GB2081707A

GB2081707A - Liquid Crystal Aromatic Nitrile Ethers

Info

Publication number: GB2081707A
Application number: GB8117580A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1980-06-09
Filing date: 1981-06-09
Publication date: 1982-02-24
Also published as: GB2081707B

Abstract

A liquid crystal compound exhibiting a positive dielectric anisotropy has a formula: R-X-CH2O-Ar-CN where R is an alkyl or alkoxy group, X is a 1,4-disubstituted bicyclo(2.2.2)octane ring or a 1,4- disubstituted benzene ring, Ar is a 1,4- disubstituted benzene ring, or a 4,4'- disubstituted biphenyl ring or a 2,6- disubstituted naphthalene ring, and CN is a cyano group, provided that where R is an alkoxy group X is a 1,4- disubstituted benzene ring.

Description

SPECIFICATION Liquid Crystal Ether Compounds Exhibiting a Positive Dielectric Anisotropy and Liquid Crystal Materials and Devices Incorporating such Compounds The present invention relates to liquid crystal ether compounds exhibiting a positive dielectric anisotropy and to liquid crystal materials and devices containing 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 (aye) of the material. This is the difference, for a given frequency and temperature, between the average dielectric constant measured parallel (") and 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-a Ikyl-or-alkoxy-4'-cyanobiphenyls and a 4"-alkyl-or-alkoxy-4-cyano-ptorphenyls 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 cholesteric to the homeotropic nematic texture.

According to the present invention a liquid crystal compound exhibiting a positive dielectric anisotropy has a formula: f3-X-CH2O-Ar-CN Formula (I) where R is an alkyl or an alkoxy group, Xis a 1 ,4-disubstituted bicyclo(2.2.2)octane ring or a 1,4disubstituted benzene ring, Ar is a 1 ,4-disubstituted benzene ring, or a 4,4'-disubstituted biphenyl ring or a 2,6-disubstituted naphthalene ring, and CN is a cyano group, provided that where R is an alkoxy group X is a 1 ,4-disubstituted benzene ring.

Preferably R has from one to ten carbon atoms and may be either an n-alkyl or branched alkyl group which may be chiral.

If R is an n-alkyl group the compound is nematogenic. If R is a chiral alkyl group the compound is cholesterogenic.

Thus, compounds of Formula (I) may be compounds having Formula (la) or Formula (Ib) as follows:

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 phase of the material by the addition and caiculating 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, preferably also with a pleochroic dye, in Fréedericksz effect devices in which the molecular arrangement may be changed from the homogeneous texture (OFF state) by an electric field; (iii) together with an optically active material giving an overall 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 surface aligned homeotropic texture (OFF state) to a strongly scattering twisted homogeneous texture (ON state) by an electric field;; (iv) 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; (v) together with a positive nematic material giving an overall positive nematic material in two frequency switching effect devices in which the dielectric anistropy 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 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.

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. The added material may be a mixture of one or more compounds having a positive dielectric anisotropy with one or more compounds having a negative dielectric anisotropy, the overall dielectric anisotropy being (eg positive) as required.

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.

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 (v) 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, one or more 4'-alkyl- or 4'-alkoxy-4-cyanobiphenyls and one or more of the following compounds:

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):

or an analogue of one or more of the compounds of formulae (VIII) to (X) 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-4- cyanobiphenyls 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

and Y is CN, or R1, or OR1 or CO.O-X-Y1 where Y' is CN, or R1 or ORt; 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 Frtedericksz 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.

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

Compounds having formula (la) may be obtained from the corresponding 4-alkylbicyclo(2,2,2)octane-1 -carboxylic acid in the method as described for R=n-alkyl in Example 1 below.

Example 1 This example describes the preparation of 4-(4'-n-pentyl-1 '-bicyclo(2,2,2)octylmethoxy)cyanobenzenes which illustrates a general method of preparation of compounds of formula (I), by the following route:

where R=n-pentyl in this example.

Step Al The production of 4-n-pentyl-l -bicyclo(2,2,2)octylmethanols.

