DE4434976A1 - Low viscosity and high dielectric anisotropy liq. crystal cpds. contg. alkenoate side chains - Google Patents

Abstract

Liq. crystal cpds. contg. alkenoate side chains of formula (I) are new; R = 2-12C alkenyl; A<1> = opt. mono- or difluoro-substd. 1,4-phenylene in which 1-2 CH gps. may be replaced by N; A<2> = trans-1,4-cyclohexenylene in which one or more non-adjacent CH2 gps. may be replaced by O and/or S, 1,4-phenylene in which 1-2 CH gps. may be replaced by N, or opt. mono- or difluoro-substd. 1,4-cyclohexenylene, 1,4-bicyclo(2.2.2)octylene, piperidin-1,4-diyl or naphthalen-2,6-diyl; Z<1>, Z<2> = -CO-O-, -O-CO-, -CH2O-, -OCH2-, -CH2CH2-, -CH=CH-, -C?=C-, -(CH2)4, -CH=CH-CH2CH2- or a single bond; m = 0-2; L<1>, L<2> = H or F; and X = F, Cl, CN, OCN, NCS or opt. mono- or difluoro-substd. 1-6C alk(en)yl or alkoxy. Also claimed are (i) cpds. of formula (II) and (III), (ii) use of (I) as a liq. crystal medium component, (iii) liq. crystal media contg. at least two liq. crystalline components and at least cpd. of formula (I), and (iv) liq. crystal or electro-optic display elements contg. the media;

Description

The invention relates to liquid crystalline compounds with alkenoate sides chains of formula I,

wherein
R is an alkenyl group with 2 to 12 carbon atoms,

unsubstituted or substituted with one or two fluorine 1,4-phenylene, which also contains one or two CH groups N can be replaced

(a) trans-1,4-cyclohexylene radical, in which one or more non-adjacent CH₂ groups can also be replaced by -O- and / or -S-,
(b) 1,4-phenylene radical, in which one or two CH groups can also be replaced by N,
(c) radical from the group 1,4-cyclohexenylene, 1,4-bicyclo (2,2,2) -octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
where the radicals (a) and (b) can be substituted by one or two fluorine,
Z¹ and Z² each independently of one another -CO-O-, -O-CO-, -CH₂O-,
-OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C-, - (CH₂) ₄-,
-CH = CH-CH₂CH₂- or a single bond,
m 0, 1 or 2,
L¹ and L² are each independently H or F, and
XF, Cl, CN, OCN, NCS, unsubstituted or substituted by one or more fluorine atoms, alkyl, alkoxy or alkenyl radical with 1 to 6 carbon atoms,
mean.

The invention further relates to the use of these compounds as Components of liquid crystalline media as well as liquid crystal and electro optical display elements, the liquid-crystalline according to the invention Media included.

The compounds of the formula I can be liquid-crystalline as components Media are used, especially for displays based on the principle the twisted cell, the guest-host effect, the effect of the deformation aligned phases or the effect of dynamic scattering are based.

The invention was based on the object, new stable liquid-crystalline or to find mesogenic compounds that are liquid as components crystalline media are suitable and in particular a ver have equally low viscosity and a relatively high dielectric trical anisotropy.

It has now been found that the introduction of an alkenoate side chain Tendency to form liquid crystal phases, especially the tendency favorably influenced to form a nematic phase. Farther it was found that compounds of the formula I as components  liquid crystalline media are particularly suitable. In particular, have they have comparatively low viscosities. With their help stable liquid-crystalline media with a wide mesophase range and above obtained partial values for the optical and dielectric anisotropy. These media also have very good low-temperature behavior.

Cyclohexylalkenoates are already known from EP 0 595 144 A1. The Phenylalkenoates according to the invention are not mentioned there.

With regard to the various areas of application of such a connection However, it was desirable to use additional connections to have high nematogenicity available on the precisely tailored properties for each application point.

With the provision of compounds of formula I is also whole generally the range of liquid crystalline substances, which can be found under ver various application-related aspects for the production liquid-crystalline mixtures are suitable, considerably broadened.

