Hironori Kaji

Purely organic electroluminescent material realizing 100% conversion from electricity to light

Efficient organic light-emitting diodes have been developed using emitters containing rare metals, such as platinum and iridium complexes. However, there is an urgent need to develop emitters composed of more abundant materials. Here we show a thermally activated delayed fluorescence material for organic light-emitting diodes, which realizes both approximately 100% photoluminescence quantum yield and approximately 100% up-conversion of the triplet to singlet excited state. The material contains electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties. The typical trade-off between effective emission and triplet-to-singlet up-conversion is overcome by fine-tuning the highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The nearly zero singlet–triplet energy gap, smaller than the thermal energy at room temperature, results in an organic light-emitting diode with external quantum efficiency of 29.6%. An external quantum efficiency of 41.5% is obtained when using an out-coupling sheet. The external quantum efficiency is 30.7% even at a high luminance of 3,000 cd m−2.

Triarylboron‐Based Fluorescent Organic Light‐Emitting Diodes with External Quantum Efficiencies Exceeding 20 %

Triarylboron compounds have attracted much attention, and found wide use as functional materials because of their electron-accepting properties arising from the vacant p orbitals on the boron atoms. In this study, we design and synthesize new donor-acceptor triarylboron emitters that show thermally activated delayed fluorescence. These emitters display sky-blue to green emission and high photoluminescence quantum yields of 87-100 % in host matrices. Organic light-emitting diodes using these emitting molecules as dopants exhibit high external quantum efficiencies of 14.0-22.8 %, which originate from efficient up-conversion from triplet to singlet states and subsequent efficient radiative decay from singlet to ground states.

A Superamphiphobic Macroporous Silicone Monolith with Marshmallow‐like Flexibility

A number of research groups have been studying the preparation of hydrophobic and oleophobic surfaces, both for pure scientific interest and industrial applications. These studies are drawing increasing attention because of the growing demands for applications such as anti-fingerprint touch panels on electronic devices and solar panels that can prevent output fall from dust and smears on the surface by the self-cleaning effect. In nature, many examples of superhydrophobic surface exist with a water contact angle of more than 1508, such as eyes of mosquitos and lotus leaves, 2] and these are important for their survival. Their non-wetting surfaces possess a combination of nanoor microscaled roughness and low surface energy, which are known for the key of creating artificial superhydrophobic surfaces. However, most of the superhydrophobic materials can easily be wetted by organic liquids because of the lower surface tension of the liquids. In recent years, techniques for creating oleophobic surfaces have been vigorously investigated. A promising way to obtain a surface with a contact angle of more than 1508 for organic liquids is to make rough microstructures covered with perfluoroalkyl groups, which are bound on some kinds of polyhedral oligomeric silsesquioxanes (POSS), monomeric silanes, and polymers. However, the reported technologies to achieve superamphiphobicity are limited in the forms of films and fibers. As far as we know, there have been no reports on monolithic superamphiphobic materials that can be prepared in a wide range of thickness and in any shapes. We have been recently investigating marshmallow-like gels derived from triand difunctional alkoxysilanes as coprecursors through a facile one-pot sol–gel reaction. These silicone-based macroporous materials have high porosity (> 90%), flexibility both for compression and bending, and built-in superhydrophobicity. The marshmallow-like gels can be used like a sponge for quick removal of organic liquids/oils from oil–water mixtures for environmental purposes and for new solid-phase extraction media in analytical chemistry. By changing the combination of the alkoxysilanes, various kinds of marshmallow-like gels with different functional groups can be obtained. For example, in the case of methyltrimethoxysilane-dimethyldimethoxysilane copolymer system, the obtained gels are composed of the cross-linked polydimethylsiloxane (PDMS)-like molecular structure. They retain the flexible mechanical properties over a wide temperature range from 130 8C to 320 8C, as evidenced from thermal and mechanical analyses. Moreover, owing to their elasticity and bendability even at temperature of under 196 8C, we can successfully absorb and squeeze-out liquid nitrogen. In the case of (3-mercaptopropyl)trimethoxysilane-(3-mercaptopropyl)methyldimethoxysilane copolymer system, gold ions can be adsorbed on the pore surface by the mercapto groups. We employed a vinyltrimethoxysilane (VTMS)-vinylmethyldimethoxysilane (VMDMS) co-precursor system to prepare the first superamphiphobic monolith. The VTMSVMDMS marshmallow-like gel can be obtained by four simple, routine steps within half a day: 1) mixing VTMS, VMDMS, urea, and surfactant n-hexadecyltrimethylammonium chloride (CTAC) in a dilute aqueous acetic acid solution, and stirring for 60 min at room temperature for acid-catalyzed hydrolysis of alkoxysilanes; 2) transferring the resulting transparent sol to an oven for gelation and aging at 80 8C over several hours to promote the siloxane network formation under basic conditions, which is brought up by the hydrolysis of urea into ammonia; 3) washing with alcohol by hand; and 4) evaporative drying under ambient conditions (Figure 1a). The obtained gel (MG1) shows enough marshmallow-like flexibility to recover their original shape from 80% uniaxial compression and 3-point bending (Figure 2; Supporting Information, Figure S1). This material has a superhydrophobic surface with a water contact angle of 1538, which is due to the negligible amount of residual hydrophilic silanol groups, as characterized by Si solid-state cross polarization/ magic angle spinning (CP/MAS) NMR spectroscopy (Supporting Information, Figure S2). However, MG1 does not show oleophobicity, but absorbs organic liquids quickly like a sponge (Figure 3a) as mentioned before. [*] G. Hayase, Dr. K. Kanamori, Prof. K. Nakanishi Department of Chemistry, Graduate School of Science, Kyoto University Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail: kanamori@kuchem.kyoto-u.ac.jp

