Molecular Trees for Collecting Sunshine -Monitoring and Controlling Energy Transfer in Dendrimers

Abstract: A continuing use of fossil fuels will accelerate global warning and result in severe consequences for the environment and thereby also us. Development of alternative, sustainable and environmentally friendly energy sources is therefore crucial. One approach is to create an artificial photosynthetic system that converts sunshine into fuel (e.g. hydrogen gas) by splitting water molecules. However, in order to split water molecules, a substantial amount of sunshine needs to be harvested. This Thesis focuses on the study of two types of light-harvesting antenna systems; (1) Zn and Cu-porphyrin-appended dendrimers, where all the chromophores are situated on the periphery, and (2) a Os-Ru transition metal complex with six additional organic pyrene ligands. Efficient energy transfer within the dendrimers is essential if they are to be implemented as antennas in an artificial photosynthetic device. To facilitate monitoring of the energy transfer dynamics, different ultrafast time-resolved spectroscopic techniques were employed, which enabled us to observe these dynamics on a sub-picosecond timescale. The energy transfer dynamics in the porphyrin-appended dendrimers were mainly monitored with excitation intensity dependent transient absorption spectroscopy (exciton-exciton annihilation). The study of the Zn-dendrimer demonstrates how the energy transfer efficiency can be greatly optimized and controlled by minor external and/or internal modifications. In the Zn-dendrimers the energy transfer involves singlet excited states. However, when replacing the Zn metal center by Cu, triplet excited states are instead dominant. The combined study of Cu-porphyrin-dendrimers therefore gives a good complementary picture of the energy transfer dynamics associated with the different spin states. The transition metal complex displays fast and efficient energy transfer towards the Os center independently of which molecular transition is excited initially. Attaching the pyrene ligands extends the light-harvesting properties both in the ultra-violet and the visible spectral region.