Contribution of the charge transfer mechanism to the surface-enhanced Raman scattering of the binuclear ion complex [Fe-2((Bpe)(CN))(10)](6-) adsorbed on a silver electrode in different solvents

A SERS (surface-enhanced Raman scattering) study of the binuclear ion compl ex [Fe2BPE(CN)(10)](6-) (BPE = trans-1,2-bis (4-pyridyl) ethylene) adsorbed on a silver electrode in different solvents is presented. The cycle voltam mogram of the complex, in the region of the Fe-II/Fe-III redox process, sho ws two oxidation and two reduction waves separated by 0.15 V, indicating th at the two iron centers are electronically coupled via the bridging ligand. The SERS measurements have shown that both SERS intensity and frequency po sition of the bridging ligand modes present strong dependence on the applie d potential. Remarkable changes in the structure of the ligand are observed for applied potentials more negative than -1.0 V, where the complex is red uced. The C=C ethylenic inter ring stretching mode shifts from 1637 to 1555 cm(-1), indicating a decrease in this bond order for the reduced molecule. The chemical interaction of the complex with the silver surface also invol ves one or more CN ligands as evidenced by an upward frequency shift of the CN stretching mode in the adsorbed complex. Upon reduction, the V(CN) freq uency shifts to lower energies, indicating that the electron transferred in the faradaic process is delocalized over the complex. On the basis of the SERS excitation profiles and their dependence on the exciting radiation, tw o potential modulated photon assisted charge-transfer processes have been c haracterized: an adsorbate to metal (HOMO(CN) --> Ag) and a metal to adsorb ate (Ag --> LUMO(BPE)), responsible for the enhancement of the v(CN) and BP E modes, respectively. Resonance between the energy of the exciting radiati on and the metal/adsorbate charge-transfer transitions is achieved at diffe rent applied potentials for different solvents, thus indicating that the po sition of the energy levels of the adsorbed complex relative to the Fermi l evel (EF) changes according to the chemical nature of the solvent and the s olvent/adsorbate interaction. Energy diagrams showing the relative position s of the donor and acceptor states of the surface complex formed by the bin uclear complex and the silver electrode surface in different solvents have also been proposed.