Thermally Activated Delayed Fluorescence Emitters Research Articles

Rational Molecular Design via Cyanobenzene Integration for Constructing Efficient Yellow‐Orange Thermally Activated Delayed Fluorescence Emitters

AbstractDeveloping efficient long‐wavelength thermally activated delayed fluorescence (TADF) emitters is a challenging issue due to inherent limitations of the energy‐gap law. In this contribution, a new molecular design of cyanobenzene‐decorated quinoxaline acceptor, combining two or four cyanobenzene acceptors and a rigid carbazole donor, is successfully utilized to construct long‐wavelength TADF emitters (tCzQx2CN and tCzQx4CN). The two additional cyanobenzene units at the 5‐ and 8‐positions of the quinoxaline acceptor are presumed to interlock the molecular rotations and improve the acceptor strength. Compared with tCzQx2CN, tCzQx4CN exhibits orange‐red emission with a considerable bathochromic shift (>35 nm), as expected. Owing to its enhanced charge transfer and well‐separated highest‐occupied and lowest‐unoccupied molecular orbitals, tCzQx4CN exhibits reduced singlet‐triplet energy splitting and improves reverse intersystem crossing. An organic light–emitting diode (OLED) of tCzQx4CN manifestes bright orange‐red emission (λEL ≈581 nm), a superior external quantum efficiency (EQE) of ≈23.7% (compared with that of tCzQx2CN), and a satisfactory operational lifetime. Furthermore, a tCzQx4CN‐sensitized red‐hyperfluorescent OLED device exhibits excellent performance with an EQE of 16.0% and a CIE(x,y) of (0.62, 0.37). This design would potentially pave the way for constructing red/deep‐red TADF emitters by using a diverse combination of donor/acceptor units.

Copper(I) TADF exciplexes via Metal-Perturbed Through-Space charge transfer for efficient OLEDs

Organo-copper(I) emitters are promising cost-effective candidates as triplet-harvesting dopants in organic light-emitting diodes (OLEDs). To date, the mainstream design of Cu(I) emitters depends on ligand engineering to modify intramolecular charge transfer and thus modulate the related optoelectronic properties. However, little is known on luminescent Cu(I) systems based on intermolecular charge transfer. For the first time, the proof-of-concept Cu(I) exciplexes were developed employing a triazine-containing molecule DMIC-TRZ as an acceptor and several Cu(I)-based carbene-metal-amide (CMA) complexes as donors. The formation of these exciplexes is strongly associated with d-orbital participation of Cu atom on the highest occupied molecular orbital of CMA complexes to match with the acceptor DMIC-TRZ. These Cu(I) exciplexes not only displayed donor-dependent emission colors spanning from sky blue to red but also exhibited distinct thermally activated delayed fluorescence (TADF) via copper(I)-perturbed through-space charge transfer (TSCT). The OLEDs adopting the Cu(I) exciplexes as emitters delivered good external quantum efficiency (EQE) of up to 10.9%. Furthermore, the use of these Cu(I) exciplexes as hosts for multi-resonance TADF emitters delivered narrowband electroluminescence with high EQEs of up to 24.9%, superior to the reference OLEDs based on the common host.

Regulation for photophysical properties of thermally activated delayed fluorescence molecules with through-space charge transfer by substitution effect

Thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) have attracted much attention in recent years. The photophysical properties of four TADF molecules with TSCT properties were studied in toluene and in solid phase. Our calculation results indicate that ortho substitution and fluorine substitution not only favor the generation of TADF, but also promote room temperature phosphorescence in solid phase. The results provide deeper understanding of substitution effect on light-emitting properties of TSCT-TADF emitters and may favor the design of TADF emitters.

Boosting Reverse Intersystem Crossing of TADF Emitter through Molecular Geometry Adaptation to Crystalline Host

AbstractRapid reverse intersystem crossing (RISC) is one of the prime concerns for blue thermally activated delayed fluorescence (TADF) emitters, as it reduces triplet exciton population, the root cause of detrimental triplet‐mediated annihilation processes that accelerate device efficiency roll‐off and degradation. This work introduces a new concept to tailor the RISC of TADF emitters through their molecular geometry adaptation to crystalline hosts bearing a similar donor‐acceptor structure. A meticulously designed crystalline host comprising isophthalonitrile acceptor (A) and carbazole‐derived donor (D) units, characterized by nearly orthogonal D‐A arrangement, has been demonstrated to alleviate singlet‐triplet energy gap (ΔEST) of the TADF dopant by forcing it to adopt a more twisted D‐A configuration, as corroborated by X‐ray diffraction (XRD) measurements. The approach not only significantly reduces the RISC activation energy, resulting in a remarkable tenfold boost of the RISC rate (above 107 s−1) and fourfold shortening of delayed FL lifetime (down to 1.5 µs), but also offers the additional benefit of suppressing conformational disorder of TADF dopant, producing a narrower emission bandwidth. The presented concept, based on crystalline host‐driven RISC engineering, is anticipated to have a profound impact on the development of high‐performance, stable blue‐emitting TADF organic light‐emitting diodes (OLEDs).

Asymmetric phosphorylation integrating host feature and blue thermally activated delayed fluorescence of thiaxanthene S,S-dioxide emitter for efficient heavily doped white organic light-emitting diodes

Significant efforts have been recently devoted to develop simply structured white organic light-emitting diodes (WOLEDs) based on purely thermally activated delayed fluorescence (TADF) with a single-emissive-layer and heavy-doping configuration. Herein, through introducing a diphenylphosphine oxide group on thiaxanthene S,S-dioxide acceptor, we integrate host characteristics to blue TADF emitter CZ-MPSPO, which is beneficial to balanced exciton allocation and quenching suppression in single emissive layers of TADF WOLEDs for high performance. After co-doping with a conventional yellow TADF emitter, TADF performance of CZ-MPSPO is preserved, but external quantum efficiencies (EQE) of WOLEDs are remarkably improved through the synergistic singlet and triplet exciton harvesting by CZ-MPSPO and 4CzTPNBu. The steric hindrance of CZ-MPSPO also mitigates emission quenching by 4CzTPNBu, leading to the favourable EQE up to ∼20 % of single-EML pure TADF WOLEDs with the doping concentration of CZ-MPSPO as high as 70 %.

Tunable Self-Referenced Molecular Thermometers via Manipulation of Dual Emission in Platinum(II) Pyridinedipyrrolide Complexes.

Optical temperature sensors based on self-referenced readout schemes such as the emission ratio and the decay time are crucial for a wide range of applications, with the former often preferred due to simplicity of instrumentation. This work describes a new group of dually emitting dyes, platinum(II) pincer complexes, that can be used directly for ratiometric temperature sensing without an additional reference material. They consist of Pt(II) metal center surrounded by a pyridinedipyrrolide ligand (PDP) and a terminal ligand (benzonitrile, pyridine, 1-butylimidazol or carbon monoxide). Upon excitation with blue light, these complexes exhibit green to orange emission, with quantum yields in anoxic toluene at 25 °C ranging from 13% to 86% and decay times spanning from 8.5 to 97 μs. The emission is attributed to simultaneous thermally activated delayed fluorescence (TADF) and phosphorescence processes on the basis of photophysical investigations and DFT calculations. Rather uniquely, simple manipulations in substituents of the PDP ligand and alteration of the terminal ligand allow fine-tuning of the ratio between TADF and phosphorescence from almost 100% TADF emission (Pt(MesPDPC6F5(BN)) to over 80% of phosphorescence (Pt(PhPDPPh(BuIm)). Apart from ratiometric capabilities, the complexes also are useful as decay time-based temperature indicators with temperature coefficients exceeding 1.5% K-1 in most cases. Immobilization of the dyes into oxygen-impermeable polyacrylonitrile produces temperature sensing materials that can be read out with an ordinary RGB camera or a smartphone. In addition, Pt(PhPDPPh)Py can be incorporated into biocompatible RL100 nanoparticles suitable for cellular nanothermometry, as we demonstrate with temperature measurements in multicellular colon cancer spheroids.

Exploring the Theoretical Foundations of Thermally Activated Delayed Fluorescence (TADF) Emission: A Comprehensive TD-DFT Study on Phenothiazine Systems.

This study conducts a thorough theoretical investigation of Thermally Activated Delayed Fluorescence (TADF) in phenothiazine-based systems, examining ten molecular configurations recognized experimentally as TADF-active. Employing Time-Dependent Density Functional Theory (TD-DFT), our analysis spans the investigation of singlet-triplet energy gaps (ΔEST), spin-orbit coupling, and excitation characteristics using Multiwfn. This approach not only validates the adherence to El Sayed's rule across these systems but also provides a detailed understanding of charge transfer dynamics, as visualized through heat maps. A significant aspect of our study is the exploration of different oxidation states of sulfur and site substitutions on phenothiazine. This systematic variation aims to identify additional TADF-active compounds, drawing parallels with properties characterizing other known TADF emitters. Our investigation into Reverse Intersystem Crossing (rISC) rates and the analysis of dihedral angles in relation to ΔEST values offer nuanced insights into the TADF behaviours of these molecules. By integrating rigorous computational analysis with practical implications, we provide a foundational understanding that enhances the design and optimization of phenothiazine-based materials for optoelectronic applications. This work not only advances our theoretical understanding of TADF in phenothiazine derivatives but also serves as a guide for experimentalists and industry professionals in the strategic design of new TADF materials.

Key requirements for ultraefficient sensitization in hyperfluorescence organic light-emitting diodes

Blue organic light-emitting diode (OLED) technology requires further advancements, and hyperfluorescent (HF) OLEDs have emerged as a promising solution to address stability and colour-purity concerns. A key factor influencing the performance of HF-OLEDs is Förster resonance energy transfer (FRET). Here we investigate the FRET mechanism in blue HF-OLEDs using contrasting thermally activated delayed fluorescence (TADF) sensitizers. We demonstrate that the molecular structure of the sensitizer profoundly impacts the FRET efficiency, exemplified by the spiro-linked TADF molecule ACRSA, which suppresses the dihedral-angle inhomogeneity and any lower-energy conformers that exhibit minimal FRET to the terminal emitter. Consequently, the FRET efficiency can be optimized to nearly 100%. Further, we demonstrate how the properties of a near-ideal sensitizer diverge from ideal TADF emitters. As a result, blue HF-OLEDs utilizing a greenish sensitizer exhibit a remarkable tripling of external quantum efficiency (~30%) compared with non-HF devices. This new understanding opens avenues for sensitizer design, indicating that green sensitizers can efficiently pump blue terminal emitters, thereby reducing device exciton energies and improving blue OLED stability.

Recent Progress in Phenoxazine-Based Thermally Activated Delayed Fluorescent Compounds and Their Full-Color Organic Light-Emitting Diodes.

Third-generation organic light-emitting diodes (OLEDs) based on metal-free thermally activated delayed fluorescent (TADF) materials have sparked tremendous interest in the last decade due to their nearly 100% exciton utilization efficiency, which can address the low-efficiency issue of the first-generation fluorescent emitters and the high-cost issue of the second-generation organometallic phosphorescent emitters. Construction of efficient and stable TADF-OLEDs requires utilizing TADF materials with a narrow singlet-triplet energy gap (ΔEST), high photoluminescence quantum yield (PLQY) and short TADF lifetime. A small ΔEST is necessary for an efficient reverse intersystem crossing (RISC) process, which can be achieved through theeffective spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). TADF emitters have been generally designed as intramolecular charge transfer (ICT) molecules with highly twisted donor-acceptor (D-A) molecular architectures. A wide variety of combinations of electron donors and acceptors have been explored. In this review, we shall focus on recent progress in organic TADF molecules incorporating strong electron-donor phenoxazine moiety and their application as emitting layer (EML) in OLEDs.

Exploring the Early Time Behavior of the Excited States of an Archetype Thermally Activated Delayed Fluorescence Molecule.

