Efficient organic light-emitting diodes with low efficiency roll-off using iridium emitter with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol as ancillary ligand

Article abstract of DOI:10.1016/j.jorganchem.2014.04.030

Using 2,4,5-trifluorophenylpyridine as a monoanionic cyclometalated ligand, 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol, 2-(5-(4-fluorophenyl)-1,3,4-oxadiazol- 2-yl)phenol and 2-(5-(4-trifluoro-methylphenyl)-1,3,4-oxadiazol-2-yl)phenol as ancillary ligands, three new heteroleptic iridium(III) complexes (Ir(F _3,4,6ppy)₂POP, Ir(F_3,4,6ppy)₂FPOP and Ir(F_3,4,6ppy)₂CF₃POP) were developed. All complexes are green phosphors (λ_max = 503-521 nm) with photoluminescence quantum efficiency yields of 12-18% in CH₂Cl ₂ solutions at room temperature, respectively. The organic light emitting diodes (OLEDs) with the structure of ITO/TAPC (1,1-bis(4-(di-p- tolylamino)phenyl)cyclohexane, 60 nm)/Ir(F_3,4,6ppy)₂POP (6, 8, 10 wt%): SimCP2 (bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane, 30 nm)/TPBi (1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl, 90 nm)/LiF (1 nm)/Al (100 nm) showed good performances. Particularly, the device with 8 wt% doped concentration exhibited superior performances with a peak current efficiency (η_c) of 61.49 cd A^-1 and a peak power efficiency (η_p) of 46.03 lm W^-1. Furthermore, the efficiency roll-off ratios from the peak current efficiency to that at the practical luminance of 100 cd m^-2 and from 100 cd m^-2 to the benchmark brightness of 1000 cd m^-2 in this device are low, which are helpful to keep high efficiency at relatively high current density and high luminance.

Full text of DOI:10.1016/j.jorganchem.2014.04.030

Accepted Manuscript
Efficient organic light-emitting diodes with low efficiency roll-off using iridium emitter
with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol as ancillary ligand
Yi-Ming Jin , Cheng-Cheng Wang , Li-Sha Xue , Tian-Yi Li , Song Zhang , Xuan Liu ,
Xiao Liang , You-Xuan Zheng , Jing-Lin Zuo
PII:
S0022-328X(14)00212-5
10.1016/j.jorganchem.2014.04.030
JOM 18565
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 February 2014
Revised Date: 28 April 2014
Accepted Date: 28 April 2014
Please cite this article as: Y.-M. Jin, C.-C. Wang, L.-S. Xue, T.-Y. Li, S. Zhang, X. Liu, X. Liang, Y.-X.
Zheng, J.-L. Zuo, Efficient organic light-emitting diodes with low efficiency roll-off using iridium emitter
with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol as ancillary ligand, Journal of Organometallic Chemistry
(2014), doi: 10.1016/j.jorganchem.2014.04.030.
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ACCEPTED MANUSCRIPT
Graphical abstract (synopsis)
Three new heteroleptic iridium(III) complexes with 1,3,4-oxadiazol containing
ancillary ligands were synthesized and the OLEDs using one emitter showed superior
performances with a peak current efficiency (ηc) of 61.49 cd A-1 and low efficiency
roll-off.

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Graphical Abstract (pictogram)
100
10
1
R
Ir(F3,4,6ppy)2POP
6 wt%
F
8 wt%
10 wt%
F
N
O
F
N
Ir
2
O
N
R = H: Ir(F3,4,6ppy)2POP
R = F: Ir(F3,4,6ppy)2FPOP
R = CF3: Ir(F3,4,6ppy)2CF3POP
0.1
100
1000
10000
Luminance (cd/m2)

