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., [Ln₆(H₃<b>L</b>)₄]⁶⁺ and [Ln₆<b>L</b>₄]^6-, from a series of lanthanide ions (Ln³⁺; Ln = Lu, Yb, Eu) and a tritopic tetradentate acylhydrazone ligand (H₆<b>L</b>) featuring multiple deprotonation states and propeller conformations. While direct self-assembly under basic conditions produced a mixture of various Δ<i>_x</i>Λ_6-<i>x</i>-[Ln₆<b>L</b>₄]^6- (<i>x</i> = 0-6) stereoisomers, racemic Δ₆- and Λ₆-[Ln₆<b>L</b>₄]^6- could be exclusively obtained from the first self-assembly of Δ₆- and Λ₆-[Ln₆(H₃<b>L</b>)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 <i>C</i>₃-symmetric Δ₃Λ₃-Ln₆<b>L</b>₄ and <i>T</i>-symmetric Δ₆/Λ₆-Ln₆<b>L</b>₄ complexes. Host-guest studies confirmed that positively charged <i>rac</i>-Δ₆/Λ₆-[Ln₆(H₃<b>L</b>)₄]⁶⁺ could bind anionic sulfonates, and negatively charged <i>rac</i>-Δ₆/Λ₆-[Ln₆<b>L</b>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 <i>rac</i>-Δ₆/Λ₆-Yb₆<b>L</b>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.