Colorimetric Sensors for Anion Recognition Based on the Proton Transfer Signaling Mechanism

Phenolic hydroxyl based sensors N,N-bi(salicylidene)-1,2-phenylenediamine (1), 2-[(4-nitrophenylimino)-methyl] phenol (2) and 2-[p-tdylimino methyl] phenol (3) bearing Schiff-base groups can act as selective colorimetric sensors for anions, which exhibit distinct color changes in the presence of fluoride, but show no response to other halogen anions. They also give response to acetate, which is clearly visible to the naked eyes. The selectivity can be rationalized on the proton transfer signaling mechanism. The sensor 1 containing plural phenolic groups undergoes stepwise deprotonation of the two O-H fragments when it interacts with excess fluoride, which is confirmed by UV-Vis, H-1 NMR and F-19 NMR spectroscopic methods.