copyright by Burkhard König
Transient Absorption on Multiple Timescales -
from Ultrafast Relaxation to Charge Transfer Processes
For chemists and physicists understanding catalysis means understanding the various chemical changes that happen to a molecule during a reaction. These start with the redistribution of electron density on a sub-femtosecond time scale, the nuclei responding to the new electronic environment during the first few- to hundreds of femtoseconds, redistribution of the additional energy into structural and solvent degrees-of-freedom on a few-picosecond time scale, changes to the molecular conformation and translation within the solvent during hundreds of picoseconds, and, finally, encounter of another reactant and the chemical formation of a product during hundreds of nanoseconds or microseconds.
Catalysis introduces energy short-cuts during the reaction, allowing a specific product to form more efficiently or more quickly, and various experimental techniques have been used to study the mechanisms over a range of different time scales. Already early on, flash photolysis has provided insight into the reaction times of photo-catalytic processes and mass spectrometry identified structure and charge of intermediates. During recent decades, optical spectroscopy has even been able to identify the very first events during chemical reactions via observation of absorption changes with femtosecond temporal resolution. Modulation of these absorption changes, which can only be observed with the most advanced techniques, further provides insight into structural changes within the reacting molecules and are thus important reporters, providing insight into molecular structure at various moments during the catalytic reaction.
In the Dynamic Spectroscopies group at the Technical University of Munich we develop novel approaches to study the connection between reaction kinetics and chemical structure. As part of the transregio collaborative research centre (SFB/TRR) “Assembly controlled chemical photocatalysis” between TUM, Universität Regensburg, LMU and Universität Leipzig, we will use optical spectroscopy with state-of-the-art temporal resolution and wavelengths ranging from the ultraviolet to the near-infrared to follow catalytic reactions in real-time, and learn about how dynamic and static changes to a molecule’s structure influence it reactivity. Our observations will identify structurally important motifs and allow to refine synthetic approaches in catalysis, which will be evaluated by the collaborating groups.
Involved group members
Miriam Jänchen
miriam.jaenchen@tum.de
Kiran Maiti
kiran.maiti@tum.de
Hongxing Hao
hongxing.hao@tum.de
Further reading
Hölzl-Hobmeier, A.; Bauer, A.; Silva, A. V.; Huber, S. M.; Bannwarth, C.; Bach, T. Catalytic deracemization of chiral allenes by sensitized excitation with visible light. Nature 2018, 564 (7735), 240, DOI: 10.1038/s41586-018-0755-1.
Masson, G.; Konig, B. Chemical Photocatalysis - Do It Right! Eur. J. Org. Chem. 2020, 2020 (10), 1191-1192, DOI: 10.1002/ejoc.201901828.
Own work
Peschel, M. T.; Kabaciński, P.; Schwinger, D. P.; Thyrhaug, E.; Cerullo, G.; Bach, T.; Hauer, J.; de Vivie-Riedle, R. Activation of 2-Cyclohexenone by BF3 Coordination: Mechanistic Insights from Theory and Experiment. Angew. Chem. Int. Ed. 2021, 60 (18), 10155-10163. DOI: 10.1002/anie.202016653.
Schwinger, D. P.; Peschel, M. T.; Rigotti, T.; Kabaciński, P.; Knoll, T.; Thyrhaug, E.; Cerullo, G.; Hauer, J.; de Vivie-Riedle, R.; Bach, T. Photoinduced B–Cl Bond Fission in Aldehyde-BCl3 Complexes as a Mechanistic Scenario for C–H Bond Activation. Journal of the American Chemical Society 2022, 144 (41), 18927-18937. DOI: 10.1021/jacs.2c06683.