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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]

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 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]

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]

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]

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]

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]

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]

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]

[Display omitted] • The first high-performance chiral polarized UV organic light-emitting diode is reported. • A UV device displays a maximum external quantum efficiency of 7.2 % peaking at 396 nm. • The devices show excellent color purity and small efficiency roll-off. • Triplet exciton utility and large horizontal dipole ratios contribute to the excellent performance. • A molecular design strategy is provided for chiral ultraviolet organic emitters. Emitting circularly polarized electroluminescence (CPEL) directly endows the CP organic light-emitting diodes (CP-OLEDs) with great advantages in 3D display. But the highly efficient CP-OLEDs are still in great need, especially for devices emitting short-wavelength light such as ultraviolet (UV) CP-OLEDs. Herein, we report the first case of CP UV-OLED, fabricated by two pairs of enantiomers, (R)-/(S)-BNP- o -CzPI and (R)-/(S)-BNP- p -CzPI, established by chiral perturbation. Devices using 10 wt% enantiomers as dopants show UV CPEL at 396 nm with external quantum efficiency, Commission Internationale de l'Eclairage y value, and full width at half maxima of 7.2/6.9 %, 0.046/0.044 and 50/50 nm for (R)-/(S)-BNP- o -CzPI, respectively. The horizontal dipole orientations of the film with 10 wt% emitters in mCPCN are measured as high as 84.1/85.2 % and 88.1/88.7 % for (R)-/(S)-BNP- o -CzPI and (R)-/(S)-BNP- p -CzPI, respectively. Experiments and theoretical calculations indicate that the high EQEs are due to the high triplet exciton utility and large horizontal dipole orientation. In addition, the enantiomers show significant circularly polarized luminescence properties both in photoluminescence and in electroluminescence, and the device using (S)-BNP- o -CzPI has a large dissymmetry factors of 3.10 × 10−3 at emission maximum. [ABSTRACT FROM AUTHOR]

Narrowband emitting fluorophores exhibit immense potentials for organic light‐emitting diodes (OLEDs) with high color purity. However, it's still hard to simultaneously realize short‐wavelength ultraviolet (UV) or near ultraviolet emission (NUV) while maintaining a narrowed full width at half maximum (FWHM) value, and rare work focus on such challenging pursuit. Herein, an ingenious synthetic method was devised to achieve emitters with coplanar structure. 11‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)indolo[3,2,1‐jk]carbazole (ICZ‐TAZ) was designed to realize narrowed UV emission both in photoluminescence (PL) and electroluminescence (EL) which benefited from the suppression of vibronic coupling. UV/NUV OLEDs based on ICZ‐TAZ achieve external quantum efficiency (EQE) maximums of 3.26 % peaks @ 388 nm and 4.02 % peaks @ 406 nm with small FWHM of 32 nm and 46 nm, respectively, corresponding with reduced efficiency roll‐off at luminance of 100 cd m−2. [ABSTRACT FROM AUTHOR]

Organic thermally activated delayed fluorescence (TADF) materials have attracted significant research interest in the field of organic electronics because of their inherent advantage of 100% exciton utilization capability in organic light‐emitting diodes (OLEDs) without the use of noble metals. However, despite their high internal electroluminescence quantum efficiencies approaching unity, broad emission spectra with sizable full width at half maxima (FWHM; 60–100 nm) present a critical issue that must be solved for their application in ultrahigh‐definition OLED displays. Recently, a new paradigm of TADF materials featuring the multiple resonance (MR) effect based on heteroatom‐doped polycyclic aromatic frameworks, referred to as MR‐TADF materials, has emerged and garnered considerable research interest owing to their remarkable features of efficient narrowband emissions with extremely small FWHMs (≤30 nm). Currently, MR‐TADF materials occupy a prominent position in the cutting‐edge research on organic light‐emitting materials from both chemical and physical perspectives. This review article focuses on recent progress in narrowband emissive MR‐TADF systems from the perspective of molecular design, photophysical properties, and electroluminescence performance in OLEDs. The current status and future prospects of this advanced material technology are discussed comprehensively. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) conjugated polymers usually show broad emission spectra due to the large structure relaxation, which is not desirable for high‐quality organic light‐emitting diode (OLED) displays. Herein, through attaching multiple resonance (MR) induced emitting moiety as pendant onto the polycarbazole backbone, TADF conjugated polymers with narrowband emission are demonstrated. The obtained conjugated polymers exhibit typical TADF characteristics with narrowband emission with full‐width at half‐maximums (FWHMs) of 33–43 nm. The solution‐processed devices employing these polymers as emissive layers realize excellent performances with maximum external quantum efficiency (EQE) of 17.5 %, emission peak at 496 nm and FWHM of 34 nm. [ABSTRACT FROM AUTHOR]

Despite the remarkable multiple resonance (MR) optoelectronic properties of organic nanographenes with boron and nitrogen atoms disposed para to each other, the synthetic procedures are sophisticated with low yields and the molecular skeletons are limited. Here, a new paradigm of easy‐to‐access MR emitter is constructed by simplifying the multiborylation through amine‐directed formation of B−N bonds while introducing an additional para‐positioned nitrogen atom to trigger the MR effect. The proof‐of‐concept molecules exhibit narrowband emissions at 480 and 490 nm, with full width at half maxima (FWHMs) of only 29 and 34 nm. The devices based on them showed external quantum efficiencies (EQE) of >33.0 %, which remained above 24.0 % even at a high brightness of 5000 cd m−2. This is the first example of MR emitters with a B−N covalent bond, not only decreasing the synthesis difficulty but also increasing the diversity of MR skeletons for emerging new optoelectronic properties. [ABSTRACT FROM AUTHOR]

In this study, we synthesized and characterized multiresonant thermally activated delayed fluorescent (TADF) materials embedded with nitrogen‐boron‐oxygen (N−B−O), exhibiting color‐tunability between blue and green, namely NBO, m‐DiNBO, and p‐DiNBO. The three emitter materials showed a high photoluminescence quantum yield (PLQY) and a state‐of‐the‐art narrow full width at half maximum (FWHM) of 96 %/25 nm, 87 %/17 nm, and 99 %/19 nm, respectively. For m‐DiNBO and p‐DiNBO, the emission color could be tuned from blue to green by regulating the nonbonding/bonding molecular orbital characters. Owing to the expanded planar molecular structure, m‐DiNBO and p‐DiNBO showed high horizontal dipole ratio (Θ) of 88 % and 92 %, respectively. OLEDs were prepared with NBO, m‐DiNBO, and p‐DiNBO, exhibiting high external quantum efficiencies of 16.8 %, 24.2 %, and 21.6 %, respectively. NBO and m‐DiNBO exhibited pure‐blue emission with CIE coordinates of (0.137, 0.142) and (0.126, 0.098), respectively. p‐DiNBO showed pure‐green emission with a CIE coordinate of (0.258, 0.665). [ABSTRACT FROM AUTHOR]

The development of boron/nitrogen‐based polycyclic heteroaromatic emitters with multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) property can efficiently promote the advancement of high‐efficiency organic light‐emitting diodes (OLEDs) with narrowband emission. Herein, a simple strategy to achieve four ternary boron/nitrogen‐based polycyclic heteroaromatic emitters (SBON, SBSN, DBON, and DBSN) from pure blue (463 nm) to yellow (553 nm) via tuning the coordination between B/N and heteroatom (O or S) is demonstrated, aiming to increase charge transfer delocalization of the polycyclic heteroaromatic emitters and adjust photo‐physical properties. This strategy endows the four emitters with full width at half maximum (FWHM) of 24, 27, 20, and 28 nm, respectively. Additionally, double‐boron type green and yellow narrowband emitters (DBON and DBSN) possess 98% photoluminescence efficiencies in doped films. Besides, considerable rate constants of reverse intersystem crossing (RISC) are achieved because of the small singlet–triplet excited state energy gap and large spin–orbit coupling values. Consequently, the OLEDs covering from blue to yellow based on these emitters show the maximum external quantum efficiency of 13.7%, 17.6%, 26.7%, and 21.8%, respectively, with low‐efficiency roll‐off. These results provide a feasible design strategy to construct boron/nitrogen‐based polycyclic heteroaromatic MR‐TADF emitters for efficient OLEDs with color‐tuning electroluminescence. [ABSTRACT FROM AUTHOR]

Narrowband emissive organoboron emitters featuring the multi‐resonance (MR) effect have now become a critical material component for constructing high‐performance organic light‐emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high‐efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet‐to‐singlet reverse intersystem crossing (RISC). However, RISC involving spin‐flip exciton upconversion is generally the rate‐limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter (CzBSe) doped with a selenium atom, demonstrating a record‐high RISC rate exceeding 108 s−1, which is even higher than its fluorescence radiation rate. Furthermore, the spin‐flip upconversion process in CzBSe can be accelerated by factors of ≈20000 and ≈800, compared to those of its oxygen‐ and sulfur‐doped homologs (CzBO and CzBS), respectively. Unlike CzBO and CzBS, the photophysical rate‐limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time‐delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR‐induced blue narrowband emission and significantly alleviated efficiency roll‐off features. [ABSTRACT FROM AUTHOR]

Herein, a ternary boron‐oxygen‐nitrogen embedded polycyclic aromatic hydrocarbon with multiple resonance thermally activated delayed fluorescence (MR‐TADF), namely DBNO, is developed by adopting the para boron‐π‐boron and para oxygen‐π‐oxygen strategy. The designed molecule presents a vivid green emission with a high photoluminescence quantum yield (96 %) and an extremely narrow full width at half maximum (FWHM) of 19 nm/0.09 eV, which surpasses all previously reported green TADF emitters to date. In addition, the long molecular structure along the transition dipole moment direction endows it with a high horizontal emitting dipole ratio of 96 %. The organic light‐emitting diode (OLED) based on DBNO reveals a narrowband green emission with a peak at 504 nm and a FWHM of 24 nm/0.12 eV. Particularly, a significantly improved device performance is achieved by the TADF‐sensitization (hyperfluorescence) mechanism, presenting a FWHM of 27 nm and a maximum external quantum efficiency (EQE) of 37.1 %. [ABSTRACT FROM AUTHOR]

We report three deep blue multiple resonant TADF emitters, m‐ν‐DABNA, 4F‐ν‐DABNA, and 4F‐m‐ν‐DABNA by incorporating methyl groups and fluorine atoms in diboron based ν‐DABNA core. The introduction of methyl groups and fluorine atoms resulted in bandgap enhancement by electron donating and electron withdrawing effects. All three emitter's exhibit pure blue emissions with high photoluminescence quantum yield around ~90%, small ΔEST (≤ 0.07 eV) values, and fast reverse intersystem crossing rate (kRISC). Fabricated OLEDs with these emitters show excellent external quantum efficiency over 33%. Among three materials, 4F‐ν‐DABNA and 4F‐m‐ν‐DABNA exhibit the CIE y coordinate of 0.08 and 0.06, respectively. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials based on multiple resonance (MR) effect exhibit enormous potentials in organic light‐emitting diodes (OLEDs) with high color purity due to their intrinsically narrow emission. However, most of MR‐TADF emitters are limited to the boron‐nitrogen‐based rigid skeleton. In this work, three novel MR‐TADF emitters, namely CzBNO, DMAcBNO and DPAcBNO, are elaborately constructed, the TADF properties of which are realized by virtue of opposite MR effect of boron and nitrogen/oxygen atom. CzBNO‐based deep blue‐emitting OLEDs achieve a maximum external quantum efficiency (EQE) of 13.6% with a small FWHM of 36 nm, as well as a Commission Internationale de l'Eclairage (CIE) coordinate of (0.14,0.08). While the other two emitter‐based devices exhibit blue emission with a maximum EQE of up to 23.0%. To further improve the OLEDs performances, DMAcBNO and DPAcBNO‐based devices assisted by a sensitizer exhibit an excellent EQE of up to 29.6% with a relatively small efficiency roll‐off. [ABSTRACT FROM AUTHOR]

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

High device efficiency and color‐purity are the two essentials for high‐quality organic light‐emitting diodes (OLEDs). Multi‐resonance (MR) molecules show great potentials for high color‐purity OLEDs due to their sharp emission bands. However, most MR molecules exhibit emission limited from deep‐blue to green spectral region. Herein, through peripherally decorating MR emitter with electron donors, a new approach enabling the emission spectra of MR emitters red‐shift while retaining narrowband emission is demonstrated. By manipulating the numbers and electron‐donating abilities of the peripheries, the first narrowband yellow emitter with emission maxima of 562 nm and a full‐width at half‐maximum (FWHM) of 30 nm is realized. Highly efficient OLEDs with an external quantum efficiency of over 24% and excellent color purity are fabricated by employing these newly developed MR molecules as emitters. [ABSTRACT FROM AUTHOR]

