GB2358870A - Liquid crystalline media having 4,4'-dialk-(1E or 3E)-enyl-1,1'-bicyclohexane & 3,4,5-trifluorophenyl-terminated components, and use in IPS displays - Google Patents

Abstract

A liquid-crystalline medium, of positive dielectric anisotropy, where the medium comprises at least one mesogenic compound of formula I and at least one bisalkenyl compound of formula I* is disclosed. There is also disclosed an electro-optical liquid-crystal display, having a realignment layer for realigning the liquid crystals whose field has a significant component parallel to the liquid-crystal layer, which comprises such a liquid-crystalline medium. The threshold voltages and the response times of in-phase switching (IPS) displays can be improved with the aid of such a medium. These media are distinguished by their relatively high clearing points and low rotational viscosity values and their increased stability to crystallization at low temperatures. [in which<BR> ```R<SP>1</SP> (terminus), A<SP>1</SP>/A<SP>2</SP> (ring) and Z<SP>1</SP>/Z<SP>2</SP> (link) are as defined in claim 1 and m is 0, 1 or 2] [in which<BR> ```R<SP>1*</SP> and R<SP>2*</SP> are each, independently of one another, 1E-alkenyl or 3E-alkenyl having C<SB>2-7</SB> or C<SB>4-7</SB> carbon atoms, respectively.]

Description

Liquid crystalline media having 4.4'-dialk-(1 E or 3E)-enyl-1,1'- bicyclohexane and 3,4,5-trifluorophenyl-terminated components, and use in IPS displays. The invention relates to an electro-optical liquid-crystal display having a realignment layer for realigning the liquid crystals whose field has a sig nificant component parallel to the liquid-crystal layer and which comprises a liquid- crystalline medium of positive dielectric anisotropy, where the medium com prises at least one mesogenic compound containing a 3,4,5-trifluorophenyl group and at least one bisalkenyl compound of the formula I*.

In conventional liquid-crystal displays (TN, STN, OMI or AMD-TN), the electric fields for realign ment are generated essentially perpendicular to the liquid-crystal layer.

International Patent Application WO 91/10936 discloses a liquid-crystal display in which the elec tric signals are generated in such a way that the elec tric fields have a significant component parallel to the liquid-crystal layer (IPS, in-plane switching). The principles of operating such a display are described, for example, by R.A. Soref in Journal of Applied Phys ics, vol. 45, No. 12, pp. 5466-5468 (1974).

EP 0 588 568 discloses various ways of address ing a display of this type.

These IPS displays can be operated with liquid crystalline materials either of positive or of negative dielectric anisotropy #AE # 0). However, using the ma terials known hitherto, relatively high threshold volt- ages and long response times are reached in IPS dis plays. In addition, the problem of crystallization of the liquid-crystal medium at low temperatures can occur in IPS displays containing materials known hitherto. The object was therefore to indicate liquid- crystalline materials which are suitable for achieving relatively low threshold voltages and short response times in IPS displays and which, in particular, have an improved low-temperature shelf life. Surprisingly, this object has been achieved by the use of liquid- crystalline materials comprising at least one compound of formula I containing a 3,4,5- trifluoropher_yl group and at least one compound of the formula I*.

Compounds of the formula I are disclosed, for example, in EP 0 387 032 and EP 0 441 932. Compounds of the formula I* are covered by the broad generic formula in EP 0 168 683.

However, there is no indication in the docu ments mentioned above that the threshold voltages and the response times of IPS displays can be improved with the aid of these substances. The novel IPS mixtures are distinguished by their relatively high clearing points. and low rotational viscosity values and their increased stability to crystallization at low temperatures.