Optical pump-probe techniques allow for an in-depth study of dark excited states. Here, we utilize them to map and gain insights into the excited states involved in the thermally activated delayed fluorescence (TADF) mechanism of a benchmark TADF emitter DMAC-TRZ. The results identify different electronic excited states involved in the key TADF transitions and their nature by combining pump-probe and photoluminescence measurements. The photoinduced absorption signals are highly dependent on polarity, affecting the transition oscillator strength but not their relative energy positions. In methylcyclohexane, a strong and vibronically structured local triplet excited state absorption (3LE → 3LEn) is observed, which is quenched in higher polarity solvents as 3CT becomes the lowest triplet state. Furthermore, ultrafast transient absorption (fsTA) confirms the presence of two stable conformers of DMAC-TRZ: (1) quasi-axial (QA) interconverting within 20 ps into (2) quasi-equatorial (QE) in the excited state. Moreover, fsTA highlights how sensitive excited state couplings are to the environment and the molecular conformation.

Two Birds with One Stone: Polymerized Thermally Activated Delayed Fluorescence Small Molecules.

Thermally activated delayed fluorescence (TADF) polymers show a great potential in low-cost, large-area and flexible full-color flat-panel displays. One of the most promising design rules is based on TADF+Linker, where a small molecular TADF unit is bonded to each other by a simple linker. Unlike the expensive vacuum deposition for small molecules, these polymerized TADF small molecules (Poly-TADF-SMs) are capable of cost-effective solution processing. Meanwhile, the good luminescent property of small molecular TADF emitters can be well inherited by Poly-TADF-SMs so as to bridge the efficiency gap between small molecules and polymers. Herein, we will highlight the recent progress of Poly-TADF-SMs, together with emphasis on their molecular design, photophysical and electroluminescence properties.

Positional isomeric effect: A simple and feasible strategy for constructing efficient red TADF materials

In order to investigate the effect of positional isomerization on the photoelectric properties of thermally activated delayed fluorescence (TADF) emitters, thereby fully stimulating the potential of the molecular backbone, herein, two red TADF isomers, oTPA-PPP and mTPA-PPP, were developed and synthesized by linking triphenylamine (TPA) donor into different positions of the pyrido [2′,3′:5,6] pyrazino [2,3-f] [1,10] phenanthroline (PPP). Their photoluminescent quantum yields (PLQYs) and external quantum efficiencies (EQEs) were significantly different by manipulating the substitution position of the donor. Compared with mTPA-PPP, oTPA-PPP exhibited higher PLQY benefiting from intramolecular hydrogen bonding and larger fluorescence radiative decay rate, as confirmed by both theoretical and experimental analyses. As a result, oTPA-PPP-based organic light-emitting diode (OLED) realized high EQEmax of 19.2 % with red emission at 603 nm, whereas mTPA-PPP achieves an EQEmax of 7.9 % at 596 nm. This work provides a simple and feasible molecular design strategy to design and select red TADF emitters with excellent properties.

The novel thermally activated delayed fluorescence compounds based on trifluoromethylpyridine were synthesized for blue OLEDs

Seeking efficient blue thermally activated delayed fluorescence (TADF) luminous materials with a simple structure and applying them to organic light-emitting diodes (OLEDs) is still an urgent challenge. Herein, two novel D-π-A type blue TADF emitters with simple molecular structure, namely p-AcBPyCF3 and m-AcBPyCF3, featuring a trifluoromethylpyridine (PyCF3) as acceptor moiety, 9,9-dimethyl-tetrahydroacridine (Ac) as donor unit and benzene (B) as π-bridge, respectively, are first developed and compared in parallel. The analysis of single crystal reveals that p-AcBPyCF3 possesses clearly twisted geometry between the donor and acceptor units, as well as expanded molecular rigidity, thus leading to a smaller singlet-triplet energy difference and greatly improved emission efficiency. The p-AcBPyCF3 exhibits a photoluminescence quantum yield (PLQY) of 28.80 %, surpassing the PLQY value of 18.70 % for the m-AcBPyCF3. Compared to m-AcBPyCF3, the TADF-OLEDs based on p-AcBPyCF3 demonstrated exceptional properties, which suppresses the rotation of molecular conformation and can achieve high PLQY by reducing non radiative transformation processes, resulting in a maximum external quantum efficiency (EQE) of 9.26 % with peak at 470 nm and blue color coordinates of (0.18, 0.25). This work highlights that the simple PyCF3 group can be a promising electron acceptor moiety for the design of blue TADF materials.

