Orange phosphorescent Ir(III) complexes consisting of substituted 2-phenylbenzothiazole for solution-processed organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2018.05.027

Orange phosphorescent iridium(III) complexes, (DMBT)₂Ir(acac) and (TBT)₂Ir(acac), based on either 2-(3,4-dimethylphenyl)benzo[d]thiazole (DMBT) or 2-(4-(trimethylsilyl)phenyl)benzo[d]thiazole (TBT) as the cyclometalated main ligands

Full text of DOI:10.1016/j.orgel.2018.05.027

Accepted Manuscript
Orange phosphorescent Ir(III) complexes consisting of substituted 2-
phenylbenzothiazole for solution-processed organic light-emitting diodes
Jae-Ho Jang, Hea Jung Park, Jeong Yong Park, Hee Un Kim, Do-Hoon Hwang
PII:
S1566-1199(18)30249-0
10.1016/j.orgel.2018.05.027
ORGELE 4687
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 21 April 2018
Revised Date: 18 May 2018
Accepted Date: 20 May 2018
Please cite this article as: J.-H. Jang, H.J. Park, J.Y. Park, H.U. Kim, D.-H. Hwang, Orange
phosphorescent Ir(III) complexes consisting of substituted 2-phenylbenzothiazole for solution-processed
organic light-emitting diodes, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.05.027.
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Orange Phosphorescent Ir(III) Complexes Consisting of Substituted
2-Phenylbenzothiazole
Light-Emitting Diodes
for
Solution-Processed
Organic
*
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Jae-Ho Jang, Hea Jung Park, Jeong Yong Park, Hee Un Kim, and Do-Hoon Hwang
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan
National University, Busan 609-735, Korea
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*Corresponding author. Tel: +82-51-510-2232; Fax: +82-51-516-7421; E-mail
address: dohoonhwang@pusan.ac.kr (D.-H. Hwang)
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Abstract
Orange phosphorescent iridium(III) complexes, (DMBT) Ir(acac) and (TBT) Ir(acac),
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based
on
either
2-(3,4-dimethylphenyl)benzo[d]thiazole
(DMBT)
or
2-(4-(trimethylsilyl)phenyl)benzo[d]thiazole (TBT) as the cyclometalated main
ligands and acetylacetone (acac) as an ancillary ligand, were synthesized for
solution-processed organic light-emitting diodes (OLEDs). The Ir(III) complex,
(BT) Ir(acac), which consists of 2-phenylbenzo[d]thiazole (BT) as the main ligand
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and acac as the ancillary ligand, was also synthesized as a reference. The
photophysical, electrochemical, and electroluminescent (EL) properties of these Ir(III)
complexes were investigated. The OLED fabricated using (TBT) Ir(acac) exhibited
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significantly improved luminance and external quantum efficiency (EQE) as
self-quenching at high doping concentrations was successfully prevented by
introducing bulky trimethylsilyl groups. The EL spectrum of (TBT) Ir(acac) exhibited
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an emission maximum at 578 nm with a full-width-at-half-maximum of 74 nm and
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Commission Internationale de L’Eclairage coordinates of (0.55, 0.44) at 1000 cd/m .
The device with 7 wt% (TBT) Ir(acac) dopant exhibited a maximum luminance of
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6,837 cd/m , maximum luminous efficiency of 25.26 cd/A, power efficiency of 12.37
lm/W, and EQE of 9.94%.
Keywords: benzothiazole; iridium(III) complex; organic light-emitting diodes;
phosphorescence; bulky substituents
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1. Introduction
In recent years, phosphorescent organic light-emitting diodes (OLEDs) have attracted
considerable attention in academia and industry because of their potential applications
as color emitters in solid-state lighting and full-color flat panel displays.
Phosphorescent OLEDs exhibit many outstanding properties such as low operating
voltages for low power consumption, high brightness, high contrast, and the
possibility to produce thin, large-area, and flexible devices [1,2].
The use of phosphorescent emitting materials such as Ir(III) [3–7], Pt(II) [8,9], Ru(II)
[10–12], and Os(II) [13,14] complexes in OLEDs has significantly improved their
performances because both singlet and triplet excitons can be harvested for light
emission. This is a result of the strong spin-orbit coupling caused by the presence of
heavy metals. Thus, high external quantum efficiencies (EQEs) can be achieved using
efficient phosphorescent materials [15–18]. Several research groups have focused on
the development of phosphorescent Ir(III) complexes because of their thermal
stability, high phosphorescence quantum efficiency, short triplet lifetime, and facile
control of the emission spectrum [3–7, 19].
