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 Ln2(OCOO−)2 unit. Each Ln2(OCOO−)2 unit links with four H₂L²⁻ ligands and four L⁴⁻ 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 Ln2(OCOO−)2 units as 8-connected nodes and L⁴⁻/H₂L²⁻ 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.