Conjugated Ligands Modulated Sandwich Structures and Luminescence Properties of Lanthanide Metal–Organic Frameworks

A conjugated ligand, 2-(carboxylic acid)-6-(2-benzimidazolyl) pyridine (Hcbmp), and a series of Lanthanide metal–organic frameworks (MOFs) [Ln₂(cbmp)(ox)₃(H₂O)₂]₂·2H₃O⁺·7H₂O (Ln = Sm (3), Eu (4), and Gd (5), H₂ox = oxalic acid) have been designed and assembled. To elucidate how the conjugated ligands modulate the structures and luminescence properties, we carried out the structural characterizations and luminescence studies of complexes 3 and 4, and their corresponding oxalate complexes [Ln(ox)_1.5(H₂O)₃]·2H₂O (Ln = Sm (1) and Eu (2)) were also investigated for comparison. The changes of luminescence behaviors upon dehydration and D₂O-rehydration processes are presented and discussed in detail. The results indicated that, the cbmp^– ligands distribute on both sides of the ox^–-Ln bilayer network to construct a sandwich structure. Moreover, the lowest triplet state of cbmp^– ligands can match well the energy levels of the Sm³⁺ and Eu³⁺ cations which allow the preparation of new Ln-MOF materials with enhanced luminescence properties. Meanwhile, the crystallinity of solid states produces more substantial change in the luminescence behaviors than removal or replacement of effective nonradiative relaxers.