New stereoregular liquid-crystalline phenylcyclosiloxanes

Representatives of stereoregular phenylcyclotetra(hexa)siloxanes with mesogenic cyanobiphenylyl groups cis-oriented relatively to the siloxane ring have been synthesized for the first time. The ability of the compounds to transit to the liquid-crystalline state was confirmed by the thermooptical, X-ray diffraction, and calorimetric methods. The temperatures and enthalpies of the phase transitions were determined, and their reversibility was shown. The bilayered molecular packing with antiparallel arrangement of molecules is the most probable for the smectic A phase of the cyclosiloxanes studied. Taking into account differences in the optical textures and interlayer distances, we cannot exclude completely the possibility of formation of liquid-crystalline phases of other types.