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Enhancing Reverse Intersystem Crossing via Secondary Acceptors: towards Sky-Blue Fluorescent Diodes with Tenfold-Improved Exter-nal Quantum Efficiency Xuefeng Fan, Chenyu Li, Zicheng Wang, Ying Wei, Chunbo Duan, Chunmiao Han, and Hui Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18041 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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ACS Applied Materials & Interfaces

Xuefeng Fan,†,‡ Chenyu Li,†,‡ Zicheng Wang,† Ying Wei,† Chunbo Duan,† Chunmiao Han† and Hui Xu*,† †

Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education) & School of Chemistry and Material Science, Heilongjiang University, 74 Xuefu Road, Harbin 150080, P. R. China. KEYWORDS. Thermally Activated Delayed Fluorescence, Reverse Intersystem Crossing, Organic Light-Emitting Diode, Blue Emission, Excited State Transition, Phosphine Oxide Acceptor

ABSTRACT: How to simply but effectively facilitate reverse intersystem crossing (RISC) transition is always the key issue for developing high-performance thermally activated delayed fluorescence (TADF) dyes. In this work, as a proof of concept, a feasible strategy named “acceptor enhancement” is demonstrated with a series of ternary blue emitters (xCzmPOnTPTZ) using diphenylphosphine oxide (PO) as secondary acceptors. Compared with its PO-free binary analogue, such simple introduction of PO groups in pCzPO2TPTZ dramatically enhances its RISC rate constant (kRISC) by 10 times to the level of ~105 s-1, accompanied by RISC efficiency (RISC) of 92%, which further improves the triplet-to-singlet upconversion for effective triplet harvesting in its devices. As the result, on the basis of a trilayer device structure, pCzPO2TPTZ realized a state-of-the-art external quantum efficiency (EQE) beyond 20% with tenfold improvement.

Thermally activated delayed fluorescence (TADF) for lightemitting diode (OLED) applications is recently paid much attention, owing to its key advantage in triplet exciton harvesting through reverse intersystem crossing (RISC) from the first triplet (T1) to the first singlet (S1) excited states.1-7 In this sense, how to enhance RISC is always one of the most important issues for developing high-performance TADF emitters. It is believed that the small singlet-triplet splitting (ΔEST) can facilitate the T1-to-S1 upconversion, which is equal to two folds of exchange energy (J) and therefore proportional to the spatial overlap between wave functions of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs).8-10 Therefore, the most common approach is thoroughly separating HOMOs and LUMOs in virtue of the twisted donor-acceptor (D-A) configurations,1, 1121 however, at the cost of radiation suppression and emission quantum efficiency (QE) decrease, especially for blue TADF dyes with relatively weak intramolecular charge transfer (ICT) interactions7 (Figure 1a, I). To relieve this contradiction, a “donor extension” strategy is demonstrated on the basis of blue emitters through adopting dendritic donor groups to delocalize the HOMOs for ΔEST reduction and RISC facilitation, accompanied by the fast fluorescence decay for high photoluminescence quantum yields (PLQY, PL), but more or less complicating the material preparation (Figure 1a, II).4, 14, 22-28 Although the acceptor development for blue TADF dyes relatively lags behind,16, 29-32 it is rational that the ICT enhancement by secondary acceptors can achieve the exact effect on

