Electrophotographic material sensitized by 3,3'-dimethylindolenine cyanine dyes

In an electrophotographic recording material comprising an electrically conductive base material and at least one photoconductive layer containing a photoconductor, a binder and at least three sensitizing dyes of which at least two are 3,3'-dimethylindolenine compounds, such as bis-(3,3'-dimethylindolenyl)-trimethinecyanines and -pentamethinecyanines, having a sensitizing action in different wavelength regions, it is possible to sensitize various organic and inorganic photoconductors within the range from about 400 to 700 nm.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrophotographic recording material 
comprising an electrically conductive base material and at least one 
photoconductive layer containing a photoconductor and 
3,3'-dimethylindolenine sensitizing dyes. 
In electrophotographic reproduction, it is known to use photoconductors 
that are sensitive to radiation in the short-wave visible portion of the 
spectrum. It is also known that the radiation-sensitivity of such 
photoconductors in the visible portion of the spectrum can be extended by 
addition of one or more sensitizing dyes capable of transferring the 
energy of longer-wave light to the photoconductor. Various classes of dye 
compounds can be used for sensitization of photoconductors in this manner. 
It is known (see German Auslegeschrift No. 2,526,720, corresponding to U.S. 
Pat. No. 4,063,948) to use in electrophotographic reproduction an 
electrophotographic recording material that contains, in the 
photoconductive layer, a cyanine dye which has a sensitizing action in the 
blue spectrum. However, such a sensitizing action does not make it 
possible also to record, for example, radiation in the green and red parts 
of the spectrum. 
It is also known (see German Offenlegungsschrift No. 1,447,907, 
corresponding to U.S. Pat. No. 3,458,310) to sensitize photoconductor 
layers to the visible red portion of the spectrum. This is done by using, 
for example, mixtures of acridine yellow, acridine orange, rhodamine dye 
and brilliant green which are added in one layer or separately, in 
different layers (see German Offenlegungsschrift No. 2,353,639, 
corresponding to U.S. Pat. No. 3,992,205), when the respective 
sensitization actions of individual dyes are added together or, 
alternatively, the resultant actions are different (see German 
Offenlegungsschrift No. 2,817,428, corresponding to U.S. Pat. No. 
4,252,880). 
Such panchromatic sensitizations provide advantages to the extent that 
high-red light sources used in reproduction technology are better 
utilized. In practice this means shorter exposure times and, hence, time 
and energy savings. Due to improved sensitivity, it is also possible to 
reduce the photoconductor content in the photoconductive layer. 
Since one sensitizer alone generally does not cover the entire visible 
spectrum, it is necessary to mix more than one sensitizer. But it is very 
difficult thereby to obtain sensitizations that meet the varied 
requirements of the reproduction industry. Different sensitizers, with 
differing chemical as well as absorptive properties, must nevertheless be 
soluble in the solvent used and in the binding agent of the 
photoconductive layer. Different sensitizers used in a mixture may also 
influence each other in such a way as to affect adversely the sensitizing 
properties of the mix. 
SUMMARY OF THE INVENTION 
With the present invention, however, electrophotographic recording material 
comprising organic or inorganic photoconductor can be sensitized for the 
spectral region covering the area of self-sensitivity of the 
photoconductor up to above approximately 700 nm, ideally by overlapping of 
individual sensitization spectra of the sensitizing dyes. The resulting 
sensitization conforms to the emission of a light source, such as a 
mercury-gallium lamp, that emits in the entire visible region, but the 
sensitization also has sensitization maxima which are within the region of 
the emission of customary lasers, such as argon ion lasers and krypton 
lasers, and LED diodes. 
Thus, it is an object of the present invention to provide 
electrophotographic recording material comprising a mixture of dyes that 
sensitizes the recording material to virtually the entire visible spectrum 
without preventing the recording material from meeting the above-discussed 
requirements of contemporary reproduction. 
It is also an object of the present invention to provide a sensitization 
mixture, for use in electrophotographic recording material, that is 
comprised of dyes which have different absorption ranges but which are 
derived from the same heterocycle of cyanine dye systems and, hence, are 
fully compatible. 
In accomplishing the foregoing objects, there has been provided, in 
accordance with one aspect of the present invention, an 
electrophotographic recording material comprising an electrically 
conductive base material and at least one photoconductive layer containing 
a photoconductor, a binder and at least three sensitizing dyes, at least 
two of the dyes being 3,3'-dimethylindolenines with a sensitizing action 
in differing wavelength regions, respectively. In preferred embodiments of 
the present invention, the photoconductive layer comprises a 
bis-(3,3'-dimethylindolenyl)-trimethinecyanine sensitizing dye and a 
bis-(3,3'-dimethylindolenyl)-pentamethinecyanine sensitizing dye. 
