Homogeneous optical dephasing and line broadening processes in an organic glass: comparison of the temperature dependences of picosecond photon echo and hole burning experiments

The homogeneous optical dephasing times (T₂) and the non-photochemical hole burning (NPHB) linewidths of resorufin in glycerol glass are reported from 1.1 to 25.5 K. At low temperature, the linewidths obtained from NPHB are substantially wider than the linewidths (1/πT₂) obtained from photon echo measurements, but the two become equal at high temperatures. The extra broadening of the NPHB is found to increase as T^0.8. A model of homogeneous dephasing including both two-level system and pseudolocal mode contributions is successful over the entire temperature range.     

Intense non-photochemical hole-burning spectra with discrete phonon structure in an aromatic organic glass (diphenylanthracene)

Hole burning in a tetracene-doped 9,10-diphenylanthracene glass deposited at 4.2 K occurs at an initial efficiency of 0.2 ± 0.1. The hole depth in the saturation limit is 0.6. The spectra show sharp phonon structures. The role of the phenyl rings for non-photochemical hole burning (NPHB) is emphasized.     

Low-energy chlorophyll states in the CP43 antenna protein complex: simulation of various optical spectra. II

The CP43 protein complex of the core antenna of higher plant photosystem II (PSII) has two quasidegenerate "red" absorption states. It has been shown in the accompanying paper I (Dang, N. C., et al. J. Phys. Chem. B 2008, 112, 9921.) that the site distribution functions (SDFs) of red-states A and B are uncorrelated and the narrow holes are burned in subpopulations of chlorophylls (Chls) from states A and B that are the lowest-energy pigments in their particular CP43 complexes and cannot further transfer energy downhill. In this work, we present the results of a series of Monte Carlo simulations using the 3.0-A structure of the PSII core complex from cyanobacteria (Loll, B., et al. Nature 2005, 303, 1040.) to model absorption, emission, persistent, and transient hole burned (HB) spectra. At the current structural resolution, we found calculated site energies (obtained from INDO/S calculations) to be only suggestive because their values are different for the two monomers of CP43 in the PS II dimer. As a result, to probe the excitonic structure, a simple fitting procedure was employed to optimize Chl site energies from various starting values corresponding to different A/B pigment combinations to provide simultaneously good fits to several types of optical spectra. It is demonstrated that the shape of the calculated absorption, emission, and transient/persistent hole-burned spectra is consistent with experimental data and our model for excitation energy transfer between two quasi-degenerate lowest-E states (A and B) with uncorrelated SDFs discussed in paper I. Calculations revealed that absorption changes observed near 670 nm in the non-line-narrowed persistent HB spectra (assigned to photoconversion involving Chl-protein hydrogen-bonding by Hughes (Biochemistry 2006, 45, 12345.) are most likely the result of nonphotochemical hole-burning (NPHB) accompanied by the redistribution of oscillator strength due to modified excitonic interactions. We argue that a unique redistribution of oscillator strength during the NPHB process helps to assign Chls contributing to the low-energy states. It is demonstrated that the 4.2 K asymmetric triplet-bottleneck (transient) hole is mostly contributed to by both A and B states, with the hole profile described by a subensemble of pigments, which are the lowest-energy pigments (B s- and A s-type) in their complexes. The same lowest-energy Chls contribute to the observed fluorescence spectra. On the basis of our excitonic calculations, the best Chl candidates that contribute to the low-energy A and B states are Chl 44 and Chl 37, respectively.     

Site-selective fluorescence and hole-burning spectroscopy of MTHF glasses doped with tetracene or pentacene

Site-selective fluorescence spectra of tetracene (TC) doped in a MTHF glass have been recorded as a function of excitation energy. Based upon the true inhomogeneous absorption profile of the TC S₁ ← S₀ origin determined via vibrational ZP emission intensities the true phonon wing has been evaluated employing an iterative numerical spectral convolution procedure. It is found that both phonon spectra and Debye-Waller factor are virtually constant (α = 0.55 ± 0.08) across the inhomogeneous absorption band as are hole-burning yields in MTHF:TC glasses if probed in the zero-phonon line fluorescence excitation mode. It is concluded that the distributions of both the rate constant for NPHB and the strength of electron-phonon coupling are uncorrelated with the electronic site excitation energies.     

Electrochemically Driven Derivatisation-Detection of Cysteine

 The electrochemical oxidation of catechol to ortho quinone at a glassy carbon electrode in the presence of cysteine is shown to lead to the deposition of a quinone-cysteine adduct at the electrode surface. Square wave voltammetry is shown to enable voltammetric resolution of unreacted orthoquinone and the accumulated adduct species such that quantification of both species is possible over a range of pH 2 – pH 6. The reaction is shown to be selective towards cysteine with little interference from cystine, homocysteine or glutathione. The foundations of a detection protocol are reported with the sensitivity shown to be capable of attenuation through the facile manipulation of the number of electro-initiated derivatisation scans. The application of six consecutive derivatisation scans is shown to provide a linear range of 2–20 μM cysteine that is well within the region required for physiological monitoring. Received December 4, 2000. Revision March 30, 2001.     

The stereochemistry of spiropiperidine cyclizations (histrionicotoxin,part I)

The spirocyclization of straingt-chain triketo-intermediates, similar to possible biogenetic precursors of histrionicotoxin, is shown to be stereoselective, generating the non-natural configuration at C-2. Aiming at an sp²-center in this position, triketo-brassylic ester was treated with ammonia at room temperature and was shown to cleanly and efficiently yield a spiroketone.     

Influence of wetting properties of the liquid on environmental stress cracking of polyethylene at high stress

It has previously been shown that, under high stress and consequently at short times to failure, a major factor governing the environmental stress cracking (ESC) of polyethylene is the ability of a liquid environment to penetrate a growing fissure at a sufficiently high speed to maintain contact with the fracture front. In this earlier study, viscosity was shown to play a significant role in this kinetic effect. The purpose of the present paper is to demonstrate that another property of the solid–liquid system influencing ESC under these high stress conditions is the spreading coefficient of the liquid on the polymer—the parameter defining the tendency of the liquid to wet the polyethylene. It has been shown that the spreading coefficient can be considered as a force and this force in conjunction with atmospheric pressure constitutes the force necessary to drive the liquid into the growing crack.     

Use of 157-nm photodissociation to probe structures of y- and b-type ions produced in collision-induced dissociation of peptide ions

y- and b-type fragment ions produced in the collisional dissociation of arginine-terminated peptide ions are photodissociated with 157-nm light in a linear trap. y-type ions are shown to have the same structure as that of intact peptides of the same sequence with the ionizing proton located at the most basic residue(s). For generic b-type ions, the ionizing proton is shown to be sequestered at the N-terminal arginine, which is consistent with the proposed oxazolone structure.     

Structural transformations of supercooled water in nanopores, studied by microwave radiation

It is shown that the state of supercooled water in nanoporous materials can be studied by measuring the attenuation of microwave radiation. By analyzing variations in the intensity of radiation transmitted through a moistened medium during the supercooling of water in the range of ?37 to ?190°C, the temperatures at which structural transformations take place can be determined. Using the example of KSKG silica gel with a mean pore size of 8 nm, it is shown that at a moisture of 3?C18%, water is found in the liquid state up to a temperature of ?130°C, at which the transition to glass occurs.     

Interfacial rheology of mixed layers of food proteins and surfactants

Mixed protein–surfactant adsorption layers at liquid interfaces are described including the thermodynamic basis, the adsorption kinetics and the shear and dilational interfacial rheology. It is shown that due to the protrusion of hydrophobic protein parts into the oil phase the adsorption layers at the water–hexane interface are stronger anchored as compared to the water-air surface. Based on the different adsorption protocols, a sequential and a simultaneous scheme, the peculiarities of complexes between proteins and added surfactants are shown when formed in the solution bulk or at a liquid interface. The picture drawn from adsorption studies is supported by the findings of interfacial rheology.     

The effect of temperature and initiator levels on the dispersion polymerization of polystyrene

Considerable interest has been generated recently by the preparation of large monodisperse polymer latices by a multistage swelling process. Dispersion polymerization in organic media has been shown to be an alternative route to the preparation of large monodisperse polymer colloids. The size of the resulting particles is known to be dependent on the polarity of the reaction medium. Several other factors are also extremely important in determining both the size and size distribution of the resulting particles. These include the temperature of the reaction and the level of initiator. The temperature affects both the rate of free radical formation and the thermodynamic properties of the polymerization system. It is shown that monodispersity can be achieved over a considerable range of temperatures provided that a particular level of initiator is used at a given temperature. It is also shown that higher levels of initiator at a particular temperature actually produced larger particles. Interestingly, the level of initiator was not found to significantly change the overall rate of polymerization.     

Conditions for the formation of superconductive polymers and possibilities for their realization

General principles of the creation of superconductivity at the polymers are considered. It is shown that high temperature superconductivity may be produced only at really one-dimensional conducting materials. It is also shown that conjugated polymers cannot have onedimensional conductivity. The new class of nonconjugated polar elastomers is proposed as a perspective one for the creation of high temperature superconductive structures–superpolarons–which appear as a result of internal selfionization of the polymer matrix and a consequent selforganization of free polarons. The practical methods of the formation of superpolarons are shown for the films of oxidized atactice polypropylene. Evidences are given which show the existence of superconductive structures in oxidized polypropylene at room temperature.     

Conformational effect on luminescence and polarization of some aniline derivatives. Intersystem crossing from higher singlet states

We present an analysis of emission spectra, obtained in polar and non-polar solvents and at different temperatures, for a series of aniline derivatives excited at different wavelengths. Important changes in the intensity and polarization of the emission are shown to be due to conformational effects of the amino group which strongly changes the molecular desactivation dynamics. This interpretation is shown to be consistent also with the absorption spectra.     

The effects of thermal processing on PVC—IX. A polymer reactive u.v. stabilizer as a thermal stabilizer for PVC

It is shown that a thiol u.v. stabilizer (4-ethoxymercaptoaceto-2-hydroxybenzophenone, EBHPT) becomes efficiently bound to PVC during normal processing at 170° in the presence of dibutyltin maleate (DBTM). This combination is an effective melt stabilizer and shows synergism as a heat stabilizer at 140°. This is shown to be due to the protection of DBTM by the PD antioxidant function in EBHPT and a synergistic optimum is observed. In the absence of DBTM or in the presence of a thiotin stabilizer binding of the u.v. stabilizer is incomplete.     

Stokes number effects in Lagrangian stochastic models of dispersed two-phase flows

The statistical properties of fluid velocities along particle trajectories in turbulent flows have a conditional dependency upon particle velocity. It is shown that the formulation of Lagrangian stochastic (LS) models for particle trajectories in terms of the well-mixed condition for these conditional velocity statistics is exactly analogous to the formulation of second-order LS models for fluid-particle trajectories. The particle aerodynamic response time is shown to be incorporated at second order, which together with the Lagrangian timescale introduced at first order, defines the Stokes number. Reynolds-number effects can be incorporated at third order. The corresponding Fokker-Planck equation is shown to be identical to that advocated by Pozorski and Minier [Phys. Rev. E 59 (1999) 855], who included the fluid velocities "seen" by a particle in the probability density function (pdf) formalism of Reeks and co-workers as a means of circumventing the closure problem (prescribing a closure for the particle flux induced by the fluid) associated with that approach. It is demonstrated that the neglect of Stokes-number effects accounts, in part, for the tendency of first-order LS models to underpredict particle deposition velocities in the diffusion-impaction regime.     

A study of gas phase and surface formaldehyde polymerisation from first principles

First principles computational studies have been carried out on the gas phase and surface polymerisation of formaldehyde. Any gas phase polymerisation in dark conditions is shown to occur by way of neutral species, as opposed to earlier suggestions that a charged species may be involved. The resulting paraformaldehyde chains are shown to be stable only when fully bonded at both chain ends, as opposed to some of the earlier suggested mechanisms. Polymerisation is shown to be thermodynamically favourable in the gas phase, but kinetically limited. Derived rate constants are insufficient to account for experimental results, in agreement with prior studies suggesting that gas phase polymerisation actually occurs on surfaces rather than directly in the gas phase. The TiO(2) (110) surface is employed as a model surface to study formaldehyde adsorption and possible polymerisation mechanisms. Formaldehyde monomers are shown to weakly bind by way of coordinate bonding through the carbonyl group to surface Ti. A particularly strong dimerisation configuration is also found where the two ends of paraformaldehyde are terminated by a surface Ti and surface O, consistent with the earlier observation that paraformaldehyde chains are only stable when saturated at both ends.     

Change of adhesion properties of epoxy oligomer modified by polyarylene ether ketone in the process of curing

The change in the adhesive strength of resin-fiber compounds modified by polyarylene ether ketone in the process of curing is studied. It is shown that the dependence of adhesive strength on the time of curing is described by a curve with a maximum. The form of the curve results from the residual stresses that arise and develop at the interface. It is also shown that using the epoxy polymer blends makes it possible to markedly increase the strength of the matrix-fiber interface.     

Diffusion of nanoparticles in dense fluids

The diffusion process of a single spherical nanoparticle immersed in a fluid solvent is studied by molecular dynamics simulations. When the nanoparticle mass stays constant, it is shown that, at short times, the decay of the nanoparticle velocity autocorrelation function is strongly modified when the particle diameter increases. It is also shown that, at large times, the characteristic algebraic decay induced by the hydrodynamic correlations between the solvated particle and the solvent presents a scaling behavior depending on the particle diameter.     

Frequency-dependent electroosmosis

This paper presents a theory for frequency-dependent electroosmosis. It is shown that for a closed capillary the electroosmosis frequency-dependent ratio of DeltaV/DeltaP is constant with increasing frequency until inertial effects become prevalent, at which time DeltaV/DeltaP starts to decrease with increasing frequency. The frequency response of the electroosmosis coupling coefficient is shown to be dependent on the capillary radius. As the capillary radius is made smaller, inertial effects start to occur at higher frequencies. As part of this paper, frequency-dependent electroosmosis is compared to frequency-dependent streaming potentials. In this comparison it is shown that inertial effects start to become more prevalent at higher frequencies for the closed capillary frequency-dependent electroosmosis case than for the frequency-dependent streaming potential case in the same capillary. It is also shown that this difference is due to a second viscosity (transverse) wave that emanates from the velocity zero within the capillary for the electroosmosis case. The second viscosity wave superposes with the viscosity wave that emanates from wall of the capillary to effectively reduce the hydraulic radius of the capillary. Data are presented for a 0.127-mm capillary to support the findings in this paper.