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

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

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties.

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

New Fields, New Opportunities and New Challenges: Circularly Polarized Multiple Resonance Thermally Activated Delayed Fluorescence Materials.

Circularly polarized luminescence (CPL) materials that concurrently exhibit high efficiency and narrowband emission are extremely promising applications in 3D and wide color gamut display. By merging the CPL optical property and multiple resonance (MR) induced thermally activated delayed fluorescence (TADF) characteristic into one molecule, a new strategy, namely CP-MR-TADF, is proposed to generate organic emitters with CPL activity, TADF and narrowband emission. High-performance red, green and blue CP-MR-TADF emitters have been developed following this strategy. Herein, the present status and progress of CP-MR-TADF materials in the field of organic light-emitting diodes (OLEDs) is summarized. Finally, for this rapidly growing new research field, the future opportunities are forecasted and the present challenges are discussed.

Frontier Molecular Orbital Engineering: Constructing Highly Efficient Narrowband Organic Electroluminescent Materials.

It is of great strategic significance to develop highly efficient narrowband organic electroluminescent materials that can be utilized to manufacture ultra-high-definition (UHD) displays and meet or approach the requirements of Broadcast Television 2020 (B.T.2020) color gamut standards. This motif poses challenges for molecular design and synthesis, especially for developing generality, diversity, scalability, and robustness of molecular structures. The emergence of multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters has ingeniously solved the problems and demonstrated bright application prospects in the field of UHD displays, sparking a research boom. This Minireview summarizes the research endeavors of narrowband organic electroluminescent materials, with emphasis on the tremendous contribution of frontier molecular orbital engineering (FMOE) strategy. It combines the outstanding advantages of MR framework and donor-acceptor (D-A) structure, and can achieve red-shift and narrowband emission simultaneously, which is of great significance in the development of long-wavelength narrowband emitters with emission maxima especially exceeding 500 nm. We hope that this Minireview would provide some inspiration for what could transpire in the future.

Solution-Processable Pure-Red Multiple Resonance-induced Thermally Activated Delayed Fluorescence Emitter for Organic Light-Emitting Diode with External Quantum Efficiency over 20 .

Developing solution-processable red organic light-emitting diodes (OLEDs) with high color purity and efficiency based on multiple resonance thermally activated delayed fluorescence (MR-TADF) is a formidable challenge. Herein, by introducing auxiliary electron donor and acceptor moieties into the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributed positions of multiple resonance skeleton simultaneously, an effective strategy to obtain red MR-TADF emitters was represented. The proof-of-the-concept molecule BN-R exhibits a narrowband pure-red emission at 624 nm, with a high luminous efficiency of 94 % and a narrow bandwidth of 46 nm. Notably, the fabricated solution-processable pure-red OLED based on BN-R exhibits a state-of-the-art external quantum efficiency over 20 % with the Commission Internationale de I'Éclairage coordinates of (0.663, 0.337) and a long operational lifetime (LT50 ) of 1088 hours at an initial luminance of 1000 cd m-2 .

Precise Regulation of Emission Maxima and Construction of Highly Efficient Electroluminescent Materials with High Color Purity.

Advanced multiple resonance induced thermally activated delayed fluorescence (MR-TADF) emitters have emerged as a privileged motif for applications in organic light-emitting diodes (OLEDs), because they furnish highly tunable TADF characteristics and high color purity emission. Herein, based on the unique nitrogen-atom embedding molecular engineering (NEME) strategy, a series of compounds BN-TP-Nx (x=1, 2, 3, 4) have been customized. The nitrogen-atom anchored at different position of triphenylene hexagonal lattice entails varying degrees of perturbation to the electronic structure. The newly-constructed emitters have demonstrated the precise regulation of emission maxima of MR-TADF emitters to meet the actual industrial demand, and further enormously enriched the MR-TADF molecular reservoir. The BN-TP-N3-based OLED exhibits ultrapure green emission, with peak of 524 nm, full-width at half-maximum (FWHM) of 33 nm, Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.71), and maximum external quantum efficiency (EQE) of 37.3 %.

Constructing Organic Electroluminescent Material with Very High Color Purity and Efficiency Based on Polycyclization of the Multiple Resonance Parent Core.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) compounds have set off an upsurge of research because of their tremendous application prospects in the field of wide color gamut display. Herein, we propose a novel MR-TADF molecular construction paradigm based on polycyclization of the multiple resonance parent core, and construct a representative multiple resonance polycyclic aromatic hydrocarbon (MR-PAH) based on the para-alignment of boron and nitrogen atoms into a six-membered ring (p-BNR). Through the retrosynthesis analysis, a concise synthesis strategy with wide applicability has been proposed, encompassing programmed sequential boron esterification, Suzuki coupling and Scholl oxidative coupling. The target model molecule BN-TP shows green fluorescence with an emission peak at 523 nm and a narrow full-width at half-maximum (FWHM) of 34 nm. The organic light-emitting diode (OLED) employing BN-TP as an emitter exhibits ultrapure green emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.70), and achieves a maximum external quantum efficiency (EQE) of 35.1 %.

Highly Efficient Electrofluorescence Material Based on Pure Organic Phosphor Sensitization*.

Pure organic room-temperature phosphorescence (RTP) materials are considered as potential candidates for replacing precious metal complexes to fabricate highly efficient organic light-emitting devices (OLEDs). However, applications of the reported RTP materials in OLEDs are seriously impeded by their low photoluminescence quantum yields (PLQYs) in a thin film state. To overcome these obstacles, we established a new strategy to construct highly efficient OLEDs based on a pure organic RTP material sensitized fluorescence emitter by selecting benzimidazole-triazine molecules (PIM-TRZ), 2,6-di(phenothiazinyl)naphthalene (ß-DPTZN), and 5,6,11,12-tetraphenylnaphthacene (rubrene) as host, phosphor sensitizer, and fluorescent emitter, respectively. The perfect combination of host, phosphorescent sensitizer, and fluorescent emitter in the emitting layer ensure the outstanding performance of the devices with an external quantum efficiency (EQE) of 15.7 %.

Constructing Charge-Transfer Excited States Based on Frontier Molecular Orbital Engineering: Narrowband Green Electroluminescence with High Color Purity and Efficiency.

The design and synthesis of organic materials with a narrow emission band in the longer wavelength region beyond 510 nm remain a great challenge. For constructing narrowband green emitters, we propose a unique molecular design strategy based on frontier molecular orbital engineering (FMOE), which can integrate the advantages of a twisted donor-acceptor (D-A) structure and a multiple resonance (MR) delayed fluorescence skeleton. Attaching an auxiliary donor to a MR skeleton leads to a novel molecule with twisted D-A and MR structure characteristics. Importantly, a remarkable red-shift of the emission maximum and a narrowband spectrum are achieved simultaneously. The target molecule has been employed as an emitter to fabricate green organic light-emitting diodes (OLEDs) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.69) and a maximum external quantum efficiency (EQE) of 27.0 %.

Frontier Molecular Orbital Engineering of Aromatic Donor Fusion: Modularly Constructing Highly Efficient Narrowband Yellow Electroluminescence.

The development of high-performance multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with narrowband yellow emission is highly critical for various applications in industries, such as the automotive, aerospace, and microelectronic industries. However, the modular construction approaches to expeditiously access narrowband yellow-emitting materials is relatively rare. Here, a unique molecular design concept based on frontier molecular orbital engineering (FMOE) of aromatic donor fusion is proposed to strategically address this issue. Donor fusion is a modular approach with a "leveraging effect"; through direct polycyclization of donor attached to the MR parent core, it is facile to achieve red-shifted emission by a large margin. As a result, two representative model molecules, namely BN-Cz and BN-Cb, have been constructed successfully. The BN-Cz- and BN-Cb-based sensitized organic light-emitting diodes (OLEDs) exhibit bright yellow emission with peaks of 560 and 556 nm, full-width at half-maxima (fwhm's) of 49 and 45 nm, Commission Internationale de L'Eclairage coordinates of (0.44, 0.55) and (0.43, 0.56), and maximum external quantum efficiencies (EQEs) of 32.9% and 29.7%, respectively. The excellent optoelectronic performances render BN-Cz and BN-Cb one of the most outstanding yellow-emitting MR-TADF materials.

Efficient circularly polarized multiple resonance thermally activated delayed fluorescence from B,N-embedded hetero[8]helicene enantiomers.

Helicene-based circularly polarized multiple resonance thermally activated delayed fluorescence (CP-MR-TADF) materials are promising for ultra-high-definition and 3D displays, but most of them encounter potential problems such as easy racemization during the thermal deposition process, low luminous efficiency, and low luminescence dissymmetry factor (g lum), making the development of efficient circularly polarized organic light-emitting diodes (CP-OLEDs) a significant challenge. Here, we report a pair of CP-MR-TADF enantiomers with high-order B,N-embedded hetero[8]helicene, (P/M)-BN-TP-ICz, by fusing two MR chromophores, DtCzB and indolo[3,2,1-jk]carbazole (ICz). BN-TP-ICz exhibits green emission in toluene with a peak of 531 nm and a full-width at half-maximum (FWHM) of 36 nm. The optimized CP-OLEDs with enantiomers (P/M)-BN-TP-ICz exhibit green emission with peaks of 540 nm, FWHMs of 38 nm and Commission Internationale de L'Eclairage coordinates of (0.33, 0.65). Moreover, they showcase maximum external quantum efficiencies (EQEs) of 32.0%, with g ELs of +6.49 × 10-4 and -7.74 × 10-4 for devices based on (P)-BN-TP-ICz- and (M)-BN-TP-ICz, respectively.

Modularly Precise Construction of B,N-Embedded Large-Sized Polycyclic Aromatic Hydrocarbon Nanographene.

A modular "fjord-stitching" reverse strategy has been disclosed to successfully prepare two large-sized B,N-embedded nanographenes: BN-TBTi and BN-TBTo. These two compounds both exhibit excellent stability, nonzero-bandgap and decent photoluminescence quantum yield. Single crystal structure of BN-TBTo features a large C78B2N4 π-skeleton with length and width of approximately 2.4 and 1.5 nm, respectively.

Structurally Tunable Donor-Bridge-Fluorophore Architecture Enables Highly Efficient and Concentration-Independent Narrowband Electroluminescence.

Luminescent materials with narrowband emission have extraordinary significance for developing ultrahigh-definition display. B-N-containing multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are strong contenders. However, their device performances pervasively encounter detrimental aggregation-caused quenching effect that is highly vulnerable to doping concentration, complicating device fabrication. Therefore, constructing highly efficient and concentration-independent MR-TADF emitters is of pragmatic importance for improving device controllability and reproducibility, simplifying manufacturing procedures, and conserving production costs. Here, by systematic arrangement of donor triphenylamine and fluorophore BNCz on distinct bridges, a spatial confinement strategy has been developed with a donor-bridge-fluorophore architecture. Structurally fine modulation and progressive evolution to construct molecular entities with congested steric hindrance effect that can suppress intermolecular interactions without substantially affecting the luminescence tone of fluorophore BNCz, resulting in highly efficient and concentration-independent narrowband emitters; through isomer engineering, two isomers BN-PCz-TPA and TPA-PCz-BN with different crystal stacking patterns are synthesized by altering the connection mode between triphenylamine and BNCz. As a result, BN-PCz-TPA-based device showcases maximum external quantum efficiency (EQE) of 36.3% with narrow full-width at half-maximum of 27 nm at 10 wt% doping concentration. Even at 20 wt% doping concentration, the maximum EQE remains at 32.5% and the emission spectrum is almost unchanged.

A multi-resonance emitter with five-membered thiophene as the π-core enables efficient, narrowband and reduced efficiency roll-off OLEDs.

Efficient, narrowband multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have recently sparked significant interest in high-resolution organic light-emitting diode (OLED) displays. However, almost all the progress in MR-TADF materials has been accomplished using a six-membered ring as the π-core to date. Herein, we present the first example of a five-membered ring π-core-based MR-TADF emitter named Th-BN developed by introducing thiophene instead of hexagonal benzene as the π-core. The introduction of thiophene significantly enhances intramolecular charge transfer intensity and the spin-orbit coupling matrix elements but does not change the intrinsic MR properties. As a result, Th-BN exhibits a narrowband green emission at 512 nm, with a high luminous efficiency of 97%, a narrow full-width at half maximum of 41 nm/0.20 eV, and a rapid reverse intersystem crossing rate of 18.7 × 104 s-1, which is 10 times higher than that of its benzenoid counterpart DtBuCzB. The corresponding green OLEDs based on Th-BN achieve excellent electroluminescence performance with an external quantum efficiency (EQE) of 34.6% and a reduced efficiency roll-off with an EQE of 26.8% at a high luminance of 1000 cd m-2.

