The blue phosphorescent iridium complexes containing the pyridyltriazole derivatives as a main ligand

Article abstract of DOI:10.1080/15421406.2012.703439

The titled iridium compounds involving the electron-withdrawing pyridyltriazole derivative as a main ligand were synthesized. The main ligand, 2-(5-methoxy-methyl)-2H-1,2,4-triazol-3-yl)pyridine (trzl-CH₂OMe), was designed for development of the blue light-emitting iridium complexes. The ligand involves the electron-withdrawing moiety, N, which may contribute to lowering the HOMO energy level of the complex. In this paper, the new homoleptic complex, tris-(2-(5-(methoxymethyl)-2H-1,2,4-triazol-3-yl)pyridinato)iridium (Ir(trzl-CH₂OMe)₃), was synthesized with trzl-CH ₂OMe as the main ligand. The heteroleptic complexes containing trzl-CH₂OMe and other ancillary ligands were also prepared, where the ancillary ligands, acac and pic, represent acetylacetonate and picolinate, respectively. The photophysical properties of the iridium complexes prepared herein, Ir(trzl-CH₂OMe)₃, Ir(trzl-CH₂OMe) ₂(acac), Ir(trzl-CH₂OMe)₂(pic) were investigated. Their photoluminescence maximum appeared at 445nm, 460nm and 472nm, respectively. We also investigated the electrochemical properties of these iridium complexes for the possible application to organic light-emitting diodes.

Full text of DOI:10.1080/15421406.2012.703439

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The Blue Phosphorescent Iridium
Complexes Containing the Pyridyltriazole
Derivatives as a Main Ligand
Hye Rim Park a , Ye Young Yi b & Yunkyoung Ha a
a Department of Information Display Engineering, Hongik University,
Seoul, 121-791, Korea
b Department of Chemical Engineering, Hongik University, Seoul,
121-791, Korea
To cite this article: Hye Rim Park , Ye Young Yi & Yunkyoung Ha (2012) The Blue Phosphorescent
Iridium Complexes Containing the Pyridyltriazole Derivatives as a Main Ligand, Molecular Crystals and
Liquid Crystals, 567:1, 156-162, DOI: 10.1080/15421406.2012.703439
To link to this article: http://dx.doi.org/10.1080/15421406.2012.703439
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Mol. Cryst. Liq. Cryst., Vol. 567: pp. 156–162, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.703439
The Blue Phosphorescent Iridium Complexes
Containing the Pyridyltriazole Derivatives
as a Main Ligand
HYE RIM PARK,1 YE YOUNG YI,2 AND YUNKYOUNG HA1,∗
1Department of Information Display Engineering, Hongik University, Seoul
121-791, Korea
2Department of Chemical Engineering, Hongik University, Seoul 121-791, Korea
The titled iridium compounds involving the electron-withdrawing pyridyltriazole deriva-
tive as a main ligand were synthesized. The main ligand, 2-(5-methoxy-methyl)-2H-
1,2,4-triazol-3-yl)pyridine (trzl-CH2OMe), was designed for development of the blue
light-emitting iridium complexes. The ligand involves the electron-withdrawing moi-
ety, N, which may contribute to lowering the HOMO energy level of the complex. In
this paper, the new homoleptic complex, tris-(2-(5-(methoxymethyl)-2H-1,2,4-triazol-
3-yl)pyridinato)iridium (Ir(trzl-CH2OMe)3), was synthesized with trzl-CH2OMe as the
main ligand. The heteroleptic complexes containing trzl-CH2OMe and other ancil-
lary ligands were also prepared, where the ancillary ligands, acac and pic, represent
acetylacetonate and picolinate, respectively. The photophysical properties of the irid-
ium complexes prepared herein, Ir(trzl-CH2OMe)3, Ir(trzl-CH2OMe)2(acac), Ir(trzl-
CH2OMe)2(pic) were investigated. Their photoluminescence maximum appeared at
445 nm, 460 nm and 472 nm, respectively. We also investigated the electrochemical
properties of these iridium complexes for the possible application to organic light-
emitting diodes.
Keywords Pyridyltriazole; Organic light-emitting diode; Iridium complex; Blue phos-
phorescence.
Introduction
A large number of octahedral 5d complexes containing iridium as a metal center and
heteroleptic (CˆN) cyclometallated ligands, have been extensively studied for multiple
emission colors by the group of Thompson [1–5]. The iridium cyclometallated complexes
have received the considerable attention because of their favorable photochemical and pho-
tophysical properties such as good stability, high photoluminescence (PL) quantum yield,
and short triplet lifetimes. Development of blue light-emitting materials is one of the im-
portant research subjects. However, blue phosphorescent iridium complexes tend to be less
attractive with respect to quantum yields and synthetic efficiency than red phosphorescent
emission materials.
∗Address correspondence to Prof. Yunkyoung Ha, Department of Information Display Engineer-
ing, Hongik University, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea (ROK). Tel. (+82)2-320-1490;
Fax: (+82)2-3142-0335. E-mail: ykha@hongik.ac.kr
[358]/156

The Blue Phosphorescent Iridium Complexes
[359]/157
We have been interested in the blue phosphorescent iridium complexes with good
color purity. The luminescent properties of iridium(III) complexes are strongly related to
the sigma-donating and electron-accepting capabilities of the coordinating ligands, and thus
prediction of the properties of the complexes containing the designed ligands is possible to a
certain extent [6]. Previously, we reported the synthesis and the luminescence of heteroleptic
Ir(F2-ppy)2(trzl-CH2OMe) which has the pyridyltriazole derivative, trzl-CH2OMe, as an
ancillary ligand [7]. The iridium complex showed the blue phosphorescence at 462 and
489(sh) nm, hypsochromically shifted from the emission maxima of Ir(F2-ppy)3 and Ir(F2-
ppy)2(pic) at 470 nm [7]. It was known that N of the pyridyl ligand is more electronegative
than C of the unsubstituted phenyl ligand. Thus, replacement of the phenyl ring with a
pyridine ring in the CˆN ligand of the iridium complex may contribute to lowering the
HOMO energy levels of the complex.
In this study, we introduced trzl-CH2OMe as a new main ligand and reported the
synthesis and luminescence of its homoleptic and heteroleptic iridium complexes.
Experimental
All of the chemical reagents were purchased from Aldrich and Strem Co., and used without
further purification. The reactions were carried out under an argon atmosphere. The solvents
were dried by standard procedures. The column chromatography was performed with the
use of silica gel (230-mesh, Merck).
Synthesis of Ligands
Synthesis of 2-(5-methoxy-methyl)-2H-1,2,4-triazol-3-yl)pyridine (trzl-CH2OMe). First,
(pyridine-2-yl) amidrazone was prepared from the reaction of the 2-cyanopyridine (10.4 g,
0.10 mol) and hydrazine monohydrate (5.5 g, 0.11 mol), yielding a cloudy mixture. Ethanol
(∼5 mL) 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. The solid was washed with cold petroleum ether, yielding
9.9 g (73%) of the amidrazone [8]. The amidrazone obtained was unstable to air and water,
and thus used immediately for the synthesis of trzl-CH2OMe.
2-(5-Methoxymethyl-2H-1,2,4-triazol-3-yl)pyridine (trzl-CH2OMe) was synthesized
using (pyridine-2-yl)amidrazone (2.0 g, 15 mmol) and methoxyacetyl chloride. (Pyridine-
2-yl) amidrazone and sodium carbonate (1.6 g, 15 mmol) were placed in a dry flask. Dry
dimethylacetamide (DMAA) (15 mL) and dry THF (5 mL) were added, yielding a pale
yellow suspension that was cooled to 0◦C. In another dry flask, methoxyacetyl chloride
(1.6g, 15 mmol) was dissolved in 5ml of DMAA. This solution was then added to precooled
amidrazone mixture dropwise, which caused it to turn bright yellow. The mixture was slowly
warmed to room temperature and stirred for additional 5 h, yielding a thick yellow mixture.
The products were filtered, the solid was washed with water and EtOH, and the resulting
pale yellow solid was allowed to air dry [8].
Synthesis of Iridium Complexes
Tris-(2-(5-(methoxymethyl)-2H-1,2,4-triazol-3-yl)pyridinato)iridium
(Ir(trzl-CH2
OMe)3). The homoleptic complex was prepared from the reaction of Ir(acac)3 and an

