Synthetic Chemistry Related to Liquid Crystals

Abstract

Because of the continued escalation of interest in liquid crystals and in their applications in display devices, the efforts of organic chemists have recently been concentrated on the synthesis of better mesogens for technological exploitation. Therefore, although the stability of available liquid crystal materials, their electrical properties and their applications will provide topics for other sections of this meeting, I feel that a review of those factors which have motivated synthetic chemists over the last two years must be given now even though some overlap may in consequence arise. I will therefore survey briefly some electro-optical effects now under active consideration with the aim of illustrating why the attention of the chemist is directed towards particular types of mesogen with particular characteristics. Whereas at one time the aim was simply to produce nematogens giving nematic mesophases in the room temperature range so that their dynamic scattering characteristics could be studied, the need is now for a wider range of mesogens with different physical properties. The reasons are that dynamic scattering, once the most widely studied electro-optical effect, is now realized to have some drawbacks, and this has led to the development of other electro-optical effects of potential and real value in display devices. I will consider five electro-optical effects: 1. The Fréedericksz Effect (F.E.) 2. The Dynamic Scattering Effect (D.S.E.) 3. The Twisted Nematic Effect (T.N.E.) 4. The Cholesteric Memory Effect (C.M.E.) 5. The Cholesteric-Nematic Phase Change Effect (P.C.E.)