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High‐efficiency multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters with narrowband emission show great potential for organic light‐emitting diodes (OLEDs). However, their inherent planar rigid structures often lead to intractable challenges of spectral broadening, self‐quenching, and low device efficiency at high dopant concentrations. Herein, two steric isomers, BN‐1TPh and BN‐2TPh, are designed by incorporating bulky shielding unit (1,3,5‐triphenylbenzene) at the para‐position of the B atom in the MR skeleton to hinder intermolecular interactions. They both show enhanced photoluminescence quantum yields (PLQYs) as compared with the model compound BCzBN. The corresponding OLEDs based on BN‐1TPh and BN‐2TPh display the maximum external quantum efficiency (EQEmax) values of up to 30.8% and 30.4% with narrow full width at half maximum (FWHM) bands of 27 and 28 nm, respectively. It is worth noting that even at the high doping ratio of 20%, the EQEs are still maintained 24.8% and 25.7% with almost unchanged emission spectra. These results show that segregating the planar MR‐TADF skeleton with spatial shielding structure can weaken the intermolecular interaction, which is one of the effective ways to resist the aggregation‐caused quenching effect and achieve high‐efficiency concentration‐indispensible MR‐TADF OLEDs. [ABSTRACT FROM AUTHOR]

In this study, three stable tetradentate Pt(II) complexes are synthesized and characterized, namely, Pt‐biPh, Pt‐biPh5tBu, and Pt‐biPh4tBu, tailored for blue phosphorescent organic light‐emitting diodes to realize high‐efficiency and narrowband emissions via ligand engineering. Biphenyl (Pt‐biPh) or tert‐butyl‐modified biphenyl (Pt‐biPh5tBu and Pt‐biPh4tBu) is introduced into the carbene unit of the ligand to control the intermolecular interactions between the Pt(II) phosphors. Pt‐biPh, Pt‐biPh5tBu, and Pt‐biPh4tBu exhibit high photoluminescence quantum yields of 74%, 84%, and 92% with exciton lifetimes of 2.2, 2.3, and 2.5 µs, respectively, demonstrating rapid and efficient light emission. Furthermore, Pt‐biPh, Pt‐biPh5tBu, and Pt‐biPh4tBu show maximum external quantum efficiency (EQE) values of 18.1%, 19.0%, and 21.8%, respectively. Pt‐biPh5tBu and Pt‐biPh4tBu exhibit narrowband emission with a full width at half maximum of 21 nm owing to the small vibrational emission because of their sterically hindered and bulky ligand structures. Moreover, phosphor‐sensitized thermally activated delayed fluorescence devices employing a Pt‐biPh4tBu sensitizer achieve a high EQE of 28.6%. In particular, Pt‐biPh4tBu performs better than the state‐of‐the‐art phosphor as the sensitizer of the blue phosphor‐sensitized thermally activated delayed fluorescence devices in terms of the EQE. [ABSTRACT FROM AUTHOR]

Organoboron compounds offer a new strategy to design optoelectronic materials with high fluorescence efficiency. In this paper, the organoboron compound B‐BNBP with double B←N bridged bipyridine bearing four fluorine atoms as core unit is facilely synthesized and exhibits a narrowband emission spectrum and a high photoluminescence quantum yield (PLQY) of 86.53 % in solution. Its polymorphic crystals were controllable prepared by different solution self‐assembly methods. Two microcrystals possess different molecular packing modes, one‐dimensional microstrips (1D‐MSs) for H‐aggregation and two‐dimensional microdisks (2D‐MDs) for J‐aggregation, owing to abundant intermolecular interactions of four fluorine atoms sticking out conjugated plane. Their structure‐property relationships were investigated by crystallographic analysis and theoretical calculation. Strong emission spectra with the full width at half maximum (FWHM) of less than 30 nm can also be observed in thin film and 2D‐MDs. 1D‐MSs possess thermally activated delayed fluorescence (TADF) property and exhibit superior optical waveguide performance with an optical loss of 0.061 dB/μm. This work enriches the diversity of polymorphic microcrystals and further reveals the structure‐property relationship in organoboron micro/nano‐crystals. [ABSTRACT FROM AUTHOR]

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have experienced great success in organic light‐emitting diodes (OLEDs) owing to their outstanding quantum efficiencies and narrow full width at half‐maximums (FWHMs). However, the reverse intersystem crossing (RISC) rates of MR‐TADF emitters are usually small, which will lead to relatively long triplet exciton lifetime and severe efficiency roll‐off. Here, we report an effective molecular design strategy to introduce multichannel RISC pathways and thus increase RISC rates without compromising the color fidelity and emission efficiency by the “hybridized long‐short axis (HLSA)” strategy. The TPA‐CN‐BN shows a near‐unity photoluminescence quantum yield, rapid RISC rate of 1.4×105 s−1, narrow FWHM of 23 nm, and small singlet‐triplet energy gap (ΔEST) of 0.06 eV in solution. The non‐sensitized OLED based on TPA‐CN‐BN exhibits a narrowband emission with the FWHM of 31 nm, in company with external quantum efficiency (EQE) of 37.9 %. Notably, the device exhibits the low efficiency roll‐off as the EQEs maintain 34.8 % and 21.8 % at 100 and 1000 cd m−2, respectively, representing the best performance for single‐host OLEDs based on the BCzBN skeleton. This study provides a fresh and promising approach to realize high‐performance OLEDs with high color purity and remarkable device efficiency. [ABSTRACT FROM AUTHOR]

Circularly polarized electroluminescence (CPEL) is highly promising in realm of 3D display and optical data storage. However, designing a groundbreaking chiral material with high comprehensive CPEL performance remains a formidable challenge. In this work, a pair of chiral polymers with self‐assembled behavior is designed by integrating a chiral BN‐moiety into polyfluorene backbone, named R‐PBN and S‐PBN, respectively. The chiral polymers show narrowband emission centered at 490 nm with full‐width half maximum (FWHM) of 29 nm and high photoluminescence quantum yield (PLQY) of 79 %. After thermal annealing treatment, the chiral polymers undergo self‐assembly, exhibiting amplified circularly polarized luminescence (CPL) with asymmetry factor (|glum|) of up to 0.11. Moreover, the solution‐processed nondoped CP‐OLEDs based on the chiral polymers as emitting layers exhibit maximum external quantum efficiency (EQEmax) of 9.8 %, intense CPEL activities with |gEL| of up to 0.07, and small FWHM of 36 nm, simultaneously. This represents the first case of self‐assembled chiral polymers that combines high EQE, large gEL value and narrowband emission. [ABSTRACT FROM AUTHOR]

