Hypsochromic Shift of Multiple-Resonance-Induced Thermally Activated Delayed Fluorescence by Oxygen Atom Incorporation

Sequential Multiple Borylation Toward an Ultrapure Green Thermally Activated Delayed Fluorescence Material

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters have emerged as an important component of organic light-emitting diodes (OLEDs) because of their narrowband emission and high exciton utilization efficiency. However, the chemical space of MR-TADF emitters remains mostly unexplored because of the lack of suitable synthetic protocols. Herein, we demonstrate a sequential multiple borylation reaction that provides new synthetically accessible chemical space. ω-DABNA, the proof-of-concept material, exhibited narrowband green TADF with a full width at half-maximum of 22 nm and a small singlet-triplet energy gap of 13 meV. The OLED employing it as an emitter exhibited electroluminescence at 512 nm, with Commission International de l'Éclairage coordinates of (0.13, 0.73) and a high external quantum efficiency (EQE) of 31.1%. Moreover, the device showed minimum efficiency roll-off, with an EQE of 29.4% at 1000 cd m^-2.

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.

Gold(I) Multi-Resonance Thermally Activated Delayed Fluorescent Emitters for Highly Efficient Ultrapure-Green Organic Light-Emitting Diodes

Acceleration of singlet-triplet intersystem crossings (ISC) is instrumental in bolstering triplet exciton harvesting of multi-resonance thermally activated delayed fluorescent (MR-TADF) emitters. This work describes a simple gold(I) coordination strategy to enhance the spin-orbit coupling of green and blue BN(O)-based MR-TADF emitters, which results in a notable increase in rate constants of the spectroscopically observed ISC process to 3×10⁹  s^-1 with nearly unitary ISC quantum yields. Accordingly, the resultant thermally-stable Au^I emitters attained large values of delayed fluorescence radiative rate constant up to 1.3×10⁵ /1.7×10⁵  s^-1 in THF/PMMA film while preserving narrowband emissions (FWHM=30-37 nm) and high emission quantum yields (ca. 0.9). The vapor-deposited ultrapure-green OLEDs fabricated with these Au^I emitters delivered high luminance of up to 2.53×10⁵  cd m^-2 as well as external quantum efficiencies of up to 30.3 % with roll-offs as low as 0.8 % and long device lifetimes (LT₆₀ ) of 1210 h at 1000 cd m^-2 .

Ultra-Narrowband Blue Multi-Resonance Thermally Activated Delayed Fluorescence Materials

Ultra-narrowband blue multi-resonance-induced thermally activated delayed fluorescence (MR-TADF) materials (V-DABNA and V-DABNA-F), consisting of three DABNA subunits possessing phenyl or 2,6-difluorophenyl substituents on the peripheral nitrogen atoms are synthesized by one-shot triple borylation. Benefiting from the inductive effect of fluorine atoms, the emission maximum of V-DABNA-F (464 nm) is blueshifted from that of the parent V-DABNA (481 nm), while maintaining a small full width at half maximum (FWHM, 16 nm) and a high rate constant for reverse intersystem crossing (6.5 × 105 s-1 ). The organic light-emitting diodes (OLEDs) using V-DABNA and V-DABNA-F as emitters are fabricated by vapor deposition and exhibit blue emission at 483 and 468 nm with small FWHMs of 17 and 15 nm, corresponding to Commission Internationale d'Éclairage coordinates of (0.09, 0.27) and (0.12, 0.10), respectively. Both devices achieve high external quantum efficiencies of 26.2% and 26.6% at the maximum with minimum efficiency roll-offs of 0.9% and 3.2%, respectively, even at 1000 cd m-2 , which are record-setting values for blue MR-TADF OLEDs.

Double boron-embedded multiresonant thermally activated delayed fluorescent materials for organic light-emitting diodes

The subclass of multi resonant thermally activated delayed fluorescent emitters (MR-TADF) containing boron atoms has garnered significant attention in the field of organic light emitting diode (OLED) research. Among boron-based MR-TADF emitters, double boron-embedded MR-TADF (DB-MR-TADF) emitters show excellent electroluminescence performances with high photoluminescence quantum yields, narrow band emission, and beneficially small singlet-triplet energy levels in all the full-color gamut regions. This article reviews recent progress in DB-MR-TADF emitters, with particular attention to molecular design concepts, synthetic routes, optoelectronic properties, and OLED performance, giving future prospects for real-world applications.

