The twist-bend phases: structure–property relationships, chirality and hydrogen-bonding

Rebecca Walker

doi:10.1080/1358314x.2020.1771841

Abstract

ABSTRACT This article will review some of our recent work concerning the relationships between molecular structure and the observation of the fascinating twist-bend nematic, NTB, phase. From a chemist’s perspective, understanding the molecular features influencing the formation and stabilisation of the NTB phase is of paramount importance, and allows for the design of new materials that have targeted properties. As such, recent work in Aberdeen has had the primary aim of enhancing our current understanding of these relationships in dimeric liquid crystals, through the synthesis and characterisation of a diverse range of materials, including a selection of supramolecular liquid crystals. A second aim has been to explore the intriguing question as to how the NTB phase, having spontaneous structural chirality, would respond at a microscopic level to the presence of intrinsic molecular chirality – the inclusion of chiral fragments in the dimers – and to obtain examples of the ‘chiral’ twist-bend nematic phase (N*TB) for comparative study alongside the conventional NTB phase. We also show that bent achiral molecules form heliconical smectic phases, as predicted by Dozov in his seminal 2001 work.

Highlights

The prediction [1,2] and subsequent experimental dis covery [3] of the twist-bend nematic phase, NTB, is unde niably one of the most significant recent developments in the field of liquid crystals
We show that bent achiral molecules form heliconical smectic phases, as predicted by Dozov in his seminal 2001 work
In 2018, we reported the first example of spontaneous chiral symmetry breaking through the mix ing of achiral components; the induction of the NTB phase through molecular recognition in mixtures con taining unlike H-bond donors and acceptors: the n-alkoxybenzoic acids (nOBAs) (n = 4,5) and 1OB6OS, respectively, Figure 12 [57]

Summary

Introduction

The prediction [1,2] and subsequent experimental dis covery [3] of the twist-bend nematic phase, NTB, is unde niably one of the most significant recent developments in the field of liquid crystals. This was subsequently selected as a candidate for further modification to explore the effects of structural changes on the incidence of the NTB phase and its proper ties This particular structure was chosen because the methylene-ether link allows for the straightforward syn thetic addition of different mesogenic units to the CB6O ‘backbone’ while maintaining the ideal molecular curva ture required for an enantiotropic, stable twist-bend nematic phase. TNI is elevated and TNTBN reduced for the diether-linked dimers compared with the CB6OABX series, with the NTB phase seen only for the butyl and butoxy-terminated materials These observa tions are attributed mainly to the influence of the spacer on molecular shape, a trend well documented for liquid crystal dimers [18,21,36,40] and completely in accord with the predictions of theoretical models [41,42]. Investigations into the photocontrolable azoben zene-based twist-bend nematogens remain ongoing, with intriguing aspects such as the effect of irradiation on the elastic constants and the possibility of lightcontrolled selective reflection in a chiral material yet to be fully addressed

Benzylideneanilines

Chirality and the NTB phase

Hydrogen-bonding and the NTB phase

Summary