In accordance with another aspect of the present invention, there has been 
provided a composition for sensitizing electrophotographic recording 
material, comprising an admixture of at least three sensitizing dyes, 
wherein at least two of the sensitizing dyes comprise 
3,3'-dimethylindolenine compounds having a sensitizing action in differing 
wavelength regions, respectively.

The following examples below are provided to illustrate the present 
invention in more detail: 
EXAMPLE 1 
The sensitizing dyes represented, respectively, by formulas (1), (2) and 
(3) were each added, in 0.04 g amounts, to a solution of 8 g of 
2-vinyl-4-(2'-chlorophenyl-5-(4'-diethylaminophenyl)-oxazole and 18 g of a 
copolymer of styrene and maleic anhydride in a mixture of 90 g of 
methylglycol, 140 g of tetrahydrofuran and 40 g of 85 percent strength 
butyl acetate. The resulting solution was applied to an aluminum foil 
which had been electrochemically roughened, surface-anodized, and then 
pretreated with polyvinylphosphonic acid as described in German 
offenlegungsschrift No. 1,621,478 (corresponding to U.S. Pat. No. 
4,153,461). Evaporation of the solvent left a layer which was 
light-sensitive within the range from about 420 to 730 nm. The 
sensitization spectrogram is depicted in FIG. 1. 
The recording material produced in this fashion was used to prepare a 
printing form for offset printing in the following manner. The 
photoconductive layer was charged in the dark to -430 V by means of a 
corona, and was then exposed for eight seconds to a mercury-gallium lamp 
(5000 W M023-Sylvania) in a repro camera set to aperture 14. The resulting 
latent charge image was developed with a commercially available dry toner 
by means of a magnetic roll, and the toner image was fixed by heat. 
Removal of the photoconductive layer in the areas not covered with toner, 
using a solution obtained by dissolving 50 g of Na.sub.2 SiO.sub.3.9 
H.sub.2 O in 250 g of glycerol (86% strength) and diluting with 390 g of 
ethylene glycol and 310 g of methanol, left a planorgraphic printing form 
with which a high edition could be printed. 
EXAMPLE 2 
To an aluminized, 100 .mu.m-thick polyester film was applied a solution 
comprised of 10 g of 
2-phenyl-4-(2'-chlorophenyl)-5-(4'-diethylaminophenyl)-oxazole, 15 g of a 
copolymer of styrene and maleic anhydride having a softening point of 
210.degree. C., 116 g of tetrahydrofuran, 33 g of butyl acetate, 76 g of 
methylglycol, and 1.5 g of the sensitizing dyes represented by formulas 
(1), (2) and (3) in a ratio of 1:1:0.5. Evaporation of the solvent left a 
photoconductive layer, approximately 5 .mu.m in thickness, that had a 
spectral sensitivity in the range from about 400 nm to about 730 nm. 
After exposure to the light of metal halide lamps, development using an 
electrophotographic developer, and removal of the photoconductor layer in 
the non-image areas via the method described in German Patent No. 
2,322,047 (corresponding to U.S. Pat. No. 4,066,453), the bared aluminum 
vapor-deposition layer was removed by treatment with 2N sodium hydroxide 
solution. A printed circuit was obtained in this way. 
Similar results were obtained when the specified oxazole was replaced by 
2,5-bis-(4'-diethylaminophenyl)-1,3,4-oxiazole as photoconductor. 
EXAMPLE 3 
A 100 .mu.m-thick aluminum foil was provided with a photoconductive layer. 
The photoconductive layer was prepared as follows: 100 parts by weight of 
photoconductive zinc oxide were mixed together with 40 parts by weight of 
a 50 percent strength solution of a modified multipolymer of vinyl acetate 
in toluene. The mixture was ball-milled for about 3 hours and was then 
applied by means of a doctor blade to a paper base material in a dry layer 
weight of about 30 g/m.sup.2. The sensitizing dyes represented by formulas 
(1), (2) and (4) had each been added beforehand to the solution, in a 
ratio of 1:1:1, to provide a 1.5 percent total concentration, based on the 
weight of the photoconductor. 
The layer was charged by means of a corona (voltage 5 kV negative, distance 
25 mm) and was imaged, using an argon ion laser within the power range 0.2 
to 0.5 mW (nominal power of 50 mW, output power 15 mW) at a forward speed 
of 400 lines/cm, in a device typically used for producing printing forms. 
The layer was processed thereafter in a manner customary with ZnO printing 
plates.