The redox properties of a molecule can be described in two ways: as a proton-electron transfer (PET; red arrows in Figure ), or as a decoupled sequence of electron transfer (ET; blue arrows in Figure ) and proton transfer (PT; green arrows in Figure ). In this study, a molecule QH 2 (Q) is oxidized (reduced) to Q (QH 2 ) by releasing (accepting) two electrons and two protons as follows: