Fuel-Controlled Reassembly of Metal-Organic Architectures

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.

Non-Equilibrium Assembly of Atomically-Precise Copper Nanoclusters

Accurate structure control in dissipative assemblies (DSAs) is vital for precise biological functions. However, accuracy and functionality of artificial DSAs are far from this objective. Herein, a novel approach is introduced by harnessing complex chemical reaction networks rooted in coordination chemistry to create atomically-precise copper nanoclusters (CuNCs), specifically Cu11(µ9-Cl)(µ3-Cl)3L6Cl (L = 4-methyl-piperazine-1-carbodithioate). Cu(I)-ligand ratio change and dynamic Cu(I)-Cu(I) metallophilic/coordination interactions enable the reorganization of CuNCs into metastable CuL2, finally converting into equilibrium [CuL·Y]Cl (Y = MeCN/H2O) via Cu(I) oxidation/reorganization and ligand exchange process. Upon adding ascorbic acid (AA), the system goes further dissipative cycles. It is observed that the encapsulated/bridging halide ions exert subtle influence on the optical properties of CuNCs and topological changes of polymeric networks when integrating CuNCs as crosslink sites. CuNCs duration/switch period could be controlled by varying the ions, AA concentration, O2 pressure and pH. Cu(I)-Cu(I) metallophilic and coordination interactions provide a versatile toolbox for designing delicate life-like materials, paving the way for DSAs with precise structures and functionalities. Furthermore, CuNCs can be employed as modular units within polymers for materials mechanics or functionalization studies.

Dissipative sequential catalysis via six-component machinery

Triphenyl phosphane (TPP) and an epoxide as a fuel system transiently transformed a non-catalytic six-component turnstile into a four-component catalytic rotor releasing N-methyl pyrrolidine and a copper(I) complex. The two latter compounds acted synergistically as catalysts to perform first a Michael addition and then a 5-exo-dig cyclization, giving rise to dissipative sequential catalysis.

Chiral Truxene-Based Self-Assembled Cages: Triple Interlocking and Supramolecular Chirogenesis

Incorporating chiral elements in host-guest systems currently attracts much attention because of the major impact such structures may have in a wide range of applications, from pharmaceuticals to materials science and beyond. Moreover, the development of multi-responsive and -functional systems is highly desirable since they offer numerous benefits. In this context, we describe herein the construction of a metal-driven self-assembled cage that associates a chiral truxene-based ligand and a bis-ruthenium complex. The maximum separation between both facing chiral units in the assembly is fixed by the intermetallic distance within the lateral bis-ruthenium complex (8.4 Å). The resulting chiral cavity was shown to encapsulate polyaromatic guest molecules, but also to afford a chiral triply interlocked [2]catenane structure. The formation of the latter occurs at high concentration, while its disassembly could be achieved by the addition of a planar achiral molecule. Interestingly the planar achiral molecule exhibits induced circular dichroism signature when trapped within the chiral cavity, thus demonstrating the ability of the cage to induce supramolecular chirogenesis.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

Orthogonal Initiation of Molecular Motion Devices by Two Chemical Fuels

Herein, we demonstrate the selective dissipative and orthogonal actuation of two distinct molecular devices controlled by alternate fuel use. When the multicomponent ensemble of [2]rotaxane 1 and turnstile [Cu(2)(3)]+ was charged with AgBF4 as chemical fuel (Fuel 1) together with NEt3/PhCH2Br (cofuels), the transiently formed [Ag(1)]+ showed a stochastic shuttling of the silver macrocycle between two degenerate triazole stations on the thread (k298 = 1.2 × 105 s-1), whereas [Cu(2)(3)]+ was unperturbed. Instead, treatment of the mixture with PPh3 as an alternative fuel (Fuel 2) in the presence of oxidant 4 (cofuel) generated the complex [Cu(3)(PPh3)2]+ and transient thermal motion in rotor 2 (k298 = 4.9 × 104 s-1), whereas rotaxane 1 stayed dormant. Thus, two distinct chemical fuels selectively and orthogonally activated two distinct transient motion devices from a multicomponent mixture. In total, four interference-free dissipative cycles were demonstrated by using alternating fuel additions.

