GB2310428A

GB2310428A - Mixture of non-cyano high birefringence laterally fluorinated terphenyl & cyclohexyl-biphenyl liquid crystalline compounds having halogenated terminal groups

Info

Publication number: GB2310428A
Application number: GB9703022A
Authority: GB
Inventors: Hideo Ichinose; Akio Takashima; Yuka Ikedo
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 1996-02-20
Filing date: 1997-02-13
Publication date: 1997-08-27
Anticipated expiration: 2017-02-13
Also published as: JPH09227868A; JP2008174747A; GB2310428B; GB9703022D0; JP4896896B2

Abstract

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it contains one or more fluorinated terphenyls having the formula I and one or more compounds of the formula II wherein R and R' are independently an alkyl or alkenyl radical having up to 12 C atoms, these radicals being unsubstituted or substituted by halogen, it also being possible for one or more CH 2 groups in these radicals to be replaced, in each case independently of one another, by -O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another; X and X' are independently F, Cl, CF 3 , OCF 3 or OCHF 2 ; and L 1 and L 2 are independently H or F.

Description

Liquid Crystal Medium The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it contains one or more fluorinated terphenyls having the formula I

and one or more compounds of the formula 11

wherein R and R' are each independently of one another and are an alkyl or alkenyl radical having up to 12 C atoms, these radicals being unsubstituted or substituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by -O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another1 and X and X' are each independently of one another F, Cl, CF3, OCF3 or OCHF2, and L1 and L2 are each independently of one another H or F.

Active matrix displays (AMD) are highly favored for commercially interesting displays with a high information content. Such AMDs are used for TV application (e.g. for projection systems) and also for displays for e.g. laptops, automobiles and aeroplanes.

AMDs have non-linear electrical switching elements which are integrated at each picture element. As non-linear driving elements thin film transistors (TFT) [Okubo, U., et al., 1982, SID 82 Digest, pp. 40A1] or diodes (e.g.: metal insulator metal: MIM) [Niwa, K, et al., 1984, SID 84, Digest, pp. 304-307j can be applied. These non-linear driving elements allow to use an electro-optical effect with a rather flat electro-optical characteristic if a good viewing angle characteristic can be obtained. So a TN-type LC cell (Schadt, M. and Helfrich, W., 1971, Appl. Phys. Lett., 18, 1271 with a twist angle of 90 can be used.To provide the good contrast over a wide viewing angle, operation in the first minimum of transmission [Pohl, L., Eidenschink, R., Pino, F. del., and Weber, G., 1980, German Pat., DBP 30 22 818, and 1981, US Pat. 4398 803; Pohl, L., Weber, G., Eidenschink, R., Baur, G., and Fehrenbach, W., 1981, Appl. Phys. Lett., 38, 497; Weber, G., Finkenzeller, U., Geelhaar, T., Plach, H.J., Rieger, B., and Pohl, L., 1988, Int. Symp. on Liq. Cryst., Freiburg, to be published in Liq. Crys.] is required. These AMDs are very well suited for TV applications and consequently are of high commercial interest. For these applications some physical properties of the liquid crystals become more important than for passive TN displays.Some of the decisive properties for the performance of an AMD are resistivity and stability of the liquid crystal [Togashi, S., Sekiguchi, K, Tanabe, H., Yamamoto, E., Sorimachi, K, Kajima, E., Watanabe, H., Shimuzu, H., Proc. Eurodisplay 84, Sept. 1984: A 21e288 Matrix LCD Controlled by Double Stage Diode Rings, p.144 ff, Paris; Stromer, M., Proc. Eurodisplay 84, Sept. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p.

145 ff, Parisl. A problem often encountered is the adverse influence of W-illumination on the resistivity and therefore on the general performance of the liquid crystal mixture in the display.

In an AMD the non-linear switching elements are addressed in a multiplex scheme. So they charge the electrodes of a pixel in the limited time they are active. Then they become inactive until they are addressed again in the next cycle. Consequently the change of the voltage on an activated (charged) pixel is a nondesired but a very decisive feature of such a display. The discharge of a pixel is determined by two factors. These are the capacity of the pixel element including liquid crystal and resistivity of the dielectric material between the electrodes, namely the liquid crystal.

