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 La 3 ⁺ 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 :La 3 ⁺) 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.