Singlet and triplet state quenching of aromatic cations by molecular oxygen

Rate constants of fluorescence quenching by molecular oxygen, k(q)(S), of 4 acridizinium ions and of 7 protonated heterocyclic compounds as well as protonated 6-aminochrysene have been determined in acetonitrile at room temperature. They adopt values in the range between 3.0x10(8) M-1 s(-1) and 2.9 x 10(10) M-1 s(-1). Singlet oxygen (O-1(2)) efficiencies for the oxygen quenching of the singlet state, phi(Delta)(S), and the tripler state, phi(Delta)(T), could be determined for most of the compounds. The results indicate that fluorescence quenching can be described here as competition between energy transfer and a process involving the second excited triplet stale. k(q)(S) shows at first approximation a correlation with the energy gap between the singlet state and the second excited triplet stale. Quenching of the first excited triplet state by oxygen is ruled mainly by energy transfer. Rate constants, k(q)(T), could be determined for 5 compounds and vary between 6.0 x 10(8) M-1 s(-1) and 1.1x10(9) M-1 s(-1).