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Recent advances in organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF)‐assisted fluorescence (TAF) attest to the great promise of this technology in practical use. However, the simultaneous realization of high efficiency and device durability in blue OLEDs remains a significant challenge. Clarification of the degradation mechanisms correlated to molecular structure and device configuration is the key to extending the device lifetime. In this study, electron transport materials incorporating two triazine units in close proximity are adopted to use in hole‐blocking and electron‐transporting layers, resulting in superior device performances. In addition, a modified photodegradation experiment reveals that the degradation origins closely relate to charge carriers. The optimization of the device according to the obtained findings leads to 4.5 times extension in the lifetime of the TAF‐OLED using a multiple resonance emitter. These results also provide guidelines for designing robust electron transport materials for blue OLEDs. [ABSTRACT FROM AUTHOR]

An ultrapure deep-blue multi-resonance-induced thermally activated delayed fluorescence material (DOB2-DABNA-A) is designed and synthesized. Benefiting from a fully resonating extended helical π-conjugated system, this compound has a small ΔEST value of 3.6 meV and sufficient spin–orbit coupling to exhibit a high-rate constant for reverse intersystem crossing (kRISC = 1.1 × 106 s–1). Furthermore, an organic light-emitting diode employing DOB2-DABNA-A as an emitter is fabricated; it exhibits ultrapure deep-blue emission at 452 nm with a small full width at half maximum of 24 nm, corresponding to Commission Internationale de l'Éclairage (CIE) coordinates of (0.145, 0.049). The high kRISC value reduces the efficiency roll-off, resulting in a high external quantum efficiency (EQE) of 21.6% at 1000 cd m–2. The realization of a BT.2020 blue emitter is challenging due to the spectral redshift associated with π-extension. Here, the authors report a multi-resonance thermally activated delayed fluorescence emitter, exhibiting ultrapure deep-blue narrow emission and CIEy ≤ 0.05 exceeding 20% efficiency. [ABSTRACT FROM AUTHOR]

Boron-based multiple-resonance (MR) emitters exhibit the advantages of narrowband emission, high absolute photoluminescence quantum yield, thermally activated delayed fluorescence (TADF), and sufficient stability during the operation of organic light-emitting diodes (OLEDs). Thus, such MR emitters have been widely applied as blue emitters in triplet–triplet-annihilation-driven fluorescent devices used in smartphones and televisions. Moreover, they hold great promise as TADF or terminal emitters in TADF-assisted fluorescence or phosphor-sensitised fluorescent OLEDs. Herein we comprehensively review organoboron-based MR emitters based on their synthetic strategies, clarify structure–photophysical property correlations, and provide design guidelines and future development prospects. [ABSTRACT FROM AUTHOR]

The investigation of organic light‐emitting diodes (OLEDs) and organic laser devices with thermally activated delayed fluorescence (TADF) molecules is emerging due to the potential of harnessing triplets. In this work, a boron/nitrogen multiple‐resonance TADF polycyclic framework fusing carbazole units (CzBNPh) was proposed. CzBNPh exhibited a narrowband emission (<30 nm), a unity photoluminescence quantum yield, and a fast radiative rate. Consequently, CzBNPh demonstrated a low distributed feedback (DFB) lasing threshold of 0.68 μJ cm−2. Furthermore, the stimulated emission zone of CzBNPh was effectively separated from its singlet and triplet absorption, thereby minimizing the singlet‐triplet annihilation under long‐pulsed excitation ranging from 20 μs to 2.5 ms. Significantly, the enhanced rigid molecular conformation, thermal stability, and photo‐stability resulted in improved lasing and electroluminescence stability compared to that of 5,9‐diphenyl‐5,9‐diaza‐13b‐boranaphtho[3,2,1‐de]anthracene (DABNA)‐core. These findings indicate the potential of CzBN‐core as a promising framework for achieving long‐pulsed wave and electrically‐pumped lasing in the future. [ABSTRACT FROM AUTHOR]

Light‐emitting electrochemical cells (LECs) have attracted considerable attention owing to their unique luminescence mechanisms and device structures, making them suitable for solution processing. However, these cells encounter problems related to color purity, which plays an important role in efficient and high‐resolution displays. Here, two types of solution‐processed LECs are reported that exhibit ultrapure blue emissions at 467 and 452 nm with narrow emission full‐widths at half‐maximum (FWHM) values of 21 and 30 nm, corresponding to the Commission Internationale de l'éclairage (CIE) coordinates of (0.12, 0.15) and (0.16, 0.10), respectively. These values are obtained using the multi‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials, namely, ν‐DABNA and DABNA‐4TBC. The suppression of structural relaxation and vibronic coupling in these MR‐TADF materials results in high‐color‐purity emissions with a small FWHM. Host‐guest‐type LECs employing these molecules as guest emitters in a spin‐coated single active layer exhibit both ultrapure blue electroluminescence (EL) and solution processability, strongly supporting their use in building a sustainable society in the future. [ABSTRACT FROM AUTHOR]

We report the synthesis and late‐stage diversification of a new class of hetero‐buckybowl, BN‐embedded dibenzocorannulenes (B2N2‐DBCs). The synthesis is achieved via one‐shot halogenative borylation, comprising the nitrogen‐directed haloboration of alkyne and an intramolecular bora‐Friedel‐Crafts reaction, which provides BN‐embedded dibenzocorannulene possessing two bromo substituents (B2N2‐DBC‐Br). B2N2‐DBC‐Br undergoes diversification via coupling reactions to provide a variety of arylated derivatives (B2N2‐DBC‐R), exhibiting strong blue fluorescence. An organic light‐emitting diode (OLED) employing one of the derivatives as an emitter exhibited a high external quantum efficiency of 6.6 % and long operational lifetime of 907 h at an initial luminance of 1000 cd m−2, indicating the significant potential for the development of efficient and stable hetero‐buckybowl‐based OLED materials. [ABSTRACT FROM AUTHOR]

