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Over the last few decades, the study of more complex, chemical systems closer to those found in nature, and the interactions within those systems, has grown immensely. Despite great efforts, the need for new, versatile, and robust chemistry to apply in CRNs remains. In this Feature Article, we give a brief overview over previous developments in the field of systems chemistry and how β′-substituted Michael acceptors (MAs) can be a great addition to the systems chemist's toolbox. We illustrate their versatility by showcasing a range of examples of applying β′-substituted MAs in CRNs, both as chemical signals and as substrates, to open up the path to many applications ranging from responsive materials, to pathway control in CRNs, drug delivery, analyte detection, and beyond. [ABSTRACT FROM AUTHOR]

Dynamic covalent (DCv) ureas have been used abundantly to design self‐healing materials. We demonstrate that apart from self‐healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile‐N‐oxides from chlorooximes, which react with acrylate‐terminated polymers to form an isoxazoline polymer gel. [ABSTRACT FROM AUTHOR]

In this contribution we describe a combined experimental and theoretical study of the relation between the molecular structure and the electronic properties of conjugated donor-acceptor type chromophores for light-harvesting applications. A series of model systems was synthesized where a central anthracene (electron donor) is connected to dicyanovinyl units (electron acceptor) through a π-conjugated spacer. The study of the redox and optical properties of these chromophores and of reference compounds without dicyanovinyl units allows us correlate the electronic properties to the presence of the electron withdrawing groups and the molecular conformation. Comparison with calculated electronic structure shows that the construction of chromophores that consist of electron donating and accepting units does not always follow the simple rules that are generally used in the design of such molecules. The results show a subtle relation between the charge transfer character and the geometry of the molecules. In some cases this leads to significant contribution of charge transfer excitation to the absorption spectra of some chromophores while such contributions are completely absent in others. [ABSTRACT FROM AUTHOR]

This review deals with recent developments in the design of switchable dopants capable of altering the pitch of cholesteric liquid crystals under the influence of light. Cholesteric liquid crystals possess many potentially useful properties owing to the helical organisation of their mesogens. For many of these applications, dynamic control over the cholesteric pitch can be achieved using chiral dopants capable of changing their shape in response to an external stimulus. The first attempts at developing such responsive systems stem from the 1970s, but major advances have been reported in recent years (2003–present), which is the subject of this review. Efficient dopants showing large changes in helical twisting power upon photo-switching have been developed, often capable of inverting the sign of the helical organisation of the surrounding liquid crystalline host. Overcrowded alkene-based molecular motors and binaphthylazobenzene-based switches have emerged as two classes of highly efficient dopants, enabling manipulation of cholesteric pitch over wide ranges using low concentrations of dopant. [ABSTRACT FROM AUTHOR]

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed. [ABSTRACT FROM AUTHOR]

Living organisms are capable of dynamically changing their structures for adaptive functions through sophisticated reaction‐diffusion processes. Here we show how active supramolecular hydrogels with programmable lifetimes and macroscopic structures can be created by relying on a simple reaction‐diffusion strategy. Two hydrogel precursors (poly(acrylic acid) PAA/CaCl2 and Na2CO3) diffuse from different locations and generate amorphous calcium carbonate (ACC) nanoparticles at the diffusional fronts, leading to the formation of hydrogel structures driven by electrostatic interactions between PAA and ACC nanoparticles. Interestingly, the formed hydrogels are capable of autonomously disintegrating over time because of a delayed influx of electrostatic‐interaction inhibitors (NaCl). The hydrogel growth process is well explained by a reaction‐diffusion model which offers a theoretical means to program the dynamic growth of structured hydrogels. Furthermore, we demonstrate a conceptual access to dynamic information storage in soft materials using the developed reaction‐diffusion strategy. This work may serve as a starting point for the development of life‐like materials with adaptive structures and functionalities. [ABSTRACT FROM AUTHOR]

Living organisms are capable of dynamically changing their structures for adaptive functions through sophisticated reaction‐diffusion processes. Here we show how active supramolecular hydrogels with programmable lifetimes and macroscopic structures can be created by relying on a simple reaction‐diffusion strategy. Two hydrogel precursors (poly(acrylic acid) PAA/CaCl2 and Na2CO3) diffuse from different locations and generate amorphous calcium carbonate (ACC) nanoparticles at the diffusional fronts, leading to the formation of hydrogel structures driven by electrostatic interactions between PAA and ACC nanoparticles. Interestingly, the formed hydrogels are capable of autonomously disintegrating over time because of a delayed influx of electrostatic‐interaction inhibitors (NaCl). The hydrogel growth process is well explained by a reaction‐diffusion model which offers a theoretical means to program the dynamic growth of structured hydrogels. Furthermore, we demonstrate a conceptual access to dynamic information storage in soft materials using the developed reaction‐diffusion strategy. This work may serve as a starting point for the development of life‐like materials with adaptive structures and functionalities. [ABSTRACT FROM AUTHOR]

The article discusses a study on the radical cation stabilization in a cucurbituril oligoaniline rotaxane. In this study, the radical cation of rotaxane which resembles the conducting emeraldine state of polyaniline is stabilized by the presence of the threaded cucurbituril macrocycle. This shows that the encapsulation of conjugated polymers can have more than just a passive insulating effect. In conclusion, researchers have synthesized rotaxane using aqueous reductive amination.

The article cites a study which reports the transfer of chirality from a single stereogenic center to a supramolecular system, by a double chiral amplification sequence. This (prototype) system not only makes it possible to assess the chirality of simple amino alcohols from the macroscopic chiral properties of the liquid crystalline (LC) phase but also shows an intriguing transfer of central chirality via axial chirality to supramolecular chirality. It is reported that liquid crystalline materials are also known to act as amplifiers of chirality, as they can lead to chiral mesophases upon doping with a small amount of a suitable chiral dopant.

Presents an integrated approach to the de novo catalytic asymmetric synthesis of saccharides uniting two protocols. Enzymatic resolution of racemic acetoxypyranones; Stereoselective palladium-catalyzed acetal bond formation; Stereochemistry; Preliminary application of iterative saccharide synthesis.

Review: [20 refs. [ABSTRACT FROM AUTHOR]

Nanomachines of the future will require molecular-scale motors that can perform work and collectively induce controlled motion of much larger objects. We have designed a synthetic, light-driven molecular motor that is embedded in a liquid-crystal film and can rotate objects placed on the film that exceed the size of the motor molecule by a factor of 10,000. The changes in shape of the motor during the rotary steps cause a remarkable rotational reorganization of the liquid-crystal film and its surface relief, which ultimately causes the rotation of submillimetre-sized particles on the film. [ABSTRACT FROM AUTHOR]

Gamma radiation triggered secondary crosslinking of dextran hydrogels leads to macroscopic hydrogel contraction. The authors use stable polymer hydrogels, prepared through azide‐alkyne crosslinking, containing surplus alkyne groups. γ‐irradiation of these gels leads to more alkyne crosslinking, enabling controlled increase of crosslink density, which in turn leads to an increase of hydrogel stiffness and macroscopic hydrogel contraction. Gel contraction scales linearly with the applied radiation dose. The same mechanism is applied to achieve γ‐radiation triggered release of the small molecule cargo, akin to wringing out a sponge. γ‐irradiation of touching hydrogel objects leads to gel fusion and the formation of a self‐supporting gel connection, demonstrating the reactivity of the excess alkyne groups. They envision applications in gel gluing and the construction of complex gel architectures, as well as in responsive materials for controlled release. [ABSTRACT FROM AUTHOR]

Stimuli‐responsive soft materials enable controlled release of loaded drug molecules and biomolecules. Controlled release of potent chemotherapeutic or immunotherapeutic agents is crucial to reduce unwanted side effects. In an effort to develop controlled release strategies that can be triggered by using Cerenkov luminescence, we have developed polymer hydrogels that can release bovine serum albumin and immunoglobulin G by using light (254 nm–375 nm) as a trigger. We describe the synthesis and photochemical characterization of two light sensitive phenacyl bis‐azide crosslinkers that are used to prepare transparent self‐supporting hydrogel patches. One crosslinker was designed to optimize the overlap with the Cerenkov luminescence emission window, bearing an π‐extended phenacyl core, resulting in a high quantum yield (14 %) of photocleavage when irradiated with 375 nm light. We used the extended phenacyl crosslinker for the preparation of protein‐loaded dextran hydrogel patches, which showed efficient and selective dosed release of bovine serum albumin or immunoglobulin G after irradiation with 375 nm light. Cerenkov‐triggered release is as yet inconclusive due to unexpected side‐reactivity. Based on the high quantum yield, efficient release and large overlap with the Cerenkov window, we envision application of these photosensitive soft materials in radiation targeted drug release. [ABSTRACT FROM AUTHOR]

