Thermotropic liquid crystals of polyesters having a mesogenic 4,4-bibenzoate unit Smectic mesophase properties and structures in dimeric model compounds

Abstract

In this study, we prepared a homologous series of dimeric compounds, di-BB-n

These compounds have two mesogenic groups linked by a flexible methylene spacer and so can be regarded as model compounds for main chain liquid crystalline BB-n polyesters. In this dimeric system, we observed distinct odd-even effects in which the smectic mesophase properties and structures are significantly affected by the number of carbon atoms in the flexible spacer. The most interesting effect among them was observed for a type of smectic phase. From detailed optical microscopy and X-ray observations, we found that the even-membered di-BB-n materials form a smectic A while the odd-membered ones form a smectic C₂ phase. Such a distinct odd-even effect was concluded to result from the confined spatial orientation of mesogenic groups which is controlled by the conformation of the flexible spacer similar to the BB-n polymeric system.