Chiral 1,2-difluorobenzene derivatives

Details Chiral 1,2-difluorobenzene derivatives Chiral 1,2-difluorobenzene derivatives

1989-05-15

1993-08-03

Stoll, Robert L.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229966, 544298, 546339, C09K 1934, C09K 1912, C07D23902, C07D21170

Patent

active

052326241

DESCRIPTION:

BRIEF SUMMARY
The invention relates to chiral 1,2-difluorobenzene derivatives of the formula I ##STR1## in which R.sup.1 is H, F, Cl, Br, CN, an alkyl or perfluoroalkyl group each having 1 to 12 C atoms and in which, in addition, one or two non-adjacent CH.sub.2 or CF.sub.2 groups may be replaced by O atoms and/or --CO-- groups and/or --CO--O-- groups and/or --CH.dbd.CH-- groups and/or --CHhalogen-- and/or --CHCN-- groups and/or --O--CO--halogen and/or --CO--O--CHCN-- groups, 1,4-phenylene which is unsubstituted or substituted by one or two F and/or Cl atoms and/or CH.sub.3 groups and/or CN groups and in which, in addition, one or two CH groups may be replaced by N, or are 1,4-cyclohexylene in which, in addition, one or two non-adjacent CH.sub.2 groups may be replaced by O atoms and/or S atoms, or are piperidine-1,4-diyl, 1,4-bicyclo(2,2,2)octylene, 1,3,4-thiadiazole-2,5-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl or 1,2,3,4-tetrahydronaphthalene-2,6-diyl, --OCH.sub.2 --, --CH.sub.2 O--, --C.dbd.C-- or a single bond,
Like similar compounds described in German Offenlegungsschrift 3,515,373, the compounds of the formula I can be used as components of chiral tilted smectic liquid-crystalline phases.
Chiral tilted smectic liquid-crystalline phases having ferroelectric properties can be prepared by adding a suitable chiral dope to base mixtures having one or more tilted smectic phases (L. A. Beresnev et al., Mol. Cryst. Liq. Cryst. 89, 327 (1982); H. R. Brand et al., J. Physique 44, (lett.), L-771 (1983). Phases of this type can be used as dielectrics for rapidly switching displays based on the principle of SSFLC technology described by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. No. 4,367,924) on the basis of the ferroelectric properties of the chiral tilted phase. In this phase, the long molecules are arranged in layers, the molecules having a tilt angle to the layer perpendiculars. On moving from layer to layer, the tilt direction changes by a small angle with respect to an axis perpendicular to the layers, thus forming a helical structure. In displays based on the principle of SSFLC technology, the smectic layers are arranged perpendicular to the plates of the cell. The helical arrangement of the tilt directions of the molecules is suppressed by a very small separation of the plates (about 1-2 .mu.m). The longitudinal axes of the molecules are therefore forced to arrange themselves in a plane parallel to the plates of the cell, thus causing two preferred tilt orientations. By applying a suitable electrical alternating field, it is possible to switch back and forth between these two states in the liquid crystalline phase exhibiting spontaneous polarization. This switching process is considerably faster than in customary twisted cells (TN-LCDs) based on nematic liquid crystals.
It is a great disadvantage for many applications of the currently available materials having chiral tilted smectic phases (such as, for example, Sc*) that they have a relatively high optical anisotropy and unacceptably short switching times, due to relatively high viscosity values, and that the dielectric anisotropy values are greater than zero or, if they are negative, have values only slightly different from zero. Negative values for the dielectric anisotropy are necessary if the planar orientation necessary is caused by overlapping the control field with an AC holding field of small amplitude (J. M. Geary, SID Congress, Orlando/Fla., April/May 1985, Paper 8.3).
It has now been found that the use of compounds of the formula I as components of chiral tilted smectic mixtures can significantly reduce the disadvantages mentioned. The compounds of the formula I are thus pre-eminently suitable as components of chiral tilted smectic liquid-crystalline phases. In particular, they can be used to prepare chiral tilted smectic liquid-crystalline phases which are particularly stable chemically and have favourable ferroelectric phase ranges, in particular broad Sc* phase ranges, negative or po

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