Ryota Kabe

Afterglow Organic Light-Emitting Diode.

An afterglow organic light-emitting diode (OLED) that displays electroluminescence with long transient decay after it is turned off is demonstrated. This OLED exhibits blue and green dual emission originating from fluorescence and phosphorescence, respectively. A phosphorescence lifetime of 4.3 s is achieved.

Blue-Light-Emitting Ambipolar Field-Effect Transistors Using an Organic Single Crystal of 1,4-Bis(4-methylstyryl)benzene

An ambipolar light-emitting organic field-effect transistor (LE-OFET) based on a 1,4-Bis(4-methylstyryl)benzene (BSB-Me) single crystal was developed. The BSB-Me single crystal has very high photoluminescence quantum efficiency (ΦPL) of 89±2%, while ΦPL of the BSB-Me vapor-deposited film is limited to a much lower value of 54±2%. Ambipolar operation with successive blue electroluminescence from the FETs based on the BSB-Me single crystals was demonstrated by realizing nearly equal electron and hole mobilities [about 0.005 cm2/(V s)] with asymmetric gold–calcium contacts. Since BSB-Me single crystals can perform light amplification, the BSB-Me-based ambipolar LE-OFET is a promising candidate for future electrically driven organic blue-emitting solid-state lasers.

Organic long persistent luminescence

Long persistent luminescence (LPL) materials—widely commercialized as ‘glow-in-the-dark’ paints—store excitation energy in excited states that slowly release this energy as light. At present, most LPL materials are based on an inorganic system of strontium aluminium oxide (SrAl2O4) doped with europium and dysprosium, and exhibit emission for more than ten hours. However, this system requires rare elements and temperatures higher than 1,000 degrees Celsius during fabrication, and light scattering by SrAl2O4 powders limits the transparency of LPL paints. Here we show that an organic LPL (OLPL) system of two simple organic molecules that is free from rare elements and easy to fabricate can generate emission that lasts for more than one hour at room temperature. Previous organic systems, which were based on two-photon ionization, required high excitation intensities and low temperatures. By contrast, our OLPL system—which is based on emission from excited complexes (exciplexes) upon the recombination of long-lived charge-separated states—can be excited by a standard white LED light source and generate long emission even at temperatures above 100 degrees Celsius. This OLPL system is transparent, soluble, and potentially flexible and colour-tunable, opening new applications for LPL in large-area and flexible paints, biomarkers, fabrics, and windows. Moreover, the study of long-lived charge separation in this system should advance understanding of a wide variety of organic semiconductor devices.

Homogeneous dispersion of organic p-dopants in an organic semiconductor as an origin of high charge generation efficiency

We report that an organic p-dopant tri[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] [Mo(tfd)3] resulted in higher density of holes than inorganic metal oxide dopants of ReO3 or MoO3 in 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine even though the metal oxide dopants possess deeper work functions compared to Mo(tfd)3. Higher charge generation efficiency results largely from the homogeneous dispersion of Mo(tfd)3 in the host. In contradistinction, the transmission electron microscopy analysis revealed a formation of metal oxide nanoclusters. This highlights the importance of homogeneous dispersion for an efficient doping.

Increased vis-to-UV upconversion performance by energy level matching between a TADF donor and high triplet energy acceptors

A question at issue in triplet–triplet annihilation-based photon upconversion (TTA-UC) has been how to maximize the anti-Stokes shift, which requires minimization of the energy losses in intersystem crossing (ISC) of donors and consequent energy transfer (TTET) to acceptors. This is resolved by the energy level matching between a thermally activated delayed fluorescence (TADF) sensitizer and emitters with the highest triplet and singlet energy levels.

Model study of CO inhibition of [NiFe]hydrogenase

We propose a modified mechanism for the inhibition of [NiFe]hydrogenase ([NiFe]H(2)ase) by CO. We present a model study, using a NiRu H(2)ase mimic, that demonstrates that (i) CO completely inhibits the catalytic cycle of the model compound, (ii) CO prefers to coordinate to the Ru(II) center rather than taking an axial position on the Ni(II) center, and (iii) CO is unable to displace a hydrido ligand from the NiRu center. We combine these studies with a reevaluation of previous studies to propose that, under normal circumstances, CO inhibits [NiFe]H(2)ase by complexing to the Fe(II) center.

Enhanced phosphorescence in dibenzophosphole chalcogenide mixed crystal

The single crystals of 9-phenyl-9-dibenzophosphole chalcogenides {oxide (1), sulfide (2) and selenide (3)} and a mixed crystal of 2 and 3 have been prepared and characterized by X-ray diffraction analysis. Several intermolecular interactions such as π–π and Se–H were observed in the mixed crystal of 2 and 3. The mixed crystal of 2 and 3 exhibited enhanced phosphorescence compared to the pure constituent crystal (2 or 3) at low temperature.

Exfoliation of Graphite into Graphene in Polar Solvents Mediated by Amphiphilic Hexa-peri-hexabenzocoronene

A water-soluble surfactant consisting of hexa-peri-hexabenzocoronene (HBC) as hydrophobic aromatic core and hydrophilic carboxy substituents was synthesized. It exhibited a self-assembled nanofiber structure in the solid state. Profiting from the π interactions between the large aromatic core of HBC and graphene, the surfactant mediated the exfoliation of graphite into graphene in polar solvents, which was further stabilized by the bulky hydrophilic carboxylic groups. A graphene dispersion with a concentration as high as 1.1 mg L(-1) containing 2-6 multilayer nanosheets was obtained. The lateral size of the graphene sheets was in the range of 100-500 nm based on atomic force microscope (AFM) and transmission electron microscope (TEM) measurements.

Wide-Range Tuning and Enhancement of Organic Long-Persistent Luminescence Using Emitter Dopants

Most long-persistent luminescent (LPL) materials, which slowly release energy absorbed from ambient light, are based on inorganic compounds. Organic long-persistent luminescent (OLPL) systems have advantages over inorganic LPL materials in terms of solubility, transparency, and flexibility. Here, the characteristics of OLPL emission are improved by doping emitter molecules into an OLPL matrix. Greenish-blue to red and even warm white emission are achieved by energy transfer from exciplex in the OLPL matrix to the emitter dopants. The dopants also improve brightness and emission duration through efficient radiative decay and the trapping of electrons, respectively. This technique will enable the development of a wide range of organic glow-in-the-dark paints.

Graphene-Pyrene Nanocomposites Obtained Using Azide Chemistry

In this study we describe a simple and fast procedure for the covalent functionalization of pristine graphene with a pyrene-terminated alkylazide, transformed in a highly reactive radical by thermal activation. The functionalized graphene sheets showed enhanced dispersibility in organic solvents compared to the pristine ones, thus enhancing their solution processability and compatibility with solvents or polymers. The relative improvement of solubility estimated form the absorption spectra was ≈60% in CHCl3 and ≈1200% in THF. The obtained materials were characterized by optical absorption spectroscopy, photoemission spectroscopy, infrared spectroscopy and X-rays photoelectron spectroscopy. The presence of the pyrene photoemitting chromophore in the grafting unit allowed to monitor the successful grafting and to confirm the effectiveness of the alkylazide to improve graphene solubility even when present in small amounts on the graphene surface.