Control of nanoparticle location in block copolymers

A simple procedure is described to incorporate gold nanoparticles and control their location within symmetric poly(styrene-b-2 vinyl pyridine) (PS-PVP) diblock copolymers. Gold nanoparticles coated with thiol-terminated PS and/or PVP homopolymer chains (Mn approximately 1300 and 1500 g/mol, respectively) are incorporated into alternating lamellar layers of PS and PVP (total Mn approximately 196 500 g/mol). The location of the particles is controlled by varying the composition of ligands on the particle surfaces. In particular, gold particles coated with 100% PS or PVP reside near the center of the respective polymer domains, while particles coated with a mixture of both homopolymers reside at the interfaces between the two blocks.     

Silicon Carbide Nanostructures from Reactions between Vapors of Organochlorosilanes and Liquid of Sodium‐Factors Affecting Morphology and Composition

Cubic shells and spherical nanoparticles of β‐SiC were produced at 1273 K by processing the ceramic precursors formed from the reactions between vapors of organochlorosilanes, Me₂SiCl₂, MeSiCl₃, MeSiHCl₂, and PhSiCl₃, and liquid Na at 523‐723 K. From Me₂SiCl₂, a flexible linear polycarbosilane precursor was synthesized and covered the NaCl byproduct surface to form a cubic shape. Hollow cubic β‐SiC shells were produced after the NaCl templates were removed. From MeSiCl₃, a rigid cross‐linked polycarbosilane was produced and phase segregated from the NaCl byproduct. The precursor was transformed into nanoparticles without special morphology. MeSiHCl₂ produced a cross‐linked polysilane precursor at low temperatures, which can be converted into a mixture of β‐SiC and Si nanoparticles. At high temperatures, the polysilane converted to polycarbosilane and produced hollow cubic β‐SiC shells. The carbon‐rich PhSiCl₃ generated cube‐like particles as the final product, which contained β‐SiC and carbon.     

Synthesis and crystal structure of a dinitrogen ruthenium(II) macrocyclic tertiary amine complex

Treatment of trans-[Ru^III(16-TMC)Cl₂]Cl (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) with zinc metal in aqueous solution under nitrogen resulted in the formation of trans-[Ru^II(16-TMC)(N₂)Cl]PF₆ (1) which has been characterized by X-ray crystal analysis. The Ru---N₂ and N---N bond distances in 1 are 1.921(6) and 1.005(10) Å, respectively.     

Optical Rotation of Time-Reversal Degenerate States

It is shown that time-reversal (doubly-) degenerate, many-electron states in molecules of point-group symmetry C₃, C₄, C₆, S₄, and S₆ and T etc., can have non-vanishing matrix elements over a time-odd (electric dipole-electric dipole) polarizability operator contributing to optical rotation. In agreement with well-known results for Kramers' doublets, the optical rotations of the two separated and oriented states of this doublet have opposite signs in this polarizability mechanism, and they have the same sign in the time-even pseudoscalar mechanism which is the usual natural optical rotation of chiral molecules. These results are proven, in an alternative formulation using time-reversal in a second-order process, to hold regardless of even or odd numbers of spins—in contrast to the first-order processes such as the Jahn-Teller effect. The universality of time-reversal in spin, orbital and rotational angular momentum, in point and continuous groups, is show in a unified treatment with consistent phases. It was shown also how time-reversal symmetry can resolve the ambiguities in lower point-groups and determine relationships for which the point-group symmetry is powerless.     

Spectroscopy and femtosecond dynamics of type-II CdTe/CdSe core-shell quantum dots.

Syntheses of CdTe/CdSe type-II quantum dots (QDs) using CdO and CdCl2 as precursors for core and shell, respectively, are reported. Characterization was made via near-IR interband emission, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). Femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core (in CdTe/CdSe) emission and CdTe/CdSe interband emission reveal that as the size of the core increases from 5.3, 6.1 to 6.9 nm, the rate of photoinduced electron separation decreases from 1.96, 1.44 to 1.07 x10(12) s(-1). The finite rates of the initial charge separation are tentatively rationalized by the small electron-phonon coupling, causing weak coupling between the initial and charge-separated states.     

Mindo/3 study of the addition of hydrogen to vinylidene

The addition of hydrogen to vinylidene is studied using the MINDO/3 method. The results obtained are then compared with those for the addition of hydrogen to methylene applying ab initio and other semi-empirical methods. As expected, the pathways of these two reactions are qualitatively similar. The insertion is a non-least-motion process in which two phases can be easily distinguished: (i) an electrophilic step in which the empty orbital of the carbene interacts with the bonding orbital of H₂, forming a three-center bond in the process, (ii) a nucleophilic step in which the lone pair orbital of the carbene interacts with the antibonding orbital of H₂. The quantitative differences between the two reactions are also discussed.     

