Dirk G. Kurth

Metallosupramolecular Polyelectrolytes Self-Assembled from Various Pyridine Ring-Substituted Bisterpyridines and Metal Ions: Photophysical, Electrochemical, and Electrochromic Properties

This work presents several metallosupramolecular coordination polyelectrolytes (MEPEs) self-assembled from rigid, pi-conjugated, pyridine ring functionalized bisterpyridines and metal ions. The MEPEs are water-soluble and display different colors spanning the entire visible regions. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. The results show that the properties are profoundly affected by the nature of the substituents at the peripheral pyridine rings. Namely, MEPEs assembled from the electron-rich OMe group modified ligands exhibit high switching reversibility and stability and show a lower switching potential than the unsubstituted and electron-deficient Br-substituted analogues. The response times can be tuned either by the design of the ligands or by the choice of the metal ions to cover a broad time scale from under 1 s to several minutes. The optical memory is enhanced from 30 s to longer than 15 min as a comparison of unsubstituted and substituted MEPEs shows. Thus, the significantly enhanced stability and the ease of tuning the properties render this type of supramolecular assembly attractive as electrochromic materials for applications in a large variety of areas. Most importantly, we presented the structure-property relationships of MEPEs, which lays the groundwork for further design of new bisterpyridine-based metallosupramolecular functional materials.

A thin-film electrochromic device based on a polyoxometalate cluster

The realization of molecular-based switching and display devices faces two entirely different challenges that require a scientific remedy. First, components possessing addressable states with distinct physical properties have to be identified and synthesized. The components must operate reversibly with long-term stability and suitable response times. The stimulus threshold and the power consumption for switching between states ought to be low. Second, one or several active components have to be positioned in a predefined way into the actual device’s structure. An efficient and rational approach towards this goal is to use strategies from supramolecular chemistry. The future fabrication of such materials may, therefore, rely on principles of molecular self-organization. [1] Most likely, thin-film technologies will play a major role in future applications. [2] In terms of possible components for electrochromic devices, polyoxometalates (POMs) are promising candidates due to their ability to act as an electron reservoir, thereby giving rise to colored mixed-valence state species while retaining their structural integrity. [3‐7] In contrast to many semiconductor nanoparticles and quantum dots, POMs are discrete, molecularly defined metal‐oxide clusters with an extensive range of structures and properties. For the realization of such devices it will, therefore, be necessary to identify existing and to synthesize novel POMs with sizeable electrochromic response and stable redox states. [8] Despite the potential of POMs, their implementation in advanced materials has remained elusive, mainly due to the fact that they are obtained as crystalline solids, which are difficult to process. [9] Traditionally, thin films of POMs are made by spin coating, the Langmuir‐Blodgett technique, electrodeposition, or simply by compressing POM solids against an indium tin oxide (ITO)-coated glass slide. [10] More recent approaches involve the use of surfactant-encapsulated POM clusters. [11] A simple

Surfactant -Encapsulated Clusters (SECs): (DODA)20(NH4)[H3Mo57V6(NO)6O183(H2O)18], a Case Study

We present a comprehensive study of the partially reduced polyoxomolybdate [H3-Mo57V6(NO)6O183(H2O)18]21-encapsulated in a shell of dimethyldioctadecylammonium (DODA) surfacmolecules. Treatment of an aqueous solution of (NH4)21[H3Mo57V6-(NO)6O183(H2O)18] . 65H2O (1a) with a trichloromethane solution of the surfactant leads to instant transfer of the encapsulated complex anion into the organic phase. Results from vibrational spectroscopy. analytical ultracentrifugation, small-angle X-ray scattering, transmission electron microscopy, elemental analysis, and Langmuir compression isotherms are consistent with a single polyoxometalate core encapsulated within a shell of 20 DODA molecules. The molar mass of the supramolecular assembly is 20249 gmol(-1) and the diameter is 3.5 nm. A material with the empirical formula (DODA)20(NH4)[H3-Mo57V6NO)6O183(H2O)18] (2) was isolated as a dark violet solid, which readily dissolves in organic solvents. Slow evaporation of solutions of 2 on solid substrates forces the hydrophobic particles to aggregate into a cubic lattice. Annealing these so-formed films at elevated temperature causes de-wetting with terrace formation similar to liquid crystals and block copolymers. Compound 2 forms a stable Langmuir monolayer at the air-water interface; Langmuir-Blodgett multilayers are readily prepared by repeated transfer of monolayers on solid substrates. The films were characterized by optical ellipsometry, Brewster angle microscopy, transmission electron microscopy, and X-ray reflectance.

