Efficient organic light-emitting diodes with low efficiency roll-off at high brightness using iridium emitters based on 2-(4-trifluoromethyl-6-fluoro phenyl)pyridine and tetraphenylimidodiphosphinate derivatives

Article abstract of DOI:10.1016/j.dyepig.2014.01.033

Using 2-(4-trifluoromethyl-6-fluorophenyl)pyridine (tfmfppy) as the cyclometalated ligand and tetraphenylimidodiphosphinate (tpip) derivatives as ancillary ligands, three iridium complexes (1: Ir(tfmfppy)₂tpip; 2: Ir(tfmfppy)₂ftpip, ftpip = tetra(4-fluorophenyl)imidodiphosphinate; 3: Ir(tfmfppy)₂tfmtpip, tfmtpip = tetra(4-trifluoromethylphenyl) imidodiphosphinate) showing phosphorescence at 514, 513 and 508 nm in CH ₂Cl₂ were synthesized, respectively. By using these complexes as emitters, the organic light emitting diodes with the concise configuration of indium tin oxides/1,1-bis(4-(di-p-tolyl-amino)phenyl) cyclohexane (60 nm)/1 or 2 or 3: bis[3,5-di(9H-carbazol-9-yl)phenyl]di-phenyl silane (8 wt%, 30 nm)/1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (90 nm)/lithium fluoride (1 nm)/aluminium (100 nm) showed bright green light with maximum luminance higher than 40 000 cd m^-2. The maximum current efficiency (η_c) values for 1, 2 and 3 based devices (GIr1, GIr2 and GIr3) were 77.49, 66.57, and 53.26 cd A^-1 at relatively high brightness of 8025, 5286 and 5169 cd m^-2, respectively. In addition, the efficiency roll-off ratios from the peak efficiency to the brightness of 10 000 cd m^-2 are less than 10% for all the devices. We believe that the good electron mobilities of the phosphorescent emitters lead to the high devices efficiency and low efficiency roll-off. The results suggest that these complexes have potential application in organic light emitting diodes.

Full text of DOI:10.1016/j.dyepig.2014.01.033

Dyes and Pigments 105 (2014) 105e113
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Efficient organic light-emitting diodes with low efficiency roll-off at
high brightness using iridium emitters based on 2-(4-trifluoromethyl-
6-fluoro phenyl)pyridine and tetraphenylimidodiphosphinate
derivatives
,
Ming-Yu Teng a b, Song Zhang a, Yi-Ming Jin a, Tian-Yi Li a, Xuan Liu a, Qiu-Lei Xu a,
,
a
a
Chen Lin a, You-Xuan Zheng a , Leyong Wang , Jing-Lin Zuo
*
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, PR China
b Faculty of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Using 2-(4-trifluoromethyl-6-fluorophenyl)pyridine (tfmfppy) as the cyclometalated ligand and tetra-
phenylimidodiphosphinate (tpip) derivatives as ancillary ligands, three iridium complexes (1: Ir(tfmfp-
py)2tpip; 2: Ir(tfmfppy)2ftpip, ftpip ¼ tetra(4-fluorophenyl)imidodiphosphinate; 3: Ir(tfmfppy)2tfmtpip,
tfmtpip ¼ tetra(4-trifluoromethylphenyl)imidodiphosphinate) showing phosphorescence at 514, 513 and
508 nm in CH2Cl2 were synthesized, respectively. By using these complexes as emitters, the organic light
emitting diodes with the concise configuration of indium tin oxides/1,1-bis(4-(di-p-tolyl-amino)phenyl)
cyclohexane (60 nm)/1 or 2 or 3: bis[3,5-di(9H-carbazol-9-yl)phenyl]di-phenyl silane (8 wt%, 30 nm)/
1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (90 nm)/lithium fluoride (1 nm)/aluminium
(100 nm) showed bright green light with maximum luminance higher than 40 000 cd mꢀ2. The
maximum current efficiency (hc) values for 1, 2 and 3 based devices (GIr1, GIr2 and GIr3) were 77.49,
66.57, and 53.26 cd Aꢀ1 at relatively high brightness of 8025, 5286 and 5169 cd mꢀ2, respectively. In
addition, the efficiency roll-off ratios from the peak efficiency to the brightness of 10 000 cd mꢀ2 are less
than 10% for all the devices. We believe that the good electron mobilities of the phosphorescent emitters
lead to the high devices efficiency and low efficiency roll-off. The results suggest that these complexes
have potential application in organic light emitting diodes.
Received 10 December 2013
Received in revised form
25 January 2014
Accepted 31 January 2014
Available online 8 February 2014
Keywords:
Iridium complex
2-(4-Trifluoromethyl-6-fluorophenyl)
pyridine
Tetraphenylimido-diphosphinate
Organic light emitting diode
Efficiency;
Efficiency roll-off
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
tuning the energy levels of the ligands through substituent effects,
which leads to a wide flexible emission color range.
Phosphorescent iridium complexes play an important part in
efficient organic light emitting diodes (OLEDs) due to their high
quantum efficiency and short lifetime of triplet excited states [1e
16]. The strong spin-orbit coupling (SOC) introduced by the cen-
tral heavy atom can promote the triplet to singlet radiative tran-
sition, resulting 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
[17], the energy level of the excited state can be controlled by
In pursuit of efficient Ir(III) complexes, many heteroleptic
complexes, Ir(C^N)2LX, have been developed, where C^N is a mono-
anionic cyclometalated ligand (e.g., 2-phenylpyridine (ppy), 2-(4-
trifluoromethylphenyl)pyridine (tfmppy)) and LX stands for a
bidentate ancillary ligand (e.g., acetylacetonate (acac), picolinate
(pic)) [18,19]. According to the density functional theory calcula-
tions, 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 ones.
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
* Corresponding author.
E-mail address: yxzheng@nju.edu.cn (Y.-X. Zheng).
http://dx.doi.org/10.1016/j.dyepig.2014.01.033
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

106
M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
iridium complexes can be tuned trough functional substitutes on
both cyclometalated and ancillary ligands.
2. Experimental section
In our previous work, we have reported some high efficient
OLEDs based on bis-cyclometalated iridium complexes with
tetraphenylimidodiphosphinate (tpip) derivatives as the ancil-
lary ligands [20e24]. The high efficiencies can be explained by
more balanced injection and transport of electrons and holes
in(to) the emitting layers (EML) due to the good electron
mobility of Ir(III) complexes by using the tpip derivatives as
ancillary ligands [25]. Therefore, further investigation of the
substituted Ir(C^N)2tpip complexes with new tpip derivatives is
possible to prepare more efficient organic light emitting mate-
rials and devices.
Fluorination can enhance the electron mobility and result in a
better balance of charge injection and transfer, lower vibrational
frequency of CeF bond can reduce the rate of radiationless deac-
tivation and the bulky trifluoromethyl substituents can affect the
molecular packing and suppress the self-quenching behavior [26].
In this article, a modification was made to ppy by introducing of F/
CF3 trifluoromethyl groups into phenyl ring at the ortho- and para-
position, as well as each phenyl ring of tpip at the para-position.
Thus, herein, we report the syntheses, photoluminescence (PL) and
electroluminescence (EL) of three novel bis-cyclometalated iridium
complexes of Ir(tfmfppy)2tpip (1, tfmfppy ¼ 2-(4-trifluoromethyl-
6-fluorophenyl)pyridine), Ir(tfmfppy)2ftpip (2, ftpip ¼ tetra(4-
fluorophenyl)imidodiphosphinate) and Ir(tfmfppy)2tfmtpip (3,
2.1. General information
Commercially available reagents were used without further
purification unless otherwise stated. 1H and 31P NMR spectra were
recorded using a Bruker spectrometer at 500 MHz using TMS as an
internal standard. Chemical shifts were recorded by ppm. Multi-
plicity was denoted by s (singlet), d (doublet), t (triplet), dd
(doublet of doublet), m (multiplet). Coupling constants (J) were in
hertz (Hz). Mass spectra (MS) were obtained with ESI (Electrospray
Ionization, Thermo Fisher Scientific) or MALDI-TOF (Matrix-Assis-
ted Laser Desorption-Ionization Time of Flight, Bruker Daltonic
Inc.). Absorption and PL spectra were recorded with a Shimazu UV-
3600 and a Hitachi F-4600 spectrometer, respectively. PL lifetime
measurements were performed with an Edinburgh FLS-920 spec-
trometer. The luminescence quantum efficiencies were calculated
by comparison of the emission intensities (integrated areas) of a
standard aerated aqueous solution of [Ru(bpy)3]Cl2 and the iridium
complexes in CH2Cl2 solutions according to the equ. (1) [27].
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Aunk
hunk
hstd
Funk
¼
Fstd
(1)
where Funk and Fstd are the luminescence quantum yields of the
iridium complexes and [Ru(bpy)3]Cl2 solutions, respectively. The
Iunk and Istd are the integrated emission intensities of the iridium
complexes and [Ru(bpy)3]Cl2 solutions, respectively. The Aunk and
tfmtpip
(Scheme 1).
¼
tetra(4-trifluoromethylphenyl)imidodiphosphinate)
CF3
CF3
CF3
CF3
Br
Cl
Cl
F
F
N
ii
F
+
i
Ir
Ir
F
N
N
N
B(OH)2
2
2
R
R
R
R
R
R
P
P
O
O
O
v
NH
iii
iv
NH
NK
+
Si
Si
P
P
O
P
Cl
R
R
R
R
R
R
R
R
CF3
CF3
CF3
P
O
O
P
O
vi
Cl
F
F
F
N
NK
Ir
+
Ir
Ir
P
P
O
Cl
N
N
N
2
2
2
R
R
R
R
1: R = H; 2: R = F; 3: R = CF3
Scheme 1. Synthetic routes of ligands and complexes. (i) Pd(PPh3)4, K2CO3,THF/H2O, 80 ꢁC, 24 h; (ii) IrCl3.3H2O, 2-EtOCH2CH2OH/H2O, reflux, 24 h; (iii) Anhydrous toluene, 105 ꢁC,
6 h; (iv) H2O2, THF; (v) KOH, methanol; (vi) 2-EtOCH2CH2OH, 120 ꢁC, 24 h.

M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
107
Astd are the absorbance of the iridium complexes and [Ru(bpy)3]Cl2
solutions at their excitation wavelengths (A < 0.1), respectively. The
hunk and hstd terms represent the refractive indices of the corre-
sponding solvents (pure solvents were assumed). The Fstd of the
aerated aqueous solution of [Ru(bpy)3]Cl2 has been revalued to be
2.8% [28].
Cyclic voltammetry (CV) was performed on an IM6ex (Zahner)
electrochemistry working station using platinum electrode, Ag/
AgNO3 in saturated KCl (aq.) and a platinum wire as working
electrode, reference electrode and counter electrode, respectively.
All CV measurements were carried out at a scan rate of 100 mV sꢀ1
[20e24,32]. Pure iridium complexes were obtained from the vac-
uum sublimation after silica column chromatography.
2.4.1. Synthesis of 2-(2-fluoro-4-(trifluoromethyl)phenyl) pyridine
(tfmfppy)
2-Bromopyridine (2.39 mL, 25.0 mmol), 2-(2-fluoro-4-
trifluoromethyl)-phenylboronic acid (5.77 g, 30.0 mmol), palla-
dium(II) acetate (0.068 g, 0.303 mmol) and K2CO3 (5.53 g, 40 mmol)
were refluxed for 24 h in 60 mL solution of THF : water (3:2, v/v).
After that, the cooled solution was poured into water, extracted
with CH2Cl2 (50 mL ꢂ 3 times) and then dried over anhydrous
magnesium sulfate. Finally, silica column purification (petroleum
ether:dichloromethane ¼ 10:1 as eluant) gave yellow color liquid in
in anhydrous CH2Cl2 containing 0.1
M tetrabutylammonium
perchlorate (TBAP) as a supporting electrolyte, purging with argon
prior to conduct the experiment. Each oxidation potential was
calibrate with ferrocene as a reference.
39% yield (2.36 g, 9.79 mmol). 1H NMR (500 MHz, CDCl3)
d 8.73e
8.52 (m, 1H), 8.01e7.82 (m, 2H), 7.75 (d, J ¼ 8.0 Hz, 1H), 7.71e7.48
(m, 2H), 7.46e7.22 (m, 1H). MS (MALDI-TOF) Calcd: 241.0515 [Mþ].
Found: m/z 241.3121 [Mþ].
2.2. X-ray crystallography
Diffraction data for 2 were collected on a Bruker SMART CCD
2.4.2. Synthesis of [(tfmfppy)2Ir(m-Cl)]2
diffractometer
using
monochromated
Mo-K
a
radiation
IrCl3,2H2O (0.80 g, 2.50 mmol) and tfmfppy (2.41 g,
10.00 mmol) in 30 mL 2-EtOCH2CH2OH: H2O (3:1, v/v) were
refluxed at 140 ꢁC for 24 h. After cooling down, yellow precipitate
was filtered and washed with ethanol, acetone, respectively. The
product was dried under vacuum with a 43% yield (0.76 g,
0.54 mmol). MS (MALDI-TOF) Calcd: m/z 1416.047 [Mþ]. Found: m/z
1416.542 [Mþ].
