Takaaki Dohmaru

Formation of colloidal silver in water by ultrasonic irradiation

Stable colloidal dispersions of silver were prepared by ultrasonic irradiations of aqueous AgClO4 or AgNO3 solutions in the presence of surfactants.

Electroluminescent Properties of a Novel σ^*-π^* Conjugated Polymer, Poly[1,1-(2,3,4,5-tetraphenylsilole)]

The optical and electronic properties of a σ*–π* conjugated polymer, poly[1,1-(2,3,4,5-tetraphenylsilole)] (PTPS), have been studied. PTPS shows emissions of σ*–π and π*–π transitions at 356 and 520 nm, respectively. PTPS light-emitting diodes (LEDs) with a single layer structure emit green-yellow light, corresponding to the π*–π transition, with an external quantum efficiency of 3×10-2%. The effect of the doping of poly(methylphenylsilane) (PMPS), a good hole transport material, into PTPS has also been studied. The shapes of the photoluminescence spectra of PMPS:PTPS films are the same as those of a PTPS film, indicating efficient energy transfer from PMPS to PTPS. PTPS:PMPS LEDs emit essentially the same green-yellow light as PTPS LEDs and exhibit better device characteristics than those without PMPS, that is, smaller turn-on voltages, larger current densities, and larger external quantum efficiencies. The PMPS:PTPS (PMPS: 30 wt%) LEDs show a luminance of 27 cd/m2 at a driving current density of 13 mA/cm2.

LIGHT-INDUCED METASTABLE STATES IN AMORPHOUS ORGANIC POLYSILANES

Ultraviolet (UV) light‐induced metastable states of transient hole transport in amorphous polymethylphenylsilane (PMPS) have been investigated with a time‐of‐flight technique. No change in the hole drift mobility is detected after UV light soaking, while the hole lifetime is reduced by UV light irradiation because of the increase in silicon (Si) dangling bond density. The photoscission of Si backbone chain is enhanced by the application of electric field to PMPS during UV light exposure. The origin of this enhancement is discussed. The annealing behavior of the light‐induced metastable state has also been investigated at various temperatures. The light soaking and annealing behavior of PMPS is found to be similar to that of hydrogenated amorphous Si.

Enhanced Ultraviolet Emission in Polysilane Light-Emitting Diodes by Inserting a SiOx Thin Layer

Bilayered polysilane light-emitting diodes (LEDs) have been fabricated by inserting a SiOx thin layer between the cathode and a poly(methylphenylsilane) (PMPS) film employed as an emission material. The SiOx layers were prepared by O2 plasma treatment of the PMPS film surfaces. It was found that the external quantum efficiency was significantly enhanced by this treatment. The enhancement may be caused by the increased electron injection due to the tunneling effect and the reduced hole current due to the blocking effect by the thin SiOx layer. Experiments also showed that the weak visible emission invariably observed from single-layer polysilane LEDs was almost completely eliminated. It is concluded that the visible emission is caused by the erosion of the PMPS surfaces occurring due to the collision with hot metal particles during the vacuum deposition of the cathode, and this erosion is avoided by the SiOx layer.

Photoluminescence study in photooxidation of poly(phenymethylsilane) films

Temporal variations of photoluminescence (PL) in poly(phenylmethylsilane) (PPMS) films under the irradiation with 325nm light at room temperature in air have been investigated. The PL intensity from the σ∗-σ transition shows a temporal decay fitting to a straight line in a log-log plot, where its slope switches from small one (−0.2) to steep one (∼ −1) at a certain time. This transfer time corresponds to the time when the PL intensity of a 530 nm peak (ascribable to the π∗-σ transition) begins to decrease and increases proportionally with increasing the molecular weight of PPMS in the range less than 2.2×105. The decay properties are discussed by introducing a thermal activated process into the photooxidation.

