INTRAMOLECULAR ELECTRON-TRANSFER IN PHOTOEXCITED RU(II)-RH(III) BINUCLEAR COMPOUNDS

Lifetimes of metal-to-ligand charge-transfer excited states, (3CT)Ru, of ruthenium(II) were measured for ruthenium(II)-rhodium(III) compounds, [Ru(bpy)2(L-L)Rh(bpy)2]5+, and its analogous compounds in a wide temperature range (170-300 K) in the fluid solvent of a mixture of propionitrile and butyronitrile. The interventing ligands (L-L) used were 2,6-bis(2'-pyridyl)benzdiimidazole (dpimbH-2), 2,2'-bis(2''-pyridyl)bibenzimidazole (bpbimH-2), 1,1'-dimethyl-2,2'-bis(2''-pyridyl)-6,6'-bibenzimidazole (dmbpbim), and bis[2-(2'-pyridyl)benzimidazoyl]ethane (dpbime). The rapid quenching of (3CT)Ru can be explained in terms of an intramolecular electron transfer (ET), (3CT)Ru-Rh(III) --> Ru(III)-Rh(II), which is followed by a fast backward ET to regenerate the ground state. The frequency factors obtained from the temperature dependence of the ET rates were found to be almost constant, (1.1-4.1) x 10(11) s-1, irrespective of the intervening ligands, indicating that the ET processes are adiabatic. These factors were in agreement with the value (4-6) x 10(11) s-1 calculated provided that the ET is influenced by the relaxation dynamics of the solvent. The activation energy (E(a)) ranged from 0.17 to 0.22 eV depending on the bridging ligands. Using the reorganization energy, lambda, determined from the metal-to-metal charge-transfer transition of a mixed-valence compound, [Ru(bpy)2(L-L)Ru(bpy)2]5+, E(a) was evaluated on the basis of the classical ET theory. The E(a) values calculated by considering the temperature dependence of lambda and the solvent motion were in good agreement with the observed values except for bpbimH-2.