Insights into the Correlation of Microscopic Motions of [c2]Daisy Chains with Macroscopic Mechanical Performance for Mechanically Interlocked Networks

Mechanically Interlocked Macrocycles on Covalent Networks for Energy and Environmental Applications

Macrocycles' unique properties of interacting with guest molecules have been an intriguing scientific endeavor for many decades. They are potentially practically useful for engineering applications, especially in energy and environmental applications. These applications are usually demanding, involving a high temperature, pH, voltage, etc., thus, finding suitable substrates that can endure working environments and sustain macrocycles' properties is highly desirable. In that sense, covalent networks are ideal as they are chemically/electrochemically/thermally stable and can be porous by design. Emerging porous materials, especially covalent organic frameworks (COFs), could be suitable as their porous spaces allow macrocycles to interact with guest species. In the past seven years, we have seen the rise of mechanically interlocked macrocycles on covalent networks (MIMc-CNs) that translate macrocycles' properties into macroscale materials. In this conceptual review, we first describe the idea of integrating MIMcs into COFs or conventional amorphous polymers. Next, we review the reported representative MIMc-CNs used in energy and environmental applications. We also provide a brief outlook for the future directions for the MIMc-CNs research.

Application of Organic Gel on Skin Realized by Hydrogel/Organic Gel Adhesion

Diversity in solvent selection bestows the organic gel with appealing characteristics embracing antidrying, anti-icing, and antifouling abilities. However, organic gel, subjected to the "toxic" inherent property of solvent, is not able to be manipulated on skin. Herein, introducing the hydrogel layer amid organic gel and skin is envisaged to realize application of organic gel on skin. Hydrogel, inserted as the medium layer, works for the coupling role between skin and organic gel, also avoids the direct contact of organic gel toward skin. First, hydrogel system composed of acrylic acid is fabricated, meanwhile organic gel is prepared employing 2-hydroxyethyl methacrylate, ethylene glycol (EG) as solvent. Organic gel is able to adhere to hydrogel by hydrogen bonding resulting from carboxyl groups of polyacrylic acid chains and hydroxyl groups occurring on 2-hydroxyethyl methacrylate or EG. Additionally, hydrogen bonding enables the hydrogel to be firmly attached to skin, thus organic gel/hydrogel/skin assembly is produced. The further application of organic gel is exploited by incorporating stimuli-responsive dyes including spiropyran and rhodamine derivative.

Anion-Coordination Foldamer-Based Polymer Network: from Molecular Spring to Elastomer

Foldamer is a scaled-down version of coil spring, which can absorb and release energy by conformational change. Here, polymer networks with high density of molecular springs were developed by employing anion-coordination-based foldamers as the monomer. The coiling of the foldamer is controlled by oligo(urea) ligands coordinating to chloride ions; subsequently, the folding and unfolding of foldamer conformations endow the polymer network with excellent energy dissipation and toughness. The mechanical performance of the corresponding polymer networks shows a dramatic increase from P-L2UCl (non-folding), to P-L4UCl (a full turn), and then to P-L6UCl (1.5 turns), in terms of strength (2.62 MPa; 14.26 MPa; 22.93 MPa), elongation at break (70 %; 325 %; 352 %), Young's modulus (2.69 MPa; 63.61 MPa; 141.50 MPa), and toughness (1.12 MJ/m3; 21.39 MJ/m3; 49.62 MJ/m3), respectively, which is also better than those without anion centers and the non-foldamer based counterparts. Moreover, P-L6UCl shows enhanced strength and toughness than most of the molecular-spring based polymer networks. Thus, an effective strategy for designing high-performance anion-coordination-based materials is presented.

