Photoredox vs. energy transfer in a Ru(II)–Fe(II) supramolecular complex built with an heteroditopic bipyridine-terpyridine ligand

International audienceA trinuclear [{RuII(bpy)2(bpy-terpy)}2FeII]6+ complex ( I) in which a FeII-bis-terpyridine-like centre is covalently linked to two RuII-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has been synthesised and characterised. Its electrochemical, photophysical and photochemical properties have been investigated in CH3CN and compared with those of mononuclear model complexes. The cyclic voltammetry of ( I) exhibits, in the positive region, two successive reversible oxidation processes, corresponding to the FeIII/FeII and RuIII/RuII redox couples. These systems are clearly separated (ΔE1/2 = 160 mV), demonstrating the lack of an electronic connection between the two subunits. The two oxidized forms of the complex, [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ and [{RuIII(bpy)2(terpy-bpy)}2FeIII]9+, obtained after two successive exhaustive electrolyses, are stable. ( I) is poorly luminescent, indicating that the covalent linkage of the RuII-tris-bipyridine to the FeII-bis-terpyridine subunit leads to a strong quenching of the RuII* excited state by energy transfer to the FeII centre. Luminescence lifetime experiments show that the process occurs within 6 ns. The nature of the energy transfer process is discussed and an intramolecular energy exchange is proposed as a preferable deactivation pathway. Nevertheless this energy transfer can be efficiently quenched by an electron transfer process in the presence of a large excess of the 4-bromophenyl diazonium cation, playing the role of a sacrificial oxidant. Finally complete photoinduced oxidation of ( I) has been performed by continuous photolysis experiments in the presence of a large excess of this sacrificial oxidant. The comparison with a mixture of the corresponding mononuclear model complexes has been made