Synthesis and photophysical studies of heteroleptic tris-cyclometalated Ir(III) complex for red OLED

Article abstract of DOI:10.1080/15421400701548316

The synthesis and photophysical study of efficient phosphorescent iridium(III) complex having two different (CN) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet-triplet (T-T) annihilation and study luminescent mec

Full text of DOI:10.1080/15421400701548316

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Synthesis and Photophysical
Studies of Heteroleptic Tris-
Cyclometalated Ir(III) Complex
for Red OLED
Gui Youn Park a , In Joon Kim a & Young Sik Kim b
a Department of Information Display Engineering ,
Hongik University , Seoul, Korea
b Department of Science and Center for Organic
Materials and Information Devices , Hongik
University , Seoul, Korea
Published online: 22 Sep 2010.
To cite this article: Gui Youn Park , In Joon Kim & Young Sik Kim (2007) Synthesis
and Photophysical Studies of Heteroleptic Tris-Cyclometalated Ir(III) Complex
for Red OLED, Molecular Crystals and Liquid Crystals, 471:1, 235-244, DOI:
10.1080/15421400701548316
To link to this article: http://dx.doi.org/10.1080/15421400701548316
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Mol. Cryst. Liq. Cryst., Vol. 471, pp. 235–244, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701548316
Synthesis and Photophysical Studies of Heteroleptic
Tris-Cyclometalated Ir(III) Complex for Red OLED
Gui Youn Park
In Joon Kim
Department of Information Display Engineering, Hongik University,
Seoul, Korea
Young Sik Kim
Department of Science and Center for Organic Materials and
Information Devices, Hongik University, Seoul, Korea
The synthesis and photophysical study of efficient phosphorescent iridium(III)
complex having two different (C^N) ligands are reported. In order to improve
the luminescence efficiency by avoiding triplet–triplet (T–T) annihilation and
study luminescent mechanism of the heteroleptic iridium complex, Ir(piq-3F)2
(dpq-4F) is designed and prepared where piq-3F and dpq-4F represent
1-(40-fluorophenyl)isoquinoline and 2-(30-fluorophenyl)-4-phenylquinoline, respect-
ively. We thought that both piq-3F and dpq-4F ligands can act as a source of
energy supply because of the relative HOMO energy level and the luminescence
lifetime of homoleptic Ir complexes Ir(piq-3F)3 Ir(dpq-4F)3. However the UV-vis
absorption spectra and the PL spectra of Ir(piq-3F)2 (dpq-4F) is more similar to
that of Ir(piq-3F)3. In spite of lower triplet energy level of dpq-4F, the excitation
energy is not intramolecular transferred from two piq-3F ligands to one dpq-4F
ligand. The luminescence occurs mainly at piq-3F ligand. It is due to the strong
MLCT characteristic of piq-3F ligand. To analyze luminescent mechanism, we
calculated these complexes theoretically by using computational method.
Keywords: DFT; Ir(piq-3F)2(dpq-4F); OLED; red phosphorescence
1. INTRODUCTION
Since Tang and coworkers reported organic light-emitting devices
(OLED) with the multi-layer structure [1,2], materials and device
This work was supported by 2007 Hongik University Research Fund.
Address correspondence to Young Sik Kim, Department of Science, Hongik
University, Seoul 121-791, Korea. E-mail: youngkim@hongik.ac.kr
235