The invention therefore relates to an electro- optical liquid-crystal display having a realignment layer for realigning the liquid crystals whose field has a significant component parallel to the liquid- crystal layer and which contains a liquid- crystalline medium of positive dielectric anisotropy, where the me dium comprises at least one compound of the formula I

in which R1 is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, mono- substituted by CN or CF, or at least monosub- stituted by halogen, where one or more CH2 groups in these radicals may also, in each case .independently of one another, be re placed by -0-, -S-, -CO-, -C0-0-, -0-C0- or -0-C0-0- in such a way that 0 atoms are not linked directly to one another, A1 and A are each, independently of one another, a (a) trans -1,4-cyclohexylene radical or 1,4-cyclohexenylene radical, in which, in addition, one or more non-adjacent CH, groups may be replaced by -0- and/or (b) 1,4-phenylene radical, in which, in ad dition, one or two CH groups may be re placed by N, (c) radical from the group consisting of 1,4-dicyclo(2,2,2)octylene, piperidine- 1,4-diyl, naphthalene -2,6-diyl, deca- hydronaphthalene-2,6-diyl and 1,2,3,4- tetrahydronaphthalene-2,6-diyl, where the radicals (a) and (b) may be substituted by one or two fluorine atoms, Z1 and Z are each, independently of one another, -C0-0-, -0-C0-, -CH2O-, -OCH2-, -CH2CH2-, -CH=CH-, -C#C-, or a single bond, or one of the radicals Z1 and Z is - (CH2)4- or - CH=CH- CH2CH2 - , and m is 0, 1 or 2, and at least one compound of the formula I*

in which Rl' and R' are each, independently of one another, 1E-alkenyl or 3E-alkenyl having 2-7 car bon atoms.

Preferred embodiments are IPS displays where the medium additionally comprises at least one compound of the formula II

in which L1 and L are each, independently of one another, H or F, R is as defined for R, A and A4 are each, independently, as defined for A1 and A2, Z and Z4 are each, independently of one another, as defined for Z and Z2, and n is 0, 1 or 2, the medium additionally comprises at least one compound of the formula III R - (As-Zs)0-A6-R4 III in which R and R4 are each, independently of one another, as defined for R, AS and A6 are each, independently, as defined for Al and A, Z5 is in each case, independently of the others, as def ined for Z and Z , and <B>0</B> is 1, 2 or 3; the medium comprises at least one compound of the formula IV

in which RS is as defined for R, A7 and A8 are each, independently of one another, as def fined for Al and A, Z7 and Z8 are each, independently of one another, as def fined for Z and Z, L and L are each, independently of one another, H or F, Q is a polyfluoroalkylene radical of the formula - (0) q- (CH2,) (C F2) in which q is 0 or 1 r is 0 or an integer between 1 and 6 and s is an integer between 1 and 6, X is H, F or C1, and p is 0, 1 or 2. Preference is furthermore given to an IPS dis play in which the pixels are addressed by means of an active matrix.

The invention furthermore relates to a liquid- crystalline medium of positive dielectric anisotropy which comprises at least one compound of the formula I and at least one compound of the formula I*, and preferably which comprises - from 10 to 60 # by weight, preferably from 30 to 600% by weight, of at least one com pound of the formula I, - from 5 to 40% by weight, preferably from 10 to 25% by weight, of at least one com pound of the formula I*, - from 0 to 40% by weight, preferably from #.0 to 25% by weight, of at least one compound of the f ormula II, - from 0 to 30% by weight, preferably from 0 to 15% by weight, of at least one compound of the formula III, and - from 5 to 50% by weight, preferably from 10 to 30% by weight, of a compound of the formula IV.

The novel licruid-crystalline medium preferably comprises: at least one compound selected from the formulae Ia and/or lb

- at least one compound selected from the formulae I*a to I*c

where R * and R4* are each, independently of one another, H, CH3 , C2H5 or n-C3H7.

Preferred compounds are compounds of the for mula I*a and compounds in which R * and R4* are H, CH, or C2H5.

Preference is furthermore given to compounds of the formulae I*a and I*c in which R * and R4* are iden tical.

- at least one compound selected from the formulae Ila to Ilc and I1'

- in a particularly preferred form, at least one compound of the formula IIal

in which z is 1-12; - at least one compound selected from the formulae IIIa, IIIb and IIIc,

and - if desired, at least one compound selected from the formulae IVa and IVb,

and R, R2, R , R4, R5, L and L are each as defined above.

In a particularly preferred embodiment, the novel media comprise at least one compound of the formula Ia in which

and at least one compound of the formula I*a and/or I*c.

In a further preferred embodiment, the novel media comprise at least one compound of the formula Ia and at least one compound of the formula Ib and a compound of the formula I*, preferably I*a, I*b or I*c.

The novel liquid- crystalline media generally have a birefringence #An) < 0.12, preferably between 0.05 and 0.11, in particular between 0.07 and 0.10.

