Liquid crystal display elements are widely used as display elements for watches, electronic desk-top computers, personal word processors, pocket-size TV sets, etc. This stems from the fact that they have such beneficial characteristics as no eyestrain because of their passive device, low electric power consumption, a thin structure etc. However, they are restricted in their practical applications because of different problems such as slow response and lack of memory effect.
In an attempt to expand their areas of application there has been found, for example, the super twisted nematic (STN) display system, which is an improvement of the twisted nematic (TN) display system. These systems, however, are not sufficient for large screen or graphic display use. Various studies have therefore been made of liquid crystal display elements which can supersede them.
One such display system [N. A. Clark et al., Applied Phys. lett., 36, 899 (1980)] utilizes ferroelectric liquid crystals [R. B. Meyer et al., J. de. Phys., 36 L-69 (1975)]. Because of its advantageous characteristics such as fast response, which is 100 times as fast as that of conventional systems, and memory effect, it is expected to expand areas of application of liquid crystal display elements. The term "ferroelectric liquid crystal" is used to mean a series of smectic liquid crystals whose molecular longitudinal axis is at a certain angle to the normal of the layer, but in practice the chiral smectic C (SmC*) phase is utilized.
Ferroelectric liquid crystals for preparing display elements are used in the form of (1) a liquid crystalline composition Obtained by blending together different compounds with the SmC* phase or (2) a liquid crystalline composition obtained by blending different compounds having the SmC phase with optically active compounds. Early research and development of ferroelectric liquid crystal display elements used liquid crystalline compositions of the formulation in accordance with (1) above. Since, with the advent of further research and development, addition of optically active compounds was found to result in the formation of ferroelectric crystals, however, the tendency is for compositions of the formulation in accordance with (2) above to be used more. This is because the formulation in accordance with (2) above is considered to be more advantageous in practical applications, for example in that it renders easier the adjustment of different properties demanded in the market (e.g. operating temperature range, response time, spontaneous polarization, helical pitch, chemical stability etc.) and the synthesis of SmC compounds is less expensive than that of SmC* compounds.
Even the formulation in accordance with (2) above, have yet to reach a stage of providing compositions having sufficient properties for putting into practical use, there thus now being a need for a variety of compounds useful as components with which to prepare ferroelectric liquid crystalline compositions being developed.