ꢀ
(
l
¼ 0.71073 A) at room temperature. Cell parameters were
retrieved using SMART software, and SAINT [29] program was used
to reduce the highly redundant data sets. Lorentz and polarization
effects were corrected. Absorption corrections were performed
with SADABS [30] supplied by Bruker. Data were collected using a
narrow-frame method with a scan width of 0.30ꢁ in
u and an
exposure time of 4 s frameꢀ1
.
The molecular structure was solved by direct methods and
refined by full-matrix least squares on F2 using the program
SHELXL-97 [31]. The positions of the metal atoms and their conjoint
coordination atoms were located from direct methods on E-maps;
other non-H atoms were found in alternating difference Fourier
syntheses and least-squares refinement cycles. All non-H atoms
were treated anisotropically. H atoms were fixed in calculated po-
sitions and they were allowed to ride on their parent C atoms and
2.4.3. General synthesis procedure of
tetraphenylimidodiphosphinate acid derivatives (R-Htpip)
A solution of chlorodiphenylphosphine derivatives (5 mmol)
and hexamethyldisilazane (2.5 mmol) in anhydrous toluene
(10 mL) was refluxed for 3 h, after which the byproduct Me3SiCl
was distilled off. The mixture was then cooled in an ice bath and a
solution of H2O2 (0.5 mL, 30 wt% in H2O) in THF (4 mL) was added
dropwise. This solution was added to diethyl ether (15 mL) and the
white precipitate was washed several times with water and
recrystallized from methanol to give the desired ancillary ligands.
Tetraphenylimidodiphosphinate acid (Htpip, 0.45 g, 1.08 mmol,
refined with a uniform value of Uiso
.
2.3. OLEDs fabrication and characterization
yield: 43%). 31P NMR (500 MHz, CDCl3)
d 19.34 ppm (s). MS (ESI)
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 U sqꢀ1. Substrate was cleaned by ultrasonic in
organic solvents followed by ozone treatment for 20 min. The
60 nm hole transport material of 1,1-bis(4-(di-p-tolyl-amino)
phenyl)cyclohexane (TAPC) film was first deposited on the ITO
glass substrate. The phosphor (x wt%) and host (SimCP2, bis(3,5-
di(9H-carbazol-9-yl)phenyl)diphenylsilane) were co-evaporated
to form 30 nm emitting layer from two separate sources. Suc-
Calcd: m/z 416.10 [Mꢀ]. Found: m/z 416.12 [Mꢀ]. Tetra(4-
fluorophenyl)imidodiphosphinate acid (Hftpip, 0.62 g, 1.28 mmol,
yield: 51%). MS (ESI) Calcd: m/z 489.07 [Mꢀ]. Found: m/z 489.10
[Mꢀ]. Tetra(4-trifluoromethylphenyl)imidodiphosphinate acid
(Htfmtpip, 0.63 g, 0.93 mmol, yield: 37%). MS (ESI) Calcd: m/z
689.05 [Mꢀ]. Found: m/z 689.08 [Mꢀ].
2.4.4. General synthesis procedure of Ir(tfmfppy)2R-tpip (1e3)
[(tfmfppy)2Ir(m-Cl)]2 (0.20 mmol) and 2.5 equivalent R-Htpip
cessively,
1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl
(0.5 mmol) and K2CO3 (0.07 g, 0.5 mmol) were dissolved in 5 mL of
2-EtOCH2CH2OH. After degassed, the solution was maintained at
140 ꢁC for 12 h under nitrogen. After cooling, the crude com-
pounds were filtered and dried under vacuum. Further purifica-
tion was taken by gradient sublimation in vacuum to give
products 1e3.
(TPBi, 90 nm) as electron transport material, LiF (1 nm) as elec-
tron injection material, and Al (100 nm) as the cathode were
evaporated. The vacuum was about 8 ꢂ 10ꢀ5 Pa during all ma-
terials deposition. The EL spectra were measured with a Hitachi
F-4600 photoluminescence spectrophotometer. The luminancee
voltage (LeV) characteristics and current efficiency versus cur-
Ir(tfmfppy)2tpip (1, 0.19 g, 0.18 mmol, yield: 44%). 1H NMR
rent density (
J) curves of the devices were measured with
controlled Keithley 2400 source meter with a calibrated silicon
diode in air without device encapsulation.
h
ceJ), power efficiency versus current density (
h
pe
(500 MHz, CDCl3)
d
9.09 (d, J ¼ 5.5 Hz, 2H), 8.18 (d, J ¼ 8.5 Hz, 2H),
a
computer
7.76 (dd, J ¼ 10.5, 8.5 Hz, 4H), 7.54 (t, J ¼ 7.8 Hz, 2H), 7.39e7.31 (m,
10H), 7.18 (t, J ¼ 7.5 Hz, 2H), 7.00 (dt, J ¼ 7.5, 2.5 Hz, 4H), 6.81 (d,
J ¼ 10.5 Hz, 2H), 6.74 (t, J ¼ 7.0 Hz, 2H), 6.02 (s, 2H). MS (MALDI-TOF)
Calcd: m/z 1089.151 [Mþ]. Found: m/z 1090.342 [Mþþ1]. Anal. calcd
for C48H32IrF8N3O2P2: C 52.94, N 3.86, H 2.96. Found: C 52.88, N
3.79, H 3.01.
2.4. Syntheses procedure
Htpip, tfmfppy, [(tfmfppy)2Ir(
m-Cl)]2 and Ir(tfmfppy)2tpip (1),
Ir(tfmfppy)2ftpip (2, 0.25 g, 0.21 mmol, yield: 53%). 1H NMR
Ir(tfmfppy)2ftpip (2), Ir(tfmfppy)2tfmtpip (3) were synthesized
according to previous published literature procedures (Scheme 1)
(500 MHz, CDCl3)
7.75 (td, J ¼ 7.5,5.0 Hz, 4H), 7.64 (t, J ¼ 7.8 Hz, 2H), 7.30e7.25 (m,
d
9.01 (d, J ¼ 6.0 Hz, 2H), 8.23 (d, J ¼ 8.0 Hz, 2H),

108
M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
4H), 7.07 (t, J ¼ 8.5 Hz, 4H), 6.84e6.80 (m, 4H), 6.70 (t, J ¼ 8.5 Hz,
4H), 6.03 (s, 2H). MS (MALDI-TOF) Calcd: m/z 1161.114 for [Mþ].
Found: m/z 669.987 [Mþ-ftpip]. Anal. calcd for C48H28IrF12N3O2P2:
C 49.66, N 3.62, H 2.43. Found: C 49.59, N 3.58, H 2.47.
from the repulsion between fluoro/trifluoromethyl groups located
on the phenyl rings.
3.2. Photophysical property
Ir(tfmfppy)2tfmtpip (3, 0.22 g, 0.16 mmol, yield: 41%). 1H NMR
(500 MHz, CDCl3)
d
8.88 (d, J ¼ 5.5 Hz, 2H), 8.22 (d, J ¼ 8.5 Hz, 2H),
The absorption spectra (CH2Cl2 solution) and emission spectra
(CH2Cl2 solution and solid state) of the three complexes 1e3 are
recorded at room temperature (Fig. 2), and the pertinent data are
summarized in Table 1. Like other 2-phenylpyridine-type hetero-
leptic iridium complexes, 1e3 exhibit two major absorption regions
(Fig. 2(a)). The intense bands in the range before 380 nm can be
7.99e7.96 (m, 4H), 7.70 (d, J ¼ 8.0 Hz, 4H), 7.57 (t, J ¼ 10.5 Hz, 2H),
7.46e7.42 (m, 4H), 7.30e7.28 (m, 4H), 6.87 (d, J ¼ 12.0 Hz, 2H), 6.69
(t, J ¼ 6.5 Hz, 2H), 6.01 (s, 2H). MS (MALDI-TOF) Calcd: m/z 1361.104
for [Mþ]. Found: m/z 636.521 [Mþ-tfmtpip]. Anal. calcd for
C
52H28IrF20N3O2P2: C 45.89, N 3.09, H 2.07. Found: C 45.82, N 3.05,
H 2.10.
assigned to the spin-allowed 1
(1LC) states localized on tfmfppy and tpip derivatives. In the range
of lower energy (390 < < 520 nm), the relatively weak absorption
p-p* transition of the ligand centered
l
3. Results and discussion
bands show subtle variations, which can be assigned to the mixing
of 1MLCT and 3MLCT (metal-to-ligand charge-transfer) states, or
LLCT (ligand-to-ligand charge-transfer) transition through strong
spin-orbit coupling of iridium atom [33], suggesting that the elec-
tronic structures of the frontier orbitals are almost the same in all
the complexes (see Table 1).
The Ir(III) complexes emit efficient and characteristic emission
of phosphorescence from MLCT state both in CH2Cl2 solution and
solid state. The emissions of 1, 2 and 3 show the broad and struc-
tureless emission maxima at 514 nm (x ¼ 0.26, y ¼ 0.64), 513 nm
(x ¼ 0.26, y ¼ 0.63) and 508 nm (x ¼ 0.24, y ¼ 0.60) with the
quantum efficiencies of 11%, 32% and 19%, respectively (Fig. 2(b)).
All the complexes exhibit emissions peaked at 551 nm (x ¼ 0.40,
y ¼ 0.58), 549 nm (x ¼ 0.37, y ¼ 0.60) and 531 nm (x ¼ 0.29,
y ¼ 0.63) in solid state (Fig. 2(c)), respectively, which are substan-
tially red-shifted compared with their corresponding solution
emissions. These low-energy emissions can be ascribed to excimer
3.1. X-ray crystallography
Selected parameters of the single crystal of Ir(tfmfppy)2ftpip (2)
and tables of atomic coordinates are listed in Fig. 1 and collected in
the Table S1, and bond lengths and bond angles are given in the
Table S2 (Supporting Information). Fig. 1 shows the Oak Ridge
Thermal Ellipsoidal plot (ORTEP) diagrams given by X-ray analysis
of the complex 2, which is crystallized in triclinic space group Pı.
The iridium center adopts a distorted coordination geometry with
two C^N cyclometalated ligands and a novel ancillary ligand ftpip
ꢁ
ꢀ
(cis-C,C and trans-N,N disposition). The Ir(1)-O(1) (2.216(5) A),
ꢀ
ꢀ
Ir(1)-O(2) (2.229(5) A), IreN (2.082(6) A) and Ir(1)-C(11) (2.001(8)
ꢀ
ꢀ
A), Ir(1)-C(23) (2.003(3) A) bond lengths are in agreement with
corresponding parameters described for other similarly constituted
complexes. It should be noted that the sterically crowded ligands
result in large deformation with regard to the phenyl rings in the
complex. The deformation is caused by steric hindrance originating
formation arising from
p-p stacking interactions in solid state,
which will lead to a broad and featureless red-shifted luminescence
compared with that in dilute solution [19,34]. The results indicate
that these ancillary ligands do not affect the electronic structures of
the frontier orbitals that engaged in the emission process signifi-
cantly, and the change of the ancillary ligand leads to a really subtle
shift in the emission color.
As we know, the phosphorescence lifetime (sp) is the crucial
factor influencing the rate of tripletetriplet annihilation in the
OLEDs. Long sp of the material usually causes the great triplete
triplet annihilation [35]. The lifetimes of complexes 1, 2, 3 are in the
range of microseconds (1.78e1.95
ms in CH2Cl2 solution and 2.17e
2.79 s in solid state) at room temperature (Table 1, Fig. S1), which
m
are indicative of the phosphorescent origin for the excited states in
each case.
3.3. Electrochemistry
The HOMO and LUMO energy levels are important for the design
of the device structure. To obtain the HOMO/LUMO energy levels of
the Ir(III) complexes 1, 2 and 3, the electrochemical properties were
investigated by cyclic voltammetry at room temperature (Fig. S2
and Table 1). Both quasi-reversible oxidation and reduction pro-
cess were observed in CH2Cl2 solution indicating 1, 2 and 3 possess
both hole- and electron-transporting characteristics. The HOMO
levels of the compounds were calculated from the oxidation po-
tentials using the ferrocene as a reference (4.8 eV) and the LUMO
levels were calculated from the HOMO and bandgap obtained from
UVevis spectra [36e38]. As listed in Table 1, the HOMO/LUMO
levels of 1, 2 and 3 are ꢀ5.66/ꢀ3.26 eV, ꢀ5.44/ꢀ3.08 eV, and ꢀ5.68/
ꢀ3.26 eV, respectively. And the HOMO and LUMO levels are all
lowered by 0.2e0.5 eV compared with that of Ir(tfmppy)tpip
(tfmppy ¼ 2-(4-trifluoromethylphenyl)pyridine) reported in our
previous work [20] due to the introduction of the
Fig. 1. ORTEP diagram of Ir(tfmfppy)2ftpip (2). The thermal ellipsoids were drawn at
the 50% probability level. Crystallographic data (CCDC 923700): Mw ¼ 1127.04, triclinic
ꢀ
space group P-1, a ¼ 12.0840(13), b ¼ 13.5660(11), c ¼ 14.2380(12) A,
a
¼ 101.076(2),
3
ꢀ
ꢁ
b
¼ 92.994(3),
g
¼ 100.549(2) , V ¼ 2218.2(4) A , Z ¼ 1, T ¼ 296(2) K, calculated density
rcalcd ¼ 1.745 g/cm3,
m
(Mo K
a
) ¼ 3.177 mmꢀ1, 8661 reflections measured, of which
6254 were unique (Rint ¼ 0.0556). Final wR ¼ 0.1316 for 6254 observed reflections with
I > 2 sigma(I).

