Pumping between phases with a pulsed-fuel molecular ratchet

Molecular Release by the Rotaxane and Pseudorotaxane Approach

The controlled release of target molecules is a relevant application in several areas, such as medicine, fragrance chemistry and catalysis. Systems which pursue this implementation require a fine-tune of the start and rate of the release, among other properties. In this scenario, rotaxane- and pseudorotaxane-based systems are postulated as ideal scaffolds to accomplish a precise cargo release, due to the special features provided by the intertwined arrangement. This short review covers advances towards the controlled release of different molecules using rotaxane- and pseudorotaxane-based systems, both in solution and in the solid state.

Functionalization of Nucleic Acid Molecular Machines under Physiological Conditions: A Review

In-situ fabrication of nucleic acid molecular machines in biological environments is desirable for smart theranostic applications. However, given the complex nature of biological environments, the integration of multiple functional modules into a coordinated machine remains challenging. Recent advances in nucleic acid nanotechnology offer solutions to these challenges. Here, we outline design principles for nucleic acid-based molecular machines tailored for physiological conditions, drawing on recent examples. We review cutting-edge technologies that facilitate their functionalization in physiological settings, particularly presynthesis modifications using unnatural bases and postsynthesis functionalization via bioorthogonal chemistry and noncovalent biological interactions. We discuss the advantages and limitations of these technologies and suggest future directions to overcome existing challenges.

Structural Influence of the Chemical Fueling System on a Catalysis-Driven Rotary Molecular Motor

Continuous directionally biased 360° rotation about a covalent single bond was recently realized in the form of a chemically fueled 1-phenylpyrrole 2,2'-dicarboxylic acid rotary molecular motor. However, the original fueling system and reaction conditions resulted in a motor directionality of only ∼3:1 (i.e., on average a backward rotation for every three forward rotations), along with a catalytic efficiency for the motor operation of 97% and a fuel efficiency of 14%. Here, we report on the efficacy of a series of chiral carbodiimide fuels and chiral hydrolysis promoters (pyridine and pyridine N-oxide derivatives) in driving improved directional rotation of this motor-molecule. We outline the complete reaction network for motor operation, composed of directional, futile, and slip cycles. Using derivatives of the motor where the final conformational step in the 360° rotation is either very slow or completely blocked, the phenylpyrrole diacid becomes enantiomerically enriched, allowing the kinetic gating of the individual steps in the catalytic cycle to be measured. The chiral carbodiimide fuel that produces the highest directionality gives 13% enantiomeric excess (e.e.) for the anhydride-forming kinetically gated step, while the most effective chiral hydrolysis promoter generates 90% e.e. for the kinetically gated hydrolysis step. Combining the best-performing fuel and hydrolysis promoter into a single fueling system results in a 92% e.e.. Under a dilute chemostated fueling regime (to avoid N-acyl urea formation at high carbodiimide concentrations with pyridine N-oxide hydrolysis promoters), the motor continuously rotates with a directionality of ∼24:1 (i.e., a backward rotation for every 24 forward rotations) with a catalytic efficiency of >99% and a fuel efficiency of 51%.

Transducing chemical energy through catalysis by an artificial molecular motor

Cells display a range of mechanical activities generated by motor proteins powered through catalysis1. This raises the fundamental question of how the acceleration of a chemical reaction can enable the energy released from that reaction to be transduced (and, consequently, work to be done) by a molecular catalyst2-7. Here we demonstrate the molecular-level transduction of chemical energy to mechanical force8 in the form of the powered contraction and powered re-expansion of a cross-linked polymer gel driven by the directional rotation of artificial catalysis-driven9 molecular motors. Continuous 360° rotation of the rotor about the stator of the catalysis-driven motor-molecules incorporated in the polymeric framework of the gel twists the polymer chains of the cross-linked network around one another. This progressively increases writhe and tightens entanglements, causing a macroscopic contraction of the gel to approximately 70% of its original volume. The subsequent addition of the opposite enantiomer fuelling system powers the rotation of the motor-molecules in the reverse direction, unwinding the entanglements and causing the gel to re-expand. Continued powered twisting of the strands in the new direction causes the gel to re-contract. In addition to actuation, motor-molecule rotation in the gel produces other chemical and physical outcomes, including changes in the Young modulus and storage modulus-the latter is proportional to the increase in strand crossings resulting from motor rotation. The experimental demonstration of work against a load by a synthetic organocatalyst, and its mechanism of energy transduction6, informs both the debate3,5,7 surrounding the mechanism of force generation by biological motors and the design principles6,10-14 for artificial molecular nanotechnology.

