Helical phases assembled from achiral molecules: Twist-bend nematic and helical filamentary B4 phases formed by mesogenic dimers

Abstract

The synthesis and characterisation of an homologous series of bent odd-membered mesogenic dimers, the 4′-(6-{4-[(E)-{[4-(alkylthio)phenyl]imino}-methyl]phenoxy}hexyl)[1,1′-biphenyl]-4-carbonitriles (CB6O.Sm) is reported. This general class of materials, despite being achiral, has a strong tendency to form helical structures, and here, for the first time we report three such chiral phases in a single homologous series. Specifically, the heliconical twist-bend nematic (NTB) phase for short terminal thioalkyl chains, and its smectic equivalent - the twist-bend smectic C (SmCTB) phase for longer chains. All the dimers showed the helical filament B4 phase, which is typically seen for rigid bent-core mesogens, but has only rarely been reported for flexible dimeric molecules. In addition, on increasing chain length, smectic behaviour emerges including the smectic A and the smectic CTB phases. We also show that these materials have the potential for their morphology to be controlled through surface interactions. The presence of the little-studied thio-linkage in the terminal chain, and the wide range of properties present in this single group of homologues, with promise of broad applicability in optics, photonics, as well as fundamental significance as a case study to achieve a better understanding of chirality and symmetry breaking in liquid crystals, ensures the importance of this new series of mesogens.