Versatile boron‐based thermally activated delayed fluorescence materials for organic light‐emitting diodes

During the last few years, organoboron‐based thermally activated delayed fluorescence (TADF) materials have received extensive attention in optoelectronic area, owing to the unique electronegativity of boron atom. Herein, many research progress of organoboron‐based TADF materials for organic optoelectronic devices is summarized. This review comprehensively documents the organoboron‐based TADF materials according to the emission colors from blue to red‐near‐infrared (red‐NIR), covering the molecular design strategies, photophysical properties, and optoelectronic performance in organic light‐emitting diodes (OLEDs). The current progress and future challenges in this fast‐growing fields are reviewed systematically, providing instructive guidance for the future research on high‐performance TADF‐OLEDs.


INTRODUCTION
In 1987, organic light-emitting diodes (OLEDs) were successfully invented by Tang and VanSlyke, which have received tremendous attentions because of their commercial application in lighting and displays. [1] Compared to the traditional light sources like inorganic light-emitting diodes and liquid crystal displays, OLEDs exhibit various advantages, including high efficiency, ultrathin thickness, self-luminous and high color purity, etc. [2][3][4][5][6][7][8][9][10] During the last two decades, extensive attempts have been contributed to exploring novel materials for high-performance OLEDs. [11][12][13][14][15][16][17][18][19][20] Based on the spin-statistics theorem, 25% singlet excitons and 75% triplet excitons were generated under the electrical stimulation. Nevertheless, only singlet excitons could be utilized directly in conventional fluorescent OLEDs (firstgeneration OLEDs), whereas triplet excitons would generally be wasted in the form of heat energy, thereby leading to the theoretical maximum internal quantum efficiency (IQE) of only 25%. Then, phosphorescent OLEDs (second-generation OLEDs) have been explored by introducing traditional-metal elements into the emitters to harvest the remaining 75% triplet excitons. [21][22][23][24] Heavy atoms could enhance the spinorbit coupling (SOC) and significantly promote the intersystem crossing (ISC) between singlet and triplet excited exci- tons, then both the triplet and singlet excitons can be utilized simultaneously, thus realizing a theoretical 100% IQE. Nevertheless, phosphorescent materials for OLEDs are usually confined to the precious noble metals, especially Pt and Ir. Subsequently, the third-generation high-efficiency OLEDs on the basis of thermally activated delayed fluorescence (TADF) emitters emerged and developed rapidly. [25][26][27][28][29][30][31][32][33][34][35] The same as the phosphorescent transitional-metal compounds, purely organic TADF luminogens could convert triplet excitons to singlet excitons by reverse intersystem crossing (RISC) due to the tiny energy gap (ΔE ST ) between singlet and triplet state, and then could achieve theoretical 100% IQE as well. Since Adachi et al. reported the first OLEDs based on the purely organic TADF luminogen in 2011, enormous attentions have been paid to achieving excellent performance of TADF-OLEDs. [36] To obtain highly efficient TADF emitters, boron-based molecules have attracted considerable interests in the last few years, which are widely utilized as promising candidates in photoelectric devices, due to the electron-deficient properties of boron atom from its vacant p-orbital. [37][38][39][40] The boron atom could form π-electron conjugation with the adjacent conjugated system through its empty p z orbital, which endows the materials with excellent photophysical and electrochemical properties, for example, high luminescence F I G U R E 1 Illustration of (A) tetracoordinate, and (B) three-coordinate organoboron compound, (C) boron/nitrogen based multiresonance TADF emitter efficiency, because of the suppressed nonradiative decays induced by the rigid molecular configuration stemming from their trigonal planar geometry. Furthermore, the boron center of the tricoordinated boron compounds features strong Lewis acidity, which enables the coordination with Lewis base to form new tetracoordinated boron compounds, accompanied by the conformation change from sp 2 hybrid planar triangle to sp 3 hybrid triangular pyramid. [39] These features make the boron-based materials exhibit rich photophysical properties. By regulating the donor and acceptor groups as well as the coordination number of the boron atom, blue, green, yellow, orange, red, and even near-infrared (NIR) luminescence could be achieved conveniently. In this review, we emphasize the systematic summary of the development of the organoboron-based TADF materials, and their monochrome device performances over the whole visible region, aiming at shedding light on future design strategy of highly efficient TADF materials. This review is not expected to be all inclusive but rather to particularly lay emphasis on the latest novel design strategies of organoboron-based TADF emitters and the corresponding photoelectric devices. Finally, the current challenges, feasible improvement, and perspectives for TADF materials and devices are illuminated.

ORGANOBORON-BASED TADF EMITTERS DESIGN
Many examples of highly efficient and chemically robust organoboron-based emitters with emission colors ranging from blue to red-NIR have been successfully constructed and widely used in OLEDs. Because of its unique electron-deficient nature or Lewis acidic nature, intrinsically reactive boron center is generally embraced by sterically bulky aryl groups, commonly bidentate chelating ligand, such as 2-phenylpyridine and its derivatives, or other bulky substituents, such as 2,4,6-trimethylphenyl and 2,4,6triisopropylphenyl, which offer enough protection and chemical stability under ambient conditions. Generally speaking, representative structures are mainly divided into two categories: tetracoordinate and three-coordinate boron compounds, as shown in Figure 1. In the construction of tetracoordinate organoboron compounds, chelate ligands with rich πelectrons, such as 2-phenylpyridine, 2-pyridylphenolate and their substituted derivatives, are often employed to coordinate with boron center with vacant p-orbitals, to allow intramolecular electron delocalization. While for three-coordinate boron compounds, steric hindrance around the boron center is rec-ognized as a universal method to prepare air-stable boronbased compounds, in which the protected empty p-orbital on boron atom serves as an efficient electron acceptor. By incorporating triarylboron or boron-based heteroaromatics into donor-acceptor structure, many three-coordinated organoboron compounds have been demonstrated to be effective light-emitting materials in OLEDs. Another alternative strategy to stabilize boron center is to embed a boron atom directly into the polycyclic heteroaromatic framework to offer sufficient structural constraints around the tricoordinate boron moiety, in which opposite resonance effect of boron and nitrogen/oxygen or other electron-rich atoms enable materials TADF characteristics. Until now, a large number of three-and four-coordinated boron-based TADF materials have been elaborated in the literature to investigate the influence of coordinate numbers, molecular configuration, and substitutions on photophysical properties and device performances.

BLUE TADF EMITTERS
In this section, blue emitters are defined as the molecules that possess maximum electroluminescence (EL) peaks less than 500 nm. This definition could help us categorize the emitters efficiently within this review. The photophysical properties and devices performances for the blue TADF materials have been summarized in Table 1.

Difluoroboron-based emitters
Difluoroboron (BF 2 )-containing chromophores are widely investigated and vastly applied as optoelectronic materials because of the strong electron-withdrawing properties and high photoluminescence quantum yields (PLQYs). For example, Zhang and coworkers reported a class of TADF emitters (1-3 in Scheme 1) by introducing BF 2 group into the donor-acceptor (D-A) structure, all of which exhibited blue emission and the photoluminescence (PL) spectra became bathochromic-shifted and broadened (68-72 nm) with increasing the electron-donating ability of the donor group. [41] Multilayer devices based on compounds 1-3 displayed blue EL peaking at 467, 471, and 483 nm, respectively. Particularly, maximum luminance and external quantum efficiency (EQE) based on compound 3 were up to 19383 cd m -2 and 15.8%, respectively. Subsequently, by simply introducing methyl groups to the phenyl moiety of the compounds 1 TA B L E 1 Summary of photophysical properties and device performance of organoboron-based blue TADF emitters

S C H E M E 1
Chemical structures of difluoroboron-based blue TADF emitters and 3, emission color can be effectively modulated from sky blue to deep blue (435-471 nm). [42] Due to the highly twisted configuration, the resulting emitters 4-6 were strongly emissive in oxygen-free toluene with high PLQYs of 94-99%, and thereby achieving the maximum EQE of up to 13.8% and 8.4% in 4 and 6-based sky-blue and deep-blue OLEDs, respectively. Similarly, a BF 2 -containing sky-blue emitter 7 was designed and synthesized, in which 2,5-diphenyl-1,3,4oxadiazole (OXD) was introduced as the second acceptor due to its good planarity and excellent electron-transport properties. [43] The results showed that the ΔE ST value of compound 7 was smaller than that of the ligand without the coordination with boron, with improved PLQY, indicating the incorporation of one more acceptor unit to the TADF molecule could not only effectively reduce ΔE ST , but also increase the oscillator strength. Based on the above mentioned results, the solution-processed device employing 7 as emitter was fabricated, and achieved a maximum EQE of 13.8% with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.43). Additionally, the EL spectrum (496 nm) of the compound 7 exhibited a large redshift compared with that of the precursor before the introduction of the BF 2 moiety (468 nm), because of stronger intramolecular charge transfer.

