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The role of quinones and hydroxyquinones as sensitizers and as quenchers in Type II photooxygenations has been examined. The second aspect is discussed here, through a systematic study, for the first time in the open literature. Quinonic compounds are excellent generators of O 2 ( 1 Δ g ) in aprotic solvents (excluding those quinones possessing substituents in positions adjacent to the carbonyl groups, in the case of anthraquinone derivatives). Benzoquinones, anthraquinones, and hydroxy derivatives are good O 2 ( 1 Δ g ) quenchers upon dye-sensitized photoirradiation. The excited oxygen species is deactivated with rate constants in the range 10 6 –10 7  M −1  s −1  depending on the solvent employed. The quenching process deactivates O 2 ( 1 Δ g ) without further destruction of the quinone. The main interaction with O 2 ( 1 Δ g ) is driven by the quinone moiety, in spite of the presence of potentially active nuclear substituents. The quenching mechanism could involve a reversible charge transfer intermediate, with the quinonic compound acting as an electron donor. Keywords: photooxidation, quenching, quinones, rose bengal, singlet oxygen.

Quinones and hydroxyquinones as generators and quenchers of singlet molecular oxygen