Quinones and hydroxyquinones as generators and quenchers of singlet molecular oxygen

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) Delta(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) Delta(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) Delta(g)) without further destruction of the quinone. The main interaction with O-2((1) Delta(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.