Simultaneously Enhanced Reverse Intersystem Crossing and Radiative Decay in Thermally Activated Delayed Fluorophors with Multiple Through-space Charge Transfers

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Enhancing Light-Emitting Efficiency of Blue Through-Space Charge Transfer Emitters via Fixing Configuration Induced by Intramolecular Hydrogen Bonding

Closely aligned configuration of the donor (D) and acceptor (A) is crucial for the light-emitting efficiency of thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) characteristics. However, precisely controlling the D-A distance of blue TSCT-TADF emitters is still challenging. Herein, an extra donor (D*) located on the side of the primary donor (D) is introduced to construct the hydrogen bonding with A and thus modulate the distance of D and A units to prepare high-efficiency blue TSCT emitters. The obtained "V"-shaped TSCT emitter presents a minimal D-A distance of 2.890 Å with a highly parallel D-A configuration. As a result, a high rate of radiative decay (>107 s-1) and photoluminescence quantum yield (nearly 90%) are achieved. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies (EQEmax) of 27.9% with a Commission Internationale de L'Eclairage (CIE) coordinate of (0.16, 0.21), which is the highest device efficiency of fluorene-based blue TSCT-TADF emitters. In addition, the TSCT-TADF emitter-sensitized OLEDs also achieve a high EQEmax of 29.3% with a CIE coordinate of (0.12, 0.16) and a narrow emission.

Organic Optoelectronic Materials: A Rising Star of Bioimaging and Phototherapy

Recently, many organic optoelectronic materials (OOMs), especially those used in organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and organic field-effect transistors (OFETs), are explored for biomedical applications including imaging and photoexcited therapies. In this review, recently developed OOMs for fluorescence imaging, photoacoustic imaging, photothermal therapy, and photodynamic therapy, are summarized. Relationships between their molecular structures, nanoaggregation structures, photophysical mechanisms, and properties for various biomedical applications are discussed. Mainly four kinds of OOMs are covered: thermally activated delayed fluorescence materials in OLEDs, conjugated small molecules and polymers in OSCs, and charge-transfer complexes in OFETs. Based on the OOMs unique optical properties, including excitation light wavelength and exciton dynamics, they are respectively exploited for suitable biomedical applications. This review is intended to serve as a bridge between researchers in the area of organic optoelectronic devices and those in the area of biomedical applications. Moreover, it provides guidance for selecting or modifying OOMs for high-performance biomedical uses. Current challenges and future perspectives of OOMs are also discussed with the hope of inspiring further development of OOMs for efficient biomedical applications.

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.

A general supramolecular strategy for fabricating full-color-tunable thermally activated delayed fluorescence materials

Developing a facile and feasible strategy to fabricate thermally activated delayed fluorescence materials exhibiting full-color tunability remains an appealing yet challenging task. In this work, a general supramolecular strategy for fabricating thermally activated delayed fluorescence materials is proposed. Consequently, a series of host-guest cocrystals are prepared by electron-donating calix[3]acridan and various electron-withdrawing guests. Owing to the through-space charge transfer mediated by multiple noncovalent interactions, these cocrystals all display efficient thermally activated delayed fluorescence. Especially, by delicately modulating the electron-withdrawing ability of the guest molecules, the emission colors of these cocrystals can be continuously tuned from blue (440 nm) to red (610 nm). Meanwhile, high photoluminescence quantum yields of up to 87% is achieved. This research not only provides an alternative and general strategy for the fabrication of thermally activated delayed fluorescence materials, but also establishes a reliable supramolecular protocol toward the design of advanced luminescent materials.

Designing thermally activated delayed fluorescence emitters with through-space charge transfer: a theoretical study

Recently, thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) features have been widely applied in developing organic light-emitting diodes with high luminescence efficiencies. The performance of TSCT-TADF molecules depends highly on their molecular structures. Therefore, theoretical investigation plays a significant role in designing novel highly efficient TSCT-TADF molecules. Herein, we theoretically investigate two recently reported TSCT-TADF molecules, 1'-(2,12-di-t-butyl[1,4]benzoxaborinino[2,3,4-kl]phenoxaborinin-7-yl)-10-phenyl-10H-spiro[acridine-9,9'-fluorene] (AC-BO) and 1-(2,12-di-t-butyl[1,4]benzoxaborinino[2,3,4-kl]phenoxaborinin-7-yl)-9',9'-dimethyl-9'H-spiro [fluorene-9,5'-quinolino[3,2,1-de]acridine](QAC-BO). The calculated photophysical properties (e.g. excited state energy levels and luminescence properties) for these two compounds are in good agreement with experimental data. Based on the systematic analysis of structure-performance relationships, we design three novel TSCT-TADF molecules with high molecular rigidity and evident TSCT features, i.e., DQAC-DBO, DQAC-SBO, and DQAC-NBO. They exhibit deep-blue light emissions and fast reverse intersystem crossing rates (KRISCs). Our calculations demonstrate that the nearly coplanar orientation of the donor and acceptor is critical to achieve remarkable KRISCs and fluorescence efficiencies in TSCT-TADF molecules.

