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Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for non-aggregated molecules. We find small Huang–Rhys factors below 0.01 for the high-frequency, 1500 cm^–1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

In contrast to regular J- and H-aggregates, thin film squaraine aggregates usually have broad absorption spectra containing both J-and H-like features, which are favorable for organic photovoltaics. Despite being successfully applied in organic photovoltaics for years, a clear interpretation of these optical properties by relating them to specific excited states and an underlying aggregate structure has not been made. In this work, by static and transient absorption spectroscopy on aggregated n-butyl anilino squaraines, we provide evidence that both the red- and blue-shifted peaks can be explained by assuming an ensemble of aggregates with intermolecular dipole–dipole resonance interactions and structural disorder deriving from the four different nearest neighbor alignments - in sharp contrast to previous association of the peaks with intermolecular charge-transfer interactions. In our model, the next-nearest neighbor dipole-dipole interactions may be negative or positive, which leads to the occurrence of J- and H-like features in the absorption spectrum. Upon femtosecond pulse excitation of the aggregated sample, a transient absorption spectrum deviating from the absorbance spectrum emerges. The deviation finds its origin in the excitation of two-exciton states by the probe pulse. The lifetime of the exciton is confirmed by the band integral dynamics, featuring a single-exponential decay with a lifetime of 205 ps. Our results disclose the aggregated structure and the origin of red- and blue-shifted peaks and explain the absence of photoluminescence in squaraine thin films. Our findings underline the important role of structural disorder of molecular aggregates for photovoltaic applications.

Subcomponent self-assembly gave access to Dy₁₂(L)₈ and Dy₆(L)₆ architectures via second-order template effects. The Dy₆(L)₆ assembly behaves as a single-molecule magnet exhibiting a high anisotropy barrier and butterfly-shaped magnetic hysteresis.

Farben verzaubern Menschen seit jeher. Sie haben wichtige Funktionen, sie verschaffen Individualität, aber auch Identität, sie dienen der Demonstration von Macht oder machen unser Leben einfach schöner und bunter.
In diesem Vortrag wird ein weiter Bogen über die Herkunft, Herstellung und Nutzung verschiedener Farbstoffe gespannt. Angefangen von den ersten anorganischen Pigmenten bis hin zu modernen organischen Hochleistungsfarbstoffen und ihrer Anwendung in opto-elektronischen Bauteilen, wird die historische Entwicklung der Farbstoffe aus der Sicht des Chemikers zur kulturgeschichtlichen Entwicklung mit einem Schwerpunkt auf dem ausgehenden 19./beginnenden 20. Jahrhundert in Beziehung gesetzt.

 

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 It is challenging to increase the rigidity of a macromolecule while maintaining solubility. Established strategies rely on templating by dendrons, or by encapsulation in macrocycles, and exploit supramolecular arrangements with limited robustness. Covalently bonded structures have entailed intramolecular coupling of units to resemble the structure of an alternating tread ladder with rungs composed of a covalent bond. We introduce a versatile concept of rigidification in which two rigid-rod polymer chains are repeatedly covalently associated along their contour by stiff molecular connectors. This approach yields almost perfect ladder structures with two well-defined π-conjugated rails and discretely spaced nanoscale rungs, easily visualized by scanning tunnelling microscopy. The enhancement of molecular rigidity is confirmed by the fluorescence depolarization dynamics and complemented by molecular-dynamics simulations. The covalent templating of the rods leads to self-rigidification that gives rise to intramolecular electronic coupling, enhancing excitonic coherence. The molecules are characterized by unprecedented excitonic mobility, giving rise to excitonic interactions on length scales exceeding 100 nm. Such interactions lead to deterministic single-photon emission from these giant rigid macromolecules, with potential implications for energy conversion in optoelectronic devices.

 

Starting from simple building blocks, six heterobimetallic cubes were obtained, following a straight-forward subcomponent self-assembly strategy. Three iron(II) based cages showed spin-crossover behaviour in solution. The crossover temperature was strongly dependent on interchangeable aldehyde components, giving rise to an efficient toolkit to manipulate magnetic behaviour.

