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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]

Employing metal‐free thermally activated delayed fluorescence (TADF) materials as novel photosensitizers (PSs) for triplet–triplet annihilation upconversion (TTA‐UC) systems has received great attention. Nevertheless, how to choose suitable TADF materials is still a gap that deserves further in‐depth research. Herein, by systematically investigating the three TADF‐based TTA‐UC systems, it is found that TADF materials with multiple resonance (MR) effect are more suitable PSs for high‐performance TTA‐UC systems than those with intramolecular charge transfer (ICT) properties. Therefore, unlike a negligible maximum UC quantum efficiency (ΦUC', with a theoretical maximum of 100%) of 0.05% for ICT‐1 with a highly twisted molecular structure, MR‐TADF materials MR‐1 and MR‐2 offer superior UC performance with ΦUC' values above 12.7%. All these results suggest that employing MR‐TADF materials as the PS is the key to achieving efficient TTA‐UC performance. [ABSTRACT FROM AUTHOR]

Constructing donor‐π‐acceptor (D‐π‐A)‐type molecular structures by employing a phenyl as the π‐bridge to link donor (D) and acceptor (A) units has been recognized as an effective way to develop highly efficient red thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs). However, flexible and relatively planar structures would open potential energy loss channels, such as nonradiative inactivation and aggregation‐induced triplet quenching processes. Here, a bulky spiro‐9,9′‐bifluorene unit is first implemented to serve as a bridging group to construct a D‐π‐A molecule, enabling it to have higher overall rigidity, more sufficient steric hindrance, prolonged molecule length, and obvious aggregation‐induced emission characteristics compared with a common phenyl bridge. As a result, energy dissipation routes are effectively relieved at the unimolecular level, together with mitigated interchromophore quenching, rendering a 100% photoluminescence quantum yield and a larger horizontal dipole ratio of 89%. The OLED based on 3‐(2‐(diphenylamino)‐9,9′‐spirobi[fluoren]‐7‐yl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile exhibits an excellent external quantum efficiency of nearly 37% at 612 nm, which is over 1.38‐fold enhancement compared with the phenyl bridge‐based control molecule. This work provides an instructive solution to design highly efficient red TADF emitters exploiting D‐π‐A‐type molecular structures. [ABSTRACT FROM AUTHOR]

Hindered by spectral broadening issues with redshifted emission, long‐wavelength (e.g. maxima beyond 570 nm) multiple resonance (MR) emitters with full width at half maxima (FWHMs) below 20 nm remain absent. Herein, by strategically embedding diverse boron (B)/nitrogen (N) atomic pairs into a polycyclic aromatic hydrocarbon (PAH) skeleton, we propose a hybrid pattern for the construction of a long‐wavelength narrowband MR emitter. The proof‐of‐concept emitter B4N6‐Me realized orange‐red emission with an extremely small FWHM of 19 nm (energy unit: 70 meV), representing the narrowest FWHM among all reported long‐wavelength MR emitters. Theoretical calculations revealed that the cooperation of the applied para B‐π‐N and para B‐π‐B/N‐π‐N patterns is complementary, which gives rise to both narrowband and redshift characteristics. The corresponding organic light‐emitting diode (OLED) employing B4N6‐Me achieved state‐of‐the‐art performance, e.g. a narrowband orange‐red emission with an FWHM of 27 nm (energy unit: 99 meV), an excellent maximum external quantum efficiency (EQE) of 35.8 %, and ultralow efficiency roll‐off (EQE of 28.4 % at 1000 cd m−2). This work provides new insights into the further molecular design and synthesis of long‐wavelength MR emitters. [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]

The conformational distribution and mutual interconversion of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utilization. However, their influence on the photophysics in amorphous film states is still not known due to the lack of a suitable quantitative analysis method. Herein, we used temperature‐dependent time‐resolved photoluminescence spectroscopy to quantitatively measure the relative populations of the conformations of a TADF emitter for the first time. We further propose a new concept of "self‐doping" for realizing high‐efficiency nondoped OLEDs. Interestingly, this "compositionally" pure film actually behaves as a film with a dopant (quasi‐equatorial form) in a matrix (quasi‐axial form). The concentration‐induced quenching that may occur at high concentrations is thus expected to be effectively relieved. The "self‐doping" OLED prepared with the newly developed TADF emitter TP2P‐PXZ as a neat emitting layer realizes a high maximum external quantum efficiency of 25.4 % and neglectable efficiency roll‐off. [ABSTRACT FROM AUTHOR]

