Photochemistry of Sulfur Dioxide Mediated by Charge Transfer through Intermolecular Chalcogen Bonds

The photochemistry of sulfur dioxide (SO₂) plays a crucial role in the Earth's atmosphere. In contrast to the well-established photochemistry of free SO₂, the electronic properties and reactivity of SO₂ complexes with other atmospherically relevant gas molecules are barely known. Herein, we demonstrate that the photochemistry of SO₂ can be dramatically influenced by the first formation of charge-transfer (CT) prereaction complexes through intermolecular chalcogen bonds. Specifically, the prototype sulfur-sulfur bonded complex between SO₂ and hydrogen sulfide (H₂S) displays a strong CT absorption at 290 nm, and photoexcitation of the complex at this CT band furnishes the elusive thiosulfurous acid (HSS(O)OH), which is a missing intermediate in the Claus sulfur recovery process. Similar charge-transfer mediated photochemistry is also applicable to the chalcogen-bonded complexes of SO₂ with the atmospherically important volatile organic sulfur compounds dimethylsulfide and methanethiol, from which sulfinic acids form efficiently through photolytic SO₂-insertion along with concerted hydrogen atom transfer. This work highlights the pivotal role of intermolecular chalcogen bonds influencing the photochemistry of SO₂ due to the formation of photolabile charge-transfer complexes as prereaction intermediates.