High-Efficiency Organic Light-Emitting Diodes Based on Sublimable

Jul 23, 2018 - Cationic iridium(III) complexes show great promise as phosphorescent materials, while their utilization in organic light-emitting diode...
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Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

High-Efficiency Organic Light-Emitting Diodes Based on Sublimable Cationic Iridium(III) Complexes with Sterically Hindered Spacers Dongxin Ma,*,† Ruihuan Liu,† Chen Zhang,† Yong Qiu,† and Lian Duan*,†,‡ †

Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084, People’s Republic of China

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S Supporting Information *

ABSTRACT: Cationic iridium(III) complexes show great promise as phosphorescent materials, while their utilization in organic light-emitting diodes is severely hindered by the inferior sublimability and low device performance. Here we devise a judicious strategy to develop high-efficiency sublimable cationic iridium(III) complexes by simultaneously introducing sterically hindered spacers into the negative counterions, major and ancillary ligands. We have exploited a novel series of yellow and red emitters, investigated their photophysical properties, electrochemical behaviors, and thermal stabilities, and finally fabricated organic light-emitting diodes by vacuum evaporation deposition. Record-high device performance has been achieved with a superior external quantum efficiency of 16%, excellent power efficiency of 49 lm/W, maximum brightness over 27.3 × 103 cd/m2, very low turn-on voltage below 2.5 V, and quite small efficiency roll-off. Our study represents a significant advance in the development of sublimable cationic iridium(III) complexes and evidences their promising applications in state-of-the-art optoelectronic devices. KEYWORDS: cationic iridium(III) complexes, luminescence, organic light-emitting diodes, steric hindrance

C

quantum yields (PLQYs). In general, a sublimable cationic iridium(III) complex contains an emissive coordinated iridium(III) cation chelated by two major and one ancillary bidentate ligands, together with a tetraphenylborate-type anion. Here we simultaneously introduce sterically hindered spacers into both the ligands and negative counterions to increase the solid-state PLQYs and devise a novel series of high-efficiency sublimable cationic iridium(III) complexes. Then we investigate their single-crystal structural characteristics, photophysical properties, electrochemical behaviors, and thermal stabilities, finally fabricating bright OLEDs thereof by vacuum evaporation deposition, realizing record-high device performance including superior efficiencies, high brightness, very low turn-on voltages, and quite small efficiency roll-off. Our results demonstrate a judicious strategy to develop brilliant sublimable cationic iridium(III) complexes, indicating their great promise in silicon-free flat-panel display and solidstate lighting technology.

ationic iridium(III) complexes have emerged as a considerable library of phosphorescent materials, featuring ease of synthesis, stable electrochemical behaviors, and excellent photophysical properties including high luminescence efficiencies and tunable emission colors ranging from the deepblue to near-infrared region, therefore indicating attractive application prospects in various optoelectronic devices.1−4 By way of example, in 2005 Plummer et al. first reported organic light-emitting diodes (OLEDs) with solution-processed emissive material layers (EMLs) based on cationic iridium(III) complexes, achieving bright yellow luminescence.5 As is known, in other cases the EMLs of OLEDs based on small molecular fluorescent materials or neutral transition-metal phosphorescent compounds are always fabricated by vacuum evaporation deposition, which allows graded dopant concentrations, multiple functional layers, and accurately patterned structures.6−17 However, the previously reported cationic iridium(III) complexes were seldom sublimable due to the ionic nature, severely restraining their practical utilization in state-of-the-art OLEDs.18−26 Later in 2016 our group put forward a feasible and versatile method to develop sublimable cationic iridium(III) complexes through facile counterion control and thus fabricated high-quality EMLs thereof by vacuum evaporation deposition. Nevertheless, the obtained OLEDs delivered quite mediocre external quantum efficiencies (EQEs) ranging from 1.2% to 8.1%.27−29 We therefore seek to improve the device performance by exploiting novel emitters with enhanced photoluminescence © XXXX American Chemical Society

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RESULTS AND DISCUSSION Material Design and Single-Crystal Structures. As described in Figure 1, initially we designed cationic iridium(III) complexes 1−4 sharing the same coordinated iridium(III) cation [Ir(ppy)2(bpy)]+ (ppy is 2-phenylpyridine and bpy is 2,2′-bipyridine) but different anions with calculated volume Received: May 25, 2018

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DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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ACS Photonics

onto the bpy segment to modify the ancillary ligand and designed complexes 5 and 6 containing a new cation, [Ir(ppy)2(dtb-bpy)]+ (dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), with [B(5FPh)4]− or [B(dCF3Ph)4]− serving as the negative counterion. Next we inserted a phenyl group into the ppy segment to exploit larger-sized major ligands and designed complexes 7−10 with bulkier cations [Ir(phq)2(dtb-bpy)]+ and [Ir(piq)2(dtb-bpy)]+ (phq is 2-phenylquinoline and piq is 2-phenylisoquinoline). The synthetic routes are shown in Figures S1 and S2. As we expected, the ancillary ligand dtb-bpy features a calculated volume of 214 cm3/mol, much larger than bpy (123 cm3/mol), while the calculated volume of the major ligands phq and piq reaches 153 cm3/mol, exceeding that of ppy (119 cm3/mol). Therefore, the four cations [Ir(ppy)2(bpy)]+, [Ir(ppy) 2 (dtb-bpy)] + , [Ir(phq) 2 (dtb-bpy)] + , and [Ir(piq)2(dtb-bpy)]+ show increasing calculated volume from 364 to 554 cm3/mol, which, combined with the bulky negative counterions, would significantly enhance the material PLQYs.30−32 All the single crystals of complexes 1−10 have been successfully grown from solutions and characterized by X-ray crystallography. As depicted in Figure 2, the cations [Ir(ppy) 2 (bpy)] + , [Ir(ppy) 2 (dtb-bpy)] + , [Ir(phq) 2 (dtbbpy)]+, and [Ir(piq)2(dtb-bpy)]+ exhibit iridium-centered distorted octahedral geometries with ppy, phq, or piq adopting C,C-cis, N,N-trans configurations. The anion [PF6]− shows a phosphorus-centered octahedral geometry, while the others, [BF4]−, [B(5FPh)4]−, and [B(dCF3Ph)4]−, display boroncentered tetrahedral geometries. As illustrated in Figures S3− S22, the single-crystal structures demonstrate the various steric hindrance of these complexes in crystal cells. For example, for complex 1 with [BF4]−, the single crystal shows a space group of Pbca with cell parameters of a = 10.6 Å, b = 15.5 Å, and c = 33.3 Å, while for complex 2 with the bulkier anion [PF6]−, the cell parameters in the same space group also become larger, with a = 10.9 Å, b = 15.8 Å, and c = 33.5 Å. Similarly, in the single crystals of complexes 1−4, the shortest Ir−Ir distance between adjacent cations is 7.8, 7.9, 9.1, and 12.5 Å,

