Versatile Indolocarbazole-Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs with Alleviated Efficiency Roll-Off

Ultra-Narrowband Organic Electroluminescence with External Quantum Efficiency of 40% from Indolocarbazole-Embedded Multiple Resonance Emitters

The demand for ultra-high-definition display technology has spurred the prosperity of multiple resonance induced thermally activated delayed fluorescence (MR-TADF) materials with narrow full-width at half-maximum (FWHM) and high efficiency, making them highly promising candidates for high-color-purity organic light-emitting diodes (OLEDs) displays. Indolocarbazole, a highly rigid aza-polycyclic aromatic hydrocarbon framework, has shown significant potential as a building block for constructing MR-TADF emitters with ultra-narrowband emission (<20 nm). However, it remains a great challenge to construct ultra-narrowband indolocarbazole-embedded MR-TADF emitters with emission maxima less than 500 nm. Here, two MR-TADF emitters, DBN-amICz and DBN-bmICz, are constructed by adopting meta-N-π-N-type indolocarbazole as core framework and achieve ultra-narrowband blue-green emission in toluene solution with peaks of both 490 nm and FWHMs of 18 and 19 nm, respectively. OLEDs incorporating emitters DBN-amICz and DBN-bmICz demonstrate excellent electroluminescence (EL) performances, with maximum external quantum efficiencies (EQEs) of 40.1% and 35.5%, and FWHMs of 21 and 24 nm, respectively. This study represents the first report of dual-boron-containing MR emitters derived from indolocarbazole with emission below 500 nm, filling a gap in the development of indolocarbazole-embedded dual-boron-containing blue-green MR-TADF emitters.

High Mobility Emissive Organic Semiconductors for Optoelectronic Devices

High mobility emissive organic semiconductors (HMEOSCs) are a kind of unique semiconducting material that simultaneously integrates high charge carrier mobility and strong emission features, which are not only crucial for overcoming the performance bottlenecks of current organic optoelectronic devices but also important for constructing high-density integrated devices/circuits for potential smart display technologies and electrically pumped organic lasers. However, the development of HMEOSCs is facing great challenges due to the mutually exclusive requirements of molecular structures and packing modes between high charge carrier mobility and strong solid-state emission. Encouragingly, considerable advances on HMEOSCs have been made with continuous efforts, and the successful integration of these two properties within individual organic semiconductors currently presents a promising research direction in organic electronics. Representative progress, including the molecular design of HMEOSCs, and the exploration of their applications in photoelectric conversion devices and electroluminescent devices, especially organic photovoltaic cells, organic light-emitting diodes, and organic light-emitting transistors, are summarized in a timely manner. The current challenges of developing HMEOSCs and their potential applications in other related devices including electrically pumped organic lasers, spin organic light-emitting transistors are also discussed. We hope that this perspective will boost the rapid development of HMEOSCs with a new mechanism understanding and their wide applications in different fields entering a new stage.

Robust Sandwich-Structured Thermally Activated Delayed Fluorescence Molecules Utilizing 11,12-Dihydroindolo[2,3-a]carbazole as Bridge

Constructing folded molecular structures is emerging as a promising strategy to develop efficient thermally activated delayed fluorescence (TADF) materials. Most folded TADF materials have V-shaped configurations formed by donors and acceptors linked on carbazole or fluorene bridges. In this work, a facile molecular design strategy is proposed for exploring sandwich-structured molecules, and a series of novel and robust TADF materials with regular U-shaped sandwich conformations are constructed by using 11,12-dihydroindolo[2,3-a]carbazole as bridge, xanthone as acceptor, and dibenzothiophene, dibenzofuran, 9-phenylcarbazole and indolo[3,2,1-JK]carbazole as donors. They hold outstanding thermal stability with ultrahigh decomposition temperatures (556-563 °C), and exhibit fast delayed fluorescence and excellent photoluminescence quantum efficiencies (86 %-97 %). The regular and close stacking of acceptor and donors results in rigidified molecular structures with efficient through-space interaction, which are conducive to suppressing intramolecular motion and reducing reorganized excited-state energy. The organic light-emitting diodes (OLEDs) using them as emitters exhibit excellent electroluminescence performances, with maximum external quantum efficiencies of up to 30.6 %, which is a leading value for the OLEDs based on folded TADF emitters. These results demonstrate the proposed strategy of employing 11,12-dihydroindolo[2,3-a]carbazole as bridge for planar donors and acceptors to construct efficient folded TADF materials is applicable.

Three-component diels-alder reaction through palladium carbene migratory insertion enabled dearomative C(sp3)-H bond activation

Owning to the versatile nature in participation of Diels-Alder (D-A) reactions, the development of efficient approaches to generate active ortho-quinodimethanes (o-QDMs) has gained much attention. However, a catalytic method involving coupling of two readily accessible components to construct o-QDMs is lacking. Herein, we describe a palladium carbene migratory insertion enabled dearomative C(sp3)-H activation to form active o-QDM species through the cross-coupling of N-tosylhydrazones with aryl halides. The in situ generated o-QDM intermediates were trapped efficiently by 3-nitroindoles and N-sulfonylaldimines to provide dihydroindolo[2,3-b]carbazole derivatives and indole alkaloids modularly. To our knowledge, this reaction represents a rare example on three-component D-A cycloaddition through in situ generation of conjugated dienes by the coupling two readily available materials. We anticipate such a reaction mode could find broad application on diversity oriented six-membered ring construction. Deuterium labeling experiments and density functional theory calculations support a pathway through reversible C(sp3)-H activation to generate heterocyclic o-QDMs.

Tailoring Ultra-Narrowband Tetraborylated Multiple Resonance Emitter for High-Performance Blue OLED

Ultra-narrowband multiple resonance (MR) emitters are a key component in the fabrication of highly efficient and stable blue organic light-emitting diodes (OLEDs). To explore the theoretical boundaries of wavelength and full width at half maximum (FWHM) in blue emitters, the currently narrowest boron-based MR emitter is carefully designed by integrating the superior v-DABNA and BBCz-DB structures under the auspices of the ingenious short-range charge-transfer region regulation strategy. The target tetraboron compound TB-PB demonstrates a blue emission with an emission maximum of 473 nm, a small FWHM of 12 nm and a CIEy coordinate of 0.14. Benefiting from the emitter's high photoluminescence quantum yield (99%), low excited-state energy (2.74 eV) and short delayed fluorescence lifetime (0.53 µs), the corresponding OLED achieves exceptional efficiencies of 36.4%, 49.1 cd A-1, and 51.4 lm W-1 with a record-high luminescence of 9.0 × 105 cd m-2, an ultra-narrow FWHM of 15 nm and a CIEy coordinate of 0.20. These breakthroughs will accelerate the development of next-generation blue emitters and lead to the advancement of OLED technology.

Molecular Engineering Towards Efficient Aggregation-Induced Delayed Fluorescence Luminogens as Emitters and Sensitizers for High-Performance Organic Light-Emitting Diodes

Exploring efficient thermally-activated delayed fluorescence materials having maximum external quantum efficiencies (ηext,maxs) exceeding 30 % for organic light-emitting diodes (OLEDs) still remains challenging because it generally requires efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (ΦPL), and large optical out-coupling efficiency (Φout) simultaneously. Herein, two green aggregation-induced delayed fluorescence (AIDF) luminogens, named XTCz-2 and XTCz-3, are designed and constructed by using xanthone (XT) as electron acceptor and phenylcarbazole-substituted carbazole as donors. XTCz-2 and XTCz-3 exhibit distinguished advantages of high thermal stability (439-560 °C), excellent ΦPLs (84-88 %) and fast RISC rates (1.9×105-4.2×105 s-1), and prefer horizontal dipole orientation and thus have high Φouts. Consequently, they can achieve the state-of-the-art electroluminescence (EL) performances with ηext,maxs of up to 35.0 %. Moreover, XTCz-3 is selected as a sensitizer for sky-blue multi-resonance delayed fluorescence emitter in hyperfluorescence OLEDs, providing narrow EL spectra and excellent ηext,maxs of up to 33.8 % with low efficiency roll-offs. The splendid comprehensive performances demonstrate the significant application potential of these AIDF luminogens as both light-emitting materials and sensitizers for OLEDs.

Rational Design of Coumarin-Based Hybridized Local and Charge-Transfer Blue Emitters for Solution-Processed Organic Light-Emitting Diodes

Hybridized local and charge-transfer (HLCT) with the utilization of both singlet and triplet excitons through the "hot excitons" channel have great application potential in highly efficient blue organic light-emitting diodes (OLEDs). The proportion of charge-transfer (CT) and locally excited (LE) components in the relevant singlet and triplet states makes a big difference for the high-lying reverse intersystem crossing process. Herein, three novel donor (D)-acceptor (A) type HLCT materials, 7-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-phenyl-1H-isochromen-1-one (pPh-7P), 7-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-methyl-1H-isochromen-1-one (pPh-7M), and 6-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-methyl-1H-isochromen-1-one (pPh-6M), were rationally designed and synthesized with diphenylamine derivative as donor and oxygen heterocyclic coumarin moiety as acceptors. The proportions of CT and LE components were fine controlled by changing the connection site of diphenylamine derivative at C6/C7-position and the substituent at C3-position of coumarin moiety. The HLCT characteristics of pPh-7P, pPh-7M, and pPh-6M were systematically demonstrated through photophysical properties and density functional theory calculations. The solution-processed doped OLEDs based on pPh-6M exhibited deep-blue electroluminescence with the maximum emission wavelength of 446 nm, maximum luminance of 8755 cd m-2, maximum current efficiency of 5.83 cd A-1, and maximum external quantum efficiency of 6.54 %. The results reveal that pPh-6M with dominant 1LE and 3CT components has better OLED performance.

Bisphosphonate-Anchored Self-Assembled Molecules with Larger Dipole Moments for Efficient Inverted Perovskite Solar Cells with Excellent Stability

In the fabrication of inverted perovskite solar cells (PSCs), the wettability, adsorbability, and compactness of self-assembled monolayers (SAMs) on conductive substrates have critical impacts on the quality of the perovskite films and the defects at the buried perovskite-substrate interface, which control the efficiency and stability of the devices. Herein, three bisphosphonate-anchored indolocarbazole (IDCz)-derived SAMs, IDCz-1, IDCz-2, and IDCz-3, are designed and synthesized by modulating the position of the two nitrogen atoms of the IDCz unit to improve the molecular dipole moments and strengthen the π-π interactions. Regulating the work functions (WF) of FTO electrodes through molecular dipole moments and energy levels, the perovskite band bends upwards with a small offset for ITO/IDCz-3/perovskite, thereby promoting hole extraction and blocking electrons. As a result, the inverted PSC employing IDCz-3 as hole-collecting layer exhibits a champion PCE of 25.15%, which is a record efficiency for the multipodal SAMs-based PSCs. Moreover, the unencapsulated device with IDCz-3 can be stored for at least 1800 h with little degradation in performance.

Modulation of luminescence properties of circularly polarized thermally activated delayed fluorescence molecules with axial chirality by donor engineering

Multifunctional thermally activated delayed fluorescence (TADF) materials are currently a trending research subject for luminescence layer materials of organic light-emitting diodes (OLEDs). Among these, circularly polarized thermally activated delayed fluorescence (CP-TADF) materials have the advantage of being able to directly achieve highly efficient circularly polarized luminescence (CPL). The simultaneous integration of outstanding luminescence efficiency and excellent luminescence asymmetry factor (glum) is a major constraint for the development of CP-TADF materials. Therefore, on the basis of first-principles calculations in conjunction with the thermal vibration correlation function (TVCF) method, we study CP-TADF molecules with different donors to explore the feasibility of using the donor substitution strategy for optimizing the CPL and TADF properties. The results indicate that molecules with the phenothiazine (PTZ) unit as the donor possess small energy difference, a great spin-orbit coupling constant and a rapid reverse intersystem crossing rate, which endow them with remarkable TADF features. Meanwhile, compared with the reported molecules, the three designed molecules exhibit better CPL properties with higher glum values. Effective molecular design strategies by donor engineering to modulate the CPL and TADF properties are theoretically proposed. Our findings reveal the relationship between molecular structures and luminescence properties of CP-TADF molecules and further provide theoretical design strategies for optimizing the CPL and TADF properties.

Enhancing OLED emitter efficiency through increased rigidity

Three new blue materials, TPI-InCz, PAI-InCz, and CN-PAI-InCz, have been developed. In the film state, TPI-InCz and PAI-InCz exhibited emission peaks at 411 and 431 nm indicating deep blue emission. CN-PAI-InCz showed a peak emission at 452 nm, within the real blue region. When these three materials were used as the emissive layer to fabricate non-doped devices, CN-PAI-InCz showed the highest current efficiency of 2.91 cd A-1, power efficiency of 1.93 lm W-1, and external quantum efficiency of 3.31%. Among the synthesized materials, CN-PAI-InCz exhibited superior charge balance due to the introduction of CN groups, as confirmed by hole-only devices and electron-only devices. PAI-InCz demonstrated fast hole mobility with a value of 1.50 × 10-3 cm2 V-1 s-1, attributed to its planar and highly rigid structure. In the resulting devices, the Commission Internationale de l'Eclairage coordinates for TPI-InCz, PAI-InCz, and CN-PAI-InCz were (0.162, 0.048), (0.0161, 0.067), and (0.155, 0.099), all indicating emission in the blue region.

A figure of merit for efficiency roll-off in TADF-based organic LEDs

Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and televisions1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. At present, materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters that usually use heavy metal atoms3,4. Although excellent progress has been made, in most TADF OLEDs there is a severe decrease of efficiency as the drive current is increased, known as efficiency roll-off. So far, much of the literature suggests that efficiency roll-off should be reduced by minimizing the energy difference between singlet and triplet excited states (ΔEST) to maximize the rate of conversion of triplets to singlets by means of reverse intersystem crossing (kRISC)5-20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new figure of merit will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality and lasing.

An Ideal Molecular Construction Strategy for Ultra-Narrow-Band Deep-Blue Emitters: Balancing Bathochromic-Shift Emission, Spectral Narrowing, and Aggregation Suppression

Narrowband emissive multiple resonance (MR) emitters promise high efficiency and stability in deep-blue organic light-emitting diodes (OLEDs). However, the construction of ideal ultra-narrow-band deep-blue MR emitters still faces formidable challenges, especially in balancing bathochromic-shift emission, spectral narrowing, and aggregation suppression. Here, DICz is chosen, which possesses the smallest full-width-at-half-maximum (FWHM) in MR structures, as the core and solved the above issue by tuning its peripheral substitution sites. The 1-substituted molecule Cz-DICz is able to show a bright deep-blue emission with a peak at 457 nm, an extremely small FWHM of 14 nm, and a CIE coordinate of (0.14, 0.08) in solution. The corresponding OLEDs exhibit high maximum external quantum efficiencies of 22.1%-25.6% and identical small FWHMs of 18 nm over the practical mass-production concentration range (1-4 wt.%). To the best of the knowledge, 14 and 18 nm are currently the smallest FWHM values for deep-blue MR emitters with similar emission maxima under photoluminescence and electroluminescence conditions, respectively. These discoveries will help drive the development of high-performance narrowband deep-blue emitters and bring about a revolution in OLED industry.

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.

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.

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.

Recent synthetic strategies for the construction of functionalized carbazoles and their heterocyclic motifs enabled by Lewis acids

This article demonstrates recent innovative cascade annulation methods for preparing functionalized carbazoles and their related polyaromatic heterocyclic compounds enabled by Lewis acid catalysts. Highly substituted carbazole scaffolds were synthesized via Lewis acid mediated Friedel-Crafts arylation, electrocyclization, intramolecular cyclization, cycloaddition, C-N bond-formations, aromatization and cascade domino reactions, metal-catalyzed, iodine catalyzed reactions and multi-component reactions. This review article mainly focuses on Lewis acid-mediated recent synthetic methods to access a variety of electron-rich and electron-poor functional groups substituted carbazole frameworks in one-pot reactions. Polyaromatic carbazole and their related nitrogen-based heterocyclic compounds were found in several synthetic applications in pharma industries, energy devices, and materials sciences. Moreover, the review paper briefly summarised new synthetic strategies of carbazole preparation approaches will assist academic and pharma industries in identifying innovative protocols for producing poly-functionalized carbazoles and related highly complex heterocyclic compounds and discovering active pharmaceutical drugs or carbazole-based alkaloids and natural products.

Non-Blinking Luminescence from Charged Single Graphene Quantum Dots

Photoluminescence blinking behavior from single quantum dots under steady illumination is an important but controversial topic. Its occurrence has impeded the use of single quantum dots in bioimaging. Different mechanisms have been proposed to account for it, although controversial, the most important of which is the non-radiative Auger recombination mechanism whereby photocharging of quantum dots can lead to the blinking phenomenon. Here, the singly charged trion, which maintains photon emission, including radiative recombination and non-radiative Auger recombination, leads to fluorescence non-blinking which is observed in photocharged single graphene quantum dots (GQDs). This phenomenon can be explained in terms of different energy levels in the GQDs, caused by various oxygen-containing functional groups in the single GQDs. The suppressed blinking is due to the filling of trap sites owing to a Coulomb blockade. These results provide a profound understanding of the special optical properties of GQDs, affording a reference for further in-depth research.

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.

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.

Controllable construction of red thermally activated delayed fluorescence molecules based on a spiro-acridine donor

Red and near-infrared (NIR) thermally activated delayed fluorescence (TADF) molecules show excellent potential applications in organic light-emitting diodes (OLEDs). Due to the lack of systematic studies on the relationship between molecular structures and luminescence properties, both the species and amounts of red and NIR TADF molecules are far from meeting the requirements for practical applications. Herein, four new efficient molecules (DQCN-2spAs, TPCN-2spAs, DPCN-2spAs and BPCN-2spAs) are proposed and their photophysical properties are theoretically predicted based on first-principles calculations and thermal vibration correlation function (TVCF) theory. The results show that all molecules exhibit red or NIR emissions and they have fast radiative decay rates and reverse intersystem crossing (RISC) rates, and the excellent TADF luminescence properties are predicted. Moreover, based on spiro-acridine (spAs) as the donor unit, the combination with different acceptors can change the dihedral angle between the ground state and the excited state, the bending degree of the donor is positively correlated with the reorganization energy, and this feature can have a great influence on the non-radiative process. Furthermore, based on these theoretical predictions, experimental verifications are performed and the synthesized BPCN-2spAs is confirmed to be an efficient NIR TADF molecule. Thus, the relationships between basic molecular structures and photophysical properties are revealed, a feasible design strategy is applied and four promising red and NIR TADF molecules are proposed. All these results could contribute to the development of red and NIR TADF emitters and OLEDs.

Progress in the Development of Imidazopyridine-Based Fluorescent Probes for Diverse Applications

Different classes of Imidazopyridine i.e., Imidazo[1,2-a]pyridine, Imidazo[1,5-a] pyridine, Imidazo[4,5-b]pyridine, have shown versatile applications in various fields. In this review, we have concisely presented the usefulness of the fluorescent property of imidazopyridine in different fields such as imaging tools, optoelectronics, metal ion detection, etc. Fluorescence mechanisms such as excited state intramolecular proton transfer, photoinduced electron transfer, fluorescence resonance energy transfer, intramolecular charge transfer, etc. are incorporated in the designed fluorophore to make it for fluorescent applications. It has been widely employed for metal ion detection, where selective metal ion detection is possible with triazole-attached imidazopyridine, β-carboline imidazopyridine hybrid, quinoline conjugated imidazopyridine, and many more. Also, other popular applications involve organic light emitting diodes and cell imaging. This review shed a light on recent development in this area especially focusing on the optical properties of the molecules with their usage which would be helpful in designing application-based new imidazopyridine derivatives.

Aggregation-Induced Intermolecular Charge Transfer Emission for Solution-Processable Bipolar Host Material via Adjusting the Length of Alkyl Chain

Molecules with donor-spacer-acceptor configuration have been developed rapidly given their peculiar properties. How to utilize intermolecular interactions and charge transfers for solution-processed organic light-emitting diodes (OLEDs) greatly relies on molecular design strategy. Herein, soluble luminophores with D-spacer-A motif were constructed via shortening the alkyl chain from nonane to propane, where the alkyl chain was utilized as a spatial linker between the donor and acceptor. The alkyl chain blocks the molecular conjugation and induces the existence of aggregation-induced intermolecular CT emission, as well as the improved solubility and morphology in a solid-state film. In addition, the length of the alkyl chain affects the glass transition temperature, carrier transport and balance properties. The mCP-3C-TRZ with nonane as the spacer shows better thermal stability and bipolar carrier transport ability, so the corresponding solution-processable phosphorescent organic light-emitting diodes exhibit superior external quantum efficiency of 9.8% when using mCP-3C-TRZ as a host material. This work offers a promising strategy to establish a bipolar host via utilizing intermolecular charge transfer process in an aggregated state.

