DE2815860A1 - LIQUID CRYSTALLINE MIXTURES - Google Patents

Description

The present invention relates to liquid crystal mixtures, specifically dye-containing liquid crystal mixtures. Such dye-containing liquid crystal mixtures are known.

It has now been found that by adding certain compounds, namely by adding the terpyrimidines shown in Formula I below, a substantial increase in the degree of order in such mixtures can be achieved. As a result, better contrasts can be obtained in liquid crystal displays in which some form of the "guest-host effect" is used.

The new dye-containing liquid crystal mixtures, which have such an increased degree of order, are particularly suitable for electro-optical displays, for example for a rotary cell.

The liquid-crystal mixtures according to the present invention are characterized in that they

a) at least three nematic liquid crystal components, at least one of which is a terpyrimidine of the formula I.

is,

where the symbols X for nitrogen and Y and Z for CH or Y for nitrogen and X and Z for CH or Z for nitrogen and X and Y for CH, and one of the substituents R [deep] 1 and R [deep] 2 die Cyano group and the other is a straight-chain alkyl group with 1 - 7 carbon atoms,

b) at least one non-ionic, pleochroic azo dye, and optionally

c) one or more isotropic, organic additives which lower the threshold voltage and / or the viscosity

contain.

Liquid-crystal mixtures in which the terpyrimidine is derived from a compound of the formula are particularly preferred here Ia is formed,

where Z is nitrogen and X is CH or X is nitrogen and Z is CH, and R [deep] 1 is a straight-chain alkyl group having 1-7 carbon atoms.

A particularly preferred subgroup of these liquid-crystal mixtures is characterized in that the terpyrimidine is derived from a compound of the general formula Ib

is formed

wherein R [deep] 1 represents a straight-chain alkyl group having 1-7 carbon atoms.

The remaining liquid-crystal components in such liquid-crystal mixtures can be derived from compounds of the general formula II

wherein R [deep] 2 is a straight-chain alkyl group with 4 - 8 carbon atoms,

and / or of compounds of the general formula III in which one of the substituents R [deep] 3 and R [deep] 4 is the cyano group and the other is a straight-chain alkyl group with 3 - 9 carbon atoms,

and / or of compounds of the general formula IV

where R [deep] 5 is ethyl, n-propyl, n-butyl, n-pentyl, iso-hexyl, n-hexyl, n-heptyl or n-octyl,

and / or of compounds of the general formula V

wherein R [deep] 6 denotes an alkyl group or an alkoxy group or a phenyl group substituted by an alkyl group or an alkoxy group,

and / or of compounds of the general formula VI

be educated

where R [deep] 7 is an alkyl group or an alkoxy group and n is the number 1 or 2.

However, other nematic and / or non-nematic compounds can also be present in the mixtures according to the invention.

A preferred embodiment of the process according to the invention is characterized in that the azo dye is formed from a compound of the following general formulas: VII VIII

or of derivatives of the formula VII or VIII in which one of the lateral positions on one of the benzene rings is substituted by one of the following substituents: a halogen atom, a methyl group, a halogen-substituted methyl group or a methoxy group;

or of derivatives of the formulas VII or VIII, in which two of the lateral positions on one of the benzene rings with a condensed aromatic ring to form a naphthalene structure are bridged,

as IX

with

A = an azo or azoxy group,

n [deep] 1, n [deep] 2 and n [deep] 3 = whole numbers from 0 to 4,

X = CN or nitro and

Y [deep] 1 and Y [deep] 2 = each hydrogen, OR [deep] 1 with R [deep] 1 = alkyl or aralkyl and ,

where R [deep] 2 and R [deep] 3 each represent simple or substituted alkyl groups or alkylene groups which form a reduced heteroaromatic system with the terminal aromatic ring; X

wherein X is hydrogen or a nitro group in the 5- or 6-position, cyano group, an alkyl group with 1-4 carbon atoms, in particular methyl, or an alkoxy group with 1-4 carbon atoms, in particular methoxy, and T is a group of the formula Xa

or the formula

-OR [deep] 10 Xb

mean in which R [deep] 8, R [deep] 9 and R [deep] 10 lower alkyl groups or R [deep] 8 and R [deep] 9 together mean a lower cycloalkyl group.

