Preparation of blue-emitting phosphorescent iridium(III) complex under ultrasound reaction

Article abstract of DOI:10.1080/15421400802618539

F2MeIrpic (Iridium(III)bis[2-(2',4'-difluorophenyl)-4-methyl pyridinato-N,C2'] picolinate) was synthesized to develop a blue-emitting phosphorescent Ir(III) complex. The synthesized F2MeIrpic showed sky blue light with the maximum PL peak at 470nm. Genera

Full text of DOI:10.1080/15421400802618539

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Preparation of Blue-Emitting
Phosphorescent Iridium(III)
Complex Under Ultrasound
Reaction
Hong Jeong Yu a , Kwanhwi Park a & Sung Hyun Kim a
a Department of Chemical and Biological
Engineering , Korea University, Sungbuk-Gu , Seoul,
Korea
Published online: 18 Mar 2009.
To cite this article: Hong Jeong Yu , Kwanhwi Park & Sung Hyun Kim (2009)
Preparation of Blue-Emitting Phosphorescent Iridium(III) Complex Under Ultrasound
Reaction, Molecular Crystals and Liquid Crystals, 499:1, 26/[348]-37/[359], DOI:
10.1080/15421400802618539
To link to this article: http://dx.doi.org/10.1080/15421400802618539
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Mol. Cryst. Liq. Cryst., Vol. 499, pp. 26=[348]–37=[359], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802618539
Preparation of Blue-Emitting Phosphorescent Iridium(III)
Complex Under Ultrasound Reaction
Hong Jeong Yu, Kwanhwi Park, and Sung Hyun Kim
Department of Chemical and Biological Engineering, Korea University,
Sungbuk-Gu, Seoul, Korea
0
F2MeIrpic (Iridium(III)bis[2-(20,40-difluorophenyl)-4-methyl pyridinato-N,C2 ]
picolinate) was synthesized to develop a blue-emitting phosphorescent Ir(III) com-
plex. The synthesized F2MeIrpic showed sky blue light with the maximum PL peak
at 470 nm. Generally the reaction time and maximum yield of F2MeIrpic were
known to be 78 hours and 28%, respectively. The reaction is composed of three
steps which are the synthesis of ligand (F2Meppy), Ir-dimer and F2MeIrpic with
Ir-dimer and picolinic acid. The synthesis of F2MeIrpic is a rate determining step.
The solubility and dispersibility of Ir-dimer in solvent are important reaction
parameters which determine reaction time and yield, because Ir-dimer is not sol-
uble in almost all solvent. Among various solvents Tetrahydrofuran(THF) was the
best solvent to solve Ir-dimer. Ir-dimer was dispersed well by an ultra-sonication
in the solvent during reaction. The reaction time was reduced to 36 hours and yield
was enhanced to 45% by ultra sound in THF.
Keywords: blue-emitting; Iridium(III); organic light-emitting diode; phosphorescent
material; ultrasound
INTRODUCTION
Blue emission in the organic light-emitting device (OLED) is an impor-
tant key in the full-color flat-panel displays, because blue is one of
three primary colors [1]. The main obstacle to its application in dis-
plays or monitors is still their lack of stability and efficiency [2]. One
of the key developments in sciences and technology for efficiency of
OLED is the discovery of electro phosphorescence which lifts the upper
This work was supported by Industrial-Educational Cooperation Program between
Korea University and LG Display.
Address correspondence to Sung Hyun Kim, Department of Chemical and Biological
Engineering, Korea University, 1 Anam-Dong, Sungbuk-Gu, Seoul 136-701, Korea
(ROK). E-mail: kimsh@korea.ac.kr
26=[348]

Preparation of Blue-Emitting Ir (III) Complex Under Ultrasound
27=[349]
limit of the internal quantum efficiency of the usual fluorescent
dopant-based devices from 25% to nearly 100%. Phosphorescence is
inherently a slower and less efficient process, but triplet states consti-
tute the majority of electro generated excited states (75%), so the
successful utilization of the triplet manifold to produce light
should undoubtedly increase the overall luminance [3]. Princeton
University group and Southern California group have demonstrated
a way to break through this efficiency limitation by using certain
organic materials which emit intense phosphorescence from triplet
states at room temperature [4].
Synthesis of heavy metal complexes such as Ir(III) [5,6,7,8] and
Pt(II) [9] complexes, which are used as phosphorescent dopants, has
a great attention because of their potential applications in the organic
light emitting devices (OLEDs) [12]. Among the complexes, F2Iracac
and F2Irpic (F2 ¼ 2-(20, 40-difluorophenyl) pyridine; acacH ¼ 2,4-
pentanedione; picH ¼ 2-picolinic acid) [5] are good examples for
phosphorescent blue-emitting materials.
Recently, F2MeIrpic (Iridium(III)bis[2-(20,40-difluorophenyl)-4-methyl
0
pyridinato-N,C2 ]picolinate) which shows sky blue light with the
maximum PL peak at 470 nm was synthesized for phosphorescent
dopant [10]. It was reported that the reaction time and maximum
yield of F2MeIrpic were generally long reaction time of 78 hours
and poor yield of 28%, respectively [10]. Especially in the synthesis
of iridium based metal complex, general precursor, IrCl3 ꢀ 3H2O and
Ir-dimmer are poor soluble and not dispersed in most of solvents.
So those are reasons of long reaction time and poor yield. As we
know the ultrasound chemical reactions are very efficient mixing
methods for high yields and short reaction times in many different
types of organic reactions compared with conventional methods [11].
In this paper we report a preparation of F2MeIrpic under ultrasound
condition and compare it with conventional method [10]. The effect of
the ultrasound and other reaction conditions were investigated to obtain
the optimum reaction conditions for the preparation of F2MeIrpic.
EXPERIMENTAL
Experimental Setup
The experimental setup for the preparation of F2MeIrpic under ultra-
sound condition is shown schematically in Figure 1. Volume of flask
reactor was 250 ml and reactor was made by Pyrex glass. The reactor
is placed in silicon oil batch and ultrasound reached to reactants
through silicon oil.

28=[350]
H. J. Yu et al.
FIGURE 1 Diagram of flask reactor assisted by ultrasound : (1) N2 purging;
(2) thermostatic bath; (3) flask reactor; (4) ultrasonic probe; (5) condenser;
(6) magnetic bar.
Synthesis
2-[2,4-difluorophenyl]-4-methylpyridine, [(F2Meppy)2IrCl]2 and F2MeIrpic
were prepared by the following methods described in the literature
[10,12]. The synthetic routes of the ligands and the complexes consist
of three steps as shown in Scheme 1. The reactions were operated
under conventional and ultrasound condition.
Synthesis of F2Meppy, (2-[2,4-difluorophenyl]-4-methylpyridine)
(Step 1)
10 mmol of 2-chloro-4-methyl pyridine, 12 mmol of 2,4-difluorophenyl-
boronic acid, and 1 mmol of triphenylphosphine were dissolved in
1,2-dimethoxyethane (50 ml). 50 ml of 2 M K2 CO3 aqueous solution
was added and the mixture was purged with nitrogen gas. 0.25 g of
palladium acetate was added and the mixture was refluxed for 18 h.
The two phases were then separated and the aqueous phase was
extracted with ethyl acetate. The combined organic phases were
washed with water and brine, successively. After evaporation of the
solvent, the pure product was obtained by column chromatography.
1H NMR (300 MHz, CDCl3), ppm: 8.51 (d, J ¼ 5.7 Hz, 1H), 7.98
(m, 1H), 7.26 (t, J ¼ 2.1 Hz, 1H), 6.94 (m, 2H), 6.78 (dd, J ¼ 2.4,
5.7 Hz, 1H), 3.87 (s, 3H).

Preparation of Blue-Emitting Ir (III) Complex Under Ultrasound
29=[351]
SCHEME 1 The reaction route for the synthesis of F2MeIrpic.
Synthesis of Ir-dimer, ([(F2Meppy)2IrCl]2) (Step 2)
A solution of IrCl3 ꢀ 3H2O (1 mmol) and 2-(20,40-difluorophenyl)-4-
methylpyridine (3 mmol) in 2-ethoxyethanol (30 ml) was refluxed for
24 h under nitrogen. The pale green mixture was cooled to room tem-
perature and 100 ml of 1 M HCl was added to precipitate the product.
The mixture was filtered and washed with 100 ml of 1 M HCl followed
by 50 ml of water, and then dried. The product was obtained as a light
green powder.
1H NMR (300 MHz, CDCl3), ppm: 8.91 (d, J ¼ 6.6 Hz, 4H), 7.80
(s, 4H), 6.43 (m, 4H), 6.31 (t, J ¼ 9.9 Hz, 4H), 5.37 (d, J ¼ 9.0 Hz, 4H),
4.01 (s, 12H).
Synthesis of F2MeIrpic, (Iridium(III)bis[2-(20,40-difluorophenyl)-
0
4-methyl pyridinato-N,C2 ]picolinate) (Step 3)
2.2 mmol of picolinic acid was added to a room temperature solution
of 0.8 mmol of [(F2Meppy)2IrCl]2 in 60 ml of THF(Tetrahydrofuran)
and 2 mmol Ag(OCOCF3) was added. The mixture was heated to
reflux under nitrogen in a silicon oil bath for 48 hours. The reaction
mixture was cooled to room temperature and the pale yellow precipi-
tate was filtered off. The pure product was obtained by a flash
chromatography.

30=[352]
H. J. Yu et al.
1H NMR (300 MHz, CDCl3): ppm 8.48 (d, J ¼ 6.9 Hz, 1H), 8.32 (d,
J ¼ 7.2 Hz, 1H), 7.92 (t, J ¼ 6.6 Hz, 1H), 7.76 (m, 3H), 7.40 (td,
J ¼ 1.5, 5.7 Hz, 1H), 7.17 (d, J ¼ 6.6 Hz, 1H), 6.73 (dd, J ¼ 2.7, 6.9 Hz,
1H), 6.53 (dd, J ¼ 2.7, 6.6 Hz, 1H), 6.42 (m, 2H), 5.88 (dd, J ¼ 2.3,
8.7 Hz, 1H), 5.64 (dd, J ¼ 2.6, 8.7 Hz, 1H), 4.06 (s, 6H).
Fabrication and Testing of OLED
30 nm of PEDOT:PSS as the hole transporting layer and 30 nm of the
complex doped (7%) PVK as the emitting layer were spin coated
sequentially onto 200 nm ITO substrate. In a vacuum chamber at a
pressure of 10ꢀ6 Torr 10 nm of 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (BCP) as a hole blocking layer, 30 nm of Alq3 as
the electron transport layer; and a cathode composed of 1 nm lithium
fluoride and 120 nm aluminum were sequentially deposited onto
the spin coated substrate to give the device structure. The current –
voltage (I–V) profiles and light intensity characteristics for the
above-fabricated devices were measured in glove box under nitrogen
condition at ambient temperature using a Keithley 2400 Source
Meter=2000 Multimeter coupled to a PR 650 Optical Meter.
RESULTS AND DISCUSSION
Optical Properties of F2MeIrpic and Performance
of OLED Device
UV-Vis spectra and PL spectra of F2MeIrpic under conventional and
ultrasound condition were shown in Figures 2 and 3. As shown in
Figures 2 and 3, the strong absorption peaks are at 375 and 450 nm
and the PL spectrum of synthesized F2MeIrpic shows emission bands
at 468 and 492 nm as same as those of reference [10]. It was clear that
no change was carried out on optical properties under ultrasound
condition compared with conventional method.
An electroluminescent device was fabricated with structure ITO
(200 nm)=PEDOT:PSS (30 nm)=PVK þ 7% Ir-compex (30 nm)=BCP
(10 nm)=Alq3 (30 nm)=LiF (1 nm)=Al (120 nm). EL device structure is
shown in Scheme 2. Figure 4 shows the EL spectrum with a maximum
at the peak wavelength of kmax ¼ 484 nm and additional subpeaks at
ksub ¼ 502 nm and 540 nm which generally agrees with the PL spectral
shape. The device of F2MeIrpic shows sky blue light because of two
additional subpeaks. This device shows a luminance value of 300 cd=m2
and luminance efficiency of 1.45 cd=A at 20 mA=cm2 current density

Preparation of Blue-Emitting Ir (III) Complex Under Ultrasound
31=[353]
FIGURE 2 UV–Vis absorption spectra for F2MeIrpic synthesized under
conventional and ultrasound condition in CH2Cl2.
FIGURE 3 Room temperature solution PL spectra for F2MeIrpic synthesized
under conventional and ultrasound condition in CH2Cl2.

32=[354]
H. J. Yu et al.
SCHEME 2 El device structure of F2MeIrpic.
and 6.4 V voltage. Luminance, current density and luminance
efficiency are plotted as functions of voltage and current density in
Figures 5 and 6.
FIGURE 4 Electroluminescence spectra of the following OLED structure:
ITO (200 nm)=PEDOT:PSS (30 nm)=PVK þ 7% Ir-complex (30 nm)=BCP
(10 nm)=Alq3 (30 nm)=LiF (1 nm)=Al (120 nm). The EL spectrum has a
maximum at the peak wavelength of kmax ¼ 484 nm and additional subpeaks
at ksub ¼ 502 nm and 540 nm which agrees with the PL spectral shape.

Preparation of Blue-Emitting Ir (III) Complex Under Ultrasound
33=[355]
FIGURE 5 Comparative plot of current density, voltage and luminance
characteristics for the device.
Effect of Ultrasound
Table 1 shows the reaction yield of each step under ultrasound
condition compared with conventional method. The reaction yield
FIGURE 6 Plot of luminescent efficiency vs. current density of device.

34=[356]
H. J. Yu et al.
TABLE 1 Effect of Ultrasound Condition on Each Reaction Step
Temperature
(ꢁC)
Conversion
Selectivity
Yield
(%)
Step
Condition
Time (h)
(%)
(%)
1
2
3
Ultrasound
Conventional
Ultrasound
Conventional
Ultrasound
Conventional
70
70
175
175
175
175
12
18
12
12
12
48
80.1
98.32
94.81
84.32
76.82
93.71
97.83
78.75
75.65
67.17
43.48
74.50
50.96
79.80
79.66
65.66
79.51
52.10
was enhanced by using ultrasound. As shown in Table 1, reaction
times of step 1 and 3 were shortened by 6 h at same conversion. In
addition, the reaction yields of step 2 and 3 were enhanced to 67%
and 74%. Figure 7 shows a relationship between conversion of step
1 and reaction time. The maximum conversion of step 1 in Figure 7
is not changed at both conditions, but the reaction time for maximum
conversion of step 1 in ultrasound condition is faster than in conven-
tional condition.
A possible reason is that ultrasound can make reactants contact
with reaction site and products move out from reaction site fast and
FIGURE 7 Relationship between yield and time under conventional and
ultrasound condition in step 1 reaction.

Preparation of Blue-Emitting Ir (III) Complex Under Ultrasound
35=[357]
efficiently. Especially for the step 2 and 3 reactions, the reactants of
IrCl3 ꢀ H2O and Ir-dimmer are not soluble in most of the solvents.
Ultrasound can help to disperse non-soluble reactants well in solvent
and replace product which have already reacted [13]. Another possible
explanation is that ultrasound can break iridium-chloride bond more
þ
easily and make Ir-dimer produce cation (F2MeIr ). Higher yield of
step 3 reaction can be obtained by combination of interaction between
Agþ and Clꢂ and ultrasound, because the break of iridium-chloride
bond and formation of cation was occurred by only interaction between
Agþ and Clꢂ in conventional method.
Effect of Reaction Temperature
Figure 8 shows a conversion of each reaction step depending on differ-
ent temperatures. Each reaction reached higher conversion with
increase of temperature. This result explains that these reactions
are endothermic and the maximum temperatures of each reaction step
are 78ꢁC, 176ꢁC and 176ꢁC. These temperatures are boiling points of
solvents used.
FIGURE 8 Relationship between conversion and reaction temperature in
each reaction step.