The characteristic time constant of the decay of the voltage at a pixel (RCtime) has to be significantly bigger than the time between two adressing cycles (t,dr ). A parameter frequently used to describe the performance of an AMD is the voltage holding ratio HR of a picture element: V(4) + V(to + tadr.) HR = ~~~~~~~~~~~~~~ 2V(t As the voltage at a pixel decays exponentially an increase of the holding ratio necessitates liquid crystal materials with exceptionally high resistivities.

There are several points of importance for the resistivity of the liquid crystal inside a display, e.g. orientation layers, curing condition of the orientation material. But by no means less important are the electrical properties of the liquid crystal used. Especially the resistivity of the liquid crystal in the display determines the magnitude of the voltage drop at the pixel.

Earlier investigations with low-An materials have shown, that the requirements with regard to resistivity and W-stability and temperature dependence of the resistivity for TFT-applications cannot be met with materials containing cyano moieties as terminal groups. Non-cyano materials containing halogenated terminal groups can show for better resistivity values and UV-stability as well as superior viscosity values than conventionally used cyano materials. However, in general these noncyano materials unfortunately show a strong tendency towards forming crystalline andlor smectic phases, especially at low temperatures. Also the clearing points and the dielectric anisotropy values of non-cyano materials with halogenated terminal groups are much lower.

Modem commercial mixtures have to operate over a wide temperature range; therefore, crystallization or formation of smectic phases at low temperatures has to be excluded. Good solubility is one of the most important preconditions for the usability of liquid crystalline materials in the development of nematic mixtures. Compounds with high melting temperatures or a tendency to form smectic phases are for this reason not suitable.

By very careful selection of the components and an appropriate mixture design it was possible to find low birefringence non-cyano mixtures having a broad nematic temperature range for first minimum application [B.

Rieger et al., Proc. 18. Freiburger Arbeitstagung Flussigkristalle, Freiburg 1989, 16 (1989)]. Non-cyano materials with high birefringence, which are essential for the mixture concept of this invention unfortunately show in many cases even more unfavourable properties such as high melting points and/or strongly smectogenic behaviour than similar materials with lower birefringence: No. Chemical structure An Mesophases (etc)

1 c {}a 0.126 K70 S 79 N 1931 2 c5Hs 0-199 K142N1921 3 CsH" { < 3a n.m. K 105 S 245 1 The broad general formula of WO 90109240 covers fluoro terphenyls of the formula I but there is no single example disclosed of these compounds nor of mixtures containing fluorinated fluoro- and chloroterphenyls.

Mixtures of the state of the art with a birefringence suited for operation in the second or a higher transmission minimum of the Gooch-Tarry curve are not acceptable for active matrix application.

There is thus still a great need for liquid-crystal composition having a high resistivity and other suitable material properties for use in AMDs.

The chloroterphenyls No. 3 as shown above are known from JP 60-056 932-A. As outlined above these compounds do not allow to meet the severe specifications from the electronic industry, especially in view of their limited solubility in other LC materials, their high melting points and their pronounced smectogenity. Accordingly there is also a need in the art for improved non-cyano high birefringence LC compounds.

The chloroterphenyls of formula I have been disclosed on the international LC Conference in Pisa/ltaly, 1992.

The invention has for one of its objectives to provide a liquid crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it contains one or more fluorinated terphenyls having the formula I

and one or more compounds of the formula ll

wherein R, R', X, X', L1 and L2 have the meaning as given in Claim 1.

In the compounds of the formulae I and II R and R' are preferably alkyl or alkoxy. X and X' are preferably Cl or F.

The invention has also for its objective to provide a matrix liquid crystal display with high temperature and UV-stability containing - two plane parallel support plates which together with a frame form a cell of the thickness d, - integrated non-linear elements for switching individual picture elements on the support plates, and - a nematic liquid crystal mixture which is present in the cell, has a positive dielectric an isotropy and a birefringence An, the display being operated in the second or a higher transmission minimum of the Gooch-Tarry curve by appropriate selection of d . dn, characterized in that the quotient of the voltage holding ratio HR20 after 20 hours exposure to UV-light (280-400 nm, 12 mW/cm2) and HRo before exposure to UV-light is larger or equal to 98 % and also liquid crystal compositions with a very high resistivity which meet also the other demands.

