Highly selective Cu²⁺ detection with a naphthalimide-functionalised pillar[5]arene fluorescent chemosensor

Ligand 1, a rim-differentiated pillar[5]arene macrocycle modified with five naphthalimide groups through click chemistry, serves as an effective ratiometric fluorescent chemosensor for Cu²⁺. In contrast to the monomeric naphthalimide control compound 2, which shows only monomer emission, ligand 1 demonstrates dual emission characteristics encompassing both the monomer and excimer of the naphthalimide moieties. The binding properties of ligand 1 toward 15 different metal ions were systematically investigated in CH₂Cl₂/CH₃CN (v/v, 1 : 1) by UV-vis and fluorescence spectroscopy. Remarkably, ligand 1 exhibits exceptional selectivity for Cu²⁺ ions. Upon complexation with Cu²⁺, the excimer emission of ligand 1 diminishes, concomitant with an enhancement of its monomer emission. The binding ratio for 1·Cu²⁺ was determined to be 1 : 1, with an association constant of (3.39 ± 0.40) × 10⁵ M^-1 calculated using a nonlinear least-squares curve-fitting method. Furthermore, the limit of detection (LOD) was found to be 185 ± 7 nM. Our results from ¹H NMR titration, high-resolution mass spectrometry analysis and density functional theory calculations of 1·Cu²⁺ suggest synergistic coordination between Cu²⁺ and the triazole groups on ligand 1.