M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
109
Fig. 2. The absorption ((a), 1 ꢂ 10ꢀ5 mol/L) and emission ((b): CH2Cl2 solution, 1 ꢂ 10ꢀ5 mol/L; (c): solid state) spectra of complexes Ir(tfmfppy)2tpip, Ir(tfmfppy)2ftpip and
Ir(tfmfppy)2tfmtpip.
electrowithdrawing fluro- and trifluoromethyl groups in 2-
phenylpyridine and tetraphenylimidodiphosphinate.
data are summarized in Table 2. All of the devices exhibit typical
emission of the Ir(III) complexes 1 (GIr1), 2 (GIr2) and 3 (GIr3) in
CH2Cl2 solution with the maximum peak at 534, 512 and 509 nm,
respectively. The CIE color coordinates are x ¼ 0.36, y ¼ 0.60 for
device GIr1, x ¼ 0.27, y ¼ 0.64 for device GIr2, and x ¼ 0.24,
y ¼ 0.63 for device GIr3, respectively, corresponding to the green
region. But it cannot be ignored that there are little residual
emissions from the host SimCP2 at 381 nm [39] for each device,
which indicates that the energy transfer from the host excited
state to the guest emitters is incomplete upon electrical
excitation.
And all of the devices have low turn-on voltages of 3.5e
3.6 V. It can be observed that the maximum luminance value of
40 000 cd mꢀ2 retained during a driving voltage range, sug-
gesting that they exceed our measurement limitation under
examined conditions. For the Ir(tfmfppy)2tpip based device GIr1,
the maximum current efficiency and power efficiency are
77.5 cd Aꢀ1 (7.6 V) and 37.5 lm Wꢀ1 (6.4 V), respectively. The
driving voltages needed to reach the practical brightness of 100
and 1000 cd mꢀ2 are 4.5 and 5.8 V, respectively. The perfor-
mances of device GIr2 based on Ir(tfmfppy)2ftpip are little
inferior to device GIr1, with the maximum hc and hp of
3.4. OLEDs performances
With the aim of evaluation the EL performances of the novel
Ir(III) emitters, OLEDs were fabricated with the structure of ITO/
TAPC (60 nm)/iridium complexes (8 wt%) : SimCP2 (30 nm)/TPBi
(90 nm)/LiF (1 nm)/Al (100 nm). The energy level diagrams of
HOMO and LUMO levels (relative to vacuum level) and molecular
structures for materials investigated in this work were shown in
Scheme 2. The devices using 1, 2 and 3 emitters were named GIr1,
GIr2 and GIr3, respectively. SimCP2 was used as the host material
due to its bipolar properties which can balance the electronehole
transport [39], and the optimized doping concentrations were
determined to be 8 wt%. By monitoring the EL performances with
increasing the thickness of the layers, the thicknesses of TAPC,
iridium complexes doped SimCP2 and TPBi layers were optimized
to be 60, 30 and 90 nm, respectively.
The EL spectra, current densityecurrent efficiency (Jehc), cur-
rent densityepower efficiency (Jehp) and luminanceevoltage (Le
V) characteristics of each device are shown in Fig. 3. The key EL
Table 1
Physical properties of complexes Ir(tfmfppy)2tpip, Ir(tfmfppy)2ftpip and Ir(tfmfppy)2tfmtpip.
Compounds
labs; CH
nm
lem; CH
nm
2 Cl2
lem, solid nm
Fa
%
sb
m
s
HOMO/LUMOc eV
2 Cl2
Ir(tfmfppy)2tpip
Ir(tfmfppy)2ftpip
Ir(tfmfppy)2tfmtpip
345/408/462
348/409/467
347/410/461
514
513
508
551
549
531
11
32
19
1.78/2.17
1.84/2.78
1.95/2.79
ꢀ5.97/ꢀ3.56
ꢀ5.60/ꢀ3.14
ꢀ5.68/ꢀ3.26
a
F
: emission quantum yield reference to an aerated aqueous solution of [Ru(bpy)3]Cl2
Measured at room temperature in solutions and solid powders.
HOMO/LUMO calculated based on the cyclovoltammety (CV) diagram with using ferrocene as the internal standard and UVeVis spectra in dichloromethane.
(
F
¼ 2.8%) as the standard solution.
b
c

110
M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
2.00
2.56
2.70
3.14
3.26
I
r
3.56
LiF/Al
(
T
t
f
A
m
P
f
C
p
T
p
y
P
N
N
)
B
2
t
i
f
ITO
m
t
p
i
p
5.50
5.60
6.12
5.68
5.97
TAPC
6.20
CF3
R
R
N
N
N
N
N
N
N
P
O
O
Si
F
N
Ir
P
N
N
N
N
2
R
R
SimCP2
TPBi
1: R = H; 2: R = F; 3: R = CF3
Scheme 2. Energy level diagram of HOMO and LUMO levels (relative to vacuum level) for materials investigated in this work and their molecular structures.
66.6 cd Aꢀ1 (7.1 V) and 35.4 lm Wꢀ1 (5.1 V), respectively. The
driving voltages needed to reach the brightness of 100 and
1000 cd mꢀ2 are 4.4 and 5.5 V, respectively. The efficiencies of
Ir(tfmfppy)2tfmtpip-based OLED device GIr3 are furthermore
degraded to a maximum hc and hp of 53.3 cd Aꢀ1 (7.5 V) and
25.1 lm Wꢀ1 (6.6 V), respectively. The driving voltages needed to
reach the brightness of 100 and 1000 cd mꢀ2 are 4.7 and 6.0 V,
respectively.
GIr1
GIr2
GIr3
(a)
1.0
4x104
3x104
2x104
(b)
GIr1
GIr2
GIr3
0.8
0.6
0.4
0.2
0.0
1x104
0
400
500
600
700
0
2
4
6
8
10
12
14
Wavelength (nm)
Voltage (V)
100
10
1
(c)
(d)
10
1
GIr1
GIr2
GIr3
GIr1
GIr2
GIr3
0.1
0.1
0
20
40
60
80
100
0
20
40
60
80
100
Current Density
(
mA cm-2
)
Current Density (mA cm-2)
Fig. 3. Characteristics of device GIr1 to GIr3: (a) EL spectra and (b) power efficiency as a function of current density, (c) current efficiency as a function of current density, and (d) Je
VeB, current density and brightness versus voltage for GIr1 to GIr3.

M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
111
Table 2
EL properties of OLEDs based on Ir(tfmfppy)2tpip, Ir(tfmfppy)2ftpip and Ir(tfmfppy)2tfmtpip.
Device
Von (V)
lem,max (nm)
CIE(x, y)a
Max.