Dynamic regulation of chemical reactivity is important in many complex chemical reaction networks, such as cascade reactions and signal transduction processes. Signal responsive catalysts could play a crucial role in regulating these reaction pathways. Recently, supramolecular encapsulation was reported to regulate the activities of artificial catalysts. We present a host‐guest chemistry strategy to modulate the activity of commercially available synthetic organocatalysts. The molecular container cucurbit[7]uril was successfully applied to change the activity of four different organocatalysts and one initiator, enabling up‐ or down‐regulation of the reaction rates of four different classes of chemical reactions. In most cases CB[7] encapsulation results in catalyst inhibition, however in one case catalyst activation by binding to CB[7] was observed. The mechanism behind this unexpected behavior was explored by NMR binding studies and pKa measurements. The catalytic activity can be instantaneously switched during operation, by addition of either supramolecular host or competitive binding molecules, and the reaction rate can be predicted with a kinetic model. Overall, this signal responsive system proves a promising tool to control catalytic activity. [ABSTRACT FROM AUTHOR]

Dynamic regulation of chemical reactivity is important in many complex chemical reaction networks, such as cascade reactions and signal transduction processes. Signal responsive catalysts could play a crucial role in regulating these reaction pathways. Recently, supramolecular encapsulation was reported to regulate the activities of artificial catalysts. We present a host‐guest chemistry strategy to modulate the activity of commercially available synthetic organocatalysts. The molecular container cucurbit[7]uril was successfully applied to change the activity of four different organocatalysts and one initiator, enabling up‐ or down‐regulation of the reaction rates of four different classes of chemical reactions. In most cases CB[7] encapsulation results in catalyst inhibition, however in one case catalyst activation by binding to CB[7] was observed. The mechanism behind this unexpected behavior was explored by NMR binding studies and pKa measurements. The catalytic activity can be instantaneously switched during operation, by addition of either supramolecular host or competitive binding molecules, and the reaction rate can be predicted with a kinetic model. Overall, this signal responsive system proves a promising tool to control catalytic activity. [ABSTRACT FROM AUTHOR]

Signal transduction in living systems is the conversion of information into a chemical change, and is the principal process by which cells communicate. In nature, these functions are encoded in non‐equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. However, man‐made catalytically controlled networks are rare. We incorporated catalysis into an artificial fuel‐driven out‐of‐equilibrium CRN, where the forward (ester formation) and backward (ester hydrolysis) reactions are controlled by varying the ratio of two organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. This fuel‐driven strategy was expanded to a responsive polymer system, where transient polymer conformation and aggregation are controlled through fuel and catalyst levels. Altogether, we show that organocatalysis can be used to control a man‐made fuel‐driven system and induce a change in a macromolecular superstructure, as in natural non‐equilibrium systems. [ABSTRACT FROM AUTHOR]

Signal transduction in living systems is the conversion of information into a chemical change, and is the principal process by which cells communicate. In nature, these functions are encoded in non‐equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. However, man‐made catalytically controlled networks are rare. We incorporated catalysis into an artificial fuel‐driven out‐of‐equilibrium CRN, where the forward (ester formation) and backward (ester hydrolysis) reactions are controlled by varying the ratio of two organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. This fuel‐driven strategy was expanded to a responsive polymer system, where transient polymer conformation and aggregation are controlled through fuel and catalyst levels. Altogether, we show that organocatalysis can be used to control a man‐made fuel‐driven system and induce a change in a macromolecular superstructure, as in natural non‐equilibrium systems. [ABSTRACT FROM AUTHOR]

Supramolecular encapsulation is known to alter chemical properties of guest molecules. We have applied this strategy of molecular encapsulation to temporally control the catalytic activity of a stable copper(I)–carbene catalyst. Encapsulation of the copper(I)–carbene catalyst by the supramolecular host cucurbit[7]uril (CB[7]) resulted in the complete inactivation of a copper‐catalyzed alkyne–azide cycloaddition (CuAAC) reaction. The addition of a chemical signal achieved the near instantaneous activation of the catalyst, by releasing the catalyst from the inhibited CB[7] catalyst complex. To broaden the scope of our on‐demand CuAAC reaction, we demonstrated the protein labeling of vinculin with the copper(I)–carbene catalyst, to inhibit its activity by encapsulation with CB[7] and to initiate labeling at any moment by adding a specific signal molecule. Ultimately, this strategy allows for temporal control over copper‐catalyzed click chemistry, on small molecules as well as protein targets. [ABSTRACT FROM AUTHOR]

