05/354183

06/10/1975

04/25/1973

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Hoechst Aktiengesellschaft

430/58.500

430/76, 430/64, 430/69, 430/58.550, 430/65

G03G5/047; G03G5/06; G03G5/14; G03G5/043; G03G5/02

96/1.6,1.5,1.8

Abstract, Belgium, 763540, A89E14G8 ASE A12-L5..

Torchin, Norman G.

Goodrow, John L.

Bryan, James E.

What is claimed is

1. Electrophotographic recording material comprising an electroconductive support and a photoconductive double layer of organic materials disposed thereon, said double layer being composed of a homogeneous, opaque, charge carrier producing dyestuff layer of a compound corresponding to one of the general formulae ##SPC4##

2. Electrophotographic material according to claim 1 in which the heterocyclic compound is selected from the group consisting of oxazoles, oxadiazoles, triazoles, imidazoles and pyrazoles.

3. Electrophotographic material according to claim 1, in which the heterocyclic compound is an oxadiazole.

4. Electrophotographic material according to claim 1, in which the heterocyclic compound is 2,5-bis(4-diethylaminophenyl)-oxadiazole-1,3,4.

5. Electrophotographic material according to claim 1, in which the heterocyclic compound is an oxazole.

6. Electrophotographic material according to claim 1, in which the heterocyclic compound is 2-phenyl-4-(2-chlorophenyl)-5-(4-diethylaminophenyl)-oxazole.

7. Electrophotographic material according to claim 1, in which the dyestuff layer has a thickness of approximately 0.005 to 2 μm and the transparent top layer has a thickness of approx. 5 to approx. 20 μm.

8. Electrophotographic material according to claim 1, in which the transparent top layer consists of an about 1 : 1 mixture (by weight) of the charge transporting compound and the binder.

9. Electrophotographic material according to claim 1, in which the binder is selected from the group consisting of polyesters, copolyesters, silicone resins, reactive resins of polyesters or polyethers and polyfunctional isocyanates, styrene/maleic anhydride copolymers, and polycarbonate resins.

10. Electrophotographic material according to claim 1, in which the binder is a copolymer of isophthalic acid and terephthalic acid with glycol.

11. Electrophotographic material according to claim 1, in which the binder is a copolymer of styrene and maleic acid anhydride.

12. Electrophotographic material according to claim 1, in which the binder is a reactive resin of polyesters or polyethers and polyfunctional isocyanates.

13. Electrophotographic material according to claim 1, in which the transparent top layer additionally contains sensitizers and/or compounds forming charge-transfer complexes.

14. Electrophotographic material according to claim 1, in which between the photoconductive double layer and the support there is an insulating intermediate layer.

This invention relates to an electrophotographic recording material consisting of an electroconductive support material and a photoconductive double layer of organic materials which consists of a homogeneous, opaque, charge carrier producing dyestuff layer of a compound corresponding to one of the general formulae ##SPC2##

Wherein

R and R ₁ may be the same or different and stand for hydrogen, alkyl or alkoxyl with 1 to 4 carbon atoms, halogen, a nitro group, a hydroxyl group, or together form a fused benzene or naphthalene group,

And of a transparent top layer of insulating materials containing at least one charge transporting compound.

It is known from German Offenlegungsschriften Nos. 1,597,877 and 1,797,342 for electrophotographic recording material to extend the spectral sensitivity of selenium layers to the red spectral range by a double layer arrangement, e.g., with phthalocyanine dispersion layers. Disadvantageous are the vacuum vapor depositions of selenium requiring high technical expenditure, the brittleness of comparatively thick selenium layers, the poor adhesion of adjacent heterogeneous constituents in these layers and the only difficulty realizable uniformly wetting coating with the corresponding dispersions. Furthermore, no optimum light-sensitivities can be achieved as a result of the absorption behavior and the different charge conducting mechanisms of selenium and phthalocyanine in the double layer arrangement.

From U.S. Pat. No. 3,573,906, for example, there are also known photoconductive double layers containing an organic, possibly photoconductive, insulating layer between the support material and the vapor-deposited selenium layer in order to impart adhesion. Such a layer construction, however, considerably hinders the necessary charge transport so that, in this case, too, no higher light-sensitivities are obtainable.

Furthermore, from German Auslegeschrift No. 1,964,817, it is known to provide vapor-deposited selenium layers with a layer of an organic, photoconductive insulating material which is substantially insensitive to light in the visible range of the spectrum. According to German Offenlegungsschrift No. 2,120,912, it has also been suggested to use those light-sensitive layer arrangements for electrophotographic recording materials which contain, as the charge carrier producing layer, an inorganic material, such as the sulfide, selenide, sulfoselenide or telluride of cadmium or zinc, and, as the charge carrier transporting layer, an organic material with at least 20 per cent by weight of 2,4,7-trinitro-9-fluorenone. A disadvantage of the production of these layers with inorganic photoconductors is the exact observation of the vapor deposition conditions of selenium or the exact adjustment of the mixtures in order to obtain a good photoconductive modification of the inorganic materials. Furthermore, the adhesion of selenium to conductive support material, such as to aluminium, is insufficient. Fatigue in repeated charge/exposure cycles does not allow the use in electrophotographic copying devices.

Japanese Pat. application No. 43-26710 already discloses photoconductive double layers of organic materials on a conductive support. According to that application, a lower, relatively thick layer of a considerably diluted homogeneous solution of a sensitizer in a binder is provided with an upper transparent light-sensitive layer. This layer construction, however, only offers a relatively low sensitivity increase only little meeting technical demands. Another known suggestion according to German Offenlegungsschrift No. 1,909,742 is to repeatedly pour a sensitizer solution over a photoconductive layer and to evaporate the solvent. A disadvantage thereof is the low mechanical resistance of the applied layer as a result of insufficient cohesion and adhesion of the applied sensitizer. Furthermore, repeated coating is cumbersome.

The construction of photoconductive double layers containing a dyestuff layer is also known, e.g., from Belgian Pat. Nos. 763,389 and 763,541, but for this layer construction, top layers are used which allow no sensitivities satisfying highest demands and, as regards adhesion between the dyestuff layer and the top layer, do not represent an optimization and are not sufficiently resistant to mechanical attack, e.g., in electrophotographic copying devices, particularly to that due to the cleaning of the photoconductive layer.

It is the object of the present invention to provide an organic photoconductor layer highly light-sensitive for the xerographic copying procedure which overcomes the described disadvantages and the adhesion of which between the various layers satisfies the highest technical demands, which exhibits no wear or fatigue and which, even after repeated use, may be used again rapidly.

The present invention provides an electrophotographic recording material consisting of an electroconductive support material with a photoconductive double layer of organic materials which consists of a homogeneous, opaque, charge carrier producing dyestuff layer of a compound corresponding to one of the general formulae ##SPC3##

wherein

R and R ₁ may be the same or different and stand for hydrogen, alkyl or alkoxyl with 1 to 4 carbon atoms, halogen, a nitro group, a hydroxyl group, or together form a fused benzene or naphthalene group,

and of a transparent top layer of insulating materials containing at least one charge transporting compound, which is characterized in that the transparent top layer consists of a mixture of a binder with a charge transporting, monomer, heterocyclic compound substituted by at least one dialkyl amino group or two alkoxy groups and having an extended π-electron system, or with a condensation product of 3-bromopyrene and formaldehyde.

By means of the invention, it is possible to obtain highly light-sensitive, photoconductive double layers for the electrophotographic recording material of the invention which have a high mechanical resistance and may be arranged on a cylindrical drum, for example, or may circulate as an endless belt without exhibiting special signs of wear and thus are very suitable for the use in electrophotographic copying devices. The high light-sensitivity particularly results from the fact that the charge transporting compound present in the transparent top layer is sensitized by the charge carrier producing dyestuff layer in that the charge carriers, i.e., electrons or defect electrons, are taken up by the top layer.

In a preferred embodiment, the organic dyestuff layer has a thickness in the range from about 0.005 to about 2 μm. High concentration of excited dyestuff molecules is achieved thereby in the dyestuff layer and at the boundary surface between the dyestuff layer and the top layer. It has been found that layers of a thickness between about 0.005 and about 1 μm may be quite satisfactory and that even layers as thin as approximately 0.001 μm may be effective. Furthermore, the adhesion between the electroconductive support material and the top layer is not impaired.

In a preferred embodiment, the transparent top layer has a thickness in the range from about 5 to about 20 μm. This assures a sufficiently high charge.

The structure of the electrophotographic recording material according to the invention is shown in the attached FIGS. 1 and 2.

