Syntheses, Structures, Near-Infrared and Visible Luminescence, and Magnetic Properties of Lanthanide-Organic Frameworks with an Imidazole-Containing Flexible Ligand

Reactions of tripodal ligand 1,3,5-tris(imidazole-1-ylmethyl)-2,4,6-trimethylbenzene (L) with lanthanide metal salts and triethyl orthoformate led to the formation of six bowl-like dinuclear compounds [Ln₂(L)(HL)(NO₃)₆(HCOO)]·3CH₃OH (Ln = Gd 1, Tb 2, Dy 3, Er 4, Yb 5, and Eu 6). The single-crystal X-ray diffraction analysis revealed that six complexes are isomorphous and isostructural and that the dinuclear molecules are further connected by hydrogen bonds and π−π interactions, resulting in 3D channel-like structures. The luminescence properties have been studied, and the results showed that the Tb(III) (2) and Eu(III) (6) complexes exhibited sensitized luminescence in the visible region and their luminescence lifetimes in powder and DMSO-d₆ solution are in the range of milliseconds. The Yb(III) complex (5) emits typical near-infrared luminescence in DMSO-d₆ solution. Variable-temperature magnetic susceptibility measurements of 1−6 showed that complex 1 (Gd) is nearly a paramagnet and complexes 2 (Tb), 3 (Dy), and 4 (Er) show the ferromagnetic coupling between magnetic centers, whereas the depopulation of the Stark levels in complexes 5 (Yb) and 6 (Eu) leads to a continuous decrease in χ_MT when the sample is cooled from 300 to 1.8 K.