Maria Ilaria Mallus

• Photosynthesis is the main energy source in plants, algae and different types of bacteria, such as purple and green sulphur bacteria. The primary step in photosynthesis is represented by the photo excitation of the light harvesting (LH) pigment present in the organism. Subsequently, the excitation delocalizes among the pigments due to the electronic couplings between them. The excitation energy is then transferred to neighboring LH systems and finally to the reaction center (RC) where charge separation occurs. During the last decades many studies have been carried out in order to understand the optical and the exciton transfer properties of the LH complexes. Nevertheless, a full understanding has not yet been achieved. In particular, the experimentally-observed long-lived coherences as well as dephasing processes have attracted the attention of the scientific community in recent years. The present thesis aims at contributing to the understanding of these processes. To this end, different methods have been employed, such as molecular dynamics simulations, quantum-chemistry methods and wave packet dynamics calculations. The combination of these methods allows a more detailed theoretical description of the studied LH systems. The dephasing phenomenon is discussed in the first half of the thesis. In this work, an analytic and a numerical methodology has been developed to relate it to the energy gap fluctuation. This formalism has been applied to the case of both single pigments and whole complexes. It can be concluded that a universal relation exists between these two entities, independently from the system and from the method used to obtain such quantities. The second half of this thesis consists of combined molecular dynamics and quantum approaches applied to different systems. Three different LH systems are discussed and compared in detail and the photo-active part of a bio-inspired solar cell is studied.