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^I₄·Nd^III·TCAS₂ in a wide pH range of 6–12. In contrast, the Ag^I-Yb^III-TCAS system gave Ag^I₂·Yb^III₂·TCAS₂ at a pH of around 6 and Ag^I₂·Yb^III·TCAS₂ 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^I₂·Yb^III₂·TCAS₂, two TCAS ligands sandwiched a cyclic array of a Ag^I–Ag^I–Yb^III–Yb^III core. In Ag^I₂·Yb^III·TCAS₂, Yb^III was accommodated in an O₈ cube consisting of eight phenolate O^– groups from two TCAS ligands linked by two S–Ag–S linkages. Crystallographic analysis of Ag^I₄·Nd^III·TCAS₂ revealed that the structure was similar to Ag^I₂·Yb^III·TCAS₂ 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^I₄·Nd^III·TCAS₂, 0; Ag^I₂·Yb^III₂·TCAS₂, 2.4; and Ag^I₂·Yb^III·TCAS₂, 0. These findings were compatible with the solution-state structures. The luminescent quantum yield (Φ) for Ag^I₄·Nd^III·TCAS₂ was 4.9 × 10^–4, which is the second largest value ever reported in H₂O. These findings suggest that the O₈ cube is an ideal environment to circumvent deactivation via O–H oscillation of coordinating water. The Φ values for Ag^I₂·Yb^III₂·TCAS₂ and Ag^I₂·Yb^III·TCAS₂ 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.