X-ray Structure Analysis and the Intervalent Electron Transfer in Organic Mixed-Valence Crystals with Bridged Aromatic Cation Radicals

• Published in: Journal of the American Chemical Society 2002-02, Vol.124 (5), p.843-855
• Main Authors: Lindeman, Sergey V, Rosokha, Sergiy V, Sun, Duoli, Kochi, Jay K
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electron density of states and band structure of crystalline solids; Electron states; Exact sciences and technology; Physics; Polymers and organic compounds
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja011579j

X-ray crystallography identifies the aromatic donor group D = 2,5-dimethoxy-4-methylphenyl to be a suitable redox center for the construction of organic mixed-valence crystals owing to its large structural change attendant upon 1e oxidation to the cation−radical (D •+). The combination of cyclic voltammetry, dynamic ESR line broadening, and electronic (NIR) spectroscopy allows the intervalence electron transfer between the redox centers in the mixed-valence system D-br-D •+ [where br can be an aliphatic trimethylene or an aromatic (poly)phenylene bridge] to be probed quantitatively. Independent measures of the electronic coupling matrix element (H) for D/D •+ electron exchange via Mulliken−Hush theory accord with the X-ray crystallographic databoth sufficient to consistently identify the various D-br-D •+ according to the Robin−Day classification. Thus, the directly coupled biaryl D−D •+ is a completely delocalized cation in class III with the charge distributed equally over both redox centers. The trimethylene- and biphenylene-bridged cations D(CH2)3 D •+ and D(ph)2 D •+ with highly localized charge distributions are prototypical class II systems involving moderately coupled redox centers with H ≈ 400 cm-1. The borderline region between class II/III is occupied by the phenylene-bridged cation D(ph)D •+; and the X-ray, CV, and NIR analyses yield ambivalent H values (which we believe to be) largely a result of an unusually asymmetric (20/80) charge distribution that is polarized between the D/D •+ redox centers.