A modern approach to intermittent illumination for the characterization of chain-propagation in photoredox catalysis

Photoredox catalysis has become an invaluable tool for the construction of organic molecules, allowing for unparalleled control over radical intermediates enabled by the mild conditions achieved through visible-light activation of a photocatalyst. These reactions can be classified under two distinct mechanistic paradigms: closed photocatalytic cycles, and photoinitiated chain reactions. While optimization strategies for each of these classes of reaction differ significantly, organic chemists still lack a straightforward means for probing chain-propagation. In this work, we report a simple and accessible approach to performing intermittent illumination studies for characterizing chain reactions in photoredox catalysis. Using modern LED technologies to precisely control the rate of sample illumination, we were able to validate the presence or absence of product-forming chain reactions in four previously reported photoredox protocols. Furthermore, this technique also allows for the determination of chain-propagating lifetimes through a simple graphical analysis. Given the operational simplicity and ease of accessibility, we believe this intermittent illumination technique will be of great value to practitioners of the field moving forward.