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Time-resolved fluorescence anisotropy measurements for a simple 2-substituted anthracene derivative (2-An-M) as a function of concentration in poly(methyl methacrylate) (PMMA) films is described. The results show an increased depolarization rate for samples containing a higher fraction of polymers labeled at the junction with anthracene chromophores.

The results of oxygen quenching experiments for four phosphorescent dyes [platinum octaethylporphyrin (PtOEP), platinum octaethylporphyrin ketone (PtOEPK), platinum tetrakis (pentafluorophenyl) porphyrin (PtTFPP), and Ru (dpp) 3Cl2 (dpp + 4,7-diphenyl-1, 10-phenanthroline)] in a low-glass-transition-temperature polymer matrix [poly [((n-butylamino) thionyl) phosphazene] (C4PATP) is compared. It is seen that oxygen diffusion in C4PATP obeys Fick's laws with diffusion coefficient of (3.7 + - 1.0) x 10 to the power -6 cm square s (sub -1).

The micelle exchange kinetics of hydrophobic pyrene derivatives between sodium dodecyl sulfate (SDS) and micelles are described in the presence of moderate concentrations of sodium counterions. The exchange rate is not very sensitive to the concentration of SDS micelles, except as the SDS concentration affects the ionic strength, which indicates that the kinetics are dominated by a first-order process.

Pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments are used for studying ligand exchange between poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMA) and trioctylphosphine oxide (TOPO) bound to the surface of CdSe/TOPO quantum dots (QDs). The studies have shown that only DMA units are directly bound to the surface and the remaining groups are present as loops or tails that protrude into the solvent and enhance the hydrodynamic diameter of the particles.

Monte Carlo simulations of resonance energy transfer experiments were studied for immobile donor and acceptor dyes confined to planar, cylindrical, and spherical restricted geometries. Euclidean spaces were examined in which the restricted geometric was a thin planar slab, a thin cylinder or cylindrical shell, or a thin spherical shell to investigate the magnitude of the coupling of the orientation parameter with the donor-acceptor separation.

Experiments that employ direct resonance energy transfer (DET) to obtain information about distances or domain sizes in polymer systems require independent information about the magnitude of the characteristic (Forster) energy transfer distance R(sub o). The experiments which determine R(sub o) values by both methods for a series of Phe-An donor-acceptro pairs in poly(methyl methacrylate) and polystyrene films are described.

The effect of concentration on photoinduced flocculation and aggregation of a nonaqueous dispersion of core-shell nanoparticles, which consist of a tightly cross-linked core composed of poly, is described. The study found that for the three different concentrations that measured average radius of gyration during aggregation scaled with time.

Isothermal titration calorimetry (ITC) is a technique to measure the stoichiometry and thermodynamics from binding experiments. Identifying an appropriate mathematical model to evaluate titration curves of receptors with multiple sites is challenging, particularly when the stoichiometry or binding mechanism is not available. In a recent theoretical study, we presented a differential binding model (DBM) to study calorimetry titrations independently of the interaction among the binding sites (Herrera, I.; Winnik, M. A. J. Phys. Chem. B 2013, 117, 8659-8672). Here, we build upon our DBM and show its practical application to evaluate calorimetry titrations of receptors with multiple sites independently of the titration direction. Specifically, we present a set of ordinary differential equations (ODEs) with the general form d[S]/dV that can be integrated numerically to calculate the equilibrium concentrations of free and bound species S at every injection step and, subsequently, to evaluate the volume-normalized heat signal (δQV = δq/dV) of direct and reverse calorimetry titrations. Additionally, we identify factors that influence the shape of the titration curve and can be used to optimize the initial concentrations of titrant and analyte. We demonstrate the flexibility of our updated DBM by applying these differentials and a global regression analysis to direct and reverse calorimetric titrations of gadolinium ions with multidentate ligands of increasing denticity, namely, diglycolic acid (DGA), citric acid (CIT), and nitrilotriacetic acid (NTA), and use statistical tests to validate the stoichiometries for the metal-ligand pairs studied. [ABSTRACT FROM AUTHOR]

Studies of micelle formation by three samples of polyisoprene-beta-poly(methyl methacrylate) in acetonitrile are reported.

