GB2260320A - Liquid crystalline fluorophenyl derivatives - Google Patents

Abstract

Optically active fluorophenyl derivatives of the formula I, <IMAGE> wherein R<1> is an unsubstituted alkyl, alkenyl, polyfluoroalkyl or polyfluoroalkenyl radical or an alkyl, alkenyl, @@@@ polyfluoroalkyl or polyfluoroalkenyl radical @@@@ monosubstituted by CN, halogen or CF3, @@@@ in which the radical contains up to 15 C atoms and @@@@ in which one or more CH2 or CF2 @@@@ groups may be replaced by a radical selected from @@@@ the group comprising -S-, O, Si(CH3)2- <IMAGE> or -CO-, -CO-O-, -O-CO- or -O-CO-O-, @@@@ two oxygen and/or sulfur atoms not being adjacent, R<2> is alkyl or alkenyl with up to 8 C atoms, in which one CH2 group may be replaced by O, R<3> and R<4> are each independently H or alkyl with up to 5 C atoms o is 1 to 8, Q<1> is CH2 or CO, Z is -CO-O-, -O-CO-, -O-CH2- or a single bond, one of the ligands L<1> and L<2> is F the other is H or F. X and Y are each independently N or CH, and m is 0 or 1 are suitable as components of ferroelectric liquid crystalline mixtures.

Description

Optically active fluorophenyl derivatives The invention relates to optically active fluorophenyl derivatives of the formula I,

wherein R1 is an unsubstituted alkyl, alkenyl, polyfluoro alkyl or polyfluoroalkenyl radical or an alkyl, alkenyl, polyfluoroalkyl or polyfluoroalkenyl radical monosubstituted by CN, halogen or CF3, in which the radical contains up to 15 C atoms and in which one or more CH2 or CF2 groups may be replaced by a radical selected from the group comprising -S-, -O-, Si(CH3)2-,

-CO-, -CO-O- -O-CO- or -O-CO-O-, two oxygen and/or sulfur atoms not being adjacent, R2 is alkyl or alkenyl with up to 8 C atoms, in which one CH2 group may be replaced by 0, R3 and R are each independently H or alkyl with up to 5 C atoms o is 1 to 8, Q1 is CH2 or Co, Z is -CO-O-, -O-CO-, -O- or a single bond, one of the ligands L1 and L2 is F the other is H or F, X and Y are each independently N or CH, and m is O or 1.

Chiral tilted smectic liquid-crystalline phases having ferroelectric properties can be prepared by adding one or more tilted smectic phases containing a suitable chiral additive to basic mixtures (L.A. Beresnev et al., Mol. Cryst. Liq.

Cryst. 89, 327 (1982); H.R. Brand et al., J. Physique 44 (lett.), L 771 (1983). Such phases can be used as dielectrics for fast switching displays which are based on the principle of SSFLC technology (Surface Stabilized Ferroelectric Liquid Crystal), described by Clark and Lagerwall (N.A. Clark and S.T. Lagerwall, Appl. Phys. Lett 36, 899 (1980); US Patent 4,367,924), based on the ferroelectric properties of the chiral tilted phase. In this phase, the elongated molecules are arranged in layers, the molecules having an angle of tilt with respect to the layer normal. On progressing from layer to layer, the direction of tilt changes by a small angle with reference to an axis perpendicular to the layers, with the result that a helical structure is formed.In displays based on the principle of SSFLC technology, the smectic layers are arranged perpendicularly to the plates of the cell. The helix-like arrangement of the directions of tilt of the molecules is suppressed by a very small spacing of the plates (approximately 1-2 Zm). This forces the long axes of the molecules to arrange themselves in a plane parallel to the plates of the cell, as a result of which two distinct tilt orientations are produced. By applying a suitable electrical alternating field it is possible to switch back and forth between these two states in the liquid-crystalline phase, which has a spontaneous polarization. This switching operation is substantially faster than in conventional twisted cells (TN LCDs) based on nematic liquid crystals.

A great disadvantage for many applications of the currently available materials having chiral tilted smectic phases (such as, for example Sic*, but also SH*, SI* SJ*E SK* SG* SF*) is their low chemical and thermal stability and low photostability. A further disadvantageous property of displays based on currently available chiral tilted smectic mixtures is that the spontaneous polarization has unduly low values, with the result that the switching time behavior of the displays is unfavorably affected and/or the pitch and/or the tilt and/or the viscosity of the phases does not meet the requirements of display technology.In addition, the temperature range of the ferroelectric phases is unduly small and is predominantly at unduly high temperatures.

It has now been found that the use of compounds of the formula I as components of chiral tilted smectic mixtures can substantially reduce the disadvantages mentioned. The compounds of the formula I are consequently eminently suitable as components of chiral titled smectic liquid-crystalline phases. In particular, it is possible to use them to prepare chiral tilted smectic liquid-crystalline phases which are particularly stable chemically and have favorable ferroelectric phase ranges, beneficial ranges for the viscosity, in particular wide S* phase ranges, outstanding supercooling capability down to temperatures below 0 OC without crystallization occurring and spontaneous polarization values which are high for such phases. Ps is the spontaneous polarization in nC/cm2.The compounds of the formula I are, however, also suitable for liquid-crystalline phases for the electroclinic effect.

Moreover, by providing the compounds of the formula I the range of liquid-crystalline substances which are suitable in various technical aspects for the preparation of ferroelectric mixtures is considerably widened.

The compounds of the formula I have a wide range of applications. Depending on the selection of the substituents, these compounds may be used as basic materials of which liquidcrystalline smectic phases are predominantly composed; however, compounds of the formula I may also be added to liquidcrystalline basic materials selected from other compound classes in order to vary, for example, the dielectric and/or optical anisotropy and/or the viscosity and/or the spontaneous polarization and/or the phase ranges and/or the angle of tilt and/or the pitch of such a dielectric.

Chiral dopants for ferroelectric mixtures are claimed in EP-A-0, 292, 954.

The compounds of the formula I are colorless in their pure state and have favorable values of optical anisotropy. Some of the compounds of the formula I have liquid-crystalline mesophases in a temperature range which is favorable for electrooptical use.

Above and below Ri, R2, R3, R4, Q1, X, Y, m, o, L1, L2 and Z have the meaning given unless expressly stated otherwise.

The compounds of the formula I accordingly comprise in particular compounds of the partial formulae Il to I4:

The compounds of the formula I1 to I4 encompass the preferred compounds of the formula Ia to Ie listed below.

Compounds of the formulae above and below which have branched wing groups R1 or R2 can be of importance. Branched groups of this type usually contain no more than two chain branchings.

R1 is preferably a straight-chain group or a branched group containing no more than one chain branching.

Preferred branched radicals for R1 are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), tert.-butyl, 2-methylbutyl, isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 2-ethylhexyl, 5-methylhexyl, 2-propylpentyl, 6-methylheptyl, 7-methyloctyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, l-methylhexoxy, 1-methylheptoxy, 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl.

The radical R1 can also be an optically active organic radical having one or more asymmetric carbon atoms.

R1 is preferably alkyl or alkenyl having up to 2 to 15 C atoms, i.e. ethyl, propyl, butyl, pently, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl. These groups can be straight-chain or branched, straight-chain alkyl groups being preferred. However, Z is preferably -0- or -CO-, preferably -0-. R3 and R4 are each independently H or alkyl having up to 5 C atoms. The alkyl group is straight-chain, i.e. methyl, ethyl, propyl, butyl, pentyl, preferably methyl. Particularly preferably R3 is H and R4 is methyl or H.

The radical

preferably has one of the following meanings:

The compounds of the formula I are prepared by methods known per se, such as are described in the literature (e.g. in the standard works such as Houben-Weyl, Methoden der Organischen Chemie (Methods of Organic Chemistry), Georg-Thieme-Verlag Stuttgart), under reaction conditions which are known and suitable for the reactions mentioned. It is also possible to use variations known per se and not mentioned here in more detail.

