Benzotriazole-containing donor-acceptor-acceptor type cyclometalated iridium(III) complex for solution-processed near-infrared polymer light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2016.04.021

A novel near-infrared-emitting cyclometalated iridium(III) complex of (CH₃OTPA-BTz-Iq)₂Irpic containing benzotriazole unit with a donor-acceptor-acceptor (D-A-A) chromophore was synthesized and characterized. The optophysical, electrochemical and electroluminescent characteristics were primarily studied. A near-infrared emission peaked at 716 nm with a shoulder at 790 nm was exhibited in the (CH₃OTPA-BTz-Iq)₂Irpic dichloromethane solution at 298 K. In its optimized solution-processed polymer light-emitting diodes, a near-infrared electroluminescent emission peaked at 723 nm with a shoulder at 780 nm was observed with a maximum external quantum efficiency of 0.41% at 8.14 mA cm^-2. This work indicates that introducing appropriate D and A units to develop D-A-A structure is an efficient approach to construct near-infrared-emitting iridium(III) complex in the polymer light-emitting diodes with suppressive efficiency roll-off.

Full text of DOI:10.1016/j.dyepig.2016.04.021

Accepted Manuscript
Benzotriazole-containing donor-acceptor-acceptor type cyclometalated iridium(III)
complex for solution-processed near-infrared polymer light emitting diodes
Junting Yu, Hua Tan, Fanyuan Meng, Kun Lv, Weiguo Zhu, Shijian Su
PII:
S0143-7208(16)30142-5
10.1016/j.dyepig.2016.04.021
DYPI 5199
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 9 January 2016
Revised Date: 7 April 2016
Accepted Date: 9 April 2016
Please cite this article as: Yu J, Tan H, Meng F, Lv K, Zhu W, Su S, Benzotriazole-containing donor-
acceptor-acceptor type cyclometalated iridium(III) complex for solution-processed near-infrared polymer
light emitting diodes, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.04.021.
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ACCEPTED MANUSCRIPT
Graphical Abstract
A novel benzotriazole-containing donor-acceptor-acceptor type cyclometalated
iridium(III) complex of (CH OTPA-BTz-Iq) Irpic was synthesized. Using it as a
3
2
dopant and a blend of PVK and OXD-7 as a host matrix, the PLEDs exhibited a
near-infrared (NIR) emission peaked at 723 nm and a shoulder at 780 nm with the
-2
maximum EQE of 0.41% at 8.14 mA cm . This work indicates that introducing
appropriate D and A unit to develop D-A-A structure is an efficient approach to
construct near-infrared-emitting iridium(III) complex and obtain high-efficiency
near-infrared polymer light-emitting diodes with suppressive efficiency roll-off.

ACCEPTED MANUSCRIPT
Benzotriazole-containing Donor-Acceptor-Acceptor Type Cyclometalated
Iridium(III) Complex for Solution-processed Near-infrared Polymer Light
Emitting Diodes
a
b
b
a
b
c
Junting Yu, * Hua Tan, Fanyuan Meng, Kun Lv, Weiguo Zhu, * and Shijian Su *
a
Key Laboratory of Theoretical Organic Chemistry and Functional Molecule,
Ministry of Education, College of Chemistry and Chemical Engineering, Hunan
University of Science and Technology, Xiangtan 411201, P. R. China
b
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application
in the Ministry of Education, Xiangtan University, Xiangtan 411105, P. R. China
c
Institute of Polymer Optoelectronic Materials and Devices, South China University
of Technology, Guangzhou 510640, P. R. China
Corresponding author: Dr. Junting Yu
Prof. Weiguo Zhu
Prof. Shijian Su
Tel: +86-731-58290097
E-mail addresses: yujuntingtt@163.com
zhuwg18@126.com
1

ACCEPTED MANUSCRIPT
Abstract: A novel near-infrared-emitting cyclometalated iridium(III) complex of
(
CH OTPA-BTz-Iq) Irpic containing benzotriazole unit with a donor-acceptor-
3 2
acceptor (D-A-A) chromophore was synthesized and characterized. The optophysical,
electrochemical and electroluminescent characteristics were primarily studied. A
near-infrared emission peaked at 716 nm with a shoulder at 790 nm was exhibited in
the (CH OTPA-BTz-Iq) Irpic dichloromethane solution at 298 K. In its optimized
3
2
solution-processed polymer light-emitting diodes, a near-infrared electroluminescent
emission peaked at 723 nm with a shoulder at 780 nm was observed with a maximum
-2
external quantum efficiency of 0.41% at 8.14 mA cm . This work indicates that
introducing appropriate D and A units to develop D-A-A structure is an efficient
approach to construct near-infrared-emitting iridium(III) complex in the polymer
light-emitting diodes with suppressive efficiency roll-off.
Keywords: donor-acceptor-acceptor; iridium(III) complexes; benzotriazole;
near-infrared emission; electrophosphorescence; polymer light-emitting devices
2

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1
. Introduction
With the rapid development of organic light-emitting diodes (OLEDs) in recent
years, the research on near-infrared (NIR)-OLEDs has gained great attentions for
diverse potential applications in night-vision-readable displays, sensors, optical com-
munication and medical systems [1-5]. To date, the developed organic NIR-emitting
materials mostly contain lanthanide complexes [6-8], fluorescent materials with a
donor-acceptor (D-A) structure [9-14], boron dipyrromethene dyes [15-20] and
transition-metal complexes [21-34]. Therein, phosphorescent transition-metal com-
plexes using osmium(II), iridium(III) or platinum(II) as the metal center are available
to exhibit higher emission efficiency due to their strong spin-orbit coupling in the
presence of heavy metal atoms, which leads to an internal quantum efficiency as high
as 100% [35,36]. For example, in a device with a sophisticated configuration, the
2
platinum(II) complex of PtL Cl realized a maximum external quantum efficiency
(EQEmax) of 14.5% at ~700 nmꢀwhich is the highest value for these NIR devices to
date [22]. However, these EQE levels were typically obtained from excimers emission,
the performance and the reproducibility of the devices cannot be well controlled [37].
Platinum (II) porphyrins are also the most typical NIR emission materials in the
reported transition metal complexes and have exhibited an EQE maxima of 2.49% for
polymer light-emitting devices (PLEDs) and 9.2% for OLEDs with NIR emission in
the 760-780 nm range. However, these EQE levels were typically obtained at very
low current densities. In high current densities, the devices exhibit a significant EQE
roll-off, which could be attributed to the increasing triplet-triplet exciton annihilation
and aggravated by long excited-state lifetimes in the planar-square platinum(II)
phosphors [25].
In contrast to the planar-square platinum (II) complexes, iridium (III) complexes
3

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with typical octahedral configuration, relatively short triplet lifetime and high phos-
phorescent efficiency have the ability to effectively prevent intermolecular aggre-
gation-induced emission (AIE) quenching and significantly alleviate the efficiency
roll-off in OLEDs [29]. In order to develop new NIR-emitting iridium (III) complex-
es, many researchers have made some contributions to this field. The earlier NIR-
emitting example of iridium (III) bis(1-pyrenyl-isoquinoinato-N,C’) acetylacetonate
was reported by Williams et al. in 2006, which displayed an electroluminescence (EL)
emission peaked at 720 nm with an EQEmax nearly 0.1% [20]. Recently, Qiu et al.
reported a series of the NIR-emitting iridium (III) phosphors [27-29], in which an
iridium (III) emitter of Ir(pbq-g) (Bphen) exhibited an extended emission peak at 850
2
2
nm by introducing an sp -hybridized N atom opposite the chelating N atom in the
phenylbenzoquinoline (pbq) ligand [28]. The Ir(pbq-g) (Bphen) based devices
2
achieved a maximum EQE up to 2.2% with negligible efficiency roll-off. In 2015, an
iridium(III) phosphor of (thdpqx) Ir(acac) (dpqx= diphenylquinoxaline, th=thienyl)
2
was reported and exhibited an EL emission peak at 704 nm in its doped devices with
an EQEmax of 3.4% [31], which is the highest value for the NIR-emitting devices
based on cyclometalated iridium (III) complexes to date.
It's worth noting that some NIR-emitting organic and polymeric fluorescent mate-
rials with D-A chromophores have been developed because the band gap levels and
photoelectronic properties can be readily tuned through a systematic variation be-
tween the D and A units [9-14]. According to this idea, introduction of the D-A
structure into phosphorescent materials should be expected to achieve NIR emission
with better device performance. In 2013, Wong et al reported a series of cyclometa-
lated iridium (III) complexes with functionalized borylated oligothiophene ligands
and their maximum emission peaks extended to 756 nm. However, the EQEmaximum
4

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is merely 0.07% as the strong D-A interaction triggered a much faster radiationless
decay, and the EL spectrum exhibited a dual emission, which indicates that the energy
transfer from the host exciton to the phosphor is not completely [30]. The similar
phenomenon was also occurred in our previous research. In 2014, our group reported
a D-A-A type cyclometalated platinum (II) complex (TPA-BT-Q)Ptpic with a
functionalized cyclometalated ligand of triphenylamine (TPA)-benzothiadia- zole
(BT)-quinoline (Q). The devices exhibited a maximum emission peak at 760 nm, and
the EQEmax is only 0.12% [38]. Therefore, selecting the appropriate D and A units is
the main subject.
Inspired by the works mentioned above, we here designed a novel benzotriazole-
containing D-A-A type cyclometalated iridium (III) complex of (CH OTPA-BTz-Iq)
3
2
Irpic, in which methoxy-triphenylamine (CH OTPA) is used as an electron-donating
–
3
unit, an alkylbenzotriazole (BTz) and an isoquinoline (Iq) are employed as electron-
withdrawing units and the alky group is introduced in order to improve molecular
solubility (Figure 1). In this complex, non-planar CH OTPA unit is available to
3
improve carrier-transporting properties and overcome the aggregation-induced emi-
ssion quenching; the BTz unit is an acceptor with moderate electron-withdrawing
intensity (compared with the benzothiadiazole) due to the substitution of the sulfur
atom in benzothiadiazole with a nitrogen atom, which may reduce the intensity of
D-A interaction and suppress the nonradiative transition to some extent [39]. Besides,
BTz is a good class of EL materials reported in recent years [40-43], the lone pair on
the nitrogen atom is more basic than the lone pair on sulfur and is more easily donated
into the triazole ring [39]. This is also contributed to form a more stable complex. The
synthetic route of (CH OTPA-BTz-Iq) Irpic is shown in Scheme 1. To prove our
3
2
tactics, we comprehensively studied its thermal, photophysical, and electrochemical
5

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properties, further EL properties of its doped PLEDs by a solution process. In the
optimized solution-processed phosphorescent PLEDs, a NIR EL emission peaked at
7
23 nm with a shoulder at 780 nm was observed. An EQE maximum of 0.41% at 8.14
-2
mA cm was obtained in the (CH OTPA-BTz-Iq) Irpic doped devices. This work
3
2
indicates that introducing appropriate D and A units to develop D-A-A structure is an
efficient approach to construct high-efficiency NIR-emitting iridium (III) complex
in PLEDs with suppressive efficiency roll-off.
2
Expermental
2
.1 Characterization
The solvents were carefully dried and distilled by standard procedures before use.
All chemicals, unless otherwise stated, were obtained from commercial sources and
1
used as received. H NMR spectra was recorded with a Bruker Dex-400 NMR
instrument using CDCl as a solvent. Elemental analysis was carried out with a vario
3
EL III elemental analysis instrument. Mass spectrum was recorded on a Bruker auto-
flexIII smartbeam MALDI-TOF spectrometer. Ultraviolet-visible (UV-vis) absorp-
tion and photoluminescent (PL) spectra at 298 K were recorded with a PE Lambda 25
spectrophotometer and a PTI Q40 luminescence spectrometer, respectively. PL
spectra at 77 K were recorded with Edinburgh analytical instrument (FLS920 fluores-
cence spectrometer). Lifetime studies both at 77 K and 298 K were performed by an
Edinburgh FLS920 transient spectrofluorimeter with time-correlated single-photon
2
s
2
r
counting technique at the peak PL wavelength. The equation of Φ = Φ (ɳ A I /ɳ A I )
s
r
r s
s r
was used to calculate the fluorescence quantum yield (Φ) of the iridium(III) complex
using complex Ru(bpy) (PF ) as the standard compound in N atmosphere, where Φ
s
3
6 2
2
is the quantum yield of the sample, Φ is the quantum yield of the reference, ɳ is the
r
6

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refractive index of the solvent, A and A are the absorbance of the sample and the
s
r
reference at the wavelength of excitation, I and I are the integrated areas of emission
s
r
bands [44]. Thermogravimetric analysis (TGA) was conducted under a dry nitrogen
-1
gas flow at a heating rate of 20°C min on a TA TQAQ50 instrument. Cyclic volta-
mmetry was performed on a CHI 600E electrochemical work station with a scan rate
-1
of 100 mV S at room temperature under argon, in which a Pt disk, Pt plate and
Ag/AgCl electrode were used as working electrode, counter electrode and reference
electrode in n-Bu NPF (0.1 M) acetonitrile solution, respectively. For calibration, the
4
6
+
redox potential of ferrocene/ferrocenium (Fc/Fc ) was measured under the same
conditions.
2
.2 PLEDs Fabrication and Measurement
Patterned indium tin oxide (ITO)-coated glass substrates with a sheet resistance of
-1
1
5-20 ꢀ square underwent a wet-cleaning course in an ultrasonic bath, beginning
with THF, following by isopropanol, detergent, deionized water and isopropanol, res-
pectively After oxygen-plasma treatment, a 40 nm thick anode hole-injection layer of
poly(ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS, Baytron P 4083,
Bayer AG) film was spin-cast on the ITO substrate and dried by baking in vacuum
o
oven at 120 C for 20 min. The 80 nm emitting layer was prepared by spin-coating
o
from chlorobenzene solution on the top of PEDOT:PSS layer, then annealed at 120 C
for 20 min to remove the solvent residue. Finally, an electron transfer layer of TPBI
(30 nm), a cathode composed of Ba (4 nm) and Al (100 nm) layer was evaporated
-4
with a shadow mask at a base pressure of 1 × 10 Pa. The thickness of the evaporated
cathode was monitored by a quartz crystal thickness /ratio monitor (Model:
STM-100/MF, Sycon). The overlapping area between the cathode and anode defined a
2
pixel size of 15 mm . Except for the spin coating of the PEDOT:PSS layer, all the
7

ACCEPTED MANUSCRIPT
fabrication processes were carried out inside a controlled atmosphere of nitrogen
dry-box (Vacuum Atmosphere Co.) containing less than 1 ppm oxygen and moisture.
All measurements were carried out at room temperature (RT) under ambient
conditions. The J-V characteristics were measured using a Keithley 236 source
measurement unit and a calibrated silicon photodiode. Radiant emittance values were
calculated assuming a Lambertian distribution accordingly. The external quantum
efficiency values were calculated from the current density, luminance and EL
spectrum, assuming a Lambertian distribution. The EL spectra were obtained by using
miniature fiber optic spectrometer (USB 2000, Ocean Optics).
2
.3 Syntheses
2
.3.1 Synthesis of N-(4-bromophenyl)-4-methoxy-N-(4-methoxyphenyl)benzenamine
(1)
A mixture of 1-iodo-4-methoxybenzene (10.00 g, 42.74 mmol), 4-bromobenzen-
amine (2.95 g, 17.15 mmol) and 1,10-phenanthroline anhydrous (0.62 g, 3.42 mmol)
in toluene (60 mL) was heated and stirred under nitrogen atmosphere. When the
o
temperature reached to 110 C, the CuI (0.65 g, 3.42 mmol) and KOH (7.66 g, 136.79
o
mmol) were added quickly, and then the mixture was stirred at 135 C for 12 h. After
cooled to RT, the mixture was poured into distilled water (50 mL) and extracted with
dichloromethane (DCM) (3×30 mL). The combined organic layer was dried over
MgSO for 2 h and filtrated. The filtrate was concentrated by rotary evaporator under
4
reduced pressure. The residue was purified by silica gel column chromatography
using DCM/ petroleum ether (PE) (1/5, V/V) as eluent to gain lyard powder (4.63 g,
1
7
0.4%). H NMR (400 MHz, CDCl , ppm): 7.25 (d, J = 4.0 Hz, 2H), 7.04 (d, J = 8.4
3
Hz, 4H), 6.83 (d, J = 8.2 Hz, 6H), 3.79 (s, 6H).
.3.2 Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)benzenamine (2)
2
8

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A mixture of compound 1 (2.03 g, 5.28 mmol), 4,4,5,5-tetramethyl-2-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane (5.34 g, 21.12 mmol), pota-
ssium acetate (2.58 g, 26.38 mmol) and [1,1′-bis(diphenylphosphino)ferrocene]di-
chloropalladium (II) (0.12 g, 0.16 mmol) in DMSO (65 mL) was heated and stirred at
o
8
0 C under nitrogen atmosphere for 20 h. After cooled to RT, the mixture was poured
into distilled water (50 mL) and extracted with DCM (3×30 mL). The combined
organic layer was washed with distilled water (4×40 mL) and dried over MgSO for 2
4
h and filtrated. The filtrate was concentrated by rotary evaporator under reduced
pressure. The residue was purified by silica gel column chromatography using
1
DCM/PE (1/1, V/V) as eluent to gain lyard powder (1.40 g, 61.5%). H NMR (400
MHz, CDCl , ppm): 7.60 (d, J = 7.0 Hz, 2H), 7.07 (d, J = 6.0 Hz, 4H), 6.84 (d, J = 8.0
3
Hz, 6H), 3.79 (s, 6H), 1.32 (s, 12H).
2
.3.3 Synthesis of N-(4-(7-bromo-2-octyl-2H-benzo[d][1,2,3]triazol-4-yl)phenyl)-4-
methoxy-N-(4-methoxyphenyl)benzenamine (3)
A mixture of compound 2 (1.00 g, 2.32 mmol), 4,7-dibromo-2-octyl-2H-benzo
[
d][1,2,3]triazole (1.17 g, 3.14 mmol), Na CO (6 mL, 2M), Pd(PPh ) (80 mg, 0.07
2 3 3 4
o
mmol) in toluene (25 mL) and methanol (6 mL) was heated and stirred at 80 C under
nitrogen atmosphere for 12 h. After cooled to RT, the mixture was poured into
distilled water (50 mL) and extracted with DCM (3×30 mL). The combined organic
layer was dried over MgSO for 2 h and filtrated. The filtrate was concentrated with
4
rotary evaporator under reduced pressure. The residue was purified by silica gel
column chromatography using DCM/PE (1/2, V/V) as eluent to gain light yellow
1
powder (0.78 g, 55.6%). H NMR (400 MHz, CDCl , ppm): 7.86 (d, J = 8.6 Hz, 2H),
3
7
.60 (d, J = 7.2 Hz, 1H), 7.37 (d, J = 7.6 Hz, 1H), 7.14 (d, J = 8.8 Hz, 4H), 7.04 (d, J
=
8.4 Hz, 2H), 6.87 (d, J = 8.8 Hz, 4H), 4.78 (t, J = 7.2 Hz, 2H), 3.81 (s, 6H),
9

ACCEPTED MANUSCRIPT
2
.16-2.12 (m, 2H), 1.36-1.25 (br, 10H), 0.87 (t, J = 6.4 Hz, 3H). MALDI-TOF MS
(
m/z) for C H BrN O , Calcd: 612.210, Found, 612.232.
34 37 4 2
2
.3.4 Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(2-octyl-7-(4,4,5,5-tetrame-
thyl-1,3,2-dioxaborolan-2-yl)-2H-benzo[d][1,2,3]triazol-4-yl)phenyl)benzenamine (4)
A mixture of compound 3 (0.26 g, 0.43 mmol), 4,4,5,5-tetramethyl-2-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane(0.42g, 1.68mol), potassium
acetate (0.33 g, 3.36 mmol) and [1,1′-bis(diphenylphosphino) ferrocene] dichloro-
palladium (II) (52 mg, 0.07 mmol) in anhydrous tetrahydrofuran (50 mL) was heated
o
and stirred at 65 C under nitrogen atmosphere for 12 h. After cooled to RT, the
mixture was poured into distilled water (50 mL) and extracted with DCM (3×30 mL).
The combined organic layer was dried over MgSO for 2 h and filtrated. The filtrate
4
was concentrated by rotary evaporator under reduced pressure. The residue was
purified by silica gel column chromatography using DCM/PE (1/1, V/V) as eluent to
1
gain yellow viscous compound (0.21 g, 73.5%). H NMR (400 MHz, CDCl , ppm):
3
7
.95-7.93 (m, 3H), 7.51 (d, J = 7.2 Hz, 1H), 7.13 (d, J = 8.8 Hz, 4H), 7.04 (d, J = 8.8
Hz, 2H), 6.86 (d, J = 8.8 Hz, 4H), 4.78 (t, J = 7.6 Hz, 2H), 3.81 (s, 6H), 2.16-2.12 (m,
H), 1.42-1.40 (br, 10H), 1.26 (s, 12H), 0.88 (t, J = 4.2 Hz, 3H). MALDI-TOF MS
m/z) for C H BN O , Calcd: 660.385, Found, 660.392.
2
(
40
49
4
4
2
.3.5 Synthesis of the CH OTPA-BTz-BT-Iq ligand
3
A mixture of compound 4 (0.15 g, 0.23 mmol), 1-bromoisoquinoline (52 mg, 0.25
mmol), K CO (2 mL, 2M), Pd(PPh ) (7 mg, 0.01 mmol) in toluene (18 mL) and
2
3
3 4
o
methanol (2 mL) was heated and stirred at 80 C under nitrogen atmosphere for 24 h.
After cooled to RT, the mixture was poured into distilled water (15 mL) and extracted
with DCM (3×15 mL). The combined organic layer was dried over MgSO for 2 h and
4
filtrated. The filtrate was concentrated by rotary evaporator under reduced pressure.
1
0

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The residue was purified by silica gel column chromatography using DCM as eluent
1
to gain yellow solid (0.13 g, 86.5%). H NMR (400 MHz, CDCl , ppm): 8.71 (d, J =
3
5
.6 Hz, 1H), 7.99-7.91 (m, 4H), 7.74-7.66 (m, 4H), 7.50 (t, J = 3.0 Hz, 1H), 7.17 (d, J
8.4 Hz, 4H), 7.09 (d, J = 8.0 Hz, 2H), 6.89 (d, J = 8.4 Hz, 4H), 4.71 (t, J = 7.0 Hz,
H), 3.82 (s, 6H), 2.06-2.02 (m, 2H), 1.31-1.22 (br, 10H), 0.84 (t, J = 3.8 Hz, 3H).
MALDI-TOF MS (m/z) for C H N O , Calcd: 661.342, Found, 661.465.
=
2
4
3
43
5
2
2
.3.6 Synthesis of (CH OTPA-BTz-Iq) Irpic
3 2
A mixture of IrCl ·H O (30 mg, 0.09 mmol), CH OTPA-BTz-Iq (150 mg, 0.23
3
2
3
mmol), 2-ethoxyethanol 12 mL and distilled water 4 mL was stirred under nitrogen
o
atmosphere at 100 C for 24 h. After cooled to RT, the mixture was poured into
distilled water (15 mL) and extracted with DCM (3×15 mL). The combined organic
layer was dried over Na SO for 4 h and filtrated. The filtrate was concentrated with
2
4
rotary evaporator under reduced pressure to gain the violet chloro-bridged precursor.
The dried chloro-bridged precursor (139 mg, 0.045 mmol) was mixed with
2
-ethoxyethanol (15 mL), picolinic acid (44 mg, 0.36 mmol) and anhydrous Na CO
2 3
(30 mg, 0.28 mmol). The resulting mixture was stirred under nitrogen atmosphere at
o
1
10 C for 24 h. After cooling to RT, the resulting precipitate was filtered off and
washed with water, ethanol and hexane, respectively. The residue was purified by
silica gel column chromatography using DCM/ethyl acetate (EA) (V/V=3/1) as eluent
1
to get an orange powder (31 mg, 41.7 %). H NMR (400 MHz, CDCl , ppm): 9.76 (d,
3
J = 8.0 Hz, 1H), 9.59 (d, J = 8.0 Hz, 1H), 8.78 (d, J = 6.4 Hz, 1H), 8.34 (d, J = 7.6 Hz,
1
H), 7.88-7.80 (m, 4H), 7.68-7.63 (m, 6H), 7.55 (d, J = 6.4 Hz, 1H), 7.45-7.39 (m,
H), 7.31 (d, J = 7.8 Hz, 2H)ꢀ7.23 (d, J = 6.6 Hz, 1H), 7.03-7.00 (m, 8H)ꢀ6.83 (d, J
4
=
7.4 Hz, 8H), 6.74 (d, J = 9.4 Hz, 2H), 6.67 (d, J = 8.2 Hz, 2H), 6.35 (s, 1H), 4.64 (t,
J = 5.6 Hz, 4H), 3.80 (s, 12H), 2.14-2.05 (m, 4H), 1.43-1.26 (br, 20H), 0.84 (t, J = 5.6
11

ACCEPTED MANUSCRIPT
Hz, 6H). MALDI-TOF MS (m/z) for C H IrN O , Calcd: 1635.655, Found,
92
88
11
6
1
635.803. Anal. Calc. for C H IrN O : C 67.54, H 5.42, N 9.42% Found: C 67.21,
92 88 11 6
H 5.34, N 9.27%
3
. Results and Discussion
3
.1 Synthesis and thermal stability
Compound 3 and CH OTPA-BTz-Iq were synthesized through Suzuki couplings.
3
The (CH OTPA-BTz-Iq) Irpic was synthesized using the previous method with
3
2
two-step procedures, which contain a cyclometalation of CH OTPA-BTz-Iq and a
3
cleavage of the chloride groups in the corresponding dimers with picolinic acid. The
1
(
CH OTPA-BTz-Iq) Irpic was characterized by H NMR, MALDI-TOF mass spectra
3
2
and element analysis.
Thermal property of (CH OTPA-BTz-Iq) Irpic was characterized by TGA. The
3
2
recorded TGA curve is shown in Figure S1 (see Electronic Supporting Information,
ESI). The onset decomposition temperature for 5 % weight loss (T ) was 302 °C,
d
which indicates the (CH OTPA-BTz-Iq) Irpic has high thermal stability.
3
2
3
.2 Photophysical Properties
The photophysical properties of (CH OTPA-BTz-Iq) Irpic in DCM solution were
3
2
investigated at 298 K (Table 1). Figure 2 shows the UV-vis spectrum of (CH OTPA-
3
BTz-Iq) Irpic. For comparison, the UV-vis spectrum of the CH OTPA-BTz-Iq ligand
2
3
in DCM is also displayed in Figure 2. Three typical absorption peaks at 304 nm, 465
nm and 535 nm were observed for (CH OTPA-BTz-Iq) Irpic. The intense high-lying
3
2
absorption peak is ascribed to the spin-allowed ligand-central (LC) π-π* transitions,
the low-lying peak with a shoulder in the region of 400-600 nm can be ascribe to a
1
3
mixture of MLCT, MLCT and intramolecular charge transfer (ICT) transitions [45].
1
2

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The low-energy absorption band cannot be distinctly resolved mainly owing to the
hiding of these transitions inside the UV region of the strong singlet transition, which
are of much lower but non-negligible absorptivity [30]. Compared to the CH OTPA-
3
BTz-Iq free ligand, (CH OTPA-BTz-Iq) Irpic exhibited a significantly red-shift low-
3
2
lying absorption peak, which implies that (CH OTPA-BTz-Iq) Irpic with D-A-A archi
3
2
-
tecture has a more intense ICT effect than the CH OTPA-BTz-Iq free ligand due to
3
incorporation of the substituent iridium(III) complex acceptor unit [38].
-6
The PL spectra of (CH OTPA-BTz-Iq) Irpic in degassed DCM (10 M) at 77 K
3
2
and 298 K are displayed in Figure 3. As a reference, CH OTPA-BTz-Iq in dilute
3
DCM at 298 K is also shown in Figure 3. The corresponding data are summarized in
Table 1. At 298 K, under photo-excitation at 460 nm, (CH OTPA-BTz-Iq) Irpic
3
2
exhibits a NIR emission with a strong peak at about 716 nm and a shoulder around
90 nm. At 77 K, the complex displays an additional emission peak at 580 nm,
although the strong emission peak and shoulder are very similar to the spectrum at
98 K. In order to further explain the species of the different emission peaks in the
7
2
spectrum, we measured the lifetimes (τ) at 298 K and 77 K in degassed DCM by using
the single-photon counting method. The decay profiles of (CH OTPA-BTz-Iq) Irpic at
3
2
7
7 K and 298 K at different emission peaks are shown in Figure S2 (see ESI). At 77 K,
(
CH OTPA-BTz-Iq) Irpic exhibits a much longer emission lifetime of 12.86 ꢀs at the
3
2
peak of 580 nm, which can be ascribe to the ligand triplet-triplet emission. Mean-
while, a strong low-temperature phosphorescence band at 708 and a shoulder around
7
83 nm are observed, which shows a little blue shift to the spectrum at 298 K due to
the prohibition of the thermal activations [49]. Using Ru(bpy) (PF ) as the standard
3
6 2
compound, (CH OTPA-BTz- Iq) Irpic shows a fluorescence quantum yield of 0.3% in
3
2
degassed DCM .
1
3

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Compared to the iridium (III) complex Ir(tpaiq) (acac) (tpa = triphenylamine, iq =
2
isoquinoline, acac = acetoacetone) reported previously by our group [46], the PL
spectrum of (CH OTPA-BTz-Iq) Irpic presents 80 nm red shift because of an
3
2
additional A unit (benzotriazole). This indicates that the intramolecular D-A effect is
available to make the iridium (III) complexes exhibit red-shifted PL spectra.
3
.3 Electrochemical Properties
The redox properties of (CH OTPA-BTz-Iq) Irpic were characterized by cyclic
3
2
voltammetry (CV) method using ferrocene as an internal standard. The recorded
voltammograms are shown in Figure S3 and the resulting CV data are summarized in
Table 1. An quasi-reversible oxidation wave (E ) at 0.75 eV and an quasi-reversible
ox
reduction wave (E ) at -0.82 eV are observed. According to the reported literature,
red
the oxidation potential is strongly dependent on the electronic environment of the
iridium(III) core [47]. On the basis of Eox and Ered values, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels (EHOMO and ELUMO) of (CH OTPA-BTz-Iq) Irpic are calculated to be
3
2
-5.08 and -3.5 eV based on the empirical formula, respectively [48]. As the LUMO
and HOMO energy levels are -2.20 eV/-5.80 eV for PVK and -2.80 eV/-6.50 eV for
OXD-7, (CH OTPA -BTz-Iq) Irpic exhibits a matched energy level with the PVK and
3
2
OXD-7 blend, which is available for (CH OTPA-BTz-Iq) Irpic to play a carrier trap
3
2
role in the PVK-OXD-7-hosted PLEDs. In order to conveniently analyze the energy
transfer of the guest and host in the PLEDs, the HOMO-LUMO energy levels of all
materials used and the device configuration are shown in Figure 4.
3
.5 Electroluminescence Properties
To investigate the applicability of this new emitter in EL devices, a series of
spin-coated PLEDs were fabricated with a single-active layer configuration: ITO/
1
4

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PEDOT:PSS (40 nm)/ PVK:OXD-7 (70:30): x wt% (CH OTPA- BTz-Iq) Irpic (x =3,
3
2
5
, 9) (80 nm)/ Ba (4 nm)/ Al (100 nm) (Device I) and a multi-active layer configu-
ration: ITO/ PEDOT:PSS (40 nm)/ PVK:OXD-7 (70:30): x wt% (CH OTPA-BTz-
3
Iq) Irpic (x = 9, 12, 14, 16) (80 nm)/ TPBI (1,3,5-tri(1-phenyl-1H-benzo[d]-
2
imidazol-2-yl)phenyl) (30 nm)/ Ba (4 nm)/ Al (100 nm) (Device II), in which PEDOT
was employed as hole-injecting layer (HIL), PVK mixed with OXD-7 was acted as
host material because of the PVK has excellent film-forming properties and high
hole-transport ability, TPBI was used as the electron-transporting/exciton-blocking
layer, and Ba was used as electron-injecting layer (EIL). The doping concentrations in
emitting layer were optimized to investigate the device performance. For convenience,
devices based on a single-active layer and a multi-active layer are described as I and
II, respectively.
Figure 5 shows the EL spectra of the (CH OTPA-BTz-Iq) Irpic-doped Device I and
3
2
II at different dopant concentrations. The detailed data are summarized in Table 2. In
Device I, at low doping concentration of 3.0 wt%, two distinct EL peaks at about 440
nm and 720 nm with a shoulder peak at about 780 nm were observed, which are
assigned to the PVK:OXD-7(70:30) and (CH OTPA-BTz-Iq) Irpic emissions, respec-
3
2
tively. The EL peak of PVK/OXD-7(70:30) is shown in Figure S4. With further
increasing dopant concentrations, the PVK:OXD-7 emission was almost quenched
and only strong NIR emission from (CH OTPA-BTz-Iq) Irpic was observed in both
3
2
Device I and II. This implies that the energy was transferred efficiently from PVK:
OXD-7 to (CH OTPA-BTz-Iq) Irpic. More importantly, we found that the platinum
3
2
(
II) complexes of (TPA-BT-Q)Ptpic and (CH OTPA-BTz-Iq)Ptpic, which have the
3
analogous D-A-A structure, displayed a broad, dual EL emission including a strong
visible emission (derived from the (TPA-BT-Q) and (CH OTPA-BTz-Iq) ligand
3
1
5

ACCEPTED MANUSCRIPT
center) and NIR emission simultaneously, especially at low dopant concentrations.
However, the (CH OTPA-BTz-Iq) Irpic shows no emission of ligand center because
3
2
the spin-orbit coupling (SOC) strength of iridium(III) complexes are much stronger
than that of platinum(II) complexes. (The EL spectra of the three complexes are
shown in Figure S5 and Figure S6).
The EQE versus current density characteristic of the Device I and II are shown in
Figure 6. The corresponding device performance data are summarized in Table 2. In
Device I, the maximum EQE of 0.20% is observed at the dopant concentrations of 9.0
wt%. Apparently, the EQEs tend to decrease firstly and then increase with the
increasing dopant ratios in Device I. In Device II, with the use of TPBI, the efficiency
enhances significantly because a broader recombination region within the emitting
layer is better for carrier balance. The maximum EQE of 0.41% at current density of
-2
8
.14 mA cm is observed at the dopant concentrations of 12.0 wt%. Furthermore, the
EQE levels display a small roll-off at high current densities. This sluggish efficiency
roll-off is favorable for practical applications of OLEDs/PLEDs [49]. Moreover, aside
from the octahedral configuration of iridium (III) complexes, the introduction of
bulky nonplanar TPA unit into (CH OTPA-BTz-Iq) Irpic can also effectively reduce
3
2
the chances of π-stacking of dopants [38].
The current densities (J)-voltage (V)-radiance (R) characteristics of the (CH OTPA-
3
BTz-Iq) Irpic doped Device I and II are shown in Figure 7 and their corresponding
2
data are summarized in Table 2. Device II acquired a preferable performance with a
-2
-2
radiant emittance of 74.1 ꢀW cm at 23.2 mA cm and 12.0 wt% doping
concentrations.
4
. Conclusions
1
6

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In summary, a novel benzotriazole-containing D-A-A type cyclometalated iridium
(
III) complex of (CH OTPA-BTz-Iq) Irpic was obtained with a NIR-EL emission
3
2
peak at 723 nm and a shoulder at 780 nm. Its NIR-emitting PLEDs showed a
suppressed efficiency roll-off with increasing operating current densities. The best
device performances were presented in the device at 12.0 wt% dopant concentration
-2
in Decive II. The maximum EQE of 0.41% at 8.14 mA cm and an irradiance
-2
intensity of 74.1 ꢀW cm at 23.2 V were observed. This work indicates that
introducing appropriate D and A unit to develop D-A-A structure is an efficient
approach to construct high-efficiency NIR-emitting iridium (III) complex in PLEDs
with insignificant efficiency roll-off.
Acknowledgements
This work was supported by Scientific Research Fund of Hunan Provincial
Education Department (15C0531), the National Natural Science Foundation of China
(51473140), the Natural Science Foundation of Hunan University of Science and
Technology (E51522) and the Open Project Program of Key Laboratory of
Theoretical Organic Chemistry and Functional Molecule, Ministry of Education
(E21507).
1
7

ACCEPTED MANUSCRIPT
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Captions of Figures
Figure 1 Molecular structure of (CH OTPA-BTz-Iq) Irpic
3
2
Scheme 1 Synthetic route of (CH OTPA-BTz-Iq) Irpic
3
2
Figure 2 Normalized UV-vis absorption spectra of CH OTPA-BTz-Iq ligand and
3
(
CH OTPA-BTz-Iq) Irpic in DCM at 298 K
3 2
Figure 3 Normalized PL spectra of CH OTPA-BTz-Iq ligand in deaerated DCM at
3
2
98 K and (CH OTPA-BTz-Iq) Irpic in deaerated DCM at 298 K and 77 K
3 2
Figure 4 Energy level diagram and device structure of the (CH OTPA-BTz-Iq) Irpic
3
2
-doped PLEDs
Figure 5 EL spectra of the (CH OTPA-BTz-Iq) Irpic-doped PLEDs. (a) EL spectra in
3
2
Device I at dopant concentrations from 3 wt % to 9 wt%; (b) EL spectra in Device II
at dopant concentrations from 9 wt% to 16 wt%.
Figure 6 The EQE versus current density characteristics of Device I (a) and Device II
(b).
Figure 7 The current density-voltage-radiance (J-V-R) curves of the (CH OTPA-BTz-
3
Iq) Irpic-doped devices at different concentrations: (a) from 3 wt% to 9 wt% in
2
Device I; (b) from 9 wt% to 16 wt% in Device II.
Table 1 Photophysical and electrochemical properties of (CH OTPA-BTz-Iq) Irpic
3
2
Table 2 The EL parameters of the (CH OTPA-BTz-Iq) Irpic-doped PLEDs
3
2
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Figure 1
Scheme 1
2
5

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Figure 2
Figure 3
2
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Figure 4
2
7

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Figure 5
2
8