Color tuning of iridium(III) complexes containing 2-phenylbenzothiazole-based cyclometalated ligands for application in highly efficient organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2019.108145

By introducing a carbazolyl or diphenylamino substituent into the cyclometalated ligand, replacing the ancillary ligand of acetylacetone into 2-picolinic acid, or employing a homoleptic molecular structure, six complexes (TM1~6) bearing a similar molecular skeleton of yellow-emissive bis(2-phenylbenzo[d]thiozolato-N,C^2')iridium (III) (acetylacetonate) (Ir-bt) have been acquired. In despite of their different types of ancillary ligand, or different homoleptic or heteroleptic molecular structures, TM1~3 bearing carbazolyl-modified cyclometalated ligands all emit yellow photoluminescence (PL) with their PL maximum (λ_PLmax) locating at 555–565 nm. Nevertheless, for TM4~6 bearing diphenylamino-modified cyclometalated ligands, they show red PL emission whose λ_PLmax = 618–636 nm, which is significantly red-shifted than that of Ir-bt. Note that the PL emission of these diphenylamino-modified iridium chelates can be fine-tuned by alteration on its ancillary ligand, or using a homoleptic structure. By taking advantage of the excellent thermal stability and relatively high PL efficiencies of TM3 and TM6, organic light-emitting diodes devices (OLEDs) using them as phosphorescent guests show relatively high electroluminescence performance. The TM3-based device can emit highly efficient yellow light with CIE coordinates of (0.43, 0.56), a maximum current efficiency (η_Lmax) of 54.2 cd·A^?1, and a maximum power efficiency (η_Pmax) of 48.9 lm·W^?1, which is one of the highest reported power efficiency data of yellow PhOLEDs; the TM6-based device can emit orange emission with CIE coordinates of (0.57, 0.42), η_Lmax of 18.6 cd·A^?1, and η_Pmax of 14.3 lm·W^?1.

Full text of DOI:10.1016/j.dyepig.2019.108145

Dyes and Pigments 175 (2020) 108145
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Color tuning of iridium(III) complexes containing 2-phenylbenzothiazole--
based cyclometalated ligands for application in highly efficient organic
light-emitting diodes
,
,
,***
Jingjing Liua *, Ming Lib, Zhiyun Lub **, Yan Huangb, Xuemei Pub, Liang Zhouc
a College of Vanadium and Titanium, Panzhihua University, Panzhihua, 617000, China
b Key Laboratory of Green Chemistry and Technology (Ministry of Education), College of Chemistry, Sichuan University, Chengdu, 610064, China
c State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
By introducing a carbazolyl or diphenylamino substituent into the cyclometalated ligand, replacing the ancillary
ligand of acetylacetone into 2-picolinic acid, or employing a homoleptic molecular structure, six complexes
(TM1~6) bearing a similar molecular skeleton of yellow-emissive bis(2-phenylbenzo[d]thiozolato-N,C2’)iridium
(III) (acetylacetonate) (Ir-bt) have been acquired. In despite of their different types of ancillary ligand, or
different homoleptic or heteroleptic molecular structures, TM1~3 bearing carbazolyl-modified cyclometalated
ligands all emit yellow photoluminescence (PL) with their PL maximum (λPLmax) locating at 555–565 nm.
Nevertheless, for TM4~6 bearing diphenylamino-modified cyclometalated ligands, they show red PL emission
whose λPLmax ¼ 618–636 nm, which is significantly red-shifted than that of Ir-bt. Note that the PL emission of
these diphenylamino-modified iridium chelates can be fine-tuned by alteration on its ancillary ligand, or using a
homoleptic structure. By taking advantage of the excellent thermal stability and relatively high PL efficiencies of
TM3 and TM6, organic light-emitting diodes devices (OLEDs) using them as phosphorescent guests show rela-
tively high electroluminescence performance. The TM3-based device can emit highly efficient yellow light with
CIE coordinates of (0.43, 0.56), a maximum current efficiency (ηLmax) of 54.2 cd⋅AÀ 1, and a maximum power
efficiency (ηPmax) of 48.9 lm⋅WÀ 1, which is one of the highest reported power efficiency data of yellow PhOLEDs;
the TM6-based device can emit orange emission with CIE coordinates of (0.57, 0.42), ηLmax of 18.6 cd⋅AÀ 1, and
Iridium(III) complex
Color-tuning
Electrophosphorescence
Light-emitting diode
ηPmax of 14.3 lm⋅WÀ 1
.
1. Introduction
material (λPLmax ¼ 557 nm, ϕPL ¼ 0.26) [11]. Recently, enormous
research effort has been devoted to the structural modification on Ir-bt,
so that complexes with other luminescent colors or further enhanced
yellow electroluminescence (EL) performance can be acquired [12–28].
The results indicated that while little substituent effect is generally
observed on the color-tuning when structural modifications are carried
on the benzothiazole moiety of the phenylbenzothiazole (bt) cyclo-
metalated ligand [12–17], distinct color-tuning can be realized by
grafting substituents on the phenyl segment of the bt ligand of Ir-bt
[18–26]. However, despite the fact that many Ir-bt-based complexes
showing diversified PL emission colors have been constructed success-
fully [12–28], so far there is only one example of Ir-bt-based complex
Electrophosphorescent iridium(III) complexes have shown great
application prospect in displays and lighting sources due to their high
luminous efficiencies as well as flexible color tunability [1–4]. As red
emission is a key component for full-color display applications, and
yellow materials play an irreplaceable role in the fabrication of binary
white organic light-emitting diodes (OLEDs) [5,6], the development of
high-performance red and yellow iridium(III) complexes is of great
significance [7–10].
Bis(2-phenylbenzothiozolato-N,C2’)iridium(III)
(acetylacetonate)
[(bt)2Ir(acac), Ir-bt] is a typical yellow-emitting phosphorescent
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: jingjingliu8610@163.com (J. Liu), luzhiyun@scu.edu.cn (Z. Lu), zhoul@ciac.ac.cn (L. Zhou).
https://doi.org/10.1016/j.dyepig.2019.108145
Received 21 September 2019; Received in revised form 12 December 2019; Accepted 15 December 2019
Available online 16 December 2019
0143-7208/© 2019 Elsevier Ltd. All rights reserved.

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
that can emit red phosphorescence via substituent effect: in 2004, by
introducing an electron-donating methoxy group (CH3O-) into the met-
a-position of the phenyl moiety of bt, Chang et al. developed an
Ir-bt-based red phosphor whose λPLmax locates at 605 nm [19].
TOF Priemier ESI mass spectrometer (Micromass, Manchester, UK).
