GB2234979A - 4-Alkyl-4'-(o-fluorophenethyl)bicyclohexanes as liquid crystal components - Google Patents

Abstract

A liquid crystal medium for use in supertwisted nematic cells with very sleep characteristic curves comprising at least three liquid crystalline components characterized in it contains at least one compound of the formula I <IMAGE> in which R<1> is straight-chain alkyl of 1 to 12 C atoms.t

Description

Liquid crystal medium The invention relates to liquid crystal medium (LC medium) having elastic and dielectric properties appropriate for use in supertwisted nematic cells.

Liquid crystal display cells can be made to support more than 900 of twist in the liquid crystal layer by a combination of offset alignment directions at the surfaces, and incorporation of chiral additives. If the twist angle is increased in this way, the slope of the threshold characteristic increases and eventually becomes vertical. Further increase of the twist angle will result in bistable behaviour, but at the onset of bistability the very steep switching curve allows high level multiplex drive using conventional RMS waveforms. The limit to the increase of twist angle is set by a scattering texture which becomes stabilised as the cholesteric pitch is shortened; the effect can be alleviated but not eli-minated by increasing the surface tilt in the cell.

By comparison with the standard twisted nematic display, the supertwist nematic cell has important differences in behaviour. Because light guiding through the highly twisted structure is imperfect, the cells are commonly operated in a variable birefringence mode and this, together with a strong dependence of threshold voltage on cell thickness, means that a high accuracy of cell spacing is required. In order to minimise the change in materials properties with temperature, a relatively high clearing point is desirable in the mixture. A near optimal optical performance can be obtained, e.g. by setting dAn - 0.73 for the 2700 twist cell and adjusting the polariser orientation if necessary to obtain the best contrast.

Because of the large number of interacting cell and material properties which contribute to the performance~-of the supertwisted nematic display, it is highly difficult to select combinations of materials to suit the parameters of the cell for best performance.

There is thus still a great need for liquid crystal media having suitable material properties for use in supertwisted nematic display cells.

The invention has for its object to prepare liquid crystal media which have a nematic phase including room temperature, exhibit elastic and dielectric properties appropriate for use in supertwist cells with very steep characteristic curves.

It has now been found that the liquid crystal media having particularly favorable combinations of material properties are obtained when they contain at least one compound of the formula I

in which R is straight-chain alkyl of 1 to 12 C atoms.

Preferable these media consist of at least three components A, B, and C, each of which contains one or more compounds, wherein component A has a viscosity of at most about 35 mPa.s at 200, a clearing point of at most 1300 and a dielectric anisotropy between about -2 and about +2, component B has a dielectric anisotropy above about +4, component C has a clearing point of more than 1300 and a dielectric anisotropy between about -2 and about +2, the compound(s) of the formula I being component A or;a part thereof.

The liquid crystal media according to the invention exhibit favorable elastic constants, i.e. relatively low K11 and K22 values, relatively high K33 values and very favorable K33/K11 and K33/K22 relationship for use in supertwisted nematic cells.

The invention thus provides the above-described liquid crystal media for use in supertwisted nematic cells which may also contain one or more suitable chiral additives to obtain the desired d/p relationship.

The invention further provides supertwisted nematic liquid crystal display elements, in particular supertwisted nematic display elements which contain such media.

Such media preferably consist of at least about 10 wt.% of component A, at least about 15 wt.% of component B and at least about 5 wt.% of component C and particularly of about 40 to 70 % wt.% (favorably about 50 wt.%) of component A, about 30 to 50 wt.% (favorably about 40 wt.%) of component B and about 5 to 30 wt.% (favorably about 10 wt.%) of component C. The amount of compounds of formula I in the total mixture conveniently amounts to at least about 10 wt.% and preferably about 15 to 60 wt.% (favorably about 20 wt.%).

Compounds having the above properties required for the components A, B and C and other additives such as chiral dopants are basically known to the person skilled in the art. The total mixture must have nematic or cholesteric properties. Component A can be nematic or of monotropic liquid crystalline behaviour or, as long as the total mixture is nematic, also smectic. However, there are preferred those mixtures in which at least one compound of component A is enantiotropic liquid crystalline and there are especially preferred those mixtures in which components A, B and C are enantiotropic liquid crystalline.

Compounds and mixtures which are suitable as component A, B and C are to a large extent known and many of them are also commercially available. The following compounds or their mixtures are especially suitable for or as components A: R1-Cy-Phe-R2 R1-Cy-Cy-R2 in which R1 is alkyl of 1 to 12 C atoms, in which in addition one or two nonadjacent CH2 groups can be replaced by 0 atoms, -CO-, -O-CO-, -CO-O- and/or -CH=CH- groups, Cy is trans-1,4-cyclohexylene, Phe is 1,4-phenylen, and R2 has one of the meanings of R1.

Compounds of the formula I are described in European Patent 0 125 563.

Component A has favorably a viscosity of at most about 30, in particular at most 25 mPa.s at 200 and a clearing point of at most 1200.

Compound A preferably contains besides one or more compounds of the formula I, one or more compounds of the formula IIIa to IIId: Rl-Cy-Cy-CH2CH2-Phe F-R2 IIIa R-Cy-COO-PheF-R IIIb R-Cy-CH2CH2-PheF-R IIIc

wherein PheF is

-or F F preferably

-0 F

or its mirror image. Compounds of the formulae IIIb and IlId are particularly preferred.

For or as component B there are suitable, for example, known compounds containing 2 or 3 p-phenylene or trans1,4-cyclohexylene groups, a polar end group and optionally a lateral halogen substituent. Such compounds are known to the skilled worker. Compounds which are suitable for or as components B are, for example, the known compounds of formula A.

in which R1 is alkyl of 1 to 12 C atoms, in which in addition one or two non adjacent CH2 groups can be re placed by 0 atoms, -CO-, -O-CO-, -CO-O- and/or -CH=CH- groups, A and A1 are each, independently of each other, Cy, Dio, Pyr or Phe, Z is -CO-O-, -O-CO-, CH2-O-, -O-CH2-, -CH2CH2-, or a single bond, X is F, Cl -OCF3, -OCHF2 or -CN, x2 is H or F, m is 0 or 1, Cy is trans-1,4-cyclohexylene, Pyr is pyrimidine-2,5-diyl or pyridine 2,5-diyl, Dio is trans-1,3-dioxane-2,5-diyl and Phe is 1,4-phenylene.

Particularly preferred are those of formula B: R1-A4 -Phe-CN B in which A4 is Cy, Phe, Cy-Cy, Cy-CH2CH2, Dio, Phe-COO-, Cy-COO, Phe-Phe, Phe-COO-Ph Phe-Phe-COO-Phe, Cy-Phe or Cy-COO-Phe and R, Cy and Phe have the meanings indicated above.

Compounds which are suitable for or as components C are, for example, the known compounds of the following formulae: R-Cy-Phe-Phe-R R1-Cy-Cy-Phe-R2 R-Cy-Cy-Cy-R R -Cy-Z-Phe-Z-Phe-R R1 -Cy-z-Cy-z-Phe-R2 R-Cy-Cy-Z-Phe-R R-Cy-Cy-Cy-Phe-R R1 -Cy-Cy-Z-Cy-R2 R1 -Cy-Z-Cy-z-Cy-R2 R-Cy-Phe-Phe-Cy-R

R1-Cy-Cy-Z-PheF-R2

Rl-Cy-Z-Phe-Phe-Cy-R2 Rl-Cy-Cy-COO-Phe-Cy-R2 Component C has favorably a clearing point of at least about 1400.Preferably component C contains at least one compound with a clearing point of at least 1800, preferably of at least 2000 Component C preferably contains one ore more compounds of formula IV R1-A2-(Z2-A2 )2-R2 IV in which Z independently is -CO-O- or -O-CO-, A is Cy or Phe and R and R independently have the meanings indicated above.

The individual compounds of the liquid crystal phases according to the invention are either known or preparable by methods which are easily derivable by those skilled in the art from the prior art, since they are based on standard methods described in the literature.

Corresponding compounds of the formula A are described for example in German Patent Applications P 34 05 914 and P 35 15 633; in European Patent Specification 0,019,665; in S.M. Kelly and Hp. Schad, Helvetica Chimica Acta, 67, 1580-1587 (1984); in S.M. Kelly, ibid, 67, 1572-1579 (1984); in Japanese Offenlegungsschrift 59-191,789 and European Offenlegungsschriften 0,099,099 and 0,119,756. Compounds of the formula II are described for example in German Offenlegungsschriften 29 44 905, 31 12 185, 31 46 249, 31 50 761, 31 50 763, 32 07 114, 32 27 916, 33 02 218, 33 34 054, in European Offenlegungsschrift 0,122,389, in U.S. Patent 4,322,354 and in Japanese Offenlegungsschrift 56-164,179.

It was found, surprisingly, that the combination according to the invention of components A, B and C gives LC phases for use in supertwisted nematic cells which on the one hand have favorable elastic properties, wide mesophase ranges with low melting points and low smectic-nematic transition temperatures and on the other hand are capable of multiplexed drive by virtue of their good electro-optic steepness and viewing angle characteristic.

The LC phases according to the invention preferably contain at least two compounds, in particular at least three compounds, of the formula I and preferably at least two compounds, in particular at least three compounds, selected from the formulae A and B. Preferably at least one compound of formula A is present.

X1is preferably F, C1 or -OCF3. R1 is preferably alkyl, alkenyl, alkoxy or alkoxymethyl in each case with 1 to o 1 7 C atoms, in particular alkyl or alkoxymethyl. (A )m-Al is preferably Phe, CyPhe, PhePhe or PyrPhe, in particular Phe or CyPhe.

Preferred compounds of the formula A are those of the partial formulae Aa to Ag:

7 R is preferably straight-chain alkyl of 2 to 7 C atoms.

Z is in the case of compounds of the formula I preferably -CO-O-, -CH2-O- or a single bond. X is preferably fluorine.

Of the foregoing partial formulae, those of the formulae Aa and Ab are preferred.

In the case of the compounds of the foregoing partial formulae, R is a straight-chain alkyl group, preferably of 1 to 7 C atoms, in which in addition a CH2 group can also be replaced by -O- or -CH=CH-. Particularly preferred groups R are methyl, ethyl, n-propyl, n-butyl, n-pentyl and n-heptyl.

In the case of compounds of the foregoing partial formulae with two groups R, both groups R each have independently of each other one of the above mentioned meanings.

In the compounds of the formula I R1 is preferably alkyl, with up to 7 C atoms, in particular n-propyl or n-pentyl.

In the compounds of the formula IV R1 and R2 have the preferred meanings indicated above. Preferred compounds of the formula IV are those of the partial formulae IVa to IVf: R1-Phe-COO-Phe-COO-Phe-R2 IVa Rl-Cy-COO-Phe-COO-Phe-R2 IVb R1-Phe-COO-Cy-COO-Phe-R2 IVc R1 -Phe-OOC-Cy-COO-Phe-R2 IVd Rl-Cy-COO-Cy-COO-Phe-R2 IVe R1 -Cy-COO-Phe-COO-Cy-R2 IVf Of the foregoing partial formulae, those of the formulae IVa, IVb and IVd are preferred.

When the alkyl groups in the compounds of formulae I to IV contain 3 or more carbon atoms, these can be arranged in a straight or branched chain. However, the phases according to the invention did not use any components which contain more than one branched alkyl group. Such branched alkyl groups for the purposes of the present invention contain not more than one chain branching; preferably this is a methyl or ethyl group in the 1- or 2-position of carbon skeleton, so that suitable branched alkyl groups are in particular: 2methylpropyl, 2-methylbutyl, l-methylpentyl, 2-methyir pentyl, 1-methylhexyl. Generally the liquid crystal dielectrics according to the invention contain only one component having a branched-chain alkyl radical in order to induce, if desired, optical activity.For this purpose, normally not more than 10 percent by weight, preferably 0.5 to 3 percent by weight, of a component having a branched alkyl radical are added.

In the compounds of the foregoing partial formulae,

Preferred phases according to the invention contain compounds of the formula III in wich R1 is n-alkyl and R2 is n-alkyl, n-alkoxy or n-alkanoyloxy.

R1 and R2 are preferably n-alkyl or n-alkoxy of in each case 2 to 7 C atoms.

The preparation of the phases according to the invention is effected in the conventional manner. In general, the desired amount of the components which is used in the smaller amount is dissolved in the components which constitutes the main constituent, preferably at elevated temperature. If this temperature is chosen to be above the clear point of the main constituent, the completeness of the process of dissolving can be observed particularly easily.

However, it is also possible to mix solutions of the-;- components in a suitable organic solvent, for example acetone, chloroform or methanol, and to remove the solvent after thorough mixing, for example by distillation under reduced pressure. It is self-evident that with this method it must be ensured that the solvent does not introduce any contaminants or undesirable dopants.

By means of suitable additives the liquid crystal media according to the invention can be modified in such a way that they can be used in any hitherto disclosed kind of supertwisted nematic liquid crystal display element.

The production of a supertwisted nematic electro-optical cell containing a medium in accordance with the invention can also be carried out in a manner known per se; for example, by evacuating a suitable cell and introducing the medium into the evacuated cell.

The examples below serve to illustrate the invention without limiting it. In the examples, the melting point and clear point of a liquid crystal substance are given in degrees Celsius. The percentages are by weight. The values of the threshold voltages relate to an angle of observation of 00, 10 % contrast and 20 OC and are voltages in a 90" twist cell.

Example 1 A liquid crystalline medium A (state of the art) consisting of 27,3 % of I eutectic Cbeing a mixture of equal parts of 1-(trans-4-propylcyclohexyl)-2-(4'-ethyl-22-fluorobiphenyl)- ethane, i-(trans-4-propylcyclohexyi )-2-(4' -pentyl-2'fluorobiphenyl) ethane and 1-(trans-4-pentylcyclohexyl)-2-(4'-ethyl-2'fluorobiphenyl)- ethane 14.95 % of trans, trans-4-propyl-4' -propoxycyclohexyl- cyclohexane, 22.59 % of 1-(trans-4-propylcyclohexyl)-2-(p-cyanophenyl) ethane, 5.21 % of 2-p-cyanophenyl-5-propyl-1,3-dioxane, 4 99 % of 4-cyano-3-fluorophenyl p-propylbenzoate 9.93 % of 4-pentyl-2-fluorophenyl trans,trans-4-pentyl cyclohexylcyclohexane-4'-carboxylate 4.97 % of 4,4' -bis-(trans-4-propylcyclohexyl)-2-fluoro- biphenyl, 5.04 % of 4,4' -bis-(trans-4-pentylcyclohexyl)-2-fluoro- biphenyl and 5.01 % of 4- (trans-4-pentylcyclohexyl )-4' - (trans-4-propyl- cyclohexyl)-2-fluorobiphenyl and a medium B according to the present invention, which is composed as medium A with the exception that 15 % of I eutectic are replaced with 15 % of l-[trans-4-(trans-4- propylcyclohexyl)-cyclohexyl)-2-(2-fluorophenyl)-ethane, show the properties given in the following table: A B N-I ( C) 93.4 92.3 Viscosity at 20 C (mPa.s) 24.6 25.3 ##/## 1.616 1.529 +6.9 +6.2 Vth (Volt)* 2.12 2.20 Vth Steepness 2.9 3.1 * measured in a 7 pm 180 twist STN cell

Claims (3)

1. Claims: 1. A liquid crystal medium for use in supertwisted nematic cells comprising at least three liquid crystalline components characterized in it contains at least one compound of the formula I

   in which R1 is straight-chain alkyl of 1 to

   12 C atoms.
2. 2. Use of a liquid crystal medium according to claim 1, for supertwisted nematic electro-optical display elements.
3. 3. Supertwisted nematic electro-optical display ele ment, characterized in that it contains a medium according to claim 1.