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The chirality of a twist–bend nematic phase identified by NMR spectroscopy

The chirality of a twist–bend nematic phase identified by NMR spectroscopy
The chirality of a twist–bend nematic phase identified by NMR spectroscopy
One of the defining characteristics of the twist–bend nematic phase, formed by the methylene-linked liquid crystal dimer 1?,7?-bis(4-cyanobiphenyl-4?-yl) heptane (CB7CB), is its chirality. This new nematic phase, predicted by Dozov, is of particular interest because although the constituent molecules are achiral the phase itself is chiral. Here, we describe the use of NMR spectroscopy to determine experimentally whether in reality the phase is chiral or not. The basis of this novel procedure is that the equivalence of the protons or deuterons in a prochiral methylene group in a nematic phase with D?h symmetry is lost in a chiral phase because its symmetry is reduced to D? on removal of the mirror plane. Recording proton-enhanced local field (PELF) NMR experiments shows that in the standard nematic phase all of the methylene groups in the heptane spacer have equivalent pairs of C–H groups but this equivalence is lost for the six prochiral methylene groups with their enantiotopic protons on passing to the twist–bend nematic. Strikingly, this equivalence is not lost for the central methylene group where the two protons are homotopic. We also show how the phase chirality can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups, using 4-octyl-4?-cyanobiphenyl-d2, perdeuteroacenaphthene-d10, and acenaphthene-d4 as examples. For the standard nematic phase deuterium, NMR shows that the deuterons in these methylene groups are equivalent but, as expected, in the twist–bend nematic phase this equivalence is lost. The deuterium NMR spectra of these probe molecules dissolved in CB7CB have been recorded from the isotropic phase, through the nematic and deep into the supercooled twist–bend nematic.
1520-6106
7940-7951
Beguin, Laetitia
6af57114-e19a-4c36-8b56-46768bf7a2a3
Emsley, James W.
9d219d5e-28c0-4a8c-bf3d-1f78cd707c17
Lelli, Moreno
4a9fb9fb-d8f5-41c3-b280-30e7ca814538
Lesage, Anne
bf3518dc-c688-4b8b-ab85-62ce2d6cbae0
Luckhurst, Geoffrey R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7
Timimi, Bakir A.
845f8bba-a04b-4e8d-adfe-51b13e9f29ca
Zimmermann, Herbert
5f846fc9-2d85-4743-88f0-20bd420a62a4
Beguin, Laetitia
6af57114-e19a-4c36-8b56-46768bf7a2a3
Emsley, James W.
9d219d5e-28c0-4a8c-bf3d-1f78cd707c17
Lelli, Moreno
4a9fb9fb-d8f5-41c3-b280-30e7ca814538
Lesage, Anne
bf3518dc-c688-4b8b-ab85-62ce2d6cbae0
Luckhurst, Geoffrey R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7
Timimi, Bakir A.
845f8bba-a04b-4e8d-adfe-51b13e9f29ca
Zimmermann, Herbert
5f846fc9-2d85-4743-88f0-20bd420a62a4

Beguin, Laetitia, Emsley, James W., Lelli, Moreno, Lesage, Anne, Luckhurst, Geoffrey R., Timimi, Bakir A. and Zimmermann, Herbert (2012) The chirality of a twist–bend nematic phase identified by NMR spectroscopy. The Journal of Physical Chemistry B, 116 (27), 7940-7951. (doi:10.1021/jp302705n).

Record type: Article

Abstract

One of the defining characteristics of the twist–bend nematic phase, formed by the methylene-linked liquid crystal dimer 1?,7?-bis(4-cyanobiphenyl-4?-yl) heptane (CB7CB), is its chirality. This new nematic phase, predicted by Dozov, is of particular interest because although the constituent molecules are achiral the phase itself is chiral. Here, we describe the use of NMR spectroscopy to determine experimentally whether in reality the phase is chiral or not. The basis of this novel procedure is that the equivalence of the protons or deuterons in a prochiral methylene group in a nematic phase with D?h symmetry is lost in a chiral phase because its symmetry is reduced to D? on removal of the mirror plane. Recording proton-enhanced local field (PELF) NMR experiments shows that in the standard nematic phase all of the methylene groups in the heptane spacer have equivalent pairs of C–H groups but this equivalence is lost for the six prochiral methylene groups with their enantiotopic protons on passing to the twist–bend nematic. Strikingly, this equivalence is not lost for the central methylene group where the two protons are homotopic. We also show how the phase chirality can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups, using 4-octyl-4?-cyanobiphenyl-d2, perdeuteroacenaphthene-d10, and acenaphthene-d4 as examples. For the standard nematic phase deuterium, NMR shows that the deuterons in these methylene groups are equivalent but, as expected, in the twist–bend nematic phase this equivalence is lost. The deuterium NMR spectra of these probe molecules dissolved in CB7CB have been recorded from the isotropic phase, through the nematic and deep into the supercooled twist–bend nematic.

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Published date: 8 June 2012
Organisations: Magnetic Resonance

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Local EPrints ID: 354785
URI: https://eprints.soton.ac.uk/id/eprint/354785
ISSN: 1520-6106
PURE UUID: ec0f9ce4-d4d0-40d9-aa08-cfeea6d965e9

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Date deposited: 23 Jul 2013 09:28
Last modified: 16 Jul 2019 21:28

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Contributors

Author: Laetitia Beguin
Author: James W. Emsley
Author: Moreno Lelli
Author: Anne Lesage
Author: Geoffrey R. Luckhurst
Author: Bakir A. Timimi
Author: Herbert Zimmermann

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