Addition of Group 14 organometallic compounds to C₆₀ via photoinduced electron transfer. Direct detection of radical ion pair intermediates

The carbon-carbon bond formation of C₆₀ with Group 14 organometallic compounds is attained through photoinduced electron transfer from Group 14 organometallic compounds acting as electron donors to the triplet excited state of C₆₀. The electron donors employed in this study are ketene silyl acetals and allylic stannanes. When an unsymmetric allylic stannane, e.g. prenyltributyltin (Me₂C=CHCH₂SnBu₃) is employed, the allylic group is introduced selectively at the α-position to yield C₆₀-1,2-CH₂CH=CMe₂ under irradiation of the visible light and no γ adduct has been formed. Such regioselectivity is characteristic of the reactions of prenyltributyltin via electron transfer, where the C-C_δ bond formation occurs after cleavage of the Sn-C bond. A comparison of the observed rate constants determined from the dependence of the quantum yields on the concentrations of electron donors, as well as the quenching of the triplet excited state of C₆₀ by the electron donors with those predicted for the electron transfer processes, indicates that the photoreduction proceeds via photoinduced electron transfer from the electron donors to the triplet excited state of C₆₀. The radical ion pair generated in the photoinduced electron transfer from ketene silyl acetal to the triplet excited state of C₆₀ has been successfully detected as transient absorption spectra in the visible and near-IR region with use of the laser flash photolysis.