Metallosupramolecular thin polyelectrolyte films.

Molecular recognition and electrostatic interaction of oppositely charged polyelectrolytes are combined in the fabrication of ultrathin metallosupramolecular multilayers [shown schematically in the picture, PEI=polyethyleneimine, PSS=poly(styrene sulfonate)]. The layers between the PSS layers are composed of an iron(II) bis(terpyridine) coordination polymer.

Transition metal ions: weak links for strong polymers

Extending macromolecular chemistry beyond carbon-based polymers offers fascinating perspectives and an enormous potential to improve the capacity of macromolecular materials with new dynamic properties.

Functional Polyoxometalate Thin Films via Electrostatic Layer-by-Layer Self-Assembly

Polyoxometalates (POMs) comprise a structurally diverse class of inorganic transition metal oxygen clusters which—owing to their unique electronic properties—hold promise for a host of technological applications such as electrochromic windows, sensors, or heterogeneous catalysts, prototypic examples of which will be briefly exemplified. The integration of POMs into functional architectures and devices, however, necessitates the development of general methods that allow positioning these clusters in well-defined supramolecular architectures, thin films, or mesophases. This short review highlights recent advances in the preparation of composite multilayers fabricated by electrostatic layer-by-layer self-assembly (ELSA) of POMs and a variety of water-soluble cationic species, including transition metal complexes, cationic surfactants, polycations and bipolar pyridine.

Self-Assembly Made Durable: Water-Repellent Materials Formed by Cross-Linking Fullerene Derivatives

Fullerene flakes: A diacetylene-functionalized fullerene derivative self-organizes into flakelike microparticles (see picture). Both the diacetylene and C(60) moieties can be effectively cross-linked, which leads to supramolecular materials with remarkable resistivity to solvent, heat, and mechanical stress. Moreover, the surface of the cross-linked flakelike objects is highly durable and water-repellent.

Layer-by-layer arrangement by protein-protein interaction of sulfite oxidase and cytochrome c catalyzing oxidation of sulfite

Layer-by-layer self-assembly of sulfite oxidase and cytochrome c was carried out without additional polymeric polyelectrolytes. The arrangement shows a linear increase in the immobilized protein mass after each deposition cycle. The modified electrodes demonstrate electrocatalytic activity for sulfite oxidation generating catalytic current with a linear increase with the number of layers. This effect shows that, in the protein assembly without a polyelectrolyte, electron transfer occurs, thus supporting the concept for direct interprotein electron exchange.

Superstructures and superhydrophobic property in hierarchical organized architectures of fullerenes bearing long alkyl tails

Formation of hierarchically self-organized architectures in organic media and their non-wetting surface features of a series of fullerene-C60 derivatives bearing different numbers of long hydrocarbon chains (1–3) and semiperfluoro-alkyl tails (4) are investigated by means of a variety of techniques, including X-ray diffraction, differential scanning calorimetry, as well as spectroscopic and microscopic methods. All derivatives self-assemble into a bilayer arrangement with their fundamental structural subunit and lamellar distance ranging from 2.88 to 4.85 nm depending on the substituents. The hydrocarbon-C60 derivatives (1–3) provide well-defined three-dimensional microparticles having a nanoflaked outer surface morphology or microparticles composed of many plate-like units in 1,4-dioxane solution, both architectures enhancing the surface water-repellency. The microparticles with a nanoflaked-outer surface obtained from a C60 derivative with semiperfluoro-alkyl chains (4) in a diethoxyethane solution exhibit a surface water-repellency comparable to objects formed from the hydrocarbon-hybrid C60 derivatives. Taking into account the moderate hydrophobic nature of the C60 surface compared to the high hydrophobicity of the hydro- or fluoro-carbons, these results suggest that the C60 moieties are exposed to the outer surface in the supramolecular objects formed in polar solvent conditions and define their non-wetting properties.

Electron Transport and Electrochemistry of Mesomorphic Fullerenes with Long-Range Ordered Lamellae

Fullerenes, C60, modified with long alkyl chains form long-range ordered lamellar mesophases permitting a high C60 content. The mesomorphic fullerenes feature reversible electrochemistry and a comparably high electron carrier mobility making them attractive components for fullerene-based soft materials.