- Email: [email protected]
Green and yellow pyridazine-based phosphorescent Iridium(III) complexes for high-efficiency and low-cost organic light-emitting diodes
Accepted Manuscript Green and yellow pyridazine-based phosphorescent Iridium(III) complexes for highefficiency and low-cost organic light-emitting diodes Xiaowen Ning, Chenyang Zhao, Bei Jiang, Shaolong Gong, Dongge Ma, Chuluo Yang PII:
S0143-7208(18)32816-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.01.031
Reference:
DYPI 7304
To appear in:
Dyes and Pigments
Received Date: 20 December 2018 Revised Date:
15 January 2019
Accepted Date: 17 January 2019
Please cite this article as: Ning X, Zhao C, Jiang B, Gong S, Ma D, Yang C, Green and yellow pyridazine-based phosphorescent Iridium(III) complexes for high-efficiency and low-cost organic lightemitting diodes, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.01.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Green
and
Yellow
Pyridazine-based
Phosphorescent
Iridium(III) Complexes for High-efficiency and Low-cost Organic Light-Emitting Diodes
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Xiaowen Ning,a,b Chenyang Zhao,c,d,e Bei Jiang,a Shaolong Gong,a Dongge Ma*c,d and Chuluo Yang*a,b
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of
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a
E-mail: [email protected] b
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Chemistry, Wuhan University, Wuhan 430072, P. R. China
Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials
Science and Engineering, Shenzhen University, shenzhen 518060, P. R. China Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of
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c
Luminescent Materials and Devices, South China University of Technology,
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Guangzhou 510640, P. R. China
d
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E-mail: [email protected]
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China e
University of Science and Technology of China, Hefei 230026, P. R. China
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Abstract Despite efficient electroluminescence performance, the high fabrication cost due to the utilization of noble metals impedes the commercialization of phosphorescent
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organic light-emitting diodes (PHOLEDs) based on iridium(III) complexes. In this paper, we designed and synthesized a series of green and yellow phosphorescent iridium(III) complexes with 3-(2,4-difluorophenyl)-6-methylpyridazine as the
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cyclometalating ligand to fabricate high-performance and low-cost PHOLEDs. By the
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introduction of a pyridazine moiety to enhance the coordination bond between iridium(III) ion and cyclometalating ligands, these iridium(III) complexes were obtained in high yields and exhibit good thermal stabilities and volatilities, which are beneficial to reduce the device manufacturing cost. Besides, high photoluminescence yields
(PLQYs)
and
short
phosphorescent
lifetimes
of
these
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quantum
pyridazine-based iridium(III) complexes endow their PHOLEDs with excellent device
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performance, which can be promising for practical applications. Particularly, the green emitter (fpdz)2Irpic exhibited a high PLQY of 0.89 in solid doped thin-film;
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PHOLED based on (fpdz)2Irpic achieved an outstanding maximum external quantum efficiency (EQEmax) of 28.7% with ultralow efficiency roll-off of 2% at the high luminance of 1000 cd m-2, which was superior to the commercial green phosphor bis(phenylpyridineyl)iridium(acetylacetonate) [(ppy)2Ir(acac), EQEmax = 24.2%].
Keywords: OLEDs, phosphorescent iridium(III) complexes, pyridazine, ancillary ligand, MLCT contribution.
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1. Introduction Organic light-emitting diodes (OLEDs) have gained widespread mindshare from both academia and industry inspiring by their promising application in flat panel displays
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and solid-state lighting.[1-5] Currently, noble-metal-containing phosphorescent iridium(III) complexes were the prevalent commercial emitters for OLEDs.[6-10] Blue, green and red phosphorescent OLEDs (PHOLEDs) based on cyclometalated
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iridium(III) complexes with external quantum efficiencies (EQEs) above 30% have
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been reported.[11-16] Nevertheless, the high fabrication cost due to the utilization of noble metals impedes the commercialization of these PHOLEDs. Despite many attempts to find more cost-effective phosphorescent emitters, such as copper(I)[17-19] and manganese(II)[20-22] complexes, their unsatisfactory device performance on
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account of EQE, efficiency roll-off, device operating lifetime and color diversity lag far behind the development of these iridium(III) counterparts. In that case, there is
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still substantial space to develop high-performance and low-cost PHOLED for practical application.
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To cut down the manufacturing cost of PHOLEDs, developing cost-effective
molecule structures and synthesis processes, decreasing wastage of phosphorescent emitters as well as reducing vacuum pressure and temperature in the device fabrication process have been demonstrated as valid and straight-forward paradigms.[23-25] Meanwhile, extensive efforts have been paid to develop decent phosphorescent emitters for highly efficient OLED application. For example, Thompson
and
Forrest
et
al.[26]
reported
a
blue
iridium(III)
complex
ACCEPTED MANUSCRIPT bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) and the optimized PHOLED with an exciplex-forming host can achieve a maximum external quantum efficiency (EQEmax) of 34.1%.[13] Later, Thompson et al. reported a blue
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emitter FIr6 by substituting the ancillary ligand of FIrpic with tetra(1-pyrazolyl)borate to improve the blue chromaticity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.26).[27] Besides, Wong et al. developed a blue emitter MS2 by
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replacement of the pyridine ring of FIrpic with pyrimidine moiety, and device based
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on MS2 achieved a superior operation lifetime (T50 ~ 2203 hr) ready for practical application.[28] It has been well demonstrated that subtle alterations in the molecular frameworks can efficiently change the photophysical properties as well as the electroluminescence (EL) performance of the corresponding iridium(III) complexes.
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In this paper, we designed and synthesized a series of phosphorescent iridium(III) complexes with 3-(2,4-difluorophenyl)-6-methylpyridazine (fpdz) as cyclometalating
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ligand to explore the potential of replacing pyridine ring in the prototypical FIrpic with pyridazine moiety. Generally speaking, pyridazine-based iridium(III) complexes
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possess a small steric hindrance of the chelating nitrogen atom due to the adjacent sp2 nitrogen atom without attached hydrogen atom, which will lead to a strong coordination bond between the chelating nitrogen atom and metal iridium atom, and further benefit to the stability and luminescent efficiency.[29-35] Despite pyridazine-based iridium(III) complexes including (fpdz)2Irpic and (fpdz)2Iracac have been reported, the inferior EL efficiencies underestimated their potentials and restricted their applications in OLEDs.[35] By the alteration of the ancillary ligand,
ACCEPTED MANUSCRIPT four pyridazine-based iridium(III) complexes, namely (fpdz)2Irtaz, (fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd, were designed and synthesized to investigate the influence of the ancillary ligand on their photophysical properties and EL
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performance. These four iridium(III) complexes were easily synthesized in high yields (67~95%) and are thermally stable and volatile, which would enhance the utilization rate of noble metal and reduce the vacuum pressure in the device
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fabrication process. Besides, the diverse ancillary ligands endowed different excited
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state characteristics to these iridium(III) complexes, which exhibited tunable emission colors with high photoluminescence quantum yields (PLQYs) in the range of 0.72~0.89. As a result, PHOLEDs based on these pyridazine-based iridium(III) complexes achieved EQEmax of 23.1%, 28.7%, 22.7% and 24.5% together with CIE
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coordinates of (0.24, 0.58), (0.30, 0.62), (0.36, 0.61) and (0.38, 0.60), respectively. It is noteworthy that the green device based on (fpdz)2Irpic achieved an EQEmax of 28.7% with an ultralow efficiency roll-off of 2% at the luminance of 1000 cd m-2, which was
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promising in the practical application of high-performance and low-cost PHOLEDs.
