Synthesis and properties of trinuclear polypyridyl complexes Ru(II)-Co(II)-Ru(II) and Ru(II)-Co(III)-Ru(II): Their photoinduced interconversion

The trinuclear [{Ru-II(bpy)(2)(bpy-terpy)}(2)Co-II](6+) complex (1(6+)) in which a Co(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. The cyclic voltammetry exhibits two successive reversible oxidation processes, corresponding to the Co-III/Co-II and Ru-III/Ru-II redox couples at E-1/2 = -0.06 and 0.91 V vs Ag/Ag+ 10 mM, respectively. The one-electron oxidized form of the complex, [{Ru-II(bpy)(2)(bpy-terpy)}(2)Co-III](7+) (1(7+)) obtained after exhaustive electrolysis carried out at 0.2 V is fully stable. 1(6+) and 1(7+) are only poorly luminescent, indicating that the covalent linkage of the Ru(II)-tris-bipyridine centre to the cobalt subunit leads to a strong quenching of the Ru-II excited state by an intramolecular process. Luminescence lifetime experiments carried out at different temperatures indicate that the transfer is more efficient for 1(7+) compare to 1(6+) due to lower activation energy. Continuous irradiation of 1(7+) performed at 405 nm in the presence of P(Ph)(3) acting as sacrificial electron donor leads to its quantitative reduction into 1(6+), whereas similar experiment starting from 1(6+) with a sulfonium salt as sacrificial electron acceptor converts 1(6+) into 1(7+) with a slower rate and a maximum yield of 80%. These photoinduced electron transfers were followed by UV-Visible spectroscopy and compared with those obtained with a simple mixture of both mononuclear parent complexes i.e. [Ru-II(bpy)(3)](2+) and [Co-II(tolyl-terpy)(2)](2+) or [Co-III(tolyl-terpy)(2)](3+) (tolyl-terpy = 4'-(4-methylphenyl)-2,2':6',2 ''-terpyridine). (C) 2009 Elsevier B. V. All rights reserved.