Molecular flexibility in the design of low molar mass liquid crystals

This Thesis investigates the influence of molecular flexibility on the meso-genic behaviour and in particular, the properties of dimeric liquid crystals and their relationship to molecular structure. Dimers possess two semi-rigid anisometric units linked through a flexible alkyl spacer, this increased overall flexibility was thought to suppress smectic behaviour. In Chapter 2, however, the properties of several series of a,u-bis (4-alkylanilinebenzylidine-4'-oxy)alkanes are presented a rich smectic polymorphism. Chapter 3 introduces a new class of dimer, the asymmetric dimeric liquid crystal, in which two different mesogenic groups are linked through a flexible spacer, and as examples, the a-(4-cyanobiphenyl- 4'-oxy)-w-(4-alkylaniline-benzylidine-4'-oxy)alkanes were synthesised. The dependence of the thermal stability of the smectic phase on the length of the terminal alkyl chain is very unusual because it does not simply increase on increasing chain length but instead passes through a minimum. This novel behaviour is rationalised by proposing that the structure of the smectic phase changes from being intercalated to interdigitated on increasing chain length. Chapter 4 investigates the properties of dimers and their relationship to molecular structure via the synthesis of a range of new compounds. In Chapter 5, the effect of a flexible core is examined in compounds which possess a mesogenic group linked via an alkyl chain to a small anisometric group. Finally, Chapter 6 presents the transitional properties of several series of mesogens which possess lateral alkyl chains; the observation of liquid- crystalline behaviour for this class of compound is rationalised by proposing that the lateral chain lies parallel to the molecular long axis and so is not in its all-trans conformation.