Edward J. Davis

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.

Self-organisation through size-exclusion in soft materials

A number of materials derived from 4-undecyloxy-4′-cyanobiphenyl but with varying terminal groups were prepared in order to better understand how such a group influences the type, and local structure of mesophases formed. Whereas electron poor terminal groups (fluoroaromatics and halogen atoms) were found to destabilise the smectic A phase through unfavourable electrostatic interactions, bulky silane, siloxane and hydrocarbon groups can be incorporated into the structure of the phase with only minor reductions in clearing point. An increase in the layer spacing of the smectic Ad phase in materials with bulky groups suggests that microphase segregation is not the driving force, but rather exists as a consequence of steric crowding at the smectic layer interface. Electrooptic studies reveal that ‘carbosilane’ end groups, such as tetramethyldisilapropane, are significantly more electrochemical stable than their siloxane counterparts whilst retaining their desirable thermal properties.

Liquid-crystalline structure–property relationships in halogen-terminated derivatives of cyanobiphenyl

In this article, we report on the liquid-crystalline properties of two series of halogen-terminated cyanobiphenyl-based materials. Unlike other rod-shaped, halogen-terminated materials reported, these materials do not exhibit the smectic A phase and instead only exhibit nematic liquid-crystal mesomorphism. Comparisons between these materials and analogous unsubstituted materials were made, and the relationships between their molecular structures and phase behaviour are discussed with the aid of molecular modelling at the B3LYP/6-31G* level of density functional theory.

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.

Liquid crystalline dihydroazulene photoswitches

A large selection of photochromic dihydroazulene (DHA) molecules incorporating various substituents at position 2 of the DHA core was prepared and investigated for their ability to form liquid crystalline phases. Incorporation of an octyloxy-substituted biphenyl substituent resulted in nematic phase behavior and it was possible to convert one such compound partly into its vinylheptafulvene (VHF) isomer upon irradiation with light when in the liquid crystalline phase. This conversion resulted in an increase in the molecular alignment of the phase. In time, the meta-stable VHF returns to the DHA where the alignment is maintained. The systematic structural variation has revealed that a biaryl spacer between the DHA and the alkyl chain is needed for liquid crystallinity and that the one aromatic ring in the spacer cannot be substituted by a triazole. This work presents an important step towards employing the dihydroazulene-vinylheptafulvene photo/thermoswitch in photoactive liquid crystalline materials.

Self-organizing behaviour of glycosteroidal bolaphiles: insights into lipidic microsegregation

In this article we describe work on the synthesis of bolaphile biomimics composed of glucose head groups and steroidal units linked together by a methylene chain of varying length. The condensed phases formed by self-organization of the products as a function of temperature were characterized by differential scanning calorimetry and thermal polarized light microscopy. The results of these studies show that the thermal stabilities of the lamellar mesophases formed vary linearly as a function of increasing aliphatic composition, which reflects a linear hydrophobic-hydrophilic balance with respect to transition temperatures.

Chapter 15:Liquid crystal glycolipids

There is a growing consciousness that the observed liquid crystallinity of many biological materials be related with their biological functions. While the survival of living systems depends on the flexibility and reformability of structures, the combination of softness and structure of the liquid crystalline state seems to determine the functionality of biological materials. The richest sources of liquid crystals derived from living systems are found in cell membranes, and glycolipids, which are important components of cell membranes, show fascinating self-assembling behaviour. In this chapter, by examining typical examples of liquid crystalline glycolipids with respect to their chemical structure, we will illustrate our understanding of the self-assembling and self-organising properties of glycolipids to the point of predictivity for rational design of mesomorphism in such systems.

Liquid crystalline glycosteroids and acyl steroid glycosides (ASG)

ABSTRACT As part of our studies on glycolipidic liquid crystals, we have investigated some molecules comprising a steroid moiety. These systems can exhibit several types of structures depending on their polarity pattern based on the number of polar and non-polar moieties and their resulting molecular shape. Therefore, to aid describing such systems, we have proposed a specific classification based on this polarity pattern. Many compounds in this family are natural products, which possess important biological properties. Some of the compounds have bolaphilic structures, with both a steroid and a fatty alkyl chain attached to the carbohydrate moiety, such as either the β-galactoside BbGL-1 or the α-glucoside α-CAG that are found in the membranes of the pathogens Borrelia burgdorferi and Helicobacter pylori, respectively. In this account, after a brief introduction on liquid crystalline glycolipids, we focus on carbohydrate–steroid hybrids, summarising our previous work on glycosteroids prepared by the CMGL-synthon strategy, and reporting our preliminary results on the thermotropic behaviour of acyl steroid glycosides (ASG), namely cholesteryl 6-O-acyl-β-gluco- and -galacto-pyranosides. Graphical Abstract