GB2121406A

GB2121406A - Liquid crystal cyanophenylethanes

Info

Publication number: GB2121406A
Application number: GB08312946A
Authority: GB
Inventors: Neil Carr; George William Gray; Damien Gerald Mcdonnell
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1982-05-11
Filing date: 1983-05-11
Publication date: 1983-12-21
Also published as: GB2121406B; GB8312946D0

Abstract

Liquid crystal ethane compounds having a positive dielectric anisotropy are represented by the general formula (I) as follows: <IMAGE> wherein A is a substituted ring structure which includes a terminal alkyl group, A being selected from: <IMAGE> where R is an alkyl group, and the cyclohexane ring is trans-, <IMAGE> where R is an alkyl group, and <IMAGE> where R1- is an alkyl group R-, an group RO- an alkylcarbonyloxy group RCO, O- or an alkoxycarbonyloxy group; and wherein X is either hydrogen or fluorine, provided that if X is hydrogen A is <IMAGE> These compounds may be prepared by routes involving known procedures, eg Friedel-Crafts acylation followed by reduction.

Description

SPECIFICATION Ethane compounds The present invention relates to liquid crystal ethane compounds exhibiting a positive dielectric anisotropy and to methods of their production.

The use of liquid crystal materials to exhibit electro-optic effects in display devices such as digital calculators or watches is now well-known. One of the parameters of a liquid crystal material which is important in relation to electro-optical operation is the dielectric anisotropy (AE) of the material. This is the difference, for a given frequency and temperature, between the average dielectric constant measured parallel (E") and perpendicular (E1) to the molecules.

The sign of the dielectric anisotropy of a given liquid crystal material is one of the major parameters which determine the kinds of electro-optic device in which that material may be used.

For example, materials having a positive dielectric anisotropy may, depending on their chirality or twist in their molecular arrangement, be used in known twisted nematic effect devices, Fréedericksz effect devices or cholesteric-to-nematic phase change effect devices. Such materials normally consist of a mixture of compounds at least one of which is a liquid crystal compound of high positive dielectric anisotropy.

According to the present invention there is provided a liquid crystal compound having a positive dielectric anisotropy which is of Formula (I) as follows:

Formula (I) wherein A is a substituted ring structure which includes a terminal alkyl group, A being selected from:

where R is an alkyl group and

is a trans-1,4-disubstituted cyclohexane ring;

where R is an alkyl group and

is a 1 ,4-disubstituted bicyclo(2,2,2)octane ring; and

where

is a 1 ,4-disubstituted benzene ring and R1- is an alkyl group R-, an alkoxy group ROan an alkyl- carbonyloxy group RCO.O- or an alkoxycarbonyioxy group ROCO.O-; and wherein X is either hydrogen or fluorine, provided that if X is hydrogen A is

The following are examples of the ethyl bridged cyanoterminated structures of Formula (I)::

Formula (II)

Formula (III) Formula (IV) Formula (V) Formula (VI) Formula (VII) Formula (VIII) Formula (IX) Formula-(X) Formula (Xl) Formula (Xla) Compounds of Formura (I) are generally speaking liquid crystal compounds having a low melting point (less than 1 000C). Where A in Formula (I) includes an alicyclic ring they have in certain cases a relatively low birefringence, particularly suitable for use in cholesteric-to-nematic phase change effect devices, and in certain cases a relatively low viscosity (which can lead to reasonably short response times in electro-optical devices).

Compounds having Formula (I) may be obtained by routes in which the individual procedures involved are known (the overall routes being new).

For example the following routes may be used:

(2) is used when A is

R' being alkyl, alkylcarbonyl or alkoxycarbonyl.

Preferably, R' or R is n-alkyl having from 1 to 8 carbon atoms inclusive.

By a 'liquid crystal compound' is meant a compound in one of the following two known categories: (i) 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 is 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 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).