Supramolecular encapsulation is known to alter chemical properties of guest molecules. We have applied this strategy of molecular encapsulation to temporally control the catalytic activity of a stable copper(I)–carbene catalyst. Encapsulation of the copper(I)–carbene catalyst by the supramolecular host cucurbit[7]uril (CB[7]) resulted in the complete inactivation of a copper‐catalyzed alkyne–azide cycloaddition (CuAAC) reaction. The addition of a chemical signal achieved the near instantaneous activation of the catalyst, by releasing the catalyst from the inhibited CB[7] catalyst complex. To broaden the scope of our on‐demand CuAAC reaction, we demonstrated the protein labeling of vinculin with the copper(I)–carbene catalyst, to inhibit its activity by encapsulation with CB[7] and to initiate labeling at any moment by adding a specific signal molecule. Ultimately, this strategy allows for temporal control over copper‐catalyzed click chemistry, on small molecules as well as protein targets. [ABSTRACT FROM AUTHOR]

Here, transient supramolecular hydrogels that are formed through simple aging‐induced seeded self‐assembly of molecular gelators are reported. In the involved molecular self‐assembly system, multicomponent gelators are formed from a mixture of precursor molecules and, typically, can spontaneously self‐assemble into thermodynamically more stable hydrogels through a multilevel self‐sorting process. In the present work, it is surprisingly found that one of the precursor molecules is capable of self‐assembling into nano‐sized aggregates upon a gentle aging treatment. Importantly, these tiny aggregates can serve as seeds to force the self‐assembly of gelators along a kinetically controlled pathway, leading to transient hydrogels that eventually spontaneously convert into thermodynamically more stable hydrogels over time. Such an aging‐induced seeded self‐assembly process is not only a new route toward synthetic out‐of‐equilibrium supramolecular systems, but also suggests the necessity of reporting the age of self‐assembling building block solutions in other self‐assembly systems. [ABSTRACT FROM AUTHOR]

Supramolecular structures with strain‐stiffening properties are ubiquitous in nature but remain rare in the lab. Herein, we report on strain‐stiffening supramolecular hydrogels that are entirely produced through the self‐assembly of synthetic molecular gelators. The involved gelators self‐assemble into semi‐flexible fibers, which thereby crosslink into hydrogels. Interestingly, these hydrogels are capable of stiffening in response to applied stress, resembling biological intermediate filaments system. Furthermore, strain‐stiffening hydrogel networks embedded with liposomes are constructed through orthogonal self‐assembly of gelators and phospholipids, mimicking biological tissues in both architecture and mechanical properties. This work furthers the development of biomimetic soft materials with mechanical responsiveness and presents potentially enticing applications in diverse fields, such as tissue engineering, artificial life, and strain sensors. [ABSTRACT FROM AUTHOR]

Supramolecular structures with strain‐stiffening properties are ubiquitous in nature but remain rare in the lab. Herein, we report on strain‐stiffening supramolecular hydrogels that are entirely produced through the self‐assembly of synthetic molecular gelators. The involved gelators self‐assemble into semi‐flexible fibers, which thereby crosslink into hydrogels. Interestingly, these hydrogels are capable of stiffening in response to applied stress, resembling biological intermediate filaments system. Furthermore, strain‐stiffening hydrogel networks embedded with liposomes are constructed through orthogonal self‐assembly of gelators and phospholipids, mimicking biological tissues in both architecture and mechanical properties. This work furthers the development of biomimetic soft materials with mechanical responsiveness and presents potentially enticing applications in diverse fields, such as tissue engineering, artificial life, and strain sensors. [ABSTRACT FROM AUTHOR]

Fuel‐driven chemical reaction networks provide an opportunity to develop chemical systems that operate out‐of‐equilibrium. There remains a need to design and develop new fuel‐driven chemical reaction networks capable of repeated operation using simple and benign chemistry. Herein, we propose a new chemical reaction network for fuel‐driven transient formation of covalent bonds, based on redox‐controlled conjugate addition and elimination chemistry. By investigating the separate reactions making up the cycle, we find that the bond formation, breaking and regeneration processes can be realized. At present, substantial side reactivity prevents achieving repeated operation of a full cycle in a single system. If such obstacles would be overcome, this chemical reaction network could be a valuable addition to the toolbox for out‐of‐equilibrium systems chemistry. [ABSTRACT FROM AUTHOR]