Rising above the surface: Bicyclophanes of different sizes with different central units have been synthesized, including species with central units pointing out of the ring plane. Scanning tunneling microscopic investigations of self-assembled monolayers of these compounds at the solid/liquid interface on graphite provide submolecularly resolved insights into the nanopatterns formed. After adsorption the central units point along the surface normal and protrude by up to 1.6 nm into the volume phase.

A biosynthetic gene cluster from Streptomyces mobaraensis encoding the first cases of a bacterial geranylfarnesyl diphosphate synthase and a type I sesterterpene synthase was identified. The structures of seven sesterterpenes produced by these enzymes were elucidated, including their absolute configurations. The enzyme mechanism of the sesterterpene synthase was investigated by extensive isotope labeling experiments.

 

 

Two bacterial diterpene synthases (DTSs) from Chryseobacterium were characterised. The first enzyme yielded the new compound chryseodiene that closely resembles the known fusicoccane diterpenes from fungi, but its experimentally and computationally studied cyclisation mechanism is fundamentally different to the mechanism of fusicoccadiene synthase. The second enzyme produced wanjudiene, a diterpene hydrocarbon with a new skeleton, besides traces of the enantiomer of bonnadiene that was recently discovered from Allokutzneria albata.
 

Fusicoccane (FC)-type diterpenes, featuring a common 5-8-5 tricyclic skeleton, possess diverse biological functions. Up to date, only FC-type diterpene synthases (DTSs) for three out of the sixteen possible stereochemically distinct sub-types of the FC skeleton have been discovered.

Herein, a (2Si,6Si,10Re,14Si)-FC-type DTS from Myrothecium gramineum is reported, that produces the diterpene alcohol myrothec-15(17)-en-7-ol. Based on homology modellings, the I189F variant was obtained by site-directed mutagenesis that prevents the penultimate carbocation from water attack, resulting in the deprotonation product myrotheca-7,15(17)-diene. VCD calculations in combination with chemical correlations through an enantioselective labeling strategy established the absolute configurations of these diterpenes. DFT calculations revealed a water-mediated proton transfer mechanism, which is supported by isotope labeling experiments, in contrast to the deprotonation/reprotonation mechanism reported in previous studies.

Two new heterobimetallic cages, a trigonal‐bipyramidal and a cubic one, were assembled from the same mononuclear metalloligand by adopting the molecular library approach, using iron(II) and palladium(II) building blocks. The ligand system was designed to readily assemble through subcomponent self‐assembly. It allowed the introduction of steric strain at the iron(II) centres, which stabilizes its paramagnetic high‐spin state. This steric strain was utilized to drive dynamic complex‐to‐complex transformations with both the metalloligand and heterobimetallic cages. Addition of sterically less crowded subcomponents as a chemical stimulus transformed all complexes to their previously reported low‐spin analogues. The metalloligand and bipyramid incorporated the new building block more readily than the cubic cage, probably because the geometric structure of the sterically crowded metalloligand favours the cube formation. Furthermore it was possible to provoke structural transformations upon addition of more favourable chelating ligands, converting the cubic structures into bipyramidal ones.

   

In organic electronics, J‐type coupling between monomers along the chain and H‐type coupling of chromophores between chains compete, which makes the two types of coupling hard to discern in the ensemble. A single‐molecule H‐type aggregate of fixed spacing and variable length of up to 10 nm is introduced, and HJ‐type aggregate formation is visualized intuitively in the scatter of single‐molecule spectra.

 

The stereochemistry of the structurally unique myxobacterial polyketides tuscolid/tuscorons was determined by a combination of high‐field NMR studies, molecular modeling, and chemical derivatization and confirmed by a modular total synthesis of tuscorons D and E. Together with the discovery of three novel tuscorons, this study provides detailed insight into the chemically unprecedented tuscolid/tuscoron rearrangement cascade.

