Extending the π-Skeleton of Multi-Resonance TADF Materials towards High-Efficiency Narrowband Deep-Blue Emission

Modulating Charge-Transfer Excited States of Multiple Resonance Emitters via Intramolecular Covalent Bond Locking

Advanced multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters with high efficiency and color purity have emerged as a research focus in the development of ultra-high-definition displays. Herein, we disclose an approach to modulate the charge-transfer excited states of MR emitters via intramolecular covalent bond locking. This strategy can promote the evolution of strong intramolecular charge-transfer (ICT) states into weak ICT states, ultimately narrowing the full-width at half-maximum (FWHM) of emitters. To modulate the ICT intensity, two octagonal rings are introduced to yield molecule m-DCzDAz-BNCz. Compounds m-CzDAz-BNCz and m-DCzDAz-BNCz exhibit bright light-green and green fluorescence in toluene, with emission maxima of 504 and 513 nm, and FWHMs of 28 and 34 nm, respectively. Sensitized organic light-emitting diodes (OLEDs) employing emitters m-CzDAz-BNCz and m-DCzDAz-BNCz exhibit green emission with peaks of 508 and 520 nm, Commission Internationale de L'Eclairage (CIE) coordinates of (0.12, 0.65) and (0.19, 0.69), and maximum external quantum efficiencies (EQEs) of 30.2 % and 32.6 %, respectively.

Theoretical Investigation and Molecular Design: A Series of Tripod-Type Cu(I) Blue Light Thermally Activated Delayed Fluorescence Materials

The photophysical properties and luminescent mechanism of a series of tripod-type Cu(I) complexes in solution and solids were comprehensively investigated through theoretical simulations. From a microscopic perspective, the experimental phenomenon is explained: (1) The intrinsic reason for the quenching of complex 1 in solution was attributed to the significant nonradiative transition caused by structural deformation; (2) In the solid, the reduced ΔEST for complex 2 effectively facilitate reverse intersystem crossing (RISC) and improves its luminescence efficiency; (3) The enhanced performance of complex 3 in solution is attributed to that its stronger steric hindrance is advantageous to decrease not only the ΔEST but also the reorganization energy through intramolecular weak interactions. Based on complex 3, the tert-butyl substituted isomeric complex 4 was designed. Complex 4 further amplifies the advantages of 3 to further promote the RISC to make full use of excitons. Meanwhile, it has an emission wavelength of 462.6 nm, which makes it an excellent candidate for high-efficiency deep-blue TADF materials. This study provides valuable information for obtaining efficient blue phosphorescence and TADF dual-channel luminescent materials.

Biomimetic Exploration and Reflection on Switchable Coordination and Narrow-Band Electrofluorochromic Devices

Electrofluorochromic (EFC) materials and devices with controllable fluorescence properties show great application potential in advanced anticounterfeiting, information storage and display. However, the low color purity caused by the broad emission spectra and underperforming switching time of the existing EFC materials limit their application. Through biomimetic exploration and the study of reversible electrochemical responsive coordination reactions, boron-nitrogen embedded polyaromatics (B,N-PAHs) with narrow-band emission and high color purity have been successfully integrated into EFC systems, which also help to better understand the role of boron in biological activity. The EFC device achieve good performance containing quenching efficiency greater than 90% within short switching time (ton: 0.6 s, toff: 2.4 s), and nearly no performance change after 200 cycles test. Three primary color (red, green, and blue) EFC devices are successfully prepared. In addition, new phenomena are obtained and discussed in this biomimetic exploration of related boron reactions. The success and harvest of this exploration are expected to provide new ideas for optimizing properties and broadening applications of EFC materials. Moreover, it may provide ideas and reference significance for further exploring and understanding the function of boron compounds in biological systems.

Nitrogen-Embedding Strategy for Short-Range Charge Transfer Excited States and Efficient Narrowband Deep-Blue Organic Light Emitting Diodes

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

Organic polaritonic light-emitting diodes with high luminance and color purity toward laser displays

Achieving high-luminescence organic light-emitting devices (OLEDs) with narrowband emission and high color purity is important in various optoelectronic fields. Laser displays exhibit outstanding advantages in next-generation display technologies owing to their ultimate visual experience, but this remains a great challenge. Here, we develop a novel OLED based organic single crystals. By strongly coupling the organic exciton state to an optical microcavity, we obtain polariton electroluminescent (EL) emission from the polariton OLEDs (OPLEDs) with high luminance, narrow-band emission, high color purity, high polarization as well as excellent optically pumped polariton laser. Further, we evaluate the potential for electrically pumped polariton laser through theoretical analysis and provide possible solutions. This work provides a powerful strategy with a material-device combination that paves the way for electrically driven organic single-crystal-based polariton luminescent devices and possibly lasers.