The viscosity (at 20 C) of the novel mixtures is generally less than 30 mm - s, in particular between 15 and 25 mm@ - s''. The specific resistivity of the novel materials at 20 C is generally between 5 x 101 and 5 x 10 #2 # cm, particularly preferably between 5 x 10l1 and 5 x 10 S# # cm. The rotational viscosity of the novel mixtures at 20 C is generally less than 150 mPa-s, in particular between 50 and 100 mPa.s.

It has been found that even a relatively small proportion of compounds of the formulae I* and I in a mixture with conventional liquid-crystal materials, but in particular with one or more compounds of the formula II, III and/or IV, results in a significant lowering of the threshold voltage, in favourable levels of rota tional viscosity y, and in fast response times, with at the same time broad nematic phases having low smectic- nematic transition temperatures being observed. The compounds of the formulae I*, I to IV are colourless, stable and readily miscible with one another and with other liquid-crystal materials.

The term "alkyl" covers straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Group [sic] having 2-5 carbon atoms are generally preferred.

The term "alkenyl" covers straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. In particular, alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C,-5-alkenyl and C7-6-alkenyl, in particular C2-C7-lE-alkenyl, C4-C7-3E-alkenyl and C5-C7-4- alkenyl. Examples of preferred alkenyl groups are vinyl, 1E-propenyl, IE-butenyl, lE-pentenyl, lE-hexenyl, 1E-heptenyl, 3E-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.

The term "fluoroalkyl" # preferably covers straight-chain groups having a terminal fluorine atom, i.e. fluoromethyl, 2-f luoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded. The term "oxaalkyl" preferably covers straight- chain radicals of the formula #"Hz",l-O- (CH2)m, in which n and m are each, independently of one another, from 1 to 6. Preferably, n = 1 and m is from 1 to 6.

Through a suitable choice of the meanings of Rl and R5, the response times, the threshold voltage, the steepness of the transmission characteristic lines, etc.. can be modified as desired. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally result in shorter response times, improved nematic tendencies and a higher ratio between the elastic constants k33 (bend) and k#l (splay) compared with alkyl and alkoxy radicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generally result in lower threshold voltages and smaller values of k#,/k,, compared with alkyl and alkoxy radicals.

A CH2CH2- group in Zl, Z2, Z3, Z4, ZS, Z7 and/or Z8 generally results in higher values of k33/kll compared with a single covalent bond. Higher values of k33/k11 facilitate, for example, flatter transmission charac teristic lines in TN cells having a 90 twist (for achieving grey shades) and steeper transmission characteristic lines in STN, SBE and OMI cells (greater multiplexibility), and vice versa.

The optimum mixing ratio between the compounds of the formulae I*, I and II + III + IV depends substantially on the desired properties, on the choice of the components of the formulae I*, I, II, III and/or IV and on the choice of any other components present. Suitable mixing ratios within the abovementioned range can easily be determined from case to case.

The total amount of compounds of the formulae I*, I to IV in the novel mixtures is not crucial. The mixtures preferably comprise 50-90 % by weight of compounds of the formulae I, I* and IV. The mixtures may also contain one or more further components in order to optimize various properties. However, the observed effect on the response times and the threshold voltage is generally higher the greater the total concentration of compounds of the formulae I*, I and IV. In a particularly preferred embodiment, the novel media comprise compounds of the formula IV in which Q-X is OCF, or OCHF2. A favourable synergistic effect with the compounds of the formula I*, I and II results in particularly advantageous properties.

The novel liquid- crystalline media preferably comprise 2 to 40, in particular 4 to 30, components as further constituents besides one or more compounds of the formulae I*, I, II, III and IV. These media very particularly preferably comprise 7 to 25 components besides one or more compounds according to the invention. These further constituents are preferably selected from nematic or nematogenic (monotropic or isotropic) sub stances, in particular substances from the classes of the azoxybenzenes, benzylideneanilines, biphenyls, ter- phenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl esters of cyclohexanecarboxylic acid, phenyl or cyclohexyl esters of cyclohexylbenzoic acid, phenyl or cyclohexyl esters of cyclohexylcyclohexanecarboxylic acid, cyclohexylphenyl esters of benzoic acid, of cyclo- hexanecarboxylic acid and of cyclohexylcyclohexane- carboxylic acid, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, cyclohexylcyclohexylcyclohexenes, 1,4-bis-cyclo- hexylbenzenes, 4,41-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclo-hexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-1,3- dithianes, 1,2-diphenylethanes, 1,2-di-cyclohexylethanes, 1-phenyl-2-cyclohexylethanes, 1-cyclohexyl-2-(4-phenyl- cyc1ohexyl)ethane s, 1-cyclohexyl-2-biphenylylethanes, 1- phenyl-2-cyc1ohexylphenylethane s optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids. The 1,4-phenylene groups in these compounds may also be fluorinated. The most important compounds suitable as further constituents of novel media can be characterized by the formulae 1. 2, 3, 4 and 5: R-L-E-R 1 R-L-COO-E-R 2 R-L-OOC-E-R 3 R-L-CH2CH2-E-R 4 R-L-C=-C-E-R 5 In the formulae 1, 2, 3, 4 and 5, L and E, which may be identical or different, are in each case, independently of one another, a bivalent radical from the group formed by -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images, where Phe is unsubstituted or fluorine- substituted 1,4-phenylene, Cyc is trans -1,4-cyclohexylene or 1,4-cyclohexylene. [sic], Pyr is pyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl and G is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl.

One of the radicals L and E is preferably Cyc, Phe or Pyr. E is preferably Cyc, Phe or Phe-Cyc. The novel media preferably comprise one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which L and E are selected from the group consisting of Cyc, Phe and Pyr and simultaneously one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which one of the radicals L and E is selected from the group consisting of Cyc, Phe and Pyr and the other radical is selected from the group consisting of -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and optionally one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which the radicals L and E are selected from the group consisting of -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G- Cyc-.

In a smaller subgroup of the compounds of the formulae 1, 2, 3, 4 and 5, R and Rare in each case, independently of one another, alkyl, alkenyl, alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy having up to 8 carbon atoms. This smaller subgroup is called group A below, and the compounds are labeled with the sub- formulae la, 2a, 3a, 4a and 5a. In most of these com pounds, R and R are different from one another, one of these radicals usually being alkyl, alkenyl, alkoxy or alkoxyalkyl.

In another smaller subgroup of the compounds of the formulae 1, 2, 3, 4 and 5 which i s known as group B, R is -F, -C1, -NCS or - (O)1CH3_(k,1)FkC1, where i is 0 or 1, and k+1 is 1, 2 or 3; the compounds in which Rhas this meaning are labeled with the sub-formulae 1b, 2b, 3b, 4b and 5b. Particular preference is given to those compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b in which R# is -F, -C1, -NCS, -CF3, -OCHF2 or -OCF3.

In the compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b, R is as defined for the compounds of the sub- formulae la-5a and is preferably alkyl, alkenyl, alkoxy or alkoxyalkyl.

In a further smaller subgroup of the compounds of the formulae 1, 2, 3, 4 and 5, R# is -CN; this sub group is known as group C below, and the compounds of this subgroup are correspondingly described by sub- formulae lc, 2c, 3c, 4c and 5c. In the compounds of the sub-formulae lc, 2c, 3c, 4c and 5c, R is as defined for the compounds of the sub-formulae la-5a and is preferably alkyl, alkoxy or alkenyl.

In addition to the preferred compounds of groups A, B and C, other compounds of the formulae 1, 2, 3, 4 and 5 having other variants of the proposed substituents are also customary. A11 these substances can be obtained by methods which are known from the literature or analo gously thereto.

Besides compounds of the formula I and I*, the novel media preferably comprise one or more compounds selected from group A and/or group B and/or group C. The proportions by weight of the compounds from these groups in the novel media are preferably Group A: 0 to 90%, preferably 20 to 90%, in particular 30 to 90% Group B: 0 to 80%, preferably 10 to 80%, in particular 10 to 65% Group C: 0 to 80%, preferably 5 to 80%, in particular 5 to 50%, the sum of the proportions by weight of the group A and/or B and/or C compounds present in the particular novel media preferably being 5%-90% and in particular 10% to 90%.

