Andrew J. Slaney

Chirality and Frustration in Ordered Fluids

Abstract Chirality in liquid crystal systems is a complex and sometimes difficult concept to understand and appreciate. In this article we review some of the reduced symmetry aspects of calamitic liquid crystal phases with reference to point symmetry, space symmetry, and helicity. We utilize these concepts in the discussion of new results obtained on Twist Grain Boundary phases, inversions in chiral dependent properties such as the helicity in cholesteric and smectic C* phases and the spontaneous polarization in ferroelectric liquid crystals, and antiferroelectric behaviour in low molar mass systems. Our observations suggest that many novel effects found in chiral liquid crystals are a result of conflicts between the effects of reduced symmetry and the desire to form normal liquid-crystalline structures, i.e., a form of frustration exists.

Chirality in liquid crystals. The remarkable phenylpropiolates

Abstract The biphenylyl esters of the 4-n-alkoxyphenylpropiolic acids are a unique family of liquid-crystalline materials. In particular, when the biphenyl moiety of the compounds carries a chiral end-group, many optically active mesophases are created which exhibit unusual structures and physical properties. For instance, when the chiral group attached to the biphenyl moiety is 1-methylheptyl then Abrikosov, twist grain boundary smectic A* and antiferroelectric smectic C* phases are observed. The wide variety of chiral phases and electrochiral properties exhibited by this family of materials makes them ideal candidates for exploring chirality in the liquid-crystalline state. These investigations allow us to contrast and compare chirality dependent phenomena in liquid crystals, thereby producing a broader view of the concept of chirality in organized fluids than is traditionally presented.

The effect of molecular chirality on the incidence of twisted smectic A phases

Abstract Four chiral chloro-esters were synthesized in order to examine systematically the effect that molecular chirality has on the temperature range and incidence of the twisted smectic A phase or the twist grain boundary phase as it has become known. We find that when the motion of the chiral centre is restricted by rotational hindrance, thereby increasing the chirality of the system, the twisted SA phase range is increased. At low chirality the twisted SA phase disappears altogether. Furthermore, it is shown that the transition temperatures of the chiral compounds are lower than their racemic analogues.

Observations of the Liquid-Crystal Analog of the Abrikosov Phase

Freeze-fracture transmission electron micrographs of the smectic A* phase confirm the twist grain boundary model of Renn and Lubensky. The fracture surface has an undulating structure with a 0.5-micrometer helical pitch parallel to 4.1-nanometer smectic layers. The layers are disrupted by a lattice of screw dislocations oriented normal to the helical axis. Optical diffraction shows that rotation of smectic blocks occurs in discrete steps of about 17�; hence, the screw dislocations are 14 to 15 nanometers apart and the grain boundaries are 24 nanometers apart. These observations show that the SmA* phase is the liquid-crystal analog of the Abrikosov phase in superconductors.

Twist inversion in a cholesteric material containing a single chiral centre

A novel liquid-crystalline material, S-2-chloropropyl 4′-(4″-n-nonyloxyphenylpropioloyloxy)biphenyl-4-carboxylate (902C13T), which undergoes an inversion of helical-twist sense in the cholesteric phase, was prepared and its physical properties were investigated. The behaviour of the pitch and the twist sense of the helical structure in the cholesteric phase are reported. It is suggested that the helix inverts because of changes in the populations of conformational isomers, and that these changes are temperature dependent. An equation that was originally formulated to account for an inversion in the sign of the spontaneous polarization (Ps) in certain smectic C* materials may be adapted for the modelling of helix inversions.

Assessing ferroelectric materials for application in τVMIN mode devices

Abstract Improved operation of FLC devices which operate in the [tacute]uVmin mode require materials with high dielectric biaxialities and low viscosities. Measurements of these properties are difficult and so a method for comparing materials using the [tacute]uV characteristic itself is proposed. Three figures of merit are suggested that allow comparison of host materials independently of the ferroelectric spontaneous polarisation, the cone angle and cell spacing. The temperature dependence for the figures of merit are analysed in detail.

Inversion of chirality-dependent properties in optically active liquid crystals

Three new low-molar-mass liquid crystals are reported, each of which shows remarkable phase behaviour. One of the materials exhibits a temperature-dependent unwinding of the helix in the cholesteric (chiral nematic) and chiral smectic C phases. This is the first example of a single, pure material to show both of these inversions. The inversion phenomena were studied in terms of the helical pitch and the electrooptic switching behaviour in the cholesteric phase and in terms of the temperature dependence of the apparent optical tilt angle and the spontaneous polarization in the chiral smectic C phase.

Mesogen exhibiting a Ch–A*–A phase sequence, a liquid-crystalline analogue of the Abrikosov phase

A number of optically active liquid-crystalline compounds have been prepared that have a chlorine atom attached to their asymmetric centres. Many of these materials exhibit unusual properties at their transitions to the liquid state. In this article we report the properties of one of these compounds: S-2-chloro-4-methylpentyl 4′-(4″-n-nonyloxybenzoyloxy)biphenyl-4-carboxylate. From thermal and optical studies it appears that this material undergoes a transition from a cholesteric phase to a smectic A phase via an intermediary twisted phase. The intermediary A* phase is belived to have a structure where there is helical ordering of the molecules occurring in the planes of the molecular layers. Theoretical models suggest that this twist is effected by the inclusion of screw dislocations in the structure of the phase in the form of a lattice of defects. This lattice is thought to be similar in nature to the Abrikosov flux lattice found in superconductors.

The synthesis and properties of fluoroterphenyls for high dielectric biaxiality ferroelectric liquid crystal mixtures

Abstract A series of liquid crystalline terphenyls with three strategically located fluoro substituents has been synthesised and evaluated for a range of physical properties. Low temperature metallations were used to prepared arylboronic acids which were employed in selective palladium-catalysed cross-coupling reactions to generate the final liquid crystals. The syntheses are discussed as efficient routes to functional, multi-substituted liquid crystals. The trifluorot-erphenyls have low melting points and exhibit the smectic C phase to moderately high temperatures. The melting points, transition temperatures and mesophase morphology are discussed and compared with analogous difluoroterphenyls. The materials have been evaluated for a range of physical properties and enable the formulation of ferroelectric mixtures with a high dielectric biaxiality which is very important in τV minimum driving schemes.

A phenomenological approach to the inversion of the helical twist sense in the chiral nematic phase

Optically active materials that show a temperature dependent inversion of the helical twist sense in the chiral smectic C* and chiral nematic phases have been known for many years. However, it has only recently been found that inversions can occur in compounds which have single chiral centres. It was found previously that the temperature range and the magnitude and sign of the helical twist in the chiral nematic phase are related to the concentration of the optically active material(s) dissolved in a nematic host. In a similar way, we propose to describe the inversion of the helical twist sense in the chiral nematic phase of pure materials containing a single chiral centre. Additionally, we seek to verify the possible validity of the latter model by means of molecular modelling on appropriate compounds, and by deriving a suitable mathematical expression to allow the direct use of experimental data.