Novel yellow phosphorescent iridium complexes with dibenzothiophene-S,S-dioxide-based cyclometalated ligand for white polymer light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2018.07.019

We have designed and synthesized two novel yellow phosphorescent iridium complexes using dibenzothiophene-S,S-dioxide-based cyclometalated ligand for the first time, which are capable of producing highly efficient yellow and white polymer light-emitting devices (PLEDs). The resulted iridium complexes display good thermal stability and high photoluminescence quantum yields. Both yellow and white PLEDs are fabricated with an identical single-emission-layer configuration of ITO/PEDOT:PSS/emission layer (EML)/CsF/Al. For the yellow phosphorescent PLEDs based on (p-CzFSOPy)₂IrPic, the best device performances with a peak luminous efficiency (LE) of 13.3 cd/A and a peak external quantum efficiency (EQE) of 5.3% are achieved. More importantly, the two-element WPLEDs containing iridium (III)bis (2-(4,6-difluorophenyl)-pyridinato-N,C^2′) picolinate (FIrpic) as blue and (p-CzFSOPy)₂IrPic as yellow phosphors doped into a PVK:OXD-7 matrix at an appropriate ratio exhibited a maximum LE of 19.2 cd/A, a maximum EQE of 9.6%, an extremely high luminance of 18717 cd/m² and Commission Internationale de L'Eclairage (CIE) coordinate of (0.317, 0.448). Moreover, at a luminance for practical application of 1000 cd/m², the LE still remains as high as 19.0 cd/A, with a very slight decrease.

Full text of DOI:10.1016/j.dyepig.2018.07.019

Dyes and Pigments 159 (2018) 637–645
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel yellow phosphorescent iridium complexes with dibenzothiophene-S,
S-dioxide-based cyclometalated ligand for white polymer light-emitting
diodes
Aihui Liang , Ming Luo , Yusheng Liu , Han Wang , Zhiping Wang , Xiaoyan Zhengc,∗∗,
a,∗
a
b
a
a
a
a
b,∗∗∗
d,∗∗∗∗
Tian Cao , Dewang Liu , Yong Zhang
College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, PR China
Institute of Optoelectronic Material and Technology, South China Normal University, Guangzhou, 510631, PR China
Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and
, Fei Huang
a
b
c
Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China
d
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou,
510640, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
We have designed and synthesized two novel yellow phosphorescent iridium complexes using dibenzothiophene-
S,S-dioxide-based cyclometalated ligand for the first time, which are capable of producing highly efficient yellow
and white polymer light-emitting devices (PLEDs). The resulted iridium complexes display good thermal stability
and high photoluminescence quantum yields. Both yellow and white PLEDs are fabricated with an identical
single-emission-layer configuration of ITO/PEDOT:PSS/emission layer (EML)/CsF/Al. For the yellow phos-
Polymer light-emitting diode
Electrophosphorescence
Iridium complexes
Dibenzothiophene-S,S-dioxide
Carbazole
phorescent PLEDs based on (p-CzFSOPy)
2
IrPic, the best device performances with a peak luminous efficiency
(
LE) of 13.3 cd/A and a peak external quantum efficiency (EQE) of 5.3% are achieved. More importantly, the
2
′
two-element WPLEDs containing iridium (III)bis (2-(4,6-difluorophenyl)-pyridinato-N,C ) picolinate (FIrpic) as
blue and (p-CzFSOPy) IrPic as yellow phosphors doped into a PVK:OXD-7 matrix at an appropriate ratio ex-
hibited a maximum LE of 19.2 cd/A, a maximum EQE of 9.6%, an extremely high luminance of 18717 cd/m and
Commission Internationale de L'Eclairage (CIE) coordinate of (0.317, 0.448). Moreover, at a luminance for
2
2
2
practical application of 1000 cd/m , the LE still remains as high as 19.0 cd/A, with a very slight decrease.
1. Introduction
for the fabrication of the low-cost devices because of its greater ease of
processing and large-area manufacturability [7,16–19]. However,
White organic light-emitting devices (WOLEDs) have become of
WPLEDs display much poorer device performance in terms of power
efficiency (PE), luminous efficiency (LE).
considerable commercialization interest in recent years for their po-
tential applications in next-generation full color displays, back lighting
of flat-panel displays and solid-state lighting [1–10]. In order to gen-
erate the desired white-light emission based on polymers and small
molecules, the hybrids of three primary colors (red, green and blue) or
two complementary colors, for example, blue and yellow, are typically
required, and several methods and technologies are adopted towards
realizing WOLEDs [6,11–15]. Compared to the WOLEDs fabricated by
vacuum-deposition technologies, white polymer light-emitting devices
In order to obtain high-efficiency WPLEDs, electrophosphorescence
is most effective due to its demonstrated potential for achieving nearly
100% internal quantum efficiency by harvesting both singlet and triplet
excitons [5,20,21]. Compared to other phosphorescent emitters, cy-
clometalated iridium complexes are of paramount importance in the
field because of their short lifetime in excited states and high phos-
phorescent efficiencies for highly efficient OLEDs [22–24]. Up to date,
most of the high-performance WPLEDs ever reported are based on so-
lution-processed dye-doped systems, in which iridium complexes are
(
WPLEDs) fabricated by solution-processing technique, is more feasible
∗
Corresponding author.
Corresponding author.
∗
∗
∗
∗
∗∗
Corresponding author.
∗∗∗
Corresponding author.
E-mail addresses: lah14god@163.com (A. Liang), zycq@scnu.edu.cn (X. Zheng), xiaoyanzheng@bit.edu.cn (Y. Zhang), msfhuang@scut.edu.cn (F. Huang).
https://doi.org/10.1016/j.dyepig.2018.07.019
Received 12 May 2018; Received in revised form 29 June 2018; Accepted 11 July 2018
0143-7208/ © 2018 Elsevier Ltd. All rights reserved.

