GB2079275A

GB2079275A - Liquid Crystal Bicyclo[2.2.2] Octane Esters

Info

Publication number: GB2079275A
Application number: GB8117579A
Authority: GB
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1980-06-09
Filing date: 1981-06-09
Publication date: 1982-01-20
Also published as: GB2079275B

Abstract

A novel liquid crystal ester compound exhibiting a low dielectric anisotropy has a structural formula: <IMAGE> where R1 is an alkyl group, R2 is an alkyl group. The compounds are useful in multiplexed twisted nematic effect devices.

Description

SPECIFICATION Liquid Crystal Ester Compounds Exhibiting a Small Dielectric Anisotropy and Liquid Crystal Materials and Devices Incorporating Such Compounds The present invention relates to liquid crystal ester compounds exhibiting a small (negative or positive) dielectric anisotropy and liquid crystal materials and devices incorporating such compounds.

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 (emu) to the molecules of the material, eg when aligned together, less the average dielectric constant measured perpendicular (E1) to the molecules.

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

For example, materials having a positive dielectric anisotropy, herein referred to as positive materials, eg mixtures of 4-alkyl-or-alkoxy-4'-cyanobiphenyls and a 4"-alkyl-or-aikoxy-4-cyano-pterphenyl may be used in known twisted nematic effect devices (if nematic) or known cholesteric-tonematic phase change effect devices (if cholesteric) in which the molecular arrangement is changed from the focal conic to the homeotropic texture.

Materials having a negative dielectric anisotropy herein referred to as negative materials (of suitable resistivity) may be used in known dynamic scattering effect devices (if nematic) or cholesteric memory mode devices (if cholesteric).

Materials having a small dielectric anisotropy (negative or positive), may also be mixed with those having a large positive or negative dielectric anisotropy to produce mixtures whose overall dielectric anisotropy is positive or negative as appropriate (but reduced in magnitude).

According to the present invention in a first aspect there is provided a novel liquid crystal ester compound exhibiting a low dielectric anisotropy having a structural formula:

Formula (I) where R1 is an alkyl group, R2 is an alkyl group,

is a 1 ,4-disubstituted bicyclo(2.2.2)octane ring and

is a trans 1,4 disubstituted cyclohexane ring.

The groups R, and R2 preferably contain eighteen or less carbon atoms, desirably ten or less carbon atoms. These groups may be normal or branched. If branched, one or both may contain a chiral centre in which case the compound may be opticaliy active. If R1 and R2 are normal or not chiral groups then the compound is nematogenic.

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 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 to vary the liquid crystal temperature ranges of the compounds or to vary the molecular helical pitch (in the case of cholesteric liquid crystals).

The compounds according to formula (I), are esters of 4-hydroxy-1 -alkylbicyclo(2.2.2)octanes and trans-4-alkylcyclohexane-1-carboxylic acids which may be prepared by known procedures. In each case the appropriate carboxylic acid is preferably converted into its acid chloride by a conventional reaction with thionyl chloride. The acid chloride is then preferably reacted with the appropriate hydroxy compound under known conditions to give the ester.

One or more compounds according to formula (I) may be used in any of the following applications: (i) together with a 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 another nematic material, preferably strongly negative, giving an overall negative material preferably also with a pleochroic dye, in Freedericksz effect devices in which the molecular arrangement may be changed from the homeotropic texture (OFF state) to the homogeneous texture (ON state): an example of such a device is given below; (iii) together with a positive nematic material giving an overall positive nematic material.