The compounds of formula I have a wide range of uses Area. Depending on the selection of the substituents, these can Compounds serve as base materials from which liquid crystalline The majority of the media are composed; but it can also compounds of the formula I from liquid-crystalline base materials other classes of compounds, for example the dielectric and / or optical anisotropy of such a dielectric to influence and / or its threshold voltage and / or optimize its viscosity.

The compounds of the formula I are colorless in the pure state and form liquid crystalline mesophases in one for electro-optical use conveniently located temperature range. Chemical, thermal and against Light they are stable.  

The invention thus relates to the compounds of the formula I. and the use of these compounds as liquid components crystalline media. The invention further relates to liquid crystalline Media containing at least one compound of formula I. and liquid crystal display elements, in particular electro-optical Display elements that contain such media.

For the sake of simplicity, A³ in the following means a remainder of formula

Cyc a 1,4-cyclohexyl radical, Che a 1,4-cyclohexenylene radical, Dio a 1,3-dioxane-2,5-diyl residue, Dit a 1,3-dithiane-2,5-diyl residue, Phe one 1,4-phenylene radical, Pyd a pyridine-2,5-diyl radical, Pyr a pyrimidine 2,5-diyl radical and Bi a bicyclo (2,2,2) octylene radical, where Cyc and / or Phe is unsubstituted or mono- or disubstituted by F or CN could be.

A² is preferably selected from the group Cyc, Che, Phe, Pyr, Pyd and Dio.

The compounds of the formula I accordingly comprise compounds with two rings of sub-formulas Ia and Ib:

R¹-A¹-A³-X Ia
R¹-A¹-Z¹-A³-X Ib

Connections with three rings of the sub-formulas Ic to If:

R¹-A¹-A²-A³-X Ic
R¹-A¹-Z¹-A²-A ³ -X Id
R¹-A¹-A²-Z²-A³-X Ie
R¹-A¹-Z¹-A²-Z²-A³-X If

as well as compounds with four rings of the sub-formulas Ig to Im:

R¹-A¹-A²-A²-A³-X Ig
R¹-A¹-Z¹-A²-A²-A³-X Ih
R¹-A¹-A²-Z²-A²-A³-X Ii
R¹-A¹-A²-A²-Z²-A³-X Ij
R¹-A¹-A²-Z²-A²-Z²-A³-X Ik
R¹-A¹-Z¹-A²-Z²-A²-Z²-A³-X Il
R¹-A¹-Z¹-A²-A²-Z²-A³-X Im

Compounds of the formula I and all sub-formulas are also preferred, in those A¹, A² and A³ mono- or disubstituted by F 1,4-pheny len means. In particular, these are 2-fluoro-1,4-phenylene, 3-fluoro 1,4-phenylene and 3,5-difluoro-1,4-phenylene.

A1 and A2 are preferred

or

A² may further preferably be a trans-1,4-cyclohexylene ring mean.

The 1,4-cyclohexenylene group preferably has the following structures:

Z¹ and Z² are preferably a single bond, -CO-O-, -O-CO- and -CH₂CH₂-, in the second place preferably -CH₂O- and -OCH₂-. If one of Z¹ and Z² is - (CH₂) ₄- or -CH = CH-CH₂CH₂-, the other Z¹ or Z² (if present) is preferably a single bond.
m is preferably 0 or 1.

If X is an alkyl radical and / or an alkoxy radical, this can straight chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and therefore preferably denotes ethyl, propyl, Butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, Tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, Undecoxy, dodecoxy, tridecoxy or tetradecoxy.

Preferred radicals for R are alkenyl radicals having 2 to 12 carbon atoms, in which the double bond is in the 1- or 3-position and in the E-con Figuration is present or alkenyl residues with a terminal double bond. Very particularly preferred radicals have 3 to 7 carbon atoms; like with for example 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 4-pentenyl, 5-hexenyl and 6-heptenyl.

If X is an alkyl radical in which a CH₂ group by -O- and one is replaced by -CO-, these are preferably adjacent. Consequently contain an acyloxy group -CO-O- or an oxycarbonyl group -O-CO-. These are preferably straight-chain and have 2 to 6 carbon atoms.

