Fluoride-selective sensing by thiocarbohydrazone derivative: Combined spectroscopic and computational investigation

• A new thiocarbohydrazone derivative selectively detects fluoride ions. • Dual-mode colorimetric and fluorescence sensing enables visual detection. • 1:2 binding stoichiometry confirmed by Job’s plot and spectroscopic analysis. • DFT calculations reveal hydrogen bonding and HOMO–LUMO gap reduction. • Sensor shows high selectivity over Cl⁻, Br⁻, I⁻, AcO⁻, ClO₄⁻, and H₂PO₄⁻. The chemosensor 1,5-bis(2-acetylpyrrole)thiocarbohydrazone was investigated as a dual-mode optical chemosensor for fluoride ions in DMSO. Upon addition of fluoride, the chemosensor solution showed a distinct color change from colorless to yellow, accompanied by significant fluorescence quenching. UV–visible and fluorescence titration experiments confirmed a 1:2 chemosensor-to-fluoride binding stoichiometry, supported by Job’s plot and Benesi–Hildebrand analysis, with association constants in the range of 10³–10⁴ M⁻¹. Density Functional Theory (DFT) calculations provided further insight into the binding mechanism, revealing strong hydrogen bonding or partial deprotonation at NH sites and a decrease in the HOMO–LUMO energy gap, consistent with an intramolecular charge-transfer process. The chemosensor displayed high selectivity toward fluoride over other common anions such as Cl⁻, Br⁻, I⁻, ClO 4 ⁻, H 2 PO 4 ⁻. AcO⁻. Importantly, the sensor also retained a measurable response in DMSO:H₂O mixtures, indicating that fluoride recognition remains effective even in aqueous environments. Together, these findings establish 1,5-bis(2-acetylpyrrole)thiocarbohydrazone as an effective and selective dual-mode chemosensor for fluoride detection, with a low detection limit of 3.2 µM and strong anti-interference performance, demonstrating its potential utility in practical applications.