Energy transfer luminescence of Tb³⁺ ion complexed with calix[4]arenetetrasulfonate and the thia and sulfonyl analogue. The effect of bridging groups

New analogues of calix[4]arenetetrasulfonate (1) were prepared, in which the methylene bridges are replaced by S (2) and SO₂ (3). The complexation ability of these calixarene ligands 1-3 toward Tb³⁺ ion and luminescence properties of the resulting complexes were examined. All the ligands formed complexes with lanthanide ions (Pr³⁺, Sm³⁺, Eu³⁺, Tb³⁺ and Dy³⁺), among which the Tb³⁺ complex exhibited strong energy transfer luminescence. The pH dependence of the luminescence intensity suggested that the analogues 2 and 3 possess higher complexation ability towards Tb³⁺ than the parent 1, which should be ascribed to the higher acidity of the phenolic OH groups as well as the coordinating ability of the bridging groups S and SO₂. The composition of the complexes of 2 and 3 was determined to be Tb³⁺·2 and Tb³⁺·3 by a molar ratio method. The photophysical properties such as emission lifetimes (τ) and quantum yields (φ) of the complexes were estimated. Both Tb³⁺·2 and Tb³⁺·3 gave comparable τ values (0.7 ms), which are larger than that of Tb³⁺·(1)₂ (0.6 ms). Comparison of their lifetimes in D₂O solution suggested that Tb³⁺·2 and Tb³⁺·3 have four to five coordinated water molecules responsible for quenching. On the other hand, complex Tb³⁺·2 gave the largest φ value (0.15), whereas Tb³⁺·(1)₂ and Tb³⁺·3 gave very close φ values (0.12 and 0.13, respectively). The results of this study showed that thiacalix[4]arenetetrasulfonate 2 and the sulfonyl analogue 3 are promising candidates for the scaffold to construct luminescence devices from the viewpoint of the higher complexation ability and luminescent performance.