Substituent‐Controlled Selective and Sensitive Potential Optical Fluoride Sensors Based on Salicylidene Schiff Base Derivatives

Abstract

A series of colorimetric chemosensors 2‐((anthracen‐9‐ylmethylene)hydrazonomethyl)phenol (1) and its derivatives with electron donating (–tBu, 2) and withdrawing groups (–Cl, 3) have been synthesized for selective colorimetric ion sensing. Receptor 1 exhibits ability to colorimetric recognition of F− ions in aqueous environment without any interference of other anions revealed in UV‐vis study. Remarkable substituents effect has been observed in selective sensing as receptors 1 and 2 are exclusive colorimetric sensor for F− ion only while probe 3 senses selectively F−, AcO− and H2PO4− ions. A significant red shift (>90 nm) in intra‐ligand charge transfer (ICT) band have been observed after F− ion addition in probes 1 and 2 results distinct color change visible by naked eyes. Proton transfer mechanism has been revealed by absorption and 1H NMR titration study. In case of 1 and 2, two‐step processes have been observed, namely, formation of hydrogen‐bonded complex [LH⋅⋅⋅F]− between receptor and F− followed by F− induced deprotonation of the complex to form L− and FHF−. In contrast, for 3, simple Brönsted acid‐base reaction has been observed. The F− selectivity is demonstrated to be finely tuned by electron push‐pull effect of the substituents on the ortho/para positions of phenyl ring. The excellent selectivity and sensitivity of 1 for F− over AcO− and H2PO4− is ascribed to the fitness in acidity of its phenolic O−H group.

Revisiting salicylidene-based anion receptors

“…In the rst step, the salicylidene -OH group which is involved in intramolecular hydrogen bonding with imine nitrogen (A in Scheme 2), forms an O-H/A À hydrogen bonded complex (B in Scheme 2). 16 The fact that the peak position of the salicylidene -OH proton in SL did not shi upon addition of halides (Cl À and Br À ) suggest that the intramolecular O-H/N hydrogen bond persists in the presence of these anions. However, broadening or disappearance of the salicylidene -OH peak in the presence of anions can be attributed to protonexchange processes in the solution-state.…”

Fluorescent Schiff base sensors as a versatile tool for metal ion detection: strategies, mechanistic insights, and applications

“…[60][61][62] The molecular switching properties of Schiff base sensors have been applied in the construction of molecular keypads and logic gates. 63,64 In the past, significant research work has been done in developing fluorescent Schiff base sensors for metal ion detection. 63,[65][66][67][68][69][70][71] However, to the best of our knowledge, no review has been reported on metal ion detection using fluorescent Schiff base sensors.…”

The sensing mechanism of a flavone-based ESIPT fluorescent chemodosimeter for selective recognition towards fluoride: a theoretical

“… 29–41 The chemosensors based on a hydrogen-bond strategy are usually disturbed by other basic anions such as H 2 PO 4 − and AcO − because they have similar surface charge density and basicity to F − . 42 In addition, the increase of the ionic strength and hydration enthalpy of F − in water often makes the hydrogen bond ineffective in aqueous medium. 43 The chemosensors based on B–F complexation can detect F − in drinking water and fluorine-containing chemical agents, but their cytotoxicity limits their biological applications.…”