One or more of compounds according to formula (I) may be used in any of the following applications (where material having a positive dielectric anisotropy is referred to as 'positive' material): (i) together with other positive nematic material giving an overall positive nematic material for use in twisted nematic effect devices particularly multiplexed devices; an example of such a device is given below; (ii) together with other nematic material to give an overall positive nematic material, preferably also with a pleochroic dye, in Fréedericksz effect devices in which the molecular arrangement may be changed from an aligned homogeneous texture (OFF state) to a homeotropic texture by an electric field; (iii) together with an optically active material giving an overall positive material which is a cholesteric, preferably together also with a pleochroic dye, in cholesteric-to-nematic phase change effect devices in which the molecular arrangement may be changed from a scattering focal conic texture (OFF state) to a clear homeotropic texture (ON state) by an electric field;; (iv) together with a positive nematic material giving an overall positive nematic material in two frequency switching effect devices in which the dielectric anisotropy of the material may be changed from (at low frequency) positive (OFF state) to negative (ON state) by the application of a high frequency electric field.

It will be apparent to those skilled in the art that in the above applications where mixtures are formed these mixtures can have their properties, eg dielectric anisotropy and liquid crystal temperature ranges, controlled as required by control of the proportions of the materials blended together to form them.

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 aliowing it to cool.

According to the present invention in a second aspect there is provided a mixture of liquid crystal compounds including at least one compound according to Formula (I) above. Such a mixture may, depending on its selected composition and properties, be used in any one of the applications (i) to (iv) above.

An example of a mixture according to the second aspect which may be used in twisted nematic devices is one including, in addition to at least one compound according to Formula (I) above together with one or more two ringed cyano compounds selected from the compounds having the following formulae:

Formula (la) Formula (ib) Formula (inc) Formula (id) Formula (le) where each R is independently n-alkyl, preferably containing up to 7 carbon atoms, and RA is n-alkyl or n-alkoxy, preferably containing up to 7 carbon atoms. These are generally low melting point, high positive dielectric anisotropy materials. Preferably, the compound(s) of Formula (I) form up to 25% by weight of the mixture with compound(s) of Formula (la) to (le).

Preferably, the positive materials of Formula (I), together with any selected from Formulae (la) to (le), are added to one or more compounds selected from the following high clearing point compounds to extend the liquid crystalline temperature range of the mixture:

Formula (XII) Formula (XIII) Formula (XIV) Formula (XV) Formula (XVI) Formula (XVII)

Formula (XVIII) Formula (XIX) Formula (XX) For multiplexed twisted nematic devices one or more compounds having a low or negative dielectric an isotropy are preferably added also, eg one of the compounds having the following formulae::

Formula (XXI) Formula (XXII) Formula (XXIII) Formula (XXIV) Formula (XXV) Formula (XXVI) Formula (XXVII) or an analogue of one or more of the compounds of formulae (XXI) to (XXVII) in which the 2 or 3 position of the right hand 1 ,4-disubstituted benzene ring as shown carries a fluorine substituent, R=alkyl, RA=alkyl or alkoxy.

Preferably the compound(s) according to Formula (I) together with any compounds selected from Formulae (la) to (le) constitute between about 10 and 90% desirably between 30 and 70%, by weight of the mixture and the compounds selected from Formulae (XII) to (XX) constitute not more than about 35% by weight in total, the remainder being one or more compounds according to Formulae (XXI) to (XXVII) which preferably constitute about 40% by weight of the mixture.

Preferably the mixture includes one or more cyanbiphenyls of Formula (la) and preferably each such compound incorporated in the mixture has five or less carbon atoms in its alkoxy or alkyl group.

Preferably this number of carbon atoms is odd when the group is an alkoxy group and is even when the group is an alkyl group.

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 trans-l ,4-disubstituted cyclohexane ring,

is a 1 4-disubstituted 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 OR1 or CO.O--XX-Y' where Y1 is CN, or R1 or OR'; the definition of R1 being the same as that of R.

Preferably, the compound(s) according to Formula (I) comprises between 5 and 80%, preferabíy not more than 30%, by weight of the mixture.

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 may be a twisted nematic effect device, which may or may not be operated in a multiplexed fashion, a cholesteric-to-nematic phase change effect device, a Fréedericksz effect device or a two-frequency switching effect device all constructed in a known manner. The various ways in which compounds according to formula (I) may be used in these devices are outlined above and will be further apparent to those skilled in the art.

