Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by ¹²⁹Xe and ²H NMR spectroscopy

Abstract

The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified as a globally uniaxial twist-bend nematic phase, N_TB. Within the temperature range 376–388 K, a classic uniaxial nematic phase, N, appears. The N_TB phase has a helical structure and the liquid crystal director, n, is tilted with respect to the helix axis; these are formed into a conglomerate of degenerate domains having opposite handedness. In a magnetic field, the helix axis orients along the field. In the present studies, the properties of CB7CB are investigated using ¹²⁹Xe NMR spectroscopy of dissolved xenon and ²H NMR of CB7CB-d₄ and the probe 4-octyl-4ʹ-cyanobiphenyl-d₂ also dissolved in CB7CB. In a uniaxial liquid crystalline environment, the ¹²⁹Xe shielding tensor is cylindrically symmetric due to the deformation of the electron cloud by anisotropic forces associated with the director. The anisotropic part of the shielding tensor depends upon the orientational order parameter of the liquid crystal with respect to the applied magnetic field and the conical (aka tilt) angle of the director. The temperature dependence of the orientational order parameter and of the conical angle has been determined independently from ¹²⁹Xe and previous ²H NMR experiments. In the N_TB phase, the averaged Saupe ordering matrix contains three off-diagonal elements. The temperature dependence of one of these, resulting from the phase chirality, has been determined from the ²H quadrupolar splittings.

Keywords:

• twist-bend nematic phase
• liquid crystal dimer
• CB7CB
• orientational order
• conical angle
• NMR
• ¹²⁹Xe
• ²H
• prochiral

Acknowledgements

Professor Geoffrey Luckhurst is extremely grateful to Dr Simon Siemianowski (Merck KGaA, Darmstadt) for working with him to create the image showing the director distribution in the twist-bend nematic phase given in Figure 1. He is also grateful to Professor Jim Emsley (University of Southampton), as well as Professor Alberta Ferrarini and Dr Cristina Greco, both from the University of Padova, for illuminating discussions.