FIG. 1 shows a material consisting of an electroconductive support 1, an organic dyestuff layer 2 disposed thereon, and an organic transparent top layer 3.

In FIG. 2, the support is a metallized plastic layer 1, 4, which carries, under the photoconductive double layer 2, 3, an intermediate layer 5 which prevents injection of the charge carriers.

Suitable electroconductive materials are materials which hitherto have been used for this purpose, for example aluminum foils or supports of plastic materials, which may be transparent and to which aluminum, gold, copper, zinc, cadmium, indium, antimony, tin, or nickel has been laminated or applied by vapor deposition.

An organic intermediate layer, or also a metal oxide layer produced by a thermal, anodic or chemical process, (such as, e.g., an aluminum oxide layer) may be applied to the electroconductive support, as shown in FIG. 2. This layer serves the purpose to reduce or present the injection of charge carriers from the electroconductive support into the organic dyestuff layer in the absence of light. In addition thereto, it improves the adhesion of the dyestuff layer to the support. Besides the inorganic oxide layers already mentioned, layers of organic materials may also be used; thus, conventional natural or synthetic resin binders may be employed which are not or not noticeably dissolved during the subsequent application of the organic top layer. Examples of suitable substances are polyamide resins or polyvinyl phosphonic acid.

In the case of an inorganic oxide layer, the intermediate layer may range in thickness from about 10 ² to 10 ⁴ Angstrom, and an organic intermediate layer should have a thickness of approximately 1 μm.

The organic dyestuff layer of the electrophotographic recording material of the invention substantially determines the spectral light-sensitivity of the photoconductive double layer of the invention.

Suitable dyestuffs are mainly those listed in the attached table of formulae. Unless otherwise stated, they are known organic compounds such as the ones mentioned in "Colour Index," Vol. 3 (1956), under No. 46,500, or described by S. S. Labana on pages 1 to 18 of "Chem. Reviews" (1967), under the heading "Quinacridones." The following compounds are listed in the table:

Linear trans-quinacridone, C. I. 46,500 (prepared as described in German Pat. Spec. No. 1 150 046) Formula 1

3,10-dichloro-quinacridone Formula 2

dimethyl-quinacridone, C. I. Pigment Red 122 Formula 3

4,11-dichloro-quinacridone and 4,11-dimethyl-quinacridone (prepared as described in German Pat. Spec. No. 1 261 106) Formula 4 and Formula 5

1,4,8,11-tetramethyl-quinacridone and 1,8-dichloro-4,11-dimethyl-quinacridone (prepared as described in German Patent Spec. No. 1 261 106) Formula 6 and Formula 7

2,9-dibromo-quinacridone (prepared as described in German Pat. Spec. No. 1 261 106) Formula 8

7,14-dioxo-5,7a,12,14a-tetraaza-7,14-dihydro-pentacene in which R and R ₁ are hydrogen: Formula 9

in which R is halogen and R ₁ is hydrogen: Formula 10

in which R is lower alkyl and R ₁ is hydrogen: Formula 11

and in which R and R ₁ together form a fused aromatic ring (prepared as described in Example 9): Formula 12.

Quinacridone itself and dihalogen-substituted quinacridone compounds have proved particularly suitable for use in the dyestuff layers producing the charge carriers.

When arranged in a double layer, the dyestuffs according to the invention have a very high degree of photosensitivity in the visible range of the spectrum. They can be easily prepared and cleaned. Moreover, they possess good thermal and photochemical resistance, so that they can be vapor deposited without decomposition and do not undergo any photochemical changes under xerographic conditions. The organic dyestuff layer must be extremely uniform, since only its uniformity guarantees a uniform injection of charge carriers into the top layer.

To achieve this object, the dyestuff layers are applied according to special coating methods. Such methods are the application by mechanically rubbing the most finely powdered dyestuff material into the electroconductive support material, the application by chemical deposition of a leucobase to be oxidized, for example, the application by electrolytical or electrochemical processes or the gun spray method. The application preferably is performed, however, by vapor depositing the dyestuff in the vacuum. A tightly packed homogeneous coating is achieved thereby.

The tightly packed coating makes it unnecessary to produce thick dyestuff layers for achieving a high absorption. The tightly packed dyestuff molecules and the extremely low layer thickness permit, in a particularly advantageous manner, the transport of charge carriers so that it is completely sufficient to produce the charge carriers at the boundary layer only.

The application of the dyestuff layer by vapor deposition in the vacuum requires dyestuffs with thermal resistivity in the temperature range to be applied for vapor deposition. The high extinction of the dyestuff allows high concentration of excited dyestuff molecules. Excitation (1) and charge separation (2) take place in the dyestuff layer according to the following reaction equations:

1. S + hv ➝ S ^x

2. S ^x + S ➝ S ⁺ + S ^-

with

S -- dyestuff molecule

S ^x -- excited dyestuff molecule, and

S ⁺ , s ^- -- dyestuff radical ions.

At the boundary surface between the organic dyestuff layer and the transparent top layer, reactions of the excited dyestuff molecules or the resulting charge carriers in the form of the dyestuff radical ions with the molecules of the charge transport effecting compound in the top layer are possible according to the following equations:

3. S ^x + F ₁ ➝ S ^- + F ₁ ⁺

4. S ^x + F ₂ ➝ S ⁺ + F ₂ ^-

5. S ⁺ + F ₁ ➝ S + F ₁ ⁺

6. S ^- + F ₂ ➝ S + F ₂ ^-

with

F ₁ -- donor molecule

F ₂ -- acceptor molecule

F ₁ ⁺ , f ₂ ^- -- donor or acceptor radical ion.

At the boundary surface, sensitizing reactions take place between the transparent top layer and the organic dyestuff layer. The top layer thus is a sensitized organic photoconductor at least in the area of the boundary surface, which leads to the surprisingly high photoconductivity.

Reactions 3 and 5 proceed preferably when the π-electron system in the top layer is a compound which, as a donor compound, easily can release electrons. This is the case with 2,5-bis-(4-diethylaminophenyl)-1,3,4-oxadiazole, for example. Reactions 4 and 6 are possible with a substance in the top layer which, as an electron acceptor, easily accepts electrons, e.g., 2,4,7-trinitrofluorenone or N-t-butyl-3,6-dinitro-naphthalimide.

By means of the specific embodiment of the invention it is sufficient for the efficiency of the dyestuff when, besides its intense absorption, it only has either electron-attracting substituents, e.g. > C = O, --NO ₂ , halogen, or electron-repelling substituents, e.g. alkyl or --O--alkyl, depending on whether it is preferably suitable for reactions 3, 5 or 4, 6. The invention permits a charge carrier transport fostered by a particularly low expenditure of energy within the tightly packed dyestuff layer according to the following reactions:

7. S ⁺ + S ➝ S + S ⁺ or

8. S + S ^- ➝ S ^- + S.

In all conventional sensitizing processes, however, transport via the dyestuff molecules present in low concentration is impeded by their large distance from one another.

Analogous is the procedure of the charge transport in the top layer with:

9. F ₁ ⁺ + F ₁ ➝ F ₁ + F ₁ ⁺ (p-conductive)

10. F ₂ ^- + F ₂ ➝ F ₂ + F ₂ ^- (n-conductive)

The practical consequence of reactions 1 to 10 is that, in the use of electron donors in the top layer, the double layer arrangement is negatively charged so that reactions 3, 5, 8, 9 can proceed. In the inverse case, layers with electron acceptors in the top layer are positively charged so that reactions 4, 6, 7, and 10 can proceed.

The transparent top layer of organic insulating materials with at least one charge transporting compound is described as follows:

The transparent top layer has a high electric resistance and prevents in the dark the flowing off of the electrostatic charge. Upon exposure to light, it transports the charges produced in the organic dyestuff layer.

If it is to be negatively charged, the transparent top layer preferably consists of a mixture of an electron donor compound and a binder. But when the electrophotographic recording material is to be used for positive charge the transparent top layer consists of a mixture of an electron acceptor compound and a binder.

Consequently, in the transparent top layer there are used compounds for charge transport which are known as electron donors or electron acceptors. They are used together with binders or adhesives adapted to the compound for charge transport as regards charge transport, film property, adhesion, and surface characteristics. Furthermore, conventional sensitizers or substances forming charge transfer complexes may be present. But they can only be used in so far as the necessary transparency of the top layer is not impaired. Finally, other usual additives such as levelling agents, plasticizers, and adhesives may also be present.

Suitable compounds for charge transport are especially those organic compounds which have an extended π-electron system, e.g., monomer aromatic heterocyclic compounds.

