PYRIDINE DERIVATIVES AND A LIQUID CRYSTAL MEDIUM

Abstract: The invention relates to pyridine derivatives of formula (I), wherein R<1>, A, Z, r, X, s and R<2> have the meaning given in claim 1, and their use as components for liquid crystal media for electrooptical display elements based on the ECB effect.

Description: PYRIDINE DERIVATIVES AND A LIQUID CRYSTAL MEDIUM The invention relates to pyridine derivatives of the formula I wherein is a straight-chain alkyl, alkoxy, alkenyl or oxaalkyl group with up to 15 C atoms in the alkyl residue, in case of X = F, the R groups may also be branched, is 1, -cyclohexylene or 1,4-phenylene which is unsubstituted or mono- or polysubstituted by F, and wherein one or two CH groups may also be replaced by N, is -CH 2 CH 2 -, -C≡C- or a single bond, is 0, 1 or 2, is 0 or 1, r+s is 1, 2 or 3, 2 R is alkyl, alkenyl, oxaalkyl or alkylcarbonyl with up to 15 C atoms in the alkyl residue and X is H or F, with the proviso that X iε a fluoro atom if R 1 and R2 are straight-chain alkyl groups, r is 1, s iε 0, A is an unsubstituted 1,4-phenylene and Z is a single bond. The invention also relates to liquid crystal media, especially for electrooptical display elements based on the ECB effect, which contain at least one compound of the pyridine derivatives of the formula I. For the sake of simplicity in the following, Phe is an unsubstituted 1, -phenylene group, PheF is a mono- or disubstituted 1, -phenylene group, Cyc is a trans-1,4- cyclohexylene group, Pyd is a pyridine-2,5-diyl group and Pyr is a pyrimidine-2,5-diyl group. The compounds of the formula I can be uεed as components of liquid crystalline media, in particular for displays which are based on the principle of the twisted cell, the guest-host effect, the effect of the deformation of aligned phases or the effect of dynamic scattering. Compounds of the formula I are preferably also suitable for the use as components in liquid crystalline media for displays which are based on the ECB effect. Furthermore, some of the compounds of the formula I are also suitable for the use as components of ST mixtures. Similar pyridine compounds are disclosed, e.g. in WO 87/ 04158 or in DE 38 14346. However, none of these disclosed pyridines show an oxygen atom near the N atom of the pyri¬ dine ring and they do not have the excellent properties of the compounds of the present invention. The ECB (electrically controlled birefringence) effect or DAP (deformation of aligned phases) effect was described for the first time in 1971 (M.F. Schieckel and K. Fahren- schon, "Deformation of nematic liquid crystals with verti- cal orientation in electrical fields", Appl. Phys. Lett. 19 (1971), 3912). Works by J.F. Kahn (Appl. Phys. Lett. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869) followed. The works by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid crystal media must have high values for the ratio of the elastic constants K 3 /K χ , high values for the optical anisotropy Δn and negative values for the dielectrical anisotropy Δε in order to be able to be used for highly informative display elements based on the ECB effect. Electrooptical display elements based on the ECB effect have a homoeotropic edge orientation, that is to say the liquid crystal medium has negative dielectric anisotropy. For technical use of this effect in electrooptical dis¬ play elements, LC media which must meet numerous require¬ ments are required. The chemical stability towards moistur air and physical influences, such as heat, infrared, visib and ultraviolet radiation and constant and alternating ele trical fields, are particularly important here. A liquid crystal mesophase in a suitable temperature range and a low viscosity are furthermore required of technically use¬ ful LC media. In none of the series of compounds with a liquid crystal mesophase which are known to date is there a single compoun which meets all these requirements. Mixtures of two to 25, preferably three to 18, compounds are therefore as a rule prepared in order to obtain substances which can be used as LC media. However, it has not been possible for optimum media to be easily prepared in this manner, since no liquid crystal materials with significantly negative dielectric anisotropy and/or correspondingly high optical anisotropy and/or particularly high values for -/K- and adquate long-term stability have as yet been available. There is thus still a great need for liquid crystal media with favorable meso ranges, high values of K 3 /K_, high optical anisotropy Δn, negative dielectric anisotropy Δε and high long-term stability. Suprisingly, it has been found that the addition of comp- ounds of the formula I produces liquid crystal media which fulfil all the above mentioned criteria very well indeed. In addition, by providing the compounds of the formula I, the whole palette of liquid crystalline substances which are suitable from a variety of technical viewpoints for the preparation of nematic mixtures, has been considerably broadened. The compounds of the formula I have a wide area of appli¬ cation. Depending on the choice of substituents, these compounds can serve as base materials from which the major part of liquid crystalline media are made up; liquid crystalline base materials from other classes of compounds can, however, also be added to compounds of the formula I, for example to optimize the dielectric and/or optical anisotropy of a dielectric of this type. The compounds of the foirmula I are also εuitable as inter¬ mediates for the preparation of other substances which can be used as constituents of liquid crystalline media. The compounds of the formula I are colourless in their pure form and form liquid crystalline meεophases in a temperature range which is favorable for electrooptical use. They are very chemically and thermally stable and are very stable to light. The invention accordingly relates to the compounds of the formula I and the use of these compounds as compo- nents of liquid crystalline media and aε components of liquid cryεtalline media for electrooptical display ele¬ ments based on the ECB effect. The invention furthermore relates to liquid crystalline media, especially for electr optical display elements based on the ECB effect which contain at least one compound of the formula I, and also to electrooptical display elements, which contain media of this type. In the foregoing and in what follows R , - A -, Z, r, X, s and R 2 have the meaning indicated, unless something else is specifically stated. In the following -( O >- is PydX. >- N X The compoundε of the formula I accordingly alεo include compounds of the partial formulae II to 13 (having two rings), 14 to 19 (having three rings) and 110 to 113 (having four rings); 0 1 2 3 The preferred compounds of the partial formula II include those of the partial formulae Ila to lid: R 1 - O >-OCH 2 -Cyc-R 2 Ila Alkyl-PydX-OCH 2 -Cyc-Alkyl lie Alkoxy-PydX-OCH 2 -Cyc-Alkyl lid Among these, those of the formulae Ila and lie are particularly preferred. The preferred compounds of the partial formula 12 include those of the partial formulae I2a to 121: R 1 -Phe-PydX-OR 2 12a R 1 -PheF-PydX-OR 2 I2b R 1 -Cyc-PydX-OR 2 I2c R 1 -Pyd-PydX-OR 2 I2d R 1 -Pyr-PydX-OR 2 I2e Alkoxy-Phe-< O >-OAlkyl 121 N Among these, those of the formulae I2a, I2c, I2g, I2k and 121 are particularly preferred. The preferred compounds of the partial formula 13 include those of the partial formulae I3a to I3v: R 1 -Phe-C≡C-PydX-OR 2 I3a R 1 -PheF-C=C-PydX-OR 2 I3b R 1 -Phe-CH 2 CH 2 -PydX-OR 2 I3c R 1 -Cyc-C=C-PydX-OR 2 I3d R 1 -Cyc-CH 2 CH 2 -PydX-OR 2 I3e R 1 -PheF-CH 2 CH 2 -PydX-OR 2 13f R 1 -Pyd-CH 2 CH 2 -PydX-OR 2 I3g R 1 -Pyr-CH 2 CH 2 -PydX-OR 2 I3h R 1 -Pyd-C≡C-PydX-OR 2 I3i R 1 -Pyr-C≡C-PydX-OR 2 I3j Alkoxy-Phe-C≡C-/ O )-OAlkyl I3k Alkyl-Phe- -CC≡≡CC--< Oθ >>--O(Alkyl 131 Alkyl-Cyc:--CC≡≡CC--( OO >>--OAlkyl I3m I3n Alkyl-Phe-C≡C-( O >-OAlkyl I3o F I3p F Alkoxy-Phe-CH 2 CH 2 -^ O )-OAlkyl I3q Alkyl-Phe-CH 2 CH 2 -/ O )-OAlkyl I3t Alkyl-Cyc-CH 2 CH 2 - 0 )-OAlkyl I3u Alkyl-Cyc-CH 2 CH 2 - O -OAlkyl I3v F Among theεe, thoεe of the formulae I3a, I3b, I3c, I3d, I3e, I3k, 131, I3m, I3n, I3o, I3q, I3ε and I3t are particularly preferred. The preferred compoundε of the partial formula 14 include thoεe of the partial formulae I4a to I4k: R 1 -Phe-PydX-OCH 2 -Cyc-R 2 I4a R 1 -PheF-PydX-OCH 2 -Cyc-R 2 I4b R 1 -Cyc-PydX-OCH 2 -Cyc-R 2 I4c Alkyl-Cyc-PydX-OCH 2 -Cyc-Alkyl I4d Alkyl-Phe-PydX-OCH 2 -Cyc-Alkyl I4e Alkyl-PheF-PydX-OCH 2 -Cyc-Alkyl I4f Alkoxy-Phe-PydX-OCH 2 -Cyc-Alkyl I4g Alkoxy-PheF-PydX-OCH 2 -Cyc-Alkyl I4h R 1 —Pyd-PydX-OCH 2 -Cyc-Alkyl I4i R 1 -Pyr-PydX-OCH 2 -Cyc-Alkyl 14j Alkyl-Phe- O >-OCH 2 -Cyc-Alkyl I4k Among these, those of the formulae I4a, I4b, I4d, I4g and I4k are particularly preferred. The preferred compounds of the partial formula 15 in¬ clude those of the partial formulae I5a to 15j: R 1 -Phe-CH 2 CH 2 -PydX-OCH 2 -Cyc-R 2 I5a R 1 -Phe-C≡C-PydX-OCH 2 -Cyc-R 2 I5b R 1 -PheF-C≡C-PydX-OCH 2 -Cyc-R 2 I5c R-^PheF-C^CH^PydX-OC^-Cyc-R 2 I5d R -Cyc-CH 2 CH 2 -PydX-OCH 2 -Cyc-R 2 I5e R 1 -Cyc-C≡C-PydX-OCH 2 -Cyc-R 2 I5f R 1 -Pyd-C≡C-PydX-OCH 2 -Cyc-R 2 I5g R 1 -Pyr-C≡C-PydX-0CH -Cyc-R 2 I5h R 1-Pyr-CH 2 CH 2 -PydX-0CH 2 -Cyc-R2 I5ι. R 1 -Pyd-CH 2 CH 2 -PydX-OCH 2 -Cyc-R 2 I5j The preferred compounds of the partial formula 16 include those of the partial formulae I6a to I6x: R 1 -Phe-Phe-PydX-OR 2 I6a R 1 -Cyc-Cyc-PydX-OR 2 I6b R 1 -Cyc-Phe-PydX-OR 2 I6c R 1 -Phe-Cyc-PydX-OR 2 I6d R 1 -Pyd-Phe-PydX-OR 2 I e R^-Pyr-Phe-PydX-OR 2 16f • R 1 -Pyd-Cyc-PydX-OR 2 I6g R 1 -Pyr-Cyc-PydX-OR 2 I6h R 1 -Phe-Pyr-PydX-OR 2 I6i R 1 -Phe-Pyd-PydX-OR 2 I6j R 1 -Cyc-Pyd-PydX-OR 2 I6k R 1 -Cyc-Pyr-PydX-OR 2 161 Alkyl-Cyc-Phe ** -<(-O>„>--OAlkyl I6n Alkyl-Phe-Phe-( O >-OAlkyl I6o Alkoxy-Phe-Phe-< O >-OAlkyl I6p Alkyl-Cyc-Phe-( O >-OAlkyl I6q N I6r I6s Alkyl-Cyc-Cyc-( 0 >-OAlkyl I6t N R 1 -PheF-Phe-PydX-OR 2 I6u R 1 -PheF-PheF-PydX-OR 2 I6v R 1 -Phe-PheF-PydX-OR 2 I6w R 1 -Cyc-PheF-PydX-OR 2 I6x Among theεe, thoεe of the formulae I6a, I6b, I6c, I6m, I6n, I6o, I6p, I6q, I6r, I6s, I6w, I6x and I6t are particu¬ larly preferred. The preferred compounds of the partial formulae 17 include those of the partial formulae I7a to I7z: R 1 -Phe.