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The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry

The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry
The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry
Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (NTB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the NTB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the NTB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1″,7″-bis-4-(4′-cyanobiphenyl-4′-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase.
twist-bend nematic, local phase bixality, moleulcar translational diffusion coefficients, macroscopic rotational diffusion, motional averaging, residual quadrupolar tensors
0267-8292
204-218
Cifelli, M.
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Domenici, V.
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Dvinskikh, S.V.
63beb0e1-83e2-4d75-9196-e2f1986674c8
Luckhurst, Geoffrey R.
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Timimi, B.A.
a1aa2a58-5d7a-4c22-9339-b8ed8e81d053
Cifelli, M.
7b7d1393-b552-48de-ac1c-8d6fbbb0ee9e
Domenici, V.
37f9f450-a9bf-420a-b6b9-35be6d53cae3
Dvinskikh, S.V.
63beb0e1-83e2-4d75-9196-e2f1986674c8
Luckhurst, Geoffrey R.
3deaf900-9562-4589-ab5e-3b1fc6fce1a9
Timimi, B.A.
a1aa2a58-5d7a-4c22-9339-b8ed8e81d053

Cifelli, M., Domenici, V., Dvinskikh, S.V., Luckhurst, Geoffrey R. and Timimi, B.A. (2016) The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry. Liquid Crystals, 44 (1), 204-218. (doi:10.1080/02678292.2016.1254292).

Record type: Article

Abstract

Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (NTB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the NTB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the NTB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1″,7″-bis-4-(4′-cyanobiphenyl-4′-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase.

Text
Translational Diffusion in NTB draft revised - Accepted Manuscript
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More information

Accepted/In Press date: 20 October 2016
e-pub ahead of print date: 20 November 2016
Keywords: twist-bend nematic, local phase bixality, moleulcar translational diffusion coefficients, macroscopic rotational diffusion, motional averaging, residual quadrupolar tensors
Organisations: Chemistry

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Local EPrints ID: 410396
URI: http://eprints.soton.ac.uk/id/eprint/410396
ISSN: 0267-8292
PURE UUID: 1b3fe216-e54d-4e9d-b4c8-3db4b284b74e

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Date deposited: 07 Jun 2017 16:31
Last modified: 07 Oct 2020 05:30

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Contributors

Author: M. Cifelli
Author: V. Domenici
Author: S.V. Dvinskikh
Author: Geoffrey R. Luckhurst
Author: B.A. Timimi

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