Heterogeneous self-assembly of thiacalixarene-p-tetrasulfonate (TCAS), Ag I, and Ln III (= Nd III, Yb III) in aqueous solutions conveniently afforded ternary complexes emitting Ln III-centered luminescence in the near-infrared (NIR) region. A solution-state study revealed that the Ag I-Nd III-TCAS system gave a complex Ag I 4•Nd III• TCAS 2 in a wide pH range of 6-12. In contrast, the Ag I-Yb III-TCAS system gave Ag I 2•Yb III 2•TCAS 2 at a pH of around 6 and Ag I 2•Yb III•TCAS 2 at a pH of approximately 9.5. The structures of the Yb III complexes were proposed based on comparison with known Ag I-Tb III-TCAS complexes that show the same self-assembly behavior. In Ag I 2•Yb III 2•TCAS 2, two TCAS ligands sandwiched a cyclic array of a Ag I-Ag I-Yb III-Yb III core. In Ag I 2•Yb III•TCAS 2, Yb III was accommodated in an O 8 cube consisting of eight phenolate O - groups from two TCAS ligands linked by two S-Ag-S linkages. Crystallographic analysis of Ag I 4•Nd III•TCAS 2 revealed that the structure was similar to Ag I 2•Yb III•TCAS 2 but that it had four instead of two S-Ag-S linkages. The number of water molecules coordinating to Ln III (q) estimated on the basis of the luminescent lifetimes was as follows: Ag I 4•Nd III• TCAS 2, 0; Ag I 2•Yb III 2•TCAS 2, 2.4; and Ag I 2• Yb III•TCAS 2, 0. These findings were compatible with the solution-state structures. The luminescent quantum yield (Φ) for Ag I 4•Nd III•TCAS 2 was 4.9 × 10 -4, which is the second largest value ever reported in H 2O. These findings suggest that the O 8 cube is an ideal environment to circumvent deactivation via O-H oscillation of coordinating water. The Φ values for Ag I 2•Yb III 2•TCAS 2 and Ag I 2•Yb III•TCAS 2 were found to be 3.8 × 10 -4 and 3.3 × 10 -3, respectively, reflecting the q value. Overall, these results indicate that the ternary systems have the potential for a noncovalent strategy via self-assembly of the multidentate ligand, Ln III, and an auxiliary metal ion to obtain a highly efficient NIR-emissive Ln III complex that usually relies on elaborate covalent linkage of a chromophore and multidentate ligands to expel coordinating water.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry