tri-carbazole derivatives

Accepted Manuscript Blue thermally activated delayed fluorescence materials based on bi/tri-carbazole derivatives Wenjuan Zhang, Ye-Xin Zhang, Xiao-Qing Zhang, Xiang-Yang Liu, Jian Fan, LiangSheng Liao PII:

S1566-1199(18)30186-1

DOI:

10.1016/j.orgel.2018.04.022

Reference:

ORGELE 4630

To appear in:

Organic Electronics

Received Date: 20 January 2018 Revised Date:

10 April 2018

Accepted Date: 10 April 2018

Please cite this article as: W. Zhang, Y.-X. Zhang, X.-Q. Zhang, X.-Y. Liu, J. Fan, L.-S. Liao, Blue thermally activated delayed fluorescence materials based on bi/tri-carbazole derivatives, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.04.022. 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.

ACCEPTED MANUSCRIPT Blue thermally activated delayed fluorescence materials based on bi/tri-carbazole derivatives Wenjuan Zhang,† Ye-Xin Zhang,† Xiao-Qing Zhang, Xiang-Yang Liu, Jian Fan,* and Liang-Sheng Liao*

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute

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a

of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China

†

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E-mail: [email protected], [email protected] The two authors contribute equally to this paper

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Three blue thermally activated delayed fluorescence materials (Cy-2Cz, Cy-3Cz and Sf-3Cz) based on bi/tri-carbazole derivatives have been designed and prepared. In Cy-2Cz, one bi-carbazole unit and one cyano group were covalently linked to the 2,2’-positions of one biphenyl motif. The molecular structure of Cy-2Cz is highly twisted due to the steric hindrance, which could efficiently break the π-conjugation

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within the molecule, thus leading to high triplet energy. The introduction of an additional carbazole unit into Cy-2Cz finished the synthesis of Cy-3Cz. The preparation

of

Sf-3Cz

electron-withdrawing

was

group

accomplished

by

biphenylcarbonitrile

the

replacement in

of

Cy-3Cz

the with

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phenylsulfonylbenzene. Due to the incorporation of big rigid π-system, Cy-3Cz and Sf-3Cz exhibited excellent thermal stability with the glass transition temperature over

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180oC. These three carbazole derivatives showed small singlet-triplet energy gaps (∆ESTs) in the range of 0.16-0.31 eV. Therefore, thermally activated delayed fluorescence

behavior

was

observed

in

these

compounds

with

their

photoluminescence quantum yields up to 97.2%. These blue TADF materials were applied in typical OLEDs as dopants and demonstrated high device efficiency. The blue OLED device based on Cy-2Cz at the doping level of 8 wt% achieved a maximum efficiency of 11.8 cd/A and external quantum efficiency (EQE) of 11.9% with the Commission Internationale de l'Eclairage (CIE) of (0.16, 0.10). An efficient blue fluorescent OLED based on Sf-3Cz was achieved with current efficiency of 19.2 cd/A and EQE of 15.8% with the CIE of (0.16, 0.14).

ACCEPTED MANUSCRIPT Introduction The last three decades have witnessed the tremendous progress in organic light emitting diodes (OLEDs), and recently OLED display demonstrates an increasing market share in the mobile market and flexible display market [1-4]. Material technology plays an important role in enabling the growth of the OLED display

organic

fluorescent

compounds

[5-10]

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market. The emissive materials have been developed rapidly from first-generation to

second-generation

heavy-metal-complex-based phosphorescent molecules [11-26] to third-generation thermally activated delayed fluorescent (TADF) materials [27-34]. TADF materials

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can efficiently harvest both singlet and triplet excitons by taking the advantage of reverse intersystem crossing (RISC) from the lowest triplet (T1) excited state to the lowest singlet (S1) excited state, which is triggered by thermal activation. The

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efficient RISC process generally requires small singlet-triplet splitting energy (typically less than 0.3 eV), which can be achieved by the spatial separation of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) [35-41]. However the negligible overlap between the HOMO and LUMO wave-functions always leads to the small S1→S0 (ground state) transition

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dipole moments, which generally results in the low photoluminescence quantum yield (PLQY) and poor device performance. So a subtle and elegant approach to control the wave-function distribution of frontier molecular orbits in TADF molecule is highly desirable. One traditional strategy is to use the highly twisted structures to break the

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conjugation between the electron-donor and acceptor parts, thus yielding the charge-transfer-type TADF compounds [42-45]. Recently, Brédas et al. reported a

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new strategy for the development of TADF materials from twisted to planar structures, where both small ∆EST and large S1→S0 transition dipole moment were achieved simultaneously via structural modulation [46].

Blue-emissive materials are of a particular interest as they are needed in the development of low-cost full-color OLED display and white OELD lighting. However, blue OLEDs showed poor efficiency relative to green and red devices. So far much effort has been devoted to the improvement of the efficiency of blue OLEDs [47-66]. Wu and coworkers reported sky-blue OLEDs with 37% external quantum efficiency (without any internal/external light-extraction enhancement structures) based on spiroacridine-triazine TADF material, which exhibited high PLQY around 100% and

ACCEPTED MANUSCRIPT high horizontal dipole ratio over 80% [67]. Although blue TADF emitters have received extensive research attention by now, yet the challenge still remains in developing efficient deep blue TADF materials with CIE y coordinate value less than 0.15 and an overall (x+y) less than 0.30 [68-70]. In this work, highly twisted structures are applied in Cy-2Cz and Cy-3Cz to separate the HOMO and LUMO

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efficiently. Furthermore, the intercalation of biphenyl between donor and acceptor could also reduce ∆EST due to the extended separation distance [71-72]. In addition, only one weak electron-withdrawing cyano group is grafted to the molecules of Cy-2Cz and Cy-3Cz, which could result in the wide bandgaps and blue emission.

