Some Developments in the Synthesis of Liquid Crystal Materials

Advances in fields such as the applications, the physics, or the theory of liquid crystals depend very critically upon the organic chemist and his role in synthesising materials that, at the one extreme, are novel and imaginative enough to exhibit new and challenging phases or sequences of phase behaviour, or at the other extreme, have preselected molecular structures tuned to make some new application possible or to test some theory. A conference lecture of this kind provides a convenient opportunity to stress the importance of the organic chemist's role and to illustrate how his artistry is brought to bear in solving a range of materials problems. Several areas of progress are reviewed and used to illustrate how the development of new synthetic methods and reagents is enhancing the chemist's ability to respond to the needs of the liquid crystal community for highly pure compounds of growing structural complexity that have increasingly exacting physical properties and are frequently, today, also chiral and totally optically pure. One particularly important area is highlighted. This refers to developments in cross-coupling reactions, whereby routes to complex linear structures have been perfected and allow small manageable units, e. g., benzene derivatives in which substituents destined to be lateral groups are pre-positioned, to be used as the synthetic precursors that are then linked together in a convergent manner by palladium-catalysed processes and under conditions that are tolerant of many functional groups that may be wanted in the finished molecules. By such and other means, many mesogenic compounds that could only have been synthesised a few years ago by impossibly low yield processes, are now readily and economically available for study or applications.