Thioxanthone-based thermally activated delayed uorescence emitters showing fast reverse intersystem crossing for ecient organic light-emitting diodes with small eciency roll-off

Thermally activated delayed uorescence-based organic light-emitting diodes (TADF OLEDs) usually suffer from severe eciency roll-off at high brightness which is considered to originate from slow reverse intersystem crossing (RISC) and resulting long-lived triplet excitons. The development of TADF molecules with very fast RISC is an effective approach to overcome this issue. Here, we report two TADF molecules (MCz-TXO, DMCz-TXO) having thioxanthone as an acceptor unit to introduce heavy atom effect. Theoretical calculations predict that both molecules will achieve close energy level matching of the charge-transfer and locally excited triplet states ( 3 CT and 3 LE, respectively), together with a small energy gap between 3 CT and the lowest excited singlet state. The newly designed molecule, MCz-TXO, showed an extremely large rate constant of RISC (k RISC ) of 6.4×10 7 s − 1 , one of the largest k RISC values among all reported pure organic TADF emitters. Moreover, DMCz-TXO showed not only a large k RISC but also a large rate constant of radiative decay both exceeding 10 7 s − 1 , offering a sub-microsecond-scale delayed lifetime (~ 0.8 µs). These thioxanthone-based emitters exhibited great device performances with suppressed eciency roll-offs at high luminance when applied to OLEDs. r ), and nonradiative decay (k nr ). d PLQY was measured under excitation wavelength at 405 nm for DMCz-TXO and 365 nm for MCz-TXO, respectively. e Photophysical properties of MCz-XT in a PPF host matrix at doping concentrations of 5 wt%, cited from ref. 34. f The rate constants were calculated using the reported method in ref. 18. g The data were for the 5 vol% DMCz-TXO:mCBP lm. h PLQY was determined under nitrogen atmosphere at the excitation wavelength of 380 nm.


Introduction
In 2012, Adachi and co-workers reported a thermally activated delayed uorescence (TADF) organic electroluminescent material, 4CzIPN, 1 which paves the way toward the design and wide applications of the lowcost and highly e cient organic light-emitting diodes (OLEDs). Most of the TADF materials were composed of electron donor (D) and acceptor (A) moieties with a large torsion angle between them, which causes well separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions on the D and A moieties, respectively. 2,3 This kind of molecular design leads to a considerably small energy gap (ΔE ST ) between the lowest excited singlet and the lowest excited triplet states (S 1 and T 1 , respectively). [4][5][6] Theoretically, TADF materials in OLEDs can utilize all electro-generated triplet excitons for luminescence via a reverse intersystem crossing (RISC) process 7,8 and realize unity internal quantum e ciencies (IQE). 1,[9][10][11] This offers a great advantage over the traditional uorescence materials, whose IQE are at most 25% as only singlet excitons could be converted into light, and over the triplet-triplet annihilation (TTA) emitters showing the highest IQE of 62.5%. 12 Moreover, metal-free and pure organic TADF materials compensate for the drawbacks of phosphorescent emitters, such as limited global resources of noble metals and the expensive material cost, whilst maintaining high IQE.
However, TADF OLEDs usually suffer from e ciency roll-off at high brightness due to the long-lived T 1 excitons, as observed from long-delayed uorescence with the lifetime ranging from several microseconds to milliseconds. [13][14][15] The delayed uorescence of TADF materials occurs via a RISC (T 1 →S 1 ) process followed by a radiative decay (S 1 →the ground state(S 0 )) pathway. The relatively long-delayed lifetime is closely related to the small rate constant of RISC (k RISC ), as reported that RISC process is the rate-limiting process of the TADF. 16,17 The slow RISC process will cause a substantial build-up of triplet excitons, leading to various types of exciton annihilations and severe e ciency roll-off. However, up to now, there have been only few TADF molecules that show k RISC over 10 7 s −1 . [18][19][20][21] According to the Femi's golden rule, as shown in equation (1), 22 k RISC depends on the magnitude of the spin-orbit coupling (SOC) and inversely depends on ΔE ST . between 1 CT and 3 CT has been often negligible. Therefore, according to the well-known El-Sayed's rule, 23 fast spin inversion cannot be expected. The inclusion of locally excited triplet state ( 3 LE) has been proposed in recent years, because larger SOCMEVs can be expected between 3 LE and 1 CT. [24][25][26][27][28][29][30][31] More recently, we have demonstrated that a robust molecular design to realize the close energy level matching of 1 CT, 3 CT, and 3 LE, leading to su cient SOCMEVs for RISC. 18,32 The rst example molecule (TpAT-tFFO) composing only of H, C, N achieved a very fast RISC rate constant over 10 7 s −1 , in spite that no heavy atom effect is expected. 18 As we mentioned in the paper, 18 far larger SOC can be expected by incorporating relatively heavier organic atoms, typically sulfur, into emitter molecules, which can pave the way toward further improvement of k RISC . For example, thioxanthone (TXO) containing sulfur atom can be a promising candidate for achieving very fast RISC and for developing e cient TADF materials owing to the heavy atom effect. 33 In this study, we developed two TXO-based TADF materials, named MCz-TXO and DMCz-TXO (Fig. 1a). Here, the sulfur is introduced in MCz-TXO instead of oxygen in MCz-XT 34 (see Supplementary Fig. S3). Therefore, heavy atom effect is expected in MCz-TXO. Theoretical calculations also indicate that MCz-TXO realizes excellent energy level matching of the three states, 1 CT, 3 CT, and 3 LE. These effects resulted in considerably large experimental k RISC of 6.4×10 7 s −1 for MCz-TXO, thirty times larger than that of MCz-XT. We also designed D-A-D type TADF molecule, DMCz-TXO, to obtain larger rate constant of radiative decay from S 1 (k r ).
