Enhanced photoinduced electron-transfer reduction of Li +@C 60 in comparison with C 60

Abstract

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ion-encapsulated C 60 (Li +@C 60) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C 60. Femtosecond laser flash photolysis measurements of Li +@C 60 revealed that singlet excited state of Li +@C 60 (λ max = 960 nm) underwent intersystem crossing to the triplet excited state [ 3(Li +@C 60)*: λ max = 750 nm] with a rate constant of 8.9 × 10 8 s -1 in deaerated benzonitrile (PhCN). The lifetime of 3(Li +@C 60)* was determined by nanosecond laser flash photolysis measurements to be 48 μs, which is comparable to that of C 60. Efficient photoinduced electron transfer from a series of electron donors to 3(Li +@C 60)* occurred to produce the radical cations and Li +@C 60 •-. The rate constants of photoinduced electron transfer of Li +@C 60 •- are significantly larger than those of C 60 when the rate constants are less than the diffusion-limited value in PhCN. The enhanced reactivity of 3(Li +@C 60)* as compared with 3C 60* results from the much higher one-electron reduction potential of Li +@C 60 (0.14 V vs SCE) than that of C 60 (-0.43 V vs SCE). The rate constants of photoinduced electron transfer reactions of Li +@C 60 and C 60 were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The reorganization energy of electron transfer of Li +@C 60 was determined from the driving force dependence of electron transfer rate to be 1.01 eV, which is by 0.28 eV larger than that of C 60 (0.73 eV), probably because of the change in electrostatic interaction of encapsulated Li + upon electron transfer in PhCN. © 2012 American Chemical Society.