Photoinduced electron transfer from nitoxide free radicals to the triplet state of C ₆₀

The photoinduced electron transfer of C ₆₀ in the presence of stable free radicals such as nitoxides has been studied by nanosecond laser photolysis by measuring the transient absorption spectra in the visible and near-IR regions. For the N-oxy-piperidine radical, the rise of the radical anion of C ₆₀ was not observed, although the decay rate of the triplet excited state of C ₆₀ was quite fast. For free radicals with 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (nitronyl nitroxide), electron transfer via the triplet excited state of C ₆₀ was confirmed from the rise of the anion radical of C ₆₀, although the quantum yield of electron transfer is not high. From the rates and efficiencies of electron transfer, which are strongly affected by the donor ability of the substituents on the phenyl moiety of the nitronyl nitroxide radical, a specific spin - spin interaction of the triplet spin state of C ₆₀ with the doublet spin state of the nitronyl nitroxide radical was revealed. In the longer time scale measurements, the radical anion of C ₆₀ persisted for a long time, suggesting that the nitronyl nitoxide cation, which was formed by donating an electron to the triplet state of C ₆₀, forms the complex with the nitronyl nitoxide radical. An electron-mediating process from the radical anion of C ₆₀ to the viologen dication was confirmed by the rise of the radical cation of the viologen, with the concomitant decay of the radical anion of C ₆₀. Final back electron transfer from the viologen radical cation to the nitronyl nitroxide cation was observed by the transient absorption measurements on additional long time scales.