Flash-Quench Studies on the One-Electron Reduction of Triiodide

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Abstract
The one-electron reduction of triiodide (I3–) 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 (I2•–). Charge recombination of the reduced ruthenium compounds and I2•– occurred with rate constants near the calculated diffusion limit of 2.6 × 1010 M–1 s–1. The reaction of the reduced ruthenium compounds with I3– was characterized spectroscopically through the addition of I3– into the experimental solution prior to the laser flash. Transient absorption data indicated that I2•– was a reaction product of I3– reduction and appeared with an average second-order rate constant of (5.0 ± 0.6) × 109 M–1 s–1 for all three compounds. The insensitivity of the rate constants for I3– 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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