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Issue 15, 2011
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Influence of ion pairing on the oxidation of iodide by MLCT excited states

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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 (KD = 1.0 ± 0.01 × 105 M−1, kq = 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.

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

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Publication details

The article was received on 22 Oct 2010, accepted on 30 Nov 2010 and first published on 17 Jan 2011


Article type: Paper
DOI: 10.1039/C0DT01447H
Citation: Dalton Trans., 2011,40, 3830-3838
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    Influence of ion pairing on the oxidation of iodide by MLCT excited states

    B. H. Farnum, J. M. Gardner, A. Marton, A. A. Narducci-Sarjeant and G. J. Meyer, Dalton Trans., 2011, 40, 3830
    DOI: 10.1039/C0DT01447H

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