Anion-Responsive Colorimetric and Fluorometric Red-Shift in a Triarylborane Derivative: Dual Role of Phenazaborine as Lewis Acid and Electron Donor

Photophysical modulation of triarylboranes (TABs) through Lewis acid-base interactions is a fundamental approach in sensing anions. Yet, design principles for anion-responsive TABs displaying significant red-shift in absorption and photoluminescence (PL) have remained elusive. Herein, a new strategy for modulating photophysics of TABs in red-shift mode has been presented, endowing a nitrogen-bridged triarylborane (phenazaborine: PAzB) with contradictory dual role as a Lewis acid and an electron donor. Following the strategy, a PAzB derivative connected with an electron-deficient azaaromatic has been developed, and it displays distinct red-shift in absorption and PL in response to an anion. Spectroscopic analyses and quantum chemical calculations have revealed the formation of tetracoordinated borate upon addition of fluoride, narrowing the HOMO-LUMO gap and enhancing charge transfer character in the excited state. This approach has also been demonstrated in solid-state films.