Force-reversible C-N bonds, resulting from the click chemistry reaction between triazolinedione (TAD) and indole derivatives, offer exciting opportunities for molecular-level engineering to design materials that respond to mechanical loads. Here, we displayed that TAD-indole adducts, acting as crosslink points in dry-state covalently crosslinked polymers, enable materials to display reversible stress-responsiveness in real time already at ambient temperature. Whereas the exergonic TAD-indole reaction results in the formation of bench-stable adducts, they were shown to dissociate at ambient temperature when embedded in a polymer network and subjected to a stretching force to recover the original products. Moreover, the nascent TAD moiety can spontaneously and immediately be recombined after dissociation with an indole reaction partners at ambient temperature, thus allowing for the adjustment of the polymer segment conformation and the maintenance of the network integrity by force-reversible behaviors. Overall, our strategy represents a general method to create toughened covalently crosslinked polymer materials with simultaneous enhancement of mechanical strength and ductility, which is quite challenging to achieve by conventional chemical methods.

Metal coordination crosslinking between stiff N-polyindole chains was constructed, and the crosslinked films exhibited high tensile strength, high heat resistance and excellent polar solvent resistance. The noncovalent crosslinking can be further removed via external pyrophosphate, which endows the crosslinked polymer with a recyclable behavior. The tensile strength and glass transition temperature of the polymers can be nondestructively detected by taking advantage of the fluorescence quenching effect of metal coordination to the adjacent bipyridine structure.

Nonequilibrium oscillation fueled by dissipating chemical energy is ubiquitous in living systems for realizing a broad range of complex functions. The design of synthetic materials that can mimic their biological counterparts in the production of dissipative structures and autonomous oscillations is of great interest but remains challenging. Here, a series of environmentally adaptable hydrogels functionalized with photoswitchable spiropyran derivatives that display a tunable equilibrium‐shifting capability, thus endowing those hydrogels with a high degree of freedom and flexibility is reported. Such nonequilibrium hydrogels are able to responsively adapt their shapes under constant light illumination due to asymmetric deswelling, which in turn generates self‐shadowing and consequently creates autonomous self‐oscillating behaviors through a negative feedback process. Diverse oscillation modes including bending, twisting, and snap‐through buckling with tunable frequency and amplitude are widely observed in three different molecular systems. Density functional theory calculations and finite element simulations further demonstrated the robustness of such a photoadaptable self‐oscillation mechanism. This study provides a useful molecular design strategy for construction of highly adaptable hydrogels with potential applications in self‐sustained soft robots and autonomous devices.

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Non-covalent crosslinking has provided versatile and affordable solutions for the design of tough soft polymer materials, but it is not applied in stiff high-performance thermosets. Here, the authors report a supramolecular approach for the design of tough high-performance thermosets by using noncovalent sandwich-structural indole-Mg-indole complexes that act as the force-reversible and energetic crosslinking points, evading the inherent tradeoff between mechanical strength and ductility for high-performance thermosets. Compared to traditional epoxy polymer materials, the indole-Mg-indole crosslinks of the network enables synchronously enhancement of mechanical strength and ductility owing to the increased interaction-energy and efficient dissociation and reassociation behaviors given by the dynamic indole-Mgindole complexes, which is quite challenging to achieve by conventional chemical methods. In addition, local manipulation of crosslinking confers the resulting thermosets with multiple fast stimuli-responsive functions, such as recyclability, healability, and adhesion.