Experimental and Computational Study of a Liquid Crystalline Dimesogen Exhibiting Nematic, Twist-Bend Nematic, Intercalated Smectic, and Soft Crystalline Mesophases.

Emily E Pocock,John W Goodby,Richard J Mandle

doi:10.3390/molecules26030532

Emily E Pocock, John W Goodby + Show 1 more

Open Access

https://doi.org/10.3390/molecules26030532

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Abstract

Liquid crystalline dimers and dimesogens have attracted significant attention due to their tendency to exhibit twist-bend modulated nematic (NTB) phases. While the features that give rise to NTB phase formation are now somewhat understood, a comparable structure–property relationship governing the formation of layered (smectic) phases from the NTB phase is absent. In this present work, we find that by selecting mesogenic units with differing polarities and aspect ratios and selecting an appropriately bent central spacer we obtain a material that exhibits both NTB and intercalated smectic phases. The higher temperature smectic phase is assigned as SmCA based on its optical textures and X-ray scattering patterns. A detailed study of the lower temperature smectic ‘’X’’ phase by optical microscopy and SAXS/WAXS demonstrates this phase to be smectic, with an in-plane orthorhombic or monoclinic packing and long (>100 nm) out of plane correlation lengths. This phase, which has been observed in a handful of materials to date, is a soft-crystal phase with an anticlinic layer organisation. We suggest that mismatching the polarities, conjugation and aspect ratios of mesogenic units is a useful method for generating smectic forming dimesogens.

Highlights

IntroductionLiquid crystals can be broadly defined as any state of matter with some degree of positional or orientational organisation intermediate between the isotropic liquid state and a crystalline solid with long-range positional and orientational order in three dimensions
Liquid crystalline dimers and dimesogens have attracted significant attention due to their tendency to exhibit twist-bend modulated nematic (NTB ) phases
[18] The twist-bend nematic phase is principally formed by liquid-crystalline dimers, [19] in which two rigid sections are adjoined by a flexible spacer

Summary

Introduction

Liquid crystals can be broadly defined as any state of matter with some degree of positional or orientational organisation intermediate between the isotropic liquid state and a crystalline solid with long-range positional and orientational order in three dimensions. [18] The twist-bend nematic phase is principally formed by liquid-crystalline dimers, [19] in which two rigid sections are adjoined by a (semi) flexible spacer This phase of matter is observed in liquid crystalline n-mers, [20,21] hydrogen bonded systems, [22,23] bent-core materials, [24] and non-linear oligomers. It is mesophases, pairing an appropriate central spacer (dictates bend angle and conformer mesophases, pairing an appropriate central spacer (dictates bend angle and conformer disdistribution) with suitable mesogenic (dictates transition temperatures); is retribution) with suitable mesogenic unitsunits this isthis reflected flected by the fact that hundreds of materials are known to exhibit this phase [34,35,36,37,38].

Mesomorphic Behaviour

Photomicrographs

A SmC the‘’X’’

Conformational Distributions

Materials and Methods

Conclusions