Weakly Coordinated Cobaltacycles: Trapping Catalytically Competent Intermediates in Cp*CoIII Catalysis

Herein, we disclose the synthesis of metallacyclic Cp*Co^III complexes containing weakly chelating functional groups. We have employed these compounds not only as an exceptional platform for accessing some of the most widely invoked transient intermediates in C?H functionalization processes but also as competent catalysts in different Cp*Co‐catalyzed transformations, including a benchmark coupling reaction.     

A Sequential Method to Prepare Polymorphs and Solvatomorphs of [Fe(1,3‐bpp)2](ClO4)2⋅nH2O (n=0, 1, 2) with Varying Spin‐Crossover Behaviour

Two polymorphs of the spin crossover (SCO) compound [Fe(1,3‐bpp)₂](ClO₄)₂ ( 1 and 2 ; 1,3‐bpp=2‐(pyrazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine) were prepared using a novel, stepwise procedure. Crystals of 1 deposit from dry solvents, while 2 is obtained from a solid‐state procedure, by sequentially removing lattice H₂O molecules from the solvatomorph [Fe(1,3‐bpp)₂](ClO₄)₂?2 H₂O ( 2 ?2 H₂O), using single‐crystal‐to‐single‐crystal (SCSC) transformations. Hydrate 2 ?2 H₂O is obtained through the same reaction as 1 , now with 2.5 % of water added. Compounds 2 and 2 ?2 H₂O are unstable in the atmosphere and absorb or lose one equivalent of water, respectively, to both yield the stable solvatomorph [Fe(1,3‐bpp)₂](ClO₄)₂?H₂O ( 2 ?H₂O), also following SCSC processes. The four derivatives have been characterised by single‐crystal X‐ray diffraction (SCXRD). Furthermore, the homogeneity of the various compounds as well as their SCSC interconversions have been confirmed by powder X‐ray diffraction (PXRD). Polymorphs 1 and 2 exhibit abrupt SCO behaviour near room temperature with T_1/2↑=279/316 K and T_1/2↓=276/314 K (near 40 K of shift) and different cooperativity.     

Ultrabroad gain in an optical parametric generator with periodically poled KTiOPO4

An optical parametric gain bandwidth of 115 THz at full-width half maximum is generated from a picosecond Ti:sapphire pumped degenerate optical parametric generator. This ultrabroad bandwidth could be obtained by first identifying the wavelength where the nonlinear optical material has zero group-velocity dispersion (GVD). By pumping at half this wavelength the degenerate signal–idler pairs can accommodate ultrabroad bandwidths. The explanation for this is that the group velocities of the signal and the idlers are approximately matched and the GVD is small. However, in order to thoroughly investigate the degeneracy region around 1700 nm we fabricated several periodically poled KTiOPO₄ (PPKTP) crystals with different periods, and also one periodically poled RbTiOPO₄ (PPRTP). Both collinear and noncollinear configurations were employed for broadband parametric generation in this region. It was found that the optimum pump wavelength is in the region between 800 nm to 850 nm for PPKTP, and we could also conclude that a similar performance was found for PPRTP. This work will allow the design of optical parametric devices for generating few-cycle pulses in the spectral region between 1.1 μm and 3.8 μm. PACS 42.65.Re; 42.65.Ky; 42.65.-k     

1H, 13C NMR studies of new 3‐aminophenol isomers linked to pyridinium salts

¹H and ¹³C NMR spectroscopic data of 20 new non‐symmetrical compounds were assigned by a combination of 1D and 2D NMR experiments (DEPT, HSQC, and HMBC). These compounds contain a 4‐(N,N‐dimethylamino)‐ or 4‐(pyrrolidin‐1‐yl)pyridinium moiety and a 3‐nitro‐, 3‐amino‐, or 3‐hydroxyphenyl ring, linked by p‐xylene, 4,4′‐dimethylbiphenyl, 1,2‐bis(p‐tolyl)ethane, or 1,4‐bis(p‐tolyl)butane. Copyright © 2013 John Wiley & Sons, Ltd.     