significant RISC and PLQY improvement (Figure 1a, III). Nevertheless, the big challenge is still remained in seeking for the suitable acceptors with the appropriate electronwithdrawing effect and limited influence on conjugation and blue emission. With respect to this concern, phosphine oxide (P=O) groups seem competent with the features of (i) moderate electronwithdrawing ability for accurate ICT modulation;33-36 (ii) conjugation blocking as insulating linkage on the basis of continuous  bonds.37-39 Actually, the outstanding modulation ability of P=O groups on intramolecular electronic communications were successfully utilized to construct host materials for blue phosphorescence and TADF diodes with top-rank performances.40-49 More importantly, the appropriate electronwithdrawing effects of P=O groups for deep-blue TADF emissions were already demonstrated in the recent reports of Yasuda’s50 and our51 groups, establishing the basis of their potential as secondary acceptors in D-A-A type blue TADF dyes. In this contribution, the “acceptor enhancement” strategy is simply demonstrated with a series of blue TADF emitters, collectively named xCzmPOnTPTZ (x = o, m and p, corresponding to ortho-, meta- and para-substitution; m/n = 1 and 2), with ternary D-A-A structures containing carbazole (Cz), triphenyltriazine (TPTZ) and diphenylphosphine oxide (PO) as donor and primary and secondary acceptors, respectively (Figure 1b). Cz with moderate electron-donating ability as one of the most common donors is incorporated to further emphasize the enhancement effect of the secondary PO acceptors on TADF performance. The substitution position of Cz and PO groups is changed to accurately adjust the influence of

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secondary PO acceptor on ICT interactions. It is showed that at para position, PO groups can simultaneously increase the molecular polarities at ground and excited states, rendering the more uniform LUMO dispersion on the primary acceptor TPTZ. Consequently, in contrast to its binary analogues pCzTPTZ, the RISC rate constant (kRISC) of pCzPO2TPTZ is dramatically raised by 10 folds to the level of ~10 5 s-1, accompanied with the RISC efficiency (RISC) as high as 92%, which further render sixfold increased PL beyond 70% and tenfold improved electroluminescent (EL) external quantum efficiency (EQE) over 20% for pCzPO2TPTZ. This state-of-the-art performance successfully manifests the effectiveness of “acceptor enhancement” strategy in RISC facilitation, pointing out a new direction for molecular design of high-performance TADF dyes.

300 oC for feasible device fabrication through vacuum evaporation. Density functional theory (DFT) calculation of xCzmPOnTPTZ and their PO-free analogues xCzmTPTZ was first performed to figure out the secondary acceptor effect on electron cloud distributions and intramolecular electronic communications (Figures 2 and S2). It is showed that the steric hindrances of PO groups at meta- and para- positions are almost negligible; while, PO groups in oCzmPOnTPTZ further distort TPTZ, which remarkably restrains the influence of PO groups in intramolecular electronic interactions. In comparison to xCzmTPTZ, the HOMO and LUMO energy levels of xCzmPOnTPTZ are simultaneously decreased due to the electronwithdrawing effect of PO groups, resulting in the limited reduction of the HOMO-LUMO gaps, which are in accord with the results of cyclic voltammetric analysis (Figures S3). Nevertheless, except for oCz2POTPTZ, the predominance of PO groups in electron withdrawing renders the bigger variation of the LUMO energy levels, which is still limited within 0.2 eV owing to the desired appropriate electron-withdrawing ability of PO groups for accurate ICT modulation. It is interesting that mCzmTPTZ with the deepest LUMOs and the smallest HOMO-LUMO gaps reveal the strongest D-A interaction among xCzmTPTZ. By contrast, the differences between mCz2POTPTZ and pCz2POTPTZ are obviously decreased; furthermore, the HOMO and LUMO energy levels of mCzPO2TPTZ and pCzPO2TPTZ with the difference ≤ 0.01 eV are almost equivalent, owing to the dominant and accumulated inductive effects of para-PO groups. This effect is not only in direct proportion to the number of PO groups, but also gives rise to the more uniform LUMO dispersion on TPTZ, especially for pCzmPOnTPTZ, whose LUMOs are remarkably delocalized to the phenyl in TPTZ bonded with P atoms, which undoubtedly increase the HOMO-LUMO separation. In addition, the electrochemical stability of pCzPO2TPTZ is relatively improved in contrast to pCzTPTZ, revealing the negligible influence of PO substitution on operation duration under field (Figures S4).

