Photoconductive material and process for producing the same

Abstract: A photoconductive material comprising a mixed crystal of at least two phthalocyanine compounds whose central substances are different from each other, the phthalocyanine compounds being represented by formula (I): wherein A represents a substance capable of bonding to the phthalocyanine ligand through a covalent bond or a coordinate bond. The photoconductive material exhibits stability to heat and light and sufficient sensitivity in the visible to near infrared region.

Description: FIELD OF THE INVENTION This invention relates to an organic photoconductive material and a process for producing the same. More particularly, it relates to an organic photoconductive material containing a mixed crystal of at least two phthalocyanine compounds and having sensitivity in the visible to near infrared region. BACKGROUND OF THE INVENTION Photoconductive materials have been intensively studied and put into practical use as electrophotographic photoreceptors, various sensors, and image pickup tubes. Known inorganic photoconductive materials include amorphous selenium, amorphous silicon, cadmium chloride, zinc oxide, and selenium-arsenic alloys. Known organic photoconductive materials include low-molecular materials such as carbazole, anthracene, pyrazolines, oxadiazoles, and hydrazones, and pigments or dyes such as phthalocyanine pigments (see e.g. GB-A-2 145 835), azo pigments, cyanine dyes, polycyclic quinone pigments, perylene pigments, and indigo dyes. Electrophotographic photoreceptors so far widespread comprise a photosensitive layer consisting mainly of an inorganic photoconductive material, e.g., selenium, cadmium, and zinc oxide. These conventional inorganic photoconductive materials are not always satisfactory in sensitivity, heat resistance or printing durability. Electrophotographic photoreceptors having a photosensitive layer consisting mainly of an organic photoconductive compound, on the other hand, have many advantages, such as relative facility in production, cheapness, non-polluting properties, and the ease on handling. In recent years, considerable attention has been devoted particularly to development of high performance organic photoreceptors in which a carrier generating function and a carrier transport function are respectively performed by different substances as described, e.g., in JP-A-60-67949 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). On the other hand, gas lasers, e.g., an Ar laser and an He-Ne laser, or semi-conductor lasers are regarded promising as a light source for the electrophotographic photoreceptor of copying machines possessing image processing functions, such as intelligent copying machines, and output printers of computers. Semi-conductor lasers, inter alia , have drawn attention because of the feasibility of reduction in size, weight, and cost. From the fact that the semi-conductor lasers are of lower output as compared with gas lasers and have oscillation wavelengths in the longer wavelength region of more than about 780 nm, some photoconductive compounds having sensitivity in the longer wavelength region have been proposed as disclosed in JP-A-60-19144 and JP-A-60-111248. However, the conventional organic photoconductive compounds, some of them having been turn into practical use, are not necessarily satisfactory in terms of sensitivity, residual potential, and stability on repeated use. Further, because of the lower output of the semi-conductor lasers are compared with gas lasers and the longer oscillation wavelength (about 780 nm or more) of the semi-conductor lasers as stated above, spectral sensitivity exhibited by the conventional photoreceptors is in the shorter wavelength side. It has been thus demanded to develop a compound having high sensitivity in the longer wavelength region corresponding to the oscillation wavelength of the semiconductor lasers. SUMMARY OF THE INVENTION One object of this invention is to provide a organic photoconductive material having stability to heat and light and sufficient sensitivity in a long wavelength region (near infrared region). This invention provides a photoconductive material having sensitivity in the visible to near infrared region, which comprises a mixed crystal of at least two phthalocyanine compounds represented by formula (I): having a different central atom or molecule A characterized in that in said at least two phthalocyanine compounds the "A"s represent one of the following combinations: H₂ and TiO; H₂ and VO; Cu and TiO; Cu and VO; or TiO and VO. This invention further provides two processes for producing the above-described photoconductive material. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is an X-ray diffraction pattern of one of the photoconductive material according to the present invention and each of the starting materials thereof as obtained in Example 3. DETAILED DESCRIPTION OF THE INVENTION The phthalocyanine compounds represented by formula (I) can be synthesized by known processes, such as the process described in G.T. Byrne, R.P. Linstead, and A.R. Lowe, J. Chem. Soc. , p1017 (1934). The photoconductive material according to the present invention comprises mixed crystals of two or more phthalocyanine compounds of formula (I) in which the respective central molecules A are different. Taking a combination of two different kinds of phthalocyanine compounds as an instance, specific examples of such a combination include a combination of H₂-phthalocyanine and TiO-phthalocyanine or VO-phthalocyanine; a combination of Cu-phthalocyanine and TiO-phthalocyanine or VO-phthalocyanine; and a combination of TiO-phthalocyanine and VO-phthalocyanine. In these cases, a mixing ratio of the two phthalocyanine compounds is arbitrary. A molar ratio of either one of them to the other of them usually ranges from 0.01 to 100, preferably from 0.1 to 10.0. The mixed crystal to be used in the present invention can be obtained by vaporizing at least two different phthalocyanine compounds either separately or simultaneously by heating at a temperature of at least the sublimation temperature of the compounds, preferably 450 to 500°C under vacuum of not more than 0.13 kPa (1 Torr), preferably not more than 0.013 kPa (0.1 Torr), most preferably not more than 0.013 Pa (1×10⁻⁴ Torr), in the respective or the same heating device and then re-aggregating the vapors on a substrate set at a temperature not higher than the sublimation temperature, preferably 300°C or lower. The substrate to be used for re-deposition includes metals, e.g., Al and Au, metal oxides, glass, and plastics in the form of a plate, a drum, a belt, etc., with metals, conductive metal oxides and conductive plastics being preferred because the substrate can be used as a conductive support as it is. The phthalocyanine compounds are usually employed in the form of a powder. It is preferable to use a mixture previously prepared by dissolving two or more kinds of phthalocyanine compounds in a solvent and then reprecipitating the mixture in a poor solvent. The heating device for sublimation in vacuo includes a vacuum evaporator and a sublimation furnace. In place of heating, vaporization of the phthalocyanine compounds can also be effected by collision of accelerated particles against the phthalocyanine compounds. This can be done with a sputtering device. The thus produced phthalocyanine mixed crystal in the form of a thin film on the substrate is usually used as a photoconductive material as such. It is also possible that the film is peeled off the substrate, powderized, and then processed to provide a photoconductive material. The phthalocyanine mixed crystals can also be obtained by dissolving two or more phthalocyanine compounds in an appropriate solubilizing agent (solvent) and then precipitating the mixed crystal in a poor solvent. In this case, the molar ratio of the starting phthalocyanine compounds is not particularly limited. For example, in case of using two kinds of phthalocyanine compounds (P₁, P₂), a P₁/P₂ molar ratio usually ranges from 0.01 to 100, preferably from 0.1 to 10.0. The kind and amount of the solubilizing agent to be used are not particularly limited as long as the two or more phthalocyanine compounds may be dissolved therein and they are not decomposed thereby. Specific examples of usable solubilizing agents are concentrated sulfuric acid, trifluoromethylsulfonic acid, and trichloromethylsulfonic acid. In usual, the solubilizing agent is used in an amount of from 5 to 100 ml, preferably from 20 to 60 ml, per gram of the phthalocyanine compounds. The phthalocyanine compounds may be dissolved in any mode, for example, at a temperature of from -20 to 30°C, preferably from 0 to 20°C. The poor solvent to be used is not particularly limited and includes, for example, water, dilute sulfuric acid, and organic solvents, e.g., methanol, ethanol, hexane, benzene, and toluene. The poor solvent is usually used in an amount of from 50 to 1,000 ml, preferably from 200 to 600 ml, per gram of the phthalocyanine compounds. Precipitation of the phthalocyanine mixed crystal can generally be effected by pouring the phthalocyanine solution in the poor solvent or vice versa . The precipitation is not particularly limited in temperature or time and usually carried out at a temperature of from -20 to 30°C, preferably from 0 to 10°C, for a period of from 1 to 3 hours. The thus formed precipitate