Construction of a series of Ln-MOF luminescent sensors based on a functional “V” shaped ligand

Abstract

It is necessary to decrease the application cost of luminescent Ln-MOF sensors to develop multiple functionalities. The ingenious design of ligands and the rational doping of Ln³⁺ ions are the main approaches to endowing Ln-MOFs with more functionalities. “V” shaped ligands can cause diamond pore channels commonly. “OC–NH” groups as hydrogen bonding sites not only can participate in supramolecular self-assembly but also can achieve molecular recognition. Based on the above considerations, a “V” shaped ligand, H₂L, with a suitable triplet state and “OC–NH” groups was designed and synthesized firstly. And the Ln-MOFs (Ln = Eu, Gd, Tb) were obtained by solvothermal reactions. Single crystal X-ray diffraction showed that Ln-MOFs had two types of diamond pore channels where “OC–NH” groups adhered to the surface. “OC–NH” groups not only played an important role in the stacking process of 2D coordinated layers but also can reduce the non-radiative transition resulting from molecular vibration. The Eu-MOF and Tb-MOF not only can emit strong “f–f” transitions characteristic of luminescence but also can detect o-phenylenediamine (OPD) and p-phenylenediamine (PPD) by luminescence quenching. Besides, Eu_xTb_1−x-MOFs (x = 0.02, 0.05, 0.1) were synthesized and can be used as ratio luminescence thermometers whose maximum relative sensitivities were 1.19% K⁻¹ at 400 K. It is pointed out specifically that the relationship between the relative sensitivities and the Eu³⁺ content was studied. What's more, our work not only developed a series of Ln-MOF luminescent sensors by designing functional ligands and doping Ln³⁺ rationally but also provided valuable knowledge for the following work.