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

Abstract

The oxidation of iodide to diiodide, I₂˙⁻, 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 × 10⁵ M⁻¹, k_q = 4.8 × 10¹⁰ M⁻¹s⁻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, I˙. 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 I₂˙⁻ could be time resolved. In acetonitrile, the rate constant for I₂˙⁻growth, 2.2 ± 0.2 × 10¹⁰ M⁻¹s⁻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 I₂˙⁻ was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I₂˙⁻, 1.3 ± 0.4 × 10¹⁰ M⁻¹s⁻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 storagevia chemical bond formation.