Synthesis of discotic star-shaped cyclosiloxanes with mesogenic cyanobiphenyl groups

Star-shaped cyclosiloxanes-first representatives of discotic liquid-crystalline compounds with different degrees of symmetry-were synthesized. cis-[Tetrakis(4'-cyanobiphen-4-yl-10-carbonyloxydecylene-8-dimethylsiloxyl-2-dimethylsiloxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxy)]-2,4,6,8-tetraphenyl-cyclotetrasiloxane, cis-[hexakis(4'-cyanobiphen-4-yl-carbonyloxydecylene-8-dimethylsiloxyl-2-dimethylsiloxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxy)]-2,4,6,8,10,12-hexaphenylcyclohexasiloxane, and cis-[hexakis(2-hexa-chloro-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxy)]-2,4,6,8,10,12-hexaphenylcyclohexasiloxane were prepared by heterofunctional condensation and hydrosilylation reactions. The structure of the compounds under consideration was established by IR and 1 H and 29 Si NMR spectroscopy. Using DSC, polarizing microscopy, and thermooptical analysis, the temperatures of phase transitions were measured and their reversibility was demonstrated. It was shown that the heptamer shows a tendency toward mesomorphism and exhibits phase transitions I ⇄ SmA ⇄ SmA' ⇄ Dh; it may be classed with columnar discotic LC compounds.