SUPERTWIST LIQUID CRYSTAL DISPLAY

Abstract: The invention relates to supertwist liquid crystal displays (STN-LCDs) having: two plane-parallel carrier plates which, with an edging, form a cell; a nematic liquid crystal mixture of positive dielectric anisotropy in the cell; electrode layers with superimposed orientation layers on the inside of the carrier plates; an angle of incidence between the longitudinal axis of the molecules on the surface of the carrier plates and the carrier plates of about 1 degree to 30 degrees and a twisting angle of the liquid crystal mixture in the cell from orientation layer to orientation layer, according to the amount, of between 160 and 720 DEG C. The nematic liquid crystal mixture contains one or more compounds of formula (I) wherein R<1> is alkyl having 1-12 C atoms, wherein one or two non-adjacent CH2 groups can also be replaced by -O-, -CH=CH-, -CO-, -O-CO- or -CO-O-, r is 0 or 1, ring A is trans-1,4-cyclohexylene or 1,4-phenylene, X is F, Cl, CF3, OCF3 or OCHF2 and Y and Z each independently of one another denotes H or F.

Description: Supertwist Liquid Crystal Display The invention relates to supertwist liquid crystal displays (STN-LCD) which exhibit short switching times and a good steepness of the electrooptical characteristic line and which can be driven at high duty ratios. The invention further relates to the novel nematic liquid crystalline mixtures used therein which exhibit a low viscosity, a high stability versus chemicals, heat and radiation, a high electrical resistivity and a low value for the ratio of the dielectrical anisotropy Δε and the dielectrical constant £ measured perpendicular with respect to the nematic director. STN-LCDs as characterized in the preamble of claim 1 are known, for example from EP 0,131,216 BI; DE 3,423,993 Al; EP 0,098,070 A2; M. Schadt and F. Leenhouts, 17th Freiburg conference on liquid crystals (8-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. .50 (21), 1468 (1987); H.A. van Sprang and H.G. Koopman, J. Appl. Phys. 12 (5), 1734 (1987); T.J. Scheffer and J. Nehring, Appl. Phys. Lett. 4jj (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 5_0 (5), 236 (1987) and E.P. Raynes, Mol. Cryst. Liq. Cryst. Letters, Vol. 4 (1), pp. 1-8 (1986) . The term SFA here includes any relatively highly twisted display element having a twist angle with a value between 160° and 720°, such as, for example, the display elements of Waters et al. (CM. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), the STN-LCDs (DE OS 3,503,259), SBE-LCDs (T.J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP OS 0,246,842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)) . Compared with standard TN displays with a twist angle of about 90° STN-LCDs of this type are distinguished by a significantly better steepness of the electrooptical cha¬ racteristic line and, associated therewith, better contrast values, and by a significantly lower angle dependence of the contrast. Of particular interest are STN-LCDs with high information content exhibiting a high multiplexing ratio. According to the "iron law of multiplexing" first formulated by Alt and Pleshko (P.M. Alt and P. Pleshko, IEEE Trans Electron Devices, ED-21, 1974, 146) the selection ratio S, i.e. the ratio of the rms voltage across a selected element to the voltage across a nonselected element, is decreasing with the number of multiplexed lines because of crosstalk. Therefore, in order to obtain a sufficient contrast ratio between the selected and nonselected state it is necessary for the STN-LCD to exhibit a high or even very high steep¬ ness of the electrooptical characteristic line. The steep¬ ness of the electrooptical characteristic line is mainly influenced by elastic and dielectric constants of the liquid crystalline medium and it has been shown quite generally that the steepness improves with decreasing Δε/ε (see for example Raynes, E.P. et al., Proc. Eurodisplay, London 1987, 100) . Since Δε must not be chosen too low in order to rea¬ lize an acceptable threshold voltage the value of ε± should be as large as possible. Furthermore, the liquid crystal media used in such STN-LCDs have to exhibit a high stability and a high electrical resistance which ideally should not deteriorate under various influences such as heat or radiation because for a fixed driving frequency the refreshing frequency at each pixel is decreasing with increasing driving duty ratio. The voltage across a pixel, however, drops with a certain decay time which is decisively influenced by the resistance of the liquid crystal layer; therefore, STN-LCDs operated with a high multiplex ratio usually require liquid crystalline media which exhibit a high stability and a high electrical resistivity. The STN-LCDs should furthermore exhibit short switching times, in particular also at relatively low temperatures, which generally require optimization of the viscosity of the liquid crystalline medium used. It is also possible, how¬ ever, to reduce the thickness of the LC layer of the STN-LCD and to use liquid crystalline media with a higher optical anisotropy Δn. Further demands for STN-LCDs are low threshold voltages, a broad temperature range of operation, an advantageous temperature dependence of the electrooptical properties, a good contrast and a good angle dependence of contrast. The liquid crystalline media developed so far do not fulfill these requirements to a desirable extent. Furthermore, optimal parameters cannot be achieved simultaneously for all the properties mentioned above because of the opposite influence of different material parameters such as, for example, dielectric and elastic properties. Therefore, there continues to be a great demand for improved STN-LCDs having a steep electrooptical characteristic line -and hence a high multiplexibility and, at the same time, short switching times, a great temperature range of opera¬ tion, a good contrast, a good viewing angle dependence of contrast and a high stability and electrical resistivity of the liquid crystal medium used therein. The invention has the object of providing STN-LCDs exhibi¬ ting a good overall combination of the properties mentioned above and especially a high multiplexability, short swit¬ ching times and a high stability and resistivity of the liquid crystalline medium used. Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from reading of the following detailed descrip- tion. It has been found that these and other advantages can be achieved if the nematic liquid crystalline mixture used contains one or more compounds of the formula I wherein R 1 is alkyl having 1-12 C atoms, wherein one or two non-adjacent CH 2 groups can also be replaced by -0-, -CH=CH-, -CO-, -O-CO- or -CO-0-, r is 0 or 1, ring A is trans-1, 4-cyclohexylene or 1,4-phenylene, X is F, Cl, CF 3 , 0CF 3 or 0CHF 2 and Y and Z each independently of one another denotes H or F. The nematic liquid-crystal mixture preferably has a nematic phase range of at least 60 °C, a viscosity of not more than 30 mPa.s and a dielectric anisotropy of at least +5, the dielectric anisotropies of the compounds and the parameters related to the nematic liquid-crystal mixture being based on a temperature of 20 °C. The invention thus relates to an SFA having - two plane-parallel carrier plates which, with an edging, form a cell, - a nematic liquid crystal mixture of positive di¬ electric anisotropy in the cell, - electrode layers with superimposed orientation layers on the inside of the carrier plates, - an angle of incidence between the longitudinal axis of the molecules on the surface of the carrier plates and the carrier plates of about 1 degree to 30 de¬ grees and - a twisting angle of the liquid crystal mixture in the cell from orientation layer to orientation layer, according to the amount, of between 100 and 720 °C, characterized in that the nematic liquid crystal mixture contains one or more compounds of the formula I wherein R 3 * is alkyl having 1-12 C atoms, wherein one or two non-adjacent CH 2 groups can also be replaced by -0-, -CH=CH-, -CO-, -O-CO- or -CO-0-, r is 0 or 1, ring A is trans-l,4-cyclohexylene or 1,4-phenylene, X is F, Cl, CF 3 , OCF 3 or OCHF 2 and Y and Z each independently of one another denotes H or F. The invention also relates to correponding liquid crystal¬ line media. The construction of the liquid-crystal display elements according to the invention from polarizers, electrode base plates and electrodes having a surface treatment such that the preferential orientation (director) of the liquid- crystal molecules in each case adjacent thereto is usually mutually twisted from one electrode to the other by a value of 100° to 720°, corresponds to the customary construction for display elements of this type. The term customary construction here is used in broad terms and also includes all derivatives and modifications of supertwist cells, in particular also matrix display elements. The surface tilt angle at the two support plates may be identical or diffe¬ rent. Identical tilt angles are preferred. An essential difference between the display elements accor¬ ding to the invention and the display elements customary hitherto based on the twisted nematic cell is, however, the choice of liquid-crystal components in the liquid-crystal layer. The mixtures according to the invention preferably contain one or more compounds of part formulae 11-13 In the compounds of part formulae 11-13 R 1 preferably is alkyl, alkoxy, oxaalkyl or alkenyl and can exhibit a straight-chain or branched structure. Alkyl or alkoxy preferably are straight-chain and have 2, 3, 4, 5, 6 or 7 C atoms. Accordingly they are preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, also methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy. Oxaalkyl is preferably straight-chain 2-oxapropyl (= methoxymethyl) , 2- (= ethoxymethyl) or 3-oxybutyl (= 2-methoxyethyl) , 2-, 3-, or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, A-, 5-, 6- or 7-oxaoctyl, 2-, 3-, A-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8-, or 9 oxadecyl. Alkenyl is preferably straight-chain and has 2 to 10 C atoms. It is accordingly, in particular, vinyl, prop-1- or prop-2-enyl, but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl. Compounds of the formula I containing a branched terminal group can occasionally be of importance because of an improved solubility in the customary liquid crystal base materials, but in particular as chiral doping substances if they are optically active. Branched groups of this type as a rule contain not more than one chain branching. Preferred branched radicals are iso- propyl, 2-butyl (= 1-methylpropyl) , isobutyl (= 2-methyl- propyl) , 2-methylbutyl, isopentyl, (= 3-methylbutyl) , 2-methylpentyl, 2-ethylhexyl, 2-propylpentyl, 2-octyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 2-methyl- hexoxy, 1-methylhexoxy, 1-methylheptoxy (= 2-octyloxy) , 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl, 6-methyloctoxcy, oxy, 2-me- thyl-3-oxapentyl and 2-,ethyl-3-oxahexyl. The