Quantum Yield of Product Formation (ΦRXN). Our proposed mechanism invokes the presence of a dark Ni(I)/Ni(III) cycle (Fig. ) in which light is only needed for initiation through Ni(II) reduction by the excited state PC to generate Ni(I). After initiation, this cycle can continuously generate product without further light input. One way to support this type of dark cycle is through ΦRXN experiments since a ΦRXN >1 can only be obtained if product formation occurs independently of a light promoted cycle. We utilized standard ferrioxalate actinometry to measure ΦRXN (See SI, Section 7.1 for further discussion) as a function of light intensity. At the lowest intensity of I0 = 5.7 * 10 -6 E L -1 s -1 , we obtained ΦRXN = 3.1, further supporting our mechanistic hypothesis and ruling out any cycle in which each product formation requires the input of a photon. The fact that ΦRXN is flux-dependent is consistent with a two-cycle mechanism as detailed above in which the PC cycle can operate with a turnover frequency distinct from that of the Ni cycle. Increasing the flux increases the turnover frequency of the PC cycle through increasing the concentration of PC*. Since the total Ni concentration has a fixed maximum, there is a light intensity threshold at which ΦPET begins to decrease with a corresponding increase in Φ for other PC* decay pathways (e.g. internal conversion and radiative decay). At high fluxes beyond this threshold value, ΦRXN decreases below 1 due to the increased prevalence of these PC* decay pathways which do not form product. Broadly, this mechanism has the consequence that the lowest flux at which the rate of product formation is practical for the process will lead to the highest quantum efficiency. Given the mechanistic findings here and in previous sections, we next sought to improve the photocatalytic coupling performance.