Blue thermally activated delayed fluorescence emitters incorporating acridan analogues with heavy group 14 elements for high-efficiency doped and non-doped OLEDs
Deep-blue thermally activated delayed fluorescence (TADF) emitters are promising alternatives for conventional fluorescence and phosphorescence materials for practical application in organic lightemitting diodes (OLEDs). However, as appropriate bipolar hosts for deep-blue TADF-OLEDs are scarce, the development of efficient deep-blue TADF emitters that are applicable to both doped and non-doped systems is an urgent task. In this study, we developed a new family of blue TADF emitters that demonstrated high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies in both doped and non-doped (neat) systems. Four new donor-acceptor (D-A)-type TADF molecules incorporating phenazasiline, phenazagermine, and tetramethylcarbazole as weak D units and phenothiaborin as a weak A unit were designed and synthesized. By varying the structural rigidity/ flexibility as well as the electron-donating ability of the D units, the resulting photophysical and TADF properties of the D-A molecules could be systematically regulated. A comprehensive photophysical investigation revealed that phenazasiline and phenazagermine-based emitters concurrently exhibit blue TADF emissions (464-483 nm), high PL quantum efficiencies ($100%), extremely fast spin-converting reverse intersystem crossing rates (>10 7 s À1 ), and suppressed concentration quenching. These fascinating features in conjunction produced high-performance doped and non-doped blue TADF-OLEDs. The doped and non-doped TADF-OLEDs using the phenazasiline-based emitter demonstrated extremely high maximum external EL quantum efficiencies (h ext ) of 27.6% and 20.9%, with CIE chromaticity coordinates of (0.14, 0.26) and (0.14, 0.20), respectively. Further, ultra-low efficiency roll-off behavior for both the doped and non-doped devices was demonstrated by their h ext as high as 26.1% and 18.2%, respectively, measured at a practically high luminance of 1000 cd m À2 .
Introduction<!>Synthesis and characterization<!>Molecular structures and conformational analyses<!>Photophysical properties in solutions<!>Photophysical properties and exciton dynamics in thin lms<!>OLED device performance<!>Conclusions<!>Conflicts of interest
<p>The development of high-performance blue organic lightemitting diodes (OLEDs) is of vital importance for practical applications in full-color at-panel displays and white lighting sources, and has been attracting growing interest both academically and commercially. 1 In recent years, considerable research efforts have been devoted to boosting the exciton utilization for electroluminescence (EL) and thereby making a leap forward in the resulting device efficiency. Phosphorescent emitters based on noble metal complexes have become indispensable for this task since 1998, 2 because of their capability of harvesting both singlet and triplet excitons with almost 100% internal quantum efficiency (h int ) in OLEDs. Among the three primary colors, green and red phosphorescent emitters have already shown high performance in terms of both efficiency and stability, and have been successfully incorporated into commercial OLED products. Recently, highly efficient deep-blue phosphorescent OLEDs with narrow EL emissions have been produced by employing rigid tetradentate Pt(II) complexes 3 and N-heterocyclic carbene coordinated Ir(III) complexes. 4 However, even state-of-the-art deep-blue phosphorescent OLEDs have still lagged behind the green and red counterparts in their operational lifetime and stability, and thus, further development of deep-blue emitters via innovative molecular design remains an ongoing challenge.</p><p>The replacement of noble metal-containing phosphors with metal-free pure organic uorophores is a challenging task, but one of high priority, for realizing future low-cost OLED applications. It was not until 2012 that purely organic thermally activated delayed uorescence (TADF) emitters were rst adopted in OLEDs 5 and demonstrated high h int of nearly 100%. Over the last few years, a number of organic TADF materials displaying various emission colors have been developed, and TADF-OLEDs with high external EL quantum efficiencies (h ext ) of up to $38%, comparable to those of phosphorescent OLEDs, have been achieved. 6 However, deep-blue TADF materials capable of exhibiting high h ext exceeding 20% (corresponding to h int approximating 100%) in OLEDs as well as appropriate color purity (with Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of x # 0.15 and y # 0.20) are still scarce. 7 To achieve deep-blue EL, TADF molecules are required to have a wide energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), as well as high lowest-excited singlet (S 1 ) and triplet (T 1 ) energies ($2.8 eV). Typically, TADF molecules adopt a donor-acceptor (D-A) electronic system, in which the HOMO and LUMO are mainly distributed on the donor and acceptor moieties, respectively, giving rise to intramolecular chargetransfer (ICT) states. Therefore, the combination of a donor having a deeper HOMO level with an acceptor having a shallower LUMO level is prerequisite for the design of deep-blue TADF emitters. While various types of acceptor units, such as triazine, 7c,f,j pyrimidine, 7d diphenylsulfone, 7a benzophenone, 7g triarylborane, 7h,i and heteraborins, 7b,k,l have been used for the design of deep-blue TADF molecules, candidates for relatively weak donors have been limited to carbazole and 9,10-dihydroacridine (acridan) derivatives, which restricts diversity in material design. In this regard, 5, 10-dihydrodibenzo[b,e][1,4]  azasiline (phenazasiline), which is a heavy-atom analogue of acridan, has recently been introduced for the design of blue TADF emitters by 8 As the C-Si bonds, which are longer than C-C bonds, reduce the electron-donating effect arising from hyperconjugation, phenazasiline can function as a weaker donor compared to acridan, and lead to blue-shied emissions. So far, there are no systematic studies on the structural, electronic, and photophysical properties of TADF emitters based on acridan-analogous donors with heavier group 14 elements.</p><p>Another inherent bottleneck for developing efficient and stable deep-blue TADF-OLEDs is the lack of suitable host materials with a high triplet energy (E T ) and balanced charge transport properties. An ideal host for deep-blue TADF emitters requires a high E T level surpassing 3.0 eV to facilitate triplet energy transfer from the host to the emitters and prevent back energy transfer. However, it is technically difficult to obtain bipolar high-E T host materials owing to the limited availability of suitable building blocks. 9 This issue can be circumvented by adopting non-doped (neat) emission layers instead of conventional guest-host systems. The non-doped devices require advanced blue TADF emitters with (i) a high photoluminescence (PL) quantum yield (F PL ) in the neat lm, (ii) short excited-state lifetime to alleviate concentration quenching, and (iii) a favorable bipolar charge transport property.</p><p>Except for a few recent examples, 10 most reported blue TADF emitters bearing carbazole-based donors undergo severe aggregation-caused emission quenching (ACQ) and red-shi or broadening of the PL emission caused by intermolecular interactions in the condensed phase. Recent works by our group 7b,11,12 and Wu, Wong, and co-workers 13 reported nondoped and heavily-doped TADF-OLEDs with acridan-based emitters, which achieved rather high maximum h ext values. We also revealed the ACQ mechanism for TADF systems and succeeded in retaining high F PL in the pure neat lms without using any host materials. 11a Hence, the utilization of acridanbased donors is regarded as a versatile design strategy for efficient non-doped TADF systems; however, achieving blue TADF with a CIE-y value (#0.20) remains a major challenge because of the strong electron-donating characteristics of the acridanbased donors. More recently, aggregation-induced delayed uorescence (AIDF) has been proposed as a new viable methodology to avoid ACQ and hence produce efficient non-doped OLEDs. 14 However, thus far, efficient deep-blue AIDF systems that are capable of fullling the aforementioned criteria have not been attained.</p><p>In this study, we focused on the electronic and structural features of a set of acridan-analogous donors with different group 14 (crystallogen) elements, and systematically designed new TADF emitters 1-4 by combining them with phenothiaborin (BS) as the common acceptor unit (Fig. 1). Having weakened electron-donating abilities, 1-4 indeed showed blue-shied TADF emissions (by 20-35 nm) in comparison to that of the parent acridan-appended TADF emitter 5. 12 In addition, effects of their unique conformational heterogeneity on the photophysical properties and EL characteristics were systematically investigated by changing the bridging group 14 elements for the phenazasiline-and phenazagermine-appended TADF emitters. Of this new family of blue TADF emitters, 1 and 3 were found to exhibit suppressed ACQ behavior and therefore high F PL values even in their neat lms. Owing to this advantageous attribute, non-doped TADF-OLEDs based on 1 and 3 demonstrated remarkable EL performances with high maximum h ext of 20.9% and 17.4%, respectively, and blue emission CIE coordinates of (0.14, 0.20), along with suppressed efficiency roll-off. This study provides a new universal guideline for the design of high-performance blue TADF materials applicable to both doped and non-doped blue OLEDs.</p><!><p>Aiming to produce efficient deep-blue TADF emitters, greenemitting MPAc-BS (5) 12 was selected as a prototypical material (Fig. 1) and its donor unit was displaced by less electrondonating analogues such as 2,8-dimethyl-10,10diphenylphenazasiline (MPASi for 1), spiro[2,8-dimethylphenazasiline-10,5 0 -dibenzo[b,d]silole] (MFASi for 2), and 2,8dimethyl-10,10-diphenylphenazagermine (MPAGe for 3), as well as 1,3,6,8-tetramethylcarbazole (MCz for 4). The target compounds 1-4 were synthesized through the procedures outlined in Scheme 1. The detailed synthetic procedures and chemical characterization data are given in ESI. † The N-benzylprotected precursors were prepared by dilithiation of N,N-bis(2bromo-4-methylphenyl)benzylamine, followed by nucleophilic substitution reactions with dichlorodiphenylsilane, 5,5dichlorodibenzo [b,d]silole, or dichlorodiphenylgermane. The catalytic hydrogenation reactions afforded the deprotected donors (MPASi, MFASi, and MPAGe). Finally, 1-4 were obtained via Buchwald-Hartwig amination of the mono-brominated phenothiaborin acceptor (Br-BS) with the corresponding donors in good yields (58-89%). Compounds 1-4 showed sufficient tolerance toward air, moisture, and heat, and could be puried by column chromatography and temperature-gradient vacuum sublimation. Thermogravimetric analyses revealed that 1-4 had good thermal stability, with 5% weight-loss temperatures (T d ) of 368, 380, 371, and 343 C, respectively  (ESI †), rendering these materials suitable for the fabrication of OLEDs using the vacuum deposition method.</p><p>The variations of the electron-donating ability for the present acridan analogues with different bridging group 14 elements along with MCz were veried by cyclic voltammetry and differential pulse voltammetry (Fig. 1 and ESI †). The half-wave oxidation potentials (E 1/2 vs. Fc/Fc + ) increased in the order of 5 (0.49 V) < 1 (0.59 V) < 2 (0.61 V) z 3 (0.61 V) < 4 (0.68 V); this trend reects the variation of their electron-donating ability. As compared to the case of acridan-appended 5, the introduction of heavier group 14 elements (i.e., Si for 1 and 2, and Ge for 3) gradually elevated E 1/2 to more positive potentials, indicating less electron-donating features of the MPASi, MFASi, and MPAGe donors. As expected, introducing MCz donor made it more difficult for 4 to be electrochemically oxidized, which was indicative of its lower electron-donating ability. These redox processes of 1-4 are essentially reversible and stable, which is a favorable attribute especially for their utilization as nondoped emitters.</p><!><p>The X-ray crystallographic analyses provided valuable structural information for 1-5 (Fig. 2 and ESI †), which was strongly inuenced by the subtle difference in the bridging moiety of the donor units. Unlike the rigid planar MCz donor in 4, all phenazasiline and phenazagermine donors in 1-3 were largely Scheme 1 Synthetic schemes for 1-4. Reagents and conditions: (a</p><p>Fig. 2 X-ray crystal structures of 1-5 (CCDC 1948037-1948040,  1825382 †) showing different conformers. Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms, solvent molecules, and disordered isopropyl groups are omitted for clarity. Atom color code: C, gray; B, pink; N, blue; S, yellow; Si, red; Ge, green.</p><p>folded along their central N/Si or N/Ge axis and adopted 'quasi-axial (QA)' conformations with small D-A torsion angles (C8-C9-N-C28) of 5. 1-15.8 in the crystalline states. The crystal structures of 1-3 were in marked contrast to that of 5, which preferentially formed a typical 'quasi-equatorial (QE)' conformer having a large torsion angle of À114.0 . 15 Such conformational heterogeneity (i.e., capability to form QA and QE conformers) of this series of D-A molecules can be attributed to the mismatch of the bridging bond lengths in each acridan-analogous donor unit; namely, Ge-C ($1.94 1.44 Å). Some TADF molecules, most frequently those with a sulfur-containing phenothiazine donor, reportedly show intriguing dual CT emissions arising from two different conformers. 15,16 Despite the similarity in the covalent radii of sulfur and silicon, effects of such conformational heterogeneity of phenazasiline and its derivatives on the optical and electronic properties have never been claried.</p><p>To gain further insight into the QA-QE conformational interconversion, we performed computational simulations using density functional theory (DFT), as presented in Fig. 3 (see ESI for details †). For the MPASi-appended 1 as a representative example, three stable conformations with different D-A torsion angles were predicted in the ground (S 0 ) state, which corresponded to one QA and two QE conformers (Fig. 3a). The torsion angle of the QA conformer for 1 was as small as 6.0 , consistent with the foregoing crystal structure. In contrast, much larger torsion angles (À93.4 and 88.3 ) were estimated for its two QE conformers (QE1 and QE2). In this case, the free energy differences (DG) between the QA and QE conformers were estimated to be considerably small (<0.6 kcal mol À1 ). The interconversion energy barriers calculated for 1 (4.8-5.7 kcal mol À1 ) appeared to be small enough for allowing free conformational changes in solution at room temperature. Likewise, the geometry optimizations for 2, 3, and 5 performed following the same protocol indicated three local minima corresponding to one QA and two QE conformers (Fig. 3b). For 5 with the less crooked MPAc donor, two QE conformers were further energetically stabilized (by 2.7-2.8 kcal mol À1 ) with respect to the corresponding QA conformer. This suggests that the QE conformer with the nearly orthogonal D-A arrangement should have a higher population at equilibrium. Contrary to 1, 2, and 5, the calculated free energies of the two QE conformers for 3 were higher than the QA one (by 2.1-2.4 kcal mol À1 ), implying that the QA conformation was thermodynamically more preferable for the phenazagermine derivative. As the interconversion energy barriers did not change signicantly with the variation of the donor units, the population ratios of the QA and QE conformers for 1, 2, 3, and 5 could be estimated using their DG values by assuming Boltzmann distribution. Consequently, for 1, 2, and 5, the orthogonal QE conformers were expected to have much higher populations (QE ¼ 70%, 87%, and 99%, respectively); in contrast, the QA conformer for 3 predominated over the QE conformer (QA ¼ 98% and QE ¼ 2%) at room temperature. These results unambiguously indicate that a structural modication in the bridging moiety of the donor units has a marked impact on the intrinsic conformational exibility and heterogeneity.</p><p>Fig. 3c depicts the HOMO and LUMO distributions of both QE and QA conformers of 1 in the S 0 state. Similar to common TADF molecules with an orthogonal D-A arrangement, there was negligible spatial HOMO-LUMO overlap in the QE conformer for 1, in which the HOMO and LUMO were localized on the phenazasiline donor and phenothiaborin acceptor moieties, respectively, exhibiting evident ICT characteristics. In contrast, the QA conformation of 1 could reduce the D-A distortion and thereby facilitate the conjugation of the lone pair of the nitrogen atom with the adjacent phenothiaborin moiety. Consequently, its HOMO and LUMO were overlapped on the acceptor moiety, giving them a more localized electronic transition character. Furthermore, this QA conformer was found to have a deeper (or more stabilized) calculated HOMO level and also wider HOMO-LUMO energy gap (E g ) compared to the QE conformer. Very similar computational results were obtained for 2 and 3 as well (ESI †), revealing the notably different electronic transition characteristics between the interconvertible QE and QA conformers.</p><!