Molecular photocatalysis has shown tremendous success in sustainable energy and chemical synthesis. However, visualizing the transient open-shell intermediates in photocatalysis is a significant and long-standing challenge. By employing our recently developed innovative time-resolved electron paramagnetic resonance technique, we directly observed all radicals and radical ions involved in the photocatalytic addition of pempidine to -butyl acrylate. The full picture of the photocatalytic cycle is vividly illustrated by the fine structures, chemical kinetics, and dynamic spin polarization of all open-shell intermediates directly observed in this prototypical system. Given the universality of this methodology, we believe it greatly empowers the research paradigm of direct observation in both photocatalysis and radical chemistry.
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