Step B1 The production of 4-n-pentyl-l bicyclo(2,2,2)octylmethyl toiuene-4-sulphonates.

Step C1 The production of 4-(4'-alkyl-l '-bicyclo(2,2,2)octylmethoxy-cyanobenzenes.

Step Al The production of 4-n-pentyl- 1 -bicyclo(2,2,2)octylmethanol from the known 4-n pentylbicyclo(2,2,2)octane- 1 -carboxylic acid.

The preparation of the acid is described in published UK Patent Application No 2,027,708A (Examples 1 and 2).

The carboxylic acid (2.69, 0.01 16 mole) is added to thionyl chloride (120cm3) and the mixture heated under reflux for 90 minutes. The excess of thionyl chloride is removed from the cooled solution in vacuo. Ether (50cm3 of sodium-dried) is then added to the residue and the solvent is distilled off again in vacuo. This procedure is carried out twice to remove the last traces of thionyl chloride. A solution of the acid chloride ether(100cm3 of sodium-dried) is added over 1 her to a suspension of lithium aluminium hydride (0.669, 0.0174 mole) in ether (80cm3 of sodium-dried); after the addition, the whole is then heated under reflux for 20 min. Water (8cm3) is added dropwise, slowly and cautiously, to the cooled solution to destroy the excess of lithium aluminium hydride; sulphuric acid (100cm3 of aqueous 10%) is then added slowly to the mixture.The aqueous layer is shaken with ether (3x80cm3); the combined ethereal extracts are washed with water (100cm3 and 50cm3) and dried over sodium sulphate. The required alcohol is then obtained by removing the ether in vacuo and distilling the product under reduced pressure. The yield of the 4-n-pentyl-l -bicyclo(2,2,2)- octylmethanol is 2.2g (90%) bp 11 00C at 0.5mm Hg.

Step B1 Cnqentyl-l -bicyclo(2,2,2)octylmethanol (2.59, 0.01 19 mole) produced in Step Al is heated at 400C in pyridine (33cm3 of sieve-dried) with toluene-4-sulphonyl chloride (3.49, 0.0179 mole) with stirring for 60 hr. The reaction mixture is poured into iced-water (60cm3) and the product extracted into ether (3x 50cm3). The combined ethereal extracts are washed with dilute hydrochloric acid (50cm3), water (3x50cm3), and dried (CaCO3). The solvent is removed in vacuo and the crude tosylate crystallised from light petroleum (bp 40-600C). The yield of Sn-pentyl-1-bicyclo(2,2,2)octylmethyl toluene-4-sulphonate is 3.lug (71%), mp 670C.

Step C1 4-n-pentyl-1-bicyclo(2,2,2)octylmethyl toluene-4-sulphonate (2.56g, 0.007 mole) and potassium carbonate (2.99, 0.021 mole) are heated under reflux in dimethylformamide (12.5cm3 of sieve-dried) for 1 6 hr with the required 4-cyanophenol (2.5g, 0.021 mole), with stirring. The reaction mixture is poured into water (60 cm3) and the organic product extracted into ether 3 x60cm3).The combined etheral extracts are washed with aqueous sodium hydroxide (3x60cm3 of 20%), water (2x60cm ), and dried (Na2SO4). The solvent is removed in vacuo and the crude product column chromatographed using gel as the stationary phase and a 2:1 (v:v) mixture of chloroform:light petroleum (bp 40-60 C) as the eluant. The final product is obtained by distillation under reduction pressure. The yield is about 50%.

The product

has the following properties: melting point 72 C, nematic to isotropic transition temperature 730C.

Further compounds embodying the invention which may be prepared by a route analogous to Example 1 are given in Table 1 as follows: Table 1 Compounds of the form:

R R continued CH3 n-C8H1, C2H5 n-C9H19 n-C3H n-C10H20 n-C4Hg 2-methylbutyl n-C6H13 3-methylpentyl n-C,H15 4-methylhexyl For example, the compounds with R=CH3 and R=n-C4Hg have melting points 89 C and 79 C and nematic to isotropic transition temperatures 4400 (virtual) and 57 C (monotropic) respectively.

