Liquid crystal materials and devices are well known, and generally exploit the useful hemeric (N) and smectic (S), especially smectic C and I (Sc, SI) liquid crystal phases, which show electro-optical characteristics. The Sc phase shows its most useful characteristics when an optically active compound is present in the material, forming a ferroelectric chiral smectic C (Sc) phase.
Liquid crystal materials are generally mixtures of compounds, and desirable requirements for such a mixture and/or its constituent compounds include ease of preparation and liquid crystal phases of a useful type which persist over a broad temperature range preferably including room temperature. Other desirable requirements may be sought for particular applications, for example a useful birefringence, dielectric anisotropy, a useful phase transition sequence such as Sc-SA with increasing temperature, and in the case of Sc materials, the ability to form a mixture with a high spontaneous polarisation (Ps).
A wide variety of compounds which show liquid crystal phases are known, for example: ##STR1## compounds where R and R' are alkyl, disclosed in EP-A-0,117,631. Another useful structural type for example is: ##STR2## where R and R' are alkyl, disclosed in PCT/GB87/00441.
It is widely acknowledged that minor structural variations in liquid crystal compounds can have a drastic and unexpected effect on their liquid crystalline properties. It is therefore an object of this invention to explore novel structural types and identify those which show liquid crystal phases, especially of a useful type.