phenyl-p-methoxystyryl)-4-[(2'-phenyl-4'-benzopyranylidene)meth yl]benzopyrylium perchlorate (compound number 7) ##SPC9##

5.5 grams of 2-(.alpha.-phenyl-p-methoxystyryl)-4-methylbenzopyrylium perchlorate and 5.5 grams of 2-phenyl-4-methoxybenzopyrylium o-nitrobenzene sulfonate are dissolved in 150 milliliters of acetic anhydride to a solution. The solution is then refluxed for 15 minutes and poured into 900 milliliters of 10 weight percent perchloric acid. The solution is filtered to obtain a precipitate. The precipitate is reprecipitated with dichloroethane ether. Precipitated crystals are filtered off, washed with ether and dried to obtain 5.5 grams (theoretical yield 69%) of blue crystals having a melting point of 162.degree. to 169.degree.C. In the procedure above mentioned the solution ios poured into an ether solution of BF.sub.3.O(C.sub.2 H.sub.5).sub.2 instead of 10 weight percent perchloric acid. In this case 2-(.alpha.-phenyl-p-methoxystyryl)-4-[(2'-phenyl-4'-benzopyranylidene)meth yl]benzopyrylium fluoroborate (compound number 88) is obtained. In the procedure when the solution is poured into a 10 weight percent zinc chloride aqueous solution, 2-(.alpha.-phenyl-p-methoxystyryl)-4-[(2'-phenyl-4'-benzopyranylidene)-met hyl]benzopyrylium chlorozincate (compound number 90) is obtained.

(c) 2-p-methoxystyryl-3-phenyl-4-[(2'-styryl-4'-benzopyranylidene)methyl]-6-me thylbenzopyrylium perchlorate (compound number 18) ##SPC10##

2-styryl-4-methoxybenzopyrylium o-nitrobenzenesulfon (melting point, 155.degree. to 157.degree. C) is obtained by a reaction of 2-styrylchromone and methyl o-nitrobenzenesulfonate in dry benzene at 50.degree. C for 24 hours. According to the same reaction as shown in procedure (a), 2-styryl-4-methoxybenzopyrylium o-nitrobenzenesulfonate reacts with 2-p-methoxystyryl-3-phenyl-4,6-dimethylbenzopyrylium perchlorate to obtain 2-p-methoxystyryl-3-phenyl-4-[(2'-styryl-4'-benzopyranylidene)methyl]-6-me thylbenzopyrylium perchlorate crystals which have a color of reddish violet in dichloroethane and have a melting point of 159.degree. to 162.degree. C. The product is obtained in 73% yield.

(d) 2-(.alpha.-phenyl-p-methoxystyryl)-4-[(4'-benzopyranylidene)-methyl]benzop yrylium perchlorate (compound number 22) ##SPC11##

4-methoxybenzopyrylium o-nitrobenzenesulfonate reacts with 2-(.alpha.-phenyl-p-methoxystyryl)-4-methyl]benzopyrylium perchlorate to 2-(.alpha.-phenyl-p-methoxystyryl)-4-[(4'-benzopyranylidene)methyl]-benzop yrylium perchlorate in the same procedure as the procedure (a). The crystals have a color of violet in dichloroethane and a melting point of 174.degree. to 178.degree.C. The product is obtained in 95% yield.

(e) 2-p-methoxystyryl-4-[(2'-3",4"-dimethoxystyryl-4'-benzopyranylidene)methyl ]benzopyrylium perchlorate (compound number 42) ##SPC12##

2-(3',4'-Dimethoxystyryl)-4-methoxybenzopyrylium o-nitrobenzenesulfonate (melting point 211.degree. to 213.degree.C) is obtained from a reaction of 2-(3',4'-dimethoxystyryl)chromone and methyl o-nitrobenzene sulfonate. 6.5 Grams of 2-(3',4'-dimethoxystyryl)-4-methoxybenzopyrylium o-nitrobenzenesulfonate and 4,5 grams of 2-p-methoxystyryl-4-methylbenzopyrylium perchlorate are dissolved in 150 milliliters of acetic anhydride to a solution. The solution is heated at 100.degree.C for 100 minutes. After cooling the solution is poured into 900 milliliters of 10 weight percent perchloric acid. Precipitates are filtered off and dried. For purification, reprecipitation is made using dichlorethane and ether. 3.2 Grams of product is obtained in 41% yield. The product has a color of drak violet in dichloroethane and a melting point of 175.degree. to 181.degree.C.

