Hans Wolfgang Spiess

Self-organization of supramolecular helical dendrimers into complex electronic materials

The discovery of electrically conducting organic crystals and polymers has widened the range of potential optoelectronic materials, provided these exhibit sufficiently high charge carrier mobilities and are easy to make and process. Organic single crystals have high charge carrier mobilities but are usually impractical, whereas polymers have good processability but low mobilities. Liquid crystals exhibit mobilities approaching those of single crystals and are suitable for applications, but demanding fabrication and processing methods limit their use. Here we show that the self-assembly of fluorinated tapered dendrons can drive the formation of supramolecular liquid crystals with promising optoelectronic properties from a wide range of organic materials. We find that attaching conducting organic donor or acceptor groups to the apex of the dendrons leads to supramolecular nanometre-scale columns that contain in their cores π-stacks of donors, acceptors or donor–acceptor complexes exhibiting high charge carrier mobilities. When we use functionalized dendrons and amorphous polymers carrying compatible side groups, these co-assemble so that the polymer is incorporated in the centre of the columns through donor–acceptor interactions and exhibits enhanced charge carrier mobilities. We anticipate that this simple and versatile strategy for producing conductive π-stacks of aromatic groups, surrounded by helical dendrons, will lead to a new class of supramolecular materials suitable for electronic and optoelectronic applications.

Catalyst-free preparation of melamine-based microporous polymer networks through Schiff base chemistry.

Recently, the synthesis of organic materials with high porosity has received considerable scientific interest, and various chemical approaches have been applied to the build-up of microporous polymer networks. In a novel catalyst-free process using Schiff base chemistry, melamine has been reacted with various di- and trivalent aldehydes to form a series of highly cross-linked microporous aminal networks with BET surface areas as high as 1377 m(2)/g and a NLDFT micropore volume of up to 0.41 cm(3)/g. It was shown that through the proper choice of the starting compounds the porosity of the final material can be fine-tuned. The materials contain up to 40 wt % of nitrogen and were also found to exhibit high thermal stability. Owing to the cheap and abundant monomers used in this study these networks are promising candidates for large-scale applications in gas storage, gas separation, catalysis, and sensing.

Physical properties of liquid crystals

Introduction and Historical Development. Guide to the Nomenclature and Classification of Liquid Crystals. Theory of the Liquid Crystalline State. Physical Properties.

Tunable and Switchable Dielectric Constant in an Amphidynamic Crystal

The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order-disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.

Determination of End-to-End Distances in a Series of TEMPO Diradicals of up to 2.8 nm Length with a New Four-Pulse Double Electron Electron Resonance Experiment

A four-pulse version of the pulsed double electron electron resonance (DEER) experiment has been applied to a series of TEMPO diradicals with well-defined interradical distances ranging from 1.4 to 2.8 nm (see picture). The new pulse sequence allows broad distributions of electron-electron distances to be measured without dead-time artifacts.

HIGHLY ORDERED COLUMNAR STRUCTURES FROM HEXA-PERI-HEXABENZOCORONENES-SYNTHESIS, X-RAY DIFFRACTION, AND SOLID-STATE HETERONUCLEAR MULTIPLE-QUANTUM NMR INVESTIGATIONS

Improved long-range ordering in the columnar mesophase of hexa(para-n-dodecylphenyl)hexabenzocoronene 1 has been achieved by inserting phenyl rings between the extended aromatic core of hexabenzocoronene and the alkyl side chains, which are needed to form liquid crystalline phases. The long-range hexagonal order of the columns is demonstrated by X-ray scattering, while the improved packing of the aromatic cores within the columns and the molecular mobility is probed by a newly developed heteronuclear multiple-quantum MAS NMR technique.

Length scale of dynamic heterogeneity in super-cooled glycerol near Tg

This letter presents the first direct measurement of the length scale of dynamic heterogeneity in a low molecular weight glass former without the perturbing effect of probe molecules or confinement. Using a multidimensional 13C solid-state exchange NMR experiment, 1 nm heterogeneities were found in glycerol for temperatures ranging from 199 K to 207 K (Tg=189 K). This small size and weak temperature dependence allow some distinctions to be made among different models of the glass transition. It is shown that the dynamics are not influenced by a low concentration (0.1 wt. %) relaxation agent Cu(NO3)2.

Photocatalytic hydrogen evolution through fully conjugated poly(azomethine) networks

Three-dimensional conjugated poly(azomethine) networks were found to be promising candidates for applications in photocatalytic water splitting. Straightforward synthetic protocols lead to fully organic photocatalysts that showed enhanced long-time stability. Furthermore, the catalytic performance of these materials was correlated to the molecular composition and the optoelectronic properties of the samples.

HIGH-RESOLUTION DOUBLE-QUANTUM NMR SPECTROSCOPY OF HOMONUCLEAR SPIN PAIRS AND PROTON CONNECTIVITIES IN SOLIDS

Abstract The NMR double-quantum response to a three-pulse sequence of dipolar coupled spin-1/2 pairs in powder samples spinning at the magic angle is investigated for arbitrary spinning frequencies. It is shown that double-quantum spinning sideband patterns result from the rotor modulation of the Hamiltonian. The existence of rotor modulated double-quantum spectra is confirmed by experiments with spin pairs of protons in barium chlorate monohydrate. The understanding of these sideband patterns now allow quantitative analysis of the double-quantum spectra to yield structural information in rigid solids. This is demonstrated by application to malonic acid. By combining single and double quantum NMR spectra proton distances within functional groups are determined, as well as connectivities between them. The 1H1H distance in the aliphatic group is 0.18 nm and the shortest distance between two carboxylic protons in the dimer structure is 0.22 nm. The closest aliphatic and carboxylic protons are separated by 0.26 nm, in agreement with neutron diffraction data.

Solid-state nuclear magnetic resonance spectra of dipolar-coupled multi-spin systems under fast magic angle spinning

A general treatment of nuclear magnetic resonance (NMR) spectra under magic-angle spinning (MAS) conditions is provided that is applicable both to homogeneously and inhomogeneously broadened lines. It is based on a combination of Floquet theory and perturbation theory, and allows the factorization of the spin system response into three factors that describe different aspects of the resulting MAS spectrum. The first factor directly reflects the Floquet theorem and describes the appearance of sidebands. The other two terms give the integral intensities of the resulting sidebands and their line shapes and depend on the specific features of the considered interaction. The analytical form of these two factors is derived for multi-spin dipolar interactions under fast MAS. The leading term in the expansion of the integral intensities involves products of only two spin operators whereas the linewidths, which are found to be different for the different sideband orders, are determined predominantly by three-spin te...