WO1995009827A1 - 3,4-difluorobenzenes and their use in liquid-crystal mixtures - Google Patents

Abstract

Described are 3,4-difluorobenzenes of formula (I) in which the symbols and indices have the following meanings: R1 is H, a straight-chain or branched-chain alkyl (with or without an asymmetric C-atom) with 1 to 15 C-atoms, whereby one or two non-adjacent -CH¿2?- groups may be replaced by -O-, -CH=CH-, -C C-, cyclopropane-1,2-diyl or -Si(CH3)2- and one or more H-Atoms in the alkyl group may be substituted by F; A?1, A2 and A3¿ which may be the same or different, are 1,4-phenylene, pyridine-2,5-diyl, pyrimidine-2,5-diyl in which one or more H-atoms may be replaced by F, trans 1,4-cyclohexylene, 1,3-dioxane-2,5-diyl or naphthalene-2,6-diyl; M?1 and M2¿, which may be the same or different, are -CH¿2?CH2-, -CH=CH-, -C C-, -CH2CH2CH2CH2-, -CH2CH2CH2-O-, -O-CH2CH2CH2-, -CH2CH2-CO-O-, -O-CO-CH2CH2-, -CH2-O-, -O-CH2, -CO-O- or -O-CO-; X is -CH2-O- or -O-CH2-; k, l, m, n, o and p are zero or one, under the condition that k+m+o is greater than zero. The compounds described have particularly low melting points. In mixtures, they improve the properties e.g. reducing the glass-transition temperature. This makes for a low viscosity and a low temperature dependence of the viscosity.

Description

 3,4-difluorobenzenes and their use in liquid crystal mixtures.

The unusual combination of anisotropic and fluid behavior of the liquid crystals has for their use in electro-optical switching and

Display devices led. Their electrical, magnetic, elastic and / or their thermal properties can increase

Orientation changes can be used. Optical effects can be achieved, for example, with the help of birefringence, the incorporation of dichroic dye molecules (guest host mode) or light scattering.

Practical requirements are constantly increasing, not least because of the increasing number of light valve types (TN, STN, DSTN, TFT, ECB, DECB, DS, GH, PDLC, NCAP, SSFLC, DHF, SBF etc.). Next

thermodynamic and electro-optical variables, such as phase sequence and

Phase temperature range, refractive index, birefringence and dielectric anisotropy, switching time, threshold voltage, steepness of the electro-optical

Characteristic curve, elastic constants, electrical resistance, multiplexability or pitch and / or polarization in chiral phases, is the stability of the

Liquid crystals against moisture, gases, temperature and

electromagnetic radiation as well as to the materials with which they are associated during and after the manufacturing process (e.g.

Orientation layers), of great importance. The toxicological and ecological harmlessness as well as the price are coming more and more

Meaning too.

The following references and the references contained therein provide a broad overview of the field of liquid crystals:

H.Kelker, H.Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, WE De Jeu, Physical Properties of Liquid Crystal Materials, Gordon and

Breach, Philadelphia, 1980; H. Kresse, Dielectric Behavior of Liquid Crystals,

Advances in Physics, Berlin 30, 10, 1982, 507-582;

 B. Bahadur, Liquid Crystals: Applications and Uses, World Scientific, Singapore,

1990;

 Landolt-Börnstein, New Series, Group IV, Volume 7 Liquid Crystals, 1992-1993 and

 J.W. Goodby et al., Ferroelectric Liquid Crystals: Principals, Properties and

Applications, Gordon Breach, Philadelphia, 1991.

The use of special derivatives of 3,4-difluorobenzene in nematic liquid crystal mixtures has been known for a long time (see:

DE-A 4108448, EP-A 0 507094, EP-A 502407, DE-A 41 12025,

DE-A 41 1 1766 and WO-A 90/05419).

However, since individual connections have so far not been able to simultaneously meet the above-mentioned requirements, there is a constant need for new improved ones

Liquid crystal mixtures and thus on a large number of mesogenic and non-mesogenic compounds of different structures, which enable the mixtures to be adapted to the most varied of applications. This applies both to the areas where nematic LC phases are used in

Find light valves, as well as for those with smectic phases.

It has now surprisingly been found that certain derivatives of

3,4-difluorobenzene is particularly suitable for use in

Liquid crystal mixtures are suitable.

