Salicylidene-based dual-responsive 'turn on' fluorometric chemosensors for the selective detection of Zn, Al and F ions: theoretical investigation and applications in the live cell imaging of zebrafish larvae and molecular logic gate operation.

Four salicylidene-based dual-responsive chemosensors 1,5-bis(5-bromosalicylaldehyde)carbohydrazone (R1), 1,5-bis(5-bromosalicylaldehyde)thiocarbohydrazone (R2), 1,5-bis(3-ethoxysalicylaldehyde)carbohydrazone (R3) and 1,5-bis(3-ethoxysalicylaldehyde)thiocarbohydrazone (R4) were synthesized and characterized. The molecular structures of R1 and R3 were confirmed by single crystal X-ray diffraction technique, which crystallized in the orthorhombic and monoclinic 2/ space groups, respectively. The chemosensor molecules were investigated for their recognition properties against the selected cations (K, Ca, Mn, Co, Ni, Cu, Zn, Fe and Al) and anions (F, Cl, Br, I, HSO, HPO, ClO, N and NO) by colorimetry, absorption spectroscopy, fluorescence spectroscopy, H NMR spectroscopy and theoretical studies. The sensor molecules showed colorimetric responses for the Co, Ni, Cu and Fe cations and the F anion. Interestingly, the Zn and Al cations showed only the 'turn on' fluorometric response, whereas the F anion showed both colorimetric and fluorometric responses. The binding constants were determined using the Benesi-Hildebrand (B-H) equation from the fluorescence titrations and found to be higher for R3 towards the Al cation (2.03 × 10 M) with a low limit of detection (1.79 μM) and for R4 towards the F anion (5.13 × 10 M) with a low limit of detection (5.23 μM). The chemosensors established 1 : 2 and 1 : 1 binding stoichiometries with the sensed cations and anion, respectively, as confirmed by Job's plots. The computational studies show a lower band gap of HOMO-LUMO when the chemosensors bind with the sensed inorganic ions compared to the free chemosensors. Furthermore, the observed fluorescent behaviour of the Zn and Al cations have motivated us to investigate the practical applications in the live cell-imaging of zebrafish larvae as well as in the development of a molecular logic gate.