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 apparently serves to suppress lateral aggregation so that 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.

Two-Fold Odd-Even Effect in Self-Assembled Nanowires from Oligopeptide-Polymer-Substituted Perylene Bisimides

Holger Frauenrath, Deborah Leite, Roman Niklaus Marty, Robin Nigon

Organic nanowires are important building blocks for nanoscopic organic electronic devices. In order to ensure efficient charge transport through such nanowires, it is important to understand in detail the molecular parameters that guide self-assembly of pi-conjugated molecules into one-dimensional stacks with optimal constructive pi-pi overlap. Here, we investigated the subtle relationship between molecular structure and supramolecular arrangement of the chromophores in self-assembled nanowires prepared from perylene bisimides with oligopeptide-polymer side chains. We observed a "two-fold" odd-even effect in circular dichroism spectra of these derivatives, depending on both the number of L-alanine units in the oligopeptide segments and length of the alkylene spacer between chromophore and oligopeptide substituents. Our results indicate that there is a complex interplay between the translation of molecular chirality into supramolecular helicity and the molecules' inherent propensity for well-defined one-dimensional aggregation into beta-sheet-like superstructures in the presence of a central chromophore. Strong excitonic coupling as expressed by the appearance of hypsochromically and bathochromically shifted UV-vis absorptions and strong CD signals was systematically observed for molecules with an odd number of L-alanines in the side chains. The latter derivatives gave rise to nanowires with a significantly higher electron mobility. Our results, hence, provide an important design rule for self-assembled organic nanowires.

Amer Chemical Soc2014

Long‐Lived Photocharges in Supramoelcular Polymers of Low‐Bandgap Chromophores

Holger Frauenrath, Daniel Görl, Regina Judith Hafner, Andrzej Sienkiewicz

Photoinduced charge separation in supramolecular aggregates of π‐conjugated molecules is a fundamental photophysical process, and a key criterion for the development of advanced organic electronics materials. Here, we report on the self‐assembly of novel low‐bandgap chromophores into helical one‐dimensional aggregates due to intermolecular hydrogen‐bonding. The chromophores confined in these supramolecular polymers show strong excitonic coupling interactions and give rise to charge‐separated states with unusually long lifetimes of several hours and charge densities of up to 5 mol% after illumination with white light. Two‐contact devices exhibit increased photoconductivity and can even show Ohmic behavior. Our findings demonstrate that the confinement of organic semiconductors into one‐dimensional aggregates results in a considerable stabilization of charge carriers for a variety of π‐conjugated systems, which may have implications for the design of future organic electronic materials.

2020

Crystallization and Organic Field-Effect Transistor Performance of a Hydrogen-Bonded Quaterthiophene

Philippe Bugnon, Piotr De Silva, Holger Frauenrath, Jan Gebers, Bilal Özen, Riccardo Petraglia, Rosario Scopelliti, Stéphane Suarez

Crystalline thin films of pi-conjugated molecules are relevant as the active layers in organic electronic devices. Therefore, materials with enhanced control over the supramolecular arrangement, crystallinity, and thin-film morphology are desirable. Herein, it is reported that hydrogen-bonded substituents serve as additional structure-directing elements that positively affect crystallization, thin-film morphology, and device performance of p-type organic semiconductors. It is observed that a quaterthiophene diacetamide exhibits a denser packing than that of other quaterthiophenes in the single-crystal structure and, as a result, displays enhanced intermolecular electronic interactions. This feature was preserved in crystalline thin films that exhibited a layer-by-layer morphology, with large domain sizes and high internal order. As a result, organic field-effect transistors of these polycrystalline thin films showed mobilities in the range of the best mobility values reported for single-crystalline quaterthiophenes. The use of hydrogen-bonded groups may, thus, provide an avenue for organic semiconducting materials with improved morphology and performance.

2020