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

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.

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition

“…13 The mesogen CB7CB now appears to be the model liquid crystal on which research about this new phase is mainly focused. [18][19][20][21][22][23][24][25][26] The heliconical twist-bend nematic phase appears as a state in which the spontaneous bend of the director is accompanied by a twist deformation in order to satisfy topological constraints. The induced twist can be left-or right-handed and the sample is expected to form a conglomerate of degenerate domains having opposite chirality even though the constituent molecules are achiral.…”

Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen

“…Recently, the twist-bend nematic (N TB ) phase, a fundamentally new type of nematic ground state of achiral molecules, exhibiting layer-free, helical liquid crystal ordering of nanoscale pitch, has been structurally identified and characterized [1,2]. The twist-bend nematic phase was initially suggested as a theoretical possibility by Meyer over 40 years ago [3] and discussed in 2000 by Dozov [4] and Memmer [5], who suggested that the tendency for local bend curvature in the director field of bent molecules (for example, Figure 1a [6,7] having nematic characteristics, spontaneous chirality [8,9], and smectic textural features [10,11] but no smectic-like lamellar x-ray diffraction peaks [12] have been interpreted in terms of the N TB structure [4]. The N TB phase is in general polar and structurally chiral despite being formed from achiral molecules.…”

Dependence of Mesomorphic Behaviour of Methylene‐Linked Dimers and the Stability of the NTB/NX Phase upon Choice of Mesogenic Units and Terminal Chain Length

“…This lower temperature ‘nematic’ phase is denoted N X or N TB ,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 in which ‘X’ and ‘TB’ refer to unknown and twist‐bend, respectively. The local structure of the N X /N TB phase is still hotly debated, with the heliconical ‘twist‐bend’ model proposed independently by Meyer19 and Dozov20 supported by 2 H NMR studies, measurement of the electroclinic effect, freeze‐fracture transmission electron microscopy (FFTEM) and carbon K‐edge SAXS 21, 22, 23, 24. All three of these methods suggest a local helical structure of extremely tight pitch, in the region of 8 nm for the well‐studied material CB7CB {4′,4′′′‐(heptane‐1,7‐diyl)bis([1,1′‐biphenyl]‐4‐carbonitrile)} 21, 22, 23, 24, 25.…”

“…The local structure of the N X /N TB phase is still hotly debated, with the heliconical ‘twist‐bend’ model proposed independently by Meyer19 and Dozov20 supported by 2 H NMR studies, measurement of the electroclinic effect, freeze‐fracture transmission electron microscopy (FFTEM) and carbon K‐edge SAXS 21, 22, 23, 24. All three of these methods suggest a local helical structure of extremely tight pitch, in the region of 8 nm for the well‐studied material CB7CB {4′,4′′′‐(heptane‐1,7‐diyl)bis([1,1′‐biphenyl]‐4‐carbonitrile)} 21, 22, 23, 24, 25. Polarised Raman spectroscopy has been used to measure order parameters in the nematic and N TB phases 26…”