ACCEPTED MANUSCRIPT
Efficient organic light-emitting diodes with low efficiency roll-off using
iridium emitter with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol as ancillary
ligand
Yi-Ming Jin, Cheng-Cheng Wang, Li-Sha Xue, Tian-Yi Li, Song Zhang, Xuan Liu, Xiao Liang,
You-Xuan Zheng,* Jing-Lin Zuo
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Center for
Molecular Organic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, P. R. China, e-mail: yxzheng@nju.edu.cn
Yi-Ming Jin and Cheng-Cheng Wang are co-first author
Abstract
Using 2,4,5-trifluorophenylpyridine as a monoanionic cyclometalated ligand, 2-(5-phenyl-1,3,4-
oxadiazol-2-yl)phenol, 2-(5-(4-fluorophenyl)-1,3,4-oxadiazol-2-yl)phenol and 2-(5-(4-trifluoro-
methylphenyl)-1,3,4-oxadiazol-2-yl)phenol as ancillary ligands, three new heteroleptic iridium(III)
complexes (Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP) were developed.
All complexes are green phosphors (λmax = 503 - 521 nm) with photoluminescence quantum
efficiency yields of 12 - 18% in CH2Cl2 solutions at room temperature, respectively. The organic
light emitting diodes (OLEDs) with the structure of ITO / TAPC (1,1-bis(4-(di-p-
tolylamino)phenyl)cyclohexane, 60 nm) / Ir(F3,4,6ppy)2POP (6, 8, 10 wt%) : SimCP2
(bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane, 30 nm) / TPBi (1,3,5-tri(1-phenyl-1H-
benzo[d]imidazol-2-yl)phenyl, 90 nm) / LiF (1 nm) / Al (100 nm) showed good performances.
Particularly, the device with 8 wt% doped concentration exhibited superior performances with a
peak current efficiency (ηc) of 61.49 cd A-1 and a peak power efficiency (ηp) of 46.03 lm W-1.
Furthermore, the efficiency roll-off ratios from the peak current efficiency to that at the practical
luminance of 100 cd m-2 and from 100 cd m-2 to the benchmark brightness of 1000 cd m-2 in this
device are low, which are helpful to keep high efficiency at relatively high current density and
high luminance.
Keywords: Iridium complex; 2,4,5-Trifluorophenylpyridine; 1,3,4-Oxadiazole; Organic light
emitting diode; Efficiency
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1. Introduction
Phosphorescent iridium complexes play an important part in efficient organic light emitting
diodes (OLEDs) due to the high quantum efficiency and short lifetime of triplet excited states
[1-20]. The strong spin-orbit coupling (SOC) introduced by the central heavy atom can promote
the triplet to singlet radiative transition, so that such complexes may exhibit unusually high
phosphorescence quantum yields at room temperature. On the other hand, since the
phosphorescence of iridium complexes primarily originates from the metal-to-ligand charge
transfers (MLCT) and the ligand-centered (LC) transitions [21], the energy level of the excited
state can be controlled by tuning the energy levels of the ligands through substituent effects, which
leads to a wide flexible emission color range.
In this context, heteroleptic cyclometalated (C^N) iridium(III) complexes ((C^N)2Ir(LX), LX =
ancillary ligand) are promising phosphorescent materials due to easy synthetic chemical
accessibility compared with the corresponding homoleptic Ir(C^N)3 complexes and good
photophysical properties [22-24]. According to the density functional theory calculation, the
highest occupied molecular orbital (HOMO) is basically centered on the Ir(III) metal while the
lowest unoccupied molecular orbital (LUMO) is generally localized on the cyclometalated
ligands. Although most ancillary ligands do not make contribution to the lowest excited state
directly, they indeed alter the energy levels of the excited states by modifying the electron density
at the metal center. Thus, the photophysical property and carrier mobility of iridium complexes
can be tuned trough functional substitutes on both cyclometalated and ancillary ligands. Our group
has reported high efficient phosphorescent OLEDs by introducing tetraphenylimidodiphosphinate
(tpip) derivatives as ancillary ligands into Ir(III) complexes [25-29]. Tpip derivatives have
stronger polar P=O bonds, which may improve the electron mobility of the Ir(III) complexes,
broaden the electron-hole recombination zone, balance the distribution of holes / electrons and
reduce leakage current, particularly for the high doping concentration, leading to the suppressed
the triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) effects in the device
[30-32]. Therefore, Ir(III) complexes attached ancillary ligands with good electron mobility are
potential efficient emitters for OLEDs.
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1,3,4-Oxadiazole derivatives are good candidates for electron injection and transport materials
due to their high electron affinity, high photoluminescence quantum yield and good
thermal/chemical stability. Yang and Ma et. al. [33,34], Tian et. al. [35], Bryce et. al. [36] and our
group [37] have reported efficient iridium complexes and devices using 1,3,4-oxadiazole
derivatives as cyclometalated ligands to increase the complexes’ electron transporting ability and
consequently facilitate charge trapping across the bulk for high performance OLEDs. However,
few heteroleptic Ir(III) complexes using 1,3,4-oxadiazole derivatives as ancillary ligands have
been reported [38,39]. In addition, fluorination can enhance the electron mobility and result in a
better balance of charge injection and transfer, lower vibrational frequency of C-F bond can
reduce the rate of radiationless deactivation. On this basis, as shown in Scheme 1, we synthesized
three heteroleptic Ir(III) complexes using 2,4,5-trifluorophenylpyridine (F3,4,6ppy) as the
cyclometalated
ligand
and
the
2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol
and
its
fluoro/trifluoromethyl substituted derivatives, 2-(5-(4-fluorophenyl)-1,3,4-oxadiazol-2-yl)phenol
and 2-(5-(4-trifluoromethyl-phenyl)-1,3,4-oxadiazol-2-yl)phenol, as the ancillary ligands. Here,
we describe the results of our investigation on the synthesis, structural characterization and
photoluminescence / electroluminescence properties of these Ir(III) complexes.
2. Experimental Section
2.1 Materials and measurements.
All reagents and chemicals were purchased from commercial sources and used without further
1
purification. H NMR spectra were measured on a Bruker AM 400 spectrometer or a Bruker AM
500 spectrometer. Mass spectrometry (MS) spectra were obtained with an electrospray ionization
(ESI) mass spectrometer (LCQ Fleet, Thermo Fisher Scientific) or matrixassisted laser desorption
ionization time-of-flight (MALDI-TOF) mass spectrometer (Bruker Daltonic Inc.). Absorption
and photoluminescence spectra were measured on a UV-3100 spectrophotometer and a Hitachi
F-4600 photoluminescence spectrophotometer, respectively. The decay lifetimes were measured
with an Edinburgh FLS-920 spectrometer in degassed CH2Cl2 solution at room temperature.
Cyclic voltammetry measurements were conducted on a MPI-A multifunctional electrochemical
and chemiluminescent system (Xi’an Remex Analytical Instrument Ltd. Co., China) at a scan rate
0.1 V s-1, with a polished Pt plate as the working electrode, platinum thread as the counter
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electrode and Ag-AgNO3 (0.1 M) in CH3CN as the reference electrode, tetra-n-butylammonium
perchlorate (0.1 M) as the supporting electrolyte, together with Fc+/Fc as the internal standard.
The luminescence quantum efficiencies were calculated by comparison of the emission
intensities (integrated areas) of a standard sample (fac-Ir(ppy)3) and the iridium complexes
according to the equation (1) [40].
I
Astd ηunk
2
unk )(
Istd Aunk ηstd
)(
)
(1)
Φunk = Φstd
(
where Φunk and Φstd are the luminescence quantum yields of the iridium complexes and
fac-Ir(ppy)3 (0.4 [41]), respectively. The Iunk and Istd are the integrated emission intensities of the
iridium complexes and fac-Ir(ppy)3 solution, respectively. The Aunk and Astd are the absorbance
values of the iridium complexes and fac-Ir(ppy)3 solution at their excitation wavelengths,
respectively. The ηunk and ηstd terms represent the refractive indices of the corresponding solvents
(pure solvents were assumed).
2.2 Crystallography.
The single crystal structures of complexes were carried out on a Bruker SMART CCD
diffractometer using monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Cell
parameters were retrieved using the SMART software and refined using the SAINT program [42]
on all observed reflections. Data were collected using a narrow-frame method with a scan width of
0.30° in ω and an exposure time of 10 s/frame. The highly redundant data sets were reduced using
the SAINT program and corrected for Lorentz and polarization effects. Absorption corrections
were applied using the SADABS program [43] supplied by Bruker. The structures were solved by
direct methods and refined by full-matrix least-squares on F2 using the SHELXS-97 program
[44]. The positions of metal atoms and their first coordination spheres were located from
direct-methods E-maps; other non-hydrogen atoms were found in alternating difference Fourier
syntheses and least-squares refinement cycles and refined anisotropically during the final cycles.
Hydrogen atoms were placed in calculated position and refined as riding atoms with a uniform
value of Uiso.
2.3 OLEDs fabrication and measurement.
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All OLEDs with the emission area of 0.1 cm2 were fabricated on the pre-patterned ITO-coated
glass substrate with a sheet resistance of 15 Ω sq-1. The 60 nm hole-transporting material of TAPC
(1,1-bis(4-(di-p-tolylamino)phenyl)cyclohexane) film was first deposited on the ITO glass
substrate. The phosphor and SimCP2 (bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane) host
were co-evaporated to form 30 nm emitting layer from two separate sources. To optimize the best
device performances, the different doped concentrations of the Ir(III) complex in the host (6, 8, 10
wt%) were applied. Successively, TPBi (1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl, 90
nm), LiF (1 nm), and Al (100 nm) were evaporated. The vacuum was less than 1×10-4 Pa during
all materials deposition. The electroluminescence spectra were measured with a Hitachi F-4600
photoluminescence spectrophotometer. The EL characteristic curves were measured with a
computer controlled Keithley 2400 source meter with a calibrated silicon diode in air without
device encapsulation.
2.4 Syntheses.
The syntheses procedures of ligands were listed in Scheme 1. The cyclometallated ligand
(F3,4,6ppy), cyclometallated Ir(III) chloro-bridged dimer ([(F3,4,6ppy)2Ir(µ-Cl)]2) and ancillary
ligand (HPOP) were synthesized according to our previous report [29,39]. All reactions were
performed under nitrogen. Solvents were carefully dried and distilled from appropriate drying
agents prior to use.
2.4.1 Synthesis of the ancillary ligands (HLX).
5-Fluoro-benzoyl chloride (1.58 g, 10 mmol) was added dropwise to a solution of
