INTRAMOLECULAR ENERGY-TRANSFER IN COVALENTLY LINKED POLYPRIDINE RUTHENIUM(II)/OSMIUM(II) BINUCLEAR COMPLEXES - RU(II)(BPY)2MEBPY-(CH2)N-MEBPYOS(II)(BPY)2(N=2, 3, 5, AND 7)

A novel series of polymethylene-linked heterobinuclear complexes of polypyridine ruthenium(II)/osmium(II) complex Ru(II)(bpy)2Mebpy-(CH2)n-MebpyOs(II)(bpy)2(bpy = 2,2'-bipyridine and n = 2, 3, 5, and 7), 1, was prepared. The photophysical behavior was examined in various solvents. The emission spectra of 1 (excitation wavelength: 455 nm) showed a nearly complete quenching of Ru(II) --> pi*(bpy) metal-to-ligand charge transfer (MLCT) emission and the enhancement of Os(II) --> pi*(bpy) MLCT emission. The luminescence lifetime measurements by a time-correlated single photon-counting method provided evidence that intramolecular energy transfer is a significant pathway for the observed emission quenching. The rate constants of the intramolecular energy transfer in ethanol are 5.3 x 10(8), 3.3 x 10(8), 1.3 x 10(8), and 1.0 x 10(8)s-1 for 1(n = 2, 3, 5, and 7), respectively. They were found to be proportional to the inverse sixth power on the center-to-center distance of the two complexes. The mechanism is discussed in terms of the Forster (a dipole-dipole interaction) mechanism.