158/[360]
H. R. Park et al.
excess of trzl-CH2OMe by a reported procedure[9]. Ir(acac)3 (1.22 g, 2.5 mmol) and
trzl-CH2OMe (2.07 g, 10 mmol) were dissolved in 50 ml of ethylene glycol and the
mixture was refluxed for 24 hr. After cooling, 1 N HCl solution was added and the resulting
precipitate was filtered off. The residue was purified by silica gel chromatography by using
CH2Cl2. Yield: 18%.
[Ir(trzl-CH2OMe)3]: Yield: 29%. FAB-MS: calculated 760; found 382(M-2∗trzl-
CH2OMe).
Ir(trzl-CH2OMe)2(acac) and Ir(trzl-CH2OMe)2(pic). The heteroleptic iridium complexes
with an ancillary ligand, acetylacetonate (acac) or a picolinate (pic) was prepared from
the reaction of the cyclometalated Ir(III) µ-chloro-bridged dimer, (trzl-CH2OMe)2Ir(µ-
Cl)2Ir(trzl-CH2OMe)2, with the corresponding ancillary ligand, according to the method
reported by Nonoyama with slight modification[10]. Yield: 19∼27%.
[Ir(trzl-CH2OMe)2(acac)]: Yield: 25%. FAB-MS: calculated 669; found 669.
Measurements
UV-visible absorption spectra were obtained from Hewlett Packard 8425A spectrometer.
Photoluminescence (PL) spectra were measured on a Perkin Elmer LS 50B spectrometer.
UV-Vis and PL spectra of iridium complexes were measured in 10−5 M dilute CH2Cl2 solu-
tion. Cyclic voltammograms were obtained at scan rate of 100mV/s with Electrochemical
Analyzer of CH Instruments, and tetrabutylammonium hexafluorophosphate was added as
an electrolyte in CH2Cl2 solution.
Results and Discussion
The synthesis of the main ligand, trzl-CH2OMe, was straightforward. The intermediate and
the ligand were subject to immediate use due to its instability in air. Its homoleptic complex,
Ir(trzl-CH2OMe)3, and the heteroleptic iridium complexes, Ir(trzl-CH2OMe)2(acac) and
Ir(trzl-CH2OMe)2(pic), were prepared according to the reported procedures [1,10]. The
reaction schemes are illustrated in Fig. 1. The overall yields of the iridium complexes were
rather low presumably due to other possible coordination of the ligand to the iridium center.
We attempted to control the band gap by lowering HOMO energy levels of the main
ligand through the change of π conjugation length and introduction of an electron withdraw-
ing nitrogen atom on the appropriate position of the aromatic ring. In addition, picolinate
(pic) which was known to be an ancillary ligand for the blue emission as well as acetylace-
tonate (acac), a typical chelating ancillary ligand, was introduced to the synthesis of the
heteroleptic iridium complexes.
The UV-Vis absorption spectra of the homoleptic and heteroleptic complexes were
compared, as shown in Fig. 2. The absorption spectra of these complexes have intense
bands appearing in the ultraviolet part of the spectrum between 230 and 300 nm. These
1
bands have been assigned to the spin-allowed (π → π∗) transitions of the ligands. The
1(π → π∗) bands are accompanied by weaker and lower energy features extending
from 300 to 400 nm that have been assigned to both allowed and spin-forbidden MLCT
transitions[11]. The absorption patterns of Ir(trzl-CH2OMe)3 and Ir(trzl-CH2OMe)2(pic)
are similar in the MLCT region with weak absorption intensities, but that of Ir(trzl-
CH2OMe)2(acac) does not show distinguishable absorption in the MLCT region.
The photoluminescence (PL) spectra of the iridium complexes prepared in this study
were shown in Fig. 3. The photoluminescence spectra of the complexes have very similar

The Blue Phosphorescent Iridium Complexes
[361]/159
(1) The main ligand, trzl-CH2OMe
O
O
N
Cl
N
H2N-NH2
EtOH
N
Na2CO3
DMAA, THF
HN
N
HN
NH
CN
N
NH2
O
(2) the ancillary ligands (AL), (AL= pic, acac)
O
O
O
N
OH
,
(3) The iridium complexes
- homoleptic complex
N
Ir
C
N
C
C^N
Ir(acac)3
ethylene glycol 50ml
N
C
C^N =
N
HN
N
N
O
-heteroleptic complex
Cl
Cl
ancillary ligands
L
(L)
2Ir(AL)
Ir(L)2
(L)
2 Ir
IrCl3nH2O
reflux
O
L =
Ancillary ligands(AL) =
O
O
N
N
OH
,
N
NH
N
O
Figure 1. The synthesis of trzl-CH2OMe, and its iridium complexes.

