Figure shows CVs of O2 (4.8 × 10 -3 mol dm -3 ) in the (a) absence and presence of H2Q (b: 4.0, and c: 20.0 × 10 -3 mol dm -3 ) in DMF. The CV of O2 dissolved in DMF in the absence of any acidic (protic) substrates demonstrates a quasireversible (cathodic and anodic currents) redox couple of O2/O2 •-(Equation (1), Supporting Information, Figure ), where O2 is one-electron reduced generating O2 •-in the cathodic scan (cathodic peak, pc1) and reoxidized to O2 in the anodic scan (anodic peak, pa1). Conversely, it is well defined that the presence of normal acidic substrates results in the overall two-electron reduction of O2 forming H2O2, which involves electro-chemical-electro (ECE) reaction and disproportionation of HO2 • (O2 •-). When H2Q is added (b), the CV of O2/O2 •-changes from reversible to irreversible with the disappearance of pa1 and the appearance of some peaks (pc2/pa2, pa3). Since H2Q is not electrolyzed within the potential range (from -0.5 to -2.1 V vs. Fc + , Fc), it is clear that the CV-modified peaks are caused by the products of a homogeneous reaction between electrogenerated O2 •-and H2Q. In a previous study, these electrochemical observations were investigated, and pc2/pa2 (Q •-/Q 2-) and pa3 (Q/Q •-) were assigned to the redox peaks of Q •-formed through 2PCET between electrogenerated O2 •-and H2Q (Equation ( )). Notably, the cathodic peak current (pc1) remained almost monoelectronic when the concentration of the added H2Q was up to 10.0 mol dm -3 . Subsequently, as the concentration of H2Q was further increased (20.0 mol dm -3 ), CV (c) exhibited an irreversible two-electronic curve. Although the CV (c) at an increased concentration of 20.0 mol dm -3 is similar to the reversible CV of Q/H2Q in an aqueous solution (not shown), it contains both O2/O2 •-and Q/H2Q, showing one additive redox curve. In CV modification, pc2 shifts to the positive potential due to the HB effect between Q •-/Q 2-and H2Q, resulting in overall two-electron reduction (pc1 and pc2). Here, pa3 is characteristic of the CV of O2 in the presence of H2Q, but the two-electron reduction (the additive wave of pc1 and pc2) is easily confused with the two-electron reduction from O2 to H2O2 observed under a proton source (Supporting Information, Figure ), or in an aqueous solution. This characteristic CV demonstrates that part of two chemical species, O2 •-and H2Q, play like one redox center toward the electrode. In other words, the electrochemical results include both the formation of a HB complex (network) between O2 •-and H2Q, and its redox response. This suggests that the details of the 2PCET, which involves three elementary steps (Equations ( )-( )), can be elucidated by analyzing the series of electrochemical mechanisms triggered by electrogeneration of O2 •-. CVs were recorded using a GC working electrode with a scan rate of 0.1 V s -1 .