Ligand-aided photoreduction of iron-porphyrin complexes probed by resonance Raman spectroscopy

Abstract

Photoreduction has been observed for the first time for an iron porphyrin with a biologically relevant axial ligand by using resonance Raman (RR) spectroscopy (for Fe/sup III/(OEP)(2-MeIm), OEP = octaethylporphyrin and 2-MeIm = 2-methylimidazole). The action spectrum for the photoreduction obtained by visible absorption spectra exhibited a broad maximum around 420-460 nm, which was appreciably shifted from the Soret band of Fe/sup III/(OEP)(2-MeIm) at 395 nm. Similar photoreduction was observed for Fe/sup III/(OEP)(1,2-Me/sub 2/Im) (1,2-Me/sub 2/Im = 1,2-dimethylimidazole) but not for Fe/sup III/(OEP)L/sub 2/ (L = imidazole and 1-methylimidazole) and Fe/sup III/(OEP)X (X = F, Cl, Br, I, and ClO/sub 4/). The coincidence of the RR spectrum of the photoreduced species with that of the ferrous porphyrin rules out the possibility of ring reduction to a porphyrin anion radical or chlorin. The dependence of the photoreduction on the concentration of 2-MeIm suggested that the ligand-free Fe/sup II/(OEP) is a likely intermediate and thus that the light-induced charge transfer from the axial ligand to the iron ion is the primary process of photoreduction.