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
doped into polymer (for example poly (N-vinylcarbazole) (PVK)) as
emissive materials [11,25–27]. As a result, the WPLEDs with a combi-
nation of efficient electrophosphorescent blue and yellow emitters will
likely optimize the efficiencies and simplify the device fabrication
process. Unfortunately, to our best knowledge, the highly efficient
yellow emitting iridium complexes have been reported rarely in the
spin-coated WPLEDs [18,28]. For example, Wu et al. reported WPLEDs
with single emitting layers containing skyblue emitting iridium (III)bis
the solid was collected. The crude product was recrystallized with
chloroform to afford a white solid (4.1 g, 95%). 1H NMR (400 MHz,
CDCl
3
, TMS) δ (ppm): 7.96 (d, J = 8.6 Hz, 2H), 7.65 (d, J = 9.8 Hz,
2H), 7.39–7.33 (m, 4H).
2.1.2. Synthesis of 3,7-dibromodibenzothiophene S,S-dioxide (2) [42]
At 0 °C, 3.89 g (18.0 mmol) of dibenzothiophene S,S-dioxide dis-
solved in 120 mL of H SO , and then 8.0 g (45 mmol) of NBS was added
2 4
to the mixture and stirred for 10 h. The reaction was quenched by ad-
dition of water (40 mL) and extracted by dichloromethane (3 × 20 mL).
2′
(
2-(4,6-difluorophenyl)-pyridinato-N,C )picolinate (FIrpic) and some
yellow emitting fluorine-based iridium complexes doped into
a
PVK:OXD-7 matrix, which exhibited an extremely high luminous effi-
ciency (LE) of 42.9 cd/A. However, the device displayed warm white
light with the Commission Internationale de L'Eclairage (CIE) co-
ordinates of (0.395, 0.452) [12]. We have synthesized several yellow
phosphorescent iridium complexes with triphenylamine- and fluorene-
functionalized cyclometalating ligands. The WPLEDs based on the
synthesized iridium complex and FIrpic exhibited the maximal LE of
2 4
The organic layer was dried with anhydride Na SO . The solvent was
removed in vacuo. The residue was purified by flash column chroma-
1
tography to afford 5.86 g of white powder with a yield of 87%. H NMR
(
2
400 MHz, CDCl
H), 7.64–7.62 (d, J = 8.4 Hz, 2H).
3
, TMS) δ (ppm): 7.93 (s, 2H), 7.78–7.76 (d, J = 10 Hz,
2.1.3. Synthesis of 2,8-dibromodibenzothiophene (3) [43]
14.7 cd/A with a CIE coordinates of (0.33, 0.42) [29]. The CIE co-
To a solution of dibenzothiophene (9.2 g, 50.0 mmol) in chloroform
ordinates are very close to the ideal pure white color, but the device
performances are not very high. Thus, the design and synthesis of new
functional organic ligands for the iridium metal core is highly desirable
in order to improve the efficiency and color purity of the WPLEDs.
On another aspect, the charge balance of electrons and holes are
also becoming an important issue for a new breakthrough in OLED
research. Along this line, we and others are interested in developing
some multi-component systems capable of performing separate func-
tional roles in a single molecule [30–32]. As we know, the electron-
deficient dibenzothiophene-S,S-dioxide moiety was demonstrated to be
beneficial for increasing electron affinity and fluorescence efficiency,
and improving the electron transporting properties of materials
(
100 mL), bromine (7.7 mL, 150 mmol) was dropwise added at 0 °C by
stirring. Under nitrogen atmosphere the reaction mixture was stirred
and kept overnight at room temperature. Saturated NaHSO aqueous
3
solution was added to the mixture. Then, the crude product was filtered
off and washed with methanol to isolate 2,8-dibromodibenzothiophene
(
(
2
1
3). The product was obtained as a white powder in 67% yield. H NMR
400 MHz, CDCl
H), 7.58–7.56 (dd, J = 10.4 Hz, 2H).
3
, TMS) δppm: 8.22 (s, 2H), 7.71–7.69 (d, J = 8.4 Hz,
2
.1.4. Synthesis of 2,8-dibromodibenzothiophene S,S-dioxide (4)
A mixture of 3 (6.8 g, 20 mmol), hydroperoxide (33%, 15.0 mL),
THF (120 mL) and glacier acetic acid (150 mL) was refluxed for 6 h. The
reaction mixture was poured into a large amount of water, filtered, and
the solid was collected. The crude product was recrystallized with
chloroform to afford a white solid (6.1 g, 81%). 1H NMR (400 MHz,
[
33–35]. As it has two reaction sites, it can be easily substituted with
various electron-donating moieties such as carbazoles and triar-
ylamines, and altered its intrinsic electronic characteristics [36,37]. On
the other hand, carbazole has a rigid molecular frame and good hole-
transport properties. Carbazole have been extensively used as the hole-
transporting (HT) components in the construction of numerous photo-
and electroluminescent devices [38,39]. However, there has been no
report on the use of structures containing carbazole and dibenzothio-
phene-S,S-dioxide moieties in iridium complexes and their corre-
sponding PLEDs.
In this paper, we have coupled carbazolyl and pyridinyl units to the
dibenzothiophene-S,S-dioxide rings as the cyclometalated ligand of ir-
idium complexes for the first time. Based on that, two novel yellow
phosphorescent iridium complexes are synthesized. The photophysical,
electrochemical and thermal properties, as well as electroluminescent
properties of the resulting iridium complexes were discussed. We also
have successfully fabricated a highly efficient WPLED based on (p-
CDCl , TMS) δ (ppm): 7.93 (s, 2H), 7.71 (s, 4H).
