Luminescent Zn(II) Coordination Polymers for Highly Selective Sensing of Cr(III) and Cr(VI) in Water

Three photoluminescent zinc coordination polymers (CPs), {[Zn2(tpeb)2(2,5-tdc)­(2,5-Htdc)2]·2H2O} n (1), {[Zn2(tpeb)2(1,4-ndc)­(1,4-Hndc)2]·2.6H2O} n (2), and {[Zn2(tpeb)2(2,3-ndc)2]·H2O} n (3) (tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene, 2,5-tdc = 2,5-thiophenedicarboxylic acid, 1,4-ndc = 1,4-naphthalenedicarboxylic acid, and 2,3-ndc = 2,3-naphthalenedicarboxylic acid) were prepared from reactions of Zn­(NO3)2·6H2O with tpeb and 2,5-H2tdc, 1,4-H2ndc, or 2,3-H2ndc under solvothermal conditions. Compound 1 has a two-dimensional (2D) grid-like network formed from bridging 1D [Zn­(tpeb)] n chains via 2,5-tdc dianions. 2 and 3 possess similar one-dimensional (1D) double-chain structures derived from bridging the [Zn­(tpeb)] n chains via pairs of 1,4-ndc or 2,3-ndc ligands. The solid-state, visible emission by 1–3 was quenched by Cr3+, CrO4 2–, and Cr2O7 2– ions in water with detection limits by the most responsive complex 3 of 0.88 ppb for Cr3+ and 2.623 ppb for Cr2O7 2– (pH = 3) or 1.734 ppb for CrO4 2– (pH = 12). These values are well below the permissible limits set by the USEPA and European Union and the lowest so far reported for any bi/trifunctional CPs sensors. The mechanism of Cr3+ luminescence quenching involves irreversible coordination to free pyridyl sites in the CP framework, while the Cr6+ quenching involves reversible overlap of the absorption bands of the analytes with those of the excitation and/or emission bands for 3.