Stephen J. Cowling

Transmission and amplification of information and properties in nanostructured liquid crystals.

In recent years the design of chemical structures of liquid-crystalline materials has developed rapidly, and in many cases changed radically. Since Reinitzers days, liquid crystals have either been classed as rodlike or disclike, with combinations of the two leading to phasmidic liquid crystals. The discovery that materials with bent molecular structures exhibited whole new families of mesophases inspired investigations into the liquid-crystal properties of materials with widely varying molecular topologies-from pyramids to crosses to dendritic molecules. As a result of conformational change, supermolecular materials can have deformable molecular structures, which can stabilize mesophase formation, and some materials that are non-mesogenic, on complexation form supramolecular liquid crystals. The formation of mesophases by individual molecular systems is a process of self-organization, whereas the mesophases of supramolecular systems are formed by self-assembly and self-organization. Herein we show 1) deformable molecular shapes and topologies of supermolecular and self-assembled supramolecular systems; 2) surface recognition processes of superstructures; and 3) that the transmission of those structures and their amplification can lead to unusual mesomorphic behavior where conventional continuum theory is not suitable for their description.

Microscopy studies of the nematic NTB phase of 1,11-di-(1′′-cyanobiphenyl-4-yl)undecane

A detailed microscopy study of the mesophases of 1,11-di-(1′′-cyanobiphenyl-4-yl)undecane (CB11CB) was undertaken, with an emphasis on attempting to relate the recent helical “twist-bend” model of the NTB phase to the observed optical textures. Our ability to draw a freestanding film indicates that the phase is unlikely to be nematic and possesses long range periodicity. No electrooptic response could be detected in the NTB phase, with or without addition of a chiral dopant although dielectric breakdown and space charge were observed at high voltage leading to field dependent pulsing of the sample. No optical rotation associated with a macroscopic helical structure could be discerned in the NTB phase. However, this observation could be taken to be in keeping with the extremely short pitch of a proposed “twist-bend” model.

Apolar Bimesogens and the Incidence of the Twist–Bend Nematic Phase

The nematic twist-bend phase (NTB) was, until recently, only observed for polar mesogenic dimers, trimers or bent-core compounds. In this article, we report a comprehensive study on novel apolar materials that also exhibit NTB phases. The NTB phase was observed for materials containing phenyl, cyclohexyl or bicyclooctyl rings in their rigid-core units. However, for materials with long (>C7) terminal chains or mesogenic core units comprising three ring units, the NTB phase was not observed and instead the materials exhibited smectic phases. One compound was found to exhibit a transition from the NTB phase to an anticlinic smectic C phase; this is the first example of this polymorphism. Incorporation of lateral substitution with respect to the central core unit led to reductions in transition temperatures; however, the NTB phase was still found to occur. Conversely, utilising branched terminal groups rendered the materials non-mesogenic. Overall, it appears that it is the gross molecular topology that drives the incidence of the NTB phase rather than simple dipolar considerations. Furthermore, dimers lacking any polar groups, which were prepared to test this hypothesis, were found to be non mesogenic, indicating that at the extremes of polarity these effects can dominate over topology.

The relationship between molecular structure and the incidence of the NTB phase

In this work, we present the first part of a study into the relationship between molecular structure and the occurrence of the ‘twist-bend nematic phase’ (NTB). Given the large amount of chemical space that might reasonably be expected to give rise to the NTB phase, this paper is only concerned with methylene-linked bimesogens bearing polar terminal groups based on the initial work of George Gray on cyanobiphenyls. As with other studies, we find that the NTB phase is observed only for materials that contain an odd number of methylene units in the spacer chain. It also appears that, in a given series of materials, there is a weak negative correlation between the dipole moment of the individual mesogenic units and the thermal stability of the NTB phase. Furthermore, we find that increasing the length–breadth ratio of the individual mesogenic units also provides a significant increase in the thermal stability of the NTB phase. The electrooptic behaviour of two materials, one with a terminal nitrile unit and one with an isothiocyanate group, was investigated. The NTB phase of the NCS-terminated material can be switched with a large applied voltage (20 V μm−1); however, the analogous nitrile-terminated material showed no electrooptic response under these conditions. Either the threshold voltage to switching is simply lower for isothiocyanate materials than nitriles or that there is more than one phase currently identified as the twist-bend nematic.

The Nematic Phases of Bent-Core Liquid Crystals

Over the last ten years, the nematic phases of liquid crystals formed from bent-core structures have provoked considerable research because of their remarkable properties. This Minireview summarises some recent measurements of the physical properties of these systems, as well as describing some new data. We concentrate on oxadiazole-based materials as exemplars of this class of nematogens, but also describe some other bent-core systems. The influence of molecular structure on the stability of the nematic phase is described, together with progress in reducing the nematic transition temperatures by modifications to the molecular structure. The physical properties of bent-core nematic materials have proven difficult to study, but patterns are emerging regarding their optical and dielectric properties. Recent breakthroughs in understanding the elastic and flexoelectric behaviour are summarised. Finally, some exemplars of unusual electric field behaviour are described.

