Substituted Squarium Compounds, Process for Preparing the Same and Electrophotographic Photoreceptors Containing the Same

This invention relates to novel squarium compounds having a flat photosensitivity in the wide range from visible ray region to the near infrared ray region as a charge generator, a process for preparing them and electrophotographic photoreceptors containing the squarium compounds.

Hitherto, inorganic photographic materials such as amorphous selenium, selenium alloys, cadmium sulfide and zinc oxide and organic photographic materials represented by polyvinylcarbazole and polyvinylcarbazole derivatives are widely known as electrophotographic photoreceptons.

It is well known that amorphous selenium or selenium alloys have superior properties as electrophotographic photoreceptors and have been put into practice use. However, in producing these materials, a complicated vacuum deposition process is necessary, and further the vacuum-deposited film obtained has a disadvantage in that the film lacks flexibility. Zinc oxide is used as a dispersed photographic material in which zinc oxide is dispersed in a resin, but such material has a problem such as a low mechanical strength and cannot be used repeatedly as is.

Polyvinylcarbazoles known as organic photoconductive materials have advantages such as transparency, good film-forming property and flexibility, but polyvinylcarbazoles per se have no photosensitivity in at visible ray region, an cannot be used practically alone. Accordingly, various sensitization methods have been proposed therefor. Although a spectral sensitization of polyvinylcarbazole using a sensitizing dye has resulted in expanded spectral sensitivities extending to visible ray region, a sufficient photographic sensitivity as electrophotographic photoreceptors cannot be obtained, and it still has a drawback that a photo-fatigue is remarkable.

On the other hand, a spectral sensitization with an electron acceptor gives rise to electrophotographic photoreceptors having a sufficient photosensitivity, and some of them has been practically used. However, there are still more problems on mechanical strength and durability.

Various extensive studies have been made on photosensitive materials and there are many reports. However, electrophotographic photoreceptors having a superior electric characteristic and a sufficient photosensitivity have not yet been obtained. At present, there are reports about phthalocyanines which show superior electrophotographic characteristics as dispersed photosensitive materials. However, their spectral sensitivity is partial to long wavelength region, and they have a drawback that reproduction of red color is inferior.

An object of this invention is to provide novel squarium compounds having a flat photosensitivity in the wide range from the visible ray region to the near infrared ray region, a process for preparing squarium compounds and electrophotographic photoreceptors containing such squarium compounds.

Another object of this invention is to provide photoconductive materials having an extremely high photosensitivity, which are possible to use in every electrophotographic process and have a spectral sensitivity in the range from the visible ray region to the near infrared ray region.

A further object of this invention is to provide extremely superior electrophotographic photoreceptors having a flexibility that inorganic photographic materials fail to have, which are free from drawbacks of organic photographic materials such as polyvinylcarbazole-trinitrofluorenones, that is a low abrasion resistance and lack of mechanical strength, and which are superior in mechanical strength such as abrasion resistance and have a flat spectral sensitivity in the wide range from visible ray region to the near infrared ray region.

As a result of extensive studies to obtain photoconductive materials having improved properties over conventional inorganic photographic materials, organic photographic materials or organic dispersed photographic materials, having superior electrophotographic properties and flexibility, and further having high photographic sensitivity in the wide range from visible ray region to the near infrared ray region, the present inventors found that the squarium compounds represented by the following general formula (I) possess extremely superior characteristics: ##STR1## wherein X represents a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, a nitro group, or a cyano group.

FIGS. 1 and 2 are profile sections of compositions of electrophotographic photoreceptors of this invention.

In figures, "1" shows an electroconductive support, "2" shows a charge generation layer, "3" shows a charge transport layer and "4" shows a photosensitive layer.

The squarium compounds of this invention represented by formula (I) above can be prepared by reacting 3,4-dihydroxy-3-cyclobutene-1,2-dione (hereinafter referred to as squaric acid) represented by formula (II): ##STR2## with an aniline derivative of formula (III): ##STR3## wherein X has the same meaning defined for formula (I) above.

In formula (I), X preferably represents a hydrogen atom, a fluorine atom, or a chlorine atom.

The reaction of squaric acid of formula (II) with an aniline derivative of formula (III) can be carried out by heating squaric acid and the aniline derivative in an inert organic solvent, such as n-butyl alcohol, amyl alcohol, or a mixed solvent of n-butyl alcohol or amyl alcohol with benzene, toluene or xylene, at about 120.degree. C. to about 140.degree. C. for about 3 hours to about 5 hours. The resulting compound can be purified by washing and then recrystallizing the compound from an appropriate solvent.

Examples of the squarium compounds of formula (I) above of this invention include the following compounds.

The spectral sensitivity of the squarium compounds of this invention show a flat photosensitivity in the range of from 400 to 850 nm, and the squarium compounds of this invention have a sufficient photosensitivity in the range from the entire visible ray region to the near infrared ray region.

The squarium compounds of formula (I) can be used in electrophotographic photoreceptors having a multilayer structure. That is, in electrophotographic photoreceptors having a double layer structure consisting of a charge generation layer and a charge transport layer, a combination of the charge generation layer containing a squarium compound of this invention and a known charge transport layer comprising a photoconductive polymer such as polyvinyldibenzothiophene, polyvinylpyrene, polyvinylanthracene, or a polyvinylcarbazole or a layer containing triallylpyrazoline, triphenylmethane, oxadiazole, tetraphenylbenzidine, or trinitofluorenone in a binder resin results in the improvement of chargibility of electrophotographic photoreceptors, lowering of residual potential, and further the improvement of mechanical strength.

As shown in FIGS. 1 and 2 which explain the construction of electrophotographic photoreceptor of this invention having a double layer structure, a photosensitive layer 4 which is composed of a lamination of charge generation layer 2 containing a squarium compound of this invention and a charge transport layer 3 containing a charge transport material is provided on an electroconductive support 1. The thickness ratio of the charge generation layer to the charge transport layer is preferably from about 1/2 to about 1/200. Further, the squarium compound and a charge transport material may be incorporated into one layer to form a single-layered photoreceptor, if desired.

The charge generation layer can consist of a squarium compound alone or a combination of the squarium compound and a binder resin. The ratio of the squarium compound of formula (I) to the binder resin used is from about 10% by weight to about 90% by weight, preferably from about 10% by weight to about 50% by weight.

A solvent coating method and a vacuum evaporation method can be used to prepare the charge generation layer with the squarium compound of this invention without using a binder resin.

The film thickness of charge generation layer is from about 0.1.mu. to about 3.mu., preferably from 0.2.mu. to 1.mu..

For the purpose of its dispersion in a binder, the compound may be ground into fine particles by a known method using any conventional mill, such as an SPEX MILL, ball mill or RED DEVIL (trade name). Particle size of the pigment is generally not more than 5.mu., preferably 0.01 to 3.mu., but the particle size is not limited thereto.

The binder used in the charge generation layer may have or may not have photoconductivity. Examples of binders having photoconductivity include photoconductive polymers such as polyvinylcarbazole, polyvinylcarbazole derivatives, polyvinylnaphthalene, polyvinylanthracene, polyvinylpyrene, or other organic matrix materials having a charge transport property.

Moreover, known insulating resins which have no photoconductive property can be used as binders. Examples of such insulating resins include polystyrene, polyesters, polyvinyltoluen, polyvinylanisole, polychlorostyrene, polyvinyl butyral, polyvinyl acetate, polyvinylbutyl methacrylate, copolystyrene-butadiene, polysulfone, copolystyrene-methyl methacrylate and polycarbonates.

In order to improve mechanical strength of the electrophotographic photoreceptors obtained, a plasticizer can also be used as in the case of ordinary polymeric materials. Examples of plasticizers which can be used include chlorinated paraffins, chlorinated biphenyl, a phosphate plasticizer and a phthalate plasticizer. These plasticizers can be used in an amount of 0 to 10% by weight based on the weight of the binder used without adversely affecting the photosensitivity and electrical properties of the electrophotographic photoreceptors thereby improving the mechanical properties.

The binder having dispersed therein a squarium compound is coated on an electroconductive support. The coating can be performed by a conventional method such as dip method, spray method, bar coater method and applicator method. A good photosensitive layer can be obtained by any of the above methods.

Usable electrically conductive supports include metals (e.g., aluminum, nickel, chromium, iron, stainless, copper, etc.), paper which is rendered electrically conductive, as well as polymeric films and glass plates having an electrically conductive coat of the above metals, Au, Ag, indium oxide, indium tin oxide, etc.

A surface layer such as a protective layer and an insulating layer may further be provided on the photosensitive layer so as to prevent mechanical damage and chemical change in properties of the photosensitive layer. The protective layer is a layer having low electric resistance of 10.sup.8 to 10.sup.14 .OMEGA.cm which can be used in so-called Carlson process, and the insulating layer is an electrically insulating layer which can be used in a process as described in U.S. Pat. Nos. 3,041,167 and 3,438,706. Both layers are substantially transparent to light for exposure and the thicknesses of the protective layer and the insulating layer are about 2 to 20.mu. and about 10 to 40.mu., respectively.

In order to prevent injection of electron from an electrically conductive support to the photosensitive layer, a barrier layer may be formed between the support and the photosensitive layer. For the purpose, aluminum oxide, nylon, epoxy resins can be used. Such a barrier layer may not be formed when the photoreceptor is used in the process of U.S. Pat. Nos. 3,041,167 and 3,438,706 as described above or when the charge transport layer is formed as a lower layer on the support in the preparation of double-layered photoreceptor (FIG. 2). An adhesive layer may also be formed between the support and the photosensitive layer to improve adhesion therebetween.

The electrophotographic photoreceptor of the present invention may be used not only with ordinary copiers but also when laser printers, as well as intelligent copiers since the photoreceptor of the present invention is sensitive to laser. Laser which can be applied to the photoreceptor of the present invention is preferably a semiconductor laser such as those of Ga-As type semiconductors (e.g., Ga-As, Al-Ga-As, Ga-As-P, etc.).

The squarium compounds of formula (I) of this invention can be applied to various materials such as optical recording media (e.g., laser disk) and organic solar batteries, in addition to the electrophotographic materials.

This invention will now be explained in more detail by the following examples.

5.76 g of N-benzyl-N-methyl-m-hydroxyaniline and 1.48 g of squaric acid were heated on an oil bath kept at 130.degree. to 140.degree. C. in a mixed solvent of 70 ml of n-butyl alcohol and 20 ml of benzene with stirring for 3 hours, whereby greenish crystals were precipitated. After cooling, the crystals were collected by filtration, washed with methanol, and then dissolved in propylamine, followed by reprecipitating from water to obtain 5.0 g (76.3% yield) of the desired squarium compound.

Decomposition point: 293.degree. C. to 295.degree. C.

Infrared absorption spectrum (KBr Tablet): .nu.c=o 1610 cm.sup.-1

Visible absorption spectrum: .lambda.max 641 nm (in dichloromethane solution)

In the same manner as described in Example 1 except using corresponding aniline derivatives, Compounds (2) and (3) were prepared.

Decomposition point: 309.degree. C. to 310.degree. C.

Infrared absorption spectrum (KBr Tablet): .nu.c=o 1610 cm.sup.-1

Visible absorption spectrum: .lambda.max 641 nm (in dichloromethane solution)

Decomposition point: 318.degree. C. to 319.degree. C.

Infrared absorption spectrum (KBr Tablet): .nu.c=o 1615 cm.sup.-1

Visible absorption spectrum: .lambda.max 640 nm (in dichloromethane solution)

The squarium compound of formula (I) wherein X is a hydrogen atom, was ground with methylene chloride and steel balls for 12 hours. The ground compound was then added to a polyester resin (Vylon 200, a tradename) in an amount of 30% by weight based on the weight of the polyester resin, followed by mixing, and then the mixture was coated by an applicator on an aluminum plate to prepare a charge generation layer having a thickness of about 0.5.mu. (dry basis). A mixture of a polycarbonate resin (Panlite, a tradename) and 1-phenyl-3-[p-diethylaminostyryl]-5-[p-diethylaminophenyl]pyrazoline in an amount of 50% by weight based on the weight of the polycarbonate resin was then coated by an applicator on the charge generation layer to prepare a charge transport layer having thickness of about 15.mu..

The photosensitive surface of this photoreceptor was negatively charged with -6 KV of corona electric discharge for 2 seconds using an electrostatic copy paper test instrument manufactured by Kawaguchi Denki Co., Ltd. After it was allowed to stand for 2 seconds in the dark, its surface potential (Vo) was measured and then a light from a tungsten-halogen lamp of illumination intensity of 10 lux was irradiated to the photosensitive layer. The time when the value of the surface potential changed to a half the original value was determined to obtain a half value of exposure (E1/2). The results obtained were Vo of 820 V and E1/2 of 2.2 lux.second.

The electrophotographic photoreceptors of these examples were prepared in the same manner as described in Example 4 except that, each of the squarium compounds (2) to (6) was used, in place of the squarium compound used in Example 4. The electric characteristics of the resulting photoreceptors were determined, and the results obtained are shown in Table 1.

Electrophotographic photoreceptors with inverted order of the charge generation layer and the charge transport layer were prepared in the same manner as those described in Examples 4 to 9 using squarium compounds (1) to (6) except that the order of the charge generation layer and the transport layer was inverted, and their electric characteristics were measured. The results obtained are shown in Table 2.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.