Blue tetradentate Pt(II) complexes, Pt‐tBuCz and Pt‐dipCz, are synthesized by introducing carbazoles with bulky substituents for improving the rigidity and inhibiting intermolecular interactions of phosphorescent emitter. tert‐Butyl and 2,6‐diisopropylphenyl groups are substituted as the blocking groups at 3 position of the carbazole in Pt‐tBuCz and Pt‐dipCz, respectively. These new phosphorescent emitters exhibit a narrow full width at half maximum (FWHM) and a high horizontal emitting dipole orientation ratio. Pt‐dipCz demonstrates a small FWHM of 24 nm, a high emitting dipole orientation ratio of 81%, and a high photoluminescence quantum yield value of 94%. As a result, the Pt‐tBuCz and Pt‐dipCz devices exhibited external quantum efficiencies (EQEs) of 23.7% and 25.0% with small FWHMs of 25 and 22 nm, respectively. For the Pt‐dipCz device, the small FWHM and high EQE of >20% are maintained even at a doping concentration of 20 wt%. Furthermore, phosphor‐sensitized organic light‐emitting diodes fabricated using Pt‐dipCz as a sensitizer achieved a high EQE of 31.4% with an FWHM of 18 nm. This result indicates that the 2,6‐diisopropylphenyl group is a effective blocking group for Pt(II) complexes to develop highly efficient, color stable, doping concentration resistant, and efficiently sensitizing blue phosphors. [ABSTRACT FROM AUTHOR]

The research on boron/nitrogen (B/N)-based multiresonance thermally activated delayed fluorescence (MR-TADF) emitters has been a prominent topic due to their narrowband emission and high luminous efficiency. However, devices derived from the common types of narrowband TADF materials often experience an efficiency roll-off, which could be ascribed to their relatively slow triplet–singlet exciton interconversion. Since inserting the heavy Se atom into the B/N scheme has been a proven strategy to address the abovementioned issues, herein, extensive density functional theory (DFT) and time-dependent DFT (TD-DFT) simulations have been employed to explore the effects of the structural modification on a series of structurally modified selenium-doped derivatives. Furthermore, the two-layered ONIOM (QM/MM) model has been employed to study the pressure effects on the crystal structure and photophysical properties of the pristine CzBSe. The theoretical results found that the introduced tert-butyl units in Cz-BSeN could result in a shorter charge transfer distance and smaller reorganization energy than the parent CzBSe. In contrast to directly incorporating the o-carborane (Cb) unit to CzBSe, incorporating the bridged phenyl units is important in order to achieve narrowband emissions and high luminous efficiency. The lowest three triplet excited states of CzBSe, Cz-BSeN and PhCb-BSeN all contribute to their triplet–singlet exciton conversions, resulting in a high utilization of triplet excitons. The pressure has an evident influence on the photophysical properties of the aggregated CzBSe and is favored for obtaining narrowband emissions. Our work is promised to provide a feasible strategy for designing selenium-doped derivatives with narrowband emissions and rapid triplet–singlet exciton interconversions. [ABSTRACT FROM AUTHOR]

The development of red phosphors with good thermal stability, high quantum yield and high color purity is still a big challenge. In this work, a series of novel red phosphors Sr3La2W2O12:xEu3+ (0≤x≤0.18, ∆x = 0.03) were successfully prepared by high‐temperature solid‐phase method. The phosphors reveal a hexagonal phase with the space group of R‐3 m. The phosphor Sr3La2W2O12:Eu3+ produced narrowband red emission from 5D0→7F2 of Eu3+ (616 nm) under excitation at 395 nm, 465 nm, and 533 nm. Importantly, phosphor Sr3La2W2O12:0.09Eu3+ exhibits astonishing quantum efficiency (IQE = 89.60%, EQE = 41.52%), and it demonstrate robust thermal performance with the emission intensity retaining 92.73% at 558 K of that at 298 K. Finally, the prepared red phosphor Sr3La2W2O12:0.09Eu3+ and yellow phosphor YAG:Ce3+ were combined with a blue LED chip (460 nm) to form a WLED device, which presented warm white light with color coordinates of (0.3523, 0.3732), color rendering index of 83.7, and color temperature of 4826 K. [ABSTRACT FROM AUTHOR]

The rigid planar architecture of multiple resonance thermally activated delayed fluorescence (MR‐TADF) molecules employing boron/nitrogen (B/N) frameworks typically results in severe aggregation‐caused quenching (ACQ) and spectral broadening. Herein, a steric modification strategy is proposed by incorporating a tetrahedral architecture of triphenylgermanium (TPhGe) into the para‐position of B/N/N, B/N/O, and B/N/S frameworks for the first time, formed three MR‐TADF emitters, BNNGe, BNOGe, and BNSGe, with narrowband emissions ranging from bluish‐green to pure blue. Consequently, these emitters exhibit high photoluminescence quantum yields of > 90% in doped films. Organic light‐emitting diodes (OLEDs) based on BNNGe, BNOGe, and BNSGe demonstrate impressive maximum external quantum efficiencies (EQEmaxs) of 30.1% to 15.5%, and 20.7%, respectively. The unique tetrahedral TPhGe moiety, with its bulky size conformation, effectively separates adjacent MR‐TADF molecules, resulting in efficient luminescence across a broad range of doping concentrations (5–30 wt%) in doped films, thereby successfully suppressing the ACQ in devices. Furthermore, OLEDs containing BNSGe display low‐efficiency roll‐offs because of higher spin‐orbital coupling and reverse intersystem crossing rates of the emitter, attributed to the heavy atom effect. Notably, the device with 5 wt% BNOGe exhibits a pure blue emission peaking at 461 nm, with a narrow full‐width at half‐maximum of 32 nm. [ABSTRACT FROM AUTHOR]

Developing multi‐resonance thermally activated fluorescence (MR‐TADF) emitters with both fast reverse intersystem crossing (RISC) rate and narrow emission bandwidth still remains a formidable challenge. Herein, a design strategy of fused MR skeleton containing heavy chalcogen (sulfur or selenium) for high‐performance MR‐TADF molecules is developed. Impressively, Se‐embedded emitter (DSeBN) shows extremely narrow full width at half maximum (FWHM) value of 16 nm and ultrafast RISC rate constant up to 2.0 × 106 s−1. The organic light‐emitting diode (OLED) based on this emitter exhibits excellent performance parameters with extremely narrow FWHM of 17 nm and high external quantum efficiency (EQE) of 35.31%. Significantly, much suppressed efficiency roll‐off is achieved, in which the EQE still stayed at 32.47% and 25.05% at the luminance of 100 and 1000 cd m−2, respectively. These results represent the state‐of‐the‐art device performance in terms of efficiency and FWHM, shedding new light on the development of practical MR‐TADF emitters. [ABSTRACT FROM AUTHOR]

The development of deep‐blue organic light‐emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh‐definition displays with wide color gamut. Herein, based on the nitrogen‐embedding strategy for modifying the short range charge transfer excited state energies of multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron‐embedded 1,3‐bis(carbazol‐9‐yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic‐based MR‐TADF molecules, Py‐BN and Pm‐BN, exhibit deep‐blue emissions with high photoluminescence quantum yields of 93 % and 94 %, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep‐blue hyperfluorescent OLED based on Py‐BN exhibited a maximum external quantum efficiency of 27.7 % and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR‐TADF emitters. [ABSTRACT FROM AUTHOR]

Sulfone group plays a pivotal role in narrowing emission spectra, while, precise modification with sulfone group for carbonyl‐based narrowband emitters that maintain high color purity remains challenging. Herein, a comprehensive exploration of the function of the sulfone group is performed with various sulfur valences and positions fused into traditional carbonyl‐based narrowband emission unit QAO, namely SpiroS‐QAO, SpiroSO2‐QAO, SpiroO‐QAO, and SpiroOSO2‐QAO. A rigid molecular skeleton with a spiro structure as an intramolecular lock enables the four emitters to exhibit narrowband emissions. After full oxidization of sulfur, the emission band of SpiroSO2‐QAO is further narrowed, with full width at half maximum of 32 nm, compared to that of 43 nm for sulfane‐decorated SpiroS‐QAO, which is attributed to the suppressed C─S bond‐length variation by the introduction of sulfone group. Nearly identical spectra of SpiroO‐QAO and SpiroOSO2‐QAO suggest that the sulfone group should be directly linked to the emission core to maximize its function. Maximum external quantum efficiencies of 30.8%, 30.3%, and 29.2% are achieved for SpiroO‐QAO, SpiroOSO2‐QAO, and SpiroS‐QAO‐based sensitized organic light‐emitting diodes with highly efficient thermally activated delayed fluorescence assistant host. These results offer a comprehensive understanding of the sulfone group embedded into the narrowband emission core with respect to the narrowing emission band. [ABSTRACT FROM AUTHOR]

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials are of vital importance for ultra‐high‐definition displays. However, developing molecular design tactics for blue MR‐TADF emitters that simultaneously achieve good TADF properties, narrowband emission, and ideal molecular orientation remains challenging. Here, an effective approach to enhance the electroluminescent (EL) performance of MR‐TADF emitter by enlarging the molecular planar through a peripheral decoration strategy is presented. The resulting proof‐of‐concept emitter, 3QCzBN, not only exhibits a blue emission with a narrow full‐width at half maximum (FWHM) of only 22 nm, but also a remarkable photoluminescent quantum yield (PLQY) of 98%. Together with the accelerated reverse intersystem crossing (RISC) processes and nearly perfect horizontal ratios of emitting dipole orientation (Θ//) of 98% compared to its parent MR molecule, the organic light‐emitting diodes (OLEDs) based on 3QCzBN demonstrate blue emission with good color purity and an outstanding maximum external quantum efficiency (EQEmax) of 34.8% with low efficiency roll‐off. [ABSTRACT FROM AUTHOR]

While the development of solution‐processed organic light‐emitting diodes (sOLEDs) utilizing multiple resonance‐induced thermally activated delayed fluorescence (MR‐TADF) is highly significant, it is restricted by the limited solubility and film‐forming property resulting from the rigid conjugate and planarity of MR‐TADF materials. Herein, an effective strategy is presented to obtain solution‐processed narrowband emitters by introducing an inert steric bulky hindrance group into the multiple resonance skeleton, thereby mitigating issues arising from intermolecular packing‐induced poor solution processing ability and quenching effects. The resulting target emitter, designed as 3CzSF‐BN, exhibits pure‐green emission with a peak at 520 nm and a small full width at half maximum (FWHM) of 30 nm (0.14) eV. Remarkably, it achieves an exceptional photoluminescence quantum yield (PLQY) of 100% and notable advancements in solution processing attributes. The optimized bottom‐emitting sOLED (BE‐sOLED) device achieves an external quantum efficiency (EQE) over 20% with Commission Internationale de I'Éclairage (CIE) coordinates of (0.214, 0.716). Notably, the top‐emitting sOLED (TE‐sOLED) device exhibits an ultra‐pure green color with FWHM of 22 nm and CIE coordinates of (0.138, 0.771), thereby highlighting the effectiveness of this strategy in designing high‐performance solution‐processed MR‐TADF materials. [ABSTRACT FROM AUTHOR]

The development of efficient, bright, and stable narrowband light‐emitting electrochemical cells (LECs) has remained a challenge. Here, intrinsically ionic multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are reported as guest emitters for narrowband LECs, which are developed by attaching an imidazolium cation onto a typical MR‐TADF emitter. In solution, the emitters show green–blue emission peaked at 486−497 nm with small full widths at half‐maximum (FWHMs) at 24−26 nm. In doped films, they show narrowband green–blue emission with high luminescent efficiencies at ≈90%. LECs using an ionic exciplex host and the ionic MR‐TADF guest emitters show green–blue emission peaked at 494−503 nm with small FWHMs at 31−34 nm, and afford high external quantum efficiencies (EQEs) up to 10% under constant‐voltage driving. With ionic TADF small‐molecule hosts, the narrowband LECs show high EQEs up to 13.0% under constant‐voltage driving, which is the highest among all reported narrowband LECs, and afford peak brightness/EQE/half lifetime at 780 cd m−2/5.6%/62.2 h under constant‐current driving. A long half‐lifetime of ≈630 h has further been achieved at 136 cd m−2. The work demonstrates the great potential for the use of intrinsically ionic MR‐TADF guest emitters and ionic TADF hosts to develop efficient, bright, and stable narrowband LECs. [ABSTRACT FROM AUTHOR]

