Luminescence Modulation, White Light Emission, and Energy Transfer in a Family of Lanthanide Metal–Organic Frameworks Based on a Planar π-Conjugated Ligand

Three isostructural lanthanide metal–organic frameworks (Ln-MOFs), {[Ln2(BDPO)1.5(DMA)3(H2O)]·5H2O }n (1-Ln, Ln = Eu, Gd, and Tb) were constructed by a less-investigated large delocalized π-electron conjugated ligand N,N′-bis(3,5-dicarboxyphenyl)-oxalamide (H4BDPO). 1-Ln reveals unprecedented trinodal (4,4,6)-connected networks based on binuclear cluster units and planar BDPO linkers. Density functional theory (DFT) and time dependent-DFT calculations were performed on the ground and excited states to provide insight into geometries, the frontier molecular orbitals, and singlet and triplet state energies of H4BDPO ligand. 1-Eu and 1-Tb exhibit strong typical red and green emission of Eu3+ and Tb3+ ions, respectively, originating from the highly efficient energy transfer from BDPO to metal centers, as demonstrated by the comparison of triplet excited energies of ligands and metal ions. Importantly, by doping different concentrations of Eu3+ and Tb3+ ions, a series of dichromatic doped 1-EuxTb1-x MOFs were fabricated, showing an unusual fluent change of luminescent color among green, yellow, orange, and red. Meanwhile, the trimetallic doped 1-Eu0.0855Gd0.6285Tb0.2860 emits white light, with the correlated color temperature of 5129 K as well as an absolute quantum yield of 22.4%.