Display element based on the electroclinic effect

Abstract

The invenion relates to a display element based on the electroclinic effect containing a liquid crystal medium being a mixture of at least two chiral components and at least one non-chiral component, characterized in that said medium has a pitch compensated cholesteric phase above the optically active smectic A phase.

Description

SUMMARY OF THE INVENTION

The invention relates to a display element based on the electroclinic effect containing a liquid crystal medium being a mixture of at least two chiral components and at least one non-chiral component, characterized in that said medium has a pitch compensated cholesteric phase above the optically active smectic A phase.

Ferroelectric smectics have recently entered the domain of liquid-crystal application in the so-called SSFLC (surface stabilized ferroelectric liquid crystal) device described by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. Specification No. 4,367,924).

This device utilizes the chiral smectic C phase or any chiral tilted smectic phase each of which has the characteristic of being ferroelectric.

The existence of a spontaneous polarization is possible only in chiral tilted smectic phases. However, in nontilted smectic phases, like smectic A, built up from chiral molecules one observes an induced tilt on applying an external electric field. Garoff and Meyer (S. Garoff and R. B. Meyer, Physical Review Letters 38, 848 (1977); Physical Review A 19, 338 (1979)) first described this so-called electroclinic effect (ECE).

The device they used shows only a small ECE and requires phase-sensitive methods for its detection.

Anderson et al. (G. Auderson et al., Appl. Phys. Lett. 51, 640 (1987); European Patent Application 0 263 225) describe a device having a geometry in which the samples are very thin and the smectic layers are perpendicular (instead of parallel) to the glass plates (this is the so-called bookshelf geometry typical of the SFLC-cells). In this device the ECE is detectable at conveniently low applied fields. This leads to an ECE which is also linear and gives a very fast response for a given substance.

Different compounds snowing an ECE are investigated by Nihiyama et al. (Jap. J. of Appl. Phys. 26, 1787 (1987)) or by Ch. Bahr and G. Heppke (Liquid Crystals, 1987, 825).

A great disadvantage for applications based on the electroclinic effect of the compounds currently available with smectic A phases is their low chemical, heat and light stability. Another adverse property of displays based on the ECE with compounds currently available is that they are difficult to align to an extent which is sufficient for practical application. But for many applications good alignment is vital although a smectic A phase does align over rubbed polymer (e.g. Patel, J. S., Leslie, T. M. and Goodby, J. W., Ferroelectrics 1984, 59, 137) and SiO, this process is much easier if a nematic phase precedes the smectic A phase.

The high cost of the chiral compounds used as single components in these displays is another disadvantage of the displays based on the ECE which are described until today.

Moreover, the temperature range of the smectic A phases showing an ECE is usually too small and is predominantly at temperatures being too high for commercial applications.

It now has been found that a display element based on the ECE with a liquid crystal medium which contains at least two chiral components and at least one non-chiral component and has a pitch compensated cholesteric phase above the optically active smectic A phase can substantially reduce the disadvantages mentioned.

Such a display element can particularly be used over a wide temperature range. The phases therein possess a high P.sub.s value and a compensated cholesteric phase which leads to a good alignment and a high induced tilt angle. P.sub.s is the spontaneous polarization in nC/cm.sup.2. Furthermore the costs of such a display containing a mixture of chiral and non-chiral compounds is essentially lower than those of one containing only chiral compounds.

The invention thus relates to a display element based on the electroclinic effect containing a liquid crystal medium being a mixture of at least two chiral components and at least one non-chiral component, characterized in that said medium has a pitch compensated cholesteric phase above the optically active smectic A phase.

The invention furthermore relates to such a display element characterized in that at least one non-chiral component is a compound of the formula I ##STR1## wherein R.sup.1 and R.sup.2 are each independently an alkyl group with 5 to 15 C atoms wherein one or two non-adjacent CH.sub.2 groups can also be replaced by --O--, --S--, --CO--O--, --O--CO-- and /or --CH.dbd.CH--, ##STR2## denotes a 1,4-phenylene group optionally substituted by fluorine and n and m are each independently 1 or 2, and

