Effective electrophosphorescence emitting devices by using new type Ir(III) complex with bluky substistuent spacers

Article abstract of DOI:10.1080/15421400500365243

The new green Ir(III) complex, fac -tris[4-methyl-2-(4'-trimethylsilylph-enyl)pyridine] [Ir(msippy) 3 ], is synthesized and phosphorescent light emitting devices fabricated by using them as triplet emissive dopants. Ir(dsippy) 3 in UV-Vis. spectra showed

Full text of DOI:10.1080/15421400500365243

Mol. Cryst. Liq. Cryst., Vol. 444, pp. 95–101, 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500365243
Effective Electrophosphorescence Emitting Devices
by Using New Type Ir(III) Complex with Bluky
Substistuent Spacers
Sung Ouk Jung
Yun-Hi Kim
Hyung-Sun Kim
Soon-Ki Kwon
School of Applied Chemical Engineering & Engineering and
Research Institute of Engineering, Gyeongsang National University,
Jinju, Korea
Hyoung-Yun Oh
LG Electronic Institute of technology, Seoul, Korea
0
The new green Ir(III) complex, fac-tris[4-methyl-2-(4 -trimethylsilylph-enyl)pyri-
dine] [Ir(msippy)
3
], is synthesized and phosphorescent light emitting devices
in UV-Vis. spec-
tra showed absorption band at 395 nm originating from MLCT. The EL devices
with configurationof ITO=CuPc (10 nm)=a-NPD(40 nm)=CBP-Ir(msippy) (20 nm)=
BCP (10 nm)=Alq (40 nm)=LiF (1 nm)=Al (100 nm).showed the emission maximum
fabricated by using them as triplet emissive dopants. Ir(dsippy)
3
1
3
3
3
at 516 nm, due to the radiative decay from MLCT state to ground state. The 6%
doped devices showed the maximum efficiency of 49.9 cd=A and power efficiency
2
5.3 lm=W. The observed PL quantum efficiency (0.43) of the [Ir(msippy)
3 2 2
] in CH Cl
shows slightly higher than that of reported Ir(ppy)
3
(U ¼ 0.40).
Keywords: annihilation; iridium; MLCT; photoluminescence
INTRODUCTION
Electrophosphorescent materials incorporating with heavy metals have
attracted great attention due to their potential application in low-cost
This work was supported by LG Electronics’ national program funded by the Ministry
of Commerce and the Information Technology Research Center (C1090-0401-0026)
under Ministry of Information & Communications.
Address correspondence to Soon-Ki Kwon, School of Applied Chemical Engineering &
Engineering and Research Institute of Engineering, Gyeongsang National University,
Jinju, 660-701, Korea. E-mail: skwon@gsnu.ac.kr
95

9
6
S. O. Jung et al.
full color displays [1]. Among the many different phosphorescent emit-
ter for use in OLED devices, iridium complexes, in particular, have been
described the most effective emitter due to their high photoluminescent
efficiency and relatively short excited state lifetime [2]. The key factor in
these complexes is an effective singlet-triplet mixing caused by the
strong spin-orbit coupling due to heavy atom effect. This leads to large
splitting of the d-orbitals which can induce emission from the low lying
3
metal-to-ligand charge-transfer ( MLCT) state. One of the important
concerns in electrophosphorescent device is to design phosphor in order
to achieve effective energy transfer between host and dye, and thus
development and synthesis of promising phosphorescent materials with
short life times in excited states and high photoluminescent quantum
efficiency have been studied continuously by many research group with
great interest so far. Especially, the study on development of Ir(III)
phosphor having cyclometallated systems because of these reasons.
Luminescnt Ir(ppy) derivatives have been described fine-tuning band
3
gap between HOMO and LOMO energy level through the modification
of ligands, leading to tune emission energy ranging from blue to red
[
3,4]. Which have been established by Thompson and Forrest groups,
emission energy of Ir(ppy) derivatives can be tuned by the introduction
3
of electron donor=acceptor as subunit into 2-phenylpyridine frame-
works [5]. For example, introduction of electron-donating groups to
pyridine ring and electron-withdrawing groups to phenyl ring that
can occurs diminution of HOMO and increasing of LUMO level, respect-
ively, allow to give lager band-gap than that of Ir(ppy) . Although exten-
3
sive research on investigation of luminescent properties of Ir(ppy)3
derivatives, containing two o-chelating ligand and bidentate ancillary
ligand (e.g., acac, tmd, bza, dbm) systems, and use as triplet emitter or
host in EL device performance has been well carried out for several
years, substituent effect depending on position of phenyl or pyridine
rings retaining 2-phenylpyridine backbone remain still scarce. The
present report aims at designing a new tris-orthometalated Ir(III)
complex as a triplet emitter by using modified phenylpyridine system,
0
that is fac-tris [4-methyl-2-(4 -trimethylsilylphenyl)pyridine]Ir(III),
[
Ir(msippy) ]. The introducing of bulky trimethylsilyl substituents is
3
expected to have inhibitors triplet annihilation due to intermolecular
interaction.
EXPERIMENTAL
Synthesis of Ligand msippy
2-bromo-4-methylpyridine (3.7 g, 0.021 mol), 3-trimethylsilylbenz-
neboronic acid (5 g, 0.026 mol), and Pd(PPh ) (0.15 g, 0.6 mol%), were
3
4

