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// = 2.186; g⊥ = 2.055; A// = 183 Gauss; A⊥ = 8 Gauss and A’Cu-N ∼ 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 (O2−), produced by the interaction of O2 with the conduction band electrons (e−CB), generated by photosensitization (λ > 420 nm) of TiO2 with the porphyrin dye. The TcPPM/TiO2 were more efficient (generation of O2−) 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.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (73 K)Download as PowerPoint slideHighlights► EPR signals are produce by unpaired electrons on oxygen and porphyrin π-system. ► TcPPM anchored on TiO2 show 300 times less intense signal than the free porphyrins. ► Free TcPPZn and TcPPH show highest photoactivity. ► TcPPCu on TiO showed the highest photoactivity.