Organic photoconductive compositions for use in electrophotography

Disclosed are organic photoconductive compositions for use in electrophotography, comprising a polymer containing as main units a naphthalene derivative and/or a naphthacene derivative; and an electron acceptor and/or a sensitizer.

BACKGROUND OF THE INVENTION 
The present invention relates to organic photoconductive compositions for 
electrophotography that comprise mainly a novel polymer. 
A recording method in which a photoconductive substance is used as a 
photosensitive material is known for the electrophotography. In summary, 
the method is an image-forming technique that combines electrostatic and 
photoconductive phenomena of substance. Namely, a highly insulated layer 
surface is electrostatically charged in the dark, then the charge is 
dissipated by raising electric conductivity of the exposed part by way of 
image-wise exposure. This results in the formation of an electrostatic 
latent image at the non-exposed part, which is then developed, for 
instance, by contacting it with a colored powder or toner to give a 
visible image. 
Prior art photoconductive layers containing a photosensitive material 
assembled in electrophotographic copying machines comprise chiefly an 
organic or inorganic photoconductive material. 
Inorganic photoconductive materials are exemplified, for instance, by 
selenium, zinc oxide, titanium oxide and cadmium sulfide. Although they 
have a superior photosensitive property, they are inferior having regard 
to transparency, flexibility, light weight, film-forming property, smooth 
surface and price. Selenium, in particular, has a disadvantage regarding 
the toxicity. 
On the other hand, organic photoconductive materials are exemplified by 
condensed polycyclic aromatic compounds such as anthracene, pyrene and 
perylene, heterocyclic compounds such as pyrazoline derivatives, imidazole 
derivatives and oxazole derivatives, and polymeric compounds such as 
poly-N-vinylcarbazole and polyvinylanthracene. They are superior to the 
above-mentioned inorganic photoconductive materials having regard to 
transparency, flexibility, light weight, film-forming property, smooth 
surface and price, hence have extensively been studied for the recent 
years. 
Photosensitive materials containing an organic photoconductive compound, 
however, have mostly quite a poor photosensitivity as compared with those 
containing an inorganic photoconductive material such as selenium and zinc 
oxide, hence only a few of them are practically employed. One of them 
comprises poly-N-vinylcarbazole (PVK), chemically sensitized with 
2,4,7-trinitro-9-fluorenone (TNF). Although it is a photoconductive 
material having superior properties such as photosensitivity, darkdecay 
rate, stability and reproducibility, it has a disadvantage owing to the 
toxicity of TNF. 
Further attempts have been made to improve photosensitivity by the 
combination of an organic photoconductive compound and a dye sensitizer. 
However, products thus obtained involve practical problems to be overcome 
such as poor darkdecay rate and reproducibility over a long period of time 
due to the instability. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide photoconductive materials having 
also a superior photosensitivity, aiming at advantages such as superior 
transparency, flexibility, light weight, film-forming property, smooth 
surface and low price that organic photoconductive materials have. In 
other words, the object of the invention is to provide organic 
photoconductive materials having a high sensitivity, low darkdecay rate, 
well-reproducible photosensitivity during and after repeated use over a 
long period of time, and good safety or low toxicity. 
According to the invention, there is provided organic photoconductive 
compositions for electrophotography comprising the following components 
(A) and (B): 
(A) A polymer having molecular weight of from 10.sup.3 to 10.sup.6 
containing as main units a naphthalene derivative represented by the 
formula (I) and/or a naphthacene derivative represented by the formula 
(II): 
##STR1## 
(wherein, R represents a hydrogen atom or a methyl group; n is an integer 
of from 0 to 3; and X represents a sulfur or selenium atom); and 
(B) an electron acceptor and/or a sensitizer. 
DETAILED DESCRIPTION OF THE INVENTION 
Compounds of formula (I) may be prepared by an esterification of an acrylic 
or methacrylic acid chloride represented by the formula (III) with a 
2-hydroxy- or 2-hydroxyalkyl-substituted naphthalene or naphthacene 
represented by the formula (IV) or (IV'). Then, the resulting acrylic or 
methacrylic ester represented by the formula (V) or (V') is subjected to 
block polymerization or suspension polymerization, solely or in 
combination with other copolymerizable monomer, in the presence of 
azobisisobutyronitrile as the catalyst to give a polymer. Finally, the 
thus obtained polymer is reacted with sulfur or selenium in 
trichlorobenzene, affording the desired polymer. 
In the final step, about 90% of the naphthacene ring may be converted to 
tetrathionaphthacene or tetraselenonaphthacene ring according to the 
present invention. Similar results may be obtained also in the case of 
naphthalene ring. 
##STR2## 
As mentioned above, the polymer (A) may be either a homopolymer or a 
copolymer. 
Naphthalene derivatives to be employed in the invention include 
tetrathionaphthalene acrylate, tetrathionaphthalene methacrylate, 
tetrathionaphthomethyl acrylate, tetrathionaphthomethyl methacrylate, 
2-tetrathionaphthoethyl acrylate, 2-tetrathionaphthoethyl methacrylate, 
3-tetrathionaphthopropyl acrylate and 3-tetrathionaphthopropyl 
methacrylate. 
Naphthacene derivatives to be employed in the invention include 
tetrathionaphthacene acrylate, tetrathionaphthacene methacrylate, 
tetrathionaphthacenomethyl acrylate, tetrathionaphthacenomethyl 
methacrylate, 2-tetrathionaphthacenoethyl acrylate, 
2-tetrathionaphthacenoethyl methacrylate, 3-tetrathionaphthacenopropyl 
acrylate and 3-tetrathionaphthacenopropyl methacrylate. 
Copolymerizable monomers are unsaturated ethylenic compounds and are 
exemplified by alkyl acrylates, alkyl methacrylates, styrene, vinyl 
chloride, vinyl acetate and acrylonitrile. The amount of such 
copolymerizable monomers in copolymers should preferably be kept up to 20 
molar % in view of photoconductivity. 
Electron acceptors to be employed in the present invention include, for 
example, inorganic compounds such as iodine, bromine, antimony 
pentachloride, zinc chloride, iron chloride, aluminium chloride, boron 
trifluoride and indium chloride; quinones such as p-benzoquinone, 
o-chloranil, p-chloranil, o-bromanil, p-bromanil, 
2,3-dichloro-5,6-dicyano-p-benzoquinone, 2,6-dinitro-p-benzoquinone, 
tetracyano-p-benzoquinone, 2,3-dicyano-p-benzoquinon, 
trichloro-p-benzoquinone, 2,6-dichloro-p-benzoquinone, 
2,5-dichloro-p-benzoquinone, 2,3-dichloro-p-benzoquinone, 
monochloro-p-benzoquinone, 2,5-dimethyl-p-benzoquinone, 
methyl-p-benzoquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, 
9,10-anthraquinone and 9,10-phenanthrenequinone; nitro compounds such as 
1,3-dinitrobenzene, 1,3,5-trinitrobenzene and tetranitromethane; and 
tetracyanoethylene, tetracyanoquinodimethane and 
2,4,7-trinitro-9-fluorenone. 
Sensitizers to be employed in the present invention may be those commonly 
used in electrophotography. They include photosensitizers or dyes 
represented by methylene blue, crystal violet, rhodamines such as 
Rhodamine G and Rhodamine 6G, and Victoria blue; and chemical sensitizers 
such as maleic acid, phthalic acid, itaconic acid, benzoic acid and acid 
anhydrides of these acids, p-nitrophenol, o-nitrophenol, 
4-chloro-2-nitrophenol and tetrachlorobisphenol A. 
Preferred mixing rate of the compositions according to the present 
invention will be described hereunder. 
Where an electron acceptor is employed, 1 to 10 moles of the polymer (A) 
(calculated in terms of the monomer I or II) are blended with 1 mole of 
the electron acceptor. Presence of 1 mole or less of the polymer (A) will 
result in a poor film-forming property, whereas presence of 10 moles or 
more thereof will result in a poor photoconductive property due to an 
insufficient formation of the charge-transfer complex. 
Where a sensitizer is employed, 1 to 10 ml of a 1% by weight solution of a 
photosensitizer or 1 to 10 g of a chemical sensitizer are blended with 10 
g of the polymer (A). Presence of a photosensitizer or a chemical 
sensitizer beyond the concentration will result in a poor film-forming 
property, whereas presence thereof below the concentration will result in 
an insufficient sensitization. 
The compositions according to the invention may contain a binder resin, if 
necessary. This is particularly preferable if the film-forming property of 
the composition consisting only of the polymer (A) and the electron 
acceptor is poor. 
Such binder resins include, for instance, poly(styrene), poly(vinyl 
chloride), vinyl chloride/vinyl acetate copolymer, poly(vinyl acetate), 
poly(vinyl acetal), phenolic resins, epoxy resins and alkyd resins. 
Incorporation into the composition of such a binder resin should be kept 
at 10% or less in order to attain a good photosensitivity. 
In order to prepare the photoconductive compositions for electrophotography 
according to the present invention, a solution of a polymer (A), an 
electron acceptor and/or a sensitizer, and a binder resin if required, at 
a suitable mixing rate is coated on a conductive support such as aluminium 
plate, then dried. 
The coating solution may be prepared by various ways. For example, a 
polymer (A) and an electron acceptor are dissolved in a suitable solvent, 
while a binder resin is dissolved in the same or different kinds of 
solvent, and finally the both solutions are blended. Such solvents 
include, for instance, benzene, trichlorobenzene, nitrobenzene, acetone, 
methanol, methylene chloride, trichloroethylene, carbon tetrachloride, 
methyl cellosolve, tetrahydrofuran, dioxane, dimethylformamide, 
dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone. 
The amount of the organic photoconductive composition of the present 
invention to be coated on a support is not critical. Usually, it is coated 
on a support so that a dried film having a 1 to 30 .mu.m thickness may be 
obtained. 
In order for the thus prepared photosensitive material to be applied for 
electrophotography, the photoconductive layer comprising the 
photosensitive material may be processed according to the conventional 
electrophotographic processes such as electrification, imagewise exposure 
and development transfer. 
The organic photoconductive compositions for electrophotography according 
to the invention described above have a superior photosensitivity, at the 
visible range, which is comparable to that of the known 
poly-N-vinylcarbazole chemically sensitized with 
2,4,7-trinitro-9-fluorenone. 
The compositions have a low darkdecay rate and a good stability that allows 
to produce images having a superior reproducibility even after the 
repeated use over a long period of time. Further, materials to be employed 
have no toxicity, hence may be used safely.

The present invention will be explained by the following working examples 
which by no means limit the scope of the invention. 
EXAMPLE 1 
10.0 g of 2-hydroxynaphthacene and 4.2 g of pyridine were added to 150 ml 
of toluene, then 5.5 g of methacryloyl chloride were added dropwise at 
10.degree. C. over 1 hour. Upon completion of addition, the mixture was 
stirred at room temperature for 5 hours. The solution was washed, in turn, 
with water and an aqueous alkaline solution, then dried over sodium 
sulfate. By removing the solvent by distillation, there was obtained 
methacrylic acid ester of 2-hydroxytetracene at 92% yield. 
A mixture of 5.0 g of the methacrylic acid ester, 0.5 g of 
azobisisobutyronitrile, 2.0 g of polyvinyl alcohol and 100 ml of water was 
refluxed, with stirring, at 90.degree.-95.degree. C. for 2 hours under 
nitrogen stream. After allowing to cool to room temperature, precipitates 
were collected by filtration and the solid product was washed with a warm 
water and dried. The product was purified by reprecipitation with 
tetrahydrofuran and methanol. 
Yield, 4.3 g (86%); 
Number average molecular weight, 190,000 (gel permeation chromatography 
with a standard poly(styrene)). 
2.9 g of the polymer and 4 g of sulfur were added to 150 ml of 
trichlorobenzene and the mixture was refluxed for 24 hours to complete the 
reaction. Precipitates were collected by filtration and the resulting 
solid was washed and dried. It was confirmed by NMR spectroscopy and 
others that 88% of the naphthacene ring were converted to 
tetrathiotetracene ring. 
Number average molecular weight, 280,000 (gel permeation chromatography 
with a standard poly(styrene)); 
Glass transition point, 121.0.degree. C.; 
IR, 800 cm.sup.-1 (C-S), 490 cm.sup.-1 (S-S). 
1 g of the polymer thus prepared and 0.3 g of tetracyanoethylene were 
dissolved in a 1:1 mixture of toluene and trichlorobenzene to give a 
uniform solution, which was coated on Mylar (Trademark for a polyester 
film) on which aluminium had been vacuum evaporated to give a thickness of 
2-8 .mu.m and dried. 
Electrification and decay of the film thus obtained were determined with an 
electrostatic paper analyzer. The result was 7.5 lux sec. when the 
sensitivity was represented in terms of exposure amount (unit: lux sec.) 
necessary for the initial surface electric charge to be decayed by 1/2. 
The photosensitive films were prepared subjected to repeated 
electrification and exposure 5,000 times in the same conditions as above, 
after which no abnormality was observed, thus suggesting the superior 
fatigue resistance. 
EXAMPLE 2 
10 g of the polymer employed in Example 1 and 1 g of 
bis(5-chloro-2-hydroxyphenyl)methane as the chemical sensitizer were added 
to trichlorobenzene, then 2 ml of 1% solution of Crystal Violet in 
dimethylformamide were added. The solution was coated on Mylar on which 
aluminium had been vacuum evaporated, and dried. Measurement of the 
sensitivity of the film thus obtained performed in the similar manner as 
in the above-mentioned Example showed the value 7.1 lux sec. The films 
prepared were then subjected to repeated electrification and exposure 
5,000 times in the similar manner as in the above-mentioned Example, after 
which no fatigue that would affect practical use was observed. 
As specifically described above, the novel organic photoconductive 
compositions according to the invention possess superior electrification 
properties such as low darkdecay rate, as well as superior photosensitive 
properties such as low half-life exposure amount. Such superior properties 
may be maintained during their use over a long period of time. 
COMATIVE EXAMPLE 1 
In comparison with the photosensitive film of the invention, 0.8 g of PVK 
and 1 g of TNF were dissolved in 18 ml of o-dichlorobenzene to give a 
uniform solution, which was coated on Mylar on which aluminium had been 
vacuum evaporated to give a thickness of 3-10 .mu.m, and dried. 
In the same manner as in Example 1, the sensitivity measurement of the 
PVK-TNF film was performed. The result was 7.0 lux sec. This film was 
subjected to repeated electrification and exposure in the same conditions 
as above, but, after 1,000 time repetitions, abnormality was observed. 
As seen from the foregoing, the photosensitive film according to the 
invention, having no toxicity as in the case of the PVK-TNF film, has a 
good photoconductivity comparable to that of the PVK-TNF film, and has far 
better reproducibility over a long period of time than the PVK-TNF film.