Won-Sik Han

Important role of ancillary ligand in the emission behaviours of blue-emitting heteroleptic Ir(III) complexes

A series of heteroleptic Ir(III) complexes composed of 2-(2,4-difluoro-3-(trifluoromethyl)phenyl)-4-methylpyridine (dfCF3) as the main ligand and such ancillary ligands as acetylacetonate [Ir(dfCF3)2(acac)] (acac), picolinate [Ir(dfCF3)2(pic)] (pic), and tetrakis-pyrazolyl borate [Ir(dfCF3)2(bor)] (bor) were prepared, and their emission behaviors depending on the ancillary ligands were systematically investigated. It was found that the Huang–Rhys factors (SMs) of the emission decrease in the order bor (0.97) > acac (0.87) > pic (0.76), while the nonradiative rate constants (knr/105 s−1) calculated from the quantum yields and lifetimes of emission were in the order acac (4.89) > pic (1.17) > bor (0.28). It was assumed that the large difference of knr for the complexes arose from important contributions of the ancillary ligands to the crossing from an emissive state (3MLCT) to a nonemissive metal-centered state (3MC). The activation energies for the crossing from 3MLCT to 3MC were estimated from the temperature dependencies of the emission lifetime and were found to be 46 meV for acac, 61 meV for pic, and >100 meV for bor. The experimental results were in line with the theoretical calculations based on integrating quantum chemical modeling methods. By the excellent emission behavior, bor was applied as a dopant to prototype deep-blue phosphorescent organic light-emitting diode devices, which revealed high emission efficiency and colour purity.

Electronic alteration of end-on phenyl groups of bis-triazolyl-silanes: electron-transport materials for blue phosphorescent OLEDs

A series of modified bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane (ST), ST-CF3, ST-Me, ST-tBu, and ST-OMe, were designed and prepared by introducing electron-withdrawing groups or electron-donating groups at the 4-position of the end-on phenyl groups on the triazole ring. Systematic investigations of their thermal-, photophysical-, and electron-transporting (ET) properties were successfully carried out. Depending on the electronic characteristics of the substituents at the end-phenyl group on the triazole, their frontier orbital energy levels were controlled while maintaining energy band gaps. Low-temperature photoluminescence spectra indicate that all the prepared ST moieties maintained high triplet energy states up to 2.85 eV owing to the suppression of electron delocalization by the silicon centre. The electron mobilities of all the compounds were investigated by fabricating electron only devices (EODs). The EOD of ST-tBu showed significantly higher current density than the other analogs. Finally, the EL device utilizing ST-tBu as an electron transporting material in phosphorescent organic light-emitting diodes with bis[(4,6-di-fluorophenyl)-pyridinate-N,C2]picolinate (FIrpic) as a dopant showed an external quantum efficiency of 15.9%.

Development of a solvent-free hydrogen storage and release system based on semi-solid-state ammonia borane (AB) fuel: high gravimetric capacity and feasibility for practical application

Ammonia borane (AB), with high hydrogen contents and favorable dehydrogenation properties, is receiving intensive attention for its potential as a hydrogen storage material. In this study, we demonstrate a new type of solvent-free AB fuel system to obtain a high hydrogen systemic gravimetric capacity needed for practical fuel cell application. The new storage material constitutes AB soaked in tetraethylene glycol dimethyl ether (TEGDE) with catalytic amounts of palladium nanoparticles. Notably, TEGDE is very essential for the successful preparation of AB fuel system in a semi-solid state. The use of a minimum amount of TEGDE in this system allows the hybrid AB catalytic system to be fabricated as an efficient solvent and catalytic reaction medium, enabling a high gravimetric and volumetric capacity. For practical applications, AB pellets with spherical shapes have been manufactured by the co-precipitation of AB/TEGDE/PdNPs, followed by the compression of semi-solid AB fuel mixture for fuel transfer from the fuel tank to the hydrogen generator. Consequently, this hybrid semi-solid state catalytic system exhibits a high gravimetric capacity of hydrogen [10.01 material weight%]. With a high hydrogen capacity, a high performance dehydrogenation is obtained because of the synergistic effects facilitated by the highly active PdNPs well-dispersed in a TEGDE medium.

