Abstract:
A photoconductor for electrophotography includes an electroconductive substrate and a photosensitive layer formed on the substrate. The photosensitive layer includes a novel bisazo compound as a charge generating substance therein. One type of bisazo compound is represented by general formula: ##STR1## wherein, A stands for a coupler residue.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to photoconductors for electrophotography, and particularly to a photoconductor for electrophotography which contains a novel charge generating substance in the photosensitive layer thereof formed on an electroconductive substrate. 
     2. Description of the Prior Art 
     Photosensitive materials which have heretofore been used in photoconductors for electrophotography include inorganic photoconductive substances such as selenium and selenium alloys, dispersions of inorganic photoconductive substances such as zinc oxide and cadmium sulfide in resin binders, organic polymeric photoconductive substances such as poly-N-vinylcarbazole and polyvinylanthracene, organic photoconductive substances such as phthalocyanine compounds and bisazo compounds, dispersions of such organic photoconductive substances in resin binder, and a sputtered film of such organic photoconductive substances. 
     Photoconductors are required to have a function of maintaining a surface electric charge in the dark, a function of generating an electric charge upon receiving light, and a function of transporting an electric charge upon receiving light. They are classified into two types of photoconductors, namely so-called monolayer type photoconductors, and so-called laminate type photoconductors. The former comprises a single layer having all of the above-mentioned three functions, and the latter comprises functionally distinguishable laminated layers, one of which contributes mainly to the generation of electric charge, and another of which contributes to the retention of surface electric charge in the dark and the electric charge transportation upon receiving light. In an electrophotographic method using a photoconductor of the kind as mentioned above, for example, the Carlson&#39;s system is applied to image formation. The image formation according to this system comprises steps of subjecting a photoconductor in the dark to corona discharge to charge the photoconductor, illuminating the surface of the charged photoconductor with imagewise light based on a manuscript or copy bearing, e.g., letters and/or pictures to form a latent electrostatic image, developing the formed latent electrostatic image with a toner, and transferring the developed toner image to a support such as a paper sheet to fix the toner image on the support. After the toner image transfer, the photoconductor is subjected to the steps of removal of the electric charge, removal of the remaining toner (cleaning), neutralization of the residual charge with light (erasion), and so on to be ready for reuse. 
     Photoconductors for electrophotography in which use is made of organic materials have recently been put into practical use by virtue of the advantageous features of flexibility, thermal stability, and/or a film forming capacity. They include a photoconductor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-on (disclosed in U.S. Pat. No. 3,484,237), a photoconductor using an organic pigment as a main component (disclosed in Japanese Patent Laid-Open No. 37,543/1972), and a photoconductor using as a main component a eutectic complex composed of a dye and a resin (disclosed in Japanese Patent Laid-Open No. 10,785/1972). A number of novel hydrazone compounds and bisazo compounds and the like have also been put into practical use for photoconductors. 
     Although organic materials have many advantageous features mentioned above with which inorganic materials are not endowed, however, the fact is that there have been obtained no organic materials fully satisfying all the characteristics required of a material to be used in photoconductors for electrophotography at the present. Particular problems involved in organic materials have been concerned with photosensitivity and characteristics in continuous repeated use. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, which has been made in view of the foregoing, is to provide a photoconductor for electrophotography for use in copying apparatuses and printers which photoconductor includes a novel organic materials not used to date as a charge generating substance in the photosensitive layer, and has a high photosensitivity and excellent characteristics in repeated use. 
     In the first aspect of the present invention, a photoconductor for electrophotography comprises: 
     an electroconductive substrate; and 
     a photosensitive layer formed on the substrate and including at least one bisazo compound represented by the following general formula (I) as a charge generating substance: ##STR2## wherein, A stands for a coupler residue represented by one of the following formulae (V) to (XI) ##STR3## wherein, Z stands for a residual group which condenses with a benzene ring to form an aromatic polycycle or a heterocycle, X 1  is selected from the group consisting of OR 1  and NR 2  R 3 , each of R 1 , R 2  and R 3  is selected from the group consisting of a hydrogen atom, an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 2  and X 5  is selected from the group consisting of an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 3  and X 6  is selected from the group consisting of a hydrogen atom, a cyano group, a carbamoyl group, a carboxyl group, an ester group and an acyl groups, each of X 4  and X 11  is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 7  and X 8  is selected from the group consisting of a hydrogen atom, a halogen atom, a nitro group, an alkyl group and an alkoxy group, the last two of which groups may be or not may be substituted, X 9  is selected from the group consisting of an alkyl group, an aryl group, a carboxyl group and an ester group, which groups may be or not may be substituted, X 10  is selected from the group consisting of an aryl group and a heterocyclic group, which groups may be or not may be substituted and Y stands for a residual group which forms an aromatic ring or a heterocycle. 
     Here, the photosensitive layer may comprise a layer including dispersion of a charge generating substance selected from bisazo compounds represented by the general formula (I) and a charge transporting substance. 
     The photosensitive layer may comprise a laminate of a charge transporting layer mainly composed of a charge transporting substance and a charge generating layer including a compound selected from bisazo compounds represented by the general formula (I). 
     In the second aspect of the present invention, a photoconductor for electrophotography comprises: 
     an electroconductive substrate; and 
     a photosensitive layer formed on the substrate and including at least one bisazo compound represented by the following general formula (II) as a charge generating substance: ##STR4## wherein, A stands for a coupler residue represented by one of the following formulae (V) to (XI) ##STR5## wherein, Z stands for a residual group which condenses with a benzene ring to form an aromatic polycycle or a heterocycle, X 1  is selected from the group consisting of OR 1  and NR 2  R 3 , each of R 1 , R 2  and R 3  is selected from the group consisting of a hydrogen atom, an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 2  and X 5  is selected from the group consisting of an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 3  and X 6  is selected from the group consisting of a hydrogen atom, a cyano group, a carbamoyl group, a carboxyl group, an ester group and an acyl group, each of X 4  and X 11  is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group and a heterocyclic group, which groups may by or not may be substituted, each of X 7  and X 8  is selected from the group consisting of a hydrogen atom, a halogen atom, a nitro group, an alkyl group and an alkoxy group, the last two of which groups may be or not may be substituted, X 9  is selected from the group consisting of an alkyl group, an aryl group, a carboxyl group and an ester group, which groups may be or not may be substituted, X 10  is selected from the group consisting of an aryl group and a heterocyclic group, which groups may be or not may be substituted and Y stands for a residual group which forms an aromatic ring or a heterocycle. 
     Here, the photosensitive layer may comprise a layer including dispersion of a charge generating substance selected from bisazo compounds represented by the general formula (II) and a charge transporting substance. 
     The photosensitive layer may comprise a laminate of a charge transporting layer mainly composed of a charge transporting substance and a charge generating layer including a compound selected from bisazo compounds represented by the general formula (II). 
     In the third aspect of the present invention, a photoconductor for electrophotography comprises: 
     an electroconductive substrate; and 
     a photosensitive layer formed on the substrate and including at least one bisazo compound represented by the following general formula (III) as a charge generating substance: ##STR6## wherein, A stands for a coupler residue represented by one of the following formulae (V) to (XI) ##STR7## wherein, Z stands for a residual group which condenses with a benzene ring to form an aromatic polycycle or a heterocycle, X 1  is selected from the group consisting of OR 1  and NR 2  R 3 , each of R 1 , R 2  and R 3  is selected from the group consisting of a hydrogen atom, an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 2  and X 5  is selected from the group consisting of an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 3  and X 6  is selected from the group consisting of a hydrogen atom, a cyano group, a carbamoyl group, a carboxyl group, an ester group and an acyl group, each of X 4  and X 11  is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 7  and X 8  is selected from the group consisting of a hydrogen atom, a halogen atom, a nitro group, an alkyl group and an alkoxy group, the last two of which groups may be or not may be substituted, X 9  is selected from the group consisting of an alkyl group, an aryl group, a carboxyl group and an ester group, which groups may be or not may be substituted, X 10  is selected from the group consisting of an aryl group and a heterocyclic group, which groups may be or not may be substituted and Y stands for a residual group which forms an aromatic ring or a heterocycle. 
     Here, the photosensitive layer may comprise a layer including dispersion of a charge generating substance selected from bisazo compounds represented by the general formula (III) and a charge transporting substance. 
     The photosensitive layer may comprise a laminate of a charge transporting layer mainly composed of a charge transporting substance and a charge generating layer including a compound selected from bisazo compounds represented by the general formula (III). 
     In the fourth aspect of the present invention, a photoconductor for electrophotography comprises: 
     an electroconductive substrate; and 
     a photosensitive layer formed on the substrate and including at least one bisazo compound represented by the following general formula (IV) as a charge generating substance: ##STR8## wherein, R 4  is selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group which may be or not may be substituted, n stands for one integer of 1 to 3 and A stands for a coupler residue represented by one of the following formulae (V) to (XI) ##STR9## wherein, Z stands for a residual group which condenses with a benzene ring to form an aromatic polycycle or a heterocycle, X 1  is selected from the group consisting of OR 1  and NR 2  R 3 , each of R 1 , R 2  and R 3  is selected from the group consisting of a hydrogen atom, an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 2  and X 5  is selected from the group consisting of an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 3  and X 6  is selected from the group consisting of a hydrogen atom, a cyano group, a carbamoyl group, a carboxyl group, an ester group and an acyl group, each of X 4  and X 11  is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 7  and X 8  is selected from the group consisting of a hydrogen atom, a halogen atom, a nitro group, an alkyl group and an alkoxy group, the last two of which groups may be or not may be substituted, X 9  is selected from the group consisting of an alkyl group, an aryl group, a carboxyl group and an ester group, which groups may be or not may be substituted, X 10  is selected from the group consisting of an aryl group and a heterocyclic group, which groups may be or not may be substituted and Y stands for a residual group which forms an aromatic ring or a heterocycle. 
     Here, the photosensitive layer may comprise a layer including dispersion of a charge generating substance selected from bisazo compounds represented by the general formula (IV) and a charge transporting substance. 
     The photosensitive layer may comprise a laminate of a charge transporting layer mainly composed of a charge transporting substance and a charge generating layer including a compound selected from bisazo compounds represented by the general formula (IV). 
     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 3 are schematic cross-sectional views of photoconductors according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The photoconductor of the present invention, which contains the specific bisazo compound as a charge generating substance in the photosensitive layer thereof, may have any one of the structures as shown in FIGS. 1, 2 and 3 according to the manner of application thereto of the bisazo compound. The specific bisazo compounds to be used in the present invention are explained later. 
     FIGS. 1, 2 and 3 are schematic cross-sectional views of different embodiments of the photoconductor of the present invention, respectively. 
     FIG. 1 shows a cross-sectional view of a monolayer type photoconductor. A photosensitive layer 2A is provided on an electroconductive substrate 1. The photosensitive layer 2A comprises the above-mentioned bisazo compound as a charge generating substance 3, and a charge transporting substance 5 both of which substances are dispersed in a resin binder matrix so that the photosensitive layer 2A functions as a photoconductor. 
     FIG. 2 shows a laminate type photoconductor. A laminated photosensitive layer 2B is provided on an electroconductive substrate 1, a lower layer of the laminate is a charge generating layer 4 including the above-mentioned bisazo compound as a charge generating substance 3 and an upper one is a charge transporting layer 6 containing a charge transporting substance 5 as a main component, so that the photosensitive layer 2B functions as a photoconductor. This photoconductor is usually used according to the negative charge mode. 
     FIG. 3 shows another laminate type photoconductor having a layer structure in reverse to that of FIG. 2. A laminated photosensitive layer 2C is provided on an electroconductive substrate 1, a lower layer of the laminate is a charge transporting layer 6 and an upper one is a charge generating layer 4 including the above-mentioned bisazo compound as a charge generating substance 3. The photosensitive layer also functions as a photoconductor. This photoconductor is usually used according to the positive charge mode. In this case, a covering layer 7 may generally be further provided as shown in FIG. 3 to protect the charge generating layer 4. 
     Thus, two kinds of layer structure are provided for laminate type photoconductors. The reason for this is that, even if any photoconductor with the layer structure as shown in FIG. 2 is to be used in the positive charge mode, no charge transporting substances adaptable to the positive charge mode have been found yet. Accordingly, when any laminate type photoconductor is to be used in the positive charge mode, the photoconductor is required of a layer structure as shown in FIG. 3 for the present. 
     A photoconductor as shown in FIG. 1 can be produced by dispersing a charge generating substance in a solution of a charge transporting substance and a resin binder and applying the resulting dispersion on an electroconductive substrate and then drying the resulting coating film. 
     A photoconductor as shown in FIG. 2 can be prepared by applying and drying a dispersion of a particulate charge generating substance in a solvent and/or a resin binder on an electroconductive substrate, followed by applying a solution of a charge transporting substance and a resin binder on the resulting layer and drying. 
     A photoconductor as shown in FIG. 3 can be prepared by applying and drying a solution of a charge transporting substance and a resin binder onto an electroconductive substrate, and coating and drying dispersion of a particulate charge generating substance in a solvent and/or a resin binder onto the resulting coating layer, followed by formation of a covering layer. 
     The electroconductive substrate 1 serves as an electrode of the photoconductor and as a support for a layer or layers formed thereon. The electroconductive substrate may be in the form of a cylinder, a plate or a film, and may be made of a metallic material such as aluminum, stainless steel or nickel, or other material having a surface treated to be electroconductive, such as glass so treated or a resin so treated. 
     The charge generating layer 4 is formed by application of a dispersion of a bisazo compound as a charge generating substance 3 in a resin binder, and this layer generates an electric charge upon receiving light. It is important that the charge generating layer 4 be high not only in charge generating efficiency but also in capability of injecting the generated electric charge into the charge transporting layer 6 and any covering layer 7, which capability is desirably as little dependent upon the electric field as possible and high even in low intensity electric fields. It also is possible to form a charge generating layer using a charge generating substance as a main component in mixture with a charge transporting substance and so on. Resin binders usable in the charge generating layer include polycarbonates, polyesters, polyamides, polyurethanes, polyvinyl chloride, epoxy resins, silicone resins, diallyl phthalate resins and homopolymers and copolymers of methacrylate esters, which may be used either alone or in appropriate combination. 
     The charge transporting layer 6, which is formed by application of a solution of dispersion of a hydrazone compound, a pyrazoline compound, a stilbene compound, a triphenyl-amine compound, an oxazole compound or an oxadiazole compound as an organic charge transporting substance in a resin binder, exhibits a function of serving as an insulating layer in the dark to retain an electric charge of the photoconductor as well as a function transporting an electric charge injected from the charge generating layer upon receiving light. Resin binders usable in the charge transporting layer include polycarbonates, polyesters, polyamides, polyurethanes, epoxy resins, silicone resins, and homopolymers and copolymers of methacrylate ester. 
     The covering layer 7 has a function of receiving and retaining an electric charge generated by corona discharge in the dark and a capability of transmitting light to which the charge generating layer should respond. It is necessary that the covering layer 7 transmits light upon exposure of the photoconductor and allows the light to reach the charge generating layer, and then undergoes the injection of an electric charge generated in the charge generating layer to neutralize and erases a surface electric charge. Materials usable in the covering layer include organic insulating film-forming materials such as polyesters and polyamides. Such organic materials may also be used in mixture with an inorganic material such as a glass resin or SiO 2 , or an electric resistance-lowering material such as a metal or a metallic oxide. Materials usable in the covering layer are not limited to organic insulating film-forming materials, and further include inorganic materials such as SiO 2 , metals, and metallic oxides, which may be formed on a covering layer by an appropriate method such as vacuum evaporation and deposition, or sputtering. From the viewpoint of the aforementioned description, it is desirable that the material to be used in the covering layer be as transparent as possible in the wavelength range wherein the charge generating substance attains maximum light absorption. 
     Although the thickness of the covering layer depends on the material or composition thereof, it can be arbitrarily set in so far as it does not produce any adverse effects including an increase in a residual potential in continuous repeated use. 
     Now, the bisazo compounds to be used as a charge generating substance in the present invention will be explained. The specific bisazo compounds are represented by one of the following general formulae (I) to (IV): ##STR10## wherein, A stands for a coupler residue represented by one of the following formulae (V) to (XI). ##STR11## wherein, Z stands for a residual group which condenses with a benzene ring to form an aromatic polycycle or a heterocycle, X 1  is selected from the group consisting of OR 1  and NR 2  R 3 , each of R 1 , R 2 , and R 3  is selected from the group consisting of a hydrogen atom, an alkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 2  and X 5  is selected from the group consisting of an alkyl group, an aryl group and a heterocyclic group, which group may be or not may be substituted, each of X 3  and X 6  is selected from the group consisting of a hydrogen atom, a cyano group, a carbamoyl group, a carboxyl group, an ester group and an acyl group, each of X 4  and X 11  is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group and a heterocyclic group, which groups may be or not may be substituted, each of X 7  and X 8  is selected from the group consisting of a hydrogen atom, a halogen atom, a nitro group, an alkyl group and an alkoxy group, the last two of which groups may be or not may be substituted, X 9  is selected from the group consisting of an alkyl group, an aryl group, a carboxyl group and an ester group, which groups may be or not may be substituted, X 10  is selected from the group consisting of an aryl group and a heterocyclic group, which groups may be or not may be substituted and Y stands for a residual group which forms an aromatic ring or a heterocycle. And in the formula (IV), R 4  is selected from the group consisting of a hydrogen atom, an alkyl group and an aryl group which may be or not may be substituted, n stands for one integer of 1 to 3. 
     Bisazo compounds, represented by the foregoing general formula (I) can be synthesized by diazotizing an amino compound represented by the following general formula (XII) with a conventional method and coupling reacting obtained diazo compound with a corresponding coupler in an appropriate solvent (for example, N,N-dimethylformamide, dimethyl sulfoxide and so on) under the presence of a base. ##STR12## 
     Bisazo compounds represented by the general formula (II) can be synthesized by diazotizing an amino compound represented by the following general formula (XIII) with a conventional method and coupling reacting obtained diazo compound with a corresponding coupler in an appropriate solvent (for example, N,N-dimethylformamide, dimethyl sulfoxide and so on) under the presence of a base. ##STR13## 
     Bisazo compounds represented by the general formula (III) can be synthesized by diazotizing an amino compound represented by the following general formula (XIV) with a conventional method and coupling reacting obtained diazo compound with a corresponding coupler in an appropriate solvent (for example, N,N-dimethylformamide, dimethyl sulfoxide and so on) under the presence of a base. ##STR14## 
     Specific examples of the bisazo compounds of the general formulae (I) to (II) prepared in the above-mentioned manner are shown in Table A. 
     
