Richard Hildner:

The efficient transport of excitation energy between organic molecules is a key step for any application of organic matter to convert light into a useful form of energy. For instance, in the initial steps in photosynthesis energy is transported over substantial distances through structurally well-defined supramolecular assemblies of chlorophyll molecules. Based on such natural systems we aim at developing artificial supramolecular nanostructures with desired functionality, e.g. long-range transport.

Employing optical microscopy and spatially resolved spectroscopy, we were able to show that individual supramolecular nanofibres, based on a carbonyl-bridged triarylamine (CBT) as building block [1], efficiently transport singlet excitons at ambient conditions over more than 4 μm, only limited by the fibre length. This long-range transport is predominantly coherent, which is achieved by one-dimensional self-assembly of CBT to well-defined cofacially stacked H-aggregates with substantial electronic coupling between CBTs [2].

Moreover, we demonstrate the formation of highly oriented nanofibers based on the prototypical conjugated polymer poly(3-hexylthiophene), P3HT, with a Shish-Kebab-like superstructure using a tailored supramolecular nucleating agent. The structural and electronic order along the P3HT nanofibers can be controlled by the processing protocol, which allows to imprint an energy gradient into the fibres, similar to what is found in photosynthetic systems.

Finally, we performed low-temperature single-molecule spectroscopy on isolated regio-regular P3HT-chains, to characterise the building blocks of the nanofibres. Surprisingly, the sum of many single-chain spectra does not result in the spectrum of a disordered P3HT ensemble (e.g. in dilute solution or film) [3]. Rather the latter is already substantially affected by inter-chain H-type electronic coupling, and thus does not reflect intrinsic properties of P3HT. A proper spectroscopic characterisation of molecular building blocks is indispensable to fully understand the photophysical properties of self-assembled nanostructures.

[1] Haedler, Beyer, Hammer, Hildner, Kivala, Köhler, Schmidt, Chem. Eur. J. 20, 11708-11718 (2014).

[2] Haedler, Kreger, Issac, Wittmann, Kivala, Hammer, Köhler, Schmidt, Hildner, Nature 523, 196-199 (2015). [3] Raithel, Baderschneider, de Queiroz, Lohwasser, Köhler, Thelakkat, Kümmel, Hildner, Macromolecules 49, 9553-9560 (2016).