Highly efficient pure-blue organic light-emitting diodes based on rationally designed heterocyclic phenophosphazinine-containing emitters

Multi-resonance thermally activated delayed fluorophores have been actively studied for high-resolution photonic applications due to their exceptional color purity. However, these compounds encounter challenges associated with the inefficient spin-flip process, compromising device performance. Herein, we report two pure-blue emitters based on an organoboron multi-resonance core, incorporating a conformationally flexible donor, 10-phenyl-5H-phenophosphazinine 10-oxide (or sulfide). This design concept selectively modifies the orbital type of high-lying excited states to a charge transfer configuration while simultaneously providing the necessary conformational freedom to enhance the density of excited states without sacrificing color purity. We show that the different embedded phosphorus motifs (phosphine oxide/sulfide) of the donor can finely tune the electronic structure and conformational freedom, resulting in an accelerated spin-flip process through intense spin-vibronic coupling, achieving over a 20-fold increase in the reverse intersystem crossing rate compared to the parent multi-resonance emitter. Utilizing these emitters, we achieve high-performance pure-blue organic light-emitting diodes, showcasing a top-tier external quantum efficiency of 37.6% with reduced efficiency roll-offs. This proposed strategy not only challenges the conventional notion that flexible electron-donors are undesirable for constructing narrowband emitters but also offer a pathway for designing efficient narrow-spectrum blue organic light-emitting diodes.

Stepwise Charge/Energy Transfer in MR-TADF Molecule-Doped Exciplex for Ultralong Persistent Luminescence Activated with Visible Light

Organic long-persistent luminescence (OLPL), which relies on energy storage for delayed light emission by the charge separation state, has attracted intense attention in various optical applications. However, charge separation (CS) is efficient only under ultraviolet excitation in most OLPL systems because it requires a driving force from the large energy difference between the local excited (LE) and charge transfer (CT) states. In this study, a multiresonance thermally activated delayed fluorescence (MR-TADF) molecule is incorporated into an exciplex system to achieve efficient OLPL in a composite material activated by visible light via a stepwise charge/energy transfer process. The enhanced absorption of the composite material facilitated a tenfold increase in the duration of the OLPL, which can last for several hours under visible light excitation. The excited state of the MR-TADF molecule tends to charge transfer to the acceptor, followed by energy transfer to the exciplex, which benefits from the small difference between the LE and CT states owing to the inherent CS characteristics of the opposing resonance effect. Afterglow displays of these composite materials are fabricated to demonstrate their considerable potential in encryption patterns and emergency lights, which take advantage of their excellent processability, visible light activation, and tunable luminescence properties.

Efficient Deep-Blue Multiple-Resonance Emitters Based on Azepine-Decorated ν-DABNA for CIEy below 0.06

Ultrapure deep-blue emitters are in high demand for organic light-emitting diodes (OLEDs). Although color coordinates serve as straightforward parameters for assessing color purity, precise control over the maximum wavelength and full-width at half-maximum is necessary to optimize OLED performance, including luminance efficiency and luminous efficacy. Multiple-resonance (MR) emitters are promising candidates for achieving ideal luminescence properties; consequently, a wide variety of MR frameworks have been developed. However, most of these emitters experience a wavelength displacement from the ideal color, which limits their practical applicability. Therefore, a molecular design that is compatible with MR emitters for modulating their energy levels and color output is particularly valuable. Here, it is demonstrated that the azepine donor unit induces an appropriate blue-shift in the emission maximum while maintaining efficient MR characteristics, including high photoluminescence quantum yield, narrow emission, and a fast reverse intersystem crossing rate. OLEDs using newly developed MR emitters based on the ν-DABNA framework simultaneously exhibit a high quantum efficiency of ≈30%, luminous efficacy of ≈20 lm W-1, exceptional color purity with Commission Internationale de l'Éclairage coordinates as low as (0.14, 0.06), and notably high operational stability. These results demonstrate unprecedentedly high levels compared with those observed in previously reported deep-blue emitters.

An electrochemically responsive B-O dynamic bond to switch photoluminescence of boron-nitrogen-doped polyaromatics

Boron-doped polycyclic aromatic hydrocarbons exhibit excellent optical properties, and regulating their photophysical processes is a powerful strategy to understand the luminescence mechanism and develop new materials and applications. Herein, an electrochemically responsive B-O dynamic coordination bond is proposed, and used to regulate the photophysical processes of boron-nitrogen-doped polyaromatic hydrocarbons. The formation of the B-O coordination bond under a suitable voltage is confirmed by experiments and theoretical calculations, and B-O coordination bond can be broken back to the initial state under opposite voltage. The whole process is accompanied by reversible changes in photophysical properties. Further, electrofluorochromic devices are successfully prepared based on the above electrochemically responsive coordination bond. The success and harvest of this exploration are beneficial to understand the luminescence mechanism of boron-nitrogen-doped polyaromatic hydrocarbons, and provide ideas for design of dynamic covalent bonds and broaden material types and applications.

Borylation and rearrangement reactions of azasilaanthracenes to afford B,N-doped nanographenes

An air-stable B3,N3-containing dibenzobisanthene (8) was prepared in 29% yield by heating a 1,3,5-tri(azasilaanthryl)benzene (5) with BBr3 (180 °C). Under these conditions, the reaction does not stop after threefold SiMe2/BBr exchange but proceeds further via two rearrangement and two intramolecular C-H borylation steps. Some mechanistic details were unveiled by using smaller model systems and applying lower reaction temperatures. According to X-ray crystallography, compound 8 has a helically distorted scaffold. Due to its multiple resonance structure, it shows a narrow-band blue-green emission (λem = 493 nm; ΦPL = 84%; FWHM = 0.20 eV; THF); samples measured in PMMA gave prompt and delayed fluorescence lifetimes of 10.7 ns and 136 μs, respectively. The optical properties of 8 and of structurally related species were also investigated by quantum-chemical means: most of these compounds exhibit a small energy gap ΔEST between the lowest excited singlet (S1) and triplet (T1) states and a non-negligible spin-orbit coupling (SOC) between S1 and T1/T2, demonstrating their potential as thermally activated delayed fluorescence (TADF) emitters.

Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

B‒N covalent bond-involved π-extension of multiple resonance emitters enables high-performance narrowband electroluminescence

Multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show promise for achieving both high color-purity emission and high exciton utilization efficiency. However, their development is often impeded by a limited synthetic scope and excessive molecular weights, which challenge material acquisition and organic light-emitting diode (OLED) fabrication by vacuum deposition. Herein, we put forward a B‒N covalent bond-involved π-extension strategy via post-functionalization of MR frameworks, leading to the generation of high-order B/N-based motifs. The structurally and electronically extended π-system not only enhances molecular rigidity to narrow emission linewidth but also promotes reverse intersystem crossing to mitigate efficiency roll-off. As illustrated examples, ultra-narrowband sky-blue emitters (full-width at half-maximum as small as 8 nm in n-hexane) have been developed with multi-dimensional improvement in photophysical properties compared to their precursor emitters, which enables narrowband OLEDs with external quantum efficiencies (EQEmax) of up to 42.6%, in company with alleviated efficiency decline at high brightness, representing the best efficiency reported for single-host OLEDs. The success of these emitters highlights the effectiveness of our molecular design strategy for advanced MR-TADF emitters and confirms their extensive potential in high-performance optoelectronic devices.

Thermally Activated Delayed Fluorescence with Nanosecond Emission Lifetimes and Minor Concentration Quenching: Achieving High-Performance Nondoped and Doped Blue OLEDs

Simultaneously achieving a high photoluminescence quantum yield (PLQY), ultrashort exciton lifetime, and suppressed concentration quenching in thermally activated delayed fluorescence (TADF) materials is desirable yet challenging. Here, a novel acceptor-donor-acceptor type TADF emitter, namely, 2BO-sQA, wherein two oxygen-bridged triarylboron (BO) acceptors are arranged with cofacial alignment and positioned nearly orthogonal to the rigid dispirofluorene-quinolinoacridine (sQA) donor is reported. This molecular design enables the compound to achieve highly efficient (PLQYs up to 99%) and short-lived (nanosecond-scale) blue TADF with effectively suppressed concentration quenching in films. Consequently, the doped organic light-emitting diodes (OLEDs) base on 2BO-sQA achieve exceptional electroluminescence performance across a broad range of doping concentrations, maintaining maximum external quantum efficiencies (EQEs) at over 30% for doping concentrations ranging from 10 to 70 wt%. Remarkably, the nondoped blue OLED achieves a record-high maximum EQE of 26.6% with a small efficiency roll-off of 14.0% at 1000 candelas per square meter. By using 2BO-sQA as the sensitizer for the multiresonance TADF emitter ν-DABNA, TADF-sensitized fluorescence OLEDs achieve high-efficiency deep-blue emission. These results demonstrate the feasibility of this molecular design in developing TADF emitters with high efficiency, ultrashort exciton lifetime, and minimal concentration quenching.

Förster Resonance Energy Transfer: Stimulus-Responsive Purely Organic Room Temperature Phosphorescence through Dynamic B-N bond

Recently, stimulus-responsive organic materials with room-temperature phosphorescence (RTP) properties have attracted significant attention owing to their potential applications in chemical sensing, anticounterfeiting, and displays. However, molecular design currently lacks systematicity and effectiveness. Herein, we report a capture-release strategy for the construction of reversible RTP via B/N Lewis pairs. Specifically, the RTP of the Lewis acid of 7-bromo-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (BrBA) can be deactivated through capturing by the Lewis base, N,N-diphenyl-4-(pyridin-4-yl)aniline (TPAPy), and reactivated by dissociation of B-N bonds to release BrBA. Reversible RTP is attributed to the exceptional self-assembly capability of BrBA, whereas the tunable RTP colors are derived from distinct Förster resonance energy transfer (FRET) processes. The potential applications of RTP materials in information storage and anti-counterfeiting were also experimentally validated. The capture-release approach proposed in this study offers an effective strategy for designing stimulus-responsive materials.

