Liquid crystal compounds are widely used as materials for display elements, and most of these liquid crystal elements are based on a TN type display system and liquid crystal materials are in the state of nematic phase.
The TN type display system has advantages such as less tiredness of the eyes and extremely small consumption of electric power because of being of the non-emissive type, whereas it has disadvantages such as slow response and disappearance of display at certain visual angles. In recent years, this system is being improved in such a direction as to keep up characteristics of flat displays, and in particular, quick response and the enlargement of the visual angle are demanded.
In order to meet these demands, improvements in liquid crystal materials have been attempted. However, as compared with other emissive type displays (e.g., electroluminescence displays and plasma displays), it is apparent that the TN type display system is much poorer with regard to response time and the extent of the visual angle.
In order that characteristics of the liquid display element such as features of the non-emissive type and the small consumption of electric power may be maintained and in order that the quick response corresponding to that of the emissive type displays may be assured, it is essential to develop a novel liquid display system in place of the TN type display system.
As one of such attempts, a display system in which the light switching phenomenon of ferroelectric liquid crystals is utilized has been suggested by N. A. Clark and S. T. Lagerwall (see Appl. Phys. Lett. 36, p 899, 1980).
The presence of the ferroelectric liquid crystals was announced for the first time in 1975 by R. B. Mayer et al. (see J. Phys., 36, p 69, 1975), and from the viewpoint of a liquid crystal structure, these crystals belong to a chiral smectic C phase, a chiral smectic I phase, a chiral smectic F phase, a chiral smectic G phase and a chiral smectic H phase (hereinafter referred to simply as "S.sub.C * phase", "S.sub.I * phase", "S.sub.F * phase", "S.sub.G * phase" and "S.sub.H * phase", respectively).
In the chiral smectic phase, molecules form a layer and tilt to the surface of the layer, and a helical axis is vertical to this layer surface. In the chiral smectic phase, spontaneous polarization takes place, and therefore, when a DC electric field is applied to this layer in parallel therewith, the molecules rotate about the helical axis in accordance with their polarity. Display elements containing the ferroelectric liquid crystals utilize this switching phenomenon.
Nowadays, of the chiral smectic phases, much attention is particularly paid to the S.sub.C * phase. As the display systems in which switching phenomenon of the S.sub.C * phase is utilized, there is a birefringence type system using two polarizers and a guest/host type system using a dichoric dye. Features of these display systems are as follows:
(1) The response is very quick. PA1 (2) Memory properties are present. PA1 (3) Display performance is not quite so affected by the visual angle. PA1 (1) further improvement in the response properties, PA1 (2) the exhibition of the S.sub.C * phase in a wide temperature range inclusive of room temperatures, and PA1 (3) improvement in alignment.
Thus, the display systems have the possibility of achieving the high-density display and are considered to be effectively utilizable in the display elements.
However, in the case that this ferroelectric liquid crystal is used in the practical display elements, there are now many problems such as slow response properties and very difficult alignment.
Therefore, requirements which are necessary for the ferroelectric liquid crystal materials are
At present, no chiral smectic liquid crystal compounds in a single state which satisfy all of such requirements are present, and thus one attempt is to provide a ferroelectric liquid crystal composition satisfying the above-mentioned requirements by mixing several chiral smectic liquid crystal compounds or several non-liquid crystal compounds which can meet a part of the above-mentioned requirements.
In addition to the ferroelectric liquid crystal compositions each comprising the ferroelectric liquid crystal compound alone, Japanese Patent Laid-open Publication No. 195,187/1986 discloses a ferroelectric liquid crystal composition which can be prepared by mixing one or more compounds assuming a ferroelectric liquid crystal phase with fundamental materials of compounds and compositions assuming achiral smectic C, F, G, H and I phases (hereinafter referred to simply as "S.sub.C and other phases").
Furthermore, another ferroelectric liquid crystal composition is also reported in which one or more compounds having optical activity but not assuming any ferroelectric liquid crystal phase are mixed with fundamental materials of compounds and compositions assuming the S.sub.C and other phases (Mol. Cryst. Liq. Cryst., 89, p 327, 1982).
The above-mentioned smectic liquid crystal mixture which comprises the fundamental material assuming at least one of the achiral S.sub.C and other phases and which has at least one of the S.sub.C and other phases will be hereinafter referred to as the base Sm mixture.
The preferable base Sm mixture is a liquid crystal mixture assuming the S.sub.C phase in an extensive temperature range inclusive of room temperatures. Components for the base Sm mixture are suitably selected from liquid crystal compounds such as phenylbenzole series, Schiff base series, phenylpyridine series and 5-alkyl-2-(4-alkoxyphenyl)pyridine.
For example, in Japanese Patent Laid-open Publication No. 291,679/1986 and the pamphlet of PCT International Publication WO86/06401, the ferroelectric liquid crystal prepared by mixing 5-alkyl-2-(4-alkoxyphenyl)pyrimidine with an optically active compound can assume the S.sub.C * phase in a wide temperature range inclusive of room temperatures. In the former publication, it is also described that when a light switching element is prepared by the use of a ferroelectric smectic liquid crystal material in which the above-mentioned pyrimidine derivative is used as the base Sm mixture, the thus prepared switching element has a shortened response time. Furthermore, in Japanese Patent Laid-open Publication No. 291,679/1986, it is disclosed that the ferroelectric liquid crystal material comprising 5-alkyl-2-(4'-alkylbiphenylyl-4)pyrimidine, 5-alkyl-2-(4-alkoxyphenyl)pyrimidine and an optically active compound also assumes the S.sub.C * phase in an extensive temperature range including room temperatures, and that it is effective for the improvement in the response time. Nevertheless, improvement in the response properties at room temperatures is further demanded.
A first object of the present invention is to provide a ferroelectric liquid crystal composition having quick response properties, and a second object thereof is to provide a switching element containing the liquid crystal composition which has quick response properties.