Boron-based multiple-resonance (MR) emitters exhibit the advantages of narrowband emission, high absolute photoluminescence quantum yield, thermally activated delayed fluorescence (TADF), and ...sufficient stability during the operation of organic light-emitting diodes (OLEDs). Thus, such MR emitters have been widely applied as blue emitters in triplet-triplet-annihilation-driven fluorescent devices used in smartphones and televisions. Moreover, they hold great promise as TADF or terminal emitters in TADF-assisted fluorescence or phosphor-sensitised fluorescent OLEDs. Herein we comprehensively review organoboron-based MR emitters based on their synthetic strategies, clarify structure-photophysical property correlations, and provide design guidelines and future development prospects.
This review highlights organoboron-based multiple-resonance compounds' synthetic strategies classified as one-pot borylation, one-shot borylation, and late-stage functionalisation, and explores material structure-photophysical property correlations.
Organic light-emitting diodes (OLEDs) are a promising light-source technology for future generations of display1,2. Despite great progress3–12, it is still challenging to produce blue OLEDs with ...sufficient colour purity, lifetime and efficiency for applications. Here, we report pure-blue (Commission Internationale de l’ Eclairage (CIE) coordinates of 0.13, 0.16) OLEDs with high efficiency (external quantum efficiency of 32 per cent at 1,000 cd m−2), narrow emission (full-width at half-maximum of 19 nm) and good stability (95% of the initial luminacnce (LT95) of 18 hours at an initial luminance of 1,000 cd m−2). The design is based on a two-unit stacked tandem hyperfluorescence OLED with improved singlet-excited-state energy transfer from a sky-blue assistant dopant exhibiting thermally activated delayed fluorescence (TADF) called hetero-donor-type TADF(HDT-1) to a pure-blue emitter. With stricter control of device fabrication and procedures it is expected that device lifetimes will further improve to rival commercial fluorescent blue OLEDs.Pure-blue organic LEDs with narrow emission and improved stability show promise for display applications.
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GEOZS, IJS, IMTLJ, IZUM, KILJ, KISLJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
Luminescent materials that exhibit narrowband emission are vital for full-colour displays. Here, we report a thermally activated delayed-fluorescence material that exhibits ultrapure blue emission ...with full-width at half-maximum of just 14 nm. The emitter consists of five benzene rings connected by two boron and four nitrogen atoms and two diphenylamino substituents. The multiple resonance effect of the boron and nitrogen atoms induces significant localization of the highest occupied and lowest unoccupied molecular orbitals on different atoms to minimize not only the vibronic coupling between the ground state (S0) and the singlet excited state (S1) but also the energy gap between the S1 state and triplet excited state (T1). Organic light-emitting diode devices employing the emitter emit light at 469 nm with full-width at half-maximum of 18 nm with an external quantum efficiency of 34.4% at the maximum and 26.0% at 1,000 cd m−2.
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GEOZS, IJS, IMTLJ, IZUM, KILJ, KISLJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
Exciplex system exhibiting thermally activated delayed fluorescence (TADF) holds a considerable potential to improve organic light‐emitting diode (OLED) performances. However, the operational ...lifetime of current exciplex‐based devices, unfortunately, falls far behind the requirement for commercialization. Herein, rationally choosing a TADF‐type electron acceptor molecule is reported as a new strategy to enhance OLEDs' operating lifetime. A comprehensive study of the exciplex system containing 9,9′,9′′‐triphenyl‐9H,9′H,9′′H‐3,3′:6′,3′′‐tercarbazole (Tris‐PCz) and triazine (TRZ) derivatives clarifies the relationship between unwanted carrier recombination on acceptor molecules, TADF property of acceptors, and the device degradation event. By employing a proposed “exciton recycling” strategy, a threefold increased operational lifetime can be achieved while still maintaining high‐performance OLED properties. In particular, a stable blue OLED that employs this strategy is successfully demonstrated. This research provides an important step for exciplex‐based devices toward the significant improvement of operational stability.
The role of reverse intersystem crossing in the acceptor molecule on the device stability of exciplex‐based organic light‐emitting diodes is revealed. The addition of an electron‐donating unit onto the acceptor core can prevent molecular degradation, and the thermally activated delayed fluorescence (TADF) ability can also recycle triplet energy as exciplex emission, resulting in significant improvement of the device lifetime.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The photophysics of multiple resonance thermally activated delayed fluorescence molecule ν-DABNA is described. We show coupling of a 285 cm–1 stretching/scissoring vibrational mode of peripheral ...phenyl rings to the S1 state, which dictates the ultimate emission full-width at half maximum. However, a separate high amplitude mode, 945 cm–1 of the N-biphenyl units, mediates the reverse intersystem crossing (rISC) mechanism. Concentration-dependent studies in solution and solid state reveal a second emission band that increases nonlinearly with concentration, independent of the environment assigned to excimer emission. Even at concentrations well below those used in devices, the excimer contribution affects performance. Using different solvents and solid hosts, rISC rates between 3–6 × 105 s–1 are calculated, which show negligible dependence on environmental polarity or host packing. At 20 K over the first 10 ns, we observe a broad Gaussian excimer emission band with energy on-set above the S1 exciton band. An optical singlet-triplet gap (ΔEST) of 70 meV is measured, agreeing with previous thermal estimates; however, the triplet energy is also found to be temperature-dependent. A monotonic increase of the exciton emission band full-width at half maximum with temperature indicates the role of hot transitions in forming vibrational excited states at room temperature (RT), and combined with an observed temperature dependency of ΔEST, we deduce that the rISC mechanism is that of thermally activated reverse internal conversion of T1 to T N (n ≥ 2) followed by rapid rISC of T N to S1. Organic light-emitting diodes with ν-DABNA as a hyperfluorescent emitter (0.5 wt % and 1 wt %) exhibit an increase of maximum external quantum efficiency, reaching 27.5% for the lower ν-DABNA concentration. On the contrary, a Förster radius analysis indicated that the energy transfer ratio is smaller because of higher donor–acceptor separation (>2.4 nm) with weak sensitizer emission observed in the electroluminescence. This indicates excimer quenching in 1 wt % devices.
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IJS, KILJ, NUK, PNG, UL, UM
Thermally activated delayed fluorescence (TADF) materials based on the multiple resonance (MR) effect are applied in organic light‐emitting diodes (OLEDs), combining high color purity and efficiency. ...However, they are not fabricated via solution‐processing, which is an economical approach toward the mass production of OLED displays. Here, a solution‐processable MR‐TADF material (OAB‐ABP‐1), with an extended π‐skeleton and bulky substituents, is designed. OAB‐ABP‐1 is synthesized from commercially available starting materials via a four‐step process involving one‐shot double borylation. OAB‐ABP‐1 presents attractive photophysical properties, a narrow emission band, a high photoluminescence quantum yield, a small energy gap between S1 and T1, and low activation energy for reverse intersystem crossing. These properties are attributed to the alternating localization of the highest occupied and lowest unoccupied molecular orbitals induced by the boron, nitrogen, and oxygen atoms. Furthermore, to facilitate charge recombination, two novel semiconducting polymers with similar ionization potentials to that of OAB‐ABP‐1 are synthesized for use as interlayer and emissive layer materials. A solution‐processed OLED device is fabricated using OAB‐ABP‐1 and the aforementioned polymers; it exhibits pure green electroluminescence with a small full‐width at half‐maximum and a high external quantum efficiency with minimum efficiency roll‐off.
A thermally activated delayed fluorescence material featuring a multiple resonance effect of boron, nitrogen, and oxygen atoms (OAB‐ABP‐1) is synthesized by one‐shot double borylation. A solution‐processed organic light‐emitting diode (OLED) device using OAB‐ABP‐1 exhibits pure green electroluminescence with a small full‐width at half‐maximum, and a high external quantum efficiency with minimum efficiency roll‐off.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ultrapure blue‐fluorescent molecules based on thermally activated delayed fluorescence are developed. Organic light‐emitting diode (OLED) devices employing the new emitters exhibit a deep blue ...emission at 467 nm with a full‐width at half‐maximum of 28 nm, CIE coordinates of (0.12, 0.13), and an internal quantum efficiency of ≈100%, which represent record‐setting performance for blue OLED devices.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The increasing attention devoted to triangulenes and their heteroatom derivatives inspired us to explore a divergent synthesis of heteroatom‐centered 4,8,12‐triazatriangulenes, which involved the ...preparation of a nitrogen‐containing macrocyclic precursor and subsequent central heteroatom introduction by electrophilic C−Li and C−H substitution. The boron‐centered triangulene has a planar structure unlike the bowl‐shaped phosphorus‐ and silicon‐centered triangulenes. The described synthetic procedure can be used to fabricate a broad range of attractive functional materials, for example, for organic light‐emitting diodes, based on heteroatom‐centered triangulenes.
Spoilt for choice: Boron‐, phosphorus‐, and silicon‐centered 4,8,12‐triazatriangulenes were synthesized. The key step involved the efficient incorporation of the heteroatom into a nitrogen‐containing macrocyclic precursor through electrophilic C−Li and C−H substitution (see scheme).
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 (kRISC). Herein, a pure green MR‐TADF material (ν‐DABNA‐CN‐Me) with high kRISC 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.
The bathochromic shift of a multiple‐resonance‐induced thermally activated delayed fluorescence material (ν‐DABNA) is achieved by introducing cyano groups. The cyano‐substituted material (ν‐DABNA‐CN‐Me) exhibits pure green emission with a small full‐width at half‐maximum. The fabricated organic light‐emitting diode device exhibits a high maximum external quantum efficiency (31.9%), which is maintained (28.5%) at 1000 cd m−2, demonstrating a minimum efficiency roll‐off.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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