Release of Cyanopyridine from a Ruthenium Complex Adsorbed on Gold: Surface-Enhanced Raman Scattering, Electrochemistry, and Density Functional Theory Analyses

The results presented in this work definitely show that the stability of the SAM formed with [Ru(NH3)(4)(CNpy)(pyS)](2+) on gold, where CNpy = 4-cyanopyridine and pyS = 4-mercaptopyridine, is dependent on the applied potential and on the chemical properties of the solution in the solid/liquid interface. By means of SERS spectroscopy, it was found that CNpy ligand is released from the coordination sphere if no reducing condition is imposed to the system, i.e., citrate solution or applied potential lower than the formal potential of the complex. Theoretical Raman spectra obtained from DFT presented reasonable correlation with the experimental spectra and gave support for the assignments. The relative intensities of the bands in the SERS spectra showed to be dependent on the applied potential as well as on the wavelength of the exciting radiation, indicating the contribution of a charge transfer process to the SERS intensification. In fact, the shift of the potential of maximum SERS intensity (E-max) to negative values as the radiation energy increases indicates a charge transfer process from the HOMO orbitals of the complex to the Fermi level.