title: Tailoring long-range energy transport in supramolecular architectures
Creators: name: Wittmann, Bernd
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: supramolecular polymers
subject: nanofibers
subject: aggregation, energy transport
subject: exciton diffusion, photophysics
subject: 500
subject: 530

description: Organic electronics, such as organic solar cells, light-emitting diodes, and field-effect transistors, are based on tailored conjugated small molecules or polymers. In virtually all organic optoelectronic devices, the material's ability to transport excitation energy plays an important role and is a constraint for device design. For instance, the distance over which energy can be transported is of great importance for organic solar cell performance, as an excitation has to reach an interface to form a charge-separated state. Artificial nanostructures with tailored properties are important model systems to investigate and understand energy transport phenomena. In this context, supramolecular chemistry offers a powerful tool for the construction of functional materials. Non-covalent interactions between molecular building blocks and advanced processing pathways are exploited to adapt the molecules' spatial arrangement within the supramolecular architecture and thus to tune the (overall) electronic properties. So far, studies on supramolecular assemblies have demonstrated that energy transport is closely related to the precise arrangement of interacting molecules and that the function typically results from the collective properties of many interacting molecules. However, reports on nanostructures that support long-range or directed transport of excitons are still rare. This might be related to the lack of model systems, since their construction is a challenge. In particular, subtle changes in the molecular framework and the processing can have a huge impact on the material properties. Moreover, numerous fundamental questions about the mechanisms of exciton transport remain unanswered. For instance, the precise role of quantum coherence (delocalization), the influence of electronic and structural disorder, the local environment, the dimensionality and morphology of the structures, and the mutual interplay of these factors are still not fully understood. In order to further advance in this direction, disentangling the different contributions of energy transport is essential. This requires methods for the direct resolution and unambiguous quantification of energy transport at the level of single objects to avoid ensemble averaging over the intrinsic structural and electronic disorder of supramolecular assemblies. To uncover the role of coherence in energy transport, supramolecular systems that allow disorder and electronic coupling to be adjusted separately are indispensable. In addition, concepts to control exciton transport, preferably externally, are needed to increase the range of possible applications. This thesis contributes to the points raised above and is thus concerned with the issue of tailoring long-range energy transport in supramolecular assemblies. The first part of this thesis (4.1) contributes to obtaining unambiguous information about spatio-temporal exciton dynamics in self-assembled materials. We use state-of-the-art spatio-temporal imaging of energy transport via detection-beam scanning of the transient photoluminescence of individual crystals based on thiophene-benzene-thiophene oligomers. In combination with numerical simulations, we address and resolve ambiguities in direct measurements of spatio-temporal exciton dynamics and are able to quantify the influence of exciton-exciton annihilation, photon recycling, and leaky mode waveguiding as sources of 'error' in such experiments. We demonstrate how these mechanisms can be discriminated by a careful design of experiments, data evaluation, and numerical simulations. Furthermore, our single crystals, in which the oligomers are stacked in an H-type arrangement, exhibit singlet exciton transport distances of up to 210 nm with a diffusivity of up to 0.2 cm²/s. Surprisingly, we observe a variation in the exciton transport lengths and exciton dynamics from crystal to crystal, which we relate to varying degrees of electronic disorder. The second part (4.2) deals with the construction and characterization of suitable supramolecular architectures that allow for tailoring coherence characteristics. Depending on the solvent, we can prepare either single supramolecular nanofibers or bundles of supramolecular nanofibers based on a carbonyl-bridged triarylamine-based (CBT) building block. We found that the excited state energy landscape, i.e., the correlation between transition energies of the building blocks, is tuned by bundling-induced electronic disorder. Consequently, excitons in single nanofibers are delocalized over more molecules than in bundles. Using optical microscopy with detection-beam scanning capability, we visualize long-range singlet exciton transport in both architectures. Due to the high degree of exciton delocalization, single nanofibers feature superior energy transport characteristics with the largest exciton diffusion coefficients of 1 cm²/s reported for H-aggregates so far. The unique combination of these supramolecular model systems and advanced optical microscopy methods thus enables us to reveal the role of morphology and coherence, i.e., exciton delocalization in energy transport. The last part of this thesis (4.3) is concerned with the demonstration of an all-optical approach to control singlet exciton transport pathways via singlet-triplet annihilation in supramolecular CBT-based bundles of nanofibers. We use two temporally and spatially separated laser pulses to generate a triplet and a subsequent singlet exciton population. The laser-generated, freely positionable triplet exciton population acts as a barrier for the mobile singlet exciton population since singlet-triplet annihilation suppresses singlet exciton transport in the direction of the triplet barrier. Our new approach paves the way towards new design principles for functional photonic nanodevices and ultimately to complete control over singlet exciton motion.