Boron‐Based Narrowband Multiresonance Delayed Fluorescent Emitters for Organic Light‐Emitting Diodes

Recently, the exploration of boron (B)/heteroatom‐embedded polycyclic nanographites featuring multiresonance thermally activated delayed fluorescence (MR‐TADF) garners astonishing attention to promote the advancement of organic light‐emitting diodes (OLEDs). Contrary to the traditional donor–acceptor (D–A)‐type TADF emitters, the MR‐TADF emitters manifest narrowband emission with full width at half maximum (FWHM ≤ 40 nm) and superior photoluminescence quantum yield (PLQY) coupled with the small singlet–triplet energy splitting, which appeal their potential as promising candidates in fabricating efficient OLEDs. Growingly, MR‐TADF emitters deliver benchmark device performance comparable to the conventional TADF/phosphorescent emitters. However, they are suffering from the major drawbacks such as difficult to realize full‐color emitters, slow exciton upconversion dynamics, aggregation‐caused emission quenching, severe efficiency roll‐off, and poor operational lifetime, which jeopardizes their practical applicability. Herein, a comprehensive review on B‐based MR‐TADF emitters reported till date is presented, focusing on the different design strategies documented for circumventing the aforementioned shortcomings. This review is divided into several subgroups based on the emission color of the materials to draw the attention of organic electronics community toward constructing efficient full‐color MR‐OLEDs. Finally, challenges and opportunities in the MR‐TADF emitters are discussed.


Introduction
Since their first demonstration, organic light-emitting diodes (OLEDs) gained tremendous impetus because of their unprecedented advantages over the traditional liquid crystal displays (LCDs) such as lightweight, color tunability, low power consumption, inexpensive, wide viewing angles, color purity, and fast response time. [1][2][3][4][5][6][7][8][9][10][11] Particularly, the capacity to fabricate OLEDs on flexible/rollable substrates appeals to their potential application in future display and solid-state lightings. [12][13][14][15][16][17][18] Growingly, OLEDs have penetrated into the commercial market and now widely using in the consumer electronics such as mobile phone displays, digital cameras, vehicle displays, and ultrahigh-definition televisions (UHDTV). [19][20][21][22][23][24][25] The OLED performance is mainly governed by the organic emitters and device engineering. Indeed, organic emitters play an essential role in controlling the color purity, operational lifetime, efficiency, and overall device performance. [26][27][28] Therefore, the development of efficient organic emitters for OLEDs is indispensable. Generally, the recombination of injected charge carriers in OLED forms a 1:3 ratio of singlet and triplet excitons. [29][30][31][32][33] Spin selection rule allows conventional organic emitters to utilize only singlet excitons for light emission, which limits their internal (IQE) and external quantum efficiencies (EQEs) to 25% and 5%, respectively. [33][34][35] To circumvent this, over the last few decades there has been an expeditious upsurge in developing triplet exciton harvesting emitters. In the initial attempts, noble metal (Ir and Pt) containing inorganic complexes were used as emitters due to their ability to exploit triplet excitons via the phosphorescence mechanism. [35,36] However, the toxicity and cost-effective of noble metals truncate their practical applicability. To this end, efforts have been devoted to the development of pure organic emitters which can exploit triplet excitons for light emission through delayed fluorescence. In this regard, thermally activated delayed fluorescence (TADF) emerged as a promising strategy because it can effectively transform the triplet excitons into a singlet state via reverse intersystem crossing (RISC) by mitigating the energy gap (ΔE ST ) between the lowest singlet (S 1 ) and triplet excited (T 1 ) states with the aid of judicious molecular design ( Figure 1). [30][31][32][33][34][35][36][37][38][39] Conventional TADF emitters are generally constructed by the donor-acceptor (D-A) configuration for effective separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), which is crucial for achieving low ΔE ST and accelerating RISC. [40][41][42][43][44][45][46][47][48][49] Although the device performance of the state-of-the-art D-A-type TADF emitters reached the level of the conventional phosphorescent emitters, their unavoidable intrinsic intramolecular charge transfer (ICT) characteristic inevitably leads to the broad emission with large full width at half maximum (FWHM ≥ 70 nm). [50,51] Such broad emission negatively impacts the color purity of the emitters and imperils their practical applicability, which needs to be resolved from the molecular design perspective. [29,[52][53][54][55][56][57][58][59][60][61] In 2016, Hatakeyama et al. proposed a unique molecular design called multiple-resonance-induced TADF (MR-TADF), in which the electron-deficient boron (B) and electron-rich heteroatom (N/O) embedded in a fused polycyclic nanographene system. [62,63] The complementary resonance effects of heteroatom and B substantially enable atomically separated HOMO and LUMO orbitals on their adjoining ortho-and para-carbon atoms, respectively, endowing them with low ΔE ST , which turn on the TADF emission. [62][63][64] The most intriguing advantage of these so-called MR-TADF emitters is their narrow emission with FWHM < 40 nm, which is enabled by the suppressed vibronic coupling and structural alterations in the excited state ( Figure 2). [62,63] These combining merits of high exciton utilization efficiency and narrow FWHM render them ideal emitters for OLEDs with high electroluminescence efficiency and unprecedented color purity comparable to the quantum dots and perovskites. [64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82] Consequently, MR-TADF emitters fascinated the researchers and the burgeoning number of MR molecules have been documented during the last 7 years. Although MR-TADF emitters delivered benchmark device performance with EQE ≥ 35%, they suffer from few major obstacles: 1) The fused molecular skeleton on which atomically separated HOMO and LUMO orbitals induce only limited ICT character, which makes them difficult to realize bathochromic-shifted emission from a blue region with the prerequisite of narrow FWHM. However, fabrication of full color OLED requires not only blue emitters, but also ultrapure green and red emitters. [83][84][85] 2) Although the conceptual advancement has been derived numerous MR-TADF emitters, unlike conventional TADF emitters, they often possess low spin-up conversion rates (k RISC % 10 4 s À1 ), which leads to the severe efficiency roll-off at practical brightness.
3) Their inherited planar molecular confirmation often prone to detrimental aggregation-caused emission quenching. 4) The MR-TADF emitters demonstrated poor operational stability, which is critical parameter for practical applications. Followed  www.advancedsciencenews.com www.adpr-journal.com by the seminal report, tremendous efforts have been devoted to overcome the aforementioned shortcomings of MR-TADF emitters. [21,[64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82] However, to the best of our knowledge no detailed review has been reported focusing on the different design strategies reported for mitigating the abovementioned obstacles of MR-TADF emitters. To fill this gap, herein we present a comprehensive review highlighting on the recent advances in the molecular designs of B-based MR-TADF materials to produce wide-color gamut emitters with concurrently achieving high k RISC , low efficiency roll-off, and long operational stability. Initially, Hatakeyama et al. unveiled the first examples of a series of wide bandgap MR-featured B-and O-embedded PAHs ( Figure 3). [63] Among them, 2a and its derivatives were used as an MR-type acceptor for construing D-A TADF emitters by tethering it with various donor units. [86][87][88][89][90][91][92][93] Albeit they demonstrated excellent EQEs > 30%, often they exhibit broad emission with FWHM > 50 nm and poor color purity ( Figure 3). Thus, efforts have been devoted to the development of MR core, which can exhibit efficient TADF property together with narrow emission and uncompromised EQE.

