GB2098986A

GB2098986A - Liquid crystal diesters containing at least two phenyl groups

Info

Publication number: GB2098986A
Application number: GB8213577A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-05-12
Filing date: 1982-05-11
Publication date: 1982-12-01

Abstract

A liquid crystal compound having the formula: <IMAGE> (where R1 is an alkyl group, A is a 1,4- disubstituted benzene ring, a trans- 1,4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring, where E is fluorine and Z is an alkyl group or where E is hydrogen and Z is an alkyl or cyano group or chlorine and wherein of the three groups W, X, Y two are chlorine or two are methyl groups and the third is hydrogen) may be used in liquid crystal mixtures in two frequency switching effect electro-optical devices.

Description

SPECIFICATION Liquid crystal diesters containing at least two phenyl groups The present invention is related to liquid crystal diester compounds, and materials and devices containing them.

Liquid crystal materials are well known organic materials comprising single compounds or mixtures of compounds which display phases, known as liquid crystal phases or mesophases, having a degree of molecular ordering intermediate between that of the fully ordered crystalline solid state and the fully disordered isotropic liquid state.

Electro-optical devices incorporating liquid crystal materials are well known and widely used as digital displays in such applications as watches, calculators and digital voltmeters. 'These devices utilise the optical contrast when an electric field is applied across a thin insulating film of suitable liquid crystal material. The molecules of the material (in a liquid crystal phase at the temperature of operation) are reorientated by the field causing a change in an optical property of the part of the film where the field is applied, e.g. a change in ambient light scattering or transmissivity.

There are several known types of electro-optical effect which are utilised in liquid crystal devices.

One is known as 'two frequency switching' and devices utilising this effect are known as two frequency switching devices.

This effect is further described in UK Patent Application No 8130622 (publication No 208591 OA). In that copending Application it is described how to make a liquid crystal mateial suitable for use in two frequency switching devices by forming a mixture composition of: i a first component A having a positive dielectric anisdtropy at low frequencies and at room temperature comprising one or more compounds containing at least three ring structures linked directly or by linkage groups to give the molecular structure a relatively rigid elongated form; ii a second component B having a low viscosity comprising one or more compounds having at low frequencies a dielectric anisotropy which is either negative, or positive with a magnitude less than one;; the composition being such that its cross-over frequency fo is not greater than 10 kHz at 200 C.

Preferably the viscosity of the composition is less than 100 cps at 200 C.

'Cross-over frequency' is the frequency of electrical signals applied to a liquid crystal material at which the dielectric anisotropy of the material changes from positive to negative.

The present invention is concerned with novel compounds suitable for use in or as the component A in the compositions described in copending Application No 8032944.

According to the present invention in a first aspect there is provided a liquid crystal compound having a formula:

formula (I) where R1 is an alkyl group, A is a 1 4-disubstituted benzene ring, a trans-i 4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring, where E is fluorine and Z is an alkyl group or where E is hydrogen and Z is an alkyl or cyano group or chlorine and wherein of the three groups W, X, Y two are chlorine or two are methyl groups and the third is hydrogen.

Examples of families of compounds of formula (I) are as follows (where R = n-alkyl and is Z' is

Z being as defined above).

By a 'liquid crystal compound' is meant a compound in one of the following two known categories: Compounds which normally exhibit a liquid crystal phase; ii Compounds which do not normally exhibit a liquid crystal phase but which nevertheless usefully affect some aspect of liquid crystal behaviour when dissolved in other liquid crystal compounds Compounds in category (ii) show a 'monotropic' or a 'virtual' liquid crystal to isotropic liquid transition at a temperature below the melting point of their solid phase.The monotropic or virtual transition may be detected respectively by rapid cooling of the liquid phase or by dissolving the compound in a material exhibiting a liquid crystal phase, observing the change in the transition to the isotropic liquid phase of the material by the addition and calculating the virtual transition temperature by extrapolation.

Compounds in category (ii) might for example be usefully dissolved in other liquid crystal compounds to extend or vary the liquid crystal temperature ranges of the compounds or to vary the molecular helical pitch (in the case of cholesteric liquid crystals).

According to the present invention in a second aspect there is provided a liquid crystal material which is a mixture of compounds comprising at least one compound having a formula (I) above.

