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

<p>Herein we present the interaction of benzimidazole based Schiff base <b>1</b> with different metal ions studied by absorption and fluorescence spectroscopies and computational modelling. Compound <b>1</b> exhibits changes in spectroscopic properties upon chelation with specific metal ions. Its strong absorption band at 425 nm is significantly red shifted (Δ<i>λ</i> = 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 <b>1</b>. 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 <b>1</b> is a promising colourimetric sensor for Al³⁺, and a fluorescence sensor for Cu²⁺ and Fe³⁺ ions. Computational analysis revealed that <b>1</b> is unionized in ethanol solution, while it becomes di-deprotonated to <b>1</b>^<b>2–</b> upon the complexation with Al³⁺. This changes the electronic structure of <b>1</b> 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 <b>1</b>^<b>–</b>, which turns out to be an insufficient electronic change to be spotted in the recorded spectra.