(f) 2-(.alpha.-phenyl-p-methoxystyryl)-4-(2'-p-methoxystyryl-3'-phenyl-6'-meth yl-4'-benzopyranylidene)methyl]-benzopyrylium perchlorate (compound number 66) ##SPC13##

2-p-Methoxystyryl-3-phenyl-4-methoxy-6-methylbenzopyrylium o-nitrobenzenesulfonate (melting point 141.degree. to 144.degree.C) is obtained from 2-p-methoxystyryl-3-phenyl-6-methylchromone and methyl o-nitrobenzenesulfonate. 2-p-methoxystyryl-3-phenyl-4-methoxy-6-methylbenzopyrylium o-nitrobenzenesulfonate reacts with 2-(.alpha.-phenyl-p-methoxystyryl)-4-methylbenzopyrylium perchlorate in the same procedure as the procedure (e). The product has a color of violet in dichloroethane and a melting point of 190.degree. to 193.degree. C.

(g) 2-p-methoxystyryl-3-phenyl-4-[(2'-phenyl-4'-benzothiopyranylidene)methyl]- 6-methyl]benzopyrylium perchlorate (compound number 85) ##SPC14##

2-phenyl-4-methoxybenzothiopyrylium o-nitrobenzenesulfonate is obtained from thioflavone and methyl o-nitrobenzenesulfonate. As the same procedure as procedure (e), the product is obtained in 62% yield. The product has a color of reddish violet in dichloroethane and a melting point of 170.degree. to 177.degree. C.

(h) 2-[(2'-phenyl-4'-benzopyranylidene)methyl]-3-phenylbenzopyrylium perchlorate (compound number 92) ##SPC15##

5.5 grams of 2-phenyl-4-methoxybenzopyrylium o-nitrobenzenesulfonate, which is obtained from flavone and methyl o-nitrobenzenesulfonate, and 4 grams of 2-methyl-3-phenylbenzopyrylium perchlorate are dissolved in 150 milliliters of acetic anhydride to a solution. The solution is heated at 100.degree. C for 15 minutes. After cooling the solution is poured into 900 milliliters of 10 weight percent perchloric acid. Precipitated product is filtered off and dried. For purification, reprecipitation is made using dichloroethane and ether. The product has a color of blue in dichloroethane and a melting point of 128.degree. to 134.degree. C. The product yields 3 grams (theoretical yield 46%). Compound number 93 (fluoborate), compound number 94 (chloroferrate) or compound number 95 (nitrate) is obtained using either solution of borontrifluoride etherate, aqueous solution of ferric chloride or diluted nitric acid respectively instead of 10 weight percent perchloric acid.

(i) 2[(2'-4-phenyl-p-methoxystyryl-4'-benzopyranylidene)methyl]-3-phenylbenzop yrylium perchlorate (compound number 11) ##SPC16##

2-(.alpha.-Phenyl-p-methoxystyryl)-4-methoxybenzopyrylium o-nitrobenzenesulfonate, which is obtained from 2-(.alpha.-phenyl-p-methoxystyryl)chromone and methyl o-nitrobenzenesulfonate, reacts with 2-methyl-3-phenylbenzopyrylium perchlorate in the same procedure as procedure (h). The reaction product yields in 43%. The product has a color of violet in dichloroethane and a melting point of 158.degree. to 163.degree.C.

It has been discovered according to the invention the compounds listed in Table 1 are sensitizing agents which can improve the photoconductivity and the spectrum characteristics of photoconductive polymeric compounds such as poly-N-vinylcarbazole, brominated poly-N-vinylcarbazoles, polyacenaphthylene, etc. These polymers, except brominated poly-N-vinylcarbazoles, are prepared in a per se well known method. The brominated poly-N-vinylcarbazole can be prepared by the following method: To the solution of 20 grams of poly-N-vinylcarbazole in 450 milliliters of chlorobenzene, there are added 18.44 grams of N-bromosuccinimide and 0.173 grams of benzoyl peroxide. The mixture is heated at 80.degree.C for 2 hours while being stirred thoroughly and is poured into methanol to obtain a white polymer. The polymer is dissolved in chlorobenzene and again poured into methanol for purification. The pure polymer thus obtained as a precipitate exhibits upon elementary analysis a halogen content of 29.87 weight percent which approximates the value calculated, i.e. 29.44 weight percent of the monobromosubstituted product from poly-N-vinylcarbazole. This indicates that the polymer obtained is a monobromosubstituted product. The degree of bromination varies from 50 mole percent to 200 mole percent according to reaction conditions.

The novel sensitizer comprising at least one compound from the group listed in Table 1 is dissolved in a suitable solvent, such as dichloroethane, methylene chloride, chloroform, or a combination thereof, and is added to the solution of the photoconductive polymer described above. The preferable amount of the sensitizer added is from 0.01 to 3.0 weight parts in connection with 100 weight parts of the photoconductive polymer. Advantageously, the amount thereof is from 0.1 to 2.0 weight parts in connection with 100 weight parts of the photoconductive polymer.

