Excitation energy transfer in tris(8-hydroxyquinolinato)aluminum doped with a pentacene derivative

We investigate the excitation energy transfer in a guest-host molecular system consisting of a pentacene derivative, namely 6,13-bis(2,6-dimethylphenyl)pentacene (DMPP), doped into tris(8-hydroxyquinolinato)aluminum (Alq(3)) using steady-state and time-resolved photoluminescence (PL) spectroscopy. The concentration dependent energy transfer rate and efficiency are calculated and analyzed in terms of the F?rster resonance energy transfer model. A relatively long excitation transfer time ( approximately 0.6-3.4 ns depending on the DMPP concentration) and a large transfer radius (31-36 A) are obtained. The F?rster radius calculated directly from the Alq(3) PL-DMPP absorption spectral overlap (26 A) is smaller than the transfer radii obtained from the PL studies, which suggests that excitation energy migration within Alq(3) plays an important role in the energy transfer process, effectively elongating the transfer radius and increasing the transfer rate and efficiency.     

Exploring the ellipsoidal and core-shell geometries of copper-seamed C-alkylpyrogallol[4]arene nanocapsules in solution

Small-angle neutron scattering (SANS) studies were used to probe the stability and geometry of copper-seamed C-alkylpyrogallol[4]arene (PgC(n)Cu; n = 11, 13, 17) hexamers in solution. Novel structural features are observed at chain lengths greater than 10 in both solid and solution phase. Scattering data for the PgC(11)Cu and PgC(13)Cu in chloroform fitted as core-shell spheres with a total spherical radius of about 22.7 and 22.9 ? respectively. On the other hand, the scattering curve for the PgC(17)Cu hexamer at both 1% and 5% mass fractions in o-xylene did not fit as a discrete sphere but rather as a uniform ellipsoid. The geometric dimensions of the ellipsoid radii are 24 ? along the minor axis and 115 ? along the major axis. It is expected that an individual hexamer with heptadecyl chains would exhibit a uniform radius of ca. 24 ?. However, an approximate ratio of 1:5 between radii lengths for the minor axis and major axis is consistent with interpenetration of the heptadceyl chains of adjacent hexamers to form a single ellipsoidal assembly.     

Relation between alkali ion affinity and ionic radius ratio by means of fast atom bombardment tandem mass spectrometry

Cationization of organic molecules has been studied using fast atom bombardment mass spectrometry and tandem mass spectrometric methods. The order of the A⁺ affinity to G (G ? H + A) and AX was determined by this tandem mass spectrometric approach, where G, A and X represent glycerol, alkali metal element and halogen element, respectively. The affinity orders of most alkali halides are divided into three classes. By analogy with the ionic radius ratio rule in solid ionic crystals, this particular parameter, ionic radius ratio, was introduced to analyse the experimental results. This classification is thought to be caused by the ionic radius ratio (X^?/A⁺). These ratios of the I, II and III classes exist between 0.657 and 1.026, 1.096 and 1.776, and 1.856 and 2.289, respectively. The ionic radius ratio plays an important role for determination of the order of alkali ion affinity.     

A nearly complete system of average crystallographic ionic radii and its use for determining ionization potentials

Available systems of empirical (crystallographic) ionic radii are compared. All these systems turn out to be compatible if the O^2? radius is taken to be 0.140 nm. The choice of the oxygen ionic radius is dictated by the equality of the metal ion-oxygen ion distances in oxide crystals and the metal ion-oxygen atom distances in crystal hydrates and concentrated aqueous solutions. In all systems of empirical ionic radii under consideration, the uncertainty of determination of ionic radii is 0.002–0.005 nm. A new method of determination of the ionic radii of elements in unusual valence states is suggested: from the empirical dependence of the electron density at an atom in a given valence state on the atomic radius, a two-parameter equation relating the ionic radii of Period 4–7 elements in two valence states is derived, which allows one to calculate the ionic radius that cannot be determined by crystallography because of the lack of stable compounds in this valence state. Ionic radii are calculated for all Period 4–7 elements in all valence states. They constitute a nearly complete system of ionic radii. There is a linear relationship between the atomic nucleus charge and the inverse ionic radius. It is shown that the square root of the ionization potential is a linear function of the inverse ionic radius. The as yet experimentally unknown ionization potentials of 78 ions of different elements are estimated.     

The effect of the spatial nonlocality of the Kirkwood g-factor on the determination of the long wavelength dielectric functions in dipolar fluids

The Kirkwood g-factor that determines the long wavelength dielectric constant of a simple, isotropic, translationally invariant dipolar fluid is given by an integral of a dipole-dipole correlation function over a spherical region of a nonzero radius R(K) chosen such that any further increase in the radius leads to no change in the value of the integral, thereby defining a Kirkwood correlation length R(K). For radii less than the correlation length the integral defines a radius dependent (nonlocal) Kirkwood g-factor, implying a nonlocal dielectric function. The nonlocal nature of these quantities has important consequences for the determination of the long wavelength dielectric function from dipole fluctuations via the Kirkwood-Fro?hlich connection. The dipole-dipole correlation function (the volume dipole auto-correlation function) commonly used in this determination involves particles residing solely within a sphere of radius R, unlike the correct correlation function which involves either a single particle with those particles in a spherical volume of radius R(K) or those particles in a spherical volume of radius R with those residing within a spherical volume of radius R+R(K). A procedure is suggested for extracting the infinite system dipole-dipole correlation function from results of simulations performed on finite spherical samples. Using some results reported in the recent literature, relative to the accurate correlation function the commonly used correlation function ranges from 27% too small for a sphere having a radius comparable to the Kirkwood correlation length to 4% too small at a radius of seven times that correlation length. As a result, the apparent dielectric constants, as determined by the conventional procedure of using the fluctuations of the sum of dipoles in a finite fixed volume, are also too small. This suggests that a dielectric constant extracted from computer simulations using a total dipole-total dipole correlation function in a given volume with other geometries and/or boundary conditions will result in similar errors.     

Structure and adsorption of water in nonuniform cylindrical nanopores

Grand canonical Monte Carlo simulations are used to examine the adsorption and structure of water in the interior of cylindrical nanopores in which the axial symmetry is broken either by varying the radius as a function of position along the pore axis or by introducing regions where the characteristic strength of the water-nanopore interaction is reduced. Using the extended simple point charge (SPC∕E) model for water, nanopores with a uniform radius of 6.0 A? are found to fill with water at chemical potentials approximately 0.5 kJ∕mol higher than the chemical potential of the saturated vapor. The water in these filled pores exists in either a weakly structured fluidlike state or a highly structured uniformly polarized state composed of a series of stacked water clusters with pentagonal cross sections. This highly structured state can be disrupted by creating hydrophobic regions on the surface of the nanopore, and the degree of disruption can be systematically controlled by adjusting the size of the hydrophobic regions. In particular, hydrophobic banded regions with lengths larger than 9.2 A? result in a complete loss of structure and the formation of a liquid-vapor coexistence in the tube interior. Similarly, the introduction of spatial variation in the nanopore radius can produce two condensation transitions at distinct points along the filling isotherm.     

Preparation and Stabilization of Emulsifier‐free Macromolecule Nanoparticle Latex Particles

By using acetone‐water as the medium and potassium persulfate (KFS) as the initiator, emulsifier‐free emulsion co‐polymerization of styrene with co‐monomers: MMA, BMA, EA and BDA under microwave irradiation has been investigated. The influence of the each co‐monomer content and hydrophilicity on the hydrodynamic radius R_h of the synthesized copolymer nanoparticles is discussed in detail. The results show mat the increase in ratio of hydrophilic copolymer causes an increase in consumption of the initiator in the initiation reaction, so an increase in the concentration of the surface ? OSO₃ groups which cause the increase in the stability of the latex and gives rise to the formation of smaller micelles. The nano‐particle radius will increase again when the co‐monomer content increases to a certain degree. The curve of the particle hydrodynamic radius vs the co‐monomer content has a minimum point.     

Separating structural heterogeneities from stochastic variations in fluorescence resonance energy transfer distributions via photon distribution analysis

We establish a probability distribution analysis (PDA) method for the analysis of fluorescence resonance energy transfer (FRET) signals to determine with high precision the originating value of a shot-noise-limited signal distribution. PDA theoretical distributions are calculated explicitly including crosstalk, stochastic variations, and background and represent the minimum width that a FRET distribution must have. In this way an unambiguous distinction is made between shot-noise distributions and distributions broadened by heterogeneities. This method simultaneously and effectively extracts highly resolved information from FRET distributions. The theoretical histograms match the exact profile of histograms generated from constant transfer efficiency experimental data with a chi2 near unity. The chi2 surface suggests an ultimate level of precision with FRET of < 1% of the F?rster radius. Distributions of FRET signals in donor-acceptor-labeled DNA were unambiguously identified as being broader than shot-noise variations could explain. A model describing a Gaussian distribution of distances was tested with the PDA method and demonstrated 5 A inhomogeneities due to dye motions. The capability of this method to recover quantitative information from FRET distributions has potential applications for studying molecular conformations and dynamics. Potential sources for artifacts such as acceptor photobleaching, spectrally different observation volumes, and fluctuations of the F?rster radius are ruled out.     

Characteristics of AOT Microemulsion Structure: a Small Angle X-ray Scattering Study

The structure of micelles has attracted renewed attention during the past decade years due to the widespread use of microemulsions in technology1 and life science1,2. To obtain information of the micellar structure, using X-ray scattering (SAXS) whose wavelength is close to the size of the micellar aggregates is one of the most direct method3-5 and SAXS has been proved to be very sensitive for the change of micellar composition6,7. SAXS experiments have been carried out to compare the st…     

Diffusion, sedimentation, and rheology of concentrated suspensions of core-shell particles

Short-time dynamic properties of concentrated suspensions of colloidal core-shell particles are studied using a precise force multipole method which accounts for many-particle hydrodynamic interactions. A core-shell particle is composed of a rigid, spherical dry core of radius a surrounded by a uniformly permeable shell of outer radius b and hydrodynamic penetration depth κ(-1). The solvent flow inside the permeable shell is described by the Brinkman-Debye-Bueche equation, and outside the particles by the Stokes equation. The particles are assumed to interact non-hydrodynamically by a hard-sphere no-overlap potential of radius b. Numerical results are presented for the high-frequency shear viscosity, η(∞), sedimentation coefficient, K, and the short-time translational and rotational self-diffusion coefficients, D(t) and D(r). The simulation results cover the full three-parametric fluid-phase space of the composite particle model, with the volume fraction extending up to 0.45, and the whole range of values for κb, and a/b. Many-particle hydrodynamic interaction effects on the transport properties are explored, and the hydrodynamic influence of the core in concentrated systems is discussed. Our simulation results show that for thin or hardly permeable shells, the core-shell systems can be approximated neither by no-shell nor by no-core models. However, one of our findings is that for κ(b - a) ? 5, the core is practically not sensed any more by the weakly penetrating fluid. This result is explained using an asymptotic analysis of the scattering coefficients entering into the multipole method of solving the Stokes equations. We show that in most cases, the influence of the core grows only weakly with increasing concentration.     

Puroindoline-a, a lipid binding protein from common wheat, spontaneously forms prolate protein micelles in solution

The self-assembly in solution of puroindoline-a (Pin-a), an amphiphilic lipid binding protein from common wheat, was investigated by small angle neutron scattering, dynamic light scattering and size exclusion chromatography. Pin-a was found to form monodisperse prolate ellipsoidal micelles with a major axial radius of 112 ± 4.5 ? and minor axial radius of 40.4 ± 0.18 ?. These protein micelles were formed by the spontaneous self-assembly of 38 Pin-a molecules in solution and were stable over a wide pH range (3.5-11) and at elevated temperatures (20-65 °C). Pin-a micelles could be disrupted upon addition of the non-ionic surfactant dodecyl-β-maltoside, suggesting that the protein self-assembly is driven by hydrophobic forces, consisting of intermolecular interactions between Trp residues located within a well-defined Trp-rich domain of Pin-a.     

New relationships connecting the dipole polarizability, radius, and second ionization potential for atoms

The atomic dipole polarizability α of the 101 elements from He to No is related to the second ionization potential I? and the Waber-Cromer radius r(WC). Our recommended model is the function α = P?·I??? + P?·r(WC)(3) I?(y). With the parameters P? = 2.26, P? = 3.912, and y = 0.439, it reproduces the polarizabilities of all 101 elements with a mean absolute deviation of 7.5 au.     