Allosteric release of cucurbit[6]uril from a rotaxane using a molecular signal

Rotaxanes can be regarded as storage systems for their wheel components, which broadens their application potential as a complement to the supramolecular systems that retain a mechanically interlocked structure. However, utilising rotaxanes in this way requires a method to release the wheel while preserving the integrity of all molecular constituents. Herein, we present simple rotaxanes based on cucurbit[6]uril (CB6), with an axis equipped with an additional binding motif that enables the binding of another macrocycle, cucurbit[7]uril (CB7). We demonstrate that the driving force behind the wheel dethreading originates from the binding of the signalling macrocycle to the allosteric site, leading to an increase in the system's strain. Consequently, the CB6 wheel leaves the rotaxane station overcoming the mechanical barrier. Portal-portal repulsive interactions between the two cucurbituril units play a crucial role in this process. Thus, the repulsive strength and the related rate of slipping off can be finely tuned by the length of the allosteric binding motif. Finally, we show that the CB6 wheel can be utilised within complexes with other guests in the mixture once released from the rotaxane.

Skeletal Editing of Mechanically Interlocked Molecules: Nitrogen Atom Deletion from Crown Ether-Dibenzylammonium Rotaxanes

Removing the nitrogen atom from secondary amines while simultaneously linking the remaining fragments is a powerful form of late-stage skeletal editing. Here, we report its use for the deletion of the nitrogen atom of the dibenzylammonium template used to assemble crown ether rotaxanes. The reaction uses an anomeric amide that activates secondary amines to generate a carbon-carbon bond that replaces the amine nitrogen. Despite the potential for dethreading of the intermediate diradical pair, the nitrogen atom was successfully deleted from a series of rotaxane axles as long as the macrocycle could access coconformations that did not inhibit the reaction of the amine group. The skeletally edited interlocked molecules were obtained directly from the parent crown ether-dibenzylammonium rotaxanes in modest yields (23-36%) and characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. One skeletally edited rotaxane shows a network of weak CH···O hydrogen bonds between the crown ether and benzylic methylene groups of the axle in the solid state, in place of the crown ether-ammonium binding motif used to assemble the parent, unedited, rotaxane.

Concise and Efficient Synthesis of Sequentially Isomeric Hetero[3]rotaxanes

Stereoisomerism, stemming from the spatial orientation of components in molecular structures, plays a decisive role in nature. While the unconventional bonding found in mechanically interlocked molecules gives rise to unique expressions of stereochemistry, the exploration of their stereoisomers is still in its infancy. Sequence isomerism, characterized by variations in the ordering of mechanically interlocked components in catenanes and rotaxanes, mirrors the sequence variations found in biological macromolecules. Herein, we report the use of artificial molecular pumps for the precise and simple production of sequentially isomeric hetero[3]rotaxanes. Utilizing redox-driven pumping cassettes with different rings, we have synthesized two hetero[3]rotaxane isomers in high isolated yields from two [2]rotaxanes. This research represents a significant advance in sequential molecular assembly, paving the way for the development of sophisticated, functionalized, mechanically interlocked materials.

Harnessing Maxwell's demon to establish a macroscale concentration gradient

Maxwell's demon describes a thought experiment in which a 'demon' regulates the flow of particles between two adjoining spaces, establishing a potential gradient without appearing to do work. This seeming paradox led to the understanding that sorting entails thermodynamic work, a foundational concept of information theory. In the past centuries, many systems analogous to Maxwell's demon have been introduced in the form of molecular information, molecular pumps and ratchets. Here we report a functional example of a Maxwell's demon that pumps material over centimetres, whereas previous examples operated on a molecular scale. In our system, this demon drives directional transport of o-fluoroazobenzene between the arms of a U-tube apparatus upon light irradiation, transiting through an aqueous membrane containing a coordination cage. The concentration gradient thus obtained is further harnessed to drive naphthalene transport in the opposite direction.

