Mechanical Bond-Induced Radical Stabilization

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

An electric molecular motor

Macroscopic electric motors continue to have a large impact on almost every aspect of modern society. Consequently, the effort towards developing molecular motors^1-3 that can be driven by electricity could not be more timely. Here we describe an electric molecular motor based on a [3]catenane^4,5, in which two cyclobis(paraquat-p-phenylene)⁶ (CBPQT⁴⁺) rings are powered by electricity in solution to circumrotate unidirectionally around a 50-membered loop. The constitution of the loop ensures that both rings undergo highly (85%) unidirectional movement under the guidance of a flashing energy ratchet^7,8, whereas the interactions between the two rings give rise to a two-dimensional potential energy surface (PES) similar to that shown by F_OF₁ ATP synthase⁹. The unidirectionality is powered by an oscillating¹⁰ voltage^11,12 or external modulation of the redox potential¹³. Initially, we focused our attention on the homologous [2]catenane, only to find that the kinetic asymmetry was insufficient to support unidirectional movement of the sole ring. Accordingly, we incorporated a second CBPQT⁴⁺ ring to provide further symmetry breaking by interactions between the two mobile rings. This demonstration of electrically driven continual circumrotatory motion of two rings around a loop in a [3]catenane is free from the production of waste products and represents an important step towards surface-bound¹⁴ electric molecular motors.

2D and 3D metal-organic frameworks constructed with a mechanically rigidified [3]rotaxane ligand

A mechanically interlocked [3]rotaxane was newly designed, synthesized, and employed as a ligand for constructing metal-organic frameworks (MOFs). The nano-confinement by macrocycles forces the soft bis-isophthalate axle into a pseudo-rigid conformation and coordinates to zinc(II) ions, affording a two- or three-dimensional MOF under controlled conditions. The 2D MOF exhibits a neutral framework with a periodic puckering sheet structure, while an anionic framework with a pts topology was observed for the 3D MOF. The phase purity of both bulk materials was confirmed by powder X-ray diffraction. Thermogravimetric analysis reveals that both materials are stable up to 250 °C. The success of applying mechanical bonds to rigidify flexible ligands provides new insights for the design of metal-organic frameworks.

Self-Assembly of Stimuli-Responsive [2]Rotaxanes by Amidinium Exchange

Advances in supramolecular chemistry are often underpinned by the development of fundamental building blocks and methods enabling their interconversion. In this work, we report the use of an underexplored dynamic covalent reaction for the synthesis of stimuli-responsive [2]rotaxanes. The formamidinium moiety lies at the heart of these mechanically interlocked architectures, because it enables both dynamic covalent exchange and the binding of simple crown ethers. We demonstrated that the rotaxane self-assembly follows a unique reaction pathway and that the complex interplay between crown ether and thread can be controlled in a transient fashion by addition of base and fuel acid. Dynamic combinatorial libraries, when exposed to diverse nucleophiles, revealed a profound stabilizing effect of the mechanical bond as well as intriguing reactivity differences between seemingly similar [2]rotaxanes.

Stereodynamics of E/Z isomerization in rotaxanes through mechanical shuttling and covalent bond rotation

The mechanical bond has opened a new world for structural and dynamic stereochemistry, which is still largely underexplored and whose significance for various applications is becoming increasingly evident. We demonstrate that designed rearrangements involving both covalent and mechanical bonds can be integrated in [2]rotaxanes, leading to interesting consequences in terms of E/Z isomerization mechanisms. Two entirely distinct and concomitant stereomutations, pertaining to the same stereogenic element but involving different kinds of linkages within the molecule, are observed and are thoroughly characterized. The rate of the two processes is affected in opposite ways upon changing solvent polarity; such a phenomenon can be used to selectively modify the rate of each motion and adjust the relative contribution of the two mechanisms to the isomerization. Although the movements are not synchronized, an analysis of the intriguing fundamental implications for transition state theory, reaction pathway bifurcation, and microscopic reversibility was triggered by our experimental observations.

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Rotaxanes that display E/Z stereoisomerism depending on the ring position

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Co-existence of two different stereomutations that yield the same product

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Mutual influence and opposite solvent dependence of the two dynamic processes

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Fundamental implications for microscopic reversibility and chemical equilibrium

The concurrence and interplay of different movements of molecular components within the same structure play a key role in providing function to naturally occurring molecular machines. Despite the progress made on artificial counterparts, the construction of molecular systems, where two (or more) motions are integrated together to produce an outcome, is still in its infancy. Molecules called rotaxanes, obtained by interlocking a ring with a dumbbell-shaped axle, are an appealing yet underexplored platform for this purpose. Here, we describe rotaxanes where two coexisting and radically different processes—rotation about a covalent bond and translation of the ring along the axle—lead to the same change in the overall molecular shape. These results are significant not only to improve our fundamental understanding of the way molecular components move but also to develop sophisticated artificial nanomachines capable of transforming or transmitting motion.

Radically Enhanced Dual Recognition

Complexation between a viologen radical cation (V^.+ ) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT^2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V^.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV^.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)^2(.+) , i.e., CBPQN^2(.+) . Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN^2(.+) suitable for binding two MV^.+ with relatively short (3.05-3.25 Å) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV^.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex - namely, [(MV)₂ ⊂CBPQN]^{4( .+)} - in MeCN was confirmed by VT ¹ H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)₂ ⊂CBPQN]^{4( .+)} reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V^.+ , suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV^.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.

The Unusual Photochromic and Hydrochromic Switching Behavior of Cellulose-Embedded 1,8-Naphthalimide-Viologen Derivatives in the Solid-State

Stimuli-responsive chromic materials such as photochromics, hydrochromics, thermochromics, and electrochromics have a long history of capturing the attention of scientists due to their potential industrial applications and novelty in popular culture. However, hybrid chromic materials that combine two or more stimuli-triggered color changing properties are not so well known. Herein, we report a design strategy that has led to a series of emissive 1,8-naphthalimide-viologen dyads which exhibit unusual dual photochromic and hydrochromic switching behavior in the solid-state when embedded in a cellulose matrix. This behavior manifests as reversible solid state fluorescence hydrochromism upon changes in atmospheric relative humidity (RH), and reversible solid state photochromism upon generation of a cellulose-stabilized viologen radical cation. In this design strategy, the bipyridinium unit serves as both a water-sensitive receptor for the hydrochromic fluorophore-receptor system, and a photochromic group, capable of eliciting its own visible colorimetric response, generating a fluorescence quenching radical cation with prolonged exposure to ultraviolet (UV) light. These dyes can be inkjet-printed onto cellulose paper or drop-cast as cellulose powder-based films and can be unidirectionally cycled between three different states which can be characteristically visualized under UV light or visible light. The material's photochromism, hydrochromism, and underlying mechanism of action was investigated using computational analysis, dynamic vapor sorption/desorption isotherms, electron paramagnetic resonance spectroscopy, and variable humidity UV-Vis adsorption and fluorescence spectroscopies.

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.

Cross dehydrogenative C-O coupling catalysed by a catenane-coordinated copper(i)

Catalytic activity of copper(i) complexes supported by phenanthroline-containing catenane ligands towards a new C(sp³)–O dehydrogenative cross-coupling of phenols and bromodicarbonyls is reported. As the phenanthrolines are interlocked by the strong and flexible mechanical bond in the catenane, the active catalyst with an open copper coordination site can be revealed only transiently and the stable, coordinatively saturated Cu(i) pre-catalyst is quickly regenerated after substrate transformation. Compared with a control Cu(i) complex supported by non-interlocked phenanthrolines, the catenane-supported Cu(i) is highly efficient with a broad substrate scope, and can be applied in gram-scale transformations without a significant loss of the catalytic activity. This work demonstrates the advantages of the catenane ligands that provide a dynamic and responsive copper coordination sphere, highlighting the potential of the mechanical bond as a design element in transition metal catalyst development.

The use of a catenane-supported copper(i) complex for the cross dehydrogenative C–O coupling of phenols and bromodicarbonyls is described.