Here we report on how metastable supramolecular gels can be formed through seeded self‐assembly of multicomponent gelators. Hydrazone‐based gelators decorated with non‐ionic and anionic groups are formed in situ from hydrazide and aldehyde building blocks, and lead through multiple self‐sorting processes to the formation of heterogeneous gels approaching thermodynamic equilibrium. Interestingly, the addition of seeds composing of oligomers of gelators bypasses the self‐sorting processes and accelerates the self‐assembly along a kinetically favored pathway, resulting in homogeneous gels of which the network morphologies and gel stiffness are markedly different from the thermodynamically more stable gel products. Importantly, over time, these metastable homogeneous gel networks are capable of converting into the thermodynamically more stable state. This seeding‐driven formation of out‐of‐equilibrium supramolecular structures is expected to serve as a simple approach towards functional materials with pathway‐dependent properties. Out‐of‐equilibrium supramolecular gels were generated by a simple seeding‐driven self‐assembly strategy. The addition of seeds forces the self‐assembly of synthetic gelators along a kinetically favoured pathway, leading to metastable gels which are capable of converting into the thermodynamically more stable gel states over time. [ABSTRACT FROM AUTHOR]

Here we report on how metastable supramolecular gels can be formed through seeded self‐assembly of multicomponent gelators. Hydrazone‐based gelators decorated with non‐ionic and anionic groups are formed in situ from hydrazide and aldehyde building blocks, and lead through multiple self‐sorting processes to the formation of heterogeneous gels approaching thermodynamic equilibrium. Interestingly, the addition of seeds composing of oligomers of gelators bypasses the self‐sorting processes and accelerates the self‐assembly along a kinetically favored pathway, resulting in homogeneous gels of which the network morphologies and gel stiffness are markedly different from the thermodynamically more stable gel products. Importantly, over time, these metastable homogeneous gel networks are capable of converting into the thermodynamically more stable state. This seeding‐driven formation of out‐of‐equilibrium supramolecular structures is expected to serve as a simple approach towards functional materials with pathway‐dependent properties. [ABSTRACT FROM AUTHOR]

Abstract: Hydrazone formation reactions from aldehydes and hydrazides have the remarkable qualities that they proceed in water and the kinetics can be controlled by organocatalysis. For these reasons, this class of reactions finds widespread use in biological as well as material settings. We recently reported a protected aniline catalyst for hydrazone formation that can be activated using a chemical signal. In our search to find a suitable hydrazone formation reaction to investigate the activation of this pro‐catalyst, we found a wide variety in reaction rates and response to catalysis. Here we report an overview of hydrazone formation reactions, their reaction rates and response to aniline catalysis, their compatibility for kinetic analysis by UV/Vis spectroscopy, and their compatibility with the reaction environment and with the pro‐catalyst pro‐aniline. [ABSTRACT FROM AUTHOR]

A generic method is used for compartmentalization of supramolecular hydrogels by using water-in-water emulsions based on aqueous multi-phase systems (AMPS). By forming the low-molecular-weight hydrogel throughout all phases of all-aqueous emulsions, distinct, micro-compartmentalized materials were created. This structuring approach offers control over the composition of each type of the compartments by directing the partitioning of objects to be encapsulated. Moreover, this method allows for barrier-less, dynamic exchange of even large hydrophilic solutes (MW≈60 kDa) between separate aqueous compartments. These features are expected to find use in the fields of, for instance, micro-structured catalysts, templating, and tissue engineering. [ABSTRACT FROM AUTHOR]

A generic method is used for compartmentalization of supramolecular hydrogels by using water-in-water emulsions based on aqueous multi-phase systems (AMPS). By forming the low-molecular-weight hydrogel throughout all phases of all-aqueous emulsions, distinct, micro-compartmentalized materials were created. This structuring approach offers control over the composition of each type of the compartments by directing the partitioning of objects to be encapsulated. Moreover, this method allows for barrier-less, dynamic exchange of even large hydrophilic solutes (MW≈60 kDa) between separate aqueous compartments. These features are expected to find use in the fields of, for instance, micro-structured catalysts, templating, and tissue engineering. [ABSTRACT FROM AUTHOR]

