Structural Variation and Switchable Nonlinear Optical Behavior of Metal–Organic Frameworks

Two europium metal-organic frameworks (MOFs) based on the same ligand, named as ZJU-23-Eu and ZJU-24-Eu, are selectively synthesized by fine-tuning solvent contents to tailor the coordination modes. Eu atoms are eight-coordinated and nine-coordinated in ZJU-23-Eu and ZJU-24-Eu respectively, and their frameworks vary in both spatial connectivity and symmetry. The ligand not only has multiphoton response but also suitable triplet energy level (19 998 cm-1 ) to sensitize Eu3+ . Thus ZJU-23-Eu exhibits characteristic emission of Eu3+ peaking at 614 nm via the energy transfer from the two-/three-photon excited ligand to Eu3+ , with its bidimensional layered structure benefiting this process. In contrast, the changed spatial connectivity in tridimensional ZJU-24-Eu narrows the distances between adjacent Eu3+ ions and reduces the density, resulting in poor two-photon excited fluorescence. Besides, noncentrosymmetric ZJU-24-Eu shows second harmonic generation (SHG) response with an intensity of ≈6.2 times relative to KH2 PO4 (KDP) microcrystalline powder while centrosymmetric ZJU-23-Eu cannot. These results have established two nonlinear optical (NLO) models based on MOFs to synchronously analyze the effects of two structural variables on different NLO behaviors, and provide ingenious ways to design MOF-based NLO devices with function on demand.