Orange red iridium complexes with good electron mobility and mild OLED efficiency roll-off

Article abstract of DOI:10.1016/j.jorganchem.2018.09.008

Two iridium(III) complexes with 1-(3,5-bis(trifluoromethyl)-pyridin-4-yl)isoquinoline (tntpiq) as main ligand, 2-(5-pyridin-4-yl)-1,3,4-oxadiazol-2-yl)phenol (pop) and 2-(5-pyridin-4-yl)-1,3,4-thiadiazol-2-yl)phenol (psp) as ancillary ligands were investigated. Both complexes emit orange red lights with different photoluminescence efficiencies (Ir(tntpiq)₂(pop): λ_em = 585 nm, Φ = 0.41 and Ir(tntpiq)₂(psp): λ_em = 590 nm, Φ = 0.59). Moreover, the electron mobility values of the two complexes are higher than that of the electron transport material Alq₃ (tris(8-hydroxyquinoline)aluminium), which are beneficial for their performances in organic light-emitting diodes (OLEDs). The devices with a structure of ITO/MoO₃ (3 nm)/TAPC (1,1-bis[4-[N,N-di(p-tolyl)amino]pyridin-4-yl]cyclohexane, 30 nm)/Ir(III) complexes (2 wt%): 26DCzPPy (2,6-bis(3-(carbazol-9-yl)pyridin-4-yl)pyridine, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-pyridin-4-yl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displayed similar performances with a maximum current efficiency of 24.3 cd A^?1 and a maximum external quantum efficiency of 11.6%, respectively, and the efficiency roll-off is very mild.

Full text of DOI:10.1016/j.jorganchem.2018.09.008

Journal of Organometallic Chemistry 876 (2018) 26e34
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Orange red iridium complexes with good electron mobility and mild
OLED efficiency roll-off
,
,
, *
Yong-Hui Zhou a 1, Dong Jiang b 1, You-Xuan Zheng c
a Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Collaborative Innovation Center for Atmospheric Environment and
Equipment Technology, College of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, PR
China
b School of Chemistry and Pharmaceutical Engineering, Taishan Medical University, Tai'an, 271016, PR China
c State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, Jiangsu Key Laboratory of Advanced
Organic Materials, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, PR
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two iridium(III) complexes with 1-(3,5-bis(trifluoromethyl)-pyridin-4-yl)isoquinoline (tntpiq) as main
ligand, 2-(5-pyridin-4-yl)-1,3,4-oxadiazol-2-yl)phenol (pop) and 2-(5-pyridin-4-yl)-1,3,4-thiadiazol-2-
yl)phenol (psp) as ancillary ligands were investigated. Both complexes emit orange red lights with
Received 16 June 2018
Received in revised form
2 September 2018
Accepted 17 September 2018
Available online 19 September 2018
different photoluminescence efficiencies (Ir(tntpiq)2(pop): lem ¼ 585 nm,
F
¼ 0.41 and Ir(tntpiq)2(psp):
lem ¼ 590 nm,
F
¼ 0.59). Moreover, the electron mobility values of the two complexes are higher than
that of the electron transport material Alq3 (tris(8-hydroxyquinoline)aluminium), which are beneficial
for their performances in organic light-emitting diodes (OLEDs). The devices with a structure of ITO/
MoO3 (3 nm)/TAPC (1,1-bis[4-[N,N-di(p-tolyl)amino]pyridin-4-yl]cyclohexane, 30 nm)/Ir(III) complexes
(2 wt%): 26DCzPPy (2,6-bis(3-(carbazol-9-yl)pyridin-4-yl)pyridine, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-
Keywords:
OLED
Orange red iridium complexes
1,3,4-Oxadiazole derivative
1,3,4-Thiadiazol derivative
Electron mobility
yl-pyridin-4-yl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displayed similar performances with
a
maximum current efficiency of 24.3 cd Aꢀ1 and a maximum external quantum efficiency of 11.6%,
respectively, and the efficiency roll-off is very mild.
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
complexes because both the charge carrier balance deterioration
and nonradioactive quenching processes increase will cause a
Organic light emitting diodes (OLEDs) have received great
attention because of their successful applications in flat-panel
displays and solid-state lighting. Cyclometalated iridium(III) com-
plexes are almost the most promising phosphorescent guest ma-
terials for highly efficient OLEDs due to their lifetime on the
microsecond time-scale, high quantum yields, flexibility in color
tuning and excellent thermal stability [1e28]. Furthermore, the
phosphorescence of Ir(III) complexes generates by the metal-to-
ligand charge transfers (MLCT) and ligand-centered (LC) transi-
tion [29], so that it is possible to control the excited state's energy
level by adjusting ligands via the substituent effect.
serious efficiency roll-off. Since the majority of hole-transporting
materials' hole mobility is much higher than the electron-
transporting materials' electron mobility, the OLEDs perfor-
mances depend on electron transport's capability. Therefore, the
use of ambipolar host materials is essential to gain phosphorescent
OLEDs with low efficiency roll-off, as well as the synthesis of
dopants with excellent electron mobility.
