Supramolecular promotion of chemosensing effect in a new amino-acid-derivative macrocyclic ligand toward Fe(III) and Pb(II) by Zn (II) polymerization

A fluorescent synthetic amino acid appended with an N2O2-donor macrocyclic ligand L was prepared and characterized by single-crystal X-ray diffraction (SCXRD), elemental analysis, IR, 1H and 13C NMR, fluorescence (FL), and UV-vis spectroscopy. L exhibited an FL emission quenching in the presence of Fe(III) ion and showed a limit of detection (LOD) of 5.98 x 10-6 M while the periodic addition of EDTA to the Fe(III)/L solution in DMF reversibly enhanced the FL emission. Based on the bridging role of a water tetramer cluster (WTC) in the crystal structure of L for supporting the formation of the self-assembly of L, an aggregation-caused mechanism was proposed for the observed concentration-based quenching phenomena, confirmed by FL, and 1H NMR spectroscopy. Moreover, we synthesized a new one-dimensional Zn(II) coordination polymer (CP) [Zn(LH)(H2O) (NO3)(DMSO)]n (1) consisting of paddle-wheel Zn2(COO)4 subunits and four bridged L linkers, characterized by SCXRD method. 1 was assumed to be formed by replacing the WTC in the crystal structure of L. A parallel FL behavior for L and 1 was determined, attributable to a ligand-based electronic transition. 1 was found as a more sensitive chemosensor for Fe(III) than L, based on an improved LOD of 9.74 x 10-8 M for Fe(III). Meanwhile, the promotional effect of Zn(II) in CP of 1 has overcome the chemosensing incapability of L towards Pb(II), maybe due to the ease of trans-metalation of Pb(II) with Zn(II) in 1 instead of the direct coordination with L.