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

Abstract

A trinuclear [{Ru^II(bpy)₂(bpy-terpy)}₂Fe^II]⁶⁺ complex (I) in which a Fe^II-bis-terpyridine-like centre is covalently linked to two Ru^II-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 Fe^III/Fe^II and Ru^III/Ru^II redox couples. These systems are clearly separated (ΔE_1/2 = 160 mV), demonstrating the lack of an electronic connection between the two subunits. The two oxidized forms of the complex, [{Ru^II(bpy)₂(bpy-terpy)}₂Fe^III]⁷⁺ and [{Ru^III(bpy)₂(terpy-bpy)}₂Fe^III]⁹⁺, obtained after two successive exhaustive electrolyses, are stable. (I) is poorly luminescent, indicating that the covalent linkage of the Ru^II-tris-bipyridine to the Fe^II-bis-terpyridine subunit leads to a strong quenching of the Ru^II* excited state by energy transfer to the Fe^II 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.