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 increasing number (2, 4 and 6) of 4-cyano-4′-oxybiphenyl mesogenic groups is described. The thermotropic behaviour of these compounds was investigated and compared with the liquid crystalline properties of the non-fullerene containing precursors 6 and 9, and the D2h-symmetric tetrakis(methano)fullerene 1 bearing eight long alkyl chains instead of mesogenic groups. Tetrakis(methano)fullerene 1 without and methanofullerene 2 with two mesogenic groups only showed an amorphous phase with glass transition temperatures Tg = 27 and 50 °C, respectively. Samples of hexakis(methano)fullerene 3 with four mesogenic groups obtained by slow crystallisation gave a nematic mesophase on the first heating (Cr 85 N 157 I) that disappeared after isotropisation. No mesogenic behaviour could be recovered for 3 in successive thermal cycles. Unlike 3, the fullerene dendrimer 4 with six 4-cyano-4′-oxybiphenyl groups showed mesogenic properties independent of the thermal history: low intensity first order phase transitions were recorded in the DSC and confirmed by optical microscopy analysis. Owing to the lack of a characteristic texture, the unambiguous identification of the mesophase was not possible. The effect of the introduction of mesogens onto C60 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 sphere.