An Original Electrochemical Method for Assembling Multilayers of Terpyridine-Based Metallic Complexes on a Gold Surface

A new method based on the electrochemical oxidation of thiols was used to easily generate multilayer assemblies of coordination complexes on a gold surface. For this purpose, two complexes bearing two anchoring groups for surface attachment have been prepared: [Ru(tpySH)2]2+ (1) and [Fe(tpySH)2]2+ (2) (tpySH = 4′-(2-(p-phenoxy)ethanethiol)-2,2′:6′,2″-terpyridine). Cyclic voltammetry of 1 in CH3CN exhibits two successive oxidation processes. The first is irreversible and attributed to the oxidation of the thiol substituents, whereas the second is reversible and corresponds to the 1 e– metal-centered oxidation. In the case of 2 both processes are superimposed. Monolayers of 1 or 2 have been formed on gold electrodes by spontaneous adsorption from micromolar solutions of the complexes in CH3CN. SAMs (self-assembled monolayers) exhibit redox behavior similar to the complexes in solution. The high surface coverage value obtained (Γ = 6 × 10–10 and 4 × 10–10 mol cm–2 for 1 and 2, respectively) is consistent with a vertical orientation for the complexes; thus, one thiol is bound to the gold electrode, with the second unreacted thiol moiety exposed to the outer surface. Successive cyclic voltammetry induced a layer-by-layer nanostructural growth at the surface of the SAMs, and this is presumably due to the electrochemical formation of disulfide bonds, where the thiol moieties play a double role of both an anchoring group and an electroactive coupling agent. The conditions of the deposition are studied in detail. Modified electrodes containing both 1 and 2 alternatively can be easily prepared following this new approach. The film proved to be stable, displaying a similar current/voltage response for more than 10 repeating cycles in oxidation up to 0.97 V vs Ag/AgNO3 (10–2 M).