Unveiling Anomalous Surface-Enhanced Resonance Raman Scattering on an Oxo-Triruthenium Acetate Cluster Complex by a Theoretical-Experimental Approach

Surface-enhanced Raman spectroscopy (SERS) has been explored by the most diverse areas of research over the past 40 years, with more special attention from analytical chemistry. Although there is a great consensus that there are two mechanisms that contribute most to the observation of the phenomenon (electromagnetic (EM) and charge transfer (CT)), several anomalies are observed, especially when studying complexes of transition metals. In this article, we present a theoretical and experimental study of Raman and SERS of an oxo-triruthenium acetate cluster ([Ru3O (CH3COO)(6)(py)(2)(pytpy)]), where it was possible to observe one of these "anomalies" of the SERS effect, in this case, an unexpected enhancement of the vibrational stretching mode of the oxo group. This cluster had its electrochemical and spectroelectrochemical properties studied by cyclic voltammetry and electronic spectroscopy in the visible and near-infrared, where it was possible to observe the fully reversible waves of the ruthenium sites (separated by 1.00 V) and the terpyridinic ligand and how the absorption of this complex varies drastically when reduced electrochemically. The molecular orbitals and electronic transitions of this cluster were calculated by time-dependent density-functional theory (TDDFT), where it can be noted that the HOMO orbitals are almost exclusively composed of the atomic orbitals of ruthenium and the LUMO orbitals can be composed of a mixture of ruthenium orbitals and acetate ligands, as well as terpyridine. From the TDDFT calculations and the transitions energies obtained, the complex resonant Raman and Raman spectra were simulated using different excitation wavelengths (696, 785, and 1064 nm), where an unexpected intensification of oxygen stretching at the Ru3O center at 700 cm(-1) was observed considering the excitation at 785 nm, even though there was no expected transition. The experimental SERS spectra of the complex in two redox states (Ru-3(III, III, III) and Ru-3(III, III, II)) were obtained experimentally using gold and silver nanoparticles with an excitation wavelength laser at 785 nm, where a very intense signal at 700 cm(-1), for the Ru-3(III, III, II) redox state, was observed. The simulation of electronic states by TDDFT and SERS spectra by DFT, considering an Au-20 cluster adduct, showed an intensification of the entire complex, with great emphasis on the Ru3O center and the 700 cm(-1) mode that, however, was not as intense as the results obtained experimentally, indicating an anomalous intensification.