Synthesis and photophysical characterization of a Schiff base as anion sensor

A colorimetric anion sensor was developed using the Schiff base formed from the condensation of salicylaldehyde and 2-amino-4-nitrophenol under microwave irradiation. The Schiff base portrayed a color change upon addition of fluoride, acetate and dihydrogen phosphate (DHP) ions with a substantial bathochromic shift from 342 to 450 nm. No significant color change was observed upon addition of other anions such as chloride, bromide, perchlorate, and hydrogen sulfate. The binding constants showed the binding order AcO− ≈ F− ≫ H2PO4−. Job plots indicated the formation of a (1:1) complex (receptor:anion) with fluoride, acetate and DHP. Single crystal X-ray structure of the Schiff base indicated that the compound exists in the keto-amine tautomer in the solid state.