Tuning the emission from local excited-state to charge-transfer state transition in quinoxaline-based butterfly-shaped molecules: Efficient orange OLEDs based on thermally activated delayed fluorescence emitter

Accepted Manuscript Tuning the emission from local excited-state to charge-transfer state transition in quinoxaline-based butterfly-shaped molecules: Efficient orange OLEDs based on thermally activated delayed fluorescence emitter Ling Yu, Zhongbin Wu, Cheng Zhong, Guohua Xie, Kailong Wu, Dongge Ma, Chuluo Yang PII:

S0143-7208(17)30138-9

DOI:

10.1016/j.dyepig.2017.02.035

Reference:

DYPI 5814

To appear in:

Dyes and Pigments

Received Date: 20 January 2017 Revised Date:

14 February 2017

Accepted Date: 17 February 2017

Please cite this article as: Yu L, Wu Z, Zhong C, Xie G, Wu K, Ma D, Yang C, Tuning the emission from local excited-state to charge-transfer state transition in quinoxaline-based butterfly-shaped molecules: Efficient orange OLEDs based on thermally activated delayed fluorescence emitter, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.02.035. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract:

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Based on quinoxaline derivatives, the emission type is tuned from local excited-state (LE) to

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charge-transfer state (CT) transition, and an orange TADF emitter is achieved.

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Tuning the emission from local excited-state to charge-transfer state

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transition in quinoxaline-based butterfly-shaped molecules: efficient orange OLEDs based on thermally activated delayed fluorescence emitter

Ling Yu a, Zhongbin Wu b, Cheng Zhong a, Guohua Xie a, Kailong Wu a, Dongge Ma bc, * and

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key

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a

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Chuluo Yang a, *

Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan, 430072, People’s Republic of China. b

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

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Chemistry, University of Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China.

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

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c

Luminescent Materials and Devices, South China University of Technology, Guangzhou,

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510640, People’s Republic of China. Corresponding Author

*(C. Yang). E-mail: [email protected]. *(D. Ma). E-mail: [email protected].

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ABSTRACT: We designed and synthesized a series of new butterfly-shaped D-A-D type compounds with quinoxaline as an electron acceptor. Their photoluminescence (PL) spectra are successfully tuned from green to orange based on the intramolecular charge transfer effect.

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Moreover, through theoretical and experimental approaches, we have verified the dihedral angles between the donor and acceptor, the value of ∆EST and the nature of T1 play crucial roles in shaping the emissive properties, and we have also successfully tuned the emission type from

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local excited-state (LE) to charge-transfer state (CT) transition to acquire a TADF molecule. A high rate constant for reverse intersystem crossing (RISC) is up to 1.5×106 s-1. The BDQDMAC-

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based orange TADF OLEDs exhibit a maximum external quantum efficiency of 7.4%, corresponding to a prominent contribution of 97% from the delayed fluorescence to the overall external quantum efficiency.

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Keywords: organic light-emitting diodes; thermally activated delayed fluorescence; charge-

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transfer state transition; high up-conversion rate constant

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1. Introduction In recent years, organic light-emitting diodes (OLEDs) based on fluorescent emitters continue

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to attract widespread attention for their promising applications in both flat-panel displays and solid-state lightings [1,2]. However, traditional fluorescence materials can utilize only 25% singlet excitons for radiation by electrical excitation [3-5]. To be inspired, some new approaches

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have been proposed to enhance the internal quantum efficiency (IQE) by efficient up-conversion process from triplet excitons to singlet excited state. Triplet-triplet annihilation (TTA), in which

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the additional 37.5% singlet yield may be acquired through the up-conversion mechanism of two triplet excitons fusion [6,7]. Thermally activated delayed fluorescence (TADF) provides a new avenue for the high-performance metal-free OLEDs [8-11], because they can theoretically achieve 100% internal quantum efficiency through reverse intersystem crossing (RISC) process to harness 75% triplet excitons [12,13]. Ma et al. put forward a hybridized local and charge-

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transfer (HLCT) state mechanism, which is able to contribute more than 25% singlet excitons resulted from the RISC occurring at the higher-lying excited energy levels [14,15]. The abovementioned mechanisms are closely related to the excited state transition type. Among them,

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it is believed that the emission of TADF is derived from the charge-transfer (CT) transition

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[11,16]. Designing a molecule with CT transition rather than local excited-state (LE) transition emission behavior is of important significance. Universally, it is necessary for TADF emitters to assure a small singlet-triplet energy gap (∆EST) for effective RISC process [17]. According to such fundamental principle, the feasible approach is to utilize a highly twisted structures between donor and acceptor units to minimize the spatial overlap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied

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molecular orbital (LUMO), since the ∆EST is dependent on the frontier orbital exchange interaction integral [18-22].

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Keeping this in mind, herein, we designed a series of new butterfly-shaped donor-accepterdonor (D-A-D) type compounds, namely, BDQDMAC, BDQ-tBuCz, BDQCz and BDQPCz (Scheme 1), in which quinoxaline is used as an electron acceptor and 9,9-dimethyl-9,10-

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dihydroacridine (DMAC), t-butyl-carbazole (t-BuCz), carbazole (Cz) and 9-phenylcarbazole (PCz) are used as electron donors, respectively. Selecting donor units with different electron-

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donating abilities [10,23-28], we anticipate to tune the emission type from local excited-state (LE) to charge-transfer state (CT) transition, and eventually acquire TADF emitter. Encouragingly, we obtained an orange TADF emitter (BDQDMAC), and the resulting OLEDs achieved an efficient EQE of 7.4%, which corresponds to a prominent contribution of 97% from

2. Experimental Section

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the delayed fluorescence to the overall EQE.