236
G. Y. Park et al.
fabrication have been extensively studied in recent years [3–5]. Lumi-
nescent materials are generally classified into two groups as fluor-
escent and phosphorescent ones. OLEDs based on phosphorescent
materials were known to be able to improve electroluminescence per-
formance significantly because both singlet and triplet excitons could
be used to harvest light emission. Theoretically, the internal quantum
efficiency of phosphorescent emitters can approach to about 100% [6].
The heavy metal complexes, particularly those containing Pt and Ir,
can induce the intersystem crossing by strong spin-orbit coupling,
leading to the mixing of the singlet and triplet excited states [7,8].
Spin-forbidden nature of radiative relaxation from the triplet excited
state then becomes allowed, resulting in high phosphorescent efficien-
cies. Thus, heavy metal complexes can serve as efficient phosphors in
OLEDs.
Unfortunately, most of phosphorescent emitters have a long radiat-
ive lifetime, which leads to the dominant triplet–triplet (T–T) annihil-
ation at high current. The occurrence of T–T annihilation decreases
the performance of a phosphorescent material, particularly its
maximum brightness and luminescence efficiency at high currents
[9,10].
In order to improve the luminescence efficiency by avoiding T–T
annihilation and study luminescent mechanism of the heteroleptic
iridium complex, the metal complex having a different species of
plural ligands has been proposed [11]. The metal complex has been
designed to transfer the energy of exciton smoothly between ligands
placed in its excited states. More specifically, when a metal complex
having one luminescent ligand among three ligands is placed in the
lowest excited state, the excited energy is transferred from the other
two ligands to one luminescent ligand. Furthermore, it is expected
that the use of one luminescent ligand decreases the probability of
energy transition between spatially adjacent molecules, leading to
the decrease of quenching or energy deactivation. In the before
article, we reported Ir complexes having this luminescent mechanism.
The complexes were designed on ligand exhibiting the great difference
of the lifetime and the MLCT characteristic.
Herein, we report efficient phosphorescent emitters having
different ligands, Ir(piq-3F)2(dpq-4F). Previously, Ir(piq-3F)3 and
Ir(dpq-4F)3 were known to have high phosphorescence efficiencies in
electroluminescent (EL) emissions near 608 and 620 nm, respectively,
because they had the metal-to-ligand charge transfer (MLCT) excited
states with the similar luminescence lifetime [12,13]. The purpose of
the present study is to design the efficient heteroleptic Ir(III) complex
having different species of ligands for the highly efficient Ir(III)

Synthesis and Photophysical Studies
237
complex suitable for red OLED devices. In addition to the high phos-
phorescent efficiency, the phosphorescent mechanism of the Ir(III)
complexes having different ligands is studied in comparison with those
having the homoleptic Ir(III) complexes having the same species of
ligands.
2. EXPERIMENTAL
2.1. Synthesis and Characterization
All reagents were purchased from Aldrich except Ir(III) trichloride
hydrate (IrCl3 ꢀ H2O) which was purchased from Strem and used with-
out further purification. All reactions were carried out under a nitro-
gen or argon atmosphere. Solvents were dried by standard
procedures. All column chromatography was performed with the use
of silica gel (230-Mesh, Merck). The synthesis method is shown in
Figure 1.
2.1.1. Synthesis of Ligand: piq-3F
1-(4-fluorophenyl)isoquinoline ligands, piq-F, were obtained from
the reaction of 1-chloroisoquinoline with the corresponding 4-fluoro-
phenylboronic acids by Suzuki coupling. 1-chloroisoquinoline (0.899 g,
5.5 mmol), 4-fluoro phenylboronic acid (0.699 g, 5 mmol) and tetrakis-
triphenylphospine palladium(0) (0.196 g, 0.17 mmol) were placed in
FIGURE 1 (a) Synthesis of C^N Ligands. (b) Synthesis of heteroleptic Iridiu-
m(III) complexes containing two different kinds of C^N ligands. (c) Synthesis
of homoleptic Iridium(III) complexes containing one kind of C^N ligand.