3
2
.1.5. Synthesis of 3-bromo-7-(9-butyl- carbazol -3-yl) dibenzothiophene
S,S-dioxide (5)
mixture of Pd(PPh (578 mg, 0.5 mmol), 3,7-di-
A
3
)
4
bromodibenzothiophene dioxide (2.17 g, 5.8 mmol), 9-butyl-3-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-carbazole (1.02 g, 2.92 mmol),
THF (150 mL) and 2.0 M K
2
CO
3
solution (20 mL) was degassed and
flow pro-
heated to 90 °C with vigorously stirring for 24 h under N
2
tection. After cooled to room temperature, the mixture was poured into
water. The organic phase was separated and the aqueous phase was
extracted with diethyl ether (3 × 30 mL). The combined organic layers
were dried over MgSO and evaporated to remove the solvents under
4
vacuum. The residue was purified by column chromatography (petro-
2
CzFSOPy) IrPic in combination with the blue phosphor FIrpic to pro-
duce a white light source with CIE coordinate of (0.317, 0.448). The
maximum LE of the WPLEDs can reach 19.2 cd/A.
leum ether/dichloromethane = 18/1) to provide 5 (1.04 g) as white
solid in 69% yield. 1H NMR (400 MHz, CDCl
, TMS) δ (ppm): 8.34 (s,
H), 8.16–8.14 (d, J = 8.0 Hz, 2H), 7.98–7.96 (dd, J = 8.0 Hz, 2H),
.82–7.80 (d, J = 8.0 Hz, 1H), 7.76–7.70 (m, 2H), 7.67–7.65 (d,
3
1
7
2
. Experimentals
.1. Materials
-butyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-carbazole
J = 8.0 Hz, 1H), 7.53–7.43 (m, 3H), 7.30–7.28 (t, J = 7.4 Hz, 1H),
4
0
.35–4.31 (t, J = 7.0 Hz, 2H), 1.92–1.85 (m, 2H), 1.48–1.38 (m, 2H),
.98–0.95 (t, J = 7.4 Hz, 3H).
2
9
was prepared according to the reported procedures [40]. All reactions
were performed under nitrogen. All solvents were carefully dried and
distilled from appropriate drying agents prior to use. Commercially
available reagents were used without further purification unless
otherwise stated.
2.1.6. Synthesis of 2-bromo-8-(9-butyl- carbazol -3-yl) dibenzothiophene
S,S-dioxide (6)
The compound
6 was prepared similarly to 5 from 2,8-di-
bromodibenzothiophene dioxide (1.58 g, 4.28 mmol) and 9-butyl-3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-carbazole (0.75 g, 2.14
mmol). Yield: 0.68 g, 62%. 1H NMR (400 MHz, CDCl
, TMS) δ (ppm):
3
2.1.1. Synthesis of dibenzothiophene S,S-dioxide (1) [41]
8.36 (s, 1H), 8.20–8.18 (d, J = 8.0 Hz, 1H), 8.06–8.04 (d, J = 8.4 Hz,
2H), 7.91–7.86 (q, 2H), 7.74–7.66 (m, 3H), 7.54–7.45 (m, 3H),
7.31–7.28 (t, J = 7.4 Hz, 1H), 4.38–4.34 (t, J = 7.2 Hz, 2H), 1.94–1.86
(m, 2H), 1.46–1.40 (m, 2H), 0.99–0.96 (t, J = 7.4 Hz, 3H).
A mixture of dibenzothiophene (3.7 g, 20 mmol), hydroperoxide
(
33%, 8.0 mL) and glacier acetic acid (50 mL) was refluxed for 3 h. The
reaction mixture was poured into a large amount of water, filtered, and
638

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
trichloride hydrate (92 mg, 0.26 mmol). Yield: 258 mg, 68%. H NMR
1
2
.1.7. Synthesis
dibenzothiophene S,S-dioxide (p-CzFSOPy) [44]
-(tributylstannyl)pyridine (0.46 g, 1.24 mmol) and
.24 mmol) were mixed in dry toluene (50 mL) and Pd(PPh
of
3-(pyridin-2-yl)-7-(9-
butyl-carbazol-3-yl)
(400 MHz, DMSO, TMS) δ (ppm): 9.00 (s, 1H), 8.96 (s, 1H), 8.74–8.68
(m, 7H), 8.32–8.18 (m, 6H), 7.96–7.90 (m, 6H), 7.84–7.81 (t,
J = 5.8 Hz, 2H), 7.78–7.76 (dd, J = 12.8 Hz, 2H), 7.69–7.64 (m, 4H),
7.54–7.48 (q, 3H), 7.29–7.25 (t, J = 7.4 Hz, 2H), 6.60 (s, 1H), 6.41 (s,
1H), 4.47–4.44 (t, J = 6.2 Hz, 4H), 1.80–1.77 (m, 4H), 1.33–1.28 (m,
2
5
(0.64 g,
1
0
3
)
4
(71.6 mg,
.06 mmol) was then added to the solution to carry out the Stille-type
coupling. The resulting mixture was stirred at 110 °C for 24 h. After
cooling to room temperature, the reaction mixture was poured into a
separating funnel and CH
with water (3 × 50 mL). The organic phase was dried over MgSO
Then the solvent was removed and the crude product was purified by
4H), 0.90–0.86 (t, J = 7.4 Hz, 6H). 13C NMR (CDCl
, 100 MHz) δ
3
2
Cl
2
(100 mL) was added followed by washing
(ppm): 172.03, 165.83, 165.24, 154.03, 152.04, 150.86, 149.81,
149.06, 148.56, 147.82, 141.20, 140.79, 140.00, 139.87, 138.43,
137.83, 133.35, 130.18, 129.71, 128.07, 126.11, 125.54, 122.67,
121.11, 119.55, 118.82, 110.34, 110.12, 42.67, 31.21, 20.26, 14.20.
4
.
column chromatography eluting with petroleum ether/di-
chloromethane = 3/2. The title product was obtained as a white solid
5 6 2
Anal. Calcd. for C70H50IrN O S : C, 64.01; H, 3.84; N, 5.33; Found: C,
1
+
(
0.48 g, 75%). H NMR (400 MHz, CDCl
3
, TMS) δ (ppm): 8.75–8.74 (d,
64.36; H, 3.78; N, 5.39. MALDI-TOF (m/z): calcd for [M] 1313.2832,
J = 4.4 Hz, 1H), 8.48 (s, 1H), 8.41–8.39 (d, J = 8.0 Hz, 1H), 8.37 (s,
found [M + Na]+ 1336.2709.