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

ABSTRACT The twist-bend phase (NTB) is most commonly observed in materials with a gross-bent shape: dimers; bent-cores; bent-oligomers. We had suggested previously that the bend-angle of such systems effectively dictates the relative thermal stability of the NTB phase. However, our earlier paper relied on the use of a single energy-minimum conformer and so failed to capture any information about flexibility and conformational distribution. In the present work, we revisit our hypothesis and examine a second set of dimers with varying linking groups and spacer composition. We have improved on our earlier work by studying the conformational landscape of each material, allowing average bend-angles to be determined as well as the conformer distribution. We observe that the stability of the NTB phase exhibits a strong dependence not only on the Boltzmann-weighted average bend-angle (rather than just a static conformer), but also on the distribution of conformers. To a lesser extent, the flexibility of the spacer appears important. Ultimately, this work satisfies both theoretical treatments and our initial experimental study and demonstrates the importance of molecular bend to the NTB phase. Graphical Abstract

Synthesis and evaluation of some novel chiral heterocyclic liquid crystalline materials exhibiting ferro‐ and antiferro‐electric phases

Two series of three‐ring heterocyclic liquid crystalline (LC) materials have been synthesized and characterized, one incorporating a thiophene ring and the other containing a thiophene‐pyrimidine moiety. This is the first study to show the effect of a thiophene‐pyrimidine moiety on the thermal and physical properties of a mesogen. The effect on the thermal stability of the mesogen by the introduction of the pyrimidine ring has been dramatic, with the thiophene‐pyrimidine‐containing mesogens exhibiting ferro‐ and antiferro‐electric phases only. The introduction of the pyrimidine ring can be compared to the effect observed by lateral fluorination; a reduction in the melting point and smectic phase thermal stability of mesogens. However, its introduction is also conducive to the formation of the SmC* (ferro‐ and antiferro‐electric) phase. Like the fluoro substituent, the pyrimidine ring could play a key role in tailoring important thermal and physical properties of LC materials. An electro‐optical study not only showed that the incorporation of a pyrimidine ring into the structure of a mesogen can produce mesogens with high P s values (200–230u2009nCu2009cm−2) and tilt angles (41°–41.5°), but also helped to identify the SmCα* phase exhibited by members of one of the series of compounds.

Side-chain liquid crystal polymers derived from oxetane monomers containing 2- or 3-fluorophenyl moieties in the core of the mesogen

Two series of side-chain liquid crystalline polymers were prepared by cationic ring-opening of oxetane substituted mesogens. Each of the terminally appended side chain polymers prepared had a flexible spacer length of six methylene units separating a mesogenic aromatic core from the polymer backbone. The core unit was designed to incorporate either a 2-fluoro- or a 3-fluoro-phenyl unit possessing either an (R)-(−)-1-methylheptyl, (S)-(+)-2-methylbutyl or a 1-propylbutyl terminal chain. The materials produced in this study were compared with the previously reported unsubstituted parent materials in order to assess the affect of having a lateral fluoro substituent in the core. Low melting points, smectic C/C* and smectic A/A* phases were exhibited by most of the monomers. The behaviour of the 3-fluorophenyl monomers was found to be similar to their unfluorinated parents, whereas the 2-fluoro analogues nwere shown to have significantly lower transition temperatures. All of the polymers exhibited smectic C/C* and smectic A/A* phases over wide temperature ranges and had relatively low Tg values. Some of the polymers were found to exhibit exceptionally large temperature ranges for the smectic C/C* phase, e.g. from 0.8u2006°C to 190u2006°C.

The unusual self-organising behaviour of a glycosteroidal bolaphile

In this communication we report on the self-organising properties of a novel asymmetric bolaphile composed of sugar and steroidal units. The material was found to exhibit unusual phase changes in that it formed a cubic phase upon heating, whereas upon cooling it formed a lamellar phase. This change in property was ascribed to kinetic versus thermodynamic behaviour at the phase transition to the liquid.

Control of free volume through size exclusion in the formation of smectic C phases for display applications

Ferroelectric liquid crystals are of interest in display devices because of their bistable operation and fast response times. However, they have not reached their potential for a number of reasons, low tilt angle and high birefringence being two of them. Although low birefringence can be achieved by incorporation of carbocyclic rings into the molecular structures of the materials this often lowers the occurrence of tilted phases, and additionally a high tilt angle is often very difficult to engineer. In this article we attempt to resolve these issues and demonstrate that through the incorporation a bulky terminal group attached to an external aliphatic chain in the molecular design, it is possible via size exclusion to achieve high tilt angles. We explore the conformational landscape of these materials with a combination of molecular modelling and NOE enhancements in 1H NMR.