Addition of Group 14 organometallic compounds to C-60 via photoinduced electron transfer. Direct detection of radical ion pair intermediates

The carbon-carbon bond formation of C-60 with Group 14 organometallic compo unds is attained through photoinduced electron transfer from Group 14 organ ometallic compounds acting as electron donors to the triplet excited state of C-60. The electron donors employed in this study are ketene silyl acetal s and allylic stannanes. When an unsymmetric allylic stannane, e.g. prenylt ributyltin (Me2C=CHCH2SnBu3) is employed, the allylic group is introduced s electively at the alpha-position to yield C-60-1,2-CH2CH=CMe2 under irradia tion of the visible light and no gamma adduct has been formed. Such regiose lectivity is characteristic of the reactions of prenyltributyltin via elect ron transfer, where the C-C, bond formation occurs after cleavage of the Sn -C bond. A comparison of the observed rate constants determined from the de pendence of the quantum yields on the concentrations of electron donors, as well as the quenching of the triplet excited state of C-60 by the electron donors with those predicted for the electron transfer processes, indicates that the photoreduction proceeds via photoinduced electron transfer from t he electron donors to the triplet excited state of C-60. The radical ion pa ir generated in the photoinduced electron transfer from ketene silyl acetal to the triplet excited state of C-60 has been successfully detected as tra nsient absorption spectra in the visible and near-IR region with use of the laser flash photolysis. (C) 1999 Published by Elsevier Science S.A. All ri ghts reserved.