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Shu Seki

Publications and source records attributed to Shu Seki.

12 recordsLinked to original sources
PubMed2006

Photoconductive coaxial nanotubes of molecularly connected electron donor and acceptor layers.

Controlled self-assembly of a trinitrofluorenone-appended gemini-shaped amphiphilic hexabenzocoronene selectively formed nanotubes or microfibers with different photochemical properties. In these nanotubes, which are 16 nanometers in diameter and several micrometers long, a molecular layer of electron-accepting trinitrofluorenone laminates an electron-donating graphitic layer of pi-stacked hexabenzocoronene. The coaxial nanotubular structure allows photochemical generation of spatially separated charge carriers and a quick photoconductive response with a large on/off ratio greater than 10(4). In sharp contrast, the microfibers consist of a charge-transfer complex between the hexabenzocoronene and trinitrofluorenone parts and exhibit almost no photocurrent generation.

Journal Article↗
PubMed2006

Customized morphologies of self-condensed multisegment polymer nanowires.

The direct formation of multisegment nanowires consisting of polymer domains by ion beam irradiation is investigated. Cross-linking reactions in the ion tracks result in localized gelation, giving isolated nanowires on substrates. It is demonstrated that the morphology of the final nanostructure is customized by appropriate selection of the ion fluence, combination of polymers, and the solvent employed for development. Octopus-like nanostructures consisting of a tangled hydrophilic polymer core and splayed hydrophobic polymer segments are successfully produced as an example of the process. The present technique provides universal feasibility for the formation of nanostructures based on "any" polymer materials in which radiation induces cross-linking reactions.

Journal Article↗
PubMed2006

Formation and decay of the triplet excited state of pyridine.

A pulse radiolysis study of the formation and decay of the triplet excited state of liquid pyridine has been performed using quenching techniques. The pyridine triplet excited state is observed with an absorption band at lambda = 310 nm and has a first-order decay with a lifetime of 72 ns. Stern-Volmer plots of the quenching of the pyridine triplet excited state with anthracene, naphthalene, and biphenyl give its yield to be 1.3 molecules/100 eV. This value is very similar to the previously determined yield of 1.25 molecules/100 eV for dipyridyl, the predominant condensed-phase product in the gamma-radiolysis of liquid pyridine. The rate coefficient for pyridine triplet excited-state scavenging by oxygen is estimated to be 6.6 x 10(9) M(-1) s(-1). Oxygen may also scavenge the electron precursor to the pyridine triplet excited state, whereas nitrous oxide is observed to have little effect. A pyridyl radical-pyridine (dimer) complex produced in the pulse radiolysis of neat liquid pyridine is detected at lambda = 390 nm and is consistent with iodine scavenging effects. Formation of the pyridiniumyl radical cation-pyridine charge-transfer complex is proposed to be insignificant in liquid pyridine.

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PubMed2006

Self-condensed nanoparticles of oligofluorenes with water-soluble side chains.

Self-condensation of the amphiphilic fluorene trimer occurs in the solvent mixture, giving pure "green" fluorescent nanoparticles with controlled radii of 20-100 nm. The origin of the green fluorescence is attributed to the excimer emission via excitation of aggregated dimer structures in the nanoparticles rather than the keto-defects. The present study also suggests the possibility of spectral tuning of emissions from oligofluorene aggregates.

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PubMed2006

Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices.

Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O(2)) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M(3)(+)) produced via the corresponding dimer radical cation (M(2)(+)) is clearly seen in the solution containing O(2) as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M(3)(+) CR absorption is observed; this is due to the large M(3)(+)/M(2)(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at approximately 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M(3)(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M(3)(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.

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PubMed2006

Experimental probe for hyperconjugative resonance contribution in stabilizing the singlet state of 2,2-dialkoxy-1,3-diyls: Regioselective 1,2-oxygen migration.

A detailed study of the regioselectivity of 1,2-oxygen migration was conducted using the unsymmetrically substituted singlet 2,2-dialkoxy-1,3-diarylcyclopentane-1,3-diyls 5. The alkoxy group selectively migrates to the electron-donating p-methoxyphenyl-substituted carbon. The regioselective migration of oxygen clearly indicates a hyperconjugative resonance structure, that is, zwitterionic characteristics, in singlet 2,2-dialkoxy-1,3-diyls. This represents the first attempt to experimentally probe the contribution of hyperconjugation to stabilizing the singlet state.

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PubMed2006

Electronic structure and optical properties of charged oligofluorenes studied by VIS/NIR spectroscopy and time-dependent density functional theory.

