Asymmetric phase-transfer reactions under base-free neutral conditions

Although quaternary onium salt-catalyzed phase-transfer reactions are generally believed to require base additives, we discovered even without any base additives conjugate additions of 3-substituted oxindoles proceeded smoothly in the presence of lipophilic quaternary onium bromide under water-organic biphasic conditions. The mechanism of this novel base-free neutral phase-transfer reaction system was investigated, and the assumed catalytic cycle was presented together with interesting effects of water and lipophilicity of the phase-transfer catalyst. The base-free neutral phase-transfer reaction system could be applied to highly enantioselective conjugate additions, aldol reaction, sulfenylation, and chlorination under the influence of chiral bifunctional onium bromides as key catalysts.     

A Base‐Free Neutral Phase‐Transfer Reaction System

Although phase‐transfer reactions catalyzed by using quaternary ammonium salts are generally believed to require base additives, we discovered that, even without any base additives, conjugate additions of 3‐substituted oxindoles to nitroolefins proceeded smoothly in the presence of lipophilic quaternary ammonium bromide under water–organic biphasic conditions. The mechanism of this novel base‐free neutral phase‐transfer reaction system is investigated and the assumed catalytic cycle is presented together with interesting effects of water and lipophilicity of the phase‐transfer catalyst. The base‐free neutral phase‐transfer reaction system can be applied to highly enantioselective conjugate addition and aldol reactions under the influence of chiral bifunctional ammonium bromides as key catalysts. The structure of the chiral ammonium enolate intermediate is discussed based on the single‐crystal X‐ray structures of relevant ammonium salts and the importance of bifunctional design of catalyst is clearly explained in the model of intermediate.     

Regulation of Multicolor Fluorescence Changes Found in Donor-acceptor-type Mechanochromic Fluorescent Dyes

The regulation of multicolor fluorescence changes in mechanochromic fluorescence (MCF) remains a challenging task. Herein, we report the regulation of MCF using a donor-acceptor structure. Two crystal polymorphs, BTD-pCHO(O) and BTD-pCHO(R) produced by the introduction of formyl groups to an MCF dye, respond to a mechanical stimulus, allowing a three-color fluorescence change. Specifically, the orange-colored fluorescence of the metastable BTD-pCHO(O) polymorph changed to a deep-red color in the amorphous-like state to finally give a red color in the stable BTD-pCHO(R) polymorph. This change occurred by mechanical grinding followed by vapor fuming. The two different crystal packing patterns were selectively regulated by the electronic effect of the introduced functional groups. The two types of selectively formed crystals in BTD(F)-pCHO bearing fluorine atoms, and BTD(OMe)-pCHO bearing methoxy groups, respond to mechanical grinding, allowing for the regulation of multicolor MCL from a three-color change to two different types of two-color changes.     

Laser-diode pumped heavy-doped Yb:YAG ceramic lasers

Laser performance of heavy-doped Yb:YAG ceramics was investigated using a two-pass pumping miniature laser configuration. Slope efficiency of 52% and optical-to-optical efficiency of 48% have been achieved for 1-mm-thick YAG ceramic doped with 20 at.% ytterbium ions. Laser spectra of Yb:YAG ceramic and single-crystal lasers were addressed under different intracavity laser intensities. Heavy-doped Yb:YAG ceramic is more suitable for a thin disk laser than a single-crystal with the same Yb(3+)-ion lasants.     

Converging-axicon-based radially polarized ytterbium fiber laser and evidence on the mode profile inside the gain fiber

The radially polarized mode is achieved from an active Yb fiber by utilizing of an intracavity converging axicon, where the axicon acts as a TM(01) mode selector based on not only its Brewster convex surface but also the distance between its vertex and plane output coupler. The polarization state of the annular laser beam is checked by using a home-made eight-hole aperture. Furthermore, an uncoated plane glass plate is inserted into the cavity, and the reflected beam points to the existence of an annular lasing mode inside the gain fiber. The issues for developing high-power radially polarized fiber lasers also are considered.     

Luminous Butterflies: Efficient Exciton Harvesting by Benzophenone Derivatives for Full‐Color Delayed Fluorescence OLEDs

Butterfly‐shaped luminescent benzophenone derivatives with small energy gaps between their singlet and triplet excited states are used to achieve efficient full‐color delayed fluorescence. Organic light‐emitting diodes (OLEDs) with these benzophenone derivatives doped in the emissive layer can generate electroluminescence ranging from blue to orange–red and white, with maximum external quantum efficiencies of up to 14.3 %. Triplet excitons are efficiently harvested through delayed fluorescence channels.     

Mapping Platinum Species in Polymer Electrolyte Fuel Cells by Spatially Resolved XAFS Techniques

There is limited information on the mechanism for platinum oxidation and dissolution in Pt/C cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) under the operating conditions though these issues should be uncovered for the development of next‐generation PEFCs. Pt species in Pt/C cathode catalyst layers are mapped by a XAFS (X‐ray absorption fine structure) method and by a quick‐XAFS(QXAFS) method. Information on the site‐preferential oxidation and leaching of Pt cathode nanoparticles around the cathode boundary and the micro‐crack in degraded PEFCs is provided, which is relevant to the origin and mechanism of PEFC degradation.     

Mechanochromic Delayed Fluorescence Switching in Propeller‐Shaped Carbazole–Isophthalonitrile Luminogens with Stimuli‐Responsive Intramolecular Charge‐Transfer Excited States

Herein, the universal design of high‐efficiency stimuli‐responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli‐triggered TADF switching for a series of carbazole–isophthalonitrile‐based donor–acceptor (D–A) luminogens is demonstrated based on systematic photophysical and X‐ray analysis, coupled with theoretical calculations. It was revealed that a tiny alteration of the intramolecular D–A twisting in the excited‐state structures governed by the solid morphologies is responsible for this dynamic TADF switching behavior. This concept is applicable to the fabrication of bicolor emissive organic light‐emitting diodes using a single TADF emitter.     

Azahelicene‐Fused BODIPY Analogues Showing Circularly Polarized Luminescence

Helical carbazole‐based BODIPY analogues were readily synthesized via aza[7]helicenes. The structures of azahelicene‐incorporated BF₂ dyes were elucidated by x‐ray diffraction analysis. DFT calculations revealed that the π‐conjugated system expanded from the helicene moiety to the BODIPY framework. The azahelicene‐fused boron complexes showed the Cotton effects and the circularly polarized luminescence (CPL) in the visible region. Furthermore, an axially chiral binaphthyl group was attached to the helically chiral dyes, which enhanced the chiroptical properties.