<i>o</i>-Azophenylboronic Acid-Based Colorimetric Sensors for <scp>d</scp>-Fructose: <i>o</i>-Azophenylboronic Acids with Inserted Protic Solvent Are the Key Species for a Large Color Change

Abstract

Many boronic acid-based chemosensors for saccharides have been developed; however, their detection mechanisms have seldom been studied. In this study, we synthesized 10 <i>o</i>-azophenylboronic acid derivatives (<b>azoB</b>s) and conducted a fundamental study on the reactivity and the sensing mechanism of <b>azoB</b>s, which undergoes a large color change, e.g., from red to yellow, upon a reaction with saccharides. Their pH-independent formation constants were determined by spectrophotometric titration and then converted to the conditional formation constant <i>K</i>' at pH 7.4. A linear free energy relationship was established between log <i>K</i>' and the p<i>K</i>_a of <b>azoB</b>. ¹¹B NMR measurements indicate that in aprotic solvents, <b>azoB</b> forms a trigonal planar structure, while in protic solvents, it forms a quasi-tetrahedral structure (<b>azoB-ROH</b>) with a protic solvent molecule (ROH) inserted between the boronic acid moiety and the azo group. In addition, UV-vis spectroscopic studies showed that the color change during the reaction between <b>azoB</b> and d-fructose in ROH was caused by the release of the ROH from <b>azoB-ROH</b> by d-fructose. Based on the findings in this study, we proposed a guideline for designing an <b>azoB</b>-based chemosensor that exhibits high reactivity toward saccharides and a sufficient color change to allow for the visual detection of saccharides.