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2. Results and Discussion 2.1 Synthesis and thermal properties Chemical structures of the pyridazine-based iridium(III) complexes were depicted in Scheme 1. The cyclometalating ligand was prepared through a simple Suzuki crossing-coupling
reaction
between
3-chloro-6-methylpyridazine
and
2,4-difluorophenylboronic acid.[35-37] The ancillary ligands of picolinic acid (Hpic),
ACCEPTED MANUSCRIPT acetylacetone (Hacac) and 2,2,6,6-tetramethylheptane-3,5-dione (Htmd) were purchased
from
commercial
sources
and
used
without
2-(5-(4-(trifluoromethyl)phenyl)-2H-1,2,4-triazol-3-yl)pyridine
purification. (Htaz)
The
ancillary
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ligand was synthesized according the literature report.[38-40] All the heteroleptic iridium(III) complexes were obtained via a conventional two-step procedure (Scheme S1).[35,41] It is noticeable that all the complexes can be easily synthesized in good
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yields of 57~95% under mild conditions, which should be attributed to the decreased
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steric hindrance of the coordination bond. The high yields and simple synthesis process are beneficial to enhance the utilization rate of noble metal and reduce producing cost. Besides, these iridium(III) complexes were further purified by temperature-gradient sublimation under vacuum. The sublimation temperatures of
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(fpdz)2Irtaz, (fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd in the vacuum of 10-5 Pa were recorded at 250, 290, 260 and 250 oC, respectively. The simple molecular skeletons and low molecular weights enabled low sublimation temperatures and high
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sublimation yields (82~96%), which enhanced the utilization rate of materials and are
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preferable for practical usage. All the iridium(III) complexes were fully characterized with
1
H NMR spectroscopy, high-resolution mass spectrometry (HRMS) and
elemental analysis (Fig. S1-S8, details were shown in the experimental section of supporting information). Thermogravimetric analysis (TGA) was performed under a stream of nitrogen to investigate the thermal properties of these pyridazine-based iridium(III) complexes (Fig. S9). The thermal decomposition temperatures (Td, with 5% weight loss) of these
ACCEPTED MANUSCRIPT complexes were over 300 oC (Table 1), and gradually depressed with the decreased rigidity of the ancillary ligand. (fpdz)2Irtmd possessed much lower Td than other three iridium(III) complexes, might being attributed to the redundant alkyl groups of
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the ancillary ligand. The high thermal stabilities and volatilities of these iridium(III) complexes make devices fabricated by vacuum evaporation more feasible.
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2.2 Photophysical properties
UV-vis absorption and photoluminescence (PL) spectra of these pyridazine-based
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iridium(III) complexes were displayed in Fig. 1; and the related data were listed in Table 1. Attributed to the identical cyclometalating ligand, these iridium(III) complexes exhibited similar absorption bands. The strong absorption bands below 300 nm were generally assigned to the ligand-centered 1π-π* (1LC) transitions. The
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absorption bands located on ca. 350 nm were conventionally assigned to the spin-allowed singlet metal-to-ligand charge transfer (1MLCT) transitions, while the
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long wavelength tails beyond 400 nm should be assigned to the spin-forbidden triplet metal-to-ligand charge transfer (3MLCT) transitions and ligand-centered 3π-π* (3LC) mixing
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transitions,
with
the
ligand-to-ligand
charge
transfer
(LLCT)
transitions.[25,42]
Under photoexcitation, these pyridazine-based iridium(III) complexes exhibited
broad and featureless emission profiles, indicating that their emissions primarily derived from
3
MLCT states.[42] The phosphorescent emission of (fpdz)2Irpic
exhibited a significant red-shift compared with the well-known phosphor of FIrpic,
ACCEPTED MANUSCRIPT which displayed a blue emission at 475 nm in dichloromethane (CH2Cl2) solution.[26] Attributed to the electronegative effect of pyridazine moiety, the 3MLCT energy level of (fpdz)2Irpic was stabilized, which led to the strong green emission at 515 nm. By
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the replacement of Hpic ancillary ligand with Hacac and Htmd, the phosphorescent emission of (fpdz)2Iracac and (fpdz)2Irtmd was further red-shifted to 544 and 554 nm, respectively. In order to offset the red-shifted effect of pyridazine moiety,
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strong-field CF3-containing pyridine-azole ancillary ligand of Htaz was adopted to
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afford the phosphorescent iridium(III) complex (fpdz)2Irtaz, which displayed an intense blue-greenish emission at 499 nm. However, it should be noted that the PL spectrum of (fpdz)2Irtaz exhibited an obscure shoulder at ca. 518 nm, suggesting the mixed MLCT/LC/LLCT character of its emissive excited state.[42] When cooled to
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77 K (Fig. S10), these four iridium(III) complexes exhibited pronounced vibronic structures and rigidochromic shifts. The maximum emissions of (fpdz)2Irtaz, (fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd at 77 K are hypsochromically shifted
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by 15, 19, 26 and 31 nm compared to those of the CH2Cl2 solutions at 298 K.
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Obviously, compared with other three iridium(III) complexes, (fpdz)2Irtaz exhibited little rigidochromism and much pronounced vibronic structures, suggesting the decreased MLCT contribution in its excited state.[43] Transient PL spectra were examined to further investigate the excited state characteristics of these pyridazine-based iridium(III) complexes (Fig. 2). All the complexes exhibited monoexponential decay with short phosphorescence lifetimes in the range of 1.26~2.26 µs (Table 1). The solid PLQYs of these complexes in 8 wt%
ACCEPTED MANUSCRIPT tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA) doped thin-films were considerably high ranging from 0.72 to 0.89. The blue-greenish phosphor (fpdz)2Irtaz exhibited longer phosphorescent lifetime and lower PLQY than other three iridium(III)
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complexes. The inferior luminescence efficiency of (fpdz)2Irtaz can be attributed to the decreased MLCT contribution in its emissive excited state, which reduced the
further reduced the transition probability.[43-45]
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2.3 Electrochemical properties
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spin-orbital coupling effect directly involving the heavy metal iridium atom and
Cyclic voltammetry (CV) was performed to investigate the electrochemical properties of these pyridazine-based iridium(III) complexes. As depicted in Fig. 3, all the complexes exhibited reversible or quasi-reversible oxidation and reduction behavior.
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The oxidation waves can be ascribed to the metal-centered IrIII/IrIV oxidation and the reduction waves were generally assigned to the reductions of the pyridazine moieties
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of the cyclometalating ligands. With regard to ferrocene/ferrocenium (Fc/Fc+), the highest occupied molecular orbital (HOMO) energy levels of (fpdz)2Irtaz,
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(fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd were deduced to be -5.59, -5.46, -5.22 and -5.13 eV, respectively, which were gradually destabilized with the decreased electron-withdrawing abilities of ancillary ligands. Nevertheless, the lowest unoccupied molecular orbital (LUMO) energy levels of these complexes almost remained and were determined to be -2.81, -2.81, -2.73 and -2.69 eV (Table 1), respectively. The decreased HOMO-LUMO energy gaps were in good accordance with the photophysical properties of these iridium(III) complexes.
