Ortho‐Azophenylboronic Acid Esters for Colorimetric Fluoride Ion Sensing Designed by Speciation

Abstract

Fluoride ions improve dental health, but their excessive intake has adverse effects on the human body. Hence, a simple system for sensing fluoride ions is required. Herein, a simple colorimetric sensing system for fluoride ions is reported, together with the detailed investigation of its sensing mechanism. The azo dye‐based chemosensors are designed bearing a boron ester moiety and a hydroxyl group at the ortho‐ and para‐positions of the azo moiety, respectively (ortho‐azophenylboronic acid esters, azoB‐esters). A series of pinacol‐ and 1,8‐naphtalenediamine‐protected azoB‐esters (azoB(pin)‐1 − azoB(pin)‐4, and azoB(dan)‐1 − azoB(dan)‐3, respectively) are synthesized. Single‐crystal X‐ray crystallography, UV–vis absorption spectroscopy, and 11B nuclear magnetic resonance studies reveal that all azoB‐esters, except azoB(pin)‐1, are mainly present as trigonal boronic acid species in tertiary butanol and aprotic solvents. Only azoB(pin)‐1 shows the presence of a solvent‐inserted species in protic solvents, resulting in an additional absorption band in the longer wavelength region. The azoB‐esters show significant changes in their absorption spectra in response to fluoride ions in tertiary butanol, with some also displaying dramatic colorimetric changes detectable by the naked eye. For example, when azoB(pin)‐2 is used to sense fluoride ions in surfactant‐containing water, a visually discernible color change is observed.