Intercomponent electronic energy transfer in heteropolynuclear complexes containing ruthenium- and rhenium-based chromophores bridged by an asymmetric quaterpyridine ligand

We have prepared the mononuclear complexes [Ru(bpy)(2)(AB)][PF6](2), [Ru(bpy)(AB)(2)][PF6](2), and [Ru(AB)(3)][PF6](2) (designated Ru-AB, Ru-AB(2), and Ru-AB(3), respectively) [where bpy is 2,2'-bipyridine and AB is the asymmetric bis(bipyridyl) bridging ligand 2,2':3',2 '':6 '',2'''-quaterpyridine] in which there are one, two, or three (respectively) bpy-type fragments pendant from the {Ru(bpy)(3)}(2+) core. In every case the less hindered site A of the ligand AB is coordinated to Ru(II)) and the more hindered site B is pendant. Reaction with Re(CO)(5)Cl affords the heteronuclear complexes [Ru(bpy)(2){ABRe(CO)(3)Cl}][PF6](2), [Ru(bpy){ABRe(CO)(3)Cl}(2)][PF6](2), and [Ru{ABRe(CO)(3)Cl)}(3)][PF6](2) (designated Ru-ABRe, Ru-ABRe(2), and Ru-ABRe(3), respectively) in which each pendant site B is now coordinated to a {Re(CO)(3)Cl} fragment. Because of the conformational properties of the AB ligand, in the tetranuclear Ru-ABRe(3) complex the Ru-based chromophore occupies an internal position in a sort of molecular ball, the three Re-based groups being located outside. Electrochemical studies show that the pendant {Re(CO)(3)Cl} fragments exert an electron-withdrawing effect on the {RU(bpy)(3)}(2+) core Such that the Ru(II)/Ru(III) redox couple moves to more positive potentials as the number of pendant {Re(CO)(3)Cl} fragments increases. We employed steady-state and time-resolved luminescence spectroscopy to investigate the Re --> Ru intercomponent energy transfer taking place in the mixed-metal complexes and found that Re --> Ru energy transfer takes place with 100% efficiency in all cases. For the tetranuclear complex, it is thus possible to convey a substantial portion of the electronic excitation energy from the molecular periphery to the center. It is also found that the peripheral Re-containing units exert a shielding effect against luminescence quenching processes at the Ru center by molecular oxygen dissolved in the solvent. During the syntheses, the unexpected byproduct [Ru(AB)(eta(3)-AB)Cl][PF6] was also isolated in which one of the AB ligands is coordinated in a hitherto unseen terdentate mode; this was crystallographically characterised. Data for [RU(AB)(eta(3)-AB)Cl][PF6].2MeCN: C44H34ClF6N10PRu; triclinic, P (1) over bar 10.578(3) Angstrom, b = 14.330(2) Angstrom, c = 14.761(3) Angstrom; alpha = 84.56(2)degrees, beta = 70.408(12)degrees, gamma 86.03(2)degrees; V = 2096.8(8) Angstrom(3); Z = 2.