Cyclohexene oxide/CO2 copolymerization catalyzed by chromium(III) salen complexes and N-methylimidazole: effects of varying salen ligand substituents and relative cocatalyst loading

A detailed mechanistic study into the copolymerization of CO2 and cyclohexene oxide utilizing CrIII(salen)X complexes and N-methylimidazole, where H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine and other salen derivatives and X = Cl or N3, has been conducted. By studying salen ligands with various groups on the diimine backbone, we have observed that bulky groups oriented perpendicular to the salen plane reduce the activity of the catalyst significantly, while such groups oriented parallel to the salen plane do not retard copolymer formation. This is not surprising in that the mechanism for asymmetric ring opening of epoxides was found to occur in a bimetallic fashion, whereas these perpendicularly oriented groups along with the tert-butyl groups on the phenolate rings produce considerable steric requirements for the two metal centers to communicate and thus initiate the copolymerization process. It was also observed that altering the substituents on the phenolate rings of the salen ligand had a 2-fold effect, controlling both catalyst solubility as well as electron density around the metal center, producing significant effects on the rate of copolymer formation. This and other data discussed herein have led us to propose a more detailed mechanistic delineation, wherein the rate of copolymerization is dictated by two separate equilibria. The first equilibrium involves the initial second-order epoxide ring opening and is inhibited by excess amounts of cocatalyst. The second equilibrium involves the propagation step and is enhanced by excess cocatalyst. This gives the [cocatalyst] both a positive and negative effect on the overall rate of copolymerization.     

Two-pump-one-probe femtosecond studies of Ni(II) porphyrins excited states

Dual excited states of nickel(II) meso-tetra(4-sulfonatophenyl)porphyrin (NiTPPS) and nickel(II) meso-tetraphenylporphyrin (NiTPP) have been investigated by two-pump-one-probe transient absorption spectrometry. By dual excited states, we mean molecular entities that have absorbed two photons to generate molecular states with electronic excitation in two distinct regions of metalloporphyrin. Two successive pulses of 400 and 550 nm were used for excitation. The first pulse (400 nm) produced an S2 state of the porphyrin pi-system, which deactivated to give rise to an S1 state and subsequently produce a metal-centered (d,d) state. The second (550 nm) pulse selectively targeted an S0 --> S1 transition of those molecules having an excited metal center and was delivered to the sample approximately 40 ps after the first excitation event. At this time, the ground state of the tetrapyrrole pi-system was already regenerated and the excitation was localized at the metal center. The kinetic profiles of the NiTPPS transients in DMSO revealed biexponential decays with time constants of 0.6 and 4 ps. Photoexcitation of NiTPP in toluene and NiTPPS in water resulted in similar behavior. A mechanism for the Ni(II) porphyrin dual excited state deactivation involving the formation of an intramolecular charge transfer state has been proposed.     

THE EFFECT OF MEDIUM POLARITY ON THE HEMATOPORPHYRIN-SENSITIZED PHOTOOXIDATION OF l-TRYPTOPHAN

Abstract The 83 μM hematoporphyrin (HP)-sensitized photooxidation of 0.1 mM tryptophan in aqueous solution buffered at pH 7.4 or in binary mixtures of phosphate buffer and organic solvents of higher (formamide) or lower (N,N-dimethylformamide, methanol, ethanol, tetrahydrofuran) polarity proceeds by a pure singlet oxygen (¹O₂) mechanism as suggested by azide quenching experiments, the rate-enhancing action of deuterated solvents, and the lack of any significant reaction between triplet HP and tryptophan. Both the first-order rate constant of the photoprocess and the photooxidation quantum yield (φ= 0.011 in phosphate buffer at pH 7.4) increase when the medium polarity is increased (e.g. φ= 0.024 in 90% formamide); this results mainly from the greater quantum yield of ¹O₂ generation and the longer lifetime of ¹O₂. The intrinsic reactivity of ¹O₂ with tryptophan is independent of formamide concentration. A moderate decrease in the medium polarity (e.g. in the range 0-30% methanol) enhances the efficiency of tryptophan photooxidation (φ= 0.014 in 30% methanol) as a result of the enhanced quantum yields of triplet HP and ¹O₂ formation. In contrast, the overall photooxidation rate is depressed at high concentrations of low-polarity organic solvents (e.g. φ= 0.0039 in 90% methanol) due to a 5.5-fold drop of the rate constant for the ¹O₂-tryptophan reaction which counteracts the enhancement of the lifetime and quantum yield of triplet HP and ¹O₂. The solvent composition also affects the equilibria between monomeric and multimeric forms of HP. However, under our experimental conditions, the aggregation state of HP appears to exert only a minor influence on the efficiency of tryptophan photooxidation.     

