Liquid crystal compounds, mixtures and devices

Details Liquid crystal compounds, mixtures and devices Liquid crystal compounds, mixtures and devices

1989-05-03

1991-08-06

Maples, John S.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229901, 359104, C09K 1930, C09K 1952

Patent

active

050375768

DESCRIPTION:

BRIEF SUMMARY
The invention relates to novel compounds which may be used as constituents of ferroelectric smectic liquid crystal mixtures, to such mixtures containing these compounds, and to electro-optical devices which use these mixtures.
Ferroelectric smectic liquid crystal materials use the ferroelectric properties of the chiral tilted smectic C, F, G, H, I and K phases (designated S.sub.C * etc, the asterisk denoting chirality). The S.sub.C phase is generally the most useful, as it is the most fluid, and it is particularly desirable that the material shows an S.sub.A or nematic (denoted N) phase at temperatures above the chiral smectic phase, to assist surface alignment in a liquid crystal device. Ferroelectric smectic liquid crystal materials desirably have low viscosity, S.sub.C phases that persist over a broad temperature range which includes ambient temperature and have a long helical pitch, chemical and photochemical stability, and a high spontaneous polarisation coefficient, Ps, measured in nC cm.sup.-2. Such materials offer the possibility of very fast switched liquid crystal display devices, with bistable characteristics.
Although some single compounds show all of the desirable properties outlined above, ferroelectric smectic liquid crystal materials in use today generally consist of a mixture of compounds at least one of which is an optically active compound, i.e. containing an asymmetrically substituted carbon atom. This asymmetric centre is generally combined with a mesogenic molecular `core`. A principal function of the optically active compound in the mixture is to induce the mixture to show a helical smectic phase, e.g. S.sub.C *, and to have a high Ps.
A wide variety of such compounds is known, but it is not usual to find compounds which result in S.sub.c * mixtures with all of the above mentioned desirable properties. For example the alpha-hydroxycarboxylic acid derivatives described in PCT/GB85/00512 which contain the asymmetric centre: ##STR1## form S.sub.c * mixtures with a very high Ps but a very short helical pitch.
It is an object of the invention to explore the possibilities of other asymmetric centres, to provide optically active compounds of increased usefulness in ferroelectric smectic liquid crystal mixtures.
According to the invention a novel compound has a formula I: ##STR2## where R.sub.1 is alkyl, Y is selected from --COO-- or --OOC--; m, n and p are independently 0 or 1, provided that (m+n+p) is no more than 2, and when Y is OOC then p is 0; and X is a mesogenic group.
The term "mesogenic" used herein with reference to the group X indicates that X is one of the molecular "cores" composed of chains of linked cyclic group that induce the molecule to show liquid crystal phases or to be miscible with liquid crystal materials, particularly those showing a smectic C liquid crystal phase, i.e. `smectogenic`.
Many such mesogenic or smectogenic cores are known, for example the groups shown attached to the group --COOR* in EP-A-0110299, and the groups represented by the symbol Q in U.S. Pat. No. 4,613,209.
The asterisk in formula I indicates the asymmetric centre.
Also the invention provides a ferrolectric smectic liquid crystal material, being a mixture of compounds, at least one of which is a compound of formula I in an optically active or racemic form, preferably the former. The structural preferences for the compound of formula I discussed below are based largely on suitability and desirability for use in such mixtures.
Preferably R.sub.1, in formula I is C.sub.1 -C.sub.5 n-alkyl, especially methyl.
Preferred structures for compounds of formula I are therefore those listed in table 1 below.


TABLE 1 ______________________________________ ##STR3## 1.1 ##STR4## 1.2 ##STR5## 1.3 ##STR6## 1.4 ##STR7## 1.5 ##STR8## 1.6 ##STR9## 1.7 ##STR10## 1.8 ##STR11## 1.9 ______________________________________
Particularly preferred structures are 1.1, 1.2, 1.3, 1.5 and 1.7 listed in table 1.
The advantages of the invention derive primarily from the nature of the novel asymmetric grou

REFERENCES:
patent: 4576732 (1986-03-01), Isogai et al.
patent: 4728458 (1988-03-01), Higuchi et al.
patent: 4874545 (1989-10-01), Heppke et al.

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