News Archive 2015-2018
The deiodination and debromination reactions are triggered either by heating or by locally applying voltage pulses with the tip. We observed a strong hierarchical behavior of the dehalogenation with respect to temperature and voltage. In connection with first-principles simulations we can determine the orientation and position of the pristine molecules as well as adsorbed mono/diradicals and the halogens. We find that the isolated radicals are chemisorbed to Cu(111) top sites, which are lifted by 16 pm (meta-position) and 32 pm (para-position) from the Cu surface plane. This leads to a strongly twisted and bent 3D adsorption structure. After heating, different types of dimers are observed whose molecules are either bound to surface atoms or connected via Cu adatoms. Such knowledge about the intermediate geometry and its interaction with the surface will open the way to rationally design syntheses on surfaces. DOI: 10.1021/acsnano.8b06283
The interdisciplinary research project was designed by Prof. Dr. Ansgar Schnurr (Institute for Art Education), Prof. Dr. Hermann Wegner (Institute for Organic Chemistry), and Jannis Neumann (student of art education and biology), Justus Liebig University (JLU) Giessen.
Sketches from the artists Patrick Borchers, Jette Flügge, and Christoph Kern are presented. Visting the exhibition will be possible until october, 14th from 8.30 am until 11.00 pm at the central library of the university Giessen.
In a ceremony held at the university's main building, JLU-president Prof. Dr. Joybrato Mukherjee handed over the award, which is also supported by the Unfallkasse Hessen, to Prof. Hermann A. Wegner and the members of his group. The prize shall reward the commitment of all members of the group for maintaining a high safety level in everydays laboratory research.
Foto: JLU-Pressestelle / Katrina Friese
For the first publication, a complex of cycloparaphenylene (CPP) with bisazafullerene (C59N)2 was investigated experimentally and computationally. Two CPP rings are bound to the dimeric azafullerene giving CPP⊃(C59N)2⊂CPP. Photophysical and redox properties support an electronic interaction between the components especially with the second CPP bound. Unlike CPP⊃C60, where there is negligible electronic communication between the two species, upon photoexcitation a partial charge transfer phenomenon was revealed between CPP and (C59N)2 reminiscent of CPP encapsulated metallofullerenes. Such an alternative electron‐rich fullerene species demonstrates C60‐like ground‐state properties and metallofullerene‐like excited‐state properties opening new avenues for construction of functional supramolecular architectures with organic materials.
In second publication the adsorption of corannulene (C20H10) on the Ag(111) surface by experimental and simulated scanning tunneling microscopy (STM), X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS) was investigated. Structural optimizations of the adsorbed molecules were performed by density functional theory (DFT) and the core excited spectra evaluated within the transition potential approach. Corannulene is physisorbed in a bowl-up orientation displaying a very high mobility (diffusing) and dynamics (tilting and spinning) at room temperature. At the monolayer saturation coverage, molecules order into a close compact phase with an average intermolecular spacing of ∼10.5±0.5Å. The lattice mismatch drives a long wavelength structural modulation of the molecular rows, which, however, could not be identified with a specific superlattice periodicity. DFT calculations indicate that the structural and spectroscopic properties are intermediate between those predicted for the limiting cases of an on-hexagon geometry (with a three-fold, ∼8.6Å unit mesh) and an on-pentagon (with a four-fold,∼11.5Å unit mesh). We suggest that molecules smoothly change their equilibrium configuration along the observed long wavelength modulation of the molecular rows by varying their tilt and azimuth in between the geometric constraints calculated for molecules in the three-fold and four-fold phases.
Its stability towards air and moisture was demonstrated by NMR studies enabling its application in organic transformations without glovebox. A one-pot procedure for its synthesis was developed starting from 1,2-bis(trimethylsilyl)benzene greatly enhancing its practicality. Comparative reactions were carried out to evaluate its catalytic activity in IEDDA reactions of diazine including phthalazine as well as 1,2,4,5-tetrazine.
