Pyridyltriazole as a main ligand in the iridium complexes for blue phosphorescence

Article abstract of DOI:10.1080/15421401003609251

New blue-phosphorescent iridium complexes containing pyridyltriazole derivatives (trzl-CH₂OMe) as main ligands and benzyldiphenylphosphines (bdp) as ancillary ligands were synthesized and their photophysical properties were investigated. As a m

Full text of DOI:10.1080/15421401003609251

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Pyridyltriazole as a Main Ligand
in the Iridium Complexes for Blue
Phosphorescence
So Youn Ahn a & Yunkyoung Ha a
a Department of Information Display Engineering, Hongik University,
Seoul, Korea
Published online: 19 Apr 2010.
To cite this article: So Youn Ahn & Yunkyoung Ha (2010) Pyridyltriazole as a Main Ligand in the Iridium
Complexes for Blue Phosphorescence, Molecular Crystals and Liquid Crystals, 520:1, 68/[344]-74/
[350], DOI: 10.1080/15421401003609251
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Mol. Cryst. Liq. Cryst., Vol. 520: pp. 68=[344]–74=[350], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003609251
Pyridyltriazole as a Main Ligand in the Iridium
Complexes for Blue Phosphorescence
SO YOUN AHN AND YUNKYOUNG HA
Department of Information Display Engineering, Hongik University,
Seoul, Korea
New blue-phosphorescent iridium complexes containing pyridyltriazole derivatives
(trzl-CH2OMe) as main ligands and benzyldiphenylphosphines (bdp) as ancillary
ligands were synthesized and their photophysical properties were investigated. As a
main ligand, 2-(5-(methoxymethyl)-2H-1,2,4-triazol-3-yl) pyridine (trzl-CH2OMe),
which has the strong electron-withdrawing ability was chosen to give the deep blue emis-
sion of its iridium complexes. As ancillary ligands, bdp and bdp-F2, were employed
where they represents benzyldiphenylphosphine and (3,5-difluorobenzyl)-diphenylpho-
sphine, respectively. The resulting iridium complexes, Ir(trzl-CH2OMe)2(bdp) and
Ir(trzl-CH2OMe)2(bdp-F2), exhibited the blue emission at 484 and 481 nm in CH2Cl2
solution, respectively. These emission was blue shifted, compared to that of the pre-
viously reported Ir complex containing a fluorinated phenylpyridine and bdp,
Ir(4-Me-40-F-ppy)2(bdp) at 491 nm. We also investigated the solid state luminescence
properties of the complexes prepared herein in a polymer film of PMMA (poly
(methylmetacrylate)) for their application to the polymeric process.
Keywords Blue phosphorescence; iridium complex; OLED; pyridyltriazole
1. Introduction
The emission colors of iridium complexes, which range from blue to red, are strongly
dependent on the choice of the cyclometallated ligands (C^N) [1–4]. Therefore,
design of C^N ligand for iridium complexes is great important in order to achieve
high efficiency and color purity for OLEDs. To date, most researches were focused
on the design and synthesis of the C^N ligand, such as changing p conjugation and
introducing electron donating or withdrawing group on the appropriate position of
the aromatic ring [5–7]. To achieve full color display applications, red, green and
blue emitting materials that have sufficient luminous efficiency and proper chroma-
ticity must be developed. While success has been achieved on the development of red
and green phosphorescent materials, development of blue emitting materials show-
ing the pure color purity and high efficiency have lagged behind.
Previously, we reported use of the pyridyltriazole ligands (trzl-CMe3,
ꢀCH2OMe) as ancillary ligands in the iridium complexes [8]. The triazolate moiety
of the pyridyltriazole has a strong electron withdrawing ability, which may contribute
Address correspondence to Yunkyoung Ha, Department of Science, College of
Engineering, Hongik University, 72-1 Sangsu-Dong, Mapo-Ku, Seoul 121-791, Korea. Tel.:
þ82-2-320-1490; Fax: þ82-2-3142-0335; E-mail: ykha@hongik.ac.kr
68=[344]

