Visible light superoxide radical anion generation by tetra(4-carboxyphenyl)porphyrin/TiO 2: EPR characterization

Abstract

The generation of superoxide radical anion O 2 − from tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbed on TiO 2 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 − and the second to the so-called nitroxide-like radical. Hyperfine coupling constants determined for the DMPO–O 2 − adduct are: a N = 14.1 G, a H β = 10.8 G and a H γ = 1.4 G, and for the nitroxide-like radical adduct a N = 14 G. 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 − 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.