Among the iridium complexes, (BT) Ir(acac) has been often used as an orange emitter
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of OLEDs, however, it suffers from problems such as poor thermal stability, low
photoluminescence (PL) quantum efficiency (Ф ), and significant concentration
PL
self-quenching at high doping concentrations [20–22]. Structural modifications of
cyclometalated ligands in Ir(III) complexes have been carried out to address these
issues. For instance, the introduction of bulky substituents such as pinene,
trimethylsilyl (TMS), or phenyl on 2-phenylpyridine (ppy) ligands not only prevented
self-quenching at high doping concentrations but also improved the thermal stability
of the devices, thus leading to enhanced device performances [24,25]. The bulky
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substituents in the Ir(III) complex suppressed bimolecular interactions and thus
effectively reduced self-quenching at high doping concentrations.
In this study, we designed new orange phosphorescent iridium(III) complexes,
(DMBT) Ir(acac)
and
(TBT) Ir(acac),
based
on
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2-(3,4-dimethylphenyl)benzo[d]thiazole
(DMBT)
and
2-(4-(trimethylsilyl)phenyl)benzo[d]thiazole (TBT) as cyclometalated main ligands
and acetylacetone (acac) as the ancillary ligand. Both (DMBT) Ir(acac) and
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(TBT) Ir(acac) exhibited improved electroluminescence (EL) performances in
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phosphorescent OLEDs as self-quenching at high doing concentrations was avoided
by the introduction of the bulky dimethyl or TMS groups. Moreover, the dimethyl and
TMS groups on the cyclometalated ligands of (DMBT) Ir(acac) and (TBT) Ir(acac)
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imparted thermal stability and improved solubility in common organic solvents
compared to (BT) Ir(acac). The detailed synthetic routes and chemical structures of
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the heteroleptic iridium(III) complexes are outlined in Scheme 1.
O
H
N
S
1
2
: R1 = H, R2 = H
: R1 = CH , R2 = CH3
R2
3
DMSO
HS
NH2
R1
R1
R2
O
H
N
S
1. t-BuLi, THF
N
S
F
Si
DMSO
2. (CH3)3SiCl
HS
NH2
3
4
F
IrCl3.H2O
Acetylacetone
S
N
S
N
Cl
Cl
N
S
O
O
1
, 2, or 4 2
-Ethoxyethanol / water
Ir
Ir
Na2CO3
Ir
2-Ethoxyethanol
R1
R1
R1
R2
R2
R2
2
2
2
(
(
(
BT) Ir(acac): R1 = H R2 = H
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6
7
: R1 = H, R2 = H
2
,
DMBT) Ir(acac): R1 = CH3, R2 = CH3
: R1 = CH , R2 = CH3
2
3
TBT) Ir(acac): R1 = H, R2 = TMS
: R1 = H, R2 = TMS
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0
1
Scheme 1. Synthesis of (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac).
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2. Materials and methods
2.1. Materials
Iridium chloride trihydrate (IrCl ·3H O), benzaldehyde, 3,4-dimethylbenzaldehyde,
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4-fluorobenzaldehyde, 2-aminobenzenethiol, tert-butyllithium, sodium carbonate, and
chlorotrimethylsilane were purchased from Sigma-Aldrich and Alfa Aesar. All
chemicals were used without further purification.
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2.2. Synthesis of ligand and phosphorescent iridium complex
2.2.1. 2-Phenylbenzo[d]thiazole (1)
Benzaldehyde (3.00 g, 28.3 mmol) and 2-aminobenzenethiol (3.54 g, 28.3 mmol)
were dissolved in DMSO (60 mL) and heated at 120 °C under nitrogen for 24 h. After
cooling to room temperature, the mixture was extracted with dichloromethane/brine.
