COMPETITIVE ENERGY AND ELECTRON-TRANSFER QUENCHING IN INTRAMOLECULAR PROCESSES OF EXCITED POLYPYRIDINE RUTHENIUM(II) OSMIUM(II) BINUCLEAR COMPLEXES

In the excited state of the covalently bridged Ru(II)/Os(II) binuclear complex, [BTFMB2Ru(II)MebpyCH2CH(OH)CH2MebpyOs(II)(bpy)2] (BTFMB = 4,4'-bis-trifluoromethyl-2,2'-bipyridine), 1, a nearly complete quenching of Ru(II) --> pi* (BTFMB) MLCT emission was observed at room temperature. Lifetime measurements were performed to evaluate the quenching rate and the mechanism on a quantitative basis. In methanol at 298 K, the rate of reductive quenching of *Ru(II) by Os(II) (k(el)) is 5.3 x 10(8) s-1 and the rate of energy transfer from *Ru(II) to Os(II) (k(en)) is 7.8 x 10(7) S-1. In n-butanol both processes are competitive. Values of k(el) and k(en) are 1.8 x 10(8) s-1 and 1.7 x 10(8) s-1, respectively. It is shown that the relaxation dynamics of solvent plays an important role in the intramolecular electron-transfer process in protic media even when the observed rate is slow with respect to the solvent motion. The dipole-dipole interaction (Forster mechanism) is proposed for the energy-transfer mechanism.