Long-time behaviour of a one-dimensional BGK model: convergence to macroscopic rarefaction waves

For one-dimensional kinetic BGK models, regarded as relaxation models for scalar conservation laws with genuinely nonlinear fluxes, we prove that the macroscopic density converges to the rarefaction wave solution of the corresponding scalar conservation law in the long time limit, and that the phase space density approaches an equilibrium distribution with the rarefaction wave as macroscopic density. The proof requires a smallness assumption on the amplitude of the rarefaction waves and uses a micro-macro decomposition of the perturbation equation.     

Fibrinogen-β-Estradiol Binding Studied by Fluorescence Spectroscopy: Denaturation and pH Effects

Fibrinogen is a blood plasma protein that plays a crucial role in hemostasis. It is known that erythrocyte aggregation increases in the presence of fibrinogen, and that β-estradiol decreases erythrocyte aggregation with a constant fibrinogen concentration. In this work, we have used intrinsic tryptophan fluorescence to obtain information on the conformational changes of fibrinogen upon the recently proposed interaction with β-estradiol. To evaluate the effect on the conformational changes during fibrinogen-β-estradiol binding, fluorescence experiments were performed using guanidine hydrochloride (0–6 M) as denaturant, at different pH values. The results obtained for pH 6.5 and 8.0 showed no effect during the binding. The main differences were observed between pH 4.2 and 7.4, in the absence and in the presence of two different denaturant concentrations (1 and 5 M). A red shift of the fluorescence emission from 344 to 354 nm is observed when denaturant concentration is above 3 M for all studied pH values. This phenomenon may be explained by the loss of compact structure of the protein in the presence of denaturant, with tryptophan residues exposure to the aqueous environment and alteration of fibrinogen-β-estradiol binding. These results demonstrate that the binding sites of fibrinogen are strongly dependent on the conformational state of the protein.     

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO₂ reduction attached onto mesoporous TiO₂ electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO₂ to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO₂ to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO₂ is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Using transient spectroelectrochemical techniques, we investigate multiply reduced states of molecular catalysts on titania photoelectrodes as a function of the applied bias and the light intensity.     

Narrowband and tunable ring optical parametric oscillator with a volume Bragg grating

We demonstrate a tunable nanosecond optical parametric oscillator with a narrowed signal spectrum. This was done by use of a volume Bragg grating based retroreflector, which makes the tuning simple and yields a compact design. Using periodically poled KTiOPO4 as the nonlinear medium, we generated 0.42 mJ of signal energy at 760 nm with a tuning range of 5 nm(2.6 THz) and a bandwidth of 0.25 nm(130 GHz) when the oscillator was pumped at 532 nm with 1.3 mJ of energy.     

Cascaded mode-locking of a spectrally controlled Yb:KYW laser

Self-starting and stable mode-locking of an Yb:KYW laser in the picoseconds pulse regime has been achieved by exploiting a positive cascaded Kerr lens in periodically poled KTP. The use of a volume Bragg grating (VBG), for locking the output spectrum of the laser, was essential to achieve a stable mode-locked operation in this wide gain bandwidth laser material. The laser emitted stable, nearly transform-limited pulses with a duration of 16 ps, at a repetition rate of 210 MHz, and with an energy of 3.2 nJ. The mode-locked spectrum was centred at 1,029.1 nm and featured a FWHM bandwidth of 85 pm, which was effectively determined by the VBG. Combination of a large cascaded Kerr nonlinearity with spectral limiting by a VBG represents so far the best opportunity for stable mode-locking of a broadband laser to produce near-transform-limited picosecond pulses.     

An all-solid-state laser source at 671 nm for cold-atom experiments with lithium

We present an all-solid-state narrow-linewidth laser source emitting 670 mW output power at 671 nm delivered in a diffraction-limited beam. The source is based on a frequency-doubled diode-end-pumped ring laser operating on the ⁴ F _3/2→⁴ I _13/2 transition in Nd:YVO₄. By using periodically poled potassium titanyl phosphate (ppKTP) in an external buildup cavity, doubling efficiencies of up to 86% are obtained. Tunability of the source over 100 GHz is accomplished. We demonstrate the suitability of this robust frequency-stabilized light source for laser cooling of lithium atoms. Finally, a simplified design based on intra-cavity doubling is described and first results are presented.