1. Field of the Invention
The present invention relates to an optically active compound and a liquid crystal composition containing said compound The compound and composition containing said compound, of the present invention show a ferroelectric liquid crystal phase, and accordingly are useful as an electrooptic switching element such as liquid crystal display device or the like and can be used in liquid crystal optical modulators.
2. Discussion on the Related Arts
Liquid crystal display devices have various excellent features such as low-voltage operability, low power consumption, being thin and light-weight, and easy on the eye to be a light-receiving type. Accordingly, they are in wide use as various display devices.
Liquid crystal display devices using a nematic liquid crystal called a twisted nematic mode (TN mode) are in use currently. However, display devices using this nematic liquid crystal have had a drawback of being very slow in response as compared to luminescent type display devices such as CRT, EL and the like. Consequently, when such display devices are applied in a display device, particularly a large scale display device capable of displaying a large amount of information, it is impossible to obtain a display of good contrast. Thus, the liquid crystal display devices using a nematic liquid crystal have had a limitation for wide applications. There has recently been developed a liquid crystal device using a nematic liquid crystal called a super twisted nematic type (STN mode) or SBE and capable of giving a display of improved contrast. Even in this STN type liquid crystal display device, however, the response is not sufficient and therefore said device finds a limitation in application to a display capable of displaying a still larger amount of information. Hence, various attempts are under way to develop a new liquid crystal display system giving an excellent response.
Ferroelectric liquid crystals have a memory function and give a high speed response, and accordingly their application to a large scale display is highly expected. As liquid crystals having ferroelectric properties, there are known those showing a chiral smectic C phase, a chiral smectic H phase, a chiral smectic J phase, etc. Of these ferroelectric liquid crystals, those showing a chiral smectic C phase are thought to have highest practical usability.
Ferroelectric liquid crystals showing a chiral smectic C phase were first synthesized in 1975 by R. B. Meyer et al., and their typical examples include 2-methylbutyl 4-(4'-n-decyloxybenzylideneamino)cinnamate (hereinafter abbreviated to DOBAMBC) [J. Physique, 36, L-69 (1975)].
A thin film liquid crystal cell was prepared response of .mu. sec order [N. A. Clark et al., Appl. Phys. Lett., 36, 89 (1980)]. Since that time, there was started the development of optical modulation devices (e.g. liquid crystal device, photo-printer head) using a ferroelectric liquid crystal showing a chiral smectic C phase (hereinafter may be referred to simply as "ferroelectric liquid crystal").
As a result, a number of ferroelectric liquid crystal compounds showing a chiral smectic C phase have been developed since then and various ferroelectric liquid crystal compounds are known currently. However, no ferroelectric liquid crystal compound is found yet which has satisfactory reliability and capability for use in a display device, particularly a large scale one, etc.
In order for a ferroelectric liquid crystal to be practically used as a liquid crystal display device, etc., the liquid crystal must be superior in high speed response, orientation, memory function, threshold, temperature dependences of these properties, etc. Also, the ferroelectric liquid crystal is required to show a chiral smectic C phase over a wide temperature range so that it can operate at a sufficiently wide temperature range including room temperature and further to have excellent physical and chemical stabilities.
In order for a ferroelectric liquid crystal to have, in particular, excellent physical and chemical stabilities, good high speed response and good memory function, the liquid crystal must have a large spontaneous polarization.
Among the so far developed ferroelectric liquid crystal, no compound is found yet which satisfies the above requirements. For example, the above mentioned DOBAMBC, being a liquid crystal of Schiff's base type, is insufficient in chemical stability to water, light, etc. and moreover, has a small spontaneous polarization of 4 nC/cm.sup.2 or below.
Ester type liquid crystals are reported as a ferroelectric liquid crystal which is chemically stable. However, these liquid crystals are not satisfactory because they have no sufficiently large spontaneous polarization and no sufficiently wide temperature range of chiral smectic C phase.
In order to obtain a large spontaneous polarization, there were synthesized those compounds having two asymmetric carbon atoms as an optically active group essential for the expression of chiral smectic C phase.
These compounds include, for example, liquid crystal compounds having a dichiral epoxide side chain [David M. Walba et al., Journal of American Chemical Society, 108, 7424 (1986)] and liquid crystal compounds having a halogen atom and a methyl group on two adjacent asymmetric carbon atoms [Japanese Patent Application Kokai (Laid-Open) Nos. 168780/1985, 218358/1985, 68449/1986, 40/1987, 46/1987, 103043/1987, 111950/1987, 142131/1987, 175443/1987, etc.].
As a typical example of the above liquid crystal compounds, there is (s)-3-methyl-2-chlorobutyl 4-(4'-octylcarbonyloxy) biphenylcarboxylate [Japanese Patent Application Kokai (Laid-Open) No. 68449/1986]. This liquid crystal compound has a very large spontaneous polarization of 180 nC/cm.sup.2 but, being an aliphatic chloro compound, has poor chemical stability. Hence, there was synthesized 4'-octylcarbonyloxy-4-[(s)-2-methoxy-(s)-3-methylpentyloxycarbonyl]bipheny l [Japanese Patent Application Kokai (Laid-Open) No. 228036/1987]. This compound has excellent chemical stability but has an insufficient spontaneous polarization of 17 nC/cm.sup.2.