Mechanically bonded macromolecules

Rotaxane Formation from Borate Ion-Containing Crown Ether and Ammonium Ion: Enhancement of Their Association through Ion-Pairing

We synthesized [2]rotaxanes featuring a catechol borate ion-containing crown ether and secondary ammonium ions. These rotaxane components show both ion-ion interactions and hydrogen bonds. X-ray crystallography and NMR spectroscopy allowed elucidation of the rotaxane structure. Moreover, 1H NMR spectroscopy revealed the rotaxane synthesis can be thermodynamically controlled. The binding affinity between the borate-containing crown ether and ammonium ions is enhanced by ion-pairing.

Highly Efficient Self-Assembly of Heterometallic [2]Catenanes and Cyclic Bis[2]catenanes via Orthogonal Metal-Coordination Interactions

Although catenated cages have been widely constructed due to their unique and elegant topological structures, cyclic catenanes formed by the connection of multiple catenane units have been rarely reported. Herein, based on the orthogonal metal-coordination-driven self-assembly, we prepare a series of heterometallic [2]catenanes and cyclic bis[2]catenanes, whose structures are clearly evidenced by single-crystal X-ray analysis. Owing to the multiple positively charged nature, as well as the potential synergistic effect of the Cu(I) and Pt(II) metal ions, the cyclic bis[2]catenanes display broad-spectrum antibacterial activity. This work not only provides an efficient strategy for the construction of heterometallic [2]catenanes and cyclic bis[2]catenanes but also explores their applications as superior antibacterial agents, which will promote the construction of advanced supramolecular structures for biomedical applications.

The Hamilton Receptor in Supramolecular Polymer Sciences

Supramolecular polymers are polymeric materials of monomeric fragments, held jointly by reversible and directional non-covalent interactions such as multiple hydrogen-bonding, charge transfer effects, host-guest interactions, metal coordination, and aromatic stacking. This review article on the Hamilton-based supramolecular polymers aims to shed light on the molecular recognition achievements by the Hamilton-based polymeric systems, evaluate Hamilton receptor's future prospects, and capitalize its potential applications in supramolecular chemistry. To the best of our knowledge, this is the first elaborative and sole manuscript in which polymeric Hamilton receptors are being exposed in detail. The first portion of this manuscript is related to the importance and urgency of polymers along with the historic background of Hamilton receptors. The middle section discloses the potential applications of Hamilton-type receptors in various fields, e.g., dendrimers, mechanically polymeric rotaxanes, and self-assemblies. The final section of the manuscript discloses the future aspects and the importance of novel polymer-based Hamilton-type receptors in the modern era. We believe that this first review in this emerging yet immature field will be useful to inspire scientists around the world to find the unseen future prospects, thereby boosting the field related to this valued artificial receptor in the province of supramolecular chemistry and also in other domains of scientific fields and technology, as well.

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.

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.

Characterization by Gel Permeation Chromatography of the Molecular Weight of Supramolecular Polymers Generated by Forming Polyrotaxanes through the Introduction of External Stoppers

Supramolecular polymers find wide applications across diverse domains, and the molecular weight exerts a critical influence on their applicability. Consequently, the measurement of molecular weight for supramolecular polymers assumes paramount significance. Gel Permeation Chromatography (GPC) requiring low-concentration condition is a common characterization employed for molecular weight determination, which is not suitable for supramolecular polymers possessing concentration-independence property. Here, to break this threshold, we synthesized M1 embodying dibenzo-24-crown-8 (DB24C8) moiety as well as dibenzylammonium salt (DBA) group, which was capable of self-assembling into supramolecular polymers terminated with aldehyde groups at its end. Upon the addition of (4- (1,2,2-Triphenylvinyl) phenyl) methylamine (TPE-NH2), supramolecular polymers underwent a transition into polyrotaxanes, for which it was led by the generation of imine bonds. By virtue of GPC, the molecular weight of polyrotaxanes was obtained, then it was available to gain the molecular weight of supramolecular polymers with the help of transformation efficiency.

