Hai Qian

Precise control over the presence and position of knots in polymers remains a long-standing synthetic challenge, with theory indicating broad implications for properties and function. Here we report on a topological synthon approach—extension and macrocyclization of an overhand knot—that enables the high yielding synthesis of closed-loop trefoiled polymers with narrow polydispersity and tunable topological parameters. From the same set of building blocks we prepared linear, cyclic and trefoiled topoisomers of matched chemical composition, allowing topology–property relationships to be examined directly. Low-temperature ultrahigh-vacuum scanning tunnelling microscopy resolved three conformers of trefoiled polystyrene, with high-, medium- and low-symmetry forms whose populations depend on chain length, local flexibility and solvation. Coarse‑grained Langevin dynamics reproduce these distributions and indicate that bending energy and conformational entropy govern conformer stability. Metal binding localizes the knotted region, reminiscent of protein/DNA knot translocation. The synthon also affords trefoiled poly(ethylene glycol) and trefoiled polystyrene–poly(ethylene glycol) diblock copolymers, demonstrating the generality of the approach across polymer backbones.

Polymer mechanochemistry enables control over the chemical reactivity through defined force-loading geometries. However, in most mechanophores, force is transmitted through only two loading points, which confines reactivity to a single dominant pathway and limits the diversity of mechanically accessible outcomes. Here, we report a fluorinated 1,2-diazetidinone-based mechanophore that enables the simultaneous activation of two orthogonal mechanochemical reaction pathways under a three-arm pulling geometry. Upon force application, parallel scission reactions occur within a single mechanophore scaffold, generating imine/isocyanate and azo/ketene products. Strategic incorporation of fluorine permits unambiguous identification and quantitative analysis of mechanically generated species by ¹⁹F NMR spectroscopy. Computational analysis reveals that pathway selectivity under three-arm pulling mirrors the relative reactivities observed under two-arm loading, with N¹–N² bond cleavage favored over C³–N² scission, while geometric constraints suppress interconversion between pathways. Together, these results establish multiarm force loading as a general strategy for accessing multiple, independent mechanochemical transformations within a single mechanophore, thereby expanding the conceptual and practical scope of force-responsive molecular design.

Mechanochemistry has emerged as a powerful strategy for controlling chemical reactivity and tuning material properties through applied force. Within this growing field, mechanochromic mechanophores have attracted particular attention as versatile molecular probes that transduce mechanical inputs into optical signals via force-induced structural transformations. By enabling direct visualization of stress and damage in real time, these systems provide unique opportunities for applications in stress sensing, damage detection, and the design of adaptive materials. This Review offers a systematic framework of classifying mechanochromic mechanophores, with a focus on their molecular design principles and activation mechanisms. We highlight how methods of force application and characterization critically shape the study of mechanochromism and examine how mechanophore scaffolds, polymer architectures, and environmental factors collectively dictate optical responses. Applications are critically assessed to underscore the role of mechanochromic systems as a bridge between fundamental mechanochemistry and functional materials engineering. Finally, we outline current challenges and emerging opportunities, with particular emphasis on the growing potential of mechanochromic systems in biological and biomedical contexts.

Modulating biological functions often relies on structural changes mediated by molecular switches. However, conventional achiral molecular switches designed to mimic biological regulation intrinsically fail to capture chirality-dependent processes. To address this limitation, engineered chiral molecular switches offer a promising strategy for elucidating the role of chirality in living organisms. Herein, we report a family of pH-sensitive, chromenoindole-based chiral switches ( CI-1 to CI-6 ) that exhibit distinct multicolor chromism—from yellow to azure—upon acid-induced ring-opening, generating the corresponding zwitterionic species ( ZI-1 to ZI-6 ). These switching processes are fully reversible and can be triggered by the addition of base, leading to original colorless solutions. Chiral chromatographic separation afforded enantiomerically pure molecular switches. By combining theoretical calculations with experimental circular dichroism (CD) analysis, we assigned the absolute configurations of (5a R ,10b R )/(5a S ,10b S )-CI-3 , CI-4 , and CI-5 . These chiral switches exhibited attenuated chiral signals upon ring opening due to the loss of one chiral center, with full signal restoration upon neutralization. This family of structurally simple yet functionally versatile chiral switches offers both mechanistic insights and practical potential for the development of smart materials.