From former work, the bulky trifluoromethyl (eCF3) sub-
stituents can affect the molecular packing and the steric protection
surrounding the metal would restrain the self-quenching impact,
and the CeF bond with low vibrational frequency can reduce
radiationless deactivation rate [30e33]. Besides that, nitrogen
heterocycle would also enhance the electron affinity and the elec-
tron mobility of the Ir(III) complexes. Therefore, the Ir(III) com-
plexes with main ligands containing 2,6-bis(trifluoromethyl)
pyridin unit always show good device performances [34e37].
Moreover, OLEDs based on Ir(III) complexes with 1,3,4-oxadiazole
It is well-known that the balance of the electron-hole injection
and transport is necessary for high efficient OLEDs using the Ir(III)
* Corresponding author.
E-mail address: yxzheng@nju.edu.cn (Y.-X. Zheng).
Zhou and Jiang have same contributions to this paper.
1
https://doi.org/10.1016/j.jorganchem.2018.09.008
0022-328X/© 2018 Elsevier B.V. All rights reserved.

Y.-H. Zhou et al. / Journal of Organometallic Chemistry 876 (2018) 26e34
27
derivatives as ancillary ligands also have good performances due to
ferrocene as internal standard at a scan rate of 50 mV/s.
their high electron mobility, high photoluminescence quantum
yield and good thermal, chemical stability [38e40], which would
enhance the electron affinity and the electron mobility of the Ir(III)
complexes.
2.2. X-ray crystallography
X-ray crystallographic measurements of the single crystals were
carried out on a Bruker SMART CCD diffractometer (Bruker Daltonic
Inc.) using monochromated Mo K
On this basis, as shown in Scheme 1, two heteroleptic Ir(III)
complexes (Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp)) were prepared
with 1-(3,5-bis(trifluoromethyl)pyridin-4-yl)isoquinoline (tntpiq)
as main ligand, 2-(5-pyridin-4-yl)-1,3,4-oxadiazol-2-yl)phenol
(pop) and 2-(5-pyridin-4-yl)-1,3,4-thiadiazol-2-yl)phenol (psp) as
ancillary ligands. These complexes containing 2,6-bis(tri-
fluoromethyl)pyridin unit and 1,3,4-oxadiazole/1,3,4-thiadiazole
derivatives would have good electron mobility and high photo-
luminescence quantum yield. Therefore, the devices based on two
emitters displayed good performances with a maximum external
quantum efficiency up to 11.6% and very low efficiency roll-off.
a
radiation (
l
¼ 0.71073 Å) at
room temperature. Cell parameters were retrieved using SMART
software and refined using SAINT [41] program in order to reduce
the highly redundant data sets. Data were collected using a narrow-
frame method with scan width of 0.30ꢁ in
u and an exposure time
of 5 s per frame. Absorption corrections were applied using SADABS
[42] supplied by Bruker. The structures were solved by Patterson
methods and refined by full-matrix least-squares on F2 using the
program SHELXS-2014 [43]. 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 dif-
ference Fourier syntheses and least-squares refinement cycles and
during the final cycles refined anisotropically. Hydrogen atoms
were placed in calculated position and refined as riding atoms with
2. Experimental section
2.1. General information
a uniform value of Uiso
.