Accordingly, they mean especially acetyloxy, propionyloxy, butyryloxy, Pentanoyloxy, hexanoyloxy, acetyloxymethyl, propionyloxymethyl, buty ryloxymethyl, pentanoyloxymethyl, 2-acetyloxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetyloxypropyl, 3-propionyloxypropyl, 4-acetyl oxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxy carbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonyl methyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2- (methoxy carbonyl) ethyl, 2- (ethoxycarbonyl) ethyl, 2- (propoxycarbonyl) ethyl, 3- (methoxycarbonyl) propyl, 3- (ethoxycarbonyl) propyl, 4- (methoxy carbonyl) butyl.  

If X is an alkyl, alkoxy or alkenyl radical, this radical is preferably straight-chain. The resulting residues also include perfluorinated residues. At Single substitution can be the fluorine substituent in any position but preferably in the ω position.

X is preferably F, Cl, CN, OCN, NCS, OCF₃, OCHF₂, OCH₂CF₃, OCH₂CHF₂, OCHFCF₃, OC₂F₅, OC₃F₇, OCH = CF₂, OCF = CF₂, CF₃, CHF₂, CF = CF₂, CH = CF₂, CH₃, C₂H₅, n-C₃H₇ and n-C₅H₁₁.

Compounds of formula I with branched wing groups R and / or X can occasionally because of better solubility in the usual liquid-crystalline base materials are of importance, in particular but as chiral dopants if they are optically active. Smectic Compounds of this type are suitable as components for ferroelectric Materials.

Compounds of the formula I with S _A phases are suitable, for example, for thermally addressed displays.

Branched groups of this type usually do not contain more than one Chain branch. Preferred branched radicals X are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), 2-methylbutyl, isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2 ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methyl butoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methyl hexoxy, 1-methylheptoxy.

If X represents an alkyl radical in which two or more CH₂ groups are replaced by -O- and / or -CO-O-, this can be straight-chain or be branched. It is preferably branched and has 3 to 12 carbon atoms. Accordingly, it means especially bis-carboxy-methyl, 2,2-bis-carboxyethyl, 3,3-bis-carboxy-propyl, 4,4-bis-carboxy-butyl, 5,5-bis-carboxy-pentyl, 6,6-bis-carboxy-hexyl, 7,7-bis-carboxy-heptyl, 8,8-bis-carboxy  octyl, 9,9-bis-carboxy-nonyl, 10,10-bis-carboxy-decyl, bis (methoxy carbonyl) methyl, 2,2-bis (methoxycarbonyl) ethyl, 3,3-bis (methoxy carbonyI) propyl, 4,4-bis (methoxycarbonyl) butyl, 5,5-bis (methoxy carbonyl) pentyl, 6,6-bis (methoxycarbonyl) hexyl, 7,7-bis (methoxy carbonyl) heptyl, 8,8-bis (methoxycarbonyl) octyl, bis (ethoxycarbonyl) - methyl, 2,2-bis (ethoxycarbonyl) ethyl, 3,3-bis (ethoxycarbonyl) propyl, 4,4-bis (ethoxycarbonyl) butyl, 5,5-bis (ethoxycarbonyl) hexyl.

Compounds of the formula I which are suitable for polycondensation Wing groups R and / or X are suitable for display liquid crystalline polycondensates.

Formula I includes both the racemates of these compounds and the optical antipodes and mixtures thereof.

Among these are compounds of formula I and the sub-formulas preferred those in which at least one of those contained therein Radicals has one of the preferred meanings indicated.

In the compounds of formula I those stereoisomers are before in which the rings Cyc and piperidine are trans-1,4-disubstituted. Those of the above formulas, the one or more Groups containing Pyd, Pyr and / or Dio each enclose the two 2,5-position isomers.

Preferred smaller groups of compounds are those of the Sub-formulas I1 to I6:

The compounds of formula I are according to methods known per se presented as they are in the literature (e.g. in the standard works such as Houben-Weyl, Methods of Organic Chemistry, Georg-Thieme-Verlag, Stuttgart) are described, namely under reaction conditions that for the implementations mentioned are known and suitable.

You can also use known ones that are not mentioned here Make use of variants.  

