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Novel Thermally Activated Delayed Fluorescence Materials–Thioxanthone Derivatives and Their Applications for Highly Efficient OLEDs Hui Wang, Lisha Xie, Qian Peng, Lingqiang Meng, Ying Wang,* Yuanping Yi, and Pengfei Wang* Recently, organic fluorescence emitters with thermally activated delayed fluorescence (TADF) have attracted much attention in that these materials use thermally activated up-conversion of triplet (T1) into singlet (S1) state, giving stable fluorescence with very high singlet yields.[1–3] TADF materials, similar to the emitter for the high efficient OLEDs by hybridized local and charge-transfer (HLCT) excited state,[4,5] require a sufficiently small energy gap between the triplet and singlet (ΔEST) to enable up-conversion of the triplet excitons to singlet excitons and realize 100% internal quantum efficiency[6,7] of the exciton formation generated by electric excitation at S1. Thus, enormous efforts have been endeavored to minimize ΔEST and then enhance the reverse intersystem crossing (RISC) by molecular structure design, leading to the maximization of TADF.[1,8–14] Versatile molecular systems with TADF have been found, including spiro-acridine,[10] triazine,[11] spirobifluorene,[12] phthalonitrile[13] and diphenyl sulfone derivates,[1] and high external quantum efficiencies (EQE) up to 20% have been reported for organic light emitting diodes (OLEDs) using TADF materials as the emitters, which are approaching the best performance of OLEDs based on organic phosphorescent materials reported.[2,15] The TADF emitters have now been accepted as the third generation of OLEDs emitters after the conventional fluorescence and phosphorescence materials. Nevertheless, there

H. Wang, L. Meng, Prof. Y. Wang, Prof. P. Wang Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected]; [email protected] H. Wang, L. Meng University of Chinese Academy of Sciences Beijing 100049, China Dr. Q. Peng, Prof. Y. Yi Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China L. Xie College of Life Science and Technology Beijing University of Chemical Technology Beijing, China

DOI: 10.1002/adma.201401393

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are still some challenges and open questions to be answered, which impede them from practical application. Firstly, although considerable efforts have been taken in the development of new TADF emitters, to date the known TADF emitters, especially purely organic emitters, are still very limited and only a few can truly realize high EQE and excellent stability for OLEDs. Rational ways to design novel TADF molecules with efficient RISC are highly desirable. Secondly, the TADF OLEDs reported generally suffer from the lower maximum brightness and serious efficiency roll-offs at high brightness related to triplettriplet annihilation.[16,17] These can be solved by the optimization of molecular structure through the deep understanding of the structure-property relationships in TADF emitters. Thus, design and synthesis of new TADF materials, one of the most challenging tasks in the OLED development today, is promising for the development of OLEDs for full-color display and white lighting application. Thioxanthone (TX) and its derivatives have been used as triplet sensitizers and photoinitiators of polymerization due to their high rate of intersystem crossing (KISC) and then the high quantum yield of triplet formation.[18,19] The energy gap between the first singlet and triplet excited state of TX had been reported to be lower than 0.3 eV.[20,21] Such a small singlet-triplet energy gap (ΔEST) facilitates the reverse intersystem crossing from T1and S1, and the quantum yield of triplet formation can be decreased due to “the inverse gap effect”.[22] Thus, it is possible that high efficient TADF materials with lower electron exchange energy and lower ΔEST can be achieved by the further molecular structure optimization. In this communication, two novel TX-based emitters with excellent TADF properties, TXO-TPA and TXO-PhCz (molecular structure as shown in Figure 1), are reported. Both of them have a typical D-A structure with 9-H-Thioxanthen-9-one-10,10-dioxide (TXO) as an electron acceptor unit and triphenylamine (TPA)/N-phenylcarbazole (N-PhCz) as an electron donor unit. The oxidation of the S atom in TX moiety can enhance the electron accepting ability, and conventional hole transporting moieties, TPA or N-PhCz, are used for their excellent hole-transporting capability. The angular-linked D-A moieties contribute to effective separation of electron densities of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in a single molecule. Small ΔEST of 52 and 73 meV for TXO-TPA and TXO-PhCz are demonstrated, affording efficient RISC and then high photoluminescence (PL) quantum efficiency. The OLEDs based on TXO-TPA and TXO-PhCz show high maximum total external quantum efficiency (EQE)

