Selective Synthesis of Planar and Curved Graphene Nanoribbons

In this thesis new synthetic protocols towards the production of atomically precise graphene nanoribbons (GNRs) were presented. After the introduction of a reliable method to synthesize planar GNRs of different sizes, new curved, helical, and bilayered systems were subsequently produced in similar fashion. One of the major tasks in graphene chemistry is the selective synthesis of GNRs with high control over the width, edge structure, and the length of the aromatic scaffold, all of which influence the physical properties of such systems. The first part of this work was to overcome these challenges and to selectively produce planar GNRs of various sizes. For that, an iterative coupling strategy, involving Suzuki coupling, reduction and Sandmeyer bromination reactions, was invented. However, since the above mentioned strategy involves many reaction steps, which are not only time consuming but also consequently lower the overall yield, the synthesis of even larger GNRs had to be modified. The increased length of the π-network is accompanied by a drastic reduction of the solubility and thus the processability. This drawback was tackled by the addition of various sterically demanding groups (dodecyl, mesitylen, spirobifluorene) to the periphery of the compounds in order to minimize intermolecular π-π-stacking.Due to the great tunability of the produced building blocks, the final GNRs could possess a broad range of substituents. All synthesized GNRs were functionalized by halogen atoms, which gave the option for post-functionalization and the production of hybrid systems. For that, porphyrin, as another exciting compound class, was chosen as coupling partner. It turned out that the solubility of the respective GNR impacts the reaction. The moderately soluble GNR-Br was successfully transformed to the Mono GNR-ZnPor conjugate in a Suzuki coupling reaction with a borylated porphyrin derivative in relatively low yields of 20 %. In the second half, the topic of the thesis shifted from planar to curved graphene nanostructures with the preparation of a series of curved hexa-peri-hexabenzocoronenes (HBCs). The synthesis of five HBCs with different substitution patterns were performed through Diels-Alder cycloaddition reactions and the FpNA approach. The curvature was introduced by steric repulsion of neighboring methyl groups, which allowed a complete closure of the aromatic network. In this way, fully conjugated, but curved, HBCs have been generated. Merging the outstanding features of graphene and its smaller model systems with the chiroptical characteristics of helicenes results in new compound classes with fascinating physical properties. Therefore, chapter 3.5 was devoted to the expansion of literature known superhelicenes by the extension of the aromatic scaffold from HBC to GNR, the addition of more nanographene units, and the introduction of a spiro linker between these superhelicenes. The helicity results from the overlapping arrangement of the sterically demanding nanographene units attached to the fluorene moiety. Due to its straightforward synthesis, usually HBC is used as model system to mimic the properties of graphene on a molecular level. In this work, not only the helical HBC-based nanographene, but also the π-extended version, as well as both spirobifluorene (SBF) derivatives SBF-HBC and SBF-GNR have been prepared and compared. In the final section, the scope of helical nanographenes was further expanded by the insertion of preformed 5- and 6-helicenes between two GNRs, which resulted in bilayer motifs. The physical properties of these systems strongly depend on the overlap of the GNR layers. Since helicenes of different sizes were used in the preparation of two GNR-bilayer motifs, the GNR layers in both molecules are shifted differently to each other. The different degree of bilayer characteristics is therefore caused by the different amount of overlapping aromatic subunits.