Figure 1. (a) RISC enhancement strategies through facilitating intramolecular charge transfer (ICT) effects: I. Distorted connection between donor (D) and acceptor (A) of binary D-A systems to suppress the overlap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO); II. HOMO location extension for HOMO-LUMO separation; III. Secondary acceptor strategy based on ternary D-A-A systems with effects on RISC enhancement; (b) Molecular design of xCzmPOnTPTZ (x = o, m and p, corresponding to ortho-, metaand para-substitution positions; m/n = 1 and 2), in which carbazole (Cz) is donor and triphenyltriazine (TPTZ) and PO are the primary and secondary acceptors; (c) Chemical structures of xCzmPOnTPTZ.

Material Design and Synthesis. xCzmPOnTPTZ can be easily synthesized through two-step coupling reactions with moderate total yields about 30%. Two para-substituted binary analogues pCzmTPTZ (m = 1 and 2) are also prepared for comparison. It is noticed that the incorporation of PO groups improves the thermostability of xCzmPOnTPTZ, especially for pCzmPOnTPTZ, whose temperatures of decomposition (Td) are about 200 oC higher than those of pCzmTPTZ (Figure S1 and Table S1). Nevertheless, Tds of all materials are beyond

Figure 2. Optimized configurations, contours and energy levels of the HOMOs and LUMOs of xCzmPOnTPTZ.


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ACS Applied Materials & Interfaces The singlet and triplet transition characteristics of xCzmPOnTPTZ and xCzmTPTZ were further compared through time-dependent DFT (TDDFT) method on the basis of natural transition orbital (NTO) analysis (Figures 3, S5 and S6). It is showed that the S1 energies (Es) of xCzmPOnTPTZ are in the range of 2.75~2.91 eV, corresponding to blue emissions, with the decrease less than 0.15 eV in comparison to those of xCzmTPTZ, which manifests the superiority of PO groups as secondary acceptors in emission color preservation. For both S0→S1 and S0→T1 excitations, “hole” of these compounds is thoroughly localized on Cz groups and Cz-bonded phenyls in TPTZ groups, accompanied with slight contributions from triazine rings in para-substituted derivatives, which is identical to the DFT-simulated HOMO distributions. On contrary, the situation of “particle” distributions is diverse that mCzmPOnTPTZ inherit the triazine and Cz-bonded phenyl localized “particle” from mCzmTPTZ, rendering the unchanged contributions of CT and localized excited (LE) components to their S1 and T1 states, as well as the similar ΔEST of ~0.15 eV and oscillator strength of singlet excitation (f) less than 0.004; whereas, the “particle” of oCzmPOnTPTZ is remarkably delocalized to the phenyl of TPTZ linked with P atoms, giving rise to their CT-predominant S1 and T1 states and the remarkably decreased ΔEST and f as the combined results of their enhanced ICT and configuration distortion. It is noteworthy that f of oCzmTPTZ is twice of that of mCzmTPTZ, but the situation is just opposite after PO-substitution, indicating the excessive sacrifice of radiation transition probability due to the redundant incorporation of steric effect from PO groups. Inspiringly, accompanied by the significant “particle” delocalization to the P-bonded phenyls of TPTZ, the CT contributions to the S 1 states of pCzmPOnTPTZ are dramatically increased at the same time of preserving the effective overlaps on Cz-linked phenyls of TPTZ for efficient radiation transitions with considerably high f of 0.30 and 0.46 (Figures 3). As the result, in comparison to pCzmTPTZ, CT characteristics of the excited states for pCzmPOnTPTZ are enhanced without weakening the radiation performance, which indicates the potential of pCzmPOnTPTZ as high-efficiency TADF emitters.

S8-S10). Estimated with Lippert-Mataga relationship, the excited-state dipole moment (e) of pCzmPOnTPTZ is beyond 9 Debye as the highest values among xCzmPOnTPTZ, which is more than 3 Debye larger than those of pCzmTPTZ, displaying the effect of PO groups on ICT enhancement (Table S1).

