Fluoride ion sensing with an acridinium borane

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 yel-low 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.

Automated summary

In this study, we synthesized a tetrafluoroborate salt and found that it can function as a fluorescent fluoride anion sensor. Through spectroscopic and computational analysis, we revealed the mechanism behind the color change, and further investigated the properties of the system through electrochemical studies.