In polymer mechanochemistry, regulating the intrinsic mechanical reactivity of a mechanophore offers extensive opportunities in material science, enabling the development of hierarchical and multifunctional polymer‐based materials. Recent advances have focused on innovating various types of mechanophores with inherent reactivity (e.g. regioselectivity and stereoselectivity). However, little attention has been given to modulating their reactivity by tailoring force transmission within mechanophores. Here, we introduce a novel approach through the implementation of a cyclic pulling geometry into an anthracene‐maleimide (AM) mechanophore. This approach manipulates force transmission within the mechanophore and effectively regulates its reactivity from 0.0160 min ⁻¹ to 0.00133 min ⁻¹ , achieving up to a 12‐fold change. Mechanochemical coupling analysis indicates that the split force transmission along ring chains contributes to the significant difference in mechanochemical reactivity. By leveraging the distinct force transmission pathways within cyclic and linear AM mechanophores, we covalently integrate them with a spiropyran mechanophore to design tandem mechanophore systems for hierarchical mechanochemical activation. These findings highlight the efficacy and versatility of the cyclic pulling strategy in modulating mechanophore reactivity, providing valuable insights for the design of tunable multifunctional polymer‐based materials.

In polymer mechanochemistry, regulating the intrinsic mechanical reactivity of a mechanophore offers extensive opportunities in material science, enabling the development of hierarchical and multifunctional polymer‐based materials. Recent advances have focused on innovating various types of mechanophores with inherent reactivity (e.g. regioselectivity and stereoselectivity). However, little attention has been given to modulating their reactivity by tailoring force transmission within mechanophores. Here, we introduce a novel approach through the implementation of a cyclic pulling geometry into an anthracene‐maleimide (AM) mechanophore. This approach manipulates force transmission within the mechanophore and effectively regulates its reactivity from 0.0160 min ⁻¹ to 0.00133 min ⁻¹ , achieving up to a 12‐fold change. Mechanochemical coupling analysis indicates that the split force transmission along ring chains contributes to the significant difference in mechanochemical reactivity. By leveraging the distinct force transmission pathways within cyclic and linear AM mechanophores, we covalently integrate them with a spiropyran mechanophore to design tandem mechanophore systems for hierarchical mechanochemical activation. These findings highlight the efficacy and versatility of the cyclic pulling strategy in modulating mechanophore reactivity, providing valuable insights for the design of tunable multifunctional polymer‐based materials.

Irreparable chemical damage to redox-active monomers, oligomers, and polymers (i.e. redoxmers) limits the lifetime of energy storage devices (e.g. redox flow batteries) by causing electrode and membrane fouling, as well as irreversible capacity loss. To predictably restore device and materials performance, it is desirable to add programmed destruction capabilities into these damage-prone materials. Here, we report the use of triggerable retrograde reactions to achieve programmable end-of-life function in redoxmers. Retrograde reactions are intended to break up damaged redoxmer materials into more soluble small-molecule constituents to prevent irreparably fouling battery interfaces. We investigated the redox-triggered mesolytic cleavage of homobenzylic ethers (HBEs) for this purpose. Combining experimental methods with simulations, we probed the influence of ring substituents on the programmable bond-scission behavior of HBEs. Variation of para-substituents on the HBEs allowed us to modify the oxidation potential, degradation pathway, and electrochemical mechanisms of the generated products. Given its ideal oxidation potential, we selected the para-methoxy-based HBE as the cleavage scaffold and integrated it with dialkoxybenzene, TEMPO, viologen, and para-nitrobenzene redox centers for compatibility, bulk deconstruction, and electrode defouling tests. Upon applying a high oxidation potential, the redox-active pendant is cleaved from the backbone via HBE bonds; of the four redox centers tested, three of them remained fully redox active. Viologen-appended redox active polymers with HBE linkers were cleaved to fully separate the redox centers from the backbone. Polymer-filmed electrodes were partially restored following HBE oxidation defouling. These studies highlight the co-design of new materials functions for developing sustainable energy storage materials.

Polymers that release functional small molecules under mechanical stress potentially serve as next-generation materials for catalysis, sensing, and mechanochemical dynamic therapy. To further expand the function of mechanoresponsive materials, the discovery of chemistries capable of small molecule release are highly desirable. In this report, we detail a nonscissile bifunctional mechanophore (i.e., dual mechano-activated properties) based on a unique mechanochemical reaction involving norborn-2-en-7-one (NEO). One property is the release of carbon monoxide (CO) upon pulsed solution ultrasonication. A release efficiency of 58% is observed at high molecular weights (M_n = 158.8 kDa), equating to ∼154 molecules of CO released per chain. The second property is the bright cyan emission from the macromolecular product in its aggregated state, resulting in a turn-on fluorescence readout coincident with CO release. This report not only demonstrates a unique strategy for the release of small molecules in a nonscissile way but also guides future designs of force-responsive aggregation-induced emission (AIE) luminogens.