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2.1. Materials and instrument The 1H NMR and

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C NMR spectra were recorded on a MERCURY-VX300 spectrometer

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with CDCl3 as the solvent. Mass spectra were measured on a ZAB 3F-HF mass spectrophotometer. Elemental analyses were performed on a Vario EL-III microanalyzer. Thermal gravity analysis (TGA) was performed on a Netzsch STA 449C instrument. Differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at a heating rate of 20 K min-1 from 20 to 500 °C under argon. The glass transition temperature (Tg) was determined from the second heating scan at a heating rate of 10 °C min-1. UV-vis absorption spectra

were

recorded

on

a

Shimadzu

UV-2700

recording

spectrophotometer.

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Photoluminescence (PL) spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer.

Cyclic

voltammetry

(CV)

was

carried

out

in

nitrogen-purged

dichloromethane (oxidation scan) at room temperature with a CHI voltammetric analyzer. N-

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Bu4PF6 (0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consists of a platinum working electrode, a platinum wire auxiliary electrode, and an Ag wire pseudoreference electrode with ferrocenium-ferrocene (Fc+/Fc) as the internal

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standard. Cyclic voltammograms were obtained at a scan rate of 100 mV s-1. The onset potential was determined from the intersection of two tangents drawn at the rising and background current

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of the cyclicvoltammogram. Through a single photon counting spectrometer from Edinburgh Instruments (FLS920), the PL lifetimes were measured with a Picosecond Pulsed UV-LASTER (LASTER377) as the excitation source. The photoluminescence quantum efficiency was measured using an absolute photoluminescence quantum yield measurement system (C9920-02,

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Hamamatsu Photonics).

2.2. Device fabrication and characterization

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The fabricated device was grown on clean glass substrates pre-coated with a 180 nm thick layer of ITO with a sheet resistance of 10 Ω per square. The ITO glass substrates were pre-

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cleaned carefully and treated by oxygen plasma for 2 min. Then the sample was transferred to the deposition system. MoO3 was firstly deposited onto the ITO substrate, consecutively followed by 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC): 20% MoO3 (50 nm), TAPC (20 nm), 1,3-bis(N-carbazoly)benzene (mCP) (10 nm), emissive layer (20 nm), and 1,3,5tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) (45 nm). Finally, a cathode composed of lithium fluoride and aluminum was sequentially deposited onto the substrate in the vacuum of 10-6 Torr. The current–voltage–brightness characteristic was measured by using a Keithley source

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measurement unit (Keithley 2400 and Keithley 2000) with a calibrated silicon photodiode. The EL spectra were measured by a Spectrascan PR650 spectrophotometer. EQE was calculated from

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the luminance and current density. 2.3. Materials

9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole

(PCz)

and

5,8-

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dibromo-2,3-diphenylquinoxaline (BDQ) were synthesized according to the literature methods

2.3.1.

Synthesis

of

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[29,30].

10,10'-(2,3-diphenylquinoxaline-5,8-diyl)bis(9,9-dimethyl-9,10-

dihydroacridine) (BDQDMAC)

A mixture of BDQ (1.00 g, 2.27 mmol), 9,9-dimethyl-9,10-dihydroacridine (1.02 g, 4.86

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mmol), Pd(OAc)2 (20 mg, 0.09 mmol), HP(t-Bu)3BF4 (64 mg, 0.22 mmol), t-BuONa (0.52 g, 5.42 mmol), and toluene (30 ml) was refluxed under argon for 24 h. After cooled, the mixture was extracted with brine and CH2Cl2, and dried over anhydrous Na2SO4. After removal of the

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solvent, the residue was purified by column chromatography on silica gel using dichloromethane/petroleum ether (2:3 by vol.) as the eluent to give an orange powder (1.54 g,

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yield: 97%). 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.11 (s, 2H), 7.53 (d, J = 7.2 Hz, 4H), 7.79 (d, J = 8.1 Hz, 4H), 7.39 (d, J = 8.1 Hz, 4H), 6.66 (m, 8H), 6.51 (t, J = 7.2 Hz, 4H), 7.23-7.15 (m, 6H), 7.10-6.95 (m, 12H), 6.46 (d, J = 7.5 Hz, 4H), 1.80 (s, 12H).

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C NMR (75 MHz,

CDCl3) δ [ppm]: 153.59, 141.02, 138.72, 138.18, 133.84, 130.59, 130.24, 128.83, 127.54, 126.28, 125.14, 120.71, 113.85, 36.17. MS (EI): m/z 697 [M+]. EA (%) for C50H40N4: C 86.17, H 5.79, N 8.04; found: C 86.25, H 5.70, N 8.01.

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2.3.2. Synthesis of 9,9'-(2,3-diphenylquinoxaline-5,8-diyl)bis(3,6-di-tert-butyl-9H-carbazole) (BDQ-tBuCz)

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A mixture of BDQ (1.00 g, 2.27 mmol), 3,6-di-tert-butyl-9H-carbazole (1.48 g, 5.01 mmol), K2CO3 (0.70 g, 5.07 mmol), CuI (0.11 g, 0.58 mmol) and 18-crown-6 (0.15 g, 0.58 mmol) were dissolved in 1,2-dichlorobenzene (4 ml) under nitrogen atmosphere. The reaction mixture was

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stirred for 48 h at 180 oC. After cooled, the mixture was extracted with brine and CH2Cl2, and dried over anhydrous Na2SO4. After removal of the solvent, the residue was purified by column

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chromatography on silica gel using dichloromethane/petroleum ether (2:3 by vol.) as the eluent to give a yellow powder (1.87 g, yield: 98%). 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.2 (d, J = 6.0 Hz, 6H), 7.5 (d, J = 7.5 Hz, 4H), 7.28 (s, 2H), 7.18 (t, J = 6.9 Hz, 6H), 7.02 (t, J = 7.5 Hz, 4H), 5.29 (s, 2H), 1.51 (s, 36H).

