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sp\(^2\)–sp\(^3\) diborane species based on bis(catecholato)diboron and N-heterocyclic carbenes (NHCs) are subjected to catechol/bromide exchange selectively at the sp\(^3\) boron atom. The reduction of the resulting 1,1-dibromodiborane adducts led to reductive coupling and isolation of doubly NHC-stabilized 1,2-diboryldiborenes. These compounds are the first examples of molecules exhibiting pelectron delocalization over an all-boron chain.
The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer
(2017)
The understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck–Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6–7 ps, in good agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.
Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate
(2019)
We describe a setup for time-resolved photoemission electron microscopy (TRPEEM) with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump–probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity (LEER) measurements. Finally, we demonstrate the in-situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.
For the rational design of new fluorophores, reliable predictions of fluorescence quantum yields from first principles would be of great help. However, efficient computational approaches for predicting transition rates usually assume that the vibrational structure is harmonic. While the harmonic approximation has been used successfully to predict vibrationally resolved spectra and radiative rates, its reliability for non-radiative rates is much more questionable. Since non-adiabatic transitions convert large amounts of electronic energy into vibrational energy, the highly excited final vibrational states deviate greatly from harmonic oscillator eigenfunctions. We employ a time-dependent formalism to compute radiative and non-radiative rates for transitions and study the dependence on model parameters. For several coumarin dyes we compare different adiabatic and vertical harmonic models (AS, ASF, AH, VG, VGF, VH), in order to dissect the
importance of displacements, frequency changes and Duschinsky rotations. In addition we analyze the effect of different broadening functions (Gaussian, Lorentzian or Voigt). Moreover, to assess the qualitative influence of anharmonicity on the internal conversion rate, we develop a simplified anharmonic model. We adress the reliability of these models considering the potential errors introduced by the harmonic approximation and the phenomenological width of the broadening function.
Animals, just like humans, can freely move. They do so for various important reasons, such as finding food and escaping predators. Observing these behaviors can inform us about the underlying cognitive processes. In addition, while humans can convey complicated information easily through speaking, animals need to move their bodies to communicate. This has prompted many creative solutions by animal neuroscientists to enable studying the brain during movement. In this review, we first summarize how animal researchers record from the brain while an animal is moving, by describing the most common neural recording techniques in animals and how they were adapted to record during movement. We further discuss the challenge of controlling or monitoring sensory input during free movement.
However, not only is free movement a necessity to reflect the outcome of certain internal cognitive processes in animals, it is also a fascinating field of research since certain crucial behavioral patterns can only be observed and studied during free movement. Therefore, in a second part of the review, we focus on some key findings in animal research that specifically address the interaction between free movement and brain activity. First, focusing on walking as a fundamental form of free movement, we discuss how important such intentional movements are for understanding processes as diverse as spatial navigation, active sensing, and complex motor planning. Second, we propose the idea of regarding free movement as the expression of a behavioral state. This view can help to understand the general influence of movement on brain function.
Together, the technological advancements towards recording from the brain during movement, and the scientific questions asked about the brain engaged in movement, make animal research highly valuable to research into the human “moving brain”.
Large Stokes shift (LSS) fluorescent proteins (FPs) exploit excited state proton transfer pathways to enable fluorescence emission from the phenolate intermediate of their internal 4 hydroxybenzylidene imidazolone (HBI) chromophore. An RNA aptamer named Chili mimics LSS FPs by inducing highly Stokes-shifted emission from several new green and red HBI analogs that are non-fluorescent when free in solution. The ligands are bound by the RNA in their protonated phenol form and feature a cationic aromatic side chain for increased RNA affinity and reduced magnesium dependence. In combination with oxidative functional-ization at the C2 position of the imidazolone, this strategy yielded DMHBO\(^+\), which binds to the Chili aptamer with a low-nanomolar K\(_D\). Because of its highly red-shifted fluorescence emission at 592 nm, the Chili–DMHBO\(^+\) complex is an ideal fluorescence donor for Förster resonance energy transfer (FRET) to the rhodamine dye Atto 590 and will therefore find applications in FRET-based analytical RNA systems.
Der Artikel gibt einen Überblick über die Forschungen zur französischen Umgangssprache von den Anfängen bis etwa 1992 in vier Abschnitten: 1. Die Anfänge, 2. die Sprachschichtenforschung, 3. die Beiträge von Soziolinguistik und Varietätenlinguistik und 4. die Arbeiten der "Gegenwart" um 1990. Eine Zusammenfassung und eine ausführliche Bibliographie schließen den Forschungsbericht ab. Berücksichtigt werden alle sprachlichen Ebenen von der Aussprache über den Wortschatz bis zur Grammatik. Wo es angebracht erscheint, sind Hinweise auf Arbeiten zu anderen Umgangssprachen angeführt.
