The reductive and electro -catalytic properties of two water -soluble cobalt porphyrins investigated by surface -enhanced Raman spectroscopy and electrochemistry

Electrochemical and surface spectroscopic techniques are utilized to investigate the reductive electrochemistry of two water soluble cobalt porphyrins, cobalt(III)-tetra(4-sulfonatophenyl) porphyrin, CoTSPP, and cobalt(III)-tetra(4-trimethylaniliniumphenyl) porphyrin Co(III)TAPP. The electrochemistry of both porphyrins, at Ag, Au, and Pt working electrodes, Surface-enhanced-Raman scattering spectra and UV/vis absorption spectra of the two redox forms of CoTSPP and CoTAPP are also reported. Voltammetric studies indicate two reductive processes at −0.35 and −1.02 V, vs SCE for CoTSPP, and a single reductive peak at −0.80 V for CoTAPP. A non Faradaic electrochemical wave was observed at −0.68 V in the CV of CoTSPP and this process was established to be an adsorption process.;The reductive intermediates generated at −0.35 and −1.02 V were characterized by potential dependent UV/visible spectroscopy using an optically transparent thin layer electrode. For CoTSPP an intense Soret band was observed at 425 nm when the electrode potential was held at 0.0 V. At −0.3 V the 425 nm Soret band disappeared, replaced by a new band at 412 nm. This 412 nm band persisted in the spectra at potentials more positive than −1.10 V. When the electrode potential was stepped to −1.10 V two bands were observed in the potential dependent UV/vis spectrum; the 412 nm band and a new band at 358 nm. For CoTAPP the Soret band was observed at 427 nm at 0.0 V and shifted to 412 nm when the electrode potential was stepped to −0.8 V. At −1.20 V a split Soret band was observed with wavelengths identical to those observed for CoTSPP. The Soret band at 425 nm has been established to be characteristic of Co porphyrins with the central metal in the 3+ oxidation state. The 412 nm band is indicative of the Co(II) species and the split Soret band at 412 and 358 nm, characteristic of the Co(I) porphyrin. The potential dependent changes are therefore suggestive of a Co(III) to Co(II) transition occurring at −0.35 V and −0.8 V for CoTSPP and CoTAPP respectively, while the transition occurring at −1.10 and −1.20 V for CoTSPP and CoTAPP indicate the Co(II) to Co(I) transition.;Potential dependent shifts in the core sensitive, oxidation state marker band ν4 were also observed for CoTSPP. The ν4 vibration was assigned to the bands at 1374 and 1343 cm−1 in the SERS spectra of CoTSPP and CoTAPP at 0.0 V. For CoTSPP, ν4 shifted from 1374 at 0.0 V to 1364 cm−1 at −0.4 V. This shift has been attributed to a change in the oxidation state of the central metal from Co(III) to Co(II). It was not possible to observe the Co(II) to Co(I) transition for CoTSPP form the potential dependent SERS data.;The catalytic activity of CoTSPP and CoTAPP towards the reduction of cystine and dithiodipropionic acid was also investigated using cyclic voltammetry. Both CoTSPP and CoTAPP catalytically reduced DPA and RSSR at −0.77 and −0.83 V, respectively, on a silver working electrode, in acidic aqueous environments. No catalytic activity was observed when the solution pH was changed from pH 3.0 to pH 10.0.;High Performance Liquid Chromatography and Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry were used to purify the porphyrins and obtain their exact molecular weights.