The one-electron reduction of triiodide (I_3 ~-) by a series of reduced ruthenium polypyridyl compounds was studied in an acetonitrile solution at room temperature using the flash-quench technique. Reductive quenching of the metal-to-ligand charge-transfer excited state of [Ru(bpy) _2(deeb)]~(2+), [Ru(deeb)_2(bpy)]~(2+), or [Ru(deeb)_3]~(2+), where bpy is 2,2′-bipyridine and deeb is 4,4′-(CO2CH2CH3)2-2, 2′-bipyridine, by iodide generated the reduced ruthenium compounds and diiodide (I_2 ?~-). Charge recombination of the reduced ruthenium compounds and I_2 ?~- occurred with rate constants near the calculated diffusion limit of 2.6 × 10~(10) M~(-1) s~(-1). The reaction of the reduced ruthenium compounds with I_3 ~- was characterized spectroscopically through the addition of I3 - into the experimental solution prior to the laser flash. Transient absorption data indicated that I_2 ?~- was a reaction product of I_3 ~- reduction and appeared with an average second-order rate constant of (5.0 ± 0.6) × 10~9 M~(-1) s~(-1) for all three compounds. The insensitivity of the rate constants for I_3 ~- reduction over an 80 meV change in the driving force was unexpected. The relevance of these findings to solar energy conversion within dye-sensitized solar cells is discussed.
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