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C NMR (75 MHz, CDCl3) δ [ppm]: 152.30, 143.04, 140.35,

137.96, 137.22, 134.77, 130.07, 129.04, 127.76, 123.78, 123.31, 116.06, 110.26, 34.72, 31.98.

N 6.76.

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MS (EI): m/z 837 [M+]. EA (%) for C60H60N4: C 86.08, H 7.22, N 6.69; found: C 86.23, H 7.12,

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2.3.3. Synthesis of 9,9'-(2,3-diphenylquinoxaline-5,8-diyl)bis(9H-carbazole) (BDQCz) A similar procedure to BDQ-tBuCz was followed, but with 9H-carbazole (1.90 g, 11.38

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mmol) to replace 3,6-di-tert-butyl-9H-carbazole. After removal of the solvent, the residue was purified by column chromatography on silica gel using dichloromethane/petroleum ether (2:3 by vol.) as the eluent to give a yellow-orange powder (2.73 g, yield: 98%). 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.21 (t, J = 6.9 Hz, 6H), 7.45 (t, J = 7.4 Hz, 4H), 7.35 (t, J = 8.3 Hz, 8H), 7.18 (d, J = 6.9 Hz, 6H), 7.06 (t, J = 7.4 Hz, 4H).

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C NMR (75 MHz, CDCl3) δ [ppm]: 152.94,

142.00, 137.86, 137.61, 134.89, 129.95, 129.16, 128.35, 127.89, 125.67, 123.91, 120.24, 120.14,

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110.64. MS (EI): m/z 612 [M+]. EA (%) for C44H28N4: C 86.25, H 4.61, N 9.14; found: C 86.43, H 4.79, N 9.16.

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2.3.4. Synthesis of 9,9'-((2,3-diphenylquinoxaline-5,8-diyl)bis(4,1-phenylene))bis(9H-carbazole) (BDQPCz)

A mixture of BDQ (1.00 g, 2.27 mmol), PCz (1.85 g, 5.01 mmol), K2CO3 (0.94 g, 6.81

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mmol), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.31 g, 0.27 mmol) were dissolved in a mixed solution which contained toluene (28 ml), ethanol (14 ml) and distilled

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water (7 ml) under nitrogen atmosphere. The reaction mixture was stirred for 48 h at 180 oC. After cooled, the mixture was extracted with brine and CH2Cl2, and dried over anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel using dichloromethane/petroleum ether (2:3 by vol.) as the eluent to give a yellow

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powder (1.65 g, yield: 95%). 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.18 (t, J = 9.0 Hz, 8H), 8.08 (s, 2H), 7.78 (d, J = 8.7 Hz, 4H), 7.68 (d, J = 7.8 Hz, 4H), 7.60 (d, J = 8.1 Hz, 4H), 7.47 (t, J = 7.65 Hz, 4H), 7.33 (t, J = 7.05 Hz, 10H). 13C NMR (75 MHz, CDCl3) δ [ppm]: 151.63, 140.79,

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138.84, 138.54, 137.15, 137.12, 132.28, 129.99, 129.85, 129.01, 128.18, 126.31, 125.89, 123.44, 120.24, 119.93, 109.94. MS (EI): m/z 764 [M+]. EA (%) for C56H36N4: C 87.93, H 4.74, N 7.32;

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found: C 88.15, H 4.85, N 7.32. 3. Results and discussion

3.1. Synthesis and thermodynamic properties The synthetic routes of the quinoxaline-containing compounds are outlined in Scheme 1. The key intermediates, BDQ and PCz were prepared according to the literature methods. The targeted

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compound BDQDMAC was obtained by palladium catalyzed C-N cross-coupling reaction between BDQ and DMAC. BDQ-tBuCz and BDQCz were synthesised via copper-catalyzed Ullmann coupling reaction of BDQ with t-BuCz and carbazole. The Suzuki cross-coupling

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reaction of BDQ with PCz afforded BDQPCz. All of products were obtained in high yields of over 95% (for the details, see supporting information). Their chemical structures were confirmed by 1H and 13C NMR, elemental analysis and mass spectrometry. All compounds exhibit excellent

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thermal stability with 5% weight loss decomposition temperatures (Td) in the range of 394-471 C and high glass-transition temperatures (Tgs) ranging from 134 to 194 oC (Fig. S1).

3.2. Theoretical calculations

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To investigate the effects of different electron-donating group, the frontier molecular orbitals and energy levels were optimized by B3LYP/6-31g(d) level of theory. The HOMOs and the

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LUMOs of the four compounds are mainly distributed on the donor moieties and the quinoxaline acceptor moiety, respectively. As shown in Fig. S2, the LUMO energy levels of BDQDMAC, BDQ-tBuCz, BDQCz and BDQPCz show similar values of -2.25, -2.21, -2.28 and -2.21 eV,

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respectively, while their HOMO energy levels display clearly different values of -4.89, -5.09, 5.31 and -5.26 eV, respectively, which are related to the difference of electron-donating ability.