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(Figure S5). Such excellent thermal properties readily endow OLEDs based on them with high morphological stability, which is favorable to improve the OLEDs performance during operation. The electrochemical properties of TXO-TPA and TXOPhCz were investigated by cyclic voltammetry (Figure S6). The HOMO and LUMO energy levels of TXO- PhCz were calculated to be −5.78 eV and −3.58 eV, which were obtained from the onset of oxidation curve and onset of reduction curve,[23] respectively. The LUMO and HOMO were distributed on the acceptor and donor moieties for both molecules according to the molecular simulation. Thus, similar LUMO energy level of −3.49 eV and higher HOMO level of −5.37 eV was expected for the TXO-TPA, which is consistent with better hole-injection ability of tripheylamine.[24] The thermal stability and frontier molecular orbital energy level of compounds TXO-TPA and TXOPhCz are summarized in Table 1. Photophysical properties of TXO-TPA and TXO-PhCz were analyzed using ultraviolet-visible (UV) and PL spectrometer. The Figure 1. Molecular structures and calculated spatial distributions of the HOMO and LUMO absorption and PL peaks of TXO-PhCz and energy densities. (a) TXO-TPA LUMO (b) TXO-TPA HOMO (c) TXO-PhCz LUMO (d) TXO- TXO-TPA in different solvents and their PhCz HOMO corresponding quantum yields are summarized in Table S1. TXO-TPA in all solutions exhibits an absorption broad band at around 415 nm, of 18.5% and 21.5% with stable emissions, which prove the which is assigned to the CT absorption mainly associated rationality of structure modification and potential advantages of with electron transfer from the TPA moiety to TXO moiety TADF materials in OLEDs. (Figure S7b). Similar absorption band centered at 400 nm can The synthesis of TXO-TPA and TXO-PhCz were achieved also be observed for TXO-PhCz in solutions (Figure S8b). No by Suzuki coupling reaction of the TXO moiety with the TPA/ obvious solvent dependence of the absorption spectra was PhCz moiety as outlined in Scheme S1 (for details of the synobserved for both compounds. All the solutions of TXO-TPA thesis, see Supporting Information). DFT calculations were and TXO-PhCz emit weak light, when excited by 365 nm, performed on TXO-TPA and TXO-PhCz to study the HOMO and even no emission can be detected in solvents with high and LUMO electronic distribution. Due to the strong electron polarity (Figure S7a, S8a). The fluorescence (FL) spectra with donating property of TPA/PhCz moiety and electron accepting unstructured peak shifted to the long wavelength in the solproperty of TXO moiety, the HOMOs of TXO-TPA and TXOvent with high polarity and the quantum yield of the solutions PhCz were mainly dispersed over the TPA and PhCz moiety, are in inverse proportion to the solvent polarity (Figure S7c, while their LUMOs were localized on TXO moiety (Figure 1). S8c). Such pronounced positive solvatochromism of the emisThere is only a small overlap between HOMO and LUMO on sion spectrum had also been demonstrated for other reported the phenyl ring in the TXO unit bridging to the donor moiety. ICT compounds[25] and TX derivatives.[26] The phosphoresThis indicates that the HOMO-to-LUMO transition has a strong CT character and a small electron exchange energy can cent emission of TXO-TPA and TXO-PhCz were examined be expected. According to the results estimated from TDDFT calculation (see suporting information for computational Table 1. Thermal stability and frontier molecular orbital energy level of details), the effective HOMO-LUMO separation induce strong compounds TXO- PhCz and TXO-TPA. charge transfer transition and lead to a small ΔEST of 0.03 and 0.14 eV for TXO-TPA and TXO-PhCz, respectively, which is HOMO/LUMOd) Compound Tda)/Tgb) Eoptce) λabsc) comparable to the reported TADF emitters.[12,15] The thermal (°C) (eV) (eV) (nm) stability of TXO-TPA/TXO-PhCz was examined by using TXO-PhCz 392.6/115.4 305,345,410 5.78/3.58 2.25 thermal gravimetric analysis (TGA) and differential scanTXO-TPA 380.9/87.4 310,375,450 5.37/3.49 2.06 ning calorimetry (DSC) under a nitrogen atmosphere. TXOTPA/TXO-PhCz showed good thermal stability with a higher a)T : decomposition temperature; b)T : glass transition temperature; c)in thin film; d g d)calculated from the empirical formula E decomposition temperatures (Td) of 380.9/392.60 °C and a HOMO = −(Eox+4.4), ELUMO = −(Ere+4.4); e) high glass transition temperatures (Tg) of 87.4/115.14 °C estimated from the onset of the absorption edge of the thin films. 2