Photophysical Properties. The secondary acceptor effect on transition characteristics of xCzmPOnTPTZ is experimentally investigated with optical analysis. The electronic absorption spectra of xCzmPOnTPTZ and pCzmTPTZ in dilute solution (10-5 M in CHCl3) contain the similar bands at 270 and 240 nm, corresponding to →* transitions of TPTZ and phenyl moieties, respectively (Figure 4a and Figure S7). However, different with their congeners, the dual-peak bands at 340 and 330 nm characteristic of n→* transition from Cz groups are excluded in the spectra of oCzmPOnTPTZ, indicating their forbidden Cz-involved transitions. Furthermore, only pCzmPOnTPTZ show the distinct CT-featured absorption bands around 375 nm with red shifts of ~10 nm in comparison to those of pCzmTPTZ, which should be owing to the high transition probabilities of their CT states and in accord with the high values of their TDDFT estimated f. The absence of CTfeature absorptions for oCzmPOnTPTZ and mCzmPOnTPTZ originate from the near-zero J of the former and the minor contribution of CT components to the S1 states of the latter, respectively. Nevertheless, similar to pCzmTPTZ, xCzmPOnTPTZ reveal the typical solvatochromic behaviour and the linear dependence between solvent orientation polarity and Stokes shift, revealing their CT-featured emissions (Figures

Figure 3. Natural transition orbital (NTO) analysis on S0→S1 transitions of pCzmPOnTPTZ and pCzmTPTZ. ES,  and f refer to the S1 energy, associated weight and oscillator strength, respectively. NTO results of other analogues are shown in Figure S5 and S6.

Photoluminescence (PL) spectra of the vacuum-evaporated xCzmPOnTPTZ films in bis[2(diphenylphosphino)phenyl]ether oxide (DPEPO) matrix are peaked in the range of 470-485 nm, corresponding to sky-blue emissions, with red shifts of ~30 nm in comparison with pCzmTPTZ, due to PO-enhanced ICT (Figure 4a). The transient emission spectra of xCzmPOnTPTZ doped consist of one ns-scaled and one s-scaled components, corresponding to prompt and delayed fluorescence (PF and DF), respectively (Table S1). The PF lifetimes (τPF) of mCzmPOnTPTZ and pCzmPOnTPTZ with planar configurations are comparable with those of pCzmTPTZ, and shorter than those of twisted oCzmPOnTPTZ. However, the DF lifetimes (τDF) of xCzmPOnTPTZ are remarkably longer than those of pCzmTPTZ, revealing the improved DF contribution by PO substitution. Their TADF characteristics are further confirmed with the distinct temperature dependence of their transient emissions (Figures S11). Significantly, in contrast to pCzmTPTZ, PL of xCzmPOnTPTZ are dramatically increased (Table S1). Espe-



cially for pCzPO2TPTZ, its PL is up to 74% as seven folds of that of PO-free pCzTPTZ (11%). To clarify the nature of secondary acceptor effect on emission characteristics, the indicators of key TADF processes, namely the rate constants (k) and efficiencies () of PF, DF and RISC, are quantitatively estimated on the basis of PL and transient emissions (see supporting information) (Figure 4b and Table S1).5, 52 PL is roughly in direct proportion to these parameters. Nevertheless, kDF and DF of xCzmPOnTPTZ are considerably larger than those of pCzmTPTZ. Especially for pCzPO2TPTZ, its PF and DF are 3 and 8 times larger than those of pCzTPTZ, respectively, revealing the dominant effect of PO groups on DF enhancement. The fundamental cause should be the dramatically facilitated RISC transitions of xCzmPOnTPTZ, as evidenced by their multiplied kRISC and RISC. In contrast to pCzTPTZ, kRISC of pCzPO2TPTZ is even improved by one order of magnitude, accompanied by nearly unitary RISC (92%), which undoubtedly can support the extremely effective triplet-to-singlet conversion.

Figure 5. (a) Structure and energy level diagram of sky-blue-emitting diodes with barrier-free carrier injection and their photographs at 6 V; (b) EL spectra (insets) and luminance-current density (J)-voltage curves of the devices; (c) efficiencies and luminance relationships of the devices.