To be competitive with other electrically rechargeable large scale energy storages, the range of active materials for redox flow batteries is currently expanded by organic compounds – this holds especially for the redox active material class of quinones that can be derived from naturally abundant resources at low cost. Here we propose the modified quinone 2,3-diaza-anthracenedione, and two of its derivatives, as promising active material for aqueous redox flow batteries. We systematically study the electrochemical performance (redox potentials, rate constants, diffusion coefficients) for these three compounds at different pH value experimentally and complement the results with density functional calculations: A positive redox potential shift of about 300 mV is achieved by the incorporation of a diaza moiety into the anthraquinone base structure. Our experiments at low pH show that the addition of a methoxy group to the base structure of the 2,3-diaza-anthracenedione strongly increases the electrochemical stability in aqueous acidic media – although the impact of the conjugate base is not clear yet. Furthermore, a functionalization with two hydroxyl groups evokes a negative redox potential shift of 54 mV in acidic and 264 mV in alkaline solution. This demonstrates that this novel class of compounds is very versatile and can be tailor-made for the use as active material in redox flow batteries – either in alkaline or acidic media. The 2,3-diaza-anthracenediones presented in this study were used as anolyte active materials in a full redox flow cell as a proof of concept; best cycling stability was achieved with 2,3-diaza-anthracenediones functionalized with a methoxy group as active material. Transferring our findings to other quinone base structures, such as naphtoquinones, could lead to even better performing catholyte and anolyte active materials for future redox flow batteries with organic active material.
This process of coordination is reversible upon addition of pyridine. The diradical species is stable above 200 °C. Computations are consistent with an open-shell triplet diradical with a very small open-shell singlet−triplet energy gap that is indicative of the electronic disjointness of the two radical sites. This opens a new way of generating stable radicals with fascinating electronic properties useful for a large variety of applications. The paper is now online available.
This work provides a deeper insight into the photoisomerization reaction of a cyclic azobenzene. We present here a combined experimental and theoretical study on the ultrafast dynamics of cyclotrisazobenzene (CTA) and demonstrate that the structural constraints in CTA prevent isomerization of the photoswitch units. In the developed molecular picture, the N=N bonds respond elastically to the motion along the isomerization coordinates, which leads to ultrafast and complete dissipation of the UV excitation as heat. Based on this property, CTA and possibly other similarly designed molecules can be utilized as UV-absorbers for example in sunscreens; other potential applications are also envisioned.
Furthermore, we like to welcome our new bachelor candidates Anne, Larissa and Daniel. We hope, that you will enjoy your time in the Wegner group.
Azobenzenes have attracted increasing attention in the past years due to their application as molecular switches. The synthetic efforts for a macrocyclic bisazobenzene as well as the successful strategy are discussed. Final ring closure under continuous irradiation gave the (Z,Z)-bisazobenzenophane. This Z-azobenzene does not show any isomerization under either heating or prolonged irradiation. The thermal stability of the Z-form has also been supported by computations. The prepared bisazobenzenophane represents one of the few azobenzenes in which the Z-isomer is more stable than the E-isomer.
The synthesis of 2,3-diaza-anthraquinones by an IEDDA reaction of 1,4-naphthoquinones (1,4-NQs) with 1,2,4,5-tetrazine (TZ) as the nitrogen source has been developed. Only due to the coordination of TZ with the bisborane catalyst, the IEDDA reaction of electron-deficient 1,4-NQs is possible. The 2,3-DAAQs themselves are suitable dienes for a second bidentate Lewis acid catalysed IEDDA reaction with various dienophiles to afford substituted anthraquinones. Anthraquinones have been discussed as privileged structures for applications in medicinal as well as materials science.
The article describes synthetic approaches for curved polycyclic aromatics such as circulenes, helicenes and cycloparaphenylenes by using a [2+2+2] cycloaddition. Several examples with optimized reaction conditions are shown to help obtaining a better understanding of the process. The online article can be accesed using this hyperlink.
The article by Luca Schweighauser and Hermann Wegner provides a nice insight into the range of applications provided by utilizing bidentate interactions in Diels–Alder reactions, carbon dioxide reduction, and ammonia-borane dehydrogenation.
The new paper with the titleshowing the dependence of the excited state lifetimes on the degree of conjugation in the multiphotochromic system of bisazobenzene derivatives is available online.
