The blue phosphorescent iridium complexes containing new triazole ligands for OLEDs

Article abstract of DOI:10.1080/15421400902939504

For the application to organic light-emitting diodes (OLEDs), the blue light emitting iridium complexes containing a new triazole (trzl) ancillary ligand were prepared and their photophysical properties was studied with respect to substituent effect at th

Full text of DOI:10.1080/15421400902939504

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The Blue Phosphorescent
Iridium Complexes Containing
New Triazole Ligands for OLEDs
So Youn Ahn a & Yunkyoung Ha a
a Department of Information Display Engineering ,
Hongik University , Seoul, Korea
Published online: 25 Jun 2009.
To cite this article: So Youn Ahn & Yunkyoung Ha (2009) The Blue Phosphorescent
Iridium Complexes Containing New Triazole Ligands for OLEDs, Molecular Crystals and
Liquid Crystals, 504:1, 59-66, DOI: 10.1080/15421400902939504
To link to this article: http://dx.doi.org/10.1080/15421400902939504
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Mol. Cryst. Liq. Cryst., Vol. 504, pp. 59–66, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902939504
The Blue Phosphorescent Iridium Complexes Containing
New Triazole Ligands for OLEDs
So Youn Ahn and Yunkyoung Ha
Department of Information Display Engineering, Hongik University,
Seoul, Korea
For the application to organic light-emitting diodes (OLEDs), the blue light emit-
ting iridium complexes containing a new triazole (trzl) ancillary ligand were pre-
pared and their photophysical properties was studied with respect to substituent
effect at the 5-position of the triazole in the complexes. As substituents, electron
donating groups, CMe3 and CH2OMe were introduced to the triazole ligand of
the complex to investigate the electronic effect on their chromacity and efficiency.
The absorption patterns of the iridium complexes, Ir(F2-ppy)2(trzl-CMe3) and
Ir(F2-ppy)2(trzl-CH2OMe) prepared in this study were very similar. Their PL spec-
tra exhibited the blue emission at 464 and 462 nm with the shoulder at 490 nm,
respectively. Previously reported Ir(F2-ppy)2(trzl-Me) showed almost identical
emission maxima at 461 nm with a shoulder at 492 nm. Such similar emission pat-
terns of these complexes indicate that the ancillary ligands, derivatives of triazole,
did not make significant influence on chromaticity of their iridium complexes.
Keywords: blue phosphorescence; iridium complex; OLED
1. INTRODUCTION
The organic light emitting diodes (OLEDs) have been extensively stu-
died for application in flat-panel displays, owing to rapid progress in
material design and device fabrication in recent years. Development
of pure red, green and blue emitting materials is the most important
issue for the advance of full-color display. OLEDs based on phos-
phorescent emitters can significantly improve electroluminescence
This work was supported by the Korea Research Foundation (KRF-2008-
531-C00036).
Address correspondence to Yunkyoung Ha, Department of Science, College of Engi-
neering, Hongik University, 72-1 Sangsu-Dong, Mapo-Ku, Seoul, 121-721, Korea.
E-mail: ykha@hongik.ac.kr
59

60
S. Y. Ahn and Y. Ha
performance because both singlet and triplet excitons can be har-
vested for light emission [1–11]. Greater success has been achieved
at development of a red and a green emitter in fluorescent and phos-
phorescent displays. However, development of the blue emitting mate-
rial is especially difficult because blue luminescence need big energy
gap between HOMO and LUMO of the material. Requirement of such
large energy gap increase means that there should be a gradual
decrease in the admixture of the ligand-centered p!pꢀ and metal-to-
ligand charge-transfer (MLCT) transition [12]. The net results are
reduction in the T1!S0 transition probability and hence a low emis-
sion quantum efficiency (QE). Moreover, increasing the energy gap
toward blue emission raises the likelihood that thermal population
of the higher lying dd, MLCT, npꢀ, or even ligand-to-ligand charge-
transfer states (LLCT) could be invoked, accounting for the fast
quenching at room temperature [13].
Recently, it was reported that 1,2,4-triazole (trzl) could be an ancil-
lary ligand for the iridium complexes with its strong r donor and weak
p acceptor characteristics [14]. Introduction of the new ancillary
legend, 1,2,4-triazole (trzl), to the blue-green phosphorescent Ir com-
plexes could lead to better blue chromacity in their electrolumines-
cence (EL) devices [14]. We thus investigated modification of the
ancillary trzl ligand in the iridium complexes with different substitu-
ents at the 5-position and studied the photophysical properties of the
complexes chelated with the trzl.
2. EXPERIMENTAL SECTION
All reagents were purchased from Aldrich Co. and Strem Co., and used
without further purification. All reactions were carried out under an
argon atmosphere. Solvents were dried by standard procedures. All
column chromatography was performed with the use of silica gel
1
(230-mesh, Merck). H NMR spectra were obtained from a 300 MHz
NMR at Sogang University, and mass spectra were determined on
JEOL, JMS-AX505WA, HP 5890 Series II Hewlett-Packard 5890A
(capillary column) at Seoul National University in Korea.
2.1. Synthesis
2.1.1. Synthesis of Ligands
Synthesis of F2-ppy [15]. 4,6-difluorophenylpyridine ligand was
obtained from the reaction of 1-chloropyridine with the corresponding
substituted phenylboronic acids by Suzuki coupling. 2-chloropyridine

The Blue Phosphorescent Iridium Complexes
61
(1.035 ml, 11 mmol), 2,4-difluorophenylboronic acid (1.58 g, 10 mmol)
and tetrakistriphenylphospine palladium(0) (0.196 g, 0.17 mmol) were
placed in a 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 8 hr at 135ꢁC. The mixture was cooled to room tempera-
ture and extracted with 20 ml of ethyl acetate. The organic fraction
was dried over anhydrous MgSO4, filtered and pumped dry. The resi-
due was chromatographed on a silica-gel column with ethyl aceta-
te=hexane (1:3). The product was collected and dried in vacuum to
yield a light yellow liquid with 60% yield. Yield: 60%.
Synthesis of ancillary ligand (trzl-CMe3, trzl-CH2OMe) (pyridine-
2-yl)amidrazone (intermediate) [14]. After melting 5.2 g (0.05 mol) of
2-cyanopyridine with gentle heating, 2.65 ml (0.055 mol) of hydrazine
monohydrate was added, yielding a cloudy mixture. Ethanol was
added until the mixture became clear, and the resulting solution
was stirred overnight at room temperature, causing a gel-like pro-
duct to form. All solvents were removed under reduced pressure,
and the solid was suspended in petroleum ether (50 ml), cooled in
an ice bath, and filtered, washing with cold petroleum ether. The
resulting amidrazone was unstable to air and water and thus used
right away for the synthesis and the functionallized ancillary ligand,
trzl. Yield: 73%.
2-(5-R-2H-1,2,4-triazol-3-yl)pyridine. General procedure [14]. (pyri-
dine-2-yl)amidrazone (2.0 g, 15 mmol) and NaCO3 (1.6 g, 15 mmol)
were placed a dry flask. 15 ml of dry dimethylacetamide (DMAA)
and 5 ml of dry THF were added, yielding a pale yellow suspension
that was cooled to 0ꢁC. In a separate, dry flask, 15 mmol of the appro-
priate acyl chloride was dissolved in 5 ml of DMAA. This solution was
then added dropwise to precooled amidrazone mixture, which caused
it to turn bright yellow. The mixture was slowly warmed to room tem-
perature and stirred for additional 5 hr, yielding a thick yellow mix-
ture. The contents were filtered and the solid was washed with
water and EtOH. The resulting pale white solid was dried under
vacuum.
2-(5-tert-butyl-2H-1,2,4-triazol-3-yl)pyridine (trzl-CMe3). Yield: 70%.
1H NMR (DMSO-d6, 300 MHz): d 8.57, 8.08, 7.88, 7.44 (m, 1H each,
aromatic Hs’); 1.21(s, 9H, C(CH3)3)
2-(5-(methoxymethyl)-2H-1,2,4-triazol-3-yl)pyridine
(trzl-CH2-
1
OMe). Yield: 66%. H NMR (DMSO-d6, 300 MHz): d 8.56, 8.05, 7.85,
7.43 (m, 1H each, aromatic Hs’); 3.32 (overlapped s, 5H, CH2OCH3)

62
S. Y. Ahn and Y. Ha
2.1.2. Synthesis of Ir(III) Complexes
Ir(F2-ppy)2(trzl-CMe3). The cyclometalated iridium m-chloro-bridged
dimer, (F2-ppy)2Ir(l-Cl)2Ir(F2-ppy)2, was prepared according to the
method reported by Nonoyama with slight modification [16]. The
resulting dimmer (1.2 g, 1 mmol) and trzl-CMe3 (0.687 g, 3.4 mmol)
were mixed with Na2CO3 (500 mg) in 2-ethoxyethanol (30 mL). The
mixture was refluxed for 6 hr. After cooling to room temperature,
the solution was filtered and concentrated under reduced pressure.
Excess of water was added to induce the precipitation of an orange
solid. This precipitate was filtered, followed by washing with methanol
and acetone and dried under vacuum. Yield: 52%. FAB-MS: calculated
774.2; found 775.
Ir(F2-ppy)2(trzl-CH2OMe). The iridium complexes were prepared
from the reaction of the corresponding dimer with the trzl-CH2OMe,
according to the procedure described above as yellow powder. Yield:
57%. FAB-MS: calculated 762.1; found 763.
2.2. Optical Measurements
UV-Vis absorption spectra were measured on a Hewlett Packard
8425A spectrometer. PL spectra were measured on a Perkin Elmer
LS 50B spectrometer. UV-Vis and PL spectra of iridium complexes
were measured in 10ꢂ5 M dilute CH2Cl2 solution.
3. RESULTS AND DISCUSSION
Synthesis of the main ligand, difluorophenylpyridine (F2-ppy), was
straightforward according to the modified Suzuki coupling method
[15]. The new ancillary ligand, 2-(5-R-2H-1,2,4-triazol-3-yl)pyridine,
was prepared from the reaction of amidrazone intermediate with the cor-
responding acyl halide, according to the reported procedure [14]. Finally,
the iridium complexes were synthesized as reported by Nonoyama [16].
The overall synthetic schemes are illustrated in Figure 1.
The UV-Vis absorption spectra of the complexes in CH2Cl2 are
shown in Figure 2. The absorption spectra of these compounds have
intense bands appearing in the UV part of the spectrum between
230 and 350 nm. These bands have been assigned to the spin-allowed
1
1(p ! pꢀ) transitions of the ligands. The (p ! pꢀ) bands are accompa-
nied by weaker and lower energy features extending into the visible
region from 350 to 400 nm. These absorption bands have been assigned
to both allowed and spin-forbidden MLCT transitions. The high inten-
sity of the MLCT bands has been attributed to effective mixing of these

The Blue Phosphorescent Iridium Complexes
63
FIGURE 1 The synthesis of F2-ppy, triazoles and their iridium complexes.
charge-transfer transitions with higher lying spin-allowed transitions
on the cyclometalating ligand [17]. As previously stated, large energy
gap leads to reduction of the MLCT transition, and as a result the
intensity of MLCT band is lower than those of red and green phosphor-
escent emitters. In detail, absorption peaks Ir(F2-ppy)2(trzl-CMe3)
appeared at 258, 280, 362 and 380 nm and those for Ir(F2-ppy)2(trzl-
CH2OMe) were shown at 257, 282, 358, 370 and 389 nm. For compar-
ison, the absorption of the previously reported Ir(F2-ppy)2(trzl-CH3)
appeared at 258, 282, 343, 374 and 451 nm [14]. The absorption pat-
terns of Ir(F2-ppy)2(trzl-CMe3) and Ir(F2-ppy)2(trzl-CH2OMe) were

64
S. Y. Ahn and Y. Ha
FIGURE 2 UV-Vis absorption spectra of Ir(F2-ppy)2(AL) (AL ¼ trzl-CMe3,
-CH2OMe).
very similar except for blue shifting of the absorption peaks in the visi-
ble region, compared with those of Ir(F2-ppy)2(trzl-CH3).
The photoluminescence (PL) spectra of the Ir complexes in 10ꢂ5M
CH2Cl2 solution are shown in Figure 3. The PL spectra of complexes
FIGURE 3 PL spectra of Ir(F2-ppy)2(AL) in a 10ꢂ5M CH2Cl2 solution.

The Blue Phosphorescent Iridium Complexes
65
have almost identical emitting pattern at the same emitting wave-
lengths within experimental errors. Ir(F2-ppy)2(trzl-CMe3) and Ir(F2-
ppy)2(trzl-CH2OMe) exhibited the emission maxima at 464 and
462 nm, respectively and a second less intense peak at 490 and
489 nm, respectively. The emission color of the complexes is a cyan-
blue because of these secondary peaks. For comparison, the emission
peak of Ir(F2-ppy)2(trzl-CH3) appeared similar at 461 and 492 nm
[14]. These results indicate that the substituent effect at the 5-position
of the triazole is negligible in PL color of their complexes. However, we
expect that the ancillary ligands might allow luminous efficiency
increase of their complexes owing to involvement of the bulky substitu-
ent, CMe3 and CH2OMe, since the steric bulkiness could reduce T-T
annihilation. Meanwhile, the CIE coordinates of the previously Ir(F2-
ppy)2(trzl-CH3) was (0.168, 0.281), which indicates more saturated blue
emission than (0.17, 0.34) by well known FIrpic [18]. Further studies are
in progress on the devices containing the complexes prepared herein
with respect to EL spectra, luminous efficiency and CIE coordinates.
4. CONCLUSION
Herein we report the detailed synthesis and photophysical properties
of the phosphorescent iridium complexes having a trzl ancillary ligand
with a different substituent. The iridium complexes, Ir(F2-ppy)2(trzl-
CMe3) and Ir(F2-ppy)2(trzl-CH2OMe), prepared in this study exhibited
the blue emission at 464 and 462 nm with the shoulder at 490 nm,
respectively. These PL results were quite similar to those of Ir(F2-
ppy)2(trzl-CH3) at 461 nm with the shoulder at 492 nm, indicating that
the substituents at 5-position of the ancillary trzl ligand did not make
significant contribution to both absorption and PL process of their iri-
dium complexes. Further studies are undergoing on device fabrication
to compare EL properties of these new Ir complexes with those of
Ir(F2-ppy)2(trzl-CH3).
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