Visible light production of superoxide anion with MCarboxyphenylporphyrins (M = H, Fe, Co, Ni, Cu, and Zn) free and anchored on TiO2: EPR characterization

Metal carboxyphenylporphyrins: TcPPM, M = H, Fe. Co, Ni, Cu, and Zn, free and anchored on TiO2 were synthesized and characterized. The EPR spectra of TcPPH, TcPPCo, TcPPNi and TcPPZn exhibited only one line attributed to free radicals. For TcPPCu, the spin Hamiltonian parameters were accurately determined: g(parallel to) = 2.186; g(perpendicular to) = 2.055; All = 183 Gauss: A(perpendicular to) = 8 Gauss and ACu-N similar to 16 Gauss. A higher level protocol, UB3LYP/6-31G(df), was applied in order to obtain accurate values of the isotropic hyperfine coupling constants (hfccs), it showed excellent agreement with experimental results. Degradation of luminol, with and without radical scavengers (SOD, Mannitol and NaN3) indicated the presence of the superoxide anion (O-2(center dot-)), produced by the interaction of O-2 with the conduction band electrons (e(-) (CB)), generated by photosensitization (lambda > 420 nm) of TiO2 with the porphyrin dye. The TcPPM/TiO2 were more efficient (generation of O-2(center dot-)) than the TcPPM in solution as evidenced by the degradation of luminol under visible light perhaps due to site isolation preventing the formation of photo-inactive dimers. (C) 2011 Elsevier B.V. All rights reserved.