Synthesis and photophysical studies of new blue phosphorescent iridium complexes of fluorinated 2-phenyl-4-methoxypyridine

Article abstract of DOI:10.1080/15421400802330473

In this study, a new Ir(III) complex containing 2-(3',5'-bistrifluoromethyl phenyl)-4-methoxypyridine have been synthesized and characterized for efficient blue organic light-emitting diodes (OLEDs). In order to tune photoluminescence (PL) spectra to a sh

Full text of DOI:10.1080/15421400802330473

Mol. Cryst. Liq. Cryst., Vol. 491, pp. 209–216, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802330473
Synthesis and Photophysical Studies of New Blue
Phosphorescent Iridium Complexes of Fluorinated
2-phenyl-4-methoxypyridine
Seung-Chan Lee1 and Young Sik Kim1,2
1Department of Information Display, Hongik University, Seoul, Korea
2Department of Science, Hongik University, Seoul, Korea
In this study,
a
new Ir(III) complex containing 2-(30,50-bistrifluoromethyl
phenyl)-4-methoxypyridine have been synthesized and characterized for efficient
blue organic light-emitting diodes (OLEDs). In order to tune photoluminescence
(PL) spectra to a shorter wavelength, ppy ligand was fluorinated by ꢀCF3 and sub-
stituted by ꢀOCH3 group. The methoxy group substituted on the 4-position of pyr-
idyl ring as electron donating group strongly affected the lowest unoccupied
molecular orbital (LUMO) and it raised the LUMO energy and increased the
energy gap. The trifluoromethyl group substituted on the 30,50-position of phenyl
ring as electron withdrawing group decreased the highest occupied molecular
orbital (HOMO), and bulky CF3 groups occurs a steric hindrance of iridium com-
plex helps decrease of self-quenching. As a result, Ir[(CF3)2OMeppy]2(acac) is more
blue-shifted emission at 471 nm and strong luminescence efficiency.
Keywords: 2-(30,50-bis-trifluoromethylphenyl)-4-metoxypyridine; blue; iridium com-
plex; OLED; phosphorescence
1. INTRODUCTION
Electroluminescent (EL) organic materials have received much of
attention due to their potential application in flat-panel displays due
to the rapid progress in material design and device fabrication in
recent years. One of the important parts of Organic light-emitting
diode (OLED) development for full-color display is to find out materi-
als emitting pure colors of red, green and blue [1–5]. Luminescent
materials for OLED devices are generally classified into two groups;
This work was supported by Seoul R&D Program (10555) and 2008 Hongik
University Research Funds.
Address correspondence to Young Sik Kim, Department of Information Display,
Hongik University, Seoul 121-791, Korea. E-mail: youngkim@wow.hongik.ac.kr
209

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S.-C. Lee and Y. S. Kim
fluorescence and phosphorescence. OLEDs based on phosphorescent
materials can greatly improve electroluminescence performance
because both singlet and triplet excitons can be harvested for light
emission. Theoretically, the internal quantum efficiency of phosphor-
escent emitters can approach 100%. Recently, Forrest, Thompson
and coworkers have developed electrophosphorescent OLEDs with
a high efficiency approaching 100% of the internal quantum
efficiency, which utilize both singlet and triplet excitons produced at
the emitting layer doped with the phosphorescent dopants [6,7]. A
heavy metal such as Ir or Pt in their complex forms is known to induce
intersystem crossing by strong spin-orbit coupling, leading to mixing
of the singlet and triplet excited states. The spin-forbidden nature of
radiative relaxation from the triplet excited state has then been chan-
ged to allow, resulting in high phosphorescent efficiencies. Thus, Ir
complexes are known to have high photoluminescence (PL) efficiency
and a relatively short excited state lifetime which minimizes quench-
ing of triplet emissive states [8,9]. Until now, greater success has been
achieved in the development of a green and red emitter in fluorescent
and phosphorescent displays. However, the purely blue-emitting com-
plex dopants are rarely found. Hence, the current effort of researchers
has been focused on the syntheses of blue-emitting iridium complexes.
Some phosphorescent blue emitter materials have been synthesized
and reported, especially 2-phenylpiridine added some electro-
withdrawing groups to the phenyl ring (fluoro, cyano group and
others) or electro-donating groups to the pyridyl ring (methoxy group
and others) were good blue emission characteristics [10–12].
In this study, Ir[(CF3)2OMeppy]2(acac) [(CF3)2OMeppy ¼ 2-(30,50-
bistrifluoromethylphenyl)-4-metoxypyridine, acac ¼ acetylacetonate]
was successfully synthesized and fully characterized as a new blue emit-
ting material by following the increasing energy gap between HOMO
(highest occupied molecular orbital) and LUMO (lowest unoccupied
molecular orbital). To analyze the color tuning by the substitution, we
have measured UV-absorption and photoluminescence (PL) spectra of
Ir[(CF3)2OMeppy]2(acac). To support these effects, we have calculated
these complexes theoretically by using computational method.
2. EXPERIMENTAL DETAILS
2.1. Synthesis
All ligands and reagents were purchased from Aldrich Co., except
Ir(III) trichloride hydrate (IrCl3ꢁH2O), which was purchased from
Strem Co. and used without further purification. All reactions were

