Helical nanofilament phases.

In the formation of chiral crystals, the tendency for twist in the orientation of neighboring molecules is incompatible with ordering into a lattice: Twist is expelled from planar layers at the expense of local strain. We report the ordered state of a neat material in which a local chiral structure is expressed as twisted layers, a state made possible by spatial limitation of layering to a periodic array of nanoscale filaments. Although made of achiral molecules, the layers in these filaments are twisted and rigorously homochiral--a broken symmetry. The precise structural definition achieved in filament self-assembly enables collective organization into arrays in which an additional broken symmetry--the appearance of macroscopic coherence of the filament twist--produces a liquid crystal phase of helically precessing layers.

Experimental demonstration, using polarized Raman and infrared spectroscopy, that both conventional and de Vries smectic-A phases may exist in smectic liquid crystals with a first-order A-C* transition.

Two models exist for the orientational distribution of the long molecular axes in smectic-A liquid crystals: the conventional unimodal distribution and the "cone-shaped" de Vries distribution. The de Vries hypothesis provides a plausible picture of how, at a molecular level, a first-order Sm-A to Sm-C* transition may occur, especially if there is little or no concomitant shrinkage of the layer spacing. This work investigates two materials with such transitions: C7 and TSiKN65. The azimuthal distribution of in-layer directors is probed using IR and polarized Raman spectroscopy, which allows us to obtain orientational order parameters. In C7, we observe a discontinuous change in the order parameter, the magnitude of which is small compared with the corresponding change in the in-layer director tilt angle Theta . Assuming that the smectic-A liquid crystal is of the de Vries type, we calculate the Theta required to reproduce the apparent order parameter app, obtained from IR, by using the true order parameter , obtained from polarized Raman scattering. The results indicate that, for C7, the tilt angle so calculated is much smaller than that in the Sm-C* angle and hence de Vries behavior may not be the appropriate explanation in this case. Conversely, we find that TSiKN65 shows a different behavior to C7, which can be explained in terms of the de Vries concept. Thus, we conclude that either type of distribution may exist in Sm-A phases which undergo a first-order transition to the Sm-C* phase. We also discuss the changes in the smectic layer spacing and the orientational order parameters across the Sm-A-Sm-C* phase transition, together with changes in birefringence with applied electric field.

Frustration between syn- and anticlinicity in mixtures of chiral and non-chiral tilted smectic-C-type liquid crystals.

We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmC(a)* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and SmC(a)*, transition via the SmC(beta)* and SmC(gamma)* subphases, or by "escaping" the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmC(a)* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected even in this case, suggesting that chirality--although necessary--does not have quite the importance that has previously been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order.

Glass forming banana-shaped compounds: vitrified liquid crystal states.

The synthesis and physical properties, in particular electro-optic switching behavior, of 3-chloro-biphenyl-3',4-bis[4-[4-(3,7-dimethyloctyloxy)-phenyliminomethyl]] benzoate are reported. The compound exhibits an antiferroelectric tilted smectic liquid crystalline phase (Sm-CP) in a broad temperature range. Below 20 degrees C the sample goes over to a glassy state and no crystallization appears down to -50 degrees C. It is observed that below the glass transition temperature both achiral and chiral structures of the Sm-CP phase can be frozen. Each of them can have three polarization states (two ferroelectric and one antiferroelectric), thus giving six different vitrified textures. This enables atomic force microscopy studies of the different liquid crystalline states and suggests possibilities for electro-optical storage devices.

Ferroelectric-chiral-antiferroelectric-racemic liquid crystal phase transition of bent-shape molecules.

Detailed dielectric, polarization current, electro-optical, and textural observations are reported on an asymmetric banana-shaped compound 1,3-biphenylene-bis[4-(3-fluoro-4-octyloxyphenyliminomethyl)benzoate]. The material possesses a chiral-ferroelectric-racemic-antiferroelectric phase transition. Our studies reveal that the higher temperature ferroelectric phase has a polar double-tilted smectic structure, where both the molecular plane and the long axis are tilted with respect to the layer normal. Accordingly, it has a chiral triclinic structure with an out-of-plane polarization component. The lower temperature phase has a monoclinic symmetry, which is higher than that of the higher temperature phase. To our knowledge, among liquid crystals such situations were previously observed only in reentrant phases.

Molecular arrangements and reorientation behavior in a dibenzopyrene-derivative ferroelectric columnar liquid crystal as studied by time-resolved Fourier-transform ir spectroscopy.

