Formation of branched calixarene aggregates-a time-resolved static light scattering study

Mixtures of a calix[4]arene and a naphthyridine derivative dissolved in 1,2-dichlorobenzene form thermoreversible aggregates. The aggregation process was followed by means of time-resolved multiangle light scattering at two different mixing ratios, 1:3 and 1:4, yielding a detailed record of the relative mass, the radius of gyration, and the particle scattering function of the growing aggregates. On the basis of these data, a conclusive model of the structure is presented for the developing aggregates: monomers aggregate to wormlike filaments which form branching points. Formation of branching points proceeds in a frequency and distribution which is similar to the polycondensation of ABC monomers toward non-randomly branched macromolecules (Burchard, W. Macromolecules 1977, 10, 919-927). Thus, aggregation results in hyperbranched-like particles with striking analogies to the polymerization of glucose to amylopectin.     

Bayesian‐type count data models with varying coefficients: estimation and testing in the presence of overdispersion

In this paper we study varying‐coefficient models for count data. A Bayesian approach is taken to model the variability of the regression parameters. Based on a Kalman filter procedure the varying coefficients are estimated as the mode of the posterior distribution. All hyperparameters, including an overdispersion parameter in the negative binomial varying‐coefficient model (NBVC), are estimated as ML‐estimators using an EM‐type algorithm. A bootstrapping test of the fixed‐coefficient hypothesis against a varying‐coefficient alternative is proposed, which is evaluated running a simulation study. The study shows that the choice of a suitable count data model is of special importance in the framework of varying‐coefficient models. The methodology is illustrated analysing the determinants of the number of individual doctor visits. Copyright © 2001 John Wiley & Sons, Ltd.     

Surface structure analysis of thin dewetted polymer blend films

The surface structure of thin polymer blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) after annealing above the glass transition temperature was investigated. With scanning force microscopy (SFM) the surface topography originated by a dewetting process is detected. The sample surface is covered with small droplets consisting of several polymer molecules. Utilizing grazing incidence small angle neutron scattering (GISANS) the topographical information as well as the in‐plane composition is probed. For thin confined blend films a substructure of the droplets resulting from an additional phase separation process at different length scales is detected.     

Probing Multiconfigurational States by Spectroscopy: The Cerium XAS L3-edge Puzzle

The Ce L₃ edge XAS spectra of CeO₂ and cerocene [Ce(C₈H₈)₂] were calculated with relativistic ab-initio multireference wavefunction approaches capable of reproducing the observed spectra accurately. The study aims to resolve the decades-long puzzle regarding the relationship between the number and relative intensities of the XAS peaks and the 4f electron occupation in the ground state (GS) versus the core-excited states (ESs). CeO₂ and cerocene exemplify the different roles of covalent bonding and wavefunction configurational composition in the observed intensity patterns. Good agreement is found between the calculated GS 4f-shell occupations and the value derived from XAS measurements using peak areas (n_f). The identity of the two-peaked Ce L₃ edge is fully rationalized from the perspective of the relaxed wavefunctions for the GS and core ESs. The states underlying the different peaks differ from each other in a surprisingly simple way that can be associated with 4f¹ vs. 4f⁰ sub-configurations. Furthermore, part of one of the cerocene spectral peaks is associated with 4f² sub-configurations. The pattern therefore reveals excited states that can be interpreted in terms of Ce IV and III oxidation numbers, as long assumed, with Ce II states additionally appearing in the cerocene spectrum. While this work demonstrates the rough accuracy of the conventional approach to determining n_f from Ce L₃-edge XAS, limitations are highlighted in terms of the ultimate accuracy of this approach and the potential of observing new types of excited states. The need to determine the sources of n_f by calculations, is stressed.     

Molecular QTAIM Topology Is Sensitive to Relativistic Corrections

The topology of the molecular electron density of benzene dithiol gold cluster complex Au₄−S−C₆H₄−S′−Au′₄ changed when relativistic corrections were made and the structure was close to a minimum of the Born–Oppenheimer energy surface. Specifically, new bond paths between hydrogen atoms on the benzene ring and gold atoms appeared, indicating that there is a favorable interaction between these atoms at the relativistic level. This is consistent with the observation that gold becomes a better electron acceptor when relativistic corrections are applied. In addition to relativistic effects, here, we establish the sensitivity of molecular topology to basis sets and convergence thresholds for geometry optimization.     

Correction to: Quantitative analysis of PET microplastics in environmental model samples using quantitative 1H-NMR spectroscopy: validation of an optimized and consistent sample clean-up method

Analytical and Bioanalytical Chemistry - The original version of this article contained a mistake.     

Tuning the excitonic and plasmonic properties of copper chalcogenide nanocrystals

The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.