The Chirality of a Twist-Bend Nematic Phase Identified by NMR Spectroscopy - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Journal of Physical Chemistry B Année : 2012

The Chirality of a Twist-Bend Nematic Phase Identified by NMR Spectroscopy

Résumé

One of the defining characteristics of the twist-bend nematic phase, formed by the methylene linked liquid crystal dimer 1 '',7 ''-bis(4-cyanobipheny1-4'-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-infinity h symmetry is lost in a chiral phase because its symmetry is reduced to D-infinity 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 chirgity can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups, using 4-octyl4'-cyanobiphenyl-d(2), perdeuteroacenaphthene-d(10), and acenaphthene-d(4) 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.
Fichier non déposé

Dates et versions

hal-00799658 , version 1 (12-03-2013)

Identifiants

Citer

Laetitia Beguin, James W. Emsley, Moreno Lelli, Anne Lesage, Geoffrey R. Luckhurst, et al.. The Chirality of a Twist-Bend Nematic Phase Identified by NMR Spectroscopy. Journal of Physical Chemistry B, 2012, 116 (27), pp.7940-7951. ⟨10.1021/jp302705n⟩. ⟨hal-00799658⟩
59 Consultations
0 Téléchargements

Altmetric

Partager

Gmail Facebook X LinkedIn More