Synthesis and characterization of new blue light emitting iridium complexes containing a trimethylsilyl group

Article abstract of DOI:10.1039/c2jm33084a

New blue iridium complexes with a trimethylsilyl group as a bulky electron donating group, Ir(F₂-p-trimethylsilyl)₂(fptp) and Ir(F₂-m-trimethylsilyl)₂(fptp), were synthesized via μ-chloro-bridged dimer and perfluoropropylated triazole-based ancillary ligands and then characterized using various spectroscopic studies. Both Ir(F₂-p-trimethylsilyl)₂(fptp) and Ir(F ₂-m-trimethylsilyl)₂(fptp) exhibited high photoluminescence quantum efficiencies of 75 ± 5% and 76 ± 5% in films, respectively. The DFT calculation demonstrated that the new blue iridium complexes have a wide bandgap compared with FIrpic and the Ir(F ₂-p-trimethylsilyl)₂(fptp) with a trimethylsilyl group in the para position has a slightly deeper HOMO level than that of the Ir(F ₂-p-trimethylsilyl)₂(fptp) with a trimethylsilyl group in the meta position. The UV-vis absorption and photoluminescent (PL) spectra of the complexes were blue shifted compared with those of FIrpic. The device using the Ir(F₂-p-trimethylsilyl)₂(fptp) exhibited a maximum external quantum efficiency of 19.3% (at 0.00152 mA cm^-2) and commission Internationale de l'Eclairage (CIE) coordinates of (0.145, 0.247).

Full text of DOI:10.1039/c2jm33084a

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PAPER
Synthesis and characterization of new blue light emitting iridium complexes
containing a trimethylsilyl group†
a
Chul Young Kim,‡ Dong-Gwang Ha,‡ Hyun Hee Kang, Hui-Jun Yun, Soon-Ki Kwon, Jang-Joo Kim
b
a
c
c
b
*
*
a
*
and Yun-Hi Kim
Received 15th May 2012, Accepted 10th September 2012
DOI: 10.1039/c2jm33084a
New blue iridium complexes with a trimethylsilyl group as a bulky electron donating group,
Ir(F2-p-trimethylsilyl)2(fptp) and Ir(F2-m-trimethylsilyl)2(fptp), were synthesized via m-chloro-bridged
dimer and perfluoropropylated triazole-based ancillary ligands and then characterized using various
spectroscopic studies. Both Ir(F2-p-trimethylsilyl)2(fptp) and Ir(F2-m-trimethylsilyl)2(fptp) exhibited
high photoluminescence quantum efficiencies of 75 ꢀ 5% and 76 ꢀ 5% in films, respectively. The DFT
calculation demonstrated that the new blue iridium complexes have a wide bandgap compared with
FIrpic and the Ir(F2-p-trimethylsilyl)2(fptp) with a trimethylsilyl group in the para position has a
slightly deeper HOMO level than that of the Ir(F2-p-trimethylsilyl)2(fptp) with a trimethylsilyl group in
the meta position. The UV-vis absorption and photoluminescent (PL) spectra of the complexes were
blue shifted compared with those of FIrpic. The device using the Ir(F2-p-trimethylsilyl)2(fptp) exhibited
a maximum external quantum efficiency of 19.3% (at 0.00152 mA cmꢁ2) and commission
Internationale de l’Eclairage (CIE) coordinates of (0.145, 0.247).
Organic light emitting diodes (OLEDs) have been successfully
employed as small size displays, and have been actively
researched for applications in large size, flexible displays and
solid state lighting. The efficiency of the OLEDs has been
improved significantly by adopting phosphorescent emitters that
can harvest triplet excitons as well as singlet excitons for light,
enabling nearly 100% internal quantum efficiencies.2–4 In
particular, iridium complexes have been very successful as
phosphorescent emitters, especially for green and red emissions,
due to their high photoluminescent (PL) efficiency and relatively
short excited state lifetime.1
metal-to-ligand charge-transfer (MLCT) transition.6 The
general approach toward the synthesis of an efficient deep blue
emitting complex has been to control the electronic structure of
the iridium complex to shift the emission from green to blue.
