Unraveling the mechanistic origins of efficiency and stereoselectivity in EDA-mediated photocatalysis.

Abstract

Visible-light-driven electron-donor-acceptor (EDA) photocatalysis holds transformative potential for asymmetric synthesis, yet its efficiency and selectivity are dictated by excited-state dynamics that have, until now, eluded direct observation. Here, we utilize femtosecond and nanosecond transient absorption spectroscopy to investigate a prototypical EDA system: asymmetric α-alkylation of aldehydes. Our real-time kinetic mapping successfully resolves the singlet charge-transfer state (¹CT), triplet charge-transfer state (³CT), and donor-acceptor radical pairs (D^•+ and A^•). We uncover two concurrent radical-generation pathways: direct decay of ¹CT and a ³CT-mediated route, which maximize reaction efficiency. Rapid Br^- departure from A^•- furnishes A^•, suppresses unproductive charge recombination with D^•+, while solvent-cage confinement preserves radical orientation, promoting in-cage coupling with high enantioselectivity. These findings establish a general mechanistic blueprint where ground-state pre-organization and excited-state dynamics are synergistically harnessed, offering a distinct paradigm to overcome diffusion-controlled limits for the rational design of advanced photocatalytic systems.