Multifunctional Lanthanide-Based Metal–Organic Frameworks with a Polyheterotopic Ligand: Doped with Ytterbium(III) for Luminescence Enhancement and Selective Dye Adsorption

The chemistry of metal–organic frameworks has been progressing fast with its exciting potential in multifunctional applications. A series of three-dimensional lanthanide-based metal–organic frameworks, {[Ln(HTPO)(NO₃)(H₂O)]⋅x(CH₃CN)⋅y(H₂O)}_n (Ln=Eu (1), Tb (2), Gd (3), Sm (4), Dy (5), Nd (6)), {[Eu(TPO)(HCOO)_0.5]⋅(H₃O)_0.5}_n (7), {[Eu(TPO)(DMF)]⋅(solv)_x}_n (8; DMF= N,N-dimethylformamide), and {[Eu(TPO)(DMA)]⋅(solv)_x}_n (9; DMA=dimethylacetamide) were synthesized with semirigid C₃-symmetric ligand tris(4-carboxylphenyl)phosphine oxide (H₃TPO). In these frameworks, the H₃TPO ligand exists in a totally different configuration. Framework 1 exhibits good breathing properties for absorbing more guest molecules through a solvent-induced single-crystal-to-single-crystal (SC–SC) transformation involving a configuration transformation of the organic linker in the framework. The ytterbium ion was doped into 1 to improve the luminescent performance (lifetime and quantum yield) of the red europium emission. Among a series of Eu_1−xYb_xTPO samples, Eu_0.88Yb_0.12TPO showed enhanced luminescence intensity (≈5.1 times that of the pure europium system), and the lifetime increased from 1073.08 to 1236.57 μs. Moreover, the porosity of these frameworks allows them to efficiently adsorb dye molecules with high selectivity and efficiency.