Embracing the Dark Side of Photoredox Catalysis─Investigating Charge Recombination across the Time Domain

Despite the rapidly expanding synthetic scope of photoredox catalysis, the scale-up of efficient reactions is limited by insufficient mechanistic understanding. The initial steps of the catalytic cycle, photon absorption and excited state quenching, are routinely interrogated and easily optimized. Subsequent steps that occur independent of irradiation, cage escape of the charge-separated encounter complex, and electron transfer to the substrate are equally important for high yields. Charge recombination reduces the efficiency of these steps and is often thermodynamically favored, but monitoring the short-lived redox intermediates involved is difficult, and these "dark" reaction steps are often assumed to occur with high efficiency. Herein, we interrogate charge recombination across the time domain of photoredox reactions by comparing two techniques for monitoring critical catalytic intermediates: nanosecond transient absorbance spectroscopy and in situ steady state spectroscopy. The respective strengths of each technique will help researchers uncover mechanistic insight into a larger range of synthetic transformations. We additionally evaluate the efficiency of dark electron transfer using nonspectroscopic techniques and our results reinforce that redox intermediates, deriving from photocatalysts or additives, strongly influence photoredox activity.