Dihydrophthalazine‐Based Molecular Fluoride Sensor in Commercial Toothpaste: Fluoride‐Induced Instant Deprotonation of the ─N─H Proton in the Receptor

ABSTRACT

Fluoride sensors that rely on hydrogen bonding, as reported so far, typically operate in protic liquids. Under these conditions, fluoride initially forms a solid hydrogen bond with the receptor's proton donor at lower concentrations and deprotonates it at higher concentrations. Conversely, such deprotonation reactions cannot occur in aqueous solutions due to competitive protonation. For the first time, we have demonstrated that a fluoride ion in water can deprotonate the ─N─H proton of 2‐(2,4‐dinitrophenyl)‐2,3‐dihydrophthalazine‐1,4‐dione ( L ) in THF with less than one equivalent, creating a visible fluoride sensor. The structure of L was confirmed by both ¹ H and ¹³ C NMR spectral techniques. The fluoride‐sensing capability of L was analyzed using UV–vis and fluorescence spectroscopy. Compound L in THF detects fluoride exclusively, even in the presence of other essential anions in water. This was confirmed by changes observed in its UV–vis spectral pattern in the presence of different anions. UV–visible absorption titration experiments with fluoride ion demonstrate that the limit of detection (LoD) is 1.16 µM. ¹ H NMR titration experiments, along with clear theoretical support, confirm that fluoride deprotonates the N─H proton in L instantaneously, rather than via a hydrogen‐bonding interaction. L detects fluoride ions in commercial toothpaste extract in water.