A SPECTROELECTROCHEMICAL INVESTIGATION OF THE REDUCTION OF THE WATER-SOLUBLE MNTSPP - SERS SPECTRA OF MN(II)TSPP

Electrochemical and surface spectroscopic techniques are used to inves tigate the reductive electrochemistry of the water-soluble manganese t etrasulfonatophenyl porphyrin. The electrocehmistry of MnTSPP at a Ag, Au, and Pt electrode and the normal Raman and surface-enhanced Raman scattering spectra of the two redox forms of MnTSPP were studied. Volt ammetric studies indicate two reductive peaks at -0.45 and -0.85 V on an Ag electrode. Potential-dependent shifts in the 466 nm UV/vis absor bance band to 432 nm at -0.5 V on a gold minigrid electrode indicate t hat reduction of the porphyrin central metal from Mn-III to Mn-II occu rs at this potential. A 32 cm(-1) downshift in the nu(4) vibration fro m 1368 to 1336 cm(-1) at -0.5 V is further evidence for the reduction of the metal from the Mn-III to the Mn-II state. The reduction of the porphyrin macrocyclic ring to the radical anion is also observed at -0 .8 V. The core-sensitive modes of aquo Mn-III TSPP are in excellent ag reement with that of a 6-coordinate species. Upon reduction to the (Mn TSPP)-T-II, the core size marker frequencies shift to those of a 5-coo rdinate species. Finally, Raman band frequencies for the solution and SERS spectra are systematically assigned, and normal Raman and surface -enhanced Raman spectra are presented for (MnTSPP)-T-III and its reduc ed (MnTSPP)-T-II.