Charge delocalization in a homologous series of α,α'-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 (H(AB)) 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.

Multistage complexation of fluoride ions by a fluorescent triphenylamine bearing three dimesitylboryl groups: controlling intramolecular charge transfer.

A propeller-shaped boron-nitrogen compound (NB(3)) with three binding sites for fluoride anions was synthesized and investigated by optical absorption, luminescence, and ((1)H, (11)B, (13)C, (19)F) NMR spectroscopy. Binding of fluoride in dichloromethane solution occurs in three clearly identifiable steps and leads to stepwise blocking of the three initially present nitrogen-to-boron charge transfer pathways. As a consequence, the initially bright blue charge transfer emission is red-shifted and decreases in intensity, until it is quenched completely in presence of large fluoride excess. Fluoride binding constants were determined from global fits to optical absorption and luminescence titration data and were found to be K(a1) = 4 × 10(7) M(-1), K(a2) = 2.5 × 10(6) M(-1), and K(a3) = 3.2 × 10(4) M(-1) in room temperature dichloromethane solution. Complexation of fluoride to a given dimesitylboryl site increases the electron density at the central nitrogen atom of NB(3), and this leads to red shifts of the remaining nitrogen-to-boron charge transfer transitions involving yet unfluorinated dimesitylboryl groups.