The use of dissipative self-assembly driven by chemical reaction networks for the creation of unique structures is gaining in popularity. In dissipative self-assembly, precursors are converted into self-assembling building blocks by the conversion of a source of energy, typically a photon or a fuel molecule. The self-assembling building block is intrinsically unstable and spontaneously reverts to its original precursor, thus giving the building block a limited lifetime. As a result, its presence is kinetically controlled, which gives the associated supramolecular material unique properties. For instance, formation and properties of these materials can be controlled over space and time by the kinetics of the coupled reaction network, they are autonomously self-healing and they are highly adaptive to small changes in their environment. By means of an example of a biological dissipative self-assembled material, the unique concepts at the basis of these supramolecular materials will be discussed. We then review recent efforts towards man-made dissipative assembly of structures and how their unique material properties have been characterized. In order to help further the field, we close with loosely defined design rules that are at the basis of the discussed examples. [ABSTRACT FROM AUTHOR]

Self-assembly of biomolecules catalytically controls the formation of natural supramolecular structures, giving highly ordered complex materials. Such desirable hybrid systems are very difficult to design and construct synthetically. A hybrid double-network hydrogel with a maximum storage modulus ( G′max) of up to 55 kPa can be synthesized by using a self-assembled hydrogel that catalyses the formation of another kinetically arrested hydrogel network. Tuning of the catalytic efficiency of the first network allowed spatiotemporal control over the evolution of the second network and the resulting mechanical properties. The distribution of active catalytic sites was optimal for catalytic fibres prepared at the minimum gelation concentration (MGC) to give the double-network hydrogel with highest storage modulus. This approach could be very useful in preparing complex hierarchical structures with tailor-made properties. [ABSTRACT FROM AUTHOR]

The article presents a new approach for pH- and temperature-triggered self-assembly of micellar aggregates. It notes that this new approach is through reversible displacement of the equilibrium among nonamphiphlic building blocks. It mentions a prototype surfactant system that can be transferred through an alteration in pH or temperature. Moreover, it also explores the potential drug delivery via reversible uptake and release of an organic dye

Conspectus: One often thinks of catalysts as chemical tools to accelerate a reaction or to have a reaction run under more benign conditions. As such, catalysis has a role to play in the chemical industry and in lab scale synthesis that is not to be underestimated. Still, the role of catalysis in living systems (cells, organisms) is much more extensive, ranging from the formation and breakdown of small molecules and biopolymers to controlling signal transduction cascades and feedback processes, motility, and mechanical action. Such phenomena are only recently starting to receive attention in synthetic materials and chemical systems. "Smart" soft materials could find many important applications ranging from personalized therapeutics to soft robotics to name but a few. Until recently, approaches to control the properties of such materials were largely dominated by thermodynamics, for instance, looking at phase behavior and interaction strength. However, kinetics plays a large role in determining the behavior of such soft materials, for instance, in the formation of kinetically trapped (metastable) states or the dynamics of component exchange. As catalysts can change the rate of a chemical reaction, catalysis could be used to control the formation, dynamics, and fate of supramolecular structures when the molecules making up these structures contain chemical bonds whose formation or exchange are susceptible to catalysis. In this Account, we describe our efforts to use synthetic catalysts to control the properties of supramolecular hydrogels. Building on the concept of synthesizing the assembling molecule in the self-assembly medium from nonassembling precursors, we will introduce the use of catalysis to change the kinetics of assembler formation and thereby the properties of the resulting material. In particular, we will focus on the synthesis of supramolecular hydrogels where the use of a catalyst provides access to gel materials with vastly different appearance and mechanical properties or controls localized gel formation and the growth of gel objects. As such, catalysis will be applied to create molecular materials that exist outside of chemical equilibrium. In all, using catalysts to control the properties of soft materials constitutes a new avenue for catalysis far beyond the traditional use in industrial and lab scale synthesis. [ABSTRACT FROM AUTHOR]

In this contribution we show that biological membranes can catalyze the formation of supramolecular hydrogel networks. Negatively charged lipid membranes can generate a local proton gradient, accelerating the acid-catalyzed formation of hydrazone-based supramolecular gelators near the membrane. Synthetic lipid membranes can be used to tune the physical properties of the resulting multicomponent gels as a function of lipid concentration. Moreover, the catalytic activity of lipid membranes and the formation of gel networks around these supramolecular structures are controlled by the charge and phase behavior of the lipid molecules. Finally, we show that the insights obtained from synthetic membranes can be translated to biological membranes, enabling the formation of gel fibers on living HeLa cells. [ABSTRACT FROM AUTHOR]

Protein immobilization on surfaces, and on lipid bilayers specifically, has great potential in biomolecular and biotechnological research. Of current special interest is the immobilization of proteins using supramolecular noncovalent interactions. This allows for a reversible immobilization and obviates the use of harsh ligation conditions that could denature fragile proteins. In the work presented here, reversible supramolecular immobilization of proteins on lipid bilayer surfaces was achieved by using the host-guest interaction of the macrocyclic molecule cucurbit[8]uril. A fluorescent protein was successfully immobilized on the lipid bilayer by making use of the property of cucurbit[8]uril to host together a methylviologen and the indole of a tryptophan positioned on the N-terminal of the protein. The supramolecular complex was anchored to the bilayer through a cholesterol moiety that was attached to the methylviologen tethered with a small polyethylene glycol spacer. Protein immobilization studies using a quartz crystal microbalance (QCM) showed the assembly of the supramolecular complexes on the bilayer. Specific immobilization through the protein N-terminus is more efficient than through protein side-chain events. Reversible surface release of the proteins could be achieved by washing with cucurbit[8]uril or buffer alone. The described system shows the potential of supramolecular assembly of proteins and provides a method for site-specific protein immobilization under mild conditions in a reversible manner. [ABSTRACT FROM AUTHOR]

Simultaneous control of the kinetics and thermodynamics of two different types of covalent chemistry allows pathway selectivity in the formation of hydrogelating molecules from a complex reaction network. This can lead to a range of hydrogel materials with vastly different properties, starting from a set of simple starting compounds and reaction conditions. Chemical reaction between a trialdehyde and the tuberculosis drug isoniazid can form one, two, or three hydrazone connectivity products, meaning kinetic gelation pathways can be addressed. Simultaneously, thermodynamics control the formation of either a keto or an enol tautomer of the products, again resulting in vastly different materials. Overall, this shows that careful navigation of a reaction landscape using both kinetic and thermodynamic selectivity can be used to control material selection from a complex reaction network. [ABSTRACT FROM AUTHOR]

The article discusses the temporary molecular self-assembly into active materials that is caused by a chemical reaction, including the role that of kinetics play in this regard. An overview of the behavior of fibers in cell organization, including their stochastic collapse, shrinkage and growth, is provided. The article offers several chemistry-related charts and diagrams on the transient assembly of active materials.

Spatial control over the self-assembly of synthetic molecular fibers through the use of light-switchable catalysts can lead to the controlled formation of micropatterns made up of hydrogel structures. A photochromic switch, capable of reversibly releasing a proton upon irradiation, can act as a catalyst for in situ chemical bond formation between otherwise soluble building blocks, thereby leading to fiber formation and gelation in water. The use of a photoswitchable catalyst allows control over the distribution as well as the mechanical properties of the hydrogel material. By using homemade photomasks, spatially structured hydrogels were formed starting from bulk solutions of small molecule gelator precursors through light-triggered local catalyst activation. [ABSTRACT FROM AUTHOR]

Spatial control over the self-assembly of synthetic molecular fibers through the use of light-switchable catalysts can lead to the controlled formation of micropatterns made up of hydrogel structures. A photochromic switch, capable of reversibly releasing a proton upon irradiation, can act as a catalyst for in situ chemical bond formation between otherwise soluble building blocks, thereby leading to fiber formation and gelation in water. The use of a photoswitchable catalyst allows control over the distribution as well as the mechanical properties of the hydrogel material. By using homemade photomasks, spatially structured hydrogels were formed starting from bulk solutions of small molecule gelator precursors through light-triggered local catalyst activation. [ABSTRACT FROM AUTHOR]

Wereport measurements of hole and electron transfer along identicaloligo-p-phenylene molecular bridges of increasinglength. Although the injection barriers for hole and electron transferare similar, we observed striking differences in the distance dependenceand absolute magnitude of the rates of these two processes. Electrontransfer is characterized by an almost distance-independent, fastcharge-transfer rate. Hole transfer presents a much slower rate thatdecreases significantly with the length of the bridge. Time-dependentdensity functional calculations show that the observed differencescan be explained by the delocalization of the respective initial excitation.The evaluation of the initial state is therefore essential when comparingcharge-transfer rates between different donor–bridge–acceptorsystems. [ABSTRACT FROM AUTHOR]