In the liquid-crystal mixtures according to the present invention, the terpyrimidine or terpyrimidines of the formula I is advantageously present in an amount of from about 1 to about 30% by weight, in particular from about 3 to about 20% by weight, based on the total weight of the mixture.

The amount of dye or dyes can generally be between about 0.05 to about 1% by weight, in particular between about 0.1 to 1% by weight, based on the total weight of the mixture.

Any isotropic, organic additives that lower the threshold voltage and / or the viscosity can be formed from one or more of the following compounds:

a) a nematogenic material with a clearing point above 60 ° C and an organic solvent or solvent mixture with a specific resistance of at least 10 [high] -7 Ohm x cm, and a dipole moment along the molecular axis of less than 3.5 D or

b) a compound of the general formula XI

where R [deep] 11 and R [deep] 12 straight-chain alkyl groups with a total of at most 14 carbon atoms or R [deep] 11 a straight-chain alkyl group and R [deep] 12 a straight-chain alkoxy group with in particular represent a maximum of 8 carbon atoms in total,

or

c) a compound of the general formula XII

where R [deep] 13 and R [deep] 14 are straight-chain alkyl groups with a total of at most 6 carbon atoms,

or

d) a compound of the general formula XIII

where R [deep] 13 and R [deep] 14 have the above meaning,

or

e) mixtures of these compounds.

As stated at the outset, the liquid-crystal mixtures according to the invention contain at least three nematic liquid-crystal components. The upper limit of the number of these nematic liquid crystal components can be up to about ten.

The number of dyes can range from 1 to about 4.

With regard to the number of isotropic, organic additives which may be present, which reduce the threshold voltage and / or the viscosity, this can be between 1 and about 3.

Unless otherwise stated, alkyl groups and alkoxy groups are to be understood as meaning lower alkyl groups or alkoxy groups with 1-7 carbon atoms, preferably those with 1-4 carbon atoms.

The following compounds in particular can be mentioned as examples of terpyrimidines of the formula I:

2- (4-cyanophenyl) -5- (4-ethylphenyl) pyrimidine,

2- (4-cyanophenyl) -5- (4-n-propylphenyl) pyrimidine,

2- (4-cyanophenyl) -5- (4-n-butylphenyl) pyrimidine,

2- (4-cyanophenyl) -5- (4-n-pentylphenyl) pyrimidine,

2- (4-cyanophenyl) -5- (4-n-hexylphenyl) pyrimidine,

2- (4-cyanophenyl) -5- (4-n-heptylphenyl) pyrimidine,

2- (4-methylphenyl) -5- (4-cyanophenyl) pyrimidine,

2- (4-ethylphenyl) -5- (4-cyanophenyl) pyrimidine,

2- (4-n-propylphenyl) -5- (4-cyanophenyl) pyrimidine,

2- (4-n-butylphenyl) -5- (4-cyanophenyl) pyrimidine,

2- (4-n-pentylphenyl) -5- (4-cyanophenyl) pyrimidine,

2- (4-n-hexylphenyl) -5- (4-cyanophenyl) pyrimidine,

2- (4-n-heptylphenyl) -5- (4-cyanophenyl) pyrimidine.

The following can be mentioned as examples of compounds of the formula II:

p-n-butylbenzoic acid p'-cyanophenyl ester,

p-n-pentylbenzoic acid p'-cyanophenyl ester,

p-n-hexylbenzoic acid p'-cyanophenyl ester,

p-n-heptylbenzoic acid p'-cyanophenyl ester,

p-n-octylbenzoic acid p'-cyanophenyl ester, p-n-pentyloxybenzoic acid p'-cyanophenyl ester,

p-n-hexyloxybenzoic acid p'-cyanophenyl ester,

p-n-heptyloxybenzoic acid p'-cyanophenyl ester,

p-n-Octyloxybenzoic acid p'-cyanophenyl ester.

