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Molecular Switches and Machines with Mechanical Bonds
Abstract
Mechanically interlocked molecules (MIMs) which contain two or more different recognition sites in the shape of noncovalent bonds between their component parts provide the perfect prototypes for the design and synthesis of molecular switches and machines. In example, namely bistable catenanes and rotaxanes, it is convenient to identify a ground state co-conformation (GSCC) and a higher energy metastable state co-conformation (MSCC) where the GSCC:MSCC ratio at room temperature in solution is ideally for most purposes in the range of at least 10:1. The chapter discusses the most of the parameters that influence ring translation in n-donor/n-acceptor mechanomolecules and discusses competitive binding interactions that lead to mechanical switching motions as a result of molecular recognition. It also describes the single-molecule experiments and many of the molecular machines that involve molecular motion in condensed phases and at interfaces, motivating the pursuit of a more fundamental understanding of how these phases influence the thermodynamics and kinetics.
... These molecules offer interesting properties that are based on the cooperative effect between their subcomponents, which can be altered at the mechanical bonds. Recently, various applications of the characteristic properties of MIMs have been actively studied by utilizing not only small molecules, but also polymer materials [2]. Although a variety of MIMs can be synthesized, research on the basic chirality of MIMs is still in its infancy. ...
... Mechanically planar chirality in rotaxane. Co-conformational stereoisomerization in [2]rotaxane having a rotationally unsymmetrical wheel and an axle with two nonequivalent ends (a), and achiral-chiral interconversion in [2]rotaxane having rotationally unsymmetrical wheel and an axle with two equivalent ends (b). ...
... Mechanically planar chirality in rotaxane. Co-conformational stereoisomerization in [2]rotaxane having a rotationally unsymmetrical wheel and an axle with two nonequivalent ends (a), and achiral-chiral interconversion in [2]rotaxane having rotationally unsymmetrical wheel and an axle with two equivalent ends (b). ...
Mechanically chiral molecules have attracted considerable attention due to their property and function based on its unique interlocked structure. This review covers the recent advances in the synthesis and function of interlocked rotaxanes with mechanical chirality along with their dynamic and complex stereochemistry. The application of mechanically chiral rotaxanes to control the polymer helical structure is also introduced, where amplification of mechanical chirality appears to cause the macroscopic polymer property change, suggesting the potential applicability of mechanical chirality in polymer systems.
... The work performed by these natural molecular motors is related to their dynamics in solution, and to the force exerted by the molecule to drive the relevant process in one direction. The invention of synthetic routes to wholly artificial molecular machines with highly precise and controlled architectures has led to the production of amazing molecules able to perform mechanical tasks (3)(4)(5)(6)(7). Their integration into materials such as metal-organic frameworks (8) or polymer gels (9, 10) has been described recently. ...
... Dynamic Force Spectroscopy. Following our previously published procedure (31), [4]-and [7]rotaxane foldamers ( Fig. 1 A and B) were grafted on gold-coated substrates to obtain isolated single molecules, which were stretched mechanically using AFM-based single-molecule force spectroscopy (Fig. 1C). The force-extension curves obtained in dimethylformamide (DMF), a good solvent for the molecules, show a characteristic unfolding profile ( Fig. 2) with regularly spaced force peaks, each separated by 2.4 nm on the basis of worm-like chain fits (see Supporting Information for details). ...
... The [4]-and [7]rotaxanes were synthesized according to protocols described previously (32). Briefly, one-pot syntheses used copper-catalyzed azidealkyne cycloadditions to thioctic ester-functionalized stoppers at both ends of the DNP-derived polyether chains in the presence of CBPQT 4+ rings. ...
Significance
Donor–acceptor oligorotaxane foldamers are a class of molecular switches elegantly incorporating mechanical bonds in a folded molecular architecture. Such examples of foldamers are very rare, leaving the question wide open regarding what might emerge from a synergistic combination of mechanically interlocked molecules and foldamers. Here, using atomic-force-microscopy-based dynamic single-molecule force spectroscopy, we mechanically drive oligorotaxanes out of equilibrium at a wide range of loading rates and observe their exceptional refolding capabilities and the very fast dynamics of the process. The near-equilibrium pulling–relaxing cycles were exploited to determine the energy required to break one single donor–acceptor interaction from the distribution of work trajectories using fluctuation theorems. Our findings highlight the importance of molecular design on the performance of artificial molecular machines.