Der Aufsatz beschäftigt sich mit der lautlichen Anpassung der arabischen Lehnwörter im Portugiesischen. Behandelt werden die Betonung, der Vokalismus und der Konsonantismus sowie verschiedene kontextabhängige Lautwandelerscheinungen.
Der Aufsatz beschreibt die portugiesischen Wörter arabischen Ursprungs, und zwar unter verschiedenen Gesichtspunkten: im Vergleich mit den Arabismen des Spanischen, Katalanischen und Italienischen; die Anzahl der portugiesischen Arabismen, ihre lexikalische Struktur, ihre Verteilung nach Sachbereichen und ihre geographische Verbreitung. Untersucht werden nur sichere und direkte Arabismen, die heute noch gebäuchlich sind. Einige Anmerkungen zum inneren Lehngut (Lehnübersetzung, Lehnbedeutung u.ä.) schließen den Artikel ab.
Energy Transfer Between Squaraine Polymer Sections: From helix to zig-zag and All the Way Back
(2015)
Joint experimental and theoretical study of the absorption spectra of squaraine polymers in solution provide evidence that two different conformations are present in solution: a helix and a zig-zag structure. This unique situation allows investigating ultrafast energy transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. We combine here femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy showing that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited helix segments to zig-zag segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of very small reorganization energy in these cyanine-like chromophores.
Die vorliegende Untersuchung gilt der Mikro- und Makrotypografie der seit 125 Jahren erscheinenden katholischen Verbandszeitschrift "Academia". Für die graphologischen und paläographischen Besonderheiten wird eine Hermeneutik vorgeschlagen. Das hier erforschte Druckbild repräsentiert mit einer Auflage von 30.000 Exemplaren im achtwöchigen Erscheinungsrhythmus den Cartellverband der Katholischen Deutschen Studentenverbindungen (CV). So illustriert und visualisiert dieses Druckbild mit seiner typografischen Performance zugleich den mitgliederstärksten Akademikerverband Europas.
The multistate metadynamics for automatic exploration of conical intersection seams and systematic location of minimum energy crossing points in molecular systems and its implementation into the software package metaFALCON is presented. Based on a locally modified energy gap between two Born–Oppenheimer electronic states as a collective variable, multistate metadynamics trajectories are driven toward an intersection point starting from an arbitrary ground state geometry and are subsequently forced to explore the conical intersection seam landscape. For this purpose, an additional collective variable capable of distinguishing structures within the seam needs to be defined and an additional bias is introduced into the off-diagonal elements of an extended (multistate) electronic Hamiltonian. We demonstrate the performance of the algorithm on the examples of the 1,3-butadiene, benzene, and 9H-adenine molecules, where multiple minimum energy crossing points could be systematically located using the Wiener number or Cremer–Pople parameters as collective variables. Finally, with the example of 9H-adenine, we show that the multistate metadynamics potential can be used to obtain a global picture of a conical intersection seam. Our method can be straightforwardly connected with any ab initio or semiempirical electronic structure theory that provides energies and gradients of the respective electronic states and can serve for systematic elucidation of the role of conical intersections in the photophysics and photochemistry of complex molecular systems, thus complementing nonadiabatic dynamics simulations.
Collective Response in DNA-Stabilized Silver Cluster Assemblies from First-Principles Simulations
(2019)
We investigate fluorescence resonant energy transfer and concurrent electron dynamics in a pair of DNA-stabilized silver clusters. For this purpose we introduce a methodology for the simulation of collective optoelectronic properties of coupled molecular aggregates starting from first-principles quantum chemistry, which can be further applied to a broad range of coupled molecular systems to study their electro-optical response. Our simulations reveal the existence of low-energy coupled excitonic states, which enable ultrafast energy transport between subunits, and give insight into the origin of the fluorescence signal in coupled DNA-stabilized silver clusters, which have been recently experimentally detected. Hence, we demonstrate the possibility of constructing ultrasmall energy transmission lines and optical converters based on these hybrid molecular systems.
The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H2, CO, alkynes, alkenes and even with N2.
