Anionic Lanthanide Metal–Organic Frameworks: Selective Separation of Cationic Dyes, Solvatochromic Behavior, and Luminescent Sensing of Co(II) Ion

Abstract

Four new microporous isostructural anionic lanthanide metal–organic frameworks (Ln-MOFs), [(CH₃)₂NH₂]_1.5[Ln_1.5(TATAT)(H₂O)_4.5]·x(solvent) {Ln = Tb, Eu, Dy, and Gd; H₆TATAT = 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)triisophthalate}, were successfully constructed. The Ln-MOFs are three-dimensional (3D) anionic frameworks and have two sizes of square channels (8.9 × 8.9 Å and 4.3 × 4.3 Å) with a Lewis basic nitrogen-decorated pore environment. The 3D frameworks of Ln-MOFs can be simplified as (4,6)-connected she networks. Because of the anionic framework properties, Ln-MOFs can efficiently select and separate cationic dyes in the presence of anionic or neutral dyes of similar sizes. The adsorption amounts of methylene blue for Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF are 147, 141, 133, and 143 mg g^–1, respectively. Moreover, Tb-MOF and Eu-MOF allow easy detection and identification of ethanol, acetonitrile, and diethyl ether through solvatochromism. Diethyl ether vapor also rapidly changes the colors of Tb-MOF and Eu-MOF. The photoluminescence experiments show that the absolute quantum yields of Tb-MOF (upon excitation at 341 nm), Eu-MOF (upon excitation at 396 nm), Dy-MOF (upon excitation at 341 nm), and Gd-MOF (upon excitation at 370 nm) are 32.5%, 11.0%, 2.1%, and 7.1%, respectively. In addition, Tb-MOF can detect Co²⁺ ion with high selectivity and quenching efficiency of 87%.