QUINONES AND HYDROXYQUINONES AS GENERATORS AND QUENCHERS OF SINGLET MOLECULAR-OXYGEN

The role of quinones and hydroxyquinones as sensitizers and as quenche rs in Type II photooxygenations has been examined. The second aspect i s discussed here, through a systematic study, for the first time in th e 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 cas e of anthraquinone derivatives). Benzoquinones, anthraquinones, and hy droxy derivatives are good O-2((1) Delta(g)) quenchers upon dye-sensit ized photoirradiation. The excited oxygen species is deactivated with rate constants in the range 10(6)-10(7) M-1 s(-1) depending on the sol vent employed. The quenching process deactivates O-2((1) Delta(g)) wit hout further destruction of the quinone. The main interaction with O-2 ((1) Delta(g)) is driven by the quinone moiety, in spite of the presen ce of potentially active nuclear substituents. The quenching mechanism could involve a reversible charge transfer intermediate, with the qui nonic compound acting as an electron donor.