Pyridine-CarbonitrileâCarbazole-Based Delayed Fluorescence

Organic Electronic Devices

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Carbonitrile-Pyridine-Carbazole-Based Delayed Fluorescence Materials with Highly Congested Structures and Excellent OLED Performance Jayachandran Jayakumar, Tien-Lin Wu, Min-Jie Huang, PeiYun Huang, Tsu-Yu Chou, Hao-Wu Lin, and Chien-Hong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04664 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Materials & Interfaces

Carbonitrile-Pyridine-Carbazole-Based Delayed Fluorescence Materials with Highly Congested Structures and Excellent OLED Performance Jayachandran Jayakumar,† Tien-Lin Wu,† Min-Jie Huang,† Pei-Yun Huang,† Tsu-Yu Chou,‡ Hao-Wu Lin,‡ and Chien-Hong Cheng*† Departments of †Chemistry and ‡Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan.

Supporting Information Placeholder Key words; pyridine-carbonitrile, TADF, dihedral-angle, carbazole, EQE nearly 30%, sky-blue, OLEDs ABSTRACT: Three carbonitrile-pyridine-carbazole-based thermally activated delayed fluorescence (TADF) materials with highly sterically congested structures have been synthesized. The donor-acceptor type TADF emitters, (26-, 246-, and 35tCzPPC) consist of a 2,6diphenylpyridine-3,5-dicarbonitrile core (PPC) as the acceptor and di(t-butyl)carbazole-substituted phenyl group attached to C4 of the PC core as the donor. The molecules show a unique structure containing two consecutive large twisted angles along the donor and acceptor groups. The structure leads to a nearly complete space separation of the HOMO and LUMO, a small ΔEST and excellent TADF property. Moreover, the 26-, and 246tCzPPC dopants favor horizontal alignment enhancing the device light outcoupling. In contrast, 35tCzPPC favors perpendicular alignment reducing the device light outcoupling efficiency. The 246tCzPPC-based devices show external quantum efficiency (EQE) as high as 29.6%, due to excellent TADF property, very high photoluminescence quantum yield, and high Θ value in the thin films. The device performance is the best among the pyridine-carbonitrile-based TADF emitters.

Introduction Recently, metal-free thermally activated delayed fluorescence (TADF) emitters have attracted great attention and have been developed as an inexpensive alternative for PhOLEDs.1-11 The advancement in this field has shown that TADF materials and devices can harvest both singlet and triplet energy to reach 100% internal quantum efficiency similar to the performance of phosphorescence materials.12-20 In general, an efficient TADF emitter requires a fast reverse intersystem crossing (RISC), and a small energy gap difference between the singlet and triplet (ΔEST) and separated HOMO and LUMO distribution in the molecule. So far, it is common to construct TADF emitters using D-A (DonorAcceptor) or D-A-D type design.16,21 Based on this strategy, a limited number of highly efficient blue TADF emitters have been reported using different acceptors, such as xanthone, sulfone, benzoylpyridine, phosphine oxide, pyrimidine, pyridinecarbonitrile, oxa-diazole, triazine, phthalonitrile and their derivatives.22-30 The pyridine-carbonitrile based TADF materials were reported by several groups. Zhang and co-workers revealed a sky-blue emitter, (2,6-di(9H-carbazol-9-yl)-4-phenylpyridine-3,5dicarbonitrile (CPC) in which pyridine-carbonitrile is designed as the acceptor and carbazole groups are as the donor and are direct linked to the pyridine core.26 The CPC-based device showed a maximum EQE of 21.2% with CIE coordinates of (0.20, 0.35). At about the same time, Huang and co-workers reported an efficient green dopant 2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy) based device showing an EQE of 11.3% and CIE of (0.34, 0.59) but with a high turn on voltage at 6.7 V.31 Later, Pan and co-workers developed several pyridine-carbonitriles based green-TADF materials with dimethylacridine as the donors. Among these, pyridine-2-carbonitrile (Py2) and pyridine-5-carbonitrile (Py5)

