Browsing by Author "Bruckner, Johanna R."

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Open Access
Amphotropic azobenzene derivatives with oligooxyethylene and glycerol based polar groups
(2015) Tan, Xiaoping; Zhang, Ruilin; Guo, Chunxiang; Cheng, Xiaohong; Gao, Hongfei; Liu, Feng; Bruckner, Johanna R.; Gießelmann, Frank; Prehm, Marko; Tschierske, Carsten
A series of amphiphilic azobenzenes with one to three lipophilic alkyl chains at one end and polar groups with oligooxyethylene (EO) and racemic 3-glyceryl units at the opposite end was synthesized and their thermotropic and lyotropic liquid crystalline self-assemblies were studied by POM, DSC and XRD. Tilted and non-tilted lamellar phases with interdigitated double layer structures (SmCd and SmAd, respectively) were found for the compounds with a single alkyl chain, whereas hexagonal columnar phases were formed by the compounds with two or three alkyl chains. The effect of protic solvents, like formamide, ethylene glycol and water, was investigated for representative examples. For the compounds with the single chain, induction and stabilization of SmA phases were observed, though broad regions of lyotropic SmC phases were retained in most cases. Depending on the structure of the polar group, the hexagonal columnar phases were either removed or drastically stabilized by the solvents. Photoisomerisation of an azobenzene chromophore was also studied.
Open Access
Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids
(2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, Sabine
Covalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.
Open Access
Cationic molybdenum imido alkylidene N‐heterocyclic carbene complexes confined in mesoporous silica : tuning transition states towards Z‐selective ring‐opening cross‐metathesis
(2022) Goldstein, Elizabeth L.; Ziegler, Felix; Beurer, Ann‐Katrin; Traa, Yvonne; Bruckner, Johanna R.; Buchmeiser, Michael R.
We recently reported a method for selective macro(mono)cyclization of dienes utilizing catalysts confined inside the pores of mesoporous silica, which we believe occurs due to suppression of oligomerization due to pore size. We hypothesized, however, that the system of cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) catalysts immobilized selectively inside the mesopores of silica materials could address much more subtle selectivity differences, such as E/Z selectivity in ring‐opening/cross‐metathesis (ROCM). Upon investigation, we observed that surface‐bound cationic molybdenum imido alkylidene NHC catalysts indeed display an increased Z‐selectivity, especially during the early stages of the reaction. This effect was present when the catalyst was confined inside a pore, as well as when the catalyst was bound to non‐porous silica, which led us to conclude it is an effect caused by the catalyst being bound directly to the surface of a silica material where the proximity of the catalyst to the surface governs the transition state. Kinetic investigations revealed that significant post‐metathesis olefin isomerization occurs, the amount of which seems to be governed by the rate of diffusion of the product away from the active catalyst, with smaller pore sizes resulting in higher Z‐selectivity at higher conversion, attributable to faster diffusion of the product out of the pore than diffusion back into the pore.
Open Access
Efficient and spatially controlled functionalization of SBA‐15 and initial results in asymmetric Rh‐catalyzed 1,2‐additions under confinement
(2021) Beurer, Ann‐Katrin; Kirchhof, Manuel; Bruckner, Johanna R.; Frey, Wolfgang; Baro, Angelika; Dyballa, Michael; Giesselmann, Frank; Laschat, Sabine; Traa, Yvonne
Selectively functionalized mesoporous silica may considerably advance heterogeneous catalysis through the controlled immobilization of highly selective complex catalysts inside the mesopores. However, spatially controlled functionalization and the precise analytical verification are still a challenge. In this publication, we report a method, which ensures a selective functionalization of the mesopore surface with a clickable linker and thus makes it possible to study confinement effects during catalyzed reactions. First, we passivate the silanol groups on the particle surface and in the pore entrances of the mesoporous silica material SBA‐15 with 1,1,1‐trimethyl‐N‐(trimethylsilyl)silanamine. Then we remove the template by solvent extraction and functionalize the pore walls with 3‐azidopropyltriethoxysilane before we click the catalyst. In initial experiments of asymmetric Rh‐catalyzed 1,2‐addition, we investigate the performance of a catalyst clicked selectively in the mesopores and compare it to the dissolved catalyst as well as to the catalyst immobilized exclusively on the external surface of SBA‐15.
Open Access
FAIR and scalable management of small‐angle X‐ray scattering data
(2023) Giess, Torsten; Itzigehl, Selina; Range, Jan; Schömig, Richard; Bruckner, Johanna R.; Pleiss, Jürgen
A modular research data management toolbox based on the programming language Python, the widely used computing platform Jupyter Notebook, the standardized data exchange format for analytical data (AnIML) and the generic repository Dataverse has been established and applied to analyze small‐angle X‐ray scattering (SAXS) data according to the FAIR data principles (findable, accessible, interoperable and reusable). The SAS‐tools library is a community‐driven effort to develop tools for data acquisition, analysis, visualization and publishing of SAXS data. Metadata from the experiment and the results of data analysis are stored as an AnIML document using the novel Python‐native pyAnIML API. The AnIML document, measured raw data and plots resulting from the analysis are combined into an archive in OMEX format and uploaded to Dataverse using the novel easyDataverse API, which makes each data set accessible via a unique DOI and searchable via a structured metadata block. SAS‐tools is applied to study the effects of alkyl chain length and counterions on the phase diagrams of alkyltrimethylammonium surfactants in order to demonstrate the feasibility and usefulness of a scalable data management workflow for experiments in physical chemistry.
