Spectroelectrochemical investigation of the pH dependent aggregation state of iron TAP

Electrochemical and surface spectroscopic techniques (surface-enhanced Raman spectroscopy, cyclic voltammetry, thin-layer spectroelectrochemistry, and UV–visible absorption spectroscopy) were utilized to investigate the pH dependent aggregation state and reductive electrochemistry of iron(III) tetrakis(N,N,N-trimethyl-4-aniliniumyl) porphyrin (FeTAP). UV–visible absorption spectroscopy indicates that at pH 3 FeTAP exists as the monomer, and as the dimer at pH 11. Cyclic voltammetry of FeTAP on a Ag electrode showed a reductive wave at −1.0 V at pH 3, and reversible waves at −0.40 V, and −0.70 V at pH 11. Potential-dependent shifts in the Soret band from 398 to 424 nm (pH 3) and 406 nm to 442 nm (pH 11), both on a gold minigrid electrode, is evidence of a reduction of the porphyrin central metal from the FeIII to FeII state. Potential dependent shifts in the wavenumber of the oxidation state marker band in the SERS spectra provided further confirmation of the oxidation state of the adsorbed FeTAP and its reduction products. The observation of the symmetric Fe–O–Fe stretching vibration at 420 cm−1 in the SERS spectrum of FeTAP at pH 11 indicates that the molecule absorbs onto the electrode surface as the μ-oxo dimer.