preferably also with a pleochroic dye, in Freedericksz effect devices in which the molecular arrangement may be changed from the homogeneous texture (OFF state) to the homeotropic texture (OFF state) by an electric field; (iv) together with an optically active and a negative material giving an overall negative material which is a cholesteric of suitable resistivity (about 109 ohm-cm). in cholesteric memory mode devices in which the molecular arrangement may be changed from a homogeneous texture (OFF state) to a turbulent scattering focal conic texture (ON state) by an dielectric field; (v) together with an optically active material and a strongly negative nematic material giving an overall strongly negative 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 weakly scattering. ie clear, surface aligned homeotropic texture (OFF state) to a strongly scattering twisted homogeneous texture (ON state) by an electric field; (vi) together with an optically active material and a positive nematic material giving an overall positive material which is a cholesteric. preferably together also with a pleochroic dye, in cholestericto-nematic phase change effect on 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;; (vii) together with a nematic material giving an overall negative nematic material of suitable resistivity (about 109 ohm-cm), in dynamic scattering effect devices in which the molecular arrangement may be changed from å clear homeotropic texture (OFF state) to a turbulent scattering texture (ON state) by an electric field; (viii) 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.

The construction and operation of the above devices and the general kinds of material which are suitable for use in them are themselves known.

It will be apparent to those skilled in the art that in the above applications where mixtures are formed 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 2 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.

The compounds according to formula (I) may usefully reduce operating voltages in the above applications as well as enhance negativity or reduce positivity of the dielectric anisotropy; this may usefully affect other properties, eg multiplexibility in twisted nematic effect devices or switching frequency in two-frequency switching effect devices.

According to the present invention in a second aspect there is provided a mixture of liquid crystal compounds inciuding 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 (viii) above.

An example of 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. one or more 4'-alkyl- or 4'-alkoxy-4-cyanobiphenyls and one or more of the following compounds:

Formula (II)

Formula (Ill)

Formula (IV)

Formula (V)

Formula (VI)

Formula (VII) For multiplexed twisted nematic devices one or more compounds having a low or negative dielectric anisotropy are preferably added also, eg one of the following compounds (between 20 and 80 per cent by weight).

Formula (VIII)

Formula (IX)

Formula (X)

Formula (Xl) or an analogue of one or more of the compounds of Formulae (VIII) to (Xl) in which the 2 or 3 position of the right hand 1 4 disubstituted benzene ring as shown carries a fluorine substituent; R,R'=alkyl.

Preferably the compound(s) according to formula (I) and the 4'-alkyl- or 4'-alkoxy-4cyanobiphenyls constitute between about 10 and 90%, desirably between 30 and 70%, by weight of the mixture and the compounds selected from formulae (II) to (VII) constitute not more than about 20% by weight in total, the remainder being one or more compounds according to formulae (VIII to XI).

Preferably each 4'-alkyl- or 4'-alkoxy-4-cyanobiphenyl 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 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

andY1 isCN, or R' or CO.O--XX-Y1 where Y' is CN, or R' or OR'; the definition of R' being the same as that of R.

Preferably, the compound(s) according to formula (I) comprises between 5 and 80%, preferably not more than 20%, 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 Freedericksz effect device or a two-frequency switching effect device all constructed in a known manner or any of the other devices mentioned above. 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.

Compounds having formula (I) may be obtained from the corresponding 4-alkyl bicyclo(2.2.2)octane-1 -carboxylic acid in the method as described for R=n-alkyl in Example 1 below.

Embodiments of the present invention will now be described by way of example.

Examples of the preparation routes, products and properties of specific compounds according to Formula (I) above will now be given.

Route 1 The intermediate compounds 4-hydroxy-1 -alkylbicyclo(2.2.2)octanes which may be used in the preparation of the esters according to Formula (I) (see Route 3 below) may be prepared by the following route:

R2-CH2-CO.CH3 STEP Al . R2 /CH2CH2CN ,\/ CH3.0C/ \CH2CH2CN J R2 CH2CH2CO2H STEP Cl R2 STEP Dl R2,C/ CH3.0C/ CH2CCO2H CH3.OC R2 < 0H STEP El R2 as defined above Step Al: The production of 3-acetyl-1 ,5-dicyano-3 substituted pentane.

Step B2: The production of 3-acetyl-3-substituted pentane-1 ,5-dicarboxylic acid.

Step Cl: The production of 4-acetyl-4-substituted cyclohexanone.

Step D1: The production of 1 -hydroxy-4-substituted bicyclo(2.2.2)octan-3-one.

Step El: The production of 1-hydroxy-4-substituted bicyclo-(2.2.2)octane.

