Multistage Complexation of Fluoride Ions by a Fluorescent Triphenylamine Bearing Three Dimesitylboryl Groups: Controlling Intramolecular Charge Transfer

A propeller-shaped boron–nitrogen compound (NB₃) with three binding sites for fluoride anions was synthesized and investigated by optical absorption, luminescence, and (¹H, ¹¹B, ¹³C, ¹⁹F) 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 × 10⁷ M^–1, K_a2 = 2.5 × 10⁶ M^–1, and K_a3 = 3.2 × 10⁴ 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 NB₃, and this leads to red shifts of the remaining nitrogen-to-boron charge transfer transitions involving yet unfluorinated dimesitylboryl groups.