Hideji Osuga

Development of a High-Performance Liquid Chromatography Method to Determine the Fagopyrin Content of Tartary Buckwheat (Fagopyrum tartaricum Gaertn.) and Common Buckwheat (F. esculentum Moench)

Abstract Buckwheat contains fagopyrin, which induces photosensitization in light-skinned livestock when exposed to sunlight. Here, we developed a high-performance liquid chromatography (HPLC) method to measure the fagopyrin content of buckwheat. The HPLC profile of the fagopyrin extract purified from Tartary buckwheat ‘Rotundatum’ had 3 apparent peaks. The ultraviolet-visible (UV-vis) absorption spectrum of each peak yielded absorbance maxima ( λmax) at 547 nm and 591 nm, indicating that these peaks corresponded to fagopyrin and unidentified fagopyrin derivatives. We considered the total content reflected by the 3 peaks to be the fagopyrin content of buckwheat. We determined the fagopyrin content in the leaves of Tartary buckwheat ‘Rotundatum’ and common buckwheat ‘Miyazakiootsubu’ both by UV-vis photometric analysis and the newly developed HPLC method. The fagopyrin content is overestimated by UV-vis photometry because the extracts contain a considerable amount of chlorophyll. Thus, HPLC analysis is more efficacious for fagopyrin-content measurements than UV-vis photometric analysis. The HPLC analysis of fagopyrin is easy, quick and efficacious for screening buckwheat varieties with trace or no fagopyrin. There are only a few reports on the accumulation sites of fagopyrin in buckwheat. We revealed that in Tartary and common buckwheat, fagopyrin is present mainly in the leaves and flowers and slightly in the stems, hulls, and groats. The fagopyrin contents of the leaves and flowers of Rotundatum were approximately 2.6 and 2.8 times higher than those in Miyazakiootsubu, respectively.

Crystallinity improvement of benzodithiophene-dimer films for organic field-effect transistors

We have significantly improved the crystallinity of vacuum-evaporated benzodithiophene (BDT)-dimer films using pentacene crystallinity control layers. The pentacene molecules evaporated on hexamethyldisilazane-treated SiO2 surfaces first form many small islands. Then, the islands grow two dimensionally. We used thin pentacene films with surface coverage of approximately 90% as crystallinity control layers, and deposited 30-nm-thick BDT-dimer films on them. Micron-scale planar grains are observed in atomic force microscopy images of the BDT-dimer films. The highly anisotropic shape of the grains indicates that the BDT-dimer films of good crystallinity have been obtained. The field-effect transistors based on the BDT-dimer films showed excellent hole mobility of 1.2cm2V−1s−1 at maximum.

Organic Field-Effect Transistors Based on Benzodithiophene-Dimer Films

We fabricated organic field-effect transistors (OFETs) based on benzodithiophene (BDT)-dimer films by vacuum evaporation. The surface of the BDT-dimer films deposited on a SiO2 gate insulator was so rough that excellent FET characteristics were not obtained. However, the surface morphology of the films deposited on poly(methyl methacrylate) buffer layers was significantly improved, and dense grains of submicron size were formed. Grain size increased from the submicron scale to the micron scale by reducing growth rate from 0.035 to 0.02 nm/s. The fabricated OFETs have shown p-type FET characteristics with a carrier mobility of 0.016 cm2 V-1 s-1. More importantly, it has been demonstrated that the performance of the BDT-dimer OFET is much more stable in air than that of the pentacene OFET.

Contact and channel resistances of organic field-effect transistors based on benzodithiophene-dimer films deposited on pentacene crystallinity control layers

We have investigated contact and channel resistances of organic field-effect transistors (FETs) based on benzodithiophene (BDT)-dimer films deposited on thin pentacene layers used as crystallinity control layers (CCLs). The contact resistance of source/drain electrodes made of conductive organic films instead of Au films has been reduced for pentacene-CCL/BDT-dimer FETs; the carrier mobility has been improved to 1.2 cm2 V−1 s−1 at maximum. Because the channel resistance of the pentacene-CCL/BDT-dimer FETs is found to be lower than that of reference pentacene FETs, the carrier transport in the BDT-dimer layers is more important than that in the pentacene CCLs for the high mobility.

