The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m, most likely because of the long delayed fluorescent lifetime (τ). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τ. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac-XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (E) and triplet state (E) energies. Among them, Ac-3MHPM, with a high E of 2.95 eV, exhibited a high external quantum efficiency (η) of 18% and an η of 10% at 100 cd m with Commission Internationale de l'Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters.

Although pyrimidine-based emitters are one of the most promising classes of compounds, a structure-property relationship for an effective molecular design strategy has not yet been elucidated. In this study, the authors develop three types of blue pyrimidine emitters for high-performance thermally activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs). 2,4,6-triphenylpyrimidine is used as an acceptor, and tri-or penta-substituted acridine as a donor unit for the blue TADF emitters. The authors investigate the effects of the nitrogen position on the photophysical properties and OLED performances, and develop a blue device with a high external quantum efficiency close to 25%. The device performances are among the best so far for blue TADF OLEDs.

1 of 5) 1600675 emitter combined with sophisticated device engineering, including an appropriate choice of neighboring materials, is absolutely imperative to achieve high η p OLEDs realizing both high η ext and low drive voltages at high brightness over 1000 cd m −2 . While we developed novel pyrimidine conjugated emitters and the devices, Yang's group reported a pyrimidinebased green TADF emitters realizing a high η ext,max close to 25% and an η p of 74 lm W −1 . [16] However, we note that this device showed a high drive voltage of 3.4 V at 1 cd m −2 and a large efficiency roll-off with η p,1000 of 37 lm W −1 at a high brightness of 1000 cd m −2 .Herein, we investigated the structure-property relationships among pyrimidine conjugate emitters revealing an effective molecular design for high-performance OLEDs. Consequently, we successfully developed a highly luminescent pyrimidine conjugate emitter named PXZ-PPM exhibiting a high η ext of over 25%, with low driving voltages at Commission Internationale de l'Eclairage chromaticity coordinates (CIE) of (0.36, 0.58), and exceptionally low efficiency roll-off realizing η ext of over 22% at a high brightness of 1000 cd m −2 . These OLEDs also exhibited an η p of over 110 lm W −1 (84.9 cd A −1 , η ext = 25.1%) while maintaining extremely low voltages of 2.2 V at 1 cd m −2 and 3.0 V at 1000 cd m −2 , and an η p,1000 of 78.3 lm W -1 (75.5 cd A -1 , η ext = 22.3%). These performances clearly exceed those of previous TADF devices and are comparable to those of their stateof-the-art phosphorescent counterparts. [17][18][19][20][21][22][23] So far, our TADF molecules, denoted as Ac-RPM (R = H, CH 3 , and phenyl), were synthesized introducing two 9,10-dihydro-9,9-dimethyl-10-phenylacridine moieties at the 4-and 6-positions with regard to the pyrimidine unit and exhibited high photoluminescent quantum yields (η PL s) of approximately 80% and strong TADF properties. However, all Ac-RPM derivatives showed similar emission peaks (λ em ) around 490 nm and ΔE ST of 0.19 eV. Thus, the structure-property relationships should be clearly identified for effective molecular design and superior OLED performances based on pyrimidine conjugate emitters. With this aim, we designed two novel pyrimidinebased emitters named PXZ-PPM and Ac-NPM (Figure 1). Ac-NPM possessed a piperidine moiety at the 2-position of pyrimidine. In this sense, stronger electron-donating properties are expected to increase the lowest unoccupied molecular orbital (LUMO) levels, thereby leading to wider energy gaps (E g ) as compared to the phenyl-based Ac-PPM emitter. On the other hand, PXZ-PPM possessed phenoxazine (PXZ) moieties

platinum and iridium, and can convert 100% of the molecular excitons into light using heavy atom effect enlarging spinorbit coupling. The 3rd generation emitter is a thermally activated delayed fluorescent (TADF) emitter that can convert 100% of the molecular excitons into light, using upconversion from an excited triplet state (E T1 ) to an excited singlet state (E S1 ). [1][2][3] To realize efficient upconversion, small energy differences between E S1 and E T1 , where ΔE ST is less than 0.20 eV, is essential. A TADF emitter consists of an electrondonor unit and an electron-acceptor unit, and the emission color is roughly related to the energy difference between the highest occupied molecular orbital (HOMO) of the electron-donor unit and the lowest unoccupied molecular orbital (LUMO) of the electron-acceptor unit. Therefore, an electronacceptor unit with a shallow LUMO level is promising to realize blue emission, that needs a wide energy gap over 2.75 eV. In this regard, imidazole is very attractive as an electron-acceptor, because of its shallow LUMO and high E T1 level. [4] Furthermore, many imidazole-based blue fluorescent emitters exhibiting high photoluminescent quantum yields (η PL ) up to 100% and high efficiency fluorescent OLEDs with external quantum efficiencies (η ext ) over 7% have been reported so far. [5,6] Surprisingly, however, the corresponding TADF emitters are rarely explored, and only two green TADF emitters have been reported, very recently. [7] Wang and co-workers reported imidazo[4,5-f]-1,10-phenanthroline/diphenylphenazine conjugate molecules named PPZTPI and PPZPPI as TADF emitters. [7] These emitters exhibited very high external quantum efficiencies over 20% and high power efficiencies of nearly 70 lm W −1 in OLEDs. This report shows that imidazole is a promising acceptor unit of TADF emitters. However, until now, no blue to sky-blue TADF emitter has been realized using imidazole unit as an acceptor. In this study, we report highly efficient, imidazole-based, sky-blue TADF emitters for the first time. Among the state-of-the-art sky-blue TADF emitters, these EQE values are not best performances, however, we would like to enhance the scientific importance of the realization in the blue to sky-blue region imidazole-based TADF emitter. We developed a series of imidazo[1,2-f]phenanthridine-based TADF emitters showing blue to sky-blue emission. A sky-blue emitter based on benzimidazole exhibited an η ext,max over 21% and CIE of (0.22, 0.37). We also reveal an effective molecular design strategy to realize imidazole-based TADF emitters with a high η PL .Although imidazole derivatives are promising as blue fluorescent emitters in organic light-emitting devices (OLEDs), the corresponding blue thermally activated delayed fluorescence (TADF) emitter is rarely explored. In this study, a series of imidazo[1,2-f ]phenanthridine-based TADF materials in the blue to sky-blue region is developed. The resultant benzimidazole (BIP)-based TADF materials, with π-expanded acceptor and electron-donor phenoxaz...

Typically, luminescent π-conjugated 2,2':6',2''-terpyridine (tpy) derivatives are versatile components for tridentate metal ligands, supramolecular materials, two-photon absorption bioimaging probes, fluorescent ion sensors, and organic light-emitting devices. However, a limited number of luminescent tpy materials, other than metal complexes, have been reported. This study introduces a series of π-conjugated tpy derivatives that exhibit strong thermally activated delayed fluorescence (TADF). We have observed that a blue tpy emitter outperforms conventional fluorescent emitters. Additionally, a green tpy emitter has exhibited a performance that is almost comparable to that of its green phosphorescent counterparts, realizing an external quantum efficiency close to 25 % and a power efficiency exceeding 80 lm W with an exceptionally low efficiency roll-off. This study demonstrates the first example of highly luminescent tpy-based TADF emitters.