Kunio Oka

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.

Fourth 3D Structure of the Chitosan Molecule: Conformation of Chitosan in Its Salts with Medical Organic Acids Having a Phenyl Group

Chitosan salts with two medical organic acids having phenyl groups (salicylic and gentisic acids) exhibited fiber diffraction patterns of a new type of crystal which does not compare with known types I and II. The crystals, called type III salts, showed a fiber repeat of 2.550 nm and a meridional reflection at the 5th layer line. These results coupled with a conformational analysis indicate the chain conformation of chitosan with the salts to be a 5/3 helix, this helix differing from those of type I (an extended two-fold helix) and type II (a relaxed two-fold helix or a 4/1 helix). The fiber patterns of all the type III salts were similar. This observation has also been found with type II salts and is an indication that the acid ions are not arranged in regular positions in the crystals. A comparison of solid-state 13C-NMR spectra of the gentisic acid salt and the aspirin salt, which could not be crystallized, suggests that, in the latter salt, the chitosan molecules also formed a 5/3 helix.

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.

X-RAY DIFFRACTION STUDY ON CHITOSAN-METAL COMPLEXES

Crystal structures of chitosan-metal complexes were studied by X-ray diffraction measurements on fiber diagrams. Stretched chitosan films immersed into Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) salt solutions, showed respective fiber patterns of the metal complexes. The crystallinity of the complex depended on metal and also on counter ion. All the complexes showed the meridional (002) reflections having similar interplanar spacings with that of the stretched chitosan film indicating that the two-fold helical conformation of chitosan did not change by the complex formation. Reflections of all the complexes were indexed

Electronic spectra of radical ions of cyclic polysilanes

The absorption spectra of the radical ions of cyclic polysilanes, (Me2Si)n, where n = 5, 6, and 7, formed by γ-irradiation have been recorded for rigid matrices at 77 K. The absorption maxima of radical anions of the cyclic polysilanes showed a red shift from 670 to 900 nm with increasing ring size. shows a broad single band at 720 nm, whereas shows two bands in the wavelength region from 500 to 1000 nm. A single peak is observed for at 900 nm. Ring-size reduction of the cyclic polysilanes with production of dimethylsilylene was not observed for the radical ions below 173 K, in sharp contrast to the behavior shown by the photoexcited states.

Reaction of phenyl(trimethylsilyl)silylene with chloromethanes; Insertion into the CCl bond and abstraction of chlorine and HCl

Abstract Photolysis of tris(trimethylsilyl)phenylsilane in chloromethanes, Cl n CX (4 - n ) , ( n = 1–4, X = H, alkyl, and Cl) produced phenyl(trimethylsilyl)silylene as a major intermediate, which afforded three types of products: 1,1-dichloro-1-phenyltrimethyldisilane via abstraction of two chlorine atoms, 1-alkyl-1-chloro-1-phenyltrimethyldisilane via insertion into the CCl bond, and 1-chloro-1-phenyl-2,2,2-trimethyldisilane via abstraction of HCl. Possible reaction mechanisms for the formation of these products were discussed based on initial formation of an 1,2-zwitterionic intermediate. A small amount of a radical-reaction product, 2-chloro-2-phenyl-hexamethyltrisilane, and a product considered to be a cyclohexadienyl derivative were detected in all cases as minor products.

Reduction of photoscission of σ bonds in polysilanes by fullerene doping

We report here an effective reduction of photoscission of σ bonds in polysilanes by C60 doping. The temporal variation of photoluminescence in polysilanes has been measured as a function of a doping amount of C60. The results indicate that the photoscission of σ bonds is related to a photogenerated electron-hole pair rather than a hole or an electron. The energy released when the electron-hole pair recombines nonradiatively is more likely required in addition to the thermal energy to scissor σ bonds. The C60 doping does not affect the activation energy of the photoscission process but decreases the photoscission cross-section. It is also demonstrated that the photoluminescence quenching by the C60 doping is observed for polysilanes not only with aromatic side groups but also with saturated hydrocarbon side groups.

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.

Hole transport in oriented polysilane films

Abstract Hole transport in oriented films of poly (di-n-pentylsilane) has been studied in terms of the effect of the orientation and conformation of Si chain on the drift mobility and disorder of hopping sites. The results support the view that (1) the interchain overlap of wave functions for a 7/3 helix is higher than that for a transoid, (2) the hopping sites have higher diagonal and off-diagonal disorder in a 7/3 helix than in a transoid, and (3) the orientation of Si chains significantly enhances the interchain overlap of wave functions and reduces the diagonal (1/1.3) and off-diagonal disorder parameter (1/2.7) in a transoid but has no effect in the case of a 7/3 helix.