Highly Efficient Phosphorescent Furo[3,2-c]pyridine Based Iridium

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1888−1896

Highly Efficient Phosphorescent Furo[3,2‑c]pyridine Based Iridium Complexes with Tunable Emission Colors over the Whole Visible Range Zhimin Yan,†,‡ Yanping Wang,§ Junqiao Ding,*,‡ Yue Wang,*,† and Lixiang Wang*,‡ †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China § School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, P. R. China S Supporting Information *

ABSTRACT: A series of highly efficient phosphorescent Ir complexes with tunable emission colors over the whole visible range have been designed and synthesized based on furo[3,2c]pyridine ligand. By mainly varying the molecular structures of the C-chelated blocks, the emission maxima of these complexes can be obviously tailored from 477 to 641 nm while keeping the considerable photoluminescence quantum yields (PLQYs) (0.55−0.78 at wavelength of 475−560 nm and 0.10−0.34 at wavelength of 590−640 nm). Correspondingly, the phosphorescent organic light-emitting diodes (OLEDs) achieve high-performance greenish-blue, green, greenish-yellow, orange, red, and deep-red electrophosphorescence, revealing state-of-art external quantum efficiences (EQEs) of 20.0% (46.6 cd/A), 31.8% (89.0 cd/A), 19.9% (71.9 cd/A), 16.6% (38.9 cd/ A), 12.0% (16.7 cd/A), and 8.5% (7.3 cd/A) as well as Commision Internationale de L’Eclairage (CIE) coordinates of (0.25, 0.48), (0.30, 0.58), (0.43, 0.54), (0.62, 0.37), (0.66, 0.32), and (0.70, 0.29), respectively. The results clearly demonstrate the great potential of furo[3,2-c]pyridine based phosphors used for full-color OLED displays. KEYWORDS: OLEDs, furo[3,2-c]pyridine, Ir complex, C-chelated block, full-color [d]thiazole,12 thieno[3,2-c]pyridine,36 pyridazine,37 and pyrimidine38 for yellow; and quinoline,21 isoquinoline,12 and benzo[h]quinoline39 for red. On the other hand, the ancillary LX ligands were also adopted to obtain the blue to red range emission control, in which excitation from the metal-to-ligand charge transfer (MLCT) transition was followed by efficient interligand energy transfer (ILET) to the “emitting ancillary ligand” so as to provide a novel channel of phosphorescence color tuning.40 This methodology was successfully employed by C.-H. Cheng to realize blue, green, and orange electrophosphorescence, revealing a peak external quantum efficiency (EQE) of 17.1%, 24.4%, and 24.9% together with Commission International De L’Eclairge (CIE) coordinates of (0.13, 0.16), (0.30, 0.62), and (0.60, 0.39), respectively.41 Despite such achievements, we note that the modification of the C-chelated blocks in the C∧N ligands has been rarely reported for the broad range color tuning based on the same N-chelated blocks.42

1. INTRODUCTION The early work1−4 of Watts and Güdel has brought Iridium (Ir) complexes to many researchers’ attention. Nowadays they have been applied to a wide scope such as organic photovoltaic cells,5 nonlinear optics,6 water photolysis,7 bioimaging,8 sensors,9,10 and so on. As one of the successful applications, organic light-emitting diodes (OLEDs),11−19 with Ir complexes as the phosphorescent emitters instead of the traditional fluorescent ones, can harvest both the generated singlet and triplet excitons to realize a theoretical 100% internal quantum efficiency.20 Therefore, many different kinds of homoleptic (Ir(C∧N)3)21 and heteroleptic (Ir(C∧N)2(LX))12 Ir complexes have been exploited for high-performance OLEDs, where C∧N is a monoanionic ligand composed of the C- and N-chelated blocks, and LX is an ancillary ligand (e.g., acetyl acetonate,22 picolinate,23 and N,N-heteroaromatic24). To fulfill the requirements of full-color OLED displays, the color regulation toward efficient blue, green, and red emissions is highly desirable for these Ir complexes.25 Through the skeletal structure changes, various C∧N ligands containing the specific N-chelated blocks have been designed for different colors, that is, pyrazole,26 triazole,27,28 and imidazole29,30 for blue; pyridine31 and benzo[d]imidazole32−35 for green; benzo© 2017 American Chemical Society