While monochrome organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have achieved over 30% external quantum efficiencies (EQEs), all‐TADF white OLEDs (WOLEDs) are still lagging behind. Herein, a simple system based on two color‐complementary TADF emitters is exploited to realize high‐performance WOLEDs. By doping a high‐performance orange–red TADF fluorophor (BPPZ‐DPXZ) into a blue TADF host (DBFCz‐Trz), energy transfer, and triplet‐to‐singlet conversion in the host‐dopant system can be optimized to simultaneously achieve full exciton utilization and color balance. With this design, all‐TADF single‐emitting‐layer WOLEDs with a maximum EQE up to 32.8% are demonstrated. This high efficiency surpasses EQEs of reported WOLEDs based on both TADF as well as phosphorescence. It is expected that this finding can provide new insight for designing highly efficient all‐TADF WOLEDs. [ABSTRACT FROM AUTHOR]

Exciplex emitters have been extensively studied owing to their natural thermally activated delayed fluorescence characteristic, and many efforts have been made to improve their performance in organic light emitting diodes (OLEDs). In this work, the authors propose a novel strategy by introducing intermolecular hydrogen bond (HB) between electron‐donating and electron‐accepting constituting molecules (D and A) to suppress non‐radiative transition of exciplex emitters and thus improve their efficiencies and stabilities in the OLEDs. Accordingly, three exciplex emitters are constructed by using 1,3‐di(10H‐phenoxazin‐10‐yl)benzene (13PXZB) as donor and 4,6‐bis(3,5‐di(pyridin‐4‐yl)phenyl)‐2‐methylpyrimidine (B4PyMPM), 4,6‐bis(3,5‐di(pyridin‐3‐yl)phenyl)‐2‐methylpyrimidine (B3PyMPM), and 4,6‐bis(3,5‐di(pyridin‐2‐yl)phenyl)‐2‐methylpyrimidine (B2PyMPM) as acceptors. With the most intermolecular HBs, 13PXZB:B4PyMPM shows the highest photoluminescence quantum yield (69.6%) and the lowest rate constant of non‐radiative process of triplet excitons (3.4 × 105 S−1). And the OLED using 13PXZB:B4PyMPM as the emitter successfully exhibits a much higher external quantum efficiency of 14.6% than other contrastive devices. Moreover, the half lifetime of 13PXZB:B4PyMPM is 4.8 and 8.2 times higher than those of 13PXZB:B3PyMPM and 13PXZB:B2PyMPM in the devices. This work not only demonstrates that intermolecular HBs between D and A molecules can improve the performance of exciplex emitters, but also paves a new route to develop efficient and stable exciplex emitters. [ABSTRACT FROM AUTHOR]

Developing red thermally activated delayed fluorescence (TADF) emitters for high‐performance OLEDs is still facing great challenge. Herein, three red TADF emitters, pDBBPZ‐DPXZ, pDTBPZ‐DPXZ, and oDTBPZ‐DPXZ, are designed and synthesized with same donor–acceptor (D‐A) backbone with different peripheral groups attaching on the A moieties. Their lowest triplet states change from locally excited to charge transfer character leading to significantly enhance reverse intersystem crossing process. In particular, oDTBPZ‐DPXZ exhibits efficient TADF feature and exciton utilization. It not only achieves an external quantum efficiency (EQE) of 20.1 % in red vacuum‐processed OLED, but also realize a high EQE of 18.5 % in a solution‐processed OLED, which is among the best results in solution‐processed red TADF OLEDs. This work provides an effective strategy for designing red TADF molecules by managing energy level alignments to facilitate the up‐conversion process and thus enhance exciton harvesting. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) provides great potential for the realization of efficient and stable organic light‐emitting diodes (OLEDs). However, it is still challenging for blue TADF emitters to simultaneously achieve high efficiency, high brightness, and low Commission Internationale de l'Eclairage (CIE) y coordinate (CIEy) value. Here, the design and synthesis of two new benzonitrile‐based TADF emitters (namely 2,6‐di(9H‐carbazol‐9‐yl)‐3,5‐bis(3,6‐diphenyl‐9H‐carbazol‐9‐yl)benzonitrile (2PhCz2CzBn) and 2,6‐di(9H‐carbazol‐9‐yl)‐3,5‐bis(3,6‐di‐tert‐butyl‐9H‐carbazol‐9‐yl)benzonitrile (2tCz2CzBn)) with a symmetrical and rigid heterodonor configuration are reported. The TADF OLEDs doped with both the emitters can achieve a high external quantum efficiency (EQE) over 20% and narrowband blue emission of 464 nm with a CIEy < 0.2. Moreover, the incorporation of a terminal tert‐butyl group can weaken the intermolecular π–π stacking in the nondoped TADF emitter, and thus significantly suppress self‐aggregation‐caused emission quenching for enhanced delayed fluorescence. A peak EQE of 21.6% is realized in the 2tCz2CzBn‐based nondoped device with an extremely low turn‐on voltage of 2.7 V, high color stability, a high brightness over 20 000 cd m−2, a narrow full‐width at half‐maximum of 70 nm, and CIE color coordinates of (0.167, 0.248). [ABSTRACT FROM AUTHOR]