Monomers employed in accordance with the invention are those which have at least one dialkyl amino group or two alkoxy groups. Particularly proved have heterocyclic compounds, such as the oxadiazole derivatives, mentioned in German Pat. No. 1,058,836. An example thereof is in particular the 2,5-bis-(p-diethylaminophenyl)-oxadiazole-1,3,4. Further suitable monomer electron donor compounds are, for example, triphenyl amine derivatives, benzo-condensed heterocycles, pyrazoline or imidazole derivatives, as well as triazole and oxazole derivatives, as disclosed in German Pat. Nos. 1,060,260 and 1,120,875.

Condensation products of formaldehyde and certain aromatic compounds, such as formaldehyde/3-bromo-pyrene condensates, may also be used.

Besides these mentioned compounds having predominantly a p-conductive character, it is also possible to use n-conductive compounds. These so-called electron acceptors are known from German Pat. No. 1,127,218, for example. Compounds such as 2,4,7-trinitrofluorenone or N-t-butyl-3,6-dinitro-naphthalimide have proved particularly suitable.

Suitable binders with regard to flexibility, film properties, and adhesion are natural and synthetic resins. Examples thereof are in particular polyester resins, e.g., those marketed under the names Dynapol (Dynamit Nobel), Vitel (Goodyear), which are copolyesters of iso- and terephthalic acid with glycol. Silicone resins as those known under the names SR of General Electric Comp. or Dow 804 of Dow Corning Corp., U.S.A., and which are three-dimensionally cross-linked phenyl-methyl siloxanes or the so-called "reactive" resins, e.g., the so-called DD lacquers consisting of an equivalent mixture of polyesters or polyethers containing hydroxyl groups and polyfunctional isocyanates, e.g., of the Desmophen or Desmodur type marketed by Bayer AG, Leverkusen, Germany, have proved particularly suitable. Furthermore, copolymers of styrene and maleic acid anhydride, e.g., those known under the name Lytron, Monsanto, and polycarbonate resins, e.g. the resins known by the name of Lexan Grade of General Electric, U.S.A. may be used.

The mixing ratio of charge transporting compound to binder may vary. Relatively certain limits are given, however, by the requirement for maximum photosensitivity, i.e., for the biggest possible portion of charge transporting compound, and for crystallization to be prevented, i.e., for the biggest possible portion of binder. A mixing ratio of about 1 : 1 parts by weight has proved preferable, but mixing ratios from about 3 : 1 to 1 : 4 or above, depending on the particular case, are also suitable.

The conventional sensitizers to be used additionally may advantageously foster charge transport. Moreover, they may produce charge carriers in the transparent top layers. Suitable sensitizers are, for example, Rhodamine B extra, "Schultz, Farbstofftabellen" (dyestuff tables), 1st volume, 7th edition, 1931, No. 864, page 365, Brilliant Green, No. 760, page 314, Crystal Violet, No. 785, page 329, and Cryptocyanine, No. 927, page 397. In the same sense as act the sensitizers may also act added compounds which form charge transfer complexes with the charge transporting compound. Thus, it is possible to achieve another increase of the photosensitivity of the described double layers. The quantity of added sensitizer or of the compound forming the charge transfer complex is so determined that the resulting donor acceptor complex with its charge transfer band still is sufficiently transparent for the organic dyestuff layer below. Optimum concentration is at a molar donor/acceptor ratio of about 10 : 1 to about 100 : 1 and vice versa.

The addition of adhesives as binders to the charge transporting compounds already yields a good photosensitivity. In this case, low-molecular polyester resin, such as Adhesive 49 000, Du Pont, has proved particularly suitable.

In the described manner, the top layers have the property to render possible a high charge with a small dark discharge. Whereas in all conventional sensitizations an increase of the photosensitivity is connected with an increase of the dark current, the arrangement of the invention can prevent this parallelity. The layers are thus usable in electrophotographic copying devices with low copying speeds and very small lamp energies as well as in those with high copying speeds and correspondingly high lamp energies.

