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 (EV) in tuning the energies of the electronic states (E) 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 [Mn-A]q complexes of different sizes (n) and charges (q). It will be demonstrated that the unexplained origin of observed huge energy gains up to G = 5 eV/V involves two different contributions: G = ΔE/ΔEV = SC. The first one is responsible for the effect of a fractional charge (qeff = q/n) on the electronic structure of [Mn-A]q complexes which tunes the CT states through the slopes S of the E vs qeff diagrams. The second contribution (C = Δqeff/ΔEV) is a kind of capacitance that quantifies the ability of the metal to convert the electrode potential EV into the atomic excess of charge qeff. The estimated capacitances of nanometer-size SERS active hot spots are 2 or 3 times larger than those measured fr...