Insights into the excited state of a phenalenyl-based photocatalyst for facile divergent synthesis of sulfoxides and sulfones

Abstract

A divergent and tunable photocatalytic protocol has been established under mild and green conditions by exploiting the excited-state pathways of a phenalenyl-based organic photocatalyst. Femtosecond transient absorption spectroscopy (fs-TAS) provides crucial mechanistic insights via the excited-state behavior, revealing the formation of a catalyst-centered radical anion via substrate-induced electron transfer, and the possible involvement of excited-state intramolecular hydrogen transfer (ESIHT) in governing reactivity. Detailed excited state investigation of the catalyst in the presence of a range of solvents (polar/nonpolar and protic/aprotic) shows, however, that the solvent plays a decisive role in determining the final products arising from the photoinduced transformation. Triplet state sensitization leads to singlet oxygen generation, with the latter reacting with the radical cation generated from the reductive quenching of the photocatalyst, offering precise control over the reaction outcome, enabling selective access to either sulfoxides or sulfones from a diverse array of thioether substrates. The protocol demonstrates broad functional group tolerance and late-stage modulation on drug-like scaffolds, thereby underscoring its synthetic utility. Furthermore, post-functionalization of the resulting sulfoxides allows access to structurally rich frameworks. This study introduces a novel excited-state modulation strategy, expanding the reactivity landscape of organic photocatalysts and providing a foundation for sustainable and switchable transformations in organic synthesis.