The ultra-high definition (UHD) displays in the market require organic light-emitting diodes (OLEDs) to meet the Rec.2020 color gamut standard, while it is still challenging to exploit ultra-deep-blue emitters that satisfy such blue standard. Herein, by substituting tCz (3,6-di(tert -butyl)carbazole) donor unit on the robust BOC (8,8,12,12-tetramethyl-8,12-dihydro-4-oxa-3a2-boradibenzo[ cd , mn ]pyrene) acceptor unit with high structural rigidity and appropriate energy levels, a novel boron-based D–A type thermally activated delayed fluorescence (TADF) ultra-deep-blue emitter, namely, tCz-BOC , was readily designed and synthesized. Inheriting from its rigid organoboron core, the emitter presents ultra-deep-blue emission with narrow spectral full width at half maximum (FWHM) of 54 nm and high photoluminescence quantum yield up to 73.7%. Additionally, tCz-BOC can effectively harvest T 1 excitons through high-lying T 2 mediated reverse intersystem crossing process. As a consequence, the fabricated OLED based on tCz-BOC exhibits a satisfactory ultra-deep-blue emission with λ EL of 424 nm, narrow emission bandwidth with FWHM of 47 nm, excellent maximum external quantum efficiency (EQE max) of 13.9%, as well as the CIE y coordinate of 0.047, which is fairly close to the Rec.2020 blue color gamut. A novel deep-blue thermally activated delayed fluorescence molecule. [Display omitted] • A robust TADF emitter tCz-BOC with ultra-deep-blue emission peaked at 428 nm and a narrow FWHM of 47 nm was obtained. • An excellent CIE y coordinate of 0.047 which is close to the Rec.2020 blue color gamut was achieved in its OLED. • The EQE max of the OLED was as high as 13.9%. [ABSTRACT FROM AUTHOR]

Organic fluorescent emitters with narrowband emissions are highly desirable for high‐resolution organic light‐emitting diode (OLED) display technology. In principle, this can be achieved by specifically controlling the intrinsic structural relaxation and vibronic coupling in the excited state. Here, a design strategy to realize narrowband emission of organic fluorescent emitters is proposed by significantly enhancing the low‐frequency vibronic coupling strength (Λ) while simultaneously reducing the high‐frequency Λ of the commonly involved stretching modes. The quinolino‐[3,2,1‐de]acridine‐5,9‐dione (QAO) species is found to be directly associated with this design principle. By introducing single bond‐linked peripheral moieties into the QAO core, the constructed QAO derivatives are shown to exhibit better performance, by achieving a full width at half‐maximum of 23 nm/0.13 eV in toluene for the narrowest band as well as 27 nm/0.15 eV in doped devices, with negligible dependence on the doping concentrations. The maximum external quantum efficiency of the fabricated blue OLED is 17.5%. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials that are able to realize electroluminescence showing simultaneously narrow emission band, high color purity, and high efficiency are highly demanded for display applications. Up to now, a few blue emissive TADF materials with such characteristics have been reported, while green TADF materials with such features are extremely scarce. Herein, two TADF compounds with highly efficient narrowband bluish green/green emission (full‐width at half‐maximum (FWHM): 22 and 21 nm in toluene) induced by multiple resonance effect are reported. An optimized organic light‐emitting diode based on the green emissive compound shows a fairly narrow electroluminescent spectrum (FWHM: 33 nm), as well as an excellent color purity with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.20, 0.65), which is a close resemblance to the standard green‐light CIE coordinates of (0.21, 0.71) defined by the National Television System Committee. Meanwhile, the maximum external quantum efficiency (EQE) up to 25.5% is achieved and a high EQE of 20.1% is maintained at a practical high luminance of 1000 cd m−2. The outstanding device performance of the compound makes it attractive for potential practical applications. [ABSTRACT FROM AUTHOR]

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials attract much attention owing to their 100 % exciton utilization, high photoluminescence quantum yield (PLQY) and narrowband emission with high color purity. However, how to regulate the energy levels and lifetimes of excited states for long-wavelength and short delayed fluorescence lifetime MR-TADF materials is still a challenge. Herein, an electron acceptor unit of pyrimidine derivative is introduced with steric-hinerance effect. The emission spectrum of BN-DPP is red shifted with 15 nm and the full width at half maximum (FWHM) is even narrower of 20 nm compared with that of DtCZB. A yellow-green MR-TADF material, called BN-DPBQ, is fabricated by extended the conjugation. Moreover, the delayed fluorescence lifetime of BN-DPBQ is half of that of BN-DPP. The BN-DPP based devices exhibit greenish blue emission with peak locating at 494 nm and maximum external quantum efficiency (EQE) of 23.6 %. BN-DPBQ based ones show much red shifted spectrum with emission peak at 538 nm and maximum EQE of 20.3 %. The results indicate that introducing electron acceptor units with steric-hinerance effect and extending the conjugation of resonance skeleton are effective ways to regulate energy levels and lifetimes of excited states for MR-TADF. [Display omitted] • Efficient energy levels and lifetimes regulation of excited states was proposed for narrowband TADF emitters. • BN-TPP was red-shifted of 15 nm with narrower FWHM by introducing electron acceptor with steric effect. • Red-shifted spectrum and decreased excited state lifetime were achieved by enlarging the conjugation of molecule. • BN-TPP and BN-DPBQ based devices demonstrated maximum EQEs of 24.3 and 22.8% with TADF or phosphorescent sensitizers. [ABSTRACT FROM AUTHOR]

