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排序方式: 共有1171条查询结果,搜索用时 15 毫秒
991.
Nico Leupold Philipp Ramming Irene Bauer Christina Witt Jennifer Jungklaus Prof. Dr. Ralf Moos Dr. Helen Grüninger Dr. Fabian Panzer 《欧洲无机化学杂志》2023,26(8):e202200736
Here, we investigate in detail the impact of the size of the methylammonium iodide (MAI) reactants in the mechanochemical powder synthesis of the halide perovskite methylammonium lead iodide (MAPbI3). Morphology and structural characterizations by scanning electron microscopy and X-ray diffraction reveal that with increasing MAI reactant size, the particle size of the perovskite powder increases, while its defect density decreases, as suggested by nuclear quadrupole resonance spectroscopy and photoluminescence investigations. The reason for this behavior seems to be associated to the sensitive influence of the MAI size on the time durations of MAPbI3 synthesis and delayed MAPbI3 crushing stage during ball milling. Thus, our results emphasize the high importance the reactant properties have on the mechanochemical synthesis of halide perovskites and will contribute to enhance the reproducibility and control of the fabrication of halide perovskites in powder form. 相似文献
992.
Dr. Fabian Eisenreich Prof. Dr. Anja R. A. Palmans 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(52):e202201322
Photoredox-catalyzed chemical conversions are predominantly operated in organic media to ensure good compatibility between substrates and catalysts. Yet, when conducted in aqueous media, they are an attractive, mild, and green way to introduce functional groups into organic molecules. We here show that trifluoromethyl groups can be readily installed into a broad range of organic compounds by using water as the reaction medium and light as the energy source. To bypass solubility obstacles, we developed robust water-soluble polymeric nanoparticles that accommodate reagents and photocatalysts within their hydrophobic interior under high local concentrations. By taking advantage of the high excited state reduction potential of N-phenylphenothiazine (PTH) through UV light illumination, the direct C−H trifluoromethylation of a wide array of small organic molecules is achieved selectively with high substrate conversion. Key to our approach is slowing down the production of CF3 radicals during the chemical process by reducing the catalyst loading as well as the light intensity, thereby improving effectiveness and selectivity of this aqueous photocatalytic method. Furthermore, the catalyst system shows excellent recyclability and can be fueled by sunlight. The method we propose here is versatile, widely applicable, energy efficient, and attractive for late-stage introduction of trifluoromethyl groups into biologically active molecules. 相似文献
993.
Fabian Mitschang Markus Langner Henning Vieker Andr Beyer Andreas Greiner 《Macromolecular rapid communications》2015,36(3):304-310
Continuous conductive gold nanofibers are prepared via the “tubes by fiber templates” process. First, poly(l‐lactide) (PLLA)‐stabilized gold nanoparticles (AuNP) with over 60 wt% gold are synthesized and characterized, including gel permeation chromatography coupled with a diode array detector. Subsequent electrospinning of these AuNP with template PLLA results in composite nanofibers featuring a high gold content of 57 wt%. Highly homogeneous gold nanowires are obtained after chemical vapor deposition of 345 nm of poly(p‐xylylene) (PPX) onto the composite fibers followed by pyrolysis of the polymers at 1050 °C. The corresponding heat‐induced transition from continuous gold‐loaded polymer tubes to smooth gold nanofibers is studied by transmission electron microscopy and helium ion microscopy using both secondary electrons and Rutherford backscattered ions.
994.
Dr. Paritosh Kumar Mohanta Masuma Sultana Ripa Fabian Regnet Dr. Ludwig Jörissen 《ChemistryOpen》2020,9(5):607-615
Performance of a low temperature polymer electrolyte membrane fuel cell (PEMFC) is highly dependent on the kind of catalysts, catalyst supports, ionomer amount on the catalyst layers (CL), membrane types and operating conditions. In this work, we investigated the influence of membrane types and CL compositions on MEA performance. MEA performance increases under all practically relevant load conditions with reduction of the membrane thickness from 50 to 15 μm, however further decrease in membrane thickness from 15 to 10 μm leads to reduction in cell voltage at high current loads. A thick anode CL is found to be beneficial under wet operating conditions assuming more pore space is provided to accommodate liquid water, whereas under dry operating conditions, an intermediate thickness of the anode CL is beneficial. When studying the impact of catalyst layer thickness, too thin a catalyst layer again shows reduced performance due to increased ohmic resistance ruled out the performance of the MEAs which have identical Pt crystallite sizes on the cathode CLs i. e. the thinnest the cathode CL, the highest the voltage were achieved at a defined current load. Adaptation of the operating conditions is highly anticipated to achieve the highest MEA performance. 相似文献
995.
Tim Mller Fabian Scholten Trung Ngo Thanh Ilya Sinev Janis Timoshenko Xingli Wang Zarko Jovanov Manuel Gliech Beatriz Roldan Cuenya Ana Sofia Varela Peter Strasser 《Angewandte Chemie (International ed. in English)》2020,59(41):17974-17983
The direct electrochemical conversion of carbon dioxide (CO2) into multi‐carbon (C2+) products still faces fundamental and technological challenges. While facet‐controlled and oxide‐derived Cu materials have been touted as promising catalysts, their stability has remained problematic and poorly understood. Herein we uncover changes in the chemical and morphological state of supported and unsupported Cu2O nanocubes during operation in low‐current H‐Cells and in high‐current gas diffusion electrodes (GDEs) using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C2+ Faradaic efficiency of around 60 % for 40 h, the dispersion on a carbon support sharply shifted the selectivity pattern towards C1 products. Operando XAS and time‐resolved electron microscopy revealed the degradation of the cubic shape and, in the presence of a carbon support, the formation of small Cu‐seeds during the surprisingly slow reduction of bulk Cu2O. The initially (100)‐rich facet structure has presumably no controlling role on the catalytic selectivity, whereas the oxide‐derived generation of under‐coordinated lattice defects, can support the high C2+ product yields. 相似文献
996.
Fabian M. Hrmann Christoph Kerzig Tim S. Chung Andreas Bauer Oliver S. Wenger Thorsten Bach 《Angewandte Chemie (International ed. in English)》2020,59(24):9659-9668
Chiral eniminium salts, prepared from α,β‐unsaturated aldehydes and a chiral proline derived secondary amine, underwent, upon irradiation with visible light, a ruthenium‐catalyzed (2.5 mol %) intermolecular [2+2] photocycloaddition to olefins, which after hydrolysis led to chiral cyclobutanecarbaldehydes (17 examples, 49–74 % yield), with high diastereo‐ and enantioselectivities. Ru(bpz)3(PF6)2 was utilized as the ruthenium catalyst and laser flash photolysis studies show that the catalyst operates exclusively by triplet‐energy transfer (sensitization). A catalytic system was devised with a chiral secondary amine co‐catalyst. In the catalytic reactions, Ru(bpy)3(PF6)2 was employed, and laser flash photolysis experiments suggest it undergoes both electron and energy transfer. However, experimental evidence supports the hypothesis that energy transfer is the only productive quenching mechanism. Control experiments using Ir(ppy)3 showed no catalysis for the intermolecular [2+2] photocycloaddition of an eniminium ion. 相似文献
997.
Dr. Fabian Eisenreich Prof. Dr. E. W. Meijer Prof. Dr. Anja R. A. Palmans 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(45):10355-10361
Photoredox catalysis has recently emerged as a powerful synthesis tool in organic and polymer chemistry. In contrast to the great achievements realized in organic solvents, performing photocatalytic processes efficiently in aqueous media encounters several challenges. Here, it is presented how amphiphilic single-chain polymeric nanoparticles (SCPNs) can be utilized as small reactors to conduct light-driven chemical reactions in water. By incorporating a phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free reduction and C−C cross-coupling reactions can be carried out upon exposure to UV light under ambient conditions. The versatility of this approach is underlined by a large substrate scope, tolerance towards oxygen, and excellent recyclability. This approach thereby contributes to a sustainable and green way of implementing photoredox catalysis. 相似文献
998.
Dr. Florian Albrecht Dulce Rey Dr. Shadi Fatayer Dr. Fabian Schulz Dr. Dolores Pérez Dr. Diego Peña Dr. Leo Gross 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(51):23189-23193
Glaser-like coupling of terminal alkynes by thermal activation is extensively used in on-surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser-like coupling between terminal alkyne carbons within a custom-synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates, providing insight into the reaction pathway. 相似文献
999.
The first total synthesis of (−)-rotundone has been accomplished from (+)-(R)-limonene and therefore for the first time from an unrelated monoterpene instead of modifying structurally closely related sesquiterpene precursors such as α-guaiene. Challenges such as intermediates with stereocenters prone to epimerization by enolization were overcome by designing a β-methyl-keto route starting from (+)-(R)-limonene which finally gave (−)-rotundone by Nazarov cyclization of a precursor 13a . Diastereomer (−)-epi-rotundone was separated from (−)-rotundone chromatographically. An alternative route from rac-citronellal provided a diastereomer mixture of racemic Nazarov precursor 13 through a TRIP-catalyzed intramolecular aldolization, thus indicating that the Nazarov cyclization precursor 13a is in principle accessible from (−)-(S)-citronellal. The 11-step synthesis from (+)-(R)-limonene with ca. 1 % overall yield confirmed the absolute configuration of (−)-rotundone and provided samples of good olfactory quality. 相似文献
1000.
A Glycan Array-Based Assay for the Identification and Characterization of Plant Glycosyltransferases
Dr. Colin Ruprecht Dr. Max P. Bartetzko Dr. Deborah Senf Dr. Anna Lakhina Dr. Peter J. Smith Dr. Maria J. Soto Hyunil Oh Dr. Jeong-Yeh Yang Dr. Digantkumar Chapla Dr. Daniel Varon Silva Prof. Dr. Mads H. Clausen Prof. Dr. Michael G. Hahn Prof. Dr. Kelley W. Moremen Prof. Dr. Breeanna R. Urbanowicz Prof. Dr. Fabian Pfrengle 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(30):12593-12598
Growing plants with modified cell wall compositions is a promising strategy to improve resistance to pathogens, increase biomass digestibility, and tune other important properties. In order to alter biomass architecture, a detailed knowledge of cell wall structure and biosynthesis is a prerequisite. We report here a glycan array-based assay for the high-throughput identification and characterization of plant cell wall biosynthetic glycosyltransferases (GTs). We demonstrate that different heterologously expressed galactosyl-, fucosyl-, and xylosyltransferases can transfer azido-functionalized sugar nucleotide donors to selected synthetic plant cell wall oligosaccharides on the array and that the transferred monosaccharides can be visualized “on chip” by a 1,3-dipolar cycloaddition reaction with an alkynyl-modified dye. The opportunity to simultaneously screen thousands of combinations of putative GTs, nucleotide sugar donors, and oligosaccharide acceptors will dramatically accelerate plant cell wall biosynthesis research. 相似文献