Photoconductive compositions sensitive to both laser light and tungsten halide light

A photoconductive composition comprises a photoconductor and a sensitizer. The sensitizer has the formula ##STR1## wherein R represents alkyl; R1 represents H, alkyl or aryl; R2 represents the atoms required to complete a hetero or carboxyclic ring; A.sup.- represents an anion; and n represents the valency of the anion. The composition is sensitive to both laser light sources and tungsten light sources and is useful in electrophotographic reproduction.

This invention relates to a photoconductive composition for use in 
electrophotographic reproduction. 
The use of photoconductive compositions comprising organic photoconductors 
having a spectral sensitivity in the range 350 to 450 nm for 
electrophotographic reproduction is proposed in GB Patent Specification 
No. 851218. This range may be extended to longer wavelengths by the 
incorporation of various sensitising dyestuffs to enable the 
photoconductive composition to be exposed to the tungsten halide lamps 
used in a graphic arts reproduction camera. Rhodamine type dyes are 
commonly used for this purpose. 
More recently, the use of lasers, particularly argon-ion lasers, as 
exposure sources has become increasingly important. Such lasers emit light 
in the blue/green region of the spectrum and have strong lines at 488 and 
514.5 nm and it has been suggested that photoconductive compositions can 
be sensitised to light of this wavelength by incorporating certain 
polymethine dyestuffs having the structure 
##STR2## 
where R1 is methyl or phenyl, R2 is hydrogen or methyl and X is a halide. 
A particular example of such a dye is CI Basic Orange 22. 
Whilst such dyes are satisfactory as regards laser exposure, they do not 
provide sufficient sensitivity to tungsten halide light sources to enable 
the compositions to be used efficiently with either source. 
It is an object of this invention to provide a photoconductive composition 
that is suitable for exposure by either argon-ion laser or tungsten halide 
light. 
According to the invention, there is provided a photoconductive composition 
comprising at least one photoconductor and, as sensitiser, a compound 
having the formula: 
##STR3## 
wherein 
R represents alkyl; 
R1 represents alkyl or aryl; 
R2 represents the atoms required to complete a hetero or carbocyclic ring; 
and 
A.sup.n- represents an anion. 
The benzene rings A and B, and the ring formed by R2 may be substituted. 
Preferred sensitisers are: 
1. 2-[2-(9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-indolium 
tetrafluoroborate. 
2. 2-[2-(9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-5-nitro-3H-indolium 
tetrafluoroborate. 
3. 2-[2-(9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-5-chloro-3H-indolium 
chloride. 
4. 2-[2-(6-bromo-9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-indolium 
hexafluorophosphate. 
5. 2-[2-(2-ethoxy-9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-indolium 
tetrafluoroborate. 
6. 
2-[2-(5,6,7,8-tetrahydro-9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-i 
ndolium tetrafluoroborate. 
7. 
2-[2-(6-diethylamino-9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-indol 
ium trifluoromethane sulphonate. 
8. 2-[2-(9-phenyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-indolium 
p-toluene sulphonate. 
9. 
2-[2-(9-propyl-6-pyrido[2,3-b]indolyl)-vinyl]-1,3,3-trimethyl-3H-indolium 
tetrafluoroborate. 
10. 
2-[2-(4-ethyl-7-thieno[3,2-b]indolyl)-vinyl]-1,3,3-trimethyl-3H-indolium c 
hloride. 
The composition preferably contains from 0.01 to 10% by weight, preferably 
from 2 to 5% by weight, of sensitiser, based on the photoconductor. 
The composition of the invention is especially intended for use in the 
electrophotographic production of printing formes and printed circuits, 
and may be applied to a support which may comprise any material suitable 
for this purpose, for example, aluminium, zinc, magnesium or copper plates 
or multi-metal plates, wherein a multi-metal plate is one which comprises 
a combination of 2 or more metals, and also cellulose products, for 
example, special papers, cellulose hydrate, cellulose acetate or cellulose 
butyrate films, especially partially saponified cellulose acetate or 
butyrate films. Some plastics material, for example, polyamides in film 
form or metal-vaporised films, are suitable for use as support. Grained 
and anodised aluminium is particularly suitable for the support. 