The Role of Zinc(II) Ion in Fluorescence Tuning of Tridentate Pincers: A Review

Tridentate ligands are simple low-cost pincers, easy to synthetize, and able to guarantee stability to the derived complexes. On the other hand, due to its unique mix of structural and optical properties, zinc(II) ion is an excellent candidate to modulate the emission pattern as desired. The present work is an overview of selected articles about zinc(II) complexes showing a tuned fluorescence response with respect to their tridentate ligands. A classification of the tridentate pincers was carried out according to the binding donor atom groups, specifically nitrogen, oxygen, and sulfur donor atoms, and depending on the structure obtained upon coordination. Fluorescence properties of the ligands and the related complexes were compared and discussed both in solution and in the solid state, keeping an eye on possible applications.

Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a Pd^II₆L₄ coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

Woven Polymer Networks via the Topological Transformation of a [2]Catenane

Weaving technology has been widely used to manufacture macroscopic fabrics to meet the artistic and practical needs of humanity for thousands of years. However, the fabrication of molecular fabrics with fascinating topologies and unique mechanical properties represents a significant challenge. Herein, we describe a topological transformation strategy to construct woven polymer networks (WPNs) at the molecular level via ring-opening metathesis polymerization (ROMP) of a zinc-template [2]catenane. The key feature of this approach is the exploitation of the pre-existing catenane crossing points that maintain the dense woven structure and the flexible alkyl chains on the [2]catenane that synergistically work with the crossing points to modulate the physicochemical and mechanical properties of the woven materials. As a result, the WPN possesses a certain degree of flexibility and stretchability, as well as high thermostability and mechanical robustness. Furthermore, we could remove the zinc ions to endow the WPN with more degrees of freedom and then enhance its mechanical behaviors by remetalation. This study not only provides a novel strategy toward woven materials with intriguing structural features and emergent mechanical adaptivities, but also highlights that mechanically interlocked molecules could offer unique opportunities for the construction of smart supramolecular materials with peculiar interlaced topologies at the molecular scale.

Chemical reactivity under nanoconfinement

Confining molecules can fundamentally change their chemical and physical properties. Confinement effects are considered instrumental at various stages of the origins of life, and life continues to rely on layers of compartmentalization to maintain an out-of-equilibrium state and efficiently synthesize complex biomolecules under mild conditions. As interest in synthetic confined systems grows, we are realizing that the principles governing reactivity under confinement are the same in abiological systems as they are in nature. In this Review, we categorize the ways in which nanoconfinement effects impact chemical reactivity in synthetic systems. Under nanoconfinement, chemical properties can be modulated to increase reaction rates, enhance selectivity and stabilize reactive species. Confinement effects also lead to changes in physical properties. The fluorescence of light emitters, the colours of dyes and electronic communication between electroactive species can all be tuned under confinement. Within each of these categories, we elucidate design principles and strategies that are widely applicable across a range of confined systems, specifically highlighting examples of different nanocompartments that influence reactivity in similar ways.

An Anionic Ring Locked into an Anionic Axle: A Metastable Rotaxane with Chemically Activated Electrostatic Stoppers

The use of the electrostatic stoppers concept in the field of mechanically interlocked molecules is reported; these stoppers are chemically sensitive end groups on a linear guest molecule that allows for the conversion of a pseudo-rotaxane species into a rotaxane complex by a change in the medium acidity. The chemical stimulus causes the appearance of negative charges on both ends of the linear component, passing from cationic to anionic, and causing a significant ring-to-axle electrostatic repulsion. This phenomenon has two different and simultaneous effects: 1) destabilizes the complex as a consequence of confining an anionic ring into an anionic axle, and 2) increases the dissociation energy barrier, thus impeding ring extrusion. This newly formed metastable rotaxane species is resistant to solvent and temperature effects and performs as a two-state degenerated molecular shuttle in solution.

Tuning radical interactions in trisradical tricationic complexes by varying host-cavity sizes

Although host–guest pairing interactions between bisradical dicationic cyclobis(paraquat-p-phenylene) (BB²⁽˙⁺⁾) and the bipyridinium radical cation (BIPY˙⁺) have been studied extensively, host molecules other than BB²⁽˙⁺⁾ are few and far between.

