Photoinduced energy and electron transfer processes in heteropolynuclear polypyridyl complexes of Ru(II) and Fe(II)

Photophysical, photochemical and electrochemical studies of a new series of heterobinuclear complexes of ruthenium and iron, [Ru-II(bpy)(2)(Ln)Fe-II(bpy)(2)](4+) (bpy = 2,2'-bipyridine, n = 2 ([Ru-II(L2)Fe-II](4+)), n = 4 ([Ru-II(L4)Fe-II](4+)), n = 6 ([Ru-II(L6)Fe-II](4+)), issued from the linkage of the Ru(bpy)(3)(2+) and Fe(bpy)(3)(2+) subunits by covalently bridging bis-bipyridine Ln ligands have been investigated. The cyclic voltammetry of the three complexes exhibits two successive metal-centered electron reversible oxidation processes, clearly separated, in the positive region. In the negative area, three successive reversible two electron waves are observed (the second and the third being strongly distorted by adsorption phenomena) corresponding to the ligand-based reduction processes. In addition, the two oxidized forms of these complexes [Ru-II(Ln)Fe-III](5+) and [Ru-III(Ln) Fe-III](6+) obtained by two successive exhaustive electrolyses exhibit high stability, which is crucial for an efficient photocatalytic operating system. Luminescence of the complexes [Ru-II(Ln) Fe-II](4+) has been observed indicating that the covalently linkage between the Ru(bpy)(3)(2+) and Fe(bpy)(3)(2+) units leads to an only partial quenching of the Ru-II* excited state by energy transfer to Fe-II. The nature of the energy transfer process involved in those heterobinuclear complexes is studied and an intermolecular electron exchange mechanism is proposed as a preferably deactivation route. On the other hand, the photoxidation of the Ru-II subunit into the Ru-III one could be easily obtained in the presence of a diazonium salt, playing the role of sacrificial oxidant. Finally photocatalytic oxidation of the complexes has been performed by continuous photolysis experiments. For each heteronuclear complexes, the multi-step oxidation process (Fe-II-->Fe-III and Ru-II-->Ru-III) has been observed. The comparison with an isoconcentrated mixture of the corresponding homonuclear parent complexes has been made.