(1,2-Dicarba-closo-dodeca­boran­yl)trimethyl­methanaminium iodide

The title compound, [1-(CH3)3NCH2-1,2-C2B10H11]+·I− or C6H22B10N+·I−, was obtained by the reaction of (1,2-dicarba-closo-dodecaboranyl)dimethylmethanamine with methyl iodide. The asymmetric unit contains two iodide anions and two (o-carboranyl)tetramethylammonium cations. The bond lengths and angles in the carborane cage are within normal ranges, but the N—Cmethylene—Ccage angle is very large [120.2 (2)°] because of repulsion between the carborane and tetramethylammonium units. In the crystal, ions are linked through C—H⋯I hydrogen bonds.

Synthesis of transition metal sulfide and reduced graphene oxide hybrids as efficient electrocatalysts for oxygen evolution reactions

The development of electrochemical devices for renewable energy depends to a large extent on fundamental improvements in catalysts for oxygen evolution reactions (OERs). OER activity of transition metal sulfides (TMSs) can be improved by compositing with highly conductive supports possessing a high surface-to-volume ratio, such as reduced graphene oxide (rGO). Herein we report on the relationship between synthetic conditions and the OER catalytic properties of TMSs and rGO (TMS–rGO) hybrids. Starting materials, reaction temperature and reaction time were controlled to synergistically boost the OER catalytic activity of TMS–rGO hybrids. Our results showed that (i) compared with sulfides, hydroxides are favourable as starting materials to produce the desired TMS–rGO hybrid nanostructure and (ii) high reaction temperatures and longer reaction times can increase physico-chemical interaction between TMSs and rGO supports, resulting in highly efficient OER catalytic activity.

Flexible and Micropatternable Triplet–Triplet Annihilation Upconversion Thin Films for Photonic Device Integration and Anticounterfeiting Applications

Triplet-triplet annihilation upconversion (TTA-UC) has recently drawn widespread interest for its capacity to harvest low-energy photons and to broaden the absorption spectra of photonic devices, such as solar cells. Although conceptually promising, effective integration of TTA-UC materials into practical devices has been difficult due to the diffusive and anoxic conditions required in TTA-UC host media. Of the solid-state host materials investigated, rubbery polymers facilitate the highest TTA-UC efficiency. To date, however, their need for long-term oxygen protection has limited rubbery polymers to rigid film architectures that forfeit their intrinsic flexibility. This study introduces a new multilayer thin-film architecture, in which scalable solution processing techniques are employed to fabricate flexible, photostable, and efficient TTA-UC thin films containing layers of oxygen barrier and host polymers. This breakthrough material design marks a crucial advance toward TTA-UC integration within rigid and flexible devices alike. Moreover, it introduces new opportunities in unexplored applications such as anticounterfeiting. Soft lithography is incorporated into the film fabrication process to pattern TTA-UC host layers with a broad range of high-resolution microscale designs, and superimposing host layers with customized absorption, emission, and patterning ultimately produces proof-of-concept anticounterfeiting labels with advanced excitation-dependent photoluminescent security features.

Incorporating two different chromophores onto a silicon atom: the crystal structure and photophysical properties of 9-{4-[(9,9-dimethyl-9H-fluoren-2-yl)dimethylsilyl]phenyl}-9H-carbazole.

The crystal structure of the title bifunctional silicon-bridged compound, C(35)H(31)NSi, (I), has been determined. The compound crystallizes in the centrosymmetric space group P2(1)/c. In the crystal structure, the pairs of aryl rings in the two different chromophores, i.e. 9-phenyl-9H-carbazole and 9,9-dimethyl-9H-fluorene, are positioned orthogonally. In the crystal packing, no classical hydrogen bonding is observed. UV-Vis absorption and fluorescence emission spectra show that the central Si atom successfully breaks the electronic conjugation between the two different chromophores, and this was further analysed by density functional theory (DFT) calculations.

CCDC 600711: Experimental Crystal Structure Determination

Related Article: Ho-Jin Son, Won-Sik Han, Hyojeong Kim, Chungkyun Kim, Jaejung Ko, Chongmok Lee, Sang Ook Kang|2006|Organometallics|25|766|doi:10.1021/om050991v