Surface-Enhanced Raman Spectroscopic (SERS and FT-SERS) Investigation of the Complex Ion [Fe2(CN)10L]6- (L = 4,4'-Bipyridine and Pyrazine) Adsorbed on Silver and Gold Electrodes

Abstract

The spectroelectrochemical behavior of the binuclear complex [Fe2(CN)l&]6-, where L is the bridging ligands 4,4’-bipyri&ne (bipy) or pyrazine (pz), has been investigated by surface-enhanced Raman spectroscopy, in the visible and in the near-infrared, on silver and gold electrodes. The vibrational frequencies of bipy were the same whether I1 or I11 was the oxidation state of the metal ions. However, shifts to higher energies of the VCN bands for both complexes have been observed as the oxidation states of the metal ions changed from I1 to 111. The SERS frequencies were also higher than the values observed in the normal Raman spectra of the complexes in solution. For potentials more negative than - 1.2 V, the ligands were reduced and the SER and m-SER spectra presented marked changes with the appearance of new bands. In the case where bipy was the bridge ligand, the results have shown that vibrational modes involving the bridge ligand and the CN or Fe-CN bonds were enhanced while for pz only the latter vibrational modes were observed. This unlike behavior was assigned to differences in the structure of the ligand in relation to metal surface. The dependence of vCN frequency on the applied potential has shown that this mode is enhanced by a charge transfer mechanism, ligand (CN) to metal, while the dependence of the SERS intensity on the applied potential and laser excitation energy has shown that a charge transfer mechanism, metal to a n* orbital of bipy ligand, is responsible for the enhancement of the bipy vibrational modes. Through the use of the data obtained from the plot of Vmax (potential of maximum SERS intensity) vs laser energy, an energy diagram for the adsorbed [Fe2(CN)l~bipy]6complex is proposed.