@article{65316, abstract = {{Metasurfaces are powerful tools for manipulating light using small structures on the nanoscale. In most metasurfaces, near-field couplings are treated as being unfavorable perturbations. Here, we experimentally investigate a structure consisting of sinusoidally modulated silicon waveguides where near-field coupling of local resonances leads to negative coupling, i.e., a negative coupling constant. This gives rise to wave-vector-dependent eigenstates of elliptical, linear, and circular polarizations. In particular, fully circular polarization states are not only present at a single point in momentum space (k-space) but also along a line. This circular polarization line, as well as a linear polarization line, emanates from a polarization degeneracy at the Dirac point. We experimentally validate the existence of these eigenstates and demonstrate the energy-, polarization-, and wave vector dependence of this metasurface as well as its sensitivity to fabrication tolerances. By tuning the incident k-vector, certain polarization-energy eigenstates are strongly reflected, allowing for uses in angle-tunable polarization filters and light sources.}}, author = {{Wetter, Helene and Wingenbach, Jan and Rehberg, Falk and Gao, Wenlong and Schumacher, Stefan and Zentgraf, Thomas}}, issn = {{2330-4022}}, journal = {{ACS Photonics}}, keywords = {{metasurface, waveguides, Dirac point, polarization, negative coupling}}, pages = {{2128--2133}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Polarization- and Wave-Vector Selective Optical Metasurface with Near-Field Coupling}}}, doi = {{10.1021/acsphotonics.5c02865}}, volume = {{13}}, year = {{2026}}, } @article{63988, abstract = {{This concept summarizes recent advances in development and application of DNP enhanced multinuclear solid-state NMR to study the molecular structure and surface functionalization of cellulose and paper-based materials. Moreover, a novel application is presented where DNP enhanced 13C and 15N solid-state NMR is used to identify structure moieties formed by cross-linking of hydroxypropyl cellulose. Given these two aspects of this concept-type of article, we thus combine both, a review on recent findings already published and unpublished recent data that complement the existing knowledge in the field of characterization of functional lignocellulosic materials by DNP enhanced solid-state NMR.}}, author = {{Höfler, Mark V. and Lins, Jonas and Seelinger, David and Pachernegg, Lukas and Schäfer, Timmy and Spirk, Stefan and Biesalski, Markus and Gutmann, Torsten}}, journal = {{Journal of Magnetic Resonance Open}}, keywords = {{solid-state nmr, dynamic nuclear polarization, Hydroxypropyl cellulose, Selective enhancement, Spin labelling}}, pages = {{100163}}, title = {{{DNP enhanced solid-state NMR – A powerful tool to address the surface functionalization of cellulose/paper derived materials}}}, doi = {{10.1016/j.jmro.2024.100163}}, volume = {{21}}, year = {{2024}}, } @article{63974, abstract = {{A versatile strategy for synthesizing tailored peptide based biradicals is presented. By labeling the protected amino acid hydroxyproline with PROXYL via the OH functionality and using this building block in solid phase peptide synthesis (SPPS), the obtained peptides become polarization agents for DNP enhanced solid-state NMR in biotolerant media. To analyze the effect of the radical position on the enhancement factor, three different biradicals are synthesized. The PROXYL spin-label is inserted in a collagen inspired artificial peptide sequence by binding through the OH group of the hydroxyproline moieties at specific position in the chain. This labeling strategy is universally applicable for any hydroxyproline position in a peptide sequence since solid-phase peptide synthesis is used to insert the building block. High performance liquid chromatography (HPLC) and mass spectrometry (MS) analyses show the successful introduction of the spin label in the peptide chain and electron paramagnetic resonance (EPR) spectroscopy confirms its activity. Dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance (NMR) experiments performed on frozen aqueous glycerol-d8 solutions containing these peptide radicals show significantly higher enhancement factors of up to 45 in 1H→13C cross polarization magic angle spinning (CP MAS) experiments compared to an analogous mono-radical peptide including this building block (ε ≈ 14). Compared to commercial biradicals such as AMUPol for which enhancement factors {\textgreater} 100 have been obtained in the past and which have been optimized in their structure, the obtained enhancement up to 45 for our biradicals presents a significant progress in radical design.