Positron lifetime in supramolecular gamma- and delta-cyclodextrin-C-60 and-C-70 compounds

Temperature dependence of positron lifetime parameters have been studied in pure cyclodextrins and their supramolecular complexes with C-60 and C-70 m olecules. It was found that while C-60 molecules are localized in molecular cavities in gamma-cyclodextrin, they are too small to be trapped by the la rger cavities of delta-cyclodextrin. On the other hand, them larger C-70 mo lecules fill these larger voids completely. In cases where fullerene molecu les are localized in molecular cavities, a low-temperature phase transition was observed. The activation energies for the phase transitions were calcu lated assuming a simple two-state annihilation model. (C) 2001 Elsevier Sci ence BN. All rights reserved.