Competitive energy and electron-transfer quenching across the peptide-bridge in polypyridine ruthenium(II)/osmium(II) binuclear systems

In the excited state of the peptide-bridged Ru(II)/Os(II) binuclear complex , [L2Ru(II)-mbCO-L-Leu-NHbpyOs(II)(bpy)(2)] and [L2Ru(II)mbCO-L-Pro-NHbpy-O s(II)(bpy)(2)] and [L = bpy ( = 2.2'-bipyridine) or BTFMB ( = 4,4'-bis-trif luoromethyl-2,2'-bipyridine) and mb = 4-methyl-bpy], a nearly complete quen ching of Ru-II --> pi* (Ligand) MLCT emission was observed at room temperat ure. Lifetime measurements were performed to evaluate the quenching rate an d the mechanism on a quantitative basis. In complexes with L = bpy, the int ramolecular energy transfer process had a unitary efficiency. In complexes with L = BTFMB, the evidence for the occurrence of competitive energy and e lectron-transfer processes was provided from the time dependence of the emi ssion spectra. The main pathway for energy-transfer in both systems could b e explained by the Forster mechanism. It was suggested that the direction o f MLCT excitation to either bridging or non-bridging bpy-ligand causes the large difference of dipole-dipole distance. (C) 2000 Elsevier Science S.A. All rights reserved.