1
7
1
2
3
H), 8.19–8.16 (d, J = 11.6 Hz, 2H), 8.00–7.98 (dd, J = 8.0 Hz, 1H),
.93–7.89 (m, 2H), 7.83–7.82 (m, 2H), 7.76–7.73 (dd, J = 10.4 Hz,
H), 7.53–7.44 (m, 3H), 7.34–7.27 (m, 2H), 4.37–4.33 (t, J = 7.2 Hz,
H), 1.93–1.86 (m, 2H), 1.46–1.40 (m, 2H), 0.99–0.95 (t, J = 7.4 Hz,
H).
2.2. Measurements and characterization
1H and 13C NMR spectra were recorded on a Bruker Avance
400 MHz spectrometer operating respectively at 400 and 100 MHz at
room temperature. Chemical shifts were reported as δ values (ppm)
relative to an internal tetramethylsilane (TMS) standard. Time-of-flight
mass spectrometry (TOF-MS) was performed in the positive ion mode
with a matrix of dithranol using a Bruker-autoflex III smartbeam.
Thermogravimetric analysis (TGA) was measured on a Diamond TG/
DTA instrument under nitrogen atmosphere at a heating rate of 20 °C
2.1.8. Synthesis
of
2-(pyridin-2-yl)-8-(9-
butyl-carbazol
-3-yl)
dibenzothiophene S,S-dioxide (m-CzFSOPy)
The compound m-CzFSOPy was prepared similarly to p-CzFSOPy
from 2-(tributylstannyl)pyridine (0.32 g, 0.87 mmol) and 6 (0.45 g,
1
0.87 mmol). Yield: 0.34 g, 77%. H NMR (400 MHz, CDCl
3
, TMS) δ
−1
(
ppm): 8.82–8.81 (d, J = 4.8 Hz, 1H), 8.73 (s, 1H), 8.41–8.39 (d,
J = 9.2 Hz, 1H), 8.28 (s, 1H), 8.22–8.20 (d, J = 7.6 Hz, 1H), 8.11–8.09
d, J = 8.0 Hz, 1H), 7.96–7.86 (m, 3H), 7.78–7.76 (dd, J = 8.4 Hz, 1H),
.71–7.69 (m, 1H), 7.53–7.49 (m, 3H), 7.44–7.40 (m, 2H), 7.30–7.27
min
d
and T was reported as the temperatures at 5% weight losses.
UV–vis absorption spectra were measured on a Hitachi U-3310 spec-
trophotometer. PL spectra were recorded on an LS55 luminescence
spectrometer (PerkinElmer, UK). Emission life times were obtained on a
FLS 980 (Edinburgh Instruments Ltd.) combined fluorescence lifetime
(
7
(
(
m, 1H), 4.38–4.34 (t, J = 7.0 Hz, 2H), 1.94–1.88 (m, 2H), 1.70–1.65
m, 2H), 0.99–0.96 (t, J = 7.2 Hz, 3H).
and steady state spectrometer in CH
2
Cl
2
solution at room temperature.
) was measured
The absolute photoluminescence quantum yield (Ф
P
2
.1.9. Synthesis of (p-CzFSOPy)
To a mixture of iridium trichloride hydrate (120 mg, 0.34 mmol)
2
IrPic
with a Hamamatsu absolute PL quantum yield spectrometer equipped
with an integrating sphere (Quantaurus-QY, C11347). Cyclic voltam-
metry was carried out on a CHI660A electrochemical workstation in a
and water (15 mL) was added p-CzFSOPy (386 mg, 0.75 mmol) and 2-
ethoxyethanol (45 mL). The mixture was heated to reflux under an inert
gas atmosphere for 20 h. After cooled to temperature, the colored
precipitate was filtered off and washed respectively with water, ethanol
and petroleum ether. The resulting dimer was obtained as a yellow solid
4 6
solution of tetrabutylammonium hexafluorophosphate (Bu NPF )
(0.1 M) in acetonitrile at a scan rate of 50 mV/s at room temperature
under the protection of argon. A platinum electrode was used as the
working electrode. A Pt wire was used as the counter electrode, and a
calomel electrode was used as the reference electrode.
(
420 mg). The dimer was directly used in the following procedure.
A mixture of the foregoing dimer (420 mg, 0.17 mmol), picolinic
acid (62 mg, 0.51 mmol) and Na
2
CO
3
(540 mg) was heated under reflux
2.3. Theoretical calculations
under an inert gas atmosphere in 2-ethoxyethanol (40 mL) for 16 h.
After cooled to room temperature, the mixture was extracted with di-
chloromethane and the combined organic layer was dried over anhy-
drous magnesium sulfate. The crude product was purified by dry flash
silica gel column (dichloromethane/ethyl acetate = 3/1 as eluent) to
The geometries of both iridium complexes (p-CzFSOPy)
2
IrPic and
(m-CzFSOPy) IrPic at the gas phase were fully optimized by the density
2
functional theory (DFT) method at the 6-31G* basis set with B3LYP
functional [45,46] without any symmetry constraints by using the
Gaussian 09 software package [47].
