Synthesis and Characterization of New Phosphorescent Heteroleptic Iridium (III) Complex by Using Tetrazole as an Ancillary Ligand for Organic Light-Emitting Diodes

Article abstract of DOI:10.1080/15421406.2015.1095788

The yellow emitting tetrazole based heteroleptic iridium(III) complex, bis(diphenylquinoline)iridium(pyridyltetrazole)[(DPQ)₂Ir(PyTz)], was synthesized and conformed by ¹H-,¹³C NMR spectral techniques. The purity was also

Full text of DOI:10.1080/15421406.2015.1095788

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and Characterization of New
Phosphorescent Heteroleptic Iridium (III) Complex
by Using Tetrazole as an Ancillary Ligand for
Organic Light-Emitting Diodes
Thota Giridhar, Woosum Cho, Yeong-Soon Gal, Jae Wook Lee & Sung-Ho Jin
To cite this article: Thota Giridhar, Woosum Cho, Yeong-Soon Gal, Jae Wook Lee & Sung-Ho
Jin (2015) Synthesis and Characterization of New Phosphorescent Heteroleptic Iridium (III)
Complex by Using Tetrazole as an Ancillary Ligand for Organic Light-Emitting Diodes, Molecular
Crystals and Liquid Crystals, 621:1, 17-25, DOI: 10.1080/15421406.2015.1095788
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1095788
Published online: 16 Dec 2015.
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Date: 01 March 2016, At: 09:44

Mol. Cryst. Liq. Cryst., Vol. 621: pp. 17–25, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1095788
Synthesis and Characterization of New
Phosphorescent Heteroleptic Iridium (III) Complex
by Using Tetrazole as an Ancillary Ligand for
Organic Light-Emitting Diodes
THOTA GIRIDHAR,1 WOOSUM CHO,1 YEONG-SOON GAL,2
JAE WOOK LEE,3,∗ AND SUNG-HO JIN1,∗
1Department of Chemistry Education, Graduate Department of Chemical
Materials, BK 21 PLUS Team for Advanced Chemical Materials, and Institute
for Plastic Information and Energy Materials, Pusan National University, Busan,
Republic of Korea
2Department of Chemistry, Dong-A University, Busan, Republic of Korea
3Polymer Chemistry Lab., Kyungil University, Hayang, Republic of Korea
The yellow emitting tetrazole based heteroleptic iridium(III) complex, bis
(diphenylquinoline)iridium(pyridyltetrazole) [(DPQ)2Ir(PyTz)], was synthesized and
1
conformed by H-,13C NMR spectral techniques. The purity was also confirmed by
HPLC. The thermal, electrochemical, photophysical and electroluminescent properties
were intrinsically investigated. The Ir(III) complex is thermally more stable having
thermal decomposition temperature (Td, 5% weight loss) more than 350◦C and it shows
very high glass transition temperature Tg-233◦C. We have followed the easy and cost
effective solution process for (DPQ)2Ir(PyTz) device fabrication and achieved better
performance yellow phosphorescent organic light-emitting diodes (PhOLEDs), maxi-
mum external quantum efficiency (EQE) of 5.68%, luminance efficiency 12.63 cd/A, and
CIE coordinate of (0.56, 0.43).
Keywords Solution process; tetrazole; yellow PhOLEDs; iridium(III) complex
Introduction
The use of phosphorescent materials has been a major revolution to enhance the electro-
luminescence efficiency due to allow for the simultaneous harvesting of both singlet and
triplet excitons [1]. For getting this phosphorescent materials mainly used third row transi-
tion metal complexes, particularly cyclometalated iridium(III) based complexes have been
actively studied [2]. In cyclometalated Ir(III) complexes the main advantage is to tune the
emission color throughout the visible region by different type of main ligands and ancillary
∗Department of Chemistry Education, Graduate Department of Chemical Materials, BK 21 PLUS
Team for Advanced Chemical Materials, and Institute for Plastic Information and Energy Materials,
Pusan National University, Busan 609-735, Korea, E-mail: shjin@pusan.ac.kr and Jae Wook Lee,
Polymer Chemistry Lab., Kyungil University, Hayang 712-701, Korea, E-mail: jlee@donga.ac.kr.
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
[187]/17

