Tunable passively Q-switched ytterbium-doped fiber laser using MoWS2/rGO nanocomposite saturable absorber

We demonstrated a tunable Q-switched ytterbium-doped fiber laser (YDFL) using MoWS₂/rGO nanocomposite as passive saturable absorber. Further, the Mo_1?xW_xS₂/rGO nanosheets, with x proportion of 0.2, are synthesized using hydrothermal exfoliation technique. The proposed nanocomposite-PVA based thin film is fabricated by mixing the MoWS₂/rGO nanosheets with polyvinyl alcohol (PVA). The fabricated thin film is sandwiched between two fiber ferrules to realize the proposed saturable absorber (SA). Further, the proposed MoWS₂/rGO-PVA based thin film SA exhibits a fast relaxation time and a high damage threshold which are suitable to realize a Q-switched pulsed laser with a tunable wavelength range of 10?nm that extends from 1028?nm to 1038?nm. For the highest pump power of 267.4?mW, the generated Q-switched pulses exhibit a narrow pulse width of 1.22 μs, the pulse repetition rate of 90.4?kHz, the highest pulse energy of 2.13?nJ and its corresponding average power of 0.193?mW. To the best of author’s knowledge, this is the first realization of a tunable Q-switching fiber laser in a 1?μm wavelength using MoWS₂/rGO nanocomposite saturable absorber.     

Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon−Hydrogen Bonds

Oxidation of the iron(II) precursor [(L¹)Fe^IICl₂], where L¹ is a tetradentate bispidine, with soluble iodosylbenzene (^sPhIO) leads to the extremely reactive ferryl oxidant [(L¹)(Cl)Fe^IV=O]⁺ with a cis disposition of the chlorido and oxido coligands, as observed in non-heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C−H abstraction and the result of a rebound of the ensuing radical to an iron-bound Cl⁻. The time-resolved formation of the halogenation product indicates that this primarily results from ^sPhIO oxidation of an initially formed oxido-bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe²⁺ and Fe³⁺ in comparison with [(L¹)Fe^IICl₂] indicate a high complex stability of the bispidine-iron complexes. DFT analysis shows that, due to a large driving force and small triplet-quintet gap, [(L¹)(Cl)Fe^IV=O]⁺ is the most reactive small-molecule halogenase model, that the Fe^III/radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.     

Stereoselective synthesis of optically active disilanes and selective functionalization by the cleavage of silicon-naphthyl bonds with bromine

Optically active disilanes with one chiral silicon center, (R)-1,2-dimethyl-1-(naphth-1-yl)-1,2,2-triphenyldisilane and (R)-1,2,2-trimethyl-2-(4-methoxynaphth-1-yl)-1-(naphth-1-yl)-1-phenyldisilane, were obtained by the reaction of (S)-methyl(naphth-1-yl)phenylchlorosilane (> 99% ee) with methyldiphenylsilyllithium or by the reaction of methyldiphenylchlorosilane with optically active (S)-methyl(naphth-1-yl)phenylsilyllithium and by the reaction of (S)-methyl(naphth-1-yl)phenylchlorosilane (> 99% ee) with dimethyl(4-methoxynaphth-1-yl)silyllithium. Under the optimized conditions, the reactions proceeded with almost complete inversion for the cholorosilanes and retention for the silyl anions. Optically active disilanes with two chiral centers, (1R,2R)-1,2-dimethyl-1,2-di(naphth-1-yl)-1,2-diphenyldisilane and (1S,2S)-1,2-di(4-methoxynaphth-1-yl)-1,2-dimethyl-1,2-diphenyldisilane, were obtained in high optical purity by the reactions of corresponding optically active halogenosilanes (Cl or F) with optically active silyllithiums. The silicon-silicon bond and the silicon-naphthyl bond of (R)-1,1,2-trimethyl-1,2-di(naphth-1-yl)-2-phenyldisilane and (1R,2R)-1,2-dimethyl-1,2-di(naphth-1-yl)-1,2-diphenyldisilane were cleaved without selectivity on bromination. The silicon-(4-methoxynaphth-1-yl) bond of (R)-1,2,2-trimethyl-2-(4-methoxynaphth-1-yl)-1-(naphth-1-yl)-1-phenyldisilane was regiospecifically cleaved, followed by the stereoselective cleavage of the remaining chiral silicon-naphthyl bond (94% inversion). Although the silicon-(4-methoxynaphth-1-yl) bonds of (1S,2S)-1,2-di(4-methoxynaphth-1-yl)-1,2-dimethyl-1,2-diphenyldisilane (> 99% ee) were regioselectively cleaved without silicon-silicon bond scission, remarkable racemization could not be avoided during the one-pot reaction.     

