Compounds Containing a Fluorobiphenylyl Group and Their Use in Liquid Crystal Material and Devices

The present invention relates to compounds containing a fluorobiphenylyl group and their use in and in the production of liquid crystal materials and devices.

The use of liquid crystal materials to exhibit electro-optical effects in display devices such as digital calculators, watches, meters and simple work displays is now well known. However, known liquid crystal materials are not ideal in all respects and a considerable amount of work is currently being carried out in the art to improve their properties.

Liquid crystal materials normally consist of specially selected mixture compositions and improved materials are obtained by forming new mixtures having an improved combination of properties.

The composition of a liquid crystal mixture for electro-optical applications depends on the kind of display effect to be utilised and the particular properties required for that effect. Examples of various display effects are given below. For all kinds however, it is desirable for the mixture to show the best possible combination of certain general properties, eg as follows:

(i) a liquid crystalline temperature range--including room temperature (20.degree. C.)--which is as wide as possible;

(ii) a melting point (solid-to-liquid crystal transition temperature) which is as low as possible;

(iii) a clearing point (liquid crystalline to isotropic liquid transition temperature) which is as high as possible;

(iv) a positive or negative (as appropriate) dielectric anisotropy (permittivity measured parallel to the molecular axis less that measured perpendicular to the molecular axis) which is as great as possible in order to minimise the display voltage;

(v) a viscosity which is as low as possible in order to minimise the display switching speeds;

(vi) an electro-optical response which varies as little as possible with temperatures;

(vii) a good chemical and photochemical stability;

Examples of further particular properties useful in specific applications are as follows:

(viii) a good multiplexibility;

(ix) an ability to switch dielectric anisotropy with frequency; and

(x) a low birefringence;

(xi) an ability to form by itself or in admixture with other compounds mainly nematic liquid crystalline phases, its tendency to form smectic liquid crystalline phases being kept to a minimum.

The required combination of properties of a liquid crystal mixture composition is obtained by blending together components of different properties in the mixture.

In order to keep production costs to a minimum it is desirable to use the smallest number of components in the mixture consistent with the achievement of a satisfactory combination of the required properties. Therefore it is desirable that the individual components used show a number of the required properties. It is desirable that the mutual solubility of the individual components is good, also in order to minimise the number of components.

The purpose of the present invention is to provide a class of novel compounds which provide suitable components for liquid crystal mixture compositions for electro-optical displays or are useful in the production of such compounds.

According to the present invention in a first aspect there is provided a compound having a formula: ##STR4## wherein: R.sub.1 is selected from H, alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy and alkoxycarbonyloxy;

R.sub.2 is selected from H, alkyl, alkoxy;

X.sub.1 is selected from H and fluorine;

X.sub.2 is selected from H and fluorine; and ##STR5## provided that one of X.sub.1 and X.sub.2 is fluorine. Compounds wherein ##STR6## are generally useful as liquid crystal compounds.

Compounds wherein ##STR7## are generally useful in the preparation of the aforementioned fluoro-p-terphenyls although they may also show liquid crystal properties themselves.

By a `liquid crystal` compound in this specification 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 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).

In the accompanying drawings:

FIGS. 1 and 2 are lists of the generalised formulae of sub-classes of compound which are examples of the general class of Formula I;

FIGS. 3 to 7 are flow diagrams respectively showing the preparation of various of the sub-classes listed in FIGS. 1 and 2;

FIGS. 8 to 11 are lists of the generalised formulae of known classes of compounds which may be mixed with compounds of Formula I to form liquid crystal mixtures;

FIGS. 12 to 18 are illustrations of examples of liquid crystal devices incorporating liquid crystal materials containing compounds of Formula I; these Figures are further described below.

Examples of sub-classes of compounds of Formula I wherein X.sub.1 is H and X.sub.2 is F are listed in FIG. 1 and the generalised formulae of these sub-classes are indicated by `Ia` to `Ik` respectively in FIG. 1. These compounds are referred to as `2'-fluoro` compounds of Formula I in this specification. In FIG. 1 each R.sub.A independently represents an alkyl group.

Examples of sub-classes of compounds of Formula I wherein X.sub.1 is F and X.sub.2 is H are listed in FIG. 2 and the generalised formulae of these sub-classes are indicated by `Il` to `Iv` respectively in FIG. 2. These compounds are referred to as `2-fluoro` compounds in this specification. Each R.sub.A independently represents an alkyl group in FIG. 2.

For use in nematic liquid crystal mixtures each group R.sub.A in any of the sub-classes listed in FIG. 1 or FIG. 2 is preferably an unbranched chain group. However, for use in chiral nematic (optically active) materials one or more groups R.sub.A contained in the molecule may independently be a chiral group, eg of the form (+)-CH.sub.3 (CH.sub.2).sub.n CH(CH.sub.3)CH.sub.2, where n is from 1 to 8 and (+)- represents a chiral group having a positive optical rotation angle, eg (+)-2-methylbutyl.

Each alkyl group included in Formula I preferably has from 1 to 12, desirably 1 to 8 carbon atoms.

Thus in the sub-classes listed in FIGS. 1 and 2 each R.sub.A may be C.sub.1-8 n-alkyl.

Preferred sub-classes are the di-alkyl terminated fluoro-p-terphenyls of Formulae Ia and Il wherein each R.sub.A independently represents a C.sub.1-8 n-alkyl group.

The compounds of Formula I, particularly those of Formulae Ia and Il are very attractive components of liquid crystal materials for electro-optical display applications because they show:

(i) a relatively wide liquid crystalline temperature range;

(ii) a relatively high clearing point (liquid crystal-to-isotropic liquid transition temperature);

(iii) a relatively low melting point;

(iv) a reduced tendency to form smectic phases by themselves and in mixtures with other nematic materials;

(v) a relatively low viscosity by themselves and a reduced mixture viscosity when used in mixtures with nematic materials;

(vii) a reasonably low birefringence;

(viii) a good chemical and photochemical stability.

Compounds of Formula A as follows are known: ##STR8## wherein R.sub.1 and R.sub.2 are for example alkyl groups. However, the fluorinated compounds of Formula I are more useful in practical devices than the corresponding compounds of Formula A because they can show:

(i) a much wider liquid crystalline temperature range;

(ii) a greater portion of the liquid crystalline phase which is nematic;

(iii) a reduced portion of the liquid crystalline phase which is smectic;

(iv) by themselves and especially in admixture with other nematic materials a depression of the temperature range at which any smectic phases are formed; as well as

(v) a lower melting point.

Examples of the comparison of specific properties of Formula I and Formula A compounds are given below (in Table 7).

According to one preferred feature of the invention a nematic or chiral nematic liquid crystal material comprises a mixture of two or more compounds of Formula I. Such a mixture may have a wide nematic or chiral nematic temperature range. The mixture may contain homologues of the same sub-class, eg 2 or more compounds of Formula Ia.

Alternatively, pairs of compounds may be formed having the same general structure but in different structural isomeric forms with regard to the groups X.sub.1 and X.sub.2. In other words one compound has X.sub.1 =F and X.sub.2 =H and the other has X.sub.2 =F and X.sub.1 =H in an otherwise identical structure (eg compounds of Formulae Ia and Il). Such isomers have similar properties but a mixture of the two can give a very useful depression of melting point.

The compounds of Formula I may be used in applications other than electro-optical applications which are known to be suitable for the use of nematic or chiral nematic liquid crystal compounds. For example, the compounds of Formula I may be incorporated as high clearing point components of temperature senstive mixtures, eg thermochromic materials, eg for use in the applications described in UK Published Patent Application Nos. 2083244A and 2085585A.

Compounds of Formula I may be prepared by synthetic routes involving steps which are known per se, the overall route being new.

For example, in FIG. 3 a generalised route, Route A is shown for the preparation of 2'-fluoro compounds of Formula I. In FIG. 3, R.sub.1 =alkyl, preferably n-alkyl, and R.sub.2 =H, alkyl, preferably n-alkyl or alkoxy preferably n-alkoxy. R' is C.sub.m-1 H.sub.2m-1 where R.sub.1 is C.sub.m H.sub.2m+1.

FIG. 4 shows a generalised route, Route B, for the preparation of 2'-fluoro compounds of Formula I wherein R" is alkyl, alkylcarbonyl or alkoxycarbonyl. The starting material in Route B may be obtained from Route A (Steps A1 to A4) wherein R'=CH.sub.3.