ACCEPTED MANUSCRIPT 2.4 Theoretical calculations To get a deeper insight into the electronic properties of these iridium(III) complexes, quantum chemical calculations were performed based on the density functional theory
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(DFT). As depicted in Fig. 4, the LUMOs of these iridium(III) complexes mainly consisted of the π* orbitals of the pyridazine moieties. Owing to the asymmetry of molecular structure, the LUMO of (fpdz)2Irtaz and (fpdz)2Irpic centered on one
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cyclometalating ligand while (fpdz)2Iracac and (fpdz)2Irtmd scattered over two
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cyclometalating ligands. The fewer moieties involved in the LUMO might be beneficial to stabilize the excited states and improve their PLQYs.[35] The HOMOs of these complexes primarily composed of the d orbitals of the central iridium(III) ion and the π orbitals of the phenyl rings of the cyclometalating ligands except for
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(fpdz)2Irtmd with the HOMO moved to the ancillary ligand. The decreased electron-withdrawing ability of ancillary ligand led to the localization of electrons on
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central iridium(III) ion, which destabilized the HOMOs of these pyridazine-based iridium(III) complexes and little affected LUMOs. These calculation results were in
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agreement with experimental electrochemical properties. To further analyze the optical properties of these pyridazine-based iridium(III)
complexes, time-dependent DFT (TD-DFT) calculations were performed on the basis of the optimized geometry. The vertical excitation energies and dominant transitions were listed in Table 2; further molecular orbital compositions in the excited states were listed in Table S1. The calculated lowest-lying triplet (T1) energy levels of these iridium(III) complexes were identical with the energies deduced from the
ACCEPTED MANUSCRIPT highest-energy vibronic bands at 77 K (Fig. S10) and demonstrated the validity of the computation. The T1 states of (fpdz)2Irpic and (fpdz)2Iracac dominantly involved the transition from HOMO to LUMO; while those of (fpdz)2Irtaz and (fpdz)2Irtmd
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were slightly different, which mainly consisted of the transition from HOMO to LUMO/LUMO+1 and the transition from HOMO-1 to LUMO, respectively. Despite different transition patterns, the T1 states of these pyridazine-based iridium(III)
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complexes could be labeled as the similar mixed state consisting of 3MLCT
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[d(Ir)→π*(pdz)], 3LC [π(ph)→π*(pdz)] and 3LLCT [π(ancillary ligand)→π*(pdz)] state, which was in good aggrement with the experimentally observed properties. Further analyses to the characters of their T1 states by the interfragment charge transfer method demonstrated that the MLCT contribution of (fpdz)2Irtaz (35.2%)
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was much lower than other three iridium(III) complexes, which further rationalized its inferior luminescent efficiency.
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2.5 Electroluminescent devices
To investigate the EL performance of these pyridazine-based iridium(III) complexes,
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PHOLEDs were fabricated with a simple configuration of ITO/MoO3 (8nm)/TAPC (60nm)/TCTA:8 wt% dopants (12 nm)/BmPyPb (60nm)/LiF (1nm)/Al (150nm), where indium tin oxide (ITO) and lithium fluoride/aluminum (LiF/Al) were used as anode and cathode, respectively. Molybdenum trioxide (MoO3) was employed as hole-injection materials. 1,1-Bis-(4-methylphenyl)-aminophenyl-cyclohexane (TAPC) and 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPb) were applied as hole- and electron-transporting materials, respectively. TCTA acted as the host material and the
ACCEPTED MANUSCRIPT pyridazine-based iridium(III) complexes were doped in the host-matrix with the concentrations of 8 wt% in the emitting layer. Devices based on different dopants were named as device A~D, respectively. The device structure, chemical structures
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and energy levels of the materials used in the devices were depicted in Fig. 5a. The EL characteristics of the devices were depicted in Fig. 5b~c and Fig. S11. Key data of the device performance were summarized in Table 3.
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Device A~D exhibited extremely low turn-on voltages (Von) of 2.6~2.8 V, which
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was due to the low energy barriers for hole/electron-injection. As shown in Fig. 5c, the (fpdz)2Irtaz-，(fpdz)2Irpic-, (fpdz)2Iracac- and (fpdz)2Irtmd-based devices displayed green or yellow emissions with the main peaks in the range of 518~540 nm, which were identical with the corresponding PL spectra in the thin-film states except
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for (fpdz)2Irtaz (Fig. S12). Compared with the PL spectrum of (fpdz)2Irtaz in TCTA doped thin-film, the EL spectrum of (fpdz)2Irtaz-based device displayed suppressed
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emission at 499 nm and enhanced shoulder peak at 518 nm. The dramatic change might be attributed to the predominant emission from LLCT state in the EL process,
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which generally not only resulted in an unwanted bathochromatic shift but also lowered the luminescent efficiency.[46] As a result, device A based on (fpdz)2Irtaz exhibited common device performance: a EQEmax of 23.1%, a maximum current efficiency (CEmax) of 73.9 cd A-1, a maximum power efficiency (PEmax) of 77.1 lm W-1 and a maximum luminance of 12270 cd m-2. Inspiringly, due to the high PLQY in TCTA doped thin-film, employing (fpdz)2Irpic as the green emitter endowed device B with better performance, which exhibited a promising EQEmax of 28.7%, CEmax of
ACCEPTED MANUSCRIPT 102.5 cd A-1, PEmax of 111.4 lm W-1 and a maximum luminance of 31159 cd m-2. More importantly, the EQE remained as high as 28.1% at the high luminance of 1000 cd m-2, corresponding to an ultralow efficiency roll-off of 2%. The high efficiency and low
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roll-off of (fpdz)2Irpic-based device were superior to the controlled device based on the commercial green emitter (ppy)2Ir(acac) with similar EL spectrum (Fig. S13 and Table S2), which revealed a EQEmax of 24.2% accompanied with efficiency roll-off of
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3% at the luminance of 1000 cd m-2. Despite the sublimation temperature of
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(fpdz)2Irpic in the vacuum of 10-5 Pa was slightly higher than that of (ppy)2Ir(acac) (ca. 240 oC), the excellent device performance with a simple device structure indicated its great potential in practical application.
With respect to the yellow devices, the (fpdz)2Iracac-based device C achieved a
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EQEmax of 22.7%, CEmax of 84.3 cd A-1, and PEmax of 84.2 lm W-1 and a maximum luminance of 26186 cd m-2. The EQE was recorded as 21.5% at the high luminance of
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1000 cd m-2, revealing a low efficiency roll-off of 5%. Besides, device D based on (fpdz)2Irtmd exhibited better performance with a EQEmax of 24.5%, CEmax of 89.0 cd
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A-1, PEmax of 88.6 lm W-1 and a maximum luminance of 22126 cd m-2, which might be attributed to the suppressed triplet-triplet annihilation by the steric hindrance of the bulky ancillary ligand.[25,47-49] At the luminance of 1000 cd m-2, device D maintained high EQE of 22.7% and the efficiency roll-off was only 7%, which was comparable to most of the reported yellow devices based on iridium(III) complexes.[6,50-54] Although these pyridazine-based iridium(III) complexes shared similar
ACCEPTED MANUSCRIPT molecular frameworks, they exihibited quite different excited state characters, photophysical properties and EL performances. Despite its blue-shifted emission, the low PLQY and long phosphorescent lifetime of (fpdz)2Irtaz resulted in inferior
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device performance and large efficiency roll-off, which could overshadow its practical application in OLEDs. These results fully demonstrated that the development of highly efficient phosphorescent iridium(III) complexes should enhance the MLCT
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contribution in their emissive excited states. All in all, these pyridazine-based
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iridium(III) complexes have great potential to fabricate high-performance and low-cost PHOLEDs for practical application: the high PLQY to ensure an intrinsically high device efficiency; the short phosphorescent lifetime and high stability to alleviate efficiency roll-off at high luminance; the high synthesis yield and sublimation yield to
device fabrication.
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3. Conclusion
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reduce the waste of noble metal; the low sublimation temperature to reduce the cost of
In conclusion, we designed and synthesized a series of pyridazine-based iridium(III)
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complexes for high-efficiency and low-cost PHOLEDs. Firstly, through the incorporation of pyridazine moiety into cyclometalating ligand to obtain strong coordination bond, these iridium(III) complexes are easily synthesized with high yields, thermal stable and volatile, which enhances the utilization rate of noble metal and reduces the device manufacturing cost. Secondly, PHOLEDs based on these iridium(III) complexes achieved promising efficiency and low roll-off with a simple device
structure,
indicating
their
great
potential
for
the
fabrication
of
ACCEPTED MANUSCRIPT high-performance and low-cost PHOLEDs. Finally, the diverse photophysical properties and EL performances of these iridium(III) complexes are demonstrated to be associated with their excited state characteristics, and we provide a valid strategy
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of enhancing MLCT contribution in emissive excited state to design highly efficient iridium(III) complexes.
