Issue Date: November 2, 2009
Computational Study Scrutinizes Mixed-Valence Systems
Researchers in Germany have devised a simple, reliable protocol for computationally evaluating organic mixed-valence systems, which are used to model electron-transfer reactions (J. Am. Chem. Soc., DOI: 10.1021/ja9070859). Organic mixed-valence systems are typically composed of two extended π systems connected by a bridge. They can be categorized as class II compounds, in which charge can be transferred optically or thermally from one redox center to the other, or as class III compounds, in which the charge is symmetrically delocalized between the two centers. Prior approaches to computational analysis of such systems often incorrectly localized the charges or were computationally costly. The German team, led by Martin Kaupp of the University of Würzburg, developed a method based on hybrid density functionals and continuum solvent models and calibrated it against a series of four experimentally characterized, mixed-valence bistriarylamine radical cations that incorporate different bridges. Kaupp and colleagues found that for such systems there is a “strikingly large” effect of polar solvents on charge localization. Solvent polarity “may indeed play the decisive role” in determining the character of the systems, they say.
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