is collected, washed, and dried to obtain the desired molecular mixture (mixed crystal). The X-ray diffraction spectrum of the resulting phthalocyanine mixed crystal shows a peak pattern different from that results from either of the starting phthalocyanine compounds. This means that the spacing of the mixed crystal differs from that of each starting phthalocyanine compound, and that the mixed crystal is different from a mere mixture of the starting phthalocyanine compounds. The thus obtained mixed crystal functions as a photoconductive material either alone or in combination with the starting phthalocyanine compounds. It is possible that the powderous phthalocyanine mixed crystal is directly press molded to obtain a photoconductive material, or it may be formed into a photoconductive material by finely grinding the powderous phthalocyanine mixed crystal by means of a ball mill, etc., dispersing the resulting fine particles in an appropriate solvent, dissolving, if desired , a binder resin in the dispersion, and coating the resulting dispersion on a substrate, followed by drying. Any kind of resins can be used as the binder resin, but insulating film-forming high polymers are preferred. Such polymers include polycarbonate, polyester, methacrylic resins, acrylic resins, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinyl acetate, styrene-butadiene copolymers, vinylidene chloride-acrylonitrile copolymers, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-maleic anhydride copolymers, silicone resins, silicon-alkyd resins, phenol-formaldehyde resins, styrene-alkyd resins, poly-N-vinylcarbazole, and polyvinyl butyral. The electrophotographic photoreceptor comprising the photoconductive material according to the present invention comprises a conductive support having provided thereon a photosensitive layer containing at least one of the above-described phthalocyanine mixed crystals as a carrier generating substance. The electrophotographic photoreceptor may have any known structural embodiment. For example, the photosensitive layer may be composed of a carrier generating layer consisting mainly of the phthalocyanine mixed crystal of the present invention and a carrier transport layer consisting mainly of a carrier transporting substance, or the photosensitive layer may be a single layer containing the carrier transporting substance having dispersed therein the carrier generating substance. The photosensitive layer may be provided on the support either directly or via an intermediate layer. In this connection, various structural embodiments as disclosed in JP-A-60-67949 are applicable. The conductive support to be used in the photoreceptor includes metallic plates, metallic drums, and composite support composed of a substrate, e.g., paper, plastics, and film, on which a conductive thin layer comprising a conductive compound (e.g., conductive polymers, and indium oxide) or a metal (e.g., aluminum, palladium, and gold) is provided by coating, vacuum deposition, laminating, and the like technique. The coating composition of forming a carrier generating layer can be prepared by finely grinding the phthalocyanine mixed crystal by means of a ball mill, etc. and dispersing the fine particles in an appropriate solvent, dissolving, if desired, a binder resin in the dispersion. The coating composition is coated on the conductive support either directly or via an intermediate layer or on a previously formed carrier transport layer, followed by drying to thereby form a carrier generating layer. The fine particles of the phthalocyanine mixed crystal usually have a particle diameter of 5 µm or less, preferably 1 µm or less. The carrier generating layer has a thickness of from 0.01 to 20 µm, preferably from 0.05 to 5 µm. The carrier generating layer contains from 10 to 100%, preferably from 30 to 95%, by weight of the phthalocyanine mixed crystal. The carrier generating layer may also be formed on a substrate by vacuum evaporation or sputtering to a film thickness of from 10⁻³ to 10 µm, preferably from 10⁻² to 1 µm. In the case of using the mixed crystal in a gaseous phase, the process may be simplified by the use of a conductive support as a substrate. The carrier transport layer can be formed by coating a carrier transport substance dissolved in an appropriate medium and drying. The carrier transporting substance includes, for example, electron accepting substances, e.g., trinitrofluorenone and tetranitrofluorenone, polymers having a heterocyclic compound in the side chain, e.g., poly-N-vinylcarbazole, and positive hole-transporting electron donating substances, e.g., triazole derivatives, oxadiazole derivatives, imidazole derivatives, pyrazoline derivatives, polyarylalkane