Directionality Reversal and Shift of Rotational Axis in a Hemithioindigo Macrocyclic Molecular Motor

Molecular motors are central driving units for nanomachinery, and control of their directional motions is of fundamental importance for their functions. Light-driven variants use easy to provide, easy to dose, and waste-free fuel with high energy content, making them particularly interesting for applications. Typically, light-driven molecular motors work via rotations around dedicated chemical bonds where the directionality of the rotation is dictated by the steric effects of asymmetry in close vicinity to the rotation axis. In this work, we show how unidirectional rotation around a virtual axis can be realized by reprogramming a molecular motor. To this end, a classical light-driven motor is restricted by macrocyclization, and its intrinsic directional rotation is transformed into a directional rotation of the macrocyclic chain in the opposite direction. Further, solvent polarity changes allow to toggle the function of this molecular machine between a directional motor and a nondirectional photoswitch. In this way, a new concept for the design of molecular motors is delivered together with elaborate control over their motions and functions by simple solvent changes. The possibility of sensing the environmental polarity and correspondingly adjusting the directionality of motions opens up a next level of control and responsiveness to light-driven nanoscopic motors.

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Redox-Regulated and Guest-Driven Transformations of Aromatic Oligoamide Foldamers in Advanced Structures

In this study, we demonstrate that an aromatic oligoamide sequence assembles into a trimeric helix-turn-helix architecture with a disulfide linkage, and upon cleavage of this linkage, it reconstructs into an antiparallel double helix. The antiparallel double helix is accessible to encapsulate a diacid guest within its cavity, forming a 2:1 host-guest complex. In contrast, hydrogen-bonding interactions between the trimeric-assembled structure and guests induce a conformational shift in the trimeric helix, resulting in a cross-shaped double-helix complex at a 2:2 host-guest ratio. Interconversions between the trimeric helix and the antiparallel double helix, along with their respective host-guest complexes, can be initiated through thiol/disulfide redox-mediated regulation.

Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.

Adaptisorption of Nonporous Polymer Crystals

Although our knowledge and understanding of adsorptions in natural and artificial systems has increased dramatically during the past century, adsorption associated with nonporous polymers remains something of a mystery, hampering applications. Here we demonstrate a model system for adaptisorption of nonporous polymers, wherein dative B-N bonds and host-guest binding units act as the kinetic and thermodynamic components, respectively. The coupling of these two components enables nonporous polymer crystals to adsorb molecules from solution and undergo recrystallization as thermodynamically favored crystals. Adaptisorption of nonporous polymer crystals not only extends the types of adsorption in which the sorbate molecules are integrated in a precise and orderly manner in the sorbent systems, but also provides a facile and accurate approach to the construction of polymeric materials with precise architectures and integrated functions.

Kinetic Asymmetry and Directionality of Nonequilibrium Molecular Systems

Scientists have long been fascinated by the biomolecular machines in living systems that process energy and information to sustain life. The first synthetic molecular rotor capable of performing repeated 360° rotations due to a combination of photo- and thermally activated processes was reported in 1999. The progress in designing different molecular machines in the intervening years has been remarkable, with several outstanding examples appearing in the last few years. Despite the synthetic accomplishments, there remains confusion regarding the fundamental design principles by which the motions of molecules can be controlled, with significant intellectual tension between mechanical and chemical ways of thinking about and describing molecular machines. A thermodynamically consistent analysis of the kinetics of several molecular rotors and pumps shows that while light driven rotors operate by a power-stroke mechanism, kinetic asymmetry-the relative heights of energy barriers-is the sole determinant of the directionality of catalysis driven machines. Power-strokes-the relative depths of energy wells-play no role whatsoever in determining the sign of the directionality. These results, elaborated using trajectory thermodynamics and the nonequilibrium pump equality, show that kinetic asymmetry governs the response of many non-equilibrium chemical phenomena.

Tunable cyclic operation of dissipative molecular switches based on anion recognition

Artificial dissipative molecular switches based on anion recognition are of great importance to simulate biological functions and construct smart materials. Five activated carboxylic acids are used as chemical fuels for dissipative molecular switches, which consist of an imidazolium macrocyclic host and a carboxylate anionic guest. By choosing different types of chemical fuels and using varied fuel concentrations, the rates of cyclic operations are tunable. The operation is capable of undergoing at least three cycles.

