Highly Efficient Deep-Red Organic Light-Emitting Devices Based on

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Highly Efficient Deep-Red Organic Light-Emitting Devices Based on Asymmetric Iridium(III) Complexes with the Thianthrene 5,5,10,10Tetraoxide Moiety Yuanhui Sun,† Xiaolong Yang,† Zhao Feng,† Boao Liu,† Daokun Zhong,† Junjie Zhang,† Guijiang Zhou,*,† and Zhaoxin Wu*,‡

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†

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Department of Chemistry, School of Science and ‡Key Laboratory of Photonics Technology for Information, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China S Supporting Information *

ABSTRACT: Highly efficient deep-red organic light-emitting devices (OLEDs) are indispensable for developing high-performance red-greenblue (RGB) displays and white OLEDs (WOLEDs). However, the shortage of deep-red emitters with high photoluminescence quantum yields (PLQYs) and balanced charge injection/transport abilities has severely restricted the performance of deep-red OLEDs. Herein, we design and synthesize four efficient emitters by combining the isoquinoline group with the thianthrene 5,5,10,10-tetraoxide group. Benefited from the introduction of the thianthrene 5,5,10,10-tetraoxide group, these Ir(III) complexes show improved electron-injection/-transport abilities. By enhancing the contribution of the triplet metal-to-ligand charge transfer (3MLCT) in emissions, the asymmetric configuration endows the related deep-red Ir(III) complexes with high PLQYs of 0.45−0.50 in solutions. More importantly, PLQYs of these Ir(III) complexes in doped host films increase up to 0.91, which is much higher than PLQYs reported for conventional deep-red Ir(III) complexes with impressive electroluminescent performance. As a result, solution-processed OLEDs based on these Ir(III) complexes exhibit deep-red emissions with Commission Internationale de L’Eclairage (CIE x, y) coordinates very close to the National Television System Committee (NTSC)-recommended standard red CIE coordinates of (0.67, 0.33). Furthermore, a deep-red OLED using the asymmetric Ir(III) complex SOIrOPh as the emitter shows outstanding performance with a peak external quantum efficiency (EQE) of 25.8%, which is the highest EQE reported for solution-processed deep-red OLEDs. This work sheds light on the great potential of utilizing the thianthrene 5,5,10,10-tetraoxide group to develop phosphorescent emitters for highly efficient OLEDs. KEYWORDS: deep red, organic light-emitting devices (OLEDs), asymmetric iridium(III) complexes, thianthrene 5,5,10,10-tetraoxide, solution-processed

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INTRODUCTION Organic light-emitting devices (OLEDs) have been widely regarded as a very promising candidate for the next-generation displays and lighting technologies because of their excellent color quality, simple construction, low cost, environmentalfriendly nature, and energy-saving properties.1−5 As a result, there is an urgent demand for organic emitters with excellent color purity and sufficient luminous efficiency for all of these applications. Phosphorescent iridium(III) complexes are among the most promising emitters for OLEDs because of their good thermal stability, high photoluminescence quantum yields (PLQYs), short phosphorescence lifetimes, and excellent color tunability.6−10 Currently, blue, green, yellow, orange, and red OLEDs based on Ir(III) complexes can show very high external quantum efficiencies (EQEs) over 30%.11−15 To meet the target of high-performance red-green-blue (RGB) displays and white OLEDs (WOLEDs) with high color quality and © XXXX American Chemical Society

purity, efficient saturated or deep-red emitters are indispensable. However, the highest EQEs of deep-red OLEDs with Commission Internationale de L’Eclairage (CIE x, y) coordinates close to the National Television System Committee (NTSC)-recommended standard red CIE coordinates of (0.67, 0.33) are usually around 20%.16−22 For example, Nagai et al. reported that the deep-red OLED based on the phosphorescent Ir(III) complex bis(2,3diphenylquinoxaline)Ir(dipivaloylmethane) [(DPQ)2Ir(dpm)] showed a peak EQE of 17.9% with CIE coordinates of (0.70, 0.29) using energy transfer from an exciplex host to the emitter.17 Jing et al. synthesized a 2,3-diphenylquinoxalinebased phosphorescent Ir(III) complex that could display a Received: April 17, 2019 Accepted: July 2, 2019 Published: July 2, 2019 A

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Synthetic Routes and Structural Drawings of Thianthrene 5,5,10,10-Tetraoxide-Based Asymmetric and Symmetric Ir(III) Complexes

of the conjugation length, which will impair the device performance. To address these issues about the deep-red color purity, luminescence efficiency, and the balanced hole and electroninjection/-transport property, we designed and synthesized symmetric and asymmetric Ir(III) complexes based on the thianthrene 5,5,10,10-tetraoxide group (Scheme 1) as phosphorescent emitters for solution-processed deep-red OLEDs. Taking advantage of its large conjugation, the isoquinoline group was used for red-shifting the emissions of these Ir(III) complexes to the deep-red region to meet the demand of color purity.27−30 The incorporation of the thianthrene 5,5,10,10-tetraoxide group into Ir(III) complexes was for two reasons. First, based on previous studies, which indicate that Ir(III) complexes containing sulfonyl groups usually can show enhanced PLQYs,31−34 we presume that Ir(III) complexes bearing thianthrene 5,5,10,10-tetraoxide groups will also display high PLQYs. Besides, by introducing one more sulfonyl group into the ligand to link the two phenyl rings, we can significantly enhance the rigidity of the molecular structure, which may suppress the nonradiative decays and thereby further improve the PLQYs.35,36 Second, with the aid of its strong electron-withdrawing property, the thianthrene 5,5,10,10-tetraoxide group can be used to improve the electron-injection/-transport abilities of emitters and thereby to balance the charge transport behavior within OLEDs. As a

deep-red electroluminescence showing CIE coordinates of (0.687, 0.313) with the maximum EQE of 19.9%.18 On the basis of a thiophene-phenylquinoline-based Ir(III) complex bearing an electron transport group, Jin et al. successfully fabricated a deep-red OLED achieving a peak EQE of 21.48% with CIE coordinates of (0.67, 0.32).19 Therefore, compared with other color-emitting OLEDs, deep-red OLEDs with higher efficiencies are really demanding. However, efficient deep-red emitters are intrinsically more difficult to develop because of the energy gap law, which indicates that PLQYs will decrease as emissions shift to the longer wavelength region.16,21,23 Thus, the development of highly efficient deepred emitters with satisfactory color purity remains a great challenge. Besides the color purity and PLQYs, the balanced hole and electron-injection/-transport behavior within OLEDs is another crucial aspect for developing high-performance deep-red OLEDs.24 However, to meet the standard red CIE coordinates, emission colors of cyclometalated Ir(III) complexes can be shifted to the deep-red region by extending the conjugation length of ligands and/or introducing electrondonor groups into the organic ligands of the complexes.22,25−27 Unfortunately, these methods may result in unbalanced charge injection/transport behaviors within the OLEDs because of the enhanced hole injection/transport ability induced by the incorporation of electron-donor groups and/or the extension B

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Perspective views of SOIrB1 and SOIrB2 (the solvent and H atoms are omitted for clarity; thermal ellipsoids are drawn at the 30% probability level).