Stretchable poly[2]rotaxane elastomers

Mechanically interlocked polymers (MIPs) are promising candidates for the construction of elastomeric materials with desirable mechanical performance on account of their abilities to undergo inherent rotational and translational mechanical movements at the molecular level. However, the investigations on their mechanical properties are lagging far behind their structural fabrication, especially for linear polyrotaxanes in bulk. Herein, we report stretchable poly[2]rotaxane elastomers (PREs) which integrate numerous mechanical bonds in the polymeric backbone to boost macroscopic mechanical properties. Specifically, we have synthesized a hydroxy-functionalized [2]rotaxane that subsequently participates in the condensation polymerization with diisocyanate to form PREs. Benefitting from the peculiar structural and dynamic characteristics of the poly[2]rotaxane, the representative PRE exhibits favorable mechanical performance in terms of stretchability (∼1200%), Young's modulus (24.6 MPa), and toughness (49.5 MJ/m3). Moreover, we present our poly[2]rotaxanes as model systems to understand the relationship between mechanical bonds and macroscopic mechanical properties. It is concluded that the mechanical properties of our PREs are mainly determined by the unique topological architectures which possess a consecutive energy dissipation pathway including the dissociation of host-guest interaction and consequential sliding motion of the wheel along the axle in the [2]rotaxane motif.

A computational study of a light-driven artificial device: a third generation rotational photo-molecular motor in dilute solutions

A third-generation artificial photo-molecular motor, featuring two photo-switchable rotating moieties in connection with a pseudoasymmetric molecular centre, is investigated by combining quantum-mechanics (QM) algorithms with classical molecular dynamics (MD) propagators. In particular, in the present contribution we have addressed such a molecular motor in different rotational isomers following the experimental observations arising from the application of multiple spectroscopic techniques in dilute solutions. At first, we focused our attention on the reproduction of the UV/Vis absorption spectrum in two solvents (acetonitrile and cyclohexane) with different gradient-corrected density functional theory (B3LYP, Cam-B3LYP, PBE, PBE0) functionals in conjunction with the conductor-like and polarizable continuum model (C-PCM). Furthermore, we refined the absorption signals by combining a classical MD sampling at room-temperature with DFT-based electronic degrees of freedom to compute perturbed excitation wavelengths driven by thermal fluctuation and solvation effects. In this respect, we have modelled the investigated artificial motor within solution nanodroplets with solvent molecules treated contextually at atomistic level and via a dielectric and polarizable continuum model.

Multiple hydrogen bonding driven supramolecular architectures and their biomedical applications

Supramolecular chemistry combines the strength of molecular assembly via various molecular interactions. Hydrogen bonding facilitated self-assembly with the advantages of directionality, specificity, reversibility, and strength is a promising approach for constructing advanced supramolecules. There are still some challenges in hydrogen bonding based supramolecular polymers, such as complexity originating from tautomerism of the molecular building modules, the assembly process, and structure versatility of building blocks. In this review, examples are selected to give insights into multiple hydrogen bonding driven emerging supramolecular architectures. We focus on chiral supramolecular assemblies, multiple hydrogen bonding modules as stimuli responsive sources, interpenetrating polymer networks, multiple hydrogen bonding assisted organic frameworks, supramolecular adhesives, energy dissipators, and quantitative analysis of nano-adhesion. The applications in biomedical materials are focused with detailed examples including drug design evolution for myotonic dystrophy, molecular assembly for advanced drug delivery, an indicator displacement strategy for DNA detection, tissue engineering, and self-assembly complexes as gene delivery vectors for gene transfection. In addition, insights into the current challenges and future perspectives of this field to propel the development of multiple hydrogen bonding facilitated supramolecular materials are proposed.

