Spectral shaping filter for broadband amplifiers

We present the design and implementation of a high-damage threshold spectral shaping filter that can be used to counteract gain narrowing in broadband laser amplifier systems. In contrast to earlier approaches to this problem, the filter has a wide range of tunability in both reflectivity and center wavelength. The shape of the reflectivity curve enables the production of an apodized output spectrum with a clean transform limit. We demonstrate the use of this filter in both regenerative and multipass Ti:Sapphire amplifier systems, yielding super-Gaussian amplified spectra of 70-80 nm width.     

Study of interaction between two oppositely charged polyelectrolytes and formation of polyelectrolyte complexes

Polyelectrolyte complex formation has been studied between oppositely charged polyelectrolytes, e.g., polyethylene-imine, polymethacrylic acid, and methacrylic acid–methacrylamide copolymer. Formation of complexes could be shown through several experimental techniques, e.g., viscometry, conductometry, potentiometry, and IR spectra. It is suggested that these complexes are perhaps formed as a result of electrostatic cooperative interaction and a “ladder-like” interaction is likely to be more favorable.     

Half-metallicity in undoped and boron doped graphene nanoribbons in the presence of semilocal exchange-correlation interactions

We perform density functional calculations on one-dimensional zigzag edge graphene nanoribbons (ZGNRs) of different widths, with and without edge doping including semilocal exchange correlations. Our study reveals that, although the ground state of edge-passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both of the edges with boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semilocal exchange correlations result in half-metallicity in edge-passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with boron edges shows half-metallic behavior irrespective of the ribbon width even in the absence of electric field and this property sustains for any field strength, opening a huge possibility of applications in spintronics.     

A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene

Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl₄ (cAAC=cyclic alkyl(amino) carbene :C(CMe₂)₂(CH₂)N(2,6‐iPr₂C₆H₃) with potassium graphite produces stable (cAAC)₃Si₃, a carbene‐stabilized triatomic silicon(0) molecule. The Si?Si bond lengths in (cAAC)₃Si₃ are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si?Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)₃Si₃ possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations.     

Singlet diradical complexes of chromium, molybdenum, and tungsten with azo anion radical ligands from M(CO)6 precursors

The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.     

Altered Membrane Mechanics Provides a Receptor-Independent Pathway for Serotonin Action

Serotonin, an important signaling molecule in humans, has an unexpectedly high lipid membrane affinity. The significance of this finding has evoked considerable speculation. Here we show that membrane binding by serotonin can directly modulate membrane properties and cellular function, providing an activity pathway completely independent of serotonin receptors. Atomic force microscopy shows that serotonin makes artificial lipid bilayers softer, and induces nucleation of liquid disordered domains inside the raft-like liquid-ordered domains. Solid-state NMR spectroscopy corroborates this data at the atomic level, revealing a homogeneous decrease in the order parameter of the lipid chains in the presence of serotonin. In the RN46A immortalized serotonergic neuronal cell line, extracellular serotonin enhances transferrin receptor endocytosis, even in the presence of broad-spectrum serotonin receptor and transporter inhibitors. Similarly, it increases the membrane binding and internalization of oligomeric peptides. Our results uncover a mode of serotonin–membrane interaction that can potentiate key cellular processes in a receptor-independent fashion.     

Superoxide dismutase mimetic properties exhibited by vacancy engineered ceria nanoparticles

In this report ceria nanoparticles are shown to act as catalysts that mimic superoxide dismutase (SOD) with the catalytic rate constant exceeding that determined for the enzyme SOD.     

Na4IrO4: Square‐Planar Coordination of a Transition Metal in d5 Configuration due to Weak On‐Site Coulomb Interactions

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition‐metal complexes. For example, square‐planar coordination geometries found in transition‐metal oxometalates such as cuprates are usually associated with the d⁸ or d⁹ electron configuration. In this work, we address an unusual square‐planar single oxoanionic [IrO₄]^4? species, as observed in Na₄IrO₄ in which Ir^IV has a d⁵ configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the Ir^IV center in ground‐state Na₄IrO₄ has square‐planar coordination geometry because of the weak Coulomb repulsion of the Ir‐5d electrons. In contrast, in its 3d counterpart Na₄CoO₄, the Co^IV center is tetrahedrally coordinated because of strong electron correlation. Na₄IrO₄ may thus serve as a simple yet important example to study the ramifications of Hubbard‐type Coulomb interactions on local geometries.     

Nonsolvent application of ionic liquids: organo-catalysis by 1-alkyl-3-methylimidazolium cation based room-temperature ionic liquids for chemoselective N-tert-butyloxycarbonylation of amines and the influence of the C-2 hydrogen on catalytic efficiency

1-Alkyl-3-methylimidazolium cation based ionic liquids efficiently catalyze N-tert-butyloxycarbonylation of amines with excellent chemoselectivity. The catalytic role of the ionic liquid is envisaged as "electrophilic activation" of di-tert-butyl dicarbonate (Boc(2)O) through bifurcated hydrogen bond formation with the C-2 hydrogen of the 1-alkyl-3-methylimidazolium cation and has been supported by a downfield shift of the imidazolium C-2 hydrogen of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][NTf(2)]) from δ 8.39 to 8.66 in the presence of Boc(2)O in the (1)H NMR and a drastic reduction of the catalytic efficiency with 1-butyl-2,3-dimethylimidazolium ionic liquids that are devoid of the C-2 hydrogen. The differential time required for reaction with aromatic and aliphatic amines has offered means for selective N-t-Boc formation during inter and intramolecular competitions. Preferential N-t-Boc formation with secondary aliphatic amine has been achieved in the presence of primary aliphatic amine. Comparison of the catalytic efficiency for N-t-Boc formation with a common substrate revealed that [bmim][NTf(2)] is superior to the reported Lewis acid catalysts.