Smectic liquid-crystalline phases

Details Smectic liquid-crystalline phases Smectic liquid-crystalline phases

1987-05-13

1989-12-12

Gron, Teddy S.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229901, 2522996, 25229962, 25229963, 25229964, 25229965, 25229966, 25229967, 350350S, G02F 113, C09K 1934, C09K 1930

Patent

active

048866202

DESCRIPTION:

BRIEF SUMMARY
The invention relates to the use of compounds of the formula I or a polyfluoroalkyl group containing 1 to 15 C atoms in each case, wherein one or more CH.sub.2 groups or CF.sub.2 groups may also be replaced by a grouping selected from the group comprising --O--, --S--, --CO--, --CHhalogen--, --CHCN--, --O--CO--, --O--COO--, --CO--O-- and --CH.dbd.CH-- or else by a combination of two suitable groupings, two hetero atoms not being directly linked to each other, and one of the radicals R.sup.1 and R.sup.2 also being halogen, ##STR2## or one of these groups, wherein one or more CH.sub.2 groups are replaced by O and/or S or aliphatic and/or aromatic CH groups are replaced by N, --(A.degree.--Z.degree.).sub.p --, where substituted singly or multiply by halogen atoms, CH.sub.3 and/or nitrile groups, wherein one or two nonadjacent CH.sub.2 groups may also be replaced by --O-- and/or --S-- and/or a ##STR3## grouping may be replaced by ##STR4## or unsubstituted 1,4-phenylene or 1,4-phenylene substituted singly or multiply by halogen atoms, CH.sub.3 -- and/or nitrile groups, wherein one or more CH groups may also be replaced by N (Ph), one of the radicals A.degree. may also stand for 2,6-naphthylene (Na) or tetrahydro-2,6-naphthylene (4H-Na), optionally substituted by halogen or CN, --CO--O--, --O--CO--, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 --, --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN-- or a single bond, and if A= ##STR5## at least one group Z.degree. denotes --CHCNCH.sub.2 or --CH.sub.2 CHCN-- and/or in at least one of the groups R.sup.1 and R.sup.2 at least one CH.sub.2 group or CF.sub.2 group is replaced by --CHCN--, as components of smectic liquid-crystalline phases, and also smectic liquid-crystalline phases, in particular chiral tilted smectic phases containing compounds of the formula I.
Chiral tilted smectic liquid-crystalline phases with ferroelectric properties can be prepared by adding a suitable chiral dopant to basic 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)). Such phases can be used as dielectrics for fast-switching displays which are 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) based on the ferroelectric properties of the chiral tilted phase. In said phase the long molecules are arranged in layers, the molecules having an angle of tilt to a line perpendicular to the layer. On proceeding from layer to layer, the direction of tilt changes by a small angle with respect to an axis situated perpendicular to the layers so that a helical structure is formed. In displays which are based on the principle of SSFLC technology the smectic layers are arranged perpendicularly to the plates of the cell. The helical arrangement of the tilt directions of the molecules is suppressed by a very small spacing of the plates (approx. 1-2 .mu.m). As a result the longitudinal axes of the molecules are forced to arrange themselves in a plane parallel to the plates of the cell, as a result of which two distinct tilt orientations are produced. By applying a suitable alternating electric field it is possible to switch back and forth between these two states in the liquid-crystalline phase which exhibits a spontaneous polarization. Said switching process is substantially faster than in the case of conventional twisted cells (TN LCDs) which are based on nematic liquid crystals.
A considerable disadvantage for many applications of the materials at present available with chiral tilted smectic phases (such as, for example, Sc*) is that the dielectric anisotropy has values greater than zero or, if negative, values only slightly different from zero. Negative values of the dielectric anisotropy are necessary if the necessary planar orientation is produced by superimposing an AC holding field with low amplitude on the drive field (J. M. Geary, SID Conference, Orlando/Fla., Apr.

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Dubois, J. C., et al., Mol. Cryst. Liq Cryst., vol. 42, No. 1-3, pp. 139-152 (1977).

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