showed high photoluminescence quantum yields (PLQYs) of 89 and 92%, with device EQEs of 23 and 24%, repectively.32 In order to obtain a small ΔEST, the molecule in general is designed to have a twisted structure to reduce the overlap between HOMO and LUMO. Furthermore, an ideal TADF emitter requires high internal quantum efficiency (IQE) and also a high horizontal orientation (Θ//) to obtain very high EQE. Recent reports have revealed the significance of having emitting dipoles in OLED emitting layers oriented preferentially along the horizontal plane to improve the optical outcoupling.33-38 Therefore, emitters showing high Θ// are highly desired to improve the efficiency of TADF OLED devices. However, so far, high efficient pyridinecarbonitrile TADF emitters were very few in the literature.30 In this paper, we designed and synthesized pyridine-carbonitrilebased three blue to green TADF molecules 26tCzPPC, 246TCzPPC, and 35tCzPPC under metal-free conditions (Scheme 1) via mutiple substitution reactions. The first two molecules contain an unusual highly sterically congested four orthosubstituted structures between the donor and the acceptor aromatic rings leading to very high PLQY in the mBCP films (Table 1). In addition, all the three molecules show a unique structure containing two consecutive large twisted angles along the donor and acceptor groups (see Figure 1 for the twisted angles). The structure leads to a nearly complete space separation of the HOMO and LUMO, a small ΔEST and excellent TADF property. Among these, the 246tCzPPC emitter gives a sky-blue emission (491 nm) with an excellent TADF property and a high PLQY of 98% with a horizontal ratio (Θ//) of 78%. As a result, this emitter-based electroluminescence device shows an EQE near 30% with CIE coordinates at (0.18, 0.40). Moreover, a reduced efficiency roll-off

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of 50% at 1000 cd m-2 can be achieved. This work successfully demonstrates a way to design TADF materials with a very high quantum efficiency in the thin film and with a high horizontal molecular orientation to realize highly efficient sky-blue device.

26tCzPPC

Page 2 of 8 246tCzPPC

35tCzPPC

LUMO

Results and Discussion

Synthesis and Physical Properties. As shown in Scheme 1, the three sky-blue TADF emitters, 26tCzPPC, 246tCzPPC, and 35tCzPPC were synthesized by a simple SNAr type substitution reaction of the corresponding fluoro substrates 26DFPPC, 246TFPPC and 35DFPPC by tbutylcarbazole (tCz) in 75, 70 and 50% yields, respectively. The detailed procedure for the synthesis of these three compounds and the starting materials and the characterization of these species are described in the Experimental Section and in the Supporting Information. The structures of 26tCzPPC and 246tCzPPC were further confirmed by the single crystal X-ray diffraction analysis (see the Supporting information). Based on the crystal structures, we calculated the dihedral angles between the planes in the molecules and we found there are two types of large dihedral angles: one is between the ortho tCz groups and the central benzene groups with the dihedral angles of 70.5, and 73.7; the other is between the central benzene and pyridine-carbonitrile groups with the dihedral angles of 71.6o for 246tCzPPC (Figures 1 and S1). The two consecutive large twisted angles along the donor and acceptor groups are likely the key for the HOMO and LUMO separation.3941

Scheme 1. Synthesis of TADF Materials. F N

N

N N

N H

N

N

NaH / DMF 120 oC,12 h

N

N

FPPC 26tCzPPC, 75%

N

N

N

N

N

N

N

N

N N N

246tCzPPC, 70%

35tCzPPC, 50%

We further employed density functional theory (DFT) and time dependent-DFT calculation at PBE0/6-31G* basis set to estimate the ΔEST, geometry and the energy gap of the emitters. Figure 1 reveals the optimized structures and the HOMO and LUMO of these three emitters. For these three compounds 26tCzPPC, 246tCzPPC, and 35tCzPPC, the HOMO is localized on all the electron-donating di-t-butyl carbazoles and the central phenyl units, while the LUMO is dispersed over the electron-deficient pyridine-nitrile unit and the two attached phenyl groups. The HOMO and LUMO spatial overlaps of these compounds are all nearly negligible. The calculated small ∆EST values are 89, 57, and 160 meV, for 26tCzPPC, 246tCzPPC, and 35tCzPPC, respectively, are in line with the calculated large dihedral angles between the ortho t-butylcarbazole and the central benzene groups, and the central benzene and the pyridine-carbonitrile groups (Figure 1). The calculated dihedral angles of these compounds are also close to those from the results of X-ray diffraction for 26tCzPPC and 246tCzPPC.