Examples of compounds of the formula III are:

5-n-propyl-2- (4-cyanophenyl) pyrimidine,

5-n-butyl-2- (4-cyanophenyl) pyrimidine,

5-n-pentyl-2- (4-cyanophenyl) -pyrimidine,

5-n-hexyl-2- (4-cyanophenyl) pyrimidine,

5-n-heptyl-2- (4-cyanophenyl) -pyrimidine,

5-n-octyl-2- (4-cyanophenyl) pyrimidine,

5-n-nonyl-2- (4-cyanophenyl) pyrimidine.

Examples of compounds of the formula IV are:

p - [(p-ethylbenzylidene) amino] benzonitrile,

p - [(p-n-propylbenzylidene) amino] benzonitrile,

p - [(p-n-Butylbenzylidene) amino] benzonitrile,

p - [(p-n-pentylbenzylidene) amino] benzonitrile,

p - [(p-n-hexylbenzylidene) amino] benzonitrile,

p - [(p-Isohexylbenzylidene) amino] benzonitrile,

p - [(p-n-heptylbenzylidene) amino] benzonitrile,

p - [(p-n-Octylbenzylidene) amino] benzonitrile.

Examples of azo dyes contained in the liquid-crystal mixtures according to the invention are in particular the substances mentioned in the following working examples.

Examples of isotropic, organic additives which lower the threshold voltage and / or viscosity are solvents such as alkanes, alkyl halides, dialkyl ethers, dialkyl carbonates, alkanecarboxylic acid alkyl esters, 1-phenylheptane or p-xylene, such as, for example, dibutyl ether, dihexyl ether, dioctyl ether or propyl butyric acid the butyric acid pentyl ester.

In the following examples, all percentages are to be understood as percentages by weight.

Furthermore, in the following examples, the terms "basic mixture A" and "dye" have the following meaning:

Basic mixture A:

10.3% p-n-butylbenzoic acid p'-cyanophenyl ester,

11.1% p-n-pentylbenzoic acid p'-cyanophenyl ester,

14.3% p-n-hexylbenzoic acid p'-cyanophenyl ester,

15.4% p-n-heptylbenzoic acid p'-cyanophenyl ester,

11.0% 5-n-pentyl-2- (4-cyanophenyl) -pyrimidine,

21.2% 5-n-heptyl-2- (4-cyanophenyl) -pyrimidine,

15.6% 5- (4-n-butylphenyl) -2- (4-cyanophenyl) pyrimidine.

dye

R = XV

example 1

Basic mixture A + 1% dye of the formula ,

wherein R is a radical of the formula XV.

Example 2

Basic mixture A + 1% dye of the formula wherein R is a radical of the formula XV.

Example 3

Basic mixture A + 1% dye of the formula wherein R is a radical of the formula XV.

Example 4

Basic mixture A + 1% dye of the formula wherein R is a radical of the formula XV.

Example 5

Basic mixture A + 5% di-n-hexyl ether + 1% of the dye used in Example 4.

Example 6

Basic mixture A + 15% 4'-methyl-4-n-pentylbiphenyl + 1% of the dye used in Example 4.

Example 7

Basic mixture A + 1% dye of the formula C [deep] 4H [deep] 9OR, in which R is a radical of the formula XV.

Example 8

Basic mixture A + 1% dye of the formula wherein R is a radical of the formula XV.

Example 9

Basic mixture A + 1% dye of the formula wherein R is a radical of the formula XV.

Example 10

Basic mixture A + 1% dye of the formula C [deep] 7H [deep] 15O-R, in which R is a radical of the formula XV.