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High-triplet-energy bi/tri-carbazole units are used as the donor part in these three TADF materials [73]. Within Sf-3Cz, sulfone, a typical acceptor for blue TADF emitters, was applied to study the effect of different acceptors on the photophysical

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properties of Cy-3Cz and Sf-3Cz. These three compounds demonstrated high thermal stability, high T1 and small ∆EST, and were applied as dopants in OLEDs. The blue device based on Cy-2Cz and Sf-3Cz demonstrated high efficiency with EQE over 10%.

General Information

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Experimental

All chemicals were purchased from J&K Scientific, Titan, Acros, and Strem chemicals. Anhydrous solvents were obtained from an Innovative Technology solvent

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purification system. 1H NMR spectra were recorded on a Bruker 400 MHz NMR instrument and

13

C NMR spectra were recorded on Agilent DD2-600 MHz NMR

instrument. Thermogravimetric analysis (TGA) was recorded on a TA SDT 2960

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instrument at a heating rate of 10 oC/min under nitrogen. Matrix-Assisted Laser Desorption/ Ionization Time of Flight Mass Spectrometry (MALDI-TOF-MS) was measured with a BRUKER ultrafleXtreme MALDI-TOF spectrometer. UV-vis absorption spectra were measured on Cary 60 spectrometer (Agilent Technologies). PL spectra and phosphorescent spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer. Cyclic voltammetry (CV) was carried out on a CHI600 voltammetric analyzer at room temperature with ferrocenium-ferrocene (Fc+/Fc) as the internal standard. A conventional three-electrode configuration consisting of a Pt-wire counter electrode, an Ag/AgCl reference electrode, and a platinum working electrode was used. The cyclic voltammograms were obtained at a

ACCEPTED MANUSCRIPT scan rate of 100 mV/s. Degassed DCM was used as solvent for oxidation scan with tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) as the supporting electrolyte.

Device Fabrication and Measurements

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The devices were fabricated on 80 nm-ITO coated glass with a sheet resistance of 15Ω per square. The ITO-coated glass was soaked in ultrasonic detergent for 30 min, followed by spraying with de-ionized water for 10 min, soaking in ultrasonic de-ionized water for 30 min, and oven-baking for 1 h. The cleaned samples were

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treated by oxygen plasma with a power of 100 W, gas flow of 50 sccm (standard cubic centimeter per minute), and pressure of 0.2 Torr for 10 s in the pretreatment chamber. The samples were transferred to the organic chamber with a base pressure of 7 × 10−7

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Torr for the deposition of NPB, emitter and TPBi, which served as hole-transport, light-emitting and electron-transport layers, respectively. Finally, the Al electrode was evaporated through a shadow mask without breaking the vacuum. The light emitting area was 4 mm2. The forward direction photons emitted from the devices were detected by a calibrated UDT PIN-25D silicon photodiode. The luminance and

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external quantum efficiencies of the devices were inferred from the photocurrent of the photodiode. The electroluminescence spectra were obtained by a PR650 spectrophotometer. All measurements were carried out under air at room temperature without device encapsulation.

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Synthesis of Cy-2Cz

9-(2'-Bromo-[1,1'-biphenyl]-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (1.6 g, 4.40 mmol), CuCN (0.47 g, 5.28 mmol) was added into DMF (30 mL) under N2. The

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reaction mixture was heated to 170 oC under microwave for 20 hours. After the removal of solvent, the residue was extracted with DCM (3×100 mL) and washed with water. The combined organic phase was dried over MgSO4. The crude product was purified by column chromatography (SiO2, DCM:PE, 1:2) to afford a white solid (0.92 g, 62.7%). 1H NMR (400 MHz, DMSO) δ 8.66 (s, 1H), 8.56 (s, 1H), 8.38 (d, J = 7.7 Hz, 1H), 8.24 (d, J = 7.6 Hz, 1H), 7.90 – 7.78 (m, 5H), 7.78 – 7.63 (m, 6H), 7.56 (t, J = 7.0 Hz, 1H), 7.49 – 7.38 (m, 3H), 7.39 – 7.29 (m, 2H), 7.24 (d, J = 4.1 Hz, 5H), 7.01 (d, J = 4.8 Hz, 1H).

13

C NMR (151 MHz, CDCl3) δ 142.10, 141.33, 140.00,

137.75, 135.43, 134.16, 133.19, 132.12, 130.58, 129.91, 128.94, 127.89, 127.04, 126.05, 125.71, 123.97, 123.22, 120.40, 120.00, 118.80, 118.50, 111.88, 109.90.

