Chiral oxiranylmethyl ethers, and their use as dopants in liquid

Details Chiral oxiranylmethyl ethers, and their use as dopants in liquid Chiral oxiranylmethyl ethers, and their use as dopants in liquid

1994-06-24

1998-01-13

Wu, Shean C.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229901, 25229963, 25229964, 25229965, 25229966, 25229967, 428 1, 549559, 549560, C09K 1934, C09K 1952, G02F 113, C07D30308

Patent

active

057075450

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to novel chiral oxiranylmethyl ethers and their use as dopants in liquid-crystal mixtures.
2. Description of the Related Art
Particularly in the last decade, liquid crystals have been introduced into various industrial areas in which electro-optical and display-device properties are required (for example in watch, calculator and typewriter displays). These display devices are based on dielectric alignment effects in the nematic, cholesteric end/or smectic phases of the liquid-crystalline compounds, where--caused by the dielectric anisotropy--the molecular long axis of the compounds adopts a preferential alignment in an applied electric field. The usual response times in these display devices are too long for many other potential areas of application of liquid crystals. This disadvantage is particularly noticeable if a large number of pixels must be addressed. Production costs of equipment containing relatively large screen areas, for example of video equipment, are then generally too high.
In addition to nematic and cholesteric liquid crystals, optically active smectic liquid crystal phases have also been increasing in importance over the last few years.
Clark and Lagerwall were able to show that the use of ferroelectric liquid-crystal systems in very thin cells results in electro-optical switching or display elements which have response times faster by a factor of 1000 compared with conventional TN ("twisted nematic") cells (cf., for example, Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, October Meeting 1985, San Diego, Calif., USA). On the basis of this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are in principle highly suitable for the abovementioned areas of application, for example via matrix addressing.
For electro-optical or fully optical components, either compounds are required which form tilted or orthogonal smectic phases and are themselves optically active, or ferroelectric smectic phases can be induced by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range.
In order to achieve good contrast in electro-optical components, a uniform planar alignment of the liquid crystals is necessary. Good alignment in the S*.sub.A and S*.sub.c phase can be achieved if the phase sequence of the liquid-crystal mixture is, with decreasing temperature:
The prerequisite is that the pitch of the helix in the N* phase is very large (greater than 10 .mu.m) or even better is fully compensated (see, for example, T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. Display Research Conf., Japan Display, Sept. 30-Oct. 2, 1986, Tokyo, Japan; M. Murakami et al., ibid. pp. 344-347). This is achieved by adding a further optically active dopant which induces a right-hand helix to the chiral liquid-crystal mixture which has, for example, a left-hand helix in the N* phase, in such amounts that the helix is just compensated.
A further prerequisite for the use of the SSFLCD effect (surface-stabilized ferroelectric liquid-crystal display) of Clark and Lagerwall for uniform planar alignment is that the pitch in the smectic C* phase is significantly greater than the thickness of the display element (Mol. Cryst. Liq. Cryst. 94 (1983), 213-134 and 114 (1984), 151-187). As in the case of the cholesteric pitch, this is achieved by using dopants having the opposite rotation of the helix.
Ferroelectric liquid-crystal displays can also be operated by utilizing the DHF (distorted helix formation) effect or the PSFLCD effect (pitch-stabilized ferroelectric liquid-crystal display, also known as SBF=short pitch bistable ferroelectric effect). The DHF effect has been described by B. I. Ostrovski in Advances in Liquid Crystal Research and Applications, Oxford/Budapest, 1980, 469 ff.

REFERENCES:
patent: 5061814 (1991-10-01), Wand et al.

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