Example 1 This example involves the preparation of 1 -(4'-alkylphenyl)-2-(4"-cyanophenyl)ethanes and 1 (4'-alkylphenyl)-2-(4"-cyano-2"-fluorophenyl)ethanes by the following route:

R C02H STEP Al ~ R R+CH2OH SIEP 81 R+CH2.CN STEP Cl STEP SI RCH2Ct STEP Ol STEP El R+CH2-C02H STEP E1, ROC.COOSr Kr STEP Fl ) < X R CH2 CH2SCN STEP G1 4--CH2 CH29 Sr x where R and X are as defined above.

Step Al The production of 4-alkylbenzyl alcohols from the known 4-alkylbenzoic acids.

This Step may be carried out essentially by the method described in Step Al in published UK Patent Application GB 2070594A (which uses the trans-4-alkylcyclohexane-1-carboxylic acids as starting materials).

An example of such a product is 4-n-pentylbenzyl alcohol, yield 90%, bp (boiling point) 1 050C at 1 mm Hg.

Step B1 The production of 4-alkylbenzyl chlorides from 4-alkylbenzyl alcohols.

The 4-alkylbenzyl alcohol (0.02 mole) is added to a solution of chloroform (60 cm3) and thionyl chloride (2.86 g) and the mixture heated for 90 min. at 8O0C. The solvent is removed in vacuo and the residual oil is taken up in ether (50 cm3). The solution is washed with 10% aqueous sodium bicarbonate (2x30 cm3), dried (Na2SO4), and the solvent removed in vacuo. The residual product is purified by distillation.

An example of such a product is 4-n-pentylbenzyl chloride, yield 95%, bp 1 200C at 2 mm Hg.

Step C1 The production of 4-alkylphenylacetonitriles from 4-alkylbenzyl chlorides.

This Step may be carried out essentially by a standard literature method for the conversion of an alkyl halide to an alkyl cyanide-see for example the method described by A I Vogel in Textbook of Practical Organic Chemistry, 4th Edition, 521(1978).

An example of such a product is 4-n-pentylphenylacetonitrile, yield 89%, bp 1 400C at 3 mm Hg.

Step D1 The production of 4-alkylphenylacetic acids from 4-alkylphenylacetonitriles.

This Step may be carried out by a standard literature method for the hydrolysis of a nitrile to a carboxylic acid-see for example the method described by A I Vogel in Textbook of Practical Organic Chemistry, 4th Edition, 479 (1978).

Step El The production of 4-alkylbenzyl 4-bromophenyl ketones and 4-alkylbenzyl 4-bromo-2-fluorophenyl ketones from 4-alkylphenylacetic acids.

This Step may be carried out essentially by the method for Friedel-Crafts acylation using the appropriate 4-alkylphenylacetic acid prepared in Step D1 (converted to the corresponding acid chloride by conventional treatment, eg using thionyl chloride) and the appropriate 1-bromo-3-fluorobenzene as described in the literature-see for example the article by D Coates and G W Gray in J Chem Soc Perkin 11, 7, 867 1 976 (where the method described uses 4-bromobiphenyl and a 4-n-alkoxyphenylacetic acid chloride as starting materials.) Step F1 The production of 1 -(4'-alkylphenyl)-2-(4"-bromophenyl)ethanes and 1 -(4'-alkylphenyl)-2-(4"- bromo-2"-fluorophenyl)ethanes from 4-alkylbenzyl 4-bromophenyl ketones and 4-alkylbenzyl 4bromo-2-fluorophenyl ketones respectively.

This Step may be carried out essentially by a standard literature method for the reduction of a carbonyl function to a hydrocarbon-see for example the article by D Coates and G W Gray in J. Chem.

Soc. Perkin 11,7,867(1976) (in which the method described uses 4-(48-n-alkoxyphenylacetyl)-4'- bromobiphenyls as starting materials).

Step G1 The production of 1 -(4'-a I kylphenyl)-2-(4"-cyanophenyl)ethanes and 1 -(4'-alkylphenyl)-2-(4"- cyano-2"-fluorophenyl)ethanes from 1 -(4'-alkylphenyl)-2-(4"-bromophenyl)ethanes and 1-(4' a Ikylphenyl)-2-(4"-bromo-2"-fluorophenyl)ethanes respectively.

