GB2098987A

GB2098987A - Liquid crystal diesters containing at least two phenyl groups

Info

Publication number: GB2098987A
Application number: GB8213579A
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 and R2 are alkyl groups, E is fluorine or hydrogen, F is fluorine and A is a 1,4-disubstituted benzene ring, a trans-1,4-disubstituted cyclohexane ring or a 2,6-disubstituted naphthalene ring) may be used in liquid crystal mixtures in two frequency switching effect electro- optical devices.

Description

SPECIFICATION Liquid crystal fluoro 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 0A). 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 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 8130622.

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

Formula (I) where R1 and R2 are alkyl groups, E is fluorine or hydrogen, F is fluorine 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 R1 and R2 are n-alkyl groups having between 1 and 10 carbon atoms.

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 Comppunds which do not normally exhibit a liquid crystal phase but which nevertheless usefully effect 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, e.g. 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, e.g. fluorine. The 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, e.q. 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'-biphenyl group

or a 2,6-naphthyl group

and Y, is CN, or R' or CO.O--XX-Y' 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 the electrodes on the respective substrate inner surfaces 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 preparation of the compounds of formula (I) above will now be given.

The preferred general route is as follows: Route 1

where R1, F, E and R2 are as defined above.

Examples of the steps in this Route are given below.

Availability of materials used in Route 1 a. Phenols 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(1972).

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

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. 2-fluoro-4-hydroxybenzoic acid

This may be prepared by Route 2 as follows: Route 2

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, 1 6 (1970).

To a cooled solution of 3-fluorophenol 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-3-fluorophenol is prepared from the 4-bromo-3-fluorophenol produced in Step A2 by the standard cyanation procedure using copper (I) cyanide and N-methylpyrrolidone as described in US Patent No 4,149,41 3.

Step C2 The crude product of Step B2 is hydrolysed for 48 hours with 1 0% aqueous sodium hydroxide.

After acidification and ether extraction the residue obtained is recrystallised from water or dilute acetic acid.

2-fluoro-4-hydroxybenzoic acid, the product, has a melting point of 207-211 OC.

Reverting to the main route, Route 1, the steps involved are as follows: Steps involved in Route 1 Step A 1 This esterification is carried out in a known way (Lowrance, Tetrahedron Lett 3453 (1971)) by dissolving the phenol

and the hydroxy acid

(prepared as in Route 2) in a solvent, e.g. 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 B 1 This esterification is carried out in a known way. The acid R1--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 O-50C. The product of 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 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 Tables 1-4 as follows: TABLE 1 Compounds of the form

C-N or C-SA SAN N-I R1 R2 (OC) (OC) (OC) n-c3H7 C2H5 58 ~ 185 n-C4Hg C2H5 68 86 180 n-C5H11 C2H5 58 106 181 CH3 n-C3H7 100 156 C2H5 n-C3H7 56 - 168 n-C3H7 n-C3H7 57 187 n-C4Hg n-C3H7 52 85 184 n-C5H11 n-C3H7 49 114 185 n-C3H7 n-CsH" 54 (53) 182 n-CsH" n-C5H11 47 128 179 TABLE 2 Compounds of the form::

C-N N-I R1, R2 ( C) ( C) n-C3H7 C2H5 89 178 n-C3H7 n-C3H7 90 182 n-C3H11 n-C3H7 78 170 n-C3H7 n-C,H11 62 175 n-C5H11 n-C5H11 53 164 TABLE 3 Compounds of the form:

C-N N-I R1, R2 ( C) ( C) n-C3H7 n-C3H7 68 174 n-C3H7 n-C5H11 51 165 n-C5H11 n-5H11 38 161 TABLE 4 Compounds of the form::

C-N N-I R, R2 ( C) ( C) n-C3H7 n-C3H7 86 166 n-C5H11 n-C5H11 49 149 In Tables 1 to 4 the following symbols are used: C-N = Crystal to nematic liquid crystal transition temperature C-SA = Crystal to smectic A liquid crystal transition temperature SA-N = Smectic A to nematic transition temperature N-I = nematic to isotropic liquid transition temperature brackets around a temperature indicate a monotropic transition at that temperature.

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:

Percentage Compound by weight 0 n-C5H11iC00 COO COO C5H11-n 35 Component A i \F "- CgHll - / \COO -I \ I \ / > o- COO -o CgHll-" 35 Component B n -C3H7 ffTh0C4H9-n 10 CH3 XC00 X c5H11-n 10 high clearing point n-C5H1f+C3H7-n ^ Q CH- 10 additive 37fl

Claims

1. A liquid crystal compound having a formula:

Formula (I) where1 and R2 are alkyl groups, E is fluorine or hydrogen, F is fluorine 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 wherein the compound has a formula:

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

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

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

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

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

8. A compound as claimed in any one of claims 1 to 7 and wherein R1 and R2 are n-alkyl groups having between 1 and 10 carbon atoms inclusive.

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

10. 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 A material as claimed in claim 10 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.

12. A material as claimed in claim 11 and which additionally includes compounds selected from diester compounds other than those of Formula (I) and high clearing point materials.

13. A two frequency switching effect liquid crystal electro-optical device incorporating as its liquid crystal material a material as claimed in claim 11 or claim 12.