Novel naked eye chromofluorogenic azomethine imine chemosensors for the detection of F-, CN-, AcO- and H2PO4- anions

Novel naked eye chromofluorogenic chemosensors for biologically important anions F-, CN-, AcO- and H2PO4- based on 5-chloro-2-hydroxy- (1), 3,5-dichloro-2-hydroxy- (2) and 3-bromo-2-hydroxy-5-methoxybenzylidene (3) azomethine imines were synthesized. Their structure and sensing properties were studied using H-1, COSY, C-13, HSQC, HMBC and N-15 NMR, UV-Vis/fluorescence spectroscopy, X-ray diffraction analysis and DFT calculations. Compounds 1-3 exhibit low-intensity emission at 537-574 nm with an anomalous Stokes shift of 7830-8420 cm(-1). In the presence of F-, CN-, AcO- and H2PO4- ions, this emission is completely quenched and a new intense fluorescence appears at 540-590 nm with values of the relative enhancement in fluorescence intensity (I/I-0) 15-79 ((F)-) and 10-137 (CN-). Simultaneously naked-eye chromogenic effect is observed with a color change from pale yellow to intense yellow or orange. These spectral responses were attributed to the OH center dot center dot center dot An(-) bonding up to the deprotonation. The association constants found for fluoride anion are 5.5 x 10(8) M-2 (1), 1.3 x 10(3) M-1 (2) and 5.3 x10(8) M-2 (3), and to cyanide anion - 2.8 x 10(3) M-1 (1), 2.0 x 10(3) M-1 (2) and 2.8 x 10(3) M-1 (3). UV-Vis limits of detection for the F- ion are 2.0-6.0 mu M (1), and for the CN- ion - 1.0-2.0 mu M. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Novel naked eye chromofluorogenic chemosensors based on 5-chloro-2-hydroxy-, 3,5-dichloro-2-hydroxy-, and 3-bromo-2-hydroxy-5-methoxybenzylidene azomethine imines were synthesized and studied for their structure and sensing properties. The compounds exhibited low-intensity emission at specific wavelengths, which were completely quenched in the presence of F-, CN-, AcO-, and H2PO4- ions. The sensing mechanism was attributed to the formation of hydrogen bonds with the anions, resulting in a color change and intense fluorescence.