Mechanically Interlocked Polyrotaxane Networks with Collective Motions of Multiple Main-Chain Mechanical Bonds.

Type I main-chain polyrotaxanes (PRs) with multiple wheels threaded onto the axle are widely employed to design slide-ring materials. However, Type II main-chain PRs with axles threading into the macrocycles on the polymer backbones have rarely been studied, although they feature special topological structures and dynamic characteristics. Herein, we report the design and preparation of Type II main-chain PR-based mechanically interlocked networks (PRMINs), based on which the relationship between microscopic motion of mechanical bonds on the PRs and macroscopic mechanical performance of materials has been revealed. The representative PRMIN-2 exhibits a robust feature in tensile tests with high stretchability (1680 %) and toughness (47.5 MJ/m3). Moreover, it also has good puncture performance with puncture energy of 22.0 mJ. Detailed rheological measurements and coarse-grained molecular dynamics (CGMD) simulation reveal that the embedded multiple [2]rotaxane mechanical bonds on the PR backbones of PRMINs could undergo a synergistic long-range sliding motion under external force, with the introduction of collective dangling chains into the network. As a result, the synchronized motions of coherent PR chains can be readily activated to accommodate network deformation and efficiently dissipate energy, thereby leading to enhanced mechanical performances of PRMINs.

Dynamically Cross-linked Oligo[2]rotaxane Networks Mediated by Metal-Coordination.

Polyrotaxanes (PRs) have attracted significant research attention due to their unique topological structures and high degrees of conformational freedom. Herein, we take advantage of an oligo[2]rotaxane to  construct a novel class of dynamically cross-linked rotaxane network (DCRN) mediated by metal-coordination. The oligo[2]rotaxane skeleton offers several distinct advantages: In addition to retaining the merits of traditional polymer backbones, the ordered intramolecular motion of the [2]rotaxane motifs introduced dangling chains into the network, thereby enhancing the stretchability of the DCRN. Additionally, the dissociation of host‒guest recognition and subsequent sliding motion, along with the breakage of metal-coordination interactions, represented an integrated energy dissipation pathway to enhance mechanical properties. Moreover, the resulting DCRN demonstrated responsiveness to multiple stimuli and displayed exceptional self-healing capabilities in a gel state. Upon exposure to PPh3, which induced network deconstruction by breaking the coordinated cross-linking points, the oligo[2]rotaxane could be recovered, showcasing good recyclability. These findings demonstrate the untapped potential of the oligo[2]rotaxane as a polymer skeleton to develop DCRN and open the door to extend their advanced applications in intelligent mechanically interlocked materials.

Polycatenanes Formed of Self-Assembled Metal-Organic Cages.

Poly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.

A Platform Approach to Cleavable Macrocycles for the Controlled Disassembly of Mechanically Caged Molecules.

Inspired by interlocked oligonucleotides, peptides and knotted proteins, synthetic systems where a macrocycle cages a bioactive species that is "switched on" by breaking the mechanical bond have been reported. However, to date, each example uses a bespoke chemical design. Here we present a platform approach to mechanically caged structures wherein a single macrocycle precursor is diversified at a late stage to include a range of trigger units that control ring opening in response to enzymatic, chemical, or photochemical stimuli. We also demonstrate that our approach is applicable to other classes of macrocycles suitable for rotaxane and catenane formation.

Molecular Recognition of Aromatics in Spherical Nanocages.

In general, due to the lack of efficient specific molecular interactions, achieving host-guest molecular recognition inside large and neutral metal-organic cages (MOCs) is challenging. Preferential molecular recognition of aromatics using the internal binding sites of interlocked icosahedral (i. e., spherical) M12 L8 MOCs within poly-[n]-catenane (1) is reported. The guest absorption was monitored directly in the solid-state by consecutive single-crystal-to-single-crystal (SCSC) reactions in a gas-solid environment, in single-crystal X-ray diffraction (SC-XRD) experiments. The preferential guest uptake was corroborated by density functional theory (DFT) calculations by determining the host-guest interaction energy (Ehost-guest ) with a nitrobenzene (NB)≫p-xylene (p-xy)≫o-dichlorobenzene (o-DCB) trend (i. e., from 44 to 25 kcal mol-1 ), assessing the XRD outcomes. Combining SC-XRD, DFT and solid-state 13 C NMR, the exceptional stability of the M12 L8 cages, together with the guest exchange/release properties were rationalized by considering the presence of mechanical bonds (efficient π-π interactions) and by the pyridine's rotor-like behaviour (with 3 kcal mol-1 rotational energy barrier). The structure-function properties of M12 L8 makes 1 a potential candidate in the field of molecular sensors.

Anion-π Catalysis Enabled by the Mechanical Bond.

We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Post-Synthetic Macrocyclization of Rotaxane Building Blocks.

Although not often encountered, cyclic interlocked molecules are appealing molecular targets because of their restrained tridimensional structure which is related to both the cyclic and interlocked shapes. Interlocked molecules such as rotaxane building blocks may be good candidates for post-synthetic intramolecular cyclization if the preservation of the mechanical bond ensures the interlocked architecture throughout the reaction. This is obviously the case if the modification does not involve the cleavage of either the macrocycle's main chain or the encircled part of the axle. However, among the post-synthetic reactions, the chemical linkage between two reactive sites belonging to embedded elements of rotaxanes still consists of an underexploited route to interlocked cyclic molecules. This Review lists the rare examples of macrocyclization through chemical connection between reactive sites belonging to a surrounding macrocycle and/or an encircled axle of interlocked rotaxanes.