                       TABLE A______________________________________FOR-  COM-MU-   POUNDLA    No.      A______________________________________I II III 1 31 61           ##STR15##I II III 2 32 62           ##STR16##I II III 3 33 63           ##STR17##I II III 4 34 64           ##STR18##I II III 5 35 65           ##STR19##I II III 6 36 66           ##STR20##I II III 7 37 67           ##STR21##I II III 8 38 68           ##STR22##I II III 9 39 69           ##STR23##I II III 10 40 70           ##STR24##I II III 11 41 71           ##STR25##I II III 12 42 72           ##STR26##I II III 13 43 73           ##STR27##I II III 14 44 74           ##STR28##I II III 15 45 75           ##STR29##I II III 16 46 76           ##STR30##I II III 17 47 77           ##STR31##I II III 18 48 78           ##STR32##I II III 19 49 79           ##STR33##I II III 20 50 80           ##STR34##I II III 21 51 81           ##STR35##I II III 22 52 82           ##STR36##______________________________________ 
    
     Bisazo compounds represented by the general formula (IV) can be synthesized by diazotizing an amino compound represented by the following general formula (XV) with a conventional method and coupling reacting obtained diazo compound with a corresponding coupler in an appropriate solvent (for example, N,N-dimethylformamide, dimethyl sulfoxide and so on) under the presence of a base. ##STR37## 
     In the formula (XV), R 4  and n is identical with those in the formula (IV). 
     Specific examples of the bisazo compounds of the general formula (IV) prepared in the above-mentioned manner are shown in Table B. 
     