The multiple resonance thermally activated delayed fluorescence (MR‐TADF) materials can meet the requirement of a high color gamut of displays due to their narrowband emission. However, most reported organic light‐emitting diodes (OLEDs) based on MR‐TADF materials suffer from severe efficiency roll‐off. Herein, three green MR‐TADF emitters, p‐ICz‐BNCz, m‐ICz‐BNCz, and dm‐ICz‐BNCz, are obtained by introducing bulky indolo[3,2,1‐jk]carbazole (ICz) units into the classical DtBuCzB skeleton at the para or meta positions relative to the boron‐substituted phenyl ring. Compared with the para‐substitution of ICz, the meta‐substitution not only increases the reverse intersystem crossing rate constants of m‐ICz‐BNCz and dm‐ICz‐BNCz by nearly three times, but also makes their configurations more twisted. The two factors work together to improve the utilization of triplet excitons of the emitters, showing photoluminescence quantum yields exceeding 90%. As a result, the two corresponding OLEDs exhibit maximum external quantum efficiency (EQEmax) values of 28% and 32.9%, respectively, with low‐efficiency roll‐off. At the high brightness of 1000 cd m−2, the EQE values of the two devices still maintain at 19.3% and 25.0%, respectively. In addition, green emissions with Commission Internationale de l'Eclairage coordinates of (0.20, 0.70) and (0.30, 0.67) are also observed. [ABSTRACT FROM AUTHOR]

The advent of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials, heralding cutting‐edge emitters with superior efficiency and color fidelity, represents a momentous stride in the realm of organic light‐emitting diodes (OLEDs). In this particular investigation, substantial advancements have been made by enhancing, synthesizing, and characterizing three distinct MR‐type emitters through meticulous control of peripheral decorations using frontier molecular orbital engineering. Through strategic attachment of various pyrimidine derivatives to the positions of the lowest unoccupied molecular orbitals (LUMO) of the parent molecule, the molecular excited state attributes are meticulously tailored, leading to the development of orange‐red MR‐TADF emitters. These designed molecules have demonstrated an emission spectrum shift from orange‐red to orange, exhibiting peak wavelengths spanning the range of 585–608 nm and full width at half‐maximums (FWHM) between 36 and 43 nm. This signifies a remarkable precision in managing the emission maxima of MR‐TADF emitters. Astonishingly, the solution‐processed OLEDs have showcased vibrant orange‐red electroluminescence, characterized by peak wavelengths ranging from 587 to 611 nm, accompanied by a notable external quantum efficiency nearing 20%. [ABSTRACT FROM AUTHOR]

1,4‐BN‐doped polycyclic aromatic hydrocarbons (PAHs) have emerged as very promising emitters in organic light‐emitting diodes (OLEDs) due to their narrowband emission spectra that may find application in high‐definition displays. While considerable research has focused on investigating the properties of these materials, less attention has been placed on their synthetic methodology. Here we developed an efficient synthetic method for 1,4‐BN‐doped PAHs, which enables sustainable production of narrowband organic emitting materials. By strategically introducing substituents, such as methyl, tert‐butyl, phenyl, and chloride, at the C5 position of the 1,3‐benzenediamine substrates, we achieved remarkable regioselective borylation in the para‐position of the substituted moiety. This approach facilitated the synthesis of a diverse range of 1,4‐BN‐doped PAHs emitters with good yields and exceptional regioselectivity. The synthetic method demonstrated excellent scalability for large‐scale production and enabled late‐stage transformation of the borylated products. Mechanistic investigations provided valuable insights into the pivotal roles of electron effect and steric hindrance effect in achieving highly efficient regioselective borylation. Moreover, the outstanding device performance of the synthesized compounds 10 b and 6 z, underscores the practicality and significance of the developed method. [ABSTRACT FROM AUTHOR]

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have attracted much attention in the academia owing to their unique photoelectrical properties. However, MR‐TADF emitters usually show slow reverse intersystem crossing (RISC) rate, resulting in high efficiency roll‐off of organic light‐emitting diodes (OLEDs) and seriously limiting their further development. Here, a peripheral selenium (Se) modification is presented for MR‐TADF molecules to promote the RISC process while keeping the narrowband emission for high‐performance blue OLEDs. Compared to the parent molecules (NBN and tBuNBN), SeNBN and SetBuNBN exhibited narrower full‐width at half maximum (FWHM) value of 23 nm and more obvious delayed fluorescence properties with a high efficiency of delayed fluorescence up to 86%, shorter delayed lifetime of 2.4 µs as well as a faster RISC rate of 3.34×105 s−1. Therefore, high‐performance OLEDs based on these two Se modified MR‐TADF emitters are achieved with a high maximum external quantum efficiency (EQE) up to 25.5% and extremely suppressed efficiency roll‐offs of 3.9% at 100 cd m−2 and 24.4% at 1000 cd m−2. This work demonstrated that the introduction of peripheral Se atom can achieve high‐performance organic semiconductors with both narrowband emission and fast RISC rate constant for high‐performance organic optoelectronic devices. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials with high photoluminescence quantum yields and fast reverse intersystem crossing (RISC) capabilities are highly desirable for applications in high-efficiency organic light-emitting diodes. Herein, we report the synthesis as well as structural and photophysical properties of 5,9-diselena-13b-boranaphtho [3,2,1-de]anthracene (SeBSe) as a narrowband-emissive TADF material. The incorporation of two selenium atoms into the boron-fused pentacyclic π-core results in a small singlet-triplet energy gap (Δ£^) and thereby significant TADF properties. Moreover, theoretical calculations revealed a noticeable spin-orbit coupling enhancement between the singlet and triplet manifolds in SeBSe by virtue of the heavy-atom effect of selenium atoms. Consequently, SeBSe allows ultrafast spin-flip RISC with the rate constant surpassing 108 s-1, which far exceeds the corresponding fluorescence radiative decay rate (~106 s-1), enabling an ideal singlet-triplet superimposed excited state. [ABSTRACT FROM AUTHOR]

