The role of ancillary ligands in iridium(lll) complexes for blue OLEDs

Article abstract of DOI:10.1080/15421400902945576

In this study, new deep blue emitting mixed ligand iridium(III) complexes comprising one cyclometalating, two phosphines trans to each other and two cis-ancillary ligands, such as Ir{(CF₃)₂Meppy}(PPh ₃)₂(H)(Cl),

Full text of DOI:10.1080/15421400902945576

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The Role of Ancillary Ligands in
Iridium(III) Complexes for Blue
OLEDs
Ho Wan Ham a , In June Kim a & Young Sik Kim a b
a Department of Information Display , Hongik
University , Seoul, Korea
b Department of Science , Hongik University , Seoul,
Korea
Published online: 26 May 2010.
To cite this article: Ho Wan Ham , In June Kim & Young Sik Kim (2009) The Role of
Ancillary Ligands in Iridium(III) Complexes for Blue OLEDs, Molecular Crystals and
Liquid Crystals, 505:1, 130/[368]-138/[376], DOI: 10.1080/15421400902945576
To link to this article: http://dx.doi.org/10.1080/15421400902945576
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Mol. Cryst. Liq. Cryst., Vol. 505, pp. 130=[368]–138=[376], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902945576
The Role of Ancillary Ligands in Iridium(III)
Complexes for Blue OLEDs
Ho Wan Ham1, In June Kim1, and Young Sik Kim1,2
1Department of Information Display, Hongik University, Seoul, Korea
2Department of Science, Hongik University, Seoul, Korea
In this study, new deep blue emitting mixed ligand iridium(III) complexes compris-
ing one cyclometalating, two phosphines trans to each other and two cis-ancillary
ligands, such as Ir{(CF3)2Meppy}(PPh3)2(H)(Cl), [Ir{(CF3)2Meppy}(PPh3)2(H)
(NCMe)]þ and Ir{(CF3)2Meppy} (PPh3)2(H)(CN), [(CF3)2Meppy ¼ 2-(30,50-bis-tri-
fluoromethylphenyl)-4-methylpyridine] were synthesized and studied to tune the
phosphorescence wavelength to the deep blue region and to enhance the lumines-
cence efficiencies. We investigate the strong field capabilities of ancillary ligands
to gain insight into the factors responsible for the emission color change and the
different luminescence efficiencies. This work represents a strategy for the design
of deep blue phosphorescent iridium complexes. The electron-withdrawing trifluor-
omethyl group substituted on the phenyl ring, the electron- donating methyl group
on the pyridyl ring and the cyano strong field ancillary ligand increase HOMO-
LUMO gap to achieve the hypsochromic shift in emission color. As a result, the max-
imum emission spectrum of Ir{(CF3)2Meppy}(PPh3)2(H)(Cl), [Ir{(CF3)2Meppy}
(PPh3)2(H)(NCMe)]þ and Ir{(CF3)2Meppy}(PPh3)2(H)(CN) were in the ranges of
436, 434 and 434 nm, respectively.
Keywords: blue; iridium complex; mixed ligand; OLED; phosphorescence
1. INTRODUCTION
Since Tang and coworkers first reported organic light-emitting devices
(OLED) with a multi-layer structure [1,2] materials and device fabri-
cation have been extensively studied [3–5]. Luminescent materials are
generally classified into two groups, either fluorescent or phosphores-
cent. OLEDs based on phosphorescent materials are able to improve
This work was supported by the Korea Research Foundation Grant funded by the
Korean Government (MOEHRD) (KRF-2007-313-D00290).
Address correspondence to Young Sik Kim, Department of Information Display,
Hongik University, Seoul, 121-791, Korea. E-mail: youngkim@wow.hongik.ac.kr
130=[368]

The Role of Ancillary Ligands in Iridium(III) Complexes
131/[369]
electroluminescence performance significantly because both singlet
and triplet excitons can be used to harvest light emission. Theoreti-
cally, the internal quantum efficiency of phosphorescent emitters
can approach almost 100% [6]. Heavy metal complexes, particularly
those containing Pt and Ir, can induce intersystem crossing by strong
spin-orbit coupling, leading to the mixing of the singlet and triplet
excited states [7,8]. The spin-forbidden nature of radiative relaxation
from the triplet excited state then becomes allowed, resulting in high
phosphorescent efficiencies. Thus, heavy metal complexes can serve as
efficient phosphors for OLEDs. Unfortunately, most of phosphorescent
emitters have a long radiative lifetime, which leads to the dominant
triplet-triplet (T-T) annihilation at high current. The occurrence of
T-T annihilation decreases the performance of a phosphorescent mate-
rial, particularly its maximum brightness and luminescence efficiency
at high currents [9,10].
Until now, greater success has been achieved in the development of
green and red emitters in fluorescent and phosphorescent displays.
However purely blue-emitting complex dopants are rarely found.
Hence, the current effort of researchers has been focused on the synth-
eses of blue-emitting iridium complexes. Some phosphorescent blue
emitter materials have been synthesized and reported; 2-phenylpyri-
dine (ppy) with some electro-withdrawing groups added to the phenyl
ring (fluoro, cyano group and others) or electro-donating groups to the
pyridyl ring (methyl group and others) had especially good blue emis-
sion characteristics [11,12].
In this study, new deep blue emitting mixed ligand iridium(III) com-
plexes comprising one cyclometalating, two phosphines trans to each
other and two cis-ancillary ligands, such as Ir{(CF3)2Meppy}(PPh3)2
(H)(Cl), [Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ, and Ir{(CF3)2Meppy}
(PPh3)2(H)(CN),
[(CF3)2Meppy ¼ 2-(30,50-bis-trifluoromethylphenyl)-4-
methylpyridine] were synthesized and studied to tune the phosphores-
cence wavelength to deep blue region and to enhance the luminescence
efficiencies. To analyze the color tuning by the effects of trans ligands
to (CF3)2Meppy ligand on the luminescent property, we have measured
UV-absorption and photoluminescence (PL) spectra of these iridium
complexes.
2. EXPERIMENTAL DETAILS
2.1. Synthesis
All reagents were purchased from Aldrich and used without further
purification. All reactions were carried out under a nitrogen or argon

132/[370]
H. W. Ham et al.
atmosphere. Solvents were dried by standard procedures. All column
chromatography was performed with the use of silica gel (230-Mesh,
Merck).
2.1.1. Synthesis of Ligand: (CF3)2Meppy
(CF3)2Meppy was obtained from the reaction of 2-chloro-4-methyl-
pyridine with the corresponding 3,5-bis(trifluoromethyl)phenylboronic
acid by Suzuki coupling. 2-Chloro-4-methylpyridine (0.438 ml,
5 mmol), 3,5-bis(trifluoromethyl)phenylboronic acid (1.29 g, 5 mmol)
and tetrakis(triphenylphospine)palladium(0) (0.196 g, 0.17 mmol)
were dissolved in 20 ml of toluene, 10 ml of ethanol and 20 ml of 2N
sodium carbonate aqueous solution. The mixture was refluxed at
110ꢀC under a nitrogen atmosphere for 8 hr. The mixture was cooled
to room temperature and extracted with 20 ml of ethyl acetate. The
organic fraction was dried over anhydrous MgSO4, filtered off and
dried under vacuum. The residue was purified by silica gel chromato-
graphy by using ethyl acetate=hexane (1:3). The product was collected
and dried under vacuum.
(CF3)2Meppy – Yield 60%, bright orange oil, MW: 302.25.
2.1.2. Synthesis of Complex: Ir{(CF3)2Meppy}(PPh3)2(H)(Cl)
Chloro(1,5-cyclooctadiene)iridium(I) dimer [Ir(COD)Cl]2 (0.1 g,
0.15 mmol), triphenylphosphine (PPh3) (0.16 g, 0.6 mmol) and
(CF3)2Meppy (0.092 g, 0.3 mmol) were dissolved in 10 ml of 2-ethox-
yethanol in a 50 ml flask. The mixture was refluxed at 110ꢀC under
a nitrogen atmosphere for 6 hr and then cooled to room temperature.
The product portion was collected and washed with 20 ml of methanol,
recrystallized in chloroform=n-hexane, and dried under vacuum.
Ir{(CF3)2Meppy}(PPh3)2(H)(Cl) – Yield 78%, yellowish crystal.
2.1.3. Synthesis of Complex:
[Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)](OTf)
Ir{(CF3)2Meppy}(PPh3)2(H)(Cl) (0.1 g, 0.1 mmol), silver trifluoro-
methane sulfonate (AgOTf) (0.026 g, 0.1 mmol) and anhydrous acetoni-
trile (MeCN) (1 ml, 20 mmol) were dissolved in 10 ml of chloroform in a
50 ml flask. The mixture was stirred under a nitrogen atmosphere for
2 hr at normal temperature. After 2 hr stirring, the white powder of
AgCl was removed by filtration and 20 mmol of MeCN was added into
the product solution and stirred under a nitrogen atmosphere for an
additional 2 hr at normal temperature. 20 ml of n-hexane was added
to yield yellowish crystals and residue was collected, washed with
methanol, and dried under vacuum.

The Role of Ancillary Ligands in Iridium(III) Complexes
133/[371]
[Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)](OTf) – Yield 65%, yellowish
crystal.
2.1.4. Synthesis of Complex: Ir{(CF3)2Meppy}(PPh3)2(H)(CN)
[Ir{(CF3)2Meppy}(PPh3)2(H)(CN)](OTf) (0.1 g, 0.08 mmol) and tetra-
butylammonium cyanide (0.027 g, 0.1 mmol) were dissolved in 10 ml of
methylene chloride in a 50 ml flask. The mixture was stirred under a
nitrogen atmosphere for 6 hr at normal temperature. After 6 hr stir-
ring, 20 ml of n-hexane was added to yield a yellowish crystal and resi-
due was collected, washed with methanol, and dried under vacuum.
Ir{(CF3)2Meppy}(PPh3)2(H)(CN) – Yield 73%, yellowish crystal.
2.2. Optical Measurements
The UV-Vis absorption spectra were measured on Hewlett Packard
8425A spectrometer. The PL spectra were obtained on Perkin Elmer
LS 55 spectrometer. UV-Vis and PL spectra of complexes were
measured in CH2Cl2.
3. RESULTS AND DISCUSSION
The colors of phosphorescent complexes used in OLEDs are tuned by
the variation of both cyclometalating and ancillary ligands. Several
small molecular and polymeric blue emitting phosphorescent com-
plexes have been reported, but the realization of highly efficient deep
blue emission is still remains a challenge. For the common 2-phenyl-
pyridine (ppy) cyclometalating ligand, the highest occupied molecular
orbital (HOMO) is localized on phenyl p and iridium 5d orbitals, while
the lowest unoccupied molecular orbital (LUMO) is localized on the pꢁ
orbitals of the pyridyl ring. To achieve deep blue emission: (1) substitu-
tion of CF3 groups on the 30,50-position of phenyl ring of the ppy ligand
as an electron withdrawing group to stabilize the HOMO and increase
HOMO-LUMO gap, (2) addition of electron-donating methyl groups on
the 4-position of the pyridyl ring of the ppy ligand to destabilize the
LUMO and increase HOMO-LUMO gap and (3) changing ancillary
ligands coordinated to the iridium atom into phosphine and cyano
groups. Both phosphine and cyano groups are known as very strong
field ligands, and their inclusion in the coordination sphere can
increase the HOMO-LUMO gap to achieve the hypsochromic shift in
the emission color.
The ligands, (CF3)2Meppy, were prepared by Suzuki coupling, as
illustrated in Scheme 1(a). The final iridium complexes were obtained
via several steps. In the first step, [Ir(COD)Cl]2, PPh3 and (CF3)2

134/[372]
H. W. Ham et al.
SCHEME 1 (a) Synthesis of (CF3)2Meppy ligand; (b) Synthesis of Iridium
complex of Ir{(CF3)2Meppy}(PPh3)2(H)(Cl); (c) Synthesis of Iridium complex
of [Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ; (d) Synthesis of Iridium complex of
Ir{(CF3)2Meppy}(PPh3)2(H)(CN).
Meppy ligand are synthesized, as illustrated in Scheme 1(b). At the
second step, the chlorine legends of Ir{(CF3)2Meppy}(PPh3)2(H)(Cl)
are replaced with MeCN legends by a precipitation reaction of Ag
and Cl ions, as illustrated in Scheme 1(c). In the third step, the MeCN
ligand is substituted with a CN ligand, as illustrated in Scheme 1(d).
The overall reaction yields were about 60 ꢂ 78%, respectively.
The solution UV–Vis absorption complexes were measured as shown
in Figure 1. The interesting characteristic of the MLCT absorption

