X-ray Structure Analysis and the Intervalent Electron Transfer in Organic Mixed-Valence Crystals with Bridged Aromatic Cation Radicals
American Chemical Society
Journal of the American Chemical Society
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 databoth 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)3D•+ and D(ph)2D•+ 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.