Thermogravimetric analysis (TGA) was performed on a TGA Q500 in-
strument. UV–Vis absorption and photoluminescence (PL) spectra were
recorded on a SHIMADZU UV-2100 spectrophotometer and a Perki-
nElmer LS55 fluorescence spectrophotometer, respectively. The abso-
lute PL quantum yield (PLQY) data of the doped films were determined
with an integrating sphere (IS80 from Labsphere) together with a digital
photometer (S370 from UDT) under excitation of 340 nm. Cyclic vol-
tammetry (CV) measurements were performed on a PARSTAT 2273
electrochemical workstation using a platinum plate, a Ag/AgNO3 (0.1
mol⋅LÀ 1 in acetonitrile) electrode and a platinum wire as the working
electrode, quasi-reference electrode and counter electrode in CH2Cl2
solution with 0.1 mol⋅LÀ 1 tetrabutylammonium perchlorate as the sup-
porting electrolyte. The crystallographic data for TM1, TM3 and TM6
were deposited into the Cambridge Structural Database (CCDC
1943633, 1943634 and 1943635). Single crystal X-ray diffraction data
of the complexes were obtained on an Xcalibur E X-ray single crystal
On the other hand, although up to date, yellow PhOLEDs with
maximum external quantum efficiency (EQEmax) approaching 30% [13],
high maximum current efficiency (ηLmax) over 137 cd⋅AÀ 1 [29], or high
maximum power efficiency (ηPmax) over 98 lm⋅WÀ 1 [30] have been
realized successfully, most of the reported yellow PhOLEDs suffer from
obvious efficiency roll-off [31], which is unfavorable for their practical
application in lighting sources. To acquire high-performance PhOLEDs,
an effective strategy is constructing double-emitting-layered devices by
doping the guest compound into two different types of host material
with good hole- and electron-transporting abilities, respectively, so that
the carrier recombination region can be limited successfully within the
two emitting layers, leading to more balanced carrier-transportation and
lowered leakage current [32,33].
In recent years, our research group has made great efforts to eluci-
date the substituent effects on the PL and EL properties of Ir-bt-based
phosphors [24,34,35]. Our experimental findings revealed that by
grafting a tert-butyl, phenyl or (4-tert-butyl)phenyl at the para-position
of the phenyl segment of bt, highly efficient yellow or orange-yellow
phosphors can be constructed [24,34]. But different from the findings
of Chang et al. when a phenyl or a 4-methoxyphenyl is grafted at the
meta-position of the phenyl of bt, the resultant iridium complexes only
show a negligible color-tuning effect, although significantly improved
film formation and morphology stability can be acquired [35]. Based on
the fact that the electron-donating capability of phenyl and 4-methoxy-
phenyl groups is lower than that of methoxy group, we infer that if a
substituent with stronger electron-donating capability is introduced into
the meta-position of the phenyl of bt, the resultant iridium complexes
may have their PL emission bands red-shifted to the red region.
As carbazole and diphenylamine units have been demonstrated to
show not only relatively strong electron-donating ability, but also good
hole-transporting capability and thermal stability [22,23,36,37], herein,
we graft a carbazol-9-yl or a diphenylamino substituent at the meta--
position of the phenyl of bt cyclometalated ligand. Moreover, to gain
insight into the impact of the ancillary ligand on the emission color of
the chelates, heteroleptic complexes bearing two carbazol-9-yl or
diphenylamino modified bt-based cyclometalated ligands and one ace-
tylacetone (acac) or 2-picolinic acid ancillary ligand, and homoleptic
complexes bearing three carbazol-9-yl or diphenylamino modified
bt-based cyclometalated ligands are constructed. For the three
carbazol-9-yl-modified objective complexes (TM1~3), a negligible
color-tuning effect is observed, and all the three complexes emit yellow
photoluminescence (PL) with their PL maximum (λPLmax) locating at
555–565 nm. However, for the three diphenylamino-modified objective
complexes (TM4~6), their PL emission bands are significantly
red-shifted than that of Ir-bt, leading to red PL emission with λPLmax of
618–636 nm. Besides, by alterating the ancillary ligand, or using a
homoleptic structure, the PL emission band of these Ir-bt-based com-
plexes can be further fine-tuned. By using the carbazol-9-yl-modified
TM3 as the guest phosphor, a high-performance yellow PhOLED with
CIE coordinates of (0.43, 0.56), a maximum current efficiency (ηLmax) of
54.2 cd⋅AÀ 1, a maximum power efficiency (ηPmax) of 48.9 lm⋅WÀ 1, and
satisfactory efficiency roll-off is realized successfully. An orange
PhOLED using the diphenylamino-modified TM6 as the guest is also
demonstrated to show moderate EL performance with CIE coordinates of
diffractometer equipped with graphite monochromator Mo-Kα (λ ¼
0.71073 Å) radiation. The data collection was executed using the Cry-
sAlisPro program. The structures were determined using direct method
and successive Fourier difference syntheses (SHELXS-97) and refined
using full-matrix least squares procedure on F2 with anisotropic thermal
parameters for all non-hydrogen atoms (SHELXL-97). Packing analysis
of the crystal cells was carried out using the Mercury program. Density
functional theory (DFT) calculations were performed using the Gaussian
09 software by employing B3LYP with LANL2DZ basis set for Ir atom,
and 6-31G(d) basis sets for C, H, S, N and O atoms. All the geometries
were confirmed as stationary structures by the presence of only real
frequencies at the same level of theory. Orbital energies were calculated
within the framework of the IEF-PCM Model in CH2Cl2 media based on
the optimized geometries.
2.2. OLED fabrication and measurements
All OLEDs were fabricated on pre-patterned indium-tin oxide (ITO)
coated glass substrate with an ITO sheet resistance of 15 Ω/sq, and the
emission area was 0.1 cm2. The ITO-coated substrate was cleaned in an
ultrasonic bath with acetone, isopropyl alcohol, de-ionized water in
sequence. After 20 min ozone treatment on ITO, a 1,1-bis[4-[N,N0-di(p-
tolyl)amino]phenyl]cyclohexane (TAPC) layer (40 nm) was deposited
on ITO. Then the two light-emitting layers of the target iridium complex
doped in 4,40,400-tri(N-carbazolyl)triphenylamine (TCTA) or 2,6-bis(3-
(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) with thickness of 10 nm
were evaporated in sequence, followed by the evaporation of 1,3,5-tris
(1-phenyl-1H-benzo[d]imidazole-2-yl)benzene (TPBi) (40 nm), LiF (1
nm), and Al (100 nm) successively. The vacuum was 1 � 10À 5 Pa during
the deposition of organic-component layers. The current density-
voltage-luminance (J-V-L) characteristics of the devices were
measured with a computer controlled KEITHLEY 2400 source meter
with a calibrated silicon diode in air without device encapsulation. The
EL spectra of the OLEDs were measured with a Hitachi F-4600 spec-
trophotometer. On the basis of the uncorrected PL and EL spectra, the
Commission Internationale de l’Eclairage (CIE) coordinates were
calculated using
spectrophotometer.
a
test program of the Spectrascan PR650
(0.57, 0.42), ηLmax of 18.6 cd⋅AÀ 1, and ηPmax of 14.3 lm⋅WÀ 1
2. Experimental
.
