Versatile Molecular Functionalization for Inhibiting Concentration Quenching of Thermally Activated Delayed Fluorescence

Benchmarking Charge-Transfer Excited States in TADF Emitters: ΔDFT Outperforms TD-DFT for Emission Energies

Charge-transfer (CT) excited states are crucial to organic light-emitting diodes (OLEDs), particularly to those based on thermally activated delayed fluorescence (TADF). However, accurately modeling CT states remains challenging, even with modern implementations of (time-dependent) density functional theory [(TD-)DFT], especially in a dielectric environment. To identify shortcomings and improve the methodology, we previously established the STGABS27 benchmark set with highly accurate experimental references for the adiabatic energy gap between the lowest singlet and triplet excited states (ΔEST). Here, we diversify this set to the STGABS27-EMS benchmark by including experimental emission energies (Eem) and use this new set to (re)-evaluate various DFT-based approaches. Surprisingly, these tests demonstrate that a state-specific (un)restricted open-shell Kohn-Sham (U/ROKS) DFT coupled with a polarizable continuum model for perturbative state-specific nonequilibrium solvation (ptSS-PCM) provides exceptional accuracy for predicting Eem over a wide range of density functionals. In contrast, the main workhorse of the field, Tamm-Dancoff-approximated TD-DFT (TDA-DFT) paired with the same ptSS-PCM, is distinctly less accurate and strongly functional-dependent. More importantly, while TDA-DFT requires the choice of two very different density functionals for good performance on either ΔEST or Eem, the time-independent U/ROKS/PCM approaches deliver excellent accuracy for both quantities with a wide variety of functionals.

Modulation of Intermolecular Interactions in Organic Emitters for Highly Efficient Organic Light-Emitting Diodes

The adverse aggregated-caused quenching (ACQ) problem of most electroluminescent materials existing in highly doped thin films is one of the key factors impeding the commercialization of high-efficiency organic light-emitting diodes (OLEDs) panel. Whereas, by delicately constructing and modulating moderate intermolecular interactions, some aggregates have been demonstrated to present distinct luminescent properties such as tunable emission spectra, improved photoluminescence quantum yields, different emission mechanism and enhanced horizontal transition dipole ratio (Θ) of emitting layer, providing feasible solution for ACQ problem. The luminescence from newly generated emissive state in aggregates is different from the traditional "isolated" molecules in organic electronics and will possess novel properties and applications. Herein, we summarize the different types of intermolecular interactions within emitter aggregates exhibiting distinct luminescent mechanisms, as well as their effects on photoluminescent and electroluminescent properties, offering reliable reference for the advancement of highly efficient OLEDs utilizing aggregated emitters.

Purely Organic Room-Temperature Phosphorescence Molecule for High-Performance Non-Doped Organic Light-Emitting Diodes

Purely organic molecules with room-temperature phosphorescence (RTP) are potential luminescent materials with high exciton utilization for organic light-emitting diodes (OLEDs), but those exhibiting superb electroluminescence (EL) performances are rarely explored, mainly due to their long phosphorescence lifetimes. Herein, a robust purely organic RTP molecule, 3,6-bis(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)-xanthen-9-one (3,2-PIC-XT), is developed. The neat film of 3,2-PIC-XT shows strong green RTP with a very short lifetime (2.9 μs) and a high photoluminescence quantum yield (72 %), and behaviors balanced bipolar charge transport. The RTP nature of 3,2-PIC-XT is validated by steady-state and transient absorption and emission spectroscopies, and the working mechanism is deciphered by theoretical simulation. Non-doped multilayer OLEDs using thin neat films of 3,2-PIC-XT furnish an outstanding external quantum efficiency (EQE) of 24.91 % with an extremely low roll-off (1.6 %) at 1000 cd m-2. High-performance non-doped top-emitting and tandem OLEDs are also achieved, providing remarkable EQEs of 24.53 % and 42.50 %, respectively. Delightfully, non-doped simplified OLEDs employing thick neat films of 3,2-PIC-XT are also realized, furnishing an excellent EQE of 17.79 % and greatly enhanced operational lifetime. The temperature-dependent and transient EL spectroscopies demonstrate the electrophosphorescence attribute of 3,2-PIC-XT. These non-doped OLEDs are the best devices based on purely organic RTP materials reported so far.

Promising interlayer sensitization strategy for the construction of high-performance blue hyperfluorescence OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are promising candidates for organic light-emitting diodes (OLEDs) with narrow electroluminescence (EL) spectra. Current researches focus on fabricating hyperfluorescence OLEDs to improve EL efficiencies of MR-TADF emitters by co-doping them with TADF sensitizers in a single host layer. However, in many cases, the polarity of the single host could be not suitable for both blue MR-TADF emitters and blue TADF sensitizers, resulting in broadened EL spectra in high-polar hosts or decreased EL efficiencies in low-polar hosts. Herein, we wish to report an efficient sensitization strategy for blue MR-TADF emitters by constructing an interlayer-sensitizing configuration, in which the blue TADF sensitizers and blue MR-TADF emitters are separated into two closely aligned host layers with high polarity and low polarity, respectively. Based on this strategy, efficient blue hyperfluorescence OLEDs are realized and verified by employing various TADF sensitizers and different MR-TADF emitters, furnishing outstanding external quantum efficiencies of up to 38.8% and narrow EL spectra. These results validate the feasibility and universality of this interlayer sensitization strategy, which provides an effective alternative to high-performance blue hyperfluorescence OLEDs.

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.

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.

Concentration Quenching of Fluorescence Decay Kinetics of Molecular Systems

Fluorescence concentration quenching occurs when increasing molecular concentration of fluorophores results in a decreasing fluorescence quantum yield. Even though this phenomenon has been studied for decades, its mechanisms and signatures are not yet fully understood. The complexity of the problem arises due to energy migration and trapping in huge networks of molecules. Most of the available theoretical work focuses on integral quantities like fluorescence quantum yield and mean excitation lifetime. In this work, we present a numerical study of the fluorescence decay kinetics of three-dimensional and two-dimensional molecular systems. We investigate the differences arising from the variations in models of trap formations. We also analyze the influence of the molecular orientations to the fluorescence decay kinetics. We compare our results to the well-known analytical models and discuss their ranges of validity. Our findings suggest that the analytical models can provide inspiration for different ways of approximating the fluorescence kinetics, yet more detailed analysis of the experimental data should be done by comparison with numerical simulations.

Deep Learning-Based Chemical Similarity for Accelerated Organic Light-Emitting Diode Materials Discovery

Thermally activated delayed fluorescence (TADF) material has attracted great attention as a promising metal-free organic light-emitting diode material with a high theoretical efficiency. To accelerate the discovery of novel TADF materials, computer-aided material design strategies have been developed. However, they have clear limitations due to the accessibility of only a few computationally tractable properties. Here, we propose TADF-likeness, a quantitative score to evaluate the TADF potential of molecules based on a data-driven concept of chemical similarity to existing TADF molecules. We used a deep autoencoder to characterize the common features of existing TADF molecules with common chemical descriptors. The score was highly correlated with the four essential electronic properties of TADF molecules and had a high success rate in large-scale virtual screening of millions of molecules to identify promising candidates at almost no cost, validating its feasibility for accelerating TADF discovery. The concept of TADF-likeness can be extended to other fields of materials discovery.

Exploring Robust Delayed Fluorescence Materials via Structural Rigidification for Realizing Organic Light-Emitting Diodes with High Efficiencies and Small Roll-Offs

Thermally activated delayed fluorescence (TADF) materials have been widely studied for the fabrication of high-performance organic light-emitting diodes (OLEDs), but the serious efficiency roll-offs still remain unsolved in most cases. Herein, it is wish to report a series of robust green TADF compounds containing rigid xanthenone acceptor and acridine-based spiro donors. The enhancement in molecular rigidity not only endows the compounds with improved thermal stability but also results in reduced geometric vibrations and thus lowered reorganization energies. These compounds exhibit distinct merits of high thermal stabilities, excellent photoluminescence quantum efficiencies (96%-97%), large horizontal dipole orientation ratios (87.4%-92.1%) and fast TADF rates (1.4-1.5 × 106 s-1 ). The OLEDs using them as emitters furnish superb electroluminescence performances with outstanding external quantum efficiencies (ηext s) of up to 37.4% and very small efficiency roll-offs. Moreover, highly efficient hyperfluorescence OLEDs are obtained by using them as sensitizers for the green mutilresonance TADF emitter BN2, delivering excellent ηext s of up to 34.2% and improved color purity. These results disclose the high potential of these TADF compounds as emitters and sensitizers for OLEDs.

Multi-resonance thermally activated delayed fluorescence molecules with intramolecular-lock: theoretical design and performance prediction

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules with narrow full width at half maximum (FWHM) have attracted much attention recently. In this work, 36 borane/amine (B/N) type MR-TADF molecules were theoretically designed by using an intramolecular-lock strategy and systematically studied based on first-principles calculations. It was found that intramolecular-lock at different positions and in different manners could induce different luminescent properties. The calculated oscillator strengths for PXZ-L2 and PTZ-L2 locking systems are weaker than that for 2DPABN (without intramolecular-lock), while the Cz-L1 and TMCZ-L1 locking could result in stronger oscillator strength. Though the calculated FWHM of all the systems with intramolecular-lock is higher than that of 2DPABN, the Cz, TMCz and DMAC locking at L1 or L2 would induce relatively small FWHM which is comparable to that of 2DPABN. Our calculation results indicate that intramolecular-lock could enhance the SOC values and decrease the energy gap between the first singlet excited state and the first triplet excited state, which is quite favorable to reverse intersystem crossing. The Cz, TMCz and DMAC locking systems could realize comparable and higher efficiency than 2DPABN, thus higher quantum efficiency could be obtained. Our calculation results indicate that the intramolecular-lock strategy is an effective method to realize the design of highly efficient MR-TADF emitters.

Positional Isomeric Cyano-Substituted Bis(2-phenylpyridine)(acetylacetonate)iridium Complexes for Efficient Organic Light-Emitting Diodes with Extended Color Range

Bis(2-phenylpyridine)(acetylacetonate)iridium, Ir(ppy)2(acac), is a benchmark green emitter for phosphorescent organic light-emitting diodes (PhOLEDs). In this work, we reported three positional isomeric cyano-substituted Ir(ppy)2(acac) complexes, i.e., Ir(3-CN), Ir(4-CN), and Ir(10-CN), with the emission in the yellow to red region (544-625 nm). Through theoretical investigation and single-crystal analysis, it was found that the introduction of cyano substitution at various positions of the ppy ligand allows for tuning the electron distribution and coordination bond length of Ir complexes. Therefore, the charge transfer property of Ir complexes is enhanced such that the energy gap of the cyano-substituted Ir(ppy)2(acac) complexes was reduced. In addition, Ir(3-CN), Ir(4-CN), and Ir(10-CN) exhibited high PLQYs of 83, 54, and 75%, respectively, with the phosphorescence lifetime in the range of 0.79-2.08 μs. Notably, the device utilizing Ir(3-CN) as the emitter exhibited a maximum external quantum efficiency (EQE) of 25.4%, current efficiency of 56.9 cd A-1, power efficiency of 68.7 lm W-1, and brightness of 61,340 cd m-2 at 8 V. The EQE of this device remained 24.3 and 19.9% at luminances of 1,000 and 10,000 cd m-2, corresponding to the efficiency roll-off of 4.3 and 21.7%, respectively. Comparing to the Ir complexes using the ligand with an extended conjugated structure, our results demonstrated a simple molecular design strategy for phosphorescence emitters with reduced molecular weight for efficient PhOLEDs in the yellow to red color region.

Rational Multidimensional Shielded Multiple Resonance Emitter Suppresses Concentration Quenching and Spectral Broadening for Solution-Processed Organic Light-Emitting Diodes

Thermally activated delayed fluorescence (TADF) emitters based on multiple resonance (MR) effects are promising for high-definition organic light-emitting diodes (OLEDs) with narrowband emission and high efficiency. However, they still face the challenges of aggregation-caused quenching (ACQ) and spectral broadening. Solution-processable MR-TADF emitters with an external quantum efficiency (EQE) of >20% and a full width at half-maximum (fwhm) of <30 nm have rarely been reported. To construct ACQ-resistant emitters without sacrificing color purity, the aggregation-induced MR-TADF material 6TBN with a rigid B,N-containing polycyclic aromatic hydrocarbon core and four carbazole substituents as well as 12 tert-butyl groups on the periphery is designed. The multidimensional shielded effect largely limits the ACQ, intermolecular interactions, and spectral broadening. Consequently, solution-processed OLEDs based on 6TBN exhibit a maximum EQE of 23.0% and high color purity with a fwhm of 25 nm. Furthermore, the nondoped device achieves a high efficiency (12.3%) and merely a slight widening of the fwhm to 27 nm. This work provides a feasible strategy to achieve MR-TADF materials with resistance to concentration quenching and high color purity.

Sterically wrapping of multi-resonant fluorophores: an effective strategy to suppress concentration quenching and spectral broadening

Multiple resonance (MR) emitters are promising for the next-generation wide color gamut organic light-emitting diodes (OLEDs) with narrowband emissions; however, they still face intractable challenges such as concentration-induced emission quenching, exciton annihilation, and spectral broadening. In this concept, we focus on an advanced molecular design strategy called "sterically wrapping of MR fluorophores" to address the above issues. By isolating the MR emission core using bulky substituents, intermolecular interactions can be significantly suppressed to eliminate the formation of unfavorable species. Consequently, using the newly designed emitters, optimized MR-OLEDs can achieve high external quantum efficiencies of >40% while maintaining extremely small full width at half maxima (FWHMs) of <25 nm over a wide range of concentrations (1-20 wt%). This strategy may shed light on the design of efficient MR emitters, which provides more room for tuning the dopant concentrations under the premise of high-efficiencies and small FWHMs, accelerating the practical application of MR-OLEDs.

Efficient pure blue hyperfluorescence devices utilizing quadrupolar donor-acceptor-donor type of thermally activated delayed fluorescence sensitizers

The hyperfluorescence (HF) system has drawn great attention in display technology. However, the energy loss mechanism by low reverse intersystem crossing rate (k_RISC) and the Dexter energy transfer (DET) channel is still challenging. Here, we demonstrate that this can be mitigated by the quadrupolar donor-acceptor-donor (D-A-D) type of thermally activated delayed fluorescence (TADF) sensitizer materials, DBA-DmICz and DBA-DTMCz. Further, the HF device with DBA-DTMCz and ν-DABNA exhibited 43.9% of high maximum external quantum efficiency (EQE_max) with the Commission Internationale de l'Éclairage coordinates of (0.12, 0.16). The efficiency values recorded for the device are among the highest reported for HF devices. Such high efficiency is assisted by hindered DET process through i) high k_RISC, and ii) shielded lowest unoccupied molecular orbital with the presence of two donors in D-A-D type of skeleton. Our current study provides an effective way of designing TADF sensitizer for future HF technology.

Optimal Dihedral Angle in Twisted Donor-Acceptor Organic Emitters for Maximized Thermally Activated Delayed Fluorescence

The twisted donor-acceptor (D-A) organic formwork with a large dihedral angle (θ_DA ) is usually adopted to narrow the singlet-triplet energy gap for obtaining excellent thermally activated delayed fluorescence (TADF) emitters. However, the dependence of overall TADF properties on θ_DA has not been systematically investigated to this day. Taking new designed CzBP, CzBP-1M and CzBP-2M via introducing methyl as investigated models, it is found that (i) with increasing θ_DA , the charge transfer component in S₁ is larger than that in T₁ in varying degrees, leading to non-monotonic spin-orbit couplings; (ii) the electron-vibration couplings between S₁ and T₁ states become the largest when θ_DA approaching 80°, facilitating phonon-driven up-conversion; (iii) the overall TADF rate reaches a peak at θ_DA ≈80°. By this, the TADF on/off switching is realized via methyl moiety for regulating θ_DA from theoretical prediction to experimental confirmation. Importantly, the θ_DA near 80° would be a good descriptor for screening excellent D-A type TADF emitters.

Either Accurate Singlet-Triplet Gaps or Excited-State Structures: Testing and Understanding the Performance of TD-DFT for TADF Emitters

The energy gap between the lowest singlet and triplet excited states (ΔE_ST) is a key property of thermally activated delayed fluorescence (TADF) emitters, where these states are dominated by charge-transfer (CT) character. Despite its well-known shortcomings concerning CT states, time-dependent density functional theory (TD-DFT) is widely used to predict this gap and study TADF. Moreover, polar CT states exhibit a strong interaction with their molecular environment, which further complicates their computational description. Addressing these two major challenges, this work studies the performance of Tamm-Dancoff-approximated TD-DFT (TDA-DFT) on the recent STGABS27 benchmark set,¹ exploring different strategies to include orbital and structural relaxation, as well as dielectric embedding. The results show that the best-performing strategy is to calculate ΔE_ST at the ground-state structure using functionals with a surprisingly small amount of Fock exchange of ≈10% and without a (complete) solvent model. However, as this approach heavily relies on error cancellation to mimic dielectric relaxation, it is not robust and exhibits large systematic deviations in excited state energies, state characters, and structures. More rigorous approaches, including state-specific solvation, do not share these systematic deviations, but their predicted ΔE_ST values exhibit larger statistical errors. We thus conclude that for the description of CT states in dielectric environments, none of the tested TDA-DFT methods is competitive with the recently presented ROKS/PCM approach regarding robustness, accuracy, and computational efficiency.

Substituent engineering of the diboron molecular architecture for a nondoped and ultrathin emitting layer

Owing to the high technology maturity of thermally activated delayed fluorescence (TADF) emitter design with a specific molecular shape, extremely high-performance organic light-emitting diodes (OLEDs) have recently been achieved via various doping techniques. Recently, undoped OLEDs have drawn immense attention because of their manufacturing cost reduction and procedure simplification. However, capable materials as host emitters are rare and precious because general fluorophores in high-concentration states suffer from serious aggregation-caused quenching (ACQ) and undergo exciton quenching. In this work, a series of diboron materials, CzDBA, iCzDBA, and tBuCzDBA, is introduced to realize the effect of steric hindrance and the molecular aspect ratio via experimental and theoretical studies. We computed transition electric dipole moment (TEDM) and molecular dynamics (MD) simulations as a proof-of-concept model to investigate the molecular stacking in neat films. It is worth noting that the pure tBuCzDBA film with a high horizontal ratio of 92% is employed to achieve a nondoped OLED with an excellent external quantum efficiency of 26.9%. In addition, we demonstrated the first ultrathin emitting layer (1 nm) TADF device, which exhibited outstanding power efficiency. This molecular design and high-performance devices show the potential of power-saving and economical fabrication for advanced OLEDs.

Diabatic Decomposition Perspective on the Role of Charge Transfer and Local Excitations in Thermally Activated Delayed Fluorescence

Thermally activated delayed fluorescence (TADF) is a phenomenon that relies on the upconversion of triplet excitons to singlet excitons by means of reverse intersystem crossing (rISC). It has been shown both experimentally and theoretically that the TADF mechanism depends on the interplay between charge transfer and local excitations. However, the difference between the diabatic and adiabatic character of the involved excited states is rarely discussed in the literature. Here we develop a diabatization procedure to implement a four-state model Hamiltonian to a set of TADF molecules. We provide physical interpretations of the Hamiltonian elements and show their dependence on the electronic state of the equilibrium geometry. We also demonstrate how vibrations affect the TADF efficiency by modifying the diabatic decomposition of the molecule. Finally, we provide a simple model that connects the diabatic Hamiltonian to the electronic properties relevant to TADF and show how this relationship translates into different optimization strategies for rISC, fluorescence, and overall TADF performance.

Distinguishing the respective determining factors for spectral broadening and concentration quenching in multiple resonance type TADF emitter systems

Multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters have attracted much recent attention due to their narrow emission spectra and high photoluminescence quantum yields (PLQYs). Spectral broadening and concentration quenching at high doping concentrations are two issues currently limiting the development of MR-TADF emitters. However, the origins of these have not been fully clarified so far. In this work, by investigating emitters with the same MR cores but peripheral groups of different steric types, we distinguished that the spectral broadening and concentration quenching are mainly caused by excimer formation and triplet exciton annihilation, respectively. This understanding on aggregated behaviors of MR emitters provides new insight for the further development of high-performance MR-TADF emitters with low concentration sensitivities.

Highly efficient and nearly roll-off-free electrofluorescent devices via multiple sensitizations

The efficiency roll-off at high luminance has hindered the wide application of organic light-emitting diodes (OLEDs) for decades. To circumvent this issue, both high exciton utilization and short exciton residence should be satisfied, which, however, faces formidable challenges. Here, we propose an advanced approach of phosphor-assisted thermally activated delayed fluorophor (TADF)-sensitized fluorescence, abbreviated as TPSF. It is proved to be a rational strategy that can realize high quantum efficiency and elaborately accelerated radiative exciton consumption simultaneously by breaking singlet-triplet spin-flip cycles on a TADF host via multiple sensitizations. On the basis of a TADF molecule exhibiting anti-accumulation-caused quenching character, a proof-of-concept device exhibits a maximum external quantum efficiency (EQE_max) of 24.2% with an ultrahigh L_90% (the luminance at which EQE drops to 90% of its maximum value) of 190,500 cd m^-2 and a greatly improved operational stability, unlocking the full potential of OLEDs for ultrahigh-luminance applications.

Novel Deep Red Thermally Activated Delayed Fluorescence Molecule with Aggregation-Induced Emission Enhancement: Theoretical Design and Experimental Validation

Thermally activated delayed fluorescence (TADF) molecules with deep red luminescence have shown great applications in organic light-emitting diodes (OLEDs). However, the development of high efficient deep red TADF emitters is full of resistance, and new design strategies are highly desired. This work theoretically predicts the luminescence properties and photophysical mechanism of a spiro-acridine based molecule DBPz-2spAc in toluene and aggregation states. Experiments further show that the solid state can effectively suppress nonradiative energy loss and thus improve luminescence efficiency. OLEDs based on DBPz-2spAc show high luminescence efficiency. In addition, studies based on spiro-acridine derivatives indicate that bending the degree of acridine in excited state will directly affect the nonradiative energy loss. This study improves the understanding of the luminous behavior of spiro-acridine derivative based TADF emitters in solution and in aggregation state, which should pave the way for the design of efficient deep red TADF materials.

Boron-Based Multi-Resonance TADF Emitter with Suppressed Intermolecular Interaction and Isomer Formation for Efficient Pure Blue OLEDs

Multi-resonance (MR) thermally activated delayed fluorescent (TADF) emitters are highly attractive due to their superior color purity as well as efficient light-harvesting ability from singlets and triplets. However, boron and nitrogen-based MR-TADF emitters suffer from their strong π-π interaction owing to their rigid flat cores. Herein, a boron-based multi-resonance blue TADF emitter with suppressed intermolecular interaction and isomer formation is developed through a simple synthetic process by introducing meta-xylene and meta-phenyphenyl groups to the core. The MR-TADF emitter, mBP-DABNA-Me, shows a narrowband blue emission with a peak at 467 nm, along with full width at half maximum of 28 nm, and photoluminescence quantum yield of 97%. Notably, highly efficient pure blue organic light-emitting diode (OLED) is realized using mBP-DABNA-Me, showing a maximum external quantum efficiency of 24.3% and a stable blue emission with a Commission Internationale de L'Eclairage coordinate of (0.124, 0.140). The color purity of the OLED is maintained at a high doping concentration of over 20%, attributed to the suppressed intermolecular interaction between the MR emitters.

Achieving Record Efficiency and Luminance for TADF Light-Emitting Electrochemical Cells by Dopant Engineering

Thermally activated delayed fluorescence (TADF) light-emitting electrochemical cells (TADF-LECs) are appealing due to their simple sandwich structure and potential applications in wearable displays and sensors. However, achieving high performance remains challenging. In this paper, we demonstrate that the use of TADF emitters with a low aggregated-caused quenching (ACQ) tendency is crucial to address this challenge. To verify it, two types of TADF-LECs are compared in parallel using different kinds of TADF emitters. The control device uses 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as the dopant, which suffers from a serious ACQ issue and thus dramatically limits the doping concentrations of 4CzIPN in these TADF-LECs. At the best doping condition (0.5 wt %), insufficient host-to-dopant energy transfer (ET) does exist, thereby displaying very limited efficiency and luminance, i.e., 2.43% and 1483 cd m^-2. By contrast, the TADF-LECs using 3,6-di(tert-butyl)-1,8-di(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl) carbazole (BPAPTC) can tolerate a much higher doping concentration because BPAPTC is a satisfactory TADF emitter featuring a low ACQ tendency. At the optimized doping condition of 18 wt %, the BPAPTC-based emissive layer possesses the best TADF property, including the longest τ_DF (2646 ns), the largest r_DF (69%), and the highest k_RISC of 7.50 × 10⁵ s^-1. Moreover, the corresponding TADF-LEC simultaneously displays the most efficient host-to-dopant ET. It thus achieves unprecedented performance, e.g., the highest external quantum efficiency (EQE_max.) of 7.6%, the highest luminance (L_max.) of 3696 cd m^-2, and an EQE of 7.01% at a practical high luminance of 1000 cd m^-2.

Highly Efficient and Stable Blue Organic Light-Emitting Diodes based on Thermally Activated Delayed Fluorophor with Donor-Void-Acceptor Motif

Thermally activated delayed fluorophores (TADF) with donor-acceptor (D-A) structures always face strong conjugation between donor and acceptor segments, rendering delocalized new molecular orbitals that go against blue emission. Developing TADF emitters with blue colors, high efficiencies, and long lifetimes simultaneously is therefore challenging. Here, a D-void-A structure with D and A moieties connected at the void-position where the frontier orbital from donor and acceptor cannot be distributed, resulting in nonoverlap of the orbitals is proposed. A proof-of-the-concept TADF emitter with 3,6-diphenyl-9H-carbazole (D) connected at the 3'3-positions of 9H-xanthen-9-one (A), the void carbon-atom with no distribution of the highest occupied molecular orbital (HOMO) of A-segment, realizes more efficient and blue-shifted emission compared with the contrast D-A isomers. The deeper HOMO-2 of A is found to participate into conjugation rather than HOMO, providing a wider-energy-gap. The corresponding blue device exhibits a y color coordinate (CIE_y ) of 0.252 and a maximum external quantum efficiency of 27.5%. The stability of this compound is further evaluated as a sensitizer for a multiple resonance fluorophore, realizing a long lifetime of ≈650 h at an initial luminance of 100 cd m^-2 with a CIE_y of 0.195 and a narrowband emission with a full-width-at-half-maxima of 21 nm.

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

Significant role of spin-triplet state for exciton dissociation in organic solids

Clarification of the role of the spin state that initiates exciton dissociation is critical to attaining a fundamental understanding of the mechanism of organic photovoltaics. Although an excited spin-triplet state with an energy lower than that of excited spin-singlet state is disadvantageous in exciton dissociation, a small electron exchange integral results in small singlet-triplet energy splitting in some material systems. This energy splitting leads to a nearly isoenergetic alignment of both excited states, raising a question about the role of excited spin states in exciton dissociation. Herein, we show that the spin-triplet rather than the spin-singlet plays a critical role in the exciton dissociation that leads to the formation of free carriers. This result indicates that the spin-triplet inherently acts as an intermediate, leading to exciton dissociation. Thus, our demonstration provides a fundamental understanding of the role of excited spin states of organic molecular systems in photoinduced charge-carrier generation.

Research Progress on Triarylmethyl Radical-Based High-Efficiency OLED

Perchlorotrityl radical (PTM), tris (2,4,6-trichlorophenyl) methyl radical (TTM), (3,5-dichloro-4-pyridyl) bis (2,4,6 trichlorophenyl) methyl radical (PyBTM), (N-carbazolyl) bis (2,4,6-trichlorophenyl) methyl radical (CzBTM), and their derivatives are stable organic radicals that exhibit light emissions at room temperature. Since these triarylmethyl radicals have an unpaired electron, their electron spins at the lowest excited state and ground state are both doublets, and the transition from the lowest excited state to the ground state does not pose the problem of a spin-forbidden reaction. When used as OLED layers, these triarylmethyl radicals exhibit unique light-emitting properties, which can increase the theoretical upper limit of the OLED's internal quantum efficiency (IQE) to 100%. In recent years, research on the luminescent properties of triarylmethyl radicals has attracted increasing attention. In this review, recent developments in these triarylmethyl radicals and their derivatives in OLED devices are introduced.

Photophysical and Electrical Properties of Highly Luminescent 2/6-Triazolyl-Substituted Push-Pull Purines

New push-pull N(9)-alkylated 6-piperidino-2-triazolylpurine and 2-piperidino-6-triazolylpurine derivatives are synthesized, and their optical and optoelectronic properties are comprehensively characterized with experimental and computational methods. The compounds possess intense violet or blue fluorescence with fluorescence quantum yields of up to 91% in solution and 40% in host-free films. Depending on their structural composition, the compounds have ionization energy in the range of 5.25-6.04 eV, electron affinity of 2.18-3.15 eV, and triplet energy of 2.52-2.95 eV. Due to the presence of hole-transporting purine and electron-transporting triazole fragments, compounds exhibit bipolar charge-transportation ability. Despite the favorable emissive properties of the studied push-pull purines, their electroluminescence in thin films is quenched owing to large current densities that are present even at a moderate driving voltage. This marks application directions related to a predominantly charge-transportation functionality as the most suitable for this compound class.

Photophysical Properties of Benzophenone-Based TADF Emitters in Relation to Their Molecular Structure

Thermally activated delayed fluorescence (TADF) materials are commonly used in various apparatus, including organic light-emitting device-based displays, as they remarkably improve the internal quantum efficiencies. Although there is a wide range of donor-acceptor-based compounds possessing TADF properties, in this computational study, we investigated TADF and some non-TADF chromophores, containing benzophenone or its structural derivatives as the acceptor core, together with various donor moieties. Following the computational modeling of the emitters, several excited state properties, such as the absorption spectra, singlet-triplet energy gaps (ΔE_ST), natural transition orbitals, and the topological Φ_S indices, have been computed. Along with the donor-acceptor torsion angles and spin-orbit coupling values, these descriptors have been utilized to investigate potential TADF efficiency. Our study has shown that on the one hand, our photophysical/structural descriptors and computational methodologies predict the experimental results quite well, and on the other hand, our extensive benchmark can be useful to pinpoint the most promising functionals and descriptors for the study of benzophenone-based TADF emitters.

A scaffold of thermally activated delayed fluorescent polymer dots towards aqueous electrochemiluminescence and biosensing applications

Thermally activated delayed fluorescent (TADF) polymer dots were prepared, which enables aqueous electrochemiluminescence of TADF polymer emitters and its biosensor application for the first time.

Quenching-Resistant Multiresonance TADF Emitter Realizes 40% External Quantum Efficiency in Narrowband Electroluminescence at High Doping Level

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters manifest great potential for organic light-emitting diodes (OLEDs) due to their high exciton-utilization efficiency and narrowband emission. Nonetheless, their tendency toward self-quenching caused by strong interchromophore interactions would induce doping sensitivity and deteriorate the device performances, and effective strategy to construct quenching-resistant emitters without sacrifycing color purity is still to be developed. By segregating the planar MR-TADF skeleton using two bulky carbazolyl units, herein a highly emissive molecule with enhanced quenching resistance is reported. The steric effect largely removes the formation of detrimental excimers/aggregates, and boosts the performance of the corresponding devices with a maximum external quantum efficiency (EQE_max ) up to 40.0% and full width at half maximum (FWHM) of 25 nm, representative of the only example of single OLED that can concurrently achieve narrow bandwidth and high EL efficiency surpassing 40% to date. Even at doping ratio of 30 wt%, the EQE_max is retained to be 33.3% with nearly unchanged emission spectrum. This work provides a viable approach to realize doping-insensitive MR-TADF devices with extreme EL efficiency and color purity for high-end OLED displays.

9,10-Phenanthrenequinone: A Promising Kernel to Develop Multifunctional Antitumor Systems for Efficient Type I Photodynamic and Photothermal Synergistic Therapy

Synergistic phototherapy provides a promising strategy to conquer the hypoxia and heterogeneity of tumors and realize a better therapeutic effect than monomodal photodynamic therapy (PDT) or photothermal therapy (PTT). The development of efficient multifunctional organic phototheranostic systems still remains a challenging task. Herein, 9,10-phenanthrenequinone (PQ) with strong electron-withdrawing ability is conjugated with the rotor-type electron-donating triphenylamine derivatives to create a series of tailor-made photosensitizers. The highly efficient Type I reactive oxygen species generation and outstanding photothermal conversion capacity are tactfully integrated into these PQ-cored photosensitizers. The underlying photophysical and photochemical mechanisms of the combined photothermal and Type I photodynamic effects are deciphered by experimental and theoretical methods and are closely associated with the active intramolecular bond stretching vibration, facilitated intersystem crossing, and specific redox cycling activity of the PQ core. Both in vitro and in vivo evaluations demonstrate that the nanoagents fabricated by these PQ-based photosensitizers are excellent candidates for Type I photodynamic and photothermal combined antitumor therapy. This study thus broadens the horizon for the development of high-performance PTT/Type I PDT nanoagents for synergistic phototheranostic treatments.

Conformational disorder enabled emission phenomena in heavily doped TADF films

Thermally activated delayed fluorescence (TADF) compounds doped in solid hosts are prone to undergo solvation effects, similar to those in the solution state. Emission peak shifts and changes in emission decay rates usually follow solid-state solvation (SSS). However, here we show that typical SSS behavior in heavily doped TADF films could be of a completely different origin, mistakenly attributed to SSS. Typically, increasing the doping load was found to redshift the emission peak wavelength and enhance the rISC rate. However, more in-depth analysis revealed that SSS actually is negligible and both phenomena are caused by the specific behavior of delayed emission. Increasing the concentration of the TADF compound was shown to enhance the concentration quenching of long-lived delayed fluorescence from conformer states with the largest singlet energy, eventually leading to a gradual redshift of the delayed emission peak wavelength. Concomitantly, the loss of long-lived delayed fluorescence entailed reverse intersystem crossing rate enhancement, though the rate-governing singlet-triplet energy gap was gradually increasing. The observed phenomena are highly unwanted, burdening molecular structure and OLED performance optimization.

Homoleptic Ir(III) Phosphors with 2-Phenyl-1,2,4-triazol-3-ylidene Chelates for Efficient Blue Organic Light-Emitting Diodes

In this report, we synthesized two series of deep-blue-emitting homoleptic iridium(III) phosphors bearing 1,2,4-triazol-3-ylidene and 5-(trifluoromethyl)-1,2,4-triazol-3-ylidene cyclometalate. Compared with reported synthetic routes using Ag₂O as the promoter, herein, we adopted a different strategy to furnish these complexes in high yields. Also, the meridional to facial isomerization was executed in the presence of trifluoroacetic acid. These phosphors were examined using NMR spectroscopies, single-crystal X-ray diffraction studies, and photophysical methods. The results revealed that electron-withdrawing trifluoromethyl substitution on the N-heterocyclic carbene fragment only gave a minor variation of photoluminescence peak wavelengths and a decrease in radiative lifetime but notable reduction in thermal stabilities. The parent 1,2,4-triazol-3-ylidene complexes have been demonstrated to be suitable for use as deep-blue phosphors, with structured emission with the peak max. located at ∼420 nm and with photoluminescence quantum yields in a range of 34.8-42.5% in degassed THF solution at RT. Fabrication of both the phosphorescent organic light-emitting diodes (OLEDs) and phosphor-sensitized OLEDs (or hyperphosphorescence) was successfully conducted, from which the OLED device based on m-tz1 showed a max. external quantum efficiency (EQE) of 10% with CIE_x,y coordinates of 0.15, 0.06, while the corresponding hyperphosphorescent OLED using m-tz2 as a sensitizer and t-DABNA as a terminal emitter afforded a significantly improved max. EQE of 19.7%, EL λ_max of 468 nm, and FWHM of 31 nm with CIE_x,y coordinates of 0.12, 0.13.

Thermally Activated Lasing in Organic Microcrystals toward Laser Displays

Thermally activated delayed fluorescent (TADF) materials are promising to overcome triplet-induced optical loss in the pursuit of electrically pumped organic lasers. However, population inversion is difficult to establish in these materials due to the severe suppression of triplet-to-singlet upconversion in their condensed states. In this work, we report thermally activated lasing in solution-processed coassembled microcrystals, where TADF dyes were uniformly dispersed into crystalline matrices to ensure an efficient reverse intersystem crossing (RISC). The dark-state triplet excitons harvested by the RISC were effectively converted into radiative singlet excitons, which subsequently participated in the population inversion to boost lasing with an unusual temperature dependence. The lasing wavelength was tuned over the full visible spectrum by doping various TADF laser dyes, owing to the excellent compatibility. Trichromatic TADF microlasers were precisely patterned into periodic pixelated arrays by a template-confined solution-growth method. With as-prepared TADF microlaser arrays as display panels, vivid laser displays were achieved under programmable excitation. These results offer valuable enlightenment to minimize triplet state-related energy losses toward high-performance lasers.