Dual-Stimulus-Driven Dynamically Controllable [3]Rotaxane with Tunable Organic Room-Temperature Phosphorescence

A dual-stimulus-driven stiff-stilbene-based dynamic [3]rotaxane has been facilely developed using the threading-stoppering strategy and exhibits reversible shuttling motions and bidirectional rotations upon encountering acid-base and distinct light stimulations, respectively. Herein, the two dibenzo-24-crown-8 macrocycles can undergo reversible switching motion between two different stations along the axle suffered from acid-base stimulation, and simultaneously, the two rotaxanes can also perform cis-trans rotations upon irradiation with distinct light. In other words, the constructed rotaxanes can conduct two modes of regular motions without interference. Interestingly, reciprocating switching motion of the rings along the axle enabled the rotaxanes to exhibit controllable and reversible photoisomerization speed, conversion yield, and quantum yield. Crucially, these rotaxanes also manifest adjustable solid-state organic room-temperature phosphorescence (RTP) and photoluminescence stimulated by dual factors (acid-base and diverse light), which are further applied in information encryption and anticounterfeiting. The presented study provides an instructive way for precisely boosting photoisomerization performances and the fabrication of dual-stimuli-induced molecular machines with functions of two-mode mechanical motions and controllable pure organic RTP switches.

Infinite Twisted Polycatenanes

Poly[n]catenanes have exceptional mechanical bonding properties that give them tremendous potential for use in the development of molecular machines and soft materials. Synthesizing these compounds has, however, proven to be a formidable challenge. Herein, we describe a concise method for the construction of twisted polycatenanes. Our approach involves using preorganized double helicates as templates, linked crosswise in a linear fashion by either silver ions or triple bonds. By using this approach, we successfully synthesized twisted polycatenanes with both coordination and covalent bonding employing Ag(I) ions and ethynylene units, respectively, as the linkages and leveraging the same Ag(I)-templated double helicate in both cases. Synthesis with Ag(I) ions formed a single-crystalline one-dimensional (1D) coordination poly[n]catenane, and synthesis using ethynylene units generated 1D fibers which self-assembled with solvents to form a gel. Our results confirm the potential of multi-stranded metallohelicates for creating sophisticated mechanically interlocked molecules and polymers, which could pave the way for exploration in the realms of molecular nanotopology and materials design.

An Interlocked Figure-of-Eight Molecular Shuttle

Here is reported the synthesis and characterization of an interlocked figure-of-eight rotaxane molecular shuttle from a dibenzo-24-crown-8 (DB24C8) derivative. This latter bears two molecular chains, whose extremities are able to react together by click chemistry. One of the two substituting chain holds an ammonium function aimed at driving the self-entanglement through the complexation of the DB24C8 moiety. In the targeted figure-of-eight rotaxane, shuttling of the DB24C8 along the threaded axle from the best ammonium station to the weaker binding site triazolium was performed through deprotonation or deprotonation-then-carbamoylation of the ammonium. This way, two discrete co-conformational states were obtained, in which the folding and size of the two loops could be changed.

Into the dynamics of rotaxanes at atomistic resolution

Mechanically-interlocked molecules (MIMs) are at the basis of artificial molecular machines and are attracting increasing interest for various applications, from catalysis to drug delivery and nanoelectronics. MIMs are composed of mechanically-interconnected molecular sub-parts that can move with respect to each other, imparting these systems innately dynamical behaviors and interesting stimuli-responsive properties. The rational design of MIMs with desired functionalities requires studying their dynamics at sub-molecular resolution and on relevant timescales, which is challenging experimentally and computationally. Here, we combine molecular dynamics and metadynamics simulations to reconstruct the thermodynamics and kinetics of different types of MIMs at atomistic resolution under different conditions. As representative case studies, we use rotaxanes and molecular shuttles substantially differing in structure, architecture, and dynamical behavior. Our computational approach provides results in agreement with the available experimental evidence and a direct demonstration of the critical effect of the solvent on the dynamics of the MIMs. At the same time, our simulations unveil key factors controlling the dynamics of these systems, providing submolecular-level insights into the mechanisms and kinetics of shuttling. Reconstruction of the free-energy profiles from the simulations reveals details of the conformations of macrocycles on the binding site that are difficult to access via routine experiments and precious for understanding the MIMs' behavior, while their decomposition in enthalpic and entropic contributions unveils the mechanisms and key transitions ruling the intermolecular movements between metastable states within them. The computational framework presented herein is flexible and can be used, in principle, to study a variety of mechanically-interlocked systems.