><p>Fig. 4a shows a comparison of the UV-vis absorption spectra of 1-4 in toluene solutions. The phenazasiline-appended 1 and 2 exhibited very weak absorption bands in a lower-energy region of 390-430 nm, in addition to major intense absorptions ranging from 330 to 390 nm. Meanwhile, the phenazagermineappended 3 exhibited much stronger absorption bands centered at 366 nm with a molar absorption coefficient (3) as high as 3.8 Â 10 4 M À1 cm À1 . To further understand the origin of their different absorption behavior, the absorption spectra of 1-4 were theoretically simulated by taking the population ratios of QA/QE conformers into account, using time-dependent DFT (TD-DFT) with the long-range corrected functional (LC-uPBE) 17 and the 6-31+G(d) basis set (Fig. 4b and ESI †). It was found that the electronic excitations of the QE conformers occurred at lower energies than those of the QA conformers in calculations, even though the oscillator strength of the former is close to zero. On this basis, the weak absorption bands observed with 1 and 2 in the range of 390-430 nm were assigned to the ICT transitions of their QE conformers. Further, the strong absorption peak at 366 nm in 3 was mainly attributable to the p-p* transition of its predominant QA conformer having a larger oscillator strength. The overall spectral proles for 1-4 could be theoretically reproduced, thus testifying that the differences in their transition energies and strengths primarily originated from the different population ratios of the QA/QE conformers in the S 0 states.</p><p>The effect of this conformational heterogeneity was also observed in the PL spectra in dilute solutions (Fig. 4c). The MCz-appended 4 exhibited a typical single emission with the peak (l PL ) at 458 nm and F PL of 22%, which originated from the twisted ICT state. In contrast, 1-3 clearly displayed dual emissions consisting of a main peak at 472-482 nm (blue region) and a shoulder peak at 403-410 nm (violet region) in deoxygenated toluene solutions (overall F PL ¼ 46%, 27%, 42%, respectively). The corresponding excitation spectra of 1-3 monitored at these two emission bands showed prominent differences (ESI †), supporting the notion that the dual emissions arose from different conformers. To verify the ICT character of the dual emissions, we compared the PL spectra of 1-3 in four solvents of different polarities (Fig. 4d and ESI †). In non-polar cyclohexane, the higher-energy emission centered at $400 nm was dominant. Upon increasing the solvent polarity from cyclohexane (E T (30) ¼ 30.9) 18 through toluene (33.9) to ethyl acetate (38.1) and dichloromethane (40.7), the relative intensity of the higherenergy emission was sharply reduced, and at the same time, the lower-energy broad emission emerged and became predominant. Furthermore, the lower-energy emission showed signicant bathochromic shi (i.e., positive solvatochromism) with an increase in the solvent polarity, indicating obvious ICT nature. On the other hand, the higher-energy emission underwent less solvatochromic effect, as a consequence of its locally excited (LE) or mixed LE and ICT character. This solventdependent dual PL behavior in 1-3 thus indicated that the higher-and lower-energy emissions primarily originated from the QA and QE conformers having different electronic characteristics, respectively.</p><p>However, one question now arises for the phenazagermineappended 3: why did the QE emission become predominant over the QA emission in spite of the negligible population of its QE conformer in the S 0 state? This may be caused by a change in the thermodynamic equilibrium between the QA and QE conformers of 3 in the S 1 state. According to the Förster cycle, 19 the energy differences (DG*) between the QA and QE conformers in the S 1 state can be estimated by the following equation:</p><p>where DE QA and DE QE are the excitation energies of the QA and QE conformers, respectively. The DE QA and DE QE values of 3 were estimated as 3.19 and 2.76 eV, respectively, from the average of the corresponding absorption and emission maxima.</p><p>Given the DG of +0.10 eV (Fig. 3b), DG* of 3 was calculated to be À0.37 eV. As schematically shown in Fig. 5, the QE conformer is energetically more stable than the QA conformer in the S 1 state, likely owing to its prominent ICT character. Therefore, the QA conformer readily undergoes excited-state structural relaxation (or photo-induced isomerization) and transforms into the QE conformer beyond a small energy barrier to emit the lowerenergy blue PL. 16g Indeed, the optimized S 1 geometries of 1-3 were calculated to have orthogonal D-A structures with the torsion angle of almost 90 (Fig. 5 and ESI †), similar to the corresponding ground-state QE conformers. This computational result supports the experimental observation of the predominant blue emissions from the QE conformers in the solution PL spectra of 1-3.</p><!