Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence.

Phosphorescent and thermally activated delayed fluorescence (TADF) blue organic light-emitting diodes (OLEDs) have been developed to overcome the low efficiency of fluorescent OLEDs. However, device instability, originating from triplet excitons and polarons, limits blue OLED applications. Here, we develop a phosphor-sensitized TADF emission system with TADF emitters to achieve high efficiency and long operational lifetime. Peripheral carbazole moieties are introduced in conventional multi-resonance-type emitters containing one boron atom. The triplet exciton density of the TADF emitter is reduced by facilitating reverse intersystem crossing, and the Förster resonant energy transfer rate from phosphor sensitizer is enhanced by high absorption coefficient of the emitters. The emitter exhibited an operational lifetime of 72.9 hours with Commission Internationale de L'Eclairage chromaticity coordinate y = 0.165, which was 6.6 times longer than those of devices using conventional TADF emitters.

A Silane-Based Bipolar Host with High Triplet Energy for High Efficiency Deep-Blue Phosphorescent OLEDs with Improved Device Lifetime.

A high triplet energy host is developed using a silane moiety, 9-(4-(triphenylsilyl)dibenzo[b,d]furan-2-yl)-9H-carbazole (SiDBFCz), is designed through extensive density functional theory (DFT) calculations to obtain appropriate hole and electron injection barriers. The chemical hardness and the charge transport characteristics are comprehensively investigated to realize a bipolar host with high triplet energy over 2.9 eV for deep blue phosphorescent organic light-emitting diodes (PHOLEDs). The synthesized SiDBFCz clearly exhibits the bipolar characteristics especially with emitter molecules doped. An external quantum efficiency over 19 % without any microcavity optimization is achieved thanks to the good charge balance in the SiDBFCz PHOLED. The device lifetime of the SiDBFCz PHOLED is improved more than 1000 %, compared to the unipolar control devices at an initial luminance of 500 cd m-2 . The dramatic enhancement of the operational stability of the deep blue PHOLED is also thoroughly investigated in terms of electrochemical stability of host molecules in charged or excited states. The results clearly indicate that the device lifetime is strongly correlated with the bond dissociation energy and the activation energy for the bond dissociation reaction in triplet excited state.

Anion Sensing with a Blue Fluorescent Triarylboron-Functionalized Bisbenzimidazole and Its Bisbenzimidazolium Salt.

A blue fluorescent p-dimesitylboryl-phenyl-functionalized 1,3-bisbenzimidazolyl benzene molecule (1) has been synthesized in high yield by Stille coupling of bisbenzimidazolyl bromobenzene with p-BMes2-SnBu3-benzene. Methylation of 1 led to the formation of the bisbenzimidazolium salt (2). The utility of both 1 and 2 in sensing CN- and halide (F-, Cl-, Br-, and I-) was examined, and it was found that only the small fluoride and cyanide anions were able to bind to the boron atom with binding constants in the range of 2.9 × 104 to 5 × 105 M-1. Computational studies provided insight into the photophysical properties of the molecules and verified that a charge-transfer process is quenched in these "turn-off" molecular sensors.

Visible light absorption and photoelectrochemical activity of colorless molecular 1,3-bis(dicyanomethylidene)indane (BDMI) by surface complexation on TiO2.

Adsorption of the colorless 1,3-bis(dicyanomethylidene)indane (BDMI) onto a nanocrystalline TiO2 surface unusually turned the BDMI a deep blue color. Upon contact of the BDMI-adsorbed TiO2 (BDMI-TiO2) with an iodide-based redox electrolyte, a photocurrent density as high as 14.9 mA cm(-2) was generated with a photovoltage of 0.42 V, leading to a power conversion efficiency of 3.63%. This unprecedented photovoltaic performance was simultaneously investigated by spectroscopic studies of BDMI-TiO2 films and density functional theory (DFT)/time-dependent DFT (TD-DFT) computational approaches for [BDMI](-)[Ti(OH)3·H2O](+) (1) as a simple model compound to inspect the light to current conversion abilities. All these results established that the color change from colorless to deep blue and the highly efficient photocurrent generation through binding on the TiO2 surface originates from interfacial charge transfer transitions from anionic BDMI to TiO2.

Blue phosphorescent N-heterocyclic carbene chelated Pt(II) complexes with an α-duryl-ß-diketonato ancillary ligand.

C^C* chelated Pt(II) compounds that contain an N-heterocyclic carbene (NHC) donor have recently attracted great interest as blue or white phosphorescent emitters for organic light-emitting devices (OLEDs). To overcome the tendency for excimer formation in Pt(II) compounds, an α-duryl-ß-diketonato ligand was selected as the ancillary ligand for blue phosphorescent C^C* chelated Pt(II) compounds. Using this approach, a series of NHC-based C^C* chelated Pt(II) compounds has been designed and synthesized. The chelate ligands used in the new C^C* chelated Pt(II) compounds include 1-phenylimidazol-2-ylidene in Pt1, 1-phenyl-1,2,4-triazol-5-ylidene in Pt2, p-TMS-1-phenylimidazol-2-ylidene in Pt3, and p-TMS-1-phenyl-1,2,4-triazol-5-ylidene in Pt4. A single-crystal X-ray diffraction analysis revealed that the presence of the α-duryl-ß-diketonato ligand in the Pt(II) compounds effectively suppresses the dimer formation in the crystal lattice. Pt1, Pt2, Pt3, and Pt4 display blue phosphorescence at room temperature. The p-TMS substituted complex Pt3 was found to display the most efficient blue phosphorescence at λ(em) = 468 nm with a Φ(PL) of 0.68. Spectroscopic and computational studies established that the blue phosphorescence in the phenyl-imidazolylidene chelated complexes originates mainly from a (3)MLCT state while that in the phenyl-triazolylidene chelated compounds arises from a (3)ILCT state. Electroluminescent devices using Pt1 and Pt3 as the dopant were fabricated which display both monomer and exciplex emission, leading to pure white light electroluminescence with CIE coordinates of (0.32, 0.31).

Selective sensitization of Eu(III) and Tb(III) emission with triarylboron-functionalized dipicolinic acids.

Three triarylboron- (Mes2BAr-) functionalized dipicolinic acids, namely, 4-(4-(dimesitylboranyl)-2,3,5,6-tetramethylphenyl)pyridine-2,6-dicarboxylic acid (H2L1), 4-(4-(4-dimesitylboranyl)-2,3,5,6-tetramethylphenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2L2), and 4-(4-(4-dimesitylboranyl) phenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2L3), have been designed and synthesized. Lanthanide(III) complexes with the general formula of [NBu4]3[LnL3] (Ln = Eu or Tb; L = L1, L2, or L3) were obtained. The new triarylboron-functionalized ligands were found to be effective in the selective sensitization of the emissions of Eu(III) and Tb(III) ions with a high quantum efficiency (e.g., 0.54 for [NBu4]3[TbL13] in the solid state) upon excitation at ∼330 nm. An intraligand charge-transfer (ILCT) transition from the mesityl or aryl group to the boron or boron-aryl unit was found to play a key role in the activation of the Eu(III) and Tb(III) emissions, based on TD-DFT computational data and luminescence titration experiments performed using fluoride and cyanide ions.

Chelation-assisted photoelimination of B,N-heterocycles.

Metal-chelation and internal H bonds have been found to greatly enhance the photoelimination quantum efficiency of B,N-heterocycles by 2 orders of magnitude. Green phosphorescent Pt(II)-functionalized 1,2-azaborines have been achieved via photoelimination. A mechanistic pathway for the PE reaction has been established.

Triarylboryl-functionalized dibenzoylmethane and its phosphorescent platinum(II) complexes.

An acetylacetonato derivative ligand, dibenzoylmethane (dbm), has been functionalized with a dimesitylboryl group. Phosphorescent N^C-chelate Pt(II) compounds with the new molecule as an ancillary ligand have been achieved and used as effective turn-on phosphorescent sensors for fluoride ions under air.

