Molecular Flexibility and Bend in Semi-Rigid Liquid Crystals: Implications for the Heliconical Nematic Ground State.

Richard J Mandle,John W Goodby

doi:10.1002/chem.201903677

Abstract

The NTB phase phases possess a local helical structure with a pitch length of a few nanometers and is typically exhibited by materials consisting of two rigid mesogenic units linked by a flexible oligomethylene spacer of odd parity, giving a bent shape. We report the synthesis and characterisation of two novel dimeric liquid crystals, and perform a computational study on 10 cyanobiphenyl dimers with varying linking groups, generating a large library of conformers for each compound; this allows us to present molecular bend angles as probability weighted averages of many conformers, rather than use a single conformer. We validate conformer libraries by comparison of interproton distances with those obtained from solution‐based 1D 1H NOESY NMR, finding good agreement between experiment and computational work. Conversely, we find that using any single conformer fails to reproduce experimental interproton distances. We find the use of a single conformer significantly overestimates the molecular bend angle while also ignoring flexibility; in addition, we show that the average bend angle and flexibility are both linked to the relative stability of the NTB phase.

Highlights

Liquid crystals (LC) are a collection of states of matter with some degree of positional or orientational organisation and are widely employed as functional materials. 1-3 Different LC mesophase are principally characterised by their degree of orientational and/or positional order; for example, the nematic LC phase possesses long range orientational order, whereas lamellar phases exhibit positional order in one dimension
The twist-bend modulated nematic phase (TB), the average orientation of the nematic phase rotates over a few nanometers giving a helical structure with a remarkably short pitch length. 4-9 The NTB phase is chiral despite being typically formed by achiral molecules, a handful of chiral materials are known to exhibit this phase. 10-12 this phase is principally exhibited by liquid-crystalline dimers, in which two rigid sections are adjoined by a flexible spacer, 13 14 it has been observed in semi-rigid bent-core materials, 15 liquid crystalline n-mers 16-19 and polymers
This is of interest as we previously suggested that a tight distribution of conformers centered about a favourable bend angle can lead to direct isotropic to NTB transitions, i.e. TNTB-N/TN-Iso of ≥1

Summary

Introduction

Liquid crystals (LC) are a collection of states of matter with some degree of positional or orientational organisation and are widely employed as functional materials. 1-3 Different LC mesophase are principally characterised by their degree of orientational and/or positional order; for example, the nematic LC phase possesses long range orientational order, whereas lamellar phases exhibit positional order in one dimension. Compounds 9 and 10 were studied as part of this work; transition temperatures and associated enthalpies were obtained from DSC at a heat/cool rate of 10 ° min-1. We use the energy of each conformer to give a Boltzmann population, allowing us to present bend angles which are the probability weighted average of many conformers (Figure 2).

Results

Conclusion