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 lipophilic [60]fullerene (C₆₀) derivatives, so-called lipo-fullerenes, have been studied by differential scanning calorimetry (DSC), deuterium nuclear magnetic resonance (²H NMR), and X-ray scattering. The lipo-fullerene studied consists of six pairs of perdeuterated C₁₈ 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 changes of the molecular arrangement of the lipo-fullerenes induced by temperature. Heating the sample from 20 to 70 °C causes it to undergo two major structural transitions. At 55 °C we observe an exothermic transition from a low-temperature, hard sphere-like packing state of the molecules, with separation distances (6.1 nm) slightly above the maximum diameter of the molecules, to a condensed one. This latter state involves partial intercalation (interdigitation) of the alkyl chains belonging to adjacent molecules and is preceded by partial melting of the chains to accommodate sterically for the (exothermic) interdigitation. The latter allows denser packing with an average separation distance of 4.8 nm. At a temperature of 64 °C, an endothermic melting transition from the interdigitated to a viscous fluidlike state is observed, with an average separation distance of 2.8 nm. Cooling the sample from 70 °C causes a direct transition from the fluid into the low-temperature state with no interdigitation of the chains.