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Jun 23, 2017 - ABSTRACT: Tetradentate chelates bearing tripodal arranged terpyr- idine and a functional pyrazole unit (i.e., L1-H and L2-H) were emplo...
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Sky Blue-Emitting Iridium(III) Complexes Bearing Nonplanar Tetradentate Chromophore and Bidentate Ancillary Yu-Sian Li,† Jia-Ling Liao,† Ke-Ting Lin,‡ Wen-Yi Hung,*,‡ Shih-Hung Liu,§ Gene-Hsiang Lee,§ Pi-Tai Chou,*,§ and Yun Chi*,† †

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan § Department of Chemistry and Instrumentation Center, National Taiwan University, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: Tetradentate chelates bearing tripodal arranged terpyridine and a functional pyrazole unit (i.e., L1-H and L2-H) were employed in preparation of Ir(III) complexes [Ir(L1)Cl2] (1) and [Ir(L2)Cl2] (2); subsequent chloride-to-bipyrazolate substitution gave [Ir(L1)(bipz)] (3) and [Ir(L2)(bipz)] (4). Single-crystal X-ray structural studies on 1 and 3 showed the possession of a tetradentate chelate, whereas the remaining cis-sites are occupied by either dual chlorides or the bipz chelate, respectively. Sky blue organic lightemitting diode with peak efficiencies (10.1%, 19.8 cd·A−1, and 20.4 lm· W−1) was successfully fabricated using 3 (or 4) as dopant emitter, highlighting the potential application of this class of Ir(III) phosphor.

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INTRODUCTION Organic light-emitting diodes (OLED) have been spurred by the rapid development of modern optoelectronic technologies such as flat panel display and solid-state lighting. Their efficiencies have been much improved during the past two decades by selecting either phosphorescent emitters1−5 or thermally activated delayed fluorescence (TADF) emitters.6,7 For the former case, the Ir(III)-based phosphors possess the heavy-atom induced rapid intersystem crossing from the lowest-lying singlet to triplet excited states and,8 hence, are capable of harvesting both singlet and triplet excitons, affording unitary internal quantum efficiency for the OLED devices.9 These Ir(III) metal phosphors were commonly assembled using three bidentate chelates arranged in either homoleptic or heteroleptic manner, cf. [Ir(ppy)3] and [Ir(ppy)2(acac)], cf. Scheme 1.10,11 The chelating chromophores may consist of 2phenylpyidine (ppyH), N-arylimidazolylidene, acetylacetone

(acacH), benzylphosphine, 2-pyridylpyrazole, and functional analogues, from which tuning of emission across the visible region were achieved by adjusting the ππ* energy gap of chelate and electron density at the metal center.12−17 Parallel to the development of tris-bidentate metal complexes, there is a growing interest of using multidentate chelate in assembly of luminescent Ir(III) phosphors. For instance, Williams,18 Haga,19 and others20−24 have examined tridentate cyclometalate in constructing Ir(III) complexes bearing ligands arranged in three distinctive coordination mode, that is, so-called 3 + 2 + 1 mode, in which the planar tridentate chelate adopted a meridional coordination arrangement, cf. [Ir(dpyx)(dfppy)Cl] (Scheme 2). By contrast, Chi and co-workers25,26 and Guerchais and Zysman-Colman27 have independently reported the isolation of a class of Ir(III) complexes with chelates arranged in a distinctive 3 + 2 + 1 mode, for example, complexes A and B, where the “nonplanar” tridentate chelate adopts a distinctive facial geometry. With this success in changing chelate from bidentate to tridentate and altering their bonding from meridional to facial coordination, it is clear that preparation of Ir(III) complexes bearing tetradentate chelate in either planar or nonplanar geometries should be also accessible. In fact, Che and co-workers have employed a four-coordinate, planar chelate for preparing Ir(III) emitters such as complex C, for which the corresponding solution-processed OLED gave a red emission with a peak

Scheme 1. Structural Drawing of [Ir(ppy)3] and [Ir(ppy)2(acac)]

Received: June 23, 2017 Published: August 10, 2017 © 2017 American Chemical Society

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dibromopyridine using literature procedure,29 followed by acylation. In contrast, the corresponding 1-(4-(tert-butyl)-6(1,1-di(pyridin-2-yl)ethyl)pyridin-2-yl)ethanone was prepared by treatment of 1,1-bis(2-pyridyl)ethane with 2-bromo-4-(tertbutyl)-6-(2-methyl-1,3-dioxolan-2-yl)pyridine, followed by acid-catalyzed deprotection. The obtained acetyl derivatives were treated with CF3CO2Et and hydrazine in two consecutive steps, yielding the desired tetradentate chelates L1-H and L2-H. Furthermore, reaction of L1-H or L2-H with IrCl3·3H2O gave dichloride Ir(III) complexes [Ir(L1)Cl2] and [Ir(L2)Cl2] (1 and 2) in refluxing 2-ethoxyethanol, while the bipyrazolate complexes [Ir(L1)(bipz)] and [Ir(L2)(bipz)] (3 and 4) were prepared by heating of 1 and 2 with the in situ generated 5,5′di(trifluoromethyl)-3,3′-bipyrazolate dianion (bipzNa2) in Nmethyl-2-pyrrolidone, followed by flash chromatography on neutral alumina. These Ir(III) complexes were characterized by mass spectrometry, 1H and 19F NMR spectroscopies, and elemental and electrochemical analyses. Single-crystal X-ray diffraction studies were executed to establish their exact molecular structure. As depicted in Figures 1 and 2, both complexes 1 and 3 possess distorted octahedral

Scheme 2. Structural Drawing of Representative Ir(III) Metal Complexes Bearing Multidentate Chelate

external quantum efficiency (EQE) of 10.5%.28 Encouraged by this finding, we proceeded to the design and preparation of Ir(III) metal complexes bearing the nonplanar tetradentate chelate of judiciously created, local π-conjugation, for example, 2,2′-(1-(6-pyrazol-5-yl)pyridin-2-yl)ethane-1,1-diyl)dipyridine (e.g., L1-H and L2-H). It is expected that such a new design of tetradentate chelate should allow the isolation of more stable and highly emissive metal phosphors and, in the meantime, provide a blue-shifted emission peak wavelength by taking the advantage of reduced π-conjugation.

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RESULTS AND DISCUSSION Syntheses and Characterization. The synthetic schemes and detailed experimental conditions of L1-H and L2-H are depicted in Scheme 3. The 1,1-bis(2-pyridyl)ethane was first prepared by coupling of 2-ethylpyridine with 2-fluoropyridine.29 The next key intermediates are 1-(6-(1,1-di(pyridin-2yl)ethyl)pyridin-2-yl)ethanone and 1-(4-(tert-butyl)-6-(1,1-di(pyridin-2-yl)ethyl)pyridin-2-yl)ethanone; the former was prepared by coupling of 1,1-bis(2-pyridyl)ethane with 2,6Scheme 3. Synthetic Protocol for Tetradentate Chelatea

Figure 1. Structural drawing of 1 with thermal ellipsoids shown at the 40% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances: Ir−N(1) = 2.045(3), Ir−N(2) = 2.025(3), Ir−N(3) = 2.000(3), Ir−N(4) = 2.034(3), Ir−Cl(1) = 2.3609(9), and Ir−Cl(2) = 2.3616(8) Å, and bond angles: N(1)−Ir−N(4) = 169.70(11), N(2)− Ir−Cl(1) = 175.04(8), and N(3)−Ir−Cl(2) = 174.93(8)°.

a Experimental conditions: (i) n-butyllithium, 2-fluoropyridine, THF, −78 °C. For derivative with R = H: (ii) n-butyllithium, 2,6dibromopyridine, THF, −78 °C. (iii) dimethylacetamide, n-butyllithium, THF, −78 °C. For derivative with R = tBu: (iv) nbutyllithium, 2-bromo-4-(tert-butyl)-6-(2-methyl-1,3-dioxolan-2-yl)pyridine, THF, −78 °C. (v) HCl, H2O, reflux. Common procedures: (vi) NaOEt, CF3CO2Et, THF, reflux. (vii) N2H4·H2O, EtOH, reflux.