Blue/Deep-Blue MR-TADF Emitters
To this end, in 2016, Hatakeyama et al. reported an enticing design strategy by replacing the oxygen in 2a with tricoordinate N-atom, resulted in B/N-containing MR-TADF chromophores (Chart 1). The opposite resonance effect of N-and B-atoms enabled atomically separated HOMO and LUMO orbitals, which endowed them with a low ΔE ST . Initially, they reported two new blue MR-TADF emitters named DABNA-1 and DABNA-2. [62] The compounds showed redshifted emission compared to the B,O-MR cores ascribed to the strong Lewis basic character of N-atom compared to O-atom. Indeed, DABNA-2 exhibited much redshifted emission than DABNA-1 because of the auxochromic effect of phenyl and diphenylamine substituents. Most importantly, unlike the conventional TADF emitters, these N/B-containing MR-TADF chromophores revealed very narrow FWHM % 33 nm. The photophysical analysis revealed low ΔE ST of 0.18 and 0.15 eV for DABNA-1 and DABNA-2, respectively (Table 1), which is sufficient to trigger the TADF emission (Table 3). Consequently, the compounds DABNA-1 and DABNA-2 showed short τ d and decent k RISC of 93.7 μs/9.3 Â 10 3 s À1 and 65.3/14.8 Â 10 3 s À1 , respectively. Further, the applicability of these emitters in OLED was evaluated in the mCBP host, and DABNA-1 showed deep-blue emission with λ EL of 459 nm and FWHM of 28 nm, CIE(x,y) of (0.13, 0.09) ( Table 2). The EQE max /CE of the DABNA-1-based device is 13.5%/ 10.6 cd A À1 . However, DABNA-2-based device demonstrated impressive performance with EQE max of 20.2%/CE of 21.1 cd A À1 , indicating its 100% IQE. Although DABNA-2 showed redshifted emission, its FWHM % 28 nm is as good as DABNA-1. These results indicate that N/B-containing MR-TADF emitters could be a promising candidate for fabricating efficient OLEDs with excellent color purity.
Later, Lee et al. developed tert-butyl-modified DABNA-1, named t-DABNA, to suppress the aggregation-caused emission quenching by reducing the intermolecular interactions with the help of tert-butyl units. [73] The t-DABNA showed a 5 nm redshifted emission and slightly reduced ΔE ST , 0.17 eV compared to DABNA-1, but it displayed a long τ d of 93.7 μs, like DABNA-1 (83.3 μs). Initially, they tested these materials as dopants in the DPEPO host and showed EQE max of 25% and 18.7% for t-DABNA and DABNA-1, respectively. The superior performance of t-DABNA was attributed to its high PLQY, and suppressed aggregation-caused emission quenching. Nonetheless, both the emitters showed severe efficiency roll-off at practical brightness because of their long τ d . To overcome this, they used DMAC-DPS as an assistant dopant to reduce the triplet exciton accumulation. The DMAC-DPS possesses a very short τ d % 5.6 μs and high k RISC of 2.53 Â 10 5 s À1 . The device with DPEPO:30% DMAC-DPS:1% t-DABNA or DABNA-1 demonstrated EQE max of 31.4% and 23.5% at 1 cd m À2 for t-DABNA and DABNA-1, respectively, and even it maintained high EQE of 19.8% and 15.6% at 1000 cd m À2 . The improved performance of these devices compared to the former device containing DPEPO host was ascribed to the reduced triplet exciton concentration in MR-TADF emitters; this was also confirmed by the TRPL analysis of the doped films with and without assistant dopant. The assistant dopant-featured t-DABNA device showed a short τ d of 6.3 μs compared to the DPEPO hosted device of 83.3 μs. The authors also presented the operational lifetime of t-DABNA with and without an assistant dopant. Although the assistant dopant-based device displayed 10 times longer lifetime than DPEPO hosted device, it showed a short device lifetime of 30 h (LT 50 ) at 100 cd m À2 may be due to the poor stability of the host and assistant dopant. Further, to improve the lifetime of DABNA-based MR-TADF cores, the same group was used t-DABNA and DABNA-1 as fluorescent dopants rather than TADF dopants to avoid triplet exciton quenching. They used anthracene-based ADN as a wide bandgap host due to its low E T % 1.73 eV than the DABNA-based emitters, E T % 2.63 eV. The triplet exciton concentration can be reduced via back energy transfer to the host. The authors also fabricated a reference device with a pyrene-based PyCN. As expected, the lifetime of t-DABNA dramatically improved with LT 90 of 608 h compared to DABNA-1 and PyCN of 203 and 33 h at 200 cd m À2 , respectively. The EQE max (%)/CE (cd/A) of t-DABNA, DABNA-1, and PyCN are of 7.0/5.8, 4.9/3.4, and 3.4/2.8, respectively.
Further, same group reported a series of asymmetric blue TADF emitters, viz., B-O-dpa, B-O-Cz, B-O-dmAc, and B-O-dpAc, respectively. [102] The design is based on replacing the one electron-rich N-atom with a relatively low electron-rich O-atom to shift the emission of the compounds into blue region.   Huang et al. modified the t-DABNA with 3,6-di-tertbutylcarbazole donor (TBN-TPA) at para-position of central phenyl unit relative to the B to improve the PL and EL performance. [75] The distinguished absorption profiles of DABNA-1 and TBN-TPA indicate the involvement of appended carbazole in the electronic transitions. The solution PL of TBN-TPA showed a 20 nm redshifted emission compared to DABNA-1 with a broad FWHM of 26 nm. The mCBP-doped film of TBN-TPA displayed improved PLQY (97%) compared to DABNA-1 (88%) is attributed to its high oscillator strength. Also, TBN-TPA witnessed improved ΔE ST and τ d and k RISC because of the increased CT with the aid of an electron-rich carbazole donor. The OLEDs were fabricated using 4 wt% of TBN-TPA in 26DCzPPy host and demonstrated EQE max of 32.1% and CE of 40.2 cd A À1 . The EL spectra of TBN-TPA displayed narrow FWHM % 27 nm and CIE(x,y) of (0. 12, 0.19). This performance is much superior than DABNA-1, though the device stack is different. Further, TBN-TPA was used as a host for the yellow Irphosphor and exhibited low V on of 3.0 V together with a high L max of 30 339 cd m À2 and EQE max of 22.2%. However, the reported chemical structure was misinterpreted, which was further confirmed by Hatakeyama et al. [66] (vide infra).