Formation of Concentrated Nanoemulsions by Extreme Shear

Abstract

We have systematically investigated the production of “nanoemulsions,” droplets of one liquid phase in another immiscible liquid phase that have diameters less than 100 nm. Our approach relies on a combination of extreme shear due to multipass, high‐pressure microfluidic injection and systematic control of the emulsion's composition. By repeatedly shearing a silicone oil‐in‐water emulsion in an inhomogeneous extensional shear flow, the multipass approach enables us to reduce the droplet polydispersity and average radius. Using dynamic light scattering, we study the changes in the average radius, ?a?, as a function of the number of passes, driving injection pressure (i.e., shear rate), droplet volume fraction, surfactant concentration, and droplet oil viscosity. The smallest nanoemulsion that we obtain has ?a?=18 nm. At large droplet volume fractions φ≥0.65, we observe phase inversion, rather than a reduction in the droplet size. This provides evidence that droplet coalescence can occur during extreme shear, even when a significant excess of a strongly stabilizing surfactant is present.     

Photophysical study of bis(naphthalimide)-amine conjugates: toward molecular design of excimer emission switching

The fluorescence properties of two bis(1,8-naphthalimides) with amino-containing spacers are investigated, giving special emphasis to the observation of excimer emission. It is found that a minor elongation of the spacer by two methylene units gives rise to a quantitative shut-down of the broad and red-shifted excimer emission. Furthermore, a switching of this emission is established through manipulation of a photoinduced electron transfer process, which involves the amino spacer. Protons as well as protic solvents lead to substantial excimer emission with lifetimes of 12 to 27 ns. The excimer quantum yield takes a maximum value of Φ(f) = 0.07 (acetonitrile with 1 equiv trifluoroacetic acid). The increased virtual Stokes shifts (ca. 150 nm) as compared to the fluorescence of monomeric 1,8-naphthalimides are an alternative approach to obtain colored, significant, and long-lived fluorescence from these chromophores. As an additional excited state pathway, the occurrence of homo-Fo?rster resonance energy transfer (homo-FRET) is established by fluorescence polarization measurements and calculation of the corresponding critical Fo?rster radius (R(0) ca. 13 ?). The average interchromophore distance between the naphthalimides is estimated as 7.5 ? and 9.5 ? for the dyad with the shorter and the longer spacer, respectively. These observations and the absence of a rise time component for excimer emission are in agreement with the formation of a "loose" ground state dimer, which upon excitation undergoes a fast geometrical adjustment to the excimer structure where the chromophores are at contact distance.     

Conformational preference of polyglycine in solution to elongated structure

Do polypeptide chains ever behave like a random coil? In this report we demonstrate that glycine, the residue with the fewest backbone restrictions, exhibits a strong preference for an extended conformation in solution when polymerized in short segments of polyglycine. A model peptide system comprised of two unique tripeptide units, between which 1 to 18 glycine residues are inserted, is characterized by NMR and by small-angle X-ray scattering (SAXS). The residual dipolar coupling (RDC) values of the two tripeptide units are insensitive to changes in number of intervening glycines, suggesting that extension of the linker does not alter the average angular relationship between the tripeptides. Polyglycine segments longer than nine residues form insoluble aggregates. SAXS measurements using synchrotron radiation provide direct evidence that polyglycine peptides adopt elongated conformations. In particular, the construct with a linker with six glycines showed a scattering profile indicative of a monomeric state with a radius of gyration and the maximum dimension of 9.1 A and approximately 34 A, respectively. The ensemble averaged global structure of this 12-mer peptide can best be approximated by a cylinder with a radius of 4 A and a length of approximately 33 A, making it intermediate in extension between a beta strand and an alpha helix.     

Derivation of the linear relationship between SWCNTs functionalization energies and sidewall curvature

A simple linear relationship between the functionalization reaction energies for the exohedral monovalent addition on the surface of an ideal, infinitely long, single-walled carbon nanotube (SWCNT) and the reciprocal SWCNT radius has been derived employing the hard?Csoft acid basis principle and the tight binding model. The slope of the derived linear relationship is a function of the effective number of valence electrons involved in the functionalization reaction. The intercept of the derived linear relationship, equal to the reaction energies on a planar graphite surface, is a function of the electrophilicity of the monovalent addend and of the condensed Fukui function of its reacting atom. The theoretical predictions of this simple formula are coherent with the computational density functional theory data reported in the literature.     

