Solution-Processed Pure Red TADF Organic Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color

Regulating the Spatially Folded Arrangement of Donor and Acceptor Units To Achieve Efficient Orange-Red Thermally Activated Delayed Fluorescence

Achieving efficient long-wavelength organic light-emitting diodes (OLEDs) remains a challenge due to the energy gap law, which leads to increased non-radiative decay rates as the emission wavelength shifts to longer regions. Herein, a strategy of constructing folded three-dimensional architectures is proposed to explore new orange-red thermally activated delayed fluorescence (TADF) emitters with through-space charge transfer characteristics. Innovatively, naphthalene is selected as a bridge to connect O-bridged triphenylamine donor and planar dibenzo[a,c]phenazine acceptor respectively via simple Suzuki-Miyaura Coupling. In this way, a series of rigid orange-red emitters with "U"-shaped, folded "Z"-shaped and "W"-shaped configurations are elaborately constructed by modifying end groups, adjusting the numbers of naphthalene and donor, and regulating the linkage sites. The excited state natures and photophysical properties of the emitters can be effectively regulated and optimized by changing three-dimensional architectures. Finally, the prepared emitter QX36 achieves a lower non-radiative transition rate, a higher radiative rate and a higher photoluminescence quantum efficiency. Solution-processed OLEDs based QX36 present the excellent electroluminescent performance with a maximum external quantum efficiency (EQE) of up to 32.3 % and EQE of 20.6 % at 1000 cd m-2, which are the leading values of solution-processed orange-red OLEDs. This work demonstrates the promising potential of folded TADF based naphthalene backbone as emitters for future efficient solution-processed long-wavelength OLEDs.

Aggregation-enhanced TADF in deep-red emitters for high-performance OLEDs

When isolated molecules undergo aggregation, their intermolecular interactions increase, potentially altering their electronic structures and affecting photophysical properties such as fluorescence lifetime. The extent of this change largely depends on the molecular structure. For thermally activated delayed fluorescent (TADF) materials, their luminescence mechanism in terms of the scale of increased lifetime from single molecules to aggregates and how that influences their optoelectronic device performance remain largely unexplored. In this study, we report a series of deep-red TADF emitters that are designed from D-A and D2-A types of molecular structures, with quinoxaline-6,7-dicarbonitrile (QCN) as the electron acceptor (A) and naphthalene-substituted N,N-diphenylamine (ND) as the electron donor (D). The D2-A type emitter (αND)2-QCN exhibits a pronounced aggregation-enhanced TADF (AE-TADF) effect compared to the D-A type emitters. This enhancement results from a significant reduction in the singlet-triplet energy gap (ΔEST) upon aggregation, driven by the formation of intermolecular hydrogen bonds in their J-aggregates. The AE-TADF effect facilitates more efficient reverse intersystem crossing (RISC), enabling the high performance of an (αND)2-QCN-based deep-red OLED (λEL,max = 622 nm) with a maximum external quantum efficiency (EQEmax) of 14.3%.

Engineering Nitrogen/Carbonyl MR-TADF Emitters: Spiro-Lock and Tert-Butyl Synergy in Narrowband Blue Emission

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with rigid planar structures are promising for their exceptional color purity and outstanding device efficiency. However, as an important MR unit, rigidly interlocked nitrogen/carbonyl-based blue materials often face challenges like spectral broadening, red-shifting, and reduced efficiency compared to nitrogen/boron system. Herein, a peripheral modification strategy incorporating tert-butyl groups via a spiro-lock framework is used to synthesize four molecules: QAO-TF, TQAO-TF, TQAO-F, and TSOQ. The spiro-lock structure solidifies the molecular framework, narrows the emission bandwidth, and elevates the photoluminescence quantum yield to over 96%. Meanwhile, the peripheral tert-butyl groups introduce steric hindrance, isolating the luminescent core and suppressing intermolecular interactions in the solid state, thereby improving device efficiency while maintaining narrowband emission. Notably, TQAO-F shows an electroluminescence peak at 476 nm with a 25 nm full width at half maximum (FWHM) and an external quantum efficiency (EQE) of 31.7%. TSOQ, with its oxygen-induced charge effect, achieves narrowband pure blue emission with an FWHM of 20 nm, surpassing 30% EQE without sensitizers. This overall performance suggests its potential to rival the classic nitrogen/boron system.

Synergetic Multiple Charge-Transfer Excited States for Anti-Quenching and Rapid Spin-Flip Multi-Resonance Thermally Activated Delayed Fluorescence Emitter

The development of multiple resonances thermally activated delayed fluorescence (MR-TADF) emitters exhibiting high efficiency, narrowband emission, rapid reverse intersystem crossing rate (kRISC), and suppressed concentration quenching simultaneously is of great significance yet a formidable challenge. Herein, an effective strategy is presented to realize the above target by synergizing multiple charge-transfer excited states, including short-range charge transfer (SRCT), through-bond charge transfer (TBCT), and through-space charge transfer (TSCT). The proof-of-concept emitter 4tCz2B exhibits a bright green emission with a narrow full width at half maximum (FWHM) of 21 nm (0.10 eV) in solution, high photoluminescence quantum yield of 97%, fast kRISC of 7.8 × 105 s-1 and significantly suppressed concentration quenching in film state. As a result, the sensitizer-free organic light-emitting diodes (OLEDs) achieve maximum external quantum efficiencies (EQEmaxS) of over 34.5% together with an unaltered emission peak at 508 nm and FWHM of 26 nm at doping concentrations ranging from 3 to 20 wt.%. Even at a doping ratio of 50 wt.%, EQEmax is still as high as 25.5%. More importantly, the non-sensitized devices exhibit significantly reduced efficiency roll-offs, with a minimum value of 13.4% at a brightness of 1000 cd m-2.

Axially Chiral TADF Imidazolium Salts for Circularly Polarized Light-Emitting Electrochemical Cells

A pair of axially chiral thermally activated delayed fluorescent (TADF) enantiomers, R-TCBN-ImEtPF6 and S-TCBN-ImEtPF6, with intrinsic ionic characteristics were efficiently synthesized by introducing imidazolium hexafluorophosphate to chiral TADF unit. The TADF imidazolium salts exhibited a high photoluminescence quantum yield (PLQY) of up to 92 %, a small singlet-triplet energy gap (▵EST) of 0.04 eV, as well as reversible redox properties. Furthermore, the enantiomers showed distinct mirror-image CD and CPL activities with glum values of -3.7×10-3 and +3.4×10-3. Notably, by doping the axial TADF imidazolium salts into achiral TADF sensitizer, sandwich-structured light-emitting electrochemical cells (LECs) without the addition of ionic liquids (ILs) or ionic transition-metal compounds (iTMCs) were fabricated. When driven at 50 A m-2, the LECs displayed an EQE of up to 5.2 % and strong circularly polarized electroluminescence (CPEL) with gEL values of +3.3×10-3 and -3.0×10-3. This represents the first CP-LEC based on TADF materials and offers a promising strategy for the development of high-performance CPEL devices.

Versatile Thermally Activated Delayed Fluorescence Material Enabling High Efficiencies in both Photodynamic Therapy and Deep-Red/NIR Electroluminescence

Thermally activated delayed fluorescence (TADF) materials have received increasing attention from organic electronics to other related fields, such as bioapplications and photocatalysts. However, it remains a challenging task for TADF emitters to showcase the versatility concurrent with high performance in multiple applications. Herein, we first present such a proof-of-concept TADF material, namely, QCN-SAC, through strategically manipulating exciton dynamics. On the one hand, QCN-SAC displays obvious aggregate-induced deep-red/near-infrared emission with a high radiative rate beyond 107 s-1, thereby demonstrating nearly 100% exciton utilization under oxygen-free conditions. In a QCN-SAC-based nondoped organic light-emitting diode (OLED), a superb external quantum efficiency of 16.4% can be reached with a peak at 708 nm. On the other hand, QCN-SAC also exhibits a high intersystem crossing rate over 108 s-1 without leveraging the heavy-atom effect, which makes QCN-SAC-based nanoparticles perform well in boosting reactive oxygen species generation for imaging-guided photodynamic therapy (PDT). This work presents a fundamental principle for designing high-performance all-in-one TADF molecules for OLED and PDT applications. This discovery holds promise for advancing the development of versatile TADF materials with a range of uses in the near future.

Lantern-Shaped Structure Induced by Racemic Ligands in Red-Light-Emitting Metal Halide with Near 100 % Quantum Yield and Multiple-Stimulus Response

Organic-inorganic metal halides (OIMHs) have become a research hotspot in recent years due to their excellent luminescent properties and tunable emission wavelengths. However, the development of efficient red-light-emitting OIMHs remains a significant challenge. This work reports three Mn-based OIMHs derived from 1-methyl-1,2,3,4-tetrahydroisoquinoline hydrobromide: racemic one (Rac-TBM) and chiral ones (R-TBM and S-TBM). As a result of the synergism of chiral organic ligands inducing a unique lantern-shaped hybrid structure containing both tetrahedra and octahedra, Rac-TBM exhibits red-light emission with near-unity luminescence quantum yield. In comparison, the chiral counterparts R/S-TBM display strong green emission and circularly polarized luminescence (CPL) with a glum value up to ±2.5×10-2. Interestingly, a mixture of R- and S-TBM can transform into Rac-TBM, successfully achieving a sensitive and reversible switch between red light of octahedra and green light of tetrahedra under external stimuli. The outstanding luminescent properties allow Rac-TBM to be utilized not only for X-ray radioluminescence with a detection limit down to 46.29 nGys-1, but also for advanced information encryption systems to achieve leak-proof decryption.

Exploration of red and deep red Thermally activated delayed fluorescence molecules constructed via intramolecular locking strategy

Red and deep red (DR) organic light-emitting diodes (OLEDs) have garnered increasing attention due to their widespread applications in display technology and lighting devices. However, most red OLEDs exhibit low luminescence efficiency, severely limiting their practical applications. To address this challenge, we theoretically design four novel TADF molecules with red and DR luminescence using intramolecular locking strategies building upon the experimental findings of DCN-DLB and DCN-DSP, and their crystal structures are predicted with the lower energy and higher packing density. The photophysical properties and luminescence mechanism of six molecules in toluene and crystal are clarified using the first principles calculation and thermal vibration correlation function (TVCF) method. The proposed design strategy is anticipated to offer several advantages: enhanced electron-donating capabilities, more rigid structures, longer emission wavelengths and higher luminescence efficiency. Specifically, we introduce oxygen atoms and nitrogen atoms as intramolecular locks, and the newly developed DCN-DBF and DCN-PHC have redshifted emission, narrow singlet-triplet energy gap (ΔEST), fast reverse intersystem crossing rate and enhanced photoluminescence quantum yield (PLQY). Notably, DCN-DBF achieves both long wavelength emission and high efficiency, with emission peaks at 598 nm and 587 nm corresponding to PLQY of 52.13 % and 43.42 % in toluene and crystal, respectively. Our work not only elucidates the relationship between molecular structures and photophysical properties, but also proposes feasible intramolecular locking design strategies and four promising red and DR TADF molecules, which could provide a valuable reference for the design of more efficient red and DR TADF emitters.

A Tailored TADF Emitter Based on Diphenylacetylene Anthracene Derivatives for Highly Efficient Solution-Processed Pure Red Organic Light-Emitting Diodes

It holds enormous significance for red thermally activated delayed fluorescence (TADF) emitters to develop organic light-emitting diodes (OLEDs) with high efficiency and high color purity, which remains challenging for highly efficient solution-processed red TADF emitters due to the limitation of severe nonradiative decays. Herein, a red TADF emitter containing space interactions, 4,4'-(9,10-bis(phenylethynyl)anthracene-1,8-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (DBP-2MOTPA), is designed and synthesized, composed of ethynyl as the acceptor and methoxytriarylamine (MOTPA) as the donor. The triphenylamine donor unit decorated with peripheral methoxy units not only improves the solubility for the solution-processed technology but also increases the electron-donating ability. The highly twisted donor-π-acceptor (D-π-A) architecture generates a small energy gap (ΔEST) of 0.14 eV, and the appropriate overlap of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) leads to a supernal photoluminescence quantum yield (ΦPL) of 68%. Consequently, the nondoped device based on DBP-2MOTPA with Commission Internationale de l'Eclairage (CIE) at (0.67, 0.33) exhibits the pure red emission, which satisfies the Rec.1931 standard red gamut (0.67, 0.33) and approaches the Rec.2020 standard (0.71, 0.29). Moreover, the doped devices employing DBP-2MOTPA as the emitter exhibit a maximum external quantum efficiency (EQE) of 6.09%, which is among the effective values in the solution-processed red TADF-OLEDs.

Efficient and Stable Narrowband Pure-Red Light-Emitting Diodes with Electroluminescence Efficiencies Exceeding 43

Advanced multiresonance-induced thermally activated delayed fluorescence (MR-TADF) materials exhibit exceptional promise for applications in state-of-the-art organic light-emitting diodes (OLEDs) owing to their unique narrowband emissions and high luminescent efficiencies. Despite substantial progress with blue and green MR-TADF materials, the development of pure-red MR-TADF emitters has lagged behind, thereby hindering the advancement toward high-performance ultrahigh-definition OLED displays. Here, we propose an effective approach for designing pure-red MR-TADF molecules based on the integration of secondary electron-donating units and π-skeleton extension into MR cores, which enables not only a redshift of narrowband emission but also an acceleration of reverse intersystem crossing (RISC) rate. The proof-of-the-concept emitter BNTPA showcases a bright and saturated red emission centered at 613 nm with a full-width at half-maximum of 0.14 eV, together with a more than twofold increase in the RISC rate. As a result, TADF OLEDs based on BNTPA achieved a record maximum external quantum efficiency (EQEmax) of up to 35.2% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.657, 0.343), approaching the coordinates of the National Television Standards Committee (NTSC) red standard. Moreover, phosphor-assisted fluorescence devices exploiting BNTPA as a terminal emitter exhibit an exceptional EQEmax of 43.3%, accompanied by improved operational stability.

Twisted Structure and Multiple Charge-Transfer Channels Endow Thermally Activated Delayed Fluorescence Devices with Small Efficiency Roll-Off and Low Concentration Dependence

Despite the rapid development of thermally activated delayed fluorescent (TADF) materials, developing organic light-emitting diodes (OLEDs) with small efficiency roll-off remains a formidable challenge. Herein, we have designed a TADF molecule (mClSFO) based on the spiro fluorene skeleton. The highly twisted structure and multiple charge-transfer channels effectively suppress aggregation-caused quenching (ACQ) and endow mClSFO with excellent exciton dynamic properties to reduce efficiency roll-off. Fast radiative rate (kr) and rapid reverse intersystem crossing (RISC) rate (kRISC) of 1.6×107 s-1 and 1.07×106 s-1, respectively, are obtained in mClSFO. As a result, OLEDs based on mClSFO obtain impressive maximum external quantum efficiency (EQEmax) exceeding 20 % across a wide doping concentration range of 10-60 wt %. 30 wt % doped OLED exhibits an EQEmax of 23.1 % with a small efficiency roll-off, maintaining an EQE of 18.6 % at 1000 cd m-2. The small efficiency roll-off and low concentration dependence observed in the TADF emitter underscore its significant potential.