><p>To unveil their potential as blue TADF emitters prior to device fabrication, the photophysical properties of 1-4 were investigated in both doped and neat lms, as presented in Fig. 6. For doped lms, bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF, E T ¼ 3.1 eV) was employed as a wide-gap high-T 1 host to effectively conne the S 1 and T 1 excitons in the TADF emitters. The detailed photophysical data are summarized in Table 1. All doped lms of 1-4 displayed intense blue PL emissions (l PL ¼ 468-483 nm) with notably high absolute F PL values of 92-100% under N 2 atmosphere (Fig. 6a and c). Interestingly, unlike dilute solutions, the emission from the QA conformers completely disappeared in the doped lms of 1 and 2. Given that large structural relaxation cannot occur easily in the solid state, the intensied QE emissions likely originated for another reason.</p><p>Because of the spectral overlap between the QA emission and QE absorption, the excited energy of the QA conformer could be transferred to the QE conformer by Förster resonance energy transfer (FRET), dissipating the emission from the QA conformer in these doped lms. However, only the doped lm of 3 exhibited a clear QA emission at $400 nm as a shoulder, which was ascribed to an incomplete FRET due to a tiny proportion of the QE conformer in its initial S 0 equilibrium. The absorption spectra of the neat lms of 1-3 (Fig. 6d) were nearly the same as those measured in toluene solution, which suggests that the neat lms were composed of mixtures of their QA and QE conformers having population ratios equivalent to those in the solution. As listed in Table 1, the DE ST values of 1-4 were estimated as small as 0.08, 0.06, 0.11, and 0.11 eV, respectively, from the energy differences between the onsets of the uorescence and low-temperature phosphorescence spectra of the doped lms (ESI †). Interestingly, the PL spectra of 1-3 were blue-shied and became narrower in the neat lms (Fig. 6b and Table 1), as compared to those of the doped lms. This behavior can be explained by the difference in the polarity of the surrounding solid host media. Compared to the highly polar PPF host, the less polar nature of 1-3 as host media implied a lower degree of stabilization for the polar ICT excited states, thus leading to hypsochromic shis of PL emissions in the neat lms. Moreover, owing to their terminal bulky substituents, 1 and 3 were capable of suppressing ACQ and thereby exhibited considerably high F PL of 74% and 83%, respectively, even in the aggregated neat lms (Fig. 6c). This feature is favorable for the fabrication of non-doped blue OLEDs (as will be discussed later). In contrast, the neat lm of 4 bearing the planar MCz donor gave a broadened PL spectrum and a much lower F PL , similar to the cases of conventional TADF emitters, presumably because of intermolecular electron-exchange interactions between the neighboring MCz moieties in the aggregated state. 11a To verify the TADF nature, the transient PL characteristics of 1-4 were examined in both doped and neat lms (Fig. 6a and b, right). All transient PL decay curves could be tted using a double-exponential model, giving prompt uorescence lifetimes in the nanosecond regime (s p ¼ 0.5-3.4 ns) and delayed uorescence lifetimes in the microsecond regime (s d ¼ 1.7-10.4 ms). It is worth noting that each decay curve of the doped lms  contained a trace portion of prompt uorescence with a very small fractional quantum yield (F p ¼ 0.5-1.5%) while most of the PL originated from delayed uorescence (F d ¼ 91-99%). This ideal TADF behavior can be attributed to their capability of fast intersystem crossing (ISC) and reverse ISC (RISC). Indeed, extremely large ISC and RISC rate constants (k ISC and k RISC ) exceeding 10 8 and 10 7 s À1 were evaluated for 1-4, as listed in Table 1. These k ISC and k RISC values were approximately one and two orders of magnitude greater, respectively, than the corresponding uorescence radiative decay rate constant (k r $ 10 6 -10 7 s À1 ). Therefore, the rate-limiting step in the TADF processes of 1-4 is now the uorescence radiative decay (S 1 / S 0 ) rather than RISC (T 1 / S 1 ), which is completely opposite to conventional TADF materials that have much lower k RISC (<10 6 s À1 ). To the best of our knowledge, the k RISC values obtained for 1-4 (>10 7 s À1 ) are the highest among all TADF emitters ever reported, irrespective of the emission colors. This result is closely associated with the fact that the F d dominated the overall F PL values, enabling nearly 100% ISC efficiencies (F ISC ¼ 1 À F p ) and 92-100% RISC efficiencies (F RISC ¼ F d /F ISC ) for 1-4. 