Solvent induced selective response to metal ions of three HNBO-based chemosensors

The 2-(2-hydroxy-3-naphthyl)-4-methylbenzoxazole (HNBO) fluorophore has been linked to three different amine scaffolds obtaining three fluorescent chemosensors (L1–L3). The ESIPT (excited-state intramolecular proton transfer) behavior was investigated in ACN and DMSO by UV–Vis spectrophotometric and spectrofluorimetric measurements, revealing the occurrence of an ESIPT process only in ACN, in the case of L1 and L2, and in both aprotic solvents, besides protic solvents (water and EtOH-water mixture), in the case of L3. The emission of L1–L3 is related to the deprotonated form of the ligand. The binding properties of L1–L3 towards Alkali, Alkaline-earth and some transition metal ions were investigated in different solvents by UV–Vis absorption, fluorescence emission and NMR spectroscopies. The fluorescence switches-ON in the presence of selected metal ions, always showing a mechanism different from ESIPT. All L1–L3 show a selectivity for Mg2+ in DMSO, while in ACN the selectivity of L1 and L2 shifts towards Zn2+ and Cd2+. L3 is also able to respond to Mg2+ in other aprotic solvents (THF and dioxane). L1 and L3 also showed the ability to signal the presence of metal ions on a paper support (L1: Zn2+; L3: Zn2+, Cd2+ and Mg2+). Therefore, L3 revealed able to signal the presence of Mg2+ both in solution, in different solvents, as well as on a paper support.