Anion‐Responsive Colorimetric and Fluorometric Red‐Shift in Triarylborane Derivatives: 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 photophysical properties of TABs in red-shift mode has been presented, endowing a nitrogen-bridged triarylborane (1,4-phenazaborine: PAzB) with a contradictory dual role as a Lewis acid and an electron donor. Following the strategy, PAzB derivatives connected with an electron-deficient azaaromatic have been developed, and these compounds display distinct red-shift in absorption and PL in response to an anion. Spectroscopic analyses and quantum chemical calculations have revealed the formation of tetracoordinate borate upon the addition of fluoride, narrowing the HOMO-LUMO gap and enhancing charge transfer character in the excited state. This approach has also been demonstrated in modulating photophysical properties of solid-state films.