The Impact of SARS-CoV-2 Infection on the Length of Stay in the Neuro-ICU:A Prospective Multicenter Cohort Study in Eight Neuro-ICU, China Between February and April 2023.

Purpose: The SARS-CoV-2 infection cases are increasing rapidly in neuro-intensive care units (neuro-ICUs) at the beginning of 2023 in China. We aimed to characterize the prevalence, risk factors, and prognosis of critically ill patients treated in neuro-ICUs. Materials and Methods: In the prospective, multicenter, observational registry study, critically ill patients with intracerebral hemorrhage (ICH), subarachnoid hemorrhage (SAH), and traumatic brain injury (TBI) admitted to eight Chinese neuro-ICUs between Feb 16, 2023, to Apr 30, 2023 were enrolled for the study. Mortality and ICU stay day were used as the primary outcomes. Results: 131 patients were finally included and analyzed (mean age 60.36 years [SD 13.81], 64.12% male, 39.69% SARS-CoV-2 infected). The mortality is higher in the SARS-CoV-2 infection group without statistical signification (7.69% vs 5.06%, p>0.05). The length of stay (LOS) in neuro-ICUs was significantly longer among the SARS-CoV-2 infection patients (7(1-12) vs 4(1-8), p<0.01), with increased viral pneumonia occurrence (58.54% vs 7.32%, p<0.01). SARS-CoV-2 infection, surgery, and low GCS scores were independent risk factors for prolonged LOS, and respiratory/renal failure were independent risk factors for death. Conclusion: Based on the present neuro-ICU cohort, SARS-CoV-2 infection was a significant risk for the prolonged LOS of neuro-critically ill patients. Trial Registration: Registered with Chictr.org.cn (ChiCTR2300068355) at 16 February 2023, Prospective registration. https://www.chictr.org.cn/showproj.html?proj=188252.

Precise Functionalization of a Multiple-Resonance Framework: Constructing Narrowband Organic Electroluminescent Materials with External Quantum Efficiency over 40.

It is of important strategic significance to develop high-efficiency narrowband organic electroluminescent materials that can be employed to fabricate ultrahigh-definition displays with wide color gamut. This topic implies a great challenge to molecular design and synthesis, especially for the development of universality, diversity, scalability, and robustness of molecular architectonics. In this work, a synthetic methodology is demonstrated for functionalizing brominated BN-containing multiple-resonance (MR) frameworks with multifarious functional groups, such as donors, acceptors, and moieties without obvious push-pull electron properties. The m-DPAcP-BNCz-based organic light-emitting diode (OLED) exhibits green emission with a full-width at half-maximum (FWHM) of 28 nm and a maximum external quantum efficiency (EQE) of 40.6%. The outstanding performance of m-DPAcP-BNCz is attributed to the perfect integration of the inherent advantages of the MR framework and the donor-acceptor configuration, which can not only achieve bathochromic shift and narrowband emission, but also obtain high photoluminescence (PL) quantum yield (ΦPL ) and horizontal emitting dipole orientation ratio (Θ// ). This straightforward and efficient approach provides insightful guidance for the construction and enrichment of more high-efficiency narrowband emitters.

Constructing Highly Efficient Circularly Polarized Multiple-Resonance Thermally Activated Delayed Fluorescence Materials with Intrinsically Helical Chirality.

Advanced circularly polarized multiple-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials synergize the advantages of circularly polarized luminescence (CPL), narrowband emission, and the TADF characteristic, which can be fabricated into highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs) with high color purity, directly facing the urgent market strategic demand of ultrahigh-definition and 3D displays. In this work, based on an edge-topology molecular-engineering (ETME) strategy, a pair of high-performance CP-MR-TADF enantiomers, (P and M)-BN-Py, is developed, which merges the intrinsically helical chirality into the MR framework. The optimized CP-OLEDs with (P and M)-BN-Py emitters and the newly developed ambipolar transport host PhCbBCz exhibit pure green emission with sharp peaks of 532 nm, full-width at half-maximum (FWHM) of 37 nm, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.29, 0.68). Importantly, they achieve remarkable maximum external quantum efficiencies (EQEs) of 30.6% and 29.2%, and clear circularly polarized electroluminescence (CPEL) signals with electroluminescence dissymmetry factors (gEL s) of -4.37 × 10-4 and +4.35 × 10-4 for (P)-BN-Py and (M)-BN-Py, respectively.

Donor-Acceptor Type of Fused-Ring Thermally Activated Delayed Fluorescence Compounds Constructed through an Oxygen-Containing Six-Membered Ring.

Nowadays, thermally activated delayed fluorescence (TADF) compounds with a fused-ring core skeleton are getting increasing research interest because of their use in high-performance organic light-emitting diodes (OLEDs). In this study, TADF compounds featuring a D-A-type fused-ring core skeleton are developed. The challenging compatibility of a planarized D-A arrangement and the TADF property is achieved through linking the D and A moieties with two oxygen atoms within a six-membered ring. Compared with a single-oxygen analogue possessing a flexible skeleton and a twisted D-A arrangement, these fused-ring compounds with higher skeleton rigidity show higher photoluminescence quantum yields and narrower emission spectra in toluene and in doped thin films. Their electroluminescent devices achieve high external quantum efficiencies (up to 19.4%), suggesting the potential of rarely achieved D-A-type fused-ring TADF systems to serve as high-performance emitters of OLEDs.

Highly Efficient Electroluminescence from Narrowband Green Circularly Polarized Multiple Resonance Thermally Activated Delayed Fluorescence Enantiomers.

Purely organic fluorescent materials that concurrently exhibit high efficiency, narrowband emission, and circularly polarized luminescence (CPL) remain an unaddressed issue despite their promising applications in wide color gamut- and 3D-display. Herein, the CPL optical property and multiple resonance (MR) effect induced thermally activated delayed fluorescence (MR-TADF) emission are integrated with high color purity and luminous efficiency together. Two pairs of highly efficient green CP-MR-TADF enantiomers, namely, (R/S)-OBN-2CN-BN and (R/S)-OBN-4CN-BN, are developed. The enantiomer-based organic light-emitting diodes (OLEDs) exhibit pure green emission with narrow full-width at half-maximums (FWHMs) of 30 and 33 nm, high maximum external quantum efficiencies (EQEs) of 29.4% and 24.5%, and clear circularly polarized electroluminescence (CPEL) signals with electroluminescence dissymmetry factors (gEL ) of +1.43 × 10-3 /-1.27 × 10-3 and +4.60 × 10-4 /-4.76 × 10-4 , respectively. This is the first example of a highly efficient OLED that exhibits CPEL signal, narrowband emission, and TADF concurrently.

Highly Efficient Orange-Red Thermally Activated Delayed Fluorescence Compounds Comprising Dual Dicyano-Substituted Pyrazine/Quinoxaline Acceptors.

The π-conjugation of molecules has a large influence on their excited state properties, especially for red thermally activated delayed fluorescence (TADF) materials. Two orange-red TADF compounds comprising dual dicyano-substituted pyrazine/quinoxaline acceptors have been designed and synthesized. TPA-2DCNQ (3,3'-((phenylazanediyl)bis(4,1-phenylene))bis(2-phenylquinoxaline-6,7-dicarbonitrile) with extended π-conjugated quinoxaline as the acceptor exhibits higher photoluminescence quantum yields (ca. 0.67-0.71) in doped films. A smaller energy splitting (ΔEst ) between the first singlet excited state and triplet excited state is also achieved, indicating that extending the π-conjugation of the acceptor rationally is an effective approach to designing highly efficient long-wavelength TADF materials. Devices with TPA-2DCNQ as the emitter display maximum external quantum efficiencies (EQEs) of 12.6-14.0 %, which are more than twice those of devices containing TPA-2DCNPZ (6,6'-((phenylazanediyl)bis(4,1-phenylene))bis(5-phenylpyrazine-2,3-dicarbonitrile).