Thermotropic behavior of lipophilic derivatized [60]fullerenes studied by deuterium NMR, X-ray diffraction, and microcalorimetry

The dynamics, structure, and thermotropic behavior of a new class of lipoph ilic [60]fullerene (C-60) derivatives, so-called lipo-fullerenes, have been studied by differential scanning calorimetry (DSC), deuterium nuclear magn etic resonance (H-2 NMR), and X-ray scattering. The lipo-fullerene studied consists of six pairs of perdeuterated C-18 alkyl chains as substituents of six covalently attached methylene groups in octahedral sites. The symmetry of this highly symmetrical hexamethanofullerene is T-h We find drastic cha nges of the molecular arrangement of the lipo-fullerenes induced by tempera ture. Heating the sample from 20 to 70 degrees C causes it to undergo two m ajor structural transitions. At 55 degrees C we observe an exothermic trans ition from a low-temperature, hard sphere-like packing state of the molecul es, with separation distances (6.1 nm) slightly above the maximum diameter of the molecules, to a condensed one. This latter state involves partial in tercalation (interdigitation) of the alkyl chains belonging to adjacent mol ecules and is preceded by partial melting of the chains to accommodate ster ically for the (exothermic) interdigitation. The latter allows denser packi ng with an average separation distance of 4.8 nm. At a temperature of 64 de grees C, an endothermic melting transition from the interdigitated to a vis cous fluidlike state is observed, with an average separation distance of 2. 8 nm. Cooling the sample from 70 degrees C causes a direct transition from the fluid into the low-temperature state with no interdigitation of the cha ins.