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In addition, we have succeeded in tuning the dihedral angles by connecting different rigid donor groups, with the dihedral angles between the two peripheral donor units and the plane of quinoxaline decreased in the order of DMAC (81o, 91o) > Cz (63o, 60o) ≈ t-BuCz (58o, 58o) > PCz (45o, 42o). For BDQ-tBuCz and BDQCz, the five-membered ring can relieve dihedral angle emerged by the steric hindrance between the carbazolyl groups and the hydrogen atoms of the benzene ring [31]. For BDQPCz having a phenylene bridge to connect carbazolyl groups and quinoxaline, the ‘bridge’ further weakens the steric repulsion, resulting in the smallest dihedral

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angles among them. It is worth noting that extending the conjugation leads to a larger HOMOLUMO orbital overlap in the BDQPCz (see Fig. S2), implying that BDQPCz may not have a small ∆EST. The above results indicate that the HOMO energy levels, and the dihedral angles

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between donor units and quinoxaline closely depend on the properties of donor units, which seem to influence the extent of HOMO-LUMO orbital overlap, and thus to determine the value

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of ∆EST.

We then calculated the ∆EST and natural transition orbitals (NTOs) for the excited states of

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these compounds by the level of TD-B3LYP/6-31g(d). As expected, BDQDMAC displays the smallest ∆EST of 0.08 eV, while BDQPCz shows the largest the ∆EST of 0.66 eV (Table S1). Moreover, in order to further investigate the relationship between different donor units and the nature of electronic transitions, NTOs at the T1 excited state for all these compounds were evaluated (Fig. 1). For the T1 excited state of BDQDMAC, the hole is predominantly located on

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the DMAC moiety, while the particle is distinctly localized on the quinoxaline moiety. Thus BDQDMAC exhibits dominant charge-transfer (CT) state character. For BDQ-tBuCz, BDQCz and BDQPCz, with the decreasing dihedral angles between donor units and the quinoxaline, the

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T1 excited states of BDQ-tBuCz and BDQCz show a major section of local excited-state (LE)

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transition of quinoxaline and a minor section of CT transition from donor units to quinoxaline unit. In contrast, the T1 excited state of BDQPCz reveals a typical LE state property, owing to the similar distribution pattern of the hole and particle, which is most localized on the quinoxaline moiety with a little on the neighboring phenyl rings. According to the reported literature [32], TADF can only occur in a bipolar system where the first triplet excited state is dominated by the CT state. BDQDMAC shows the almost orthogonal dihedral angles, the very small ∆EST and the

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dominant CT character of the first triplet excited state, suggesting that it is a potential TADF molecule.

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3.3. Electrochemical and photophysical properties We investigated their electrochemical properties by cyclic voltammetry (CV) (Fig. S3). The HOMO levels of BDQDMAC, BDQ-tBuCz, BDQCz and BDQPCz are established from the

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onsets of their oxidation waves to be -5.00, -5.07, -5.06 and -5.14 eV, respectively. UV-vis absorption and photoluminescence (PL) spectra for all compounds are shown in Fig. 2. The

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lowest-energy broad and weak absorption bands in the absorption spectra can be assigned to intramolecular charge-transfer (ICT) from the donor units to the electron-acceptor of quinoxaline core. With the increasing electron-donating ability from phenyl to carbazole to DMAC, the ICT absorption bands gradually red-shift from 370 nm (BDQPCz) to 482 nm (BDQDMAC) (Table 1).

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The fluorescence spectra of BDQPCz, BDQCz, BDQ-tBuCz and BDQDMAC exhibit a broad and structureless emission bands with the peaks of 502, 537, 557 and 603 nm, respectively, which cover a wide range of luminescence from green to orange. Meanwhile, phosphorescence

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spectra were measured to experimentally evaluate the ∆EST. Considering that the first triplet excited state is the dominant CT character for BDQDMAC, to avoid the solvatochromism effect,

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the triplet energy is calculated to be 1.99 eV with the emission peak of 622 nm from its phosphorescent spectrum in film at 77 K. While the triplet energy for BDQ-tBuCz, BDQCz and BDQPCz are determined to be 2.05, 2.11 and 2.14 eV from phosphorescence spectra in 2methyl-tetrahydrofuran at 77 K. Thus their ∆ESTs can be estimated following the order of 0.07 eV (BDQDMAC) < 0.18 eV (BDQ-tBuCz) ≈ 0.20 eV (BDQCz) < 0.33 eV (BDQPCz), which agree well with the results from theoretical analysis. Detailed data of thermal, photophysical, and electrochemical properties of all compounds are shown in Table 1.

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We further investigated the transient PL features of these compounds in toluene under the presence of oxygen and argon conditions. As exemplified in Fig. 3, BDQ-tBuCz, BDQCz and BDQPCz only display the normal prompt fluorescence without delayed one after degassing with

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argon. In contrast, in an argon-purged solution, the emission decay of BDQDMAC exhibits both prompt and delayed components with the lifetimes of 22 ns and 15.7 µs, respectively. Furthermore, the delayed component immediately disappeared in a non-degassed condition due

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to the oxygen sensitive behavior [33]. TADF property of BDQDMAC was further confirmed in pure film. The prompt and delayed fluorescence are shown in Fig. 4a. Moreover, the delayed

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emission are gradually intensified with the temperature from 77 to 300 K, demonstrating that the RISC process from T1 to S1 is an accelerative trend by the thermal activation, which is a clear evidence of TADF (Fig. 4b and Table S2).

