Geoffrey R. Luckhurst

The twist-bend nematic (N_TB) phase of bent-shaped molecules has recently attracted much attention due to the spontaneous bend of its director field and the doubly-degenerate chirality of its heliconical structure. Despite intensive experimental and theoretical investigation worldwide, the main structural characteristics (pitch and conical angle) and elastic properties of the phase are still barely understood. This is mainly due to the difficulty in growing large single domains of the N_TB phase, which prevents the application of the powerful electro-optical techniques developed for the nematic (N) phase. Moreover, the twist and bend distortions of the optic axis are forbidden by the pseudo-layered structure of the N_TB phase, which makes its response to the field smectic-like instead of nematic-like. Therefore, the only macroscopic electric effect that can be observed deep in the N_TB phase is the smectic-like “electroclinic” effect (ECE_NTB). Here, we achieve large monochiral N_TB domains which remain uniform over a wide temperature range (20–60 °C) in thin (1.5 µm) planar cells, thus avoiding the so-called stripe- and rope-like textural instabilities. This allowed us to experimentally determine, using electro-optical measurements, the temperature dependence of the ECE_NTB response in four different N_TB materials: namely the dimers CB7CB, CB9CB, CB6OCB, and BNA76. For all compounds, the thermal dependences of conical angle and pitch in the vicinity of the N-N_TB transition follow the theoretically predicted power law behaviour. However, the agreement between the measured and predicted power law exponents remains only qualitative, which calls for improvement of the theoretical models.

The Luckhurst-Samulski Prize was announced by the present Editor, Corrie Imrie [1], in 2009 when he explained that with the Publishers, Taylor & Francis, it had been decided to inaugurate a new Awa...

Here we discuss the use of deuteriated probe molecules in NMR studies of the nematic (N) and twist-bend nematic (N_TB) phases formed by the generic liquid crystal dimer CB7CB. The key properties of three probes dissolved in the N_TB phase investigated are the orientational order S_zz ^B and the conical angle, θ₀, of the director with respect to the spectrometer field. In addition for one probe 5CB-d₂ it is possible to obtain a measure of the phase chirality from the difference in the quadrupolar splittings given by the prochiral pair of deuterons. The relative simplicity of the probes facilitates the determination of the key properties which are obtained using two different approaches. GRAPHICAL ABSTRACT

Stimulated by the paper that precedes this, our aim is to correct an error made concerning the twist-bend nematic phase and to place its development, both theoretical and experimental, in their proper context. The prediction starts with R.B.Meyer in 1973 but then jumps to 2001 and a different direction with I. Dozov and to 2002 with R. Memmer. There was then a gap until 2011 when a team of thirteen European scientists, while studying a liquid crystal dimer formed from achiral bent molecules, identified a nematic phase as the missing twist-bend nematic. This was based on a detailed study of its defining characteristics such as the chirality of the phase and its helicoidal structure with its degenerate handedness. This discovery proved to be of considerable interest and has prompted numerous investigations resulting in the discovery of many other mesogens forming the twist-bend nematic phase. However, as we shall see this view has not been universally shared; indeed, amongst others, Samulski, Vanakaras and Photinos claim that cyanobiphenyl mesogens instead of producing the N_TB nematic, form the polar twisted phase, N_PT so that the twist-bend nematic has yet to be discovered. It is this error that we shall correct.The Doubly Degenerate Heliconical Structure The Doubly Degenerate Heliconical Structure

In this article, we explore those factors that influence the orientational order in both nematic and twist-bend nematic phases. The systems used for our NMR experiments are binary mixtures of a dimer and a related monomer thus enabling its behaviour to be controlled continuously, through its composition rather than chemical structure. Earlier studies have revealed that as the temperature is lowered in the nematic phase, the orientational order frequently passes through a maximum prior to the formation of the N_TB phase. A confirmative indication of this unusual behaviour is that the Haller function cannot fit this dependence over the entire nematic range; such behaviour is intriguing. In the N_TB phase, the measured order parameter is often insensitive to decreasing temperature for systems with strong N_TB–N transitions often associated with a narrow nematic range. For those mesogens with longer nematic ranges and hence weaker N_TB–N transitions, the order parameter decreases more markedly with temperature. As expected for the weak transitions, the behaviour of the order parameter is observed to be essentially continuous across the transition. Associated with the orientational order in the N_TB phase is the conical angle and an off-diagonal element of the Saupe ordering matrix.

Liquid crystal dimers with odd spacers are good candidates as materials for biaxial nematic phases (N_B). The dimers are flexible molecules sustaining biaxial conformations, and couplings between the conformational and orientational distributions could be expected to stabilise N_B. We apply a molecular field theory for flexible molecules developed elsewhere to study a simple system made up of dimers composed of two cylindrically symmetric mesogenic groups. Our model allows for two idealised conformations: one linear and one bent at a tetrahedral angle. For a restricted set of chain lengths, the model predicts a first-order reentrant phase transition from the N_B phase into a low temperature uniaxial nematic phase (N_U). However the formation of the biaxial nematic could be blocked by the appearance of a twist-bent nematic.

The name Eric Hulley, like his picture, will not be known to most readers of Liquid Crystals. He did, however, play a significant role in the creation and success of this Journal but more of that l...