Developing red thermally activated delayed fluorescence (TADF) emitters, attainable for both high‐efficient red organic light‐emitting diodes (OLEDs) and non‐doped deep red/near‐infrared (NIR) OLEDs, is challenging. Now, two red emitters, BPPZ‐PXZ and mDPBPZ‐PXZ, with twisted donor–acceptor structures were designed and synthesized to study molecular design strategies of high‐efficiency red TADF emitters. BPPZ‐PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with a photoluminescence quantum yield (ΦPL) of 100±0.8 % and external quantum efficiency (EQE) of 25.2 % in a doped OLED. Its non‐doped OLED has an EQE of 2.5 % owing to unavoidable intermolecular π–π interactions. mDPBPZ‐PXZ releases two pyridine substituents from its fused acceptor moiety. Although mDPBPZ‐PXZ realizes a lower EQE of 21.7 % in the doped OLED, its non‐doped device shows a superior EQE of 5.2 % with a deep red/NIR emission at peak of 680 nm. [ABSTRACT FROM AUTHOR]

A blue fluorescent compound, 9-[4-(4,6-diphenoxy-1,3,5-triazin-2-yl)phenyl]-9 H-carbazole (POTC), the triplet energy level of which reaches 2.76 eV, has been designed and synthesized. POTC is an excellent blue emitter as well as host for green and red phosphors, and therefore, matches the requirements of the host for single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes (OLEDs). The blue, green, red, and white devices based on POTC show maximum external quantum efficiencies (EQEs) of 2.4, 22.4, 13.0, and 8.1 %, respectively. Even at a high brightness of 1000 cd m−2, these values maintain EQEs of 2.3, 22.1, 11.1, and 7.0 %, respectively, indicating less than 15 % roll-offs from the maxima. [ABSTRACT FROM AUTHOR]

Due to the feature of the strong intermolecular interactions causing aggregation‐caused excitons quenching (ACQ) of solution process, realizing high‐performance solution‐processed organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge nowadays. To address this issue, herein, a novel strategy is proposed to construct solution‐processable ternary exciplex emitters by utilizing the dendritic oligomer as the substrate, simultaneously realizing excellent solution‐process adaptability, good dispersibility to restrain harmful ACQ, and improved exciton utilization with multiple reverse intersystem crossing channels. Accordingly, dendritic oligomer‐based exciplex systems are constructed by combining small‐molecule donor and acceptor components with the multi‐carbazole‐substituted tetraphenylsilane‐core analog as substrate. And their film morphologies, photoluminescence quantum yields (ΦPLs), and non‐radiative decay rates of triplet excitons (knrTs) are significantly improved. Among them, DMAC‐DPS:PO‐T2T:SimCP3 exhibits the highest ΦPL of 97.5% and the lowest knrT of 1.0 × 103 s−1, and realizes the best maximum external quantum efficiency of 29.7% in solution‐processed OLED, which is comparable with the highest electroluminescence efficiencies of all solution‐processable emitters. Therefore, this work will provide a prospective pathway to develop efficient solution‐processable exciplex emitters and promote the progress of solution‐processed OLEDs. [ABSTRACT FROM AUTHOR]

Flexible organic light‐emitting diodes (OLEDs) are attracting tremendous attention due to their promise as a key element in bendable display and curved lighting applications. However, their performance in terms of efficiency and bendability is limited, since flexible transparent electrodes with superior electrical, optical, and mechanical properties are rare. Here, a multifunctional electrode architecture that is based on flexible plastic, and consists of electrically conductive silver nanowires, a nanopatterned ZnO outcoupling layer, and a hole‐injection polymer layer, is proposed for the actualization of high‐performance flexible OLEDs. The trapped light in the waveguide and substrate modes is effectively released by integrating aperiodic nanostructures into high‐refractive‐index ZnO layers on both sides of the plastic substrate. A maximum external quantum efficiency of 61.7% and a power efficiency of 126.6 lm W−1 are achieved for the white‐emission flexible OLEDs with broadband and angle‐independent outcoupling enhancement. In addition, the proposed approach allows for high‐level mechanical flexibility, retaining over 80% of the initial efficiency after 3000 cycles of repeated bending. [ABSTRACT FROM AUTHOR]

Sind gründlich studiert worden, allerdings verlief die Entwicklung von roten/nahinfraroten TADF-Emittern viel langsamer als die von grünen und blauen Emittern (TADF: thermisch aktivierte verzögerte Fluoreszenz). Sind gründlich studiert worden, allerdings verlief die Entwicklung von roten/nahinfraroten TADF-Emittern viel langsamer als die von grünen und blauen Emittern (TADF: thermisch aktivierte verzögerte Fluoreszenz). Zhang et al. präsentieren in ihrem Forschungsartikel auf S. 14808 zwei neuartige rote TADF-Emitter. [Extracted from the article]

With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light‐emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT‐SADF:PO‐T2T:CDBP is constructed with three RISC channels respectively on DBT‐SADF, DBT‐SADF:PO‐T2T, and CDBP:PO‐T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT‐SADF:PO‐T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters. [ABSTRACT FROM AUTHOR]

Low-cost and efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are crucial for organic light-emitting diodes (OLEDs) applications. In this study, 1,8-naphthalimide (NAI)-based D-A-type molecules were explored with a focus on donor engineering. A green-emitting non-TADF original control, the NAI/triphenylamine (TPA) hybrid NAI-TPA, was subjected to structural locking by oxygen-atom bridges on the TPA unit and further adopted an adjacent two-donor configuration to obtain two efficient orange-red/red TADF emitters, NAI-DPXZ and NAI-2DPXZ. The two compounds exhibited both significantly redshifted electroluminescence and improved external quantum efficiency (EQE), reaching 22.12 % at 583 nm and 11.17 % at 622 nm, respectively, which are approximately 2–3 times greater than those achieved by the O-bridges-free controls NAI-TPA (7.38 % at 517 nm) and NAI-2TPA (5.80 % at 564 nm). This work may provide an effective design strategy for the development of high-performance and low-cost orange-red/red TADF materials. Low-cost and efficient 1,8-naphthalimide-based orange‒red/red TADF materials were obtained by reasonable step by step donor engineering. [Display omitted] • Low-cost 1,8-naphthalimide-based orange‒red/red TADF emitters were obtained by donor engineering. • The oxygen-bridging strategy on the TPA unit turned on the TADF characteristics. • The adjacent two-donors configuration and the oxygen-bridging strategy resulted in a double emission redshift. [ABSTRACT FROM AUTHOR]

Multi-resonance (MR) thermally activated delayed fluorescence (TADF) materials with planar rigidity show great potential as emitters in organic light-emitting diodes (OLEDs) due to their high photoluminescence quantum yields and narrowband emission. However, their strong intermolecular interaction trends usually result in redshifted and broadened emission spectra as well as low device efficiencies due to the aggregation-caused quenching (ACQ) effect. In this paper, a bulky spirobifluorene unit was appended onto an asymmetric carbazole/diphenylamine-embedded MR skeleton to relieve such trends, thus affording a new MR-TADF blue emitter, BNSF-1. Introducing spirobifluorene significantly relieves the intermolecular interaction trends of MR backbones without affecting their original photophysical properties. The corresponding sensitizer-free OLED devices based on BNSF-1 exhibit narrowband sky-blue electroluminescence peaking at 475 nm with a full width at half-maximum of 25 nm, as well as a maximum external quantum efficiency of 25.9 %. More importantly, BNSF-1 exhibited negligible spectral broadening and ACQ trends as the doping ratio increased. Our work provides a feasible method for constructing doping concentration-insensitive MR-TADF emitters. [Display omitted] • A novel multi-resonance TADF emitter with spirobifluorene unit is developed. • The emitter exhibits blue emission at 475 nm with a small FWHM of 25 nm. • Maximum external quantum efficiency of 25.9 % and PLQY of 92.6 % are realized. [ABSTRACT FROM AUTHOR]

