Electropolymerization Mechanism of Aminophenyl-Bis(terpyridine) Metal Complexes Featuring Electrochromic Properties

Two M(II) bis(terpyridine) complexes (M = Fe²⁺, Ru²⁺), functionalized with aminophenyl groups at the para position of the terpyridine ligands, [M(tpy-Ph-NH₂)₂]²⁺, were synthesized to introduce terminal electroactive sites capable of undergoing electrochemical oxidation-induced polymerization. The resulting metallopolymers, poly[M(tpy-Ph-NH₂)₂]²⁺, were grown on indium-tin oxide (ITO) substrates through oxidative electropolymerization of the aminophenyl-substituted monomers, as performed by multisweep cyclic voltammetry (CV) to obtain films with various thicknesses. Both metal complexes behaved in a similar way, and a detailed investigation has been given here only for the iron complex [Fe(tpy-Ph-NH₂)₂]²⁺ (illustrative data for [Ru(tpy-Ph-NH₂)₂]²⁺ are given as Supporting Information). CV, associated with other physicochemical techniques (UV-vis, IR, Raman, and XPS), was used to characterize the electropolymerized films in order to elucidate the mechanism involved in the electropolymerization process and to determine the nature of the bonds formed between the monomeric units. This enabled us to point out the formation of azo bonds resulting from the oxidation of the starting monomers and the intermediate hydrazo derivatives. The poly[Fe(tpy-Ph-NH₂)₂]²⁺ films exhibited a reversible electrochemical behavior involving the Fe³⁺/Fe²⁺ redox couple according to a mixed diffusion/adsorption charge transfer mechanism. The different colors of their oxidized and reduced forms made possible the elaboration of a solid-state electrochromic device where the redox-responsive optical properties of the polymer arising from metal-to-ligand charge transfer (MLCT) transitions can be exploited. From the strong MLCT absorption band at 580 nm and biasing the electrode successively at -0.5 V (coloration step) and 2.5 V (bleaching step), one can reach quite good optical contrast, with a transmittance change (Δ<i>T</i>%) of 20% and short response times (0.7 s for coloration and 1.8 s for bleaching).