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. "OC-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 "OC-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 "OC-NH" groups adhered to the surface. "OC-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, EuxTb1-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.