Thermotropic behaviour of covalent fullerene adducts displaying 4-cyano-4 '-oxybiphenyl mesogens

The synthesis of the covalent fullerene derivatives 2, 3 and 4 with an incr easing number (2, 4 and 6) of 4-cyano-4'-oxybiphenyl mesogenic groups is de scribed. The thermotropic behaviour of these compounds was investigated and compared with the liquid crystalline properties of the non-fullerene conta ining precursors 6 and 9, and the D-2h-symmetric tetrakis(methano)fullerene 1 bearing eight long alkyl chains instead of mesogenic groups. Tetrakis(me thano)fullerene 1 without and methanofullerene 2 with two mesogenic groups only showed an amorphous phase with glass transition temperatures T-g = 27 and 50 degrees C, respectively. Samples of hexakis(methano)fullerene 3 with four mesogenic groups obtained by slow crystallisation gave a nematic meso phase on the first heating (Cr 85 N 157 I) that disappeared after isotropis ation. No mesogenic behaviour could be recovered for 3 in successive therma l cycles. Unlike 3, the fullerene dendrimer 4 with six 4-cyano-4'-oxybiphen yl groups showed mesogenic properties independent of the thermal history: l ow intensity first order phase transitions were recorded in the DSC and con firmed by optical microscopy analysis. Owing to the lack of a characteristi c texture, the unambiguous identification of the mesophase was not possible . The effect of the introduction of mesogens onto C-60 on the stability of the mesophases and on the preservation of liquid crystalline properties is discussed with respect to the number of mesogenic moieties per fullerene sp here.