WO1995004306A1 - Organic charge transfer compounds with liquid crystal properties - Google Patents

Abstract

Low molecular or polymer tricycloquinazolines with liquid crystal properties are used as organic charge transfer compounds. These charge transfer compounds are suitable for producing electrophotographic recording materials.

Description

Organic charge transport compounds with liquid crystalline properties

description

The invention relates to organic charge transport compounds with liquid crystalline properties which have an increased photoconductivity in the liquid crystalline state. The terms “state” and “phase” are used interchangeably below.

Photoconductive polymers are an interesting class of materials and are widely used technically in copiers, laser printers and offset printing plates.

Several attempts are known to improve the charge transport properties of liquid-crystalline materials by orientation in the liquid-crystalline state. There are two approaches to this.

On the one hand, low-molecular liquid crystals, which form nematic phases at room temperature, are doped with carbazole. However, the absorption capacity of the liquid-crystalline matrix is very limited. The carbazole begins to crystallize even at very low concentrations (a few% by weight). Accordingly, there is only a small photocurrent of the liquid crystal samples doped with carbazole (cf. LL Chapoy, DK Munck, KH Rasmussen, E. Juul-Diekmann, RK Sethi, D. Biddle, in Molecular Crystals, Liquid Crystals, Vol. 105, pp. 353ff. (1984)).

The second approach is illustrated in EP-A 254 060. EP-A 254 060 discloses photoconductive films with a thickness below 20 μm which are produced from concentrated lyophasic solutions of a polymer which has a repeating unit according to the following general formula,

NH-CH-CO

R

where R is an alkylene radical having 1 to 20 carbon atoms and P is a photoconductive group. Electrophotographic photoreceptors which contain tricycloquinazoline trisazo pigments as a charge-generating substance can be found in JP-OS 01 076 060. In Liquid Crystals, 1992, Vol. 11, No. 2, 157-173, discotic liquid-crystalline substances are described which contain a tricycloquinazoline nucleus.

JP 04 182 485-A and JP 04 182 486-A describe liquid-crystalline phthalocyanines and their transition metal complex compounds which are suitable for LCDs (liquid-crystal displays) or as a photo-conductive material, but which have extremely high absorption coefficients in the optical range and therefore as e.g. Charge transport material in photocopiers are unsuitable.

There is a constant need in the art for new organic charge transport compounds, e.g. Photoconductors which have improved photoconductive properties or methods for improving the photoconductive properties.

From DE-A-41 26 496 it is known that the photoconductivity of triphenylenes in the liquid-crystalline phase is increased compared to the other phases.

The object of the present invention is to provide new organic charge transport compounds with improved properties.

The invention thus relates to the use of liquid-crystalline tricycloquinazolines or liquid-crystalline mixtures containing tricycloquinazolines as charge transport compounds. Tricycloquinazolines of the general formula (I) are particularly preferred

(I)

worin R¹ bis R¹² untereinander gleich oder verschieden sind und für Wasserstoff, Halogen, 0-CH=CH₂, S-CH=CH₂, 0-CO-CH=CH₂, 0-CO-C(CH₃)=CH₂, S-CO-CH=CH₂, S-CO-C(CH₃)=CH₂, Alkyl, O-Alkyl, S-Alkyl, NH-Alkyl oder N(Alkyl)₂ mit 1 bis 20 Kohlenstoffatomen 5 im Alkylrest stehen und die Alkylreste zusätzlich funktionelle Gruppen tragen können, wobei COOH, OH, NCO, NH₂, NHR¹³, NR₂ ¹³ mit R¹³ = Alkyl mit 1 bis 20 Kohlenstoffatomen, Aryl mit 6 bis 12 Kohlenstoffatomen, Heteroaryl, Vinyl, Allyl, Ethinyl, COOR¹⁴ mit R^{14 =} Alkyl mit 1 bis 20 Kohlenstoffatomen, Aryl mit 6 bis 10 12 Kohlenstoffatomen oder Heteroaryl, oder Oxiranyl an die Alkyl¬ reste gebundene funktionelle Gruppen sein können.