The present invention therefore relates to new ones

wobei die Symbole und Indizes folgende Bedeutungen haben: 3,4-difluorobenzene derivatives of the formula (I),

where the symbols and indices have the following meanings:

R ¹ is H, a straight-chain or branched (with or without asymmetrical C atom) alkyl having 1 to 15 C atoms, one or two non-adjacent -CH ₂ groups also being represented by -O-, -CH = CH-, -C≡C-, cyclopropane-1, 2-diyl or -Si (CH ₃ ) ₂ - can be replaced, and one or more H atoms of the alkyl radical can also be substituted by F;

A ¹ , A ² , A ³ are the same or different 1, 4-phenylene, pyridine-2,5-diyl,

Pyrimidine-2,5-diyl, in which one or two H atoms can be replaced by F, trans-1, 4-cyclohexylene, 1, 3-dioxane-2,5-diyl or naphthalene-2,6-diyl;

M ¹ , M ² are the same or different -CH ₂ CH ₂ -, -CH = CH-, -C≡C-,

-CH ₂ CH ₂ CH ₂ CH ₂ -, -CH ₂ CH ₂ CH ₂ -O-, -O-CH ₂ CH ₂ CH ₂ -, -CH ₂ C _H2 -CO-O-,

-O-CO-CH ₂ CH ₂ -, -CH ₂ -O-, -O-CH ₂ -, -CO-O- or -O-CO-;

X is -CH ₂ -O- or -O-CH ₂ -; k, I, m, n, o, p are zero or one, provided that the sum k + m + o is greater than zero.

In a preferred embodiment of the invention, the symbols and indices in the formula (I) have the following meanings:

R ¹ is a straight-chain alkyl having 1 to 15 carbon atoms;

A ¹ , A ² , A ³ are identical or different 1, 4-phenylene, in which one or two H atoms can be replaced by F, or trans-1, 4-cyclohexylene;

M ¹ , M ² are identical or different -CH ₂ CH ₂ -, -C≡C-, -CH ₂ -O-, -O-CH ₂ -, -CO-O- or -O-CO-; X is -CH ₂ -O- or -O-CH ₂ -; k, I, m, n, o, p are zero or one, provided that the sum k + m + o is greater than zero.

In particular, the 3,4-difluorobenzenes (11) to (14) listed below are preferred:

wobei

in which

R ^{1 is} methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl.

The substances according to the invention are distinguished by particularly low melting points. In mixtures, they have an advantageous, for example lowering, effect on the glass transition temperature. This is favorable for a low viscosity and its low temperature dependence. The compounds according to the invention can be prepared according to

methods known from the literature and familiar to the person skilled in the art generally take place without problems.

Starting from commercially available 3,4-difluorophenol, species of the formula (I) are obtained by etherification with hydroxymethyl or halomethyl derivatives of Z ¹ (see scheme 1).

The etherification of also commercially available 3,4-difluorobenzyl alcohol or 3,4-difluorobenzyl bromide with alcohols from Z ^{1 also} provides

Compounds of formula (I) (see Scheme 2).

For the implementation of the syntheses see, among others:

- Journal of the American Chemical Society 1947, 69, 2451;

- Synthesis 1981, 1st

Scheme 1

Die Synthese des Restes R¹ (-A¹)_k(-M¹ ),(-A²)_m(-M²)_n(-A³)_o bzw. eines geeigneten Vorläufers erfolgt nach an sich bekannten, dem Fachmann geläufigen Methoden. Scheme 2

The synthesis of the radical R ¹ (-A ¹ ) _k (-M ¹ ), (- A ² ) _m (-M ² ) _n (-A ³ ) _o or a suitable precursor is carried out according to methods known per se and familiar to the person skilled in the art Methods.

The preparation takes place under reaction conditions which are known and suitable for the reactions mentioned. Use can also be made of variants which are known per se and are not mentioned here in detail.

For example, reference is made to DE-A 23 44 732, 24 50 088, 24 29 093, 25 02 94, 26 36 684, 27 01 591 and 27 52 975 for connections with

1,4-cyclohexylene and 1,4-phenylene groups; DE-A 26 41 724 for

Compounds with pyrimidine-2,5-diyl groups; DE-A 40 26 223 and

EP-A 03 91 203 for compounds with pyridine-2,5-diyl groups;

WO-A 92/16500 for Naρhthalin-2,6-diyl groups.

