A Novel Fluorescent Probe for Dual-Channel Detection of Cu(II) and Zn(II): Experimental and Theoretical Study

The development of selective fluorescent probes for biologically and environmentally relevant metal ions remains a significant challenge in coordination chemistry. Herein, we report a novel fluorescent probe, H₂L·DMSO, for the discriminative detection of Cu(II) and Zn(II). In solution, the free probe exhibits a yellow color arising from intramolecular charge transfer (ICT) and emits medium-green fluorescence ((Formula presented) ) via excited keto-state radiative decay. The probe displays pronounced selectivity for Cu(II) and Zn(II) over other competing metal ions, with both inducing comparable colorimetric shifts from yellow to orange-red upon coordination, attributed to ligand-to-metal charge transfer and ligand-centered transitions, respectively. Critically, the fluorescence responses at 525 nm diverge orthogonally between the two ions: Cu(II) quenches the emission ((Formula presented) ) via a chelation-enhanced quenching (CHEQ) mechanism, attributable to its paramagnetic nature and high intersystem crossing (ISC) efficiency, whereas Zn(II) enhances the emission ((Formula presented) ) via a chelation-enhanced fluorescence (CHEF) mechanism, arising from its diamagnetic character and correspondingly low ISC efficiency. Both metals form 1:1 complexes with binding constants of 10.00 μM⁻¹ for Cu(II) and 6.67 μM⁻¹ for Zn(II), and limits of detection of 0.056 μM and 0.0038 μM, respectively, both well below WHO-permissible limits for drinking water. Binding equilibria are established within ten seconds of metal-ion addition, and both interactions are fully reversible upon EDTA treatment. Furthermore, DFT computations confirm that H₂L·DMSO binding to Cu(II) and Zn(II); and their divergent fluorescence arises from the excited-state dynamics, rather than from the ground-state orbital gaps. The probe operates efficiently in buffered ethanol, highlighting its practical utility for environmental monitoring applications.