Encapsulation and Sensitization of Ln³⁺ within Indium Metal-Organic Frameworks for Ratiometric Eu³⁺ Sensing and Linear Dependence of White-Light Emission

Four novel compounds {[In(FDA)(HFDA)(H₂O)₄]·2H₂O} (In1) and {[Ln₂(FDA)₂(H₂O)₁₀]·FDA·6H₂O}_n (Ln = Dy2, Eu3, Gd4) have been hydrothermally synthesized by using the ligand furan-2,5-dicarboxylic acid (H₂FDA). Driven by hydrogen bonding and π⋯π stacking, In1 displays a supramolecular metal-organic framework (MOF) with 6-connected pcu topology. Isostructural Dy2, Eu3, and Gd4 possess one-dimensional "wave-like" chains and further exhibit bnn topological structures with the Schläfli symbol of (4⁶·6⁴) via a hydrogen bonding network. The triplet state (T₁ = 22371 cm^-1) of H₂FAD studied by compound Gd4 indicates that the energy transition from the H₂FDA ligand to the Eu³⁺ ion in compound Eu3 is efficient but inefficient to the Dy³⁺ ion in compound Dy2. Interestingly, the uncoordinated carbonyl group of compound In1 can sensitize Ln³⁺ ions to obtain Dy³⁺@In1 and Eu³⁺@In1 materials for enhancing luminescence. The possible sensitization mechanism is studied by X-ray photoelectron spectroscopy and surface photovoltage spectroscopy. When the Eu³⁺ ion is increased from 0.0 to 1.6 μM in aqueous solution, compound In1 exhibits a distinguishable luminescence color change from blue to red with a fast detection time (<2 min), and the ratio of luminescence intensity at 617 nm to that at 408 nm (I₆₁₇/I₄₀₈) enhances linearly with a low detection limit of 0.87 μM. Additionally, with careful adjustment of lanthanide(III) ions, white-light emission material could be achieved with metal content of Dy_0.87Eu_0.13.