The dynamics of energy transfer (ET) for a lamellar block copolymer system in which donor dyes have been attached to the block junction has been studied. The research seeks to find out if ET can determine the shape of a polymer-polymer interface and discover the limits in the quantitative recovery of system parameters. Results indicate ET experiments can give reliable estimates of the ratio of the period spacing to interface thickness but not the individual parameters H and delta. H can be determined with scattering techniques while delta can be estimated with an accuracy better than 5%.

We present a set of model-independent differential equations toanalyze isothermal titration calorimetry (ITC) experiments. In contrastwith previous approaches that begin with specific assumptions aboutthe number of binding sites and the interactions among them (e.g.,sequential, independent, cooperative), our derivation makes more generalassumptions, such that a receptor with multiple sites for one typeof ligand species (homotropic binding) can be studied with the sameanalytical expression. Our approach is based on the binding polynomialformalism, and the resulting analytical expressions can be extendedto account for any number of binding sites and any type of bindinginteraction among them. We refer to the set of model-independent differentialequations to study ITC experiments as a differential binding model(DBM). To demonstrate the flexibility of our DBM, we present the analyticalexpressions to study receptors with one or two binding sites. TheDBM for a receptor with one site is equivalent to the Wiseman isothermbut with a more intuitive representation that depends on the bindingpolynomial and the dimensionless parameter c= K·MT, where Kis the bindingconstant and MTthe total receptor concentration.In addition, we show how to constrain the general DBM for a receptorwith two sites to represent sequential, independent, or cooperativebinding interactions between the sites. We use the sequential bindingmodel to study the binding interaction between Gd(III) and citrateanions. In addition, we simulate calorimetry titrations of receptorswith positive, negative, and noncooperative interactions between thetwo binding sites. Finally, we derive a DBM for titrations of receptorswith n-independent binding sites. [ABSTRACT FROM AUTHOR]

Experiments that employ direct resonance energy transfer (DET) to obtain information about distances or domain sizes in polymer systems require independent information about the magnitude of the characteristic (Förster) energy transfer distance R0. Values of R0 are relatively straightforward to obtain by the traditional spectral overlap method (R0SO) for dyes in fluid solution, but are much more difficult to obtain for dyes in rigid polymer films. Here one can obtain a value for R0 as a fitting parameter (R0FF) for donor fluorescence decay experiments for samples containing a random distribution of donor and acceptor dyes in the polymer film. In previous experiments from our group, we needed values of R0 for various phenanthrene (Phe, donor) and anthracene (An, acceptor) derivatives. In this paper, we describe experiments which determine R0 values by both methods for a series of Phe-An donor−acceptor pairs in poly(methyl methacrylate) and polystyrene films. Both the location of substituents on the donor and acceptor as well as the choice of the medium had an effect on the measured R0, which varied between 2.0 and 2.6 nm. We also ascertained that there is some unknown factor, also prevalent in the work of others, which results in the Förster radius being larger when determined by the Förster fit method than by the method of spectral overlap. [ABSTRACT FROM AUTHOR]

Scattering techniques (i.e., static light scattering, small angle neutron scattering,11 or small angle X-ray scattering) are excellent tools to study nanoscopic objects in solution. However, to interpret the experimental data, one needs to use the appropriate form factor. While recent progress has been made in the writing of form factors for complex structures, there is still a need to develop a method to evaluate the form factor of inhomogeneous elongated scatterers. Here, we propose an approach based on the principle of "Russian dolls". Multiblock rods are represented as multi generations of rods (mother, daughter, granddaughter, etc.), where each rod is nested within the rod of the previous generation, like Russian dolls. A shift parameter is used to introduce asymmetry in the rod along its long axis. This approach not only allowed us to write the form factor of multiblock rods, but it also gave us the possibility to account for the polydispersity in length of each block and of the shift parameter. Finally, we applied these equations to the case of a series of solutions of triblock comicelles slightly polydisperse in length.

We describe simultaneous static (SLS) and dynamic light scattering (DLS) measurements on dilute solutions of a series of poly(ferrocenyldimethylsilane-b-isoprene) (PFS(50)-PI(1000)) block copolymer micelles of uniform length in tert-butyl acetate (tBA) and in decane. The subscripts in the term PFS(50)-PI(1000) refer to the mean degree of polymerization of each block. The SLS experiments show that in both solvents the micelles formed are elongated and rigid. We also observed that the large length of the PI block (1000 units) contributes to the SLS signal. From the SLS data, we calculated the mass per unit length (linear aggregation number), as well as the cross section of the micelles in both solvents. Interestingly, the linear aggregation number and the micelle cross sections, as deduced by SLS, were the same in decane and in tBA. However, the fitting of DLS data indicates that the hydrodynamic cross section of the micelles in tBA is much larger than that in decane, and both values are larger than the values determined by SLS. We hypothesize that the difference between cross sections deduced from SLS and DLS data fitting is related to the shape of the segment density profile of the corona. In tBA, the PI chains are more stretched than in decane, increasing the hydrodynamic radius of the micelle cross section.

A series of poly(ethylene oxide)s labeled at both ends with pyrene, (PEO(X)-Py(2), where X represents the number average molecular weight (M(n)) of the PEO chains and equals 2, 5, 10, and 16.5 K) was prepared together with one-pyrene-monolabeled PEO (PEO(2K)-Py). The process of end-to-end cyclization (EEC) was investigated by monitoring intramolecular excimer formation in seven organic solvents with viscosities (η) ranging from 0.32 to 1.92 mPa·s. The steady-state fluorescence spectra showed that excimer formation of PEO(X)-Py(2) decreased strongly with increasing η and M(n). The monomer and excimer time-resolved fluorescence decays were analyzed according to the traditional Birks' scheme. Birks' scheme analysis indicated that the decrease in excimer formation with increasing M(n) and η was due partly to a decrease in the rate constant of EEC, but most importantly, to a large increase in the fraction of pyrenes that did not form excimer (f(Mfree)). This result is in itself incompatible with Birks' scheme analysis which requires that f(Mfree) be the molar fraction of chains bearing a single pyrene at one chain end; in short, f(Mfree) does not depend on M(n) and η within the framework of Birks' scheme analysis. In turn, this unexpected result agrees with the framework of the fluorescence blob model (FBM) which predicts that quenching takes place inside a blob, which is the finite volume probed by an excited chromophore during its lifetime. Increasing M(n) and η results in a larger fraction of chains having a conformation where the quencher is located outside the blob resulting in an increase in f(Mfree). Equations were derived to apply the FBM analysis, originally designed to study randomly labeled polymers, to the end-labeled PEO(X)-Py(2) series. FBM analysis was found to describe satisfyingly the data obtained with the longer PEO(X)-Py(2) samples.

Pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments have been used to examine ligand exchange between poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMA) (Mn = 12,000, Mw/Mn = 1.20, Nn = 78) and trioctylphosphine oxide (TOPO) bound to the surface of CdSe/TOPO quantum dots (QDs). We show that PFG 1H NMR can quantify the displacement of TOPO by PDMA through its ability to differentiate signals due to TOPO bound to the QDs versus those from TOPO molecules free in solution. For CdSe QDs with a band edge absorption maximum at 558 nm (diameter 2.7 nm by transmission electron microscopy), we determined that, at saturation, 8 polymer chains on average displace greater than 90% of the surface TOPO groups. At partial saturation, with an average of 6 polymer chains/QD, each TOPO displaced requires 28 DMA repeat units. Assuming that one Me2N- group binds to a surface Cd2+ for each TOPO displaced, we infer that only about 3% of the DMA units are directly bound to the surface. The remaining groups are present as loops or tails that protrude into the solvent and increase the hydrodynamic diameter of the particles.

