Dimesogenic liquid crystalline organosiloxanes

Liquid crystalline (LC) organosiloxanes with two terminal cyanobiphenylyl groups attached to a linear or cyclic siloxane center through an aliphatic spacer (CH₂) _n with i = 10 were synthesized. The ability of compounds to pass into the LC state was confirmed by thermooptical, X-ray diffraction, and calorimetric measurements. The temperatures and the enthalpies of phase transitions were determined. The types of LC structures and the capability of one compound for polymesomorphism to form the chiral S_mC* phase without a chiral center in the mesogenic group were established. The temperatures and the enthalpies of the reversible phase transitions, crystal S_mC S_mA melt and crystal S_mA melt, for linear and cyclic LC organosiloxanes, respectively, were determined. Models of molecular packing in the S_mA and S_mC* phases were proposed based on X-ray diffraction data. A specific feature of the S_mA phases of new LC organosiloxanes is a negative gradient of the temperature dependence of the interlayer spacing.