Structure and Thermodynamics of Linear, Ring, and Catenane Polymers in Solutions and at Liquid-Liquid Interfaces

In recent decades, advances in the syntheses of mechanically interlocked macromolecules, such as catenanes, have led to much greater interest in the applications of these complexes, from molecular motors and actuators to nanoscale computational memory and nanoswitches. Much remains to be understood, however, regarding how catenated ring compounds behave as a result of the effects of different solvents as well as the effects of solvent/solvent interfaces. In this work, we have investigated, using molecular dynamics simulations, the effects of solvation of poly(ethylene oxide) chains of different topologies─linear, ring, and [2]catenane─in two solvents both considered favorable toward PEO (water, toluene) and at the water/toluene interface. Compared to ring and [2]catenane molecules, the linear PEO chain showed the largest increase in size at the water/toluene interface compared to bulk water or bulk toluene. Perhaps surprisingly, observations indicate that the tendency of all three topologies to extend at the water/toluene interface may have more to do with screening the interaction between the two solvents than with optimizing specific solvent-polymer contacts.

Thermodynamics, Kinetics, and Optical Properties of Rotaxane: A First-Principles Molecular Dynamics Study

Through molecular mechanics using the force field along with the quantum dynamical aspect of mechanically interlocked compounds, rotaxanes (defined as macromolecular rings that are threaded on a dumbbell-shaped axle molecule) are investigated with advanced quantum mechanical methods, including the atom-centered density matrix propagation simulation technique, at different temperatures like 300, 500, 700, 900, 2000, and 2500 K for 1.2 ps. Ab initio molecular dynamics simulation is carried out. In addition to, we investigate the noncovalent interaction present in the rotaxane compound 2R-D-2PF₆ with the help of reduced density gradient, average reduced density gradient, density overlap region indicator, and interaction region indicator as well as Hirshfeld surface analyses. Furthermore, the stability of 2R-D-2PF₆ at room temperature and higher temperatures is elucidated by analyzing the thermal fluctuation index through a dynamic process. In order to check the optical behavior of our selected rotaxane compound, an evaluation of the electronic dipole moment, static and frequency-dependent average polarizability, and first- and second-order hyperpolarizability is carried out. The rotaxane compound shows very promising linear and nonlinear optical responses, which indicates its utility as a very good optical material. The calculation of the time-dependent density-functional theory highlights the broad absorption band of rotaxane spanning the UV-visible domain. Therefore, we also unravel that this can tap into solar radiation or harnessing of solar energy.

The effect of thread-like monomer structure on the synthesis of poly[n]catenanes from metallosupramolecular polymers

The main-chain poly[n]catenane consists of a series of interlocked rings that resemble a macroscopic chain-link structure. Recently, the synthesis of such intriguing polymers was reported via a metallosupramolecular polymer (MSP) template that consists of alternating units of macrocyclic and linear thread-like monomers. Ring closure of the thread components has been shown to yield a mixture of cyclic, linear, and branched poly[n]catenanes. Reported herein are studies aimed at accessing new poly[n]catenanes via this approach and exploring the effect the thread-like monomer structure has on the poly[n]catenane synthesis. Specifically, the effect of the size of the aromatic linker and alkenyl chains of the thread-like monomer is investigated. Three new poly[n]catenanes (with different ring sizes) were prepared using the MSP approach and the results show that tailoring the structure of the thread-like monomer can allow the selective synthesis of branched poly[n]catenanes.

Advanced supramolecular design for direct ink writing of soft materials

The exciting advancements in 3D-printing of soft materials are changing the landscape of materials development and fabrication. Among various 3D-printers that are designed for soft materials fabrication, the direct ink writing (DIW) system is particularly attractive for chemists and materials scientists due to the mild fabrication conditions, compatibility with a wide range of organic and inorganic materials, and the ease of multi-materials 3D-printing. Inks for DIW need to possess suitable viscoelastic properties to allow for smooth extrusion and be self-supportive after printing, but molecularly facilitating 3D printability to functional materials remains nontrivial. While supramolecular binding motifs have been increasingly used for 3D-printing, these inks are largely optimized empirically for DIW. Hence, this review aims to establish a clear connection between the molecular understanding of the supramolecularly bound motifs and their viscoelastic properties at bulk. Herein, extrudable (but not self-supportive) and 3D-printable (self-supportive) polymeric materials that utilize noncovalent interactions, including hydrogen bonding, host-guest inclusion, metal-ligand coordination, micro-crystallization, and van der Waals interaction, have been discussed in detail. In particular, the rheological distinctions between extrudable and 3D-printable inks have been discussed from a supramolecular design perspective. Examples shown in this review also highlight the exciting macroscale functions amplified from the molecular design. Challenges associated with the hierarchical control and characterization of supramolecularly designed DIW inks are also outlined. The perspective of utilizing supramolecular binding motifs in soft materials DIW printing has been discussed. This review serves to connect researchers across disciplines to develop innovative solutions that connect top-down 3D-printing and bottom-up supramolecular design to accelerate the development of 3D-print soft materials for a sustainable future.

