Synthesis and photophysical studies of iridium complexes of fluorinated phenylisoquinolines

Article abstract of DOI:10.1080/15421400500365326

There have been reported materials emitting green color in both fluorescence and phosphorescence with great success, however efficient red luminescence materials are rare. Recently, it has been reported iridium (III) complexes of 1-(phenyl)isoquinoline (

Full text of DOI:10.1080/15421400500365326

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Synthesis and Photophysical
Studies of Iridium
Complexes of Fluorinated
Phenylisoquinolines
Hyeon Hee Rho a , Yong Hwan Park b , Young Hee Lee
b , No Gill Park c , Yunkyoung Ha d & Young Sik Kim d
a Department of Chemical Engineering , Hongik
University , Seoul, Korea
b Department of Molecular Electronics Engineering ,
Hongik University , Seoul, Korea
c Research Institute for Science and Technology,
Hongik University , Seoul, Korea
d Department of Science , Hongik University , Seoul,
Korea
Published online: 20 Aug 2006.
To cite this article: Hyeon Hee Rho , Yong Hwan Park , Young Hee Lee , No Gill Park ,
Yunkyoung Ha & Young Sik Kim (2006) Synthesis and Photophysical Studies of Iridium
Complexes of Fluorinated Phenylisoquinolines, Molecular Crystals and Liquid Crystals,
444:1, 145-155, DOI: 10.1080/15421400500365326
To link to this article: http://dx.doi.org/10.1080/15421400500365326
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Mol. Cryst. Liq. Cryst., Vol. 444, pp. 145–155, 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500365326
Synthesis and Photophysical Studies of Iridium
Complexes of Fluorinated Phenylisoquinolines
Hyeon Hee Rho
Department of Chemical Engineering, Hongik University, Seoul, Korea
Yong Hwan Park
Young Hee Lee
Department of Molecular Electronics Engineering, Hongik University,
Seoul, Korea
No Gill Park
Research Institute for Science and Technology, Hongik University,
Seoul, Korea
Yunkyoung Ha
Young Sik Kim
Department of Science, Hongik University, Seoul, Korea
There have been reported materials emitting green color in both fluorescence and
phosphorescence with great success, however efficient red luminescence materials
are rare. Recently, it has been reported iridium (III) complexes of 1-(phenyl)isoqui-
noline (piq) and 1-(40-fluorophenyl) isoquinoline (piq-F) show strong electrolumi-
nescence (EL) brightness and efficiency, even at high currents. These complexes
show the strong intensity of band around 450 nm assigned to the spin-forbidden
3MLCT (metal to ligand charge transfer) transition [1]. In this study, Ir(III) com-
plexes of fluorinated piqs (piq-F, piq-2F, piq-CF3) as cyclometalated ligand were
prepared and photonic properties were investigated to improve the EL efficiency.
The Ir(III) complex with piq-CF3 ligands exhibits the largest emission efficiency
with maximum at 612 nm. The result of ab. Initio calculation using the Time-
dependent density functional theory (TD-DFT) shows that the strong 3MLCT tran-
sition of the complex is due to the strong coupling between the 5d orbital of the Ir
atom and the highest occupied molecular orbitals (HOMOs) of ligands.
Keywords: DFT; Ir complex; Ir(piq-CF3)2(acac); OLED; phosphorescence; red
This work was supported by 2005 Hongik University Research Fund.
Address correspondence to Young Sik Kim, Department of Science, Hongik Univer-
sity, Seoul 121-791, Korea. E-mail: youngkim@hongik.ac.kr
145