1H NMR spectra were measured on a Bruker AM 500 spec-
trometer. Electrospray ionization mass spectra (ESI-MS) were ob-
tained with ESI-MS (LCQ Fleet, Thermo Fisher Scientific) and Matrix
Assisted Laser Desorption Ionization Time of Flight Mass Spec-
trometry (autoflex TOF/TOF, Bruker Daltonics). Elemental analyses
for C, H and N were performed on an Elementar Vario MICRO
analyzer. TGA measurements were carried out on a DSC 823e
analyzer (METTLER). UVevis absorption and photoluminescence
spectra were measured on a Shimadzu UV-3100 and a Hitachi F-
4600 spectrophotometer at room temperature, respectively. A
conventional three-electrode configuration, consisting of Glassy
Carbon Electrode (GCE) as working electrode, a Pt wire counter
electrode, and a reference electrode of Ag/AgNO3 (0.1 M), was used
to record cyclic voltammetry data in nitrogen-deaerated CH3CN
solution with 0.1 M [Bu4N]ClO4 as the supporting electrolyte and
2.3. OLEDs fabrication and measurement
All OLEDs were fabricated on the pre-patterned ITO-coated glass
substrate with a sheet resistance of 15
U
sqꢀ1. The deposition rate
for organic compounds is 1e2 Å sꢀ1. The phosphor and host were
co-evaporated from two separate sources. The cathode consisting
of LiF/Al was deposited by evaporation of LiF with a deposition rate
of 0.1 Å sꢀ1 and then by evaporation of Al metal with a rate of
3 Å sꢀ1. The effective area of the emitting diode is 0.1 cm2. The
characteristics of the devices were measured with a computer
controlled KEITHLEY 2400 source meter with a calibrated silicon
diode in air without device encapsulation. On the basis of the un-
corrected PL and EL spectra, the CIE coordinates were calculated
using a test program of the spectra scan PR650 spectrophotometer.
Scheme 1. Synthetic routes of the ligand and the complexes.

28
Y.-H. Zhou et al. / Journal of Organometallic Chemistry 876 (2018) 26e34
2.4. Syntheses
3. Results and discussion
All the reagents were used with commercial grade. The ligands
and complexes were synthesized under nitrogen atmosphere and
the synthetic routes were listed in Scheme 1.
3.1. Synthesis and characterization
Scheme 1 shows the chemical structures and synthetic routes of
the ligands and Ir(III) complexes. The ligand (tntpiq) was synthe-
sized using a Suzuki coupling reaction. The 2-(5-pyridin-4-yl)-
1,3,4-oxadiazol-2-yl)phenol (pop) and 2-(5-pyridin-4-yl)-1,3,4-
thiadiazol-2-yl) phenol (psp) ancillary ligands and their potas-
sium salts were prepared according to our previous publications
[40,44e46]. The Ir(III) complexes were obtained in two steps with
popular methods via Ir(III) chloro-bridged dimer. Purification of the
mixture by silica gel chromatography provided crude products,
which were further purified by vacuum sublimation. Both com-
pounds were characterized by 1H NMR, the electrospray ionization
mass spectra (ESI-MS) and the elemental analyses for C, H, and N.
Furthermore, the molecular structures of Ir(tntpiq)2(pop) and
Ir(tntpiq)2(psp) complexes were also proved via the single crystals
and their crystal diagrams are displayed in Fig. 1. The molecular
parameters and atomic coordinates are collected in Table S1,
Table S2 (Supporting Information), respectively. From the structure
diagrams of crystals it can be found that the iridium atom is
embraced by C, N and O atoms from tntpiq or pop/psp, with twisted
octahedral coordination geometry. For Ir(tntpiq)2(pop) and
Ir(tntpiq)2(psp), angles of [NeIreO] are 85.8(2)ꢁ - 86.8(3)ꢁ, and the
angles of [CeIreN] are 79.4(4)ꢁ - 80.9(3)ꢁ, respectively. The lengths
of IreC bonds range from 2.020(8) to 2.041(10) Å. The IreN and
IreO bonds have the lengths of 2.010(9) - 2.088(8) Å and 2.087(6) -
2.098(7) Å, respectively. These results are similar to the parameters
of the cyclometalated Ir(III) complexes that have been reported.
The thermal stability of the material is crucial for efficient and
stable OLEDs. If a complex can be applied in practical OLEDs, the
decomposition temperature (Td) and the melting points (Tm) needs
to be high enough to guarantee that the complex could be depos-
ited onto the solid face without any decomposition on sublimation.
From the thermogravimetric analysis (TGA, Fig. 2(a)) curves of
Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) it is can be figured out that
the decomposition temperatures (5% loss of weight) are as high as
342 ꢁC for Ir(tntpiq)2(pop) and 377 ꢁC for Ir(tntpiq)2(psp),
respectively. From the DSC curves in Fig. 2(b) it can be seen that the
melting points (Tm) of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) are as
high as 350 ꢁC and 338 ꢁC, respectively. Both complexes can be
vacuum evaporated easily without decomposition, suggesting that
both complexes have the potential for use in OLEDs.