1
gain the target product (902 mg) as yellow solid in 65% yeild. H NMR
(
(
400 MHz, DMSO, TMS) δ (ppm): 8.73–8.69 (m, 6H), 8.63 (s, 1H), 8.34
s, 1H), 8.31 (s, 1H), 8.29–8.26 (dd, J = 12.0 Hz, 2H), 8.21–8.19 (m,
2.4. PLEDs fabrication and measurements
4
H), 8.07–8.04 (t, J = 6.0 Hz, 2H), 7.88–7.83 (t, J = 9.2 Hz, 3H), 7.76
s, 1H), 7.66–7.63 (m, 4H), 7.59–7.57 (d, J = 8.4 Hz, 2H), 7.54–7.52 (d,
J = 8.0 Hz, 1H), 7.48–7.42 (m, 4H), 7.24–7.20 (t, J = 7.4 Hz, 2H), 6.92
(
The ITO-coated glass substrates were ultrasonically cleaned with
deionized water, acetone, detergent, deionized water, and isopropyl
(
s, 1H), 6.66 (s, 1H), 4.40–4.36 (t, J = 6.8 Hz, 4H), 1.75–1.71 (m, 4H),
alcohol. Then
ythiophene): poly (styrene sulfonic acid) (PEDOT:PSS) (H.C.Stack,
4083) was spin-coated onto the pre-cleaned and O -plasma-treated ITO
a layer of 40 nm thick poly (3,4-ethylenediox-
1
3
1
1
1
1
1
1
.29–1.22 (m, 4H), 0.86–0.82 (t, J = 8.6 Hz, 6H). C NMR (DMSO,
00 MHz) δ (ppm): 172.12, 165.88, 165.29, 160.15, 158.14, 150.75,
49.30, 147.56, 146.96, 143.90, 141.03, 140.68, 140.64, 139.77,
32.36, 131.27, 130.50, 130.10, 128.82, 128.34, 126.57, 124.98,
22.75, 121.20, 119.37, 117.97, 110.32, 109.98, 42.62, 31.16, 20.23,
2
substrates. After that, the PEDOT:PSS layer was baked at 150 °C for
20 min to remove residual water, and then the devices were moved into
a glove box under the argon-protected environment. A mixture of the
−
1
4.14. Anal. Calcd. for C70
H
50IrN
5
O
6
2
S : C, 64.01; H, 3.84; N, 5.33+;
iridium complex with PVK + PBD or OXD-7 (30%) (30 mg mL
in o-
Found: C, 63.95; H, 3.65; N, 5.45. MALDI-TOF (m/z): calcd for [M]
1
DCB) were spin-coated onto PEDOT:PSS at the speed of 2000 rpm to
yield 90 nm thickness light emitting layers. The samples were trans-
+
313.2832, found [M + Na] 1336.2686.
−4
ferred into a chamber and kept under vacuum (3.0 × 10 Pa) for 2 h.
Then caesium fluoride (CsF) with a thickness of 2.5 nm and aluminium
with a thickness of 100 nm were subsequently deposited on top of the
EML to form the cathode. The current density (J) and brightness (L)
2
.1.10. Synthesis of (m-CzFSOPy)
The compound (m-CzFSOPy)
CzFSOPy) IrPic from m-CzFSOPy (296 mg, 0.58 mmol) and iridium
2
IrPic
2
IrPic was prepared similarly to (p-
2
639

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
2 2
Scheme 1. Synthetic routes to (p-CzFSOPy) IrPic and (m-CzFSOPy) IrPic.
versus voltage (V) data were collected using a Keithley 236 source
meter and silicon photodiode. After typical encapsulation with UV
epoxy and cover glass, the devices were taken out from the dry box and
mixtures by silica chromatography furnished (p-CzFSOPy)
2
IrPic and (m-
CzFSOPy) IrPic as an air-stable yellow powder in high purity. Both the
2
1
13
desired iridium complexes were confirmed by H and C nuclear
magnetic resonance (NMR) spectroscopy, elemental analysis (EA) and
mass (MS) spectrometry.
the luminance was calibrated by
a
PR-705 SpectraScan
Spectrophotometer (Photo Research) with simultaneous acquisitions of
the EL spectra.
3.2. Photophysical properties
3. Results and discussion
The photophysical properties of iridium complexes (p-
CzFSOPy)
vestigated. The absorption and photoluminescence (PL) spectra of (p-
CzFSOPy) IrPic and (m-CzFSOPy) IrPic at room temperature are shown
in Fig. 1 and relevant data are presented in Table 1. The intense peaks
2 2 2 2
IrPic and (m-CzFSOPy) IrPic in CH Cl solution were in-
3
.1. Synthesis and characterization
2
2
2
The synthesis of iridium complexes (p-CzFSOPy) IrPic and (m-
CzFSOPy)
1) was synthesized by oxidation of dibenzothiophene into S,S-dioxide
derivative after refluxing for 3 h in a mixture of hydrogen peroxide and
acetic acid (H /HOAc), and then 3,7-dibromodibenzothiophene S,S-
dioxide (2) was obtained with a bromination reaction using sulphuric
acid (H SO ) and N-bromosuccinimide (NBS) at 0 °C in 87% yield.
2
IrPic are shown in Scheme 1. Dibenzothiophene S,S-dioxide
(
2 2
O
2
4
Contrast to the synthesis of 2, 2,8-dibromodibenzothiophene S,S-di-
oxide (3) could be prepared by first bromination of dibenzothiophene
with bromine and then oxidation of the resulting bromide with H O .
2 2
Compound 5 was obtained by a Pd-catalyzed Suzuki coupling of com-
pouned 2 with 0.5 equiv of 9-butyl-3-(4,4,5,5-tetramethyl-1,3,2-diox-
aborolan-2-yl)-carbazole. Stille-type cross-coupling of 5 with 2-(tribu-
tylstannyl)pyridine in toluene, in the presence of Pd(PPh
the cyclometalated ligand of p-CzFSOPy in 75% yield. The final iridium
complex (p-CzFSOPy) IrPic was prepared in a two-step procedure
48–50]: the first step is IrCl ·nH O reacted with an excess of p-
CzFSOPy to give a chloride-bridged iridium dimmer [(CˆN) IrCl] ; the
second step is a chloride-cleavage of the iridium dimer with 3 equiv of
picolinic acid. The iridium complex (m-CzFSOPy) IrPic was synthesized
by the similar procedure of (p-CzFSOPy) IrPic. Purification of the
3 4
) afforded
2
[
3
2
2
2
Fig. 1. Normalized UV–vis absorption and PL spectra of (p-CzFSOPy)
m-CzFSOPy) IrPic in CH Cl solution at room temperature.
2
IrPic and
2
(
2
2
2
2
640

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
Table 1
Photophysical, electrochemical and thermal data of (p-CzFSOPy)
2
IrPic and (m-CzFSOPy)
2
IrPic.