wherein R ¹ to R ^{12 are the} same or different from each other and for hydrogen, halogen, 0-CH = CH ₂ , S-CH = CH ₂ , 0-CO-CH = CH ₂ , 0-CO-C (CH ₃ ) = CH ₂ , S-CO-CH = CH ₂ , S-CO-C (CH ₃ ) = CH ₂ , alkyl, O-alkyl, S-alkyl, NH-alkyl or N (alkyl) ₂ with 1 to 20 carbon atoms 5 are in the alkyl radical and the alkyl radicals can additionally carry functional groups, where COOH, OH, NCO, NH ₂ , NHR ¹³ , NR ₂ ¹³ with R ¹³ = alkyl with 1 to 20 carbon atoms, aryl with 6 to 12 carbon atoms, heteroaryl, vinyl, Allyl, ethynyl, COOR ¹⁴ with R ^{14 =} alkyl with 1 to 20 carbon atoms, aryl with 6 to 10 12 carbon atoms or heteroaryl, or oxiranyl can be functional groups bonded to the alkyl radicals.

Preferred is the use of such tricycloquinazolines which are in the 2,3-, 6,7- and 10, 11-position, ie as R ² , R ³ , R ⁶ , R ⁷ , R ¹⁰ 15 and R ^11, electron-donating substituents, such as S-alkyl or O-alkyl.

The present invention also relates to photoconductive layers which contain, as charge transport compound, one or more tricycloquinazoline of the aforementioned type having liquid-crystalline properties; these layers can have thicknesses between 2 and 100 μm.

The present invention also relates to photoconductive layers, for which mixtures which contain liquid-crystalline properties and contain tricycloquinazolines are used as photoconductors.

The present invention also relates to electrophotographic recording materials consisting of an electrically conductive substrate and a photoconductive layer, photoconductive layers according to the invention containing the above-mentioned tricycloquinazolines being used, and electrophotographic recording materials 5 consisting of an electrically conductive substrate , a charge carrier-producing sensitizer layer and a photoconductive layer, the photoconductive layers used being those which contain the photoconductors according to the invention.

40

Suitable low-molecular liquid-crystalline tricycloquinazolines or mixtures of liquid-crystalline properties include, in particular, those of the general formula (I)

45

Methyldodecylamino, methyltetradecylamino, methylhexadecylamino, methylheptadecylamino and methyloctadecylamino.

The alkyl radicals can additionally carry functional groups, such as COOH, OH, OR ⁱ³ , NCO, NH ₂ , NHR ⁱ³ , NR ₂ ¹³ , where R ^{i3 is} straight-chain or branched alkyl, oxaalkyl and thialkyl having 1 to 12 carbon atoms, aryl having 6 up to 12 carbon atoms, alkaryl with 7 to 12 carbon atoms, heteroaryl such as furyl, thiophenyl, pyridinyl, vinyl, allyl, ethinyl, CO-R ⁱ⁴ with B. ¹ - = CH = CH ₂ , CHRi ⁵ = CH ₂ with R ⁱ⁵ = straight-chain or branched alkyl having 1 to 6 carbon atoms, COOR ¹⁶ with Rl ⁶ = straight-chain or branched alkyl having 1 to 12 carbon atoms, aryl or alkaryl having up to 12 carbon atoms or oxiranyl. Examples of these additional functional groups are diphenylamino, ethylphenylamino, acrylate, methacrylate, vinyl ether, epoxy and glycidyl groups.

The production of tricycloquinazolines can be seen, for example, from the following work: MW Partridge, SA Slorach and AJ Vipond, J. Chem. Soc. 1964, 3670; HG Dean, RJ Grout, MW Partridge and HJ Vipond, J. Chem. Soc. C 1968, 142; ME Suh, Yakhak Hoechi 30, 203 (1986); VP Reddy, VB Rao and CV Ratnam, Indian J. Chem., Sect. B 23B, 560 (1984); F. Yoneda and K. Mera, Chem. Pharm. Bull. 21, 1610 (1973); HG Dean, RJ _. Grout, MW Partridge and HJ Vipond, J. Chem. Soc. C 1968, 142.

