Photoinduced Electron Transfer from Nitoxide Free Radicals to the Triplet State of C₆₀

Abstract

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.

Ultrafast Intersystem Crossing and Spin Dynamics of Zincmeso-Tetraphenylporphyrin Covalently Bound to Stable Radicals

“…We have previously discussed in detail the most common radical-induced excited state quenching mechanisms . To briefly summarize, radical-induced quenching of excited states usually proceeds by one or more of the following mechanisms: electron transfer, Förster and/or Dexter energy transfer, electron-exchange-induced enhanced intersystem crossing (EISC), and enhanced internal conversion (EIC), i.e., rapid vibrational relaxation. − ,,,− ,,,, …”

Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO−Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical−Triplet Interaction

“…For example, electron transfer from a TEMPO free radical to the triplet state of a covalently linked 1,8:4,5-naphthalene-bis(dicarboximide) (NI) chromophore has been reported to occur with τ cs < 15 ns in acetonitrile, resulting in a long-lived charge-separated state with a lifetime of >200 μs . In addition, several studies have reported the oxidation of nitroxide radicals by fullerene triplet states, but they are limited to intermolecular interactions. − Fluorescence quenching as a result of electron transfer to or from a stable radical has often been considered unlikely due to the lack of a solvent dependence, which is often key evidence for assigning this mechanism . However, it has been argued that a lack of solvent dependence is not definitive evidence for ruling out electron transfer, since the process could be diffusion controlled and not appear solvent dependent .…”

Visible Light Activated Benzimidazolequinone Alkoxyamines of 1,1,3,3‐Tetramethylisoindolin‐2‐yloxyl (TMIO)

“…The retardation in alkoxyamine homolysis caused by free TEMPO can be attributed to the well‐known effects of nitroxide quenching of excited triplet states [17,20,24] . Further, the UV‐Vis absorption spectrum shows strong absorbance of TEMPO in the blue region (Figure 4), thus competitive absorption plays a significant role in TMIO‐Vis homolysis rates, particularly when TEMPO is present at 100‐fold excess.…”