terminal group can be laterally unsubstituted or laterally mono- or difluorinated: -(θ)-X (1) °£ • X (2) r F X (4) Liquid crystalline compounds according to part formulae 11-13 are generally characterized by advantageous values of εj_ and Δε/εj_. Furthermore, they exhibit a high resistivity and a high stability versus heat and radiation, especially UV-light. Compounds according to part formulae 11-13 with a terminal group according to formulae (l)-(3) are generally preferred. Compounds of part formulae 11-13 with a terminal group (1) generally exhibit a somewhat lower value of Δε than the coresponding compounds with a terminal group (2) or (3) while the value of εj . is more or less the same. Compounds of part formulae 11-13 with a terminal group (1) consequently exhibit a lower value of the quotient Δε/ε± which is favou¬ rable in view of a steep electrooptical characteristic line; on the other hand a relatively high or high value of Δε results in a relatively low or low value of the threshold voltage which is often desired. In the compounds according to part formulae 11-13 X prefer¬ ably is F, Cl, CF 3 , OCF 3 and OCHF 2 and especially F, Cl, CF 3 and OCF 3 . By varying the percentage of the compounds of part formulae 11-13 with the terminal groups (l)-(4) the expert can easily accentuate and optimize certain properties with respect to the intended application. At any rate the liquid crystalline media of this invention are very stable and have a high resistivity. They allow the realization of SFAs with a steep characteristic electrooptical line, a multiplex ratio, a broad temperature range of operation, a low thre¬ shold voltage and a short switching time. The following smaller groups of compounds according to part formulae 11-13 are especially preferred. Compounds according to part formula II: _0 R i - _ ^-(o -O HFz 11-5 0 F F F Compounds according to part formulae 12 : --, , --0υ R -^ -H ^ 00 ^ I2_4 0 F F F F Ri -( " . ^® "CF3 I2~10 0 Compounds according to part formulae 13: R- 0)-(_ *-(ό θCF 3 13-4 0 F R1 - C o K°λ F I3~5 R- θ}- _>»- ό -Cl 13-6 F F Preferred liquid-crystalline media which can be used accord- ing to the invention contain one or more compounds from group A preferably in a proportion of 5 % to 60 % and especially of 10 % to 40 %. These compounds or this compound from group A have a dielectric anisotropy of more than +8 (preferably of more than +12) and form the component A of the LC mixtures of this invention. Preferably group A comprises compounds selected from the following group: _N R- θ>-Q-CN AllI N R-( " θ>-Q-CN AIV N wherein R is alkyl having 1-12 C atoms, wherein one or two non- adjacent CH 2 groups can also be replaced by -0-, -CH=CH-, -CO-, -O-CO- or -CO-0-, Q is the formula ~-( t ~ v Zi is -_ "' ~ _ - H 2 CH2-, a single bond, -CH 2 CH 2 -, -CO-O- or -O-CO- and Preferably the mixtures comprise one ore more compounds of the formula Al in the range of 5 to 50 %. Preferred are those compounds wherein Zi denotes a single bond, -CH 2 CH 2 - or -CO-0- and especially preferred are the following compounds: R-(θ)-(θ -CN AI3 Also preferred are the following compounds: R- H^- O -C AI4 Group A preferably comprises one ore more compounds selected from the formulae All to AI3 and optionally also one or more compounds of the formula AI4. Preferably the mixtures further contain one or more polar compounds with a higher clearing point, e.g. selected from the following compounds: R -^SK K ) "CN AI5 In the above four formulae the 1,4-phenylene rings can also be laterally substituted by one fluorine atom, e.g. AI5a AV2a These polar compounds with a higher clearing point are preferably used in the range of 2 to 15 %. In another preferred embodiment the mixtures comprise one or more compounds with a very high dielectric anisotropy. Such compounds are preferably used in the range of 2 to 50 %. Preferred compounds of this type are those of the formulae All, AIII and AV, wherein Q denotes 1,4-phenylene or 3- fluoro-l,4-phenylene or compounds of the formula AI7: Preferred liquid-crystal mixtures contain one or more compounds from group B. preferably in a proportion of 5 % to 40 % and especially of 5 % to 30 %. These compounds or this compound from group B have a clearing point of more than 120° and are dielectrically neutral ( (Δε) < 2) or medium polar (Δε in the range of +2 to +10, preferably +4 to +8) and form component B of the LC mixtures of this invention. Preferably group B comprises compounds selected from the following group: R1- H - O -Q-<^H^-R2 BI R1 "(2 ~Zθ "(i)" Q "® R2 BIV Ri-^H^-Q-(θ -R2 BV R1- H^-Q-Z 0 -^ -R2 BVI R-ζH *Z 0 -Q-/O R2 BVII wherein R and R 2 each independently of one another are R as defined in claim 3, Z° is -CO-0-, -O-CO- or -CH 2 CH 2 -, Q is as defined in claim 3 and the 1,4-phenylene ring in BI to BVII may also be substituted in 2- or 3-position by fluorine. Preferably the mixtures comprise one or more compounds with four rings of the formulae BI to BIV in order to achieve a high clearing point. These compounds are preferably selected from the group of the following four-ring compounds: Ri-(H 0/- θ - H -R BII Preferably the mixtures comprise also one or more compounds selected from the following group of three-ring compounds: R1 " E D" CH CH 2-{£}" R2 BVI1 (F) R1-(H -^)-COO- H)-R2 BVIII1 R1-(H - 0)-OOC- H - R2 BVIII2 BVTII3 BVIII4 The liquid crystalline media according to the invention can contain dielectrically neutral compounds with -1.5 < Δε < +1.5 the proportion of which generally depends on the