derivatives, phenylenediamine derivatives, hydrazone derivatives, amino-substituted chalcone derivatives, triarylamine derivatives, carbazole derivatives, and stilbene derivatives. The carrier transport layer has a film thickness of from 1 to 100 µm, preferably from 5 to 50 µm. Binders which can be used, if desired, in the formation of the carrier generating layer or carrier transport layer are arbitrary. In particular, the above-described insulating film-forming high polymers are preferred as a binder. The disperse single layer system combining the function of a carrier-generating layer and the function of a carrier-transport layer can be formed by coating the above-described dispersion for a carrier generating layer having dissolved or dispersed therein the carrier transporting substance on a conductive support either directly or via an intermediate layer. Any other known techniques can be used for the production of the electrophotographic photoreceptor. For example, the photosensitive layer may contain a third component, such as a sensitizer. Suitable sensitizers include Lewis acids capable of forming a charge transfer complex with an organic photoconductive substance and dyestuffs. The photosensitive layer may further contain a plasticizer for improving film-formability, flexibility and mechanical strength. The photoconductive material according to the present invention functions as a charge generating substance for an infrared sensor or an electrophotographic photoreceptor, exhibiting sufficient sensitivity to longer wavelength light sources, such as semi-conductor lasers and is therefore widely applicable in the field of electrophotography including laser printers and laser facsimiles. The present invention is now illustrated in greater detail with reference to the following Examples and Comparative Examples, but it should be understood that the present invention is not deemed to be limited thereto. EXAMPLE 1 Cu-Phthalocyanine (100 mg) and TiO-phthalocyanine (100 mg) were separately put in each of two tungsten boats in a bell jar, and the bell jar was evaculated to a degree of vacuum of 2.67×10⁻³ Pa (2×10⁻⁵ Torr). Each evaporation source was heated to about 450°C and, after controlling the rate of deposition constant at 200 Å/min (as measured with a film thickness monitor), the shutter was opened to thereby deposit Cu-phthalocyanine and TiO-phthalocyanine on an aluminum substrate set at room temperature to a deposite thickness of 2000 Å for over a period of about 10 minutes. EXAMPLE 2 In 50 ml of sulfuric acid were dissolved 1.8 mmol of Cu-phthalocyanine and 1.8 mmol of TiO-phthalocyanine. After any insoluble matter was removed by filtration, the filtrate was poured into 600 ml of water to cause re-precipitation. The resulting powder was thoroughly washed with a large quantities of water and ethanol and dried in vacuo at 70°C to obtain a molecular mixture. A hundred milligrams of the resulting molecular mixture in a tungsten boad was evaporated by heating in an ordinary vacuum evaporation apparatus under vacuum of 2.67×10⁻³ Pa (2×10⁻⁵ Torr) at a sublimation temperature of the molecular mixture (about 450°C) to thereby deposit the mixture on an aluminum plate set at room temperature to a thickness of 2000 Å. EXAMPLE 3 A deposited film was prepared in the same manner as in Example 1, except for using H₂-phthalocyanine in place of Cu-phthalocyanine. The X-ray diffraction pattern of the film having been peeled off the substrate is shown in Fig. 1. For comparison, the X-ray diffraction pattern of a deposited film obtained from each of TiO-phthalocyanine and H₂-phthalocyanine alone by the use of an ordinary vacuum evaporation apparatus is also shown in Fig. 1. In Fig. 1, the film prepared by the method of the present invention exhibited new peaks at 7.56°, 10.22°, and 22.52° in addition to the peaks assigned to each of the two kinds of phthalocyanine compounds. It can thus be proved that the film of this example comprises one kind of crystal in which the two kinds of phthalocyanine compounds are mixed in a molecular state. APPLICATION EXAMPLE 1 On each of the deposited film on an aluminum plate as prepared in Examples 1 to 3 was coated a solution consisting of 200 mg of p-diethylaminobenzaldehyde diphenylhydrazone and 200 mg of a polycarbonate resin "Iupilon E-2000" produced by Mitsubishi Gas Chemical Industries Ltd., and 2.5 ml of tetrahydrofuran to a dry thickness of 15 µm, followed by drying to obtain an electrophotographic photoreceptor. The resulting photoreceptors were evaluated for electrophotographic characteristics according to the following test method. The results obtained are shown in Table 1 below. Test Method: The photoreceptor was statically