A blue tetradentate Pt(II) complex named Pt‐tmCyCz is developed by introducing a cycloalkyl unit fused to carbazole to improve the rigidity and bulkiness of the complex. The introduction of the tetramethylcyclohexyl (tmCy) group results in a narrow full width at half maximum (FWHM), a high horizontal emitting dipole orientation, doping concentration resistant stable spectrum, and extremely small efficiency roll‐off, and little concentration quenching effect. Phosphorescent organic light‐emitting diodes (OLEDs) doped with Pt‐tmCyCz achieve a high external quantum efficiency (EQE) of 21.5%, with a small EQE roll‐off of 3.8% up to 1000 cd m−2, a small FWHM of 24 nm, and a color coordinate of (0.132, 0.138). Moreover, Pt‐tmCyCz is investigated as a sensitizer in phosphor‐sensitized OLEDs using N7,N7,N13,N13,5,9,11,15‐octaphenyl‐5,9,11,15‐tetrahydro‐5,9,11,15‐tetraaza‐19b,20b‐diboradinaphtho[3,2,1‐de:1′,2′,3′‐jk]pentacene‐7,13‐diamine (νDABNA) as a terminal emitter. The Pt‐tmCyCz:νDABNA device achieves a high EQE of 33.9%, with a small EQE roll‐off of only 8.0% up to 1 000 cd m−2. The results demonstrate that fused tmCy group in carbazole can be an effective building block for the development of high‐performance Pt(II) complexes, which can be utilized as efficient phosphors or sensitizers in OLEDs. [ABSTRACT FROM AUTHOR]

The development of thermally activated delayed fluorescence (TADF) polymers with narrowband emission, particularly in the blue region, remains a formidable challenge. Herein, a new approach is demonstrated for blue TADF conjugated polymers by integrating multiresonance TADF moiety into the conjugated backbone. All the new polymers exhibit TADF characteristics with narrowband blue emission with full‐width at half‐maximums (FWHMs) of 23 nm. The solution‐processed OLEDs based on these polymers achieve a maximum external quantum efficiency (EQEmax) of 17.9% with the emission peak at 479 nm and FWHM of 28 nm. The main‐chain type MR‐TADF polymer shed light on the development of narrowband emissive TADF conjugated polymers. [ABSTRACT FROM AUTHOR]

Purely organic sulfone‐embedded materials play a significant role in thermally activated delayed fluorescence (TADF) emitters with theoretical 100 % exciton utilization for highly efficient organic light‐emitting diodes. However, intrinsic emission mechanism featuring twisted intramolecular charge transfer design strategy leads to severe structural distortion between ground state and excited state, disabling traditional sulfone‐based TADF emitters to achieve narrowband emission and high colour purity for wide colour gamut display. Recently, sulfone‐embedded narrowband TADF emitters have gradually emerged. Herein, we summarize the currently reported sulfone‐based narrowband emitters, with emphasis on the perspectives of molecular design, role of sulfone group in fused skeleton, photophysical property as well as future application prospect of this sort of materials. [ABSTRACT FROM AUTHOR]

Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters exhibit enormous potential for use in organic light‐emitting diodes (OLEDs), owing to their exceptional external quantum efficiencies (EQEs) and narrowband emission spectra. However, planar MR‐TADF emitters often suffer from aggregation‐caused quenching (ACQ) and spectral broadening at high doping concentrations because of strong interchromophore π–π interactions. A method of sterically encapsulating the planar MR skeleton with four bulky 9‐phenyl‐fluorene (Fl) units is devised, resulting in the development of a bright bluish‐green emitter (4FlCzBN). This steric shielding effectively reduces intermolecular interactions, suppresses ACQ, and improves solubility. Consequently, by utilizing 4FlCzBN as a doping‐insensitive MR emitter, solution‐processed OLEDs are fabricated with doping concentrations of 2–16 wt.%, and they show EQEs of 10.1–10.9% with a bandwidth of 28–30 nm. Furthermore, a TADF‐sensitizer‐based device using 4FlCzBN demonstrates a significantly reduced efficiency roll‐off while achieving an EQE of 12.2%. This is a remarkable improvement that overcomes the disadvantages that are difficult to achieve in previously reported MR‐TADF OLEDs. The current work provides valuable insights into the design of efficient MR‐TADF emitters with minimized aggregation and reduced efficiency roll‐off for solution processing. [ABSTRACT FROM AUTHOR]

Platinum phosphorescent materials are emerging as promising candidates for efficient blue organic light‐emitting diodes. Especially, tetradentate ligands with a 5/6/6 configuration have the advantage of having twisted structure to minimize intermolecular interactions. However, the reported platinum complexes in the 5/6/6 configuration still exhibit relatively broad luminescence spectra and low efficiency at high doping concentration due to insufficient intermolecular interaction suppression. It is necessary to introduce the appropriate blocking group at the proper position on the ligand to ensure that intermolecular interactions are inhibited. In this work, we propose an ideal design introducing blocking group for platinum (II) complexes with small changes in efficiency and emission spectra at high doping concentration. The bulky blocking group, 2,6‐diisopropylphenyl, introduced at the 3‐position of the carbazole is distorted vertically due to steric hindrance, so the energy is still retained. However, the photophysical and device properties were improved by suppression of intermolecular interaction. As a result, Pt‐dipCz showed high efficiency of 25.0% with narrow full width at half maximum (FWHM) of 22 nm at 5 wt.% doping concentration. When doping concentration increased to 20 wt.%, high efficiency and narrow FWHM were maintained with small changes of only 4.8% reduction and 2 nm increase, respectively. These values indicate that 2,6‐diisopropylphenyl group introduced at carbazole is efficient for suppression of intermolecular interaction. [ABSTRACT FROM AUTHOR]

In this paper, a method for further improving the efficiency of top emission organic light‐emitting diodes (TE‐OLEDs) by phosphorescent sensitized fluorescent organic light‐emitting diodes (PSF‐OLEDs) was provided. The maximum current efficiency (CE) of PSF‐OLEDs realized an improvement of nearly 40% compared with the phosphorescent device, which exhibited great potential in high‐efficiency OLEDs. [ABSTRACT FROM AUTHOR]

The research on boron/nitrogen (B/N)-based multiresonance thermally activated delayed fluorescence (MR-TADF) emitters has been a prominent topic due to their narrowband emission and high luminous efficiency. However, devices derived from the common types of narrowband TADF materials often experience an efficiency roll-off, which could be ascribed to their relatively slow triplet–singlet exciton interconversion. Since inserting the heavy Se atom into the B/N scheme has been a proven strategy to address the abovementioned issues, herein, extensive density functional theory (DFT) and time-dependent DFT (TD-DFT) simulations have been employed to explore the effects of the structural modification on a series of structurally modified selenium-doped derivatives. Furthermore, the two-layered ONIOM (QM/MM) model has been employed to study the pressure effects on the crystal structure and photophysical properties of the pristine CzBSe. The theoretical results found that the introduced tert-butyl units in Cz-BSeN could result in a shorter charge transfer distance and smaller reorganization energy than the parent CzBSe. In contrast to directly incorporating the o-carborane (Cb) unit to CzBSe, incorporating the bridged phenyl units is important in order to achieve narrowband emissions and high luminous efficiency. The lowest three triplet excited states of CzBSe, Cz-BSeN and PhCb-BSeN all contribute to their triplet–singlet exciton conversions, resulting in a high utilization of triplet excitons. The pressure has an evident influence on the photophysical properties of the aggregated CzBSe and is favored for obtaining narrowband emissions. Our work is promised to provide a feasible strategy for designing selenium-doped derivatives with narrowband emissions and rapid triplet–singlet exciton interconversions.

