This invention relates to 2-(3',5'-difluoro-4'-cyanophenyl)pyrimidine derivatives, and more particularly to novel liquid crystal compositions including 2-(3',5'-difluoro-4'cyanophenyl)pyrimidine derivatives suitable for use in electro-optical displays.
Liquid crystal display devices utilize electro-optical effects possessed by liquid crystals. The liquid crystal materials used in these devices have a nematic phase, a cholesteric phase and a smectic phase. The most widely used display system uses liquid crystal materials in a twisted nematic mode.
Liquid crystal display devices have several advantages. The devices are small in size and can be made thin, and can be driven at low voltage with low power consumption. The liquid crystal material is a light receiving element so that when a liquid crystal display is viewed over a long time, eye strain does not occur.
In view of these advantages, liquid crystal display technology has been applied to watches, cameras, electronic counters, audio equipment, automobile dashboard indicators, telephone equipment, measuring devices, and the like. More particularly, liquid crystal display devices are also used presently in personal computers and word processor displays and in other devices including displays which require high resolution and many pixels, including black and white and color pocket televisions, and the like. Thus, liquid crystal display devices continue to attract attention as potentially replacing cathode ray tubes. As a result, liquid crystal display devices are widely used in various areas and it is likely that their use will be broadened further.
For practical use, liquid crystal compositions must possess the following characteristics.
1. The liquid crystal materials must be colorless and thermally, optically, electrically and chemically stable; PA1 2. Have a wide nematic temperature range; PA1 3. A rapid electro-optical response speed; PA1 4. Require a low driving voltage; PA1 5. A steep rise in voltage-light transmittance; PA1 6. The temperature dependency of threshold voltage be small; and PA1 7. A wide visual angle.
Many liquid crystal materials possess the first of the above-desired properties, however, no single compound satisfies all of the remaining characteristics. Thus, liquid crystal compositions are formed of several different nematic liquid crystal compounds or liquid crystal compositions are obtained by mixing liquid crystal compounds with non-liquid crystal compounds to obtain the desirable properties.
In order to decrease the driving voltage of a liquid crystal display device, it is necessary to reduce the threshold voltage. However, the following relationship exist between threshold voltage (V.sub.th), the elasticity constant (K) and dielectric constant anisotropy (.DELTA..epsilon.): EQU V.sub.th .varies.(K/.sub..DELTA..epsilon.)1/2.
Thus, in order to decrease V.sub.th, a liquid crystal compound having large .DELTA..epsilon. and K is required.
Accordingly, it is desirable to provide an improved liquid crystal material having a low threshold voltage and overcomes disadvantages in present liquid crystal compositions.