Influence of In-Situ Synthesized Tetrazole Coordination on the TNP Detection Efficiency of Schiff Base Zn(II) Complexes

In this study, we report the design and synthesis of three luminescent dinuclear and polynuclear Zn(II) Schiff base complexes featuring azide and tetrazole bridges. Complex JU2 has been constructed from the Schiff base HL1 (derived from 5-chlorosalicylaldehyde and 2-aminomethylpiperidine) and exhibits a 1D polymeric structure where Zn(II) centers are interconnected via azide linkages. Complexes JU3 and JU4 are synthesized using HL1 and HL2 (derived from 5-bromosalicylaldehyde and 2-aminomethylpiperidine), respectively, with tetrazole serving as a bridging unit. Single-crystal X-ray diffraction confirmed that complexes JU3 and JU4 are isostructural, with each Zn(II) ion adopting a distorted trigonal bipyramidal geometry through coordination with one deprotonated Schiff base ligand and two nitrogen atoms from the tetrazole group. The tetrazole unit links two Zn(II) centers. The luminescence properties of all complexes are systematically explored to evaluate their sensing ability toward nitroaromatic compounds. Both optical and spectroscopic studies revealed the highly sensitive detection of trinitrophenol (TNP), with limits of detection (LOD) in the nanomolar range. Mechanistic investigations, supported by DFT calculations, indicate that the fluorescence quenching process arises from a synergistic combination of photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), and charge transfer (CT) pathways. Among the three complexes, tetrazole-coordinated complexes, especially JU3, exhibit a comparatively stronger interaction with picric acid, consistent with its higher experimental sensing efficiency. The enhanced stabilization arises from the chloro-substituted phenyl ring, which provides a more favorable electronic environment for π–π stacking and noncovalent binding. These findings highlight the potential of azide- and tetrazole-bridged Zn(II) Schiff base complexes as promising candidates for the development of advanced luminescent sensors for environmental and security applications.