18−26%, which are among the highest efficiencies reported for deep-red OLEDs so far.

result, relative to the well-known deep-red emitter, bis(1phenylisoquinoline)Ir(acetylacetonate) [(piq)2Ir(acac)],37 complex SOIrSO possessed a reduced lowest unoccupied molecular orbital (LUMO) level of ca. −3.28 eV and improved electron mobility. Meanwhile, SOIrSO showed a broad photoluminescence (PL) spectrum in the deep-red region with the PLQY of 0.56 in THF solution. With the intention to delicately manipulate the charge carrier injection/transport properties and further fine-tune the properties of Ir(III) complexes, we also synthesized three asymmetric Ir(III) complexes SOIrOPh, SOIrB1, and SOIrB2 (Scheme 1) with one thianthrene 5,5,10,10-tetraoxide-based ligand. On the one hand, the asymmetric structure can increase the solubility of resultant Ir(III) complexes. Especially, ligands L-B1 and L-B2 with large dimesitylboron groups can further improve the solubility to benefit the device fabrication process by the solution method. On the other hand, the different electronwithdrawing abilities of L-OPh, L-B1, and L-B2 will further adjust the emission energies and the charge injection/transport properties. The inherent electron-deficient boron atoms in dimesitylboron groups are very useful to enhance the electroninjection/-transport ability of related organometallic complexes.38,39 Consequently, these asymmetric Ir(III) complexes showed improved electron-injection/-transport behaviors and displayed much deeper red emissions with PLQYs remaining higher than 0.45 in THF solutions. The PLQYs of doped films containing these complexes were even increased up to 0.91. Finally, highly efficient deep-red OLEDs were fabricated with emission CIE coordinates very close to NTSC-recommended standard red CIE coordinates of (0.67, 0.33). Moreover, the maximum EQEs of these deep-red OLEDs were in the range of

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes and structural drawing of thianthrene 5,5,10,10-tetraoxidebased symmetric and asymmetric Ir(III) complexes are depicted in Scheme 1. The synthesis and characterization details of organic ligands are given in the Supporting Information (Schemes S1 and S2). Using the one-pot method, that is, heating the thianthrene 5,5,10,10-tetraoxide-based ligand, the pyridine- or thiazole-based ligand, and IrCl3· nH2O together in a mixture of THF/H2O, the symmetric and asymmetric Ir(III) complexes were obtained at the same time with acceptable yields. Because of their large difference in polarity, the symmetric and asymmetric Ir(III) complexes could be conveniently separated by column chromatography on silica gel. These four Ir(III) complexes were fully characterized by NMR and MS (Figures S1−S4 in the Supporting Information). The single-crystal X-ray diffraction results of SOIrB1 and SOIrB2 further confirmed the structures of these complexes. As shown in Figure 1, asymmetric structures of SOIrB1 and SOIrB2 can be observed clearly. The Ir centers in SOIrB1 and SOIrB2 are coordinated with two different cyclometalating ligands and an acetylacetonate (acac) auxiliary ligand, resulting in a distorted octahedral coordination geometry. Besides, the thianthrene 5,5,10,10-tetraoxide group bends along the two sulfur atoms to form dihedral angles of 133.5° and 139.5° in SOIrB1 and SOIrB2, respectively. The detailed crystallographic data and selected bond lengths and angles are summarized in the Supporting Information (Tables S1−S3). C

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) UV−vis absorption and PL spectra of these complexes in THF and (b) PL spectra of TCTA films doped with these complexes at the concentration of 8 wt %.

Table 1. Photophysical, Thermal, and Electrochemical Data for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO λabsa (nm) SOIrOPh SOIrB1 SOIrB2 SOIrSO

236 233 233 236

(4.73), (4.67), (4.69), (4.68),

276 305 276 276

(4.65), (4.59), (4.54), (4.60),

348 366 324 346

(4.36), (4.49), (4.49), (4.42),

498 491 499 476

λem (nm)/τ (μs)/PLQYb

λem (nm)/τ (μs)/PLQYc

Td (°C)

HOMO/LUMOd (eV)

652/0.23/0.48 646/0.19/0.45 660/0.22/0.50 621/0.22/0.56

644/0.40/0.91 641/0.43/0.84 648/0.43/0.86 621/0.44/0.66

351 330 331 381

−5.46/−3.17 −5.47/−3.14 −5.40/−3.13 −5.70/−3.28

(3.50), 572 (3.24) (3.50), 566 (3.13) (3.66), 575 (3.28) (3.80)

Measured in THF at room temperature, log ε values are shown in parentheses. bMeasured in THF at room temperature, PLQY measured relative to (piq)2Ir(acac) (PLQY = 0.20) with 420 nm excitation. cMeasured in doped TCTA films at the concentration of 8 wt % (the doping level for SOIrB2 was 6 wt %). dCalculated from the onset potentials of oxidation (Eox) and reduction (Ered) according to EHOMO = −(Eox + 4.8) eV and ELUMO = −(Ered + 4.8) eV. a

featureless PL spectra in THF solutions, and the asymmetric complexes displayed obviously red-shifted emission, which was in good agreement with their red-shifted low energy absorption bands (see the enlarged region of the absorption spectra in Figure 2a). Relative to the well-known deep-red emitter (piq)2Ir(acac) (PLQY = 0.20), PLQYs of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO were determined to be as high as 0.48, 0.45, 0.50, and 0.56, respectively. The photophysical behaviors of these complexes doped in the 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) host were also investigated (Figure S6). As shown in Figures 2b and S7, complexes in doped TCTA films showed PL spectra with featured shoulders, indicating that the mixed 3MLCT and 3π−π* states were responsible for these emissions.30,42 Different from that in solutions, the free intramolecular motions of complexes would be restricted in doped films, and the nonradiative decays would be suppressed to some extent, resulting in improved emission efficiency.6,43 Therefore, these complexes could display higher PLQYs in doped TCTA films. At relatively low doping levels, very high PLQYs in the range of 0.68−0.91 were achieved, which were among the highest efficiencies reported for deepred emitters.16−21 It was interesting that the improvement of the PLQY for SOIrSO was not as high as those for asymmetric complexes. The possible reason is that SOIrSO already possessed a relatively rigid structure due to the two rigid thianthrene 5,5,10,10-tetraoxide-based ligands; thus, the change degree of restriction on intramolecular motions for SOIrSO from the solution to the doped film was relatively lower than those for asymmetric complexes. However, at the higher concentration of 10 wt %, PLQYs of all films were decreased due to the triplet−triplet annihilation (TTA) effect.44 In doped TCTA films, all of these complexes showed excitation lifetimes shorter than 0.5 μs (Table 1 and Figure S6), which would benefit the related OLED performance by reducing the efficiency roll-off.44 When recorded at 77 K, the PL spectra of these complexes displayed blue-shifted peaks

