Dissipative out-of-equilibrium assembly of man-made supramolecular materials

Reduction-Induced Self-Propelled Oscillatory Motion of Perylenediimides on Water

The emergence of macroscopic self-propelled oscillatory motion based on molecular design has attracted continual attention in relation to autonomous systems in living organisms. Herein, a series of perylenediimides (PDIs) with various imide side chains was prepared to explore the impact of molecular design and alignment on the self-propelled motion at the air-water interface. When placed on an aqueous solution containing a reductant, a solid disk of neutral PDI was reduced to form the water-soluble, surface-active PDI dianion species, which induces a surface tension gradient in the vicinity of the disk for self-propelled motion. We found that centimeter-scale oscillatory motion could be elicited by controlling the supply rate of PDI dianion species through the reductant concentration and the structure of the imide side chains. Furthermore, we found that the onset and speed of the self-propelled motion could be changed by the crystallinity of PDI at the water surface. This design principle using π-conjugated molecules and their self-assemblies could advance self-propelled, non-equilibrium systems powered by chemical energy.

Non-equilibrium self-assembly for living matter-like properties

The soft and wet machines of life emerged as the spatially enclosed ensemble of biomolecules with replicating capabilities integrated with metabolic reaction cycles that operate at far-from-equilibrium. A thorough step-by-step synthetic integration of these elements, namely metabolic and replicative properties all confined and operating far-from-equilibrium, can set the stage from which we can ask questions related to the construction of chemical-based evolving systems with living matter-like properties - a monumental endeavour of systems chemistry. The overarching concept of this Review maps the discoveries on this possible integration of reaction networks, self-reproduction and compartmentalization under non-equilibrium conditions. We deconvolute the events of reaction networks and transient compartmentalization and extend the discussion towards self-reproducing systems that can be sustained under non-equilibrium conditions. Although enormous challenges lie ahead in terms of molecular diversity, information transfer, adaptation and selection that are required for open-ended evolution, emerging strategies to generate minimal metabolic cycles can extend our growing understanding of the chemical emergence of the biosphere of Earth.

Supramolecular fibrillation in coacervates and other confined systems towards biomimetic function

As in natural cytoskeletons, the cooperative assembly of fibrillar networks can be hosted inside compartments to engineer biomimetic functions, such as mechanical actuation, transport, and reaction templating. Coacervates impose an optimal liquid-liquid phase separation within the aqueous continuum, functioning as membrane-less compartments that can organise such self-assembling processes as well as the exchange of information with their environment. Furthermore, biological fibrillation can often be controlled or assisted by intracellular compartments. Thus, the reconstitution of analogues of natural filaments in simplified artificial compartments, such as coacervates, offer a suitable model to unravel, mimic, and potentially exploit cellular functions. This perspective summarises the latest developments towards assembling fibrillar networks under confinement inside coacervates and related compartments, including a selection of examples ranging from biological to fully synthetic monomers. Comparative analysis between coacervates, lipid vesicles, and droplet emulsions showcases the interplay between supramolecular fibres and the boundaries of the corresponding compartment. Combining inspiration from natural systems and the custom properties of tailored synthetic fibrillators, rational monomer and compartment design will contribute towards engineering increasingly complex and more realistic artificial protocells.

Polymer Free Volume-Controlled Molecular Clock and Emitter for Multicolored Transient Data Display in Advanced Photonic Cryptography

The booming demand on data security has aroused great interest for developing smart materials with temporal display feature and dynamic multicolor fluorescence. However, it remains challenging to implement both features on most responsive molecules. Herein, we construct a polymer free volume-controlled "molecular clock and emitter" via covalently embedding a multi-stimuli responsive molecular switch (i.e., spiropyran) into a polymer network (i.e., poly(pentafluorophenyl acrylate)) with programmable crosslink density and free volume. By the aminolysis of pentafluorophenyl ester with different amount of diamine crosslinkers, pPFPA-co-SP networks with controllable crosslink densities are generated, which have different confinement effects on the rate constant of SP/MC isomerization, thus leading to time-dependent photochromism. In addition, PTF1, a fluorescent probe that is sensitive to polymer rigidity, is introduced to further endow pPFPA-co-SP system with phototunable dynamic full-color emission. Therefore, relying on their synergistical responses to the rigidity of the polymer network, we have successfully developed a versatile molecular clock and emitter via an "one stone two birds" manner, which shows time-dependent data display along with dynamic multicolor fluorescence switching, providing great potential for advanced encryption and anticounterfeiting with a high security level.

Driven Self-Assembly of Patchy Particles Overcoming Equilibrium Limitations

Bridging biological complexity and synthetic material design, we investigate dissipative self-assembly in patchy particle systems. Utilizing Monte Carlo and Molecular Dynamics simulations, we demonstrate how external driving forces mitigate equilibrium trade-offs between assembly time and structural stability, traditionally encountered in self-assembly processes. Our findings also extend to biological-mimicking environments, where we explore the dynamics of patchy particles under crowded conditions. This comprehensive analysis offers insights into advanced material design, opening avenues for innovations in nanotechnology applications.

Self-Assembly in Living Cells: Bottom-Up Syntheses in Natural Factory

In situ self-assembly in living systems is referred to as the processes that regulate assembly by stimuli-responsive reactions at target sites under physiological conditions. Due to the advantages of precisely forming well-defined nanostructures at pathological lesions, in situ-formed assemblies with tailored bioactivity are promising for the development of next-generation biomedical agents. In this Perspective, we summarize the progress of in situ self-assembly of peptides in living cells with an emphasis on the state-of-the-art strategies regulating assembly processes, establishing complexity within assembly systems, and exploiting their applications in biomedicines. We also provide our forward conceiving perspectives on the challenges in the development of in situ assembly in living cells to demonstrate its great potential in creating biomaterials for healthcare in the future.