CROCETIN, A WATER SOLUBLE CAROTENOID MONITOR FOR SINGLET MOLECULAR OXYGEN

Abstract The water soluble carotenoid crocetin has been studied as a singlet molecular oxygen monitor in D₂O solution, pD 8.4. Crocetin reacts chemically with singlet molecular oxygen with a rate constant of 4 x 10⁸ M ^-1 s^-1. The rate constant for total quenching, chemical and physical, is 2.5 x 10⁹ M ^-1 s^-1. Crocetin shows evidence for a reversible reaction with singlet molecular oxygen, as demonstrated by a fairly rapid absorption recovery after bleaching.     

The growth and physics of ultra-high-mobility two-dimensional hole gas on (311)A GaAs surface

We report on the molecular beam epitaxy growth of modulation-doped GaAs-(Ga,Al)As heterostructures on the (311)A GaAs surface using silicon as the acceptor. Two-dimensional hole gases (2DHGs) with low-temperature hole mobility exceeding 1.2×10⁶ cm² V⁻¹ s⁻¹ with carrier concentrations as low as 0.8×10¹¹ cm⁻² have been obtained. This hole mobility is the highest ever observed at such low densities by any growth technique. We also report the first observation of persistent photoconductivity in a 2DHG. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above 4 K and interface scattering at lower temperatures.     

High efficiency, phase-locked operation of the harmonic-multiplyinginverted gyrotwystron oscillator

The inverted gyrotwystron (phigtron) is a millimeter wave frequency-doubling amplifier that has been demonstrated to produce over 300 kW peak power at twice the input frequency (centered at f_in =16.85 GHz and f_out=33.7 GHz) over a 0.5% bandwidth with a saturated gain of 30 dB and efficiency greater than 35%. The device has also been studied both theoretically and experimentally in a different operating regime where frequency-doubled, phase-locked oscillation is possible. A signal, injected via a fundamental gyro-traveling wave tube input section, modulated a 55 kV, 10 A electron beam. After transit through a drift section, the prebunched electron beam produced phase-locked, second harmonic oscillations in a TE₀₃ mode output cavity. RF output centered at either of two frequencies, 34.42 and 34.62 GHz, with a maximum output power of 180 kW, an efficiency of 32% and a locked signal gain of 35 dB was measured. A theoretical prediction of locking bandwidth, a design overview, and the experimental results are presented followed by a summary and discussion of the results     

DEPOLARIZATION OF MOUSE MYELOMA CELL MEMBRANES DURING PHOTODYNAMIC ACTION

The effect of photodynamic action on plasma membranes was examined using a fluorescent potentiometric indicator [di-SBA-C2(3)] to measure alterations in the plasma membrane potential of mouse myeloma cells treated with zinc phthalocyanine sulfonate and light. Plasma membrane depolarization was observed to be an early event in photodynamic action, showing both photosensitizer concentration and light dose dependence. Depolarization occurred while membrane integrity was retained and appears to be an early event preceding cell death.     

Reactivities in olefin-difluorocarbene reactions

The relative rates of addition of difluorocarbene to a series of methyl-substituted olefins have been determined and correlated with similar data for dichlorocarbene, chlorofluorocarbene and ground-state oxygen atoms. The electrophilic nature and stabilization of difluorocarbene by the fluorine substituents is discussed. Relative activation energies for the difluorocyclopropane-forming reaction have been estimated and correlated with properties of the olefins as derived from molecular orbital theory.