18/[188]
T. Giridhar et al.
ligands [3]. The high quantum yields, short triplet lifetimes, and emission wavelength that
can be tuned from blue to deep red are the added advantages of these cyclometalated Ir(III)
complexes. The cyclometalated Ir(III) complexes are classified in to two types homoleptic
encircled by all same type of ligands and heteroleptic surrounded by two main ligands and
one ancillary ligand [4].
In the case of heteroleptic Ir(III) complexes, the emitting colour can be tuned mainly
by changing the structure of the main ligands, although in some rare cases ancillary ligand
has also an ability to tune the colour [5]. The well-known various ancillary ligands used in
solution process phosphorescent organic light-emitting diodes (PhOLEDs) are acetylace-
tone, picolinic acid, picolinic acid N-oxide and 4-Cl-picolinic acid [6]. Solution processes
technologies such as spin-coating, inkjet and screen printing, represents good alternative for
PhOLEDs fabrication because of their cost competitiveness and ready scalability. Never-
theless, the use of thermal deposition makes the fabrication process relatively complicated,
expensive and maximum wastage of the materials. Thus, much attention in the area of
solution-processed PhOLEDs has been given to prepare as well as optimization of device
architectures.
Particularly, the electron-donating or accepting units in the ligands can tune the color
of the emitters. The yellow emitting Ir(III) complexes have appealed more research interest
due to their utility in the fabrication of two color emitting component for white OLEDs with
blue emitters [7]. In earlier days for preparing white OLEDs people relied on the mixture
of either the three primary colors (red, green and blue) or two complementary colors (blue
and yellow).
The main research related to quinoline-based compounds is used as a dopant or host
materials to produce PhOLEDs with high efficiencies. These compounds have been used
due to their higher electron affinities in opto-electronic materials [8]. Additionally, the
quinolone compounds possess short phosphorescence life times to suppress triplet–triplet
annihilation and stable at high current density, thus improving device quantum efficiencies
[9]. The ease of synthetic variation in tetrazole based ancillary ligand containing Ir(III)
complexes over that of the normal heteroleptic Ir(III) complexes. Using (DPQ)2Ir(PyTz) as
a dopant, we achieved PhOLEDs that emits in the yellow (CIE coordinates (0.64, 0.34)),
very high luminous current efficiency of ∼12.63 cd/A and an external quantum efficiency
(EQE) of ∼5.68%, which is one of the competent current efficiency and EQE of a solution
processed yellow-emitting PhOLEDs.
Experimental
All chemicals and reagents were purchased from Aldrich Chemical Co., and it was used
without further purification. The 5-(2ꢀ-Pyridyl)tetrazole (2) is a known compound and it can
be prepared easily in single step from commercially available 2-cyanopyridine and sodium
azide [10]. The compound 3 was synthesized according to the method in our previous
reports [11]. The UV-visible absorption and the fluorescence spectra were recorded with a
JASCO V-570 and Hitachi F-4500 fluorescence spectrophotometers at room temperature,
respectively. High resolution mass spectra were obtained from the Korea Basic Science
and Institute, Daegu Center (HR-ESI Mass). TGA and DSC thermograms were obtained
with Mettler Toledo TGA/SDTA 851e and DSC 822e analyzer under an N2 atmosphere at a
heating rate of 10 ◦C/min, respectively. 1H- and 13C- NMR spectra were recorded on a Var-
ian Mercury Plus 300 MHz spectrometer in CDCl3 using tetramethylsilane as an internal

New Phosphorescent Heteroleptic Iridium (III) Complex
[189]/19
standard. CV measurements were performed with a CHI 600C potentiostat (CH Instru-
ments), which is equipped with a platinum disc as the working electrode, a platinum wire
as the counter electrode, and a Ag/AgCl as the reference electrode, at a scan rate of 100 mV
s−1 using anhydrous MC and 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as
the solvent and electrolyte, respectively. All electrochemical experiments were carried out
at room temperature. The potentials were referenced to the ferrocene/ferrocenium redox
couple (Fc/Fc+). It was assumed that the redox potential of Fc/Fc+ had an absolute energy
level of -4.8 eV under vacuum.
Synthesis of bis[2,4-diphenylquinoline]iridium(III) [5-(2ꢀ-pyridyl)tetrazolate]
The cyclometalated Ir(III) μ-chloro-bridged dimer (2) (0.50 g, 0.32 mmol) was placed
in a flask and dispersed in dichloromethane and ethanol. 5-(2ꢀ-Pyridyl)tetrazole (0.23 g,
1.60 mmol) was added to the solution, and the mixture was stirred at room temperature for
8 h. After completion of the reaction crude product was washed with deionized water and
extracted with dichloromethane. The solvent was removed under reduced pressure and the
crude product was purified by column chromatography on silica gel using EA/MC/hexane
(2:3:5 v/v) as an eluent to give orange solid (0.28 g, 54%). 1H- NMR (300 MHz, CDCl3):
δ (ppm): 8.11-7.91 (m, 7H), 7.74-7.67 (m, 3H), 7.56 (s, 10H), 7.41-7.39 (d, 1H), 7.26-7.04
(m, 6H), 6.95-6.75 (m, 4H), 6.64-6.62 (d, 1H). 13C-NMR (CDCl3): δ 170.4, 169.3, 163.0,
154.6, 151.4, 151.0, 149.7, 148.5, 147.9, 147.7, 147.5, 146.9, 138.5, 137.4, 136.2, 134.6,
131.5, 130.5, 130.1, 129.9, 129.8, 129.5, 129.3, 129.1, 129.0, 128.9, 127.8, 127.3, 126.9,
126.5, 126.1, 125.8, 125.2, 122.5, 122.4, 117.9, 117.3. HRESI-MS [M+H]+: m/z calcd for
899.05, found 899.05.
PhOLEDs Fabrication and Measurements
The indium tin oxide (ITO) was coated onto the glass substrate (20 ꢀ/squre and
110 nm). The ITO glass was UV-ozone treated for 20 min, the PEDOT:PSS ((poly-
ethylenedioxythiophene)-polystyrenesulfonate (H.C.Starck, ◦Clevios P VP AI4083)) was
spin-cast onto the substrate and immediately baked at 150 C for 20 min on a hot plate
in air condition. The emitting layer solutions composed of dopant and PVK:PBD:SPPO1
(40:40:20 or 50:30:20) blends in chlorobenzene were spin coated on top of the PEDOT:PSS
in a glove box and annealed at 80 ◦C for 30 min. Then, Bphene (20 nm), LiF (1 nm) and
Al (100 nm) were thermally deposited under high vacuum (< 5.0 × 10−6 Torr). The film
thickness was measured with α-Step IQ surface profiler (KLA Tencor, SanJose, CA). To
characterize the performance of PhOLEDs, the current density-voltage-luminance (J-V-L)
characteristic were measured by using a current/voltage source meter (Keithley 236) and
an optical power meter (CS-1000).
Results and Discussion
The target heteroleptic yellow emitting (DPQ)2Ir(PyTz) was synthesized according to the
synthetic route as shown in Scheme 1. The ancillary ligand 5-(2ꢀ-Pyridyl)tetrazole is a
known compound and it can be prepared easily in single step from commercially available
2-cyanopyridine and sodium azide and the compound 2 was synthesized according to the
previous report. The compound 2 can be easily converted to heteroleptic Ir(III) complex
by simple approach rather than sophisticated and high temperature reaction conditions.

