Surface-Enhanced Resonance Raman Scattering from Water-Soluble Porphyrins Adsorbed on a Silver Electrode

Resonance Raman (RR) and surface-enhanced resonance Raman (SERR) spectroscopy have been used to study two water-soluble porphyrins, tetrasodium meso-tetrakis(4-sulfonatophenyl)porphine (TSPP) and meso-tetrakis(4-carboxy-phenyl)porphine (TCPP). Both compounds were found to adsorb spontaneously at a silver electrode and exhibit intense SERR spectra. The combination of the surface-enhanced and resonance effects allows the observation of high-quality spectra of the adsorbed porphyrins using bulk solution concentrations of only 10⁻⁶ M. Evidence which supports a surface-enhancement process includes the increase in signal intensity by a factor of approximately 10⁵ above that observed under resonance conditions alone, depolarization ratios greater than 0.5 for all of the Raman vibrations, and the dependence of signal intensity on the anodization step and the static electrode potential during data acquisition. An analysis of the vibrational spectra of the adsorbed porphyrins shows that both undergo partial metal incorporation during the electrode anodization step. However, the two exhibit marked differences in photostability and adsorption behavior. The differences are discussed in terms of the possible effects of the sulfonate and carboxylate groups on the porphyrin-electrode interactions.