Ft-epr study of light induced energy and electron transfer from porphyrin triplets to C_(60)

摘要

Energy and electron transfer reactions from porphyrins in the triplet excited state (~3P) to C_(60) have been studied with Fourier Transform EPR (FT-EPR). Photoexcitation of the systems magnesium tetraphenylporphyrin (MgTPP)/C_(60) and free base octaethylporphyrin (OEP)/C_(60) in benzonitrile gives rise to resonance peaks attributed to C_(60) triplets and the radical anion C_(60)~-. From the time profile of the ~3C_(60) signal it can be deduced that C_(60)~- is generated, at least in part, by the reductive triple quenching reaction ~3C_(60) + MgTPP (OEP) -> C_(60)~- + MgTPP~+ (OEP~+). With toluene as solvent, ~3C_(60) is generated, in part by the energy transfer process ~3MgTPP (~3OEP) + C_(60) ->MgTPP (OEP) + ~3C_(60) as is evident from the emissive spin polarization of the ~3C_(60) signal at early delay times. For MgTPP/C_(60) the energy transfer contribution is minor, which is consistent with an endothermic process (triangle openG_(ent) approx 3kT). In this nonpolar solvent no redox products could be detected. However, the time profiles of the ~3C_(60) signal show clear evidence of an electron transfer quenching reaction.