Although host–guest pairing interactions between bisradical dicationic cyclobis(paraquat-p-phenylene) (BB²⁽˙⁺⁾) and the bipyridinium radical cation (BIPY˙⁺) have been studied extensively, host molecules other than BB²⁽˙⁺⁾ are few and far between. Herein, four bisradical dicationic cyclophanes with tunable cavity sizes are investigated as new bisradical dicationic hosts for accommodating the methyl viologen radical cation (MV˙⁺) to form trisradical tricationic complexes. The structure–property relationships between cavity sizes and binding affinities have been established by comprehensive solution and solid-state characterizations as well as DFT calculations. The association constants of the four new trisradical tricationic complexes are found to range between 7400 and 170 000 M^–1, with the strongest one being 4.3 times higher than that for [MV⊂BB]³⁽˙⁺⁾. The facile accessibility and tunable stability of these new trisradical tricationic complexes make them attractive redox-controlled recognition motifs for further use in supramolecular chemistry and mechanostereochemistry.

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Neighboring Component Effect in a Tri-stable [2]Rotaxane

The redox properties of cyclobis(paraquat- p-phenylene)cyclophane (CBPQT⁴⁺) render it a uniquely variable source of recognition in the context of mechanically interlocked molecules, through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT⁴⁺) and radical-pairing interactions in its partially reduced state (CBPQT^2(•+)). Although it is expected that the fully reduced neutral state (CBPQT⁽⁰⁾) might behave as a π-donating recognition unit, resulting in a dramatic change in its binding properties when compared with the other two redox states, its role in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis of a tri-stable [2]rotaxane in which a CBPQT⁴⁺ ring is mechanically interlocked with a dumbbell component containing five recognition sites-(i) a bipyridinium radical cation (BIPY^(•+)) located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT⁴⁺ ring and the triazole units, and (v) the CBPQT^2(•+) ring and the reduced BIPY^(•+) unit in the dumbbell component, investigations in solution have now confirmed the presence of additional non-covalent bonding interactions between the CBPQT⁽⁰⁾ ring, acting as a donor in its neutral state, and the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler, covalently linked, organic molecules of neighboring group participation (anchimeric assistance giving way to transannular interactions) in small-, medium-, and large-membered rings.

Shuttling Rates, Electronic States, and Hysteresis in a Ring-in-Ring Rotaxane

The trisradical recognition motif between a 4,4'-bipyridinium radical cation and a cyclo-bis-4,4'-bipyridinium diradical dication has been employed previously in rotaxanes to control their nanomechanical and electronic properties. Herein, we describe the synthesis and characterization of a redox-active ring-in-ring [2]rotaxane BBR·8PF6 that employs a tetraradical variant of this recognition motif. A square-shaped bis-4,4'-bipyridinium cyclophane is mechanically interlocked around the dumbbell component of this rotaxane, and the dumbbell itself incorporates a smaller bis-4,4'-bipyridinium cyclophane into its covalently bonded structure. This small cyclophane serves as a significant impediment to the shuttling of the larger ring across the dumbbell component of BBR8+ , whereas reduction to the tetraradical tetracationic state BBR4(+•) results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, BBR·8PF6 exhibits qualitatively similar behavior to its predecessors that interconvert between hexacationic and trisradical tricationic states. The rigid preorganization of two bipyridinium groups within the dumbbell of BBR·8PF6 confers, however, two distinct properties upon this rotaxane: (1) the rate of shuttling is reduced significantly relative to those of its predecessors, resulting in marked electrochemical hysteresis observed by cyclic voltammetry for switching between the BBR8+/BBR4(+•) states, and (2) the formally tetraradical form of the rotaxane, BBR4(+•) , exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within BBR4(+•) , has a long enough lifetime to be characterized by 1H NMR spectroscopy.

Introducing Stable Radicals into Molecular Machines

Ever since their discovery, stable organic radicals have received considerable attention from chemists because of their unique optical, electronic, and magnetic properties. Currently, one of the most appealing challenges for the chemical community is to develop sophisticated artificial molecular machines that can do work by consuming external energy, after the manner of motor proteins. In this context, radical-pairing interactions are important in addressing the challenge: they not only provide supramolecular assistance in the synthesis of molecular machines but also open the door to developing multifunctional systems relying on the various properties of the radical species. In this Outlook, by taking the radical cationic state of 1,1'-dialkyl-4,4'-bipyridinium (BIPY^•+) as an example, we highlight our research on the art and science of introducing radical-pairing interactions into functional systems, from prototypical molecular switches to complex molecular machines, followed by a discussion of the (i) limitations of the current systems and (ii) future research directions for designing BIPY^•+-based molecular machines with useful functions.

Macrocyclic shape-persistency of cyclo[6]aramide results in enhanced multipoint recognition for the highly efficient template-directed synthesis of rotaxanes

Examples of using two-dimensional shape-persistent macrocycles, i.e. those having noncollapsible and geometrically well-defined skeletons, for constructing mechanically interlocked molecules are scarce, which contrasts the many applications of these macrocycles in molecular recognition and functional self-assembly. Herein, we report the crucial role played by macrocyclic shape-persistency in enhancing multipoint recognition for the highly efficient template-directed synthesis of rotaxanes. Cyclo[6]aramides, with a near-planar conformation, are found to act as powerful hosts that bind bipyridinium salts with high affinities. This unique recognition module, composed of two macrocyclic molecules with one bipyridinium ion thread through the cavity, is observed both in the solid state and in solution, with unusually high binding constants ranging from ∼10¹³ M^-2 to ∼10¹⁵ M^-2 in acetone. The high efficacy of this recognition motif is embodied by the formation of compact [3]rotaxanes in excellent yields based on either a "click-capping" (91%) or "facile one-pot" (85%) approach, underscoring the great advantage of using H-bonded aromatic amide macrocycles for the highly efficient template-directed synthesis of mechanically interlocked structures. Furthermore, three cyclo[6]aramides bearing different peripheral chains 1-3 demonstrate high specificity in the synthesis of a [3]rotaxane from 1 and 2, and a [2]rotaxane from 3via a "facile one-pot" approach, in each case as the only isolated product. Analysis of the crystal structure of the [3]rotaxane reveals a highly compact binding mode that would be difficult to access using other macrocycles with a flexible backbone. Leveraging this unique recognition motif, resulting from the shape-persistency of these oligoamide macrocycles, in the template-directed synthesis of compact rotaxanes may open up new opportunities for the development of higher order interlocked molecules and artificial molecular machines.

Click-chemistry approaches to π-conjugated polymers for organic electronics applications

Given the wide utility of click-chemistry reactions for the preparation of simple moieties within large architecturally complex materials, this minireview article aims at surveying papers exploring their scope in the area of π-conjugated polymers for application in organic electronics to enable advanced functional properties.

Cooperative capture synthesis: yet another playground for copper-free click chemistry

Click chemistry describes a family of modular, efficient, versatile and reliable reactions which have acquired a pivotal role as one of the most useful synthetic tools with a potentially broad range of applications. While copper(i)-catalysed alkyne-azide cycloaddition is the most widely adopted click reaction in the family, the fact that it is cytotoxic restricts its practice in certain situations, e.g., bioconjugation. Consequently, researchers have been exploring the development of copper-free click reactions, the most popular example so far being strain-promoted alkyne-azide cycloadditions. An early example of copper-free click reactions that is rarely mentioned in the literature is the cucurbit[6]uril (CB6) catalysed alkyne-azide cycloaddition (CB-AAC). Despite the unique ability of CB-AAC to generate mechanically interlocked molecules (MIMs) - in particular, rotaxanes - its slow reaction rate and narrow substrate acceptance limit its scope. In this Tutorial Review, we describe our efforts of late in developing the fundamental principles and practical applications of a new copper-free click reaction - namely, cooperative capture synthesis, whereby introducing a cyclodextrin (CD) as an accelerator in CB-AAC, hydrogen bonding networks are formed between the rims of CD and CB6 in a manner that is positively cooperative, giving rise to a high level of pre-organisation during efficient and quick rotaxane formation. For example, [4]rotaxanes can be prepared nearly quantitatively within a minute in water. Furthermore, we have demonstrated that CB-AAC can accommodate a wider substrate tolerance by introducing pillararenes as promoters. To date, we have put cooperative capture synthesis into practice by (i) preparing polyrotaxanes containing up to 200 rings in nearly quantitative yields, (ii) trapping conformational isomers of polymacrocycles as rings in rotaxanes, (iii) demonstrating solid-state fluorescence and Förster resonance energy transfer (FRET) processes by fixing the fluorophores in a rotaxane and (iv) establishing the principle of supramolecular encryption in the preparation of dynamically and reversibly tunable fluorescent security inks.

Oligorotaxane Radicals under Orders

A strategy for creating foldameric oligorotaxanes composed of only positively charged components is reported. Threadlike components-namely oligoviologens-in which different numbers of 4,4'-bipyridinium (BIPY(2+)) subunits are linked by p-xylylene bridges, are shown to be capable of being threaded by cyclobis(paraquat-p-phenylene) (CBPQT(4+)) rings following the introduction of radical-pairing interactions under reducing conditions. UV/vis/NIR spectroscopic and electrochemical investigations suggest that the reduced oligopseudorotaxanes fold into highly ordered secondary structures as a result of the formation of BIPY(•+) radical cation pairs. Furthermore, by installing bulky stoppers at each end of the oligopseudorotaxanes by means of Cu-free alkyne-azide cycloadditions, their analogous oligorotaxanes, which retain the same stoichiometries as their progenitors, can be prepared. Solution-state studies of the oligorotaxanes indicate that their mechanically interlocked structures lead to the enforced interactions between the dumbbell and ring components, allowing them to fold (contract) in their reduced states and unfold (expand) in their fully oxidized states as a result of Coulombic repulsions. This electrochemically controlled reversible folding and unfolding process, during which the oligorotaxanes experience length contractions and expansions, is reminiscent of the mechanisms of actuation associated with muscle fibers.

Quantum Mechanical and Experimental Validation that Cyclobis(paraquat-p-phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene

The promiscuous encapsulation of π-electron-rich guests by the π-electron-deficient host, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge-transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT(4+), is an emerald-green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas-phase calculations that reinterpreted this CT band in terms of an intermolecular side-on interaction of TTF with one of the bipyridinium (BIPY(2+)) units of CBPQT(4+), rather than the encapsulation of TTF inside the cavity of CBPQT(4+). We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT(4+) arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY(2+) units of a CBPQT(4+) ring residing on a separate [2]rotaxane in a side-on fashion. This [2]rotaxane has similar UV/Vis and (1)H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT(4+). The [2]rotaxane exists as an equimolar mixture of cis- and trans-isomers associated with the disubstituted TTF unit in its dumbbell component. Solid-state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT(4+).

Nanoarchitectonics for Dynamic Functional Materials from Atomic-/Molecular-Level Manipulation to Macroscopic Action

Objects in all dimensions are subject to translational dynamism and dynamic mutual interactions, and the ability to exert control over these events is one of the keys to the synthesis of functional materials. For the development of materials with truly dynamic functionalities, a paradigm shift from "nanotechnology" to "nanoarchitectonics" is proposed, with the aim of design and preparation of functional materials through dynamic harmonization of atomic-/molecular-level manipulation and control, chemical nanofabrication, self-organization, and field-controlled organization. Here, various examples of dynamic functional materials are presented from the atom/molecular-level to macroscopic dimensions. These systems, including atomic switches, molecular machines, molecular shuttles, motional crystals, metal-organic frameworks, layered assemblies, gels, supramolecular assemblies of biomaterials, DNA origami, hollow silica capsules, and mesoporous materials, are described according to their various dynamic functions, which include short-term plasticity, long-term potentiation, molecular manipulation, switchable catalysis, self-healing properties, supramolecular chirality, morphological control, drug storage and release, light-harvesting, mechanochemical transduction, molecular tuning molecular recognition, hand-operated nanotechnology.