20 We further investigated the excited-state electronic properties of 1-4 using TD-DFT (LC-uPBE/6-31+G(d)) calculations. As can be seen from Fig. 7, the optimized S 1 states of 1-4 (with the emissive QE conformation) were dominated by the HOMO / LUMO ICT transitions ( 1 CT). In contrast, their optimized lowest T 1 states were mainly associated with the HOMOÀ1 (or HOMOÀ2) / LUMO transitions ( 3 LE), corresponding to the pp* transition of the phenothiaborin acceptor moiety. Indeed, the measured E T of the phenothiaborin precursor (Br-BS, 2.78 eV) was nearly the same as those of 1-4 (E T ¼ 2.76-2.81 eV, ESI †), supporting the implication that the p-p* transition of the phenothiaborin moiety was from their most relevant T 1 states. The adiabatic DE ST values for 1-4 were calculated to be 37, 61, 146, and 112 meV, respectively, which are in satisfactory agreement with the experimental DE ST values. It was also found that the upper-lying T 2 states, which are represented by triplet ICT ( 3 CT), lay at slightly higher energies than the corresponding T 1 states. One can anticipate that RISC is intrinsically allowed between the 3 LE and 1 CT states with effective spin-orbit coupling (SOC), h 1 CT|H SOC | 3 LEi, because of the vanishing of the SOC between 3 CT and 1 CT states having the same spatial orbital occupation (i.e., h 1 CT|H SOC | 3 CTi z 0). 12,21 The predominant 3 LE component (79-97%) in the lowest T 1 states of 1-4 is thus benecial to the spin-converting RISC processes, in addition to having small DE ST values.</p><!><p>To evaluate the EL performances of 1-4 as blue TADF emitters, we rst fabricated doped OLEDs (devices A-D) employing 50 wt%-emitter:PPF doped lms as the emission layer (EML). The fabricated device structure was indium tin oxide (ITO, 50 nm)/HAT-CN (10 nm)/TAPC (50 nm)/CCP (10 nm)/EML (20 nm)/ PPF (10 nm)/B3PyPB (30 nm)/Liq (1 nm)/Al (100 nm), where 2,3, 6,7,10,11-hexacyano-1,4,5,8,9,12-   phenyl]benzene (B3PyPB), and 8-hydroxyquinolinolato lithium (Liq) layers play the roles of hole injection, hole transport, exciton and electron block, electron transport, and electron injection, respectively. A detailed energy level diagram of the devices is provided in ESI. † As the HOMO and LUMO energy levels of emitters 1-4 (E HOMO ¼ À5.7 to À5.9 eV and E LUMO ¼ À2.7 to À3.0 eV) are shallower and deeper, respectively, than those of the PPF host (À6.7 and À2.7 eV), electrically injected holes and electrons can be directly trapped on the guest emitter molecules to effectively generate excitons with in the EML. Fig. 8a-c shows the EL spectra, current density-voltageluminance (J-V-L) characteristics, and h ext versus L plots for the doped devices A-D, while Table 2 compiles the key EL parameters. Devices A-D displayed blue to sky-blue EL with the emission peaks (l EL ) ranging from 476 to 484 nm, similar to the corresponding PL spectra. The turn-on voltages (V on ) of these devices were as low as 2.8-3.2 V, which were comparable to the photo-excited S 1 energies of 1-4 (E S ¼ 2.8-2.9 V), suggesting a direct charge recombination and exciton generation mechanism. Comparing the performances of the TADF emitters 1-4 among devices A-D, the maximum h ext values were in the order of 1 (27.6%) > 2 (23.9%) > 4 (21.6%) > 3 (15.7%). Particularly, device A using 1 achieved the best EL performance with a maximum h ext of 27.6%, current efficiency (h c ) of 48.7 cd A À1 , power efficiency (h p ) of 48.7 lm W À1 , and CIE coordinates of (0.14, 0.26). Furthermore, device A demonstrated ultra-low efficiency roll-off; the h ext values remained as high as 27.5% at 100 cd m À1 (for display), 26.1% at 1000 cd m À1 (for lighting), and 18.7% even at 10000 cd m À1 , corresponding to approximately 0.3%, 5%, and 32% roll-offs, respectively, relative to the maximum h ext value. This suppressed efficiency roll-off in device A is even smaller than those of state-of-the-art blue TADF-OLEDs. 7 In spite of the same F PL values for the doped lms of 3 and 4, device C using 3 exhibited much lower h ext (#15.5%) over the entire luminance range, which was attributed to the low population of its emissive QE conformer causing a reduced exciton generation probability, even in the heavily-doped EML. In device C, the excitons are formed mainly on the major QA conformer of 3, and then excited energy transfer to the emissive QE conformer takes place. As the population of the QE conformer is signicantly low, the generated excitons are lost to some extent via incomplete energy transfer, resulting in lower EL efficiencies. 