Liquid crystal trimers.: The synthesis and characterisation of the 4,4′-bis [ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls

The synthesis and characterisation of a homologous series of liquid crystal trimers, the 4,4'-bis [omega-(4-cyanobiphenyl-4'-yloxy)alkoxy]biphenyls, is reported in which the length of the flexible spacers is varied from 3 to 12 methylene units. All the members of the series exhibit enantiotropic nematic behaviour and, in addition, monotropic smectic A behaviour is observed for compounds with spacers containing from 4 to 11 methylene units. The formation of a smectic phase in these compounds is attributed to a specific interaction between the unlike mesogenic groups, namely, the central biphenyl and the terminal cyanobiphenyl units. The nematic-isotropic transition temperatures and the associated entropy changes exhibit a dramatic odd-even effect as the length and parity of the spacers is varied, in which the even members exhibit the higher values. This behaviour is interpreted in terms of the geometry as well as the flexibility of the spacers and how these control the average molecular shape. A comparison of the magnitudes of these odd-even effects with those for the analogous dimeric series suggests that in the nematic phase the mesogenic units in the trimers are correlated to the same extent as in the dimers but that their orientational ordering is significantly higher.