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Measuring Exciton Migration Length in Tubular Cyanine Dye Aggregates

Hooff, C.J.S. van (2017) Measuring Exciton Migration Length in Tubular Cyanine Dye Aggregates. Master's Thesis / Essay, Applied Physics.

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Natural light-harvesting systems exhibit extreme quantum efficiencies, made possible due to efficient long range transport of excitation energy. Knowledge of the underlying processes might lead to improved photovoltaics and electronic energy transport systems. A known model system is based on the amphiphilic cyanine dye 3,3' bis(2-sulfopropyl)- 5,5',6,6'-tetra- chloro-1,1' dioctylbenzimidacarbocyanine (C8S3-Cl). By self-assembly they aggregate into highly ordered supramolecular light-harvesting nanotubes of up to µm’s in length. Single C8S3-Cl molecules were embedded in PMMA thin films in different concentrations, resulting in intermolecular distances of ~4 - 8nm. A clear red shift in absorption and fluorescence spectra is seen in comparison to disperse solution, as well as for decreasing intermolecular distances. We attribute this to increased J-dimerization. Moreover, time resolved spectroscopy yields a multi-exponential fluorescence decay with lifetimes of 80 - 150ps and 850 - 1050ps, in contrast to solution with a single exponential decay of 115ps, from which we conclude that the rigid matrix suppresses vibrational relaxation. Aggregates based on the Bromide derivative C8S3-Br are investigated by embedding them in a sugar thin film. We report minor spectral changes opposed to aqueous solution and a high degree of photo stability. Fluorescence microscopy with wide field illumination confirms non-destructive immobilization, supported by comparison to independent cryogenic TEM data. With the aid of focused excitation of bundled aggregates, exciton migration is directly probed by imaging the amount of broadening of the excitation spot. Limited by precise knowledge of the initial excitation spot, we can resolve broadening with a resolution of ~30nm, with which we can exclude exciton migration lengths of >50nm. Refinements on the fluorescence microscopy setup might increase the resolution of the migration length measurement. Moreover, fluorescent anisotropy measurements and single aggregate spectra, in combination with molecular and quantum mechanical simulations, further strengthen our understanding of artificial light-harvesting systems, which might lead to next-generation photovoltaics.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Applied Physics
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 08:30
Last Modified: 15 Feb 2018 08:30

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