Origins of Stereoselectivity in Mannich Reactions Catalyzed by Chiral Vicinal Diamines

The origins of the enantio- and diastereoselectivities in the Mannich reactions between aldehydes and ketimines catalyzed by chiral vicinal diamines have been determined by density functional theory calculations and distortion–interaction analysis. Computational results indicate a strong energetic preference for hydrogen-bonded nine-membered cyclic transition states. The favored transition states involve eight heavy atoms in the crown (chair–chair) conformation using the nomenclature of the analogous cyclic hydrocarbons. Energetic discrimination in the chirality-imparting step arises from pseudogauche-butane-type interactions in the disfavored transition states, as well as steric clashes between the <i>N</i>-Boc protecting group and the ammonium <i>N</i>-substituents

Improved Efficiency and Enhanced Color Quality of Light-Emitting Diodes with Quantum Dot and Organic Hybrid Tandem Structure

Light-emitting diodes based on organic (OLEDs) and colloidal quantum dot (QLEDs) are widely considered as next-generation display technologies because of their attractive advantages such as self-emitting and flexible form factor. The OLEDs exhibit relatively high efficiency, but their color saturation is quite poor compared with that of QLEDs. In contrast, the QLEDs show very pure color emission, but their efficiency is lower than that of OLEDs currently. To combine the advantages and compensate for the weaknesses of each other, we propose a hybrid tandem structure which integrates both OLED and QLED in a single device architecture. With ZnMgO/Al/HATCN interconnecting layer, hybrid tandem LEDs are successfully fabricated. The demonstrated hybrid tandem devices feature high efficiency and high color saturation simultaneously; for example, the devices exhibit maximum current efficiency and external quantum efficiency of 96.28 cd/A and 25.90%, respectively. Meanwhile, the full width at half-maximum of the emission spectra is remarkably reduced from 68 to 44 nm. With the proposed hybrid tandem structure, the color gamut of the displays can be effectively increased from 81% to 100% NTSC. The results indicate that the advantages of different LED technologies can be combined in a hybrid tandem structure

Efficient Red/Green/Blue Tandem Quantum-Dot Light-Emitting Diodes with External Quantum Efficiency Exceeding 21%

Highly efficient tandem quantum-dot light-emitting diodes (QLEDs) are developed by using an interconnecting layer (ICL) with the structure of ZnMgO/Al/HATCN/MoO₃. The developed ICL exhibits high transparency, efficient charge generation/injection capability, and high robustness to resist solvent damage during deposition of the upper functional layers. With the proposed ICL, full color (red/green/blue, R/G/B) tandem QLEDs are demonstrated with extremely high current efficiency and external quantum efficiency (EQE): 17.9 cd/A and 21.4% for B-QLEDs, 121.5 cd/A and 27.6% for G-QLEDs, 41.5 cd/A and 23.1% for R-QLEDs. To the best of our knowledge, these are the highest values ever reported. In addition, the EQEs of R-, G-, and B-QLEDs all exceed 21%. The high efficiency can be well maintained over a wide range of luminance from 10² to 10⁴ cd/m². For example, even at a high brightness of 20 000 cd/m², the EQE of R-, G-, and B-QLEDs can still sustain its 96%, 99%, and 78% maximum value, respectively. The demonstrated full-color tandem QLEDs, with extremely high efficiency, long operational lifetime, low roll-off efficiency, and high color purity, would be ideal candidates to bring QLEDs into the next generation of full-color displays and the solid-state lighting market

Very Bright and Efficient Microcavity Top-Emitting Quantum Dot Light-Emitting Diodes with Ag Electrodes

The microcavity effect in top-emitting quantum dot light-emitting diodes (TQLEDs) is theoretically and experimentally investigated. By carefully optimizing the cavity length, the thickness of the top Ag electrode and the thickness of the capping layer, very bright and efficient TQLEDs with external quantum efficiency (EQE) of 12.5% are demonstrated. Strong dependence of luminance and efficiency on cavity length is observed, in good agreement with theoretical calculation. By setting the normal-direction resonant wavelength around the peak wavelength of the intrinsic emission, highest luminance of 112 000 cd/m² (at a driving voltage of 7 V) and maximum current efficiency of 27.8 cd/A are achieved, representing a 12-fold and a 2.1-fold enhancement compared to 9000 cd/m² and 13.2 cd/A of the conventional bottom emitting devices, respectively, whereas the highest EQE of 12.5% is obtained by setting the resonant wavelength 30 nm longer than the peak wavelength of the intrinsic emission. Benefit from the very narrow spectrum of QDs and the low absorption of silver electrodes, the potential of microcavity effect can be fully exploited in TQLEDs

Facile Rh(III)-Catalyzed Synthesis of Fluorinated Pyridines

A Rh­(III)-catalyzed C–H functionalization approach was developed for the preparation of multisubstituted 3-fluoropyridines from α-fluoro-α,β-unsaturated oximes and alkynes. Oximes substituted with aryl, heteroaryl, and alkyl β-substituents were effective coupling partners, as were symmetrical and unsymmetrical alkynes with aryl and alkyl substituents. The first examples of coupling α,β-unsaturated oximes with terminal alkynes was also demonstrated and proceeded with uniformly high regioselectivity to provide single 3-fluoropyridine regioisomers. Reactions were also conveniently set up in air on the benchtop

Inverted Quantum-Dot Light-Emitting Diodes Fabricated by All-Solution Processing

All-solution processed, multilayer, and inverted quantum-dot light-emitting diodes (QD-LEDs) are developed in this work. To protect the QDs from dissolving by the solvents of upper layers, the solvents of poly­(9-vinlycarbazole) (PVK) hole transporting layer are first investigated. The QD layer has been less affected by <i>o</i>-dichlorobenzene solvent than other typical solvents like chloroform and chlorobenzene. Second, to deposit a hydrophilic poly­(ethylenedioxythiophene)/polystyrenesulfonate (PEDOT:PSS) hole injection layer on top of hydrophobic PVK, the surface energy of the PEDOT:PSS is reduced by using isopropanol as the additive. With optimized conditions, the demonstrated QD-LEDs exhibit a maximum luminance of 16290 cd/m² and a peak current efficiency of 4.1 cd/A, which is the highest among the reported values. These results may offer a practicable platform for further research, leading to the achievement of all-solution processed, multilayer, and efficient inverted QD-LEDs

Performance of Inverted Quantum Dot Light-Emitting Diodes Enhanced by Using Phosphorescent Molecules as Exciton Harvesters

Exciton harvesters based on blue phosphorescent molecules bis­(4,6-difluorophenylpyridinato-<i>N</i>,C2)­picolinatoiridium (FIrpic) doped in 4,4′,4″-tris­(carbazol-9-yl)­triphenylamine (TCTA) are used to enhance the performance of inverted quantum dot light-emitting diodes (QD-LEDs). In the proposed device structures, electrons that leak to the TCTA layer can be effectively captured by FIrpic and subsequently can recombine in the TCTA:FIrpic layer. The harvested energy is then nonradiatively transferred to the adjacent QDs via Förster dipole–dipole coupling mechanism. Because of effective harvest of leaked electrons and complete energy transfer from FIrpic to the adjacent QDs, the demonstrated QD-LEDs exhibit pure QD emission, higher efficiency (1.62-fold improvement), and longer lifetime

Ni-Catalyzed 1,1- and 1,3-Aminoboration of Unactivated Alkenes

Alkene borylfunctionalization reactions have emerged as useful methods for chemical synthesis. While much progress has been made on 1,2-borylamination reactions, the related 1,1- and 1,3-borylaminations have not been reported. Herein, a Ni-catalyzed 1,1-borylamination of 1,1-disubstituted and monosubstituted alkenes and a 1,3-borylamination of cyclic alkenes are presented. Key to development of these reactions was the identification of an alkyllithium activator in combination with Mg salts. The utility of the products and the mechanistic details are discussed

Regio- and Diastereoselective Synthesis of Highly Substituted, Oxygenated Piperidines from Tetrahydropyridines

Diastereoselective epoxidation and regioselective ring-opening methods were developed for the synthesis of densely substituted, oxygenated piperidines from two classes of tetrahydropyridines with distinct stereochemical displays of functionalities. A new and practical in situ prepared epoxidation reagent was developed for the diastereoselective epoxidation of one class of sterically hindered tetrahydropyridines. The novel bifunctional epoxidation reagent, 2-carboperoxy-3,4,5,6-tetrafluorobenzoic acid, was designed to incorporate highly reactive percarboxy acid and pendant carboxylic acid groups, which through hydrogen bonding to the amino group successfully overrode steric effects and directed epoxidation to occur at the more hindered face of the tetrahydropyridine. Nucleophilic ring-opening of the epoxides with water, alcohols, and HF proceeded with high regioselectivity, affording piperidinol products with adjacent tetrasubstituted carbons

Arylketone π‑Conjugation Controls Enantioselectivity in Asymmetric Alkynylations Catalyzed by Centrochiral Ruthenium Complexes

The origin of enantio­selectivity in the asymmetric alkyn­yl­ation of trihalo­methyl ketones catalyzed by octahedral stereogenic-at-ruthenium complexes has been investigated through density functional theory calculations. Computational results support a mechanism involving formation of a ruthenium acetylide, followed by pre-coordination of the trihalomethyl ketone through the carbonyl oxygen and intra­molecular attack of the acetylide via a compact four-membered transition state. Differences in computed free energies of activation for the formation of the major and minor propargyl alcohol enantiomers are in good agreement with the experimentally observed levels of asymmetric induction. Analysis of fragment distortion energies shows that disfavored transition states are destabilized due to the more severe distortion and loss of π-conjugation in the coordinated aryl­ketone fragments. Examination of the different substitution patterns in the ketone substrate and the catalyst reveals the key steric factors that control the enantio­selectivity. Finally, calculations indicate promising directions for the simplification of the catalyst scaffold while preserving the high levels of enantio­selectivity of these alkyn­yl­ation reactions