Synthesis and Photophysical Studies
211
carried out under a 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).
2.1.1. Synthesis of Ligands [L ¼ (CF3)2ppy, (CF3)2OMeppy]
Synthesis of (CF3)2OMeppy. A synthetic of phenylpyridine process
use Suzuki coupling [13]. 2-chloro-4-methoxypyridine (1 eq.) and 3,5-
Bis(trifluoromethyl)phenylboronic acid (1.05 eq.) and tetrakistriphe-
nylphospine palladium (0) (0.196 g, 0.17 mmol) were dissolved in the
mixture of toluene (20 ml), ethanol (10 ml) and 2 N sodium carbonate
aqueous solution (20 ml). The reaction mixture was heated to reflux
for 12 hr at 105ꢂC. The mixture was cooled to 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 a silica gel column with ethyl acetate=hexane
(1:3). The product were collected and dried. (Yield 60%, orange sticky
oil, MW:321.24)
Synthesis of (CF3)2ppy. We use 2-bromo pyridine (Aldrich Co. Pro-
duct number ¼ B80100) (1 eq.) and 3,5-Bis(trifluoromethyl) phenyl-
boronic acid (1.05 eq.). The reaction and purification method is same
to (30,50-bis-trifluoromethylphenyl)-4-metoxypyridine. (Yield 62%,
red sticky oil, MW:291.18)
2.1.2. Synthesis of Complexes Ir(L)2(acac)
Cyclometalated Ir(III) m-chloro-bridged dimers of the general for-
mula, (L)2Ir(m-Cl)2Ir(L)2, were synthesized by the method reported
by Nonoyama with slight modification [14]. To a flask containing
IrCl3ꢁH2O (1.49 g, 5 mmol) and ligand (12.5 mmol (2.5 eq)) was added
a 3:1 mixture of 2-ethoxyethanol and water. The mixture was refluxed
for 15 hr and cooled to room temperature. The solution mixture was
evaporated under vacuum slowly to obtain the crude product
(L)2Ir(m-Cl)2Ir(L)2. The yellow to yellowish green solid was filtered
and washed with ethanol. This chloride-bridged dimer (2 mmol) was
then placed in 50 ml two-neck flask filled with 2-ethoxyethanol
(30 mL). 2,4-pentanedione (0.68 ml, d ¼ 0.975, 6.8 mmol (3.4 eq)) was
added and the reaction mixture was refluxed for 2 hr at 135ꢂC. The sol-
ution was cooled to room temperature and poured into 30 ml of 2 N
HCl. The yellow solid was filtered and washed with water, followed
by purification by silica gel column chromatography using CH2Cl2 to
afford a powder of Ir(L)2(acac).
Ir[(CF3)2OMeppy]2(acac) – Yield 45%, yellowish green powder.
MW: 931.75

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S.-C. Lee and Y. S. Kim
Ir[(CF3)2ppy]2(acac) – Yield 48%, light yellow powder. MW:
871.69
Ir(F2ppy)2(acac) – Yield 52%, light yellow powder. MW: 671.67
2.2. Optical Measurements
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(L)2(acac) were measured
with a 10ꢀ5 M dilute solution in CH2Cl2.
2.3. Theoretical Calculations
Calculations on the electronic ground states Ir[(CF3)2OMeppy]2(acac),
Ir[(CF3)2ppy]2(acac), Ir(ppy)2(acac) and Ir(F2ppy)2(acac) were carried
out using the B3LYP density functional theory (DFT). LANL2DZ
[15] and 6-31G (d) [16] basis sets were employed for Ir and the other
atoms, respectively. For the calculated ground state geometries, the
electronic structure is examined in terms of the highest occupied mol-
ecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs).
3. RESULTS AND DISCUSSION
The ligands, (CF3)2OMeppy, (CF3)2ppy and F2ppy were prepared by
the Suzuki coupling [13], as illustrated in Figure 1(a). The bis-cyclo-
metalated Ir(III) complexes containing these ligands and an ancillary
ligand (acac) were prepared. Ir(L)2(acac) were obtained via two steps.
In the first step, Ir(III) l-chloro-bridged dimer were prepared accord-
ing to Nomoyama method [14]. The second step to make the final
monomeric Ir(L)2(acac) complex with 2,4-pentandione requires a short
reaction time compared to that reported previously. Synthesis of irid-
ium complexes is shown in Figure 1(b). The overall reaction yields
were about 40%.
The solution UV–Vis absorption complexes have been measured as
shown in Figure 2. The interesting characteristic of the MLCT absorp-
tion spectra for Ir[(CF3)2ppy]2(acac) resembles that of Ir[(CF3)2OMep-
py]2(acac). Both 1MLCT and 3MLCT peaks are observed for these
complexes. MLCT absorption peaks of Ir[(CF3)2ppy]2(acac) are
observed at 381, 397, and 406 nm. While, MLCT absorption peaks of
Ir[(CF3)2OMeppy]2(acac) are observed at 378, 393, and 405 nm. On
the other hand, Ir(F2ppy)2(acac)’s absorption peaks are observed at
388, 405, 422 and 430 nm. CF3 groups substituted on the 30,50-position

Synthesis and Photophysical Studies
213
FIGURE 1 (a) Synthesis of (CF3)2ppy and (CF3)2OMeppy ligands; (b) Synthesis
of Iridium complexes of Ir[(CF3)2ppy]2(acac) and Ir[(CF3)2OMeppy]2(acac).
FIGURE 2 UV–Vis absorption spectrum of Ir[(CF3)2OMeppy]2(acac),
Ir[(CF3)2ppy]2(acac) and Ir(F2ppy)2(acac) in 10ꢀ5 M CH2Cl2 solution of room
temperature.

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S.-C. Lee and Y. S. Kim
of phenyl ring as electron withdrawing group, tune the MLCT absorp-
tion peak to bluish. F atom of Ir(F2ppy)2acac have similar effect the
MLCT absorption peak. In addition, in the case of Ir[(CF3)2OMeppy]2(a-
cac), the methoxy group substituted on the 4-position of pyridyl ring as
electron donating group strongly affected the lowest unoccupied mol-
ecular orbital (LUMO) and it raised the LUMO energy and increased
the energy gap. Ir[(CF3)2ppy]2(acac) and Ir[(CF3)2OMeppy]2(acac)’s
MLCT peaks are 7 nm and 12 nm blue-shifted than Ir(F2ppy)2(acac),
respectively. It is suggest that CF3 groups are more strong withdrawing
force than F groups, while F2ppy ligands shows more strong MLCT
peaks. And methoxy group of Ir[(CF3)2OMeppy]2(acac) helps more
blue-shifted (6–7 nm) MLCT absorption spectrum than Ir[(CF3)2p-
py]2(acac).
The PL spectra of iridium complexes in CH2Cl2 also reflected
their absorption patterns. As shown in Figure 3, the maximum emis-
sion spectrum of Ir[(CF3)2OMeppy]2(acac) and Ir[(CF3)2ppy]2(acac)
were 471 and 475 nm. In the case of Ir[(CF3)2OMeppy]2(acac) and
Ir[(CF3)2ppy]2(acac), the substitution of CF3 groups at the ortho and
para positions to the iridium atom blue shifted the emission spectrum.
This shift is attributed to the fact that the HOMO level is lower
than that ppy ligand as judged from their electrochemical properties.
Calculated HOMO and LUMO energy levels of iridium complexes are
FIGURE 3 PL spectrum of Ir[(CF3)2OMeppy]2(acac), Ir[(CF3)2ppy]2(acac) and
Ir(F2ppy)2(acac) in 10ꢀ5 M CH2Cl2 solution of room temperature.

Synthesis and Photophysical Studies
215
FIGURE 4 Calculated HOMO and LUMO energy level of Ir(ppy)2(acac),
Ir[(CF3)2ppy]2(acac), Ir[(CF3)2OMeppy]2(acac), and Ir(F2ppy)2(acac).
shown in Figure 4. CF3 or F group in the phenyl ring pull the HOMO
level of iridium complexes, and added methoxy group in the pyridyl
ring up the LUMO level of iridium complex, it helps enlarge energy
level and tune the color to blue. Moreover, CF3 group in the phenyl
ring is stronger electron withdrawing than F group, it realize more
blue-shifted emission wavelength.
In the view of the luminance efficiency, the bulky CF3 groups in
phenyl ring occurs a steric hindrance of iridium complex, less self-
quenching effects, increasing luminance efficiency. Even F groups of
ppy shows stronger MLCT, F groups are can’t prevent self-quenching
effect, less luminance efficiency than CF3 groups in phenyl ring. As a
result, Ir[(CF3)2OMeppy]2(acac) can decrease a self-quenching and
achieve good luminance efficiency compared to Ir(F2ppy)2(acac).
4. CONCLUSIONS
In summary, we synthesized and measured optical properties of the
phosphorescent Ir[(CF3)2OMeppy]2(acac) in order to find a new blue
emitting material for the application in OLEDs. The PL of device
with Ir[(CF3)2OMeppy]2(acac) showed the strongest emission spectra
with maximum at 471 nm. Significant improvements of the new blue
iridium complex, Ir[(CF3)2OMeppy]2(acac), were achieved in the

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S.-C. Lee and Y. S. Kim
blue-shifted luminance spectrum and good luminance efficiency due
to effect of steric hindrance. It was suggested that the Ir[(CF3)2OMep-
py]2(acac) be a good radiation characteristic blue dopant for new
phosphorescent OLEDs.
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