The invention will be described in more detail by reference to the following examples:

APPLICATION OF DIFFERENT DYESTUFFS:

For the preparation of multi-layer photoconductor materials, the following dyestuffs are vapor deposited at a reduced pressure of 10 ^{- 3} to 10 ^{- 4} mm Hg (pump type A-1 of Pfeiffer, Wetzlar, Germany) on a 100 μm thick aluminium foil mounted at a distance of approximately 15 cm. The following conditions are employed for the different dyestuffes:

Table I ______________________________________ Dyestuff Time Temperature Formula No. (min.) (°C) ______________________________________ 1 2 to 3 370 2 2.5 430 to 440 3 3.0 380 4 2.5 440 5 3.0 420 6 2.0 430 7 4.0 430 ______________________________________

EXAMPLE 1

A layer containing quinacridone (Formula 1) as the dyestuff on a support consisting of a 100 μm thick aluminium foil is whirl-coated with a solution composed of

a. 1 part by weight of 2,5-bis-(4-diethylaminophenyl)-oxadiazole-1,3,4, and 1 part by weight of a polyester resin (e.g., Dynapol L 206, a product of Dynamit Nobel AG., Troisdorf, Germany) in tetrahydrofurane as the solvent;

b. the same components as in (a), plus 0.1 part by weight of 3,5-dinitrobenzoic acid; and

c. the same components as in (a), plus 10 ^{- 3} part by weight of Brilliant Green (C. I. 42,040).

The homogeneous, glossy layers thus produced are then dried for 5 minutes at 120°C. The top layers have a thickness of approximately 10 μm.

For comparison of the photosensitivity, a top layer is analogously produced on an aluminium foil without a dyestuff (zero layer).

The thickness of the top layers was determined by means of a measuring device marketed by Carl Mahr, Esslingen, Germany. In order to measure their photo-sensitivity, the double layer photoconductor materials are negatively charged by passing them three times through a charging apparatus (Type AG 56 of Kalle Aktiengesellschaft, Wiesbaden-Biebrich, Germany) adjusted to 7.5 kV. Subsequently, the layers are exposed with an Osram xenon lamp type XBO 150. The intensity of illumination in the plane of measurement is approximately 300 lux. Height of charge and the curves of the photo-induced light decay of the photoconductive layers are measured through a probe by means of an electrometer (type 610 B of Keithley Instruments, USA). The layers are characterized by stating their altitude of charge (U ₀ ) and their half time (T 1/2), i.e., the time in which the charge drops to half its initial value (in msec).

______________________________________ Top Layer Dyestuff Layer Zero Layer U _O (V) T 1/2 U _O (V) T 1/2 (neg.charge) (msec) neg.charge (msec) ______________________________________ a 1150 25 1200 525 b 1025 42 1350 1710 c 1000 35 1400 280 ______________________________________

COMPARATIVE EXAMPLE

By way of comparison, a 20% solution containing 0.5 part by weight of 2,4,7-trinitrofluorenone and 0.5 part by weight of a polyester resin (Dynapol L 206) in tetrahydrofurane, and having 0.1 part by weight of quinacridone C.I. 46,500 (e.g. Cinquasia Red B, a product of DuPont, USA) dispersed therein as the pigment, is thoroughly milled during 30 minutes in a Perl Mill apparatus (type PM1 of Draiswerke GmbH, Mannheim, Germany) at 3,000 revolutions per minute. The dispersion solution is then whirl-coated onto an aluminium foil of 100 μm thickness. A homogeneous, glossy layer of about 12 μm thickness is formed. It is dried for 30 minutes at 105°C in a recirculated air drier.

When determining the photosensitivity of the dispersion layer by the above described method of measurement (300 lux in the plane of measurement), the following values result:

negative charge: 1125 V T ₁ /2 = 900 msec

Upon comparing the photosensitivity of this layer with that of the above described double layer, it becomes apparent that the double layer is far superior in phototensitivity

EXAMPLE 2

A tetrahydrofurane solution containing 1 part by weight of 2,5-bis-(4-diethylaminophenyl)-oxadiazole-1,3,4, 1 part by weight of a polyester resin (e.g., Dynapol L 206) and 10 ^{- 3} part by weight of Brilliant Green (C.I. 42,040) is whirl-coated upon the dyestuff layers mentioned in the following table. After drying, the resulting layer has a thickness of about 10 μm.

The photosensitivity is determined by the method described in Example 1, and the values obtained are listed in the table.

Using a Dyntest-90 apparatus of ECE, Giessen, Germany, the dark decay Δ U _D of the double layers was measured. The dark decay is of particular importance when the photoconductor materials are to be employed in xerographic copying machines. It indicates how fast the photoconductive layer is discharged in the absence of light. The results show that the dark decay is in the order of 40-50 V/sec.