[Display omitted] • PYHB and PYHP deliver excellent thermal stability and morphological stability. • PYHB and PYHP can inherit the narrowband blue emission and HLCT characteristics. • PYHB and PYHP remove the limitations of traditional polymer luminescent materials. • PYHB and PYHP outperform previously reported polymer luminescent materials. • Our strategy provides a promising platform for PIs as a photoelectric material. Blue light-emitting materials with excellent film-forming ability, superior morphological stability, outstanding color purity, and high device efficiency are in urgent demand for new-generation display and solid-state lighting devices fabricated by low-cost wet processing. However, their current performances are far from satisfactory. Herein, two polyimide (PI)-based blue polymers are first precisely constructed and synthesized through a "Hinge-type Linking" strategy. A small molecular is introduced into the PI backbone via non-conjugated aliphatic imide ring linker. These polymers not only show superb thermal stability and morphological stability, but also maintain narrowband blue emission. Furthermore, PIs are prepared by polycondensation method, precipitated and purified in ethanol solvent, which avoids the use of metal catalysts. The resulting solution-processable devices achieve high performance with a maximum external quantum efficiency (EQE) of 9.94 % and maximum emission peak at 460 nm. To the best of our knowledge, this is the first time that solution-processable polymer light-emitting diodes (PLEDs) based on the "hot exciton" mechanism achieving splendid performance with maximum EQE close to 10 %. The superior performance is attributed to the validity of the "Hinge-type Linking" strategy in PLEDs, opening up a new avenue for PLED development and application. [ABSTRACT FROM AUTHOR]

Multi-resonance emitters are attractive for organic light-emitting diode (OLED) applications because of their narrowband emissions and high photoluminescent efficiencies. Nevertheless, their tendency to aggregation by intermolecular interactions could cause doping sensitivity and reduce color purity at high concentrations. Here, we report a novel boron, sulfur (B, S)-containing multi-resonance emitter (BSS-Ph-TBCz) consisting of 1,8-diphenyl-carbazole substituent at para- position of boron atom of B, S-doped polycyclic aromatic skeleton. Different from control compound (BSS-TBCz) containing conventional carbazole substituents showing aggregation-caused spectral broadening, BSS-Ph-TBCz with 1,8-diphenyl-carbazole group exhibits weak intermolecular aggregation behavior owing to the large twist angle between polycyclic skeleton and carbazole plane caused by two steric phenyl groups on carbazole group, leading to nearly unchanged emissions with small full width at half-maximum of 30–31 nm at doping concentration ranging from 1 to 100 wt%. OLEDs employing BSS-Ph-TBCz as emitter reveal blue electroluminescence at 468 nm and maximum external quantum efficiency of 21.4% at 5 wt% doping concentration, which remains at 20.1% at high doping concentration of 25 wt%. [Display omitted] • Novel multi-resonance emitter with 1,8-diphenyl-carbazole substituent is developed. • Aggregation emission is suppressed by steric hindrance of 1,8-diphenyl-carbazole. • Rapid reverse intersystem crossing is realized by heavy-atom effect of sulfur. • EQE of >20% and FWHM of 31 nm are achieved for OLEDs at high doping concentration. [ABSTRACT FROM AUTHOR]

