Manganese is an attractive earth-abundant metal for chromophores because it has multiple oxidation states, making it well-suited for photochemical applications involving electron transfer reactivity. ...

Photoredox catalysis is important in modern organic chemistry and the development of new synthetic methods. Mechanistic insights, particularly with photoredox chain reactions, are underdeveloped. This study combines quantum yield (QY) measurements, transient absorption spectroscopy (TAS), and electrochemical analysis to rigorously characterize the mechanism and rate constants of a ruthenium-catalyzed photoredox chain [4 + 2] cyclization between trans-anethole and isoprene. TAS and steady-state photochemical measurements (QY studies) reveal the role of oxygen in chain propagation as a secondary electron acceptor. The key kinetic competition between chain propagation and termination is identified by TAS and explored with kinetic modeling.