Synthesis and liquid-crystalline properties of novel laterally connected twins: The influence of the connecting topology and the length of the rigid core

Laterally ligated p-terphenyl twins containing 2-oxapropylene spacers have been synthesized. Their liquid-crystalline properties were investigated by polarizing microscopy, calorimetry and X-ray scattering. The mesophase stability depends strongly on the topology of connection. It was found that the connection of the mesogenic units at the peripheries of the rigid cores gave twins with the highest clearing temperatures and the broadest smectic C (S-C) ranges. Shifting the connecting units to the middle of the rigid cores diminished their mesophase stability. The X-ray investigations indicated that the thickness of the smectic layer was nearly independent of the position of the lateral connecting group when the same mesogens were linked. Also, twins incorporating a 1,4-disubstituted phenyl ring and a biphenyl rigid core have been prepared and they were found to exhibit broad mesomorphic ranges. Another kind of twins was obtained by the lateral-terminal connection of a p-terphenyl rigid core and a 2,5-diphenyl-1,3,4-thiadiazole calamitic unit. Again, a remarkable influence of the connecting position was observed.