Thermally activated delayed fluorescence (TADF) materials with high photoluminescence quantum yields and fast reverse intersystem crossing (RISC) capabilities are highly desirable for applications in high-efficiency organic light-emitting diodes. Herein, we report the synthesis as well as structural and photophysical properties of 5,9-diselena-13b-boranaphtho[3,2,1-de]anthracene (SeBSe) as a narrowband-emissive TADF material. The incorporation of two selenium atoms into the boron-fused pentacyclic π-core results in a small singlet–triplet energy gap (ΔEST) and thereby significant TADF properties. Moreover, theoretical calculations revealed a noticeable spin-orbit coupling enhancement between the singlet and triplet manifolds in SeBSe by virtue of the heavy-atom effect of selenium atoms. Consequently, SeBSe allows ultrafast spin-flip RISC with the rate constant surpassing 108 s−1, which far exceeds the corresponding fluorescence radiative decay rate (∼106 s−1), enabling an ideal singlet–triplet superimposed excited state.

It is of great strategic significance to develop highly efficient narrowband organic electroluminescent materials that can be utilized to manufacture ultra‐high‐definition (UHD) displays and meet or approach the requirements of Broadcast Television 2020 (B.T.2020) color gamut standards. This motif poses challenges for molecular design and synthesis, especially for developing generality, diversity, scalability, and robustness of molecular structures. The emergence of multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters has ingeniously solved the problems and demonstrated bright application prospects in the field of UHD displays, sparking a research boom. This Minireview summarizes the research endeavors of narrowband organic electroluminescent materials, with emphasis on the tremendous contribution of frontier molecular orbital engineering (FMOE) strategy. It combines the outstanding advantages of MR framework and donor‐acceptor (D−A) structure, and can achieve red‐shift and narrowband emission simultaneously, which is of great significance in the development of long‐wavelength narrowband emitters with emission maxima especially exceeding 500 nm. We hope that this Minireview would provide some inspiration for what could transpire in the future. [ABSTRACT FROM AUTHOR]

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have drawn considerable attention because of their remarkable optoelectronic properties of high emission efficiency and narrow emission profile, and represent an active subject of cutting‐edge research in the organic electroluminescence (EL). However, the slow reverse intersystem crossing (RISC) rate of MR‐TADF emitter caused by the large energy gap (ΔEST) and small spin‐orbit coupling (SOC) matrix elements between the singlet and triplet excited states limits their further development in organic EL devices. Currently, innovative molecular design strategies have been developed including heavy atom integration, π‐extended MR framework and metal perturbation, and so on to improve the RISC process of MR‐TADF emitters for high‐performance EL devices. Here, an overview is presented on the recent progress of MR‐TADF emitters with fast RISC rate (> 10−5 s−1), with particular attention to the molecular design, optoelectronic properties, and device performance of organic light‐emitting diodes (OLEDs), which intends to systematize the knowledge in this subject for the thriving development of highly efficient MR‐TADF emitters. Finally, the challenges and future prospects of MR‐TADF materials are discussed comprehensively. [ABSTRACT FROM AUTHOR]

Nonbenzenoid polycyclic arenes have attracted great attention because of their unique topological structures and appealing properties; however, they are generally considered as poor luminescent materials due to the ring puckering behavior of the nonhexagons. Acepleiadylene (APD), a nonbenzenoid isomer of pyrene, has been demonstrated as an excellent building block for optoelectronic applications, but its poor photoluminescence quantum yield (PLQY) has hampered its application in luminescent materials. Considering that the ring puckering character can be suppressed by increasing the rigidity of the nonhexagon rings, herein, we propose a novel strategy for enhancing the aromaticity of the nonhexagons to improve the PLQYs of APD derivatives. Electron-withdrawing cyano groups are introduced on the five-membered ring of APD to enhance the charge-separated character and thus the aromaticity of the nonhexagons, endowing the cyano-substituted APDs (CNAPD and 2CNAPD) with better rigidity. Therefore, the cyano substitution successfully suppresses the nonradiative energy dissipation caused by the ring puckering, improving the PLQY from 2.4% for APD to 14% for CNAPD, and to 63% for 2CNAPD. In addition, the enhanced rigidity also suppresses the vibration sideband of the photoluminescence spectra, leading to an ultra-narrowband emission from 2CNAPD with a full-width at half-maximum (FWHM) of 13 nm (47 meV), which is a new record in organic molecules. These results demonstrate that APD derivatives have great potential in highly efficient luminescent materials with high color purity via the aromaticity regulation strategy, which provides a novel concept for designing nonbenzenoid luminescent materials. [ABSTRACT FROM AUTHOR]

Organic light‐emitting diodes (OLEDs) using conventional fluorescent emitters are currently attracting considerable interests due to outstanding stability and abundant raw materials. To construct high‐performance narrowband fluorophores to satisfy requirements of ultra‐high‐definition displays, a strategy fusing multi‐resonance BN‐doped moieties to naphthalene is proposed to construct two novel narrowband fluorophores. Green Na−sBN and red Na−dBN, manifest narrow full‐width at half‐maxima of 31 nm, near‐unity photoluminescence quantum yields and molecular horizontal dipole ratios above 90 %. Their OLEDs exhibit the state‐of‐the‐art performances including high external quantum efficiencies (EQE), ultra‐low efficiency roll‐off and long operational lifetimes. The Na−sBN‐based device achieves EQE as high as 28.8 % and remains 19.8 % even at luminance of 100,000 cd m−2, and Na−dBN‐based device acquires a record‐high EQE of 25.2 % among all red OLEDs using pure fluorescent emitters. [ABSTRACT FROM AUTHOR]