A pair of radial [4]catenane isomers interlocked with two CB[6]s and one β-CD is reported. Due to the different positions of the tightly bound CB[6]s, shuttling dynamics of the β-CD between the two biphenyl stations are different in the isomers.
We report the stepwise assembly of supramolecular daisy chain rotaxanes (DCR) made of double-stranded DNA: Small dsDNA macrocycles bearing an axle assemble into a pseudo-DCR precursor that was connected to rigid DNA stoppers to form DCR with the macrocycles hybridized to the axles. In presence of release oligodeoxynucleotides (rODNs), the macrocycles are released from their respective hybridization sites on the axles, leading to stable mechanically interlocked DCRs. Besides the expected threaded DCRs, certain amounts of externally hybridized structures were observed, which dissociate into dumbbell structures in presence of rODNs. We show that the genuine DCRs have significantly higher degrees of freedom in their movement along the thread axle than the hybridized DCR precursors. Interlocking of DNA in DCRs might serve as a versatile principle for constructing functional DNA nanostructures where the movement of the subunits is restricted within precisely confined tolerance ranges.
Nature's molecular machines are a constant source of inspiration to the chemist. Many of these molecular machines function within lipid membranes, allowing them to exploit potential gradients between spatially close, but chemically distinct environments to fuel their work cycle. Indeed, the realisation of such principles in synthetic transmembrane systems remains a tantalising goal. This tutorial review opens by highlighting seminal examples of synthetic molecular machines. We illustrate the importance of surfaces for facilitating the extraction of work from molecular switches and motors. We chart the development of man-made transmembrane systems; from passive to machine-like stimuli-responsive channels, to fully autonomous transmembrane molecular machines. Finally, we highlight higher-order compartmentalised systems that exhibit emergent properties. We suggest that such higher-order architectures could serve as platforms for sophisticated devices that co-ordinate the activity of numerous transmembrane molecular machines.
The past quarter of a century has witnessed an increasing engagement on the part of physicists and chemists in the design and synthesis of molecular machines de novo. This minireview traces the development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy in the form of chemical fuels, electrochemical potentials and light activations become a minimum requirement for them to function away-from-equilibrium. The challenge facing this rapidly growing community of scientists and engineers today is one of putting wholly synthetic molecules to work, both as individuals and as collections. Here, we highlight some of the recent conceptual and practical advances relating to the operation of wholly synthetic rotary and linear motors.
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 (BIPY2+) subunits are linked by p-xylylene bridges, are shown to be capable of being threaded by cyclobis(paraquat-p-phenylene) (CBPQT4+) 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.
We present an operationally simple approach to 2,2`-bipyridine macrocycles. Our method uses simple starting materials to produce these previously hard to access rotaxane precursors in remarkable yields (typically >65%) across a range of scales (0.1 - 5 mmol). All of the macrocycles reported are efficiently converted (>90%) to rotaxanes under AT-CuAAC conditions. With the requisite macrocycles finally available in sufficient quantities, we further demonstrate their long term utility through the first gram-scale synthesis of an AT-CuAAC [2]rotaxane and extend this powerful methodology to produce novel Sauvage-type molecular shuttles.
Subcellular organelle-specific reagents for simultaneous targeting, imaging and treatment are highly desirable for cancer therapy. However, it remains a challenge to fabricate a single molecular platform containing a targeting group, imaging and therapeutic agents through traditional synthesis. Due to their superior sensitivity and photostability, fluorescent probes with aggregation-induced emission (AIE) characteristics have attracted more and more attention in studying the process of translocation, drug release, and excretion of nanomedicines in vitro or in vivo. We construct a pillar[5]arene-based [2]rotaxane (R1) by employing tetraphenylethene (TPE) and triphenylphosphonium (TPP) moieties as stoppers; the TPE unit retains the aggregation-induced emission (AIE) attribute and the TPP group is used as a mitochondria-targeting agent. R1 exhibits enhanced AIE, high specificity to mitochondria, and superior photostability. By introducing doxorubicin (DOX) into R1, prodrug R2 is constructed as a dual-fluorescence-quenched Förster resonance energy transfer (FRET) system, in which the TPE-based axle acts as a donor fluorophore and the DOX unit acts as the acceptor. Upon hydrolysis of R2 in endo/lysosomes, the fluorescences of the carrier and the drug recover. R1 is further utilized as a drug delivery platform to conjugate other anticancer drugs containing amine groups through imine formation to prepare prodrugs. The anticancer drugs are released from these prodrugs in the cells upon hydrolysis of the pH-responsive imine bonds.
The transport of substrates is one of the main tasks of biomolecular machines in living organisms. We report a synthetic small-molecule system designed to catch, displace, and release molecular cargo in solution under external control. The system consists of a bistable rotaxane that behaves as an acid–base controlled molecular shuttle, whose ring component bears a tether ending with a nitrile group. The latter can be coordinated to a ruthenium complex that acts as the load, and dissociated upon irradiation with visible light. The cargo loading/unloading and ring transfer/return processes are reversible and can be controlled independently. The robust coordination bond ensures that the cargo remains attached to the device while the transport takes place.
We all learn - in schools, factories, bars and streets. We gather, store, process and transmit information in society. Molecular systems involved in our senses and within our brains allow all this to happen and molecular systems allow living things of all kinds to handle information for the purpose of survival and growth. Nevertheless, the vital link between molecules and computation was not generally appreciated until a few decades ago. Semiconductor-based information technology had penetrated society at many levels and the interest in maintaining momentum of this revolution led to the consideration of molecules, among others, as possible information handlers. Such an overlap between the recent engineering-oriented revolution with the ancient biology-oriented success story is very interesting and George Boole's times in Ireland 150 years ago produced the logic ideas that provide the foundations of computation to this day. Molecular logic and computation is a field which is 17 years young, has had a healthy growth and is a story which deserves to be told. It is a growing branch of chemical science which highlights the connection between information technology (engineering and biological) and chemistry. The author and co-workers of this publication launched molecular logic as an experimental field by publishing the first research in the primary literature in 1993 and are uniquely placed to recount how the field has grown. There is no other book at present on molecular logic and computation and is more comprehensive than that found in any review available so far. It shows how designed molecules can play the role of information processors in a wide variety of situations, once we are educated by those information processors already available in the semiconductor electronics business and in the natural world. Following a short history of the field, is a set of primers on logic, computing and photochemical principles which are an essential basis in this field. The book covers all of the Boolean logic gates driven by a single input and all of those with double inputs and the wide range of designs which lie beneath these gates is a particular highlight. The easily-available diversity of chemical systems is another highlight, especially when it leads to reconfigurable logic gates. Further on in the book, molecular arithmetic and other more complex logic operations, including those with a memory and those which stray beyond binary are covered. Then follows molecular computing approaches which lie outside the Boolean blueprint, including quantum phenomena and finally, the book catalogues the useful real-life applications of molecular logic and computation which are already available. This book is an authoritative, state of the art, reference and a 'one-stop-shop' concerning the current state of the field for scientists, academics and postgraduate students.
The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties—namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.
The field of molecules in motion, for which movements and shape changes are triggered and controlled from outside, has been indisputably one of the most rapidly developing areas of the last decade. Clearly, molecular chemists in general are able to elaborate more and more complex species, as beautifully demonstrated by the synthesis of amazingly complicated natural products. For instance, the total synthesis of compounds such as taxol and brevetoxin B represents a formidable tour de force, as does that of many other recent examples. However, most of the time, once the compound has been made, the target has been reached. The synthesis of molecules for which given functions are to be expected and explored once sufficient amounts of the compounds are available, requires the interaction of several fields. The multidisciplinary aspect, involving various methodologies from synthesis to electro and photochemistry, from surface science to spectroscopy and magnetic properties, allows the design and elaboration of molecular objects displaying new functions. Amongst the new functions that chemists want to introduce into their systems, motion is particularly important. In addition, a geometrical change will generally be accompanied by a modification of one or more properties (colour, lumines cence, catalytic properties, etc...) which could be used as a testimony to the motion and also be of practical interest for future applications.
Monomolecular assemblies on substrates, now termed Langmuir-Blodgett (LB) films, have been studied for over half a century. Their development can be viewed in three stages. Following the pioneering work of Irving Langmuir and Katharine Blodgett in the late 1930s there was a brief flurry of activity just before and just after the Second World War. Many years later Hans Kuhn published his stimulating work on energy transfer. This German contribution to the field, made in the mid-1960s, can be regarded as laying the foundation for studies of artificial systems of cooperat ing molecules on solid substrates. However, the resurgence of activity in academic and industrial laboratories, which has resulted in four large international con ferences, would not have occurred but for British and French groups highlighting the possible applications of LB films in thefield of electronics. Many academic and industrial establishments involved in high technology are now active in or maintaining a watching brief on the field. Nevertheless this impor tant area of solid state science is still perhaps largely unfamiliar to many involved in materials or electronic device research. The richness of the variety of organic molecular materials suitable for LB film deposition offers enormous scope for those interested in their basic properties or their practical applications. LB films are now an integral part of the field of molecular electronics. It seems inevitable that they will play some role in replacing inorganic materials in certain areas of application.
