Research into stimuli-responsive self-assembly and transformation behaviors in supramolecular isomers/quasi-isomers, which are more similar to those in natural macromolecules, has received a great of attention recently, because it is important to understand and further mimic the structural reorganization processes in biological systems.

The C-alkylpyrogallol[4]arenes(PgC_n)-based metal-organic nanocapsules (MONCs) have been studied for more than ten years, but their metal centers are limited to Mg, Co, Ni, Cu, Zn and Ga, and the structural shapes are limited to hexameric octahedral, dimeric spherical and hexameric “rugby ball”.

In a study published in Nature Communications, the research group led by Prof. YUAN Daqiang from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported an interesting example of solvent responsive assembly and interconversion of V₂₄ MONC quasi-isomers.

Single crystal X-ray experiments revealed that the V₂₄ quasi-isomers are constructed from the same number of subcomponents, but adopt distinct geometries, which include a contracted octahedral capsule (V₂₄-oct) with the inner cavity of 1000 , and an expanded ball-shaped capsule (V₂₄-ball) with the inner cavity of 1400 .

These two V₂₄ capsules represent the first example of a metal displaying versatility and forming different PgC_n-based hexamer capsules.

Further studies revealed that their structural differences are due to the coordination geometry differences in V centers. Specifically, the V centers adopt five-coordinated square-pyramidal and six-coordinated octahedral geometries for V₂₄-ball and V₂₄-oct, respectively.

More interestingly, it was found that the five-coordinate square pyramidal and six-coordinated octahedral V centers in the V₂₄ MONCs could interconvert by associating and disassociating the axial molecules. Such changes in V centers lead to the interconversions of inner cavities between contracted octahedra and expanded ball in V₂₄ capsules.

Following such structural interconversions, their magnetic properties are significantly changed. Specifically, the V⁴⁺ centers in V₂₄ ball are antiferromagnetic coupled, while V₂₄ octahedron ferromagnetic at higher temperature.

This study  provided a new strategy for efficient construction of MONC quasi-isomers by controlling the coordination environments of the metal centers,  and improved the understanding of the transformation process and their structure-property relationships in stimuli-responsive assembly system.

Schematic representation of the controlled self-assembly and interconversions of V₂₄ capsules. (Image by Prof. YUAN’s Group)

Contact:

Prof. YUAN Daqiang

Fujian Institute of Research on the Structure of Matter

Chinese Academy of Sciences

Email: ydq@fjirsm.ac.cn