Fusing condensed aromatics into multi‐resonance (MR) frameworks has been an exquisite strategy to modulate the optoelectronic properties, which, however, always sacrifices the small full width at half maxima (FWHM). Herein, we strategically embed B−N/B−O contained heterocycles as fusion locker into classical MR prototypes, which could enlarge the π‐extension and alleviate the steric repulsion for an enhanced planar skeleton to suppress the high‐frequency stretching/ scissoring vibrations for ultra‐narrowband emissions. Sky‐blue emitters with extremely small FWHMs of 17–18 nm are thereafter obtained for the targeted emitters, decreased by (1.4–1.9)‐fold compared with the prototypes. Benefiting from their high photoluminescence quantum yields of >90 % and fast radiative decay rates of >108 s−1, one of those emitters shows a high maximum external quantum efficiency of 31.9 % in sensitized devices, which remains 25.8 % at a practical luminance of 1,000 cd m−2 with a small FWHM of merely 19 nm. Notably a long operation half‐lifetime of 1,278 h is also recorded for the same device, representing one of the longest lifetimes among sky‐blue devices based on MR emitters. [ABSTRACT FROM AUTHOR]

Organic semiconductors that emit by the process of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) can deliver narrowband and efficient electroluminescence while being processable from solvents and metal‐free. This renders them attractive for use as the emitter in sustainable light‐emitting electrochemical cells (LECs), but so far reports of narrowband and efficient MR‐TADF emission from LEC devices are absent. Here, this issue is addressed through careful and systematic material selection and device development. Specifically, the authors show that the detrimental aggregation tendency of an archetypal rigid and planar carbazole‐based MR‐TADF emitter can be inhibited by its dispersion into a compatible carbazole‐based blend host and an ionic‐liquid electrolyte, and it is further demonstrated that the tuning of this active material results in a desired balanced p‐ and n‐type electrochemical doping, a high solid‐state photoluminescence quantum yield of 91%, and singlet and triplet trapping on the MR‐TADF guest emitter. The introduction of this designed metal‐free active MR‐TADF material into a LEC, employing air‐stabile electrodes, results in bright blue electroluminescence of 500 cd m−2, which is delivered at a high external quantum efficiency of 3.8% and shows a narrow emission profile with a full‐width‐at‐half‐maximum of 31 nm. [ABSTRACT FROM AUTHOR]

Highly efficient near‐infrared (NIR) emitters have significant applications in medical and optoelectronic fields, but the development stays a great challenge due to the energy gap law. Here, we report two NIR phosphorescent Ir(III) complexes which display emission peaks around 730 nm with a narrow full width at half maximum of only 43 nm. Therefore, pure NIR luminescence can be obtained without having a very long emission wavelength, thus alleviating the restriction of the energy gap law, and obtaining impressively high photoluminescence quantum yield up to 0.70. More importantly, the pure NIR organic light‐emitting diode (OLED) fabricated by the solution‐processed mothed shows outstanding device performance with the highest external quantum efficiency of 16.43 %, which sets a new record for solution‐processed NIR‐OLEDs based on different emitters. This work sheds light on the development of Ir(III) complexes with narrowband emissions as highly efficient pure NIR‐emitters. [ABSTRACT FROM AUTHOR]

Chiral B,N-doped polycyclic aromatic hydrocarbons with circularly polarized luminescence (CPL) and small full-width at half-maxima (FWHM) are promising multiple resonance (MR) emitters for CP organic light-emitting diodes (CP-OLEDs). This work presents a pair of chiral MR enantiomers, P/M-o[B-N]2N2, featuring B-N bonds incorporated within a [7]-helicene skeleton. These enantiomers exhibit narrow 0–0 and 0–1 electronic transition bands, whose relative intensity can be fine-tuned by increasing doping concentrations, resulting in redshifts of the emission peak from 542 to 592 nm. The enantiomers show mirror-symmetric CPL spectra with an asymmetry factor (∣gPL∣) of 1.0 × 10−3. The hyperfluorescent CP-OLEDs with double-sensitized emitting layers display a FWHM of 33 nm, an external quantum efficiency of 25.1%, and a ∣gEL∣ factor of 7.7 × 10−4. Notably, the CP-OLEDs realize color-tunable CP electroluminescence peak from 553 to 613 nm by regulating the vibrational coupling. This work provides a novel concept for the design of helical CP-MR materials and CP-OLEDs, highlighting their potential for future applications in advanced optoelectronic devices. [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]

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have been studied to address the issue of the broadband emission in organic light‐emitting diodes (OLEDs). Herein, the authors have systematically investigated the effect of electron‐donating or ‐withdrawing units in the para position of B atom on the optoelectronic emission modulation BCz‐BN MR‐TADF emitters. Due to the enhanced spin‐orbit coupling (SOC) effect, BN(p)SCH3 with electron structure of para‐D‐π‐B is synthesized by introducing a heavy S atom into the para position of B atom of BCz‐BN. By oxidizing BN(p)SCH3, BN(p)SOCH3 and BN(p)SO2CH3 with electron structure of para‐A‐π‐B have been synthesized. The quantum simulations and photophysical studies have illustrated BN(p)SCH3 can exhibit large reverse intersystem crossing rate constant (kRISC) of 6.4 × 104 s−1 due to the large SOC constants and small singlet‐triplet energy splitting (ΔEST) of 0.12 eV. BN(p)SOCH3 and BN(p)SO2CH3 with electron structure of para‐A‐π‐B displayed red‐shift emissions with smaller full‐width at half‐maximum (FWHM) values of ≈21 nm and kRISC values owing to enhanced ΔEST and the low emission contribution of the triplet excitons in contrast to those of BN(p)SCH3 with electron structure of para‐D‐π‐B. Consequently, BN(p)SCH3‐based OLEDs show highly efficient blue emission with an external quantum efficiency (EQE) of 26.2% and excellent color purity. [ABSTRACT FROM AUTHOR]

Narrowband emitters based on the multiple resonance (MR) effect have shown great promises for applications in high‐definition displays. Though most reported MR emitters adopt the boron‐ and nitrogen‐doped triangulene‐like skeleton following the design of the DABNA series, MR materials with the nitrogen‐only indolocarbazole structures have recently emerged and demonstrated good color tunability from violet to deep red and impressive electroluminescence performances in terms of both efficiency and stability. Herein, we will summarize indolocarbazole‐based MR materials in recent reports, with emphasis on their molecular design, synthesis, photophysical and electroluminescence properties as well as some future research directions to unlock the full potential of this fascinating class of materials. [ABSTRACT FROM AUTHOR]

Most multi‐resonance (MR) induced thermally activated delayed fluorescence (TADF) emitters generally exhibit strong aggregation and relatively worse solubility due to their rigid and planar molecule structures, which is highly undesirable for solution‐processible devices. Herein, a simple but feasible approach for solution‐processible small‐molecule MR‐TADF emitters is developed by incorporating two MR‐TADF units onto carbazole bridge bearing long alkyl chains. The obtained emitters demonstrate supreme film‐forming capability and narrowband emissions with full‐width at half‐maximums (FWHMs) of 22 nm. The resulting solution‐processed narrowband electroluminescent devices achieve maximum external quantum efficiency of 27.1 %, which represents the highest efficiency among the solution‐processed OLEDs based on MR‐TADF emitters. This simple approach reveals great potential of developing solution‐processible emitters for rigid and planar molecular structures. [ABSTRACT FROM AUTHOR]

The S1 state and high‐lying triplet excited states (ΔES1Tn) offer insight into clarifying the mechanism of efficiency roll‐off of organic light‐emitting diodes (OLEDs). However, experimental detection of the ΔES1Tn is challenging due to Kasha's rule. Here, two emitters, PhCz‐O‐DiKTa and PhCz‐DiKTa, showing multi‐resonant thermally activated delayed fluorescence (MR‐TADF) are reported. By modulating the conjugation between the MR‐TADF DiKTa emissive center and donor substituent, emission directly from the T2 state is for the first time observed in MR‐TADF emitters. Single crystal and reduced density gradient analyses reveal the origin of the reduced observed concentration‐quenching results from weak CH···π and slipped π···π stacking interactions, which suppress nonradiative transitions. Theoretical and photophysical investigations reveal that the ΔES1T2 difference influences the reverse intersystem crossing rate. The OLEDs employing PhCz‐O‐DiKTa and PhCz‐DiKTa as emitters show maximum external quantum efficiencies (EQEmax) of over 20%, but very different efficiency roll‐off behavior (54.5% vs 13.6% at 100 cd m−2). Thus, this design provides a possible solution to mitigating device efficiency roll‐off by designing MR‐TADF emitters with degenerate S1 and T2 states. [ABSTRACT FROM AUTHOR]

Developing deep‐blue thermally activated delayed fluorescence (TADF) emitters with both high efficiency and color purity remains a formidable challenge. Here, we proposed a design strategy by integrating asymmetric oxygen‐boron‐nitrogen (O−B−N) multi‐resonance (MR) unit into traditional N−B−N MR molecules to form a rigid and extended O−B−N−B−N MR π‐skeleton. Three deep‐blue MR‐TADF emitters of OBN, NBN and ODBN featuring asymmetric O−B−N, symmetric N−B−N and extended O−B−N−B−N MR units were synthesized through the regioselective one‐shot electrophilic C−H borylation at different positions of the same precursor. The proof‐of‐concept emitter ODBN exhibited respectable deep‐blue emission with Commission International de l′Eclairage coordinate of (0.16, 0.03), high photoluminescence quantum yield of 93 % and narrow full width at half maximum of 26 nm in toluene. Impressively, the simple trilayer OLED employing ODBN as emitter achieved a high external quantum efficiency up to 24.15 % accompanied by a deep blue emission with the corresponding CIE y coordinate below 0.1. [ABSTRACT FROM AUTHOR]

Sulfone‐embedded heterocyclics are of great interest in organic light‐emitting diodes (OLEDs), however, exploring highly efficient narrowband emitters based on sulfone‐embedded heterocyclics remains challenging. Herein, five emitters with different sulfur valence state and molecular rigidity, namely tP, tCPD, 2tCPD, tPD and tPT, are thoroughly analysed. With restricted twisting of flexible peripheral phenyl by strengthening molecular rigidity, molecular emission spectra can be enormously narrowed. Further, introducing the sulfone group with bending vibration in low‐frequency region that suppresses high‐frequency vibration, sharp narrow full‐widths at half‐maximum of 28 and 25 nm are achieved for 2tCPD and tPD, respectively. Maximum external quantum efficiencies of 22.0 % and 27.1 % are successfully realized for 2tCPD‐ and tPD‐based OLED devices. These results offer a novel design strategy for constructing narrowband emitters by introducing sulfone group into a rigid molecular framework. [ABSTRACT FROM AUTHOR]

Multiple resonance (MR) emitters are promising for the next-generation wide color gamut organic light-emitting diodes (OLEDs) with narrowband emissions; however, they still face intractable challenges such as concentration-induced emission quenching, exciton annihilation, and spectral broadening. In this concept, we focus on an advanced molecular design strategy called “sterically wrapping of MR fluorophores” to address the above issues. By isolating the MR emission core using bulky substituents, intermolecular interactions can be significantly suppressed to eliminate the formation of unfavorable species. Consequently, using the newly designed emitters, optimized MR-OLEDs can achieve high external quantum efficiencies of >40% while maintaining extremely small full width at half maxima (FWHMs) of

Circularly polarized luminescence (CPL) materials that concurrently exhibit high efficiency and narrowband emission are extremely promising applications in 3D and wide color gamut display. By merging the CPL optical property and multiple resonance (MR) induced thermally activated delayed fluorescence (TADF) characteristic into one molecule, a new strategy, namely CP‐MR‐TADF, is proposed to generate organic emitters with CPL activity, TADF and narrowband emission. High‐performance red, green and blue CP‐MR‐TADF emitters have been developed following this strategy. Herein, the present status and progress of CP‐MR‐TADF materials in the field of organic light‐emitting diodes (OLEDs) is summarized. Finally, for this rapidly growing new research field, the future opportunities are forecasted and the present challenges are discussed. [ABSTRACT FROM AUTHOR]

Developing solution‐processable red organic light‐emitting diodes (OLEDs) with high color purity and efficiency based on multiple resonance thermally activated delayed fluorescence (MR‐TADF) is a formidable challenge. Herein, by introducing auxiliary electron donor and acceptor moieties into the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributed positions of multiple resonance skeleton simultaneously, an effective strategy to obtain red MR‐TADF emitters was represented. The proof‐of‐the‐concept molecule BN‐R exhibits a narrowband pure‐red emission at 624 nm, with a high luminous efficiency of 94 % and a narrow bandwidth of 46 nm. Notably, the fabricated solution‐processable pure‐red OLED based on BN‐R exhibits a state‐of‐the‐art external quantum efficiency over 20 % with the Commission Internationale de I'Éclairage coordinates of (0.663, 0.337) and a long operational lifetime (LT50) of 1088 hours at an initial luminance of 1000 cd m−2. [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]

Abstract 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.