Long-lived Photo-Charges in 1D Helical Organic Semiconductor Nanostructures

The formation of photoexcited states in organic semiconductors, their separation into long-lived or stable charge carriers, as well as the charge transport are critical processes relevant to several important research fields and emerging technologies. While it is already well established that ordered and strongly π-π interacting systems improve the lifetime of charge-separated states, the confinement to one-dimensional structures can further lead to intriguing new optoelectronic properties. Supramolecular self-assembly has proven to be a powerful tool to obtain such one-dimensional nanostructures by using non-covalent interactions such as hydrogen bonding and π-π interactions. In the present thesis, we provide a series of examples of π-conjugated chromophores with terminal poly(isobutylene)-oligopeptide substituents that universally self-assemble into one-dimensional aggregates with high aspect ratio, uniform lateral dimensions, and a single stack of helically arranged chromophores at their core. The resulting nanostructures can hence be regarded as supramolecular polymers with the π-conjugated chromophores as the repeating units and a helical secondary structure. We show that, under these conditions and irrespective of the nature of the chromophores, the latter exhibit strong electronic interactions with their neighbors and consequently show unprecedented spectroscopic and electric properties. As a first example, we investigated a thieno[3,2-b]thiophene derivative that self-assembles into such well-defined one-dimensional nanostructures. We demonstrate that the strong excitonic coupling and the helical arrangement of the chromophores within these aggregates are the structural prerequisites for a facile generation of a high concentration of photo-charges with unusually long lifetimes on the order of hours. This high photo-charge generation results in macroscopic photoconductivity and electric properties typical for doped polymer semiconductors. The relationship between the supramolecular arrangement of the chromophores and the generation of long-lived charge carriers was further investigated by employing π-conjugated donor-acceptor chromophores reminiscent of segments of low bandgap polymers as well as a series of oligothiophenes ranging from dimer to octamer. All molecules give rise to charge-separated states with lifetimes of hours upon aggregation and illumination with white light. Charge carrier concentrations of up to 5 mol% have been observed for the low-bandgap derivatives. Finally, the applicability of our approach to electron-poor systems will be shown by the introduction of a dicyanoperylene bisimide chromophores into the one-dimensional helical nanostructures, which show the reversible formation of air-stable radical anions upon illumination that results in an increase in conductivity by up to three orders of magnitude. Our findings hence establish a clear structure-property relationship between the presence of one dimensional helical aggregates with strongly electronically coupled chromophores at their core and the formation of exceptionally stable photoinduced charge carriers for a broad variety of π-conjugated molecules.

Frauenrath, Holger
Lausanne, EPFL

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 Record created 2018-04-25, last modified 2019-05-09

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