20/[190]
T. Giridhar et al.
The ligand and Ir(III) complex are readily soluble in common organic solvents suchlike
methylene chloride (MC), chloroform (CF), and ethylacetate (EA), etc. The chemical
structure of the Ir(III) complexes were confirmed by the 1H-, 13C-NMR and HR-ESI mass
spectral techniques.
Scheme 1. Synthesis of Ir(III) complex.
The excellent thermal stability (Td) with high Tg values allows the preparation of
homogeneous and stable amorphous thin films which is critical factor for the operation of
solution processing PhOLEDs. The thermal stability of (DPQ)2Ir(PyTz) was identified by
thermo gravimetric analysis (TGA) and differential scanning calorimeter (DSC) analysis
under N2 atmosphere at a heating rate of 10 ◦C/min. The thermal decomposition temperature
◦
(Td) of (DPQ)2Ir(PyTz) at 5% mass loss (ꢁT5%) was 354 C and the glass transition
temperature (Tg) was 233 ◦C, respectively. This indicates (DPQ)2Ir(PyTz) has good thermal
stability for evaluation high temperature device studies.
The photophysical properties of (DPQ)2Ir(PyTz) were measured using UV–visible
absorption and photoluminescence (PL) spectra at room temperature in CF solution. The
four absorption maxima of the Ir(III) complex show in the range of 291-496 nm. The higher
energy region below 350 nm absorption bands are corresponding to the spin-allowed 1π-π∗
transitions of the cyclometalated ligands. The singlet metal-to-ligand charge-transitions
(1MLCT), 3MLCT and 3π-π∗ weak spin forbidden absorption bands are located at lower
energy region 400-456 nm. Additionally, the (DPQ)2Ir(PyTz) showed a PL emission
maximum at 578 nm in CF solution. When compared with the other ancillary ligands
like picolinic acid and acetylacetone [(DPQ)2Ir(pic-N-O) and (DPQ)2Ir(acac)] the 5-(2ꢀ-
Pyridyl)tetrazole ancillary ligand shows little hypsochromic shift in PL maxima due to
non-uniform electron density in tetrazole ring.
The electrochemical properties of the (DPQ)2Ir(PyTz) were studied by cyclic voltame-
try (CV) analysis. The onset oxidation potential is located at 1.10 V and the corresponding
highest occupied molecular orbital (HOMO) energy level was found to be -5.48 eV and the
reduction potential located at −1.52 V and the corresponding lowest unoccupied molecular
orbital (LUMO) was found −2.76 eV with ferrocene/ferrocenium as a reference redox
system (4.8 eV below vacuum). The Ir(III) complex showed reversible redox waves over

New Phosphorescent Heteroleptic Iridium (III) Complex
[191]/21
Figure 1. Energy level diagram of the materials.
both the anodic and cathodic ranges. Hence, it suggests that the stable Ir(II) and Ir(IV)
radicals are formed.
To investigate the EL properties of solution-processed PhOLEDs using
(DPQ)2Ir(PyTz) fabricated with the following four configuration devices I, II:
ITO/PEDOT:PSS/PVK (40 wt%):PBD (40 wt%):SPPO1 (20 wt%):(DPQ)2Ir(PyTz)
(8 wt%, device I), (5 wt%, device II)/Bphen/LiF (1 nm)/Al (100 nm); devices III,
IV: ITO/PEDOT:PSS/PVK (50 wt%):PBD (30 wt%):SPPO1 (20 wt%):(DPQ)2Ir(PyTz)
(8 wt%, device III), (5 wt%, device IV)/Bphen/LiF (1 nm)/Al (100 nm). Generally, it is
necessary to select a suitable host for efficient energy transfer and/or charge trapping
from the host to the phosphorescent dopant and to avoid concentration quenching to
maximize the performance of PhOLEDs. One of the most commonly used polymers in
PhOLEDs is poly(N-vinylcarbazole) (PVK) due to its excellent film-forming properties,
wide band gap, high glass transition temperature, and good hole mobility at typical electri-
cal fields. In this work, we have used (DPQ)2Ir(PyTz) into a host polymer matrix of PVK
blended with an electron-transport material, 2-(4-biphenyllyl-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole) (PBD).
The device configuration with the HOMO and LUMO energy levels were calculated
from electrochemical measurements and showed in Figure 1. The energy band gap of
(DPQ)2Ir(PyTz) at 2.6 eV, HOMO 5.3 eV, and LUMO 2.7 eV are between the HOMO
and LUMO energy levels of PVK:PBD:SPPO1, suggesting that dopant can possibly trap
both electrons and holes in PhOLEDs with the device configuration. In view of the HOMO
and LUMO energy levels of PVK and PBD, we attempted to use them as the host- and
electron-transporting materials in the fabrication of PhOLEDs because of their good hole-
and electron transporting ability. These energy band diagrams indicated that the HOMO
and LUMO of PVK is little higher than that of PBD and SPPO1 indicates that the efficient
hole and electron transportation can occur in right direction. This good alignment of
energy level is important to facilitate carrier injection into both the transport molecules and
(DPQ)2Ir(PyTz).

