GB2249309A - Polyfluorobiphenyls - Google Patents

Abstract

Polyfluorobiphenyls of the formula <IMAGE> wherein A=B=A'=B=F and C=D=C'=D''=H, or B=C=B'=C'=F and A=A'=D=D'=H, or A=A'=D=D'=F and B=C=B'=C'=H, or A=B=C=A'=B'=C'=F and D=D'=H or A=B=D=A'=B'=D'=F and C=C'=H, and R<1> and R<2> each signify R-(A<1>-Z<1>)p-)A<2>-Z<2>) q- in which R is F, OH, Cl, Br, CN, NCO, NCS, NO2, NC or alkyl or perfluoroalkyl,wherein one or two non-adjacent @@@ CH2 or CF2 groups, respectively, may be replaced by -O-, -CO-, -O-CO-, -CO-O-, @@@ -O-CO-O-, -C IDENTICAL 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 CH2 groups may be replaced by -O- and/or -S-, or denote a 1,4-bicyclo @@@ [2,2,2]octylene group, or a 1,4-phenylene group which is unsubstituted or substituted by one to four @@@ F and/or one or two Cl atoms and/or CH3 @@@ groups and/or CN groups, wherein one or two CH groups may be replaced by N, Z<1> denotes -CO-O-, -O-CO-, -CH2CH2-, -CHCN-CH2-, -CH2-CHCN-, -CH=CH-, @@@ -C IDENTICAL C-, -OCH2-, -CH2O-, -N=N-, @@@ -NO=N-, -N=NO- or a single bond, Z<2> denotes -CO-O-, -O-CO-, -CH2CH2-, -CHCN-CH2-, -CH2-CHCN-, -CH=CH-, @@@ -C IDENTICAL C-, -OCH2-, -CH2O-, -N=N-, @@@ -NO=N-, -N=NO-, -(CH2)k-CO-O @@@ or a single bond, @

Description

Polyfluorobiphenyls DESCRIPTION The invention relates to polyfluorobiphenyls of the formula I

wherein A=B=A'=B'=F and C=D=C'=D'=H, or B=C=B'=C'=F and A=A'=D=D'=H, or A=A'=D=D'=F and B=C=B'=C'=H, or A=B=C=A'=B'=C'=F and D=D'=H, or A=B=D=A'=B'=D'=F and C=C'=H, and Rl and R2 each signify a group of formula R-(A-Z)p - (A-Z)q in which R is F, OH, Cl, Br, CN, NCO, NCS, NO2, NC or alkyl or perfluoroalkyl wherein alkyl each has 1-15 C atoms, wherein one or two non-adjacent CH2 or cF2 groups, respectively, may be replaced by -O-, -CO-, -O-CO-, -CO-O-, -O-CO-O-, -C-C and/or -CH=CH-, Al and A2 independently are an unsubstituted or mono- or polysubstituted 1,4-cyclohexylene group, wherein one or two non-adjacent CH2 groups may be replaced by -0- andlor -S-, or denote a 1,4-bicyclo [2,2,2]octylene group, or a 1,4-phenylene group which is unsubstituted or substituted by one or up to four F andlor one or two Cl atoms and/or CH3 groups and/or CN groups, wherein one or two CH groups may be replaced by N, Z1 denotes -CO-O-, -O-CO-, -CH2CH2-, -CHCN-CH2-, -CH2 CHCN-, -CH=CH-, -C-c-, -OCH2-, -CH2O-, -N=N-, -NO=N-, -N=NO- or a single bond, z2 denotes -CO-O-, -O-CO-, -CH2CH2-, -CHCN-CH2-, -CH2 CHCN-, -CH=CH-, -C=C-, -OCH2-, -CH2O-, -N=N-, -NO=N-, -N=NO-, -(CH2)k-CO-O- or a single bond, k is 1 to 6, p is O, 1 or 2, and q is O or 1, and in particular means whereby liquid crystalline mixtures 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 crystalline solid and of an isotropic liquid respectively. The wide-spread use of liquid crystalline materials in electrooptic 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 device 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 application 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 electrooptic display 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 U = 2DAn/A where D is the thickness of the display cell, An is the birefringence of the liquid crystal and A is the average wavelength of visible light and optimal performance in the display cell is obtained for values of U equal to 3, ;15, etc. To allow use of a liquid crystalline material in a display cell it is therefore desirable that its birefringence should be adjusted so that the above equation is satisfied for the particular cell thickness which is chosen.In liquid crystal display cells containing dichroic-dyed liquid cystal materials and operating in the cholesteric to nematic phasechange mode otherwise known as the White-Taylor mode of operation, the birefringence 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 therefore desirable when designing liquid crystal materials for use in this type of display, to adjust the birefringence 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 waveguide 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 refractive index intermediate between the two refractive indices 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 refractive 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 polymers than would otherwise be possible.