All five of these steps may be carried out by methods essentially analogous to those for R=methyl and ethyl described by H D Holtz and L M Stock in the Preparation of 1 -Carboxy-4-Substituted Bicyclo(2.2.2) octanes, J. Am. Chem. Soc. 86, 5183 (1964).

Route 2 The intermediate compounds trans-4-alkyl-cyclohexane-1 -carboxylic acids which may be used in the preparation of the esters according to Formula (I) (see Route 3 below) may be prepared by the following route:

STEPA2 R1 STEP B2 R1 CO.CH3 XCOOH STEP D;1 R1- j" EP C2 RlCOOHSTEPD2Rl CO0H R, as defined above (alkyl- which may be normal, branched or branched and chiral).

Step A2: The production of alkybenzenes.

Step B2: The production of 4-alkylacetophenones.

Step C2: The production of 4-alkylbenzoic acids.

Step D2: The production of trans-4-alkyl-cyclohexane-1 -carboxylic acids.

All four of these Steps may be carried out by the methods described in US Patent No. 4195916 (Example 4) where the case R2=(+)-2 methylbutyl is described as a specific example.

Route 3 Esters according to Formula (I) above may be prepared by the following route:

R1 CO2H STEP A3 R?- CO.C1 rlSBT3EP R2 OH R1 {i3 CO.O w R2 Step A3: The production of trans-4-alkylcyclohexane-1 carboxylic acid chloride.

A solution of the appropriate trans-4-alkylcyclohexane-1 carboxylic acid (0.001 8 mole) (prepared as in Route 2) in dry toluene (10 cm3) is heated under reflux with thionyl chloride (1 cm3) for one hour under anhydrous conditions and then allowed to cool. The mixture is evaporated to dryness under vacuum and then more dry toluene (10 cm3) is added and the resultant solution is again evaporated to dryness under vacuum. The acid chloride residue is used in the next Step B3 without further purification.

Step B3: The production of esters according to Formula (I).

A solution of the appropriate 1-alkyl-4-hydroxybicyclo(2.2.2)octane (0.0018 mole) in freshly distilled pyridine (4 cm3 of sieve-dried) is added dropwise to a boiling solution of the appropriate trans4-alkylcyclohexane-1 -carboxylic acid chloride (0.0018 mole) in dry toluene (20 cm3). The reaction mixture is heated under reflux for 64 hr with the exclusion of moisture.

The excess of toluene is removed in vacuo and the residue column chromatographed using a 2:1 (V:V) mixture of chloroform: Ight petroleum (bp 4060 ) as the eluent with silica gel as the stationary phase. The solvent is removed in vacuo from the appropriate combined fractions and the final product is obtained by a reduced pressure, short path distillation. The yield is 65%.

Examples of product of Step B3, ie compounds according to Formula (I) are given in Tables 1 to 10 as follows where R1 and R2 are given by

Table 1: R,=CH3 Table 2: R,=C2Hs R2 R2 CH3 CH3 C2H5 C2H5 n-C3H7 nC3H7 n-C4Hg n-C4H9 n-C5-H11 n-C5H11 n-C5H12 n-C6H13 n-C7H15 n-C7H15 n-C8H17 N-C8H17 nCgH19 n-CgH19 n-C10H21 n-C10H21 CH3CH(CH3)CH2 CH3CH(CH3)CH2 CH3CH(CH3)(CH2)2 CH3CH(CH3)(CH2)2 CH3CH(CH3)(CH2)3 CH3CH(CH3)(CH2)3 Table 3: R1=n-C3H7 Table 4: R1=n-C4H9 R2 R2 Same list as for R2 Same list as for R2 in Table 1. in Table 1.

Table 5: R1=n-C5H11 Table 6: R1=n-C6H13 R2 R2 Same list as for R2 Same list as for R2 in Table 1. in Table 1.

Table 7: R1=n-C7H15 Table 8: B1=n-C8H17 R2 R2 Same list as for R2 Same list as for R2 in Table 1. in Table 1.