Synthesis and structural study of thiacyclophanes utilizing dibromides and methane dithiolate

The synthesis of a series of thiacyclophanes and optically active binaphthol-based chiral thiacyclophanes is reported with XRD structure. Two diastereomeric tetrathiacyclophanes are designed and synthesized. The two diastereomers are evidenced by crystal structure; the single-crystal X-ray studies reveal that one of the isomers possesses an inherent property of self-assembling into a vertical stack of tunnel-like structures.     

Optimization of the reversed-phase high-performance liquid chromatographic separation of the enantimers of a cationic chiral drug (tolperisone) on a heptakis(6-azido-6-deoxy) perphenylcarbamated β-cyclodextrin column

Summary Heptakis(6-azido-6-deoxy) perphenylcarbamated β-cyclodextrin has been synthesized and chemically immobilized on silica gel for use as a chiral stationary phase (PC-CSP) for analytical separation of the enantiomers of chiral drugs. Separation of the enantiomers of tolperisone was studied by high-performance liquid chromatography under reversed-phase conditions. The chromatographic conditions were optimized by varying mobile phase pH, composition, ionic strength, and velocity; 40:60 methanol-1% triethylammonium acetate (TEAA) buffer, pH 5.5, was found to be the most suitable for this separation.     

Crystal and Molecular Structure of an Acridinedione

The title compound, 10‐(4‐hydroxybenzoylamino)‐3,4,6,7,9,10‐hexahydro‐1,8‐(2H,5H)‐acridinedione monohydrate,C₂₀H₂₂N₂O₄.H₂O, consists of partially hydrogenated acridine moiety with one benzoylamino substituent on the central ring. The compound crystallizes in monoclinic system with P2₁/c space group and the unit cell constants are: a = 11.142(4), b = 12.266(2), c = 13.320(2) Å; β = 91.76(2)° and V = 1819.6(8) ų.The central ring (B) adopts boat and the outer rings (A and C) adopt sofa conformations. The water molecule takes part in OW‐H...O and N‐H...OW hydrogen bond formation with acridinedione and benzoylamino group. The oxygen atom O1 interacts through O–H...O bond with the hydroxyl group and in a head‐to‐tail link up of molecules that results in the formation of an infinite “supra molecular“ chain.     

Use of iterative OASIS with low resolution SAD laboratory X-ray source (Cr Kα) data sets

Anomalous scattering methods are widely used for phasing macromolecular structures. OASIS program works on a direct methods procedure and is used to break the phase ambiguity intrinsic in a single wavelength anomalous diffraction (SAD) experiment. An iterative phasing/model-building procedure for processing SAD data includes the initial direct method phasing of SAD data, density modification, automatic model building and direct methods phasing of SAD data with feed back from partial structure information. This dual-space procedure has been tested on two experimental Cr Kα SAD data sets, 2.3 ? for S-SAD and 2.4 ? for I-SAD, of an enzyme glucose isomerase with an approximate molecular weight of 44 kDa (388 residues). HYSS in PHENIX was used to locate the anomalous scatterers for both the data sets (11 sulfurs for 2.3 ? data and 9 iodines for 2.4 ? data). Phasing and density modification were carried out using OASIS, DM and model building was then carried out using ARP/wARP. Two iterative cycles were necessary for the successful structure solution and refinement of the 2.3 ? data set which built 384 residues out of 388. Regular ARP/wARP failed for the 2.4 ? resolution data and hence the modified version of REFMAC5 was used in ARP/wARP. A successful model could be built after three iterative cycles of OASIS using this modified version which built 382 residues out of 388 residues. Minimal manual model building was required at this stage and the structure determination was completed using regular REFMAC5. All the computations mentioned here were carried out using Pentium IV PC.