Voltage selection of physisorbed or chemisorbed 4-cyanobenzoate on a nanostructured silver electrode and the dual electronic structure of charged metal–molecule hybrids

Applied electric potentials tune SERS wavenumbers due to vibrational Stark effect, but some modes of 4CNB- show two differentiated regions, being redshifted at more negative potentials than the potential of zero charge of the electrode but remain unshifted at positive potentials. DFT calculations have been carried out for a model where 4CNB- is linked through the carboxylate [Agnq-OOC(4CNB-)]q-1 or the nitrile [Agnq-NC(4CNB-)]q-1 to stick-like silver clusters [Agn]q with different densities of charge (qeff =q/n). The comparison between calculated and experimental wavenumber shifts points out that 4CNB- is always adsorbed through the carboxylate. The dual behaviour of the wavenumbers is due to the existence of two types of electronic structure of the metal-molecule hybrid. Physisorbed (P-hybrid, repulsive) or chemisorbed (C-hybrid, attractive) surface complexes are selected by the sign of the surface excess of charge of the electrode. The electronic structure of weakly bonded P-hybrid is very sensitive to the voltage and their wavenumbers are continuously shifted, while the wavenumbers of the strong C-hybrid remain almost unshifted. This result proves the dual nature of the electronic structure of molecules bonded to charged metal electrodes or nanoparticles which can be responsible of the qualitative changes observed in electrochemical or molecular electronics experiments.