Table 9: R1=n-C9H19 Table 10:R1=n-C10H21 R2 R2 R2 R2 Same list as for R2 Same list as for R2 in Table 1. in Table 1.

Table 11: R,=CH3CH(CH3)CH2 R2 Same list as for R2 in Table 1.

Table 12: R,=CH3CH(CH3)(CH2)2 R2 Same list as for R2 in Table 1.

Table 13: R,=CH3CH(CH3)(CH2)3 R2 Same list as for R2 in Table 1.

Examples of specific properties of particular compounds of the form

are given in Table 14 as follows.

In Table 14 C-SB=crystal to smectic B transition temperature ( C) SBN/l=smectic B to nematic or isotropic liquid transition (as appropriate) ( C) N-l=nematic to isotropic liquid transition temperature ( C) Table 14 Specific Properties of Compounds

R1 R2 C-SB SB SB-N/l N-I CH3 n-C5H11 22 49 - C2H5t nC5H11 4 73 n-C3H7 n-C5H11 1 88 1851* n-C4H9 n-CH11 9. 104-5 - n-CH11 n-C5H11 3 107 [92J* n-C6H13t n-C8H11 19 115 n-C7H151 n-C5H11 11 114 14 n-C5H11 n-C2H7 18 79 92 *This is a virtual N-I value which was obtained on a smectogen by using a nematic host (the commercially available material E7)- latent SAwhich strongly represses the smectic tendencies of the compound (SB) and produces an enantiotroptic nematic phase, even at high concentratrions (90%) of the smectogen.

tThese CSB values were determined by differential thermal analysis. The other CSB values were determined by optical microscopy.

For the compound in Table 14 where R1=n-C5H11, R2=n-C3H7, the birefrigence has been measured as 0.036 at a temperature of 0.98x(N-l).

represents a virtual transition temperature.

C-I represents crystal to isotropic liquid transition.

N-I represents nematic to isotropic liquid transition temperature.

Examples of materials and devices embodying the invention 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 ym 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 polarizer 5 arranged with its axis of polarization horizontai. A reflector 7 is arranged behind the slide 3. A rear polarizer 6 or analyzer is arranged between the slide 3 and reflector 7.

Electrodes 8, 9 of tin oxide typically 100 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 x1, 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 polarizers, ie so that the polarizers 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 polarizer 5, through the cell 1 (whilst having its plane of polarization rotated 900) through its rear polarizer 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 Yin 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 12 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 that 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 timer intervals in a linescan fashion. An electric 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 applying3V/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 multiplex display because the electrodes are shared between ON and OFF intersections or display elements.

A material embodying the invention which is suitable for use as the material 1 2 in the above device is as in Table 14 as follows (Mixture 1).

Table 14 Mixture 1 Compound Weight percentage

C2H5 MM CN 15 n- C4 H9 X CN 23 C2H5 {ss} COO E MCN 12 ncsHllMMMCN 10 10 n-C5H11 XCOOX C5H11-n 20 F n-C5H11 COO < C5lQ11-n 20 Alternatively the last mentioned compound may be substituted partially or wholly in the Mixture by one or more of the other compounds according to Formula (I) described above.

Small amounts of 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: C 1 5 about 0.1 to 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 contract eg one of the anthraquinone dyes described in UK Patent Application No 42,810/77.

In another embodiment mixtures embodying the second aspect of the invention may be used in a Freedericksz effect cell. Such a cell may be constructed by sandwiching the liquid crystal material between glass slides having electrode films deposited on their inner surfaces as in the above device.

However, in this case the polarizers are not necessary, the glass slide inner surfaces are treated with a coating of lecithin and the liquid crystal material is a negative material whose molecules are aligned in the OFF state perpendicular to the slide substrates (homeotropic texture) by the licithin coating.

Application of an appropriate electric field across the material in the ON state re-arranges the molecules parallel to the slide surfaces (homogeneous texture). A pleochroic dye may be incorporated in the liquid crystal material to enhance the contrast between the ON and OFF states.

A Freedericksz effect cell made in the above way may incorporate Mixture 2 below, the cell spacing being 10 ,us.