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.

Mechanically interlocked polymers based on rotaxanes

The nature of mechanically interlocked molecules (MIMs) has continued to encourage researchers to design and construct a variety of high-performance materials. Introducing mechanically interlocked structures into polymers has led to novel polymeric materials, called mechanically interlocked polymers (MIPs). Rotaxane-based MIPs are an important class, where the mechanically interlocked characteristic retains a high degree of structural freedom and mobility of their components, such as the rotation and sliding motions of rotaxane units. Therefore, these MIP materials are known to possess a unique set of properties, including mechanical robustness, adaptability and responsiveness, which endow them with potential applications in many emerging fields, such as protective materials, intelligent actuators, and mechanisorption. In this review, we outline the synthetic strategies, structure-property relationships, and application explorations of various polyrotaxanes, including linear polyrotaxanes, polyrotaxane networks, and rotaxane dendrimers.

Cyclodextrin-pillar[n]arene hybridized macrocyclic systems

Cyclodextrin (CD) and pillar[n]arene are significant macrocyclic host molecules in supramolecular chemistry, and have either similar or contrasting physicochemical properties, for example, both can provide capable cavities available for recognizing various favorite guest molecules, while they usually possess different solubility in aqueous solutions, and exhibit diverse chiral characteristics. To balance their similarity and differences inherited from each chemical structure and incorporate both advantages, the CD-pillar[n]arene hybrid macrocyclic system was recently developed. In this review, we will focus on the preparation and application of CD-pillar[n]arene hybrid macrocyclic systems. Both noncovalent interactions and covalent bonds were employed in the synthesis strategies of building the hybrid macrocyclic system, which was in the form of host-guest inclusion, self-assembly, conjugated molecules, and polymeric structures. Furthermore, the CD-pillar[n]arene hybrid macrocyclic system has been primarily applied for the removal of organic pollutants from water, induced chirality, as well as photocatalysis due to the integration of both cavities from CD and pillar[n]arene as hybrid hosts and chiral characteristics inherited from their chemical structures.

Ring-Over-Ring Deslipping From Imine-Bridged Heterorotaxanes

Ring-over-ring slippage and ring-through-ring penetration are important processes in the construction of ring-in-ring multiple interlocked architectures. We have successfully observed “ring-over-ring deslipping” on the rotaxane axle by exploiting the dynamic covalent nature of imine bonds in imine-bridged heterorotaxanes R1 and R2 with two macrocycles of different ring sizes on the axle. When the imine bridges of R1 were cleaved, a hydrolyzed hetero[4]rotaxane [4]R1′ was formed as an intermediate under dynamic equilibrium, and the larger 38-membered macrocycle M was deslipped over the 24-membered ring (24C8 or DB24C8) to dissociate into a [3]rotaxane [3]R3 and a macrocycle M. The time dependent NMR measurement and the determined thermodynamic parameters revealed that the rate-limiting step of the deslipping process was attributed to steric hindrance between two rings and reduced mobility of M due to proximity to the crown ether, which was bound to the anilinium on the axle molecule.

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Supramolecular systems prepared using terpyridine-containing pillararene

Recent progresses about the preparation of terpyridine-containing pillararene, as well as the utilization of those building blocks for making external stimulud-responsive supramolecular systems were summarized in this review.