160/[362]
H. R. Park et al.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Ir(trzl-CH2OMe)3
Ir(trzl-CH2OMe)2(acac)
Ir(trzl-CH2OMe)2(pic)
300
400
500
Wavelength (nm)
Figure 2. UV-Vis absorption spectra of the homoleptic and heteroleptic iridium complexes of trzl-
CH2OMe.
patterns with 80–100 nm of full-width at half maxima (FWHM) which is related to color
purity. Ir(trzl-CH2OMe)3, Ir(trzl-CH2OMe)2(acac) and Ir(trzl-CH2OMe)2(pic) exhibited
the blue PL with the emission peaks at 445, 460 and 472 nm, respectively. The lumines-
cence maxima of homoleptic Ir(trzl-CH2OMe)3 underwent the most blue-shift among the
1.0
Ir(trzl-CH2OMe)3
Ir(trzl-CH2OMe)2(acac)
Ir(trzl-CH2OMe)2(pic)
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure 3. PL spectra of the homoleptic and heteroleptic iridium complexes of trzl-CH2OMe.

The Blue Phosphorescent Iridium Complexes
[363]/161
Table 1. Physical parameters for the iridium complexes
Sample
Eox/Va
0.696
HOMO/eVb
LUMO/eVc
ꢀE/eVd
3.125
Ir(trzl-CH2OMe)3
−5.496
−2.371
ascan rate: 100 mV/s, Electrolyte: tetrabutylammonium hexafluorophosphate. The potentials are
quoted against the internal ferrocene standard. bDeduced from the equation HOMO = −4.8 − Eox.
cCalculated from the optical edge ꢀE = LUMO − HOMO.
iridium complexes of trzl-CH2OMe prepared herein. Thus, the iridium complex contain-
ing trzl-CH2OMe as both a main ligand and an ancillary ligand showed the deepest blue
phosphorescence with high color purity.
We also investigated electrochemical properties of the iridium complexes with the
cyclic voltammetry(CV) which could reveal their positions of the HOMOs and LUMOs
[12,13]. The oxidation potential which indicates the HOMO of Ir(trzl-CH2OMe)3 was
irreversible at 0.69 V relative to an internal ferrocenium/ferrocene reference (Fc+/Fc). The
LUMOs of the complexes were estimated from their respective absorption spectra and
electrochemical data, using the optical edge and band gap equation (ꢀE = Eox − Ered). The
band gap of Ir(trzl-CH2OMe)3 was estimated to be 3.13 eV, and its reduction potential was
calculated to be −2.37 eV. The detailed CV data of the homoleptic iridium complex was
summarized in Table 1.
According to our previous study [7], the iridium complex of F2-ppy main ligand
in which trzl-CH2OMe was introduced as an ancillary ligand showed the blue-shifted
emission, compared to the complexes containing pic or acac as an ancillary ligand. In this
study, we also observed the hypsochromic shift in the luminescence of the homoleptic
complex containing trzl-CH2OMe ligands. The ancillary ligands, acac and pic, are not
luminescent ligands while trzl-CH2OMe can be luminescent itself. Thus, the complexes
containing acac and pic ligands cannot significantly influence luminescence color, just
leading to the minor tuning in the blue emission of their complexes. On the other hand,
trzl-CH2OMe is a blue-luminescent ligand and trzl-CH2OMe could cause relative blue-shift
in the luminescence of its complex.
In fact, we were able to demonstrate the most blue-shifted luminescence spectra of
the homoleptic complex, Ir(trzl-CH2OMe)3, as expected. Therefore, we have shown that
the luminescence properties of the iridium complexes can be tuned to achieve deep blue
emission with high color purity by employing the pyridyltriazole derivative not only as a
main ligand but also as an ancillary ligand.
Conclusions
We reported the synthesis of the new homoleptic and heteroleptic iridium(III) complexes co-
ordinated with an electron withdrawing pyridyltriazole derivative, trzl-CH2OMe, as a main
ligand. The iridium complexes, Ir(trzl-CH2OMe)3, Ir(trzl-CH2OMe)2(acac) and Ir(trzl-
CH2OMe)2(pic), prepared in this study exhibited the blue emission at 445, 460 and 472 nm,
respectively. We found that the homoleptic iridium complex containing the trzl-CH2OMe
exhibited the more hypsochromic shift than the heteroleptic complexes of trzl-CH2OMe
containing acac or pic ligand, displaying deep blue phosphorescence with narrow color
range. Though the other ancillary ligands, pic and acac, did not show a significant effect on

162/[364]
H. R. Park et al.
the modulating the emission wavelengths of their iridium complexes, they could also influ-
ence the minor tuning in the blue photoluminescence of their complexes. The photophysical
and electrochemical properties of the complexes supported that deep blue phosphorescence
could be exhibited by the Ir complexes chelated with trzl-CH2OMe.
Acknowledgment
This work was supported by National Research Foundation of Korea Grant funded by the
Korean Government (NO. 2011-0003765).
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