Enhanced photoinduced electron-transfer reduction of Li⁺@C ₆₀ in comparison with C₆₀

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ion-encapsulated C₆₀ (Li⁺@C₆₀) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C₆₀. Femtosecond laser flash photolysis measurements of Li⁺@C₆₀ revealed that singlet excited state of Li⁺@C₆₀ (λ_max = 960 nm) underwent intersystem crossing to the triplet excited state [ ³(Li⁺@C₆₀)*: λ_max = 750 nm] with a rate constant of 8.9 × 10⁸ s^-1 in deaerated benzonitrile (PhCN). The lifetime of ³(Li ⁺@C₆₀)* 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⁺@C₆₀)* occurred to produce the radical cations and Li⁺@C₆₀ ^•-. The rate constants of photoinduced electron transfer of Li⁺@C₆₀^•- are significantly larger than those of C₆₀ when the rate constants are less than the diffusion-limited value in PhCN. The enhanced reactivity of ³(Li ⁺@C₆₀)* as compared with ³C ₆₀* results from the much higher one-electron reduction potential of Li⁺@C₆₀ (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⁺@C₆₀ and C₆₀ 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⁺@C₆₀ 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 C₆₀ (0.73 eV), probably because of the change in electrostatic interaction of encapsulated Li⁺ upon electron transfer in PhCN.