Aggregation-induced emission of a Eu(III) complex via ligand-to-metal charge transfer

Thiacalix[4]arene-Sandwiched Sandglass-like Ln9 Clusters (Ln = Tb and Eu): Insights into the Selective Luminescence Quenching Properties by p-Nitrobenzene Derivatives

Nonanuclear lanthanide clusters Ln₉ (Ln = Tb and Eu) based on p-tert-butylthiacalix[4]arene (H₄TC4A) have been synthesized by the solvothermal reaction and were structurally determined by single-crystal X-ray diffraction. The framework of Ln₉ can be termed as a sandglass-like structure whose two Ln₄-TC4A polynuclear secondary building units are bridged by one octa-coordinate {Ln(μ₃-O)₈} unit. Efficient TC4A-to-Ln energy transfer was observed for Tb₉ but not for Eu₉. The luminescence quantum yield (QY) of Tb₉ in the solid state at room temperature was determined to be 17.6%, while its highest QY in a methanolic solution (2 × 10^-5 mol/L) is 59.2% upon excitation at 318 nm. The luminescence of Tb₉ was quenched selectively by derivatives of p-nitrobenzene, as demonstrated by the results of photoluminescence and UV-vis titration experiments and supported by density functional theory calculations. We believe that the interactions between the analyte molecules and the pocket of Tb₉ are primarily responsible for the observed quenching. As such, this work represents one of the few examples of utilizing structurally interesting lanthanide cluster complexes as a sensory platform for the recognition of meaningful analytes and portends the further development of lanthanide-calixarene-complex-based functional materials.

Bright red emission with high color purity from Eu(iii) complexes with π-conjugated polycyclic aromatic ligands and their sensing applications

Eu(iii) complexes emit red light with a high color purity and have consequently attracted attention for development toward display and physical sensing applications. The characteristic pure color emission originates from the intra-4f-4f transition, and the brightness strongly depends on the electronic and steric structures of organic ligands. A large π-conjugated ligand design with a large absorption coefficient has been actively studied for achieving bright emission. The π-conjugated Eu(iii) luminophores also provide oxygen and temperature sensing properties by controlling their excited state dynamics based on π-electron systems. A comprehensive understanding of the design strategy of large π-conjugated ligands is crucial for the further development of luminescent Eu(iii) complexes. In this review, we summarize the research progress on π-conjugated Eu(iii) luminophores exhibiting bright emission and their physical sensing applications.