Triplet Energy Transfer as a Handle to Tune 1,2-Dialkyldiazene Fragmentation in Radical C(sp³)-C(sp²) Cross-Coupling

Mechanistic investigation of light-induced processes is paramount as it not only offers an overall mechanistic picture but also provides information about the efficiency and associated rate constants of the different reaction steps. In some cases, study of systematic series of photosensitizers or quenchers also allows to determine ground-state redox potentials or triplet energy levels of unknown species. Herein, through a combination of steady-state and time-resolved spectroscopic techniques, we elucidate the mechanism of geminate triplet radical pair formation from 1,2-dialkyldiazenes operating via energy transfer from excited photocatalysts. Stern-Volmer and Rehm-Weller analyses confirmed the energy-transfer pathway and provided access to the triplet energy level of two model 1,2-dialkyldiazenes, which were found to be around 2.3 eV. Further evidence was gained by mediator-enhanced triplet energy transfer to an anthracene substrate, showcasing that the excited diazene can serve as an energy shuttle that can be intercepted before bond fragmentation to release N₂ and the corresponding radicals. This activation mechanism confers clear advantages over conventional high-energy UV photofragmentation as triplet sensitization was shown to promote a more efficient solvent-cage escape of the resulting geminate radical pairs relative to direct excitation. Additionally, the structure of the 1,2-dialkyldiazenes was found to profoundly influence the kinetics of fragmentation following energy transfer. These mechanistic insights were leveraged to improve C(sp³)-C(sp²) cross-coupling efficiency with challenging electron-rich aryl bromides by slowing alkyl radical generation through photocatalyst selection to match the rate of Ni oxidative addition, thereby demonstrating the tunability of energy-transfer-based dual catalytic systems.