Klaus Kreger

Long-range energy transport in single supramolecular nanofibres at room temperature.

Efficient transport of excitation energy over long distances is a key process in light-harvesting systems, as well as in molecular electronics. However, in synthetic disordered organic materials, the exciton diffusion length is typically only around 10 nanometres (refs 4, 5), or about 50 nanometres in exceptional cases, a distance that is largely determined by the probability laws of incoherent exciton hopping. Only for highly ordered organic systems has the transport of excitation energy over macroscopic distances been reported—for example, for triplet excitons in anthracene single crystals at room temperature, as well as along single polydiacetylene chains embedded in their monomer crystalline matrix at cryogenic temperatures (at 10 kelvin, or −263 degrees Celsius). For supramolecular nanostructures, uniaxial long-range transport has not been demonstrated at room temperature. Here we show that individual self-assembled nanofibres with molecular-scale diameter efficiently transport singlet excitons at ambient conditions over more than four micrometres, a distance that is limited only by the fibre length. Our data suggest that this remarkable long-range transport is predominantly coherent. Such coherent long-range transport is achieved by one-dimensional self-assembly of supramolecular building blocks, based on carbonyl-bridged triarylamines, into well defined H-type aggregates (in which individual monomers are aligned cofacially) with substantial electronic interactions. These findings may facilitate the development of organic nanophotonic devices and quantum information technology.

Holographic Gratings and Data Storage in Azobenzene-Containing Block Copolymers and Molecular Glasses

This review covers synthesis, materials development, and photophysics of azobenzene-containing block copolymers as potential media for reversible volume holographic data storage. For high-density holographic data storage, volume gratings must be inscribed in millimeter-thick samples to achieve efficient angle multiplexing. It is demonstrated that block copolymers with azobenzene side-groups in the minority block develop no detrimental surface relief structures and exhibit superior performance regarding volume gratings, compared to homopolymers and statistical copolymers. Several material concepts for optimizing the refractive index modulation and the stability of volume gratings are presented. Stabilities of more than 2 years were achieved. Most important is the development of polymer blends comprising the azobenzene-containing block copolymer and an optically transparent homopolymer. This enables the preparation of millimeter-thick samples with the required optical density of ∼ 0. 7 at the writing wavelength by conventional injection molding techniques. The inscription of up to 200 holograms at the same lateral position was demonstrated. In addition, more than 1,000 write/erase cycles can be performed. This is the first time that the inscription and erasure of the long-term stable angle-multiplexed volume gratings in a rewritable polymeric medium have been achieved by purely optical means. A second important application for azobenzene-containing materials is the controlled preparation of surface relief structures. It is demonstrated that azobenzene-containing molecular glasses are an ideal class for efficient formation of surface relief gratings (SRGs) with amplitude heights of more than 600 nm. Clear relationships can be established between the chemical structure of the molecules and the behavior of SRG formation. All results are in agreement with the gradient force model by Kumar et al. The surface patterns are stable enough to be transferred to a polymer surface via replica molding.

Tailoring Supramolecular Nanofibers for Air Filtration Applications

The demand of new materials and processes for nanofiber fabrication to enhance the performance of air filters is steadily increasing. Typical approaches to obtain nanofibers are based on top-down processes such as melt blowing, centrifugal spinning, and electrospinning of polymer materials. However, fabrication of polymer nanofibers is limited with respect to either a sufficiently high throughput or the smallest achievable fiber diameter. This study reports comprehensively on a fast and simple bottom-up process to prepare supramolecular nanofibers in situ inside viscose/polyester microfiber nonwovens. Here, selected small molecules of the materials class of 1,3,5-benzenetrisamides are employed. The microfiber-nanofiber composites exhibit a homogeneous nanofiber distribution and morphology throughout the entire nonwoven scaffold. Small changes in molecular structure and processing solvent have a strong influence on the final nanofiber diameter and diameter distribution and, consequently, on the filtration performance. Choosing proper processing conditions, microfiber-nanofiber composites with surprisingly high filtration efficiencies of particulate matter are obtained. In addition, the microfiber-nanofiber composite integrity at elevated temperatures was determined and revealed that the morphology of supramolecular nanofibers is maintained compared to that of the utilized polymer nonwoven.

Athermal Azobenzene‐Based Nanoimprint Lithography

A novel nanoimprint lithography technique based on the photofluidization effect of azobenzene materials is presented. The tunable process allows for imprinting under ambient conditions without crosslinking reactions, so that shrinkage of the resist is avoided. Patterning of surfaces in the regime from micrometers down to 100 nm is demonstrated.

Photo-induced molecular alignment of trisazobenzene derivatives

Optically active small-molecular trisazobenzene derivatives are explored that allow facile photo-induced fabrication of holographic volume gratings which are unusually stable over time compared to structures based on other small-molecular organic compounds. The origin of this favorable characteristic of such architectures is investigated with three compounds that structurally differ only in the length of an alkyl spacer positioned between the molecular core and the active azobenzene chromophores. Species comprising spacers of sufficient length, and that exhibit a latent liquid-crystalline phase, feature efficient formation of stable, ordered domains in which the three side groups orient perpendicular to the polarization of the inscribing light beam. It is demonstrated that molecular order in these domains can be significantly improved by annealing at temperatures between the glass transition temperature and the clearing point of the specific compounds. This phenomenon allows combining the highly desirable processing characteristic of relatively short holographic writing times with small molecules in the fabrication of stable volume gratings, the latter feature so far having been reserved predominantly for polymeric species.

