August 24, 2015

Supramolecular nanofibers could create super efficient solar energy

The conversion of sunlight into electricity at low cost becomes increasingly important to meet the world's fast growing energy consumption. This task requires the development of new device concepts, in which particularly the transport of light-generated energy with minimal losses is a key aspect. An interdisciplinary group of researchers from the Universities of Bayreuth and Erlangen-Nuremberg (Germany) report in Nature on nanofibers, which enable for the first time a directed energy transport over several micrometers at room temperature. This transport distance can only be explained with quantum coherence effects along the individual nanofibers.

The research groups of Richard Hildner (Experimental Physics) and Hans-Werner Schmidt (Macromolecular Chemistry) at the University of Bayreuth prepared supramolecular nanofibers, which can comprise more than 10,000 identical building blocks. The core of the building block is a so-called carbonyl-bridged triarylamine. This triarylamine derivative was synthesized by the research group of Milan Kivala (Organic Chemistry) at the University of Erlangen-Nuremberg and chemically modified at the University of Bayreuth. Three naphthalimidbithiophene chromophores are linked to this central unit. Under specific conditions, the building blocks spontaneously self-assemble and form nanofibers with lengths of more than 4 micrometers and diameters of only 0.005 micrometer. For comparison: a human hair has a thickness of 50 to 100 micrometers.

This is a supramolecular nanofiber consisting of more than 10,000 perfectly ordered building blocks, which enables an energy transport over a distance of more than 4 micrometers at room temperature. Credit Picture by A. T. Haedler.

Nature - Long-range energy transport in single supramolecular nanofibres at room temperature



With a combination of different microscopy techniques the scientists at the University of Bayreuth were able to visualize the transport of excitation energy along these nanofibers. To achieve this long-range energy transport, the triarylamine cores of the building blocks, that are perfectly arranged face to face, act in concert. Thus, the energy can be transferred in a wave-like manner from one building block to the next: This phenomenon is called quantum coherence.

"These highly promising nanostructures demonstrate that carefully tailoring materials for the efficient transport of light energy is an emerging research area" says Dr. Richard Hildner, an expert in the field of light harvesting at the University of Bayreuth. The research area light harvesting aims at a precise description of the transport processes in natural photosynthetic machineries to use this knowledge for building novel nanostructures for power generation from sunlight. In this field interdisciplinary groups of researchers work together in the Bavarian initiative Solar Technologies Go Hybrid and in the Research Training Group Photophysics of synthetic and biological multichromophoric systems (GRK 1640) funded by the German Research Foundation (DFG).


Self-assembly of compound

Abstract - Long-range energy transport in single supramolecular nanofibres at room temperature

Efficient transport of excitation energy over long distances is a key process in light-harvesting systems, as well as in molecular electronics. However, in synthetic disordered organic materials, the exciton diffusion length is typically only around 10 nanometers, or about 50 nanometers in exceptional cases a distance that is largely determined by the probability laws of incoherent exciton hopping. Only for highly ordered organic systems has the transport of excitation energy over macroscopic distances been reported—for example, for triplet excitons in anthracene single crystals at room temperature, as well as along single polydiacetylene chains embedded in their monomer crystalline matrix at cryogenic temperatures (at 10 kelvin, or −263 degrees Celsius). For supramolecular nanostructures, uniaxial long-range transport has not been demonstrated at room temperature. Here we show that individual self-assembled nanofibres with molecular-scale diameter efficiently transport singlet excitons at ambient conditions over more than four micrometres, a distance that is limited only by the fibre length. Our data suggest that this remarkable long-range transport is predominantly coherent. Such coherent long-range transport is achieved by one-dimensional self-assembly of supramolecular building blocks, based on carbonyl-bridged triarylamines, into well defined H-type aggregates (in which individual monomers are aligned cofacially) with substantial electronic interactions. These findings may facilitate the development of organic nanophotonic devices and quantum information technology.

Supplemental information

SOURCES- Eurekalert, Nature, University of Bayreuth

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