Photoredox catalysis offers powerful new synthetic methods at the cost of technically challenging mechanistic validation, typically requiring laser-based transient absorption methods with microsecond time resolution. While great progress has been made in investigating and understanding photoredox mechanisms with long-lived photosensitizer excited states and their nanosecond to millisecond timescale interactions with the substrate in solution, the development of even more redox active radical photoredox catalysts presents a new challenge to studying their catalytic mechanisms due to their inherent instability and unknown photophysical properties. Since many of the recently proposed radical photoredox catalysts are generated in situ either photochemically or electrochemically, clear assignment of the catalytically active photosensitizer has been typically assumed by invoking one electron oxidation or reduction of a neutral radical precursor. We evaluate the mechanistic hypotheses of radical photoredox catalysis by bulk electrolyzing neutral forms of various radical photosensitizers and subsequently measuring their excited state lifetimes using ultrafast transient absorption spectroscopy. There was no radical photosensitizer measured that had an excited-state lifetime capable of bimolecular photoinduced electron transfer as originally proposed, including in cases where substrate-radical photosensitizer precomplexation was used to justify excited-state radical reactivity. We attempt to explain the seemingly pervasive observation of short (&lt;100 ps) excited state lifetimes of stable radical cations and anions by measuring a series of acene radical cations of increasing core size, with an explicit aim to not rely on possibly misleading electronic structure calculations of systems with doublet spin multiplicity. We contextualize these results with current literature hypotheses on the origin of short excited-state lifetimes of radical excited states, which range from invoking conical intersections between the excited and ground state potential energy surfaces to molecular plasmonicity. This work highly motivates the discovery of the closed-shell super-reducing or super-oxidizing photoreagents that likely give rise to the extreme reactivity originally proposed to originate from radical excited states and also inspires new exploratory photophysical investigations to illuminate the striking ultrafast dynamics in radical excited states.

Photophysics of radical photosensitizers: Ultrafast internal conversion precludes excited-state bimolecular reactivity

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.