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.