The specialized pigment-protein complexes involved in the first process of photosynthesis are light-harvesting structures that are composed of networks of chromophores in protein scaffolds. Though light-harvesting complexes vary in chromophore composition and protein structure, they are capable of transferring the absorbed energy as molecular excitation energy from chromophore to chromophore with maximal efficiency. Thus, numerous interdisciplinary studies focus on elucidating energy transfer mechanisms in these biological complexes and how the same principles can be applied to artificial photosynthetic and photovoltaic machines. From advanced spectroscopic measurements and theoretical models, the interaction between the excited electronic states and the nuclear vibrational degrees of freedom is now established to be crucial for efficient energy transfer. In light-harvesting complexes of plants and bacteria, it is now understood that the classical-like vibrational modes of the protein and solvent environment drive energy transfer between the energetically close electronic states of the chromophores. On the other hand, recent spectroscopic measurements on algae light-harvesting complexes discovered signatures of quantized, high frequency vibrational modes of the chromophore. Unfortunately, a deterministic interpretation of the data and the underlying Hamiltonian is hindered due to significant inhomogeneous spectral line-broadening. Though numerous model Hamiltonians have been proposed from theoretical work, various computational approximations employed in these studies necessitate empirical parameter tuning in order to obtain agreement with benchmark linear optical spectra. Thus in this work, we present a simple, but improved, computational prescription to compute the ensemble of Hamiltonians for four closely-related algae light-harvesting complexes. We verify the reliability of our proposed models by comparing simulated optical spectra with experimental measurements. We show that static disorder and inhomogeneous broadening are significant for phycobiliproteins due to large site energy fluctuations. We also show that the nuclear environment plays an important role in defining the trapping state, or the final energy acceptor. Finally, our work for the first time suggests that EET dynamics can be tuned by varying the titration states of the chromophores.

Das Verstehen der elektronischen Prozesse in Nano-Hybridsystemen, bestehend aus Molekülen und Halbleiterstrukturen, eröffnet neue Möglichkeiten für optoelektronische Bauteile. Dafür benötigt es nanoskopische und gleichzeitig atomare Modelle und somit angepasste Rechenmethoden. Insbesondere "Standard"-Ansätze für die Dynamik offener Quantensysteme werden mit zunehmender Systemgröße jedoch sehr ineffizient. In dieser Arbeit wird eine neue Methode basierend auf einer stochastischen Schrödinger-Gleichung etablieren. Diese umgeht die numerischen Limits der Quanten-Mastergleichung und ermöglicht Simulationen von imposanter Größe. Ihr enormes Potenzial wird hier in Studien zu Anregungsenergietransfer und Ladungsseparation an zwei realistischen Nano-Hybridsystemen demonstriert: para-sexiphenyl Moleküle auf einer flachen ZnO Oberfläche (6P/ZnO), und ein tubuläres C8S3 Farbstoffaggregat gekoppelt an einen CdSe Nanokristall (TFA/NK). Im 6P/ZnO System findet nach optischer Anregung Energietransfer vom 6P Anteil zum ZnO statt. Direkt an der Grenzfläche können Frenkel-Exzitonen zusätzlich Ladungsseparation initiieren, wobei Elektronen ins ZnO transferiert werden und Löcher im 6P Anteil verbleiben. Beide Mechanismen werden mittels laserpulsinduzierter ultraschneller Wellenfunktionsdynamik simuliert. Danach wird die langsamere dissipative Lochkinetik im 6P Anteil studiert. Hierfür wird die eigene Simulationstechnik der stochastischen Schrödinger-Gleichung verwendet. Die Studie an der TFA/NK Grenzfläche basiert auf einer gigantischen equilibrierten Aggregatstruktur aus 4140 Molekülen. Ein generalisiertes Frenkel-Exzitonenmodell wird benutzt. Der Ansatz der stochastischen Schrödinger-Gleichung ermöglicht bemerkenswerte Einblicke in die Aggregat-interne Exzitonenrelaxation. Danach werden inkohärente Raten des Exzitonentransfers zum NK berechnet. Unterschiedliche räumliche Konfigurationen werden untersucht und es wird diskutiert, warum das Förster-Modell hier keine Gültigkeit besitzt. / Understanding the electronic processes in hybrid nano-systems based on molecular and semiconductor elements opens new possibilities for optoelectronic devices. Therefore, it requires for models which are both nanoscopic and atomistic, and so for adapted computational methods. In particular, "standard" methods for open quantum system dynamics however become very inefficient with increasing system size. In this regard, it is a key challenge of this thesis, to establish a new stochastic Schrödinger equation technique. It bypasses the computational limits of the quantum master equation and enables dissipative simulations of imposing dimensionality. Its enormous potential is demonstrated in studies on excitation energy transfer and charge separation processes in two realistic nanoscale hybrid systems: para-sexiphenyl molecules deposited on a flat ZnO surface (6P/ZnO), and a tubular dye aggregate of C8S3 cyanines coupled to a CdSe nanocrystal (TDA/NC). After optical excitation, the 6P/ZnO system exhibits exciton transfer from the 6P part to the ZnO. Close to the interface, Frenkel excitons may further initiate charge separation where electrons enter the ZnO and holes remain in the 6P part. Both mechanisms are simulated in terms of laser-pulse induced ultrafast wave packet dynamics. Afterwards, slower dissipative hole motion in the 6P part is studied. For this purpose, the own stochastic Schrödinger equation simulation technique is applied. The study on the TDA/NC interface is based on a gigantic equilibrated nuclear structure of the aggregate including 4140 dyes. A generalized Frenkel exciton model is employed. Thanks to the stochastic Schrödinger equation approach, energy relaxation in the exciton band of the TDA is simulated in outstanding quality and extend. Then, incoherent rates for exciton transfer to the NC are computed. Different spatial configurations are studied and it is discussed why the Förster model possesses no validity here.