Photocatalysis is a promising route to generate solar fuels with the help of sun light. In this work time resolved spectroscopy is applied to understand the behavior of photocatalytic systems and to elucidate the elementary steps like absorption of light, intramolecular relaxation, charge generation and transfer, as well as the formation of intermediate species. Beside of ultrafast absorption spectroscopy we perform time resolved photoluminescence measurements with a streak camera and transient absorption experiments using a YAG laser system. In this way all time scales between femto- and milliseconds can be covered spanning eleven orders of magnitude.

To investigate the behavior of photosensitizers Ir- and Cu-complexes are studied in a hydrogen evolving system which consists beside of the light absorbing sensitizer, of an iron complex as catalyst, triethylamine (TEA) as electron donor, and water as hydrogen source. Directly after the absorption event ultrafast intersystem crossing occurs in the sensitizer. The subsequent interaction between the absorber and the electron donor as well as the catalyst is characterized by photoluminescence quenching as well as time resolved absorption on the nano- and microsecond timescale. In the case of the Ir-complex electron transfer from TEA is found to have a surprisingly low probability of 0.4% per collision. Calculations indicate that electron transfer is only possible for specific encounter geometries providing a convincing explanation for the experimental findings. In the case of the copper sensitizerin an acetonitrile solution 20 vol% of TEA reduce the luminescence lifetime of the sensitizer from 284 ns only to 210 ns indicating that an even less efficient quenching process occurs. The catalyst quenches more strongly leading to a lifetime of 108 ns at the concentration applied in the catalytic system. In the absorption measurements a long-living signal is observed in the presence of TEA or the catalyst after the quenching step which results probably from electron transfer products. The studies are extended to systems with semiconductors as catalytic active supports. Here we try to use plasmonic nanoparticles as sensitizers deposited on TiO2. After optical excitation ultrafast absorption spectroscopy reveals signatures which are characteristic for relaxation processes and cooling of the nanoparticles. Signals due to electron transfer processes are not yet identified.

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[2] A. Neubauer et al., J. Phys. Chem. Lett. 5 (2014), 1355.