Precise Regulation of Multiple Resonance Distribution Regions of a B,N-Embedded Polycyclic Aromatic Hydrocarbon to Customize Its BT2020 Green Emission

Recently, boron (B)/nitrogen (N)-embedded polycyclic aromatic hydrocarbons (PAHs), characterized by multiple resonances (MR), have attracted significant attention owing to their remarkable features of efficient narrowband emissions with small full width at half maxima (FWHMs). However, developing ultra-narrowband pure-green emitters that comply with the Broadcast Service Television 2020 (BT2020) standard remains challenging. Precise regulation of the MR distribution regions allows simultaneously achieving the emission maximum, FWHM value, and spectral shape that satisfy the BT2020 standard. The proof-of-concept molecule TPABO-DICz exhibited ultrapure green emission with a dominant peak at 515 nm, an extremely small FWHM of 17 nm, and Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.76). The corresponding bottom-emitting organic light-emitting diode (OLED) exhibited a remarkably high CIEy value (0.74) and maximum external quantum efficiency (25.8 %). Notably, the top-emitting OLED achieved nearly BT2020 green color (CIE: 0.14, 0.79) and exhibited a state-of-the-art maximum current efficiency of 226.4 cd A-1 , thus fully confirming the effectiveness of the above strategy.

Tailored Design of π-Extended Multi-Resonance Organoboron using Indolo[3,2-b]Indole as a Multi-Nitrogen Bridge

Extending the π-skeletons of multi-resonance (MR) organoboron emitters can feasibly modulate their optoelectronic properties. Here, we first adopt the indolo[3,2-b]indole (32bID) segment as a multi-nitrogen bridge and develop a high-efficiency π-extended narrowband green emitter. This moiety establishes not only a high-yield one-shot multiple Bora-Friedel-Crafts reaction towards a π-extended MR skeleton, but a compact N-ethylene-N motif for a red-shifted narrowband emission. An emission peak at 524 nm, a small full width at half maximum of 25 nm and a high photoluminescence quantum yield of 96 % are concurrently obtained in dilute toluene. The extended molecular plane also results in a large horizontal emitting dipole orientation ratio of 87 %. A maximum external quantum efficiency (EQE) of 36.6 % and a maximum power efficiency of 135.2 lm/W are thereafter recorded for the corresponding device, also allowing a low efficiency roll-off with EQEs of 34.5 % and 28.1 % at luminance of 1,000 cd/m2 and 10,000 cd/m2 , respectively.

Alignment of Heptagonal Diimide and Triazine Enables Narrowband Pure-Blue Organic Light-Emitting Diodes with Low Efficiency Roll-Off

The endeavor to develop high-performance narrowband blue organic light-emitting diodes (OLEDs) with low efficiency roll-off represents an attractive challenge. Herein, we introduce a hetero-acceptor design strategy centered around the heptagonal diimide (BPI) building block to create an efficient thermally activated delayed fluorescence (TADF) sensitizer. The alignment of a twisted BPI unit and a planar diphenyltriazine (TRZ) fragment imparts remarkable exciton dynamic properties to 26tCz-TRZBPI, including a fast radiative decay rate (kR ) of 1.0×107  s-1 and a swift reverse intersystem crossing rate (kRISC ) of 1.8×106  s-1 , complemented by a slow non-radiative decay rate (kNR ) of 6.0×103  s-1 . Consequently, 26tCz-TRZBPI facilitates the fabrication of high-performance narrowband pure-blue TADF-sensitized fluorescence OLEDs (TSF-OLEDs) with a maximum external quantum efficiency (EQEmax ) of 24.3 % and low efficiency roll-off even at a high brightness level of 10000 cd m-2 (EQE10000 : 16.8 %). This showcases a record-breaking external quantum efficiency at a high luminance level of 10000 cd m-2 for narrowband blue TSF-OLEDs.

Recent advances in the design of afterglow materials: mechanisms, structural regulation strategies and applications

Afterglow materials are attracting widespread attention owing to their distinctive and long-lived optical emission properties which create exciting opportunities in various fields. Recent research has led to the discovery of many new afterglow materials featuring high photoluminescence quantum yields (PLQY) and lifetimes of up to several hours under ambient conditions. Afterglow materials are typically categorized according to their luminescence mechanism, such as long-persistent luminescence (LPL), room temperature phosphorescence (RTP), or thermally activated delayed fluorescence (TADF). Through rational design and novel synthetic strategies to modulate spin-orbit coupling (SOC) and populate triplet exciton states (T1), luminophores with long lifetimes and bright afterglow characteristics can be realized. Initial research towards afterglow materials focused mainly on pure inorganic materials, many of which possessed inherent disadvantages such as metal toxicity or low energy emissions. In recent years, organic-inorganic hybrid afterglow materials (OIHAMs) have been developed with high PLQY and long lifetimes. These hybrid materials exploit the tunable structure and easy processing of organic molecules, as well as enhanced SOC and intersystem crossing (ISC) processes involving heavy atom dopants, to achieve excellent afterglow performance. In this review, we begin by briefly discussing the structure and composition of inorganic and organic-inorganic hybrid afterglow materials, including strategies for regulating their lifetime, PLQY and luminescence wavelength. The specific advantages of organic-inorganic hybrid afterglow materials, including low manufacturing costs, diverse molecular/electronic structures, tunable structures and optical properties, and compatibility with a variety of substrates, are emphasized. Subsequently, we discuss in detail the fundamental mechanisms used by afterglow materials, their classification, design principles, and end applications (including sensing, anticounterfeiting, and photoelectric devices, among others). Finally, existing challenges and promising future directions are discussed, laying a platform for the design of afterglow materials for specific applications.

Phosphine-Oxide-Balanced Intra- and Interchain Through-Space Charge Transfer in Thermally Activated Delayed Fluorescence Polymers: Beyond 30% External Quantum Efficiency

Through-space charge transfer (TSCT) is crucial for developing highly efficient thermally activated delayed fluorescence polymers. The balance of intra- and interchain TSCT can markedly improve performance, but it is still a big challenge. In this work, an effective strategy for "intra- and interchain TSCT balance" is demonstrated by way of a series of non-conjugated copolymers containing a 9,9-dimethylacridine donor and triazine-phosphine oxide (PO)-based acceptors. Steady-state and transient emission spectra indicate that compared to the corresponding blends, the copolymers can indeed achieve balanced intra- and interchain TSCT by accurately optimizing the inductive and steric effects of the acceptors. The DPOT acceptor with the strongest electron-withdrawing ability and the second bigger steric hindrance endows its copolymers with state-of-the-art photoluminescence and electroluminescence quantum efficiencies beyond 95% and 32%, respectively. This demonstrates that, compared to other congeners, the synergistic inductive and steric effects effectively enhance TSCT in DPOT-based copolymers for radiation, and suppress singlet and triplet quenching. The record-high efficiencies of its devices make this kind of copolymers hold the potential for low-cost, large-scale, and high-efficiency applications.

Highly Twisted Thermally Activated Delayed Fluorescence (TADF) Molecules and Their Applications in Organic Light-Emitting Diodes (OLEDs)

Thermally activated delayed fluorescence (TADF) materials have attracted great potential in the field of organic light-emitting diodes (OLEDs). Among thousands of TADF materials, highly twisted TADF emitters have become a hotspot in recent years. Compared with traditional TADF materials, highly twisted TADF emitters tend to show multi-channel charge-transfer characters and form rigid molecular structures. This is advantageous for TADF materials, as non-radiative decay processes can be suppressed to facilitate efficient exciton utilization. Accordingly, OLEDs with excellent device performances have also been reported. In this Review, we have summarized recent progress in highly twisted TADF materials and related devices, and give an overview of the molecular design strategies, photophysical studies, and the performances of OLED devices. In addition, the challenges and perspectives of highly twisted TADF molecules and the related OLEDs are also discussed.

A Feasible Strategy for a Highly Efficient Thermally Activated Delayed Fluorescence Emitter Over 900 nm Based on Phenalenone Derivatives

Near-infrared (NIR) organic light-emitting diodes (OLEDs) suffer from the low external electroluminescence (EL) quantum efficiency (EQE), which is a critical obstacle for potential applications. Herein, 1-oxo-1-phenalene-2,3-dicarbonitrile (OPDC) is employed as an electron-withdrawing aromatic ring, and by incorporating with triphenylamine (TPA) and biphenylphenylamine (BBPA) donors, two novel NIR emitters with thermally activated delayed fluorescence (TADF) characteristics, namely OPDC-DTPA and OPDC-DBBPA, are first developed and compared in parallel. Intense NIR emission peaks at 962 and 1003 nm are observed in their pure films, respectively. Contributed by the local excited (LE) characteristics in the triplet (T1 ) state in synergy with the charge transfer (CT) characteristics for the singlet (S1 ) state to activate TADF emission, the solution processable doped NIR OLEDs based on OPDC-DTPA and OPDC-DBBPA yield EL peaks at 834 and 906 nm, accompanied with maximum EQEs of 0.457 and 0.103 %, respectively, representing the state-of-the-art EL performances in the TADF emitter-based NIR-OLEDs in the similar EL emission regions so far. This work manifests a simple and effective strategy for the development of NIR TADF emitters with long wavelength and efficiency synchronously.

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

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

Efficient Spin-Flip between Charge-Transfer States for High-Performance Electroluminescence, without an Intermediate Locally Excited State

Thermally activated delayed fluorescence (TADF) materials with both high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC) are strongly desired to realize efficient and stable organic light-emitting diodes (OLEDs). Control of excited-state dynamics via molecular design plays a central role in optimizing the PLQY and RISC rate of TADF materials but remains challenging. Here, 3 TADF emitters possessing similar molecular structures, similar high PLQYs (89.5% to 96.3%), and approximate energy levels of the lowest excited singlet states (S₁), but significantly different spin-flipping RISC rates (0.03 × 10⁶ s^-1 vs. 2.26 × 10⁶ s^-1) and exciton lifetime (297.1 to 332.8 μs vs. 6.0 μs) were systematically synthesized to deeply investigate the feasibility of spin-flip between charge-transfer excited states (³CT-¹CT) transition. Experimental and theoretical studies reveal that the small singlet-triplet energy gap together with low RISC reorganization energy between the ³CT and ¹CT states could provide an efficient RISC through fast spin-flip ³CT-¹CT transition, without the participation of an intermediate locally excited state, which has previously been recognized as being necessary for realizing fast RISC. Finally, the OLED based on the champion TADF emitter achieves a maximum external quantum efficiency of 27.1%, a tiny efficiency roll-off of 4.1% at 1,000 cd/m², and a high luminance of 28,150 cd/m², which are markedly superior to those of the OLEDs employing the other 2 TADF emitters.

