Alternated bimetallic [Ru–M] (M = Fe2+, Zn2+) coordination polymers based on [Ru(bpy)3]2+ units connected to bis-terpyridine ligands: synthesis, electrochemistry and photophysics in solution or in thin film on electrodes

Abstract

Two alternated bimetallic Ru–Fe and Ru–Zn coordination polymers, [{Ru^II(bpy)₂(L2)M^II}_n]⁴ⁿ⁺ (M = Fe²⁺, Zn²⁺), were synthesized using the [Ru(bpy)₂(L2)]²⁺ (bpy = 2,2′ bipyridine) complex as a building block, in which L2 is a bipyridine ligand substituted by two terpyridine sites. The [Ru(bpy)₃]²⁺ like-subunits provide the assemblies with photoredox properties whereas the second metal allows the build-up of the polymer structure by coordination of the free terpyridine units, associated with additional redox activities. Thin robust films of these metallo supramolecular structures can be easily obtained as a coating on electrode surfaces (C, Pt, and ITO) by a simple electrochemical procedure based on an electroreductive precipitation adsorption process. The morphology of the films has been characterized by AFM. Electrochemical and photophysical properties of these coordination polymers were investigated in CH₃CN solution as well as thin films deposited on an electrode. The Ru(II)–Zn(II) film, deposited on a transparent ITO electrode, displays luminescence properties. On the other hand, the Ru(II)–Fe(II) film exhibits electrochromic properties under continuous cycling over the Fe^II/Fe^III and Ru^II/Ru^III waves, shifting from reddish dark at the Fe(II)–Ru(II) reduced state to orange-yellow at the Fe(III)–Ru(II) state and to pale green at the more oxidized state Fe(III)–Ru(III). These oxidation processes can be also driven by visible light. Indeed, upon continuous irradiation of an CH₃CN solution of the Ru–Fe polymer in the presence of a diazonium salt as a sacrificial electron acceptor, the fast quantitative one-electron oxidation of the Fe(II) centers followed by that of the Ru(II) ones occurs, despite the strong quenching of the luminescence of the Ru(II) moieties by the Fe(II) center. The photoinduced oxidation of the Fe(II) center is still efficient when the Ru(II)–Fe(II) polymer is electrodeposited as a thin film on ITO leading to the storage of an oxidative equivalent on an electrode.