The compounds according to the invention can be prepared by the following reaction scheme:

(J.G. Hill, K.B. Sharpless, C.M. Exon, R. Regenye, Org. Synthesis 1984, 63, 6)

(i) Ti (OPr-i) 4/diethyl (2R, 3R) tartrate/t-butylhydroperoxide (ii) FeSO4/D-(+)-tartaric acid The invention further relates to ferroelectric liquid-crystalline phases having a content of at least one compound of the formula I and also to liquid-crystal display components, in particular ferroelectric electrooptical display components, which contain such phases. The compounds of the formula I show a slight increase in the Sc*-SA transition temperature so phase stability is maintained.

The phases according to the invention contain preferably at least two, and in particular, at least three compounds of the formula I. Particularly preferred are chiral tilted smectic liquid-crystalline phases according to the invention whose achiral basic mixture contains, in addition to compounds of the formula I, at least one other component having negative dielectric anisotropy or positive dielectric anisotropy which is small in magnitude. Preferably the inventive ferroelectric mixtures consist of an achiral base mixture with a broad 5c phase, preferably exhibiting the following phase transitions: K-Sc-S-N-I and a chiral dopant inducing ferroelectricity to this base mixture.

The chiral dopant contains at least one of the compounds of the formula I, preferably one or two compounds of the formula I. The chiral dopant(s) form 2.5 to 25 % by weight, preferably 5 to 15 % by weight of the ferroelectric mixture.

The achiral base mixture preferably contains one or more, in particularly three, four, five, sic or seven compounds of the formula III,

wherein alkyl is straight-chain or branched alkyl with 5 to 12 C atoms, L3 and L4 are each independently H or F and n and p are each independently 0 or 1.

Preferably the achiral base mixture contains one or more compounds of the formula IIIa,

one or more compounds of the formula IIIb, and

one or more compounds of the formula IIIc.

This further component or these further components of the chiral basic mixture may form 1 to 50 %, preferably 10 to 25 %, of the basic mixture. Suitable further components having positive or negative dielectric anisotropy which is small in magnitude are compounds of the formula IV

wherein

are in each case, independently of each other 1,4-phenylene or trans-1,4-cyclohexylene, m is 1 or 2, and n is 1 or 2.

Formula IV encompasses compounds of the subformulae IVa to IVi:

R5 and R6 are in each case preferably straight-chain alkyl, alkoxy, alkanoyloxy or alkoxycarbonyl containing in each case 3 to 12 carbon atoms. X is preferably sulphur or oxygen. In the compounds of the formulae IVa, IVb, IVd, IVe, IVf and IVg, a 1,4-phenylene group may also be laterally substituted by halogen or CN, in particular, preferably by fluorine.

Particularly preferred are the compounds of the subformulae IVa, IVb, IVd and IVf in which R5 and R6 are in each case straight-chain alkyl or alkoxy containing in each case 5 to 10 carbon atoms.

The compounds of the subformulae IVc, IVh and IVi are suitable as additives for lowering the melting point and are normally added to the basic mixtures in an amount of not more than 5 %, preferably 1 to 3 %. R5 and R6 in the compounds of the subformulae IVc, IVh and IVi are preferably straightchain alkyl containing 2 to 7, preferably 3 to 5, carbon atoms. A further class of compound which is suitable for lowering the melting point in the phases according to the invention is that of the formula:

in which Rs and R6 have the preferred meaning specified for IVc, IVh and IVi.

Further suitable components having negative dielectric anisotropy are furthermore compounds containing structural element A, B or C.

Preferred compounds of this type correspond to the formulae Va, Vb and Vc:

R'-Q4-Q5~Rt,, Vc R' and R" are in each case preferably straight-chain alkyl or alkoxy groups containing in each case 2 to 10 carbon atoms. Q2 and Q3 are in each case 1,4-phenylene, trans-1,4cyclohexylene, 4,4'-biphenylyl, 4-(trans-4-cyclohexyl)phenyl, trans,trans-4,4'-bicyclohexyl or one of the groups Q2 and Q3 is also a single bond.

Q4 and Q5 are in each case 1,4-phenylene, 4,4'-biphenylyl or trans-1,4-cyclohexylene. One of the groups Q4 and Q5 may also be 1,4-phenylene in which at least one CH group is replaced by N. R"' is an optically active radical containing an asymmetric carbon atom of the structure

Particularly preferred compounds of the formula Vc are those of the formula Vc':

in which A is 1,4-phenylene or trans-l,4-cyclohexylene and n is 0 or 1.