To achieve green solution-processed thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs) with low turn-on voltages and high power efficiencies (PEs), an interfacial exciplex system 4,4′-N,N′-dicarbazolylbiphenyl (CBP)/(1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl)tris(diphenylphosphineoxide) (PO-T2T) was used as the host to sensitize TADF emitters. Due to barrier-free hole-electron injection and effectively energy transfer, green solution-processed OLED with interfacial CBP:PO-T2T host exhibits a low turn-on voltage of 3.0 V and maximum current efficiency (CE), PE and external quantum efficiency (EQE) of 53.4 cd/A, 41.9 lm/W and 16.2% by using our previous reported TADF molecule 2'-(10H-phenoxazin-10-yl)-[1,1':3′,1″-terphenyl]-4,4″,5′-tricarbonitrile (oPTBC) as the emitter. And the compared OLEDs based on the bulk CBP:PO-T2T host and single CBP host exhibit relatively high turn-on voltages of 4.3 and 3.7 V and low maximum PEs of 20.8 and 33.0 lm/W. Moreover, the OLED based on classic green TADF emitter 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) achieves high performance with an ultralow turn-on voltage of 2.9 V and high maximum CE, PE and EQE of 67.2 cd/A, 62.1 lm/W and 20.0%, respectively. These results are remarkable among the reported green solution-processed TADF-OLEDs and demonstrate the superiority of interfacial exciplex host in solution-processed OLEDs. Image 1061 • Green solution-processed TADF-OLEDs were successfully developed by using an interfacial exciplex host CBP/PO-T2T. • Green solution-processed TADF-OLED achieves a low turn-on voltage of 2.9 V and maximum PE and EQE of 62.1 lm/W and 20%. • Our work demonstrates the superiority of the interfacial exciplexes as the host of solution-processed TADF-OLEDs. [ABSTRACT FROM AUTHOR]

Abstract Thermally activated delayed fluorescence (TADF) mechanism has been considered as the most attractive approach for organic light emitting diodes (OLEDs) due to its full exciton utilization with pure organic emitters. However, most TADF-based OLEDs still suffer two important issues — low maximum luminance and severe efficiency roll-off, which hinder their further development. Herein, a novel star-shaped yellow TADF emitter benzene-1,3,5-triyltris((4-(10H-phenoxazin-10-yl)phenyl) methanone) (TBP-PXZ) was designed and synthesized. Owing to the well separated frontier molecular orbitals, TBP-PXZ exhibits a tiny singlet–triplet energy gap and thus realizes significant TADF characteristic. Using TBP-PXZ as the emitter, the OLED presents excellent electroluminescence performances with a high maximum EQE of 17.7%. Moreover, an ultraslow efficiency roll-off was observed at high luminance with an impressively EQE remaining 16.0% at a luminance of 1000 cd m−2 and 10.5% at a luminance of 30000 cd m−2, respectively. These results suggest TBP-PXZ is an ideal model to develop TADF emitters with high maximum luminance and low efficiency roll-off. Graphical abstract Image Highlights • A novel yellow TADF emitter benzene-1,3,5-triyltris((4-(10H-phenoxazin-10-yl)phenyl) methanone) (TBP-PXZ) exhibits an extremely small ΔE ST of 0.02 eV and realizes significant TADF characteristic. • The TBP-PXZ-based OLED presents a high maximum EQE of 17.7% and a quite high maximum luminance of 49240 cd m−2 • The TBP-PXZ-based OLED realizes a quite high maximum luminance of 49240 cd m−2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Developing red TADF emitters by simultaneously employing polycyclic large-conjugation donor and acceptor units. • The first example of delicate management of 3LE D and 3LE A for constructing a high-performance red TADF emitter. • DBBPZ-IDPZ delivers fast k RISC and high PLQY of 98.1% thanks to the joint contribution of 3LE D and 3LE A. • DBBPZ-IDPZ based OLED affords an improved EQE of 26.0 %, peaking at 628 nm. Accelerating reverse intersystem crossing (RISC) rates of red thermally activated delayed fluorescence (TADF) emitters is crucial yet remains ongoing challenges. Herein, we disclose an ingenious molecular design to dramatically boost such an up-conversion process by leveraging effective participation of localized excited triplet states from both donor and acceptor segments (3LE D and 3LE A). Inspiringly, energy level alignment with 3LE A -3LE D close to the charge-transfer singlet and triplet (1CT and 3CT) states are obtained in the case of DBBPZ-IDPZ, by simultaneously employing polycyclic large-conjugation donor and acceptor units. Due to the joint contribution of 3LE D and 3LE A , DBBPZ-IDPZ shows 15.7 folds accelerated RISC rate and delivers a higher photoluminescence quantum yield of 98.1 % relative to the control molecule (36.7 %). Eventually, the red OLED based on DBBPZ-IDPZ presents a significantly improved external quantum efficiency of 26.0 % with an emission maximum at 628 nm, which is approximately triple that of the control device (8.9 %). This is the first example of a delicate management of 3LE D and 3LE A to simultaneously participate in RISC process for developing highly efficient red TADF emitters. Our work provides new insights into the molecular design tactic of high-performance red TADF OLEDs. [ABSTRACT FROM AUTHOR]