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Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural
of
China
(973
Program
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Science Foundation of China (No. 91433201), the National Basic Research Program 2015CB655002),
Shenzhen
Peacock
Plan
(KQTD20170330110107046) and the key Technological Innovation Program of
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Phosphorescent Neutral Tetrahedral Manganese(II) Complexes for Organic Light-Emitting Diodes. Adv. Opt. Mater. 2018; 1801160.
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Thieno[2,3-d]pyrimidine
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Materials
for
Blue-Light-Emitting
Phosphorescent
Organic
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Electroluminescent Devices. Adv. Mater. 2005; 17(3): 285-9. [39] Feng Y, Zhuang X, Zhu D, Liu Y, Wang Y, Bryce MR. Rational design and characterization of heteroleptic phosphorescent iridium(iii) complexes for highly
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Mater. Chem. C 2014; 2(12): 2150-9. [43] Lai PN, Brysacz CH, Alam MK, Ayoub NA, Gray TG, Bao J, et al. Highly
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Efficient Red-Emitting Bis-Cyclometalated Iridium Complexes. J. Am. Chem.
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[44] Chang C-F, Cheng Y-M, Chi Y, Chiu Y-C, Lin C-C, Lee G-H, et al. Highly Efficient Blue-Emitting Iridium(III) Carbene Complexes and Phosphorescent OLEDs. Angew. Chem. Int. Ed. 2008; 47(24): 4542-5.
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[47] Kim DH, Cho NS, Oh H-Y, Yang JH, Jeon WS, Park JS, et al. Highly Efficient Red Phosphorescent Dopants in Organic Light-Emitting Devices. Adv. Mater. 2011; 23(24): 2721-6.
ACCEPTED MANUSCRIPT [48] Kim J-B, Han S-H, Yang K, Kwon S-K, Kim J-J, Kim Y-H. Highly efficient deep-blue
phosphorescence
from
heptafluoropropyl-substituted
iridium
complexes. Chem. Commun. 2015; 51(1): 58-61.
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[49] Song Y-H, Yeh S-J, Chen C-T, Chi Y, Liu C-S, Yu J-K, et al. Bright and Efficient, Non-Doped, Phosphorescent Organic Red-Light-Emitting Diodes. Adv.
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Funct. Mater. 2004; 14(12): 1221-6.
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furo[3,2-c]pyridine ligand for efficient phosphorescent OLEDs. Org. Electron. 2018; 63: 244-9.
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electroluminescence of iridium complexes with good electron mobility. Mater. Chem. Front. 2018; 2(7): 1284-90.
[52] Cui L-S, Liu Y, Liu X-Y, Jiang Z-Q, Liao L-S. Design and Synthesis of
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Pyrimidine-Based Iridium(III) Complexes with Horizontal Orientation for
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Orange and White Phosphorescent OLEDs. ACS Appl. Mater. Interfaces 2015; 7(20): 11007-14.
[53] Tao P, Li W-L, Zhang J, Guo S, Zhao Q, Wang H, et al. Facile Synthesis of Highly Efficient Lepidine-Based Phosphorescent Iridium(III) Complexes for Yellow and White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2016; 26(6): 881-94.
ACCEPTED MANUSCRIPT [54] Fan C, Zhu L, Jiang B, Li Y, Zhao F, Ma D, et al. High Power Efficiency Yellow Phosphorescent
OLEDs
by
Using
New
Iridium
Complexes
with
Halogen-Substituted 2-Phenylbenzo[d]thiazole Ligands. J. Phys. Chem. C 2013;
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117(37): 19134-41.
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Figures and Tables Scheme 1. Chemical structures of the pyridazine-based iridium(III) complexes. Fig. 1. Normalized (a) UV-vis absorption and (b) PL spectra of the pyridazine-based
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iridium(III) complexes measured in CH2Cl2 solution (10-5 M) at room temperature.
Fig. 2. Normalized transient phosphorescence decays of the pyridazine-based
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iridium(III) complexes in degassed CH2Cl2 solution (10-5 M).
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Fig. 3. CV curves of the pyridazine-based iridium(III) complexes.
Fig. 4. Calculated energy levels, HOMO-LUMO gaps and orbital composition distribution of HOMO and LUMO for the pyridazine-based iridium(III) complexes. Fig. 5. (a) Energy level diagranm (left) and chemical structures of the materials used
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in the decices (right). (b) J-V-L characteristics of devices; (c) EQE versus luminance and the EL spectra (inset) of the devices.
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Table 1. Thermal, photophysical and electrochemical data for the pyridazine-based
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iridium(III) complexes.
Table 2. Theoretical calculations for the pyridazine-based iridium(III) complexes. Table 3. Summary of device performance based on the pyridazine-based iridium(III) complexes.
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Scheme 1. Chemical structures of the pyridazine-based iridium(III) complexes.
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Fig. 1. Normalized (a) UV-vis absorption and (b) PL spectra of the pyridazine-based
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iridium(III) complexes measured in CH2Cl2 solution (10-5 M) at room temperature.
Fig. 2. Normalized transient phosphorescence decays of the pyridazine-based iridium(III) complexes in degassed CH2Cl2 solution (10-5 M).
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Fig. 3. CV curves of the pyridazine-based iridium(III) complexes.
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Fig. 4. Calculated energy levels, HOMO-LUMO gaps and orbital composition distribution of HOMO and LUMO for the pyridazine-based iridium(III) complexes.
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Fig. 5. (a) Energy level diagranm (left) and chemical structures of the materials used
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in the decices (right). (b) J-V-L characteristics of devices; (c) EQE versus luminance
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and the EL spectra (inset) of the devices.
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Table 1. Thermal, photophysical and electrochemical data for the pyridazine-based iridium(III) complexes. Td
λabs a)
λPL a)
[oC]
[nm]
[nm]
(fpdz)2Irtaz
388
258, 342
499
(fpdz)2Irpic
365
263, 345
(fpdz)2Iracac
354
(fpdz)2Irtmd
301
Compounds
a)
τobs c)
HOMO d)
LUMO d)
(µs)
[eV]
[eV]
0.72
2.26
-5.59
-2.81
515
0.89
1.72
-5.46
-2.81
259, 350, 471
544
0.81
1.75
-5.22
-2.73
261, 350, 481
554
0.78
1.26
-5.13
-2.69
Ф b)
Measured in CH2Cl2 solution; b) Measured in 8 wt% doped TCTA films; c) Measured in degassed CH2Cl2 solution; d) Estimated from the onset values of oxidation potentials and reduction potentials.
ACCEPTED MANUSCRIPT Table 2. Theoretical calculations for the pyridazine-based iridium(III) complexes. E
λcal a)
(eV)
(nm)
T1
2.61
475
(fpdz)2Irpic
T1
2.50
495
HOMO→LUMO: 82.4%
42.0
(fpdz)2Iracac
T1
2.46
505
HOMO→LUMO: 86.0%
45.0
(fpdz)2Irtmd
T1
2.45
506
HOMO-1→LUMO: 85.8%
44.8
State
(fpdz)2Irtaz
Character c)
(%)
HOMO→LUMO: 58.3 %
d(Ir)/π(ph)/π(taz)→π*(pdz)
35.2
HOMO→LUMO+1: 12.4%
b)
d(Ir)/π(ph)/π(taz)→π*(pdz) d(Ir)/π(ph)/π(pic)→π*(pdz) d(Ir)/π(ph)/π(acac)→π*(pdz) d(Ir)/π(ph)/π(tmd)→π*(pdz)
Calculated wavelength; The compos that are less than 10% are not listed; c) Ph represents for the phenyl fragment and pdz for the pyridazine fragment.