Table 14 Mixture 2 Compound Percentage by weight

Compound A: n -C4H9 COO < C5 H11-n 42% F Compound B: n -C6H13O COO C5H11-n 42% F n-C5H11 COOC5H11-n 10% Compound E: C2H5 {t3Coo ){)NOOC C2H5 c 2H5 6% CN CN The preparation of Compound E is described in UK Patent Application No 7934129, whilst that of Compounds A and B is described in UK Patent Application No 7934127.

1.2% by weight of a known pleochroic dye eg 1 ,5-bis-4'-n-butylphenylaminoanthraquinone may be added to Mixture 2 to give a dyed mixture Mixture 2a.

When a voltage is applied across the cell, the colour changes from a weakly absorbing state to a strongly absorbing state.

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

A cell is prepared containing a long helical pitch cholesteric material sandwiched between electrode-bearing glass slides as in the twisted nematic cell described above. However the polarizers 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 anistropy (AE) 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.

If the inner glass slide surfaces are treated, eg with a coating of lecithin, to give alignment perpendicular to those surfaces, and the liquid crystal material has AE negative the material in the OFF state is in a homeotropic texture which has little scattering effect on incident light. If an electric field is applied between a pair of electrodes on the respective inner surfaces of the glass slides the region of liquid crystal material between the electrodes is converted to a twisted homogeneous texture which scatters light (the ON state). This is a 'positive contrast type of phase change effect device.

The contrast between the two states in each case 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-butylphenylamino anthraquinone (in the case where AE iS positive) to the liquid crystal material.

A suitable positive dielectric anisotropy material embodying the invention for use in a phase change effect device is: Table 16 Mixture 3 Material Percentage by weight

C2H5 MM CN 37.5% Mixture B n-C4 H9 CN 37.5% . 60% n-C387o X CN 25% CiS (known) RCO - CN (Rc=2-methylbutyl) 10% F n- C4Hg OOC - CgHII- n 20% C5H11 COO C5H11-n 10% A suitable negative dielectric anisotropy material embodying the invention for use in a phase change effect device, Mixture 4, is as follows: : Table 17 Mixture 4 Compound Percentage by weight

z C8HS } COO SC5H11-n 11% n-C5H11 DOC CLHq-n 40% F n-C5H11 DOe C5H11-n 40% 'F Compound E C2H5 < COO H COO - CNCN HC2H5 8% CN CN RC Mco RC 1X (Rc=2-methylbutyl)

Claims

1. A liquid crystal ester compound exhibiting a low dielectric anisotropy having a structural formula:

where R1 is an alkyl group, R2 is an alkyl group,

is a 1,4 disubstituted bicyclo(2.2.2)octane ring and

is a trans 1,4 disubstituted cyclohexane ring.

2. A compound as claimed in claim 1 and wherein R, and R2 have 18 or less carbon atoms.

3. A compound as claimed in claim 1 and wherein B1 and R2 have 10 or less carbon atoms.

4. A compound as claimed in claim 3 and which is any one of the specific compounds listed in Tables 1 to 13 hereinbefore.

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

6. A liquid crystal material as claimed in claim 5 and which is suitable for use in a twisted nematic effect device and includes one or more 4'-alkyl-or 4'-alkoxy-4-cyanobiphenyls and one or more compounds of Formulae (11) to (VII) given hereinbefore.

7. A liquid crystal material as claimed in claim 6 and which s suitable for use in a multiplexed twisted nematic effect device and which also includes, to an extent of between 20 and 80 per cent by weight. one or more compounds of Formulae (Vll) to (XI) given hereinbefore.

8. 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 as claimed in any one of claims 1 to 4.

9. A device as claimed in claim 8 and wherein the device is a twisted nematic effect device.

1 0. A device as claimed in claim 9 and wherein the device is a twisted nematic effect device which may be operated in a multiplexed fashion.

11. A device as claimed in claim 8 and wherein the device is a cholesteric-to-nematic phase change effect device, a Freedericksz effect device or a two-frequency switching effect device.