2.3. Synthesis
All the reagents involved in the synthetic procedure were commer-
cially available and used without further purification unless otherwise
stated. All the solvents were of analytical grade and freshly distilled
prior to use. The synthetic routes of objective molecules TM1~TM6 are
shown in Scheme 1. The synthetic details of ligands and intermediates
are provided in Supporting Information.
2.1. General information
1H NMR and 13C NMR spectra were recorded on a Bruker AVANCE-
400 spectrometer. High resolution MS spectra were obtained from a Q-
2

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
2.3.1. General procedure for the synthesis of dichloro-bridged iridium(III)
complexes [24a]
corresponding cyclometalated ligands (0.8 mmol), AgCF3SO3 (0.4
mmol), and xylene (30 mL) were placed in a pressure-resistant reaction
�
A mixture of IrCl3⋅nH2O (1 mmol), the corresponding cyclometalated
ligands (2.4 mmol), 2-ethoxyethanol (30 mL) and water (10 mL) was
refluxed under argon for 24 h. Then the mixture was cooled down to
room temperature, and the precipitate was filtered and washed with 10
mL of 1 mol⋅LÀ 1 HCl and 3 � 15 mL methanol in sequence, and dried in
vacuum. The crude product was used in the next step without further
purification.
vessel and stirred for 12 h at 130 C. Then the solvent was removed
under reduced pressure, the precipitates were collected and purified by
flash chromatography through silica column using n-hexane/dichloro-
methane (1/2) as the eluent, followed by recrystallization for more than
three times to afford the objective complexes with satisfied purity. The
complexes were dried at 100 �C under vacuum of 1.5 kPa for 24 h.
2.3.5. Bis[2-(3-(9H-carbazol-9-yl)phenyl)benzothiazole-N,C2’]iridium
(III) (acetyl acetonate) (TM1)
2.3.2. General procedure for the synthesis of (C^N)2Ir(acac) complexes
[24a]
Orange solid, recrystallized from a mixture of dichloromethane and
methanol. Yield: 56.2%, m.p.: 305–310 �C. 1H NMR (400 MHz, CDCl3) δ
(ppm): 8.22 (d, J ¼ 8.0 Hz, 2H, ArH), 8.13 (d, J ¼ 8.0 Hz, 4H, ArH), 7.93
(d, J ¼ 8.0 Hz, 2H, ArH), 7.87 (d, J ¼ 1.6 Hz, 2H, ArH), 7.56–7.46 (m,
8H, ArH), 7.41–7.36 (m, 6H, ArH), 7.28–7.24 (m, 2H, ArH), 6.90 (dd, J
¼ 8.0 Hz, 4.0 Hz, 2H, ArH), 6.68 (d, J ¼ 8.0 Hz, 2H, ArH), 5.24 (s, 1H,
–CH), 1.89 (s, 6H, –CH3). 13C NMR (100 MHz, CDCl3) δ (ppm): 186.0,
179.7, 150.8, 147.6, 143.1, 141.0, 136.4, 131.5, 131.3, 128.9, 127.7,
125.8, 125.6, 124.0, 123.1, 122.6, 120.40, 120.2, 119.6, 110.1, 101.9,
28.4. ESI-MS: m/z 1065.1880 (M þ Na)þ; Calcd. for (Mw þ Na)þ:
1065.1885.
The chloro-bridged dimer complex (0.2 mmol), acetylacetone (0.6
mmol), sodium carbonate (2 mmol) and 30 mL 2-ethoxyethanol were
refluxed under argon for 12 h. After the mixture was cooled down, the
precipitates were collected and purified by flash chromatography
through silica column using n-hexane/dichloromethane (1/2) as the
eluent, followed by recrystallization for more than three times to afford
the objective complexes with satisfied purity. The complexes were dried
at 100 �C under vacuum of 1.5 kPa for 24 h.
2.3.3. General procedure for the synthesis of (C^N)2Irpic complexes [38]
The corresponding chloro-bridged dimer complex (0.2 mmol), so-
dium carbonate (2 mmol) and picolinic acid (0.6 mmol) were dissolved
in 30 mL 1,2-dicholorethane, and the reaction mixture was stirred for 6
h at 90 �C under argon. Then the solvent was removed under reduced
pressure, and the precipitates were collected and purified by flash
chromatography through a silica column using n-hexane/EtOAc (5/1) as
the eluent, followed by recrystallization for more than three times to
afford the objective complexes with satisfied purity. The complexes
were dried at 100 �C under vacuum of 1.5 kPa for 24 h.
2.3.6. Bis[2-(3-(9H-carbazol-9-yl)phenyl)benzothiazole-N,C2’]iridium
(III) (picolin ate) (TM2)
Orange solid, recrystallized from a mixture of dichloromethane and
methanol. Yield: 62.5%, m.p.: >280 �C. 1H NMR (400 MHz, DMSO‑d6)
δ(ppm): 8.42 (d, J ¼ 8.4 Hz, 2H, ArH), 8.33–8.18 (m, 8H, ArH), 8.13 (d,
J ¼ 5.2 Hz, 1H, ArH), 8.06 (d, J ¼ 7.6 Hz, 1H, ArH), 7.87 (t, J ¼ 6.0 Hz,
1H, ArH), 7.62–7.40 (m, 10H, ArH), 7.30–7.21 (m, 4H, -picH þ ArH),
7.15–7.11 (m, 2H, -picH þ ArH), 6.83 (d, J ¼ 8.0 Hz, 1H, ArH), 6.59 (d,
J ¼ 8.0 Hz, 1H, ArH), 6.19 (d, J ¼ 8.4 Hz, 1H, ArH), 5.76 (s, 2H, ArH).
ESI-MS: m/z 1066.1858 (M þ Na)þ; Calcd. for (Mw þ Na)þ: 1066.1789.
2.3.4. General procedure for the synthesis of fac-Ir(C^N)3 complexes [39]
A mixture of chloro-bridged dimer complex (0.2 mmol), the
Scheme 1. Molecular structures and synthetic routes of the objective iridium complexes TM1~6.
3

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
2.3.7. fac-Tris[2-(3-(9H-carbazol-9-yl)phenyl)benzothiazole-N,C2’]
iridium(III) (TM3)
TM2 is not available due to its poor solubility in common solvents.
The single crystal samples of TM1, TM3 and TM6 are obtained by
slow vapor diffusion of methanol into their CH2Cl2 solution. The ORTEP
drawing of the crystal structure and the crystallographic refinement
parameters as well as selected bond length/angle data of the three
complexes are given in Fig. S1, Tables S1 and S2. All the three complexes
display a distorted octahedral geometry around the metal center. In the
heteroleptic complex TM1, its two C^N ligands show a C,C-cis, N,N-trans
configuration; but in the case of homoleptic complexes TM3 and TM6,
fac-configurations are observed. As generally, iridium(III) complexes
bearing a fac-configuration show better thermal stability than those with
a mer-configuration [41], TM3 and TM6 are expected to display better
thermostability, hence better photoelectric performance than the other
four complexes. In TM1, the average bond lengths of Ir–C, Ir–N and Ir–O
are 2.000(4) Å, 2.059(3) Å and 2.142(3) Å respectively. The longer Ir–O
bond is due to the strong trans-effect of Ir–C bond. On the other hand, the
average bond lengths of Ir–C and Ir–N in TM3 and TM6 are 2.015(4) Å,
2.162(3) Å and 2.017(3) Å, 2.164(3) Å, respectively. The similarity of
Ir–C and Ir–N in their bond lengths suggests a nearly identical coordi-
nation environment around the Ir center in these homoleptic complexes
[42].
Yellow solid, recrystallized from a mixture of dichloromethane and
methanol. Yield: 42%, m.p.: >280 �C. 1H NMR (400 MHz, CDCl3) δ
(ppm): 8.13 (d, J ¼ 8.0 Hz, 6H, ArH), 7.92–7.90 (m, 6H, ArH), 7.43 (d, J
¼ 8.0 Hz, 6H, ArH), 7.39–7.31 (m, 9H, ArH), 7.27–7.24 (m, 6H, ArH),
7.17 (dd, J ¼ 10.0, 2.0 Hz, 3H, ArH), 7.05–7.01 (m, 6H, ArH), 6.84 (d, J
¼ 8.4, 3H, ArH). 13C NMR (100 MHz, CDCl3) δ (ppm): 178.6, 156.6,
152.3, 142.0, 141.2, 137.4, 132.7, 131.1, 130.6, 127.5, 125.8, 125.2,
124.6, 123.1, 122.8, 120.3, 119.9, 119.6, 110.0. ESI-MS: m/z 1341.2404
(M þ Na)þ; Calcd. for (Mw þ Na)þ: 1341.2395.
2.3.8. Bis[3-(benzo[d]thiazol-2-yl)-N,N-diphenylaniline-N,C2’]iridium
(III) (acetyl acetonate) (TM4)
Red solid, recrystallized from a mixture of dichloromethane and
methanol. Yield: 51.3%, m.p.: 212–215 �C. 1H NMR (400 MHz, CDCl3) δ
(ppm): 8.09 (d, J ¼ 7.6 Hz, 2H, ArH), 7.82 (d, J ¼ 7.6 Hz, 2H, ArH),
7.46–7.40 (m, 6H, ArH), 7.22–7.15 (m, 8H, ArH), 7.06–7.00 (m, 8H,
ArH), 6.92 (t, J ¼ 8.0 Hz, 4H, ArH), 6.46 (dd, J ¼ 8.4 Hz, 2 Hz, 2H, ArH),
6.27 (d, J ¼ 8.4, 2H, ArH), 5.17 (s, 1H, –CH), 1.81 (s, 6H, –CH3). 13
C
NMR (100 MHz, CDCl3) δ (ppm): 185.7, 179.9, 150.8, 147.9, 142.7,
142.4, 141.3, 135.8, 131.5, 129.1, 127.9, 127.3, 125.1, 123.4, 122.3,
122.19, 122.0, 120.2, 101.7, 28.5. ESI-MS: m/z 1069.2193 (M þ Na)þ;
Calcd. for (Mw þ Na)þ: 1069.2198.
The intermolecular packing pattern of the three objective com-
pounds in their crystals is depicted in Fig. 1. In the single crystal
structure of TM1, only a weak “edge-to-face” interaction between the
two C^N ligands (3.518(2) Å) is discernible, suggesting that no severe
2.3.9. Bis[3-(benzo[d]thiazol-2-yl)-N,N-diphenylaniline-N,C2’]iridium
(III) (picolin ate) (TM5)
π-π interactions exist in the crystal sample of TM1. In the case of TM3,
relatively weak π-π interactions are observed, because the distances
Red solid, recrystallized from a mixture of dichloromethane and
methanol. Yield: 50.6%, m.p.: >280 �C. 1H NMR (400 MHz, DMSO‑d6) δ
(ppm): 8.27 (d, J ¼ 8.0 Hz, 1H, ArH), 8.18 (d, J ¼ 8.0 Hz, 1H, ArH),
8.14–8.10 (m, 2H, ArH), 7.99 (d, J ¼ 7.6 Hz, 1H, ArH), 7.94 (d, J ¼ 5.2
Hz, 1H, ArH), 7.79 (t, J ¼ 7.2 Hz, 1H, ArH), 7.54–7.44 (m, 4H, ArH),
7.41 (t, J ¼ 8.0, 1H, ArH), 7.28–7.21 (m, 8H, -picH þ ArH), 7.15–7.11
(m, 1H, ArH), 7.01–6.94 (m, 12H, ArH), 6.62 (dd, J ¼ 10.4, 2.0 Hz, 1H,
ArH), 6.54 (dd, J ¼ 8.0, 2.0 Hz, 1H, ArH), 6.44 (d, J ¼ 8.0 Hz, 1H, ArH),
6.19 (d, J ¼ 8.0, 1H, ArH), 6.05 (d, J ¼ 8.4, 1H, ArH). 13C NMR (100
MHz, CDCl3) δ (ppm): 181.4, 178.7, 173.1, 153.2, 149.9, 149.8, 148.7,
147.9, 147.7, 143.7, 142.5, 142.0, 141.9, 141.4, 138.0, 135.7, 135.1,
131.9, 131.2, 129.3, 129.1, 128.9, 128.4, 128.1, 127.9, 127.1, 125.8,
125.0, 123.5, 123.3, 123.1, 122.6, 122.4, 122.2, 122.0, 121.2, 117.9,
37.1, 32.0, 30.1, 29.7, 29.4, 27.1, 22.7, 14.2. ESI-MS: m/z 1092.1991
(M þ Na)þ; Calcd. for (Mw þ Na)þ: 1092.1994.
between the benzothiazole and benzothiazole rings, as well as carbazole
and carbazole rings, are 3.846(2) Å and 3.798(3) Å, respectively.
However, in TM6, relatively strong
π-π stacking can be observed,
because the intermolecular interplanar distance between its benzothia-
zole and benzothiazole rings is as short as 3.648(2) Å. Accordingly, the
carbazolyl-modified complexes should show more alleviated concen-
tration quenching than their diphenylamino-modified counterparts.