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

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

Confining donor conformation distributions for efficient thermally activated delayed fluorescence with fast spin-flipping

Fast spin-flipping is the key to exploit the triplet excitons in thermally activated delayed fluorescence based organic light-emitting diodes toward high efficiency, low efficiency roll-off and long operating lifetime. In common donor-acceptor type thermally activated delayed fluorescence molecules, the distribution of dihedral angles in the film state would have significant influence on the photo-physical properties, which are usually neglected by researches. Herein, we find that the excited state lifetimes of thermally activated delayed fluorescence emitters are subjected to conformation distributions in the host-guest system. Acridine-type flexible donors have a broad conformation distribution or bimodal distribution, in which some conformers feature large singlet-triplet energy gap, leading to long excited state lifetime. Utilization of rigid donors with steric hindrance can restrict the conformation distributions in the film to achieve degenerate singlet and triplet states, which is beneficial to efficient reverse intersystem crossing. Based on this principle, three prototype thermally activated delayed fluorescence emitters with confined conformation distributions are developed, achieving high reverse intersystem crossing rate constants greater than 10⁶ s^-1, which enable highly efficient solution-processed organic light-emitting diodes with suppressed efficiency roll-off.

Thermally Enhanced and Long Lifetime Red TADF Carbon Dots via Multi-Confinement and Phosphorescence Assisted Energy Transfer

Thermally activated delayed fluorescence (TADF) materials, which can harvest both singlet and triplet excitons for high-efficiency emission, have attracted widespread concern for their enormous applications. Nevertheless, luminescence thermal quenching severely limits the efficiency and operating stability in TADF materials and devices at high temperature. Herein, a surface engineering strategy is adopted to obtain unique carbon dots (CDs)-based thermally enhanced TADF materials with ≈250% enhancement from 273 to 343 K via incorporating seed CDs into ionic crystal network. The rigid crystal network can simultaneously boost reverse intersystem crossing process via enhancing spin-orbit coupling between singlet and triplet states and suppressing non-radiative transition rate, contributing to the thermally enhanced TADF character. Benefiting from efficient energy transfer from triplet states of phosphorescence center to singlet states of CDs, TADF emission at ≈600 nm in CDs displays a long lifetime up to 109.6 ms, outperforming other red organic TADF materials. Thanks to variable decay rates of the delayed emission centers, time and temperature-dependent delayed emission color has been first realized in CDs-based delayed emission materials. The CDs with thermally enhanced and time-/temperature-dependent emission in one material system can offer new opportunities in information protection and processing.

Precise Manipulation of Excited-State Intramolecular Proton Transfer via Incorporating Charge Transfer toward High-Performance Film-Based Fluorescence Sensing

Excited-state intramolecular proton transfer (ESIPT) has been widely employed for the design of a variety of functionality-led molecular systems. However, precise manipulation of the excited-state reaction is challenging. Herein, we report a new tactic for tuning ESIPT via incorporating an excited-state intramolecular charge transfer (ESICT) process. Specifically, three o-carborane derivatives, NaCBO, PaCBO, and PyCBO, were designed, where the 2-(2'-hydroxyphenyl)-benzothiazole is a typical ESIPT unit functioning as an electron acceptor, and the electron-donating units are naphthyl-(Na), phenanthrenyl-(Pa), and pyrenyl-(Py), respectively. The architectures of the molecules are featured with a face-to-face alignment of the two units. Spectroscopy and theoretical calculation studies revealed that the electron-donating capacity of the donors and solvent polarity continuously modulate the ESIPT/ESICT energetics and dynamics, resulting in distinct emissions. Moreover, the molecules depicted not only highly porous structures but also very different fluorescent colors in the solid state, enabling highly selective film-based fluorescence sensing of mustard gas simulant, 2-chloroethyl ethyl sulfide, with a detection limit of 50 ppb and a response time of 5 s. This work thus provides a reliable strategy for the creation of high-performance sensing fluorophores via ESIPT manipulation.