The compounds of the formula I are also suitable as components of nematic liquid-crystalline phases, e.g. for avoiding reverse twist.

These liquid-crystalline phases according to the invention consist of 2 to 25, preferably 3 to 15, components, of which at least one is a compound of the formula I. The other components are preferably selected from nematic or nematogenic substances, in particular known substances, from the classes of azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl- or cyclohexylbenzoates, phenylcyclohexanecarboxylates or cyclohexyl cyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexylcyclohexanes, cyclohexylnaphthalenes, 1,4-bis-cyclohexylbenzenes, 4,4' -bis (cyclo- hexyl)biphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridazines, and their N-oxides, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-1,3-dithianes, 1,2diphenylethanes, 1,2-dicyclohexylethanes, 1-phenyl-2cyclohexylethanes, halogenated or non-halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids.

The most important compounds suitable as components of these liquid-crystalline phases can be described by the formula I' R' -L-G-E-R" I' in which L and E are each a carbocyclic or heterocyclic ring system from the group consisting of 1,4-disubstituted benzene and cyclohexane rings, 4,4'-disubstituted biphenyl, phenylcyclohexane and cyclohexylcyclohexane systems, 2,5-disubstituted naphthalene, di- and tetrahydronaphthalene, quinazoline and tetrahydroquinazoline.

G is -CH=CH- -N(O)=N -CH=CY- -CH=N(O)- -C3c- -CH2-CH2- -CO-O- -CH2-O -CO-S- -CH2-S -CH=N- -COO-Phe-COOor a C-C single bond, Y is halogen, preferably chlorine, or -CN, and R' and R" are alkyl, alkoxy, alkanoyloxy, alkoxycarbonyl or alkoxycarbonyloxy having up to 18, preferably up to 8, carbon atoms, or one of these radicals is also CN, NO2, CF3, F, Cl or Br.

In most of these compounds, R' and R" are different from one another, one of these radicals being in most cases an alkyl or alkoxy group. However, other variations of the intended substituent are also customary. Many of these substances or even mixtures thereof are commercially available. All these substances are available by methods known in the literature. The phases according to the invention contain about 0.1 to 99, preferably 10 to 95, % of one or more compounds of the formula I. Liquid-crystalline phases according to the invention which contain 0.1-40, preferably 0.5-30 %, of one or more compounds of the formula I are furthermore preferred.

The preparation of the phases according to the invention is carried out in a manner customary per se. As a rule, the components are dissolved in one another, advantageously at elevated temperature. The liquid-crystalline phases can be modified according to the invention by means of suitable additives in such a manner that they can be used in all previously known types of liquid crystal display elements.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees celsius and unless otherwise indicated, all parts and percentages are by weight.

M.P. = melting point, c.p. = clear point.

"Customary work-up means: water is added, the mixture is extracted with methylene chloride, the organic phase is separated off, dried and evaporated, and the product is purified by crystallization and/or chromatography.

Furthermore C denotes crystalline solid state, S denotes smectic phase (the index designates the phase type), N denotes nematic phase, Ch cholesteric phase, I isotropic phase. The number between two symbols indicates the conversion temperature in degrees celsius.

Exaxnple 1 Preparation of 2-(4'-octoxy-2'-fluorophenyl)-5-(2,3-difluoro- hexoxy)-1,3-diazine steP 1.1 2-(4'-octoxy-2'-fluorophenyl)-5-((2S,3S)-3-pronl- oxiranemethoxy) -1, 3-diazine

To a stirred solution of 2-(4'-octoxy-2'fluorophenyl)-5-hydroxy-1,3-diazine (0.50 mmol) in dry dimethyl formamide under an atmosphere of dry nitrogen at 0-5 C was added sodium hydride as an 80 % dispersion in oil (0.50 mmol). When the gas evolution had died down, (2S,3S)-3-Propyl oxiranemethyl-p-toluene sulphonate (0.70 mmol) was added. The reaction mixture was quenched in ice, extracted with dichloromethane, washed with water and dried.Purification of the crude yellow oil by flash column chromatography on silica. gel using ethyl acetate: petroleum fraction (40-60 C bp) (1:3) as the eluent gave compound A as a white crystalline solid. C 63 Ch 76 I.