Current efficiency (cd Aꢀ1) voltage (V), luminance (cd mꢀ2
)
Max. Power efficiency
Luminance
(Voltage, V) (lm Wꢀ1
)
Max.
10 000 cd mꢀ2
20 000 cd mꢀ2
(cd mꢀ2
)
GIr1
GIr2
GIr3
3.5
3.5
3.6
534
512
509
0.36, 0.60
0.27, 0.64
0.24, 0.63
>40 000
>40 000
>40 000
77.5 (7.6, 8025)
66.6 (7.1, 5286)
53.3 (7.5, 5169)
73.5
65.8
50.0
66.4
60.0
43.1
37.5 (6.4)
35.4 (5.1)
25.1 (6.6)
a
Commission Internationale de L’Eclairage coordinates.
With current density increasing, the EL efficiency increases until
it reaches the maximum. Then, the EL efficiency decreases gradu-
ally with current density further increasing. This phenomenon is
called EL efficiency roll-off effect due to the tripletetriplet annihi-
lation (TTA) of the phosphor-bound excitons increasing [36e38]
and field-induced quenching effects [40,41]. For device GIr1, the
hc at the practical luminance of 100 and 1000 cd mꢀ2 is 35.4 and
67.4 cd Aꢀ1, respectively, and it reaches the maximum current ef-
ficiency (77.5 cd Aꢀ1) at 8025 cd mꢀ2. The hc at the high luminance
of 10 000 and 20 000 cd mꢀ2 is 73.5 and 66.4 cd Aꢀ1, respectively.
The roll-off ratios of efficiency from the peak value (77.5 cd Aꢀ1) to
that at the brightness of 10 000 cd mꢀ2 and from the brightness of
10 000 to 20 000 cd mꢀ2 are as low as 5.12% and 9.73%, respectively.
For device GIr2, the hc at the practical luminance of 100 and
1000 cd mꢀ2 is 36.31 and 59.58 cd Aꢀ1, respectively, and it reaches
the maximum current efficiency (66.6 cd Aꢀ1) at 5286 cd mꢀ2. The
hc at the high luminance of 10 000 and 20 000 cd mꢀ2 is 65.8 and
60.0 cd Aꢀ1, respectively. The roll-off ratios of efficiency from the
peak value (66.6 cd Aꢀ1) to that at the brightness of 10 000 cd mꢀ2
and from the brightness of 10 000 to 20 000 cd mꢀ2 are as low as
1.14% and 8.83%, respectively. For device GIr3, the hc at the practical
built-in electric field between organic/organic interface is negli-
gible. Bearing this in mind, the electron mobility can be roughly
calculated with the equation of
me ¼ d2ðtd$VÞꢀ1
(2)
where d is the thickness of the emitting layer, V is the driving
voltage and td is the delay time. We chose the popular electron
transport material Alq3 (tri(8-hydroxyquinoline)aluminum) as the
reference because its electron mobility was reported (Fig. S4) [44].
The transient EL signals for Ir(tfmfppy)2tpip-based, Ir(tfmfppy)2ft-
pip-based devices and electric field dependence of charge electron
mobilities in the thin films of Ir(tfmfppy)2tpip, Ir(tfmfppy)2ftpip
and Alq3 were shown in Fig. 4.
The experimental results showed that the 60 nm Ir(tfmfp-
py)2tpip and Ir(tfmfppy)2ftpip layers have good electron mobilities
between 2.71e3.18
ꢂ
10ꢀ6 cm2 Vꢀ1 sꢀ1 and 2.47e
2.71 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1 under electric field from 1150 (V cmꢀ1 1/2
)
to 1300 (V cm )
ꢀ1 1/2, a litter lower than those of Alq3 (4.74e
4.86 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1 vs 1150e1300 (V cm
)
ꢀ1 1/2). In the OLEDs,
the dopants act 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 SimCP2 and TPBi in
OLEDs [45], the excitons accumulation is expected in hole blocking
layer (TPBi) near the interface of emitting layer (Ir complexes (x wt
%) : SimCP2)/TPBi due to the energy barrier between TPBi and
SimCP2 [46]. The accumulation of excitons is expected to cause the
serious TTA and triplet-polaron annihilation (TPA) of the iridium
complexes, and high efficiency roll-off consequently. Therefore, the
excitation lifetime of OLEDs relies on the capability of electron
transport. In our case, the good electron mobility of the phospho-
rescent emitters 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 sup-
pressed the TTA, TPA effects [47,48], improved recombination
probability and high device efficiency, low efficiency roll-off.
luminance of 100 and 1000 cd mꢀ2 is 16.60 and 41.08 cd Aꢀ1
,
respectively, and it reaches the maximum current efficiency
(53.3 cdAꢀ1) at 5169 cd mꢀ2. The hc at the high luminance of 10 000
and 20 000 cd mꢀ2 is 50.0 and 43.1 cd Aꢀ1, respectively. The roll-off
ratios of efficiency from the peak value (53.26 cd Aꢀ1) to that at the
brightness of 10 000 cd mꢀ2 and from the brightness of 10 000 to
20 000 cd mꢀ2 are 6.16% and 13.73%, respectively. All the devices
maintain high efficiencies at relative high brightness suggesting
they are useful for practical application, especially for lighting.
3.5. Electron mobility
The high efficiency of devices at high brightness and low effi-
ciency roll-off effects can be interpreted by the more balanced in-
jection and transport of carriers in(to) the emitting layer, due to the
more balanced hole and electron mobility of the SimCP2 and the
good electron transport ability of Ir(III) complexes. The respective
hole
(mh) and electron mobility (me) of the SimCP2 are
mh ¼ 6.4 ꢂ 10ꢀ4 and me ¼ 4.6 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1, which are higher
4. Conclusion
than that of the popular host of mCP (1,3-bis(carbazol-9-yl)ben-
zene, mh ¼ 3.2 ꢂ 10ꢀ4
,
me ¼ 2.0 ꢂ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1) [39,42].
Three Ir(III) complexes using 2-(4-trifluoromethyl-6-
fluorophenyl)pyridine as a cyclometalated ligand and tetrapheny-
limidodiphosphinate derivatives as ancillary ligands were synthe-
sized. The complexes emit green phosphorescence at 514, 513 and
508 nm in CH2Cl2 solution, respectively. The OLEDs with these Ir(III)
complexes doped SimCP2 as missive layers exhibited high EL effi-
Meanwhile, the electron mobilities of Ir(tfmfppy)2tpip (1) and
Ir(tfmfppy)2ftpip (2) were measured by the transient EL method
based on the device structure of ITO/TAPC (50 nm)/1 or 2 (60 nm)/
LiF (1 nm)/Al (100 nm). The TAPC is the hole-transport layer while 1
and 2 perform as both emissive and electron-transport layers. The
experimental setup and the process for transient EL measurements
were listed in Fig. S3. Due to the serious TTA effect of the
Ir(tfmfppy)2tfmtpip (3), the EL intensity of ITO/TAPC (50 nm)/3
(60 nm)/LiF (1 nm)/Al (100 nm) was too weak to calculate the
electron mobility. Because the hole mobility of TAPC
ciency at high brightness with the maximum current efficiency (hc
)
value of 77.49 cd Aꢀ1 at relatively high brightness of 8025 cd mꢀ2. In
addition, the efficiency roll-off ratios from the peak efficiency to the
brightness of 10 000 cd mꢀ2 are less than 10% for all the devices. We
have utilized the transient EL method to measure the electron
mobilities in Ir(tfmfppy)2tpip and Ir(tfmfppy)2ftpip films directly,
which are a little lower than that in Alq3. We believe that the good
(w1.0 ꢂ 10ꢀ2 cm2 Vꢀ1
s
ꢀ1) [43] is much higher than the electron
mobility of 1 and 2, the voltage drop across the TAPC layer and the

112
M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
Fig. 4. The transient EL signals for the devices based on Ir(tfmfppy)2tpip (a), Ir(tfmfppy)2ftpip (b) and electric field dependence of charge electron mobilities in the thin films of
Ir(tfmfppy)2tpip, Ir(tfmfppy)2ftpip and Alq3.
[7] Du B-S, Lin C-H, Chi Y, Hung J-Y, Chung M-W, Lin T-Y, et al. Diphenyl(1-
electron mobility of the phosphorescent emitter leads to the high
devices efficiency and low efficiency roll-off effect. All the devices
maintain high efficiencies at relative high brightness suggesting
they are useful for practical application, especially for lighting.
naphthyl)phosphine ancillary for assembling of red and orange-emitting
Ir(III) based phosphors: strategic synthesis, photophysics, and organic light-
emitting diode fabrication. Inorg Chem 2010;49:8713e23.
[8] Schneidenbach D, Ammermann S, Debeaux M, Freund A, Zöllner M, Daniliuc C,
et al. Efficient and long-time stable red iridium(III) complexes for organic
light-emitting diodes based on quinoxaline ligands. Inorg Chem 2010;49:
397e406.
Acknowledgments
[9] Ren XF, Kondakova ME, Giesen DJ, Rajeswaran M, Madaras M, Lenhart WC.
Coumarin-based, electron-trapping iridium complexes as highly efficient and
stable phosphorescent emitters for organic light-emitting diodes. Inorg Chem
2010;49:1301e3.
[10] Xiao LX, Chen ZJ, Qu B, Luo JX, Kong S, Gong QH, et al. Recent progresses on
materials for electrophosphorescent organic light-emitting devices. Adv
Mater 2011;23:926e52.
This work was supported by the National Natural Science
Foundation of China (21371093) and the Major State Basic Research
Development Program (2013CB922101, 2011CB808704).
Appendix A. Supplementary data
[11] Hsieh CH, Wu FI, Fan CH, Huang MJ, Lu KY, Chou PY, et al. Design and syn-
thesis of iridium bis(carbene) complexes for efficient blue electro-
phosphorescence. Chem-Eur J 2011;17:9180e7.
[12] Chen SM, Tan GP, Wong WY, Kwok HS. White organic light-emitting diodes
with evenly separated red, green, and blue colors for efficiency/color-
rendition trade-off optimization. Adv Funct Mater 2011;21:3785e93.
[13] Fernández-Hernández JM, Yang C-H, Beltrán JI, Lemaur V, Polo F, Fröhlich R,
et al. Control of the mutual arrangement of cyclometalated ligands in cationic
iridium(III) complexes. synthesis, spectroscopy, and electroluminescence of
the different isomers. J Am Chem Soc 2011;133:10543e58.
[14] Rai VK, Nishiura M, Takimoto M, Zhao S, Liu Y, Hou ZM. Bis-cyclometalated
iridium(III) complexes bearing ancillary guanidinate ligands. synthesis,
structure, and highly efficient electroluminescence. Inorg Chem 2012;51:
822e35.
[15] Costa RD, Ortí E, Bolink HJ, Monti F, Accorsi G, Armaroli N. Luminescent ionic
transition-metal complexes for light-emitting electrochemical cells. Angew
Chem Int Ed 2012;51:8178e211.
[16] Monti F, Kessler F, Delgado M, Frey J, Bazzanini F, Accorsi G, et al. Charged bis-
cyclometalated iridium(III) complexes with carbene-based ancillary ligands.
Inorg Chem 2013;52:10292e305.
[17] Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Lee HE, Adachi C, et al.
Highly phosphorescent bis-cyclometalated iridium complexes: synthesis,
photophysical characterization, and use in organic light emitting diodes. J Am
Chem Soc 2001;123:4304e12.
[18] You Y, Park SY. Inter-ligand energy transfer and related emission change in
the cyclometalated heteroleptic iridium complex: facile and efficient color
tuning over the whole visible range by the ancillary ligand structure. J Am
Chem Soc 2005;127:12438e9.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.01.033.
References
[1] Kang DM, Kang JW, Park JW, Jung SO, Lee SH, Park HD, et al. Iridium com-
plexes with cyclometalated 2-cycloalkenyl-pyridine ligands as highly efficient
emitters for organic light-emitting diodes. Adv Mater 2008;20:2003e7.
[2] Lo SC, Bera RN, Harding RE, Burn PL, Samuel IDW. Solution-processible
phosphorescent blue dendrimers based on biphenyl-dendrons and fac-
tris(phenyltriazolyl)iridium(III) cores. Adv Funct Mater 2008;18:3080e90.
[3] Zhou GJ, Wang Q, Wong WY, Ma DG, Wang LX, Lin ZY. A versatile color tuning
strategy for iridium(III) and platinum(II) electrophosphors by shifting the
charge-transfer states with an electron-deficient core. J Mater Chem 2009;19:
1872e83.
[4] Wong WY, Ho CL. Functional metallophosphors for effective charge carrier
injection/transport: new robust OLED materials with emerging applications.
J Mater Chem 2009;19:4457e82.
[5] Kim JJ, You Y, Park YS, Kim JJ, Park SY. Dendritic Ir(III) complexes function-
alized with triphenylsilylphenyl groups: synthesis, DFT calculation and
comprehensive structure-property correlation. J Mater Chem 2009;19:8347e
59.
[6] Chen ZQ, Bian ZQ, Huang CH. Functional Ir(III) complexes and their applica-
tions. Adv Mater 2010;22:1534e9.

M.-Y. Teng et al. / Dyes and Pigments 105 (2014) 105e113
113
[19] Kwon T-H, Cho HS, Kim MK, Kim J-W, Kim J-J, Lee KH, et al. Color tuning of
cyclometalated iridium complexes through modification of phenylpyrazole
derivatives and ancillary ligand based on ab initio calculations. Organome-
tallics 2005;24:1578e85.
[20] Zhu YC, Zhou L, Li HY, Xu QL, Teng MY, Zheng YX, et al. Highly efficient green
and blue-green phosphorescent OLEDs based on iridium complexes with the
tetraphenylimidodiphosphinate ligand. Adv Mater 2011;23:4041e6.
[21] Li HY, Zhou L, Teng MY, Xu QL, Lin C, Zheng YX, et al. Highly efficient green
phosphorescent OLEDs based on a novel iridium complex. J Mater Chem C
2013;1:560e5.
[34] Chan SC, Chan MCW, Wang Y, Che CM, Cheung KK, Zhu NY. Organic light-
emitting materials based on bis(arylacetylide)platinum(II) complexes
bearing substituted bipyridine and phenanthroline ligands: photo- and elec-
troluminescence from 3MLCT excited states. Chem Eur J 2001;7:4180e90.
[35] Baldo MA, Adachi C, Forrest SR. Transient analysis of organic electro-
phosphorescence. II. Transient analysis of triplet-triplet annihilation. Phys Rev
B 2000;62:10967e77.
[36] Ashraf RS, Shahid M, Klemm E, Al-Ibrahim M, Sensfuss S. Thienopyrazine-
based low-bandgap poly(heteroaryleneethynylene)s for photovoltaic devices.
Macromol Rapid Commun 2006;27:1454e9.
[22] Xu QL, Wang CC, Li TY, Teng MY, Zhang S, Jing YM, et al. Syntheses, photo-
luminescence and electroluminescence of a series of iridium complexes with
trifluoromethyl-substituted 2-phenylpyridine as main ligands and tetraphe-
nylimidodiphosphinate as ancillary ligand. Inorg Chem 2013;52:4916e25.
[23] Wang CC, Jing YM, Li TY, Xu QL, Zhang S, Li WN, et al. Syntheses, photo-
luminescence and electroluminescence of novel iridium(III) complexes with
fluorinated substituted 2-phenylpyridine as main ligands and ter-
traphenylimidodiphosphinate as ancillary ligand. Eur J Inorg Chem; 2013:
5683e93.
[24] Li HY, Li TY, Teng MY, Xu QL, Zhang S, Jin YM, et al. Syntheses, photo-
luminescence and electroluminescence of four heteroleptic iridium com-
plexes with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-phenol derivatives as ancillary
ligands. J Mater Chem C 2014;2:1116e24.
[37] Thelakkat M, Schmidt HW. Synthesis and properties of novel derivatives of
1,3,5-tris(diarylamino)benzenes for electroluminescent devices. Adv Mater
1998;10:219e23.
[38] Kang J-W, Lee D-S, Park H-D, Park Y-S, Kim JW, Jeong W-I, et al. Silane- and
triazine-containing hole and exciton blocking material for high-efficiency
phosphorescent organic light emitting diodes.
J Mater Chem 2007;17:
3714e9.
[39] Wu M-F, Yeh S-J, Chen C-T, Murayama H, Tsuboi T, Li W-S, et al. The quest for
high-performance host materials for electrophosphorescent blue dopants.
Adv Funct Mater 2007;17:1887e95.
[40] Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4.
[25] Teng MY, Zhang S, Jiang SW, Yang X, Lin C, Zheng YX, et al. Electron mobility
determination of efficient phosphorescent iridium complexes with tetraphe-
nylimidodiphosphinate ligand via transient electroluminescence method.
Appl Phys Lett 2012;100:073303.
[26] Tao YT, Yang CL, Qin JG. Organic host materials for phosphorescent organic
light-emitting diodes. Chem Soc Rev 2011;40:2943e70.
[27] Rillema DP, Taghdiri DG, Jones DS, Keller CD, Worl LA, Meyer TJ, et al.
Structure and redox and photophysical properties of a series of ruthenium
heterocycles based on the ligand 2,3-bis(2-pyridyl)quinoxaline. Inorg Chem
1987;26:578e85.
[28] Suzuki K, Kobayashi A, Kaneko S, Takehira K, Yoshihara T, Ishida H, et al.
Reevaluation of absolute luminescence quantum yields of standard solutions
using a spectrometer with an integrating sphere and a back-thinned CCD
detector. Phys Chem Chem Phys 2009;11:9850e60.
[41] Baldo MA, O’Brien DF, Thompson ME, Forrest SR. Excitonic singlet-triplet ratio
in a semiconducting organic thin film. Phys Rev B 1999;60:14422e8.
[42] Jou J-H, Wang W-B, Chen S-Z, Shyue J-J, Hsu M-F, Lin C-W, et al. High-effi-
ciency blue organic light-emitting diodes using a 3,5-di(9H-carbazol-9-yl)
tetraphenylsilane host via a solution-process. J Mater Chem 2010;20:8411e6.
[43] Lee J, Lee JI, Lee JW, Chu HY. Effects of charge balance on device performances
in deep blue phosphorescent organic light-emitting diodes. Org Electron
2010;11:1159e64.
[44] Barth S, Muller P, Riel H, Seidler PF, Rieß W, Vestweber H, et al. Electron
mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via
transient electroluminescence from single- and multilayer organic light-
emitting diodes. J Appl Phys 2001;89:3711e9.
[45] Fong HH, Lun KC, So SK. Hole transports in molecularly doped triphenylamine
derivative. Chem Phys Lett 2002;353:407e13.
[29] Madison, WI SAINT-Plus, version 6.02. Bruker Analytical X-ray System; 1999.
[30] Sheldrick GM. SADABS an empirical absorption correction program. Madison,
WI: Bruker Analytical X-ray Systems; 1996.
[31] Sheldrick GM. SHELXTL-97. Göttingen, Germany: Universität of Göttingen; 1997.
[32] Magennis SW, Parsons S, Pikramenou Z. Assembly of hydrophobic shells and
shields around lanthanides. Chem Eur J 2002;8:5761e71.
[33] Wong WY, Ho CL, Gao ZQ, Mi BX, Chen CH, Cheah KW, et al. Multifunctional
iridium complexes based on carbazole modules as highly efficient electro-
phosphors. Angew Chem Int Ed 2006;45:7800e3.
[46] Parker ID. Carrier tunneling and device characteristics in polymer light-
emitting diodes. J Appl Phys 1994;75:1656e66.
[47] Kalinowski J, Stampor W, Mezyk J, Cocchi M, Virgili D, Fattori V, et al.
Quenching effects in organic electrophosphorescence. Phys Rev B 2002;66:
235321e35.
[48] Jeon WS, Park TJ, Kim SY, Pode R, Jang J, Kwon JH. Low roll-off efficiency green
phosphorescent organic light-emitting devices with simple double emissive
layer structure. Appl Phys Lett 2008;93:063303.