Scheme 1

The synthesis of some particularly preferred compounds is described in described the following in more detail:  

Scheme 2

The liquid-crystalline media according to the invention preferably contain in addition to one or more compounds according to the invention as further Components 2 to 40, especially 4 to 30 components. Very special more preferably, these media contain one or more inventions compounds according to the invention 7 to 25 components. These others Components are preferably selected from nematic or nematogenic (monotropic or isotropic) substances, in particular Substances from the classes of azoxybenzenes, benzylidene anilines,  Biphenyls, terphenyls, phenyl or cyclohexyl benzoates, cyclohexane carboxylic acid phenyl or cyclohexyl ester, phenyl or cyclohexyl ester of cyclohexylbenzoic acid, phenyl or cyclohexyl ester of cyclo hexylcyclohexane carboxylic acid, cyclohexyl phenyl ester of benzoic acid, cyclohexane carboxylic acid or cyclohexylcyclohexane carboxylic acid, Phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, Cyclohexylcyclohexanes, cyclohexylcyclohexylcyclohexenes, 1,4-biscyclo hexylbenzenes, 4,4'-bis-cyclohexylbiphenyls, phenyl- or cyclohexyl pyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyl dioxanes, phenyl- or cyclohexyl-1,3-dithiane, 1,2-diphenylethanes, 1,2-dicyclohexylethane, 1-phenyl-2-cyclohexylethane, 1-cyclohexyl 2- (4-phenylcyclohexyl) ethane, 1-cyclohexyl-2-biphenylylethane, 1-phenyl-2-cyclohexyl-phenylethane, optionally halogenated Stilbenes, benzylphenyl ethers, tolanes and substituted cinnamic acids. The 1,4-phenylene groups in these compounds can also be fluorinated.

The most important as further components of media according to the invention in Compounds in question can be represented by formulas 1, 2, 3, 4 and 5 characterize:

R'-LER '' 1
R'-L-COO-ER '' 2
R'-L-OOC-ER '' 3
R'-L-CH₂CH₂-ER '' 4
R′-LC≡CER ′ ′ 5

In formulas 1, 2, 3, 4 and 5, L and E mean the same or ver can be different, each independently a bivalent Balance from the from -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- as well as their mirror images formed group, where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc trans-1,4-cyclohexylene or 1,4-cyclohexylene, pyr pyrimidine-2,5-diyl or pyridine-2,5-diyl, bio 1,3-dioxane-2,5-diyl and G 2- (trans-1,4-cyclo hexyl) ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl mean.  

One of the radicals L and E is preferably Cyc, Phe or Pyr. E is in front preferably Cyc, Phe or Phe-Cyc. Preferably contain the inventions media according to the invention selected one or more components the compounds of formulas 1, 2, 3, 4 and 5, wherein L and E are selected are from the group Cyc, Phe and Pyr and simultaneously one or more Components selected from the compounds of the formulas 1, 2, 3, 4 and 5, in which one of the radicals L and E is selected from the group Cyc, Phe and Pyr and the rest of the rest is selected from the group -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and optionally one or more components selected from the Compounds of the formulas 1, 2, 3, 4 and 5, in which the radicals L and E are selected from the group -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-.

R 'and R' 'mean in a smaller subset of the compounds of Formulas 1, 2, 3, 4 and 5 each independently of the other alkyl, alkenyl, Alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy with up to 8 carbon atoms. This smaller subgroup is called Group A below and the compounds are represented by sub-formulas 1a, 2a, 3a, 4a and 5a designated. For most of these compounds, R 'and R' 'are the same others different, one of these radicals mostly alkyl, alkenyl, alkoxy or alkoxyalkyl.

In another smaller subgroup, referred to as group B, of the compounds of the formulas 1, 2, 3, 4 and 5, R ′ ′ -F, -Cl, -NCS or - (O) _i CH _{3- (k + 1)} F _k Cl₁, where i is 0 or 1 and k + 1 is 1, 2 or 3; the compounds in which R '' has this meaning are designated by the sub-formulas 1b, 2b, 3b, 4b and 5b. Such compounds of the sub-formulas 1b, 2b, 3b, 4b and 5b are particularly preferred in which R '' has the meaning -F, -Cl, -NCS, -CF₃, -OCHF₂ or -OCF₃.

