Singlet-state electron transfer between a porphyrin and ubiquinone: A transient resonance Raman and quantum chemical study

Radical ion pairs resulting from bimolecular electron transfer between excited singlet porphyrin donors and quinone accepters have such short lifetimes that they are rarely observed. Here we report the results of a spectroscopic investigation of the photodynamics of a noncovalent complex of a free base meso-tetrakis (4-sulfonatophenyl)porphine (P) and 2,3-dimethoxy-5-methyl-1,4-benzoquinone (ubiquinone, UQ(0)). P and UQ(0) form a weak ground-state complex (K-a approximate to 1.0 x 10(3) M-1), which decays via a CT intermediate after photoexcitation of a local porphyrin pi-pi* excited state. Transient resonance Raman spectroscopy (TRRS) was employed to identify and characterize singlet-correlated electron-transfer intermediates in this process. The transient porphyrin cation modes are well-characterized and conform to previous assignments. While some of the modes expected of the ubiquinone radical anion are obscured by the porphyrin vibrations, the predominantly C=C and C=O modes are clearly discernible in the transient RR spectra. Ab initio calculations were used to help assign the observed modes of the UQ(0) and calculate "self-exchange" reorganizational energy (approximate to 0.60 eV) for the UQ(0)/UQ(0)(.-) radical anion pair. These results are discussed in the context of modern theories of electron transfer.