Kiichi Nakajima

Ultrapure Blue Thermally Activated Delayed Fluorescence Molecules: Efficient HOMO–LUMO Separation by the Multiple Resonance Effect

Ultrapure blue-fluorescent molecules based on thermally activated delayed fluorescence are developed. Organic light-emitting diode (OLED) devices employing the new emitters exhibit a deep blue emission at 467 nm with a full-width at half-maximum of 28 nm, CIE coordinates of (0.12, 0.13), and an internal quantum efficiency of ≈100%, which represent record-setting performance for blue OLED devices.

One‐Step Borylation of 1,3‐Diaryloxybenzenes Towards Efficient Materials for Organic Light‐Emitting Diodes

The development of a one-step borylation of 1,3-diaryloxybenzenes, yielding novel boron-containing polycyclic aromatic compounds, is reported. The resulting boron-containing compounds possess high singlet-triplet excitation energies as a result of localized frontier molecular orbitals induced by boron and oxygen. Using these compounds as a host material, we successfully prepared phosphorescent organic light-emitting diodes exhibiting high efficiency and adequate lifetimes. Moreover, using the present one-step borylation, we succeeded in the synthesis of an efficient, thermally activated delayed fluorescence emitter and boron-fused benzo[6]helicene.

Preparation of zirconia-toughened bioactive glass-ceramics

Bioactive glass-ceramics toughened by tetragonal zirconia polycrystal (TZP) were prepared by hot-pressing mixed powders of the MgO-CaO-P2O5-SiO2 glass and TZP containing 20 to 80% alumina. The bending strength and the fracture toughness of the composite materials were improved compared with those of the material without TZP. These composites showed high bending strengths (400 to 500MPa) and high fracture toughness (∼ 2.8MPa m1/2). The existence of a crack deflection mechanism was observed by scanning electron microscopy. After soaking in simulated physiological solution at 100 °C, no phase transformation from tetragonal to monoclinic of TZP in the composites and no degradation in bending strength occurred.

Synthesis of Boronate-Based Benzo[fg]tetracene and Benzo[hi]hexacene via Demethylative Direct Borylation.

A demethylative direct borylation is reported, which was applied to the synthesis of benzo[fg]tetracenes containing boronate ester, amide, and thioester substructures. Depending on the heteroatom adjacent to boron, the molecules showed characteristic photophysical properties, molecular arrangements, and chemical stabilities. The key to the successful synthesis is the appropriate choice of the boron source and Brønsted base. The versatility of the direct borylation was demonstrated by the synthesis of a boronate-based benzo[hi]hexacene.

One-Shot Multiple Borylation toward BN-Doped Nanographenes

One-shot double, triple, and quadruple borylation reactions of triarylamines were developed through a judicious choice of boron source and Brønsted base. With the aid of borylation reactions, a variety of BN-doped nanographenes were synthesized in two steps from commercially available starting materials. An organic light-emitting diode device employing BN-doped nanographene as an emitter exhibited deep pure-blue emission at 460 nm, with CIE coordinates of (0.13, 0.11), and an external quantum efficiency of 18.3%.

Newly developed bioactive glass-ceramic composite toughened by tetragonal zirconia

Abstract A new type of bioactive glass-ceramic toughened by the dispersion of tetragonal zirconia polycrystal (TZP) grains was developed. This bioceramic exhibited high bending strength ( 500–800 MPa) and toughness. The high strength and toughness can be attributed to the inhibition of the crack propagation by deflection, branching and pinning, and the wastage of the fracture energy in propagating through TZP grains. It was shown that in composites containing TZP (especially up to 50 % vol.) a new apatite layer could form on the surface after immersion in simulated body fluid.

Efficient HOMO-LUMO separation by multiple resonance effect toward ultrapure blue thermally activated delayed fluorescence

Organic light-emitting diodes (OLEDs) play an important role in the new generation of flat-panel displays. Conventional OLEDs employing fluorescent materials together with triplet–triplet annihilation suffer from a relatively low internal quantum efficiency (IQE) of ~62.5%. On the other hand, the IQE of OLEDs employing phosphorescent or thermally activated delayed fluorescence (TADF) materials can reach ~100%. However, these materials exhibit very broad peaks with a full-width at half-maximum (FWHM) of 70–100 nm and cannot satisfy the color-purity requirements for displays. Therefore, the latest commercial OLED displays employ blue fluorescent materials with a relatively low IQE, and efficient blue emitters with a small FWHM are highly needed. In our manuscript, we present organic molecules that exhibit ultrapure blue fluorescence based on TADF. These molecules consist of three benzene rings connected by one boron and two nitrogen atoms, which establish a rigid polycyclic framework and significant localization of the highest occupied and lowest unoccupied molecular orbitals by a multiple resonance effect. An OLED device based on the new emitter exhibits ultrapure blue emission at 467 nm with an FWHM of 28 nm, Commission Internationale de l’Eclairage (CIE) coordinates of (0.12, 0.13), and an IQE of ~100%, which represent record-setting performance for blue OLED devices.