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Atsuko Kosaka

Publications and source records attributed to Atsuko Kosaka.

9 recordsLinked to original sources
PubMed2007

Formation of water-dispersible nanotubular graphitic assembly decorated with isothiouronium ion groups and its supramolecular functionalization.

A newly designed Gemini-shaped hexabenzocoronene amphiphile (1), carrying an isothiouronium ion-appended side chain, self-assembles in CH2Cl2 to form a nanotubular object, whose graphitic wall is densely covered by a positively charged molecular layer of isothiouronium ion pendants. The graphitic nanotube can be dispersed uniformly in aqueous media owing to effective hydration as well as electrostatic repulsion. Post-supramolecular functionalization of the nanotube surface is possible, without disruption of the tubular morphology, by taking advantage of a specific interaction of the isothiouronium ion pendants with oxoanion guests. Mixing with sodium poly(4-styrenesulfonate) results in wrapping of the nanotube, while complexation with an electron-accepting oxoanion such as anthraquinone carboxylate allows photoinduced electron transfer from the graphitic wall to the bound guest molecules.

Anthraquinones↗
PubMed2006

Stabilization of a kinetically favored nanostructure: surface ROMP of self-assembled conductive nanocoils from a norbornene-appended hexa-peri-hexabenzocoronene.

Newly designed norbornene-appended hexabenzocoronene 1 self-assembles, upon diffusion of an Et(2)O vapor into its CH(2)Cl(2) solution, to form either graphitic nanocoils or nanotubes, depending on the self-assembling conditions. The coiled assembly, selectively formed at 15 degrees C, is a kinetic intermediate for the tubular assembly and transforms into nanotubes on standing at 25 degrees C. However, post-ring-opening metathesis polymerization of the norbornene pendants of 1 enhances the thermal stability of the coiled assembly as well as the tubular one and disables a thermodynamic coil-to-tube transition. The polymerized nanocoils show an electroconductivity of 1 x 10(-)(4) S cm(-)(1) upon doping with I(2), while the nonpolymerized nanocoils are disrupted upon being doped.

Diffusion↗
PubMed2006

Dramatic effect of dispersed carbon nanotubes on the mechanical and electroconductive properties of polymers derived from ionic liquids.

Free-radical polymerization of an imidazolium ion-based ionic liquid bearing a methacrylate group, gelling with single-walled carbon nanotubes (SWNTs), allows fabrication of a mechanically reinforced, electroconductive soft material (bucky plastic). A film sample of this material displays an excellent conductivity of 1 S cm(-1) and a 120-fold enhancement of the Young's modulus at a 7 wt % content of SWNTs. The conductivity is temperature-dependent in the range 5-300 K, suggesting that the conductive process involves carrier hopping. Scanning electron and atomic force micrographs of a bucky plastic film display the presence of crosslinked networks consisting of finely dispersed SWNTs. Such nanotube networks, developed in the polymer matrix, likely suppress slipping of entrapped polymer molecules via a strong interfacial interaction and also facilitate intertubular carrier transport. Although a bucky plastic derived from a vinylimidazolium ion-based ionic liquid monomer shows a comparable conductivity to that of the methacrylate version, the film is brittle irrespective of the presence or absence of SWNTs.

Colloids↗
PubMed2005

Self-assembled graphitic nanotubes with one-handed helical arrays of a chiral amphiphilic molecular graphene.

Self-assembly of a Gemini-shaped, chiral amphiphilic hexa-peri-hexabenzocoronene having two chiral oxyalkylene side chains, along with two lipophilic side chains, yields graphitic nanotubes with one-handed helical chirality. The nanotubes are characterized by an extremely high aspect ratio of >1,000 and have a uniform diameter of 20 nm and a wall thickness of 3 nm. The nanotubes with right- and left-handed helical senses were obtained from the (S)- and (R)-enantiomers of the amphiphile, respectively, due to an efficient translation of point chirality into supramolecular helical chirality. The (S)- and (R)-enantiomers coassemble at varying mole ratios to give nanotubes, whose circular dichroism profiles are almost unchanged over a wide range of the enantiomeric excess of the amphiphile (100-20%). The high level of chirality amplification thus observed indicates a long-range cooperativity in the self-assembling process. In sharp contrast, a hexabenzocoronene amphiphile with chiral lipophilic side chains did not form nanotubular assemblies. The present work demonstrates the majority rule in noncovalent systems and also may provide a synthetic strategy toward realization of molecular solenoids.

