Takashi Isoshima

Stimulus-Responsive Azobenzene Supramolecules: Fibers, Gels, and Hollow Spheres

Novel, stimulus-responsive supramolecular structures in the form of fibers, gels, and spheres, derived from an azobenzene-containing benzenetricarboxamide derivative, are described. Self-assembly of tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxamide (Azo-1) in aqueous organic solvent systems results in solvent dependent generation of microfibers (aq DMSO), gels (aq DMF), and hollow spheres (aq THF). The results of a single crystal X-ray diffraction analysis of Azo-1 (crystallized from a mixture of DMSO and H2O) reveal that it possesses supramolecular columnar packing along the b axis. Data obtained from FTIR analysis and density functional theory (DFT) calculation suggest that multiple hydrogen bonding modes exist in the Azo-1 fibers. UV irradiation of the microfibers, formed in aq DMSO, causes complete melting while regeneration of new fibers occurs upon visible light irradiation. In addition to this photoinduced and reversible phase transition, the Azo-1 supramolecules display a reversible, fiber-to-sphere morphological transition upon exposure to pure DMSO or aq THF. The role played by amide hydrogen bonds in the morphological changes occurring in Azo-1 is demonstrated by the behavior of the analogous, ester-containing tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxylate (Azo-2) and by the hydrogen abstraction in the presence of fluoride anions.

Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq)3] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq)3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq)3/Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices.

Phase measurement of second-harmonic generation reveals the directional sense of a 2-docosylamino-5-nitropyridine (DCANP) molecule in Langmuir-Blodgett films

Abstract Molecules of 2-docosylamino-5-nitropyridine, NO 2 -C 5 H 3 N-NH-C 22 H 45 in a multilayer Langmuir-Blodgett film are ordered in the same direction with the same directional sense. All over the film the -NO 2 group is pointing opposite to the dipping direction. This ordering is solely determined by the dipping process.

Solubilization of single-walled carbon nanotubes using a peptide aptamer in water below the critical micelle concentration.

The solubilizing ability of single-walled carbon nanotubes (SWCNTs) in water with several dispersants was investigated. Among the dispersants, including low-molecular-weight surfactants, peptides, DNA, and a water-soluble polymer, the peptide aptamer, A2 (IFRLSWGTYFS), exhibited the highest dispersion capability below the critical micelle concentration at a concentration of 0.02 w/v%. The dispersion of supernatant aqueous solution of SWCNTs containing aptamer A2 was essentially unchanged for several months after high-speed ultracentrifugation and gave rise to an efficient and stable dispersion of the SWCNTs in water. From the results of isothermal titration calorimetry and molecular dynamics simulations, the effective binding capability of A2 was due to π-π interaction between aromatic groups in the peptide aptamer and the side walls of SWCNTs. Interestingly, the peptide aptamer showed the possibility of diameter separation of semiconducting SWCNTs using a uniform density gradient ultracentrifuge. These phenomena are encouraging results toward an effective approach to the dispersion and separation of SWCNTs.

Photoinduced reduction and pattern preservation of giant surface potential on tris(8-hydroxyquinolinato) aluminum(III) thin films

The characteristics and mechanisms of photoinduced reduction in giant surface potential (gSP) on tris(8-hydroxyquinolinato) aluminum(III) (Alq3) thin films were investigated and discussed from the results of the reduction dependences on exposure time/intensity and the preservation of photopatterned gSP. The reduction dependences are explained well with numerical-model calculation of surface potential reduction due to the drift of photoexcited carriers assuming the Poole–Frenkel formula for electron mobility. The preservation of patterned gSPs suggests the existence of deep traps involved in the carrier-drift mechanism.

Positive Giant Surface Potential of Tris(8-hydroxyquinolinolato) Aluminum (Alq

To examine the origin of spontaneous noncentrosymmetric molecular dipole orientation of an evaporated film of tris(8-hydroxyquinolinolato) aluminum (Alq3), overlayer deposition of Alq3 on the backside of a peeled Alq3 film was investigated. Although the surface potential (SP) of the backside of the Alq3 film was negative, the SP showed a positive shift by overlayer deposition of Alq3. This means that the spontaneous dipole orientation of Alq3 molecules in the overlayer is opposite to that in the underlayer. A molecular geometric effect due to a bulky molecular shape is proposed as the origin of the noncentrosymmetry.

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Long-term measurement of noncentrosymmetric molecular orientation of vacuum-evaporated tris(8-hydroxyquinolinato) aluminum (III) (Alq3) thin film was performed by means of first-order electroabsorption (EA) spectroscopy. This molecular orientation is responsible for the giant surface potential of Alq3 film, and it might be utilized to nonlinear optical devices. Although the EA response presented reduction, the relaxation time constant was on the order of year. Possible mechanisms for the reduction are discussed.

) Film Evaporated onto Backside of Alq

We examined influence of an organic overlayer deposition on giant surface potential (GSP) of tris-(8-hydroxyquinolinato)aluminum (III) (Alq3) using Kelvin probe method. Before deposition of organic overlayer, surface potential (SP) of an Alq3 film on a silicon wafer was +13 ± 2 V under the dark condition. When we deposited copper phthalocyanine (CuPc) on the Alq3 film, the SP value remained the same at first and then rapidly decreased with time. It is suggested that the charge transportation through the CuPc layers causes the reduction of SP value. On the other hand, the SP value did not change by deposition of n-hexatriacontane. These results indicate that the GSP possibly vanishes without the photoabsorption of the Alq3 molecules by organic overlayer deposition, depending on the electronic/optoelectronic properties of the overlayer.

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Abstract In order to improve the spatial resolution in scanning near-field optical microscopy (SNOM), we designed a hybrid system of illumination mode SNOM integrated with scanning tunnelling microscopy (STM) by introducing a metal-coated optical fiber tip with a nanometer-scale aperture. The aperture is also covered with a thin metal layer, thus the top of the aperture acts as an STM tip. We employed several materials to show the capability of our hybrid SNOM/STM system as a tool to investigate nanoscopic phenomena. We present a simultaneous SNOM/STM imaging of Au(111), where the SNOM image has a lateral resolution of the order of nanometers, quite comparable to that of the STM image. An azobenzene dye molecule embedded in liquid crystalline cyanobiphenyl molecules on MoS2 was also investigated.

Film Showing Negative Giant Surface Potential

We report a direct measurement of two-dimensional (2D) transfer functions of conductive probes used in Kelvin probe force microscopy (KFM). The 2D transfer functions are obtained by measuring a well-defined step pattern of surface potentials, prepared on thin films of tris(8-hydroxyquinolinato)aluminum(III) (Alq3) by contact-mask exposure. The experimentally obtained 2D transfer functions are highly asymmetric and are spread over 50 µm. A new finding is the observation of negative values in the KFM transfer function, which cannot be explained by conventional KFM formula. The reconstruction of true surface-potential profiles by model-fitting calculation is demonstrated. The technique presented in this study, i.e., the preparation of surface-potential patterns on Alq3 thin films, is ideal for determining the KFM transfer functions experimentally.