Zinc(II)-Based Ring-and-Rod Coordination Layer as an Excitation-Wavelength-dependent Dual-Emissive Chemosensor for Discriminating Fe3+, Cr3+, and Al3+ in Water

Abstract

The reactions of Zn(NO₃)₂, 3,6-bis(pyridin-3-yl)-9H-carbazole (bpycz), and 2,5-dihydroxyterephthalic acid (H₄dhbdc) or 2-bromoterephthalic acid (Br-1,4-H₂bdc) under hydro(solvo)thermal conditions yielded corresponding coordination polymers (CPs) {[Zn(H₂dhbdc)(bpycz)]•0.5H₂O}_n (1) and [Zn(Br-1,4-bdc)(bpycz)]•2DMAc•H₂O (2), respectively, with high thermal stability approaching 350 °C. CP 1 adopts a ring-and-rod layer structure, which is topologically described as a 4-connected net with the point symbol of 2•6⁵. Two layers are interpenetrated in parallel interlocking mode to form a double 2D → 2D polyrotaxane entanglement with extra-framework void space of 19.6%. CP 2 has a non-interpenetrating ring-and-rod layer structure of 4-connected 2•6⁵ net topology, with extra-framework void space of 16.6%. Thermally activated 1 and 2 revealed CO₂ uptakes of 101.1 and 98.6 cm³ g^-1, respectively, at P/P₀ = 1 and 195 K. X-ray powder diffraction (XRPD) patterns confirmed that 1 and 2 both possessed high chemical stability in H₂O, CH₃OH, acetone, and DMF, and framework stability during gas adsorption-desorption. The H₂O suspension of 1 displayed excitation-dependent dual-emissive properties, appearing at 432 nm upon excitation at 300 nm and at 528 nm upon excitation at 365 nm. Of note, 1 was capable of detection of Fe³⁺, Cr³⁺, and Al³⁺ ions in H₂O, showing good anti-interference ability, excellent selectivity, and high sensitivity. More interesting, the dual-emissive properties make 1 to be an excellent luminescence chemosensor to screen Fe³⁺, Cr³⁺, and Al³⁺ from a pool of metal ions in H₂O upon excitation at 300 nm via luminescence quenching effect and then discriminate Fe³⁺, Cr³⁺, and Al³⁺ upon excitation at 365 nm via luminescence quenching, unaltered, and enhancement responses, respectively. On the other hand, the H₂O suspension of 2 demonstrated an excitation-independent emission appearing at around 430 nm, which could be utilized to sensitively detect Fe³⁺ and Cr³⁺ ions with good anti-interference ability and excellent selectivity via luminescence quenching effect. Further, 1 and 2 were recyclability and possessed cycling stability. The plausible sensing mechanisms for 1 and 2 toward Fe³⁺, Cr³⁺, and Al³⁺ were also explored in detail.