We describe time-resolved fluorescence anisotropy measurements for a simple 2-substituted anthracene derivative (2-An-M) as a function of concentration in poly(methyl methacrylate) (PMMA) films. The anisotropy decays via energy migration among 2-An-M molecules in the matrix. The data were interpreted in terms of the survival probability of the initially excited chromophores and are in good agreement with the plot predicted using R0 = 1.8 nm, the value of the Förster radius determined independently by the spectral overlap method. For all concentrations, we found r0 = 0.20 +/- 0.01 for the initial anisotropy at time zero, whereas the residual anisotropy, r(infinity), was concentration-dependent and higher in magnitude than the theoretically predicted value, 4% of r0. The higher residual anisotropy values may originate from the possibility that not all of the anthracene molecules are involved in the energy migration process. Similar anthracene anisotropy experiments were performed on anthracene-labeled poly(isoprene-b-methyl methacrylate) (PI-PMMA). The results show an increased depolarization rate for samples containing a higher fraction of polymers labeled at the junction with anthracene chromophores.

We describe Monte Carlo simulations of resonance energy transfer (RET) experiments for immobile donor (D) and acceptor (A) dyes confined to planar, cylindrical, and spherical restricted geometries. We compare values of the quantum efficiency (PhiET) evaluated through consideration of individual donor-acceptor pairs, with values calculated assuming a pre-averaged value of the orientation parameter /kappa/2 = 0.476 appropriate for infinite three dimensional (3D) space. For dyes confined to restricted geometries where the length scale of the confining dimension is less than or equal to the Förster radius R0, the coupling of the orientation parameter and the donor-acceptor distance becomes noticeable. Values of Phi(ET) obtained by proper consideration of the orientation parameter are smaller than those calculated using /kappa/2 = 0.476. We use this Monte Carlo method to reanalyze the fluoresce decay measured from dye-labeled poly(isoprene-b-methyl methacrylate) diblock copolymer with lamellar structure,(1) from which the interface thickness for PI-PMMA lamella can be retrieved. We found the retrieved interface thickness is sensitive to the choice of dipole orientation. If all dipoles in the confined polymer interface have a random orientation, the value of interface thickness was found to be 0.9 +/- 0.2 nm through consideration of individual dipole orientations. Assumption of /kappa/2 = 0.476 in the FRET calculations leads to a larger value of interface thickness (1.3 +/- 0.2 nm) due to the neglect of the coupling between dipole orientation and D-A distance for the dyes confined to lamellar interfaces.

Experiments that employ direct resonance energy transfer (DET) to obtain information about distances or domain sizes in polymer systems require independent information about the magnitude of the characteristic (Förster) energy transfer distance R(0). Values of R(0) are relatively straightforward to obtain by the traditional spectral overlap method (R(0)(SO)) for dyes in fluid solution, but are much more difficult to obtain for dyes in rigid polymer films. Here one can obtain a value for R(0) as a fitting parameter (R(0)(FF)) for donor fluorescence decay experiments for samples containing a random distribution of donor and acceptor dyes in the polymer film. In previous experiments from our group, we needed values of R(0) for various phenanthrene (Phe, donor) and anthracene (An, acceptor) derivatives. In this paper, we describe experiments which determine R(0) values by both methods for a series of Phe-An donor-acceptor pairs in poly(methyl methacrylate) and polystyrene films. Both the location of substituents on the donor and acceptor as well as the choice of the medium had an effect on the measured R(0), which varied between 2.0 and 2.6 nm. We also ascertained that there is some unknown factor, also prevalent in the work of others, which results in the Förster radius being larger when determined by the Förster fit method than by the method of spectral overlap.