Energy- and Electron-Transfer Quenching of Porphyrin Triplets by C60

Abstract

Energy- and electron-transfer from porphyrins in the triplet excited state to C60 in toluene and benzonitrile has been studied with Fourier transform EPR (FT-EPR). Pulsed-laser excitation of the systems magnesium tetraphenylporphyrin (MgTPP)/C60 and free-base octaethylporphyrin (OEP)/C60 in toluene gives rise to a resonance peak from C60 triplets (3C60) generated primarily by the energy-transfer process 3MgTPP (3OEP) + C60 → MgTPP (OEP) + 3C60 as evident from the emissive spin polarization of the 3C60 signal observed at early times. No FT-EPR signals from redox products could be detected. In benzonitrile, triplet energy transfer from 3MgTPP or 3OEP to C60 is a minor process. In this polar solvent photoexcitation of MgTPP/C60 produces FT-EPR spectra with signal contributions from MgTPP+ and C60- in addition to the 3C60 resonance. From the spin polarization and time profile of the signal from the anion radical, it can be deduced that the primary route of electron transfer is oxidative quenching of the porphyrin triplets, 3MgTPP (3OEP) + C60 → MgTPP+ (OEP+) + C60-. The reduction in 3C60 lifetime indicates that C60- is generated as well by the reductive triplet quenching reaction 3C60 + MgTPP (OEP) → C60- + MgTPP+ (OEP+).