Figure 1. Schematic material design strategy and chemical structures.

increasing from 35 to 425 cm3/mol, namely, tetrafluoroborate ([BF4]−), hexafluorophosphate ([PF6]−), tetrakis(2,3,4,5,6pentafluorophenyl)borate ([B(5FPh)4]−), and tetrakis[3,5bis(trifluoromethyl)phenyl]borate ([B(dCF3Ph)4]−), respectively. Then we introduced two bulky tert-butyl substituents

Figure 2. Schematic single-crystal structures with the calculated volume labeled. B

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 3. PL spectra of complexes 1−10 in solutions (top) and neat films (bottom).

respectively, quite shorter than those in solutions (573 nm). Nevertheless, complexes 7−10 behave differently, possibly due to their triplet excited states with more LC 3π−π* character, while the underlying causes remain unknown and require further investigation. Note that the solid-state PLQYs are markedly improved along with the increasing steric hindrance of both cations and anions. For example, complexes 3 and 4 with [B(5FPh)4]− and [B(dCF3Ph)4]− show superior PLQYs approaching 0.50, significantly surpassing complexes 1 and 2 with smaller anions [BF4]− and [PF6]−. Owing to the large-sized tert-butyl substituents, complexes 5 and 6 with [Ir(ppy)2(dtb-bpy)]+ show even higher PLQYs of 0.59 and 0.52, respectively. For complexes 7 and 8 with [Ir(phq)2(dtb-bpy)]+, the PLQYs further increase to 0.70. Besides, complexes 9 and 10 with [Ir(piq)2(dtb-bpy)]+ also show high PLQYs over 0.40, although their PL spectra are red-shifted to 600 nm. These experimental results demonstrate that introduction of sterically hindered spacers could effectively reduce the dye aggregation, mitigate the concentration quenching effect, and therefore improve the material PLQYs. Electrochemical and Thermal Stabilities. Electrochemical behaviors of complexes 1−10 were then performed by cyclic voltammetry (CV) in oxygen-free anhydrous solutions, and energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) were calculated according to the redox potentials. Experiments reveal that all these complexes show decent electrochemical stabilities with reversible redox process in oxygen-free anhydrous solutions. As depicted in Figure S35, complexes 1−4, with the same cation [Ir(ppy)2(bpy)]+, exhibit quite similar oxidation potentials of ca. 0.86 V (EHOMO is −5.68 eV) and reduction potentials of ca. −1.78 V (ELUMO is −3.02 eV). For complexes 5 and 6, introducing electrondonating tert-butyl substituents onto the ancillary ligands destabilizes the LUMOs and leads to lower reduction potentials of approximate −1.86 V (Figure S36). For complexes 7−10, introducing phenyl groups onto the major ligands enhances the HOMO energy levels, therefore the oxidation potentials appear cathodically shifted to 0.82 and 0.76 V, respectively (Figures S37 and S38). Next thermogravimetric analysis (TGA) suggests that complexes 1−10 are thermally stable with the 5% weight reduction temperatures (ΔT5%) over 320 °C. As described in Figure S39, complexes 1 and 2 with [BF4]− and [PF6]− show the highest ΔT5% of 375 and 384 °C, respectively, while complexes 3, 5, 7, and 9 with [B(5FPh)4]− show quite higher ΔT5% than complexes 4, 6, 8, and 10 with [B(dCF3Ph)4]−. Vacuum sublimation purification then proves that complexes

respectively, successively enhanced along with the increasing anion steric hindrance. Photophysical Properties. As expressed in Figure S23, complexes sharing the same cation show analogous absorption spectra in acetonitrile solutions. For complexes 1−4 with [Ir(ppy)2(bpy)]+ and complexes 5 and 6 with [Ir(ppy)2(dtbbpy)]+, the intense band at about 255 nm derives from the spin-allowed ligand-centered (LC) 1π−π* transitions, while the quite weaker bands around 310 nm are attributed to the singlet metal-to-ligand charge-transfer (1MLCT) and ligand-toligand charge-transfer (1LLCT) transitions due to the promotion of electrons from the major ligand ppy to the ancillary ligand bpy or dtb-bpy. For complexes 7−10 with [Ir(phq)2(dtb-bpy)]+ or [Ir(piq)2(dtb-bpy)]+, three absorbance bands are observed, including intense peaks at 270 or 290 nm from LC 1π−π* transitions, lower peaks at about 340 nm from 1MLCT and 1LLCT transitions, and even weaker bands around 440 nm from the spin-forbidden 3MLCT, 3LLCT, and LC 3π−π* transitions, owing to the strong spin−orbit coupling endowed by the heavy iridium atom.33,34 Upon the excitation wavelength of 380 nm, complexes 1−6 show broad featureless photoluminescence (PL) spectra not only in solutions and neat films at room temperature (Figure 3) but also in acetonitrile glass at 77 K (Figure S24), indicating that their emissive excited states have predominantly 3MLCT or 3LLCT character. On the contrary, complexes 7−10 exhibit vibronically structured spectra, pointing out that their emissive excited states have more LC 3π−π* character.35,36 In dilute acetonitrile solutions (1 × 10−5 mol/L), complexes with the same emissive coordinated iridium(III) cation show similar PL spectra, nearly independent of their negative counterions. For example, complexes 1−4 with [Ir(ppy)2(bpy)]+ exhibit yellow emission peaked at approximately 590 nm, while for complexes 5 and 6, introduction of the electron-donating tert-butyl substituents onto the ancillary bpy ligand leads to blue-shifted PL spectra with shorter emission wavelengths of 573 nm. However, introducing phenyl groups onto the ppy ligands would cause different spectral shifts. For complexes 7 and 8 with [Ir(phq)2(dtb-bpy)]+, the PL spectra are peaked at about 560 nm, while for complexes 9 and 10 with [Ir(piq)2(dtb-bpy)]+, the major peaks are redshifted to 593 nm with a shoulder at 625 nm. By contrast, in neat films the PL spectra also depend on the negative counterions. The peak wavelengths of complexes 1−4 are 581, 570, 560, and 553 nm, respectively, obviously blueshifted in sequence, probably because the bulkier anions are less able to stabilize the polar triplet excited states arising from 3 MLCT or 3LLCT transitions.27 Similarly, the peak wavelengths of complexes 5 and 6 are 550 and 547 nm in neat films, C

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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S40−S43, which explicitly explain the different emissive excited state character of complexes 1−10. For complexes 1−4 with [Ir(ppy)2(bpy)]+, the T1 state with a calculated energy of 2.18 eV is assigned from the excitation from HOMO to LUMO, while the T2 state (2.40 eV) derives from HOMO−1 to the LUMO, both due to the 3MLCT transition from the iridium center to bpy or the 3LLCT transition from ppy to bpy. For complexes 5 and 6 with [Ir(ppy)2(dtb-bpy)]+, the T1 state comes from the 3MLCT or 3LLCT transition, while the T2 state is mainly attributed to the LC 3π−π* transitions of ppy itself. Since the T1 state shows a calculated energy of only 2.29 eV, much lower than the T2 state (2.69 eV), the emissive excited states of complexes 5 and 6 predominantly have 3 MLCT or 3LLCT character, similar to complexes 1−4. By contrast, the cations [Ir(phq)2(dtb-bpy)]+ and [Ir(piq)2(dtbbpy)]+ show quite smaller excitation energy gaps between the T1 and T2 states, only 0.08 and 0.03 eV, respectively, while the two triplet excited states both derive from mixed 3MLCT, 3 LLCT, and LC 3π−π* transitions; therefore the emissive excited states of complexes 7−10 feature not merely 3MLCT or 3LLCT but also LC 3π−π* character. High-Performance OLEDs. Using sublimable cationic iridium(III) complexes 3−10 as phosphorescent dyes, we finally fabricated vacuum-evaporated-deposited OLEDs in the structure of indium tin oxide (ITO, anode)/dipyrazino[2,3f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN, hole-injection layer, HIL, 5 nm)/N,N′-bis(naphthalen-1-yl)N,N′-bis(phenyl)benzidine (NPB, hole-transport layer, HTL, 40 nm)/4,4′,4″-[tris(9-carbazolyl)phenyl]amine (TCTA, exciton-blocking layer, EBL, 10 nm)/EML (12 nm)/9,10-bis[4(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl]anthracene (BPBiPA, electron-transport layer, ETL, 50 nm)/LiF (1 nm)/ Al (150 nm). In the EMLs, complexes 3−10 were doped into the thermally activated delayed fluorescence (TADF) bipolar host material, 2,4-diphenyl-6-bis(12-phenylindolo[2,3-α]carbazole-11-yl)-1,3,5-triazine (DIC-TRZ), which bears

3−10 are easy to sublimate, owing to the utilization of tetraphenylborate-type negative counterions with large steric hindrance and well-dispersed charges, which effectively reduces the ionic interaction and improves the material sublimability, therefore permitting the preparation of devices by vacuum evaporation deposition.26 Quantum Chemical Calculations. To gain deep insights into the photophysical and electrochemical characteristics of complexes 1−10, we then investigated the ground and lowlying triplet states of the emissive cations by density functional theory (DFT) and time-dependent DFT (TD-DFT), respectively, on the basis of the optimized geometries calculated from the single-crystal structures. As expressed in Figure 4, all the

Figure 4. Plots of the HOMO and LUMO of each emissive coordinated iridium(III) cation, corresponding to an isocontour value of |Ψ| = 0.02. All the hydrogen atoms are omitted for clarity.

HOMOs stand on the iridium center and major ligands, while the LUMOs only stay on the bpy segments of the ancillary ligands. The frontier molecular orbital (MO) surfaces and the contribution of each monoelectronic excitation to the T1 → S0 and T2 → S0 transitions are expressed in Table S1 and Figures

Figure 5. OLED characteristics. (a) Device architecture and energy level diagram, (b) current efficiency−luminance curves, and (c) EL spectra of the optimal devices. D

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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ACS Photonics Table 1. Photophysical Properties, Electrochemical Behaviors, and Thermal Stabilities of Complexes 1−10 absorptiona λPL [nm] (ε [10 M cm−1])

λPL [nm]

1

254 (4.63), 309 (1.89)

590

0.19

2

255 (4.84), 310 (2.27)

591

0.19

3

255 (5.02), 309 (2.13)

590

0.30

4

254 (5.02), 309 (2.14)

590

0.31

5

258 (5.52), 309 (2.23)

573

0.34

6

256 (5.51), 309 (2.28)

573

0.33

7

265 (5.57), 335 (2.38), 437 (0.52)

560

0.65

8

9

10

PL in neat filmsb

PL in solutionsb −1

4

271 (6.19), 337 (2.52), 437 (0.60)

291 (4.30), 340 (2.06), 440 (0.82)

291 (4.20), 339 (1.71), 440 (0.54)

562

593, 625 (sh)

593, 625 (sh)

λPL [nm] (τ [ns])

φ

0.63

0.34

0.32

581 (341, 823 [57.0%]) 570 (405, 1122 [39.2%]) 560 (294, 824 [91.0%]) 553 (232, 756 [93.7%]) 550 (230, 734 [90.0%]) 545 (242, 687 [88.3%]) 574 (281, 645 [68.8%])

573 (409, 862 [79.8%])

595 (423, 898 [76.4%]) 624 (452, 854 [82.4%]) 593 (529, 1079 [67.6%]) 621 (594, 1157 [59.6%])

PL at 77 Kc λPL [nm] (τ [ns])

φ 0.02 0.03 0.52 0.49 0.59 0.52 0.69

0.71

0.40

0.41

558 (2287, [36.9%]) 566 (2222, [56.3%]) 550 (1864, [72.3%]) 550 (1690, [78.0%]) 547 (1939, [69.3%]) 544 (1897, [68.9%]) 555 (1697, [36.2%]) 598 (1972, [47.8%]) 555 (1803, [36.7%]) 598 (2003, [45.4%]) 586 (1688, [80.4%]) 639 (1860, [82.7%]) 584 (2451, [67.6%]) 632 (2711, [60.3%])

electrochemistry in solutionsd

TGAe

ET [eV]

Eox [V]

Ered [V]

EHOMO [eV]

ELUMO [eV]

Egap [eV]

ΔT5% [°C]

7682

2.23

0.86

−1.78

−5.66

−3.02

2.64

375

4546

2.20

0.86

−1.78

−5.66

−3.02

2.64

384

4157

2.26

0.85

−1.78

−5.65

−3.02

2.63

360

4092

2.26

0.86

−1.77

−5.66

−3.03

2.63

323

4205

2.27

0.88

−1.86

−5.68

−2.94

2.74

363

4219

2.28

0.86

−1.85

−5.66

−2.95

2.71

356

4250

2.24

0.82

−1.86

−5.62

−2.94

2.68

369

2.24

0.82

−1.86

−5.62

−2.94

2.68

338

2.12

0.77

−1.85

−5.57

−2.95

2.62

373

2.13

0.76

−1.85

−5.56

−2.95

2.61

333

5070 4447 4843 3791 3829 4690 4977

In anhydrous acetonitrile solutions (1 × 10−5 mol/L), ε denotes the molar extinction coefficient. bIn degassed anhydrous acetonitrile solutions (1 × 10−5 mol/L) or neat films at room temperature, and the excited state lifetimes in neat films are shown in Figures S25a−S34a. cIn acetonitrile glass at 77 K, the percentage in brackets denotes the proportion of each lifetime, the excited state lifetimes are shown in Figures S25b−S34b, and the triplet excited state energy levels were estimated according to the PL spectra at 77 K. dRedox in oxygen-free anhydrous solutions (1 × 10−3 mol/L) at room temperature with ferrocence as the internal standard. The HOMO and LUMO energy levels were calculated according to the formula EHOMO = −4.80 eV − eEox and ELUMO = −4.80 eV − eEred. eUnder nitrogen-flow conditions. a

Table 2. Device Characteristics of the Optimal OLEDs Based on Complexes 3−10

3

xa [%]

Vonb [V]

max CEc [cd/A]

CEc at 103 cd/m2 [cd/A]

CEc at 104 cd/m2 [cd/A]

max EQEd [%]

max PEe [lm/W]

max Lf [cd/m2]

2.0

2.34

40.9

39.5

35.3

13.5

40.4

>27.3 × 103

548

3

λELg [nm]

4

1.5

2.38

35.1

33.7

25.3

11.7

36.7

>27.3 × 10

558

5

1.9

2.31

46.5

42.8

40.4

14.8

37.9

>27.3 × 103

556

6

1.4

2.48

40.7

38.5

29.5

13.5

31.8

>27.3 × 103

564

7

1.7

2.19

40.6

36.5

21.7

13.8

40.9

>27.3 × 103

8

1.6

2.29

48.9

44.2

28.9

15.8

49.0

>27.3 × 103

9

1.7

2.40

16.5

15.4

8.9

10.7

16.6

>27.3 × 103

10

2.4

2.42

18.0

16.0

6.1

11.1

19.1

18.8 × 103

556, 586 (sh) 558, 586 (sh) 588 (sh), 626 588 (sh), 624

CIEh (x, y) (0.41, 0.55) (0.44, 0.53) (0.43, 0.55) (0.44, 0.53) (0.48, 0.51) (0.48, 0.51) (0.60, 0.40) (0.59, 0.40)

a x, dopant ratio. bVon, turn-on voltage (at the luminance of 1 cd/m2). cCE, current efficiency. dEQE, external quantum efficiency. ePE, power efficiency. fL, luminance. gλEL, EL wavelength, sh denotes the shoulder peak. hCIE, Commission Internationale de I’Elairage.

donor/acceptor moieties and suitable frontier energy levels.37 Molecular structures of the above materials are displayed in Figure S44), while the schematic device architecture and energy level diagram are illustrated in Figure 5a. By controlling the dopant concentration, we successfully obtained highperformance optimal OLEDs with yellow and red luminescence, of which the current efficiency versus luminance curves

and electroluminescence (EL) spectra are presented in Figure 5b,c, with detailed data in Tables S2 and S3 and Figures S45− S52. As listed in Table 2, complexes 3−8 show excellent yellow EL behaviors in OLEDs at very low dopant concentrations. The optimal device doped with 2.0% complex 3 reaches a maximum current efficiency (CE) of 40.9 cd/A, power E

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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Table 3. Summary of the Reported Device Performance of Selected OLEDs Based on Cationic Iridium(III) Complexes polychromic dopant [Ir(dFppy)2(pzpy)][PF6] [Ir(dFppy)2(dppmmi)][PF6] [Ir(ppy)2(Phpyim)][PF6] [Ir(ppy)2(EHCAF)][PF6] [Ir(L)2(N^N)][PF6] [Ir(ppy)2(bpy)][B(5FPh)4] [Ir(ppy)2(bpy)][B(dCF3Ph)4] [Ir(ppy)2(dtb-bpy)][B(5FPh)4] [Ir(ppy)2(dtb-bpy)] [B(dCF3Ph)4] [Ir(phq)2(dtb-bpy)][B(5FPh)4] [Ir(phq)2(dtb-bpy)] [B(dCF3Ph)4] [Ir(piq)2(dtb-bpy)][B(5FPh)4] [Ir(piq)2(dtb-bpy)] [B(dCF3Ph)4] [Ir(npy)2(c-phen)][PF6] [Ir(npy)2(o-phen)][PF6]

Vona [V]

max CEb [cd/A]

max EQEc [%]

max PEd [lm/W]

max Le [cd/m2]

458 478 526 540 565 548 558 556 564

(0.16, (0.20, (0.34, (0.37, (0.44, (0.41, (0.44, (0.43, (0.44,

λELf [nm]

CIEg (x, y)

ref

4.1 7.2 4.4 8.4 5.0 2.3 2.4 2.3 2.5

2.5 3.4 25.3 23.7 19.7 40.9 35.1 46.5 40.7

8.1 6.8 6.5 13.5 11.7 14.8 13.5

5.3 18.4 40.4 36.7 37.9 31.8

2.0 × 10 7.3 × 102 38.5 × 103 11.9 × 103 15.6 × 103 >27.3 × 103 >27.3 × 103 >27.3 × 103 >27.3 × 103

2.2 2.3

40.6 48.9

13.8 15.8

40.9 49.0

>27.3 × 103 >27.3 × 103

556, 586 (sh) 558, 586 (sh)

(0.48, 0.51) (0.48, 0.51)

this work this work

2.4 2.4

16.5 18.0

10.7 11.1

16.6 19.1

>27.3 × 103 18.8 × 103

588 (sh), 626 588 (sh), 624

(0.60, 0.40) (0.59, 0.40)

this work this work

6.8 8.6

10.0 9.1

7.1 6.5

3.2 × 103 2.3 × 103

618 620

(0.57, 0.40) (0.57, 0.40)

20 20

3

0.27) 0.38) 0.56) 0.58) 0.47) 0.55) 0.53) 0.55) 0.53)

18 21 19 26 25 this this this this

work work work work

a

Von, turn-on voltage (at the luminance of 1 cd/m2). bCE, current efficiency. cEQE, external quantum efficiency. dPE, power efficiency. eL, luminance. fλEL, EL wavelength; sh denotes the shoulder peak. gCIE, Commission Internationale de I’Elairage.

DFT and TD-DFT calculations were also simulated for each emissive cation and corroborated with the experimental results. Finally we have fabricated yellow- and red-emitting devices by vacuum evaporation deposition of these new phosphorescent materials, accomplishing superior EQEs over 16%, high brightness exceeding 27.3 × 103 cd/m2, quite low turn-on voltages below 2.5 V, and very small efficiency roll-off. Our study has achieved a record performance of OLEDs based on cationic iridium(III) complexes, predicting their brilliant promise for future flat-panel displays and solid-state lighting technology.

efficiency (PE) of 40.4 lm/W, and EQE of 13.5%, transcending the optimal device based on 1.5% complex 4 with CE, PE, and EQE of 35.1 cd/A, 36.7 lm/W, and 11.7%, respectively. By contrast, owing to the introduction of larger-sized ancillary ligands, complexes 5 and 6 show superior EL performance, accomplishing a peak CE of 46.5 cd/A, PE of 37.9 lm/W, and EQE of 14.8%. Analogously, due to the bulkier major ligands, complexes 7 and 8 express even higher efficiencies with a CE, PE, and EQE of 48.9 cd/A, 49.0 lm/W, and 15.8%. All the brightness maxima exceed 27.3 × 103 cd/m2, and the turn-on voltages are below 2.5 V. Besides, the red-emitting diodes based on complexes 9 and 10 also exhibit decent EQEs over 11%. It is noteworthy that complexes 3, 5, 7, and 9 with [B(5FPh)4]− show quite smaller efficiency roll-off at high luminance than complexes 4, 6, 8, and 10 with [B(dCF3Ph)4]−. For example, in the optimal device based on 1.9% complex 5, the CE measured at the luminance of 103 cd/ m2 is 42.8 cd/A and reduces to 40.4 cd/A at 104 cd/m2 with a decay ratio of only 5.6%, while in the optimal device based on 1.4% complex 6, the CEs at 103 and 104 cd/m2 are 38.5 and 29.5 cd/A, respectively, indicating a much larger decay ratio of 23.4%. We tentatively rationalized this phenomenon as a consequence of the different ion distribution in the EMLs,38 while a clearer explanation and deeper investigation are still on the way. As far as we know, the above results are among the top device performances of yellow- and red-emitting OLEDs based on cationic iridium(III) complexes, as summarized in Table 3.5,18−29

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EXPERIMENTAL SECTION Synthesis and Structural Characterization. All the reactants and organic solvents in this article were purchased from commercial sources and used as received unless otherwise stated. [Ir(ppy)2Cl]2. IrCl3 hydrate (2.4288 g, 6.888 mmol) was utilized as the metal source reacting with a 50% excess of ppy (3.2060 g, 20.657 mmol) ligand in 2-ethoxyethanol and deionized water (v/v = 3:1) mixed solvents, then refluxing for 24 h under a nitrogen atmosphere to produce the intermediate dichloro-bridged diiridium complex [Ir(ppy)2Cl]2 (2.7151 g, 2.533 mmol), yield: 74%. [Ir(phq)2Cl]2. IrCl3 hydrate (2.2232 g, 6.305 mmol) was utilized as the metal source reacting with a 50% excess of phq (4.1368 g, 20.154 mmol) ligand in 2-ethoxyethanol and deionized water (v/v = 3:1) mixed solvents, then refluxing for 24 h under a nitrogen atmosphere to produce the intermediate dichloro-bridged diiridium complex [Ir(phq)2Cl]2 (3.3187 g, 2.608 mmol), yield: 83%. [Ir(piq)2Cl]2. IrCl3 hydrate (1.7615 g, 4.995 mmol) was utilized as the metal source reacting with a 50% excess of piq (3.0862 g, 15.036 mmol) ligand in 2-ethoxyethanol and deionized water (v/v = 3:1) mixed solvents, then refluxing for 24 h under a nitrogen atmosphere to produce the intermediate dichloro-bridged diiridium complex [Ir(piq)2Cl]2 (2.6026 g, 2.046 mmol), yield: 82%.

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CONCLUSION In summary, we have exploited a novel series of high-efficiency sublimable cationic iridium(III) complexes by simultaneously introducing sterically hindered spacers into the negative counterions, major and ancillary ligands. All the single crystals have been successfully obtained and investigated; then photophysical properties, electrochemical behaviors, and thermal stabilities were fully characterized and discussed. F

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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with a = 17.282(4) Å, b = 22.589(5) Å, c = 29.070(6) Å, α = 88.65(3)°, β = 74.22(3)°, γ = 69.69(3)°, V = 10210(4) Å3, Z = 103, dcalcd = 1.686 g/cm3, R1 = 0.0622, wR2 = 0.1168 for 66 885 observed reflections [I ≥ 2σ(I)] (CCDC 1046825). [Ir(ppy)2(bpy)][B(dCF3Ph)4] (Complex 4). Complex 4 was synthesized through an ion-exchange reaction at room temperature in air between complex 1 and Na[B(dCF3Ph)4], then purified by column chromatography on silica gel (200− 300 mesh) with dichloromethane as the eluent, producing a yellow powder, yield: 86%. 1H NMR (600 MHz, DMSO-d6, δ): 8.89 (d, J = 8.2 Hz, 2H), 8.27 (t, J = 7.2 Hz, 4H), 7.95− 7.90 (m, 4H), 7.88 (d, J = 4.7 Hz, 2H), 7.71 (s, 4H), 7.69 (s, 2H), 7.63 (s, 2H), 7.62 (s, 8H), 7.16 (t, J = 6.0 Hz, 2H), 7.03 (dd, J = 12.6 Hz, 5.5 Hz, 2H), 6.90 (t, J = 7.5 Hz, 2H), 6.20 (d, J = 7.2 Hz, 2H). 19F NMR (564 MHz, DMSO-d6, δ): −61.74 (s, 24F). MS (ESI) [m/z]: [M − B(dCF3Ph)4]+ calcd for C32H24IrN4, 657.16+; found, 657.16+; [M − Ir(ppy)2(bpy)]− calcd for C32H12BF24, 863.06−; found, 863.06−. A single crystal was successfully grown from acetone and methanol mixed solutions and characterized by X-ray crystallography. Space group of P1 with a = 12.677(3) Å, b = 15.014(3) Å, c = 17.707(4) Å, α = 77.20(3)°, β = 83.63(3)°, γ = 84.77(3)°, V = 3258.6(11) Å3, Z = 1, dcalcd = 1.637 g/cm3, R1 = 0.0421, wR2 = 0.1177 for 28 020 observed reflections [I ≥ 2σ(I)] (CCDC 1046398). [Ir(ppy) 2 (dtb-bpy)][B(5FPh) 4 ] (Complex 5). [Ir(ppy)2Cl]2 (0.5446 g, 0.508 mmol) and dtb-bpy (0.2727 g, 1.016 mmol) were dissolved in 1,2-ethanediol and refluxed for 16 h under a nitrogen atmosphere to form a clear orange solution. After cooling to room temperature, an aqueous solution of Na[B(5FPh)4] (0.7520 g, 1.071 mmol, in 20 mL of deionized water) was slowly added into the reaction mixture under stirring, resulting in a yellow suspension. The suspension was then extracted by dichloromethane, dried under vacuum overnight, and purified by column chromatography on silica gel (200−300 mesh) with dichloromethane as the eluent, producing a yellow powder (1.1214 g, 0.774 mmol), yield: 76%. 1H NMR (600 MHz, DMSO-d6, δ): 8.85 (d, J = 1.6 Hz, 2H), 8.25 (d, J = 8.2 Hz, 2H), 7.94−7.87 (m, 4H), 7.74 (d, J = 5.9 Hz, 2H), 7.69 (dd, J = 5.9, 1.8 Hz, 2H), 7.59 (d, J = 5.4 Hz, 2H), 7.15 (td, J = 6.6, 1.2 Hz, 2H), 6.99 (td, J = 7.2, 1.2 Hz, 2H), 6.87 (td, J = 7.5, 1.1 Hz, 2H), 6.16 (d, J = 7.3 Hz, 2H), 1.37 (s, 18H). 19F NMR (564 MHz, DMSO-d6, δ): −132.96 (s, 8F), −161.70 (s, 4F), −165.94 (s, 8F). MS (ESI) [m/z]: [M − B(5FPh)4]+ calcd for C40H40IrN4, 769.29+; found, 769.30+; [M − Ir(ppy)2(dtb-bpy)]− calcd for C24F20B, 678.98−; found, 678.95−. A single crystal was successfully grown from acetone and methanol mixed solutions and characterized by X-ray crystallography. Space group of P1̅ with a = 12.525(3) Å, b = 14.818(3) Å, c = 17.676(4) Å, α = 88.45(3)°, β = 83.15(3)°, γ = 77.94(3)°, V = 3185.3(13) Å3, Z = 2, dcalcd = 1.510 g/cm3, R1 = 0.0996, wR2 = 0.3370 for 11 056 observed reflections [I ≥ 2σ(I)] (CCDC 1552813). [Ir(ppy)2(dtb-bpy)][B(dCF3Ph)4] (Complex 6). The synthetic route of complex 6 is similar to that of complex 5, while Na[B(5FPh)4] was replaced by Na[(dCF3Ph)4], producing a yellow powder, yield: 87%. 1H NMR (600 MHz, DMSO-d6, δ): 8.86 (s, 2H), 8.26 (d, J = 8.1 Hz, 2H), 7.93 (dd, J = 15.1, 7.6 Hz, 4H), 7.77 (d, J = 5.9 Hz, 2H), 7.70 (s, 6H), 7.61 (d, J = 12.8 Hz, 10H), 7.17 (t, J = 6.5 Hz, 2H), 7.02 (t, J = 7.5 Hz, 2H), 6.90 (t, J = 7.4 Hz, 2H), 6.19 (d, J = 7.8 Hz, 2H), 1.38 (s, 18H). 19F NMR (564 MHz, DMSO-d6, δ): −61.57 (s, 24F). MS (ESI) [m/z]: [M − B(dCF3Ph)4]+

[Ir(ppy)2(bpy)][BF4] (Complex 1). [Ir(ppy)2Cl]2 (1.1002 g, 1.026 mmol) and bpy (0.3208 g, 2.054 mmol) were dissolved in 1,2-ethanediol and refluxed for 16 h under a nitrogen atmosphere to form a clear orange solution. After cooling to room temperature, an aqueous solution of sodium tetrafluoroborate (Na[BF4], 1.0013 g, 9.120 mmol, in 20 mL of deionized water) was slowly added to the reaction mixture under stirring, resulting in a yellow suspension. The suspension was then filtered, washed, dried under vacuum overnight, and purified by column chromatography on silica gel (200−300 mesh) with dichloromethane and methanol mixed solvents (v/ v = 10:1) as the eluent, producing a yellow powder (1.2322 g, 1.657 mmol), yield: 81%. 1H NMR (600 MHz, DMSO-d6, δ): 8.88 (d, J = 8.2 Hz, 2H), 8.30−8.21 (m, 4H), 7.95−7.89 (m, 4H), 7.86 (d, J = 5.4 Hz, 2H), 7.69 (t, J = 6.6 Hz, 2H), 7.61 (d, J = 5.8 Hz, 2H), 7.15 (t, J = 6.0 Hz, 2H), 7.01 (t, J = 7.5 Hz, 2H), 6.89 (dd, J = 10.6, 4.2 Hz, 2H), 6.18 (d, J = 7.6 Hz, 2H). 19 F NMR (564 MHz, DMSO-d6, δ): −148.15 (s, 4F). MS (ESI) [m/z] [M − BF4]+ calcd for C32H24IrN4, 657.16+; found, 657.16+; [M − Ir(ppy)2(bpy)]− calcd for BF4, 87.00−; found, 87.00−. A single crystal was successfully grown from acetone and methanol mixed solutions, then characterized by X-ray crystallography. Space group of Pbca with a = 10.579(2) Å, b = 15.509(3) Å, c = 33.275(7) Å, α = 90°, β = 90°, γ = 90°, V = 5459.4(19) Å3, Z = 8, dcalcd = 1.809 g/cm3, R1 = 0.1137, wR2 = 0.2953 for 4801 observed reflections [I ≥ 2σ(I)] (CCDC 1813954). [Ir(ppy)2(bpy)][PF6] (Complex 2). The synthetic route of complex 2 is similar to that of complex 1, while Na[BF4] was replaced by ammonium hexafluorophosphate ([NH4][PF6]), producing a yellow powder, yield: 84%. 1H NMR (600 MHz, DMSO-d6, δ): 8.88 (d, J = 8.1 Hz, 2H), 8.27 (dd, J = 10.4, 4.2 Hz, 4H), 7.93 (dd, J = 13.9, 7.8 Hz, 4H), 7.87 (d, J = 5.4 Hz, 2H), 7.71−7.67 (m, 2H), 7.62 (d, J = 5.8 Hz, 2H), 7.15 (t, J = 6.6 Hz, 2H), 7.02 (t, J = 7.5 Hz, 2H), 6.90 (t, J = 7.4 Hz, 2H), 6.19 (d, J = 7.6 Hz, 2H). 19F NMR (564 MHz, DMSO-d6, δ): −69.48 (s, 3F), −70.68 (s, 3F). MS (ESI) [m/z]: [M − PF6]+ calcd for C32H24IrN4, 657.16+; found, 657.16+; [M − Ir(ppy)2(bpy)]− calcd for PF6, 144.96−; found, 144.96−. A single crystal was successfully grown from acetone and methanol mixed solutions and characterized by X-ray crystallography. Space group of Pbca with a = 10.8913(17) Å, b = 15.838(3) Å, c = 33.478(5) Å, α = 90°, β = 90°, γ = 90°, V = 5774.8(16) Å3, Z = 8, dcalcd = 1.844 g/cm3, R1 = 0.0691, wR2 = 0.1236 for 8352 observed reflections [I ≥ 2σ(I)] (CCDC 1046402). [Ir(ppy)2(bpy)][B(5FPh)4] (Complex 3). Complex 3 was synthesized through an ion-exchange reaction at room temperature in air between complex 1 and Na[B(5FPh)4], then purified by column chromatography on silica gel (200− 300 mesh) with dichloromethane as the eluent, producing a yellow powder, yield: 65%. 1H NMR (600 MHz, DMSO-d6, δ): 8.89 (d, J = 8.4 Hz, 2H), 8.28 (dd, J = 11.0, 4.8 Hz, 4H), 7.96−7.91 (m, 4H), 7.89−7.87 (m, 2H), 7.70 (ddd, J = 7.6, 5.5, 1.1 Hz, 2H), 7.62 (dd, J = 5.8, 0.7 Hz, 2H), 7.18−7.15 (m, 2H), 7.05−7.00 (m, 2H), 6.91 (td, J = 7.5, 1.3 Hz, 2H), 6.20 (dd, J = 7.6, 0.6 Hz, 2H). 19F NMR (564 MHz, DMSO-d6, δ): −132.95 (s, 8F), −161.19 (s, 4F), −165.80 (s, 8F). MS (ESI) [m/z]: [M − B(5FPh)4]+ calcd for C32H24IrN4, 657.16+; found, 657.16+; [M − Ir(ppy)2(bpy)]− calcd for C24F20B, 678.98−; found, 678.98−. A single crystal was successfully grown from acetone and methanol mixed solutions and characterized by X-ray crystallography. Space group of P1̅ G

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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678.98−; found, 678.95−. A single crystal was successfully grown from acetone and methanol mixed solutions, then characterized by X-ray crystallography. Space group of P1̅ with a = 11.932(2) Å, b = 17.247(3) Å, c = 17.562(3) Å, α = 81.43(3)°, β = 75.31(3)°, γ = 89.13(3)°, V = 3456.0(12) Å3, Z = 2, dcalcd = 1.488 g/cm3, R1 = 0.0675, wR2 = 0.2188 for 15 784 observed reflections [I ≥ 2σ(I)] (CCDC 1813955). [Ir(piq)2(dtb-bpy)][B(dCF3Ph)4] (Complex 10). The synthetic route of complex 10 is similar to that of complex 5, while [Ir(ppy)2Cl]2 and Na[B(5FPh)4] were replaced by [Ir(piq)2Cl]2 and Na[B(dCF3Ph)4], respectively, producing a red powder, yield: 83%. 1H NMR (600 MHz, DMSO-d6, δ): 9.00−8.97 (m, 2H), 8.90 (s, 2H), 8.35 (d, J = 8.1 Hz, 2H), 8.08−8.04 (m, 2H), 7.92−7.85 (m, 4H), 7.69 (s, 4H), 7.66 (dd, J = 5.9, 1.6 Hz, 2H), 7.61 (d, J = 5.5 Hz, 10H), 7.59 (d, J = 6.1 Hz, 2H), 7.48 (d, J = 6.5 Hz, 2H), 7.11 (t, J = 7.6 Hz, 2H), 6.90 (t, J = 7.4 Hz, 2H), 6.16 (d, J = 7.6 Hz, 2H), 1.34 (s, 18H). 19F NMR (564 MHz, DMSO-d6, δ): −61.59 (s, 24F). MS (ESI) [m/z]: [M − B(dCF3Ph)4]+ calcd for C48H44IrN4, 869.32+; found, 869.31+; [M − Ir(piq)2(dtb-bpy)]− calcd for C32H12BF24, 863.06−; found, 863.05−. A single crystal was successfully grown from acetone and methanol mixed solutions, then characterized by X-ray crystallography. Space group of P21/n with a = 13.391(3) Å, b = 14.571(3) Å, c = 41.441(8) Å, α = 90°, β = 91.39(3)°, γ = 90°, V = 8084(3) Å3, Z = 4, dcalcd = 1.423 g/cm3, R1 = 0.0849, wR2 = 0.2667 for 16 473 observed reflections [I ≥ 2σ(I)] (CCDC 1813963). CCDC 1813954, 1046402, 1046825, 1046398, 1552813, 1552814, 1835311, 1813974, 1813955, and 1813963 contain the supplementary crystallographic data here and can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

calcd for C40H40IrN4, 769.29+; found, 769.29+; [M − Ir(ppy)2(dtb-bpy)]− calcd for C32H24F20B, 863.06−; found, 863.04−. A single crystal was successfully grown from acetone and methanol mixed solutions and characterized by X-ray crystallography. Space group of Pna21 with a = 26.597(3) Å, b = 13.1746(3) Å, c = 19.012(2) Å, α = 90°, β = 90°, γ = 90°, V = 6661.6(14) Å3, Z = 62, dcalcd = 1.609 g/cm3, R1 = 0.0469, wR2 = 0.1149 for 21 452 observed reflections [I ≥ 2σ(I)] (CCDC 1552814). [Ir(phq)2(dtb-bpy)][B(5FPh)4] (Complex 7). The synthetic route of complex 7 is similar to that of complex 5, while [Ir(ppy)2Cl]2 was replaced by [Ir(phq)2Cl]2, producing a yellow powder, yield: 74%. 1H NMR (600 MHz, DMSO-d6, δ): 8.55 (q, J = 9.0 Hz, 4H), 8.39 (d, J = 2.0 Hz, 2H), 8.28 (d, J = 7.0 Hz, 2H), 8.03−7.88 (m, 4H), 7.70 (dt, J = 16.6, 8.2 Hz, 2H), 7.43 (dd, J = 17.9, 10.9 Hz, 2H), 7.20−7.08 (m, 4H), 7.03 (ddd, J = 8.5, 6.8, 1.3 Hz, 2H), 6.85−6.75 (m, 2H), 6.40 (d, J = 7.6 Hz, 2H), 1.26 (s, 18H). 19F NMR (564 MHz, DMSO-d6, δ): −132.70 (s, 8F), −161.48 (s, 4F), −166.03 (s, 8F). MS (ESI) [m/z]: [M − B(5FPh)4]+ calcd for C48H44IrN4, 869.32+; found, 869.30+; [M − Ir(phq)2(dtb-bpy)]− calcd for C24BF20, 678.98−; found, 678.99−. A single crystal was successfully grown from acetone and methanol mixed solutions, then characterized by X-ray crystallography. Space group of P1̅ with a = 15.084(3) Å, b = 19.566(4) Å, c = 22.203(4) Å, α = 105.00(3)°, β = 90.11(3)°, γ = 90.42(3)°, V = 6329(2) Å3, Z = 2, dcalcd = 1.655 g/cm3, R1 = 0.0635, wR2 = 0.1273 for 28 619 observed reflections [I ≥ 2σ(I)] (CCDC 1835311). [Ir(phq)2(dtb-bpy)][B(dCF3Ph)4] (Complex 8). The synthetic route of complex 8 is similar to that of complex 5, while [Ir(ppy)2Cl]2 and Na[B(5FPh)4] were replaced by [Ir(phq)2Cl]2 and Na[B(dCF3Ph)4], respectively, producing a yellow powder, yield: 75%. 1H NMR (600 MHz, DMSO-d6, δ): 8.55−8.46 (m, 4H), 8.34 (t, J = 8.3 Hz, 2H), 8.23 (dd, J = 15.2, 6.6 Hz, 2H), 7.98−7.92 (m, 2H), 7.87 (t, J = 11.1 Hz, 2H), 7.72−7.63 (m, 6H), 7.57 (d, J = 14.3 Hz, 8H), 7.37 (t, J = 7.5 Hz, 2H), 7.11 (dd, J = 15.9, 7.8 Hz, 4H), 7.03−6.94 (m, 2H), 6.77 (t, J = 7.4 Hz, 2H), 6.38 (t, J = 11.4 Hz, 2H), 1.21 (s, 18H). 19F NMR (564 MHz, DMSO-d6, δ): −61.56 (s, 24F). MS (ESI) [m/z]: [M − B(dCF3Ph)4]+ calcd for C48H44IrN4, 869.32+; found, 869.29+; [M − Ir(phq)2(dtbbpy)]− calcd for C32H12BF24, 863.06−; found, 863.09−. A single crystal was successfully grown from acetone and methanol mixed solutions, then characterized by X-ray crystallography. Space group of C2/c with a = 36.392(7) Å, b = 12.255(3) Å, c = 34.626(7) Å, α = 90°, β = 108.42(3)°, γ = 90°, V = 14651(6) Å3, Z = 8, dcalcd = 1.571 g/cm3, R1 = 0.0760, wR2 = 0.2585 for 12 876 observed reflections [I ≥ 2σ(I)] (CCDC 1813974). [Ir(piq)2(dtb-bpy)][B(5FPh)4] (Complex 9). The synthetic route of complex 9 is similar to that of complex 5, while [Ir(ppy)2Cl]2 was replaced by [Ir(piq)2Cl]2, producing a red powder, yield: 77%. 1H NMR (600 MHz, DMSO-d6, δ): 8.98− 8.95 (m, 2H), 8.89 (d, J = 1.7 Hz, 2H), 8.32 (d, J = 8.0 Hz, 2H), 8.05 (dd, J = 5.3, 4.3 Hz, 2H), 7.89−7.83 (m, 4H), 7.64 (dd, J = 5.9, 1.8 Hz, 2H), 7.59 (d, J = 6.5 Hz, 2H), 7.56 (d, J = 5.9 Hz, 2H), 7.46 (d, J = 6.4 Hz, 2H), 7.08 (td, J = 7.5, 0.6 Hz, 2H), 6.87 (td, J = 7.5, 0.6 Hz, 2H), 6.13 (dd, J = 7.8, 1.2 Hz, 2H), 1.32 (s, 18H). 19F NMR (564 MHz, DMSO-d6, δ): −132.96 (s, 8F), −161.78 (s, 4F), −165.76 (s, 8F). MS (ESI) [m/z]: [M − B(5FPh)4]+ calcd for C48H44IrN4, 869.32+; found, 869.33+; [M − Ir(piq)2(dtb-bpy)]− calcd for C24BF20,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00716.

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Synthetic routes, single-crystal structures, excited state lifetimes, CV, TGA, the frontier MO surfaces, and detailed device characteristics (PDF) Crystallographic data files (ZIP)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lian Duan: 0000-0001-7095-2902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Tsinghua University Initiative Scientific Research Program, the National Natural Science Foundation of China (Grant Nos. U1601651, 51525304), and the National Key Basic Research and Development Program of China (Grant Nos. 2015CB655002, 2016YFB0400702, 2016YFB04001003) for financial support. H

DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsphotonics.8b00716 ACS Photonics XXXX, XXX, XXX−XXX