Jan H. van Esch

Biocatalytic induction of supramolecular order

Supramolecular gels, which demonstrate tunable functionalities, have attracted much interest in a range of areas, including healthcare, environmental protection and energy-related technologies. Preparing these materials in a reliable manner is challenging, with an increased level of kinetic defects observed at higher self-assembly rates. Here, by combining biocatalysis and molecular self-assembly, we have shown the ability to more quickly access higher-ordered structures. By simply increasing enzyme concentration, supramolecular order expressed at molecular, nano- and micro-levels is dramatically enhanced, and, importantly, the gelator concentrations remain identical. Amphiphile molecules were prepared by attaching an aromatic moiety to a dipeptide backbone capped with a methyl ester. Their self-assembly was induced by an enzyme that hydrolysed the ester. Different enzyme concentrations altered the catalytic activity and size of the enzyme clusters, affecting their mobility. This allowed structurally diverse materials that represent local minima in the free energy landscape to be accessed based on a single gelator structure. Supramolecular gels show promise in diverse areas, including healthcare and energy technologies, owing to tunable properties that arise directly from the organization of their building blocks. Researchers have now been able to control this behaviour by combining enzymatic catalysis with molecular self-assembly. Although it seems counter-intuitive, gels that assembled faster showed fewer defects.

Chiral Recognition in Bis-Urea-Based Aggregates and Organogels through Cooperative Interactions

Chiral recognition in organogels occurs in the coassembly of the chiral gelator 1 with the chiral guest molecule 2. The diastereomeric aggregates formed from (R,R)-2 with (R,R)-1 or (S,S)-1 were found to be truly different in structure and strength. Cooperativity makes a major contribution to the chiral recognition in this system.

Efficient Intermolecular Charge Transport in Self‐Assembled Fibers of Mono‐ and Bithiophene Bisurea Compounds

Hydrogen bonds between urea units allow self-organization of π systems in mono- and bithiophenes into fibers as shown schematically. In these fibers there is a surprisingly high mobility of charge carriers as determined by pulse-radiolysis time-resolved microwave conductivity measurements.

We Can Design Molecular Gelators, But Do We Understand Them?†

The recent application of supramolecular principles to the design of molecular gelators has led to an enormous variety of new gelators and functional gels but has contributed relatively little to the understanding of molecular gelation phenomena. How do we progress from here?

Transient assembly of active materials fueled by a chemical reaction

Nonequilibrium transient self-assembly In biology, the constant supply of energy can drive a system to be far from its equilibrium conditions and allow for useful work to be done. In contrast, in most synthetic systems, there is a drive toward lower energy states. Boekhoven et al. made a molecule that can switch between a nonassociating state and an associating state through the addition of a chemical fuel (see the Perspective by Van der Zwagg and Meijer). The lifetime, stiffness, and regenerative behavior of the self-assembled state could be controlled and tuned by the kinetics of fuel conversion. Science, this issue p. 1075; see also p. 1056 A synthetic chemical system exhibits transient self-assembly fueled by a chemical reaction. [Also see Perspective by Van der Zwagg and Meijer] Fuel-driven self-assembly of actin filaments and microtubules is a key component of cellular organization. Continuous energy supply maintains these transient biomolecular assemblies far from thermodynamic equilibrium, unlike typical synthetic systems that spontaneously assemble at thermodynamic equilibrium. Here, we report the transient self-assembly of synthetic molecules into active materials, driven by the consumption of a chemical fuel. In these materials, reaction rates and fuel levels, instead of equilibrium composition, determine properties such as lifetime, stiffness, and self-regeneration capability. Fibers exhibit strongly nonlinear behavior including stochastic collapse and simultaneous growth and shrinkage, reminiscent of microtubule dynamics.

Dissipative Self-Assembly of a Molecular Gelator by Using a Chemical Fuel†

The construction of energy-dissipating self-assembling systems, which, like self-assembled structures found in nature are formed transiently, far from equilibrium, and under the constant influx of chemical energy, still represents a frontier in nanoscale assembly. The self-assembly of small molecules, polymers, proteins, nanoparticles, colloids, and particles with sizes that approach the mesoscale under thermodynamic equilibrium conditions has been a powerful approach for the construction of a variety of structures of nanoto micrometer dimensions, like vesicles, capsules, and nanotubules. The reversible nature of selfassembly processes has been exploited in switchable, adaptive, and autopoietic self-assembling systems, which lead to novel responsive materials and artificial systems that are capable of self-replication and compartmentalization. Recently, there has also been a strongly growing interest in self-assembled materials obtained under non-equilibrium conditions. For instance, the formation of hierarchically structured membranes in a reaction-diffusion field, and the orthogonal self-assembly of molecular gels with surfactants, liquid crystals, or other components can be controlled through the processing conditions, thus leading to a much richer structural diversity compared to equilibriumprocessed materials. These self-assembled structures offer new and intriguing opportunities for functional materials and biomimetic cellular structures. Nevertheless, in all these cases, the final self-assembling systems reside in a (local) thermodynamic minimum state. Despite these advances, the permanent nature of these synthetic self-assembled structures does not compare well to the complex spatiotemporally confined self-assembly processes seen in natural systems, which for instance allow the dynamic compartmentalization of incompatible processes, responsiveness, and self-healing. Natural self-assembled structures such as the cytoskeleton and phospholipid membranes are formed by dissipative self-assembly (DSA). In general, DSA systems consist of non-assembling entities which, through activation by an energy source, assemble into ordered structures. Energy dissipation causes deactivation of the building blocks, hence leading to a collapse of the formed structures. A typical example is microtubule assembly that uses guanosine-5’-triphosphate (GTP) as an energy source, which in turn catalyzes the hydrolysis of GTP and therefore its own collapse. The microtubule assembly process is controlled by feedback loops that lead to self-organization, including oscillatory behavior and nonlinear responses of microtubule formation, which are essential for rapid morphogenic alterations, self-healing, and self-replication. These fascinating properties of natural DSA systems have motivated research on their artificial counterparts. Several artificial DSA systems based on natural building blocks have been reported. Examples of fully artificial DSA systems are most commonly found in the top-down engineered mesoscopic regime with hard inorganic or polymeric objects. The few examples that concern soft matter are mostly fueled by light, whereas the dissipation of “chemical fuels” has been used to drive mechanical motion. It remains a challenge to develop a DSA system that is chemically fueled. A first step towards the development of a self-organizing self-assembly system is the construction of a simple DSA system without feedback control loops. Such a simple system typically follows a sequence of processes. Firstly, an energy source activates the precursor building blocks so that selfassembly is favored. Upon self-assembly, the activated building block can dissipate its energy, thus resulting in the formation of the initial building block and disassembly of the architecture. A requirement is that the rate of energy dissipation (Pd) should be lower than the consumption of fuel (Pc) to allow the formation of self-assembled architectures (Figure 1). Herein we present a synthetic DSA fibrous network that uses chemical fuel as an energy source. A gelator precursor is converted into a gelator by reaction with a chemical fuel, thus leading to self-assembly. Hydrolysis of the gelator, which is labile under ambient conditions, leads to energy dissipation and disassembly of the formed structures. Reactive gels have been previously reported and the hydrolysis of ester functions has been exploited to achieve an enzymatically controlled gel–sol phase transition. The design of the dissipative self-assembling system presented here is based on dibenzoyl-(l)-cystine (DBC; Bz = benzoyl), a well-known pH-responsive hydrogelator . Above their pKa value (ca. 4.5), intermolecular repulsion occurs between the anionic carboxylic acid groups of DBC, and therefore DBC [*] J. Boekhoven, Dr. A. M. Brizard, K. N. K. Kowlgi, Dr. G. J. M. Koper, Dr. R. Eelkema, Prof. Dr. J. H. van Esch Department of Chemical Engineering Delft University of Technology Julianalaan 136, 2628 BL, Delft (The Netherlands) Fax: (+ 31)15-278-4289 E-mail: j.h.vanesch@tudelft.nl Homepage: http://www.dct.tudelft.nl/sas

Syntheses of dithienylcyclopentene optical molecular switches

Properly functionalized dithienylethenes show promise for light-induced switching processes. To prevent cis/trans isomerization from competing with conrotatory 6π-electron ring closure, the ethene segment is usually incorporated in a (perfluorinated) cyclopentene. In the present article syntheses of perhydrocyclopentene 1 and perfluorocyclopentene 2 are described, which are amenable for large-scale conversions. Both compounds have chloro substituents at the 5-position of the thiophene rings to allow further functionalization. The conversion of the chloro substituents of 1 to formyl, carboxylate, boronyl, and hydrogen groups by halogen/lithium exchange at room temperature is described, and examples are given of further elaboration of 1 and 2 by attachment, both in a symmetrical as well as unsymmetrical fashion, of additional functionality by condensation, Friedel−Crafts or Suzuki reactions. The newly prepared thienylperhydrocyclopentene derivatives show reversible photochromism if the substituents at the 5-postions allow for conjugation with the thiophene π-system. (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003)

Catalytic control over supramolecular gel formation

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.

Large tunable image-charge effects in single-molecule junctions

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 metals 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.

Geminal Bis-ureas as Gelators for Organic Solvents : Gelation Properties and Structural Studies in Solution and in the Gel State

Several geminal bis-urea compounds were synthesised by means of an acid-catalysed condensation of various benzaldehydes with different monoalkylureas. Many of these compounds form thermoreversible gels with a number of organic solvents at very low concentrations (<3mM) and which are stable to temperatures higher than 100 degrees C. Electron microscopy revealed a three-dimensional (3D) network of intertwined fibres, which are several tens of micrometers long and have a width ranging from approximately 30 to 300 nm. The possible aggregate forms and aggregate symmetries were evaluated by means of molecular mechanics calculations. 1H NMR, 2D NMR, 13C NMR and 13C-CP/MAS NMR techniques were used to obtain information about the aggregation and possible aggregate symmetry of geminal bis-ureas in solution, in the gel state, and in the solid state.