Tunable Emission Color of Iridium(III) Complexes with Phenylpyrazole

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Tunable Emission Color of Iridium(III) Complexes with Phenylpyrazole Derivatives as the Main Ligands for Organic LightEmitting Diodes Zhi-Gang Niu,†,‡ Hua-Bo Han,† Min Li,‡ Zheng Zhao,‡ Guang-Ying Chen,‡ You-Xuan Zheng,*,† Gao-Nan Li,*,‡ and Jing-Lin Zuo*,†

Organometallics Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/13/18. For personal use only.

†

State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China ‡ College of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, P. R. China S Supporting Information *

ABSTRACT: Seven cyclometalated iridium(III) complexes Ir1−Ir7 based on phenylpyrazole derivatives as main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand were synthesized and fully characterized. The proligands of 1-[4(trifluoromethyl)phenyl]-1H-pyrazole (cf3ppz, 2a), substituted phenyl-2H-indazole {2phenyl-2H-indazole (h-1-pidz, 2b), 2-(4-fluorophenyl)-2H-indazole (f-1-pidz, 2c), and 2[4-(trifluoromethyl)phenyl]-2H-indazole (cf3-1-pidz, 2d)}, and substituted phenyl-1Hindazole {1-phenyl-1H-indazole (h-7-pidz, 2e), 1-(4-fluorophenyl)-1H-indazole (f-7-pidz, 2f), and 1-[4-(trifluoromethyl)phenyl]-1H-indazole (cf3-7-pidz, 2g)} were prepared with good yields. The emission maxima of all Ir(III) complexes can be tuned from 453 to 576 nm with different photoluminescence quantum yields (10.4−70.9%) by varying the type of substituent on the different main ligand frameworks. The organic light-emitting diodes using Ir(III) complexes as the emitters with blue, bluish-green, and yellow colors exhibit a maximum current efficiency and an external quantum efficiency of 39.2 cd A−1 and 14.8%, respectively.

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INTRODUCTION Iridium(III) complexes have attracted considerable attention because of their remarkable photophysical properties, such as their relatively short phosphorescence lifetimes, high quantum efficiencies, and various emission colors.1 Because of these unique photophysical features, Ir(III) complexes have been applied in many fields, for example, organic photoredox catalysis,2 organic photovoltaic cells,3 water photolysis,4 oxygen sensors,5 and bioimaging.6 In particular, they are employed in the fabrication of organic light-emitting diodes (OLEDs) and full-color displays.7 To realize full-color displays, color tuning via the molecular design of Ir(III) complexes has been studied extensively for many years.8 Generally, one strategy is to alter the heterocycle, the degree of conjugation in the C^N ligand, and/or the ancillary ligand.9 Another strategy is to vary substituents on the cyclometalated phenyl, the coordination heterocycle, or the ancillary ligands.9c,10 Meanwhile, to obtain Ir(III) complexes for efficient OLEDs, tetraphenylimidodiphosphinate (tpip) derivatives with polar PO bonds and phenyl rings were used as ancillary ligands in our group because they could increase the electron mobility of the complexes and improve their corresponding electroluminescent performances.11 Moreover, the fluorine groups were also always introduced to modify the ligands because they could not only modify the electronic © XXXX American Chemical Society

properties but also decrease the rate of nonradioactive deactivation and improve phosphorescence quantum yields.12 Among the most frequently used main ligands, phenylpyrazole (ppz) is one typical ligand framework for constructing Ir(III) complexes, and its derivatives can be used to fine-tune the emission color of complexes by judicious modification.13 For instance, Thompson’s group first reported tris-cyclometalated Ir(III) complexes with fluoro/trifluoromethylsubstituted ppz ligands in 2003, and their emissions covered a broad range from 390 to 440 nm.14 Subsequently, five ionic Ir(III) complexes [Ir(R-ppz)2(bipy)][PF6] (R = H, Me, CF3, NO2, or OMe) were synthesized and exhibited a wide range of emission wavelengths (497−615 nm) with a change in the substituents on the cyclometalated ligands.9c More recently, a series of Ir(III) complexes based on ppz derivatives with electron-withdrawing groups (-F, -CF3, -OCF3, and -SF5) were prepared and revealed a broad, featureless emission peak that ranged between 510 and 560 nm.15 However, to the best of our knowledge, there have been no additional reports about the tunability of colors achieved by different conjugation positions on the ppz-based cyclometalated ligand. Received: July 14, 2018

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DOI: 10.1021/acs.organomet.8b00491 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthetic Routes of Main Ligands and Ir(III) Complexes Ir1−Ir7

Figure 1. ORTEP views of Ir(III) complexes Ir2 (CCDC 1578682), Ir3 (CCDC 1578683), Ir4 (CCDC 1578686), Ir5 (CCDC 1578684), Ir6 (CCDC 1578685), and Ir7 (CCDC 1578687) with the atom numbering scheme at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for the sake of clarity.

Herein, as shown in Scheme 1, on the basis of the precursor ligand 1-[4-(trifluoromethyl)phenyl]-1H-pyrazole (2a), we designed ppz derivatives (2b−2g) as the main ligands by introducing a benzene group to increase the π system and a fluorine group to enhance the photophysical quantum yields.

According to the different positions of the benzene group on the main ligands, they can be divided into two series: ligands 2b−2d (looking like the number 1) and ligands 2e−2g (looking like the number 7). Then we synthesized seven Ir(III) complexes Ir1−Ir7 (Scheme 1) with ppz derivatives 2a−2g, B

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metric analysis (TGA) under a nitrogen stream with a heating rate of 10 °C min−1 (Figure 2). From the TG data in Table 1,

respectively, as the main ligands and tpip as the ancillary ligand. The emission maximum wavelengths of these complexes can be tuned from 453 to 576 nm in a CH2Cl2 solution at room temperature. The monochrome phosphorescent OLED devices D1, D3, D4, D6, and D7 using Ir(III) complexes Ir1, Ir3, Ir4, Ir6, and Ir7, respectively, as the emitters emit blue, yellow, and bluish-green light.