Photoinduced annihilation of σ bonds in amorphous poly(methylphenylsilane)

Abstract The photoinduced annihilation of σ bonds in poly(methylphenylsilane) (PMPS) films has been studied by prolonged exposure to 325 nm light at room temperature. The transparency of the film begins to increase significantly after an exposure time, t T . The decay profile of the absorption after t T confirms the presence of a thermally activated process in the photoscission of σ bonds. Photoluminescence measurements show a decay of the σ∗−σ emission and a slight increase in a peak at 530 nm (ascribable to the π∗−σ emission) before t T . The former is explained by the diffusion of photogenerated holes and the creation of nonradiative recombination centers by them. The latter is discussed by the accumulation of σ bonds which interact with phenyl-π electrons and have higher activation energy for the photoscission than the others.

Thermochromism and solvatochromism of non-ionic polar polysilanes

Abstract Polar polysilanes bearing ethereal side groups [{CH 3 Si(CH 2 ) m O(CH 2 CH 2 O) n CH 3 } x , m =3–5 and n =0–3}] were prepared and were found to be soluble in a wide range of polar solvents. UV spectroscopic behavior is highly dependent on the nature of the solvents used but no simple relationship between λ max and specific solvent parameter is found. Small molar absorbance ( e 3 ) 2 CHOH (HFIP) in non-polar solvent (benzene, CH 2 Cl 2 ) or even in (CH 3 ) 2 CHOH solution, where higher HFIP concentration brings about longer wavelength absorption. This type of solvatochromism is originated by strong hydrogen bond formation between the ethereal side groups and HFIP. Because non-dissociative nature of HFIP-concerned hydrogen bonding, increased bulkiness of the side moieties brings about disentanglement. Polysilanes 1 – 5 , though having long side chains, show thermochromism of continuous spectral shift on cooling. The same λ max value in thermochromism and solvatochromism indicates that nearly the same degree of disentanglement is caused by HFIP and thermally.

Photoelectric properties of organic polysilane containing carbazolyl side groups

The photoelectric properties of a series of N-carbazolyl-substituted silane copolymers have been studied by optical absorption, photoluminescence, and time-of-flight transient photocurrent measurements. It is found in N-carbazolyl-substituted silane copolymers that the optical absorption and the photoluminescence bands of the carbazolyl side groups, superimposed on those of the Si backbones, are observed in the blue to ultraviolet spectrum region. The hole drift mobilities of the copolymers are essentially the same as those of polymethylphenylsilane (PMePhSi) and are higher than those of poly(N-vinylcarbazole), and the photocarrier generation efficiencies of the copolymers are higher than those of PMePhSi. These results indicate that the photogenerated charge carriers on the carbazolyl side groups transfer to the Si backbones and then proceed through the Si backbones.

γ-Ray induced reduction of acid chlorides with trichlorosilane

Abstract The γ-ray induced reduction of various acid chlorides (RCOCl) with trichlorosilane was studied at ambient temperature. Trichlorosilyl radicals were found to attack acid chloride selectively at chlorine atom to produce acyl radicals at the first step. The preference of chlorine-atom attack to carbonyl-oxygen-atom attack is explained in terms of molecular orbital calculations (STO-3G); the SOMO (singly occupied molecular orbital) of trichlorosilyl radicals is more likely to interact with the energetically closer σ-HOMO (C-Cl) than π-HOMO (CO) of acid chlorides. The acyl radicals thus formed yield two kinds of products depending on the nature of the alkyl moiety involved. When the alkyl moeity (R) is tertiary, the acyl radical undergoes decomposition into carbon monoxide and an alkyl radical which finally yields an alkane (RH). When R is primary, the acyl radical abstracts a hydrogen atom from trichlorosilane producing an aldehyde, which, by subsequent hydrosilation, gives an alkoxysilane (RCH2OSiCl3) as a final product. Thus, the present reaction can be used as a complementary synthesis to the reported method in which initiators are used.

Transient electron transport in organic polysilane containing anthracene units

Electron and hole transport in polymethylphenylsilane (PMPS) containing anthracene units in the polymer backbone (PMPS-AN) have been studied by time-of-flight and electrophotographic measurements. It is found that the hole drift mobility of PMPS-AN is slightly decreased because of the increase in energetic and geometric disorder, and that the electron transport, which has not been observed in PMPS, is detected. We suggest that the electron transport is due to electron hopping in a site percolation cluster formed by the anthracene units.