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.

Chemically Fueled Dissipative Cross-Linking of Protein Hydrogels Mediated by Protein Unfolding

Cells assemble dynamic protein-based nanostructures far from equilibrium, such as microtubules, in a process referred to as dissipative assembly. Synthetic analogues have utilized chemical fuels and reaction networks to form transient hydrogels and molecular assemblies from small molecule or synthetic polymer building blocks. Here, we demonstrate dissipative cross-linking of transient protein hydrogels using a redox cycle, which exhibit protein unfolding-dependent lifetimes and mechanical properties. Fast oxidation of cysteine groups on bovine serum albumin by hydrogen peroxide, the chemical fuel, formed transient hydrogels with disulfide bond cross-links that degraded over hours by a slow reductive back reaction. Interestingly, despite increased cross-linking, the hydrogel lifetime decreased as a function of increasing denaturant concentration. Experiments showed that the solvent-accessible cysteine concentration increased with increasing denaturant concentration due to unfolding of secondary structures. The increased cysteine concentration consumed more fuel, which led to less direction oxidation of the reducing agent and affected a shorter hydrogel lifetime. Increased hydrogel stiffness, disulfide cross-linking density, and decreased oxidation of redox-sensitive fluorescent probes at a high denaturant concentration provided evidence supporting the unveiling of additional cysteine cross-linking sites and more rapid consumption of hydrogen peroxide at higher denaturant concentrations. Taken together, the results indicate that the protein secondary structure mediated the transient hydrogel lifetime and mechanical properties by mediating the redox reactions, a feature unique to biomacromolecules that exhibit a higher order structure. While prior works have focused on the effects of the fuel concentration on dissipative assembly of non-biological molecules, this work demonstrates that the protein structure, even in nearly fully denatured proteins, can exert similar control over reaction kinetics, lifetime, and resulting mechanical properties of transient hydrogels.

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.

Transformation networks of metal-organic cages controlled by chemical stimuli

The flexibility of biomolecules enables them to adapt and transform as a result of signals received from the external environment, expressing different functions in different contexts. In similar fashion, coordination cages can undergo stimuli-triggered transformations owing to the dynamic nature of the metal-ligand bonds that hold them together. Different types of stimuli can trigger dynamic reconfiguration of these metal-organic assemblies, to switch on or off desired functionalities. Such adaptable systems are of interest for applications in switchable catalysis, selective molecular recognition or as transformable materials. This review highlights recent advances in the transformation of cages using chemical stimuli, providing a catalogue of reported strategies to transform cages and thus allow the creation of new architectures. Firstly we focus on strategies for transformation through the introduction of new cage components, which trigger reconstitution of the initial set of components. Secondly we summarize conversions triggered by external stimuli such as guests, concentration, solvent or pH, highlighting the adaptation processes that coordination cages can undergo. Finally, systems capable of responding to multiple stimuli are described. Such systems constitute composite chemical networks with the potential for more complex behaviour. We aim to offer new perspectives on how to design transformation networks, in order to shed light on signal-driven transformation processes that lead to the preparation of new functional metal-organic architectures.

Dissipative Formation of Covalent Basket Cages

Living systems use chemical fuels to transiently assemble functional structures. As a step toward constructing abiotic mimics of such structures, we herein describe dissipative formation of covalent basket cage CBC 5 by reversible imine condensation of cup‐shaped aldehyde 2 (i.e., basket) with trivalent aromatic amine 4. This nanosized [4+4] cage (V=5 nm³, M_w=6150 Da) has shape of a truncated tetrahedron with four baskets at its vertices and four aromatic amines forming the faces. Importantly, tris‐aldehyde basket 2 and aliphatic tris‐amine 7 undergo condensation to give small [1+1] cage 6. The imine metathesis of 6 and aromatic tris‐amine 4 into CBC 5 was optimized to bias the equilibrium favouring 6. Addition of tribromoacetic acid (TBA) as a chemical fuel perturbs this equilibrium to result in the transient formation of CBC 5, with subsequent consumption of TBA via decarboxylation driving the system back to the starting state.