Boron, sulfur-doped polycyclic aromatic hydrocarbon emitters with multiple-resonance-dominated lowest excited states are developed by tuning energy level alignment between 5,9-dithia-13b-boranaphtho[3,2,1- de ]anthracene skeleton and peripheral arylamine moieties, which reveal narrowband deep-blue electroluminescence with full-width at half maximum of 29 nm, Commission Internationale de l'Éclairage color coordinates of (0.13, 0.09) and maximum external quantum efficiency of 21.8%. [Display omitted] • Novel B,S-doped polycyclic aromatic multiple resonance emitters are developed. • Sulfur atoms enhance spin-orbital coupling and reverse intersystem crossing. • The lowest excited state characteristics are regulated by arylamine substituents. • The emitters exhibit narrowband deep-blue electroluminecence with EQE of 21.8%. Different from boron, nitrogen-based multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters, boron (B), sulfur (S)-doped polycyclic aromatic hydrocarbons (PAHs) containing 5,9-dithia-13b-boranaphtho[3,2,1- de ]anthracene (BSS) skeleton and arylamine moieties in para -positions of boron atoms are developed as a novel kind of MR-TADF emitters for narrowband deep-blue electroluminescence. The molecular design not only utilizes heavy-atom effect of sulfur atoms in BSS skeleton to enhance spin-orbital coupling and promote reverse intersystem crossing, but also uses arylamines as peripheral substituents to modify excited state characteristics. By decreasing electron-donating ability and downshifting HOMO level of arylamines, the lowest excited state for resultant emitter is converted from intersegmental charge transfer state to multiple resonance state localized on BSS skeleton, leading to blue-shifted and narrowed emissions. The B,S-doped PAH emitter bearing carbazole moiety exhibits deep-blue electroluminecence with full-width at half maximum of 29 nm, Commission Internationale de l'Éclairage color coordinates of (0.13, 0.09), and the maximum external quantum efficiency of 21.8 %, representing a new approach toward efficient narrowband deep-blue emission beyond boron, nitrogen-based MR-TADF emitters. [ABSTRACT FROM AUTHOR]

[Display omitted] • Deep Blue MR-TADF emitters were designed by incorporating rigid DOBNA into MR skeletons. • Both emitters possess narrowband deep blue emission with FWHM of 19 nm. • Small ΔE ST (0.05 eV) and high PLQY are realized in the emitters. • MR-TADF OLEDs demonstrate high EQEs (>30 %) with CIE y coordinate below ≤ 0.07. The simultaneous achievement of high efficiency and deep blue narrowband emission in multi resonant thermally activated delayed fluorescence (MR-TADF) materials are crucial and challenging. Herein we report two deep blue MR-TADF emitters, namely, TPD4PA and tBu-TPD4PA , using double boron, three nitrogen and two oxygen atoms. The design is based on amalgamating the high charge transfer (CT) characteristic moiety into MR-type fragments towards efficient MR-TADF emitters with improved CT characteristics. Both the materials show deep blue photo luminescent emissions of ∼450 nm with a high photoluminescence quantum yield (PLQY) of ∼90 %. These materials showed very small singlet–triplet gap (≤0.06 eV) and a high rate of reverse intersystem crossing of ∼2.5 × 105 s−1. The TADF devices based on TPD4PA and tBu-TPD4PA showed maximum external quantum efficiencies of 30.7 and 32.5 %, respectively. Furthermore, both devices exhibited narrow band deep blue emissions and corresponding CIE y coordinates 0.06 and 0.07, which match near NTSC and BT2020 blue color requirements. [ABSTRACT FROM AUTHOR]

[Display omitted] • Newly synthesized diboron embedded deep blue MR-TADF emitters. • Narrow spectra with FWHM of 14 nm and CIE y coordinate below 0.10. • Small △E ST (≤0.07 eV), high PLQY, and good TADF performances. • All these materials show high EQEs (>33%) We report three deep blue multiple resonant TADF emitters, m-ν-DABNA, 4F-ν-DABNA, and 4F-m-ν-DABNA by incorporating methyl groups and fluorine atoms in diboron based ν-DABNA core. The introduction of methyl groups at para positions to the boron atoms and fluorine atoms at ortho positions to the nitrogen atoms resulted in bandgap enhancement by electron donating and electron withdawing effects. All three emitters exhibit pure blue emissions with high photoluminescence quantum yield around ∼ 90%, small ΔE ST (≤0.07 eV) values, and fast reverse intersystem crossing rate (k RISC). Fabricated OLEDs with these emitters show excellent external quantum efficiency over 33%. Among three materials, 4F-ν-DABNA and 4F-m-ν-DABNA exhibit the CIE y coordinate of 0.08 and 0.06, respectively. Our current material design approach guides a path for the development of deep blue TADF emitters for highly efficient narrowband OLEDs. [ABSTRACT FROM AUTHOR]