Mesogenic dimers composed of bent-core molecules with flexible alkylene spacer.

The synthesis and characterization of six symmetrical mesogenic dimers composed of bent-core molecules containing alkylene spacers are reported. The mesophase was characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electric field experiments. The X-ray diffraction studies revealed a columnar structure with an oblique lattice for the mesophase. The electric field experiments indicated a ferroelectric behaviour for the mesophase. On the basis of obtained experimental data, a possible structural model for the mesophase has been proposed.

Relationships between molecular structure and physical properties in bent-core mesogens.

New five-ring bent-core mesogens that possess only ester connecting groups between the aromatic rings and different lateral substituents at the central phenyl ring are presented. The mesophases have been assigned by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical measurements. It is shown that the mesophase behaviour depends strongly on the position of the lateral substituents. Compounds, which are derived from 4-cyano-, 4-chloro- and 4,6-dichloro-resorcinol, show polymorphism variants where polar phases (SmAP, SmCP) occur together with nematic and conventional smectic phases, e.g. SmA-SmAP, SmA-SmC(S)P(A)-Col(ob)-SmC(S)P(A), N-SmA-SmCP(A), SmA-SmC-SmCP(A) and SmC-SmCP(A). On the basis of the behaviour of two series of materials, the occurrence of different polar-switching mechanisms could be demonstrated. Apart from the usual mechanism by director rotation around the tilt cone, the polar switching can also take place through collective rotation of the molecules around their long axes, which corresponds to a field-induced switching of the layer chirality. A remarkable finding is the polar switching in the crystalline modification of long-chain, bent-core compounds with a methyl group in 2-position, which is accompanied by a clear change of the optical texture and by a relatively high switching polarization (approx. 600 nC cm(-2)). It was found for selected bent-core compounds that, above the transition temperature of a polar to a non-polar phase, the non-polar phase can be transformed to the polar phase by application of an electric field, which was proved for the transitions isotropic-SmCP(F), SmA-SmCP(F) and isotropic-CrII polar.