Tracking Photocatalytic Sulfide Oxidation Reaction with Dioxygen Inside Water-Soluble Nanocages

Spectroscopic identification of on-pathway reaction intermediates during the photocatalytic oxidation of organic sulfides with O 2 has been a challenge, due to the transient nature of the reactive intermediates. Here, using a combination of femtosecond transient absorption spectroscopy along with low-temperature electron paramagnetic resonance spectroscopy, we identified the formation of an aromatic sulfide radical cation intermediate and a putative aromatic sulfide radical cation–O 2 adduct formed inside a water-soluble supramolecular nanocavity. Our work demonstrates that cage-confined sulfide radical cation intermediates can lead to chemoselective sulfoxide formation driven by host–guest charge-transfer (CT) chemistry.