Albert P. H. J. Schenning

Pathway complexity in supramolecular polymerization

Self-assembly provides an attractive route to functional organic materials, with properties and hence performance depending sensitively on the organization of the molecular building blocks. Molecular organization is a direct consequence of the pathways involved in the supramolecular assembly process, which is more amenable to detailed study when using one-dimensional systems. In the case of protein fibrils, formation and growth have been attributed to complex aggregation pathways that go beyond traditional concepts of homogeneous and secondary nucleation events. The self-assembly of synthetic supramolecular polymers has also been studied and even modulated, but our quantitative understanding of the processes involved remains limited. Here we report time-resolved observations of the formation of supramolecular polymers from π-conjugated oligomers. Our kinetic experiments show the presence of a kinetically favoured metastable assembly that forms quickly but then transforms into the thermodynamically favoured form. Quantitative insight into the kinetic experiments was obtained from kinetic model calculations, which revealed two parallel and competing pathways leading to assemblies with opposite helicity. These insights prompt us to use a chiral tartaric acid as an auxiliary to change the thermodynamic preference of the assembly process. We find that we can force aggregation completely down the kinetically favoured pathway so that, on removal of the auxiliary, we obtain only metastable assemblies.

Chemistry: Material marriage in electronics

Self-organizing molecules can form structures with useful electronic properties. These supramolecular materials combine the benefits of polymers with those of organic crystalline systems.

Emerging Solvent‐Induced Homochirality by the Confinement of Achiral Molecules Against a Solid Surface

The unique handedness of chiral molecules affects chemical, physical, and biological phenomena. While observed in solution for helical polymers and self-assembled stacks of molecules, transmission of chiral information is particularly selective at ordered interfaces as a result of geometrical restrictions introduced by two-dimensional (2D) confinement. Chiral amplification of enantiomerically enriched mixtures has been demonstrated either by chemical reactions at the air–water interface, or upon self-assembly on solid surfaces. Homochirality in achiral enantiomorphous monolayers can be realized by merging chiral modifiers in the monolayer or by exposing monolayers to magnetic fields. Alternatively, the potential role of solvents in amplification of chirality and emergence of homochirality at surfaces remains unexplored to date. Herein we show how solvent-induced macroscopic chirality emerges within self-assemblies of achiral molecules on achiral surfaces. It is an exclusive surface-confined process, and as such it differs from “chiral-solvent-” or “chiral-guestinduced” chirality of supramolecular systems in solution. To demonstrate that homochirality emerges at the interface between a chiral liquid and the surface of highly oriented pyrolytic graphite (HOPG), we selected a hydrogen-bonding achiral diamino triazine oligo-(p-phenylenevinylene) oligomer (A-OPV4T, Figure 1). The chiral analogue, ((S)-OPV4T, Figure 1), was recently shown to assemble exclusively in a counter-clockwise (CCW) hydrogen-bonded rosette motif at the liquid–solid interface, with 1-phenyloctane as the achiral solvent. Molecular homochirality is expressed at the supramolecular level as a result of the 2D packing of the chiral rosette. The chiral solvent in the current study,

Model-driven optimization of multicomponent self-assembly processes

Significance The molecular organization of semiconducting molecules is extremely important for the performance of functional organic materials in electronic devices. The processing of these materials often leads to multiple assembly pathways toward different types of molecular organizations. Hence, directing the assembly process toward the desired type of organization requires many trial-and-error optimization steps. In this paper, we introduce an approach to optimize these self-assembly processes. Based on experiments with a system that assembles into 1D helices in solution, we have developed models to describe the dynamics of multicomponent self-assembly processes. These models simulate the experimental data very well and allow us to understand and avoid, or alternatively attenuate, the entrapment of materials in nonequilibrium assemblies. Here, we report an engineering approach toward multicomponent self-assembly processes by developing a methodology to circumvent spurious, metastable assemblies. The formation of metastable aggregates often hampers self-assembly of molecular building blocks into the desired nanostructures. Strategies are explored to master the pathway complexity and avoid off-pathway aggregates by optimizing the rate of assembly along the correct pathway. We study as a model system the coassembly of two monomers, the R- and S-chiral enantiomers of a π-conjugated oligo(p-phenylene vinylene) derivative. Coassembly kinetics are analyzed by developing a kinetic model, which reveals the initial assembly of metastable structures buffering free monomers and thereby slows the formation of thermodynamically stable assemblies. These metastable assemblies exert greater influence on the thermodynamically favored self-assembly pathway if the ratio between both monomers approaches 1:1, in agreement with experimental results. Moreover, competition by metastable assemblies is highly temperature dependent and hampers the assembly of equilibrium nanostructures most effectively at intermediate temperatures. We demonstrate that the rate of the assembly process may be optimized by tuning the cooling rate. Finally, it is shown by simulation that increasing the driving force for assembly stepwise by changing the solvent composition may circumvent metastable pathways and thereby force the assembly process directly into the correct pathway.

Merging of Hard Spheres by Phototriggered Micromanipulation

By careful design of the dendritic mol. the phys. properties of the self-assembled structure can be tuned in such a way that phototriggered micromanipulation and merging of hard spheres is possible. This possibility leads to the bottom-up synthesis of micrometer-sized objects through a combination of covalent synthesis and supramol. organization followed by micromanipulation.

Water-responsive dual-coloured photonic polymer coatings based on cholesteric liquid crystals

This work describes a straightforward method to prepare patterned photonic coatings which alter their colour when exposed to water. Various kinds of dual-coloured patterns were made, which become visible or fade away when placed in water. These effects are reversible and can be repeated many times.

Liquid crystal elastomer coatings with programmed response of surface profile

Stimuli-responsive liquid crystal elastomers with molecular orientation coupled to rubber-like elasticity show a great potential as elements in soft robotics, sensing, and transport systems. The orientational order defines their mechanical response to external stimuli, such as thermally activated muscle-like contraction. Here we demonstrate a dynamic thermal control of the surface topography of an elastomer prepared as a coating with a pattern of in-plane molecular orientation. The inscribed pattern determines whether the coating develops elevations, depressions, or in-plane deformations when the temperature changes. The deterministic dependence of the out-of-plane dynamic profile on the in-plane orientation is explained by activation forces. These forces are caused by stretching-contraction of the polymer networks and by spatially varying molecular orientation. The activation force concept brings the responsive liquid crystal elastomers into the domain of active matter. The demonstrated relationship can be used to design coatings with functionalities that mimic biological tissues such as skin.Liquid crystal elastomers are anisotropic rubbers which can be actuated by an external trigger. Here the authors develop elastomer coatings with pre-patterned molecular orientation that induces deterministic topography changes in response to changes in temperature.

Supramolecular chemistry at the liquid/solid interface probed by scanning tunnelling microscopy

The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunnelling microscopy (STM) is one of the preferred methodologies to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It is important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions to direct the ordering of both achiral and chiral molecules on the atomically flat surface. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.

Sub-5 nm structured films by hydrogen bonded siloxane liquid crystals and block copolymers

This paper describes the synthesis and characterization of a novel class of hydrogen-bonding oligo(dimethylsiloxane)-based thermotropic liquid crystals (LCs), with which polymeric supramolecules were obtained with glassy smectic and columnar sub-5 nm features when combined with poly(4-vinylpyridine) homopolymer or poly(styrene)-b-poly(4-vinylpyridine) block copolymer (BCP). The hierarchical self-assembly afforded by the LC/BCP complexes further resulted in the vertical orientation of the LC features in thin films.

On the Dimensional Control of 2D Hybrid Nanomaterials

Abstract Thermotropic smectic liquid crystalline polymers were used as a scaffold to create organic/inorganic hybrid layered nanomaterials. Different polymers were prepared by photopolymerizing blends of a hydrogen bonded carboxylic acid derivative and a 10 % cross‐linker of variable length in their liquid crystalline phase. Nanopores with dimensions close to 1 nm were generated by breaking the hydrogen bonded dimers in a high pH solution. The pores were filled with positively charged silver (Ag) ions, resulting in a layered silver(I)‐polymeric hybrid material. Subsequent exposure to a NaBH4 reducing solution allowed for the formation of supported hybrid metal/organic films. In the bulk of the film the dimension of the Ag nanoparticles (NPs) was regulated with subnanometer precision by the cross‐linker length. Ag nanoparticles with an average size of 0.9, 1.3, and 1.8 nm were produced inside the nanopores thanks to the combined effect of spatially confined reduction and stabilization of the nanoparticles by the polymer carboxylic groups. At the same time, strong Ag migration occurred in the surface region, resulting in the formation of a nanostructured metallic top layer composed of large (10–20 nm) NPs.