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The Role of Quantum Coherence and Geometry on the Excitonic Energy Transfer in Photosynthetic Structures

Baghbanzadeh, Sima | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48490 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Karimipour, Vahid; Rezakhani, Ali
  7. Abstract:
  8. It has been of great interest to know whether quantum effects can play an essential role in the biological processes. Considering that quantum features, such as coherence and entanglement,can be harnessed to perform quantum information processing tasks, which are believed to be hard classically, the question is whether nature has also exploited these features in order to enhance the performance of biological systems. Although the presence of warm and wet environment around the biological systems results in the rapid destruction of quantum effects, recent experiments reported the existence of quantum coherence in light-harvesting complexes of some photosynthetic systems.This coherence lasts for a long time compared to the relevant time scales of the system. In this thesis, we study the role of quantum coherence and geometry on the efficiency of exciton transfer in primary steps of photosynthesis process in biological systems.There are two properties that contribute to the efficiency of exciton transfer: the energy landscape of the subsystems that leads to an energy funnel towards the desired direction, and coherent excitonic delocalization that can enhance transfer rates through supertransfer process. We show that the relative importance of these two properties can be determined by comparing the energy transfer efficiencies in the natural light-harvesting apparatus and structures in which delocalization and energy landscape are altered. By considering the photosynthetic structure of purple bacteria as an example, we show that although supertransfer can enhance the rates, the energetic funnelling plays a more important role. As delocalization has a minor role (and sometimes detrimental), this implies that it is a side- effect of the dense pigment packing that presumably evolved to increase the light absorption. Moreover, we show that the geometry of pigments within light-harvesting complexes could have a significant impact on the efficiency.Our results suggest that probably the configurations which result in efficient excitonic energy transfer in photosynthetic structure of purple bacteria form a small subspace in the space of all configurations and the natural configuration of this structure belongs to this subspace. We further show that the feature which makes the natural configuration of this structure an outlier is the coherent excitonic delocalization and the resulting supertransfer process. Our results enable a better understanding of the mechanisms of photosynthetic energy transfer,and can help to design more efficient artificial light-harvesting devices
  9. Keywords:
  10. Quantum Coherence ; Open Quantum System ; Excitonic Energy Transfer ; Photosynthetic Structures

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