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a)
MLCT
Orbital: Compos b)
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Compounds
Table 3. Summary of device performance based on the pyridazine-based iridium(III) complexes. L b)
EQE c)
[V]
[cd m-2]
[%]
(fpdz)2Irtaz
2.8
12270
23.1/20.6
(fpdz)2Irpic
2.6
31159
(fpdz)2Iracac
2.8
(fpdz)2Irtmd
2.8
CE c)
PE c)
λmax
CIE e)
off d)
[cd A-1]
[lm W-1]
[nm]
[x, y]
11%
73.9/65.8
77.1/41.3
518
(0.24, 0.58)
28.7/28.1
2%
102.5/100.0
111.4/71.4
520
(0.30, 0.62)
26186
22.7/21.5
5%
84.3/80.0
84.2/57.1
532
(0.36, 0.61)
22126
24.5/22.7
7%
89.0/82.5
88.6/58.9
540
(0.38, 0.60)
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Turn on voltages at 1 cd m-2; b) Maximum luminance; c) Order of measured efficiency values: maximum, then values at 1000 cd m−2; d) Efficiency roll-off at 1000 cd m-2; e) CIE coordinates.
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a)
Roll-
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Von a)
dopants
ACCEPTED MANUSCRIPT Highlights: Four pyridazine-based iridium(III) complexes were synthesized with high yields.
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Organic light-emitting diodes (OLEDs) achieve high efficiency and low roll-off. These complexes offer greatly potential for high-efficiency and low-cost OLEDs.
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We propose a valid strategy to design highly efficient iridium(III) complexes.
S0143-7208(18)32816-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.01.031
Reference:
DYPI 7304
To appear in:
Dyes and Pigments
Received Date: 20 December 2018 Revised Date:
15 January 2019
Accepted Date: 17 January 2019
Please cite this article as: Ning X, Zhao C, Jiang B, Gong S, Ma D, Yang C, Green and yellow pyridazine-based phosphorescent Iridium(III) complexes for high-efficiency and low-cost organic lightemitting diodes, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.01.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
ACCEPTED MANUSCRIPT
Green
and
Yellow
Pyridazine-based
Phosphorescent
Iridium(III) Complexes for High-efficiency and Low-cost Organic Light-Emitting Diodes
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Xiaowen Ning,a,b Chenyang Zhao,c,d,e Bei Jiang,a Shaolong Gong,a Dongge Ma*c,d and Chuluo Yang*a,b
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of
SC
a
E-mail: [email protected] b
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Chemistry, Wuhan University, Wuhan 430072, P. R. China
Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials
Science and Engineering, Shenzhen University, shenzhen 518060, P. R. China Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of
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c
Luminescent Materials and Devices, South China University of Technology,
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Guangzhou 510640, P. R. China
d
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E-mail: [email protected]
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China e
University of Science and Technology of China, Hefei 230026, P. R. China
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Abstract Despite efficient electroluminescence performance, the high fabrication cost due to the utilization of noble metals impedes the commercialization of phosphorescent
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organic light-emitting diodes (PHOLEDs) based on iridium(III) complexes. In this paper, we designed and synthesized a series of green and yellow phosphorescent iridium(III) complexes with 3-(2,4-difluorophenyl)-6-methylpyridazine as the
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cyclometalating ligand to fabricate high-performance and low-cost PHOLEDs. By the
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introduction of a pyridazine moiety to enhance the coordination bond between iridium(III) ion and cyclometalating ligands, these iridium(III) complexes were obtained in high yields and exhibit good thermal stabilities and volatilities, which are beneficial to reduce the device manufacturing cost. Besides, high photoluminescence yields
(PLQYs)
and
short
phosphorescent
lifetimes
of
these
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quantum
pyridazine-based iridium(III) complexes endow their PHOLEDs with excellent device
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performance, which can be promising for practical applications. Particularly, the green emitter (fpdz)2Irpic exhibited a high PLQY of 0.89 in solid doped thin-film;
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PHOLED based on (fpdz)2Irpic achieved an outstanding maximum external quantum efficiency (EQEmax) of 28.7% with ultralow efficiency roll-off of 2% at the high luminance of 1000 cd m-2, which was superior to the commercial green phosphor bis(phenylpyridineyl)iridium(acetylacetonate) [(ppy)2Ir(acac), EQEmax = 24.2%].
Keywords: OLEDs, phosphorescent iridium(III) complexes, pyridazine, ancillary ligand, MLCT contribution.
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1. Introduction Organic light-emitting diodes (OLEDs) have gained widespread mindshare from both academia and industry inspiring by their promising application in flat panel displays
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and solid-state lighting.[1-5] Currently, noble-metal-containing phosphorescent iridium(III) complexes were the prevalent commercial emitters for OLEDs.[6-10] Blue, green and red phosphorescent OLEDs (PHOLEDs) based on cyclometalated
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iridium(III) complexes with external quantum efficiencies (EQEs) above 30% have
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been reported.[11-16] Nevertheless, the high fabrication cost due to the utilization of noble metals impedes the commercialization of these PHOLEDs. Despite many attempts to find more cost-effective phosphorescent emitters, such as copper(I)[17-19] and manganese(II)[20-22] complexes, their unsatisfactory device performance on
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account of EQE, efficiency roll-off, device operating lifetime and color diversity lag far behind the development of these iridium(III) counterparts. In that case, there is
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still substantial space to develop high-performance and low-cost PHOLED for practical application.
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To cut down the manufacturing cost of PHOLEDs, developing cost-effective
molecule structures and synthesis processes, decreasing wastage of phosphorescent emitters as well as reducing vacuum pressure and temperature in the device fabrication process have been demonstrated as valid and straight-forward paradigms.[23-25] Meanwhile, extensive efforts have been paid to develop decent phosphorescent emitters for highly efficient OLED application. For example, Thompson
and
Forrest
et
al.[26]
reported
a
blue
iridium(III)
complex
ACCEPTED MANUSCRIPT bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) and the optimized PHOLED with an exciplex-forming host can achieve a maximum external quantum efficiency (EQEmax) of 34.1%.[13] Later, Thompson et al. reported a blue
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emitter FIr6 by substituting the ancillary ligand of FIrpic with tetra(1-pyrazolyl)borate to improve the blue chromaticity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.26).[27] Besides, Wong et al. developed a blue emitter MS2 by
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replacement of the pyridine ring of FIrpic with pyrimidine moiety, and device based
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on MS2 achieved a superior operation lifetime (T50 ~ 2203 hr) ready for practical application.[28] It has been well demonstrated that subtle alterations in the molecular frameworks can efficiently change the photophysical properties as well as the electroluminescence (EL) performance of the corresponding iridium(III) complexes.
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In this paper, we designed and synthesized a series of phosphorescent iridium(III) complexes with 3-(2,4-difluorophenyl)-6-methylpyridazine (fpdz) as cyclometalating
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ligand to explore the potential of replacing pyridine ring in the prototypical FIrpic with pyridazine moiety. Generally speaking, pyridazine-based iridium(III) complexes
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possess a small steric hindrance of the chelating nitrogen atom due to the adjacent sp2 nitrogen atom without attached hydrogen atom, which will lead to a strong coordination bond between the chelating nitrogen atom and metal iridium atom, and further benefit to the stability and luminescent efficiency.[29-35] Despite pyridazine-based iridium(III) complexes including (fpdz)2Irpic and (fpdz)2Iracac have been reported, the inferior EL efficiencies underestimated their potentials and restricted their applications in OLEDs.[35] By the alteration of the ancillary ligand,
ACCEPTED MANUSCRIPT four pyridazine-based iridium(III) complexes, namely (fpdz)2Irtaz, (fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd, were designed and synthesized to investigate the influence of the ancillary ligand on their photophysical properties and EL
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performance. These four iridium(III) complexes were easily synthesized in high yields (67~95%) and are thermally stable and volatile, which would enhance the utilization rate of noble metal and reduce the vacuum pressure in the device
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fabrication process. Besides, the diverse ancillary ligands endowed different excited
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state characteristics to these iridium(III) complexes, which exhibited tunable emission colors with high photoluminescence quantum yields (PLQYs) in the range of 0.72~0.89. As a result, PHOLEDs based on these pyridazine-based iridium(III) complexes achieved EQEmax of 23.1%, 28.7%, 22.7% and 24.5% together with CIE
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coordinates of (0.24, 0.58), (0.30, 0.62), (0.36, 0.61) and (0.38, 0.60), respectively. It is noteworthy that the green device based on (fpdz)2Irpic achieved an EQEmax of 28.7% with an ultralow efficiency roll-off of 2% at the luminance of 1000 cd m-2, which was
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promising in the practical application of high-performance and low-cost PHOLEDs.
