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.

Although the liquid-crystal research is well established in science, there are newly emerging exciting systems, that deserve extensive basic studies. One of these areas is the research of the bent-shaped molecules (so-called "banana liquid crystals"), which have delicate chirality and polarity properties. In this paper we show that these materials also have very unusual rheological features, such as the formation of stable fluid fibers and bridges. Under electric fields, these objects present striking mechanical effects, such as horizontal and transversal vibrations. Studies indicate that the research of banana-liquid-crystal fibers may lead to new type of artificial muscle systems.

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.

Discotic cholesteric phases with extremely small pitches were obtained with cellobiose derivatives as chiral dopants. These binary mixtures tend to form up to three distinct blue phases. An interesting property of these mixtures is that the blue phases can be supercooled to a glass-like state. Microscopic studies, reflection spectra, and Kossel diagrams all indicate that the three discotic blue phases BP D I, BP D II and BP D III are analogous to the well-known calamitic modifications. In addition to the optical studies, we investigated the free surfaces of the frozen blue phases using atomic force microscopy.Blue phases are formed by calamitic mesogens of a high chirality in a narrow temperature interval close to the clearing point. They have been studied intensively theoretically and experimentally.1 Up to three blue phases can be distinguished. Two of these phases have a periodic orientational order that can be characterised by crystallographic space-group symmetries. The low temperature modifications BPI and BPII possess a cubic lattice symmetry with the space group I4 1 32 (O8) and P4 2 32 (O2), respectively. According to the theory of Meiboom et al.,2 these structures can be described as a combination of double twist cylinders and disclinations ( Fig. 1). Alternatively, a Landau theory has been developed by Hornreich and Shtrikman which shows that blue phases can be thermodynamically stable in certain regions of the temperature-chirality plane.3 Blue phases are optically active Fig. 1 (a) Schematic representation of a double twist cylinder (the lines but, unlike the cholesteric phase, they are optically isotropic represent orientation of the director). Arrangement of the disclination and are not birefringent.4 lines ( left) and the double twist tubes (right) for (b) the unit cell of The high temperature modification (BPIII) appears on BPI and (c) the unit cell of BPII. (Figures from E. Dubois-Violette cooling from the isotropic phase as an amorphous 'blue fog'. and B. Pansu.12)BPI, BPII and BPIII exhibit selective reflection of circularly polarised light and strong optical activity indicating a local dopants.11 Three blue phases could be observed for concenhelical structure with correlations over distances of the helical trations close to the solubility limit of the dopants. The textures pitch.5 But in contrast to BPI and BPII modifications, BPIII of these modifications are very similar to the textures of the does not exhibit the Bragg scattering of a lattice structure. Its corresponding calamitic modifications. structure is not thoroughly understood and still subject to In order to investigate the complicated structure of the blue scientific discussion.6,8 phases by scanning probe microscopy, some attempts were A considerable amount of the ordering can occur at the made for calamitic systems to freeze the structures into glass isotropic-BPIII transition. For many compounds the major like states with chiral polymers, oligomers and networks.13,14 part of the latent heat in the phase sequence cholesteric phas...

We have synthesized a number of new achiral and chiral triphenylene derivatives bearing laterally substituted phenylene units in the side chains. Their polymorphism includes the ®rst reported discotic blue phase of a pure compound and a ferroelectrically switchable columnar mesophase.