Rotaxanes have garnered considerable 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, 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 was assembled into polymeric rotaxane by using the “axle-donor···ring-acceptor” mode. Therefore, the controlled stepwise preparation of polymeric rotaxanes based on hybrid macrocycles remains a challenge.

In a study published inAngew. Chem. Int. Ed., the 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 proposed aninverse "ring-donor···axle-acceptor" pattern.

This new pattern breaks the conventional fixed binding mode, significantly enhancing adjustability for the axle acceptors.Using Al8(OH)8(NA)16 (abbreviated as Al8, HNA= naphthoic acid) as the macrocycle platform, researchers achieved the gradual coordination growth of inorganic metal ions and organic ligands within the ring’s confined cavity. The flexibility and adaptability of the Al8 macrocycle facilitate axle growth from molecules, complexes to polymers.

The Al8 macrocycle’s interior resembles a tubular cavity with one hydrophilic Al8(OH)8ring 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 point towards the Al8(OH)8 plane, suggesting the potential for constructing an axle structure by introducing a metal cation. Here, Ag+ and Na+ are 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+, a 1D infinite polyrotaxane can be obtained, in which neighboring [2]-rotaxanes are tightly held together by Ag+ cation via Ag–N bonds. The Al8 macrocycles are strung on the [Agnbpyn]n+ chain through multiple non-covalent interactions, resembling a Chinese “sugar gourd”.

These host-guest complexes exhibit typically reverse saturable absorption responses in nonlinear optical (NLO) measurement. After a quantitative evaluation, researchers found that the NLO responses can be significantly enhanced by introducing heavy metal cations, increasing the conjugation of organic guests, and promoting polymerization. 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 rotaxanes and polyrotaxane constructions. The unique "ring-donor···axle-acceptor" pattern in the Al8 macrocycle enables it to encapsulate a variety of guests, facilitating the gradual growth of coordination axle.

Schematic diagram ofthe assembly of aluminum molecular ring into rotaxane and polyrotaxane(Image by Prof. ZHANG’s group)

Contact:

Prof. FANG Weihui

Fujian Institute of Research on the Structure of Matter

Chinese Academy of Sciences

Email: fwh@fjirsm.ac.cn