Homostructural two-ring mesogens:: a comparison of p-carboranes, bicyclo [2.2.2]octane and benzene as structural elements

Mesogenic properties of diheptyl and diheptynyl derivatives of 12- and 10-vertex bi-p-carboranes have been studied in the pure state and as binary mixtures with a nematic host. The results are compared with those for analogous derivatives of bibicyclo[2.2.2]octane and biphenyl. The clearing temperature for 10,10'-diheptyl-bi-p-carborane B2 is significantly lower than that for the 12-vertex analogue B1 and still lower than that for the corresponding bicyclo [2.2.2] octane derivative B3. The observed order of mesophase stability is opposite to the trend expected from shape and polarizability anisotropy. Introduction of an acetylene link into bicarborane derivatives dramatically lowers the clearing temperatures by about 150 degrees C. The results are rationalized in terms of molecular conformations and intramolecular rotational barriers.