Near-infrared emission of dinuclear iridium complexes with hole/electron transporting bridging and their monomer in solution-processed organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2017.09.061

To obtain efficient near-infrared (NIR) emitting materials, a novel iridium (III) complex based on a triphenylamine functionalized pyridylpyrene ligand, namely, Mono-Ir, as well as its two corresonding dinuclear iridium (III) complexes possessing π-conjugated bridging linkages of hole-transporting carbazole (Caz) unit or electron-transporting 2,5-diphenyl-1,3,4-oxadiazole (OXD) unit, namely D-Ir-Caz and D-Ir-OXD, were successfully synthesized and characterized. Under photo-excitation, similar phosphorescence spectrum were obtained for the three complexes with intense NIR emission peak at approximately 698 nm and emission wavelength ranging from 650 to 900 nm. Also, in their single-emissive-layer OLEDs with a device structure of ITO/PEDOT:PSS/TFB/CBP:PBD:complexes/TmPyPB/Liq/Al, similar NIR electroluminescent (EL) emission peaked at 698 nm with a shoulder at 762 nm were obtained. The Mono-Ir-based device showed an external quantum efficiency (EQE) of 1.29% at low current density of 3.5 mA cm^?2, while a relatively lower efficiency of 0.27% for the D-Ir-Caz-based device and 0.41% for the D-Ir-OXD-based device, accompanied with negligible efficiencies roll-off at high current densities were achieved, respectively. The better device performance should be achieved when coupled with electron transporting bridging, especially at high currents.

Full text of DOI:10.1016/j.dyepig.2017.09.061

Accepted Manuscript
Near-infrared emission of dinuclear iridium complexes with hole/electron transporting
bridging and their monomer in solution-processed organic light-emitting diodes
Zhaoran Hao, Min Li, Yajun Liu, Yafei Wang, Guohua Xie, Yu Liu
PII:
S0143-7208(17)31828-4
10.1016/j.dyepig.2017.09.061
DYPI 6287
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 28 August 2017
Revised Date: 23 September 2017
Accepted Date: 25 September 2017
Please cite this article as: Hao Z, Li M, Liu Y, Wang Y, Xie G, Liu Y, Near-infrared emission of dinuclear
iridium complexes with hole/electron transporting bridging and their monomer in solution-processed
organic light-emitting diodes, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.09.061.
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ACCEPTED MANUSCRIPT
Near-Infrared Emission of Di-nuclear Iridium Complexes with
hole/electron transporting bridging and Their Monomer in
Solution-Processed Organic Light-Emitting Diodes
Zhaoran Hao a, Min Li a, Yajun Liu a, Yafei Wang a,c, Guohua Xie b*, Yu Liu a,c*
Graphical abstracts
Two dinuclear iridium (III) complexes D-Ir-Caz and D-Ir-OXD grafting hole or
electron transporting π-conjugated bridges, and their corresponding monomer
Mono-Ir were successfully synthesized and characterized. Under photo-excitation, all
these complexes exhibit similar NIR emission peaked at approximately 698 nm in
CH2Cl2. Due to a lower degree of spin-orbit coupling (SOC) in a system with
centrosymmetric environment at the ancillary ligand, both dinuclear iridium (III)
complexes D-Ir-Caz and D-Ir-OXD exhibited a much lower luminescence quantum
yields (ΦL) in comparison with their corresponding monomer complex Mono-Ir.
Furthermore, the Mono-Ir-based device shows a EQE of 1.29% at 3.5 mA/cm2, while
D-Ir-Caz- and D-Ir-OXD-based devices suffer relatively lower efficiencies of 0.27%
and 0.41% accompanied with a negligible efficiency roll-off.

ACCEPTED MANUSCRIPT
Near-Infrared Emission of Dinuclear Iridium Complexes with
hole/electron transporting bridging and Their Monomer in
Solution-Processed Organic Light-Emitting Diodes
Zhaoran Hao a, Min Li a, Yajun Liu a, Yafei Wang a,c, Guohua Xie b*, Yu Liu a,c*
a College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application
in Ministry of Education, Xiangtan University, Xiangtan 411105, China
b Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials Department of
Chemistry, Wuhan University, Wuhan 430072, China
c Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, School
of Materials Science & Engineering, Changzhou University, Changzhou 213164, PR
China
* Corresponding authors. Tel: +86-731-58293377; fax: +86-731-58292251 (Y. Liu).
Email addresses:
(G. X.) guohua.xie@whu.edu.cn
(Y. L.) liuyu03b@126.com
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Abstract: To obtain efficient near-infrared (NIR) emitting materials, a novel iridium
(III) complex based on a triphenylamine functionalized pyridylpyrene ligand, namely,
Mono-Ir, as well as its two corresonding dinuclear iridium (III) complexes possessing
π-conjugated bridging linkages of hole-transporting carbazole (Caz) unit or
electron-transporting 2,5-diphenyl-1,3,4-oxadiazole (OXD) unit, namely D-Ir-Caz and
D-Ir-OXD, were successfully synthesized and characterized. Under photo-excitation,
similar phosphorescence spectrum were obtained for the three complexes with intense
NIR emission peak at approximately 698 nm and emission wavelength ranging from
650-900 nm. Also, in their single-emissive-layer OLEDs with a device structure of
ITO/PEDOT:PSS/TFB/CBP:PBD:complexes/TmPyPB/Liq/Al,
similar
NIR
electroluminescent (EL) emission peaked at 698 nm with a shoulder at 762 nm were
obtained. The Mono-Ir-based device showed an external quantum efficiency (EQE) of
1.29% at low current density of 3.5 mA cm-2, while a relatively lower efficiency of
0.27% for the D-Ir-Caz-based device and 0.41% for the D-Ir-OXD-based device,
accompanied with negligible efficiencies roll-off at high current densities were
achieved, respectively. The better device performance should be achieved when
coupled with electron transporting bridging, especially at high currents.
Keywords: Cyclometalated dinuclear iridium (III) complexes; Near-infrared; Organic
light-emitting diodes; Synthesis
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1. Introduction
1
Since the pioneer work by Tang and Vanslyke in 1980s , great progress have
been made on developing high-efficiencies visible organic light-emitting diodes
(OLEDs), where has potential applications in low cost, efficient, flexible full-color
displays and lighting panels, et al 2-8. As an extension of visible light, near-infrared
(NIR) organic luminescent materials (700-2500 nm) also have emerging applications
9-11
in night-vision displays, medical diagnosis, sensor, optical communication, et al
.
However, as for their device performance, NIR materials always suffer low
efficiencies and sharp efficiencies roll-off at high currents, which should always be
attributed to their decreased energy gap12,13 and long excited-state lifetimes, and that
14
commonly occurs in square-planar Pt(II)-porphyrin phosphors . Furthermore, in
persuit of increasing emission wavelength, many materials are always characterized
by their strong donor/acceptor (D/A) or conjugate rigid plane structure, which are
15
easily prone to fluorescence quenching that caused by close molecular packing
.
Hence, it is still a tough issue for the design of high-efficient NIR emitters for OLEDs
application.
Classical phosphorescent transition metal materials are available to exhibit high
luminescence quantum efficiencies and narrow energy gap, resulting from their triplet
emissive character through strong spin orbit coupling in the presence of heavy metal.
26
Therefore, materials based on the Pt 14,16-20, Ir 21-24, or Os
complexes are of
particular relevance in NIR applications. Of these, Pt complexes have shown record
external quantum efficiencies (EQE) up to 8.8~24% with peak emission in the range
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of 650~800 nm 14,19-20. However, most of these platinum systems are constructed by
square-planar configuration, and the accompanying easy aggregation and relatively
long phosphorescence lifetime always generate noticeable efficiency roll-offs at high
current density 27.
As another major category in phosphorescent transition metal materials, Ir(III)
complexes exhibit typical octahedral configuration and relatively short triplet lifetime,
which is favor for their efficiency stabilities in OLEDs 21,24. Besides, in a C^N
bidentate cyclometalated system, spin-orbit coupling (SOC) seems to be more
efficient for octahedral d6 Ir complexes than that for square-planar d8 Pt complexes 28.
These materials always exhibit slightly reduced device efficiencies at high currents,
besides, their EQEs can reach up to as high as 3.4% at 702 nm 21, 3.1% at 714 nm 22,
and 4.5% at 760 nm 24, respectively. Qiao etc, reported a cationic iridium emitter
based on benzophthalazine in solution-processed NIR OLED, and shows negligible
efficiency roll-off even over a wide range of currents of 1-100 mA cm-2 23. On the
basis of the foregoing work, recently, their group designed a neutral homoleptic
iridium material, which may has the highest EQEs in the Ir complexes, of that 4.5% at
760 nm with small efficiency roll-off remaining of 3.5% at 100 mA cm-2 24. Similarly,
Kesarkar etc, demonstrated a kind of benzothiophenyl-isoquinolinate-based Ir
complexes with different β-diketonate ancillary ligands, and obtained a remarkable
EQE of 3.07% in a solution processed OLED with negligible efficiency roll-off 22.
Through the introduction of triphenylamine appendices at the peripheral side of
the pyridylpyrene (PyPyr) ligand 29 to extend the π-conjugation, an effectively narrow
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energy gap with longer emission wavelength was achieved in a new complex Mono-Ir.
This material exhibit an efficient NIR emission peaked at 698 nm with a vibronic
shoulder, while the spin-coated OLEDs achieved a maximum external quantum
efficiency of 1.29%. Besides, carbazole and oxadiazole derivatives usually exhibt
high hole-transporting and electron-injecting abilities, and are ideal component in
30
constructing OLEDs materials . To gain an insight into the performance change of
materials that have these functional group act as bridging linkages, two dinuclear
iridium (III) complexes incorporating a hole-transporting carbazole (Caz) or an
electron-transporting oxadiazole (OXD) bridging unit were also designed and
synthesized, namely D-Ir-Caz and D-Ir-OXD, respectively. The synthetic routes of
the three complexes are shown in Scheme 1. Their thermal, electrochemical, and
optophysical properties were also investigated. The strategy is focused on the
introduction of the bulky structure to suppress molecular aggregation and provide
with efficient hole/electron transporting abilities in device. It is interesting that the
similar PL spectra were obtained in the three iridium (III) complexes. However, much
lower luminescence quantum yields were detected in the dinuclear iridium (III)
complexes. We think that it should be caused by a lower degree of spin-orbit coupling
(SOC) in a system with centrosymmetric environment at the ancillary ligand.
Therefore, the D-Ir-Caz- and D-Ir-OXD-based devices suffer relatively lower
efficiencies of 0.27% and 0.41%, along with negligible efficiency roll-off values.
2. Experimental section
2.1. Materials and Synthesis
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All reagents were purchased from Aldrich and Acros Inc. Tetrahydrofuran (THF)
was distilled over sodium before used. The other reagents were directly used without
further purification. All reactions were performed under nitrogen atmosphere and
were monitored by thin-layer chromatography. As shown in Scheme 1, both dinuclear
iridium (III) complexes D-Ir-Caz and D-Ir-OXD with hole-transporting carbazole- or
electron-transporting oxadiazole bridging, as well as their corresponding monomer
complex Mono-Ir were designed and synthesized, respectively. Both the ligand
t-BuTPAPyPyr and the target dinuclear iridium (III) complexes were obtained simply
16,18
through Suzuki coupling reactions which are similarly to the previous reports
.
The yields of dinuclear iridium (III) complexes are low due to steric hindrance, and
the reaction environment was carried out in two kinds of large polarity solvents. The
target iridium (III) complexes were synthesized according to the literature procedures
21-24. t-BuPyr-B was prepared followed by the literature reported 31.
2.2. Apparatus and Procedures
1H and 13C NMR spectra were measured with a Bruker Dex-400 NMR
instrument using CDCl3 as a solvent. Mass spectra (MS) were recorded on a Bruker
Autoflex MALDI-TOF instrument using dithranol as a matrix. The thermogravimetric
analysis (TGA) was undertaken using a NETZSCH STA449 measurement under
nitrogen atmosphere at a heating rate of 20 °C min−1. The ultraviolet-visible (UV-vis)
absorption and photoluminescence (PL) spectra of the compound were recorded on a
Varian Cray 50 and Perkin-Elmer LS50B luminescence spectrometer, respectively.
The optical band gap (Eg) energy level was estimated from the onset of the absorption
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edge. Excited-state lifetimes were recorded at excitation wavelengths of 440 nm for
three complexes on FLS920 with time-corrected single-photon-counting (TCSPC)
measurement, while the low temperature phosphorescence spectra were obtained on
an Edinburgh FLS920 Spectrometer at 77 K in DCM solution. The Φp values of
complexes in degassed DCM solutions were measured with tris(2,2'-bipyridine)
ruthenium(II) (Φp=0.062 in acetonitrile solution at 298 K) as the standard. The
equation of Φs = Φr(ηs2ArIs/ηr2AsIr) was used to calculate the fluorescence quantum
yields where Φs is the quantum yield of the sample, Φr is the quantum yield of the
reference, η is the refractive index of the solvent, As and Ar are the absorbance of the
sample and the reference at the wavelength of excitation, and Is and Ir are the
32
integrated areas of emission bands . Cyclic voltammetry (CV) measurements were
performed on a CHI600E electrochemical workstation with platinum as the working
electrode, platinum wire as the counter electrode, and KCl saturated Ag/AgCl as the
reference electrode at a 100 mV s−1 scanning rate. Tetrabutylammonium
hexafluorophosphate (Bu4NPF6, 0.10 M) was used as the supporting electrolyte in
acetonitrile solvent. Standard ferrocenium/ferrocene (Fc+/Fc) system was used to
calibrate the experiment data.
2.3. Device fabrication.
The OLEDs structure employed in this study is ITO/PEDOT:PSS (30 nm)/TFB
(10 nm)/CBP:PBD:complexes (60:30:10, 40 nm)/TmPyPB (40 nm)/Liq (1 nm)/Al
(100 nm), where PEDOT is poly(3,4-ethylenedionythiophene) and PSS is
poly(styrenesulfonic acid) (Batron-P 4083, purchased from Bayer AG). The ITO
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surface was pre-cleaned in an ultrasonic solvent bath, and dried in an oven at 120 °C
for more than 3 h, then treated with UV ozone for 15 min. After that, the substrates
were transferred to the spin-coating process, and then different functional layers
(PEDOT: PSS layer and transporting layer and emissive layer) were sequentially
spin-coated onto the substrates and were dried in vacuum. Then the samples were
transferred to the deposition system, and TmPyPB and Liq/Al layers were
sequentially deposited onto the substrate by thermal deposition in the vacuum of 10-6
Torr. The emitting layer consisted of the host of CBP:PBD and dopant of the complex.
The dopant concentration was 10 wt %. The electroluminescent (EL) spectra were
measured by a PR650 spectrometer. The current density-voltage-brightness (J-V-R)
features of the OLEDs were measured with a Keithey 2400 Source meter and Konica
Minolta chromameter CS-200 with a calibrated silicon photodiode. The EQEs values
were calculated from the current densities, current efficiencies, and the corresponding
EL spectra. All measurements were carried out at room temperature under ambient
conditions.
2.4. Synthesis
2.4.1. Synthesis of 7-tert-butyl-1-(5-bromopyridine-2-yl)pyrene (t-BuPyrPy)
A mixture of 7-tert-butyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene
(0.86 g, 2.2 mmol), 2,5-dibromopyridine (0.63 g, 2.7 mmol), aqueous Na2CO3 (4.5 M,
5 mL), and Pd(PPh3)4 (0.05 g, 0.045 mmol) in 30 mL THF was flushed with N2 while
stirring at reflux temperature for 24 h. After cooled to RT, the mixture was extracted
with CH2Cl2 and the combined organic layer was dried over MgSO4. The solvent was
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removed by rotary evaporation and the residue was passed through a flash silica gel
column using PE-DCM (V/V, 2/1) as the eluent, then recrystallized using hexane to
1
afford a yellowish solid (0.48 g, 52 %). H NMR (400 MHz, CDCl3) δ 8.94 (s, 1H),
8.34 (d, J = 9.2 Hz, 1H), 8.25 (d, J = 10.1 Hz, 3H), 8.09 (t, J = 7.8 Hz, 4H), 8.03 (d, J
= 8.3 Hz, 1H), 7.66 (d, J = 8.3 Hz, 1H), 1.59-1.54 (m, 9H). MALDI-TOF MS (m/z)
for C43H34N2. Calcd: 413.078; Found, 414.112.
2.4.2. Synthesis of N,N-diphenyl-4-(2-(1-tert-butyl-pyrene-7-yl)-pyridine-5-yl)-
benzene amine (t-BuPyrPyTPA)
A mixture of compound 5 (0.62 g, 1.5 mmol), N,N-diphenyl-4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine (0.72 g, 1.9 mmol), aqueous
Na2CO3 (5 M, 10 mL), and Pd(PPh3)4 (35 mg, 0.03 mmol) was added into 30 mL dry
THF under N2 atmosphere. The reaction was heated at reflux for 24 h under stirring.
After cooled to RT, the mixture was extracted with CH2Cl2 and the combined organic
layer was dried over MgSO4. The solvent was removed by rotary evaporation and the
residue was passed through a flash silica gel column using PE-DCM (V/V, 2/1) as the
eluent, then recrystallized using acetone to afford a yellow solid (0.45 g, 55 %). 1H
NMR (400 MHz, CDCl3) δ 9.10 (s, 1H), 8.49 (d, J = 9.4 Hz, 1H), 8.23 (dd, J = 20.7,
12.6 Hz, 4H), 8.13 (s, 1H), 8.10 (s, 1H), 8.06 (d, J = 7.9 Hz, 2H), 7.81 (d, J = 8.0 Hz,
1H), 7.60 (d, J = 8.3 Hz, 2H), 7.32 (t, J = 7.7 Hz, 4H), 7.21 (dd, J = 15.6, 8.1 Hz, 6H),
7.09 (t, J = 7.2 Hz, 2H), 1.60 (s, 9H). 13C NMR (101 MHz, CDCl3) δ 157.77, 149.26,
148.17, 147.80, 147.59, 135.32, 134.23, 133.96, 131.38, 131.18, 130.90, 129.44,
128.57, 128.23, 128.11, 127.81, 127.31, 125.53, 125.15, 124.85, 124.64, 123.68,
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123.41, 123.22, 122.65, 122.34, 35.25, 31.96. MALDI-TOF MS (m/z) for C43H34N2.
Calcd: 578.272; Found, 578.237.
2.4.3. Synthesis of Mono-Ir.
A mixture of t-BuPyrPyTPA (200mg, 0.33 mmol) and IrCl3·H2O (30 mg, 0.095
mmol) were dissolved in 2-ethoxyethanol/H2O (12mL/4 mL) (V/V) and heated at
100 °C for 24 h under N2 atmosphere. The suspension was filtered and the red solid
was washed with water and a small amount of methanol then dried under high
vacuum. The product was directly thrown into the next step without any purification.
The compounds Ir(t-BuPyrPyTPA)2(µ-Cl)2 (128mg, 0.046 mmol), picolinic acid (147
mg, 1.2 mmol), and Na2CO3 (100 mg) were suspended in 2-ethoxyethanol (25 mL)
and stirred at 80 °C for 7 h under N2 atmosphere. After cooled to RT, the mixture was
directly poured into methanol and filtered, evaporated in vacuum to give a red solid,
the residue was purified by silica gel column chromatography using PE-DCM (V/V,
2/1) as eluent to afford Mono-Ir as an orange red solid (85 mg, 60.9%). 1H NMR (400
MHz, CDCl3) δ 9.29 (s, 1H), 8.84 (m, 4H), 8.39 (d, J = 7.8 Hz, 1H), 8.10(m, 8H),
7.98 (s, 1H), 7.91 (s, 1H), 7.86 (t, J = 7.6 Hz, 1H), 7.77 (d, J = 8.9 Hz, 1H), 7.70 (d, J
= 8.9 Hz, 1H), 7.55 (d, J = 8.2 Hz, 2H), 7.47 (d, J = 8.8 Hz, 2H), 7.40 (d, J = 9.0 Hz,
13
1H), 7.28 (s, 3H), 7.08 (m, 24H), 6.81 (s, 1H), 1.53 (d, J = 7.8 Hz, 18H). C NMR
(101 MHz, CDCl3) δ 173.05, 167.66, 166.06, 152.60, 148.96, 148.65, 148.44, 147.81,
147.33, 146.30, 138.29, 137.92, 134.84, 134.47, 133.84, 131.86, 131.58, 130.23,
129.33, 129.30, 128.25, 127.76, 127.24, 126.91, 124.93, 123.79, 123.43, 123.25,
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122.67, 122.15, 35.06, 31.91. MALDI-TOF MS (m/z) for C92H70IrN5O2. Calcd:
1469.516; Found, 1469.700.
2.4.4. Synthesis of Mono-Ir-Br
Similar synthetic procedures was adopted to afford Mono-Ir-Br as an orange red
solid (80 mg, 54.4%). 1H NMR (400 MHz, CDCl3) δ 9.22 (s, 1H), 8.85 (m, 4H), 8.28
(d, J = 8.3 Hz, 1H), 8.08(m, 11H), 7.78 (d, J = 8.7 Hz, 1H), 7.71 (d, J = 8.6 Hz, 1H),
7.53 (m, 4H), 7.38 (d, J = 8.6 Hz, 1H), 7.29 (s, 3H), 7.12 (m, 23H), 6.77 (s, 1H), 1.53
(d, J = 10.1 Hz, 18H). MALDI-TOF MS (m/z) for C92H69BrIrN5O2. Calcd: 1547.426;
Found, 1547.632.
2.4.5. Synthesis of D-Ir-Caz
A
mixture
of
Mono-Ir-Br
(200
mg,
0.13
mmol),
2,5-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazole (28
mg, 0.058 mmol), aqueous Na2CO3 (5 M, 3 mL), and Pd(PPh3)4 (5 mg, 0.004 mmol)
was added into a mixture of 10 mL dry THF and DMF under N2 atmosphere. The
reaction was heated at 80 °C for 24 h under stirring. After cooled to RT, the mixture
was extracted with CH2Cl2 and the combined organic layer was dried over MgSO4.
The solvent was removed by rotary evaporation and the residue was passed through a
flash silica gel column using PE-DCM-THF (V/V/V, 1/1/0.04) as the eluent several
1
times to afford an orange red solid (70 mg, 37.5 %). H NMR (400 MHz, CDCl3) δ
9.29 (s, 1H), 8.82 (m, 4H), 8.43 (d, J = 8.0 Hz, 1H), 8.11 (m, 9H), 7.99 (d, J = 5.4 Hz,
2H), 7.89 (s, 1H), 7.71 (d, J = 8.8 Hz, 2H), 7.54 (d, J = 8.2 Hz, 2H), 7.42 (m, 3H),
7.16 (m, 14H), 7.01 (m, 11H), 6.90 (m, 2H), 6.66 (t, J = 8.5 Hz, 1H), 3.27 (s, 1H),
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13
1.52 (s, 18H), 1.03 (m, 8H). C NMR (101 MHz, CDCl3) δ 173.10, 167.67, 166.01,
150.79, 148.67, 148.33, 147.67, 147.61, 147.44, 147.41, 147.23, 147.19, 146.53,
141.48, 138.19, 137.82, 134.79, 134.51, 133.91, 131.88, 131.57, 130.28, 129.45,
129.40, 129.24, 129.12, 128.78, 128.70, 128.65, 128.24, 128.17, 127.72, 127.17,
127.00, 126.93, 124.85, 123.96, 123.92, 123.77, 123.55, 123.40, 123.34, 123.29,
123.05, 122.68, 122.61, 122.46, 122.40, 122.29, 122.17, 121.88, 121.29, 106.99,
61.95, 53.48, 35.05, 31.90, 29.76, 22.80, 10.80. MALDI-TOF MS (m/z) for
C204H161Ir2N11O4. Calcd: 3214.204; Found: 3215.289.
2.4.6. Synthesis of D-Ir-OXD
Similar synthetic procedures was adopted to afford D-Ir-OXD as an orange red
solid (65 mg, 35.5%). 1H NMR (400 MHz, CDCl3) δ 9.28 (s, 1H), 8.85 (m, 4H), 8.47
(d, J = 7.9 Hz, 1H), 8.13 (m, 11H), 7.73 (m, 5H), 7.55 (d, J = 8.0 Hz, 2H), 7.43 (m,
13
2H), 7.29 (s, 2H), 7.24 (s, 3H), 7.08 (m, 23H), 6.85 (s, 1H), 1.52 (s, 18H). C NMR
(101 MHz, CDCl3) δ 172.71, 167.63, 165.93, 163.81, 151.90, 148.74, 148.43, 147.81,
147.49, 147.23, 147.20, 146.56, 146.24, 139.36, 138.17, 137.83, 135.83, 134.96,
131.62, 130.30, 130.24, 129.49, 129.36, 129.28, 129.06, 128.78, 128.60, 128.32,
127.74, 127.66, 127.24, 127.03, 124.96, 124.93, 124.00, 123.77, 123.59, 123.37,
123.29, 122.11, 35.05, 31.90. MALDI-TOF MS (m/z) for C199H146Ir2N12O5. Calcd:
3157.084; Found, 3158.101.
3. Results and discussion
3.1. Thermal properties
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The thermogravimetry analysis (TGA) curves of the iridium (III) complexes
under nitrogen atmosphere are depicted in Figure 1 and the corresponding data are
summarized in Table 1. Three iridium (III) complexes exhibited good thermal
stabilities, indicating these iridium (III) complexes are thermally suitable candidates
o
for application in OLEDs. Decomposition temperature (Td) values of 446 C for
o
D-Ir-Caz, 437 C for D-Ir-OXD and 414 °C for Mono-Ir were observed at a 5%
weight loss, respectively. Obviously, the Td values of both dinuclear iridium (III)
complexes are higher than that of the corresponding mononuclear iridium (III)
complex, which may mainly due to their larger conjugation systems.
3.2. Optical properties
The UV-vis absorption spectra of the iridium (III) complexes in dilute DCM
solution at RT is shown in Figure 2a, and the detailed photophysical characteristics
are summarized in Table 2. The similar absorption spectra of these iridium (III)
complexes exhibit quite typical profiles of many other iridium (III) complexes 21-24, 33
,
and their optical properties can be accordingly interpreted. The high intensity
absorption bands in the range of 260 to 360 nm can be normally ascribed to
ligand-centered (1LC) transitions. The structured bands centered at approximately 417
nm can be ascribed to a mixture of 1LC and 1CT (either metal to ligand charge transfer
(1MLCT) or intraligand charge transfer (1ILCT) type) transitions 29. The broad and
weak absorptions at wavelengths higher than 470 nm are arise from formally triplet
3LC and 3CT transitions that facilitated by the spin-orbit coupling of iridium atom. In
terms of the ε values, the light-harvesting capability of both dinuclear complexes were
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found to be enhanced in comparison with their parent complex, resulting from the
expanded conjugation bridging and as each metal-based unit possesses its own
34
absorption properties in a polynuclear specie . The photoluminescence (PL) spectra
of three iridium (III) complexes in DCM solution at 298 k and 77 k under
opto-excitation are presented in Figure 2b, and the corresponding data are
summarized in Table 2. At 298 k, complexes Mono-Ir, D-Ir-Caz and D-Ir-OXD
exhibited a remarkable NIR emission peak at 698, 699 and 700 nm with a shoulder
around 792 nm, respectively. It indicates that these iridium (III) complexes possess
the same 3LC/3ILCT transition on the phosphor ligand 27. Furthermore, the
luminescence quantum yields (ΦL)and lifetime (τ) of the mononuclear and dinuclear
iridium (III) complexes were investigated (Figure S1, and the resulting data are
summarized in Table 1). As results, a ΦL of 2.25% and a τ of 1.04 µs were observed in
the mononuclear complex Mono-Ir, whereas a large decrease in ΦL is detected of 0.7%
for D-Ir-Caz and 1.22% for D-Ir-OXD, respectively. Meanwhile, the large extended τ
value of 1.41 µs for D-Ir-Caz and 1.42 µs for D-Ir-OXD were observed, respectively.
Obviously, these nonradiative rate constants (knr) are essentially identical for the three
iridium (III) complexes, and indicating that the same nonradiative deactivation
pathway occurred in the iridium (III) complexes. Furthermore, a more than two-fold
decreased radiative rate constants (kr) was found in dinuclear iridium (III) complexes
3
due to the relatively decreased CT character 35. As compared to the observed
increased kr value in the dimetallic complexes (with centrosymmetric environment at
36
the phosphor ligand) , apparently, centrosymmetric environment at the ancillary
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ligand may provide less metal component in the excited states and give rise to a lower
degree of spin-orbit coupling (SOC), in spite of an additional metal center. When
cooled to 77 K in DCM matrix, the monomer complex Mono-Ir shows much more
structured emission spectra with a small rigid chromic blue-shift of 5 nm in
comparison to its emission spectra in DCM at 298 k, while D-Ir-Caz and D-Ir-OXD
exhibit a little rigidochromic red-shift of 3 and 4 nm, respectively. This indicates that
3
these dimetallic iridium (III) complexes have stronger LC character on the emissive
excited states than that of the monomer complex 35,37
.
3.3. Electrochemical Properties
The electrochemical properties of the iridium (III) complexes were investigated
through cyclic voltammetry (CV). Figure 3 shows the recorded CV curves of the
complexes using an Ag/AgCl electrode as a reference and the redox potential of
ferrocene/ferrocenium (Fc/Fc+) as a calibrated standard, and the resulting
electrochemical data are summarized in Table 1. The oxidation potentials of these
complexes are similar to those of many other TPA-based complexes 38-40, shows a
similar HOMO localization on the TPA portion. The onset of oxidation potentials (Eox)
of Mono-Ir, D-Ir-Caz, and D-Ir-OXD are observed at 0.41 V, 0.47 V and 0.56 V vs
Fc/Fc+, respectively. As the energy level of ferrocene/ferrocenium (Fc/Fc+) is
approximately -4.8 eV below the vacuum level, the HOMO energy levels (EHOMO) of
three complexes can be calculated according to the following equation: EHOMO
=
on
on
-(Eox + 4.80) eV, where Eox is the recorded onset oxidation potentials of these
complexes. As a calculate result, the EHOMO values are -5.21, -5.27 and -5.34 eV for
15

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the Mono-Ir, D-Ir-Caz, and D-Ir-OXD, respectively. Meanwhile, the LUMO energy
18
levels of these complexes can be estimated based on their optical energy gap
,
calculated to be -2.96, -3.02 and -3.09 eV for Mono-Ir, D-Ir-Caz, and D-Ir-OXD,
respectively, trapped well in PBD-CBP matrix. It indicates that the grafted different
π-conjugated bridges into monomer iridium (III) complex presents a little influence
on electrochemical properties in the dinuclear iridium (III) complexes.
3.4. Electroluminescent properties
To investigate these complexes’ application in electroluminescence (EL) devices,
the single-emissive-layer OLED was prepared as shown in Figure 4. We choose TFB
act as an interfacial layer in these devices since its HOMO level is aligned with those
of these complexes at around 5.2-5.3 eV 41. TmPyPB is a suitable candidate for
42
charge transporting with high electron mobility and high triplet energy level , and
act as an electron transporting layer here. CBP/PBD are efficient hole/electron-
transporting host, the carrier mobility could be balanced by tuning the blend ratio of
the two host materials, and achieve high efficiency device performance 43. The
optmized doping concentration for the three complexes is 10 wt % in the emitting
layer.
Figure 5 shows the EL properties of the iridium (III) complexes with 10 wt %
doping concentration in CBP/PBD hosts. Obviously, NIR emissions peak at 698 nm
along with a shoulder at 762 nm are observed in the given complexes. All complexes
exhibit similar EL spectra and are almost identical to those of their PL counterparts,
negligible fluorescence from the CBP/PBD hosts could be observed, indicating the
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efficient energy transfer from hosts to dopants.
The current densities-voltage-radiant emittance (J-V-R) characteristics and
external quantum efficiency-current density curves (EQE-J) of the iridium (III)
complexes- doped devices at 10 wt % dopant concentration are shown in Figure 6 and
Figure 7, respectively. For comparison, the device performances are summarized in
Table 3. As results, the almost same turn-on voltage of these devices is observed to be
around 7.0 V. A maximum EQE of 1.29% at 3.4 mA cm-2 with a radiant emittance of
0.59 µW sr-1 cm-2 was achieved in the Mono-Ir-based device. Unfortunately, the
D-Ir-Caz-based device exhibited a relatively lower EQE of 0.27% at 11.6 mA cm−2,
as well as the D-Ir-OXD-based devices provided a EQE of 0.41% at 5.0 mA cm−2,
respectively, correspond well with their poorer ΦL values in solution. Furthermore, a
radiant emittance of 0.54 µW sr-1 cm-2 for the D-Ir-Caz-based device and 0.62 µW sr-1
cm-2 for the D-Ir-OXD-based device were obtained, respectively. Distinctly, with
increasing the current density, the Mono-Ir-based device show apparent
efficiency-roll off at high currents and the EQE value was dropped down comparable
to that in D-Ir-Caz- and D-Ir-OXD-based device at 100 mA cm-2. The relative small
values of efficiency roll-off of the D-Ir-Caz- and D-Ir-OXD-based devices should be
attributed to their bulky structure that efficiently inhibite molecular aggregation.
Besides, the better device performance should be achieved when coupled with
electron transporting bridging at high currents.
4. Conclusions
In summary, two novel dinuclear iridium (III) complexes with carbazole-hole or
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oxadiazole-electron transporting bridging, and their analogue iridium (III) complex
were synthesized and characterized. All the iridium (III) complexes show similar
emission spectra peaked at around 700 nm in DCM solution. Furthermore, the
dinuclear iridium (III) complexes present lower ΦL which may be caused by the
centrosymmetric environment at the ancillary ligand. As expected, the NIR EL
emission peaked at 698 nm with a shoulder at 762 nm was observed in the iridium (III)
complexes-doped SEL OLEDs. A maximum EQE of 1.29% at 3.4 mA cm−2 with a
radiant emittance of 0.59 µW sr-1 cm-2 was achieved in the Mono-Ir-based device.
However, a relatively lower EQE of 0.27% at 11.6 mA cm−2 for the D-Ir-Caz-doped
device and a EQE of 0.41% at 5.0 mA cm−2 for the D-Ir-OXD-based device were
observed, respectively, accompanied with negligible efficiency roll-off values. This
also demonstrate that the structure of bulky ancillary ligand can efficiently inhibite
molecular aggregation.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(51273168, 51573154), the Hunan Natural Science Foundation (2015JJ2139), and the
Collaborative Innovation Center of New Chemical Technologies for Environmental
Benignity and Efficient Resource Utilization (2011).
Appendix A. Supplementary data
The following is the supplementary data related to this article:
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Figures and Tables
Scheme 1. Synthetic routes of the iridium (III) complexes.
Figure 1. TGA curves of the iridium (III) complexes..
Figure 2. UV-vis absorption spectra (a) and PL spectra (b) of the iridium (III)
complexes in CH2Cl2 solution (10-5 M) at 298 K (solid line) and 77 K (dash line).
Figure 3. Cyclic voltammograms of the iridium (III)complexes coated on a platinum
electrode in acetonitrile solution containing 0.1 mol L-1 Bu4NPF6 at a scan rate of 100
mV s-1 using ferrocene as an internal standard
Figure 4. EL device structure and energy level diagram of the devices.
Figure 5. The EL spectra of the complexes-doped devices.
Figure 6. The current density-voltage-radiance (J-V-R) curves of devices.
Figure 7. The EQE versus current density characteristics of the devices.
Table 1. Physical properties of the iridium (III) complexes in degassed CH2Cl2 at 298
K except where stated otherwise.
Table 2. Electrochemical properties and thermal parameters of the iridium (III)
complexes.
Table 3. EL characteristics of the iridium (III) complexes.
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Scheme 1.
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Figure 1.
Figure 2.
28