Stabilization and destabilization of the SA phase in mixtures of 4-nitrobenzyl-2,5-bis(4-n-alkyloxybenzoyloxy)benzoates

4-Nitrobenzyl-2,5-bis(4-n-alkyloxybenzoyloxy)benzoates exhibit S_A phases which are characterized by an antiparallel packing of the molecules and a partial intercalation of the terminal alkyl chains. In binary systems of selected rod-like components the S_A mixed phase can be stabilized or destabilized depending on the molecular length of the rod-like molecules. This unusual behaviour is mainly due to steric interaction and can be interpreted by a simple packing model.     

Unusual electro-optical response of an oblique columnar phase formed by a bent-core mesogen

Mesophases formed by bent-core mesogens have attracted special attention because they can organise into fluid phases with polar order and supramolecular chirality. In this paper, a new five-ring bent-core mesogen is presented which forms a columnar mesophase. The structure is built up by layer fragments and possesses an oblique two-dimensional lattice with a layer group of the type p112/a. The columnar phase can be transformed into a ferroelectric SmCP phase (SmCP_F) by application of a sufficiently high electric field (25 V µm^-1). This field-induced transition was found to be reversible. The mechanism of the polar switching depends on the frequency of the applied electric field. The switching takes place in the usual way by a collective rotation of the molecules around the tilt cone. At very low frequencies (0.1 Hz and lower), the polar switching is based on a collective rotation of the molecules around their long axes. In the latter case, the switching is accompanied by an inversion of the layer chirality.     

The B2-B7 phase transition in symmetrical bent-shaped mesogens with methoxy substitution

New mesogens composed of achiral bent molecules with thermally stable ester linkages, and laterally substituted by a methoxy group symmetrically near the central benzene ring, were synthesized. Texture, calorimetric, electro-optical, X-ray and dielectric measurements were performed. In most of studied compounds the antiferroelectric B₂ phase was found on cooling from the isotropic phase, followed by the B₇ phase at lower temperatures. Undulation of layers in the B₇ phase was confirmed by precise synchrotron studies.     

Synthesis and liquid crystalline properties of novel laterally connected trimesogens and tetramesogens

Three types of laterally connected triplet mesogens and one quadruplet mesogen incorporating rigid p-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy, differential scanning calorimetry and X-ray scattering. The lateral fixation of three rod-like 4,4'-didecyloxy-p-terphenyl units mostly gives liquid crystalline materials with considerably increased mesophase stabilities with respect to the parent 4,4'-didecyloxy-2'-methyl-p-terphenyl. The mesophase stability strongly depends on the type of connection. The highest clearing temperatures were observed for triplets which are connected in line with each other (type I) and triplets which are laterally connected in a peripheral manner. Only the oligomesogens of type III are not liquid crystalline. All compounds incorporating exclusively decyloxy chains exhibit smectic phases (S_A and S_c). For the ethoxy derivatives the nematic phase was found.     

New banana-shaped mesogens with bromine-substituted central core

Banana-shaped five-ring mesogens derived from either isophthalic acid or 4-bromoisophthalic acid have been synthesized. It is shown that the melting and clearing points of the bromine-substituted compounds are markedly decreased in comparison with the analogous unsubstituted compounds. On the basis of X-ray diffraction studies and NMR measurements, as well as dielectric and electro-optical investigations, the mesophase of these compounds could be identified as a B₂ phase.     

Kojic acid—a new fragment for the preparation of bent‐core mesogens

Kojic acid was used to synthesize bent‐core mesogens. Antiferroelectric polar smectic mesophases, an oblique and a rectangular columnar mesophase were found. The mesophases were characterized by X‐ray scattering, electro‐optical measurements, polarizing microscopy and DSC. On the base of these measurements, models of the mesophases are proposed.