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 shor t 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 spectr oscopy (TRRS) was employed to identify and characterize singlet-correl ated electron-transfer intermediates in this process. The transient po rphyrin cation modes are well-characterized and conform to previous as signments. 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 (approx imate to 0.60 eV) for the UQ(0)/UQ(0)(.-) radical anion pair. These re sults are discussed in the context of modern theories of electron tran sfer.