Selenium-Doped Polycyclic Aromatic Hydrocarbon Multiresonance Emitters with Fast Reverse Intersystem Crossing for Narrowband Blue Emission

Two kinds of boron- (B), selenium- (Se), and nitrogen-doped (N) polycyclic aromatic hydrocarbon (PAH) emitters (Cz-BSeN and DCz-BSeN) with a multiresonance effect are developed for narrowband blue emission by embedding boron as an electron-deficient atom and selenium and nitrogen as electron-donating atoms into a benzo[a]naphtho[1,2,3-hi]aceanthrylene skeleton. It is found that both emitters exhibit strong spin-orbit coupling and fast reverse intersystem crossing (rate constant of 7.5-8.8 × 106 s-1) due to the heavy-atom effect of selenium, which is 2 orders of magnitude faster than its B, N-doped PAH analogue. Meanwhile, compared to parent B, Se, N-doped PAH emitter Cz-BSeN, incorporating carbazole moieties on the para position of the boron atom in DCz-BSeN not only blueshifts the emission by 7 nm without broadening its spectra but also results in an enhanced photoluminescent quantum efficiency of 93% in the doped film. The organic light-emitting diode (OLED) employing DCz-BSeN as emitter revealed narrowband blue emission at 481 nm with a small full-width at half-maximum (fwhm) of 32 nm, as well as a maximum external quantum efficiency of 22.3%, accompanied by alleviated efficiency roll-off compared to its B, N-containing counterpart.

Manipulating Spectral Width and Emission Wavelength towards Highly Efficient Blue Asymmetric Carbazole Fused Multi-Resonance Emitters

The novel carbazole-based multiresonance types of thermally activated delayed fluorescence (MR-TADF) emitters of mICz-DABNA and BFCz-DABNA are reported, and their spectroscopic properties are investigated with the inductive effect on the central nitrogen atom for pure and deep blue emission. With the introduction of electron-donating/-withdrawing properties of substituents, emitters exhibited the bathochromic/hypsochromic shifted emission, respectively, compared to simple carbazole-based MR-TADF. Moreover, their spectral bandwidths became narrower. Theoretical calculation indicated that the meta-positioned bulky moiety restricts the molecular geometry discrepancy and reduces the Huang-Rhys factors. Particularly, the organic light-emitting diode (OLED) with 3% BFCz-DABNA exhibited the maximum external quantum efficiency of 28.0% with the Commission International de l'Éclairage (CIE) of (0.13, 0.09), which is the best record value among single-boron MR-TADF devices of CIE y < 0.10.

Ultrafast Triplet-Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens

Narrowband emissive organoboron emitters featuring the multi-resonance (MR) effect have now become a critical material component for constructing high-performance organic light-emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high-efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet-to-singlet reverse intersystem crossing (RISC). However, RISC involving spin-flip exciton upconversion is generally the rate-limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter (CzBSe) doped with a selenium atom, demonstrating a record-high RISC rate exceeding 10⁸  s^-1 , which is even higher than its fluorescence radiation rate. Furthermore, the spin-flip upconversion process in CzBSe can be accelerated by factors of ≈20000 and ≈800, compared to those of its oxygen- and sulfur-doped homologs (CzBO and CzBS), respectively. Unlike CzBO and CzBS, the photophysical rate-limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time-delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR-induced blue narrowband emission and significantly alleviated efficiency roll-off features.

Development of Pure Green Thermally Activated Delayed Fluorescence Material by Cyano Substitution

Multiple resonance (MR)-effect-induced thermally activated delayed fluorescence (TADF) materials have garnered significant attention because they can achieve both high color purity and high external quantum efficiency (EQE). However, the reported green-emitting MR-TADF materials exhibit broader emission compared to those of blue-emitting ones and suffer from severe efficiency roll-off due to insufficient rate constants of reverse intersystem crossing process (k_RISC ). Herein, a pure green MR-TADF material (ν-DABNA-CN-Me) with high k_RISC of 10⁵ s^-1 is reported. The key to success is introduction of cyano groups into a blue-emitting MR-TADF material (ν-DABNA), which causes remarkable bathochromic shift without a loss of color purity. The organic light-emitting diode employing it as an emitter exhibits green emission at 504 nm with a small full-width at half-maximum of 23 nm, corresponding to Commission Internationale d'Éclairage coordinates of (0.13, 0.65). The device achieves a high maximum EQE of 31.9% and successfully suppresses the efficiency roll-off at a high luminance.

Extending the π-Skeleton of Multi-Resonance TADF Materials towards High-Efficiency Narrowband Deep-Blue Emission

Multi-resonance TADF (MR-TADF) emitters are promising for high-resolution OLEDs, but the concurrent optimization of excited-state dynamics and color purity remains a tough challenge. Herein, three deep-blue MR-TADF compounds (BN1-BN3) featuring gradually enlarged ring-fused structures and increased rigidity are accessed by lithium-free borylation in high yields from the same precursor, with all the emitters possessing CIE_y coordinates below 0.08. Structure-property investigations demonstrate a strategic improvement of the oscillator strength (f_osc ) and acceleration of the reverse intersystem crossing (RISC) process by extending the π-skeleton, where BN3 realizes a maximum external quantum efficiency (EQE) of 37.6 % and reduced roll-off, thus showing the best efficiency reported for deep-blue TADF OLEDs. The internal regulation of the efficiency and color purity of these compounds validate the general effectiveness to achieve advanced deep-blue narrowband emitters with higher-order boron/nitrogen-based MR motifs.

Carbazole-2-carbonitrile as an acceptor in deep-blue thermally activated delayed fluorescence emitters for narrowing charge-transfer emissions

This work reports a new acceptor for constructing donor–acceptor type (D–A type) blue thermally activated delayed fluorescence (TADF) emitters with narrowed charge-transfer (CT) emissions. A new acceptor core, carbazole-2-carbonitrile (CCN), is formed by the fusion of carbazole and benzonitrile. Three D–A type TADF emitters based on the CCN acceptor, namely 3CzCCN, 3MeCzCCN, and 3PhCzCCN, have been successfully synthesized and characterized. These emitters show deep-blue emissions from 439 to 457 nm with high photoluminescence quantum yields of up to 85% in degassed toluene solutions. Interestingly, all CCN-based deep-blue TADF emitters result in narrow CT emissions with full-width at half-maximums (FWHMs) of less than 50 nm in toluene solutions, which are pretty narrower compared with those of typical D–A type TADF emitters. Devices based on these emitters show high maximum external quantum efficiencies of up to 17.5%.

Deep-blue donor–acceptor thermally activated delayed fluorescence emitters based on carbazole-2-carbonitrile are synthesized, resulting in narrow emission with full-width at half-maximums of less than 50 nm and a maximum OLED EQE of up to 17.5%.

Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters─Properly Accounting for Electron Correlation Is Key!

With the surge of interest in multiresonant thermally activated delayed fluorescent (MR-TADF) materials, it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the spin-component scaling second-order approximate coupled-cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔE_ST (i.e., the energy difference between the lowest singlet S₁ and triplet T₁ excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-dependent density functional theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔE_ST as a result of a poorer account of Coulomb correlation as compared to SCS-CC2. Very interestingly, the use of a metric to assess the importance of higher order excitations in the SCS-CC2 wavefunctions shows that Coulomb correlation effects are substantially larger in the lowest singlet compared to the corresponding triplet and need to be accounted for a balanced description of the relevant electronic excited states. This is further highlighted with coupled cluster singles-only calculations, which predict very different S₁ energies as compared to SCS-CC2 while T₁ energies remain similar, leading to very large ΔE_ST, in complete disagreement with the experiments. We compared our SCS-CC2/cc-pVDZ with other wavefunction approaches, namely, CC2/cc-pVDZ and SOS-CC2/cc-pVDZ leading to similar performances. Using SCS-CC2, we investigate the excited-state properties of MR-TADF emitters showcasing large ΔE_T2T1 for the majority of emitters, while π-electron extension emerges as the best strategy to minimize ΔE_ST. We also employed SCS-CC2 to evaluate donor-acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge-transfer singlet state (S₂). This work highlights the importance of using wavefunction methods in relation to MR-TADF emitter design and associated photophysics.

The Rise of 1,4-BN-Heteroarenes: Synthesis, Properties, and Applications

BN-heteroarenes, which employ both boron and nitrogen in aromatic hydrocarbons, have gained great attention in the fields of organic chemistry and materials science. Nevertheless, the extensive studies on BN-heteroarenes are largely limited to 1,2-azaborine-based compounds with B-N covalent bonds, whereas 1,3- and 1,4-BN-heteroarenes are relatively rare due to their greater challenge in the synthesis. Recently, significant progresses have been achieved in the synthesis and applications of BN-heteroarenes featuring 1,4-azaborines, especially driven by their significant potential as multiresonant thermally activated delayed fluorescence (MR-TADF) materials. Therefore, it is timely to review these advances from the chemistry perspective. This review summarizes the synthetic methods and recent achievements of 1,4-azaborine-based BN-heteroarenes and discusses their unique properties and potential applications of this emerging class of materials, highlighting the value of 1,4-BN-heteroarenes beyond MR-TADF materials. It is hoped that this review would stimulate the conversation and cooperation between chemists who are interested in azaborine chemistry and materials scientists working in the fields of organic optoelectronics, metal catalysis, and carbon-based nanoscience etc.

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 %.

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

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

Deep-Blue OLEDs with Rec.2020 Blue Gamut Compliance and EQE Over 22% Achieved by Conformation Engineering

To achieve high-efficiency deep-blue electroluminescence satisfying Rec.2020 standard blue gamut, two thermally activated delayed fluorescent (TADF) emitters are developed: 5-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (TDBA-PAS) and 10-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-9,9-diphenyl-9,10-dihydroacridine (TDBA-DPAC). Inheriting from their parented organoboron multi-resonance core, both emitters show very promising deep-blue emissions with relatively narrow full width at half-maximum (FWHM, ≈50 nm in solution), high photoluminescence quantum yield (up to 92.3%), and short emission lifetime (≤2.49 µs) with fast reverse intersystem crossing (>10⁶ s^-1 ) in doped films. More importantly, replacing the spiro-centered sp³ C atom (TDBA-DPAC) with the larger-radius sp³ Si atom (TDBA-PAS), enhanced conformational heterogeneities in bulky-group-shielded TADF molecules are observed in solution, doped film, and device. Consequently, OLEDs based on TDBA-PAS retain high maximum external quantum efficiencies ≈20% with suppressed efficiency roll-off and color index close to Rec.2020 blue gamut over a wide doping range of 10-50 wt%. This study highlights a new strategy to restrain spectral broadening and redshifting and efficiency roll-off in the design of deep-blue TADF emitters.

Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations

Molecules that exhibit multiple resonance (MR) type thermally activated delayed fluorescence (TADF) are highly efficient electroluminescent materials with narrow emission spectra. Despite their importance in various applications, the emission mechanism is still controversial. Here, a comprehensive understanding of the mechanism for a representative MR-TADF molecule (5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene, DABNA-1) is presented. Using the equation-of-motion coupled-cluster singles and doubles method and Fermi's golden rule, we quantitatively reproduced all rate constants relevant to the emission mechanism; prompt and delayed fluorescence, internal conversion (IC), intersystem crossing, and reverse intersystem crossing (RISC). In addition, the photoluminescence quantum yield and its prompt and delayed contributions were quantified by calculating the population kinetics of excited states and the transient photoluminescence decay curve. The calculations also revealed that TADF occurred via a stepwise process of 1) thermally activated IC from the electronically excited lowest triplet state T1 to the second-lowest triplet state T2, 2) RISC from T2 to the lowest excited singlet state S1, and 3) fluorescence from S1.

Ultrafast Photophysics of Multiple-Resonance Ultrapure Blue Emitters

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are becoming increasingly attractive due to their applications in high-resolution organic light-emitting diode (OLED) display technology. Here, we present an investigation on the photophysics of two MR-TADF emitters (t-DABNA and TBN-TPA) by using quantum chamical calculation and ultrafast transient absorption (TA) spectroscopy. Compared with one-step structural planarization of t-DABNA, TBN-TPA undergoes two-step relaxation in S₁ state, i.e., fast twisting of the peripheral group and subsequent restrained planarization of the B-N framework. The efficient twisting motion of the peripheral group largely reduces the energy level of the TBN-TPA system and correspondingly increases the barrier for subsequent planarization, which is favored for the narrowband emission. Our work provides a detailed picture for the excited-state deactivation of peripheral group-modified MR-TADF emitters without a pronounced charge-transfer (CT) characteristic mixed in the lowest-lying fluorescent state, which might be helpful for the future design of narrowband OLED emitters.

Highly Efficient Asymmetric Multiple Resonance Thermally Activated Delayed Fluorescence Emitter with EQE of 32.8 % and Extremely Low Efficiency Roll-Off

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show great potentials for high color purity organic light-emitting diodes (OLEDs). However, the simultaneous realization of high photoluminescence quantum yield (PLQY) and high reverse intersystem crossing rate (k_RISC ) is still a formidable challenge. Herein, a novel asymmetric MR-TADF emitter (2Cz-PTZ-BN) is designed that fully inherits the high PLQY and large k_RISC values of the properly selected parent molecules. The resonating extended π-skeleton with peripheral protection can achieve a high PLQY of 96 % and a fast k_RISC of above 1.0×10⁵  s^-1 , and boost the performance of corresponding pure green devices with an outstanding external quantum efficiency (EQE) of up to 32.8 % without utilizing any sensitizing hosts. Remarkably, the device sufficiently maintains a high EQE exceeding 23 % at a high luminance of 1000 cd m^-2 , representing the highest value for reported green MR-TADF materials at the same luminescence.

A Novel Strategy toward Thermally Activated Delayed Fluorescence from a Locally Excited State

It is well-known that thermally activated delayed fluorescence (TADF) is always generated from charge-transfer (CT) excited states in donor-acceptor (D-A) systems, which limits its application owing to a slow radiative process and a small stimulated emission cross section. Herein, a design strategy is proposed for realizing TADF from a locally excited (LE) state without a typical donor-acceptor type structure through controlling the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes between the lowest excited singlet with LE character and higher triplet states. Using this strategy, a boron difluoride derivative is theoretically predicted and experimentally synthesized to exhibit locally excited TADF (LE-TADF) with a fairly large radiative rate of 1.12 × 10⁸ s^-1, extremely fast RISC rate of 5.09 × 10¹⁰ s^-1, and a large stimulated emission cross section of 4.35 × 10^-17 cm², making this a promising organic amplified spontaneous emission (ASE) material. This work might open a new avenue to extend TADF materials, especially TADF laser emitters.

Achieving Ultimate Narrowband and Ultrapure Blue Organic Light-Emitting Diodes Based on Polycyclo-Heteraborin Multi-Resonance Delayed-Fluorescence Emitters

To achieve an ultimate wide color gamut for ultrahigh-definition displays, there is great demand for the development of organic light-emitting diodes (OLEDs) enabling monochromatic, ultrapure blue electroluminescence (EL). Herein, high-efficiency and ultrapure blue OLEDs based on polycyclo-heteraborin multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, BOBO-Z, BOBS-Z, and BSBS-Z, are reported. The key to the design of the present luminophores is the exquisite combination and interplay of multiple boron, nitrogen, oxygen, and sulfur heteroatoms embedded in a fused polycyclic π-system. Comprehensive photophysical and computational investigations of this family of MR-TADF materials reveal that the systematic implementation of chalcogen (oxygen and sulfur) atoms can finely modulate the emission color while maintaining a narrow bandwidth, as well as the spin-flipping rates between the excited singlet and triplet states. Consequently, OLEDs based on BOBO-Z, BOBS-Z, and BSBS-Z demonstrate narrowband and ultrapure blue EL emission, with peaks at 445-463 nm and full width at half maxima of 18-23 nm, leading to Commission Internationale de l'Éclairage-y coordinates in the range of 0.04-0.08. Particularly, for OLEDs incorporating sulfur-doped BOBS-Z and BSBS-Z, notably high maximum external EL quantum efficiencies of 26.9% and 26.8%, respectively, and small efficiency roll-offs are achieved concurrently.

Boron-, Sulfur- and Nitrogen-Doped Polycyclic Aromatic Hydrocarbon Multiple Resonance Emitters for Narrow-Band Blue Emission

Two boron-, sulfur- and nitrogen-doped polycyclic aromatic hydrocarbon multiple resonance thermally activated delayed fluorescence emitters with high photoluminescent quantum efficiency (88 %) and rapid reverse intersystem crossing (k_RISC = 1.0×10⁵  s^-1 ) are designed and synthesized, enabling efficient narrow-band blue electroluminescence at 473 nm with full width at half maximum of 29 nm and maximum external quantum efficiency of 22.0 %, which provides an avenue to expand the structure library for multiple resonance emitters and an approach to regulate their emission properties.

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.

One-Shot Synthesis of Expanded Heterohelicene Exhibiting Narrowband Thermally Activated Delayed Fluorescence

An expanded heterohelicene consisting of three BN₂-embedded [4]helicene subunits (V-DABNA-Mes) has been synthesized by one-shot triple borylation. The key to success is the excessive use of boron tribromide in an autoclave. Based on the multiple resonance effect of three boron and six nitrogen atoms, V-DABNA-Mes exhibited a narrowband sky-blue thermally activated delayed fluorescence with a full width at half-maximum of 16 nm. The resonating π-extension minimized the singlet-triplet energy gap and enabled rapid reverse intersystem crossing with a rate constant of 4.4 × 10⁵ s^-1. The solution-processed organic light-emitting diode device, employed as an emitter, exhibited a narrowband emission at 480 nm with a high external quantum efficiency of 22.9%.