Precision synthesis of linear oligorotaxanes and polyrotaxanes achieving well-defined positions and numbers of cyclic components on the axle

Interest in macromolecules has increased because of their functional properties, which can be tuned using precise organic synthetic methods. For example, desired functions have been imparted by controlling the nanoscale structures of such macromolecules. In particular, compounds with interlocked structures, including rotaxanes, have attracted attention because of their unique supramolecular structures. In such supramolecular structures, the mobility and freedom of the macrocycles are restricted by an axle and dependent on those of other macrocycles, which imparts unique functions to these threaded structures. Recently, methods for the ultrafine engineering and synthesis, as well as functions, of "defined" rotaxane structures that are not statistically dispersed on the axle (i.e., control over the number and position of cyclic molecules) have been reported. Various synthetic strategies allow access to such well-defined linear oligo- and polyrotaxanes, including [1]rotaxanes and [n]rotaxanes (mostly n > 3). These state-of-the-art synthetic methods have resulted in unique functions of these oligo-and polyrotaxane materials. Herein, we review the effective synthetic protocols and functions of precisely constructed one-dimensional oligomers and polymers bearing defined threaded structures, and discuss the latest reports and trends.

Macrocyclic vs. [2]catenane btp structures: influence of (aryl) substitution on the self templation of btp ligands in macrocyclic synthesis

The synthesis of four 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) olefin based ligands 3, 4, 11 and 12 is described and their attempted use to form mechanically interlocked molecules using ring closing metatheses (RCM) reactions. The btp ligands were modified in two ways, in 3 and 4 the aryl substitution pattern was changed from 4^th position to 3^rd position and in the case of 11 and 12, the arms were replaced with aliphatic chains. Our study demonstrates that for all four ligands, the RCM reactions only result in the formation of macrocyclic structures, which in three of the cases, were structurally characterised in both solution (using NMR and HRMS) and in the solid-state using X-ray crystallography. NMR studies were also carried out to investigate if these ligands could preorganise in solution via hydrogen bonding interactions. This study provides a handle of how such precursor substitution can be used to direct the formation of macrocycles or mechanically interlocked structures.

Direct enhancement of intercomponent interactions in polyrotaxane and its pronounced effects on glass state properties

Strong interactions between the host cyclodextrin and the threading guest polymer were introduced by selective modifications to the polymer of a polybutadine-based polyrotaxane. The changes in the intercomponent interactions influenced the mobility of the threading polymer that was confined in the glassy host framework, resulting in different mechanical properties.

Self-Assembly Synthesis of a [2]Catenane CoII Single-Molecule Magnet

We describe herein the self-assembly synthesis of an octanuclear Co^II [2]catenane {[Co₄ (H₂ L)₆ ]₂ ¹⁶⁺ } formed by the mechanical interlocking of two {[Co₄ (H₂ L)₆ ]⁸⁺ } rectangles of unprecedented topology. Subtle manipulation of the synthetic conditions allows the isolation of a mixed-valence [Co₂ ^III /Co₂ ^II ]¹⁰⁺ non-catenated rectangle. The Co^II centers in the [2]catenane exhibit slow relaxation of their magnetic moment, i. e. single-molecule magnet properties, dominated by quantum tunneling and Raman relaxation processes. This work shows that metallo-supramolecular chemistry can precisely control the organization of single-molecule magnets in topologically complex arrangements.

Complementarity of 2,6-Dimethanolpyridine and Di(ethylene glycol) in the Complexation of Na+ Ions: Attaching Multiple Copies of [2]Catenane Branches to Isophthalaldehyde-Containing Cores