The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties—namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.
A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the π-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a RuII polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4′-bipyridinium unit and a 3,3′-dimethyl-4,4′-bipyridinium unit as π-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called “west-side” comprised of the RuII polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called “east-side” comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3′-dimethyl-4,4′-bipyridinium unit. Next, 3) the “west-side” and “east-side” were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4′-bipyridinium unit, in keeping with the fact that this station is a better π-electron acceptor than the other station. This observation raises the question—can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed—one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would appear that the electronic reset of the system is faster than the ring displacement.
A mechanical switch in a [2]catenane, made up of a cyclobis(paraquat-p-phenylene) tetracation interlocked with a macrocyclic polyether containing a redox-active tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene ring system, can be thrown either chemically or electrochemically. The neutral TTF unit resides “inside” the tetracationic cyclophane in the reduced state and “alongside” it in the oxidized species (TTF⁺/ TTF²⁺). Switching between the reduced (I⁴⁺) and oxidized state (I⁵⁺(I⁶⁺)) is accompanied by a dramatic color change.
We report a CB6 rotaxane for the (129)Xe hyperCEST NMR detection of matrix metalloprotease 2 (MMP-2) activity. MMP-2 is overexpressed in cancer tissue, and hence is a cancer marker. A peptide containing an MMP-2 recognition sequence was incorporated into the rotaxane, synthesized via CB6-promoted click chemistry. Upon cleavage of the rotaxane by MMP-2, CB6 became accessible for (129)Xe@CB6 interactions, leading to protease-responsive hyperCEST activation.
Hierarchically organized myosin and actin filaments found in biological systems exhibit contraction and expansion behaviours that produce work and force by consuming chemical energy. Inspired by these naturally occurring examples, we have developed photoresponsive wet- and dry-type molecular actuators built from rotaxane-based compounds known as [c2]daisy chains (specifically, [c2]AzoCD2 hydrogel and [c2]AzoCD2 xerogel). These actuators were prepared via polycondensation between four-armed poly(ethylene glycol) and a [c2]daisy chain based on α-cyclodextrin as the host component and azobenzene as a photoresponsive guest component. The light-induced actuation arises from the sliding motion of the [c2]daisy chain unit. Ultraviolet irradiation caused the gels to bend towards the light source. The response of the [c2]AzoCD2 xerogel, even under dry conditions, is very fast (7° every second), which is 10,800 times faster than the [c2]AzoCD2 hydrogel (7° every 3 h). In addition, the [c2]AzoCD2 xerogel was used as a crane arm to lift an object using ultraviolet irradiation to produce mechanical work.
The production of molecular machines1,2 that might be able to function as information processing systems presents3,4 a considerable challenge to the chemical community. The so-called bottom-up approach5 to device manufacture has intrigued physical scientists6 and electronic engineers7 for many years. Only recently are chemists8–13 beginning finally to learn how to self-assemble molecular4 and supramolecular systems such that information might ultimately be written into them, stored in them, processed in them, and eventually read back out of them.
Up and down the dumbbell: Photoisomerization of a stilbene rotaxane results in relocation of the cyclodextrin macrocycle in just one direction (see scheme). This behavior, and that of related rotaxanes, sheds light on the mechanism of shuttling in molecular machines.
A new strategy for phototriggered supramolecular polymerization of the [c2]daisy chain is demonstrated. A daisy chain rotaxane containing two coumarin protected 2-ureido-4-pyrimidinone (Upy) terminals at two ends was designed, whose supramolecular polymerization can be initiated by light irradiation that can remove the photocleavable protecting coumarin groups and uncage the two terminal Upy functional units.
