Mechanistic insights into heavy metal ion sensing by NOS2-macrocyclic fluorosensors via the structure-function relationship: influences of fluorophores, solvents and anions

Abstract

To obtain a mechanistic understanding of the effects derived from fluorophores, solvents and anions on heavy metal sensing, two NOS2-macrocycle-based fluorosensors with different fluorophores (L-1: 9-methylanthracene, L-2: benzothiazolyl) were synthesised. In this regard, particular attention was given to monitoring the cation-ligand, cation-anion and cation-solvent interactions from the detailed complexation processes in both the solution and solid states while considering the structure-function relationship. L-1 showed turn-on type silver(I) selectivity over other metal ions, including mercury(II), in ethanol. According to the complexation results obtained by titration (UV-vis, fluorescence and NMR), cold-spray ionization mass spectrometry and X-ray crystallography, the observed silver(I) sensing by L-1 is mainly due to its 1 : 1 complexation with silver(I) via the Ag-N-tert bond and the strong solvation of mercury(II). Thus, the turn-on sensing for silver(I) can be explained by the CHEF effect, in which the coordination of silver(I) to the receptor unit effectively prevents PET quenching. As a dual-probe (UV-vis and fluorescence) chemosensor, L-2 showed fluorescence turn-off type selectivity for both silver(I) and mercury(II) in ethanol. In acetonitrile, L-2 showed improved fluorescence turn-off type selectivity for mercury(II) with ClO4- and NO3-; however, no such responses were observed with other anions, such as Cl-, Br-, I-, SCN-, OAc- and SO42-. Together with the complexation results by titration, the crystal structures of an endocyclic mercury(II) perchlorate complex and an exocyclic mercury(II) iodide complex revealed that the anion-controlled mercury(II) sensing by L-2 arises from the endo- and exo-coordination modes depending on the anion coordinating ability, which induces either metal-receptor/fluorophore binding (Hg-N-tert and Hg-N-fl) or no binding. Taken collectively, the photophysical, thermodynamic and structural results of the complexations herein suggest that the sensing properties of heavy metal ions by macrocycle-based fluorosensors are very sensitive not only to the cation-receptor and cation-fluorophore interactions but also to the cation-anion (endo/exo-coordination modes) and/or cation-solvent interactions.