Triarylboron-based emitters
Triarylboron-based compounds have attracted considerable interest and been widely used as optoelectronics materials because of the electron-withdrawing properties of the central boron atom. The incorporation of triarylboron groups into the donor-acceptor framework can offer a newly tactic to construct blue TADF emitters. Two triarylboron-based compounds (8 and 9 in Scheme 2) were designed as blue TADF emitters. [44] Both compounds exhibited a remarkable solvatochromism effect, and the emission peak of compound

S C H E M E 2 Chemical structures of triarylboron-based blue TADF emitters
8 (495 nm) in toluene solution was redshifted by up to 18 nm compared with that of compound 9 (477 nm), arising from the former possessing the relatively strong donor. Furthermore, the multilayer OLEDs using 16 wt% of compound 8 or 9 codeposited with DPEPO as emitting layers were fabricated and exhibited sky-blue EL peaking at 487 and 477 nm, respectively. In addition, the maximum EQEs of these two devices were up to 21.6% and 14.0%, respectively. In another study, a class of blue emitters (10-12) were designed and prepared by introducing methoxy groups to the carbazole donor at different positions. [45] Among these compounds, only compounds 11 and 12 exhibited pronounced TADF properties, and achieved the maximum EQE of 12.5% and 13.3% in corresponding OLED.
Using the similar strategy, Lee and coworkers developed a D-A type emitter 13 via attaching carbazole donor to the ortho-position of the phenyl ring on the dimesitylphenylboron to induce the formation of a orthogonal configuration between the donor and acceptor, thereby reducing the ΔE ST value. [46] The results showed that the carbazole plane was nearly orthogonal to the central phenyl plane, accompanied by a tiny ΔE ST of 0.12 eV as well as a high PLQY of 79% in degassed toluene. Then the device using DPEPO as host material and compound 13 as emitter was fabricated and showed blue emission with the peak of 466 nm with CIE coordinates of (0.139, 0.150), as well as achieving a high maximum EQE of 22.6%. Based on the above experimental observations, the authors continued to design a set of ortho-carbazole-appended triarylboron compounds (14)(15)(16)(17) by introducing various substituents to the dimesitylphenylboron acceptor and/or the carbazole donor to modulate the emission spectra from sky-blue to deep-blue region. [47] All the compounds exhibited blue emission, and the spectra gradually blueshifted from sky blue (λ em = 481 nm for 14) to ultradeep blue (λ em = 438 nm for 17) in toluene solution, while the PLQY of the compound exhibited a dramatic decrease with the increase of the emission energy. A highperformance blue device (λ EL = 479 nm) using compound 15 as emitter was achieved with EQE as high as 32.8%, as well as CIE coordinates of (0.135, 0.266). Furthermore, the deep blue device (λ EL = 449 nm) based on 17 exhibited a maximum EQE value of 14.9% along with the CIE coordinates of (0.151, 0.058). Similarly, two blue emitters of 18 and 19 were successfully developed by attaching alkyl substituents on the ortho-donor-acceptor framework to effectively inhibit concentration quenching. [48] From the results of crystal structures (Figure 2A [48] triarylboron-based TADF molecules (20 and 21) by employing multiple donors with mild donating abilities and suitable numbers. [49] The utilization of 9,9-diphenyl-9,10dihydroacridine and 10H-spiro[acridine-9,9′-fluorene] as electron donors endowed molecules 20 and 21 with rigid structures. 20 and 21 showed blue emission peaking at 464 and 479 nm in doped films, accompanied with high PLQYs of over 70%. The solution-processed OLEDs utilizing 20 and 21 as emitters exhibited blue emissions, and abtained the maximum EQE of 12.8% and 17.3%, respectively. Despite moderate device efficiencies, the design strategy and relationship between chemical structure and device performance could provide instructive guidance to the future construction of highly efficient blue TADF molecules.

Dibenzoheteraborin-based emitters
The introduction of dibenzoheteraborins as acceptor groups bearing electron-deficient boron center and a heteroatombased bridge into the TADF emitter is a practicable tatic for obtaining highly efficient OLEDs with excellent luminescence efficiency and good color purity. In this way, two luminescent compounds (22 and 23 in Scheme 3) containing 10H-phenoxaboryl substituents as electron acceptor and carbazole or 9,9-dimethylacridane as electron donor have been designed, and both of them showed blue emission. [50] However, only compound 23 exhibited TADF properties with relatively short delayed lifetime of 2.36 μs. Given that the luminescence efficiency of compound in doped DPEPO film was as high as 98%, the device based on the emit-ter 23 exhibited blue EL with the peak of 466 nm, and obtained the EQE of 15.1% with a maximum luminance of 8216 cd m -2 . In another study, a class of high-efficient blue TADF emitters (24)(25)(26)(27) containing acridan or 1,3,6,8tetramethylcarbazole donor units has been designed and showed high PLQY (Φ = 56-100%). [51] The emission spectra of compounds 25-27 (λ em = 456, 451, and 443 nm, respectively) were blueshifted compared with that of compound 24 (λ em = 475 nm), which is because the rotation and bending were restricted by the spiro-type acridan moieties for compounds 25 and 26. The results of the optimized device performance demonstrated that the maximum EQEs of compounds 24-27 were 21.7%, 19.0%, 20.0%, and 13.3%, respectively. Although the EL spectra slightly redshifted in comparison with corresponding PL spectra in toluene, the CIE coordinates of the devices based on compounds 26 and 27 were both (0.14, 0.16), which approached the national standards of blue color. Instead of the widely used carbazole-or acridine-based donors, Kwon and coworkers recently prepared two blue emitters 28 and 29 by introducing highly conjugated rigid donor based on fused carbazole. [52] The introduction of bulky and rigid ring could efficiently enhance oscillator strength and reduce vibronic coupling by large steric hindrance, thereby improving PLQY. The results showed that compound 28 exhibited a slightly redshift of emission peak (λ em = 458 nm) compared to compound 29 (λ em = 438 nm), along with the high PLQY of up to 97.1% for compound 28 and 63.1% for compound 29 in doped film. Notably, both compounds exhibited excellent device performances, with the maximum EQE of up to 37.4% in the skyblue region for compound 28 (CIE coordinates of (0.16, 0.34)), and 12.5% with CIE coordinates of (0.15, 0.08) for compound 29.
Another method to develop high-efficient boron-contained TADF materials is realized by introducing a bridging heteroatom into the acceptor groups. Yasuda and coworkers reported a class of high-performance emitters (30-33) based on dibenzoheteraborins including phenothiaborin, phenoxaborin, and phenazaborin. [53,54] All these compounds exhibited blue emission with the peak from 465 to 485 nm, and high PLQYs of over 99% in doped films. Especially, there was no obvious concentration quenching for compounds 30 and 32, both exhibiting PLQY as high as 99% even in the neat films, possibly because of the suppressed vibrational nonradiative relaxation by their twisted configuration. The multilayer TADF devices based on these emitters were fabricated and exhibited redshifted EL (473-500 nm) in comparison with the corresponding PL spectra. Among the four devices, 30-containing device showed the highest maximum EQE of 24.9% and current efficiency of 57 cd A -1 . In another study, donor-acceptor-donor type compounds 34-37 incorporating carbazole or acridine units at 2 and 8 positions of azaborine unit were prepared. [55] Surprisingly, only compounds 36 and 37 exhibited pronounced TADF properties with extremely high PLQY up to 99%. 37-based device displayed deep-blue EL with the CIE coordinates of (0.14, 0.19), and achieved a maximum EQE of over 21.0%.
Aside from the desirable luminescence efficiency, the color purity is also a critical factor for the design of blue-emitting OLEDs. Yasuda and coworkers designed two blue TADF emitters 38 and 39 by adopting a pentacyclic

S C H E M E 3 Chemical structures of dibenzoheteraborin-based blue TADF emitters
ladder-heteraborin acceptor. [56] The incorporation of the πextended acceptor not only makes these two compounds exhibit extraordinarily high PLQY of nearly 100%, but also possess an ultrashort TADF lifetime of 0.78 μs and a minimal ΔE ST value of only 0.01 eV, which translate into the excellent performance in TADF devices, with the impressive maximum EQE as high as 20.1% and 25.9%, CIE coordinates of (0.13, 0.20) and (0.14, 0.33) for the compound 38 (λ EL = 473 nm) and 39 (λ EL = 484 nm), respectively ( Figure 3A and B).

Oxygen-bridged boron-based emitters
The substitution of nonconjugated boron-based acceptors with oxygen-bridged triphenylboron acceptors provides a novel strategy to realize efficient blue emission. In 2015, Hatakeyama and coworkers first prepared a class of novel boron-containing polycyclic aromatic compounds (40)(41)(42)(43)(44) in Scheme 4), all of which exhibited blue emission with the maximum wavelength ranging from 418 to 477 nm, along with the relatively high luminescence efficiency. [57] F I G U R E 3 (A) Photographs of EL emissions and (B) EQE versus L characteristics. Reproduced with permission. Copyright 2020, American Chemical Society [56] Although only compound 44 exhibited desirable features for a TADF emitter, the ΔE ST values could be further reduced by the introduction of other donor groups to obtain an ideal TADF emitter. Using the similar strategy, two Copyright 2020, Wiley-VCH [59] deep-blue emitters 45 and 46 were designed and synthesized through employing 1,3,6,8-tetramethyl-9H-carbazole as a donor moiety. [58] Both compounds exhibited remarkable solvatochromic effects upon changing the polarity of the solvents from cyclohexane to dichloromethane, with large bathochromicshifts of maximum emission wavelength up to 96 nm and 101 nm for compounds 45 and 46, respectively. Notably, the doped films of 45 and 46 exhibited blue emission peaking at 467 and 477 nm, respectively, both of which were slightly bathochromic-shifted in comparison with those in toluene solution (λ em = 446 and 455 nm for 45 and 46, respectively). The devices based on compounds 45 and 46 exhibited blue EL peaking at 471 and 479 nm, with the CIE coordinates of (0.14, 0.18) and (0.14, 0.26), respectively, both of which obtained the maximum EQEs of over 20%. A series of compounds (47-49) based on boron-based electron acceptor by tethering carbazole derivatives with different electron-donating ability were developed to modulate PL spectra from deep-blue to sky-blue region. [59] As the electron-donating ability increased, the emission spectra of 47-49 exhibited gradually redshifts with emission maxima going from 433 (47) to 494 nm (49) in the neat film, accompanied with the PLQY as high as 99%. However, all the compounds exhibited relatively weak emission in solution. When the ratio of poor solvent increased, a significant increase in the emission intensity was observed, resulted from the formation of aggregation and thus benefiting from the suppression of the nonradiative decay. Consequently, nondoped solutionprocessed devices ( Figure 4A) containing compounds 47-49 were fabricated and showed deep-blue to sky-blue EL peaking at 424, 448, and 492 nm, respectively, and the maximum EQE and current efficiency were 9.90%/4.0 cd A -1 (47), 6.13%/4.59 cd A -1 (48), and 6.04%/15.0 cd A -1 (49), respectively ( Figure 4B-D). In another study, the blue emitter 50 (λ em = 445 nm) was obtained from 48 by substituting methyl groups on the 3 and 6 positions of the carbazole units. As a result, better device performances were achieved for emitter 50 with improved maximum EQE and luminance, which was ascribed to the shorter delayed lifetime and higher delayed portion. [60] Similarly, through introducing electrondonating substituents, such as tert-butyl and tert-butylphenyl, into the 3 and 6 positions of the carbazole moiety, two ultradeep-blue emitters (51 and 52) featuring aggregationinduced emission (AIE) characteristics were designed and synthesized by Choi and coworkers. [61] These two compounds exhibited broad and relatively weak emission band at 450 nm in solution. However, fluorescence intensity rapidly enhanced with increasing the ratio of poor solvent, due to the formation of aggregation thereby leading to the blocking of the nonradiative decay. Additionally, the twisted donoracceptor geometry suppresses the spatial overlap of HOMO and LUMO, resulting in a smaller ΔE ST for 52 between these two compounds. Then solution-processed nondoped TADF-OLEDs were fabricated, and both 51 and 52-based devices displayed ultradeep-blue electroluminescence peaking at 416 and 428 nm, with CIE coordinates of (0.17, 0.06) and (0.16, 0.05), respectively. Noticeably, both devices showed narrowband emission with FWHM about 45 nm, and the device with the emitter 52 achieved an unprecedentedly high maximum EQE of reaching 15.8%. Subsequently, the authors continued to developed two novel deep-blue emitters possessing AIE properties (53 and 54), by linking two carbazole derivatives as electron donors and an oxygen-bridged triarylboron unit as an acceptor to the benzene center. [62] Likewise, the AIE properties endowed the nondoped solution-processed devices containing 53 and 54 with deep-blue emission and CIE color Reproduced with permission. Copyright 2020, Wiley-VCH [63] coordinates of (0.17, 0.07) and (0.16, 0.08), respectively, as well as the highest maximum EQE of 10.11%.

S C H E M E 4 Chemical structures of oxygen-bridged boron-based blue TADF emitters
The factor of high horizontal dipole orientation is often taken into account for the development of high-performance TADF materials. For example, two blue-emitting compounds 55 and 56 were designed and prepared by employing spiroacridine derivatives as donor units. [63] The introduction of the spiro-type donor not only helps provide a relatively deep energy level of the highest occupied molecular orbital (HOMO) compared with the dimethyl acridine structure, thereby leading to the both compounds exhibiting blue emission with the peak at 450 (55) and 460 nm (56), respectively, but also offers an elongated molecule structure, which resulted in high ratios of horizontal emitting dipole moment (Θ) over 80% with high PLQY of nearly 90%. Meanwhile, the devices emitted deep-blue (λ EL = 445 nm for 55) and blue (λ EL = 456 nm for 56) EL emission, and achieved the maximum EQE of 28.2% and 25.7% for 55 and 56 ( Figure 5A-C). In addition, both devices showed a narrowband emission with a small FWHM of lower than 65 nm, which benefited from reduced vibronic transitions caused by high rigid structures ( Figure 5D). Later, upon the modification of 55 and 56, Yang and coworkers prepared two similarly linear TADF emitters (57 and 58), both of which exhibited high Θ values of over 86%, and thereby obtaining the maximum EQE of 29.3%, and a maximum luminance of 27663 cd cm -2 in blue TADF-OLEDs based on 58. [64] Similarly, two oxygenbridged triarylboron compounds 59 and 60 modified with the dimethyl acridine donor or rigid diindolocarbazole groups were designed to show high PLQY and narrow-band blue emission. [65] The results showed that both compounds 59 and 60 exhibited narrow-band deep-blue emission peaking at 458 nm (FWHM: 50 nm) and 456 nm (FWHM: 55 nm) in solution, accompanied by relatively short delayed lifetimes of less than 1.0 μs, resulting from the well-separated HOMO and LUMO induced by the large twisted configu-ration. Surprisingly, compound 60 exhibited sky-blue emission with an extremely high EQE of 38.15 ± 0.42%, which was mainly ascribed to its high PLQY close to 100% and the large Θ of up to 0.89. Additionally, the device exhibited maximum luminance of 47680 cd m -2 , and the value could be still maintained at 25.2% at 5000 cd m -2 . In another study, upon the modification of the rigid acceptor on the compound 60 with t-butyl groups, a diindolocarbazole-based boron derivative 61 was obtained, and the single host device based on 61 showed blue EL with the CIE coordinates of (0.16, 0.39). Additionally, the device achieved an excellent maximum EQE of 28.1%, and a long device operational lifetime (LT 50 ) of up to 329 h, whereas the mixed hostbased device exhibited a relatively low EQE of 26.4% but with two times longer lifetime (LT 50 : 540 h). [66] Moreover, when incorporating an oxygen or sulfur atom into the rigid donor of the compound 60, the resulting emitters (62 and 63) not only exhibited excellent EQEs of 33.2% and 32.8% in TADF-OLEDs devices, respectively, but also achieved extremely high EQEs of 38.8% and 37.3%, respectively, in hyperfluorescence devices with using them as TADF sensitizing host. [67] Additionally, a series of oxygen-bridged boronbased compounds 64-66 were designed by attaching various donors to the central phenyl ring at the meta position of the boron atom, which showed highly efficient blue emission (444-470 nm). [68] For compound 66, the introduction of Br atom accelerated RISC, thereby resulting in a shorter delayed lifetime. Compared to the compounds 64 and 65, compound 66 with Br substituent exhibited better device performance with the maximum EQE approaching 22.5%. Different from the above-mentioned modification on the meta/para position of the boron atom in the central phenyl ring, Wang and coworkers designed three isomeric conjugated derivatives (67-69) by incorporating two dimethylacridine groups at various positions of two pheripheral phenyl units in the electron-accepting boron-embedded rigid  [69] framework. [69] These compounds showed orthogonal donoracceptor configuration, which endowed these molecules with an effective separation of HOMO/LUMO, thereby leading to a tiny ΔE ST . All the compounds exhibited obvious solvatochromism effect, and the emission spectra were redshifted over 100 nm when the solvent changed from cyclohexane to acetonitrile, which was because of the enhanced intramolecular charge transfer between electron donor and acceptor ( Figure 6A). Additionally, compound 68 exhibited the highest PLQY up to 96% ( Figure 6B) and the shortest delayed fluorescence lifetime of 1.17 μs among these three compounds, all of which made the emitter exhibit excellent performance in doped and nondoped sky-blue OLEDs with the highest maximum EQE reaching 20.5% ( Figure 6C and D). In another study, two organoboron compounds 70 and 71 substituting weak electron-donating groups of the dimethylacridine units with carbazole units were reported with tunable emission colors and highly efficient blue TADF devices. [70] Compared to the widely used acridan derivatives, 10,10-diphenyl-phenazasiline is a weaker and more stable donor unit, thus resulting in blueshifted emissions. For example, Yasuda and coworkers reported a deep-blue TADF emitter 72, in which the polycyclic organoboron was employed as a weak electron acceptor and phenazasiline was used as a weak electron donor. [71] When doped in 2,8bis(diphenylphosphinyl)dibenzo[b,d]furan (PPF) films, 72 exhibited gradually redshifted emissions with increasing doping concentrations, while retaining high PLQYs even in the neat film (Φ PL = 73%). In addition, the emitter showed quite short delayed fluorescence lifetimes of about 1 μs regardless of doped ratios. The TADF-OLEDs containing 72 achieved extremely high maximum EQE approaching 33.8% in the blue region, and the EQE still maintained at 26.2% even at the luminance of 1000 cd m -2 . Significantly, even the 72-based nondoped devices achieved the maximum EQE of up to 23.1%, together with suppressed efficiency roll-off.
Recently, Wang and coworkers designed and synthesized a class of blue polymers (73-78) by connecting acridan donor and oxygen-bridged boron-fused acceptors by a nonconjugated polystyrene. [72] Through attaching different substituents on the acceptor moiety, emission colors could be effectively modulated from deep blue (444 nm) to sky blue (480 nm) for these polymers. In addition, all the polymers exhibited weak emission in the good solvent. However, the emission intensity was significantly enhanced as the the ratios of poor solvent increased. Solution-processed devices using these polymers as emitters achieved excellent performance with the highest maximum EQE and current efficiency of 15.0%, and 30.7 cd A -1 , respectively.