Efficient Blue Thermally Activated Delayed Fluorescence Molecules with Sandwich‐Structured Through‐Space Charge Transfer for Fabricating High‐Performance OLEDs

AbstractExploring robust blue luminescent materials is of high significance but challenging for the commercialization application of organic light‐emitting diodes (OLEDs). In this work, it is wished to report two blue thermally activated delayed fluorescence (TADF) molecules with a through‐space charge transfer feature. They have sandwich‐structured molecular backbones comprised of one triphenyltriazine acceptor and two 9‐phenylcarbazole or 3,6‐di(tert‐butyl)‐9‐phenylcarbazole donors that are linked at the 1,8,9‐positions of a 3,6‐di(tert‐butyl)carbazole holder, and exhibit high thermal stability and strong blue delayed fluorescence with high photoluminescence quantum yields and fast reverse intersystem crossing. The blue OLEDs use 2TBCTC as an emitter and provide outstanding maximum external quantum efficiencies (EQEmaxs) of up to 32.76% with Commission Internationale de I'Eclairage coordinates of (0.160, 0.206). To the best of knowledge, 2TBCTC is the most efficient blue TADF emitter based on through‐space charge transfer in the literature. Moreover, the interlayer‐sensitizing OLED employs 2TBCTC as a sensitizer for blue multi‐resonance TADF (MR‐TADF) molecule (v‐DABNA) as guest emitter attains an excellent EQEmax of 32.69% with a narrow full width at half maximum of 18 nm, validating the excellent potential of 2TBCTC as sensitizer for MR‐TADF emitters and the feasibility and effectiveness of the interlayer sensitization strategy for blue hyperfluorescence OLEDs.

Rational Design of Bipolar Host and Thermally Activated Delayed Fluorescence Materials in Donor-Acceptor Molecular Architecture: A Theoretical Study.

Donor-acceptor (D-A) molecules have drawn massive attention recently in the design of high-performance materials, but the underlying reasons for the magic abilities of D-A architecture in building very different organic semiconductors are still unclear. Here, based on a series of experimentally bipolar host and thermally activated delayed fluorescence (TADF) molecules with the same donor but different acceptor units, it was found that TADF emitters have more effective charge transfer between donor and acceptor units than bipolar host molecules. More efficient conjugation effects between the donor and acceptor units of host materials were identified from the lower dihedral angles of the D-A structure, smaller and even negative charge transfer amount, shorter charge-transfer length, and larger hole-electron overlap extent. These findings with in-depth insights into different interaction models of donor and acceptor units shed important light on the molecular design of TADF emitters and bipolar materials in a D-A architecture.

A perspective on next-generation hyperfluorescent organic light-emitting diodes.

Hyperfluorescence, also known as thermally activated delayed fluorescence (TADF) sensitized fluorescence, is known as a next-generation efficient and innovative process for high-performance organic light-emitting diodes (OLEDs). High external quantum efficiency (EQE) and good color purity are crucial parameters for display applications. Hyperfluorescent OLEDs (HF-OLEDs) take the lead in this respect as they utilize the advantages of both TADF emitters and fluorescent dopants, realizing high EQE with color saturation and long-term stability. Hyperfluorescence is mediated through Förster resonance energy transfer (FRET) from a TADF sensitizer to the final fluorescent emitter. However, competing loss mechanisms such as Dexter energy transfer (DET) of triplet excitons and direct charge trapping on the final emitter need to be mitigated in order to achieve fluorescence emission with high efficiency. Despite tremendous progress, appropriate guidelines and fine optimization are still required to address these loss channels and to improve the device operational lifetime. This perspective aims to provide an overview of the evolution of HF-OLEDs by reviewing both molecular and device design pathways for highly efficient narrowband devices covering all colors of the visible spectrum. Existing challenges and potential solutions, such as molecules with peripheral inert substitution, multi-resonant (MR) TADF emitters as final dopants, and exciplex-sensitized HF-OLEDs, are discussed. Furthermore, the operational device lifetime is reviewed in detail before concluding with suggestions for future device development.

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.

Narrowband TADF emitters with high utilization of triplet excitons: theoretical insights and molecular design.