FIG. 5 shows a generalised route, Route C, for the preparation of 2'-fluoro compounds of Formula I wherein R.sub.1 is alkyl, C.sub.m H.sub.2m+1, R' is alkyl C.sub.m-1 and R.sub.2 is H, alkoxy or alkyl.

FIG. 6 shows a generalised route, Route D, for the preparation of 2'-fluoro compounds of Formula I wherein R" is alkyl, alkylcarbonyl or alkoxycarbonyl.

2-Fluoro compounds of Formula I may be made by routes analogous to Routes A to D for the corresponding 2'-fluoro compounds.

For example Route E (Steps E3 to E5) shown in FIG. 7 is analogous to Route C (Steps C3 to C5). In Route E R.sub.1 =alkyl, preferably n-alkyl, and R.sub.2 =H, alkyl, preferably n-alkyl, or alkoxy, preferably n-alkoxy. Steps E1 and E2 of Route E illustrate production of the appropriate starting material of 2-fluoro-4-alkyl-4'-bromobiphenyl for Steps E3 to E5.

The compounds of Formula I have a relatively small negative dielectric anisotropy and may be added to liquid crystal materials of positive or negative dielectric anisotropy, known and referred to herein respectively as "positive" or "negative" materials in order to produce a mixture having amongst other things a suitable dielectric anisotropy. As is well known to those skilled in the art the dielectric anisotropy of the liquid crystal material is necessary for electro-optical operation and its sign (for a given frequency) is chosen according to the kind of electro-optical device in which the material is to be used.

Compounds of reasonably low melting point are preferred as positive high dielectric anisotropy components. For example, the compounds of the classes represented by the formulae listed in FIG. 8, labelled IIa to IIi respectively, are suitable as positive materials.

In FIG. 8 each R is independently n-alkyl or n-alkoxy and each R.sub.A is independently n-alkyl.

Alternatively, or additionally, the compounds of Formula I may be added to other small dielectric anisotropy compounds, eg to reduce mixture melting point, viscosity or to improve multiplexibility. The classes represented by the Formulae listed in FIG. 9 and labelled IIIa to IIIn are examples of such other compounds.

In FIG. 9

each R is independently n-alkyl or n-alkoxy

each R.sub.A is independently n-alkyl

each R' is independently n-alkyl, n-alkoxy or hydrogen;

X=H or F; and

Q=halogen, eg Cl or F.

Thus, one or more compounds of Formula I may be added to one or more compounds of Formula IIa to IIi listed in FIG. 8 optionally together with one or more compounds of Formula IIIa to IIIn listed in FIG. 9.

Additional high clearing point compounds may be included in such mixtures eg one or more compounds selected from the classes represented by the formulae listed in FIG. 10 labelled IVa to IV.

In FIG. 10 R, R.sub.A and X are as defined above for FIG. 9.

Other specific known additives, eg chiral additives, such as ##STR9## where C.sub.C =(+)-2-methylbutyl and R.sub.D =(+)-2-methylbutoxy, may be incorporated in the mixture where required.

The liquid crystal material obtained by blending together compounds of Formula I with those of the other classes as specified may be any one of the following:

(i) a positive nematic material for use in twisted nematic effect devices including multiplexed devices; an example of such a device is given below:

(ii) a negative material preferably also with a pleochroic dye, for use in Freedericksz effect devices (negative nematic type) in which the molecular arrangement may be changed from the homeotropic texture (OFF state) to the homogeneous texture (ON state) by an electric field; an example of such a device is given below:

(iii) a positive nematic material, preferably also with a pleochroic dye, for use in Freedericksz effect devices (positive nematic type) in which the molecular arrangement may be changed from the homogeneous texture (OFF state) to the homeotropic texture (ON state) by an electric field:

(iv) a negative material which is a cholesteric (chiral nematic) of suitable resistivity (about 10.sup.9 ohm-cm), for use in cholesteric memory mode devices in which the molecular arrangement may be changed from a homogeneous texture (OFF state) to a scattering focal conic texture (ON state) by an electric field:

(v) a strongly negative material which is a cholesteric, preferably together also with a pleochroic dye, for use in cholesteric-to-nematic phase change effect devices (positive contrast type) in which the molecular arrangement may be changed from a weakly scattering, is clear, surface aligned homeotropic texture (OFF state) to a strongly scattering twisted homogeneous texture (ON state) by an electric field:

(vi) a positive material which is a cholesteric, (chiral nematic) preferably together also with a pleochroic dye, in cholesteric-to-nematic phase change effect devices (negative contrast type) 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) a negative nematic material of suitable resistivity (about 10.sup.9 ohm-cm), in dynamic scattering effect devices in which the molecular arrangement may be changed from a clear homeotropic texture (OFF state) to a turbulent scattering texture (ON state) by an electric field:

(viii) a positive nematic material in two frequency switching effect devices (which may be twisted nematic effect devices) in which the dielectric anisotropy of the material may be changed in the ON state (at low frequency) from positive to negative by the application of a high frequency electric field:

(ix) a material suitable for the device described in copending UK Patent Application No. 8218821.

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

Where a liquid crystal material is for use in a twisted nematic effect, cholesteric to nematic phase change effect (negative contrast type) or Freedericksz effect (positive nematic type) device the material preferably contains:

Component A: one or more compounds of Formula I; plus

Component B: one or more compounds of Formula IIa to IIi

optionally together with one or more of the following:

Component C: one or more compounds of Formula IIIa to IIIn;

Component D: one or more compounds of Formula IVa to IVo;

Component E: one or more chiral additives.

For the twisted nematic effect and Freedericksz (positive nematic) effect the following percentages of the various components may be used in the material (the overall sum of the percentages adding to 100%).

Component A: 5 to 95% by weight (typically 5 to 40% by weight)

Component B: 5 to 60% by weight (typically 15% to 40% by weight)

Component C: 0 to 90% by weight (typically 5 to 30% by weight)

Component D: 0 to 40% by weight (typically 5 to 30% by weight)

Component E: 0 to 5% by weight (typically 0 to 1% by weight)

For the phase change (negative contrast type) the following proportions may be used:

Components A to D: in the percentages as specified above;

Component E: 2 to 20% (typically 4 to 5%) by weight.

For the Freedericksz (positive nematic) and phase change (negative contrast type) effects a pleochroic dye forming from 1.5 to 15% of the overall mixture is preferably added to the liquid crystal material. Suitable dyes are described in published UK Patent Application Nos. 2081736A, 208219A and 2093475A. Typically, each dye compound incorporated forms 1 to 3% by weight of the overall mixture.

Liquid crystal mixtures including compounds of Formula I may be formed in a known way, eg simply by heating the constituent compounds together in the correct weight proportion to form an overall isotropic liquid (eg about 100.degree. C.).

Examples of further generalised formulae of liquid crystal compounds which may be mixed with the components of the present invention are given in FIG. 11. Each R and R.sub.A is as defined above.

According to the present invention in a second 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 second 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.

Examples of the preparation and properties of compounds of Formula I embodying the present invention will now be given. In these Examples the following symbols are used:

C-I, mp=melting point

bp=boiling point

ir=infra-red spectroscopy

.sup.1 Hnmr=proton nuclear magnetic resonance

Rf=rate factor

ms=mass spectrometry

tlc=thin layer chromatogry

glc=gas liquid chromatography

hplc=high pressure liquid chromatography

g=grammes

h=hours

DSC=differential scanning calorimetry

Hg=mercury

Brackets around a transition temperature eg (N-I) indicate a monotropic transition.

In the following Examples spectroscopic analysis (ir, .sup.1 H nmr and ms), tlc, glc and hplc were carried out on all the final products (and, where necessary, on the intermediates) to verify their structure and/or purity.

An example of Route A specified above wherein R.sub.1 =n-C.sub.5 H.sub.11 and R.sub.2 =n-C.sub.3 H.sub.7.

As an example, the following procedure describes the preparation of cis-/trans-4-n-propyl-1-(2'-fluorobiphenyl-4'-yl)cyclohexanol.

About a third of a solution of 4-bromo-2-fluorobiphenyl (25.1 g, 0.10 mol) in sodium-dried ether (60 cm.sup.3) was added to a flask containing magnesium turnings (2.40 g, 0.10 g atom). The reaction was initiated by the addition of a crystal of iodine and gentle warming. The remaining ethereal solution of 4-bromo-2-fluorobiphenyl was added dropwise so that the reaction mixture gently boiled. After the addition, the reaction mixture was gently heated under reflux for 1 hour. When the reaction mixture had cooled to room temperature, a solution of 4-n-propylcyclohexanone (0.10 mol) in sodium-dried ether (50 cm.sup.3) was added over a period of 1/2-hour, and the resulting reaction mixture was then heated under reflux for a further 1/2-hour. The cooled solution was carefully poured onto a vigorously stirred mixture of concentrated sulphuric acid (25 cm.sup.3) and crushed ice (300 g). The product was extracted into ether, and the combined ethereal extracts were washed with water and dried (MgSO.sub.4). The crude residual alcohol was recrystallised from hexane.

From the results obtained from tlc and spectroscopic analyses (ir, .sup.1 Hnmr and ms), it was evident that the purified samples were isomeric mixtures, containing the cis- and trans-isomers in about equal amounts.

The pure cis- and trans-isomers of these compounds were isolated by column chromatography on silica gel, eluting with chloroform. The cis- and trans-isomers were recrystallised from hexane, but a positive identification as to which of the isomers was cis- and which trans- was not made.

The isomeric compounds prepared by Step A1 are given in Table 1 as follows:

The preparation of 4-n-propyl-1-(2'-fluorobiphenyl-4'-yl)cyclohexene.

A solution of the isomeric mixture of 4-n-propyl-1-(2'-fluorobiphenyl-4'-yl)cyclohexanols (0.06 mol) in sodium-dried benzene (100 cm.sup.3) was added dropwise to a vigorously stirred mixture of phosphorus pentoxide (0.15 mol) in sodium-dried benzene (200 cm.sup.3) over a period of 1/2-hour. The resulting reaction mixture was then stirred for 3 hours at room temperature. When cooled to 0.degree. C., ice-water (300 cm.sup.3) was added carefully. The aqueous phase was treated with benzene and the combined organic extracts were washed with 5% sodium carbonate solution and water, and then finally dried (MgSO.sub.4). The crude residual cyclohexene was recrystallised from ethanol. A small sample of the pure material was obtained by column chromatography on silica gel, eluting with a 1:9 mixture of chloroform: petroleum ether (b p 60.degree.-80.degree. C.). The product was recrystallised from ethanol.

The product had the following properties:

C-I=63.5.degree. C., N-I=(57.2.degree. C.).

The preparation of 2-fluoro-4-n-propyl-p-terphenyl.

A solution of the 4-n-propyl-1-(2'-fluorobiphenyl-4'-yl)cyclohexene (0.05 mol), prepared as in Step A2, in sodium-dried toluene (100 cm.sup.3) was added dropwise to a vigorously stirred mixture of 2,3-dichloro-5,6-dicyanobenzoquinone (0.11 mol) in sodium-dried toluene (200 cm.sup.3) over a period of 1/2-hour. The reaction mixture was then heated under reflux at 110.degree. C. for 3 hours. On cooling to room temperature, any precipitate of the quinol derived from quinone was filtered off, and the filtrate was washed with 15% sodium metabisulphite solution and with water. Finally, the solution in toluene was dried (MgSO.sub.4).

The residual solid was sublimed (120.degree.-140.degree. C./0.01 mm Hg) and recrystallised from ethanol. This afforded 2-fluoro-4-n-propyl-p-terphenyl mp 74.degree.-76.degree. C. (72%) as a white crystalline solid.

Compounds of formula ##STR11## which may be prepared by a procedure analogous to Step A3 are those wherein R.sub.2 is respectively: CH.sub.3, CH.sub.3 O, C.sub.2 H.sub.5, C.sub.2 H.sub.5 O, n-C.sub.3 H.sub.7 O, n-C.sub.4 H.sub.9 , C.sub.4 H.sub.9 O, n-C.sub.5 H.sub.11, n-C.sub.5 H.sub.11 O, n-C.sub.6 H.sub.13, n-C.sub.6 H.sub.13 O, n-C.sub.7 H.sub.15, n.sub.7 H.sub.15 O, n-C.sub.8 H.sub.17, n-C.sub.8 H.sub.17 O, n-C.sub.9 H.sub.19, n-C.sub.9 H.sub.19 O, n-C.sub.10 H.sub.21, n-C.sub.10 H.sub.21 O, n-C.sub.11 H.sub.23, n-C.sub.11 H.sub.23 O, n-C.sub.12 H.sub.25, n-C.sub.12 H.sub.25 O.

The preparation of 2'-fluoro-4"-n-pentanoyl-4-n-propyl-p-terphenyl

To a cooled stirred mixture of finely powdered anhydrous aluminium chloride (2.0 g, 0.015 mol) and 2'-fluoro-4-n-propyl-p-terphenyl (0.0125 mol) (prepared as in Step A3) in sieve-dried nitrobenzene (25 cm.sup.3), a solution of pentanoyl chloride (0.015 mol) in sieve-dried nitrobenzene (5 cm.sup.3) was added over a period of 15 minutes, the temperature being kept below 0.degree. C. After stirring the reaction mixture at room temperature for 20 hours, it was poured onto a mixture of crushed ice (50 g), water (10 cm.sup.3), and concentrated hydrochloric acid (10 cm.sup.3), and stirred vigorously for 1/2-hour. The aqueous phase was treated with chloroform and the combined organic extracts were washed with water. The chloroform and nitrobenzene were removed by steam distillation and the product was extracted into ether, the ethereal solution being washed with water prior to drying (MgSO.sub.4). The resulting solid residue was purified by column chromatography on silica gel, eluting with a 2:1 mixture of chloroform/petroleum ether (bp 60.degree.-80.degree. C.). The product was recrystallised from 5% benzene/propan-2-ol (yield 66%).

The preparation of 2'-fluoro-4"-n-pentyl-4-n-propyl-p-terphenyl.

The 2'-Fluoro-4"-n-pentanoyl-4-n-propyl-p-terphenyl (0.007 mol) prepared in Step A4 was added to a stirred mixture of diethylene glycol (50 cm.sup.3), hydrazine hydrate (0.035 mol), and potassium hydroxide (0.0175 mol), and the reaction mixture was heated at 130.degree. C. for 2 hours. The excess of hydrazine was distilled off and the temperature of the reaction mixture was raised to 180.degree. C. for 4 hours. On cooling to room temperature, the reaction mixture was poured into water (1000 cm.sup.3), and the product was extracted into ether. The combined ethereal extracts were then washed with water and dried (MgSO.sub.4). The pale yellow residue was purified by column chromatography on silica gel, eluting with a 1:5 mixture of chloroform/petroleum ether (bp 60.degree.-80.degree. Recrystallisation from propan-2-ol afforded the product as a white crystalline solid.

The product, 2'-fluoro-4"-n-pentyl-4-n-propyl-p-terphenyl had the following properties: C-N=50.degree. C.; N-I=140.6.degree. C. The yield was 38%.

Examples of products prepared by a procedure analogous to Example 1 are given in Table 2 as follows.

As examples of properties of the compounds listed in Table 2 where R.sub.1 =C.sub.2 H.sub.5 and R.sub.2 =n-C.sub.5 H.sub.11 the compound has the properties C-S=64.0.degree. C.; S-N=90.degree. C.; N-I=128.7.degree. C.; where R.sub.1 =n-C.sub.3 H.sub.7 and R.sub.2 =n-C.sub.5 H.sub.11 the compound* has the properties C-S=61.0.degree. C.; S-N=99.5.degree. C. and N-I=141.5.degree. C. and where R.sub.1 =n-C.sub.4 H.sub.9 and R.sub.2 =n-C.sub.5 H.sub.11 the compound has the properties C-S=68.degree. C.; S-N=106.8.degree. C. and N-I=131.2.degree. C.

The clearing transition temperatures of various examples of compounds of Formula ##STR13## are as given in Table 3a as follows:

The main transition temperatures of various compounds of Formula ##STR14## are as given in Table 3b as follows:

Additional transition temperatures shown by compounds listed in Table 3b are given in Table 3c as follows:

An example of Route B specified above wherein R.sub.1 =C.sub.4 H.sub.9 O and R.sub.2 =n-C.sub.3 H.sub.7.

