Researchers achieve aluminum molecular ring-based rotaxane and polyrotaxane

Rotaxanes have garnered interest for their unique structures consisting of mechanically interlocked axles and macrocycles. Numerous organic macrocycles have been employed to construct rotaxanes, including crown ether, cyclobis(paraquat-p-phenylene), calixarene, pillararene, cyclodextrin, and cucurbituril.

Inorganic metal ions with unique electronic configurations have been employed to expand and precisely manipulate macrocycles at the molecular level. However, only one case has been assembled into polymeric rotaxane using the “axle-donor···ring-acceptor” mode. The controlled stepwise preparation of polymeric rotaxanes based on hybrid macrocycles remains a challenge.

In a study published in Angewandte Chemie International Edition, a research group led by Prof. Zhang Jian and Prof. Fang Weihui from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences propose an inverse “ring-donor···axle-acceptor” pattern utilizing Al₈ molecular rings, enabling the stepwise assembly of molecules, complexes, and polymers through tailored coordination chemistry.

The Al₈ macrocycle’s interior resembles a tubular cavity with one hydrophilic ring Al₈(OH)₈ and two hydrophobic ports. The inward OH groups bestow the Al₈ macrocycle with a unique “ring-H/axle-acceptor” binding pattern, providing abundant interaction sites for a wide range of acceptors, including anions, metal cations, and organic ligands.

Researchers encapsulated a variety of aromatic compounds within the Al₈ macrocycle, including single-site carboxylic acid (HNA, HBA) and dual-site bipyridine (bpy). The coordination sites of these aromatic guests all pointed towards the Al₈(OH)₈ plane, suggesting the potential for constructing an axle structure by introducing a metal cation.

Ag⁺ and Na⁺ were well-suited due to their linear coordination geometry. Consequently, researchers obtained a series of [2]-rotaxanes, each with inner linear complex axles: Ag(bpy)₂⁺, Ag(NA)₂^–, HAg(BA)₂ and Na(AQS)₂^–.

Considering the residual uncoordinated N sites in Ag(bpy)₂⁺ axle, researchers carried out further polymerization chemistry research. By increasing the amount of Ag⁺, they obtained a 1D infinite polyrotaxane in which neighboring [2]-rotaxanes were tightly held together by Ag⁺ cation via Ag–N bonds. The Al₈ macrocycles were strung on the [Agnbpyn]ⁿ⁺ chain through multiple non-covalent interactions, resembling a famous Beijing snack: Tanghulu (sugar coated hawthorn).

These host-guest complexes exhibit typically reverse saturable absorption responses in nonlinear optical (NLO) measurement. A quantitative evaluation showed that the NLO responses can be significantly enhanced by introducing heavy metal cations, increasing the conjugation of organic guests, and promoting polymerization.

Notably, the Ag(NA)₂^– embedded [2]-rotaxane exhibits the best NLO performance with the highest nonlinear absorption coefficient and the lowest limiting threshold, outperforming some reported organic molecules, graphene oxide materials, and traditional crystalline compounds.

This study demonstrates a universal assembly strategy to achieve rotaxane and polyrotaxane constructions. The unique “ring-donor/axle-acceptor” pattern breaks the conventional fixed binding mode, significantly enhancing adjustability for the axle acceptors.