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) [Ln<sub>2</sub>(cbmp)(ox)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub>2</sub>·2H<sub>3</sub>O<sup>+</sup>·7H<sub>2</sub>O (Ln = Sm (<b>3</b>), Eu (<b>4</b>), and Gd (<b>5</b>), H<sub>2</sub>ox = 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 <b>3</b> and <b>4</b>, and their corresponding oxalate complexes [Ln(ox)<sub>1.5</sub>(H<sub>2</sub>O)<sub>3</sub>]·2H<sub>2</sub>O (Ln = Sm (<b>1</b>) and Eu (<b>2</b>)) were also investigated for comparison. The changes of luminescence behaviors upon dehydration and D<sub>2</sub>O-rehydration processes are presented and discussed in detail. The results indicated that, the cbmp<sup>–</sup> ligands distribute on both sides of the ox<sup>–</sup>-Ln bilayer network to construct a sandwich structure. Moreover, the lowest triplet state of cbmp<sup>–</sup> ligands can match well the energy levels of the Sm<sup>3+</sup> and Eu<sup>3+</sup> 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.