1,720,986 research outputs found
Self-Diffusion in Uniaxial Liquid Crystals
No full textExpressions are given for the translational diffusion parameters of non-spherical solutes in nematic and smectic-A phases, as measured by magnetic field gradient spin-echo NMR
Void size distribution in MD-modelled silica glass structures
No full textThe void distribution of different models of the silica glass have been analyzed in terms of the Voronoi-Delaunay description of the void space into a disordered system. The silica glass has been modelled by making use of different pair-pair and three body potentials, previously proposed. The glass structural parameters (bond lengths and bond angles) are compared with known experimental data. A computer algorithm has been implemented to apply the Voronoi-Delaunay approach to a system of non-uniform spheres, and it is fully described. The algorithm furnishes the size distribution of interstices into the bulk structure. From this result, the solubility of noble gasses into the glass is computed, and the values obtained are successfully compared to known experimental data. The correlation between void size distribution and ring size distribution in silica glass is discussed. Both the glass structure and void distribution give better agreement to the experimental data when the pair pair interaction is modelled with the potential developed by Beest et al. [B.W.H. Beest, G.J. Kramer, R.A. Santen, Phys. Rev. Lett. 64 (1990) 1955]
Intermolecular electron transfer in merocyanine aggregates studied by optical and transient EPR methods
No full textExcited states of two merocyanine chromophores have been studied by means of optical and magnetic resonance techniques. The dye molecules were dissolved in solvents of different polarity and cast in thin films on quartz surfaces. The optical absorption and emission spectra of both molecules indicate a little charge-transfer character in the S-0-S-1 transition. The cast films contain monomers and H type aggregates. EPR spectra have been obtained by time resolved techniques at low temperature after illumination of the sample. EPR spectra of isolated molecules in frozen solutions are typical of triplet excited states generated by spin-orbit promoted intersystem crossing. Two signals are observed in EPR spectra of the cast films, with a narrow line in emission superimposed on a very weak molecular triplet lineshape. The polarization and lineshape analysis suggest that a radical ion pair with a lifetime of the order of microseconds is formed by intermolecular charge migration following the photoinduced electron-transfer reaction between the donor and acceptor moieties of the chromophore
EPR study of in-plane anisotropy in the LB films of a Cu(II)[tetra alkoxy carbonyl] phthalocyanine deposited on mylar sheets
No full textElectron paramagnetic resonance spectroscopy has been applied to obtain information on the in-plane anisotropy of Cu(II) [tetrakis-(3,3-dimethyl-1-butoxy carbonyl)] phthalocyanine (CuPcBC) molecules deposited as a Langmuir-Blodgett film on a biaxially oriented mylar substrate. We have found that the in-plane distribution of orientations of the film molecules depends on the angle between the asis of preferential order identified in the substrate and the dipping direction. The in-plane orientational anisotropy of the material in the film is mainly due to the ability of the substrate to induce oriented growth of CuPcBC molecules which reflects its surface order
Model for Triplet-Triplet Energy Transfer in Natural Clusters of Peridinin Molecules Contained in "Dinoflagellate"'s Outer Antenna Proteins.
No full textTriplet-triplet energy transfer among peridinin molecules in peridinin-chlorophyll-protein (PCP) complexes from two different dinoflagellate species is quantitatively evaluated by the use of the optically detected magnetic resonance technique supplemented by triplet-state decay rate measurements. The two complexes are related by a dimer-monomer relationship in the sense that the four peridinins-one chlorophyll cluster of the Heterocapsa pygmeae PCP (H-PCP) is a copy of the two clusters contained in the larger protein Amphidinium carterae PCP (A-PCP), which are related by a local 2-fold symmetry axis. The system is conveniently discussed as a multistate Frenkel exciton system performing stochastic jumps from one exciton state to another. The rate of triplet-triplet energy exchange among peridinin molecules is obtained through the simulation of the spectral line shapes of optically detected magnetic resonance in zero field. Measurements were collected in a range of temperatures between 2 and 40 K and supplemented by the determination of the decay rate of the triplet to ground state in the same temperature range. The latter is of the order of 0.005 MHz and is relevant in determining the line shapes, especially at the lowest temperatures where the exchange rates are low. Exchange rates within the four-member peridinin cluster of H-PCP exactly match those within each of the two clusters of A-PCP and span a range from 0 to 20 MHz for the uphill rate and from 10 to 300 MHz for the downhill rate, corresponding to an energy difference of 41 cm(-1). The intercluster exchange rate in A-PCP goes from 0 to 12 MHz uphill and from 0.05 to 30 MHz downhill, with an energy difference of only 12.5 cm(-1). The kinetics conform to phonon-stimulated radiationless transition theory, and coupling parameters and reorganization energies are evaluated and discussed
Carotenoid interactions in peridinin chlorophyll-a proteins from dinoflagellates. Evidence for optical excitons and triplet migration.
No full textOptical and magnetic resonance spectroscopy has been used to investigate two peridinin-chlorophyll alpha-protein (PCP) complexes constituting the peripheral antenna of the photosystem of two dinoflagellate species. One protein contains a cluster of four peridinins and one chlorophyll molecule, the other contains two such clusters in a 'quasi' dimer of the former. Evidence of intra-cluster excitonic interactions between peridinins is provided by low-temperature absorption and circular dichroism spectra. Peridinin triplets are detected in zero-field magnetic resonance spectroscopy by both optical emission and absorption. Interaction among peridinin molecules causes a fast intra-cluster and a slow inter-cluster triplet migration. The origin of the interactions is discussed by correlating optical and magnetic spectroscopic data
Phenomenological representation of triplet-exciton kinetics
No full textA general density-matrix formulation is derived in a way suitable for the analysis of optical and magnetic effects due to the triplet-exciton kinetics in molecular crystals, in the presence of a microwave field inducing transitions among the spin levels. The expansion of the density matrix as a power series of the microwave field intensity provides a hierarchy of systems of equations, the knowledge of higher-order corrections requiring the solutions of all lower-order terms. The solutions of the zeroth-, first- and second-order equations are relevant for the calculation of polarization and delayed fluorescence effects. ESR lineshapes and DF ODMR spectral profiles, respectively
Elastic and dynamical properties of alkali silicate glasses from computer simulations techniques
No full textThis paper shows recent progresses in the field ofcomputer simulations of inorganic glasses. Moleculardynamics simulations and energy minimization methods havebeen applied to calculate the elastic and transport proper-ties of alkali silicate glasses of compositions xM2 O · (100 −x)SiO2 (with x = 0, 10, 15, 20, 25, 30 % mol for M = Li,Na and K) and of a soda-lime glass with composition 15Na2 O·10CaO · 75SiO2 , which has been employed to ascertain theeffect of the replacement of CaO for Na2 O. The excellentagreement of the computed results with the experimentaldata highlights the important predictive and interpretativerole reached by computer simulations techniques
CaO and ZnO in soda-silicate glasses: a molecular dynamics simulation study and experimental characterization
No full textCaO and ZnO in soda-silicate glasses: a molecular dynamics simulation study and experimental characterizatio
An ab initio parameterized interatomic force field for hydroxyapatite
No full textA classical interatomic force field for hydroxyapatite has been parameterized from periodicab initio calculations carried out on the hexagonal structure (space group P63). The GULPprogram has been used for fitting geometry and phonon frequencies computed with theCRYSTAL06 program using the B3LYP hybrid functional and Gaussian-type basis set ofpolarized double zeta quality. Polarization effects and covalent bonding have been includedthrough the shell-ion model potential. Excellent agreement has been found in reproducingstructural features, lattice dynamics, the OH stretching vibrations and relative phase stabilitiesbetween the monoclinic structure (space group P21/b) and the hexagonal one. Transferability fromhydroxyapatite to other calcium phosphates has also been demonstrated
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