2-Aminophenols based Schiff bases as fluorescent probes for selective detection of cyanide and aluminium ions - Effect of substituents.

Three Schiff base based probes are designed and synthesized by facile condensation of a commercially available fluorophore 2,6-diformyl-4-tert-butylphenol with 4-nitro-2-aminophenol (KP1), 2-aminophenol (KP2) and 4-tert-butyl-2-aminophenol (KP3) and are characterized using various spectral techniques. The probes exhibit high selectivity and sensitivity CN and Al(III) ions with striking fluorescent signaling responses in HO-DMSO (1:1, v/v) medium. The mechanism of the probes' detection of CN involves deprotonation of the phenolic OH group(s) followed by nucleophilic addition of CN onto imine C-atom. The H NMR chemical shifts of the OH protons of 2-aminophenol moiety exhibits a linear correlation with the Hammett's substituent constants (σ), yielding a positive reaction constant (ρ). In KP1, the electron-withdrawing nitro substituent polarizes the imine bond to a larger extent than in KP2, resulting in easier addition of CN to imine C-atom. The electron releasing tert-Bu substituent in KP3 produces the opposite effect leading to a sluggish addition reaction. The separately populated HOMO and LUMO in KP1 and a relatively lower HOMO-LUMO energy gap indicate substantial intramolecular charge transfer (ICT) character, leading to weak fluorescence emission. The large reduction in HOMO-LUMO energy gap, in KP1, upon addition of cyanide is responsible for the greater enhancement in fluorescence with blue shift upon addition of CN. Formation of tetrahedral Probe-Al(III) complex prevents the isomerization of imine bond, leading to enhancement in fluorescence and contribution from chelation enhanced fluorescence. As these probes show very low limits of detection of these ions, their practical utility has also been demonstrated.