Ru-Pt and Ru-Pd heterobimetallic complexes based on a new ligand with two distinct chelate sites

A new ligand p-[N-2-(2'-pyridyl)benzimidazolyl]-[N-2-(2'-pyridyl)indolyl]-benzene (L1) has been synthesized and fully characterized. L1 has two distinct chelating sites: one N,N-chelate site and one N,C-chelate site. This ligand has been found to be very effective in selective binding to two different metal ions. Two new heterobimetallic complexes Ru-Pt and Ru-Pd using L1 as the bridging ligand have been successfully synthesized and fully characterized. To understand the mutual influence of the two metal centers on electronic and photophysical properties, the corresponding monometallic Ru(II), Pt(II) and Pd(II) compounds have also been synthesized and investigated. All Ru(II)-containing complexes have been found to be luminescent. Electronic communication between the two different metal centers in the heterobimetallic compounds was found to be weak. The Pt(II) moiety appears to enhance the phosphorescent efficiency of the Ru(II) unit while the Pd(II) analogue has little influence.     

Atomistic band gap engineering in donor-acceptor polymers

We have synthesized a series of cyclopentadithiophene-benzochalcogenodiazole donor-acceptor (D-A) copolymers, wherein a single atom in the benzochalcogenodiazole unit is varied from sulfur to selenium to tellurium, which allows us to explicitly study sulfur to selenium to tellurium substitution in D-A copolymers for the first time. The synthesis of S- and Se-containing polymers is straightforward; however, Te-containing polymers must be prepared by postpolymerization single atom substitution. All of the polymers have the representative dual-band optical absorption profile, consisting of both a low- and high-energy optical transition. Optical spectroscopy reveals that heavy atom substitution leads to a red-shift in the low-energy transition, while the high-energy band remains relatively constant in energy. The red-shift in the low-energy transition leads to optical band gap values of 1.59, 1.46, and 1.06 eV for the S-, Se-, and Te-containing polymers, respectively. Additionally, the strength of the low-energy band decreases, while the high-energy band remains constant. These trends cannot be explained by the present D and A theory where optical properties are governed exclusively by the strength of D and A units. A series of optical spectroscopy experiments, solvatochromism studies, density functional theory (DFT) calculations, and time-dependent DFT calculations are used to understand these trends. The red-shift in low-energy absorption is likely due to both a decrease in ionization potential and an increase in bond length and decrease in acceptor aromaticity. The loss of intensity of the low-energy band is likely the result of a loss of electronegativity and the acceptor unit's ability to separate charge. Overall, in addition to the established theory that difference in electron density of the D and A units controls the band gap, single atom substitution at key positions can be used to control the band gap of D-A copolymers.     

Structural and algorithmic properties for parametric minimum cuts

We consider the minimum s, t-cut problem in a network with parametrized arc capacities. Following the seminal work of Gallo et?al. (SIAM J. Comput. 18(1):30–55, 1989), classes of this parametric problem have been shown to enjoy the nice Structural Property that minimum cuts are nested, and the nice Algorithmic Property that all minimum cuts can be computed in the same asymptotic time as a single minimum cut by using a clever Flow Update step to move from one value of the parameter to the next. We present a general framework for parametric minimum cuts that extends and unifies such results. We define two conditions on parametrized arc capacities that are necessary and sufficient for (strictly) decreasing differences of the parametric cut function. Known results in parametric submodular optimization then imply the Structural Property. We show how to construct appropriate Flow Updates in linear time under the above conditions, implying that the Algorithmic Property also holds under these conditions. We then consider other classes of parametric minimum cut problems, without decreasing differences, for which we establish the Structural and/or the Algorithmic Property, as well as other cases where nested minimum cuts arise.     

Experimental demonstration of subtleties in subharmonic intermittency

Experiments on the Belousov-Zhabotinsky reaction in a well-stirred flow reactor elucidate one of the routes to chaos, subharmonic intermittency (type III intermittency). Measurements conducted as a function of two independent control parameters demonstrate how difficult it is in practice to distinguish this route to chaos from subcritical period doubling transitions with similar chaotic time signatures. Necessary criteria for the establishment of a second order (continuous) transition, such as the absence of hysteresis, are discussed in the context of one-dimensional maps.     

Selective determination of organofluorine compounds by capillary column gas chromatography with an atmospheric pressure helium microwave-induced plasma detector

Fluorinated analogs of compounds typical of those found in metabolic and other biological studies are detected with high selectivity using a gas chromatograph/microwave-induced plasma detector (GC-MIPD), which permits fluorine-selective detection by monitoring the emission at 685.6 nm. Using the described atmospheric pressure helium-sustained plasma detector, the minimum detectable level, fluorine selectivity (relative to carbon), and linear dynamic range of this GC-MIPD system were determined to be 4.8 pg-F/s, 1060, and 5000, respectively. The utility of this GC-MIPD system for the selective detection of organofluorine compounds is demonstrated by its application in the analysis of the metabolic fate of a fluorinated substrate administered to a mixture of wheat germ phosphatase and potato apyrase, as well as by analysis of synthetic mixtures.     

Prediction of adsorption of xenon in zeolite NaA with molecular density functional theory

The Tarazona model of molecular density functional theory is used to determine the adsorption isotherm and the density distribution of xenon atoms in zeolite NaA. We consider the alpha cage of NaA to be fully three-dimensional and we introduce a basis set for the density distribution to make the solution computationally tractable. The results of this density functional theory model are compared to previous results of grand canonical Monte Carlo simulations for the same system.We dedicate this paper to Professor Herbert Wagner, whose excellent contributions to density functional theory typify his valuable role in the advancement of statistical physics     

Aqueous solution properties of pH‐responsive AB diblock acrylamido–styrenic copolymers synthesized via aqueous reversible addition–fragmentation chain transfer

Here we report the synthesis and solution characterization of a novel series of AB diblock copolymers with neutral, water‐soluble A blocks consisting of N,N‐dimethylacrylamide and pH‐responsive B blocks of N,N‐dimethylvinylbenzylamine. To our knowledge, this represents the first example of an acrylamido–styrenic block copolymer prepared directly in a homogeneous aqueous solution. The best blocking order [with poly(N,N‐dimethylacrylamide) as a macro‐chain‐transfer agent] yielded well‐defined block copolymers with minimal homopolymer impurities. The reversible aggregation of these block copolymers in aqueous media was studied with ¹H NMR spectroscopy and dynamic light scattering. Finally, an example of core‐crosslinked micelles was demonstrated by the addition of a difunctional crosslinking agent to a micellar solution of the parent block copolymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1724–1734, 2004     

More Than 12 % Polarization and 20 Minute Lifetime of 15N in a Choline Derivative Utilizing Parahydrogen and a Rhodium Nanocatalyst in Water

Hyperpolarization techniques are key to extending the capabilities of MRI for the investigation of structural, functional and metabolic processes in vivo. Recent heterogeneous catalyst development has produced high polarization in water using parahydrogen with biologically relevant contrast agents. A heterogeneous ligand‐stabilized Rh catalyst is introduced that is capable of achieving ¹⁵N polarization of 12.2±2.7 % by hydrogenation of neurine into a choline derivative. This is the highest ¹⁵N polarization of any parahydrogen method in water to date. Notably, this was performed using a deuterated quaternary amine with an exceptionally long spin‐lattice relaxation time (T₁) of 21.0±0.4 min. These results open the door to the possibility of ¹⁵N in vivo imaging using nontoxic similar model systems because of the biocompatibility of the production media and the stability of the heterogeneous catalyst using parahydrogen‐induced polarization (PHIP) as the hyperpolarization method.