A [2]rotaxane, a [3]rotaxane and the corresponding thread containing two succinamide (succ) binding stations and a central redox-active pyromellitimide (pmi) station were studied. Infrared spectroelectro-chemical experiments revealed the translocation of the macrocycle between the succinamide station and the electrochemically reduced pmi station (radical anion and dianion). Remarkably, in the [3]rotaxane, the rings can be selectively translocated. One-electron reduction leads to the translocation of one of the two macrocycles from the succinamide to the pyromellitimide station, while activation of the shuttle via two-electron reduction results in the translocation of both macrocycles: the dianion, due to its higher electron density and hence greater hydrogen bond accepting affinity, is hydrogen-bonded to both macrocycles. Systems with such an on-command contraction are known as molecular muscles. The relative strengths of the binding between the macrocycle and the imide anions could be estimated from the hydrogen bond induced shifts in the C=O stretching frequencies of hydrogen bond accepting amide groups of the macrocycle.
In recent years, many dynamic molecular systems have been elaborated. They constitute the key elements of the emerging field of "molecular machine". In these systems, large amplitude motions can be triggered by various external signals. Among them caténanes and rotaxanes play an important role since the inherent mechanical bonds restrict the degree of freedom of their components but permit motions like pirouetting, rotation or translation. Several examples of electrochemically triggered molecular machine prototypes based on caténanes and rotaxanes are described in this article. Recent spectacular results obtained with such systems are also discussed in the context of possible applications.
An acid/base-responsive polyrotaxane system was constructed, in which the side chain was modified with a diferrocene-functionalized bistable rotaxane molecular switch via “CuAAC” click reaction. The reversible subunit shuttling movement of the rotaxane unit in the polymer system in response to external acid/base stimuli was accompanied with visual fluorescence changes.
The reversible helical pitch change of polyphenylacetylenes by a thermoresponsive rotaxane switch inserted into the side chain was demonstrated through an accompanying color change. Similar to an ethynyl rotaxane monomer, the corresponding polyphenylacetylene having a rotaxane moiety in its side chain exhibited a reversible helical pitch change induced by the thermoresponsive rotaxane switch, i.e., the amine/ammonium salt was converted by treatment with trichloroacetic acid (TCA) and subsequent heating, which led to thermal decomposition of TCA to chloroform and carbon dioxide. Such a rotaxane switch caused the helical pitch change and the accompanying color change of the polymer main chain in both solution and solid state.
Acting as hosts, cationic cyclophanes, consisting of π-electron-poor bipyridinium units, are capable of entering into strong donor–acceptor interactions to form host–guest complexes with various guests when the size and electronic constitution are appropriately matched. A synthetic protocol has been developed that utilizes catalytic quantities of tetrabutylammonium iodide to make a wide variety of cationic pyridinium-based cyclophanes in a quick and easy manner. Members of this class of cationic cyclophanes with boxlike geometries, dubbed ExⁿBoxm⁴⁺ for short, have been prepared by altering a number of variables: (i) n, the number of “horizontal” p-phenylene spacers between adjoining pyridinium units, to modulate the “length” of the cavity; (ii) m, the number of “vertical” p-phenylene spacers, to modulate the “width” of the cavity; and (iii) the aromatic linkers, namely, 1,4-di- and 1,3,5-trisubstituted units for the construction of macrocycles (ExBoxes) and macrobicycles (ExCages), respectively.
A branched [4]rotaxane containing three switching arms with both secondary ammonium cation and aniline binding sites for threaded crown ethers was prepared. 1H NMR spectroscopy was used to show the translational isomerisation of the [4]rotaxane, and different absorption maxima and the oxidation potential for the [4]rotaxane possessing a 1,3,5-tris(4-aminophenyl)benzene centre were monitored by ultraviolet spectroscopy and electrochemical studies, respectively, as the [4]rotaxane was exposed to encircling dibenzo[24]crown-8 species.
Macromolecular [2]rotaxanes comprising a polymer axle and crown ether wheel were synthesized to evaluate the effect of component mobility on the properties of the axle polymer, especially its crystallinity. Living ring-opening polymerization of δ-valerolactone with a pseudorotaxane initiator with a hydroxy group at the axle terminus was followed by end-capping with a bulky isocyanate. This yielded macromolecular [2]rotaxanes (M2Rs) possessing polyester axles of varying molecular weights. The crystallinity of the axle polymers of two series of M2Rs, with either fixed and movable components, was evaluated by differential scanning calorimetry. The results revealed that the effect of component mobility was significant in the fixed and movable M2Rs with a certain axle length, thus suggesting that the properties of the axle polymer depend on the mobility of the polyrotaxane components.
Recently, the development of rotaxane-based architectures for biological applications has started to attract interest. In this short review, we summarize the main advances that have been realized in this burgeoning area of research, including the design of squaraine-rotaxane dyes for optical bioimaging applications, host-rotaxanes as cellular transport agents, enzyme-responsive rotaxanes and mechanized mesoporous silica nanoparticles coated with “nanovalves” as drug delivery systems.
We report a method for blocking interactions between 129Xe and cucurbit[6]uril (CB6) until activation by a specific chemical event. We synthesized a CB6-rotaxane that allowed no 129Xe interaction with the CB6 macrocycle component until a cleavage event released the CB6, which then produced a 129Xe@CB6 NMR signal. This contrast-upon-activation 129Xe NMR platform allows for modular synthesis and can be expanded to applications in detection and disease imaging.
Mechanically-interlocked molecules have been considered for many years only aesthetically appealing structures but nowadays they are finding interesting applications in the field of artificial molecular machines and electronic devices. Improved template-directed protocols have allowed highly programmable and efficient syntheses of rotaxanes and catenanes. In particular those compounds which exhibit bi- and multistability are reliable candidates for the construction of molecular switches. Recently, great efforts have been directed towards the incorporation of these molecules into integrated nanosystems. The first step of this challenge consists in transferring such mechanically-interlocked molecules from solution to surfaces without altering their switching properties. An overview of [2] catenanes that have been synthesized and deposited into surfaces with the aid of various techniques is presented, furthermore some attempts of incorporating such compounds into molecular devices are discussed.
A switchable [2]rotaxane based on the pyridinediamide crown ether macrocycle and the thread bearing phosphine oxide, urea, and dibenzylammonium functional groups was developed successfully, and characterized by 1H NMR, 2D NMR spectra, mass spectra and single crystal analyses. The three recognition sites in the [2]rotaxane were sorted from the strong to the weak according to their bonding abilities, so that the macrocycle could move along the thread from one side to the other side in directional way and operated as a multistable molecular shuttle.
Artificial molecular machines have received significant attention from chemists because of their unique capability in mimicking the behaviors of biological systems. Meanwhile, artificial molecular machines can be easily modified with functional groups to construct new type of functional molecular switches. However, the practical application for artificial molecular machines is still challenging because the working platform of artificial molecular machines is mostly in solution. Artificial molecular machines immobilized surface (AMMIS) has been considered as a promising platform to construct functional materials. Herein, we provide a Minireview of some recent advances of functional AMMISs. The functions of AMMISs are highlighted and the strategies of constructing AMMISs are also discussed. Meanwhile, a brief perspective of the development of artificial molecular machines towards functional materials is given.
Ferrocene is the prototypical organometallic sandwich complex and despite over 60 years passing since the discovery and elucidation of ferrocene's structure, research into ferrocene-containing compounds continues to grow as potential new applications in catalysis, biology and the material sciences are found. Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability function as a "molecular ball bearing" has led to the use of ferrocene as a component in wide range of interlocked and non-interlocked synthetic molecular machine systems. This review will focus on the exploitation of ferrocene (and related sandwich complexes) for the development of non-interlocked synthetic molecular machines.
We describe the incorporation of endo-pyridine units into the tetralactam ring of di(acylamino)pyridine-based rotaxanes. This macrocycle strongly associates with the linear interlocked component as confirmed by X-Ray diffraction studies of the rotaxane 2b. Dynamic NMR studies of 2b in solution afforded a rotational energy barrier higher than that of the related rotaxane 2a, which lacks of pyridine rings in the macrocycle. The macrocycle distribution of the molecular shuttle 4b, containing two endo-pyridine rings, shows that the major co-conformer is that with the cyclic component sitting over the di(acylamino)pyridine station. DFT calculations also support the marked preference of the ring for occupying the heterocyclic binding site. The association of N-hexylthymine with the di(acylamino)pyridine binding site of 4b led to the formation of a rare S-shaped co-conformer in which the tetralactam ring interacts simultaneously with both stations of the thread.
Modern-day factory assembly lines often feature robots that pick up, reposition and connect components in a programmed manner. The idea of manipulating molecular fragments in a similar way has to date only been explored using biological building blocks (specifically DNA). Here, we report on a wholly artificial small-molecule robotic arm capable of selectively transporting a molecular cargo in either direction between two spatially distinct, chemically similar, sites on a molecular platform. The arm picks up/releases a 3-mercaptopropanehydrazide cargo by formation/breakage of a disulfide bond, while dynamic hydrazone chemistry controls the cargo binding to the platform. Transport is controlled by selectively inducing conformational and configurational changes within an embedded hydrazone rotary switch that steers the robotic arm. In a three-stage operation, 79-85% of 3-mercaptopropanehydrazide molecules are transported in either (chosen) direction between the two platform sites, without the cargo at any time fully dissociating from the machine nor exchanging with other molecules in the bulk.
Mass production at the nanoscale requires molecular machines that can control, with high fidelity, the spatial orientation of other reactive species. The demonstration of a synthetic system in which a molecular robotic arm can be used to manipulate the position of a chemical cargo is a significant step towards achieving this goal.
Smart catalysts offer the control of chemical process and sequences of transformations, and catalysts with unique catalytic behavior can afford chiral products or promote successive polymerization. To meet advanced demands, the key to construct smart catalysts is to incorporate traditional catalytic functional groups with trigger-induced factors. Molecular machines with dynamic properties and particular topological structures have typical stimulus-responsive feature. Since recent years, scientists have made efforts to utilize molecular machines (molecular switches, rotaxanes, motors, etc.) as scaffolds to develop smart catalysts. This minireview focuses on the achievements of catalysts encapsulated in molecular machines and their remarkable specialities. We believe this strategy will provide more potential applications in switchable reaction, asymmetric synthesis and processive catalysis.
The long-awaited second edition of the successful book covering molecular switches now in two volumes! Providing principles and applications this book brings you everything you need to know about molecular switches - a hot topic in the nanoworld. The major classes of molecular switches including catenanes, rotaxanes, azobenzenes together with polymer and biomolecular switching systems and DNA based switches are covered. Chemists and material scientists interested in one of the most innovative areas of their science will benefit greatly from reading this book. "This book will appeal most to organic chemists, because of the way new structures are introduced through their synthesis, but it will also provide a useful introduction for other scientists, provided they are conversant with molecular structures." (Organic and Biomolecular Chemistry). "... a comprehensive and up-to-date insight..." (Chemistry & Industry).
Synthesis of an electrochemically addressable [2]catenane has been achieved following formation by templation of a [2]pseudorotaxane employing radically enhanced molecular recognition between the bisradical dication obtained on reduction of the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and the radical cation generated on reduction of a viologen disubstituted with p-xylylene units, both carrying tetraethylene glycol chains terminated by allyl groups. This inclusion complex was subjected to olefin ring-closing metathesis, which was observed to proceed under reduced conditions, to mechanically interlock the two components. Upon oxidation, Coulombic repulsion between the positively charged and mechanically interlocked components results in the adoption of a co-conformation where the newly formed alkene resides inside the cavity of the tetracationic cyclophane. (1)H NMR spectroscopic analysis of this hexacationic [2]catenane shows a dramatic upfield shift of the resonances associated with the olefinic and allylic protons as a result of them residing inside the tetracationic component. Further analysis shows high diastereoselectivity during catenation, as only a single (Z)-isomer is formed.
Biological molecular machines operate far from equilibrium by coupling chemical potential to repeated cycles of dissipative nanomechanical motion. This principle has been exploited in supramolecular systems that exhibit true machine behavior in solution and on surfaces. However, designed membrane-spanning assemblies developed to date have been limited to simple switches or stochastic shuttles, and true machine behavior has remained elusive. Herein, we present a transmembrane nanoactuator that turns over chemical fuel to drive autonomous reciprocating (back-and-forth) nanomechanical motion. Ratcheted reciprocating motion of a DNA/PEG copolymer threaded through a single α-hemolysin pore was induced by a combination of DNA strand displacement processes and enzyme-catalyzed reactions. Ion-current recordings revealed saw-tooth patterns, indicating that the assemblies operated in autonomous, asymmetric cycles of conformational change at rates of up to one cycle per minute.
We report the efficient preparation of an A2D2 (A = acceptor and D = donor) metallacycle 2 = [(en)2Pd2(1)2](NO3)4, using the coordination driven self-assembly of trans-azobenzene based bispyridyl ligand 1 and (en)Pd(NO3)2 (en = ethylenediamine). In the metallacycle, the trans-azobenzene units serve both as a structural element and as sites for subsequent host-guest chemistry with β-cyclodextrin, leading to the formation of a [2] catenane 3. This catenation process is reversible and can be switched off and on in a photocontrollable manner via the trans-cis isomerization of the azobenzene units.