Hatakeyama et al. developed a series of MR-TADF chromophores (B2-B4) containing double or triple B-atoms with extended conjugated skeleton of DABNA, which was accomplished by the one-shot borylation reaction. [74] The photophysical properties were investigated in 1 wt% PMMA-doped films. The λ PL (nm) of B2, B2-F, B3, and B4 is 455, 467, 441, and 450 nm, respectively. The observed redshift (%17 nm) with broad FWHM of 44 nm for B2-F compared to B2 was attributed to the electronwithdrawing F atom in the former. Although B3 showed deep-blue emission, it displayed relatively low PLQY (33%). The compounds B2 and B3 revealed low ΔE ST of 0.15 and 0.19 eV, which is sufficient to accomplish efficient RISC. The authors explored the applicability of B2 as an emitter in an OLED and found to exhibit EQE max of 18.3% and CE of 11.5 cd A À1 . However, its EL displayed 5 nm redshift compared to PL with broad FWHM % 37 nm, which resulted in CIE(x,y) of (0.13, 0.11).
To expand the chemical space of MR-TADF emitters, in 2021, Yasuda et al. reported a ternary (B/N/S)-doped MR-TADF framework and designed an acyclic emitter, BSBS-N1. [101] The two S atoms in the phenothiaborin subunits of BSBS-N1 are expected to induce the MR effect with N atoms due to its electron richness and facilitate the RISC owing to the heavy atom effect. It displayed sky-blue emission with λ PL of 473 FWHM of 21 nm. The BSBS-N1 revealed a small stoke shift of 15 nm owing to the reduced structural reorganizations in the excited state due to its fully fused molecular geometry. The estimated S 1 /T 1 / ΔE ST /PLQY of doped film of BSBS-N1 are 2.59 eV/2.45 eV/ 0.14 eV/89%. The BSBS-N1 revealed short τ d and fast k RISC of 5.6 and 1.9 Â 10 6 s À1 due to the heavy atom effect of S. Further, its OLED exhibited sky-blue emission, λ EL of 478 nm and FWHM of 25 nm. Although BSBS-N1 showed a low EQE max of 21.0% compared to the typical sky-blue emitter BBCz-SB (27%), it showed improved efficiency roll-off at high brightness attributed to its fast RISC and suppressed bimolecular exciton quenching (TTA or TPA).
Kim et al. reported a new pure blue emitter named mBP-DABNA-Me, wherein xylene and meta-phenyl groups were introduced into the DABNA core to effectively suppress aggregation-caused quenching and prevent the isomer formation in the reaction. [100] The mBP-DABNA-Me manifested pure blue emission with λ PL of 464 nm and FWHM of 28 nm in the toluene solution. Notably, the doped film exhibited only a 3 nm redshift compared to the solution attributed to the suppressed π-π interactions, and as a result, the PLQY yielded in 97%. The OLED fabricated using mBP-DABNA-Me demonstrated EQE max of 24.3% and CE of 24.2 cd A À1 with pure blue emission (468 nm). Interestingly, the color purity of mBP-DABNA-Me was unchanged even at above 20 wt% doping concentration, highlighting the importance of the bulky substituent addition strategy for effective suppression of aggregation-caused emission quenching. Xu et al. reported self-host featured MR-TADF emitter, namely, tCBNDADPO, by integrating MR-core (tCBN) with the ambipolar host segment (DADPO) (Chart 2). [104] Notably, the tCBNDADPO displayed improved TADF properties with accelerated k r % 2.11 Â 10 8 s À1 and suppressed k nr . Also, tCBNDADPO manifested narrowband blue emission with FWHM % 28 nm even at high doping concentration of 30 wt%. Further, the nondoped OLED demonstrated an EQE max of 30% attesting the superiority of self-host strategy in constructing the efficient host-free MR-TADF emitters.
In 2022, Lee et al. reported MR-TADF blue emitter (t-DABNA-dtB) by introducing di-tert-butyl benzene substituent in the MR core (t-DABNA) to minimize the quenching mechanisms by restricting the intermolecular interaction. [105] The designed emitter showed a high PLQY and small FWHM % 22 nm, which realized high EQE of 11.4% in the single unit OLED with operational lifetime LT 95 of 208 h at 1000 cd m À2 and over 10 000 h at 100 cd m À2 . The t-DABNA-dtB displayed pure blue emission λ EL % 470 nm with a CIEy coordinate of %0.13. The optimized tandem device of the new blue emitter achieves a high EQE > 25% and extremely long LT 95 > 500 h at 1000 cd m À2 .
Notably, in the case of a tandem device, the CIEy coordinate is significantly decreased to 0.11. The lifetime of this work is one of the best reported blue OLEDs.
Generally, MR-TADF chromophores are susceptible to the aggregation-caused emission quenching because of their planar geometries. To overcome this, recently Jiang et al. reported a new design featuring the integration of spirobifluorene (SBF) building block with MR-core (SF1BN and SF3BN) either via C1 or C3 positions, respectively. [106] As expected, the SBF-substituted derivatives prevented the aggregation even at high doping ratio and manifested excellent PLQY with a narrow FWHM of %27 nm. Particularly, the C1-substituted SBF in SF1BN acts as a shield to prevent aggregation at high doping ratios compared to the parent molecule and SF3BN. Compared to the parent core, the designed emitters showed 1.5 times increased EQE (32.5-35%). These results indicate that the spirostrategy is an effective method for preventing the aggregation-caused emission quenching in MR-TADF emitters.
In 2022, Yang et al. reported three deep blue MR-TADF emitters (BN1, BN2, and BN3) by extending the π-skeleton by increasing the number of phenyl rings from 7 to 13. [95] The designed emitters were synthesized by a one-pot lithium-free cyclization method. All three emitters exhibited high PLQY of over 90%, with increased k RISC from BN1 to BN3. The emitters showed a deep-blue emission with CIEy coordinate below 0.08. The OLED fabricated with BN3 exhibited a EQE max of 37.6% and reduced roll-off, representing the best efficiency reported for deep-blue TADF OLEDs. These results highlight that the extinction of fused π-conjugated skeleton can improve the RISC and enhance the OLED performance.
The same group further reported a new approach for narrowing the EL spectra of MR-TADF emitters by peripheral decoration strategy. They developed three new MR-TADF emitters by attaching one or two diphenylamine units on the parent CzBN core (DPACzBN1, DPACzBN2, and DPACzBN3). [107] Interestingly, with increasing the number of diphenylamine units, the FWHM of EL spectra decreased. In the series, the DPACzBN3 exhibited deep-blue emission with FWHM of 20 nm. Also, the diphenylamine decoration benefited to promote the PLQY and k RISC of the emitters. The fabricated OLED of these compounds displayed blue to sky-blue emission. In the series, the DPACzBN3 exhibited best performance with EQE max of 27.7%.