charged by corona discharge to -6 kV and, after allowing to stand in dark for 10 seconds, the initial surface potential was measured. The photoreceptor was then exposed to monochromatic light of 800 nm which was isolated from light emitted from a xenone lamp by means of a monochrometer, and the time (second) required for the surface potential to decrease to half was measured to obtain an exposure amount (µJ/cm²). EXAMPLE 4 In 50 ml of sulfuric acid were dissolved 1.8 mmol of Cu-phthalocyanine and 1.8 mmol of TiO-phthalocyanine. After any insoluble matter was removed by filtration, the filtrate was poured into 600 ml of water to cause re-precipitation. The resulting powder was thoroughly washed with a large quantities of water and ethanol and dried in vacuo at 70°C to obtain a molecular mixture. Fifty milligrams of the resulting phthalocyanine mixed crystal and 50 ml of a polycarbonate resin "Iupilon E-2000" were added 2.5 ml of tetrahydrofuran, and the mixture was dispersed in a ball mill for 12 hours. The dispersion was coated on an aluminum plate to a dry thickness of 1 µm to form a carrier generating layer. A solution consisting of 200 mg of p-diethylaminobenzaldehyde diphenylhydrazone, 200 mg of a polycarbonate resin "Iupilon E-2000", and 2.5 ml of tetrahydrofuran was coated on the carrier generating layer to a dry thickness of 15 µm to form a carrier transport layer. The thus produced electrophotographic photoreceptor was evaluated in the same manner as in Application Example 1. As a result, the initial surface potential was 970 V, and the exposure amount at 800 nm was 1.6 µJ/cm². APPLICATION EXAMPLE 2 An electrophotographic photoreceptor was produced in the same manner as in Application Example 1. except for using 200 mg of 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole as a carrier transporting substance in place of p-diethylaminobenzaldehyde diphenylhydrazone. The photoreceptor was evaluated in the same manner as in Application Example 1. As a result, the initial surface potential was 1065 V and the exposure amount at 800 nm was 1.4 µJ/cm².

Claims: A photoconductive material comprising a mixed crystal of at least two phthalocyanine compounds represented by formula (I): having a different central atom or molecule A characterized in that in said at least two phthalocyanine compounds, the "A"s represent one of the following combinations: H₂ and TiO; H₂ and VO; Cu and TiO; Cu and VO; or TiO and VO. A photoconductive material as claimed in claim 1, wherein the molar ratio of either one of said two phthalocyanine compounds to the other of them ranges from 0.01 to 100. A process for producing a photoconductive material according to claim 1, which comprises vaporizing said at least two phthalocyanine compounds and re-depositing the vapors on a substrate. A process as claimed in claim 3, wherein said vaporizing occurs by heating at a temperature not lower than the sublimation temperature of the phthalocyanine compounds under vacuum of not more than 0.13 kPa (1 Torr) and said substrate is set at a temperature not higher than the sublimation temperature. A process as claimed in claim 4, wherein said vaporizing occurs by collision of accelerated particles against the phthalocyanine compounds. A process for producing a photoconductive material according to claim 1, which comprises dissolving said at least two phthalocyanine compounds in a common solubilizing agent and precipitating the obtained mixed crystal in a poor solvent. A process as claimed in claim 6, wherein said common solubilizing agent is selected from concentrated sulfuric acid, trifluoromethylsulfonic acid, and trichloromethylsulfonic acid. A process as claimed in claim 6, wherein said poor solvent is selected from water, dilute sulfuric acid, methanol, ethanol, hexane, benzene, and toluene. An electrophotographic photoreceptor comprising a conductive support having provided thereon a photosensitive layer containing a photoconductive material according to claim 1 as a carrier generating substance. An electrophotographic photoreceptor as claimed in claim 9, wherein said photosensitive layer further contains a carrier transporting substance.

Inventors: GOUDA JUNKO MITSUBISHI PETROCH, ITSUBO AKIRA MITSUBISHI PETROC, SASAKI TOMOKO MITSUBISHI PETRO, SUZUKI SHINICHI MITSUBISHI PET, TODA HIDEO MITSUBISHI PETROCHE, TOKURA YOSHINORI, GOUDA, JUNKO MITSUBISHI PETROCHEMICAL CO. LTD., ITSUBO, AKIRA MITSUBISHI PETROCHEMICAL CO. LTD., SASAKI, TOMOKO MITSUBISHI PETROCHEMICAL CO. LTD., SUZUKI, SHINICHI MITSUBISHI PETROCHEMICAL CO. LTD., TODA, HIDEO MITSUBISHI PETROCHEMICAL CO. LTD., TOKURA, YOSHINORI
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1989
Main classification: G03G5
IPC codes: G03G5, C09B67