Ultra‐narrowband blue multi‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials (V‐DABNA and V‐DABNA‐F), consisting of three DABNA subunits possessing phenyl or 2,6‐difluorophenyl substituents on the peripheral nitrogen atoms are synthesized by one‐shot triple borylation. Benefiting from the inductive effect of fluorine atoms, the emission maximum of V‐DABNA‐F (464 nm) is blueshifted from that of the parent V‐DABNA (481 nm), while maintaining a small full width at half maximum (FWHM, 16 nm) and a high rate constant for reverse intersystem crossing (6.5 × 105 s−1). The organic light‐emitting diodes (OLEDs) using V‐DABNA and V‐DABNA‐F as emitters are fabricated by vapor deposition and exhibit blue emission at 483 and 468 nm with small FWHMs of 17 and 15 nm, corresponding to Commission Internationale d'Éclairage coordinates of (0.09, 0.27) and (0.12, 0.10), respectively. Both devices achieve high external quantum efficiencies of 26.2% and 26.6% at the maximum with minimum efficiency roll‐offs of 0.9% and 3.2%, respectively, even at 1000 cd m−2, which are record‐setting values for blue MR‐TADF OLEDs. [ABSTRACT FROM AUTHOR]

There is increasing interest in thermally activated delayed fluorescence (TADF) in materials, and to understand its mechanism in the excited state dynamics. Recent challenges include color purity, efficient deep‐blue emission, fast exciton decay lifetimes, high reverse intersystem crossing rates (kRISC), low‐efficiency roll‐off in organic light‐emitting diodes (OLEDs), and long device lifetimes. Here, a series of compounds having benzonitrile and carbazole rings are examined, that provide a detailed understanding of the excited states, and a guideline for high‐performance TADF. A dense alignment of the excited states with several different characters within a small energy range results in high kRISC of >2 × 106 s−1, while maintaining radiative rate constants (kr) >107 s−1. OLEDs based on the optimum compound exhibit a low‐efficiency roll‐off and a CIEy (y color coordinate of Commission Internationale de l'Éclairage) <0.4. TADF‐assisted fluorescence (TAF) OLED exhibits a maximum external quantum efficiency of 22.4% with CIE coordinates (0.13,0.15). This work also provides insights for device engineering and molecular designs. [ABSTRACT FROM AUTHOR]

Herein, we reported an ultrapure blue multiple‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) material (ν‐DABNA‐O‐Me) with a high photoluminescence quantum yield and a large rate constant for reverse intersystem crossing. Because of restricted π‐conjugation of the HOMO rather than the LUMO induced by oxygen atom incorporation, ν‐DABNA‐O‐Me shows a hypsochromic shift compared to the parent MR‐TADF material (ν‐DABNA). An organic light‐emitting diode based on this material exhibits an emission at 465 nm, with a small full‐width at half‐maximum of 23 nm and Commission Internationale de l'Eclairage coordinates of (0.13, 0.10), and a high maximum external quantum efficiency of 29.5 %. Moreover, ν‐DABNA‐O‐Me facilitates a drastically improved efficiency roll‐off and a device lifetime compared to ν‐DABNA, which demonstrates significant potential of the oxygen atom incorporation strategy. [ABSTRACT FROM AUTHOR]

By simple modification of the functional groups on the boron–nitrogen‐containing skeleton, the energy level of the highest occupied molecular orbital (EHOMO) of emitters can be easily adjusted. Blue‐emission derivatives are developed, which are capable of showing small full width at half maximums and high photoluminescence quantum yields. Blue thermally activated delayed fluorescence (TADF)‐assisted fluorescence organic light‐emitting diodes (TAF‐OLEDs) based on two new emitters as the terminal emitter are fabricated, resulting in high external quantum efficiency (EQE) of up to 21.9%, high color purity, and high brightness (Lmax = 63 777 cd m−2). By analyzing the transient electroluminescence spectra of the TAF‐OLEDs, it is found that a smaller EHOMO difference between TADF‐assistant dopant (TADF‐AD) and terminal emitter efficiently helps to decrease hole trapping inside the emitting layer, hence resulting in a lower efficiency rolloff and a longer operational device lifetime. TAF‐OLEDs based on CzBNCz as a terminal emitter having the closest EHOMO to that of TADF‐AD show a maximum EQE of 21.9% together with a reduced efficiency rolloff (EQEs of 21.2% and 19.8% at 100 and 1000 cd m−2, respectively). This research provides a designing principle for a terminal emitter in TAF‐OLEDs with well‐matched energy levels towards reaching the requirements of commercial displays. [ABSTRACT FROM AUTHOR]

Carbazole‐based DABNA analogues (CzDABNAs) were synthesized from triarylamine by regioselective one‐shot single and double borylation. The reaction proceeded selectively at the ortho position of the carbazolyl group, where the highest occupied molecular orbital is mainly localized owing to the difference in the electron‐donating abilities of the diarylamino and carbazolyl groups. The facile and scalable method enabled synthesis of CzDABNAs, exhibiting narrowband thermally activated delayed fluorescence with emission spectra ranging from deep blue to green. The organic light‐emitting diode devices employing these products as emitters exhibited deep‐blue, sky‐blue, and green emission with high external quantum efficiencies of 19.5, 21.8, and 26.7 %, respectively. [ABSTRACT FROM AUTHOR]