Examples of this method include the addition of fluorine
atoms to phenyl rings, and the introduction of electron-
donating groups to the ortho and para positions of the
pyridine.7
Recently, our group reported that a trimethylsilyl group could
be easily incorporated as a substituent in the pyridine ring of the
phenylpyridine ligand. Since the trimethylsilyl group is a minor
electron-donating group, the incorporation modifies the HOMO
and/or the LUMO level, as well as the 3MLCT state of the
complex.8 Moreover, the incorporation improves the solubility,
thermal stability, and steric bulk through having higher volumes
that can inhibit the triplet–triplet annihilation.8–18
However, iridium complexes with blue emissions are rare,
compared with those of the phosphorescent green and red.5
Deep blue emitters have a low PL quantum efficiency due to
their high band gap. Furthermore, the T1 / S0 transition
probability is reduced as the band gap increases due to
decreases in the admixture of the ligand-centered p / p* and
In this research, we designed a new iridium complex using the
dippy ligand combined with a trimethylsilyl group as a bulky
electron donating group for OLEDs. The ancillary ligand is
extended to include the perfluoropropyl-substituted triazole. We
recognized that the C-linked 2-pyridyl-azoles were capable of
using two adjacent nitrogen atoms to form a stable chelate
interaction. Moreover, perfluoropropyl-substituted triazole has
strong acidity, which is also reinforced further with the electron
withdrawing CF2CF2CF3 substituent. The perfluoropropylated
triazole-based ancillary ligand has an added advantage of easy
purification through sublimation when compared with picolinic
acid and acetylacetone.18–25
aDepartment of Chemistry and Research Institute of Natural Sciences
(RINS), Gyeongsang National University, Jinju, 660-701, South Korea.
E-mail: ykim@gnu.ac.kr; Fax: +82-55-772-1489; Tel: +82-55-772-1491
bDepartment of Materials and Science and Engineering, and OLED Center,
Seoul National University, Seoul, 151-744, South Korea
cSchool of Materials Science and Engineering & Engineering Research
Institute (ERI), Gyeongsang National University, Jinju, 660-701, South
Korea. E-mail: skwon@gnu.ac.kr
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm33084a
‡ Chul Young Kim and Dong-Gwang Ha equally contributed to be the
first authors of this work.
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This report provides the syntheses and characterization of two
new blue Ir(III) complexes that contain the perfluoropropylated-
triazole ancillary ligand and trimethylsilyl substituted dippy
ligand. Furthermore, the substituent effect of trimethylsilyl
group on the para and meta orientations of the phenyl ring was
also studied.
The Ir(F2-m-trimethylsilyl)2(fptp) and Ir(F2-p-trimethylsi-
lyl)2(fptp) depicted in Scheme 1 were obtained in high yields. The
ancillary ligand, 2-(3-(perfluoropropyl)-1H-1,2,4-triazole-5-yl)
pyridine, was obtained via cyclization of the acetamidine
hydrochloride and adduct of the ethyl pentafluoropanoate and
hydrazine monohydrate. The m-chloro-bridged dimer was
formed through the reaction of the cyclometalated ligand
precursor with IrCl3$H2O in a mixture of 2-ethoxyethanol and
water. The new Ir complexes were obtained in the presence of
sodium carbonate via the reaction of the m-chloro-bridged dimer
and ancillary ligand. The resulting complexes were air-stable,
sublimable and sufficiently soluble for photophysical and elec-
trochemical measurements. The Ir(F2-m-trimethylsilyl)2(fptp)
and Ir(F2-p-trimethylsilyl)2(fptp) were characterized by NMR,
mass spectra and elemental analysis.
Fig. 1 Electronic density distributions of the HOMO and LUMO
orbitals of (a) Ir(F2-p-trimethylsilyl)2(fptp), (b) Ir(F2-m-trimethylsi-
lyl)2(fptp) and (c) FIrpic.
results imply that the perfluoropropyl triazole unit affects the
lower HOMO level of the Ir(F2-m-trimethylsilyl)2(fptp) and
Ir(F2-p-trimethylsilyl)2(fptp), and it leads to a larger bandgap.
Moreover, the Ir(F2-p-trimethylsilyl)2(fptp) with the trime-
thylsilyl group in the para position of the pyridine exhibited
slightly lower HOMO levels than that of the Ir(F2-m-trime-
thylsilyl)2(fptp) with the trimethylsilyl group in the meta position
of the pyridine.