Oxidation-Responsive Supramolecular Hydrogels Based on Glucosamine Derivatives with an Aryl Sulfide Group

Supramolecular hydrogels can be obtained via self-assembly of small molecules in aqueous environments. In this study, we describe the development of oxidation-responsive supramolecular hydrogels comprising glucosamine derivatives with an aryl sulfide group. We demonstrate that hydrogen peroxide can induce a gel-sol transition through the oxidation of the sulfide group to the corresponding sulfoxide. Furthermore, we show that this oxidation responsiveness can be extended to photo-responsiveness with the aid of a photosensitizer.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Non-Equilibrium Dissipative Assembly with Switchable Biological Functions

Natural dissipative assembly (DSA) often exhibit energy-driven shifts in natural functions. However, creating man-made DSA that can mimic such biological activities transformation remains relatively rare. Herein, we introduce a cytomembrane-like dissipative assembly system based on chiral supramolecules. This system employs benzoyl cysteine in an out of equilibrium manner, enabling the shifts in biofunctions while minimizing material use. Specifically, aroyl-cystine derivatives primarily assemble into stable M-helix nanofibers under equilibrium conditions. These nanofibers enhance fibroblast adhesion and proliferation through stereospecific interactions with chiral cellular membranes. Upon the addition of chemical fuels, these functional nanofibers temporarily transform into non-equilibrium nanospheres, facilitating efficient drug delivery. Subsequently, these nanospheres revert to their original nanofiber state, effectively recycling the drug. The programmable function-shifting ability of this DSA establishes it as a novel, fuel-driven drug delivery vehicle. And the bioactive DSA not only addresses a gap in synthetic DSAs within biological applications but also sets the stage for innovative designs of 'living' materials.

Repurposing a Catalytic Cycle for Transient Self-Assembly

Life operates out of equilibrium to enable various sophisticated behaviors. Synthetic chemists have strived to mimic biological nonequilibrium systems in such fields as autonomous molecular machines and dissipative self-assembly. Central to these efforts has been the development of new chemical reaction cycles, which drive systems out of equilibrium by conversion of chemical fuel into waste species. However, the construction of reaction cycles has been challenging due to the difficulty of finding compatible reactions that constitute a cycle. Here, we realize an alternative approach by repurposing a known catalytic cycle as a chemical reaction cycle for driving dissipative self-assembly. This approach can overcome the compatibility problem because all steps involved in a catalytic cycle are already known to proceed concurrently under the same conditions. Our repurposing approach is applicable to diverse combinations of catalytic cycles and systems to drive out of equilibrium, which will substantially broaden the scope of out-of-equilibrium systems.

Surface Tension Manipulation with Visible Light through Sensitized Disequilibration of Photoswitchable Amphiphiles

Azoarene isomerization lies at the heart of numerous applications, from catalysis or energy storage to photopharmacology. While efficient switching between their E and Z isomers predominantly relies on UV light, a recent study by Klajn and co-workers introduced visible light sensitization of E azoarenes and their subsequent isomerization as a tool coined disequilibration by sensitization under confinement (DESC) to obtain high yields of the Z isomer. This host-guest approach is, however, still constrained to minimally substituted azoarenes with limited applicability in advanced molecular systems. Herein, we expand DESC for the assembly of surfactants at the air-water interface. Leveraging our expertise with photoswitchable amphiphiles, we induce substantial alterations of the water surface tension through reversible arylazopyrazole isomerization. After studying the binding of charged surfactants to the host, we find that the surface activity differences upon visible light switching for both isomers are comparable to those obtained by UV light excitation. The method is demonstrated on a large concentration range and can be activated using green or red light, depending on the sensitizer chosen. The straightforward implementation of photoswitch sensitization in a complex molecular network showcases how DESC enables the improvement of existing systems and the development of novel applications driven by visible light.

Photoinitiated Transient Self-Assembly in a Catalytically Driven Chemical Reaction Cycle

Chemically fueled chemical reaction networks (CRNs) are essential in controlling dissipative self-assembly. A key challenge in the field is to store chemical fuel-precursors or "pre-fuels" in the system that are converted into activating or deactivating fuels in a catalytically controlled CRN. In addition, real-time control over catalysis in a CRN by light is highly desirable, but so far not yet achieved. Here we show a catalytically driven CRN that is photoinitiated with 450 nm light, producing activated monomers that go on to perform transient self-assembly. Monomer activation proceeds via photoredox catalysis, converting the monomer alcohol groups into the corresponding aldehydes that self-assemble into large supramolecular fibers. Monomer deactivation is achieved by organometallic catalysis that relies on pre-fuel hydrolysis to release formate (i.e. the deactivating fuel). Additionally, irradiation with 305 nm light accelerates the release of formate by photo-uncaging the pre-fuel, leading to a factor of ca. 2 faster deactivation of the monomer. Overall, we show transient self-assembly upon visible light photoactivation, and tunable life-times by ultraviolet light.

Handedness-Inverted and Stimuli-Responsive Circularly Polarized Luminescent Nano/Micromaterials Through Pathway-Dependent Chiral Supramolecular Polymorphism

The precise manipulation of supramolecular polymorphs has been widely applied to control the morphologies and functions of self-assemblies, but is rarely utilized for the fabrication of circularly polarized luminescence (CPL) materials with tailored properties. Here, this work reports that an amphiphilic naphthalene-histidine compound (NIHis) readily self-assembled into distinct chiral nanostructures through pathway-dependent supramolecular polymorphism, which shows opposite and multistimuli responsive CPL signals. Specifically, NIHis display assembly-induced CPL from the polymorphic keto tautomer, which become predominant during enol-keto tautomerization shifting controlled by a bulk solvent effect. Interestingly, chiral polymorphs of nanofiber and microbelt with inverted CPL signals can be prepared from the same NIHis monomer in exactly the same solvent compositions and concentrations by only changing the temperature. The tunable CPL performance of the solid microbelts is realized under multi external physical or chemical stimuli including grinding, acid fuming, and heating. In particular, an emission color and CPL on-off switch based on the microbelt polymorph by reversible heating-cooling protocol is developed. This work brings a new approach for developing smart CPL materials via supramolecular polymorphism engineering.

Light-Driven, Dynamic Assembly of Micron-To-Centimeter Parts, Micromachines and Microbot Swarms

This work describes light-driven assembly of dynamic formations and functional particle swarms controlled by appropriately programmed light patterns. The system capitalizes on the use of a fluidic bed whose low thermal conductivity assures that light-generated heat remains "localized" and sets strong convective flows in the immediate vicinity of the particles being irradiated. In this way, even low-power laser light or light from a desktop slide projector can be used to organize dynamic formations of objects spanning four orders of magnitude in size (from microns to centimeters) and over nine orders of magnitude in terms of mass. These dynamic assemblies include open-lattice structures with individual particles performing intricate translational and/or rotational motions, density-gradient particle arrays, nested architectures of mechanical components (e.g., planetary gears), or swarms of light-actuated microbots controlling assembly of other objects.