The methyl p-terphenylylketone (R.sup.1 =CH.sub.3, R.sub.2 =n-C.sub.3 H.sub.7) produced by the procedure of Step A4 of Route A was converted into the corresponding hydroxy compound using 1.25 equivalents m-chloroperoxybenzoic acid in dry dichloromethane (Baeyer-Villiger Oxidation); the resulting ester was hydrolysed to the appropriate phenol with 10% ethanolic potassium hydroxide.

The resulting hydroxy compound produced by Step B1 was alkylated (R.sub.1 =n-C.sub.5 H.sub.11) using a standard method from the literature, eg as described in J. Chem. Soc., P1467 (1954) by G. W. Gray and B. Jones.

Analogous compounds with the 4-substituent being either alkoxy (OR) or hydrogen (H) which may be prepared by an analogous procedure are listed in Table 3 as follows.

An example of Route C specified above wherein R.sub.1 =n-C.sub.5 H.sub.11 and R.sub.2 =n-C.sub.3 H.sub.7.

The preparation of 4'-n-pentanoyl-4-brono-2-fluoro-biphenyl.

The procedure carried out in Step C1 was similar to that described in Step A4 above.

The preparation of 4'-n-pentyl-4-bromo-2-fluorobiphenyl.

The ketone produced by Step C1 was reduced using the standard LiAlH.sub.4 -AlCl.sub.3 /(C.sub.2 H.sub.5).sub.2 O/CHCl.sub.3 method from the literature described in J. Org. Chem. 37, 3355 (1972) by W. L. Albrecht, D. H. Gustafson and S. W. Horgan. Any resulting alkene was hydrogenated using 10% Pt-C/hydrogen (H.sub.2) in ethyl acetate:

4'-n-pentyl-4-bromo-2-fluorobiphenyl, bp=110.degree. C./0.01 mmHg, was the product obtained by this procedure.

The method of preparing cis/trans-4-n-Pentyl-1-(4"-n-propyl-2'-fluorobiphenyl-4'yl)cyclohexanol was similar to that described in Step A1 above.

The isomeric compounds prepared by Step C3 are given in Table 4 below:

The mp's and transition temperatures were confirmed by DSC.

The method of preparation of 4-n-pentyl-1-1-4"-n-propyl-1-2'-fluorobiphenyl-4'-yl)cyclohexenes was similar to that described in Step A2 above.

The product of Step C4 was converted into the corresponding 2'-fluoro-4-n-pentyl-4"-n-propyl-p-terphenyl by a procedure similar to Step A3 above.

Compounds which may be prepared by an analogous procedure are as listed in Table 2 above.

Further examples of the products of Steps C1 to C5 are as listed in Table 5 as follows:

An example of Route D specified above wherein R.sub.1 =n-C.sub.4 H.sub.9 and R.sub.2 =n-C.sub.3 H.sub.7.

The method of preparation of 4-bromo-2-fluoro-4'-hydroxybiphenyl was similar to that described in Step B1 above.

The method of preparation of 4-bromo-4'-butoxy-2-fluorobiphenyl was similar to that described in Step B2 above.

The method of preparation of cis/trans-4-n-butoxy-1-(4"-n-propyl-2'-fluorobiphenyl-4'-yl)cyclohexanol was similar to that described in Step A1 above.

The method of preparation of 4-n-butoxy-1-14"-n-propyl-2'-fluorobiphenyl-4'-yl)cyclohexene was similar to that described in Step A2 above.

The method of preparation of 2'-fluoro-4-n-butoxy-4"-n-propyl-p-terphenyl was similar to that described in Step A3 above.

The fluoro-p-terphenyl compounds which may be prepared analogously to Steps D1-D5 are as listed in Table 3 above.

As mentioned above, compounds of Formula I which are 2-fluoro compounds may be prepared by methods analogous to Examples 1, 2, 3 or 4 using the appropriate 2-fluoro starting material. Examples of 2-fluoro compounds which may be made in this way are listed in Table 6 as follows.

The superiority of the fluorinated compounds of Formula I compared with known analogues containing no fluorine which are of Formula A given above is illustrated in Table 7 as follows.

As seen from Table 7 the known Compound No. PA1 has a high melting point, a narrow liquid crystalline temperature range and no nematic liquid crystalline phase. Its liquid crystalline phases are entirely smetic in nature.

In contrast, Compound Nos. 9 and 19 embodying the invention have a considerably lower melting point, a wide liquid crystalline temperature range in which the smectic region is suppressed or eliminated, but in any event is considerably lowered in temperature, and the nematic region is considerably enhanced.

The improvements illustrated in Table 7 in the properties of the Compounds Nos. 9 and 19 per se lead also to corresponding improvements in the properties of mixtures in which the Compounds Nos. 9 and 19 are incorporated, eg reduced viscosity, and reduced tendency to form injected smectic phases, as compared with corresponding mixtures containing Compound No PA1.

In fact, incorporation of fluorine in the basic terphenyl molecule of Compound No. PA1 to form Compound Nos. 9 and 19 effectively converts the structure from one having little practical use to one which is highly attractive for use in commercial devices.

Examples of the main transition temperatures of various alkoxy-terminated compounds of Formula I are as given in Table A1 as follows:

Examples of other transition temperatures of various compounds listed in Table A1 are given in Table A2 as follows:

Examples of electro-optical devices embodying the invention will now be described by way of example only with reference to the following drawings wherein:

FIG. 11 is a sectional view of a twisted nematic digital display:

FIG. 12 is a sectional view of the display shown in FIG. 11.

FIG. 13 shows a rear electrode configuration for FIG. 1;

FIG. 14 shows a front electrode configuration for FIG. 1;

FIGS. 15, 16 and 17 show schematic views of the device of FIGS. 11 to 14 with typical addressing voltages.

Examples of electro-optical devices embodying the invention will now be described by way of example only with reference to the following drawings wherein:

FIG. 12 is a sectional view of a twisted nematic digital display;

FIG. 13 is a sectional view of the display shown in FIG. 12.

FIG. 14 shows a rear electrode configuration for FIG. 1:

FIG. 15 shows a front electrode configuration for FIG. 1:

FIGS. 16, 17 and 18 show schematic views of the device of FIGS. 12 to 15 with typical addressing voltages.

The display of FIGS. 11 to 14 comprises a cell 1, formed of two, front and back, glass slides 2, 3 respectively, spaced about 7 .mu.m 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 100 .ANG. thick are deposited on the inner faces of the slides 2, 3 as a complete layer and etched to the shapes shown in FIGS. 3, 4. The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in FIG. 13 the rear electrode structure is formed into three electrodes x.sub.1, x.sub.2, x.sub.3. Similarly the front electrode structure is formed into three electrodes per digit and decimal point y.sub.1, y.sub.2, y.sub.3 . . . 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 90.degree.) through its rear polariser 6 to the reflector 7 where it is reflected back again to an observer (shown in FIG. 11 at an angle of 45.degree. 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 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 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 FIGS. 15, 16 and 17 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 FIGS. 12 to 18 is a multiplexed 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 12 in the above device is in Table as follows (Mixture 1).

Small amounts of an optically active 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 U.K. Pat. Ser. Nos. 1,472,247 and 1,478,592.

Suitable optically active materials are:

C15: about 0.1-0.5% by weight and CB15: about 0.01% to 0.05% by weight. ##STR29##

Small amounts of pleochroic dye may be added to enhance the display contrast, eg 2% by weight of dye Mixture 2 specified in U.K. Patent Specification No. 2093475A. One polariser is removed in this case.

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 polarisers 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 lecithin 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 3 below, the cell specing being 10 .mu.m.

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 (chlolesteric-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 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 (.DELTA..epsilon.) the liquid crystal material is in a twisted focal conic molecular configuration 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 .DELTA..epsilon. 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 the dye mixture specified above in the case where .DELTA..epsilon. is positive) to the liquid crystal material.

A suitable positive dielectric anisotropy material, Mixture 4, embodying the invention for use in a phase change effect (negative contrast type) device is:

A suitable negative dielectric anisotropy material embodying the invention for use in a phase change effect (positive contrast type) device, Mixture 5, is as follows:

As an alternative to the chiral compound specified in Table 11 a chiral compound of Formula I may be used.