Dissipative Acid-Fueled Reprogrammable Supramolecular Materials

Smart materials reversibly changing properties in response to a stimuli are promising for a broad array of applications. In this article, we report the use of trichloroacetic acid (TCA) as fuel to create new types of time-controlled materials switching reversibly from a gel to a solution (gel-sol-gel cycle). Applying various neutral amines as organogelators, TCA addition induces amine protonation, switching the system to a solution, while TCA decarboxylation over time enables a return to the initial gel state. Consequently, the newly obtained materials possess interesting time-dependent properties applied in the generation of remoldable objects, as an erasing ink, as chiroptical switches, or for the generation of new types of electrical systems.

Temporal Control of the Host-Guest Properties of a Calix[6]arene Receptor by the Use of a Chemical Fuel

The host–guest interaction of a 1,3,5-trisaminocalix[6]arene receptor with N-methylisoquinolinium trifluoromethanesulfonate (K_ass of 500 ± 30 M^–1 in CD₂Cl₂) can be dissipatively driven by means of 2-cyano-2-(4′-chloro)phenylpropanoic acid used as a convenient chemical fuel. When the fuel is added to a dichloromethane solution containing the above complex, the host is induced to immediately release the guest in the bulk solution. Consumption of the fuel allows the guest to be re-uptaken by the host. The operation can be satisfactorily reiterated with four subsequent additions of fuel, producing four successive release–reuptake cycles. The percentage of the guest temporarily released in the bulk solution by the host and the time required for the reuptake process can be finely regulated by varying the quantities of added fuel.

Cooperativity of steric bulk and H-bonding in coordination sphere engineering: heteroleptic PdII cages and bowls by design

Recently developed self-assembly strategies allow to rationally reduce the symmetry of metallosupramolecular architectures. In addition, the combination of multiple ligand types without creating compound mixtures has become possible. Among several approaches to realize non-statistical heteroleptic assembly, Coordination Sphere Engineering (CSE) makes use of secondary repulsive or attractive interactions in direct vicinity of the metal nodes. Previously, we used steric congestion to turn dinuclear [Pd₂L₄] cages with fourfold symmetry into [Pd₂L₃X₂] (X = solvent, halide) bowl structures. Here, we introduce a new subtype of this strategy based on balancing hydrogen bonding and repulsive interactions between ligands carrying quinoline (L^Qu) and 1,8-naphthyridine (L^Na) donors to generate trans-[Pd₂L₂] and [Pd₂L₃L′] cages, assisted by templation of encapsulated fullerenes. Combined with steric congestion caused by acridine (L^Ac) donors, we further report the first example of a heteroleptic [Pd₂L₂L′X₂] bowl. Formation, structure and fullerene binding ability of these metallo-supramolecular hosts were studied by NMR, mass spectrometry and single crystal X-ray diffraction.

Coordination Sphere Engineering (CSE) allows non-statistical assembly of heteroleptic supramolecular architectures by fine adjustment of steric and electronic features around square-planar Pd(ii) cations with naphthyridine donors.

Carbodiimide-fueled catalytic reaction cycles to regulate supramolecular processes

Using molecular self-assembly, supramolecular chemists can create Gigadalton-structures with angstrom precision held together by non-covalent interactions. However, despite relying on the same molecular toolbox for self-assembly, these synthetic structures lack the complexity and sophistication of biological assemblies. Those assemblies are non-equilibrium structures that rely on the constant consumption of energy transduced from the hydrolysis of chemical fuels like ATP and GTP, which endows them with dynamic properties, e.g., temporal and spatial control and self-healing ability. Thus, to synthesize life-like materials, we have to find a reaction cycle that converts chemical energy to regulate self-assembly. We and others recently found that this can be done by a reaction cycle that hydrates carbodiimides. This feature article aims to provide an overview of how the energy transduced from carbodiimide hydration can alter the function of molecules and regulate molecular assemblies. The goal is to offer the reader design considerations for carbodiimide-driven reaction cycles to create a desired morphology or function of the assembly and ultimately to push chemically fueled self-assembly further towards the bottom-up synthesis of life.

