Michael Ryan Hansen

Towards high charge-carrier mobilities by rational design of the shape and periphery of discotics

Discotic liquid crystals are a promising class of materials for molecular electronics thanks to their self-organization and charge transporting properties. The best discotics so far are built around the coronene unit and possess six-fold symmetry. In the discotic phase six-fold-symmetric molecules stack with an average twist of 30 degrees, whereas the angle that would lead to the greatest electronic coupling is 60 degrees. Here, a molecule with three-fold symmetry and alternating hydrophilic/hydrophobic side chains is synthesized and X-ray scattering is used to prove the formation of the desired helical microstructure. Time-resolved microwave-conductivity measurements show that the material has indeed a very high mobility, 0.2 cm(2) V(-1) s(-1). The assemblies of molecules are simulated using molecular dynamics, confirming the model deduced from X-ray scattering. The simulated structures, together with quantum-chemical techniques, prove that mobility is still limited by structural defects and that a defect-free assembly could lead to mobilities in excess of 10 cm(2) V(-1) s(-1).

Two‐Dimensional Sandwich‐Type, Graphene‐Based Conjugated Microporous Polymers

There are several classes ofmicro-/mesoporous polymers, such as hyper-crosslinked poly-mers(HCPs),polymersofintrinsicmicroporosity(PIMs),andcovalent organic frameworks (COFs). Porous polymers canbe also classified according to their structural conformationsas amorphous- (HCPs and PIMs) or crystalline-type (COFs)materials.

Conjugated Microporous Polymers with Dimensionality‐Controlled Heterostructures for Green Energy Devices

Dimensionality for conjugated micro-porous polymers (CMP-nD, n = 0, 1, 2) is proven to be of great importance for tailoring their photophysical properties. Moreover, CMP-nD can further be converted into boron and nitrogen co-doped porous carbons (nDBN, n = 0, 1, 2) with maintained 0D, 1D, and 2D nano-structures and highly efficient catalytic performance.

High-resolution 13C NMR spectra and long-range 13C1H spin coupling constants in pyridine and 2-bromopyridine

Abstract High-resolution proton undecoupled 13 C NMR spectra (continuous wave mode) have been obtained for pyridine and 2-bromopyridine using the 13 C isotope in natural abundance. Analysis of these spectra has provided all possible long-range 13 CH coupling constants including their relative signs, all directly bonded 13 CH coupling constants, and in some cases differences in the 13 C isotope effects on the 1 H chemical shifts for these compounds. The 13 CH coupling constants in pyridine and the effects of a 2-bromo substituent on these parameters are discussed with respect to corresponding data in previously reported compounds and with values obtained from CNDO/2 and INDO calculations. Contributions from incomplete quadrupolar removal of the 13 C 14 N coupling constants to the 13 C line widths have been calculated and compared with experimental values.

Use of X-Ray Diffraction, Molecular Simulations, and Spectroscopy to Determine the Molecular Packing in a Polymer-Fullerene Bimolecular Crystal

The molecular packing in a polymer: fullerene bimolecular crystal is determined using X-ray diffraction (XRD), molecular mechanics (MM) and molecular dynamics (MD) simulations, 2D solid-state NMR spectroscopy, and IR absorption spectroscopy. The conformation of the electron-donating polymer is significantly disrupted by the incorporation of the electron-accepting fullerene molecules, which introduce twists and bends along the polymer backbone and 1D electron-conducting fullerene channels.

Bottom-Up Synthesis of Liquid-Phase-Processable Graphene Nanoribbons with Near-Infrared Absorption

Structurally defined, long (>100 nm), and low-band-gap (∼1.2 eV) graphene nanoribbons (GNRs) were synthesized through a bottom-up approach, enabling GNRs with a broad absorption spanning into the near-infrared (NIR) region. The chemical identity of GNRs was validated by IR, Raman, solid-state NMR, and UV-vis-NIR absorption spectroscopy. Atomic force microscopy revealed well-ordered self-assembled monolayers of uniform GNRs on a graphite surface upon deposition from the liquid phase. The broad absorption of the low-band-gap GNRs enables their detailed characterization by Raman and time-resolved terahertz photoconductivity spectroscopy with excitation at multiple wavelengths, including the NIR region, which provides further insights into the fundamental physical properties of such graphene nanostructures.

Solid-State NMR in Macromolecular Systems: Insights on How Molecular Entities Move

The function of synthetic and natural macromolecularsystems critically depends on the packing and dynamics of the individual components of a given system. Not only can solid-state NMR provide structural information with atomic resolution, but it can also provide a way to characterize the amplitude and time scales of motions over broad ranges of length and time. These movements include molecular dynamics, rotational and translational motions of the building blocks, and also the motion of the functional species themselves, such as protons or ions. This Account examines solid-state NMR methods for correlating dynamics and function in a variety of chemical systems. In the early days, scientists thought that the rotationalmotions reflected the geometry of the moving entities. They described these phenomena as jumps about well-defined axes, such as phenyl flips, even in amorphous polymers. Later, they realized that conformational transitions in macromolecules happen in a much more complex way. Because the individual entities do not rotate around well-defined axes, they require much less space. Only recently researchers have appreciated the relative importance of large angle fluctuations of polymers over rotational jumps. Researchers have long considered that cooperative motions might be at work, yet only recently they have clearly detected these motions by NMR in macromolecular and supramolecular systems. In correlations of dynamics and function, local motions do not always provide the mechanism of long-range transport. This idea holds true in ion conduction but also applies to chain transport in polymer melts and semicrystalline polymers. Similar chain motions and ion transport likewise occur in functional biopolymers, systems where solid-state NMR studies are also performed. In polymer science, researchers have appreciated the unique information on molecular dynamics available from advanced solid-state NMR at times, where their colleagues in the biomacromolecular sciences have emphasized structure. By contrast, following X-ray crystallographers, researchers studying proteins using solution NMR introduced the combination of NMR with computer simulation before that became common practice in solid-state NMR. Todays simulation methods can handle partially ordered or even disordered systems common in synthetic polymers. Thus, the multitechnique approaches employed in NMR of synthetic and biological macromolecules have converged. Therefore, this Account will be relevant to both researchers studying synthetic macromolecular and supramolecular systems and those studying biological complexes.

Empty helical nanochannels with adjustable order from low-symmetry macrocycles.

Natural channel-forming structures are mandatory for connecting different compartments within a living organism. For instance, transmembrane proteins function as ion channels, transporters, or antibiotics. Biomacromolecules that are formed during evolution self-assemble into tubular structures with precisely defined positions of functional groups. The stimuli-responsive activity of these molecules has inspired the search for artificial channel-forming structures that can mimic the functionality of the natural systems. Artificial channel systems may even include new functionalities in advanced chemical applications. Several attempts, including templating methods, have been reported for the de novo design of pore-forming structures that are stable both in solution and in the bulk state. In particular, macrocycles have an attractive topology for the formation of supramolecular channels if they organize in a columnar mesophase with close packing of successive rings. If properly designed, a channel is created with tight walls that do not allow the penetration of small molecules. In contrast to macrocycles that are held together by strong intermolecular forces, such as hydrogen bonds in cyclopeptides or cyclosaccharides, the increased mobility in the liquid-crystalline (LC) phase allows for self-healing and orientation of the channels by external forces (shear, electromagnetic fields, surface properties, etc.). When the channels are appropriately functionalized, the inclusion and manipulation of nano-objects becomes feasible. Columnar mesophases have indeed been found in macrocyclic polyamines. However, because of their flexibility, the macrocyclic rings assume a folded conformation and stable phases with large open voids have not been reported to date. This problem might be overcome by using shapepersistent macrocycles, and columnar liquid-crystalline compounds based on cyclic phenylene and phenylene–ethynylene oligomers with inner diameters of up to 1 nm, as deduced from X-ray diffraction studies (XRD), have been reported (Figure 1a). Powder XRD cannot provide details about the packing of the macrocycles on the molecular level, 12c] whereas solid-state NMR spectroscopy can provide this information with the help of quantum-chemical calculations. In contrast to diffraction techniques, NMR spectroscopy does not require strict periodicity and is therefore particularly suited to probe the local structure in LC phases. Moreover, NMR spectroscopy can be used to reveal the presence of guest molecules inside the channels, including back-folded side chains. Herein, we describe two phenylene–ethynylene–butydiynylene macrocycles 1a and 1b (Figure 1b), each of which contains two benzo[1,2-b:4,3-b’]dithiophene units that include a nanoscale interior with a diameter as large as approximately 1.3 nm (Figure 1a). At the expense of symmetry, we have introduced groups with different electron affinities. Both macrocycles 1a and 1 b were obtained by the statistical oxidative Glaser coupling of the appropriate “half-rings” under Pd/Cu catalysis, and were obtained in yields of 33% (1a) and 50% (1 b) after purification by recycling gel-permeation chromatography (recGPC) using THF as eluent, and subsequent precipitation from methanol and drying under vacuum. The compounds were obtained as slightly yellow powders that do not contain residual solvents as shown by NMR spectroscopy of solutions in dichloromethane (see the Supporting Information). Upon heating above room temperature, the compounds become waxy materials that are birefringent under optical microscopy (crossed polarizers). Differential scanning calorimetry (DSC) investigations (2nd heating; 10 8C min ) showed reversible endothermic transitions for both compounds, thus indicating different LC phases over a broad temperature range (1a : 22 8C (138.9 kJ mol ), 109 8C (85.2 kJmol ), 151 8C (2.8 kJ mol ); 1b : 33 8C (116.6 kJmol ), 160 8C (2.6 kJ mol )). The type of LC phases formed and the lattice parameters were determined by XRD. Upon cooling from the isotropic [*] Dr. M. Fritzsche, Dr. G. Richardt, Prof. Dr. S. H ger Kekul -Institut f r Organische Chemie und Biochemie Rheinische Friedrich-Wilhelms-Universit t Bonn Gerhard-Domagk-Str. 1, 53121 Bonn (Germany) Fax: (+ 49)228-73 5662 E-mail: hoeger@uni-bonn.de

Cooperative molecular motion within a self-assembled liquid-crystalline molecular wire: the case of a TEG-substituted perylenediimide disc.

Always on the move: Molecular dynamics of perylene cores in columnar structures influences the processability and self-healing of these materials. A combination of X-ray scattering and advanced solid-state NMR methods show that these systems have restricted angular mobility of the cores even in the frozen phase, and a cooperative spiral type of motion in the liquid crystalline phase (see picture).

Probing the Relation Between Charge Transport and Supramolecular Organization Down to Ångström Resolution in a Benzothiadiazole‐Cyclopentadithiophene Copolymer

Molecular modeling shows that longitudinal displacement of the backbones by a couple of ångströms has a profound impact on the electronic coupling mediating charge transport in a conjugated copolymer. These changes can be probed by monitoring the calculated X-ray scattering patterns and NMR chemical shifts as a function of sliding of the polymer chains and comparing them to experiment.