                                           TABLE B__________________________________________________________________________ ##STR38##COMPOUND                     COMPOUNDNo.     n  A                 No.     n  A__________________________________________________________________________ 91 114 137   1 2 3       ##STR39##         97 120 143                                1 2 3                                    ##STR40## 92 115 138   1 2 3       ##STR41##         98 121 144                                 1 2 3                                    ##STR42## 93 116 139   1 2 3       ##STR43##         99 122 145                                1 2 3                                    ##STR44## 94 117 140   1 2 3       ##STR45##        100 123 146                                1 2 3                                    ##STR46## 95 118 141   1 2 3       ##STR47##        101 124 147                                1 2 3                                    ##STR48## 96 119 142   1 2 3       ##STR49##        102 125 148                                1 2 3                                    ##STR50##103 126 149   1 2 3       ##STR51##        109 132 155                                1 2 3                                    ##STR52##104 127 150   1 2 3       ##STR53##        110 133 156                                1 2 3                                    ##STR54##105 128 151   1 2 3       ##STR55##        111  134 157                                1 2 3                                    ##STR56##106 129 152   1 2 3       ##STR57##        112 135 158                                1 2 3                                    ##STR58##107 130 153   1 2 3       ##STR59##        113     1                                    ##STR60##                        136     2  or108 131 154   1 2 3       ##STR61##        159     3                                    ##STR62##__________________________________________________________________________ ##STR63##COM-                         COM-POUND                        POUNDNo.  R     A                 No.  R     A__________________________________________________________________________160  CH.sub.3       ##STR64##        165  CH.sub.3                                    ##STR65##161  CH.sub.3       ##STR66##        166  CH.sub.3                                    ##STR67##162  CH.sub.3       ##STR68##        167  CH.sub.3                                    ##STR69##163 ##STR70##       ##STR71##        168  CH.sub.3                                    ##STR72##164  C.sub.2 H.sub.5       ##STR73##        169  CH.sub.3                                    ##STR74##170 ##STR75##       ##STR76##        175  CH.sub.3                                    ##STR77##171  CH.sub.3       ##STR78##        176  CH.sub.3                                    ##STR79##172  CH.sub.3       ##STR80##        177  CH.sub.3                                    ##STR81##173 ##STR82##       ##STR83##        178                              ##STR84##                                    ##STR85##174  CH.sub.3       ##STR86##        179  CH.sub.3                                    ##STR87##180  CH.sub.3       ##STR88##        182  CH.sub.3                                    ##STR89##181 ##STR90##       ##STR91##                                    ##STR92##__________________________________________________________________________ 
    
