Influence of ion pairing on the oxidation of iodide by MLCT excited states

The oxidation of iodide to diiodide, I-2(center dot-), by the metal-to-ligand charge-transfer (MLCT) excited state of [Ru(deeb)(3)](2+), where deeb is 4,4'-(CO2CH2CH3)(2)-2,2'-bipyridine, was quantified in acetonitrile and dichloromethane solution at room temperature. The redox and excited state properties of [Ru(deeb)(3)](2+) were similar in the two solvents; however, the mechanisms for excited state quenching by iodide were found to differ significantly. In acetonitrile, reaction of [Ru(deeb)(3)](2+)* and iodide was dynamic (lifetime quenching) with kinetics that followed the Stern-Volmer model (K-D = 1.0 +/- 0.01 x 10(5) M-1, k(q) = 4.8 x 10(10) M-1 s(-1)). Excited state reactivity was observed to be the result of reductive quenching that yielded the reduced ruthenium compound,[Ru(deeb(-))(deeb)(2)](+), and the iodine atom, IS. In dichloromethane, excited state quenching was primarily static (photoluminescence amplitude quenching) and [Ru(deeb(-))(deeb)(2)](+) formed within 10 ns, consistent with the formation of ion pairs in the ground state that react rapidly upon visible light absorption. In both solvents the appearance of I-2(center dot-) could be time resolved. In acetonitrile, the rate constant for I-2(center dot-) growth, 2.2 +/- 0.2 x 10(10) M-1 s(-1), was found to be about a factor of two slower than the formation of [Ru(deeb(-))(deeb)(2)](+), indicating it was a secondary photoproduct. The delayed appearance of I-2(center dot-) was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I-2(center dot-), 1.3 +/- 0.4 x 10(10) M-1 s(-1), was similar to that in acetonitrile, yet resulted from iodine atoms formed within the laser pulse. These results are discussed within the context of solar energy conversion by dye-sensitized solar cells and storage via chemical bond formation.