Step 1.2 2- (4' -octoxy-2' -fluorophenyl) -5- (2-fluoro-3-hydroxyhex- oxy) -1, 3-diazine

To a stirred solution of HF/Pyridine (70 % HF) (1.2 ml) in CHCl3 (10 ml) at room temperature in a polyethylene reaction flask was added dropwise over 30 min compound A (0.48 mmol) in CHCl3 (5 ml). The reaction mixture was stirred at room temperature for 24 h and then quenched in ice. Diethyl ether was added and the organic product was extracted with diethyl ether, washed with water and dried.Purification of the crude product by column chromatography on silica gel using ethyl acetate : petroleum fraction (60-80 OC bp) (1:2.5) as the eluent followed by crystallization from ethanol gave compound B as a white crystalline solid. mp = 104-105 OC Step 1.3 2-(4'-octoxy-2'-fluorophenyl)-5-(2,3-difluorohexoxy)-1,3diazine

To a stirred solution of 4-morpholino sulphur trifluoride (Morpho-DAST) (0.1 ml) in CHCl3 (10 ml) at 0 C under an atmosphere of dry nitrogen in a polyethylene reaction flask was added dropwise over 30 min compound B (0.23 mmol) in CHCl3 (10 ml). The reaction mixture was allowed to warm up to room temperature and stirred (20 h). The reaction mixture was then quenched in ice and extracted with diethyl ether.Purification of the crude product by crystallization from ethanol gave C as a white crystalline solid. mp = 80-81 C Analogously the following compounds of the formula

are obtained.

R L1 L2 n-C3H7 H F n-C3H7 F H n-C3H7 F F n-C5H11 H F n-C5H11 F H n-C5H11 F F n-C6H13 H F n-C6H13 F H n-C6Hl3 F F R L1 L2 n-C7H15 H F n-C7H15 F H n-C7H15 F F n-C8H17 F H n-C8H17 F F n-C9H15 H F n-C9H15 F H n-C9H15 F F n-C1oH21 H F n-C1oH21 F H n-C1oH21 F F n-C12H25 H F n-C12H25 F H n-C12H25 F F Example 2 Preparation of 2-(4'-octoxy-2'-fluorophenyl)-5-(2,3-difluoro- 3,6-dimethyloct-6-enyloxy)-1,3-diazine SteP 2.1 2-(4'-octoxy-2'-fluorophenyl)-5-((2S,3S)-3emethyl-3-(4- : :nethylpent-3-enyl) oxirane methoxy)-1,3-diazine

Compound D was prepared by a similar method to that described for the preparation of compound A except using the p-toluene sulphoxide of (2S,3S)-3-methyl-3-(4-methylpent-3-enyl)oxirane methanol (0.92 mmol), 2-(4' -octoxy-2' -fluorophenyl) -5-hy- droxy-1,3-diazine (0.66 mol), dimethyl formamide (10 ml) and sodium hydride as an 80 % dispersion in oil (0.66 mmol).

Purification of the crude product by flash column chromatography on silica gel using ethyl acetate ; petroleum fraction (60-80 C bp) (1:4) as the eluent followed by crystallization from ethanol gave compound D as a white crystalline solid.

mp = 47 OC.

SteD 2.2 2-(4'-octoxy-2'-fluorophenyl)-5-(2-fluoro-3-hydroxy-3,6dimethyloct-6-enyloxy)-1,3-diazine

Compound E was prepared by a similar method to that described for the preparation of compound B except using HF/Pyridine (70 %) (0.5 ml) and compound D (0.2 mmol). Purification of the crude product by gravity column chromatography on silica gel using ethyl acetate : petroleum fraction (60-80 C bp) (1:5) as the eluent gave compound E as a white crystalline solid. mp = 77 OC.

