Solution-Processable Pure-Red Multiple Resonance-induced Thermally Activated Delayed Fluorescence Emitter for Organic Light-Emitting Diode with External Quantum Efficiency over 20 %

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.     

B−N/B−O Contained Heterocycles as Fusion Locker in Multi-Resonance Frameworks towards Highly-efficient and Stable Ultra-Narrowband Emission

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.     

A Quadruple-Borylated Multiple-Resonance Emitter with para/meta Heteroatomic Patterns for Narrowband Orange-Red Emission

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.     

Boron-Dipyrromethene-Based Fluorescent Emitters Enable High-Performance Narrowband Red Organic Light-Emitting Diodes

Narrowband organic light-emitting diodes (OLEDs) are receiving significant attention and have demonstrated impressive performance in blue and green OLEDs. However, developing high-performance narrowband red OLEDs remains a highly desired yet challenging task. Herein, we have developed narrowband red fluorescent emitters by utilizing a boron-dipyrromethene (BODIPY) skeleton in combination with a methyl-shield strategy. These emitters exhibit small full-width at half-maxima (FWHM) ranging from 21 nm (0.068 eV) to 25 nm (0.081 eV) and high photoluminescence quantum yields (Φ_PL) ranging from 88.5 % to 99.0 % in toluene solution. Using BODIPY-based luminescent materials as emitters, high-performance narrowband red OLEDs have been assembled with external quantum efficiency as high as 18.3 % at 623 nm and 21.1 % at 604 nm. This work represents, to our knowledge, the first successful case of achieving NTSC pure-red OLEDs with the Commission Internationale de l’Éclairage (CIE) coordinates of [0.67, 0.33] based on conventional fluorescent emitters.     

Combining Carbazole Building Blocks and ν-DABNA Heteroatom Alignment for a Double Boron-Embedded MR-TADF Emitter with Improved Performance

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.     

Multi‐Resonance Induced Thermally Activated Delayed Fluorophores for Narrowband Green OLEDs

High‐color‐purity emissions with small a full‐width at half‐maximum (FWHM) are an ongoing pursuit for high‐resolution displays. Though the flourishment of narrow‐band emissive materials with multi‐resonance induced thermally activated delayed fluorescence (MR‐TADF) in the blue region, such materials have not validated their potential in other color regions. By amplifying the influence of skeleton and peripheral units, a series of highly efficient green‐emitting MR‐TADF materials are firstly reported. Peripheral units with electron‐deficit properties can significantly narrow the energy gap for bathochromic emission without compromising the color fidelity. MR‐TADF emitters with photo‐luminance quantum yields of above 90 % with FWHMs of ≤25 nm are developed. The corresponding organic light‐emitting diodes show maximum external quantum efficiency/ power efficiency of 22.02 %/ 69.82 lm W^?1 with excellent long‐term stability.     

Achieving High-Performance Pure-Red Electrophosphorescent Iridium(III) Complexes Based on Optimizing Ancillary Ligands

Two new iridium(III) complexes were synthesized by introducing two trifluoromethyl groups into an ancillary ligand to develop pure-red emitters for organic light-emitting diodes (OLEDs). The electron-donating ability of the ancillary ligands is suppressed, owing to the electron-withdrawing nature of trifluoromethyl groups, which can reduce the HOMO energy levels compared with those of compounds without trifluoromethyl groups. However, the introduction of trifluoromethyl groups into the ancillary ligand has little impact on the LUMO energy levels. Therefore, a well-tuned, pure-red, excited-state energy was achieved by regulating the relative energy level between the HOMO and LUMO. OLEDs with these complexes as emitters showed high external quantum efficiencies (EQEs) of 26 % and realized high EQEs of about 25 % and fairly low driving voltages of 3.3–3.6 V for practical luminance of 1000 cd m⁻², as well as excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.66, 0.33) and (0.67, 0.33); thus, this demonstrates the successful molecular design strategy by modifying the electron-donating ability of ancillary ligand.     