Orienting Group Directed Cascade Borylation for Efficient One-Shot Synthesis of 1,4-BN-Doped Polycyclic Aromatic Hydrocarbons as Narrowband Organic Emitters

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

Highly efficient multi-resonance thermally activated delayed fluorescence material toward a BT.2020 deep-blue emitter

An ultrapure deep-blue multi-resonance-induced thermally activated delayed fluorescence material (DOB2-DABNA-A) is designed and synthesized. Benefiting from a fully resonating extended helical π-conjugated system, this compound has a small ΔEST value of 3.6 meV and sufficient spin-orbit coupling to exhibit a high-rate constant for reverse intersystem crossing (kRISC = 1.1 × 106 s-1). Furthermore, an organic light-emitting diode employing DOB2-DABNA-A as an emitter is fabricated; it exhibits ultrapure deep-blue emission at 452 nm with a small full width at half maximum of 24 nm, corresponding to Commission Internationale de l'Éclairage (CIE) coordinates of (0.145, 0.049). The high kRISC value reduces the efficiency roll-off, resulting in a high external quantum efficiency (EQE) of 21.6% at 1000 cd m-2.

A figure of merit for efficiency roll-off in TADF-based organic LEDs

Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and televisions1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. At present, materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters that usually use heavy metal atoms3,4. Although excellent progress has been made, in most TADF OLEDs there is a severe decrease of efficiency as the drive current is increased, known as efficiency roll-off. So far, much of the literature suggests that efficiency roll-off should be reduced by minimizing the energy difference between singlet and triplet excited states (ΔEST) to maximize the rate of conversion of triplets to singlets by means of reverse intersystem crossing (kRISC)5-20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new figure of merit will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality and lasing.

An Ideal Molecular Construction Strategy for Ultra-Narrow-Band Deep-Blue Emitters: Balancing Bathochromic-Shift Emission, Spectral Narrowing, and Aggregation Suppression

Narrowband emissive multiple resonance (MR) emitters promise high efficiency and stability in deep-blue organic light-emitting diodes (OLEDs). However, the construction of ideal ultra-narrow-band deep-blue MR emitters still faces formidable challenges, especially in balancing bathochromic-shift emission, spectral narrowing, and aggregation suppression. Here, DICz is chosen, which possesses the smallest full-width-at-half-maximum (FWHM) in MR structures, as the core and solved the above issue by tuning its peripheral substitution sites. The 1-substituted molecule Cz-DICz is able to show a bright deep-blue emission with a peak at 457 nm, an extremely small FWHM of 14 nm, and a CIE coordinate of (0.14, 0.08) in solution. The corresponding OLEDs exhibit high maximum external quantum efficiencies of 22.1%-25.6% and identical small FWHMs of 18 nm over the practical mass-production concentration range (1-4 wt.%). To the best of the knowledge, 14 and 18 nm are currently the smallest FWHM values for deep-blue MR emitters with similar emission maxima under photoluminescence and electroluminescence conditions, respectively. These discoveries will help drive the development of high-performance narrowband deep-blue emitters and bring about a revolution in OLED industry.

Stereo effects for efficient synthesis of orange-red multiple resonance emitters centered on a pyridine ring

Despite theoretical difficulties, we herein demonstrate an effective strategy for the inaugural synthesis of an orange-red multiple resonance (MR) emitter centered on a pyridine ring via stereo effects. Compared to conventional benzene-centered materials, the pyridine moiety in the novel MR material acts as a co-acceptor. This results in a significant spectral redshift and a narrower spectrum, as well as an improved photoluminescence quantum yield (PLQY) due to the formation of intramolecular hydrogen bonds. As envisioned, the proof-of-concept emitter Py-Cz-BN exhibits bright orange-red emission peaking at 586 nm with a small full width at half maximum (FWHM) of 0.14 eV (40 nm), exceeding both the wavelength and FWHM achieved with benzene-centered BBCz-Y. Benefiting from high PLQYs (>92%) and suppressed chromophore interactions, the optimized organic light-emitting diodes achieved high maximum external quantum efficiencies of 25.3-29.6%, identical small FWHMs of 0.18 eV (54 nm), and long lifetimes over a wide range of dopant concentrations (1-15 wt%). The performance described above demonstrates the effectiveness of this molecular design and synthesis strategy in constructing high performance long-wavelength MR emitters.

Narrowband Near-Infrared Multiple-Resonance Thermally Activated Delayed Fluorescence Emitters towards High-Performance and Stable Organic Light-Emitting Diodes

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) materials are highly coveted for their high efficiency and narrowband emission in organic light-emitting diodes (OLEDs). Nevertheless, the development of near-infrared (NIR) MR-TADF emitters remains a formidable challenge. In this study, we design two new NIR MR-TADF emitters, PXZ-R-BN and BCz-R-BN, by embedding 10H-phenoxazine (PXZ) and 7H-dibenzo[c,g]carbazole (BCz) fragments to increase the electron-donating ability or extending π-conjugation on the framework of para-boron fusing polycyclic aromatic hydrocarbons (PAHs). Both compounds emit in the NIR region, with a full-width at half-maximum (FWHM) of 49 nm (0.13 eV) for PXZ-R-BN and 43 nm (0.11 eV) for BCz-R-BN in toluene. To sensitize the two NIR MR-TADF emitters in OLEDs, a new platinum complex, Pt-1, is designed as a sensitizer. The PXZ-R-BN-based sensitized OLEDs achieve a maximum external quantum efficiency (EQEmax ) of nearly 30 % with an emission band at 693 nm, and exceptional long operational stability with an LT97 (time to 97 % of the initial luminance) value of 39084 h at an initial radiance of 1000 mW sr-1  m-2 . The BCz-R-BN-based OLEDs reach EQEmax values of 24.2 % with an emission band at 713 nm, which sets a record value for NIR OLEDs with emission bands beyond 700 nm.