______________________________________ Dyestuff in Form- U _O (V) T 1/2 Dyntest-90 Double layer ula neg. (msec) U _O (V) U _D No. charge neg. after charge 2 sec ______________________________________ Zero Layer -- 1400 280 1275 -- a) quina- 1 1100 43 1190 100 cridone b) 4,11-di- methyl- quina- 5 1125 95 1470 80 cridone c) 4,11-di- chloro- quina- 4 1200 46 1350 75 cridone d) 1,4,8,11- tetra- methyl-quin- acridone 6 1100 120 1350 85 e) 1,8-di- chloro- 4,11-di- methyl- quina- 7 1125 38 1310 80 cridone ______________________________________

EXAMPLE 3

A solution containing 1 part by weight of 2-phenyl-4-(2-chlorophenyl)-5-(4-diethylaminophenyl)-oxazole (German Pat. No. 1,120,875) and 1 part by weight of a polyester resin (e.g., Dynapol L 206) in tetrahydrofurane as the solvent is whirl-coated upon a quinacridone layer (Formula 1, e.g., Cinquasia Red B, a product of DuPont, USA). After drying, the top layer has a thickness of approximately 10 μm.

The photosensitivity of this layer is determined by a modified process:

The photoconductor layer is placed on a slowly rotating disk and passed through a charging apparatus (corona adjusted to 7.0 kV, grid 1.5 kV) to the exposure station, where it is exposed to the light emitted by an Osram xenon lamp type XBO 150. A heat absorbing glass (type KG3, Schott and Gen., Mainz, Germany) and a neutral filter of 15% transparency are interposed between the photoconductor layer and the lamp, so that the light-intensity in the plane of measurement is 270 μW/cm ² . Height of charge and the curve of the photoinduced light decay are measured through a transparent probe by an electrometer (type 610 CR, Keithley Instruments, USA) and recorded by means of an oszillograph. Measurement of the height of charge (U ₀ ) and of the half time (T ₁ /2) yields the following values:

Layer acc. to the invention:

negative charge: 850 V T ₁ /2 = 19 msec

Zero layer (without dyestuff):

negative charge: 800 V T ₁ /2 = 750 msec

EXAMPLE 4

A 100 μm thick aluminium foil carrying a vacuum deposited layer of quinacridone (C.I. 46,500, e.g., Hostaperm-Rotviolett ER 01, a product of Farbwerke Hoechst AG, Frankfurt, Germany) is whirl-coated with a solution containing 81.4% of polyvinyl carbazole (e.g., Luvican M 170, a product of BASF, Ludwigshafen, Germany) and 18.6% of a polyester resin (e.g., Adhesive 49000, a product of DuPont, USA) in tetrahydrofurane as the solvent. After drying, the layer has a thickness of approximately 6 μm. Upon measuring the homogeneous double layer by the method described in Example 3 (270 μW/cm ² in the plane of measurement), the following light-sensitivity was determined:

Layer acc. to the invention:

negative charge: 325 V T ₁ /2 = 180 msec

Zero layer (without dyestuff):

negative charge: 550 V T ₁ /2 = much more than 1000 msec

EXAMPLE 5

A dyestuff layer consisting of Cinquasia Red B is coated with a solution containing 1 part by weight of a polyester resin and 2 parts by weight of 3-bromopyrene resin, which is produced by condensing 3-bromopyrene (Org. Synth., Vol.48, 1968, page 30) with formaldehyde in glacial acetic acid. The top layer has a thickness between about 5 and 10 μm, depending on the adjustment of the centrifugue.

The photosensitivity is measured as described in Example 3 (270 μW/cm ² in the plane of measurement). The following values result: Thickness of U _O (V) T ₁ /2 the layer neg.charge msec ______________________________________ approx. 5 μm 525 57 approx. 10 μm 750 88 ______________________________________

The photosensitivity of the zero layer (top layer of about 10 μm thickness without dyestuff layer) is as follows, when measured under the same conditions:

negative charge: T ₁ /2 (msec) 650 V 465

EXAMPLE 6

For the introduction of an intermediate layer (Numeral 5 in FIG. 2), a 2% solution of a polyamide resin (e.g., Elvamide 8061, a product of DuPont, USA) in a 1:1 mixture of trichloroethylene and methanol is applied to a 100 μm thick polyester film (e.g., Hostaphan film of Kalle Aktiengesellschaft, Wiesbaden-Biebrich, Germany) carrying a vacuum deposited aluminium layer. The intermediate layer has a thickness of less than 1 μm and a weight of 0.2 g/m ² .

