Quantum chemical elucidation of the turn-on luminescence mechanism in two new Schiff bases as selective chemosensors of Zn²⁺: synthesis, theory and bioimaging applications

Abstract

We report the synthesis and characterization of two new selective zinc sensors (<i>S</i>,<i>E</i>)-11-amino-8-((2,4-di-<i>tert</i>-butyl-1-hydroxybenzylidene) amino)-11-oxopentanoic acid (A) and (<i>S</i>,<i>E</i>)-11-amino-8-((8-hydroxybenzylidene)amino)-11-oxopentanoic acid (B) based on a Schiff base and an amino acid. The fluorescent probes, after binding to Zn²⁺ ions, presented an enhancement in fluorescent emission intensity up to 30 times (<i>ϕ</i> = A 50.10 and B 18.14%). The estimated LOD for compounds A and B was 1.17 and 1.20 μM respectively (mixture of acetonitrile : water 1 : 1). Theoretical research has enabled us to rationalize the behaviours of the two selective sensors to Zn²⁺ synthesized in this work. Our results showed that in the free sensors, PET and ESIPT are responsible for the quenching of the luminescence and that the turn-on of luminescence upon coordination to Zn²⁺ is mainly induced by the elimination of the PET, which is deeply analysed through EDA, NOCV, molecular structures, excited states and electronic transitions <i>via</i> TD-DFT computations. Confocal fluorescence microscopy experiments demonstrate that compound A could be used as a fluorescent probe for Zn²⁺ in living cells.