Pyridyltriazole as a Main Ligand in the Iridium Complexes 69=[345]
to lowering the HOMO energy level of the complex [9]. Using this concept, we
designed the new Ir complexes containing pyridyltriazole as a main ligand for the blue
emission. In addition, we introduced a non-conjugating benzyldiphenylphosphine
(bdp) ancillary ligand to the complexes for true blue emission [10]. We expect that
modification of the this strong-field ligand might allow further color purity improve-
ment [11]. Thus, new iridium complexes, Ir(trzl-CH2OMe)2(bdp) and Ir(trzl-CH2
OMe)2(bdp-F2), were synthesized and their PL properties were compared in this
study. In addition, the PMMA film spin coated with iridium complexes were
fabricated and their film PL properties were also studied.
2. Experimental Section
2.1. Synthesis and Characterization
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 per-
formed with the use of silica gel (230-mesh, Merck). Mass spectra were determined
on JEOL, JMS-AX505WA, HP 5890 Series II Hewlett-Packard 5890A (capillary
column) at Seoul National University in Korea.
2.2. Synthesis of Ligands
Synthesis of Emitting Ligand (trzl-CH2OMe).
(Pyridine-2-yl)amidrazone (intermediate) [12]. 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 product 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 directly for the synthesis and the derivatized ancillary
ligand, trzl. Yield: 73%.
2-(5-(Methoxymethyl)-2H-1,2,4-triazol-3-yl)pyridine (trzl-CH2OMe) [12]. The
intermediate, (pyridine-2-yl)amidrazone (2.0 g, 15 mmol), and NaCO3 (1.6 g,
15 mmol) were placed a dry flask. Dry dimethylacetamide (DMAA) (15 ml) and
dry THF (5 ml) were added, yielding a pale yellow suspension that was cooled to
0ꢁC. In a separate, dry flask, methoxyacetyl chloride (1.6 g, 15 mmol) 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 temperature and stirred for additional 5 hr, yielding a thick yellow mixture.
The contents were filtered and the solid was washed with water and EtOH. The
1
resulting white solid was dried under vacuum. Yield: 61%. 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).
Synthesis of Ancillary Ligand (bdp-F2).
(3,5-Difluorobenzyl)diphenylphosphine (bdp-f2) [13]. Diphenylphosphine (0.88 ml,
5 mmol) and cesium hydroxide monohydrate (0.84 g, 5 mmol) were placed in

70=[346]
S. Y. Ahn and Y. Ha
anhydrous N,N-dimethylformamide (20 ml), the reaction mixture was stirred for 1 hr
resulting in a dark red-orange solution. 3,5-Difluorobenzyl bromide (0.77 ml,
6 mmol) was added in one portion, at which point, the mixture turned immediately
to white. The reaction mixture was stirred for 26 hr under a dry argon atmosphere at
room temperature. Degassed basic water (30 ml) was added and the mixture was sub-
sequently extracted with CH2Cl2 (3 ꢂ 30 ml). The combined organic layers were then
washed with degassed basic water (3 ꢂ 30 ml), dried using anhydrous sodium sulfate,
decanted and the solvent was removed in vacuum. Purification via recrystallization
from benzene afforded bdp-F2 as an air-sensitive white powder. Yield: 21%.
2.3. Synthesis of Ir(III;) Complexes
.
Ir(trzl-CH2OMe)2(bdp) [10]. Ir(tht)3Cl3 [14] (1.12 g, 2 mmol) and bdp (0.55 g,
2 mmol) were combined in degassed decahydronaphthalene (30 mL), and the mixture
was refluxed for 1 hr. Upon cooling to room temperature, Na2CO3 (2.12 g, 20 mmol)
and trzl-CH2OMe (0.8 g, 4.2 mmol) were added, and the mixture was again refluxed
for another 3 hr. After cooling to room temperature and removal of solvent, the solid
was chromatographed on a silica gel column with dichloromethane (CH2Cl2) to give
Ir(trzl-CH2OMe)2(bdp). Yield: 19%. FAB-MS: calculated 846.22; found 847.
Ir(trzl-CH2OMe)2(bdp-F2). This compound was prepared from the reaction of
trzl-CH2OMe with Ir(tht)3Cl3 and bdp-F2, according to the similar procedure described
for Ir(trzl-CH2OMe)2(bdp). Yield: 17%. FAB-MS: calculated 882.2; found 883.
2.4. 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 the iridium complexes were measured in 10ꢀ5 M dilute
CH2Cl2 solution and in the PMMA film. The PMMA film was fabricated by the
spin-coating onto the glass substrate with 10 wt% Ir complexes of PMMA in
1,2-dichloroetane solution and following solvent evaporation.
3. Results and Discussion
The synthesis of the main ligand, the ancillary ligand and their iridium complexes
was straightforward. Synthesis of the main ligand, 2-(5-(methoxymethyl)-2H-1,2,4-
triazol-3-yl)pyridine (trzl-CH2OMe), was prepared from the reaction of amidrazone
precursor with methoxyacetyl chloride, according to the reported procedure [12].
The ancillary ligand, (3,5-difluorobenzyl)diphenylphosphine (bdp-F2) was synthe-
sized according to the procedure reported previously [13]. Their iridium complexes
were obtained from the reaction of Ir(tht)3Cl3 [14] with bdp followed by addition
of trzl-CH2OMe. 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 strong absorption bands
appearing at the ultraviolet region of the spectrum between 230 and 330 nm. These
1
bands were assigned to the spin-allowed (p!pꢃ) transitions of the ligands. The
1(p!pꢃ) bands are accompanied by weaker and lower energy features extending