The precoated material produced in this manner is then provided by vapor deposition with a dyestuff layer containing quinacridone, e.g., Cinquasia Red B, and the dyestuff layer is, in turn, coated with a top layer consisting of 1 part by weight of 2,5-bis-(4-diethylaminophenyl)-oxadiazole-1,3,4 and 1 part by weight of a polyester resin (e.g., Dynapol L 206). After drying, the top layer has a thickness of approximately 9 μm.

The photosensitivity is measured by the method described in Example 3 (light-intensity in the plane of measurement: 270 μW/cm ² ) and yields the following value:

negative charge: 1350 V T ₁ /2 = 18 msec.

EXAMPLE 7

Under the usual vacuum depositing conditions and at a temperature of approximately 420°C, an aluminium foil of 100 μm thickness is coated, for 3 minutes, with a layer of 2,9-dibromo-quinacridone (Formula 8). The resulting homogeneous dyestuff layer is coated with an about 10 μm thick layer of a solution containing 1 part by weight of 2,5-bis-(4-diethylaminophenyl)-oxadiazole-1,3,4 and 1 part by weight of a polyester resin (e.g., Dynapol L 206).

The photosensitivity of the material is determined as described in Example 3, the light-intensity in the plane of measurement being 750 μW/cm ² , however:

negative charge: 1050 V T 1/2 = 26 msec.

By way of comparison, a zero layer (without a dyestuff layer) yielded the following values:

negative charge: 900 V T 1/2 = 240 msec.

The dioxo-tetraaza-pentacene compounds already mentioned are prepared in accordance with the reaction pattern attached hereto; the following prescription refers to a compound in which R and R ₁ stand for hydrogen and R ₂ is ethyl: 25.6 g of succinylo-succinic acid ethyl ester (0.1 mol) is heated with 20.7 g (0.22 mol) of 2-amino-pyridine and 24 g (0.4 mol) of glacial acetic acid for 8 hours to 140°-145°C. The precipitating tetrahydro compound I is filtered off, washed with alcohol, and dried.

20 g of the weakly yellow colored powder are heated for 6 hours, with reflux, with 200 g of nitrobenzene, 20 g of glacial acetic acid, and 0.5 g of piperidine. The reaction product is drawn off at 50°C, washed with alcohol, and dried; red crystals are obtained which decompose at temperature above 360°C.

The substituted derivatives are prepared analogously.

EXAMPLE 8

Analogously to the conditions stated in Example 1, the dyestuff corresponding to formula 9, 10 and 11, i.e.,

7,14-dioxo-5,7a,12,14a-tetraaza-7,14-dihydropentacene,

2,9-dichloro,7,14-dioxo-5,7a,12,14a-tetraaza-7,14-dihydr opetacene, and

1,2,8,9-dibenzo-7,14-dioxo-5,7a,12,14a-tetraaza-7,14-dih ydropentacene

are vapor-deposited on 100 μm thick aluminium foils. Dyestuff layers of good covering power and weighing approx. 0.15 g/m ² are produced within 1 minute at 300°C when using the dyestuff according to Formula 9 (e.g., Pigment Red 1650 of Farbwerke Hoechst A.G.), and within 2 minutes at 350°C when using the dyestuff according to Formula 10 (e.g., Pigmentorange 1963/1 of Farbwerke Hoechst A.G.) or the dyestuff according to Formula 11 (e.g., Pigment Yellow 1966 of Farbwerke Hoechst A.G.).

In order to test their photo-sensitivity, the dyestuff layers thus obtained are coated with a solution containing 1 part by weight of 2,5-bis-(4-diethylaminophenyl)-oxadiazole-1,3,4 and 1 part by weight of a polyester resin (e.g., Dynapol L 206) in tetrahydrofurane. After drying, the top layers have a thickness of approximately 10 μm. The photosensitivity was measured by the method described in Example 3, a light -intensity in the plane of measurement of approx. 500 μW/cm ² being produced by a xenon lamp.

______________________________________ Dyestuff contained U _O (V) T ₁ /2 in the Double Layer neg. charge (msec) ______________________________________ Formula 9 1025 34 Formula 10 1025 112 Formula 11 1025 98 Zero Layer 900 380 ______________________________________

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from its spirit, and the invention includes all such modifications.