Coupled electron-proton transfer in interactions of triplet C60 with hydrogen-bonded phenols:: Effects of solvation, deuteration, and redox potentials

The quenching of tripler C-60 by phenols is greatly enhanced by addition of pyridines, which also lower the phenol voltammetric oxidation potentials. Flash photolysis shows that the products of this quenching reaction are the C-60(.-) anion radical, neutral phenoxy (or naphthoxy) radicals, and protonated pyridines. Analysis of the second-order kinetics gives quenching rate constants and values of formation constants of hydrogen-bonded phenol-pyridine pairs. The latter agree with those derived from absorption spectra over a wide range of phenols, pyridines, and solvents. Significant deuterium kinetic isotope effects are observed, indicating the importance of both proton transfer and hydrogen bonding in enhancing the rate. Quenching rates and radical yields both increase with solvent polarity. It is concluded that this quenching process involves interaction between C-3(60) and a hydrogen-bonded phenol-pyridine complex, in which electron transfer from the phenol to C-3(60) is concerted with proton transfer from phenol to hydrogen-bonded pyridine.