In this study we found that 2,6-dimethanolpyridine displays good complementarity toward di(ethylene glycol) for the complexation of Na⁺ ions, allowing us to use this recognition system for the efficient synthesis of hetero[2]catenanes; indeed, it allowed us to attach multiple copies of [2]catenanes to branched systems presenting multiple isophthalaldehyde units. When we attempted to form a catenane from a preformed macrocycle featuring only a single di(ethylene glycol) unit, reacting it with a di(ethylene glycol) derivative presenting two amino termini, isophthalaldehyde, and templating Na⁺ ions [i.e., with the aim of using di(ethylene glycol)·Na⁺·di(ethylene glycol) recognition to template the formation of the interlocked imino macrocycle], the yields of the hetero[2]catenane and homo[2]catenane, comprising two imino macrocyclic units, were both poor (14% and 7%, respectively). In contrast, when one or two 2,6-dimethanolpyridine units were present in the preformed macrocycles, their reactions with the same diamine, dialdehyde, and Na⁺ ions provided the hetero[2]catenanes with high selectivity and efficiency (44% and 64% yields, respectively), with minimal formation of the competing homo[2]catenane. The high complementary of the 2,6-dimethanolpyridine·Na⁺·di(ethylene glycol) ligand pair allowed us to synthesize [2]catenane dimers and trimers directly from corresponding isophthalaldehyde-presenting cores, with yields, after subsequent reduction and methylation, of 42% and 31%, respectively.

High-component reactions (HCRs): An overview of MCRs containing seven or more components as versatile tools in organic synthesis

Recently, multi-component reactions (MCRs) have gained special attention due to their versatility for the synthesis of polycyclic heterocycles. Moreover, their applicability can become more widespread as they can be combined together as a union of MCRs. In this overview, the authors have tried to collect the MCRs containing more than seven components that can lead to effectual heterocycles in organic and/or pharmaceutical chemistry. The review contains papers published up to the end of 2020. The subject is classified based on the number of substrates, such as seven-, eight-, nine-, ten-, and more components. The authors expect their report to be helpful for researchers to clarify their route to significant MCRs.

Chemoresponsive Supramolecular Polypseudorotaxanes with Infinite Switching Capability

Chemoresponsive supramolecular systems with infinite switching capability are important for applications in recycled materials and intelligent devices. To attain this objective, here a chemoresponsive polypseudorotaxane is reported on the basis of a bis(p-phenylene)-34-crown-10 macrocycle (H) and a cyano-substituted viologen guest (G). H and G form a [2]pseudorotaxane (H⊃G) both in solution and in the solid state. Upon addition of AgSF₆ , a polypseudorotaxane (denoted as [H⋅G⋅Ag]_n ) forms as synergistically driven by host-guest complexation and metal-coordination interactions. [H⋅G⋅Ag]_n depolymerizes into a [3]pseudorotaxane (denoted as H₂ ⋅G⋅Ag₂ ⋅acetone₂ ) upon addition of H and AgSF₆ , while it reforms with successive addition of G. The transformations between [H⋅G⋅Ag]_n and H₂ ⋅G⋅Ag₂ ⋅acetone₂ can be switched for infinite cycles, superior to the conventional chemoresponsive supramolecular polymeric systems with limited switching capability.

Effect of metallosupramolecular polymer concentration on the synthesis of poly[n]catenanes

Poly[n]catenanes are a class of polymers that are composed entirely of interlocked rings. One synthetic route to these polymers involves the formation of a metallosupramolecular polymer (MSP) that consists of alternating units of macrocyclic and linear thread components. Ring closure of the thread components has been shown to yield a mixture of cyclic, linear, and branched poly[n]catenanes. Reported herein are investigations into this synthetic methodology, with a focus on a more detailed understanding of the crude product distribution and how the concentration of the MSP during the ring closing reaction impacts the resulting poly[n]catenanes. In addition to a better understanding of the molecular products obtained in these reactions, the results show that the concentration of the reaction can be used to tune the size and type of poly[n]catenanes accessed. At low concentrations the interlocked product distribution is limited to primarily oligomeric and small cyclic catenanes . However, the same reaction at increased concentration can yield branched poly[n]catenanes with an ca. 21 kg mol-1, with evidence of structures containing as many as 640 interlocked rings (1000 kg mol-1).