HOMO

Dihedral Angel

Figure 1. The LUMO (top) and HOMO (middle) distributions, and the dihedral angels (bottom) of 26tCzPPC, 246tCzPPC and 35tCzPPC calculated from single crystal X-ray diffraction and DFT (parenthesis). The absorption and emission spectra of these three compounds in various solvents including cyclohexane, toluene, THF and CH2Cl2 and in mCPB film (5 wt% doped in mCPB) are shown in Figures S2-4, where mCBP is 3,3-di(9H-carbazol-9-yl)biphenyl, a high-triplet-energy host.42 The related photophysical properties are listed in Table 1. All three compounds show a weak lowest-energy absorption band around 330–350 nm, which can be attributed to the intramolecular charge transfer (ICT) transition from the tbutylcarbazole groups to the pyridine-carbonitrile moiety. In addition, the emission spectra are bathochromically shifted in polar solvents, and the emission maxima change from 488 nm in toluene to 546 nm in DCM. The fluorescence (phosphorescence) spectra of 26tCzPPC, 246tCzPPC, and 35tCzPPC, in mCPB films give broad emission spectra centered around 494 (504), 488 (492), and 494 nm (499 nm), respectively (Table 1 and Figure S3). Among these emitters, 246tCzPPC gives most blue emission compared to 26tCzPPC and 35tCzPPC in the same host or solution (Figure S2 and Table 1). This may be due to the slightly larger dihedral angles between the planes of ortho-carbazolyl and the central phenyl groups and between the central phenyl and pyridine planes of 246tCzPPC compared with those in 26tCzPPC (Figures 1 and S1). The large dihedral angle is expected to increase both singlet and triplet energy gaps of the emitter.41 The steric effect of the carbazole group at 4 position of the central benzene moiety likely pushes the other two ortho carbazoles closer to pyridinecarbonitrile plane leading to slightly larger dihedral angles.40,41 From the difference of the onset of fluorescence and phosphorescence spectra, relatively small ΔEST of 90, 63, and 120 meV (in 5 wt% dopped film) were found for 26tCzPPC, 246tCzPPC, and 35tCzPPC, respectively (Table 1, and Figure S4). The lowest ΔEST value for 246tCzPPC relative to the other two materials in the mCBP thin films supports that the emitter has fastest RISC. The absolute PLQYs of 26tCzPPC, 246tCzPPC, and 35tCzPPC 5 wt% doped in the mCBP films under N2 atmosphere were measured using an integrating sphere. The values are 96, 98 and 86% respectively. The high PLQYs and the low ΔEST value of these emitters suggested that they are promising candidates for high-performance TADF OLEDs. Table 1. Summary of the Physical and Photo-physical Properties.a-i