Achieving Ultra-Narrow-Band Deep-Red Electroluminescence By a Soliton-type Dye Squaraine

Due to the soliton-like electronic structural characteristics, cyanine dyes typically exhibit spectral behaviors such as large molar extinction coefficients, narrow spectra, and high fluorescence efficiency. However, their extensive applications as emitters in electroluminescence are largely ignored due to their serious emission quenching in the aggregation state. Herein, it is reported a squaraine dye (a type of cyanine) SQPhEt. At different solution concentrations, the unusual decrease in full-width at half-maxima (FWHM) with increasing Stokes shift indicates the fluorescence quenching of SQPhEt in the aggregated state is because of the strong self-absorption effect. A sensitized device structure can help to reduce the doping concentration of dye, which can effectively suppress self-absorption. Benefitting from the large molar extinction coefficient of SQPhEt, even at low doping concentrations of 0.1 wt%, efficient Förster energy transfer can be achieved. The corresponding spin-coating sensitized device based on SQPhEt as the dopant exhibits favorable deep-red emission at 668 nm with a small FWHM of 0.10 eV.

Theoretical insights on highly efficient X-shaped near-infrared thermal activation delayed fluorescence emitter

The near-infrared (NIR) thermally activated delayed fluorescence (TADF) molecules hold practical application value in various fields, including biological imaging, anti-counterfeiting, sensors, telemedicine, photomicrography, and night vision display. These molecules have emerged as a significant development direction in organic electroluminescent devices, offering exciting possibilities for future technological advancements. Despite the remarkable potential of NIR-TADF molecules in various applications, the development of molecules that exhibit both long-wavelength emission and high efficiency remains a significant challenge. Herein, based on T-type and Y-type TADF molecules BCN-TPA and ECN-TPA, a novel X-type TADF molecule X-ECN-TPA is theoretically designed through a molecular fusion strategy. Utilizing first-principles calculations and the thermal vibration correlation function (TVCF) method, the photophysical properties and luminescent mechanisms of these three molecules in both solvent and solid (doped films) are revealed. A comparison of the luminescent properties of isomeric BCN-TPA and ECN-TPA shows that the enhanced luminescence efficiency of BCN-TPA in the solid states is attributed to higher radiative rates and lower non-radiative rates. Furthermore, compared to BCN-TPA and ECN-TPA, X-ECN-TPA exhibits significant conjugation extension, resulting in a pronounced redshift, reaching 831 nm and 813 nm in solvent and solid states, respectively. Importantly, molecular fusion significantly increases the transition dipole moment density between the donor and acceptor, leading to a substantial increase in radiative transition rates. Additionally, molecular fusion effectively reduces the energy gap between the first singlet excited state (S1) and the first triplet excited state (T1), facilitating the improvement of the reverse intersystem crossing (RISC) process. In addition, the calculation of Marcus formula shows that the triplet energy transfer from CBP to BCN-TPA, ECN-TPA and X-ECN-TPA is very effective. This work not only designs a novel efficient NIR-TADF molecule but also proposes a strategy for designing efficient NIR-TADF molecules. This principle offers unique insights for optimizing traditional molecular frameworks, opening up new possibilities for future advancements.

An A-D-A-Type Thermally Activated Delayed Fluorescence Emitter with Intrinsic Yellow Emission Realizing Record-High Red/NIR OLEDs upon Modulating Intermolecular Aggregations

Realizing efficient red/near-infrared (NIR) electroluminescence (EL) by precisely modulating molecular aggregations of thermally activated delayed fluorescence (TADF) emitters is an attractive pathway, yet the molecular designs are elusive. Here, a new approach is proposed to manage molecular aggregation via a mild-twist acceptor-donor-acceptor (A-D-A)-type molecular design. A proof-of-concept TADF molecule, QCN-PhSAC-QCN, is developed that furnishes a fast radiative rate and obvious aggregation-induced emission feature. Its emission bands can be facilely shifted from intrinsic yellow to the red/NIR region via fine-tuning doping levels and molecular aggregates while maintaining elegant photoluminescence quantum yields benefiting from suppressed exciton annihilation processes. As a result, a QCN-PhSAC-QCN-based organic light-emitting diode (OLED) exhibits a record-setting external quantum efficiency (EQE) of 39.1% at a doping ratio of 10 wt.%, peaking at 620 nm. Moreover, its nondoped NIR OLED affords a champion EQE of 14.3% at 711 nm and retains outstanding EQEs of 5.40% and 2.35% at current densities of 10 and 100 mA cm-2 , respectively, which are the highest values among all NIR-TADF OLEDs at similar density levels. This work validates the feasibility of such mild-twist A-D-A-type molecular design for precisely controlling molecular aggregation while maintaining high efficiency, thus providing a promising pathway for high-performance red/NIR TADF OLEDs.