Theoretical study on thermally activated delayed fluorescent molecules based on space charge transfer

Thermally Activated Delayed Fluorescent (TADF) molecules with through-space charge transfer (TSCT) have broad application potential in organic light-emitting diodes. In this paper, five TPA-ace based molecules with different electron-withdrawing groups and TSCT property are investigated using polarizable continuum model (PCM) combined with density functional theory (DFT) and time-dependent functional theory (TD-DFT) in Methylcyclohexane, Toluene and Dichloromethane. It is found that stronger electron-withdrawing ability of acceptors could induce redshift of emission and smaller energy gap between the first singlet excited state (S₁) and the first triplet excited state (ΔE_ST). The ratio of TSCT to through bond charge transfer (TBCT) for S₁ of TPA-ace-TRZ is calculated quantitatively, which further confirmed the TSCT character of TPA-ace-TRZ. The TADF property is also analyzed based on the calculation of spin-orbit coupling and the (reverse) intersystem crossing rates between S₁ and T₁. Our calculation results would favor the understanding of TSCT-TADF.

Isomeric thermally activated delayed fluorescence emitters for highly efficient organic light-emitting diodes

The isomeric strategy is an important design concept in molecular design that has a non-negligible influence on molecular properties. Herein, two isomeric thermally activated delayed fluorescence (TADF) emitters (NTPZ and TNPZ) are constructed with the same skeleton consisting of an electron donor and electron acceptor but different connection sites. Systematic investigations show that NTPZ exhibits a small energy gap, large up-conversion efficiency, low non-radiative decay, and high photoluminescence quantum yield. Further theoretical simulations reveal that the excited molecular vibrations play a key role in regulating the non-radiative decays of the isomers. Therefore, an NTPZ based OLED achieves better electroluminescence performances, such as a higher external quantum efficiency of 27.5% compared to a TNPZ based OLED (18.3%). This isomeric strategy not only provides an opportunity to deeply understand the relationship between substituent locations and molecular properties, but also affords a simple and effective strategy to enrich TADF materials.

Metal-Perturbed Multiresonance TADF Emitter Enables High-Efficiency and Ultralow Efficiency Roll-Off Nonsensitized OLEDs with Pure Green Gamut

Multiresonance (MR)-induced thermally activated delayed fluorescence (TADF) emitters based on B- and N-embedded polycyclic aromatics are desirable for ultrahigh-definition organic light-emitting diodes (OLEDs) due to their high photoluminescence quantum yield (PLQY) and narrow bandwidth. But the reverse intersystem crossing (RISC) rates of MR-TADF emitters are usually small, resulting in severe device efficiency roll-off at high brightness. To solve this issue, a sensitizer for the MR-TADF emitter has been required. Herein, a new MR-TADF emitter is developed through coordination of Au with B/N-embedded polycyclic ligand. Benefitting from the Au perturbation, the RISC rate is dramatically accelerated to 2.3 × 10⁷ s^-1 , leading to delayed fluorescence lifetime as short as 4.3 µs. Meanwhile, the PLQY of 95% and full width at half maximum of 39 nm (0.18 eV) are essentially unchanged after metal coordination. Therefore, a high PLQY, short delayed fluorescence lifetime, and high color purity are concurrently realized in a single TADF emitter. Accordingly, vacuum-deposited OLEDs exhibit high-performance electroluminescence with a maximum external quantum efficiency (EQE) of 35.8% without sensitization. The EQE is maintained as high as 32.3% at 10 000 cd m^-2 . Furthermore, solution-processed OLED based on the emitter also achieves excellent performance with a maximum EQE of 25.7% and a small efficiency roll-off.

Multiple Fusion Strategy for High-Performance Yellow OLEDs with Full Width at Half Maximums Down to 23 nm and External Quantum Efficiencies up to 37.4

The pursuit of ideal narrowband yellow multiple resonance (MR) emitters is hampered by the mutual constraints of effective spectral redshift and maintaining a small full width at half maximum (FWHM) value. Here, a novel multiple fusion molecular design strategy is reported to break this trade-off. Compared with the selected narrowband parent core, the specific multiple MR effects in target molecules can simultaneously extend the π-conjugation length, increase the rigidity of the structure, and reduce the vibrational frequency. Proof-of-the-concept emitters BN-DICz and DBN-ICz show bright yellowish green to yellow emissions in dilute toluene solutions with peaks at 533-542 nm and extremely small FWHMs of ≤20 nm. Notably, BN-DICz-based electroluminescent device exhibits excellent efficiencies of 37.4%, 136.6 cd A^-1 , and 119.2 lm W^-1 with an FWHM of merely 23 nm, representing the best performance for yellow MR organic light-emitting diodes.

Wide-range color-tunable polycyclo-heteraborin multi-resonance emitters containing B-N covalent bonds

Boron- and nitrogen (BN)-fused polycyclic aromatic frameworks with amine-directed formation of B-N covalent bonds have the potential to form a new family of facile-synthesis multi-resonance luminophores, which, however, still face imperative challenges in diversifying the molecular design to narrow the emission bandwidth and tune the emission colors. Here, we demonstrate a strategic implementation of B-N bond containing polycyclo-heteraborin multi-resonance emitters with wide-range colors from deep-blue to yellow-green (442-552 nm), small full-width at half-maxima of only 19-28 nm and high photoluminescence efficiencies, by stepwise modifying the basic para B-π-B structures with heteroatoms. The corresponding electroluminescent devices show superior maximum external quantum efficiencies with an exceptional low-efficiency roll-off, retaining 21.0%, 23.6% and 22.1% for the sky-blue, green and yellow-green devices at a high luminance of 5000 cd m^-2, respectively.

Hyperbranched Conjugated Polymer with Multiple Charge Transfer Enables High-Efficiency Nondoped Red Electroluminescence with Low Driving Voltage

Conjugated polymers featuring thermally activated delayed fluorescence (TADF) attract tremendous attention in both academic and industry communities due to their easy solution processing for fabricating large-area and low-cost high-performance polymer light-emitting diodes (PLEDs). However, current nondoped solution-processed PLEDs frequently encounter significant efficiency roll-offs and unreasonably high operating voltages at high brightness, especially for red-emitting polymers. Herein, we design hyperbranched conjugated polymers (HCPs) with D-A-D type TADF characteristics for high-performance red-emitting PLEDs. Multiple intramolecular charge transfer (ICT) channels induced by quasi-equivalent donors of the TADF core strongly boost the reverse intersystem crossing (RISC) process and singlet excitons radiative transition. Coupling with the efficient energy transfer process generated by structure advantages of HCPs, the strongly electron-withdrawing oxygen atoms located on the TADF cores further accelerate hole transportation from the host chains to the TADF cores. Under a rational regulation of the TADF core ratio, the related nondoped red-emitting device performs an outstanding performance with an EQE_max of 8.39% and exhibits no roll-off while the luminance is less than 100 cd/m² and only 3.3% decrease at 500 cd/m². Simultaneously, the EQE can maintain 7.4% under 1000 cd/m². Furthermore, the corresponding nondoped device exhibits a low turn-on voltage of around 2.5 V and achieves a luminance of 500 cd/m² at 3.5 V and even 1000 cd/m² at 3.9 V. To our knowledge, this is the best performance among all nondoped red PLEDs with high brightness obtained at low operating voltage.

Theoretical perspective for substitution effect on luminescent properties of through space charge transfer-based thermally activated delayed fluorescence molecules

Recently, through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules have shown advantages in achieving high efficiencies and tunable emissions. However, the relationships between basic molecular structures and luminescent properties are unclear. Theoretical investigations to reveal the substitution effects with different numbers and positions on excited-state properties are highly desired. Herein, by taking TSCT-based TADF molecules S-CNDF-S-tCz, S-CNDF-D-tCz and T-CNDF-T-tCz as skeletons, a series of promising TADF molecules are designed by adopting ortho, meta and para substitutions with different numbers and positions. Photophysical properties of total 16 molecules are theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods in chloroform combined with polarizable continuum model. Results indicate that molecules with ortho-substitution possess small geometric changes and short Donor-Acceptor distances which are induced by the intramolecular van der Waals interactions. Decreased non-radiative consumption and increased TSCT ratio and therefore excellent performance for them can be expected. For molecules with large substitution numbers, twist structures facilitate them to realize small adiabatic energy gaps between the lowest singlet excited state (S₁) and the lowest triplet excited state (T₁), this designing strategy is consistent with the TADF dendrimers. Thus, the relationships between molecular structures and luminescent properties are revealed and promising TSCT-based TADF molecules with high efficiencies are theoretically proposed. Our investigations provide theoretical perspectives for inner mechanisms of substitution effect, which could further afford meaningful guidance to design new efficient TSCT-based TADF molecules.

Carbonyl (CO)/N-based thermally activated delayed fluorescent materials with high efficiency and fast reverse intersystem crossing rate: a theoretical design and study

Two novel highly efficient organic TADF materials based on MR structures have been theoretically designed and investigated.

Highly Efficient Green and Red Narrowband Emissive Organic Light-Emitting Diodes Employing Multi-Resonant Thermally Activated Delayed Fluorescence Emitters

Herein, we demonstrate how judicious selection of the donor decorating a central multi-resonant thermally activated delayed fluorescence (MR-TADF) core based on DiKTa can lead to very high-performance OLEDs. By decorating the DiKTa core with triphenylamine (TPA) and diphenylamine (DPA), 3TPA-DiKTa and 3DPA-DiKTa exhibit bright, narrowband green and red emission in doped films, respectively. The OLEDs based on these emitters showed record-high performance for this family of emitters with maximum external quantum efficiencies (EQEmax ) of 30.8 % for 3TPA-DiKTa at λEL of 551 nm and 16.7 % for 3DPA-DiKTa at λEL =613 nm. The efficiency roll-off in the OLEDs was improved significantly by using 4CzIPN as an assistant dopant in hyperfluorescence (HF) devices. The outstanding device performance has been attributed to preferential horizontal orientation of the transition dipole moments of 3TPA-DiKTa and 3DPA-DiKTa.

Constructing high-efficiency orange-red thermally activated delayed fluorescence emitters by three-dimension molecular engineering

Preparing high-efficiency solution-processable orange-red thermally activated delayed fluorescence (TADF) emitters remains challenging. Herein, we design a series of emitters consisting of trinaphtho[3,3,3]propellane (TNP) core derivatized with different TADF units. Benefiting from the unique hexagonal stacking architecture of TNPs, TADF units are thus kept in the cavities between two TNPs, which decrease concentration quenching and annihilation of long-lived triplet excitons. According to the molecular engineering of TADF and host units, the excited states can further be regulated to effectively enhance spin-orbit coupling (SOC) processes. We observe a high-efficiency orange-red emission at 604 nm in one instance with high SOC value of 0.862 cm^-1 and high photoluminescence quantum yield of 70.9%. Solution-processable organic light-emitting diodes exhibit a maximum external quantum efficiency of 24.74%. This study provides a universal strategy for designing high-performance TADF emitters through molecular packing and excited state regulation.

Stable Tetradentate Gold(III)-TADF Emitters with Close to Unity Quantum Yield and Radiative Decay Rate Constant of up to 2 × 106 s-1 : High-Efficiency Green OLEDs with Operational Lifetime (LT90 ) Longer than 1800 h at 1000 cd m-2

High maximum external quantum efficiency (EQE_max ), small efficiency roll-offs, and long operational lifetime at practical luminances are three crucial parameters for commercialization of organic light-emitting diodes (OLEDs). To simultaneously achieve these goals, it is desirable to have the radiative decay rate constant (k_r ) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S₁ →S₀ radiative decay rate constant (k_r ^S ) and a small singlet-triplet energy gap (ΔE_ST ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the ΔE_ST while keeping the k_r ^S large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and k_r approaching 2 × 10⁶ s^-1 in thin films. The vacuum-deposited green OLEDs based on 1 and 4 demonstrate maximum EQEs of up to 24 and 27% with efficiency roll-offs of 5.5 and 2.2% at 1000 cd m^-2 , respectively; the EQEs maintain high at 10 000 cd m^-2 (19% (1) and 24% (4)). A long LT₉₀ device lifetime of 1820 h at 1000 cd m^-2 for complex 1 is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.

Acceptor-Donor-Acceptor π-Stacking Boosts Intramolecular Through-Space Charge Transfer towards Efficient Red TADF and High-Performance OLEDs

Organic push-pull systems featuring through-space charge transfer (TSCT) excited states have been disclosed to be capable of exhibiting thermally activated delayed fluorescence (TADF), but to realize high-efficiency long-wavelength emission still remains a challenge. Herein, we report a series of strongly emissive orange-red and red TSCT-TADF emitters having (quasi)planar and rigid donor and acceptor segments which are placed in close proximity and orientated in a cofacial manner. Emission maxima (λ_em) of 594−599 nm with photoluminescence quantum yields (PLQYs) of up to 91% and delayed fluorescence lifetimes of down to 4.9 μs have been achieved for new acceptor-donor-acceptor (A-D-A) molecules in doped thin films. The presence of multiple acceptors and the strong intramolecular π-stacking interactions have been unveiled to be crucial for the efficient low-energy TSCT-TADF emissions. Organic light-emitting diodes (OLEDs) based on the new A-D-A emitters demonstrated electroluminescence with maximum external quantum efficiencies (EQEs) of up to 23.2% for the red TSCT-TADF emitters. An EQE of 18.9% at the brightness of 1000 cd m⁻² represents one of the highest values for red TADF OLEDs. This work demonstrates a modular approach for developing high-performance red TADF emitters through engineering through-space interactions, and it may also provide implications to the design of TADF emitter with other colours.

Secondary through-space interactions facilitated single-molecule white-light emission from clusteroluminogens

Clusteroluminogens refer to some non-conjugated molecules that show visible light and unique electronic properties with through-space interactions due to the formation of aggregates. Although mature and systematic theories of molecular photophysics have been developed to study conventional conjugated chromophores, it is still challenging to endow clusteroluminogens with designed photophysical properties by manipulating through-space interactions. Herein, three clusteroluminogens with non-conjugated donor-acceptor structures and different halide substituents are designed and synthesized. These compounds show multiple emissions and even single-molecule white-light emission in the crystalline state. The intensity ratio of these emissions is easily manipulated by changing the halide atom and excitation wavelength. Experimental and theoretical results successfully disclose the electronic nature of these multiple emissions: through-space conjugation for short-wavelength fluorescence, through-space charge transfer based on secondary through-space interactions for long-wavelength fluorescence, and room-temperature phosphorescence. The introduction of secondary through-space interactions to clusteroluminogens not only enriches their varieties of photophysical properties but also inspires the establishment of novel aggregate photophysics for clusteroluminescence.

Although mature and systematic theories of molecular photophysics have been developed, it is still challenging to endow clusteroluminogens (CLgens) with designed photophysical properties by manipulating through-space interactions. Here, the authors design three CLgens that show multiple emissions and white-light emission in the crystalline state, and emphasize the important role of secondary through-space interactions between the acceptor and non-conjugated donor units.

Fusion of Multi-Resonance Fragment with Conventional Polycyclic Aromatic Hydrocarbon for Nearly BT.2020 Green Emission

Herein, we report a general strategy for achieving ultra-pure green emissions by suppressing the shoulder peaks in the emission spectra of conventional polycyclic aromatic hydrocarbons (PAHs). Through precise synthetic fusion of multi-resonance (MR) fragments with conventional PAH, extended π-conjugation lengths, increased molecular rigidity, and reduced vibrational frequency could be simultaneously realized. The proof-of-concept emitters exhibited ultra-pure green emissions with dominant peaks at ca. 521 nm, photoluminescence quantum yields that are greater than 99 %, a small full-width-at-half-maximum of 23 nm, and CIE coordinates of (0.16, 0.77). The bottom-emitting organic light-emitting diode (OLED) exhibited a record-high CIEy value of 0.74 and a high maximum external quantum efficiency of 30.5 %. The top-emitting OLED not only achieved a BT.2020 green color (CIE: 0.17, 0.78) for the first time but also showed superior performance among all green OLED devices, with a current efficiency of 220 cd A^- .

Achieving 37.1% Green Electroluminescent Efficiency and 0.09 eV Full Width at Half Maximum Based on a Ternary Boron-Oxygen-Nitrogen Embedded Polycyclic Aromatic System

Herein, a ternary boron-oxygen-nitrogen embedded polycyclic aromatic hydrocarbon with multiple resonance thermally activated delayed fluorescence (MR-TADF), namely DBNO, is developed by adopting the para boron-π-boron and para oxygen-π-oxygen strategy. The designed molecule presents a vivid green emission with a high photoluminescence quantum yield (96 %) and an extremely narrow full width at half maximum (FWHM) of 19 nm/0.09 eV, which surpasses all previously reported green TADF emitters to date. In addition, the long molecular structure along the transition dipole moment direction endows it with a high horizontal emitting dipole ratio of 96 %. The organic light-emitting diode (OLED) based on DBNO reveals a narrowband green emission with a peak at 504 nm and a FWHM of 24 nm/0.12 eV. Particularly, a significantly improved device performance is achieved by the TADF-sensitization (hyperfluorescence) mechanism, presenting a FWHM of 27 nm and a maximum external quantum efficiency (EQE) of 37.1 %.

Two-Photon Excited Near-Infrared Phosphorescence Based on Secondary Supramolecular Confinement

Organic phosphorescence materials have received wide attention in bioimaging for bio-low toxicity and large Stokes. Herein, a design strategy to achieve near-infrared (NIR) excitation and emission of organic room-temperature phosphorescence through two-stage confinement supramolecular assembly is presented. Via supramolecular macrocyclic confinement, the host-guest complexes exhibit phosphorescence with two-photon absorption (excitation wavelength up to 890 nm) and NIR emission (emission wavelength up to 800 nm) in aqueous solution, and further nano-confinement assembly significantly strengthens phosphorescence. Moreover, the nano-assemblies possess color-tunable luminescence spanning from the visible to NIR regions under different excitation wavelengths. Intriguingly, the prepared water-soluble assemblies maintain two-photon absorption and multicolor luminescence in cells or vivo.

Accelerating Radiative Decay in Blue Through-Space Charge Transfer Emitters by Minimizing the Face-to-Face Donor-Acceptor Distances

Thermally activated delayed fluorophors (TADF) featuring through-space charge transfers (TSCT) suffer from low radiative decay rates (k_r s), especially for blue emitters. Here, a xanthene bridge is adopted to construct space-confined face-to-face donor-acceptor alignment and minimize their distances down to 2.7-2.8 Å, even shorter than the interlayer distance of graphite and thus strengthening the electronic interactions. The resulting blue TSCT-TADF emitters exhibit peaks around ≈460 nm, photoluminescence quantum yields of >90 %, and k_r s of nearly 10⁷  s^-1 , almost 2-10 times higher than previously observed values with comparable reverse intersystem crossing rates. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies of 27.8 % and 34.7 % with Commission Internationale de L'Eclairage y coordinates of 0.29 and 0.15 using those molecules as emitters and sensitizers, respectively.

Carbene-Metal Complexes As Molecular Scaffolds for Construction of through-Space Thermally Activated Delayed Fluorescence Emitters

The through-space charge transfer (CT) process is observed in Cu(I) carbene-metal-amide complexes, where conventional imidazole or imidazoline N-heterocyclic (NHC) carbene fragments act as inert linkers and CT proceeds between a metal-bound carbazole donor and a distantly situated carbene-bound phenylsulfonyl acceptor. The resulting electron transfer gives a rise to efficient thermally activated delayed fluorescence (TADF), characterized with high photoluminescence quantum yields (Φ_PL up to 90%) and radiative rates (k_r) up to 3.32 × 10⁵ s^-1. The TADF process is aided by fast reverse intersystem crossing (rISC) rates of up to 2.56 × 10⁷ s^-1. Such emitters can be considered as hybrids of two existing TADF emitter design strategies, combining low singlet-triplet energy gaps (ΔE_ST) met in all-organic exciplex-like emitters (0.0062-0.0075 eV) and small, but non-negligible spin-orbital coupling (SOC) provided by a Cu atom, like in TADF-active organometallic complexes.

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.