Masahide Yamamoto

Surface improvements of aramid fibers by physical treatments

The application of the low-temperature plasma method, the excimer laser treatment method and the corona-discharge method to aramid) were discussed, presenting an overview of current trends and developments in this area.

Spatial distribution of electrons ejected by two-photon excitation of aromatic molecules in polymer systems

Abstract The spatial distribution of electron–cation pairs formed in polymer films through two-photon ionization of aromatic dopants was evaluated from the decay of charge recombination luminescence at 20xa0K according to Hamas theory. The distribution was also examined directly using a layer-structure of ultrathin polymer films where the separation distance between donor and acceptor moiety was controlled on a nanometer scale. The results in both systems show that electrons ejected by two-photon ionization are distributed over 30xa0A away from the parent cations.

Charge delocalization of naphthalene cations formed in triad molecules studied by near-IR transient absorption spectroscopy

Charge delocalization of radical cations formed in naphthalene monomer, dyad, and triad molecules in solutions was studied by near-IR transient absorption spectroscopy. The triad molecules were 1,5-di(2-naphthyl)-3-phenylpentane (NBN), 1,3-di(2-naphthyl)-5-phenylpentane (NNB), and 1,3,5-tri(2-naphthyl)pentane (NNN). Charge resonance bands were observed around 900 nm for NBN, 1250 nm for NNB, and 1300 nm for NNN. The absorption bands for the triad molecules were ascribed to hetero-dimer cation of naphthalene and benzene, dimer radical cation, and trimer radical cation, respectively.

Photoinduced intramolecular charge separation in a polymer solid below the glass transition temperature

Photoinduced intramolecular charge separation (CS) in a polar polymer glass, cyanoethylated pullulan (CN-PUL), was studied below the glass transition temperature (Tg=395 K). A series of three carbazole (Cz: donor)-cyclohexane (S: spacer)-acceptor (A: acceptor) molecules (Cz-S-A) was used as intramolecular donor-acceptor dyads. The photoinduced CS rate was evaluated by the fluorescence decay measurement at temperatures from 100 to 400 K. The CS rate (kCS) increased above 200 K even far below Tg where micro-Brownian motions of the whole polymer chain are frozen. Below 200 K, on the other hand, kCS showed weak dependence on temperature. The temperature dependence of kCS is discussed in terms of the dielectric relaxation time of the polymer matrix. Consequently, CS below Tg was well explained by a thermally nonequilibrium electron transfer (ET) formula above 200 K and by a two-mode quantum-mechanical ET formula below 200 K. The increase in kCS above 200 K is mainly caused by a thermally activated low-frequency matrix mode originating from the side-chain relaxation of polar cyano groups. The weak temperature dependence of kCS can be explained by a nuclear-tunneling effect caused by a high-frequency matrix mode (variant Plancks over 2piomegH=250 cm-1) and an intramolecular vibrational mode (variant Plancks over 2piomegaQ=1300 cm-1). The high-frequency mode of the polymer matrix was attributed to a vibrational or librational motion of polar groups in the CN-PUL glassy solid.

Interplanar interactions in the excited triplet states of carbazole dimers by means of time-resolved EPR spectroscopy

The interplanar interactions for fully overlapped (FO) and partially overlapped (PO) carbazole dimers were investigated using zero field splitting (ZFS) parameters D and E in the excited triplet (T1) states. The observed D value decreases from the monomer to the dimers due to electron delocalization in the T1 dimers. The highest electron delocalization was observed within the two planes of the FO dimer. The interplanar interactions were analysed in terms of exciton (EX) and charge resonance (CR) types interactions by comparing the D values of monomer and the dimers. It is found that about 90% of excitation is delocalized over the two carbazole units in the T1 state of the FO dimer whereas for PO this contribution is 40%. In a model system, DCR and ECR were calculated using the two approximations, point charge and half point charge, for the evaluation of the ZFSs obtained.