Enhanced luminescence in multivariate metal-organic frameworks through an isolated-ligand strategy

Numerous luminescent metal-organic frameworks (LMOFs) with desired functionalized ligands often suffer from solid-state fluorescence quenching, which greatly restricts their practical applications. The multivariate LMOFs and isolated-ligand strategy provide an effective platform to address the above defect. In this work, two new zinc-based MOFs constructed with isolated ligands [1,1 ':4 ',1 ''-terphenyl]-4,4 ''-dicarboxylic acid (H2TPDC) or desired functionalized ligands 4,4 '-(benzo[c][1,2,5]-thiadiazole-4,7-diyl)dibenzoic acid (H2BTDD), denoted as ZJU-235 and ZJU-236, were synthesized and their structures were well characterized. Then, a series of isostructural multivariate LMOFs with different ratios of H2BTDD with a solid-state fluorescence quenching feature and H2TPDC were synthesized and used to enhance the photoluminescence quantum yield of H2BTDD in ZJU-236. Thanks to the isolation of H2TPDC, the solid-state fluorescence quenching of the LMOF is suppressed, the quantum yield of the LMOF significantly increases from similar to 0.00% to as high as 80.92% and a high efficiency luminescent material is obtained. Furthermore, preliminary studies based on steady-state and time-resolved photoluminescence experiments also reveal that the solid-state fluorescence quenching can be systematically tuned by an isolated-ligand strategy. We believe that the general approach can be extended to other linker systems for the development of well-functionalized LMOFs.