GB2074182A

GB2074182A - Liquid crystal composition containing pleochroic material for an electro-optical device

Info

Publication number: GB2074182A
Application number: GB8107303A
Authority: GB
Original assignee: Asulab AG
Current assignee: Asulab AG
Priority date: 1980-03-21
Filing date: 1981-03-09
Publication date: 1981-10-28
Also published as: CH643291A5; JPS56145969A; FR2478664B1; DE3109088A1; GB2074182B; FR2478664A1

Abstract

A liquid crystal composition contains an anthraquinone pleochroic colouring agent of formula (I> <IMAGE> in which R1 is substituted or unsubstituted amino or hydroxy group; each of R2, R3 and R4 is a hydrogen or halogen atom, an amino group which may be alkylated or acylated, a substituted or unsubstituted hydroxy group or a cyano or nitro group; X is a group of formula -COOR5 or OOCR5 in which R5 is a substituted or unsubstituted alkyl, cycloalkyl or phenyl group and Y is a hydrogen atom or a group as defined for X. The compounds (I) have a high order parameter and are stable, thereby imparting to the composition the properties required for use for example in a display device.

Description

SPECIFICATION Liquid crystal-base composition for an electro-optical device The present invention relates to a liquid crystal-base composition which contains an anthraquinone pleochroic colouring agent in solution and which can be used in electro-optical devices, more particularly in display devices.

A colouring agent can be suitably used in solution in a liquid crystal for a display device, if it has at least the following properties: it must be sufficiently soluble in the liquid crystal; it must be stable chemically (in particular, inert with respect to the liquid crystal) and especially photochemically (stable in regard to UV-light; it must have a sufficient intensity of absorption; and it must not contain ionic or ionisable groups.

In addition, such a colouring agent must have a high order parameter (S), which parameter corresponds to the capability of orientation of the colouring agent by the molecules of the liquid crystal, in order to be able to produce a high-contrast display. Finally, the base structure of the compounds envisaged must obviously correspond to a suitable colour for use in a display device, preferably blue and red.

In their Swiss patent No (application No 7543/79 of 1 7th August 1979), the applicants describe a liquid crystal-base composition for use in electro-optical devices, especially display devices. The composition contains in solution an anthraquinone derivative having the following general formula:

which can be used as a pleochroic colouring agent and which has in particular a high order parameter, and a high level of stability with respect to light.

Continuing their research, the present inventors have discovered that other anthraquinone colouring agents of the same type but with an ester substituent are also valuable pleochroic colouring agents which can be used in liquid crystal-base compositions.

The present invention is therefore concerned with a liquid crystal-base composition containing at least one pleochroic colouring agent of formula (I):

wherein P1 is an amino group which can be alkylated or acylated or a substituted or unsubstituted hydroxy group; R2, R3 and R4 which are indentical or different are each a hydrogen atom, an amino group which may be alkylated or acylated, a substituted or unsubstituted hydroxy group, a nitro group, a halogen atom or a cyano group;X is a --COOR, group or a --OOCR, group in which R5 is a cyclic, branched or straight alkyl chain, an alkoxyalkyl chain, a cyanoalkyl chain, the alkyl residue having at most 1 6 carbon atoms, or a group

in which R6 is a hydrogen atom, an alkyl chain, an alkoxy chain, a cyano group, an amino group, or a dialkylamino group, the alkyl residue containing from 1 to 1 6 carbon atoms, Z is a hydrogen atom, a halogen atom, a hydroxy group, a lower alkyl group or a lower alkoxy group and n is an integer of from 1 to 4; and Y has the same meaning as X or is a hydrogen atom.

Preferably, the group Y is in position 6 when X is substituted in position 2 on the anthraquinone ring and in position 7 when X is substituted in position 3.

The order parameter as measured in a liquid crystal of the colouring agent of formula (I) is generally higher when Y = X than when Y = H. This is particularly true when the mixture of liquid crystals used is based on esters or contains an ester component.

The length of the side chain or chains, determines at least in part the solubility of the compound of formula (I) and consequently should not exceed about 24 carbon atoms. The substituents P5 and R6 each preferably have 10 carbon atoms at most, but when a plurality of such groups are present simultaneously, the total of the carbon atoms does not in principle exceed about 24. Above this limit, the compound (I) may not be sufficiently soluble in the liquid crystal and consequently cannot be used as a colouring agent therefor.

The anthraquinone colouring agents described in Swiss patent No (application No 7543/79 of 16th August 1979) mentioned hereinbefore have a high order parameter in the mixtures of liquid crystals based on biphenyl, pyrimidine or phenylcyclohexane. However, the values of order parameter of the same colouring agents decrease substantially when they are dissolved in a mixture containing an ester liquid crystal. Now, liquid crystals with negative dielectric anisotropy, which essentially comprise esters, have been used in particular in positive-contrast dichroic display devices such as those described for example in Swiss patent No (Application No 1272/78 of 6th February 1978).

The ester derivatives which are represented by the following general formula:

in which R is an alkyl chain or a substituted phenyl group, which are also described in the abovementioned Swiss patent No (Application No 7543/79 of 17th August 1979) are however exceptions, as well as the ester derivatives represented by the general formula (I). The latter are therefore of great practical interest, especially because they are relatively easy to synthesize.

The invention will now be illustrated by means of an example of a compound of formula (I), the studied properties of which are set forth in the Table hereinafter. More particularly, the solubility and the order parameter of such compounds have been measured, using the following known methods.

SOLUBILITY A saturated solution of the compound (I) is prepared in a liquid crystal, and the resulting solution is filtered. Then 100,us of the filtrate are taken off and dissolved in 50 cc of chloroform.