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2. Results and Discussion 2.1 Synthesis and thermal properties Chemical structures of the pyridazine-based iridium(III) complexes were depicted in Scheme 1. The cyclometalating ligand was prepared through a simple Suzuki crossing-coupling
reaction
between
3-chloro-6-methylpyridazine
and
2,4-difluorophenylboronic acid.[35-37] The ancillary ligands of picolinic acid (Hpic),
ACCEPTED MANUSCRIPT acetylacetone (Hacac) and 2,2,6,6-tetramethylheptane-3,5-dione (Htmd) were purchased
from
commercial
sources
and
used
without
2-(5-(4-(trifluoromethyl)phenyl)-2H-1,2,4-triazol-3-yl)pyridine
purification. (Htaz)
The
ancillary
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ligand was synthesized according the literature report.[38-40] All the heteroleptic iridium(III) complexes were obtained via a conventional two-step procedure (Scheme S1).[35,41] It is noticeable that all the complexes can be easily synthesized in good
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yields of 57~95% under mild conditions, which should be attributed to the decreased
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steric hindrance of the coordination bond. The high yields and simple synthesis process are beneficial to enhance the utilization rate of noble metal and reduce producing cost. Besides, these iridium(III) complexes were further purified by temperature-gradient sublimation under vacuum. The sublimation temperatures of
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(fpdz)2Irtaz, (fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd in the vacuum of 10-5 Pa were recorded at 250, 290, 260 and 250 oC, respectively. The simple molecular skeletons and low molecular weights enabled low sublimation temperatures and high
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sublimation yields (82~96%), which enhanced the utilization rate of materials and are
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preferable for practical usage. All the iridium(III) complexes were fully characterized with
1
H NMR spectroscopy, high-resolution mass spectrometry (HRMS) and
elemental analysis (Fig. S1-S8, details were shown in the experimental section of supporting information). Thermogravimetric analysis (TGA) was performed under a stream of nitrogen to investigate the thermal properties of these pyridazine-based iridium(III) complexes (Fig. S9). The thermal decomposition temperatures (Td, with 5% weight loss) of these
ACCEPTED MANUSCRIPT complexes were over 300 oC (Table 1), and gradually depressed with the decreased rigidity of the ancillary ligand. (fpdz)2Irtmd possessed much lower Td than other three iridium(III) complexes, might being attributed to the redundant alkyl groups of
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the ancillary ligand. The high thermal stabilities and volatilities of these iridium(III) complexes make devices fabricated by vacuum evaporation more feasible.
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2.2 Photophysical properties
UV-vis absorption and photoluminescence (PL) spectra of these pyridazine-based
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iridium(III) complexes were displayed in Fig. 1; and the related data were listed in Table 1. Attributed to the identical cyclometalating ligand, these iridium(III) complexes exhibited similar absorption bands. The strong absorption bands below 300 nm were generally assigned to the ligand-centered 1π-π* (1LC) transitions. The
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absorption bands located on ca. 350 nm were conventionally assigned to the spin-allowed singlet metal-to-ligand charge transfer (1MLCT) transitions, while the
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long wavelength tails beyond 400 nm should be assigned to the spin-forbidden triplet metal-to-ligand charge transfer (3MLCT) transitions and ligand-centered 3π-π* (3LC) mixing
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transitions,
with
the
ligand-to-ligand
charge
transfer
(LLCT)
transitions.[25,42]
Under photoexcitation, these pyridazine-based iridium(III) complexes exhibited
broad and featureless emission profiles, indicating that their emissions primarily derived from
3
MLCT states.[42] The phosphorescent emission of (fpdz)2Irpic
exhibited a significant red-shift compared with the well-known phosphor of FIrpic,
ACCEPTED MANUSCRIPT which displayed a blue emission at 475 nm in dichloromethane (CH2Cl2) solution.[26] Attributed to the electronegative effect of pyridazine moiety, the 3MLCT energy level of (fpdz)2Irpic was stabilized, which led to the strong green emission at 515 nm. By
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the replacement of Hpic ancillary ligand with Hacac and Htmd, the phosphorescent emission of (fpdz)2Iracac and (fpdz)2Irtmd was further red-shifted to 544 and 554 nm, respectively. In order to offset the red-shifted effect of pyridazine moiety,
SC
strong-field CF3-containing pyridine-azole ancillary ligand of Htaz was adopted to
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afford the phosphorescent iridium(III) complex (fpdz)2Irtaz, which displayed an intense blue-greenish emission at 499 nm. However, it should be noted that the PL spectrum of (fpdz)2Irtaz exhibited an obscure shoulder at ca. 518 nm, suggesting the mixed MLCT/LC/LLCT character of its emissive excited state.[42] When cooled to
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77 K (Fig. S10), these four iridium(III) complexes exhibited pronounced vibronic structures and rigidochromic shifts. The maximum emissions of (fpdz)2Irtaz, (fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd at 77 K are hypsochromically shifted
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by 15, 19, 26 and 31 nm compared to those of the CH2Cl2 solutions at 298 K.
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Obviously, compared with other three iridium(III) complexes, (fpdz)2Irtaz exhibited little rigidochromism and much pronounced vibronic structures, suggesting the decreased MLCT contribution in its excited state.[43] Transient PL spectra were examined to further investigate the excited state characteristics of these pyridazine-based iridium(III) complexes (Fig. 2). All the complexes exhibited monoexponential decay with short phosphorescence lifetimes in the range of 1.26~2.26 µs (Table 1). The solid PLQYs of these complexes in 8 wt%
ACCEPTED MANUSCRIPT tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA) doped thin-films were considerably high ranging from 0.72 to 0.89. The blue-greenish phosphor (fpdz)2Irtaz exhibited longer phosphorescent lifetime and lower PLQY than other three iridium(III)
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complexes. The inferior luminescence efficiency of (fpdz)2Irtaz can be attributed to the decreased MLCT contribution in its emissive excited state, which reduced the
further reduced the transition probability.[43-45]
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2.3 Electrochemical properties
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spin-orbital coupling effect directly involving the heavy metal iridium atom and
Cyclic voltammetry (CV) was performed to investigate the electrochemical properties of these pyridazine-based iridium(III) complexes. As depicted in Fig. 3, all the complexes exhibited reversible or quasi-reversible oxidation and reduction behavior.
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The oxidation waves can be ascribed to the metal-centered IrIII/IrIV oxidation and the reduction waves were generally assigned to the reductions of the pyridazine moieties
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of the cyclometalating ligands. With regard to ferrocene/ferrocenium (Fc/Fc+), the highest occupied molecular orbital (HOMO) energy levels of (fpdz)2Irtaz,
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(fpdz)2Irpic, (fpdz)2Iracac and (fpdz)2Irtmd were deduced to be -5.59, -5.46, -5.22 and -5.13 eV, respectively, which were gradually destabilized with the decreased electron-withdrawing abilities of ancillary ligands. Nevertheless, the lowest unoccupied molecular orbital (LUMO) energy levels of these complexes almost remained and were determined to be -2.81, -2.81, -2.73 and -2.69 eV (Table 1), respectively. The decreased HOMO-LUMO energy gaps were in good accordance with the photophysical properties of these iridium(III) complexes.