For the preparation of the photoconductive insulating layer, a said solution of the photoconductive polymer and the sensitizer in a suitable solvent is applied to the electroconductive support in per se usual manner, for example, by spraying, by means of bladed coating, by means of whirler coating, etc., and then dried so as to produce a homogeneous photoconductive insulating layer on the electroconductive support. Operable solvents are benzene, toluene, chlorobenzene, dioxane, methylene chloride, dichloroethane and combinations thereof. Said solution may be incorporated with suitable plasticizers and/or organic colloids for improving the flexibility and strength of the photoconductive polymer. Operable plasticizers are as follows: chlorinated diphenyl, dimethyl phthalate, diethyl phthalate and octyl phthalate. Operable organic colloids are as follows: natural and synthetic resins, e.g. phenol resin, phenol resin modified with resin, polyvinyl acetal, polyvinyl butyral, polyvinyl cinnamate, polycarbonate resin. Operable materials for electroconductive supports may be made of any materials which satisfy the requirement of the electrophotographic art, e.g. metal plate or glass plate having NESA coating, plate or foil made of electrically conductive resin or coated with evaporated thin metal layer. If paper is to be used as a support for the photoconductive layer, pretreatment of the paper against penetration of the coating solution in advisable. The transparent support can produce a transparent electrophotographic plate, foil or film. After an electrostatic charge has been applied, i.e. after the layer has been charged positively or negatively by means of a corona discharge, the layer becomes light sensitive.

The reproduction of images by the electrophotographic method is carried out as follows: when the photoconductive layer has been charged by means of a corona discharge apparatus, the support with the sensitized layer is exposed to light under a master and is then dusted over in a per se known manner with a resin powder colored with carbon black. The image that now becomes visible can easily be wiped off. It can also be fixed by heating at about 120.degree.C. From positive masters, positive images characterized by good contrast the producted.

This invention is still further illustrated with reference to the following illustrative examples.

1 Gram of polyacenaphthylene and 0.6 gram of, as a plasticiser, chlorinated diphenyl (commercially available as "Kanechlor"), are dissolved in 8 milliliters of chlorobenzene. To the solution are added 0.5 milliliters of dichloroethane containing 0.006 gram of a sensitizer corresponding to compound number listed in Table 1. The solution is applied to an aluminum plate by means of whirler coating and is dried to form a layer of 7.mu. in thickness. After said aluminum plate provided with the layer is charged negatively by means of corona discharge with a charging device maintained at approximately 6000 volts in the dark, it is placed under a positive master and is exposed to a 100W tungsten lamp at an illumination of 50 luxes, and the said plate is powdered over with a developer in a per se known manner. This developer consists of toner and carrier. The toner consists of low melting point polystyrene, colophony and carbon black. The toner is mixed with a carrier substance such that the toner becomes triboelectrically charged with a charge opposite to that produced on the plate. A positive image is produced and is fixed by slight heating. In Table 2, there are shown the optimum amounts of exposure in lux-second units.

1 Gram of poly-N-vinylcarbazole, chlorinated diphenyl (commercially available as "Kanechlor") and 0.006 gram of a sensitizer, coresponding to compound number listed in Table 1, in 10 milliliters of dichloroethane to a solution. The solution is applied to an aluminum plate by means of blade coating and is dried to form a layer of 10.mu. in thickness. An electrophotographic image in the same way as that described in Example 1. In Table 3, there are shown the optimum amounts of exposure in lux-second units to produced exactly the original images.

It is clear from Table 3 that the novel sensitizers improve the photoconductivity of poly-N-vinylcarbazole.

1 Gram of brominated poly-N-vinylcarbazole (monobromo-substituted product), 0.5 gram of polycarbonate resin (commercially available as "Panlite-C"), 0.3 gram of chlorinated diphenyl (commercially available as "Kanechlor") and 0.002 gram of sensitizer listed in Table 1 are dissolved in a mixed-solvent of 8 milliliters of chlorobenzene and 2 milliliters of dichloroethane. This solution is applied to an aluminum plate by means of a blade coating and dried to form a layer of 14.mu. in thickness. On this support, electrophotographic images are produced in the same way as that described in Example 1. Table 4 shows the optimum amounts of exposure in lux-second units to reproduce exactly the original images.

It is clear from Table 4 and FIGS. 1 to 9 that the novel sensitizers improve the photoconductivity and the spectrum characteristics of brominated poly-N-vinylcarbazole.

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 hereinabove and as defined in the appended claims.