The combined organic extracts were dried over MgSO , filtered, and concentrated in
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vacuo. The resulting solid was recrystallized in methanol to obtain compound 1 as a
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pale violet solid (4.20 g, 70.4%). H NMR (300 MHz, CDCl ) δ (ppm): 8.08 (m, 3H),
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7.89 (d, 1H, J = 7.5 Hz), 7.48 (m, 4H), 7.36 (dd, 1H, J = 8.4, 7.5 Hz); C NMR (75
MHz, CDCl ) δ (ppm): 168.1, 154.2, 135.0, 133.4, 131.1, 128.8, 127.5, 126.1, 125.1,
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+
123.2, 121.7; MALDI-TOF (M , C H NS): calcd. 211.05, found 211.05.
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2.2.2. 2-(3,4-Dimethylphenyl)benzo[d]thiazole (2)
Compound 2 (4.20 g, 78.5%) was prepared from 3,4-dimethylbenzaldehyde using the
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same procedure as that for the synthesis of 1. H NMR (300 MHz, CDCl ) δ (ppm):
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8.11 (m, 2H), 7.91 (d, 1H, J = 7.5 Hz), 7.50 (m, 4H), 2.08 (s, 3H), 2.01 (s, 3H); C
NMR (75 MHz, CDCl ) δ (ppm): 168.2, 154.0, 138.9, 135.3, 133.5, 131.8, 128.8,
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128.0, 126.3, 125.0, 124.5, 123.0, 121.7, 21.5, 21.3; MALDI-TOF (M , C H NS):
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calcd. 239.08, found 239.10.
2.2.3. 2-(4-Fluorophenyl)benzo[d]thiazole (3)
Compound 3 (3.60 g, 64.9%) was prepared from 4-fluorobenzaldehyde using the
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same procedure as that for the synthesis of compound 1. H NMR (300 MHz, CDCl )
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δ (ppm): 8.07 (d, 1H, J = 8.4 Hz), 7.96 (d, 2H, J = 8.7 Hz), 7.91 (d, 1H, J = 7.5 Hz),
7.63 (d, 2H, J = 8.7 Hz), 7.51 (dd, 1H, J = 8.4, 7.5 Hz), 7.40 (dd, 1H, J = 8.4, 7.5 Hz);
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C NMR (75 MHz, CDCl ) δ (ppm): 166.5, 164.4, 154.0, 136.0, 130.0, 129.6, 126.4,
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125.2, 123.2, 121.7, 116.1; MALDI-TOF (M , C H FNS): calcd. 229.04, found 229.
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03.
2.2.4. 2-(4-(Trimethylsilyl)phenyl)benzo[d]thiazole (4)
2-(4-Fluorophenyl)benzo[d]thiazole (3) (3.5 g, 15.3 mmol) was dissolved in
tetrahydrofuran and stirred at –70 °C. Addition of t-BuLi (10.6 mL, 23.0 mmol, 1.7 M
in pentane) over 30 min resulted in a red solution. After stirring for 1 h at this
temperature, chlorotrimethylsilane (2.3 mL, 18.1 mmol) was added into the reaction
mixture and the solution was stirred for an additional hour. The mixture was then
allowed to warm to room temperature and stirred for 4 h. Subsequently, the mixture
was extracted with ethyl acetate/brine and the combined organic extracts were dried
over MgSO . After filtration, the solvent was removed under reduced pressure and the
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resulting liquid was purified by column chromatography on silica gel (ethyl
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acetate/hexane = 1/7) to obtain compound 4 as a pale yellow liquid (2.73 g, 63%). H
NMR (300 MHz, CDCl ) δ (ppm): 8.10 (m, 3H), 7.90 (d, 1H, J = 8.4 Hz), 7.67 (d,
3
2H, J = 7.5 Hz ), 7.50 (dd, 1H, J = 8.4, 7.5 Hz), 7.40 (dd, 1H, J = 8.4, 7.5 Hz), 0.34 (s,
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9H); C NMR (75 MHz, CDCl ) δ (ppm): 168.0, 154.6, 154.2, 135.2, 131.0, 127.4,
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126.2, 125.7, 125.1, 123.3, 121.4, 5.82; MALDI-TOF (M , C H NSSi): calcd.
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283.09, found 283.07.