3.2. Photophysical properties
The UV–Vis absorption spectra and photoluminescence (PL) spectra
of all the complexes studied here are depicted in Fig. 2 and Fig. 3, and
representative data are summarized in Table 1. For the purpose of
comparison, the absorption spectrum of Ir-bt is also given. All these
complexes exhibit two distinguishable absorption bands in their ab-
sorption spectra, namely, an intense absorption band at 280–380 nm
and a weak one at 380–550 nm. For the intense absorption band,
TM1~3 show quite analogous spectral profiles, and TM4~6 show
similar absorption spectra. But the latter three complexes all possess a
red-shifted absorption band than that of TM1~3. Consequently, this
2.3.10. fac-Tris[3-(benzo[d]thiazol-2-yl)-N,N-diphenylaniline-N,C2’]
iridium(III) (TM6)
Red solid, recrystallized from a mixture of dichloromethane and
1
�
methanol. Yield: 28.6%, m.p.: >280 C. H NMR (400 MHz, CDCl3) δ
(ppm): 7.80 (d, J ¼ 8.0 Hz, 3H, ArH), 7.42 (d, J ¼ 2.4 Hz, 3H, ArH),
7.23–7.16 (m, 15H, ArH), 7.02 (d, J ¼ 7.6 Hz, 12H, ArH), 6.93–6.88 (m,
9H, ArH), 6.70–6.68 (m, 6H, ArH), 6.59 (d, J ¼ 8.0, 3H, ArH). 13C NMR
(100 MHz, CDCl3) δ (ppm): 178.8, 155.8, 152.4, 147.8, 141.5, 141.4,
140.9, 136.9, 132.7, 129.8, 129.0, 127.0, 124.6, 123.2, 123.1, 122.4,
absorption band is safely assigned to the 1π
-
π
* transition of the C^N
ligand [11]. In the case of the weak absorption band, TM1~3 shows
similar absorption onsets with that of Ir-bt, but the absorption onsets of
TM4~6 are all observed to be bathochromically shifted than those of
TM1~3 and Ir-bt, indicative of the lower optical bandgaps (Eg) of
TM4~6 than those of TM1~3 and Ir-bt. Besides, for complexes bearing
either carbazolyl- or diphenylamino-modified C^N ligands, the ones
with a homoleptic-structure show the largest Egs, while the ones bearing
an ancillary ligand of acac show the smallest Egs.
121.8, 119.8. ESI-MS: m/z 1347.2879 (M þ Na)þ; Calcd. for (Mw
þ
Na)þ: 1347.2864.
3. Results and discussion
Consistent with the absorption characteristics, with regard to the PL
spectra, the carbazolyl-substituted complexes TM1~3 show yellow
emission with their λPLmax locating at 555–565 nm, which is similar to
3.1. Synthesis and characterization
The synthetic procedure of the parent compound Ir-bt can be found
in our previous report [24a]. The six objective complexes of TM1~6 are
synthesized following the similar procedures reported in references[40]
the parent compound Ir-bt (λ
¼ 557 nm). However, TM4~6
PLmax
bearing stronger electron-donating diphenylamino substituents display
red or even deep-red emission with λ
of 617–636 nm, which is
[24a,38-40]. Their molecular structure is characterized by 1H NMR, 13
C
significantly red-shifted than that PLomfax Ir-bt. Additionally, the
heteroleptic-structured TM1 and TM4 using acac as the ancillary ligand
are both observed to show the utmost red-shifted PL band, but the
homoleptic-structured TM3 and TM6 have their PL bands located at the
NMR, and HR-ESI-MS spectrometry. Besides, the molecular structure of
TM1, TM3 and TM6 is further verified by single crystal X-ray diffraction.
It should be pointed out that the 13C NMR spectrum data of compound
4

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
Fig. 1. Crystal packing diagrams of TM1, TM3 and TM6.
highest energy region among TM1~3 and TM4~6, respectively. The
more red-shifted λ
of the acac-comprising complexes should be
attributed to the loPwLmeraxligand field of acac than that of either the C^N
ligand or picolinic acid, through which a low crystal field splitting en-
ergy, hence a reduced energy gap of the complexes can be achieved [43].
Based on these experimental findings, it can be deduced that the
introduction of strong electron-donating substituents into the meta-site
of the phenyl moiety of bt ligand would result in complexes with red-
shifted PL emission, and the magnitude of the spectral redshift corre-
lates highly with the electron-donating ability of the substituents.
Further fine-tuning on the PL emission band could be realized through
replacing one of the C^N ligand in homoleptic complexes with picolinic
acid or acetylacetone to construct corresponding heteroleptic-structured
complexes.
Subsequently, the absolute photoluminescence quantum yields
(PLQYs) of the complexes are measured by doping 5 wt% of TM1~6 into
a poly(methyl methacrylate) (PMMA) matrix (data summarized in
Table 1). The results show that except for TM1 (PLQY: 8.9%), TM2~6
all show a relatively high PLQY (23–25%), which is propitious to their
EL performance.
Fig. 2. Normalized absorption spectra of the complexes studied here in 10À 5
mol LÀ 1 CH2Cl2 solution at 298 K.
3.3. Electrochemical and thermal properties
The electrochemical properties of these complexes are investigated
by cyclic voltammetry (CV) in an argon purged 5 � 10À 4 mol LÀ 1 CH2Cl2
solution with the Fc/Fcþ redox couple as reference, and the voltam-
mograms are shown in Fig. 4, data are summarized in Table 2. During
the anodic scan, all the six complexes exhibit reversible oxidation waves,
implying that they have good electrochemical stability. In the scanning
region of À 0.2–0.85 V, the carbazolyl-modified TM1~3 all display a
one-electron oxidation characteristics. Consequently, the HOMO energy
levels of TM1~3 are estimated to be À 5.37, À 5.42, À 5.25 eV, respec-
tively. According to the corresponding data of HOMO energy level and
Fig. 3. Normalized PL spectra of the complexes studied here in 10À 5 mol LÀ 1
CH2Cl2 solution at 298 K under irradiation of 420 nm.
Table 1
Photophysical data of the complexes studied here.
Compounds
λabs (nm)a
λPLmax (nm)a
ϕPL
b
Ir-bt
TM1
TM2
TM3
TM4
TM5
TM6
313, 327, 358, 447, 488
293, 310, 330, 415, 452, 490
292, 327, 396, 447
557
565
559
555
636
631
618
–
8.9%
26.5%
23.1%
25.0%
25.3%
25.2%
293, 325, 345, 404, 436, 480
308, 416, 520
309, 319, 398, 506
308, 438, 505
a
Determined in 10À 5 mol LÀ 1 CH2Cl2 solution at 298 K.
b
Absolute PL quantum yields of the 5 wt% doped film samples (with poly
(methyl methacrylate) (PMMA) as the host) determined using an integrating
sphere at 298 K.
Fig. 4. Cyclic voltammograms of TM1~6. The oxidation potentials are deter-
mined relative to Ag/Agþ in 5 � 10À 4 mol⋅LÀ 1 CH2Cl2 solution, using ferrocene
as external reference.
5

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
Table 2
Electrochemical and thermal data of the complexes studied here.
