Release of Cyanopyridine from a Ruthenium Complex Adsorbed on Gold: Surface-Enhanced Raman Scattering, Electrochemistry, and Density Functional Theory Analyses Dieric dos S. Abreu, † Te ́ rcio de F. Paulo, †,‡ Rômulo A. Ando, ‡ Ma ́ rcia L. A. Temperini, ‡ Elisete A. Batista, ∥ Elisane Longhinotti, § and Izaura C. N. Dió genes* ,† † Departamento de Química Orgâ nica e Inorgâ nica, Universidade Federal do Ceara ́ , Cx. Postal 6021, Fortaleza, Ceara ́ , Brasil 60455-970 ‡ Instituto de Química, Universidade de Sã o Paulo, Cx. Postal 26077, Sã o Paulo-SP, Brasil 05508-000 § Departamento de Química Analítica e Físico-Química, Universidade Federal do Ceara ́ , Cx. Postal 12200, Fortaleza, Ceara ́ , Brasil 60455-960 ∥ Departamento de Físico-Química, Instituto de Química, Universidade Estadual Paulista, Cx. Postal 355, Araraquara, Sã o Paulo, Brasil 14800-060 * S Supporting Information ABSTRACT: The results presented in this work definitely show that the stability of the SAM formed with [Ru- (NH 3 ) 4 (CNpy)(pyS)] 2+ on gold, where CNpy = 4-cyanopyr- idine 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. ■ INTRODUCTION It is widely accepted that self-assembling of sulfur-containing molecules on gold usually results in ordered arrays of monolayer dimensions (∼10 −11 mol cm −2 ). Because of the typical thickness (1−3 nm) of these so-called self-assembled monolayers (SAMs), they are considered as the most elementary form in the area of thin film materials (nanoscale). In the field of molecular electronics, where the charge transfer is a crucial process and has to be controlled, the insertion of redox compounds in SAMs formed on metallic surfaces represents a great advantage as it can serve as model for studying the fundamentals of electron transfer since the distance to the underlying gold surface, for instance, is known. 1−8 The insertion of redox moieties in SAMs can be achieved by the formation of coordination compounds on surface which, in comparison to the organic counterpart, are likely to form more robust molecular assemblies due to the metal-to-ligand back-donation. This interaction enhances the electron density on the adsorption fragment, thereby strengthening the bonding with the surface. 9−15 Coordination compounds have been used as SAMs on gold to study electron transfer of metalloproteins, coordination reactions on surface, conduction as molecular wires, and so on. 6−19 The SAM formed with [Ru(NH 3 ) 4 (CNpy)(pyS)] 2+ on gold, where CNpy = 4-cyanopyridine and pyS = 4-mercaptopyridine, was first reported by our group in 2006 and was used to assess the heterogeneous electron transfer of the metalloprotein cytochrome c (Cyt c). 16 For this SAM, SERS (surface-enhanced Raman scattering) spectra, which were obtained in air and without applied potential, showed, as expected, that the molecule adsorbs on gold through the sulfur atom of the pyS moiety. By accounting that the ammonia ligands, which are located in the equatorial plane, do not possess orbitals of π symmetry (at least not at appropriate energy), all the back- Received: September 11, 2014 Revised: November 11, 2014 Published: November 12, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 27925 dx.doi.org/10.1021/jp5092152 | J. Phys. Chem. C 2014, 118, 27925−27932