ACCEPTED MANUSCRIPT MALDI-TOF (m/z), calculated for: 585.221, found: 584.950. Anal. calcd for: C43H27N3 : C, 88.18, H, 4.65; N, 7.17; found: C, 88.16; H, 4.77; N, 7.25%. Synthesis of Cy-3Cz Cy-3Cz was prepared with the similar procedure as that for Cy-2Cz. (1.1 g,

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65.0%). 1H NMR (400 MHz, DMSO) δ 8.71 (d, J = 13.0 Hz, 4H), 8.39 (d, J = 7.7 Hz, 2H), 7.92 – 7.77 (m, 8H), 7.77 – 7.64 (m, 9H), 7.57 (t, J = 6.8 Hz, 2H), 7.45 (dt, J = 15.9, 8.2 Hz, 6H), 7.40 – 7.18 (m, 6H), 7.11 (d, J = 7.1 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ 141.32, 140.00, 137.75, 134.12, 132.15, 130.61, 129.89, 127.97, 127.42,

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127.04, 126.02, 125.70, 123.97, 123.56, 119.98, 118.79, 111.90, 110.00. MALDI-TOF (m/z), calculated for: 826.310, found: 826.247. Anal. calcd for:

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C61H38N4 : C, 88.59; H, 4.63; N, 6.77; found: C, 88.59; H, 4.85; N, 6.79%. Synthesis of Sf-3Cz

9,9''-Diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole (1.1 g, 1.69 mmol), t-BuONa (0.32 g, 3.38 mmol), 1-chloro-4-(phenylsulfonyl)benzene (0.51 g, 2.03 mmol), Pd2(dba)3 (0.062 g, 0.068 mmol) and s-Phos (0.11 g, 0.272 mmol) were mixed in

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toluene (100mL) under N2. The reaction mixture was heated to 110 oC for 2 days. After cooling down to the room temperature, the solvent was removed under reduced pressure. The crude product was purified by column chromatography (SiO2, DCM) to afford a pale yellow solid (1.2 g, 81.9 %). 1H NMR (400 MHz, DMSO) δ 8.88 (m,

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2H), 8.74 (m, 2H), 8.40 (d, J = 8.0 Hz, 2H), 8.30 (d, J = 8.6 Hz, 2H), 8.12 (d, J = 7.3 Hz, 2H), 8.05 (d, J = 8.5 Hz, 2H), 7.95 – 7.88 (m, 4H), 7.80 – 7.64 (m, 13H), 7.57 (t, 13

C NMR (151 MHz, CDCl3) δ

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2H), 7.53-7.40 (m, 6H), 7.34 (t, J = 7.3 Hz, 2H).

141.35, 140.13, 139.45, 135.46, 133.82, 129.90, 129.48, 127.84, 127.47, 127.05, 126.22, 125.71, 120.40, 120.00, 119.13, 118.88, 110.06, 109.90. MALDI-TOF (m/z), calculated for: 865.276, found: 865.259. MALDI-TOF (m/z), calculated for: 865.276, found: 865.259. Anal. calcd for: C60H39N3O2S : C, 83.21; H, 4.54; N, 4.85; found: C, 83.47; H, 4.85; N, 4.79%.

Results and discussion Preparation and characterization The synthetic routes of Cy-2Cz, Cy-3Cz and Sf-3Cz were shown in Scheme 1. The di/tri-carbazole

precursors

9-phenyl-3,3'-bi-9H-carbazole

and

ACCEPTED MANUSCRIPT 9,9''-diphenyl-3,3':6',3''-ter-9H-carbazole were prepared according to the literature [74-75]. The Ullmann reaction between the dibromide and di/tri-carbazole derivatives with CuI as catalyst followed by substitution reaction with CuCN gave compounds Cy-2Cz and Cy-3Cz, respectively. Sf-3Cz was prepared by Ullmann reaction between tri-carbazole and 1-chloro-4-(phenylsulfonyl)benzene with high yield. The 13

C NMR, MALDI-TOF mass

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three compounds were fully characterized by 1H and

spectrometry, and elemental analysis. All these compounds demonstrated good thermal properties with the decomposition temperature (corresponding to 5% weight loss) over 390 oC as determined by thermogravimetric analyses (TGA) and glass

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transition temperature over 130 oC as measured by differential scanning calorimetry (DSC) (Figure 1 and Figure 2). HOMOs of these compounds are estimated with cyclic voltammograms measurement by determining the onset oxidization potential (Figure

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S2). Since the compounds contain the similar electron-donating carbazole derivatives, their HOMOs are located in the narrow range from -5.25 to -5.29 eV. Their LUMOs (-1.92, -1.90 and -2.02 eV for Cy-2Cz, Cy-3Cz and Sf-3Cz, respectively) are calculated with the difference between HOMO and the optical bandgap.

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Photophysical properties

The absorption spectra, fluorescence spectra in toluene at room temperature (RT) and phosphorescence spectra in toluene at 77 K of Cy-2Cz, Cy-3Cz and Sf-3Cz were shown in Figure 3. Photophysical data of the compounds were given in Table 1.

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Cy-2Cz and Cy-3Cz adopted twisted conformation due to the steric repulsion, resulting in the limited conjugation between the acceptor and donor within these two

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compounds. So Cy-2Cz and Cy-3Cz demonstrated the similar absorption edges at around 370, identifying their bandgaps of about 3.35 eV. There is a red-shift of about 12 nm in the absorption of Sf-3Cz as compared to those of Cy-2Cz and Cy-3Cz due to the relatively large conjugation system in Sf-3Cz (vide infra). The singlet energy and triplet energy values of the compounds were estimated from the emission peaks in their fluorescence spectra and phosphorescence spectra, respectively. The ∆EST values were calculated to be 0.18, 0.31 and 0.16 eV for Cy-2Cz, Cy-3Cz and Sf-3Cz, respectively, which could be small enough to allow TADF emission. Considering that the singlet energy values of these compounds are all above 2.8 eV, they can be used as blue-emissive materials. Since there is a red-shift in the absorption spectrum of Sf-3Cz, it showed the expected red-shift in phosphorescence relative to Cy-2Cz and

