Lanthanide\nMetal–Organic Framework Isomers\nwith Novel Water-Boosting Lanthanide Luminescence Behaviors

Abstract

Lanthanide\nmetal–organic frameworks (Ln-MOFs) with exceptional\noptical performance and structural diversity offer a unique platform\nfor the development of luminescent materials. However, Ln-MOFs often\nsuffer from luminescence quenching by high-vibrating oscillators,\nespecially in aqueous solution. Thus, multiple strategies have been\nadopted to improve the luminescence of Ln³⁺. Anomalous\nresearch about water-induced lanthanide luminescence enhancement of\nLn-MOFs is in the primary stage. Here, two Eu-based metal–organic\nframework (Eu-MOF) isomers named QXBA-Eu-1 and QXBA-Eu-2 were constructed\nby using the same ligand under different solvent thermal conditions,\nwhich exhibited distinctive water- and methanol-boosting emission\nbehaviors. As for QXBA-Eu-1, water and methanol molecules replaced\nthe free <i>N</i>,<i>N</i>-dimethyl­acetamide\n(DMA) molecules in the framework, repressed the rotation or libration\nsuppression of the QXBA linker, and formed hydrogen bonds with the\ncoordinated water molecules, which suppressed the O–H high-energy\nvibrations, reduced nonradiative transitions, stabilized the T₁ state, and facilitated the intersystem crossing (ISC) process.\nFor QXBA-Eu-2, water molecules tended to replace the coordinated DMA\nligands, which altered the S₁ and T₁ energy\nlevels of the ligand and facilitated the ligand-to-metal energy transfer\n(LMET) process and strengthened the luminescence of Eu³⁺. Importantly, free solvent molecules and the hydroxylation of Eu³⁺ centers also restrained the rotation or libration of the\nQXBA linker, by which the nonradiative transition was further inhibited\nand the lanthanide luminescence enhanced. Thus, this work not only\nopened an unprecedented path to enhance lanthanide luminescence in\naqueous solution but also expanded its application scope.