2.4.1. Synthesis of tntpiq ligand
A stirred solution of 2,6-bis-(trifluoromethyl)pyridine (0.22 g,
10 mmol) in diethyl ether (20 mL) was cooled to ꢀ78 ꢁC. LDA
(lithium diisopropylamide, 6.0 mL, 10 mmol) was added over
20 min and stirred for 1 h, and then B(OPr-i)3 (2.89 mL, 12.4 mmol)
was injected. The mixture was warmed to room temperature and
stirred for another 1 h. The pH was adjusted to 10 by the slow
addition of 10% aqueous NaOH solution (20 mL). After 1 h, the
organic phase was acidified to pH ¼ 4 by the dropwise addition of
3 N HCl. The extraction with ethyl acetate and evaporation of the
organic phase gave the crude corresponding aryl boronic acids. 1-
Chloroisoquinoline and 1,10-bis(diphenylphosphion)ferrocene
palladium(II) dichloride (0.3 mmol) and the boronic acids were
added in 50 mL THF. After 20 mL of aqueous 2 N K2CO3 was
delivered, the reaction mixture was heated at 70 ꢁC for 1 day. The
mixture was poured into water and extracted with CH2Cl2
(10 mL ꢂ 3 times). Finally, silica column purification (PE: EA ¼ 10:
1) gave the 1-(2,6-bis(trifluoromethyl)pyridin-4-yl)isoquinoline
in 40% yield. 1H NMR (400 MHz, CDCl3)
d
8.69 (d, J ¼ 5.6 Hz, 1H),
8.26 (s, 2H), 8.00 (d, J ¼ 8.3 Hz, 1H), 7.94 (dd, J ¼ 8.5, 0.8 Hz, 1H),
7.81 (ddd, J ¼ 8.2, 5.9, 1.2 Hz, 2H), 7.69 (ddd, J ¼ 8.3, 6.9, 1.2 Hz, 1H).
MS(ESI) m/z calcd for C16H8F6Nþ2 : 352.06 [M]þ, found: 353.06
[MþH]þ.
2.4.2. General syntheses of iridium complexes
A mixture of IrCl3 (1 mmol) and tntpiq (2.5 mmol) in 2-
ethoxyethanol and water (20 mL, 3:1, v/v) was refluxed for 24 h.
After cooling, the solid precipitate was filtered to give the crude
cyclometalated Ir(III) chloro-bridged dimer. Then, the slurry of
crude chloro-bridged dimer (0.2 mmol) and 2-(5-pyridin-4-yl)-
1,3,4-oxadiazol-2-yl)phenol or 2-(5-pyridin-4-yl)-1,3,4-thiadiazol-
2-yl)phenol potassium salt (0.5 mmol) in 2-ethoxyethanol (20 mL)
was refluxed for 24 h. The solvent was evaporated at low pressure
and the mixture was poured into water and extracted with CH2Cl2
(10 mL ꢂ 3 times), and then chromatographed to give complexes
Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp), which were further purified
by sublimation in vacuum.
Ir(tntpiq)2(pop). 72% yield. 1H NMR (400 MHz, CDCl3)
d 8.84
(dd, J ¼ 8.3, 4.9 Hz, 2H), 8.63 (d, J ¼ 6.5 Hz, 1H), 8.57 (d, J ¼ 11.9 Hz,
2H), 7.94e7.89 (m, 1H), 7.88e7.74 (m, 6H), 7.72e7.66 (m, 2H),
7.55e7.39 (m, 5H), 7.23 (d, J ¼ 6.5 Hz, 1H), 6.95 (ddd, J ¼ 8.7, 6.9,
1.8 Hz, 1H), 6.45 (d, J ¼ 8.1 Hz, 1H), 6.42e6.34 (m, 1H). MS(ESI) m/z
calcd for C46H23F12IrN6O2: 1111.93 [M]þ, found 1113.01 [MþH]þ.