A density function theory (DFT) calculation of the Ir(F2-m-
trimethylsilyl)2(fptp), Ir(F2-p-trimethylsilyl)2(fptp) and bis(4,6-
difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) was
carried out in order to estimate the energy level and electron
density distribution of the orbitals. The Dmol3 module installed
within Materials Studio 4.2 was used for the simulation. The
double numerical plus polarization (DNP) basis set and Perdew–
Burke–Ernzerhof (PBE) function were chosen.26 The optimized
structures and the highest occupied molecular orbital/ lowest
unoccupied molecular orbital (HOMO/LUMO) for the Ir(F2-m-
trimethylsilyl)2(fptp), Ir(F2-p-trimethylsilyl)2(fptp) and FIrpic
are shown in Fig. 1. The HOMO orbitals of the Ir(F2-m-trime-
thylsilyl)2(fptp) and Ir(F2-p-trimethylsilyl)2(fptp) were mostly
distributed over the main ligand, but those of the FIrpic were
observed at the ancillary ligand as well as the main ligand. The
HOMO levels of Ir(F2-m-trimethylsilyl)2(fptp) and Ir(F2-p-tri-
methylsilyl)2(fptp) were ꢁ5.33 and ꢁ5.34 eV, respectively, which
are lower than that of ꢁ5.17 eV for the FIrpic. However, the
LUMO orbitals of the molecules were almost identical. These
The thermal stabilities of the Ir(F2-m-trimethylsilyl)2(fptp) and
Ir(F2-p-trimethylsilyl)2(fptp) were studied using thermo gravim-
etry analysis (TGA) and differential scanning calorimetry (DSC)
under a nitrogen atmosphere. (Fig. 2) The degradation temper-
ature (Td) was 350 ꢂC for Ir(F2-p-trimethylsilyl)2(fptp) and
333 ꢂC for Ir(F2-m-trimethylsilyl)2(fptp). The glass transition
temperatures of the Ir(F2-m-trimethylsilyl)2(fptp) and Ir(F2-p-
trimethylsilyl)2(fptp) were 140 ꢂC and 136 ꢂC, respectively.
Fig. 3 shows the UV-vis absorption and PL spectra of the
complexes in the CH2Cl2 solution at room temperature. The two
complexes exhibited very similar absorption spectra, which
suggest that the position of the trimethylsilyl group does not
contribute to the absorption process. The high energy bands
around 256 nm were assigned to the intraligand p–p* transition
Scheme 1 Synthetic scheme of Ir(F2-m-trimethylsilyl)2(fptp) and Ir(F2-
p-trimethylsilyl)2(fptp).
Fig. 2 TGA and DSC thermograms of (a) Ir(F2-m-trimethylsilyl)2(fptp),
(b) Ir(F2-p-trimethylsilyl)2(fptp).
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The HOMO levels of the complexes were measured using
cyclic voltammetry (CV). Pt was used as the working and counter
electrode, and an Ag/AgCl electrode was used as the reference
electrode. NPB was used for the potential calibration (HOMO
level of ꢁ5.4 eV) (Fig. 5).24 The LUMO energy levels of the
materials were estimated using the HOMO level and the UV-Vis
absorption spectra. The HOMO levels of the Ir(F2-m-trime-
thylsilyl)2(fptp) and Ir(F2-p-trimethylsilyl)2(fptp) were identical
at ꢁ6.1 eV. This value was slightly lower than that of the FIrpic
(ꢁ6.0 eV). The trend is consistent with the prediction of the DFT
calculation. This result can be explained by the electron with-
drawing ability of the perfluorinated triazole ancillary ligand of
the Ir(F2-m-trimethylsilyl)2(fptp) and Ir(F2-p-trimethylsi-
lyl)2(fptp). The LUMO levels of the Ir(F2-p-trimethylsilyl)2(fptp)
and Ir(F2-m-trimethylsilyl)2(fptp) were ꢁ3.1 eV, which is the
same as FIrpic. In Table 1 we summarise the physical properties
of Ir(F2-m-trimethylsilyl)2(fptp), Ir(F2-p-trimethylsilyl)2(fptp)
and FIrpic.
Fig. 3 UV-vis absorption and PL spectra of complexes in CH2Cl2
solution.
of the ligand. The spin allowed ligand-centered absorptions to
appear at approximately 306 nm and the spin enabled the metal-
to-ligand charge transfer (1MLCT) absorptions to be clearly
distinguished at approximately 365 nm. In addition, the spin-
forbidden triplet 3LC and/or 3MLCT transitions appeared as
low-energy absorption shoulders at approximately 450 nm.
Apparently, the absorption spectra of the new complexes were
overall blue shifted compared with that of the FIrpic. The
emission spectra of the complexes in the solution at room
temperature were dominated by phosphorescence in the region of
450–500 nm with maximum values of 464 and 495 nm for the
Ir(F2-m-trimethylsilyl)2(fptp), and 464 and 492 nm for the Ir(F2-
p-trimethylsilyl)2(fptp). The optical bandgaps of the Ir(F2-m-
trimethylsilyl)2(fptp) and Ir(F2-p-trimethylsilyl)2(fptp) deduced
from the absorption edges, were the same at 3.0 eV, which is
larger than that of the FIrpic (2.9 eV). The triplet energy levels
were 2.69 eV for the new iridium complexes, and 2.65 eV for
FIrpic. The PL efficiencies of the iridium complexes were
measured using the TCTA : 3TPYMB (1 : 1) films doped with
the iridium complexes using an integrating sphere. Both the
Ir(F2-p-trimethylsilyl)2(fptp) and Ir(F2-m-trimethylsilyl)2(fptp)
exhibited very high PL quantum efficiencies of 75 ꢀ 5% and 76 ꢀ
5%, respectively, which were lower than that of the FIrpic (87 ꢀ
3%). The lifetimes of the excited states were determined to be
1.8 ms for Ir(F2-p-trimethylsilyl)2(fptp) and 2.1 ms for the Ir(F2-
m-trimethylsilyl)2(fptp), which are a little longer than FIrpic
(1.4 ms) (Fig. 4).
OLEDs were fabricated using the iridium complexes as the
phosphorescent emitter. The structure of the OLEDs was as
follows: ITO (150 nm)/2TNATA (30 nm)/TAPC (40 nm)/
TCTA : 3TPYMB (3 : 1), and iridium complex 10% (20 nm)/
TCTA : 3TPYMB (1 : 3), and iridium complex 10% (20 nm)/
3TPYMB (30 nm)/LiF (1 nm)/Al (100 nm) (Fig. 6). 4,40,400-Tris(N-
(2-naphthyl)-N-phenyl-amino)triphenylamine (2TNATA) was
used as the hole injection material. 1,1-Bis[(di-4tolylamino)phenyl]
Fig.
4
Transient PL of Ir(F2-m-trimethylsilyl)2(fptp) and Ir(F2-p-
Fig.
5 CV of (a) Ir(F2-m-trimethylsilyl)2(fptp) and (b) Ir(F2-p-
trimethylsilyl)2(fptp).
trimethylsilyl)2(fptp).
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Table 1 Basic properties of the three dopants
PL peak PL efficiency Exciton lifetime T1 level Band gap HOMO LUMO HOMOa LUMOa Tg
Td
(ꢂC) (ꢂC)
b
c
(nm)
(%)
(ms)
1.8
2.1
1.4
(eV)
2.69
2.69
2.65
(eV)
(eV)
ꢁ6.1
ꢁ6.1
ꢁ6.0
(eV)
ꢁ3.1
ꢁ3.1
ꢁ3.1
(eV)
(eV)
Ir(F2-p-
trimethylsilyl)2(fptp)
Ir(F2-m-
trimethylsilyl)2(fptp)
FIrpic
462, 491 75
461, 493 76
3.0
ꢁ5.34
ꢁ5.33
ꢁ5.17
ꢁ2.94
ꢁ2.92
ꢁ2.91
140
136
192
350
333
377
3.0
468, 494 87
2.9
a
b
Obtained from DFT calculation. Glass transition temperature. c Decomposition temperature.
(TCTA) and 3TPYMB were used as a co-host of the emission layer
to remove the energy barrier for charge injection from the charge
transporting layers to the emission layer. TAPC, TCTA, and
3TPYMB have higher triplet and singlet levels than the iridium
complexes.
Fig. 7 shows the current density–voltage–luminance (J–V–L)
characteristics and external quantum efficiency of OLEDs. The
device using the Ir(F2-p-trimethylsilyl)2(fptp) as a dopant,
exhibited the maximum external quantum efficiency of 19.3%,
which is slightly lower than the FIrpic based device (EQE ¼ 23%)
with the same structure. However, the emission color is much
deeper blue than the FIrpic based device. The commission
Internationale de l’Eclairage (CIE) coordinate of the EL was
(0.145, 0.247) compared to the FIrpic based device of (0.16,
0.38).27 The device using the Ir(F2-m-trimethylsilyl)2(fptp) as a
dopant showed lower maximum EQE (14.2%) with a CIE
coordinate of (0.151, 0.256) and it had a higher turn on voltage.
Since the emitting material was the only one difference between
the two devices and both emitters had the same energy level and
high PL efficiency we surmised that the position of the trime-
thylsilyl unit affected the charge trapping and transport. The
bulky trimethylsilyl unit might hinder the charge transport from
the host material to the iridium complexes, and the particular
position of the unit obstructs the charge flow more effectively.
The devices showed high roll-off of efficiency at high current
density due to instability of the devices, which is frequently
observed in blue phosphorescent OLEDs (Fig. 8).
Fig. 6 Structure of the device and the molecular structures of the
compounds used in the device.
cyclohexane (TAPC) as the hole transport material, and tris[3-(3-
pyridyl)-mesityl]borane (3TPYMB) as the electron transport
material, respectively. 4,40,400-Tris(N-carbazolyl)triphenylamine
In conclusion, new blue iridium complexes with trimethylsilyl
groups and perfluoropropylated triazole-based ancillary ligands
were synthesized. The new Ir complexes have high photo-
luminescence quantum efficiencies over 75% and blue-shifted
Fig. 7 J–V–L characteristics and external quantum efficiency of devices.
22724 | J. Mater. Chem., 2012, 22, 22721–22726
Fig. 8 EL spectra of devices.
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emission compared with FIrpic, and the OLEDs fabricated using
the new Ir complexes exhibited deeper blue emission than FIrpic.
In particular, Ir(F2-p-trimethylsilyl)2(fptp) with the trimethylsilyl
group in the para position exhibited the maximum EQE of 19.3%
with color coordinates of (0.145, 0.247) while the Ir(F2-m-tri-
methylsilyl)2(fptp) with the trimethylsilyl group in the meta
position exhibited and a maximum EQE of 14.2% with a CIE
index of (0.151, 0.256).
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(F2-5-Trimethylsilyl)2Ir(m-Cl)Ir(F2-5-trimethylsilyl)2 (0.5 g, 0.33
mmol), ancillary ligand (fptp) (0.27 g, 0.86 mmol) and Na2CO3
(0.35 g, 3.32 mmol) were dissolved in 2-ethoxyethanol (30 mL)
and the mixture was stirred under N2 at 135 ꢂC overnight. After it
cooled to room temperature, the solvent was removed under
reduced pressure to give a dark yellow oil. The crude product was
purified by chromatography on silica gel (MC/hexane, 5/1, v/v)
ꢂ
to obtain a yellow solid. Yield (0.41 g, 61%). Tm ¼ 292 C. IR
1
(KBr): 3021, 1609, 1112, 721. H NMR (300 MHz, CDCl3, d):
8.38 (d, 1H), 8.23 (t, 2H), 7.93 (t, 1H), 7.80 (m, 4H), 7.33 (s, 1H),
7.28 (m, 1H), 6.52 (m, 2H), 5.77 (d, 1H), 5.68 (d, 1H), 0.14 (s,
9H), 0.05 (s, 9H) ppm. 13C NMR (85 MHz, CDCl3, d): ꢁ1.5,
ꢁ1.4, 97.7, 97.9, 98.1, 98.2, 98.4, 98.6, 104.5, 114.0, 114.2, 114.0,
114.2, 114.4, 114.5, 119.9, 122.3, 122.5, 122.7, 122.8, 122.9, 128.9,
133.8, 135.6, 138.5, 142.5, 142.9, 149.8, 150.4, 151.7, 154.3, 154.8,
154.9, 155.8, 160.1, 163.0, 164.8 HRMS (FAB+) m/z calcd for
C38H32F11IrN6Si2 (M+) 1030.07, found 1030.3.
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Org. Electron., 2011, 12, 785.
The procedure used is the same as that for the preparation of
compound Ir(F2-m-trimethylsilyl)2(fptp). Yield (0.49 g, 73%). Tm
1
ꢂ
¼ 295 C. (KBr): 3022, 1610, 1113, 722. H NMR (300 MHz,
CDCl3, d): 8.38 (d, 3H), 7.91 (t, 1H), 7.77 (d, 1H), 7.61 (d, 1H),
7.31 (d, 1H), 7.22 (d, 1H), 7.03 (d, 1H), 6.96 (d, 1H), 6.49 (m,
2H), 5.86 (d, 1H), 5.68 (d, 1H), 0.33 (s, 18H) ppm. 13C NMR (85
MHz, CDCl3, d): ꢁ1.45, ꢁ1.41, ꢁ1.24, 97.7, 97.9, 98.1, 98.12,
98.4, 98.6, 104.7, 114.29, 114.3, 114.6, 114.8, 116.2, 116.5, 120.2,
123.1, 126.5, 127.4, 127.5, 127.6, 127.8, 128.0, 138.5, 146.7, 148.8,
149.7, 150.7, 153.0, 153.6, 154.3, 155.6, 159.7, 163.5, 164.3
HRMS (FAB+) m/z calcd for C38H32F11IrN6Si2 (M+) 1030.07,
found 1030.4.
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This research was financially supported by the National
Research Foundation of Korea Grant founded by the Korean
Government (2012-0000145 and 2012-047047). Chul Young Kim
and Hui-Jun Yun thank the support by MKE and KIAT through
the Workforce Development Program in Strategic Technology.
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