ACCEPTED MANUSCRIPT Cy-3Cz. The structureless emission spectra indicated their lowest excited states have the charge-transfer character [76] which was further confirmed by their solvent polarity-dependent photoluminescent (PL) properties (Figure 4) [77]. Compared with Cy-2Cz and Cy-3Cz, Sf-3Cz showed a large red-shift in their PL spectra from low-polarity hexane to high-polarity dichloromethane (DCM), which could be due to

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its relatively large conjugation [78]. These three compounds are highly emissive in degassed DCM with photoluminescence quantum yields (PLQYs) of 56.7, 55.4, 97.2% for Cy-2Cz, Cy-3Cz and Sf-3Cz, respectively. Their transient PL decay profiles showed a delayed component with long lifetimes in the range of 9.36-11.4 µs (Figure

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S1).

DFT calculations

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In order to understand the photophysical properties of these TADF emitters, density functional theory (DFT) calculations were carried out. Figure 5 indicated that the HOMOs of these compounds were mainly distributed on the carbazole motifs and the LUMOs were mainly located on the biphenylcarbonitrile group (for Cy-2Cz and Cy-3Cz) and diphenylsulfone (for Sf-3Cz). The spatial separation of HOMO and

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LUMO suggested small singlet–triplet splitting energies of these compounds. The dihedral angle between the donor (the carbazole unit directly attached to the acceptor part) and the acceptor (the phenyl group directly attached to the donor part) in Cy-2Cz (69.49o) is comparable with that in Cy-3Cz (70.07o). So the introduction of

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an additional carbazole into Cy-3Cz didn’t change the connection pattern between donor and acceptor significantly. The disruption of conjugation via the introduction of

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twisted structure led to the large band gaps and high triplet energies of the hosts. On the other hand, in Sf-3Cz the dihedral angle between the donor and the acceptor (50.62 o) is much smaller than those in Cy-2Cz and Cy-3Cz. Thus the conjugation between donor and acceptor in Cy-2Cz and Cy-3Cz was limited as compared to that in Sf-3Cz. Furthermore, the orbital overlap between HOMO and LUMO levels in Sf-3Cz (0.28) is much larger than those in Cy-2Cz (0.10) and Cy-3Cz (0.13), which could account in part for the high PLQY of Sf-3Cz.

Electroluminescence properties In order to study the electroluminescent (EL) properties of Cy-2Cz, Cy-3Cz and Sf-3Cz, OLEDs based on these dopants were fabricated with a normal sandwiched

ACCEPTED MANUSCRIPT structure: ITO/HAT-CN (10 nm)/NPB (30 nm)/mCBP(10 nm)/DPEPO:dopant (8 wt%, 20 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (1.5 nm)/Al (100 nm); HAT-CN = 1, 4, 5, 8, 9, 11-hexaazatriphenylene-hexacarbonitrile, NPB = N, N′-bis(naphthalen-1-yl)-N, N′-bis(phenyl)benzidine, mCBP = 3,3-di(9H-carbazol-9-yl)biphenyl, DPEPO = bis-(2-(diphenylphosphino)phenyl)ether

oxide,

TPBi

=

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1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene, Liq = 8-hydroxyquinolinolato lithium. The EL spectra of these dopants were depicted in Figure 6. Compared with other two compounds, Cy-2Cz produced somewhat less emission in the long-wavelength region, which could result in its small CIE y coordinate value. So a narrow emission band

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was observed in the EL spectrum of Cy-2Cz relative to that of Sf-3Cz. Their EL spectra resembled the corresponding PL spectra, suggesting an effective exciton recombination and confinement in the emitting layer of these devices.

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The current efficiency (CE)- and external quantum efficiency (EQE)-luminance curves of these dopants-based devices are showed in Figures 7-9. The maximum device efficiency of 11.8 cd/A, 8.5 lm/w and 11.9% with CIE of (0.16, 0.10) was achieved for Cy-2Cz, and 19.2 cd/A, 13.7 lm/w and 15.8% with CIE of (0.16, 0.14) for Sf-3Cz, which were comparable with the best deep blue TADF emitters [79]. For

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example, Lee et al. reported deep blue OLEDs with quantum efficiency of 14.0% based on the interlocked molecular structure [80]. The high EQE observed in Sf-3Cz based device could be due to its PLQY. The EQEs of Cy-2Cz and Sf-3Cz are much higher than that of traditional fluorescence-type OLEDs, indicating the significant

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contribution of triplet excitons to the electroluminescence. In addition, the linear relationship between current density and luminescence (Figure S6) for these

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compounds indicated the TADF emission character rather than triplet–triplet annihilation (TTA) emission mode [81]. Cy-3Cz showed the relatively poor device performance with the maximum efficiency of 6.6 cd/A, 5.1 lm/w and 5.4% with CIE of (0.17, 0.14), which could be due to its relatively large ∆EST and difficult RISC process from T1 to S1. Sf-3Cz-based device suffered from large efficiency roll-off at high luminance, which could be due to unbalanced charge injection and transport at high current density [82]. On the other hand, the combination of electro-oxidation and photo-oxidation could be one of the important reasons for the rapid degradation of blue TADF emitters in their devices [83].