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at 77 K in oxygen-free 2-MeTHF (Figure S9). TXO-TPA and TXO-PhCz exhibit unstructured phosphorescent emission spectra with peaks centered at 546 nm and 503 nm, and then their triplet energy levels were obtained to be 2.46 eV and 2.27 eV from the emission peaks. While TXO-TPA and TXOPhCz in 2-MeTHF at 77 K have fluorescence peak centered at 555 nm and 524 nm, respectively. The ΔEST can be estimated to be 0.04 eV for TXO-TPA and 0.09 eV for TXO-PhCz estimated from fluorescence and phosphorescence measurements, which are much lower than other TX derivatives[20,21] and comparable to those reported TADF materials.[12,27] Thus, these low ΔEST make them possibly possess TADF properties. UV-vis absorption and PL spectra of TXO-PhCz and TXOTPA in thin films are also depicted in Figure 2. TXO-PhCz emits yellow light with an emission peak at 570 nm and TXOTPA has a red emission centered at 625 nm, and the absolute fluorescence quantum yields (ΦPL) of TXO-PhCz and TXOTPA film were 0.93 ± 0.02 and 0.36 ± 0.02, which are much higher than the counterparts in solutions. Such remarkable PL enhancement in solid states indicates that the two compounds may be aggregation-induced emission (AIE) active. Thus, we examined the AIE behavior of TXO-TPA and TXO-PhCz in acetonitrile/water mixtures following a literature method.[28] By mixing acetonitrile and water with different water fractions, we observed the change trend of emission spectra as the quantitative change of the water content (Figure S10 and S11). The spectral intensity of TXO-TPA and TXO-PhCz started to increase abruptly when the water fraction goes up to 50 vol%, which suggests that molecules begin to aggregate in the solvent mixture with this composition. Confocal laser scanning microscopy (CLSM) gives us a visualized proof of the existence of the aggregations (Figure S12). TXO-TPA aggregated to form closepacking nanowires and emits intense red fluorescence with 408 nm excitation. While, TXO-PhCz showed smooth belt-like structures with widths in the range of 1.0–5.0 µm and emits yellow light. To decipher the different emission behavior of TXO-TPA and TXO-PhCz in solid state, the single crystal structures and molecular packing are further explored (Figure 3). The single crystals were obtained by slow evaporation of solvent from a mixture of dichloromethane and methanol at room

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Figure 2. UV-vis absorption and PL spectra of TXO-PhCz and TXO-TPA thin films.