In the SOIrB1 crystal, the bond length of Ir1−N2 (2.030(4) Å) is shorter than that of Ir1−N1 (2.037(4) Å), and the bond length of Ir1−C42 (1.974(5) Å) is shorter than that of Ir1−C9 (2.010(5) Å), indicating that the thianthrene 5,5,10,10tetraoxide-based ligand has a stronger interaction with the Ir center than the thiazole-based ligand. The same conclusion can be drawn for SOIrB2 for a similar reason. The differences in the bond lengths can influence the thermal stability of these complexes. The thermal properties of these complexes were investigated by thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 20 K min−1. As shown in Figure S5a, the decomposition temperature (Td) of the symmetric complex SOIrSO (Td = 381 °C) was higher than those of asymmetric complexes SOIrOPh (Td = 351 °C), SOIrB1 (Td = 330 °C), and SOIrB2 (Td = 331 °C). As indicated by the differential scanning calorimetry (DSC) curves (Figure S5b), no obvious glass-transition temperatures were observed for these Ir(III) complexes. Nevertheless, with Tds higher than 330 °C, these complexes showed thermal stabilities high enough for OLEDs in both fabrication and operation processes. Photophysical Properties. The UV−vis absorption and photoluminescence (PL) spectra of these complexes in THF solutions are shown in Figure 2a, and the related data are summarized in Table 1. The strong absorption bands in the high-energy region (230−380 nm) can be assigned to the spinallowed ligand-centered singlet π−π* transition of two different cyclometalating ligands, and the relatively weak and broad absorption bands in the range of 400−550 nm are attributed to the spin-allowed singlet metal-to-ligand charge transfer (1MLCT) and the spin-forbidden triplet metal-toligand charge transfer (3MLCT) mixed with the ligand-toligand charge transfer (LLCT).18,30,40,41 Under UV irradiation, these complexes emitted bright deep-red phosphorescence with emission peaks in the range from 621 to 660 nm. As shown in Figure 2a, all of these complexes showed broad and D

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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complexes should be mainly attributed to the strong electron-withdrawing thianthrene 5,5,10,10-tetraoxide segments. Besides, the electron-deficient dimesitylboron group could also help enhance the electron mobilities.38,39 We also fabricated the hole-only devices, which implied that the neat films of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO possessed hole mobilities (μh) of ca. 2.4 × 10−6, 2.3 × 10−6, 2.0 × 10−6, and 1.8 × 10−6 cm2 V−1 s−1, respectively. At the same condition, the μh of the (piq)2Ir(acac) neat film was deduced to be ca. 2.5 × 10−6 cm2 V−1 s−1. Apparently, the incorporation of the thianthrene 5,5,10,10-tetraoxide group favored electron transport more over hole transport, which would lead to relatively more balanced charge transport properties. To further investigate the charge transport property in the OLED, the hole-only and electron-only devices based on SOIrOPh-doped TCTA films were fabricated. As shown in Figure S10, with the doping concentration increasing from 0, 6, 8 to 10 wt %, the hole-only devices showed reduced current densities, whereas the electron-only devices displayed increased current densities. This result could be explained by the frontier molecular orbital energy-level alignment of TCTA and SOIrOPh. On the one hand, with a higher HOMO level of −5.46 eV than TCTA (HOMO level of −5.7 eV), SOIrOPh might facilitate the hole injection process by reducing the hole injection barrier.45 However, SOIrOPh molecules might also act as traps for holes injected to the SOIrOPh-doped TCTA film to retard the hole transport process, and the trap density would be increased at a higher doping level.46,47 Therefore, the 10 wt % doped TCTA film showed a relatively lower current density among the hole-only devices. On the other hand, compared with TCTA (LUMO level of −2.4 eV), SOIrOPh possessed a lower LUMO level of −3.17 eV, which apparently would improve the electron injection due to the small injection barrier. In the meantime, the multiple reversible reduction waves observed in the CV test (Figure S9) indicated that SOIrOPh preferred to exhibit good electron-transport ability rather than act as electron traps. Therefore, higher current densities of doped TCTA films were observed at the higher doping concentration. The above results demonstrated that although the doping concentrations of the Ir(III) complex were not higher than 10 wt %, still the charge transport property was affected by the doped Ir(III) complex to some extent. More importantly, the doped films showed reduced hole-transport ability and enhanced electron transport behavior, which was of vital importance for improving the charge transport balance and the final device performance.48,49 Theoretical Calculation. To gain a deep understanding of photophysical and electrochemical properties of these Ir(III) complexes, we performed the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The calculated bond lengths and angles for SOIrB1 and SOIrB2 were very close to those detected by single-crystal X-ray diffraction (Tables S2 and S3), confirming the validation of the theoretical calculation method. The theoretical calculation results are presented in Figure 4 and Table 2. The calculation results indicated that HOMO → LUMO transitions made a dominant contribution (>93%) to S1 states and significant contribution (>63%) to T1 states. For the symmetric Ir(III) complex SOIrSO, its HOMO was distributed on the Ir center and two phenyl rings chelating to the Ir center, whereas its LUMO mainly located on the two isoquinoline groups and the two phenyl rings chelating to the Ir center. Therefore, both the lowest singlet transition (S0 → S1) and the lowest triplet