Influence of fluorine side-group substitution on the crystal structure formation of benzene-1,3,5-trisamides

By a combination of powder X-ray diffraction, multidimensional and multinuclear solid-state NMR spectroscopy and quantum chemical calculations, we were able to determine the crystal structure of 1,3,5-tris(2-fluoro-2-methylpropionylamino)benzene. Solid-state NMR experiments guided the structure solution by predicting the content of the asymmetric unit and the presence of a NH⋯OC hydrogen bond network. In addition to real-space structure solution and Rietveld refinement, quantitative symmetry-based 19F19F double-quantum recoupling experiments provided a cost function to determine the positions of the methyl groups and fluorine atoms. The structure solution of this particular fluorine-substituted trisamide illustrates the impact of fluorine side-group substitution on the common columnar packing motif of benzene-1,3,5-tricarboxamides. As also in the case 1,3,5-tris(2,2-dimethylpropionylamino)benzene, the supramolecular aggregation is then guided by the formation of triple helical NH⋯OC hydrogen bond networks within the individual columns. In contrast, the substitution of one methyl group by a fluorine atom in each side chain results in a two-dimensional NH⋯OC hydrogen bond pattern, leading to a lamellar crystal structure with only van der Waals interactions between the layers. Since fluorine is not involved in the hydrogen bond network and both chemical units exhibit a similar steric demand, the fundamental differences of the packing are most probably caused by changes in the molecular polarity.

Mesoscale Polarization by Geometric Frustration in Columnar Supramolecular Crystals

Abstract Columnar supramolecular phases with polarization along the columnar axis have potential for the development of ultrahigh‐density memories as every single column might function as a memory element. By investigating structure and disorder for four columnar benzene‐1,3,5‐trisamides by total X‐ray scattering and DFT calculations, we demonstrate that the column orientation, and thus the columnar dipole moment, is receptive to geometric frustration if the columns aggregate in a hexagonal rod packing. The frustration suppresses conventional antiferroelectric order and heightens the sensitivity towards collective intercolumnar packing effects. The latter finding allows for the building up of mesoscale domains with spontaneous polarization. Our results suggest how the complex interplay between steric and electrostatic interactions is influenced by a straightforward chemical design of the molecular synthons to create spontaneous polarization and to adjust mesoscale domain size.

Refractive-index determination of solids from first- and second-order critical diffraction angles of periodic surface patterns

We present two approaches for measuring the refractive index of transparent solids in the visible spectral range based on diffraction gratings. Both require a small spot with a periodic pattern on the surface of the solid, collimated monochromatic light, and a rotation stage. We demonstrate the methods on a polydimethylsiloxane film (Sylgard® 184) and compare our data to those obtained with a standard Abbe refractometer at several wavelengths between 489 and 688 nm. The results of our approaches show good agreement with the refractometer data. Possible error sources are analyzed and discussed in detail; they include mainly the linewidth of the laser and/or the angular resolution of the rotation stage. With narrow-band light sources, an angular accuracy of ±0.025∘ results in an error of the refractive index of typically ±5 ⋅ 10−4. Information on the sample thickness is not required.

Photo-Oriented Trisazobenzene Layers for Patterned Liquid-Crystal Alignment

Photosensitive trisazobenzene-based layers, after irradiation with polarized light, are employed to align the common liquid-crystal (LC) compound 4-cyano-4′-heptylbiphenyl (7CB) into predetermined patterns. Surface analysis of the trisazobenzene films indicate that ordering of the LC material is not caused by illumination-induced surface relief structures, but through “molecular mesoepitaxial” phenomena. This finding can be exploited to produce in one-step in-plane patterns of randomly oriented and ordered macroscopic LC domains by irradiation of the substrates.

Improved compression properties of polypropylene extrusion foams by supramolecular additives

Owing to the high lightweight design potential polymer foams become increasingly important. For rigid polymer foams, requiring high dimensional stability under load, a high compression modulus is a key feature. Here, we demonstrate how supramolecular additives can be utilized to control the foam morphology and to significantly improve the compression behavior of extruded foams made of linear isotactic polypropylene. Three different 1,3,5-benzenetrisamides were selected as supramolecular additives. These additives are soluble in the polymer melt and form a supramolecular nanofiber network upon cooling, acting as nucleating sites for both, foam cells and polymer crystals. It is shown that the in situ formed nanofiber network is very effective in reducing the cell size of extruded foams. Depending on the molecular structure and the concentration of the supramolecular additives, the compression modulus of polypropylene-polymer foams can be significantly increased compared to a reference foam with talc. Unexpectedly, an improvement of 100% with a concentration of only 0.02 wt% of a supramolecular additive compared to the neat polypropylene foam featuring similar densities is achieved. This improvement cannot be correlated with the foam morphology and is most likely attributed to the presence of the supramolecular nanofiber network.