Polymer-assisted dispersion of single-walled carbon nanotubes in alcohols and applicability toward carbon nanotube/sol-gel composite formation

Single-walled carbon nanotubes (SWCNTs) were directly dispersed into various alcohols by sonicating the nanotubes in the presence of poly(4-vinylpyridine) (P4VP). Depending upon the alcohol, it was possible to disperse up to 0.3 g of SWCNTs per liter of alcohol using only 0.6 g of P4VP, and with solution stability greater than 6 weeks. Scanning electron microscopy of "bucky" paper prepared from the polymer-treated nanotubes revealed reduced bundle size compared to the corresponding untreated nanotube paper. Additionally, the applicability of the dispersion system in the formation of SWCNT/silica composites is demonstrated.     

Use of Ag(II) as a removable template in porphyrin chemistry: diol cleavage products of [meso-tetraphenyl-2,3-cis-diolchlorinato]silver(II)

The use of Ag^II as a removable template in synthetic porphyrin chemistry is described. Mild procedures for the insertion of Ag^II into chlorins and the demetallation of the [chlorinato]Ag^II complexes are delineated. The UV-vis spectra of the novel [chlorinato]Ag^II complexes are discussed. The diol cleavage products of [meso-tetraphenyl-2,3-diolchlorinato]silver(II) under a number of conditions are characterized and compared to those resulting from the cleavage of the corresponding free base diol chlorin or its Ni^II complex, highlighting the unique templating effect of Ag^II. The scopes and limits of electrospray ionization mass spectrometry (ESI-MS) for the analysis of Ag^II chlorins is described. The use of Ag^II as a templating metal is superior over Ni^II or Zn^II for the preparation of free base pyrrole-modified porphyrins along metal templated pathways.     

Surface-confined nickel mediated cross-coupling reactions: characterization of initiator environment in Kumada catalyst-transfer polycondensation

Kumada catalyst-transfer polycondensation (KCTP) has proven to be an excellent strategy toward the synthesis of well-defined conjugated polymers. In this report, Ni(0) species are reacted with surface-bound aryl bromides to yield KCTP initiators of structure (aryl)Ni(II)-Br. Surface-confined Kumada reactions are carried out with a ferrocene functionalized Grignard reagent to quantify initiator coverage, ligand exchange, and Kumada reaction kinetics. In addition, surface-initiated Kumada catalyst-transfer polycondensation (SI-KCTP) is carried out from the fabricated initiators to modify SiO(2) and ITO surfaces. Uniform poly(3-methylthiophene) films with thicknesses between 40 and 65 nm were characterized using a variety of spectroscopic and electrochemical techniques.     

Anomalous ionic conductivity increase in Li2S + GeS2 + GeO2 glasses

Numerous studies of the ionic conductivities in oxide-doped chalcogenaide glasses have shown the anomalous result that the ionic conductivity actually increases significantly (by more than a factor of 10 in some cases) by the initial addition of an oxide phase to a pure sulfide glass. After this initial sharp increase, the conductivity then monotonically decreases with further oxide addition. While this behavior is important to the application of these glasses for Li batteries, no definitive understanding of this behavior has been elucidated. To examine this effect further and more completely, the ionic conductivities of 0.5Li(2)S + 0.5[(1 - x)GeS(2) + xGeO(2)] glasses have been measured on disc-type bulk glasses. The ionic conductivity of the 0.5Li(2)S + 0.5GeS(2) (x = 0) glass was observed to increase from 4.3 x 10(-5) (Omega cm)(-1) to 1.5 x 10(-4) (Omega cm)(-1) while the activation energy decreased to 0.358 eV from 0.385 eV by the addition of 5 mol % of GeO(2). Further addition of GeO(2) monotonically decreased the conductivity and increased the activation energy. On the basis of our previous studies of the structure of this glass system, the Anderson and Stuart model was applied to explain the decrease in the activation energy and increase in the conductivity. It is suggested that the "doorway" radius between adjacent cation sites increases slightly (from approximately 0.29(+/-0.05) A to approximately 0.40(+/-0.05) A) with the addition of oxygen to the glass and is proposed to be the major cause in decreasing the activation energy and thereby increasing the conductivity. Further addition of oxides appears to contract the glass structure (and the doorway radius) leading to an increase in the conductivity activation energy and a decrease in the conductivity.     

What is syncrystallization? States of the pH indicator methyl red in crystals of phthalic acid

The concept of syncrystallization was reinvestigated by focusing on phthalic acid (PA) grown with methyl red (MR). Crystals are alternately red and yellow in adjacent growth sectors. X-ray structures of MR and its cocrystals, revealing MR in the neutral, zwitterionic, and protonated states, as well as measurements of linear birefringence and linear dichroism of mixed crystals, were used to investigate mechanisms of PA coloring. These experiments were complemented by force field calculations of the lowest energy stable surfaces of expressed facets and energies of MR on and in crystals, as well as molecular orbital calculations of MR. Two MR species were detected in PA having distinct energies, polarizations, and face selectivities. Assignments of structures to these MRs, previously thought to be neutral and protonated, required a nuanced analysis of hydrogen bonds. The essential difference between yellow and red species is whether the MR carboxylic acid proton is inter- or intramolecularly hydrogen bound. Inferences about mixed crystal structure drawn from an examination of cocrystals of PA and MR are inconsistent with polarization spectroscopy signaling caution when using stoichiometric compounds as models of dilute solid solutions. Upon heating mixed crystals, linear dichroism diminishes and oriented, elongated pools of MR separate and pass through the bulk in directions perpendicular to the direction of elongation. These batonnets subsequently crystallize leaving macroscopic oriented crystals of a MR-rich phase within PA. No evidence was found for the simultaneous crystallization of MR and PA; however, the MR reorientation on heating as well as the separation and recrystallization of a MR-rich phase are distinct processes that could be embraced by the literal meaning of syncrystallization.     

The direct reaction of the tBu-DAB ligand with SeCl4: a redox route to selenium-nitrogen heterocycles

The reaction of SeCl4 with the ubiquitous tert-butyl-substituted diazabutadiene ligand results in the isolation of a rare example of a 1,2,5-selenadiazolium cation, representing a novel route to Se-N ring formation; these heterocycles can be derivatised at selenium, which has led to the identification of a short Se...N secondary bonding interaction.     

Fabrication of spiropyran-containing thin film sensors used for the simultaneous identification of multiple metal ions

In this article, a methacrylate-based spiropyran-containing copolymer was used as a colorimetric sensor to identify multiple metal ions simultaneously. Through UV-vis absorption spectroscopy, the relative binding affinity of merocyanine to each metal ion was investigated by displacement studies of a bound metal ion with a second metal ion of a higher binding affinity. We also show that because each metal ion gives rise to a distinct spectral response, partial least-squares discriminant analysis (PLS-DA) can be used to analyze the UV-vis absorbance spectra to identify the two metal ions that are present in solution at varying concentrations simply by dipping a coated polymer substrate into solution after irradiation. Partial least-squares regression analysis (PLS) was used to determine the metal ions in solution for several binary mixtures quantitatively. We also demonstrate that the quantitative determination depends on the relative binding preference of merocyanine to each metal ion.     

Exploring the top and bottom of the quantum control landscape

A controlled quantum system possesses a search landscape defined by the target physical objective as a function of the controls. This paper focuses on the landscape for the transition probability P(i → f) between the states of a finite level quantum system. Traditionally, the controls are applied fields; here, we extend the notion of control to also include the Hamiltonian structure, in the form of time independent matrix elements. Level sets of controls that produce the same transition probability value are shown to exist at the bottom P(i → f)=0.0 and top P(i → f)=1.0 of the landscape with the field and/or Hamiltonian structure as controls. We present an algorithm to continuously explore these level sets starting from an initial point residing at either extreme value of P(i → f). The technique can also identify control solutions that exhibit the desirable properties of (a) robustness at the top and (b) the ability to rapidly rise towards an optimal control from the bottom. Numerical simulations are presented to illustrate the varied control behavior at the top and bottom of the landscape for several simple model systems.     

Diastereoselectivity control in formal nucleophilic substitution of bromocyclopropanes with oxygen- and sulfur-based nucleophiles

A diastereoconvergent formal nucleophilic substitution of bromocyclopropanes with oxygen- and sulfur-based nucleophiles is described. The reaction proceeds via in situ formation of a highly reactive cyclopropene intermediate and subsequent diastereoselective addition of a nucleophile across the strained C═C bond. Three alternative means of controlling the diastereoselectivity of addition have been demonstrated: (1) thermodynamically driven epimerization of enolizable carboxamides, (2) steric control by bulky substituents, and (3) directing effect of carboxamide or carboxylate functions.     

Nucleobase-containing transition metal complexes as building blocks for biological markers and supramolecular structures

The incorporation of metal complexes into nucleobases, nucleosides and nucleotides has provided a focus for the development of many novel compounds with a wide range of applications. In this perspective article the different methods to incorporate transition metal complexes into these species will be described. Applications of these compounds as biological markers, catalysts and how the hydrogen bonding properties may be employed in directing supramolecular assembly in both the solid state and solution will be discussed.