This Step may be carried out essentially by the method of cyanation described by D Coates and G W Gray in J. Chem. Soc. Perkin II, 7, 867 (1976) (which uses 4-(4'-n-alkoxyphenylethyl)-4'-bromobiphenyls as the starting materials).

An example of such a compound is 1 -(4'-cyanophenyl)-2-(4"-n-pentylphenyl)ethane, yield 63%, mp 620 C, virtual nematic to isotropic liquid transition temperature=-250C.

The viscosity of this compound at 200C by logarithmic extrapolation using 20% of the compound as a solution with the material ZLI 1132 obtained from E Merck Co is 1 7c St.

Example 2 The Example involves the preparation of 1 -(trans-4'-alkylcyclohexyl)-2-(4"-cyano-2"-fluoro- phenyl)ethanes by the following route:

R-Q -C02H STEP A2 ~ R{+CH2-0H STEP 82 R-G CH2CN CN STEP C2 R+}CH2Br RCH2Br | STEP 02 R+CH2-C02H STEP E2 RA}CH2-CO+Br STEP F2 F R{0}CH2 - CH2+CN STEP G2 R {P}CH2 - CH2 9 QCN STEP (32 F F where R is as defined above.

Step A2 The production of trans-4-alkylcyclohexylmethanols from the corresponding known trans-4-alkyl cyclohexane-1-carboxylic acids.

This Step is identical to Step Al described in published UK Patent Application GB 2070594.

An example of such a product is trans-4-n-propylcyclohexylmethanol, yield 89% bp 1 550C at 3 mm Hg.

Step B2 The production of trans-4-alkylcyclohexylmethyl bromides from trans-4-alkyl- cyclohexylmethanols.

This Step is identical to Step B1 described in published UK Patent Application GB 2070594.

An example of such a product is trans-4-propylocyclohexylmethyl bromide, yield 82%, bp 1 550C at 15 mm Hg.

Step C2 The production of trans-4-alkylcyclohexylacetonitri les from trans-4-al kylcyclohexylmethyl bromides.

This Step may be carried out essentially by the method as used in Step C1 above (which uses the 4-alkylbenzyl chlorides as starting material).

Step D2 The production of trans-4-alkylcyclohexylacetic acids from trans-4-alkylcyclohexylacetonitriles.

This Step may be carried out essentially by the method given in Step D1 above which uses the 4alkylphenylacetic acids as starting material.

Examples of such products are trans-4-n-pentyicyclohexylacetic acid, yield 84%, mp 59CC and trans-4-n-hexyicyclohexylacetic acid, yield 82%, mp (melting point) 660C.

Step E2 The production of trans-4-alkylcyclohexylmethyl 4-bromo-2-fluorophenyl ketones from trans-4alkylcyclohexylacetic acids.

This Step may be carried out essentially by the method as in Step El above (which uses 4alkylphenylacetic acids as starting material).

Step F2 The production of 1 -(trans-4'-alkylcyclohexyl)-2-(4"-bromo-2"-fluorophenyl)ethanes from trans 4-alkylcyclohexylmethylbromo-2-fl uorophenyl ketones respectively.

This Step may be carried out essentially by the method given in Step F1 above (which uses 4alkylbenzyl 4-bromo-2-fluorophenyl ketones.

Step G2 The production of 1 -(trans-4'-alkylcyclohexyl)-2-(4"-cyano-2"-fluorophenyl)ethanes from 1 (trans-4'-alkylcyciohexyl)-2-(4"-bromo-2"-fluorophenyl)ethanes.

This Step may be carried out essentially by the method given in Step G 1 above (which uses 1 (4'-alkylphenyl)-2-(4"-bromo-2"-f luorophenyl)ethanes as starting materials).