Catalyst-assisted self-assembly is widespread in nature to achieve spatial control over structure formation. Reported herein is the formation of hydrogel micropatterns on catalytic surfaces. Gelator precursors react on catalytic sites to form building blocks which can self-assemble into nanofibers. The resulting structures preferentially grow where the catalyst is present. Not only is a first level of organization, allowing the construction of hydrogel micropatterns, achieved but a second level of organization is observed among fibers. Indeed, fibers grow with their main axis perpendicular to the substrate. This feature is directly linked to a unique mechanism of fiber formation for a synthetic system. Building blocks are added to fibers in a confined space at the solid-liquid interface. [ABSTRACT FROM AUTHOR]

Catalyst-assisted self-assembly is widespread in nature to achieve spatial control over structure formation. Reported herein is the formation of hydrogel micropatterns on catalytic surfaces. Gelator precursors react on catalytic sites to form building blocks which can self-assemble into nanofibers. The resulting structures preferentially grow where the catalyst is present. Not only is a first level of organization, allowing the construction of hydrogel micropatterns, achieved but a second level of organization is observed among fibers. Indeed, fibers grow with their main axis perpendicular to the substrate. This feature is directly linked to a unique mechanism of fiber formation for a synthetic system. Building blocks are added to fibers in a confined space at the solid-liquid interface. [ABSTRACT FROM AUTHOR]

Low-molecular-weight gels show great potential for application in fields ranging from the petrochemical industry to healthcare and tissue engineering. These supramolecular gels are often metastable materials, which implies that their properties are, at least partially, kinetically controlled. Here we show how the mechanical properties and structure of these materials can be controlled directly by catalytic action. We show how in situ catalysis of the formation of gelator molecules can be used to accelerate the formation of supramolecular hydrogels, which drastically enhances their resulting mechanical properties. Using acid or nucleophilic aniline catalysis, it is possible to make supramolecular hydrogels with tunable gel-strength in a matter of minutes, under ambient conditions, starting from simple soluble building blocks. By changing the rate of formation of the gelator molecules using a catalyst, the overall rate of gelation and the resulting gel morphology are affected, which provides access to metastable gel states with improved mechanical strength and appearance despite an identical gelator composition. [ABSTRACT FROM AUTHOR]

Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions. [ABSTRACT FROM AUTHOR]

Molecular electronics: Charge transport through a single benzene ring is studied using the mechanically controlled break‐junction technique. The low‐bias conductance for a meta‐coupled benzene ring is more than an order of magnitude smaller than that of a para‐coupled benzene ring. This difference can be explained by a destructive quantum interference effects in the meta‐coupled benzene (see picture). [ABSTRACT FROM AUTHOR]

Molekulare Elektronik: Der Ladungstransport durch einen Benzolring wurde mit der mechanisch kontrollierten Bruchkontakt ‐ Methode untersucht. Die niedrige Eingangsleitfähigkeit eines meta ‐ substituierten Benzolrings ist mehr als eine Größenordnung kleiner als die eines para ‐ substituierten. Dieser Unterschied wird mit einem destruktiven Quanteninterferenzeffekt in meta ‐ substituiertem Benzol erklärt (siehe Bild). [ABSTRACT FROM AUTHOR]

Come together: Self‐assembly can drive the formation of conjugated imine polymers in water, and stabilization of otherwise unstable imine bonds is used to obtain fully π‐conjugated, responsive dynamic covalent polyimines in aqueous environments. Both the optical properties and the aggregate morphology can be tuned by varying the aromatic monomers. [ABSTRACT FROM AUTHOR]

Stabile Organisation: Dynamische kovalente, vollständig π ‐ konjugierte responsive Iminpolymere entstehen in wässriger Umgebung, wenn die reversiblen kovalenten Bindungen durch molekulare Selbstorganisation stabilisiert werden. Sowohl die optischen Eigenschaften als auch die Aggregatmorphologie lassen sich über die Art des aromatischen Monomers gezielt einstellen. [ABSTRACT FROM AUTHOR]

Dynamic covalent chemistry is a powerful tool for theconstruction of adaptive and stimulus-responsive nanosystems. Herewe report on the spontaneous formation of dynamic covalent wormlikemicelles from imine-based gemini surfactants, formed upon mixing aqueoussolutions of two complementary non-surface-active precursors. Resultingfrom the reversibility of the dynamic covalent imine bond, the wormlikemicelles can be switched between an isotropic solution and the assembledstate, triggered by pH and temperature. Thermodynamic modeling ofthe reaction equilibria shows that, although mixtures of single- anddouble-tailed surfactants are formed, it is mainly the double-tailedsurfactant that assembles into the wormlike micelles. [ABSTRACT FROM AUTHOR]