Journal Article↗
PubMed2005

Controlled self-assembly triggered by olefin metathesis: cross-linked graphitic nanotubes from an amphiphilic hexa-peri-hexabenzocoronene.

Acyclic diene metathesis (ADMET) in CH2Cl2 of a Gemini-shaped nonionic hexabenzocoronene amphiphile (2), bearing triethylene glycol chains with terminal allylic functionalities, resulted in spontaneous formation of graphitic nanotubes with a cross-linked surface. Without ADMET, 2 did not self-assemble to form a tubular structure due to a high solubility in CH2Cl2. Although 2 formed nanotubes in THF, ADMET on the surface of the preformed nanotubes in THF proceeded only sluggishly and resulted in partial disruption of the tubular structure. The cross-linked nanotubes showed a softening temperature (244 degrees C) higher than that of the uncross-linked version (195 degrees C) and preserved their hollow structure much longer upon heating.

Journal Article↗
PubMed2004

Self-assembled hexa-peri-hexabenzocoronene graphitic nanotube.

An amphiphilic hexa-peri-hexabenzocoronene self-assembles to form a pi-electronic, discrete nanotubular object. The object is characterized by an aspect ratio greater than 1000 and has a uniform, 14-nanometer-wide, open-ended hollow space, which is an order of magnitude larger than those of carbon nanotubes. The wall is 3 nanometers thick and consists of helical arrays of the pi-stacked graphene molecule, whose exterior and interior surfaces are covered by hydrophilic triethylene glycol chains. The graphitic nanotube is redox active, and a single piece of the nanotube across 180-nanometer-gap electrodes shows, upon oxidation, an electrical resistance of 2.5 megohms at 285 kelvin [corrected]. This family of molecularly engineered graphite with a one-dimensional tubular shape and a chemically accessible surface constitutes an important step toward molecular electronics.

Journal Article↗
PubMed2003

Molecular ordering of organic molten salts triggered by single-walled carbon nanotubes.

When mixed with imidazolium ion-based room-temperature ionic liquid, pristine single-walled carbon nanotubes formed gels after being ground. The heavily entangled nanotube bundles were found to untangle within the gel to form much finer bundles. Phase transition and rheological properties suggest that the gels are formed by physical cross-linking of the nanotube bundles, mediated by local molecular ordering of the ionic liquids rather than by entanglement of the nanotubes. The gels were thermally stable and did not shrivel, even under reduced pressure resulting from the nonvolatility of the ionic liquids, but they would readily undergo a gel-to-solid transition on absorbent materials. The use of a polymerizable ionic liquid as the gelling medium allows for the fabrication of a highly electroconductive polymer/nanotube composite material, which showed a substantial enhancement in dynamic hardness.

Journal Article↗
PubMed2002

Absolute asymmetric photoreactions of aliphatic amino acids by circularly polarized synchrotron radiation: critically pH-dependent photobehavior.

The absolute asymmetric photoreaction (AAP) of racemic aliphatic amino acids, such as alanine (Ala) and leucine (Leu), by left- and right-handed circularly polarized light (l- and r-CPL) irradiation was investigated in aqueous solutions at various pHs, by using the Onuki-type polarizing undulator installed in an electron storage ring. The magnitude of the optical purity (op) generated and the enantiomer-enriching mechanism operative in the AAP were found to be entirely dependent on the ionic state (and thus pH) of the amino/carboxylic acid moieties. At pH 1, the op of Ala and Leu determined by circular dichroism (CD) spectral measurement gradually developed with CPL irradiation, according to Kagan's equation. In contrast, irradiation at pH 7 gave op's much smaller than the theoretical values predicted by Kagan's equation. However, it turned out that the photodecomposition at pH 7 produces the corresponding alpha-hydroxycarboxylic acids stereoselectively, the CD sign of which is just opposite to that of the remaining amino acid, thus affording the apparently small op. It is concluded that, irrespective of solution pH, the AAP of amino acid proceeds upon CPL irradiation. At pH 1, the photodecomposition of valine, Leu, and isoleucine occurs via a Norrish type II mechanism, which is also applicable to other amino acids possessing a gamma-hydrogen. In the case of amino acids lacking a gamma-hydrogen, such as glycine and Ala, the photodecomposition mechanism is a photodeamination/hydroxylation and a Norrish type I reaction. At pH 7, the main photoproducts were ammonia and alpha-hydroxycarboxylic acids that were produced via photodeamination.

Alanine↗