It has now been found that such values for the HR are even possible for mixtures with higher birefringence by using laterally fluorinated and/or ethyl-linked non-cyano materials. Very high RC time values can be obtained in AMDs. These mixtures also show a reduced viscosity and allow short switching times at reasonable threshold voltages.

The thickness of the AMDs is preferably in the range of 3 to 10 pm.

Especially preferred is the range from 3 to 7 pm.

The following preferred embodiments concern the nematic liquid crystal mixture which is present in the AMD: The liquid crystal mixture preferably comprises one or more compounds of the formula Ill

wherein is is an alkyl or alkenyl radical having up to 15 C atoms, these radicals being unsubstituted or substituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by -O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another1 r isOor1, is is F, Cl, CF3, OCF3 or OCHF2, and L*, Y* and Z* are each H or F, and one of Q and Q2 is 1,4-phenylene or 3-fluoro-1,4-phenylene and the other residue is a single bond.

The compounds of the formula Illa are particularly preferred:

The liquid crystal mixture preferably comprises one or more compounds selected from the formulae IV to VII

wherein R is an alkyl or alkenyl radical having up to 12 C atoms, these radicals being unsubstised or substituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced. in each case independently of one another, by -O-, -O0- or OCO in such a manner that oxygen atoms are not linked directly to one another, and wherein the benzene ring can be further substituted by fluorine, X1 is F, Cl, CF OCF3, OCHF2, fluoroalkyl or fluoroalkoxy in each case having up to 7 carbon atoms, Yl, y2 and Y3 are each H or F, Q is -C2H4-, -C4H8- or -CO-O-,

is trans-1,4-cyclohexylene or 1,4-phenylene. and r isOor1.

The liquid crystal mixture preferably comprises one or more compounds of the formula VIII to XIII:

in which R, r, X1, Y1, y2, Y3 are each, independently from one another, as defined in claim 4. R' has the same meaning as given for R.

The mixture essentially comprises compounds selected from the group comprising the general formulae I to Vil.

In the following preferred embodiments are given: - The mixtures contain at least 10 % by weight of one or more compounds of the formula I.

- The mixtures contain at least 5 % by weight of one or more compounds of the formula II.

- The mixtures can contain additionally at least one compound of the formula

The compounds shown above are known from e.g. DOS 30 42 391, DOS 39 02 328, DOS 39 13 554, DOS 39 09 802, WO 89/02884, WO 90/15113, WO 90/09420, the International Patent Appln. No. PCT/EP 90/01292, No. PCT/EP 91/00411, No. PCT/EP 90/01471, No. PCT/EP 90/0 2109 and the European Patent Appin. No. 9 1 100 675.7 or can be prepared in analogy to known compounds.

The mixtures according to the present invention usually are based on the medium polar components having the indicated core structure and other non-cyano components. Of course, however, such mixtures can also additionally contain known cyano LC components, preferably compounds of the formula

wherein R, r,

Yl and Y2 have the meaning given above, if extremely high values for the HR are not needed, e.g. for TN or STNuse. The resulting mixtures are important for achieving very broad nematic phase ranges including very low temperatures (outdoor use).

The mixtures are preferably based on halogenated components of medium polarity and/or are essentially free of cyano components.

The novel compounds of the formula I can be prepared in analogy to the methods described in EP 0 439 089A1, WO 90/09420, WO 90/15113 and WO 91/13850, WO 91/00411 the disclosure of which is incorporated here by reference.

In the components of the formulae I to XIII R, R' and R' are preferably a straight-chained alkyl radical of 1 to 7 carbon atoms or is straight-chained methoxy alkyl (methoxymethyl, methoxyethyl, methoxypropyl, methoxybutyl, methoxypentyi, methoxyhexyl, methoxyheptyl).

The preparation of the mixtures according to the invention is effected in the conventional manner. In general, the desired amount of the components which is used in the smaller amount is dissolved in the components which constitutes the main constituent, preferably at elevated temperature. If this temperature is chosen to be above the clearing point of the main constituent, the completeness of the process of dissolving can be observed particularly easily.

However, it is also possible to mix solutions of the components in a suitable organic solvent, for example acetone, chloroform or methanol, and to remove the solvent does not introduce any contaminants or undesirable dopants.