Organoboron-based multiple-resonance emitters: synthesis, structure-property correlations, and prospects

Boron-based multiple-resonance (MR) emitters exhibit the advantages of narrowband emission, high absolute photoluminescence quantum yield, thermally activated delayed fluorescence (TADF), and sufficient stability during the operation of organic light-emitting diodes (OLEDs). Thus, such MR emitters have been widely applied as blue emitters in triplet-triplet-annihilation-driven fluorescent devices used in smartphones and televisions. Moreover, they hold great promise as TADF or terminal emitters in TADF-assisted fluorescence or phosphor-sensitised fluorescent OLEDs. Herein we comprehensively review organoboron-based MR emitters based on their synthetic strategies, clarify structure-photophysical property correlations, and provide design guidelines and future development prospects.

Switching thermally activated delayed fluorescence to room temperature phosphorescence for oxygen sensing: Effect of donor substituents of trifluoromethylphenyl

Two compounds consisting of electron-accepting trifluoromethylphenyl moiety and electron-donating phenoxazine and phenothiazine moieties were designed and synthesized via Buchwald-Hartwig coupling reaction. Thermal, photophysical, and electrochemical properties of the compounds are discussed. Only compound with phenothiazine form molecular glass, with glass transition temperatures of 90 °C. The geometry and electronic characteristics of the compounds were substantiated within density functional theory (DFT). 10,10'-(2-(Trifluoromethyl)-1,4-phenylene)bis(10H-phenoxazine) shows efficient thermally activated delayed fluorescence with high spin-orbit coupling values. 10,10'-(2-(Trifluoromethyl)-1,4-phenylene)bis(10H-phenothiazine) as efficient room-temperature phosphor shows high oxygen sensitivity.

Pericyclic Cascade Reactions Affording Thienyl- and Benzo[b]thienyl-Fused Architectures: A Synthetic and Density Functional Theory Analysis of Asynchronous Diels-Alder Reactions of Thioarylmaleimides

Herein, we report the synthesis and characterization of a series of thioarylmaleimides and their varied propensity toward highly selective domino Diels-Alder (D-A)/rearrangement, D-A/ene/elimination, and D-A/oxidation reactions to give three types of thienyl-fused architectures. Stereochemical assignment was achieved using a combination of nuclear magnetic resonance (NMR) studies, gauge independent atomic orbital (GIAO) NMR chemical shift calculations, and DP4+ analysis. Transition-state calculations support an asynchronous concerted mechanism and provide support for rationalizing the observed regio- and stereoselectivity.

Green-Emitting Extended B3,N2-Doped Polycyclic Aromatic Hydrocarbon with Multiple Resonance Structure

An air-stable B3,N2-PAH (B3N2; nine annulated six-membered rings) was synthesized from 1-X-2,6-di(azasilaanthryl)benzenes (X = Cl, I) via lithiation/borylation, electrophilic aromatic borylation, and Si/B exchange. The heteroatom distribution in B3N2 meets the requirements for multiple resonance thermally activated delayed fluorescence (MR-TADF). Indeed, B3N2 emits green light (λem = 523 nm; ΦPL = 85%; CHCl3) with a small fwhm of 0.15 eV. Lifetimes for prompt (7.8 ns) and delayed (60 μs) fluorescence were measured in PMMA.

Deep-Blue Narrowband Hetero[6]helicenes Showing Circularly Polarized Thermally Activated Delayed Fluorescence Toward High-Performance OLEDs

Helicenes exhibit substantial potential as circularly polarized luminescence (CPL) active molecules. However, their application in circularly polarized organic light-emitting diodes (CP-OLEDs) is typically hindered by the challenge of integrating both high color purity and efficient triplet-harvesting capability, particularly in the blue spectral region. Herein, a series of hetero[6]helicene-based emitters that is strategically engineered through the helical extension of a deep-blue double-boron-based multiple resonance thermally activated delayed fluorescence (MR-TADF) motif, is introduced. Importantly, the helical extension does not cause apparent structural deformation or perturb frontier molecular orbitals; thus, preserving the deep-blue emission and MR-TADF characteristics of the parent molecule. This approach also leads to reduced reorganization energy, resulting in emitters with narrower linewidth and higher photoluminescence quantum yield. Further, the helical motif enhances the racemization barrier and leads to improved CPL performance with luminescence dissymmetry factor values up to 1.5 × 10-3 . Exploiting these merits, devices incorporating the chiral dopants demonstrate deep-blue emission within the Broadcast Service Television 2020 color-gamut range, record external quantum efficiencies (EQEs) up to 29.3%, and have distinctive circularly polarized electroluminescence (CPEL) signals. Overall, the authors' findings underscore the helical extension as a promising strategy for designing narrowband chiroptical materials and advancing high-definition displays.