Example 11

Basic mixture A + 0.25% dye of the formula

+ 0.25% dye of the formula C [deep] 7H [deep] 15-OR, in which R is a radical of the formula XV.

Example 12

Basic mixture A + 1% dye of the formula (CH [deep] 3) [deep] 2NR, in which R is a radical of the formula XV.

Example 13

Basic mixture A + 0.3% dye of the formula (CH [deep] 3) [deep] 2NR, in which R is a radical of the formula XV, and 0.2% dye of the formula

Example 14

Basic mixture A + 1% dye of the formula

Example 15

Basic mixture A + 1% dye of the formula

Example 16

Basic mixture A + 1% dye of the formula

Example 17

Basic mixture A + 1% dye of the formula

Example 18

Basic mixture A + 1% dye of the formula

Example 19

Basic mixture A + 1% dye of the formula

Example 20

Basic mixture A + 1% dye of the formula

Example 21

Basic mixture A + 1% dye of the formula

Example 22

Basic mixture A + 1% dye of the formula

Example 23

Basic mixture A + 1% dye of the formula

Example 24

Basic mixture A + 1% dye of the formula

Example 25

Basic mixture A + 1% dye of the formula

Example 26

Basic mixture A + 1% dye of the formula

Example 27

Basic mixture A + 1% dye of the formula

Example 28

Basic mixture A + 1% dye of the formula

Example 29

Basic mixture A + 1% dye of the formula

Example 30

Basic mixture A + 1% dye of the formula

Example 31

Basic mixture A + 1% dye of the formula

Example 32

Basic mixture A + 1% dye of the formula

Example 33

Basic mixture A + 1% dye of the formula

Example 34

Basic mixture A + 1% dye of the formula

Example 35

Basic mixture A + 1% dye of the formula

Example 36

Basic mixture A + 1% dye of the formula

Example 37

Basic mixture A + 1% dye of the formula

Example 38

Basic mixture A + 1% dye of the formula

Example 39

Basic mixture A + 1% dye of the formula

Example 40

Basic mixture A + 1% dye of the formula

Example 41

Basic mixture A + 1% dye of the formula

Example 42

Basic mixture A + 1% dye of the formula

Example 43

Basic mixture A + 1% dye of the formula

Example 44

Basic mixture A + 1% dye of the formula

Example 45

Basic mixture A + 1% dye of the formula

Example 46

Basic mixture A + 1% dye of the formula

C [low] 10H [low] 21OR

Example 47

Basic mixture A + 1% dye of the formula

Example 48

Basic mixture A + 1% dye of the formula

Example 49

Basic mixture A + 1% dye of the formula

Example 50

Basic mixture A + 1% dye of the formula

Example 51

Basic mixture A + 1% dye of the formula

The dyes mentioned in Examples 1, 4, 12, 14, 15, 21, 22, 23, 24, 25, 26, 30, 34, 35, 36, 38 and 49 are known substances.

The dyes mentioned in the following examples are new substances which can be obtained by processes known per se; Properties (color, melting point) are specified in each case, as well as a reference to the production in analogy to the production of the known dyes:

Example 2: Analogous to example 4; black crystals, m.p. 218 ° (dec.); UV (ethanol): small epsilon [deep] 584 = 47670.

Example 3: Analog 1 or 12; golden brown crystals, m.p. 246 ° (dec.); UV (ethanol): small epsilon [low] 573 = 49570; small epsilon [deep] 270 = 11120, shoulder at 299 µm.

Example 7: Analogous to Example 4; red crystals, m.p. 108-109 °. UV (ethanol): small epsilon [low] 434 = 29580; small epsilon [deep] 260 = 8400, shoulders at 220 and 300 µm.

Example 8: Analogous to Example 4; violet-black crystals, m.p. 155-158 °; UV (ethanol): small epsilon [deep] 586 = 59290.

Example 9: Analogous to Example 4; violet black crystals, m.p. 185-190 °; UV (ethanol): small epsilon [deep] 587 = 54040.