     As for the use of the bisazo compounds represented by the general formulae given above in photosensitive layers, there has been no precedent before. In the course of the intensive study of various organic materials as made in an attempt to achieve the above object, the present inventors conducted a number of experiments with those bisazo compounds and, as a result, found that the use of such specific bisazo compounds represented by the above general formulae (I) to (IV) as charge generating substances is very effective in improving electrophotographic characteristics. Based on this finding, photoconductors having high sensitivity and good repeated use characteristics are obtained. 
     Examples will now be given, wherein various compounds represented by the general formulae (I) to (IV) were respectively used to produce photoconductors. 
     EXAMPLE 1 
     50 parts by weight of the bisazo compound No. 1, 100 parts by weight of a polyester resin (Vylon 200, manufactured by Toyobo Co., Ltd.) and 100 parts by weight of 1-phenyl-3-(p-diethylaminaostyryl)-5-(p-diethylaminophenyl)-2-pyrazoline (ASPP) are kneaded with tetrahydrofuran (THF) as a solvent with a mixer for 3 hours to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film (Al-PET) as an electroconductive substrate by means of the wire bar technique to form a photosensitive layer having a dry thickness of 15 μm. Thus, a photoconductor with the structure shown in FIG. 1 was produced. 
     EXAMPLE 2 
     First, 100 parts by weight of p-diethylaminobenzaldehyde-diphenylhydrazone (ABPH) and 100 parts by weight of polycarbonate resin (Panlite L-1250 manufactured by Teijin chemicals Co., Ltd.) were solved in methylene chloride to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film substrate by the wire bar technique to form a charge transporting layer having a dry thickness of 15 μm. 50 parts by weight of the compound No. 2 and 50 parts by weight of a polyester resin (Vylon 200) were kneaded with a mixer for 3 hours together with THF as a solvent to prepare a coating liquid, which was then applied onto the charge transporting layer by the wire bar technique to form a charge generating layer having a dry thickness of 0.5 μm. Thus, a photoconductor with a structure corresponding to that shown in FIG. 3 was produced. The covering layer, which is not directly related to the present invention was not provided. 
     EXAMPLE 3 
     A charge transporting layer was formed in substantially the same manner as in Example 2 except that α-phenyl-4&#39;-N,N-dimethylaminostilbene, which is a stilbene compound, was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 2, thus a photoconductor was produced. 
     EXAMPLE 4 
     A charge transporting layer was formed in substantially the same manner as in Example 2 except that tri(p-tolyl)amine, which is a triphenylamine compound, was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 2, thus a photoconductor was produced. 
     EXAMPLE 5 
     A charge transporting layer was formed in substantially the same manner as in Example 2 except that 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, which is an oxadiazole compound, was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 2, thus a photoconductor was produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured by utilizing an electrostatic recording paper testing apparatus (Kawaguchi Denki Model SP-428). The result obtained are shown in Table 1. 
     The surface potential V s  (volts) of each photoconductor is an initial surface potential which was measured when the surface of the photoconductor was positively charged in the dark by corona discharge at +6.0 KV for 10 seconds. After the discontinuation of the corona discharge, the photoconductor was allowed to stand in the dark for 2 seconds, after which the surface potential V d  (volts) of the photoconductor was measured. Subsequently, the surface of the photoconductor was irradiated with white light at an illuminance of 2 luxes and the time (seconds) required for the irradiation to decrease the surface potential of the photoconductor to half of the V d  was measured, than from which the half decay exposure amount E 1/2  (lux·sec) was calculated. Also, the surface potential of the photoconductor after 10 seconds of irradiation thereof with white light at an illuminance of 2 luxes was measured as a residual potential V r  (volts). 
     