Tuning the Energy Gap by Supramolecular Approaches: Towards Near-Infrared Organic Assemblies and Materials

Near-infrared (NIR) organic materials are of great importance for both fundamental research and practical applications. While much effort has been taken to covalently design and synthesize NIR organic materials with a low energy gap, there are supramolecular approaches for tuning the energy gap to noncovalently fabricate NIR organic assemblies and materials. In this concept article, we summarize and discuss several supramolecular approaches, including the fabrication of charge transfer supramolecular complexes, the fabrication of supramolecular J-aggregates, and the fabrication of supramolecularly stabilized organic radicals. The nature of noncovalent interactions in supramolecular approaches can provide NIR organic assemblies and materials with unique properties such as reversibility, stimuli-responsiveness, recyclability, and adaptive abilities.

Supramolecular radical polymers self-assembled from the stacking of radical cations of rod-like viologen di- and trimers

A series of π-conjugated oligomeric viologens have been synthesized, from which supramolecular radical polymers were constructed through the stacking of their radical cations.

Supramolecular chemistry: from aromatic foldamers to solution-phase supramolecular organic frameworks

This mini-review covers the growth, education, career, and research activities of the author. In particular, the developments of various folded, helical and extended secondary structures from aromatic backbones driven by different noncovalent forces (including hydrogen bonding, donor-acceptor, solvophobicity, and dimerization of conjugated radical cations) and solution-phase supramolecular organic frameworks driven by hydrophobically initiated aromatic stacking in the cavity of cucurbit[8]uril (CB[8]) are highlighted.

Design and Synthesis of Nonequilibrium Systems

The active transport of ions and molecules across cell membranes is essential to creating the concentration gradients that sustain life in all living organisms, be they bacteria, fungi, plants, animals or Homo sapiens. Nature uses active transport everywhere for everything. Molecular biologists have long been attracted to the study of active transport and continue to this day to investigate and elucidate the tertiary structures of the complex motor proteins that sustain it, while physicists, interested in nonequilibrium statistical mechanics, have developed theoretical models to describe the driven ratcheting motions that are crucial to its function. The increasingly detailed understanding that contemporary science has acquired relating to active transport, however, has yet to lead to the design and construction of artificial molecular motors capable of employing ratchet-driven motions that can also perform work against concentration gradients. Mechanically interlocked molecules (MIMs) in the form of pseudo- and semirotaxanes are showing some encouraging signs in meeting these goals. This review summarizes recent progress in making artificial molecular motors that can perform work by "pumping" tetracationic rings into high-energy states. The launching pad is a bistable [2]rotaxane whose dumbbell component contains two electron-donating recognition sites, one, a tetrathiafulvalene (TTF) unit, which interacts more strongly with the ring component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), containing two electron-accepting bipyridinium units, than does the other 1,5-dioxynaphthalene (DNP) unit. Switching can be induced electrochemically by oxidizing the TTF unit to a TTF(•+) radical cation, whereupon Coulombic repulsion takes care of moving the ring to the DNP unit. Reduction of the radical cation resets the switch. Molecular switches operate at, or close to, equilibrium. Any work done during one switching event is undone during the reset. Molecular motors, on the other hand, rely on a flux of energy, and a ratchet mechanism to make periodic changes to the potential energy surface of a system in order to move molecules uphill to higher energy states. Forging a path from molecular switches to motors involved designing a molecular pump prototype. An asymmetric dumbbell with a 2-isopropylphenyl (neutral) end and a 3,5-dimethylpyridinium (charged) end with a DNP recognition site to entice CBPQT(4+) rings out of solution exhibits relative unidirectional movement of the rings with respect to the dumbbell. Redox chemistry does the trick. During the oxidative cycle, the rings enter the dumbbell by passing over the neutral end onto the recognition site; in the reduction cycle, much of the recognition is lost and the rings find their way back into solution by leaving the dumbbell from the charged end. This on-one-end, off-the-other process can be repeated over and over again using light as the energy source in the presence of a photosensitizer and a compound that shuttles electrons back and forth. Although this prototype demonstrates ratchet-driven translational motion, no work is done. A ring enters the dumbbell from one end and leaves from the other end. Another deficiency of the prototype is the fact that, although the recognition site is muted on reduction, it retains some attraction for the ring. What if the recognition site was attractive initially and then became repulsive? This question was answered by turning to radical chemistry and employing the known stabilization behavior of a bipyridinium radical cation and the bisradical dication, generated on reduction of the CBPQT(4+) ring, to pluck rings out of solution and thread them over the charged end of the pump portion of a semidumbbell. On subsequent oxidation, the pump is primed and the rings pass through a one-way door, given a little thermal energy, onto a collecting-chain where they find themselves accumulating where they would rather not be present. In this manner, an artificial molecular pump mimics the pumping machinery commonplace in biological systems. Looking beyond this state-of-the-art artificial molecular pump, we discuss, from a theoretical standpoint, the measures that would need to be taken in order to render its operation autonomous.