at least one chiral component is a compound of the formula II ##STR3## wherein R.sup.3 is an alkyl or alkenyl group each with 5 to 15 C atoms, optionally substituted by CN or at least one halogen atom wherein one or two non-adjacent CH.sub.2 groups can be replaced by --O--, --CO--, --CO--O--, --O--CO--, --S-- and/or --C.ident.C--, R.sup.4 is an alkyl group with up to 8 C atoms wherein one CH.sub.2 group can be replaced by --O--, --CO--O--or --O--CO--, A.sup.1 and A.sup.2 independently are an unsubstituted or mono- or polysubstituted 1,4-cyclohexylene group, wherein one or two non-adjacent CH.sub.2 groups may be replaced by --O-- and/or --S--, or denote a 1,4-bicyclo[2,2,2]octylene group, or a 1,4-phenylene group optionally substituted by fluorine, and wherein one or two CH groups may be replaced by N, X is CN, Cl, F, Br, CH.sub.3 or CF.sub.3, Y is H or CH.sub.3, Z.sup.1 denotes --CO--O--, --O--CO--, --CH.sub.2 CH.sub.2 --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN--, --CH.dbd.CH--, --C.ident.C--, --OCH.sub.2 --, --CH.sub.2 O, --CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--, --NO.dbd.N--, --N.dbd.NO-- or a single bond, o is 1, 2, or 3, and is an alkylene or alkenylene group with 1 to 7 C atoms wherein one CH.sub.2 group can be replaced by --O--, --CO--O--or --O--CO--, with the proviso that X, Y and R.sup.4 being different groups.

The invention relates furthermore to such a display element, characterized in that at least one component is a compound of the formula III ##STR4## wherein R.sup.5 and R.sup.6 are each independent an alkyl group with 5 to 15 C atoms wherein one or two CH.sub.2 groups can also be replaced by --O--, --S-- --CO--O--, --O--CO-- and/or --CH.dbd.CH-- ##STR5## denotes a 1,4 phenylene group optionally substituted by fluorine, ##STR6## denotes a 1,4-phenylene group optionally substituted by fluorine or a trans-1,4-cyclohexylene group,

q is 0 or 1, and r is 1 or 2, and at least one chiral component is a compound of the formula II wherein R.sup.3, R.sup.4, A.sup.1, A.sup.2, Y and Q have the meaning given.

In particular the invention relates to such a display element, characterized in that in formula I at least one group ##STR7## denotes a 1,4-phenylene group which is substituted by fluorine, and to such a display element, characterized in that R.sup.1 and R.sup.2 in formula I are each alkyl or alkoxy with 5 to 15 C atoms.

Furthermore the invention relates to such a display element characterized in that R.sup.5 and R.sup.6 in formula III are each alkyl or alkoxy with 5 to 15 C atoms.

Preferred display elements according to the invention are those in which the non-chiral base mixture comprises compounds of the formula I.

Particularly preferred compounds of the formula I are those of the part formulae Ia to Ik: ##STR8##

Particularly preferred display elements according to the invention are those in which the non-chiral base mixture comprises compounds of the formula III. Preferred compounds of the formula III are those of the part formulae IIIa to IIIi: ##STR9##

Particularly preferred display elements according to the invention are those in which the both chiral components are of the formula II. Preferred compounds of the formula II are those of the part formulae IIa to IIf: ##STR10## wherein R.sup.7 is alkyl or alkoxy with 5 to 15 C atoms, R.sup.8 is alkyl with 1 to 8 C atoms, 0 is 1 or 2, s is 0 or an integer value between 1 and 6 and A.sup.1 is preferably 1,4-phenylene, pyridine-2,5-diyl pyrimidine-2,5-diyl, 2-fluoro-1,4-phenylene or trans-1,4-cyclohexylene. R.sup.1 and R.sup.2 (in the compounds of the formula I) and R.sup.5 and R.sup.6 (in the compounds of the formula III) independently of one another are each preferably alkyl, alkoxy, alkanoyl, alkanoyloxy, alkoxycarbonyl or alkoxycarbonyloxy, each preferably having 5 to 12, in particular 6 to 10, C atoms. Alkyl and alkoxy are particularly preferred. A particularly preferred combination in the compounds of the formula I is one in which R.sup.1 is alkoxy and R.sup.2 is alkyl. A particularly preferred combination in the compounds of the formula III is one in which R.sup.5 is alkyl and R.sup.6 is alkoxy. R.sup.1 and R.sup.2 groups with a straight-chain alkyl radical are particularly preferred.

