Enhancing Light-Emitting Efficiency of Blue Through-Space Charge Transfer Emitters via Fixing Configuration Induced by Intramolecular Hydrogen Bonding.

Closely aligned configuration of the donor (D) and acceptor (A) is crucial for the light-emitting efficiency of thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) characteristics. However, precisely controlling the D-A distance of blue TSCT-TADF emitters is still challenging. Herein, an extra donor (D*) located on the side of the primary donor (D) is introduced to construct the hydrogen bonding with A and thus modulate the distance of D and A units to prepare high-efficiency blue TSCT emitters. The obtained "V"-shaped TSCT emitter presents a minimal D-A distance of 2.890 Å with a highly parallel D-A configuration. As a result, a high rate of radiative decay (>107 s-1) and photoluminescence quantum yield (nearly 90%) are achieved. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies (EQEmax) of 27.9% with a Commission Internationale de L'Eclairage (CIE) coordinate of (0.16, 0.21), which is the highest device efficiency of fluorene-based blue TSCT-TADF emitters. In addition, the TSCT-TADF emitter-sensitized OLEDs also achieve a high EQEmax of 29.3% with a CIE coordinate of (0.12, 0.16) and a narrow emission.

Design, Synthesis and Second-Order Nonlinear Optical Properties of Azobenzene-Based Polysiloxanes.

To study the effect of polymeric structures on second-order nonlinear optical properties, polysiloxanes materials based on azobenzene as chromophore have been designed and synthesized successfully. Herein, the siloxane monomer is directly bonded to azobenzene units by palladium catalysis, which avoids the influence of flexible chains on the photoelectric properties of azobenzene. According to the different positions of azobenzene units in the polymers, it is divided into side-chain, main-chain, and alternative-type polymers. The chemical structures of obtained polysiloxanes are confirmed by nuclear magnetic resonance spectra and mass spectra. Three polymers present high thermal decomposition temperatures and the medium glass transition temperatures. The effects of polymeric structures on the second-order nonlinear properties are compared. The main-chain polysiloxane possesses the highest thermal stability because of its rigid architecture. The side-chain polysiloxane shows the fastest isomerization transformation rate due to the large free volume. Besides, the alternative polysiloxane displays the best second-order nonlinear performance with second harmonic generation coefficient (d33 ) value of 47.6 pm V-1 , which is 3 times higher than the side-chain one.

Hyperbranched Conjugated Polymer with Multiple Charge Transfer Enables High-Efficiency Nondoped Red Electroluminescence with Low Driving Voltage.

Conjugated polymers featuring thermally activated delayed fluorescence (TADF) attract tremendous attention in both academic and industry communities due to their easy solution processing for fabricating large-area and low-cost high-performance polymer light-emitting diodes (PLEDs). However, current nondoped solution-processed PLEDs frequently encounter significant efficiency roll-offs and unreasonably high operating voltages at high brightness, especially for red-emitting polymers. Herein, we design hyperbranched conjugated polymers (HCPs) with D-A-D type TADF characteristics for high-performance red-emitting PLEDs. Multiple intramolecular charge transfer (ICT) channels induced by quasi-equivalent donors of the TADF core strongly boost the reverse intersystem crossing (RISC) process and singlet excitons radiative transition. Coupling with the efficient energy transfer process generated by structure advantages of HCPs, the strongly electron-withdrawing oxygen atoms located on the TADF cores further accelerate hole transportation from the host chains to the TADF cores. Under a rational regulation of the TADF core ratio, the related nondoped red-emitting device performs an outstanding performance with an EQEmax of 8.39% and exhibits no roll-off while the luminance is less than 100 cd/m2 and only 3.3% decrease at 500 cd/m2. Simultaneously, the EQE can maintain 7.4% under 1000 cd/m2. Furthermore, the corresponding nondoped device exhibits a low turn-on voltage of around 2.5 V and achieves a luminance of 500 cd/m2 at 3.5 V and even 1000 cd/m2 at 3.9 V. To our knowledge, this is the best performance among all nondoped red PLEDs with high brightness obtained at low operating voltage.

Enhanced Upconversion of Triplet Excitons for Conjugated Polymeric Thermally Activated Delayed Fluorescence Emitters by Employing an Intramolecular Sensitization Strategy.

Endowed by a thermally activated delayed fluorescence (TADF) sensitizer with a high constant rate of reverse intersystem crossing, the singlet excitons could be accumulated and then delivered to emitting states through favorable Förster resonance energy transfer, bypassing the inefficient intersystem transition processes of emitters. However, the conventional intermolecular sensitization strategies suffer from inherent aggregation-induced quenching and inevitable phase segregation of TADF sensitizers and emitters. In this context, we proposed a novel intramolecular sensitization strategy by covalently incorporating the TADF sensitizer into conjugated polymeric emitters. After rationally regulating the proportions of sensitizer and emitter units in polymers, the intramolecular sensitized conjugated TADF polymers with anticipated photophysical properties and stable device performance were obtained. A superior kRISC value over 106 s-1 accompanied by a suppressed nonradiative transition of the triplet exciton could be gained; therefore, the photoluminescence quantum yield (PLQY) could reach nearly 90%. In accord with the superior PLQY values enhanced by our intramolecular sensitization strategy, the solution-processed organic light-emitting diodes (OLEDs) can achieve a maximum external quantum efficiency (EQE) value of 17.8% while still maintaining 16.0% at 1000 cd/m2 with extremely low efficiency roll-off. These results convincingly manifest the significance of an intramolecular sensitization strategy for designing high-efficiency polymeric TADF emitters.

A dual role for acetohydrazide in Pd-catalyzed controlled C(sp3)-H acetoxylation of aldehydes.

The palladium catalyzed aldehyde directed acetoxylation of C(sp3)-H bonds was realized by a transient directing group approach for the first time. Crucial to the successful outcome of this reaction is the dual role of acetohydrazide as a directing group for the catalytic C(sp3)-H activation process and as a protecting group for the CHO functional group. The applicable methodology exhibits good functional group tolerance and occurs readily under mild conditions.