• A stereoscopic aromatic macrocycle was used to form a new-type TSCT-TADF emitter. • The rigid electron-deficient macrocycle clips the triphenylamine unit. • OC2A2T-TPA shows an extremely small ΔE ST of 18 meV. • OC2A2T-TPA behaves like an intramolecular exciplex. • Sensitizing the high-lying LE states is required to realize high exciton utilization. Through-space charge transfer (TSCT) has been increasingly utilized for the design of efficient thermally activated delayed fluorescence (TADF) emitters. TSCT molecular skeletons with spatially and electronically confined D/A subunits can provide a small ΔE ST , and also ease for fine control of CT properties within a single molecule. However so far TSCT-TADF molecules have not been fully investigated. Herein, we report a new ortho -position molecular pattern for TSCT-TADF design, using a stereoscopic V-shaped aromatic macrocycle of electron-deficient oxacalix[2]arene[2]triazine to "clip" the electron-rich triphenylamine unit. The new emitter OC2A2T-TPA exhibits a small ΔE ST of 18 meV, and the OLED device that uses high-energy-level host DPEPO realizes the first-time electroluminescence application of OC2A2T-type macrocyclic compounds by achieving an impressive external quantum efficiency (EQE) of 14.6 %. Further host-dependence investigation reveals that OC2A2T-TPA actually behaves like an intramolecular exciplex, exhibiting high exciton utilization once its high-lying locally excited (LE) states on D/A could be sensitized first in the case of DPEPO host, otherwise low exciton utilization in the case of lower-energy-level mCP host. This work opens a way to enrich TSCT-TADF design and clarifies the importance to involve the high-lying LE states in the energy transfer routes for high-efficiency intramolecular-exciplex molecules. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three TADF emitters were synthesized by finely modifying the acceptor units. • The donor–acceptor intramolecular hydrogen bond is formed in TPA-APm. • TPA-APm exhibits both twofold higher k F and an order of magnitude lower k nr. • TPA-APm-based red OLEDs achieved multiply increasing efficiency >20%. As one of the three primary colors, red organic light-emitting diodes (OLEDs) are indispensable in practical applications. However, red emitters are generally subject to severe non-radiative exciton loss due to their narrow energy gap. In this work, three new thermally activated delayed fluorescence (TADF) emitters were developed, namely 4-(acenaphtho[1,2-b]quinoxalin-9-yl)-N,N-diphenylaniline (TPA-AP), 4-(acenaphtho[1,2-b]pyrido[2,3-e]pyrazin-10-yl)-N,N-diphenylaniline (TPA-APy), and 4-(acenaphtho[1,2-b]pyrazino[2,3-e]pyrazin-9-yl)-N,N-diphenylaniline (TPA-APm), employing a series of finely modified acenaphtho[1,2-b]quinoxaline (AP) derivatives as acceptor units. Among three TADF emitters, an intramolecular hydrogen bond is formed between the donor (D) and acceptor (A) units in TPA-APm. Consequently, the overlap of the frontier molecular orbitals (FMOs) of TPA-APm can increase appropriately, and the fluorescence radiative rate (k F) of TPA-APm is nearly twofold than that of TPA-AP and TPA-APy. Furthermore, the non-radiative decay rate (k nr) of TPA-APm is less than that of TPA-AP and TPA-APy by an order of magnitude, which is attributed to the improved molecular rigidity caused by intramolecular hydrogen bond. As a result, TPA-APm-based OLEDs achieved a multiplied external quantum efficiency (EQE) of 21.1% with the electroluminescence peak at 590 nm, comparing to only 7.0% and 11.5% for the TPA-AP-based and TPA-APy-based devices, respectively. These results demonstrate appropriate intramolecular hydrogen bond can suppress the influence of non-radiative decay by simultaneously enhancing molecular rigidity and facilitating the fluorescence process, and have great potential in the design of efficient red TADF emitters. [ABSTRACT FROM AUTHOR]