Example 10: Analogous to Example 4; red crystals, m.p. 95-96 °; UV (ethanol): small epsilon [low] 433 = 30185; small epsilon [deep] 261 = 8545.

Example 13: Analogous to example 4; M.p. 212-214 °, dark red crystals; UV (ethanol): small epsilon [low] 507 = 48180; small epsilon [deep] 320 = 8930; small epsilon [deep] 276 = 26660.

Example 16: Analogous to example 30; red crystals, m.p. 221-224 °; UV (ethanol): small epsilon [low] 515 = 46940; small epsilon [deep] 284 = 7890.

Example 17: Analogous to Example 4; red crystals, m.p. 131-132 °; UV (ethanol): small epsilon [low] 432 = 37370; small epsilon [deep] 266 = 8830.

Example 18: Analogous to Example 4; red-violet crystals, m.p. 200 ° (decomp.); UV (ethanol): small epsilon [low] 505 = 9170; small epsilon [deep] 381 = 15980; small epsilon [deep] 261 = 22010.

Example 19: This dye can be obtained by heating an equimolar mixture of p-cyano cinnamaldehyde with p-n-butoxyaniline. M.p. 87.8-88.9 °, clp. 174.3-174.5 ° (nematic), yellow crystals; UV (ethanol): small epsilon [low] 296 = 25400; small epsilon [deep] 238 = 8100; Shoulders at 376 and 394 µm.

Example 20: This dye can be obtained by heating p-nitrosobenzonitrile and p-n-butoxyaniline in glacial acetic acid for 1 hour; 119.4-119.7 °, clp. 127.3-127.4 ° (nematic); red crystals; UV (ethanol): small epsilon [deep] 363 = 36260; small epsilon [deep] 252 = 14660.

Example 27: Analogous to Example 4; red crystals, m.p. 208 ° (dec.); UV (ethanol): small epsilon [deep] 605 = 51000.

Example 28: Analogous to Example 12; blue crystals, m.p. 234 ° (dec.); UV (ethanol): small epsilon [low] 567 = 47575; small epsilon = 7830.

Example 29: Analogous to Example 12; purple crystals, m.p. 201 ° (dec.); UV (ethanol): small epsilon [low] 570 = 29750; small epsilon [deep] 285 = 8360.

Example 31: Analogously to Example 19 from p-nitrocinnamaldehyde and p-n-butoxyaniline; orange crystals, m.p. 103.4-103.5 °, clp. 138.2-139.2 °; UV (ethanol): small epsilon [low] 379 = 23700; small epsilon [deep] 297 = 18900; small epsilon [deep] 236 = 9750; Shoulder at 390 µm.

Example 32 Analogous to Example 20, by reacting p-nitrosobenzonitrile with p-n-hexoxyaniline in glacial acetic acid; red crystals, m.p. 100.5-100.8 °, clp. 117 ° (nematic); UV (ethanol): small epsilon [low] 364 = 31,000; small epsilon [deep] 252 = 12450.

Example 33: Analogous to Example 30; purple crystals, m.p. 191-194 °; UV (ethanol): small epsilon [low] 545 = 56050; small epsilon [deep] 308 = 10130.

Example 37: Analogous to Example 36; blue-black crystals, m.p. 204-205 °; UV (ethanol): small epsilon [low] 542 = 58310; small epsilon [deep] 308 = 8980.

Example 39: Analogous to example 38; green-black crystals, m.p. 208-210 °; UV (ethanol): small epsilon [low] 543 = 61330; small epsilon [deep] 297 = 9530.

Example 40: Analogous to Example 38; purple crystals, m.p. 194.0-194.6 °; UV (ethanol): small epsilon [low] 514 = 52130; small epsilon [deep] 286 = 9380.

Example 41: Analogous to Example 38; dark green crystals, m.p. 190.0-190.5 °; UV (ethanol): small epsilon [low] 548 = 60040; small epsilon [deep] 294 = 8750.