Electrochemically addressable trisradical rotaxanes organized within a metal-organic framework

The organization of trisradical rotaxanes within the channels of a Zr6-based metal-organic framework (NU-1000) has been achieved postsynthetically by solvent-assisted ligand incorporation. Robust Zr(IV)-carboxylate bonds are forged between the Zr clusters of NU-1000 and carboxylic acid groups of rotaxane precursors (semirotaxanes) as part of this building block replacement strategy. Ultraviolet-visible-near-infrared (UV-Vis-NIR), electron paramagnetic resonance (EPR), and 1H nuclear magnetic resonance (NMR) spectroscopies all confirm the capture of redox-active rotaxanes within the mesoscale hexagonal channels of NU-1000. Cyclic voltammetry measurements performed on electroactive thin films of the resulting material indicate that redox-active viologen subunits located on the rotaxane components can be accessed electrochemically in the solid state. In contradistinction to previous methods, this strategy for the incorporation of mechanically interlocked molecules within porous materials circumvents the need for de novo synthesis of a metal-organic framework, making it a particularly convenient approach for the design and creation of solid-state molecular switches and machines. The results presented here provide proof-of-concept for the application of postsynthetic transformations in the integration of dynamic molecular machines with robust porous frameworks.

Using click chemistry toward novel 1,2,3-triazole-linked dopamine D3 receptor ligands

The dopamine D3 receptor (D3R) is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, Parkinson's disease, restless leg syndrome, and drug addiction. A common molecular template used in the development of D3R-selective antagonists and partial agonists incorporates a butylamide linker between two pharmacophores, a phenylpiperazine moiety and an extended aryl ring system. The series of compounds described herein incorporates a change to that chemical template, replacing the amide functional group in the linker chain with a 1,2,3-triazole group. Although the amide linker in the 4-phenylpiperazine class of D3R ligands has been previously deemed critical for high D3R affinity and selectivity, the 1,2,3-triazole moiety serves as a suitable bioisosteric replacement and maintains desired D3R-binding functionality of the compounds. Additionally, using mouse liver microsomes to evaluate CYP450-mediated phase I metabolism, we determined that novel 1,2,3-triazole-containing compounds modestly improves metabolic stability compared to amide-containing analogues. The 1,2,3-triazole moiety allows for the modular attachment of chemical subunit libraries using copper-catalyzed azide-alkyne cycloaddition click chemistry, increasing the range of chemical entities that can be designed, synthesized, and developed toward D3R-selective therapeutic agents.

Enhanced Squaraine Rotaxane Endoperoxide Chemiluminescence in Acidic Alcohols

Squaraine rotaxane endoperoxides (SREPs) are storable chemiluminescent compounds that undergo a clean cycloreversion reaction that releases singlet oxygen and emits near-infrared light when warmed to body temperature. This study examined the effect of solvent on SREP chemiluminescence intensity and found that acidic alcohols, such as 2,2,2-trifluoroethanol, α-(trifluoromethyl)benzyl alcohol, and 1,1,1,3,3,3-hexafluoroisopropanol, greatly increased chemiluminescence. In contrast, aprotic solvents, such as trifluoroethylmethyl ether, had no effect. The interlocked rotaxane structure was necessary as no chemiluminescence was observed when the experiments were conducted with samples containing a mixture of the two non-interlocked components (squaraine thread and macrocycle endoperoxide). Spectroscopic analyses of the enhanced SREP chemiluminescent reactions showed a mixture of products. In addition to the expected squaraine rotaxane product caused by cycloreversion of the endoperoxide, a diol derivative was isolated. The results are consistent with an endoperoxide O–O bond cleavage process that is promoted by the hydrogen bonding solvent and produces light emission from a squaraine excited state.