Semicarbazone and thiosemicarbazone appended 4-diethylamino-2‑hydroxybenzyl compounds as highly selective bifunctional chemosensors: An experimental and computational approach

Three bifunctional chemosensors, 2-(4-(diethylamino)-2-hydroxybenzylidene)hydrazinecarboxamide (S1), 2-(4-(diethylamino)-2-hydroxybenzylidene)hydrazinecarbothioamide (S2) and 2-(4-(diethylamino)-2-hydroxybenzylidene)-N-methylhydrazinecarbothioamide (S3) have been designed and synthesized for recognition of inorganic cations and anions in solution at room temperature. The structure of the receptor S2 was confirmed by single crystal X-ray diffraction technique, which crystallizes in the monoclinic P21/n space group. The selectivity and sensitivity of cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ and Pb2+) and anions (F−, Cl−, Br−, I−, HSO4− and H2PO4−) were investigated using colorimetric, UV–vis absorption, emission and 1H NMR studies. The receptors (S1–S3) with ONO/ONS donors allow bare-face recognition of Co2+, Ni2+ and Cu2+ cations, and F− anion in a mixture of solvent, CH3CN:CH3OH (8:2, v/v). The receptor S3 possesses high sensitivity and selectivity in comparison with the receptors S1 and S2. The binding constant and limit of detection of the receptors with sensed cations and anion showed high sensitivity and selectivity towards Co2+ cation with order of magnitude 6, greater than the other sensed cations. In addition, the receptors S3 also shows 6 order of magnitude for Cu2+ cation and F− anion. The optimized geometry determined by DFT studies shows that the receptors in its conjugated state provide binding sites for cations and F− anion. The HOMO-LUMO energy gap between the receptors and the Co2+ cation was found to be the least among the studied receptors-cations binding interaction, possibly due to charge transfer effect.