entry

26tCzPPC

246tCzPPC

35tCzPPC

λmax,abs (nm)a

320, 394

330, 390

335, 391

λmax, fl (nm) a

497

492

495

(nm)b

508

498

501

(meV)c

λmax,ph ΔEST

90

63

120

ηPL (%)d

96

98

85

PF (ns)e

39.8

22.1

50.0

DF

268

107

237

(μs)e

Θ// (%)f

75

78

58

396/120

450/209

477/202

HOMO (eV)h

5.78

5.76

5.68

LUMO (eV)i

2.78

2.82

2.88

Td / Tg (oC)g

aAbsorption

and emission maxima of mCBP: 5 wt% emitter thin films at room temperature; bThe phosphoresence maxima of the above thin films at 77 K. cΔEST = ESET. dThe ηPL is the photoluminescence quantum yield of a 5 wt% emitter doped in a mCBP film. eThe transient PL lifetimes of the codoped (mCBP:emitters (5 wt%)) thin film at 300 K. fAngle-dependent PL intensity of the p-polarized light from a 30 nm thick film composed of mCBP:5 wt% emitter. gTd, were measured by TGA, and Tg was measured by DSC. hThe HOMO was measured using cyclic voltammetry. iLUMO was calculated using EHOMO  Eg. To confirm the TADF property, the transient PL decay characteristics of three compounds were measured in the mCBP thin films. The temperature dependence transient PL curves of 5 wt% 246tCzPPC doped in mCBP film from 100 K to 300 K were also measured and are shown in Figure 2a. The transient decay curves can be described by the prompt and delayed fluorescence43,44 and the calculated lifetimes for the fast component of 39.8, 22.1 and 50.0 ns for the slow component of 268, 107, and 237 μs, for 26tCzPPC, 246tCzPPC, and 35tCzPPC, respectively. The lifetimes of the slow components support strongly that the materials possess TADF property.21-25 Notably, the temperature dependent transient PL decays of 26tCzPPC, 246tCzPPC, and 35tCzPPC doped in the mCBP films were measured at various temperatures (Figure S5). In addition, the RISC rate constants (kRISC) determined by using a previously reported method44 (EqS1-6) were summarized in Table S1. The small ΔEST of these emitters appears to account well the moderate kRISC. The delayed components exhibit noticeable temperature dependence. The ratio of delayed to prompt components was also gradually increased with increasing temperature, indicating the presence of TADF features in these emitters. The thermal stability of these compounds was examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere and the results were shown in Table 1 (Figures S6 and S7). The decomposition temperatures for 246tCzPPC, and 35tCzPPC exhibit at 450 and 477 oC, respectively, higher than that of 26tCzPPC at 396 oC. In addition, these molecules showed high glass transition temperatures at 120~ 209 oC, suggesting that they could form morphologically stable and even amorphous films by vacuum deposition for OLED fabrication. The HOMO level is measured by cyclic voltammetry (Figure S8) and the LUMO level is calculated from EHOMO  Eg and the values are summarized in Table 1, where Eg is the optical band gap and determined from the onset of the absorption spectrum.

a) Normalized intensity (a.u)

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ACS Applied Materials & Interfaces

4

10

246tCzPPC 26tCzPPC 35tCzPPC

3

10

2

10

1

10

0

10

200

400

600

800

1000

Time (µs)

b) 1.5

Intensity (a.u.)

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-------- mCBP Isotroic (Q=0.67) mCBP Horizontal (Q=1)

mCBP:26tCzPPC (Q=0.73) mCBP:35tCzPPC (Q=0.58) mCBP:246tCzPPC (Q=0.78)

1.0

0.5

0.0

0

10

20

30

40

50

60

70

80

90

Angle (degree)

Figure 2. (a) Transient PL decay curves of 26tCzPPC, 246tCzPPC, and 35tCzPPC doped in the mCBP host measured at 300 K (Excitation wavelength was 300 nm). (b) Variable-angle PL measurements of 26tCzPPC, 246tCzPPC, and 35tCzPPC (5 wt%) in the mCBP films.

Variable-angle PL measurement. In addition, the orientation factors, (Θ//) of 26tCzPPC, 246tCzPPC, and 35tCzPPC (5 wt%) in the mCBP film (30 nm) were measured using the angledependent p-polarized light on a fused glass substrate. The horizontal transition dipole ratio (Θ) of 0.78 was obtained for 246tCzPPC as shown in Figure 2b.33-38 The Θ of 246tCzPPC is higher than the isotropic value (0.67), indicating that a high outcoupling efficiency OLED can be achieved by using 246tCzPPC as the dopant. The Θ values for the other two compounds were also mesured. For 26tCzPPC, its Θ is 0.73, while for 35tCzPPC, the Θ is only 0.58 smaller than the isotropic value indicating that 35tCzPPC prefers to lie vertically (Figure 2). Lastly, we measure a angular radiation patterns for highly efficient devices 246tCzPPC and 26tCzPPC (Figure S9). The results suggest that the angular distribution of the two devices are nearly Lambertian. Device Fabrication. Next, we examined the electroluminescence (EL) of 26tCzPPC, 246tCzPPC, and 35tCzPPC in OLEDs. The optimized device has the following structure (Figure S10): ITO/NPB (10 nm)/TAPC (30 nm)/mCBP:emitter (5 wt %) (30 nm)/ PPT (10 nm)/TmPyPB (60 nm)/LiF (0.8 nm)/Al (100 nm). In this device, N,N′-bis(1naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) acts as a hole injection material and 1,1-bis[4-[N,N′-di(ptolyl)amino]phenyl] cyclohexane (TAPC) as a hole-transporting