                       TABLE 1______________________________________   V.sub.s   V.sub.r E.sub.1/2   (volt)    (volt)  (lux · sec)______________________________________Example 1 650         80      4.1Example 2 630         100     3.3Example 3 630         20      5.8Example 4 670         100     4.4Example 5 670         60      5.9______________________________________ 
    
     As can be seen in Table 1, the photoconductors of Examples 1 to 5 have good characteristics in the half decay exposure amounts E 1/2  and the residual potentials V r . 
     EXAMPLE 6 
     100 parts by weight of each of respective bisazo compound Nos. from 3 to 22 and 100 parts by weight of polyester resin (Vylon 200) were kneaded with THF as a solvent with a mixer for 3 hours to prepare a coating liquid. The respective coating liquids were applied onto aluminum substrates to form a charge generating layer having a dry thickness of about 0.5 μm. Further, the coating liquid of ABPH prepared in substantially the same manner as in Example 2 was applied on the respective charge generating layer having a thickness of about 15 μm, thus photoconductors were produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured by utilizing an electrostatic recording paper testing apparatus SP-428. The results of the measurements are shown in Table 2. 
     The surface potential V s  (volts) of each photoconductor is an initial surface potential which was measured when the surface of the photoconductor was positively charged in the dark by corona discharge at -6.0 kV for 10 seconds. After the discontinuation of the corona discharge, the photoconductor was allowed to stand in the dark for 2 seconds, after which the surface potential V d  (volts) of the photoconductor was measured. Subsequently, the surface of the photoconductor was irradiated with white light at an illuminance of 2 luxes and the time (seconds) required for the irradiation to decrease the surface potential of the photoconductor to half of the V d  was measured, then from which the half decay exposure amount E 1/2  (lux·sec) was calculated. 
     As can be seen in Table 2, the photoconductors using the bisazo compound Nos. 3 to 22 as a charge generating substance have good characteristics in the half decay exposure amount E 1/2 . 
     
                       TABLE 2______________________________________Compound  E.sub.1/2    Compound  E.sub.1/2No.       (lux · sec)                  No.       (lux · sec)______________________________________3         6.7          13        6.14         6.3          14        4.55         5.2          15        3.66         5.4          16        4.77         5.0          17        3.98         6.1          18        5.09         8.1          19        3.910        5.7          20        5.711        5.3          21        6.012        5.9          22        4.4______________________________________ 
    
     EXAMPLE 7 
     50 parts by weight of the bisazo compound No. 31, 100 parts by weight of a polyester resin (Vylon 200) and 100 parts by weight of 1-phenyl-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)-2-pyrazoline (ASPP) are kneaded with tetrahydrofuran (THF) as a solvent with a mixer for 3 hours to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film (Al-PET) as an electroconductive substrate by means of the wire bar technique to form a photosensitive layer having a dry thickness of 15 μm. Thus, a photoconductor with the structure shown in FIG. 1 was produced. 
     EXAMPLE 8 
     100 parts by weight of p-diethylaminobenzaldehyde-diphenylhydrazone (ABPH) and 100 parts by weight of polycarbonate resin (Panlite L-1250) were solved in methylene chloride to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film substrate by the wire bar technique to form a charge transporting layer having a dry thickness of 15 μm. 50 parts by weight of the bisazo compound No. 32 and 50 parts by weight of a polyester resin (Vylon 200) were kneaded with a mixer for 3 hours together with THF as a solvent to prepare a coating liquid, which was then applied onto the charge transporting layer by the wire bar technique to form a charge generating layer having a dry thickness of 0.5 μm. Thus, a photoconductor with a structure corresponding to that shown in FIG. 3 was produced. The covering layer, which is not directly related to the present invention was not provided. 
     EXAMPLE 9 
     A charge transporting layer was formed in substantially the same manner as in Example 8 except that α-phenyl-4&#39;-N,N-dimethylaminostilbene was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 8, thus a photoconductor was produced. 
     EXAMPLE 10 
     A charge transporting layer was formed in substantially the same manner as in Example 8 except that tri(p-tolyl)amine was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 8, thus a photoconductor was produced. 
     EXAMPLE 11 
     A charge transporting layer was formed in substantially the same manner as in Example 8 except that 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 8, thus a photoconductor was produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured in the same manner as in Examples 1 to 5 by utilizing an electrostatic recording paper testing apparatus Model SP-428. The result obtained are shown in Table 3. 
     