Example 3 This example involves the preparation of 1-(4'-alkylbicyclo(2',2',2')octyl)-2-(4"-cyano- phenyl)ethanes and 1 -(4'-alkylbicyclo(2' ,2' ,2')octyl)-2-(4"-cyano-2"-fl uorophenyl)ethanes by the following route:

RC02H H STEP A3 R+CH20H STEP 83 /7 STEP CN STEP C3 R--f-CH2CN STEP 03 'CH2. STEP E3 +CH2 CO+Br STEP F3 X R vCH2-CH29Br R-2 R+CH2- C H2+CN CH2CN where R and X are as defined above.

Step A3 The production of 4-alkylbicyclo(2,2,2)octylmethanols from the corresponding known 4 alkylbicyclo(2,2,2)octane-1-carboxylicacids. The method used may be as described by H D Holtz and L M Stock in J. Am. Chem. Soc., 92, 1582 (1970).

An example of such a product is 4-n-pentylbicyclo(2,2,2)octylmethanol, yield 86%, bp 1 850C at 4 mm Hg.

Step B3 The production of 4-alkylbicyclo(2,2,2)octylmethyl toluene-4-sulphonates from 4-alkylbicyclo(2,2,2)octylmethanols.

This Step may be carried out in the same way as Step A described in published UK Patent Application GB 2070594A.

An example of such a product is 4-n-pentylbicyclo(2,2,2)octyl toluene-4-sulphonate, yield 82%, mp 670C.

Step C3 The production of 4-alkylbicyclo(2,2,2)octylacetonitriles from 4-alkylbicyclo(2,2,2)octylmethyl toluene-4-sulphonates.

The 4-alkylbicyclo(2,2,2)octylmethyl toluene-4-sulphonate (0.01 mole) is heated under reflux for 16 hour with potassium cyanide (0.012 mole) in dry dimethylformamide (40 cm3).

Iron (III) chloride (2.0 g in 1 50 cm3 water) is added to the cooled solution followed by concentrated hydrochloric acid (10 cm3) at 50--60"C, and the solution is stirred for 20 min. Water (100 cm3) is added and the whole is shaken with ether (3x80 cm3). The combined ethereal extracts are washed with brine (2x60 cm3) and dried (Na2SO4). The solvent is removed in vacuo and the residual oil is purified by reduced pressure distillation.

An example of such a product is 4-n-butylbicyclo(2,2,2)octylacetonitrile, yield 69%, bp 1 700C at 2 mm Hg.

Step D3 The production of 4-alkylbicyclo(2,2,2)octylacetic acids from 4-alkylbicyclo(2,2,2)octylacetonitriles.

A mixture of 4-alkylbicyclo(2,2,2)octylacetonitrile (0.005 mole), sulphuric acid (10 cm3 of a 50% aqueous solution) and glacial acetic acid (25 cm3) is heated under reflux for 24 hours. The cooled solution is poured onto ice (100 g) and the organic product extracted into ether (3 x80 cm3). The ethereal extracts are boiled with decolourising charcoal (2 g) and filtered whilst still hot to remove coloured impurities. The ethereal solution is dried (Na2SO4) and the solvent removed in vacuo. The product is crystallised from methanol using cardice as refrigerant.

Examples of such materials are 4-n-butylbicyclo(2,2,2)octylacetic acid, yield 68%, mp 850C and 4-n-propylbicyclo(2,2,2)octylacetic acid, yield 62%, mp 920C.

Step E3 The production of 4-alkylbicyclo(2,2,2)octylmethyl 4-bromophenyl ketones and 4-alkylbicyclo(2,2,2)octylmethyl 4-bromo-24luorophenyl ketones from 4-alkylbicyclo(2,2,2)octylacetic acids.

This Step may be carried out essentially by the method given in Step El above (which uses 4alkylphenylacetic acids as starting material).

Step F3 The production of 1 -(4'-alkylbicyclo(2',2',2')octyl)-2-(4"-bromophenyl)ethanes and 1-(4'-alkyl- bicyclo(2',2',2')octyl)-2-(4"-bromo-2"-fluorophenyl)ethanes from 4-alkylbicyclo(2,2,2)octylmethyl 4bromophenyl ketones and 4-alkylbicyclo(2,2,2)octylmethyl 4-bromo-2-fluorophenyl ketones respectively.

