Temperature-induced self-assembly of two kinds Zn(ii)-based coordination polymers with luminescence properties for application in sensing and adsorption

Abstract

Temperature-induced self-assembly of coordination polymers (CPs) is of great importance for structural tunability and is quite necessary for further research on structure–property relationships. Two types of CPs {[Zn(L)(DMF)]·3DMF}_n (1) (H₂L = 4,4′-(trans-cyclohexane-1,2 diyl)bis(azanediyl)bis(carbonyl)dibenzoic acid, DMF = N,N-dimethylformamide) and {ZnL}_n (2) are synthesised by the reaction of Zn(NO₃)₂·6H₂O with H₂L at different reaction temperatures. CP 1 with a one-dimensional structure is obtained at room temperature, 35 °C and 45 °C. CP 2 with a two-dimensional structure is prepared by heating at 75 °C. When the temperature is further increased to 85 °C, 95 °C, 105 °C and 120 °C, CP 2 could still be obtained. Considering the decomposition of ligands and carbonization of solvents at high temperature, 120 °C is chosen as the temperature endpoint in the experiment. Single crystal X-ray diffraction analysis indicates that 1 has a chain structure formed by binuclear metal clusters and a bridged carboxylate group. It also indicates that 2 shows a layered structure formed through intermolecular interactions between two adjacent chain motifs. The luminescence properties of 1 and 2 are explored at room temperature in the solid-state. The luminescence intensities of 1 and 2 are weak due to the N–H vibration of the ligand. In order to improve their luminescence properties, we attempt to introduce lanthanide(III) ions into their channels. Only 2 can accept lanthanide ions Eu³⁺ and Tb³⁺ and still maintain its framework, as confirmed by PXRD. Besides, 2 can also sensitize them well. Therefore, the luminescence properties of 2 are improved vastly. The doped material can emit bright red and green light of Eu³⁺ and Tb³⁺. A series of Eu_xTb_(1−x)@ZnL coordination polymers are prepared using a post-synthesis method to realize white light emission. The luminescent material Tb³⁺@ZnL shows a highly sensitive quenching effect to acetone among common solvents. The characteristic emission of Tb³⁺ could be quenched in the presence of 8 vol% acetone. In addition, 2 could selectively adsorb methylene blue (MB) due to its porosity and can act as a green adsorbent. Compared to 1, 2 is a promising multifunctional material for applications in photoluminescence, luminescent detection and dye adsorption.