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Modelling exciton dynamics in light-harvesting molecules Ruocco, Leonard


I investigate the dynamics of multi-state central systems coupled bilinearly to an external oscillator bath within the noninteracting-blip approximation. I focus on both a 3-site configuration, as well as a 2-site model for the central systems of interest. The 2-site model, dubbed the dual-coupling spin-boson (DCSB) model, includes both diagonal and non-diagonal system-bath couplings, whereas the 3-site model considers only diagonal couplings. The bath spectral densities considered in this work include both Ohmic and super-Ohmic forms, as well as single optical phonon peaks. This work is motivated by the recent observance of long-lived quantum coherence effects in the photosynthetic organism known as the Fenna-Matthews-Olson (FMO) complex. The models investigated in this thesis are applied to this system in an attempt to explain its remarkably efficient exciton transfer mechanism, as well as to shed light on the functionality of coherence. The DCSB model is shown to reproduce the rapid exciton transfer times as well as the relatively long coherence times observed in the FMO complex. The non-diagonal system-bath coupling is shown to play a crucial role in this process. This can be attributed to the inelastic phonon-assisted tunnelling (IPAT) mechanism arising from the presence of significant non-diagonal system-bath interactions. Conversely, the 3-site model predicts rapid but incoherent exciton transfer. This can be attributed to the presence of a resonant state in the 3-site architecture, resulting in a relatively slow exciton transfer mode in the system. Therefore efficient exciton transfer requires a careful configuration of the chromophore energy landscape to avoid a resonant 3-site-V configuration. Furthermore, I conclude that coherence effects arising from excitons delocalised across multiple chromophores, promotes IPAT processes arising from non-diagonal system-bath couplings, producing rapid exciton transfer between chromophores. This offers a potential explanation as to the functional role that coherence plays in the energy transfer mechanism of photosynthesis.

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