Selective Detection of Picric Acid Using 2D Zinc-Coordination Polymer (Zn-CP): “Turn-On” Fluorescence Response Triggered by PET Suppression

A Zn(II)-based coordination polymer {[Zn(L)(DEF)]}_n (Zn-CP), with the 2,4-c network, was synthesized under solvothermal conditions. Zn(II) coordinated with the ligand H₂L (5-((anthracen-9-ylmethyl)amino)isophthalic acid) in the presence of diethylformamide (DEF) to form a 2D coordination polymer. The Zn-CP was utilized for selective detection of mutagenic explosive picric acid (PA) in the presence of other nitroaromatics, viz., nitrobenzene (NB), meta-dinitrobenzene (MDNB), trinitrotoluene (TNT), 4-nitrophenol (4-NP), and 4-nitrotoluene (4-NT). The probe Zn-CP showed enhanced fluorescence selectively in the presence of picric acid with a detection limit of 9.72 ppm and a binding constant of 3 × 10⁴ M^–1. This fluorescence ″turned on″ mechanism is due to the suppression of photoinduced electron transfer (PET) and the rigidification of the Zn-CP framework, as revealed by DFT studies. Additionally, the nuclear magnetic resonance (NMR) titration of PA with the Zn-CP revealed the formation of a Zn-CP···PA adduct, which was further confirmed by atom-in-molecule (AIM) analysis. PXRD studies confirmed the stability of the Zn-CP under water, various solvents, and ambient conditions.