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Electronic Excitation Energy Transport in Self-Assembled Molecular Nanotubes

Kriete, B. (2015) Electronic Excitation Energy Transport in Self-Assembled Molecular Nanotubes. Master's Thesis / Essay, Physics.

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Self-assembled tubular aggregates of cyanine dyes are promising candidates for synthetic light harvesting systems as potentially deployable in new generation solar cells due to their remarkably efficient excitation energy transport. In that regard such aggregates are comparable to natural antenna-complexes enhancing photosynthesis in plants, such as Green Sulphur bacteria for instance. In the frame of this thesis tubular aggregates of the amphiphilic cyanine dye C8S3 were investigated using spectroscopic techniques as well as cryogenic transmission electron microscopy. The latter has proven that upon aggregation of C8S3 molecules in aqueous solution double-walled tubules are formed. The length of these tubes extends up to several μm while maintaining remarkably uniform inner and outer diameters of approximately 6nm and 15 nm, respectively. Linear absorption spectra reflected this morphological change due to aggregation in form of a strong spectral redshift and the evolution of two narrow peaks referring to the inner and outer cylinder, as predicted by theoretical J-aggregate models. Besides, by comparing fluorescence emission and absorption spectra, strong indications for efficient intercylinder energy transfer could be found. Furthermore, long-term absorption measurements revealed that once the aggregates had formed, they were stable over the course of weeks up to months upon storage. Together with the spectral redshift this finding underlined the proposed strong intermolecular coupling that is responsible for efficient excitation energy migration. An interesting side effect of storage was the formation of bundles consisting of several tubules. Bundling was accompanied by spectral changes, which were not solely explainable by assuming strands of double-walled tubules. Instead, bundles consisted only of the inner cylinder. To date experiments could not explicitly explain exciton behaviour in tubular C8S3 aggregates. Therefore, the ultimate goal is to establish a well-founded understanding of exciton dynamics in C8S3 aggregates using advanced experimental techniques, such as 2D spectroscopy. Having good control of the system and obtaining reproducible results is the indispensable basis for these experiments.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Physics
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 08:10
Last Modified: 15 Feb 2018 08:10

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