Radical-pairing-induced molecular assembly and motion

Radical-pairing interactions between conjugated organic π-radicals are relative newcomers to the inventory of molecular recognition motifs explored in supramolecular chemistry. The unique electronic, magnetic, optical and redox-responsive properties of the conjugated π-radicals render molecules designed with radical-pairing interactions useful for applications in various areas of chemistry and materials science. In particular, the ability to control formation of radical cationic or anionic species, by redox stimulation, provides a flexible trigger for directed assembly and controlled molecular motions, as well as a convenient means of inputting energy to fuel non-equilibrium processes. In this Review, we provide an overview of different examples of radical-pairing-based recognition processes and of their emerging use in (1) supramolecular assembly, (2) templation of mechanically interlocked molecules, (3) stimuli-controlled molecular switches and, by incorporation of kinetic asymmetry in the design, (4) the creation of unidirectional molecular transporters based on pumping cassettes powered by fuelled switching of radical-pairing interactions. We conclude the discussion with an outlook on future directions for the field.

Rapid and High-Yield Synthesis of [23]Crown Ether: Applied as a Wheel Component in the Formation of Pseudo[2]rotaxane and Synthesis of [2]Catenane with a Dibenzylammonium Dumbbell

A facile, rapid, and high yield synthesis of [23]crown ether (X23C7) has been developed from commercially available starting materials, in one step with good to excellent yield. The reaction is completed in 6 h under room temperature conditions, with the highest yield being 81%. The X23C7 macrocycle formed pseudo[2]rotaxane with a dibenzylammonium ion (DBA⁺) dumbbell, exhibiting strong association (K_a = 2.61 × 10³ M^-1). Consequently, a [2]catenane was synthesized from a DBA⁺-based diolefin terminated salt and X23C7 in 81% yield, using a threading-followed-ring-closing-metathesis approach.

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings*

Poly(ethylene oxide)s (PEOs) are useful polymers with good water solubility, biological compatibility, and commercial availability. PEOs with various end groups were threaded into pillar[5]arene rings in a mixture of water and methanol to afford pseudopolyrotaxanes. Corresponding polyrotaxanes were also constructed by capping COOH-terminated pseudopolyrotaxanes with bulky amines, in which multiple hydrogen bonds involving the pillar[5]arene OH groups were critically important to prevent dethreading. The number of threaded ring components could be rationally controlled in these materials, providing a simple and versatile method to tune the mechanical and thermal properties. Specifically, a polyrotaxane with a high-molecular-weight axle became elastic upon heating above the melting point of PEOs and exhibited temperature-dependent shape memory property because of the topological confinement and crosslinked hydrogen bonds.

Acid-Activated Motion Switching of DB24C8 between Two Discrete Platinum(II) Metallacycles

The precise operation of molecular motion for constructing complicated mechanically interlocked molecules has received considerable attention and is still an energetic field of supramolecular chemistry. Herein, we reported the construction of two tris[2]pseudorotaxanes metallacycles with acid-base controllable molecular motion through self-sorting strategy and host-guest interaction. Firstly, two hexagonal Pt(II) metallacycles M1 and M2 decorated with different host-guest recognition sites have been constructed via coordination-driven self-assembly strategy. The binding of metallacycles M1 and M2 with dibenzo-24-crown-8 (DB24C8) to form tris[2]pseudorotaxanes complexes TPRM1 and TPRM2 have been investigated. Furthermore, by taking advantage of the strong binding affinity between the protonated metallacycle M2 and DB24C8, the addition of trifluoroacetic acid (TFA) as a stimulus successfully induces an acid-activated motion switching of DB24C8 between the discrete metallacycles M1 and M2. This research not only affords a highly efficient way to construct stimuli-responsive smart supramolecular systems but also offers prospects for precisely control multicomponent cooperative motion.

Advances in applied supramolecular technologies

Supramolecular chemistry has successfully built a foundation of fundamental understanding. However, with this now achieved, we show how this area of chemistry is moving out of the laboratory towards successful commercialisation.

Efficient one-pot synthesis of [3]catenanes based on Pt(ii) metallacycles with a flexible building block

Three [3]catenanes were fabricated in high efficiency through the self-assembly of a 90° platinum(ii) receptor, a flexible bis(4,4′-bipyridinium) donor and a crown ether (DB24C8 or DB30C10).

Orientation of the α-CD component of [2]rotaxanes affects their specific molecular recognition behaviour

An α-CD component enhanced the anion recognition ability of the urea moiety and the deprotonation of the phenol moiety in the axle component in orientationally isomeric [2]rotaxanes with the OH groups on the wide rim of the α-CD, respectively.