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RESULTS AND DISCUSSION Cyclometalated ligands 2a−2g and their corresponding Ir(III) complexes, Ir1−Ir7, respectively, were prepared and are shown in Scheme 1, and the details are provided in the Supporting Information. Compounds 2a and 2e−2g were conveniently synthesized via an efficient Ullmann reaction under CuI catalysis, which used 1H-pyrazole or 1H-indazole and the corresponding substituted iodobenzene as raw materials. Compounds 2b−2d were prepared through a two-step reaction. First, 2-nitrobenzaldehyde reacted with the substituted aniline by the condensation reaction in ethanol to give 1b−1d in good yields (over 95%). Subsequently, 2b−2d were prepared by the cyclization reaction using P(OEt)3 as the solvent and catalyst. Then Ir(III) complexes Ir1−Ir7 were prepared by the reaction of chloride-bridged dimer [(C^N)2Ir(μ-Cl)]2 with Ktpip (2.5 equiv) in anhydrous 2-ethoxyethanol.16 All the desired products were obtained in good yield and characterized using 1H nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (see the Experimental Section and Supporting Information). Crystal Structures. The ORTEP diagrams of six representative complexes, Ir2−Ir7, are shown in Figure 1. Their crystallographic data and structure refinement details are listed in Table S1; the selected bond lengths and bond angles are listed in Table S2. As shown in Figure 1, the coordination geometry of the iridium center in each complex is a distorted octahedral geometry with cis-C,C, cis-O,O, and trans-N,N chelate disposition.17 The lengths of Ir−C and Ir−N bonds are in the range of 1.978(1)−2.010(1) and 1.938(1)−2.048(1) Å, respectively, close to those in previously reported Ir(III) complexes.18 It is noteworthy that the Ir−O bond lengths [2.172(4)−2.227(3) Å] are slightly longer than the mean Ir− O value of 2.088 Å reported in the Cambridge Crystallographic Database, which may be attributed to the strong trans influence of the carbon donors.19 In particular, the Ir−C bond lengths of two series of Ir(III) complexes, Ir2−Ir4 and Ir5−Ir7, exhibit the same trend: Ir3 < Ir4 < Ir2 and Ir6 < Ir7 < Ir5. Namely, a stronger electron-withdrawing ability of substituted groups would result in shorter the Ir−C bond lengths. In addition, the C−Ir−C, O−Ir−O, C−Ir−O, and N−Ir−O angles increase in the following order: Ir2−Ir4 < Ir5−Ir7. The order of C−Ir−N and N−Ir−N angles is reversed. Among them, the O−Ir−O bite angles are 87.1(3)−87.70(1)° for Ir2−Ir4 and 90.1(2)− 91.63(8)° for Ir5−Ir7. This may be consistent with the steric hindrance, originating from different connection positions between the indazole and phenyl group.20 Thermal Properties. If an Ir(III) complex used as an emitter is suitable for OLED application, it should have sufficiently high melting points (Tm) and decomposition temperatures (Td, 5% weight loss) to ensure that the material could be deposited onto the solid face and survive long periods of application without any decomposition.21 Therefore, the thermal properties of these Ir(III) complexes are very important for efficient OLEDs. The thermal properties of investigated complexes were characterized by thermogravi-

Figure 2. TGA curves of Ir(III) complexes Ir1−Ir7.

it can be clearly seen that the decomposition temperatures are 345 °C for Ir1, 364 °C for Ir2, 374 °C for Ir3, 355 °C for Ir4, 373 °C for Ir5, 380 °C for Ir6, and 365 °C for Ir7. They all possess decomposition temperatures of >300 °C, suggesting that they have good thermal stability that improves their application in OLEDs. Photophysical Properties. The ultraviolet−visible (UV− vis) absorption spectra in CH2Cl2 solutions at room temperature for complexes Ir1−Ir7 are depicted in Figure 3, and the data are listed in Table 1. All complexes exhibit intense absorption bands between 220 and 350 nm, mostly ascribed to spin-allowed ligand-centered (LC) π−π* transitions involving both the cyclometalated and ancillary ligands. The broad and less intense bands at longer wavelengths [λ > 350 nm (Figure 3, inset)] are associated with metal-to-ligand charge transfer (MLCT).22 In comparison with Ir1, the lowest-lying absorption bands for complexes Ir2−Ir7 are remarkably redshifted, resulting from the different extended π conjugation of the C^N ligands.23 Furthermore, the bathochromic shifts observed in the series of Ir2−Ir4 are more pronounced than those of analogues Ir5−Ir7, which is related to the different positions of the phenyl ring on the indazole framework. Specifically, the trend of the absorption onset in the first series is Ir3 < Ir2 < Ir4, while the order of another series is Ir6 < Ir7 < Ir5. These results suggest that -F and -CF3 groups on the cyclometalated ligands also could modulate the lower-lying transitions, which will be proved by density functional theory (DFT) calculations discussed below. Photoluminescence (PL) emission spectra of complexes Ir1−Ir7 in degassed CH2Cl2 solutions at 298 and 77 K are shown in Figure 3 and Figure S22, respectively. The corresponding data are also summarized in Table 1. Complexes Ir1−Ir7 are emissive in the blue-yellow spectral region upon photoexcitation in CH2Cl2 solutions at 298 K, and each of the emission profiles is a structured band, indicative of a pronounced 3MLCT character with a weaker 3LC contribution.25 As expected, emission maxima of complexes Ir2−Ir7 (494−576 nm) appear at higher energies with respect to that of complex Ir1 (453 nm), and the series of Ir2−Ir4 complexes are even more red-shifted than another series of Ir5−Ir7. This observation is also in line with those of the lowest-energy electronic transition in the UV−vis spectra mentioned above. Upon cooling to 77 K (Figure S22), all the Ir(III) complexes show well-resolved vibronic structures with a predominant C

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Organometallics Table 1. Photophysical and Thermal Stability Data of Complexes Ir1−Ir7 thermal properties complex

Tm (°C)a

Td (°C)b

Ir1 Ir2 Ir3 Ir4 Ir5 Ir6 Ir7

332 334 352 308 360 369 350

345 364 374 355 373 380 365

emission absorption (nm)c 228, 228, 230, 229, 230, 231, 230,

253, 252, 252, 250, 251, 248, 251,

274, 307, 304, 304, 311, 309, 311,

282, 318, 315, 317, 356, 352, 354,

321, 358, 352, 363, 393, 387, 390,

354, 396, 388, 398, 412 402 409

298 K (nm)c 398 426, 465 414, 451 428, 462

433, 566, 556, 576, 507, 494, 496,

453 605 594 620 528 520 521

ΦPL (%)d

τ (μs)c

kr (×105 s−1)e

knr (×105 s−1)e

10.4 48.7 70.9 50.5 23.2 30.9 36.3

2.15 2.80 2.75 2.72 2.63 2.75 2.96

0.48 1.74 2.58 1.86 0.88 1.12 1.23

4.17 1.83 1.06 1.82 2.92 2.51 2.15

77 K (nm)f 440, 563, 555, 572, 524, 488, 485,

467 607 596 622 557, 607 519, 558 518, 557

a

Tm, melting temperature. bTd, decomposition temperature corresponding to a 5% weight loss. cData were collected from degassed CH2Cl2 solutions at room temperature. dfac-Ir(ppy)3 as the reference standard (0.4).24 eThe radiative decay rate (kr) and nonradiative decay rate (knr) were estimated from the measured quantum yields and lifetimes. fData were collected from degassed CH2Cl2 solutions at 77 K.

Figure 3. (a) Absorption and (b) emission spectra of complexes Ir1−Ir7 in degassed CH2Cl2 solutions at 298 K.

contribution of ligand-centered 3π−π* transitions to the excited state because LC excited states are almost unaffected in frozen medium, because of their low dipole character.26 In the meantime, it is found that the emission maxima further undergo a hypsochromic shift of approximately 3−11 nm from 298 to 77 K (Table 1), due to the rigidochromic effect of the frozen media.27 Phosphorescence relative quantum yields (Φem) of Ir1−Ir7 in dichloromethane solutions were measured to be 10.4− 70.9% (Table 1) at room temperature by using typical phosphorescent fac-Ir(ppy)3 as a standard (Φem = 40%). Compared with that of parent complex Ir1 (Φ = 10.4%), the more extended π conjugation of the phenylpyrazole moiety in other complexes led to the dramatic increase in the phosphorescence quantum yield. As for the two sets of Ir(III) complexes, fluorinated complexes Ir3 and Ir4 and fluorinated complexes Ir6 and Ir7 always display a strongly increase relative to fluorine-free analogues Ir2 and Ir5, respectively. This is dependent on the fact that the presence of the C−F bonds could lead to the reduced radiationless deactivation rate in comparison with that seen with C−H bonds.28 Remarkably, the emission quantum yield for Ir3 (Φem = 70.9%) is the highest in the family of complexes, which would improve its device performance. For all the investigated complexes, the luminescence lifetimes (τ) are in the range of 2.15−2.96 μs, indicating that their emitting excited states have triplet character (Table 1).29 From the Φem and τ values, the radiative decay rate (kr) and the nonradiative decay rate (knr) were estimated. As one can see, the kr value of Ir1 is the lowest among those of the investigated complexes. Complexes Ir5− Ir7 all possess much smaller kr values in comparison to those of another series of complexes (Ir2−Ir4). In contrast, complex

Ir1 has the highest knr value in all complexes. In comparison to those of complexes Ir2−Ir4, complexes Ir5−Ir7 show much higher knr values. Consequently, the largest kr/knr ratio is observed for complex Ir3 and the smallest for complex Ir1, in line with the highest and the lowest quantum yields, respectively. Electrochemical Properties and Theoretical Calculations. The electronic properties of Ir1−Ir7 were investigated by cyclic voltammetry, and the electrochemical waves are shown in Figure 4. On the basis of their oxidation potentials and absorption spectra, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels are estimated and included in Table 2, as well as DFT-calculated values for the purpose of comparison.

Figure 4. Cyclic voltammogram curves of CH2Cl2 solutions of Ir1− Ir7 containing n-Bu4NClO4 (0.1 M) at a sweep rate of 100 mV s−1. D

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determined by comparison of the two series, complexes Ir2− Ir4 are more easily oxidized than complexes Ir5−Ir7 as a result of the different π conjugation positions. The energy levels of the LUMO were obtained from the HOMO and Eopt,g values. It can be observed that the LUMO energy levels and the HOMO energy levels have the same tendency to vary based on experimental and theoretical data. Remarkably, the HOMO/ LUMO energy levels inferred from the CV data are systematically slightly lower than those from the DFT data, as reported in ref 30 (Table 2). Time-dependent DFT (TD-DFT) is an excellent tool for accurately assigning the origin of each band in ultraviolet absorption spectra of these complexes. The most representative molecular frontier orbital diagrams and the energy gap are presented in Figure 5. The calculated spin-allowed electronic transitions, oscillator strengths, and related molecular orbitals are listed in Table 3 and compared with the experimental absorption spectral data. The electron density distributions are summarized in Table S3. In all cases, the HOMOs are mainly dominated by iridium d orbitals and π orbitals of cyclometalated ligands. Meanwhile, the LUMOs and the LUMO+1s are mostly located on π* orbitals of the two cyclometalated ligands. As demonstrated in Table 3, the lowest-lying energy transitions of all Ir(III) complexes arise from a HOMO → LUMO+1 orbital electronic transition (except the HOMO → LUMO transition for Ir1). Therefore, these transitions are assigned to MLCT in combination with partial ligand-to-ligand charge transfer (LLCT), in good agreement with the experimental absorption spectra. With respect to pristine complex Ir1, the introduction of the extended conjugation in complexes Ir2−Ir7 gives rise to the destabilization of the HOMO levels and the stabilization of the LUMO levels. The results lead to a shorter HOMO−LUMO energy gap, thereby causing a red shift in the actual absorption spectra (Table 3). In analogy to the rule, the red shift observed in Ir2−Ir4 agrees well with the lower HOMO−LUMO gap with regard to those in Ir5−Ir7. In the case of their HOMO energy values, the orders are Ir2 > Ir3 > Ir4 and Ir5 > Ir6 > Ir7, which are in accordance with the electron-withdrawing ability of -H < -F < -CF3. Similar results have also been obtained for their LUMO energy values. Different decreasing trends of the HOMO/LUMO levels induce distinct HOMO− LUMO energy gaps. For instance, the energy bandgaps of fluorine-substituted complexes Ir3 and Ir6 are larger than those of trifluoromethyl-substituted complexes Ir4 and Ir7, respectively, which further proves that the nature of the substituents of the cyclometalated ligands can also affect the absorption spectra. Interestingly, the LUMOs have a topology analogous to that of the LUMO+1s in complexes Ir2−Ir7 (Figure 5). Thus, it is not surprising that the LUMOs and

Table 2. Electrochemical and Theoretical Data of Complexes Ir1−Ir7 complex

Eox (eV)

Eopt,g (eV)a

HOMO/LUMO (eV)b

HOMO/LUMO (eV)c

Ir1 Ir2 Ir3 Ir4 Ir5 Ir6 Ir7

1.24 0.92 1.06 1.14 0.96 1.09 1.22

3.12 2.67 2.75 2.69 3.01 3.09 3.04

−6.04/−2.92 −5.72/−3.05 −5.86/−3.11 −5.94/−3.25 −5.76/−2.75 −5.89/−2.80 −6.02/−2.98

−5.46/−1.26 −5.23/−1.61 −5.37/−1.64 −5.48/−1.85 −5.14/−1.38 −5.26/−1.42 −5.40/−1.56

a

Calculated from the UV−vis absorption edges (Eopt,g = 1240/λonset). Deduced from the equations HOMO = −(Eox + 4.8 eV) and LUMO = HOMO + Eopt,g, respectively. cObtained from theoretical calculations. b

All complexes exhibit reversible oxidation peaks in the region of 0.92−1.24 V, which are attributed to the Ir(III)/Ir(IV) redox couples with contributions from the phenyl rings of cyclometalated fragments, as evidenced by DFT calculations (Figure 5). Compared to that of complex Ir1, the oxidation

Figure 5. Frontier molecular orbital energy level diagrams of Ir1−Ir7 from DFT calculations.

waves for complexes Ir2−Ir7 are shifted to less negative potentials. These results reflect the fact that the π conjugation system on the cyclometalated ligand can enrich the electron density around the iridium center and destabilize the corresponding HOMO orbital (Table 2 and Figure 5). Notably, the trends in Eox are Ir2 < Ir3 < Ir4 and Ir5 < Ir6 < Ir7, contrary to the order of the HOMO energy levels: Ir2 > Ir3 > Ir4 and Ir5 > Ir6 > Ir7 (Table 2). These findings indicate the electron-withdrawing fluorine atoms can lower the HOMO energy levels, especially that of the CF3 group. As

Table 3. Main Experimental and Calculated Optical Transitions for Complexes Ir1−Ir7 complex Ir1 Ir2 Ir3 Ir4 Ir5 Ir6 Ir7

orbital excitations HOMO HOMO HOMO HOMO HOMO HOMO HOMO

→ → → → → → →

LUMO LUMO+1 LUMO+1 LUMO+1 LUMO+1 LUMO+1 LUMO (62%), HOMO → LUMO+1 (−34%)

nature of the transition

oscillation strength

calcd (nm)

exptl (nm)

Ir(dπ)/Lcf3ppz(π) → Lcf3ppz(π*) Ir(dπ)/Lh‑1‑pidz(π) → Lh‑1‑pidz(π*) Ir(dπ)/Lf‑1‑pidz(π) → Lf‑1‑pidz(π*) Ir(dπ)/Lcf3‑1‑pidz(π) → Lcf3‑1‑pidz(π*) Ir(dπ)/Lh‑7‑pidz(π) → Lh‑7‑pidz(π*) Ir(dπ)/Lf‑7‑pidz(π) → Lf‑7‑pidz(π*) Ir(dπ)/Lcf3‑7‑pidz(π) → Lcf3‑7‑pidz (π*)

0.0456 0.0808 0.0864 0.0698 0.0430 0.0496 0.0535

361 422 404 418 407 393 396

398 465 451 462 412 402 409

E

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Figure 6. Energy level diagram of phosphorescent OLEDs and the molecular structures of used materials.

Figure 7. Characteristics of devices D1, D3, D4, D6, and D7: (a) normalized EL spectra at 8 V, (b) luminance−voltage−current density curves, (c) current efficiency−luminance curves, and (d) external quantum efficiency−luminance curves.

injecting layer (HIL) and an electron-injecting layer (EIL), respectively. TAPC [4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-ptolylaniline)] acts as a hole-transporting layer (HTL), while TmPyPB {1,3,5-tri[(3-pyridyl)phen-3-yl]benzene} acts as an electron-transporting layer (ETL). Herein, 2,6-DCzPPy layers doped with Ir(III) complexes act as the emissive layer. Simultaneously, bipolar material 2,6-DCzPPy is used as the buffer layer to achieve the cascade hole injection from HTL to the emitting layer (EML) due to its nearly equal electron mobility (μe) and hole mobility (μh) values (1−8 × 10−5 cm2 V−1 s−1 at an electric field between 6.0 × 105 and 1.0 × 106 V cm−1), which improves the electron−hole balance in the EML.32

LUMO+1s have very close energy values. The very small orbital overlap between the HOMO and the LUMO/LUMO +1 orbitals may explain the absence of the corresponding absorptions.31 OLED Performance. To evaluate their electroluminescence (EL) properties, monochrome devices D1, D3, D4, D6, and D7 using complexes Ir1, Ir3, Ir4, Ir6, and Ir7, respectively, as emitters with colors of blue, green, andyellow were fabricated with a simple ITO/MoO3 (molybdenum oxide, 5 nm)/TAPC (30 nm)/2,6-DCzPPy:Ir(III) complex (8 wt %, 10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) architecture. The schematic energy level diagrams of phosphorescent OLEDs and the molecular structures of their materials are shown in Figure 6. MoO3 and LiF act as a holeF

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Organometallics Table 4. EL Performance of Single-EML Devices D1, D3, D4, D6, and D7 device

emitter

Vturn‑ona (V)

D1 D3 D4 D6 D7

Ir1 Ir3 Ir4 Ir6 Ir7

4.0 3.9 3.9 3.8 3.9

Lmaxb [cd m−2 (V)] 3135 14127 9535 6689 9568

(12.6) (14.5) (11.5) (13.4) (12.2)

ηcc (cd A−1)

ηpc (lm W−1)

EQEc (%)

ELd (nm)

CIEe (x, y)

8.6/8.0 26.8/23.9 26.2/23.5 39.2/26.3 28.7/23.3

3.4/3.0 18.7/9.3 16.4/12.3 27.4/10.2 17.0/10.6

2.6/2.4 9.0/8.0 10.7/9.6 14.8/9.9 10.3/8.3

450 559, 595 577, 618 496 491, 519

(0.16, (0.47, (0.55, (0.21, (0.25,

0.17) 0.49) 0.44) 0.48) 0.53)

a Turn-on voltage recorded at a luminance of 1 cd m−2. bMaximum luminance. cData at maximum and 1000 cd m−2 for current efficiency (ηc), power efficiency (ηp), and EQE, respectively. dValues were collected at 8 V. eCIE (Commission Internationale de l’Eclairage) coordinates (CIE) at 8 V.

practical luminance of 1000 cd m−2, the EQEs of devices D6 and D7 were reduced by ∼33.1 and ∼19.4%, respectively, but the efficiencies of D3 and D4 decayed from their maximum values by only 11.1 and 10.3%, respectively. At the same time, the trends in efficiency roll-off are D3 > D4 (11.1% vs 10.3%) and D6 > D7 (33.1% vs 19.4%) in the same types of luminescent materials. These results suggest that the bulky -CF3 series can affect the packing of molecules and the steric protection around the metal can restrain the self-quenching of luminescence to improve device performances.

The EL spectra and current density−luminance−voltage (J− L−V), current efficiency−luminance (ηc−L), and external quantum efficiency−luminance (EQE−L) characteristics of devices are shown in Figure 7. The key EL data are summarized in Table 4. It should be noted that all devices emit blue, yellow, or bluish-green light with peaks at 450, 559, 577, 496, and 491 nm for devices D1, D3, D4, D6, and D7, respectively. Their CIE coordinates are (0.16, 0.17), (0.47, 0.49), (0.55, 0.44), (0.21, 0.48), and (0.25, 0.53), respectively (Table 4). The EL spectra match well with the PL spectra of these complexes in solution, suggesting that the generated excitons are mainly deactivated by phosphorescence emission of Ir(III) complexes and the energy can be transferred from 2.6-DCzPPy to the emitters efficiently. To further investigate the reason for the different PL and EL spectra, the PL emissions of complexes Ir3 and Ir6 in the solid state were measured and the spectra are shown in Figure S23. It is clear that the PL emissions of complexes Ir3 and Ir6 in the solid state are similar to the EL emissions of the devices, which indicate that the difference is due to the solid effect. Because of the similar molecular structures of the complexes, their EL performances mainly depend on their photophysical (PLQY especially) and electrochemical properties (energy HOMO/LUMO levels especially). Because of the lowest PLQY (10.4%) and blue emission (433 and 453 nm) of Ir1, device D1 reveals the weakest performance with a maximum luminance (Lmax) of 3135 cd m−2 at 12.6 V, a maximum current efficiency (ηc,max) of 8.6 cd A−1 with an EQEmax of 2.6%, and a maximum power efficiency (ηp,max) of 3.4 lm W−1. Compared with device D1, devices D3, D4, D6, and D7 displayed much better performances. The EQEmax values (Lmax, ηc,max, ηp,max) for devices D3, D4, D6, and D7 were measured to be 9.0% (14127 cd m−2, 26.8 cd A−1, 18.7 lm W−1), 10.7% (9535 cd m−2, 26.2 cd A−1, 16.4 lm W−1), 14.8% (6689 cd m−2, 39.2 cd A−1, 27.4 lm W−1), and 10.3% (9568 cd m−2, 28.7 cd A−1, 17.0 lm W−1), respectively. From the results, it can also be observed that the EL performance of device D6 is obviously better than that of D7, though Ir7 has a photoluminescence quantum efficiency that is higher than that of Ir6 (Ir6, 30.9%; Ir7, 36.3%). Because the HOMO level of Ir6 (−5.89 eV) is higher than that of Ir7 (−6.02 eV), the hole trapped by the emitter from TAPC becomes easier, resulting in a lower turnon voltage and better usage of the carriers. Furthermore, the shorter excited state lifetime of complex Ir6 (2.75 μs) compared to that of Ir7 (2.96 μs) also contributes to improving the device performances because the longer lifetime will lead to exciton quenching through triplet−triplet annihilation (TTA) and triplet−polaron annihilation (TPA) effects, which would result in relatively poor EL performances. It is noteworthy that devices D6 and D7 show efficiency rolloff that is more severe than that of devices D3 and D4. At the

