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

Abstract

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 Dy0.87Eu0.13.