Later, same group strategically developed three new blue emitters, PTZBN1, PTZBN2, and PTZBN3. [108] By virtue of peripheral modification with diphenylamine donor units, the emission wavelengths of the compounds were tuned from 490 nm (PTZBN1) to 468 nm (PTZBN3). The incorporation of sulfone functionality in PTZBN3 benefited to restrict the emission in deep-blue region with high PLQY of 98% and narrowband emission, FWHM of 30 nm. In the series, the PTZBN2-based device exhibited EQE of 34.8%. Impressively, the PTZBN3-based device demonstrated high EQE % 32.0% in deep-blue region (λ EL % 468 nm) indicates the importance of sulfone modification in the development of efficient deep-blue MR-TADF emitters.
As mentioned above, k RISC associated with spin-flip from T 1 ! S 1 is a rate limiting factor in the TADF emitters. To promote the k RISC in MR-TADF emitters, Yasuda et al. proposed a heavy atom doping approach and consequently they developed three blue MR-TADF emitters by doping the different heteroatoms (O, S, and Se), viz., CzBO, CzBS, and CzBSe. [109] With the aid of prominent MR-effect, the compounds showed low ΔE ST below 0.15 eV. Further, the SOC of the compounds improved from O to Se attributed to the heavy atom effect of Se. In the series, CzBSe displayed ultrafast T 1 ! S 1 exciton upconversion with high k RISC rate exceeding 10 8 s À1 , which is higher than its k r . Even the k RISC of CzBSe is accelerated by 20 000 and 800 times than the O-and S-doped emitters, respectively ( Figure 4). As a result, the CzBSe demonstrated best OLED performance with EQE max of 23.9% and CIEy % 0.24.
In 2019, Hatakeyama et al. developed an expanded DABNAbased linear conjugated MR-TADF emitter consisting of four benzene rings containing two B and four N atoms and two peripheral diphenylamine groups, ν-DABNA (Chart 3). [94] Intriguingly, the ν-DABNA showed a narrow FWHM of 14 nm in solution, which is the narrowest FWHM reported so www.advancedsciencenews.com www.adpr-journal.com far for MR-TADF emitters. In the same report, the authors also developed a new DOBNA-based host material named DOBNA-OAr. The 1 wt% ν-DABNA-doped DOBNA-OAr films exhibited λ PL of 469 nm, PLQY of 90%, and ΔE ST of 0.017 eV, which is the smallest among the reported MR-TADF emitters. The ν-DABNA also exhibited a high k RISC of 2.0 Â 10 5 s À1 , which is 3 times higher than the reported DABNA-1 and DABNA-2, [62] attributed to its low ΔE ST . The OLED demonstrated ultrapure blue emission with λ EL of 469 nm and FWHM of 18 nm, and CIE(x,y) of (0.12, 0.11). In addition, the device displayed excellent performance with EQE max of 34.4% at 15 cd m À2 and a suppressed efficiency roll-off of 26.6% at 1000 cd m À2 . This is attributed to the reduced bimolecular exciton quenching with the aid of small ΔE ST . Although ν-DABNA exhibited excellent TADF property, narrow FWHM, and high efficiency, its emission restricted to the sky-blue emission, λ PL % 469 nm, and deviated its CIEy coordinate %0.11 from the NTSC/ITU BT.2020 standards. To address thus, same group has proposed a new molecular design by replacing the N-atom with the O in the ν-DABNA core, yielded ν-DABNA-O-Me. [67] They envisaged that, because of the low atomic energy of O atom, it restricts the π-conjugation of the HOMO rather than LUMO orbital. As expected, the HOMO energy level of ν-DABNA-O-Me showed a large deviation (%0.16 eV) than LUMO energy (%0.13 eV), leading to the wide bandgap for the former. The photophysics was analyzed in 1 wt% emitter-doped PMMA film, and ν-DABNA-O-Me displayed 5 nm blueshifted emission compared to ν-DABNA. However, its FWHM (24 nm) was broadened compared to the ν-DABNA. The ν-DABNA-O-Me displayed a relatively large ΔE ST /long τ d /slow k RISC of 0.29 eV/7.7 μs/1.6 Â 10 5 s À1 compared to the ν-DABNA. Further, the OLED fabricated using ν-DABNA-O-Me demonstrated decent performance with EQE max of 29.5% and CE of 24.6 cd A À1 . Notably, ν-DABNA-O-Me displayed alleviated efficiency roll-off only 2.6% at 1000 cd m À2 compared to ν-DABNA (8.4%) may be due to the reduced bimolecular quenching process. Also, ν-DABNA-O-Me exhibited an improved device lifetime, LT 50 , of 314 h   To further improve the color purity of the ν-DABNA, Yasuda and co-workers developed a new design comprised of the replacement of intracyclic N-atoms with the less electron rich O and/or S atoms, BOBO-Z, BOBS-Z, and BSBS-Z ( Figure 5). [97] As expected, these luminophores showed conspicuous blueshifted emission in solution in the order of BOBO-Z (λ PL % 441 nm), BOBS-Z (λ PL % 453 nm), and BSBS-Z (λ PL % 460 nm) compared to ν-DABNA with retaining narrow FWHM of 15-21 nm. The calculated CIE(x,y) of BOBO-Z (0.15, 0.03), BOBS-Z (0.15, 0.06), and BSBS-Z (0.14, 0.07) is encouraged as they are closely matched with the requirement of standard UHD displays. Even the 3 wt% mCBP-doped films of BOBO-Z, BOBS-Z, and BSBS-Z showed deep-blue emission with a maximum of λ PL % 445, 457, and 464 nm, respectively, compared to ν-DABNA of λ PL % 474 nm. The S-containing derivatives, BOBS-Z (k RISC % 8.6 Â 10 5 s À1 ) and BSBS-Z (k RISC 1.6 Â 10 6 s À1 ), displayed 2.4 and 4.2 times faster k RISC than the ν-DABNA (3.3 Â 10 5 s À1 ) may be attributed to their heavy atom effect of S, which is also supported by the DFT computations. Later, the applicability of these materials as dopants in OLEDs was tested in mCBP host. Consistent with their PL, the EL of the compounds showed deep-blue emission, 445 nm for BOBO-Z, 456 nm for BOBS-Z, and 463 nm for BSBS-Z, and narrow FWHM falls in the range of 18-23 nm, which indicates distinct hypsochromic shift compared to the ν-DABNA (472 nm). Consequently, OLEDs displayed excellent color purity with CIE(x,y) of (0.15, 0.04) for BOBO-Z, (0.14, 0.06) for BOBS-Z, and (0.13, 0.08) for BSBS-Z fulfilling the requirements of Rec.2020 standard of UHD OLEDs. The S-containing derivatives BOBS-Z and BSBS-Z exhibited best performance with EQE max of 26.9% and 26.8% over BOBO-Z (13.6%) and ν-DABNA (26.6%). Initially, the device stability was analyzed in unipolar host, mCBP and showed a very short lifetime; further, it was improved (LT 50 > 30 h at 100 cd m À2 ) by replacing the mCBP with bipolar host mCBP-CN.
