Visible light superoxide radical anion generation by tetra(4-carboxyphenyl)porphyrin/TiO2: EPR characterization

The generation of superoxide radical anion O-2(center dot-) from tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbed on TiO2 in DMSO and irradiated by visible light was studied using EPR spectroscopy and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin trap. A chemical filter was used to remove light with wave lengths <500 nm. A multiline EPR spectrum was observed, characteristic of a mixture of two adducts, the first corresponding to DMPO-O-2(center dot-) and the second to the so-called nitroxide-like radical Hyperfine coupling constants determined for the DMPO-O-2(center dot-) adduct are a(N) = 14 1 G, a(H)(beta) =10 8 G and a(H)(gamma) = 1 4G, and for the nitroxide-like radical adduct a(N) = 14G An increased intensity of the EPR lines corresponding to the nitroxide-like radical adduct was observed under irradiation without chemical filter, which suggests a possible DMPO-O-2(center dot-) decomposition. No singlet oxygen could be detected by EPR spectroscopy using 2,2,6,6-tetramethyl-4-piperidone (TEMP) as spin trap and by chemical trapping using anthracene as the trap (C) 2010 Elsevier B.V All rights reserved