Photoswitchable Imines Drive Dynamic Covalent Systems to Nonequilibrium Steady States

Coupling a photochemical reaction to a thermal exchange process can drive the latter to a nonequilibrium steady state (NESS) under photoirradiation. Typically, systems use separate motifs for photoresponse and equilibrium-related processes. Here, we show that photoswitchable imines can fulfill both roles simultaneously, autonomously driving a dynamic covalent system into a NESS under continuous light irradiation. We demonstrate this using transimination reactions, where E-to-Z photoisomerism generates a more kinetically labile species. At the NESS, energy is stored both in the metastable Z-isomer of the imine and in the system's nonequilibrium constitution; when the light is switched off, this stored energy is released as the system reverts to its equilibrium state. The system operates autonomously under continuous light irradiation and exhibits characteristics of a light-driven information ratchet. This is enabled by the dual-role of the imine linkage as both the photochromic and dynamic covalent bond. This work highlights the ability and application of these imines to drive systems to NESSs, thus offering a novel approach in the field of systems chemistry.

Transient Covalent Polymers through Carbodiimide-Driven Assembly

Biochemical systems make use of out-of-equilibrium polymers generated under kinetic control. Inspired by these systems, many abiotic supramolecular polymers driven by chemical fuel reactions have been reported. Conversely, polymers based on transient covalent bonds have received little attention, even though they have the potential to complement supramolecular systems by generating transient structures based on stronger bonds and by offering a straightforward tuning of reaction kinetics. In this study, we show that simple aqueous dicarboxylic acids give poly(anhydrides) when treated with the carbodiimide EDC. Transient covalent polymers with molecular weights exceeding 15,000 are generated which then decompose over the course of hours to weeks. Disassembly kinetics can be controlled using simple substituent effects in the monomer design. The impact of solvent polarity, carbodiimide concentration, temperature, pyridine concentration, and monomer concentration on polymer properties and lifetimes has been investigated. The results reveal substantial control over polymer assembly and disassembly kinetics, highlighting the potential for fine-tuned kinetic control in nonequilibrium polymerization systems.

Kinetic Trapping of an Out-of-Equilibrium Dynamic Library of Imines by Changing Solvent

A well-behaved dynamic library composed of two imines and corresponding amines was subjected to the action of an activated carboxylic acid (ACA), whose decarboxylation is known to be base promoted, in different solvents, namely CD2Cl2, CD3CN, and mixtures of them. Two non-equilibrium systems are consequently obtained: i) a dissipative (CD2Cl2) and ii) an out-of-equilibrium (CD3CN) dynamic library whose composition goes back to equilibrium after a given time. In the former case, the library is fully coupled with the decarboxylation of the ACA, while in the latter, an energy ratchet operates. In the mixed solvents, the library exhibits a mediated behavior. Interestingly, in the presence of an excess of added ACA, the different behavior of the imine library in the two solvents is expected to manifest only when the excess acid is consumed.

Chemically fueled dynamic switching between assembly-encoded emissions

Self-assembly provides access to non-covalently synthesized supramolecular materials with distinct properties from a single building block. However, dynamic switching between functional states still remains challenging, but holds enormous potential in material chemistry to design smart materials. Herein, we demonstrate a chemical fuel-mediated strategy to dynamically switch between two distinctly emissive aggregates, originating from the self-assembly of a naphthalimide-appended peptide building block. A molecularly dissolved building block shows very weak blue emission, whereas, in the assembled state (Agg-1), it shows cyan emission through π stacking-mediated excimer emission. The addition of a chemical fuel, ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC), converts the terminal aspartic acid present in the building block to an intra-molecularly cyclized anhydride in situ forming a second aggregated state, Agg-2, by changing the molecular packing, thereby transforming the emission to strong blue. Interestingly, the anhydride gets hydrolyzed gradually to reform Agg-1 and the initial cyan emission is restored. The kinetic stability of the strong blue emissive aggregate, Agg-2, can be regulated by the added concentration of the chemical fuel. Moreover, we expand the scope of this system within an agarose gel matrix, which allows us to gain spatiotemporal control over the properties, thereby producing a self-erasable writing system where the chemical fuel acts as the ink.

Spatiotemporal control over self-assembly of supramolecular hydrogels through reaction-diffusion

Supramolecular self-assembly is ubiquitous in living system and is usually controlled to proceed in time and space through sophisticated reaction-diffusion processes, underpinning various vital cellular functions. In this contribution, we demonstrate how spatiotemporal self-assembly of supramolecular hydrogels can be realized through a simple reaction-diffusion-mediated transient transduction of pH signal. In the reaction-diffusion system, a relatively faster diffusion of acid followed by delayed enzymatic production and diffusion of base from the opposite site enables a transient transduction of pH signal in the substrate. By coupling such reaction-diffusion system with pH-sensitive gelators, dynamic supramolecular hydrogels with tunable lifetimes are formed at defined locations. The hydrogel fibers show interesting dynamic growing behaviors under the regulation of transient pH signal, reminiscent of their biological counterpart. We further demonstrate a proof-of-concept application of the developed methodology for dynamic information encoding in a soft substrate. We envision that this work may provide a potent approach to enable transient transduction of various chemical signals for the construction of new colloidal materials with the capability to evolve their structures and functionalities in time and space.

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.

Enzymatic Reaction-Coupled, Cooperative Supramolecular Polymerization

Nature employs sophisticated mechanisms to precisely regulate self-assembly and functions within biological systems, exemplified by the formation of cytoskeletal filaments. Various enzymatic reactions and auxiliary proteins couple with the self-assembly process, meticulously regulating the length and functions of resulting macromolecular structures. In this context, we present a bioinspired, reaction-coupled approach for the controlled supramolecular polymerization in synthetic systems. To achieve this, we employ an enzymatic reaction that interfaces with the adenosine triphosphate (ATP)-templated supramolecular polymerization of naphthalene diimide monomers (NSG). Notably, the enzymatic production of ATP (template) plays a pivotal role in facilitating reaction-controlled, cooperative growth of the NSG monomers. This growth process, in turn, provides positive feedback to the enzymatic production of ATP, creating an ideal reaction-coupled assembly process. The success of this approach is further evident in the living-growth characteristic observed during seeding experiments, marking this method as the pioneering instance where reaction-coupled self-assembly precisely controls the growth kinetics and structural aspects of supramolecular polymers in a predictive manner, akin to biological systems.

Reassembly of the vesicular structure of niosomes after their destruction in a mechanical field

This paper presents, for the first time, evidence for vesicle destruction and payload loss at the stage of purification of niosome dispersions by centrifugation, an important operation in the assembly of vesicular materials. The ability of niosomes of different compositions to reassemble, i.e., to restore the vesicular structure after destruction in the field of centrifugal forces, was demonstrated by dynamic light scattering and fluorescence spectroscopy. The kinetics of reassembly of vesicular structures is determined by the strength of the centrifugal field and the composition of niosomes. In contrast to ternary compositions, where particle size and modality are essentially unchanged after redispersion of the precipitate resulting from centrifugation, niosome dispersions containing anionic dicetyl phosphate includes micron-sized particles after redispersion, which vary in size over a wide range throughout the observation period. The reassembly process is complicated by the presence of charge on the surface of the niosomes. Elastic niosomes - ethosomes have been synthesised which, due to the high deformability of the shells, are less susceptible to destruction in the centrifugal field and retain the contents of the aqueous core. Using the "energy landscape" approximation, it is shown that vesicular structures assembled during hydration and reassembled after their centrifugation occupy different positions in the energetic pathway of their preparation. The results obtained should also be taken into account when determining the entrapment efficiency, since this procedure uses centrifugation to separate the load. It is important to note that the physical stability of niosomes, which is usually considered in terms of the functional activity of particles, is manifest and should be considered at the material preparation stage.

Building a stable and robust anti-interference DNA dissipation system by eliminating the accumulation of systemic specified errors

BACKGROUND

The emergence of DNA nanotechnology has enabled the systematic design of diverse bionic dissipative behaviors under the precise control of nucleic acid nanodevices. Nevertheless, when compared to the dissipation observed in robust living systems, it is highly desirable to enhance the anti-interference for artificial DNA dissipation to withstand perturbations and facilitate repairs within the complex biological environments.

RESULTS

In this study, we introduce strategically designed "trash cans" to facilitate kinetic control over interferences, transforming the stochastic binding of individual components within a homogeneous solution into a competitive binding process. This approach effectively eliminates incorrect binding and the accumulation of systemic interferences while ensuring a consistent pattern of energy fluctuation from response to silence. Remarkably, even in the presence of numerous interferences differing by only one base, we successfully achieve complete system reset through multiple cycles, effectively restoring the energy level to a minimum.

SIGNIFICANCE

The system was able to operate stably without any adverse effect under conditions of irregular interference, high-abundance interference, and even multiplex interferences including DNA and RNA crosstalk. This work not only provides an effective paradigm for constructing robust DNA dissipation systems but also greatly broadens the potential of DNA dissipation for applications in high-precision molecular recognition and complex biological reaction networks.

Observing the Dynamics of an Electrochemically Driven Active Material with Liquid Electron Microscopy

Electrochemical liquid electron microscopy has revolutionized our understanding of nanomaterial dynamics by allowing for direct observation of their electrochemical production. This technique, primarily applied to inorganic materials, is now being used to explore the self-assembly dynamics of active molecular materials. Our study examines these dynamics across various scales, from the nanoscale behavior of individual fibers to the micrometer-scale hierarchical evolution of fiber clusters. To isolate the influences of the electron beam and electrical potential on material behavior, we conducted thorough beam-sample interaction analyses. Our findings reveal that the dynamics of these active materials at the nanoscale are shaped by their proximity to the electrode and the applied electrical current. By integrating electron microscopy observations with reaction-diffusion simulations, we uncover that local structures and their formation history play a crucial role in determining assembly rates. This suggests that the emergence of nonequilibrium structures can locally accelerate further structural development, offering insights into the behavior of active materials under electrochemical conditions.

Reversibly Tuning the Viscosity of Peptide-Based Solutions Using Visible Light

Light can be used to design stimuli-responsive systems. We induce transient changes in the assembly of a low molecular weight gelator solution using a merocyanine photoacid. Through our approach, reversible viscosity changes can be achieved via irradiation, delivering systems where flow can be controlled non-invasively on demand.

Soft Materials with Time-Programmed Changes in Physical Properties through Lyotropic Phase Transitions Induced by pH-Changing Reactions

We present the development of time-programmable functional soft materials. The materials undergo reversible phase transitions between lyotropic phases with different topologies and symmetries, which in turn have very different physical properties: viscosity, diffusion, and optical transparency. Here, this behavior is achieved by combining pH-responsive lyotropic phases made from the lipid monoolein doped with 10% oleic acid, with chemical reactions that have well-defined controllable kinetics: autocatalytic urea-urease and methyl formate hydrolysis, which increase and decrease pH, respectively. In this case, we use small-angle X-ray scattering (SAXS) and optical imaging to show temporally controlled transitions between the cloudy hexagonal phase, which is a two-dimensional (2D) array of cylindrical inverse micelles, and the transparent, highly viscous three-dimensional (3D) bicontinuous cubic phases. By combining these into a single reaction mixture where the pH increases and then decreases again, we can induce a sequential transformation cycle from hexagonal to cubic and back to hexagonal over several hours. The sample therefore changes from cloudy to transparent and back again as a proof-of-concept demonstration for a wider range of soft materials with time-programmable changes in physical properties.

Understanding multicomponent low molecular weight gels from gelators to networks

BACKGROUND

The construction of gels from low molecular weight gelators (LMWG) has been extensively studied in the fields of bio-nanotechnology and other fields. However, the understanding gaps still prevent the prediction of LMWG from the full design of those gel systems. Gels with multicomponent become even more complicated because of the multiple interference effects coexist in the composite gel systems.

AIM OF REVIEW

This review emphasizes systems view on the understanding of multicomponent low molecular weight gels (MLMWGs), and summarizes recent progress on the construction of desired networks of MLMWGs, including self-sorting and co-assembly, as well as the challenges and approaches to understanding MLMWGs, with the hope that the opportunities from natural products and peptides can speed up the understanding process and close the gaps between the design and prediction of structures.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review is focused on three key concepts. Firstly, understanding the complicated multicomponent gels systems requires a systems perspective on MLMWGs. Secondly, several protocols can be applied to control self-sorting and co-assembly behaviors in those multicomponent gels system, including the certain complementary structures, chirality inducing and dynamic control. Thirdly, the discussion is anchored in challenges and strategies of understanding MLMWGs, and some examples are provided for the understanding of multicomponent gels constructed from small natural products and subtle designed short peptides.

Electrically Powered Dissipative Hydrogel Networks Reveal Transient Stiffness Properties for Out-of-Equilibrium Operations

Living systems use dissipative processes to enable precise spatiotemporal control over various functions, including the transient modulation of the stiffness of tissues, which, however, is challenging to achieve in soft materials. Here, we report a new platform to program hydrogel films with tunable, time-dependent mechanical properties under out-of-equilibrium conditions, powered by electricity. We show that the lifetime of the transient network of a surface-confined hydrogel film can be effectively controlled by programming the generation of an electrochemically oxidized mediator in the presence of a chemical or photoreducing agent in solution. It is, therefore, electrically possible to direct the transient stiffening or softening of the hydrogel film, enabling high modularity of the material functions with precise spatiotemporal control. Temporally controlled operations of the hydrogel films are demonstrated for the on-demand, dose-controlled release of multiple model protein payloads from electrode arrays using the present electrically powered dissipative system. This demonstration of electrically driven transient modulation of the stiffness properties of hydrogel films represents an important step toward the engineering of dissipative materials for developing future biomedical applications that can harness the temporal, adaptive properties of this new class of materials.

Novel Acid-Driven Bioinspired Self-Resettable Bilayer Hydrogel Actuator Mimicking Natural Muscles

Soft robots have great potential applications in manufacturing, disaster rescue, medical treatment, etc. Artificial muscle is one of the most important components of a soft robot. In previous years, hydrogel actuators that can be controllably deformed by the stimuli of external signals have been developed as good candidates for muscle-like materials. In this article, we successfully prepared a chemical fuel-driven self-resettable bilayer hydrogel actuator mimicking natural muscles with the aid of a new negative feedback reaction network. The actuator can temporarily deform upon the addition of H+ (chemical fuel). Subsequently, H+ accelerated the reaction between BrO3- and Fe(CN)64-, which consume H+. It resulted in the spontaneous recovery of the pH as well as the shape of the actuator. Such an actuator exhibits a great similarity with natural muscles in actuation mechanisms and automaticity in the manipulation compared to the widely reported stimuli-responsive hydrogel actuators. This illustrates that fuel-driven self-resettable hydrogel is a promising dynamic material for mimicking the functions of living creatures.

Temporal (Dis)Assembly of Peptide Nanostructures Dictated by Native Multistep Catalytic Transformations

Emergence of complex catalytic machinery via simple building blocks under non-equilibrium conditions can contribute toward the system level understanding of the extant biocatalytic reaction network that fuels metabolism. Herein, we report temporal (dis)assembly of peptide nanostructures in presence of a cofactor dictated by native multistep cascade transformations. The short peptide can form a dynamic covalent bond with the thermodynamically activated substrate and recruit cofactor hemin to access non-equilibrium catalytic nanostructures (positive feedback). The neighboring imidazole and hemin moieties in the assembled state rapidly converted the substrate to product(s) via a two-step cascade reaction (hydrolase-peroxidase like) that subsequently triggered the disassembly of the catalytic nanostructures (negative feedback). The feedback coupled reaction cycle involving intrinsic catalytic prowess of short peptides to realize the advanced trait of two-stage cascade degradation of a thermodynamically activated substrate foreshadows the complex non-equilibrium protometabolic networks that might have preceded the chemical emergence of life.

Pumping Small Molecules Selectively through an Energy-Assisted Assembling Process at Nonequilibrium States

In living organisms, precise control over the spatial and temporal distribution of molecules, including pheromones, is crucial. This level of control is equally important for the development of artificial active materials. In this study, we successfully controlled the distribution of small molecules in the system at nonequilibrium states by actively transporting them, even against the apparent concentration gradient, with high selectivity. As a demonstration, in the aqueous solution of acid orange (AO7) and TMC10COOH, we found that AO7 molecules can coassemble with transient anhydride (TMC10CO)2O to form larger assemblies in the presence of chemical fuel 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide hydrochloride (EDC). This led to a decrease in local free AO7 concentration and caused AO7 molecules from other locations in the solution to move toward the assemblies. Consequently, AO7 accumulates at the location where EDC was injected. By continuously injecting EDC, we could maintain a stable high value of the apparent AO7 concentration at the injection point. We also observed that this process which operated at nonequilibrium states exhibited high selectivity.

Thermo-Chemical Cues-Mediated Strategy to Control Peptide Self-Assembly and Charge Transfer Complexation

Peptide amphiphiles (PAs) are known for their remarkable ability to undergo molecular self-assembly, a process that is highly responsive to the local microenvironment. Herein, we design a pyrene tethered peptide amphiphile Py-VFFAKK, 1 that exhibits pathway-driven self-assembly from metastable nanoparticles to kinetically controlled nanofibers and thermodynamically stable twisted bundles upon modulations in pH, temperature, and chemical cues. The presence of the pyrene moiety ensures donation of the electron to an electron acceptor, namely, 7,7,8,8-tetracyanoquinodimethane (TCNQ), to form a supramolecular charge transfer complex in aqueous solution that was studied in detail with microscopic and spectroscopic techniques. Excitation of the donor species in its excimer state facilitates electron donation to the acceptor moiety, paving away a long-lived charge-separated state that persists for over a nanosecond, as ascertained through transient absorption spectroscopy. Finally, the self-assembled charge transfer complex is explored toward antimicrobial properties with Escherichia coli while maintaining biocompatibility toward L929 mice fibroblast cells.

Systems theory, thermodynamics and life: Integrated thinking across ecology, organization and biological evolution

In this paper we explore the relevance and integration of system theory and thermodynamics in terms of the Earth system. It is proposed that together, these fields explain the evolution, organization, functionality and directionality of life on Earth. We begin by summarizing historical and current thinking on the definition of life itself. We then investigate the evidence for a single unit of life. Given that any definition of life and its levels of organization are intertwined, we explore how the Earth system is structured and functions from an energetic perspective, by outlining relevant thermodynamic theory relating to molecular, metabolic, cellular, individual, population, species, ecosystem and biome organization. We next investigate the fundamental relationships between systems theory and thermodynamics in terms of the Earth system, examining the key characteristics of self-assembly, self-organization (including autonomy), emergence, non-linearity, feedback and sub-optimality. Finally, we examine the relevance of systems theory and thermodynamics with reference to two specific aspects: the tempo and directionality of evolution and the directional and predictable process of ecological succession. We discuss the importance of the entropic drive in understanding altruism, multicellularity, mutualistic and antagonistic relationships and how maximum entropy production theory may explain patterns thought to evidence the intermediate disturbance hypothesis.

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.

Chemical Information Processing by a Responsive Chemical System

Nature has an extraordinary capacity to precisely regulate the chemical reactivity in a highly complex mixture of molecules that is present in the cell. External stimuli lead to transient up- and downregulation of chemical reactions and provide a means for a cell to process information arriving from the environment. The development of synthetic chemical systems with life-like properties requires strategies that allow likewise control over chemical reactivity in a complex environment. Here, we show a synthetic system that mimics the initial steps that take place when a natural signal transduction pathway is activated. Monophosphate nucleosides act as chemical triggers for the self-assembly of nanoreactors that upregulate chemical reactions between reagents present at low micromolar concentrations. Different nucleotides template different assemblies and hence activate different pathways, thus establishing a distinct connection between input and output molecules. Trigger-induced upregulation of chemical reactivity occurs for only a limited amount of time because the chemical triggers are gradually removed from the system by enzymes. It is shown that the same system transiently produces different output molecules depending on the chemical input that is provided.

CryoEM reveals the complex self-assembly of a chemically driven disulfide hydrogel

Inspired by the adaptability of biological materials, a variety of synthetic, chemically driven self-assembly processes have been developed that result in the transient formation of supramolecular structures. These structures form through two simultaneous reactions, forward and backward, which generate and consume a molecule that undergoes self-assembly. The dynamics of these assembly processes have been shown to differ from conventional thermodynamically stable molecular assemblies. However, the evolution of nanoscale morphologies in chemically driven self-assembly and how they compare to conventional assemblies has not been resolved. Here, we use a chemically driven redox system to separately carry out the forward and backward reactions. We analyze the forward and backward reactions both sequentially and synchronously with time-resolved cryogenic transmission electron microscopy (cryoEM). Quantitative image analysis shows that the synchronous process is more complex and heterogeneous than the sequential process. Our key finding is that a thermodynamically unstable stacked nanorod phase, briefly observed in the backward reaction, is sustained for ∼6 hours in the synchronous process. Kinetic Monte Carlo modeling show that the synchronous process is driven by multiple cycles of assembly and disassembly. The collective data suggest that chemically driven self-assembly can create sustained morphologies not seen in thermodynamically stable assemblies by kinetically stabilizing transient intermediates. This finding provides plausible design principles to develop and optimize supramolecular materials with novel properties.

Dynamic monitoring of an enzymatically driven dissipative toehold-mediated strand displacement reaction

A general strategy to program self-resettable and dissipative toehold-mediated strand displacement reactions was proposed, using DNA strands as the fuel and lambda exonuclease as the fuel-consuming unit. This non-equilibrium system is reversible and temporally controllable. Furthermore, it can be well integrated into a DNA network to temporally control its cascade reaction or dynamic behaviour.

Organelle-Mediated Dissipative Self-Assembly of Peptides in Living Cells

Implementing dissipative assembly in living systems is meaningful for creation of living materials or even artificial life. However, intracellular dissipative assembly remains scarce and is significantly impeded by the challenges lying in precisely operating chemical reaction cycles under complex physiological conditions. Here, we develop organelle-mediated dissipative self-assembly of peptides in living cells fueled by GSH, via the design of a mitochondrion-targeting and redox-responsive hexapeptide. While the hexapeptide undergoes efficient redox-responsive self-assembly, the addition of GSH into the peptide solution in the presence of mitochondrion-biomimetic liposomes containing hydrogen peroxide allows for transient assembly of peptides. Internalization of the peptide by LPS-stimulated macrophages leads to the self-assembly of the peptide driven by GSH reduction and the association of the peptide assemblies with mitochondria. The association facilitates reversible oxidation of the reduced peptide by mitochondrion-residing ROS and thereby dissociates the peptide from mitochondria to re-enter the cytoplasm for GSH reduction. The metastable peptide-mitochondrion complexes prevent the thermodynamically equilibrated self-assembly, thus establishing dissipative assembly of peptides in stimulated macrophages. The entire dissipative self-assembling process allows for elimination of elevated ROS and decrease of pro-inflammatory cytokine expression. Creating dissipative self-assembling systems assisted by internal structures provides new avenues for the development of living materials or medical agents in the future.

Solvent-Induced Transient Self-Assembly of Peptide Gels: Gelator-Solvent Reactions and Material Properties Correlation

Herein, we introduce a new methodology for designing transient organogels that offers tunability of the mechanical properties simply by matching the protective groups of the precursor to that of the solvent. We developed solvent-induced transient materials in which the solvent chemically participates in a set of reactions and actively supports the assembly event. The activation of a single precursor by an acid (accelerator) yields the formation of two distinct gelators and induces gelation. The interconversion cycle is supplied by the secondary solvent (originating from hydrolysis of the primary solvent by the accelerator), which then progressively solubilizes the gel network. We show that this gelation method offers a direct correlation between the mechanical and transient properties by modifying the chemical structure of the precursors and the presence of an accelerator in the system. Such a method paves the way for the design of self-abolishing and mechanically tunable materials for targeted purposes. The biocompatibility and versatility of amino acid-based gelators can offer a wide range of biomaterials for applications requiring a controllable and definite lifetime such as drug delivery platforms exhibiting a burst release or self-abolishing cell culture substrates.

