Verena Görtz

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.

Thermotropic biaxial nematic order parameters and phase transitions deduced by Raman scattering

Raman Scattering was used to investigate biaxiality in the nematic phase formed by the bent-core material, C5-Ph-ODBP-Ph-OC12. Linearly polarised light was normally incident on a homogeneously aligned sample, and the depolarisation ratio was measured over a 360° rotation of the incident polarisation for the Raman-active phenyl stretching mode. By modeling the bent-core structure and fitting to the depolarisation data, both the uniaxial (P200 and P400) and biaxial (P220, P420 and P440) order parameters, are deduced. We show unequivocally the presence of a uniaxial to biaxial nematic phase transition approximately 30 °C above the underlying smectic phase. Further, we report the temperature evolution of the biaxial and uniaxial order parameters, which increase in magnitude continuously with reducing temperature, reaching values of 0.1, −0.15 and −0.18 for P220, P420 and P440, respectively.

Enantioselective segregation in achiral nematic liquid crystals

The elusive biaxial nematic liquid crystal phase was recently discovered in a family of substituted oxadiazoles. Our investigations of these materials show that the achiral biaxial nematic phase can segregate into chiral domains of opposite handedness, thereby demonstrating that the liquid-like nematic phase exhibits the properties of a conglomerate.

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.

Revealing the uniaxial to biaxial nematic liquid crystal phase transition via distinctive electroconvection

Electroconvection (EC) phenomena are reported in the uniaxial and biaxial nematic phases of a bent-core liquid crystal. Two EC instabilities are observed. Prewavy_1, which exists only in the uniaxial phase, has stripes parallel to the rubbing direction, a periodicity of order of the device thickness, and an onset voltage that diverges near the uniaxial to biaxial transition. The prewavy_2 instability, characterized by stripes perpendicular to the rubbing direction and a periodicity of twice the device thickness, exists across the entire nematic regime. Our observations are explained through changes in the electrical conductivity anisotropy at the uniaxial to biaxial transition.

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

Unusual behaviour of the dark conglomerate (DC) phase seen in an oxadiazole-based achiral bent-core liquid crystal, which has not previously been reported for the DC phase of other liquid crystals, is described. Under polarising optical microscopy, we see no domains of opposite handedness in the ground state of the DC phase. However, it shows unusual transformations when an electric field is applied to the system. On increasing the electric field, at first the domains of opposite handedness become visible and then they grow in size and slowly the sample transforms to a monochiral or single-handed form which is followed by a nonchiral state at very high fields. The threshold electric fields required to achieve these changes are temperature dependent and the transformations are seen irrespective of the frequency of the applied electric field (100 Hz to 5 kHz), type of the waveform (sine, square and triangular) and the thickness (1.5 μm to 15 μm) or the geometry (planar and twisted) of the device used. Further, there is no field-induced high birefringence texture observed even though sufficiently large electric field (~22 V/μm) has been applied across the devices. The nature of the behaviour is investigated by various techniques such as optical microscopy, conoscopy, circular dichroic and Raman spectroscopies, electro-optics and dielectric spectroscopy. The possible physical phenomena behind these changes are discussed in detail.

Chiral resolution in bent-core nematic liquid crystals

This paper presents the currently available experimental evidence for the occurrence of chiral resolution in bent-core nematic liquid crystals. The observation of chiral domains in these systems is discussed in context with other indications for the unique character of the nematic phase in bent-core materials, which challenge our understanding of nematic phase behaviour. Two possible mechanisms for the emergence of macroscopically chiral domains are exposed; the spontaneous resolution of chiral bent-core conformers and the occurrence of helical twist bend deformations due to a unique propensity for spontaneous bending of the nematic director in bent-core materials. The latter proposition is put into context with the recently developed cluster model, as overall it appears more appropriate to consider bent-core nematic systems as self-assembling-self-organising fluids with a hierarchical domain structure.

Optical measurements of orientational order in uniaxial and biaxial nematic liquid crystals

The use of optical methods to study the orientational order in nematic liquid crystals is both convenient and powerful. Some of the earliest approaches made use of measurements of the refractive index anisotropy, following the methodology of Halle [1] to measure the order parameter, . This method is reviewed briefly and some typical areas of application are presented. Raman spectroscopy is a second approach discussed in this paper. Although technically a more complex methodology, Raman spectroscopy has the significant advantage that in addition to being able to measure it is also possible to determine . A review of current approaches is presented. Very recently, it has been shown that polarised Raman spectroscopy can be employed to determine biaxial order parameters in liquid crystals. The methodology is described, and data are presented for such measurements on a series of bent-core nematic materials that could exhibit the elusive biaxial nematic phase.

The influence of structure on the elastic, optical and dielectric properties of nematic phases formed from bent-core molecules

The physical properties of the nematic phases formed by four bent-core oxadiazole based materials are reported. In particular, the splay (K11), twist (K22) and bend (K33) elastic constants, the birefringence and the dielectric anisotropy of the materials are described and the effect of chain length and the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure on these parameters is determined. The birefringence and order parameter are found to be independent of the modification of molecular structure. The dielectric anisotropy is quite strongly dependent on molecular structure; the fluoro-substituted material has the largest magnitude of dielectric anisotropy while the alkyl-substituted compound has the smallest. Changes in the molecular length and fluoro-substitution in the bent-core materials are found to have little influence on the splay, twist and bend elastic constants at equivalent reduced temperatures. However, the material substituted with an alkyl terminal chain exhibits both smaller elastic constants and a less marked dependence on temperature than the alkoxy-substituted compounds. A possible insight into the behaviour of the elastic constants relevant to the formation of the dark conglomerate phase, which underlies the nematic phase in one of the compounds studied, is suggested by following the analysis proposed by Berreman and Meiboom. Importantly, using molecular field theory and atomistic modelling, we calculate elastic constants that are in excellent agreement with the experimental values. Our conclusion that the elasticity in the nematic phase formed from bent-core molecules is not strongly influenced by changes to the terminal chains or the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure is in agreement with our previous work showing that the dominant parameter is the bend angle.

Liquid crystal blue phases: stability, field effects and alignment

The blue phases are fascinating structures in liquid crystals, fluids that exhibit cubic structures that have true crystalline order. The blue phases were discovered in the 1970s and were the subject of extensive research in the 1980s, when a deep understanding of many of their properties was established. The discovery that the blue phases could be stabilised to exist over wide temperature ranges meant that they became more than scientific curiosities and led to a recent resurgence in research into them as they offer some promise in applications. This paper considers some important aspects of the blue phases that are recurrent topics in their research. It describes factors affecting blue phase stability, demonstrating on the role of the bend elastic constant; field effects, including the Kerr effect, electrostriction and relaxation phenomena; and alignment, in particular production and control of blue phase monodomains. The dependence of these phenomena on the physical properties of the liquid crystalline system, including the twist and bend elastic constants and the dielectric anisotropy, is emphasised wherever possible. The paper links work carried out in the 1980s with contemporary research, using a few key examples to show how there is still much to understand in this beautiful topic.