skeleton and a nonplanar tetradentate chelate, whereas the remaining cis-sites are occupied by either two chlorides or bipz chelate, respectively. Moreover, in 1, both Ir−Cl distances are approximately equal, while the internal pyridyl unit of tetradentate chelate exhibits shorter Ir−Npy distance (Ir− N(3) = 2.000(3) Å) vs other Ir−Npy distances (2.025(3)− 2.045(3) Å). When the chlorides were replaced with bipz, giving complex 3, both the terminal and internal pyridyl fragments trans to the bipz now showed increased Ir−N distances, that is, Ir−N(3) = 2.0177(16) Å and Ir−N(1) = 2.0585(18) Å, which are consistent with the stronger transinfluence exerted by the bipz versus chloride in 1. Notably, all pyridyl units in both 3 and 4 are located at the mutual trans disposition versus pyrazolate fragments (e.g., facgeometry, Scheme 4). This is in sharp contrast to the trisbidentate analogue mer-[Ir(fppz)3] that showed the meridional geometry and poor emission efficiency observed at room temperature (RT).30,31 This photophysical character of mer[Ir(fppz)3] is also in agreement with the structure−photo10055

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Figure 3. UV−vis absorption of chelates and all studied Ir(III) complexes in CH2Cl2 and the emission of 3 and 4 recorded in degassed CH2Cl2 solution.

contribution from the ligand ππ* transition. As for the emission, both complexes 3 and 4 are only weakly emissive in solution at RT due to the enhanced deactivation induced by solvent collision. The efficiency improved substantially upon changing to solid state, for which the rigid media has effectively destabilized the metal-centered dd excited state by preventing the switching and elongation of metal−ligand distances upon excitation. Overall, emission of 4 occurred at the slightly higher energy than that of 3, which is consistent with the existence of t-butyl substituent that enlarged the energy gap by destabilizing the π*-level of chelate. Figure S1 depicted the emission profiles recorded at 77 K, in which the emission of both 1 and 2 are found to be red-shifted compared to that of 3 and 4, confirming the destabilization of metal dπ orbitals exerted by the πdonating chlorides versus bipyrazolate. Cyclic voltammograms of Ir(III) complexes 1−4 were also recorded, exhibiting quasi-reversible oxidation and irreversible reduction processes (cf. Table 1 and Figure S2). In general, the anodic peak potentials (Epa) of 1 and 2 were notably more positive than those of bipz analogues 3 and 4, suggesting less electron density at the metal center, due to the poor electron donating character of chlorides versus bipz chelate. Furthermore, the small Epa differences between the pair of Ir(III) complexes 1 and 2 (Δ = 0.01 V) and 3 and 4 (Δ = 0.02 V) are consistent with the metal-centered oxidation, which is relatively insensitive to the t-butyl substituent introduced. In sharp contrast, the cathodic peak potential (Epc) occurred at −2.20 and −2.26 V for Ir(III) complexes 1 and 2 and at −2.35 and −2.50 V for 3 and 4, respectively. Since the reduction mainly occurred at the unique, conjugated pyridyl azolate fragment of the tetradentate chelate, it will be more sensitive to the t-butyl substituent added at the unique, central pyridyl unit. Theoretical Investigation. We then performed the timedependent density functional theory (TD-DFT) to gain further insight into the transition properties. Figure 4 and Figures S3− S6 of Supporting Information depict the frontier molecular orbital (MO) involved in the lower-lying transitions of Ir(III) complexes 1−4. All pertinent energy wavelengths and corresponding assignments of each transition were listed in Table 2 and Tables S1−S4. The calculated emission wavelengths of the T1→S0 transition for 3: 430.9 nm and 4: 425.1 nm were also in agreement with the trend of the onset of their phosphorescence spectra in Figure 3. These simulation results indicate that the TD-DFT calculation is suitable in predicting

Figure 2. Structural drawing of 3 with thermal ellipsoids shown at the 40% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances: Ir−N(1) = 2.0585(18), Ir−N(2) = 2.0395(18), Ir− N(3) = 2.0177(16), Ir−N(4) = 2.0277(18), Ir−N(7) = 2.0158(18), and Ir−N(8) = 2.0443(17) Å, and bond angles: N(1)−Ir−N(7) = 175.11(7), N(2)−Ir−N(4) = 167.89(7), and N(3)−Ir−N(8) = 172.83(7)°.

