Reduction of Manganese(III) Protoporphyrin IX Dimethyl Ester Studied by Electrochemistry and Surface-Enhanced Raman Scattering Spectroscopy

Abstract

The reductive electrochemistry of manganese(III) protoporphyrin IX dimethyl ester, MnIII(PPDME), in an aprotic solvent was studied using cyclic voltammetry, surface-enhanced Raman scattering spectroscopy (SERS), and thin layer potential dependent UV/vis absorption spectroscopy. Good quality SERS spectra are reported for an electrochemically roughened silver electrode in contact with Mn(PPDME) dissolved in acetonitrile. A reversible one-electron reduction was observed at ca. −0.44 V/SCE in the cyclic voltammetric scans on silver, gold, and platinum electrodes. In thin layer UV/vis spectroelectrochemistry, the Soret band at 473 nm at 0.0 V undergoes a 36 nm blue shift to 437 nm when the electrode potential was stepped to −0.5 V. Resonance Raman and SERS spectral frequencies of Mn(PPDME) are assigned and tabulated. The shift of the Soret band along with the downshift in the ν4 oxidation state marker band from 1373 to 1360 cm-1 in the SERS spectra identify the process occurring at ca. −0.5 V to be the reduction of the porphyrin central metal ion from the MnIII to MnII state. Core sizes for MnIII(PPDME) and MnII(PPDME) adsorbed on an anodized silver electrode surface were calculated to be 1.993 and 2.072 Å, respectively. Both the MnII and MnIII complexes are adsorbed as high spin, five-coordinate species. On the basis of relative intensities of the SERS bands, the orientation of Mn(PPDME) adsorbed onto the silver electrode surface is proposed to be face-on.