Experimentally, DMCz-TXO exhibited k r = 1.2×10 7 s −1 and k RISC = 3.4×10 7 s −1 , the rst TADF molecule realizing both k r and k RISC exceeding 10 7 s −1 to the best of our knowledge. Due to the very large k r and k RISC , DMCz-TXO showed a very short lifetime (~0.8 μs) of the delayed uorescence. The DMCz-TXO-based OLED exhibited maximum external quantum e ciency (EQE MAX ) of > 20% and maximum luminance of > 20,000 cd m −2 with suppressed e ciency roll-off (EQE of 20.5% and 19.1% with e ciency roll-off of only 0.2% and 7.0% at 100 cd m −2 and 1,000 cd m −2 , respectively) without any outcoupling treatment.

Results And Discussion
Theoretical calculations. The two TADF molecules were designed using TXO moiety as an acceptor coupled with one or two 1,3,6,8-tetramethylcarbazole (MCz) as electron-donating units, namely, MCz-TXO and DMCz-TXO (Fig. 1a), respectively. Here, TXO fragment is likely to realize stronger SOC due to the heavy atom effect introduced by sulfur, thus endowing the molecules with larger k RISC and higher TADF e ciency. Considering that k r is also important to control the exciton lifetimes, 22 the D-A-D structure is adopted to expect higher k r . 16,35 To verify our design strategy, density functional theory (DFT) and time-dependent DFT with Tamm-Dancoff approximation (TDA-DFT) calculations were carried out by Amsterdam Density Functional program package (ADF2019.302). 36 We used PBE0 functional with triple zeta basis set for geometry optimization of S 0 . The highest occupied natural transition orbitals (HONTOs) and the lowest unoccupied natural transition orbitals (LUNTOs) were visualized to understand the nature of multiple excited states, together with the corresponding energy levels of the states (Fig. 1, Supplementary Fig. S4 and Supplementary Table S1). SOCMEVs between S 1 and T n (where n = 1-5) were also calculated as shown in Table 1.
MCz-TXO and DMCz-TXO have very small ΔE ST of 0.03 eV and 0.01 eV, respectively, due to the well-separated HOMO and LUMO as shown in Fig. 1a. Both molecules possess CT-type S 1 and T 1 . Figure 1b shows HONTO and LUNTO of MCz-TXO. T 2 has LE character and thus large SOCMEV between T 2 ( 3 LE) and S 1 ( 1 CT) is expected. In addition, T 2 is close to S 1 in energy with the downhill energy gap from T 2 to S 1 (ΔE S1−T2 ) of 0.20 eV. As shown in Table 1, a considerably large SOCMEV of 4.68 cm − 1 is found between the T 2 ( 3 LE) and S 1 ( 1 CT) in MCz-TXO. For DMCz-TXO (Fig. 1c), both T 1 and T 2 are CT-type but T 3 is LE with a small downhill energy gap between T 3 and S 1 , ΔE S1−T3 , of 0.18 eV. A simiarly large SOCMEV of 6.01 cm − 1 is found between T 3 ( 3 LE) and To investigate the heavy atom effect introduced by the sulfur atom in TXO, we also conducted calculations on a reported molecule, MCz-XT (see Supplementary Fig. S3), for comparison. MCz-XT has the same chemical structure as MCz-TXO except that the oxygen atom is replaced with sulfur. 34 Therefore, the heavy atom effect of sulfur can be investigated directly by the comparison. As shown in Supplementary Fig. S3, similar to MCz-TXO, MCz-XT exhibited well separated HOMO and LUMO with a similarly large torsion angle of 81°. For MCz-XT (see Supplementary Table S1), S 1 and T 1 are CT-type. T 2 is hybridized local and charge-transfer (HLCT) type, but is a CT-dominant character. In contrast, T 3 is LE-dominant HLCT type, showing the largest SOCMEV in MCz-XT of 1.04 cm − 1 for T 3 ( 3 LE)→S 1 ( 1 CT) transition (Table 1). For MCz-TXO, HONTO and LUNTO of S 1 and T 1 are very similar to those of MCz-XT, but HONTO of T 2 is signi cantly different. T 2 of MCz-TXO is LE-type and the distribution on sulfur is very large (see Supplementary Table S1) To understand the origin of the very fast delayed uorescence, we performed a detailed analysis of related rate constants for the MCz-TXO and DMCz-TXO according to the reported equation, which is suitable for the TADF molecules showing large k RISC . 18 The experimental and the analyzed data are summarized in Table 2. For comparison, the reported photophysical performance of 5 vol% MCz-XT:PPF lm was also cited in Table 2. As we expected, both MCz-TXO and DMCz-TXO showed very large k RISC , exceeding 10 7 s − 1 . When compared to the k RISC value of MCz-XT (~ 2×10 6 s − 1 ), MCz-TXO achieved thirty times larger k RISC of 6.4×10 7 s − 1 , con rming the heavy atom effect introduced by sulfur. We assume that our strategy can be widely adopted in various TADF molecular designs for further RISC enhancement. DMCz-TXO also showed a large k RISC of 3.4×10 7 s − 1 and large k r of 1.2×10 7 s − 1 , providing a sub-microsecond-scale τ DF of 0.8 µs. To our best knowledge, DMCz-TXO is the rst organic molecule showing both k r and k RISC exceeding 10 7 s − 1 , simultaneously.
The photophysical performance of the doped lms were also examined. Here, MCz-TXO and DMCz-TXO were doped in 9-(4-tertbutylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) and 3,3′-di(9H-carbazol-9-yl)-1,1′biphenyl (mCBP), respectively. As shown in Supplementary Table S2, Supplementary Table S3), Ф PL of 100% was obtained for 5 vol% DMCz-TXO:mCBP lm with λ MAX of 492 nm and CIE of (0.21, 0.40). The transient PL decay curves (Fig. 2c) showed a very fast prompt uorescence with nanosecond-scale τ PF of 3.0 ns and a very fast delayed uorescence with a sub-microsecond-scale τ DF of 0.8 µs. The detailed analysis provides rate constants of k r = 1.5×10 7 s − 1 , k RISC = 2.9×10 7 s − 1 , and k ISC = 3.0×10 8 s − 1 with completely suppressed non-radiative decay from S 1 (PLQY of 100%) (see Supplementary Table S3). From an Arrhenius plot of k RISC for the 5 vol% DMCz-TXO:mCBP lm (Fig. 2d), the activation energy was estimated to be 53 meV. Moreover, DMCz-TXO still possessed a high k RISC of 8.5×10 6 s − 1 even at low temperature of 200 K (see Supplementary Table S4), which is higher than that of most of TADF materials at 300 K, indicating that the inclusion of 3 LE, together with the heavy atom effect by sulfur, enhance RISC process effectively. Table 2 The    Fig. S7). The structure of DMCz-TXO based device (as shown in Fig. 3a) is ITO   Fig. 3 and Table 3. Previously, there are two reports about the TXO-based TADF molecules and the related OLED performances were also summarized in Table 3. 40 Among these OLEDs using TXO as an acceptor of TADF emitters, MCz-TXO-based OLED showed blue emission with slightly reduced e ciency roll-off. Besides, the DMCz-TXO-based OLED also exhibited greatly improved roll-off suppression, clearly exemplifying the great advantage of simultaneous realization of very fast RISC and radiative decay with the rate constants both over 10 7 s − 1 .
In conclusion, a newly-designed molecule, MCz-TXO, showed (1) good energy level matching of the three states, 1 CT, 3 CT, 3 LE; (2) heavy atom effect introduced by sulfur, thus possessing considerably large k RISC of 6.4×10 7 s − 1 , one of the largest k RISC among all reported pure organic TADF emitters. The value is thirty times larger than that of the molecule which has the same chemical structure as MCz-TXO except that the oxygen atom is replaced with sulfur, con rming the effectiveness of our design concept. We believe that this concept, namely, curves at atmospheric pressure and the points corresponding to 50% (sublimation temperature, T sub ) weight loss of the TGA curves at 100 Pa are indicated in Supplementary Fig. S1. The glass transition temperatures were determined from the onset of the differential scanning calorimetry (DSC) curves (see Supplementary Fig.  S2). were measured under N 2 atmosphere. UV-vis absorption spectra and PL spectra were collected using a UV-vis spectrophotometer (UV-2600, SHIMAZU, Japan) and a spectro uorometer (FluoroMax Plus, HORIBA, Japan), respectively. The temperature-dependent transient PL decay measurements were conducted with a uorescence lifetime measurement system (Quantaurus-Tau C11367-01, Hamamatsu Photonics, Japan) equipped with a cryostat (Oxford Instruments, Optistat DN2, UK).
Device fabrication and characterization. The ITO substrates were precleared carefully before use. Multi-layers OLEDs with active areas of 4 mm 2 were fabricated under vacuum at a pressure of ~10 −4 Pa with a deposition apparatus (SE-4260, ALS Technology, Japan). OLED characteristics were measured with a source meter (2400, Keithley, Japan) and an absolute EQE measurement system (C9920-12, Hamamatsu Photonics, Japan). The applied voltage during device performance tests was increased with an increment of 200 meV.

Competing interests
The authors declare no competing financial interests.