Pyridyltriazole as a Main Ligand in the Iridium Complexes 71=[347]
Figure 1. The synthesis of trzl-CH2OMe, bdp-F2 and their iridium complexes.
into the visible region from 330 to 400 nm. These absorption bands have been
assigned to both allowed and spin-forbidden metal-to-ligand charge transfer
(MLCT) transitions. The high intensity of the MLCT bands has been attributed
to effective mixing of these charge-transfer transitions with higher lying spin-allowed
transitions on the cyclometallating ligand [15]. MLCT is important with respect to
the emission efficiency of the complexes. The absorption patterns of Ir(trzl-CH2-
OMe)2(bdp) and Ir(trzl-CH2OMe)2(bdp-F2) are almost identical in the MLCT
region, suggesting that substituent change of its ancillary ligands does not make
significant contribution to the absorption process.
The photoluminescence (PL) spectra of the Ir complexes in 10ꢀ5 M CH2Cl2 sol-
ution are shown in Figure 2. Ir(trzl-CH2OMe)2(bdp) and Ir(trzl-CH2OMe)2(bdp-F2)
exhibited the emission maxima at 484 and 481 nm, respectively. The PL spectra of
two complexes have similar emitting pattern, which are consistent with their UV-vis
absorption results. These results also support that the substituent effect at the 3 and
5-position of the benzyldiphenylphosphine (bdp) is negligible in PL color of their
complexes. For comparison, the iridium complex containing 2-phenylpyridine(ppy)-
based ligands, Ir(4-Me-40-F-ppy)2(bdp), was reported to exhibit emission at 491 nm
[16]. Thus, we were able to show further blue shift of the Ir complexes by employ-
ment of pyridyltriazole instead of the fluorinated phenylpyridine as a main ligand.

72=[348]
S. Y. Ahn and Y. Ha
Figure 2. UV-Vis absorption and PL spectra of Ir(trzl-CH2OMe)2(AL) in a 10ꢀ5 M CH2Cl2
solution (AL ¼ bdp, bdp-F2).
Additionally, we attempted to investigate the UV absorption and PL emission of
new Ir complexes in PMMA (poly(methylmethacrylate)) film for the polymer light-
emitting device (PLED) application. PMMA was chosen as a host because its non-
emitting property within the visible range could provide the PL of the iridium
complexes only [17]. The UV absorption pattern of Ir(trzl-CH2OMe)2(bdp-F2) in
solution and film are somewhat different. The UV spectra of film has weak band
at the visible region from 400 to 500 nm may be originated from the formation of
Figure 3. UV-Vis absorption and PL spectra of Ir(trzl-CH2OMe)2(AL) on the PMMA film.