Artificial cells with viscoadaptive behavior based on hydrogel-loaded giant unilamellar vesicles

Viscoadaptation is an essential process in natural cells, where supramolecular interactions between cytosolic components drive adaptation of the cellular mechanical features to regulate metabolic function. This important relationship between mechanical properties and function has until now been underexplored in artificial cell research. Here, we have created an artificial cell platform that exploits internal supramolecular interactions to display viscoadaptive behavior. As supramolecular material to mimic the cytosolic component of these artificial cells, we employed a pH-switchable hydrogelator based on poly(ethylene glycol) coupled to ureido-pyrimidinone units. The hydrogelator was membranized in its sol state in giant unilamellar lipid vesicles to include a cell-membrane mimetic component. The resulting hydrogelator-loaded giant unilamellar vesicles (designated as HL-GUVs) displayed reversible pH-switchable sol-gel behavior through multiple cycles. Furthermore, incorporation of the regulatory enzyme urease enabled us to increase the cytosolic pH upon conversion of its substrate urea. The system was able to switch between a high viscosity (at neutral pH) and a low viscosity (at basic pH) state upon addition of substrate. Finally, viscoadaptation was achieved via the incorporation of a second enzyme of which the activity was governed by the viscosity of the artificial cell. This work represents a new approach to install functional self-regulation in artificial cells, and opens new possibilities for the creation of complex artificial cells that mimic the structural and functional interplay found in biological systems.

Photoswitchable coordination cages

Stimuli-responsive behaviour is key to the design of smart materials, surfaces, nano-systems and effector molecules, allowing their application as switchable catalysts, molecular transporters, bioimaging probes or caged drugs. Supramolecular chemistry has embraced the widespread integration of photoswitches because of their precise spatiotemporal addressability and waste-free nature. In the vibrant area of discrete metal-mediated self-assembly, however, photoswitches are still rarely employed. Only recently has it been shown that embedding photoswitches into the organic backbones of coordination cages enables control of their host and material properties and thus unlocks the hitherto unexploited dynamic adaptivity of such systems. Here we discuss four cases where triggering ligand-integrated photoswitches leads to (1) control over disassembly/reassembly, (2) bi-stable switching between defined states, (3) interplay with thermal processes in metastable systems and (4) light-fuelled dissipative self-assembly. We highlight first clues concerning the relationship between fundamental photophysics and dynamic assembly equilibria and propose directions for future development.

Nonequilibrium Colloids: Temperature-Induced Bouquet Formation of Flower-like Micelles as a Time-Domain-Shifting Macromolecular Heat Alert

Climate change requires enhanced autonomous temperature monitoring during logistics/transport. A cheap approach comprises the use of temperature-sensitive copolymers that undergo temperature-induced irreversible coagulation. The synthesis/characterization of pentablock copolymers (PBCP) starting from poloxamer PEO130-b-PPO44-b-PEO130 (poly(ethylene oxide)130-b-poly(propylene oxide)44-b-poly(ethylene oxide)130) and adding two terminal qPDMAEMA85 (quaternized poly[(2-dimethylamino)ethyl methacrylate]85) blocks is presented. Mixing of PBCP solutions with hexacyanoferrate(III)/ferricyanide solutions leads to a reduction of the decane/water interfacial tension accompanied by a co/self-assembly toward flower-like micelles in cold water because of the formation of an insoluble/hydrophobic qPDMAEMA/ferricyanide complex. In cold water, the PEO/PPO blocks provide colloidal stability over months. In hot water, the temperature-responsive PPO block is dehydrated, leading to a pronounced temperature dependence of the oil-water interfacial tension. In solution, the sticky PPO segments exposed at the micellar corona cause a colloidal clustering above a certain threshold temperature, which follows Smoluchowski-type kinetics. This coagulation remains for months even after cooling, indicating the presence of a kinetically trapped nonequilibrium state for at least one of the observed micellar structures. Therefore, the system memorizes a previous suffering of heat. This phenomenon is linked to an exchange of qPDMAEMA-blocks bridging the micellar cores after PPO-induced clustering. The addition of ferrous ions hampers the exchange, leading to the reversible coagulation of Prussian blue loaded micelles. Hence, the Fe2+ addition causes a shift from history monitoring to the sensing of the present temperature. Presumably, the system can be adapted for different temperatures in order to monitor transport and storage in a simple way. Hence, these polymeric "flowers" could contribute to preventing waste and sustaining the quality of goods (e.g., food) by temperature-induced bouquet formation, where an irreversible exchange of "tentacles" between the flowers stabilizes the bouquet at other temperatures as well.

Self-assembly of cyclic peptide monolayers by hydrophobic supramolecular hinges

Supramolecular polymerisation of two-dimensional (2D) materials requires monomers with non-covalent binding motifs that can control the directionality of both dimensions of growth. A tug of war between these propagation forces can bias polymerisation in either direction, ultimately determining the structure and properties of the final 2D ensemble. Deconvolution of the assembly dynamics of 2D supramolecular systems has been widely overlooked, making monomer design largely empirical. It is thus key to define new design principles for suitable monomers that allow the control of the direction and the dynamics of two-dimensional self-assembled architectures. Here, we investigate the sequential assembly mechanism of new monolayer architectures of cyclic peptide nanotubes by computational simulations and synthesised peptide sequences with selected mutations. Rationally designed cyclic peptide scaffolds are shown to undergo hierarchical self-assembly and afford monolayers of supramolecular nanotubes. The particular geometry, the rigidity and the planar conformation of cyclic peptides of alternating chirality allow the orthogonal orientation of hydrophobic domains that define lateral supramolecular contacts, and ultimately direct the propagation of the monolayers of peptide nanotubes. A flexible 'tryptophan hinge' at the hydrophobic interface was found to allow lateral dynamic interactions between cyclic peptides and thus maintain the stability of the tubular monolayer structure. These results unfold the potential of cyclic peptide scaffolds for the rational design of supramolecular polymerisation processes and hierarchical self-assembly across the different dimensions of space.

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.

