A series of benzothiazole based Schiff bases for colorimetric sensing of fluoride and acetate ions: acetate induced turn-on fluorescence for selectivity
Three benzothiazole functionalized Schiff bases have been synthesized and applied as colorimetric sensors for fluoride and acetate ions in acetonitrile. Absorption spectroscopy based sensing aptitude of L1, [(E)-1-((2-(benzo[d]thiazol-2-yl)hydrazineylidene)methyl)naphthalen-2-ol] has been enhanced by introducing coumarin moiety in L2, [(E)-3-((2-(benzo[d]thiazol-2-yl)hydrazineylidene)methyl)-4-chloro-2H-chromen-2-one]. L2 gets better of L1 with rapid naked eye detection of both fluoride and acetate ion via a palpable change in colour and 147 nm red shift. Selectivity between the two anions are accomplished by using emission spectroscopy as another Schiff base L3, with its dimeric cage like structure, can selectively sense acetate ion based on its shape complementarity emitting turn-on fluorescence at 520 nm in presence of the aforementioned ion. L2 exhibits the lowest detection limit for fluoride ions with a limit of detection as low as 3.35x10-8 M via colorimetric responses while L3 detects acetate ions down to 2.907x10-8 M with turn-on fluorescence signal. 1H NMR studies confirm the sensing mechanism to be the initial formation hydrogen bonds with –OH and –NH protons and the subsequent deprotonation of the most acidic protons upon addition of excess fluoride and acetate ions.