Preferred photoconductors for use in the composition are those 
amino-phenyl-substituted oxazoles mentioned in British Patent 
Specification No. 874,634, for example 
2-phenyl-4-(2'chlorophenyl-5-(4"diethylaminophenyl)-oxazole. Other 
suitable photoconductors include, for example triphenylamine derivatives, 
higher condensed aromatic compounds, such as anthracene, benzo-condensed 
heterocyclic compounds, and pyrazoline or imidazole derivatives. Also 
suitable are triazole and oxadiazole derivatives, as disclosed in British 
Patent Specification Nos. 836,148 and 851,218; 
2,5-bis-(4'-diethylaminophenyl)-1,3,4-oxadiazole is particularly suitable 
here. In addition, vinyl-aromatic polymers such as polyvinyl anthracene, 
polyacenaphthylene, poly-N-vinylcarbazole and copolymers of these 
compounds are suitable, particularly those that contain hydrophilic 
groups. Also suitable are polycondensates of aromatic amines and 
aldehydes, as described in British Patent Specification No. 977,399 and 
resins as described in British Patent Specification No. 1404829. 
The composition preferably also contains one or more natural or synthetic 
resin binders. In addition to having film-forming and electrical 
properties, and also adhesion to the support, the resins should also have 
good solubility properties. For practical purposes, the resin binders 
which are particularly suitable are those which are soluble in essentially 
aqueous or solvent systems. Aromatic or aliphatic, easily combustible 
solvents are excluded for physiological and safety reasons. The most 
suitable resin binding agents are high-molecular substances carrying 
alkali-solubilising groups. Such groups are, for example, carboxyl, 
phenol, sulphonic acid, sulphonamide sulphonimide groups and also acid 
anhydride groups. 
Partial esters of copolymers of styrene and maleic acid anhydride, for 
example, those known under the name Scripset (Registered Trade Mark), 
Monsanto Co., United States, are especially suitable; also phenol resins. 
for example those known under the name Alnovol (Registered Trade Mark), 
Hoechst AG., Germany, have proved very satisfactory. 
Additional sensitising dyes for example, triarylmethane dyes, xanthene 
dyes, polymethine dyes, phthalein dyes, pyrylium and thiopyrylium dyes, 
quinoline dyes, thiazine dyes, acridine dyes, and quinone dyes may be 
included in the composition to extend the spectral response. 
The anion may be halide, e.g. chloride or bromide, tetrafluoroborate, 
toluene sulphonate, hexafluorophosphate, trifluoromethane sulphonate, 
lauryl sulphate, methyl sulphate and methyl sulphonate. 
The sensitisers may be prepared by reacting a Fischer's base with a 
suitable aldehyde in glacial acetic acid and heating under reflux. After 
cooling, the product may be precipitated by the addition of an aqueous 
solution containing a suitable anion. 
Thus, for example, 
2-[2-(9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-3H-indolium 
tetrafluoroborate was prepared by mixing 1,3,3-trimethyl-2-methylene 
indoline (0.01 mole) and 9-ethyl-3-carbazolecarboxaldehyde (0.01 mole) in 
glacial acetic acid (30 ml) and heating under reflux for 2 hours. The 
solution was allowed to cool to room temperature and then poured into 
water (600 ml). The product was precipitated by the addition of a solution 
of sodium tetrafluoroborate (12 g) in water (40 ml) and then filtered, 
washed with water and dried at 60 degC. The product had a melting point of 
216-218 degC. Similarly 
2-[2-(9-ethyl-3-carbazolyl)-vinyl]-1,3,3-trimethyl-5-chloro-3H-indolium 
chloride (melting point 226-228 degC.) may be prepared using the 5-chloro 
derivative of the above Fischer's base and sodium chloride solution as the 
precipitating agent.

The following examples illustrate the invention: 
EXAMPLE 1 
A coating solution was prepared by dissolving 
2,5-bis-(4'-diethylaminophenyl)-1,3,4-oxadiazole (40 g), Scripset 540 (60 
g) and sensitiser 1 (1.5 g) in methyl ethyl ketone (850 ml). This was 
applied to a 0.30 mm thick aluminium substrate which had been 
electrochemically grained and anodised. 
After evaporation of the solvent the coated substrate was baked at 120 
degC. for 5 minutes. This produced a photoconductive layer with a coating 
weight of 5-6 g/m2. The region of electrophotographic sensitivity was 
440-620 nm having a broad peak centred at 508 nm. The layer was charged, 
using a corona wire, to a surface potential of -550 V. The device was 
imagewise exposed to 25 microjoules/cm2 of light energyat 488 nm from an 
argon-ion laser. The resulting latent electrostatic image was developed 
using a conventional magnetic brush toner. Radiant heat was used to fuse 
the toner powder in the image areas and an aqueous alkali wash removed the 
background layer. The resulting lithographic plate was washed with water, 
gummed, dried and used to produce several thousand prints on an offset 
printing press. 