In 2020, Colman et al. developed a new ladder-type B,N-containing heptacene (3BNOH), and studied its photophysical properties. [99] The compound showed violet emission in toluene solution with λ PL of 390 nm, a small FWHM of 31 nm, and PLQY of 50%. 3BNOH displayed a large ΔE ST of 0.31 eV in solution, which is larger than the reported MR-TADF emitters. The temperature dependent PL study suggests the formation of aggregates in the THF matrix, which induced delayed fluorescence from the TTA channel. However, the TADF component is dominant at a high temperature, 300 K. The direct calculated activation energy for delayed emission was 70 meV, and its ΔE ST is 0.31 eV. This may be due to the contribution of higher triplets for the RISC, which is evident by the DFT studies. In 1 wt% PMMA film, 3BNOH exhibited 5 nm redshifted emission and FWHM of 32 nm, and its ΔE ST became narrow by 0.22 eV compared to the solution. Interestingly, it showed impressive CIEy % 0.01 in the PMMA matrix. But the delayed lifetime in the film state (260 μs) was longer than the solution state (0.45 μs). The activation energy in the film state matches with the ΔE ST indicating that the high lying triplet contribution for RISC in the film state is negligible.
Hatakeyama et al. developed an expanded helicene-type MR-TADF emitters consisting of three B and six nitrogen atoms (ν-DABNA-Mes) employing a one-shot triple borylation reaction. [96] The PL and EL performance of ν-DABNA-Mes were compared with the reference material ν-DABNA. The 1 wt% dispersed PMMA films showed sharp emission at 484 nm with a narrow FWHM of 16 nm, which was redshifted compared to the parent core due to the extended π-skeleton. As a result of extended π-conjugation, the ν-DABNA-Mes showed an improved MR effect and resulted in a narrow ΔE ST of 0.005 eV, short τ d of 2.4 μs, and fast k RISC of 4.4 Â 10 5 s À1 compared to the ν-DABNA core. Further, the applicability of ν-DABNA-Mes was examined in solution-processed OLED in a polymer host. The device exhibited sky blue emission with narrow emission FWHM of 27 nm, λ EL of 480 nm, and CIE(x,y) of (0.09, 0.21). In addition, the V-DABNA-Mes demonstrated an EQE max of 22.9%, CE of 26.7 cd A À1 , but it displayed severe efficiency roll-off at 1000 cd m À2 with EQE of 10.9%, and CE of 12.9 cd A À1 was attributed to the bimolecular quenching of triplet excitons via TTA or TSA process.

Green MR-TADF Emitters
As discussed above, the DABNA-based MR-TADF emitters flourished with very high EQE and narrowband emission; however, they are limited to the blue region. Nonetheless, it is highly desirable to develop full-color emitters especially ultrapure primaries, i.e., green and red, for fabricating wide color gamut UHD displays. To address this, Duan et al.  with hybridized MR and ICT (HMCT) effect are feasible to construct efficient MR-TADF emitters with tunable emission colors. Inspired by the above report, the same group was developed another molecular design based on MR core fused with a difficult to access aza-aromatic skeleton, which can contribute to the MR effect. The target material AZA-BN was synthesized in a one-shot multiple cyclization method. [79] To understand electronic properties and validate the HMCT concept, the authors presented the DFT computations of the model compounds. The addition of a fused aza cyclic system to the MR-core increased the extension of π-conjugation. It improved the HOMO/LUMO orbital distribution, which strengthen the ICT character and benefited to stabilize the S 1 energy. The S 1 energy is effectively modulated by the HMCT design without affecting the FWHM and oscillator strength. As expected, AZA-BN showed pure green emission with λ PL of 522 nm and narrow FWHM of 28 nm in toluene, and it was restored in 4 wt% emitter-doped mCBP film, λ PL % 528 nm with CIE(x,y) of (0.25, 0.70). But the doped film exhibited a slightly broad FWHM % 36 nm. The transient PL analysis indicates a relatively long τ d of 163 μs and a slow k RISC of 7.53 Â 10 3 s À1 . Further, its potential as an emitter in OLED was tested in phosphorescence-sensitized MR-TADF device. The device showed low V on of 2.6 V, pure green emission of λ EL % 526 nm, FWHM of 30 nm, and CIE(x,y) of (0.27, 0.69). The device exhibited EQE max /PE of 28.2%/121.7 lm W À1 and maintained stable efficiency of 26.5% and 19.1% at 100 and 1000 cd m À2 , respectively.
Later, Wang and co-workers developed a series of green MR-TADF emitters by modifying the para-carbon of the central phenyl unit w.r.t. to the B atom with various electron-withdrawing units (DPTRZ, TPTRZ, PPm, CNPm). [112] It is expected that the substitution of acceptor units at the para-carbon of the B atom can significantly depress the LUMO energy with negligible effect on the HOMO energy, which would narrow the bandgap and www.advancedsciencenews.com www.adpr-journal.com results bathochromic shift. This is also evident from the DFT studies, where the HOMO orbital of the acceptor substituted MR-core is showed like that of parent core. In contrast, the LUMO orbital was largely delocalized on the acceptor core. As expected, all the compounds showed moderate to large bathochromic-shifted emission compared to the parent core depends on their acceptor unit. The DPTRZ displayed a shoulder emission peak, which negatively impacts color purity. To avoid this, the phenyl or pyrimidine unit was introduced between the parent core and acceptor unit. In the series, the phenylsubstituted derivative (TPTRZ) showed the most redshifted emission with λ PL of 521 nm. Usually, the compounds displayed narrow emission with FWHM ≤ 35 nm. Further, the compounds were applied as dopants in the PhCzBCz host, and the TPTRZ-based device demonstrated superior performance with EQE max of 30.6% and CE of 105.8 cd A À1 , and green CIE color coordinates of (0.23, 0.68). Later, Wang et al. extended the π-conjugation of BCz-BN by introducing tert-butylphenyl units on the 3,6-positions of carbazole for developing narrowband green MR-TADF emitter (DtBuPhCzB) and compared its performance with the reference material DtBuCzB. [76] The DtBuPhCzB exhibited λ PL of 496 nm, which is 16 nm redshifted compared to DtBuCzB, showed λ PL % 481 nm in toluene with maintaining narrow FWHM of 21 nm. The DtBuPhCzB revealed small ΔE ST of 0.09 eV compared to DtBuCzB (0.13 eV) attributed to its reduced singlet energy caused by the extended conjugation. The PLQY of DtBuPhCzB (97%) was slightly improved compared to The EQE max of DtBuCzB and DtBuPhCzB is 21.6% and 23.4%, respectively. However, the compounds revealed severe efficiency roll-off at high brightness due to their long τ d . Thus, to avoid this, the authors modified the device structure by replacing the mCBP host with the exciplex cohost (TCTA:PIM-TRZ) system and achieved an improved EQE max of 25.5% in a green color region (0.20, 0.65). Further, the same group proposed a new design strategy based on frontier molecular engineering, which can couple the advantages of D-A structure and MR-property ( Figure 5). They have introduced an auxiliary donor 3,6-di-tert-butyl-carbazole on the HOMO localized meta-carbon of central phenyl ring relative to the B-atom (BCz-BN). It is expected that the attached carbazole could contribute for the HOMO delocalization and stabilize the respective energy, which results in redshifted emission. [77] As expected, the mCzBNCz showed pure green emission (λ PL % 519 nm) and a narrow FWHM of 38 nm in solution. Its PL was preciously matched with the solid-state PL, indicating that the aggregation was prevented in the solid-state. In addition, mCzBNCz displayed strong positive solvatochromism in emission, indicating its ICT character in the excited state. The ΔE ST /PLQY/τ d /k RISC of mCzBNCz are 97%/0.08 eV/ 1.0 Â 10 6 s À1 , suggesting its potential as an efficient TADF emitter. Further, OLED performance was tested in PhCzBCz host by varying the doping connotation from 1 to 50 wt%. The EL showed a slight redshift with broadened FWHM as increasing doping concentration. Interestingly, all the devices exhibited EQE over 26%, indicating its potential as an efficient green MR-TADF emitter.
Generally, the intrinsic planar nature of the MR-TADF emitters leads to undesirable aggregation-caused emission quenching, exciton annihilation, and spectral broadening, which deteriorates the color purity and device performance. To mitigate this, Duan et al. proposed an advanced molecular design featuring sterical protection of MR-core with bulky substituents. As a result, the intermolecular interactions between the molecules can significantly suppress and effectively inhibit the aggregationcaused emission quenching. To validate the design concept, authors developed two new MR-TADF emitters, viz., s-Cz-BN and d-Cz-BN, containing one and two carbazole donors substituted at meta-carbon relative to the B-atom of DtBuCzB, respectively, and compared their physicochemical and device properties. [110] DFT computations revealed that the substituted donors have a negligible effect on the FMO distributions, which suggests that the attached donors can act as electronically inert units and not influence the photophysics of the emitters. The s-Cz-BN and d-Cz-BN displayed slightly redshifted emission (λ PL % 490 nm) compared to the parent core DtBuCzB (480 nm). However, the FWHM of s-Cz-BN (23 nm) and d-Cz-BN (22 nm) was reduced compared to the DtBuCzB (FWHM % 25 nm), highlights the merit of design strategy. The compounds s-Cz-BN and d-Cz-BN showed small ΔE ST of 0.15 and 0.13 eV and high PLQY of 94% and 98%, respectively. As the doping concentration increased from 1 to 30 wt% in mCBP film, the emission of the parent compound significantly redshifted with broad emissions and decreased PLQY, but s-Cz-BN-and d-Cz-BN-doped films maintained narrow emission and less significant redshift and high PLQY. Especially, d-Cz-BN displayed a narrow FWHM of 26 nm in the pristine film compared to s-Cz-BN (40 nm) and DtBuCzB (60 nm), indicating the more promising steric effect of d-Cz-BN. Further, the materials were applied as dopants in TADF-sensitized device. Upon increasing the dopant concentration, the parent compound showed slightly redshifted emission and spectral broadening with FWHM from 29 to 36 nm. Still, the s-Cz-BN and d-Cz-BN maintained narrow emissions, 26-30 nm over the doping ratios, like the photophysical properties. The s-Cz-BN and d-Cz-BN showed EQE max of 30.5% and 37.2%, respectively. Notably, S-Cz-BN and d-Cz-BN displayed suppressed efficiency roll-off at practical brightness of 28.8% and 34.3% compared to the parent compound (43.6%) attributed to the interrupted long-range DET process and suppressed aggregation-caused quenching.
Further, Yang et al. validated the abovementioned design concept by synthesizing two new MR-TADF emitters, BN-CP1 and BN-CP2. In BN-CP1, the two carbazole donors were substituted at ortho-positions of phenyl linker with respect to the parent MR-TADF core, while in control molecule BN-CP2, the carbazole donor positions were switched on to the meta-position of phenyl linker. [111] Both the compounds showed monomeric photophysical properties in solution and 1 wt% doped film in the mCP host, but in the neat film, they revealed different behavior. In solution and doped film, the compounds showed sky-blue emission with λ PL of 490 and 496 nm, respectively. In the case of neat film, the BN-CP2 displayed significant redshift (λ PL % 521 nm), broad FWHM 48 nm, and reduced PLQY % 25% compared to BN-CP1, showing λ PL 502 nm, narrow FWHM % 26 nm, and high PLQY of 40%. This is attributed to the severe aggregation in BN-CP2, compared to BN-CP1. Nonetheless, both the compounds BN-CP1 and BN-CP2 revealed short τ d of 79.6 and 83.6 μs and fast k RISC of 1.56 Â 10 4 and 1.43 Â 10 5 s À1 , respectively, indicative of their TADF nature. The OLED performance of these emitters was characterized in DMIC-TRZ host. As the doping concentration increases from 1 to 30 wt%, the EL maximum (496 nm) and FWHM (25 nm) were unchanged in BN-CP1, but in the control molecule, BN-CP2 a dramatic redshift with broadened emission was observed, attesting the rationality of the molecular design for suppressing the aggregation-caused emission quenching. The EQE max of BN-CP1 was maintained 33-40% over the doping concentrations of 1-30 wt%. However significant EQE drop was noticed in the case of BN-CP2 from 34% to 23% as doping concentration increased. The optimized device in 5 wt% of BN-CP1 demonstrated best EQE max of 40% reported to date.
In 2022, Zheng et al. proposed a modular design by fusing the rigid hole-transporting type units such as dibenzofuran (NBO) and carbazole (NBNP) with the MR-core for shifting the emission wavelengths to the red region without compromising their color purity and accomplishing the high k RISC . [113] DFT studies showed that the HOMO orbitals for the compounds are completely delocalized in the entire molecular backbone, while the LUMO orbitals are mainly localized on the parent MR core. www.advancedsciencenews.com www.adpr-journal.com As a result, the HOMO energy of the NBO and NBNP improved with maintaining similar LUMO energy for the materials, which lead to the low bandgap for the compounds and bathochromicshifted emission. As a result, the compounds NBO and NBNP endowed low ΔE ST of 0.12 and 0.09 eV, suitable for applying as efficient TADF emitters in OLEDs. Consequently, their k RISC rate was improved to 10 6 s À1 in the doped film compared to the parent compound. This is mainly assigned to their enhanced SOC (S 1 -T n ) in NBO of 0.1-0.13 cm À1 compared to the parent core of 0.02 cm À1 . As a result, the compounds exhibited a very high EQE max of 28.0% for NBNP and 26.5% for NBO. Furthermore, due to the high k RISC , the compounds showed excellent efficiency stability at high brightness (1000 cd m À2 ) of 25.6% for NBNP and 22.4% for NBO. These results highlight the importance of fusion strategy in shifting the emission wavelengths to the red region with suppressed efficiency roll-off. Later Wang et al. reported interesting approach by fusing MR core with polycyclic aromatic phenylene unit, BN-TP, featuring para-aligned B and N into the six-membered ring ( Figure 6). [114] The photophysical properties revealed that the BN-TP showed a very intense absorption band at 506 nm corresponding to the ICT transition and vivid green emission with a peak wavelength of 523 nm. The observed small Stokes shift %22 nm and narrow FWHM % 34 nm indicates the structural rigidity of BN-TP in the excited state. The calculated ΔE ST , PLQY, and k RISC are of 0.14 eV, 96%, and 2.09 Â 10 4 s À1 , indicating the promising TADF characteristic of BN-TP. Then BN-TP applied as a dopant in the PhCzBCz host; it exhibited an EQE max of 35.1%, CE of 139.3 cd A À1 , and FWHM of 36 nm with green CIE color coordinates of (0.26, 0.70).
Lu and co-workers developed a series of green MR-TADF emitters, TPh-BN, TW-BN, pCz-BN, and mCz-BN, by modifying the parent core with either phenyl or carbazole substituents. [115] As a result of the direct connection of substituents through the single bond, the compounds showed very narrow emission with FWHM of %20 nm and a small Stokes shift of ≤21 nm. All the compounds exhibited high PLQY of ≥88% and small ΔE ST of ≤0.15 eV. Indeed, the TPh-BN displayed small ΔE ST , superior PLQY, and a short τ d of 0.09 eV, 94%, and 62 μs. Consequently, TPh-BN exhibited the best performance in the series with EQE of 28.9% and CE of 54.8 cd A À1 . Notably, it exhibited alleviated efficiency roll-off of 13% at practical brightness without any assistant dopants attributed to its short τ d . Also, the compounds demonstrated decent operational stability. In the series, TPh-BN displayed the best lifetime with LT 50 of 36.5 h (L 0 ¼ 500 cd m À2 ).

Orange/Red MR-TADF Emitters
As discussed above, the development of pure red emitters is highly demanding for the fabrication of full-color OLEDs and imaging applications. [10,21,130,131] Despite of the great potential of MR-TADF emitters in blue and green regions, the narrowband red emitters are scarcely reported because of their intrinsic tradeoff between the bathochromic-shifted emission and color purity. [132][133][134][135] [81] As a result of significantly enhanced donor and acceptor strengths of N and B, the HOMO and LUMO energies are well stabilized and lead to the very narrow bandgap ≤1.7 eV. Consequently, the R-BN and R-TBN showed deep-red emission with λ PL of 662 and 692 nm, respectively, without compromising their MR-effect. Notably, both the materials displayed unity PLQY and very short τ d and high k RISC % 10 4 s À1 . These results highlight the superiority of para-B-π-B and para-N-π-N approach for constructing deep-red MR-TADF emitters. Further, the performance of these materials in OLEDs was tested in ternary system using Ir(mphmq) 2 tmd as sensitizer and exhibited deep-red emission with λ EL of 664 and 686 nm and FWHM of 48 and 49 nm, respectively. As a result, they exhibited deep-red color coordinates of (0.71, 0.28) and (0.72, 0.27), respectively. Unprecedentedly, the EQE max of the R-BN and R-TBN was demonstrated to be 28.1% and 27.6%, respectively. It is also noteworthy to mention that the devices displayed exceptional operational stability with LT 90 of 125 and 151 h at L 0 of 2000 cd m À2 .
Inspired by this report, Yang et al. constructed a series of pure red-MR TADF emitters by replacing the two N-atoms with arylated O-atoms by para-positioning O-π-O and N-π-N and B-π-B (BNO1-BNO3). [121] The emission wavelengths of the materials are fine-tuned by peripheral decoration of aryl units on O-atom ( Figure 7). In the series, BNO3 exhibited pure-red emission with λ PL of 616 nm and FWHM of 33 nm. Photophysical studies revealed marked PLQY ≥ 95% and narrow ΔE ST and short τ d , indicating their ability to exhibit TADF property. Further, they fabricated phosphor sensitized OLED and BNO3 outperformed with EQE max of 36.1% and it maintained very high EQE of 28.6% at 10 000 cd m À2 . Importantly, all the materials exhibited purered emission with CIEx % 0.64-0.67. Further, BNO1 showed decent operational stability with LT 90 of 49.8 h.
In 2021, Bin et al. reported a new molecular design by introducing CN acceptor and carbazole donors on LUMO and HOMO located carbons of the central phenyl unit in the MR-skeleton (CNCz-BNCz) (Chart 6). [70] As expected, the emission wavelength of CNCz-BNCz realized in orange-red region (λ PL % 583 nm). Notably, benefited by the simple molecular structure of CN, the FWHM of the CNCz-BNCz was unchanged compared to the reference material due to the suppressed structural relaxations in the excited state. The photophysical properties indicating that the CNCz-BNCz possesses prominent MR-TADF effect with the aid of its small ΔE ST of 0.18 eV and high k RISC of 4.2 Â 10 5 s À1 . The optimized OLED of CNCz-BNCz in TADF sensitized system demonstrated EQE max of 33.7% and pure orange-red emission (λ PL % 584 nm), CIE(x,y) of (0.54, 0.46). These results indicate that the attachment of simple CN acceptor at para-carbon relative to B-atom is advantageous for shifting toward red region without compromising color fidelity.