temperature, and the structures were determined by X-ray single crystal analysis. Both molecules show the asymmetric molecule geometry and all the atoms in the π-system of TXO unit, except the two oxygen atoms connecting to sulfur atom, is almost in one plane. However, the TPA and PhCz units adopt a highly twisted conformation, which prevents both molecules to form dentrimental species such as excimers and exciplexes in solid state. When dissolved in solutions, their donor units with twisted conformation will undergo active intramolecular rotations and then quench their emissions.[29] While, in the aggregated state, such intramolecular rotations are impeded by the intermolecular short contacts and local interactions. All these multiple interactions have locked and rigidified the molecular conformation, largely reducing the loss via non-radiative relaxation and affording the highly emissive.[30] Both TPA and N-PhCz units are joint with the TXO unit by the C−C bond (1.496 and 1.481 Å). The twist angle of TXO unit and phenyl group of TPA connecting to TXO unit is about 2°. While, the twist angle in TXO-PhCz is 18°, larger than the TXO-TPA. Interestingly, although only one single bond is changed from TXO-TPA to TXO-PhCz, the packing arrangements were totally different from each other. Helding together through the intermolecular π−π interaction and C−H···π (2.364∼2.761Å) short contacts, TXO-TPA formed a columnar stacking along the crystallographic a–axis direction with an interplanar separation of 3.49 Å, affording a large intermolecular π-overlap. Two TXOTPA molecules, with nearly parallel TXO units, form a flatly spread dimer with anti conformation to minimize the dipolar interactions.[31] The intermolecular C−H···π short contacts held the dimers, displaying a one-dimensional supermolecular chain along crystallographic c-axis direction. Differently, TXOPhCz has dimers formed by two adjacent molecules with parallel configurations, helding together by weaker π-π interaction and the intermolecular C−H···π (2.587 Å) short contacts. Its dimers are packed tightly in the crystal with the so-called sandwiched herringbone structure,[32] and short C−H···π short contacts exist in a face-to-edge manner between dimers, indicative of the two-dimensional electronic structure of the crystal. The aromatic stacking could offer a charge-transfer pathway and enhance the mobility, which is essential for excellent electroluminescence materials.[33] The weaker intermolecular π–π interaction for TXO-PhCz, noticeable discrepancy of the molecular packing in their single crystals, can explain its higher ΦPL in solid film.[34] As a step further to confirm that TADF occurs in TXO-TPA and TXO-PhCz in solid film, the doped films in the 1,3-bis (9H-carbazol-9-yl) benzene (mCP) host (5±1 wt%) were fabricated and their transient PL decay and temperature dependence were measured using a Edinburgh Instruments FLS920 spectrometer. The mCP was chosen as the host due to its high T1 state, which suppresses back energy transfer from guest to the host materials and confines the triplet excitons within the guest molecules.[35] The 5 ± 1 wt% TXO-TPA:mCP film gave yellow emission with a peak emission wavelength of 580 nm, significantly blue shifted compared with the counterpart of the pure film. Its PL efficiency increased up to 83%, almost 2.5 times higher than the pure film (36%), indicating that an effective up-conversion of TXO-TPA triplet excitons did occur. The transient PL decay characteristics of the 5 ± 1 wt%