Boron-, Sulfur- and Nitrogen-Doped Polycyclic Aromatic Hydrocarbon Multiple Resonance Emitters for Narrow-Band Blue Emission

Two boron-, sulfur- and nitrogen-doped polycyclic aromatic hydrocarbon multiple resonance thermally activated delayed fluorescence emitters with high photoluminescent quantum efficiency (88 %) and rapid reverse intersystem crossing (k_RISC= 1.0×10⁵ s⁻¹) are designed and synthesized, enabling efficient narrow-band blue electroluminescence at 473 nm with full width at half maximum of 29 nm and maximum external quantum efficiency of 22.0 %, which provides an avenue to expand the structure library for multiple resonance emitters and an approach to regulate their emission properties.     

A Simple Approach to Solution-Processible Small-Molecule Multi-Resonance TADF Emitters for High-Performance Narrowband OLEDs

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.     

Tailored Design of π-Extended Multi-Resonance Organoboron using Indolo[3,2-b]Indole as a Multi-Nitrogen Bridge

Extending the π-skeletons of multi-resonance (MR) organoboron emitters can feasibly modulate their optoelectronic properties. Here, we first adopt the indolo[3,2-b]indole (32bID) segment as a multi-nitrogen bridge and develop a high-efficiency π-extended narrowband green emitter. This moiety establishes not only a high-yield one-shot multiple Bora–Friedel–Crafts reaction towards a π-extended MR skeleton, but a compact N-ethylene-N motif for a red-shifted narrowband emission. An emission peak at 524 nm, a small full width at half maximum of 25 nm and a high photoluminescence quantum yield of 96 % are concurrently obtained in dilute toluene. The extended molecular plane also results in a large horizontal emitting dipole orientation ratio of 87 %. A maximum external quantum efficiency (EQE) of 36.6 % and a maximum power efficiency of 135.2 lm/W are thereafter recorded for the corresponding device, also allowing a low efficiency roll-off with EQEs of 34.5 % and 28.1 % at luminance of 1,000 cd/m² and 10,000 cd/m², respectively.     

Medium-Ring Strategy Enables Multiple Resonance Emitters with Twisted Geometry and Fast Spin-Flip to Suppress Efficiency Roll-Off

Suppressing aggregation-caused quenching (ACQ) effect and reducing device efficiency roll-off are both crucial yet challenging for multi-resonance (MR) emitters. Herein, we put forward a medium-ring strategy to design efficient MR emitters that feature heptagonal tribenzo[b,d,f]azepine (TBA) donors. The highly twisted conformation enlarges the intermolecular distances between the MR-emitting cores, and thus suppresses ACQ effect. Meanwhile, the introduction of heptagonal donors enhances spin-orbital coupling, so as to accelerate reverse intersystem crossing (RISC) process. This medium-ring strategy gives rise to the first example of blue MR emitter that simultaneously possesses radiative decay rate as fast as 10⁸ s⁻¹ and RISC rate as fast as 10⁶ s⁻¹. Accordingly, DTBA-B2N3 enables to assemble high-performance blue organic light-emitting diodes (OLEDs) with maximum external quantum efficiency (EQE_max) of 30.9 % and alleviated efficiency roll-off (EQE₁₀₀₀: 20.5 %).     

Constructing Charge-Transfer Excited States Based on Frontier Molecular Orbital Engineering: Narrowband Green Electroluminescence with High Color Purity and Efficiency

The design and synthesis of organic materials with a narrow emission band in the longer wavelength region beyond 510 nm remain a great challenge. For constructing narrowband green emitters, we propose a unique molecular design strategy based on frontier molecular orbital engineering (FMOE), which can integrate the advantages of a twisted donor–acceptor (D-A) structure and a multiple resonance (MR) delayed fluorescence skeleton. Attaching an auxiliary donor to a MR skeleton leads to a novel molecule with twisted D-A and MR structure characteristics. Importantly, a remarkable red-shift of the emission maximum and a narrowband spectrum are achieved simultaneously. The target molecule has been employed as an emitter to fabricate green organic light-emitting diodes (OLEDs) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.69) and a maximum external quantum efficiency (EQE) of 27.0 %.     