Scheme 4. Structural Drawing of Ir(III) Metal Complexes 1−4 and mer-[Ir(fppz)3]

physical behaviors of fac- and mer-[Ir(46dfppy)3], 46dfppyH = 4,6-difluorophenylpyridine, for which the fac-derivative is known to exhibit better photoluminescence at RT versus the mer isomer.32 We expected that the destabilized metal dπ orbitals within the fac isomer might relax, in part, the radiationless deactivation and hence afford better emission efficiency than that of mer counterpart. Figure 3 shows the absorption of both chelates and complexes 1−4 and emission spectra of 3 and 4 in CH2Cl2, as 1 and 2 are totally nonemissive under all states at RT. As can be seen, the absorption onset of 1 and 2 appears at ∼440 nm, while it is blue-shifted to ∼425 nm for 3 and 4 in CH2Cl2 solution. The result manifests the involvement of different lower-lying transitions between 1 and 3 (or 2 and 4), and the absorption extinction coefficient gradually increased upon shifting to the shorter wavelength, indicative of the increasing 10056

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Inorganic Chemistry Table 1. Selected Photophysical and Electrochemical Properties for the Studied Ir(III) Complexes photophysical data abs λ, nm (ε × 10−3 M−1·cm−1)a 267 (14.3), 283 268 (17.1), 283 253 (25.3), 266 321 (11.9) 252 (23.9), 264 318 (10.7)

1 2 3 4

PL λmax (nm)b

electrochemical data Q.Y. (%)b

τobs (μs)b

Q.Y. (%)c 10 wt % in mCP

Q.Y. (%)c 10 wt % in mCP:POT2T

Epaox (V)d

Epcred (V)d −2.20 −2.26 −2.35 −2.50

(12.6) (15.4) (25.2),

490 [475]

1.8e [80]

0.12 [4.5]

51

45

1.16 1.15 0.97

(22.8),

484 [488]

1.4e [52]

0.13 [5.1]

47

41

0.95

a UV−vis spectra were recorded in CH2Cl2 at 1 × 10−5 M. bPhotoluminescence, quantum yields, and lifetime were recorded in degassed CH2Cl2 at 1 × 10−5 M; those recorded as powder were marked with square bracket. cQuantum efficiency in films was recorded with an integration sphere coupled with a photonic multichannel analyzer (Hamamatsu C9920). dAll electrochemical potentials were measured in 0.1 M TBAPF6/CH2Cl2 and THF for oxidation and reduction and reported in volts using FcH/FcH+ as reference; the Pt electrode and Au/Hg electrode were selected as the working electrode of oxidation and reduction processes, respectively. eCoumarin 102 (C102) in MeOH (Φ = 87% and λmax = 480 nm) was employed as standard.

