Photoinduced Electron-Transfer of Inclusion Complexes of Fullerenes (C60and C70) inγ-Cyclodextrin

Abstract Photophysical and photochemical properties of 1:2 inclusion complexes of C60 and C70 in γ-cyclodextrin (γ-CD) have been studied by laser flash photolysis and steady photolysis methods. The triplet-triplet annihilations of C60/γ-CD and C70/γ-CD are retarded, compared with those of the pristine fullerenes. The rate constant for the fluorescence-quenching of C70/γ-CD by 1,4-diazabicyclo[2.2.2]octane was larger than that of C60/γ-CD, indicating incomplete hindrance by γ-CD due to larger size of C70 than C60. A similar trend was also observed in triplet-triplet annihilation processes of the encapsulated fullerenes. Generations of radical ions by photoinduced electron transfer between amines and 3C60*/γ-CD or 3C70*/γ-CD were confirmed by observing the transient absorption bands due to radical anions in the near-IR region. The electron transfer rates for 3C60*/γ-CD or 3C70*/γ-CD were slower than those of corresponding 3C60* or 3C70* in solution by ca. 1/100. For reversible systems in which the cation radicals of amines are stable (for example, p-anisidine), C60•-/γ-CD and C70•-/γ-CD decayed slowly by back electron transfer. For an irreversible system in which the cation radical of amine (nitrilotriethanol) is degraded, the high concentration of C60•-/γ-CD or C70•-/γ-CD was persistent. On addition of methylviologen (MV2+), persistent MV•+ was generated in high concentration as equilibrium with C60•-/γ-CD, suggesting that C60/γ-CD acts as an efficient photosensitizer and an electron mediator to produce MV•+ using nitrilotriethanol as a sacrificial donor.