                       TABLE 3______________________________________   V.sub.s   V.sub.r E.sub.1/2   (volt)    (volt)  (lux · sec)______________________________________Example 7 630         80      4.9Example 8 670         80      5.3Example 9 590         50      5.7Example 10     630         70      6.3Example 11     660         60      5.1______________________________________ 
    
     As can be seen in Table 3, the photoconductors of Examples 7 to 11 have good characteristics in the half decay exposure amounts E 1/2  and the residual potentials V r . 
     EXAMPLE 12 
     100 parts by weight of each of respective bisazo compounds Nos. from 33 to 52 and 100 parts by weight of polyester resin (Vylon 200) were kneaded with THF as a solvent with a mixer for 3 hours to prepare a coating liquid. The respective coating liquids were applied onto aluminum substrates to form a charge generating layer having a dry thickness of about 0.5 μm. Further, the coating liquid of ABPH prepared in substantially the same manner as in Example 8 was applied on the respective charge generating layer having a thickness of about 15 μm, thus photoconductors were produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured by utilizing an electrostatic recording paper testing apparatus SP-428 and in the same manner as in Example 6. The results of the measurements are shown in Table 4. 
     As can be seen in Table 4, the photoconductors using the bisazo compound Nos. 33 to 52 as a charge generating substance have good characteristics in the half decay exposure amount E 1/2 . 
     
                       TABLE 4______________________________________Compound  E.sub.1/2    Compound  E.sub.1/2No.       (lux · sec)                  No.       (lux · sec)______________________________________33        6.2          43        6.234        5.8          44        7.335        4.8          45        4.636        4.6          46        5.737        3.9          47        5.638        5.1          48        5.339        5.2          49        5.840        5.2          50        4.441        6.1          51        5.342        7.0          52        5.2______________________________________ 
    
     EXAMPLE 13 
     50 parts by weight of the bisazo compound No. 61, 100 parts by weight of a polyester resin (Vylon 200) and 100 parts by weight of 1-phenyl-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)-2-pyrazoline (ASPP) are kneaded with tetrahydrofuran (THF) as a solvent with a mixer for 3 hours to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film (Al-PET) as an electroconductive substrate by means of the wire bar technique to form a photosensitive layer having a dry thickness of 15 μm. Thus, a photoconductor with the structure shown in FIG. 1 was produced. 
     EXAMPLE 14 
     100 parts by weight of p-diethylaminobenzaldehyde-diphenylhydrazone (ABPH) and 100 parts by weight of polycarbonate resin (Panlite L-1250) were solved in methylene chloride to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film substrate by the wire bar technique to form a charge transporting layer having a dry thickness of 15 μm. 50 parts by weight of the bisazo compound No. 62 and 50 parts by weight of a polyester resin (Vylon 200) were kneaded with a mixer for 3 hours together with THF as a solvent to prepare a coating liquid, which was then applied onto the charge transporting layer by the wire bar technique to form a charge generating layer having a dry thickness of 0.5 μm. Thus, a photoconductor with a structure corresponding to that shown in FIG. 3 was produced. The covering layer, which is not directly related to the present invention was not provided. 
     EXAMPLE 15 
     A charge transporting layer was formed in substantially the same manner as in Example 14 except that α-phenyl-4&#39;-N,N-dimethylaminostilbene was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 14, thus a photoconductor was produced. 
     EXAMPLE 16 
     A charge transporting layer was formed in substantially the same manner as in Example 14 except that tri(p-tolyl)amine was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 14, thus a photoconductor was produced. 
     EXAMPLE 17 
     A charge transporting layer was formed in substantially the same manner as in Example 14 except that 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 14, thus a photoconductor was produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured by utilizing an electrostatic recording paper testing apparatus Model SP-428 and in the same manner as in Examples 1 to 5. The result obtained are shown in Table 5. 
     
                       TABLE 5______________________________________   V.sub.s   V.sub.r E.sub.1/2   (volt)    (volt)  (lux · sec)______________________________________Example 13     630         80      4.3Example 14     590         50      5.9Example 15     620         40      5.3Example 16     660         30      4.8Example 17     600         20      5.1______________________________________ 
    
     As can be seen in Table 5, the photoconductors of Examples 13 to 17 have good characteristics in the half decay exposure amounts E 1/2  and the residual potentials V r . 
     EXAMPLE 18 
     100 parts by weight of each of respective bisazo compound Nos. from 63 to 82 and 100 parts by weight of polyester resin (Vylon 200) were kneaded with THF as a solvent with a mixer for 3 hours to prepare a coating liquid. The respective coating liquids were applied onto aluminum substrates to form a charge generating layer having a dry thickness of about 0.5  μm. Further, the coating liquid of ABPH prepared in substantially the same manner as in Example 14 was applied on the respective charge generating layer having a thickness of about 15 μm, thus photoconductors were produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured by utilizing an electrostatic recording paper testing apparatus SP-428 and in the same manner as in the Example 6. The results of the measurements are shown in Table 6. 
     As can be seen in Table 6, the photoconductors using the bisazo compound Nos. 63 to 82 as a charge generating substance have good characteristics in the half decay exposure amount E 1/2 . 
     
                       TABLE 6______________________________________Compound  E.sub.1/2    Compound  E.sub.1/2No.       (lux · sec)                  No.       (lux · sec)______________________________________63        4.8          73        5.364        4.6          74        5.765        4.9          75        5.266        6.1          76        6.667        4.3          77        4.068        5.8          78        6.969        6.2          79        5.070        6.9          80        5.871        6.2          81        5.772        4.4          82        5.0______________________________________ 
    
     EXAMPLE 19 
     50 parts by weight of the bisazo compound No. 91, 100 parts by weight of a polyester resin (Vylon 200) and 100 parts by weight of 1-phenyl-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)-2-pyrazoline (ASPP) are kneaded with tetrahydrofuran (THF) as a solvent with a mixer for 3 hours to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film (Al-PET) as an electroconductive substrate by means of the wire bar technique to form a photosensitive layer having a dry thickness of 15 μm. Thus, a photoconductor with the structure shown in FIG. 1 was produced. 
     EXAMPLE 20 
     100 parts by weight of p-diethylaminobenzaldehyde-diphenylhydrazone (ABPH) and 100 parts by weight of polycarbonate resin (Panlite L-1250) were solved in methylene chloride to prepare a coating liquid. The coating liquid was applied onto an aluminum-deposited polyester film substrate by the wire bar technique to form a charge transporting layer having a dry thickness of 15 μm. 50 parts by weight of the bisazo compound No. 92 and 50 parts by weight of a polyester resin (Vylon 200) were kneaded with a mixer for 3 hours together with THF as a solvent to prepare a coating liquid, which was then applied onto the charge transporting layer by the wire bar technique to form a charge generating layer having a dry thickness of 0.5 μm. Thus, a photoconductor with a structure corresponding to that shown in FIG. 3 was produced. The covering layer, which is not directly related to the present invention was not provided. 
     EXAMPLE 21 
     A charge transporting layer was formed in substantially the same manner as in Example 20 except that α-phenyl-4&#39;-N,N-dimethylaminostilbene was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 20, thus a photoconductor was produced. 
     EXAMPLE 22 
     A charge transporting layer was formed in substantially the same manner as in Example 20 except that tri-(p-tolyl)amine was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 20, thus a photoconductor was produced. 
     EXAMPLE 23 
     A charge transporting layer was formed in substantially the same manner as in Example 20 except that 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole was used to replace ABPH as a charge transporting substance. Then a charge generating layer was formed on the charge transporting layer in the same manner as in Example 20, thus a photoconductor was produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured in the same manner as in Examples 1 to 5 by utilizing an electrostatic recording paper testing apparatus Model SP-428. The result obtained are shown in Table 7. 
     
                       TABLE 7______________________________________   V.sub.s   V.sub.r E.sub.1/2   (volt)    (volt)  (lux · sec)______________________________________Example 19     600         60      6.5Example 20     680         60      5.8Example 21     650         50      5.2Example 22     670         90      5.3Example 23     700         100     4.9______________________________________ 
    
     As can be seen in Table 7, the photoconductors of Examples 19 to 23 have good characteristics in the half decay exposure amounts E 1/2  and the residual potentials V r . 
     EXAMPLE 24 
     100 parts by weight of each of respective bisazo compounds Nos. from 93 to 182 and 100 parts by weight of polyester resin (Vylon 200) were kneaded with THF as a solvent with a mixer for 3 hours to prepare a coating liquid. The respective coating liquids were applied onto aluminum substrates to form a charge generating layer having a dry thickness of about 0.5 μm. Further, the coating liquid of ABPH prepared in substantially the same manner as in Example 20 was applied on the respective charge generating layer having a thickness of about 15 μm, thus photoconductors were produced. 
     The electrophotographic characteristics of the photoconductors thus produced were measured in the same manner as in Example 6 by utilizing an electrostatic recording paper testing apparatus SP-428. The results of the measurements are shown in Table 8. 
     As can be seen in Table 8, the photoconductors using the bisazo compound Nos. 93 to 182 as a charge generating substance have good characteristics in the half decay exposure amount E 1/2 . 
     
                       TABLE 8______________________________________Compound  E.sub.1/2    Compound  E.sub.1/2No.       (lux · sec)                  No.       (lux · sec)______________________________________ 93       6.1          110       5.3 94       5.7          111       5.2 95       5.1          112       6.4 96       4.8          113       4.7 97       4.2          114       5.8 98       6.5          115       5.2 99       6.3          116       5.9100       6.2          117       6.7101       5.2          118       5.7102       6.8          119       3.7103       4.9          120       4.8104       5.0          121       6.3105       4.6          122       5.7106       5.1          123       3.8107       5.5          124       4.6108       4.8          125       6.0109       5.0          126       4.8127       5.2          144       5.7128       4.2          145       5.1129       5.6          146       3.6130       5.7          147       4.7131       6.0          148       4.6132       7.4          149       6.6133       4.6          150       5.7134       5.2          151       5.5135       5.3          152       3.8136       4.3          153       4.1137       4.9          154       6.1138       5.0          155       5.4139       3.7          156       3.6140       4.9          157       4.8141       4.2          158       4.7142       4.9          159       5.0143       5.6          160       5.7161       5.1          172       5.7162       5.3          173       5.4163       4.7          174       8.0164       6.0          175       4.7165       5.0          176       6.6166       6.1          177       6.7167       4.7          178       5.7168       6.3          179       5.3169       4.4          180       4.6170       6.1          181       4.4171       4.8          182       5.5______________________________________ 
    
     As described above, according to the present invention, since a bisazo compound represented by any one of the aforementioned chemical formulae is used as a charge generating substance in a photosensitive layer formed on an electroconductive substrate, a photoconductor shows a high sensitive and excellent characteristics in repeated use when adapted to either a positive charge mode or a negative charge mode. If necessary, a covering layer may be provided on the surface of a photoconductor to improve the durability thereof. 
     The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the invention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.