B-N/B-O Contained Heterocycles as Fusion Locker in Multi-Resonance Frameworks towards Highly-efficient and Stable Ultra-Narrowband Emission

Spectral Narrowing Strategies in Multiple Resonance Thermally Activated Delayed Fluorescence Materials

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials possess unique advantages of high-efficiency and narrowband emission, which have rapidly occupied an important position in the field of organic light-emitting diodes (OLEDs). In recent years, significant advancements have been made in the development of MR-TADF materials, particularly in achieving spectral narrowing for high-color-purity OLED applications. Based on diverse MR-TADF molecular skeletons, this review summarizes the primary molecular strategies to narrow spectrum by suppressing structural relaxation and intermolecular interactions. Key strategies include π-conjugation extension, increased molecular rigidity, and the introduction of bulky substituents and intramolecular hydrogen bonds. Additionally, effects of these strategies on photophysical properties are discussed. These molecular design strategies are expected to offer valuable insights for the future design of high-efficiency, narrowband OLED emitters.

Facile access to spirobifluorene-fused chiral multi-resonance materials with ultra-narrowband blue emission

Narrowband multiple resonance (MR) emitters with circularly polarized luminescence (CPL) hold significant promise for three dimension organic light-emitting diode (3D-OLED) displays. In this work, we present a simple method for developing a new type of 9,9'-spirobifluorene-based CP-MR emitters by a recrystallization resolution technology and chlorine (Cl)-directed electrophilic borylation reaction, both of which significantly enhance the efficiency and scalability for mass production. This approach allows for integrating an asymmetric 9,9'-spirobifluorene core between two MR frameworks to achieve optically active CP-MR emitters. The highly rigid spirobifluorene framework minimizes structural relaxation, and meanwhile, the introduction of a cyano (CN) group enhances the MR emission characteristics, leading to an ultra-narrowband blue emission with full width at half maximum (FWHM) of 14 nm. As a result, these molecules facilitate the assembly of narrowband blue CPL-OLEDs with a maximum external quantum efficiency (EQEmax) of 30.7% and an impressive narrow FWHM of 16 nm, representing the first instance of an FWHM below 20 nm in CPL-OLEDs.

The Combination of a Donor-Acceptor TADF and a MR-TADF Emitting Core Results in Outstanding Electroluminescence Performance

Here the utility and potential of an emitter design are demonstrated, consisting of a narrowband-emitting multiresonant thermally activated delayed fluorescent (MR-TADF) core that is decorated with a suitably higher energy donor-acceptor TADF moiety. Not only does this D-A TADF group offer additional channels for triplet exciton harvesting and confers faster reverse intersystem crossing (RISC) kinetics but it also acts as a steric shield, insulating the emissive MR-TADF core from aggregation-caused quenching. Two emitters, DtCzBN-CNBT1 and DtCzBN-CNBT2, demonstrate enhanced photophysical properties leading to outstanding performance of the organic light-emitting diodes (OLEDs). DtCzBN-CNBT2, containing a D-A TADF moiety, has a faster kRISC (1.1 × 105 s-1) and higher photoluminescence quantum yield (ΦPL: 97%) compared to DtCzBN-CNBT1 (0.2 × 105 s-1, ΦPL: 90%), which contains a D-A moiety that itself is not TADF. The sensitizer-free OLEDs with DtCzBN-CNBT2 achieve a record-high maximum external quantum efficiency (EQEmax) of 40.2% and showed milder efficiency roll-off (EQE1000 of 20.7%) compared to the DtCzBN-CNBT1-based devices (EQEmax of 37.1% and EQE1000 of 11.9%).

Peripheral Substitution Engineering of MR-TADF Emitters Embedded With B‒N Covalent Bond Towards Efficient BT.2020 Blue Electroluminescence

Compared with the classical boron/nitrogen (B/N) doped ones, multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters embedded with B-N covalent bond behave a significantly blue-shifted narrowband TADF, and thus show a greater potential in ultrapure blue organic light-emitting diodes (OLEDs). As a proof of concept, herein a peripheral substitution engineering is demonstrated based on such a B‒N embedded parent core. The simple approach is found to ensure easy synthesis via a one-pot lithium-free borylation-annulation, manipulate the excited states through different electronic coupling between core and substituent, and introduce the steric hindrance to minimize the unwanted spectral broadening. Impressively, ultrapure blue OLEDs are realized to give a high external quantum efficiency of 20.3% together with Commission Internationale de l'Éclairage coordinates of (0.152, 0.046). The performance is well competent with those of B/N doped MR-TADF emitters, clearly highlighting that the B‒N embedded framework is a novel promising paradigm towards efficient BT.2020 blue standard.

Twisted Structure and Multiple Charge-Transfer Channels Endow Thermally Activated Delayed Fluorescence Devices with Small Efficiency Roll-Off and Low Concentration Dependence

Despite the rapid development of thermally activated delayed fluorescent (TADF) materials, developing organic light-emitting diodes (OLEDs) with small efficiency roll-off remains a formidable challenge. Herein, we have designed a TADF molecule (mClSFO) based on the spiro fluorene skeleton. The highly twisted structure and multiple charge-transfer channels effectively suppress aggregation-caused quenching (ACQ) and endow mClSFO with excellent exciton dynamic properties to reduce efficiency roll-off. Fast radiative rate (kr) and rapid reverse intersystem crossing (RISC) rate (kRISC) of 1.6×107 s-1 and 1.07×106 s-1, respectively, are obtained in mClSFO. As a result, OLEDs based on mClSFO obtain impressive maximum external quantum efficiency (EQEmax) exceeding 20 % across a wide doping concentration range of 10-60 wt %. 30 wt % doped OLED exhibits an EQEmax of 23.1 % with a small efficiency roll-off, maintaining an EQE of 18.6 % at 1000 cd m-2. The small efficiency roll-off and low concentration dependence observed in the TADF emitter underscore its significant potential.

Organoboron Polymorphs with Different Molecular Packing Modes for Optical Waveguides

Organoboron compounds offer a new strategy to design optoelectronic materials with high fluorescence efficiency. In this paper, the organoboron compound B-BNBP with double B←N bridged bipyridine bearing four fluorine atoms as core unit is facilely synthesized and exhibits a narrowband emission spectrum and a high photoluminescence quantum yield (PLQY) of 86.53 % in solution. Its polymorphic crystals were controllable prepared by different solution self-assembly methods. Two microcrystals possess different molecular packing modes, one-dimensional microstrips (1D-MSs) for H-aggregation and two-dimensional microdisks (2D-MDs) for J-aggregation, owing to abundant intermolecular interactions of four fluorine atoms sticking out conjugated plane. Their structure-property relationships were investigated by crystallographic analysis and theoretical calculation. Strong emission spectra with the full width at half maximum (FWHM) of less than 30 nm can also be observed in thin film and 2D-MDs. 1D-MSs possess thermally activated delayed fluorescence (TADF) property and exhibit superior optical waveguide performance with an optical loss of 0.061 dB/μm. This work enriches the diversity of polymorphic microcrystals and further reveals the structure-property relationship in organoboron micro/nano-crystals.

Multiple-resonance thermally activated delayed fluorescence materials based on phosphorus central chirality for efficient circularly polarized electroluminescence

Circularly polarized organic light-emitting diodes (CP-OLEDs) hold great potential for naked-eye 3D displays, necessitating efficient chiral luminescent materials with an optimal CP luminescence (CPL) dissymmetry factor (g). Herein, we present the first chiral multiple resonance thermally activated delayed fluorescence (MR-TADF) materials containing a phosphorus chiral center by incorporating 5-phenylbenzo[b]phosphindole-5-oxide into the para-position of two MR-TADF cores. The compounds, NBOPO and NBNPO, exhibit photoluminescence peaks at 462 and 498 nm with narrow full-width at half-maximum values of 25 and 24 nm in toluene, respectively. Notably, (R/S)-NBOPO and (R/S)-NBNPO enantiomers display high quantum yields of 87% and 93% and symmetric CPL with |gPL| factors of 1.18 × 10-3 and 4.30 × 10-3, respectively, in doped films. Moreover, the corresponding CP-OLEDs show impressive external quantum efficiencies of 16.4% and 28.3%, along with symmetric CP electroluminescence spectra with |gEL| values of 7.0 × 10-4 and 1.4 × 10-3, respectively.

Mechanical Bond Induced Enhancement and Purification of Pyrene Emission in the Solid State

To address key concerns on solid-state pyrene-based luminescent materials, we propose a novel and efficient mechanical bond strategy. This strategy results in a transformation from ACQ to AIE effect and a remarkable enhancement of pyrene emission in the solid state. Moreover, an unusual purification of emission is also achieved. Through computational calculation and experimental characterisation, finally determined by X-ray diffraction analysis, we prove that the excellent emissions result from mechanical bond induced refinement of molecular arrangements, including reduced π-π stacking, well-ordered packing and enhanced structural stability. This work demonstrates the potential of mechanical bond in the field of organic luminescent molecules, providing a new avenue for developing high-performance organic luminescent materials.

New Application of Multiresonance Organic Delayed Fluorescence Dyes: High-Performance Photoinitiating Systems for Acrylate and Epoxy Photopolymerization and Photoluminescent Pattern Preparation

The primary focus of photopolymerization research is to advance highly efficient visible photoinitiating systems (PISs) as alternatives to conventional ultraviolet (UV) photoinitiators. We developed four multiresonance emitters (BIC-pCz, BNO1, BO-DICz, and TPABO-DICz) to sensitize iodonium salt (Iod) and initiate free-radical and cationic photopolymerization under visible light for the first time. The TPABO-DICz/Iod system achieved a double-bond conversion of over 70% within just 4 s of exposure to green light (520 nm), while the BNO1/Iod system achieved a double-bond conversion exceeding 50% with 10 s of exposure to red light (630 nm). The photochemical properties were studied through thermodynamic research, steady-state photolysis, and electron spin resonance. Photolithography techniques were employed to fabricate photoluminescent films and micrometer-scale patterns utilizing the blue-emitting BIC-pCz dye, showcasing the potential of photolithography in the production of photoluminescent pixels. Additionally, the BIC-pCz/Iod and TPABO-DICz/Iod systems have been employed to rapidly fabricate photoluminescent polymer patterns using a digital-light-processing 3D printer with a low-intensity light (3.2 mW cm-2). These multiresonance emitters show exceptional photosensitizing effects and can act as fluorescent dyes in photoluminescent patterns, highlighting the potential of utilizing photopolymerization for OLED applications.

B‒N covalent bond-involved π-extension of multiple resonance emitters enables high-performance narrowband electroluminescence

Multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show promise for achieving both high color-purity emission and high exciton utilization efficiency. However, their development is often impeded by a limited synthetic scope and excessive molecular weights, which challenge material acquisition and organic light-emitting diode (OLED) fabrication by vacuum deposition. Herein, we put forward a B‒N covalent bond-involved π-extension strategy via post-functionalization of MR frameworks, leading to the generation of high-order B/N-based motifs. The structurally and electronically extended π-system not only enhances molecular rigidity to narrow emission linewidth but also promotes reverse intersystem crossing to mitigate efficiency roll-off. As illustrated examples, ultra-narrowband sky-blue emitters (full-width at half-maximum as small as 8 nm in n-hexane) have been developed with multi-dimensional improvement in photophysical properties compared to their precursor emitters, which enables narrowband OLEDs with external quantum efficiencies (EQEmax) of up to 42.6%, in company with alleviated efficiency decline at high brightness, representing the best efficiency reported for single-host OLEDs. The success of these emitters highlights the effectiveness of our molecular design strategy for advanced MR-TADF emitters and confirms their extensive potential in high-performance optoelectronic devices.

Stable pure-green organic light-emitting diodes toward Rec.2020 standard

Manipulating dynamic behaviours of charge carriers and excitons in organic light-emitting diodes (OLEDs) is essential to simultaneously achieve high colour purity and superior operational lifetime. In this work, a comprehensive transient electroluminescence investigation reveals that incorporating a thermally activated delayed fluorescence assistant molecule with a deep lowest unoccupied molecular orbital into a bipolar host matrix effectively traps the injected electrons. Meanwhile, the behaviours of hole injection and transport are still dominantly governed by host molecules. Thus, the recombination zone notably shifts toward the interface between the emissive layer (EML) and the electron-transporting layer (ETL). To mitigate the interfacial carrier accumulation and exciton quenching, this bipolar host matrix could serve as a non-barrier functional spacer between EML/ETL, enabling the distribution of recombination zone away from this interface. Consequently, the optimized OLED exhibits a low driving voltage, promising device stability (95% of the initial luminance of 1000 cd m-2, LT95 > 430 h), and a high Commission Internationale de L'Éclairage y coordinate of 0.69. This indicates that managing the excitons through rational energy level alignment holds the potential for simultaneously satisfying Rec.2020 standard and achieving commercial-level stability.

High-Efficiency and Narrowband Green Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Based on Two Diverse Boron Multi-Resonant Skeletons

Up to now, highly efficient narrowband thermally activated delayed fluorescence (TADF) molecules constructed by oxygen-bridged boron with an enhancing multiple resonance (MR) effect have been in urgent demand for solid-state lighting and full-color displays. In this work, a novel MR-TADF molecule, BNBO, constructed by the oxygen-bridged boron unit and boron-nitrogen core skeleton as an electron-donating moiety, is successfully designed and synthesized via a facile one-step synthesis. Based on BNBO as an efficient green emitter, the organic light-emitting diode (OLED) shows a sharp emission peak of 508 nm with a full-width at half-maximum (FWHM) of 36 nm and realizes quite high peak efficiency values, including an external quantum efficiency (EQEmax) of 24.3% and a power efficiency (PEmax) of 62.3 lm/W. BNBO possesses the intramolecular charge transfer (ICT) property of donor-acceptor (D-A) materials and multiple resonance characteristics, which provide a simple strategy for narrowband oxygen-boron materials.