This Step may be carried out essentially by the method given in Step F1 above (which uses alkylphenyl)-2-(4"-bromophenyl)ethanes and 1 -(4'-alkylphenyl)-2-(4"-bromo-2"-fluorophenyi)ethanes as starting materials).

Step G3 The production of 1 -(4'-alkylbicyclo(2',2',2')octyl)-2-(4"-cyanophenyl)ethanes and 1 -(4'-alkyl- bicyclo(2',2',2')octyl)-2-(4"-cyano-2"-fluorophenyl)ethanesfrom 1 -(4'-alkylbicyclo(2',2',2')octyl)-2-(4"- bromophenyl)ethanes and 1 -(4'-alkylbicyclo(2',2',2')octyl)-2-(4"-bromo-2"-fluorophenyl)ethanes respectively. This Step may be carried out essentially by the method given in Step G 1 above. An example of such a product is 1 -(4'-n-pentylbicyclo(2',2',2')octyl)-2-(4"-cyanophenyl)ethane, yield 60%, C-N, 760C; N--l, 1 130C. The viscosity of this compound at 200C by logarithmic extrapolation using 20% by weight of the compound as a solution with the material ZLI 1132 (80% by weight of the solution) obtained from E Merck Co is 101 c St.

Example 4 This example involves the preparation of compounds having the structures:

by the following route:

HO F-- STEP A4 < > CH2 CO2H HOCH2.CO2H STEPA4 cH3O #-QCH2. CO2H j STEP 84 CH3OCH2 CO Br STEP STEP4 CH3aCH2 CH2 Br STEP 04 HO CH2, Cm2 bur |STEP E4 R1O CH2. CH2 Sr STEP F4 XI g > --CH2 STEP F4 R1OCH2 CH2CN where R1 is an alkyl group (-R) or an alkylcarbonyl group (-CO.R) or an alkoxycarbonyl group (-CO.OR) where R is an alkyl group as defined above and X is as defined above.

Step A4 The production of 4-methoxyphenylacetic acid from the known, commercially available, 4- hydroxyphenylacetic acid.

This Step may be carried out essentially by a standard literature method for the alkylation of a phenol-see for example the method used in the article by G W Gray and Brynmor Jones, in J. Chem Soc.,678(1954).

Step B4 The production of 4-methoxybenzyl 4-bromophenyl ketone and 4-methoxybenzyl 4-bromo-2fluorophenyl ketone from 4-methoxyphenylacetic acid.

This Step may be carried out essentially by the method given in Step El above (which uses 4alkylphenylacetic acid as starting material).

Step C4 The production of 1 -(4'-methoxyphenyl)-2-(4"-bromophenyl)ethane and 1 -(4'-methoxyphenyl)- 2-(4"-bromo-2"-fluorophenyl)ethane from 4-methoxybenzyl 4-bromophenyl ketone and 4-methoxybenzyl 4-bromo-2-fluorophenyl ketone respectively.

This Step may be carried out essentially by the method given in Step F1 above (which uses 4alkylbenzyl 4-bromophenyl ketones and 4-alkylbenzyl 4-bromo-2-fluorophenyl ketones as starting materials).

Step D4 The production of 1 -(4'-hydroxyphenyl)-2-(4"-bromophenyl)ethane and 1 -(4'-hydroxyphenyl)-2- (4"-bromo-2"-fluorophenyl)ethane from 1 -(4'-methoxyphenyl)-2-(4"-bromophenyl)ethane and 1 -(4' methoxyphenyl)-2-(4g-bromo-2"-fluorophenyl)ethane respectively.

This Step may be carried out essentially by a standard literature method for the demethylation of methyl ethers involving the use of constant boiling hydrobromic acid and acetic acid-see for example Step C4 of UK Patent Application No. 8203798.

Step E4 The production of the bromo-terminated ethyl bridged compounds mentioned above.

This Step may be carried out essentially by a known method for the alkylation of the phenol produced in produce of Step D4 above see for example the method used in Step D4 in UK Patent Application No. 8203798 and the method described in the article by G W Gray and Brynmor Jones in J. Chem. Soc. 678 (1954).

Step F4 The production of the cyano terminated ethyl bridged compounds mentioned above from the corresponding bromo-compounds prepared in Step E4.

