Reinhard Stößer’s research while affiliated with Humboldt-Universität zu Berlin and other places

The first coupled operando EPR/UV-Vis/ATR-IR spectroscopy setup for mechanistic studies of gas-liquid phase reactions is presented and exemplarily applied to the well-known copper/TEMPO-catalyzed (TEMPO=(2,2,6,6-tetramethylpiperidin-1-yl)oxyl) oxidation of benzyl alcohol. In contrast to previous proposals, no direct redox reaction between TEMPO and CuI/CuII has been detected. Instead, the role of TEMPO is postulated to be the stabilization of a (bpy)(NMI)CuII-O2⋅−-TEMPO (bpy=2,2′-bipyridine, NMI=N-methylimidazole) intermediate formed by electron transfer from CuI to molecular O2.

Hydrogen atoms were generated in the matrices AlF3 and CaF2 at room temperature by ionizing radiation (γ and UV) and were investigated by electron paramagnetic resonance spectroscopy. To gain information on the nature of the hydrogen precursors and hydrogen traps within the solid matrices, studies regarding defined thermal and chemical influence, as well as mechanical impact on the fluoridic matrices before and after irradiation were the focus of this work. Impurities such as HO– and hydrocarbons were identified as potential precursors. The hydrogen traps were characterized by means of their spatial distribution within the different samples and by their energetic states (“trap-depth”). In this respect, the rate constants, which were determined for different temperatures, turned out to be a valuable tool. Similarities and differences regarding the generation and stabilization of atomic hydrogen in AlF3 and CaF2 were worked out. Also, the influence of the type of ionizing radiation was elucidated. Whereas the comparison of AlF3 and CaF2 was dominated by similarities, significant differences were found for rather molecular siloxane cages R8Si8O12, especially regarding their reaction upon UV excitation and the possibility to resolve super-hyperfine splittings.

The first coupled operando EPR/UV-Vis/ATR-IR spectroscopy setup for mechanistic studies of gas-liquid phase reactions is presented and exemplarily applied to the well-known copper/TEMPO-catalyzed (TEMPO=(2,2,6,6-tetramethylpiperidin-1-yl)oxyl) oxidation of benzyl alcohol. In contrast to previous proposals, no direct redox reaction between TEMPO and Cu(I) /Cu(II) has been detected. Instead, the role of TEMPO is postulated to be the stabilization of a (bpy)(NMI)Cu(II) -O2 (⋅-) -TEMPO (bpy=2,2'-bipyridine, NMI=N-methylimidazole) intermediate formed by electron transfer from Cu(I) to molecular O2 . © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work, the new organic-inorganic hybrid compound Ph4P[(V2O3)(2)(PhAsO3)(4)subset of F] (VAsF) has been prepared and characterized by single-crystal XRD and multinuclear magnetic resonance (H-1, F-19, P-31, and V-51). Redox properties and thermal stability have been investigated by EPR, cyclic voltammetry, and thermal analysis in comparison to its Ph4P-[(V2O3)(2)(PhPO3)(4)subset of F] (VPF) analogue. The VAsF cluster has a lower redox potential and higher electrochemical stability in solution, while it is thermally less stable in the solid state. Density functional theory (DFT) calculations showed that the difference in the redox potential is due to the different electron affinities of VPF and VAsF. With this approach of modifying the type of the ligand of the molecular vanadium cage, we hope to enhance the utility of such compounds as building blocks for the design of new hybrid materials with desirable properties.

The photolysis of 9-diazofluorene-derivatives yields various paramagnetic species. By EPR-spectroscopy at continuous and modulated optical excitation these species have been identified as radicals as well as triplets in ground- and excited states. In the simulation and interpretation of the spectra comparison with structure analogous compounds and products was helpfull. On the basis of the results the possible role of the paramagnetic intermediates in the reaction-scheme is discussed.

The modulation of the electron-donor properties of tetrathiafulvalene (TTF) with light is attempted by two molecular designs: the integration of the TTF core as aryl unit(s) into the hexatriene system of diarylethene (DAE) derivatives and the attachment of terminal TTF unit(s) to a conventional 1,2-dithienylcyclopentene chromophore via conjugated ethynylene spacers. While photochromism is inhibited in the first approach, presumably by a twisted intramolecular charge transfer mechanism, the second approach yields photoswitchable TTF derivatives. Remarkably, the extension of the conjugated π-system upon ring closure affects the first oxidation potential values of the TTF–DAEs only slightly. Nevertheless, the electronic changes in the photochromic core are foreseen to play a crucial role in the intramolecular electron transfer process in donor–DAE–acceptor architectures.

Novel thermally-reversible photochromic spiro-acridans are prepared and their photoactivity is reported.

The Goldschmidt reaction (2Al+α-Fe2O3) was investigated both under mechanical and thermal activation by employing a combination of analytical methods such as ESR and Mössbauer spectroscopy, magnetic susceptibility, X-ray powder diffraction, and thermal analysis (TA). Both the mechanical treatment and the TA runs under air caused the reaction conditions of “quiet redox reactions” due to a retarding effect and enabled a reaction study in a “slow motion mode”. This allowed to establish distinct partial steps of the integral reaction process depending on the intensity of the mechanical impact, the educt composition, the gas phase composition, and, finally, the thermal excitation in the TA runs. Particular attention was payed to the role of the gas phase and to the reaction of aluminum with differently activated oxygen species. The phlegmatizing effect was traced back to the formation of Al2O3 coverages of the Al particles.

In order to present the relationship between ESR spectroscopy and isotope effects three levels are considered: (i) ESR spectroscopy is described on a general level up to the models for interpretation of the experimental spectra, which go beyond the usually used time and mass independent spin-Hamilton operator, (ii) the main characteristics of the generalized isotope effects are worked out, and finally (iii) the basic, mainly quantum mechanical effects are used to describe the coupling of electron spins with the degrees of freedom, which are accessible under the selected conditions, of the respective paramagnetic object under investigation. The ESR parameters and the respective models are formalized so far, that they include the time and mass depending influences and reflect the specific isotope effects. Relations will be established between the effects in ESR spectra to spin relaxation, to spin exchange, to the magnetic isotope effect, to the Jahn-Teller effects, as well as to the influence of zero-point vibrations. Examples will be presented which demonstrate the influence of isotopes as well as the kind of accessible information. It will be differentiated with respect to isotope effects in paramagnetic centres itself and in the respective matrices up to the technique of ESR imaging. It is shown that the use of isotope effects is indispensable in ESR spectroscopy.