Life of Excitons in Artificial Photosynthetic Antennas

Abstract: Due to the predicted energy crisis when fossil fuels will be exhausted it is important to search for alternative energy sources. Utilizing sunlight via artificial photosynthesis is an interesting possibility. The long-term goal for research in this area is to design and build molecular and/or solid state systems that can use sunlight to split water and produce fuel (e.g. hydrogen). Nature tells us that such a system should consist of a light-harvesting antenna coupled to the photochemical reaction center, for highest efficiency. In this Thesis both reaction center and antenna functions are studied, with the emphasis on artificial antennas. We have studied two types of antennas, dendrimers of transition metal polypyridine complexes and Zn-porphyrin dendrimers. In order to study the ultrafast energy transfer and relaxation processes in these molecules we have employed polarization sensitive time resolved absorption and fluorescence spectroscopy. In the study of transition metal dendrimers we used very short, ~30 fs, laser pulses to elucidate the processes involved in energy migration within a tetramer transition metal complex, Os{(µ-2,3-dpp (Ru(bpy)2)3}8+. This complex is funneling all deposited photon energy to the central Os core. From our experiments we can identify two pathways of energy transfer from the Ru-moieties to the Os-core, a singlet pathway characterized by a time constant of < 60 fs and a more than ten times slower triplet pathway. Experiments on monomeric building blocks were important in order to obtain a complete picture of the processes in the denrimers, and showed that excited state localization occurs in < 50 fs, intersystem crossing in the Ru-complexes takes ~80 fs, but is faster (< 50 fs) in the Os-complexes. Thermally activated interligand electron transfer occurs with a characteristic time of ~10 ps in complexes with equivalent ligands; in complexes with dissimilar ligands the excitation is trapped on the lowest energy ligand. Zn-porphyrin dendrimers of various sizes (4-64 chromophores) and the monomeric Zn-porphyrin building block were studied by a fluorescence streak-camera. Energy transfer of Förster type between the four nearest neighboring Zn- porphyrins within a dendron was observed. Transfer time is on a timescale of 100 ps at 200 K. The fact that energy transfer only occur over four chromophores, even in the largest molecule of 64 Zn-porphyrins, shows that the structure of the dendrimer has to be optimized to obtain an efficient light-harvesting and transporting system.