Figure 4. (a) Electronic absorption (hollow) of xCzmPOnTPTZ and pCzmTPTZ in chloroform (10-6 M) and room-temperature emission (solid) and time-resolved phosphorescence (line, at 77 K after 300 s) spectra and time decay curves of their vacuum-evaporated DPEPO-hosted films (10 wt%, 100 nm); (b) Relationship between photoluminescence quantum yield (PL) and constants (k) and efficiencies () of the transitions in TADF processes for xCzmPOnTPTZ and pCzmTPTZ, which are linear fitted. The subscripts of PF, DF and RISC refer to prompt and delayed fluorescence and reverse intersystem crossing, respectively.

OLED Performance. The effectiveness of PO groups on EL performance was finally evaluated through fabricating blue-emitting diodes of ITO |MoO 3 (6 nm)|mCP (80 nm)|DPEPO: xCzmPOnTPTZ (80 wt%, 20 nm)|pTPOTPZ (40 nm)|LiF (1 nm)|Al, in which mCP and pTPOTZ53 are 1,3bis(N-carbazolyl)benzene and 2,4,6-tris(4(diphenylphosphoryl) phenyl)-1,3,5-triazine for hole and electron transportation, respectively, with the excellent energy level matching for barrier-free charge injection (Figure 5a). The control devices employing pCzmTPTZ were also fabricated for comparison (Figures S11). All the devices showed the sky-blue emissions with peaks of 472-484 nm (inset of Figure 5b and Table 1). Furthermore, the angle-correlated EL emission intensity curves of pCzPO2TPTZ and pCzTPTZbased devices manifested their nearly equivalent superLambertain characteristics, indicating the negligible influence of PO groups on dipole orientation (Figure S13). Except for oCz2POTPTZ, the operation voltages of xCzmPOnTPTZbased devices were decreased, owing to their


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ACS Applied Materials & Interfaces Table 1. EL performance of of sky-blue OLEDs using xCzmPOnTPTZ and pCzmTPTZ as Emitters. ηc

Emitter

Va (V)

Lmaxb (cd m-2)

ηCE (cd A )

ηPE (lm W )

ηEQE (%)

oCzPO2TPTZ

3.3, 5.3, 9.3

2609

13.3, 11.4, 4.5

12.7, 6.8, 1.5

6.7, 5.7, 2.3

(0.17, 0.27)

mCzPO2TPTZ

3.0, 5.0, 8.0

3138

17.6, 11.0, 4.5

18.4, 6.9, 1.8

11.6, 7.2, 3.0

(0.16, 0.20)

pCzPO2TPTZ

2.9, 4.4,6.4

8286

40.4, 31.6, 19.8

43.7, 22.6, 9.7

20.9, 16.4, 10.2

(0.15, 0.27)

oCz2POTPTZ

5.3, -, -

64

0.8, -, -

0.5, -, -

0.5, -, -

(0.17, 0.21)

mCz2POTPTZ

3.4, 5.9, 9.4

2914

10.3, 6.8, 3.3

9.6, 3.6, 1.1

5.1, 3.4, 1.6

(0.16, 0.29)

pCz2POTPTZ

3.6, 5.6, 8.1

9975

22.3, 17.3, 11.1

19.5, 9.7, 4.3

11.2, 8.7, 5.6

(0.15, 0.28)

pCzTPTZ

3.9, 6.8,12.0

1561

4.6, 3.3, 0.9

3.3, 1.5, 0.2

2.0, 1.4, 0.4

(0.15, 0.23)

pCz2TPTZ

3.9, 6.7, 9.0

6953

3.8, 3.6, 3.1

2.8, 1.6, 1.1

2.2, 2.1, 1.8

(0.16, 0.23)

a

-2 b

-1

CIE -1

c

-2

At 1, 100 and 1000 cd m ; the maximum luminance; EL efficiencies at the maximum, 100 and 1000 cd m .