Nitrogen-bridged boron-based emitters
Unlike conventional donor-acceptor-based TADF molecules, multiresonance (MR)-induced TADF molecules exhibit an alternating HOMO/LUMO distribution induced by opposite resonance effect of boron and nitrogen or oxygen atoms in a fused polycyclic aromatic skeleton. Benefiting from the rigid polycyclic skeleton, these so-called MR-induced TADF materials usually show extremely high PLQY, fairly small FWHM, as well as high oscillator strength, which eventually translate into the satisfied performances in TADF-based OLED. The rigid polycyclic aromatic framework featuring the multiresonance (MR) effects affords a novel strategy to construct high-performance blue materials. For example, in 2016, Hatakeyama and coworkers reported the first example of boron-based MR-TADF emitters (79 and 80 in Scheme 5) with two nitrogen atoms incorporated into the triphenylboron to form a rigid framework. [73] The results indicated that the Wiley-VCH [73] resonance effect located LUMO on the boron atom and its ortho and para positions, whereas HOMO on the nitrogen atoms and the meta position of the boron atom, thereby resulting in an effective separation of HOMO and LUMO and thus a tiny ΔE ST value ( Figure 7A). Both the compounds 79 and 80 exhibited strong and sharp emission spectra peaking at 460 (FWHM: 30 nm) and 469 nm (FWHM: 28 nm) in thin films of 3,3′-Di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP), respectively, accompanied by high PLQYs ranging from 88% to 90%. By using mCBP as host material, the corresponding OLEDs were fabricated, which exhibited pure and sharp blue emission (λ EL = 459 nm; FWHM = 28 nm) with the CIE coordinates of (0.13, 0.09), but with the maximum EQE of 13.5%. Upon modification by substituents, the device performance was dramatically improved. Compared with 79, the device based on compound 80 exhibited a slight redshift (λ EL = 467 nm) along with a FWHM of 28 nm and a maximum EQE of 20.2% ( Figure 7B and C). Modification of the compound 79 with diphenylamine, carbazole, methyl, or tbutyl units afforded a class of MR-TADF emitters 81-85, all of which exhibited blue emission with the wavelength ranging from 454 to 466 nm accompanied by amplified oscillator strength and improved device performance. Particularly, the device based on the compound 85 with a TADF assistant dopant exhibited a higher EQE of over 30% and a lower efficiency roll-off. [74,75] In another study, carbazole moiety was introduced to the para position of the B atom on MR skeleton to obtain MR-TADF emitter 86 ( Figure 8A). [76] Compared with the reported compound 79, the emission spectrum of the compound 86 (λ em = 470 nm) ( Figure 8B) slightly redshifted by about 10 nm and showed a slightly smaller FWHM of 26 nm in toluene accompanied by an extremely high PLQY up to 100%, which can be attributed to the improved oscil-lator strength and a fast RISC. Furthermore, the device employing the compound 86 as emitter exhibited ultrapure blue emission and superior performance with the maximum EQE approaching 32.1% as well as a suppressed efficiency roll-off ( Figure 8C and D). Subsequently, two compounds 87 and 88 by introducing additional aromatic rings into the MR framework were also designed for solutionprocessed TADF devices. [77] The results showed that the extension of π-conjugation could enhance MR effect, and the solution-processed devices doped with these two compounds exhibited efficient sky-blue EL with the maximum EQE of 16.3%. Recently, after replacing two carbazole subunits of 87 with imine-containing γ-carborine (pyrido [4,3-b]indole), the resulting compound 89 possessed much higher S 1 and T 1 energies and thereby leading to an obvious increase of band gap and thus a shorter-wavelength emission, while still maintaining the intrinsic MR-dominant characteristics. [78] Consequently, the corresponding OLEDs not only showed narrowband deep-blue emission peaking at 461 nm with a small FWHM value of 28 nm, but also achieved moderate maximum EQE of 19.0%.
Since most MR emitters usually suffer from severe aggregation-caused quenching and could only function well in OLEDs at extremely low doping concentrations, thereby encountering incomplete energy transfer and serious phase separation, the design of quenching-resistant emitters is of paramount importance and provides key benefits to mass production and manufacturing cost from the practical viewpoint of application. Recently, Yang and coworkers prepared a sterically wrapped MR emitter 90 with a MR-emitting core 88, in which the intermolecular distance between the MR-emitting core and the adjacent molecules can be significantly enlarged by steric hindrance, thereby leading to quenching-resistant PL in comparison with the reference molecules (91 and 92). [79,80] The corresponding OLEDs devices based on 90 exhibited superior performances with EQE of 40.0%, PE of 109.7 lm W -1 , and FWHM of only 25 nm. Because of slight insensitiveness of 90 to doping concentration, the maximum EQE was still retained at 33.3% with almost unchanging emission spectrum in 30 wt% doped devices. Additionally, a series of MR analogs (93)(94)(95)(96)(97) functionalized with the compound 79 have been designed to exhibit highly efficient blue emission. [81] The results indicated that the emission color can be effectively modulated from pure blue (λ em = 460 nm) to sky blue (λ em = 478 nm) by the introduction of the alkyl substituents or extension of the π-conjugation. The device employing compound 97 as emitter showed sky-blue EL (λ EL = 477 nm) with the CIE coordinates of (0.11, 0.23), and a FWHM of 27 nm, along with the maximum EQE of 21.8%. Incorporation of one more diphenylamine group into 93 afforded emitter 99, which still maintained blue emission and high PLQY, but achieved more desirable maximum EQE approaching 27.7% and narrower FWHM of only 24 nm in comparison with 93 and parent core 98. [82] Instead of introducing only one boron atom in MR system, Hatakeyama and coworkers reported an ultrapure blueemitting compound 100 containing five benzene rings linked by two boron and four nitrogen atoms. [83] Opposite MR effect between boron and nitrogen atoms induced the alternating distribution of HOMO and LUMO, as well as minimized the vibronic coupling and vibrational relaxation in the material, thereby resulting in a small ΔE ST and a  [76] narrow emission spectrum with FWHM of 14 nm. Furthermore, 100-based device exhibited an ultrapure blue EL peaking at 469 nm accompanied by a FWHM of 18 nm, and achieved an extremely high EQE value of 34.4% with a small efficiency roll-off. Upon replacing a nitrogen atom in 100 with an oxygen atom, a novel MR-TADF material 101 was reported with blueshifted emission spectra. [84] As expected, the emission spectrum of 101 (λ em = 464 nm) was slightly blueshifted by 5 nm than that of 100, which was attributed to the weaker electron-donating ability of oxygen than nitrogen atom. Undoubtedly, corresponding OLEDs obtained superior device performances with the maximum EQE of up to 29.5%, improved efficiency roll-off and a long device lifetime at the initial brightness of 100 cd m -2 (LT 50 = 314 h). In another similar design, with the same polycyclic boronfused MR-TADF skeleton, a class of ultrapure blue emitters (102)(103)(104) were successfully constructed by incorporating boron, nitrogen, oxygen, and sulfur atoms into the same polycyclic π-system. [85] Through substituting two nitrogen atoms in 100 for oxygen and/or sulfur atoms, emission color could be precisely tuned from 440 to 470 nm while retaining MR-induced narrowband spectra as well as high PLQY. The OLEDs based on 102, 103, and 104 showed sharp blue emission, with EL peaks at 445-463 nm, FWHM of 18-23 nm, and small changes in CIE-y coordinates. Especially, for the 103 and 104-based OLEDs, high maximum EQEs of 26.9% and 26.8%, respectively, as well as relative low efficiency roll-offs, were realized concurrently. Later, a set of compounds (105-107) that contained two, three, or four boron atoms were successfully designed and synthesized. [86] Interestingly, the emission maximum of the compound 106 (λ em = 441 nm) was blueshifted compared to that of the compound 105 (λ em = 455 nm) and 107 (λ em = 450 nm). Given the CIE coordinates of the compound 105 perfectly satisfied the NTSC standard for pure blue, an OLED employing compound 105 as the emitter was fabricated, which exhibited excellent performance with the maximum EQE about 18.3%, accompanied by a slight redshift of 5 nm and broadening of 5 nm in FWHM.
With the similar method, two polycyclic aromatic compounds 108 and 109 bearing carbazole unit were designed, both of which showed efficient blue emission peaking at about 466-483 nm with the high luminescence efficiency. [81,87,88] Furthermore, the compound 108 was used as the blue emitter in OLEDs devices, and the device exhibited a very similar but slightly broadened EL spectra (λ em = 469 nm, FWHM = 27 nm) compared with that of the emission in solution, and an impressively high maximum EQE of 29.3%, probably due to the occurrence of interaction between the host and guest molecule and the adoption of horizontal molecular orientation within the doped films. In the related works, a series of symmetrical and unsymmetrical blue-emitting compounds (110)(111)(112)(113)(114)(115)(116) obtained by introducing various substituents into the polyaromatic hydrocarbons skeleton were designed and prepared. [89,90] The unsymmetric derivatives (111 and 113) exhibited similar photophysical properties compared with their symmetrical counterparts, and functionalization with trifluoromethyl or carbazolyl groups resulted in similar emission band albeit with higher PLQYs of 85% (112) and 81% (114). Considering that the compounds 115 (λ em = 482 nm, FWHM = 33 nm, Φ = 89%) and 116 (λ em = 479 nm, FWHM = 34 nm, Φ = 88%) showed strong emission and narrow FWHM in doped film, the devices based on these two compounds were fabricated, and showed skyblue EL at 480 nm and a narrow FWHM of 33 nm as well as the maximum EQE of up to 21.4%.

