Anatomy of a Discovery: The Twist–Bend Nematic Phase

Abstract

New fluid states of matter, now known as liquid crystals, were discovered at the end of the 19th century and still provide strong themes in scientific research. The applications of liquid crystals continue to attract attention, and the most successful so far has been to the technology of flat panel displays; this has diversified in recent years and LCDs no longer dominate the industry. Despite this, there is plenty more to be uncovered in the science of liquid crystals, and as well as new applications, novel types of liquid crystal phases continue to be discovered. The simplest liquid crystal phase is the nematic together with its handed or chiral equivalent, named the cholesteric phase. In the latter, the aligned molecules of the nematic twist about an axis perpendicular to their alignment axis, but in the 1970s a heliconical phase with a tilt angle of less than 90° was predicted. The discovery of this phase nearly 40 years later is described in this paper. Robert Meyer proposed that coupling between a vector order parameter in a nematic and a splay or bend elastic distortion could result in spontaneously splayed or bent structures. Later, Ivan Dozov suggested that new nematic phases with splay–bend or twist–bend structures could be stabilised if the appropriate elastic constants became negative. Theoretical speculation on new nematic phases and the experimental identification of nematic–nematic phase transitions are reviewed in the paper, and the serendipitous discovery in 2010 of the nematic twist–bend phase in 1″,7″-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB) is described.

13C Nuclear Magnetic Resonance Investigations of Molecular Mesogens with a Terminal Phenyl Ring Linked via a Spacer: Odd–Even Effect

“…A liquid crystal dimer may be described as a molecule containing two mesogenic units connected by a flexible spacer. , They are broadly categorized into even dimers and odd dimers depending on the number of atoms in the flexible spacer that connects two mesogenic core units . There is increasing attention on the investigations of odd liquid crystal dimers due to the recent discovery of twist-bend (N TB ) nematic mesophase. − The occurrence of the N TB phase with reduced short-range positional and quasi-long-range heliconical orientational order is attributed to the molecular bend of the odd-membered dimers and is of considerable academic interest in the current liquid crystal research . In particular, the study of the flexoelectro-optic effect in these materials could open interesting paths for display applications .…”

Effortless Extraction of Structural and Orientational Information from 13C–1H Dipolar Couplings for Thiophene Mesogens

“…18−20 The discovery of the twist-bend (N TB ) nematic mesophase in odd dimers has drawn the attention of researchers in which a spacer is incorporated between two mesogenic core units. 21,22 In this context, mesogens comprising bulky terminal groups attached to mesogenic cores through a flexible spacer are considered model systems and their studies attained significant importance. 23,24 Nuclear magnetic resonance spectroscopy played a central role in probing the geometry and dynamics of molecules in the liquid crystalline phase.…”

“…Thermotropic liquid crystals are generally built with core units made up of phenyl rings directly linked or via connecting units along with terminal chains on both sides or at one terminus. , This design framework led to numerous molecular mesogens displaying various nematic and a wide range of smectic mesophases . The replacement of phenyl rings with five-membered heterocyclic rings led to molecular architectures slightly deviating from rod-like shapes. , Among the five-membered heterocyclic rings exploited for generating the mesogens, thiophene, a five-membered ring with a sulfur atom, engrossed principal attention due to exciting reasons. − The interest in π-conjugated liquid crystalline semiconductors unequivocally motivated the researchers to incorporate thiophene with other aromatic rings for applications in organic electronics. , Fundamentally, thiophene as well as phenyl rings are aromatic systems by sp 2 -hybridized carbons with significant differences in their geometry. , When thiophene is part of the core unit, besides contributing to a change in the geometry of the molecule, it greatly influences the material properties. − It has been well recognized that the replacement of the phenyl ring with thiophene in a mesogenic core leads to changes in properties like an increase in optical anisotropy and a decrease in melting point, offers negative dielectric anisotropy, reduces the viscosity, and favors fast switching times. , In these molecular systems, thiophene can connect to other segments of the core unit directly or through the linking groups or via a flexible spacer and greatly impact the geometry and shape of the molecule. − The discovery of the twist-bend (N TB ) nematic mesophase in odd dimers has drawn the attention of researchers in which a spacer is incorporated between two mesogenic core units. , In this context, mesogens comprising bulky terminal groups attached to mesogenic cores through a flexible spacer are considered model systems and their studies attained significant importance. , …”

Molecular Conformations of Shape Anisometrically Variant Mesogens in Liquid Crystalline Phase Studied by 13C NMR Spectroscopy

“…[12] The recent discovery of a twist-bend nematic phase in the odd dimers, where the molecules fill the space efficiently when twist and bend deformations combine to form heliconical correlation lengths, clearly illustrates the nature of unusual mesophases. [13,14] Further, super mesogens, dendrons, and taper-shaped dendritic molecules completely deviate from the rigid rod-like shape and exhibit a wide range of supramolecular organization leading to complex mesophase morphologies. [1,2,9,15] Star-like mesogens are another class of molecules grouped as non-conventional mesogens that display mesophase diversity driven by molecular architecture.…”

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Mesogens that vary in shape anisometry have been investigated by 13 C solid-state NMR in the liquid crystalline phase to inspect the conformations. The molecules examined comprise of (i) rod-like mesogen with three-phenyl ring core and terminal hexyloxy chains, (ii) three-ring core linked to the fourth phenyl ring via a spacer, and (iii) trimesic acid connected to three side arms core units through a spacer.

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