Eg (eV)b
HOMOc (eV)
LUMOd (eV)
HOMOe (eV)
LUMOe (eV)
Eg (eV)
ox
a
f
Td (�C)
Compounds
E
1/2
Ir-bt
TM1
TM2
TM3
TM4
TM5
TM6
0.51
0.57
0.62
0.45
0.13
0.20
0.08
2.23
2.18
2.25
2.21
1.97
2.00
2.14
À 5.31
À 5.37
À 5.42
À 5.25
À 4.93
À 5.00
À 4.88
À 3.08
À 3.19
À 3.17
À 3.04
À 2.96
À 3.00
À 2.74
À 5.28
À 5.23
À 5.33
À 5.28
À 4.77
À 4.87
À 4.82
À 1.80
À 1.96
À 2.05
À 1.94
À 1.86
À 1.95
À 1.82
3.48
3.27
3.28
3.34
2.91
2.92
3.00
326
356
338
443
313
349
342
a
Oxidation potential values are measured in CH2Cl2 solution containing 5 � 10À 4 mol LÀ 1 of the iridium complexes, potential values are reported versus Fc/Fcþ.
b
c
d
e
f
Eg are estimated from the onset wavelength of the optical absorption bands.
HOMO energies are deduced from the equation HOMO ¼ -(4.8 þ Eox 1/2).
LUMO energies are obtained from the equation LUMO ¼ HOMO þ Eg.
Obtained from B3LYP calculations within the framework of the IEF-PCM model in CH2Cl2.
Temperature with 5 wt % loss.
ox
Eg, the LUMO energy levels of TM1~3 are estimated to be À 3.19, À 3.17
Consequently, on the basis of the first oxidation potentials (E ), the
1/2
and À 3.04 eV in sequence. Note that the energy level data of both the
HOMO and LUMO of TM1~3 are close to that of the parent complex Ir-
bt (HOMO: À 5.31 eV; LUMO: À 3.08 eV) [24a]. Therefore, the absorp-
tion onset and PL emission color of TM1~3 are similar to those of Ir-bt.
However, in similar scanning region of À 0.1–0.75 V, the
diphenylamino-decorated TM4~6 all display a two-electron oxidation
profile. The ones with lower oxidation potentials are assigned to the
oxidation of iridium (III/IV) metal center as well as the C^N ligand;
whereas the ones with higher oxidation potentials are attributed to the
oxidation of the triphenylamine moiety of the C^N ligand [44].
HOMO energy levels of TM4~6 are calculated to be À 4.93, À 5.00 and
À 4.88 eV, respectively; and the LUMO energy levels of TM4~6 are
estimated to be À 2.96, À 3.00 and À 2.74 eV in sequence. In comparison
with TM1~3 and Ir-bt, TM4~6 all show dramatically elevated HOMO
and slightly elevated LUMO energy levels. Hence the reduced Eg and
red-shifted PL band of TM4~6 than those of TM1~3 should be mainly
attributed to the more elevated HOMO energy levels of TM4~6 than
TM1~3 originating from the stronger electron-donating capability of
diphenylamino group than the carbazol-9-yl group at the meta-site of the
phenyl moiety of bt.
Fig. 5. Isodensity plots of the HOMOs and LUMOs for TM1~6.
6

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
Thermogravimetric analysis (TGA) (data summarized in Table 2)
results, TM3 and TM6 are selected as the phosphorescent guests to
fabricate PhOLEDs with single or double light-emitting-layer (EML). The
configuration of the single-EML devices is: ITO/TAPC (40 nm)/
26DCzPPy:TM3 or TM6 (x wt%) (10 nm)/TPBi (40 nm)/LiF (1 nm)/Al
(100 nm); and the configuration of the double-EML devices is: ITO/
TAPC (40 nm)/TCTA: TM3 or TM6 (x wt%) (10 nm)/26DCzPPy: TM3 or
TM6 (x wt%) (10 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm), where
TAPC serves as the hole-transporting layer; 26DCzPPy and TCTA serve
as the host materials for TM3 or TM6 with electron- and hole-
transporting capability, respectively; TPBi serves as the electron-
transporting layer. The two emission layers in double-EML devices
have identical doping levels. The devices based on TM3 and TM6 are
named as device I and II, respectively; a~e represents single-EML de-
vices, and f~j stands for double-EML devices with different doping
levels. For TM3-based devices, the doping levels are 3 wt%, 4 wt%, 5 wt
%, 6 wt%, 7 wt%; for TM6-based devices, the doping levels are 2 wt%, 3
wt%, 4 wt%, 5 wt%, 6 wt%, respectively. The relative energy level
alignments of the devices I and II, and the molecular structure of the
materials used are shown in Fig. 6.
shows that except for TM4 (Td ¼ 313 �C), all these objective complexes
show better thermal stability than the reference compound of Ir-bt (Td:
>330 �C vs 326 �C) [24a], implying that the incorporation of carbazole
and diphenylamine units with the C^N ligand can effectively improve
the thermal stability of the resultant chelates. Furthermore, the homo-
leptic complexes TM3 and TM6 are both found to show higher Tds than
their acetylacetone-comprising counterparts, confirming that the
homoleptic complexes show better thermal stability.
3.4. DFT theoretical calculation
To gain insights into the effect of substituents and auxiliary ligands
on the Eg and energy levels of the complexes, density functional theory
(DFT) calculations are performed on TM1~6 with the Gaussian09
software. The electronic density distribution of the HOMOs and LUMOs
of TM1~6 is depicted in Fig. 5, and the calculated HOMO and LUMO
energy level data are summarized in Table 2. Although the calculated
LUMOs are significantly shallower than those estimated from experi-
mental data, the similar variation trends in both LUMO energy level and
Eg of the complexes between the calculation results and experiment
findings illustrates the reliability of our computation results.
For single- and double-EML OLEDs based on the same guest phos-
phor with similar doping levels, their EL spectra are almost identical
(Fig. 7), implying that the presence and absence of a hole-transporting
EML has negligible impact on the EL process of the guests. All the
TM3-based devices Ia~j, regardless of their different doping levels of
3–7 wt%, exhibit yellow EL with CIE coordinates of (0.43, 0.56). The
dopant-concentration independent EL spectra suggest that there only
exist weak intermolecular interactions in TM3. The EL spectra of devices
As depicted in Fig. 5, the carbazolyl and diphenylamino substituents
are both observed to participate in the HOMO of the chelates, whereas
the LUMOs mainly locate on the 2-phenylbenzothiazole segment, indi-
cating that the substituents should have a major effect on the HOMO
energy levels of the complexes. This is consistent with the electro-
chemical characterization results. Further calculation on the orbital
compositions of the HOMO of TM1~6 (shown in Table S1) reveals that
the substituents have an obvious contribution to the HOMOs
(41.5–64.2%), while the contributions from Ir-d orbitals are only
11.7–26.3%. In addition, for all the target compounds, the HOMO-
Ia~j (λ
¼ 558 nm) match well with the corresponding PL spectra of
ELmax
TM3 in diluted CH2Cl2 solution (λ
¼ 555 nm), suggestive of the
PLmax
analogous luminescence processes of EL and PL. In contrast, the λELmax of
the TM6-based devices II is concentration-dependent. With increasing
dopant concentration from 2 wt % to 6 wt %, the corresponding EL
spectrum is bathochromic-shifted gradually from 590 nm to 601 nm.