2.2.5. Synthesis of Ir(III) µ–chloro-bridged dimer complexes (5–7)
Typical procedure: A mixture of iridium(III) chloride trihydrate (0.50 g, 1.7 mmol)
and 2-phenylbenzo[d]thiazole (1) (0.85 g, 4.0 mmol) in 2-ethoxyethanol/water (40
mL, 3/1, v/v) was refluxed under nitrogen for 18 h at 120 °C. After cooling to room
temperature, the precipitate was collected and washed several times with water and
petroleum ether to afford a red powder. The solid was dried in vacuo to obtain the
iridium(III) µ-chloro-bridged dimer 5 in 77% yield. The other dimers, complexes 6
(64%) and 7 (59%), were prepared from the corresponding 2-phenylbenzothiazole
derivatives 2 and 4, respectively, by following the same procedure. The crude Ir(III)
µ-chloro-bridged dimers were used directly in the next step without further
purification.
2.2.6. Synthesis of Ir(III) complexes
Typical procedure: A mixture of iridium(III) µ-chloro-bridged dimer 5 (0.5 g, 0.39
mmol), 2,4-pentadione (0.15 g, 1.5 mmol), and sodium carbonate (0.38 g, 3.6 mmol)
was heated at 135 °C in 2-ethoxyethanol for 24 h under nitrogen atmosphere. After
cooling to room temperature, the solvent was removed under reduced pressure. The
mixture was then extracted with dichloromethane. The organic extract was dried over
anhydrous MgSO and the filtrate was evaporated in vacuo. The resulting solid was
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purified by column chromatography on silica gel using ethyl acetate/hexane (1/5, v/v)
as the eluent. The resulting solid was recrystallized from dichloromethane/ hexane to
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obtain (BT) Ir(acac) as a red solid (0.35 g, 64%). (DMBT) Ir(acac) and
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(TBT) Ir(acac) were prepared from the corresponding Ir(III) µ-chloro-bridged dimers
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6 and 7, respectively, by following the same procedure.
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(BT) Ir(acac) (64%): H NMR (300 MHz, CDCl ) δ (ppm): 8.08 (m, 2H), 7.90 (m,
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2H), 7.64 (d, 2H, J = 7.5 Hz), 7.43 (m, 4H), 6.85 (dd, 2H, J = 7.5, 7.4 Hz), 6.63 (dd,
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2H, J = 7.8, 7.4 Hz), 6.40 (d, 2H, J = 7.8 Hz), 5.12 (s, 1H), 1.75 (s, 6H); C NMR (75
MHz, CDCl ) δ (ppm): 185.5, 180.2, 150.9, 147.6, 142.5, 142.3, 140.2, 131.5, 127.1,
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+
125.8, 125.1, 124.4, 122.2, 120.5, 101.6, 28.4; MALDI-TOF (M , C H IrN S O ):
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calcd. 712.08, found 712.00 (HPLC purity: 99.8%).
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(DMBT) Ir(acac) (64%): H NMR (300 MHz, CDCl ) δ (ppm): 8.05 (m, 2H), 7.86
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(m, 2H), 7.35 (m, 6H), 6.16 (s, 2H), 5.09 (s, 1H), 2.08 (s, 6H), 1.91 (s, 6H), 1.73 (s,
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6H); C NMR (75 MHz, CDCl ): 185.5, 181.0, 151.0, 144.9, 143.2, 139.7, 136.1,
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131.1, 129.5, 127.1, 126.7, 124.6, 122.1, 119.9, 101.6, 28.5, 20.4, 19.5; MALDI-TOF
+
(M , C H IrN O S ): calcd. 768.15, found 768.10 (HPLC purity: 99.8%).
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2 2
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(TBT) Ir(acac) (58%): H NMR (300 MHz, CDCl ) δ (ppm): 8.15 (m, 2H), 7.92 (m,
2
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2H), 7.58 (d, 2H, J = 7.5 Hz), 7.42 (m, 4H), 6.97 (d, 2H, J = 7.2 Hz), 5.51 (s, 2H),
1
3
5.18 (s, 1H), 1.79 (s, 6H), 0.35 (s, 18H); C NMR (75 MHz, CDCl ): 185.5, 180.2,
3
150.9, 147.6, 142.5, 142.3, 140.2, 131.5, 127.1, 125.8, 125.1, 124.4, 122.2, 120.1,
+
101.6, 30.2, 28.5; MALDI-TOF (M , C H IrN O S Si ): calcd. 856.16, found
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2
2
2
856.16 (HPLC purity: 99.7%).
