Abstract:

Via stepwise assembly method, discrete mononuclear Ln(Ln=Tb/Sm/Yb) complexes were synthesized from 3- or 4-pyridyl modified tripodal ligands, which were then assembled into three dimensional Ln-M(M=Ag or Cd) metal-organic frameworks(MOFs) with different topologies through the linkage of pyridyl ends. The heteronuclear MOFs sustain a characteristic emission of Ln³⁺ coordination centers, but the energy levels of the ligands are lowered in the three-dimensional packing structures, which affects the energy transfer efficiency to lanthanide centers. As a result, a stronger near-infrared luminescence is obtained in 2-Yb-Ag compared with its mononuclear counterpart, and the decay lifetime increases from 4.3 μs to 6.7 μs. While 4-Tb-Cd simultaneously emits metal-centered and ligand-based emissions, in which the decay lifetime for Tb-centered emission shortens from 2.91 ms to 0.62 ms compared with the corresponding mononuclear complex.

Key words: lanthanide emission, metal-organic framework, heteronuclear complex