Simulation of Low-Lying Singlet and Triplet Excited States of Multiple-Resonance-Type Thermally Activated Delayed Fluorescence Emitters by Delta Self-Consistent Field (ΔSCF) Method

The delta self-consistent field (ΔSCF) method was applied to the simulation of low-lying singlet and triplet excited states of multiple-resonance (MR)-type thermally activated delayed fluorescence (TADF) molecules, which form a promising group for organic light-emitting diode (OLED) emitters. A comparison with the experimental values of 13 emitters from the literature showed that ΔSCF gave fairly accurate S₁ and T₁ excitation energies (mean absolute errors (MAEs) of 0.092 and 0.055 eV, respectively) as well as quite accurate ΔE_ST (S₁-T₁ gap, MAE of 0.041 eV), which could not be calculated with sufficient accuracy by the conventional time-dependent density functional theory (TDDFT). ΔSCF also demonstrated its utility for the analysis of photophysical properties through a simulation of the reverse intersystem crossing (RISC) process of an MR-type emitter.

An Air- and Water-Stable B4 N4 -Heteropentalene Serving as a Host Material for a Phosphorescent OLED

Replacement of the carbon-carbon bonds of antiaromatic compounds with polar boron-nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B₄ N₄ -heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B₄ N₄ -heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

Novel Synthetic Approach to Heteroatom Doped Polycyclic Aromatic Hydrocarbons: Optimizing the Bottom-Up Approach to Atomically Precise Doped Nanographenes

The success of the rational bottom-up approach to nanostructured carbon materials and the discovery of the importance of their doping with heteroatoms puts under the spotlight all synthetic organic approaches to polycyclic aromatic hydrocarbons. The construction of atomically precise heteroatom doped nanographenes has evidenced the importance of controlling its geometry and the position of the doping heteroatoms, since these parameters influence their chemical-physical properties and their applications. The growing interest towards this research topic is testified by the large number of works published in this area, which have transformed a once "fundamental research" into applied research at the cutting edge of technology. This review analyzes the most recent synthetic approaches to this class of compounds.

Wide-Range Color Tuning of Narrowband Emission in Multi-resonance Organoboron Delayed Fluorescence Materials through Rational Imine/Amine Functionalization

Establishing a simple and versatile design strategy to finely modulate emission colors while retaining high luminescence efficiency and color purity remains an appealing yet challenging task for the development of multi-resonance-induced thermally activated delayed fluorescence (MR-TADF) materials. Herein, we demonstrate that the strategic introduction of electron-withdrawing imine and electron-donating amine moieties into a versatile boron-embedded 1,3-bis(carbazol-9-yl)benzene skeleton enables systematic hypsochromic and bathochromic shifts of narrowband emissions, respectively. By this method, effective electroluminescence color tuning was accomplished over a wide visible range from deep-blue to yellow (461-571 nm), using the same MR molecular system, without compromising very narrow spectral features. Deep-blue to yellow organic light-emitting diodes with maximum external quantum efficiencies as high as 19.0-29.2 % and superb color purity could be produced with this family of color-tunable MR-TADF emitters.

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

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

Asymmetric Blue Multiresonance TADF Emitters with a Narrow Emission Band

In this study, we synthesized and characterized multiple resonance (MR) type blue thermally activated delayed fluorescence (TADF) emitters. Unlike many boron-based MR-TADF materials, the blue TADF emitters of this work had an asymmetric molecular structure with one boron, one oxygen, and one nitrogen. The aromatic units linked to the nitrogen were changed into diphenylamine, carbazole, dimethylacridine, and diphenylacridine to manage the light emission properties of the emitters. The TADF emitters exhibited a blue emission due to the weak electron-donating oxygen atom and the emission color was controlled by the aromatic unit connected to the nitrogen. The simple diphenylamine unit was effective in achieving real deep-blue emission for the BT2020 standard with a high external quantum efficiency (EQE), while the electron-rich nitrogen-based dimethylacridine and diphenylacridine accelerated the reverse intersystem crossing for high EQE and small EQE roll-off. Among the emitters, a diphenylamine-substituted emitter, 7-(tert-butyl)-9-phenyl-9H-5-oxa-9-aza-13b-boranaphtho[3,2,1-de]anthracene (B-O-dpa), showed a maximum external quantum efficiency of 16.3%, a small full width at half-maximum of 32 nm, and a real deep-blue color coordinate of (0.15, 0.05).