Self-assembled monolayers of 1,4-dicyanobenzene on Au(111) electrodes are studied by cyclic voltammetry, in-situ STM and ex-situ XPS. High-resolution STM images reveal a long-range order of propeller-like assemblies each of which consists of three molecules, all lying flat on the gold substrate with the cyano groups oriented parallel to the metal surface. It is demonstrated that both functional groups can act as complexation sites for metal ions from solution. Surprisingly, such arrangements still allow the metal to be deposited on top of the molecules by electrochemical reduction despite the close vicinity to the Au surface. The latter is demonstrated by angle-resolved XPS which unequivocally shows that the metal indeed resides on top of the organic layer rather than underneath, despite the flat arrangement of the molecules.
A new cyano substituted bis(terpyridine) derivative CN-BTP was synthesized and its adsorption on highly oriented pyrolytic graphite (HOPG) and Au(111) was investigated. CN-BTP is closely related to the previously investigated 2,4'-BTP, where the cyanophenyl groups are replaced by pyridine moieties. The scanning tunneling microscopy (STM) investigation of CN-BTP at the liquid|HOPG interface shows a highly ordered herringbone structure that is stabilized by double weak intermolecular C-HN hydrogen bonds, partially through the -CN substituents, which is different from the most stable square structure of 2,4'-BTP. The adsorption processes were investigated using cyclic voltammetry (CV) on Au(111) in a neutral phosphate buffer. A fast and full adlayer formation could be observed with CN-BTP, whereas an extremely slow process with 2,4'-BTP under the same conditions was found. Our data show that the CN substituents on BTP not only change the structure of the monolayer at the liquid|HOPG interface, but also accelerate the phase transition process in the electrolyte dramatically. This could be explained by the adlayer-substrate interactions, which is supported by DFT calculations. Our findings might be extended more generally to further pyridine comprising self-assembling molecules to fine-tune the adlayer structure and phase transition/adsorption kinetics by replacing pyridine by cyanophenyl moieties.
Motivated by our previous studies on metallic substrates, in the present work we addressed the functionalization and the subsequent metallization of a hydrogen-terminated n-Si(111) electrode. DFT provides atomistic insights on the grafting mechanism of 4-pyridinecarboxaldehyde (C6H5NO) what encouraged electrochemical investigations, i. e. cyclic voltammetry and in-situ STM, combined with XPS measurements which together provide evidence for a successful transfer of the so far obtained knowledge from metal single crystal to semiconductor surfaces.
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