Complexes
λabs [a] (nm)
λ
PL[a] (nm)
Ф
p
(%) (μs)
τ
p
Eox[b] (V)
HOMO (eV)
LUMO (eV)
E
g
(eV)
d
T (°C)
(
(
p-CzFSOPy)
m-CzFSOPy)
2
IrPic
IrPic
275, 322, 394, 456
293, 338, 476
558
554
45.6
38.1
0.62
0.64
0.78
0.72
−5.58
−5.52
−3.00
−2.91
2.58
2.61
443
434
2
[
a]Measured in CH
2
Cl
2
solution at room temperature. [b] Measured in a solution of Bu
4
6
NPF (0.1 M) in acetonitrile at a scan rate of 50 mV/s at room temperature.
CzFSOPy)
Py) IrPic absorption are much smaller, which means that the Förster
energy transfer would be inefficient.
2
IrPic, the overlaps between hosts' emission and (m-CzFSO-
2
3.3. Electrochemical properties and theoretical calculations
The electrochemical behaviours of (p-CzFSOPy)
2
IrPic and (m-
CzFSOPy) IrPic were studied by cyclic voltammetry (CV) using ferro-
2
cene as the internal standard and the results are listed in Table 1. Both
the iridium complexes show reversible anodic waves at 0.78 V and
0
.72 V, respectively, recorded relative to the ferrocence/ferrocenium
+
(
Fe/Fe ) couple, due to the oxidation of the iridium center. On the
basis of these onset oxidation potentials (Eox) and the formula of
HOMO = −(Eox + 4.8) eV [55], the highest occupied molecular orbital
energies (EHOMO) were calculated to be −5.58 eV for (p-CzFSOPy) IrPic
and −5.52eV for (m-CzFSOPy) IrPic, respectively. Otherwise, duo to
the reduction waves of the newly synthesized iridium complexes are
quite poor, the lowest unoccupied molecular orbital energies (ELUMO
had to be calculated based on the equation of ELUMO = EHOMO + E , in
which energy gap (E ) was estimated from the edge of UV–vis absorp-
tion spectra. As a result, the ELUMO are −3.00 eV for (p-CzFSOPy) IrPic
and −2.91 eV for (m-CzFSOPy) IrPic.
Fig. 2. The electron density distributions of HOMO and LUMO orbitals of (p-
CzFSOPy) IrPic (left) and (m-CzFSOPy) IrPic (right) complexes.
E
2
2
2
2
in the short wavelength region below 420 nm are attributed to the spin-
allowed ligand-centered 1π-π transition and the weaker absorption
tails that appeared above 420 nm are assigned to an admixture of spin-
allowed singlet and spin-forbidden triplet metal-ligand-charge-transfer
∗
)
g
g
1
3
1
(
MLCT and MLCT) excited states [48,50,51]. The admixture of MLCT
2
3
and MLCT band was generated by the strong spin-orbital coupling
which was induced by heavy-metal effect of Ir center between the
singlet and triplet manifolds. Minor differences of PL profiles are ob-
2
To get deep insights into the structure-property relationship of the
iridium complexes at the molecular level, the geometrical and elec-
tronic structures of the complexes were performed using density func-
tional theory (DFT). The electron density distributions of both HOMO
served for (p-CzFSOPy)
p-CzFSOPy) IrPic and (m-CzFSOPy)
phosphorescence emission and exhibited maximal emission at 558 and
54 nm, respectively, which are mainly characteristic of MLCT emis-
sion. Compared to (m-CzFSOPy) IrPic, (p-CzFSOPy) IrPic exhibited a
little red-shifted emission with ca. 4 nm. This is owing to the longer
conjugated π system in the cyclometalating ligand of (p-CzFSOPy) IrPic
than that of (m-CzFSOPy) IrPic. The absolute photoluminescence
quantum yields (Ф ) for (p-CzFSOPy) IrPic and (m-CzFSOPy) IrPic in
CH Cl solution are measured by integrating sphere method (Table 1).
The values of (p-CzFSOPy) IrPic and (m-CzFSOPy) IrPic were recorded
2
IrPic and (m-CzFSOPy)
2
IrPic in neat film. Both
(
2
2
IrPic displayed strong yellow
2 2
and LUMO for complexes (p-CzFSOPy) IrPic and (m-CzFSOPy) IrPic are
5
illustrated in Fig. 2. It is clear that the HOMO orbitals are mainly
contributed by the carbazole group, while the LUMO orbitals are
mainly delocalized on the dibenzothiophene S,S-dioxide moiety and the
central Iridium atom. The calculated energies of HOMOs for (p-
2
2
2
CzFSOPy)
respectively. And, the corresponding energies of LUMOs for (p-CzFSO-
Py) IrPic and (m-CzFSOPy) IrPic are −2.42 eV and −2.24 eV, respec-
2 2
IrPic and (m-CzFSOPy) IrPic are −5.46 eV and −5.38 eV,
2
P
2
2
2
2
2
2
tively. Based on the calculated HOMO and LUMO energies for both
iridium complexes, we found that the calculated HOMO–LUMO energy
2
2
to be 45.6 and 38.1%, respectively, suggesting they are evidently af-
fected by the substituted positions of the pyridine- and carbazole-group
at the aromatic ring of dibenzothiophene S,S-dioxide cores. The emis-
gap of (p-CzFSOPy)
2
IrPic (ca. 3.04 eV) is smaller than that of (m-
CzFSOPy) IrPic (ca. 3.14 eV), which is qualitatively consistent with the
2
sion lifetimes (τ
Py) IrPic were well fitted to a single-exponential decay at room tem-
perature and measured as 0.62 μs and 0.64 μs, respectively (Fig. S1,
p
) in CH
2
Cl
2
of (p-CzFSOPy)
2
IrPic and (m-CzFSO-
experimental evidence from CV and absorption measurement.