Pyridyltriazole as a Main Ligand in the Iridium Complexes 73=[349]
the aggregation. The solid PL peak of the Ir(trzl-CH2OMe)2(bdp) and Ir(trzl-CH2
OMe)2(bdp-F2) in the PMMA had emission maxima at 477 and 474 nm, respectively,
as shown in Figure 3. The solid PL spectra of the films underwent hypsochromic
shift with respect to its solution PL maxima. However, such shift is not conclusive
due to the low intensity of PL in the film state. We presume that the energy transfer
did not seem to be efficient from the host, PMMA, to the dopant, the iridium com-
plex because energy levels are not well matched between PMMA and Ir complexes
studied herein. Further investigation of the complexes in other host materials such
as UGH2 is in progress for its PLED application having high efficiency.
4. Conclusion
The pyridyltriazole ligand having strong electron withdrawing character was
introduced as main ligands in the iridium complexes, and the photoabsorption
and photoluminescence properties of its iridium complexes were studied. The iridium
complexes prepared in this study, Ir(trzl-CH2OMe)2(bdp) and Ir(trzl-CH2OMe)2
(bdp-F2), exhibited blue emission at 484 and 481 nm, respectively in its solution
PL. The hypsochromic shifts were observed in the PL spectra of Ir(trzl-CH2OMe)2
(bdp) and Ir(trzl-CH2OMe)2(bdp-F2), compared with that of Ir(4-Me-40-F-ppy)2
(bdp) containing fluorinated phenylphyridine main ligands. On the other hand,
the substituent in the bdp-based ancillary ligand did not show a significant effect
on the modulating the emission wavelength of the iridium complexes. The PL spectra
in the solution and the film states had similar patterns, but the emission maxima of
the complexes in the PMMA film were somewhat blue-shifted and had low intensi-
ties. Further studies are in progress on the film fabrication with other host materials.
Acknowledgement
This work was supported by National Research Foundation of Korea Grant funded
by the Korean Government (2009-0065382).
References
[1] Lamansky, S., Djurovich, P., Murphy, D., Abdel-Razzaq, F., Lee, H.-E., Adachi, C.,
Burrows, P. E., Forrest, S. R., & Thompson, M. E. (2001). J. Am. Chem. Soc., 123, 4304.
[2] Adachi, C., Kwong, R. C., Djurovich, P., Adamovich, V., Baldo, M. A., Thompson,
M. E., & Forrest, S. R. (2001). Appl. Phys. Lett., 79, 2082.
[3] Duan, J.-P., Sun, P.-P., & Cheng, C.-H. (2003). Adv. Mater., 15, 224.
[4] Wu, F.-I., Su, H.-J., Shu, C.-F., Luo, L., Diau, W.-G., Cheng, C.-H., Duan, J.-P., &
Lee, G.-H. (2005). J. Mater. Chem., 15, 1035.
[5] Rayabarapu, D. K., Paulose, B. M. J. S., Duan, J. P., & Cheng, C. H. (2005). Adv.
Mater., 17, 349.
[6] Avilov, I., Minoofar, P., Cornil, J., & Cola, L. D. (2007). J. Am. Chem. Soc., 129, 8247.
[7] Li, C., Zhang, G., Shih, H.-H., Jiang, X., Sun, P., Pan, Y., & Cheng, C.-H. (2009). J.
Organomet. Chem., 694, 2415.
[8] Ahn, S. Y. & Ha, Y. (2009). Mol. Cryst. Liq. Cryst., 504, 59.
[9] Yu, J.-K., Hu, Y.-H., Cheng, Y.-M., Chou, P.-T., Peng, S.-M., Lee, G.-H., Carty, A. J.,
Tung, Y.-L., Lee, S.-W., Chi, Y., & Liu, C.-S. (2004). Chem. Eur. J., 10, 6255.
[10] Chiu, Y.-C., Hung, J.-Y., Chi, Y., Chen, C.-C., Chang, C.-H., Wu, C.-C., Cheng, Y.-M.,
Yu, Y.-C., Lee, G.-H., & Chou, P.-T. (2008). Adv. Mater., 20, 1.

74=[350]
S. Y. Ahn and Y. Ha
[11] Eum, M.-S., Chin, C. S., Kim, S., Kim, C., Kang, S. K., Hur, N. H., Seo, J. H., Kim,
G. Y., & Kim, Y. K. (2008). Inorg. Chem., 47, 6289.
[12] Orselli, E., Kottas, G. S., Konradsson, A. E., Coppo, P., Fro¨hlich, R., De Cola, L.,
van Dijken, A., Buchel, M., & Bo¨rner, H. (2007). Inorg. Chem., 46, 11082.
¨
[13] Honaker, M. T., Sandefur, B. J., Hargett, J. L., McDaniel, A. L., & Salvatore, R. N.
(2003). Tetrahedron letters, 44, 8373.
[14] John, K. D., Salazar, K. V., Scott, B. L., Baker, R. T., & Sattelberger, A. P. (2001).
Organometallics, 20, 296.
[15] Holmes, R. J., D’Andrade, B. W., Forrest, S. R., Ren, X., Li, J., & Thompson, M. E.
(2003). Appl. Phys. Lett., 83, 3818.
[16] Ahn, S. Y. & Ha, Y. Manuscript Submitted to J. Phys. Chem. Solids
[17] Lee, J., Chu, H. Y., Kim, S. H., Do, L. M., Zyung, T., & Hwang, D. H. (2003). Opt.
Mater., 21, 205.