Fuel Acid Drives Base Catalysis and Supramolecular Cage-to-Device Transformation under Dissipative Conditions

In State-I, a mixture comprising a DABCO-bridged tris(zinc-porphyrin) double decker and a free biped (=slider), catalysis was OFF. Acid addition (TFA or Di-Stefano fuel acid) to State-I liberated DABCO-H⁺ while generating a highly dynamic slider-on-deck device (State-II). The released DABCO-H⁺ acted as a base organocatalyst for a Knoevenagel reaction (catalysis ON). The system was reversed to State-I (catalysis OFF) by reducing the acidity in the system (by adding DBU or via the fuel-derived base).

Dissipative operation of pH-responsive DNA-based nanodevices

We demonstrate here the use of 2-(4-chlorophenyl)-2-cyanopropanoic acid (CPA) and nitroacetic acid (NAA) as convenient chemical fuels to drive the dissipative operation of DNA-based nanodevices. Addition of either of the fuel acids to a water solution initially causes a rapid transient pH decrease, which is then followed by a slower pH increase. We have employed such low-to-high pH cycles to control in a dissipative way the operation of two model DNA-based nanodevices: a DNA nanoswitch undergoing time-programmable open–close–open cycles of motion, and a DNA-based receptor able to release-uptake a DNA cargo strand. The kinetics of the transient operation of both systems can be easily modulated by varying the concentration of the acid fuel added to the solution and both acid fuels show an efficient reversibility which further supports their versatility.

We demonstrate here the use of 2-(4-chlorophenyl)-2-cyanopropanoic acid (CPA) and nitroacetic acid (NAA) as convenient chemical fuels to drive the dissipative operation of DNA-based nanodevices.

Chemically Fueled Three-State Chiroptical Switching Supramolecular Gel with Temporal Control

The recent discovery of temporally controlled gels opens broad perspectives to the field of smart functional materials. However, to obtain fully operative systems, the design of simple and robust gels displaying complex functions is desirable. Herein, we fuel dissipative gelating materials through iterative additions of trichloroacetic acid (TCA). This simple fuel enables to switch over time an acid/base-dependent commercially available amino acid gelator/DBU combination between three distinct states (anionic, cationic, and neutral), while liberating volatile CO₂ and CHCl₃ upon fuel consumption. Of interest, the anionic resting state of the system is obtained through trapping of 1 equiv of CO₂ through the formation of a carbamate. The system is tunable, robust, and resilient over time with over 25 consecutive sol-gel-sol cycles possible without significant loss of properties. Most importantly, because of the chiral nature of the amino acid gelator, the system features chiroptical switching properties moving reversibly between three distinct states as observed by ECD. The described system considerably enhances the potential of smart molecular devices for logic gates or data storage by adding a time dimension based on three states to the gelating materials. It is particularly simple in terms of chemical components involved, but it enables sophisticated functions.

A dissipative pathway for the structural evolution of DNA fibres

Biochemical networks interconnect, grow and evolve to express new properties as different chemical pathways are selected during a continuous cycle of energy consumption and transformation. In contrast, synthetic systems that push away from equilibrium usually return to the same self-assembled state, often generating waste that limits system recyclability and prevents the formation of adaptable networks. Here we show that annealing by slow proton dissipation selects for otherwise inaccessible morphologies of fibres built from DNA and cyanuric acid. Using single-molecule fluorescence microscopy, we observe that proton dissipation influences the growth mechanism of supramolecular polymerization, healing gaps within fibres and converting highly branched, interwoven networks into nanocable superstructures. Just as the growth kinetics of natural fibres determine their structural attributes to modulate function, our system of photoacid-enabled depolymerization and repolymerization selects for healed materials to yield organized, robust fibres. Our method provides a chemical route for error-checking, distinct from thermal annealing, that improves the morphologies and properties of supramolecular materials using out-of-equilibrium systems.

Multitasking with Chemical Fuel: Dissipative Formation of a Pseudorotaxane Rotor from Five Distinct Components

A 3-fold completive self-sorted library of dynamic motifs was integrated into the design of the pseudorotaxane-based rotor [Zn(2·H⁺)(3)(4)]²⁺ operating at k₂₉₈ = 15.4 kHz. The rotational motion in the five-component device is based on association/dissociation of the pyridyl head of the pseudorotaxane rotator arm between two zinc(II) porphyrin stations. Addition of TFA or 2-cyano-2-phenylpropanoic acid as a chemical fuel to a zinc release system and the loose rotor components 2-4 enabled the liberated zinc(II) ions and protons to act in unison, setting up the rotor through the formation of a heteroleptic zinc complex and a pseudorotaxane linkage. With chemical fuel, the dissipative system was reproducibly pulsed three times without a problem. Due to the double role of the fuel acid, two kinetically distinct processes played a role in both the out-of-equilibrium assembly and disassembly of the rotor, highlighting the complex issues in multitasking of chemical fuels.

The Transient Covalent Bond in Abiotic Nonequilibrium Systems

Biochemical systems accomplish many critical functions with by operating out-of-equilibrium using the energy of chemical fuels. The formation of a transient covalent bond is a simple but very effective tool in designing analogous reaction networks. This Minireview focuses on the fuel chemistries that have been used to generate transient bonds in recent demonstrations of abiotic nonequilibrium systems (i.e., systems that do not make use of biological components). Fuel reactions are divided into two fundamental classifications depending on whether the fuel contributes structural elements to the activated state, a distinction that dictates how they can be used. Reported systems are further categorized by overall fuel reaction (e.g., hydrolysis of alkylating agents, carbodiimide hydration) and illustrate how similar chemistry can be used to effect a wide range of nonequilibrium behavior, ranging from self-assembly to the operation of molecular machines.

Electron-rich Coordination Receptors Based on Tetrathiafulvalene Derivatives: Controlling the Host-Guest Binding

The coordination-driven self-assembly methodology has emerged over the last few decades as an extraordinarily versatile synthetic tool for obtaining discrete macrocyclic or cage structures. Rational approaches using large libraries of ligands and metal complexes have allowed researchers to reach more and more sophisticated discrete structures such as interlocked, chiral, or heteroleptic cages, and some of them are designed for guest binding applications. Efforts have been notably produced in controlling host-guest affinity with, in particular, an evident interest in targeting substrate transportation and subsequent delivering. Recent accomplishments in this direction were described from functional cages which can be addressed with light, pH, or through a chemical exchange. The case of a redox-stimulation has been much less explored. In this case, the charge state of the redox-active cavity can be controlled through an applied electrical potential or introduction of an appropriate oxidizing/reducing chemical agent. Beyond possible applications in electrochemical sensing for environmental and medical sciences as well as for redox catalysis, controlling the cavity charge offers the possibility to modulate the host-guest binding affinity through electrostatic interactions, up to the point of disassembly of the host-guest complex, i.e., releasing of the guest molecule from the host cavity.This Account highlights the key studies that we carried out at Angers, related to discrete redox-active coordination-based architectures (i.e., metalla-rings, -cages, and -tweezers). These species are built upon metal-driven self-assembly between electron-rich ligands, based on the tetrathiafulvalene (TTF) moiety (as well as some of its S-rich derivatives), and various metal complexes. Given the high π-donating character of those ligands, the corresponding host structures exhibit a high electronic density on the cavity panels. This situation is favorable to bind complementary electron-poor guests, as it was illustrated with bis(pyrrolo)tetrathiafulvalene (BPTTF)-based cavities, which exhibit hosting properties for C₆₀ or tetrafluorotetracyanoquinodimethane (TCNQ-F₄). In addition to the pristine tetrathiafulvalene, which was successfully incorporated into palladium- or ruthenium-based architectures, the case of the so-called extended tetrathiafulvalene (exTTF) appears particularly fascinating. A series of related polycationic and neutral M₄L₂ ovoid containers, as well as a M₆L₃ cage, were synthesized, and their respective binding abilities for neutral and anionic guests were studied. Remarkably, such structures allow to control of the binding of the guest upon a redox-stimulation, through two distinctive processes: (i) cage disassembling or (ii) guest displacement. As an extension of this approach, metalla-assembled electron-rich tweezers were designed, which are able to trigger the guest release through an original process based on supramolecular dimerization activated through a redox stimulus. This ensemble of results illustrates the remarkable ability of electron-rich, coordination-based self-assembled cavities to bind various types of guests and, importantly, to trigger their release through a redox-stimulus.

