Rate of Electron Transfer from PC* to Ni-OMebpy Complexes. Bimolecular rate constants associated with the photoreduction of Ni(II) to Ni(I) by both Ir(ppy)3 and the PhenO PCs (Fig. ) were determined by conducting a Stern-Volmer quenching study and monitoring lifetime shortening via ns-TA. Rate constants of 1.2 * 10 9 and 6.6 * 10 8 M -1 s -1 were obtained for the triplet excited states of the Ir and PhenO PCs, respectively. Despite the relatively high triplet quantum yield of 0.91 for PhenO, significant quenching of the PhenO singlet excited state was detectable via time-correlated single photon counting (TCSPC) at total Ni concentrations relevant to catalysis (i.e. 0.2-14.5 mM), giving a rate constant of 3.4 * 10 9 M -1 s -1 (Fig. ). Recent collaborative work by several of the authors suggests that singlet states are only expected to be significant contributors to catalysis when the quencher is present in sufficiently high concentration. Given the known triplet quantum yield of PhenO, we calculated the quantum yield for photoinduced electron transfer, ΦPET, occurring from the singlet ( 1 PhenO) or triplet ( 3 PhenO) excited states and from MLCT for Ir(ppy)3 (Fig. ). At high Ni loadings (e.g. 40 mM Ni), the ΦPET calculated for Ir(ppy)3 is 0.99 (all from the 3 MLCT), while for PhenO ΦPET = 0.95 (0.46 from 1 PhenO and 0.49 from 3 PhenO). Thus, it is expected that Ir(ppy)3 may exhibit slightly better performance at high Ni loadings. The results are consistent with (but may not be the sole contributing factor to) the higher rate of product formation when Ir(ppy)3 is used as opposed to PhenO under the standard Ni loading of 20 mM (See SI for details, Section 6.2). Importantly, PhenO state quenching as observed here, despite the relatively few studies elsewhere in dual Ni/photoredox catalysis, should be considered more commonly as a relevant mechanistic pathway when experimental conditions permit (i.e. when organic PCs are used with high quencher concentrations).