Synthesis, loading, and application of individual nanocapsules for probing single-cell signaling

This paper describes the synthesis and loading of silica and polystyrene-acrylic based nanocapsules with small molecules. The nanocapsules are used for delivering defined packages of stimuli to single cells with both high spatial and temporal resolutions. To introduce molecules into the capsules, we characterized two approaches. The first approach is based on a base-swell process in which the shell of the capsule is swelled so small molecules can diffuse into the interior of the capsule and be trapped inside once the capsules are de-swelled. The second approach is based on a dry-swell-dry process in which the solution containing the molecules of interest and the nanocapsules is physically dried to promote more molecules to enter into the interior of the capsule. We characterized both methods by monitoring the content of and the release from individual capsules with confocal microscopy and wide-field imaging. To illustrate the biological applications of such nanocapsules, we used optical trapping to position a single carbachol-loaded capsule adjacent to a single CHO cell that has been transfected with muscarinic acetylcholine (M1) receptors, released the carbachol from the capsule with a single 3-ns N2 laser pulse, and then monitored the subsequent intracellular signaling triggered by the binding of carbachol to the M1 receptors.     

An acid-base switchable [2]rotaxane

A chemically addressable, bistable [2]rotaxane, which incorporates a dumbbell-shaped component containing both secondary dialkylammonium and 1,2-bis(pyridinium)ethane recognition sites for its ring component, dibenzo[24]crown-8 (DB24C8), has been assembled. (1)H NMR spectroscopy has demonstrated that deprotonation (and reprotonation) of the secondary dialkylammonium (dialkylamine) recognition site induces the DB24C8 ring to move away from this site to the 1,2-bis(pyridinium)ethane one (and back again) in a discrete manner, particularly when the experiment is performed in CDCl(3) solution.     

Post-assembly processing of [2]rotaxanes

The concept of using [2]rotaxanes that carry one or more surrogate stoppers which can subsequently be converted chemically into other structural units, resulting in the formation of new interlocked molecular compounds, is introduced and exemplified. Starting from simple NH2(+)-centered/crown-ether-based [2]rotaxanes, containing either one or two benzylic triphenylphosphonium stoppers, the well-known Wittig reaction has been employed to make, 1) other [2]rotaxanes, 2) higher order rotaxanes, 3) branched rotaxanes, and 4) molecular shuttles--all isolated as pure compounds, following catalytic hydrogenations of their carbon-carbon double bonds, obtained when aromatic aldehydes react with the ylides produced when the benzylic triphenylphosphonium derivatives are treated with strong base. The two starting [2]rotaxanes were characterized fully in solution and also in the solid state by X-ray crystallography. The new interlocked molecular compounds that result from carrying out post-assembly Wittig reactions on two [2]rotaxanes were characterized by (dynamic) 1H NMR spectroscopy. In the case of a molecular shuttle in which the crown ether component is dibenzo[24]-crown-8 (DB24C8), shuttling is slow on the 1H NMR timescale, even at high temperatures. However, when DB24C8 is replaced by benzometaphenylene[25]-crown-8 as the ring component in the molecular shuttle, the frequency of the shuttling is observed to be around 100 Hz in [D4]methanol at 63 degrees C.     

A Semiempirical Study of Carbon Nanotubes with Finite Tubular Length and Various Tubular Diameters

A semiempirical PM3 quantum computational method has been used to generate the electronic and optimized geometrical structure of SWNT of zigzag and armchair types. We shed light on the electronic structures of SWNT with various diameters and lengths of the tube. Particularly, the calculated HOMO, LUMO and band‐gap of SWNT are not monotonic but exhibit a well‐defined oscillation, which depends on the tubular diameter and the tubular length. Calculated HOMO, LUMO and band‐gap of the zigzag SWNTs have oscillated with tubular diameter as they contain an odd or even number of benzenoids in the circular plane of the carbon nanotube. The zigzag SWNTs with an odd number of benzenoids have a higher band‐gap than those of SWNTs with an even number of benzenoids in the circular plane of the carbon nanotube. Calculated results also reveal that the tubular length in the zigzag SWNTs influences the band‐gaps very little. For the armchair SWNT, calculated HOMO, LUMO and band‐gap contained the oscillate depending on the number of carbon sections in the tubular length axis. Their repeat sections are 3n‐1, 3n and 3n+1. The armchair SWNT with 3n+1 sections has a high band‐gap while the SWNTs with 3n‐1 sections have a low band‐gap. The tubular diameters of armchair SWNT influence the HOMO, LUMO and band gap very little.