MEASUREMENT AND DFT CALCULATION OF Fe(cp) 2 REDOX POTENTIAL IN MOLECULAR MONOLAYERS COVALENTLY BOUND TO H-Si(100) M. Cossi *§ , M. F. Iozzi § , A. G. Marrani # , T. Lavecchia # , P. Galloni # , R. Zanoni # , F. Decker # § Dipartimento di Chimica, Università degli Studi di Napoli “Federico II”, Complesso Monte S. Angelo, 80126 Naples, Italy; # Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”, p.le Aldo Moro 5, 00185 Rome, Italy * to whom correspondence should be addressed: maurizio.cossi@unina.it Abstract The electron transfer to self-assembled molecular monolayers carrying a ferrocene (Fc) center, grafted on a flat Si(100) surface, is a recent subject of experimental investigation. We report here the DFT ab-initio calculation of Fc-silicon hybrids redox potential. The systems were modeled with a slab of H-terminated Si(100) 1x1 and 2x1 surfaces: geometries were optimized using the ONIOM method, and solute-solvent interactions were included through the polarizable continuum model (PCM) method. Two new routes for Si functionalization with ethyl- (EtFC) and ethynyl-Fc (EFC) differing only for the unsaturation degree of the anchoring arm have been successfully explored, and the redox potential of the resulting hybrids has been measured by cyclic voltammetry: 0.675 and 0.851 V vs. NHE for the EtFC and EFC derivatives, respectively. These values, along with the previously measured potential (0.700 V) for the mono-unsaturated derivative, vinyl-Fc, allow to study the relation between the unsaturation degree and the adduct redox potential. The comparison among the measured and computed potentials allows to discriminate between different adduct isomers for the saturated species, and more importantly provides strong indications that the carbon- carbon unsaturation initially present in the molecular arm used for anchoring to the surface is preserved upon addition, in contrast with the commonly accepted reaction mechanism. Keywords: Cyclic voltammetry, DFT calculations, ferrocene-silicon hybrids, molecular monolayers, Si(100)