Particularly preferred display elements according to the invention are those in which the achiral base mixture contains, in addition to compounds of the formula I or III, at least one other component with a negative or comparatively low positive dielectric anisotropy. This/these other component(s) of the achiral base mixture can make up 1 to 95%, preferably 5 to 40 , of the base mixture. Suitable further components with a comparatively low positive or negative dielectric anisotropy are compounds of the formulae IV to VII. ##STR11## wherein R.sup.9 and R.sup.10 are each alkyl or alkoxy with 5 or 15 C atoms, L is hydrogen or fluorine, A.sup.3 and A.sup.4 each independently are 1,4-phenylene or trans-1,4-cyclohexylene, u and v are each independently 0, 1 or 2, the sum of u+v is 1 or 2 and t is 1 or 2.

All components of the phases according to the invention are either known or can be prepared in a manner known per se, analogously to known compounds.

Esters of the formula I can be obtained by esterification of corresponding carboxylic acids (or their reactive derivatives) with phenols (or their reactive derivatives).

The corresponding carboxylic acids and phenols are known or can be prepared by processes analogous to known processes.

Particularly suitable reactive derivatives of the carboxylic acids mentioned are the acid halides, above all the chlorides and bromides, and furthermore the anhydrides, for example also mixed anhydrides, azides or esters, in particular alkyl esters with 1-4 C atoms in the alkyl group.

Possible reactive derivatives of the alcohols or phenols mentioned are, in particular, the corresponding metal phenolates, preferably of an alkali metal, such as sodium or potassium.

The esterification is advantageously carried out in the presence of an inert solvent. Particularly suitable solvents are esters, such as diethyl ether, di-n-butyl ether, THF, dioxane or anisole, ketones, such as acetone, butanone or cyclohexanone, amides, such as dimethylformamide or phosphoric acid hexamethyltriamide, hydrocarbons, such as benzene, toluene or xylene, halogenohydrocarbons, such as dichloromethane carbon tetrachloride or tetrachloroethylene, and sulfoxides, such as dimethylsulfoxide or sulfolane. Water-immiscible solvents can simultaneously be advantageously used for azeotropic distillation of the water formed during the esterification. An excess of an organic base, for example pyridine, quinoline or triethylamine, can occasionally also be used as the solvent for the esterificaton. The esterification can also be carried out in the absence of a solvent, for example by heating the components in the presence of sodium acetate. The reaction temperature is usually between -50.degree. and +250.degree., preferably between -20.degree. and +80.degree.. At these temperatures, the esterification reactions have as a rule ended after 15 minutes to 48 hours.

In detail, the reaction conditions for the esterification depend largely on the nature of the starting substances used. Thus, a free carboxylic acid is as a rule reacted with a free alcohol or phenol in the presence of a strong acid, for example a mineral acid, such as hydrochloric acid or sulfuric acid. A preferred reaction procedure is the reaction of an acid anhydride or, in particular, an acid chloride with an alcohol, preferably in a basic medium, bases which are of importance being, in particular, alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, alkali metal carbonates or bicarbonates, such as sodium carbonate, sodium bicarbonate, potassium carbonate or potassium bicarbonate, alkali metal acetates, such as sodium acetate or potassium acetate, alkaline earth metal hydroxides, such as calcium hydroxide, or organic bases, such as triethylamine, pyridine, lutidine, collidine or quinoline. Another preferred embodiment of the esterification comprises first converting the alcohol or phenol into the sodium alcoholate or phenolate or potassium alcoholate or phenolate, for example by treatment with ethanolic sodium hydroxide solution or potassium hydroxide solution, isolating this product and suspending it in acetone or diethyl ether, together with sodium bicarbonate or potassium carbonate, with stirring, and adding a solution of the acid chloride or anhydride in diethyl ether, acetone or dimethylformamide to this suspension, advantageously at temperatures between about -25.degree. and +20.degree..

A further preferred reaction procedure is the reaction of an acid with an alcohol in an inert solvent in the presence of a water binding material, e.g. molecular sieves or carbodiimides, preferably dicyclohexylcarbodiimide (DCC).

The phases according to the invention are prepared in a manner which is customary, for example by mixing the components together, preferably at elevated temperatures.

The liquid crystal phases according to the invention can be modified by suitable additives so that they can be used in all the types of liquid crystal display elements hitherto disclosed.

The geometry of the display according to the invention is, for example, that described by Anderson et al. (Appl. Phys. Lett. 51, 640 (1987); European Patent Application 0 263 225).

The following examples are intended to illustrate the invention without limiting it. Percentages above and below are percentages by weight; all the temperatures are stated in degrees Celsius. The values given for spontaneous polarization are applicable to room temperature. The symbols are furthermore as follows: Cr: crystalline solid state, S: smectic phase (the index characterizes the phase type), N: nematic state, Ch: cholesteric phase, I: isotropic phase. The figure between two symbols indicates the transition temperature in degrees Celsius.

EXAMPLES

Example 1

A liquid crystal medium consisting of

20.3% of 4-pentyl-2-fluorophenyl 4'-octyloxybiphenyl-4-ylcarboxylate 20.3% of 4-heptyl-2-fluorophenyl 4'-heptyloxybiphenyl-4-ylcarboxylate 20.3% of 4-heptyl-2-fluorophenyl 4'-octyloxy-3'-fluoro-biphenyl-4-ylcarboxylate 16% of 4-pentyl-2-fluorophenyl 4'-octyloxybenzolate 16% of optically active 1-cyano-3-methylpropyl 4'-octyloxy-3'-fluorobiphenyl-4-ylcarboxylate and 7% of optically active 1-cyanoethyl 4'-octyloxy-3'-fluorobiphenyl-4-ylcarboxylate

exhibits S.sub.c * 28.degree. S.sub.A 73.degree. Ch 89-92.degree. I, a significant field induced tilt angle in the smectic A phase and a helix compensated cholesteric phase. Due to this feature this medium can be readily aligned to produce a good alignment of the smectic A phase.

The following tables illustrate the effect of applying a dc field to this liquid crystal medium aligned with rubbed polyamide and contained in a 2,4 .mu.m thick cell.

______________________________________TILT ANGLE vs TEMPERATURE at 20 v/.mu.mTemperature (.degree.C.) Tilt Angle (.degree.)______________________________________29 1430 1333 11.538 940 743 648 5.553 5______________________________________TILT ANGLE (.degree.) vs APPLIED FIELDat Various Temperatures Temperature (.degree.C.)Field (v/.mu.m) 29 31 33 38 46______________________________________ 5 8 7 6.5 3 1.510 10.5 9 8.5 5 2.515 12 11 10 6.5 420 14 12.5 11.5 9 525 13 12 10.5 10 6______________________________________

EXAMPLE 2

A liquid crystal medium consisting of

3.9% of 2-p-hexyloxyphenyl-5-heptylpyrimidine 3.9% of 2-p-heptyloxyphenyl-5-heptylpyrimidine 3.9% of 2-p-octyloxyphenyl-5-heptylpyrimidine 3.9% of 2-p-nonyloxyphenyl-5-heptylpyrimidine 24.0% of 2-p-heptyloxyphenyl-5-hexylpyrimidine 2.0% of 2-p-hexyloxyphenyl-5-nonylpyrimidine 26.0% of 2-p-nonyloxyphenyl-5-nonylpyrimidine 9.8% of r-1-cyano-cia-4-(trans-4-pentylcyclohexyl)-1-(trans-4-pentylcyclohexyl)-cyclohexane 2.6% of optically active 2-(4-(3,7-dimethyloctyloxy)-phenyl)-5-nonylpyrimidine and 20% of optically active 4-(5-heptylpyrimidine-2-yl)-phenyl 2-chloro-4-methylbutyrate)

exhibits C -20.degree. S.sub.A 62.degree. CH 66.degree. I and a cholesteric pitch of -14 .mu.m at 63.degree. C.

EXAMPLE 3

A liquid crystal medium consisting of

17% of 4-pentyl-2-fluorophenyl 4'-octyloxybiphenyl-4yl-carboxylate 17% of 4-heptyl-2-fluorophenyl 4'-heptyloxybiphenyl-4-yl-carboxylate 17% of 4-heptyl-2-fluorophenyl 4'-octyloxy-3'-fluorobiphenyl-4-yl carboxylate 16% of 4-pentyl-2-fluorophhenyl-4'-octyloxybenzoate 10% of 4-pentylphenyl 4'-heptylbenzoate 16% of optically active 1-cyano-3-methylpropyl 4'-octyl-oxy-3'-fluorobiphenyl-4-yl carboxylate and 7% of optically active 1-cyanoethyl 4'-octyloxy-3'-fluorobiphenyl 4-yl carboxylate

exhibits S*.sub.c -1.6.degree. A.sub.A 60.degree. Ch 75.7.degree. I, a significant field induced tilt angle in the smectic A phase and a helix compensated cholesteric phase.

