(Resonanz)-Raman- und Redox-Sensitive Spektroskopie zur Prozessaufklärung in funktionellen Molekülen

Komplexe dynamische Reaktionsmechanismen bilden die Grundlage natürlicher und artifizieller Prozesse. Ein tiefgehendes Verständnis dieser Reaktionsmechanismen ist deshalb von enormer Bedeutung, z.B. für die Entwicklung neuartiger Photokatalysatoren zur Umwandlung von Sonnenenergie in chemische Energie. Diese Arbeit widmet sich der Aufklärung der während dynamischer Prozesse auftretenden strukturellen Änderungen in Abhängigkeit von Prozessvariablen wie Ort, Temperatur, Reaktionsdauer oder Anregungswellenlänge mithilfe (Resonanz)-Raman-(Mikro)-spektroskopischer Techniken. Dafür werden die die Variation der Prozessgrößen begleitenden Strukturänderungen komplexer Moleküle analysiert und daraus Rückschlüsse auf den Reaktionsverlauf und den zugrunde liegenden Reaktionsmechanismus im Hinblick auf die Funktion einzelner struktureller Komponenten gezogen. Den Schwerpunkt der Arbeit bildete die Konzeption und Konstruktion einer Messapparatur für die redoxsensitive Resonanz-Raman-Spektroskopie, mit deren Hilfe erstmals gezeigt werden konnte, dass (i) breitbandig absorbierende 4H-Imidazol-Rutheniumkomplexe durch Modifikation der Substituenten an den Aryliminoresten zwei Elektronen lokal in einer Ligandensphäre speichern können, was einen großen Fortschritt auf dem Weg zur Entwicklung funktionaler supramolekularer Systeme für die Mehrelektronenkatalyse darstellt. (ii) Für einen Ru-Polypyridyl-Tetrapyridophenazin-Komplex konnte nachgewiesen werden, dass die Ladungstransfereffizienz von der sequentiellen Abfolge der einzelnen Ladungstransferprozesse abhängt. Die Photoanregung von Bipyridin-Liganden, Intra-Ligand-Übergänge im Brückenliganden sowie die Rückreduktion des Photozentrums vor Abschluss des Elektronentransfers auf das katalytische Zentrum stellen die Effizienz limitierende Konkurrenzprozesse zum gerichteten (Mehr)-Elektronentransfer auf das Katalysezentrum dar

Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways

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

Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis

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

Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways

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

Disruption-free imaging by Raman spectroscopy reveals a chemical sphere with antifouling metabolites around macroalgae

Investigations of the surface chemistry of marine organisms are essential to understand their chemically mediated interactions with fouling organisms. In this context, the concentration of natural products in the immediate vicinity of algal surfaccs, as well as their biological activity, are of particular importance. Howevcr, due to lack of appropriate methods, the distribution of Compounds within the Chemical sphere around marine algae is unknown. This study demonstrates the suitability of confocal resonance Raman microspcctroscopy for the determination of metabolites around algal surfaces with a micrometer resolution

In Situ Localization and Structural Analysis of the Malaria Pigment Hemozoin

Raman microspectroscopy was applied for an insitu localization of the malaria pigment hemozoin in Plasmodium falciparum-infected erythrocytes. The Raman spectra (λexc = 633 nm) of hemozoin show very intense signals with a very good signal-to-noise ratio. These in situ Raman signals of hemozoin were compared to Raman spectra of extracted hemozoin, of the synthetic analogue β-hematin, and of hematin and hemin. β-Hematin was synthesized according to the acid-catalyzed dehydration of hematin and the anhydrous dehydrohalogenation of hemin which lead to good crystals with lengths of about 5−30 μm. The Raman spectra (λexc = 1064 nm) of hemozoin and β-hematin show almost identical behaviors, while some low wavenumber modes might be used to distinguish between the morphology of differently synthesized β-hematin samples. The intensity pattern of the resonance Raman spectra (λexc = 568 nm) of hemozoin and β-hematin differ significantly from those of hematin and hemin. The most striking difference is an additional band at 1655 cm-1 which was only observed in the spectra of hemozoin and β-hematin and cannot be seen in the spectra of hematin and hemin. Raman spectra of the β-hematin dimer were calculated ab initio (DFT) for the first time and used for an assignment of the experimentally derived Raman bands. The calculated atomic displacements provide valuable insight into the most important molecular vibrations of the hemozoin dimer. With help from these DFT calculations, it was possible to assign the Raman band at 1655 cm-1 to a mode located at the propionic acid side chain, which links the hemozoin dimers to each other. The Raman band at 1568 cm-1, which has been shown to be influenced by an attachment of the antimalarial drug chloroquine in an earlier study, could be assigned to a CC stretching mode spread across one of the porphyrin rings and is therefore expected to be influenced by a π−π-stacking to the drug

Electron transfer in a covalent dye-cobalt catalyst assembly - a transient absorption spectroelectrochemistry perspective.

International audienceVarious oxidation states of the catalytically active cobalt center in a covalent dyad were electrochemically prepared and the light-induced excited-state processes were studied. Virtually identical deactivation processes are observed, irrespective of the oxidation state of the cobalt center, varying from CoIII to CoI, indicating the absence of oxidative quenching within the dye-catalyst assembly

Which bridge to cross, which mountain to climb – Supramolecular Photocatalysis Outpacing Conventional Catalysis

Unequivocal assignment of rate limiting steps in supramolecular photocatalysts is of utmost importance to rationally optimize photocatalytic activity. By spectroscopic and catalytic analysis of a series of three structurally similar [(tbbpy) 2 Ru-BL-Rh(Cp*)Cl] 3+ photocatalysts just differing in the central part (alkynyl, triazole or phenazine) of the bridging ligand (BL) we were able to derive design strategies for improved photocatalytic activity of this class of compounds (tbbpy = 4,4´-tert-butyl- 2,2´-bipyridine, Cp* = pentamethylcyclopentadienyl). Most importantly, not the rate of the transfer of the first electron towards the Rh III center but rather the rate at which a two-fold reduced Rh I species is generated can directly be correlated with the observed photocatalytic formation of NADH from NAD + . Interestingly, the complex which exhibited the fastest intramolecular electron transfer kinetics for the first electron is not the one that allowed the fastest photocatalysis. With the photocatalytically most efficient alkynyl linked system, it was even possible to overcome the rate of thermal NADH formation. Moreover, for this photocatalyst loss of the alkynyl functionality under photocatalytic conditions was identified as an important deactivation pathway

A new RuCo dyad for dye-sensitized hydrogen-evolving photocathode applications – synthesis, photo- and spectroelectrochemical studies.

National audienc

Noble metal-free covalent dye catalyst assemblies for H2-evolving dye-sensitized photocathodes: improved performance and transient absorption spectroelectrochemistry

International audienc