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CONCLUSION Seven bis-cyclometalated phenylpyrazole-based Ir(III) complexes Ir1−Ir7 with tpip as the ancillary ligand were synthesized and investigated in detail. Via variation of the main ligands, the emission colors can be adjusted from blue to yellow in CH2Cl2 solutions. The extended π conjugation within the C^N ligand can lead to a red shift, and then the conjugated system introduced at difference positions in the center unit can lead to different photoelectric properties. The phosphorescent OLEDs comprising Ir1, Ir3, Ir4, Ir6, and Ir7 as blue, yellow, and bluish-green dopants realize device performances with EQEmax values of 2.6, 9.0, 10.7, 14.8, and 10.3%, respectively. At a luminance of 1000 cd m−2, the EQE values of devices D3, D4, D6, and D7 were reduced from their maximum values by ∼11.1, ∼10.3, ∼33.1, and ∼19.4%, respectively. Furthermore, the efficiency roll-off of D4 and D7 is lower than that of D3 and D6, suggesting the bulky -CF3 series can improve device performance.

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EXPERIMENTAL SECTION

All reactions were performed under a nitrogen atmosphere, and all reagents were purchased and used without further purification unless otherwise stated. Potassium tetraphenylimidodiphosphinate (Ktpip) was synthesized according to our previous work.33 1H NMR spectra were recorded on a Bruker AM 400 MHz instrument, and chemical shifts are reported in parts per million relative to Me4Si as an internal standard. High-resolution mass spectrometry (HRMS) was performed on an Agilent 6540 UHD Accurate-Mass Q-TOF LC/MS instrument. UV−vis spectra were recorded on a Hitachi U3900/3900H spectrophotometer. Photoluminescence spectra were recorded on a Hitachi F7000 spectrophotometer as deaerated CH2Cl2 solutions at room temperature and 77 K. Luminescence lifetime curves were measured on an Edinburgh Instruments FLS920P fluorescence spectrometer, and the data were subjected to one-order exponential fitting using OriginPro 8 software. Thermogravimetric analysis (TGA) was performed with a simultaneous NETZSCH STA 449C thermal analyzer. Elemental analyses were performed on a Vario EL Cube Analyzer system. 1-[4-(Trifluoromethyl)phenyl]-1H-pyrazole (cf3ppz, 2a). To a mixture of 1H-pyrazole (1.0 g, 14.7 mmol), 1-iodo-4(trifluoromethyl)benzene (4.8 g, 14.6 mmol), N,N′-dimethylethylenediamine (130 mg, 1.47 mmol), and K3PO4 (6.2 g, 29.4 mmol) in G

