Fluoride ion sensing with an acridinium borane

Abstract

With our continuing interest in the chemistry of cationic boranes, we have synthesized the tetrafluoroborate salt of 1-dimesitylboron-4-( N-methyl-9-acridinium)-phenylene, which acts as a turn-on fluoride anion sensor, visibly changing from yellow to orange upon binding fluoride. To understand this reactivity, we spectroscopically and computationally analyzed the cation and fluoroborate adduct. Ultraviolet–visible spectroscopy and time-dependent density functional theory calculations revealed the basis of the color change to be a low energy redshift in a intramolecular charge transfer band. Electrochemical studies were undertaken to further probe this system. Cyclic voltammetry indicated a reversible one-electron reduction for the cation and a cathodic shift of −0.12 V in the first reduction wave upon fluoride binding. Chemical reduction of the cation yielded the acridine borane radical, which was verified by electron paramagnetic resonance spectroscopy.