Charge Delocalization in a Homologous Series of a,a’-Bis(dianisylamino)-Substituted Thiophene Monocations

A homologous series of three molecules containing thiophene, bithiophene, and terthiophene bridges between two redox-active tertiary amino groups was synthesized and explored. Charge delocalization in the one-electron-oxidized forms of these molecules was investigated by a combination of cyclic voltammetry, near-infrared optical absorption spectroscopy, and EPR spectroscopy. All three cation radicals can be described as organic mixed-valence species, and for all of them the experimental data are consistent with strong delocalization of the unpaired electron. Depending on what model is used for analysis of the optical absorption data, estimates for the electronic coupling matrix element (HAB) range from ∼5000 to ∼7000 cm–1 for the shortest member of the homologous series. According to optical absorption and EPR spectroscopy, even the terthiophene radical appears to belong either to Robin–Day class III or to a category of radicals commonly denominated as borderline class II/class III systems. The finding of such a large extent of charge delocalization over up to three adjacent thiophene units is remarkable.