This Step is identical to Step G1 above (which uses 1-(4'-alkylphenyl)-2-(4"-bromo- phenyl)ethanes and 1 -(4'-alkylphenyl)-2-(4"-bromo-2"-fluorophenyl)ethanes as starting materials).

Examples of compounds which may be prepared by the above routes are compounds of Formulae (II) to (Xl) above wherein R in those Formulae is as given in the following Table: Table 1 Compounds of the form specified in Formulae (II) to (Xl) above

R R R CH3 n-C6ii13 n-C11H23 C2H5 n-C7H15 n-C12H25 n-C3H7 n-C8H17 CH3 n-C4H n-C9H19 CH3CH2CH(CH2)n n-C5H11 n-C10H21 n=O, 1,2,3 Examples of compositions incorporating compounds of Formula (I) and of devices containing such compositions will now be described by way of example only with reference to the accompanying drawings wherein:: Figure 1 is a sectional view of a twisted nematic digital display; Figure 2 is a sectional view of the display shown in Figure 1; Figure 3 shows a rear electrode configuration for Figure 1; Figure 4 shows a front electrode configuration for Figure 1; Figures 5, 6, 7 show schematic views of the device of Figures 1 to 4 with typical addressing voltages; The display of Figures 1 to 4 comprises a cell 1, formed of two, front and back, glass slides 2, 3 respectively, spaced about 7 ,um apart by a spacer 4 all held together by an epoxy resin glue. A liquid crystal material 12 fills the gap between the slides 2, 3 and the spacer 4. In front of the front glass slide 2 is a front polariser 5 arranged with its axis of polarisation axis horizontal. A reflector 7 is arranged behind the slide 3.A rear polariser 6 or analyser is arranged between the slide 3 and reflector 7.

Electrodes 8, 9 of tin oxide typically 1 00 A thick are deposited on the inner faces of the slides 2, 3 as a complete layer and etched to the shapes shown in Figures 3, 4. The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in Figure 3 the rear electrode structure is formed into three electrodes xa, x2, x3. Similarly the front electrode structure is formed into three electrodes per digit and decimal point y1, y2,y3.... Examination of the six electrodes per digit shows that each of the eight elements can independently have a voltage applied thereto by application of suitable voltage to appropriate x, y electrodes.

Prior to assembly the slides 2, 3 bearing the electrodes are cleaned then dipped in a solution of 0.2% by weight of poly-vinyl alcohol (PVA) in water. When dry, the slides are rubbed in a single direction with a soft tissue then assembled with the rubbing directions orthogonal to one another and parallel to the optical axis of the respective adjacent polarisers, ie so that the polarisers are crossed.

When the nematic liquid crystal material 12 is introduced between the slides 2,3 the molecules at the slide surfaces lie along the respective rubbing directions with a progressive twist between the slides.

When zero voltage is applied to the cell 1 light passes through the front polariser 5, through the cell 1 (whilst having its plane of polarisation rotated 900) through its rear polariser 6 to the reflector 7 where it is reflected back again to an observer (shown in Figure 1 at an angle of 450 to the axis Z normal to axes X and Y in the plane of the slides 2, 3). When a voltage above a threshold value is applied between two electrodes 8, 9 the liquid crystal layer 1 2 loses its optical activity, the molecules being re-arranged to lie perpendicular to the slides 2, 3, ie along the axis Z. Thus light at the position does not reach the reflector 7 and does not reflect back to the observer who sees a dark display of one or more bars of a digit 10.

Voltages are applied as follows as shown in Figures 5, 6 and 7 for three successive time intervals in a linescan fashion. An electrical potential of 3V/2 is applied to, ie scanned down, each x electrode in turn whilst -V/2 is applied to the remaining x electrodes. Meanwhile -3V/2 or V/2 is applied to the y electrodes. A coincidence of 3V/2 and -3V/2 at an intersection results in a voltage 3 V across the liquid crystal layer 12. Elsewhere the voltage is V or -V. Thus by applying -3V/2 to appropriate y electrodes as 3V/2 is scanned down the x electrodes selected intersections are turned ON as indicated by solid circles. The electric voltage V is an ac signal of eg 100 Hz square wave, and the sign indicates the phase.