Indolo[3,2-b]indole-based multi-resonance emitters for efficient narrowband pure-green organic light-emitting diodes

Organic light-emitting diodes (OLEDs) utilizing multi-resonance (MR) emitters show great potential in ultrahigh-definition display benefitting from superior merits of MR emitters such as high color purity and photoluminescence quantum yields. However, the scarcity of narrowband pure-green MR emitters with novel backbones and facile synthesis has limited their further development. Herein, two novel pure-green MR emitters (IDIDBN and tBuIDIDBN) are demonstrated via replacing the carbazole subunits in the bluish-green BCzBN skeleton with new polycyclic aromatic hydrocarbon (PAH) units, 5-phenyl-5,10-dihydroindolo[3,2-b]indole (IDID) and 5-(4-(tert-butyl)phenyl)-5,10-dihydroindolo[3,2-b]indole (tBuIDID), to simultaneously enlarge the π-conjugation and enhance the electron-donating strength. Consequently, a successful red shift from aquamarine to pure-green is realized for IDIDBN and tBuIDIDBN with photoluminescence maxima peaking at 529 and 532 nm, along with Commission Internationale de l'Eclairage (CIE) coordinates of (0.25, 0.71) and (0.28, 0.70). Furthermore, both emitters revealed narrowband emission with small full width at half-maximum (FWHM) below 28 nm. Notably, the narrowband pure-green emission was effectively preserved in corresponding devices, which afford elevated maximum external quantum efficiencies of 16.3% and 18.3% for IDIDBN and tBuIDIDBN.

High-Efficiency Ultraviolet Electroluminescence from Multi-Resonance Phosphine Oxide Polycyclic Aromatics

Efficient ultraviolet (UV) electroluminescent materials remain a great challenge, since short peak wavelength <400 nm and narrow full width at half maximum (FWHM) <50 nm are simultaneously required. In this sense, multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters featuring narrow-band emissions hold the promise for UV applications. Herein, a novel MR-TADF skeleton featuring carbazole-phosphine oxide (P=O) fused aromatics is developed to construct the first two UV MR emitters named CzP2PO and tBCzP2PO. In addition to synergistic resonance effects of P=O and N atom, sp3 -hybrid P atom renders the curved polycyclic planes of CzP2PO and tBCzP2PO, giving rise to their narrowband UV emissions with peak wavelengths <390 nm and FWHM<35 nm. Besides configuration quasi-planarization for radiation enhancement and quenching suppression, P=O moiety further enhances singlet-triplet coupling to facilitate reverse intersystem crossing, resulting in the state-of-the-art photoluminescence quantum yield of 62 % in tBCzP2PO doped films. As consequence, tBCzP2PO endowed its UV organic light-emitting diodes with the peak at 382 nm and FWHM of 32 nm, and especially the record-high external quantum efficiency (EQE) of 15.1 % among all kinds of UV devices. Our results demonstrate great potential of P=O based MR emitters in practical applications including optoelectronics, biology and medicine science.

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.

Color-Tunable Dual-Mode Organic Afterglow for White-Light Emission and Information Encryption Based on Carbazole Doping

Dual-mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white-light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white-light emission. In this study, a carbazole-based host-guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long-lived white-light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which facilitating the delayed fluorescence properties of host-guest system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white-light afterglow.

Naphthalene-Embedded Multi-Resonance Emitters Enabling Efficient Narrow Emissive Blue OLEDs

Great achievements have been made in the development of organic light-emitting diodes in recent decades. However, achieving high color purity for blue emitters remains a challenge. In this study, we have designed and synthesized three naphthalene (NA)-embedded multi-resonance (MR) emitters, named SNA, SNB and SNB1, based on N-B-O frameworks with isomer variations for finely adjusting the photophysical properties. These emitters show tunable blue emission with emission peaks of 450-470 nm. Small full width of half maximum (FWHM) of 25-29 nm are achieved in these emitters, indicating the well maintaining of molecular rigidity and MR effect with NA extension. Such design also ensures a fast radiative decay. However, no obvious delayed fluorescence is observed in all three emitters due to the relatively large energy differences between the first singlet and triplet excited states. Both SNA and SNB enable high electroluminescent (EL) performance in doped devices with external quantum efficiency (EQE) of 7.2 and 7.9 %, respectively. When applying the sensitized strategy, devices based on SNA and SNB show huge improvement with EQE of 29.3 and 29.1 %. More importantly, SNB with twist geometry enables stable EL spectra with almost unchanged FWHM under different doping concentrations. This work demonstrates the potential of NA extension design in constructing narrowband emissive blue emitters.

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.

RGB Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes toward Realizing the BT.2020 Standard

With the surging demand for ultra-high-resolution displays, the International Telecommunication Union (ITU) announce the next-generation color gamut standard, named ITU-R Recommendation BT.2020, which not only sets a seductive but challenging milestone for display technologies but also urges researchers to recognize the importance of color coordinates. Organic light-emitting diodes (OLEDs) are an important display technology in current daily life, but they face challenges in approaching the BT.2020 standard. Thermally activated delayed fluorescence (TADF) emitters have bright prospects in OLEDs because they possess 100% theoretical exciton utilization. Thus, the development of TADF emitters emitting primary red (R), green (R), and blue (B) emission is of great significance. Here, a comprehensive overview of the latest advancements in TADF emitters that exhibit Commission Internationale de l'Éclairage (CIE) coordinates surpassing the National Television System Committee (NTSC) and approaching BT.2020 standards is presented. Rational strategies for molecular designs, as well as the resulting photophysical properties and OLED performances, are discussed to elucidate the underlying mechanisms for shifting the CIE coordinates of both donor-acceptor and multiple resonance (MR) typed TADF emitters toward the BT.2020 standard. Finally, the challenges in realization of the wide-color-gamut BT.2020 standard and the prospects for this research area are provided.