Characterizing the Conformational Distribution in an Amorphous Film of an Organic Emitter and Its Application in a "Self-Doping" Organic Light-Emitting Diode

The conformational distribution and mutual interconversion of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utilization. However, their influence on the photophysics in amorphous film states is still not known due to the lack of a suitable quantitative analysis method. Herein, we used temperature-dependent time-resolved photoluminescence spectroscopy to quantitatively measure the relative populations of the conformations of a TADF emitter for the first time. We further propose a new concept of "self-doping" for realizing high-efficiency nondoped OLEDs. Interestingly, this "compositionally" pure film actually behaves as a film with a dopant (quasi-equatorial form) in a matrix (quasi-axial form). The concentration-induced quenching that may occur at high concentrations is thus expected to be effectively relieved. The "self-doping" OLED prepared with the newly developed TADF emitter TP2P-PXZ as a neat emitting layer realizes a high maximum external quantum efficiency of 25.4 % and neglectable efficiency roll-off.

Sterically Wrapped Multiple Resonance Fluorophors for Suppression of Concentration Quenching and Spectrum Broadening

Multiple resonance (MR) emitters are promising for highly efficient organic light-emitting diodes (OLEDs) with narrowband emission; however, they still face intractable challenges with concentration-caused emission quenching, exciton annihilation, and spectral broadening. In this study, sterically wrapped MR dopants with a fluorescent MR core sandwiched by bulk substituents were developed to address the intractable challenges by reducing intermolecular interactions. Consequently, high photo-luminance quantum yields of ≥90 % and small full width at half maximums (FWHMs) of ≤25 nm over a wide range of dopant concentrations (1-20 wt %) were recorded. In addition, we demonstrated that the sandwiched MR emitter can effectively suppress Dexter interaction when doped in a thermally activated delayed fluorescence sensitizer, eliminating exciton loss through dopant triplet. Within the above dopant concentration range, the optimal emitter realizes remarkably high maximum external quantum efficiencies of 36.3-37.2 %, identical small FWHMs of 24 nm, and alleviated efficiency roll-offs in OLEDs.

PCM-ROKS for the Description of Charge-Transfer States in Solution: Singlet-Triplet Gaps with Chemical Accuracy from Open-Shell Kohn-Sham Reaction-Field Calculations

The adiabatic energy gap between the lowest singlet and triplet excited states ΔE_ST is a central property of thermally activated delayed fluorescence (TADF) emitters. Since these states are dominated by a charge-transfer character, causing strong orbital-relaxation and environmental effects, an accurate prediction of ΔE_ST is very challenging, even with modern quantum-chemical excited-state methods. Addressing this major challenge, we present an approach that combines spin-unrestricted (UKS) and restricted open-shell Kohn-Sham (ROKS) self-consistent field calculations with a polarizable-continuum model and range-separated hybrid functionals. Tests on a new representative benchmark set of 27 TADF emitters with accurately known ΔE_ST values termed STGABS27 reveal a robust and unprecedented performance with a mean absolute deviation of only 0.025 eV (∼0.5 kcal/mol) and few deviations greater than 0.05 eV (∼1 kcal/mol), even in electronically challenging cases. Requiring only two geometry optimizations per molecule at the ROKS/UKS level in a compact double-ζ basis, the approach is computationally efficient and can routinely be applied to molecules with more than 100 atoms.

Fused-Nonacyclic Multi-Resonance Delayed Fluorescence Emitter Based on Ladder-Thiaborin Exhibiting Narrowband Sky-Blue Emission with Accelerated Reverse Intersystem Crossing

Developing organic luminophores with unique capability of strong narrowband emission is both crucial and challenging for the further advancement of organic light-emitting diodes (OLEDs). Herein, a nanographitic fused-nonacyclic π-system (BSBS-N1), which was strategically embedded with multiple boron, nitrogen, and sulfur atoms, was developed as a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter. Narrowband sky-blue emission with a peak at 478 nm, full width at half maximum of 24 nm, and photoluminescence quantum yield of 89 % was obtained with BSBS-N1. Additionally, the spin-orbit coupling was enhanced by incorporating two sulfur atoms, thereby facilitating the spin-flipping process between the excited triplet and singlet states. OLEDs based on BSBS-N1 as a sky-blue MR-TADF emitter achieved a high maximum external electroluminescence quantum efficiency of 21.0 %, with improved efficiency roll-off.

A methyl-shield strategy enables efficient blue thermally activated delayed fluorescence hosts for high-performance fluorescent OLEDs

Here, we report a novel methyl-shield strategy to design ideal TADF hosts for the improvement of the performance of TSF-OLEDs. The methyl group on the xanthone acceptor acts like a shield to protect the luminance center from close intermolecular hydrogen bonding with adjacent molecules, thus alleviating exciton quenching, and meanwhile the small size of the methyl group almost does not disturb the π-π stacking between acceptors, thus maintaining fast electron-transport pathways. dMeACRXTO having two methyl shields is exploited as the host to achieve a record-high EQE of 32.3%, which represents the first report of an EQE above 30% in TSF-OLEDs.

Non-toxic near-infrared light-emitting diodes

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided.