Electrochemical measurement was conducted in a three-electrode system comprising working electrode: a 1.0-mm-diameter 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 for calibration of the reference electrode. The working electrode was polished with alumina paste, rinsed with deionized water and acetone, and air-dried before the experiments. Data collection was conducted using an ECstat-301 electrochemical analyzer (EC-frontier Co., Ltd., Kyoto, Japan) at 25°C (Supporting Information, Table ). ESR spectra were obtained using a JES-X320 X-band spectrometer (JEOL Ltd., Tokyo, Japan). Controlled-potential electrolysis was performed in an ESR cell with a Pt working electrode (straight Pt wire with 0.5-mm-diameter sealed in a glass capillary) at 20°C (Supporting Information, Figure ). Samples were prepared in a glovebox filled with dried N2 gas to prevent moisture contamination of samples. Weakly basic DMF was used as the solvent for the aprotic electrochemistry of O2/O2 •-to avoid free proton-derived electrode noise.