Luminescence properties of europium (III)-based metal–organic frameworks: Influence of varied organic linkers

Four new functionalized europium (Eu) based MOFs; Eu-BDC-PC, Eu-BDC-MA, Eu-BDC-SA, and Eu-BDC-DPC were synthesized with different organic linker's through post synthetic modification (PSM) strategy using Eu-BDC-NH2. The obtained Eu-MOFs were investigated for photoluminescence studies. Pristine Eu-BDC-NH2, functionalized Eu-BDC-PC and Eu-BDC-DPC depicted very intense red emission. While, Eu-BDC-MA and Eu-BDC-SA depicted fairly low red emission, this is attributed to differences in energy transfer from linkers to europium metal. The asymmetry ratio values of pristine Eu-BDC-NH2 and functionalized Eu-BDC-PC suggested that very low symmetry around europium ion. On the other hand, asymmetry value is relatively lower in Eu-BDC-DPC indicates the high symmetry around europium metal compared to Eu-BDC-NH2 and Eu-BDC-PC. Luminescence lifetime measurements in all the samples depicted monoexponential decay suggested homogenous distribution of europium ions in the MOF without inversion symmetry. The fluorescence lifetime values of all the functionalized Eu-MOFs (<0.57 ms) are less than the pristine Eu-BDC-NH2 (0.87 ms). The color correlated temperature (CCT) value of Eu-BDC-MA (10,000 K) and Eu-BDC-SA (4800 K) suggested near white emission with bluish hue know as cool white. Judd-Ofelt analysis also performed for all the Eu-MOFs. The trend in JO parameter and branching ratio suggests high color purity of pristine and Eu-BDC-PC samples where the Eu-BDC-DPC samples has higher excited state lifetime. Therefore, the work explained here will surely give a path to synthesize varied Ln-based MOFs with high efficiency for optoelectronic devices and LEDs by further tuning the light via PSM.