MALDI-TOF, m/z: calcd for C46H23F12IrN6O2, 1111.928 [M], found
1111.880 [M]. Anal. Calcd. For C46H23F12IrN6O2: C 49.69, H 2.09, N
7.56. Found: C 49.41, H 2.31, N 7.45.
3.2. Electrochemical properties and theoretical calculation
The frontier molecular orbitals (FMOs), especially the highest
occupied molecular orbital (HOMO) and lowest unoccupied mo-
lecular orbital (LUMO) energy levels of the dopants are pretty
crucial to the device structure design. Therefore, aiming to measure
energy levels of the HOMO/LUMO, the electrochemistry measure-
ments by cyclic voltammetry (CV) were adopted with ferrocene as
the internal standard in CH3CN (Fig. 3). The HOMO level was ob-
tained via the oxidation potential and then the LUMO level was
calculated by the HOMO and the band gap observed from UVevis
absorption spectra. During the progress of anodic oxidation, an
obvious oxide peak can be observed for both complexes with the
oxidation potential of 0.99 and 1.04 V, which can be ascribed to the
metal-centered Ir(III)/Ir(IV) oxide couple, consistent with the
cyclometalated Ir(III) system reported [47]. The cyclic voltammo-
grams of complexes Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) show
strong oxidation peaks, while the reduction peaks are not obvious,
Ir(tntpiq)2(psp). 81% yield. 1H NMR (400 MHz, CDCl3)
d 8.83 (t,
J ¼ 7.8 Hz, 2H), 8.72 (d, J ¼ 6.5 Hz, 1H), 8.58 (d, J ¼ 10.1 Hz, 2H),
7.96e7.87 (m, 1H), 7.88e7.67 (m, 6H), 7.47 (d, J ¼ 6.5 Hz, 1H),
7.46e7.40 (m, 1H), 7.40e7.31 (m, 4H), 7.18 (d, J ¼ 6.5 Hz, 1H), 7.08
(dd, J ¼ 8.0, 1.6 Hz, 1H), 6.83 (ddd, J ¼ 8.6, 6.9, 1.7 Hz, 1H), 6.39 (dd,
J ¼ 8.6, 0.9 Hz, 1H), 6.33e6.27 (m, 1H). MS(ESI) m/z calcd for
C
46H23F12IrN6OS: 1127.99 [M]þ, found 1129.17 [MþH]þ. MALDI-TOF,
m/z: calcd for C46H23F12IrN6OS, 1127.989 [M], found 1129.083 [M].
Anal. Calcd. For C46H23F12IrN6OS: C 48.98, H 2.06, N 7.45. Found: C
48.86, H 2.37, N 7.31.

Y.-H. Zhou et al. / Journal of Organometallic Chemistry 876 (2018) 26e34
29
Fig. 1. The crystal diagrams of Ir(tntpiq)2(pop) (CCDC No.1826720) and Ir(tntpiq)2(psp) (CCDC No. 1826721) shown at 30% probability level. The hydrogen atoms are omitted for
clarity.
Fig. 2. The TGA (a) and DSC (b) curves of Ir(tfmpiq)2(pop) and Ir(tfmpiq)2(psp) complexes.
Fig. 3. (a) The cyclic voltammogram curves and (b) contour plots of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp).
demonstrating that the redox process of the complexes Ir(tnt-
piq)2(pop) and Ir(tntpiq)2(psp) is not completely reversible, which
is also observed in our former publication for Ir(III) complexes with
psp ancillary ligand [44] and other related Ir(III) complexes con-
taining oxadiazole units [48e51]. By comparison between two
complexes, the Ir(tntpiq)2(psp) exhibits a lower oxidation poten-
tial (0.99 V) and HOMO/LUMO energy levels are ꢀ5.65/-3.62 eV.
The oxidation potential of Ir(tntpiq)2(pop) increases to 1.04 V and
the HOMO/LUMO energy levels are ꢀ5.70/-3.66 eV.
In order to gain insights into the electronic state and the orbital
distribution, the density functional theory (DFT) calculations [52]
for both Ir(III) complexes were conducted employing Gaussian 09
software with B3LYP function [53]. Plots of the HOMO/LUMO and
the molecular orbital energy levels are presented in Fig. 3(b). The
basis set used for C, H, N, O and F atoms was 6e31G(d, p) while the
LanL2DZ basis set was employed for iridium atoms [54,55]. The
solvent effect of CH2Cl2 was taken into consideration using
conductor like polarizable continuum model (C-PCM). It can be

30
Y.-H. Zhou et al. / Journal of Organometallic Chemistry 876 (2018) 26e34
observed from the theoretical calculation that nearly all LUMOs are
on tntpiq ligands (94.80%/93.84%), and rarely situated on iridium
atom (3.25%/3.33%) and ancillary ligand (1.96%/2.83%). While HO-
MOs almost situated on ancillary ligands (80.44%/78.99%), and
rarely situated on iridium atom (13.59%/14.59%) and tntpiq ligand
(5.97%/6.42%). The rising compositions of HOMOs on the ancillary
ligands suggest the photophysical properties of the complexes can
be controlled by the modification on the 1,3,4-oxadiazole/1,3,4-
thiadiazole derivatives.
profiles between Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) suggest
that the ancillary ligand variation has effects on their excited state
energy. The phosphorescence lifetime (
determines the rate of triplet-triplet annihilation in OLEDs. Longer
values usually cause greater triplet-triplet annihilation. The life-
times of the complexes are in the range of microseconds (0.95 and
1.08 s in CH2Cl2 solution, 1.35 and 1.40 s in 26DCzppy, respec-
t) is the crucial factor that
t
m
m
tively) at room temperature (Fig. S2 and Table 1) and are indicative
of the phosphorescent origin for the excited states in each case.