Tunable Nano-Bending Structures of Loosened/Tightened Lassos with Bi-Stable Vibration-Induced Emissions for Multi-Manipulations of White-Light Emissions and Sensor Applications

The first tunable nano-bending structures of [1]rotaxane containing a single-fluorophoric N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) moiety (i.e., [1]RA) are developed as a loosened lasso structure to feature the bright white-light emission [CIE (0.27, 0.33), Φ = 21.2%] in THF solution, where bi-stable states of bending and twisted structures of DPAC unit in [1]RA produce cyan and orange emissions at 480 and 600 nm, respectively. With acid/base controls, tunable loosened/tightened nano-loops of corresponding [1]rotaxanes (i.e., [1]RA/[1]RB) can be achieved via the shuttling of macrocycles reversibly, and thus to adjust their respective white-light/cyan emissions, where the cyan emission of [1]RB is obtained due to the largest conformational constraint of DPAC moiety in its bending form of [1]RB with a tightened lasso structure. Additionally, the non-interlocked analog M-Boc only shows the orange emission, revealing the twisted form of DPAC fluorophore in M-Boc without any conformational constraint. Moreover, the utilization of solvents (with different viscosities and polarities), temperatures, and water fractions could serve as effective tools to adjust the bi-stable vibration-induced emission (VIE) colors of [1]rotaxanes. Finally, tuning ratiometric emission colors of adaptive conformations of DPAC moieties by altering nano-bending structures in [1]rotaxanes and external stimuli can be further developed as intelligent temperature and viscosity sensor materials.

Highly robust supramolecular polymer networks crosslinked by a tiny amount of metallacycles

Supramolecular polymeric materials have exhibited attractive features such as self-healing, reversibility, and stimuli-responsiveness. However, on account of the weak bonding nature of most noncovalent interactions, it remains a great challenge to construct supramolecular polymeric materials with high robustness. Moreover, high usage of supramolecular units is usually necessary to promote the formation of robust supramolecular polymeric materials, which restrains their applications. Herein, we describe the construction of highly robust supramolecular polymer networks by using only a tiny amount of metallacycles as the supramolecular crosslinkers. A norbornene ring-opening metathesis copolymer with a 120° dipyridine ligand is prepared and self-assembled with a 60° or 120° Pt(II) acceptor to fabricate the metallacycle-crosslinked polymer networks. With only 0.28 mol% or less pendant dipyridine units to form the metallacycle crosslinkers, the mechanical properties of the polymers are significantly enhanced. The tensile strengths, Young's moduli, and toughness of the reinforced polymers reach up to more than 20 MPa, 600 MPa, and 150 MJ/m3, respectively. Controllable destruction and reconstruction of the metallacycle-crosslinked polymer networks are further demonstrated by the sequential addition of tetrabutylammonium bromide and silver triflate, indicative of good stimuli-responsiveness of the networks. These remarkable performances are attributed to the thermodynamically stable, but dynamic metallacycle-based supramolecular coordination complexes that offer strong linkages with good adaptive characteristics.

Mechanically Interlocked Polymers with Dense Mechanical Bonds

ConspectusMechanically interlocked polymers (MIPs) such as polyrotaxanes and polycatenanes are polymer architectures that incorporate mechanical bonds, which represent a compelling frontier in polymer science. MIPs with cross-linked structures are known as mechanically interlocked networks (MINs) and are widely utilized in materials science. Leveraging the motion of mechanical bonds, MINs hold the potential for achieving a combination of robustness and dynamicity. Currently, the reported MINs predominantly consist of networks with discrete mechanical bonds as cross-linking points, exemplified by well-known slide-ring materials and rotaxane/catenane cross-linked polymers. The motion of these mechanically interlocked cross-linking points facilitates the redistribution of tension throughout the network, effectively preventing stress concentration and thereby enhancing material toughness. In these instances, the impact of mechanical bonds can be likened to the adage "small things can make a big difference", whereby a limited number of mechanical bonds substantially elevate the mechanical performance of conventional polymers. In addition to MINs cross-linked by mechanical bonds, there is another type of MIN in which their principal parts are polymer chains composed of dense mechanical bonds. Within these MINs, mechanical bonds generally serve as repeating units, and their unique properties stem from integrating and amplifying the function of a large amount of mechanical bonds. Consequently, MINs with dense mechanical bonds tend to reflect the intrinsic properties of mechanical interlocked polymers, making their exploration critical for a comprehensive understanding of MIPs. Nevertheless, investigations into MINs featuring dense mechanical bonds remain relatively scarce.This Account presents a comprehensive overview of our investigation and insights into MINs featuring dense mechanical bonds. First, we delve into the synthetic strategies employed to effectively prepare MINs with dense mechanical bonds, while critically evaluating their advantages and limitations. Through meticulous control of the core interlocking step, three distinct strategies have emerged: mechanical interlocking followed by polymerization, supramolecular polymerization followed by mechanical interlocking, and dynamic interlocking. Furthermore, we underscore the structure-property relationships of MINs with dense mechanical bonds. The macroscopic properties of MINs originate from integrating and amplifying countless microscopic motions of mechanical bonds, a phenomenon we define as an integration and amplification mechanism. Our investigation has revealed detailed motion characteristics of mechanical bonds in bulk mechanically interlocked materials, encompassing the quantification of motion activation energy, discrimination of varying motion distances, and elucidation of the recovery process. Additionally, we have elucidated their influence on the mechanical performance of the respective materials. Moreover, we have explored potential applications of MINs, leveraging their exceptional mechanical properties and dynamicity. These applications include enhancing the toughness of conventional polymers, engineering mechanically adaptive and multifunctional aerogels, and mitigating Li protrusion as interfacial layers in lithium-ion batteries. Finally, we offer our personal perspectives on the promises, opportunities, and key challenges in the future development of MINs with dense mechanical bonds, underscoring the potential for transformative advancements in this burgeoning field.

BP23C7: high-yield synthesis and application in constructing [3]rotaxanes and responsive pseudo[2]rotaxanes

A biphenyl-23-crown-7 ether (BP23C7) is synthesized in 86% yield from commercially available starting materials. BP23C7 forms pseudo[2]rotaxane with a dibenzylammonium ion (DBA+), exhibiting a good association constant value (ka = 1 × 103 M-1). Subsequently, fluorophoric properties of BP23C7 and anthracene terminated axles are blended to create responsive pseudo[2]rotaxanes. The "turn-on" fluorescence response of BP23C7 due to the addition of fluoride and chloride anions to pseudo[2]rotaxane systems has been investigated. Concomitant fluorescence quenching of the anthracene moiety of corresponding axles due to ion-pair formation has been addressed. Furthermore, two variants of [23]crown ethers, i.e. BP23C7 and o-xylene-23-crown-7 ether (X23C7), are applied for constructing homo[3]rotaxane architectures. A half-axle comprising of DBA+ moiety and a terminal olefin is mixed separately with two [23]crown ethers and subjected to self-metathesis using Grubbs' first-generation catalyst.

Mechanically interlocked [c2]daisy chain backbone enabling advanced shape-memory polymeric materials

The incorporation of mechanically interlocked structures into polymer backbones has been shown to confer remarkable functionalities to materials. In this work, a [c2]daisy chain unit based on dibenzo-24-crown-8 is covalently embedded into the backbone of a polymer network, resulting in a synthetic material possessing remarkable shape-memory properties under thermal control. By decoupling the molecular structure into three control groups, we demonstrate the essential role of the [c2]daisy chain crosslinks in driving the shape memory function. The mechanically interlocked topology is found to be an essential element for the increase of glass transition temperature and consequent gain of shape memory function. The supramolecular host-guest interactions within the [c2]daisy chain topology not only ensure robust mechanical strength and good network stability of the polymer, but also impart the shape memory polymer with remarkable shape recovery properties and fatigue resistance ability. The incorporation of the [c2]daisy chain unit as a building block has the potential to lay the groundwork for the development of a wide range of shape-memory polymer materials.

Assembly and Utility of a Drawstring-Mimetic Supramolecular Complex

Inspired by the drawstring structure in daily life, here we report the development of a drawstring-mimetic supramolecular complex at the molecular scale. This complex consists of a rigid figure-of-eight macrocyclic host molecule and a flexible linear guest molecule which could interact through three-point non-covalent binding to form a highly selective and efficient host-guest assembly. The complex not only resembles the drawstring structure, but also mimics the properties of a drawstring with regard to deformations under external forces. The supramolecular drawstring can be utilized as an interlocked crosslinker for poly(methyl acrylate), and the corresponding polymer samples exhibit comprehensive enhancement of macroscopic mechanical performance including stiffness, strength, and toughness.