146
H. H. Rho et al.
1. INTRODUCTION
Electroluminescent (EL) organic materials have received a great deal
of attention due to their potential for application in flat-panel displays
because of their luminous efficiency and the simplicity to obtain low
cost device fabrication. Recently, Forrest and Thompson have
developed electro-phosphorescent OLEDs with high efficiency
approaching 100% of internal quantum efficiency which utilize both
singlet and triplet excitons occurring at the emitting layer doped with
the phosphorescent dopants [2,3]. The heavy metal forms such as Ir or
Pt in the complex were known to induce the intersystem crossing by
strong spin-orbit coupling, leading to mixing of the singlet and triplet
excited states. Spin-forbidden nature of radiative relaxation from the
triplet excited state then changed to be allowed, resulting in high
phosphorescent efficiencies. Thus, Ir complexes were known to have
high photoluminescence efficiency and relatively short excited state
lifetime which minimizes quenching of triplet emissive states [4,5].
However, so far satisfactory luminescence efficiency in blue and red
phosphorescent material has not been obtained enough to guarantee
commercialization. Ir(btp)2(acac) (btp ¼ 2-(2-benzo[4.5-a]thienyl)pyri-
dinato; acac ¼ acetylacetonate) were well known efficient red phos-
phorescence [6,7], however external quantum efficiency was only
about 2.5% at high current (100 mA=cm) because of increase of triplet–
triplet (T–T) annihilation phenomenon at high current. Recently, it
was reported that Ir(piq)2(acac) with piq representing the 1-(phenyl)-
isoquinoline ligand showed high external quantum efficiency of
nearly 7.00 at J ¼ 400 mA=cm2, whose EL emission band at 620 nm
[1]. Unlike other Ir(III) red phosphor complexes, remarkably high
efficiency could be maintained at high currents with a negligible effect
from either the T–T annihilation or saturation of the excited state. The
emission spectra for Ir(btp)2(acac) closely resemble the phosphor-
escence spectrum reported for the free organic ligand, supporting
the assignment of the lowest energy excited state to be predominantly
ligand-based. The vibronic fine structure is clearly observed for the btp
ligand [6,7]. However, the emission spectrum of piq based iridium
complexes are broad and featureless and it denoted that Ir(piq)2
(acac) emits from an excited state that is predominantly due to the
MLCT transition [1]. Hence, we wish to synthesize and characterize
more efficient red luminance Ir(III) complexes of fluorinated
piqs (piq-F, piq-2F, piq-CF3) to improve EL efficiency, where
piq, piq-F, piq-2F, and piq-CF3 representing 1-(phenyl)isoquinoline,
1-(40-fluorophenyl)isoquinoline, 1-(3,5-difluorophenyl)isoquinoline and
1-(40-trifluoromethylphenyl)isoquinoline). All complexes emit bright

Photophysical Studies of Iridium Complexes
147
red light and the peak of emission wavelengths can be tuned depend-
ing on the position of the substituent. In absorption spectra, the
3
1MLCT and MLCT transitions have been resolved and characterized
in the range 400–450 nm using the time-dependent density functional
theory (TD-DFT) with B3LYP=6-31G(d). The results of the DFT calcu-
lation on the ground state of Ir(III) complexes indicated that the 5d
orbitals of the Ir atom and the highest occupied molecular orbitals
(HOMOs) of ligands were strongly mixed. The strong coupling
between the central Ir atom and the HOMOs of ligands allows the
strong MLCT transition. The photoluminescent (PL) characteristics
for the substituted and the unsubstituted complexes have been com-
pared and discussed.
2. EXPERIMENTAL SECTION
2.1. Synthesis and Characterization
All reagents were purchased from Aldrich Co. except Ir(III) trichloride
hydrate(IrCl3 ꢀ H2O) which was purchased from Strem Co. and used
without further purification. All reactions were carried out under
nitrogen or argon atmosphere. Solvents were dried by standard
procedures. All column chromatography was performed with the use
of silica gel (230-Mesh, Merck. co) Mass spectra were determined on
JEOL, JMS-AX505WA, HP 5890 Series II Hewlett-Packard 5890 A
(capillary column) at Seoul National University, Korea.
2.1.1. Synthesis of Ligands (L=piq-F, piq-2F, piq-CF3)
Phenylisoquinoline ligands, 1-(40-fluorophenyl) isoquinoline, 1-(3,5-
difluorophenyl) isoquinoline, 1-(40-trifluoromethylphenyl) isoquinoline
were obtained from the reaction of 1-chloroisoquinoline and
corresponding phenylboronic acids by Suzuki coupling.
Synthesis of piq-F. 1-chloroisoquinoline (0.899 g, 5.5 mmol), 4-fluoro-
phenylboronic acid (0.699 g, 5 mmol) and tetrakistriphenylphospine
palladium(0) (0.196 g, 0.17 mmol) was placed in 20 ml of toluene,
10 ml of ethanol and 20 ml of 2 N sodium carbonate aqueous solution.
The mixture were charged with nitrogen atmosphere and heated to
reflux for 15hr. The mixture was cooled room temperature and
extracted with 20 ml of ethyl acetate. The organic fraction were dried
over anhydrous MgSO4, filtered and pumped dry. The residue was
chromatographed on silica gel column with ethyl acetate=hexane
(1:7). The pure product was collected and dried in vacuum, to afford
white powder in 64% yield.

148
H. H. Rho et al.
Ligands of piq-2F and piq-CF3 were prepared from the reaction
of 1-chloroisoquinoline and corresponding phenylboronic acids (3,5-
difluorophenylboronicacid, 4-trifluoromethylphenyl-boronic acid) by a
similar procedure.
piq-F: Yield (0.72 g, 0.64)
piq-2F: Yield (0.82 g, 0.68)
piq-CF3: Yield (0.78 g, 0.57)
2.1.2. Synthesis of Complexes
Ir(L)2(acac) ¼ Ir(piq-F)2(acac),
Ir(piq-2F)2(acac),
Ir(piq-CF3)2
(acac). Cyclometalated Ir(III) m-chloro-bridged dimers of general for-
mula, (L)2Ir(m-Cl)2Ir(L)2, where L represents the cyclometalated phe-
nylquinoline derivatives, were synthesized by the method reported by
Nonoyama with slight modification. The synthesis of Ir(L)2(acac) were
described in detail as follows.
To a flask containing IrCl3ꢀH2O (0.298 g, 1 mmol) and L (piq-F, piq-
2F, piq-CF3 ¼ 2.5 mmol) was added a 3:1 mixture of 2-ethoxyethanol
and water. The mixture was refluxed for 12 hr and cooled to room tem-
perature. The mixture solution was evaporated under vacuum slowly
to obtain the crude product (L)2Ir(m-Cl)2Ir(L)2. The red solid was fil-
tered and washed with ethanol. (L)2Ir(m-Cl)2Ir(L)2 (0.3 mmol) was then
mixed with Na2CO3 (210 mg) in a two-neck flask. 2, 4-pentanedione
(0.21 ml, d ¼ 0.975, 2.1 mmol) in 2-ethoxyethanol (10 mL) were
added and the mixture was refluxed for 1 hr. The solution was cooled
to room temperature and the red solid was filtered. Ir(L)2(acac) was
obtained after chromatographed using CH2Cl2 to afford bright red
powder.
Ir(piq-F)2(acac): Yield (0.335 g, 0.76)MS calced for C35H25F2IrN2O2
m=e 735.8, found m=e 736.
Ir(piq-2F)2(acac): Yield (0.287 g, 0.62)MS calced for C35H23F4IrN2O2
m=e 772.13, found m=e 772.
Ir(piq)-CF3)2(acac): Yield (0.302 g, 0.60) MS calced for C37H25F6IrN2O2
m=e 836.14, found m=e 836.
2.2. Photoluminescence Property Measurement
UV-Vis absorption spectra were measured on Hewlett Packard 8425 A
spectrometer. The PL spectra were measured on Perkin Elmer LS 50B
spectrometer. UV-Vis and PL spectra of ligandsn (L) and Ir(L)2(acac)
were measured in 10ꢁ5 M dilute solution in CH2Cl2.

Photophysical Studies of Iridium Complexes
149
2.3. Computational Method
The ground and low-lying excited electronic states in the Ir(III) com-
plexes were calculated using Gaussian 03 (ab-initio method). The
ppy (biphenyl pyridine) ligand has been known as an excellent ligand
for MLCT transitions. In order to study the MLCT transition, we have
compared the HOMO energy level of ppy ligand with piq ligand deri-
vatives. Density functional theory (DFT) utilizing Hatree-Fork (HF)
with 3–21G(d) basic set and B3LYP with the 6–31G(d) basis set were
used for the geometry optimization and the energy level calculation of
the ground state of those ligands, respectively. The HOMO energy
level and the energy gap between HOMO and the lowest unoccupied
molecular orbital (LUMO) of ppy ligand and piq ligand derivatives
were compared to explain the strong MLCT transitions. Calculations
on the electronic ground states of the Ir(III) complexes have been car-
ried out using the same calculation method and LANL2DZ basis set
for Ir atom. Excitation energies of triplet and singlet state were calcu-
lated from the TD-DFT utilizing the B3LYP functional with the
6–31G(d) basis set. The electronic populations on the central
atom were calculated to show the significant admixture of ligand p
character with the amount of metal 5d character in occupied molecular
orbitals related to those MLCT transitions [8].
3. RESULTS AND DISCUSSION
Fluorinated piq ligands were prepared by palladium (0) catalyzed
Suzuki coupling reaction, as illustrated in (a) of Scheme 1. Using
the ligands prepared above, the bis-cyclometalated Ir(III) complexes
containing an ancillary ligand were synthesized. Ir(L)2(acac) were
obtained via two step synthesis. Ir(III) m-chloro-bridged dimer,
(L)2Ir(m-Cl)2Ir(L)2 were synthesized according to Nonoyama method.
(Scheme 1(b)) Reaction of IrCl3 ꢀ H2O with piq ligand forms chloride-
bridged dimer, (L)2Ir(m-Cl2)Ir(L)2, and the replacement of bridging
chlorides with b-diketonate gives Ir(L)2(acac). Synthetic procedure to
make the final monomeric Ir(III) complex with the ancillary ligand
required a shorter reaction time than the previously reported time.
These reactions result in 60–74% yield of the desired Ir complexes.
Originally, piq ligands were modified from ppy ligand to decrease
the energy gap, increasing the conjugation length which leads to red
shift of the emission spectrum to the longer wavelengths. Fluorinated
piq ligands were designed to decrease the HOMO energy level of piq
to the HOMO energy level of ppy. Because the HOMO of ppy and the
5d orbital of the iridium atom have similar orbital energies, it causes

150
H. H. Rho et al.
SCHEME 1 Synthesis of ligands and complexes of Ir(L)2(acac).
strong coupling between them. Thus, their strong coupling creates
MLCT states and gives a strong 3MLCT absorption and emission spec-
tra in the Ir(III) complexes. F-, 2F- or CF3- were introduced an electron
withdrawer in HOMO of piq ligand to decrease HOMO level. Figure 1
shows calculated HOMO and LUMO energy levels of ligands. HOMO
and LUMO levels of piq-CF3 were lowered down to ꢁ6.179 eV and
ꢁ1.767 eV from ꢁ5.906 eV and ꢁ1.466 eV of those of piq, respectively,
by the effect of electron withdrawing of CF3-. The stronger electron
withdrawers were introduced, the lower HOMO level of ligand was
obtained. HOMO levels of piq-2F and piq-CF3 are much close to
the one of ppy to induce strong MLCT transition.
Absorption spectra of free ligands and Ir(III) complexes of fluori-
nated piqs are shown in Figure 2. Ir(piq-CF3)2(acac) shows strong
MLCT absorption band around 450 nm assigned to the 3MLCT.
However, Ir(piq-2F)2(acac) shows weak 3MLCT character. This
trend was also found in calculation results.

Photophysical Studies of Iridium Complexes
151
FIGURE 1 Comparison of the HOMO and LUMO energy levels of piq deriva-
tives and ppy ligands.
FIGURE 2 Absorption spectra of ligands (inserted) and Ir(L)2(acac), where
L’s are -F, -2F, and -CF3.

152
H. H. Rho et al.
After optimizations of molecular structures of three ligands, using
density functional theory energy levels and d-orbital characteristics
of HOMOs and LUMOs of these Ir(III) complexes were investigated.
Three highest HOMOs are denoted d1, d2, and d3. HOMOs of these
Ir(III) complexes has the strong metallic character of the 5d orbital
of the centric Ir atom and the electronic population confined to 5d
orbital is more than about 30%. For Ir(piq)2(acac), the resultant
5d character from population analysis are 51%, 48% and 43% for
d1, d2 and d3, respectively. Three lowest LUMOs are combinations of
the first pꢂ orbital of piq. For the convenience, all virtual pꢂ orbitals
will be labeled starting with p1ꢂ denote orbitals on piq ligand. For
Ir(piq-F)2(acac), a population analysis of each orbital shows the
following 5d percentages: d1 (48.4%), d2 (46.8%), and d3 (30.9%). As
compared with three Ir(III) complexes, Ir(piq-CF3)2(acac) has the
highest 5d orbital characters, which agrees with the experimental
result that Ir(piq-CF3)2(acac) shows the highest emission intensity
among four Ir(III) complexes.
TD-DFT calculation was employed to examine the low-lying singlet
and triplet states of the complexes and results are shown in Table 1.
For those molecules, we typically give the vertical excitation energies
for the lowest ten triplet and singlet states calculated at the optimized
structure for the ground states. For each of triplet and singlet excita-
tions, a dominant excitation is achieved among several excitations. All
correspond to excitations from an electron in the three HOMOs with
the significant 5d character to virtual MOs with pꢂ character. Accord-
ing to this assessment, it is certain that every excitations calculated in
this study are MLCT states. For the comparison with experimental
data, we plotted the results obtained through the TD-DFT in the
absorption spectra of the Ir(piq-CF3)2(acac) having the highest oscil-
lation strengths, as shown in Figure 3. It is certain that peaks distrib-
uted from 374 nm to 602 nm would be assigned to MLCT absorptions
that singlets (S1 ꢃ S7) and triplets (T1 ꢃ T7) are separately located
in the region of 374 nm ꢃ 507 nm and 444 nm ꢃ 602 nm, respectively.
Ir(piq-2F)2(acac) has weak MLCT character because energy transfer
from the carbon atom linked iridium atom to the metal atom was
hindered by F atom at para position.
Particularly, the strong intensity of band around 450 nm was shown
3
spin-forbidden MLCT. Measured and calculated values are similar,
as shown in Figure 3. The emission spectra of complexes are
shown in Figure 4. The emission wavelength of Ir(piq-F)2(acac),
Ir(piq-CF3)2(acac) in CH2Cl2 was 597 nm and 612 nm, respectively.
The PL spectrum of Ir(piq)2(acac) shows emission band at 622 nm
in CH2Cl2 [3], whereas Ir(III) complexes of fluorinated piqs were blue

153

154
H. H. Rho et al.
FIGURE 3 Comparison of absorption spectrum and values of TD-DFT calcu-
lations for Ir(piq-CF3)2(acac).
FIGURE 4 PL spectra of Ir(L)2(acac), where L’s are -F, -2F, and -CF3.

Photophysical Studies of Iridium Complexes
155
shifted. It is because the energy gap of ligands increase as HOMO level
goes down due to withdrawer like as F and CF3.
4. CONCLUSION
To improve luminescence efficiency of reported piq-based Ir(III) com-
plexes, we have designed and synthesized fluorinated piq ligands such
as piq-F, piq-2F, and piq-CF3 to decrease the HOMO energy level of
piq to the HOMO energy level of ppy. Because the HOMO of ppy
and the 5d orbital of the iridium atom have similar orbital energies,
it causes strong coupling between them. Thus, their strong coupling
3
creates MLCT states and gives a strong MLCT absorption and emis-
sion spectra in the Ir(III) complexes. Complexes containing these
ligands were prepared and photophysical properties were investi-
gated. Among these two complexes, Ir(piq-CF3)2(acac) exhibits the
largest emission efficiency with maximum emission at 612 nm. All of
the low-lying transitions calculated in this study are categorized as
3MLCT transitions for Ir(piq-CF3)2(acac) because the metal 5d
character of three highest MOs are considerably high as 52.8%,
47.7% and 49.1%, respectively. The calculational result shows that
the strong 3MLCT transition of the complex is due to the strong
coupling between the 5d orbital of the Ir atom and the HOMOs of
ligands.
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