Selective sensing of picric acid using a Zn(ii)-metallacycle: experimental and theoretical validation of the sensing mechanism and quantitative analysis of sensitivity in contact mode detection

Abstract

A combination of N,N′,N′′-tris(3-pyridyl)-1,3,5-benzenetricarboxamide (L1) and p-chlorobenzoic acid (HL2) with Zn(NO₃)₂·6H₂O resulted in the formation of a dinuclear metallacycle [ZnL1(L2)₂(DMF)₂]₂ (1(DMF)₄). In 1(DMF)₄, the Zn(II) centre adopts a square pyramidal geometry, while one of the pyridyl N out of the three pyridyl groups in L1 remained uncoordinated. Solvated DMF molecules are present in 1(DMF)₄. The structural and chemical nature of 1(DMF)₄ is effective for it to act as a potential fluorescent probe for the detection of nitroaromatic compounds. It is observed that the probe, 1(DMF)₄, could selectively detect picric acid (PA) among various aromatic compounds in solution (DMSO), while the solid state (contact mode) detection showed a positive sensing response for the nitrophenols (PA: 87% quenching efficiency, 2,4-dinitrophenol (2,4-DNP): 57% quenching efficiency and 4-nitrophenol (4-NP): 40% quenching efficiency). The limit of detection (LOD) of PA by the probe in DMSO was found to be 6.8 × 10⁻¹¹ M while the LOD in contact mode detection was estimated to be 0.49 ng cm⁻². The mechanism of selective detection of PA by 1(DMF)₄ in DMSO was analyzed through photophysical studies, ¹H-NMR experiments and also by density functional theory (DFT) calculations. The effective overlap of the absorption spectrum of 1(DMF)₄ and emission spectrum of PA in DMSO suggests that the Förster resonance energy transfer (FRET) is responsible for quenching phenomena in DMSO. The DFT calculations and molecular docking studies showed the adduct formation due to the favorable interactions between 1(DMF)₄ and PA in DMSO, while negligible interactions were observed between 1(DMF)₄ with other aromatic compounds. The experimental and DFT studies showed that the efficient sensing ability of PA by 1(DMF)₄ in the solid-state was due to photoelectron transfer (PET) and FRET phenomena described herein.