2D Lanthanide Coordination Polymers: Synthesis, Structure, Luminescent Properties, and Ratiometric Sensing Application in the Hydrostable PMMA-Doped Hybrid Films

Three LnIII coordination polymers [LnL(NO3)3]n·2C4H8O2 (LnL, Ln = Nd, NdL; Eu, EuL; Tb, TbL) based on an asymmetric polymonodentate ligand L, which has abundant branched end groups for chelating LnIII ions and strong light-harvesting ability to sensitize lanthanide luminescence, have been prepared and well characterized. Single-crystal X-ray diffraction studies reveal that they are isomorphic and feature unique undulated two-dimensional (2D) networks where ligand L acted as asymmetric trimonodentate connectors to bind three LnIII ions. Solid-state luminescence determination indicates that TbL emits bright green lights while EuL shows both EuIII-based red emission and L-based blue emission. For EuL, white emission with CIE coordinates of (0.3280, 0.3471) and (0.3441, 0.3236) was obtained upon excitation at 285 and 345 nm, which are close to the NTSC (National Television System(s) Committee) standard value for white emission. In addition, poly(methyl methacrylate) (PMMA) films doped with EuL and TbL (EuL@PMMA and TbL@PMMA) were developed to improve their luminescence performance, thermal stability, and water resistance. With the aid of the PMMA matrix, the emission with the CIE coordinates of (0.3304, 0.3119), which was much closer to the standard white light than that of EuL, resulted when EuL@PMMA was excited at 296 nm. Prominently, both EuL@PMMA and TbL@PMMA exhibited ratiometric emission responses toward Zn2+ selectively in water and could in-site detect Zn2+ to render them as ratio luminescence sensors for Zn2+ in aqueous solution.