(637, 630, 647, and 616 nm for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO, respectively) with obvious shoulders (Figure S8), again implying that the phosphorescence of these complexes originated from a mixture of 3MLCT and 3π−π* states.30,42 Nevertheless, these results have clearly demonstrated the success of our conception of combining the isoquinoline group and the thianthrene 5,5,10,10-tetraoxide group to construct deep-red phosphorescent Ir(III) complexes with high PLQYs. Electrochemical Properties. The highest occupied molecular orbital (HOMO) and LUMO levels were estimated by cyclic voltammetry (CV) in CH3CN solutions. During the anodic scan, these Ir(III) complexes showed irreversible or reversible metal-related oxidation processes with the oxidation potential (versus Fc/Fc+) onsets of 0.66, 0.67, 0.60, and 0.90 V for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO (Figure S3), respectively. Thus, corresponding HOMO levels of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO were calculated to be −5.46, −5.47, −5.40, and −5.70 eV, respectively. Containing the strong electron-withdrawing thianthrene 5,5,10,10-tetraoxide group, these Ir(III) complexes displayed several irreversible or reversible reduction processes during the cathodic scan (Figure S9). Based on the first reduction potential (versus Fc/Fc+) onsets of −1.63, −1.66, −1.67, and −1.52 V for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO, respectively, the LUMO levels of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO were calculated to be −3.17, −3.14, −3.13, and −3.28 eV, respectively, which were lower than that of (piq)2Ir(acac) (LUMO level of 2.84 eV). 37 The electrochemical HOMO−LUMO gaps for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO were calculated to be 2.29, 2.33, 2.27, and 2.42 eV, respectively. Therefore, it is reasonable that asymmetric Ir(III) complexes SOIrOPh, SOIrB1, and SOIrB2 could show much deeper red emissions. Charge Mobilities. With deeper LUMO levels, SOIrOPh, SOIrB1, SOIrB2, and SOIrSO would have a better chance to trap and transport electrons. As shown in Figure 3, the

Figure 3. Current density−voltage (J−V) curves of hole-only and electron-only devices based on neat films of these complexes.

electron-only devices based on neat films of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO showed higher current densities compared with that based on (piq)2Ir(acac), which implied the enhanced electron-injection/-transport abilities of these thianthrene 5,5,10,10-tetraoxide-based Ir(III) complexes. Based on the space-charge-limited-current (SCLC) method, electron mobilities (μe) of neat films of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO were determined to be ca. 6.8 × 10−7, 7.7 × 10−7, 7.9 × 10−7, and 7.1 × 10−7 cm2 V−1 s−1, respectively, which were higher than that of the (piq)2Ir(acac) neat film (μe = 5.8 × 10−7 cm2 V−1 s−1). The improved electron mobilities of these newly synthesized Ir(III) E

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Representative molecular orbitals (MOs) for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO.

Table 2. Calculation Results for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO-Based Optimized S0 Geometries complexes

MOs

SOIrOPh L H H−1 H−3 SOIrB1 L H H−6 SOIrB2 L H H−3 SOIrSO L+1 L H H−3

contribution from metal center orbitals and ligand orbitals to MOs (%) Ir 4.23 34.25 28.76 48.11 Ir 4.10 38.13 28.52 Ir 4.11 40.98 18.22 Ir 4.36 3.17 42.32 2.25

L-SO 94.61 14.06 6.53 36.54 L-SO 94.39 16.34 45.91 L-SO 94.46 17.84 34.12 L-SO 47.41 48.24 26.25 43.95

L-OPh 0.53 48.09 25.34 8.94 L-B1 0.85 40.38 15.57 L-B2 0.79 36.11 40.01 L-SO 47.44 48.21 26.25 43.80

main contribution for S0 → S1 excitation/Ecal/f a

acac 0.62 3.60 39.37 6.41 acac 0.58 5.04 8.01 acac 0.64 5.06 7.66 acac 0.70 0.30 4.75 8.97

main contribution for S0 → T1 excitation/Ecala

H → L (93.3%) 2.297 eV (540 nm) 0.0212

H → L (68.9%) H−1 → L (9.0%) H−3 → L (9.2%) 2.042 eV (607 nm)

H → L (95.6%) 2.334 eV (531 nm) 0.0225

H → L (69.6%) H−6 → L (16.0%) 2.067 eV (600 nm)

H → L (95.8%) 2.285 eV (543 nm) 0.0277

H → L (77.7%) H−3 → L (11.3%) 2.031 eV (611 nm)

H → L (97.0%) 2.502 eV (496 nm) 0.0618

H → L (63.7%) H−3 → L+1 (16.3%) 2.145 eV (578 nm)

a

H and L stand for HOMO and LUMO, respectively. Ecal and f denote the calculated excitation energy (in wavelength) and oscillator strength, respectively.

transition (S0 → T1) of SOIrSO could be assigned to a mixture of MLCT and π−π* transitions. Unlike the symmetric Ir(III) complex SOIrSO that distributed the HOMO/LUMO equally on the two identical cyclometalating ligands, asymmetric Ir(III) complexes SOIrOPh, SOIrB1, and SOIrB2 possessed HOMOs mainly locating on the Ir centers and the cyclometalating ligands without thianthrene 5,5,10,10-tetraoxide

groups (L-OPh, L-B1, and L-B2), and LUMOs contributed mainly from the cyclometalating ligands with thianthrene 5,5,10,10-tetraoxide groups (L-SO). This could explain the CV results that ligands L-OPh, L-B1, and L-B2 in the asymmetric Ir(III) complexes exerted a stronger influence on HOMO levels than LUMO levels. Therefore, both S0 → S1 and S0 → T1 of SOIrOPh, SOIrB1, and SOIrB2 could be assigned to the F

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. NTO distributions of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO based on optimized T1 geometries.

