Lanthanide Metal-Organic Framework Isomers with Novel Water-Boosting Lanthanide Luminescence Behaviors

Lanthanidemetal-organic frameworks (Ln-MOFs) with exceptionaloptical performance and structural diversity offer a unique platformfor the development of luminescent materials. However, Ln-MOFs oftensuffer from luminescence quenching by high-vibrating oscillators,especially in aqueous solution. Thus, multiple strategies have beenadopted to improve the luminescence of Ln(3+). Anomalousresearch about water-induced lanthanide luminescence enhancement ofLn-MOFs is in the primary stage. Here, two Eu-based metal-organicframework (Eu-MOF) isomers named QXBA-Eu-1 and QXBA-Eu-2 were constructedby using the same ligand under different solvent thermal conditions,which exhibited distinctive water- and methanol-boosting emissionbehaviors. As for QXBA-Eu-1, water and methanol molecules replacedthe free N,N-dimethyl-acetamide(DMA) molecules in the framework, repressed the rotation or librationsuppression of the QXBA linker, and formed hydrogen bonds with thecoordinated water molecules, which suppressed the O-H high-energyvibrations, reduced nonradiative transitions, stabilized the T-1 state, and facilitated the intersystem crossing (ISC) process.For QXBA-Eu-2, water molecules tended to replace the coordinated DMAligands, which altered the S-1 and T-1 energylevels of the ligand and facilitated the ligand-to-metal energy transfer(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 theQXBA linker, by which the nonradiative transition was further inhibitedand the lanthanide luminescence enhanced. Thus, this work not onlyopened an unprecedented path to enhance lanthanide luminescence inaqueous solution but also expanded its application scope.