Conclusions

ACCEPTED MANUSCRIPT In summary, three blue TADF materials Cy-2Cz, Cy-3Cz and Sf-3Cz have designed and synthesized. These three compounds demonstrated small ∆ESTs, excellent thermal properties, and high PLQYs. The introduction of an additional carbazole unit into Cy-3Cz led to high thermal stabilities and broad EL emission band due to the conjugation

effect.

The

replacement

of

biphenylcarbonitrile

with

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phenylsulfonylbenzene resulted in large HOMO and LUMO overlap and high PLQY, which could be partially due to the extended conjugation between the donor and acceptor units. These compounds were applied as blue dopants in OLEDs, and demonstrated high device performance. Particularly, the blue OLED device based on

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Sf-3Cz showed a maximum device efficiency of 19.2 cd/A, 13.7 lm/w and 15.8%. The small ∆EST (0.16 eV) and high PLQY (97.2%) could be responsible for its high

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efficiency.

Acknowledgements

We acknowledge financial support from the National Key R&D Program of China (2016YFB0400703).

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References

[1] C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C. W. Tang, S. A. VanSlyke and C. H. Chen, J. Appl. Phys. 65 (1989) 3610. [3] J. Wang, C. Song, Z. Zhong, Z. Hu, S. Han, W. Xu, J. Peng, L. Ying, J. Wang and

EP

Y. Cao, J. Mater. Chem. C. 5 (2017) 5005.

[4] J.-H. Lee, B.-Y. Lin, Y.-C. Shih, K.-F. Lin, L. Wang, T.-L. Chiu, C.-F. Lin, Org.

AC C

Electron. 48 (2017) 330.

[5] S.-W. Wen, M.-T. Lee, and C. H. Chen, J. Display Technol. 1 (2005) 90. [6] S. Zhang, L. Yao, Q. Peng, W. Li, Y. Pan, R. Xiao, Y. Gao, C. Gu, Z. Wang, P. Lu, F. Li, S. Su, B. Yang, and Y. Ma, Adv. Funct. Mater. 25 (2015) 1755. [7] S. O. Kim, K. H. Lee, G. Y. Kim, J. H. Seo, Y. K. Kim and S. S. Yoon, Synth. Met., 160 (2010) 1259. [8] Y. Wei and C.-T. Chen J. Am. Chem. Soc. 129 (2007) 7478. [9] K. H. Lee, L. K. Kang, J. Y. Lee, S. Kang, S. O. Jeon, K. S. Yook, J. Y. Lee, and S. S. Yoon, Adv. Funct. Mater. 20 (2010) 1345. [10] G. L. Mao, Z. Wu, Q. He, B. Jiao, G. Xu, X. Hou, Z. Chen and Q. Gong, Appl. Surf. Sci. 257 (2011) 7394.

ACCEPTED MANUSCRIPT [11] Y. Ma, H. Zhang, J. Shen and C.-M. Che, Synth. Met. 94 (1998) 245. [12] M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature. 395 (1998) 151. [13] B. Wang, F. Liang, H. Hu, Y. Liu, Z. Kang, L.-S. Liao and J. Fan, J. Mater. Chem., C. 3 (2015) 8212.

RI PT

[14] G. Liu, F. Liang, Y. Zhao, H. Hu, J. Fan and L.-S. Liao, J. Mater. Chem. C. 5 (2017) 1944.

[15] J. Li, F. Liang, Y. Zhao, X.-Y. Liu, J. Fan and L.-S. Liao, J. Mater. Chem. C. 5 (2017) 6202.

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[16] W. Zhang, J. Ma, G.-J. Liu, X.-Y. Liu, J. Fan and L.-S. Liao, J. Mater. Chem. C. 5 (2017) 9496.

[17] X.-C. Zeng, J.-Y. Wang, L.-J. Xu, H.-M. Wen and Z.-N. Chen, J. Mater. Chem.

M AN U

C. 4 (2016) 6096.

[18] C.-L. Ho and W.-Y. Wong, Coord. Chem. Rev. 257 (2013) 1614. [19] D. Xia, B. Wang, B. Chen, S. Wang, B. Zhang, J. Ding, L. Wang, X. Jing and F. Wang, Angew. Chem., Int. Ed. 53 (2014) 1048.

[20] J. Lee, H.-F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich, M. E. Thompson

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and S. R. Forrest, Nat. Mater. 15 (2016) 92.

[21] C. Shi, H. Sun, X. Tang, W. Lv, H. Yan, Q. Zhao, J. Wang and W. Huang, Angew. Chem., Int. Ed. 52 (2013) 13434.

[22] D. Ma, L. Duan, Y. Wei, L. He, L. Wang and Y. Qiu, Chem. Commun. 50 (2014)

EP

530.

[23] Y.-C. Zhu, L. Zhou, H.-Y. Li, Q.-L. Xu, M.-Y. Teng, Y.-X. Zheng, J.-L. Zuo,

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H.-J. Zhang and X.-Z. You, Adv. Mater. 23 (2011) 4041. [24] C.-Y. Kuei, W.-L. Tsai, B. Tong, M. Jiao, W.-K. Lee, Y. Chi, C.-C. Wu, S.-H. Liu and G.-H. Lee, P-T. Chou, Adv. Mater. 28 (2016) 2795. [25] L. Ying, C.-L. Ho, H. Wu, Y. Cao and W.-Y. Wong, Adv. Mater. 26 (2014) 2459. [26] H. Xu, Q. Sun, Z. An, Y. Wei, X. Liu, Coord. Chem. Rev. 293–294 (2015) 228. [27] H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature. 492 (2012) 234. [28] L.-S. Cui, H. Nomura, Y. Geng, J. U. Kim, H. Nakanotani, and C. Adachi, Angew. Chem. 129 (2016) 1593. [29] T. Hosokai, H. Matsuzaki, H. Nakanotani, K. Tokumaru, T. Tsutsui, A. Furube, K. Nasu, H. Nomura, M. Yahiro and C. Adachi, Sci. Adv. 3 (2017) e1603282.

ACCEPTED MANUSCRIPT [30] D. Zhang, L. Duan, C. Li, Y. Li, H. Li, D. Zhang, and Y. Qiu, Adv. Mater. 26 (2014) 5050. [31] P. Chen, L.-P. Wang, W.-Y. Tan, Q.-M. Peng, S.-T. Zhang, X.-H. Zhu and F. Li, ACS Appl. Mater. Interfaces. 7 (2015) 2972. [32] D. Zhang, M. Cai, Y. Zhang, D. Zhang and L. Duan, Mater. Horiz. 3 (2016) 145.

RI PT

[33] S. Shao, J. Hu, X. Wang, L. Wang, X. Jing and F. Wang, J. Am. Chem. Soc. 139 (2017) 17739.

[34] K. Wang, C.-J. Zheng, W. Liu, K. Liang, Y.-Z. Shi, S.-L. Tao, C.-S. Lee, X.-M. Ou and X.-H. Zhang, Adv. Mater. (2017) 1701476.

SC

[35] Y. Yu, L. Ma, D. Wang, H. Zhou, B. Yao and Z. Wu, Org. Electron. 51 (2017) 173.

[36] L.-S. Cui, S.-B. Ruan, F. Bencheikh, R. Nagata, L. Zhang, K. Inada, H.

M AN U

Nakanotani, L.-S. Liao and C. Adachi, Nat. Commun. 8 (2017) 2250.

[37] D. Zhang, X. Cao, Q. Wu, M. Zhang, N. Sun, X. Zhang and Y. Tao, J. Mater. Chem., C. (2018) DOI: 10.1039/C7TC04969B.

[38] J. Zhang, D. Ding, Y. Wei and H. Xu, Chem. Sci. 7 (2016) 2870. [39] G. Xie, J. Luo, M. Huang, T. Chen, K. Wu, S. Gong and C. Yang, Adv. Mater.

TE D

(2017) 1604223.

[40] W. Zeng, H.-Y. Lai, W.-K. Lee, M. Jiao, Y.-J. Shiu, C. Zhong, S. Gong, T. Zhou, G. Xie, M. Sarma, K.-T. Wong, C.-C. Wu and C. Yang, Adv. Mater. (2017) 1704961. [41] L. Yu, Z. Wu, G. Xie, W. Zeng, D. Ma and C. Yang, Chem. Sci. (2017) DOI:

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10.1039/C7SC04669C.

[42] Y. Li, T. Chen, M. Huang, Y. Gu, S. Gong, G. Xie and C. Yang, J. Mater. Chem.

AC C

C. 5 (2017) 3480.

[43] J. Pei, X. Du, C. Li, C. Wang, C. Fan, H. Tan, B. Cao, F. Huang, S. Tao and J. Li, Org. Electron. 50 (2017) 153. [44] M. Liu, R. Komatsu, X. Cai, H. Sasabe, T. Kamata, K. Nakao, K. Liu, S.-J. Su and J. Kido, Adv. Optical Mater. (2017) 1700334. [45] X. Cao, D. Zhang, S. Zhang, Y. Tao and W. Huang, J. Mater. Chem. C. 5 (2017) 7699. [46] X.-K. Chen, Y. Tsuchiya, Y. Ishikawa, C. Zhong, C. Adachi and J.-L. Brédas, Adv. Mater. (2017) 1702767. [47] I. Lee and J. Y. Lee, Org. Electron. 29 (2016) 160. [48] J. W. Sun, J. Y. Baek, K.-H. Kim, C.-K. Moon, J.-H. Lee, S.-K. Kwon, Y.-H.

ACCEPTED MANUSCRIPT Kim and J.-J. Kim, Chem. Mater. 27 (2015) 6675. [49] C. Duan, J. Li, C. Han, D. Ding, H. Yang, Y. Wei, and H. Xu, Chem. Mater. 28 (2016) 5667. [50] M. Kim, S. K. Jeon, S.-H. Hwang, S.-S. Lee, E. Yu and J. Y. Lee, J. Phys. Chem. C. 120 (2016) 2485.

Cheng, ACS Appl. Mater. Interfaces. 8 (2016) 27026.

RI PT

[51] P. Rajamalli, N. Senthilkumar, P. Gandeepan, C.-C. R.-Wu, H.-W. Lin, and C.-H.

[52] Y. H. Lee, S. Park, J. Oh, J. W. Shin, J. Jung, S. Yoo and M. H. Lee, ACS Appl. Mater. Interfaces. 9 (2017) 24035.

Adachi, J. Mater. Chem. C. 2 (2014) 421.

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[53] S. Wu, M. Aonuma, Q. Zhang, S. Huang, T. Nakagawa, K. Kuwabara and C.

[54] Y. Kitamoto, T. Namikawa, D. Ikemizu, Y. Miyata, T. Suzuki, H. Kita, T. Sato

M AN U

and S. Oi, J. Mater. Chem. C. 3 (2015) 9122.

[55] M. Kim, S. K. Jeon, S.-H. Hwang, S.-s. Lee, E. Yu and J. Y. Lee, Chem. Commun. 52 (2016) 339.

[56] I. S. Park, J. Lee and T. Yasuda, J. Mater. Chem. C. 4 (2016) 7911. [57] P. Rajamalli, V. Thangaraji, N. Senthilkumar, C.-C. R.-Wu, H.-W. Lin and C.-H.

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Cheng, J. Mater. Chem. C. 5 (2017) 2919.

[58] Y. J. Cho, B. D. Chin, S. K. Jeon and J. Y. Lee, Adv. Funct. Mater. 25 (2015) 6786. [59] T. Hatakeyama, K. Shiren, K. Nakajima, S. Nomura, S. Nakatsuka, K. Kinoshita, J. Ni, Y. Ono and T. Ikuta, Adv. Mater. 28 (2016) 2777.

EP

[60] P. Rajamalli, N. Senthilkumar, P.-Y. Huang, C.-C. R.-Wu, H.-W. Lin, and C.-H. Cheng, J. Am. Chem. Soc. 139 (2017) 10948.

AC C

[61] M. Kim, S. K. Jeon, S.-H. Hwang and J. Y. Lee, Adv. Mater. 27 (2015) 2515. [62] D. R. Lee, M. Kim, S. K. Jeon, S.-H. Hwang, C. W. Lee and J. Y. Lee, Adv. Mater. 27 (2015) 5861.

[63] M. Y. Wong and E. Z.-Colman, Adv. Mater. 29 (2017) 1605444. [64] Y. J. Cho, K. S. Yook, J. Y. Lee, Sci. Rep. 5 (2015) 7859. [65] S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang and C. Adachi, Angew. Chem. 126 (2014) 6520. [66] Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka and C. Adachi, Nature Photon. 8 (2014) 326. [67] T.-A. Lin, T. Chatterjee, W.-L. Tsai, W.-K. Lee, M.-J. Wu, M. Jiao, K.-C. Pan, C.-L. Yi, C.-L. Chung, K.-T. Wong and C.-C. Wu, Adv. Mater. 28 (2016) 6976.

ACCEPTED MANUSCRIPT [68] X. Cai, B. Gao, X.-L. Li, Y. Cao and S.-J. Su, Adv. Funct. Mater. 26 (2016) 8042. [69] K. Bai, S. Wang, L. Zhao, J. Ding and L. Wang, Polym. Chem. 8 (2017) 2182. [70] B. Chen, J. Ding, L. Wang, X. Jing and F. Wang, Chem. Commun. 48 (2012) 8970. [71] Q. Zhang, H. Kuwabara, W. J. Potscavage Jr, S. Huang, Y. Hatae, T. Shibata, C.

RI PT

Adachi, J. Am. Chem. Soc. 136 (2014) 18070. [72] Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki and C. Adachi, J. Am. Chem. Soc. 134 (2012) 14706.

[73] W. Jiang, L. Duan, J. Qiao, G. Dong, D. Zhang, L. Wang and Y. Qiu, J. Mater.

SC

Chem. 21 (2011) 4918.

[74] C. Xia, Y. Wu, S. Layek, J. Fiordeliso, U.S. Pat. Appl. Publ. 20110260138, 2011. [75] T. Yamaki, K. Ikeda, H. Kawakami, PCT Int. Appl. 2015137472, 2015.

M AN U

[76] M. Numata, T. Yasuda and C. Adachi, Chem. Commun. 51 (2015) 9443. [77] J.-J. Huang, Y.-H. Hung, P.-L. Ting, Y.-N. Tsai, H.-J. Gao, T.-L. Chiu, J.-H. Lee, C.-L. Chen, P.-T. Chou, M.-k. Leung, Org. Lett. 18 (2016) 672. [78] H. Xu, P. Sun, K. Wang, J. Li, F. Wang, Y. Miao, H. Wang, B. Xu, W.-Y. Wong, J. Mater. Chem. C. 5 (2017) 4455.

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[79] N. Jürgensen, A. Kretzschmar, S. Höfle, J. Freudenberg, U. H. F. Bunz, and G. H.-Sosa, Chem. Mater. 29 (2017) 9154.

[80] Y. J. Cho, S. K. Jeon, S.-S. Lee, E. Yu and J. Y. Lee, Chem. Mater. 28 (2016) 5400.

EP

[81] C. Ganzoring and M. Fujihira, Appl. Phys. Lett. 81 (2002) 3137. [82] M. Liu, R. Komatsu, X. Cai, H. Sasabe, T. Kamata, K. Nakao, K. Liu, S.-J. Su,

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and J. Kido, Adv. Optical Mater. (2017), 1700334. [83] L.-S. Cui, Y.-L. Deng, D. P.-K. Tsang, Z.-Q. Jiang, Q. Zhang, L.-S. Liao, and C. Adachi, Adv. Mater. 28 (2016) 7620.

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Scheme 1 The synthetic route of Cy-2Cz, Cy-3Cz and Sf-3Cz

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o

540 C

0.8

520 C

o

430 C

0.6 0.4

Cy-2Cz Cy-3Cz Sf-3Cz

0.0 100

200

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0.2

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Weight Loss

o

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1.0

300

400

500

600

700

o

Temperature( C)

Fig. 1. The TGA curves of Cy-2Cz, Cy-3Cz and Sf-3Cz at a heating rate of 10 oC

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/min under N2.