Integrating Asymmetric O−B−N Unit in Multi-Resonance Thermally Activated Delayed Fluorescence Emitters towards High-Performance Deep-Blue Organic Light-Emitting Diodes

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.     

Rigid Bridge-Confined Double-Decker Platinum(II) Complexes Towards High-Performance Red and Near-Infrared Electroluminescence

A molecular design to high-performance red and near-infrared (NIR) organic light-emitting diodes (OLEDs) emitters remains demanding. Herein a series of dinuclear platinum(II) complexes featuring strong intramolecular Pt???Pt and π–π interactions has been developed by using N-deprotonated α-carboline as a bridging ligand. The complexes in doped thin films exhibit efficient red to NIR emission from short-lived (τ=0.9–2.1 μs) triplet metal-metal-to-ligand charge transfer (³MMLCT) excited states. Red OLEDs demonstrate high maximum external quantum efficiencies (EQEs) of up to 23.3 % among the best Pt^II-complex-doped devices. The maximum EQE of 15.0 % and radiance of 285 W sr^?1 m^?2 for NIR OLEDs (λ_EL=725 nm) are unprecedented for devices based on discrete molecular emitters. Both red and NIR devices show very small efficiency roll-off at high brightness. Appealing operational lifetimes have also been revealed for the devices. This work sheds light on the potential of intramolecular metallophilicity for long-wavelength molecular emitters and electroluminescence.     

Constructing Charge‐Transfer Excited States Based on Frontier Molecular Orbital Engineering: Narrowband Green Electroluminescence with High Color Purity and Efficiency

The design and synthesis of organic materials with a narrow emission band in the longer wavelength region beyond 510 nm remain a great challenge. For constructing narrowband green emitters, we propose a unique molecular design strategy based on frontier molecular orbital engineering (FMOE), which can integrate the advantages of a twisted donor–acceptor (D‐A) structure and a multiple resonance (MR) delayed fluorescence skeleton. Attaching an auxiliary donor to a MR skeleton leads to a novel molecule with twisted D‐A and MR structure characteristics. Importantly, a remarkable red‐shift of the emission maximum and a narrowband spectrum are achieved simultaneously. The target molecule has been employed as an emitter to fabricate green organic light‐emitting diodes (OLEDs) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.69) and a maximum external quantum efficiency (EQE) of 27.0 %.     

Solution-processed multi-resonance organic light-emitting diodes with high efficiency and narrowband emission

With excellent color purity(full-width half maximum(FWHM) 40 nm) and high quantum yield,multiresonance(MR) molecules can harvest both singlet and triplet excitons for highly efficient narrowband organic light-emitting diodes(OLEDs) owing to their thermally activated delayed fluorescence(TADF)nature.However,the highly rigid molecular skeleton with the oppositely positioned bo ron and nitrogen in generating MR effects results in the intrinsic difficulties in the solution-processing of MR-OLEDs.Here,we demonstrate a facile strategy to increase the solubility,enhance the efficiencies and modulate emission color of MR-TADF molecules by extending aromatic rings and introducing tert-butyls into the MR backbone.Two MR-TADF emitters with smaller singlet-triplet splitting energies(ΔE~(ST))and larger oscillator strengths were prepared conveniently,and the solution-processed MR-OLEDs were fabricated for the first time,exhibiting efficient bluish-green electroluminescence with narrow FWHM of 32 nm and external quantum efficiency of 16.3%,which are even comparable to the state-of-the-art performances of the vacuum-evaporated devices.These results prove the feasibility of designing efficient solutionprocessible MR molecules,offering important clues in developing high-performance solution-processed MR-OLEDs with high efficiency and color purity.     

