Electrochemical measurements were conducted in a threeelectrode system comprising working electrode: a 1.0-mmdiameter planar glassy carbon (GC), counter electrode: a coiled platinum (Pt, 99.99 %), and reference electrode: a silver/silver nitrate (Ag/AgNO3) containing tetrabutylammonium perchlorate (0.1 mol dm -3 ) in an acetonitrile solution and AgNO3 (0.01 mol dm -3 ). A ferrocenium ion/ferrocene couple (Fc + /Fc) was used to calibrate the reference electrode. The working electrode was polished with alumina paste, rinsed with deionized water and acetone, and air-dried before the experiments. Data were collected using an ECstat-301 electrochemical analyzer (ECfrontier Co., Ltd., Kyoto, Japan) at 25°C. Samples were prepared in a glovebox filled with dried N2 gas to prevent contamination by moisture. DMF was used as the solvent for the aprotic electrochemistry of O2/O2 •-to avoid free proton-derived electrode noise. DMF solutions containing TPAP (0.1 mol dm -3 ) were saturated with O2 by air-bubbling the gas for ca. 2-3 min. During the electrochemical measurements, O2 gas was passed over the solutions to maintain a constant concentration (4.8 × 10 -3 mol dm -3 ).