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Colorimetric and fluorimetric metal ion chemosensor based on a benzimidazole functionalised Schiff base

Benzimidazole based Schiff bases exhibit structural similarity with biomolecules and due to their attractive photophysical properties offer an interesting molecular scaffold for development of chemosensor molecules. The interaction of benzimdazole based Schiff base 1 with different metal ions has been studied by absorption and fluorescence spectroscopy and computational modelling. Compound 1 exhibits changes in spectroscopic properties upon chelation with specific metal ions. The strong absorption band of 1 at 425 nm is significantly red shifted (Δλ = 30 nm) only upon complexation with Al3+, an effect observable by naked eye as a colour change from light- to dark-yellow. Of the metal ions investigated, only Al3+ caused any increase in the fluorescence intensity of 1. All other metal ions, such as K(I), Na(I), Ag(I), Li(I), Zn(II), Co(II) and Hg(II) showed negligible fluorescence response, while Cu(II) and Fe(III) significantly decreased fluorescence intensity. Compound 1 is therefore a promising colorimetric sensor for Al3+ and a fluorescence sensor for Cu2+ and Fe3+ ions. Computational analysis revealed that 1 is unionized in ethanol solution, while it becomes di-deprotonated to 12– upon the complexation with Al3+. This changes the electronic structure of 1 and leaves no acidic hydrogen atoms to allow the ESIPT process, which helps explain the changes in the photophysical properties. On the other hand, the presence of monovalent metal cations induces only mono- deprotonation to 1–, which turns out to be insufficient to be spotted in the recorded spectra.

chemosensor ; fluorescence sensor ; colorimetric sensor ; benzimidazole ; Schiff base ; DFT calculations

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