Effective Electrophosphorescence Emitting Devices
97
dissolved in THF (45 mL). A solution of 2M-K CO (15 mL) was added,
2
3
and the mixture refluxed with stirring for 18 hr in an atmosphere of
nitrogen. After being cooled, the mixture was poured into aqueous
EDTA and extracted with ether, the organic layer was dried (MgSO4).
The solvent was removed under reduced pressure to give a yellow oil.
The crude product was purified by chromatography on silica gel
(
EtOAc=hexane, 1=5, v=v) to obtain a colorless oil (4.83 g, 93%).
Synthesis of Complex [Ir(msippy)3]
msippy (3 g, 0.012 mol) and Ir(acac) (1.01 g, 2.07 mmol) were dis-
3
solved in glycerol(50 mL), and the mixture was heated to reflux under
nitrogen for 25 h. The reaction mixture was then cooled to room tem-
perature, 1 N HCl was added, and the mixture filtered to give a crude
product, and then flash chromatographed on silica column using
dichloromethane to yield 28%. The compound was then further purified
by sublimation. 1H-NMR (CDCl3): d 7.62 (2H), 7.25 (1H), 6.84 (1H), 6.65
þ
(
1H), 6.55 (1H), 2.35 (3H), 0.01 (9H) ppm. MS (EI): m=z ¼ 913[M ].
Fabrication of Electroluminescent Device
A glass substrate precoated with indium tin oxide (ITO) was
cleaned in an ultrasonic bath of acetone, followed by 2-propanol. Sur-
face treatment was carried out by exposing ITO to a UV ozone plasma.
OLEDs with CBP as host material were fabricated as follows. The
hole-injecting layer, a 10 nm thick film of CuPc was deposited on the
ITO surface by high-vacuum thermal evaporation and a 40 nm thick-
ness a-NPD as hole-transporting layer was deposited onto the CuPc.
An [Ir(msipy) ]-doped host CBP layer was thermally co-evaporated
3
3
FIGURE 1 Reaction Scheme of [Ir(msippy) ].

9
8
S. O. Jung et al.
onto the a-NPD layer. The [Ir(msippy) ] doping concentration was 6%
3
in CBP. A 10 nm thick BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenan-
throline) layer was then deposited as the exciton-blocking layer, fol-
lowed by a 40 nm thick Alq layer as an electron-transporting layer.
3
Finally, LiF (1 nm) and Al (100 nm) were deposited on top of the
organic layers by thermal evaporation. The fabricated multilayer
organic light-emitting devices have the structure of ITO=CuPc
(
(
10 nm)=a -NPD (40 nm)=CBP-Ir(msippy) (20 nm)=BCP (10 nm)=Alq3
40 nm)=LiF (1 nm)=Al (100 nm).
3
RESULTS AND DISCUSSION
The newly desiged [Ir(msippy) ] was successfully synthesized. The
3
obtained material was comfirmed by spectroscopic analysis.
The UV-Vis absorption and PL spectra of [Ir(mppy) ] in degassed
3
dichloromethane solution at room temperature are displayed in
Figure 2. The strong absorption bands around 286 nm can be assigned
ꢀ
to spin allowed p-p transitions on the cyclometalated ligands. The
broad absorption bands at lower energies are typical for spin allowed
1
metal to ligand charge-transfer ( MLCT) transitions, and the weaker
3
FIGURE 2 UV visible and PL spectra of [Ir(msippy) ].

Effective Electrophosphorescence Emitting Devices
99
absorption bands extending into the visible region are assigned to the
3
formally spin-forbidden MLCT transitions (480 nm). The emission of
the complex, centered at 510 nm is due to the radiative decay from
the triplet manifold to the ground state. The observed PL quantum
efficiency (0.43) of the [Ir(msippy) ] in CH Cl shows slightly higher
3
2
2
than that of reported Ir(ppy) (U ¼ 0.40) [6].
3
NPD=CBP-Ir(msippy) =BCP=Alq =LiF-Al
3
3
The EL spectrum of a green phosphorescent device based on [Ir
msippy) ] is shown in Figure 3. The emission peak was located at
(
3
5
16 nm with a sub-peak 541 nm. The EL spectra originating from
the triplet-excited states of the phosphors as shown in Figure 2 is
coincident with this of the phosphors in a dilute solution. This is no
characteristic emission peaks from CBP or Alq , indicating that the
3
emission originates mostly from the dopant. Furthermore, this result
support the proposal that effective energy transfer from the host
(
CBP) to the dopant (Ir complex) occurs in the emissive layer. The
Commission Internationale de L’Eclairage (CIE) coordinates were
0.29, 0.62).
(
3
FIGURE 3 EL spectra of [Ir(msippy) ], device structure: ITO=CuPc.

1
00
S. O. Jung et al.
3
FIGURE 4 Voltage versus current density characteristics of [Ir(msippy) ].
FIGURE 5 Luminance and power efficiency versus current density charac-
teristics of [Ir(msippy) ].
3

Effective Electrophosphorescence Emitting Devices
101
2
The turn-on voltage (defined as 1 mA=cm ) of [Ir(msippy) ] is 7.2 V
3
(
Figure 4). The ionization potential (Ip) level of [Ir(msippy) ] was
3
investigated cyclic voltammetry. The HOMO, LUMO and band gap
of [Ir(msippy) ] was 5.15 eV, 2.67 eV and 2.47 eV, respectively. The
3
2
luminance reached 20540 cd=m at an applied voltage of 12.3 V.
The luminance efficiency and power efficiency of the green device
are shown as a function of current density in Figure 5. The maximum
luminance efficiency was 49.9 cd=A, and the power efficiency was
2
5.3 lm=W.
In conclusion, we have successfully synthesis new ligand, msippy and
have used this ligand to synthesis the iridium complex [Ir(msippy)3].
The EL device based on this complex emit green light with high lumi-
nance and power efficiency. We believe the efficiency can be further
improved by using a more suitable host material for the emitting layer
or a more suitable material for the hole blocking layer [7].
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[
[
[
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[
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