ACCEPTED MANUSCRIPT 2.4 Theoretical calculations To get a deeper insight into the electronic properties of these iridium(III) complexes, quantum chemical calculations were performed based on the density functional theory
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(DFT). As depicted in Fig. 4, the LUMOs of these iridium(III) complexes mainly consisted of the π* orbitals of the pyridazine moieties. Owing to the asymmetry of molecular structure, the LUMO of (fpdz)2Irtaz and (fpdz)2Irpic centered on one
SC
cyclometalating ligand while (fpdz)2Iracac and (fpdz)2Irtmd scattered over two
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cyclometalating ligands. The fewer moieties involved in the LUMO might be beneficial to stabilize the excited states and improve their PLQYs.[35] The HOMOs of these complexes primarily composed of the d orbitals of the central iridium(III) ion and the π orbitals of the phenyl rings of the cyclometalating ligands except for
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(fpdz)2Irtmd with the HOMO moved to the ancillary ligand. The decreased electron-withdrawing ability of ancillary ligand led to the localization of electrons on
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central iridium(III) ion, which destabilized the HOMOs of these pyridazine-based iridium(III) complexes and little affected LUMOs. These calculation results were in
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agreement with experimental electrochemical properties. To further analyze the optical properties of these pyridazine-based iridium(III)
complexes, time-dependent DFT (TD-DFT) calculations were performed on the basis of the optimized geometry. The vertical excitation energies and dominant transitions were listed in Table 2; further molecular orbital compositions in the excited states were listed in Table S1. The calculated lowest-lying triplet (T1) energy levels of these iridium(III) complexes were identical with the energies deduced from the
ACCEPTED MANUSCRIPT highest-energy vibronic bands at 77 K (Fig. S10) and demonstrated the validity of the computation. The T1 states of (fpdz)2Irpic and (fpdz)2Iracac dominantly involved the transition from HOMO to LUMO; while those of (fpdz)2Irtaz and (fpdz)2Irtmd
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were slightly different, which mainly consisted of the transition from HOMO to LUMO/LUMO+1 and the transition from HOMO-1 to LUMO, respectively. Despite different transition patterns, the T1 states of these pyridazine-based iridium(III)
SC
complexes could be labeled as the similar mixed state consisting of 3MLCT
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[d(Ir)→π*(pdz)], 3LC [π(ph)→π*(pdz)] and 3LLCT [π(ancillary ligand)→π*(pdz)] state, which was in good aggrement with the experimentally observed properties. Further analyses to the characters of their T1 states by the interfragment charge transfer method demonstrated that the MLCT contribution of (fpdz)2Irtaz (35.2%)
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was much lower than other three iridium(III) complexes, which further rationalized its inferior luminescent efficiency.
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2.5 Electroluminescent devices
To investigate the EL performance of these pyridazine-based iridium(III) complexes,
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PHOLEDs were fabricated with a simple configuration of ITO/MoO3 (8nm)/TAPC (60nm)/TCTA:8 wt% dopants (12 nm)/BmPyPb (60nm)/LiF (1nm)/Al (150nm), where indium tin oxide (ITO) and lithium fluoride/aluminum (LiF/Al) were used as anode and cathode, respectively. Molybdenum trioxide (MoO3) was employed as hole-injection materials. 1,1-Bis-(4-methylphenyl)-aminophenyl-cyclohexane (TAPC) and 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPb) were applied as hole- and electron-transporting materials, respectively. TCTA acted as the host material and the
ACCEPTED MANUSCRIPT pyridazine-based iridium(III) complexes were doped in the host-matrix with the concentrations of 8 wt% in the emitting layer. Devices based on different dopants were named as device A~D, respectively. The device structure, chemical structures
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and energy levels of the materials used in the devices were depicted in Fig. 5a. The EL characteristics of the devices were depicted in Fig. 5b~c and Fig. S11. Key data of the device performance were summarized in Table 3.
SC
Device A~D exhibited extremely low turn-on voltages (Von) of 2.6~2.8 V, which
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was due to the low energy barriers for hole/electron-injection. As shown in Fig. 5c, the (fpdz)2Irtaz-，(fpdz)2Irpic-, (fpdz)2Iracac- and (fpdz)2Irtmd-based devices displayed green or yellow emissions with the main peaks in the range of 518~540 nm, which were identical with the corresponding PL spectra in the thin-film states except
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for (fpdz)2Irtaz (Fig. S12). Compared with the PL spectrum of (fpdz)2Irtaz in TCTA doped thin-film, the EL spectrum of (fpdz)2Irtaz-based device displayed suppressed
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emission at 499 nm and enhanced shoulder peak at 518 nm. The dramatic change might be attributed to the predominant emission from LLCT state in the EL process,
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which generally not only resulted in an unwanted bathochromatic shift but also lowered the luminescent efficiency.[46] As a result, device A based on (fpdz)2Irtaz exhibited common device performance: a EQEmax of 23.1%, a maximum current efficiency (CEmax) of 73.9 cd A-1, a maximum power efficiency (PEmax) of 77.1 lm W-1 and a maximum luminance of 12270 cd m-2. Inspiringly, due to the high PLQY in TCTA doped thin-film, employing (fpdz)2Irpic as the green emitter endowed device B with better performance, which exhibited a promising EQEmax of 28.7%, CEmax of
ACCEPTED MANUSCRIPT 102.5 cd A-1, PEmax of 111.4 lm W-1 and a maximum luminance of 31159 cd m-2. More importantly, the EQE remained as high as 28.1% at the high luminance of 1000 cd m-2, corresponding to an ultralow efficiency roll-off of 2%. The high efficiency and low
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roll-off of (fpdz)2Irpic-based device were superior to the controlled device based on the commercial green emitter (ppy)2Ir(acac) with similar EL spectrum (Fig. S13 and Table S2), which revealed a EQEmax of 24.2% accompanied with efficiency roll-off of
SC
3% at the luminance of 1000 cd m-2. Despite the sublimation temperature of
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(fpdz)2Irpic in the vacuum of 10-5 Pa was slightly higher than that of (ppy)2Ir(acac) (ca. 240 oC), the excellent device performance with a simple device structure indicated its great potential in practical application.
With respect to the yellow devices, the (fpdz)2Iracac-based device C achieved a
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EQEmax of 22.7%, CEmax of 84.3 cd A-1, and PEmax of 84.2 lm W-1 and a maximum luminance of 26186 cd m-2. The EQE was recorded as 21.5% at the high luminance of
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1000 cd m-2, revealing a low efficiency roll-off of 5%. Besides, device D based on (fpdz)2Irtmd exhibited better performance with a EQEmax of 24.5%, CEmax of 89.0 cd
AC C
A-1, PEmax of 88.6 lm W-1 and a maximum luminance of 22126 cd m-2, which might be attributed to the suppressed triplet-triplet annihilation by the steric hindrance of the bulky ancillary ligand.[25,47-49] At the luminance of 1000 cd m-2, device D maintained high EQE of 22.7% and the efficiency roll-off was only 7%, which was comparable to most of the reported yellow devices based on iridium(III) complexes.[6,50-54] Although these pyridazine-based iridium(III) complexes shared similar
ACCEPTED MANUSCRIPT molecular frameworks, they exihibited quite different excited state characters, photophysical properties and EL performances. Despite its blue-shifted emission, the low PLQY and long phosphorescent lifetime of (fpdz)2Irtaz resulted in inferior
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device performance and large efficiency roll-off, which could overshadow its practical application in OLEDs. These results fully demonstrated that the development of highly efficient phosphorescent iridium(III) complexes should enhance the MLCT
SC
contribution in their emissive excited states. All in all, these pyridazine-based
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iridium(III) complexes have great potential to fabricate high-performance and low-cost PHOLEDs for practical application: the high PLQY to ensure an intrinsically high device efficiency; the short phosphorescent lifetime and high stability to alleviate efficiency roll-off at high luminance; the high synthesis yield and sublimation yield to
device fabrication.
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3. Conclusion
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reduce the waste of noble metal; the low sublimation temperature to reduce the cost of
In conclusion, we designed and synthesized a series of pyridazine-based iridium(III)
AC C
complexes for high-efficiency and low-cost PHOLEDs. Firstly, through the incorporation of pyridazine moiety into cyclometalating ligand to obtain strong coordination bond, these iridium(III) complexes are easily synthesized with high yields, thermal stable and volatile, which enhances the utilization rate of noble metal and reduces the device manufacturing cost. Secondly, PHOLEDs based on these iridium(III) complexes achieved promising efficiency and low roll-off with a simple device
structure,
indicating
their
great
potential
for
the
fabrication
of
ACCEPTED MANUSCRIPT high-performance and low-cost PHOLEDs. Finally, the diverse photophysical properties and EL performances of these iridium(III) complexes are demonstrated to be associated with their excited state characteristics, and we provide a valid strategy
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of enhancing MLCT contribution in emissive excited state to design highly efficient iridium(III) complexes.
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Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural
of
China
(973
Program
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Science Foundation of China (No. 91433201), the National Basic Research Program 2015CB655002),
Shenzhen
Peacock
Plan
(KQTD20170330110107046) and the key Technological Innovation Program of
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Orange and White Phosphorescent OLEDs. ACS Appl. Mater. Interfaces 2015; 7(20): 11007-14.
[53] Tao P, Li W-L, Zhang J, Guo S, Zhao Q, Wang H, et al. Facile Synthesis of Highly Efficient Lepidine-Based Phosphorescent Iridium(III) Complexes for Yellow and White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2016; 26(6): 881-94.
ACCEPTED MANUSCRIPT [54] Fan C, Zhu L, Jiang B, Li Y, Zhao F, Ma D, et al. High Power Efficiency Yellow Phosphorescent
OLEDs
by
Using
New
Iridium
Complexes
with
Halogen-Substituted 2-Phenylbenzo[d]thiazole Ligands. J. Phys. Chem. C 2013;
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117(37): 19134-41.
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Figures and Tables Scheme 1. Chemical structures of the pyridazine-based iridium(III) complexes. Fig. 1. Normalized (a) UV-vis absorption and (b) PL spectra of the pyridazine-based
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iridium(III) complexes measured in CH2Cl2 solution (10-5 M) at room temperature.
Fig. 2. Normalized transient phosphorescence decays of the pyridazine-based
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iridium(III) complexes in degassed CH2Cl2 solution (10-5 M).
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Fig. 3. CV curves of the pyridazine-based iridium(III) complexes.
Fig. 4. Calculated energy levels, HOMO-LUMO gaps and orbital composition distribution of HOMO and LUMO for the pyridazine-based iridium(III) complexes. Fig. 5. (a) Energy level diagranm (left) and chemical structures of the materials used
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in the decices (right). (b) J-V-L characteristics of devices; (c) EQE versus luminance and the EL spectra (inset) of the devices.
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Table 1. Thermal, photophysical and electrochemical data for the pyridazine-based
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iridium(III) complexes.
Table 2. Theoretical calculations for the pyridazine-based iridium(III) complexes. Table 3. Summary of device performance based on the pyridazine-based iridium(III) complexes.
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Scheme 1. Chemical structures of the pyridazine-based iridium(III) complexes.
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Fig. 1. Normalized (a) UV-vis absorption and (b) PL spectra of the pyridazine-based
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iridium(III) complexes measured in CH2Cl2 solution (10-5 M) at room temperature.
Fig. 2. Normalized transient phosphorescence decays of the pyridazine-based iridium(III) complexes in degassed CH2Cl2 solution (10-5 M).
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Fig. 3. CV curves of the pyridazine-based iridium(III) complexes.
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Fig. 4. Calculated energy levels, HOMO-LUMO gaps and orbital composition distribution of HOMO and LUMO for the pyridazine-based iridium(III) complexes.
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Fig. 5. (a) Energy level diagranm (left) and chemical structures of the materials used
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in the decices (right). (b) J-V-L characteristics of devices; (c) EQE versus luminance
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and the EL spectra (inset) of the devices.
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Table 1. Thermal, photophysical and electrochemical data for the pyridazine-based iridium(III) complexes. Td
λabs a)
λPL a)
[oC]
[nm]
[nm]
(fpdz)2Irtaz
388
258, 342
499
(fpdz)2Irpic
365
263, 345
(fpdz)2Iracac
354
(fpdz)2Irtmd
301
Compounds
a)
τobs c)
HOMO d)
LUMO d)
(µs)
[eV]
[eV]
0.72
2.26
-5.59
-2.81
515
0.89
1.72
-5.46
-2.81
259, 350, 471
544
0.81
1.75
-5.22
-2.73
261, 350, 481
554
0.78
1.26
-5.13
-2.69
Ф b)
Measured in CH2Cl2 solution; b) Measured in 8 wt% doped TCTA films; c) Measured in degassed CH2Cl2 solution; d) Estimated from the onset values of oxidation potentials and reduction potentials.
ACCEPTED MANUSCRIPT Table 2. Theoretical calculations for the pyridazine-based iridium(III) complexes. E
λcal a)
(eV)
(nm)
T1
2.61
475
(fpdz)2Irpic
T1
2.50
495
HOMO→LUMO: 82.4%
42.0
(fpdz)2Iracac
T1
2.46
505
HOMO→LUMO: 86.0%
45.0
(fpdz)2Irtmd
T1
2.45
506
HOMO-1→LUMO: 85.8%
44.8
State
(fpdz)2Irtaz
Character c)
(%)
HOMO→LUMO: 58.3 %
d(Ir)/π(ph)/π(taz)→π*(pdz)
35.2
HOMO→LUMO+1: 12.4%
b)
d(Ir)/π(ph)/π(taz)→π*(pdz) d(Ir)/π(ph)/π(pic)→π*(pdz) d(Ir)/π(ph)/π(acac)→π*(pdz) d(Ir)/π(ph)/π(tmd)→π*(pdz)
Calculated wavelength; The compos that are less than 10% are not listed; c) Ph represents for the phenyl fragment and pdz for the pyridazine fragment.
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a)
MLCT
Orbital: Compos b)
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Compounds
Table 3. Summary of device performance based on the pyridazine-based iridium(III) complexes. L b)
EQE c)
[V]
[cd m-2]
[%]
(fpdz)2Irtaz
2.8
12270
23.1/20.6
(fpdz)2Irpic
2.6
31159
(fpdz)2Iracac
2.8
(fpdz)2Irtmd
2.8
CE c)
PE c)
λmax
CIE e)
off d)
[cd A-1]
[lm W-1]
[nm]
[x, y]
11%
73.9/65.8
77.1/41.3
518
(0.24, 0.58)
28.7/28.1
2%
102.5/100.0
111.4/71.4
520
(0.30, 0.62)
26186
22.7/21.5
5%
84.3/80.0
84.2/57.1
532
(0.36, 0.61)
22126
24.5/22.7
7%
89.0/82.5
88.6/58.9
540
(0.38, 0.60)
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Turn on voltages at 1 cd m-2; b) Maximum luminance; c) Order of measured efficiency values: maximum, then values at 1000 cd m−2; d) Efficiency roll-off at 1000 cd m-2; e) CIE coordinates.
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a)
Roll-
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Von a)
dopants
ACCEPTED MANUSCRIPT Highlights: Four pyridazine-based iridium(III) complexes were synthesized with high yields.
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Organic light-emitting diodes (OLEDs) achieve high efficiency and low roll-off. These complexes offer greatly potential for high-efficiency and low-cost OLEDs.
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We propose a valid strategy to design highly efficient iridium(III) complexes.