In the second aspect one or more of the compounds of formula (I) may in particular be mixed together with a low viscosity liquid crystal material to form mixtures as described in UK Patent Application No 8032944, eg with compounds of the form

or of the form

where R and R' are alkyl groups, n is O or 1, and Ar is a benzene ring which may or may not contain a lateral substituent, eg fluorine. These mixtures may optionally contain additional (other) diester compounds and other additives such as materials having a high clearing point These mixtures are suitable for use in two frequency switching devices.

It will be apparent to those skilled in the art that where mixtures are formed as in the second aspect these mixtures can have the value and sign of their dielectric anisotropy controlled as required by control of the proportions of the materials blended together to form them.

Where a material is added to one or more compounds according to formula (I) the material may itself be a mixture of two or more compounds.

Mixtures may be formed in a known way, eg simply by heating the constituent compounds to form an overall isotropic liquid, stirring the liquid and allowing itto cool.

To provide more general examples of a mixture according to the second aspect at least one compound according to formula (I) above may be mixed together with one or more compounds in the following known families for use in one or more of the applications given above (the actual application(s) depending on the mixture's properties):

where

is a cyclohexane ring,

is bicyclo(2.2.2)octane fing, X is a 1,4-phenylene group

or a 4,4'-biphenylyl group,

or a 2,6-naphthyl group

and Yr is CN, or R' or CO.O-X-Y1 where Y1 is CN, or R' or OR'; where R and R' are alkyl groups.

According to the present invention in a third aspect a liquid crystal device includes two dielectric substrates at least one of which is optically transparent, a layer of liquid crystal material sandwiched between the substrates and electrodes on the inner surfaces of the substrates to enable an electric field to be applied across the layer of liquid crystal material to provide an electro-optic effect therein, characterised in that the liquid crystal material consists of or includes a compound according to formula (I) above.

The device according to the third aspect is preferably a two-frequency switching effect device constructed in a known manner.

Such a device includes means for switching the frequency of electric fields applied between electrodes on the respective substrate inner surface to cause the dielectric anisotropy of the liquid crystal material to be switched between states giving different optical response.

An example of a method of the preparation of the compound of formula (I) above will now be given.

The preferred general route is Route 1 as follows:

W HOXCOOH + HO-ZI STEP Al R1-A-COOH + HO+COO ZI )0 ZI STEP B1 R1-A-COO9COO zl This is described further below.

Availability of starting materials for Route 1 a. Phenols HO--Z' 4-chlorophenol is commercially available 4-alkylphenols are either commercially available or can be prepared by the standard method described by Van der Veen, de Jeu, Grobben and Boven in Mol. Cryst. Liq. Cryst. 17, 291 (1 972).

4-alkyl-2-fluorophenols may be prepared as described in UK Patent Application No 8031248.

4-cyanophenol is commercially available.

b. Acids R,A--COOH 6-alkyl-2-naphthoic acids may be prepared as described in UK Patent Application No 33861/76.

4-alkylbenzoic acids are commercially available.

Trans-4-n-alkylcyclohexane-1-carboxylic acids are commercially available.

c. Hydrnxybenzoic acids These may be prepared by Route 2 as follows:

STEPA2 WSTEPB2 W STEP C2 W HOA HOXBrHO+CNs HOXCOOH X Y X Y X Y X Y 2,3-Dichloro-4-hydroxybenzoic acid may be prepared from 2,3-dichlorophenol by using carbon tetrachloride in a Reimer-Tiemann reaction (Ger 2743469 (1977/78)).

An example of the application of this Route to the preparation of

will now be given.

Step A2 This step involves preparing 4-bromo-2,5-dimethylphenol by the fast bromination procedure described by O'Bara, Balsley and Starter, J. Org. Chem. 35, 16 (1970). To a cooled solution of commercially available 2,5-dimethylphenol (e1 g, 0.5 mol) in chloroform (300 ml) is added a solution of bromine (80g, 0.5 mol) in chloroform (250 ml), followed by a solution of sodium hydroxide (609) in water (300 ml). The reaction mixture is transferred to a separating funnel and shaken; then the aqueous layer is removed and acidified with concentrated hydrochloric acid. The organic product is extracted into ether and the residue obtained by removal of the ether by evaporation is recrystallised from cyclohexane. The 4-bromo-2,5-dimethylphenol has a melting point of 8 7--89 OC.

Step B2 4-cyano-2,5-dimethylphenol may be prepared by the standard cyanation procedure using copper(l) cyanide and N-methyl-pyrrolidone described in US Patent 4,149,413.