Cy-2Cz Cy-3Cz Sf-3Cz

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188 °C

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Heat Flow

130 °C

100

183 °C

150 Temperature (°C)

200

Fig. 2. The DSC thermograms of Cy-2Cz, Cy-3Cz and Sf-3Cz at a heating rate of 10 o

C /min under N2.

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Fig. 3. The fluorescence spectra in toluene at room temperature (RT) and phosphorescence spectra in toluene at 77 K of Cy-2Cz (a), Cy-3Cz (b), Sf-3Cz (c)

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and the absorption spectra of these compounds in toluene at RT (d).

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0.4 0.2 0.0

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0.6 0.4 0.2 0.0

350

400

450

500

550

600

650

350

400

450

500

550

Wavelength (nm)

Wavelength (nm)

600

n-hex Tol THF DCM

0.8

Intensity (a.u.)

0.6

1.0 Sf-3Cz

n-hex Tol THF DCM

0.8

Intensity (a.u.)

Intensity (a.u.)

0.8

Cy-3Cz

1.0

n-hex Tol THF DCM

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Cy-2Cz

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0.6 0.4 0.2 0.0

650

350

400

450

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Fig. 4. The photoluminescence spectra of Cy-2Cz (left), Cy-3Cz (middle) and Sf-3Cz (right) in different solvents.

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Wavelength (nm)

600

650

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Fig. 5. Theoretically calculated spatial distributions and energies of the HOMO and

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LUMO levels of Cy-2Cz, Cy-3Cz and Sf-3Cz as well as their values of Eg.

ACCEPTED MANUSCRIPT Table 1. Physical properties of Cy-2Cz, Cy-3Cz and Sf-3Cz. Compounds

Tg

Td

S1

T1

∆EST HOMO LUMO

Eg

τ1

τ2

Φso

[µs]

[%]

[eV]

[eV]

[eV]

[eV]

[ns]

Cy-2Cz

130

430

2.91 2.73

0.18

-5.29

-1.92

3.37

2.35 9.36 56.7

Cy-3Cz

183

520

3.04 2.73

0.31

-5.25

-1.90

3.35

2.02 11.4 55.4

Sf-3Cz

188

540

2.88 2.72

0.16

-5.26

-2.02

3.24

1.18 9.62 97.2

Tg = glass-transition temperature;

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Td = temperature for 5% weight loss;

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[oC] [oC] [eV] [eV]

Eg = energy gap calculated from the onset absorption wavelength;

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HOMO estimated by cyclic voltammetry, LUMO deduced from Eg and HOMO; τ1 The prompt PL lifetimes in degassed CH2Cl2 at room temperature; τ2 The delayed PL lifetimes in degassed CH2Cl2 at room temperature;

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Φso Measured in degassed CH2Cl2 at room temperature.

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Cy-2Cz

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Cy-3Cz

EL intensity (a.u.)

Sf-3Cz

400

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-2 @10 mA cm

500

600

700

Wavelength (nm)

Fig. 6.The EL spectra of Cy-2Cz, Cy-3Cz and Sf-3Cz in 8% doping ratios at 10

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mA/cm2.

12 9 6 3 0 12 9 6 3 0

Cy-2Cz

10

100

1000

2

Luminance (cd/m )

7.

The

current

efficiency

(CE)-

and

10000

external

quantum

efficiency

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Fig.

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EQE (%)

CE (cd /A)

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(EQE)-luminance curves of Cy-2Cz at 8% doping ratios.

8

CE (cd /A)

6 4

Cy-3Cz

2

4 2 0

1

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EQE (%)

0 6

10

100

1000

2

Luminance (cd/m )

Fig. 8 The current efficiency (CE)- and external quantum efficiency (EQE)-luminance

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curves of Cy-3Cz at 8% doping ratios.

CE (cd /A)

15

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15

EQE (%)

20 10

5

0 20

Sf-3Cz

10

5 0

1

10

100 2

1000

Luminance (cd/m )

Fig. 9 The current efficiency (CE)- and external quantum efficiency (EQE)-luminance curves of Sf-3Cz at 8% doping ratios

ACCEPTED MANUSCRIPT Table 2. Electroluminescence characteristics of the devices. Device

Vona

CEb [cd/A]

PEb [lm/W]

EQEb [%]

CIEc (x, y)

(V) 3.7

11.8, 10.8

8.5, 6.2

11.9, 10.9

(0.16, 0.10)

Cy-3Cz

4.0

6.6, 4.6

5.1, 2.3

5.4, 3.7

(0.17, 0.14)

Sf-3Cz

4.4

19.2, 7.3

13.7, 3.2

15.8, 6.1

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Cy-2Cz

(0.16, 0.14)

a

Turn-on voltage (Von) at 1 cd/m2.

b

Current efficiency (CE), power efficiency (PE) or external quantum efficiency (EQE)

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Commission International de I’Eclairage coordinates measured at 5 mA/cm2.

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in the order of maximum, at 100 cd/m2.

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ISC

S1

Small ∆ES1-T1

DF

PF

Excitation

RISC

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N

N

N

O S

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EQE=15.8% CIE (0.16, 0.14)

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Three blue-emitting TADT materials based on bi/tri-carbazole derivatives have been

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prepared as efficient dopants for blue OLEDs.

ACCEPTED MANUSCRIPT Highlights Twisted structures led to high triplet energy and small singlet–triplet splitting energy.

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An efficient OLED was achieved with EQE of 15.8% and the CIE of (0.16, 0.14).

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The introduction of rigid π-system into dopants could improve their thermal properties.