Open Access
From equilibrium liquid crystal formation and kinetic arrest to photonic bandgap films using suspensions of cellulose nanocrystals
(2020) Schütz, Christina; Bruckner, Johanna R.; Honorato-Rios, Camila; Tosheva, Zornitza; Anyfantakis, Manos; Lagerwall, Jan P. F.
The lyotropic cholesteric liquid crystal phase developed by suspensions of cellulose nanocrystals (CNCs) has come increasingly into focus from numerous directions over the last few years. In part, this is because CNC suspensions are sustainably produced aqueous suspensions of a fully bio-derived nanomaterial with attractive properties. Equally important is the interesting and useful behavior exhibited by solid CNC films, created by drying a cholesteric-forming suspension. However, the pathway along which these films are realized, starting from a CNC suspension that may have low enough concentration to be fully isotropic, is more complex than often appreciated, leading to reproducibility problems and confusion. Addressing a broad audience of physicists, chemists, materials scientists and engineers, this Review focuses primarily on the physics and physical chemistry of CNC suspensions and the process of drying them. The ambition is to explain rather than to repeat, hence we spend more time than usual on the meanings and relevance of the key colloid and liquid crystal science concepts that must be mastered in order to understand the behavior of CNC suspensions, and we present some interesting analyses, arguments and data for the first time. We go through the development of cholesteric nuclei (tactoids) from the isotropic phase and their potential impact on the final dry films; the spontaneous CNC fractionation that takes place in the phase coexistence window; the kinetic arrest that sets in when the CNC mass fraction reaches ∼10 wt.%, preserving the cholesteric helical order until the film has dried; the ’coffee-ring effect’ active prior to kinetic arrest, often ruining the uniformity in the produced films; and the compression of the helix during the final water evaporation, giving rise to visible structural color in the films.
Open Access
Liquid crystalline self-assembly of mixtures of rod- and wedge-shaped MIDA boronates
(2025) Schilling, Christopher; Bauch, Soeren; Wuckert, Eugen; Zens, Anna; Bruckner, Johanna R.; Laschat, Sabine
Rod-like MIDA boronates form smectic mesophases, while wedge-shaped MIDA boronates self-assemble into columnar mesophases. However, the phase behavior of mixtures is less understood. In order to obtain further insight on the molecular self-assembly of MIDA boronate mixtures two series of binary mixtures of rod-like and wedge-shaped mesogens were prepared. The phase behavior was studied using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). The study revealed a strong dependency of the mesophase structure on the mesogen composition. Usage of a less bulky columnar mesogen suppressed the formation of columnar mesophases in the mixture and led to a decrease in melting and clearing temperatures. The phase behavior is discussed in terms of the packing parameter model typically applied for lyotropic liquid crystals.
Open Access
Macrocyclization of dienes under confinement with cationic tungsten imido/oxo alkylidene N‐heterocyclic carbene complexes
(2023) Ziegler, Felix; Bruckner, Johanna R.; Nowakowski, Michal; Bauer, Matthias; Probst, Patrick; Atwi, Boshra; Buchmeiser, Michael R.
Macrocyclization reactions are still challenging due to competing oligomerization, which requires the use of small substrate concentrations. Here, the cationic tungsten imido and tungsten oxo alkylidene N-heterocyclic carbene complexes [[W(N-2,6-Cl2-C6H3)(CHCMe2Ph(OC6F5)(pivalonitrile)(IMes)+ B(ArF)4-] (W1) and [W(O (CHCMe2Ph(OCMe(CF3)2)(IMes)(CH3CN)+ B(ArF)4-] (W2) (IMes=1,3-dimesitylimidazol-2-ylidene; B(ArF)4-=tetrakis(3,5-bis(trifluoromethyl)phenyl borate) have been immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 3.3 and 6.8 nm, respectively, using a pore-selective immobilization protocol. X-ray absorption spectroscopy of W1@OMS showed that even though the catalyst structure is contracted due to confinement by the mesopores, both the oxidation state and structure of the catalyst stayed intact upon immobilization. Catalytic testing with four differently sized α,ω-dienes revealed a dramatically increased macrocyclization (MC) and Z-selectivity of the supported catalysts compared to the homogenous progenitors, allowing high substrate concentrations of 25 mM. With the supported complexes, a maximum increase in MC-selectivity from 27 to 81 % and in Z-selectivity from 17 to 34 % was achieved. In general, smaller mesopores exhibited a stronger confinement effect. A comparison of the two supported tungsten-based catalysts showed that W1@OMS possesses a higher MC-selectivity, while W2@OMS exhibits a higher Z-selectivity which can be rationalized by the structures of the catalysts.