Positron lifetime in supramolecular gamma- and delta-cyclodextrin–C60 and –C70 compounds

Temperature dependence of positron lifetime parameters have been studied in pure cyclodextrins and their supramolecular complexes with C60 and C70 molecules. It was found that while C60 molecules are localized in molecular cavities in gamma-cyclodextrin, they are too small to be trapped by the larger cavities of delta-cyclodextrin. On the other hand, the larger C70 molecules fill these larger voids completely. In cases where fullerene molecules are localized in molecular cavities, a low-temperature phase transition was observed. The activation energies for the phase transitions were calculated assuming a simple two-state annihilation model.