Derivatives of diiminoquinones useful as electron transport agents in electrophotographic elements
TECHNICAL FIELD 
     The present invention relates generally to electrophotographic printing, and, more particularly, to specific electron transport agents useful in electrophotographic printing. 
     BACKGROUND ART 
     Electrophotographic (EP) laser printing employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (conducting or insulative), or an intermediate transfer medium. 
     There is a demand in the laser printer industry for multi-colored images. The image quality can be enhanced by a large number of approaches, including the technique which utilizes small particle developer including dry toner having an average particle size less than 5 μm; see, e.g., U.S. Pat. Nos. 4,927,727; 4,968,578; 5,037,718; and 5,284,731. However, it has also been known that the electrophotographic dry toner having particle size less than 1 μm is very hard to prepare due to increased specific area, and consequently, liquid toner has become one of the solutions for practical preparation of sub-micrometer xerographic developer. 
     Liquid toners comprise pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate transfer medium to produce a multi-colored image. 
     The organic photoconductor products in the market today, generally speaking, are dual layer OPCs, which comprise a charge generation layer (CGL) and a charge transport layer (CTL) as key components. In addition to these layers, the photoconductor body can be undercoated or overcoated with other materials to improve adhesion to the substrate or to improve surface wear resistance or to reduce the surface adhesion for improved image transfer efficiency. The organic photoconductor (OPC) with an additional undercoating layer or overcoating layer becomes an organic photoreceptor (OPR) and ready for use in various designs of electrophotographic systems. 
     Most of the multilayer OPRs in the market are negative charging OPCs in which a thick hole transport layer is located on the top of a thin CGL. This is called the standard, or conventional, dual layer OPC. In the conventional case, the CGL usually comprises a photoconductive pigment or dye dispersed in an inert binder, with a pigment/dye content ranging up to about 90 wt %. 100% pigment in the CGL is possible where the pigment CGL is vacuum-evaporated in the format of a thin film; see, e.g., U.S. Pat. No. 4,578,334. Besides dispersion stabilizing functions, the CGL binder also plays an important role of adhesion. 
     Positive charging OPCs are also known, in which a thick electron transport layer is located on top of the thin CGL. Electron transport molecules are molecules which can transport an electron under a positive bias. 
     The advantages of the electron transport agent can be found in the design of a positive charging photoreceptor, in which the major carder is the electron. In this design, the electron transport agent is also expected to provide excellent electrical stability of the photoreceptor, since it exhibits the least surface charge injection. 
     On the other hand, the challenges of the design of the electron transport molecules are associated with the solubility and the compatibility in various types of binders, inasmuch as electron transport agents, in general, are bulky. 
     A variety of electron transport agents have been disclosed, including derivatives of 4-thiopyran, dicyanofluorenone, imines, diphenobenzoquinone, and stilbene diphenobenzoquinone; see, e.g., U.S. Pat. No. 5,013,849; 5,034,293; and 5,213,923. However, 4-thiopyrans are expensive, most of the afore-mentioned compounds evidence poor compatibility with binders used to form the CTL, and most of these compounds suffer from a limited electron mobility range. 
     Thus, an electron transport agent is required which avoids most, if not all, of the problems associated with prior art electron transport agents. 
     DISCLOSURE OF INVENTION 
     In accordance with the invention, derivatives of diiminoquinones are effective as electron transport agents. The diiminoquinones of the present invention are represented by formula (I): ##STR1## where A is a moiety selected from the group consisting of ═CH--CH═, ##STR2## B 1  and B 2  are independently selected from the group consisting of O, S, Se, Te, dicyano, and alkoxy, and R 1  to R 23  are independently selected from the group consisting of hydrogen, alkyl, alkoxy, alkene, aryl, hydroxy, halogen, cyano, nitro, and sulfuryl, n is an integer within the range of 0 to 3, and ##STR3## are independently selected from the group consisting of ##STR4## The diiminoquinone derivatives of the invention are inexpensive materials and have excellent solubility and compatibility with most binders due to the presence of long alkyl chains (n=0,1,2). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of one embodiment of a photoconductive generation and transport configuration, using the electron transport agents of the present invention; 
     FIG. 2 is a cross-sectional view of another embodiment of a photoconductive generation and transport configuration, using the electron transport agents of the present invention; 
     FIG. 3 is a cross-sectional view of yet another embodiment of a photoconductive generation and transport configuration, using the electron transport agents of the present invention; 
     FIG. 4 is a cross-sectional view of still another embodiment of a photoconductive generation and transport configuration, using the electron transport agents of the present invention; and 
     FIG. 5 is a cross-sectional view of a still further embodiment of a photoconductive generation and transport configuration, using the electron transport agents of the present invention. 