The production of disubstituted pyridines and disubstituted pyrimidines

can also be found, for example, in the corresponding volumes of the series "The Chemistry of Heterocyclic Compounds" by A. Weissberger and E.C. Taylor (editor).

Dioxane derivatives are useful by reacting an appropriate aldehyde (or one of its reactive derivatives) with one

corresponding 1,3-diol (or one of its reactive derivatives)

prepared, preferably in the presence of an inert solvent such as benzene or toluene and / or a catalyst e.g. a strong acid like

Sulfuric acid, benzene or p-toluenesulfonic acid, at temperatures between about 20 ° C and about 150 ° C, preferably between 80 ° C and 120 ° C. Acetals are primarily suitable as reactive derivatives of the starting materials.

Some of the aldehydes and 1,3-diols mentioned and their reactive derivatives are known, and some of them can be copied without difficulty

Standard methods of organic chemistry from literature

Connections are made. For example, the aldehydes are through Oxidation of corresponding alcohols or by reduction of nitriles or corresponding carboxylic acids or their derivatives, the diols obtainable by reduction of corresponding diesters.

Compounds in which an aromatic ring has at least one F atom as

Can also be substituted from the corresponding diazonium salts by exchanging the diazonium group for a fluorine atom, e.g. according to the methods of Balz and Schiemann.

Regarding the connection of the ring systems with each other, reference is made to:

 N. Miyaura, T. Yanagai and A. Suzuki, Synth. Comm. 1981, 1 1, 513-519; DE-C-39 30 663; M.J. Sharp, W. Cheng, V. Snieckus, Tetrahedron Letters 1987, 28, 5093; G.W. Gray, J. Chem. Soc. Perkin Trans II 1989, 2041 and Mol. Cryst. Liq. Cryst. 1989, 172, 165; 1991, 204, 43 and 91; EP-A 0 449 015; WO-A 89/12039; WO-A 89/03821 and EP-A 0 354 434 for direct

Linking aromatics and heteroaromatics; DE-A 32 01 721 for

Connections with -CH ₂ CH ₂ bridge members and Koji Seto et al., Liquid Crystals 1990, 8, 861-870 for connections with -C≡C bridge members.

Esters of the formula (I) can also be correspondingly esterified

Carboxylic acids (or their reactive derivatives) with alcohols or

Phenols (or their reactive derivatives) or by the DCC method (DCC = dicyclohexylcarbodiimide) can be obtained.

The corresponding carboxylic acids and alcohols or phenols are known and can be prepared in analogy to known processes.

Suitable reactive derivatives of the carboxylic acids mentioned are

in particular the acid halides, especially the chlorides and bromides, furthermore the anhydrides, for example also mixed anhydrides, azides or esters, in particular alkyl esters with 1-4 C atoms in the alkyl group. Reactive derivatives of the alcohols or phenols mentioned include, in particular, the corresponding metal alcoholates or phenolates,

preferably an alkali metal such as sodium or potassium.

The esterification is advantageously carried out in the presence of an inert solvent. Particularly suitable are ethers, such as diethyl ether,

Di-n-butyl ether, THF, dioxane or anisole, ketones such as acetone, butanone or cyclohexanone, amides such as DMF or phosphoric acid hexamethyltriamide,

Hydrocarbons such as benzene, toluene or xylene, halogenated hydrocarbons such as carbon tetrachloride, dichloromethane or tetrachlorethylene and sulfoxides such as dimethyl sulfoxide or sulfolane.

Ethers of the formula (I) are more appropriate by etherification

Hydroxy compounds, preferably corresponding phenols, are available, the hydroxyl compound expediently first in a corresponding

Metal derivative, for example by treatment with NaH, NaNH ₂ , NaOH, KOH, Na ₂ CO ₃ or K ₂ CO _{3 is converted} into the corresponding alkali metal alcoholate or alkali metal phenolate. This can then be reacted with the corresponding alkyl halide, sulfonate or dialkyl sulfate, advantageously in an inert solvent such as acetone, 1, 2-dimethoxyethane, DMF or dimethyl sulfoxide, or with an excess of aqueous or aqueous-alcoholic NaOH or KOH at temperatures between about 20 and 100 ° C.

With regard to the synthesis of special R ¹ radicals, reference is also made, for example, to EP-A 0 355 008 for compounds with silicon-containing compounds

Side chains and EP-A 0 292 954 and EP-A 0 398 155 for compounds with cyciopropyl groups in the side chain.