36=[358]
H. J. Yu et al.
FIGURE 9 Relationship between n (IrCl3 ꢀ 3H2O)=n (F2Meppy) and conversion
or selectivity.
Effect of IrCl3 ꢀ 3H2O versus F2Meppy Molar Ratio
For step 2 reaction, an ideal schiometric ratio of IrCl3 ꢀ 3H2O to
F2Meppy is 1:2. But the n (IrCl3 ꢀ 3H2O)=n (F2Meppy) is generally
reported 0.3 for higher conversion [10]. The influence of
n
(IrCl3 ꢀ 3H2O)=n (F2Meppy) on the conversion and selectivity of Ir-
dimer under the standard reaction conditions was shown in Figure 9.
It was clear that the conversion of IrCl3 ꢀ 3H2O increased with dec-
reasing n (IrCl3 ꢀ 3H2O)=n (F2Meppy) ratio from 0.5 to 0.2, whereas
the selectivity of Ir-dimer decreased. The yield was 51.9% at 0.44 of
n (IrCl3 ꢀ 3H2O)=n (F2Meppy) and that was maximum yield. Further-
more two reactants of IrCl3 ꢀ 3H2O and F2Meppy were too expensive
or took much time to prepare generally. These results are expected
that the synthesis of Iridium complex under ultrasound reaction is
beneficial for research and industry with its good efficiency.
CONCLUSIONS
F2MeIrpic of sky blue light with the maximum PL peak at 470 nm was
synthesized successfully under conventional and ultrasound con-
ditions. EL performance of a fabricated OLED with F2MeIrpic shows
a maximum at the peak wavelength of kmax ¼ 484 nm and this result
is reasonable for a blue emitting phosphorescent material. In the

Preparation of Blue-Emitting Ir (III) Complex Under Ultrasound
37=[359]
synthesis of F2MeIrpic, the reaction time and yield are long and poor,
because reactants, IrCl3 ꢀ 3H2O and Ir-dimer, are not soluble in most of
solvents. Higher yield was obtained under ultrasound condition. The
ultrasound could help to disperse non-soluble reactants well in solvent
and replace product which have already reacted. The total reaction
yield was enhanced to 45% and the total reaction time was shortened
to 36 hours under ultrasound condition. Other optimized operation
conditions were the reaction temperatures of 78ꢁC, 176ꢁC, and
176ꢁC, and the molar ratio (IrCl3 ꢀ 3H2O=F2Meppy) of 0.44.
REFERENCES
[1] Burrows, P. E., Bulovic, G. Gu, V., Shen, Z., Forrest, S. R., & Thompson, M. E.
(1997). IEEE Trans. Electron., 44, 1188.
[2] Wang, S., Zenga, P. J., Liu, Y. Q., Yu, G., Sun, X. B., Niu, H. B., & Zhu, D. B. (2005).
Synth. Met., 150, 33.
[3] Hung, L. S. & Chen, C. H. (2002). Mat. Sci. Eng. R., 39, 143.
[4] Baldo, M. A., Lamansky, S., Burrows, P. E., Thompson, M. E., & Forrest, S. R.
(1999). Appl. Phys. Lett., 75, 4.
[5] Thompson, M. E., Lamansky, S., Adamovich, V., Djurovich, P. I., Adachi, C.,
Kwong, R., Forrest, S. R., & Baldo, M. A. (2002). Patent US 0182441 A1.
[6] Lamansky, S., Djurovich, P., Murray, 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.
[7] Adachi, C., Kwong, R. C., Djurovich, P., Adamovich, V., Baldo, M. A.,
Thompson, M. E., & Forrest, S. R. (2001). Appl. Phys. Lett., 79, 2082.
[8] Baldo, M. A., Thompson, M. E., & Forrest, S. R. (2000). Nature, 403, 750.
[9] Liu, Q. D., Jia, W. L., Wu, G., & Wang, S. (2003). Organometallics, 22, 3781.
[10] Samsung SDI Co. (2005). Patent KR 0522702.
[11] Boldyrev, V. V. (1995). Ultrason. Sonochem., 2, S143.
[12] Laskar, I. R., Hsu, S., & Chen, T. (2005). Polyhedron, 24, 189.
[13] Zhao, Y., Feng, R., Shi, Y., & Hu, M. (2005). Ultrason. Sonochem., 12, 173.