Five emitters CzAZB, tBuCzAZB, tmCzAZB, dmAcAZB, and PxzAZB based on dibenzo-1,4-azaborine as the electron acceptors and two identical amine groups as the donors were designed and synthesized. The dihedral angles between the planes of dibenzo-1,4-azaborine acceptors and amine-based donors greatly affect the thermally activated delayed fluorescence (TADF) property of these materials. A simple concept "steric switching" is introduced to predict whether the emitter possesses TADF property. CzAZB and tBuCzAZB, with very high photoluminescence quantum yields (PLQYs) but small dihedral angles, do not show TADF. In contrast, tmCzAZB reveals a PLQY of only 56% but with a large dihedral angle due to the presence of two methyl groups at C1 and C8 of the carbazole groups, the steric switching operates, and the compound shows TADF property with a deep-blue color having CIE coordinates of (0.14, 0.15). In a similar manner, in dmAcAZB and PxzAZB with high PLQYs and large dihedral angles between the donor and acceptor planes, the "TADF steric switch" readily operates to achieve device external quantum efficiencies as high as 20.8 ± 1.2 and 27.5 ± 1.9% with blue and green emissions, respectively.
Hydroarylation of bicyclic alkenes has been developed using a low-valent Re -catalyzed, directing group-assisted C-H bond activation strategy. The addition of sodium acetate significantly improves the reaction efficiency; moreover, bicyclic alkenes such as 7-oxa and aza benzonorbornadienes worked efficiently under this reaction condition. Preliminary mechanistic studies suggest that, after the alkene insertion, the rhenacycle preferentially undergoes protonolysis rather than reductive elimination.
Re(i)-Catalyzed ortho alkenylation of arylpyridines and N-pyrimidyl indoles with alkynes provides excellent regio- and stereoselectivity with lower catalytic loadings.
A nickel‐catalyzed synthesis of trisubstituted acrylic acids from alkynes, Grignard reagents, and CO2 is reported. The reaction proceeds through carbomagnesiation of the alkyne with Grignard reagent followed by carboxylation with CO2 under mild reaction conditions in short time. Various unsymmetrical alkynes were transformed into the corresponding acid products in good yields with high stereoselectivity.
Continued efforts are made on the development of earth-abundant
metal catalysts for dehydrogenation/hydrolysis of amine boranes. In
this study, complex [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)(μ-CO)Fe(NO)2] (3-K-crown, MePyr = 3-methylpyrazolate) was explored as a pre-catalyst
for the dehydrogenation of dimethylamine borane (DMAB). Upon evolution
of H2(g) from DMAB triggered by 3-K-crown,
parallel conversion of 3-K-crown into [(NO)2Fe(N,N′-MePyrBH2NMe2)]− (5) and
an iron-hydride intermediate [(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2]− (A) was evidenced by X-ray
diffraction/nuclear magnetic resonance/infrared/nuclear resonance
vibrational spectroscopy experiments and supported by density functional
theory calculations. Subsequent transformation of A into
complex [(NO)2Fe(μ-CO)2Fe(NO)2]− (6) is synchronized with the deactivated
generation of H2(g). Through reaction of complex [Na-18-crown-6-ether][(NO)2Fe(η2-BH4)] (4-Na-crown) with CO(g) as an alternative synthetic route, isolated
intermediate [Na-18-crown-6-ether][(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2] (A-Na-crown) featuring catalytic reactivity
toward dehydrogenation of DMAB supports a substrate-gated transformation
of a pre-catalyst [(NO)2Fe(μ-MePyr)(μ-CO)Fe(NO)2]− (3) into the iron-hydride
species A as an intermediate during the generation of
H2(g).
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