Narrowband emitters with thermally activated delayed fluorescence (TADF) features, known as multi-resonant TADF (MR-TADF) emitters, are drawing increasing research interest owing to their properties of high efficiency and excellent color purity. However, MR-TADF-based devices often face serious efficiency roll-off at high luminance intensity, which could be attributed to undesired triplet-triplet annihilation (TTA) caused by the structural planarity and relatively small reverse intersystem crossing rate constants (krisc) of MR-TADF emitters. Herein, combining a sp3-C inserted strategy to suppress harmful bimolecular interactions and chalcogens to improve the krisc, a series of asymmetric narrowband emitters, namely, DMAC-O, DMAC-S, and DMAC-Se, have been theoretically designed to break the slow rate-limiting step of krisc of experimental BN-DMAC. For comparison, both O and Se atoms were doped into the MR skeleton to substitute two sp3-inserted units, yielding BN-O-Se. The combination of TD-DFT and the wavefunction-based STEOM-DLPNO-CCSD approach exhibits that those asymmetric molecules are promising for simultaneously exhibiting narrow emission spectral full-width at half-maximums (FWHMs) and high luminous efficiencies. The contributions of chalcogens to hole distributions result in red-shifted fluorescent peaks, and the asymmetric strategy also helps with twisted molecular configuration, which is beneficial for suppressing unfavorable TTA. Furthermore, the incorporation of chalcogens is sufficient to promote the intersystem crossing and reverse intersystem crossing channels of asymmetric emitters. More importantly, the doped heavy Se atom results in a significantly increased krisc of 2.32 × 106 s-1 for DMAC-Se, which is more than 200 times larger than 1.09 × 104 s-1 of pristine BN-DMAC. These results suggest that the combination of the heavy Se atom and an sp3-inserted unit is a feasible strategy for achieving poor planarity and significantly enhancing krisc, which will help in harvesting triplet excitons, thereby inhibiting efficiency roll-off in corresponding narrowband devices.

Theoretical design and performance prediction of deep red/near-infrared thermally activated delayed fluorescence molecules with through space charge transfer.

Thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) have attracted much attention in recent years because of their ability to simultaneously reduce the energy difference (ΔEST) and enlarge the spin-orbit coupling (SOC). In this paper, 40 molecules are theoretically designed by changing the different substitution positions of the donors and acceptors, and systematically investigated based on the first-principles calculations and excited-state dynamics study. It is found that the emission wavelengths of v-shaped molecules with intramolecular TSCT are larger than those of the molecules without TSCT. Therefore, the intramolecular TSCT can induce the red-shift of the emission and realize the deep-red/near-infrared emission. Besides intramolecular TSCT can simultaneously increase the SOC as well as the oscillator strength and reduce the ΔEST. In addition, PXZ or PTZ can also favor the realization of smaller ΔEST and red-shift emission. Our calculations suggest that intramolecular TSCT and suitable donors (-PXZ or -PTZ) are an effective strategy for the design of efficient deep red/near-infrared TADF emitters.

Efficient orange and red thermally activated delayed fluorescence materials based on 1,8-naphthalimide derivatives.

Thermally activated delayed fluorescence (TADF) molecules have emerged as a promising class of third-generation organic light-emitting diode (OLED) emitters that can achieve 100% internal quantum efficiency without the use of noble metals. However, the design of high-efficiency red TADF materials has been challenging due to limitations imposed by the energy-gap law. To overcome this challenge, two new TADF emitters, namely, 6-(4-(diphenylamino)phenyl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (NI-TPA) and 6-(10H-phenothiazin-10-yl)-2-phenyl-1H-benzo[de]-isoquinoline-1,3(2H)-dione (NI-Pz), have been synthesized and characterized. These compounds exhibit strong TADF characteristics with a small energy gap (ΔEST) between the lowest excited singlet and triplet states, short delayed fluorescence lifetimes, high thermal stability, and high photoluminescence quantum yields. The OLED devices fabricated using NI-TPA and NI-Pz as emitters show orange and red electroluminescence with emission peaks at 593 nm and 665 nm, respectively, and maximum external quantum efficiencies (EQEs) of 11.3% and 7.6%, respectively. Furthermore, applying NI-TPA to cell imaging yielded excellent imaging results, indicating the potential of red TADF materials in the field of biological imaging.