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 χMT when the sample is cooled
from 300 to 1.8 K.