15b In addition, the different length dependence of the energy transfer processes for singlet and triplet excitons seems to be relevant to the discrepancy of the trend between the PL and EL efficiencies. 22 Since only the singlet excitons are initially generated upon photoexcitation, effective energy transfer from the QA to QE conformers takes place via longrange Förster mechanism and hence, efficient TADF emission from the QE conformer can be achieved in the PL process with a high F PL value. In contrast, under electrical excitation in OLEDs, additional short-range Dexter energy transfer, which is much more sensitive to the interchromophore distance, should cause incomplete energy transfer to the emissive QE conformer and lowering the EL efficiency. This can be supported by the fact that the non-doped OLED (device F) with a higher content of the QE conformer within the EML exhibited a higher EL efficiency than the corresponding doped OLED (device C).</p><p>The specically low ACQ characteristics of 1 and 3 in the neat lms prompted us to further evaluate the EL performance in the host-free non-doped devices. To this end, we fabricated the nondoped TADF-OLEDs (devices E and F) by adopting the neat lms of 1 and 3 instead of the doped lms as the EML, with the same device conguration. Devices E and F exhibited slightly blue-shied l EL of 473 nm with narrower bandwidths (E FWHM ¼ 0.31-0.32 eV and l FWHM ¼ 57-59 nm, Fig. 8d) compared to the corresponding doped devices (A and C). Consequently, both devices E and F achieved blue CIE coordinates of (0.14, 0.20). Remarkably, h ext of the non-doped devices E and F reached 20.9% and 17.4%, respectively (Fig. 8e and f and Table 2). Moreover, the efficiency roll-off of device E was rather low, retaining high h ext values of 20.7% and 18.2% even when driven at practically high luminances of 100 and 1000 cd m À1 , respectively, which are among the best performing values for the reported non-doped blue TADF-OLEDs. [10][11][12] It is also noteworthy that device F using 3 exhibited superior EL performance to the corresponding doped device C. This result may be explained by the increased emissive QE conformers within the non-doped EML.</p><!><p>In this study, we systematically designed and synthesized new blue TADF emitters featuring acridan-analogous donors containing silicon and germanium bridging atoms. Incorporating larger silicon and germanium atoms enhanced the structural exibility of the donor units, leading to conformational heterogeneity as well as dual uorescence capability of the a Emission layer consisting of a codeposited 50 wt%-emitter:PPF doped lm (for devices A-D) or a non-doped neat lm (for devices E and F). b EL emission maximum at 10 mA cm À2 . c Full width at half-maximum of the EL spectrum given in energy (eV) or wavelength (nm). d CIE chromaticity coordinates. e Turn-on voltage at a luminance above 1 cd m À2 . f Maximum external EL quantum efficiency. g External EL quantum efficiency at the luminance of 100, 1000, and 10000 cd m À2 . h Maximum current efficiency. i Maximum power efficiency. resulting TADF emitters. While all these TADF molecules adopted dual conformations in both solution and thin lm states, efficient excited energy transfer from the QA to QE conformers took place and the latter displayed obvious TADF characteristics. As phenazasiline and phenazagermine acted as weaker electron donors than the commonly used acridan unit, bluer TADF was attained. Additionally, the direct electronic coupling between the close-lying local triplet ( 3 LE) and chargetransfer singlet ( 1 CT) excited states via SOC greatly enhanced the RISC rates (k RISC > 10 7 s À1 ) and thereby TADF efficiencies of the emitters. It was also found that blue TADF emitters 1 and 3 with the diphenyl-substituted phenazasiline and phenazagermine donors demonstrated surprisingly small concentration quenching behavior and were applicable to both doped and non-doped systems as efficient bifunctional emitters. The doped and non-doped blue TADF-OLEDs fabricated using 1 demonstrated excellent EL performances, with maximum h ext as high as 27.6% and 20.9%, respectively, and suppressed efficiency roll-off at a practically high luminance. We believe that this systematic study can offer a general guideline toward precise control of the structures and photophysical and TADF properties. By utilizing the design concept demonstrated here and improving the intrinsic stability and device architectures, outstanding deep-blue TADF-OLEDs can be developed.</p><!><p>There are no conicts to declare.</p>