The optical density of each solution is then measured spectroscopically and the concentration of the colouring agent at saturation is deduced from that measurement, the absorption coefficient in cholorform being known.

The results obtained are set forth in the following Table. With regard to the value of solubility, it should be noted that the solution was considered as being saturated when it contained grains of colouring agent after heating in the isotropic phase and agitation for 3 minutes.

ORDER PARAMETER (S) A 0.5% solution of the compund studied, in a liquid crystal, is introduced into a 30 8 thick glass cell which is covered with an alignment film (SiOx evaporated tangentially in rubbed polyimide). The optical density of the solution is then measured at its maximum absorption polarised light respectively when the direction of the polarising means is parallel (D//) and perpendicular (D1 ) to the alignment direction. The order parameter is calculated from the following relationship: r- 1 S r+2 in which r is the dichroic ratio corresponding to the ratio DI The result obtained is also set forth in the following Table.

TABLE Properties of the compound of formula (1)

h cO ...

Po * EE~I ro o co co Order parameter O 605 aD I 261-262,7 0,6 : > G 641 0,65 0,68 l,0, O O O O O, .Hn Q Qg o ,o oz CoO cvi E CQ 9 E t N.B.The liquid crystal (LC) in which solubility, nematic-isotropic transition temperature (TN-1) and order parameter have been measured is "E7" of BDH Chemicals Limited (mixture of cyanobiphenyl and cyanoterphenyl with positve dielectric anisotropy), except for the values accompanied by an asterisk (*), which designate measurements taken in "Lixon EN 17 H" of Chisso Corp. (mixture of ester and Schiff base with negative dielectric anisotropy).

The compounds of formula (I) which can be used as colouring agents in solution in liquid crystals for display devices can be prepared by the known methods used for synthesizing the anthraquinone compounds.

In the composition of the invention, it is possible to use either a single type of liquid crystal or, preferably, a mixture of liquid crystals such as those delivered by BDH Chemicals Limited under the references E3, E7, E8 or E9, the respective compositions of which are disclosed in published European patent application No 78 300 487.2 or by Chisso Corp. under the references "EN 1 7" or "EN 1 8" (see Takashi Innukai, J.E.E. sepa.1979, page 84). These are preferably nematic liquid crystals with positive or negative anisotropy, which may or may not also contain an optically active agent.

In addition, the composition of the invention may comprise a plurality of colouring agents of formula (I) or possibly other types of anthraquinone colouring agents, and also a cholesterisation agent and/or an alignment agent. As an example, the cholesteric agent may be that sold by BDH Chemicals Limited under the reference "CB 1 5", (about 3% with respect of the total composition). The alignment agent (intended to disperse the molecules of the liquid crystals perpendicularly to the walls of the electro-optical device) may be a surfactant, for example a sorbitol monoester, sorbitol monolaurate being particularly suitable (about 2% with respect to the total composition).

Finally, the pleochroic colouring agent of formula (I) is generally present in the composition according to the invention in an amount corresponding to about 0.5 to 5% of the total composition, and preferably about 1.5%.

It will be appreciated that the composition according to the invention may contain a plurality of compounds respresented by formula (I), and one or more anthraquinone colouring agents of a different formula.

Claims

1. A liquid crystal-base composition containing a pleochroic colouring agent for use in an electrooptical device, wherein said colouring agent is an anthraquinone compound represented by the general formula (I):

in which R1 is an amino group which can be alkylated or acylated or a substituted or unsubstituted hydroxy group:R2, R3 and R4 which are identical or different are each a hydrogen atom, an amino group which may be alkylated or acylated, a substituted or unsubstituted hydroxy group, a nitro group, a halogen atom or a cyano group; X is a -COOP5 group or a --OOCR, group in which P5 is a cyclic, branched or straight alkyl chain, an alkoxyalkyl chain, a cyanoalkyl chain, the alkyl residue having at most 16 carbon atoms, or a group

2. A composition according to claim 1 wherein Y is in position 6 of the anthraquinone ring when X is substituted in position 2 and in position 7 when X is substituted in position 3.

3. A composition according to claim 1 or claim 2 wherein the anthraquinone base structure which is substituted in position 2 or 2 and 6/7 or 3 or 3 and 6/7 is selected from the group comprising 1 aminoanthraquinone, 1-4 diaminoanthraquinone, 1 -5-diaminoanthraquinone, 1,8- diaminoanthraquinone, 1 -amino-4-hydroxyanthraquinone, 1 ,8-diamino-4,5-dihydroxyanthraquinone, 1 ,4-diamino-5-nitroanthraquinone, 1 ,4,5,8-tetraminoanthraquinone, 1 ,5-diamino-4,8- dihydroxyanthraquinone and 1 -amino-4-nitroanthraquinone.

4. A composition according to claim 1 wherein the colouring agent is represented by the formula:

5. A composition according to claim 1 comprising one or more nematic liquid crystals with positive or negative anisotropy.

6. A composition according to claim 5 comprising an optically active agent inducing a cholesteric structure and/or an alignment agent.

7. A composition according to claim 1 comprising a solution of from 0.5 to 5% of an anthraquinone colouring agent of formula (I).

8. A composition according to claims 6 and 7 comprising about 0.5% of an anthraquinone colouring agent of formula (I), about 3% of an opitcally active agent inducing a cholesteric structure and about 2% of an alignment agent.

9. A composition according to claim 1 at least one other anthraquinone colouring agent of formula (I).

10. A composition according to claim 1 comprising at least one other anthraquinone colouring agent different from those represented by formula (I).