Elucidating the Mechanism of Photoinduced Sulfur Monoxide Extrusion from Dianthryl Sulfoxide

Light-induced bond cleavage reactions are powerful tools for constructing complex molecular structures under mild conditions. Among these, extrusion processes enable skeletal reorganization through the elimination of small fragments, granting access to otherwise synthetically challenging frameworks. Photoextrusion that results in the formation of carbon-carbon bonds is particularly intriguing due to the possibility of broad applicability in synthetic chemistry. 9,9'-Dianthrylsulfoxide (9-AnSO) undergoes photoinduced sulfur monoxide (SO) extrusion, resulting in C-C bond formation between the two anthracene groups. This transformation could proceed via a concerted SO elimination or stepwise radical pathway, however the excited-state dynamics remain unclear. SO-trapping and spin-trapping experiments on 9-AnSO, supported by EPR and mass spectrometry, confirm the formation of sulfinyl radicals and rule out pathways involving sulfur extrusion as S or SO2. Triplet sensitization experiments and DFT calculations support a triplet-mediated pathway, consistent with the reduced reactivity observed in the presence of oxygen. In contrast, 1,1'- and 2,2'-bridged isomers show no photochemical reactivity, demonstrating strong positional dependence of the C-S homolysis reaction. These findings provide direct evidence for a stepwise radical mechanism for SO extrusion, establish the structural features controlling photochemical C-S bond cleavage in dianthryl sulfoxides, and offer design principles for new light-responsive molecules.