Organic Phosphorescence Lasing Based on a Thermally Activated Delayed Fluorescence Emitter

Organic phosphorescence materials provide an opportunity to use triplets for lasing. However, population inversion based on phosphorescence is hard to establish, owing to low luminescent quantum efficiency and intensive optical loss. By comparison, thermally activated delayed fluorescence emitters exhibit excellent optical gain with the aid of the reverse intersystem crossing (RISC) process. In this work, we designed a multifunctional gain material, not only serving as a thermally activated delayed fluorescence (TADF) emitter with excellent optical gain but also working as a phosphorescence source with high utilization of triplets. The lone pair of electrons in oxygen substitutions promotes a fast spin-flip and high delayed fluorescence quantum yield (Φ_DF = 55%), enabling TADF amplified spontaneous emissions (ASE) of CH₂Cl₂ solution. Single-crystalline nanowires of H-aggregates effectively lower triplet energy levels with high phosphorescence quantum yield (Φ_P = 27%), demonstrating Fabry-Perot mode phosphorescence lasing at 630 nm.

Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence

Phosphorescent and thermally activated delayed fluorescence (TADF) blue organic light-emitting diodes (OLEDs) have been developed to overcome the low efficiency of fluorescent OLEDs. However, device instability, originating from triplet excitons and polarons, limits blue OLED applications. Here, we develop a phosphor-sensitized TADF emission system with TADF emitters to achieve high efficiency and long operational lifetime. Peripheral carbazole moieties are introduced in conventional multi-resonance-type emitters containing one boron atom. The triplet exciton density of the TADF emitter is reduced by facilitating reverse intersystem crossing, and the Förster resonant energy transfer rate from phosphor sensitizer is enhanced by high absorption coefficient of the emitters. The emitter exhibited an operational lifetime of 72.9 hours with Commission Internationale de L'Eclairage chromaticity coordinate y = 0.165, which was 6.6 times longer than those of devices using conventional TADF emitters.

Tuning the Diradical Character of Indolocarbazoles: Impact of Structural Isomerism and Substitution Position

In this study, a set of 10 positional indolocarbazole (ICz) isomers substituted with dicyanomethylene groups connected via para or meta positions are computationally investigated with the aim of exploring the efficiency of structural isomerism and substitution position in controlling their optical and electronic properties. Unrestricted density functional theory (DFT), a spin-flip time-dependent DFT approach, and the multireference CASSCF/NEVPT2 method have been applied to correlate the diradical character with the energetic trends (i.e., singlet-triplet energy gaps). In addition, the nucleus-independent chemical shift together with ACID plots and Raman intensity calculations were used to strengthen the relationship between the diradical character and (anti)aromaticity. Our study reveals that the substitution pattern and structural isomerism represent a very effective way to tune the diradical properties in ICz-based systems with meta-substituted systems with a V-shaped structure displaying the largest diradical character. Thus, this work contributes to the elucidation of the challenging chemical reactivity and physical properties of diradicaloid systems, guiding experimental chemists to produce new molecules with desirable properties.

Color-Tunable All-Fluorescent White Organic Light-Emitting Diodes with a High External Quantum Efficiency Over 30% and Extended Device Lifetime

White organic light-emitting diodes (WOLEDs) with high efficiencies and tunable colors attracts considerable interest from the industry and academia. Thermally activated delayed-fluorescence (TADF) emitters can revolutionize such WOLED devices; however, they still suffer from poor performances. In this study, an advanced double-emissive-layer device architecture capable of hole-trapping TADF-sensitized emissions is proposed to not only achieve a recombination zone shift for the tunable colors but also accelerate exciton emission dynamics for high efficiency and alleviated roll-off. The proof-of-concept WOLEDs exhibit significant shifts in their Commission Internationale de l'Eclairage (CIE) coordinates and correlated color temperatures from (0.40, 0.47) and 4088 K at 100 cd m^-2 to (0.27, 0.33) and 9269 K at 5000 cd m^-2 . Additionally, the maximum external quantum efficiency (EQE) reaches 30.7% and remains >25% over a wide luminance range of 500-5000 cd m^-2 , along with an extended LT80 of over 20 000 h at an initial luminance of 100 cd m^-2 . This is the first time that all-fluorescent WOLEDs have been used to realize an EQE exceeding 30%, thereby establishing a new benchmark in this field.

Structurally Nontraditional Bipolar Hosts for RGB Phosphorescent OLEDs: Boosted by a "Butterfly-Shaped" Medium-Ring Acceptor

The emitting layer based on a host-guest system plays a crucial role in organic light-emitting diodes (OLEDs). While emitters have witnessed rapid progress in structural diversity, hosts still rely heavily on traditional structures and are underdeveloped. Herein a "medium-ring" strategy has been put forward to design structurally nontraditional host molecules, which are not only rotatable enough to suppress close π-π stacking, thus reducing exciton annihilation, but also rigid enough to prevent excessive conformational flipping, thus inhibiting non-radiative decay. Accordingly, a brand-new type of bipolar hosts with a twisted "butterfly-shaped heptagonal acceptor (EtBP), which features an electron-deficient benzophenone fragment with a flexible ethylidene bridge, has been developed. With satisfactory morphological stability and well-balanced hole- and electron-transporting properties, the EtBP-based bipolar hosts enable high-performance RGB phosphorescent OLEDs with small efficiency roll-off, which are superior to those of acyclic benzophenone-based devices.

Efficient Ultraviolet Organic Light-Emitting Diodes with a CIEy of 0.04 and Negligible-Efficiency Roll-Off

Organic light-emitting diodes (OLEDs) with ultraviolet (UV) emission (λ_EL ≤ 400 nm) have attracted special attention in commercial and civil fields owing to their special functions. Nevertheless, the lack of high-quality ultraviolet emitters restricts the practical application of UV OLEDs. Herein, a novel organic molecule with desirable UV emission, 2Na-CzCN, is developed for UV OLEDs. Theoretical investigation indicates that it is equipped with hybridized local and charge-transfer (HLCT) characteristics, which is in favor of the high-lying reverse intersystem crossing (RISC) process, thus remarkably boosting the exciton utilization in electroluminescence (EL). Significantly, the nondoped device derived from the 2Na-CzCN emitter exhibits an EL emission peak of 398 nm with a maximum external quantum efficiency (EQE) of 5.92%, which represents the record-high result among nondoped UV OLEDs. The doped UV OLED of 2Na-CzCN radiates robust UV emission at a peak of 392 nm with a maximum EQE of 6.15%. Coupled with the narrow full width at half-maximum (FWHM) of the EL spectra, desirable color purities with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.06) and (0.16, 0.04) for nondoped and doped OLEDs are presented, respectively. Additionally, the potential of 2Na-CzCN adopted as the host material is demonstrated with phosphorescent OLEDs (PhOLEDs), and all of the devices show good EL performances with low-efficiency roll-offs. An orange PhOLED with 2Na-CzCN acquires a maximum current and external quantum efficiency of 84.9 cd A^-1 and 25.3%, respectively. These findings may pave an avenue for the development of high-performance UV emitters.

Multi-resonant thermally activated delayed fluorescence emitters based on tetracoordinate boron-containing PAHs: colour tuning based on the nature of chelates

Multi-resonant thermally activated delayed fluorescence (MR-TADF) materials have attracted considerable attention recently. The molecular design frequently incorporates cycloboration. However, to the best of our knowledge MR-TADF compounds containing nitrogen chelated to boron are still unknown. Reported herein is a new class of tetracoordinate boron-containing MR-TADF emitters bearing C^N^C- and N^N^N-chelating ligands. We demonstrate that the replacement of the B–C covalent bond in the C^N^C-chelating ligand by the B–N covalent bond affords an isomer, which dramatically influences the optoelectronic properties of the molecule. The resulting N^N^N-chelating compounds show bathochromically shifted absorption and emission spectra relative to C^N^C-chelating compounds. The incorporation of a tert-butylcarbazole group at the 4-position of the pyridine significantly enhances both the thermal stability and the reverse intersystem crossing rate, yet has a negligible effect on emission properties. Consequently, high-performance hyperfluorescent organic light-emitting diodes (HF-OLEDs) that utilize these molecules as green and yellow-green emitters show a maximum external quantum efficiency (η_ext) of 11.5% and 25.1%, and a suppressed efficiency roll-off with an η_ext of 10.2% and 18.7% at a luminance of 1000 cd m⁻², respectively.

A new class of tetra-coordinate boron-containing MR-TADF emitters and their corresponding high-performance hyperfluorescent organic light-emitting diodes have been successfully achieved.

Tandem rigidification and π-extension as a key tool for the development of a narrow linewidth yellow hyperfluorescent OLED system

Hyperfluorescence (HF), a relatively new phenomenon utilizing the transfer of excitons between two luminophores, requires careful pairwise tuning of molecular energy levels and is proposed to be the crucial step towards the development of new, highly effective OLED systems. To date, barely few HF yellow emitters with desired narrowband emission but moderate external quantum efficiency (EQE < 20%) have been reported. This is because a systematic strategy embracing both Förster resonance energy transfer (FRET) and triplet to singlet (TTS) transition as complementary mechanisms for effective exciton transfer has not yet been proposed. Herein, we present a rational approach, which allows, through subtle structural modification, a pair of compounds built from the same donor and acceptor subunits, but with varied communication between these ambipolar fragments, to be obtained. The TADF-active dopant is based on a naphthalimide scaffold linked to the nitrogen of a carbazole moiety, which through the introduction of an additional bond leads not only to π-cloud enlargement, but also rigidifies and inhibits the rotation of the donor. This structural change prevents TADF, and guides bandgaps and excited state energies to simultaneously pursue FRET and TTS processes. New OLED devices utilizing the presented emitters show excellent external quantum efficiency (up to 27%) and a narrow full width at half maximum (40 nm), which is a consequence of very good alignment of energy levels. The presented design principles prove that only a minor structural modification is needed to obtain commercially applicable dyes for HF OLED devices.

The rigidification with simultaneous π-extension of TADF-active dye leads to fluorescent dopant with fine-tuned energy levels. These used as hyperfluorescent OLED device shows extraordinary EQE and brightness due to effective FRET and TTS processes.

Realization of Efficient Phosphorescent Organic Light-Emitting Devices Using Exciplex-Type Co-Host

High-performance phosphorescent organic light-emitting devices with an exciplex-type co-host were fabricated. The co-host is constituted by 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene, and 4,4,4-tris (N-carbazolyl) triphenylamine, and has obvious virtues in constructing efficient devices because of the thermally activated delayed fluorescence (TADF) resulting from a reverse intersystem crossing (RISC) process. The highest external quantum efficiency and luminance are 14.60% and 100,900 cd/m2 for the optimal co-host device. For comparison, 9.22% and 25,450 cd/m2 are obtained for a device employing 4,4,4-tris (N-carbazolyl) triphenylamine as a single-host. Moreover, the efficiency roll-off is notably alleviated for the co-host device, indicated by much higher critical current density of 327.8 mA/cm2, compared to 120.8 mA/cm2 for the single-host device. The alleviation of excitons quenching resulting from the captured holes and electrons, together with highly sufficient energy transfer between the co-host and phosphorescent dopant account for the obvious boost in device performances.

High-efficiency hyperfluorescent white light-emitting diodes based on high-concentration-doped TADF sensitizer matrices via spatial and energy gap effects

Despite the success of monochromatic hyperfluorescent (HF) organic light-emitting diodes (OLEDs), high-efficiency HF white OLEDs (WOLEDs) are still a big challenge. Herein, we demonstrate HF WOLEDs with state-of-the-art efficiencies, featuring a quasi-bilayer emissive layer (EML) composed of an ultrathin (0.1 nm) blue fluorescence (FL) emitter (TBPe) layer and a layer of thermally activated delayed fluorescence (TADF) sensitizer matrix heavily doped with a yellow FL emitter (TBRb, 3%). Based on an asymmetric high-energy-gap TADF sensitizer host (PhCzSPOTz), such an “ultrathin blue emitting layer (UTBL)” strategy endowed the HF WOLEDs with a record power efficiency of ∼80 lm W⁻¹, approaching the level of fluorescent tubes. Transient photoluminescence (PL) and electroluminescence (EL) kinetics demonstrate that the spatial separation of TBPe from the TADF sensitizer and TBRb, and the large energy gap between the latter two effectively suppress triplet leakage, in addition to suppressing triplet diffusion in the PhCzSPOTz matrix with anisotropic intermolecular interactions. These results provide a new insight into the exciton allocation process in HF white light-emitting systems.

A thermally activated delayed fluorescence host was developed to realize high-efficiency fluorescence white organic light-emitting diodes (WOLED) through spatial and energy gap effects.

Effective Design Strategy for Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Emitters Achieving 18% External Quantum Efficiency Pure-Blue OLEDs with Extremely Low Roll-Off

High-color purity organic emitters with a simultaneous combination of aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in great demand due to their excellent comprehensive performances toward efficient organic light-emitting diodes (OLEDs). In this work, two D-π-A-structure emitters, ICz-DPS and ICz-BP, exhibiting AIE and TADF properties were developed, and both the emitters have narrow singlet (S₁)-triplet (T₁) splitting (ΔE_ST) and excellent photoluminescence (PL) quantum yields (Φ_PL), derived from the distorted configurations and weak intra/intermolecular interactions, suppressing exciton annihilation and concentration quenching. Their doped OLEDs based on ICz-BP provide an excellent electroluminescence external quantum efficiency (η_ext) and current efficiency (η_C) of 17.7% and 44.8 cd A^-1, respectively, with an η_ext roll-off of 2.9%. Their nondoped OLEDs based on ICz-DPS afford high efficiencies of 11.7% and 30.1 cd A^-1, with pure-blue emission with Commission Internationale de l'Éclairage (CIE) coordinates of (0.15, 0.08) and a low roll-off of 6.0%. This work also shows a strategy for designing AIE-TADF molecules by rational use of steric hindrance and weak inter/intramolecular interactions to realize high Φ_PL values, fast reverse intersystem crossing process, and reduced nonradiative transition process properties, which may open the way toward highly efficient and small-efficiency roll-off devices.

Realizing Record-High Electroluminescence Efficiency of 31.5 % for Red Thermally Activated Delayed Fluorescence Molecules

Tailor-made red thermally activated delayed fluorescence (TADF) molecules comprised of an electron-withdrawing pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile core and various electron-donating triarylamines are developed. They can form intramolecular hydrogen-bonding, which is conducive to improving emission efficiency and promoting horizontal orientation and show near infrared (NIR) emissions (692-710 nm) in neat films and red delayed fluorescence (606-630 nm) with high photoluminescence quantum yields (73-90%) in doped films. They prefer horizontal orientation with large horizontal dipole ratios in films, rendering high optical out-coupling factors (0.39-0.41). Their non-doped OLEDs exhibit NIR lights (716-748 nm) with maximum external quantum efficiencies (η_ext,max ) of 1.0-1.9%. And their doped OLEDs radiate red lights (606-648 nm) and achieve record-beating η_ext,max of up to 31.5%. These new red TADF materials should have great potentials in display and lighting devices.

Crystal structure, Hirshfeld surface and photo-physical analysis of 2-nitro-3-phenyl-9H-carbazole

The title compound, C18H12N2O2, was synthesized from a di-nitro-biphenyl-benzene derivative using a novel modification of the Cadogan reaction. The reaction has several possible ring-closed products and the title compound was separated as the major product. The X-ray crystallographic study revealed that the carbazole compound crystallizes in the monoclinic P space group and possesses a single closed Cadogan ring. There are two independent mol-ecules in the asymmetric unit. In the crystal, the mol-ecules are linked by N-H⋯O hydrogen bonding.

Molecular design of thermally activated delayed fluorescent emitters for narrowband orange-red OLEDs boosted by a cyano-functionalization strategy

The establishment of a simple molecular design strategy to realize red-shifted emission while maintaining good color purity for multi-resonance induced thermally activated delayed fluorescent (MR-TADF) materials remains an appealing yet challenging task. Herein, we demonstrate that the attachment of a cyano (CN) functionality at the lowest unoccupied molecular orbital location of the MR-TADF skeleton can promote attractive red-shifted emission due to the exceptional electron-withdrawing capacity of the CN group, which represents the first example of orange-red MR-TADF emitters. Meanwhile, the linear CN group adopts a coplanar conformation with the MR-framework to restrict structure relaxation associated with rotation, which is beneficial to maintain a small full-width at half-maximum and thus a good color purity. The CNCz-BNCz-based OLED device, which utilizes a TADF sensitized mechanism to accelerate the up-conversion process of triplet excitons in the emitting layer, exhibits an outstanding external quantum efficiency (EQE) as high as 33.7%, representing the state-of-the-art performance for orange-red TADF-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 (g_EL ) 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.

Triazolotriazine-based thermally activated delayed fluorescence materials for highly efficient fluorescent organic light-emitting diodes (TSF-OLEDs)

Thermally activated delayed fluorescence (TADF) sensitized fluorescent organic light-emitting diodes (TSF-OLEDs) have shown great potential for the realization of high efficiency with low efficiency roll-off and good color purity. However, the superior examples of TSF-OLEDs are still limited up to now. Herein, a trade-off strategy is presented for designing efficient TADF materials and achieving high-performance TSF-OLEDs via the construction of a new type of triazolotriazine (TAZTRZ) acceptor. The enhanced electron-withdrawing ability of TAZTRZ acceptor, fused by triazine (TRZ) and triazole (TAZ) together, enables TADF luminogens with small singlet-triplet energy gap (ΔE_ST) values. Meanwhile, the increased planarity from the TRZ-phenyl linkage (6:6 system) to the TAZ-phenyl linkage (5:6 system) can compensate the decrease of oscillator strength (f) while lowing ΔE_ST, thus achieving a trade-off between small ΔE_ST and high f. As a result, the related TSF-OLED achieved an extremely low turn-on voltage of 2.1 V, an outstanding maximum external quantum efficiency (EQE_max) of 23.7% with small efficiency roll-off (EQE₁₀₀₀ of 23.2%; EQE₅₀₀₀ of 20.6%) and an impressively high maximum power efficiency of 82.1 lm W^-1, which represents the state-of-the-art performance for yellow TSF-OLEDs.

A Brief History of OLEDs-Emitter Development and Industry Milestones

Organic light-emitting diodes (OLEDs) have come a long way ever since their first introduction in 1987 at Eastman Kodak. Today, OLEDs are especially valued in the display and lighting industry for their promising features. As one of the research fields that equally inspires and drives development in academia and industry, OLED device technology has continuously evolved over more than 30 years. OLED devices have come forward based on three generations of emitter materials relying on fluorescence (first generation), phosphorescence (second generation), and thermally activated delayed fluorescence (third generation). Furthermore, research in academia and industry toward the fourth generation of OLEDs is in progress. Excerpts from the history of green, orange-red, and blue OLED emitter development on the side of academia and milestones achieved by key players in the industry are included in this report.

Enhanced Upconversion of Triplet Excitons for Conjugated Polymeric Thermally Activated Delayed Fluorescence Emitters by Employing an Intramolecular Sensitization Strategy

Endowed by a thermally activated delayed fluorescence (TADF) sensitizer with a high constant rate of reverse intersystem crossing, the singlet excitons could be accumulated and then delivered to emitting states through favorable Förster resonance energy transfer, bypassing the inefficient intersystem transition processes of emitters. However, the conventional intermolecular sensitization strategies suffer from inherent aggregation-induced quenching and inevitable phase segregation of TADF sensitizers and emitters. In this context, we proposed a novel intramolecular sensitization strategy by covalently incorporating the TADF sensitizer into conjugated polymeric emitters. After rationally regulating the proportions of sensitizer and emitter units in polymers, the intramolecular sensitized conjugated TADF polymers with anticipated photophysical properties and stable device performance were obtained. A superior k_RISC value over 10⁶ s^-1 accompanied by a suppressed nonradiative transition of the triplet exciton could be gained; therefore, the photoluminescence quantum yield (PLQY) could reach nearly 90%. In accord with the superior PLQY values enhanced by our intramolecular sensitization strategy, the solution-processed organic light-emitting diodes (OLEDs) can achieve a maximum external quantum efficiency (EQE) value of 17.8% while still maintaining 16.0% at 1000 cd/m² with extremely low efficiency roll-off. These results convincingly manifest the significance of an intramolecular sensitization strategy for designing high-efficiency polymeric TADF emitters.

Disentangling Multiple Effects on Excited-State Intramolecular Charge Transfer among Asymmetrical Tripartite PPI-TPA/PCz Triads

By utilizing the bipolarity of 1,2-diphenylphenanthroimidazole (PPI), two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) were designed with triphenylamine (TPA) and 9-phenylcarbazole (PCz). These triads are deep-blue luminescent materials with a high fluorescence quantum yield of nearly 100 %. To trace the photophysical behaviors of these triads, their excited-state evolution channels and interchromophoric interactions were investigated by ultrafast time-resolved transient absorption and excited-state theoretical calculations. The results suggest that the electronic nature, asymmetrical tripartite structure, and electron-hole distance of these triads, as well as solvent polarity, determine the lifetime of intramolecular charge transfer (ICT). Interestingly, PPI-PCz triads show anti-Kasha ICT, and the charge-transfer direction among the triads is adjustable. For the PPI-TPA triad, the electron is transferred from TPA to PPI, whereas for the PPI-PCz triad the electron is pushed from PPI to PCz. Exploration of the excited-state ICT in these triads may pave the way to design better luminescent materials in the future.