The compounds of the general formula (I) according to the invention are chemically and photochemically stable. They have low melting points and generally broad liquid-crystalline phases, in particular broad nematic ones. Compounds of the formula (I) can be used, for example, for the preparation of nematic or chiral nematic liquid-crystal mixtures

use that are for use in electro-optical or fully optical elements, e.g. Display elements, switching elements,

Light modulators, elements for image processing, signal processing or generally in the field of nonlinear optics are suitable. This also applies to compounds that have no liquid-crystalline phases as the pure substance. In general, the compounds of formula (I) are for introduction or

Suitable for widening a nematic phase in LC mixtures.

The invention therefore also relates to the use of compounds of the formula (I) in liquid-crystal mixtures.

The invention further relates to liquid crystal mixtures which contain one or more compounds of the formula (I).

The liquid crystal mixtures according to the invention generally consist of 2 to 20, preferably 2 to 15 components, including at least one, preferably 1 to 5, particularly preferably 1 to 3, compounds of the formula (I). The LC mixtures according to the invention can be, for example, nematic, chiral nematic, smectic and / or ferroelectric. Of the compound or compounds of formula (I) according to the invention contain the

Liquid crystal mixtures in general 0.1 to 70 mol%, preferably 0.5 to 50 mol%, in particular 1 to 25 mol%.

Further constituents of the mixtures according to the invention are preferably selected from the known compounds with nematic or

cholesteric phases; these include, for example, biphenyls, terphenyls, phenylcyclohexanes, bicyclohexanes, cyclohexylbiphenyls, mono-, di- and

Trifluorophenyls. Generally, there are those available commercially

Liquid crystal mixtures before the addition of the invention Compound (s) as mixtures of various components, at least one of which is mesogenic.

Suitable further constituents of nematic or chiral nematic liquid crystal mixtures according to the invention are, for example, 4-fluorobenzenes, as for example in EP-A 494 368, WO 92/06 148, EP-A 460 436, DE-A 4 111 766, DE-A 4 1 12 024, DE-A 4 1 12 001, DE-A 4 100 288, DE-A 4 101 468, EP-A 423 520, DE-A 392 3064, EP-A 406 468, EP-A 393 577, EP -A 393 490, - 3,4-difluorobenzenes, as described for example in DE-A 4 108 448, EP-A 507 094 and EP-A 502 407, - 3,4,5-trifluorobenzenes, as for example in DE- A 4 108 448, EP-A

 387 032, 4-benzotrifluoride, as described for example in DE-A 4 108 448, phenylcyclohexane, as described for example in DE-A 4 108 448.

Liquid-crystalline mixtures which contain compounds of the general formula (I) are particularly suitable for use in electro-optical switching and display devices (displays). Such switching and

Display devices (LC displays) generally have, inter alia, the following

Components on: a liquid-crystalline medium, carrier plates (e.g. made of glass or plastic), coated with transparent electrodes, at least one orientation layer, spacers, adhesive frames, polarizers and thin color filter layers for color displays. Other possible components are antireflection, passivation, compensation and barrier layers as well as electrically non-linear elements, such as thin-film transistors (TFT) and metal-insulator-metal (MIM) elements. The structure of liquid crystal displays is in detail already described in relevant monographs (e.g. E. Kaneko, "Liquid Crystal TV Displays: Principles and Applications of Liquid Crystal Displays", KTK Scientific Publishers, 1987, pages 12-30 and 63-172).

Examples

Various measurement methods are used to physically characterize the compounds according to the invention.

The phase transition temperatures are determined with the help of a polarizing microscope on the basis of the texture changes. The

The melting point, however, is determined with a DSC device

carried out. The indication of the phase transition temperatures between the phases

Isotropic (I)

Nematic (N or N ^* )

Smectic-C (S _c or S _c ^* )

Smectic-A (S _A or S _A ^* )

 Crystalline (X)

 Glass transition (Tg) takes place in ° C and the values are between the phase names in the phase sequence.

With different values for heating and cooling, the latter are in

Parentheses are set, or the phase sequence is given ascending and descending in temperature.

Electro-optical investigations are carried out according to methods known from the literature (for example B. Bahadur: Liquid Crystals Application and Uses, Vol. I, World Scientific, Singapore, 1990). For nematic liquid crystals (pure or in a mixture), the values for the optical and dielectric anisotropy and the electro-optical characteristic are recorded at a temperature of 20 ° C.