238
G. Y. Park et al.
20 ml of toluene, 10 ml of ethanol and 20 ml of 2 N sodium carbonate
aqueous solution. The mixture was heated to reflux for 15 hr. The mix-
ture was cooled to room temperature and extracted with 20 ml of ethyl
acetate. The organic fraction were dried over anhydrous MgSO4, fil-
tered and pumped dry. The residue was chromatographed on a silica
gel column with ethyl acetate=hexane (1:3). The product were collected
and dried in vacuum to yield white powder in 64% yield.
2.1.2. Synthesis of Ligand: dpq-3F
dpq-3F ligand was obtained from the Friedlander reaction [14,15].
The concentrated sulfuric acid of 1.0 ml was added to a solution of
1.972 g (10.0 mmol) of 2-aminobenzophenone and 1.381 g (10.0 mmol)
of 30-fluoroacetophenone (for dpq-4F) in 30 ml of glacial acetic acid.
The solution was heated to reflux for 4 h. The reaction mixture
was then cooled and dripped slowly with stirring into an icy concen-
trated ammonium hydroxide solution of 15 ml of in 40 ml of water.
The resultant yellow precipitate was filtered, washed with water,
and dissolved in dichloromethane. The organic fraction was sepa-
rated and dried over anhydrous MgSO4, and pumped to dry. The
solid was chromatographed on a silica gel column with dichloro-
methane (CH2Cl2).
2.1.3. Synthesis of Heteroleptic Ir(III) Complex: Ir(piq-3F)2 (dpq-4F)
Cyclometalated Ir(III) l-chloro-bridged dimers of the general for-
mula, (piq-3F)2Ir(l-Cl)2Ir(piq-3F)2, were synthesized by the method
reported by Nonoyama with slight modification [16]. IrCl3 ꢀ H2O
(1.490 g, 5 mmol) and H2O (10 ml) were added to a solution of piq-3F
(12.5 mmol) in 2-ethoxyethanol (30 ml). The mixture was refluxed at
120ꢁC under the argon atmosphere for 12 h and then cooled to room
temperature. The solution mixture was evaporated under the vacuum
slowly to obtain the crude product (piq-3F)2Ir(l-Cl)2Ir(piq-3F)2. The
resultant precipitate was dissolved in dichloromethane and was
filtered chromatographically on silica gel column with dichloromethane.
The product portion was collected and dried in vacuum. (piq-3F)2Ir(l-
Cl)2Ir(piq-3F)2 (0.51 mmol) and 2,4-pentanedione (d ¼ 0.975, 1.73 mmol)
were mixed with Na2CO3 (250 mg) in 2-ethoxyethanol (30 ml). The mix-
ture was refluxed for 2 h. The solution was cooled to room temperature
and the yellow solid was filtered. Ir(piq-3F)2(acac) was obtained after the
chromatographing on silica gel column with dichloromethane to yield a
bright yellow powder. Ir(piq-3F)2(acac) (1 mmol) and dpq-3F ligand
(2 mmol) were dissolved in 20 ml of glycerol and refluxed for 10 h. After
cooling, 20 ml of 1 N HCl solution was added and the resulting

Synthesis and Photophysical Studies
239
precipitate was filtered off. The residue was purified by silica gel
chromatography using CH2Cl2.
2.1.4. Synthesis of Homoleptic Ir(III) Complex: Ir(piq-3F)3,
Ir(dpq-4F)3
These complexes were prepared from Ir(acac)3 and the corresponding
ligand by a reported procedure [9]. Ir(acac)3 (245 mg, 0.5 mmol) and
piq-3F (or dpq-4F) (2 mmol) were dissolved in 20 ml of glycerol and
refluxed for 12 h. After cooling, 20 ml of 1 N HCl solution was added
and the resulting precipitate was filtered off. The residue was purified
by silica gel chromatography using CH2Cl2.
2.2. UV-Absorption and Photoluminescence (PL)
Measurement
UV-Vis absorption spectra were measured on Hewlett Packard 8425 A
spectrometer. The PL spectra were obtained on Perkin Elmer LS 50B
spectrometer. UV-Vis and PL spectra of Ir(piq-3F)3, Ir(dpq-4F)3 and
Ir(piq-3F)2(dpq-4F) were measured with a 10ꢂ5 M dilute solution in
CH2Cl2.
2.3. Theoretical Calculation
Calculations on the electronic ground states of piq-3F, dpq-4F
ligands and Ir(piq-3F)2(dpq-4F) were carried out using the B3LYP
density functional theory (DFT) and compared with those of Ir(piq-
3F)3 and Ir(dpq-4F)3. LANL2DZ [17] and 6–31G(d) [18] basis sets
were employed for Ir and the other atoms, respectively. Through
the calculation of the ground state geometry, the electronic structure
is examined in terms of the highest occupied molecular orbitals
(HOMOs) and the lowest unoccupied molecular orbitals (LUMOs).
To obtain the vertical excitation energies of the low-lying singlet
and triplet excited states of the complexes, time-dependent density
functional theory (TD-DFT) calculations using the B3LYP functional
was performed at the respective ground-state geometry, where the
basis set of ligands was changed to 6–31 þ G(d). Typically, the lowest
10 triplet and 10 singlet roots of the nonhermitian eigenvalue equa-
tions were obtained to get the vertical excitation energies and com-
pared with the absorption spectra to examine each peak. The
ground-state B3LYP and excited-state TD-DFT calculations were
carried out using Gaussian 98. We noted that our results were per-
formed to the molecules only in the ground state geometry. If there

