Singlet oxygen quenching by stable nitroxy radicals

Abstract

The quenching mechanism of singlet oxygen luminescence at 1.27 μm by different aliphatic and aromatic nitroxy radicals has been studied. The 1O 2 was generated in solutions of anthracene or protoporphyrin IX by excitation with a 347 nm laser flash. The luminescence quenching rate constants have been found to be about 10 5 – 10 6 M −1 s −1 for aliphatic radicals and about 10 6 – 10 7 M −1 s −1 for aromatic radicals and are weakly dependent upon the nature of the solvent. The maximum value k q = 1.6 × 10 7 M −7 s −1 was obtained for di- tert-butyldiphenyl nitroxide which is characterized by a considerable delocalization of the unpaired electron over the molecular fragment and a lower steric screening of the ▪ centre in comparison with other radicals. From an analysis of the experimental data it has been concluded that 1O 2 quenching by radicals occurs exclusively via the electron exchange interaction in collision complexes.