-CH 2 CH 2 -Phe-PydX-OR 2 I7a R 1 -Cyc-CH 2 CH 2 -Phe-PydX-OR 2 I7b R 1 -Pyd-CH 2 CH 2 -Phe-PydX-OR 2 17c R 1 -Pyr-CH 2 CH 2 -Phe-PydX-OR 2 I7d R 1 -Phe-CH 2 CH 2 -Cyc-PydX-OR 2 I7e R 1 -Cyc-CH 2 CH 2 -Cyc-PydX-OR 2 I7f R 1 -Phe-C≡C-Phe-PydX-OR 2 I7g R 1 -Cyc-C≡C-Phe-PydX-OR 2 I7h R 1 -Pyd-C≡C-Phe-PydX-OR 2 I7i R 1 -Pyr-C≡C-Phe-PydX-OR 2 17j R 1 -Phe-C≡C-Cyc-PydX-OR 2 I7k R 1 -Cyc-C≡C-Cyc-PydX-OR 2 171 R 1 -Phe-C≡C-Pyd-PydX-OR 2 I7m R 1 -Phe-C≡C-Pyr-PydX-OR 2 I7n R 1 -Phe-CH 2 CH 2 -Pyr-PydX-OR 2 I7o R 1 -Phe-CH 2 CH 2 -Pyd-PydX-OR 2 I7p Alkyl-Phe-CH 2 CH 2 -Phe- O )-OAlkyl I7q Alkoxy-Phe-CH CH»-Phe-( O >-OAlkyl I7r Δ Δ \ N I7s Alkoxy-Phe-CH 9 CH,-Phe-( O ) -OAlkyl I7t N Alkyl-Cyc-CH 2 CH 2 -Phe- O )-OAlkyl I7u R 1 -PheF-Z-Phe-PydX-OR 2 I7w R 1 -PheF-Z-PheF-PydX-OR 2 I7x R 1 -Phe-Z-PheF-PydX-OR 2 I7y R 1 -Cyc-Z-PheF-PydX-OR 2 I7z Among these, those of the formulae I7a, I7b, I7e, I7f, I7g, I7h, I7k, I7q, I7r, I7s, I7t, I7u, I7w, I7y, I7v and I7z are particularly preferred. The preferred compounds of the partial formulae 18 include thoεe of the partial formulae I8a to I8p: R 1 -Phe-CH 2 CH 2 -Phe-Z-PydX-OR 2 I8a R 1 -Phe-C≡C-Phe-Z-PydX-OR 2 I8b R 1 -Phe-Z-Cyc-Z-PydX-OR 2 I8c R 1 -Cyc-Z-Cy:-CH 2 CH 2 -PydX-OR 2 I8d R 1 -Cyc-C≡C-Phe-Z-PydX-OR 2 I8e R 1 -Pyd-Z-Phe-Z-PydX-OR 2 18f R 1 -Pyr-Z-Phe-Z-PydX-OR 2 I8g R 1 -Cyc-Z-Pyd-Z-PydX-OR 2 I8h R 1 -Phe-Z-Pyr-Z-PydX-OR 2 I8i R 1 -Phe-Z-Pyd-Z-PydX-OR 2 I8j R 1 -Cyc-CH 2 CH 2 -Pyr-Z-PydX-OR 2 I8k R 1 -PheF-Z-Phe-Z-PydX-OR 2 181 R 1 -PheF-Z-PheF-Z-PydX-OR 2 I8m R 1 -Phe-Z-Phe^-Z-PydX-OR 2 I8n R 1 -PheF-Z-Cyc-Z-PydX-OR 2 I8o R 1 -Cyc-Z-PheF-Z- ydX-OR 2 I8p The preferred compoundε of the partial formula 19 include thoεe of the partial formulae I9a to I9ab: R 1 -Phe-Phe-CH 2 CH 2 -PydX-OR 2 19a R 1 -Phe-Phe-C≡C-PydX-OR 2 I9b R 1 -Phe-Cyc-C≡C-PydX-OR 2 I9c R 1 -Phe-Cyc-CH 2 CH 2 -PydX-OR 2 I9d R 1 -Pyd-Phe-C≡C-PydX-OR 2 I9e R 1 -Pyr-Phe-CH 2 CH 2 -PydX-OR 2 I9f R 1 -Cyc-Phe-CH 2 CH 2 -PydX-OR 2 I9g R 1 -Cyc-Phe-C≡C-PydX-OR 2 I9h R 1 -Cyc-Cyc-C≡C-PydX-OR 2 I9i R 1 -Cyc-Cyc-CH 2 CH 2 -PydX-OR 2 19j Alkyl-Phe-Phe-C≡C- O >-OAlkyl I9k Alkyl-Phe-Phe-CH 2 CH 2 - O >-° Al kyl 191 Alkyl-Cyc-ήie --CC≡≡CC--/ OO )>--OAlkyl 19m Alkyl-Cyc-Cyc--CC≡≡CC--( OO >)--OAlkyl I9n Alkyl-Cyc-Phe-CH 2 CH 2 -/ O )-OAlkyl I9o Alkyl-Cyc-Cyc-CH 2 CH 2 - O )-OAlkyl 19p Alkoxy-Phe-Phe-C≡C-PydX-OAlkyl I9q Alkyl-Cyc-Cyc-C≡C-( O )-OAlkyl I9u N Alkyl-Cyc-Phe-C≡C-( O >-OAlkyl I9v N R 1 -PheF-Phe-Z-PydX-OR 2 I9x R 1 -PheF-PheF-Z-PydX-OR 2 I9y R^-Phe-PheF-Z-PydX-OR 2 I9z R 1 -Cyc-PheF-Z-PydX-OR 2 I9aa R 1 -PheF-Cyc-Z-PydX-OR 2 I9ab The preferred compounds of the partial formulae 110 to 113 include those of the partial formulae 110a to IlOr: R 1 -Phe-Phe-PydX-OCH 2 -Cyc-R 2 110a R 1 -Cyc-Phe-PydX-OCH 2 -Cyc-R 2 I10b R 1 -Cyc-Cyc-PydX-OCH 2 -Cyc-R 2 ilOc R 1 -Phe-CH 2 CH 2 -Phe-PydX-OCH 2 -Cyc-R 2 IlOd R 1 -Phe.-C≡C-Phe-PydX-OCH 2 -Cyc-R 2 IlOe R 1 -Cyc-Z-Phe-PydX-OCH 2 -Cyc-R 2 IlOf R 1 -Cyc-Cyc-CH 2 CH 2 -PydX-OCH 2 -Cyc-R 2 IlOg R 1 -Phe-Phe-Z-PydX-OCH 2 -Cyc-R 2 IlOh R 1 -Cyc-Phe-Z-PydX-OCH 2 -Cyc-R 2 IlOi R 1 -Cyc-Z-Phe-PydX-OCH 2 -Cyc-R 2 110j R 1 -Phe-Z-Phe-CH 2 CH 2 -PydX-OCH 2 -Cyc-R 2 I10k R 1 -PheF-Phe-PydX-OCH 2 -Cyc-R 2 1101 R 1 -PheF-PheF-PydX-OCH 2 -Cyc-R 2 I10m R 1 -Phe-PheF-PydX-OCH 2 -Cyc-R 2 IlOn R 1 -Cyc-PheF-PydX-OCH 2 -Cyc-R 2 IlOo R 1 -Cyc-PheF-Z-PydX-OCH 2 -Cyc-R 2 I1Op R 1 -Cyc-Z-PheF-PydX-OCH 2 -Cyc-R 2 IlOq R 1 -PheF-Phe-Z-PydX-OCH 2 -Cyc-R 2 IlOr In the compounds of the formulae above and below R is preferably a straight-chain alkyl, alkoxy or oxaalkyl group. If X is a fluor atom, then R is preferably also a branched alkyl, alkoxy or oxaalkyl group. R 2 i.s preferably an alkyl, oxaalkyl or alkylcarbonyloxy group, which also may be branched. Furthermore, compounds of the formula I are preferred in which R 1 and/or R2 are alkenyl groups. However, straight-chain alkyl groups are particularly preferred. A is preferably 1,4-cyclohexylene or 1,4-phenylene, which can be substituted by fluorine (PheF). PheF in the compound of the formulae above and below has the following preferred meanings: Furthermore, compounds of formula I are preferred in which A denotes 1,4-phenylene wherein one or two CH groups are replaced by N atoms. Accordingly pyridine-2,5-diyl and pyrimidine-2,5-diyl groups are particularly preferred. Z is preferably a εingle bond or a -CH 2 CH 2 - group. Alεo preferred is -C≡C-. r is preferably 1 or 2, ε iε 0 or 1, preferably 0, and r+ε iε preferably 1 or 2. X is H or F, preferably H. When r is 2 the two rings A may be identical or different from one another. Ir R 1 and/or R2 are alkyl radi.cals i.n whi.ch one ("alkoxy" or "oxaalkyl") CH_ group can also be replaced by an O atom, they can be straight-chain or branched. They are preferably straight-chain having 1, 2, 3, 4, 5, 6, 7, 8 or 9 C atomε and are accordingly preferably methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, ethoxy, propoxy, butoxy pentoxy, hexoxy, heptoxy, octoxy, nonoxy, 2-oxapropyl (= met oxymethyl), 2- (= ethoxymethyl) or 3-oxabutyl (= 2-methoxy- ethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, furthermore decyl, uridecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, decoxy, undecoxy, dodecoxy, tridecoxy, tetradecoxy, pentadecoxy, 2-, 3-, 4-, 5.-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 7-oxanonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl. Compounds of the formula I having branched wing groups R 2 or R can occasionally be of significance because of their better, solubility in the customary liquid crystalline base materials, but in particular as chiral dopes, if they are optically active. Branched groups of this type do not, as a rule, contain more than one chain branch. Preferred branched radicals are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), 2-methylbutyl, isopentyl (= 3-methyl- butyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, 2-octyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methyl- pentoxy, 2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy (= 2-octyloxy), 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl, 6-methyloctoxy, 6-methyloctanoyl, 5-methylheptylcarbonyl, 2-methylbutyryl, 3-methylvaleryl, 4-methylhexanoyl, 2-methyl-3-oxapentyl or 2-methyl-3-oxahexyl. For compoundε having branched wing groups, formula I en¬ compasses both the optical antipodes and racemates and also mixtures thereof. Compounds of the partial formulae I2a, I2k, 121, I6a, I6r and I6s having branched or/and optical active radicals are particularly preferred. If R1 and/or R2 are an alkenyl radical the tranε form iε preferred. Thiε alkenyl radical may be εtraight-chain or branched. It is preferably straight-chain and has 2 to 9 C atoms. Accordingly, it is particularly 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. Preferred compounds of the formula I and their εubformulae are thoεe in which at leaεt one of the radicals contained therein haε one of the indicated preferred meanings. The compounds of the formula I are prepared by methods known per se, aε are deεcribed in the literature (for example, in the standard reference workε such aε Houben- Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag, Stuttgart), and, indeed, under reaction conditionε which are known and suitable for the reactions mentioned. Vari¬ ants may alεo be used which are known per se but are not mentioned in more detail here. If desired, the starting substances can also be formed in situ, in εuch a manner that they are not iεolated from the reaction mixture, but are immediately reacted further to the compoundε of the formula I. Preferred routeε for the εyntheεiε of compoundε of the formula la are εhown in the following reaction εchemes. A preferred route for the preparation of compounds of the formulae 12, 14, 16, 17, 110 and 111, wherein the ring A adjacent to the pyridine is an aromatic ring, is shown in scheme 1. In the following εchemeε R3 meanε R-? or -CH_-Cyc-R . LDA means lithium-diiεopropylamide. Scheme 1 OR- A preferred route for the synthesis of compounds of the formulae 13, 15, 18, 19, 112 and 113 iε shown in scheme 2 Scheme 2 b ) -4- H, Pd-C A preferred route for the synthesis of compounds of the formulae 12, 14, 16, 17, 110 and 111 wherein the ring A adjacent to the pyridine is 1,4-cyclohexylene, iε shown in scheme 3. Scheme 3 1. Lithium/ZnBr 2 2. coupling with Br -{ - OR~ Pd catalyst \—N A preferred route to syntheεize the pyridine productε according to scheme 1 but having a fluorine subεtituent at the pyridine ring (X = F) iε εhown in εcheme 4. Scheme 4 + NaOR~ A preferred route for the synthesis of compoundε of the formulae 13, 15, 18, 19, 112 and 113 having a fluorine εubεtituent of the pyridine ring (X = F) iε given in Scheme 5. catalysts : PdCl 2 (P 3 P ) 2 CuJ A preferred route for the synthesis of products according to scheme 3 but having a fluorine substituent at the pyri¬ dine ring is shown in εcheme 6. Scheme 6 . lithium/ZnBr . coupling with F NaOR ~ The compounds of the formula II can be prepared, for example, by coupling Br-^O^-OCH,-Cyc-R with R Br in the presence of lithium/ZnBr 2 and a catalyst, or by coupling (HO) 2 B-^0 -F with R Br in the presence of a F Pd catalyεt and subsequent reaction with NaOCH 2 ~Cyc-R . All starting materials are known or can be prepared in analogy to known compounds. Other routeε are apparent to the skilled worker. All these steps and the corresponding reaction conditions are known to the skilled worker. The liquid crystal media according to the invention for electrooptical display elements based on the ECB effect contain at least one compound of the formula I, these substances have a significantly negative dielectric anisotropy. In addition to one or more compounds of the formula I (A) the liquid crystal media preferably contain components (B) having a pronounced nematogenicity and a low viscoεity, and if appropriate further components (C) with a high clear point or components (D) with not more than a weakly positive dielectric anisotropy of at least 0.2. These additional components are preferably chosen from the following group of formulae II to VI R 1 -(A 0 -Z°) -A-R 2 II wherein R 1 and. R2 i-n each case independently of one another are an alkyl group with in each case 1 to 15 C atoms, wherein one or more CH_ groups can also be re¬ placed by a grouping chosen from the group comprising -0-, -S-, -CO-, -CH-halogen-, -CHCN-, -0-CO-, -0-COO-, -CO-O- and -CH=CH- or also by a combination of two suitable groupings, two hetero atoms not being linked directly to one another, is CN or one of these groups wherein one or more CH 2 groups is/are replaced by 0 and/or S or ali¬ phatic and/or aromatic CH groups are replaced by N, A in each caεe independently of the other radi¬ cals A is 1,4-cyclohexylene which is unsub¬ stituted or mono- or polysubεtituted by halogen, CH 3 groups and/or nitrile groups, wherein one or two non-adjacent CH 2 groups can also be I replaced by -O- and/or -S- and/or a -CH-CH-- 1 grouping can be replaced by -C=N- (Cy), or 1,4- phenylene which is unsubstituted or mono- or polysubstituted by halogen atoms, CH_ groups and/or nitrile groups, wherein one or more CH groups can also be replaced by N (Ph), and one of the radicals A° iε also 2,6-naphthylene (Na) or tetrahydro-2,6-naphthylene (4H-Na), optionally substituted by halogen or CN, Z in each case independently of the other radicals Z° iε -CO-0-, -0-CO-, -CH 2 θ7 -OCH 2 -, -CH 2 CH 2 - -CHCN-CH 2 -, -CH 2 -CHCN- or a single bond, and p is 1, 2 or 3, or in the case where A = tetra- or octahydrophenanthrene, alεo 0, wherein, in the caεe where A = " at least one group Z° is -CHCNCH 2 ~ or -CH 2 CHCN- and/or at least one CH 2 group in at least one of the groups R and R is replaced by -CHCN-. R 1- A 5. 2 1. A 6. R 2 III wherein R 1 and R2 each independently of one another are an alkyl group with in each case 1 to 15 C atoms, where- in one or more CH 2 groups can also be replaced by a grouping chosen from the group comprising -0-, -S-, -CO-, -CH-halogen-, -CHCN-, -0-CO-, -O-COO-, -CO-O- and -CH=CH- or also by a combination of two εuitable groupings, two hetero atomε not being linked directly to one another, Z 1 iε -CO-0-, -0-CO-, -CH 2 CH 2 -, -OCH 2 -, -CH 2 0- or a εingle bond and A and A in each caεe independently of one another are trans-1,4-cyclohexylene, or 1,4-phenylene which iε unεubεtituted or substituted by fluorine. R 3 -(A 1 -Z 1 ) I m„-A-(Z 2 -A 2 )n„-R 4 IV wherein R 3 and R4 i.n each case independently of one another are an alkyl group with in each case 1 to 15 C atomε, wherein one or more CH 2 groups can also be replaced by a grouping chosen from the group comprising -0-, -S-, -CO-, -CH-halogen-, -CHCN-, -0-CO-, -O-COO-, -CO-O- and -CH=CH- or by a combination of two suitable groupings, two hetero atoms not being linked directly to one another, A1 and A2 i.n each case independently of one another are 1,4-cyclohexylene which is unsubεtituted or morip- or polyεubstituted by halogen atoms, CH 3 groups and/or nitrile groups, wherein one or two.non-adjacent CH 2 groups can also be I replaced by -O- and/or -S- and/or a -CH-CH.- I grouping can be replaced by -C=N- (Cy), or 1,4- phenylene which is unsubstituted or mono- or polysubεtituted by halogen atomε, CH 3 groupε and/or nitrile groups, wherein one or more CH groups can also be replaced by N (Ph), and one of the radicals A 1 and A2 is also 2,6-naphthylene (Na) or tetrahydro-2,6-naphthylene (4H-Na), optionally substituted by halogen or CN, A is 2-fluoro-l,4-phenylene, 2,3-difluoro-l,4- phenylene or 4,4 f -biphenylyl which is mono- or polyεubstituted in the 2-, 3-, 2 1 - and/or 3'-position by fluorine, Z and Z^ are each -CO-O-, -0-CO-, -CH 2 CH 2 -, -0CH 2 -, -CH 2 0- or a single bond, m ic 1 or 2 and n iε 0 or 1, wherein, where m = 2, the two groupε A and Z can be identical or different from one another. R 3 - (A 3 -Z 1 ) 0 -Q 1 -C≡C-Q 2 - ( Z 2 -A 4 ) p -R 4 V wherein R 3 and R4 in each case i.ndependently of one another are an alkyl group with in each case 1 to 15 C atoms, wherein one or more CH 2 groupε can alεo be replaced by a grouping choεen from the group compriεing -0-, -S-, -CO-, -CH-halogen-, -CHCN-, -0-CO-, -O-COO-, -CO-O- and -CH=CH- or alεo by a combination of two suitable groupingε, two hetero atomε not being linked directly to one another, Q 1 and 2 in each case independently of one another are 1,4-phenylene which iε unεubstituted or mono- or polysubstituted by halogen atoms, CH 3 groups and/or nitrile groups, and one of the groups Q 1 and Q2 is also pyridine-2,5-diyl, A 3 and A4 in each case independently of one another are trans-1,4-cyclohexylene, wherein one or two non-adjacent CH 2 groupε can also be replaced by -O- and/or -S-, or 1,4-phenylene, wherein one or more CH groups can also be replaced by N, o and p in each case independently of one another are 0 or 1, and Z 1 and Z2 have the meaning given in the case of formula IV. 3 1 1 1 5 R - (A -Z X ) m -Q -C≡C-R VI wherein R 5 to 15 C atoms and R 3, A1, Z1, mentioned meaning. The media according to the invention are prepared in a manner which is customary per se. As a rule, the desired amount of the components used in a relatively εmall amount are diεεolved in the component which makes up the main constituent, advantageously at elevated temperature. It is also possible to mix solutions of the componentε in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing. The dielectrics can also contain other additives which are known to the expert and are described in the litera¬ ture- For example, 0-15 % of pleochroic dyestuffs can be added, as well as conductive salts, preferably ethyl- dimethyldodecylammonium 4-hexoxybenzoate, tetrabutyl- ammonium tetraphenylboranate or complex saltε of crown etherε (compare, for example, Haller et al., Mol. Cryst. Liq. Cryst. Volume 24, pages 249-258 (1973)), to improve the conductivity, or substances to modify the dielectric anisotropy, the viscosity and/or the orientation of the nematic phaseε. Such substanceε are described, for example, in DE-OS 2,209,127, 2,240,864, 2,321,632, 2,338,281, 2,450,088, 2,637,430 and 2,853,728. The individual compounds of the formulae II to VI are either known or have preparation procedures which can easily be deduced from the prior art by the appropriate expert, since they are based on standard procesεes des¬ cribed in the literature. The liquid cryεtal media according to the invention preferably conεiεt of 2 to 15, preferably 3 to 18, components. J^ε well aε compoundε of the formulae I to VI, other constituentε can also additionally be present, for example in an amount of up to 45 % of the total mixture, but preferably up to 34 % and in particular up to 10 %. The media according to the invention preferably contain 1 to 40 %, in particular 5 to 30 % of the compounds according to the invention. Media containing more than 40 %, in particular 45 to 90 % of the compounds according to the invention, are further preferred. The media con¬ tain preferably 3, 4 or 5 compounds according to the invention. The content of the compounds of the formulae II to VI can be chosen in each case in a broad range to give totally 10 to 90 %, in particular 20 to 80 %. The additional constituentε mentioned above are preferably choεen from the nematic or nematogenic (monotropic or isotropic) εubstances; in particular from the classes of azoxybenzenes, benzylideneanilines, biphenylε, terphenylε, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylateε, phenyl or cyclohexyl cyclohexyl- benzoates, phenyl or cyclohexyl cyclohexylcyclohexane- carboxylates, cyclohexylphenylbenzoateε, cyclohexyl- phenyl cyclohexanecarboxylateε, cyclohexylphenyl cyclo- hexylcyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, cyclohexylcyclohexeneε, cyclo- hexylcyclohexylcyclohexene, 1,4-bis-cyclohexylbenzenes, 4,4'-biε-cyclohexylbiphenylε, phenyl- or cyclohexyl- pyriraidines, phenyl- or cyclohexylpyridineε, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-l,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethaneε, 1-phenyl- 2-cyclohexylethaneε, l-cyclohexyl-2-(4-phenyl-cyclohexyl)- ethaneε, l-cy-clohexyl-2-biphenylethaneε, l-phenyl-2-cyclo- hexyl-phenylethanes, optionally halogenated εtilbeneε, benzyl phenyl ethers, tolaneε and substituted cinnamic acids. The 1,4-phenylene groupε of these compounds may be fluorinated. The most important compounds which are posεible consti- tuents of liquid crystal media according to the invention can be characterized by the formulae 1, 2, 3, 4 and 5: R'-L-U-R" 1 R ! -L-COO-U-R" 2 R'-L-OOC-U-R" 3 R « -L-CH 2 CH 2 -U-R" 4 R'-L-C≡C-U-R" 5 In the formulae 1, 2, 3, 4 and 5 L and ϋ may be equal or different from each other. L and U independently from each other denote a bivalent residue selected from the group consiεting of -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe-, -G-Cyc- and their mirror imageε; in thiε compilation of residues Phe denotes un- substituted or fluorinated 1,4-phenylene, Cyc trans- 1,4- cyclohexylene or 1,4-cyclohexenylene, Pyr pyrimidine-2,5- diyl or pyridine-2,5-diyl, Dio 1,3-dioxane-2,5-diyl and G 2-(trans-l,4-cyclohexyl)-ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl. One of the residues L and U is preferably Cyc, Phe or Pyr. U preferably denotes Cyc, Phe or Phe-Cyc. The liquid crystal medi^ according to the invention preferably con¬ tain one or more components selected from the compounds of formulae 1, 2, 3, 4 and 5 with L and U meaning Cyc, Phe and Pyr, said liquid crystal media further containing at the same time one ore more components selected from the compounds of formulae 1, 2, 3, 4 and 5 with one of the residueε L and U denoting Cyc, Phe and Pyr and the other reεidue being selected from the group consiεting of -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Cyc-, εaid liquid cryεtal media containing in addition to thiε optionally one or more componentε selected from the compounds of formulae 1, 2, 3, 4 and 5 with L and U being εelected from the group conεiεting of -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc. Jn a preferred εubgroup of the compounds of formulae 1, 2, 3, 4 and 5 (subgroup 1) R' and R" are independent¬ ly from each other alkyl, alkenyl, alkoxy, alkenoxy with up to 8 carbon atoms. R' and R" differ from one another in most of these compounds, one of the residueε uεually being alkyl or alkenyl. In another preferred subgroup of the compounds of formulae 1, 2, 3, 4 and 5 (subgroup 2) R" denotes -CN, -CF 3 , -F, -Cl or -NCS while R 1 has the meaning indicated in εubgroup 1 and iε preferably alkyl or alkenyl. Other variantε of the enviεaged εubstituents in the compounds of formulae 1, 2, 3, 4 and 5 are alεo customary. Many such substances are commercially available. All these substanceε are obtainable by methods which are known from the literature or by analogous methods. The liquid crystal media according to the invention pre¬ ferably contain in addition to components selected from subgroup 1 also components of subgroup 2, the percentage of these components being as follows: εubgroup 1: 20 to 90 %, in particular 30 to 90 % subgroup 2: 10 to 50 %, in particular 10 to 50 % In theεe liquid cryεtal media the percentageε of the com¬ poundε according to the invention and the compounds of sub¬ group 1 and 2 and further the compounds of the formulae II to VI may add up to give 100 %. The following examples are intended to illustrate the invention without limiting it. Percentageε above and below are percentages by weight; all the temperatures are quoted in degree Celsius. Further are: C: crystalline-solid state, S: smectic phase (the index denoting the type of smectic phase), N: nematic phase, Ch: cholesteric phase, I: iεotropic phase. The number between two symbols indicates the transition temperature in degree Celsius. Example 1 A mixture of 0.05 m 4-pentylphenyl boronic acid (obtained by reacting 4-pentylbiphenyl-4'-yl bromide with n-butyl- lithium and trimethylborate at low temperatures and subse¬ quent hydrolyzing with HC1/H 2 0), 0.05 m of 5-bromo-2-ethoxy pyridine (obtained by reaction of 2,5-dibromopyridine with NaOC - -), 1,25 g (0.001 m) of tetrakiε (triphenylphosphine) palladium (Pd (Ph 3 P) 4 ), 100 ml of toluene, 40 ml of ethanol and 50 ml of sodium carbonate solution (2 m) is stirred an refluxed for 3 hours. After cooling the organic layer iε separated, washed with water and the solvent evaporated. The crude 2-erhoxy-5-(4-pentylbiphenyl-4'-yl)pyridine is purified by chromatography on silica and/or crystallizatio The following compounds of the formulae VII and VIII arran in the tables 1 and 2 are obtained analogously: Table 1 m A Z A * n m n m A Z A ' n Further combinations of m and n are certainly posεible and εuch compounds are obtained analogously. Table 2 m n m A Z A' n m n Further combinations of m and n are certainly possible and such compounds are obtained analogously. Example 2 0.05 m of ethylmagnesium bromide in ether are added to a mixture of 0.05 m of 4-propylphenylacetylene at 0-10°. The mixture iε εtirred for 4 hours at room temperature and then a solution of 0.05 m of 2-ethoxy-5-bromopyridine and 0.5 g of NiCl 2 (Ph 3 P) 2 in ether is added. After stirrin for 6 hours at room temperature and cuεtomary work-up l-(4-propylphenyl)-2-(2-ethoxy-pyridine-5-yl)-acetylene is obtained. The following compounds of the formulae IX and X listed in the table 3 are obtained analogously, whereby further combinationε of m and n are certainly poεεible. Table 3 < IX > C m H 2m + l-€^ 2 - A, - C≡C {^-° C n H 2n + l and X > C m H 2m + l-°-<I>- Z - A '' C≡C °} N - 0C n..-.:2n+l m n m n Hydrogenation of 1-(4-propylbiphenyl-4'-yl)-2-(2-ethoxy- pyridine-5-yl)acetylene in THF in the presence of Pd-C at room temperature and under normal presεure yieldε after cuεtomary work-up l-(4-propylbiphenyl-4'-yl)-2-(2-ethoxy- pyridine-5-yl)ethane. The following compounds of the formulae XI and XII listed in the table 4 are obtained analogously and further comp¬ ounds having different combinations of m and n are certainly obtainable analogously. Table 4 < XI > C m H 2m^i<5 - Z" ®- CH 2 CH 2 θ OC n H 2n + l < XI τ > C m H 2m + l-° ^ Z" ^ CT 2 CH 2-<°^ OC n H 2n + l m A Z A 1 n 5-(4-Ethylphenyl)-pyridine-2-yl-(trans-4-propylcyclohexyl- methyl)ether iε obtained analogously to example 1 by react¬ ing 4-ethylphenyl boronic acid with 5-bromo-2-(trans-4- propylcyclohexylmethyloxy)-pyridine. The following compounds of the formulae XIII and XIV arranged in table 5 are obtained analogously. Table 5 (XIV) C m H 2m+1 -θ ^Z^T {7 N >-OCH 2 -Cyc-C n „H2n+l m n 2 2 2 2 3 4 5 2 Cyc m n The definitions of m and n in Table 5 may be combined at discretion. Example 5 0.02 m of trans-4-(tranε-4-propylcyclohexyl)-cyclohexyl- bromide, 0.04 m of lithium, 0.01 m of ZnBr 2 , 40 ml of toluene and 12 ml of THF are stirred at 0-10° for 2 hours under ultraεonic. Then 0.02 m of 5-bromo-2-ethoxy-pyridine and 0.5 g Pd (Ph 3 P) 4 as the catalyst are added, and the mixture is stirred for further 4 hours at room temperature. Customary work-up yields 2-ethoxy-5-[trans-4-(trans-4-propyl- cyclohexyl)cyclohexyl]-pyridine. The following compoundε of the formula XV liεted in table 6, the formula XVI listed in table 7 and the formula XVIII in table 8 are obtained analogously. The definitions of m and n in these tables may be combined at discretion. . . . Table 6 <**> c m^ + l^^- Z) r- cyc -0 x— N 'OC n H : 2n+l m n Table 7 m n Table 8 Example 6 a) 0.6 of n-butyllithium are added to 0.6 m of diiso- propylamine in 600 ml of THF at -10°. Then the mix¬ ture is cooled to -60 to -70° and 0.6 m of 2,6-di- fluoropyridine are added. The mixture is stirred for further 15 minutes and 0.6 m of trimethylborate is added at -60°. The mixture is allowed to warm to -30° and is hydrolyzed by addition of 170 ml of HCl (25 %) and 100 ml of H 2 0. Ether is added and after customary work-up 2,6-difluoropyridine-3-boronic acid iε obtained. b) A mixture of 0.05 m of 4-(trans-4-pentylcyclohexyl)- bromobenzene, 0.06 m of 2,6-difluoropyridine-5-boronic acid, 100 ml of toluene, 40 ml of ethanol, 1.25 g of Pd(Ph 3 P) 4 and 50 ml of a solution of Na 2 C0 3 (2 m) iε εtirred and heated for hourε at 60°. After custom- ary work-up and recryεtallization from ethanol 2,6- difluoro-3-[4-(tranε-4-pentylcyclohexyl)phenyl]pyri¬ dine with C 58° N 107° I iε obtained. c) 0.06 m of NaH iε added to a εuεpension of 0.06 m of ethanol in 50 ml THF at 20-25°. When the gas develop¬ ment has finished 0.03 m of the above 2,6-difluoro- pyridine derivative iε added. After 15 minutes the mixture is poured in 150 ml of H_0, extracted with petroleum ether and worked up as usual. Recrystalli- zation from methanol/ethanol yields 6-ethoxy-2-fluoro- 3-[trans-4-pentylcyclohexyl)phenyl)pyridine with C 72° N 147.3° I. The following compounds of the formulae XVIII and XIX listed in the tables 9 and 10 are obtained