Controlled Self-Assembly, Isomerism, and Guest Uptake/Release of Charge-Reversible Lanthanide–Organic Octahedral Cages

Abstract

Charge plays a crucial role in the function of molecular and supramolecular systems, but coordination hosts capable of orthogonal charge regulation remain elusive so far. In this study, we report the condition-dependent self-assembly of charge-reversible lanthanide–organic tetra-capped octahedral cages, i.e., [Ln 6 (H 3 L ) 4 ] 6+ and [Ln 6 L 4 ] 6–, from a series of lanthanide ions (Ln 3+; Ln = Lu, Yb, Eu) and a tritopic tetradentate acylhydrazone ligand (H 6 L ) featuring multiple deprotonation states and propeller conformations. While direct self-assembly under basic conditions produced a mixture of various Δ x Λ 6– x -[Ln 6 L 4 ] 6– ( x = 0–6) stereoisomers, racemic Δ 6 - and Λ 6 -[Ln 6 L 4 ] 6– could be exclusively obtained from the first self-assembly of Δ 6 - and Λ 6 -[Ln 6 (H 3 L ) 4 ] 6+ under neutral conditions followed by post-assembly deprotonation. Rich isomerism on the tetra-capped octahedral cages arising from the coupling between the metal-centered Δ/Λ chirality and the ligand conformations has been discussed based on X-ray single-crystal structures of the C 3 -symmetric Δ 3 Λ 3 -Ln 6 L 4 and T -symmetric Δ 6 /Λ 6 -Ln 6 L 4 complexes. Host–guest studies confirmed that positively charged rac -Δ 6 /Λ 6 -[Ln 6 (H 3 L ) 4 ] 6+ could bind anionic sulfonates, and negatively charged rac -Δ 6 /Λ 6 -[Ln 6 L 4 ] 6– exhibited strong encapsulation ability toward ammonium guests, where acid/base-triggered guest uptake/release could be realized taking advantage of the charge reversibility of the cage. Moreover, photophysical studies revealed visible-light-sensitized and guest-encapsulation-enhanced NIR emissions on the rac -Δ 6 /Λ 6 -Yb 6 L 4 cage. This work not only enriches the library of functional lanthanide–organic cages but also provides a promising candidate with charge reversibility for the development of smart supramolecular materials.

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