The new paper with the title "Connectivity matters - ultrafast isomerization dynamics of bisazobenzene photoswitches" showing the dependence of the excited state lifetimes on the degree of conjugation in the multiphotochromic system of bisazobenzene derivatives is from now on available online.
A new paper with the title "Bidentate Lewis Acid Catalyzed Domino Diels-Alder Reaction of Phthalazine for the Synthesis of Bridged Oligocyclic Tetrahydronaphthalenes" showing a domino process consisting of an inverse and a normal electron demand Diels-Alder reaction to form bridged tri- and tetracyclic 1,2,3,4-tetrahydronaphthalenes is from now on available online.
The Wegner group would like to welcome our new postdoc who will be working on Boron catalyzed Diels Alder reactions and investigate Molecular Structure-Properties Relationship in electronic chemistry.
The article with the title "Subsurface Controlled Angular Rotation: Triphenylene Molecules on Au(111) Substrates" is now online available at the Journal of Physical Chemistry C.
We are pleased to inform you about new Job Offers in the Wegner group. In cooperation with the Janek group, there is a new PhD position available in our group for the synthesis and electrochemical characterisation of substituted aromatic systems especially quinones. For further details, please visit the JLU job market.
For a PostDoc position in the Wegner group, please apply directly to Professor Dr. Hermann A. Wegner.
The Young Chemist Award at the chemical instituts of the Justus Liebig university Giessen offers high school students the chance to listen to lectures, solve tasks and work in a chemical laboratory. The program covers basic chemical knowledge and trains laboratory skills. After the first round of the award, some participants are selected by an exam for the second round which takes place around august/september. For more detailed information follow this link: Young Chemist Award 2016.
An article about the winners of this year can be found in the Giessener Allgemeine Zeitung.
We congratulate Zhenpin Lu that his paper "Metal-free ammonia borane dehydrogenation catalyzed by a bisborane Lewis acid" was recently accepted by the Angewandte Chemie Int. Ed. and we are looking forward to its publication.
Starting from september 18th, 2015, the Wegner group is now located in the new chemistry building of the Justus Liebig university giessen. The new rooms of the group are located on the upper second floor in building part B ranging from number 228 to 233.
Institut Organische Chemie
Prof. Dr. Hermann A. Wegner
Room B 229, 2. Floor
A new paper with the title "Attraction or Repulsion? London Dispersion Forces Control Azobenzene Switches" dealing with the influence of bulky electronically neutral alkyl residues on the cis-trans isomerization rates of azobenzenes is from now on available online in an english and a german version.
Silvia Belloto took part in an interdisciplinary PhD project at the EPF Lausanne and the Justus Liebig University Giessen with the title "Quantifying the activity contribution of individual matrix metalloproteinases (MMPs) to the overall MPP activity in the extracellular space using highly specific and photo-switchable inhibitors". This project was supervised by Prof. Christian Heinis, Laboratory of Therapeutic Proteins and Peptides, EPF Lausanne and Prof. Hermann A. Wegner, Institute for Organic Chemistry, Justus Liebig University Giessen and ended in january 2015. If you want to learn more about the project, please visit the SystemsX.ch website.
After their visit in March, the produced videos from Beilstein TV about the developed in the Wegner group and the application of such t a bidentate catalyst in organic synthesis are now online. The methodology and experiments are presented by Prof. Dr. Hermann A. Wegner, Luca Schweighauser and Sebastian Ahles.
We congratulate Zhenpin Lu that his paper "Aromaticity as Stabilizing Element in the Bidentate Activation for the Catalytic Reduction of Carbon Dioxide" was recently accepted by the Journal of American Chemical Society and is now as ASAP article available.
On March 17th & 18th, Beilstein TV was visiting the Wegner group, producing some short videos of specific synthesis procedures. The synthesis of the bidentate boron-based Lewis acid catalyst as well as Inverse Electron Demand Diels-Alder reactions catalysed by these bidentate Lewis acids were shown. We expect to be able to post the final video here in mid-april.
New support for the Wegner group as a new research associate and PhD student. He will be working on azo switches and boron based bidentate Lewis-acid catalysis.
The Wegner group would like to congratulate Silvia Bellotto to her doctoral degree. We hope you enjoyed your time in the Wegner group and wish you all the best for your future.