Flash-quench studies on the one-electron reduction of triiodide

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'-(CO(2)CH(2)CH(3))(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 I(3)(-) 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.