22/[192]
T. Giridhar et al.
Figure 2. Current density-voltage-luminance (J-V-L) characteristics of (DPQ)2Ir(PyTz).
The current density–voltage–luminance (J–V–L) characteristics of (DPQ)2Ir(PyTz)
was investigated in different doping concentration of dopant as well as different ratios of
hole transporting materials (HTM) and shown in Figure 2 and Figure 3 and the values are
depicted in Table 1. However, the dopant at 8% weight ratio performances of PhOLEDs is
better than 5% weight ratio. We found that the performances of PhOLEDs not improved as
the concentration of carrier-transporting material increased but the performances gradually
decreased if the percentage of weight decreases. We expected that there should be an
efficient exothermic energy transfer from the host triplet state to the Ir(III) complexes
triplet state within the emitting layer due to an excellent triplet energy confinement on the
Ir(III) complexes.
Moreover, the SPPO1 serves as carrier block/electron transporting layer effectively
enable electron injection/transport and charge carrier balance within the emitting layer. The
LUMO energy of Ir(III) complex is close to the electron transport layer (ETL) SPPO1, an
Figure 3. Luminance efficiency-luminance-power efficiency characteristics of (DPQ)2Ir(PyTz).

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24/[194]
T. Giridhar et al.
Figure 4. External quantum efficiency (E.Q.E) characteristics of (DPQ)2Ir(PyTz).
important feature that ensures efficient electron injection into both the transport molecules
and the Ir(III) complexes. Additionally the SPPO1 layer acts as a hole blocking layer to pre-
vent frequent consignment of holes. The maximum EL emission peaks of (DPQ)2Ir(PyTz)
is observed at 585 nm, the shift of the exciton recombination zones, the EL spectra of
PhOLEDs display a greater 7 nm red shift than those seen in the PL spectra. The maximum
luminance of PhOLEDs using (DPQ)2Ir(PyTz) is 12.63 cd/A and EQE of 5.68% as shown
in Figure 4 with the CIE coordinates of (0.64, 0.34).
Conclusions
We designed and synthesized phenyl-quinoline and 2-pyridyltetrazole based heteroleptic
Ir(III) complex, [(DPQ)2Ir(PyTz)], as a yellow emitter for OLEDs. It was synthesized using
simple approach rather than using sophisticated and high temperature reaction conditions.
Due to the introduction of 2-pyridyltetrazole as ancillary ligand, the color and perturbation
of electron density was altered significantly. Thus, (DPQ)2Ir(PyTz) complex was shown
external quantum efficiency of 5.68% with CIE coordinates of (0.56, 0.43) and can use as
a key color component for fabricating white PhOLED.
Acknowledgments
This work was supported by a grant from the National Research Foundation of Korea
(NRF) of the Ministry of Science, ICT & Future Planning (MSIP) of Korea (NRF-2011-
0028320) and the Pioneer Research Center Program through the National Research Foun-
dation of Korea funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea
(NRF-2013M3C1A3065522).
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