ratio between compounds of formula I and carbonitriles of, for example, formulae AI-AIV. If the proportion of carboni¬ triles exceeds the proportion of the compounds of formula I by more than a factor of 1.5 and especially 2.5 the propor¬ tion of dielectrically neutral compounds preferably is not too small and especially more than 10 % and in particular more than 20 %. If, however, the ratio between the propor¬ tion of the carbonitriles and the compounds of formula is less than 1.5 or especially if the proportion of the com¬ pounds of formula I is equal or greater than the proportion of carbonitriles the proportion of dielectrically neutral compounds can be smaller and is preferably smaller than 25 % and especially not larger than 15 %. In case the proportion of compounds of formula I distinctively exceeds the propor¬ tion of carbonitriles the proportion of dielectrically compounds can be smaller than 10 % and preferably smaller than 5 %. In another preferred embodiment the LC mixtures also contain one or more compounds from the following group: R- __^-CH 2 CH-<^ ^ό Hal 0 >-Hal / R < £^ Hal wherein Hal is F or Cl and L and R are as defined above. The LC media according to the invention also preferably comprise an optically active component C, in an amount such that the ratio between the layer thickness (separation of the plane-parallel carrier plates) and the natural pitch of the chiral nematic liquid crystal mixture is more than 0.2 suitable to the desired twist angle. Suitable dopants can be selected from a wide variety of known chiral materials and commercially available dopants such as cholesteryl nonan- oate, S 811 (E. Merck, Darmstadt, FRG) and CB 15 (BDH, Poole, UK) . The choice thereof is not crucial per se. The liquid crystalline media according to the invention are especially suited for STN-LCDs exhibiting a high twisting angle. Preferably the mixtures also contain one or more compounds from group BI comprising the compounds of the formulae BII to B1IV: in which R and R 2 , in each case independently of one ano- ther, are as defined for R, Z is -CH 2 CH 2 -, -CO-0-, -O-CO- or a single bond, and -< >- -O-' -O « -©- and/or at least one component selected from group B2 compri¬ sing the compounds of the formulae B1V to B1VII: in which Ri is as defined for R, Z° is -CH 2 CH 2 - or a single bond, and where n is 1 to 9, X is CN or F and Y is H or F; and/or at least one component selected from group B3 comprising the compounds of the formulae BVIII and BIX: BVIII in which R and R 2 , in each case independently of one another, are as defined for R, and -©- is -O-' -O-' -d or ■ ©"- ■ The proportion of component (s) from group BI is preferably 5 % to 45 %, in particular preferably about 10 % to 40 %. Components of the formulae B1III and B1IV are preferred. Particularly preferred compounds of the formula BIIII are those of the following sub-formulae: RI O R2 in which Ri is CH 3 -(CH 2 ) n -0-, -CH 3 -(CH 2 ) t -, trans-H-(CH 2 ) r - CH=CH-(CH 2 CH 2 ) s -CH 2 0- or trans-H-(CH 2 ) r -CH=CH-(CH 2 CH 2 ) s -, R2 is CH 3 -(CH 2 ) t -, n is 1, 2, 3 or 4, r is 0, 1, 2 or 3, s is 0 or 1 , and t is 1, 2, 3 or 4. Furthermore preferred compounds are those of the sub-formula R i -(^-COO- ^-R 2 in which Ri and R 2 are as defined above. The proportion of the compounds of the formula BlIII of the abovementioned sub-formulae is preferably about 5 % to 45 %, in particular preferably about 10 % to 35 %. Particularly preferred compounds of the formula B1IV are those of the following sub-formula: R1 >- - R2 in which Ri is CH 3 -(CH 2 ) n -0- or trans-H-(CH 2 ) r -CH=CH-(CH 2 CH 2 ) s -CH 2 0- and R 2 is CH 3 -(CH 2 ) t -, where n is 1, 2, 3 or 4, r is 0, 1, 2 or 3, s is 0 or 1, and t is 1, 2, 3 or 4 . The proportion of these compounds or of the compounds of the formula B1IV is preferably about 5 % to 40 %, in particular preferably about 10 % to 35 %. The mixtures preferably contain compounds of the formula III, in particular those of the sub-formula In a particularly preferred embodiment, the mixtures simul- taneously contain compounds of the formulae BlIII and B1IV, the total proportion for components of group BI being observed. If compounds of the formulae BII and BlIII are present, Ri and R 2 are preferably, in each case independently of one another, n-alkyl having 1 to 7 C atoms or (trans)-n-alkenyl having 3 to 7 C atoms. Z 2 is preferably a single bond. BI is particularly preferred. Furthermore preferred mixtures according to the invention are those which contain one or more compounds of the formula B1IV in which is -ζ~/~ or and R i and R 2 have one of the abovementioned preferred meanings, in particular preferably n-alkyl having 1 to 7 C atoms. In all cases, the total proportion for components of group BI is observed. The proportion of the compounds of group B2 is preferably about 5 % to 45 %, in particular preferably 5 % to 20 %. The proportion (preferred ranges) for B1V to BIVII is as fol¬ lows: B1V about 5 % to 30 %, preferably about 5 % to 15 % Total of B1VI and BIVII: about 5 % to 25 %, preferably about 10 % to 20 %. Preferred compounds of group B2 are indicated below: BIVII R L-^HH^t--CCHH 22 CCHH 22 --<HH (θ0 )--FF B1VI11 Ri is preferably n-alkyl having 1 to 7 C atoms or (trans)-n- alkenyl having 3 to 7 C atoms. Z° is preferably a single bond. R preferably has the preferred meaning mentioned above for R or is fluorine. Y is preferably fluorine. The mixtures according to the invention preferably contain one or more compounds selected from the group comprising B1V3, BIVII and B1VII1 in a total proportion of about 5 to 35 %. In a particularly preferred embodiment, the mixtures accord¬ ing to the invention contain, besides B1V3, BIVII, B1VII1 and B1V2 (R = F) , further terminally fluorinated compounds, for example selected from the group comprising: N ^Y R 1 -(-(θV) y -{θ)-{θ)-F N R 1 oV O in which R is preferably n-alkyl having 1 to 7 C atoms or (trans)-n-alkenyl having 3 to 7 C atoms, X is 1 or 2, y is 0 or 1 and Y is H or F. The total proportion of all terminally fluorinated compounds is preferably about 5 % to 65 %, in particular about 15 % to 10 40 %. The proportion of the compounds from group B3 is preferably about 5 % to 30 %, in particular preferably about 10 % to 20 %. Ri is preferably n-alkyl or n-alkoxy, each having 1 to - .. 