For simplicity, in the following text PheF2 is

BiphF is

PheF4 is a tetrafluoro-1,4phenylene group, Cy is a 1,4-cyclohexylene group, Dio is a 1,3-dioxane-2,5-diyl group, Bi is a bicyclo[2.2.2]octylene-1,4-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 andlor Phe to be unsubstituted or substituted by one or up to four F and/or one or two C1 atoms andlor one or two CH3 groups and/or one or two CM 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 characterised 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 birefrin gence 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 octafluorobiphenyl are also known (D. Demus et al.; Flussige Kristalle in Tabellen, VEB Deutscher Verlag fir Grundstoffindustrie, 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 electrooptic 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 crystalline 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 anisotropy of such a dielectric.

The compounds of the formula I are colourless in the pure state. They are very stable towards chemicals, heat and light.

The invention thus relates to liquid crystalline compounds 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.

Above and below, R1, R2, R, p, q, k, A1, A2, Z1 and z2 have the meaning given, unless expressly indicated otherwise.

The compounds of the formula I accordingly include preferred compounds with two rings of the part formula Ia: R-BiphF4-R Ia compounds with three rings of part formulae Ib R-BiphF4-Z2-A2-R Ib and compounds with four rings of part formulae Ic and Id R-BiphF4-Zl-Al-Zl-Al-R Ic R-A-Z-BiphF4-Z1-A1-R Id In the compounds of the formulae above and below, R preferably denotes independently F, alkyl, or furthermore alkoxy.

The groups A1 and A2 are preferably Cy, Phe, Dio, 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 preferably -CO-O-, -O-CO- or -CH2CH2-groups. z2 may also preferably denote -(CH2)k-CO-O-, k is preferably 1 or 2. Particularly preferred are compounds wherein z2 denotes -CO-O-. Particularly preferred are compounds wherein A1 and A2 are Phe or Cy.

If R is an alkyl radical andlor 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 (= methoxymethyl), 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 CH2 group is replaced 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-l-. -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-l-, -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 (= l-methylpropyl), isobutyl (= 2-methylpropyl), 2-methylbutyl, isopentyl (= 3-methyl-butyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 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, Ia to Ie 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 I1 to I32: R-BiphF-R' I 1 R-BiphF-Phe-R' 12 R-BiphF-Cyc-R' 13 R-BiphF4-PheF4-R' I 4 R-BiphF4-Z-Phe-R' I 5 R-BiphF4-Z-Cyc-R' I 6 R-BiphF4-Phe-Phe-R' I 7 R-Phe-BiphF4-Phe-R' I 8 R-Cyc-BiphF4-Phe-R' I 9 R-Cy-BiphF4-Cy-R' I 10 R-BiphF4-Cy-Cy-R' I 11 R-BiphF4-Phe-Cy-R' I 12 R-BiphF4-Cy-Phe-R' I 13 R-BiphF4-Phe4-Cy-R' I 14 R-BiphF4-Phe4-Phe-R' I 15 R-BiphF4-BiphF4-R' I 16 R-BiphF4-Z-Phe-Phe-R' I 17 R-BiphF4-Z-Cy-Phe-R' I 18 R-BiphF4-Z-Phe-Cy-R' I 19 R-BiphF4-Z-Cy-Cy-R' I 20 R-BiphF4-Phe-Z-Phe-R' I 21 R-BiphF4-Phe-Z-Cy-R' I 22 R-Phe-Z-BiphF4-Phe-R' I 23 R-Cy-Z-BiphF4-Phe-R' I 24 R-Phe-BiphF4-Cy-R' I 25 R-Cy-Z-BiphF4-Cy-R' I 26 R-BiphF4-Z-Phe-R' I 27 R-BiphF4-PheF4-Z-Cy-R' I 28 R-BiphF4-Z-BiphF4-R' I 29 R-Phe-Z-BiphF4-Z-Phe-R' I 30 R-Cy-Z-BiphF4-Z-Cy-R' I 31 R-Cy-Z-BiphF4-Z-Phe-R' I 32 In the preferred compounds of the part formulae I1 to I32 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 CH2 group is replaced by -O-CO- or -CO-O. A particularly preferred meaning of R' is -(CH2)r-0-CO-CH2)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 I1 to I32 the bridging groups Z denote each independently -CO-O-, -OCO-, -CH2-CH2-, -CC-, -OCH2- or -CH20-, preferably -CO-O-, -O-CO- or -CH2 CH2-. Particularly preferred are compounds wherein the PheF4groups are linked by an O-atom to the bridging group Z.

Particularly preferred smaller groups of compounds of the formula I are those of formula II to XVI II: alkyl-BiphF4-alkyl alkyl-BiphF4-alkoxy alkyl-BiphF4-CO-O-alkyl alkyl-BiphF4-O-CO-alkyl alkoxy-BiphF4-alkoxy III: alkyl-BiphF4-A-alkyl alkyl-BiphF4-A-alkoxy alkoxy-BiphF4-A-alkoxy a lkoxy-BiphF4-A-alkyl F-BiphF4-A-alkyl F-BiphF4-A-alkoxy IV: alkyl-BiphF4-CO-O-A-alkyl alkyl-BiphF4-CO-O-A-alkoxy alkoxy-BiphF4-CO-O-A-alkoxy alkoxy-BiphF4-CO-O-A-alkyl F-BiphF4-CO-O-A-alkyl F-BiphF4-CO-O-A-alkoxy V: alkyl-BiphF4-O-CO-A-alkyl alkyl-BiphF4-O-CO-A-alkoxy alkoxy-BiphF4-O-CO-A-alkoxy alkoxy-BiphF4-O-CO-A-alkyl F-BiphF4-O-CO-A-alkyl F-BiphF4-O-CO-A-alkoxy F-BiphF4-O-CHz-A-alkyl F-BiphF4-O-CH2-A-alkoxy alkoxy-BiphF4-O-CH2-A-alkyl alkoxy-BiphF4-O-CH2-A-alkoxy alkyl-BiphF4-O-CH2-A-alkyl alkyl-BiphF4-O-CH2-A-alkoxy VI: alkyl-BiphF4-A-A-alkyl alkyl-BiphF4-A-A-alkoxy alkoxy-BiphF4-A-A-alkyl alkoxy-BiphF4-A-A-alkoxy F-BiphF4-A-A-alkyl F-BiphF-A-A-alkoxy VII: alkyl-A-BiphF4-A-alkyl alkoxy-A-BiphF4-A-alkyl alkoxy-A-BiphF4-A-alkoxy VIII: alkyl-BiphF4-PheF4-A-alkyl alkyl-BiphF4-PheF4-A-alkoxy alkoxy-BiphF4-PheF4-A-alkyl alkoxy-BiphF4-PheF4-A-alkoxy F-BiphF4-PheF4-A-alkyl F-BiphF4-PheF4-A-alkoxy IX: alkyl-BiphF4-BiphF4-alkyl alkoxy-BiphF4-BiphF4-alkyl alkoxy-BiphF4-BiphF4-alkoxy F-BiphF4-BiphF4-alkoxy F-BiphF4-BiphF4-alkyl F-BiphF4-BiphF4-F X: alkyl-BiphF4-O-CO-A-A-alkyl alkyl-BiphF4-O-CO-A-A-alkoxy alkoxy-BiphF4-O-CO-A-A-alkyl alkoxy-BiphF4-O-CO-A-A-alkoxy F-BiphF4-O-CO-A-A-alkyl F-BiphF4-O-CO-A-A-alkoxy XI: alkyl-BiphF4-O-CO-A-A-alkyl alkyl-BiphF4-O-CO-A-A-alkoxy alkoxy-BiphF4-CO-O-A-A-alkyl alkoxy-BiphF4-CO-O-A-A-alkoxy F-BiphF4-CO-O-A-A-alkyl F-BiphF4-CO-O-A-A-alkoxy XII: alkyl-A-CO-O-BiphF4-CO-O-A-alkyl alkyl-A-CO-O-BiphF4-CO-O-A-alkoxy alkoxy-A-CO-O-BiphF4-CO-O-A-alkyl alkoxy-A-CO-O-BiphF4-CO-O-A-alkoxy alkoxy-A-O-CO-BiphF4-CO-O-A-alkyl alkoxy-A-O-CO-BiphF4-CO-O-A-alkoxy alkyl-A-O-CO-BiphF4-CO-O-A-alkyl alkoxy-A-CO-O-BiphF4-O-CO-A-alkoxy alkyl-A-CO-O-BiphF4-O-CO-A-alkoxy alkyl-A-CO-O-BiphF4-O-CO-A-alkyl XIII: alkyl-BiphF-PheF4-CO-O-A-alkyl alkyl-BiphF4-PheF4-CO-O-A-alkoxy alkoxy-BiphF4-PheF4-CO-O-A-alkyl alkoxy-BiphF4-PheF4-CO-O-A-alkoxy F-BiphF4-PheF4-CO-O-A-alkyl F-BiphF4-PheF4-CO-O-A-alkoxy XIV: alkyl-BiphF4-PheF4-O-CO-A-alkyl alkyl-BiphF4-PheF4-O-CO-A-alkoxy alkoxy-BiphF4-PheF4-O-CO-A-alkyl alkoxy-BiphF4-PheF4-O-CO-A-alkoxy F-BiphF4-PheF4-O-CO-A-alkyl F-BiphF4-PheF4-O-CO-A-alkoxy XV: alkyl-BiphF4-PheF4-O-CO-PheF4-alkyl alkyl-BiphF4-PheF4-O-CO-PheF4-alkoxy alkoxy-BiphF4-PheF4-O-CO-PheF4-alkyl alkoxy-BiphF4-PheF4-O-CO-PheF4-alkoxy F-BiphF4-PheF4-O-CO-PheF4-alkyl F-BiphF4-PheF4-O-CO-PheF4-alkoxy F-BiphF4-PheF4-CO-O-PheF4-alkyl F-BiphF4-PheF4-CO-O-PheF4-alkoxy alkoxy-BiphF4-PheF4-CO-O-PheF4-alkyl alkoxy-BiphF4-PheF4-CO-C-PheF4-alkoxy alkyl-BiphF4-PheF4-CO-C-PheF4-alkyl alkyl-BiphF4-PheF4-CO-C-PheF4-alkoxy XVI: alkyl-BiphF4-CO-O-BiphF4-alkyl alkyl-BiphF4-CO-O-BiphF4-alkoxy alkoxy-BiphF4-CO-O-BiphF4-alkyl alkoxy-BiphF4-CO-O-BiphF4-alkoxy F-BiphF4-CO-O-BiphF4-alkyl F-BiphF4-CO-O-BiphF4-alkoxy F-BiphF4-O-CO-BiphF4-alkyl F-BiphF4-O-CO-BiphF4-alkoxy F-BiphF4-CH2-O-BiphF4-alkyl F-BiphF4-CHz-O-BiphF4-alkoxy F-BiphF4-O-CH2-BiphF4-alkyl F-BiphF4-O-CH2-BiphF4-alkoxy alkoxy-BiphF4-CH2-O-BiphF4-alkyl alkoxy-BiphF4-CH2-O-BiphF4-alkoxy alkyl-BiphF4-CH2-O-BiphF4-alkyl alkyl-BiphF4-CH2-O-BiphF4-alkoxy Especially preferred are the following compounds where R is n-alkoxy or n-alkyl and n in CnH2n+1 is 1 to 5:

These compounds may be prepared as indicated in the following schemes: Scheme 1:Preparation of

F F O /O FF O p F B.V. F )-COCH3 -, C-CH3 e Br ------, ) Br-OH F F F Oxidation F 1 I i) BuLi ii) iPB (HO) 2B-OR F

Scheme 2: Preparation of

Scheme 3: Preparation of

Scheme 4: Preparation of

Particularly preferred are the polyfluorobiphenyls of formula I, wherein one of R and R denotes R and the other is

wherein n is 1 to 12.

In these preferred compounds Z is preferably -CH2-CH2-, -CO-O-, -O-CO-, -CH2-O- or -OCH2- and R is alkyl or alkoxy with in each case up to 12 carbon atoms 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-1,4-disubstituted andlor Dio are trans-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 A1 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 carboxylic 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 dimethylformamide 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 pyridine, quinoline or triethylamine, can occasionally also be used as the solvent for the esterification. 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 reactions 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 particular, alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, alkali metal carbonates or bicarbonates, 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 triethylamine, pyridine, lutidine, collidine or quinoline.

Another preferred embodiment of the esterification comprises 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, advantageously at temperatures between about -25" and +20 .

The liquid crystalline mixtures according to the invention 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 nematic or nematogenic substances, in particular the known substances, from the classes of azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexylcyclohexanes, cyclohexylcyclohexenes, cyclohexylnaphthalenes, 1,4-bis-cyclohexylbenzenes, 4,4'-biscyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenylpyridines, phenyl-or cyclohexyldioxanes, phenyl- or cyclohexyldithianes, 1 2-bisphenylethanes, 1,2-biscyclohexylethanes, l-phenyl-2-cyclohexylethanes, optionally halogenated stilbenes, benzyl phenyl ethers, tolanes and substituted cinnamic acids.

The most important compounds which are possible constituents of such liquid crystalline mixtures can be characterized by the formula 1 R3-L-G-E-R4 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-cyanocyclohexane rings, 4,4-disubstituted biphenyl, phenylcyclohexane and cyclohexylcyclohexane systems, 2,5-disubstituted naphthalene, di- and tetra-hydronaphthalene, quinazoline and tetrahydroquinazoline, G is -CH=CH -CH=CY- -CH=N(O) -C-c- -CH2-CH2- -CO-S- -CH2-S- -CH=N- -COO-Phe-COOor a C-C single bond, Y is halogen, preferably chlorine, or -CN and R3 and R4 are alkyl, alkoxy, alkanoyloxy or alkoxycarbonyloxy with up to 18, preferably up to 8, carbon atoms, it also being possible for one CH2 group non-adjacent to an oxygen atom to be replaced by -O-, -CH=CH- or -CC-, or that one of the radicals R3 and R4 may also denote CN, N02, CF3, NCS, F, C1 or Br.

In most of these compounds, R3 and R4 are different from one another, one of these radicals usually being an alkyl or alkoxy group. However, other variants of the substituents 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 invention 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.

Idle 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, preferably 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 ethyldimethyldodecylammonium 4-hexyloxybenzoate, tetrabutylammonium tetraphenylboranate 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 andlor 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 furthermore as follows: Cr: crystalline 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-3,3',5,5'-tetrafluoro-4'-n-alkoxy-biphenyls 4,4'-Dihydroxy-3,3',5,5'-tetrafluorobiphenyl (3 g), (preparation as described in scheme 4) methylated spirit (10 cm3) and n-pentyl bromide (1.36 g) are gently heated under reflux, with stirring. Potassium hydroxide (0.6 g) in water (10 cm3) 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 cm3), the extract is dried over anhydrous magnesium sulphate, and the solvent removed under reduced pressure. 4-Hydroxy-3,3',5, 5' -tetraflu- oro-4'-n-pentyloxybiphenyl is obtained as a clear oil.

The following compounds are obtained analogously: 4-hydroxy-3,3',5,5'-tetrafluoro-4'-ethoxybiphenyl 4-hydroxy-3,3',5,5'-tetrafluoro-4'-n-propyloxybiphenyl 4-hydroxy-3,3',5,5'-tetrafluoro-4'-n-butyloxybiphenyl 4-hy droxy-3,3',5,5'-tetrafluoro-4'-n-hexyloxybiphenyl 4-hydroxy-3,3',5,5'-tetrafluoro-4'-n-heptyloxybiphenyl 4-hydroxy-3,3',5,5'-tetrafluoro-4'-n-octyloxybiphenyl 4-hy droxy-3,3',5,5'-tetrafluoro-4'-n-nonyloxybiphenyl 4-hydroxy-3,3',5,5'-tetrafluoro-4'-n-decyloxybiphenyl Example 2 3,3',5,5'-Tetrafluoro-4'-alkvloxvbishenvl-4-vl trans-4-nalkvlcvclohexane-l-carboxvlates Trans-4-n-pentylcyclohexane-l-carboxylic acid (0.7 g), and 4-hydroxy-3,3',5,5'-tetrafluoro-4'-n-pentyloxybiphenyl (1.35 g) are added with stirring to dry methylene chloride (20 cm3) contained in a 50 cm3 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 3,3',5,5'-tetrafluoro-4'-n-pentyloxybiphenyl-4-yl trans-4-npentylcyclohexane-l-carboxylate as a white crystalline solid.

ExamDle 3 Preparation of 1- (4'-n-decyloxy-3, 3'' 5, 5'-tetrafluorobiphe- nyl-4-yl)-2-(trans-4-n-pentylcyclohexyl)ethane 3, 4, 5-Trifluoronhenvlboronic acid A solution of the Grignard reagent in dry ether (50 cm3, prepared from 1-bromo-3,4,5-trifluorobenzene (0.04 mol/l) and magnesium (0.045 g.atom) was added, dropwise, over 30 min. to a rapidly stirred, cooled (-75 OC) solution of triisopropylborate (0.085 mol/l) in dry THF (50 cm3). The reaction mixture was stirred at -75 C for 15 min. and then allowed to warm to room temperature. After 4-8 h, 4M-aqueous hydrochloric acid (100 cm3) was added to the reaction mixture and then separated aqueous layer was washed with diethyl ether. After work-up the 3,4,5-trifluorophenylboronic acid was isolated from the combined organic phases.

3.3' .4,5.5' -Pentafluorobinhenvl 2M-Sodium carbonate (7.5 cm3) was added to a solution of tetrakis (triphenylphosphino)palladium(O) (0.35 mmole) and l-bromo-3,5-difluorobenzene (0.0075 mol) in benzene (15 cm3) under nitrogen. A solution of 3,4,5-trifluorophenylboronic acid (0.008 mol) in ethanol (3.5 cm3) was added to the vigorously stirred reaction mixture which was then heated under reflux for 2 h. The reaction mixture was cooled, treated with 30 t hydrogen peroxide (0.5 cm3), and then stirred at room temperature for 1 h. Diethyl ether (50 cm3) was added to the reaction mixture. After work-up of the organic phase, the 3,3' ,4,5,5'-pentafluorobiphenyl was purified by column chromatography on silica gel.

3,3',4' 5,5'-PentafluorobiDhenyl-4-carbaldehvde This compound was prepared via reaction of 3,3' ,4,5,5'-pen- tafluorobiphenyl with an equimolar quantity of butyllithium in ether solution, followed by treatment with solid CO2, affording 3,3',4',5,5'-pentafluorobiphenyl-4-carboxylic acid.

The acid was reduced with borane dimethyl sulphide and the resulting alcohol then oxidised with pyridine chlorochromate affording the required 3,3',4',5,5'-pentafluorobiphenyl-4- carbaldehyde.

1-(3,3',4',5,5'-Pentafluorobiphenyl-4-yl)-2-(trans-4-n-pentvlcvclohexvl)ethene Triphenylphosphine was quaternised with trans-4-n-pentylcyclohexyl chloride in 1,2-dichlorobenzene. The resulting dry phosphonium chloride (0.02 mol) was suspended in a mixture of dry ThF (15 cm3) and dry ether (15 cm3), and, in an atmosphere of nitrogen, 1.6 M butyllithium in hexane (0.022 mol) was added during 15 min. The mixture was then stirred for 4 h, affording the orange solution ot the phosphorane. The reaction mixture was then cooled to -30 C and 3,3',4',5,5'-pen- ta fluorobiphenyl-4-carbaldeyde (0.01 mol) in ether (10 cm3) was added with vigorous stirring at this temperature. After stirring for 2 h, 4M-aqueous hydrochloric acid (25 cm3) was added to the cold reaction mixture.The organic phase was separated and the 1-(3,3',4',5,5'-pentafluorobiphenyl-4- yl)-2-(trans-4-n-pentylcyclohexyl)ethene was isolated from the ethereal solution.

1- (4'-n-Decyloxv-3,3' 5,5'-tetrafluorobihenvl-4- vl) -2- (trans-4-n-pentylcvclohexyl) ethene 1-(3,3',5,5'-Pentafluorobiphenyl-4-yl)-2-(trans-4-n-pentylcyclohexyl)ethene (0.001 mol) was dissolved in dry pyridine (10 cm3) at -10 "C. A slight molar excess of sodium decoxide, prepared by the addition of the appropriate quantity of sodium hydride to decanol (1 cm3), was then added, dropwise, with stirring. The reaction mixture was maintained at -10 C for 1 h, then poured into cold 4M-aqueous hydrochloric acid (25 cm3). The crude product was isolated by ether extraction of the aqueous solution and was purified by column chromatography on silica gel, eluting with light petroleum (b.p.

60-80 C) followed by crystallisation from ethanol. The 1-(4'-n-decloxy-3,3',5,5'-tetrafluorobiphenyl-4- yl)-2-(trans-4-n-pentylcyclohexyl)ethene gives the following liquid crystal transition temperatures: C-SA,50, SA-N, 130.0; N-I, 144.8 C Example 4 1-(4'-n-Pentyloxy-3,3',5,5'-tetrafluorobiphenyl-4 vl)-2-(trans-4-n-Pentvlcvclohexvl)ethene This compound was prepared similarly and shows the following liquid crystal transition temperatures: C-SA, 36.2; SA-N, 131.4; N-I, 167.5 C Example 5 1-(4'-n-Decyloxv-3,3' 5,5'-tetrafluorobishenyl-4- yl)-2-(trans-4-n-pentylcyclohexyl)ethane Hydrogenation of the alkene was effected at atmospheric pressure with hydrogen on a 5 % Pd/C catalyst and afforded the alkane which shows the following liquid crystal transition temperatures: C-SA, 36.7; SA-N, 62.0;N-I, 71.4 C Example 6 1-(4-n-Pentyloxy-3,3',5,5'-tetrafluorobiphneyl-4yl)-2-(trans-4-n-pentylcyclohexyl)ethane This compound was prepared similarly and gives the following liquid crystal transition temperatures: C-SA, 38.4; SAN, 55.2; N-I, 76.7 C The following compounds were prepared in analogy to the above examples or the reaction schemes given in the specification.

All groups R are straight-chained alkyl with n carbon atoms.

n CSA SA-N N-I 3 ~ 30 118.2 178.4 4 - 30 125.0 177.3 6 - 30 136.3 164.9 7 - 30 136.1 156.6 8 - 30 125.9 145.3 9 - 30 119.0 136.2

n C-N C-SA SA-N N-I 3 56.9 [45.3] 93.6 4 6 35.4 77.1 91.0 7 32 77.1 85.5 8 33.5 77.4 86.5 9 38.4 61.8 75.4

n C-N N-I 5 85.3 136.4 6 77.6 136.2 7 69.2 121.4 8 67.1 123.6 9 74.2 118.1 10 61.7 118.2

n C-N N-I n C-I C-N N-I 5 69.8 [65.9] 9 60.1 61.2 6 55.0 65.8 10 58.1 62.7 7 53.4 60.8 8 58.9 63.4

C-N N-I 75 106

R = CnH2n+1 n C-I C-N N-I n C-S, SA-N N-I 5 64 64.7 69.1 5 53.4 112.8 141.8 7 63 [60.3] 7 57 114.3 129.1 8 59.9 66 8 58.7 114.9 129.1 9 64.5 [59.6] 9 54.7 96.5 119 10 64 [63.4] 10 52 101.3 122.4 Example 7 1-(4-Ethoxy-2,2',6,6'-tetrafluorobiphenyl-4-yl)-2-(trans-4-npentylcyclohexyl) ethane is prepared according to the following route:

F F (ester via Baeyer CH3CO.O < - + I > COCH3 Villiger oxidation F F of ketone) Cu. Ullmann FF CH3CO.O-COCH3 FF (1) (1) hydrolysis (2) (2) RBr/Na2CO3 1 (3) NaOBr F RO eCO2H FF (1) LiAlH4 $ (2) PCC F RO+CHO FF Witting F F RO + CH=CH < C5H FF $ tH] F RO'CH2CH2 sH11 FF

Claims (1)

1. Claims: 1. Polyfluorobiphenyls of the formula I

   wherein A=B=A'=B'=F and C=D=C'=D'=H, or B=C=B'=C'=F and A=A'=D=D'=H, or A=A'=D=D'=F and B=C=B'=C'=H, or A=B=C=A'=B'=C'=F and D=D'=H, or A=B=D=A' =B' =D' F and C=C' =H, and R and R2 each signify a group of formula R-(A-Z)p-(A-Z)q in which R is F, OH, Cl, Br, CN, NCO, NCS, NO2, NC or alkyl or perfluoroalkyl wherein alkyl each has 1-15 C atoms, wherein one or two non-ad jacent CH2 or CF2 groups, respectively, may be replaced by -O-, -CO-, -O-CO-, -CO-O-, -O-CO-O-, -C#C and/or -CH=CH-, Al and A2 independently are an unsubstituted or mono or polysubstituted 1,4-cyclohexylene group, wherein one or two non-adjacent CH2 groups may be replaced by -0- and/or -S-, or denote a 1,4-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 CH3 groups and/or CN groups, wherein one or two CH groups may be replaced by N, Z1 denotes -CO-O-, -O-CO-, -CH2CH2-, -CHCE-CH2-, -CH2-CHCN-, -CH=CH-, -C#C-, -OCH2-, -CH2O-, -N=N-, -NO=N-, -N=NO- or a single bond, z2 denotes -CO-O-, -O-CO-, -CH2CH2-, -CHCN-CH2-, -CH2-CHCN-, -CH=CH-, -CmC-, -OCH2-, -CH2O-, -N=N-, -NO=N-, -N=NO-, -(CH2)k-CO-O- or a single bond, k is 1 to 6, p is 0, 1 or 2, and q is 0 or 1,