Rienk Eelkema

Molecular machines: Nanomotor rotates microscale objects

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.

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

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.

Triggered Self-Assembly of Simple Dynamic Covalent Surfactants

A prototype surfactant system was developed with the unique feature that it can be switched between an aggregated, amphiphilic state and a nonaggregated, nonamphiphilic state using external stimuli. This switchable surfactant system uses the reversible formation of a dynamic covalent bond for pH- and temperature-triggered on/off self-assembly of micellar aggregates by reversible displacement of the equilibrium between nonamphiphilic building blocks and their amphiphilic counterparts. The potential for application in controlled-release systems is shown by reversible uptake and release of an organic dye in aqueous media.

Signatures of Quantum Interference Effects on Charge Transport Through a Single Benzene Ring

Inthis description, the width and height of the energy barrier,and the electronic coupling between the molecule and theelectrodes are the main parameters that characterize theefficiency of charge transport. Electron transfer througha molecule then depends exponentially on the length of theconductance pathway and this has indeed been observed inmany experiments.

Spatial Structuring of a Supramolecular Hydrogel by using a Visible‐Light Triggered Catalyst

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.

A self-assembled delivery platform with post-production tunable release rate.

Self-assembly of three molecular components results in a delivery platform, the release rate of which can be tuned after its production. A fluorophore-conjugated gelator can be hydrolyzed by an enzyme, resulting in the release of a fluorescent small molecule. To allow the release to be tunable, the enzyme is entrapped in liposomes and can be liberated by heating the system for a short period. Crucially, the heating time determines the amount of enzyme liberated; with that, the release rate can be tuned by the time of heating.

Photoinduced Reorganization of Motor-Doped Chiral Liquid Crystals : Bridging Molecular Isomerization and Texture Rotation

We recently reported that the photoisomerization of molecular motors used as chiral dopants in a cholesteric liquid crystal film induces a rotational reorganization which can be observed by optical microscopy and produces the motion of microscopic objects placed on top of the film (Feringa, B. L.; et al. Nature 2006, 440, 163; J. Am. Chem. Soc. 2006, 128, 14397). The mechanism underlying the mesoscopic manifestation of the molecular process was not fully understood, and here we present a joint theoretical and experimental investigation, which provides a detailed insight into the mechanism of texture rotation. This description allows us to identify the interplay between the chemical structure of the chiral dopant and the material properties of the liquid crystal host, and to quantify their role in the observed dynamic phenomenon. We have found that a crucial role is played by the hybrid anchoring of the liquid crystal, with the director parallel to the substrate and perpendicular to the interface with air; in this configuration an almost unperturbed cholesteric helix, with its axis normal to the substrate, is present in most of the film, with strong deformations only close to the free interface. The texture rotation observed in the experiment reflects the rotation of the director during the unwinding of the cholesteric helix, produced by the change in shape of the chiral dopant under photoisomerization. The rotational reorganization is controlled by the photochemical process, via the coupling between the chirality of the dopant and the elastic properties of the liquid crystal host.