Examples of such tricycloquinazolines having liquid crystalline properties are:

with R ¹ C ₅ H ₉ , C ₈ Hι ₇ , Cι ₂ H ₂₅ , Cι ₈ H ₃₇

To increase the sensitivity of the layers to light, sensitizers, ie compounds which generate charge carriers, can be added. Compounds of this type are, for example, those from DE-A 22 37 539 and DE-A 31 10 955 known perylene tetracarboxylic acid derivatives. The addition of liquid-crystalline compounds as charge carrier generators is particularly preferred.

The formation of charge transfer complexes for charge carrier generation is also possible.

The photoconductors according to the invention are generally used in the form of thin photoconductive layers, it also being possible to separate the charge transport from the charge generation in the sense of a two-layer arrangement as used in electrophotography. Here, the photoconductor according to the invention is located in the photoconductive charge transport layer, which is adjacent to a conventional and known charge carrier-producing sensitizer layer. Charging is usually carried out by a high-voltage corona.

The layers according to the invention can be produced on a carrier surface by applying a melt or in a customary and known manner, e.g. by scraping a solution of the compounds onto a carrier surface. Various auxiliaries, e.g. to improve the leveling properties.

For example, tetrahydrofuran or dichloromethane are used as solvents.

These photoconductive layers generally have a layer thickness between 2 and 100, preferably between 4 and 50 and particularly preferably between 4 and 30 μm.

The photoconductor or the photoconductive layers can be arranged between or on conductive coated, transparent substrates, for which glass plates or plates made of optically transparent plastics (for example polymethyl methacrylate, polycarbonate etc.) can be used. The conductive coating of the substrate can consist of electrically conductive polymers, semiconductors or metals. The thickness of the coating should, however, be chosen so that the optical transmission is not impaired too much. Particularly advantageous coatings consist of ITO ("indium tin oxide").

To generate a photocurrent, a DC voltage of between 5 and 50 V is generally applied to the electrically conductive coated plates. According to the invention, the liquid-crystalline (discotic) state in which the photoconductivity is higher than in the unordered state is set. This can be done in a number of ways. Orientation can be achieved, for example, mechanically (by stretching or shearing) or by electrical or magnetic fields. Orientation can also be set by means of orienting sublayers, which contain or consist of polyimides, for example. The simplest option is thermal treatment (tempering).

The photoconductors and photoconductive films according to the invention can be used in electrophotography, in laser printers, in offset printing or in microelectronics for photosensitive switches.

In addition, the invention can be used in all areas in which the increase in photoconductivity can be exploited by molecular order.

The tricycloquinazolines described can also be used for the construction of solar cells.

Examples

Examples 1 to 3 are examples of inventive

Charge transport compounds and the method according to the invention for increasing the photoconductivity.

In Examples 1 to 3, the organic photoconductors were examined as follows.

The organic photoconductors were arranged between two electrically conductive, coated, transparent glass plates of a glass measuring cell.

The distance between the glass plates was set between 5 and 15 μm using spacers. A voltage was applied to the sample via the conductive layer and the current was measured.

For the measurement, the glass measuring cell with the sample layer to be examined was located in a microscope heating table, which could be temperature-controlled at a constant heating rate from room temperature to 300 ° C. The cell was irradiated perpendicularly to the sample surface by a halogen incandescent lamp through a window in the heating table cover of approximately 5 mm in diameter (intensity approximately 0.02 watt / cm ² ). The incident light beam was through a chopper ("light chopper") with a frequency of 10 Hz modulated, ie broken down into light pulses of 50 msec in length and in the same dark phase.

The measuring cell was connected in series with a picoammeter to a controllable voltage source via its two connection electrodes (transparent ITO electrodes). 10 V DC voltage was thus applied to the measuring cell. The picoammeter measured the electrical current generated thereby through the sample, i.e. alternating with the frequency of the light modulation, the dark current during the dark phase of the lighting or the sum of the dark and photocurrent during the light phase.

