Holger Frauenrath

Nanostructured Carbonaceous Materials from Molecular Precursors

Nanostructured carbonaceous materials, that is, carbon materials with a feature size on the nanometer scale and, in some cases, functionalized surfaces, already play an important role in a wide range of emerging fields, such as the search for novel energy sources, efficient energy storage, sustainable chemical technology, as well as organic electronic materials. Furthermore, such materials might offer solutions to the challenges associated with the on-going depletion of nonrenewable energy resources or climate change, and they may promote further breakthroughs in the field of microelectronics. However, novel methods for their preparation will be required that afford functional carbon materials with controlled surface chemistry, mesoscopic morphology, and microstructure. A highly promising approach for the synthesis of such materials is based on the use of well-defined molecular precursors.

A General Concept for the Preparation of Hierarchically Structured π‐Conjugated Polymers

Typical biopolymers exhibit structures and order on different length scales. By contrast, the number of synthetic polymers with a similar degree of hierarchical structure formation is still limited. Starting from recent investigations on the structures of amyloid proteins as well as research activities toward nanoscopic scaffolds from synthetic oligopeptides and their polymer conjugates, a general strategy toward hierarchically structured pi-conjugated polymers can be developed. The approach relies on the supramolecular self-assembly of diacetylene macromonomers based on beta-sheet forming oligopeptides equipped with hydrophobic polymer segments. Polymerization of these macromonomers proceeds under retention of the previously assembled hierarchical structure and yields pi-conjugated polymers with multi-stranded, multiple-helical quaternary structures.

Functional carbon nanosheets prepared from hexayne amphiphile monolayers at room temperature

Carbon nanostructures that feature two-dimensional extended nanosheets are important components for technological applications such as high-performance composites, lithium-ion storage, photovoltaics and nanoelectronics. Chemical functionalization would render such structures better processable and more suited for tailored applications, but typically this is precluded by the high temperatures needed to prepare the nanosheets. Here, we report direct access to functional carbon nanosheets of uniform thickness at room temperature. We used amphiphiles that contain hexayne segments as metastable carbon precursors and self-assembled these into ordered monolayers at the air/water interface. Subsequent carbonization by ultraviolet irradiation in ambient conditions resulted in the quantitative carbonization of the hexayne sublayer. Carbon nanosheets prepared in this way retained their surface functionalization and featured an sp(2)-rich amorphous carbon structure comparable to that usually obtained on annealing above 800 °C. Moreover, they exhibited a molecularly defined thickness of 1.9 nm, were mechanically self-supporting over several micrometres and had macroscopic lateral dimensions on the order of centimetres.

Hierarchically Structured Microfibers of “Single Stack” Perylene Bisimide and Quaterthiophene Nanowires

Organic nanowires and microfibers are excellent model systems for charge transport in organic semiconductors under nanoscopic confinement and may be relevant for future nanoelectronic devices. For this purpose, however, the preparation of well-ordered organic nanowires with uniform lateral dimensions remains a challenge to achieve. Here, we used the self-assembly of oligopeptide-substituted perylene bisimides and quaterthiophenes to obtain well-ordered nanofibrils. The individual nanofibrils were investigated by spectroscopic and imaging methods, and the preparation of hierarchically structured microfibers of aligned nanofibrils allowed for a comprehensive structural characterization on all length scales with molecular level precision. Thus, we showed that the molecular chirality resulted in supramolecular helicity, which supposedly serves to suppress lateral aggregation. We also proved that, as a result, the individual nanofibrils comprised a single stack of the π-conjugated molecules at their core. Moreover, the conformational flexibility between the hydrogen-bonded oligopeptides and the π-π stacked chromophores gave rise to synergistically enhanced strong π-π interactions and hydrogen-bonding. The result is a remarkably tight π-π stacking inside the nanofibrils, irrespective of the electronic nature of the employed chromophores, which may render them suitable nanowire models to investigate one-dimensional charge transport along defined π-π stacks of p-type or n-type semiconductors.

Synthesis of Diacetylene-Containing Peptide Building Blocks and Amphiphiles, their Self-Assembly and Topochemical Polymerization in Organic Solvents

A series of functional iodoacetylenes was prepared and converted into the corresponding diacetylene-substituted amino acids and peptides via Pd/Cu-promoted sp-sp carbon cross-coupling reactions. The unsymmetrically substituted diacetylenes can be incorporated into oligopeptides without a change in the oligopeptide strands directionality. Thus, a series of oligopeptide-based, amphiphilic diacetylene model compounds was synthesized, and their self-organization as well as their UV-induced topochemical polymerizability was investigated in comparison to related polymer-substituted macromonomers. Solution-phase IR spectroscopy, gelation experiments, and UV spectroscopy helped to confirm that a minimum of five N-H...O=C hydrogen-bonding sites was required in order to obtain reliable aggregation into stable beta-sheet-type secondary structures in organic solvents. Furthermore, the non-equidistant spacing of these hydrogen-bonding sites was proven to invariably lead to beta-sheets with a parallel beta-strand orientation, and the characteristic IR-spectroscopic signatures of the latter in organic solution was identified. Scanning force micrographs of the organogels revealed that compounds with six hydrogen-bonding sites gave rise to high aspect ratio nanoscopic fibrils with helical superstructures but, in contrast to the related macromonomers, did not lead to uniform supramolecular polymers. The UV-induced topochemical polymerization within the beta-sheet aggregates was successful, proving parallel beta-strand orientation and highlighting the effect of the number and pattern of N-H...O=C hydrogen-bonding sites as well as the hydrophobic residue in the molecular structure on the formation of higher structures and reactivity.

A convenient Negishi protocol for the synthesis of glycosylated oligo(ethynylene)s.

A convenient and efficient sp-sp carbon heterocoupling protocol based on the Negishi reaction was developed, in which the required zinc diacetylide was generated from 1,4-bis(trimethylsilyl)butadiyne in situ and reacted with a bromoacetylene in apolar solvent mixtures. The method has been applied to the synthesis of unsymmetric glycosylated and symmetric diglycosylated oligo(ethynylene)s up to the octa(ethynylene).

Perfluorophenyl–Phenyl Interactions in the Crystallization and Topochemical Polymerization of Triacetylene Monomers

A series of symmetrically and unsymmetrically substituted octa-2,4,6-triyne-1,8-diol derivatives with benzoyl, 4-dodecyloxybenzoyl, as well as perfluorobenzoyl substituents were prepared and investigated with respect to their crystal structures and topochemical polymerizability. Single-crystal structures for several of these triacetylene monomers have been obtained and proved that the perfluorophenyl-phenyl interactions played a decisive role in the molecular packing. As a consequence of the geometric requirements imposed by the perfluorophenyl-phenyl interactions, packing parameters appropriate for a topochemical triacetylene polymerization in the sense of either a 1,6- or a 1,4-polyaddition along different crystallographic axes were observed in two cases, and UV irradiation led to successful polymerization. Raman as well as solid-state (13)C NMR spectra of the obtained polymers revealed that the polymerization had predominantly proceeded in the form of a 1,4-polyaddition.

Consecutive Conformational Transitions and Deaggregation of Multiple-Helical Poly(diacetylene)s

The polymerization of diacetylene macromonomers based on oligopeptide-polymer conjugates yields conjugated polymers with multiple-helical quaternary structures. These polymers exhibit a rich dynamic folding behavior upon the addition of protic cosolvents. Thus, a helix-helix transition under helix-sense inversion was followed by a reversible helix-coil transition. Both transitions involved changes in the aggregation state of the multiple-helical superstructures. The resemblance of the observed consecutive and cooperative conformational transitions to those of biopolymers underlines the importance of supramolecular self-assembly as a pathway toward biofunctional materials with optoelectronic activity.

Coordination-Driven Self-Assembly of PEO-Functionalized Perylene Bisimides: Supramolecular Diversity from a Limited Set of Molecular Building Blocks

A limited number of poly(ethylene oxide)-substituted perylene bisimides, some of which are equipped with terpyridine ligands for transition-metal coordination (see structure), combine different types of noncovalent interactions to yield optoelectronically active organic materials with different types of supramolecular morphologies.

A multistep single-crystal-to-single-crystal bromodiacetylene dimerization

Packing constraints and precise placement of functional groups are the reason that organic molecules in the crystalline state often display unusual physical or chemical properties not observed in solution. Here we report a single-crystal-to-single-crystal dimerization of a bromodiacetylene that involves unusually large atom displacements as well as the cleavage and formation of several bonds. Density functional theory computations support a mechanism in which the dimerization is initiated by a [2 + 1] photocycloaddition favoured by the nature of carbon-carbon short contacts in the crystal structure. The reaction proceeded up to the theoretical degree of conversion without loss of crystallinity, and it was also performed on a preparative scale with good yield. Moreover, it represents the first synthetic pathway to (E)-1,2-dibromo-1,2-diethynylethenes, which could serve as synthetic intermediates for the preparation of molecular carbon scaffolds. Our findings both extend the scope of single-crystal-to-single-crystal reactions and highlight their potential as a synthetic tool for complex transformations.