Compounds having formula (Ib) above may be obtained from the corresponding acids

where R=alkyl or alkoxy as in Example 2 as follows: Example 2 This example describes the preparation of 4-(4'-alkylbenzyloxy)cyanobenzenes, 2-(4'alkylbenzyloxy)-6-cyanonaphthalenes and 4-(4"-alkylbenzyloxy)-4"-cyanobiphenyls by the following route:

where, R1 is an alkyl group as defined above and Ar is a 1,4-disubstituted benzene ring or a 2,6disubstituted naphthalene ring or a 4,4'-disubstituted biphenyl ring.

STEP A2: The production of 4-alkylbenzyl alcohols.

STEP B2:The production of 4-alkylbenzyl chlorides.

STEP C2:The production of 4-(4'-aíkylbenzyloxy)cyanobenzenes,2-(4'-aíkylbenzyioxy)-6- cyanonaphthalenes, and 4-(4"-alkylbenzyloxy)-4'-cyanobisphenyls, Step A2 The production of 4-alkylbenzyl alcohols from the known 4-alkylbenzoic acids.

This step may be carried out essentially by the method described in Patent Application No 803129 (starting with trans-4-alkylcyclohexane-1-carboxylic acids in that case).

An example of such a product is 4-n-pentylbenzyl alcohol, yield 90%, bp 105 C at 1 mm Hg.

Step B2 The production of 4-alkylbenzyl chlorides from 4-alkylbenzyl alcohols.

4-Alkylbenzyl alcohol (0.02 mole) is added to chloroform (60cm3) and thionyl chloride (2.86g) and the mixture heated for 90 min at 80 C. The solvent is removed in vacuo and the residual oil taken up in ether (50 cm ). The solution is washed with 10% aqueous sodium bicarbonate (2 x 30 cm ), 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 120 C at 2mm Hg.

Step C2 The production of 4-(4'-alkylbenzyloxy)cyanobenzenes, 2-(4'-alkylbenzyloxy)-6cyanonaphthalenes, and 4-(4"-alkylbezyloxy)-4'-cyanobisphenyls.

4-Alkylbenzyl chloride (0.02 mole) is added to a mixture of 4-cyanophenol, 6-cyanonaphth-2-ol or 4-cyano-4'-hydroxybiphenyl, as appropriate, and anhydrous potassium carbonate (0.10 mole) in dry butan-2-one (60cm3). The mixture is stirred and heated under reflux for 24 hr. On cooling, the mixture is poured into water (15cm3) and the whole shaken with ether (3 x 80cm3). The combined ethereal extracts are washed with water (3x60cm3) and dried (Na2SO4). The solvent is removed in vacuo and the crude product purified by column chromatography using silica gel as the stationary phase and chloroform as the eluant. The final product is obtained by crystallisation from a smail volume of light ptroleum (bp, 40-60 C).

Examples of such products are given in Tables 2 to 5 below (Table 4 giving properties of specific products).

In the Tables the following symbols are used: N-I=nematic to isotropic liquid transition temperature (OC) C-N=crystalline solid to nematic transition temperature (OC) CSA=crystal to smectic A transition temperature (OC) SAN=smectic A to nematic transition temperature (OC) SA-I=smectic A to isotropic liquid transition temperature ( C) ()=monotropic transition temperature - [J=virtual transition temperature (+)-=optically active compound having a positive optical rotation angle Further compounds embodying the invention which may be prepared by the method described in Example 2 are given in Tables 2 to 4, as follows.

