GB2055803A - Hydrazones of 9-ethylcarbazole-3- aldehyde and their use in electrophotography - Google Patents

Abstract

The invention provides hydrazones of the general formula: <IMAGE> (in which R is a methyl or benzyl group) and which are useful as charge transport materials for use in electrophotographic photoconductors of the type comprising an electroconductive support material and a photosensitive layer containing a charge generation material and a charge transport material. The hydrazones may be prepared by the reaction of 9-ethylcarbazole-3-aldehyde with a 1-substituted phenylhydrazine of the general formula: <IMAGE> (in which R is methyl or benzyl), suitably in a molar ratio of hydrazine to aldehyde of 1:1 or slightly higher and in an organic solvent in the presence of an acid catalyst.

Description

SPECIFICATION Novel hydrazones suitable for use as charge transfer materials in electrophotographic elements This invention is concerned with certain hydrazones, hydrogens, the preparation thereof and their use as charge transfer materials in electrophotographic elements.

Electrophotographic elements comprising a photoconductive material supported on an electroconductive support are well known.

Generally, in the art of electrophotography, the photoconductor is electrically charged, for example by corona charging in the dark and is then imagewise exposed to light, whereby the charge on the photoconductive is selectively dissipated in the illuminated areas of the photoconductorwhile leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing on the photoconductor finely divided electroscopic marking particles (toner) which generally comprise colouring materials, such as dyestuffs or pigments, and a binder material comprising polymeric compounds.The following basic characteristics are required for photoconductors for use in electrophotography: (1) The photoconductors should be capable of being electrically charged to a predetermined potential in the dark.

(2) The photoconductor should retain the charge sufficiently in the dark. In other words, the dark decay of the photoconductors must be low.

(3) The charge on the photoconductors should be dissipated quickly under illumination. In other words, the light decay of the photoconductor must be high and accordingly its photosensitivity must be high.

Additionally, it is required that the photoconductors have a high mechanical strength and be capable of being worked into the desired shape.

Inorganic materials which have been conventionally employed as photoconductive materials include selenium, cadmium sulphide and zinc oxide. Such conventional inorganic photoconductors have some advantages, but, at the same time, they have several drawbacks. For example, the selenium photoconductor, which is now widely used, can to some extent satisfy the above mentioned requirements (1) (2) and (3).

However, difficulties are encountered when producing the photoconductor and its production cost is high.

More specifically, since its flexibility is poor, it is difficult to form it into various shapes. Furthermore, it is highly susceptible to heat and mechanical shocks, so care must be taken when handling it. Cadmium sulphide and zinc oxide are generally used as dispersions in a binder resin. However, since they are poor in mechanical characteristics, such as smoothness, hardness, tensile strength and durability, they cannot be used repeatedly as they are, so that, for example, a protective layer is required, which complicates the process of producing the electrophotographic element using these materials.

Recently, in order to eliminate the above mentioned drawbacks of the inorganic photoconductors, a variety of electrophotographic organic photoconductors have been studied and developed and used in practice: for example, a photoconductor comprising a support material and a photosensitive layer containing poly-N-vinycarbazole and 2,4,7-trinitro-fluorene-9-on (as disclosed in United States Patent No.

3,484,237), which is formed on the support material; a photoconductor comprising a photosensitive layer containing poly-N-vinylcarbazole sensitized by a pyryllium salt dyestuff (as is disclosed in Japanese Patent No.48-25658); a photoconductor having a photosensitive layer consisting essentially of an organic pigment (as described in Japanese Laid-Open Patent Application No.47-37543); or a photoconductor having a photosensitive layer which contains as a main component an eutectic crystal complex consisting of a dyestuff and a resin (as described in Japanese Laid-Open Patent Application No. 47-10735). These electrophotographic organic photoconductors generally have improved mechanical characteristics and working properties as compared with those of the inorganic photoconductors.However, the organic photoconductors generally have a low photosensitivity and accordingly do not sufficiently satisfy the requirements for electrophotographic photoconductors. Furthermore, the characteristics of the electrophotographic photoconductors depend significantly upon the materials used and the preparative methods employed, in particular upon the photoconductive materials, and, therefore, photoconductive materials have been actively studied.In addition to the previously mentioned inorganic and organic photoconductors, the photoconductive materials having high photosensitivities have been studied and developed, in which a material which readily generates charge carriers upon absorption of light (hereinafter referred to as a charge generation material) is used in combination with a material which receives the generated charge carriers and transports them (hereinafter referred to as a charge transfer material).

It has now been found, in accordance with the present invention, that certain hydrazones, as hereinafter defined, are particularly suitable for use as charge transfer materials.

According to one embodiment of the invention, therefore, there are provided, as new compounds, hydrazone compounds of the formula:

in which R is a methyl or benzyl group.

The invention also provides a process for preparing the hydrazones of formula (I) which comprises reacting 9-ethylcarbazole-3-aldehyde of the formula:

with a 1 -substituted phenylhydrazine of the formula:

in which R has the meaning defined above; the reaction suitably being carried out in an appropriate solvent in the presence of an acid catalyst. The thus prepared hydrazones may be purified by recrystallization form a suitable solvent.

The hydrazone compounds according to the present invention, 9-ethylcarbazole-3-aldehyde 1 -methyl-l - phenylhydrazone and 9-ethylcarbazole-3-aldehyde 1 -benzyl-1 -phenylhydrazone, are colourless or yellow crystals at room temperature which can easily be obtained by reacting 9-ethylcarbazole-3-aldehyde with an appropriate 1-substituted phenylhydrazine, generally in molar ratios of 1: 1 or using a slight excess of hydrazine, in an appropriate organic solvent. In this reaction the amino group of the hydrazine condenses with the carbonyl group of the aldehyde to give the hydrazone and water As is well known, such condensation reactions are promoted by acid catalysts including inorganic acids (such as hydrochloric acid or dilute sulphuric acid), or organic acids (such as acetic acid).Organic solvents which can be used as the reaction solvents are those in which the reaction components are soluble. Examples of such solvents include lower alcohols, such as methanol and ethanol; cyclic ethers, such as 1,4-dioxan and tetrahydrofuran; cellosolves (ethylene glycol alkyl ethers) such as methyl cellosolve (ethylene glycol methylether) and ethyl cellosolve (ethylene glycol monobutyl ether); N, N-dimethylformamide; and acetic acid. The reaction temperature may vary, depending upon the solvent employed. When N, N-dimethylformamide is used since the above mentioned reaction components are very soluble in the solvent, the reaction proceeds sufficiently at room temperature. On the other hand, in the case of ethanol, since the above mentioned reaction components are less soluble in ethanol, it is preferable to head the reaction mixture under reflux.In any case, the reaction is completed in one to five hours Depending upon the solvent employed the hydrazone may separate out from the reaction mixture on its formation, or if it does not, it may be precipitated by the addition of a diluent, such as a mixture of methanol and water, to the reaction solution. The precipitated material is filtered off and is then recrystallized from an appropriate solvent to give a pure hydrazone.

The thus obtained pure hydrazone compounds may be used as charge transport materials in combination with a variety of charge generation materials to form electrophotographic photoconductive materials, wherebyelectrophotographic photoconductors having high photosensitivities can be obtained.

In the following description reference will be made to the accompanying drawings in which: Figures 1, 2 and 3 are schematic sectional views of the electrophotographicphotoconductors in which the novel hydrazone compounds according to the present invention may be employed, the section being on a larger scale in the direction of the thickness of each electrophotographic photoconductor; and Figures 4 and 5 are the infrared spectra of the novel hydrazone compounds according to the present invention.

The photoconductor shown in Figure 1 comprises an electroconductive support material 1 and a photosensitive layer 2a comprising a hydrazone compound, a sensitizer dyestuff and a binder resin, the photosensitive layer 2a beng formed on the electroconductive support material 1. The photoconductor shown in Figure 1 comprises an electroconductive support material 1 and a photosensitive layer 2b in which a charge generation material 3 is dispersed in a charge transport medium 4 consisting of a hydrazone compound and a binder material, the photosensitive layer 2b being formed on the electroconductive support material 1. The photoconductor shown in Figure 3 comprises an electroconductive support material 1 and a photosensitive layer 2c consisting of a charge generation layer 5 consisting essentially of the charge generation material 3 and a charge transport layer 6 containing a hydrazone compound.

In order that the invention may be well understood the following examples are given by way of illustration only.

Example 1 22.3 Grams (0.1 mole) of 9-ethylcarbazole-3-aldehyde was dissolved in 100 ml of N, N-dimethylformamide, and the solution was acidified with 5 ml of one normal hydrochloric acid. 13.4 g (0.11 mole) of 1methyl1 -phenylhydrazine was added dropwise to the acidified solution, with stirring, over a period of 30 minutes. The reaction mixture was then stirred for a further hour at room temperature and 50 ml of methanol and 50 ml of water were then added to the mixture. The crystals which separated out were filtered on a suction funnel and dried. The product was recrystallized from a mixture of ethyl acetate and ethanol, whereby a pure 9-ethylcarbazole-3-aldehyde 1-methyl 1-phenylhyrazone was obtained.

Table 1 below shows the yield, melting point and analysis of the hydrazone compound.

Figure 4 of the drawings shows the infrared spectrum of the hydrazone compound.

Example 2 Following the procedure of Example 1, 0.1 mole of 9-ethylcarbazole-3-aldehyde was reacted with 0.1 mole of 1 -benzyl-1 -phenylhydrazine to give 9-ethylcarbazole-3-aldehyde 1 -benzyl-1 -phenylhydrazone. The yield, melting point and analysis of the hydrazone compound are shown in Table 1.

Figure 5 of the drawings shows the infrared spectrum of the hydrazone compound.

TABLE 1 Example Hydrazone compound Melting point Yield Elemental analysis No. Found Calculated 1 9-Ethylcarbazole-3 aldehyde-1-methyl-1- 128"C C 80.98 80.69 phenylhydrazone (Formula I,R = methyl) 76.5% H 6.47 6.48 N 12.60 12.84 2 9-Ethylcarbazole-3 aldehyde-1-methyl-1- 178"C C 83.67 83.88 phenylhydrazone (Formula I, R = benzyl) 75.6% H 10.08 10.41 N 10.08 10.41 Example 3 This example illustrates an electrophotographic photoconductor employing the hydrazone compound prepared in Example 1.

A mixture of 3 parts by weight of 4', 4"-bis(2-hydroxy-3-phenylcarbamoyl)-1 -naphthylazo)-1,4distyrylbenzene, 1 part by weight of a polyester resin (Trade name: polyester adhesive 4900 made by Du Pont) and 96 parts by weight of tetrahydrofuran was ground in a ball mill. This dispersion was coated onto a polyester film having a vacuum deposited layer of aluminium by means of a doctor blade and was then dried at 80"C for 5 minutes, so that a charge generation layer about 1 micron thick was formed on the aluminium surface of the coated polyester film.

A solution consisting of 1 part by weight of 9-ethylcarbazole-3-aldehyde 1 -methyl-1 -phenylhydrazone (prepared as in Example 1), 1 part by weight of a polycarbonate resin and 8 parts by weight of tetrahydrofuran was coated on the charge generation layer by means of a doctor blade and was then dried at 1 00 C for 10 minutes so that a charge transport layer about 10 microns thick was formed on the charge generation layer.

The thus prepared electrophotographic photoconductor was negatively charged in the dark under the application of -6 kV or corona charge for 20 seconds by means of a commercially available electrostatic copying sheet testing apparatus and was then allowed to stand in the dark for 20 seconds without applying any charge thereto, and the surface potential Vpo (V) of the photoconductor was measured. The photoconductor was then illuminated by a tungsten lamp so that the illuminance on the illuminated surface of the photoconductor was 20 lux and the exposure E1/2 (lux second) required to reduce the initial surface potential Vpo (V) to half its initial value was measured. The results showed that Vpo was 847 and E1/2 = 2.0 lux.

second.

The charge retention property in the dark and the photosensitivity of this photoconductor were excellent.

Claims (5)

1. As new compounds, hydrazones of the general formula

in which R is a methyl or benzyl group.

2. A process for the preparation of a hydrazone as claimed in claim 1 which comprises reacting 9-ethylcarbazole-3-aldehyde with a 1-substituted phenylhydrazine of the general formula:

(in which R has the meaning defined in claim 1) in the presence of an organic solvent.

3. A process as claimed in claim 2, in which the organic solvent is a lower alcohol, a cyclic ether, an ethylene glycol alkyl ether, N,N-dimethylformamide or acetic acid.

4. A process as claimed in claim 2 or claim 3 in which the reaction is carried out in the presence of an acid catalyst.

5. A process as claimed in claim 2 substantially as hereinbefore described with reference to Examples 1 and 2.