It will be apparent to those skilled in the art that the device shown in Figures 1 to 7 is a multiplexed display because the electrodes are shared between ON and OFF intersections or display elements.

A material embodying the second aspect of the invention which is suitable for use as the material 12 in the above device is in Table 6 as follows (Mixture 1).

Table 6 Mixture 1

Compound Weight percentage C2HThThOOCN 15 n--CI Ho- C4H9m CN t3 C2H5 COO ThffCN 12 " -C5H11ffmThCN 10 n CsH114}CH2 CH24 10 n CsH114}CH2 CH2+0c2HS 20 nC5H11+CH2. -n2' CH2+CN 20 Small amounts of a cholesteric material may be added to the nematic material to induce a preferred twist in the molecules in the liquid crystal layer. This and the use of appropriate slide surface treatment removes the problems of display patchiness as taught in UK Patent Serial Numbers 1,472,247 and 1,478,592.

Suitable cholesteric materials are: C15: about 0.1 0.5% by weight and CB 15: about 0.01% to 0.05% by weight.

Small amounts of pleochroic dye may be added to enhance the display contrast, eg one of the anthraquinone dyes described in UK Patent Specification No 2011 940A. One polariser is removed in this case.

In an alternative embodiment of the invention a (choiesteric-to-nematic) phase change effect device incorporates a material as defined above.

A cell is prepared containing a iong helical pitch cholesteric material sandwiched between electrode-bearing glass slides as in the twisted nematic cell described above. However the polarisers and surface preparations for homogeneous alignment, eg treatment of the glass slide surfaces with SiO, are not used in this case.

If the glass slides are untreated and the liquid crystal material has a positive dielectric anisotropy (E) the liquid crystal material is in a twisted focal conic molecular texture in the OFF state which scatters light. The effect of an electric field applied between a pair of electrodes on the respective inner surface of the glass slides is to convert the region of liquid crystal material between the electrodes into the ON state which is a homeotropic nematic texture which is less scattering than the OFF state. This is a 'negative contrast' type of phase change effect device.

The contrast between the two states may be enhanced by the addition of a small amount of a suitable pleochroic dye (eg 1% by weight of 1 ,5-bis-4'n-butylphenylaminoanthraquinone to the liquid crystal material.

A suitable positive dielectric anisotropy material, Mixture 2, embodying the invention for use in a phase change effect (negative contrast type) device is: Table 2 Mixture 2

Compound Weight percentage nC6H13 X CN 37-5% n C4H9 C CN 37.5% 1 61 n C3H70 X CN 25% 5 1hex Cm 10 CH3 (+)H3 CH2, CI CH-L'--CN 4 n H7ffThC4H9-n 15 nC5H1T+CH2- CH2CN 10 An example of a high birefringence, low viscosity material of positive dielectric anisotropy suitable for simple twisted nematic displays and which includes a compound of Formula (I) is Mixture 3 defined in Table 3 as follows: : Table 3 Mixture 3

Compound Weight percentage n -C5H1MffCN 29 nC7H1 CN 17 n5H11M < OH2 CH2+CN 15 n C3H7{9}CH2 CH2 < 11 n-C5 Hii w CN 8 n C5H1193 < CN 10 n C3H7{0}Co H11-n 10

Claims

1. A liquid crystal compound having a positive dielectric anisotropy which is of Formula (I) as follows:

where R is an alkyl group and

is a trans-1,4-disubstituted cyclohexane ring;

where R is an alkyl group and

is a 1 ,4-disubstituted bicyclo(2,2,2)octane ring, and where

is a 1,4aisubstituted benzene ring and R1- is an alkyl group R-, an alkoxy group RO- an alkylcarbobyloxy group RCO.O- or an alkoxycarbonyloxy group ROCO.O-; and wherein X is either hydrogen or fluorine, provided that if X is hydrogen A is

2. A compound as claimed in claim 1 and wherein the group R or R1 contained within the radical A is an n-alkyl group having from 1 to 8 carbon atoms inclusive.

3. A compound as claimed in claim 1 and having the formula:

4. A compound as claimed in claim 1 and having the formula: