The effect of molecular anisotropy on the properties of liquid crystals

In this Thesis we present the results of investigations into the effect of structural changes on the liquid crystalline properties of poly-aromatic molecules . Following a brief introduction to various aspects of liquid crystalline phases, we describe the synthesis and characterisation of two homologous series of halo substituted 4"-n-alkyloxy-p-perphenyls, namely the 4-alkyloxy-4"-fluoro-p-terphenyls and 4-alkyloxy-4"-choloro-p-terphenyls. The physical properties of these novel compounds are compared with those of the analogous cyano substituted compounds. In Chapter 3 we describe the synthesis of a number of specifically deuteriated 4-alkyloxy-4"-cyano-p-terphenyls and 4-alkyl-4"-cyano-p-terphenyls. The effect of the linking oxygen atom on the liquid crystalline phases formed by these materials is investigated using deuterium NMR spectroscopy.The remainder of this work is concerned with dimeric liquid crystals, where two mesogenic moieties are connected via a flexible alipahtic chain. Chapter 4 describes the synthesis and characterisation of a number of novel homologous series in which the 4-cyanophenyl group is linked to the 4-cyanobiphenyl and 4-cyano-p-terphenyl moieties, namely the α-(4-cyanophenyloxy)-ω-(4'cyanobiphenyl-4-yloxy) alkanes, the α-(4-cyanophenyloxy)-ω-(4"-cyano-p-terphenyl-4-yloxy) alkanes and the α-(4-cyanophenyloxy)-ω-(4"cyano-p-terphenyl-4-yl) alkanes. In Chapter 5 we describe the synthesis and characterisation of the biphenyl analogues to the compounds described in Chapter 4, the α-(4'-cyanobiphenyl-4-yloxy)-ω-(4"cyano-p-terphenyl-4-yloxy) alkanes and the α-(4'-cyanophenyl-4-yloxy)-ω-(4"cyano-p-terphenyl-4-yl) alkanes. The unusual smectic phase that is observed for two of these materials is then investigated using ESR and NMR. These results are compared with those obtained for model compounds known to form a smectic A antiphase.