Lanthanide-organic frameworks based on terphenyl-tetracarboxylate ligands: syntheses,structures, optical properties and selective sensing of nitro explosives

Three isostructural three-dimensional (3D) lanthanide-based metal-organic frameworks Ln₂L(H₂L)(NMP)₂]·H₂O (Ln=Sm (1), Eu (2), Gd (3); H₄L=1,1′:4′,1″-terphenyl-2′,4,4″,5′-tetracarboxylic acid; NMP=N-methyl-2-pyrrolidone) have been synthesized and structurally characterized. In 1–3, two Ln³⁺ ions are doubly-bridged by two oxygen atoms of two carboxylate groups to form the dinuclear \(L{n_2}{\left( {{O_{CO{O^ - }}}} \right)_2}\) unit. Each \(L{n_2}{\left( {{O_{CO{O^ - }}}} \right)_2}\) unit links with four H₂L^2? ligands and four L^4? ligands to lead to the 3D framework, which can be rationalized as a new trinodal 4,4,8-connected (4⁴.6²)(4⁵.6)(4¹².6¹⁶) topological network by considering the dinuclear \(L{n_2}{\left( {{O_{CO{O^ - }}}} \right)_2}\) units as 8-connected nodes and L^4?/H₂L^2? ligands as planar 4-connected nodes, respectively. 1 and 3 exhibit blue emission originated from the ligand with the emission maximum at 384 nm, while 2 shows intense characteristic red emission of Eu³⁺ ions and weak ligand-centered emission. Moreover, 2 has fluorescent quenching response towards the aromatic nitro compounds, especially for the 3,4-dinitrotoluene (3,4-DNT) with the linear Stern-Volmer relationship in the concentration range of 0–1 mM and the quenching constant (K _sv) of 2.084×10³ M^?1.