The electronic structure and optical properties of charged oligofluorenes were studied experimentally and theoretically. Measurements of the optical absorption spectra of charged oligofluorenes in dilute solutions have been performed by using the pulse radiolysis technique. In addition, optical absorption spectra of radical cations and anions in a solid matrix were measured after gamma-irradiation at 77 K. The optical absorption spectra were measured in the range of 440-2100 nm (0.6-2.8 eV) and compared with results from time-dependent density functional theory (TDDFT) calculations. The calculated charge induced deformations and charge distribution do not indicate the occurrence of polaronic effects. The potential energy profiles for rotation around the inter-unit bond show that oligofluorenes are nonplanar in their neutral state, while they tend to more planar structures in their charged state. The optical absorption spectra of charged oligofluorenes are dependent on the angle between neighboring units. TDDFT absorption energies shift to lower values with increasing chain length, which suggests that the charge delocalizes along the oligomer chain.

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PubMed2005

Photogeneration of charge carriers and their transport properties in poly[bis(p-n-butylphenyl)silane].

The photocarrier generation mechanism and mobility in poly[bis(p-n-butylphenyl)silane] (PBPS) thin films doped with a variety of electron acceptors are studied by time-resolved microwave conductivity (TRMC) measurements. It was found that fullerene is a suitable electron acceptor for PBPS as it provides the highest product of photocarrier generation yield phi and mobility Sigmamu under excitation at 532 and 355 nm. The observed high phiSigmamu value of 4.5 x 10(-3) cm(2)/(V s) under excitation at 193 nm (6.39 eV) can be attributed to the direct ionization of PBPS molecules. The photoinduced electron transfer between C(60) and PBPS was investigated in a solution sample by laser flash photolysis under excitation at 532 nm. On the basis of the extinction coefficient of PBPS(*+), transient absorption of PBPS(*+) provides a maximum value of phi of 0.83% for the electron-transfer reaction from PBPS to (3)C(60). On the basis of this value of phi, the intrinsic intrachain mobility of holes on the PBPS backbone is estimated to be higher than 1.7 x 10(-2) cm(2)/(V s), suggesting the presence of a high conducting path along the Si backbone of PBPS.

Journal Article↗
PubMed2005

Requirements for laser-induced desorption/ionization on submicrometer structures.

Laser-induced and matrix-free desorption/ionization on various submicrometer structures was investigated. First, to examine the effect of surface roughness on ionization, a silicon wafer or stainless steel was scratched with sandpaper. The fluences of a 337-nm nitrogen laser, required for ionization of synthetic polymers and reserpine, were markedly reduced on the scratched stainless steel or silicon as compared to the corresponding untreated surface. Next, arrays of submicrometer grooves, which had been lithographically fabricated on a silicon wafer, yielded protonated angiotensin, and the morphologic orientation demonstrated the positive relation between the laser and groove directions for promoting ionization. The fabricated structure also suggested the submicrometer, but not smaller, or nanometer, structures to be a key factor in direct desorption/ionization on rough surfaces. Finally, submicrometer porous structures of alumina or polyethylene yielded intense molecular ion signals of angiotensin and insulin, in response to direct UV irradiation, when the surface was coated with Au or Pt. The coating provided the additional advantage of prolonged activity for a porous alumina chip, exceeding a month even when the chip was left in the open air. These results indicate that laser-induced desorption/ionization of organic compounds can be implemented on submicrometer structures with an Au- or Pt-coated surface irrespective of the basal materials.

Adsorption↗
PubMed2005

Increase in the mobility of photogenerated positive charge carriers in polythiophene.

We report the increase in the mobility of charge carriers in regioregular poly 3-hexyl thiophene (RR-P3HT) films by mixing them with tetracyanoethylene (TCNE), which is examined by in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS). TCNE acts not only as an electron acceptor which increases the number of charge carriers on photoexposure but also as a functional additive which enhances the mobility of the charge carriers. This conclusion was deduced from the results of fluorescence quenching, transient optical absorption and photobleaching, and comparison of the TRMC signal with the TOS signal. The combination of the TRMC and TOS techniques represents a comprehensive and fully experimental approach to the determination of the intrinsic carrier mobility in conjugated polymers.

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PubMed2004

Dynamics of positive charge carriers on Si chains of polysilanes.

The transient absorption of radical cations of a variety of substituted polysilanes is discussed quantitatively in terms of the molar extinction coefficient and oscillator strength by nanosecond pulse radiolysis. Oxygen-saturated polysilane solutions in benzene exhibit a strong transient absorption band ascribed to the polysilane radical cation. The transient species react with N,N,N',N'-tetramethyl-p-phenylene-diamine (TMPD) to produce TMPD radical cations. On the basis of the molar extinction coefficient of the TMPD radical cation, the molar extinction coefficients for the radical cations of polysilanes are found to increase in the range 3.3 x 10(4) to 2.0 x 10(5) M(-)(1) cm(-)(1) with increasing polymer segment length. The stepwise increase in the total oscillator strength with an increase in the number of phenyl rings directly bonded to the Si skeleton suggests the delocalization of the positive polaron state and/or the SOMO state over the phenyl rings, indicating the importance of phenyl rings in intermolecular hole transfer processes.

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