Table 2 Compounds of the form:

R R continued CH3 n-C8H17 02H5 n-CgH,g n-c3H7 n-C10H21 n-C4H9 (+)-2-methylbutyl n-05H11 (+)-3-methylpentyl n-06H13 (+)-4-methylhexyl n-C,H15 Table 3 Compounds of the form:

R" R" continued CH3 n-C8H17 C2H5 n-C9H19 n-C3H, n-C1OH21 n-C4Hg (+)-2-methylbutyl n-C5H11 (+)-3-methylpentyl n-C6H13 (+)-4-methylhexyl n-C7H15 Table 4 Compounds of the form:

R''' R''' continued CH3 n-C8H17 C2H5 n-C9H19 n-C3H7 n-C10H21 n-C4H9 (+)-2-methylbutyl n-C5H11 (+)-3-methylpentyl n-C6H13 (+)-4-methylhexyl n-C7H15 Table 5::Products of Step C2

Product Yield% C-I/N/SA SA-N SA-I N-I n-C5H11-#-CH2O-#-CN 21 49 C - - [-20 C] n-C6H13-#-CH2O-#-CN 35 48 - - [-3] n-C9H19-#-CH2O-#-CN 40 60 - - [5] #-CN 49 82 - - (62) n-C5H11-#-CH2O-# #-CN 41 82 - - (56) n-C6H13-#-CH2O-# #-CN 43 74 - - (71) n-C9H19-#-CH2O-# n-C5H11-#-CH2O-#-#-CN 58 91 - - 147 n-C6H13-#-CH2O-#-#-CN 52 85 129 C - 142 n-C9H19-#-CH2O-#-#-CN 60 60 - 142 C Examples of a material and device embodying the invention (in the second and third aspects above) 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 space 4 all held together by an epoxy resin glue. A liquid crystal material 1 2 fills the gap between the slides 2, 3 and the space 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 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.

Simiiarly the front electrode structure is formed into three electrodes per digit and decimal point y1, Y .... .. 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 offs0 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 whilstV/2 is applied to the remaining x electrodes. Meanwhile3V/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 3V 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 c 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 2 as follows (Mixture 1).

Table 3 : Mixture 1

The preparation of the last mentioned compound is described in UK Patent Application No 8003129.

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.

Claims

1. A liquid crystal compound exhibiting a positive dielectric anisotropy having a formula: R-X-CH2O-Ar-CN where R is an alkyl or alkoxy group, X is a 1,4-disubstituted bicyclo(2.2.2)octane ring or a 1,4disubstituted benzene ring, Ar is 1,4-disubstituted benzene ring, or a 4,4'-disubstituted biphenyl ring or a 2,6-disubstituted naphthalene ring, and CN is a cyano group, provided that where R is an alkoxy group X is a 1 ,4-disubstituted benzene ring.

2. A compound as claimed in claim 1 and wherein R is an alkyl group and X is a 1 ,4-disubstituted bicyclo(2.2.2)octane ring.

3. A compound as claimed in claim 1 and wherein R is an alkyl or alkoxy group and X is a 1,4disubstituted benzene ring.

4. A compound as claimed in claim 2 and wherein Ar is a 1,4-disubstituted benzene ring.

5. A compound as claimed in claim 3 and wherein Ar is a 1 ,4-disubstituted benzene ring.

6. A compound as claimed in claim 2 and wherein Ar is a 4,4'-disubstituted biphenyl ring.

7. A compound as claimed in claim 3 and wherein Ar is a 4,4'-disubstituted benzene ring.

8. A compound as claimed in claim 2 and wherein Ar is a 2,6-disubstituted naphthaline ring.

9. A compound as claimed in claim 3 and wherein Ar is a 2,6-disubstituted naphthalene ring.

1 0. A compound as claimed in claim 2 and which has been produced by a method substantially the same as that in Example 1 described hereinbefore.

11. A compound as claimed in claim 3 and which has been produced by a method substantially the same as that in Example 2 described hereinbefore.

12. A compound as claimed in claim 2 and which has the formula

is a bicyclo(2.2.2)octane ring.

13. A compound as claimed in claim 2 and which is any one of the compounds listed in Table 1 given hereinbefore.

14. A compound as claimed in claim 3 and which is any one of the compounds listed in Tables 2 to 4 given hereinbefore.

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

1 6. A liquid crystal device including 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 14.