2
†
ESI ). Accordingly, the radiative lifetimes (τ
were 1.36 μs for (p-CzFSOPy) IrPic and 1.68 μs for (m-CzFSOPy)
deduced from τ = τ /Ф . The triplet radiative and non-radiative rate
constant (k and knr) are calculated from Ф and τ , using the expres-
sions Ф = ФISC [k /(k + knr)] and τ = (k
tersystem-crossing yield (ФISC) is safely assumed to be 1.0 for metal
phosphors with strong heavy-atom effect. We obtained k
and knr o5f
IrPic and 5.95 × 10
IrPic, respectively. Since the τ of
IrPic are very short, triplet–triplet
r
) of the triplet excited state
3.4. Thermal properties
2
2
IrPic,
The thermal stabilities of these iridium complexes were evaluated
r
p
p
by thermogravimetric analysis (TGA) under a stream of N
2
with a
scanning rate of 20 °C min . Their TGA curves are shown in Fig. 3 and
their degradation temperatures (T ) for 5% weight loss are listed in
Table 1. The recorded T level is 443 °C for (p-CzFSOPy) IrPic and
434 °C for (m-CzFSOPy) IrPic, respectively. Compared to (m-CzFSO-
Py) IrPic, (p-CzFSOPy) IrPic exhibits a higher decomposition tem-
r
p
p
−1
−1
p
r
r
p
r
+ knr
)
, where the in-
d
r
d
2
5
5
−1
7
.35 × 10 and 8.78 × 10
s
for (p-CzFSOPy)
and 9.68 × 10 s for (m-CzFSOPy)
p-CzFSOPy) IrPic and (m-CzFSOPy)
2
2
5
−1
2
p
2
2
(
2
2
perature, which indicates the different substituted-situation of di-
benzothiophene S,S-dioxide with pyridine and carbazole can distinctly
affect the thermal stability of the resulting iridium complexes. The data
indicate that these functionalized iridium complexes possess good
thermal properties, which is very important for electroluminescent
purpose.
annihilation could be avoided in the OLEDs at high brightness [52]. The
PL emission spectra of hosts (PVK, PBD and OXD-7) show good overlap
with the absorption spectrum of (p-CzFSOPy)
2
IrPic, which indicates
IrPic
guest can be expected (Fig. S2, ESI ) [53,54]. Compared to (p-
that efficient Förster energy transfer from hosts to (p-CzFSOPy)
2
†
641

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
2 2
Fig. 3. TGA curves of (p-CzFSOPy) IrPic and (m-CzFSOPy) IrPic under nitrogen
atmosphere.
3.5. Electroluminescent devices
Monochromic yellow-light-emitting OLEDs with high efficiencies
are indispensable for developing highly efficient WPLEDs. Hence, in
order to investigate the electroluminescent performances of (p-
CzFSOPy)
2
IrPic and (m-CzFSOPy)
2
IrPic as dopant emitters, two yellow-
emitting PLEDs with
a
configuration of ITO/PEDOT:PSS/
dopant + PVK:PBD/CsF/Al were fabricated. Poly (3,4-ethylenediox-
ythiophene) poly (styrene sulfonate) (PEDOT:PSS) was used as a hole-
injection layer. PVK was chosen as a host material due to its higher-
lying triplet state (3.0 eV), excellent film-forming properties, high glass-
Fig. 5. (A) Current density (J) and brightness (L) versus voltage (V) char-
acteristics (J–L–V); (b) The luminous efficiency–current density (LE–J) char-
transition temperature (T
g
≈ 160 °C) and hole-transport characteristics
[
11]. In order to balance electron/hole injection in the emitting layer,
acteristics of the yellow-emitting PLEDs with (p-CzFSOPy)
CzFSOPy) IrPic.
2
IrPic and (m-
2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole
(PBD)
was
2
blended into PVK. The emitting layer consists of the dopants ((p-
CzFSOPy) IrPic or (m-CzFSOPy) IrPic) and polymeric host matrix of
PVK:PBD blend, in which the weight concentration of the dopant is 2 wt
, PBD weight ratio is 30 wt % in the PVK-PBD blend, respectively. A
2
2
relative to PL spectrum, the presence of an additional emission peak at
ca. 430 nm in the EL spectrum of (m-CzFSOPy) IrPic indicates an in-
%
2
complete energy transfer from the PVK:PBD host to iridium complex
guest, which is consistent with results of photophysical measurement
vacuum-deposited caesium fluoride (CsF) layer was used as an electron-
injection layer.
[
56,57].
Fig. 5 shows the current density–voltage-brightness characteristics
and the luminous efficiency (LE) versus current density for those de-
vices from (p-CzFSOPy) IrPic and (m-CzFSOPy) IrPic, the relevant data
are presented in Table 2. The devices based on (p-CzFSOPy) IrPic and
(m-CzFSOPy) IrPic can be turned on at very low voltage (4.8 V for (p-
CzFSOPy) IrPic and 4.1 V for (m-CzFSOPy) IrPic), and show maximum
luminance (Lmax) with 9144 cd/m2 and 2344 cd/m , respectively. The
device of (p-CzFSOPy) IrPic shows a maximal LE of 13.3 cd/A with CIE
coordinates of (0.486, 0.486) at a current density of 6.2 mA/cm , while
the peak LE of device of (m-CzFSOPy) IrPic is 4.2 cd/A with CIE co-
ordinates of (0.347, 0.392) at 12.4 mA/cm . Compared to (m-CzFSO-
Py) IrPic, (p-CzFSOPy) IrPic exhibits much better device performances,
which may be attributed to that (p-CzFSOPy) IrPic has higher Ф than
m-CzFSOPy) IrPic, and the energy transfer from PVK:PBD blend to (p-
Fig. 4 showed the electroluminescent (EL) spectra of the resulting
iridium complexes. The EL spectra of (p-CzFSOPy)
2
IrPic and (m-
CzFSOPy) IrPic displayed yellow emission with a maximal phosphor-
2
escent peak at ca. 560 nm and 550 nm, respectively, which arose solely
from the iridium complexes. Moreover, the EL spectrum of (p-CzFSO-
2
2
2
Py)
2
IrPic is identical to the corresponding PL emission. Nevertheless,
2
2
2
2
2
2
2
2
2
2
2
p
(
2
Table 2
The device performances of (p-CzFSOPy)
2
IrPic and (m-CzFSOPy)
2
IrPic.
Dopants
V
on[a] (V) V [b] J [b] (mA QE Lmax (cd LEmax
−
2
m
−2)
(cd A−1)
(V)
cm
)
[b]
%)
(
(
(
p-CzFSOPy)
m-CzFSOPy)
2
IrPic
4.8
8.1
6.9
6.2
12.4
5.3
1.7
9144
2344
13.3
4.2
2
IrPic 4.1
Fig. 4. EL spectra of the yellow-emitting PLEDs with (p-CzFSOPy)
CzFSOPy) IrPic.
2
IrPic and (m-
a] The turn-on voltage at which luminescence reach 1 cd m−2. [b] Device data
at maximum LE.
[
2
642

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
Fig. 7. (A) Current density (J) and brightness (L) versus voltage (V) char-
acteristics (J–L–V); (b) The luminous efficiency–current density (LE–J) char-
acteristics of double-doped WPLEDs at different concentrations of (p-
2
CzFSOPy) IrPic:FIrpic.
Fig. 6. (a) EL spectra of the double-doped WPLEDs at different concentrations
5 mA/cm2 fluctuate from (0.283, 0.429) to (0.346, 0.450), all are
within the white light-emission region. Of which, the CIE coordinate of
of (p-CzFSOPy)
2
IrPic:FIrpic; (b) EL spectra under different applied voltages
IrPic:FIrpic is 1:20.
when the concentration of (p-CzFSOPy)
2
(
0.317, 0.448) is very close to the ideal CIE coordinate of (0.333, 0.333)
for pure white color when the doping concentration of (p-CzFSO-
Py) IrPic:FIrpic is 1:20. In order to investigate the EL stability of
WPLEDs, the EL spectra of devices was measured under different ap-
plied voltages when the mass ratio of (p-CzFSOPy) IrPic:FIrpic is 1:20.
CzFSOPy)
CzFSOPy)
2
2
IrPic is more efficient than that from PVK:PBD blend to (m-
IrPic.
2
On the basis of these efficient monochromatic devices, a series of
highly efficient WPLEDs with a configuration of ITO/PEDOT:PSS/
emitting layer/CsF/Al were fabricated by simultaneously doping blue
2
As shown in Fig. 6(b), it can be seen that the EL CIE coordinates only
shift from (0.334, 0.447) to (0.307, 0.439) with Δ(x, y) = ± (0.014,
emitter FIrpic and yellow phosphors (p-CzFSOPy)
matrix. The emitting layer is PVK (63 wt%):OXD-7 (27 wt%):dopants
FIrpic and (p-CzFSOPy) IrPic, 10 wt%). PVK was used as host, and
2
IrPic into PVK:OXD-7
0.004), while the driving voltages increasing from 6 V to 9 V. Therefore,
the WPLEDs exhibited stable white emission under different driving
voltages. As shown in Fig. 7, when the mass ratio of (p-CzFSO-
(
2
OXD-7 was included into the host matrix to facilitate electron transport.
We tuned the emission color by controlling the blending ratio of FIrpic/
Py)
2
IrPic:FIrpic is 1:20, the device exhibit best EL performances: the
turn-on voltage (Von) is 5.3 V, the maximum luminance (Lmax) is
(
2
p-CzFSOPy) IrPic within the range of 10:1 to 30:1. Fig. 6(a) depicts the
2
2
2
18717 cd/m , the peak LE is 19.2 cd/A at 6.86 mA/cm , and the cor-
responding EQE is 9.6%. At a high forward-view luminance of 1000 cd/
EL spectra of these WPLEDs at the current density of 5 mA/cm . Two
emission region of 440–530 nm and 530–700 nm were observed, which
are assigned to the emission from FIrpic and (p-CzFSOPy)
spectively. Moreover, there is a monotonous increase of the yellow
emission as the doping concentration of (p-CzFSOPy) IrPic:FIrpic in-
2
m , the LE slightly decrease to 19.0 cd/A. Compared to the tripheny-
2
IrPic, re-
lamine- and fluorene-functionalized iridium complexes which we have
reported early, the iridium complex (p-CzFSOPy)
2
IrPic displayed better
2
EL properties in WPLEDs [29]. As we know, most phosphorescent
creases from 1:30 to 1:10. As shown in Table 3, the CIE coordinates at
Table 3
Device performances of the double-doped WPLEDs.
Composition [a]
V
on
Peak LE
(cd/A)
Peak EQE
(%)
Peak PE
(lm/W)
Peak L
Device performance at 1000 cd/m2
CIE[b]
(x, y)
(V)
(cd/m2)
LE (cd/A)
EQE (%)
PE (lm/W)
1
2
3
0:1
0:1
0:1
5.5
5.3
4.6
16.9
19.2
9.1
8.4
9.6
4.6
7.43
8.44
3.26
17950
18717
13087
16.8
19.0
5.1
8.3
9.5
2.5
7.36
8.23
2.27
(0.346, 0.450)
(0.317, 0.448)
(0.283, 0.429)
[
2
a]FIrpic: (p-CzFSOPy) IrPic ratio. [b] Observe: 2 Degrees; obtained at 5 mA.
643

A. Liang et al.
Dyes and Pigments 159 (2018) 637–645
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Acknowledgements
The work was financially supported by the National Natural Science
Foundation of China (No. 51763013 and 51403088), the China
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Postdoctoral
Science
Foundation
(No.
2018T110659
and
[
[
[
[
[
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25] Niu Y-H, Liu MS, Ka J-W, Bardeker J, Zin MT, Schofield R, Chi Y, Jen AKY.
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2
016M592111), the Postdoctoral Science Foundation of Jiangxi
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Appendix A. Supplementary data
Supplementary data related to this article can be found at https://
doi.org/10.1016/j.dyepig.2018.07.019.
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