Ingo Schnell

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.

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...

SPINNING-SIDEBAND PATTERNS IN MULTIPLE-QUANTUM MAGIC-ANGLE SPINNING NMR SPECTROSCOPY

Recent interest has focused on solid-state NMR experiments which excite multiple-quantum (MQ) coherences in the presence of magic-angle spinning (MAS). Such experiments have been applied to both dipolar-coupled spin I = 1/2 and half-integer quadrupolar systems. A feature common to both cases is the observation of interesting spinning sideband patterns in the indirect (MQ) dimension. In this paper, the origin of these patterns is reviewed in terms of two distinct mechanisms: first, rotor encoding of the dipolar or quadrupolar interaction caused by the change in the Hamiltonian active during the MQ reconversion period relative to the excitation period (reconversion rotor encoding, RRE); and, second, rotor modulation of the interaction during the evolution of the MQ coherences in the t 1 dimension (evolution rotor modulation, ERM). Only the first mechanism is present for total spin coherences, while for lower-order MQ coherences both mechanisms contribute to the pattern. For dipolar and quadrupolar model sys...

The competing effects of π–π packing and hydrogen bonding in a hexabenzocoronene carboxylic acid derivative: A 1H solid-state MAS NMR investigation

1 H solid-state NMR methods employing fast (νR=30 kHz) magic-angle spinning (MAS) are applied to the investigation of both the solid and columnar liquid-crystalline (LC) phases of HBC-C10COOH, a discotic molecule consisting of a hexa-peri-hexabenzocoronene core with six symmetrically attached alkyl chains, each chain being capped by a COOH group. In the solid phase at T=320 K, the presence of hydrogen-bonded COOH dimers is demonstrated by the observation of an auto COOH peak in the 1H double-quantum (DQ) MAS NMR two-dimensional spectrum, with the interproton distance being determined to be 0.279±0.09 nm. Considering the COOH peak in the one-dimensional 1H MAS spectra, both a shift to high field of the peak position as well as an initial increase followed by a subsequent decrease in the linewidth are observed upon heating. These observations are interpreted in terms of a chemical exchange process involving the making and breaking of hydrogen bonds, with the coalescence point corresponding to T=362 K. The equilibrium constant at a given temperature is calculated from the observed chemical shift, and a thermodynamic analysis yields for the opening of the hydrogen-bonded dimers: ΔH=45±4 kJ mol−1 and ΔS=113±11 J K−1 mol−1. In 1H DQ-filtered (DQF) MAS spectra, the intensity of the COOH peak is observed, upon heating, to reduce faster than expected from thermodynamic factors alone, and above T=380 K, no signal is detected. Using the fact that the observation of a DQ signal relies on the existence of a DQ coherence for the duration of the experiment, the kinetics of dimer opening are determined: the extracted activation energy and Arrhenius parameter equal 89±10 kJ mol−1 and 4.2×1016 s−1, respectively. On the transition to the LC phase, a marked narrowing of all resonances is observed, with the COOH chemical shift initially having a very high-field value, 9.0 ppm, which is indicative of free COOH groups. An analysis of DQ MAS spinning-sideband patterns shows that the dipolar coupling in the LC phase is reduced by a factor of 0.43±0.04.

Triple-Quantum NMR Spectroscopy in Dipolar Solids

Abstract Triple-quantum (TQ) NMR spectroscopy in dipolar solids under fast magic-angle spinning is introduced. Proton dipolar connectivities derived from two-dimensional high-resolution TQ NMR spectra are shown for bisphenol-A-polycarbonate, fully protonated and OH-deuterated dimethylglyoxime and acetonitrile trapped in the cages of perdeuterated hydroquinone. For spin-1/2-triads, fast rotating about their threefold symmetry axis, MAS-induced TQ spinning-sideband patterns are evaluated. Analytical treatment of isolated spin-1/2-triads is compared to numerical spin-dynamics simulations including influences of surrounding spins with respect to effects on both the integral intensities of TQ coherences and the signal distribution on TQ sideband patterns. Using the above samples, which contain methyl groups in different surroundings, the perturbing effects of neighboring protons are elucidated.

Quadruple hydrogen bonds of ureido-pyrimidinone moieties investigated in the solid state by 1H double-quantum MAS NMR spectroscopy

The structure of the quadruple hydrogen bond formed by ureido-pyrimidinone moieties is investigated in dimerised model compounds, as well as in a supramolecular polymer, by solid-state 1H double-quantum (DQ) NMR spectroscopy under fast magic-angle spinning (MAS). This NMR method combines the sensitivity of 1H NMR chemical shifts to the strengths of hydrogen bonds with quantitative information about dipole–dipole couplings between pairs of protons. Thus, two-dimensional 1H DQ MAS spectra provide particularly detailed insight into the arrangement of hydrogen bonds and allow proton–proton distances to be measured. For the supramolecular polymer, a thermally induced irreversible tautomeric rearrangement of the hydrogen-bonded moieties is elucidated in the bulk material. This process is associated with an Arrhenius activation energy of (145 ± 15) kJ mol−1, which can be rationalised in terms of hydrogen-bond dissociation and the reorientation of the supramolecular polymer chain.

NMR chemical shifts in periodic systems from first principles

A recently developed ab-initio method for the calculation of NMR chemical shifts and magnetic susceptibilities in systems under periodic boundary conditions is presented and applied to a hydrogen-bonded molecular crystal. The calculations can unambiguously assign the chemical shifts to individual atoms in experimental spectra, and can further serve for the validation of simulated atomic trajectories and geometries. Apart from the example presented, the method can be applied to crystalline and amorphous insulators, as well as to isolated molecules using a supercell technique. The results are in good agreement with experiment.

NMR chemical shifts in proton conducting crystals from first principles

Abstract We compute the hydrogen 1 H NMR chemical shift spectra of proton conducting crystals using a recently developed density functional theory method for systems under periodic boundary conditions. Comparison with experimental spectra yields an excellent agreement. Thus, besides of unambiguously assigning the chemical shifts to individual atoms, the calculations can also characterize the microscopic hydrogen bonding structure of this class of materials. Apart from the example presented, the method can be applied to crystalline and amorphous insulators and semiconductors, as well as to isolated molecules using a supercell technique. It is implemented in CPMD, a state-of-the-art pseudopotential plane wave DFT package.

Erratum: Self-organization of supramolecular helical dendrimers into complex electronic materials (Nature (2002) 419 (384-387))

This corrects the article DOI: nature01072

2H Chemical-shift resolution and dipolar2H-1H,2H-15N correlations in solid-state MAS NMR spectroscopy for structure determination and distance measurements in hydrogen-bonded systems

In solid-state NMR, deuteron (2H) spectroscopy can be performed in full analogy to1H spectroscopy, including2H chemical-shift resolution and2H-X dipolar correlation schemes, when the NMR experiments are conducted in a “rotor-synchronized” fashion under fast magic-angle spinning. Here, 2H-X NMR experiments of this type, including2H-15N and2H-1H chemical-shift correlations and distance measurements, are introduced and demonstrated on cytosine monohydrate, whose acidic protons can readily be replaced by deuterons by recrystallization from D2O. In this way,2H NMR spectroscopy provides information complementary to1H NMR data, which is particularly useful for studying hydrogen bonds in supra- or biomolecular systems.