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Turning now to the drawings wherein like numerals of reference depict like elements throughout, FIG. 1 depicts one photoconductive generation and transport configuration 10, in which the electron transport agents of the present invention find use. In this embodiment, a conductive support 12 comprises an electrically conductive layer 14, typically of aluminum, formed on a substrate 16, such as a web or subbing layer to improve adhesion to an underlying web (not shown). The web, e.g., drum, is used as a component in electrophotographic printers and copiers, as is well-known. A charge generation layer (CGL) 18 is formed on the electrically conductive layer 14. The CGL 18 typically comprises a photoconductive pigment or dye, either dispersed in a binder or deposited as a thin film, or other well-known photoconducting inorganic material, including amorphous selenium (a-Se), a-As 2  Se 3 , a-AsSeTe, amorphous Si, ZnO, CdS, and TiO 2 . 
     Examples of suitable photoconductive pigments and dyes include: 
     (a) the metastable form of phthalocyanine pigments: x-form, tau-form of metal-free phthalocyanine pigment (x-H 2  Pc), alpha-, epsilon-, beta-form of copper phthalocyanine pigment (CuPc), titanyl phthalocyanine pigments (TiOPcX 4 , where X is H, F, Cl, Br, I), vanadyl phthalocyanine pigment (VOPc), magnesium phthalocyanine pigment (MgPc), zinc phthalocyanine pigment (ZnPc), chloroindium phthalocyanine pigment (ClInPc), bromoindium phthalocyanine pigment (BrInPc), chloroaluminum phthalocyanine pigment (ClAlPc), hydroxy gallium phthalocyanine, and the like; 
     (b) pyrollo pyrole pigments; 
     (c) tetracarboximide perylene pigments; 
     (d) anthanthrone pigments; 
     (e) bis-azo, -trisazo, and -tetrakisazo pigments; 
     (f) zinc oxide pigment; 
     (g) cadmium sulfide pigment; 
     (h) hexagonal selenium; 
     (i) squarylium dyes; and 
     (j) pyrilium dyes. 
     Examples of suitable binders for the pigments and dyes include polyvinyl carbazoles, polystyrenes, polysilanes, polycarbonates, polyimides, polygermanes, polyesters, polyvinyl butyral (PVB), fluoropolymers, silicone resins, and other such materials well-known in this art. Additional suitable binders include thermoset and thermoplastic polymers having a large degree of flexibility in the polymer conformation due to its flexible backbone, and having a glass transition temperature lower than about 120° C., as disclosed in co-pending application Ser. No. 08/287,437, filed Aug. 8, 1994, entitled &#34;Reusable Inverse Composite Dual-Layer Organic Photoconductor Using Specific Polymers Available for Diffusion Coating Process with Non-Chlorinated Solvents&#34; by Khe C. Nguyen et al and assigned to the same assignee as the present application. These additional binders comprise specific vinyl polymers. In use, the concentration range of the pigment or dye in the binder ranges from about 10 to 80 wt %. 
     The charge generation layer 18 can also be a thin film of the above-mentioned photoconductive materials. The thin film charge generation layer 18 is conveniently prepared by vacuum technology techniques, including vacuum evaporation, sputtering, glow discharge, and the like. If such thin films are used, then no binders are required. 
     A charge transport layer (CTL) 20 is formed on top of the CGL 18 and includes one or more of the electron transport agents of the present invention in a binder. The binder may comprise any of the conventional binders listed above, as well as poly-condensation product polymers or specific vinyl polymers having a glass transition temperature greater than about 120° C., as also described in the above-referenced patent application by K. C. Nguyen et al. 
     As shown in FIG. 1, light hv passes through the electron transport layer 20 and creates electron (-)/hole (+) pairs in the charge generation layer 18. The electrons are transported through the electron transport layer 20 to its outer surface, where they selectively discharge the electrostatic surface charge 21 (denoted as &#34;+&#34;); the holes migrate to the electrically conductive layer 14. 
     In FIG. 2, another photoconductive generation and transport configuration 10a is depicted. A hole transport layer 24 is shown formed on the electrically conductive substrate 16. The hole transport layer 24 typically comprises any of the conventional hole transport molecules, including, but not limited to, triaryl methanes, triarylamines, hydrazones, pyrazolines, oxadiazoles, styryl derivatives, carbazolyl derivatives, and thiophene derivatives, polysilanes, polygermanes, and the like. In this embodiment, the electron transport and charge generation functions are provided by a single layer 26, which is formed on the CTL 24. The electron transport/charge generation layer 26 contains the electron transport agent(s) of the present invention in a suitable binder. Light hv generates electron/hole pairs in the electron transport/charge generation layer 26. The electrons are transported to the surface of this layer 26, where they selectively discharge the electrostatic surface charge 21; the holes are transported through the hole transport layer 24 to the electrically conductive layer 14. 