On the use of catalysis to bias reaction pathways in out-of-equilibrium systems

Catalysis is an essential function in living systems and provides a way to control complex reaction networks. In natural out-of-equilibrium chemical reaction networks (CRNs) driven by the consumption of chemical fuels, enzymes provide catalytic control over pathway kinetics, giving rise to complex functions. Catalytic regulation of man-made fuel-driven systems is far less common and mostly deals with enzyme catalysis instead of synthetic catalysts. Here, we show via simulations, illustrated by literature examples, how any catalyst can be incorporated in a non-equilibrium CRN and what their effect is on the behavior of the system. Alteration of the catalysts' concentrations in batch and flow gives rise to responses in maximum conversion, lifetime (i.e. product half-lives and t90 - time to recover 90% of the reactant) and steady states. In situ up or downregulation of catalysts' levels temporarily changes the product steady state, whereas feedback elements can give unusual concentration profiles as a function of time and self-regulation in a CRN. We show that simulations can be highly effective in predicting CRN behavior. In the future, shifting the focus from enzyme catalysis towards small molecule and metal catalysis in out-of-equilibrium systems can provide us with new reaction networks and enhance their application potential in synthetic materials, overall advancing the design of man-made responsive and interactive systems.

Transient Self-assembly Processes Operated by Gaseous Fuels under Out-of-Equilibrium Conditions

Herein we report transient out-of-equilibrium self-assembly of molecules operated by gaseous fuel mixtures. The combination of an active gaseous chemical fuel and an inert gas or compressed air, which assists the degassing of the gaseous fuel from the solution, drives the transient self-assembly process. The gaseous nature of the fuel as well as the exhaust helps in their easy removal and thereby prevents their accumulation within the system and helps in maintaining the efficiency of the transient self-assembly process. The strategy is executed with a rather simple experimental set up and operates at ambient temperatures. Our approach may find use in the development of smart materials suitable for applications such as temporally active gas sensing and sequestration.

Light-fueled dynamic covalent crosslinking of single polymer chains in non-equilibrium states

While polymer synthesis proceeds predominantly towards the thermodynamic minimum, living systems operate on the reverse principle – consuming fuel to maintain a non-equilibrium state. Herein, we report the controlled formation of 3D macromolecular architectures based on light-fueled covalent non-equilibrium chemistry. In the presence of green light (525 nm) and a bivalent triazolinedione (TAD) crosslinker, naphthalene-containing polymers can be folded into single chain nanoparticles (SCNPs). At ambient temperature, the cycloaddition product of TAD with naphthalene reverts and the SCNP unfolds into its linear parent polymer. The reported SCNP is the first example of a reversible light triggered folding of single polymer chains and can readily be repeated for several cycles. The folded state of the SCNP can either be preserved through a constant supply of light fuel, kinetic trapping or through a chemical modification that makes the folded state thermodynamically favored. Whereas small molecule bivalent TAD/naphthalene cycloaddition products largely degraded after 3 days in solution, even in the presence of fuel, the SCNP entities were found to remain intact, thereby indicating the light-fueled stabilization of the SCNP to be an inherent feature of the confined macromolecular environment.

