Highly efficient near-infrared-emitting lanthanide(III) complexes formed by heterogeneous self-assembly of Ag ^I, Ln ^III, and thiacalix[4]arene-p-tetrasulfonate in aqueous solution (Ln ^III = Nd ^III, Yb ^III)

Heterogeneous self-assembly of thiacalix[4]arene-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.