     In FIG. 3, yet another photoconductive generation and transport configuration 10b is depicted. The hole transport layer 24 is formed on the electrically conductive layer 14 and in turn supports a separate charge generation layer 28, which typically comprises any of the charge generation molecules (pigments or dyes) in a binder, as described above, and an electron transport layer 30, which is formed on top of the charge generation layer. The electron transport layer 30 contains the electron transport agents of the present invention, again, in a suitable binder and performs as the positive charge injection blocking layer. Light hv generates electron/hole pairs in the charge generation layer 28. The electrons are transported through the electron transport layer 30 to its outer surface, where they selectively discharge the electrostatic surface charge 21; the holes are transported through the hole transport layer 24 to the electrically conductive layer 14. 
     In FIG. 4, still another photoconductive generation and transport configuration 10c is depicted. A layer 32 which contains one or more hole transport molecules, one or more electron transport molecules of the present invention, and provides charge generation, is formed on top of the hole transport layer 24. Light hv generates electron/hole pairs in the charge generation layer 32. The electrons migrate to the outer surface of the charge generation layer 32, where they selectively discharge the electrostatic surface charge 21; the holes are transported through the hole transport layer 24 to the electrically conductive layer 14. 
     In FIG. 5, yet a still further photoconductive generation and transport configuration 10d is depicted. A single layer 34 contains both the charge transport molecules, including one or more of the electron transport agents of the present invention, and charge generator molecules in a binder. This single layer 34 is formed directly on the conductive layer 14. The nature of the charge (21a for positive charge, 2lb for negative charge) is indicated on the surface of this single layer 34, and may be bipolar, depending on the predominance of the charge transport molecule. 
     The electron transport agents of the present invention comprise derivatives of diiminoquinones represented by formula (I): ##STR5## where A is a moiety selected from the group consisting of ═CH--CH═, ##STR6## B 1  and B 2  are independently selected from the group consisting of O, S, Se, Te, dicyano, and alkoxy, and R 1  to R 23  are independently selected from the group consisting of hydrogen, alkyl, alkoxy, alkene, aryl, hydroxy, halogen, cyano, nitro, and sulfuryl, n is an integer within the range of 0 to 3, and ##STR7## are independently selected from the group consisting of ##STR8## 
     The diiminoquinone derivatives of the invention are inexpensive materials, requiring only two steps to synthesize, have excellent solubility and compatibility with most binders due to the presence of long alkyl chains, and evidence high electron mobility. Many of these derivatives are commercially available. A time-of-flight technique described elsewhere detects an electron mobility of this class of material in the range of about 10 -3  to 10 -5  V/sec.cm 2 . Therefore, the diiminoquinone derivatives of the invention are comparable or better than dicyano methylene fluorenone derivatives, 4-thiopyran, and the like. 
     Particularly preferred compounds include: ##STR9## 
     EXAMPLES 
     Example 1. 
     Preparation of ##STR10## 
     A slurry of 2,6-dimethyl-4-aminophenol (5.15 g, 37.54 mmol) in chloroform (0.57 g) was degassed for 1/2 hr under dry nitrogen. Then, glyoxal (2.7 g of 40 wt % solution in water, 18.6 mmol) was added. The reaction mixture was heated to 50° C. and heating was discontinued. The mixture was stirred at ambient temperature for 22 hrs and reheated to 60° C. for 3 hrs. This solution was washed with dilute hydrochloric acid (20 ml), followed by water (2×100 ml). The organic layer was dried over anhydrous magnesium sulfate and then filtered. The solvent from one filtrate was evaporated to yield the desired phenolic compound (A) shown above (5.27 g, 95.7% yield based on glyoxal). 
     Example 2. 
     Preparation of ##STR11## 
     The phenolic compound (A) (4.67 g, 15.78 mmol) from Example 1 was mixed with potassium permanganate (13.0 g, 82.3 mmol) in chloroform (71 g). This reaction mixture was heated to 60° C. for 18 hrs and then filtered. The potassium permanganate mixture was extracted with dichloromethane (4×50 ml) and filtered. The combined filtrate was eluted through a silica gel column. The solvent from one eluate was evaporated to obtain the desired compound (B) (2.3 g, 49.6% yield). The melting point of this compound was found to be 290° C. 
     Example 3. 
     20 g of the x-form metal-free phthalocyanine pigment, 10 g of polyvinylbutyral B-76(Monsanto Chemical Co.), 500 g of dichloromethane (DCM) and stainless steel beads (3 mm diameter) were milled together using a ball mill for 72 hours. The viscosity was adjusted by diluting the solution down to 1% solids. The suspension was coated onto aluminum-coated Mylar using a doctor blade to achieve a 1 μm thick coating after being dried in an oven at 80° C. for a few seconds to form the charge generation layer (CGL). 
     Next, 40 g of any of compounds (1) to (24), 60 g of polycarbonate Panlite L (Teijin Chemical), and 900 g of DCM may be stirred together until completely dissolve. This was the electron transport solution to form the charge transport layer (CTL). The solution was coated on top of the above-mentioned CGL using a doctor blade to achieve a thickness of 20 μm after being dried in an oven at 80° C. for two hours, forming a full construction of a conventional dual layer photoreceptor. 
     The photoconductor was tested by a drum tester system known as Cynthia 1000, developed by Gentek Co. In this test, the well-grounded photoreceptor specimen was charged by corona charger at +6 kV, rested in dark for 10 seconds, and then exposed to 780 nm light source provided by a combination of halogen lamp, interference filter, and 10 ms electrical shutter. Typical results obtained for these compounds are summarized in Table 1. 
     
                       TABLE 1                                                     
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XEROGRAPHIC PERFORMANCE DATA                                              
                                Residual                                  
                                Voltage Residual                          
                      E.sub.1/2 (energy re-                               
                                after   voltage                           
                      quired to dis-                                      
                                closing the                               
                                        after                             
Com-  V.sub.0                                                             
             Dark de- charge 50% of                                       
                                shutter eraser                            
pound (V)    cay (%)  V.sub.0 (ergs/cm.sup.2)                             
                                V.sub.r (V)                               
                                        V.sub.er (V)                      
______________________________________                                    
(1)   700    96       10.0      100     2                                 
(2)   650    94       5.5       40      0                                 
(3)   720    96       12.0      105     2                                 
(4)   632    92       8.0       75      2                                 
(5)   635    95       7.5       120     15                                
(10)  650    93       6.6       80      5                                 
(14)  645    92       4.5       45      0                                 
(17)  642    94       6.8       80      10                                
(19)  650    95       5.5       60      6                                 
(22)  674    96       4.6       43      2                                 
(23)  660    97       11.0      100     17                                
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     Comparison Example 3a. 
     40 g of hole transport molecule ##STR12## 60 g of polycarbonate Panlite L (Teijin Chemical, Japan) and 900 g of dichloromethane were stirred together until completely dissolved. The solution was coated directly onto Al-coated Mylar using a doctor blade and dried in an oven at 80° C. for 2 hours to achieve a hole transport layer (CTL) having thickness of 20 μm. Next, 3 g of alpha form titanyl phthalocyanine (α-TiOPc), 97 g of polycarbonate and 900 g of DCM were milled together for 72 hours using a ball milling process employing stainless steel beads (4 mm diameter, special burning grade) as milling media. The viscous suspension was diluted into a solution having 5 wt % of solid content. This solution was coated on the top of the above-mentioned hole transport molecule using a doctor blade to give rise to a thickness of 3 μm after being dried at 80° C. for 2 hours. This coating layer is a charge generation layer (CGL). The photoconductor is called an inverted dual layer (IDL) photoconductor, compared to conventional composite dual layer photoconductor described in Example 1. 
     The photoconductor was tested by the method described in Example 1. Typical results obtained are summarized below: 
     V o  =780 V 
     dark decay rate (DDR) =98% 
     E 1/2  (energy required to discharge 50% of V o ) =123 ergs/cm2 
     residual voltage after closing the shutter V r  =300 V 
     residual voltage after erasure V re  =200 V. 
     Example 4. 
     The formulation of the IDL described in the Comparison Example 3a was repeated, except that the CGL was formulated as described below: 
     3 g of alpha form titanyl phthalocyanine (α-TiOPc) 
     37 g of electron transport compound (1) 
     60 g of polycarbonate Panlite L 
     900 g of DCM 
     were used. 
     Typical results, obtained by the method described in Example 1, are summarized below: 
     V o  =750 V 
     dark decay rate (DDR) =96% 
     E 1/2  (energy required to discharge 50% of V o ) =7 ergs/cm2 
     residual voltage after closing the shutter V r  =60 V 
     residual voltage after erasure V re  =0 V. 
     So, it is obvious that by adding the electron transport molecule in the CGL of an inverted dual layer, it is possible to provide a significant improvement of the photo-discharge due to the increase of electron transport effect in CGL. 
     INDUSTRIAL APPLICABILITY 
     The derivatives of diiminoquinones disclosed herein are expected to find use in electrophotographic printing, especially in color electrophotographic printing. 
     Thus, there has been disclosed improved electron transport agents comprising derivatives of diiminoquinones for electrophotographic printing. It will be readily apparent to those skilled in this art that various changes and modifications of an obvious nature may be made without departing from the scope of the invention, which is defined by the appended claims.