Fullerene encapsulation with calix[5]arenes

This paper presents the synthesis of the fullerene hosts based on the calix[5]arenes and their binding properties. Calix[5]arenes , , bind C60 or C70 in organic solvents. The solvent effect of the fullerene complexation was clearly observed; the association constant decreases in a solvent with high solubility for C60. Covalently linked double-calix[5]arenes were also investigated on their binding properties for fullerenes in organic solvents. Their binding abilities for both C60 and C70 are extremely high in toluene solution. Higher binding selectivity toward C70 is observed by all the double-calix[5]arenes. The selectivity of toward C70/C60 is highest in toluene with a value of 10. The structures of the supramolecular complexes of the calix[5]arene hosts and C60 or C70 were investigated by using 1H and 13C NMR studies. The molecular mechanics calculation and X-ray structure reveal that the interior of the calix[5]arene is complementary to the exterior of C60 molecule. In contrast, the host–guest complexes of C70 with the simple calix[5]arenes take many conformational options due to its less symmetric shape. The molecular mechanics calculation and our chemical shift simulation nicely worked to estimate the reliable structures; the calix[5]arene cavity takes up C70 molecule, and the C70 molecule tilts significantly from the C5 axis of the calix[5]arene. In the case of the host–guest complex of C70 with the double-calix[5]arene, the molecular dynamics simulation of the host–guest complex represented the realistic movement of the bound C70 inside the cavity. The combination of the molecular dynamics simulation and the chemical shift simulation of the host–guest complex suggested that the C70 molecule rapidly moves inside the cavity.