Synthesis and liquid crystalline properties of novel laterally connected twins

Several laterally connected twin mesogens incorporating rigid p-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy and differential scanning calorimetry. If one compares the twins with their respective halves, the mesophase stabilizing influence of the lateral fixation becomes evident. It obviously pre-organizes the calamitic units and therefore stabilizes smectic phases. The mesophase behaviour of these twins depends strongly on the length and the structure of the spacer. Short as well as highly polar and rigid spacer units give rise to smectic phases with high transition temperatures. By increasing the spacer length the mesophase stability is drastically diminished. Dimesogenic 4,4 ''-didecyloxyterphenyl derivatives containing spacers consisting of two connecting atoms exhibit smectic C phases exclusively. Elongation of the spacer to three connecting atoms gives an S-A-S-C dimorphism, and in the case of compounds with longer spacers mostly the S-A phase and in one case the nematic phase were found.