In this thesis we examine key structure-property relationships associated with eumelanin's photophysical properties. This has involved characterizing both the molecular and supramolecular structure of eumelanin, and examining how these relate to eumalanin's key optical properties that are relevant to their their role in the biosphere as photoprotectants. Using low-voltage high resolution transmission electron microscopy we definitively show that sheets of proto-molecules stack to form nanostructures. The inter-sheet spacings within these structures are between 3.7 and 4.0 Å consistent with non-covalent pie-pie stacking in heteroaromatic systems. Stacking interactions in similarly structured aromatic macromolecules play an important role in non-radiative energy dissipation and we propose that this may also be the case in the eumelanin system. We also examine the recently proposed hypothesis that excited state intramolecular proton transfer may play a role in the photophysics of 5,6-dihydoxyindole carboxylic acid, a key eumelanin monomer. The experimental results acquired in this study indicate that this hypothesis needs to be carefully re-examined and its justification would require more extensive experimental support. Key optical properties of 5,6-dihydoxyindole carboxylic acid are re-evaluated in an inert aprotic solvent and these have provided some insight into the electronic structure as well as the rates of radiative and non-radiative decay in this important eumelanin monomer. We go on to show how the structure of eumelanin can be manipulated to produce eumelanin thin films. These films display electrical conductivities comparable with amorphous silicon, as well as a host of other interesting and potentially useful optoelectronic properties. The results show great promise for eumelanin-based applications such as chemi-sensors (in a variety of architectures including organic field effect transistors with chemi-sensitive channels) and bolometric photon detectors.
| Identifer | oai:union.ndltd.org:ADTP/286124 |
| Creators | Jacques Bothma |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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