Intraligand charge transfer enables visible-light-mediated nickel-catalyzed cross-coupling reactions

We demonstrate that several visible-light mediated carbon–heteroatom cross-couplings can be carried out using a photoactive NiII precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl2). The activation of this precatalyst towards cross-couplings follows a hitherto undisclosed mechanism that is different from previously reported light-responsive nickel complexes that were reported to undergo metal-to-ligand charge transfer. Theoretic and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl2 with visible-light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-couplings. The heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation

The Conformational Equilibrium of the Neuropeptide Y2 Receptor in Bilayer Membranes

Dynamic structural transitions within the seven-transmembrane bundle represent the mechanism by which G-protein-coupled receptors convert an extracellular chemical signal into an intracellular biological function. Here, the conformational dynamics of the neuropeptide Y receptor type 2 (Y2R) during activation was investigated. The apo, full agonist-, and arrestin-bound states of Y2R were prepared by cell-free expression, functional refolding, and reconstitution into lipid membranes. To study conformational transitions between these states, all six tryptophans of Y2R were(13)C-labeled. NMR-signal assignment was achieved by dynamic-nuclear-polarization enhancement and the individual functional states of the receptor were characterized by monitoring(13)C NMR chemical shifts. Activation of Y2R is mediated by molecular switches involving the toggle switch residue Trp281(6.48)of the highly conserved SWLP motif and Trp327(7.55)adjacent to the NPxxY motif. Furthermore, a conformationally preserved "cysteine lock"-Trp116(23.50)was identified

Intraligand Charge Transfer Enables Visible Light Mediated Nickel Catalyzed Cross Coupling Reactions

We demonstrate that several visible light mediated carbon heteroatom cross coupling reactions can be carriedout using a photoactive Ni II precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups Ni Czbpy Cl2 . The activation of this precatalyst towards cross coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light responsive nickel complexes that undergo metal to ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni Czbpy Cl2 with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross coupling reactions. The heterogeneous catalyst shows stable performance in a packed bed flow reactor during a week of continuous operatio

Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array

Abstract: We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments. Hübers, "Sub-megahertz frequency stabilization of a terahertz quantum cascade laser to a molecular absorption line," Appl. Phys. Lett. 96(7), 071112 (2010). ©2010 Optical Society of Americ

Two-color picosecond and continuous-wave experiments on anti-Stokes and Stokes carrier-transfer phenomena in GaAslAl(x)Ga(1-x)As and InGaP2/AlxGa1-xAs heterostructures

We present direct evidence of the two-step absorption process in anti-Stokes photoluminescence in both GaAs/AlxGa1-xAs and InGaP2/AlxGa1-xAs heterostructures using two-color picosecond and continuous-wave photoluminescence experiments. We show information about the lifetime of the defect states that participate in the two-step absorption process. As a result, we conclude that the long-lived states rather than excitons play the dominant role in the two-step absorption process. We also study the possible contribution of the two-step absorption process to Stokes carrier transfer in GaAs/AlxGa1-xAs asymmetric double quantum well structuresclos

Investigation of MIR-pumped quantum-cascade structures as emitters of THz radiation

One proposal to obtain THz radiation from a compact semiconductor source at room temperatures is based on an optically pumped, electrically driven (OPED) quantum-cascade structure. It has been theoretically shown that such a structure may allow for overcoming the fundamental Manley-Rowe limit of frequency conversion by recycling the pump photons. Furthermore, quantum coherence effects and gain without inversion have been predicted. However, the realization of such a structure has proven to be difficult, which is due to nonlinear transport effects such as the formation of electric-field domains. We investigate the influence of MIR pumping on the electrical transport properties in different OPED quantum-cascade structure. Experiments are performed using free-electron laser (FEL) radiation in the range of 10 to 12 μm. Although the effect of the FEL pumping on the transport properties is rather small and ambiguous, in particular with respect to different polarization directions, a small emission signal in the THz range is observed. Additional experiments with a pulsed CO2 laser show a clear influence of the pumping being on or off and of its polarization direction on the dc current-voltage characteristics

Development of a 4.7-THz front end for the GREAT heterodyne spectrometer on SOFIA

Heterodyne spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and planetary research. It allows for the investigation of the chemical composition, the evolution, and the dynamical behaviour of astronomical objects such as molecular clouds and star-forming regions. For frequencies beyond 2 THz, SOFIA, the Stratospheric Observatory for Infrared Astronomy, is currently the only platform which allows for heterodyne spectroscopy at these frequencies. One example is the OI fine structure line at 4.7 THz, which is a main target to be observed with GREAT, the German Receiver for Astronomy at Terahertz Frequencies, on board of SOFIA. We report on the progress toward a 4.7-THz front end for the GREAT heterodyne spectrometer on SOFIA. The local oscillator (LO) combines a quantum-cascade laser (QCL) with a compact, low-input-power Stirling cooler. The 4.7-THz QCL is based on a two-miniband design and has been developed for continuous-wave operation, high output powers, and low electrical pump powers. The mixer is a phonon-cooled NbN hot electron bolometer (HEB). It consists of a 2-μm-wide, 0.2-μm-long, and 5.5-nm-thin NbN stripe on a high-resistivity (> 5 kΩ) silicon substrate located in the center of a planar logarithmic spiral antenna and glued onto the flat side of an extended hemispherical 12-mm-diameter silicon lens. In addition, we will present results obtained with a liquid-cryogen-free front end, which is integrated in a pulse-tube cooler (PTC). It is based on a QCL-LO, which is mounted on the first cold stage of the PTC, and a HEB mixer mounted on the second cold stage