QUENCHING OF SINGLET OXYGEN BY OXYGEN-CENTERED AND SULFUR-CENTERED RADICALS - EVIDENCE FOR ENERGY-TRANSFER TO PEROXYL RADICALS IN SOLUTION

Quenching of singlet oxygen luminescence at 1.27 mu m by PhS., PhSO., and peroxyl radicals phOO(.), t-BuOO., PhCH2OO., Ph2CHOO., and Ph3COO. was studied in liquid solution. The quantum yields of decomposition o f different initiators which lead to the formation of free radicals we re measured by using nanosecond transient absorption. This allowed det ermination of singlet oxygen O-2((1) Delta g) quenching rate constants by the radicals. They are <2 x 10(8) M-1 s(-1) for the sulfur-centere d radicals and (2-7) x 10(9) M-1 s(-1) for peroxyl radicals in acetoni trile. The rapid quenching is attributed to energy transfer quenching by the peroxyls, which have an n --> pi transition leading to a low-l ying (2)A' state above their (2)A '' ground state. PhSO. is shown comp utationally not to have such a low-lying 2A' state. There may be a ver y low-lying B-2(1) state, forPhS(.), but it is apparently not an effic ient acceptor of electronic energy from O-2((1) Delta g).