A novel pyrene derivative 1,6-bis[2-(3,5-diphenylphenyl)phenyl]pyrene (DPPP) was successfully designed and synthesized. X-ray analysis shows the pyrene core in this compound is fully protected by the introduced 3,5-diphenylphenyl groups, resulting no π–π stacking between pyrene units, and the dihedral angle between pyrene ring and adjacent benzene ring is as large as 80.1°. This structure character leads to DPPP achieving a violet emission both in solution and as a thin solid film. Furthermore, DPPP exhibits high thermal properties due to its non-coplanar structure and large molecular size. The non-doped electroluminescence device employing DPPP as emitting layer shows a stable and efficient violet emission with a maximum external quantum efficiency of 2.2% and a CIE coordinate of (0.16, 0.04), which is remarkable in reported violet devices. [ABSTRACT FROM AUTHOR]

By controlling the conjugation extension of acceptor (A) frameworks, making 3LE A energy levels gradually reduced while their CT energy levels are well maintained with semblable photophysical and electrochemical properties. In the case of pDPBPZ-PXZ, the ideal molecular energy level alignments are obtained with near-degenerate 1CT, 3CT and 3LE A states. Consequently, resulting in enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. [Display omitted] • Two red TADF emitters were designed and synthesized. • Modulating conjugation extension can shift 3LE A states without affecting CT ones. • Ideal energy alignments achieved with improved k RISC. • Both emitters show similar CT properties and Δ E ST , but different exciton harvests. Increasing the reverse intersystem crossing (RISC) rates remains a challenge for red thermally activated delayed fluorescence (TADF) emitters, and energy level alignments play an important role. In this work, we demonstrate that locally excited triplet energy levels from acceptor segments (3LE A) can be well modulated by changing the conjugation extension inside acceptors. Our newly designed compounds pDPAPQ-PXZ and pDPBPZ-PXZ show nearly identical charge-transfer (CT) properties because of similar donor and acceptor strengths. On the other hand, the 3LE A state of pDPBPZ-PXZ was significantly lower than that of pDPAPQ-PXZ only because of its greater π-delocalization. In the case of pDPBPZ-PXZ, ideal molecular energy level alignments are obtained with near-degenerate CT featuring singlet and triplet states (1CT, 3CT) and 3LE A. As a result, pDPBPZ-PXZ simultaneously achieves an approximately 10-fold improved k RISC and a dramatically improved photoluminescence quantum yield of 85.6% compared with pDPAPQ-PXZ. The corresponding OLED successfully doubles the external quantum efficiency to 20.3%. This work demonstrates that the conjugation extensions of acceptors play a fatal role in determining 3LE A levels and that they can be isolated from controlling the acceptor strengths. This provides a promising approach toward enhancing RISC efficiency and developing highly efficient red TADF emitters. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fullerene fragments are used as acceptor to prepare TADF emitters for the first time. • OLED using one of the fullerenes fragment-based TADF emitter shows high EQE of 20.2%. • Acceptor without heteroatom can be used for designing efficient TADF emitter. • This strategy may open a new path for enhancing operation stability of TADF emitters. Fragments of fullerene are used as acceptor segments for the first time to construct two thermally activated delayed fluorescence (TADF) compounds DBCP and FAP. Both compounds exhibit similar highly twisted geometries and separated frontier molecular orbital distributions. While on the other hand, their locally exited triplet energy levels from acceptor moieties (3LE A) are fine-tunable owning to the different structural stress in the fragments, resulting in different lowest triplet excited state (T 1) features. In DBCP, the 3LE A is the high lying state and thus DBCP exhibits a small singlet–triplet energy difference of 0.10 eV and a high photoluminescence quantum yield (Φ PL) of 89% with excellent TADF characteristics; while T 1 of FAP is 3LE A , and thus, FAP displays poor triplet exciton utilization with Φ PL of only 54%. In organic light-emitting diodes (OLEDs) based on DBCP and FAP as dopants, maximum external quantum efficiencies of 20.2% and 12.8% are respectively achieved. This work not only demonstrates for the first time that fullerene fragments can be used as key building blocks for TADF emitters, it also proves in principle that pure hydrocarbon mobilities without any electronegative heteroatom can also be employed as acceptor segment in high performance TADF emitters. [ABSTRACT FROM AUTHOR]

