Short Radiative Lifetime and Non-Triplet Sensitization in Near-Infrared-Luminescent Yb(III) Complex with Tripodal Schiff Base

Emergent mutual separation strategy for rare-earth elements based on crystallization of coordination compounds

Over the past several decades, rare-earth (RE) elements have been widely used in industrial products due to their exceptional physical, chemical, and mechanical properties. The discovery and industrial applications of RE elements have largely been driven by advances in their mutual separation, as RE ions exhibit highly similar chemical properties. In recent years, the growing production of RE-based products has highlighted concerns over supply risks. Recovering RE ions from end-of-life industrial products, often referred to as "urban mines," offers a sustainable solution and supports a circular RE economy. To promote RE recycling, there is a pressing need to develop separation methods with higher selectivity and lower environmental impact. This mini-review focuses on a novel strategy for RE separation based on the crystallization of coordination compounds, as reported over the past decade. First, separation systems utilizing the crystallization of coordination polymers constructed with RE ions and organic bridging ligands are summarized. Then, separation through the crystallization of discrete RE complexes with organic ligands is described, with particular emphasis on ligand structure, separation factors, and crystallization conditions.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some Tb^III and Eu^III complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of Tb^III /Eu^III emission, Eu^III /Sm^III emission, Eu^III /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

Emergence of the super antenna effect in mixed crystals of ytterbium and lutetium complexes showing near-infrared luminescence

The synthesis of luminescent molecular crystalline materials requires a good understanding of the luminescence properties of crystals in which many molecules are densely packed. Previously, we studied the near-infrared (NIR) luminescence of a trivalent ytterbium (Yb(iii)) complex with a Schiff base ligand, tris[2-(5-methylsalicylideneimino)ethyl]amine (H₃L). Herein, we extended our study on the Yb complex (YbL) to enhance and understand its solid-state luminescence via mixed crystallization with the lutetium complex (LuL). We prepared (YbL) _x (LuL)_1-x mixed crystals (x = 0.01, 0.05, 0.1, 0.2, 0.3, 0.5, and 0.7) and studied their NIR luminescence properties. The NIR luminescence intensity per Yb(iii) ion for (YbL)_0.01(LuL)_0.99 was determined to be two orders of magnitude larger than that for YbL. The excitation spectral shape of (YbL)_0.01(LuL)_0.99 was different from the absorption spectral shape of YbL but similar to that of LuL. We attribute this observation to the emergence of an intermolecular energy-migration path. In the mixed crystals, LuL molecules acted as a light-harvesting super antenna for Yb(iii) luminescence. Decay measurements of the NIR luminescence for (YbL) _x (LuL)_1-x with x > 0.2 showed mono-exponential decay, while (YbL) _x (LuL)_1-x with x < 0.1 showed a grow-in component, which reflected the lifetime of the intermediate state for energy migration. The decay lifetime values tended to increase with decreasing x, suggesting that Yb(iii) isolation resulted in a reduction in concentration quenching. We propose that the luminescence enhancement in the highly Yb-diluted conditions was mainly caused by an increase in the super antenna effect.

S-Heptazine N-ligand based luminescent coordination materials: synthesis, structural and luminescent studies of lanthanide-cyamelurate networks

Various series of lanthanide metal-organic networks denoted Ln-Cy (Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), were synthesized under solvothermal conditions using potassium cyamelurate (K₃Cy) and lanthanide nitrate salts. All obtained materials were fully characterized, and their crystal structures were solved by single-crystal X-ray diffraction. Four types of coordination modes were elucidated for the Ln-Cy series with different Ln³⁺ coordination geometries. Structural studies were performed to compare the various coordination compounds of the Ln-Cy series. Moreover, the cyamelurate linkers of rich π-conjugated and uncoordinated Lewis basic sites were used as an absorbing chromophore to enhance the luminescence quantum efficiency, the band emission and the luminescence lifetime of the coordinated Ln metal centers. Solid-state UV-visible measurements combined with density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to further explore luminescent features of the Ln-Cy series and their origins.

Selective crystallization of dysprosium complex from neodymium/dysprosium mixture enabled by cooperation of coordination and crystallization

Designing a molecular-level Ln³⁺ separation system remains a challenge for developing next-generation separation methodologies. Herein, we report crystallization-based Nd³⁺/Dy³⁺ separation using a tripodal Schiff base ligand. Highly selective crystallization of the Dy³⁺ complex was enabled by cooperation between the coordination and crystallization processes.