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For multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters, electron cloud distributions of their π‐conjugated planes are crucial for determining their eventual performance. Currently, modulation attempts of MR‐TADF emitters are mainly inside the π‐conjugated planes. Possible out‐of‐plane interactions may also significantly impact the photophysical properties, but the exploration is quite limited. Here, a novel concept of using out‐of‐plane (e.g., π–π and lone pair‐π) interactions to introduce spatial perturbation (SPPT) to improve TADF performance is proposed. Two newly developed MR‐TADF emitters, namely, o‐BNPO and BNPO, which both consist of a popular MR framework, DtBuCzB, and diphenylphosphine oxide (DPPO), are compared in depth. In particular, for o‐BNPO, evident π–π interaction is observed between one side of the DtBuCzB π‐conjugated plane and a phenyl ring from DPPO, and lone pair‐π interaction with the oxygen atom from DPPO is noticed on the other side, resulting in significantly accelerated reverse intersystem crossing and better TADF without sacrificing narrowband emission features. Ultimately, in organic light‐emitting diodes with sensitizer‐free emitting layers, both emitters achieve similar narrowband emissions, while the o‐BNPO‐based device demonstrates a much higher external quantum efficiency of 36% and milder efficiency roll‐off. [ABSTRACT FROM AUTHOR]

Multiple resonance (MR)‐type thermally activated delayed fluorescence (TADF) emitters have promising prospects for high‐color‐purity organic light‐emitting diodes (OLEDs), but they are seldom attempted in the fabrication of solution‐processed devices. In addition, another issue with MR‐TADF emitters is that their heteroatom patterns are very limited, hampering their diversity. Herein, a novel double boron (B)‐containing MR‐TADF paradigm that merges an unembedded organoboron unit and a B‐embedded π‐fused MR framework into one system, furnishing a unique para‐B‐π‐B′/meta‐N (nitrogen)‐π‐N molecular pattern is proposed. Based on this, a proof‐of‐concept molecule, BNB′‐1, is developed, simultaneously achieving a bright sharp emission peaking at 540 nm with a full width at half maximum (FWHM) of only 24.5 nm/98 meV, a nearly unity photoluminescence quantum yield and excellent organic solubility. A solution‐processed OLED using BNB′‐1 emitter delivers an impressive external quantum efficiency (EQE) as high as 36.2% with an emissive FWHM of only 30.0 nm/0.13 eV at ≈540 nm; both parameters set new records among the ever‐reported solution‐processed MR‐OLEDs. Moreover, BNB′‐1 also obtains a superhigh EQE of 40.3% in vacuum‐processed OLEDs, surpassing all MR‐OLEDs in the similar emission region. This work provides an interesting solution to develop high‐performance solution‐processable MR‐TADF emitters with diverse heteroatom patterns. [ABSTRACT FROM AUTHOR]

Building blocks and heteroatom alignments are two determining factors in designing multiple resonance (MR)‐type thermally activated delayed fluorescence (TADF) emitters. Carbazole‐fused MR emitters, represented by CzBN derivatives, and the heteroatom alignments of ν‐DABNA are two star series of MR‐TADF emitters that show impressive performances from the aspects of building blocks and heteroatom alignments, respectively. Herein, a novel CzBN analog, Π‐CzBN, featuring ν‐DABNA heteroatom alignment is developed via facile one‐shot lithium‐free borylation. Π‐CzBN exhibits superior photophysical properties with a photoluminescence quantum yield close to 100 % and narrowband sky blue emission with a full width at half maximum (FWHM) of 16 nm/85 meV. It also gives efficient TADF properties with a small singlet‐triplet energy offset of 40 meV and a fast reverse intersystem crossing rate of 2.9×105 s−1. The optimized OLED using Π‐CzBN as the emitter achieves an exceptional external quantum efficiency of 39.3 % with a low efficiency roll‐off of 20 % at 1000 cd m−2 and a narrowband emission at 495 nm with FWHM of 21 nm/106 meV, making it one of the best reported devices based on MR emitters with comprehensive performance. [ABSTRACT FROM AUTHOR]

Multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters are very promising in the high‐resolution and high‐efficiency displays, due to their narrow and highly efficient optical emissions. Early MR‐TADF cores that show only short‐range charge‐transfer (CT) electronic excitations hardly afford ideal performances (e.g., show low efficiencies) in organic light‐emitting diodes (OLEDs). This work thus designs and synthesizes two MR‐TADF emitters (TCzBN‐BP and TCzBN‐FP), where the same MR core TCzBN is chemically modified by the acceptor fragments benzophenone/9‐fluorenone (BP/FP) to incorporate long‐range CT excitations in the two molecules. OLEDs exploiting TCzBN‐BP as emitter, in which short‐range CT excitation is dominant in the first singlet (S1) excited state, achieve a maximum external quantum efficiency (EQE) of 35.6% and a narrow emission bandwidth of 35 nm. In contrast, OLEDs exploiting TCzBN‐FP with an overloaded long‐range CT excitation in the S1 state exhibit a maximum EQE of 27.2% and a broadened emission bandwidth of 56 nm. This work not only shows the importance of careful management of long‐ and short‐range CT excitations, but also provides a new insight into the structure–property relationship in the MR‐TADF emitters, which thus promotes the design of more novel MR‐TADF emitters with high efficiencies and high color purity. [ABSTRACT FROM AUTHOR]

Multiple resonance (MR) type thermally activated delayed fluorescence (TADF) material is currently a research hotspot in organic light‐emitting diodes (OLEDs) due to their high color purity and high exciton utilization. However, there are only a handful of MR‐TADF emitters with emissions beyond the blue‐to‐green region. The very limited emission colors for MR‐TADF emitters are mainly caused by the fact that so far molecular modifications of MR‐TADF do not offer much change in the emission colors. Here, we report a new approach to modifying a prototypical MR core of DABNA by fusing carbazoles to the MR framework. The carbazole‐fused molecule (TCZ‐F‐DABNA) basically maintains the MR‐dominated features of DABNA while red‐shifting the emission. Its OLED achieves an external quantum efficiency of 39.2 % with a peak at 588 nm, which is a record‐high efficiency for OLEDs with peaks beyond 560 nm. This work provides a new approach for significantly tunning emission colors of MR‐TADF emitters. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two multiple resonance derivatives with heavy atom-containing chains are developed. • A new heavy atom effect involvement method for enhancing RISC of MR emitters. • TCzBN-S based evaporated OLEDs afford a low roll-off of 44% at 1000 cd m−2. • Both emitters are feasible for solution-process OLEDs with high performance. Multiple resonance (MR)-based thermally activated delayed fluorescence (TADF) emitters are promising candidates for ultra-high definition (UHD) organic light-emitting diodes (OLEDs). However, the device performances of MR-TADF emitters are usually encumbered with their slow reverse intersystem crossing (RISC) rates (k RISC s). Recently, applying heavy atom effect to MR emitters has been proved to be a plausible solution for enhancing their k RISC and improving TADF properties. Nevertheless, to date, heavy atoms have all been directly fused in MR frameworks, leading to limitations in molecular design and additional structure relaxation from heavy atoms. Herein, we incorporate heavy atom effect via facilely hanging heavy atom-containing chains onto a MR framework and thus synthesize two MR emitters (sulfur ether group for TCzBN-S and sulfone group for TCzBN-SO). An enhanced k RISC of 1.40 × 105 s−1 is reached for TCzBN-S due to sufficient natural transition orbital (NTO) contributions from the heavy atom, which is absent for TCzBN-SO, with a relatively slow k RISC of 5.6 × 104 s−1. Consequently, OLEDs employing TCzBN-S as emitter achieve a maximum external quantum efficiency (EQE) of 30.0% and an alleviated efficiency roll-off of 44% at 1000 cd m−2. In contrast, OLEDs based on TCzBN-SO achieve a maximum EQE of 33.0% but a more severe efficiency roll-off 1000 of 65%. Moreover, solution-process OLEDs based on TCzBN-S and TCzBN-SO are also fabricated and achieved high EQEs of 23.3% and 25.5%, respectively, demonstrating that peripheral flexible and hanging heavy atom chains are also beneficial to solution processing. Our work not only extends heavy atom involving modes in MR emitters, which are important for further developing MR derivatives by using the heavy atom effect, but also confirms that such MR derivatives with hanging chains are applicable to both vacuum-evaporation and solution-process OLEDs. [ABSTRACT FROM AUTHOR]