Clathrate formation by and self-assembled supramolecular structures of a “molecular spring”

The inclusion properties and self-assembly of racemic and optically active helicenediols have been thoroughly investigated. The racemic helicenediol (PM-1) crystallizes with ethanol or 1,2-dichloroethane in different host–guest stoichiometric ratios to form (PM-1)·(ethanol) or (PM-1)2·(1,2-dichloroethane), respectively. Single crystal X-ray analyses of the clathrates show that the helicenediol 1 has greater flexibility to accommodate guest molecules than might have been anticipated. The helical pitch of the helicenediol, which controls the interplanar angle between the terminal thiophene rings, ranges from 38.0, for the ethanol clathrate, to 54.5° for the 1,2-dichloroethane clathrate. This represents an increase of 16.5° or 44%. Testosterone is selectively incorporated into the left-handed helicenediol (M-1) to afford a 1∶1 inclusion complex, (M-1)·(testosterone), in which the interplanar angle decreases from 54.5 to 46.2°. Without guest molecules, racemic helicenediols self-assemble through a unique supramolecular network of hydrogen bonds to form an alternate-leaf motif, while right-handed helicenediols form a four-leaf clover motif in projection. In the self-assembled structures, the interplanar angle of the helicenediol 1 changes from 44.7, for the racemic case, to 33.8° for the right-handed helical case. All of the above evidence points to the surprising conclusion that helicenediol 1 can expand and contract as a “molecular spring”. The maximum elongation of the spring is about 61%.

Carrier transport in benzodithiophene-dimer field-effect transistors with pentacene crystallinity control layers

The carrier transport in the benzodithiophene (BDT)-dimer films plays significant role for high carrier mobility of BDT-dimer field-effect transistors (FETs) using very thin pentacene layers as crystallinity control layers (CCLs). When the grains of the BDT-dimer films are small, the carrier mobility is not only low but also depends on the average thickness of the pentacene-CCLs. However, it is independent from the thickness of the CCLs when the grain size of the BDT-dimer films is large enough, and becomes as high as 0.76 cm2 V−1 s−1. This result shows that the high carrier mobility of pentacene-CCL/BDT-dimer FETs is due to the carrier transport in the BDT-dimer films.

Synthesis and Properties of Polyalkylated Thiaheterohelicenes

Abstract

Synthesis and Optical Properties of Fluorescent Materials Derived from Tricyclic Heterocycles

We have recently found that 1,2-diarylethylene 1a derived from ortho-fused tricyclic benzo[1,2-b:4,3-b′]dithiophene is thermally stable and highly fluorescent in toluene solution. We now wish to report the synthesis and characterization of highly fluorescent materials like dimeric (2a, 2b) and trimeric (3a, 3b) compounds consisting of dihexylbenzodithiophenes. The introduction of hexyl groups is very important in the synthesis and purification of these compounds, because longer alkyl groups in the benzodithiophene skeleton increase the solubility in organic solvents. The oligomeric derivatives such as 4a (n = 2) and 5a (n = 3) were also prepared and characterized. The fluorescence band maxima of 4a and 5a in toluene solution undergo red-shifting with increasing conjugation length of the benzanthracene moiety. A significant red shift (∼29 nm) is observed in the thin film of 3a, compared to the spectrum in toluene. This is probably because of the aggregation, planarization, and excimer formation in the solid state.

Organic thin-film transistors based on solution-processable benzodithiophene dimers modified with hexyl groups

Benzodithiophene dimers modified with hexyl groups (2C6-BDT-dimer) were investigated as solution-processable organic semiconductors for organic thin-film transistors (OTFTs). Since 2C6-BDT-dimer crystals have an anisotropic shape, flow coating was adopted to grow polycrystalline films. The flow-coated films were inferior to the vacuum-evaporated ones in terms of their crystallinity estimated from X-ray diffraction data. However, the hole mobility of the OTFTs with the flow-coated films, which was 1.7 cm2 V−1 s−1 at maximum, was higher than that of the OTFTs with vacuum-evaporated films because the one-dimensional thin crystals of the flow-coated films were aligned in the flow-coating direction.