2-methoxybenzohydrazine (1.66 g, 10 mmol) and triethylamine (1.01 g, 10 mmol) in chloroform
(20 mL) at room temperature. The mixture was stirred for 2 h and then filtered. The solid was
washed with water and ethanol to give N-(4-fluorobenzoyl)-2-methoxybenzohydrazine (2.59 g,
90% yield). A mixture of N-(4-fluorobenzoyl)-2-methoxybenzo-hydrazine and POCl3 (20 mL) in a
50 mL flask was refluxed under nitrogen for 5 h. The excessive POCl3 was then distilled out, and
the residue was poured into water. The crude solid product was purified by recrystallization from
chloroform/hexane to give 2-(4-fluorophenyl)-5-(2-methoxyphenyl)-1,3,4-oxadiazole (2.30 g,
85% yield). Then, to a mixture of 2-(4-fluorophenyl)-5-(2-methoxyphenyl)-1,3,4-oxadiazole (2.30
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g) in 50 mL CH2Cl2 at -78 oC was added BBr3 (12.5 g, 50 mmol in 20 mL CH2Cl2 ) dropwise. The
o
mixture was stirred for 24 h at -78 C and the resulting solution was poured into water, extracted
with CH2Cl2 (50 mL × 3 times) and then dried over anhydrous sodium sulfate. The solvent was
removed under reduced pressure, and recrystallization of the residue from ethanol gave colorless
1
crystal of 2-(5-(4-fluoro-phenyl)-1,3,4-oxadiazol-2-yl)phenol (HFPOP) with a 87% yield. H
NMR (500 MHz, DMSO-d6) δ 8.10 (dd, J = 8.4, 5.5 Hz, 2H), 7.74 (d, J = 7.6 Hz, 1H), 7.49 – 7.39
(m, 2H), 7.25 (t, J = 7.5 Hz, 1H), 6.90 (d, J = 8.4 Hz, 1H), 6.63 (t, J = 7.3 Hz, 1H).
2-(5-(4-Trifluoromethyl-phenyl)-1,3,4-oxadiazol-2-yl)phenol (HCF3POP) was synthesized
with the same procedure with 92% yield. 1H NMR (500 MHz, CDCl3) δ 8.29 (d, J = 8.1 Hz, 2H),
8.06 (dd, J = 7.7, 1.6 Hz, 1H), 7.82 (d, J = 8.2 Hz, 2H), 7.57 (t, J = 10 Hz, 2H), 7.16 – 7.10 (m,
2H).
2.4.2 General syntheses of complexes.
A mixture of IrCl3·3H2O (1 mmol) and F3,4,6ppy (2.5 mmol) in 2-ethoxyethanol and water (20
mL, 3 : 1, v/v) was refluxed for 24 h. After cooling, the yellow solid precipitate was filtered to
give the crude cyclometalated Ir(III) chloro-bridged dimer ([(F3,4,6ppy)2Ir(µ-Cl)]2). Then the
slurry of crude chloro-bridged dimer (0.2 mmol) and ancillary potassium salt (KLX, 0.5 mmol) in
2-ethoxyethanol (20 mL) was refluxed for 24 h. After the mixture was cooled, the solvent was
evaporated at low pressure. The crude product was washed by water, and the column
chromatography using CH2Cl2 as the eluent gave Ir(III) complexes which were further purified
again by sublimation in vacuum.
Ir(F3,4,6ppy)2POP (yield: 56%): M.p. > 310 oC. 1H NMR (500 MHz, DMSO-d6) δ 8.65 (d, 1H),
8.44 (d, 1H), 8.17 (dd, J = 21.3 Hz, 2H), 8.01 – 7.86 (m, 4H), 7.83 (d, J = 6.5 Hz, 1H), 7.59 (dd, J
= 16.0, 7.2 Hz, 1H), 7.33 (t, J = 6.4 Hz, 2H), 7.21 (t, J = 7.0 Hz, 1H), 7.12 (t, J = 6.6 Hz, 1H),
7.09 – 6.99 (m, 2H), 6.63 (d, J = 8.1 Hz, 1H), 6.53 (t, J = 7.5 Hz, 2H). MS(MALDI-TOF), m/z:
calcd for C36H19F6IrN4O2, 845.77 [M]; found 844.82 [M-1].
Ir(F3,4,6ppy)2FPOP (yield: 45%): M.p. >310 oC. 1H NMR (500 MHz, DMSO-d6) δ 8.64 (d, J =
5.6 Hz, 1H), 8.44 (d, J = 5.6 Hz, 1H), 8.18 (dd, J = 20.5, 8.5 Hz, 2H), 8.01 – 7.86 (m, 4H), 7.83
(d, J = 6.9 Hz, 1H), 7.42 (t, J = 8.7 Hz, 2H), 7.33 (t, J = 6.4 Hz, 1H), 7.21 (t, J = 7.0 Hz, 1H), 7.12
(t, J = 6.6 Hz, 1H), 7.09 – 6.99 (m, 2H), 6.63 (d, J = 8.6 Hz, 1H), 6.53 (t, J = 7.5 Hz, 1H).
MS(MALDI-TOF), m/z: calcd for C36H18F7IrN4O2, 863.76 [M]; found 862.75 [M-1].
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Ir(F3,4,6ppy)2CF3POP (yield: 30%): M.p.: 291 oC. 1H NMR (500 MHz, DMSO-d6) δ 8.64 (d, J
= 5.6 Hz, 1H), 8.45 (d, J = 5.7 Hz, 1H), 8.18 (dd, J = 21.9, 8.4 Hz, 2H), 8.04 (d, J = 8.2 Hz, 2H),
7.93 (t, J = 8.9 Hz, 4H), 7.85 (d, J = 8.0 Hz, 1H), 7.33 (t, J = 6.5 Hz, 1H), 7.22 (t, J = 7.3 Hz, 1H),
7.12 (t, J = 6.6 Hz, 1H), 7.06 (td, J = 11.0, 6.0 Hz, 2H), 6.64 (d, J = 8.7 Hz, 1H), 6.54 (t, J = 7.4
Hz, 1H). MS(MALDI-TOF), m/z: calcd for C37H18F9IrN4O2, 913.76 [M]; found 912.70[M-1].
3. Results and Discussion
3.1 Crystallography and thermal stability.
The single crystals suitable for X-ray diffraction study of two iridium phosphors,
Ir(F3,4,6ppy)2POP and Ir(F3,4,6ppy)2FPOP, were obtained by controlled sublimation under high
vacuum atmosphere. The ORTEP diagrams of these two complexes are depicted in Fig. 1. The
corresponding crystallographic data were collected in Table 1 and the selected bond lengths and
angles were listed in Table S1. The central iridium atom is chelated by two anionic C^N main
ligands and one monoanionic bidentate N^O ancillary ligand. The coordination sphere presents a
distorted octahedral geometry, with the cis-C,C and trans-N,N in chelating disposition.
Concerning the designed ancillary ligands with three rings, the phenol ring and the oxadiazole ring
chelate with the iridium center via the O atom and a N atom, respectively, forming a relatively
rigid hexatomic coordination ring. The left free phenyl and 4-fluorophenyl ring present a rather
obvious dihedral angel towards the other two rings, in order to minimizing the steric effect. The
Ir-C and Ir-N bonds between iridium center and C^N main ligands are about 2.0 Å and the Ir-O1
bonds (2.1 Å) are the longest among all the coordination bonds.
The melting points of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP are
o
o
322, 326 and 291 C, respectively. And they are thermally stable up to 406, 408 and 393 C (5
wt% loss, Fig. 2), respectively, indicating that the compounds are potential emitting materials used
for fabricating efficient and stable OLEDs with long lifetime.
3.2 Photophysical and electrochemical property.
The UV-vis absorption spectra of the Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP complexes in degassed CH2Cl2 at 5 × 10-5 mol.L-1 are shown in Fig. 3(a),
and the electronic absorption data are listed in Table 2. The complexes showed almost the similar
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absorption spectra in the range of 230-450 nm. The intense bands at high energy (250-350 nm) are
1
assign to spin-allowed ligand-centered LC (π-π*) transitions of F3,4,6ppy and 1,3,4-oxadiazole
derivative ligands. The absorption bands observed at lower energies extending into the spectral
region (350-450 nm) can be assigned to spin-allowed (π-π*) ligands along with contribution from
metal-to ligand charge transfer MLCT transition. These MLCT bands are attributed to an effective
mixing of charge-transfer transitions with higher lying spin-allowed transition on the ligands
through strong spin-orbit coupling of iridium atom [45].
Photoluminescence measurements were conducted in deaerated CH2Cl2 solutions at room
temperature (Fig. 3(b), Table 2). The complexes Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP emit intense green phosphorescence with peaks maxima at 504, 503 and
521 nm, respectively, which are belong to the mixed MLCT and CT transitions. The major
emission peaks of Ir(F3,4,6ppy)2POP and Ir(F3,4,6ppy)2FPOP are similar, indicating that the
introducing of F atom in the ancillary ligand doesn’t affect the excited state of the complex greatly.
However, when the CF3 moiety was introduced to the ancillary ligand, the maximum peak of
Ir(F3,4,6ppy)2CF3POP red-shifts about 20 nm, suggesting the strong electron withdrawing unit
CF3 in the ancillary ligand has effect on the charge transfer process. Moreover, the quantum yields
of the three complexes Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP in
solution are 18%, 12% and 13%, respectively. In addition, the lifetimes of complexes
Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP are in the range of
microseconds of 1.83, 2.14 and 1.91 µs in CH2Cl2 solution (Table 2, Fig. S1), which are indicative
of the phosphorescent origin for the excited states in each case.
The redox properties and HOMO/LUMO energy levels of the dopants are relative to the charge
transport ability and the OLED structure. To calculate the HOMO/LUMO energy levels of the
present heteroleptic iridium complexes, the electrochemical behaviours of them in degassed
CH2Cl2 solution were investigated by using cyclic voltammetry (Fig. 4). The HOMO levels were
calculated from the oxidation potentials and the LUMO levels were obtained from the HOMO
data and band gap got from UV-vis spectra. The cyclic voltammograms of the complexes
Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP in the positive range show
strong oxidation peaks, while the reduction peaks are not obvious, demonstrating that the redox
process of the complexes is not reversible completely, which is also observed in related Ir(III)
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complexes containing oxadiazole units [33-39]. From the Fig. 4 and Table 2 it is observed that the
differences of the oxidation peaks and HOMO/LUMO values of the three complexes are
neglectable suggesting that the introduction of the F/CF3 units has little effects on the the
HOMO/LUMO levels.
3.3 OLEDs characterization
Because HOMO/LUMO levels of the three complexes are similar and the complex
Ir(F3,4,6ppy)2POP shows the highest PL efficiency, the OLEDs using Ir(F3,4,6ppy)2POP as the
emitter were fabricated with the structure of ITO / TAPC (60 nm) / Ir(F3,4,6ppy)2POP (6, 8, 10
wt%) : SimCP2 (30 nm) / TPBi (90 nm) / LiF (1 nm) / Al (100 nm) named G1 – G3, respectively.
TAPC and SimCP2 were employed as the hole-transporting layer and bipolar host material,
respectively. TPBi was used as an electron-transporting and hole-blocking layer. The energy
diagrams of the devices and the molecular structures of the materials used are shown in Fig. 5.
Obviously, the HOMO/LUMO levels of Ir(F3,4,6ppy)2POP are all within that of host material
SimCP2. So, a good carrier trapping is expected in these devices, which is the dominated EL
mechanism. Furthermore, holes and electrons will be well confined within the doped
light-emitting layer.
The current efficiency (ηc) versus current density (J), power efficiency (ηp) versus J, J versus
driving voltage (V), and ηc versus luminance (L) characteristics of devices are shown in Fig. 6.
The key EL data are summarized in Table 3. All devices emit green light with the EL emission
peaks at 504 nm for G1 – G3, closing to the PL spectrum of the Ir(F3,4,6ppy)2POP complex in
CH2Cl2 solution. The EL emission spectrum shapes are almost invariant of the current density and
also do not show any concentration dependent, indicating that the EL emission of the device
originates from the triplet excited states of the phosphor and the energy can be transferred from
SimCP2 to the emitter [33].
The OLEDs using Ir(F3,4,6ppy)2POP as the emitter exhibit encouraging EL performances,
displaying low turn-on voltages of 3.4 - 3.5 V and high efficiency. The maximum ηc,max andηp,max
of G1 – G3 are in the range of 55.13 – 61.49 cd A-1 and 39.52 – 46.03 lm W-1, respectively.
According to the respective device, the ηc,max and ηp,max of 57.27 cd A-1 and 41.82 lm W-1 were
obtained in G1 (6 wt%) at 4.3 V. The device G2 with 8 wt% dopant concentration showed the best
9

ACCEPTED MANUSCRIPT
performances with a ηc,max of 61.49 cd A-1 (4.9 V) and a ηp,max of 46.03 lm W-1 (4.1 V). For the
device G3, it showed the comparative values to that of G1 with a ηc, max of 55.13 cd A-1 at 4.7 V
and a ηp, max of 39.52 lm W-1 at 3.9 V. The results suggested that for this device structure, the
optimized doped concentration is 8 wt%.
Gratifyingly, from the Fig. 6 and Table 3 it is also can be observed that the efficiency roll-off in
these devices is low, which is helpful to keep high efficiency at relatively high current density and
high luminance. For device G1, the roll-off ratios of efficiency from the peak value (57.27 cd A-1)
to that at the practical brightness of 100 cd m-2 (49.59 cd A-1) and from 100 cd m-2 to the
benchmark brightness of 1000 cd m-2 (46.63 cd A-1) are 13.4% and 6.0%, respectively. For device
G2, efficiency roll-off ratios from the peak value (61.49 cd A-1) to that at the brightness of 100 cd
m-2 (59.81 cd A-1) and from the brightness of 100 cd m-2 to 1000 cd m-2 (54.56 cd A-1) are as low
as 2.7% and 8.8%, respectively. For device G3, the efficiency roll-off ratios from the peak value
(55.13 cd A-1) to that at the brightness of 100 cd m-2 (51.21 cd A-1) and from the brightness of 100
cd m-2 to 1000 cd m-2 (43.73 cd A-1) are as low as 2.0% and 2.3%, respectively.
The good EL properties should be partly due to the application of
2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol as the ancillary ligand and 2,4,5-trifluorophenylpyridine
as cyclometalated ligand. The dopant acts as the hole and electron traps to retard the motion of
both types of carriers. The lower LUMO levels of the dopants are particularly important for the
reason that the hole mobility of the TAPC is high than the electron mobility of the TPBi in OLEDs
[46], the exciton accumulation is expected in hole blocking layer (TPBi) near the interface of
emitting layer (Ir(F3,4,6ppy)2POP : SimCP2) / TPBi) due to the high energy barrier between TPBi
and SimCP2 [47]. The accumulation of exciton will cause the serious triplet-triplet annihilation
(TTA), triplet-polaron annihilation (TPA) of the iridium complexes, and high efficiency roll-off
consequently. In our case, the 1,3,4-oxadiazole units, as part of the ligand framework, will benefit
the electron transport properties. The good electron mobility of the phosphorescent emitter would
facilitate the injection and transport of electrons, which broaden the recombination zone, balance
the distribution of holes and electrons, and reduce leakage current, particularly for the high doping
concentration, leading to the suppressed the TTA, TPA effects [48,49], improved recombination
probability and high device efficiency, low efficiency roll-off.
10

ACCEPTED MANUSCRIPT
4. Conclusion
In
2,4,5-trifluorophenylpyridine as a monoanionic cyclometalated ligand, 2-(5-phenyl-1,3,4-
oxadiazol-2-yl)phenol, 2-(5-(4-fluorophenyl)-1,3,4-oxadiazol-2-yl)phenol and 2-(5-(4-
conclusion,
three
new
heteroleptic
iridium(III)
complexes
using
trifluoromethylphenyl)-1,3,4-oxadiazol-2-yl)phenol as ancillary ligands were synthesized.
Efficient EL devices ITO / TAPC (60 nm) / Ir(F3,4,6ppy)2POP (6, 8, 10 wt%) : SimCP2 (30 nm) /
TPBi (90 nm) / LiF (1 nm) / Al (100 nm) were fabricated. A ηp,max of 46.03 lm W-1 and a ηc, max of
61.49 cd A-1 are achieved at 8 wt% dopant concentration. Furthermore, the efficiency roll-off
ratios from the peak values to that at the practical luminance are low, which is helpful to keep high
efficiency at relatively high current density and high luminance. The study demonstrates that the
iridium complexes with 1,3,4-oxadiazole containing ancillary ligand are potential emitters for
efficient OLEDs.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant 21371093),
the Major State Basic Research Development Program (2011CB808704, 2013CB922101) and the
Natural Science Foundation of Jiangsu Province (BK20130054).
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13

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Table captions
Table 1 Crystallographic data for Ir(F3,4,6ppy)2POP and Ir(F3,4,6ppy)2FPOP.
Table 2. The photophysical properties of complexes Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP.
Table 3. EL performances of devices G1 - G3.

ACCEPTED MANUSCRIPT
Table 1 Crystallographic data for Ir(F3,4,6ppy)2POP and Ir(F3,4,6ppy)2FPOP.
Ir(F3,4,6ppy)2POP
Ir(F3,4,6ppy)2FPOP
Formula
FW
C36H19F6IrN4O2
C36H18F7IrN4O2
845.77
296(2)
0.71073
863.76
296(2)
0.71073
T (K)
Wavelength (Å)
Cryst syst
Monoclinic
Monoclinic
Space group
P21/n
P21/c
a (Å)
11.2437(5)
15.6194(7)
17.6047(8)
90.00
11.3459(4)
18.0095(7)
15.7264(6)
90.00
b (Å)
c (Å)
a (deg)
β (deg)
1100.1100(10)
90.00
107.3820(10)
90.00
γ (deg)
V (Å3)
3043.7(2)
4
3066.7(2)
4
Z
ρcalcd (g/cm3)
ꢀ (Mo Kα) (mm-1)
F (000)
1.846
1.871
4.465
4.439
1640
1672
Range of transm factors (deg)
Reflns collected
Unique
1.76 -28.42
21911
1.77 -28.32
21789
7629
7636
GOF on F2
1.009
1.024
a
R1 ,wR2b (I>2s(I))
0.0238, 0.0546
0.0317, 0.0578
934801
0.0293, 0.0654
0.0404, 0.0678
934802
a
R1 ,wR2b (all data)
CCDC NO.
a
2
2
R1 = Σ||Fo| − |Fc||/ΣFo|. wR2b = [Σw(Fo − Fc2)2/Σw(Fo )]1/2

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Table 2. The photophysical properties of complexes Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP.
a)
b)
b)
Φc)
[%]
18
τ b)
Complex
Tm/Td
[oC]
HOMO/LUMO d)
[eV]
λAbs
λem, RT
[nm]
[nm]
[µs]
1.83
2.14
1.92
322/406
326/408
291/393
252/394
253/393
253/307
504/530
503/530
521
-5.59/-2.95
-5.58/-2.92
-5.58/-2.91
Ir(F3,4,6ppy)2POP
Ir(F3,4,6ppy)2FPOP
12
13
Ir(F3,4,6ppy)2CF3POP
a)
b)
c)
Tm: melting point; Td: decomposed temperature; Measured in CH2Cl2 solution at room temperature;
Φ
= 0.4);；
d )
Measured in degassed CH2Cl2 solution at room temperatureusing fac-Ir(ppy)3 as the standard sample (
HOMO/LUMO calculated based on the cyclovoltammety (CV) diagram using ferrocene as the internal standard
and UV-vis spectra in dichloromethane. HOMO (eV) = -(Eox-E1/2,Fc)-4.8, LUMO (eV) = HOMO + Ebandgap
.

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Table 3. EL performances of devices G1 - G3.
a)
Current efficiency roll-off (%)
100–1000 cd/m2 g)
6.0
Vturn-on
(V)
ηp, max b) (lm W-1)
V
ηc, max (cd A-1)
Device
ηc, max –100 cd/m2 f)
13.4
ηc, max c), V
100d)
49.59
59.81
51.21
1000 e)
46.63
54.56
43.73
G1
G2
G3
3.5
41.82, 4.3
46.03, 4.1
39.52, 3.9
57.27, 4.3
61.49, 4.9
55.13, 4.7
2.7
2.0
8.8
2.3
3.4
3.4
a)
b)
c)
Vturn-on: turn-on voltage recorded at a brightness of 1 cd m-2; ηp, max: maximum power efficiency; ηc, max
:
d)
h)
f)
maximum current efficiency; current efficiency at 100 cd m-2; current efficiency at 1000 cd m-2; Current
efficiency roll-off (%) from peak value to 100 cd m-2; g) Current efficiency roll-off (%) from 100 cd m-2 to 1000 cd
m-2.

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Scheme and figure captions
Scheme 1. The synthetic route of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP.
Fig. 1. ORTEP diagrams of Ir(F3,4,6ppy)2POP (left) and Ir(F3,4,6ppy)2FPOP (right) with thermal
ellipsoids shown at 40% probability level. Labels are added on the atoms bonded to iridium
center. Hydrogen atoms are omitted for clarity.
Fig. 2. The TG curves of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP
complexes.
Fig. 3. UV-vis absorption (a) and emission (b) spectra of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP
and Ir(F3,4,6ppy)2CF3POP complexes in degassed dichloromethane (5 × 10-5 mol.L-1) at room
temperature.
Fig. 4. Cyclic voltammograms of complexes in the positive range.
Fig. 5. Energy level diagrams of HOMO and LUMO levels (relative to vacuum level) and
molecular structures for materials investigated.
Fig. 6. Characteristics of devices with configuration ITO / TAPC (60 nm) / Ir(F3,4,6ppy)2POP :
SimCP2 (30 nm) / TPBi (90 nm) / LiF (1 nm) / Al (100 nm): (a) current efficiency (
function of current density (J), (b) power efficiency ( p) as a function of J, and (c) J versus
driving voltage (V), (d) c versus luminance (L).
ηc) as a
η
η
1

ACCEPTED MANUSCRIPT
F
F
F
F
F
F
F
F
+
IrCl3.3H2O
Pd(PPh3)4/K2CO3
Cl
Cl
F
F
F
N
Ir
Ir
EtOCH2CH2OH/H2O
THF/H2O
F
N
N
N
R
Br
B(OH)2
O
2
2
O
O
O
POCl3
Et3N
NHNH2
R
Cl
NHNH
+
CHCl3
OCH3
H3CO
R
N N
O
N N
O
N N
O
BBr3
KOH
R
R
CH2Cl2
HO
R=H: HPOP KO
R=F: HFPOP
H3CO
R
R=CF3: HCF3POP
F
F
F
F
F
F
R = H: Ir(F3,4,6ppy)2POP
R = F: Ir(F3,4,6ppy)2FPOP
R = CF3: Ir(F3,4,6ppy)2CF3POP
N
N
O
Cl
Cl
KRPOP
F
F
F
Ir
Ir
Ir
EtOCH2CH2OH
N
N
O
N
2
2
2
Scheme 1. The synthetic route of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP.
2

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Fig. 1. ORTEP diagrams of Ir(F3,4,6ppy)2POP (left) and Ir(F3,4,6ppy)2FPOP (right) with thermal
ellipsoids shown at 40% probability level. Labels are added on the atoms bonded to iridium
center. Hydrogen atoms are omitted for clarity.
3

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100
90
80
70
60
50
40
30
20
10
0
Ir(F3,4,6ppy)2POP
Ir(F3,4,6ppy)2FPOP
Ir(F3,4,6ppy)2CF3POP
100
200
300
400
500
600
Temperature (oC)
Fig. 2. The TG curves of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP
complexes.
4

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Ir(F
Ir(F
Ir(F
ppy) POP
2
Ir
(
F
F
F
ppy
)
POP
(a)
(b)
3,4,6
3,4,6
3,4,6
2
2
2
3,4,6
3,4,6
3,4,6
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Ir
Ir
(
(
ppy
ppy
)
)
FPOP
ppy) FPOP
2
CF POP
3
ppy) CF POP
2
3
250
300
350
400
450
500
550
600
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. UV-vis absorption (a) and emission (b) spectra of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP
and Ir(F3,4,6ppy)2CF3POP complexes in degassed dichloromethane (5 × 10-5 mol.L-1) at room
temperature.
5

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10
8
Ir(F
Ir(F
Ir(F
ppy) POP
2
3,4,6
3,4,6
3,4,6
ppy) FPOP
2
ppy) CF POP
2
3
6
4
2
0
-2
0.5
0.6
0.7
0.8
0.9
E (V)
1.0
1.1
1.2
1.3
Fig. 4. Cyclic voltammograms of complexes in the positive range.
6

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2.00
2.56
2.70
2.95
LiF/Al
ITO
N
N
5.50
F
5.59
6.12
TAPC
6.20
F
N
N
N
N
N
N
N
N
O
Si
F
Ir
2
N
N
N
O
N N
Ir(F3,4,6ppy)2POP
SimCP2
TPBi
Fig. 5. Energy level diagrams of HOMO and LUMO levels (relative to vacuum level) and
molecular structures for materials investigated.
7

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100
(b)
100
10
1
(a)
wt 6%
wt 8%
wt 10%
wt 6%
wt 8%
wt 10%
10
1
0.1
0.1
0.1
0.1
1
10
100
1
10
100
Current Density (mA/cm2)
Current Density (mA/cm2)
100
10
1
1000
100
10
(d)
(c)
wt 6%
wt 8%
wt 10%
6 wt%
8 wt%
10 wt%
1
0.1
0.01
1E-3
0.1
0
2
4
6
8
10
12
14
100
1000
10000
Luminance (cd/m2)
Voltage (V)
Fig. 6. Characteristics of devices with configuration ITO / TAPC (60 nm) / Ir(F3,4,6ppy)2POP :
SimCP2 (30 nm) / TPBi (90 nm) / LiF (1 nm) / Al (100 nm): (a) current efficiency ( c) as a
function of current density (J), (b) power efficiency ( p) as a function of J, and (c) J versus
driving voltage (V), (d) c versus luminance (L).
η
η
η
8

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Research highlights
•
•
•
We synthesize Ir(III) complexes with 1,3,4- oxadiazol derivatives.
The complexes own green emission peaks with various intensities.
One OLED shows a peak current efficiency of 61.49 cd A-1.

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Supporting Information
Efficient organic light-emitting diodes with low efficiency roll-off using
iridium emitter with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol as ancillary
ligand
Yi-Ming Jin, Cheng-Cheng Wang, Li-Sha Xue, Tian-Yi Li, Song Zhang, Xuan Liu, Xiao Liang,
You-Xuan Zheng,* Jing-Lin Zuo
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Center for
Molecular Organic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, P. R. China, e-mail: yxzheng@nju.edu.cn
Yi-Ming Jin and Cheng-Cheng Wang are co-first author
Table S1. The selected bond lengths (Å) and angles (°) of Ir(F3,4,6ppy)2POP and
Ir(F3,4,6ppy)2FPOP complexes.
Fig. S1. The lifetime decay curves of Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and
Ir(F3,4,6ppy)2CF3POP in CH2Cl2 solution.