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.