Spontaneous and Selective Peptide Elongation in Water Driven by Aminoacyl Phosphate Esters and Phase Changes

Nature chose phosphates to activate amino acids, where reactive intermediates and complex machinery drive the construction of polyamides. Outside of biology, the pathways and mechanisms that allow spontaneous and selective peptide elongation in aqueous abiotic systems remain unclear. Herein we work to uncover those pathways by following the systems chemistry of aminoacyl phosphate esters, synthetic counterparts of aminoacyl adenylates. The phosphate esters act as solubility tags, making hydrophobic amino acids and their oligomers soluble in water and enabling selective elongation and different pathways to emerge. Thus, oligomers up to dodecamers were synthesized in one flask and on the minute time scale, where consecutive additions activated autonomous phase changes. Depending on the pathway, the resulting phases initially carry nonpolar peptides and amphiphilic oligomers containing phosphate esters. During elongation and phosphate release, shorter oligomers dominate in solution, while the aggregated phase favors the presence of longer oligomers due to their self-assembly propensity. Furthermore we demonstrated that the solution phases can be isolated and act as a new environment for continuous elongation, by adding various phosphate esters. These findings suggest that the systems chemistry of aminoacyl phosphate esters can activate a selection mechanism for peptide bond formation by merging aqueous synthesis and self-assembly.

Bio-Inspired Far-From-Equilibrium Hydrogels: Design Principles and Applications

Inspired from dynamic living systems that operate under out-of-equilibrium conditions in biology, developing supramolecular hydrogels with self-regulating and autonomously dynamic properties to further advance adaptive hydrogels with life-like behavior is important. This review presents recent progress of bio-inspired supramolecular hydrogels out-of-equilibrium. The principle of out-of-equilibrium self-assembly for creating bio-inspired hydrogels is discussed. Various design strategies have been identified, such as chemical-driven reaction cycles with feedback control and physically oscillatory systems. These strategies can be coupled with hydrogels to achieve temporal and spatial control over structural and mechanical properties as well as programmable lifetime. These studies open up huge opportunities for potential applications, such as fluidic guidance, information storage, drug delivery, actuators and more. Finally, we address the challenges ahead of us in the coming years, and future possibilities and prospects are identified.

Dynamic supramolecular hydrogels mediated by chemical reactions

Supramolecular self-assembly in a biological system is usually dominated by sophisticated metabolic processes (chemical reactions) such as catalysis of enzymes and consumption of high energy chemicals, leading to groups of biomolecules with unique dynamics and functions in an aqueous environment. In recent years, increasing efforts have been made to couple chemical reactions to molecular self-assembly, with the aim of creating supramolecular materials with lifelike properties and functions. In this feature article, after summarising the work of chemical reaction mediated supramolecular hydrogels, we first focus on a typical example where dynamic self-assembly of molecular hydrogels is activated by in situ formation of a hydrazone bond in water. We discuss how the formation of the hydrazone-based supramolecular hydrogels can be controlled in time and space. After that, we describe transient assembly of supramolecular hydrogels powered by out-of-equilibrium chemical reaction networks regulated by chemical fuels, which show unique properties such as finite lifetime, dynamic structures, and regenerative capabilities. Finally, we provide a perspective on the future investigations that need to be done urgently, which range from fundamental research to real-life applications of dynamic supramolecular hydrogels.

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.

Metallosupramolecular polymers: current status and future prospects

Metallo-supramolecular polymers have gained increasing attention and witnessed continuous development as a vibrant new research interest in the domain of soft materials. These nonconventional polymers have found widespread application in materials and biology owing to their well-defined and diversified topologies and the distinct dynamic nature of the metallosupramolecular interactions against various stimuli. Because of the intriguing redox, photonic, electronic, and magnetic properties, these stimuli-responsive supramolecular structures have attracted considerable interest for optoelectronic device fabrication. However, it still remains challenging to develop stimuli responsive systems with offbeat applications. Furthermore, achieving spatiotemporal control remains elusive with thermoresponsive and sono-responsive metallosupramolecular polymers, which encounter the disadvantage of poor precision control. Additionally, controlling the morphology of these soft materials on the mesoscale, both in solution and on substrates, has many challenges. In this review, we discuss the recent developments and future directions for the construction of stimuli responsive metallosupramolecular systems targeting practical applications. Furthermore, we discuss the synthetic methodologies that have been used to regulate the mesoscale morphology of these materials, such as coordination modulation and pseudomorphic replication. Finally, we briefly cover the burgeoning field of programmed synthesis of metallosupramolecular polymers, emphasizing techniques, such as living polymerization and chemical fuel-driven transiently active systems, which we believe will be the major research directions in the future.

Mechanistic studies of isomeric [2]rotaxanes consisting of two different tetrathiafulvalene units reveal that the movement of cyclobis(paraquat-p-phenylene) can be controlled

Controlling the movement in artificial molecular machines is a key challenge that needs to be solved before their full potential can be harnessed. In this study, two isomeric tri-stable [2]rotaxanes 1·4PF6 and 2·4PF6 incorporating both a tetrathiafulvalene (TTF) and a monopyrrolotetrathiafulvalene (MPTTF) unit in the dumbbell component have been synthesised to measure the energy barriers when the tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) ring moves across either a TTF2+ or an MPTTF2+ dication. By strategically exchanging one of the thiomethyl barriers on either the TTF unit or the MPTTF unit with the bulkier thioethyl group, the movement of the CBPQT4+ ring in 14+ and 24+ can be controlled to take place in only one direction upon tetra-oxidation. Cyclic voltammetry and 1H NMR spectroscopy were used to investigate the switching mechanism and it was found that upon tetra-oxidation of 14+ and 24+, the CBPQT4+ ring moves first to a position where it is located between the TTF2+ and MPTTF2+ dications producing high-energy co-conformations which slowly interconvert into thermodynamically more stable co-conformations. The kinetics of the movement occurring in the tetra-oxidised [2]rotaxanes 18+ and 28+ were studied at different temperatures allowing the free energy of the transition state, when CBPQT4+ moves across TTF2+ (21.5 kcal mol-1) and MPTTF2+ (20.3 kcal mol-1) at 298 K, to be determined. These results demonstrate for the first time that the combination of a TTF and an MPTTF unit can be used to induce directional movement of the CBPQT4+ ring in molecular machines with a 90% efficiency.