ELECTRON-TRANSFER QUENCHING OF THE EXCITED MLCT STATE OF RU(II) IN THE DOUBLE COMPLEX SALTS [RU(BPY)(3)](2)[M(CN)(6)]CL-CENTER-DOT-8H2O (M=CO AND FE)

The lifetimes of the excited MLCT states of ruthenium(II) diimine complex ions in the double complex salts, [Ru(bpz)(3)](2)[Fe(CN)(6)]Cl.14H(2)O, [Ru(bpy)(3)](2)[Fe(CN)(6)]Cl.8H(2)O, [Ru(bpy)(3)](2)[Co(CN)(6)]Cl.8H(2)O and [Ru(bpy)(3)](4)[Co(CN)(6)]2SO4. 15H(2)O (bpz=2,2'-bipyrazine; bpy=2,2'-bipyridine), were studied in the temperature range 77-353 K. The excited states of Ru(bpy)(3)(2+) and Ru(bpz)(3)(2+) were very rapidly quenched in the double complex salts [Ru(bpz)(3)](2)[Fe(CN)(6)]Cl.14H(2)O and [Ru(bpy)(3)](2)[Fe(CN)(6)]Cl.8H(2)O even at 77 K. The lifetime of excited Ru(bpy)(3)(2+) in the double complex salts containing Co(CN)(6)(3-) with an excitation energy higher than 2.2 eV was shortened to 30-40 ns at 300 K. Three possible quenching mechanisms, electron transfer, energy transfer and internal conversion to a dissociative state, are examined. The high activation energy (230 meV) of the quenching process is ascribed to the large reorganization energy of electron transfer between the moieties of an excited ruthenium(II) ion and a cobalt(III) ion. Excitation energy transfer is unlikely because of the absence of sensitized emission and the high activation energy of the quenching process. Internal conversion to a dissociative state is less probable because it needs more activation energy (400 meV) than that obtained for the single salts Ru(bpy)(3)Cl-2.6H(2)O and Ru(bpy)(3)SO4.H2O.