9 C atoms. However, it is also possible to employ analogous compounds containing alkenyl or alkenyloxy groups. Compounds of the formula BVIII are preferred. is preferably 1,4-phenylene. 0 The mixtures according to the invention contain compounds from at least one of groups BI, B2 and B3. They preferably contain one or more compounds from BI and one or more compounds from group B2 and/or B3. 5 The proportion of compounds of component C is preferably about 5 % to 60 %, in particular about 20 % to 50 %. Those skilled in the art can easily adjust this proportion to produce the threshold voltage desired, it being possible to 0 use, in principle, all customary liquid-crystal compounds where Δε > +2. If predominantly less-positive terminally fluorinated compounds are used, the total proportion can be above the upper range (about 35 % to 80 %) , while the proportion may be lower (about 10 % to 35 %) if terminally cyano-substituted compounds are used. In a particularly preferred embodiment, the mixtures accord¬ ing to the invention preferably contain about 5 % to 20 % of one or more compounds having a dielectric anisotropy of less than -2 (component D) . Compounds of this type are known, for example derivatives of 2,3-dicyanohydroquinone or cyclo- hexane derivatives containing the structural element -^yas in DE-OS 3,231,707 or DE-OS 3,407,013. However, compounds containing the structural element 2,3- difluoro-l,4-phenylene are preferably chosen, for example compounds as in DE-OS 3,807,801, 3,807,861, 3,807,863, 3,807,864 or 3,807,908. Particularly preferred are tolans containing these structural elements, as in International Patent Application PCE/DE 88/00133, in particular those of the formulae F F F F Ri*- H zo--(0)-C^C- θ -OR in which R 1 and R 2 , in each case independently of one another, are preferably n-alkyl having 1 to 7 C atoms or n-alkenyl having 3 to 7 C atoms, and Z° is -CH 2 CH 2 - or a single bond. Component D causes, in particular, an improvement in the steepness of the characteristic line. In a particularly preferred embodiment, the mixtures contain about 5 % to 35 %, in particular preferably about 10 % to 20 %, of liquid-crystalline tolan compounds. These make it possible to work with smaller cell thicknesses (about 5-6 μm) , which significantly shorten the switching times. Particularly preferred tolans are indicated below: R-(θ)-CsC-Q R I - H^-Z°- O -Q R 1 is preferably n-alkyl having 1 to 7 C atoms, Z° is -CH 2 CH 2 - or a single bond, Q is -(θ)-R2, -(θ)-F or -ζo -F, where R 2 is n-alkyl or n-alkoxy, each having 1 to 7 C atoms, or n-alkenyl or n-alkenyloxy, each having 3 to 7 C atoms. In further particularly preferred embodiments, the mixtures contain - a component D which contains one or more compounds having a l-cyano-trans-l,4-cyclohexylene group or a 2,3-difluoro-1,4-phenylene group, - at least two compounds of the formulae AllI or AV, - compounds of the formulae AllI and AV, - at least one compound from the following group: X Alkyl- ])-(0)-CH 2 CH 2 -(0)-F X Alkyl-T V(0)- in which alkyl is a straight-chain alkyl group having 2-7 C atoms, and X is H or F, - one or more compounds in which R is a trans-alkenyl group or a trans-alkenyloxy group. one or more compounds selected from the following group RI S °XI>- R2 in which R i and R 2 have the preferred meanings indica¬ ted in the case of component B, and one of the two 1,4-phenylene groups may also be substituted by fluorine; the proportion of these compounds is 0 % to 25 %, preferably about 5 % to 15 %. The liquid-crystal mixtures which can be used according to the invention are prepared in a manner customary per se. In general, the desired amount of the components used in a relatively small amount is dissolved in the components making up the principal constituent, expediently at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, after mixing, for example by distillation. The dielectrics may also contain further additives known to those skilled in the art and described in the literature. For example, 0-15 % of pleochroic dyes may be added. The mixtures of this invention are very stable and have a high resistivity. They allow the realization of STN-LCDs with a steep characteristic electrooptical line, a high multiplex ratio, a broad temperature range of operation, a low threshold voltage and a short switching time. The mixtures of this invention thus are of considerable economic importance. The examples below are intended to illustrate the invention without representing a limitation. Above and below all temperatures are given in °C. The percentages are percent by weight. The STN displays given in the examples below exhibit advantageous properties. The liquid-crystal compounds which are comprised by the liquid crystalline media are indicated to acronyms, with the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals C n H 2n+1 are straight-chain alkyl radicals containing n or m carbon atoms. The coding in Table B is self-evident. In Table A, only the acronym for the base structure is given. In individual cases, the acronym for the base structure is followed, separated by a hyphen, by a code for the substituents Ri, R2, L , L 2 and L 3 ; R2 L L 2 L3 TgfcteA: PYP PYRP CBC CCH CCP CP CPTP CEPTP ECCP CECP EPCH HP Li L2 Li L2 PDX PTP L -(H)-C 2 H 4 BCH ME ET BEP FET PCH CEDP-nX CEDP-nX.F Table B: T15 K3n M3n G3n BCH-nXF C n H 2n+1 - C 2 H 4 - 0)- 0)-C m H 2m+1 C n H 2n+1 -(H)-<H}-00C-C m H 2m+1 Inm C-nm CH 3 CH 3 C 2 H 5 -CH-CH 2 -0- θ}-^£ -CN C 2 H 5 -CH-CH 2 - 0 -(θ}-CN C15 CB15 C n H 2n+1 -(H)- -C I -H 2m+1 CBC-ntnF CCN-n C n H 2n+ ι H)-^^COO-<2-{H)-C m H 2m+1 CCPC-nm C n H 2n+1 -(H)-(H)-COO-(H)-C m H 2ιn+1 C n H 2n -(H)-(θ)-OOC-^H)-C 1B H 2n+1 CH-nm HD-nm HH-nm NCB-nm C 2 H 5 -(H)-COO-(O)-(O)-CN OS-nm CHE C n H 2n+ i-<^C 2 H 4 -{0 -0)-(H^C m H 2m+1 ECBC-nm ECCH-nm CCH-nlEM F T-nFn CP-nOCF 3 C n H 2n+1 - H)-(θ)-COO--(0)-OCF 3 C n H 2n+1 -{H)-COO- θ> -OCF 3 HP-nOCF 3 D-nOCF 3 Example 1 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F.F 12.0 PDX-5F.F 10.0 PDX-7F.F 10.0 CCP-2OCF 3 10.0 CCP-3OCF 3 12.0 CCP-4OCF 3 10.0 CCP-5OCF 3 12.0 BCH-3F.F 12.0 BCH-5F.F 12.0 and exhibits the following physical parameter Δn = 0.0808 (589 nm, 20 °C) is characterized by advantageous properties. Example 2 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F 12.0 PDX-5F 10.0 PDX-7F 10.0 CCP-2OCF 3 10.0 CCP-3OCF 3 12.0 CCP-4OCF 3 10.0 CCP-5OCF 3 12.0 BCH-3F .F 12 . 0 BCH-5F .F 12 .0 and exhibits the following physical parameter Δn = 0.08.66 (589 nm, 20 °C) is characterized by advantageous properties. Example 3 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F.F 16.0 PDX-5F 12.0 CCP-20CF 3 8.0 CCP-30CF 3 13.0 CCP-4OCF 3 5.0 CCP-50CF 3 12.0 BCH-3F.F 12.0 BCH-5F.F 14.0 ECCP-2F.F 8.0 and exhibits the following physical parameter Δn = 0.0874 (589 nm, 20 °C) is characterized by advantageous properties. Example 4 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3C1.F 16.0 PCH-5F 12.0 PCH-6F 0.0 PCH-7F 0.0 CCP-2OCF 3 8.0 CCP-30CF 3 13.0 CCP-4OCF 3 5.0 CCP-5OCF 3 12.0 BCH-3F.F 12.0 BCH-5F.F 14.0 ECCP-2F.F 8.0 and exhibits the following physical parameter Δn = 0.0934 (589 nm, 20 °C) N 66.6 I is characterized by advantageous properties. Example 5 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F 15.5 PCH-5F 14.6 CCP-2OCF 3 7.8 CCP-3OCF 3 12.6 CCP-4OCF 3 4.9 CCP-5OCF 3 11.6 BCH-3F.F 11.6 BCH-5F.F 13.6 ECCP-2F.F 7.8 and exhibits the following physical parameter Δn = 0.0868 (589 nm, 20 °C) N 65.5 I is characterized by advantageous properties. Example 6 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3CF 3 8.0 PDX-5CF 3 6.0 PDX-3CF 3 0.0 PCH-5F 12.0 PCH-6F 8.0 PCH-7F 0.0 CCP-2OCF 3 8.0 CCP-30CF 3 13.0 CCP-4OCF 3 5.0 CCP-50CF 3 12.0 BCH-3F.F 14.0 BCH-5F.F 14.0 is characterized by advantageous properties. Example 7 A STN display containing a liquid crystal medium which comprises the following compounds PCH-5F 8.0 PCH-6F 0.0 PCH-7F 0.0 PDX-7F 8.0 PDX-3F 10.0 CCP-2OCF 3 10.0 CCP-3OCF 3 14.0 CCP-4OCF 3 10.0 CCP-5OCF 3 12.0 BCH-3F.F 8.0 BCH-5F.F 8.0 ECCP-3F.F 12.0 and exhibits the following physical parameter Δn = 0.0857 (589 nm, 20 °C) N 75 I is characterized by advantageous properties. Example 8 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F 10.0 PDX-5F 8.0 PDX-7F 6.0 CCP-2OCF 3 8.0 CCP-3OCF 3 10.0 CCP-4OCF 3 7.0 CCP-5OCF 3 10.0 BCH-3F.F.F 12.0 BCH-5F.F.F 10.0 ECCP-3OCF 3 5.0 ECCP-5OCF 3 5.0 CBC-33F 3.0 CBC-53F 3.0 CBC-55F 3.0 is characterized by advantageous properties. Example 9 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F 10.0 PDX-5F 8.0 PDX-7F 6.0 CCP-3OCF 3 11.0 CCP-4OCF 3 6.0 CCP-5OCF 3 10.0 CCP-3F.F.F 10.0 BCH-3F.F 12.0 BCH-5F.F 10.0 ECCP-30CF 3 5.0 ECCP-5OCF 3 5.0 CBC-33F 3.0 CBC-53F 2.0 CBC-55F 2.0 is characterized by advantageous properties. Example 10 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F.F 10.0 PDX-5F 10.0 PDX-7F 10.0 CCP-30CF 3 13.0 CCP-5OCF 3 12.0 ECCP-3OCF 3 10.0 ECCP-3F.F 13.0 ECCP-5F.F 10.0 CBC-33 4.0 CBC-53F 4.0 CBC-55F .0 and exhibits the following physical parameters N 91.3 I η = 16.8 mm 2 s ~ ι (20 °C) Δn = 0.0840 (20 °C, 589 nm) Δε = 5.6 (20 °C, 1 kHz) is characterized by advantageous properties. Example 11 A STN display containing a liquid crystal medium which comprises the following compounds PCH-32 17.6 PCH-301 16.0 PCH-302 12.0 BCH-32 15.2 BCH-52 11.2 CBC-33 4.0 CBC-53 .0 PDX-3CF 3 10.0 PDX-5CF 3 10.0 is characterized by advantageous properties. Example 12 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F.F 16.3 PDX-5F 10.2 CCP-2OCF 3 8.2 CCP-3OCF 3 13.3 CCP-4OCF 3 5.1 CCP-5OCF 3 12.2 BCH-3F.F 12.2 BCH-5F.F 14.3 ECCP-2F.F 8.2 and exhibits the following physical parameter Δn = 0.0886 (20 °C, 589 nm) is characterized by advantageous properties. Example 13 A STN display containing a liquid crystal medium which comprises the following compounds PCH-32 17.6 PCH-301 16.0 PCH-302 12.0 BCH-32 15.2 BCH-52 11.2 CBC-33 4.0 CBC-53 4.0 PDX-30CF 3 10.0 PDX-5OCF 3 10.0 is characterized by advantageous properties. Example 14 A STN display containing a liquid crystal medium which comprises the following compounds IS-3968 12.0 PDX-7F 10.0 PCH-7F 10.0 CCP-3OCF 3 13.0 CCP-5OCF 3 12 . 0 ECCP-3OCF 3 20 .0 ECCP-3F .F 13. 0 CBC-33F 3.0 CBC-53F 4 . 0 CBC-55F 3.0 and exhibits the following physical parameter Δn = 0 . 