By means of suitable additives the liquid crystal phases according to the invention can be modified in such a way that they can be used in any hitherto disclosed kind of AMD.

The examples below serve to illustrate the invention without limiting it. In the examples, the melting point and clearing point of a liquid crystal substance are given in degrees Celsius. The percentages are by weight.

The measurement of HR was performed as described by S. Matsumoto et al. (Liquid Crystals 5,1320 (1989)) in standard 6 pm TN-displays without spacers. Standard floatglass with conductive ITO layers (Balzers) and a robbed polyimide layer (AL-1051 of Japan Synthetic Rubber) as orientation layer was used. The cells were sealed with an UV-curable adhesive (NOA-61 of Norland) and filled under standard conditions. The liquid crystal mixture was composed of components being carefully purified under standard procedures. UV exposure was performed in a Heraeus Suntest with a Xenon lamp (1.1 kW, 0.082 W/cm2, UV cutoff 310 nm).

In the present patent application and in the following examples all chemical structures of LC compounds are given by acronyms the transformation of which into chemical formulae is done as shown in the following. All residues CnH2n+1 and C TH2m+1 are straight-chained alkyl groups with n resp. m carbon atoms. The code of Table B is self-explanatory. In Table A only the acronym for the core structure is given.In a concrete this acronym is followed by a dash and a code for the substituents R1, R2, LI1 L2 and L3 as follows: Code for R1, R1 R2 Lt L2 L3 R2, L1, L2, L3 nm CnH2n+1 CmH2m+1 H H H nOm CnH2n+1 OCmH2m+1 H H H nO.m OCnH2n+1 CmH2m+1 H H H n CnH2n+1 CN H H H nN.F CnH2n+1 CN F H H nF CnH2n+1 F H H H nOF OCnH2n+1 F H H H nCI CnH2n+1 Cl H H H nF.F CnH2n+1 F F H H nOmFF CnH2n+1 OCmH2m+i F H F nmF CnH2n+1 CmH2m+1 H H F nCF3 CnH2n+i CF3 H H H nOCF3 CnH2n+1 OCF3 H H H nOCF3 CnH2n+1 OCHF2 H H H nS CnH2n+1 NCS H H H rVsN CrH2r+i-CH=CH-CsH2s- CN H H H rEsN CrH2t+i-O-CsH2s CN H H H nNF CnH2n+1 CN H H F nAm CnH2n+1 COOCmH2m+i H H H nF.F.F CnH2n+1 F F F H nF.F CnH2n+1 F F H H Table A:

CfiHtmpO%H4MSX FET-nX.F CaHo mC2H,+ CFET-nX F L3 L1 Rt X R2 BCH L2 F C"H2- < x BCH-nX.F F C*H2, mx BCH-nX.F.F F aHiO)H2CH 3 i}X PEUP-n(O)X F F F cH2"+1 o) eCH2CH2 Qox PEUP-n(O)X.F F c H HOO0x CGPP-nX.F C*H2-1 --X CGPP-nX.F F F F F Cho,, Mx CUP-nX.F.F F F

F F Cm, , HOox CUP-nX.F F F F C.H2,.1 v 9+C7H4{X CFET-nX.F F F C,Hh,( CIH, CFET.nX.F,F F F F C,H,,, < C2H, < x FET nX.F.F F F F C,Ha,v x CC U-n-X F Table B:

Examples ExamDle 1 FET-3CI 7.0 % FET-SCI 6.0 % Clearing point [ C]: +98 C FT-5.FCI 14.0 % An [589 nm, 20 C]: +0.1493 CCP-30CF3 5.0 % V10,0,20): 1.53 V CCP-40CF3 5.0 % CCP-50CF3 5.0 % BCH-3F.F 13.0 % BCH-5F.F 13.0 % CCP-3F.F.F 8.0 % CCP-5F.F.F 4.0 % CGU-3-F 8.0 % CGU-5-F 9.0 % CBC-33F 3.0 % ExamDle 2 FET-2Cl 5.0 % FT-3.FCI 10.0 % Clearing point [ C]: +96 C CCP-20CF3 7.0% An [589 nm, 20 C]: +0.1359 CCP-30CF3 7.0 % V(100.20): 1.45 V 1.45 V CCP-40CF3 6.0 % BCH-2F.F 15.0 % BCH-3F.F 15.0 % CCP-2F.F.F 6.0 % CCP-3F.F.F 8.0 % CCP-5F.F.F 4.0 % CGU-2-F 4.0 % CGU-3-F 5.0 % BCH-32 5.0 % CBC-33F 3.0 % Example 3 FET-SCI 6.0 % FT-3.FCI 11.0 % Clearing point ["C]: +98 C CCP-30CF3 6.0 % An [589 nm, 20 C]: +0.1346 CCP-40CF3 7.0 % V(10.0,20): 1.53 V CCP-50CF3 7.0 % BCH-3F.F 14.0 % BCH-5F.F 14.0 % CCP-2F.F.F 6.0 % CCP-3F.F.F 8.0 % CCP-5F.F.F 4.0 % CGU-3-F 7.0 % CGU-5-F 8.0 % CBC-33F 2.0 % Examole 4 FET-3CI 8.0 % FET-SCI 5.0 % Clearing point [ C]: +95 C FT-5.FCI 13.0 % An [589 nm, 20 C]: +0.1488 CCP-30CF3 5.0 % V(10.0.20): 1.52 V CCP-40CF3 4.0 % CCP-50CF3 5.0 % BCH-3F.F 13.0 % BCH-5F.F 13.0 % CCP-3F.F.F 8.0 % CCP-5F.F.F 3.0 % CGU-3-F 10.0 % CGU-5-F 10.0 % CBC-33F 3.0 % ExamDle 5 FET-3CI 6.0 % FET-SCI 4.0 % Clearing point [0C]: +95 C FT-5.FCI 12.0 % An [589 nm, 20 C]: +0.1390 CCP-30CF3 6.0 % V(10.0.20): 1.48 V CCP-40CF3 5.0 % CCP-50CF3 5.0 % BCH-3F.F 11.0 % BCH-5F.F 11.0 % CCP-2F.F.F 3.0 % CCP-3F.F.F 11.0 % CCP-5F.F.F 5.0 % CGU-3-F 9.0 % CGU-5-F 9.0 % CBC-33F 3.0 % ExamDle 6 FT-5.FCI 14.65 % CCP-20CF3 3.25 % Clearing point [ C]: +97 C CCP-30CF3 5.00 % An [589 nm, 20 C]: +0.1399 CCP-40CF3 5.00 % V(100A20): 1.45 V CCP-50CF3 5.00 % BCH-2F.F 7.15 % BCH-3F.F 11.05 % BCH-5F.F 11.05 % CCP-2F.F.F 3.90 % CCP-3F.F.F 8.00 % CCP-5F.F.F 4.00 % CGU-2-F 2.60 % CGU-3-F 5.40 % CGU-5-F 5.10 % BCH-32 1.95 % CBC-33F 2.35 % FET-3Ct 2.45 % FET-SCI 2.10 % Example 7 FT-5.FCI 14.5 % CCP-20CF3 2.5 % Clearing point ["C]: +97 C CCP-30CF3 5.0 % An [589 nm, 20 "C]: +0.1424 CCP-40CF3 5.0 % CCP-50CF3 5.0 % BCH-2F.F 5.5 % BCH-3F.F 11.5 % BCH-5F.F 11.5 % CCP-2F.F.F 3.0 % CCP-3F.F.F 8.0 % CCP-5F.F.F 4.0 % CGU-2-F 2.0 % CGU-3-F 6.0 % CGU-5-F 6.0 % BCH-32 1.5 % CBC-33F 2.5 % FET-3CI 3.5 % FET-SCI 3.0 % ExamDle 8 FT-5.FCI 6.0 % Clearing point [ C]: +95 C FET-3CI 12.0 % An [589 nm, 20 C]: +0.1346 FET-5C1 5.0 % V(10020): 1.47 V CCP-20CF3 5.0 % CCP-30CF3 6.0 % CCP-40CF3 6.0 % CCP-50CF3 6.0 % BCH-3F.F.F 9.0 % BCH-5F.F.F 9.0 % CCP-3F.F.F 12.0 % CCP-5F.F.F 5.0 % CGU-3-F 7.0 % CGU-5-F 7.0 % CBC-33F 3.0 % CBC-53F 2.0 % ExamDle 9 FET-3C1 10.0 % Clearing point [ C]: +95 "C FET-SCI 6.0 An [589 nm, 20 C]: +0.1503 FT-5.CI 14.0 % V(100.20): 1.51 V CCP-30CF3 5.0 % CCP-40CF3 5.0 % CCP-S0CFs 5.0 % BCH-3F.F 5.0 % BCH-5F.F 4.0 % BCH-3F.F.F 9.0 % BCH-5F.F.F 9.0 % CCP-3F.F.F 10.0 % CCP-5F.F.F 4.0 % CGU-3-F 5.0 % CGU-5-F 5.0 % CBC-33F 4.0 % Examole 10 FET-3Cl 11.0 % Clearing point [ C]: +95 C FET-SCI 6.0 % An [589 nm, 20 "C]: +0.1497 FT-5.FCI 14.0 % V(10,0,20): 1.49 V CCP-30CF3 6.0 % CCP-40CF3 5.0 % CCP-SOCF3 5.0 % BCH-3F.F.F 12.0 % BCH-5F.F.F 12.0 % CCP-3F.F.F 10.0 % CCP-5F.F.F 4.0 % CGU-3-F 5.0 % CGU-5-F 5.0 % CBC-33F 3.0 % CBC-53F 2.0 % Examole 11 FET-3CI 10.0 % Clearing point ["C]: +94 C FET-SCI 5.0 % An [589 nm, 20 C]: +0.1490 FT-5.FCl 14.0 % V(10,0,20): 1.54 V CCP-30CF3 6.0 % CCP-40CFs 5.0 % CCP-50CF3 5.0 % BCH-3F.F.F 12.0 % BCH-5F.F.F 12.0 % CCP-3F.F.F 11.0 % CCP-5F.F.F 4.0 % BCH-3F.F 5.0 % CGU-5-F 5.0 % CBC-33F 2.0 % 132 4.0 %