resulted from the 3π−π* transition with minor contribution from 3MLCT. As for SOIrOPh, SOIrB1, and SOIrB2, the contribution from Ir centers to hole orbitals was increased to around 25%, and the cyclometalating ligands without thianthrene 5,5,10,10-tetraoxide groups (L-OPh, L-B1, and L-B2) also made a little contribution to hole orbitals. The particle orbitals of SOIrOPh, SOIrB1, and SOIrB2 were dominantly contributed from the thianthrene 5,5,10,10tetraoxide-based cyclometalating ligand (>93%). Therefore, the phosphorescent emissions of SOIrOPh, SOIrB1, and SOIrB2 mainly resulted from the 3π−π* transitions with increased contribution from 3MLCT and a little contribution from 3LLCT. These calculation results are very helpful for explaining the experimental phenomena. As aforementioned, the PL spectra of these complexes became structured at 77 K compared with those at room temperature. This is because their phosphorescence emissions were composed of dominant 3 π−π* characters. In addition, the emission peaks of PL spectra at 77 K were blue-shifted, indicating the charge transfer feature, which could be attributed to 3MLCT. Furthermore, because of the increased contribution from 3MLCT and a little extra contribution from 3LLCT, the blue shifts of emission peaks for SOIrOPh, SOIrB1, and SOIrB2 were larger than that for SOIrSO (ca. 15 nm for SOIrOPh, SOIrB1, and SOIrB2 versus ca. 5 nm for SOIrSO). The energy law indicates that low energy emissions are usually accompanied by low PLQYs;50 thus, even deep-red Ir(III) complexes with outstanding electroluminescent performance showed PLQYs below 0.60.18−22,25,51 Accordingly, with longer emission wavelengths, SOIrOPh, SOIrB1, and SOIrB2 showed lower PLQYs compared with SOIrSO. If SOIrOPh, SOIrB1, and SOIrB2 possessed contributions from the 3MLCT to emissions similar to SOIrSO, then SOIrOPh, SOIrB1, and SOIrB2 might exhibit PLQYs less than 0.60 because of the energy law. However, SOIrOPh, SOIrB1, and SOIrB2 doped in TCTA films could show high PLQYs in the range of 0.84− 0.91, which should associate with the enhanced 3MLCT features since improving the contribution of the 3MLCT to emissions is helpful to increase PLQYs of Ir(III) complexes.30,52 In addition, although SOIrOPh, SOIrB1, and SOIrB2 showed particle orbitals (mainly contributed by the isoquinoline-based ligand L-SO) similar to SOIrSO, they exhibited increased contribution from Ir centers and ligands without thianthrene 5,5,10,10-tetraoxide groups (L-OPh, L-

MLCT/LLCT transition. Considering the notable contributions from HOMO−3 → LUMO (9.2%), HOMO−6 → LUMO (16.0%), and HOMO−3 → LUMO (11.3%) to S0 → T1 transitions for SOIrOPh, SOIrB1, and SOIrB2, respectively, the π−π* transitions were also partially responsible for the S0 → T1 transitions of SOIrOPh, SOIrB1, and SOIrB2. Therefore, low energy absorptions of SOIrOPh, SOIrB1, and SOIrB2 were composed of MLCT/LLCT characters mixed with notable π−π* transitions. The calculated energies of T1 states for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO were 2.04, 2.07, 2.03, and 2.14 eV, respectively. This trend was in good agreement with the low energy absorptions as well as the electrochemical HOMO−LUMO gaps. We also calculated the natural transition orbitals (NTOs) based on optimized T1 geometries to gain deep insight into the origination of phosphorescent emissions of these complexes. The calculation results are presented in Figure 5 and Table 3. Table 3. NTO Results for SOIrOPh, SOIrB1, SOIrB2, and SOIrSO Based on Optimized T1 Geometries complexes

NTOs

SOIrOPh hole particle SOIrB1 hole particle SOIrB2 hole particle SOIrSO hole particle

contribution from metal center orbitals and ligand orbitals to NTOs (%) Ir 25.69 5.53 Ir 23.31 5.23 Ir 26.21 5.70 Ir 14.56 4.19

L-SO 65.44 93.15 L-SO 69.23 93.31 L-SO 63.66 93.07 L-SO 83.50 94.45

L-OPh 6.85 0.55 L-B1 5.24 0.70 L-B2 7.71 0.41 L-SO 1.93 1.36

acac 2.02 0.77 acac 2.12 0.69 acac 2.42 0.82 acac 0.80 0.57

For all of these Ir(III) complexes, the NTO calculation results showed that hole → particle transitions made dominant contribution (>99.0%) to their T1 states. As listed in Table 3, for SOIrSO, the hole orbital was mainly distributed on one cyclometalating ligand (83.50%) with minor contribution from the Ir center (14.56%), and the particle orbital was dominantly contributed from the same cyclometalating ligand (94.45%). Therefore, the phosphorescent emission of SOIrSO mainly G

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Figure 6. AFM images (10 μm × 10 μm) for TCTA films doped with SOIrOPh, SOIrB1, SOIrB2, and SOIrSO at concentrations of 6, 8, and 10 wt %.

Figure 7. Energy levels and the chemical structures of materials used in OLEDs.

B1, and L-B2) to the hole orbitals, which were clearly caused by the second cyclometalating ligands and thus significantly influenced the emission color. It was interesting that SOIrOPh, SOIrB1, and SOIrB2 displayed similar emission wavelengths despite the significant difference in structures of the second cyclometalating ligands L-OPh, L-B1, and L-B2. Hence, adopting an asymmetric structure can provide a flexible scope to finely optimize other properties, such as electron-

transport abilities (Figure 3), of Ir(III) complexes while maintaining their desired deep-red emission color. It is unlikely for conventional symmetric Ir(III) complexes to optimize other properties without notably changing emission colors.7,8 These results demonstrated the advantages of designing an asymmetric configuration in fine-tuning properties of Ir(III) complexes. H

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Figure 8. Best EL performance of OLEDs based on the complexes: (a) EL spectra, (b) J−V−L characteristics, (c) curves of EQE vs luminance, and (d) curves of CE and PE vs luminance.

Table 4. Key EL Performance of OLEDs Based on SOIrOPh, SOIrB1, SOIrB2, and SOIrSO devices

λEL (nm)

Vturn‑on (V)

Lmax (cd m−2)

EQEa (%)

CEa (cd A−1)

PEa (lm W−1)

CIE (x, y)

A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3

636 640 640 636 636 636 644 644 644 620 620 620

3.7 3.5 4.1 3.5 3.6 3.8 3.5 3.5 3.9 5.1 3.9 4.1

5443 4450 2615 3662 4458 2932 5007 5906 6560 3645 4757 3346

13.4/11.1 25.8/23.6 10.4/9.40 12.0/11.1 19.0/18.0 7.65/5.16 18.5/15.9 10.8/9.54 8.43/7.31 9.10/7.20 10.4/9.57 6.34/5.30

7.41/6.24 12.8/11.7 4.96/4.49 7.02/6.53 10.6/10.0 4.45/3.00 9.07/7.80/5.12 5.10/5.53/3.10 4.35/3.77/2.58 8.70/6.79 9.92/9.13 5.88/5.84

5.65/2.19 9.14/4.12 3.76/1.88 5.35/2.75 7.62/3.75 3.26/1.04 7.21/3.13/1.41 4.26/2.03/0.97 3.19/1.60/0.78 6.52/1.96 7.62/3.30 4.30/1.54

(0.66, (0.68, (0.67, (0.65, (0.68, (0.68, (0.69, (0.68, (0.68, (0.65, (0.65, (0.65,

0.31) 0.30) 0.30) 0.30) 0.31) 0.31) 0.31) 0.31) 0.32) 0.33) 0.33) 0.33)

Efficiencies on the order of the maximum value/at a luminance of 100 cd m−2.

a

Morphology of Doped Films. Since the morphology of the emissive layer is also important for OLED device performance,53 we investigated this property of TCTA films doped with SOIrOPh, SOIrB1, SOIrB2, and SOIrSO at concentrations of 6, 8, and 10 wt % using an atomic force microscope (AFM). As shown in Figure 6, all of the doped films showed significantly small root-mean-square (RMS) roughness below 0.4 nm. In addition, except some slight wrinkles, no pinholes were observed. In detail, due to the better solubilities of SOIrOPh, SOIrB1, and SOIrB2, the related films displayed smoother surfaces and uniform morphologies. Especially, the SOIrOPh-doped film showed the smallest RMS roughness of only 0.210 nm without any needlelike projection at low doping concentrations. However, obvious bulky projections, likely the emitter aggregation, were recorded for films containing SOIrSO at different concentrations. Therefore, the SOIrOPh-doped film could show the best film morphology to avoid self-quenching effects and thus benefit the device performance.53

Electroluminescence Performance. Because of their attractive emission color purity, high PLQYs, improved electron-injection/-transport abilities as well as good solubility, these complexes were used as emitters to fabricate deep-red OLEDs by the solution-processed method. The devices had a conventional structure of ITO/PEDOT:PSS (45 nm)/x wt % Ir complexes: TCTA (30 nm)/TPBI (45 nm)/LiF (1 nm)/Al (100 nm) in which poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), TCTA, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI), and LiF played the roles of hole transport, host, electron transport, and electron injection, respectively (Figure 7). The devices were optimized by adjusting the doping concentration (6, 8, and 10 wt % of SOIrOPh, SOIrB1, SOIrB2, and SOIrSO in TCTA for devices A1−3, B1−3, C1−3, and D1−3, respectively). Figures 8, S11, and S12 show the electroluminescence (EL) spectra, curves of current density (J)−voltage (V)−luminance (L), and curves of efficiencies versus luminance of all devices. The related key EL data is summarized in Table 4. The profiles of I

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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practical luminance of 100 cd m−2. The high EL performance of these Ir(III) complexes could be attributed to the suitable HOMO/LUMO levels, high PLQYs, and improved electroninjection/-transport ability ensuring a more balanced injection/transport behavior within the devices.59 These results demonstrate that the thianthrene 5,5,10,10-tetraoxide group is a very useful block to develop high EL performance phosphorescent emitters.

all EL spectra were similar to those of the respective PL spectra of Ir(III) complexes doped in TCTA films, and no typical TCTA emissions peaking at ca. 390 nm were detected. The triplet energy (ET) of TCTA is 2.98 eV, which is high enough to prevent the inverse energy transfer from Ir(III) complexes to the TCTA host.54 Besides, as shown in Figure 2a, these complexes showed strong absorptions at ca. 390 nm, illustrating an effective spectral overlap between the emission of the TCTA host and the absorption of these complexes. Therefore, the high ET of the TCTA host and good overlap between the emission of the TCTA host and absorptions of dopants could guarantee efficient energy transfer from the TCTA host to these complexes and effective confinement of the triplet excitons within the emitting layers. As shown in Figure 7, compared with those of TCTA, these Ir(III) complexes possessed higher HOMO levels and lower LUMO levels. Therefore, besides the energy transfer from the TCTA host to these Ir(III) complexes, direct charge trapping on these Ir(III) complexes could possibly happen due to the energetically favored energy levels.47 Actually, this had been confirmed by the dependence of current densities on the doping concentration of SOIrOPh in TCTA films (Figure S10).46,47 Charge trapping followed by direct recombination of holes and electrons occurred on emitters can avoid the loss of energy transfer from the host to dopant and thus increase the electroluminescent efficiency.46,55,56 The turn-on voltages of these OLEDs were in the range of 3.5−5.1 V, which were comparable to those of previously reported efficient deep-red OLEDs fabricated by the solution-processed method.19,20,26 More importantly, all of these devices could show deep-red emissions with CIE coordinates very close to (0.67, 0.33) (Table 4). At a doping level of 8 wt %, devices based on asymmetric Ir(III) complexes SOIrOPh, SOIrB1, and SOIrB2 displayed impressive EL performance in terms of both color purity and efficiencies. The device A2 based on SOIrOPh showed peak EQE, current efficiency (CE), power efficiency (PE), and CIE coordinates of 25.8%, 12.8 cd A−1, 9.14 lm W−1, and (0.68, 30), respectively. To the best of our knowledge, the EQE of 25.8% is the highest EQE reported for solutionprocessed deep-red OLEDs and is comparable to the highest EQE of 25.9% reported for a deep-red OLED fabricated by the vacuum-deposited method.16−22,26,51,57,58 Besides the good electron-injection/-transport ability as well as efficient energy transfer from the host to the emitter, such high EQE of the device A2 should also relate to the impressive advantages of SOIrOPh such as direct charge trapping effect induced by suitable energy levels, higher PLQY (Table 1), and better emissive layer morphology (Figure 6). Under the nitrogen atmosphere, the operational lifetime of the unencapsulated device A2 was tested at an initial luminance (L0) of 500 cd m−2 (Figure S13). The curve of relative luminance vs operation time indicated the half luminance lifetime LT50 to be ca. 470 h. Devices based on SOIrB1 and SOIrB2 exhibited bright deepred emissions with CIE coordinates around (0.68, 0.31) and peak EQEs close to 19%, which are also among the highest efficiencies reported for solution-processed deep-red OLEDs. The device D2 showed the maximum luminance (Lmax), EQE, CE, and PE of 4757 cd m−2, 10.4%, 9.92 cd A−1, and 7.62 lm W−1, respectively. Although the EL emission spectrum had a peak at 620 nm, a large full width at half-maximum of 90 nm and an intense emission shoulder peaking at ca. 660 nm guaranteed its deep-red character. In addition, the efficiency roll-offs of these deep-red OLEDs were relatively low at the

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CONCLUSIONS In summary, we successfully developed four efficient deep-red phosphorescent emitters by combining the isoquinoline and thianthrene 5,5,10,10-tetraoxide groups to synthesize Ir(III) complexes with symmetric and asymmetric structures for the first time. The incorporation of the thianthrene 5,5,10,10tetraoxide group effectively increased the PLQYs and enhanced the electron-injection/-transport abilities of these Ir(III) complexes. The asymmetric configuration could fine-tune the properties while maintaining the desired deep-red emissions. Consequently, solution-processed OLEDs based on these Ir(III) complexes displayed bright deep-red emissions with CIE coordinates very close to (0.67, 0.33). Furthermore, the device based on SOIrOPh showed the best performance with the peak EQE, CE, PE, and CIE coordinates of 25.8%, 12.8 cd A−1, 9.14 lm W−1, and (0.68, 30), respectively, which are among the best performances reported for deep-red OLEDs. This work demonstrates that the thianthrene 5,5,10,10tetraoxide group has a great potential for developing highly efficient emitters.

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EXPERIMENTAL SECTION

General Procedure for Synthesizing These Ir(III) Complexes. Under a N2 atmosphere, the mixture solution of THF and H2O (3:1, v/v) (30 mL), ligand L-SO (1.0 equiv), ligand L-SO, L-OPh, L-B1, or L-B2 (1.0 equiv), and IrCl3·nH2O (1.0 equiv) was heated to 110 °C and stirred for ca. 12 h. After being cooled to room temperature, the mixture was extracted with CH2Cl2 several times. The collected organic layers were dried over anhydrous MgSO4. After the removal of the solvent, the residual was readily dissolved in CH2Cl2 (20 mL). After the addition of thallium(I) acetylacetonate (1.0 equiv), the mixture was stirred at room temperature for ca. 16 h under a N2 atmosphere. Then, the solvent was removed with a rotary evaporator, and the residual was purified on self-made silica TLC to give the desired Ir(III) complexes. SOIrOPh (Yield, 36.0%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.89 (s, 1H), 8.84 (d, J = 8.4 Hz, 1H), 8.41 (d, J = 6.0 Hz, 1H), 8.36 (d, J = 4.2 Hz, 1H), 8.20 (d, J = 6.4 Hz, 1H), 8.10 (d, J = 7.6 Hz, 1H), 7.95 (d, J = 7.6 Hz, 1H), 7.87−7.78 (m, 4H), 7.74−7.72 (m, 2H), 7.56 (d, J = 6.0 Hz, 1H), 7.50 (d, J = 8.8 Hz, 1H), 7.27 (s, 1H), 7.23 (t, J = 4.0 Hz, 1H), 7.02 (t, J = 7.6 Hz, 2H), 6.83 (t, J = 7.2 Hz, 1H), 6.77 (d, J = 7.6 Hz, 2H), 6.44 (d, J = 6.8 Hz, 1H), 5.46 (s, 1H), 5.26 (s, 1H), 1.86 (s, 3H), 1.74 (s, 3H); 13C NMR (100 MHz, CDCl3): δ (ppm) 185.20, 185.17, 167.36, 166.11, 165.59, 158.46, 155.42, 152.14, 147.76,146.54, 140.52, 140.35, 139.50, 139.06, 137.98, 137.20, 135.95, 133.17, 133.00, 131.44, 129.77, 129.53, 129.45, 129.15, 127.78, 126.66, 125.63, 125.52, 125.49, 125.09, 124.33, 123.50, 122.59, 121.50, 121.02, 120.15, 118.79, 111.16, 100.79, 28.62, 28.50. ESI-MS (m/z): [M]+, 944.1348. SOIrB1 (Yield, 18.8%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.90 (s, 1H), 8.88 (d, J = 7.2 Hz, 1H), 8.52 (d, J = 6.0 Hz, 1H), 8.20 (t, J = 4.4 Hz, 1H), 8.14 (d, J = 4.4 Hz, 1H), 8.00 (d, J = 8.4 Hz, 1H), 7.88−7.79 (m, 2H), 7.76−7.74 (m, 2H), 7.64−7.62 (m, 3H), 7.47 (d, J = 3.6 Hz, 1H), 6.72 (s, 4H), 6.67 (d, J = 8.0 Hz, 1H), 5.98 (d, J = 7.6 Hz, 1H), 5.26 (s, 1H), 2.45 (s, 6H), 1.91 (s, 12H), 1.87 (s, 3H), 1.76 (s, 3H); 13C NMR (100 MHz, CDCl3): δ (ppm) 185.66, 185.35, 178.57, 164.79, 153.03, 152.09, 141.36, 140.67, 140.54, 140.50, J

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ACS Applied Materials & Interfaces 139.53, 138.98, 138.34, 137.97, 137.23, 136.08, 133.19, 133.03, 132.55, 131.65, 130.02, 129.57, 127.94, 127.72, 126.70, 125.59, 125.53, 125.12, 124.39, 122.60, 117.27, 100.73, 28.48, 28.24, 23.51, 21.13. ESI-MS (m/z): [M]+, 1106.2252. SOIrB2 (Yield, 21.0%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.90 (s, 1H), 8.89 (d, J = 7.6 Hz, 1H), 8.51 (d, J = 6.0 Hz, 1H), 8.39 (s, 1H), 8.16 (dd, J = 5.2, 3.2 Hz, 1H), 8.16 (dd, J = 5.6, 3.2 Hz, 1H), 8.01 (d, J = 7.6 Hz, 1H), 7.90−7.78 (m, 4H), 7.71−7.63 (m, 4H), 7.25 (d, J = 2.4 Hz, 1H), 6.85 (s, 5H), 6.66 (t, J = 7.2 Hz, 1H), 6.06 (d, J = 7.2 Hz, 1H), 5.12 (s, 1H), 2.30 (s, 6H), 2.11 (s, 12H), 1.83 (s, 3H), 1.20 (s, 3H); 13C NMR (100 MHz, CDCl3): δ (ppm) 185.42, 184.77, 169.88, 166.34, 165.72, 155.25, 151.95, 146.49, 145.18, 144.13, 140.99, 140.58, 139.68, 139.46, 137.27, 136.16, 133.09, 133.01, 132.92, 131.46, 130.06, 129.74, 129.44, 128.55, 127.76, 126.78, 125.62, 125.27, 125.02, 124.33, 122.61, 122.14, 118.59, 100.84, 28.44, 27.83, 23.59, 21.26. ESI-MS (m/z): [M]+, 1100.2660. SOIrSO (Yield, 40.0%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.97 (s, 2H), 8.94 (d, J = 8.4 Hz, 2H), 8.39 (d, J = 6.4 Hz, 2H), 8.18 (d, J = 7.6 Hz, 2H), 8.08 (d, J = 8.0 Hz, 2H), 8.02 (d, J = 7.2 Hz, 2H), 7.97−7.88 (m, 4H), 7.79 (d, J = 6.4 Hz, 2H), 7.73−7.66 (m, 4H), 7.13 (s, 2H), 5.23 (s, 1H), 1.76 (s, 6H); 13C NMR (100 MHz, CDCl3): δ (ppm) 185.70, 165.28, 161.45, 151.84, 140.15, 139.85, 139.26, 137.61, 136.34, 133.27, 133.19, 132.18, 131.54, 130.02, 129.69, 127.98, 126.93, 125.78, 125.65, 125.29, 124.73, 123.39, 28.39. ESI-MS (m/z): [M + Na]+, 1127.0407.

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acterization assistance from the Instrument Analysis Center of Xi’an Jiaotong University and High Performance Computing Platform of Xi’an Jiaotong University is also acknowledged.

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(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151−154. (2) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234−238. (3) Kim, S.; Kwon, H.-J.; Lee, S.; Shim, H.; Chun, Y.; Choi, W.; Kwack, J.; Han, D.; Song, M.; Kim, S.; Mohammadi, S.; Kee, I.; Lee, S. Y. Low-Power Flexible Organic Light-Emitting Diode Display Device. Adv. Mater. 2011, 23, 3511−3516. (4) Wang, Z. B.; Helander, M. G.; Qiu, J.; Puzzo, D. P.; Greiner, M. T.; Hudson, Z. M.; Wang, S.; Liu, Z. W.; Lu, Z. H. Unlocking the Full Potential of Organic Light-Emitting Diodes on Flexible Plastic. Nat. Photonics 2011, 5, 753−757. (5) Hirata, S.; Sakai, Y.; Masui, K.; Tanaka, H.; Lee, S. Y.; Nomura, H.; Nakamura, N.; Yasumatsu, M.; Nakanotani, H.; Zhang, Q.; Shizu, K.; Miyazaki, H.; Adachi, C. Highly Efficient Blue Electroluminescence Based on Thermally Activated Delayed Fluorescence. Nat. Mater. 2014, 14, 330−336. (6) You, Y.; Park, S. Y. Phosphorescent Iridium(III) Complexes: Toward High Phosphorescence Quantum Efficiency Through Ligand Control. Dalton Trans. 2009, 1267−1282. (7) Fan, C.; Yang, C. Yellow/Orange Emissive Heavy-Metal Complexes as Phosphors in Monochromatic and White Organic Light-Emitting Devices. Chem. Soc. Rev. 2014, 43, 6439−6469. (8) Yang, X.; Zhou, G.; Wong, W.-Y. Functionalization of Phosphorescent Emitters and Their Host Materials by Main-Group Elements for Phosphorescent Organic Light-Emitting Devices. Chem. Soc. Rev. 2015, 44, 8484−8575. (9) Chi, Y.; Chang, T.-K.; Ganesan, P.; Rajakannu, P. Emissive Bistridentate Ir(III) Metal Complexes: Tactics, Photophysics and Applications. Coord. Chem. Rev. 2017, 346, 91−100. (10) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. The Triplet State of Organo-Transition Metal Compounds. Triplet Harvesting and Singlet Harvesting for Efficient OLEDs. Coord. Chem. Rev. 2011, 255, 2622−2652. (11) Udagawa, K.; Sasabe, H.; Igarashi, F.; Kido, J. Simultaneous Realization of High EQE of 30%, Low Drive Voltage, and Low Efficiency Roll-Off at High Brightness in Blue Phosphorescent OLEDs. Adv. Opt. Mater. 2016, 4, 86−90. (12) Kuei, C.-Y.; Tsai, W.-L.; Tong, B.; Jiao, M.; Lee, W.-K.; Chi, Y.; Wu, C.-C.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T. Bis-tridentate Ir(III) Complexes with Nearly Unitary RGB Phosphorescence and Organic Light-Emitting Diodes with External Quantum Efficiency Exceeding 31%. Adv. Mater. 2016, 28, 2795−2800. (13) Kim, K.-H.; Ahn, E. S.; Huh, J.-S.; Kim, Y.-H.; Kim, J.-J. Design of Heteroleptic Ir Complexes with Horizontal Emitting Dipoles for Highly Efficient Organic Light-Emitting Diodes with An External Quantum Efficiency of 38%. Chem. Mater. 2016, 28, 7505−7510. (14) Lee, S.; Shin, H.; Kim, J.-J. High-Efficiency Orange and Tandem White Organic Light-Emitting Diodes Using Phosphorescent Dyes with Horizontally Oriented Emitting Dipoles. Adv. Mater. 2014, 26, 5864−5868. (15) Kim, K.-H.; Lee, S.; Moon, C.-K.; Kim, S.-Y.; Park, Y.-S.; Lee, J.-H.; Woo Lee, J.; Huh, J.; You, Y.; Kim, J.-J. Phosphorescent DyeBased Supramolecules for High-Efficiency Organic Light-Emitting Diodes. Nat. Commun. 2014, 5, No. 4769. (16) Chen, C.-T. Evolution of Red Organic Light-Emitting Diodes: Materials and Devices. Chem. Mater. 2004, 16, 4389−4400. (17) Nagai, Y.; Sasabe, H.; Takahashi, J.; Onuma, N.; Ito, T.; Ohisa, S.; Kido, J. Highly Efficient, Deep-Red Organic Light-Emitting

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06749. Experimental procedures including synthesis, characterization, and device fabrication, and characterization data, such as 1H, 13C, and MS spectra, TGA and DSC curves, PL of doped films, PL at 77 K, CV curves of these Ir(III) complexes, hole-only and electron-only devices based on the SOIrOPh-doped TCTA film, EL characteristics of other devices, curve of relative luminance vs operation time for device A2 (PDF) Crystal data of SOIrB1 (CCDC 1870975) (CIF) Crystal data of SOIrB2 (CCDC 1870974) (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Z.). *E-mail: [email protected] (Z.W.). ORCID

Xiaolong Yang: 0000-0002-0243-6698 Guijiang Zhou: 0000-0002-5863-3551 Zhaoxin Wu: 0000-0003-2979-3051 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51803163, 21572176, 21602170, and 21875179), the China Postdoctoral Science Foundation (Grant Nos. 2015M580831 and 2016M600778), the Shaanxi Province Postdoctoral Science Foundation (Grant Nos. 2017BSHYDZZ02 and 2017BSHEDZZ03), the Fundamental Research Funds for the Central Universities (Grant Nos. xjj2017099, xjj2016061, and cxtd2015003), and Key Laboratory Construction Program of Xi’an Municipal Bureau of Science and Technology (201805056ZD7CG40). The charK

DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.9b06749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX