Columnar mesomorphic properties of new macrocyclic mesogens

Abstract

The mesomorphic properties of a new class of columnar liquid crystals, consisting of a bowl-shaped metacyclophane core surrounded by twelve flexible alkyl chains are reported. These mesogens form D_ho columnar mesophases having negative optical and dielectric anisotropy. Both planar and homeotropic alignment is obtained. An antiferroelectric pairing of the molecules within the columns is observed, ruling out the possibility of a ferroelectric self-organization of the cores. For all the mesogens a complex thermal behaviour is observed. Two different solid phases C₁ and C₂ have been characterized and their thermal behaviour elucidated: C₁ is the mesophase precursor, while C₂, more ordered, melts directly to an isotropic liquid. Mesophase stability over time depends on the length of the side chains. The overall mesomorphic complexity is attributed to the combination of the different possible core packing geometries with the numerous conformations available to the alkyl chains.