To assess the potential application of BDQDMAC as the TADF emitter for OLEDs, similar

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transient PL decay spectra are observed for 4,4´-N,N´-dicarbazolebiphenyl (CBP): BDQDMAC (10 wt.%) in doped film at room temperature (Fig. 5a). The contribution of delayed component to the overall emission is evaluated by comparing the steady state emission intensity in degased

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and non-degassed doped film (Fig. 5b). The normalized emission spectra are in accordance with

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that in degassed and non-degassed doped film, proving the delayed and prompt fluorescence stem from the same 1CT state. The CT emission intensity is increased by 1.79 times without oxygen compared with that in non-degassed doped film, thus the total ΦPL is 39.6% based on the ΦPL of 22.1% in non-degassed doped film, and the contribution of delayed fluorescence to total photoluminescence (PL) quantum efficiency (ΦPL) is 36% [34]. According to the total ΦPL of 39.6%, we calculated the rate constant of RISC is up to 1.5×106 s-1 (Table 2). 3.4. EL performance

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The vacuum-processed OLEDs with a configuration of ITO/MoO3/TAPC: 20% MoO3 (50 nm)/TAPC (20 nm)/mCP (10 nm)/CBP: 10% BDQDMAC (20 nm)/TmPyPB (45 nm)/LiF/Al was fabricated (Fig. 6a), where TAPC and TmPyPB were introduced as the hole- and electron-

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transporting layer (HTL and ETL), respectively. An additional 20% MoO3 was added into the TAPC layer to improve the hole-injection ability. CBP served as a host for the TADF emitter in the emitting layer (EML). mCP with a high T1 energy was inserted as a blocking layer to confine

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T1 excitons [11]. The electroluminescence (EL) spectra, external quantum efficiency, power and current efficiency versus luminance, and current density–voltage–luminance (J–V–L)

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characteristics for the device are shown in Fig. 6b, S4a and S4b. For the BDQDMAC-based device, an orange EL emission peaking at 580 nm is obtained and the EL spectra exhibit negligible change with the CIE coordinates being close to (0.45, 0.47) when the voltage increases from 6.0 to 8.0 V, indicating the quite good spectral stability in a wide luminance

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range. Additionally, as shown in Table 3, the device turns on at 3.8 V and displays a high luminance of 11456 cd m−2. The maximum current efficiency, power efficiency and EQE of the TADF-based device are 19.7 cd A−1, 16.3 lm W−1 and 7.4%, respectively. The EQE is exceeding

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the 5% theoretical limit for the traditional fluorescent emitters. Similar to most previously reported TADF emitters, especially blue and orange/red devices, the efficiency roll-off problem

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of BDQDMAC-based device is also somewhat severe. Better device performance could be expected by further device engineering. In order to ascertain the contribution of the delayed component to the overall EQE, we specified the value via the relevant equations presented in the supporting information. Using a hypothesis of γηout = 0.2 [35], the theoretical maximum EQE is calculated to be 7.6%, which is in well accordance with the experimental obtained value (7.4%). The contribution from the

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delayed fluorescence to the overall EQE can reach up to 97% based on the EQEs of 0.2% and 7.4% , respectively, contributed from the prompt and delayed components. This indicates a

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highly-efficient RISC process. 4. Conclusions

In conclusion, a series of new butterfly-shaped D-A-D type compounds was designed and

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synthesized with the combination of quinoxaline-based electron acceptor and different electrondonors. Their PL spectra span a wide range from green to orange based on the tunable ICT

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effect. The dihedral angles, the value of ∆EST and the nature of T1 play a crucial role in determining the emission properties. We have successfully tuned the emission type from local excited-state (LE) to charge-transfer state (CT) transition to acquire a TADF molecule. And the rate constant for reverse intersystem crossing (RISC) is up to 1.5×106 s-1. Although the

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BDQDMAC-based orange TADF device exhibited an EQE of only 7.4% due to a relatively low ΦPL, the contribution from the delayed fluorescence to the overall EQE can reach up to 97%. The systematic study of these quinoxaline emitters reveals a feasible strategy for designing high-

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performance red/orange TADF emitters for OLEDs applications, and further study is under way.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project No. 91433201 and 61575146), the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655002

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and 2013CB834805), the National Key Research Program (2016YFB0401002), and the Innovative Research Group of Hubei Province (No. 2015CFA014).

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Reference

[1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987; 51; 913-

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915.

[2] Reineke S, Lindner F, Schwartz G, Seidler N, Walzer K, Lussem B, Leo K. White organic

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light-emitting diodes with fluorescent tube efficiency. Nature 2009; 459; 234-238. [3] Liu C, Fu Q, Zou Y, Yang C, Ma D, Qin J. Low Turn-on Voltage, High-Power-Efficiency, Solution-Processed

Deep-Blue

Organic

Light-Emitting

Diodes

Based

on

Starburst

Oligofluorenes with Diphenylamine End-Capper to Enhance the HOMO Level. Chem Mater

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2014; 26; 3074-3083.

[4] Zou Y, Ye T, Ma D, Qin J, Yang C. Star-shaped hexakis(9,9-dihexyl-9H-fluoren-2yl)benzene end-capped with carbazole and diphenylamine units: solution-processable, high Tg

AC C

23491.

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hole-transporting materials for organic light-emitting devices. J Mater Chem 2012; 22; 23485-

[5] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR. Highly efficient Phosphorescent emission from organic electroluminescent devices. Nature 1998; 395; 151-154.

15

ACCEPTED MANUSCRIPT

[6] Fukagawa H, Shimizu T, Ohbe N, Tokito S, Tokumaru K, Fujikake H. Anthracene derivatives as efficient emitting hosts for blue organic light-emitting diodes utilizing triplet–

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triplet annihilation. Org Electron 2012; 13; 1197-1203. [7] Kondakov DY, Pawlik TD, Hatwar TK, Spindler JP. Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes. J Appl Phys 2009;

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106; 124510-7.

[8] Endo A, Ogasawara M, Takahashi A, Yokoyama D, Kato Y, Adachi C. Thermally Activated

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Delayed Fluorescence from Sn4+-Porphyrin Complexes and Their Application to Organic LightEmitting Diodes-A Novel Mechanism for Electroluminescence. Adv Mater 2009; 21; 4802-4806. [9] Kim YH, Wolf C, Cho H, Jeong SH, Lee TW. Highly Efficient, Simplified, Solution-

Mater 2016; 28; 734-741.

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Processed Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes. Adv

[10] Kim M, Jeon SK, Hwang SH, Lee SS, Yu E, Lee JY. Highly efficient and color tunable

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thermally activated delayed fluorescent emitters using a “twin emitter’’ molecular design. Chem Commun 2016; 52; 339-342.

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[11] Sun JW, Baek JY, Kim KH, Moon CK, Lee JH, Kwon SK, Kim YH, Kim JJ. Thermally Activated Delayed Fluorescence from Azasiline Based Intramolecular Charge-Transfer Emitter (DTPDDA) and a Highly Efficient Blue Light Emitting Diode. Chem Mater 2015; 27; 66756681. [12] Hirata S, Sakai Y, Masui K, Tanaka H, Lee SY, Nomura H, Nakamura N, Yasumatsu M, Nakanotani H, Zhang Q, Shizu K, Miyazaki H, Adachi C. Highly efficient blue

16

ACCEPTED MANUSCRIPT

electroluminescence based on thermally activated delayed fluorescenc. Nat Mater 2015; 14; 330336.

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[13] Seino Y, Inomata S, Sasabe H, Pu YJ, Kido J. High-Performance Green OLEDs Using Thermally Activated Delayed Fluorescence with a Power Efficiency of over 100 lm W Mater 2016; 28; 2638-2643.

−1

. Adv

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[14] Li W, Pan Y, Xiao R, Peng Q, Zhang S, Ma D, Li F, Shen F, Wang Y, Yang B, Ma Y. Employing ∼100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized

M AN U

Local and Charge-Transfer Excited State. Adv Funct Mater 2014; 24; 1609-1614. [15] Zhang S, Yao L, Peng Q, Li W, Pan Y, Xiao R, Gao Y, Gu C, Wang Z, Lu P, Li F, Su S, Yang B, Ma Y. Achieving a Signifi cantly Increased Efficiency in Nondoped Pure Blue

1755-1762.

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Fluorescent OLED: A Quasi-Equivalent Hybridized Excited State. Adv Funct Mater 2015; 25;

[16] Yu L, Wu Z, Xie G, Zhong C, Zhu Z, Cong H, Ma D, Yang C. Achieving a balance between

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small singlet–triplet energy splitting and high fluorescence radiative rate in a quinoxaline-based orange-red thermally activated delayed fluorescence emitter. Chem Commun 2016; 52; 11012-

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11015.

[17] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic lightemitting diodes from delayed fluorescence. Nature 2012; 492; 234-238. [18] Tanaka H, Shizu K, Miyazaki H, Adachi C. Efficient green thermally activated delayed fluorescence (TADF) from a phenoxazine–triphenyltriazine (PXZ–TRZ) derivative. Chem Commun 2012; 48; 11392-11394.

17

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[19] Tanaka H, Shizu K, Nakanotani H, Adachi C. Twisted Intramolecular Charge Transfer State for Long-Wavelength Thermally Activated Delayed Fluorescence. Chem Mater 2013; 25; 3766-

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3771. [20] Shizu K, Tanaka H, Uejima M, Sato T, Tanaka K, Kaji H, Adachi C. Strategy for Designing Electron Donors for Thermally Activated Delayed Fluorescence Emitters. J Phys Chem C 2015;

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119; 1291-1297.

[21] Wong MY, Hedley GJ, Xie G, Kölln LS, Samuel IDW, Pertegás A, Bolink HJ, Zysman-

M AN U

Colman E. Light-Emitting Electrochemical Cells and Solution-Processed Organic Light-Emitting Diodes Using Small Molecule Organic Thermally Activated Delayed Fluorescence Emitters. Chem Mater 2015; 27; 6535-6542.

[22] Endo A, Sato K, Yoshimura K, Kai T, Kawada A, Miyazaki H, Adachi, C. Efficient up-

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conversion of triplet excitons into a singlet state and its application for organic light emitting diodes. Appl Phys Lett 2011; 98; 083302-3.

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[23] Li Y, Xie G, Gong S, Wu K, Yang C. Dendronized Delayed Fluorescence Emitters for NonDoped, Solution-Processed Organic Light-Emitting Diodes with High Efficiency and Low

5447.

AC C

Efficiency Roll-Off Simultaneously: Two Parallel Emissive Channels. Chem Sci 2016; 7; 5441-

[24] Wu Z, Luo J, Sun N, Zhu L, Sun H, Yu L, Yang D, Qiao X, Chen J, Yang C, Ma D. HighPerformance Hybrid White Organic Light-Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll-Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter. Adv Funct Mater 2016; 26; 3306-3313.

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[25] Wu Z, Yu L, Zhou X, Guo Q, Luo J, Qiao X, Yang D, Chen J, Yang C, Ma D. Management of Singlet and Triplet Excitons: A Universal Approach to High-Efficiency All Fluorescent

Mater 2016; 4; 1067-1074.

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WOLEDs with Reduced Efficiency Roll-Off Using a Conventional Fluorescent Emitter. Adv Opt

[26] Park IS, Lee SY, Adachi C, Yasuda T. Full-Color Delayed Fluorescence Materials Based on Phthalonitriles

and

Dicyanopyrazines:

Systematic

Design,

Tunable

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Wedge-Shaped

Photophysical Properties, and OLED Performance. Adv Funct Mater 2016; 26; 1813-1821.

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[27] Wada Y, Shizu K, Kubo S, Suzuki K, Tanaka H, Adachi C, Kaji H. Highly efficient electroluminescence from a solution-processable thermally activated delayed fluorescence emitter. Appl Phys Lett 2015; 107; 183303-4.

[28] Xie G, Li X, Chen D, Wang Z, Cai X, Chen D, Li Y, Liu K, Cao Y, Su SJ. Evaporation- and

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Solution-Process-Feasible Highly Efficient Thianthrene-9,9 ´ ,10,10 ´ -Tetraoxide-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Efficiency Roll-Off. Adv

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Mater 2016; 28; 181-187.

[29] Jang HG, Song W, Lee JY, Hwang SH. Synthesis of pyrimidine-cored host materials

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bearing phenylcarbazole for efficient yellow phosphorescent devices: effect of linkage position. RSC Adv. 2015; 5; 17030-17033. [30] Jo S, Kim J, Noh J, Kim D, Jang G, Lee N, Lee E, Lee TS. Conjugated Polymer Dots-onElectrospun Fibers as a Fluorescent Nanofibrous Sensor for Nerve Gas Stimulant. Appl. Mater. Interfaces 2014; 6; 22884-22893.

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[31] Kitamoto Y, Namikawa T, Suzuki T, Miyata Y, Kita H, Sato T, Oi S. Dimesitylarylboranebased luminescent emitters exhibiting highly-efficient thermally activated delayed fluorescence

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for organic light-emitting diodes. Org Electron 2016; 34; 208-217. [32] Zhang Q, Li J, Shizu K, Huang S, Hirata S, Miyazaki H, Adachi C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light Emitting

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Diodes. J Am Chem Soc 2012; 134; 14706-14709.

[33] Nakagawa T, Ku SY, Wong KT, Adachi C. Electroluminescence based on thermally

Commun 2012; 48; 9580-9582.

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activated delayed fluorescence generated by a spirobifluorene donor–acceptor structure. Chem

[34] Santos PL, Ward JS, Data P, Batsanov AS, Bryce MR, Dias FB, Monkman AP. Engineering

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the singlet-triplet energy splitting in a TADF molecule. J Mater Chem C 2016; 4; 3815-3824. [35] Lee J, Shizu K, Tanaka H, Nomura H, Yasuda T, Adachi C. Oxadiazole- and triazole-based highly-efficient thermally activated delayed fluorescence emitters for organic light-emitting

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diodes. J Mater Chem C 2013; 1; 4599-4604.

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Figures and Tables Scheme 1. Synthetic routes of BDQDMAC, BDQ-tBuCz, BDQCz and BDQPCz. Fig. 1. Natural transition orbitals of the hole-particle contribution for these four compounds at

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the T1 excited state.

Fig. 2. Normalized UV/vis absorption of (a) BDQDMAC, (b) BDQ-tBuCz, (c) BDQCz and (d) BDQPCz in toluene. Fluorescence spectra of these four compounds in film. Phosphorescence

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spectrum of BDQDMAC in film and other phosphorescence spectra in 2-methyl-tetrahydrofuran at 77 K. Pink line and red line represent UV/vis and fluorescence spectra, respectively, while

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blue line represents phosphorescence spectra.

Fig. 3. Transient PL characteristics of (a) BDQDMAC, (b) BDQ-tBuCz, (c) BDQCz and (d) BDQPCz in toluene (10-4 M) under the presence of oxygen and argon conditions at room temperature. IRF: the instrument response function.

Fig. 4. (a) The transient PL decay of BDQDMAC in pure film after the deoxygenation. Inset:

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prompt fluorescence. (b) Temperature-dependent transient PL decays of BDQDMAC from 77 to 300 K in pure film after degassing.

Fig. 5. (a) Transient PL decay of CBP:BDQDMAC (10 wt.%) in film. (b) Photoluminescence

spectra.)

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spectra (PL) of BDQDMAC in degased and non-degassed doped film. (Inset: normalized PL

Fig. 6. (a) Energy level diagram of the TADF device based on BDQDMAC. (b) EQE versus

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luminance curves of the device. Inset: EL spectra and Commission Internationale de I´Éclairage (CIE) coordinates of the device at different voltages. Table 1. Thermal, photophysical, and electrochemical data of all compounds. Table 2. The lifetimes, quantum efficiencies and rate constants of CBP: 10% BDQDMAC in film. Table 3. The EL characteristic data of the BDQDMAC-based device.

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Scheme 1.

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Fig. 1

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Fig. 2 Abs. 1.0 Fl. at RT Ph. at 77K 0.5

1.0 (a) BDQDMAC

0.0

Abs. 1.0 Fl. at RT Ph. at 77K 0.5

BDQ-tBuCz

0.5 0.0 1.0 (c)

0.0

1.0 Abs. Fl. at RT Ph. at 77K 0.5

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BDQCz

0.5 0.0 1.0 (d)

0.0 300

400

0.0

1.0 Abs. Fl. at RT Ph. at 77K 0.5

BDQPCz

0.5

PL Inensity(a.u.)

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0.0 1.0 (b)

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Absorbance(a.u.)

0.5

500

600

700

0.0 800

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Wavelength(nm)

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Fig. 3 4

(b)

Ar O2

3

10

IRF BDQDMAC

2

10

Ar O2

4

10

BDQ-tBuCz

3

10

1

10

2

10 0

20

30

Time(us)

40

Ar O2

4

10

BDQCz 3

10

200

5

10

(d)

400

600

Time(ns)

800

1000

Ar O2

BDQPCz

4

10

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Intensity(Counts.)

(c)

50

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10

Intensity(Counts.)

10

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(a)

Intensity(Counts.)

Intensity(Counts.)

10

3

10

2

10

2

10

400

600

Time(ns)

800

1000

200

400

600

800

1000

Time(ns)

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200

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Fig. 4(a)

Film IRF

Ar 2

3

10

BDQDMAC

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10

2

10

1

10

0

20

40

60

80

100

Time(ns) 1

10

0

10

10

20

30

40

50

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Time(us)

Fig. 4(b) 4

(b)

77K 100K 150K 200K 250K 300K

3

10

2

1

10

0

10

20

30

40

50

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10

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10

Time(us)

AC C

Intensity(Counts.)

10

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Intensity(Counts.)

(a)

Intensity(Counts.)

4

10

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Fig. 5(a) 4

(a)

CBP:BDQDMAC(10 wt.%) IRF

3

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10

2

10

1

10

0

10

10

20

30

40

50

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Time(us)

Fig. 5(b) 1400

(b)

Ar 1.0 O2 0.8 0.6

1000

0.4

800

0.2 0.0

600

600

650

700

750

Idegas / Inon-degas = 1.79

400 200 CBP:BDQDMAC(10 wt.%)

550

600

650

700

750

800

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Wavelength(nm)

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0

550

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PL Inensity(a.u.)

1200

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Intensity(Counts.)

10

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Fig. 6(a)

(b)

1

EQE (%)

10

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Fig. 6(b) the vacuum-deposited device

6.0 V CIE (0.45, 0.47) 6.5 V (0.45, 0.47) 7.0 V (0.46, 0.46) 7.5 V (0.46, 0.46) 8.0 V (0.46, 0.46)

0

10

-1

500

550

600

650

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10

700

750

800

Wavelength (nm) 1

2

10 2

Luminance (cd/m )

3

10

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10

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0

10

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Table 1. λabs[c]

λFl,max[d]

PLQY[e]

ES/ET[f]

∆EST[g]

∆Eg[h]

HOMO[i]/LUMO[j]

[oC]

[nm]

[nm]

%

[eV]

[eV]

[eV]

[eV]

BDQDMAC

394/134

348/482

603

14.0

2.06/1.99

0.07

BDQ-tBuCz

419/194

345/465

557

42.7

2.23/2.05

0.18

BDQCz

414/150

367/443

537

47.8

2.31/2.11

0.20

BDQPCz

471/157

343/370

502

67.3

2.47/2.14

[d]

[b]

Obtained by DSC.

[c]

-5.00/-2.74

2.38

-5.07/-2.69

2.52

-5.06/-2.54

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Obtained by TGA.

2.26

-5.14/-2.30

0.33

2.84

Measured in toluene at room tempurature.

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[a]

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Td[a]/Tg[b]

Compound

Measured in film. [e]PLQY: photoluminescence (PL) quantum efficiency. Measured in their

neat films under air condition.

[f]

Measured in film for BDQDMAC and in 2-methyl-

tetrahydrofuran for other compounds at 77 K.

[g]

∆EST= ES- ET. [h]Calculated from the absorption

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from HOMO and Eg.

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edge of the UV/Vis spectrum. [i]Determined from the onset of the oxidation potential. [j]Deduced

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Table 2.

[a]

τd

Ratio of τd ΦF[a]

ΦTADF[a]

kP[b]

kd[b]

krS[c]

kISC[d]

kRISC[d]

[s−1]

[s−1]

[s−1]

[s−1]

[s−1]

[ns] [µs]

[%]

[%]

[%]

22

91

3.6

36

7.4

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τF

4.5×107 1.4×105 1.6×106 4.3×107 1.5×106

The prompt and delayed fluorescence quantum yield, respectively.

prompt fluorescence and delayed fluorescence, respectively.

The rate constant for

represents the radiative decay

The rate constant for intersystem crossing (ISC) and

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rate constants from S1 to S0 transition.

[d]

[c] S kr

[b]

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reverse intersystem crossing (RISC) between the S1 and T1 states, respectively.

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Table 3 EQEmax

CEmax

PEmax

LEmax

Peak

[V]

[%]

[cd A-1]

[lm W-1]

[cd m-2]

(nm)

V (V)

EQE [%]

CE [cd A-1]

PE [lm W-1]

3.8

7.4

19.7

16.3

11456

580

5.0

3.6

9.5

6.0

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At a luminance of 1 cd m−2.

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[a]

Performance at the brightness of 100 cd m-2

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Von[a]

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Research Highlights A series of new butterfly-shaped D-A-D type compounds with the combination of

and synthesized.

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quinoxaline-based electron acceptor and different electron donors have been designed

The emission types have been tuned from local excited-state (LE) to charge-transfer

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state (CT) transition and acquire an orange TADF molecule.

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A maximum EQE of 7.4% was achieved in TADF OLEDs, corresponding to a ultra-high contribution of 97% from the delayed fluorescence to the overall external quantum efficiency.

This work would be instructive in the rational molecule design of orange TADF

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materials.