B. Göricke, M. F. Bieber, K. E. Mohr, D. Menche*, Stereochemical Determination of Tuscolid/Tuscorons and Total Synthesis of Tuscoron D and E: Insights into the Tuscolid/Tuscoron Rearrangement
Angew. Chem. 2019, 131, 13153–13157; Angew. Chem. Int. Ed. 2019, 58, 13019-13023. (link)
 

 

Indolines and tetrahydroquinolines from the same substrates by titanocene catalysis! The absolute configuration of the cyclopentadienyl ligand and the inorganic ligand determine the regioselectivity of ring opening and diastereoselectivity of radical arylation in the regiodivergent radical arylation of epoxides.

 

Mühlhaus, F.; Weißbarth, H.; Dahmen, T.; Schnakenburg, G.; Gansäuer. A., Angew. Chem. 2019, 131, 14346-14350; Angew. Chem. Int. Ed. 2019, 58, 14208-14212. DOI: 10.1002/anie.201908860 and 10.1002/ange.201908860 (link)

 

This Minireview summarises recent developments in the biosynthesis of diterpenes by diterpene synthases in bacteria. It is structured by the class of enzyme involved in the first committed step towards diterpenes, starting with type I diterpene synthases, followed by type II enzymes and the more recently discovered UbiA‐related diterpene synthases. A special emphasis lies on the reaction mechanisms of diterpene synthases that convert simple linear precursors through cationic cascades into structurally complex, usually polycyclic carbon skeletons with multiple stereogenic centres. A further main focus of this Minireview is a discussion of how these mechanisms can be unravelled. Downstream modifications to bioactive molecules are also covered.

The skeletons of some classes of terpenoids are unusual, because they contain a larger number of Me groups (or their biosynthetic equivalents such as olefinic methylene groups, hydroxymethyl groups, aldehydes or carboxylic acids and their derivatives) than provided by their oligoprenyl diphosphate precursor. This is sometimes the result of an oxidative ring opening reaction at a terpene cyclase derived molecule containing the regular number of Me group equivalents, as observed for picrotoxan sesquiterpenes. In this study a sesquiterpene cyclase from Trichoderma is described that can convert farnesyl diphosphate (FPP) directly via a remarkable skeletal rearrangement into trichobrasilenol, a new brasilane sesquiterpene with one additional Me group equivalent compared to FPP. A mechanistic hypothesis for the formation of the brasilane skeleton is supported by extensive isotopic labelling studies.

Der Theodor-Laymann-Stiftungsfonds hat im Sommersemester 2019 ein Stipendium an eine förderungswürdige Studentin aus der Organischen Chemie für die letzte Phase des Studiums vergeben. Das sechsmonatige Stipendium wird für die Zeit der Bearbeitung der Masterarbeit im Studiengang Chemie vergeben. Die Förderungswürdigkeit bestimmt sich einmal nach der Qualifikation und zu anderen nach sozialen Gesichtspunkten. Das Stipendium wird monatlich mit 735 Euro finanziert.
Wir gratulieren der Stipendiatin zu ihrem Erfolg.

We describe an epoxide hydrogenation that delivers the less-substituted anti-Markovnikov alcohols. The key-feature of our reaction is the coupling of titanocene catalyzed epoxide opening with chromium catalyzed hydrogen activation. Titanocene catalysis enforces the formation of the higher substituted radical and, hence, the anti-Markovnikov regioselectivity. Chromium catalysis enables the highly unusual stepwise transfer of H₂ as H⁺, e^-, and H●. Our unique dual catalysis approach sidesteps the typical regioselectivity of epoxide ring-opening by an S_N2 mechanism that leads to the formation of the common Markovnikov alcohols by preventing the formation of hydric intermediates. The atom-economy of the reaction is excellent. Each step of the catalytic cycles has been observed separately.