DOI: 10.1021/acs.organomet.8b00491 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(m, 6H), 6.35 (d, J = 1.2 Hz, 2H), 6.30 (t, J = 2.8 Hz, 2H); HRMS calcd for C44H32F6IrN5O2P2 1031.156 ([M + H]+), found 1032.156. Anal. Calcd for C44H32F6IrN5O2P2: C, 51.26; H, 3.13; N, 6.79. Found: C, 51.21; H, 3.12; N, 6.80. Ir2 (104 mg, 49% yield): yellow powder; 1H NMR (400 MHz, CDCl3) δ 8.34 (s, 2H), 8.17 (d, J = 8.8 Hz, 2H), 7.77−7.82 (m, 4H), 7.45 (d, J = 8.4 Hz, 2H), 7.38 (d, J = 7.6 Hz, 2H), 7.28−7.32 (m, 6H), 7.10−7.15 (m, 4H), 6.98 (t, J = 7.6 Hz, 2H), 6.83−6.89 (m, 4H), 6.64−6.70 (m, 6H), 6.54 (t, J = 7.2 Hz, 2H), 6.08 (d, J = 7.2 Hz, 2H); HRMS calcd for C50H38IrN5O2P2 995.213 ([M + H]+), found 996.225. Anal. Calcd for C50H38IrN5O2P2: C, 60.35; H, 3.85; N, 7.04. Found: C, 60.28; H, 3.82; N, 7.11. Ir3 (104 mg, 66% yield): orange powder; 1H NMR (400 MHz, CDCl3) δ 8.29 (2s, 2H), 8.10 (d, J = 8.8 Hz, 2H), 7.78−7.82 (m, 4H), 7.44 (d, J = 7.2 Hz, 2H), 7.32−7.39 (m, 8H), 7.08−7.13 (m, 4H), 6.97 (t, J = 7.6 Hz, 2H), 6.88 (t, J = 7.6 Hz, 2H), 6.57−6.69 (m, 8H), 5.68−5.71 (m, 2H); HRMS calcd for C50H36F2IrN5O2P2 1031.194 ([M + H] + ), found 1032.223. Anal. Calcd for C50H36F2IrN5O2P2: C, 58.25; H, 3.52; N, 6.79. Found: C, 58.22; H, 3.48; N, 6.83. Ir4 (138 mg, 61% yield): orange powder; 1H NMR (400 MHz, CDCl3) δ 8.30 (s, 2H), 8.09 (d, J = 8.8 Hz, 2H), 7.77−7.82 (m, 4H), 7.45 (d, J = 8.4 Hz, 2H), 7.31−7.39 (m, 8H), 7.07−7.12 (m, 4H), 6.99 (t, J = 8.0 Hz, 2H), 6.86−6.90 (m, 2H), 6.65−6.77 (m, 8H), 5.82 (d, J = 1.2 Hz, 2H); HRMS calcd for C52H36F6IrN5O2P2 1131.188 ([M + H]+), found 1164.186 ([MS + H + MeOH]+). Anal. Calcd for C52H36F6IrN5O2P2: C, 55.22; H, 3.21; N, 6.19. Found: C, 55.17; H, 3.32; N, 6.25. Ir5 (128 mg, 57% yield): yellow powder; 1H NMR (400 MHz, CDCl3) δ 8.17 (s, 2H), 8.12 (d, J = 8.8 Hz, 2H), 7.77−7.83 (m, 4H), 7.68 (d, J = 7.2 Hz, 2H), 7.48−7.53 (m, 4H), 7.28−7.40 (m, 10H), 7.21−7.25 (m, 2H), 6.92 (t, J = 7.2 Hz, 2H), 6.76 (t, J = 7.6 Hz, 2H), 6.65−6.69 (m, 4H), 6.54 (t, J = 7.2 Hz, 2H), 6.13 (dd, J1 = 1.6 Hz, J2 = 7.2 Hz, 2H); HRMS calcd for C50H38IrN5O2P2 995.213 ([M + H]+), found 996.219. Anal. Calcd for C50H38IrN5O2P2: C, 60.35; H, 3.85; N, 7.04. Found: C, 60.40; H, 3.79; N, 7.07. Ir6 (146 mg, 63% yield): yellow powder; 1H NMR (400 MHz, CDCl3) δ 8.13 (s, 2H), 8.05 (d, J = 8.8 Hz, 2H), 7.76−7.81 (m, 4H), 7.62 (dd, J1 = 4.8 Hz, J2 = 8.4 Hz, 2H), 7.48−7.54 (m, 4H), 7.32− 7.38 (m, 6H), 7.27−7.29 (m, 2H), 7.22−7.25 (m, 4H), 6.76 (t, J = 7.6 Hz, 2H), 6.62−6.69 (m, 6H), 5.78 (dd, J1 = 2.8 Hz, J2 = 9.6 Hz, 2H); HRMS calcd for C50H36F2IrN5O2P2 1031.194 ([M + H]+), found 1032.201. Anal. Calcd for C50H36F2IrN5O2P2: C, 58.25; H, 3.52; N, 6.79. Found: C, 58.20; H, 3.47; N, 6.86. Ir7 (113 mg, 49% yield): yellow powder; 1H NMR (400 MHz, CDCl3) δ 8.19 (s, 2H), 8.11 (d, J = 8.4 Hz, 2H), 7.72−7.81 (m, 6H), 7.57 (t, J = 8.0 Hz, 2H), 7.51 (d, J = 8.0 Hz, 2H), 7.34−7.41 (m, 6H), 7.28 (t, J = 7.6 Hz, 2H), 7.21−7.25 (m, 6H), 6.76 (t, J = 7.2 Hz, 2H), 6.62−6.67 (m, 4H), 6.36 (d, J = 1.6 Hz, 2H); HRMS calcd for C52H36F6IrN5O2P2 1131.188 ([M + H]+), found 1132.196. Anal. Calcd for C52H36F6IrN5O2P2: C, 55.22; H, 3.21; N, 6.19. Found: C, 55.27; H, 3.20; N, 6.21.