The following table illustrates the effect of applying a dc field to this liquid crystal medium:

______________________________________Induced Tilt Angle at 20.degree. C.Field (V/.mu.m) Induced Tilt (.degree.)______________________________________5 210 415 620 825 9______________________________________

EXAMPLE 4

A liquid crystal medium consisting of

13.19% of 4-n-octyloxy-2'-fluoro-4"-n-pentyl-p-terphenyl 13.23% of 4-n-pentyloxy-2'-fluoro-4"-(4methylhexyl)-p-terphenyl 13.17% of 4-n-heptyloxy-2'-fluoro-4"-(4-methylhexyl)-p-terphenyl 15.47% of 4n-heptylphenyl 4'-pentylbenzoate 14.99% of 4-octyloxy-3-fluorophenyl 4'-octyloxybenzoate 4.98% of optically active 1-cyano-3-methylpropyl 4'-octyl-oxy-3'-fluoro biphenyl-4-yl carboxylate 9.86% of optically active 1-cyanoethyl 4'-octyloxy-3'-fluoro biphenyl-4-yl-carboxylate

exhibits S*.sub.C 23.6.degree. S .sub.A 52.6.degree. Ch 71.7.degree. I.

Claims

1. In an electroclinic effect display element containing a liquid crystal medium, the improvement wherein:

said medium is a mixture of at least two chiral components and at least one non-chiral component, said medium has a compensated cholesteric phase above an optically active smectic A phase, wherein said medium is capable of exhibiting an electroclinic effect, and

wherein said at least one non-chiral component is a compound of formula I ##STR12## wherein R.sup.1 and R.sup.2 are each independently an alkyl group with 5 to 15 C atoms wherein one or two non-adjacent CH.sub.2 groups can also be replaced by --O--, --S--, --CO--O--, --O--CO-- and/or --CH.dbd.CH, ##STR13## is a 1,4-phenylene group optionally substituted by fluorine wherein in Formula I, at least one group ##STR14## is a 1,4-phenylene group which is substituted by fluorine, and n and m are each independently 1 or 2, and

at least one of said chiral components is a compound of formula II ##STR15## wherein R.sup.3 is an alkyl or alkenyl group each with 5 to 15 C atoms, optionally substituted by CN or at least one halogen atom wherein one or two non-adjacent CH.sub.2 groups can be replaced by --O--, --CO--, --CO--O--, --O--CO--, --S-- and/or --C.dbd.C--,

R.sup.4 is an alkyl group with up to 8 C atoms wherein one CH.sub.2 group can be replaced by --O--, --CO--O-- or --O--CO--,

A.sup.1 and A.sup.2 are independently each an unsubstituted or mono- or polysubstituted 1,4-cyclohexylene group wherein one or two non-adjacent CH.sub.2 groups may be replaced by --O--and/or --S--, a 1,4-bicyclo[2,2,2]octylene group, or a 1,4-phenylene group optionally substituted by fluorine, and wherein one or two CH groups may be replaced by N,

X is CN, Cl, F, Br, CH.sub.3 or CF.sub.3,

Y is H or CH.sub.3,

Z.sup.1 is --CO--O--, --O--CO--, --CH.sub.2 CH.sub.2 --, --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN--, --CH.dbd.CH--, --C.dbd.C--, --OCH.sub.2 --, --CH.sub.2 O--, --CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--, --N.dbd.NO-- or a single bond,

is 1, 2, or 3, and

is an alkylene or alkenylene group with 1 to 7 C atoms wherein one CH.sub.2 group can be replaced by --O--, --CO--O-- or --O--CO--,

with the proviso that X, Y and R.sup.4 are different groups.

2. A display element according to claim 1, wherein at least one component is a compound of the formula III ##STR16## wherein R.sup.5 and R.sup.6 are each independently an alkyl group with 5 to 15 C atoms wherein one or two CH.sub.2 groups can also be replaced by --O--, --S--, --CO--O--, --O--CO-- and/or --CH.dbd.CH--, ##STR17## denotes a 1,4-phenylene group optionally substituted by fluorine, ##STR18## denotes a 1,4-phenylene group optionally substituted by fluorine or a trans-1,4-cyclohexylene group,

q is 0 or 1, and

r is 1 or 2,

and

said at least one chiral component is a compound of the formula II wherein ##STR19## R.sup.3 is an alkyl or alkenyl group each with 5 to 15 C atoms, optionally substituted by CN or at least one halogen atom wherein one or two nonadjacent CH.sub.2 groups can be replaced by --O--, --CO--, --CO--O--, --O--CO--, --S-- and/or --C.dbd.C--,