Polymethylsilsesquioxane-cellulose nanofiber biocomposite aerogels with high thermal insulation, bendability, and superhydrophobicity.

Polymethylsilsesquioxane-cellulose nanofiber (PMSQ-CNF) composite aerogels have been prepared through sol-gel in a solvent containing a small amount of CNFs as suspension. Since these composite aerogels do not show excessive aggregation of PMSQ and CNF, the original PMSQ networks are not disturbed. Composite aerogels with low density (0.020 g cm(-3) at lowest), low thermal conductivity (15 mW m(-1) K(-1)), visible light translucency, bending flexibility, and superhydrophobicity thus have been successfully obtained. In particular, the lowest density and bending flexibility have been achieved with the aid of the physical supporting effect of CNFs, and the lowest thermal conductivity is comparable with the original PMSQ aerogels and standard silica aerogels. The PMSQ-CNF composite aerogels would be a candidate to practical high-performance thermal insulating materials.

Reversible generation of a carbon-centered radical from alkyl iodide using organic salts and their application as organic catalysts in living radical polymerization.

A new method of producing carbon-centered radicals was discovered through the reaction of an alkyl iodide (R-I) with organic salts to reversibly generate the corresponding alkyl radical (R(•)). Via this new reaction, the organic salts were used as new and highly efficient organic catalysts in living radical polymerization. The catalysts included common and inexpensive compounds such as tetrabutylammonium iodide and methyltributylphosphonium iodide. Notably, the catalysts were highly reactive. They enabled the synthesis of high-molecular-weight polymers (up to Mn = 140,000) and the control of acrylate polymerization, which had been difficult with other organic catalysts. The organic salt catalysts were highly versatile, reacting with methacrylate, acrylate, styrene, acrylonitrile, and functional methacrylate monomers. Well-defined block copolymers were also prepared by using this method. A kinetic study quantitatively confirmed the high reactivity of these catalysts. Attractive features of this system include its low cost, its ease of operation, and its ability to access a wide range of polymer designs.