240
G. Y. Park et al.
are significant geometry changes in the excited state, the lumi-
nescence properties of complex could change significantly from the
presented results. In addition, the TD-DFT results do not provide
information on the oscillator strength of the triplet excited state
since the spin-orbit coupling effect is not included in current
TD-DFT results.
3. RESULTS AND DISCUSSION
In order to improve the luminescence efficiency by avoiding T–T
annihilation and to study the luminescent mechanism of the hetero-
leptic iridium complex, Ir(piq-3F)2(dpq-4F) is designed for the appli-
cation in OLEDs. The iridium complexes prepared herein can be
classified into two groups. The first group includes the iridium com-
plexes having two different C^N ligand structures (heteroleptic com-
plexes) such as Ir(piq-3F)2(dpq-4F) and the second one involves the
iridium complexes having only one kind of C^N ligand (homoleptic
complexes) such as Ir(piq-3F)3 and Ir(dpq-4F)3.In homoleptic complex
cases, luminescence efficiency may decrease because of the saturated
quenching effect caused by the energy transfer between the same spe-
cies of ligands. Therefore, we have prepared and characterized hetero-
leptic Ir(III) complexes having two piq-3F ligands and one of dpq-4F.
The syntheses of ligands, heteroleptic complexes and homoleptic com-
plexes are shown in Figure 1.
The HOMO and LUMO energy levels of the ground state of homo-
leptic Ir(III) complexes of piq-3F and dpq-4F ligands were shown in
Figure 2. Comparing HOMO and LUMO energy levels of piq-3F and
dpq-4F complexes, we confirmed that HOMO and LUMO energy level
of dpq-4F complex are located between those of piq-4F. So, in the case
of Ir(piq-3F)2(dpq-4F), we expected that the excited energy is absorbed
in piq-3F because of strong MLCT characteristic, that the excitation
energy is transferred from piq-3F to dpq-4F, because the dpq-4F has
lower LUMO energy level than that of the piq-3F. But experimental
result does not agree perfectly with expectation.
The UV-vis absorption spectra of heteroleptic and homoleptic Ir(III)
complexes, Ir(piq-3F)3, Ir(dpq-4F)3 and Ir(piq-3F)2(dpq-4F) in 10ꢂ5
M
1
CH2Cl2 at room temperature were shown in Figure 3. Both MLCT
3
and MLCT peaks are observed for these complexes. The absorption
peaks of Ir(piq-3F)3 is located at 400, 460, 480 and 520 nm. The absorp-
1
tion bands below 420 nm can be assigned to the spin-allowed p ꢂ pꢃ
transition and the band around 460 nm to a spin-allowed 1MLCT band
and the band around 480 and 520 nm can be assigned to a spin-forbid-
3
den MLCT band. MLCT absorption is allowed by the strong mixing

Synthesis and Photophysical Studies
241
FIGURE 2 HOMO and LUMO energy levels of Ir(piq-3F)3 and Ir(dpq-4F)3
complexes.
between the p character of the ligand and the 5d character of the
centric metal in HOMOs. The MLCT absorption peaks of Ir(dpq-4F)3
are observed at 410, 460, 480 and 550 nm. The overall profile of
FIGURE 3 UV-vis absorption spectra of Ir(piq-3F)3, Ir(dpq-4F)3 and
Ir(piq-3F)2(dpq-4F) in 10 ꢂ 5M CH2Cl2.