ACS Applied Materials & Interfaces material.44 mCBP acts as the host material, 2,8bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT), as the exciton blocker and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) the electron-transporter.45-48 All the organic materials near to or in the emission layer have high triplet energy (ET) gaps of 2.98, 3.00 and 2.85 eV for TAPC, mCBP and PPT, respectively, higher than the triplet energy of emitters (2.72 – 2.80 eV). Consequently, the triplet excitons of the emitter can be well confined within the emitting layer (EML).

Table 2. EL Performances of the Devices.a entry

26tCzPPC

b

Vd

3.6

4500 (13.0)

EQE (%, V) c

25.4, 4.0

29.6, 4.0

18.2, 4.0

CE (cd A-1, V) c

67.5, 4.0

70.8, 4.0

46.0, 4.0

53.0, 4.0

55.6, 4.0

36.2, 4.0

497

491

494

74

73

75

PE (lm

W-1,

V) c

(nm) c

10

1.0

0.1

26tCzPPC 246tCzPPC 35tCzPPC

0.5

0.0 400

1

1 500

10

600

100

1000

(0.20, 0.47)

(0.18, 0.40)

(0.19, 0.42)

aDevice

Current efficiency (cd A -1)

1

3.5

7993 (13.5)

CIE (x, y), 8V 100

3.8

35tCzPPC

8505 (13.5)

max

10

246tCzPPC

L (cd m-2, V) c

fwhm (nm) c

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10000

Luminance (cd m -2)

Figure 3. The EL characteristic plots of 26tCzPPC, 246tCzPPC, and 35tCzPPC-based devices: EQE and current efficiency vs luminance. Inset; Electroluminescent spectra. The devices based on 26tCzPPC, 246tCzPPC, and 35tCzPPC as the dopants exhibited blue to greenish-blue EL with emission peaks at 497, 491 and 494 nm, respectively, nearly the same as their PL spectra. The EL properties of the 26tCzPPC, 246tCzPPC, and 35tCzPPC devices are shown in Figures 3 and S11 and summarized in Table 2. The turn-on voltages were observed at 3.5 - 3.8 V for these devices, indicating that the devices have similar carrier injection barrier as a result of the similar energy levels. The maximum EQEs for the 26tCzPPC, 246tCzPPC, and 35tCzPPC devices all with a 5 wt % dopant concentration are 25.4, 29.6 and 18.2%, respectively. The device performances are very high compared to the reported CPC-based emitters which required a high doping concentration of 13 wt % to achieve a maximum EQE of 21.2 % with a Commission Internationale de L’Eclairage (CIE) coordinates of (0.20 0.35).26 In addition, the 26tCzPPC, and 246tCzPPC devices show high current efficiency of 67.5, and 70.8 cd A-1 and power efficiencies of 53.0, and 55.6 lm W-1, respectively. The EL spectrum of 246tCzPPC device exhibits a sky-blue emission with the emission maximum (max) at 491 nm and CIE coordinates of (0.18; 0.40) as shown in Table 2. The EL and PL spectra are the same within experimental errors indicating both emissions are all from 246tCzPPC. The excellent performance of this 246tCzPPC device is likely due to the highly substituted structure of the material leading to high PLQY, large dihedral angles, small Δ EST and fast RISC compared to the other two materials. It is noteworthy that the full width at half maximum (fwhm) of the EL spectrum is 74, 73 and 75 nm for 26tCzPPC, 246tCzPPC, and 35tCzPPC, respectively, narrower than most blue TADF devices (>80 nm).34,35 This fwhm value is also less than that of the well-known blue phosphorescent emitter, FIrpic, based devices.49,50 In addition, we fabricate hole-only and electron-only devices of 26tCzPPC and 246tCzPPC indicating that they are excellent hole transporting materials, but are less effective as electron transporters (Figure S11b).

configuration; ITO/NPB (10)/TAPC (30)/mCBP: emitter (5 wt%) (30)/ PPT (10)/TmPyPB (60)/LiF (0.8)/Al (100), thickness in nm. bVd is the operating voltage at a brightness of 1 cd m-2. cV is the applied voltage where the maximum luminance L, maximum EQE, maximum external quantum efficiency, maximum current efficiency CE, or maximum power efficiency PE is observed. λmax is the wavelength where the EL spectrum has the highest intensity and fwhm is the full width at half maximum of the EL spectrum. The 35tCzPPC device exhibited a lower EQE of 18.2%, current efficiency (ηc) of 46.0 cd A-1 and power efficiency of 36.2 lm W-1 compared to those of the 26tCzPPC and 246tCzPPC devices. Similarly, the brightness decreased to 4500 cd m-2 with the CIE of (0.19, 0.42). The low performance of 35tCzPPC device is likely due to the relatively larger ΔEST of 35tCzPPC compared with 26tCzPPC and 246tCzPPC and the unusual low horizontal transition dipole ratio (Θ) of 0.58 (Figure 2b). In Table S2, we summarized the EL performances of the reported blue TADF emitters. There are several pyridine-carbonitrile based TADF materials known, but the device efficiencies are all less than 29%.30 Conclusion In summary, we have designed and synthesized highly congested, sky-blue TADF emitters consisting of an electronaccepting pyridine-carbonitrile core, and electron-donating di-tbutyl carbazole groups attached to the bridging phenyl moiety at various positions. The molecules show a unique structure containing two consecutive large twisted angles along the donor and acceptor groups. The molecules have high PL quantum yields of 85-98% in the thin films along with a narrow emission band. The structure of the molecules appear to affect the orientation in the films and thus the emission out-coupling efficiency of the EL device. 246tCzPPC favors a horizontal orientation and shows a small ΔEST of 63 meV and high PLQY in the film. By combining these properties, the 246tCzPPC-based EL device demonstrates a high EQE of nearly 30%. Experimental Section Synthesis of 4-(2,6-bis(3,6-di-tert-butyl-9H-carbazol-9yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (26tCzPPC).51 To a two-necked round bottom flask (250 mL) containing sodium hydride (0.295 g, 12.3 mmol, 3.0 equiv) was added tert-butylcarbazole (2.52 g, 9.0 mmol, 2.2 equiv) in 40 mL anhydrous DMF under nitrogen atmosphere and stirred for 30 minutes at room temperature. To this solution, 26DFPPC (1.60 g, 4.1 mmol, 1.0 equiv.) in DMF (10 mL) was added dropwise. After addition, the reaction mixture was stirred for 12 h at 120 oC. Then, the reaction solution was poured into cold water, and the yellow precipitate was filtered and dried under vacuum. The crude product


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ACS Applied Materials & Interfaces was purified by column chromatography to obtain a greenish solid in 75% yield. 1H NMR (400 MHz, CDCl3): δ 8.03 (s, 5 H), 7.92 (d, J = 7.6 Hz, 2 H), 7.53 (d, J = 8.0 Hz, 4 H), 7.46 (d, J = 8.0 Hz, 4 H),7.40 (t, J = 7.2 Hz, 2 H), 7.28 (t, J = 7.2 Hz, 4 H), 7.16 (d, J = 8.0 Hz, 4 H), 1.44 (s, 36 H); 13C NMR (100 MHz, CDCl3): δ 162.0, 152.9, 143.5, 139.6, 139.5, 135.7, 133.2, 130.7, 129.1, 128.1, 123.5, 123.4, 116.5, 115.8, 111.2, 111.1, 111.0, 106.5, 34.7, 32.0; HRMS (ESI+): calc. for [(C65H61N5)H] (M+H) 912.5005, found 912.5001.

We thank the Ministry of Science and Technology of Republic of China (MOST 107-2113-M-007-004) for support of this research and the National Center for High-Performance Computing (Account number: u32chc04) of Taiwan for providing the computing time.

Synthesis of 2,6-diphenyl-4-(2,4,6-tris(3,6-di-tert-butyl-9Hcarbazol-9-yl)phenyl)pyridine-3,5-dicarbonitrile (246tCzPPC). To a two-necked round bottom flask (250 mL) containing sodium hydride (0.445 g, 18.5 mmol, 4.2 equiv.) was added tertbutylcarbazole (3.80 g, 13.5 mmol, 3.3 equiv.) in 40 mL anhydrous DMF under nitrogen atmosphere and stirred for 30 minutes at room temperature. To this solution, 246TFPPC (1.68 g, 4.1 mmol, 1.0 equiv.) in DMF (10 mL) was added dropwise. After addition, the reaction mixture was stirred for 12 h at 120 oC. Then, the reaction solution was poured into cold water, and the yellow precipitate was filtered and dried under vacuum. The crude product was purified by column chromatography to obtain a greenish solid in 70% yield. 1H NMR (400 MHz, CDCl ): δ 8.14 (s, 2 H), 8.10 (s, 2 H), 7.98 (s, 3 4 H), 7.66 (d, J = 8.8 Hz, 2 H), 7.59 (d, J = 8.8 Hz, 4 H),7.48 (dd, J = 6.8, 1.6 Hz, 2 H), 7.44 (dd, J = 6.8, 2.0 Hz, 4 H), 7.38 (t, J = 7.6 Hz, 2 H), 7.25 (d, J = 6.8 Hz, 4 H), 7.15 (dd, J = 6.8, 1.2 Hz, 4 H), 1.39 (s, 18 H), 1.35 (s, 36 H); 13C NMR (100 MHz, CDCl3): δ 162.5, 153.0, 144.5, 144.1, 139.9, 138.2, 136.1, 131.1, 130.5, 129.5, 128.5, 127.0, 124.5, 124.1, 123.9, 116.9, 116.8, 116.7, 116.3, 116.2, 111.4, 111.3, 109.6, 107.1, 35.2, 32.3. HRMS (ESI+); calc. for [(C85H84N6)H] (M+H) 1189.6836, found 1189.6830.

(2) Lee, M.-T.; Liao, C.-H.; Tsai, C.-H.; Chen, C. H., Highly Efficient, Deep-Blue Doped Organic Light-Emitting Devices. Adv. Mater. 2005, 17, 2493-2497.

Synthesis of 4-(3,5-bis(3,6-di-tert-butyl-9H-carbazol-9yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (35tCzPPC). According to the procedure similar to that of 26tCzPPC, compound 35tCzPPC was synthesized using 35DFPPC (1.60 g, 4.1 mmol, 1.0 equiv.) to obtain an orange solid in 50% yield. 1H NMR (400 MHz, CDCl3): δ 8.23-8.19 (m, 10 H), 8.19 (dm, J = 1.6 Hz, 1 H), 7.80 (dm, J = 2.0 Hz, 1 H), 7.77 (d, J = 8.0 Hz, 2 H), 7.66-7.25 (m, 10 H), 1.54 (s, 36 H); 13C NMR (100 MHz, CDCl3): δ 163.1, 158.5, 143.5, 140.6, 138.5, 136.5, 135.9, 131.4, 130.5, 129.4, 128.7, 124.0, 123.9, 123.7, 116.2, 115.8, 109.4, 105.7, 34.8, 32.0; HRMS (ESI+): calc. for [(C65H61N5)H] (M+H) 912.5005, found 912.5002.

(8) Chou, H.-H.; Cheng, C.-H. A Highly Efficient Universal Bipolar Host for Blue, Green, and Red Phosphorescent OLEDs. Adv. Mater. 2010, 22, 2468-2471.

ASSOCIATED CONTENT

Supporting Information Experimental procedures, characterizations, spectral data, and CIF files. The Supporting Information is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION

Corresponding Author

(15) Wang, C.; Li, X.; Pan, Y.; Zhang, S.; Yao, L.; Bai, Q.; Li, W.; Lu, P.; Yang, B.; Su, S.; Ma, Y. Highly Efficient Nondoped Green Organic Light-Emitting Diodes with Combination of High Photoluminescence and High Exciton Utilization. ACS Appl. Mater. Interfaces 2016, 8, 3041-3049.

*[email protected].

ORCID ID C.-H. Cheng: 0000-0003-3838-6845 J. Jayakumar: 0000-0003-3135-1535

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

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Pyridine-CarbonitrileâCarbazole-Based Delayed Fluorescence

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