Formation of azaborines by photoelimination of B,N-heterocyclic compounds.

Highly fluorescent π-conjugated polycyclic azaborines can be prepared from B,N-heterocyclic compounds with a BR2 -CH2 unit through the elimination of an R-H molecule (see scheme). These clean photoelimination reactions occur both in solution and in polymers doped with the precursors.

Tuning the photoisomerization of a N

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes2 compounds (ppy = 2-phenyl pyridine, Mes = mesityl), three metal-functionalized B(ppy)Mes2 compounds, Re-B, Au-B, and Pt-B, have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO)3(tert-Bu2 bpy)(C≡C), Au(PPh3)(C≡C), and trans-Pt(PPh3)2(C≡C)2, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re(I) unit completely quenches the photoisomerization of the boron unit because of a low-lying intraligand charge transfer/MLCT triplet state. The Au(I) unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes2 , upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au-B is comparable to that of the silyl-alkyne-functionalized B(ppy)Mes2 (TIPS-B), which was attributable to a triplet-state-sensitization effect by the Au(I) unit. The Pt(II) unit links two B(ppy)Mes2 together in Pt-B, thereby extending the π-conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt-B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low-energy absorption band of the isomerized boron unit. TD-DFT computations and spectroscopic studies of the three metal-containing boron compounds confirm that the photoisomerization of the B(ppy)Mes2 chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system.

Decorating BODIPY with three- and four-coordinate boron groups.

Two new BODIPY derivative molecules decorated by a Lewis acidic BMes(2)(vinyl) group and a photochromic four-coordinate boryl chromophore, respectively, have been synthesized. Significant mutual influence on photophysical and photochemical properties by the different boron-containing units has been observed.

Modulating the photoisomerization of N,C-chelate organoboranes with triplet acceptors.

Triplet acceptors such as naphthalene, pyrene, and anthracene have been found to be highly effective in controlling the photoisomerization efficiency of N,C-chelate boryl chromophores, establishing the involvement of a photoactive triplet state in the isomerization of this class of photochromic compounds.

Improvement of mass transport of the [Co(bpy)3](II/III) redox couple by controlling nanostructure of TiO2 films in dye-sensitized solar cells.

Mass transport of the [Co(bpy)(3)](II/III) redox electrolyte is found to strongly depend on porosity and pore size of mesoporous TiO(2) films. Photocurrent density is improved by nearly two times with increasing the porosity from 0.52 to 0.59 despite 23% decreased dye loading. Optimized nanostructure sensitized with MK-2 dye shows efficiency of 5.5% at 1 sun.

Highly enantioselective dynamic kinetic resolution of 1,2-diarylethanols by a lipase-ruthenium couple.

A practical procedure has been developed for the dynamic kinetic resolution of 1,2-diarylethanols. This procedure employs a highly enantioselective lipase from Pseudomonas stutzeri (trade name, lipase TL) as the resolution catalyst and a ruthenium complex as the racemization catalyst. Sixteen 1,2-diarylethanols have been efficiently resolved to provide their acetyl derivatives with good yields (95-97%) and high enantiomeric excesses (96-99%).

Air-stable racemization catalysts for the dynamic kinetic resolution of secondary alcohols.

The substitution of a carbonyl ligand with PPh(3) in cyclopentadienylruthenium dicarbonyl complexes produces a new class of recyclable alcohol racemization catalysts. The catalysts are active at room temperature under aerobic conditions in the presence of silver oxide. Furthermore, the catalysts are compatible with the use of a lipase and isopropenyl acetate for the dynamic kinetic resolution (DKR) of secondary alcohols under ambient conditions.

Air-stable racemization catalyst for dynamic kinetic resolution of secondary alcohols at room temperature.

[reaction: see text] A novel racemization catalyst was synthesized for the dynamic kinetic resolution (DKR) of alcohols with a lipase at room temperature in the air. Furthermore, a polymer-supported derivative was also synthesized and tested as a recyclable catalyst for the aerobic DKR of alcohols.