3.4. Electron mobility
3.3. Photophysical property
According to previous studies, an excellent electron mobility of
emitters could benefit electron transport and improve the device
performances [25,59]. To determine the electron mobility of two
complexes, the transient electroluminescence (TEL) measurement
was carried out via the devices with a structure of ITO (indium-tin-
oxide)/TAPC (1,1-bis[4-[N,N-di(p-tolyl)amino]pyridin-4-yl]cyclo-
hexane, 50 nm)/Ir(III) complexes (60 nm)/LiF (1 nm)/Al (100 nm).
The Ir(III) complexes act as not only the emissive layer (EML) but
also electron-transport layer. The experimental results (Fig. 5,
Fig. S1) indicated that the electron mobility values of Ir(tntpiq)2(-
pop) and Ir(tntpiq)2(psp) are 7.20e7.84 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1 and
7.56e8.00 ꢂ 10ꢀ6 cm2 Vꢀ1sꢀ1, respectively, under the electric field
The UVevis absorption and photoluminescence spectra of the
complexes Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) in CH2Cl2
(5 ꢂ 10ꢀ5 M) are shown in Fig. 4. The broad and intensive absorp-
tion bands below 300 nm are due to the spin-allowed intraligand
1LC (
p-p*) transition of tntpiq and pop/psp ligands. The weak ab-
sorption lasting to 550 nm can be assigned to spin-allowed metal-
ligand charge transfer band (1MLCT) and spin forbidden 3MLCT
transition bands caused by the large spin orbital coupling (SOC)
which was introduced by Ir(III) center indicating an efficient spin-
orbit coupling that is prerequisite for phosphorescent emission.
Excited by 397 nm, the strongest emissions peak at 585 and 590 nm
in CH2Cl2, produced by the electronic transition between the lowest
triplet excited state and the ground state, which makes Ir(tnt-
piq)2(pop) and Ir(tntpiq)2(psp) orange red phosphors. Similar
with our former research [44], the psp ancillary ligand resulted the
emission red-shift. The emissions in the scope of lower energy
around 625 nm may generate by the overlapping vibrational sat-
ellites [56]. From Fig. S1(a) (Supporting Information) it can be
observed that the PL spectra of two complexes in one popular used
host 26DCzPPy (2,6-bis(3-(carbazol-9-yl)pyridin-4-yl)pyridine) are
similar with that in CH2Cl2 solutions, suggesting the energy transfer
between host and dopants. From Fig. S1(b) and Table 1, it can be
seen that the PL spectra of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) at
77 K remain almost the same. In general, the emission bands from
the MLCT states are broad and featureless, whereas a highly
from 1040 (V cmꢀ1 1/2
)
to 1300 (V cm )
ꢀ1 1/2, higher than of the
electron transport material Alq3 (tris-(8-hydroxyquinoline)
aluminium, 4.74e4.87 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1) [60]. Additionally, the
electron mobility of Ir(tntpiq)2(psp) is a little better than that of
Ir(tntpiq)2(pop). These results are exactly fitting our design intent
to improve the electron transport ability of the emitter. The good
electron transport ability of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp)
will reinforce the recombination chance of electrons and holes, so
that their devices may have good performances.
3.5. OLEDs performance
To confirm our assumption, we researched the single emitting
layer (EML) devices G1 and G2 using Ir(tntpiq)2(pop) and Ir(tnt-
piq)2(psp) emitters, respectively, with a structure of ITO/MoO3
(molybdenum oxide, 3 nm)/TAPC (30 nm)/Ir(III) complexes (x wt%):
26DCzPPy (2,6-bis(3-(carbazol-9-yl)pyridin-4-yl)pyridine, 10 nm)/
TmPyPB (1,3,5-tri(m-pyrid-3-yl-pyridin-4-yl)benzene, 40 nm)/LiF
(1 nm)/Al (100 nm) (Fig. 6). MoO3 and LiF served as hole- and
electron-injecting interface modified materials, respectively. TAPC
owning high hole mobility (1 ꢂ 10ꢀ2 cm2 Vꢀ1 sꢀ1) and high-lying
LUMO level (ꢀ2.0 eV) was used as hole transport/electron block
layer (HTL/EBL), while TmPyPB with high electron mobility
(1 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1) and low-lying HOMO level (ꢀ6.7 eV) was
structured emission band mainly originates from the 3p
-p* state.
Accordingly, the emission of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp)
is mainly generated from MLCT states. This indicates that the MLCT
characters involved in the emitting T1 states of different complexes
are various but significant, since a dominant MLCT character in T1
usually leads to large inhomogeneity and low-energy lying metal-
ligand vibrational satellites, smearing out the spectrum below the
electronic original emission.
Furthermore, the complexes show different quantum effi-
ciencies as 0.41 and 0.59 for Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp),
respectively [57,58]. The differences of the absorption and emission
Fig. 4. (a) The UVevis absorption and (b) emission spectra of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) complexes in degassed CH2Cl2 solutions (5.0 ꢂ 10ꢀ5 mol Lꢀ1) at room
temperature.

Y.-H. Zhou et al. / Journal of Organometallic Chemistry 876 (2018) 26e34
31
Table 1
Physical properties of Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp) complexes.
F[d]
te
(m
s)
CIE (x,y)
HOMO/LUMO[f]
[a]
[b]
[c]
Complex
Td
l
l
abs
em
298 K
77 K
Ir(tntpiq)2(pop)
Ir(tntpiq)2(psp)
342
377
235/255/291/353/380
226/256/294/352/427
585
590
588/639
591/642
0.41
0.59
0.95
1.08
0.558,0.406
0.580,0.401
ꢀ5.70/-3.66
ꢀ5.65/-3.62
[a] Td: decomposition temperature. [b][e] Measured in degassed CH2Cl2 solution at a concentration of 5 ꢂ 10ꢀ5 mol Lꢀ1 at room temperature. [c] Measured in degassed CH2Cl2
solution at a concentration of 5 ꢂ 10ꢀ5 mol Lꢀ1 at 298 K and 77 K, respectively. [d] Measured in degassed CH2Cl2 solution at a concentration of 5 ꢂ 10ꢀ5 mol Lꢀ1 at room
temperature emission relative to Ir(ppy)3
and the optical band gap from the absorption spectra in degassed solution (CH3CN).
(
F
¼ 0.4). [f] From the onset of oxidation potentials of the cyclovoltammetry (CV) diagram using ferrocene as the internal standard
emission spectra are almost invariant of the current density and
there is no dependence on concentration. From Fig. 7(a) it is can be
seen that the EL spectra are almost identical to the PL spectra of the
complexes, suggesting that the devices' EL emissions come from
the triplet excited states of the phosphors. Apart from the charac-
teristic emissions of Ir(III) complexes, both devices also displayed
weak emissions range from 350 nm to 500 nm, which originate
from the host 26DCzPPy attributed to the accumulation of holes
and electrons within the EML and few holes/electrons would
recombine on 26DCzPPy molecules. And the Commission Inter-
nationale de 10Eclairage (CIE) color coordinates of G1 and G2 are
(0.558, 0.406) and (0.580, 0.401), respectively.
For the device G1,
31 413 cd mꢀ2, a maximum current efficiency (hc max
(5.6 V) with a maximum external quantum efficiency (EQEmax) of
11.9%, a maximum power efficiency (hp max
) of 14.3 lm Wꢀ1 (5.4 V)
are obtained. While the device G2 displays similar EL performances
with a hc max of
of 24.3 cd Aꢀ1 (5.8 V), an EQEmax of 11.6%, a hp max
a
maximum luminance (Lmax
)
of
,
) of 24.9 cd Aꢀ1
,
Fig. 5. Electric field dependence of charge electron mobility in the thin films of
Ir(tntpiq)2(pop), Ir(tntpiq)2(psp) and Alq3.
,
,
15.2 lm Wꢀ1 (5.0 V) and a Lmax of 27 348 cd mꢀ2 at 13.9 V. It is
noteworthy that both devices show very mild efficiency roll-off. For
device G1, the hc and EQE values at the practical brightness of
100 cd mꢀ2 are 21.9 cd Aꢀ1 and 10.5%, respectively. Even at the high
luminance of 1000 cd mꢀ2 these values are still kept at 21.7 cd Aꢀ1
and 10.3%, respectively. Device G2 even exhibits lower efficiency
roll-off with these values as 24.1 cd Aꢀ1 and 11.5% at 100 cd mꢀ2 and
23.1 cd Aꢀ1 and 11.0% at 1000 cd mꢀ2, respectively.
The introduction of 1,3,4-oxadiazole and 1,3,4-thiadiazole de-
rivatives in the ancillary ligand not only increased the electron
mobility of the Ir(III) complexes, but also enhanced their PL quan-
tum efficiencies, which will benefit the OLED performances. The
good electron mobility of the phosphorescent emitter would
facilitate the injection and transport of electrons, which would
broaden the recombination zone, balance the distribution of holes
used as electron transport/hole block layer (ETL/HBL). Ambipolar
material 26DCzPPy was chosen as the host because its' nearly equal
electron mobility
(
me
)
and hole mobility
(
mh
)
values
(1e8 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 at an electric field between 6.0 ꢂ 105 and
1.0 ꢂ 106 V cmꢀ1), which benefits the electron-hole balance in the
EML [61e63]. The optimal device performances are achieved at
doping level of 2 wt% for Ir(tntpiq)2(pop) and Ir(tntpiq)2(psp)
complexes.
The EL spectra, luminance-voltage-current density (L-V-J)
curves, current efficiency-luminance (s -L) curves, power
c
efficiency-luminance (s -L) curves for G1 and G2 are shown in
p
Fig. 7, and the crucial EL data are summarized in Table 2. The peaks
of EL emission are 585 and 587 nm for G1 and G2, respectively. The
Fig. 6. Energy level diagram of HOMO and LUMO levels (relative to vacuum level) for materials investigated in this work and their molecular structures.

32
Y.-H. Zhou et al. / Journal of Organometallic Chemistry 876 (2018) 26e34
Fig. 7. Characteristics of devices G1 and G2: (a) electroluminescence spectra (8 V); (b) luminance e voltage e current density (L e V e J) curves; (c) current efficiency e luminance
hc e L) curves; (d) power efficiency e luminance (hp e L) curves.
(
Table 2
EL performances of G1 and G2.
Lmaxb (cd mꢀ2
)
hc,maxc (cd Aꢀ1
)
hc,L100d (cd Aꢀ1
)
hc,L1000 (cd Aꢀ1
)
EQEmaxf (%)
EQEL100g (%)
EQEL1000h (%)
hp,maxi (lm Wꢀ1
)
a
e
device
Vturn-on (V)
G1
G2
4.3
4.2
31413
27348
24.9
24.3
21.9
24.1
21.7
23.1
11.9
11.6
10.5
11.5
10.3
11.0
14.3
15.2
a
Vturn-on: turn-on voltage recorded at a luminance of 1 cd mꢀ2
Lmax: maximum luminance.
.
b
c
d
e
f
hc max
hc,L100: current efficiency at 100 cd mꢀ2
hc,L1000: current efficiency at 1000 cd mꢀ2
,
: maximum current efficiency.
.
.
EQEmax: maximum external quantum efficiency.
g
h
i
EQEL100: external quantum efficiency at 100 cd mꢀ2
.
EQEL1000: external quantum efficiency at 1000 cd mꢀ2
.
hp,max: maximum power efficiency.
and electrons, particularly at a high doping concentration, leading
to the suppressed TTA and TPA effects. The high electron mobility of
complexes would leads to a good-balanced charge carrier transport
and efficient recombination, which is agree with the design
intension.
The EQE values of11.5% and 11.0% still can be obtained at the
practical luminance of 100 cd mꢀ2 and 1000 cd mꢀ2, respectively.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51773088), the Natural Science Foundation of
Jiangsu Province (BY2016075-02) and the Fundamental Research
Funds for the Central Universities (020514380131).
4. Conclusion
In conclusion, two bis-cyclometalated Ir(III) complexes (Ir(tnt-
piq)2(pop) and Ir(tntpiq)2(psp)) with 2-(5-pyridin-4-yl)-1,3,4-
oxadiazol-2-yl)phenol,
Appendix A. Supplementary data
2-(5-pyridin-4-yl)-1,3,4-thiadiazol-2-yl)
phenol as ancillary ligands and 1-(3,5-bis(trifluoromethyl)pyridin-
4-yl) isoquinoline as the main ligand were investigated. Due to the
trifluoromethyl substituted nitrogen heterocyclic structure of the
main ligand and 1,3,4-oxadiazole/1,3,4-thiadiazol moieties in the
ancillary ligands, both complexes emit orange red phosphores-
cence with high quantum efficiency and good electron mobility.
The good electron mobility would improve a good-balanced charge
carrier transport and efficient recombination of both complexes,
resulting in an EQEmax of 11.6% with very mild efficiency roll-off.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2018.09.008.
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