Precisely regulating the double-boron-based multi-resonance framework towards pure-red emitters: high-performance OLEDs with CIE coordinates fully satisfying the BT. 2020 standard

Here, we propose a new simple and effective strategy for designing pure-red multi-resonance (MR) emitters through precisely regulating the double-boron-based MR framework. The two designed emitters exhibit ultrapure red emission together with superb photophysical properties, and further enable high-performance, high color-purity red OLEDs.

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.

Precise modulation of multiple resonance emitters toward efficient electroluminescence with pure-red gamut for high-definition displays

Multiple resonance (MR) compounds have garnered substantial attention for their prospective utility in wide color gamut displays. Nevertheless, developing red MR emitters with both high efficiency and saturated emission color remains demanding. We herein introduce a comprehensive strategy for spectral tuning in the red region by simultaneously regulating the π-conjugation and electron-donating strengths of a double boron-embedded MR skeleton while preserving narrowband characteristics. The proof-of-concept materials manifested emissions from orange-red to deep red, with bandwidths below 0.12 eV. The pure-red device based on CzIDBNO displayed superior color purity with CIE coordinates of (0.701, 0.298), approaching the Broadcast Television 2020 standard. In concert with high photoluminescence quantum yield and strong horizontal dipole orientation, CzIDBNO also achieved a maximum external quantum efficiency of 32.5% and a current efficiency of 20.2 cd A^-1, outstripping prior reported organic light-emitting diodes (OLEDs) with CIE_x exceeding 0.68. These findings offer a roadmap for designing high-performance emitters with exceptional color purity for future OLED material research advancements.

Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ_// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQE_max) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQE_max of 15.4% and CIE coordinates of (0.14, 0.09). The EQE_max of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S₁ state to give lower ΔE_ST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Charge Transfer Excited State Promoted Multiple Resonance Delayed Fluorescence Emitter for High-Performance Narrowband Electroluminescence

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising candidates for narrowband organic light-emitting diodes, but their electroluminescent performance is typically hindered by the slow reverse intersystem crossing rate (k_RISC). Herein, we present an effective strategy to introduce a multichannel reverse intersystem crossing (RISC) pathway with large spin-orbit coupling by orthogonally linking an electron-donating unit to the MR framework. Through delicate manipulation of the excited-state energy levels, an additional intersegmental charge transfer triplet state could be "silently" induced without perturbing the MR character of the lowest excited singlet state. The proof-of-concept emitter CzBN3 not only affords 23-fold increase of k_RISC compared with its prototypical MR skeleton but also realizes close-to-unity photoluminescence quantum yield, large radiative rate constant, and very narrow emission spectrum. These merits enable high maximum external quantum efficiency (EQE_max) of up to 37.1% and alleviated efficiency roll-off in the sensitizer-free device (EQE₁₀₀₀ = 30.4%), and a further boost of efficiency (EQE_max/1000 = 42.3/34.1%) is realized in the hyperfluorescent device. The state-of-the-art electroluminescent performance validates the superiority of our molecular design strategy.

One-Shot Construction of BN-Embedded Heptadecacene Framework Exhibiting Ultra-narrowband Green Thermally Activated Delayed Fluorescence

BN-embedded nonacene, tridecacene, and heptadecacene frameworks were constructed using one-shot quadruple, sextuple, and octuple borylation reactions, respectively. The key to success is the judicious choice of borylating reagents and long-chain alkyl-substituted carbazolyl groups as boron-trapping groups, which suppressed the decrease in HOMO energy and insolubilization associated with borylation. Based on the product yields, each electrophilic C-H borylation proceeded in >99% yield, which is the best efficiency reported so far for C-H borylation reactions. Owing to the multiple resonance effects of boron and nitrogen, the prepared acenes exhibited ultra-narrowband green thermally activated delayed fluorescence with full-width at half-maximum of 12-16 nm; moreover, their k_RISC values were in the order of 10⁵ s^-1. We fabricated an organic light-emitting diode by employing the nonacene as an emitter, which exhibited high external quantum efficiency (EQE) of 28.7%. The device also showed a minimum efficiency roll-off with an EQE of 25.8% at 1000 cd m^-2.