The Role of Ancillary Ligands in Iridium(III) Complexes
135/[373]
FIGURE 1 UV-Vis absorption spectra of Ir{(CF3)2Meppy}(PPh3)2(H)(Cl),
[Ir{(CF3)2Meppy}(PPh3)2(H)(MeCN)]þ and Ir{(CF3)2Meppy}(PPh3)2(H)(CN) in
CH2Cl2 solution of room temperature.
spectra for [Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ is that it resembles
that of Ir{(CF3)2Meppy}(PPh3)2(H)(CN). Both 1MLCT and 3MLCT
peaks are observed for these complexes. MLCT absorption peaks of
[Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ are observed at 345 and 403 nm,
while MLCT absorption peaks of Ir{(CF3)2Meppy}(PPh3)2(H)(CN) are
observed at 337 and 385 nm. On the other hand, absorption peaks of
Ir{(CF3)2Meppy}(PPh3)2(H)(Cl) are observed at 365, 405 and 433 nm.
This can be explained by the strong field ancillary ligand such as
CN- that lowered the HOMO energy level more than did the weak field
ancillary ligand Cl-. Ancillary ligands alter the MLCT energy mainly
by changing the HOMO energy level. The HOMO energy level may
then be lowered by strong-field ancillary ligands, which causes a large
d-orbital energy splitting. It is also likely that strong-field ancillary
ligands lengthen the bond lengths of Ir–C (ppy) trans to the ancillary
ligand to lower the energy levels of the dp-orbitals of the metal as
strong-field ligands are known to show a high trans effect. The med-
ium field ancillary ligand MeCN is located between these. CF3 groups,
substituted on the 30,50-position of phenyl ring as electron withdraw-
ing groups, tune the MLCT absorption peak to bluish. In addition,
the methyl group substituted on the 4-position of pyridyl ring as an

136/[374]
H. W. Ham et al.
electron donating group strongly affected LUMO and raised the
LUMO energy and increased the energy gap.
The PL spectra of iridium complexes are shown in Figure 2, the
maximum emission spectra of Ir{(CF3)2Meppy}(PPh3)2(H)(Cl), [Ir
{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ and Ir{(CF3)2Meppy}(PPh3)2(H)
(CN) were 436, 434, and 434 nm, respectively. Knowing that d orbitals
of iridium are involved in the HOMO of C^N complexes such as
Ir(C^N)3 and Ir(C^N)2LL0, one could expect the HOMO energy level
to be lowered by strong field ligands more than by weak field ligands.
Table 1 summarizes the photoluminescence data for iridium com-
plexes. A significantly longer wavelength emission band is measured
for the complex that contains a chlorine ligand trans to the carbon
of the C^N ligand, whereas the emission band is measured at much
shorter wavelengths for all other complexes with a strong field ligand
such as CNꢃ. The trans and axial ligand PPh3 is not involved in the
HOMO of iridium complexes. The effects of CF3 groups in phenyl ring
on luminescence wavelength are clearly shown in comparison with the
study of Ir(F2Meppy)(PPh3)2(H)(CN) [13]. The complex with (CF3)2
Meppy ligand has an about 5 nm blue-shifted emission band compared
to the complex with F2Meppy ligand.
As a result, the PL spectrum of Ir{(CF3)2Meppy}(PPh3)2(H)(CN)
shows a significant blue shifted phosphorescence emission due to the
FIGURE 2 PL spectrum of Ir{(CF3)2Meppy}(PPh3)2(H)(Cl), [Ir{(CF3)2Meppy}
(PPh3)2(H)(NCMe)]þ and Ir{(CF3)2Meppy}(PPh3)2(H)(CN) in CH2Cl2 solution
of room temperature.

The Role of Ancillary Ligands in Iridium(III) Complexes
137/[375]
TABLE 1 UV-Vis Absorption and Photoluminescence Data of Ir{(CF3)2
Meppy}(PPh3)2(H)(Cl), [Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ and Ir{(CF3)2
Meppy}(PPh3)2(H)(CN)
Complex
kabs. (nm)
kem. (nm)
Ir{(CF3)2Meppy}(PPh3)2(H)(Cl)
[Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ
Ir{(CF3)2Meppy}(PPh3)2(H)(CN)
365, 435
345, 403
337, 385
436, 462
434, 463
434, 460
substitution of CF3 groups in the phenyl ring that pulls the HOMO
level of iridium complexes, and the added methyl group in the pyridyl
ring that pulls up the LUMO level of iridium complex, and the strong
field ancillary, it helps enlarge HOMO-LUMO energy gap to achieve
the hypsochromic shift in emission color.
4. CONCLUSIONS
We synthesized and measured optical properties of Ir{(CF3)2Meppy}
(PPh3)2(H)(Cl), [Ir{(CF3)2Meppy}(PPh3)2(H)(NCMe)]þ and Ir{(CF3)2-
Meppy}(PPh3)2(H)(CN) in order to find a new blue emitting material
for the application in OLEDs by tuning the ancillary ligand and mod-
ified emit ligand. The PL spectra of Ir{(CF3)2Meppy}(PPh3)2(H)(CN)
shows a significant blue shifted phosphorescence due to replacing
chlorine with CN, MeCN ligand. We suggest that the Ir{(CF3)2-
Meppy}(PPh3)2(H)(CN) might serve as a good radiation characteristic
blue dopant for new phosphorescent OLED materials.
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