Received: September 30, 2017 Accepted: December 22, 2017 Published: December 22, 2017 1888

DOI: 10.1021/acsami.7b14906 ACS Appl. Mater. Interfaces 2018, 10, 1888−1896

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ACS Applied Materials & Interfaces

Scheme 1. (a) Synthetic Route and (b) Molecular Structures for Developed Furo[3,2-c]pyridine Based Ir Complexes with Tunable Emission over the Whole Visible Range

Earlier, we developed a new N-chelated block furo[3,2c]pyridine, and the resultant heteroleptic Ir complex (pfupy)2Ir(acac) showed an unexpected EQE of 30.5% without any outcoupling technology.43 Promoted by this outstanding performance, here, we further demonstrate its color tuning in the whole visible region. Starting from (pfupy)2Ir(acac) (Scheme 1), by mainly varying the molecular structures of the C-chelated blocks, the emission maxima of the newly synthesized furo[3,2c]pyridine based Ir complexes can be continuously tailored from 477 to 641 nm while keeping considerable photoluminescence quantum yields (PLQYs: 0.55−0.78 at wavelength of 475−560 nm and 0.10−0.34 at wavelength of 590− 640 nm). The corresponding electrophosphorescent devices emit bright greenish-blue, green, greenish-yellow, orange, red, and deep-red lights and give a record-high EQE of 20.0% (46.6 cd/A), 31.8% (89.0 cd/A), 19.9% (71.9 cd/A), 16.6% (38.9 cd/ A), 12.0% (16.7 cd/A), and 8.5% (7.3 cd/A) as well as CIE coordinates of (0.25, 0.48), (0.30, 0.58), (0.43, 0.54), (0.62, 0.37), (0.66, 0.32), and (0.70, 0.29), respectively. The results indicate that furo[3,2-c]pyridine is a potential N-chelated block

for the construction of highly efficienct Ir complexes with tunable emission colors over the whole visible range.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. The synthetic route and chemical structures of furo[3,2-c]pyridine based Ir complexes are depicted in Scheme 1. At first, the key C∧N ligands were prepared through a Suzuki cross-coupling between 4-chlorofuro[3,2-c]pyridine and the corresponding precursors of boric acid or boric acid pinacol ester. Then a typical two-step procedure was performed,22 where the μ-chloro-bridged dimers were initially synthesized via a Nonoyama reaction44 between the C∧N ligands and IrCl3·3H2O, followed by the further ligand exchange with 2,4-pentanedione or 2-picolinic acid in 2ethoxyethanol. Under such conditions, the other seven Ir complexes could be produced with acceptable yields of 24− 51% except for (dfpyfupy)2Ir(pic). This is because the fluorine attached to the ortho-position of pyridyl tended to be nucleophilic substituted by 2-ethoxyethanol in a base condition owing to the electron-deficient nature. Therefore, 2-ethoxyethanol was replaced by tetrahydrofuran (THF) as the 1889


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Figure 1. ORTEP diagrams of (a) (thfupy)2Ir(acac), (b) (pthfupy)2Ir(acac), (c) (Cz-N4-pfupy)2Ir(acac), and (d) (3-Czfupy)2Ir(acac) with thermal ellipsoids drawn at the 50% probability level, and H atoms, solvent molecules are removed for clarity.

(pthfupy)2Ir(acac), the C(1)−C(3)−C(4) angles in the fivemembered chelated ring are about 118.46° and 118.97°, respectively, larger than those of (Cz-N4-pfupy)2Ir(acac) (114.84°) and (3-Czfupy)2Ir(acac) (113.59°). This would inevitably reinforce the tension of the five-membered chelated ring, leading to the average longer Ir−N bond lengths of (thfupy) 2 Ir(acac) (2.059(3) Å) and (pthfupy) 2 Ir(acac) (2.060(4) Å) with respect to (Cz-N4-pfupy) 2 Ir(acac) (2.045(3) Å) and (3-Czfupy)2Ir(acac) (2.030(3) Å). As previously reported in furo[3,2-c]pyridine based Ir complexes,43,45 furo[3,2-c]pyridine tends to pack with the Cchelated block (e.g., phenyl) unless the environment of phenyl is crowded. Thus, (pthfupy)2Ir(acac) and (3-Czfupy)2Ir(acac) exhibit a distinct π−π stacking between furo[3,2-c]pyridine and benzo[b]thiophene or carbazole due to the planar structures of these C-chelated blocks (Figures S10 and S12). Nonetheless, the π−π interactions between two furo[3,2-c]pyridine parts are observed for (thfupy)2Ir(acac) and (Cz-N4-pfupy)2Ir(acac) due to the nonplanar character of thiophene and the repulsion to phenyl substituted by carbazole, respectively (Figures S9 and S11). 2.2. Thermal Properties. The thermal properties of these Ir complexes were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). They all possess the decomposed temperatures (Td: corresponding to a 5% weight loss) higher than 300 °C, indicative of their good thermal stability (Figure S13). Noticeably, when a rigid carbazole is incorporated into the C∧N ligands, an even higher Td of 440, 421, and 408 °C is found for (Cz-N4-pfupy)2Ir(acac), (2-Czfuy)2Ir(acac), and (3-Czfuy)2Ir(acac), respectively. The only exception is (Cz-N3-pfupy)2Ir(acac), which

complexation solvent to afford (dfpyfupy)2Ir(pic) with a yield of 52%. According to their chemical structures (Scheme 1b), the resultant furo[3,2-c]pyridine based Ir complexes can be divided into four groups. The first group (GI) includes (dfpyfupy)2Ir(pic) and (dfpfupy)2Ir(pic) by introducing the electronwithdrawing F atom and sp2 hybridized N atom into phenyl; the second group (GII) includes (thfupy)2Ir(acac) and (pthfupy)2Ir(acac) with thienyl and benzothienyl as the Cchelated blocks; the third group (GIII) includes (Cz-N3pfupy)2Ir(acac) and (Cz-N4-pfupy)2Ir(acac) contain the electron-donating carbazole moiety at different positions in phenyl; and the fourth group (GIV) includes (2-Czfupy)2Ir(acac) and (3-Czfupy)2Ir(acac), where the conjugation is extended with 2- and 3-linked carbazoles, respectively. The structures of all the Ir complexes in GI-GIV were confirmed by 1 H NMR spectroscopy, MALDI-TOF mass spectrometry, and elemental analysis (Figures S1−S8). As for GII, GIII, and GIV with acetyl acetonate as the auxiliary ligand, the 1H signals from the two C∧N ligands have the same chemical shifts, manifesting that these complexes possess a configuration of C2 symmetry. Nevertheless, as for GI with picolinate as the auxiliary ligand, the corresponding 1H signals differ in the same two C∧N ligands due to the dissymmetry of picolinate. The single crystals of (thfupy)2Ir(acac), (pthfupy)2Ir(acac), (Cz-N4-pfupy)2Ir(acac), and (3-Czfupy)2Ir(acac) were successfully grown from the dichloromethane and menthol/n-hexane mixed solvents by gradually evaporating dichloromethane. They all adopt a distorted octahedral coordination geometry with trans-N,N- and cis-C,C-chelate disposition around Ir atom (Figure 1 and Tables S1−S3). In (thfupy)2Ir(acac) and 1890


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Figure 2. UV−vis absorption and PL spectra for Ir complex in (a) GI, (b) GII, (c) GIII, and (d) GIV compared with the reference (pfupy)2Ir(acac).

gives a low Td of 344 °C caused by the meta linkage of carbazole relative to furo[3,2-c]pyridine.46 In addition, no obvious glass transition is detected before decomposition for all the complexes (Figure S14). We assume that their glass transition temperatures may be beyond the corresponding decomposition ones, in accordance with the literature.43 2.3. Photophysical Properties. Figure 2 shows the UV− vis absorption in dichloromethane and photoluminescence (PL) spectra in toluene for GI-GIV with (pfupy)2Ir(acac) as the reference complex. In GI (Figure 2a), both (dfpyfupy)2Ir(pic) and (dfpfupy)2Ir(pic) display two absorption bands: the strong one in the range of 250−350 nm is assigned to the ligand-centered (LC) transition, and the other weak one above 350 nm is related to the MLCT transition.12 Benefiting from fluorination and the usage of the auxiliary ligand picolinate other than acetyl acetonate, the MLCT absorption of (dfpfupy)2Ir(pic) moves to a shorter wavelength without obviously affecting the LC band. Correspondingly, the emission peak is blue-shifted from 538 nm of (pfupy)2Ir(acac) to 497 nm of (dfpfupy)2Ir(pic). Further replacing the 3-carbon by the sp2 hybridized N atom results in a more hypsochromic shift of the maximum emission for (dfpyfupy)2Ir(pic) (477 nm). Given the structured PL spectra of (dfpyfupy)2Ir(pic) and (dfpfupy)2Ir(pic), the trend can be reasonably attributed to the lifted 3 MLCT state, which means the 3LC state mainly contributes to the emission.21,47,48 By contrast, when α-thienyl and αbenzothienyl are used as the C-chelated blocks in place of phenyl, a bathochromic shift is observed for both the absorption and PL spectra of (thfupy) 2 Ir(acac) and (pthfupy)2Ir(acac) in GII (Figure 2b). Consequently, they

achieve orange and deep-red emissions peaked at 590 and 641 nm, respectively. Owing to the electron-donating capability as well as the diversity of chemical modification, carbazole is one of the most popular functional groups used in organic semiconductors.49−54 On one hand, carbazole is attached to the meta- and parapositions of phenyl to constitute (Cz-N3-pfupy)2Ir(acac) and (Cz-N4-pfupy)2Ir(acac) in GIII. They exhibit similar absorption to (pfupy)2Ir(acac) except that a novel band corresponding to the intraligand charge transfer from carbazole to furo[3,2-c]pyridine appears at 360 nm for (Cz-N4-pfupy)2Ir(acac) (Figure 2c). Moreover, their emissive wavelengths are dependent on the substitution position of carbazole in phenyl. Compared with (Cz-N3-pfupy)2Ir(acac) (557 nm), a slightly blue shift of 16 nm is obtained for (Cz-N4-pfupy)2Ir(acac) (541 nm). On the other hand, carbazole is directly used as the C-chelated block through 2- and 3-linkage to furo[3,2c]pyridine. The resultant Ir complexes (2-Czfupy)2Ir(acac) and (3-Czfupy)2Ir(acac) in GIV have quite different photophysical properties (Figure 2d). As for the UV−vis spectra, the LC absorption of both (2-Czfupy)2Ir(acac) and (3-Czfupy)2Ir(acac) is found to be red-shifted because of the extended conjugation length induced by carbazole. And the MLCT absorption of (3-Czfupy)2Ir(acac) seems to be close to that of (pfupy)2Ir(acac), whereas a significant bathochromic shift is observed for the MLCT absorption of (2-Czfupy)2Ir(acac). As for the PL spectra, the emission maximum is blue-shifted to 533 nm for (3-Czfupy)2Ir(acac) and oppositely red-shifted to 633 nm for (2-Czfupy)2Ir(acac). The distinct color shifting may be caused by the larger conjugation extent of the C∧N ligand in (2-Czfupy)2Ir(acac) than in (3-Czfupy)2Ir(acac) because of the 1891


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ACS Applied Materials & Interfaces Table 1. Photophysical, Electrochemical, and Thermal Properties of Furo[3,2-c]pyridine Based Ir Complexes Ir complexes h

(pfupy)2Ir(acac) (dfpyfupy)2Ir(pic) (dfpfupy)2Ir(pic) (3-Czfupy)2Ir(acac) (Cz-N4-pfupy)2Ir(acac) (Cz-N3-pfupy)2Ir(acac) (thfupy)2Ir(acac) (2-Czfupy)2Ir(acac) (pthfupy)2Ir(acac)

λabsa [nm] 279, 276, 274, 328, 260, 267, 321, 280, 280,

355, 350, 359, 360, 287, 295, 475 355, 347,

426, 479 381 400 451 349, 433 343, 488 544 491

λemb [nm] 538 477 497 533 541 557 590 633 641

ΦPLc 0.80 0.55 0.73 0.78 0.68 0.56 0.34 0.14 0.10

τd [μs]

HOMO/LUMOe [eV]

1.04 1.37 1.33 1.27 1.47 1.45 2.39 2.10 3.06

−5.15/−2.40 −6.00/−2.76 −5.63/−2.64 −4.97/−2.40 −5.24/−2.53 −5.16/−2.56 −5.11/−2.39 −4.78/−2.53 −5.06/−2.56

Tdf [°C]

krg [s−1]

354 362 329 408 440 344 323 421 381

× × × × × × × × ×

7.7 4.0 5.5 6.1 4.6 3.9 1.4 6.7 3.3

5

10 105 105 105 105 105 105 104 104

knrg [s−1] 1.9 3.3 2.0 1.7 2.2 3.0 2.8 4.1 2.9

× × × × × × × × ×

105 105 105 105 105 105 105 105 105

Measured in 10−5 M dichloromethane solution. bMeasured in 10−5 M toluene solution. cMeasured in N2-saturated toluene solution with Ir(ppy)3 (ΦPL = 0.97) as the reference. dEstimated from the transient PL spectrum measured in N2-saturated toluene solution excited by a 355 nm pulse. e HOMO = −e(Eox, onset + 4.8 V), LUMO = −e(Ered, onset + 4.8 V), where Eox, onset and Ered, onset are the onset values of the first oxidation and reduction waves, respectively. fDecomposition temperature corresponding to a 5% weight loss. gCalculated according to the equations: τ = 1/(kr + knr) and Φ = kr/(kr + knr). hData from ref 43. a

“full conjugation” linkage, consistent with the literature.55 It is worthy to note that (2-Czfupy)2Ir(acac) displays a blue residue in the range of 400−500 nm. In consideration of the nanosecond scale lifetime monitored at 455 nm (Figure S15), we tentatively ascribe this blue emission to the fluorescence from the C∧N ligand.56,57 Additionally, the PLQYs were measured in N2-saturated toluene solutions with Ir(ppy)3 (ΦPL = 0.97)58 as the reference (Table 1). They are first up from 0.55 of (dfpyfupy)2Ir(pic) to 0.78 of (3-Czfupy)2Ir(acac) along with the increased wavelength from 477 to 533 nm, and then down to 0.10 of (pthfupy)2Ir(acac) when the wavelength is further extended to 641 nm. According to the literature,59 the high-lying nonradiative metal-centered (MC) state is easy to be occupied in short wavelength, leading to the decreased PLQYs. In long wavelength, it is more likely to decay to the ground state by some nonemissive ways due to the energy gap law,60 and thus, the low PLQYs for red phosphors are within our expectation. Also, we examined the transient PL spectra for all the Ir complexes (Figure S16). As a result of low spin−orbital coupling, the lifetimes of (thfupy)2Ir(acac), (2-Czfupy)2Ir(acac), and (pthfupy)2Ir(acac) are estimated to be 2.39, 2.10, and 3.06 μs, respectively.61 The other Ir complexes show similar lifetimes in the range of 1.27−1.47 μs. 2.4. Electrochemical Properties. Cyclic voltammetry (CV) was used to study the electrochemical properties of these furo[3,2-c]pyridine based Ir complexes. As plotted in Figure 3, they all show a reversible or quasi-reversible oxidation and reduction waves during the anodic sweeping in dichloro-

methane and the cathodic sweeping in N,N-dimethylformamide. With regard to ferrocene/ferrocenium (Fc/Fc+: 4.8 eV under vacuum), their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels can be determined, and the related data are listed in Table 1. Consistent with the photophysical properties, it is found that, the electrochemical properties could be effectively tailored by the C-chelated block in the C∧N ligand. For example, in GI containing the electron-withdrawing moieties (F and N atoms), (dfpyfupy)2Ir(pic), and (dfpfupy)2Ir(pic) shows both the deep HOMO/LUMO levels of −6.00/−2.76 eV and −5.63/−2.64 eV, respectively, which are much lower than those of (pfupy)2Ir(acac) (−5.15/−2.40 eV). In GIV directly with carbazole as the C-chelated block, the HOMO levels are increased to −4.97 eV for (3-Czfupy)2Ir(acac) and −4.78 eV for (2-Czfupy)2Ir(acac). Meanwhile, the LUMO level of (3Czfupy)2Ir(acac) (−2.40 eV) keeps nearly unchanged, while (2-Czfupy)2Ir(acac) possesses a decreased LUMO level of −2.53 eV. Theoretical calculations were simultaneously performed to explain such variations. As can be clearly seen for all the Ir complexes (Table S4), the HOMO mainly distributes on the Cchelated block and Ir center, and the LUMO is localized on the C-chelated block and furo[3,2-c]pyridine. In this case, the substitution of F in phenyl could stabilize the LUMO and HOMO orbitals, thus decreasing both the LUMO and HOMO levels for (dfpyfupy)2Ir(pic) and (dfpfupy)2Ir(pic) in GI. As for (3-Czfupy)2Ir(acac) and (2-Czfupy)2Ir(acac) in GIV, the HOMO can be destabilized due to the extension to the whole carbazole unit, leading to the elevation of the HOMO levels. Furthermore, the 2-linked carbazole is found to contribute more to the LUMO distribution than does the 3linked one. Hence, the LUMO level of (2-Czfupy)2Ir(acac) turns out to be reduced, different from (3-Czfupy)2Ir(acac) with the same LUMO level relative to (pfupy)2Ir(acac). Similar agreements between the CV experiments and theoretical calculations are also observed for the other two groups GII and GIII. 2.5. Electroluminescent Properties. To evaluate the electroluminescent (EL) properties, the phosphorescent OLEDs were fabricated with a configuration of ITO/MoO3 (10 nm)/TAPC (60 nm)/TCTA (5 nm)/Host (TCTA or CBP):Ir complex (x wt %) (20 nm)/BmPyPB (35 nm)/LiF (1 nm)/Al (Figure 4). Herein, TAPC (4,4′-(cyclohexane-1,1diyl)bis(N,N-di-p-tolylaniline)) and BmPyPB (1,3-bis(3,5-

Figure 3. CV curves for Ir complex in GI−GIV compared with the reference (pfupy)2Ir(acac). 1892


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Figure 4. Energy level diagram of phosphorescent OLEDs and chemical structures of used materials.

Table 2. Device Performance of Furo[3,2-c]pyridine Based Ir Complexes Ir complexes a

(dfpyfupy)2Ir(pic) (15 wt %) (dfpfupy)2Ir(pic) (20 wt %)a (3-Czfupy)2Ir(acac) (8 wt %)a (Cz-N4-pfupy)2Ir(acac) (8 wt %)a (thfupy)2Ir(acac) (6 wt %)a (2-Czfupy)2Ir(acac) (2 wt %)b (pthfupy)2Ir(acac) (4 wt %)b a

b

Vonc [V]

ηcd [cd/A]

ηpd [lm/W]

EQEd [%]

3.0 2.8 2.6 4.6 2.8 3.8 3.4

46.6/39.7/25.5 89.0/70.2/55.8 79.6/73.8/60.7 71.9/67.9/52.7 38.9/20.3/3.6 16.7/9.7/6.4 7.3/2.5/1.2

43.0/19.5/8.3 99.8/40.8/21.9 93.3/41.4/24.4 42.9/29.6/16.9 43.6/7.1/0.7 13.1/3.0/1.4 6.7/0.63/0.25

20.0/17.0/10.9 31.8/24.6/20.0 21.8/20.2/16.7 19.9/18.8/14.6 16.6/8.7/1.6 12.0/6.9/4.5 8.5/2.9/1.4

c

λEL [nm]

CIEe [x, y]

476, 496, 536, 541, 592, 636 640,

(0.25, (0.30, (0.41, (0.43, (0.62, (0.66, (0.70,

511 532 569 580 640 702

0.48) 0.58) 0.58) 0.54) 0.37) 0.32) 0.29)

2 d

TCTA is used as the host. CBP is used as the host. Turn-on voltage at a brightness of 1 cd/m . Data at maximum, 1000, and 5000 cd/m2 for current efficiency (ηc), power efficiency (ηp), and EQE, respectively. eCIE at 6 V.

because of the above-mentioned low Td as well as high molecular weight. Therefore, we do not report the device performance of (Cz-N3-pfupy)2Ir(acac), and the related data for the other seven complexes are summarized in Table 2. It should be noted that the EL covers the whole visible range (Figure 5a), and the wavelength peaks at 476, 496, 541, 592, 636, and 640 nm following a sequence of (dfpyfupy)2Ir(pic), (dfpfupy)2Ir(pic), (Cz-N4-pfupy)2Ir(acac), (thfupy)2Ir(acac), (2-Czfupy)2Ir(acac), and (pthfupy)2Ir(acac). Their corresponding CIE coordinates are (0.25, 0.48), (0.30, 0.58), (0.43, 0.54), (0.62, 0.37), (0.66, 0.32), and (0.70, 0.29), respectively. In addition, the EL spectra match well with the PL counterparts, suggesting the emission is mainly from Ir dopant. Especially, (dfpyfupy)2Ir(pic) and (dfpfupy)2Ir(pic) exhibit the EL with well-defined vibronic structures including 0−0, 0−1, and 0−2

dipyrid-3-yl-phenyl) benzene) serve as the hole-transporting layer (HTL) and electron-transporting layer (ETL), respectively; TCTA (tris(4-(9H-carbazol-9-yl)phenyl)amine) is used as the buffer layer to realize the cascade hole injection from HTL to the emitting layer (EML); and the developed Ir complexes are doped into TCTA to form the EML. To optimize the device performance, the doping concentration is varied in the range of 10−20 wt % for (dfpyfupy)2Ir(pic) and (dfpfupy)2Ir(pic), and 2−8 wt % for the others. Meanwhile, to avoid the inefficient energy transfer from TCTA to the red phosphors (e.g., (2-Czfupy)2Ir(acac) and (pthfupy)2Ir(acac)), their corresponding devices are assembled using a low-tripletenergy CBP (4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl) as the host instead of TCTA. During device fabrication, unfortunately, (Cz-N3-pfupy)2Ir(acac) is found to be partly decomposed 1893


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(2-Czfupy)2Ir(acac), and (pthfupy)2Ir(acac) reveal a recordhigh EQE of 20.0% (46.6 cd/A, 43.0 lm/W), 31.8% (89.0 cd/A, 99.8 lm/W), 19.9% (71.9 cd/A, 42.9 lm/W), 16.6% (38.9 cd/A, 43.6 lm/W), 12.0% (16.7 cd/A, 13.1 lm/W), and 8.5% (7.3 cd/ A, 6.7 lm/W), respectively. It is noteworthy that (thfupy)2Ir(acac), (2-Czfupy)2Ir(acac), and (pthfupy)2Ir(acac) exhibit more severer efficiency roll-off at high luminance than (dfpyfupy)2Ir(pic), (dfpfupy)2Ir(pic), and (Cz-N4-pfupy)2Ir(acac). For instance, at a luminance of 1000 cd/m2, the EQE of (pthfupy)2Ir(acac) is reduced by about 66% from its maximum value, while a much lower decay of 21% is observed for (dfpfupy)2Ir(pic). As discussed above, the longer lifetime of (pthfupy)2Ir(acac) compared with (dfpfupy)2Ir(pic) (3.06 μs Vs 1.33 μs) is responsible for their difference in efficiency rolloff, which would lead to the exciton quenching through triplet− triplet annihilation (TTA) and triplet−polaron annihilation (TPA).62 Irrespective of this, the obtained promising EQEs are among the highest ever reported for blue, green, and red phosphors and demonstrate the successful realization of efficient electrophosphorescence covering the whole visible region based on furo[3,2-c]pyridine.

3. CONCLUSIONS In summary, we report the synthesis and characterization of a series of highly efficient phosphorescent furo[3,2-c]pyridine based Ir complexes with tunable emission colors over the whole visible range. The corresponding phosphorescent OLEDs comprising (dfpyfupy)2Ir(pic), (dfpfupy)2Ir(pic), (Cz-N4pfupy)2Ir(acac), (thfupy)2Ir(acac), (2-Czfupy)2Ir(acac), and (pthfupy)2Ir(acac) as greenish-blue, green, greenish-yellow, orange, red, and deep-red dopants realize state-of-art EQEs of 20.0%, 31.8%, 19.9%, 16.6%, 12.0%, and 8.5%, respectively. Without any out-coupling, the obtained promising efficiencies over such a wide light-emitting region clearly indicate the great potential of furo[3,2-c]pyridine based Ir complexes possibly due to their favorable horizontal orientation in the host matrix.43 Additionally, as an exception, we note that (Cz-N3-pfupy)2Ir(acac) peaked at 557 nm is unable to be vacuum-deposited, highlighting the urgence of the development of efficient yellow phosphors based on furo[3,2-c]pyridine. This work is under way in our lab.

Figure 5. Selected device performance for (dfpyfupy)2Ir(pic), (dfpfupy)2Ir(pic), (Cz-N4-pfupy)2Ir(acac), (thfupy)2Ir(acac), (2Czfupy)2Ir(acac), and (pthfupy)2Ir(acac): (a) EL spectra at 6 V; (b) CIE chromaticity diagram; (c) EQE as a function of luminance.

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transitions. Because of the contribution from the 0−1 and 0−2 emissions, the color of (dfpyfupy)2Ir(pic) becomes greenishblue, deviating from the typical sky-blue of FIrpic23 with a similar 0−0 peak. In addition, the color of (dfpfupy)2Ir(pic) locates into the green region, whose CIE coordinates approach to the s-RGB standard for green (0.30, 0.60). As an exception of (2-Czfupy)2Ir(acac), the observed blue residue in the PL spectrum disappears in the EL. This indicates that, under electric excitation, the blue fluorescence from the C∧N ligand could down-converted to the red phosphorescence from the Ir complex by additional charge trapping besides the energy transfer. As a result, it turns out to be broad and structureless and shows close CIE coordinates to the s-RGB standard for red (0.64, 0.33). For (pthfupy)2Ir(acac), the considerable shoulder related to the 0−1 emission at 702 nm further shifts the color to the deep-red region. Most interestingly, high device performance is attained at the same time when the emission maxima are varied from 476 to 640 nm. As one can see in Figure 5b, (dfpyfupy)2Ir(pic), (dfpfupy)2Ir(pic), (Cz-N4-pfupy)2Ir(acac), (thfupy)2Ir(acac),

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14906. Experimental section; 1H NMR, MALDI-TOF spectra and π−π stacking in crystals; TGA, DSC, transient PL spectra, and device performance of Ir complexes; DFT calculation results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Junqiao Ding: 0000-0001-7719-6599 Yue Wang: 0000-0001-6936-5081 Lixiang Wang: 0000-0002-4676-1927 1894


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author acknowledge the support from the National Key Research and Development Program of China (No. 2016YFB0400701).

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