Liquid crystals which do not have a nematic phase at 20 ° C. are mixed in 10% by weight in ZLI-1565 and / or in 20% by weight in ZLI-4792 (commercial nematic liquid crystal mixtures from E. Merck, Darmstadt) and the values are extrapolated from the results of the mixture.

Electro-optical characteristics are determined on the basis of the transmission of a measuring cell. For this purpose, the cell is positioned between crossed polarizers in front of a light source. There is a light detector behind the cell, the sensitivity of which is optimized by filters for the visible area of the light. Analogous to the gradual increase in the voltage applied to the cell, the change in transmission is recorded. Values such as threshold voltage and slope are determined from this.

The optical anisotropy is determined using an Abbέ refractometer (from Zeiss). To orient the liquid crystal, a

Orientation layer, obtained from a 1% by weight lecithin-methanol solution.

To determine the dielectric anisotropy, a measuring cell with homeotropic and planar orientation is produced and its capacities and dielectric losses are determined with a multi-frequency LCR meter (Hewlett Packard 4274 A). The dielectric constants are calculated as described in the literature (W. Maier, G. Meier, Z. Naturforsch. 1961, 16a, 262 and W.H. de Jeu, F.Leenhonts, J.Physique 1978, 39, 869).

The electrical variable HR (holding ratio) is in accordance with the

References determined (M.Schadt, Linear and nonlinear liquid crystal materials, Liquid Crystals 1993, 14, 73-104). To determine the switching speed (T) and contrast (K), the measuring cell is attached to the turntable of a polarizing microscope between the crossed analyzer and polarizer. To determine the contrast, the measuring cell is positioned by turning it so that a photodiode indicates minimal light transmission (dark state). The microscope illumination is controlled so that the photodiode shows the same light intensity for all cells. After a switching operation, the light intensity (bright state) changes, and the contrast is calculated from the ratio of the light intensity of these states.

example 1

3,4-difluorophenyl (trans-4-pentylcyclohexyl) methyl ether

1.97 g (7.50 mmol) triphenylphosphine are at 0 ° C in 30 ml

Tetrahydrofuran was mixed with 1.31 g (7.5 mmol) of diethyl azodicarboxylate and stirred at room temperature for 0.5 h. Then 0.98 g (7.50 mmol) of 3,4-difluorophenol and 0.92 g (5.00 mmol) of trans-4-pentylcyclohexylmethanol are added and the mixture is stirred at room temperature for 18 h. After evaporating the

Solvent and chromatography on silica gel with hexane give 0.79 g of product.

Phase sequence: Tg -80 × 21 I -32 S _x -42 × -87 Tg

The following are prepared as in Example 1:

Example 2

3,4-difluorophenyl (trans-4-ethylcyclohexyl) methyl ether

Beispiel 3

Example 3

3,4-difluorophenyl (trans-4-propylcyclohexyl) methyl ether

Example 4

3,4-difluorophenyl (trans-4-butylcyclohexyl) methyl ether

 Example 5

3,4-difluorophenyl- [trans-4- (trans-4-ethylcyclohexyl) cyclohexyl] methyl ether

 Example 6

3,4-difluorophenyl- [trans-4- (trans-4-propylcyclohexyl) cyclohexyl] methyl ether

Example 7

3,4-difluorophenyl- [trans-4- (trans-4-butylcyclohexyl) cyclohexyl] methyl ether

Beispiel 8

Example 8

3,4-difluorophenyl- [trans-4- (trans-4-pentylcyclohexyl) cyclohexyl] methyl ether

Phase sequence: X 60 N 109 1 109 N 37 S _x 1 1 X

Example 9

3,4-difluorophenyl- [4- (trans-4-ethylcyclohexyl) phenyl] methyl ether

Example 10

3,4-difluorophenyl- [4- (trans-4-propylcyclohexyl) phenyl] methyl ether

Example 1 1

3,4-difluorophenyl- [4- (trans-4-butylcyclohexyl) phenyl] methyl ether

Example 12

3,4-difluorophenyl- [4- (trans-4-pentylcyclohexyl) phenyl] methyl ether

Phase sequence: X 63 (44) Example 13

3,4-difluorophenylmethyl [4- (trans-4-ethylcyclohexyl) phenyl] ether

 Phase sequence: X 60 (34) I

Example 14

3,4-difluorophenylmethyl [4- (trans-4-propylcyclohexyl) phenyl] ether

Phase sequence: X 75 (43) N (57) I

Example 15

3,4-difluorophenylmethyl [4- (trans-4-butylcyclohexyl) phenyl] ether

Phase sequence: X 68 (33) N (56) I

Example 16

3,4-difluorophenylmethyl [4- (trans-4-pentylcyclohexyl) phenyl] ether

Phase sequence: X 62 (31) N 69 I Examples of use

Table 1 shows application examples. The substances from Examples 1 and 8 are used as pure substance and in a mixture (10% by weight) with ZLI 1565

(commercial nematic liquid crystal mixture from E. Merck,

Darmstadt) examined and compared with known reference substances.

It shows how low the melting points of the substances according to the invention are in comparison and how they can induce even lower glass transitions when mixed.

In Table 2 the substance from Example 8 is compared with a reference substance in the commercial nematic liquid crystal mixture ZLI-4792 (E. Merck, Darmstadt). It turns out that the substance according to the invention is able to lower the higher-order smectic phase considerably, quite in contrast to the reference substance.

In addition, the electro-optical threshold voltages are increased less by the substance from Example 8, which is also advantageous.

Claims

Claims: 1. 3,4-difluorobenzene derivatives of the formula (I),

where the symbols and indices have the following meanings: R ¹ is H, a straight-chain or branched (with or without asymmetrical C atom) alkyl having 1 to 15 C atoms, one or two non-adjacent -CH ₂ groups also being represented by -O-, -CH = CH-, -C≡C-, cyclopropane-1, 2-diyl or -Si (CH ₃ ) ₂ - can be replaced, and one or more H atoms of the alkyl radical can also be substituted by F; A ¹ , A ² , A ³ are the same or different 1, 4-phenylene, pyridine-2,5-diyl, Pyrimidine-2,5-diyl, in which one or two H atoms can be replaced by F, trans-1, 4-cyclohexylene, 1, 3-dioxane-2,5-diyl or naphthalene-2,6-diyl; M ¹ , M ² are the same or different -CH ₂ CH ₂ -, -CH = CH-, -C≡C-, -CH ₂ CH ₂ CH ₂ CH ₂ -, -CH ₂ CH ₂ CH ₂ -O-, -O-CH ₂ CH ₂ CH ₂ -, -CH ₂ CH ₂ -CO-O-, -O-CO-CH ₂ CH ₂ -, -CH ₂ -O-, -O-CH ₂ -, -CO-O- or -O-CO-; X is -CH ₂ -O- or -O-CH ₂ -; k, I, m, n, o, p are zero or one, provided that the sum k + m + o is greater than zero. 2. 3,4-difluorobenzenes according to claim 1, characterized in that the symbols and indices in the formula (I) have the following meanings: R ¹ is a straight-chain alkyl having 1 to 15 carbon atoms; A ¹ , A ² , A ³ are identical or different 1, 4-phenylene, in which one or two H atoms can be replaced by F, or trans-1, 4-cyclohexylene; M ¹ , M ² are identical or different -CH ₂ CH ₂ -, -C≡C-, -CH ₂ -O-, -O-CH ₂ -, -CO-O- or -O-CO-; X is -CH ₂ -O- or -O-CH ₂ -; k, I, m, n, o, p are zero or one, provided that the sum k + m + o is greater than zero. 3. 3,4-difluorobenzenes of the formula (I) according to claim 1 and / or 2, characterized by the formulas (11) to (14),

in which R ^{1 is} methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl. 4. Use of 3,4-difluorobenzenes according to one or more of claims 1 to 3 in liquid crystal mixtures. 5. Liquid crystal mixture containing at least one compound of the formula (I) according to one or more of claims 1 to 3. 6. Liquid crystal mixture according to claim 5, characterized in that the liquid crystal mixture is nematic. 7. Liquid crystal mixture according to claim 5 or 6, characterized in that it contains 1 to 8 compounds of formula (I). 8. Liquid crystal mixture according to one or more of claims 5 to 7, characterized in that it contains 0.1 to 70 mol% of at least one Contains compound of formula (I). 9. switching and / or display device, comprising carrier plates, Electrodes, at least one polarizer, at least one orientation layer and a liquid-crystalline medium, characterized in that the liquid-crystalline medium is a liquid-crystal mixture according to one or more of claims 5 to 8.