In the compounds of sub-formulas 1b, 2b, 3b, 4b and 5b, R 'has the the compounds of the sub-formulas 1a-5a and meaning preferably alkyl, alkenyl, alkoxy or alkoxyalkyl.  

In a further smaller subgroup of the compounds of the formulas 1, 2, 3, 4 and 5 means R ′ ′ - CN; this subgroup is hereinafter referred to as Group C designated and the compounds of this subgroup described accordingly with sub-formulas 1c, 2c, 3c, 4c and 5c. In the Compounds of sub-formulas 1c, 2c, 3c, 4c and 5c has R 'in the Compounds of the sub-formulas 1a-5a given meaning and is before preferably alkyl, alkoxy or alkenyl.

In addition to the preferred compounds of groups A, B and C are also other compounds of formulas 1, 2, 3, 4 and 5 with other variants of the proposed substituents in use. All of these substances are obtainable according to methods known from the literature or in analogy.

The media according to the invention contain, in addition to ver compounds of the formula I preferably one or more compounds, which are selected from group A and / or group B and / or group C. The mass fractions of the compounds from these Groups on the media according to the invention are preferred

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%

where the sum of the mass fractions of the invention in each according to media contained compounds from groups A and / or B and / or C preferably 5 to 90% and in particular 10 to 90% is.

The media according to the invention preferably contain 1 to 40%, ins particularly preferably 5 to 30% of compounds according to the invention In addition, media containing more than 40% are preferred  special 45 to 90% of compounds according to the invention. The media preferably contain three, four or five verb according to the invention fertilize.

The media according to the invention are produced in a manner which is customary per se Wise. As a rule, the components are dissolved into one another, for this purpose moderate at elevated temperature. With suitable additives, the liquid crystalline phases are modified according to the invention so that it in all types of liquid crystal display that have become known elements can be used. Such additives are the subject known and extensively described in the literature (H. Kelker / R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980). For example, pleochroic dyes can be used to produce colored ones Guest host systems or substances for changing the dielectric Anisotropy, the viscosity and / or the orientation of the nematic Phases are added.

In the present application and in the following examples, the structures of the liquid crystal compounds are indicated by acronyms, the transformation into chemical formulas taking place according to Tables A and B below. All radicals C _n H _{2n + 1} and C _m H _{2m + 1} are straight-chain alkyl radicals with n or m C atoms. The coding according to Table B goes without saying. In Table A only the acronym for the basic body is given. In individual cases, there is a line for the substituents R¹, R², L¹ and L² separated from the acronym for the basic body:

Preferred liquid containing the compounds of the invention Crystalline mixtures preferably consist of one or more in the compounds mentioned in Tables A and B.

Table A

Table B

The following examples are intended to illustrate the invention without limiting it limit. Percentages above and below mean percentages by weight percent. All temperatures are given in degrees Celsius. Mp. Means Melting point, Kp. = Clearing point. Furthermore, K = crystalline state, N = nematic phase, S = smectic phase and I = isotropic phase. The Information between these symbols represents the transition temperatures Δn means optical anisotropy (589 nm, 20 ° C) and the viscosity (mm² / sec) was determined at 20 ° C.

"Normal work-up" means: if necessary, water is added, extracted with dichloromethane, diethyl ether, methyl tert-butyl ether or Toluene, separates, dries the organic phase, evaporates and cleanses the product by distillation under reduced pressure or crystallization and / or chromatography. The following abbreviations are used:

DAST Diethylaminosulfur trifluoride
DCC N, N'-dicyclohexylcarbodiimide
DMAP 4- (dimethylamino) pyridine
DMEU 1,3-dimethyl-2-imidazolidinone
KOT potassium tertiary butanolate
THF tetrahydrofuran
pTsOH p-toluenesulfonic acid

example 1

Step 1.1

In a nitrogen atmosphere, 1.25 mol of 3-fluoro-4-bromoanisole 800 ml of diethyl ether dissolved and at -70 ° C with 1.3 mol BuLi (15% Solution in n-hexane) added. The mixture is stirred for 0.5 h, cooled to -90 ° C and 1.45 mol of trimethyl borate dropwise to the reaction mixture. You let go Warm up to -20 ° C, drop 350 ml conc. HCl and then works as usual.

Step 1.2

0.46 mol of 4-methoxy-2-fluorophenylboronic acid, 0.5 mol of bromobenzonitrile, 1200 ml toluene, 500 ml ethanol, 13.8 g tetrakis (triphenylphosphine) palla dium (O) and 1600 ml sodium carbonate solution (2 mol Na₂CO₃ / lH₂O) stirred at boiling temperature for 24 h. The suspension is then left on Cool down to room temperature, separate the organic phase and work as usual.

Step 1.3

0.133 mol of 1-methoxy-3-fluorophenyl-4'-benzonitrile, 0.16 mol of aluminum chloride and 290 ml of xylene are refluxed overnight. Man allows to cool to room temperature, sucks the resulting precipitate and concentrates the filtrate. After adding methyl tert-butyl ether washed with water and then dried, filtered and concentrated tight.  

Step 1.4

0.047 mol of p-cyanophenyl-4 '- (3'-fluoro) phenol, 0.056 mol of 4-pentenoic acid and 0.047 mol of DMAP are initially introduced and, in a nitrogen atmosphere, first mixed with 100 ml of dichloromethane and then with 0.056 mol of DCC in 50 ml of dichloromethane. The mixture is stirred at room temperature overnight and then filtered through a silica gel column. The filtrate is concentrated and recrystallized in n-hexane. K 53 N (37.0) l;
Δn = +0.197; Δε = 21.47

The following compounds of the formula become analogous

manufactured:

Claims (9)

1. Liquid-crystalline compounds with alkenoate side chains of the formula I, wherein
R is an alkenyl group with 2 to 12 carbon atoms, 1,4-phenylene which is unsubstituted or substituted by one or two fluorine, in which one or two CH groups can also be replaced by N, (a) trans-1,4-cyclohexylene radical, in which one or more non-adjacent CH₂ groups can also be replaced by -O- and / or -S-,
A¹ 1,4-phenylene which is unsubstituted or substituted by one or two fluorine, in which one or two CH groups can also be replaced by N,
A² (a) trans-1,4-cyclohexylene radical, in which one or more non-adjacent CH₂ groups can also be replaced by -O- and / or -S-,
(b) 1,4-phenylene radical, in which one or two CH groups can also be replaced by N,
(c) residue from the group 1,4-cyclohexenylene, 1,4-bicyclo (2,2,2) -octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
where the radicals (a) and (b) can be substituted by one or two fluorine,
Z¹ and Z² each independently of one another -CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C-, - (CH₂) ₄-, - CH = CH-CH₂CH₂- or a single bond,
m 0, 1 or 2,
L¹ and L² are each independently H or F, and
XF, Cl, CN, OCN, NCS, unsubstituted alkyl, alkoxy or alkenyl radical having 1 to 6 carbon atoms and substituted by one or more fluorine atoms,
mean. 2. Compounds of the formula wherein X, L¹ and L² have the meaning given in claim 1 and L³ is H or F and R * H or CH₃. 3. Compounds of the formula wherein X, L¹ and L² have the meaning given in claim 1 and L³ is H or F and R * H or CH₃. 4. Compounds according to any one of claims 1 to 3, characterized records that X F, CN, OCF₃, OCHF₂, OCH₂CF₃, OCH₂CHF₂, OCHFCF₃, OC₂F₅, OC₃F₇, OCH = CF₂, OCF = CF₂, CH₃ or C₂H₅ means. 5. Compounds according to any one of claims 1 to 4, characterized indicates that L¹ and / or L² is fluorine. 6. Use of compounds of formula I as components liquid crystalline media. 7. Liquid-crystalline medium with at least two liquid-crystalline ones Components, characterized in that there is at least one Contains compound of formula I. 8. Liquid crystal display element, characterized in that it is a contains liquid-crystalline medium according to claim 7. 9. Electro-optical display element, characterized in that it as a dielectric, a liquid-crystalline medium according to claim 7 contains.