Model-driven optimization of multicomponent self-assembly processes.

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.

White-light emitting hydrogen-bonded supramolecular copolymers based on pi-conjugated oligomers.

Three different pi-conjugated oligomers (a blue-emitting oligofluorene, a green-emitting oligo(phenylene vinylene), and a red-emitting perylene bisimide) have been functionalized with self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at both ends. The molecules self-assemble in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random noncovalent copolymers are formed that contain all three types of chromophores, resulting in energy transfer upon excitation of the oligofluorene energy donor. At a certain mixing ratio, a white emissive supramolecular polymer can be created in solution. In contrast to their unfunctionalized counterparts, bis-UPy-chromophores can easily be deposited as smooth thin films on surfaces by spin coating. No phase separation is observed in these films, and energy transfer is much more efficient than in solution, giving rise to white fluorescence at much lower ratios of energy acceptor to donor. Light emitting diodes based on these supramolecular polymers have been prepared from all three types of pure materials, yielding blue, green, and red devices, respectively. At appropriate mixing ratios of these three compounds, white electroluminescence is observed. This approach yields a toolbox of molecules that can be easily used to construct pi-conjugated supramolecular polymers with a variety of compositions, high solution viscosities, and tuneable emission colors.

Preparation and characterization of helical self-assembled nanofibers.

This tutorial review serves as an introduction to helical self-assembled systems, illustrated by a specific class of helical aggregates: helical nanofibers. The common ways by which these systems are designed, prepared, and characterized will be presented.

Influence of the film thickness and morphology on the colorimetric properties of spray-coated electrochromic disubstituted 3,4-propylenedioxythiophene polymers.

Variation of the colorimetric properties as a function of the film thickness and morphology has been investigated for two spray-coated electrochromic disubstituted 3,4-propylenedioxythiophene polymers. Changes in the luminance, hue, and saturation have been tracked using CIE 1931 Lxy chromaticity coordinates, with CIELAB 1976 color space coordinates, L*, a*, and b*, being used to quantify the colors. For (precycled) neutral PProDOT-(Hx)(2) films, with an increase in the thickness, L* is seen to decrease, with a* and b* coordinates moving in positive and negative directions, respectively, with quantification of the pink/purple (magenta) color as the summation of red and blue. For all thicknesses, L* is comparable, pre- and postcycling, with a* decreasing (less red) and b* becoming more negative (more blue) and the film now appearing as purple in the neutral state. Color coordinates for the reverse (reduction) direction exhibited hysteresis in comparison with the initial oxidation, with the specific choice of perceived color values depending not only on the film thickness but also on both the potential applied and from which direction the potential is changed. Neutral PProDOT-(2-MeBu)(2) films appear blue/purple to the eye both as-deposited and after potential cycling to the transparent oxidized state. For the neutral, colored state, with an increase in the thickness, L* is seen to decrease, with a* and b* coordinates moving in positive and negative directions, respectively. For PProDOT-(2-MeBu)(2) films, the a* coordinates are lower positive values and the b* coordinates are higher negative values, thus quantifying the high dominance of the blue color in the blue/purple films compared to the pink/purple PProDOT-(Hx)(2) films. As for the PProDOT-(Hx)(2) films, the tracks of the color coordinates show that the specific choice of perceived color values depends on the film thickness. Unlike the PProDOT-(Hx)(2) films, hysteresis is absent in the oxidation/reduction track of the x-y coordinates for the PProDOT-(2-MeBu)(2) films, although slight hysteresis is present in the luminance. Characterization of the film morphologies through atomic force microscopy reveals a much rougher, higher surface area morphology for the PProDOT-(2-MeBu)(2) films versus the PProDOT-(Hx)(2) films. The branched repeat unit in the PProDOT-(2-MeBu)(2) films provides a structure that allows ions to ingress/egress more effectively, thus removing hysteresis from the optical response.

Chiral ethylhexyl substituents for optically active aggregates of pi-conjugated polymers.

We report an efficient synthesis of chiral (2S)-ethylhexanol for functionalizing and solubilizing conjugated polymers. The alpha-substituted chiral ethylhexyl side chains were obtained through a powerful and flexible asymmetric synthesis using pseudoephedrine as a chiral auxiliary. The dependence of the properties of conjugated polymers on molecular structure is investigated by circular dichroism, fluorescence, and absorption spectroscopy on two new chiral conjugated polymers, poly(3,3-bis((S)-2-ethylhexyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-ethylhexyl)(2)) and poly(3,3-bis((S)-2-methylbutyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-methylbutyl)(2)). The properties of PProDOT((2S)-ethylhexyl)(2)) differ significantly from those of its methylbutyl analog as investigated by chiral aggregation providing insight into the role of interchain interactions in these subsecond switching electrochromic polymers.