FLUORINATED OLIGOPHENYL DERIVATIVES

Abstract: The invention relates to chiral tilted smectic liquid crystal media comprising at least two liquid crystalline compounds, wherein at least one compound is of formula (I), wherein R<1>, R<2>, L<1>, L<2>, L<3>, L<4>, L<5>, L<6> and n have the meanings indicated in Patent Claim 1 and to ferroelectric display devices which contain such media.

Description: Fluorinated Oligophenyl Derivatives The invention relates to chiral tilted smectic liquid crystal medium comprising at least two liquid crystalline compounds, wherein at least one compound is of the formula I wherein R 1 and R2 are each alkyl with 1-15 C atoms, and wherein one or two non-adjacent CH- groups can also be replaced by a bridging element selected from the group consisting of -0-, -S-, -CO-, -0-CO-, -CO-0-, -CO-S-, -S-CO-, -CHHalogen-, -CHCN-, -CH=CH-, and combinations of these bridging elements, with the proviso that two hetero atoms are not directly linked, Z is -CO-0-, -0-CO-, -CH-0-, -0-CH-- or -CH-CH--, n is 0 or 1, and 1 through 6 are each hydrogen or fluorine, at least one of L 1 through L4 being fluorine wi.th the provisos that a) one of L and L is fluorine and the other is hydrogen and one of 1 and 2 is hydrogen and the other is fluorine or hydrogen, if n = 0 and Z is -CO-0-, and b) one of L 1 and L2 is fluorine and the other is hydrogen and one of L and L is hydrogen and the other is fluorine or hydrogen, if n = 0 and Z is -O-CO-. Chiral tilted smectic liquid crystal phases with ferro- electric properties can be prepared by adding a suitable chiral doping substance to base mixtures with one or more tilted smectic phases (L.A. Beresnev et al., Mol. Cryst. iq. Cryst. 89, 327 (1982); H.R. Brand et al., J. Physique 44 (Lett.), L-771 (1983)). Such phases can be used as dielectrics for rapidly switching displays based on the principle of SSFLC technology described by Clark and Lagerwall (N.A. Clark and S.T. Lagerwall, Appl. Phys. Lett. 3 , 899 (1980); U.S. Patent Specification 4,367,924) on the basis of ferroelectric properties of the chirally tilted phase. In this phase, the longitudin¬ ally stretched molecules are arranged in layers, the molecules having a tilt angle relative to the normal of the layers. On advancing from layer to layer, the tilt direction changes by a small angle in respect of an axis vertical to the layers, so that a helix structure is formed. In displays based on the principle of SSFLC technology, the smectic layers are arranged vertically to the plates of the cell. The helical arrangement of the tilt direction of the molecules is suppressed by a very small distance between the plates (about 1-2 μ ). The longi¬ tudinal axes of the molecules are thereby forced to align themselves in a plain parallel to the plates of the cell, which means that two distinct tilt orientations result. By applying a suitable alternating electrical field, a system can be switched back and forth between these two states in the liquid crystal phase which exhibits spon- taenous polarization. This switching process is consider- ably more rapid than in conventional twisted cells (TN-LCDs) based on nematic liquid crystals. A great disadvantage for many applications of the materials currently available with chirally tilted smectic phases (such as, for example, S_*, but also S *, S *, S_.*, S κ *, S G * and S *) is their low chemical, heat and light stability. Another adverse property of displays based on the chirally tilted smectic mixtures currently available is that values of the spontaneous polarization are too low, so that the switch time properties of the displays are adversely influenced and/or the pitch and/or the tilt and/or the viscosity of the phases do not meet the requirements of display technology. Moreover, the temperature range of the ferroelectric phases is usually too small and is predominantly at temperatures which are too high. It has now been found that the use of compounds of the formula I as components of chirally tilted smectic mixtures can substantially reduce the disadvantages mentioned. The compounds of the formula I are thus out¬ standingly suitable as components of chirally tilted « smectic liquid crystal phases. In particular, chirally tilted smectic liquid crystal phases which are particu¬ larly chemically stable, have favourable ferroelectric phase ranges and favourable viscosity values, in particu¬ lar broad S * phase ranges, have an outstanding behaviour on supercooling down to temperatures below 0 °C without crystallization occurring and high spontaneous polarization values for such phases can be prepared with the aid of these compounds. P is the spontaneous polarization in nC/cm 2 . Partially the compounds of the formula I are known as components of nematic liquid crystalline phases; par¬ tially the compounds of the formula I are new. In the Japanese laid open applications J6 1229842 and J6 2030740 optically active fluorinated phenyl biphenylcarboxylates are claimed as dopants for ferroelectric liquid crystal¬ line phases. Those compounds are not suitable for the development of modern ferroelectric mixtures, because their spontaneous polarization and mesogenic properties don't fulfill today's requirements. In the German laid open application DE 2939782 a broad general formula of non-optically active compounds is claimed embracing partially the compounds according to the invention, but no single species of formula I is disclosed. No information at all is given there which indicates that these compounds can be used as components of chirally tilted smectic mixtures for ferroelectric displays based on, for example, the SSFLC technology described by Clark and Lagerwall. The compounds of the formula I have a wide range of appli¬ cations. Depending on the choice of substituents, these compounds can be used as base materials from which liquid crystal smectic phases are predominantly composed; however, it is also possible for compounds of the formula I to be added to liquid crystal base materials from other classes of compounds for example in order to vary the dielectric and/or optical anisotropy and/or the viscosity and/or the spontaneous polarization and/or the phase ranges and/or the tilt angle . and/or the pitch of such a dielectric. The invention relates to chiral tilted smectic liquid crystal media containing at least one compound of the formula I and liquid crystal display elements, in parti- cular ferroelectric electrooptical display elements, containing such media. Furthermore the invention relates to compounds of the formulae la and lb alkyl-O- alkyl la X J _f x ~ R ll -- <(_O} )--(( _O}> -- « CO_---©( O >--R 2 lb wherein R 1 and R2 have the meaning given, and X 1 through X3 are each hydrogen or fluorine. In the compounds of the formula la X is fluorine, in the compounds of the formula lb at least two of X 1 to X3 are fluorine. For simplicity, in the following text Cy is a 1,4 cyclo- hexylene group, Dio is a l,3-dioxane-2,5-diyl group, Phe is a 1,4-phenylene group, Pym is a pyrimidine-2,5-diyl group and Pyr is a pyridine-1,5-diyl group, it being possible for Cy and/or Phe to be unsubstituted or substi¬ tuted by one or two F and/or Cl atoms and/or CH- groups and/or CN groups. PheF denotes a 2(3)-monofluoro-1,4-phenylene group. Particularly preferred chiral tilted smectic liquid crystal phases according to the invention are those in which the achiral base mixture contains, in addition to compounds of the formula I, at least one other com¬ ponent with a negative or comparatively low positive dielectric anisotropy. This/these other component(s) of the achiral base mixture can make up 10 to 99 %, preferably 50 to 95 %, of the base mixture. Suitable further components with a comparatively low positive or negative dielectric anisotropy are compounds of the formulae 1 to 6. m m R 3.0. -t)- R 3 6 wherein R and R are each alkyl or alkoxy with 5 to 15 C atoms, Z is -CO-O- or a single bond, X 1, X2 and L3 i.s hydrogen or fluorine, R 4 and R5 are each alkyl with 5 to 15 C atoms, m is 1 or 2, n is 0 or 1 and A is 1,4-cyclohexylene or l,4-bicyclo[2.2.2]octylene. The phases according to the invention contain preferably at least three and in particular at least five compounds of the formula I. Particularly preferred chirally tilted smectic liquid crystal phases according to the invention are those in which the achiral base mixture contains, in addition to compounds of the formula I, at least one other component with a negative or comparatively low positive dielectric anisotropy. This/these other component(s) of the chiral base mixture can make up 1 to 50 %, prefer¬ ably ' 10 to 25 %, of the base mixture. Suitable further components with a comparatively low positive or negative dielectric anisotropy are compounds of the formula II, which include the compounds of the part formula Ila to Hi: R 4 - VcOX- Hg 4 5 R and R are in each case preferably straight-chain alkyl, alkoxy, alkanoyloxy or alkoxycarbonyl with in each case 3 to 12 C atoms. X is preferably 0. In the compounds of the formulae Ha, lib, Hd, He, Hf and Hg, it is also possible for a 1,4-phenylene group to be laterally sub¬ stituted by halogen or CN, particularly preferably by fluorine. Particularly preferred compounds are those of the part formulae Ha, lib, Hd and Hf, wherein R 4 and R 5 are each straight-chain alkyl or alkoxy with in each case 5 to 10 C atoms. Particularly preferred individual compounds are shown in the following table: Formula X The compounds of the part formulae He, Hh and Hi are suitable as additives for lowering the melting point and are usually added to the base mixtures in an amount of not more than 5 %, preferably 1 to 3 %. R 4 and R5 m. the compounds of the part formulae He, Hh and Hi are preferably straight-chain alkyl with 2 to 7, preferably 3 to 5 C atoms. Another class of compound which is suit¬ able for lowering the melting point in the phases according to the invention is that of the formula wherein R 4 and R5 have the preferred meaning given f_•or lie, Hh and Hi. Suitable further components with negative dielectric anisotropy are furthermore compounds containing the structural element A, B or C CN CN Cl B Preferred compounds of this type correspond to the formulae Ilia, Hlb and HIc: CN R « -Q 1 -CH 2 -CH- 2 -R « ' IIlb I CN R'-Q 3 -Q 4 -R"' Hie R' and R' ' are each preferably straight-chain alkyl or alkoxy groups with in each case 2 to 10 C atoms. Q and are each 1,4-phenylene, trans-l,4-cyclohexylene, 4,4'-biphenylyl, 4-(trans-4-cyclohexyl)-phenyl or trans, 1 2 trans-4,4'-bicyclohexyl, or one of the groups Q and Q may also be a single bond. Q 3 and Q 4 are each 1,4-phenylene, 4,4'-biphenylyl or trans- 1,4-cyclohexylene. One of the groups Q 3 and Q4 can also be i 4-phenylene, at least one CH group being replaced by N. R 1 ' ' is an optically active radical with an asym¬ metric carbon atom of the structure Cl * CN I I -CH*-? or -CH*-. Particularly preferred compounds of the formula Hie are those of the formula Hie 1 : IIIc 1 n wherein A is 1, -phenylene or trans-1,4-cyclohexylene and n is O or 1. The compounds of the formula I include preferred compounds with two rings of the part formulae Ia-Ic R 1 -PheF-Z-Phe-R 2 la R 1 -Phe-Z-PheF-R 2 lb R 1 -PheF-Z-PheF-R 2 Ic and compounds with three rings of the part formulae Ic-li R 1 -PheF-Phe-Z-Phe-R 2 Ic R 1 -Phe-PheF-Z-Phe-R 2 Id R 1 -Phe-Phe-Z-PheF-R 2 Ie R 1 -PheF-PheF-Z-Phe-R 2 If R 1 -PheF-Phe-Z-PheF-R 2 Ig R 1 -Phe-PheF-Z-PheF-R 2 Ih R 1 -PheF-PheF-Z-PheF-R 2 Ii Therefore, the compounds of the formula I include dinuclear and trinuclear materials. Of the dinuclear materials, tthhoossee wwlherein R 1 is n-alkyl and R2 is n-alkyloxy are pre- ferred. Compounds of the formula I where R = n-heptyl or n-octyl impart good low temperature properties to the phases according to the invention, whilst the corresponding com¬ pounds with R = n-nonly are capable of increasing the S-/S_ transition temperature of the phases according to the invention. Preferred phases according to the invention contain at least one compound of the formula I wherein R is n-nonyl and at least one compound of the formula I wherein R is n-heptyl or n-octyl. Particularly preferred phases accord- ing to the invention are those containing compounds of the formula I wherein R is n-heptyl, n-octyl or n-nonyl. 2 R in the dinuclear compounds of the formula I is prefer- alby n-alkoxy with 6 to 12, in particular with 7 to 10 C atoms. The phases according to the invention preferably contain at least one compound of the formula I wherein R 2 is n-hexyloxy, n-heptyloxy or n-octyloxy (preferably n-heptyloxy or n-octyloxy) and at least one compound of 2 the formula I wherein R is n-nonyloxy or n-decyloxy. The sum of the C atoms in the groups R 1 and R2 of the preferred dinuclear compounds of the formula I is prefer¬ ably 15 or more, particularly preferably in the range from 15 to 20. Particularly preferred dinuclear indivi¬ dual compounds of the formula I are listed in the follow¬ ing table: R J R' n-octyl n-heptyloxy COO n-octyloxy n-nonyloxy COO n-octyloxy n-octyl COO Compounds of the formula I which are furthermore pre¬ ferred are those wherein R is n-alkyl with 7 to 10 C 2 atoms and R is n-alkanoyloxy, n-alkoxycarbonyl or n-alkylthio with in each case 5 to 10 C atoms. Phases according to the invention which are further¬ more preferred are those containing at least one di¬ nuclear compound of the formula I wherein R is n-alkyl with -7 to 10 C atoms and R 2 is -CH 2 0-(CH~) -CH- or -O- (CH-) -0-(CH 2 ) r -CH 3 .p is preferably 4 to 10, in parti- cular 5 to 9. q is preferably 1 or 2, particularly pre¬ ferably 2. r is 4 to 10, particularly preferably 5 to 9. q can also be > 2, for example 3 to 5. These phases according to the invention preferably contain at least one trinuclear compound of the formula I and/or 2 one compound of the formula I wherein R is n-alkylthio. These phases are distinguished by particularly high S~/S A transition temperatures. Smaller preferred groups of trinuclear compounds of the formula I are those of the formula II to 17: II alkyl-PheF-Phe-CO-O-Phe-alkyl alkyl-PheF-Phe-CO-O-Phe-alkoxy alkyl-PheF-Phe-CO-O-Phe-CO-O-alkyl alkoxy-PheF-Phe-CO-O-Phe-alkyl 12 alkyl-Phe-PheF-CO-O-Phe-alkyl alkyl-Phe-PheF-CO-O-Phe-alkoxy alkyl-Phe-PheF-CO-O-Phe-CO-O-alkyl alkoxy-Phe-PheF-CO-O-Phe-alkyl 13 alkyl-Phe-Phe-CO-O-PheF-alkyl alkyl-Phe-Phe-CO-O-PheF-alkoxy alkyl-Phe-Phe-CO-O-PheF-CO-O-alkyl alkoxy-Phe-Phe-CO-O-PheF-alkyl 14 alkyl-PheF-Phe-CO-0-PheF-alkyl alkyl-PheF-Phe-CO-O-PheF-alkoxy alkyl-PheF-Phe-CO-O-PheF-CO-O-alkyl alkoxy-PheF-Phe-CO-O-PheF-alkyl 15 alkyl-Phe-PheF-CO-O-PheF-alkyl alkyl-Phe-PheF-CO-O-PheF-alkoxy alkyl-Phe-PheF-CO-O-PheF-CO-O-alkyl alkoxy-Phe-PheF-CO-6-PheF-alkyl 16 F F alkyl-PheF-Phe-CO-0-( O )-alkoxy F F alkoxy-PheF-Phe-CO-0- 0 )-alkyl F F 17 F F alkyl-Phe-PheF-CO-0- 0 )-alkyl F F F F alkoxy-Phe-PheF-CO-0-( 0 )-alkyl F F alkoxy-Phe-PheF-CO-0-( 0 )-alkoxy Phases according to the invention which are furthermore preferred are those containing at least one compound of the formula V 0-< )_0_ Rb o. ø -H-, wherein Ra is alky ■ -l with 3 to 12 C atoms and R,b i•s al,kyl 1 or alkoxy with in each case 5 to 12 C atoms. R and R, together contain at least 15 C atoms, preferably at least 17 C atoms. A particularly preferred chiral tilted smectic liquid crystal phase with at least one compound of the formula I according to-claim 1 is characterized in that it contains a liquid crystal component with negative dielectric aniso- tropy, and in particular is a phase according to the invention, characterized in that it contains at least one compound containing the structural element A, B or C as component with negative dielectric anisotropy. CN CN Cl B A particular preferred phase according to the invention is furthermore charcterized in that it contains at least one compound of the formula II R 4 -A 1 -COX-A 2 R 5 II wherein R 4 and R5 are each alkyl with 1-15 C atoms, it also being possible for one or two non-adjacent CH 2 groups to be replaced by -0-, -CO-, -0-CO-, -CO-O- and/or -CH=CH-, X is O or S and A 1 and A2 are each 1,4-phenylene or trans-1,4-cyclo- hexylene, or one of the groups A 1 and A2 " is optionally also 4,4'-biphenylyl or trans, trans-4,4'-bicyclohexyl. Other preferred ferroelectric phases according to the invention are those which contain at least one compound of the formula V 1 1 2 2 R 1 -Q -A-(Q^) g -R^ V wherein R 1 and R2 independently of one another are each a straight-chain alkyl group having 1 to 15 C atoms, wherein furthermore one or more non-ad¬ jacent CH- groups may be replaced by -0-, -S-, -CO-, CHCH--0-, CHCH--, -CH-halogen-, -CHCN-, -0-C0-, 0-COO-, -CO-O- and/or -CH=CH-, A is CN -0-. -0 /-, -_ _>- 2 - or CN CN q is O or 1, Q 1 and 2 i•ndependently of another are each (A°-Z°) p -, wherein A° is 1,4-cyclohexylene which is unsubstituted or monosubstituted or polysubstituted by halogen atoms, CH- groups and/or nitrile groups, and wherein futhermore one or two non-adjacent CH- groups may be replaced by -O- and/or -S- I and/or a -CH-CH--grouping may be replaced by -C=N-(Cy), or is 1,4-phenylene which is un¬ substituted or monosubstituted or polysubsti¬ tuted by halogen atoms, CH- groups and/or nitrile groups, wherein furthermore one or more CH groups may be replaced by N (Phe), and one of the radicals A° may furthermore be 2,5- naphthylene (Na) or tetrahydro-2,6-naphthylene (4H-Na), if appropriate substituted by halogen or CN, Z°, Z 1 and Z2 i.ndependently of one another are each -CO-O-, -0-CO-, CΕ-O-, OCH 2 -, CH-CH--, -CHCNCH--, CH--CHCN- or a single bond, and p is 1, 2 or 3 or, where A is tetra- or octahydro- phenanthrene, also O, and where A is - at least one group Z° is -CHCNCH- or -CH-CHCN- and/or at least one CH- group in at least one of the groups R and R is replaced by -CHCN-. The compounds of the formula V can have straight-chain or branched wing groups R 1 and/or R2. Compounds possessing branched wing groups can be used in the form of the race- mate or as optically active compounds. Achiral base mix¬ tures of compounds of the formula V and, if appropriate, further archiral components can be doped with chiral compounds of the formula I or additionally with other chiral compounds, in order to obtain chirally tilted smectic phases. Particularly preferred smaller groups of compounds are those of the formula VI to V8: CN CN CN CN CN R 1 -Cy-Z°-Phe- HV-R 2 V6 CN CN R 1 -Cy-Phe-Z°-( H y-R 2 Vll CN V12 CN R -Phe-Phe - / - Z°-Cy-R V13 CN Another particularly preferred smaller group of compounds comprises those of the formulae V19 to V22: R 1 --AA°°--Ccyy--((CCHH--)) r --CcHHCCNN--Cc s HH-- s _ + . 1 - V19 R -. A o _ A o . Cy . (CH2 ) r -CHCN-C 3 H- s+1 v2 - R 1 -A°-A°-CHCN-CH 2 -Cy-R 2 V21 R 1 -A°-A°-CH 2 CHCN-Cy-R 2 V22 wherein r is 0, 1, 2 or 3 and (r+s) is 1 to 14. Particularly preferred chiral dopants are those of formula VI R > 7 '-- < (--<<rA>>-- > )-- m ø- 0>>--<C00-CHCN-R 8 VI wherein R 7 is alkyl or alkoxy with 5 to 15 C atoms. R8 is alkyl with 1 to 8 C atoms, m is 1 or 2, and - A - is 1,4-phenylene optionally substituted by fluorine, pyridine-2,5-diyl, pyrimidine-2,5-diyl or trans-1,4- cyclohexylene. All components of the phases according to the invention are either known or can be prepared in a manner known per se, analogously to known compounds. Compounds of the formula I which do not possess any S c phases are likewise suitable as components of smectic phases according to the invention. The phases according to the invention may furthermore contain compounds of the formula wherein R 1 and R2 have the meani.ng given in the case of formula V preferred phases according to the invention are those which contain in addition to the compounds of the formula I further fluorinated components, in particular compounds, which exhibit the following structure elements of the formulae D to G: All components of the phases according to the invention are either known or can be prepared in a manner known per se, analogously to known compounds. Esters of the formula I can be obtained by esterifica- tion of corresponding carboxylic acids (or their reac¬ tive derivatives) with phenols (or their reactive derivates) . The corresponding carboxylic acids and phenols are known or can be prepared by processes analogous to known processes. Particulary suitable reactive derivatives of the carboxylic acids mentioned are the acid halides, above all the chlorides and bromides, and furthermore the an¬ hydrides, for example also mixed anhydrides, azides or esters, in particular alkyl esters with 1 - 4 C atoms in the alkyl group. Possible reactive derivatives of the alcohols or phenols mentioned are, in particular, the corresponding metal phenolates, preferably of an alkali metal, such as sodium or potassium. The esterification is advantageously carried out in the presence of an inert solvent. Particularly suitable solvents are ethers, such as diethyl ether, di-n-butyl ether, THF, dioxane or anisole, ketones, such as acetone, butanone or cyclohexanone, amides, such as dimethylform- amide or phosphoric acid hexamethyltriamide, hydrocar¬ bons, such as benzene, toluene or xylene, halogenohydro- carbons, such as dichlormethane carbon tetrachloride or tetrachloroethylen, and sulfoxides, such as dimethyl- sulfoxide or sulfolane. Water-immiscible solvents can simultaneously be advantageously used for azeotropic distillation of the water formed during the esterifica- tion. An excess of an organic base, for example pyridine, quinoline or triethylamine, can occasionally also be used as the solvent for the esterification. The esterifica¬ tion can also be carried out in the absence of a solvent, for example by heating the components in the presence of sodium acetate. The reaction temperature is usually between -50° and +250°, preferable between -20° and +80°. At these temperatures, the esterification reac¬ tions have as a rule ended after 15 minutes to 48 hours. In detail, the reaction conditions for the esterification depend largely on the nature of the starting substances used. Thus, a free carboxylic acid is as a rule reacted with a free alcohol or phenol in the presence of a strong acid, for example a mineral acid, such as hydrochloric acid or sulfuric acid. A preferred reaction procedure is the reaction of an acid anhydride or, in particular, an acid chloride with an alcohol, preferably in a basic medium bases which are of importance being, in parti¬ cular, alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, alkali metal carbonates or bi- carbonates, such as sodium carbonate, sodium bicarbonate, potassium carbonate or potassium bicarbonate, alkali metal acetates, such as sodium acetate or potassium acetate, alkaline earth metal hydroxides, such as calcium hydroxide, or organic bases, such as triethyl¬ amine, pyridine, lutidine, collidine or quinoline. Another preffered embodiment of the esterification comprises first converting the alcohol or phenol into the sodium alcoho- late or phenolate or potassium alcoholate or phenolate, for example by treatment with ethanolic sodium hydroxide solution or potassium hydroxide solution, isolating this product and suspending it in acetone or diethyl ether, together with sodium bicarbonate or potassium carbonate, with stirring, and adding a solution of the acid chloride or anhydride in diethyl ether, acetone or dimethylform- amide to this suspension, advantageously at temperatures between about -25° and +20°. A further preferred reaction procedure is the reaction of an acid with an alcohol in an inert solvent in the presence of a water binding material, e.g. molecular sieves or carbodiimides, preferably dicyclohexylcarbo- diimide (DCC). The compounds of the groups 16 to 17 are obtained by esterification of the corresponding fluorinated biphenylcarboxylic acid with 4-alkyl- or alkoxy-2,3-difluorophenols. These are prepared by oxidation of the corresponding 3-alkyl- or alkoxy-l,2-difluorbenzene. Oxidation is achieved, for example, by treating the corresponding 4-alkyl- or alkoxy-2,3-difluorophenyl- lithium (Gilman, H. , Soddy, T.S., J. Org. Chem., 22, 1715, (1957)) with trialkoxyborates and cleaving the boronic acid with H_0_, or treating it with lithium tert. butoxide or with oxygen. The phases according to the invention are prepared in a manner with is customary per se. As a rule, the components are dissolved in one another, advantageously at elevated temperature. The liquid crystal phases according to the invention can be modified by suitable additives so that they can be used in all the types of liquid crystal display elements hitherto disclosed. The liquid crystal materials of this invention may be used in liquid crystal electrooptical display devices of known construction (for example U.S. Patent 4,367,924) or in liquid crystal electro-optical display devices which are modifications of the known ones. The following examples are intended to illustrate the invention without limiting it. Percentages above and below are percentages by weight; all the temperatures are stated in degrees Celsius. The symbols are furthermore as follows: Cr: crystalline solid state; S: smectic phase (the index charcterizes the phase type), N: smectic state, Ch: cholesteric phase, I: isotropic phase. The figure between two symbols indicates the transition temperature in degrees Celsius. P indicates the spontaneous polari- sation in nC cm -2 at the given temperature. Example 1 Preparation of (4-octoxy-3-fluorophenyl)-boronic acid A mixture of 4-bromo-2-fluoro-octoxybenzene (95 g) (ob¬ tained by bromination of 2-fluoro-octoxybenzene) and tetrahydrofurane (250 ml) is slowly added to a mixture of magnesium turnings (8.5 g) and tetrahydrofurane (50 ml) containing a crystal of iodine. After refluxing and stiring for 1 hour the reaction mixture is transferred via a flexi- needle into a solution of tri-isopropylborate (123 g) in tetrahydrofuran (10 ml) at -70 °C in a nitrogen atmos- phere over 30 minutes. After warming up to room temperature hydrochloric acid (10 %) is added and the organic layer is spearated. The organic layer is washed with brine, dried and the solvent is removed by distilliation. After drying in vacuo 80.4 g of (4-octoxy-3-fluorophenyl)-boronic acid are obtained as a crude solid. Example 2 Preparation of methyl 4'-octoxy-3'-fluorobiphenyl-4-yl carboxylate A mixture of the product of example 1 (32.2 g) and methy- lated spirits (50 ml) is added to a mixture of palladium tetra(triphenylphosphine) (0.06 g), toluene (200 ml), methyl 4-bromobenzoate (21.5 g) and a solution of sodium carbonate (2 M, 100 ml). The mixture is heated under reflux and stirred for 16 hours. After cooling the reaction mix- ture is poured into water and the organic layer is separated. The aqueous layer is extracted twice with toluene (25 ml) each time and the combined organic layers are washed once with water (50 ml). The solvent is removed, and the crude product purified by chromatography on crystallization from methylated spirits. Example 3 Preparation of 4'-octoxy-3'-fluorobiphenyl-4-yl-carboxylic acid A mixture of 19.5 g of the product from example 2, 8.4 g of potassium hydroxide, 30 ml of water and 100 ml of methylated spirits is stirred and heated under reflux for 2 hours. The mixture is poured into water and acidified with concentrated hydrochloric acid. The solid is filtered off, washed with water and dried in vacua. After crystallization from acetic acid 15 g of the acid are obtained as a white solid. Example 4 Preparation of 4-heptoxy-2,3-difluorophenol 2,3-Difluorobenzene is lithiated using butyl lithium at -70 °C and converted to the boronic acid at -70 °C using trimethyl borate. Oxidation of the boronic acid gives 2,3-difluorophenol which is alkylated using an heptyl bromide, potassium carbonate and butanone. A repeat lithiation, boronation and oxidation gives the required product. Example 5 Preparation of 4-alkyl-2-fluorophenyl 4'-alkyloxy-3'- fluorobiphenyl-4-ylcarboxylates. The product from Example 3 (5 g) and 2-fluoro-4-octyl- phenol (3.5 g) are added to dichloromethane (70 ml), and dicyclohexyl carbodii ide (3.3 g) is added and the mixture is stirred at room temperature for 16 hours. The reaction mixture is filtered and the solid is washed with dichloromethane (10 ml). The combined di¬ chloromethane filtrates are washed with water and dried. After purifying by chromatography and crystallization from methylated spirits 6.4 g of 4-octyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl-4-ylcarboxylate are combined as a solid, Cr 42 S_ 118 N 135.0 I. The following compounds are obtained analogously: 4-ethyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-propyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-butyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 46 S c 105.6 N 144.5 I 4-hexyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 34 S c 112.1 N 137.9 I 4-heptyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-nonyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-undecyl-2-fluorophenyl 4 l -octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-ethyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-propyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-butyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 63.2 S c 87.0 N 151.3 I 4-hexyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 48.0 S_ 97.5 N N 143.9 I 4-heptyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 50.0 S c 100.8 N 143.8 I 4-octyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 44.7 S c 106.0 N 138.7 I 4-nonyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-undecyl-2-fluorophenyl 4'-hexyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-2-fluorophenyl 4'-hexyloxy-3 • -fluorobiphenyl- 4-ylcarboxylate 4-ethyl-2-fluorophenyl '-heptyloxy-3 • -fluorobiphenyl- 4-ylcarboxylate 4-propyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-butyl-2-fluorophenyl 4'-heptyloxy-3*-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-2-fluorophenyl '-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 61.0 S_ 98.4 N 146.0 I 4-hexyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 37.0 S_ 106.0 N 138.9 I 4-heptyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 36.0 S 108.6 N 139.5 I 4-octyl-2-fluorophenyl 4*-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 54 S c 112.0 N 134.7 I 4-nonyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-undecyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-2-fluorophenyl 4'-heptyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-ethyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-propyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-butyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-hexyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-heptyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-octyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-nonyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-undecyl-2-fluorophenyl 4'-nonyloxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-2- luorophenyl '-nonyloxy-3 '-fluorobiphenyl- 4-ylcarboxylate With 3-fluoro-4-alkylphenols as starting materials the following compounds are obtained analogously: 4-ethyl-3-fluorophenyl 4*-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-propyl-3-fluorophenyl 4'-octoxy-3•-fluorobiphenyl- 4-ylcarboxylate 4-butyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-hexyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-heptyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate Cr 65 Sc_ 119.5 S Ά. 165.7 I 4-butyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-nonyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-undecyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-ethoxy-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-propoxy-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-butoxy-3-fluorophenyl 4'-octoxy-3*-fluorobiphenyl- 4-ylcarboxylate 4-pentoxy-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-hexyloxy-3-fluorophenyl 4'-octoxy-3 '-fluorobiphenyl- 4-ylcarboxylate 4-heptyloxy-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-octyloxy-3-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-ethyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-propyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-butyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-2-fluorophenyl '-octoxy-3•-fluorobiphenyl- 4-ylcarboxylate 4-hexyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-heptyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-octyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-nonyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-y1carboxylate 4-undecyl-2-fluorophenyl 4'-octoxy-3*-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-2-fluorophenyl 4'-octoxy-3'-fluorobiphenyl- 4-ylcarboxylate 4-ethyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-propyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-butyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-pentyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-hexyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-heptyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-octyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-nonyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-decyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-undecyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate 4-dodecyl-2-fluorophenyl 4'-heptyl-3'-fluorobiphenyl- 4-ylcarboxylate The fluorinated phenyl biphenylcarboxylates of Table I are obtained analogously: Table I R 2 R J R' Transition points heptyl heptyl H H Example 6 Preparation of 4-alkyloxy-3-fluorophenyl 4-alkyloxy- benzoates p-Hexyloxybenzoic acid (3.3 g) and 3-fluoro-4-octyloxy- phenol (3.7 g) are added to a mixture of dicyclohexyl carbodiimide (3.3 g) and dichloromethane (70 ml). After storage at room temperature for 16 hours the mixture is filtered and the solid is washed with dichlorome¬ thane (10 ml). The combined filtrates are washed with water and dried over anhydrous magnesium sulfate. After purifying by chromatography and crystallization 3.5 g of 3-fluoro-4-octyloxyphenyl p-hexyloxybenzoate are obtained as a solid, Cr. 44.6 °C S 69.0 °C N 71.6 °C I. The following compounds are obtained analogously: 3-fluoro-4-octyloxyphenyl p-ethyloxybenzoate 3-fluoro-4-octyloxyphenyl p-propyloxybenzoate 3-fluoro-4-octyloxyphenyl p-butyloxybenzoate 3-fluoro-4-octyloxyphenyl p-pentyloxybenzoate 3-fluoro-4-octyloxyphenyl p-heptyloxybenzoate Cr 57.7 S A (47.8) S_ 72.1 I 3-fluoro-4-octyloxyphenyl p-octyloxybenzoate Cr 56.2 S A (55.2) S_ 77.7 I 3-fluoro-4-octyloxyphenyl p-nonyloxybenzoate Cr 63.1 S A 65.6 S_ 78.2 I 3-fluoro-4-octyloxyphenyl p-decyloxybenzoate 3-fluoro-4-octyloxyphenyl p-undecyloxybenzoate 3-fluoro-4-octyloxyphenyl p-dodecyloxybenzoate 3-fluoro-4-octyloxyphenyl p-ethylbenzoate 3-fluoro-4-octyloxyphenyl p-propylbenzoate 3-fluoro-4-octyloxyphenyl p-butylbenzoate 3-fluoro-4-octyloxyphenyl p-pentylbenzoate 3-fluoro-4-octyloxyphenyl p-hexylbenzoate 3-fluoro-4-octyloxyphenyl p-heptylbenzoate 3-fluoro-4-octyloxyphenyl p-octylbenzoate Cr 42.5 I 3-fluoro-4-octyloxyphenyl p-nonylbenzoate 3-fluoro-4-heptyloxyphenyl p-ethylbenzoate 3-fluoro-4-heptyloxyphenyl p-propylbenzoate 3-fluoro-4-heptyloxyphenyl p-butylbenzoate 3-fluoro-4-heptyloxyphenyl p-pentylbenzoate 3-fluoro-4-heptyloxyphenyl p-hexylbenzoate 3-fluoro-4-heptyloxyphenyl p-heptylbenzoate 3-fluoro-4-heptyloxyphenyl p-octylbenzoate 3-fluoro-4-heptyloxyphenyl p-nonylbenzoate 3-fluoro-4-hexyloxyphenyl p-ethyloxybenzoate 3-fluoro-4-hexyloxyphenyl p-propyloxybenzoate 3-fluoro-4-hexyloxyphenyl p-butyloxybenzoate 3-fluoro-4-hexyloxyphenyl p-pentyloxybenzoate 3-fluoro-4-hexyloxyphenyl p-hexyloxybenzoate Cr 39.5 S_ (32.5) S. 66.1 N 70 I 3-fluoro-4-hexyloxyphenyl p-heptyloxybenzoate • Cr 52 S (49) S A 70.2 I 3-fluoro-4-hexyloxyphenyl p-octyloxybenzoate Cr 53.1 S 53.6 S A 76.7 I 3-fluoro-4-hexyloxyphenyl p-nonyloxybenzoate Cr 51.4 S 61.4 S A 77.4 I 3-fluoro-4-hexyloxyphenyl p-decyloxybenzoate 3-fluoro-4-hexyloxyphenyl p-undecyloxybenzoate 3-fluoro-4-hexyloxyphenyl p-dodecyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-ethyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-propyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-butyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-pentyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-hexyloxybenzoate Cr 51.3 S A 67.9 N 69.9 I 3-fluoro-4-heptyloxyphenyl p-heptyloxybenzoate Cr 66.9 S (49.0) S Λ 70.6 I 3-fluorb-4-heptyloxyphenyl p-octyloxybenzoate Cr 60.3 S c (52) S A 73.3 I 3-fluoro-4-heptyloxyphenyl p-nonyloxybenzoate Cr 67 S c (64.5) S A 77 I 3-fluoro-4-heptyloxyphenyl p-decyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-undecyloxybenzoate 3-fluoro-4-heptyloxyphenyl p-dodecyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-ethyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-propyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-butyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-pentyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-hexyloxybenzoate Cr 58.5 S A 67.1 N 70 I 3-fluoro-4-nonyloxyphenyl p-heptyloxybenzoate Cr 70.5 S 71.7 I 3-fluoro-4-nonyloxyphenyl p-octyloxybenzoate Cr 64.5 S (51.7) S_ 77.0 I 3-fluoro-4-nonyloxyphenyl p-nonyloxybenzoate Cr 73.2 Sc (65.6) SA Λ 77.6 I 3-fluoro-4-nonyloxyphenyl p-decyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-undecyloxybenzoate 3-fluoro-4-nonyloxyphenyl p-dodecyloxybenzoate Example 7 Preparation of 4-alkyloxyphenyl 4-alkyloxy-3-fluoro- benzoates 4-Octyloxy-3-fluorobenzoic acid (obtained from 2-fluoro- phenol 1 ) (2.86 g) and 4-octyloxyphenol (2.25 g) are treated with dicyclohexyl carbodiimide as described in Example 3. 3.48 g of 4-octyloxyphenyl 4-octyloxy-3-fluoro-benzoate are obtained as a solid, Cr 63.5 S _ 69.7 N 75.4 I The following compounds are obtained analogously: 4-ethyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-propyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-butyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-pentyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-hexyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-heptyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-nonyloxyphenyl 4-octyloxy-3-fluorobenzoate 4-octylphenyl 4-octyloxy-3-fluorobenzoate Cr 62 I 4-heptyl-2-fluorophenyl 4-octyloxy-3-fluorobenzoate Cr 37.1 N (24.3) I 4-ethyloxyphenyl 4-heptyloxy-3-fluorobenzoate 4-propyloxyphenyl 4-heptyloxy-3-fluorobenzoate 4-butyloxyphenyl 4-heptyloxy-3-fluorobenzoate 4-pentyloxyphenyl 4-heptyloxy-3-fluorobenzoate 4-hexyloxyphenyl 4-heptyloxy-3-fluorobenzoate 4-heptyloxyphenyl 4-heptyloxy-3-fluorobenzoate 4-nonyloxyphenyl 4-heptyloxy-3-fluorobenzoate Lit 1 : D. Coates Liquid Crystal Vol. 2 (4) p 423-428, 1987 and refs therein The following fluorinated phenyl benzoates are obtained analogously: Table II -Θ- COO- ■ R 2 R J R < X' Transition temperatures octyl heptyl H octyloxy heptyl H octyl octyloxy F octyloxy octyloxy F octyloxy pentyloxy F Example 8 A liquid crystal mixture is formulated containing the following compounds 33.3 % of 4-pentyl-2-fluorophenyl 4'-octoxy-3 '-fluoro- biphenyl-4-ylcarboxylate 33.3 % of 4-hexyl-2-fluorophenyl 4'-octoxy-3 '-fluoro¬ biphenyl-4-ylcarboxylate 33.3 % of 4-heptyl-2-fluorophenyl 4'-octoxy-3 '-fluoro- biphenyl-4-ylcarboxylate Example 9 A liquid crystal phase consisting of 30.3 % of 4-pentyl-2-fluorophenyl-4'-octyloxybiphenyl-4- ylcarboxylate 30.3 % of 4-pentyl-2-fluorophenyl-4'-octylbiphenyl-4- ylcarboxylate 30.3 % of 4-heptyl-2-fluorophenyl-4'-heptyloxybiphenyl-4- ylcarboxylate 14.0 % of 4-(2-methylbutyl)-2-fluorophenyl-4'-octyloxy- biphenyl-4-ylcarboxylate 5.0 % of optically active S-l-cyanoethyl-4-nonyloxy- bipheny1-4-ylcarboxylate 20.0 % of Cr < -20 °C S 4 °C S* 83 °C S Λ 105 Ch 125 I and spontaneous polarization of 12.89 nC.cm at 30 °C. Example 10 A liquid crystal phase (coded MBF 609) consisting of 33.3 % of 4-pentyl-2-fluorophenyl-4'-octylbiphenyl-4- ylcarboxylate 33-.3 % of.4-pentyloxy-2- luorophenyl-4'-octylbiphenyl- 4-ylcarboxylate 33.3 % of 4-he tyloxy-2-fluorophenyl-4'-heptylbiphenyl- 4-ylcarboxylate is doped with different fluorophenyl benzoates. The doped mixture consists of 10 % of the following addi¬ tives (Al to A4) each time: Al 4-octyloxy-3-fluorophenyl-4-octyloxybenzoate A2 4-octyloxy-3-fluorophenyl-4-octylbenzoate A3 4-hexyloxyphenyl-4-octyloxybenzoate A4 4-pentyl-2-fluorophenyl-4-octyloxybenzoate Table HI Transition temperatures (°C) of host (MBF 609) with different additives Table HI illustrates the advantage of compounds Al com- pared to a) other fluoro compounds (A2, A4), b) non- fluoro compounds (A3). Al depresses the S--S-, while still retaining on high S_,-S-. Example 11 A liquid crystal mixture is formulated containing the following compounds: 25.67 % of 4-heptyl-2-fluorophenyl 4'-octyloxy 3'-fluorobiphenyl-4-ylcarboxylate 21.37 % of 4-heptyl-2-fluorophenyl 4'-octyloxy 2' ,3'-difluorobiphenyl-4-ylcarboxylate 25.67 % of 3-methylpentyl 4'-octyloxy-2• ,3'-difluoro- biphenyl-4-ylcarboxylate 12.67 % of 4'-octyloxy-2' ,3'-difluorobiphenyl-4-yl trans-4-heptylcyclohexylcarboxylate 3.84 % of 4-heptyl-3-fluorophenyl 4-octylbenzoate 7.74 % of 4-pentyl-2-fluorophenyl 4-octyloxybenzoate 2.4 • % of optically active 1-cyanoethyl 4'-octyloxy- 3'-fluorobiphenyl 4-yl carboxylate 0.55 % of optically active l-cyano-2-methylbutyl 4'-octyloxy-3'-fluorobiphenyl-4-ylcarboxylate and exhibits the following properties: S * 61.4 S Λ 75.3 Ch 89 I, P_ 8.4 nC cm "2 and S c *-tilt of 22.5 ° at 30 °C Response times (μsec) 89 (10 v/μm), 52 (15 v/μm) and 37 (20 v/μm) Examples 12 A liquid crystal mixture is formulated containing the following compounds: 17.2 % of 4-pentyloxy-4 « l -(4-methylhexyl)-2'-fluoro- p-terphenyl 17.19 % of 4-heptyloxy-4"-(4-methylhexyl)-2 « -fluoro- p-terphenyl 17.19 % of 4-octyloxy-4' '-pentyl-2'-fluoro-p-terphenyl 14.17 % of 4'-octyloxy-3'-fluoro-biphenyl-4-yl trans- 4-heptylcyclohexylcarboxylate 14.5 % of 4-pentyl-2-fluorophenyl octyloxybenzoate 10.0 % of 4-octyloxy-3-fluorophenyl octyloxybenzoate 6.5 % of otpically active S-1-cyanoethyl 4'-octyloxy- 3'-fluorobiphenyl-4-ylcarboxylate 3.25 % of optically active S-l-cyano2-methylpropyl 4'-octyloxy-3'-fluorobiphenyl-4-ylcarboxylate and exhibits the following properties: S c * 60.8 S A 86.3 Ch 100.8 I Temp. (°C) Ps (nC cm "2 ) Sv_ * tilt (°) 0 34.4 23 10 32.2 22.5 20 27.6 22 30 25.6 21.5 40 21.3 19.5 50 15.6 18.5 60 4.2 10 Using a liquid crystal electro-optic cell of known construction the mixture of example 10 shows the following response times (switching times are measured in a cell of 2 μm space) Response Times Example 13 A liquid crystal mixture is formulated containing the following compounds: 17 % of 4-heptyl-2-fluorophenyl 4'-octyloxy-3'-fluoro- biphenyl-4-ylcarboxylate 17 % of '-octyloxy-2',3'-difluorobiphenyl-4-yl trans- 4-heptylcyclohexylcarboxylate 17 % of 4'-octyloxy-3'-fluorobiphenyl-4-yl trans-4- heptyl-cyclohexylcarboxylate 17 % of 4'-octyloxy-2' ,3*-difluorobiphenyl-4-yl 4-methyl- hexanoate 11 % of 4-octyloxy-3-fluorophenyl 4-octyloxybenzoate 14 % of 4-pentyl-2-fluorophenyl 4-octyloxybenzoate 5.6 % of optically active S-1-cyanoethyl .4'-octyloxy-3'- fluorobiphenyl-4-ylcarboxylate 1.4 % of optically active S-l-cyano-2-methylpropyl 4'- octyloxy-3'-fluorobiphenyl-4-ylcarboxylate and exhibits the following properties: Response time (μsec) at 10 V/μm 35 24

Claims: Patent Claims: Chiral tilted smectic liquid crystal medium comprising at least two liquid crystalline compounds, wherein at least one compound is of the formula I wherein R 1 and R2 are each alkyl with 1-15 C atoms, and wherein one or two non-adjacent CH- groups can also be replaced by a bridging element selected from the group consisting of -0-, -S-, -CO-, -0-CO-, -CO-O-, -CO-S-, -S-CO-, -CHHalogen-, -CHCN-, -CH=CH-, and combinations of these bridging elements, with the proviso that two hetero atoms are not directly linked, Z is -CO-O-, -0-CO-, -CH O-, -0-CH-- or -CH-CH--, n is 0 or 1, and -1 c. L through L are each hydrogen or fluorine, at least one of L 1 through L4 being fluorine with the provisos that a) one of L and L is fluorine and the other is hydrogen and one of L 1 and L2 i.s hydrogen and the other is fluorine or hydrogen, if n = 0 and Z is -CO-O-, and b) one of L " I and L 7 is fluorine and the other s hydrogen and one of L and L is hydrogen and the other is fluorine or hydrogen, if n 0 and Z is -0-CO-. 2. Medium according to Claim 1, characterized in that in formula I Z is -CO-O-, R is alkyl or alkoxy with 5 2 to 15 C atoms and R is alkoxy with 5 to 15 C atoms. 3. Medium according to Claim 1 characterized in that L in formula I is fluorine. 4. Medium of Claim 1, wherein at least one compound is of ffoorrmmuullaa ]I, wherein at least two of L through L are fluorine. 5. Medium of Claim 1 or 4, wherein at least one compound is 5 6 of the formula I, wherein one of L and L is fluorine 5 6 and the other radical L or L is hydrogen, one of L 1 and L4 i.s fluori.ne and the other radical L1 or L4 i.s hydrogen, and L 2 and L3 are hydrogen. 6. Medium of at least one of Claims 1, 4 and 5, wherein at least one compound is of the formula I, wherein Z is -CO-O- or -O-CO-. 7. Medium of Claim 6, wherein at least one compound is of the formula I, wherein Z is -CO-O-. 8. Medium of Claim 1 comprising non-chiral compounds of formula I. 9. Medium of Claim 1 comprising at least one chiral compound of ormula I. 10. Medium of Claim 8, wherein at least one compound is of 1 2 the formula I, wherein R and R are selected from the group consisting of alkyl, alkyl-O, alkyl-CO-, alkyl-CO-O alkyl-O-CO- and alkyl-O-CO-O-. 11. Medium of Claim 10, wherein alkyl is straight-chain alkyl of 6 to 12 carbon atoms. 12. Medium of Claim 8, wherein Rl is alkyl-O- and R2 i.s alkyl. 13. Medium according to at least one of the claims 1 to 12, characterized in that it contains at least one compound selected from the formulae 1 to 6, F F R3 _X_X_ R' 3 6 wherein R , R are each alkyl or alkoxy with 5 to 15 C atoms, Z is -CO-O-, -O-CO- or a single bond, X 1, X2 and L 3 are each hydrogen or fluori.ne, R4 and R5 i.s alkyl with 5 to 15 C atoms, m is 1 or 2, n is O or 1, R are each and A is 1,4-cyclohexylene or 1,4-bicyclo [2.2.2.]octylene. 14. Medium according to at least one of the claims 1 to 13, characterized in that it contains at least one chiral dopant of the formula VI m wherein R is alkyl or alkoxy with 5 to 15 C atoms, • R is alkyl with 1 to 8 C atoms, m is 1 or 2, and A is 1, -phenylene optionally substituted by fluorine, pyridine-2,5-diyl, pyrimidine-2,5-diyl or trans-1,4- cyclohexylene. 15. Medium according to Claim 1 characterized in that it contains a trifluorinated biphenylcarboxylates of the formula Ic wherein R 1 and R2 have the meaning given in Claim 1 and X 1 and X2 are each hydrogen or fluorine at least one of X 1 and X2 being fluorine. 16. Fluorinated phenyl benzoates of the formula la wherein each alkyl independently from each other denotes an alkyl group with 5 to 15 C atoms, and wherein one CH--group can also be replaced by -0-, -S-, -CO-, -CO-O-, -O-CO- or -CH=CH, with the proviso that two heteroatoms are not directly linked, and X is fluorine or hydrogen. 17. Phenyl benzoates according to Claim 16, character¬ ized in that each alkyl independently from each other denotes a straight-chain alkyl group with 5 to 15 C atoms and L 3 and L4 are each hydrogen. 18. Fluorinated biphenylcarboxylates of the formula lb wherein R1 and R2 have the meaning given m Claim 1 and X 1 through X3 are each hydrogen or fluorine at least two of X 1 through X1 and X3 being fluorine. 19. Ferroelectric electrooptical display element, charac¬ terized in that it contains a medium according to at least one of the claims 1 to 15 as the dielectric.

Inventors: COATES DAVID, GREENFIELD SIMON, SAGE IAN CHARLES, COATES, DAVID, GREENFIELD, SIMON, SAGE, IAN, CHARLES
Applicants: MERCK PATENT GMBH, MERCK PATENT GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG
Application year: 1989
Main classification: C09K19
IPC codes: C07C69, C09K19, G02F1