New mesomorphic organocyclosiloxanes I. Thermal behaviour and mesophase structure of organocyclotetrasiloxanes

cis-Cyclotetrasiloxanes of the formula cis-[PhSi(O)(OSiMe2R)](4) with R = M e, CH2Cl, CH=CH2 and cis-[ClC6H4Si(O)(OSiMe3)](4) were synthesized and inve stigated in terms of their thermotropic phase transitions. Two ordered phas es were observed for the cis-cyclotetrasiloxanes, one at lower temperature exhibiting the properties of a crystal and one at higher temperature exhibi ting the properties of a plastically crystalline (3D) mesophase. A detailed examination of the mesophase behaviour and mesophase structure of octaphen ylcyclotetrasiloxane was also carried out. It was shown that the thermal pr operties and structural characteristics of the mesophase are influenced by the structural characteristics of the substituent attached at the silicon a tom in the tetracyclosiloxane. The new mesomorphic cis-cyclotetrasiloxanes are by far the largest molecules reported to date as forming plastic crysta ls, and the temperature region of the mesophase is much broader than in oth er plastic crystals. All five cyclotetrasiloxanes studied were found to be isomorphous in the 3D-mesophase and the low temperature forms of the two ci s-cyclotetrasiloxanes: PhSi(O)(OSiMe2R)(4) (R = Me, CH=CH2) were also isomo rphous.