analogously. The definitions of m and n in these tables may be combined at discretion in order to obtain further compounds of these types. Table 9 XVIIΪ n m A Z A' S n m A Z A* S n 2 4 3 8 3 - 5 4 6 2 Phe 2 Phe 1 Phe 4 Phe m n a) Analogouεly to example 6 a) 2,6-difluoropyridine is reacted with lithiumdiisopropylamide. Then J 2 is added to the mixture at -60° to -70° and cuεtomary work-up yieldε 2,6-difluoro-3-iodopyridine. b) 0.2 mM of PdCl 2 (Pb 3 P) 2 and 0.1 m of CuJ are added to a suspenεion of 0.01 M of 2,6-difluoro-3-iodopyri- dine, 0.01 M of 4-pentylphenylacetylene and 200 ml of triethylamine. The mixture iε stirred for 8 hours at room tempera¬ ture and customary work-up yields l-(4-pentlyphenyl)- 2-(2,6-difluoropyridine-3-yl)-acetylene. Thiε compound is transferred into l-(4-pentylphenyl)- 2-(6-ethoxy-2-fluoropyridine-3-yl)-acetylene analo¬ gouεly to example 6 c). The following compounds of the formulae XX and XXI listed in the table 11 are obtained analogously. The definitions of m and n may be combined at discretion. Table 11 XXI C m H 2m + l-°-<£)- Z" < m n m n Analogously to example 3 l-(4-pentylphenyl)-2-(6-ethoxy- 2-fluoropyridine-3-yl)-acetylene is hydrogenated to ob¬ tain 1-(4-pentylphenyl)-2-(6-ethoxy-2-fluoropyridine-3- yl)ethane. The following compounds of the formulae XXII and XXIII listed in table 12 are obtained analogously. The defini- tions of m an n may be combined at diεcretion. Table 12 m n 2 4 3 . 5 3 Phe 4 Phe m A Z A » n Analogously to example 5 2,6-difluoro-3-[tranε-4-(tranε- 4-propylcyclohexyl)cyclohexyl]pyridine iε obtained by reacting 2,6-difluoro-3-iodopyridine with trans-4-(trans- 4-propylcyclohexyl)cyclohexylbromide. 6-Ethoxy-2-fluoro- 3-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pyridine is then obtained analogously to example 6 c). The following compounds of the formulae XXIV and XXV listed in the tables 13 and 14 are obtained analogously. The definitions of m and n may be combined at discretion. Table 13 m r Z s n >s- C n H 2n + l m n A liquid crystalline medium consisting of a) 90 % of a mixture N consisting of 22 % trans-l-p-ethylphenyl-4-propylcyclo- hexane, 20 % tranε-l-p-methoxyphenyl-4-propylcylo- hexane, 15 % tranε-l-p-ethoxyphenyl-4-propylcyclo- hexane, 19 % 4-ethyl-4 l -(tranε-4-propylcyclohexyl)- biphenyl, 14 % 4-ethyl-4'-(tranε-4-pentylcyclohexyl) biphenyl, 5 % 4-(trans-4-pentylcyclohexyl)-4'-(trans- 4-propylcyclohexyl)biphenyl, and 5 % 4,4'-bis-(trans-4-propylcyclohexyl)- biphenyl and b) 10 % o 2-octyloxy-5-(3-fluoro-4-octyloxyphenyl)- pyridine haε a clearing point of 65.3°, Δε = -0.06 and the viεco- εity iε 14.3 mm 2 /εec at 20°. Example 11 A liquid cryεtalline medium conεiεting of a) 90 % of a mixture N (deεcribed in example 10) and b) 10 % of 2-octyloxy-5-(4-octyloxyphenyl)-pyridine haε a viεcoεity of 14.3 mm 2 /εec at 20° and a clearing point of 69.5°.

Claims: _ Γ-J _ Patent claims 1. Pyridine derivatives of the formula I wherein R is a straight-chain alkyl, alkoxy, alkenyl or oxaalkyl group with up to 15 C atoms in the alkyl residue, in case of X = F, the R groups may also be branched. is 1,4-cyclohexylene or 1,4-phenylene which is unsubstituted or mono- or polysubstituted by F, and wherein one or two CH groups may also be replaced by N, is -CH 2 CH 2 -, -C≡C- or a single bond, is 0, 1 or 2, is 0 or 1, r+ε is 1, 2 or .3, -_ 5 " 3 - 2 R is alkyl, alkenyl, oxaalkyl or alkylcarbonyl with up to 15 C atomε in the alkyl residue and X is H or F, with the proviεo that X iε a fluoro atom if R 1 and R2 are εtraight-cham alkyl groups, r is 1, s i A is an unsubεtituted 1,4-phenylene and Z iε a εingle 2. Pyridine derivativeε of claim 1 of the formulae la to 3. Pyridine derivativeε of claim 1 of the formula Id 4. Pyridine derivativeε of claim 1 of the formulae le to I X X 5. Pyridine derivatives of claim 1 of the formulae Ij to In R t i --0( H >-- ( CH 2 CH 2 -<: O >-OR 2 In X - 1 _ 6. Pyridine derivatives of claim 1 of the formulae lo to I ιl O-i 0 >--C C H H 2C C H H 2--<<2O >>--(< O° >-OR 2 lo X Pyridine derivatives of claim 1 of the formulae Iq to I 8. Pyridine derivatives of at least one of claims 1 to 7 wherein X is H. 9. Pyridine derivatives of at least one of claims 1 to 7 wherein X is F. 10. Pyridine derivatives of at least one of claims 1 to 7 wherei .*n R1 and R2 are each alkyl with 2 to 9 C atoms. 11. Liquid crystalline medium being a mixture of at leaεt two compounds, characterized in that at leaεt one compound is a pyridine derivative according to claim 1. 12. Liquid crystalline medium being a mixture of at least two compounds for electrooptical display elements based on the ECB effect, characterized in that at least one compound is a pyridine derivative according to claim 1. 13. Electrooptical display element, characterized in that it contains a liquid crystalline medium according to claim 11. 14. Electrooptical display element on the ECB effect, characterized in that it contains a liquid crystalline medium according to claim 12.

Inventors: MEYER VOLKER, PLACH HERBERT, POETSCH EIKE, REIFFENRATH VOLKER, MEYER, VOLKER, PLACH, HERBERT, POETSCH, EIKE, REIFFENRATH, VOLKER
Applicants: MERCK PATENT GMBH, MERCK PATENT GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG
Application year: 1990
Main classification: C07D213
IPC codes: C07D213, C09K19