OCTAFLUOROBIPHENYLS

Abstract: The invention relates to liquid crystalline compounds of formula (I), wherein R<1>, R<2> and n have the meanings indicated in Patent Claim 1, and to liquid crystalline mixtures with at least two liquid crystalline compounds characterized in that at least one of them is a compound of formula (I).

Description: Octafluorobiphenyls The invention relates to liquid crystalline compounds of formula I F F F F wherein n is 2, 3 or 4 and R 1 and R2 each signify a group of formula R-( A 1 -Z 1 ) p -(A 2 -Z 2 ) g - in which R is F, OH, Cl, Br, CN, NCO, NCS, N0 2 , NC or alkyl or perfluoroalkyl wherein alkyl each has 1-15 C atoms, wherein one or two non-adjacent CH ? or CF~ groups, respectively, may be replaced by -O-, -CO-, -0-CO-, -CO-O-, -0-CO-0-, -C≡C- and/or -CH=CH-, A1 and A2 independently are an unsubstituted or mono- or polysubstituted 1,4-cyclohexylene group, wherein one or two non-adjacent CH groups may be replaced by -O- and/or -S-, or denote a l,4-bicyclo[2,2,2]octylene group, or a 1, -phenylene group which is unsubstituted or substituted by one or up to four F and/or one or two Cl atoms and/or CH_ groups and/ or CN groups, wherein one or two CH groups may be replaced by N, z- denotes -CO-O-, -O-CO-, -CH 2 CH 2 -, -CHCN-CH ' - , -CH 2 -CHCN-, -CH=CH-, -C=C-, -OCH 2 -, -CH O-, -N=N-, -NO=N-, -N=NO- or a single bond. Z 2 denotes -CO-O-, -O-CO-, -CH 2 CH 2 -, -CHCN-CH 2 -, -CH 2 -CHCN-, -CH=CH-, -C≡C-, -OCH 2 ~, -CH^-, -N=N-, -NO=N-, -N=NO-, -(CH 2 > k -CO-0- or a single bond, k is 1 to 6, p is 0, 1 or 2, and q is 0 or 1, with the proviso that in the case n = 2 (a) the sum of p and q is 1, 2 or 3 and/or (b) the sum of p and q is 0 and at least one of the rests R is alkyl or perfluoralkyl wherein alkyl has 1-15 C atoms, wherein one or two non-adja¬ cent CH_ or CF 2 groups, respectively, may be replaced by -O-, -CO-, -O-CO-, -CO-O-, -O-CO-O-, -C≡C- and/or -CH=CH-, and in particular means whereby liquid crystalline mix¬ tures can be provided having physical properties optimised for practical applications. Liquid crystal phases are commonly exhibited by organic compounds having extended rod-like molecules, and are characterised in their liquid crystalline state by a degree of order intermediate between those of a crystal¬ line solid and of an isoptropic liquid respectively. The wide-spread use of liquid crystalline materials in electro- optic devices arises from their combination of fluid-like flow with an anisotropy of their physical properties that is typical for a crystalline material. When a liquid cystal material is utilised in an electro-optic device, optimal performance can only be obtained when the physical properties of the material are adjusted to extreme or optimal values to suit the particular application and de¬ vice geometry in use. Examples of the physical properties which may beneficially be changed to improve the utility of a liquid crystalline material for a particular appli- cation include such properties as the mesogenic phase range, the dielectric constants, the elastic constants, the viscosity coefficient, and the refractive indices of the material. The present invention provides a novel family of liquid crystal compounds which may be used by themselves or in admixture with other liquid crystal compounds classes to provide mixtures having advantageous combinations of liquid crystal properties, in particular low values of refractive indices and/or birefringence. It is well known that the performance of known optical and electro-optic devices can be improved by the reduction of the refractive indices or birefringence of the liquid crystalline material contained therein. For example, in the electroop ic dis- play based upon the twisted nematic mode of operation, the off-state transmission of light only achieves its ideal value for discreet values of the parameter U where U is defined by the relation ϋ = 2DΔn/λ where D is the thickness of the display cell, Δn is the birefringence of the liquid crystal and λ is the average wavelength of visible light and optimal performance in the display cell is obtained for values of U equal to , l5, etc. To allow use of a liquid crystalline material in a display cell it is therefore desirable that its bire¬ fringence should be adjusted so that the above equation is satisfied for the particular cell thickness which is chosen. In liquid crystal display cells containing di- chroic-dyed liquid cystal materials and operating in the cholesteric to nematic phase-change mode otherwise known as the White-Taylor mode of operation, the bire¬ fringence of the liquid crystalline host mixture leads to.the undesirable propagation of eliptically polarised light rays in the display cell which diminishes the optical efficiency of the display devices. It is there¬ fore desirable when designing liquid crystal materials for use in this type of display, to adjust the bire¬ fringence of the liquid crystalline phase to the smallest practical value in order to obtain the best performance from the display. In electro-optic switching devices in which the liquid crystal material is used on an overlay on a planar optical waveguide, or as a cladding material on a fibre waveguide, it is essential that at least one of the refractive indices of the liquid crystalline material used is lower in value than that of the wave¬ guide material or else the structure will no longer sustain the propagation of light within the waveguide. According to the mode of operation chosen for such a device, it may be required to have the waveguide refrac¬ tive index intermediate between the two refractive in¬ dices of the liquid crystalline overlayer, or to have the refractive index of the waveguide higher than either of the refractive indices of the liquid crystalline material. In liquid crystal mixtures intended for use in the NCAP display mode the clarity of the "ON" state of the device depends upon the accurate matching of the ordinary refractive index of the LC material to the re¬ fractive index of the supporting polymer matrix. The ability to alter the absolute refractive indices of the liquid crystal therefore both facilitates the formulation of mixtures for use in this device, and offers the opportunity to utilise a wider range of supporting poly¬ mers than would otherwise be possible. For simplicity, in the following text PheF. is a tetra- fluoro-l,4-phenylene group, Cy is a 1,4-cyclohexylene group, Dio is a l,3-dioxane-2,5-diyl group, Bi is a bicyclo[2,2,2]octylene-l,4-diyl group, Phe is a 1,4- phenylene group, Pym is pyrimidine-2,5-diyl group and Pyr is a pyridine-2 ,5-diyl group, it being possible for Cy and/or Phe to be unsubstituted or substituted by one or up to four F and/or one or two Cl atoms and/or one or two CH_ groups and/or one or two CN groups. The birefringence and refractive indices of organic compounds and in particular those of liquid crystalline compounds are dependent on the electronic polarisability of the constituent molecules. Liquid crystal compounds having a largely aromatic character are commonly charac¬ terised by large values of the refractive indices and the birefringence. It has now been found unexpectedly that the compounds of formula I uniquely combine a moderately large birefringence with low absolute values of the refractive indices and further uniquely provide liquid crystalline compounds with wide nematic phase ranges having an ordinary refractive index lower than that of a fused silica wave guide at ordinary temperatures and wavelengths. The compounds can therefore be used by themselves or in combination with other classes of liquid crystalline compounds to provide mixtures in which the physical properties, particularly the refractive indices are optimised for electro-optic device applications. The compounds of formula I can be used as components of liquid crystalline mixtures in particular for displays based on the twisted nematic and super twisted nematic cell, the effect of deformation of aligned phases or the NCAP effect,- and for optical switching devices using a liquid crystal overlayer on a waveguiding substrate. The invention was based on the object of discovering new stable partially fluorinated materials which are suitable as components of conventional liquid crystalline mixtures. Liquid crystals derived from 4,4'-disubstituted octa- fluorobiphenyl are also known (D. Demus et al.; Flϋssige Kristalle in Tabellen, VEB Deutscher Verlag fϋr Grund- stoffindustrie, Leipzig 1974 (Vol I) and 1984 (Vol II)), but the known materials are derivatives of Schiff bases which suffer from photochemical, hydrolytic and oxidative instability rendering them unsuitable for use in electro- optic devices. Further these compounds of the prior art show their liquid crystalline phase behaviour at very high temperature, which also makes them unsuitable for device applications. The compounds of formula I provide liquid crystalline compounds having their liquid crystalline phase ranges at conveniently low temperature and the standard mixing techniques known to those skilled in the art may be applied to formulate mixtures which are liquid crystal¬ line in the room temperature region and below. The compounds of the formula I have a wide range of application. Depending on the choice of the substituents, these compounds can be used as the base materials from which liquid crystalline mixtures are composed up to 50 % of the total constitutents; however, it is also possible for compounds of the formula I to be added to liquid crystalline base materials of other classes of compounds, in order to influence the optical aniso- tropy of such a dielectric. The compounds of the formula I are colourless in the pure state. They are vary stable towards chemicals, heat and light. The invention thus relates to liquid crystalline co - pounds of formula I. Furthermore the invention relates to liquid crystalline mixtures with at least two components characterized in that at least one component is a compound of the formula I Preferably the invention relates to liquid crystalline mixtures with at least two liquid crystalline compounds characterized in that at least one compound is a compound of formula I. The invention furthermore relates to the use of compounds of formula I as components of liquid crystalline mixtures, and electro-optic display devices containing such mixtures. 1 2 1 2 1 Above and below, R , R , R, n, p, q, k, A , A , Z and 2 Z have the meaning given, unless expressly indicated otherwise. The compounds of the formula I accordingly include pre¬ ferred compounds with two rings of the part formula la: R-PheF 4 -PheF 4 -R la compounds with three rings of part formulae lb R-PheF 4 -PheF 4 -Z 2 -A 2 -R lb compounds with four rings of part formulae Ic and Id R-PheF 4 -PheF 4 -Z 1 -A 1 -Z 1 -A 1 -R Ic R-A 2 -Z 2 -PheF 4 -PheF 4 -Z 1 -A 1 -R Id and compounds with five rings of part formulae le to If R-PheF 4 -PheF 4 -Z 2 -A 2 -(Z^A 1 ) 2 ~R le R-A 1 -Z ϊ -PheF 4 -PheF 4 -Z 2 -A 2 -Z i -A 1 -R If In the compounds of the formulae above and below, R preferably denotes independently F, alkyl, or further¬ more alkoxy. The groups A 1 and A2 are preferably Cy, Phe, Di;o, Pym, or Pyr; the compounds of the formula I preferably contain not more than one of the radicals Dio, Bi, Pym, or Pyr. Z1 and Z2 are preferably single bonds, and secondly 2 preferably -CO-O-, -O-CO- or -CH 2 CH 2 -groups. Z may also preferably denote -(CH 2 ),-CO-0-, k is preferably 1 or 2. Particularly preferred are compounds wherein 2 Z denotes -CO-O-. Particularly preferred are compounds wherein A 1 and A2 are Phe or Cy. If R is an alkyl radical and/or alkoxy radical, this radical can be straight-chain or branched. Preferably, it is straight-chain and has 2, 3, 4, 5, 6 or 7 C atoms and is accordingly preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propyloxy, butyloxy, pentyloxy, hexyloxy or heptyloxy, also methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy. Oxaalkyl is preferably straight-chain 2-oxapropyl (= me hoxymethyl), 2-(= ethoxymethyl) or 3-oxabutyl (= 2- methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6-, or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8- oxanonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl. If R is an alkyl radical in which a CH 2 group is re¬ placed by -CH=CH-, it can be straight-chain or branched. Preferably, it is straight-chain and has 2 to 10 C atoms It is accordingly, in particular, 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 or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl. Compounds of the formula I with branched terminal groups R can occasionally be of importance because of an improved solubility in the customary liquid crystal base materials, but in particular as chiral doping substances if they are optically active. Branched groups of this type as a rule contain not more than one chain branching. Preferred branched radicals R are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), 2-methylbutyl, isopentyl (= 3-methyl- butyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2- propylpentyl, isopropoxy, 2-methylpropoxy, 2-m thylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethyl- hexoxy, 1-methylhexoxy and 1-methylheptoxy. Formula I includes both the racemates of these compounds and the optical antipodes, as well as mixtures thereof. Those of the compounds of the formulae I, la to le in which at least one of the radicals contained therein has one of the preferred meanings mentioned are preferred. Particular preferred compounds of the formula I are those of the part formulae II to 132: R-PheF 4 -PheF 4 -R « I 1 R-PheF 4 -PheF 4 -Phe-R' I 2 R-PheF 4 -PheF 4 -Cyc-R f I 3 R-PheF 4 -PheF 4 -PheF 4 -R' I 4 R-PheF 4.-PheF4.-Z-Phe-R' I 5 R-PheF 4 -PheF 4 -Z-Cyc-R' I 6 R-PheF.-PheF.-Phe-Phe-R' I 7 4 4 R-Phe-PheF 4 -PheF 4 -Phe-R' I 8 R-Cyc-PheF 4 -PheF 4 -Phe-R' I 9 R~Cy-PheF 4 -PheF 4 -Cy-R « 110 R-PheF 4 -PheF 4 -Cy-Cy-R* 111 R-PheF 4 -PheF 4 -Phe-Cy-R' 112 R-PheF 4 -PheF 4 -Cy-Phe-R' I13 R-PheF 4 -PheF 4 -Phe 4 -Cy-R" 114 R-PheF 4 -PheF 4 -Phe 4 -Phe-R' 115 R-PheF 4 -PheF 4 -PheF 4 -PheF 4 -R' 116 R-PheF 4 -PheF 4 -Z-Phe-Phe-R » 117 R-PheF 4 -PheF 4 -Z-Cy-Phe-R « 118 R-PheF 4 -PheF 4 -Z-Phe-Cy-R' 119 R-PheF 4 -PheF 4 -Z-Cy-Cy-R' 120 R-PheF 4 -PheF 4 -Phe-Z-P e-R « 121 R-PheF 4 -PheF 4 -Phe-Z-Cy-R' 122 R-Phe-Z-PheF 4 -PheF 4 -Phe-R ! 123 R-Cy-Z-PheF 4 -PheF 4 -Phe-R' 124 R-Phe-Z-PheF 4 -PheF 4 -Cy-R ! 125 R-Cy-Z-PheF 4 -PheF 4 -Cy-R' 126 R-PheF 4 -PheF 4 -PHeF 4 -Z-Ehe-R' 127 R-PheF 4 -PheF 4 -PheF 4 -Z-Cy-R « 128 R-PheF 4 -PheF 4 -Z-PheF 4 -PheF 4 -R « 129 R-Phe-Z-PheF 4 -PheF 4 -Z-Phe-R « 130 R-Cy-Z-PheF 4 -PheF 4 -Z-Cy-R' 131 R-Cy-Z-PheF 4 -PheF 4 -Z-Phe-R' 132 In the preferred compounds of the part formulae II to 132 R and R' independently have one of the meanings of R in formula I. Preferably R is alkanoyloxy with up to 15 C atoms and R* denotes preferably alkyl with 2-15 C atoms, wherein one CH ? group is replaced by -O-CO- or -CO-O. A particularly preferred meaning or R' is -(CH 2 ) r -0-CO- CH 2 ) s -H, wherein r is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13 and s is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13 (r + s) is 2 to 13. In the compounds of the part formulae II to 132 the bridging groups Z denote each independently -CO-O-, -OCO-, -CH,-CH ? -, -C≡C-, -OCH 2 - or -CH 2 0-, preferably -CO-O-, -O-CO- or -CH_-CH Particularly preferred are compounds wherein the PheF.-groups are linked by an O-atom to the briding group Z. Particluarly preferred smaller groups of compounds of the formula I are those of formula II to XVI II: alkyl-PheF 4 -PheF 4 -alkyl alkyl-PheF 4 -PheF 4 -alkoxy alkyl-PheF 4 -PheF 4 -CO-0-alkyl alkyl-PheF 4 -PheF 4 -0-C0-alkyl alkoxy-PHeF 4 -PheF 4 -alkoxy III: alkyl-PheF 4 -PheF 4 -A-alkyl alkyl-PheF 4 -PheF 4 -A-alkoxy alkoxy-PheF 4 -PheF 4 -A-alkoxy alkoxy-PheF 4 -PheF 4 -A-alkyl F-PheF 4 -PheF 4 -A-alkyl • F-PheF 4 -PheF 4 -A-alkoxy IV: alkyl-PheF 4 -PheF 4 -C0-0-A-alkyl alkyl-PheF 4 -PheF 4 -CO-0-A-alkoxy alkoxy-PheF 4 -PheF 4 -CO-0-A-alkoxy alkoxy-PheF 4 -PheF 4 -C0-0-A-alkyl F-PheF 4 -PheF 4 -C0-0-A-alkyl F-PheF 4 -PheF 4 -C0-0-A-alkoxy V: alkyl-PheF 4 -PheF 4 -0-C0-A-alkyl alkyl-PheF 4 -PheF 4 -0-C0-A-alkoxy alkoxy-PheF 4 -PheF 4 -0-C0-A-alkoxy alkoxy-PheF 4 -PheF 4 -0-C0-A-alkyl F-PheF 4 -PheF 4 -0-CO-A-alkyl F-PheF 4 -PheF 4 -0-C0-A-alkoxy F-PheF 4 -PheF 4 -0-CH 2 -A-alkyl F-PheF 4 -PheF 4 -0-CH 2 -A-alkoxy alkoxy-PheF 4 -PheF 4 -0-CH 2 -A-alkyl alkoxy-PheF 4 -PheF 4 -0-CH 2 -A-alkoxy alkyl-PheF 4 -PheF 4 -0-CH 2 -A-alkyl alkyl-PheF 4 -PheF 4 -0-CH 2 -A-alkoxy VI: alkyl-PheF 4 -PheF 4 -A-A-alkyl alkyl-PheF 4 -PheF 4 -A-A-alkoxy alkoxy-PheF 4 -PheF 4 -A-A-alkyl alkoxy-PheF 4 -PheF 4 -A-A-alkoxy F-PheF 4 -PheF 4 -A-A-alkyl F-PheF 4 -PheF 4 -A-A-alkoxy VII: alkyl-A-PheF 4 -PheF 4 -A-alkyl alkoxy-A-PheF 4 -PheF 4 -A-alkyl alkoxy-A-PheF 4 -PheF 4 -A-alkoxy VIII: alkyl-PheF 4 -PheF 4 -PheF 4 -A-alkyl alkyl-PheF 4 -PheF 4 -PheF 4 -A-alkoxy alkoxy-PheF 4 -PheF 4 -PheF 4 -A-alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -A-alkoxy F-PheF 4 -PheF 4 -PheF 4 -A-alkyl F-PheF 4 -PheF 4 -PheF 4 -A-alkoxy IX: alkyl-PheF 4 -PheF 4 -PheF 4 -PheF 4 -alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -PheF 4 -alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -PheF 4 -alkoxy F-PheF 4 -PheF 4 -PheF 4 -PheF 4 -alkoxy F-PheF 4 -PheF 4 -PheF 4 -PheF 4 -alkyl F-PheF 4 -PheF 4 -PheF 4 -PheF 4 -F alkyl-PheF 4 -PheF 4 -O-CO-A-A-alkyl alkyl-PheF 4 -PheF 4 -0-C0-A-A-alkoxy alkoxy-PheF 4 -PheF 4 -0-CO-A-A-alkyl alkoxy-PheF 4 -PheF 4 -0-C0-A-A-alkoxy F-PheF 4 -PheF 4 -0-C0-A-A-alkyl F-PheF 4 -PheF 4 -0-CO-A-A-alkoxy XI: alkyl-PheF 4 -PheF 4 -CO-0-A-A-alkyl alkyl-PheF 4 -PheF 4 -CO-0-A-A-alkoxy alkoxy-PheF 4 -PheF 4 -C0-0-A-A-alkyl alkoxy-PheF 4 -PheF 4 -CO-0-A-A-alkoxy F-PheF 4 -PheF 4 -C0-0-A-A-alkyl F-PheF 4 -PheF 4 -C0-0-A-A-alkoxy XII: alkyl-A-CO-0-PheF 4 -PheF 4 -CO-0-A-alkyl alkyl-A-CO-0-PheF 4 -PheF 4 -CO-0-A-alkoxy alkoxy-A-C0-0-PheF 4 -PheF 4 -C0-0-A-alkyl alkoxy-A-C0-0-PheF 4 ~PheF 4 -C0-0-A-alkoxy alkoxy-A-0-C0-PheF 4 -PheF 4 -C0-0-A-alkyl alkoxy-A-0-C0-PheF 4 -PheF 4 ~C0-0-A-alkoxy alkyl-A-0-C0-PheF 4 -PheF 4 -C0-0-A-alkyl alkoxy-A-CO-O-PheF 4 -PheF 4 -0-CO-A-alkoxy alkyl-A-C0-0-PheF 4 -PheF 4 -0-C0-A-alkoxy alkyl-A-C0-0-PheF 4 -PheF 4 -0-C0-A-alkyl XI11: alkyl-PheF 4 -PheF 4 -PheF 4 -C0-0-A-alkyl allr^l-PheF 4 -PheF 4 -PheF 4 -C0-0-A-alkoxy alkoxy-PheF 4 -PheF 4 -PheF 4 -C0-0-A-alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -CO-0-A-alkoxy F-PheF 4 -PheF 4 -PheF 4 -C0-0-A-alkyl F-PheF 4 -PheF 4 -PheF 4 -C0-0-A-alkoxy XIV: alkyl-PheF 4 -PheF 4 -PheF 4 -0-C0-A-alkyl alkyl-PheF 4 -PheF 4 -PheF 4 -O-CO-A-alkoxy alkoxy-PheF 4 -PheF 4 -PheF 4 -O-C0-A-alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -0-CO-A-alkoxy F-PheF 4 -PheF 4 -PheF 4 -0-CO-A-alkyl F-PheF 4 -PheF 4 -PheF 4 -0-C0-A-alkoxy XV: alkyl-PheF 4 -PheF 4 -PheF 4 -0-CO-PheF 4 -alkyl alkyl-PheF 4 -PheF 4 -PheF 4 -0-CO-PheF 4 -alkoxy alkoxy-PheF 4 -PheF 4 -PheF 4 -O-C0-PheF 4 -alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -0-CO-PheF 4 -alkoxy F-PheF 4 -PheF 4 -PheF 4 -0-C0-PheF 4 -alkyl F-PheF 4 -PheF 4 -PheF 4 -0-C0-PheF 4 -alkoxy F-PheF 4 -PheF 4 -PheF 4 -C0-0-PheF 4 -alkyl F-PheF 4 -PheF 4 -PheF 4 -C0-0-PheF 4 -alkoxy alkoxy-PheF 4 -PheF 4 -PheF 4 -CO-0-PheF 4 -alkyl alkoxy-PheF 4 -PheF 4 -PheF 4 -CO-C-PheF 4 -alkoxy alkyl-PheF 4 -PheF 4 -PheF 4 -CO-0-PheF 4 -alkyl alkyl-PheF 4 -PheF 4 -PheF 4 -CO-C-PheF 4 -alkoxy XVI: alkyl-PheF 4 -PheF 4 -C0-0-PheF 4 -PheF 4 -alkyl alkyl-PheF 4 ~PheF 4 -C0-0-PheF 4 -PheF 4 -alkoxy alkoxy-PheF 4 -PheF 4 -C0-0-PheF 4 -PheF 4 -alkyl alkoxy-PheF 4 -PheF 4 -C0-0-PheF 4 -PheF 4 -alkoxy F-PheF 4 -PheF 4 -C0-0-PheF 4 -PheF 4 -alkyl F-PheF 4 -PheF 4 -C0-0-PheF 4 -PheF 4 -alkoxy F-PheF 4 -PheF 4 -0-CO-PheF 4 -PheF 4 -alkyl F-PheF 4 -PheF 4 -0-C0-PheF 4 -PheF 4 -alkoxy F-PheF 4 -PheF 4 -CH 2 -0-PheF 4 -PheF 4 -alkyl F-PheF 4 -PheF 4 -CH 2 -0-PheF 4 -PheF 4 ~alkoxy F-PheF 4 -PheF 4 -0-CH 2 -PheF 4 -PheF 4 -alkyl F-PheF 4 -PheF 4 -0-CH 2 -PheF 4 -PheF 4 -alkoxy alkoxy-PheF 4 -PheF 4 -CH 2 -0-PheF 4 -PheF 4 -alkyl alkoxy-PheF 4 -PheF 4 -CH 2 -0-PheF 4 ~PheF 4 -alkoxy alkyl-PheF 4 -PheF 4 -CH 2 -0-PheF 4 -PheF 4 -alkyl alkyl-PheF 4 -PheF 4 -CH 2 -0-PheF 4 -PheF 4 -alkoxy In the preferred compounds of the groups II to XVI the rings A denote each independently a Phe and/or a Cy-group. The structure element A-A of the compounds of the subgroups VI, X and XI denotes Phe-Phe, Cy-Cy, Phe-Cy or Cy-Phe, preferably Phe-Phe or Cy-Phe. In the compounds of the formula I, those stereoisomers in which the rings Cy are trans-l,4-disubstituted and/or Dio are txans-2,5-disubstituted are preferred. Those of the abovementioned formulae which contain one or more groups Dio, Pym and/or Pyr include in each case the two 2,5-position isomers. In the compound of the formula I in which A represents a Pym or Pyr ring which is substituted in the 2-position by R, R is preferably alkyl. The compounds of the formula I are prepared by methods which are known per se, such as are described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), and in particular under reaction conditions which are known and suitable for the reactions mentioned in more detail here can also be used in this connection. If desired, the starting substances can also be formed in situ, such that they are not isolated from the reaction mixture but are immediately reacted further to give the compounds of the formula I. Esters of the formula I can be obtained by esterification of corresponding carboxylic acids (or their reactive derivatives) with alcohols or phenols (or their reactive derivatives). The corresponding carboxylic acids and alcohols or phenols are known or can be prepared by processes analogous to known processes. Particularly suitable reactive derivatives of the carb- oxylic acids mentioned are the acid halides, above all the chlorides and bromides, and furthermore the anhydrides, for example also mixed anhydrides, preferably those of the corresponding carboxylic acids and trifluoroacetic acid formed in situ by mixing these carboxylic acids with trifluoroacetic anhydride, 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 alcoholates or 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, hydrocarbons, such as benzene, toluene or xylene, halogenohydrocarbons, such as carbon tetrachloride or tetrachloroethylene, and ' sulfoxides, such as dimethylsulfoxide or sulfolane. Water- immiscible solvents can simultaneously be advantageously used for azeotropic distillation of the water formed during the esterification. An excess of an organic base, for example pyri ine, quinoline or triethylamine, can occasionally also be used as the solvent for the esterifi- cation. The esterification 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. An¬ other preferred embodiment of the esterification com¬ prises first converting the alcohol or phenol into the sodium alcoholate 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 diemthylformamide to this suspension, ad¬ vantageously at temperatures between about -25° and +20°. The liquid crystalline mixtures according to the inven¬ tion consist of 3 to 25, preferably 4 to 15, components, at least one of which is a compound of the formula I. The other constitutents are preferably chosen from nema- tic or nematogenic substances, in particular the known substances, from the classes of azoxybenzenes, benzyli- deneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexylcyclo- hexanes, cyclohexylcyclohexenes, cyclhexylnaphthalenes, 1 ,4-bis-cyclohexylbenzenes, 4,4'-biscyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines , phenylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyldithianes, 1,2-bis-phenylethanes, 1,2-biscyclohexylethanes, 1-phenyl- 2-cyclohexylethanes , optionally halogenated stilbenes, benzyl phenyl ethers, tolanes and substituted cinnamic acids. The most important compounds which are possible consti¬ tuents of such liquid crystalline mixtures can be charac- terized by the formula 1 ~ 4 R -L-G-E-R 1 wherein L and E are each an unsubstituted or laterally fluoro- or cyano- substituted carbo- or hetero-cyclic ring system from the group comprising 1,4-disubstituted benzene and cyclohexane rings, 1,4-disubstituted 1-cyano- cyclohexane rings, 4,4-disubstituted biphenyl, phenyl - cyclohexane and cyclohexylcyclohexane systems, 2, 5-di- substituted naphthalene, di- and tetra-hydronaphthalene, quinazoline and tetrahydroquinazoline, G is -CH=CH- -N(0)=N- -CH=CY- -CH=N(0)- -C≡C- -CH 2 -CH 2 - -CO-O- -CH 2 -0- -CO-S- -CH 2 -S- -CH=N- -COO-Phe-COO- or a C-C single bond, Y is halogen, preferably chlorine, or -CN and R 3 and R4 are alkyl, alkoxy, alkanoyloxy or alkoxycarbonyloxy with up to 18, preferably up to 8, carbon atoms, it also being possible for one CH 2 group non-adjacent to an oxygen atom to be replaced by -0-, -CH=CH- or -C≡C-, or that one of the radicals R 3 and R4 may also denote CN, N0 2 , CF-, NCS, F, Cl or Br. In most of these compounds, R 3 and R4 are different from one another, one of these radicals usually being an alkyl or alkoxy group. However, other variants of the substi¬ tuents envisaged can also be used. Many such substances or mixtures thereof are commercially available. All of these substances can be prepared by methods which are known from the literature. The liquid crystalline mixtures according to the inven¬ tion contain about 0.5 to 100, preferably 15 to 100 %, of one or more compounds of the formula I. Liquid crystalline mixtures which contains 25 - 100, particular 30 - 90 %, of one or more compounds of the formula I can be used advantageously in the mixtures according to the invention. The liquid crystalline mixtures according to the invention are prepared in a manner which is customary per se. As a rule, the components are dissolved in one another, pre- ferably at elevated temperature. The liquid crystalline mixtures according to the invention can be modified by suitable additives such that they can be used in all the types of liquid crystal display elements disclosed to date. Such additives are known to the expert and are described in detail in the literature. For example, it is possible to add conductive salts, preferably ethyldimethyldodecyl- ammonium 4-hexyloxybenzoate, tetrabutylammonium tetra- phenylboranate or complex salts of crown ethers (compare, for example, I. Haller et al., Mol.Cryst.Liq.Cryst. Volume 24, pages 249-258 (1973)) for improving the conductivity, dichoric dyestuffs for the production of coloured guest/ host systems or substances for changing the dielectric anisotropy, the viscosity and/or the orientation of the nematic phases. Such substances are described, for example, in German Offenlegungsschrift 2,209,127, 2,240,863, 2,321,632, 2,338,281, 2,450,088, 2,637,430, 2,853,728 and 2,902,177. The following examples are intended to illustrate the invention without limiting it. Percentages above and below are percentages by weight. All the temperatures are given in degrees Centrigrade. The symbols are further¬ more as follows: Crϊ crystlline solid state, S: smectic phase (the index characterizes the phase type), N: nematic phase, Ch: cholesteric phase, I: isotropic phase. The figure between two symbols indicates the transition temperature. Example 1 4-Hydroxy-2,2 t ,3,3 T ,5,5 f ,6,6 t -octafluoro-4'-n-alkyloxy- biphenyls 4,4'-Dihydroxy-2,2 , ,3,3 I 5,5 , ,6,6 « -octafluorobiphenyl (3 g), (preparation described by Lit. ) methylated spirit (10 cm 3 ) and n-pentyl bromide (1.36 g) are gently heated under reflux, with stirring. Potassium hydroxide (0.6 g) in water (10 cm 3 ) is added dropwise to the mixture over 2 h. On completion of the addition, the reaction mixture is stirred and heated under reflux for a further 3 h. The reaction mixture is then cooled, acidified with 4M- aqueous hydrochloric acid, extracted with dichloromethane (3 x 20 cm 3 ), the extract is dried over anhydrous magnesium sulphate, and the solvent removed under reduced pressure. 4-Hydroxy-2,2 ■ ,3,3' ,5,5',6,6'-octafluoro-4 r - n-pentyloxybiphenyl is obtained as a clear oil, b.p. 163-164 °C/0.04 torr. ' Reference I.L. Knunyants and G.G. Yakobson, Synthesis of Fluoroorganic Compounds, Springer Verlag, Berlin, Heidelberg, New York, Tokyo, 1985, p. 172 The.following compounds are obtained analogously: 4-hydroxy-2,2',3,3' ,5,5' ,6,6'-octafluoro-4'-ethoxy- biphenyl 4-hydroxy-2,2' ,3,3' ,5,5' ,6,6*-octafluoro-4'-n-propyl- oxybiphenyl 4-hydroxy-2 , 2 - ,3,3' ,5,5' ,6,6'-octafluoro-4'-n-butyloxy- biphenyl 4-hydroxy-2,2' ,3,3' ,5,5' ,6,6'-octafluoro- '-n-hexyloxy- biphenyl 4-hydroxy-2,2 I ,3,3',5,5 I ,6,6'-octafluoro-4'-n-heptyloxy- biphenyl 4-hydroxy-2,2',3,3 , ,5,5 , ,6,6'-octafluoro-4'-n-octyloxy- biphenyl 4-hydroxy-2,2' ,3,3' ,5,5' ,6,6'-octafluoro-4'-n-nonyloxy- biphenyl 4-hydroxy-2,2 , ,3,3 t ,5,5 , ,6,6'-octafluoro-4'-n-decyloxy- biphenyl Example 2 2,2',3,3',5,5' ,6,6*-Octafluoro-4'-alkyloxybiphenyl- 4-yl trans-4-n-alkylcyclohexane-l-carboxylates Trans-4-n-pentylcyclohexane-l-carboxylic acid (0.7 g), and 4-hydroxy-2,2 l ,3,3 , ,5,5',6,6 t -octafluoro-4'-n- pentyloxybiphenyl (1.35 g) are added with stirring to dry methylene chloride (20 cm 3 ) contained in a 50 cm 3 round-bottomed flask equipped with a calcium chloride guard tube. Trifluoroacetic anhydride (0.95 g) is added and the reaction mixture stirred at room temperature for 15 h. The solvent is removed under reduced pressure and the residue recrystallised from ethanol affording 2,2 l ,3,3 , 5,5 , ,6,6 « -octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-pentylcyclohexane-l-carboxylate, 1.25 g, as a white crystalline solid, Cr 41.5 °C N 112 °C I. The following are obtained analogously: 2,2 , -3,3',5,5 , ,6,6'-octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-ethylcyclohexane-l-carboxylate 2,2'-3,3' ,5,5' ,6,6'-octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-propylcyclohexane-l-carboxylate Cr 58.5 ° N 105.3 ° I 2,2 , -3,3 , ,5,5 , ,6,6 l -octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-butylcyclohexane-l-carboxylate 2,2 t -3,3',5,5 , ,6,6 f -octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-hexylcyclohexane-l-carboxylate 2,2 l -3,3 , ,5,5*,6,6'-octafluoro-4*-n-pentyloxybiphenyl- 4-yl trans-4-n-heptylcyclohexane-l-carboxylate Cr. 59.5 °, N 102 ° I 2, 2 --3,3' ,5,5',6,6'-octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-octylcyclohexane-l-carboxylate 2,2'-3,3' ,5,5*,6,6'-octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-nonylcyclohexane-l-carboxylate 2,2'-3,3' ,5,5',6,6'-octafluoro-4'-n-pentyloxybiphenyl- 4-yl trans-4-n-decylcyclohexane-l-carboxylate 2,2 , -3,3',5,5 , ,6,6 l -octafluoro-4 « -ethoxybiphenyl-4-yl trans-4-ethylcyclohexane-l-carboxylate 2,2 l -3,3 , ,5,5 , ,6,6'-octafluoro-4'-ethoxybiphenyl-4-yl trans-4-n-butylcyclohexane-l-carboxylate 2, 2 --3,3',5,5* ,6,6'-octafluoro-4 • -ethoxybiphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate 2,2 r -3,3 , ,5,5',6,6'-octafluoro-4•-ethθ3jybiphenyl-4-yl trans-4-n-hexylcyclohexane-l-carboxylate 2,2*-3,3',5,5 , ,6,6'-octafluoro-4'-ethoxybiphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate 2,2'-3,3' ,5,5',6,6'-octafluoro-4 • -ethoxybiphenyl-4-yl trans-4-n-octylcyclohexane-l-carboxylate 2,2'-3,3' ,5,5',6,6'-octafluoro-4'-ethoxybiphenyl-4-yl trans-4-n-nonylcyclohexane-l-carboxylate 2,2'-3,3',5,5*,6,6*-octafluoro-4'-ethoxybiphenyl-4-yl trans-4-n-decylcyclohexane-l-carboxylate 2, 2 --3,3',5,5',6,6'-octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-ethylcyclohexane-l-carboxylate 2,2'-3,3',5,5',6,6'-octafluoro-4 ! -n-octyloxybiphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate 2,2'-3,3 f ,5,5 l ,6,6 l -octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-n-butylcyclohexane-l-carboxylate 2,2 l -3,3',5,5',6,6'-octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate 2,2 , -3,3 , ,5,5 , ,6,6'-octafluoro-4*-n-octyloxybiphenyl-4-yl trans-4-n-hexylcyclohexane-l-carboxylate Cr. 48.5 ° N 94.3° I 2,2'-3,3 l ,5,5 l ,6,6'-octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate 2,2'-3,3',5,5 l ,6,6 l -octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-n-octylcyclohexane-l-carboxylate 2,2 , -3,3' r 5,5',6 / 6 l -octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-n-nonylcyclohexane-l-carboxylate 2, 2 ~-3,3',5,5',6,6'-octafluoro-4'-n-octyloxybiphenyl-4-yl trans-4-n-decylcyclohexane-l-carboxylate Example 3 4-n-Alkyl-2,2',3,3* ,4',5,5' ,6,6'-nonafluorobiphenyls A vigorously stirred solution of decafluorobiphenyl (20 g) in dry ether (280 cm 3 ) is cooled to -10 °C. In an atmos- phere of dry nitrogen 1.6 M n-butyllithium (40 cm 3 ) is added at such a rate that the temperature does not rise above -5 °C. After completion of the addition, the reac¬ tion mixture is maintained at -10 °C for 1 h and the temperature is then allowed to rise. When room tempera- ture is reached water (200 cm 3 ) is added, at first drop- wise to destroy any unreacted n-butyllithium. The ether layer is separated and the aqueous layer extracted with ether (3 x 120 cm 3 ). The ether solutions are combined, washed with water (200 cm 3 ), dried over anhydrous magnesium sulphate, and the solvent removed under re¬ duced pressure. When the residual oil is purified by short-path distillation 4-n-butyl-2 2 , ,3,3 , ,4',5,5 , , 6,6*-nonafluorobiphenyl is obtained as a clear oil, 12.8 g, b.p. 90-95 °C/0.01 torr. The following compounds are obtained analogously 4-methyl-2,2',3,3',4* ,5,5',6,6'-nonafluorobiphenyl 4-ethyl-2,2*,3,3* ,4 f ,5,5* ,6,6*-nonafluorobiphenyl 4-n-propyl-2,2 , ,3,3',4 , ,5,5 I ,6,6'-nonafluorobiphenyl 4-(1-methylpropyl)-2,2 , ,3,3 , ,4',5 r 5',6,6 f -nonafluorobi¬ phenyl 4-n-pentyl-2,2',3,3',4* ,5,5' ,6,6'-nonafluorobiphenyl 4-n-hexyl-2,2',3,3',4 I ,5,5 , ,6,6*-nonafluorobiphenyl 4-n-heptyl-2,2 l ,3,3 t ,4',5,5 , ,6,6'-nonafluorobiphenyl 4-n-octyl-2,2* ,3,3',4' ,5,5' ,6,6'-nonafluorobiphenyl Example 4 4-n-Alkyl-4 f -hydroxy-2,2',3,3',4',5,5',6,6'-octafluoro- biphenyls 4-n-Butyl-2,2' ,3,3' ,4' ,5,5*,6,6'-nonafluorobiphenyl (12 g), potassium hydroxide (18 g) and tert-butyl alcohol (240 cm 3 ) are placed in a 500 cm 3 round-bottomed flask equipped with a reflux condenser. The mixture is heated under re¬ flux for 1 h, then water (200 cm 3 ) is added and an aqueous solution of tert-butyl alcohol distilled off until the temperature at the still-head reaches 99-100 °C. The reaction mixture is cooled and acidified with 4M-aqueous hydrochloric acid and extracted with ether (3 x 150 cm 3 ). The combined extracts are dried over anhydrous magnesium sulphate and the solvent removed under reduced pressure. The crude phenol is purified by flash chromatography on silica gel eluted with 5:1 light petroleum (b.p. 60-80 °C) ethyl acetate. The solvents are removed under reduced . ,. pressure whereafter short-path distillation, of the pro¬ duct affords the pure 4-n-butyl-4'-hydroxy-2,2',3,3',5,5', 6,6*-octafluorobiphenyl as a clear oil, 4.0 g, b.p. 190- 192/0.1 torr. The following compounds are obtained analogously: 4-methyl-4 « -hydroxy-2,2',3,3',5,5',6,6 , -octafluorobi¬ phenyl 4-ethyl-4*-hydroxy-2,2',3,3 I ,5,5 I ,6,6'-octafluorobi¬ phenyl 4-n-propyl-4 « -hydroxy-2,2',3,3',5,5',6,6'-octafluorobi¬ phenyl 4-(1-m thylpropyl)-4 « -hydroxy-2,2',3,3* ,5,5',6,6'-octa- fluorobiphenyl 4-n-pentyl-4 « -hydroxy-2,2',3,3',5,5',6,6'-octafluorobi- phenyl 4-n-hexyl-4'-hydroxy-2,2',3,3',5,5 , ,6,6 t -octafluorobi¬ phenyl 4-n-heptyl-4'-hydroxy-2,2',3,3',5,5',6,6 « -octafluorobi¬ phenyl 4-n-octyl-4'-hydroxy-2,2' ,3,3* ,5,5',6,6'-octafluorobi¬ phenyl Example 5 2,2* ,3,3* ,5,5',6,6*-Octafluoro-4'-n-alkylbiphenyl-4-yl trans-4-n-alkylcyclohexane-l-carboxylates 2,2',3,3',5,5 , ,6,6*-Octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate is prepared and purified in an analogous manner to that described for the esterification of trans-4-n-pentylcyclohexane- 1-carboxylic acid with 4-hydroxy-2,2',3,3',5,5',6,6'- octafluoro-4'-n-pentyloxybiphenyl in the presence of trifluoroacetic anhydride (cp. example 3) in this case with 4-n-butyl-4'-hydroxy-2,2',3,3',5,5',6,6'-octa- fluorobiphenyl as starting material. The ester is ob¬ tained as a white crystalline solid, Cr 63 °, N 88.5 ° I. The following compounds are obtained analogously: 2,2 « ,3,3 « ,5,5',6,6*-octafluoro-4'-n-butylbiphenyl-4-yl trans-4-ethylcyclohexane-l-carboxylate 2,2',3,3',5,5 « ,6,6'-octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate Cr 77.5° N 93° I 2,2',3,3',5,5 « ,6,6'-octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-butylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6*-octafluoro-4*-n-butylbiphenyl-4-yl trans-4-n-hexylcyclohexane-l-carboxylate 2,2',3,3*,5,5',6,6'-octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate Cr 64 ° N 82.5° I 2,2',3,3',5,5',6,6'-octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-octylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-nonylcyclohexane-l-carboxylate 2,2',3,3',5,5*,6,6 I -octafluoro-4'-n-butylbiphenyl-4-yl trans-4-n-decylcyclohexane-l-carboxylate 2,2*,3,3',5,5 « ,6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-ethylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-butylcyclohexane-l-carboxylate 2,2',3,3*,5,5 , ,6,6 I -octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6 I -octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-hexylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate 2,2',3,3*,5,5 , ,6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-octylcyclohexane-l-carboxylate 2,2 « ,3,3',5,5',6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-nonylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-ethylbiphenyl-4-yl trans-4-n-decylcyclohexane-l-carboxylate 2,2',3,3',5,5 , ,6,6*-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-ethylcyclohexane-l-carboxylate 2,2',3,3*,5,5 « ,6,6'-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-butylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6'-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate 2,2',3,3',5,5* ,6,6'-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-hexylcyclohexane-l-carboxylate 2,2' ,3,3',5,5',6,6'-octafluoro-4 » -n-oct lbiphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate 2,2',3,3',5,5',6,6 I -octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-octylcyclohexane-l-carboxylate 2,2',3,3',5,5*,6,6*-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-nonylcyclohexane-l-carboxylate 2,2',3,3',5,5*,6,6*-octafluoro-4'-n-octylbiphenyl-4-yl trans-4-n-decylcyclohexane-l-carboxylate Example 6 A mixture is formulated containing 40 % of 2,2',3,3',5,5',6,6'-octafluoro-4'-n-pentyloxy- biphenyl-4-yl trans 4-n-pentylcyclohexane-l-carboxylate 30 % of 2,2',3,3',5,5' 6,6'-octafluoro-4'-n-pentyloxγ- biphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate 30 % of 2,2',3,3',5,5',6,6'-octafluoro-4'-n-pentyloxy- biphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate The nematic to isotropic transition temperature of this mixture is measured and found to be 102 °C. Further the physical properties of the mixture are measured at 20 °C and found to be Dielectric anisotropy -0.5 Refractive indices at 589 nm Δn = 0.1224 Example 7 A mixture is formulated containing 36 % of 2,2',3,3' ,5,5' ,6,6'-octafluoro-4 « -n-pentyloxy- biphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate 27 % of 2,2',3,3 , ,5,5',6,6 « -octafluoro-4'-n-pentyloxy- biphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate 26 % of 2,2' ,3,3' ,5,5* ,6,6 « -octafluoro-4•-n-pentyloxy- biphenyl-4-yl trans-4-n-heptylcyclohexane-l-carboxylate 10 % of trans-4-n-propylcyclohexyl-4-benzonitrile The refractive indices of this mixutre at 20 °C and 577 nm are measured and found to be Δn = 0.1019 Example 8 A mixture is formulated containing 33.3 % of 2,2' ,3,3' ,5,5' ,6,6'-octafluoro-4'-n-butyl- biphenyl-4-yl trans-4-n-propylcyclohexane-l-carboxylate, 33.3 % of 2,2 « ,3,3' ,5,5' ,6,6'-octafluoro-4'-n-butyl- biphenyl-4-yl trans-4-n-pentylcyclohexane-l-carboxylate, 33.4 % of 2,2' ,3,3" ,5,5' ,6,6'-octafluoro-4'-n-butyl- biphenyl-4-yl trans-4-n-hep ylcyclohexane-1-carboxylate The mixture forms a nematic liquid crystal phase which crystallises slowly on standing on room temperature. The physical properties of the mixture are measured and found to be: Cr. 44.0 ° N 83 ° I Δε +0.05 no at 20 °C and 589.6 nm = 1.4515 Example 9 An electro-optic display cell constiructed of 2 parallel sheets of glass carrying conduction indium tin oxide electrodes on the inner surfaces is filled with the mix¬ ture described in example 7. The glass surfaces are further treated with a rubber polymer aligning layer to from a twisted nematic structure within the display cell. The cell is placed between crossed sheets of polarising film and the electro-optic characteristics of the display are evaluated at 20 °C by measuring the percentage transmission of light under different applied voltages. The following results are found V 90,0,20 - • - - VOltS V 10,0,20 4*57 VOltS V 50,10,20 3.38 volts V 90,45,20 2.55 volts M 20 1.77 M '20 1.50 The significance of the voltage above is described by the three subscripts appended to each voltage, in which the first subscript describes the percentage transmission of light through the electro-optic cell when viewed be¬ tween crossed polarising film, the second subscript gives the angle in degrees away from normal incidence in the quadrant of the cell having the lowest threshold voltage at which the measurement is made, and the third subscript gives the temperature in degrees centigrade at which the measurement is made. The figures 2Q and M ' Λ defined as below are figures of merit relating to the variation of contrast in the cell under different conditions of angular view. By virtue of the near ideal match of the optical thickness of the cell to the ideal value a good viewing angle characteristic can be obtained. M 20 = V 10,0,20^90,45,20 M '20 = ^0,10,20^90,45,20

Claims: Octafluorobiphenyls Claims: 1. A liquid crystalline compound of formula I wherein n is 2, 3 or 4 and R 1 and R2 each signify a group of formula R-(A 1 -Z 1 ) p -(A 2 -Z 2 ) g - in which R is F, OH, Cl, Br, CN, NCO, NCS, 0 2 , NC or alkyl or perfluoroalkyl wherein alkyl each has 1-15 C atoms, wherein one or two non- djacent CH^ or CF 2 groups, respectively, may be replaced by -O-, -CO-, -O-CO-, -CO-O-, -O-CO-O-, -C≡C- and/or -CH=CH-, A1 and A2 i.ndependently are an unsubstituted or mono- or polysubstituted 1,4-cyclohexylene group, wherein one or two non-adjacent CH ? groups may be replaced by -O- and/or -S-, or denote a l,4-bicyclo[2,2,2]octylene group, or a 1,4-phenylene group which is unsubstituted or substituted by one or up to four F and/or one or two Cl atoms and/or CH 3 groups and/or CN groups, wherein one or two CH groups may be replaced by N, 1 Z denotes -CO-O-, -O-CO-, -CH 2 CH 2 -, -CHCN-CH 2 -, -CH 2 -CHCN-, -CH=CH-, -C≡C-, -OCH 2 ~, -CH 2 0-, -N=N-, -NO=N-, -N=NO- or a single bond, Z 2 denotes -CO-O-, -O-CO-, -CH 2 CH -, -CHCN-CH 2 -, -CH 2 -CHCN-, -CH=CH-, -C≡C-, -OCH 2 -, -CH 2 0-, -N=N-, -NO=N-, -N=NO-, -(CH 2 ) k -CO-0- or a single bond, k is 1 to 6, p is 0, 1 or 2, and q is 0 or 1, with the proviso that in the case n = 2 (a) the sum of p and q is 1, 2 or 3 and/or (b) the sum of p and q is 0 and at least one of the rests R is alkyl or perfluoralkyl wherein alkyl has 1-15 C atoms, wherein one or two non-adjacent CH 2 or CF 2 groups, respectively, may be replaced by -0-, -CO-, -O-CO-, -CO-O-, -O-CO-O-, -C≡C- and/or -CH=CH-. 2. A liquid crystalline mixture with at least two liquid crystalline compounds characterized by the fact that at least one compound is of the formula I. 3. A means of reducing the refractive indices of a liquid crystalline mixture by adding one or more compounds of formula I. 4. An electro-optic display device containing a liquid crystalline mixture according to claim 2. 5. An electro-optic display device containing a liquid crystalline mixture according to claim 3. 6. An optical waveguide switching device containing a liquid crystalline mixture according to claim 2. 7. An optical waveguide switching device containing a liquid crystalline mixture according to claim 3.

Inventors: BROWN JOHN WILLIAM, BYRON DAVID JOHN, SAGE IAN CHARLES, WILSON ROBERT CHARLES, BROWN, JOHN, WILLIAM, BYRON, DAVID, JOHN, SAGE, IAN, CHARLES, WILSON, ROBERT, CHARLES
Applicants: MERCK PATENT GMBH, MERCK PATENT GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG
Application year: 1989
Main classification: C07C69
IPC codes: C07C17, C07C25, C07C39, C07C43, C07C69, C07C255, C07C331, C07D319, C09K19, G02F1