Erlangen 2018 – wissenschaftliches Programm
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MO: Fachverband Molekülphysik
MO 4: Photochemistry
MO 4.5: Vortrag
Montag, 5. März 2018, 15:00–15:15, PA 1.150
The Femtochemistry of the Ferrioxalate Actinometer — •Steffen Straub, Jörg Lindner, and Peter Vöhringer — Institut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstrasse 12, 53177 Bonn, Germany
The ferrioxalate actinometer is an aqueous solution of the complex anion, potassium trisoxalatoferrat(III), which is frequently used in preparative photochemistry to determine the internal radiant flux of photoreactors. Actinometers are needed to quantify the quantum yield and hence, the total efficiency of a photochemical reaction of interest. In the presence of light, ferrioxalate is reduced to ferrous oxalate and a neutral carbon dioxide together with a free oxalate anion is released from the complex. Despite having served as the "gold Standard" of liquid-phase actinometry for many years, the primary processes of ferrioxalate are entirely unknown. Previous femtosecond (fs) UV-pump/Vis-probe experiments and most recent ultrafast x-ray absorption spectroscopy have yielded ambiguous and partly contradictory results. Here, we report on fs-UV-pump mid-infrared probe spectroscopy, which is able to provide unique structural-dynamical information about the complex following its initial ultrafast optical excitation. The fs-spectra demonstrate that a hot CO2 molecule is expelled from coordination sphere of the ferric center within 500 fs to generate a penta-coordinated ferrous species that bears an intriguing carbon dioxide radical anion ligand. A subsequent structural isomerization on a 10 PS-time scale and the loss of the CO2− moiety yields the final ferrous product of the actinometer.