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is from TADF, the temperature dependent of the transient PL delcay was conducted from 100 K to 300 K (Figure 4a). The overall fluorescence efficiency (ΦPL), prompt fluorescence efficiency (Φprompt) and delayed fluorescence efficiency (Φdelayed) in different temperature are shown in Figure 4c. The delayed component showed remarkable temperature dependence, and Φdelayed increased from 42% to 82% when the temperature increased from 150 to 300 K, consistent with other TADF materials. These results proved the presence of thermal activation energy for the delayed fluorescence.[14] While, Φprompt decreased down to 2% as the temperature increased from 100 to 170 K, and almost no temperature effect can be observed above 170 K. This is attributed to the acceleration of the non-radiation decay from the S1 state at high temperature. These negative and positive effects of temperature on Φprompt and Φdelayed compromised, leading to the unusual temperature dependence of ΦPL as other TADF materials: No temperature dependence of ΦPL can be observed until 170 K, and then ΦPL increased linearly with the temperature. Similar blue shift of the peak emission wavelength can also be observed for 5 ± 1 wt% TXO-PhCz:mCP film, and its high PL efficiency was remained to be 90% with λmax = 520 nm. The co-deposited film of 5 ± 1 wt% TXO-PhCz:mCP exhibited similar transient PL decay characteristics and sensitivity of emission to oxygen (Figure 4b). The increasing trend of both ΦPL and Φdelayed with the temperature was accordant (Figure 4d). While, Φprompt remained to be below 2% Figure 3. Single crystal structure of TXO-TPA and TXO-PhCz: molecular structure (a) and without temperature dependence. All those packing structure (b and c) of TXO-TPA; molecular structure (d) and packing structure (e and above also affirmed the TADF property of TXO-PhCz. Noticeably, both co-doped films f) of TXO-PhCz. showed very high ratio of the delayed component (Φdelayed/ΦPL) even at 100 K, over 60% for TXO-TPA and TXO-TPA:mCP film showed clear second-order exponential decays in the time range of 200 µs at 300 K, where the delay 98% for TXO-PhCz, suggesting that most of the excited singlets component decays completely within 78.0 µs. The prompt comdecayed, accompanying by emission of radiation, via efficient ponent (τ) was estimated as 25.0 ns in the time range of 100 ns ISC and RISC between the T1 state and S1 state. corresponding to the S1 relaxation to S0 (Figure S13). The lifeTo get solid evidence for the efficient ISC and RISC in both co-doped films, the triplet formation efficiency (ΦT) and ΔEST time of the delayed component is shorter than the counterpart of heptazine derivatives,[3] suggesting the more efficient RISC were derived using a Berberan-Santos plot from the temperature dependence results (Figure S15).[36] The intersystem for TXO-TPA. Similar to other TADF emitters reported, the delayed component's contribution was significantly suppressed crossing rate constant (KISC), the radiative rate constants (Kr) and the overall PL decays completely within much shorter time and the non-radiactive rate constant (Knr) were also calculated after flowing oxygen gas over a sample in air for 30 minites assuming that KISC was independent of temperature and sum(Figure 4a inset).[10] The presence of the delayed component marized in Table S2. KISC of TXO-PhCz and TXO-TPA were calwith the long emission decay time and the sensitivity of the culated to be 5.13 × 107 and 3.79 × 107 s−1, about two magnitudes emission to oxygen are typical of TADF materials. The nice higher than Kr and three magnitudes higher than Knr, respecoverlap between the photoluminescence spectra for the prompt tively. These illustrated most of the singlet excitons by optical and delayed component also clarified that the delayed compoexcitation will convert to the triplet excitons via ISC, affording nent can be assigned to the delayed fluorescence occurring via the high ΦT of 99% and 98%. Such high ΦT had been typical RISC (Figure S14). To further confirm the delayed fluorescence for the reported aromatic carbonyl compounds derivatives.[37]

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COMMUNICATION Figure 4. Transient PL characteristics of TXO-TPA and TXO-PhCz films. (a) Temperature-dependence of the transient PL spectra for 5 ± 1 wt% TXO-TPA doped in mCP and (b) 5 ± 1 wt%TXO-PhCz doped in mCP. Inset: Emission decay of 5 ± 1 wt% TXO-TPA doped in mCP and 5 ± 1 wt%TXO-PhCz doped in mCP after oxygen flow for 30 mins. (c) Temperature-dependence of the total photoluminescence (black squares), prompt fluorescence (red circles), and delayed fluorescence (blue triangles) for 5 ± 1 wt% TXO-TPA doped in mCP and (d) 5 ± 1 wt%TXO-PhCz doped in mCP.

From the slop of Berberan-Santos plot we obtained ΔEST = 52 meV for TXO-TPA and 73 meV for TXO-PhCz, which are consistent with the results calculated from phosphorescent and fluorescent emission. These ΔEST are much lower than the conventional TX derivatives[20,21] and comparable to the high efficient TADF materials.[10–13] These demonstrated that ΔEST of TX derivatives had been reduced by the structural optimization. Thus, high efficient up-conversions from T1 exciton to S1 exciton were expected because these conversions were dominated by an Arrhenius process.[2] The high ΦPL and efficient TADF component enable them to be the emissive molecules in high efficiency OLEDs. To investigate the characteristics of OLEDs based on TXOTPA and TXO-PhCz, multilayer OLEDs were fabricated using these doped films as the emitting layers (EML). The molecular structures and energy levels used in the devices were shown in Figure S16. The OLED structure was: ITO/PEDOT (30 nm)/ TAPC (20 nm)/EML (35 nm) /TmPyPB (55 nm)/LiF(0.9 nm)/ Al, where poly(3,4-ethylenedioxythiophene) (PEDOT) was used as the hole-injection layer (HIL), 1,1-bis[4-[N,N′-di(ptolyl)amino]phenyl] cyclohexane (TAPC) was used as the holetransporting layer (HTL), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) was used as electron-transporting layer (ETL) and hole-blocking layer (HBL), and 5 ± 1 wt% TXO-TPA:mCP or 5 ± 1 wt% TXO- PhCz:mCP is used as the emitting layer (EML). The electroluminescent (EL) characteristics of the devices are shown in Figure 5 and Figure S17. The device containing TXO-TPA gave yellow electroluminescence centered at 552 nm with color coordinates of CIE (0.45, 0.53), and showed a turnon voltage of 5.3 V, a current efficiency of 43.3 cd/A, a power