dry dimethylformamide (15 mL) was added CuI (140 mg, 0.73 mmol). The mixture was heated at 80 °C for 20 h, diluted with EtOAc (50 mL), and filtered through a pad of Celite. The filtrate was washed with H2O, and the aqueous layer was extracted with EtOAc. The combined organic extract was concentrated, and the residue was purified by column chromatography on silica gel and eluted with EtOAc/hexanes [1:100 (v/v)] to give compound 2 (2.7 g, 87%) as a white solid: 1H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 2.4 Hz, 1H), 7.83 (d, J = 8.4 Hz, 2H), 7.77 (d, J = 1.2 Hz, 1H), 7.72 (d, J = 8.4 Hz, 2H), 6.52 (t, J = 2.0 Hz, 1H). Anal. Calcd for C10H7F3N2: C, 56.61; H, 3.33; N, 13.20. Found: C, 56.60; H, 3.32; N, 13.22. 2-Phenyl-2H-indazole (h-1-pidz, 2b). A solution of phenylamine (1.0 g, 10.7 mmol) and 2-nitrobenzaldehyde (1.6 g, 10.7 mmol) in EtOH (20 mL) was refluxed for 8 h. After removal of most of the solvent, the yellow solid was collected by filtration to afforded compound 1b, which was used in the next step without further purification. Compound 1b (1.2 g, 5.3 mmol) was dissolved in P(OEt)3 (15 mL) and heated at 110 °C for 10 h. The solvent was removed, and the resulting residue was purified by column chromatography on silica gel and eluted with EtOAc/hexanes [1:50 (v/v)] to give compound 2b (760 mg, 86%) as a white solid: 1H NMR (400 MHz, CDCl3) δ 8.40 (s, 1H), 7.90 (d, J = 7.6 Hz, 2H), 7.81 (d, J = 8.8 Hz, 1H), 7.70 (d, J = 8.8 Hz, 1H), 7.52 (t, J = 7.6 Hz, 2H), 7.31−7.41 (m, 2H), 7.12 (dd, J1 = 7.2 Hz, J2 = 8.0 Hz, 1H). Anal. Calcd for C13H10N2: C, 80.39; H, 5.19; N, 14.42. Found: C, 80.40; H, 5.18; N, 14.43. Compounds 2c and 2d (2e−2g) were obtained in good to excellent yields by following the procedure described for 2b (2a). 2-(4-Fluorophenyl)-2H-indazole (f-1-pidz, 2c). White solid: 750 mg (83% yield); 1H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H), 7.84−7.86 (m, 2H), 7.78 (d, J = 8.8 Hz, 1H), 7.69 (d, J = 8.4 Hz, 1H), 7.31−7.35 (m, 1H), 7.19−7.23 (m, 2H), 7.12 (dd, J1 = 6.8 Hz, J2 = 8.0 Hz, 1H). Anal. Calcd for C13H10FN2: C, 73.57; H, 4.27; N, 13.20. Found: C, 73.55; H, 4.26; N, 13.23. 2-[4-(Trifluoromethyl)phenyl]-2H-indazole (cf3-1-pidz, 2d). White solid: 820 mg (89% yield); 1H NMR (400 MHz, CDCl3) δ 8.45 (s, 1H), 8.05 (d, J = 8.8 Hz, 2H), 7.77−7.79 (m, 3H), 7.70 (d, J = 8.4 Hz, 1H), 7.33−7.37 (m, 1H), 7.11−7.15 (m, 1H). Anal. Calcd for C13H10F3N2: C, 64.12; H, 3.46; N, 10.68. Found: C, 64.14; H, 3.45; N, 10.69. 1-Phenyl-1H-indazole (h-7-pidz, 2e). White solid: 980 mg (92% yield); 1H NMR (400 MHz, CDCl3) δ 8.23 (s, 1H), 7.74−7.83 (m, 4H), 7.55 (t, J = 7.6 Hz, 2H), 7.42−7.46 (m, 2H), 7.35−7.39 (m, 1H), 7.22−7.25 (m, 2H). Anal. Calcd for C13H10N2: C, 80.39; H, 5.19; N, 14.42. Found: C, 80.41; H, 5.19; N, 14.41. 1-(4-Fluorophenyl)-1H-indazole (f-7-pidz, 2f). White solid: 1.05 g (90% yield); 1H NMR (400 MHz, CDCl3) δ 8.10 (s, 1H), 7.71 (d, J = 8.0 Hz, 1H), 7.56−7.60 (m, 3H), 7.31−7.35 (m, 1H), 7.11− 7.16 (m, 3H). Anal. Calcd for C13H10FN2: C, 73.57; H, 4.27; N, 13.20. Found: C, 73.57; H, 4.28; N, 13.21. 1-[4-(Trifluoromethyl)phenyl]-1H-indazole (cf3-7-pidz, 2g). White solid: 1.14 g (88% yield); 1H NMR (400 MHz, CDCl3) δ 8.25 (s, 1H), 7.91 (d, J = 8.0 Hz, 2H), 7.82 (t, J = 8.4 Hz, 4H), 7.49 (t, J = 8.0 Hz, 1H), 7.28−7.32 (m, 1H). Anal. Calcd for C13H10F3N2: C, 64.12; H, 3.46; N, 10.68. Found: C, 64.13; H, 3.47; N, 10.70. General Synthetic Procedure for Ir(III) Complexes Ir1−Ir7. The cyclometalated Ir(III) dichloro-bridged dimers, [(C^N)2Ir(μCl)]2, were synthesized by IrCl3 hydrate with 2.2 equiv of 2a−2g in a 3:1 mixture of 2-ethoxyethanol and deionized water according to a method similar to that reported by Nonoyama.34 Then Ir(III) complexes Ir1−Ir7 were prepared by the reaction of Ir(III) dichlorobridged dimers with 2.2 equiv of Ktpip in 2-ethoxyethanol at 120 °C. After the reaction was completed, the reaction mixture was diluted with water and extracted multiple times with dichloromethane. The combined organic phase was concentrated, and the residue was purified by column chromatography to provide the crude product, which was further purified by vacuum sublimation. Ir1 (124 mg, 53% yield): yellow powder; 1H NMR (400 MHz, CDCl3) δ 7.88 (d, J = 2.8 Hz, 2H), 7.67−7.72 (m, 4H), 7.61 (d, J = 2.0 Hz, 2H), 7.27−7.36 (m, 10H), 7.15−7.23 (m, 4H), 7.06−7.11

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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/acs.organomet.8b00491. OLED fabrication and measurement, 1H NMR spectra and mass spectrometry of main ligands 2a−2g and iridium complexes Ir1−Ir7, emission spectra of iridium(III) complexes Ir1−Ir7 in degassed CH2Cl2 solutions at 77 K, PL spectra of Ir3 and Ir6 in solution and solid state, EL spectra of D3 and D6, crystallographic data and selected bonds and angles of complexes Ir2−Ir7, and frontier orbital energy and electron density distributions of complexes Ir1−Ir7 (PDF) H

DOI: 10.1021/acs.organomet.8b00491 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Accession Codes

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CCDC 1578682−1578687 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Zhi-Gang Niu: 0000-0003-4842-1464 You-Xuan Zheng: 0000-0002-1795-2492 Jing-Lin Zuo: 0000-0003-1219-8926 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51773088 and 21501037), the Natural Science Foundation of Jiangsu Province (BY2016075-02), the Natural Science Foundation of Hainan Province (218QN236), and the Program for Innovative Research Team in University (IRT-16R19).

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REFERENCES

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DOI: 10.1021/acs.organomet.8b00491 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00491 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00491 Organometallics XXXX, XXX, XXX−XXX