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

Abstract

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 D_2h-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 T_g = 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.