Note that the all the TM6-based devices have their λELmax blue-shifted
for 16–26 nm than the PL spectrum of TM6 in diluted CH2Cl2 solution
(λPLmax ¼ 618 nm), which is tentatively ascribed to the higher envi-
ronmental polarity of CH2Cl2 than either the TCTA or the 26DCzPPy
host. To validate this conjecture, PL spectra of binary host-guest films of
TM3 or TM6 in 26DCzPPy or TCTA with similar doping levels of the
→LUMO transitions show a d(Ir) þ
π
(sub þ C^N)→
π
*(C^N) configura-
tion, implying that the transition process possesses a metal-to-ligand
charge-transfer (MLCT) characteristics.
3.5. Electroluminescence properties
Based on the photophysical and thermal stability experimental
Fig. 6. The relative energy level alignments of the devices I and II and the molecular structure of the materials used.
7

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
the concentration quenching of TM6 [24b], or the higher molecular
polarity of TM6 than 26DCzPPy or TCTA. It is noteworthy that when
doped in 26DCzPPy, the TM6-based films show slightly red-shifted
λ
than those doped in TCTA with similar doping levels (e.g., at
dPoLpmianxg level of 4 wt%, λPLmax is 598 nm in TCTA host, but 603.5 nm in
26DCzPPy host). Consequently, the concentration-dependent EL spectra
of these TM6-based devices II should originate from the concentration
quenching of TM6, the higher molecular polarity of the guest TM6 than
the hosts, leading to orange EL with their CIE coordinates shifted from
(0.53, 0.42) to (0.59, 0.41).
In addition, for the 3 wt%-doped devices Ia and If and 2 wt%-doped
devices IIa and IIf, an extra EL band at the blue-light region of 380–450
nm is observed. According to Fig. S2, the blue EL of these low doping-
level devices should be assigned to the emission from the hosts of
26DCzPPy and/or TCTA due to the inefficient host-guest energy trans-
fer. It should be pointed out that the energy transfer efficiency in the
TM3-based samples are much higher than that in the TM6-based ones,
because more effective spectral overlap exists between the absorption
spectrum of TM3 and PL emission spectra of 26DCzPPy or TCTA (vide
Fig. S3). Additionally, according to the device energy level diagram,
TM3 shows a better trapping effect on both hole and electron carriers
[45], while TM6 only acts as a trap for holes, and no distinct electrons
trapping may occur on it.
The corresponding current density-voltage-luminance (J-V-L) and
current efficiency-current density-power efficiency characteristics of
these devices are shown in Fig. 8 and Fig. 9, some representative data are
summarized in Table 3. All the TM3-based devices I exhibit low turn-on
voltage of 3.1–3.2 V, suggesting that the presence of an extra 10 nm-
thick EML and the different guest concentrations have negligible effects
on the turn-on voltages of these devices. The single-EML devices of Ia~e
can emit bright yellow light with the maximum brightness (Lmax) of
33390–44910 cd⋅mÀ 2, and the highest brightness is obtained at a
doping-level of 5 wt%. In the case of the double-EML devices, they all
show much higher maximum brightness than their single-EML coun-
terparts. Note that an expressive Lmax of 60860 cd⋅mÀ 2 is achieved in the
5 wt% doped device Ih. In addition to Lmax, the maximum current effi-
ciency (ηLmax) and power efficiency (ηPmax) of the 5 wt%-doped double-
EML device are also observed to be much higher than those of the 5 wt
%-doped single-EML device (ηLmax: 54.2 vs 50.6 cd⋅AÀ 1
; ηPmax: 42.9 vs
39.5 lm⋅WÀ 1), while the ηPmax data of 41.8 lm⋅WÀ 1 and 48.9 lm⋅WÀ 1 are
separately obtained in single-EML device Ie (7 wt%) and double-EML
device Ig (4 wt%), confirming that the double-EML device structure
can lead to much improved EL performance. Note that to our knowledge,
the ηPmax value of 48.9 lm⋅WÀ 1 is one of the highest reported power
efficiency data of yellow PhOLEDs [5,20,30,31,46–52].
It should be pointed out that all these yellow devices, either single-
EML or double-EML, display rather low efficiency roll-off. In fact, a
high ηL of 51.4 cd⋅AÀ 1 could be even achieved at 1000 cd⋅mÀ 2 for device
Ih (5 wt%), and the efficiency roll-off is only as low as 5.17%, indicative
of its potential applications for white OLED lighting sources.
Analogous to devices I, the TM6-based devices II also display low
turn-on voltage of 3.2–3.4 V (vide Table 3), regardless of their different
device structures and dopant concentrations. The Lmax of the five devices
IIa~e with single EML is 9960–14350 cd⋅mÀ 2, which are much inferior
to the corresponding Lmax data of the double-EML devices
(14330–19870 cd⋅mÀ 2). For single-EML and double-EML devices, the
best performance is both acquired in the 4 wt%-doped devices. The ηLmax
of devices IIc and IIh is 14.9 cd⋅AÀ 1 and 18.6 cd⋅AÀ 1, respectively. In the
double-EML device IIg, relatively low efficiency roll-off is observed,
Fig. 7. EL spectra of device I and device II.
PhOLEDs are recorded. As depicted in Fig. S2, in both the two hosts,
since its ηL is determined to be 14.1 cd⋅AÀ 1 at 1000 cd⋅mÀ 2
.
All these EL characterization results indicate that TM3 is a very
promising yellow phosphorescent emitter for white OLEDs applications.
Although TM6 could also display moderate EL performance, it shows
inferior EL performance to that of TM3. This may be ascribed to the
more properly matched energy levels and higher energy transfer effi-
ciency between the selected hosts and TM3, and more alleviated
regardless of the different doping levels, the λ
of the TM3-based
PLmax
films locates at 558 nm; but in both 26DCzPPy and TCTA, with
increasing concentration of TM6 from 2 wt% to 6 wt%, the resultant
films have their λPLmax slightly red-shifted, which should be ascribed to
8

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
Fig. 8. Current density-voltage-luminance(J-V-L) characteristics of device I (a,
b) and device II (c,d).
Fig. 9. Current efficiency-current density-power efficiency characteristics of
device I (a,b) and device II (c,d).
intermolecular interactions of TM3 than TM6.
ability of the substituents, but also be fine-tuned through the alteration
of the acetylacetone into picolinic acid or the corresponding C^N ligand.
