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

A series of colorimetric chemosensors 2‐((anthracen‐9‐ylmethylene)hydrazonomethyl)phenol ( 1 ) and its derivatives with electron donating (– t Bu, 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 H 2 PO 4 − 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 1 H 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 H 2 PO 4 − is ascribed to the fitness in acidity of its phenolic O−H group.