GB2111974A

GB2111974A - Liquid crystal chlorophenyl arylcarbonyloxybenzoates

Info

Publication number: GB2111974A
Application number: GB08130742A
Authority: GB
Inventors: George William Gray; Dr Alan Mosley
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1980-10-13
Filing date: 1981-10-12
Publication date: 1983-07-13
Also published as: GB2111974B

Abstract

Liquid crystal compounds having the formula: <IMAGE> where R1 is an alkyl group, X is a methyl group or chlorine, and A is a 1,4-disubstituted benzene ring, a trans-1,4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring are suitable for use in or as the diester component in two frequency switching material compositions.

Description

SPECIFICATION Liquid crystal diester compounds and materials containing them 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 re-orientated by the field causing a change in an optical property of the part of the film where the field is applied, eg 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 a copending UK Patent Application. In that copending Application it is described how to make a liquid crystal material suitable for use in two frequency switching devices by forming a mixture composition of: a a first component A having a positive dielectric anisotropy 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 that 10 kHz at 20"C.

Preferably the viscosity of the composition is less than 100 cps at 20"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 the copending Application.

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

Formula (I) where R, is an alkyl group, X is a methyl group or chlorine, and A is a 1 ,4-disubstituted benzene ring, a trans-1 ,4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring.

Examples of families of compounds of formula (I) are as follows:

Preferably R, is CH3, C2H5 or n-CmH2m+, where mis between 3 and 10.

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 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 0 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 it to 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 a bicyclo(2.2.2)octane ring, X is a 1,4-phenylene group

or a 4,4'-biphenylyl group,

or a 2,6-naphthyl group

and Y, is CN, or R' or CO.O-X-Y' where Y' 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 surfaces to cause the dielectric anisotropy of the liquid crystal metarial to be switched between states giving different optical response.

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

The preferred general route is as follows: Route 1

HOXCOOH + HO eCI x rSTEP A1 H OCOOCt x + R1 -A-COOH

|5TEP B1 R1 -A--COOO-COO- I x Availability of materials used in Route 1 a. The phenol

This compound 4-chlorophenol 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. Hydroxybenzoic acids

2-chloro-4-hydroxybenzoic acid is commercially available.

2-methyl-4-hydroxybenzoic acid may be prepared by Route 2 as follows: Route 2

HO EH3 Br sThPQ2 HN SThPA2 HO- Ho < B STEP B2 HO-- -CN XSTEP U HO < QDOH CH3 Steps involved in Route 2 Step A2 This involves the fast bromination procedure described by O'Bara, Balsley and Starter in J.

Org. Chem 35,16(1970).

To a cooled solution of 3-methylphenol in chloroform is added a solution of bromine in chloroform followed by a solution of sodium hydroxide in water. 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.

Step B2 4-cyano-e-methylphenol is prepared from the 4-bromo-3-methylphenol produced in Step A2 by the standard cyanation procedure using copper(l) cyanide and N-methylpyrrolidone as described in US Patent No 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 is recrystallised from water or dilute acetic acid.

2-methyl-4-hydroxybenzoic acid, the product, has a melting point of 177-178"C.

Reverting to the main route, Route 1, the steps involved are as follows: Steps involved in Route 1 Step Al This esterification is carried out in a known way by dissolving the phenol

and the acid

in a solvent and heating them. For example the method described by Lowrance (Tetrahedron Lett 3453 (1971)) is used.

For example, the hydroxy-ester

is prepared as follows: 2-chloro-4-hydroxybenzoic acid and 4-chlorophenol in a molar ratio of 1:1.23 are dissolved in toluene and heated in a Dean and Stark apparatus according to the method referred to above together with sulphuric acid and boric acid as catalysts. After isolation and crystallisation of the product from aqueous ethanol the melting point of the hydroxyester is found to be 157-160"C.

Another hydroxy-ester prepared in this way is:

Step B1 This esterification is preferably carried out in a known way. The acid R,-A-COOH is first converted into its chloride R,-A-COCI using an excess of thionyl chloride. After removal of excess of thionyl chloride by distillation the acid chloride is mixed with dry pyridine (40 ml) and cooled to 0-5"C. The product from Step Al is then added and the solution stirred at room temperature (20-25"C) 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 di-ester is purified by column chromatography on silica gel using chloroform or chloroform/petroleum ether (boiling point 40-60 C) (1.5:1) mixture as eluant.The product is isolated and crystallised from ethanol.

For example the diester n-C5H"

is made by this method using the acid

and the hydroxy-ester

This has the following properties C-N (crystalline solid to nematic liquid crystal transition) = 75"C and N-l (clearing point or nematic to isotropic liquid transition) = 1 62 C.

Further examples of compounds which are products of Step B1 are:

C-N 137'C; N-l=210'C

C-N = 78.5"C; N-l = 1 50 C An example of a liquid crystal material (suitable for use in two frequency devices) in which the compounds of formula (I) are used is as follows:

Percentage Compound by weight n-4Hll-COO-COO H3 Component A nCsH11{+ COtCOO+Cl 35 3 mm 0 C4 n 10 Component B CH3- COO- 10 high clearing 1 0 point additive n C5H11 < > < 3H < n n--C5H110 >

Claims

1. A liquid crystal compound having a formula:

Formula (I) where R, is an alkyl group, X is a methyl group or chlorine, and A is a 1,4-disubstituted benzene ring, a trans-1 ,4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring.

2. A compound as claimed in claim 1 and which has a formula:

3. A compound as claimed in claim 1 and which has a formula:

4. A compound as claimed in claim 1 and which has a formula:

5. A compound as claimed in claim 1 and which has a formula:

6. A compound as claimed in claim 1 and which has a formula:

7. A compound as claimed in claim 1 and which has a formula:

8. A compound as claimed in any one of the preceding claims and wherein R, is CH3, C2H5 or n-CmH2m+" where m is between 3 and 10 inclusive.

9. A compound as claimed in claim 1 and which is any one of the specific compounds mentioned hereinbefore.

10. A compound as claimed in any one of the preceding claims and which is prepared substantially by one of the methods described herein.

11. A liquid crystal material which is a mixture of compounds comprising at least one compound as claimed in any one of the preceding claims.

1 2. A material as claimed in claim 11 and wherein the mixture is substantially the same as the specific 5-component mixture specified herein.