donating chloride ligands for 1 and 2 would reduce the crystal field stabilization energy and give relatively lower-lying metalcentered dd states that, upon thermal activation, accelerate radiationless deactivation from the excited molecules.33,34 For complexes 3 and 4, the electron density distributions of HOMO are mainly delocalized at the bipz ligand and partially at iridium (∼4%). Therefore, the S0→S1 and S0→T1 transitions were mainly ascribed to ligand-to-ligand charge transfer (LLCT) in combination with partial metal-to-ligand charge transfer (MLCT). The involvement of iridium for MLCT and S 1 (MLCT)→T 1 (LLCT) (or vice versa S 1 (LLCT)→ T1(MLCT)) incorporating orbital flipping greatly facilitate both S1→T1 intersystem crossing and T1→S0 radiative decay,12 giving appreciable phosphorescence intensity in solution and solid for the Ir(III) complexes 3 and 4 (see Table 1). Electroluminescence. To investigate their electroluminescence (EL), we selected a simple architecture: indium tin oxide (ITO)/4% ReO3:mCP (60 nm)/mCP (15 nm)/mCP:POT2T: 3 or 4 (50:50:10 wt %) (20 nm)/ PO-T2T (50 nm)/ Liq (0.5 nm)/ Al (100 nm), for which the phosphors 3 and 4 were vacuum-deposited into the cohost of N,N′-dicarbazolyl-3,5benzene (mCP)35 and ((1,3,5-triazine-2,4,6-triyl)-tris(benzene3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T)36 to achieve the optimized carrier transport. Rhenium oxides (ReO3) in mCP is applied as the ohmic contact,37 while mCP, PO-T2T, 8-hydroxyquinolinolatolithium (Liq), and Al are used as the hole-transporting layer (HTL), electrontransporting layer (ETL), electron injection layer (EIL), and cathode, respectively. The device structure and the energylevels diagram are presented in Figure 5a, which follows a previous design for blue-emitting OLED.38 Figure 5b−d depicts the current density-voltage-luminance (J-V-L) characteristics, device efficiencies, and EL spectra, respectively. The device of 3 reveals a maximum luminance (Lmax) of 43 056 cd·m−2 at 13.8 V (2680 mA·cm−2) and CIE coordinates of (0.17, 0.31). The maximum EQE, current (CE), and power efficiencies (PE) were measured to be 10.1%, 19.8 cd·A−1, and 20.4 lm·W−1, respectively; all of them are higher than those obtained using the second dopant 4, cf. 9.0%, 17.4 cd·A−1, and 16 lm·W−1 with comparable CIE coordinates of (0.16, 0.31). In addition, the efficiency roll-off is relatively severe, exhibiting maximum EQE of 10.1% (3.6 V) and EQE of 8.8% at 1 × 103 cd·m−2 (5.4 V) and 5.4% at 1 × 104 cd·m−2 (9.0 V) for dopant 3 and maximum EQE of 9.0% (4.0 V) and EQE of 7.6% at 1 × 103 cd·m−2 (6.0 V) and 4.5% at 1 × 104 cd·m−2 (9.8 V) for dopant 4, respectively. Apparently, the emission lifetime of the emitters

Figure 4. Frontier molecular orbital HOMO and LUMO for Ir(III) complexes 1−4. “Ir” indicates the relative electron density distribution at Ir atom.

Table 2. Calculated Wavelengths, Transition Probabilities, and Main Charge Transfer Characters of the Lowest Optical Transitions S1 and T1 for Ir(III) Complexes 1−4 in CH2Cl2 complex 1 2

3 4

state λ (nm)

f

T1 S1 T1

437.3 375 432.6

0 0.011 0

S1 T1 S1 T1 S1

370.4 430.9 418.6 425.1 410.3

0.008 0 0.0008 0 0.0046

main assignments HOMO→LUMO(59%) HOMO→LUMO(90%) HOMO→LUMO +1(43%) HOMO→LUMO(87%) HOMO→LUMO(53%) HOMO→LUMO(97%) HOMO→LUMO(38%) HOMO→LUMO(96%)

MLCT 18.61% 22.46% 20.23% 21.23% 4.34% −0.40% 5.02% 0.62%

the lowest Franck−Condon excited state for both absorption and emission based on the ground-state S0 optimized geometries of the studied Ir(III) complexes. The optical S0→ S1 and S0→T1 transition assignments were highest occupied molecular orbital (HOMO)→lowest unoccupied molecular orbital (LUMO) mainly for Ir(III) complexes 1−4 (except HOMO→LUMO+1 in S0→T1 transition for 2). For complexes 1 and 2, the electron density distributions of HOMO were mainly delocalized at iridium (∼31%) and chlorides. The π10057