A review of nanomaterials for biosensing applications

A biosensor is a device that reacts with the analyte to be analyzed, detects its concentration, and generates readable information, which plays an important role in medical diagnosis, detection of physiological indicators, and disease prevention. Nanomaterials have received increasing attention in the fabrication and improvement of biosensors due to their unique physicochemical and optical properties. In this paper, the properties of nanomaterials such as the size effect, optical and electrical properties, and their advantages in the field of biosensing are briefly summarized, and the application of nanomaterials can effectively improve the sensitivity and reduce the detection limit of biosensors. The advantages of commonly used nanomaterials such as gold nanoparticles (AuNPs), carbon nanotubes (CNTs), quantum dots (QDs), graphene, and magnetic nanobeads for biosensor applications are also reviewed. Besides, the two main types of biosensors using nanomaterials involved in their construction and their working principles are described, and the toxicity and biocompatibility of nanomaterials and the future direction of nanomaterial biosensors are discussed.

Enhancing the Toughness and Strength of Polymers Using Mechanically Interlocked Hydrogen Bonds

The energy dissipative features of hydrogen bonds under conditions of mechanical strain have provided an ongoing incentive to explore hydrogen bonding units for the purpose of controlling and customizing the mechanical properties of polymeric materials. However, there remains a need for hydrogen bond units that (1) possess directionality, (2) provide selectivity, (3) dissipate energy effectively, and (4) can be incorporated readily into polymeric materials to regulate their mechanical properties. Here, we report mechanically interlocked hydrogen bond units that incorporate multiple hydrogen bonds within a [2]catenane structure. The conformational flexibility and associated spatial folding characteristics of the [2]catenane units allow for molecular scale motion under external stress, while the interlocked structure serves as a pivot that maintains the directionality and selectivity of the resultant hydrogen bonding units. When incorporated into polymers, these interlocked hydrogen bond motifs serve to strengthen and toughen the resulting materials. This study not only presents a novel hydrogen bond unit for creating polymeric materials with improved mechanical properties but also underscores the unique opportunities that mechanically interlocked hydrogen bond structures may provide across a diverse range of applications.

Negative Enthalpy Variation Drives Rapid Recovery in Thermoplastic Elastomer

The mechanism behind the resilience of polymeric materials, typically attributed to the well-established entropy elasticity, often ignores the contribution of enthalpy variation (ΔH), because it is based on the assumption of an ideal chain. However, this model does not fully account for the reduced resilience of thermoplastic polyurethane (TPU) during long-range deformation, which is mainly caused by the dynamics of physical crosslink networks. Such reduction is undesirable for long-range stretchable TPU considering its wide application range. Therefore, a negative ΔH effect is established in this work to facilitate instant recovery in long-range stretchable TPU, achieved by constructing a reversible interim interface via strain-induced phase separation. Consequently, the newly constructed dual soft segmental TPU shows resilience efficiency exceeding 95%, surpassing many synthetic high-performance TPUs with typical efficiencies below 80%, and comparable to biomaterials. Moreover, a remarkable hysteresis loop with a ratio exceeding 50%, makes it a viable candidate for applications such as artificial ligaments or buffer belts. The research also clarifies structural factors influencing resilience, including the symmetry of the dual soft segments and the content of hard segments, offering valuable insights for the design of highly resilient long-range stretchable elastomers.

Molecular Clogging Organogels with Excellent Solvent Maintenance, Adjustable Modulus, and Advanced Mechanics for Impact Protection