 

Three diterpene synthases from actinomycetes were studied. The first enzyme from Streptomyces cattleya produced the novel compound cattleyene, and the other two enzymes from Nocardia testacea and Nocardia rhamnosiphila were identified as phomopsene synthases. The cyclisation mechanism of cattleyene synthase and the EI-MS fragmentation mechanism of its product were extensively studied by incubation experiments with isotopically labelled precursors. Oxidative transformations expanded the chemical space of these unique diterpenes.
 

sequently activates fast‐responding voltage‐gated sodium channels. The dielectric top coating on the organic layer ensures sufficient capacitive charge injection efficiency while maintaining the rapid response of the device. Due to the high irradiance level required for photocapacitive stimulation, another slower, independent, and unintended, nonelectrical signaling pathway is identified. This activates voltage‐gated potassium channels, presumably by photothermal effects. The present study provides the basis for further improvements on standalone photovoltaic neurostimulating platforms based on organic photoactive layers.

A facile route to soft matter self-powered bulk heterojunction photodiode detectors sensitive to the circular polarization state of light is shown based on the intrinsic excitonic circular dichroism of the photoactive layer blend. As light detecting materials, enantiopure semiconducting small molecular squaraine derivates of opposite handedness are employed. Via Mueller matrix ellipsometry, the circular dichroism is proven to be of H-type excitonic nature and not originating from mesoscopic structural ordering. Within the green spectral range, the photodiodes convert circular polarized light into a handedness-dependent photocurrent with a maximum dissymmetry factor of ±0.1 corresponding to 5% overall efficiency for the polarization discrimination under short circuit conditions. On the basis of transfer matrix optical simulations, it is rationalized that the optical dissymmetry fully translates into a photocurrent dissymmetry for ease of device design. Thereby, the photodiode's ability to efficiently distinguish between left and right circularly polarized light without the use of external optical elements and voltage bias is demonstrated. This allows a straightforward and sustainable future design of flexible, lightweight, and compact integrated platforms for chiroptical imaging and sensing.

Based on a terpenoid overproduction platform in yeast for genome mining, a chimeric diterpene synthase from the endophytic fungus Colletotrichum gloeosporioides ES026 was characterized as the (5R,12R,14S)‐dolasta‐1(15),8‐diene synthase. The absolute configuration was independently verified through the use of enantioselectively deuterated terpene precursors, which unequivocally established the predicted C1‐III‐IV cyclization mode for this first characterized clade II‐D enzyme. Extensive isotopic labeling experiments and isolation of the intermediate (1R)‐δ‐araneosene supported the proposed cyclization mechanism.

First the cage, then the ring: A BODIPY‐based bis(3‐pyridyl) ligand undergoes self‐assembly upon coordination to tetravalent palladium(II) cations to form a Pd₆L₁₂ metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane‐like cage‐in‐ring arrangement. This assembly is not mechanically interlocked in the sense of a catenation but it is stabilized only by attractive noncovalent interactions.

M. Käseborn, J. J. Holstein, G. H. Clever, A. Lützen, A Rotaxane‐like Cage‐in‐Ring Structural Motif for a Metallosupramolecular Pd₆L₁₂ Aggregate, Angew. Chem. Int. Ed. 2018, 57, 12171-12175. (link) 

 

Monday, Mai 14th 2018

09:15 - 17:00, Chemische Institute Endenich, Hörsaal 2

In this workshop young female scientists will present their individual scientific results in the field of molecular or synthetic chemistry.

 

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Y.-Q. Zhang, E. Vogelsang, Z.-W, Qu, S. Grimme, A. Gansäuer, Angew. Chem. Int. Ed. 2017, 56, 12654-12657. (link)

 

The first catalytic generation of β-amido substituted radicals from N-acyl aziridines is described. The radicals that are generated with excellent regioselectivity are useful intermediates for intermolecular additions, reductions, and cyclizations to yield functionalized amides. Through calculations we have shown that the ring opening is concerted and not step-wise as previously postulated.

Electron-deficient trifluoromethyl-substituted sub-components affect the properties of M₄L₄ tetrahedral cages 

N. Struch, F. Topić, K. Rissanen,  A. Lützen, Dalton Trans. 2017, 46, 10809-10813. (link)