enhanced ambipolar characteristics (Figure 5b). The highly distorted configuration of oCz2POTPTZ restrains the carrier injection and transportation in the emissive layer (EML), which further induced the lowest current density (J) at the same driving voltages.40, 53 pCzPO2TPTZ endowed its devices with the lowest driving voltages of 2.9 V for turn on and 4.4 and 6.4 V at 100 and 1000 cd m-2, which were 1~5 V lower than those of pCzTPTZ-based analogues. The substantial reduction of driving voltage can also be partially attributed to the improved radiation transition of pCzPO2TPTZ,54 which was further validated by the maximum luminance of its devices beyond 8000 cd m-2 as 5 folds of that of pCzTPTZ-based analogues. Significantly, in contrast to pCzmTPTZ, the EL efficiencies of xCzmPOnTPTZ were dramatically improved and directly corresponded to their RISC efficacies, reflecting the crucial role of RISC process as the single triplet-exciton harvesting channel. As the result, pCzPO2TPTZ successfully provided its devices with the state-of-the-art efficiencies with the maxima of 40.4 cd A-1 for current efficiency (CE), 43.7 lm W-1 for power efficiency (PE) and 20.9% for EQE, which were the best results achieved by trilayer-structured sky-blue TADF diodes to date (Figure 5c). It is interesting that the maximum EL efficiencies of pCzPO2TPTZ are about 10 folds of that of pCzTPTZ, which is exactly equivalent to the ratio of their kRISC. Furthermore, the maximum luminance of pCz2TPTZbased devices were larger than that of pCzTPTZ-based analogues, but their efficiencies were comparable, revealing the effect of increasing donor groups on enhancing radiation rather than RISC transition. On the contrary, accompanied with the comparable maximum luminance, EL efficiencies of xCzPO2TPTZ were about twice of those of the corresponding xCz2POTPTZ, manifesting a direct correlation between PO number and triplet exciton harvesting efficiency. In this sense, PO substitution can indeed drastically improve exciton utilization through RISC facilitation, standing as a proof of concept for how effective “acceptor enhancement” strategy is for constructing TADF emitters. Despite the remarkable roll-offs due to the high exciton density worsened concentration quenching, the efficiencies of pCzmPOnTPTZ at the practical luminance were still higher than those of pCzmTPTZ-based analogues, which is in accord with the situation of the TPTZ-type emitters featured “donor extension”.4

A series of ternary D-A-A type sky-blue TADF emitters xCzmPOnTPTZ are developed through incorporating PO groups as the secondary acceptors to demonstrate the effectiveness of “acceptor enhancement” strategy in highperformance TADF emitter development. In comparison to its PO-free congener pCzTPTZ, by virtue of the appropriate electron withdrawing effect from PO groups, kRISC (> 105 s-1) of pCzPO2TPTZ with the most strengthened ICT interactions is dramatically improved by 10 times without sacrificing radiation probability, which manifests the secondary acceptor effect on RISC enhancement and radiation preservation, leading to the state-of-the-art EQE beyond 20% of its trilayer devices. Since Cz is one of the most common donors, this high performance realized by such simple system not only demonstrates the superiority of “acceptor enhancement” strategy in TADF facilitation, but also leaves a huge space for further molecular design and performance improvement.

Materials and Synthesis. The detailed information of Ullmann coupling reaction and the characteristics of the intermediates xCzmTPTZBrn were described in supporting information. The materials for OLED fabrication were purchased from Xi’an Polymer Light Technology Corp. General Procedure of Pd2+-Catalyzed Phosphorylation Coupling Reaction. The mixture of xCzmTPTZBrn (1 mmol), diphenylphosphine (2n mmol), palladium acetate (0.02n mmol) and sodium acetate trihydrate (2n mmol) were dissolved in DMF (5 mL) and stirred at 130 oC for 24 h. After cooling to room temperature, water (10 mL) was added to quench the reaction. Then, the mixture was extracted with dichloromethane (3 × 10 mL). The organic phase was concenrtated to 10 mL and then added with 30% H2O2 (3.1 mL, 24 mmol) at 0 °C and stirred for 4 h. The system was extracted again with dichloromethane (3 × 10 mL). The organic phase was combined and dried with anhydrous Na2SO4. The solvent was then removed in vacuo. The crude product was purified by flash column chromatography to afford the title compounds as yellow solids. ((6-(2-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine-2,4diyl)bis(2,1-phenylene))bis(diphenylphosphine