R.sup.4 is an alkyl group with up to 8 C atoms wherein one CH.sub.2 group can be replaced by --O--, --CO--O--or --O--CO,

A.sup.1 and A.sup.2 independently are an unsubstituted or mono--or polysubstituted 1,4-cyclohexylene group, wherein one or two non-adjacent CH.sub.2 groups may be replaced by --O-- and/or --S--, or denote a 1,4-bicyclo octylene group, or a 1,4-phenylene group optionally substituted by fluorine, and wherein one or two CH groups may be replaced by N,

X is CN, Cl, F, Br, CH.sub.3 or CF.sub.3,

Y is H or CH.sub.3,

Z.sup.1 denotes --CO--O--, --O--CO--, --CH.sub.2 CH.sub.2 --, --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN--, --CH.dbd.CH--, --C.dbd.C--, --OCH.sub.2 --, --CH.sub.2 O, --CH.dbd.N--, --N.dbd.CH--, --N.dbd.N, --NO.dbd.N--, --.dbd.NO-- or a single bond,

o is 1, 2, or 3, and

O is an alkylene or alkenylene group with 1 to 7 C atoms wherein one CH.sub.2 group can be replaced by --O--, --CO--O or --O--CO--,

with the proviso that X, Y and R.sup.4 are different groups.

3. A display element according to claim 1 wherein R.sup.1 and R.sup.2 in formula I are each alkyl or alkoxy with 5 to 15 C atoms.

4. A display element according to claim 2, wherein R.sup.5 and R.sup.6 in formula III are each alkyl or alkoxy with 5 to 15 C atoms.

5. A display element according to claim 1, wherein both said at least two chiral components are compounds according to Formula II.

6. A display element according to claim 2, wherein both said at least two chiral components are compounds according to Formula II.

7. A display element according to claim 1, wherein said chiral compound according to Formula II is of Formulae IIa-IIf ##STR20## wherein R.sup.7 is alkyl or alkoxy with 5 to 15 C atoms,

R.sup.8 is alkyl with 1 to 8 C atoms,

o is 1 or 2,

s is 0 or an integer value between 1 and 6, and is preferably 1,4-phenylene, pyridine-2,5-diyl pyrimidine-2,5-diyl, 2-fluoro-1,4-phenylene or trans-1,4-cyclohexylene.

8. A display element according to claim 2, wherein said chiral compound according to Formula II is of Formulae IIa-IIf ##STR21## wherein R.sup.7 is alkyl or alkoxy with 5 to 15 C atoms,

R.sup.8 is alkyl with 1 to 8 C atoms,

o is 1 or 2,

s is 0 or an integer value between 1 and 6, and is preferably 1,4-phenylene, pyridine-2,5-diyl pyrimidine-2,5-diyl, 2-fluoro-1,4-phenylene or trans-1,4-cyclohexylene.

9. A display element according to claim 1, wherein said liquid crystal mixture further contains at least one ##STR22## wherein R.sup.9 and R.sup.10 are each alkyl or alkoxy with 5 or 15 C atoms,

L is hydrogen or fluorine,

A.sup.3 and A.sup.4 each independently are 1,4-phenylene or trans-1,4-cyclohexylene,

u and v are each independently 0, 1 or 2, the sum of u+v being 1 or 2, and

t is 1 or 2.

10. A display element according to claim 2, wherein said liquid crystal mixture further contains at least one ##STR23## wherein R.sup.9 and R.sup.10 are each alkyl or alkoxy with 5 or 15 C atoms,

L is hydrogen or fluorine,

A.sup.3 and A.sup.4 each independently are 1,4-phenylene or trans-1,4-cyclohexylene,

u and v are each independently 0, 1, or 2, the sum of

is 1 or 2.

11. A display element according to claim 1, wherein said liquid crystal medium contains 4-pentyl-2-fluorophenyl-4-octyloxybiphenyl-4-yl-carboxylate; 1-cyano-3-methylpropyl-4'-octyloxy-3'-fluorobiphenyl-4-ylcarboxylate; and 2-p-hexyloxyphenyl-5-heptylpyrimidine.