Synthetic polymers consume green light as fuel for intramolecular crosslinking, yielding non-equilibrium single chain nanoparticles that can be light-stabilised, kinetically and chemically trapped, or else unfold in the absence of light fuel.

Enzyme-Regulated Healable Polymeric Hydrogels

The enzyme-regulated healable polymeric hydrogels are a kind of emerging soft material capable of repairing the structural defects and recovering the hydrogel properties, wherein their fabrication, self-healing, or degradation is mediated by enzymatic reactions. Despite achievements that have been made in controllable cross-linking and de-cross-linking of hydrogels by utilizing enzyme-catalyzed reactions in the past few years, this substrate-specific strategy for regulating healable polymeric hydrogels remains in its infancy, because both the intelligence and practicality of current man-made enzyme-regulated healable materials are far below the levels of living organisms. A systematic summary of current achievements and a reasonable prospect at this point can play positive roles for the future development in this field. This Outlook focuses on the emerging and rapidly developing research area of bioinspired enzyme-regulated self-healing polymeric hydrogel systems. The enzymatic fabrication and degradation of healable polymeric hydrogels, as well as the enzymatically regulated self-healing of polymeric hydrogels, are reviewed. The functions and applications of the enzyme-regulated healable polymeric hydrogels are discussed.

Force generation by a propagating wave of supramolecular nanofibers

Dynamic spatiotemporal patterns that arise from out-of-equilibrium biochemical reactions generate forces in living cells. Despite considerable recent efforts, rational design of spatiotemporal patterns in artificial molecular systems remains at an early stage of development. Here, we describe force generation by a propagating wave of supramolecular nanofibers. Inspired by actin dynamics, a reaction network is designed to control the formation and degradation of nanofibers by two chemically orthogonal stimuli. Real-time fluorescent imaging successfully visualizes the propagating wave based on spatiotemporally coupled generation and collapse of nanofibers. Numerical simulation indicates that the concentration gradient of degradation stimulus and the smaller diffusion coefficient of the nanofiber are critical for wave emergence. Moreover, the force (0.005 pN) generated by chemophoresis and/or depletion force of this propagating wave can move nanobeads along the wave direction.

Dissipative Assembly of Macrocycles Comprising Multiple Transient Bonds

Dissipative assembly has great potential for the creation of new adaptive chemical systems. However, while molecular assembly at equilibrium is routinely used to prepare complex architectures from polyfunctional monomers, species formed out of equilibrium have, to this point, been structurally very simple. In most examples the fuel simply effects the formation of a single short-lived covalent bond. Herein, we show that chemical fuels can assemble bifunctional components into macrocycles containing multiple transient bonds. Specifically, dicarboxylic acids give aqueous dianhydride macrocycles on treatment with a carbodiimide. The macrocycles are assembled efficiently as a consequence of both fuel-dependent and fuel-independent mechanisms; they undergo slower decomposition, building up as the fuel recycles the components, and are a favored product of the dynamic exchange of the anhydride bonds. These results create new possibilities for generating structurally sophisticated out-of-equilibrium species.

Devising Synthetic Reaction Cycles for Dissipative Nonequilibrium Self-Assembly

Fuel-driven reaction cycles are found in biological systems to control the assembly and disassembly of supramolecular materials such as the cytoskeleton. Fuel molecules can bind noncovalently to a self-assembling building block or they can react with it, resulting in covalent modifications. Overall the fuel can either switch the self-assembly process on or off. Here, a closer look is taken at artificial systems that mimic biological systems by making and breaking covalent bonds in a self-assembling motif. The different chemistries used so far are highlighted in chronological order and the pros and cons of each system are discussed. Moreover, the desired traits of future reaction cycles, their fuels, and waste management are outlined, and two chemistries that have not been explored up to now in chemically fueled dissipative self-assembly are suggested.

