Synthesis and characterization of red phosphorescent iridium(Ⅲ) complexes based on electron-acceptor modulation of main ligand for high efficiency organic light-emitting diodes

Article abstract of DOI:10.1080/15421406.2016.1200941

A new series of red phosphorescent iridium(Ⅲ) complexes, (PT-TFP)2Ir(tmd), (PT-P)2Ir(tmd), and (PT-MP)2Ir(tmd) were synthesized for highly efficient red emitter of the organic light-emitting diodes (OLEDs). The main ligands consisted of electron-donor (phenanthren) and electron-acceptor (trifluoromethyl pyridine, pyridine, and methyl pyridine) were synthesized by Suzuki coupling reaction. The iridium(Ⅲ) complexes based on main ligands and 2,2,6,6-tetramethyl-3,5-heptanedione(tmd) ancillary ligand were synthesized by Nonoyama reaction. Their luminescence properties were investigated by UV-visible spectroscopy and photoluminescence (PL) spectroscopy. The OLEDs devices were manufactured by vacuum deposition and These OLEDs devices were characterized by investigation of current density-voltage-luminance (J-V-L), current efficiency, power efficiency, external quantum efficiency (EQE), EL spectrum. The Ir(Ⅲ) complexes will be used as a solution process dopant for the hybrid OLED devices.

Full text of DOI:10.1080/15421406.2016.1200941

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and characterization of red
phosphorescent iridium(Ⅲ) complexes based on
electron-acceptor modulation of main ligand for
high efficiency organic light-emitting diodes
Sang-Yong Park, Hyun-Kyung Kim & Dong-Myung Shin
To cite this article: Sang-Yong Park, Hyun-Kyung Kim & Dong-Myung Shin (2016) Synthesis
and characterization of red phosphorescent iridium(Ⅲ) complexes based on electron-acceptor
modulation of main ligand for high efficiency organic light-emitting diodes, Molecular Crystals
and Liquid Crystals, 636:1, 38-44, DOI: 10.1080/15421406.2016.1200941
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1200941
Published online: 01 Nov 2016.
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Date: 23 November 2016, At: 00:37

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Synthesis and characterization of red phosphorescent
iridium(Ⅲ) complexes based on electron-acceptor
modulation of main ligand for high efficiency organic
light-emitting diodes
Sang-Yong Parka, Hyun-Kyung Kimb, and Dong-Myung Shina
aDepartment of Chemical Engineering, Hong-ik University, Seoul, Korea; bKorea Institute for Curriculum and
Evaluation, Seoul, Korea
ABSTRACT
KEYWORDS
red dopant; iridium(III)
complexes; phosphorescent
organic light-emitting diodes
A new series of red phosphorescent iridium(Ⅲ) complexes, (PT-
TFP)2Ir(tmd), (PT-P)2Ir(tmd), and (PT-MP)2Ir(tmd) were synthesized for
highly efficient red emitter of the organic light-emitting diodes
(OLEDs). The main ligands consisted of electron-donor (phenanthren)
and electron-acceptor (trifluoromethyl pyridine, pyridine, and methyl
pyridine) were synthesized by Suzuki coupling reaction. The irid-
ium(Ⅲ) complexes based on main ligands and 2,2,6,6-tetramethyl-3,5-
heptanedione(tmd) ancillary ligand were synthesized by Nonoyama
reaction. Their luminescence properties were investigated by UV-visible
spectroscopy and photoluminescence (PL) spectroscopy. The OLEDs
devices were manufactured by vacuum deposition and These OLEDs
devices were characterized by investigation of current density-voltage-
luminance(J-V-L), currentefficiency, powerefficiency, externalquantum
efficiency (EQE), ELspectrum. TheIr(Ⅲ) complexes will be used as asolu-
tion process dopant for the hybrid OLED devices.
1. Introduction
Active matrix phosphorescence organic light emitting diodes (AMPhOLED) attracting atten-
tion as the next generation display have been evolving into high dfficiency, high definition,
large area, and flexible display. OLED has a lot of difficulties in the development process
because materials of OLED are required to have ultra high purity than materials of other
displays. But because of Lambertian reflectance characteristic, AMPhOLED has less visual
stimuli and high illumination. Therefore It is more friendly human body[1–8].
In this paper, phosphorescent red dopants which are used as solution process was investi-
gated. Red dopants are required to emit a red long-wavelength red light and high efficiency
as the core of the light-emitting material at the same time. In this study, by combining the
design of a phosphorescent iridium compound was studied for the implementation of a com-
plete high-efficiency devices for reddish solution process was carried out basic research, which
enables a large area device fabrication using less processes.
CONTACT Dong-Myung Shin
Wausan-ro, Mapo-gu, Seoul , Korea; Hyun-Kyung Kim
shindm@hongik.ac.kr
Department of Chemical Engineering, Hong-ik University, ,
kimhk@kice.re.kr Korea Institute for Curriculum and Evalu-
ation, Jeongdong Bldg., -, Jeong-dong, Jung-gu, Seoul -, Korea.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[229]/39
Figure . Device structure and energy band-gap of materials used in the device.
2. Experimental details
2.1. General information
All reagents and solvents were purchased from Aldrich Chemical Co., TCI, and Alpha Aesar,
and they were used without further purification. All column chromatography was performed
under a standard atmosphere using silica gel (230–400 mesh, Sigma-Aldrich). The 1H NMR
spectra were measured using a UNITY-INOVA 500MHz Nuclear Magnetic Resonance Spec-
trometer of VARIAN. INC. in a deuterated chloroform solution. UV–visible absorption spec-
tra were recorded on an Agilent 8453 UV–Visible spectrophotometer. Photoluminescence
(PL) spectra were obtained using an LS55 fluorescence spectrometer. The current density-
voltage (J-V) and luminance-voltage (L-V) data of OLEDs were measured by the Keithley
SMU 238 and Minolta CS-100A, respectively. The OLED area was 4 mm2 for all the samples
studied in this work. Electroluminescence (EL) spectra and CIE coordinates were obtained
using a Minolta CS-2000 spectroradiometer.
2.2. Synthesis of the main ligands
4-methyl-2-(phenanthren-9-yl)pyridine (PT-MP).
A
mixture of phenanthren-9-
ylboronic acid (1.00 g, 4.503 mmol), 2-bromo-4-methylpyridine (0.85 g, 4.954 mmol),
tetrakis(triphenylphosphine)palladium (0.05 g, 0.045 mol, 1 mol%), potassium carbonate
(15 ml, 2M aqueous solution), and tetrahydrofuran (30 mL) was headed under a nitrogen
atmosphere at 80°C for 24 h. This reaction is the Suzuki coupling reaction. After the reaction,
the mixture was cooled to room temperature for 1 h. The compound was extracted by
liquid-liquid separation (water and dichloromethane). The compound was purified by a
celite-silica gel filtration (solvent: toluene) and column chromatography on silica gel (eluent:
hexane/ethyl acetate, 15:1). Yield: 98.9% (1.20 g); 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.93
(d, J = 7.3 Hz, 2H), 8.44(s, 1H), 8.39(d, J = 6.7 Hz, 1H), 8.12 (d, J = 7.1 Hz, 2H), 7.96(s, 1H),
7.88(dd, J = 18.2 Hz, 2H), 7.82(dd, J = 19.1 Hz, 2H), 7.05(d, J = 5.3 Hz, 1H), 2.36(s, 3H).
2-(phenanthren-9-yl)pyridine (PT-P). A PT-P ligand was obtained from the reaction of
phenanthren-9-ylboronic acid (1.00 g, 4.503 mmol) and 2-bromopyridin (0.78 g, 4.954 mmol)
by Suzuki coupling. Yield: 99.1% (1.14 g); 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.93(d,
J = 7.3 Hz, 2H), 8.50(d, J = 7.8 Hz, 1H), 8.44(s, 1H), 8.12 (d, J = 7.1 Hz, 2H), 7.88

40/[230]
S.-Y. PARK ET AL.
(dd, J = 18.2 Hz, 2H), 7.82(dd, J = 19.1 Hz, 2H), 7.51(dd, J = 14.7 Hz, 1H), 7.26(d, J =
4.8 Hz, 1H), 7.00(dd, J = 17.1 Hz, 1H).
2-(phenanthere-9-yl)-4-(trifluoromethyl)pyridine (PT-TFP). A PT-TFP ligand was
obtained from the reaction of phenanthren-9-ylboronic acid (1.00 g, 4.503 mmol) and 2-
bromo-4-(trifluoromethyl)pyridin (1.12 g, 4.954 mmol) by Suzuki coupling. Yield: 97.5%
(1.42 g); 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.93(d, J = 7.3 Hz, 2H), 8.44(s, 1H), 8.38(d,
J = 6.9 Hz, 1H), 8.12 (d, J = 7.1 Hz, 2H), 8.06(s, 1H), 7.88(dd, J = 18.2 Hz, 2H), 7.82(dd, J =
19.1 Hz, 2H), 7.15(d, J = 5.9 Hz, 1H).
2.3. Synthesis of the cyclometalated iridium(III) µ-chloride-bridged dimer
Dimer [(PT-MP)2IrCl]2 diiridium(III). The cyclometalated iridium(III) μ-chloride-
bridged dimer, [(PT-MP)2Ir(μ-Cl)]2 (II), was prepared using a slightly modified Nonoyama
reaction step 1. Iridium(III) chloride hydrate (0.665 g, 2.228 mmol) and PT-MP (1.20 g,
4.45 mmol) were dissolved in a mixed solution of 2-ethoxyethanol and water (36 ml: 12 ml).
The reaction was required under a nitrogen atmosphere. The mixture was refluxed overnight
at 135°C. The solution was cooled to room temperature for 1 h. The crude product was puri-
fied via liquid-liquid separation (dichloromethane and water). The precipitation was collected
by a vacuum evaporator. Yield: 98.7%.
Dimer [(PT-P)2IrCl]2 diiridium(III). The cyclometalated iridium(III) μ-chloride-
bridged dimer, [(PT-P)2Ir(μ-Cl)]2 (II), was obtained from the reaction of iridium(III) chlo-
ride hydrate (0.667 g, 2.233 mmol) and PT-P (1.60 g, 4.465 mmol) by the Nonoyama reaction
step 1. Yield: 97.3%.
Dimer [(PT-TFP)2IrCl]2 diiridium(III). The cyclometalated iridium(III) μ-chloride-
bridged dimer, [(PT-TFP)2Ir(μ-Cl)]2 (II), was obtained from the reaction of iridium(III)
chloride hydrate (0.656 g, 2.196 mmol) and PT-TFP (1.42 g, 4.392 mmol) by the Nonoyama
reaction step 1. Yield: 95.5%.
2.4. Synthesis of the iridium(III) complexes (Scheme 1)
(PT-MP)2Ir(tmd) complex: [(PT-MP)2Ir(μ-Cl)]2 (II) (1.68 g, 1.099 mmol), 2,2,6,6-
tetramethyl-3,5-heptanedione (1.013 g, 5.495 mmol), and sodium carbonate (1.519 g,
10.99 mmol) were dissolved in 2-ethoxyethanol (67.20 ml) and refluxed under a nitrogen
atmosphere overnight at 130°C. After cooling to room temperature for 2 h. The compound
was extracted by liquid-liquid separation (water and dichloromethane). The compound was
purified by a celite-silica gel filter (solvent: toluene) and column chromatography on silica
gel (eluent: methanol/CH2Cl2, 1:40). The product was further purified by recrystallization to
the Yield: 89.7%; 1H NMR (500 MHz, CDCl3, δ), δ(ppm): 8.62(dd, J = 29 Hz, 4H), 8.45(d,
J = 9.9 Hz, 2H), 8.37(s, 2H), 7.62 (dd, J = 19.85 Hz, 4H), 7.50(t, J = 17.55 Hz, 2H), 7.24
(dd, J = 16.8 Hz, 2H), 6.62(t, J = 19.1 Hz, 2H), 6.53(d, J = 7.6 Hz, 2H), 6.37(d, J = 9.15 Hz,
2H), 5.38(s, 1H), 2.53(s, 6H), 0.859(s, 18H).
The (PT-P)2Ir(tmd) complex was obtained from the reaction of [(PT-P)2Ir(μ-Cl)]2 (II)
(1.60 g, 1.087 mmol) and 2,2,6,6-tetramethyl-3,5-heptanedione (1.001 g, 5.433 mmol) by
the Nonoyama reaction step 2. Yield: 88.9%; 1H NMR (500 MHz, CDCl3, δ), δ(ppm): 8.62
(dd, J = 29 Hz, 4H), 8.44(d, J = 9.9 Hz, 2H), 8.37(s, 2H), 7.62 (dd, J = 19.85 Hz, 4H), 7.51(dd,
J = 17.55 Hz, 2H), 7.24(dd, J = 16.8 Hz, 2H), 6.62(t, J = 19.1 Hz, 2H), 6.53(d, J = 7.6 Hz,
2H), 6.43(d, J = 8.2 Hz, 2H), 6.32(d, J = 9.15 Hz, 2H), 5.38(s, 1H), 0.859(s, 18H).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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Table . Photophysical properties of Ir-complexes and the Device performance of solution-processed red
phosphorescent OLEDs.
Current
Efficiency
(Ca/A)
Power
Efficiency
(lm/W)
External
Quantum
Efficiency (%) Color Index
Abs λMax
(nm)
PL λMax
(nm)
EL λMax
(nm)
Red Dpants
(PT-MP)Ir(tmd)















.a, .b
.a, .b
.a, .b
.a, .b
.a, .b
.a, .b
.a, .b
.a, .b
.a, .b
(., .)c
(., .)c
(., .)c
(PT-P)Ir(tmd)
(PT-TFP)Ir(tmd)
a Maximum efficiency; b Efficiency at , cd/m; c Color coordinates were obtained at , cd/m.
The (PT-TFP)2Ir(tmd) complex was obtained from the reaction of [(PT-TFP)2Ir(μ-Cl)]2
(II) (1.83 g, 1.049 mmol) and 2,2,6,6-tetramethyl-3,5-heptanedione (0.967 g, 5.245 mmol) by
the Nonoyama reaction step 2. Yield: 82.2%; 1H NMR (500 MHz, CDCl3, δ), δ(ppm): 8.62(dd,
J = 29 Hz, 4H), 8.37(s, 2H), 8.16(d, J = 7.9 Hz, 2H), 7.62 (dd, J = 19.85 Hz, 4H), 7.60(dd,
J = 15.23 Hz, 2H), 7.24(dd, J = 16.8 Hz, 2H), 6.62(t, J = 19.1 Hz, 2H), 6.53(d, J = 7.6 Hz,
2H), 6.47(d, J = 9.15 Hz, 2H), 5.38(s, 1H), 0.859(s, 18H).
2.5. Device fabrication
To fabricate solution-processed OLED devices, 150-nm thick patterned indium-tin oxide
(ITO) glasses covered by a bank layer with an open emission area of 4 mm2 were used. The ITO
glasses were cleaned in acetone and isopropyl alcohol with a sonication process and rinsed in
deionized water. Then, ITO glass substrates were treated in UV-ozone to eliminate all the
organic impurities from the previous fabrication processes. A PEDOT:PSS as a hole injection
layer was spin-coated on ITO glass in an ambient condition and annealed at 120°C for 15 min
in an inert atmosphere. Subsequently, HTL as a hole transport layer dissolved in chloroben-
zene was spin-coated and cross-linked by a standard process. In the case of a red EML host,
red dopants were dissolved in toluene to produce a 5 wt% solution, respectively. A red EML
was spin coated and dried at 100°C for 10 min. All solution processes were performed in
a nitrogen atmosphere at room temperature, except PEDOT:PSS. After spin coating of red
EML, the hybrid-connecting layer, blue EML, and TPBI were thermally deposited at a vac-
˚
uum condition under 10-7 Torr with 0.5 A/s. Then, LiF and Al were deposited successively
˚
˚
with 0.3 A/s and 3 A/s, respectively (Fig. 1).
3. Results and discussion
Figure 2 shows the UV-visible absorption and photoluminescence (PL) spectra of the Ir(III)
complex dissolved in a Dichloromethane solution.
The main absorption bands are located in between 300 nm and 500 nm. The distinct
vibronic features are assigned to the spin-allowed 1π-π∗ transition of the ligands in the Ir(III)
complexes. Due to the perturbation of the Ir(III) metal center, these transitions were shifted
with respect to those of the free ligands. The absorption peaks in the region of 450 nm–600 nm
can be assigned to the spin-allowed metal-ligand charge transfer band (1MLCT) and the spin-
forbidden metal-to-ligand (3MLCT) transition. The absorption bands with peak wavelengths

42/[232]
S.-Y. PARK ET AL.
Scheme . Synthetic scheme and chemical structure of complexes.
3
around 600 nm suggest that the S0 → MLCT transition is greatly enhanced and becomes
partially allowed due to strong spin-orbital coupling[9–12].
The PL spectra of complexes show the emission of red light with main peaks at 624–
649 nm, as shown in Figure 2 (A). The phosphorescence peak wavelength can be tuned by
aromatic amine moiety to the main ligand. Complexes with –CH3 functional group of pyri-
dine ring have a shorter phosphorescence wavelength than complexes with –CF3, because
the high electron-density of pyridine ring in the complexes increased the LUMO energy
level.
Figure 2 (B) shows the EL emission spectra at a brightness of 1,000 cd/m2 for the
red PhOLEDs. The EL emission spectra of all the complexes are slightly (3–5 nm) red-
shifted relative to the solution PL spectra. The devices give very deep red EL spectra with
much narrower full width at half maximum (FWHM) compared to commercial OLED
devices.
Figure 3 (a) shows the current density-voltage-luminance (J-V-L) curves of the fabricated
red-phosphorescent OLEDs. At a constant voltage of 5.0 V, current density values of 0.10, 0.71
and 1.42 mA/cm2 are observed in the fabricated PhOLEDs, respectively. The driving voltage
Figure . (A) UV/PL spectra of Ir-complexes and (B) Electroluminescence spectra of solution-processed red
phosphorescent OLEDs.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[233]/43
Figure . (A) J-V-L Characteristics of the red phosphorescent OLEDs (B) Current efficiency-luminance charac-
teristics (C) Power efficiency-luminance characteristics (D) External quantum efficienty of fabricated phos-
phorescent OLEDs.
to reach 1,000 cd/m2 is 7.14, 7.21 and 7.03V for devices, respectively. Also, a low turn-on
voltage of 2.0 V for devices was observed.
The Photophysical properties of Ir-complexes and the device performance of solution-
processed red PhOLEDs is summarized in Table 1. Figure 3. (B), (C) and (D) showed a similar
trend of device performance. The (PT-MP)2Ir(tmd) device showed a higher power efficiency,
current efficiency, and external quantum efficiency than efficiency of other devices.
4. Conclusion
The –CF3 is an efficient electron-withdrawing group. Therefore, the electron density in the
pyridine ring is decreased and the lowest unoccupied molecular orbital (LUMO) energy level
is decreased. Therefore, the HOMO-LUMO energy gap of (PT-TFP)2Ir(tmd) is decreased and
the emission wavelength is increased. Alternatively, the –CH3 functional group increases the
electron density of the pyridine ring. Therefore, the LUMO energy level is increased and the
HOMO-LUMO energy gap of (PT-MP)2Ir(tmd) is increased. When energy levels of all mate-
rials in devices was arranged properly and structure of dopants is similar, the phosphorescent
red dopant having emission of low wavelength was proved to have a higher efficiency. Also,
these dopants is highly proper for solution process.
Acknowledgments
This work was supported by the Technology Innovation Program (201210940004, Materials Devel-
opment for 50-inches UD OLED TV Using Super Hybrid Process) and funded by the Ministry of

44/[234]
S.-Y. PARK ET AL.
Knowledge Economy (MKE, Korea). We specially thank Professor M. C. Suh of Kyung-Hee Univer-
sity for the device performance experiments.
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