Akito Masuhara

Fullerene Fine Crystals with Unique Shapes and Controlled Size

The preparation of fullerene fine crystals with uniform size and shape would permit the control of their specific electronic energy levels and the fabrication of materials with completely new properties. To this end, we have successfully fabricated, for the first time, shape- and size-controlled C60 fine crystals using a reprecipitation method developed in our laboratory. The C60 fine crystals obtained were clearly monodisperse and came in an interesting diversity of shapes such as spherical, rodlike, fibrous, disk, and octahedral. We were able to selectively control these sizes and shapes by simply changing the combination of solvents used and the reprecipitation conditions.

Single Benzene Green Fluorophore: Solid‐State Emissive, Water‐Soluble, and Solvent‐ and pH‐Independent Fluorescence with Large Stokes Shifts

Benzene is the simplest aromatic hydrocarbon with a six-membered ring. It is one of the most basic structural units for the construction of π conjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal. Presented herein is 2,5-bis(methylsulfonyl)-1,4-diaminobenzene as a novel architecture for green fluorophores, established based on an effective push-pull system supported by intramolecular hydrogen bonding. This compound demonstrates high fluorescence emission and photostability and is solid-state emissive, water-soluble, and solvent- and pH-independent with quantum yields of Φ=0.67 and Stokes shift of 140 nm (in water). This architecture is a significant departure from conventional extended π-conjugated systems based on a flat and rigid molecular design and provides a minimum requirement for green fluorophores comprising a single benzene ring.

Multibranched C60 Micro/Nanocrystals Fabricated by Reprecipitation Method

We found that monodispersed C60 micro/nanocrystals (M/NCs) with unique multibranched structures can be fabricated by reprecipitation method, using m-xylene and 2-propanol as good and poor solvents, respectively. The resulting C60 M/NCs had a hexagonal crystal structure and was found to be a kind of crystal solvates in which the molar ratio of C60 to m-xylene was 3:2. C60 M/NCs seem to be important nanoparts in an integrated device.

Solution-processed inorganic-organic hybrid electron injection layer for polymer light-emitting devices.

A lithium quinolate complex (Liq) has high solubility in polar solvents such as alcohols and can be spin-coated onto emitting polymers, resulting in a smooth surface morphology. A polymer light-emitting device fabricated with spin-coated Liq as an electron injection layer (EIL) exhibited a lower turn-on voltage and a higher efficiency than a device with spin-coated Cs₂CO₃ and a device with thermally evaporated Ca. The mixture of ZnO nanoparticles and Liq served as an efficient EIL, resulting in a lower driving voltage even in thick films (∼10 nm), and it did not require a high-temperature annealing process.

Evaluation of Thermoelectric Properties of Polythiophene Films Synthesized by Electrolytic Polymerization

Polythiophene films were synthesized by electrolytic polymerization, and the synthetic parameters and their thermoelectric properties, i.e., Seebeck coefficient and electric conductivity, were investigated in detail. The Seebeck coefficient tended to decrease with increasing electric conductivity. However, the thermoelectric power factor increased with electric conductivity, and was 1.03×10-5 Wm-1K-2 at 23 µVK-1 and at 201 Scm-1. This value is large compared with those of other conductive polymers, and the figure of merit reached to one thirty against Bi–Te system. In addition, the elaborated surface morphology and internal structure, i.e., crystallinity, of the films were also evaluated by scanning electron microscopy, X-ray diffraction, and elemental analysis, and the relationship between thermoelectric properties and structure was discussed.

Laser flash photolysis study on photophysical and photochemical properties of C60 fine particles

Abstract Photophysical and photochemical properties of the C 60 fine particles (C 60 FP) which were prepared by the reprecipitation method were examined by the time-resolved fluorescence and laser flash photolysis methods. Fluorescence decay of the fine particles can be divided into two components: emission from free exciton and self-trapped exciton. Rapid decay of the broad absorption after nanosecond-laser excitation was attributed to triplet–triplet annihilation due to migration of energy of triplet state within the fine particle. After the annihilation, the energy of triplet state localized on C 60 molecule. Energy transfer from the localized C 60 triplet state in the fine particle was confirmed for energy acceptors such as O 2 and β-carotene. Photoexcitation of the C 60 FP in the presence of electron donor in solution resulted in photoinduced electron transfer. The reaction rate constant was one order smaller than that in solution, suggesting small collision frequency of the localized triplet state in C 60 FP. Oxidation of the fine particle was also observed by using methylacridinuim ion and biphenyl as a sensitizer and a cosensitizer, respectively.

Photon Antibunching in the Emission from a Single Organic Dye Nanocrystal

We demonstrated that the emission from a single organic nanocrystal consisting of organic dye molecules shows photon antibunching. That is, an assembling structure consisting of many chromophores can also be made to behave as a single-photon source by controlling the size. We found that most of single nanocrystals within a size range from 35 to 85 nm show photon antibunching even when more than one exciton in a crystal is generated by an intense single excitation pulse. On the other hand, single micrometer-sized crystals showed no antibunching. The present results indicate that molecular assemblies can also be considered as candidates for new single-photon sources.

Hybridized Microcrystals Composed of Metal Fine Particles and π-Conjugated Organic Microcrystals

We have successfully prepared hybridized microcrystals composed of polydiacetylene microcrystals and silver fine particles by means of the co-reprecipitation method, and investigated their hybridized structure and optical properties. It was revealed experimentally for the first time that the core domain of metal fine particles may interact optoelectronically with the shell layer of polydiacetylene in the hybridized microcrystals to give different electronic states from those of the original compounds.

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices.

Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.

Diacetylene Nanowire Crystals Prepared by Reprecipitation/Microwave-Irradiation Method

The effects of microwave irradiation on the crystal sizes and morphology of diacetylene nanocrystals have been investigated carefully by controlling experimental conditions such as temperature and concentration in a reprecipitation/microwave-irradiation method. As a result, the fabrication condition of diacetylene nanowire crystals has been clarified. The formation mechanism of such nanowire crystals is also discussed.