Full-Color MR-TADF Emitters
It is imperative to develop a feasible molecular design for tuning the emission colors of MR-TADF emitters from deep-blue to red region for full-color display applications ( Figure 8). Later, the same group proposed a straightforward design for constructing color-tunable MR-TADF emitters by modifying the parent core, CzB, either with electron-withdrawing imine or electron-donating amine units (Cb-B-DACzB). [71] The emission wavelengths of the compounds were tuned from blue (460 nm) to yellow (576 nm) depending on the donor strength of the appended chromophores. In the series, the imine containing derivative Cz-B exhibited deep-blue emission because of electron-withdrawing imine N-atom in the HOMO-distributed carbon, which depressed the HOMO energy and increased the bandgap. However, the carbazole or diphenylamine substituted derivatives displayed more redshifted emission because of the stabilized HOMO energy and reduced bandgap. Indeed, the diphenylamine featured derivative exhibited redshifted emission in the series compared to the carbazole due to its strong donor strength. All the compounds displayed narrow emission with FWHM ≤ 35 nm and low ΔE ST . Their OLED performance was investigated either in oCBP or mCBP hosts. In the series, the carbazole-substituted derivative exhibited the best performance with EQE max of 29.2% and CE of 100.7 cd A À1 .
Further, Yang et al. reported peripheral decoration strategy of MR-fragment with the strong electron donors. They modified the well-known CzB core with carbazole (BN1), monodiphenylamine (BN2), and di-diphenylamine (BN3) units. [116] The emission wavelengths of the compounds tuned from blue (λ PL % 496 nm) to yellow (λ PL % 562 nm) depending on the donor strength and number of appended chromophores. In the series, the BN3 exhibited yellow emission and low ΔE ST . These results attest the superiority of peripheral decoration strategy for color tuning the MR-TADF emitters without effecting on the MR-property. When applied these materials as dopant emitters in OLED, the BN3 outperformed with maximum EQE of 24.7% and CE of 92.6 cd A À1 in the yellow region with CIE color coordinates of (0.47, 0.52).
Recently, Zheng et al. reported four ternary B/N-based MR-TADF emitters (SBON, SBSN, DBON, and DBSN) employing B-π-C and E-π-E strategy (E ¼ O or S) for tuning the emission wavelengths from blue (λ PL % 463 nm) to yellow (λ PL % 553 nm) region. [120] The coordination between the B/N and S/O atoms played a vital role in regulating the CT delocalization and emission colors. This approach endowed narrow band emission for all the compounds with FWHM ≤ 28 nm. The diboron embedded derivatives DBON and DBSN showed high PLQY of 98% is attributed to its extended π-conjugation and suppressed nonradiative deactivations. The S-containing materials SBSN and DBSN revealed high k RISC of 1.5 Â 10 5 and 1.9 Â 10 5 s À1 , compared to the O-derivatives, SBON and DBON, because of the strong SOC with the aid of heavy-atom effect of S atom. The OLED fabricated with DBON demonstrated best performance in the series with EQE max of 26.7% and (CIE (x,y) % 0.17, 0.68) (Chart 7).

Conclusion and Outlook
B-based MR-TADF emitters fascinated the researchers with their unprecedented color purity (FWHM < 40 nm) and excellent EQE > 35%. As a result, numerous MR-TADF materials have been derived to expand their structural diversity and understanding the emission mechanism. Although the MR-TADF emitters have potentially overcome the major obstacles of state-of-the-art D-A-type TADF emitters, they are suffering from low k RISC , severe efficiency roll-off, poor operational stability, and aggregationcaused emission quenching and difficult to realize ultrawide color gamut emitters. In this review, we have summarized the different design strategies reported till date to mitigate the aforementioned shortcomings of MR-TADF emitters ( Figure 9).
First, the production of ultrawide colour gamut MR-materials is essential for the fabrication of full-color high-definition OLED displays. However, the fused molecular framework with enfeebled ICT characteristic restricts the emission in blue region (470-500 nm). To shift the emission into deep-blue region (<460 nm), the incorporation of electron-deficient imine N or the replacement of amine N with O/S was reported to be an ideal strategy. It is also reported that the meta-B-π-B and E-π-E (E ¼ N/O/S) arrangement could interrupt the effective CT delocalization and increase the bandgap of emitters. The peripheral modification of MR-skeleton with either D or A units judiciously tunes the emission beyond blue region. For example, substitution of D and A units on HOMO-and LUMO-dominated carbons of MR-core can stabilize respective energies, and reduce the bandgap, which leads to the redshifted emission. The emission wavelength can be toggled between green and yellow by managing the strength of appended donor or acceptor units. Also, the integration of MR-core with the rigid PAHs is another promising strategy to construct the narrowband green emitters.  The red MR-TADF emitters associated with narrow FWHM are scarcely reported due to their limited scope for molecular design and structural diversity. Till date only one design has been reported for devising red MR-TADF emitters, i.e., para-B-π-B and N-π-N arrangement coupled with the extended fused π-skeleton. Nonetheless, it is highly desirable to develop the new molecular designs for red MR-TADF emitters. The linearly extended MR-TADF emitters can improve the CT delocalization and reduce the ΔE ST and τ d . Most of the reported MR-TADF emitters exhibit inferior k RISC in the order of %10 4 s À1 , which is detrimental for stability and efficiency roll-off. Construction of emitters with heavy atoms such as S and Se is important for high k RISC and low τ d because of their strong SOC. The linearly π-extended molecular geometries with multiple B-atoms can benefit high k RISC . Also, PAH-integrated MR-TADF emitters realized high k RISC because of their low ΔE ST . The FWHM of the emitters can be narrowed by fully fused molecular conformation and multiple B encapsulation in the molecular framework. The aggregation-caused emission quenching is one of the critical parameters in MR-TADF emitters attributed to their planar geometries. The employment of nonplanar molecular confirmation and protection of MR-core with shielding groups can restrain the formation of aggregation. Further, efficiency rolloff at high brightness can be alleviated by the high k RISC and short τ d of the emitters. Finally, the operational lifetime is one of the most important parameters of MR-TADF emitters. So far, no ubiquitous design has been reported in material design perspective; however, construction of MR-core with rigid PAHs together with high BDE of B-C bond could improve the chemical stability of the emitters. The high k RISC and short τ d are vital to suppress the triplet exciton accumulation, which reduces the undesirable TTA or TPQ. Besides, employment of MR-emitters as terminal dopant in hyperfluorescent devices is proficient approach for improving the operational lifetime. Recently, Lee et al. proposed a tandem device engineering strategy to promote the operational lifetime of MR-TADF emitters. This result would expatiate the new device engineering approaches for improving the operational stability of MR-OLEDs.
Kenkera Rayappa Naveen received his bachelor's degree from Regional Institute of Education, Mysore, Karnataka, India, in 2017 and his master's degree in chemistry from National Institute of Technology-Tiruchirappalli (NITT), Tamil Nadu, India, in 2019. Currently, he is pursuing his doctoral research under the supervision of Prof. Jang Hyuk Kwon in the Department of Information Display at Kyung Hee University, Seoul, South Korea. His research interests include the synthesis of thermally activated delayed fluorescent (TADF) molecules and boron-based multiple resonance TADF (MR-TADF) emitters and their application in optoelectronics.