Electrochemical Synthesis of a Nitrogen‐Rich Imidazolylpyrazine for Selective “Turn‐On” Fluorescent Detection of Lanthanum Ions

A series of nitrogen-rich heterocyclic compounds were designed and synthesized as novel fluorescent ligands for the specific detection of rare earth elements. Monoimidazolylpyrazine and bisimidazolylpyrazine species (1 and 2) were synthesized via a green electrochemical approach. This method utilized platinum electrodes under constant-current conditions, achieving optimized yields while minimizing environmental impact. Subsequent tetrazole ring functionalization yielded derivatives 3 and 4, which exhibited enhanced metal coordination capabilities. Structural analyses revealed a two-dimensional planar architecture for 1 and a three-dimensional framework for 2. Compound 3 demonstrated exceptional performance as a "turn-on" fluorescent probe for La³⁺ ions, showing remarkable selectivity and significant fluorescence enhancement over other metal ions. Combined experimental and theoretical studies confirmed a unique 10:1 binding stoichiometry (3:La³⁺) revealed by isothermal titration calorimetry (ITC). The probe 3 shows a low detection limit of 2.32 µM. The efficiency stems from minimized energy dissipation and optimal ligand-to-metal energy transfer (LMET), underscoring its potential for practical sensing applications in environmental monitoring and rare-earth separation.