Harnessing Valence Tautomerism and CHEF Effect for Distinctive Sensing of Hg(II) and Cu(II) Using Dyad Fluorophores

We report a compelling structure–property relationship in a series of four naphthalimide-derived ligands, engineered for the selective and differential detection of Hg(II) and Cu(II) ions. By integrating a dansyl moiety, we induce intramolecular electron transfer that gives rise to two non-redox-innocent dyad systems. Notably, the detection of Hg(II) operates through a rare chelation-enhanced fluorescence (CHEF) mechanism—an unusual pathway considering the heavy atom effect typically associated with mercury. In contrast, the Cu(II) dyads reveal striking valence tautomerism in solution, driven by ligand-to-metal electron transfer that reduces Cu(II) to Cu(I), as conclusively demonstrated via EPR and XPS analysis. Furthermore, introducing a catalytic substrate probe highlights that this redox transformation is the bottleneck in catalytic efficiency. These findings provide deep mechanistic insights and position these ligands as versatile platforms for dual-mode ion sensing and redox-responsive catalysis.