Novel "lipid-flow chip" configuration to determine donor-to-acceptor ratio-dependent fluorescence resonance energy transfer efficiency

We report on the determination of fluorescence resonance energy transfer (FRET) efficiency, which is dependent on the donor-to-acceptor (D-A) ratio, by using a new type of microchannel device called a "lipid-flow chip". The chip comprises two supported lipid bilayers (SLBs) that self-spread from either side of 10 microm wide straight lines and carry molecules embedded in them. We first show that the diffusion process that occurs when the two SLBs collide with each other in the channel and form a unified SLB can be expressed by a one-dimensional diffusion equation. Next we describe a method for determining the FRET efficiency between NBD (donor) and Texas Red (acceptor) from observations using the lipid-flow chip by employing a one-dimensional diffusion model. The advantages of our method are that all the D-A ratios are achieved in one chip, and a large number of data are recorded in one chip. The FRET efficiency varies depending on the D-A ratio under conditions whereby the concentration of the sum of the donors and acceptors is constant. The F?rster radius is also estimated from our results using a known model describing two-dimensional FRET systems, which yields a radius consistent with the previously reported value for NBD and Texas Red.     

A scattering study on intermicellar interactions and structures of polymeric micelles

The micellar structure is usually considered to be composed of a hard sphere (liquid) core and a heavily solvated corona. Therefore, the correction for intermicellar interactions at finite concentrations can be relatively complicated. In this article, small-angle neutron scattering of a copolymer, known as Pluronic L64 (PEO₁₃PPO₃₀PEO₁₃), in o-xylene in the presence of D₂O is used to demonstrate that, based on the hard sphere approximation, intermicellar interactions can be corrected by representing the micellar size as having an equivalent hard sphere radius. The procedure remains valid even if the micellar shape becomes asymmetric, with axial ratios of 3 ? 4. For the present system, the equivalent hard sphere radius corresponds to the micellar core radius plus one-half of the micellar shell thickness. With the equivalent hard sphere approach, the scattering behavior of the micelle could be described by using a core-shell structure. © 1994 John Wiley & Sons, Inc.     

Impact of Na- and K-C pi-interactions on the structure and binding of M3(sol)n(BINOLate)3Ln catalysts

Shibasaki's heterobimetallic complexes M3(THF)n(BINOLate)3Ln [M = Li, Na, K; Ln = lanthanide(III)] are among the most successful asymmetric Lewis acid catalysts. Why does M3(THF)n(BINOLate)3Ln readily bind substrates when M = Li but not when M = Na or K? Structural studies herein indicate Na- and K-C cation-pi interactions and alkali metal radius may be more important than even lanthanide radius. Also reported is a novel polymeric [K3(THF)2(BINOLate)3Yb]n structure that provides the first evidence of interactions between M3(THF)n(BINOLate)3Ln complexes.     

How big are atoms in crystals?

Atoms and bonds are central concepts in structural chemistry, but neither are concepts that arise naturally from the physics of condensed phases. It is ironic that the internuclear distances in crystals that are readily measured depend on the sizes of atoms, but since atoms in crystals can be defined in many different ways, all of them arbitrary and often incompatible, there is no natural way to express atomic size. I propose a simple coherent picture of Atoms-in-Crystals which combines properties selected from three different physically sound definitions of atoms and bonds. The charge density of the free atom that is used to construct the procrystal is represented by a sphere of constant charge density having the quantum theory of atoms in molecules (QTAIM) bonded radius. The sum of these radii is equal to the bond length that correlates with the bond flux (bond valence) in the flux theory of the bond. The use of this model is illustrated by answering the question: How big are atoms in crystals? The QTAIM bonded radii are shown to be simple functions of two properties, the number of quantum shells in the atomic core and the flux of the bond that links neighbouring atoms. Various radii can be defined. The univalent bonded radius measures the intrinsic size of the atom and is the same for all cations in a given row of the periodic table, but the observed bonded radius depends also on the bond flux that reflects the chemical environment.