Synthesis and electroluminescent property of highly efficient phosphorescent red dopants based on modulated main ligands

Article abstract of DOI:10.1080/15421406.2017.1350041

Novel red phosphorescent iridium(III) complexes, (DPAP-MQ)₂Ir(tmd), (DMAP-MQ)₂Ir(tmd), (PN-MQ)₂Ir(tmd) were synthesized for red emitter of the phosphorescent organic light-emitting diodes (phOLEDs). The ligands have sites of both the electron donor and acceptor in a molecule. The main ligands consisted of electron-donors ((dimethylamino)phenyl, (diphenylamino)phenyl, (dimethylamino)naphthalene) and electron-acceptor methyl quinoline 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 property was investigated by UV-visible spectroscopy and photoluminescence (PL) spectroscopy. The manufactured PHOLEDs were characterized by investigation of current density-voltage-luminance, luminance efficiency, external quantum efficiency, electroluminescence spectrum. These phosphorescent iridium complexes will be promising candidates for highly efficient red emitters.

Full text of DOI:10.1080/15421406.2017.1350041

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and electroluminescent property of
highly efficient phosphorescent red dopants based
on modulated main ligands
Seo Yun Lee, Song Eun Lee, Ye Na Oh, Young Kwan Kim & Dong Myung Shin
To cite this article: Seo Yun Lee, Song Eun Lee, Ye Na Oh, Young Kwan Kim & Dong Myung Shin
(2017) Synthesis and electroluminescent property of highly efficient phosphorescent red dopants
based on modulated main ligands, Molecular Crystals and Liquid Crystals, 653:1, 118-124, DOI:
10.1080/15421406.2017.1350041
To link to this article: http://dx.doi.org/10.1080/15421406.2017.1350041
Published online: 07 Nov 2017.
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Date: 14 November 2017, At: 04:14

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Synthesis and electroluminescent property of highly efficient
phosphorescent red dopants based on modulated main ligands
Seo Yun Leea, Song Eun Leeb, Ye Na Oha, Young Kwan Kimb, and Dong Myung Shina
aDepartment of Chemical Engineering, Hongik University, Seoul, Republic of Korea; bDepartment of Information
Display, Hongik University, Seoul, Republic of Korea
ABSTRACT
KEYWORDS
Iridium(III) complexes;
phosphorescent organic
light-emitting diodes; red
dopant
Novel red phosphorescent iridium(III) complexes, (DPAP-MQ)2Ir(tmd),
(DMAP-MQ)2Ir(tmd), (PN-MQ)2Ir(tmd) were synthesized for red emitter
of the phosphorescent organic light-emitting diodes (phOLEDs).
The ligands have sites of both the electron donor and acceptor in a
molecule. The main ligands consisted of electron-donors ((dimethy-
lamino)phenyl, (diphenylamino)phenyl, (dimethylamino)naphthalene)
and electron-acceptor methyl quinoline 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 property
was investigated by UV-visible spectroscopy and photoluminescence
(PL) spectroscopy. The manufactured PHOLEDs were characterized
by investigation of current density-voltage-luminance, luminance
efficiency, external quantum efficiency, electroluminescence spectrum.
These phosphorescent iridium complexes will be promising candidates
for highly efficient red emitters.
1. Introduction
Organic light-emitting diodes (OLEDs) have attracted extensive attention because of their
several advantages, such as high efficiency, wider viewing angles, high sense of realness, high
definition, fast response time and flexible display. One important requirement for OLED
application is the low power consumption and high efficiency. Phosphorescence active matrix
organic light emitting diodes (PHAMOLED), expecting as the next generation display because
of many advantage. Phosphorescent transition metal complexes as triplet dopant emitters offer
a great promise, since the transition metal ions can induce efficient spin–orbit coupling (SOC),
which facilitates efficient intersystem crossing (ISC), leading to a higher quantum efficiency
[1–6].
Hole and electron transporting layer, an appropriate emission characteristics and the amor-
phous properties of the organic material, was known to be useful in the manufacture of a
light emitting diode. The excitons formed in a device operation are composed of 25% singlets
and 75% triplets. The efficiency of the devices limits that contain organic materials as emitter
CONTACT Dong Myung Shin
Wausan-ro, Mapo-gu, Seoul , Korea; Young Kwan Kim
Hong-ik University, , Wausan-ro, Mapo-gu, Seoul , Korea.
shindm@hongik.ac.kr
Department of Chemical Engineering, Hong-ik University, ,
kimyk@hongik.ac.kr Department of Information Display,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
119
Figure . Synthetic and chemical structure of complexes.
to 25%. The cyclometalated iridium(III) complexes are considered attractive candidates for
electroluminescent devices as they enhance the possibility of reaping both singlet and triplet
excitons formed during the device operation. So, phosphorescence might eventually lead to
higher efficiency than fluorescence [4–7].
In this paper, we report the synthesis and photo-physical properties of novel phosphores-
cent red dopants using methyl quinolone and aromatic amine moiety modification of main
ligand and 2,2,6,6-tetramethyl-3,5-heptanedione(tmd) of ancillary ligand. Also, red-emitting
devices was measured characteristics of electroluminescent properties, respectively
2. Experimental details
Total synthesis of three new red materials is shown in Figure 1. Experimental details are
described below.
2.1. Synthesis of the main ligands
All the main ligands were prepared by essentially following the same procedure.
-(-methylquinolin--yl)-diphenylaniline (DPAP-MQ)
DPAP-MQ was synthesized by Suzuki coupling reaction.
A mixture of (4-
(diphenylamino)phenyl)boronic acid (2.00 g, 7 mmol), 2-chloro-4-methylquinoline (1.35 g,
7.6 mmol), tetrakis(triphenylphosphine)palladium (0.08 g, 0.07 mmol, 1 mol%), potas-
sium carbonate (10.38 ml, 2M aqueous solution), and tetrahydrofuran (20.75 mL) was
handed under a nitrogen atmosphere at 80°C for 24 h. After the reaction, the mixture was
cooled to room temperature for 1 h and extracted by liquid-liquid separation (water and
dichloromethane) and dried over anhydrous Na2SO4, filtered concentrated under reduced
pressure [2–3]. 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: 90%
(2.4 g); 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.15(d, J = 9.5 Hz, 1 H), 8.05(d, J = 9.2 Hz,

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S. Y. LEE ET AL.
2 H), 7.92(d, 8.4 Hz, 1 H), 7.75(t, J = 8.5 Hz,1 H), 7.58(t, J = 7.5 Hz, 1 H), 7.25(t, J = 7.3 Hz,
4 H),7.03(s, 1 H), 6.81(t, J = 4.2 Hz, 2 H), 6.62(dd, J = 10.4, 3.4 Hz, 6 H), 2.62(s, 3 H)
N,N-dimethyl--(-methylquinolin--yl)aniline (DMAP-MQ)
Yield: 91%; 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.11(t, J = 8.9 Hz, 3H), 7.95(d, J = 8.3 Hz,
1 H), 7.67(t, J = 5.7 Hz, 2 H), 7.47(t, J = 7.1, 1 H), 6.83(d, J = 8.8, 2 H), 3.05(s, 6 H), 2.73
(s, 3 H)
-methyl--(-phenylnaphthalen--yl)quinolone (PN-MQ)
Yield: 93%; 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.67(d, J = 8.5 Hz,1H), 8.47(s, 1H), 8.11
(d, J = 8.4 Hz, 1H), 7.98(dd, J = 5.4, 2.9 Hz, 2H), 7.95(d, J = 7.4 Hz, 1H), 7.67(q, J = 1.7 Hz,
3H), 7.47(m, 5 H), 7.26(t, J = 3.9 Hz, 1H), 7.18(s, 1H), 2.35(s, 3H)
2.2. Synthesis of the iridium(III) complexes
All the cyclometalated iridium(III) complexes were prepared by essentially following the same
procedure.
(DPAP-MQ)Ir(tmd)
Dimer [(DPAP-MQ)2IrCl]2 was synthesized using a slightly modified Nonoyama reaction
step 1. Iridium(III) chloride hydrate (0.74 g, 2.5 mmol) and DMP-MDCPQ (2.4 g, 6.2 mmol)
were dissolved in a mixed solution of 2-ethoxyethanol (36 mL) and water (12 mL). The reac-
tion should be required under a nitrogen atmosphere, refluxed overnight at 135°C. After the
reaction, the mixture was cooled to room temperature for 1 h and column chromatography
on silica gel (eluent: hexane/ethyl acetate, 15:1). The crude product dimer was obtained by
using a solvent (chloroform/methanol, 1:1).
Complex (DPAP-MQ)2Ir(tmd) using a slightly modified Nonoyama reaction step 2.
[(DPAP-MQ)2IrCl]2 dimer (2.8 g, 1.11 mmol), 2,2,6,6-tetramethyl-3,5-heptanedione(tmd)
(1.2 mL), 2-Ethoxyethanol (84 mL) and fine powder of sodium carbonate (1.84 g, 13.3 mmol)
is mixed under a nitrogen atmosphere at 130°C for 24 h. After the reaction, the mixture
was cooled to room temperature for 1 h. The compound was extracted by liquid-liquid
separation (water and dichloromethane) and dried over anhydrous Na2SO4, washed with
brine for the combining organic phase, and filtered concentrated under reduced pressure.
Product purified column chromatography on silica gel (eluent: hexane/ethyl acetate, 2:1).
The insoluble products were filtered out with methanol and dried. The product was further
purified by recrystallization several times for high purity [2–4]. Yield: 74% (1.4 g); 1H NMR
500 MHz, CDCl3, δ), δ(ppm): 8.17(d, J = 9.1 Hz, 2 H), 8.05(d, J = 8.2 Hz, 2 H), 7.92(d, J
= 8.7 Hz, 2 H), 7.75(t, J = 7.5 Hz,2H), 7.58(t, J = 7.3 Hz, 2H), 7.18(t, J = 7.6 Hz, 8H),7.02
(s, 2 H), 6.87(tt, J = 4.2, 1.7 Hz, 4H), 6.60(dd, J = 9.2, 2.4 Hz, 12H), 2.62(s, 6H), 0.98(s, 18H)
(DMAP-MQ)Ir(tmd)
Yield: 65%; 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 8.11(t, J = 8.2 Hz, 4H), 7.92(d, J = 7.9 Hz,
2H), 7.66(t, J = 5.3 Hz, 4H), 7.15(s, 1H), 6.83(d, J = 8.9 Hz, 4H), 3.12(s, 12H), 2.63(s, 6H),
1.13(s, 18H)
(PN-MQ)Ir(tmd)
Yield: 68%; 1H NMR 500 MHz, CDCl3, δ), δ(ppm): 71(d, J = 8.8 Hz,2H), 8.49(s, 2H), 8.21
(d, J = 9.0 Hz, 2H), 7.90(d, J = 8.1 Hz, 2H), 7.76(d, J = 8.2 Hz,2H), 7.52(t, J = 0.9 Hz, 2H),

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
121
Figure . Schematic illustration and energy level diagram of the red PHOLEDs.
7.47(t, J = 1.0 Hz, 2H), 7.26(p, J = 6.5 Hz, 6H), 7.15(t, J = 2.4 Hz, 6H), 5.3(s, 1H), 4.9(s, 1H),
2.88(s, 6H), 0.69(s,18H)
2.3. Device fabrication
Indium-tin-oxide (ITO) coated glass substrates, which have a sheet resistance of 10 ꢀ/sq and
a thickness of 180 nm, were used to fabricate the red PHOLEDs. The cleaning procedure
included sequential ultra-sonication in acetone, methyl alcohol, distilled water, and ethyl alco-
hol at room temperature for 15 min before depositing the organic layers. The pre-cleaned
ITO-coated glass substrates were dried in a convection oven at 120°C for 10 min. The ITO
substrates were then treated with oxygen plasma at 2 × 10−2 Torr at 125 W for 2 min. The
organic layers were deposited by thermal evaporation through a shadow mask under high
vacuum (8 × 10−7 Torr) condition. The fabricated devices were sealed with UV-epoxy resin
and a glass cap in a nitrogen-purged glove box under presence of O2 and H2O at a concen-
tration below 1 ppm. A getter was used to absorb the residue moisture in the encapsulated
device.
Figure 2 presents three types of red PHOLEDs with the following structures: ITO (150 nm)
as an anode / 4,4ꢀ,4ꢀꢀ-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine (2-TNATA,
30 nm) as the hole injection layer (HIL) / N,Nꢀ-bis(1-naphyl)-N,Nꢀ-diphenyl-1,1ꢀ-biphenyl-
4,4ꢀ-diamine (NPB, 20 nm) as a hole transporting layer (HTL) / 4,4ꢀ-bis(N-carbazolyl)-1,1ꢀ-
biphenyl (CBP) doped with three kinds of dopant 8 wt.% as an emitting layer (EML, 30 nm) /
2,2ꢀ,2ꢀꢀ-(1,3,5-tris)tris(1-phenyl)-1H-benzimidazol (TPBi, 40 nm) as an electron transporting
layer (ETL) / lithium quinolate (Liq, 2 nm) as an electron injection layer (EIL) / aluminum
(Al, 100 nm) as a cathode.
2.4. Measurement
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). UV–visible
absorption spectra were recorded on an Agilent 8453 UV–Visible spectrophotometer. Photo-
luminescence (PL) spectra were obtained using an LS55 fluorescence spectrometer. The 1H
NMR spectra were measured using a UNITY-INOVA 500 MHz Nuclear Magnetic Resonance
Spectrometer of VARIAN. INC. in a deuterated chloroform solution.
The current density–voltage (J-V) characteristics of the OLEDs were measured using a
source measure unit (model 2400, Keithley Instruments Inc., Cleveland, OH, USA). The

122
S. Y. LEE ET AL.
Figure . UV/PL spectra of complexes.
electroluminescence (EL) performance of the devices including the luminance, luminous
efficiency (LE), external quantum efficiency (EQE), Commission International de L’Éclairage
(CIE) chromaticity coordinate, and EL spectra were analyzed using a Chroma meter
(model CS-1000A, Konica Minolta Holdings, Inc., Chiyoda, Tokyo, Japan). All measure-
ments were carried out at room temperature (about 23°C) under ambient conditions.
3. Results and discussion
Figure 3 is displayed the absorption spectra measured for solutions of (DPAP-MQ)2Ir(tmd),
(DMAP-MQ)2Ir(tmd), (PN-MQ)2Ir(tmd) in Chloroform at room temperature. The main
absorption bands are located between 300 and 400 nm. The absorption spectrum can be
1
assigned to the spin-allowed π–π∗ transition of the ligands in the Ir(III) complexes. The
weak absorption bands were observed in the lower energy range from 400 to 500 nm.
The lowest energy absorption peaks are assigned to the spin-allowed metal-ligand charge
transfer band (1MLCT). And the spin-forbidden metal-to-ligand (3MLCT) was tran-
3
sited. Cross point of the MLCT absorption and PL emission spectrum was assigned to
Figure . EL spectra of CBP doped with (DPAP-MQ)Ir(tmd) or (DMAP-MQ)Ir(tmd) or (PN-MQ)Ir(tmd)
within red PHOLED EMLs at  V.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
123
Figure . (a) Current density-voltage-luminance, (b) LE-luminance, and (inset) EQE-luminance characteris-
tics of the red PHOLEDs.
correspond to the first triplet excited energy (T1) level. Because the strong spin-orbital
coupling [4, 8–11].
The PL spectra have (DPAP-MQ)2Ir(tmd), (DMAP-MQ)2Ir(tmd), and (PN-MQ)2Ir(tmd)
emission with peak wavelength at 590, 598 and 647 nm in Chloroform at room tempera-
ture, as shown also in Figure 3. The tuned main ligand could change the phosphorescence
peak wavelength. Because of phenyl group is stronger electron-donor group than amino
substituents, the electron density of naphtalane in PN-MQ ligand is more increased than
the electron density of phenyl in DPAP-MQ, DMAP-MQ ligand. That is why the highest
occupied molecular orbital (HOMO) energy level of PN-MQ ligand is higher than others.
Therefore, the HOMO-LUMO energy bandgap of (PN-MQ)2Ir(tmd) is more decreased than
others. So, PL of (PN-MQ)2Ir(tmd) is more red-shifted than others.
The normalized EL spectra at 12 V of fabricated red PHOLEDs are shown in Figure 4. The
EL spectra have clear (DPAP-MQ)2Ir(tmd), (DMAP-MQ)2Ir(tmd), and (PN-MQ)2Ir(tmd)
emission with peak wavelength at 593, 594 and 649 nm for red PHOLEDs. There are no
other emission peak from the host or charge transport materials, indicating complete energy
transfer from the host of CBP to the (DPAP-MQ)2Ir(tmd) or (DMAP-MQ)2Ir(tmd) or
(PN-MQ)2Ir(tmd). In other study, redPHOLEDs are indicated emission of 610∼630 nm gen-
erally. So, (PN-MQ)2Ir(tmd) was observed large wavelength, (DPAP-MQ)2Ir(tmd), (DMAP-
MQ)2Ir(tmd) was observed small wavelengths as compared with other research [2–7].
Figure 5(a) illustrates the current density-voltage-luminance (J-V-L) characteristics of
three red PHOLEDs. The turn-on voltages of red devices using (DPAP-MQ)2Ir(tmd),
(DMAP-MQ)2Ir(tmd), and (PN-MQ)2Ir(tmd) were observed with 2.64, 4.30, and 3.60 V at
1.0 cd/m2. The maximum luminance of (DPAP-MQ)2Ir(tmd), (DMAP-MQ)2Ir(tmd), and
(PN-MQ)2Ir(tmd) red devices were 14,720, 4,526, and 1,770 cd/m2. The luminance and LE,
EQE characteristic of fabricated devices are shown in Fig. 5(b). The maximum LEs and EQEs
of three red PHOLEDs were 16.32, 9.33, and 1.14 cd/A and 9.42, 5.00, and 3.97%, respectively.
The device using (DPAP-MQ)2Ir(tmd) show lower turn-on voltage, higher luminance and
EQE compared two other devices. It is indicated that the device using (DPAP-MQ)2Ir(tmd)
in more efficient than two others.
4. Conclusion
The novel phosphorescent red dopants of Ir(III) complexes are using three main ligands.
(DPAP-MQ)2Ir(tmd), (DMAP-MQ)2Ir(tmd), and (PN-MQ)2Ir(tmd) were synthesized for
highly efficient of PHOLEDs. Ir(III) complexes had emission maxima at 590, 598 and 647 nm.

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S. Y. LEE ET AL.
Because of smaller HOMO-LUMO band gap energy, (PN-MQ)2Ir(tmd) had more long wave-
length than the others. EL emissions of fabricated red PHOLEDs were 593, 594 and 649 nm.
It is indicated completely energy transfer. (DPAP-MQ)2Ir(tmd) was higher performance than
the others. The maximum luminance is 14,720 cd/m2, maximum LE is 16.32 cd/A and maxi-
mum EQE is 9.42%. Therefore, it was concluded that (DPAP-MQ)2Ir(tmd) is highest dopant
in those three dopants and it will be able to use a strong candidate dopant for PHOLEDs.
Funding
This research was supported by Basic Science Research Program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education (No. 2015R1A6A1A03031833)
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