“Naked-eye” detection of inorganic fluoride ion in aqueous media using base labile proton: A different approach

Abstract

Two new receptors R1 and R2 were designed and synthesized based on 1-naphthohydrazide Schiff's bases for the colorimetric detection of fluoride ion. The receptor R1 was selective toward fluoride ion over other anions in organic media. The presence of carbonyl group in 1-naphthohydrazide makes –NH proton acidic and therefore it could deprotonate with addition of basic anion such as fluoride. However, the acidic –NH proton easily gets solvated even with the trace amount of water. Alternatively, the receptor R2, encompasses highly base labile, hydroxyl (–OH) functionality which detects basic fluoride ions via deprotonation mechanism not only in organic solutions but also in aqueous media. The mechanism involved in the color change of receptor R1 is deprotonation of acidic –NH followed by stabilization of complex through intramolecular charge transfer (ICT) transition which was evidenced by the formation of HF2 peak in 1H NMR titration. The color change of receptor R2 involves initial hydrogen bond formation of F− ion with –NH group and deprotonation at higher concentration of F− ions which leads to intramolecular charge transfer transition in organic media. On the other hand, in aqueous solution of receptor R2, addition of the basic fluoride ion leads to the deprotonation of base labile hydroxyl (–OH) proton, which is responsible for colorimetric detection.