}}, author = {{Herr, Kevin and Höfler, Mark V. and Heise, Henrike and Aussenac, Fabien and Kornemann, Felix and Rosenberger, David and Brodrecht, Martin and Oliveira, Marcos and Buntkowsky, Gerd and Gutmann, Torsten}}, journal = {{Journal of Magnetic Resonance Open}}, keywords = {{solid-state nmr, dynamic nuclear polarization, peptides, Biradicals, Spin labeling}}, pages = {{100152}}, title = {{{Biradicals based on PROXYL containing building blocks for efficient dynamic nuclear polarization in biotolerant media}}}, doi = {{10.1016/j.jmro.2024.100152}}, volume = {{20}}, year = {{2024}}, } @article{63970, abstract = {{Abstract Recent advances in solid-state nuclear magnetic resonance (NMR) spectroscopy, combined with dynamic nuclear polarization (DNP), quantum chemical DFT calculations, and gas-phase NMR spectroscopy investigating the structure and reactivity of heterogeneous catalysts are reviewed. The investigated catalysts range from classical mononuclear catalysts, like immobilized derivates of Wilkinson’s catalysts over binuclear catalysts such as the dirhodium paddlewheel catalyst to catalytic nanoparticles, employing various support materials, such as mesoporous silica gels, coordination polymers, and biomaterials such as cellulose.}}, author = {{Haro Mares, Nadia and Logrado, Millena and Kergassner, Jan and Zhang, Bingyu and Gutmann, Torsten and Buntkowsky, Gerd}}, issn = {{1867-3880}}, journal = {{ChemCatChem}}, keywords = {{solid-state nmr, heterogeneous catalysis, dynamic nuclear polarization, Nanocatalysis, Surface-reactions}}, pages = {{e202401159}}, publisher = {{John Wiley & Sons, Ltd}}, title = {{{Solid-State NMR of Heterogeneous Catalysts}}}, doi = {{10.1002/cctc.202401159}}, year = {{2024}}, } @article{64052, abstract = {{Abstract Estuaries are key ecosystems with unique biodiversity and are of high economic importance. Along the estuaries, variations in environmental parameters, such as salinity and light penetration, can modify the characteristics of dissolved organic matter (DOM). Nevertheless, there is still limited information about the atomic-level transformations of DOM in this ecosystem. Solid-state NMR spectroscopy provides unique insights into the nature of functional groups in DOM. A major limitation of this technique is its lack of sensivity, which results in experimental time of tens of hours for the acquisition of 13C NMR spectra and generally precludes the observation of 15N nuclei for DOM. We show here how the sensitivity of solid-state NMR experiments on DOM of Seine estuary can be enhanced using dynamic nuclear polarization (DNP) under magic-angle spinning. This technique allows the acquisition of 13C NMR spectra of these samples in few minutes, instead of hours for conventional solid-state NMR. Both conventional and DNP-enhanced 13C NMR spectra indicate that the 13C local environments in DOM are not strongly modified along the Seine estuary. Furthermore, the sensitivity gain provided by the DNP allows the detection of 15N NMR signal of DOM, in spite of the low nitrogen content. These spectra reveal that the majority of nitrogen is in the amide form in these DOM samples and show an increased disorder around these amide groups near the mouth of the Seine.}}, author = {{Venel, Florian and Nagashima, Hiroki and Rankin, Andrew G. M. and Anquetil, Christelle and Klimavicius, Vytautas and Gutmann, Torsten and Buntkowsky, Gerd and Derenne, Sylvie and Lafon, Olivier and Huguet, Arnaud and Pourpoint, Frédérique}}, issn = {{1439-4235; 1439-7641}}, journal = {{Chemphyschem}}, keywords = {{dynamic nuclear polarization, 13C, 15N, dissolved organic matter, Seine estuary}}, number = {{18}}, pages = {{1907–1913}}, publisher = {{John Wiley & Sons, Ltd}}, title = {{{Characterization of Functional Groups in Estuarine Dissolved Organic Matter by DNP-enhanced 15N and 13C Solid-State NMR}}}, doi = {{10.1002/cphc.202100334}}, volume = {{22}}, year = {{2021}}, } @article{63969, abstract = {{A number of Ir-N-heterocyclic carbene (Ir-NHC) complexes with asymmetric N-heterocyclic carbene (NHC) ligands have been prepared and examined for signal amplification by reversible exchange (SABRE). Pyridine was chosen as model compound for hyperpolarization experiments. This substrate was examined in a solvent mixture using several Ir-NHC complexes, which differ in their NHC ligands. The SABRE polarization was created at 6mT and the H-1 nuclear magnetic resonancesignals were detected at 7T. We show that asymmetric NHC ligands, because of their favorable chemistry, can adapt the SABREactive complexes to different chemical scenarios.}}, author = {{Hadjiali, S. and Savka, R. and Plaumann, M. and Bommerich, U. and Bothe, S. and Gutmann, Torsten and Ratajczyk, T. and Bernarding, J. and Limbach, H. H. and Plenio, H. and Buntkowsky, G.}}, issn = {{1613-7507}}, journal = {{Applied Magnetic Resonance}}, keywords = {{dynamic nuclear-polarization, hyperpolarization, enhancement, hydrogen induced polarization, olefin-metathesis catalysts, parahydrogen-induced polarization, peptides, Physics, sabre, spectroscopy}}, number = {{7}}, pages = {{895–902}}, title = {{{Substituent Influences on the NMR Signal Amplification of Ir Complexes with Heterocyclic Carbene Ligands}}}, doi = {{10.1007/s00723-019-01115-x}}, volume = {{50}}, year = {{2019}}, } @article{63960, abstract = {{Recent advances in solid-state nuclear magnetic resonance (NMR) spectroscopy and dynamic nuclear polarization (DNP) of nanostructured materials are reviewed. A first group of materials is based on crystalline nanocellulose (CNC) or microcrystalline cellulose (MCC), which are used as carrier materials for dye molecules, catalysts or in combination with heterocyclic molecules as ion conducting membranes. These materials have widespread applications in sensorics, optics, catalysis or fuel cell research. A second group are metal oxides such as V-Mo-W oxides, which are of enormous importance in the manufacturing process of basic chemicals. The third group are catalytically active nanocrystalline metal nanoparticles, coated with protectants or embedded in polymers. The last group includes of lead-free perovskite materials, which are employed as environmentally benign substitution materials for conventional lead-based electronics materials. These materials are discussed in terms of their application and physico-chemical characterization by solid-state NMR techniques, combined with gas-phase NMR and quantum-chemical modelling on the density functional theory (DFT) level. The application of multinuclear 1H, 2H, 13C, 15N and 23Na solid state NMR techniques under static or MAS conditions for the characterization of these materials, their surfaces and processes on their surfaces is discussed. Moreover, the analytic power of the combination of these techniques with DNP for the identification of low-concentrated carbon and nitrogen containing surface species in natural abundance is reviewed. Finally, approaches for sensitivity enhancement by DNP of quadrupolar nuclei such as 17O and 51V are presented that enable the identification of catalytic sites in metal oxide catalysts.}}, author = {{Gutmann, Torsten and Groszewicz, Pedro B. and Buntkowsky, Gerd}}, journal = {{Annual Reports on NMR Spectroscopy}}, keywords = {{solid-state nmr, heterogeneous catalysis, dynamic nuclear polarization, Ferroelectrics, Nanocatalysis, Surface reactions}}, pages = {{1–82}}, title = {{{Solid-state NMR of nanocrystals}}}, doi = {{10.1016/bs.arnmr.2018.12.001}}, volume = {{97}}, year = {{2019}}, } @article{63956, abstract = {{The synthesis of novel robust and stable iridium-based immobilized catalysts on silica-polymer hybrid materials (Si-PB-Ir) is described. These catalysts are characterized by a combination of 1D P-31 CP-MAS and 2D P-31-H-1 HETCOR and J-resolved multinuclear solid state NMR experiments. Different binding situations such as singly and multiply coordinated phosphines are identified. Density functional theory (DFT) calculations are performed to corroborate the interpretation of the experimental NMR data, in order to propose a structural model of the heterogenized catalysts. Finally, the catalytic activity of the Si-PB-Ir catalysts is investigated for the hydrogenation of styrene employing para-enriched hydrogen gas.}}, author = {{Gutmann, Torsten and Alkhagani, S. and Rothermel, N. and Limbach, H. H. and Breitzke, H. and Buntkowsky, G.}}, issn = {{0942-9352}}, journal = {{Zeitschrift Fur Physikalische Chemie-International Journal of Research in Physical Chemistry & Chemical Physics}}, keywords = {{Chemistry, dynamic nuclear-polarization, solid-state nmr, DFT, heterogeneous catalysis, hydrido complexes, hydrogenation, immobilized catalyst, inorganic hybrid, iridium, materials, mesoporous, molecular-orbital methods, PHIP, phosphine complexes, reusable catalysts, silica, solid-state-NMR, wilkinsons catalyst}}, number = {{3}}, pages = {{653–669}}, title = {{{P-31-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts}}}, doi = {{10.1515/zpch-2016-0837}}, volume = {{231}}, year = {{2017}}, } @article{63920, abstract = {{Coordinatively unsaturated sites (CUS) present a key feature of alumina based catalysts as they are believed to act as Lewis-acid sites in heterogeneously catalyzed reactions. In the present study, the direct observation of active species on a fluoride-doped aluminum oxide catalyst is demonstrated. This new fluoride-doped aluminum oxide exhibits strong Lewis-acid sites and superior catalytic activity as compared to gamma-Al2O3. To emphasize the labile state of Lewis-acid sites, two distinctive states of the catalysts surface are addressed using H-1-Al-27 cross polarization (CP) MAS NMR. On the one hand, the highly dehydrated and active state after calcination at 700 degrees C and on the other hand the rehydrated and catalytically inactive surface (produced by contact to air) are probed. These experiments revealed the presence of significant amounts of coordinatively unsaturated sites in the form of 4-and 5-fold coordinated Al-sites on the highly dehydrated surface. In contrast to this, the rehydrated sample exhibited a severely restructured surface caused by the chemisorption of H2O which is ’constituted in a manner that was proposed in earlier models for gamma-Al2O3 surfaces.}}, author = {{Ahrem, L. and Scholz, G. and Gutmann, Torsten and Calvo, B. and Buntkowsky, G. and Kemnitz, E.}}, issn = {{1932-7447}}, journal = {{Journal of Physical Chemistry C}}, keywords = {{al-27 nmr, characterization, Chemistry, cross-polarization, dynamic nuclear-polarization, eta-alumina, gamma-alumina, hydroxy fluorides, ions, Materials Science, pentacoordinated al3+, Science & Technology - Other Topics, solid-state nmr, spectroscopic, structural insights}}, number = {{22}}, pages = {{12206–12213}}, title = {{{Direct Observation of Coordinatively Unsaturated Sites on the Surface of a Fluoride-Doped Alumina Catalyst}}}, doi = {{10.1021/acs.jpcc.7b02535}}, volume = {{121}}, year = {{2017}}, } @article{4246, abstract = {{Spins in semiconductor quantum dots have been considered as prospective quantum bit excitations. Their coupling to the crystal environment manifests itself in a limitation of the spin coherence times to the microsecond range, both for electron and hole spins. This rather short-lived coherence compared to atomic states asks for manipulations on timescales as short as possible. Due to the huge dipole moment for transitions between the valence and conduction band, pulsed laser systems offer the possibility to perform manipulations within picoseconds or even faster. Here, we report on results that show the potential of optical spin manipulations with currently available pulsed laser systems. Using picosecond laser pulses, we demonstrate optically induced spin rotations of electron and hole spins. We further realize the optical decoupling of the hole spins from the nuclear surrounding at the nanosecond timescales and demonstrate an all-optical spin tomography for interacting electron spin sub-ensembles.}}, author = {{Varwig, S. and Evers, E. and Greilich, A. and Yakovlev, D. R. and Reuter, Dirk and Wieck, A. D. and Meier, Torsten and Zrenner, Artur and Bayer, M.}}, issn = {{0946-2171}}, journal = {{Applied Physics B}}, keywords = {{Spin Polarization, Pump Pulse, Trion, Spin Component, Coherence Time}}, number = {{1}}, publisher = {{Springer Nature}}, title = {{{Advanced optical manipulation of carrier spins in (In,Ga)As quantum dots}}}, doi = {{10.1007/s00340-015-6274-y}}, volume = {{122}}, year = {{2016}}, } @article{63963, abstract = {{A novel heterogeneous dirhodium catalyst has been synthesized. This stable catalyst is constructed from dirhodium acetate dimer (Rh2(OAc)4) units, which are covalently linked to amine- and carboxyl-bifunctionalized mesoporous silica (SBA-15NH2COOH). It shows good efficiency in catalyzing the cyclopropanation reaction of styrene and ethyl diazoacetate (EDA) forming cis- and trans-1-ethoxycarbonyl-2-phenylcyclopropane. To characterize the structure of this catalyst and to confirm the successful immobilization, heteronuclear solid-state NMR experiments have been performed. The high application potential of dynamic nuclear polarization (DNP) NMR for the analysis of binding sites in this novel catalyst is demonstrated. Signal-enhanced 13C CP MAS and 15N CP MAS techniques have been employed to detect different carboxyl and amine binding sites in natural abundance on a fast time scale. The interpretation of the experimental chemical shift values for different binding sites has been corroborated by quantum chemical calculations on dirhodium model complexes.}}, author = {{Gutmann, Torsten and Liu, Jiquan and Rothermel, Niels and Xu, Yeping and Jaumann, Eva and Werner, Mayke and Breitzke, Hergen and Sigurdsson, Snorri T. and Buntkowsky, Gerd}}, journal = {{Chemistry A European Journal}}, keywords = {{heterogeneous catalysis, immobilized catalyst, dynamic nuclear polarization, hyperpolarization, NMR spectroscopy}}, number = {{9}}, pages = {{3798–3805}}, publisher = {{WILEY-VCH Verlag}}, title = {{{Natural Abundance 15N NMR by Dynamic Nuclear Polarization: Fast Analysis of Binding Sites of a Novel Amine-Carboxyl-Linked Immobilized Dirhodium Catalyst}}}, doi = {{10.1002/chem.201405043}}, volume = {{21}}, year = {{2015}}, } @inproceedings{13326, abstract = {{Communication within online social network applications enables users to express and share sentiments electronically. Existing studies examined the existence or distribution of sentiments in online communication at a general level or in small-observed groups. Our paper extends this research by analyzing sentiment exchange within social networks from an ego-network perspective. We draw from research on social influence and social attachment to develop theories of node polarization, balance effects and sentiment mirroring within communication dyads. Our empirical analysis covers a multitude of social networks in which the sentiment valence of all messages was determined. Subsequently we studied ego-networks of focal actors (ego) and their immediate contacts. Results support our theories and indicate that actors develop polarized sentiments towards individual peers but keep sentiment in balance on the ego-network level. Further, pairs of nodes tend to establish similar attitudes towards each other leading to stable and polarized positive or negative relationships}}, author = {{Hillmann, Robert and Trier, Matthias}}, booktitle = {{AMCIS 2012 Proceedings}}, editor = {{Joshi, K.D. and Yoo, Youngjin}}, keywords = {{Social Network Analysis, Ego-Network Analysis, Node Polarization, Sentiment Dissemination}}, publisher = {{Association for Information Systems. AIS Electronic Library (AISeL)}}, title = {{{Sentiment Polarization and Balance among Users in Online Social Networks}}}, volume = {{24}}, year = {{2012}}, }