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

Abstract

Herein we present the interaction of benzimidazole based Schiff base 1 with different metal ions studied by absorption and fluorescence spectroscopies and computational modelling. Compound 1 exhibits changes in spectroscopic properties upon chelation with specific metal ions. Its strong absorption band at 425 nm is significantly red shifted (Δλ = 30 nm) only upon the complexation with Al3+, an effect observable by a 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, including K(I), Na(I), Ag(I), Li(I), Zn(II), Co(II) and Hg(II), showed a negligible fluorescence response, while Cu(II) and Fe(III) significantly decreased the fluorescence intensity. Therefore, system 1 is a promising colourimetric 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. On the other hand, the presence of monovalent metal cations induces only mono-deprotonation to 1–, which turns out to be an insufficient electronic change to be spotted in the recorded spectra.