0844 (20 °C, 589 nm) is characterized by advantageous properties . Example 15 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F 12.0 IS-3968 10.0 PDX-7F 10.0 CCP-3OCF 3 13.0 CCP-5OCF 3 12.0 ECCP-3OCF 3 20.0 ECCP-3F.F 13.0 CBC-33F 3.0 CBC-53F 4.0 CBC-55F 3.0 and exhibits the following physical parameters N 89 I Δn = 0.0835 (20 °C, 589 nm) is characterized by advantageous properties. Example 16 A STN display containing a liquid crystal medium which comprises the following compounds IS-3968 12.0 PDX-7F 10.0 PCH-7F 4.0 CCP-30CF 3 11.0 CCP-5OCF 3 11.0 ECCP-3OCF 3 11.0 ECCP-3F.F 11.0 BCH-3F.F 11.0 BCH-5F.F 10.0 CBC-33F 3.0 CBC-53F 3.0 CBC-55F 3.0 and exhibits the following physical parameters N 93 I Δn = 0.0992 (20 °C, 589 nm) is characterized by advantageous properties. Example 17 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F 12.0 IS-3968 10.0 PDX-7F 8.0 CCP-3OCF 3 13.0 CCP-5OCF 3 12.0 ECCP-3OCF 3 10.0 ECCP-3F.F 13.0 BCH-3F.F 11.0 CBC-33F 4.0 CBC-53F .0 CBC-55F 3.0 and exhibits the following physical parameters N 86 I Δn = 0.0924 is characterized by advantageous properties. Example 18 A STN display containing a liquid crystal medium which comprises the following compounds IS-3968 12.0 PDX-7F 10.0 PCH-7F 5.0 CCP-3OCF 3 11.0 CCP-5OCF 3 11.0 ECCP-3OCF 3 11.0 ECCP-3F.F 12.0 BCH-3F.F 10.0 BCH-5F.F 9.0 CBC-33F 3.0 CBC-53F 3.0 CBC-55F 3.0 and exhibits the following physical parameters N 91 I Δn = 0.0964 (20 °C, 589 nm) is characterized by advantageous properties. Example 19 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3F 12.0 IS-3968 10.0 PDX-7F 7.0 CCP-30CF 3 13.0 CCP-50CF 3 12.0 ECCP-30CF 3 10.0 ECCP-3F.F 13.0 BCH-3F.F 11.0 CBC-33F 4.0 CBC-53F 4.0 CBC-55F 4.0 and exhibits the following physical parameters N 90 I Δn = 0.0935 (20 °C, 589 nm) is characterized by advantageous properties. Example 20 A STN display containing a liquid crystal medium which comprises the following compounds ECCP-3C1 12.00 ECCP-5C1 17.00 ECCP-3C1.F 6.00 ECCP-5C1.F 13.00 G9 14.00 CCPC-33 6.00 PTP-102 7.00 PTP-35 10.00 PDX-3C1 10.00 PDX-2C1 5.00 and exhibits the following physical parameters N 94 I Δn = 0.1393 (20 °C, 589 nm) is characterized by advantageous properties. Example 21 A STN display containing a liquid crystal medium which comprises the following compounds ECCP-3C1 8.32 ECCP-5C1 12.5 ECCP-3C1.F 4.25 ECCP-5C1.F 9.80 K6 6.9 G9 11.82 PCH-3 6.82 PCH-5F 8.34 PCH-7F 4.17 PTP-102 4.19 PTP-35 5.95 CCPC-33 4.21 CCPC-35 2.92 PDX-3C1 9.81 and exhibits the following physical parameters N 75 I Δn = 0.1250 (20 °C, 589 nm) is characterized by advantageous properties. Example 22 A STN display containing a liquid crystal medium which comprises the following compounds ECCP-3C1 8.49 ECCP-5C1 12.69 ECCP-5C1.F 8.46 K6 10.19 G9 11.87 PCH-3 6.96 PCH-5F 8.52 PCH-7F 4.24 PTP-102 6.18 CCPC-33 6.46 CCPC-35 5.98 PDX-3C1 9.96 and exhibits the following physical parameters N 82 I Δn -= 0.1292 (20 °C, 589 nm) is characterized by advantageous properties. Example 23 A STN display containing a liquid crystal medium which comprises the following compounds ECCP-3C1 8.44 ECCP-5C1 12.71 ECCP-5C1.F 8.49 K6 10.08 G9 11.90 PCH-3 6.77 PCH-5F 5.64 PCH-7F 4.24 PTP-102 6.15 CCPC-33 6.48 CCPC-35 5.98 PDX-3C1 10.00 ME2N.F 3.12 and exhibits the following physical parameters N 83 I Δn = 0.1331 (20 °C, 589 nm) η = 19.5 mm 2 s-ι (20 °C) is characterized by advantageous properties. Example 24 A STN display containing a liquid crystal medium which comprises the following compounds ECCP-3C1 15.73 ECCP-5C1 20.23 ECCP-5C1F 17.16 PDX-3C1 11.15 PDX-5C1 12.48 EPCH-3C1 16.49 CCH303 6.76 and exhibits the following physical parameters N 76.7 I Δn = 0.0967 (20 °C, 589 nm) η = 16.02 mm 2 s-ι (20 °C) is characterized by advantageous properties. Example 25 A STN display containing a liquid crystal medium which comprises the following compounds ME3N.F 9.00 D-33 6.24 D-35 7.62 D-37 5.94 CHE 15.30 CP-55F 9.00 PCH-3 8.10 K6 3.60 PDX-3 12.60 PDX-5 12.60 PDX-3C1 10.00 and exhibits the following physical parameter Δn = 0.1246 (20 °C, 589 nm) is characterized by advantageous properties. Example 26 A STN display containing a liquid crystal medium which comprises the following compounds ME3N.F 8.50 D-33 5.89 D-35 7.20 D-37 5.61 CHE 14.44 CP-55F 8.50 PCH-3 7.65 K6 3.40 PDX-3 11.89 PDX-5 11.89 PDX-3C1 10.02 CCPC-33 5.01 and exhibits the following physical parameters N 71 I Δn = 0.1257 (20 °C, 589 nm) is characterized by advantageous properties. Example 27 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3C1 9.97 ME3N.F 4.96 D-33 13.17 D-35 16.09 D-37 12.54 CHE 10.18 CP-55F 9.96 PCH-3 10.00 K6 5.13 PDX-3 8.00 and exhibits the following physical parameters η = 27 mm 2 s- (20 °C) Δn = 0.1063 (20 °C, 588 nm) s characterized by advantageous properties. Example 28 A STN display containing a liquid crystal medium which comprises the following compounds 132 11.33 135 11.34 152 11.33 K6 6.80 K9 5.95 K15 8.5 PCH-3 21.25 CBC-33 4.25 CBC-53 4.25 PDX-3C1 10.00 CCPC-33 5.00 and exhibits the following physical parameters N 85.3 I η = 24 mm 2 s- (20 °C) Δn = 0.1554 (20 °C, 589 nm) is characterized by advantageous properties. Example 29 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3C1 10 D-33 14.17 D-35 17.33 D-37 13.5 K6 15 K12 15 CHE 10 CBC-53 5 and exhibits the following physical parameters Δn = 0.1314 (20 °C, 589 nm) η = 28 mm 2 s-ι (20 °C) is characterized by advantageous properties. Example 30 A STN display containing a liquid crystal medium which comprises the following compounds PDX-3C1 10 ME15 14.48 ME35 17 E55 8.52 K6 15 K12 15 CHE 10 CBC-53 5 CBC-55F 5 and exhibits the following physical parameters Δn = 0.1566 (20 °C, 589 nm) is characterized by advantageous properties. Example 31 A STN display containing a liquid crystal medium which comprises the following compounds PCH-5F 8.0 PDX-7F 8.0 PDX-3F 10.0 CCP-2OCF 3 10.0 CCP-3OCF 3 14.0 CCP-4OCF 3 10.0 CCP-5OCF 3 12.0 BCH-3F .F 8 . 0 BCH-5F .F 8. 0 ECCP-3F .F 12 . 0 and exhibits the following physical parameters N 75 I Δn = 0.086 (20 °C, 589 nm) is characterized by advantageous properties. Example 32 A STN display containing a liquid crystal medium which comprises the following compounds ECCP-32 12.000 ECCP-35 12.000 PCH-3 12.000 G9 10.000 PDX-3C1 8.000 FET-5C1 10.000 BECH-3C1.F 10.000 CCPC-33 7.000 K6 10.000 PYP-3N.F 4.000 PYP-5N.F 5.000 is characterized by advantageous properties. Example 33 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F 10.0 PDX-7F 8.0 PDX-3F 6.0 CCP-2OCF 3 8.0 CCP-3OCF 3 12.0 CCP-4OCF 3 7.0 CCP-5OCF 3 11.0 BCH-3F.F 12.0 BCH-5F.F 10.0 ECCP-3OCF 3 5.0 ECCP-5OCF 3 5.0 CBC-33F 2.0 CBC-53F 2.0 CBC-55F 2.0 is characterized by advantageous properties. Example 34 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F.F 24.0 PDX-7F.F 23.0 PDX-3F.F 23.0 CBC-33 5.0 CBC-53 5.0 CBC-55 5.0 CBC-33F 5.0 CBC-53F 5.0 CBC-55F 5.0 is characterized by advantageous properties. Example 35 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F 24.0 PDX-7F 23.0 PDX-3F 23.0 CBC-33 5.0 CBC-53 5.0 CBC-55 5.0 CBC-33F 5.0 CBC-53F 5.0 CBC-55F 5.0 is characterized by advantageous properties. Example 36 A STN display containing a liquid crystal medium which comprises the following compounds PDX-5F.F 10.0 PDX-7F.F 8.0 PDX-3F.F 6.0 CCP-2OCF 3 8.0 CCP-3OCF 3 12.0 CCP-4OCF 3 7.0 CCP-5OCF 3 11.0 BCH-3F.F 12.0 BCH-5F.F 10.0 ECCP-3OCF 3 5.0 ECCP-5OCF 3 5.0 CBC-33F 2.0 CBC-53F 2.0 CBC-55F 2.0 is characterized by advantageous properties.

Claims: Patent Claims 1. Supertwist liquid crystal display having - two plane-parallel carrier plates which, with an edging, form a cell, - a nematic liquid crystal mixture of positive dielectric anisotropy in the cell, - electrode layers with superimposed orientation layers on the inside of the carrier plates, - an angle of incidence between the longitudinal axis of the molecules on the surface of the carrier plates and the carrier plates of about 1 degree to 30 degrees and - a twisting angle of the liquid crystal mixture in the cell from orientation layer to orientation layer, according to the amount, of between 160 and 720 °C, characterized in that the nematic liquid crystal mix¬ ture contains one or more compounds of the formula I wherein R 1 is alkyl having 1-12 C atoms, wherein one or two non-adjacent CH 2 groups can also be replaced by -0-, -CH=CH-, -CO-, -O-CO- or -CO-0-, r is 0 or 1, ring A is trans-1,4-cyclohexylene or 1,4-phenylene, X is F, Cl, CF 3 , OCF 3 or OCHF 2 and Y and Z each independently of one another denotes H or F. 2. Display according to claim 1, characterized in that the nematic liquid crystal mixture further comprises a) 5-60 % by weight of a liquid crystalline component A consisting of one or more compounds having a dielectric anisotropy of more than +8, b) 10-40 % by weight of a liquid crystalline component B consisting of one or more compounds having a clearing point of more than 120 °C, and c) an optically active component C, in an amount such that the ratio between the layer thickness (separation of the plane-parallel carrier plates) and the natural pitch of the chiral nematic liquid crystal mixture is more than 0.2 suitable to the desired twist angle, and in that the nematic liquid crystal mixture has a nematic phase range of at least 60 °C, a viscosity of not more than 30 mPa.s and a dielectric anisotropy of at least +5, the dielectric anisotropies of the compounds and the parameters relating to the nematic liquid crystal mixture being based on a temperature of 20 °C. 3. Display according to claim 2, characterized in that compo¬ nent A comprises compounds selected from the following group: R-^A -ZI-Q-CN Al _N RH 0>-Q-CN AllI wherein R is alkyl having 1-12 C atoms, wherein one or two non- adjacent CH 2 groups can also be replaced by -0-, -CH=CH-, -CO-, -O-CO- or -CO-0-, Q is the formula Zi is - " ~VcH 2 CH 2 -, a single bond, -CH 2 CH 2 -, -CO-O- or -O-CO- and 4. Display according to one of claims 1 to 3, characterized in that component B comprises compounds selected from the following group: R-- H) ^O)-Q-^H)^R 2 BI R BIH wherein R 1 and R 2 each independently of one another are R as defined in claim 3, Z° is -CO-0-, -O-CO- or -CH 2 CH 2 -, Q is as defined in claim 3 and the 1,4-phenylene ring in BI to BVII may also be substituted in 2- or 3-position by fluorine. 5. Display according to one of claims 1 to 4, characterized in that the liquid crystal mixture contains at least one compound from the following group: R - C °y- Hal wherein Hal is F or Cl and L and R are as defined above. 6. Display according to at least one of claims 1 to 5, cha¬ racterized in that the liquid crystal mixture contains one or more compounds wherein R is a trans-alkenyl group or a trans-alkenyloxy group. 7. Liquid crystal mixture of the composition defined in at least one of Claims 1 to 6.

Inventors: HITTICH REINHARD, REIFFENRATH VOLKER, WEBER GEORG, HITTICH, REINHARD, REIFFENRATH, VOLKER, WEBER, GEORG
Applicants: MERCK PATENT GMBH, MERCK PATENT GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG
Application year: 1991
Main classification: C09K19
IPC codes: C09K19