324 research outputs found
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Cationic palladium allyl complexes [η3-C3H5)Pd(k2P^O)]+SbF6- (2[SbF6],P^O= Ph2P(CH2)2C-(=O)OEt; 3[SbF6], o-Ph2PC6H4C(=O)OEt; 4[SbF6], Ph2P(CH2)2P(=O)Ph2) have been prepared. In all complexes the oxygen donor can be displaced by other ligands such as carbon monoxide and ethylene. Displacement of an ester donor occurs much more readily than displacement of the phosphine oxide function. Above 0°C, the resulting ethylene complexes [η3-C3H5)Pd(C2H4)(k1P~O)]+ react to give (1,2,5-η3)-pent-1-en-5-yl complexes [(H2C=CH(CH2)3Pd(k2P^O)]+. A rate constant of e.g. k(17°C) = (2.27 ± 0.11) × 10-4 s-1 was determined for P,O ≡ Ph2P(CH2)2C(O)OEt by 1H NMR spectroscopy. Using 2-4 as catalyst precursors for ethylene dimerization, the allyl moiety is ultimately cleaved from the metal center as 1,4-pentadiene
Electric-field fluctuations as the cause of spectral instabilities in colloidal quantum dots
Spectral diffusion (SD) represents a substantial obstacle towards
implementation of solid-state quantum emitters as a source of indistinguishable
photons. By performing high-resolution emission spectroscopy for individual
colloidal quantum dots at cryogenic temperatures, we prove the causal link
between the quantum-confined Stark effect and SD. Statistically analyzing the
wavelength of emitted photons, we show that increasing the sensitivity of the
transition energy to an applied electric field results in amplified spectral
fluctuations. This relation is quantitatively fit to a straightforward model,
indicating the presence of a stochastic electric field on a microscopic scale
whose standard deviation is 9 kV/cm, on average. Compensating the commonly
observed intrinsic electric bias with an external one, we find that SD can be
suppressed by up to a factor of three in CdSe/CdS core/shell nanorods. The
current method will enable the study of SD in multiple types of quantum
emitters, such as solid-state defects or organic lead-halide perovskite quantum
dots, for which spectral instability is a critical barrier for applications in
quantum sensing
Mechanistic features of isomerizing alkoxycarbonylation of methyl oleate.
The weakly coordinated triflate complex
[(P∧P)Pd(OTf)]+(OTf)− (1) (P∧
P = 1,3-bis(di-tert-butylphosphino)
propane) is a suitable reactive precursor for
mechanistic studies of the isomerizing alkoxcarbonylation of
methyl oleate. Addition of CH3OH or CD3OD to 1 forms the
hydride species [(P∧P)PdH(CH3OH)]+(OTf)− (2-CH3OH)
or the deuteride [(P∧P)PdD(CD3OD)]+(OTf)− (2DCD3OD),
respectively. Further reaction with pyridine cleanly
affords the stable and isolable hydride [(P∧P)PdH-
(pyridine)]+(OTf)− (2-pyr). This complex yields the hydride
fragment free of methanol by abstraction of pyridine with BF3·OEt2, and thus provides an entry to mechanistic observations
including intermediates reactive toward methanol. Exposure of methyl oleate (100 equiv) to 2D-CD3OD resulted in rapid
isomerization to the thermodynamic isomer distribution, 94.3% of internal olefins, 5.5% of α,β-unsaturated ester and <0.2% of
terminal olefin. Reaction of 2-pyr/BF3·OEt2 with a stoichiometric amount of 1-13C-labeled 1-octene at −80 °C yields a 50:50
mixture of the linear alkyls [(P∧P)Pd13CH2(CH2)6CH3]+ and [(P∧P)PdCH2(CH2)6
13CH3]+ (4a and 4b). Further reaction with
13CO yields the linear acyls [(P∧P)Pd13C(=O)12/13CH2(CH2)6
12/13CH3(L)]+ (5-L; L = solvent or 13CO). Reaction of 2-pyr/
BF3·OEt2 with a stoichiometric amount of methyl oleate at −80 °C also resulted in fast isomerization to form a linear alkyl
species [(P∧P)PdCH2(CH2)16C(=O)OCH3]+ (6) and a branched alkyl stabilized by coordination of the ester carbonyl group as
a four membered chelate [(P∧P)PdCH{(CH2)15CH3}C(=O)OCH3]+ (7). Addition of carbon monoxide (2.5 equiv) at −80 °C
resulted in insertion to form the linear acyl carbonyl [(P∧P)PdC(=O)(CH2)17C(=O)OCH3(CO)]+ (8-CO) and the fivemembered
chelate [(P∧P)PdC(=O)CH{(CH2)15CH3}C(=O)OCH3]+ (9). Exposure of 8-CO and 9 to 13CO at −50 °C
results in gradual incorporation of the 13C label. Reversibility of 7 + CO ⇄ 9 is also evidenced by ΔG = −2.9 kcal mol−1 and
ΔG‡ = 12.5 kcal mol−1 from DFT studies. Addition of methanol at −80 °C results in methanolysis of 8-L (L = solvent) to form
the linear diester, 1,19-dimethylnonadecandioate, whereas 9 does not react and no branched diester is observed. DFT yields a
barrier for methanolysis of ΔG‡ = 29.7 kcal mol−1 for the linear (8) vs ΔG‡ = 37.7 kcal mol−1 for the branched species (9)
Influence of Inner Shell Structure on the Encapsulation Behavior of Dexamethasone and Tacrolimus
We here present the synthesis and characterization of a set of biodegradable
core–multishell (CMS) nanocarriers. The CMS nanocarrier structure consists of
hyperbranched polyglycerol (hPG) as core material, a hydrophobic (12, 15, 18,
19, and 36 C-atoms) inner and a polyethylene glycol monomethyl ether (mPEG)
outer shell that were conjugated by ester bonds only to reduce the toxicity of
metabolites. The loading capacities (LC) of the drugs, dexamethasone and
tacrolimus, and the aggregate formation, phase transitions, and degradation
kinetics were determined. The intermediate inner shell length (C15) system had
the best overall performance with good LCs for both drugs as well as a
promising degradation and release kinetics, which are of interest for dermal
deliver
Tailored Interface Energetics for Efficient Charge Separation in Metal Oxide-Polymer Solar Cells.
Hybrid organic-inorganic heterointerfaces in solar cells suffer from inefficient charge separation yet the origin of performance limitations are widely unknown. In this work, we focus on the role of metal oxide-polymer interface energetics in a charge generation process. For this purpose, we present novel benzothiadiazole based thiophene oligomers that tailor the surface energetics of the inorganic acceptor TiO2 systematically. In a simple bilayer structure with the donor polymer poly(3-hexylthiophene) (P3HT), we are able to improve the charge generation process considerably. By means of an electronic characterization of solar cell devices in combination with ultrafast broadband transient absorption spectroscopy, we demonstrate that this remarkable improvement in performance originates from reduced recombination of localized charge transfer states. In this context, fundamental design rules for interlayers are revealed, which assist the charge separation at organic-inorganic interfaces. Beside acting as a physical spacer in between electrons and holes, interlayers should offer (1) a large energy offset to drive exciton dissociation, (2) a push-pull building block to reduce the Coulomb binding energy of charge transfer states and (3) an energy cascade to limit carrier back diffusion towards the interface
Reactor blending with early/late transition metal catalystcombinations in ethylene polymerization
Ethylene is polymerized by heterobimetallic combinations of early and late transition metal catalysts. Dual combinations of zirconocenes and cationic nickel and iron catalysts with multidentate nitrogen-donor ligands are described. Reactor blends of linear and branched ethylene homopolymers are obtained. With zirconocene / nickel complex catalyst combinations, addition of hydrogen selectively reduces the molecular weight of the linear polyethylene formed by the metallocene catalyst
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