Exceptionally long-lived luminescence emitted from Tb^III ion caged in an Ag^I-Tb^III-thiacalix[4]arene supramolecular complex in water

The compositions and photophysical properties of luminescent ternary complexes of thiacalix[4]arene-p-sulfonate (TCAS), Tb^III, and Ag ^I ions were determined. At pH 6, Ag^I₂· Tb^III₂·TCAS₂ formed. Moreover, at pH 10, in the presence of a 20-fold excess of Ag^I and a 50-fold excess of TCAS with respect to Tb^III, Ag^I₂·Tb ^III·TCAS₂ formed as the main luminescent species. The structure of these complexes was proposed: two TCAS ligands are linked by two S-Ag^I-S linkages to adopt a double-cone supramolecular structure. Furthermore, each Tb^III ion in the former complex accepts O ^-, S, O^- donation, whereas in the latter, the Tb ^III center accepts eightfold O^- donation. The luminescence quantum yield (Φ) of Ag^I₂·Tb ^III₂·TCAS₂ (0.16) was almost equal to that of Tb^III·TCAS, but the luminescence lifetime τ of the former (= 1.09 ms) was larger than that of the latter. For Ag^I ₂·Tb^III·TCAS₂, the yield Φ (= 0.11) was small, which is attributed to the low efficiency of photosensitization (η = 0.11). However, the τ value (4.61 ms) was exceptionally large and almost equal to the natural luminescence lifetime of Tb^III (4.7 ms), which is due to the absence of coordinating water molecules (q = 0.1). This is compatible with the proposed structure in which the Tb^III ion is shielded by a supramolecular cage that expels coordinated water molecules responsible for luminescence quenching.