Claims

Patent Claims 1. A liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it contains one or more fluorinated terphenyls having the formula I

and one or more compounds of the formula II

wherein R and R' are each independently of one another and are an alkyl or alkenyl radical having up to 12 C atoms1 these radicals being unsubstituted or substituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by -O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another, and X and X' are each independently of one another F, Cl, CF3, OCF3 or OCHF2, and L' and L2 are each independently of one another H or F.
2. Mixture according to Claim 1 characterized in that X and X' are chlorine or fluorine.
3. Mixture according to at least one of the Claims 1 to 2, characterized in that the liquid crystal mixture comprises additionally one or more compounds of the formula Ill

wherein R- is an alkyl or alkenyl radical having up to 15 C atoms1 these radicals being unsubstituted or substituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another1 by -O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another1 r isOor1, X is F, Cl, CF3, OCF3 or OCHF2, and L*, V* and Z* are each H or F, and one of Q1 and Q2 is 1 ,4-phenylene or 3-fluoro-1,4-phenylene and the other residue is a single bond.
4. Mixture according to at least one of Claims 1 to 3, characterized in that the liquid crystal mixture comprises one or more compounds selected from formulae IV to VII:

wherein R is each an alkyl or alkenyl radical having up to 12 C atoms, these radicals being unsubstituted or substituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by-O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another1 and wherein the benzene ring can be further substituted by fluorine, X1 is F, Cl, CF3, OCF3, OCHF2, fluoroalkyl or fluoroalkoxy in each case having up to 7 carbon atoms, yl, y2 and Y3 are each H or F, O is -C2H4-, -C4Hr or-CO-O-,

is trans-I ,4-cyclohexylene or 1,4-phenylene, and r isOor1.
5. Mixture according to at least one of daims 1 to 4, characterized in that the liquid crystal mixture additionally contains one or more compounds of the formula

wherein R, r,

Y1 and Y2 have the meaning given in daim 4.
6. Mixture according to at least one of Claims 1 to 4, characterized in that it essentially comprises compounds selected from the group comprising the general formulae I to VII.
7. Use of the liquid crystal mixture according to Claim 1 for electrooptical purposes.
8. Electrooptical liquid-crystal display containing a liquid-crystal mixture according to claim 1.