A main-chain [6]rotaxane: from synthesis to multi-stimuli-responsive motions

Abstract

The synthesis of higher-order oligo[n]rotaxanes (usually n ≥ 3) featuring multi-stimuli responsive motions at the molecular level is critical for advancing artificial molecular machines and smart materials; yet, achieving this goal remains a significant challenge. Herein, a main-chain [6]rotaxane and control [n]rotaxanes (n = 2, 3, and 4) were constructed with the assistance of templation by bipyridinium- or dialkylammonium-threaded two dimensional hydrogen-bonded (H-bonded) azo-macrocycles via a Cu-mediated azide–alkyne cycloaddition reaction. The [6]rotaxane exhibits reversible multi-stimuli-responsive motions triggered by acid–base reactions, solvent polarity changes, and light irradiation. Notably, three macrocycles (one ring A component and two ring B components), which are compactly stacked at the bipyridinium (BP²⁺) station owing to the large π-surface, act as a masked binding site to facilitate shuttling of the remaining two macrocycles (ring C components) driven by cooperative π–π stacking interactions. Morphological studies by SEM on the [6]rotaxane reveal that the supramolecular assemblies undergo reversible transitions between amorphous and spherical or nanorod nanostructures in solution. This work presents a rare example of using H-bonded aramide macrocycles for the construction of multi-stimuli-responsive higher-order rotaxanes, expanding the toolkit of supramolecular elements with controlled supramolecular structures.