242
G. Y. Park et al.
absorption spectrum and the peak position of the MLCT absorption of
the heteroleptic Ir(piq-3F)2(dpq-4F) are very similar to those of Ir(piq-
3F)3. It provides the evidence that MLCT absorption of the heterolep-
tic Ir(piq-3F)2(dpq-4F) occurs mainly at the piq-3F ligand. Because
MLCT characteristic of Ir(piq-3F)3 is stronger than that of Ir(dpq-
4F)3, MLCT absorption is allowed mainly by the piq-3F ligand.
The PL spectra of the homoleptic and heteroleptic complexes on
CH2Cl2 are shown in Figure 4. The PL spectra of Ir(piq-3F)2(dpq-4F)
and Ir(piq-3F)3 show an emission band at 594 nm, while Ir(dpq-4F)3
show the maximum emission peak at 604 nm. We expected that the
heteroleptic complex Ir(piq-3F)2(dpq-4F) has red-shifted emission
peak than that of Ir(piq-3F)3 by emitting from the mixed state of
piq-3F and dpq-4F ligands. However, the PL spectra of Ir(piq-
3F)2(dpq-4F) is similar to that of Ir(piq-3F)3. Therefore, the lumi-
nescence occurs mainly at piq-3F ligand. The dpq-4F ligand acts not
as an emitting ligand but as an ancillary ligand to increase the lumi-
nescence efficiency by avoiding T–T annihilation.
In order to examine which ligand mainly contributes to the MLCT
transition process of Ir(piq-3F)2(dpq-4F), the d-orbital characteristics
of HOMOs and LUMOs were investigated after geometry optimization
of the molecular structure of this complex using the density functional
theory (DFT). In Figure 5, contour plots of the three highest HOMOs
and three lowest LUMOs of Ir(piq-3F)2(dpq-4F) are shown. These
FIGURE 4 PL spectra of Ir(piq-3F)3, Ir(dpq-4F)3 and Ir(piq-3F)2(dpq-4F).

Synthesis and Photophysical Studies
243
FIGURE 5 Contour plots of HOMOs and LUMOs of Ir(piq-3F)2(dpq-4F)
where dpq-4F ligand is located at the left side.
orbitals are important because dominant excitations and emissions
mainly occur due to the electronic transition among those orbitals.
Three HOMOs and LUMOs show to have orbitals that are localized
mainly in piq-3F ligands, where the calculated electron population
in centric Ir atom is 49.34%. It concluded that three HOMOs and
three LUMOs have different characters with respect to each other,
as the former has mainly the strong metallic character of the 5d orbital
of the centric Ir atom while the latter has the character of the piq-3F
ligand. It shows that the electronic transition between those orbitals
would be the MLCT. MLCT has been known to occur when the Ir(III)
complex illuminates intensified phosphorescent light. Ir(piq-3F)2(dpq-4F)
has a large electron population of 49.34%, which is rather similar to
that of Ir(ppy)3 (51%) as known strong MLCT material. Therefore, it
is expected that Ir(piq-3F)2(dpq-4F) may have a high phosphorescence
emitting characteristics.
In the luminescent mechanism of Ir(piq-3F)2(dpq-4F), the excited
energy absorption and the luminescence occur mainly at the piq-3F
ligand because of strong MLCT characteristic of piq-3F ligand. Also,
because the radiative lifetime of Ir(piq-3F)3 is shorter than that of
Ir(dpq-4F)3. As a result, light emits with the red emission from piq-3F
ligand. Thus it allows a monochromatic luminescent color. The lumi-
nescence intensity of Ir(piq-3F)2(dpq-4F) is stronger than that of
Ir(piq-3F)3 and Ir(dpq-4F)3 because it is improved the luminescent
by the decrease of quenching or energy deactivation.

244
G. Y. Park et al.
4. CONCLUSIONS
In summary, we report the detailed syntheses and photophysical
properties of the phosphorescent iridium(III) complex having a differ-
ent species of plural (C^N) ligands in order to improve the lumi-
nescence efficiency by avoiding T–T annihilation and study
luminescent mechanism of the heteroleptic iridium complex. We have
synthesized the iridium complex such as Ir(piq-3F)2(dpq-4F) and stud-
ied their photophysical properties for the application in OLEDs. As a
result, in the case of Ir(piq-3F)2(dpq-4F), the excited energy is
absorbed mainly from piq-3F ligands and the excitation energy is
not intramolecular transferred to dpq-4F ligand but emits in piq-3F,
because of stronger MLCT characteristic and shorter lifetime of
piq-3F. Therefore the absorption and luminescence is concerned on
piq-3F. The dpq-3F ligand increases the luminescence efficiency by
avoiding T–T annihilation.
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