Protein kinase D1 deletion in adipocytes enhances energy dissipation and protects against adiposity
(2018)
Nutrient overload in combination with decreased energy dissipation promotes obesity and diabetes. Obesity results in a hormonal imbalance, which among others, activates G-protein coupled receptors utilizing diacylglycerol (DAG) as secondary messenger. Protein kinase D1 (PKD1) is a DAG effector which integrates multiple nutritional and hormonal inputs, but its physiological role in adipocytes is unknown. Here, we show that PKD1 promotes lipogenesis and suppresses mitochondrial fragmentation, biogenesis, respiration, and energy dissipation in an AMP-activated protein kinase (AMPK)-dependent manner. Moreover, mice lacking PKD1 in adipocytes are resistant to diet-induced obesity due to elevated energy expenditure. Beiging of adipocytes promotes energy expenditure and counteracts obesity. Consistently, deletion of PKD1 promotes expression of the β3-adrenergic receptor (ADRB3) in a CCAAT/enhancerbinding protein (C/EBP)-α and δ-dependent manner, which leads to the elevated expression of beige markers in adipocytes and subcutaneous adipose tissue. Finally, deletion of PKD1 in adipocytes improves insulin sensitivity and ameliorates liver steatosis. Thus, loss of PKD1 in adipocytes increases energy dissipation by several complementary mechanisms and might represent an attractive strategy to treat obesity and its related complications.
General and efficient tools for site-specific fluorescent or bioorthogonal labeling of RNA are in high demand. Here, we report direct in vitro selection, characterization, and application of versatile trans-acting 2'-5' adenylyl transferase ribozymes for covalent and site-specific RNA labeling. The design of our partially structured RNA pool allowed for in vitro evolution of ribozymes that modify a predetermined nucleotide in cis (i.e. intramolecular reaction), and were then easily engineered for applications in trans (i.e. in an intermolecular setup). The resulting ribozymes are readily designed for specific target sites in small and large RNAs and accept a wide variety of N6-modified ATP analogues as small molecule substrates. The most efficient new ribozyme (FH14) shows excellent specificity towards its target sequence also in the context of total cellular RNA.
The lability of B=B, B-P and B-halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes demonstrate cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange.
We demonstrate two-quantum (2Q) coherent two-dimensional (2D)electronic spectroscopy using a shot-to-shot-modulated pulse shaper and fluorescence detection. Broadband collinear excitation is realized with the supercontinuum output of an argon-filled hollow-core fiber, enabling us to excite multiple transitions simultaneously in the visible range. The 2Q contribution is extracted via a three-pulse sequence with 16-fold phase cycling and simulated employing cresyl violet as a model system. Furthermore, we report the first experimental realization of one-quantum−two-quantum (1Q-2Q) 2D spectroscopy, offering less congested spectra as compared with the 2Q implementation. We avoid scattering artifacts and nonresonant solvent contributions by using fluorescence as the observable. This allows us to extract quantitative information about doubly excited states that agree with literature expectations. The high sensitivity and background-free nature of fluorescence detection allow for a general applicability of this method to many other systems.
Die Europäische Union (EU) ist derzeit ernsthaft in Gefahr. Zahlreiche Krisen wie der fast-Grexit letztes Jahr und der mögliche Brexit, die Euro- und Flüchtlingskrise könnten die EU scheitern lassen. Außerdem wenden sich immer mehr Bürger von der EU ab und wählen nationalistische bzw. populistische Parteien mit ihren simplizistischen Parolen. Der Beitrag analysiert zunächst diesen alarmierenden Krisenkontext. Es wird herausgearbeitet, dass die EU und ihre Mitgliedstaaten bislang keine ausreichenden Antworten auf diese gravierenden Herausforderungen zu geben wissen und es all überall am Willen, an Entschlossenheit, an Solidarität und Loyalität mangelt.
Der Beitrag argumentiert, dass Europa nicht scheitern darf, weil allzu viel auf dem Spiel steht. Er appelliert an die europäischen Nationen, sich daran zu erinnern, dass nur ein geeintes Europa dem Kontinent den inneren und äußeren Frieden, gemeinsamen Wohlstand und Zukunfts- und Überlebensfähigkeit sichern kann. Daher wäre ein Scheitern Europas ein aberwitziger Irrweg und ein tödlicher Fehler.
Um den europäischen Mitbürgern die aktuelle Gefahrenlage anschaulich vor Augen zu führen und sie wachzurütteln, nimmt der Beitrag in seiner zweiten Hälfte den Duktus eines Aufrufs an. Die meisten der EU-Nationen werden direkt angesprochen und auf ihre sträfliche Gleichgültigkeit sowie ihre je eigenen nationalen Dünkel und Egoismen im Umgang mit Europa befragt. Besonders kritisch wird mit jenen EU-Bürger umgegangen, die durch ihr Wahlverhalten nationalistischen Kräften mit ihrer Absage an Europa Vorschub leisten.
Der Aufruf endet mit einem als „EU-Märchen“ bezeichneten fiktiv-optimistischen Ausblick.