Adv. Mater. 2014, DOI: 10.1002/adma.201401393

efficiency of 47.4 lm/W, a high external quantum efficiency (EQE) of 18.5% and a maximum luminance up to 16300 cd/cm2 without any light out-coupling enhancement. The device based on TXO-PhCz emitted green light with color coordinates of CIE (0.31, 0.56) and performed even better. It turned on at 4.7 V and achieved a maximum brightness of 21000 cd/cm2 at 18.3 V, and the current efficiency and power efficiency can be up to 76.0 cd/A and 70.0 lm/W. It is noteworthy that the OLED exhibited a maximum EQE up to 21.5% at the luminance of 1 cd/cm2 and the low current density of 1 × 10−3 mA/cm2, which is among the best of the reported OLEDs based on TADF materials.[2,15,38] The performance of the devices can be further improved by the optimization of the device structure. Those high EQE of the devices are beyond the theoretical limit of conventional fluorescent OLEDs if we assume that TXOTPA and TXO-PhCz are the normal fluorescent emitter.[39] This phenomenon indicates the efficient up-conversion of triplet excitons from T1 to S1 through TADF, which coincides with the results of transient PL measurement above. Noticeably, both devices showed high EQE and excellent EL stability. The value of EQE decreased slowly with increasing luminance and the high EQE up to 6% of the devices can be remained at 1000 cd/cm2; this effect can be ascribed to the fast reverse ISC and then suppressed triplet-triplet or triplet-singlet annihilation. The EL spectra were recorded at different driving voltage to explore the EL spectra stability. The EL spectra of both devices are identical at driving voltages ranging from 7 to 19 V, and, even at high voltage, no derivation or new peaks can be observed, indicating the balanced charge-carrier injection and transportation into the emissive layer.[40]

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Figure 5. (a) Current density-voltage-luminance characteristics of the OLEDs based on compounds TXO-TPA and TXO-PhCz. (b) The EQE-current density characteristics. (c) The EL spectra operated at different voltages of the device incorporating TXO-TPA. (d) The EL spectra operated at different voltages of the device incorporating TXO-PhCz.

In summary, we have designed and synthesized two TADF emitters, TXO-PhCz and TXO-TPA, which have a TXO core as an electron-accepting unit in common and PhCz/TPA as an electron-donating unit. Both compounds show excellent thermal stability and positive solvatochromic behavior. Pronounced aggregation-induced emission (AIE) of TXO-PhCz and TXO-TPA in water/acetonitrile mixture can be observed, and the fluorescence quantum efficiency of their films is 0.93 ± 0.02 and 0.36 ± 0.02, respectively. More specificially, their small ΔEST for efficient up-conversion induces the thermally activated delayed fluorescence (TADF), facilitating their great application for highly efficient OLEDs. OLEDs using TXOPhCz and TXO-TPA as an emitter achieved a maximum EQE value of 21.5% and 18.5%, respectively, which is among the best of TADF-based OLEDs. These made TXO-PhCz or TXOTPA promising for the application in the future organic flat panel display and lighting.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was financially supported by the “Hundred Talents Program” of the Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant No. 61178061, No.51373189 and No. 61227008), the National Basic Research Program of China (973) (No. 2014CB932600), the National High Technology Research and Development Program of China (863) (Grant No. 2011AA03A110), and

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the Start-Up Fund of the Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences. Received: March 28, 2014 Revised: May 12, 2014 Published online:

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Novel thermally activated delayed fluorescence materials-thioxanthone derivatives and their applications for highly efficient OLEDs.

Thermally activated delayed fluorescence emitters with small energy gap between the triplet and singlet (ΔEST ), TXO-PhCz and TXO-TPA, have been succe...
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