Structural variation of liquid crystalline trioxadecalins

Synthesis and mesogenic properties of new liquid crystals bearing a chiral trioxadecalin system are described. Boron-containing three-ring systems with a lateral methoxy group show cholesteric, TGB(A) and smectic A phases. Molecules containing four or five rings show mostly smectic C* phases. The insertion of a triple bond leads to ferroelectric smectic C* phases, but compounds with a flexible spacer between the rings show only monotropic smectic A phases. Lateral fluorination of the aromatic rings leads, depending on the position of the fluorine, either to stabilised smectic phases with lower transition temperatures or to cholesteric phases with complete suppression of all smectic phases.