Tsuyoshi Tsujioka

Red organic light-emitting diodes using an emitting assist dopant

We propose an emitting assist (EA) dopant system for obtaining organic light-emitting diodes (OLEDs) with pure red emission. The EA dopant (rubrene) did not itself emit but assisted the energy transfer from the host (Alq3) to the red emitting dopant (DCM2). The cell structure used was {indium tin oxide/hole injection layer [(20 nm), CuPc/hole transport layer (50 nm), NPB/emitting layer (40 nm), Alq3+DCM2 (2%)+rubrene (5 wt %)]/MgIn}. (CuPc: Copper (II) phthalocyanine, NPB: N, N′-Di(naphthalen-1-yl)-N, N′-diphenyl-benzidine, DCM2: 4-Dicyanomethylene- 2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl)vinyl]-4H-pyran). A stable red emission (chromaticity coordinates: x=0.64, y=0.36) was obtained in this cell within the luminance range of 100–4000 cd/m2. When the cell was not doped with rubrene, the emission color changed from red to orange as the luminance increased. The EA dopant system is a promising method for obtaining red OLEDs.

Organic bistable molecular memory using photochromic diarylethene

A principle of organic memory device using a bistable photochromic molecule is presented that allows extremely high bit densities and very low power consumption. This device is based on an isomerization reaction of photochromic diarylethene molecule via its excited state by an electric carrier injection, not by photon absorption. Experimental data show that the reversible writing and nondestructive reading of information by the carrier injection is feasible. The advantages and properties of such an organic semiconductor memory using a bistable molecule are discussed.

Nondestructive readout of photochromic optical memory using photocurrent detection

We proposed and demonstrated a nondestructive readout method using photocurrent detection for photon-mode photochromic memory. The principle of this readout method, which utilized the ionization potential change according to photoisomerzation reaction, was confirmed by using a medium with a photochromic diarylethene layer and phthalocyanine photoabsorbing layer, and by using a near-infrared readout light. We demonstrated perfect nondestructive readout operations over 106 times.

Selective metal deposition on photoswitchable molecular surfaces.

We report here a novel phenomenon: selective metal deposition on photoswitchable diarylethene (DAE) surfaces. Magnesium vapor was deposited by vacuum evaporation on the colored DAE but not on the uncolored surface. The selective deposition originates in the change of the glass transition temperature of the amorphous DAE film resulting from photoisomerization and therefore from changes of surface molecular motion. We clarified that Mg atoms on the uncolored surface actively migrated on the surface and were desorbed from the surface. The possibility of depositing other metals is also discussed. Light-controllable metal-integrated deposition was demonstrated as a new function of the photoswitchable molecular surfaces. This study reveals new features of the photoswitchable molecular surfaces, and their potential suggests bright prospects for future applications in organic electronics.

Superlow-Power Readout Characteristics of Photochromic Memory

Readout characteristics have been investigated with a superlow-power readout method in order to solve the problem of information destruction caused by repeated readout operations in photon-mode photochromic memories. For ordinary photochromic diarylethene media, it was possible to read signals of about 105–106 cycles with this method. This method was also applied successfully to wavelength multiplexed recording.

Electrical carrier-injection and transport characteristics of photochromic diarylethene films

Electrical carrier-injection and transport characteristics of photochromic diarylethene thin films were investigated. Injected and transported carriers (injected current) to the diarylethene film were increased with the existence ratio of the closed-ring form, which was one isomerization state of the diarylethene molecule. A dramatic increase was observed at a 12-percent existence ratio of the closed-ring molecules. A lowering effect of the potential barrier, which is caused by a Poole–Frenkel-like effect with a short distance interaction potential for hole transportation by increasing the existence ratio of the closed-ring molecules is proposed as the mechanism behind this phenomenon.

Photochromic reactions of a diarylethene derivative in polymer matrices

Abstract The sensitivity (product of molar extinction coefficient and quantum yield) of 2-(1,2-dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl)-maleic anhydride is dependent on its concentration in poly(vinyl butyral) film and on the irradiation wavelength (515 nm and 633 nm). Energy transfer from the open-ring to the closed-ring form is suggested as a possible mechanism for the time dependence of the sensitivity in the polymer film containing high chromophore concentration on irradiation with 515 nm light.

Crosstalk in Photon-Mode Photochromic Multi-Wavelength Recording

The mechanism of crosstalk in photon-mode multi-wavelength recording has been theoretically analyzed, and a new method to reduce the crosstalk is proposed in this paper. The crosstalk has linear components of superposition and nonlinear components due to intermodulation. Both can be reduced by linear and nonlinear mixing of the readout signals at λi (i=1,2,...). This reduction method was applied to photochromic optical media having two broad absorption bands which were superimposed with each other. Thus 2-wavelength recording (writing and reading) was achieved.

Super-Resolution Disk with a Photochromic Mask Layer

Super-resolution readout for read-only optical disks was demonstrated using a photochromic mask layer. Nonlinear transmittance change, which is desirable for effective super-resolution readout, was achieved using a the photochromic mask layer with an optical density of 0.5. The photochromic super-resolution method is expected to increase the recording density of not only read-only disks, but also magneto-optical and phase change disks.

Optical Density Dependence of Write/Read Characteristics in Photon-Mode Photochromic Memory

When photobleaching reactions are used for writing in a photochromic optical memory, the initial optical density in the recording layer plays an important role in the various write/read characteristics; i.e., secondary harmonics, repeatedly readout characteristics, and crosstalks in a multi-wavelength recording. Better characteristics, that is, lower secondary harmonics, higher readout repeatability and lower nonlinear component of crosstalks, were obtained for media with higher optical density.