Theoretical Research and Photodynamic Simulation of Aggregation-Induced Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

The discovery and utilization of pure organic thermally activated delayed fluorescence (TADF) materials provide a major breakthrough in obtaining high-performance and low-cost organic light-emitting diodes (OLEDs). In spite of recent research progress in TADF emitters, highly efficient and stable TADF emitters in high-concentration solutions and in the solid state have been rarely reported, and most of them suffer from aggregation-induced quenching (ACQ). To resolve this issue, the aggregation-induced delayed fluorescence (AIDF) mechanism was studied in depth by the simulation of excited-state dynamic processes, and the effect of geometric modifications on optical properties was minutely investigated based on molecular modeling. TD-DFT calculations demonstrate that it is the key point for the transformation between prompt fluorescence and TADF to effectively regulate singlet-triplet energy difference and electron-vibration coupling by the aggregation effect. Then, excellent green and red TADF materials with very small singlet-triplet energy differences of 0.05 and 0.06 eV, high TADF quantum yields up to 57.53% and 39.19%, and suitable fluorescence lifetimes of 0.99 and 1.67 us, respectively, were designed and obtained, which demonstrate the potential application of these two TADF materials in OLEDs.

Highly Efficient Luminescence from a Red Thermally Activated Delayed Fluorescence Emitter with Flexible Conformation of Ancillary Groups

Robust scaffolds were typically applied in thermally activated delayed fluorescence (TADF) molecules to suppress the non-radiative decay, trigger the fast spin-flipping, and enhance the light out-coupling efficiency. Herein, we disclosed for the first time the positive effect of flexible conformation of ancillary groups on the photophysical properties of TADF emitter. The red TADF emitter Ph-TPA with flexible conformation demonstrated small excited-state structural distortion and low reorganization energy compared to the counterpart Mc-TPA with a rigid macrocycle. Consequently, Ph-TPA showed an excellent photoluminescent quantum yield (PLQY) of 92 % and a state-of-the-art external quantum efficiency (EQE) of 30.6 % at 630 nm. This work could deepen our understanding of structure-property relationships of organic luminophores and help us to rationalize the design of efficient TADF materials.

Ultra-Narrow-Bandwidth Deep-Red Electroluminescence Based on Green Plant-Derived Carbon Dots

Deep-red light-emitting diodes (DR-LEDs, >660 nm) with high color-purity and narrow-bandwidth emission are promising for full-color displays and solid-state lighting applications. Currently, the DR-LEDs are mainly based on conventional emitters such as organic materials and heavy-metal based quantum dots (QDs) and perovskites. However, the organic materials always suffer from the complicated synthesis, inferior color purity with full-width at half-maximum (FWHM) more than 40 nm, and the QDs and perovskites still suffer from serious problems related to toxicity. Herein, this work reports the synthesis of efficient and high color-purity deep-red carbon dots (CDs) with a record narrow FWHM of 21 nm and a high quantum yield of more than 50% from readily available green plants. Moreover, an exciplex host is further established using a polymer and small molecular blend, which has been shown to be an efficient strategy for producing high color-purity monochrome emission from deep-red CDs via Förster energy transfer (FET). The deep-red CD-LEDs display high color-purity with Commission Internationale de l'Eclairage (CIE) coordinates of (0.692, 0.307). To the best of the knowledge, this is the first report of high color-purity CD-LEDs in the deep-red region, opening the door for the application of CDs in the development of high-resolution light-emitting display technologies.