Selective and colorimetric fluoride chemosensors containing phenol hydroxyl and 1,3,4-oxadiazole groups

Two novel anion sensors, methyl 4-(5-(5-chloro-2-hydroxyphenyl)-1,3,4-oxadiazol-2-yl) benzoate (L1) and methyl 4-(5-(2-hydroxyphenyl)-1,3,4-oxadiazol-2-yl) benzoate (L2) were reported. Their spectroscopic and colorimetric properties in CH2Cl2/CH3CN (1:2, v/v) solution for F− sensing were investigated by naked-eye, UV–vis, and fluorescence measurements. Both molecules with coplanar structure possess adjacent phenolic hydroxyl and 1,3,4-oxadiazole units, resulting in a sufficiently strong intramolecular hydrogen bond to hinder the association of most anions, but not F−, with phenolic hydroxyl hydrogen. They displayed the same color changes from colorless to yellow but different optical shifts upon addition of F−.