Graphitic Nanoribbons with Dibenzo[e,l]pyrene Repeat Units: Synthesis and Self-Assembly

A novel homologous series of five monodisperse ribbon-type polyphenylenes, with rigid dibenzo[e,l]pyrene cores in the repeat units, are synthesized by a microwave-assisted Diels-Alder reaction. The size of the obtained polyphenylene ribbons ranges from 132 to 372 carbon atoms in the aromatic backbone which incorporates Lip to six dibenzo[e,l]pyrene units, thus Showing quite different aspect ratios. Because of the flexibility of the backbone and the peripheral substitution with dodecyl chains, the polyphenylene ribbons are well soluble in organic solvents and can be fully characterized by standard analytical techniques. Their unique structure is especially designed to produce a series of giant ribbon-type polycyclic aromatic hydrocarbons (PAHs) in a single further reaction step by cyclodehydrogenation. Therefore, the incorporated dibenzo[e,l]pyrene cores are an important feature because they facilitate the dehydrogenation and improve the reaction yields. The smallest representative of the completely dehydrogenated planarized (2D) PAH ribbons. prepared from the polyphenylene ribbon with 132 aromatic carbon atoms, is still sufficiently soluble and shows a lambda(max) value of 644 nm. Most. importantly, these graphitic molecules self-organize into 2D columns when adsorbed on highly oriented pyrolytic graphite (HOPG), thus rendering them attractive candidates for future applications in organic electronic devices such as e.g. field effect transistors. A deeper insight into possible conformations of the sterically demanding side chains and their influence on the packing behavior Of Such giant PAHs into columnar arrangements is additionally obtained by molecular dynamics simulations. Using Marcus theory for a nonadiabatic temperature-activated charge transfer, we discuss the advantages of an extended, anisotropic in shape, a-system as compared to traditional one-dimensional molecular arrangements of typical discotics.