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Article

EXPERIMENTAL SECTION

General Information and Materials. All reactions were performed under nitrogen atmosphere, and solvents were distilled from appropriate drying agents prior to use. Commercially available reagents were used without further purification unless otherwise stated. The starting materials, 1-(6-bromopyridin-2-yl)ethanone,40 2bromo-4-(tert-butyl)-6-(2-methyl-1,3-dioxolan-2-yl)pyridine, and di(trifluoromethyl)-3,3′-bipyrazole,41 were synthesized according to literature procedures. 1H and 19F NMR spectra were measured with a Varian Mercury-400 instrument. UV−vis spectra were recorded on a HITACHI U-3900 spectrophotometer. Details of measurement of steady-state emission in both solution and solid state was described in our previous reports. Lifetime studies were measured with Edinburgh FL 900 photon-counting system. The elemental analysis was performed on a Heraeus CHN-O Rapid Elementary Analyzer. Mass spectra were recorded on a JEOL SX-102A instrument operating in electron impact (EI) or fast atom bombardment (FAB) mode. Preparation of (1) and (2). A mixture of IrCl3·3H2O (1.03 g, 2.92 mmol) and L1-H (1.12 g, 2.83 mmol) in 2-ethoxyethanol (50 mL) was heated at 140 °C for 12 h. After the solution cooled to RT, the solvent was removed, and the residue was washed with water and diethyl ether to afford a light yellow powder of 1 (1.81 g, 97%). Crystals suitable for X-ray diffraction study were obtained from slow evaporation of an acetonitrile solution at RT. The corresponding tbutyl derivative [Ir(L2)Cl2] (2) was obtained from a similar reaction of IrCl3·3H2O and chelate L2-H; yield: 92%. Spectral Data of 1. 1H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d6)): δ 9.22 (d, J = 5.4 Hz, 1 H), 9.13 (d, J = 5.4 Hz, 1 H), 8.20−8.14 (m, 2 H), 8.07−7.99 (m, 3 H), 7.93 (d, J = 7.9 Hz, 1 H), 7.84 (d, J = 7.9 Hz, 1 H), 7.62 (t, J = 6.3 Hz, 1 H), 7.48 (t, J = 6.3 Hz, 1 H), 7.25 (s, 1 H), 2.75 (s, 3 H). 19F NMR (376 MHz, DMSOd6): δ −58.54 (s, 3 F). MS (FAB, 193Ir): m/z 657.9 [M+]. Anal. Calcd for C21H15Cl2F3IrN5: C, 38.36; H, 2.30; N, 10.65. Found: C, 38.41; H, 2.57; N, 10.60%. Selected Crystal Data of 1. C23H18Cl2F3IrN6; M = 698.53; T = 150(2) K; trigonal; space group = R3c; a = 22.9503(5) Å, b = 22.9503(5) Å, c = 23.5928(5) Å; γ = 120°; V = 10761.8(5) Å3; Z = 18; ρcalcd = 1.940 Mg·m−3; F(000) = 6048; crystal size = 0.215 × 0.093 × 0.087 mm3; λ(Mo Kα) = 0.710 73 Å; μ = 5.855 mm−1; 24 610 reflections collected, 5455 independent reflections (Rint = 0.0216), max and min transmission = 0.7456 and 0.5792, restraints/parameters = 25/328, goodness-of-fit (GOF) = 1.035, final R1 [I > 2σ(I)] = 0.0122 and wR2(all data) = 0.0272, largest difference peak and hole = 0.680 and −0.509 e·Å−3. Spectral Data of 2. 1H NMR (400 MHz, DMSO-d6): δ 9.22 (d, J = 5.3 Hz, 1 H), 9.13 (d, J = 5.3 Hz, 1 H), 8.17 (d, J = 4.9 Hz, 2 H), 8.05−7.98 (m, 3 H), 7.70 (s, 1 H), 7.63 (dd, J = 6.5, 5.3 Hz, 1 H), 7.48 (t, J = 6.5 Hz, 1 H), 7.32 (s, 1 H), 2.80 (s, 3 H), 1.33 (s, 9 H). 19F NMR (376 MHz, DMSO-d6): δ −58.54 (s, 3 F). MS (FAB, 193Ir): m/z 713.0 [M+]. Anal. Calcd for C25H23Cl2F3IrN5: C, 42.08; H, 3.25; N, 9.81. Found: C, 41.97; H, 3.41; N, 9.81%. Preparation of (3) and (4). A suspension of NaH (6.05 mg, 0.25 mmol) in anhydrous tetrahydrofuran (THF) was treated with 5,5′di(trifluoromethyl)-3,3′-bipyrazole (30.9 mg, 0.11 mmol) at 0 °C, and stirred for 30 min. After removal of unreacted NaH, the resulting pyrazolate salt was transferred to a 50 mL round-bottom flask, together with 1 (50.1 mg, 0.08 mmol) and 30 mL of anhydrous Nmethyl-2-pyrrolidone (NMP), and the mixture was brought to reflux

Figure 5. (a) Schematic OLED structure with the cohost of mCP:POT2T. (b) Current density−voltage−luminance (J-V-L) characteristics, (c) EQE and PE as a function of luminance, and (d) EL spectra, respectively.

(3: 4.5 μs and 4: 5.1 μs) is still not short enough to reduce the quenching at higher driving voltages.39 These device characteristics are summarized in Table 3. For comparison, the second OLEDs were fabricated using pure mCP host; cf. ITO/4% ReO3:mCP (60 nm)/ mCP (15 nm)/ mCP: 3 or 4 (10 wt %) (20 nm)/3TPYMB (50 nm)/ Liq (0.5 nm)/ Al (100 nm). As shown in Figure S7 of Supporting Information, they exhibited notable inferior efficiencies. This result clearly infers the more balanced carrier transports of mCP:POT2T cohost as well as better exciton confinement in the emissive layer.

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CONCLUSION In summary, we synthesized a new class of tetradentate 2,2′-(1(6-pyrazol-5-yl)pyridin-2-yl)ethane-1,1-diyl)dipyridine chelates (L1-H and L2-H). Subsequently, addition to IrCl3·3H2O gave isolation of Ir(III) complexes 1 and 2, respectively, with a 4 + 1 + 1 geometry, in which L1 (L2) adopted a nonplanar, facial-like arrangement. It is expected that the tripodal arranged terpyridine offered the main driving force for initial ligand coordination. Complexes 1 and 2 showed no perceptible emission in both solution and solid states at RT, which is exerted by the π-donating terminal chlorides and destabilized energy level of metal centered dπ quenching state. Then, substitution with bipz gives the 4 + 2 coordinate Ir(III) complexes 3 and 4, which show drastic increase in emission at RT due to the stronger metal−ligand bonding interaction. As a result, highly efficient OLEDs have been fabricated using both 3 and 4 as dopant emitters, paving a new research direction in the quest of Ir(III) complexes for lighting materials. Table 3. EL Performances of the Studied OLED Devices host

dopanta

Von [V]b

Lmax [cd·m−2]

Imax [mA·cm−2]

max EQE, CE, PE [%, cd·A−1, lm·W−1]

EQE,c (V)

CIEx,y

mCP:PO-T2T mCP:PO-T2T mCP mCP

3 4 3 4

2.6 3.0 4.4 4.4

43 056 (13.8 V) 39 007 (14.0 V) 4714 (14.0 V) 4179 (13.4 V)

2680 2514 1120 940

10.1, 19.8, 20.4 9.0, 17.4, 16.0 9.1, 17.2, 10.78 7.0, 12.8, 8.35

8.8, (5.4) 7.6, (6.0) 3.45, (7.8) 2.93, (8.0)

0.17,0.31 0.16,0.31 0.16,0.30 0.16,0.31

Dopant concentration was 10 wt %. bTurn-on voltage at which emission became detectable, that is, 1 cd·m−2. cThe EQE and driving voltage of device at 1000 cd·m−2. a

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Article

Inorganic Chemistry Notes

for 8 h. After removal of solvent, the residue was purified by Al2O3 (neutral) column chromatography and eluted with acetone and CH2Cl2 (1:10) to afford a light yellow powder of 3 (66.1 mg, 50.8%). Sublimation: 380 °C (78%). Crystals suitable for X-ray diffraction study were obtained from slow evaporation of a mixed acetone and methanol (5:1) solution at RT. The corresponding t-butyl derivative [Ir(L2)(bipz)] (4) was obtained from a similar reaction of 2 and bipzNa2; yield: 78%. Spectral Data of 3. 1H NMR (400 MHz, acetone-d6): δ 9.68 (d, J = 5.7 Hz, 1 H), 8.39 (d, J = 7.9 Hz, 1 H), 8.24 (t, J = 7.9 Hz, 1 H), 8.20− 8.15 (m, 2 H), 8.12−8.08 (m, 2 H), 7.97 (d, J = 7.9 Hz, 1 H), 7.64 (t, J = 6.6 Hz, 1 H), 7.41 (d, J = 5.7 Hz, 1 H), 7.30 (t, J = 6.6 Hz, 1 H), 7.00 (s, 1 H), 6.83 (s, 1 H), 6.52 (s, 1 H), 3.06 (s, 3 H). 19F NMR (376 MHz, acetone-d6): δ −60.20 (s, 3 F), −60.56 (s, 3 F), −60.81 (s, 3 F). MS (FAB, 193Ir): m/z 854.1 [M+]. Anal. Calcd for C29H17F9IrN9: C, 40.75; H, 2.00; N, 14.75. Found: C, 40.83; H, 2.49; N, 14.72%. Selected Crystal Data of 3. C29H17F9IrN9; M = 854.72; T = 150(2) K; triclinic; space group = P1̅; a = 11.6027(5) Å, b = 11.9046(5) Å, c = 12.1167(5) Å; α = 64.9420(12)°, β = 80.1304(15)°, γ = 79.6747(15)°; V = 1482.96(11) Å3; Z = 2; ρcalcd = 1.914 Mg·m−3; F(000) = 824; crystal size = 0.274 × 0.161 × 0.137 mm3; λ(Mo Kα) = 0.710 73 Å; μ = 4.597 mm−1; 30 442 reflections collected, 6812 independent reflections (Rint = 0.0253), max and min transmission = 0.7456 and 0.5756, restraints/parameters = 19/465, GOF = 1.087, final R1 [I > 2σ(I)] = 0.0169 and wR2(all data) = 0.0371, largest difference peak and hole = 1.613 and −0.927 e·Å−3. Spectral Data of 4. 1H NMR (400 MHz, acetone-d6): δ 9.67 (d, J = 5.6 Hz, 1 H), 8.37 (d, J = 8.2 Hz, 1 H), 8.22 (d, J = 8.2 Hz, 1 H), 8.16 (t, J = 7.9 Hz, 1 H), 8.09−8.04 (m, 3 H), 7.62 (t, J = 6.8 Hz, 1 H), 7.41 (d, J = 5.6 Hz, 1 H), 7.29 (t, J = 6.8 Hz, 1 H), 7.05 (s, 1 H), 6.83 (s, 1 H), 6.53 (s, 1 H), 3.10 (s, 3 H), 1.47 (s, 9 H). 19F NMR (376 MHz, acetone-d6): δ −60.15 (s, 3 F), −60.38 (s, 3 F), −60.76 (s, 3 F). MS (FAB, 193Ir): m/z 910.3 [M+]. Anal. Calcd for C33H25F9IrN9: C, 43.52; H, 2.77; N, 13.84. Found: C, 43.47; H, 2.86; N, 13.81%. Crystallography. Crystallographic data of Ir(III) complexes 1 and 3 correspond to CCDC Nos. 1553886 and 1553887, respectively. Additional crystallographic information is available in the Supporting Information.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Technology of Taiwan, under the Grant No. MOST 1052811-M-007-028.

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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.inorgchem.7b01583. Details of electrochemical and photophysical data, computation method, and OLED fabrication method (PDF) Accession Codes

CCDC 1553886−1553887 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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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (W.Y.H.) *E-mail: [email protected]. (P.T.C.) *E-mail: [email protected]. (Y.C.) ORCID

Wen-Yi Hung: 0000-0003-1761-2743 Pi-Tai Chou: 0000-0002-8925-7747 Yun Chi: 0000-0002-8441-3974 10059

DOI: 10.1021/acs.inorgchem.7b01583 Inorg. Chem. 2017, 56, 10054−10060

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01583 Inorg. Chem. 2017, 56, 10054−10060