# Investigations of the Energy Transfer
## 1 Introduction
platensis_ as photosensitizers, a better understanding of intra- and inter-molecular energy transfer of both PBPs and the PBS is required. Time-resolved absorption and fluorescence measurements have been employed to investigate the mechanism of energy transfer in PBPs on the femtosecond (fs) to nanosecond (ns) time scale [17; 18; 19]. Several theories have been proposed for the ultrafast dynamics observed in PBPs, with the most likely involving Forster energy transfer based on the energy resonant interaction of transition dipoles of donor and acceptor molecules, which can be applied for donor-acceptor distances \(\geq\)20 A. For shorter distances, this mechanism was replaced by the exciton interaction, which assumes a strong electronic interaction between the electronic states of two closed-lying chromophores, making them behave as a single super chromophore with electronic states delocalized over both chromophores [20]. PC and APC have been isolated from various organisms, and the excited state kinetics have been investigated for their specific aggregation states [21; 22; 23]. PC is usually found in three states, monomeric, trimeric and hexameric, the last one being the most stable [24]. APC is found in similar assembly states as PC, but with the notable mention that the trimeric state is usually the stable form [9]. In PC, fluorescence decays in the range of tens of picosecond (ps) were identified and assigned to the energy transfer from short-wavelength sensitizing chromophores to long-wavelength absorbing fluorescing ones [21]. However, previous results showed that in APC, the \(\alpha\)-84 and \(\beta\)-84 chromophores are excitonically coupled, and a short 14 ps time component was assigned to the relaxation from the upper to the lower exciton level [21]. Energy flow investigations in PBSs have resolved kinetics as fast as a few hundreds of fs along the PC rod subcomplex to the APC core [25]. The excitation energy transfer (EET) in the PBS extracted from _A. marina_ was described as a four-step process: (1) the excitonic relaxation between \(\alpha\)-84 and \(\beta\)-84 chromophores in approximately 400 fs, followed by (2) excitation energy equilibration of PC and energy transfer to APC in 3 ps, (3) energy transfer from APC to the terminal emitter pigment in 14 ps, and (4) to the photosystem in 100 ps [26]. More recent studies have focused on the interaction between pigment and proteins and how they modify to adapt to changes in light conditions [27; 28] or to the type of water medium (e.g., seawater or freshwater) [29]. It was observed that even if the energy transfer from the PBS to the photosystems was modified depending on the light quality and quantity, the total amount of energy transferred to photosystem I remained constant. Cyanobacteria can sense the wavelength and energy of the ambient light, process the information, and modulate the PBS construction by changing the size, form, and PBP composition [30], therefore showing variations of the spectral absorption region and the duration of the energy transfer. With this in mind, we aim to understand the ultrafast excitation energy transfer in the PBS extracted from _A. platensis_ and in its sub-components PC and APC. We plan to further use this insight for building biomimetic hybrid systems based on PBS/PBPs and metal or semiconductor nanoparticles optimized for solar energy collection and photocurrent amplification. To our knowledge, the energy transfer investigations on light-harvesting antenna from _A. platensis_ strain AICB 49 have not been performed before in a bottom-up approach, from isolated PBPs to the highly assembled PBS.