Ratcheting synthesis

Synthetic chemistry has traditionally relied on reactions between reactants of high chemical potential and transformations that proceed energetically downhill to either a global or local minimum (thermodynamic or kinetic control). Catalysts can be used to manipulate kinetic control, lowering activation energies to influence reaction outcomes. However, such chemistry is still constrained by the shape of one-dimensional reaction coordinates. Coupling synthesis to an orthogonal energy input can allow ratcheting of chemical reaction outcomes, reminiscent of the ways that molecular machines ratchet random thermal motion to bias conformational dynamics. This fundamentally distinct approach to synthesis allows multi-dimensional potential energy surfaces to be navigated, enabling reaction outcomes that cannot be achieved under conventional kinetic or thermodynamic control. In this Review, we discuss how ratcheted synthesis is ubiquitous throughout biology and consider how chemists might harness ratchet mechanisms to accelerate catalysis, drive chemical reactions uphill and programme complex reaction sequences.

Switched "On" Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

We report the synthesis and operation of a molecular energy ratchet that transports a crown ether from solution onto a thread, along the axle, over a fluorophore, and off the other end of the thread back into bulk solution, all in response to a single pulse of a chemical fuel (CCl3CO2H). The fluorophore is a pyrene residue whose fluorescence is normally prevented by photoinduced electron transfer (PET) to a nearby N-methyltriazolium group. However, crown ether binding to the N-methyltriazolium site inhibits the PET, switching on pyrene fluorescence under UV irradiation. Each pulse of fuel results in a single ratchet cycle of transient fluorescence (encompassing threading, transport to the N-methyltriazolium site, and then dethreading), with the onset of the fluorescent time period determined by the amount of fuel in each pulse and the end-point determined by the concentration of the reagents for the disulfide exchange reaction. The system provides a potential alternative signaling approach for artificial molecular machines that read symbols from sequence-encoded molecular tapes.

Artificial Molecular Ratchets: Tools Enabling Endergonic Processes

Non-equilibrium chemical systems underpin multiple domains of contemporary interest, including supramolecular chemistry, molecular machines, systems chemistry, prebiotic chemistry, and energy transduction. Experimental chemists are now pioneering the realization of artificial systems that can harvest energy away from equilibrium. In this tutorial Review, we provide an overview of artificial molecular ratchets: the chemical mechanisms enabling energy absorption from the environment. By focusing on the mechanism type-rather than the application domain or energy source-we offer a unifying picture of seemingly disparate phenomena, which we hope will foster progress in this fascinating domain of science.

Daisy chain architectures: from discrete molecular entities to polymer materials

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

Light-responsive rotaxane-based materials: inducing motion in the solid state

Light-responsive rotaxane-based solid-state materials are ideal scaffolds in order to develop smart materials due to the properties provided by the mechanical bond, such as control over the dynamics of the components upon application of external stimuli. This perspective aims to highlight the relevance of these materials, by pointing out recent examples of photoresponsive materials prepared from a rotaxanated architecture in which motion of the counterparts and/or macroscopic motion of the interlocked materials are achieved. Although further development is needed, these materials are envisioned as privileged scaffolds which will be used for different advanced applications in the area of molecular machinery.

Photoactivated Artificial Molecular Motors

Accurate control of long-range motion at the molecular scale holds great potential for the development of ground-breaking applications in energy storage and bionanotechnology. The past decade has seen tremendous development in this area, with a focus on the directional operation away from thermal equilibrium, giving rise to tailored man-made molecular motors. As light is a highly tunable, controllable, clean, and renewable source of energy, photochemical processes are appealing to activate molecular motors. Nonetheless, the successful operation of molecular motors fueled by light is a highly challenging task, which requires a judicious coupling of thermal and photoinduced reactions. In this paper, we focus on the key aspects of light-driven artificial molecular motors with the aid of recent examples. A critical assessment of the criteria for the design, operation, and technological potential of such systems is provided, along with a perspective view on future advances in this exciting research area.

Formation of Direction-Controllable Pseudorotaxane and Catenane Using Chemically Cyclized Oligodeoxynucleotides and Their Noncovalent RNA Labeling

The formation of interlocked structures, such as rotaxane and catenane, enables noncovalent conjugations. We previously confirmed that the chemically cyclized pseudorotaxane-forming oligodeoxynucleotides (prfODNs) with double-tailed parts formed a pseudorotaxane structure with the target DNA and RNA via the slipping process. Here, we report the one-step synthesis of cyclized prfODNs from alkyne-modified ODNs, after which we investigated the properties and mechanism of the slipping process and performed noncovalent RNA labeling with prfODNs. Additionally, the catenane structure was formed by the combination of pseudorotaxane formation with a 5'-end-phosphorylated RNA and enzymatic ligation. The newly synthesized prfODN represents a new tool for achieving the noncovalent conjugation of various functional moieties to RNAs.

Transient self-assembly of metal-organic complexes

Implementing transient processes in networks of dynamic molecules holds great promise for developing new functional behaviours. Here we report that trichloroacetic acid can be used to temporarily rearrange networks of dynamic imine-based metal complexes towards new equilibrium states, forcing them to express complexes otherwise unfavourable in their initial equilibrium states. Basic design principles were determined for the creation of such networks. Where a complex distribution of products was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily yielded a simplified output, forcing a more structured distribution of products. Where a single complex was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily modified the properties of this complex. By doing so, the mechanical properties of an helical macrocyclic complex could be temporarily altered by rearranging it into a [2]catenane.

Design and Manipulation of a Minimalistic Hydrocarbon Nanocar on Au(111)

Nanocars are carbon-based single-molecules with a precise design that facilitates their atomic-scale control on a surface. The rational design of these molecules is important in atomic and molecular-scale manipulation to advance the development of molecular machines, as well as for a better understanding of self-assembly, diffusion and desorption processes. Here, we introduce the molecular design and construction of a collection of minimalistic nanocars. They feature an anthracene chassis and four benzene derivatives as wheels. After sublimation and adsorption on an Au(111) surface, we show controlled and fast manipulation of the nanocars along the surface using the tip of a scanning tunneling microscope (STM). The mechanism behind the successful displacement is the induced dipole created over the nanocar by the STM tip. We utilized carbon monoxide functionalized tips both to avoid decomposition and accidentally picking the nanocars up during the manipulation. This strategy allowed thousands of maneuvers to successfully win the Nanocar Race II championship.

A tape-reading molecular ratchet

Cells process information in a manner reminiscent of a Turing machine¹, autonomously reading data from molecular tapes and translating it into outputs^2,3. Randomly processive macrocyclic catalysts that can derivatise threaded polymers have been described^4,5, as have rotaxanes that transfer building blocks in sequence from a molecular strand to a growing oligomer^6-10. However, synthetic small-molecule machines that can read and/or write information stored on artificial molecular tapes remain elusive^11-13. Here we report on a molecular ratchet in which a crown ether (the 'reading head') is pumped from solution onto an encoded molecular strand (the 'tape') by a pulse^14,15 of chemical fuel¹⁶. Further fuel pulses transport the macrocycle through a series of compartments of the tape via an energy ratchet^14,17-22 mechanism, before releasing it back to bulk off the other end of the strand. During its directional transport, the crown ether changes conformation according to the stereochemistry of binding sites along the way. This allows the sequence of stereochemical information programmed into the tape to be read out as a string of digits in a non-destructive manner through a changing circular dichroism response. The concept is exemplified by the reading of molecular tapes with strings of balanced ternary digits ('trits'²³), -1,0,+1 and -1,0,-1. The small-molecule ratchet is a finite-state automaton: a special case²⁴ of a Turing machine that moves in one direction through a string-encoded state sequence, giving outputs dependent on the occupied machine state^25,26. It opens the way for the reading-and ultimately writing-of information using the powered directional movement of artificial nanomachines along molecular tapes.

Tuning the Force, Speed, and Efficiency of an Autonomous Chemically Fueled Information Ratchet

Autonomous chemically fueled molecular machines that function through information ratchet mechanisms underpin the nonequilibrium processes that sustain life. These biomolecular motors have evolved to be well-suited to the tasks they perform. Synthetic systems that function through similar mechanisms have recently been developed, and their minimalist structures enable the influence of structural changes on machine performance to be assessed. Here, we probe the effect of changes in the fuel and barrier-forming species on the nonequilibrium operation of a carbodiimide-fueled rotaxane-based information ratchet. We examine the machine’s ability to catalyze the fuel-to-waste reaction and harness energy from it to drive directional displacement of the macrocycle. These characteristics are intrinsically linked to the speed, force, power, and efficiency of the ratchet output. We find that, just as for biomolecular motors and macroscopic machinery, optimization of one feature (such as speed) can compromise other features (such as the force that can be generated by the ratchet). Balancing speed, power, efficiency, and directionality will likely prove important when developing artificial molecular motors for particular applications.

Precise Non-Equilibrium Polypropylene Glycol Polyrotaxanes

Traditionally, the synthesis of polyrotaxanes has been limited by synthetic methods that rely on an innate affinity between the rings and the polymer chains. The use of rotaxane-forming molecular pumps allows this limitation to be circumvented in the production of non-equilibrium polyrotaxanes in which rings are trapped on polymer chains for which they have little or no affinity. Pumping cassettes, each composed of a bipyridinium unit linked (i) by a bismethylene bridge to a terminal 2,6-dimethylpyridinium cationic unit and (ii) by a methylene group to an isopropylphenylene steric barrier, were attached using copper-catalyzed azide-alkyne cycloadditions to the ends of a polypropylene glycol (PPG) chain of number-average molecular weight M_n ≈ 2200. Using a one-pot electrosynthetic protocol, a series of PPG-based polyrotaxanes with cyclobis(paraquat-p-phenylene) as the rings were synthesized. Despite the steric bulk of the PPG backbone, it was found to be a suitable collecting chain for threading up to 10 rings. The pumping of two rings is sufficient to render these hydrophobic polymers soluble in aqueous solution. Their hydrodynamic diameters and diffusion constants vary according to the number of pumped rings. The non-equilibrium nature of these polyrotaxanes is manifested in their gradual degradation and dethreading at elevated temperatures.

Controlling dynamics in extended molecular frameworks

Molecular machines are essential dynamic components for fuel production, cargo delivery, information storage and processing in living systems. Scientists have demonstrated that they can design and synthesize artificial molecular machines that operate efficiently in isolation - for example, at high dilution in solution - fuelled by chemicals, electricity or light. To organize the spatial arrangement and motion of these machines within close proximity to one another in solid frameworks, such that useful macroscopic work can be performed, remains a challenge in both chemical and materials science. In this Review, we summarize the progress that has been made during the past decade in organizing dynamic molecular entities in such solid frameworks. Emerging applications of these dynamic smart materials in the contexts of molecular recognition, optoelectronics, drug delivery, photodynamic therapy and water desalination are highlighted. Finally, we review recent work on a new non-equilibrium adsorption phenomenon for which we have coined the term mechanisorption. The ability to use external energy to drive directional processes in mechanized extended frameworks augurs well for the future development of artificial molecular factories.

Transamidation-Driven Molecular Pumps

We report a new class of synthetic molecular pumps that use a stepwise information ratchet mechanism to achieve the kinetic gating required to sequester their macrocyclic substrates from bulk solution. Threading occurs as a result of active template reactions between the pump terminus amine and an acyl electrophile, whereby the bond-forming reaction is accelerated through the cavity of a crown ether. Carboxylation of the resulting amide results in displacement of the ring to the collection region of the thread. Conversion of the carbamate to a phenolic ester provides an intermediate rotaxane suitable for further pumping cycles. In this way rings can be ratcheted onto a thread from one or both ends of appropriately designed molecular pumps. Each pumping cycle results in one additional ring being added to the thread per terminus acyl group. The absence of pseudorotaxane states ensures that no dethreading of intermediates occurs during the pump operation. This facilitates the loading of different macrocycles in any chosen sequence, illustrated by the pump-mediated synthesis of a [4]rotaxane containing three different macrocycles as a single sequence isomer. A [5]rotaxane synthesized using a dual-opening transamidation pump was structurally characterized by single-crystal X-ray diffraction, revealing a series of stabilizing CH···O interactions between the crown ethers and the polyethylene glycol catchment region of the thread.

Mechanically interlocked derivatives of carbon nanotubes: synthesis and potential applications

An introduction to mechanically interlocked derivatives of single-walled carbon nanotubes: their main structural features, their potential advantages compared to covalent and supramolecular derivatives, how to synthesize them, and their most promising fields for application.

Polyrotaxanes and the pump paradigm

The year 2022 marks the 30th anniversary of the first reports of polyrotaxanes in the scientific literature.