The novel media preferably comprise 1 to 40%, particularly preferably 5 to 30%, of compounds of the formulae I and I*. Further preferred media are those which comprise more than 40%, in particular 45 to 90%, of compounds of the formulae I and I*. The media preferably comprise three, four or five compounds of the formulae I and I*.

The structure of the IPS display according to the invention corresponds to the usual construction for such displays, as described, for example, in w0 91/10936 or EP 0 588 568. The term conventional construction is broadly drawn here and also covers all derivatives and modifications of the IPS display, in particular, for example, including matrix display elements based on poly- Si TFT or MIM.

However, an essential difference between the displays according to the invention and those conventional hitherto is in the choice of the liquid- crystal parameters of the liquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner conventional per se. In general, the desired amount of the components used in lesser 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, after mixing, to remove the solvent again, for example by distillation.

The dielectrics may also comprise further additives known to the person skilled in the art and described in the literature. For example, 0-15% of pleochroic dyes or chiral dopants can be added.

C denotes a crystalline phase, S a smectic phase, S# a smectic B phase, N a nematic phase and I the isotropic phase.

Vo denotes the capacitive threshold voltage. #n denotes the optical anisotropy and n0 the ordinary refractive index (in each case at 589 nm). #E denotes the dielectric anisotropy (#E = E1 - #_, where El denotes the dielectric constant parallel to the longitudinal axes of the molecules and E, denotes the dielectric constant perpendicular thereto). The electro-optical data were measured in a planar cell at 20 C, unless expressly stated otherwise. The optical data were measured at 20 C, unless expressly stated otherwise.

The cells are dark in the "off" state.

In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by acronyms, the transformation into chemical formulae taking place as in Tables A and B below. All the radicals CnH2n+l are straight-chain alkyl radicals containing n or m carbon atoms, respectively. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is given. In individual cases, the acronym for the parent structure is followed, separated by a hyphen, by a code for the substituents R1, R2, Ll and L2

R2, L1, L2 nm C@Hz,., CmHa"#, H H nOm CnH2n., OCmHa"., H H nO.m OC<B>A#</B>r,., C,H2m., H H n CnH2n., CN H H nN.F C@Hz,., CN F H nN.F.F C"H2,., CN F F nF C"H2n., F H H nOF OC,H2n., F H H nCI CnH2r," CI H H nF.F CnH2n., F F H nCF3 C@H2r,., CF3 H H nOCF3 C,H2r,., OCF3 H H nOCF3 CnH2n., OCF3 H H nOCF2 C"H2,., OCHF2 H H nS C"H2,., NCS H H rVsN C,H2r.,-CH=CH-CSH2,- CN H H rEsN C,H2r.,-0-C2H2s- CN H H nAm CnH2n., COOC,,Han., H H nF.F.F CnH2n., F F F nCI.F.F C"H2r,., CI F F nCF3.F.F CnH2"., CF3 F F nOCF3.F.F C"H2"., OCF3 F F nOCF2.F.F CnH2,., OCHF2 F F nOCF3.F C"H2,., OCF3 F H Preferred media comprise, in particular, one or more compounds from Tables A and B in addition to the compounds of the formulae I and I*.

Example 1 An IPS display comprises a nematic mixture having clearing point +73 C #n [589 nm, 20 C] 0.0768 nn 1.4732 rotational viscosity (20 C) 80 mPas and comprising CCP-20CF3 8.0 % CCP-30CF3 6.0 % CCP-40CF3 5.0 % CCP-50CF3 5.0 % CCP-2F. F.F 9.0 % CCP-3F. F.F 8.0 % CCP-5F. F.F 6.0 % CCZU-2-F 6.0 % CCZU- 3 - F 9.0 % CCZU-5-F 6.0 % PCH-2N. F.F 5.0 % PCH-3N. F.F 5.0 % PCH-5N.F.F 4.0 % CC-V-V1 18.0 % and has high contrast and short response times.

The shelf life at -30 C up to which no crystal lization of the mixture occurs is 890 hours.

