Femtosecond-to-Second Time-Resolved Spectroscopy Brings Unparalleled Insight Into the Life Cycle of the Versatile Manganese Photocatalyst [Mn2(CO)10]

Visible-light-activated radical photoinitiators are pivotal in the efficient construction of complex molecular architectures and the precision synthesis of advanced polymeric materials. At the heart of their function lies the formation of reactive radical species that drive selective homolytic bond cleavage, but elucidating the fundamental mechanisms of these processes is notoriously difficult due to the fleeting nature of key intermediates. In this study, time-resolved infrared (TRIR) spectroscopy provides a powerful window into the complete reaction profile of the versatile photocatalyst [Mn2(CO)10], including the observation of [Mn(O2)(CO)5], which is a long-lived (ms) reservoir of the reactive 17-electron complex [Mn(CO)5]. We give unprecedented structural and mechanistic insights concerning the formation of [Mn(CO)5] in electronically and vibrationally excited states (fs–ps), its quenching by O2 (ns), regeneration of the ground-state catalyst, and C–I bond activation (ms). New avenues for the rational design of next-generation metal–metal photocatalysts are provided, as [Mn(O2)(CO)5] significantly extends the catalyst longevity.