oxide)(oCzPO2TPTZ): 0.35 g with a yield of 41%. 1H NMR (CDCl3, 400 MHz): δ = 8.018-7.980 (m, 5H), 7.685-7.648 (t, J= 7.2 Hz, 3H), 7.574-7.526 (dd, J1 = 12 Hz, J2 = 7.2 Hz, 16H), 7.460-7.423 (t, J = 7.6 Hz, 2H), 7.391-7.354 (t, J = 7.2 Hz, 6H), 7.282-7.238 ppm (m, 8H); LDI-TOF: m/z (%) 874 (100) [M+]; Elemental analysis for C57H40N4O2P2: C 78.25, H 4.61, N 6.40, found: C 78.28, H 4.62, N 6.45. ((6-(3-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine-2,4diyl)bis(3,1-phenylene))bis(diphenylphosphine oxide) (mCzPO2TPTZ): 0.36 g with a yield of 41%. 1H NMR(TMS, CDCl3, 400 MHz): δ = 8.879-8.818 (m, 4H), 8.710-8.684 (m, 1H), 8.590 (s, 1H), 8.239 (d, J = 7.6 Hz, 2H), 8.021-7.973 (dd, J1 = 11.6 Hz, J2 = 7.6 Hz, 2H), 7.773-7.760 (m, 2H), 7.7057.643(m, 10H), 7.419-7.345 ppm (m, 18H); LDI-TOF: m/z (%) 874 (100) [M+]; Elemental analysis for C57H40N4O2P2: C 78.25, H 4.61, N 6.40, found C 78.27, H 4.64, N 6.43. ((6-(4-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine-2,4diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide) (pCzPO2TPTZ): 0.61 g with a yield of 70%. 1H NMR (CDCl3, 400 MHz): δ = 8.991 (d, J = 8.4 Hz, 2H), 8.884-8.856 (dd, J1 = 8.0 Hz, J2 = 2.0 Hz, 4H), 8.170 (d, J = 8.0 Hz, 2H), 7.9497.899 (dd, J1 = 11.2 Hz, J2 = 8.0 Hz, 4H), 7.828 (d, J = 8.4 Hz, 2H), 7.758-7.711 (dd, J1 = 12.0, J2 = 7.2 Hz, 8H), 7.610-7.484 (m, 16H), 7.464-7.426 ppm (t, J = 8.0 Hz, 2H); LDI-TOF: m/z (%) 874 (100) [M+]; Elemental analysis for C57H40N4O2P2: C 78.25, H 4.61, N 6.40, found C 78.28, H 4.61, N 6.44. (2-(4,6-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,5-triazin-2yl)phenyl)diphenylphosphine oxide (oCz2POTPTZ): 0.39 g with a yield of 47%. 1H NMR (400 MHz, CDCl3): δ = 8.055 (d, J = 8.0 Hz, 2H), 8.006-7.977 (dd, J1 = 7.6 Hz, J2 = 4.0 Hz, 2H), 7.678-7.641 (t, J = 7.2 Hz, 3H), 7.608-7.513 (m, 14H), 7.4637.426 (t, J = 7.6 Hz, 2H), 7.380-7.340 (td, J1 = 7.2 Hz, J2 = 1.2 Hz, 5H), 7.303-7.228 ppm (m, 10H); LDI-TOF: m/z (%) 839 (100) [M+]. Elemental analysis for C57H38N5OP: C 81.51, H 4.56, N 8.34, found C 81.54, H 4.58, N 8.38. (3-(4,6-bis(3-(9H-carbazol-9-yl)phenyl)-1,3,5-triazin-2yl)phenyl)diphenylphosphine oxide (mCz2POTPTZ): 0.43 g with a yield of 51%. 1H NMR (CDCl3, 400 MHz): δ = 8.8868.864 (dd, J1 = 7.6 Hz, J2 = 1.2 Hz, 1H), 8.794-8.727 (m, 5H), 8.201-8.182 (d, J = 7.6 Hz, 4H), 8.090-8.043 (dd, J1 = 11.2 Hz, J2 = 7.6 Hz, 1H)，7.786-7.770 (m, 4H), 7.701-7.591 (m, 5H), 7.427-7.370 (dd, J1 = 14.8 Hz, J2 = 8.0 Hz, 8H), 7.335-7.296 (td, J1 = 7.6 Hz, J2 = 1.6 Hz, 4H), 7.207-7.149 ppm (m, 6 H); LDI-TOF: m/z (%) 839 (100) [M+]. Elemental analysis for C57H38N5OP: C 81.51, H 4.56, N 8.34, found C 81.55, H 4.57, N 8.37. (4-(4,6-bis(4-(9H-carbazol-9-yl)phenyl)-1,3,5-triazin-2yl)phenyl)diphenylphosphine oxide (pCz2POTPTZ): 0.60 g with a yield of 72%. 1H NMR (CDCl3, 400 MHz): δ = 9.047 (d, J = 8.4 Hz, 4H), 8.939-8.914 (dd, J1 = 8.0 Hz, J2 = 2.0 Hz, 2H), 8.182-8.163 (d, J = 7.6 Hz, 4H), 7.979-7.930 (dd, J1 = 11.6 Hz, J2 = 8.4 Hz, 2H), 7.855 (d, J = 8.4 Hz, 4H),7.778-7.730 (dd, J1 = 12.0 Hz, J2 = 7.2 Hz, 4H), 7.586-7.441 (m, 14H), 7.3557.318 ppm (t, J = 7.6 Hz, 4H) ); LDI-TOF: m/z (%) 839 (100) [M+]. Elemental analysis for C57H38N5OP: C 81.51, H 4.56, N 8.34, found C 81.53, H 4.56, N 8.37. Device Fabrication and Testing. Before loading into a deposition chamber, the ITO substrate was cleaned with detergents and deionized water, dried in an oven at 120 °C for 4 h, and treated with oxygen plasma for 3 min. Devices were fabri-