The electrophotographic layer was also exposed in a repro camera, to a 
positive original, after charging with a corona wire to a surface 
potential of -550 V. An exposure time of 21s was required when using 
4.times.1000 watt tungsten halide lamps. The layer was then processed in 
the manner previously mentioned. 
EXAMPLE 2 
A coating solution was prepared by dissolving 
2,5-bis-(4'-diethylaminophenyl)-1,3,4 oxadiazole (50 g) Scripset 540 (50 
g), sensitiser 1 (1 g) and CI Basic Violet 16 (1 g) in methyl ethyl ketone 
(850 cm). This was applied to a 0.03 mm aluminium substrate which had been 
electrochemically grained and anodised. After evaporation of the solvent 
the coated substrate was baked at 120 degC. for 5 minutes. This produced a 
photoconductive layer with a coating weight of 5-6 g/m. The region of 
sensitivity was 440-650 nm. After charging the layer, with a corona wire, 
and exposure by Argon Ion laser or repro camera a lithographic printing 
plate was prepared by the method described for example 1. 
EXAMPLE 3 
Example 1 was repeated except that sensitiser 2 was used and the 
photoconductor was 2-phenyl-4-(2'chlorophenyl)-5-(4" diethyl amino 
phenyl)-oxazole. 
Results similar to those of Example 1 were obtained. 
EXAMPLE 4 
A series of solutions was prepared consisting of 
2-phenyl-4-82'-chlorophenyl)-5-(4"-diethyl aminophenyl)-oxazole (4 g), 
Scripset 540 (6 g), a sensitiser (0.1 g) and 85 ml ethyl methyl ketone. 
The solutions were coated on electrochemically grained and anodised 
aluminium substrates as described in Example 1 and the various 
characteristics of the plates as indicated in Table I were investigated as 
follows; 
the wavelength of maximum absorbance (.lambda.max) and the absorption 
spectral range (.lambda.range) were measured by reflectance on a Perkin 
Elmer spectrophotometer; 
the light energy in microjoules/cm2 (E1/2) required to discharge the 
surface potential to one half its initial value was measured on a 
Princeton Electrodynamics Inc Static Analyser. The samples were charged in 
the dark to a surface voltage of -550 volts and then exposed to an 
unfiltered tungsten lamp of colour temperature 2810 deg K. at an 
illumination of (269 lux.). 
The plates contained sensitiser as follows: 
______________________________________ 
Plate 1 Sensitiser 1 
Plate 2 Sensitiser 2 
Plate 3 Sensitiser 3 
Plate 4 Sensitiser 4 
Plate 5 Sensitiser 5 
Plate 6 Sensitiser 6 
Plate 7 CI Basic Orange 22 
Plate 8 Solvent Red 49 (Rhodamine Base FB) 
Plate 9 No sensitiser 
______________________________________ 
TABLE I 
______________________________________ 
.lambda. max .lambda. range 
E 1/2 
Plate (nanometers) (nanometers) 
(micro J/cm2) 
______________________________________ 
1 508 440-620 23 
2 550 455-660 29 
3 524 450-625 23 
4 490 440-590 29 
5 527 440-615 26 
6 518 440-610 28 
7 495 440-550 34 
8 564 470-620 23 
9 -- -- 400 
______________________________________ 
Two further samples of each of plates 1, 3, 7 and 8 were charged as above. 
One sample of each plate was exposed in a reproduction camera to 
4.times.1000 watt tungsten halide lamps and the other sample of each plate 
was exposed to an argon-ion laser. The camera exposure time and the laser 
energy required for the exposed plates to reach a voltage at which they 
did not accept toner when processed as described in Example I were 
measured and the results are shown in Table II. 
TABLE II 
______________________________________ 
Camera Exposure Time 
Laser energy 
Plate (seconds) (micro J/cm2) 
______________________________________ 
1 21 25 
3 22 25 
7 37 25 
8 25 67 
______________________________________ 
These results clearly show that the sensitisers of the invention (1) are 
comparable in terms of argon-ion laser exposure to the polymethine dyes, 
(2) are comparable in terms of camera exposure to the rhodamin dyes 
usually used to sensitise photoconductive compositions to tungsten halide 
light, (3) are superior to the polymethine dyes for camera exposure and 
(4) are superior to the rhodamins for laser exposure. 
The sensitisers used in this Example have the following structures: 
##STR4##