Attaching perylene dyes to polyfluorene: three simple,efficient methods for facile color tuning of light-emitting polymers

The emission color of fluorene-based polymers can be facilely tuned across the whole visible spectrum by copolymerization with perylene dyes. Methods are demonstrated for incorporation of the dyes in the polymer mainchain, at the chain termini, or as side chains. Efficient energy transfer causes the emission to come solely from the dye units. Efficient LEDs have been made from the copolymers with dyes in the mainchain.     

Mechanistic study of an improbable reaction: alkene dehydrogenation by the delta12 acetylenase of Crepis alpina

The mechanism by which the fatty acid acetylenase of Crepis alpina catalyzes crepenynic acid ((9Z)-octadeca-9-en-12-ynoic acid) production from linoleic acid has been probed through the use of kinetic isotope effect (KIE) measurements. This was accomplished by incubating appropriate mixtures of regiospecifically deuterated isotopomers with a strain of Saccharomyces cerevisiae expressing a functional acetylenase. LC/MS analysis of crepenynic acid obtained in these experiments showed that the oxidation of linoleate occurs in two discrete steps, since the cleavage of the C12-H bond is very sensitive to isotopic substitution (k(H)/k(D) = 14.6 +/- 3.0) while a minimal isotope effect (k(H)/k(D) = 1.25 +/- 0.08) was observed for the C13-H bond breaking step. These data suggest that crepenynic acid is produced via initial H-atom abstraction at C12 of a linoleoyl substrate. The relationship between the mechanism of enzymatic acetylenation and epoxidation is discussed.     

A kinetic study of metallocene‐catalyzed ethylene polymerization using different aluminoxane cocatalysts

The kinetics of ethylene polymerization using homogeneous Cp₂ZrCl₂/aluminoxane catalysts in toluene has been investigated at 70 °C with an ethylene pressure of 30 psi. Four aluminoxanes were used: methylaluminoxane, modified methylaluminoxanes with a fraction of methyl groups substituted with isobutyl (MMAO‐4) or octyl (MMAO‐12) groups, and polymethylaluminoxane (PMAO‐IP). The cocatalyst‐to‐catalyst ratio, [Al]/[Zr], varied from 1000 to 10,000. The experimental results obtained using the four cocatalysts were compared and a model was proposed to fit the rate of polymerization as a function of polymerization time and [Al]/[Zr] ratio. Molecular weight distributions with polydispersities between three and four indicate the presence of more than one active site type. We proposed a model that explained these broad molecular weight distributions using an unstable active complex that is formed in the early stages of the reaction and is transformed over time to a more stable active complex via an intermediate. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1677–1690, 2007     

Zur Bestimmung der Kieselfluorwasserstoffs?ure

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New Insights in Frustrated Lewis Pair Chemistry with Azides

The geminal frustrated Lewis pair (FLP) tBu₂PCH₂BPh₂ ( 1 ) reacts with phenyl-, mesityl-, and tert-butyl azide affording, respectively, six, five, and four-membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six-membered ring structure, which is thermally stable with an N-phenyl group, but rearranges when sterically more encumbered Mes−N₃ and tBu−N₃ are used. The reaction of 1 with Me₃Si−N₃ is believed to follow the same course, yet subsequent N₂ elimination occurs to afford a four-membered heterocycle ( 5 ), which can be considered as a formal FLP-trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.     

Diastereomeric Xe chemical shifts in tethered cryptophane cages

Cryptophane cages serve as host molecules to a Xe atom. Functionalization of cryptophane-A has permitted the development of Xe as a biosensor. Synthetic routes used to prepare cryptophanes result in racemic mixtures of the chiral cages. In the preparation of a tethered cryptophane-A cage for biosensor applications, some achiral and chiral substituents such as left-handed amino acids have been used. When the substituent is achiral, the NMR signal of the Xe atom in the functionalized cage in solution is a single isotropic peak, since the Xe shielding tensor components in the R and L cages differ by no more than the signs of the off-diagonal elements. Chiral substituents can split the cage-encapsulated Xe NMR signal into one or more sets of doublets, depending on the number of asymmetric centers in the substituent. We carry out quantum mechanical calculations of Xe nuclear magnetic shielding for the Xe atom at the same strategic position within an L cryptophane-A cage, under the influence of chiral potentials that represent r or l substituents outside the cage. Calculations of the Xe shielding response in the Lr and Ll diastereomeric pairs permit the prediction of the relative order of the Xe chemical shifts in solutions containing the Rl and Ll diastereomers. Where the substituent itself possesses two chiral centers, comparison of the calculated isotropic shielding responses in the Llr, Lrl, Rll, and Lrr systems, respectively, permits the prediction of the Xe spectrum of diastereomeric systems in solutions containing Llr, Rlr, Lll, and Rll systems. Assignment of the peaks observed in the experimental Xe NMR spectra is therefore possible, without having to undertake the difficult synthetic route that produces a single optically pure enantiomer.     

Hot electron production using the Texas Petawatt Laser irradiating thick gold targets

We present data for relativistic hot electron production by the Texas Petawatt Laser irradiating solid Au targets with thickness between 1 and 4 mm. The experiment was performed at the short focus target chamber TC1 in July 2011, with intensities on the order of several ×10¹⁹ W/cm² and laser energies around 50 J. We discuss the design of an electron-positron magnetic spectrometer to record the lepton energy spectra ejected from the Au targets and present a deconvolution algorithm to extract the lepton energy spectra. We measured hot electron spectra out to ～50 MeV, which show a narrow peak around 10–20 MeV, plus high energy exponential tail. The hot electron spectral shapes appear significantly different from those reported for other PW lasers.     

Bis(phosphine imide)s: easily tunable organic electron donors

[graph: see text] The electrochemical, structural, and spectroscopic properties of bis(phosphine imide)s have been investigated. p-Phenylenebis(phosphine imide)s Ar3PNC6H4NPAr3 (1a-d) have two reversible single-electron oxidations. The first oxidation potentials can be varied from -0.05 to 0.15 V (versus SCE) by modification of the substituents on phosphorus (Ar). Electron-donating substituents lower the oxidation potential, while electron-withdrawing substituents increase the oxidation potential. The difference between the first and second oxidation potential (deltaE, 0.41-0.50) and the electronic coupling (Hab, 1.1 eV) are similar for 1a-d. Computational (DFT) and UV-visible-NIR spectroscopic investigations of 1a-d suggest that the first oxidation leads to a delocalized radical cation 1a*+ while the second oxidation leads to a quinonoidal dicationic state 1a2+. The aromatic linker between phosphine imides has also been modified. Upon oxidation, N,N'-4,4'-biphenylene(bis(triphenyl)phosphine imide) (3) forms radical cationic and a dicationic species similar to 1a-d. While deltaE (0.18 V) and Hab (0.63 eV) are smaller, suggesting weaker electronic communication between the two P=N units in the radical cationic state, the presence of NIR absorptions with vibrational fine structure (768, 861, and 983 nm) supports the formation of delocalized radical cation for 3*+.     

Precipitation of nickel and palladium dimethylglyoximates from homogeneous solution

A comparison of the reaction mechanisms by which nickel or palladium dimethylglyoximate is precipitated from homogeneous solution by the reactions of biacetyl and hydroxylamine discloses many differences. Knowledge of the kinetics and mechanism of the reactions can be used to avoid formation of excess dimethylglyoxime, a limitation of the conventional methods of precipitation.