ABSTRACT
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.
Subject(s)
Molecular Structure , Nanostructures , Crystallization , Microscopy, Atomic Force , Microscopy, Electron, Transmission , Microscopy, Polarization , X-Ray DiffractionABSTRACT
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.
ABSTRACT
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. %
ABSTRACT
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.