The analog output signal of the picoamperimeter was applied to a lock-in amplifier, which received its reference frequency from the chopper. The voltage component of the picoammeter output, which changes with the modulation frequency, was measured here. It was directly proportional to the difference of the measured cell current in the light or dark phase of the exposure, and was therefore proportional to the photocurrent.

For the temperature-dependent measurement of the photocurrent, the measuring cell in the heating table was heated up to the isotropic state at a heating rate of 5 ° C./min. The measuring electronics described above were active during this temperature cycle. The measured values of the lock-in amplifier could be read from the device as a function of the temperature and converted into units of the photocurrent.

Examples 1 to 3

Compounds I, II and III purified by repeated recrystallization were each arranged in the melt between two electrically conductive coated, transparent glass plates of a glass measuring cell (layer thickness: 8 μm) and the photocurrent as described during heating up to a temperature just above the Clarification temperature (isotropic melt) measured. The measured values found are shown in the following table.

As can be seen from the table, in the discotic-liquid-crystalline area of the sample just below the clarification temperature, the photocurrent is increased compared to the isotropic phase (melt). Photocurrent (nA)

Compound phase behavior in the liquid in the isotropic crystalline phase melt

I with R ¹ = C ₈ Hι ₇ k81 D _ho 207i 1.50 (at 190 ° C) 0.46 (at 200 ° C)

II with R ¹ = Cι H ₂₅ k54 D _ho 170i 0.122 (at 150 ° C) 0.019 (at 165 ° C)

K = transition from the crystalline to the liquid-crystalline phase D _ho

Claims

claims 1. Use of liquid-crystalline tricycloquinazolines or of liquid-crystalline mixtures containing tricycloquinazolines as charge transport compounds. 2. Use according to claim 1, characterized in that the tricycloquinazolines correspond to the general formula (I)

wherein R ¹ to R ^{12 are the} same or different from each other and for hydrogen, halogen, 0-CH = CH _/ S-CH = CH ₂ , 0-CO-CH = CH ₂ , 0-CO-C (CH ₃ ) = CH , S-CO-CH = CH ₂ , S-CO-C (CH ₃ ) = CH ₂ , alkyl, O-alkyl, S-alkyl, NH-alkyl or (alkyl) ₂ with 1 to 20 carbon atoms in the alkyl radical stand and the alkyl radicals can also carry functional groups. 3. Use according to claim 2, characterized in that COOH, OH, OR ¹³ , NCO, NH ₂ , HR ¹³ , R ₂ ³ with R ¹³ = straight-chain or branched alkyl, aryl, alkaryl, oxaalkyl and thialkyl, vinyl, allyl, Ethinyl, CO-R ¹⁴ with R = CH = CH ₂ , CHR ⁱ⁵ = CH ₂ with R ¹⁵ = straight-chain or branched alkyl, COOR ¹⁶ with R ¹⁶ = straight-chain or branched alkyl, aryl or alkaryl, or oxiranyl functional groups bonded to the alkyl radicals Groups are. 4. Use according to one of the preceding claims, characterized in that the tricycloquinazolines have electron-donating substituents in the 2,3-position. 5. Photoconductive layer, characterized in that it contains, as photoconductor, a liquid-crystalline tricycloquinazoline according to one of the preceding claims. 6. Photoconductive layer according to claim 5, characterized gekennzeich¬ net that the layer has a thickness between 2 and 100 microns. 7. Electrophotographic recording material consisting of an electrically conductive support and a photoconductive layer, characterized in that photoconductive layers according to claim 5 or 6 are used. 8. Electrophotographic recording material, consisting of an electrically conductive layer support, a charge carrier-generating sensitizer layer and a photoconductive layer, characterized in that photoconductive layers according to claim 5 or 6 are used. 9. Hold the use of liquid-crystalline Tricyclochinazolinen or liquid crystalline ^mixtures' Tricyclochinazoline ent, in an organic solar cell.