Huge Energy Gain in Metal-to-Molecule Charge Transfer Processes: A Combined Effect of an Electrical Capacitive Enhancement in Nanometer-Size Hot Spots and the Electronic Structure of the Surface Complex

A comprehensive analysis of the effectiveness of the electrode potential (<i>E</i>_V) in tuning the energies of the electronic states (<i>E</i>) of hybrid systems composed by molecules (A) bonded to discrete metal clusters (M) is carried out for the first time by combining experimental SERS data with TDDFT calculations on [M_<i>n</i>-A]^<i>q</i> complexes of different sizes (<i>n</i>) and charges (<i>q</i>). It will be demonstrated that the unexplained origin of observed huge energy gains up to <i>G</i> = 5 eV/V involves two different contributions: <i>G</i> = Δ<i>E</i>/Δ<i>E</i>_V = <i>SC</i>. The first one is responsible for the effect of a fractional charge (<i>q</i>_eff = <i>q</i>/<i>n</i>) on the electronic structure of [M_<i>n</i>-A]^<i>q</i> complexes which tunes the CT states through the slopes <i>S</i> of the <i>E</i> vs <i>q</i>_eff diagrams. The second contribution (<i>C</i> = Δ<i>q</i>_eff/Δ<i>E</i>_V) is a kind of capacitance that quantifies the ability of the metal to convert the electrode potential <i>E</i>_V into the atomic excess of charge <i>q</i>_eff. The estimated capacitances of nanometer-size SERS active hot spots are 2 or 3 times larger than those measured from classical experiences, and therefore, SERS can also be used as a probe for electric capacitance measurements at the nanometric scale. Given that SERS signals come mainly from very especial surface sites usually called “hot spots”, the estimated properties from SERS spectra must be restricted to these special locations. Summarizing, surface nanostructures perform a double enhancement mechanism in SERS: the well-known electromagnetic enhancement based on localized surface plasmon resonances (LSPR), which is responsible for the enormous intensification of the Raman signal, and this new capacitive mechanism characterized by its unusually high ability for charge storage