By enhancing electron-withdrawing abilities of the secondary acceptor attachments into the main acceptor core, their charge transfer triplet (3CT) energy levels are gradually reduced, while the locally excited triplet (3LE A) levels are well maintained. The well-aligned excited states, consequently, results in the enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. [Display omitted] • A facile strategy to modulate CT states while maintaining LE state is proposed. • CT energy levels are gradually reduced, while the 3LE A ones are well maintained. • Fast k RISC , small ΔE ST and high PLQY can be realized simultaneously. • mDPBPZ-DPXZ-based red OLED exhibits much higher EQE compared to its counterparts. Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet (3LE A) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3CT and the 3LE A states are almost degenerate, and gives rise to a small singlet–triplet energy difference (ΔE ST) of 0.02 eV, a fast RISC rate of 1.54 × 106 s−1 and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3LE A state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1CT and 3CT state to lower energies without affecting the 3LE A energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters have only realized limited performance in solution-processed organic light-emitting diodes (OLEDs) comparing to in evaporation-processed OLEDs. To address this issue, a novel D-D′-A structure, where A is the electron-accepting group, D is the primary electron-donating group and D′ is the secondary electron-donating group, was proposed to develop efficient solution-processable TADF emitters in this work. As the intermediate D′ spacer weakens the direct intramolecular interaction between D and A groups, D-D′-A structure molecules simultaneously possess intramolecular and intermolecular charge-transfer transition channels, suppressing the aggregation-caused quenching induced by solution process. Accordingly, a novel TADF emitter 2-(3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DMAC-Cz-TTR) was designed and synthesized. In the optimized evaporation- and solution-processed OLEDs, DMAC-Cz-TTR successfully realized similar maximum external quantum efficiencies (EQEs) of 21.1% and 20.6%, respectively. To the best of our knowledge, this is the first TADF emitter realizing nearly equal performance in both evaporation- and solution-processed OLEDs with over 20% EQEs. The outstanding performance of DMAC-Cz-TTR successfully demonstrates the feasibility of the D-D′-A structure to develop efficient solution-processable TADF emitters. [Display omitted] • A novel D-D′-A structure was proposed to develop efficient solution-processed TADF emitters. • A new TADF emitter DMAC-Cz-TTR was developed accordingly with intra- and inter-CT transition channels simultaneously. • In the evaporation- and solution-processed OLEDs, DMAC-Cz-TTR realized maximum EQEs of 21.1% and 20.6%, respectively. [ABSTRACT FROM AUTHOR]

To develop high-performance solution-processed thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) based on exciplex emitters, a novel strategy was present to develop ternary polymer-based exciplex emitter by introducing the third small-molecule constituent material into binary polymer-based exciplex in this work. Compared with common binary polymer-based exciplex emitters, such ternary polymer-based exciplex emitters could not only enhance up-conversion of nonradiative triplet excitons and realize higher exciton utilization, but also maintain the superiorities of polymers in solution processing, like excellent film-forming ability and thermal stability. Based on our strategy, a highly efficient ternary polymer-based exciplex PVK:PO-T2T:mCP was developed. As expected, PVK:PO-T2T:mCP successfully realizes much higher photoluminescence quantum yield of 82.0% and rate constant of RISC process of 2.62 × 105 than PVK:PO-T2T. In the solution-processed OLEDs, interfacial PVK:PO-T2T:mCP exhibits a record-high maximum external quantum efficiency of 21.9% for polymer-based exciplexes, demonstrating the superiority of our strategy for further development of solution-processed TADF-OLEDs. Image 1 • Solution-processed TADF-OLEDs were successfully developed by using ternary polymer-based exciplex PVK:PO-T2T:mCP as the emitters. • Ternary exciplex PVK:PO-T2T:mCP exhibits much higher Φ PL (82.0%), k R ISC (2.62 × 105) and lower k n r T (2.07 × 104) than PVK:PO-T2T. • This work is the first report with an over 20% EQE for the OLEDs using polymer-based exciplexes as the emitters. [ABSTRACT FROM AUTHOR]