Photoinduced electron transfer between metal octaethylporphyrins and fullerenes (C60/C70) studied by laser flash photolysis: electron-mediating and hole-shifting cycles

Abstract

Electron transfer (ET) processes of fullerenes (C₆₀/C₇₀) with metal octaethylporphyrins (MOEP) in a polar solvent have been investigated by a nanosecond laser photolysis technique in the visible/near-IR regions. By the selective excitation of C₆₀/C₇₀ using OPO laser light it has been proved that electron transfer takes place from the ground states MOEP to the triplet excited states ³C₆₀*/³C₇₀*. By selective excitation of MOEP the electron transfer processes via³MOEP* to C₆₀/C₇₀ were also confirmed. In nonpolar toluene the lifetimes of ³C₆₀*/³C₇₀* decreased in the presence of MOEP without evidence for electron and/or energy transfer processes. On adding benzonitrile to toluene the ion-radical formation was increased, suggesting that the triplet exciplexes ³[MOEP^(δ+)—C₆₀^(δ−)/C₇₀^(δ−)]* would be dominant prior to the ion-pair formation in a less-polar solvent. On addition of a viologen dication the electron of the anion radicals of C₆₀/C₇₀ transfers to the viologen dication yielding the viologen radical cation. In the system MOEP–C₆₀/C₇₀–aromatic amine (DTQH), as a second donor having lower oxidation potential than MOEP, the hole shift from the radical cation of MOEP to the aromatic amine was observed. These observations proved that the photosensitized electron-transfer/electron-mediating and photosensitized electron-transfer/hole-shift cycles have been confirmed by the transient absorption spectral method.