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Author: Dietzek, Benjamin × Author: Zedler, Linda × End date: 2019 × Start date: 2019 ×

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    Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis
    (Weinheim : Wiley-VCH, 2019) Zedler, Linda; Mengele, Alexander Klaus; Ziems, Karl Michael; Zhang, Ying; Wächtler, Maria; Gr-fe, Stefanie; Pascher, Torbjörn; Rau, Sven; Kupfer, Stephan; Dietzek, Benjamin
    Understanding photodriven multielectron reaction pathways requires the identification and spectroscopic characterization of intermediates and their excited-state dynamics, which is very challenging due to their short lifetimes. To the best of our knowledge, this manuscript reports for the first time on in situ spectroelectrochemistry as an alternative approach to study the excited-state properties of reactive intermediates of photocatalytic cycles. UV/Vis, resonance-Raman, and transient-absorption spectroscopy have been employed to characterize the catalytically competent intermediate [(tbbpy)2RuII(tpphz)RhICp*] of [(tbbpy)2Ru(tpphz)Rh(Cp*)Cl]Cl(PF6)2 (Ru(tpphz)RhCp*), a photocatalyst for the hydrogenation of nicotinamide (NAD-analogue) and proton reduction, generated by electrochemical and chemical reduction. Electronic transitions shifting electron density from the activated catalytic center to the bridging tpphz ligand significantly reduce the catalytic activity upon visible-light irradiation. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
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    Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways
    (Basel : MDPI, 2019) Zedler, Linda; Krieck, Sven; Kupfer, Stephan; Dietzek, Benjamin
    Electron transfer reactions play a key role for artificial solar energy conversion, however, the underlying reaction mechanisms and the interplay with the molecular structure are still poorly understood due to the complexity of the reaction pathways and ultrafast timescales. In order to investigate such light-induced reaction pathways, a new spectroscopic tool has been applied, which combines UV-vis and resonance Raman spectroscopy at multiple excitation wavelengths with electrochemistry in a thin-layer electrochemical cell to study [RuII(tbtpy)2]2+ (tbtpy = tri-tert-butyl-2,2′:6′,2′′-terpyridine) as a model compound for the photo-activated electron donor in structurally related molecular and supramolecular assemblies. The new spectroscopic method substantiates previous suggestions regarding the reduction mechanism of this complex by localizing photo-electrons and identifying structural changes of metastable intermediates along the reaction cascade. This has been realized by monitoring selective enhancement of Raman-active vibrations associated with structural changes upon electronic absorption when tuning the excitation wavelength into new UV-vis absorption bands of intermediate structures. Additional interpretation of shifts in Raman band positions upon reduction with the help of quantum chemical calculations provides a consistent picture of the sequential reduction of the individual terpyridine ligands, i.e., the first reduction results in the monocation [(tbtpy)Ru(tbtpy•)]+, while the second reduction generates [(tbtpy•)Ru(tbtpy•)]0 of triplet multiplicity. Therefore, the combination of this versatile spectro-electrochemical tool allows us to deepen the fundamental understanding of light-induced charge transfer processes in more relevant and complex systems.