Challenges and Opportunities in the Post-Synthetic Modification of Interlocked Molecules.

Several strategies have been successfully utilised to obtain a wide range of interlocked molecules. However, some interlocked compounds are still not obtained directly and/or efficiently from non-interlocked components because the requisites for self-assembly cannot always be enforced. To circumvent such a synthetic problem, a strategy that consists of synthesizing an isolable and storable interlocked building block in a step that precedes its modification is an appealing chemical route to more sophisticated interlocked molecules. Synthetic opportunities and challenges are closely linked to the fact that the mechanical bond might greatly affect the reactivity of a functionality of the encircled axle, but that the interlocked architecture needs to be preserved during the synthesis. Hence, the mechanical bond plays a fundamental role in the strategy employed. This Review focuses on the challenging post-synthetic modifications of interlocked molecules, sometimes through cleavage of the axle's main chain, but always with conservation of the mechanical bond.

From Molecular Machines to Stimuli-Responsive Materials.

Artificial molecular machines are able to produce and exploit precise nanoscale actuations in response to chemical or physical triggers. Recent scientific efforts have been devoted to the integration, orientation, and interfacing of large assemblies of molecular machines in order to harness their collective actuations at larger length scale and up to the generation of macroscopic motions. Making use of such "hierarchical mechanics" represents a fundamentally new approach for the conception of stimuli-responsive materials. Furthermore, because some molecular machines can function as molecular motors-which are capable of cycling a unidirectional motion out of thermodynamic equilibrium and progressively increasing the work delivered to their environment-one can expect unique opportunities to design new kinds of mechanically active materials and devices capable of autonomous behavior when supplied by an external source of energy. Recently reported achievements are summarized, including the integration of molecular machines at surfaces and interfaces, in 3D self-assembled materials, as well as in liquid crystals and polymer materials. Their detailed functioning principles as well as their functional properties are discussed along with their potential applications in various domains such as sensing, drug delivery, electronics, optics, plasmonics, and mechanics.

A Visible-Light-Induced Dynamic Mechanical Bond as a Linkage for Dynamic Materials.

Exploring dynamic bonds and their applications in fabricating dynamic materials has received great attention. A photoinduced [2]rotaxane-based dynamic mechanical bond (DMB) features visible-light-triggered dynamic bonding behavior that is essentially distinguished from conventional dynamic chemical bonds. In this DMB, a photoisomerizable ortho-fluoroazobenzene unit is introduced as a steric-controllable stopper, the visible-light-induced dynamic wagging movement of which enables the photoregulated threading of the macrocycle. This allows reversible in situ de-/reforming of the mechanical bond without involving dynamic chemical linkage. The DMB-cross-linked polymeric gel shows interesting photoinduced degradation behavior upon visible light irradiation. Benefiting from the distinctive dual dynamic nature of reversible bonding behavior and mechanical interlocked structure, this DMB is expected to serve as a new type of dynamic bond that can be applied in designing dynamic soft materials.

Nanohoop Rotaxanes from Active Metal Template Syntheses and Their Potential in Sensing Applications.

The unique optoelectronic properties and smooth, rigid pores of macrocycles with radially oriented π systems render them fascinating candidates for the design of novel mechanically interlocked molecules with new properties. Two high-yielding strategies are used to prepare nanohoop [2]rotaxanes, which owing to the π-rich macrocycle are highly emissive. Then, metal coordination, an intrinsic property afforded by the resulting mechanical bond, can lead to molecular shuttling as well as modulate the observed fluorescence in both organic and aqueous conditions. Inspired by these findings, a self-immolative [2]rotaxane was then designed that self-destructs in the presence of an analyte, eliciting a strong fluorescent turn-on response, serving as proof-of-concept for a new type of molecular sensing material. More broadly, this work highlights the conceptual advantages of combining compact π-rich macrocyclic frameworks with mechanical bonds formed via active-template syntheses.

Chemical Consequences of the Mechanical Bond: A Tandem Active Template-Rearrangement Reaction.

We report the unexpected discovery of a tandem active template CuAAC-rearrangement process, in which N2 is extruded on the way to the 1,2,3-triazole product to give instead acrylamide rotaxanes. Mechanistic investigations suggest this process is dictated by the mechanical bond, which stabilizes the CuI -triazolide intermediate of the CuAAC reaction and diverts it down the rearrangement pathway; when no mechanical bond is formed, the CuAAC product is isolated.

A Fluorescent Ditopic Rotaxane Ion-Pair Host.

We report a rotaxane based on a simple urea motif that binds Cl- selectively as a separated ion pair with H+ and reports the anion binding event through a fluorescence switch-on response. The host selectively binds Cl- over more basic anions, which deprotonate the framework, and less basic anions, which bind more weakly. The mechanical bond also imparts size selectivity to the ditopic host.

Nucleic Acid Nanostructures and Topology.

Knots, polyhedra, and Borromean rings with specific structural and topological features can be made from DNA. Biotechnologists have been exploiting the programmability of DNA intermolecular associations for a quarter of a century. These operations have now been applied successfully to branched DNA species to produce complex target structures (for example, the cube shown in the picture) and a nanomechanical device. The assembly of two-dimensional crystals with programmed topographic characteristics demonstrates the simplicity of translating design into surface structures.