Measurement and DFT Calculation of Fe(cp)₂ Redox Potential in Molecular Monolayers Covalently Bound to H−Si(100)

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 density functional theory (DFT) ab initio calculation of Fc-silicon hybrid redox potentials. The systems were modeled with a slab of H-terminated Si(100) 1 x 1 and 2 x 1 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 in 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 versus 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 the relation between the unsaturation degree and the adduct redox potential to be studied. The comparison among the measured and computed potentials allows one 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.

Functionalization of Si(100) with ferrocene derivatives via “click” chemistry

“…Recently, different routes for grafting a family of ferrocenebased derivatives have been successfully explored on Si(1 0 0), and the redox potentials of the resulting hybrids, measured by cyclic voltammetry, have been shown to agree well with those theoretically computed at the DFT level [16]. Such results indicate that the adduct geometries on the two possible surface reconstructions of Si(1 0 0) (2 × 1 and 1 × 1) are both structurally and energetically equivalent, and thus identical for the sake of the electrochemical investigations.…”

“…The electrochemical formal redox potential E o , taken as (E p,c + E p,a )/2, and estimated from the CV's at the slowest scan rates was 169 mV vs. Ag/Ag + , a significantly lower value compared to the redox potential measured for the hybrid material obtained on the same p-Si substrate with pure ethynylferrocene [16]. This indicates that a good overlapping of the valence band of the p-Si with the redox level of the grafted redox molecules, and that the EFC group is more easily oxidized when attached by "click" reaction on the N 3 -Si surfaces, rather than when it is directly grafted to the Si(1 0 0) surface.…”

Chemical routes to fine tuning the redox potential of monolayers covalently attached on H–Si(100)

“…Ferrocenes anchored to gold substrates have been widely studied [5][6][7][8][9][10][11], while their anchoring to Si has been seldom attempted, maybe due to the difficulty of establishing robust Si-C or Si-O bonds without oxidation of the Si substrate. Ferrocenes substituted by short linkers anchored directly on Si(1 0 0) [1,[12][13][14][15][16][17] or coupled to pre-assembled acid- [18], ethynyl- [19,20] or azideterminated [21] alkyl monolayers on both Si(1 1 1) and Si(1 0 0) have been recently reported. Generally, short-chain redox hybrids on Si(1 0 0) have been found to have higher electron transfer rates but shorter lifetimes [16] than mixed Fc-alkyl layers.…”

“…In our previous studies [12][13][14][15][16], light-photoactivated functionalization of H-Si(1 0 0) was investigated on ethylferrocene (EtFC) and vinylferrocene (VFC), resulting in the formation of a covalent bond between the silicon surface and the molecules. Nevertheless, the hybrids obtained by visible light activation bear an unknown degree of unsaturation in the C-C tethering arm, which is assumed to be reduced (from triple to double and from double to single C-C bond) upon reaction, suggesting that photochemical activation would bring different adducts on the surface.…”

“…Nevertheless, the hybrids obtained by visible light activation bear an unknown degree of unsaturation in the C-C tethering arm, which is assumed to be reduced (from triple to double and from double to single C-C bond) upon reaction, suggesting that photochemical activation would bring different adducts on the surface. We recently compared experimental and theoretical results on these systems [13], 1 demonstrating that a close relationship lies between the unsaturation degree of the lateral tethering arm in the grafted molecule and the redox potential of the corresponding hybrid. In the light of such findings, we have explored reaction routes commonly used for Si surface derivatization, but for the first time used here to attach ferrocene derivatives.…”