Liquid crystal tetramers

Abstract

The first homologous series of liquid crystal tetramers in which four mesogenic groups are linked via three alkyl spacers has been synthesised and characterised. The length of the outer two spacers is varied from 3 to 12 methylene units while the central spacer is held at 8 methylene units. All the members of the series exhibit enantiotropic nematic behaviour except for the decyl and dodecyl homologues which exhibit exclusively smectic A behaviour. In addition, smectic A behavior is observed for the hexyl to dodecyl homologues. The transitional properties, namely the clearing temperatures and associated entropy changes, exhibit a dramatic odd–even effect as the length and parity of the outer spacers are varied. This behaviour is interpreted in terms of how the spacers control the average molecular shape. The ratio of the smectic periodicity to the estimated all trans molecular length for the nonyl homologue is just 0.27. Three differing local packing arrangements are suggested to account for this surprising result.