Liquid crystal compounds, mixtures and devices

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

1996-12-31

1998-08-25

Kelly, C H

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229963, 25229966, 560 65, C09K 1934, C09K 1912, C09K 1930, C07C 6976

Patent

active

057980586

DESCRIPTION:

BRIEF SUMMARY
This application is a 35 U.S.C. 371 of PCT/GB95/01525 filed Jun. 30, 1995.
This invention relates to novel compounds suitable for use in liquid crystal mixtures and their inclusion in liquid crystal devices.
Liquid crystals can exist in various phases. In essence there are three different classes of liquid crystalline material, each possessing a characteristic molecular arrangement. These classes are nematic, cholesteric and smectic. A wide range of smectic phases exists, for example smectic A and smectic C. Some liquid crystal materials possess a number of liquid crystal phases on varying the temperature, others have just one phase. For example, a liquid crystal material may show the following phases on being cooled from the isotropic phase:- isotropic - nematic - smectic A - smectic C - solid. If a material is described as being smectic A then it means that the material possesses a smectic A phase over a useful working temperature range.
Materials possessing a smectic A (S.sub.A) phase may exhibit an electroclinic effect. The electroclinic effect was first described by S. Garoff and R. Meyer, Phys. Rev. Lett., 38, 848 (1977). An electroclinic device has also been described in UK patent application GB 2 244 566 A. This particular device helps to overcome the poor alignment problems of electroclinic (EC) devices using a surface alignment that gives a surface tilt within a small range of angles. When a smectic A phase is composed of chiral molecules, it may exhibit an electroclinic effect, ie a direct coupling of molecular tilt to applied field. The origin of the electroclinic effect in a smectic A phase composed of chiral polar molecules has been described by Garoff and Meyer as follows. The application of an electric field parallel to the smectic layers of such a smectic A phase biases the free rotation of the transverse molecular dipoles and therefore produces a non-zero average of the transverse component of the molecular polarization. When such a dipole moment is present and coupled to the molecular chirality, a tilt of the long molecular axis (the director) is induced in a plane perpendicular to the dipole moment.
In thin samples for example 1-3 .mu.m and with the smectic layers tilted or perpendicular with respect to the glass plates the electroclinic effect is detectable at low applied fields.
In an aligned smectic A sample a tilt of the director is directly related to a tilt of the optic axis. The electroclinic effect results in a linear electro-optic response. The electro-optic effect can manifest itself as a modulation of the effective birefringence of the device.
Electroclinic (EC) devices are useful, for example, in spatial light modulators having an output that varies linearly with applied voltage. A further advantage of EC devices is that they have high speed response times, much faster than twisted nematic type devices. One known type of ferroelectric device is bistable, in contrast the EC device is not bistable and has an output that varies linearly with applied voltage.
The electroclinic effect is sometimes referred to as the soft-mode effect see G. Andersson et al in Appl. Phys. Lett., 51, 9, (1987). In general terms, regarding the electroclinic effect, it is advantageous if on applying a small voltage there results a large induced tilt. An increase in induced tilt may result in an increase in contrast ratio. It is also advantageous if a large induced tilt can be obtained at as low a voltage as possible.
It is also advantageous if the relationship between molecular induced tilt and applied voltage is temperature independent. When an increase in applied voltage results in little or no change in induced tilt then the material being tested is generally referred to as exhibiting a saturation voltage effect.
By S.sub.A * is meant a S.sub.A phase which contains some proportion of chiral molecules.
Documents EP-A-0401522, EP-A-0501849, EP-A-0343487 and Liquid Crystals, 1994, vol. 16(4), 625-641 are concerned with liquid crystal materials containing a cyclohexyl group. Cholesteric or chiral nemat

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