Step C2 The crude product of Step B2 is hydrolysed for 48 hours with 10% aqueous sodium hydroxide.

After acidification and ether extraction, the residue obtained by removal of the ether by evaporation is recrystallised from water or dilute acetic acid. This product, 2,5-dimethyl-4-hydroxybenzoic acid, has a melting point of 183-1 840C.

2,3- and 3,5-dimethyl-4-hydroxybenzoic acids may be made analogously starting from commercially available 2,3- and 2,6-dimethylphenols respectively. These products have melting points of 167-1690C and 220-221 0C respectively.

Reverting to the main diester synthesis Route 1 shown above, the steps Al and B1 in that Route are preferably carried out as follows: Step Al This esterification is carried out in a known way (Lowrance, Tetrahedron Lett 3453 (1971)) by dissolving the phenol HO--Z and the hydroxy acid

(prepared as in Step A2) in a solvent, eg toluene, and heating them in a Dean and Stark apparatus together with sulphuric acid and boric acid catalysts. The product is isolated and crystallised from aqueous ethanol.

Step B1 This esterification is carried out in a known way. The acid R,--AA-COOH is first converted into its chloride using an excess of thionyl chloride. After removal of excess of thionyl chloride by distillation the acid chloride is mixed with dry pyridine and cooled to 0--50C. The product of Step Al is then added and the solution stirred at room temperature (20--250C) for 24 hours. The reaction mixture is protected from atmospheric moisture during this time using a calcium chloride guard tube. The mixture is then rotary evaporated to remove pyridine. The residual diester product is purified by column chromatography on silica gel using chloroform or chloroform/petroleum ether as eluant. The product is isolated and crystallised from ethanol.

Examples of products of Step B1 are given in Table 1 as follows: TABLE 1 Compounds of the form

where two of the groups W, X and Y are Me (Methyl).

R A W X Y Z C-N C SA SA N N I ( C) (etc) ( C) ( C) Compound 1 n-C5H11 e Me H Me n-C6Hs3 74 128.5 Compound 2 n-C5H,l X Me H Me n-C6H,3 56.5 62.6 Compound 3 n-C5H11 H Me Me n-C6H13 75 130 Compound 4 n-C,H,, Me H Me n-C6H,3 63.4 90.3 Compound 5 n-CsH" H H Me Me n-C6H,3 63 69.5 151.5 Compound 6 n-CsH,l &commat; Me Me H n-C6H,3 95 126 where C-N - crystalline solid to nematic liquid crystal transition temperature.

C-SA - crystalline solid to smectic A liquid crystal transition temperature.

SA-N - smectic A to nematic liquid crystal transition temperature.

N-I - nematic to isotropic liquid transition temperature.

TABLE 2 Compounds of the form

where two of the groups W, X and Y and Cl.

R A W X Y Z C-N N-I ( C) (C) Compound 7 n-C5H" F H Cl CI n-C5H,l 97 145 An example of a liquid crystal material (suitable for use in two frequency switching devices) in which the compounds of formula (I) are used is as follows: :

Compound Percentage by weight t n C5H11 COO COO-oo 4 C6H13-n 35 (Component A) | n-C5H1d}COOCOOC6H13 n 35 o- C3H1HmHQOC4lI9 -n 10 (Component B) CH3COO C5H11-n 10 high clearing n-C5Ht ) +C3H7 n 10 point additive

Claims

1. A liquid crystal compound having a formula:

Formuia (I) where R, is an alkyl group, A is a 1,4-disubstituted benzene ring, a trans-1,4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring, where E is fluorine and Z is an alkyl group or where E is hydrogen and Z is an alkyl or cyano group or chlorine and wherein of the three groups W, X, Y two are chlorine or two are methyl groups and the third is hydrogen.

2. A compound as claimed in claim 1 and which is any one of the particular compounds specified herein.

3. A liquid crystal material which is a mixture of compounds comprising at least one compound as claimed in claim 1 or claim 2.

4. A material as claimed in claim 3 and which is suitable for use in a two frequency switching effect device the mixture also including one or more compounds of the formula:

or of the formula

where R and R' are alkyl groups, n is O or 1, and Ar is a benzene ring optionally carrying a lateral substituent.

5. A material as claimed in claim 4, and which additionally includes compounds selected from diester compounds other than those of formula (i) and high clearing point materials.

6. A two frequency switching effect liquid crystal electro-optical device incorporating as its liquid crystal material a material as claimed in claim 4 or claim 5.