Improving the stability and color purity of a BT.2020 blue multiresonance emitter by alleviating hydrogen repulsion

Stable deep blue multiresonance emitters with small full width at half maximum (FWHM) are attractive for wide color-gamut organic light-emitting diodes (OLEDs). However, the steric repulsion from the spatially close hydrogens would twist the multiresonance skeletons, causing spectral broadening and molecular instability issues. Here, we strategically introduce a mesitylboron locking unit into a carbazole-embedded multiresonance model emitter, alleviating the hydrogen repulsions and also strengthening the para-positioned weak carbon-nitrogen bond in anionic states. An emission peaking at 452 nm with an FWHM of merely 14 nm and nearly BT.2020 blue chromaticity coordinates are obtained in toluene, affording a high maximum external quantum efficiency of 33.9% in a sensitizing device. Moreover, an impressive LT97 (time to decay to 97% of the initial luminance) of 178 hours at a constant current density of 12 mA/cm² was achieved in a stable device with a small y coordinate of 0.057, nearly 20 times longer than the model emitter with even a substantially red-shifted emission.

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 .

Sequential Multiple Borylation Toward an Ultrapure Green Thermally Activated Delayed Fluorescence Material

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters have emerged as an important component of organic light-emitting diodes (OLEDs) because of their narrowband emission and high exciton utilization efficiency. However, the chemical space of MR-TADF emitters remains mostly unexplored because of the lack of suitable synthetic protocols. Herein, we demonstrate a sequential multiple borylation reaction that provides new synthetically accessible chemical space. ω-DABNA, the proof-of-concept material, exhibited narrowband green TADF with a full width at half-maximum of 22 nm and a small singlet-triplet energy gap of 13 meV. The OLED employing it as an emitter exhibited electroluminescence at 512 nm, with Commission International de l'Éclairage coordinates of (0.13, 0.73) and a high external quantum efficiency (EQE) of 31.1%. Moreover, the device showed minimum efficiency roll-off, with an EQE of 29.4% at 1000 cd m^-2.

One-Shot Synthesis of B/N-Doped Calix[4]arene Exhibiting Narrowband Multiple Resonance Fluorescence

A novel macrocycle of B/N-doped calix[4]arene (C-BN) was synthesized by a one-shot double boronation. Owing to the structural tension and electron-donating properties of the nitrogen atoms in the macrocycle, reaction selectively proceeds between the adjacent benzene rings outside the macrocycle. C-BN shows a highly centrosymmetric structure with two multiple resonance (MR) fragments bridged by tertiary amine groups at the 1,3 positions of the benzene ring. Benefiting from the large intermolecular distance (>4.6 Å) between adjacent MR-emitting cores, C-BN also exhibits excellent narrowband emitting features against aggregation-induced quenching and spectrum broadening. Optimized organic light-emitting diode devices based on C-BN exhibit high maximum external quantum efficiencies of 24.7-26.6 % and small full width at half maximums of 25-28 nm over a wide doping range of 1-12 wt %.

Recent Progress in Blue Thermally Activated Delayed Fluorescence Emitters and Their Applications in OLEDs: Beyond Pure Organic Molecules with Twist D-π-A Structures

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy under the reverse inter-system crossing owing to the small singlet-triplet splitting energy (ΔE_ST). This results from a specific molecular design consisting of minimal overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, due to the spatial separation of the electron-donating and electron-releasing part. When a well-designed device structure is applied, high-performance blue-emitting TADF organic light-emitting diodes can be realized with an appropriate molecular design. Unlike the previous literature that has reviewed general blue-emitting TADF materials, in this paper, we focus on materials other than pure organic molecules with twist D-π-A structures, including multi-resonance TADF, through-space charge transfer TADF, and metal-TADF materials. Cutting-edge molecules with extremely small and even negative ΔE_ST values are also introduced as candidates for next-generation TADF materials. In addition, OLED structures used to exploit the merits of the abovementioned TADF emitters are also described in this review.

Ultra-Narrowband Blue Multi-Resonance Thermally Activated Delayed Fluorescence Materials

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.

Selenium-Doped Polycyclic Aromatic Hydrocarbon Multiresonance Emitters with Fast Reverse Intersystem Crossing for Narrowband Blue Emission

Two kinds of boron- (B), selenium- (Se), and nitrogen-doped (N) polycyclic aromatic hydrocarbon (PAH) emitters (Cz-BSeN and DCz-BSeN) with a multiresonance effect are developed for narrowband blue emission by embedding boron as an electron-deficient atom and selenium and nitrogen as electron-donating atoms into a benzo[a]naphtho[1,2,3-hi]aceanthrylene skeleton. It is found that both emitters exhibit strong spin-orbit coupling and fast reverse intersystem crossing (rate constant of 7.5-8.8 × 106 s-1) due to the heavy-atom effect of selenium, which is 2 orders of magnitude faster than its B, N-doped PAH analogue. Meanwhile, compared to parent B, Se, N-doped PAH emitter Cz-BSeN, incorporating carbazole moieties on the para position of the boron atom in DCz-BSeN not only blueshifts the emission by 7 nm without broadening its spectra but also results in an enhanced photoluminescent quantum efficiency of 93% in the doped film. The organic light-emitting diode (OLED) employing DCz-BSeN as emitter revealed narrowband blue emission at 481 nm with a small full-width at half-maximum (fwhm) of 32 nm, as well as a maximum external quantum efficiency of 22.3%, accompanied by alleviated efficiency roll-off compared to its B, N-containing counterpart.