Example 42: Analogous to Example 36; dark red crystals, m.p. 262.7-266.5 °; UV (ethanol): small epsilon [low] 518 = 521700; small epsilon [deep] 290 = 8870.

Example 43: Analogous to Example 36; purple crystals, m.p. 198.3-199.8 °; UV (ethanol): small epsilon [low] 552 = 62790; small epsilon [deep] 308 = 10660.

Example 44: Analogous to Example 36; red crystals, m.p. 166 ° (dec.); UV (ethanol): small epsilon [low] 489 = 29950; small epsilon [deep] 282 = 18540; small epsilon [deep] 222 = 34170; small epsilon [deep] 203 = 44780.

Example 45: Analogous to Example 36; violet crystals, m.p. 220-222 °; UV (ethanol): small epsilon [low] 528 = 54950; small epsilon [deep] 299 = 8780.

Example 46: Analogous to Example 4; orange crystals, m.p. 92-94 °; UV (ethanol): small epsilon [low] 433 = 29795; small epsilon [deep] 259 = 8480.

Example 47: Analogous to Example 36; violet crystals, m.p. 230-232 °; UV (ethanol): small epsilon [low] 519 = 51720; small epsilon [deep] 285 = 11100; small epsilon [deep] 221 = 31130; Shoulder at 246 µm.

Example 48: Analogous to Example 22; red crystals, m.p. 223-226 °; UV (ethanol): small epsilon [low] 482 = 29630; small epsilon [deep] 303 = 15850; small epsilon [deep] 247.5 = 10230.

Example 50: Analogous to Example 36; red black crystals, m.p. 247.5 °; UV (ethanol): small epsilon [low] 516 = 40320; small epsilon [deep] 318 = 4920; small epsilon [deep] 292 = 6210; small epsilon [deep] 242 = 11020.

Example 51: Analogous to Example 22; black-violet crystals, m.p. 200 ° (decomp.); UV (ethanol): small epsilon [low] 570 = 63550; small epsilon [deep] 313 = 11760; small epsilon [deep] 210 = 38570.

Claims (11)

1. Liquid-crystal mixture, characterized in that it a) at least three nematic liquid crystal components, at least one of which is a terpyrimidine of the formula I. is, where the symbols X for nitrogen and Y and Z for CH or Y for nitrogen and X and Z for CH or Z for nitrogen and X and Y for CH, and one of the substituents R [deep] 1 and R [deep] 2 die Cyano group and the other is a straight-chain alkyl group with 1 - 7 carbon atoms, b) at least one non-ionic, pleochroic azo dye, and optionally c) one or more isotropic, organic additives which lower the threshold voltage and / or the viscosity contains. 2. Liquid-crystal mixture according to claim 1, characterized in that the terpyrimidine is derived from a compound of the formula Ia is formed, where Z is nitrogen and X is CH or X is nitrogen and Z is CH, and R [deep] 1 is a straight-chain alkyl group having 1-7 carbon atoms. 3. Liquid-crystal mixture according to claim 1 or 2, characterized in that the terpyrimidine is derived from a compound of the general formula Ib is formed wherein R [deep] 1 represents a straight-chain alkyl group having 1-7 carbon atoms. 4. Liquid crystal mixture according to claim 1, 2 or 3, characterized in that the remaining liquid crystal components of compounds of the general formula II wherein R [deep] 2 is a straight-chain alkyl group with 4 - 8 carbon atoms, and / or of compounds of the general formula III wherein one of the substituents R [deep] 3 and R [deep] 4 is the cyano group and the other is a straight-chain alkyl group having 3 - 9 carbon atoms, and / or of compounds of the general formula IV where R [deep] 5 is ethyl, n-propyl, n-butyl, n-pentyl, iso-hexyl, n-hexyl, n-heptyl or n-octyl, and / or of compounds of the general formula V wherein R [deep] 6 denotes an alkyl group or an alkoxy group or a phenyl group substituted by an alkyl group or an alkoxy group, and / or of compounds of the general formula VI where R [deep] 7 is an alkyl group or an alkoxy group and n is the number 1 or 2, are formed. 5. Liquid-crystal mixture according to one of claims 1-4, characterized in that the azo dye is formed from a compound of the following general formulas: VII VIII or of derivatives of the formula VII or VIII in which one of the lateral positions on one of the benzene rings is substituted by one of the following substituents: a halogen atom, a methyl group, a halogen-substituted methyl group or a methoxy group; or of derivatives of the formula VII or VIII in which two of the lateral positions on one of the benzene rings are bridged with a condensed aromatic ring to form a naphthalene structure, as IX with A = an azo or azoxy group, n [deep] 1, n [deep] 2 and n [deep] 3 = whole numbers from 0 to 4, X = CN or nitro and Y [deep] 1 and Y [deep] 2 = each hydrogen, OR [deep] 1 with R [deep] 1 = alkyl or aralkyl and , where R [deep] 2 and R [deep] 3 each represent simple or substituted alkyl groups or alkylene groups which form a reduced heteroaromatic system with the terminal aromatic ring; X wherein X is hydrogen or a nitro group in the 5- or 6-position, cyano group, an alkyl group with 1-4 carbon atoms, in particular methyl, or an alkoxy group with 1-4 carbon atoms, in particular methoxy, and T is a group of the formula Xa or the formula Xb mean in which R [deep] 8, R [deep] 9 and R [deep] 10 lower alkyl groups or R [deep] 8 and R [deep] 9 together mean a lower cycloalkyl group. 6. Liquid-crystal mixture according to one of claims 1-5, characterized in that the terpyrimidine or the terpyrimidines of the formula I in an amount of about 1 to about 30% by weight, in particular from about 3 to about 20% by weight, based on the Total weight of the mixture is or are present. 7. Liquid-crystal mixture according to one of claims 1-6, characterized in that the dye or dyes in an amount of about 0.05 to about 1 wt.%, In particular from about 0.1 to 1 wt.%, Based on the total weight of the mixture is present. 8. Liquid-crystal mixture according to one of claims 1 - 7, characterized in that the isotropic, organic, the threshold voltage and / or the viscosity-lowering additives are formed from one of the following compounds: a) a nematogenic material with a clearing point above 60 ° C and an organic solvent or solvent mixture with a specific resistance of at least 10 [high] -7 Ohm x cm, and a dipole moment along the molecular axis of less than 3.5 D or b) a compound of the general formula XI where R [deep] 11 and R [deep] 12 are straight-chain alkyl groups with a total of at most 14 carbon atoms or R [deep] 11 is a straight-chain alkyl group and R [deep] 12 is a straight-chain alkoxy group with a total of at most 8 carbon atoms, or c) a compound of the general formula XII where R [deep] 13 and R [deep] 14 are straight-chain alkyl groups with a total of at most 6 carbon atoms, or d) a compound of the general formula XIII where R [deep] 13 and R [deep] 14 have the above meaning, or e) mixtures of these compounds. 9. Process for the preparation of liquid-crystal mixtures according to one of claims 1-8, characterized in that one a) at least three nematic liquid crystal components, at least one of which is a terpyrimidine of the formula I. is, where the symbols X for nitrogen and Y and Z for CH or Y for nitrogen and X and Z for CH or Z for nitrogen and X and Y for CH, and one of the substituents R [deep] 1 and R [deep] 2 die Cyano group and the other is a straight-chain alkyl group with 1 - 7 carbon atoms, b) at least one non-ionic, pleochroic azo dye, and optionally c) one or more isotropic, organic additives which lower the threshold voltage and / or viscosity are heated above the clearing point of the mixture and mixed thoroughly. 10. Use of a mixture according to any one of claims 1-8 for electro-optical purposes. 11. Use of a mixture according to any one of claims 1-8 in a rotating cell.