Inspired by mechanically interlocking supramolecular materials, exploiting the size difference between the bulky solvent and the cross-linked network mesh, a molecular clogging (MC) effect is developed to effectively inhibit solvent migration in organogels. A bulky solvent (branched citrate ester, BCE) with a molecular size above 1.4 nm is designed and synthesized. Series of MC-Gels are prepared by in situ polymerization of crosslinked polyurea with BCE as the gel solvent. The MC-Gels are colorless, transparent, and highly homogeneous, show significantly improved stability than gels prepared with small molecule solvents. As solvent migration is strongly inhibited by molecular clogging, the solvent content of the gels can be precisely controlled, resulting in a series of MC-Gels with continuously adjustable mechanics. In particular, the modulus of MC-Gel can be regulated from 1.3 GPa to 30 kPa, with a variation of 43 000 times. The molecular clogging effect also provides MC-Gels with unique high damping (maximum damping factor of 1.9), impact resistant mechanics (high impact toughness up to 40.68 MJ m-3 ). By applying shatter protection to items including eggs and ceramic armor plates, the potential of MC-Gels as high strength, high damping soft materials for a wide range of applications is well demonstrated.

Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness

Sacrificial chemical bonds have been used effectively to increase the toughness of elastomers because such bonds dissociate at forces significantly below the fracture limit of the primary load-bearing bonds, thereby dissipating local stress. This approach owes much of its success to the ability to adjust the threshold force at which the sacrificial bonds fail at the desired rate, for example, by selecting either covalent or noncovalent sacrificial bonds. Here, we report experimental and computational evidence that a mechanical bond, responsible for the structural integrity of a rotaxane or a catenane, increases the elastomer's fracture strain, stress, and energy as much as a covalent bond of comparable mechanochemical dissociation kinetics. We synthesized and studied 6 polyacrylates cross-linked by either difluorenylsuccinonitrile (DFSN), which is an established sacrificial mechanochromic moiety; a [2]rotaxane, whose stopper allows its wheel to dethread on the same subsecond time scale as DFSN dissociates when either is under tensile force of 1.5-2 nN; a structurally homologous [2]rotaxane with a much bulkier stopper that is stable at force >5.5 nN; similarly stoppered [3]rotaxanes containing DFSN in their axles; and a control polymer with aliphatic nonsacrificial cross-links. Our data suggest that mechanochemical dethreading of a rotaxane without failure of any covalent bonds may be an important, hitherto unrecognized, contributor to the toughness of some rotaxane-cross-linked polymers and that sacrificial mechanical bonds provide a mechanism to control material fracture behavior independently of the mechanochemical response of the covalent networks, due to their distinct relationships between structure and mechanochemical reactivity.

Daisy chain architectures: from discrete molecular entities to polymer materials

Daisy chain architectures, made by the self-complementary threading of an axle covalently linked to a macrocycle, represent a particularly intriguing family of supramolecular and mechanically interlocked (macro)molecules. In this review, we discuss their recent history, their modular chemical structures, and the various synthetic strategies to access them. We also detail how their internal sliding motions can be controlled and how their integration within polymers can amplify that motions up to the macroscopic scale. This overview of the literature demonstrates that the peculiar structure and dynamics of daisy chains have already strongly influenced the research on artificial molecular machines, with the potential to be implemented from nanometric switchable devices to mechanically active soft-matter materials.

Mechanically Interlocked [2]Rotaxane Aerogels with Tunable Morphologies and Mechanical Properties

Mechanical bonds have been utilized as promising motifs to construct mechanically interlocked aerogels (MIAs) with mechanical adaptivity and multifunctionality. However, fabricating such aerogels with not only precise chemical structures but also dynamic features remains challenging. Herein, we present MIAs carrying dense [2]rotaxane units, which bestow both the stability and flexibility of the aerogel network. Owing to the stable chemical structure of a [2]rotaxane, MIAs possessing a precise and full-scale mechanically interlocked network could be fabricated with the aid of diverse solvents. In addition, the dynamic nature of the [2]rotaxane resulted in morphologies and mechanical performances of the MIAs that can be dramatically modulated under chemical stimuli. We hope that the structure-property relationship in MIAs will facilitate the development of mechanically interlocked materials and provide novel opportunities toward constructing smart materials with multifunctionalities.

Control of the assembly of a cyclic hetero[4]pseudorotaxane from a self-complementary [2]rotaxane

The self-association of a ditopic [2]rotaxane with two macrocycles mainly leads to a [4]pseudorotaxane which can be reversibly disassembled by adding competitive binders, varying the solvent polarity and changing a binding site affinity.