Controlled emission colors and singlet–triplet energy gaps of dihydrophenazine-based thermally activated delayed fluorescence emitters

We have developed thermally activated delayed fluorescence (TADF) emitters containing 5,10-dihydrophenazine as an electron donor and various electron-acceptor units. The TADF emitters exhibit wide ranges of emission colors from green to orange, singlet–triplet energy gaps ΔEST of ∼0–0.19 eV, and delayed fluorescence lifetimes τd of 0.1–50 μs. An organic light-emitting diode containing one of the TADF emitters exhibits a maximum external quantum efficiency (EQE) of 12%, which is higher than those obtained with conventional fluorescent emitters. Time-resolved photoluminescence measurements of the compounds in a host matrix reveal that TADF makes a large contribution to the EQE of the devices. Our findings provide guidelines for modulating ΔEST and τd of TADF emitters.

π-Extended planarized triphenylboranes with thiophene spacers.

Planarized triphenylboranes extended with thiophene or bithiophene spacers were synthesized, which showed intense fluorescences in solution and reversible redox waves for reduction in cyclic voltammetry. Organic light-emitting diodes (OLEDs) using these compounds as an electron-transporting material were fabricated.

Highly efficient electroluminescence from a solution-processable thermally activated delayed fluorescence emitter

We developed a thermally activated delayed fluorescence (TADF) emitter, 2,4,6-tris(4-(9,9-dimethylacridan-10-yl)phenyl)-1,3,5-triazine (3ACR-TRZ), suitable for use in solution-processed organic light-emitting diodes (OLEDs). When doped into 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host at 16 wt. %, 3ACR-TRZ showed a high photoluminescence quantum yield of 98%. Transient photoluminescence decay measurements of the 16 wt. % 3ACR-TRZ:CBP film confirmed that 3ACR-TRZ exhibits efficient TADF with a triplet-to-light conversion efficiency of 96%. This high conversion efficiency makes 3ACR-TRZ attractive as an emitting dopant in OLEDs. Using 3ACR-TRZ as an emitter, we fabricated a solution-processed OLED exhibiting a maximum external quantum efficiency of 18.6%.

On‐Top π‐Stacking of Quasiplanar Molecules in Hole‐Transporting Materials: Inducing Anisotropic Carrier Mobility in Amorphous Films

Dimers of partially oxygen-bridged triarylamines were designed and synthesized as hole-transporting materials. X-ray structural analyses revealed that these compounds form on-top π-stacking aggregates in the crystalline state. TRMC measurements showed that high levels of anisotropic charge transport were induced in the direction of the π-stacking. Surprisingly, even in vacuum-deposited amorphous films, these compounds retained some of the face-on π-stacking, thus facilitating an out-of-plane carrier mobility.

Solid-state 13C and 1H spin diffusion NMR analyses of the microfibril structure for bacterial cellulose.

To obtain further information about the cause for the rather large splitting of the C4 resonance line into the downfield (C4D) and upfield (C4U) lines in CP/MAS 13C NMR spectra for native cellulose, 13C and 1H spin diffusion measurements have been conducted by using different types of bacterial cellulose samples. In 13C spin diffusion measurements, the C4D resonance line is selectively inverted by the Dante pi pulse sequence and the 13C spin diffusion is allowed to proceed from the C4D carbons to other carbons including the C4U carbons with use of the 13C4-enriched bacterial cellulose sample. The analysis based on the simple spin diffusion theory for the process experimentally observed reveals that the C4U carbons may be located at distances less than about 1 nm from the C4D carbons. In 1H spin diffusion measurements, poly(vinyl alcohol) (PVA) films in which ribbon assemblies of bacterial cellulose are dispersed are employed and the 1H spin diffusion process is examined from the water-swollen PVA continuous phase to the dispersed ribbon assemblies by the 13C detection through the 1H-13C CP technique. As a result, it is found that the C4D and C4U carbons are almost equally subjected to the 1H spin diffusion from the PVA phase, indicating that the C4U carbons are not localized in some limited area, e.g. in the surfacial region, but are distributed in the whole area in the microfibrils. These experimental results suggest that the C4U carbons may exist as structural defects probably due to conformational irregularity associated with disordered hydrogen bonding of the CH(2)OH groups in the microfibrils.