Effect of a Pendant Acceptor on Thermally Activated Delayed Fluorescence Properties of Conjugated Polymers with Backbone‐Donor/Pendant‐Acceptor Architecture

Three sets of conjugated polymers with backbone‐donor/pendant‐acceptor architectures, named PCzA3PyB, PCzAB2Py, and PCzAB3Py, are designed and synthesized. The three isomeric benzoylpyridine‐based pendant acceptor groups are 6‐benzoylpyridin‐3‐yl (3PyB), 4‐((pyridin‐2‐yl)carbonyl)phenyl (B2Py) and 4‐((pyridin‐3‐yl)carbonyl)phenyl (B3Py), whereas the identical backbone consists of 3,6‐carbazolyl and 2,7‐acridinyl rings. One acridine ring and each acceptor group constitute a definite thermally activated delayed fluorescence (TADF) unit, incorporated into the main chain of the polymers through the 2,7‐position of the acridine ring with the varied content. All of the polymers display legible TADF features with a short microsecond‐scale delayed lifetime (0.56–1.62 μs) and a small singlet/triplet energy gap (0.10–0.19 eV). Progressively redshifted emissions are observed in the order PCzAB3Py, PCzA3PyB, and PCzAB2Py owing to the different substitution patterns of the pyridyl group. Photoluminescence quantum yields can be improved by regulating the molar content of the TADF unit in the range 0.5–50 %. The non‐doped organic light‐emitting devices (OLEDs) fabricated by solution‐processing technology emit yellow‐green to orange light. The polymers with 5 mol % of the TADF unit exhibit excellent comprehensive electroluminescence performance, in which PCzAB2Py5 achieves a maximum external quantum efficiency (EQE) of 11.9 %, low turn‐on voltage of 3.0 V, yellow emission with a wavelength of 573 nm and slow roll‐off with EQE of 11.6 % at a luminance of 1000 cd m^?2 and driving voltage of 5.5 V.     

高分子热活化延迟荧光材料研究进展

高分子热活化延迟荧光材料能够利用热活化的反向系间窜越过程将三线态激子转变为单线态激子而发出荧光,理论上可以实现100%的内量子效率,突破了传统高分子荧光材料内量子效率不超过25%的极限,因而代表了未来低成本高效率高分子发光材料的发展方向。 近年来,高分子热活化延迟荧光材料在分子设计方面取得了重要进展,形成了主链型、侧链型和树枝状高分子热活化延迟荧光材料等材料体系,同时其器件性能得到了大幅提升,部分材料的器件效率达到了高分子磷光材料的水平。 本文从材料和器件两个方面,围绕高分子热活化延迟荧光材料的分子结构、光物理特性和器件性能,总结和评述了国内外研究者在该领域方向的研究进展,并分析了未来发展面临的机遇和挑战。     

Molecular and Device Design Strategies for Ideal Performance White Organic Light‐Emitting Diodes

The progress of white organic light‐emitting diodes (WOLEDs) via adopting fluorescent and phosphorescent organic materials have attracted commercial interest for their broad range of visible spectrum and potential of 100 % internal quantum efficiency. In this account, smart molecular designs for developing efficient phosphorescent host and good color purity blue fluorescent emitters are prepared to be discussed, especially donor‐acceptor modification to regulate their triplet states and bipolar transport properties. Rational device configuration design strategies were also introduced by cooperating with efficient conventional fluorescent and thermally activated delayed fluorescent emitting molecules to achieve full exciton utilization and simplified device structures, further suggesting perspectives of potentially low‐cost, ideal performance and promoted operational lifetime in WOLED devices.     

Dicyano-Imidazole: A Facile Generation of Pure Blue TADF Materials for OLEDs

A series of dicyano-imidazole-based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain pure blue-emitting organic light-emitting diodes (OLEDs). The targeted molecules used dicyano-imidazole with a short-conjugated system as the electron acceptor to strong intermolecular π-π interactions, and provide a relatively shallow energy level of the lowest unoccupied molecular orbital (LUMO). The cyano group was selected to improve imidazole as an electron acceptor due to its prominent electron-transporting characteristics. Four different electron donors, that is, 9,9-dimethyl-9,10-dihydroacridine (DMAC), 10H-spiro(acridine-9,9’-fluoren) (SPAC), and 9,9-diphenyl-9,10-dihydroacridine (DPAC), were used to alternate the highest occupied molecular orbital (HOMO) energy level to tune the emission color further. The crowded molecular structure in space makes the electron donor and acceptor almost orthogonal, reducing the energy gap (ΔE_ST) between the first excited singlet (S₁) and the triplet (T₁) states and introducing significant TADF property. The efficiencies of the blue-emissive devices with imM-SPAC and imM-DMAC obtained in this work are the highest among the reported imidazole-based TADF-OLEDs, which are 13.8 % and 13.4 %, respectively. Both of Commission Internationale de l′Eclairage (CIE) coordinates are close to the saturated blue region at (0.17, 0.18) and (0.16, 0.19), respectively. Combining these tailor-made TADF compounds with specific device architectures, electroluminescent (EL) emission from sky-blue to deep-blue could be achieved, proving their great potential in EL applications.     

Optimizing Optoelectronic Properties of Pyrimidine‐Based TADF Emitters by Changing the Substituent for Organic Light‐Emitting Diodes with External Quantum Efficiency Close to 25 % and Slow Efficiency Roll‐Off

A series of green butterfly‐shaped thermally activated delayed fluorescence (TADF) emitters, namely PXZPM , PXZMePM , and PXZPhPM , are developed by integrating an electron‐donor (D) phenoxazine unit and electron‐acceptor (A) 2‐substituted pyrimidine moiety into one molecule via a phenyl‐bridge π linkage to form a D –π–A–π–D configuration. Changing the substituent at pyrimidine unit in these emitters can finely tune their emissive characteristics, thermal properties, and energy gaps between the singlet and triplet states while maintaining frontier molecular orbital levels, and thereby optimizing their optoelectronic properties. Employing these TADF emitters results in a green fluorescent organic light‐emitting diode (OLED) that exhibits a peak forward‐viewing external quantum efficiency (EQE) close to 25 % and a slow efficiency roll‐off characteristic at high luminance.     

Efficient Exciplex-Based Deep-Blue Organic Light-Emitting Diodes Employing a Bis(4-fluorophenyl)amine-Substituted Heptazine Acceptor

The realization of a deep-blue-emitting exciplex system is a herculean task in the field of organic light-emitting diodes (OLEDs) on account of a large red-shifted and broadened exciplex emission spectrum in comparison to those of the corresponding single compounds. Herein, 2,5,8-tris(di(4-fluorophenyl)amine)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3FDPA) was designed as an electron acceptor by integrating three bis(4-fluorophenyl)amine groups into a heptazine core, while 1,3-di(9H-carbazol-9-yl)benzene (mCP) possessing two electron-donating carbazole moieties was chosen as the electron donor. Excitingly, the exciplex system of 8 wt% HAP-3FDPA:mCP exhibited deep-blue emission and a high photoluminescence quantum yield of 53.2%. More importantly, an OLED containing this exciplex system as an emitting layer showed deep-blue emission with Commission Internationale de l’Eclairage coordinates of (0.16, 0.12), a peak luminance of 15,148 cd m⁻², and a rather high maximum external quantum efficiency of 10.2% along with a low roll-off. This study not only reports an efficient exciplex-based deep-blue emitter but also presents a feasible pathway to construct highly efficient deep-blue OLEDs based on exciplex systems.     

High‐Performance Non‐doped OLEDs with Nearly 100 % Exciton Use and Negligible Efficiency Roll‐Off

Non‐doped organic light‐emitting diodes (OLEDs) possess merits of higher stability and easier fabrication than doped devices. However, luminescent materials with high exciton use are generally unsuitable for non‐doped OLEDs because of severe emission quenching and exciton annihilation in neat films. Herein, we wish to report a novel molecular design of integrating aggregation‐induced delayed fluorescence (AIDF) moiety within host materials to explore efficient luminogens for non‐doped OLEDs. By grafting 4‐(phenoxazin‐10‐yl)benzoyl to common host materials, we develop a series of new luminescent materials with prominent AIDF property. Their neat films fluoresce strongly and can fully harvest both singlet and triplet excitons with suppressed exciton annihilation. Non‐doped OLEDs of these AIDF luminogens exhibit excellent luminance (ca. 100000 cd m^?2), outstanding external quantum efficiencies (21.4–22.6 %), negligible efficiency roll‐off and improved operational stability. To the best of our knowledge, these are the most efficient non‐doped OLEDs reported so far. This convenient and versatile molecular design is of high significance for the advance of non‐doped OLEDs.     