Introducing Spiro-locks into the Nitrogen/Carbonyl System towards Efficient Narrowband Deep-blue Multi-resonance TADF Emitters

The current availability of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials with excellent color purity and high device efficiency in the deep-blue region is appealing. To address this issue in the emerged nitrogen/carbonyl MR-TADF system, we propose a spiro-lock strategy. By incorporating spiro functionalization into a concise molecular skeleton, a series of emitters (SFQ, SOQ, SSQ, and SSeQ) can enhance molecular rigidity, blue-shift the emission peak, narrow the emission band, increase the photoluminescence quantum yield by over 92 %, and suppress intermolecular interactions in the film state. The referent CZQ without spiro structure has a more planar skeleton, and its bluer emission in the solution state redshifts over 40 nm with serious spectrum broadening and a low PLQY in the film state. As a result, SSQ achieves an external quantum efficiency of 25.5 % with a peak at 456 nm and a small full width at half maximum of 31 nm in a simple unsensitized device, significantly outperforming CZQ. This work discloses the importance of spiro-junction in modulating deep-blue MR-TADF emitters.     

Regulation of Multiple Resonance Delayed Fluorescence via Through-Space Charge Transfer Excited State towards High-Efficiency and Stable Narrowband Electroluminescence

B- and N-embedded multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters usually suffer from slow reverse intersystem crossing (RISC) process and aggregation-caused emission quenching. Here, we report the design of a sandwich structure by placing the B−N MR core between two electron-donating moieties, inducing through-space charge transfer (TSCT) states. The proper adjusting of the energy levels brings about a 10-fold higher RISC rate in comparison with the parent B−N molecule. In the meantime, a high photoluminescence quantum yield of 91 % and a good color purity were maintained. Organic light-emitting diodes based on the new MR emitter achieved a maximum external quantum efficiency of 31.7 % and small roll-offs at high brightness. High device efficiencies were also obtained for a wide range of doping concentrations of up to 20 wt % thanks to the steric shielding of the B−N core. A good operational stability with LT₉₅ of 85.2 h has also been revealed. The dual steric and electronic effects resulting from the introduction of a TSCT state offer an effective molecular design to address the critical challenges of MR-TADF emitters.     

长波敏化发光铕配合物纳米粒子的制备与表征

以牛血清白蛋白(BSA)为保护剂, 利用沉淀法制备了平均粒径为35 nm的Eu(tta)3dpbt (dpbt = 2-(N,N-二乙基苯胺-4-基)-4,6-二(3,5-二甲基吡唑-1-基)-1,3,5-三嗪, tta = 噻吩甲酰三氟丙酮负离子)荧光纳米粒子. BSA保护Eu(tta)3dpbt纳米粒子在水中分散稳定性高, 光稳定性好, 长波敏化发光性能优良. 其在可见光区激发峰位于415 nm, 激发峰尾部延展至470 nm, 发光量子产率为0.20 (λex=415 nm, 25 ℃). 在近红外双光子激发下可发出纯正的红光, Eu(tta)3dpbt纳米粒子最大双光子激发作用截面为2.4×10⁵ GM (λex=830 nm, 1 GM=10^-50 cm⁴·s·photo^-1·particle^-1).     

Triptycene-Fused Sterically Shielded Multi-Resonance TADF Emitter Enables High-Efficiency Deep Blue OLEDs with Reduced Dexter Energy Transfer

Designing multi-resonance (MR) emitters that can simultaneously achieve narrowband emission and suppressed intermolecular interactions is challenging for realizing high color purity and stable blue organic light-emitting diodes (OLEDs). Herein, a sterically shielded yet extremely rigid emitter based on a triptycene-fused B,N core (Tp-DABNA) is proposed to address the issue. Tp-DABNA exhibits intense deep blue emissions with a narrow full width at half maximum (FWHM) and a high horizontal transition dipole ratio, superior to the well-known bulky emitter, t-DABNA. The rigid MR skeleton of Tp-DABNA suppresses structural relaxation in the excited state, with reduced contributions from the medium- and high-frequency vibrational modes to spectral broadening. The hyperfluorescence (HF) film composed of a sensitizer and Tp-DABNA shows reduced Dexter energy transfer compared to those of t-DABNA and DABNA-1. Notably, deep blue TADF-OLEDs with the Tp-DABNA emitter display higher external quantum efficiencies (EQE_max=24.8 %) and narrower FWHMs (≤26 nm) than t-DABNA-based OLEDs (EQE_max=19.8 %). The HF-OLEDs based on the Tp-DABNA emitter further demonstrate improved performance with an EQE_max of 28.7 % and mitigated efficiency roll-offs.