GREEN TADF EMITTERS
In this section, green TADF emitters are defined as those whose maximum emission of EL lies 500-555 nm. The photophysical properties and devices performance of the TADF emitters are summarized and shown in Table 2.

Boron-nitrogen coordination based emitters
The incorporation of intramolecular B-N coordination between a N-heteroarene and a boryl unit is a feasible strategy for acquiring high PLQYs and bathochromic shift of the emission spectra, due to the increase of the electronaccepting abilities and the enhancement of the coplanarity of the π-conjugated skeleton. For example, Chi and coworkers designed two boron four-coordinate complexes (117 and 118 in Scheme 6), in which electron-accepting phenylpyridine groups and the electron-donating triphenylamine were connected by a core boron atom. [91] Both compounds emitted strong green light at around 530 nm in toluene solution, and exhibited obvious solvatochromism upon increasing the polarity of the solvent, ascribed to the presence of the interligand charge transfer. Using the identical device patterns, the device performances containing compound 118 (26.6%, 88.2 cd A -1 , 81.5 lm W -1 ) had competitive advantage over those of the device containing compound 117 (20.2%, 63.9 cd A -1 , 66.9 lm W -1 ), owing to the high quantum efficiency of the former. Using the similar strategy, a series of fourcoordinate boron complexes (119-122) merged with various donors and acceptors were designed to tune the emission wavelength. [92] The emission spectra were gradually redshifted with the emission peak ranging from 503 to 558 nm with the increase of the strength of the donor groups. Given that the PLQYs of these complexes in doped host films were less than 65%, OLEDs achieved the highest maximum EQE of only up to 8.1%.
Another research demonstrated that the utilization of 2-phenylpyridinatoboron derivative 123 modified with a diphenylether bridge can improve the efficiency of the exciplexes. [93] The emission spectra of the doped films showed large redshifts compared with that of the neat films of the compound, confirming the formation of exciplexes in the blend films, accompanied by the PLQYs as high as 60%. The device using the blend film as emitting layer exhibited green EL peaking at 518 nm, but with the maximum EQE of only 10.5% because of charge carrier imbalance and carrier traps. In another study, upon the treatment of the introduction of heavy atom, compound 124 with structure of donorspiro-acceptor was obtained from 123, along with the slightly bathochromic shift of the emission spectra and the increasing intensity of emission, which was attributed to the enhanced strength of the donor unit and suppression of the nonradiative deactivation by the distorted configuration. [94]

Triarylboron-based emitters
The introduction of the triarylborane acceptor into D-A system affords another strategy to realize highly efficient green emission. In this way, two triarylboron-based compounds (125 and 126 in Scheme 7) bearing an identical nonplanar phenoxazine group were designed and synthesized. These emitters exhibited high PLQYs of up to 92% due to the distorted configuration. [44,95,96] For the devices performances, the results indicated that the devices doped with these compounds showed strong EL emission peaking at 502-525 nm, together with high maximum EQEs of up to 22.4%. Similarly, a set of nido-carborane-appended triarylboranes (127-134) were designed to endow these compounds with a distorted molecular configuration by introducing various substituents. [97] The large torsion angle of compound 128 indicated a highly distorted linker between the plane of nido-carborane and phenylene bridge, which can be mainly attributed to steric effect of the 8-Me unit. All nido compounds exhibited broad emission with peaks ranging from 502 to 548 nm, and the emission bands of compounds 127-130 and 131-134 were blueshifted as electron-donating abilities of the 8-alkyl substituents increased. Interestingly, the compounds 127-130 exhibited weak luminescence in solution, while strong emission was observed in doped film. For nido compounds 131-134, all compounds exhibited slightly high PLQYs in solution, but with small changes in film state in comparison with corresponding meta derivatives. However, a little increase of the PLQY in the films was TA B L E 2 Summary of photophysical properties and device performance of organoboron-based green TADF emitters

S C H E M E 7 Chemical structures of triarylboron-based green TADF emitters
observed much compared with that of the meta derivatives. Additionally, introduction of the nido-carborane endowed these compounds with a highly twisted configuration, thereby resulting in a very tiny ΔE ST , which endowed nido-carborane compounds effective TADF properties. In another study, the same group developed a class of ortho-carbazoleappended triarylboron compounds 135-138 by introducing perfluoroalkyl or perfluoroaryl groups as the secondary acceptors. [98] Interestingly, the emission spectra of these compounds exhibited gradually bathochromic shifts from 512 to 536 nm with the increase of the electron-withdrawing ability of the perfluoro-substituent, accompanied by extremely high PLQYs, particularly for compound 136 of nearly 100%. The device based on the compound 136 achieved excellent performance with a maximum EQE of 29.9% and an extremely low turn-on voltage of 2.35 V. Particularly, the device showed a high maximum power efficiency of 123.9 lm W -1 , which could still retain at high values (82.3 lm W -1 ) at the brightness of 1000 cd m -2 . Another research demonstrated that the utilization of the triarylboron acceptor combining ortho-donor group could limit the free rotation of the molecule, thereby leading to efficient TADF. [46] The results indicated that the incorporation of steric hindrance of the acceptor endowed the compound with inherent steric locking for a highly skewed configuration, resulting in a minimal ΔE ST value and thus displaying efficient TADF properties with lifetimes in the range of microseconds. Additionally, the resulting compound 139 exhibited a broad emission spectrum in orange region peaking at 585 nm, accompanied by a high PLQY in oxygenfree toluene solution, which was about fourfold higher than that in air-saturated toluene solution, indicating that the T 1 state was quenched by triplet oxygen in air-saturated solution. When the ortho-D-A compound 139 was used as emit-

S C H E M E 8
Chemical structures of dibenzoheteraborin-based green TADF emitters ter, highly efficient orange device was realized with a high maximum EQE of 16.3% and the CIE coordinates of (0.46, 0.51).
Additionally, a donor-acceptor-donor compound 140 with acridan derivative as the electron donor and dimesitylborane as the acceptor moiety was designed to efficiently reduce the efficiency roll-off. [99] The results showed that the device obtained a maximum EQE of 19.3%, which can be retained at 18.2% at the brightness of 102 cd m -2 . Further observations of materials based on triarylboron were performed by integrating cyano or other electron-accepting groups into the phenoxazine/triarylboron hybrids. For example, a class of triarylboron-based compounds 141-143 was designed and synthesized, in which the acceptor triarylboron and donor phenoxazine were connected by a highly sterically hindered tetramethylphenyl group. [100,101] All the compounds exhibited highly distorted configuration, which rendered the tiny ΔE ST values and enhancement of PLQYs because of the effectively inhibition of the nonradiative decay by rigid molecular structures. Additionally, introduction of the cyano groups strengthened the intramolecular interaction thereby increasing the intensity of the charge-transfer transition. As a result, PLQYs of the compounds 142 and 143 were 1.1 and 1.15 times higher than that of the noncyano counterpart (Φ = 65%). Corresponding devices were successfully fabricated by solution-process technology, and achieved efficient green emission with the wavelength ranging from 510 to 533 nm, and maximum EQEs of 13.9% along with relatively low-efficiency roll-off. In a related work, Wang and coworkers reported two boron-containing compounds 144 and 145 by integrating the bismesitylboryl and triazine or sulfonyl acceptor to construct donor-acceptor-acceptor type TADF emitters. [102] The expansion of acceptor rendered dispersive LUMO distribution on the two acceptor units, thereby leading to a tiny overlap of the HOMO and LUMO and relatively large oscillator strengths. 144 and 145 exhibited structureless emission spectra peaking at 546 and 533 nm in toluene solution, respectively, while the spectra were slightly bathochromic-shifted in doped film, accompanied by a significant enhancement of PLQY because of the blocking of the nonradiative decays by the rigid environment. The merits of tiny ΔE ST and high PLQY endowed the OLEDs of these compounds with high EQE of 24.8% and a low-efficiency roll-off.

Dibenzoheteraborin-based emitters
Inspired by the idea that the steric hindrance between donor and acceptor moiety could efficiently segregate the HOMO and LUMO thereby leading to a tiny ΔE ST , Oi and coworkers reported a green emitter 146 as shown in Scheme 8, in which phenoxazine donor and 10H-phenoxaborin acceptor were connected by a phenylene bridge. [50] The dihedral angle between phenylene and acceptor was 52.21 • , while the phenylene and the donors were determined to be 87.51 indicating the presence of the steric hindrance could efficiently decrease the overlap of the HOMO and LUMO. The OLEDs using this compound as emitter exhibited green EL emission peaking at 503 nm, and a maximum EQE of over 22.0%. In a related study, Cheng and coworkers designed an organoboron emitter 147 by introducing two electrondonating amino groups to the 2 and 8 positions of azaborine group. [55] Surprisingly, the compound displayed highly efficient green emission at 518 nm accompanied by an extremely high PLQY nearly 100%, attributed to the large dihedral angles between the donor and acceptor, which made corresponding device based on the compound 147 exhibit a green emission (λ EL = 512 nm) with the CIE coordinates of (0.26, 0.57) and the maximum EQE of over 27.5%. Taking into account the shorter TADF lifetime induced by the heavy atom, a compound 148 by introducing a phenothiaborin moiety instead of a phenoxaborin moiety was designed to show highly efficient green emission. [53] The results indicated that the introduction of sulfur atom largely could raise the spinorbit coupling (SOC) between the lowest singlet (S 1 ) and triplet (T 1 ) states, and thus accelerating RISC and shortening delayed fluorescence lifetime close to 1.0 μs. Additionally, compound 148 exhibited extremely an ultrahigh PLQY of nearly 100% in heavily doped films and showed negligible concentration quenching even in the neat films, because of the effective suppression of triplet excitons-related quenching. The multilayer-doped OLEDs based on compound 148 emitted stable green EL with emission peak of 503 nm and exhibited a maximum EQE of 25.3%, a current efficiency of 72.1 cd A -1 and a power efficiency of 77.4 lm W -1 . Although the device performances of nondoped OLEDs were less than those of the optimal doped devices, both doped and nondoped devices retained high EQE values over 20% even at a high luminance.
In another study, two diboron-based compounds 149 and 150 containing a 9,10-dihydro-9,10-diboraanthracene (DBA) derivative linked to two electron-donating units by a distorted ortho-dimethylphenylene bridge were designed and synthesized. [103] The emission spectrum of 150 exhibited a redshift of about 20 nm in comparison with compound 149 due to the stronger electron-donating ability of di-tbutylcarbazole compared with carbazole groups, but with low PLQYs (lower than 12%) for both compounds in degassed toluene. Surprisingly, PLQYs of compounds 149 (λ em = 524 nm) and 150 (λ em = 553 nm) significantly increased to 100% and 86.0% in doped film, respectively, which made the devices using these two emitters as dopants exhibit highly efficient green emission with the peaks ranging from 528 to 542 nm. Most notably, both devices exhibited extraordinary efficiencies, particularly the device based on the compound 149 achieved the highest maximum EQE of 37.8 ± 0.6%, current efficiency of 139.6 ± 2.8 cd A -1 , and power efficiency of 121.6 ± 3.1 lm W -1 and revealed lowefficiency roll-off because of suppression of self-quenching. In a related work, Lee and coworkers designed another DBA derivative 151 by introducing ortho-donor groups to the acceptor, which showed strong green emission with the PLQYs nearly 100% in both solution and film state. [104] Although emission spectra of the doped films exhibited a small redshift with the increase of the doping concentration of compound 151, the PLQY still as high as 100%. A green OLED device based on the emitter 151 (λ EL = 544 nm) achieved a maximum EQE of up to 26.6% and a desirable power efficiency over 100 lm W -1 .

Nitrogen-bridged boron-based emitters
Besides D-A type molecules, TADF materials with MR effect can also be designed to obtain high-performance OLEDs with high efficiency, narrowband emission, and high color purity. For example, Wang and coworkers designed a compound 152 (Scheme 9) with highly efficient narrowband green emission induced by MR effect. [105] Compound 152 exhibited a sharp and featureless emission band with the peak of 496 nm in toluene with a narrow FWHM of only 21 nm, accompanied by an extremely high PLQY of up to 97% because of the rigid core skeleton and high oscillating strength. Benefiting from the narrowband emission and the small ΔE ST value, an optimized OLED based on compound 152 showed a fairly narrow EL spectrum (λ EL = 508 nm, FWHM: 33 nm) and achieved a maximum EQE approaching 25.5% with the CIE coordinates of (0.20, 0.65). The device performances were further elevated by attaching an additional donor to the MR skeleton of 152. [106] The resulting compound 153 was endowed with a twisted D-A characteristics and MR skeleton, which made the PL spectra exhibit a bathochromic shift with the emission mamima at 519 nm ( Figure 9A), and the device achieved a maximum EQE of 27.0% at the CIE coordinates of (0.23, 0.69) when employed as an emitter ( Figure 9B). In another study, two MR-TADF emitters (154 and 155) employing the simple bis(acridan)phenylene skeleton with two sp 3 carbon atoms were designed for redshifts of the emission. [107] The results indicated that insertion of sp 3 carbon atoms served as locks to rigidify the molecular backbone, and thus extending πconjugation and ultimately inducing the redshifts of emission to green region. Of the two green-emitting devices, the OLED based on 155 achieved the higher maximum EQE of 28.2% with a small efficiency roll-off.
Recently, a hybridized MR-based charge transfer molecule 156 containing boron-nitrogen skeleton fused with azaaromatic rings was prepared. [108] The introduction of fused aza-rings endowed the molecule with twisted conformation, and thus tuned spatial distributions of HOMO and LUMO, thereby leading to the enhancement of intramolecular charge transfer for a small ΔE ST value ( Figure 10A and B). Compound 156 showed green emission peaking at 522 nm with a narrow FWHM of 28 nm, and exhibited significant solvatochromic effect, with redshifted emission and broadened FWHM on changing the solvents from less polar n-hexane to more polar dichloromethane. Additionally, the doped host film (Φ = 94%) also displayed a green emission peaking at 526 nm but with the FWHM broadening by 8 nm, probably because of the presence of intermolecular and intramolecular interactions. The corresponding green-emitting device achieved excellent performances with maximum EQE of 28.2%, maximum power efficiency of 121.7 lm W -1 , as well as a small FWHM of merely 30 nm (Figure 10C and D). Another research demonstrated that the utilization of more than one additional 3,6-di-tert-butylcarbazole units attached to the MR skeleton could realize narrowband EL and improve the OLEDs performance. [88] The results indicated that the devices using the compounds 157 and 158 as emitters displayed narrowband EL with the peak at 515-549 nm, and FWHM values of 54 and 48 nm, respectively. Impressively, compound 157 achieved a higher maximum EQE of 31.8% in comparison with that of 158, derived not only from the high PLQY but also from its spontaneous horizontal molecular orientation within doped film. [106] To further amplify the effect of MR-based skeleton and peripheral substituents, a class of green-emitting MR-TADF emitters (159-161) were reported through attaching the electron-withdrawing groups on the para position of the Bsubstituted phenyl ring. [109] Interestingly, after peripheral functionalization, the distributions of the HOMO were almost unchanged, while the LUMO was clearly extended to the peripheral fluorobenzene groups, which indicated that such distributions showed little influence on the MR effect; that is, the unique properties of the small FWHM for these MR compounds could be maintained. The results showed that all the doped films based on compounds 159, 160, and 161 exhibited strong green emission peaking at 502, 503, and 501 nm, and small FWHMs of 24, 24, and 25 nm, respectively. Particularly, the corresponding OLED based on compound 159 achieved a maximum EQE of 22.02% and power efficiency of 69.82 lm W -1 , which were comparable to those of 160- Based on the similar strategy, a class of efficient narrowband green TADF emitters (162-166) were successfully constructed by introducing the electron-withdrawing 1,3,5-triazine, pyrimidine or benzonitrile derivatives into MR frameworks. [110,111] According to the ability of peripheral electron-withdrawing units, emission wavelength can be modulated precisely from 508 to 540 nm in OLEDs. Among these emitters, 163 exhibited pure green electroluminescence with CIE coordinates of (0.23, 0.68), highest maximum EQE of 30.6%, and relatively low-efficiency roll-off. More recently, Yang and coworkers demonstrated that peripheral decoration of MR skeleton with electron-donating moieties could extend the peripheral conjugation, thereby resulting in the redshifts of emission. [112] Through controlling the S C H E M E 9 Chemical structures of multiresonant boron-based green TADF emitters numbers and electron-donating abilities of peripheral donors, designed MR emitters 167 and 168 enabled the color-tunable narrowband emission from bluish-green (167, 496 nm) to green (168, 534 nm) in toluene solution. The exciplex-hosted OLEDs using 167 and 168 as the emitters showed green emission peaking at 506 and 545 nm, respectively, and both achieved the maximum EQE of over 24.0%. Subsequently, upon replacing tert-butyl substituent group with carbazole moiety in compound 167, compound 169 was developed, with narrowband and slightly redshifted emission. [78] Compared to 167, the maximum EQE was raised to 29.2% for 169-based OLED.
Additionally, the incorporation of oxygen or sulfur atoms into B-N based framework was reported to redshift the emission to long wavelength region. Two MR-TADF emitters 170 and 171 were therefore developed, both of which exhibited green emission whether in solution or doped films. [113] Because of larger size of sulfur atom in comparison with oxygen atom, emitter 171 exhibited a more twisted configuration, thereby resulting in larger geometry relaxation, and thus slightly broadened FWHM. Moreover, heavy atom effect of sulfur atom accelerated the RISC process in 171, and hence achieved high-performance green EL with maximum EQE reaching 25.5%.  [108] Different from the strategy mentioned above, Hatakeyama and coworkers designed and synthesized a solution-processed MR-TADF material 172 (Scheme 9) containing more than one boron atom and bulky mesityl substituents. [114] Although the compound exhibited distorted configuration induced by the steric hindrance ( Figure 11A), the HOMO and LUMO were located on various atoms due to MR effect. The compound displayed strong and sharp green emission with the peak of 506 nm and a small FWHM of about 34 nm ( Figure 11B), accompanied by the significantly high PLQY in doped film, because of the suppression of the vibronic couplings and stretching/scissoring vibrations. The green OLED was fabricated by solution-processed method, which emitted at 505 nm with a FWHM of 33 nm, and achieved the maximum EQE about 21.8% at the CIE coordinates of (0.12, 0.63) ( Figure 11C and D). Similarly, a compound 173 containing carbazole and diphenylamine groups linked by two boron atoms was synthesized by the regioselective one-shot borylation. The device based on this emitter exhibited excellent device performances with a high maximum EQE of 26.7% and the CIE coordinates of (0.12, 0.57), which proved significant potential of MR skeleton containing more than one boron in high-performance OLEDs. [81]

YELLOW-ORANGE TADF EMITTERS
In this section, yellow-orange TADF emitters are defined as those whose EL emission peak lies at 555-600 nm. Compared to green TADF emitters, there are only a few  examples of yellow-orange emitters, which are limited to diarylboron/triarylboron-based or diboron-based structures. The photophysical properties and device performances of the TADF emitters are summarized as shown in Table 3.

Diarylboron/triarylboron-based emitters
Chi and coworkers developed a boryl compound 174 (Scheme 10) bearing a nonplanar phenoxazine moiety and dimesitylboryl acceptor. [96] This compound exhibited remarkable solvatochromism effect, and the emission maximum was redshifted by up to 38 nm upon increasing the solvent polarity from less polar toluene to more polar acetonitrile. The reason could be ascribed to photoinduced charge transfer between the electron donor and acceptor. The utilization of compound 174 as an emitter afforded moderate device performance, giving orange emission with the maximum EQE of 10.9%, current efficiency of 30.3 cd A -1 , and power efficiency of 18.7 lm W -1 , originating from the acceleration of radiationless deactivation induced by the twisting configuration.
In addition to the dimesitylboryl as acceptor, triarylboron can also serve as an efficient acceptor for TADF emitters.

S C H E M E 1 0 Chemical structures of yellow-orange emitters
For example, a donor-acceptor-donor borylated compound 175 was designed and synthesized by using acridan derivatives as the electron donors and triarylboron as the acceptor, which showed near-orthogonal configuration to generate an extremely tiny ΔE ST of less than 30 meV. [99] The emission of compound 175 (λ em = 549 nm) in doped film exhibited a large blueshift up to 56 nm, compared with its emission in toluene solution, originating from the change of dipole moment for the whole system between ground state and excited state. Moreover, the borylated compound was used as dopant to construct OLEDs, which exhibited yellow emission with maximum EQE of 7.6% and maximum current efficiency of 11.6 cd A -1 .

Diboron-based emitters
Benefiting from the high horizontal dipole orientations of the rod-like structures based on the 9,10-diboraanthracene (DBA) derivatives, a series of diboron-based compounds (176 and 177 in Scheme 10) were designed by introducing different electron-donating groups to achieve efficient orange emission. [115] The emission maxima of these compounds in doped host film ranged from 570 to 587 nm, and gradually redshifted with increasing electron-donating ability, accompanied by high PLQYs as well as high horizontal dipole ratios of 84-86%. The EL of these diboron compounds exhibited maximum emission wavelength at 583 and 567 nm for compounds 176 and 177, respectively. Additionally, the devices based on compounds 176 and 177 obtained outstanding performance with the highest EQEs of 24.9 ± 0.5%, and 30.0 ± 0.8%, respectively. Particularly, the device with compound 176 exhibited a relatively long operational lifetime (LT 50 : 124 h) at an initial brightness of 1000 cd m -2 .

Other-type emitters
Yasuda and coworkers developed two organic-inorganic conjugate compounds 178 and 179 (Scheme 10) containing ocarboranes tethered with donor and acceptor groups. [116] Both compounds showed broad emission band in THF solution with low PLQYs of lower than 3%. While the increase of ratios of water resulted in fluorescence enhancement and remarkable redshifts of emission spectra, because of inhibition of the nonradiative decay by formation of aggregation. Benefiting from the unique properties of the aggregationinduced emission, nondoped OLEDs employing 178 and 179 as emitting layers achieved yellow emission peaking at 586 and 590 nm, respectively, both of which were bathochromicshifted by 29 and 19 nm compared to their respective PL emission in neat films. The doped device incorporating 178 exhibited a higher EQE of 11.0% and a maximum luminance of 4530 cd m -2 in these two devices, which was because of the higher PLQY of 178 in neat film. In another study, borylated 2-phenylpyridines derivatives were merged into the pyridyl groups to form donor-spiroacceptor compounds 180 and 181. [94] Both compounds showed weak orange emission peaking at about 600 nm in toluene solution, while the emission intensity was greatly improved with blueshifted spectra in film state, probably because of the rigid environment in film state and weak stability of the excited state. Moreover, the photoluminescence lifetimes of these two compounds in toluene solutions were determined to be several microseconds, and the delayed lifetimes immediately decrease considerably upon the contact with air. Although spiro structures suppresses undesired interconversion of the two conformers, rigid architectures are in urgent need to inhibit the nonradiative decay and improve the stability of the compounds, thereby obtaining highly efficient TADF devices.
Aside from the aforementioned design strategy, boron compounds featuring a linear D-A-D or A-D-A molecular configuration also redshifts the emission to the longer wavelength region. Additionally, such molecular structures could increase the horizontal dipole orientation of emitters and thus enhance PLQY and device efficiency. For example, Kwon and coworkers reported two new A-D-A-type orangered TADF emitters (182 and 183), with rigid boron acceptors and dihydrophenazine donor moieties. [117] Benefiting from the long molecular configuration and rigid skeleton, these two materials not only exhibited high PLQYs (182: 99.8%; 183: 85.4%) and high RISC rates at the order of 10 6 s -1 , but also high horizontal dipole orientation in doped films. Consequently, 182and 183-based devices emitted at 595 and 576 nm achieved the maximum EQE of 30.3% and 21.8%, accompanied by low-efficiency roll-off of only 3.6% and 3.2% at 1000 cd m -2 , respectively. Moreover, these devices showed long operating device lifetimes (LT 50 ) of over 100 h at the initial brightness of 1000 cd m -2 .
For MR-TADF molecules, peripheral decoration of MR skeleton is another versatile strategy for realizing longwavelength emission. For example, by peripherally decorating the parent BNCz molecule with carbazole and diphenylamine moieties, Yang and coworkers reported the first narrowband yellow emitter 184 peaking at 562 nm with a small FWHM of 30 nm. [112] Accordingly, the device with an maximum EQE of 24.7% and excellent color purity was realized. Subsequently, by introducing methyl moieties into peripheral diphenylamine of compound 184, the resulting compound 185 exhibited slightly bathochromic-shifted emission, and the corresponding device ultimately achieved maximum EQE of 19.6%. [78] Similarly, an electron-withdrawing cyano (CN) group was introduced to expand LUMO distribution of the MR-induced TADF skeleton to reduce LUMO energy level thereby redshifting emission. [118] Benefiting from the coplanar conformation between the MR-skeleton and CN group, the resulting emitter 186 exhibited orange-red emission while maintaining a small FWHM of only 42 nm in toluene. By utilizing a TADF sensitizing host to accelerate up-conversion process between triplet and singlet excitons, the OLED based on 186 emitter achieved a maximum EQE of 33.7%, accompanied by the low-efficiency roll-off.

RED-NIR TADF EMITTERS
In this section, red-to-NIR emitters are defined as those whose maximum emission of EL is larger than 600 nm. Similar to yellow-orange TADF emitters, red-NIR emitters are still under development. The photophysical properties and devices performance of the representative red-to-NIR TADF emitters are summarized as shown in Table 4.

Difluoroboron-based emitters
Given the strong electron-deficient properties of the borondifluoride group, a borondifluoride curcuminoid complex 187 (Scheme 11) containing two triphenylamine donor groups and one acetylacetonate borondifluoride acceptor unit was designed to realize efficient NIR emission. [119] The emission spectra exhibited large redshifts peaking at 706-782 nm upon increasing doping concentration of the compound 187 in host blends ( Figure 12A-C). The reason can be ascribed to different singlet triplet dipole moments in film of different doped ratios as well as the occurrence of large aggregates in blend film. While the PLQY gradually increased as the doping concentration decreased, reaching a maximum value of 70% in the 6 wt% doped film ( Figure 12D). When applied as an emitter, the solution-processed NIR-OLEDs showed far red/near-infrared electroluminescence with tunable emission wavelength ranging from 700 to 780 nm via regulating doping ratios. The OLEDs with doping concentration of 6 wt% achieved the highest maximum EQE approaching 10% at the emission wavelength of 721 nm. Such excellent device performance not only benefited from the high PLQY of the doped film but also its TADF activity ( Figure 12E and F). In another study, the same group demonstrated the utilization of the dimeric borondifluoride curcuminoid derivative 188 as efficient NIR TADF emitter. [120] The compound exhibited strong solvatochromism effect, and the emission peak was redshifted by up to 139 nm accompanied by the gradual decrease of the PLQY from 41% in cyclohexane (λ em = 649 nm) to 1.3% in dichloromethane (λ em = 788 nm). Steady-state and time-resolved photoluminescence measurements demonstrated that the compound showed highly efficient NIR TADF emission with the PLQYs of 45% in doped film at the emission maxima of 760 nm, which endowed the corresponding NIR TADF OLEDs with excellent device performances, achieving the maximum EQE of up to 5.1% for the maximum emission wavelength of 758 nm, which is the best performance for NIR electroluminescence among conventional NIR fluorescent emitters. the maximum EQE of 10.1%, serious efficiency roll-off was observed with increasing current density, which was mainly because of the increase of the exciton quenching induced by long delayed lifetime of the emitter. In another study, Yasuda and coworkers designed an organic-inorganic conjugated compound 190 containing electron-deficient icosahedral boron cluster. [116] The introduction of the o-carboranes endowed the compound with a twisted configuration and the aggregation-induced emission, which enabled the corresponding nondoped OLEDs to exhibit efficient red emission (λ EL = 631 nm) with the maximum EQE of up to 10.1%. In another study, a polycyclic aromatic hydrocarbon derivative 191 merged into two para-positioned boron atoms and two nitrogen atoms in the central ring was designed to form multiple resonance skeleton induced by boron and nitrogen atoms, thereby leading to large redshifts of emission spectra. [88] The results indicated that the compound showed intense red emission peaking at 615 nm, accompanied by the high PLQY of up to 89%. Given that the emission intensity in the film was almost the same as that in solution, the device employing compound 191 as emitter exhibited narrowband EL emission (λ EL = 616 nm) with the FWHM of 26 nm, accompanied by the impressively high EQE of 22.0% and CIE coordinates of (0.67, 0.33). Similarly, two deep-red emitters 192 and 193 containing multiple boron (B) and nitrogen (N) atoms embedded polycyclic heteroaromatics were developed. [121] The adoption of multiple linear B-phenyl-B and N-phenyl-N structures featuring ortho-positioned B and N atoms not only could induce MR effect on the whole skeleton and thus narrowing the FWHM of emission spectrum but also enhance the electronic coupling of those para-positioned atoms, forming delocalized excited states for narrowing energy gap to facilitate redshifted emission. The corresponding deep-red TADF-OLEDs exhibited narrow emission spectra peaking at 664 and 686 nm for 192 and 193, and high maximum EQE of about 28%. Very recently, Kwon and coworkers utilized a C-C bond type connection instead of conventional C-N bond in well-known polycyclic skeleton to develop novel red emitters with MR effect. [122] The resulting emitters (194 and 195) not only exhibited high PLQY of over 90%, but also realized pure and narrowband red emis-sion peaking at 605 (FWHM: 42 nm) and 617 nm (FWHM: 44 nm), together with a maximum EQE of 11.3% and 15.1%, respectively.

CONCLUSION
Over the past decade, tremendous progress has been made for tuning the emission color of boron-based TADF emitters for highly efficient OLEDs by vacuum deposition or solutionprocessed methods. Hence, many of the developed organic luminescent materials with organoboron skeleton exhibit novel optical properties by tuning molecular configuration, composition, or aggregation for desired emission profiles, which make them overly potential candidates for highly efficient TADF-OLEDs. In this review, we provide a summary of the recent advances of boron-based TADF molecules with different colors, as well as their corresponding design strategy, photophysical properties, and device performances. The boron-based acceptors help the materials achieve excellent TADF performances with extremely high EQEs of over 30% for blue and green devices, particularly for the blue devices with the maximum EQE value of up to 40.0%, indicating the tremendous potential of the organic boron compounds for future use in TADF-OLEDs. In contrast, the lack of highperformance and chemically stable yellow and deep-red/nearinfrared organicboron compounds is still a weakness in developing the highly efficient organic light-emitting diodes. Compared with blue and green organoboron-based TADF emitters, yellow and deep-red/near-infrared organoboron compounds are still rarely reported since electron-deficient boronbased moiety is recognized as a weak acceptor. Yellow and red organoboron compounds could be achieved by employing strong donors or multiple donors to intensify charge transfer characteristics. When placing boron atom at para position with respect to another boron atom, or attaching secondary acceptor at the para position to boron atom in a polycyclic aromatic skeleton, not only high PLQY and narrow FWHM but also significant bathochromic shifts of emission spectra could be simultaneously obtained. From this view, it could be envisioned that MR-TADF-type materials could be promising for the construction of red and NIR emitters.
Despite the increasing research on organoboron-based TADF materials and applications in OLEDs, the existing material systems for OLEDs require essentially rigid donors and acceptors modified with a blocking group to inhibit intermolecular interaction or enhance the RISC rate thereby suppressing efficiency roll-off. The RISC rate could be increased by reducing delayed fluorescence lifetime via rational molecular design and engineering, while the intermolecular interaction is usually associated with quenching processes such as triplet-triplet annihilation, which need to be avoided by introducing large steric hindrance groups in donor or acceptor segments or employing distorted configuration, so as to efficiently stabilize the triplet exciton or to protect the triplet luminogens from external interference.
Another challenge on the path toward practical applications of boron-based TADF materials is the dilemma for achieving chemically robust materials and long lifetime in the devices simultaneously. In term of OLED materials, researchers could develop chemically stable functional materials by incorporating robust substituents. The device structure should have an injection barrier as small as possible, as well as an effective carrier balance between holes and electrons so as to elevate the efficiency of light generation/extraction. In other words, the operating voltage and aggregation of the carriers and excitons should be reduced, thereby improving EQE and operational lifetimes of TADFbased OLEDs. The combination of the high-efficiency materials and reasonable device construction would contribute to the improvement of the device stability and thus prolong the operating lifetime.
Although challenges facing TADF-OLEDs are significant, a series of breakthrough progress show the way forward OLEDs based on organoboron compounds and offer immense confidence in the development of TADF-OLEDs for future lighting and displays over the years. We hope that this review may offer a comprehensive exposition of these novel boronbased materials for both scientists and engineers toward further development.

C O N F L I C T O F I N T E R E S T S
The authors declare no conflict of interest.

E T H I C S S TAT E M E N T
This review article does not involve any human investigation and animal experiment.

D ATA AVA I L A B I L I T Y S TAT E M E N T
We have provided the Ethics Statement and Data Availability as follows.