蓝色延迟荧光材料及器件的研究进展

有机发光二极管(OLEDs)作为新一代的显示技术,因其启亮电压低、厚度薄、能效高、响应速度快等优点,而被世界各国研究人员所关注。 有机电致发光材料及器件的发展一直备受关注,根据发光机理的不同,有机发光材料可以分为荧光材料和磷光材料。 蓝色荧光材料作为OLEDs发展中的重要部分,拥有磷光材料无法比拟的优势,近年来成为研究热点。 本文介绍了近年来国内外蓝色延迟荧光材料的研究进展,并展望了其发展前景和趋势。     

Acceptor Derivatization of the 4CzIPN TADF System: Color Tuning and Introduction of Functional Groups

We demonstrate modular modifications of the widely employed emitter 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) by replacing one or both nitrile acceptors with oxadiazole groups via a tetrazole intermediate. This allows the introduction of various functional groups including halides, alkynes, alkenes, nitriles, esters, ethers and a protected amino acid while preserving the thermally activated delayed fluorescence (TADF) properties. The substituents control the emission maximum of the corresponding emitters, ranging between 472–527 nm, and show high solid-state photoluminescence quantum yields up to 85 %. The TADF emission of two compounds, 4CzCNOXDtBu and 4CzdOXDtBu, a mono- and a bis-oxadiazole substituted 4CzIPN is characterized in detail by time- and temperature-dependent photoluminescence. Solution-processed OLEDs comprising 4CzCNOXDtBu and 4CzdOXDtBu show a significant blue-shift of the emission compared to the reference 4CzIPN, with external quantum efficiencies of 16 %, 5.9 % and 17 % at 100 cd m⁻², respectively.     

Phenoxazine-Dibenzothiophene Sulfoximine Emitters Featuring Both Thermally Activated Delayed Fluorescence and Aggregation Induced Emission

In this work, we demonstrate dibenzothiophene sulfoximine derivatives as building blocks for constructing emitters featuring both thermally activated delayed fluorescent (TADF) and aggregation-induced emission (AIE) properties, with multiple advantages including high chemical and thermal stability, facile functionalization, as well as tunable electron-accepting ability. A series of phenoxazine-dibenzothiophene sulfoximine structured TADF emitters were successfully synthesized and their photophysical and electroluminescent properties were evaluated. The electroluminescence devices based on these emitters displayed diverse emissions from yellow to orange and reached external quantum efficiencies (EQEs) of 5.8% with 16.7% efficiency roll-off at a high brightness of 1000 cd·m⁻².     

Recent progress in thermally activated delayed fluorescence dendrimers for solution-processed organic light-emitting diodes

During the past decade, the discovery of thermally activated delayed fluorescence (TADF) materials has significantly boosted the development of organic light-emitting diode (OLED) technology. Compared with small-molecule and polymeric TADF materials, TADF dendrimers have emerged as promising emitters for solution-processed OLEDs because they have the integrated advantages of TADF small molecules and polymers in achieving high efficiency, excellent solution processability, and precise molecular structures. In recent years, TADF dendrimers have experienced important advances in molecular design, mechanism exploration and device performance. Herein, we present a comprehensive review of solution-processable TADF dendrimers, mainly focusing on their molecular design principles and structure–property correlations. The advanced device performances of these dendrimers are also summarized. Finally, we proposed the prospects and challenges on the development of TADF dendrimers.     

Mechanochromic Delayed Fluorescence Switching in Propeller‐Shaped Carbazole–Isophthalonitrile Luminogens with Stimuli‐Responsive Intramolecular Charge‐Transfer Excited States

Herein, the universal design of high‐efficiency stimuli‐responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli‐triggered TADF switching for a series of carbazole–isophthalonitrile‐based donor–acceptor (D–A) luminogens is demonstrated based on systematic photophysical and X‐ray analysis, coupled with theoretical calculations. It was revealed that a tiny alteration of the intramolecular D–A twisting in the excited‐state structures governed by the solid morphologies is responsible for this dynamic TADF switching behavior. This concept is applicable to the fabrication of bicolor emissive organic light‐emitting diodes using a single TADF emitter.     

Unique Solid‐State Emission Behavior of Aromatic Difluoroboronated β‐Diketones as an Emitter in Organic Light‐Emitting Devices

Aromatic difluoroboronated β‐diketone ( BF₂DK ) derivatives are a widely known class of luminescent organic materials that exhibit high photoluminescent quantum efficiency and unique aggregation‐dependent fluorescence behavior. However, there have been only a few reports on their use in solid‐state electronic devices, such as organic light‐emitting devices (OLEDs). Herein, we investigated the solid‐state properties and OLED performance of a series of π‐extended BF₂DK derivatives that have previously been shown to exhibit intense fluorescence in the solution state. The BF₂DK derivatives formed exciplexes with a carbazole derivative and exhibited thermally activated delayed fluorescence (TADF) behavior to give orange electroluminescence with a peak external quantum efficiency of 10 % that apparently exceeds the theoretical efficiency limit of conventional fluorescent OLEDs (7.5 %), assuming a light out‐coupling factor of 30 %.     

Nonbonding/Bonding Molecular Orbital Regulation of Nitrogen-Boron-Oxygen-embedded Blue/Green Multiresonant TADF Emitters with High Efficiency and Color Purity

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).     

Simply Structured Near-Infrared Emitters with a Multicyano Linear Acceptor for Solution-Processed Organic Light-Emitting Diodes

Near-infrared (NIR) organic light-emitting diodes (OLEDs) show great potential in a variety of applications including sensors, night vision, and information security. Despite the superiority of thermally activated delayed fluorescence (TADF) in 100 % exciton harvesting, the development of NIR TADF OLEDs is still a great challenge, especially in terms of solution-processing technology. In this work, a multicyano acceptor of 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofurance (TCF) with strong electron-withdrawing ability was employed to construct solution-processible NIR emitters, CzTCF and tBCzTCF, with the feature of donor–π–acceptor (D –π–A) structure. The significantly enhanced intermolecular charge transfer effects not only render the deep-red and NIR emissions of CzTCF and tBCzTCF films, respectively, but also lead to their typical TADF characteristics. Consequently, the nondoped solution-processed NIR OLED based on tBCzTCF was successfully demonstrated with the peak wavelength of 715 nm, which paves the way for developing NIR emitters without polycyclic aromatic cores and heavy-metal ions.     

Acridan‐Grafted Poly(biphenyl germanium) with High Triplet Energy,Low Polarizability,and an External Heavy‐Atom Effect for Highly Efficient Sky‐Blue TADF Electroluminescence

We propose the novel σ–π conjugated polymer poly(biphenyl germanium) grafted with two electron‐donating acridan moieties on the Ge atom for use as the host material in a polymer light‐emitting diode (PLED) with the sky‐blue‐emitting thermally activated delayed fluorescence (TADF) material DMAC‐TRZ as the guest. Its high triplet energy (E_T) of 2.86 eV is significantly higher than those of conventional π–π conjugated polymers (E_T=2.65 eV as the limit) and this guest emitter (E_T=2.77 eV). The TADF emitter emits bluer emission than in other host materials owing to the low orientation polarizability of the germanium‐based polymer host. The Ge atom also provides an external heavy‐atom effect, which increases the rate of reverse intersystem crossing in this TADF guest, so that more triplet excitons are harvested for light emission. The sky‐blue TADF electroluminescence with this host/guest pair gave a record‐high external quantum efficiency of 24.1 % at maximum and 22.8 % at 500 cd m^?2.     

Aggregation‐Induced Delayed Fluorescence Based on Donor/Acceptor‐Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic–inorganic conjugated systems based on o‐carboranes directly bonded to electron‐donating and electron‐accepting π‐conjugated units were elaborated as novel optoelectronic materials. These o‐carborane derivatives simultaneously possessed aggregation‐induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow‐to‐red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light‐emitting diodes utilizing these o‐carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.     

Efficient Red Thermally Activated Delayed Fluorescence Emitters Based on a Dibenzonitrile-Substituted Dipyrido[3,2-a:2′,3′-c]phenazine Acceptor

How to construct efficient red-emitting thermally activated delayed fluorescence (TADF) materials is a challenging task in the field of organic light-emitting diodes (OLEDs). Herein, an electron acceptor moiety, 3,6-DCNB-DPPZ, with high rigidity and strong acceptor strength was designed by introducing two cyanobenzene groups into the 3,6-positions of a dipyrido[3,2-a:2′,3′-c]phenazine unit. A red-emitting compound, 3,6_R, has been designed and synthesized by combining the rigid acceptor unit with two triphenylamine donors. Due to high molecular rigidity and strong intramolecular charge transfer characteristic in donor–acceptor–donor skeleton, 3,6_R exhibited a red emission with a high photoluminescence quantum yield of 86% and distinct TADF nature with short delayed fluorescence lifetime of about 1 microsecond. Accordingly, the OLED using 3,6_R as the guest emitter gained a high external quantum efficiency of 12.0% in the red region with an electroluminescence peak of 619 nm and favorable Commission Internationale de l’Eclairage coordinates of (0.62, 0.38).