PhOLEDs with the carbazol-9-yl-modified TM3 or the diphenylamino-
modified TM6 as phosphors show relatively high EL performance. The
4. Conclusions
In summary, a series of novel Ir-bt derivatives are developed by
introducing an electron-donating carbazolyl or diphenylamino substit-
uent into its cyclometalated ligand, and using 2-picolinic acid instead of
acetylacetone as the ancillary ligand or employing a homoleptic struc-
ture. Through this strategy, the emission color of the objective iridium
complexes could be tuned from yellow to red, and the magnitude of the
spectral red-shift can be not only adjusted through the electron-donating
TM3-based yellow devices show ηLmax of 54.2 cd⋅AÀ 1
, ηPmax of 48.9
lm⋅WÀ 1 and CIE coordinates of (0.43, 0.56), together with satisfactory
efficiency roll-off. Note that the ηPmax data of 48.9 lm⋅WÀ 1 is one of the
highest reported power efficiency data for yellow PhOLEDs. The TM6-
based orange devices show ηLmax of 18.6 cd⋅AÀ 1
,
ηPmax of 14.3 lm⋅WÀ 1
9

J. Liu et al.
Dyes and Pigments 175 (2020) 108145
Table 3
EL characteristics of device I and device II based on TM3 and TM6.
TM3
device Ia
device Ib
device Ic
device Id
device Ie
device If
device Ig
device Ih
device Ii
device Ij
Dopant ratio (wt%)
3
4
5
6
7
3
4
5
6
7
λELmax (nm)
557
558
557
557
557
558
558
558
559
559
V
turn-on (V)a
3.2
3.1
3.1
3.1
3.1
3.3
3.2
3.2
3.2
3.2
b
L
max (cd⋅mÀ 2
)
33390
31.5
39420
46.0
44910
50.6
41670
44.6
38660
45.1
47230
44.0
55420
53.9
60860
54.2
55240
48.0
52800
45.9
ηLmax (cd⋅AÀ 1
)
d
c
ηPmax (lm⋅WÀ 1
)
18.4
33.4
39.5
37.7
41.8
37.3
48.9
42.9
42.7
38.8
e
ηL (cd⋅AÀ 1
CIE (x, y)
TM6
)
31.4
43.7
46.7
42.1
41.2
42.1
50.1
51.4
43.8
43.6
0.42, 0.56
device IIa
0.43, 0.57
device IIb
0.42, 0.57
device IIc
0.42, 0.57
device IId
0.42, 0.57
device IIe
0.42, 0.56
device IIf
0.43, 0.57
device IIg
0.43, 0.56
device IIh
0.43, 0.56
device IIi
0.43, 0.56
device IIj
Dopant ratio (wt%)
2
3
4
5
6
2
3
4
5
6
λELmax (nm)
592
596
597
602
601
590
598
598
603
601
V
turn-on (V)a
3.2
3.2
3.1
3.2
3.2
3.3
3.4
3.4
3.3
3.3
b
Lmax (cd⋅mÀ 2
)
9960
12.6
8.2
12600
13.8
10.8
9.4
14350
14.9
10.3
12.2
0.58, 0.42
13640
13.5
10.9
10.6
0.58, 0.41
12240
13.4
11.1
10.4
0.58, 0.41
14330
18.4
14.4
10.3
0.53, 0.42
18030
16.7
13.8
12.0
0.56, 0.43
19870
18.6
14.3
14.1
0.57, 0.42
18540
16.2
14.1
12.2
0.59, 0.41
16620
15.6
14.1
11.4
0.59, 0.41
ηLmax (cd⋅AÀ 1
)
d
c
ηPmax (lm⋅WÀ 1
)
ηL (cd⋅AÀ 1
)
7.7
e
CIE (x, y)
0.54, 0.42
0.57, 0.42
a
The driven voltages of the device at 1 cd⋅mÀ 2
.
b
c
Maximum brightness of the device.
Maximum current efficiencies of the device.
Maximum power efficiencies of the device
d
e
current efficiencies of the device at 1000 cd⋅mÀ 2
.
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and CIE coordinates of (0.57, 0.42). All these preliminary studies reveal
that this rational molecular design strategy is a simple yet effective
means to realize color tunability as well as the enhancement of EL
performance of iridium complexes.
Declaration of competing interest
[6] Zhang T, Liu D, Wang Q, Wang R, Ren H, Li J. Deep-blue and white organic light-
emitting diodes based on novel fluorene-cored derivatives with
naphthylanthracene endcaps. J Mater Chem 2011;21:12969–76.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[7] Hu YX, Xia X, He WZ, Tang ZJ, Lv YL, Li X, et al. Recent developments in
benzothiazole-based iridium(III) complexes for application in OLEDs as
electrophosphorescent emitters. Org Electron 2019;66:126–35.
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CRediT authorship contribution statement
[9] Fan CH, Sun P, Su TH, Cheng CH. Host and dopant materials for idealized deep-red
organic electrophosphorescence devices. Adv Mater 2011;23:2981–5.
[10] Feng Z, Yu Y, Yang X, Zhong D, Song D, Yang H, et al. Isomers of coumarin-based
cyclometalated Ir(III) complexes with easily tuned phosphorescent color and
features for highly efficient organic light-emitting diodes. Inorg Chem 2019;58:
7393–408.
Jingjing Liu: Investigation, Writing - original draft, Data curation.
Ming Li: Formal analysis, Visualization, Validation. Zhiyun Lu:
Conceptualization, Methodology, Funding acquisition, Writing - review
& editing. Yan Huang: Project administration, Supervision. Xuemei Pu:
Software, Methodology, Investigation. Liang Zhou: Resources,
Investigation.
[11] (a) Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Kwong R, Tsyba I, et al.
Synthesis and characterization of phosphorescent cyclometalated iridium
complexes. Inorg Chem 2001;40:1704–11.(b) 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;123:4304–12.
[12] (a) Wang R, Liu D, Ren H, Zhang T, Yin H, Liu G, et al. Highly efficient orange and
white organic light-emitting diodes based on new orange iridium complexes. Adv
Mater 2011;23:2823–7.(b) Wang R, Deng L, Zhang T, Li J. Substituent effect on
the photophysical properties, electrochemical properties and electroluminescence
performance of orange-emitting iridium complexes. Dalton Trans 2012;41:
6833–41.
Acknowledgements
We acknowledge the support from the National Natural Science
Foundation of China (No. 21672156 and 21771172). We thank Dr D.
Luo of Analytical & Testing Center, Sichuan University for crystallo-
graphic measurements.
[13] Wang R, Liu D, Ren H, Zhang T, Wang X, Li J. Homoleptic tris-cyclometalated
iridium complexes with 2-phenylbenzothiazole ligands for highly efficient orange
OLEDs. J Mater Chem 2011;21:15494–500.
Appendix A. Supplementary data
[14] Wang R, Liu D, Zhang R, Deng L, Li J. Solution-processable iridium complexes for
efficient orange-red and white organic light-emitting diodes. J Mater Chem 2012;
22:1411–7.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2019.108145.
[15] Wang JS, Xu XJ, Tian Y, Yao C, Li LD. Obtaining highly efficient single-emissive-
layer orange and two-element white organic light-emitting diodes by the solution
process. J Mater Chem C 2014;2:5036–45.
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