Polarized, time-resolved Fourier-transform infrared spectroscopy was employed to study the orientational order and the reorientation dynamics of a diskotic ferroelectric liquid crystal. In the shear oriented cell the dibenzopyrene derivative forms two different field-dependent columnar phases that show a tripling in the spontaneous polarization. These field-dependent phases are analyzed with respect to the dependence of the infrared absorbance from the polarization plane. In this way it was confirmed that the high-field phase is characterized by a homogeneous orientation of the tilt-plane formed by the core normals n and the column axis N. In contrast, in the low-field phase the columns exhibit several different tilt-planes. The orientational order parameter of the columns is determined. It was also detected that the average orientation of the alkyl tails of the molecules is not lying in the plane of the disklike core. By monitoring the evolution of the infrared bands in the course of the electric-field-induced reorientation, we found that the reorientation process is divided into three steps: A fast initial response followed by a slowing down of the reorientation is observed, which then is followed by an acceleration of the reorientation. In the high-field phase the fast initial electrical induced process can be assigned to a rotation of the molecules around the column axis by a few degrees. During the subsequent ferroelectriclike response the molecules rotate around the column axis by approximately 180 degrees. Other models for this switching mechanisms could be excluded.

Glasslike character of molecular ordering in discotic lyomesophases.

We studied dynamic light scattering from isotropic solutions of triphenylene derivative in dodecane at concentrations close to the isotropic-cholesteric phase transition that takes place at c=25 wt. % at room temperature. The correlation decay g((2))(t) reveals the presence of three dynamic modes, from which two appear in polarized (VV) and one in depolarized (VH) scattering. The VH mode, which is independent of the scattering wave vector q, exhibits an Arrhenius type temperature dependence and shows a strong deviation from the exponential relaxation associated with the stretching-exponent parameter beta(VH) approximately 0.5 for all the investigated concentrations. The VV modes both exhibit a quadratic dispersion of the inverse relaxation time 1/tau on the scattering wave vector q. For concentrations 5 wt. %

Evidence for triclinic symmetry in smectic liquid crystals of bent-shape molecules.

The first experimental evidence for triclinic symmetry of bulk smectic liquid-crystal samples of achiral banana-shaped molecules is presented. This phase corresponds to the so-called Sm-CG phase consisting of biaxial molecules and characterized by two tilt directions with respect to the layer normal: tilt of the molecular plane (clinic) and tilt of the molecular kink direction (leaning). Each smectic layer has a polarization component normal to the smectic layers (C1 symmetry). The observations suggest that the phase tentatively labeled as B7 is identical with the Sm-CG phase.

Wetting behavior above the liquid-crystal-isotropic transition in a homologous series

An ellipsometric study of the wetting behavior at the free surface above the isotropic to nematic or isotropic to smectic-A transition of nine homologous compounds with even alkyl chain lengths n in the range from four to twenty carbon atoms is presented. All compounds show a pretransitional increase of the nematic or smectic surface coverage as the bulk isotropic to liquid-crystal transition is approached from above. The behavior of the nematic compounds (n=4 to 10) can be interpreted, within the framework of a Landau model, as complete wetting. In short nematic homologs the divergence of the nematic coverage is strongly reduced by a decrease of the nematic susceptibility of the isotropic phase. The elastic coefficient L of the Landau model shows a pronounced increase with increasing n, resulting in the occurrence of a discontinuous prewetting transition in the shortest smectic homolog (n=12) that is still describable by the nematic Landau model. In the longer smectic homologs (n=14 to 20), layering steps appear in the pretransitional increase of the coverage. The results indicate probable partial wetting for the longest homolog, whereas for the other smectic compounds the distinction between complete and partial wetting is difficult on the basis of ellipsometry.

Electric-field-induced chiral separation in liquid crystals.

Recently it was shown that smectic liquid crystal phases formed by achiral banana-shaped molecules are chiral. Films of such phases generally contain both homochiral (consisting of layers of the same chirality) and racemic (the chirality alternates in subsequent layers) domains. So far it has not been clear how to control the overall chirality of the sample. By observing the effects of the application of suitable electric fields, it was noted that chirality of a film of banana shaped molecules was interchanged between homochiral and racemic. We present dielectric, electro-optic, and polarization current measurements on both the racemic and chiral states. The observations indicate that the synclinic states have minimum free energies. The observed dielectric modes can be explained by a simple model assuming that the antiferroelectric state is very weak.

Slow mode of the smectic-A-smectic-C(*)(alpha) phase transition.

Unusual slow fluctuations as revealed recently by dynamic light scattering close to the Sm-A-Sm-C(alpha)(*) phase transition in the antiferroelectric liquid crystal 4-(1-methylheptyloxy-carbonyl)phenyl 4(')-octyloxy biphenyl-4-carboxylate can be explained by the electrostatic coupling between impurity ions and director fluctuations. Within the vicinity of Sm-A-Sm-C(alpha)(*) transition, the relaxation rate of the slow mode depends linearly on temperature, but with a different slope in each phase. The square root of its intensity shows a clear Curie-Weiss divergence at the phase transition, which is a direct confirmation of the electrostatic coupling mechanism.