WO1986000067A1

WO1986000067A1 - Pyrimidines

Info

Publication number: WO1986000067A1
Application number: PCT/GB1985/000266
Authority: WO
Inventors: George William Gray; David Lacey; Kenneth Johnson Toyne; Richard Michael Scrowston; Adam Jackson; Edward Peter Raynes
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1984-06-13
Filing date: 1985-06-13
Publication date: 1986-01-03
Also published as: GB2171696B; GB2171696A; GB8602796D0; DE3590260T1; JPS61503033A; GB8415039D0

Abstract

Pyrimidine esters which are suitable for use as liquid crystal materials. The esters have a nucleus comprising a 1.3-pyrimidine ring linked at the 2- or 5- position by an ester linkage to an optionally substituted phenyl, cyclohexyl, bicyclo-octyl or dioxane ring, or to an optionally substituted biphenylyl group. Preferred substituents on the pyrimidine are alkyl or alkoxy. The pyrimidine is preferably linked to a phenyl or biphenylyl group which may carry a lateral fluoro substituent and a terminal alkyl, alkoxy or cyano group. The alkyl or alkoxy group is preferably an n- alkyl or n- alkoxy group with 1 to 12 carbon atoms. The pyrimidine esters of the invention may be mixed with various other liquid crystal materials, suitable mixtures being described. An electro-optical device suitable for use with the pyrimidines of the invention is described. A number of suitable preparative routes for the pyrimidines esters are described.

Description

PYRIMIDINES

The present invention relates to pyrimidines suitable for use in liquid crystal materials.

The use of liquid crystal materials to exhibit electrooptical effects in display devices such as digital calculators, watches, meters and simple word 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 mixtures of compounds and new materials may be obtained by forming new compounds for use in such mixtures.

According to the present invention in a first aspect there is provided a pyrimidine ester having a formula:

R₁-(A)_n-B-Z I wherein: n is 0 or 1;

R₁ represents a group R_1a or R_1b wherein R_1a is selected from alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl and alkoxycarbonyloxy; and R_1b is selected from hydrogen, cyano, fluoro, bromo, chloro and CF₃;

A represents a group A₁(E₁)_m, wherein A₁ is selected from

Ph, Ch, Bco and Dx, E₁ is selected from -CO.O-, -O.OG-, -CH₂O-, -OCH₂ and -CH₂.CH₂-; and m is 0 or 1; B represents a 2,5 disubstituted 1,3 pyrimidine ring carrying the substituent Z in either its 2 or 5 position; and

Z represents a group selected from O.OCW₁ and CO.OW₂ wherein each of W₁ and W₂ is independently selected from the following groups:

-Ph-R₂; -Ph-Ph-R₂; -Ch-Ph-R₂. -Ch-Ph-Ph-R₂; -Ph-Ch-Ph- R₂; -Ph-Ph-Ch-R₂; -Ch-Ch-Ph-R₂, -Ch-Ph-Ch-R₂; -Ph-Ch-Ch-R₂; -Ph-Ph-Ph-R₂; -Bco-Ph-R₂; Dx-Ph-R₂ and -A₂-E₂-Ph-, wherein A₂ represents a group selected from Ph, Ch, Bco and Dx and wherein E₂ represents a group selected from -CO.O-, -O.OC-, -CH₂O-, OCH₂-, and -CH₂.CH₂-; wherein each Ph represents 1,4-disubstituted benzene optionally carrying a fluoro, chloro, cyano or methyl substituent in any one or more of its 2, 3, 4 and 5 positions, each Ch represents trans 1,4-disubstituted cyclohexane, each Bco represents 1,4-disubstituted bicyclo (2,2,2) octane, and each Dx represents trans -2,5-disubstituted 1,3 dioxan; and wherein each R₂ independently represents a group R_2a or R_2b, R_2a being selected from alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl and alkoxycarbonyloxy; and R_2b being selected from hydrogen, cyano, fluoro, bromo, chloro and CF₃;

The compounds of Formula I are useful components of liquid crystal materials, eg for electro-optical displays. Where the group R₁ and/or R₂ contains an n-alkyl group the compounds are, generally speaking, nematogenic and are useful as components of nematic liquid crystal materials. Where R₁ and/or R₂ contains a chiral alkyl group, eg (+)-2-methylbutyl, the compounds are, generally speaking, chiral nematogenic. Chiral nematogenic compounds (alternatively known as cholesterogenics) may be used as chiral additives to nematic materials or in other applications, eg temperature sensing applications taking advantage of their selective reflection of radiation. The principal reflected Wavelengths can be varied as a function of temperature.

A preferred species of compounds of Formula I are those wherein R₁ represents alkyl or alkoxy n is zero and -B- represents ^**""\_2^' .

Preferred sub-classes of compounds of this species of

Formula I are those having the following formulae:

^" R_A-Py-CO.O-Ph-R_B la

R_A-Py-CO.O-Ph.Ph-R_B lb wherein R_A represents alkyl or alkoxy, Py represents

—\0-" and Ph represents \j y or v and R_B represents

alkyl, alkoxy or cyano.

In Formula I, particularly Formulae la and lb, each alkyl group included in the terminal group(s) R₁ and/or R₂ (or R_A and/or R_B) is preferably a group having from 1 to 12 carbon atoms. It is preferably an n-alkyl group although it may alternatively be a chiral group eg (+)-2-methylbutyl. Where the group R₁ and/or R₂ includes an alkyl group it preferably has from 1 to 12 carbon atoms. Particularly preferred compounds are those of Formula la wherein R_A and R_B are both n-alkyl or n-alkoxy groups having from 1 to 12 carbon atoms, especially those wherein R_A and R_B are both n-alkyl or n-alkoxy groups having from 1 to 12 carbonatoms, especially those wherein R_A and R_B are both n-alkyl groups having from 3 to 7 carbon atom s and Ph =

"~\2/ In the accompanying drawings:

Figure 1 to 4 are schematic flow diagrams of preliminary routes which may be used for the preparation of intermediates.

Figures 5 and 6 are schematic flow diagrams of routes which may be used for the preparation of compounds of various sub-classes of formula I; Figures 7 to 13 are diagrams illustrating the construction and operation of an electro-optical device embodying the invention. These Figures are described further below.

The dielectric anisotropy of a liquid crystal compound or mixture is an important parameter which determines the operation voltage of an electro-optical cell incorporating the compound or mixture. It also determines, together with the cell may be used. Examples of known effects are described below. The compounds of Formula I may have a strongly positive, strongly negative or small dielectric anisotropy depending on any terminal and/or lateral substituent groups contained within the Formula I structure.

For example compounds having a terminal cyano or halo group are known to be strongly positive compounds. Those having a lateral cyano group as a substituent on a benzene ring are known to be strongly negative.

The compounds of Formula I 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 and suitable liquid crystal transition temperatures.

If one or more compounds of Formula I are mixed with one or more compounds of high positive dielectric anisotropy Δε, ie then compounds of reasonably low melting point are

preferred for use as the high dielectric anisotropy components to which the formula I compounds are added. For example, the compounds of the classes whose generalised formulae are listed in Table.1 are suitable as positive materials.

In Table 1 each R is independently n-alkyl or n-alkoxy and each R_A is independently n-alkyl.

Alternatively, or additionally, the compounds of Formula I may be added to small dielectric anisotropy compounds, which may also be low melting, ie having a melting point less than

80ºC, eg in order to reduce mixture melting point, viscosity or to improve multiplex!bility. Examples of the generalised formulae of such small dielectric anisotropy compounds are listed in Table 2. In Table 2, each R is independently n-alkyl or n-alkoxy; each R_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. Generally one or more compounds of Formula I may be added to one or more compounds of Formula Ila to Ili listed in Table 1 optionally together with one or more compounds of Formula IlIa to Illn listed in Table 2.

Additional high clearing point (clearing point 100°C) compounds may be included in such mixtures, eg one or more compounds selected from the known classes whose generalised formulae are listed in Table 3.

In Table 3 R, R_A, and X are as specified above.

Other specific known additives, eg chiral additives, such as R_c and R_D

where R_C = (+)-2-methylbutyl and R_D = (+)-2-methylbutoxy, may be incorporated in the mixture where required, eg for twisted nematic operation as described below.

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 arangement may be changed from the homeotropic texture (OFF state) to the homogenous 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 10⁹ 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, ie clear, surface aligned hometropic texture (OFF state) to a strongly scattering twisted homeogenous texture (ON state) by an electric field; (vi) a positive material which is a cholesteric, preferably together also with a pleochroic dye, in cholestericto-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 10⁹ 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 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 from (at low frequency) positive (OFF state) to negative (ON state) by the application of a high frequency electric field; (ix) a material suitable for the device described in copending UK Patent Application No 8317355.

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 Ila to IIi optionally together with one or more of the following: Component C: one or more compounds of Formula IlIa to IIIn; Component D: one or more compounds of Formula IVa to IVl; 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 75% by weight) Component B: 5 to 95% by weight (typically 10 to 50% by weight) Component C: 0 to 90% by weight (typically 5 to 25% by weight) Component D: 0 to 30% by weight (typically 0 to 20% 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 (postive nematic) and phase change (negative contrast type) effects a pleochroic dye forming from1.5 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 incorported 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 togeher in the correct weight proportion to form an overall isotropic liquid (eg about 100°C).

To provide a more general example of a mixture embodying the invention at least one compound according to Formula I above may be mixed together with one or more compounds in any one or more of the known families listed in Table 4 for use in one or more of the applications given above (the actual applications) depending on the mixture's properties):

In Table 4 each X is a 1,4 phenylene group, a 4,4' biphenylyl group, a 2,6 naphthyl group or a trans-1,4- disubstituted cyclohexane ring, and Y₁ is CN, or R' or halogen or CO.O-X-Y¹ where Y¹ is CN, or R' or OR'; where R and R' are alkyl groups; the compound may alternatively be a derivative of one of those listed in Table 4 wherein H is replaced by a halogen, eg F, in one or more of the benzene rings of the structure.

Preferably, the compound(s) of Formula I comprises between 5 and 95% by weight of the mixture.

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,

TABLE 4

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.

Although liquid crystal compounds which are esters are known esters having a -CO.O- group attached directly to a pyrimidine ring have not previously been published. The compounds of Formula I, particularly di-n-alkyl substituted compounds of Formula la, can show certain advantageous-properties. For example, they can show a low ratio k₃₃/k₁₁ where k₃₃ and k₁₁ are elastic constants well understood in the art. Such a low ratio provides a basis for good multiplexibility in liquid crystal materials suitable for multiplexed twisted nematic operation.

The compounds of Formula la above are particularly preferred for use in materials for multiplexed twisted nematic electro-optical devices when mixed together with the compounds of Formulae Ila to Ili in Table 1, particularly the compounds of Formula Ila where in R = n-alkyl. For this purpose each alkyl or alkoxy group R_A or R_B in Formula la is n-alkyl or n- alkoxy. Desirably, both R_A and R_B are n-alkyl.

Especially preferred mixtures for multiplexed twisted nematic operation comprise:

(i) one or more compounds of Formula Ila to III comprising from 40% to 60% by weight and

(ii) one or more compounds of Formula la comprising from 60% to 40% by weight of the mixtures. Optional additives such as high clearing point materials may be added to these mixtures.

The ester compounds of Formula I may be produced using procedures which are known per se the overall production route being new. Where Z = CO.OW₂ they may be produced by an esterification reaction of the carboxylic acid R₁-(A)_n-B-COOH, or a derivative thereof eg its chloride, and the appropriatehydroxy compound W₂OH. Where Z = O.OCW₁ they may be produced from the appropriate hydxoxy compound R₁-(A)_n-OH by esterification with the appropriate carboxylic acid W₁COOH or a suitable derivative thereof.

Generally speaking, methods of introduction of the radicals -O.OCW₁ or -CO.OW₂ into esters from the corresponding precursors W^OH and W₂C00H are known and such methods may be employed to introduce these radicals into the esters of Formula

I.

Similarly, methods of attachment of radicals R₁(A)_n to pyrimidine rings are known and such methods may be employed in the course of the formation of the appropriate esters or their precursors -R₁(A)_n-B-CO.OH and R₁(A)₁-B-OH.

Examples of preparative routes illustrating the formation of precursors of the esters of Formula I are Routes 1 to 4 shown in Figures 1 to 4. Examples of the use of these precursors to form esters are Routes 5 and 6 as shown in Figures 5 and 6. In Figures 1 to 6 R,R' = alkyl eg n-alkyl. Examples of methods of carrying out Routes 1 to 6 are as follows.

ROUTE 1

Step 1a The production of 2-alkyl-3-ethoxyprop-2-enals from the corresponding alkylmalondialdehyde tetraethyl acetals. This Step can be carried out by a standard literature method - as for example described by A BOLLER, M CEREGHETTI, M SCHADT and H SCHERRER, in Mol.Cryst .Liq .Cryst; 42, 215 (1977) Step 1b The production of 5-alkylpyrimidin-l-ylmethanols from the corresponding 2-alkyl-3-ethoxyprop-2-enals. This Step can be carried out by a standard literature method using 2-hydroxyacetamidine hydrochloride prepared from glycollic acid nitrile via the hydrochloride of HOCH₂. OC₃H₇ -

as for example described by A BOLLER, M CEREGHETTI, M SCHADT and H SCHERRER, in Mol.Cryst.Liq.Cryst; 42, 215 (1977)

Step lc The production of 5-alkyl-2-carboxypyrimidines from the corresponding 5-alkylpyrimidin-l-ylmethanoxs.

This Step can be carried out by first oxidising the 5- alkylprimidin-l-ylmethanol to the 5-alkylprimidine-2- carboxyaldehyde. This is achieved using toluene as solvent and manganese dioxide as the oxidising agent according to the method described by A BOLLER, M CEREGHETTI, M SCHADT and M SCHERRER, in Mol.Cryst.Liq.Cryst; 42, 215 (1977). The aldehyde is isolated, but not purified before it is converted by mild oxidation - for example by known literature methods using dilute nitric acid - into the 5-alkyl-2-carboxypyrimidine which is readily purified by crystallisation from ethanol.

ROUTE 2 Step 2a The production of 5-alkoxy-4, 6-dihydroxy-2- methylprimidines from the corresponding diethyl alkoxymalonates (prepared as described in Chem.Abstr ; 64 , 12, 673 ( 1966) ) .

This Step can be carried out using a standard literature method for the production of dihydroxypyrimidines from amidinehydrochlorides - in this case acetamidine hydrochloride - as for example described in East German Patent, 95892.

Step 2b The production of 5-alkoxy-4,6-dichloro-2-methyl- pyrimidines from the corresponding 5-alkoxy-4,6-dihydroxy-2- methylpyrimidines.

This Step can be carried out by a standard literature method for the conversion of dihydroxypyrimidines into dichloropyrimidines using phosphorous oxychloride - as for example described in East German Patent, 95892. Step 2c The production of 5-alkoxy-4,6-dichloro-2- tribromomethylpyrimidines from the corresponding 5-alkoxy-4,6- dichloro-2-methylpyrimidines.

This Step can be carried out by a standard literature methods using N-bromosuccinimide or bromine with irradiation. Step 2d The production of 5-alkoxy-2-carboxy- 4,6-dichloropyrimidines from the corresponding 5-alkoxy-4,6- dichloro-2-tribromomethyl-pyrimidines.

This Step can be carried out by hydrolysis procedures of a standard nature or by treatment with silver nitrate in acetic acid - as for example described in USSR Patent 104411 (1956) - [Chem.Abstr; 51, 6709 (1957)]

Step 2e The production of 5-alko y-2-carboxypyrimidines from the corresponding 5-alkoxy-2-carboxy-4,6- dichloropyrimidines.

This Step can be carried out by a standard literature procedure for the catalytic dechlorination of dichloropyrimidines - as for example described in East German Patent 95892. ROUTE 3

Step 3a The production of 5-alkyl-2-hydroxypyrimidines from the corresponding 2-alkyl-3-ethoxyprop-2-enals.

This Step can be carried out using the method of A BOLLER, M CEREGHETTI, M SCHADT and M SHERRER for the production of pyrimidines from ethoxyacroleins, but replacing the 2-hydroxyacetamidine hydrochloride by urea - as for example described in Mol.Cryst.Liq.Cryst; 42, 215 (1977). The 5- alkyl-2-hydroxypyrimidines are readily purified by crystallisation from common organic solvents such as methanol. ROUTE 4

Step 4a The production of 5-alkoxy-4, 6-dihydroxy-2- thioethylpyrimidines from the corresponding diethyl alkoxymalonates. This Step can be carried out using a standard literature method for the production of dihydroxypyrimidines from amidine hydrochlorides - as for example described in East German Patent 95892. In this case the amidine hydrochloride used is S- ethylisothiourea hydrochloride (C₂H₅S.C.NH₂.HCl) - see the

classical literature procedure of Wheeler and Johnson (1903) for the synthesis ofcytosine.

Step 4b The production of 5-alkoxy-4,6-dichloro-2- thioethylpyrimidines from the corresponding 5-alkoxy-4,6- dihydroxy-2-thioethylpyrimidines.

This Step can be carried out as described for STEP 2b of Route 2.

Step 4c The production of 5-alkoxy-2-thioethylpyrimidines from the corresponding 5-alkoxy-4,6-dichloro-2- thioethylpyrimidines.

This step can be carried out as described for STEP-2c of Route 2.

Step 4d The production of 5-alkoxy-2-hydroxyprimidines from the corresponding 5-alkoxy-2-thioethylpyrimidines. This Step can be carried out by treatment of the 5- alkoxy-2-thioethylpyrimidines with hydrogen bromide as described in the classical synthesis of cytosine by Wheeler and Johnson (1903). The lactams formed are tautomeric with the required 5-alkoxy-2-hydroxypyrimidines which are readily purified by crystallisation from a range of organic solvents. ROUTE 5

The production of esters from 5-alkyl- or 5-alkoxy-2- carboxypyrimidines and 4-alkyl- or 4-alkoxy-phenols (typical examples of compounds of formula W₁OH). Step 5a This Step for the production of these esters can be carried out by standard methods for the conversion of carboxylic acids (via their acid chlorides) into esters by interaction with hydroxy compounds. The esters are isolated and purified by column chromatography on silica gel. Fractions of each of the esters (single spot on tic) are combined and crystallised from either ethyl acetate or light petroleum (bp 40-60°C). The pure esters give infra-red spectra consistent with their structures and mass spectrometry gives the correct mass ion in each case.

ROUTE 6

The production of esters from 4-alkyl- or 4- alkoxy-henzoic acids (typical examples of compounds of formula W₂CO.OH) and 5-alkyl- or 5-alkoxy-2-hydroxypyrimidines. Step 6a This Step may be carried out in the same manner as Step 5a.

Examples of compounds which may be prepared by the methods of Routes 5 and 6 are listed in Table 5 as follows:

O CH₃O

Examples of materials and devices embodying the invention will now be described b,y way of example only with reference to the accompanying drawings wherein:

Figure 7 is a sectional view of a twisted nematic digital display; Figure 8 is a sectional view of the display shown in Figure 7 ;

Figure 9 shows a rear electrode configuration for Figure

7;

Figure to shows a front electrode configuration for Figure7 ;

Figures 11, 12 and 13 show schematic views of the device of Figures 7 to 9 with typical addressing voltages.

The display of Figures 7 to 10 comprise a cell 1, formed of two, front and back, glass slides 2, 3 respectively, spaced about 7 μm 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 Å thick are deposited on the inner faces of the slides 2, 3 as a complete layer and etched to the shapes shown in Figures 9, 10 . The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in Figure 9 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 ... as shown in Figure 10. 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 voltages 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°) through its rear polariser 6 to the reflector 7 where it is reflected back again to an observer (shown in Figure 1 at an angle of 45° 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 incident light at the position shown does not reach the reflector 7 and does not reflect back to the observer who seeks a dark display of one or more bars of a digit 10. Voltages are applied as follows as shown in Figures 11 , 12 and 13 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 3V across the liquidcrystal 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 3g 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 7 to 10 is a multiplexed display because the electrodes are shared between ON and OFF intersections or display elements.

Material embodying the invention which are suitable for use as the material 12 in the above device is Mixture 1 specified in Table 6 as follows.

TABLE 6 - MIXTURE 1

Compound Weight Percentage .

A small amount of an optically active material may be added to the nematic material to indiuce 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 optically active materials are:

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

Small amounts of pleochroic dye may be added to enhance the diplay contrast, eg 2% by weight of dye Mixture 2 specified in UK 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 filmes 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 2 below, the cell spacing being 10 μm.

TABLE 7 - MIXTURE 2

Compound Weight Percentage ( C r

Compound

may optionally be added to Mixture 2 (up to 3% by weight of Mixture 2) as a negative additive.

The preparation of Compound J is described in published UK Patent Application No 2061256A. About 1% by weight of a the dye mixture specified above 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 alterative 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 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 anisotopy (Δε) 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 on the respective inner surface of the glass slides is to convert the region of lquid 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 Δε 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 beteen 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 Δε is positive) to the liquid crystal material.

A suitable positive dielectric anisotropy material, Mixture 3, embodying the invention for use in a phase change effect (negative contrast type) device is:

The preparation of pyrimidine esters according to the present invention will now be described by way of example only, using Routes 2 and 5 illustrated in Fig 2 and Fig 5 of the accompanying drawings.

Step A Alkylation of Diethl Malonate Diethl Malonate (50 mmol) was added during 20 min to a rapidly stirred solution of sodium (50 mg-atom) in super-dry ethanol (100 ml). The n-alkyl bromide (n-octyl) (50 mmol) was then added and the solution was heated under reflux for 2h. The solvent was removed, crushed ice (100 g) was added, and the alkylmalonate layer was separated and distilled under reduced pressure. Yields: ca 78%

Step B Condensation of the Diethyl Alkylmalonates with Acetamidine Acetamidine hydrochloride (50 mmol) was added to a solution of sodium (150 mg-atom) in super-dry ethanol (300 ml), and the precipitated soduim chloride was filtered off. The diethyl alkylmalonate (50 mmol) was added to the filtrate, the mixture was stirred for 48 h, and the precipitated pyrimidine -4, 6 dione was filtered off. It was purified by dissolution in aqueous ammonia (d 0.88) and boiling to expel ammonia and crystallise the pyrimidine -4, 6- dione.

Yields: ca 54% Step C Chlorination of the Pyrimidine -4, 6- diones

A mixture of the dione (50 mmol), freshly distilled phosphoryl chloride (300 mmol) and NN - dimethylaniline (catalytic amount) was heated under reflux for several hours until a homogeneous solution was obtained. The remaining phosphoryl chloride was then distilled off and crushed ice (100 g) was added to the oily residue with efficient cooling and with the utmost caution. The resulting solution was basified by the careful addition of solid sodium hydrogen carbonate, then organic material was extracted as quickly as possible with several portions of ether. The dried extracts were evaporated, and the product was distilled. Step D Hydrodehaloqenation of the 4, 6- Dichloropyrimidines

A solution of the 4, 6- dichloropyrimidine (10 mmol) in ethanol (200 ml) was hydrogenated in the presence of 10% pallαdised charcoal (90 mg) and potassium acetate (22 mmol). When the calculated amount of hydrogen had been taken up the catalyst was filtered off, most of the solvent was removed, water was added, and the product was extracted into ether. The dried ethereal extracts were evaporated, to give a pale yellow oil which was purified to distillation.

Yields: ca 80% Step E 5 - Alkylpyrimidine - 2-carboxylic Acids A stirred solution of the 2, 5 - dialkylprimidine (10 mmol) in dry pyridine (20 ml) was heated under reflux for 2-3 hr with selenium dioxide (12 mmol). The precipitated selenium was then removed and the solvent was evaporated, to yield a semi-solid product which was crystallised from acetic acid.

Yields: ca 60% Step F Esterification of the 5- Alkylpyrimidine - 2- carboxylic Acids A solution of the 5 - alkyl pyrimidine - 2- carboxylic acid (10 mmol) in dry dichlorometlane was stirred with the appropriate phenol (4-n-pentyl phenol or hydroxybiphenyl (4-n-pentyl hydroxy diphenyl and 4-n-butoxy hydroxy biphenyl) (10 mmol) in the presence of dicyclohaxylcarbodi-imide (10 mmol) and 4-pyrrolidinopyridine (catalyst; 1 mmol) until reaction was complete (t.l.c). The precipitated NN'-dicyclohexylurea was filtered off and the organic layer was washed successively with water (3 x 50 ml), aqueous 5% acetic acid solution (3 x 50 ml) and water (3 x 50 ml). Evaporation of the dried solution gave the crude ester, which was purified by column chromatography on alumina.

Yields 85%

Other solvents including Ethylene Glycol Dimethyl Ether, Di-n-Propyl Urther and the commercially available solvent Ethyl Cellosolve (Trade Mark) may also be used.

Claims

1. A pyrimidine ester having a formula:

R₁-(A)_n-B-Z I wherein: n is 0 or 1;

R₁ represents a group R_1a or R_1b wherein R_1a is selected from alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl and alkoxycarbonyloxy; and R_{1 b} is selected from hydrogen, cyano, fluoro, bromo, chloro and CF₃;

A represents a group A₁ (E₁)_m , wherein A₁ is selected from Ph, Ch, Bco and Dx, E₁ is selected from -CO.O-, -O.OC-, -CH₂O-, OCH₂- and -CH₂.CH₂-; and m is O or 1;

B represents a 2,5 disubstituted 1,3 pyrimidine ring carrying the substituent Z in either its 2 or 5 position; and

Z represents a group selected from O.OCW₁ and CO.OW₂ wherein each of W1 and W2 is independently selected from the following groups:

-Ph-R₂; -Ph-Ph-R₂; -Ch-Ph-R₂; -Ch-Ph-Ph-R₂; -Ph-Ch-Ph-R₂; -Ph-Ph-Ch-R₂; -Ch-Ch-Ph-R₂, -Ch-Ph-Ch-R₂; -Ph-Ch-Ch-R₂; -Ph-Ph-Ph-R₂; -Bco-Ph-R₂; Dx-Ph-R₂ and -A₂-E₂-Ph-R₂,wherein A₂ represents a group selected from Ph, Ch, Bco and Dx and wherein E₂ represents a group selected from -CO.O-, -O.OC-, -CH₂O-, OCH₂-, and -CH₂.CH₂-; wherein each Ph represents 1,4-disubstituted benzene optionally carrying a fluoro, chloro, cyano or methyl substituent in any one or more of its 2, 3, 4 and 5 positions, each Ch represents trans 1,4-disubstituted cyclohexane, each Bco represents 1,4-disubstituted bicyclo (2,2,2) octane, and each Dx represents trans 2,5-disubstituted 1,3 dioxan; and wherein each R₂ independently represents a group R_2a or R_2b, R_2a being selected from alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl and alkoxycarbonyloxy; and R_2b being selected from hydrogen, cyano, fluoro, bromo, chloro and CF₃;

2. A pyrimidine ester as claimed in claim 1 and wherein the ester has the formula:

wherein each of R¹ and R² independently represents an n-alkyl or n-alkoxy group having up to 12 carbon atoms.

3. A pyrimidine ester as claimed in claim 1 and wherein the ester has the formula

4. A pyrimidine ester as claimed in any one of claims 1 to 3 wherein either or both of R¹ and R² contain a chiral group.

5. A pyrimidine ester as claimed in claim 2 wherein R¹ is n-Octyl and R² is n-Pentyl.

6. A pyrimidine ester as claimed in claim 3 wherein R¹ is n-Octyl and R² is n-Pentyl.

7. A pyrimidine ester as claimed in claim 3 wherein R¹ is n-Octyl and R² is n-Butyloxy.

8. A liquid crystal composition comprising a mixture of compounds at least one of which is a compound as claimed in claim 1.

9. A composition as claimed in claim 8 characterised in that it comprises one or more compounds selected from the families having the following generalised formulae:

mixed together with at least one compound as claimed in claim 1 and having a formula:

wherein each of R¹ and R² independently represents C_1-12 n- alkyl or C_1-12 n-alkoxy; and each R_B independently represents C_1-12 n-alkyl.

10. A liquid crystal electro-optical device characterised in that it incorporates a liquid crystal composition as claimed in claim 8 or claim 9.