Selective Formation of S4- and T-Symmetric Supramolecular Tetrahedral Cages and Helicates in Polar Media Assembled via Cooperative Action of Coordination and Hydrogen Bonds

We report on the synthesis and self-assembly study of novel supramolecular monomers encompassing quadruple hydrogen-bonding motifs and metal-coordinating 2,2'-bipyridine units. When mixed with metal ions such as Fe²⁺ or Zn²⁺, the tetrahedron cage complexes are formed in quantitative yields and full diastereoselectivity, even in highly polar acetonitrile or methanol solvents. The symmetry of the complexes obtained has been shown to depend critically on the flexibility of the ligand. Restriction of the rotation of the hydrogen-bonding unit with respect to the metal-coordinating site results in a T-symmetric cage, whereas introducing flexibility either through a methylene linker or rotating benzene ring allows the formation of S₄-symmetric cages with self-filled interior. In addition, the possibility to select between tetrahedral cages or helicates and to control the dimensions of the aggregate has been demonstrated with a three-component assembly using external hydrogen-bonding molecular inserts or by varying the radius of the metal ion (Hg²⁺ vs Fe²⁺). Self-sorting studies of individual Fe²⁺ complexes with ligands of different sizes revealed their inertness toward ligand scrambling.

Exploiting the labile site in dinuclear [Pd2L2]n+ metallo-cycles: multi-step control over binding affinity without alteration of core host structure

Synthetic metallosupramolecular systems have generally been binary (on/off) when they have control over molecular recognition. This report details a dipalladium(ii) system with four-step graduated control over recognition for a guest.

Structure-Property Effects in the Generation of Transient Aqueous Benzoic Acid Anhydrides by Carbodiimide Fuels

The design of dissipative systems, which operate out-of-equilibrium by consuming chemical fuels, is challenging. As yet, there are a few examples of privileged fuel chemistries that can be broadly applied in abiotic systems in the same way that ATP hydrolysis is exploited throughout biochemistry. The key issue is that designing nonequilibrium systems is inherently about balancing the relative rates of coupled reactions. The use of carbodiimides as fuels to generate transient aqueous carboxylic anhydrides has recently been used in examples of new nonequilibrium materials and supramolecular assemblies. Here, we explore the kinetics of formation and decomposition of a series of benzoic anhydrides generated from the corresponding acids and EDC under typical conditions (EDC = N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride). The reactions can be described by a simple mechanism that merges known behavior for the two processes independently. Structure-property effects in these systems are dominated by differences in the anhydride decomposition rate. The kinetic parameters allow trends in concentration-dependent properties to be simulated, such as reaction lifetimes, peak anhydride concentrations, and yields. For key properties, there are diminishing returns with the addition of increasing amounts of fuel. These results should provide useful guidelines for the design of functional systems making use of this chemistry.

Selective and reversible interconversion of nanosliders commanded by remote control via metal-ion signaling

A multi-device network mainly consisting of two two-component nanosliders was formed by self-sorting of six components. Addition/removal of zinc(ii) ions reversibly reorganized the network by chemical signaling involving the translocation of copper(i) from a relay station followed by the selective disassembly/assembly of one of both multi-component devices. The thus liberated machine parts served to erect a three-component nanoslider alongside the other unchanged two-component nanoslider.

Fuel-Driven Transient Crystallization of a Cucurbit[8]uril-Based Host-Guest Complex

Transient self-assembling systems often suffer from accumulation of chemical wastes that interfere with the formation of pristine self-assembled products in subsequent cycles. Herein, we report the transient crystallization of a cucurbit[8]uril-based host-guest complex, preventing the accumulation of chemical wastes. Base-catalyzed thermal decarboxylation of trichloroacetic acid that chemically fuels the crystallization process dissolves the crystals, and produces volatile chemical wastes that are spontaneously removed from the solution. With such self-clearance process, no significant damping in the formation of the crystals was observed. The morphology and structural integrity of the crystals was also maintained in subsequent cycles. The concept may be further extended to obtain other temporally functional materials, quasicrystals, etc., based on stimuli-responsive guest molecules.