Claims (3)

1. claims 1. Electro-optical liquid-crystal display - having a realignment layer for realigning the liquid crystals whose field has a significant component parallel to the liquid-crystal layer - and containing a liquid crystalline medium of positive dielectric anisotropy, characterized in that the medium comprises at least one compound of the formula I

   in which R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF, or at least monosubstituted by halogen, where one or more <B>CH,</B> groups in these radicals may also, in each case independently of one another, be replaced by -0-, -S-, -CO-, -C0-0-, -0-C0- or -0-CO-0- in such a way that 0 atoms are not linked directly to one another, A and A are each, independently of one another, a (a) 1,4-cyclohexenylene radical or trans-1,4- cyclohexylene radical, in which, in addition, one. or more non-adjacent CH2 groups may be replaced by -0- and/or -S-, (b) 1,4-phenylene radical, (c) radical from the group consisting of 1,4- dicyclo[2.2.2]octylene, piperidine-1,4- diyl, naphthaline-2,6-diyl decah ydro- naphthalene-2,6-diyl and 1,2,3,4- tetrahydronaphthalene-2,6-diyl, where the radicals (a) and (b) may be substituted by one or two fluorine atoms, Z1 and Z2 are each, independently of one another, -C0-0-, -0-C0-, -CH20-, -OCH2-, -CH2CH2-, -CH=CH-, -C#-C-, or a single bond, and m is 0, 1 or 2, and at least one compound of the formula I*

   in which R1 and R2* are each, independently of one another, lE- alkenyl or 3E-alkenyl having 2-7 carbon atoms.
2. 2. Liquid-crystal display according to one of claims 1 and 2, characterized in that the medium comprises at least one compound of the formula I*a, I*b or I*c:

   in which R3* and R3* are each independently of one another, H, CH3, C2H5 or n-C3H7. Liquid-crystal display according to one of claims 1 to 3, characterized in that the medium comprises at least one compound of the formula II

   in which L1 and L2 are each, independently of one another, H or F. R2 is as defined for R1, A3 and A4 are each, independently, as defined for A1 and A2, Z3 and Z4 are each, independently of one another, as defined for Z1 and Z2, and n is 0, 1, or 2. Liquid-crystal display according to one of claims 1 to 3, characterized in that the medium comprises at least one compound of the formula III R3- (A 5-Z5) o-A6-R4 III in which R3 and R9 are each, independently of one another, as defined for R1, AS and A6 are each, independently as defined for A1 and A2, Z` is in each case, independently of the others, as defined for Z1 and Z2, and o is 1, 2 or
3. 3. 5. Liquid-crystal display according to one of claims 1 to 4, characterized in that the medium comprises at least one compound of the formula IV

   in which R5 is as defined for R1, A7 and A8 are each, independently of one another, as defined for A1 and A2, Z7 and Z8 are each, independently of one another, for Z1 and Z2, L1 and L2 are each, independently of one another, H or F, Q is a polyfluoroalkyklene radical of the formula -(0)q-(CH2)r-(CF2z)s- in which q is 0 or 1 r is 0 or an integer between 1 and 6 and s is an integer between 1 and 6, X is H, F or C1, and p is 0, 1 or 2. 6. Liquid-crystal display according to one of claims 1 to 5, characterized in that the medium comprises at least one compound of the formula IIa1

   in which z is 1-12. 7. Display according to one of claims 1 to 6, characterized in that the pixels are addressed by means of an active matrix. 8. Liquid-crystalline medium of positive dielectric anisotropy which has a composition according to claims 1 to 7. 9. Liquid crystalline-medium according to claim 8, characterized in that it comprises - from 10 to 60% by weight, preferably from 30 to 60% by weight, of at least one compound of the formula I, - from 5 to 40% by weight, preferably from 10 to 25% by weight, of at least one compound of the formula I*, - from 0 to 40% by weight, preferably from 0 to 25% by weight, of at least one compound of the formula II, - 0 to 30% by weight, preferably from 0 to 15% by weight, of at least one compound of the formula III, and - from 5 to 50% by weight, preferably from 10 to<B>30%</B> by weight, of a compound of the formula IV. 10. Liquid-crystalline medium according to claim 8 or claim 9, characterized in that it comprises: - at least one compound selected from the formulae Ia and/or Ib

   - if desired, at least one compound selected from the formulae IIa to IIc and II#,

   - if desired, at least one compound selected from the formulae IIIa, IIIb and IIIc,

   - at least one compound selected from the formulae IVa and IVb,

   and R1, R2 R3, R4, R5, L1 and L are each as defined above.