Step 2.3 2-(4'-octoxy-2'-fluorophenyl)-5-(2,3-difluorohexoxy)-3,6- dimethyloct-6-enyloxy)-1,3-diazine

To a stirred solution of 4-morpholino sulphur trifluoride (Morpho-DAST) (0.1 ml) in CHCl3 (10 ml) at 0 C under an atmosphere of dry nitrogen in a polyethylene flask was added dropwise over 30 min compound E (0.23 mmol) in CHCl3 (10 ml).

The reaction mixture was allowed to warm up to room temperature and stirred (20 h). The reaction mixture was then quenched in ice and extracted with diethyl ether. Purification of the crude product by crystallization from ethanol gave compound F as a white crystalline solid.

Analogously the following compounds of the formula

are obtained.

R L1 L2 n-C3H7 H F n-C3H7 F H n-C3H7 F F n-C5H11 H F n-C5Hl1 F H n-C5H1i F F n-C6H13 H F n-C6H13 F H n-C6H13 F F R L1 L2 n-C7H15 H F n-C7H15 F H n-C7H15 F F n-C8H17 F H n-C8Hl7 F F n-C9H15 H F n-C9H15 F H n-C9H15 F F n-C1oH21 H F n-C1oH21 F H n-C1oH21 F F n-C12H25 H F n-C12H25 F H n-C12H25 F F The example which follows relates to a liquid-crystalline phases according to the invention.

Example A A liquid crystalline phase H (host) was used consisting of 20,00 % 2-p-heptoxyphenyl-5-nonylpyrimidine 20,00 % 2-p-octoxyphenyl-5-nonylpyrimidine 20,00 % 2-p-nonoxyphenyl-5-nonylpyrimidine 6,67 % 2- [p-heptoxy-2,3-difluorophenyl]-5-nonylpyrimidine 6,67 % 2-[p-octoxy-2,3-difluorophenyl]-5-nonylpyrimidine 6, 67 % 2-[p-nonoxy-2,3-difluorophenyl]-5-nonylpyrimidine 20,00 % 2-p-hexoxyphenyl-5-hexoxypyrimidine To this phase H there was added 5 % by weight of compound C.

The phase transition points and the values of the physical properties are shown in Table 1: Table 1: SC SA N I Ps [nC/cm2] at 20 C 30 C phase H 64 70 74 H + 5 wt % C 65 69 74 +8.2 +6.7 (Step 1.2) Table 2 shows the response times of compound C (5 wt% in H).

Table 2: 20 C 30 C 5 V/ m 130 s 110 s 10 V/ m 70 s 60 s 15 V/ m 35 s 30 s A 5 % mixture of compound C shows enhanced Sc phase stability and good switching / P5 data.

Claims (5)

Claims

1. Optically active fluorophenyl derivatives of the formula I,

wherein R1 is an unsubstituted alkyl, alkenyl, polyfluoroal kyl or polyfluoroalkenyl radical or an alkyl, alkenyl, polyfluoroalkyl or polyfluoroalkenyl radical monosubstituted by CN, halogen or CF3, in which the radical contains up to 15 C atoms and in which one or more CH2 or CF2 groups may be replaced by a radical selected from the group comprising -S-, -O-, Si(CH3)2-,

-CO-, -CO-O- -O-CO- or -O-CO-O-, two oxygen and/or sulfur atoms not being adjacent, R2 is alkyl or alkenyl with up to 8 C atoms, in which one CH2 group may be replaced by O, R3 and are each independently H or alkyl with up to 5 C atoms o is 1 to 8, Q1 is CH2 or CO, Z is -CO-O-, -O-CO-, -O- or a single bond, one of the ligands L1 and L2 is F the other is H or F, X and Y are each independently N or CH, and m is 0 or 1.

2. Optically active derivative of the formula Ia',

wherein L1 and L2 have the meaning given in claim 1 and alkyl is straight-chain having up to 5 C atoms.

3. Optically active derivative of the formula Ib',

wherein L1, L2 and alkyl have the meaning given in claim 1 or 2.

4. Ferroelectric liquid-crystalline medium containing at least two liquid-crystalline components, characterized in that at least one component is a compound of one of claim 1 to 3.

5. Electrooptical display, characterized in that it contains a medium according to claim 4 as dielectric.