cated by evaporating organic layers at a rate of 0.1-0.2 nm s-1 onto the ITO substrate sequentially at a pressure below 4×10-4 Pa. Onto the electron-transporting layer, a layer of LiF with 1 nm thickness was deposited at a rate of 0.1 nm s-1 to improve electron injection. Finally, a 100-nm-thick layer of Al was deposited at a rate of 0.6 nm s-1 as the cathode. The emission area of the devices was 0.09 cm2 as determined by the overlap area of the anode and the cathode. After fabrication, the devices were immediately transferred to a glove box for encapsulation with glass cover slips using epoxy glue. The EL spectra and CIE coordinates were measured using a PR655 spectra colorimeter. The current-density-voltage and brightness– voltage curves of the devices were measured using a Keithley 4200 source meter and a calibrated silicon photodiode. All the measurements were carried out at room temperature under ambient conditions. For each structure, four devices were fabricated in parallel to confirm the performance repeatability. To make conclusions reliable, the data reported herein were most close to the average results.

Supporting Information. Experimental details, thermal properties, DFT and TDDFT calculation results, electrochemical properties, absorption spectra, solvatochromic effect, transient emission spectra in different temperatures and angular EL emission intensity curves. This material is available free of charge via the Internet at http://pubs.acs.org.

* [email protected] ‡These authors contributed equally. The authors declare no competing financial interest.

This study was supported by Changjiang Scholar Program of Chinese Ministry of Education (Q2016208), NSFC (21672056, 61605042, 21602048 and 51873056), Science and Technology Bureau of Heilongjiang Province (QC2016072), Harbin Science and Technology Bureau (2015RAYXJ008) and National Postdoctoral Program for Innovative Talents (BX201600048).

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