2.3. Fabrication of OLED devices
The device structure of the fabricated OLEDs was indium tin oxide (ITO; 50
nm)/PEDOT:PSS (40 nm)/TCTA (20 nm)/TCTA:TPBi:orange dopant (50:50:x%) (40
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nm)/TPBi (40 nm)/LiF (1 nm)/Al (120 nm). In these devices, the dopants were
(BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac); the doping ratio of
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tris(4-carbazoyl-9-ylphenyl)amine
(TCTA)
to
2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) was 1:1, and the
concentrations of the dopants (x%) were 3, 5, 7, and 11%. The ITO substrate was
cleaned by sonication in acetone and isopropyl alcohol followed by ultraviolet/ozone
treatment
for
10
min.
After
doped
treatment
with
with
oxygen
plasma,
acid)
poly(3,4-ethylenedioxythiophene)
poly(styrenesulfonic
(PEDOT:PSS, AI4083) was spin-coated onto the cleaned ITO substrates. TCTA,
which served as the hole-transport layer (HTL), was deposited onto the PEDOT:PSS
layer. The emissive layer consisting of TCTA, TPBi, and the Ir(III) complex in the
composition ratio of TCTA:TPBi:Ir(III) = 50:50:x was spin-coated onto TCTA layer
and then annealed at 120 °C for 30 min to remove the residual solvent. TPBi, which
acted as a high triplet-energy exciton blocking layer (HBL) with electron transport
properties, was then deposited on the emissive layer. Finally, a layer of lithium
fluoride (LiF) as the electron injecting layer (EIL) and that of aluminum (Al) as the
cathode were sequentially deposited by thermal evaporation on top of the film through
−
6
a mask under high vacuum (below 2.0 × 10 Torr, 0.27 mPa).
2.4. Measurements
1
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H and C NMR spectra were recorded on a Varian Mercury 300 MHz (75 MHz)
spectrometer in CDCl . Thermogravimetric analysis (TGA) was performed using an
3
SDT Q600 V20.9 Build 20 instrument under nitrogen atmosphere at a heating rate of
−
1
10 °C min . Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF)
mass spectra were obtained using a ZMS-DX303 mass spectrometer (JEOL Ltd.).
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High-performance liquid chromatographic analysis (HPLC) was performed on a
Shimadzu Prominence instrument. UV-Visible absorption spectra were recorded on a
Shimadzu UV-3600 spectrophotometer, whereas photoluminescence (PL) spectra
were recorded on a Shimadzu RF 5301 PC fluorometer. Cyclic voltammetry (CV)
measurements were carried out with a CHI600D system at potential scan rate of
−
1
50–100 mV·s
in a dichloromethane (CH Cl ) solution containing 0.1 M
2 2
tetrabutylammonium perchlorate as the supporting electrolyte at room temperature
(Ag/AgNO as reference electrode, Pt wire as counter electrode, Pt disc as working
3
+
electrode (2 mm diameter), and Cp Fe/Cp Fe redox couple as a secondary internal
2
2
reference). The current density–voltage–luminance (I–V–L) characteristics were
measured and electroluminescence (EL) spectra of the phosphorescent OLEDs were
recorded using a Keithley 2400 source measurement unit and CS-1000
spectrophotometer. All devices were fabricated by solution processing and
encapsulated with a glass lid and CaO getter before device measurements. A Keithley
2400 source measurement unit and CS-1000 spectroradiometer were used to evaluate
the device performances.
3. Results and discussion
3.1. Synthesis and characterization
The heteroleptic Ir(III) complexes, (BT) Ir(acac), (DMBT) Ir(acac), and
2
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(TBT) Ir(acac), were synthesized according to the procedure shown in Scheme 1. The
2
cyclometalated main ligands, BT, DMBT, and TBT were synthesized via a
cycloaddition reaction between benzaldehyde derivatives and 2-aminobenzenethiol
[20–22]. The Ir(III) µ-chloro-bridged dimers were formed by the reaction of the
cyclometalating ligands, BT, DMBT, and TBT, with IrCl ·3H O in a mixture of
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2-ethoxyethanol and water [26]. The Ir(III) µ-chloro-bridged dimers were then reacted
with acac as an ancillary ligand in 2-ethoxyethanol to afford the heteroleptic Ir(III)
1
13
complexes. These complexes were characterized by H- and C-NMR, and
MALDI-TOF mass spectroscopy measurements (Figure S1 and S2). The materials
were purified by vacuum sublimation before device fabrication. The purity after
sublimation was confirmed to be >99.5% based on HPLC (Figure S3). The thermal
properties of (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) were investigated
2
2
2
by TGA under a nitrogen atmosphere. The 5% weight-loss temperatures (T ) of
d
(BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) were 327, 361, and 338 °C,
2
2
2
respectively (Figure S4), indicating that they would be thermally stable under the
OLED fabrication conditions. The T values showed that the Ir(III) complexes with
d
either DMBT or TBT as the cyclometalated ligands were thermally more stable than
the corresponding Ir(III) complex containing BT as the main ligand.
3.2. X-ray Crystallography
The crystal structure of (DMBT) Ir(acac) was investigated using single-crystal X-ray
2
crystallography. The crystallographic refinement parameters, selected bond distances
as well as bond angles are summarized in Tables S1–S6 and the crystal structure is
shown in Figure S11. The crystallographic data for (DMBT) Ir(acac) has been
2
deposited in the Cambridge Structural Database (CCDC 1824937). The iridium atom
adopts a distorted octahedral coordination geometry with two C^N cyclometalated
ligands and one O^O ligand. The geometrical configuration of (DMBT) Ir(acac) was
2
N,N-trans, which is the same configuration as that of (BT) Ir(acac) standard material
2
[22]. Although the single crystal sample of (TBT) Ir(acac) has not been obtained, a
2
similar conformation can be assumed in view of its structural similarity to
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(DMBT) Ir(acac). Detailed crystallographic data (cif) of (DMBT) Ir(acac) can be
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found in the Supporting Information.
3.3. Photophysical properties
The photophysical properties of the Ir(III) complexes were investigated by conducting
UV-Vis absorption and PL spectroscopic measurements of their dilute
–
5
dichloromethane solutions (10 M) and thin films at room temperature. The UV-Vis
absorption spectra of the Ir(III) complex solutions were almost the same as those of
the corresponding films (Figure S5). All the Ir(III) complexes showed a strong
absorption band below 350 nm and a weak broad absorption band in the longer
wavelength region above 380 nm. The absorption band in the shorter wavelength
1
region corresponds to spin-allowed ligand-centered transitions ( π-π*) of main ligands
(Figure 1(a)), and the weak and broad absorption bands in the range of 380–550 nm to
1
3
metal-to-ligand charge transfer ( MLCT and MLCT) and spin-orbit coupling
3
enhanced π-π*; these assignments were made by comparison with those of
previously reported Ir(III) complexes [20–23]. The optical properties of the Ir(III)
complexes are summarized in Table 1. Complexes (DMBT) Ir(acac) and
2
(TBT) Ir(acac) exhibited red-shifted absorption in comparison to that of (BT) Ir(acac)
2
2
because of the substitution of electron-donating dimethyl or trimethylsilyl groups on
opt
the phenyl ring of the main ligand. The optical band gap energies (Eg ) of
(BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) were determined to be 2.25,
2
2
2
2.21, and 2.19 eV, respectively, according to the onset of the absorption edge.
The PL maxima of (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) were
2
2
2
observed at 563, 569, and 576 nm, respectively. These measurements were conducted
with dilute dichloromethane solutions of the complexes at room temperature (Figure
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1(b)). (DMBT) Ir(acac), and (TBT) Ir(acac) exhibited 7–12 nm red-shifted emission
2
2
compared to that of (BT) Ir(acac) with the same effect as observed in the absorption
2
spectra. The PL quantum yields (ФPL) of (DMBT) Ir(acac) and (TBT) Ir(acac)
2 2
−
5
measured using their 10 M degassed dichloromethane solution and (BT) Ir(acac) as
2
a reference (Ф = 0.26) were determined to be 0.38 and 0.49, respectively [20–22].
PL
The higher ФPL values of (DMBT) Ir(acac) and (TBT) Ir(acac) than that of
2
2
(BT) Ir(acac) were attributed to the reduced free rotation of the compounds because
2
of the bulky dimethyl or trimethylsilyl substituents.
3.4. Theoretical calculations
We performed theoretical calculations based on density functional theory (DFT),
using the B3LYP/6-31G(d) hybrid exchange-correlation functional and LANL2DZ
basis sets, for a better understanding of the effects of DMBT and TBT cyclometalated
ligands in iridium(III) complexes. The calculations were conducted using the
Gaussian 09 package [27]. Figure 2 shows the electron densities and calculated
energy levels of the highest occupied molecular orbitals (HOMOs) and lowest
unoccupied molecular orbitals (LUMOs) of (BT) Ir(acac), (DMBT) Ir(acac), and
2
2
(TBT) Ir(acac). The HOMOs of (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac)
2
2
2
2
were similarly localized on the phenyl ring of the main ligand and the d orbital of the
Ir metal. Thus, electron donating substituents on the phenyl ring of the main ligand
could be expected to raise the HOMO energy level. The LUMOs in all synthesized
Ir(III) complexes were largely localized over the benzothiazole and phenyl rings.
The calculated HOMO/LUMO energy levels of (BT) Ir(acac), (DMBT) Ir(acac), and
2
2
(TBT) Ir(acac) were –5.15/–1.88, –4.94/–1.72 eV, and –5.25/–2.04 eV, respectively.
2
The calculated HOMO energy level of (DMBT) Ir(acac) was higher than that of
2
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(BT) Ir(acac) because of the electron donating dimethyl groups on the phenyl ring.
2
The increased HOMO and decreased LUMO energy levels of the synthesized Ir(III)
complexes using DMBT and TBT led to a narrower band gap, and therefore,
(DMBT) Ir(acac) and (TBT) Ir(acac) could be expected to exhibit red-shifted
2
2
emission compared to that of (BT) Ir(acac).
2
3.5. Electrochemical properties
The electrochemical properties of the synthesized Ir(III) complexes were investigated
using CV. The measurements were conducted using their dichloromethane solutions
with tetrabutylammonium perchlorate as the electrolyte and the results are shown in
Figure 3. Complexes (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) showed
2
2
2
reversible electron oxidations at positive potentials in the region of 0.50–0.65 V. The
oxi
onset potentials of the first oxidations (E
) of (BT) Ir(acac), (DMBT) Ir(acac),
2 2
onset
+
oxi
and (TBT) Ir(acac) were 0.65, 0.54, and 0.60 V vs. Ag/Ag , respectively. The E
2
onset
values of (DMBT) Ir(acac) and (TBT) Ir(acac) are clearly lower than that of the
2
2
parent complex (BT) Ir(acac). The HOMO energy levels of (BT) Ir(acac),
2
2
(DMBT) Ir(acac), and (TBT) Ir(acac) were estimated to be –5.35, –5.24, and –5.30
2
2
+
eV vs. Fc/Fc , respectively. The LUMO energy levels of (BT) Ir(acac),
2
(DMBT) Ir(acac), and (TBT) Ir(acac), obtained from the differences between the
2
2
optical band gaps and the HOMO energy levels (∆Eband gap= ELUMO-EHOMO), were
found to be –3.10, –3.03, and –3.11 eV, respectively. The HOMO and LUMO energy
levels of the synthesized Ir(III) complexes were well matched with those of TCTA
(HOMO: –5.8 eV, LUMO: –2.50 eV) and TPBi (HOMO: –6.30 eV, LUMO: –2.70
eV) as host materials. The electrochemical properties of the Ir(III) complexes are
summarized in Table 1.
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3.6. Electrophosphorescent OLEDs
The performances of the Ir(III) complexes in the phosphorescent OLEDs were
evaluated by fabricating multi-layered devices with the structure ITO (50
nm)/PEDOT:PSS (40 nm)/TCTA (20 nm)/TCTA:TPBi:dopant (50:50:x%) (40
nm)/TPBi (40 nm)/LiF (1 nm)/Al (120 nm). We fabricated devices using TCTA and
TPBi as mixed-hosts in the emissive layer for efficient carrier injection and charge
balance [28]. Doping concentrations of 5 wt% for (BT) Ir(acac), 7 wt% for
2
(DMBT) Ir(acac), and 7 wt% for (TBT) Ir(acac) gave the optimal device
2
2
performances (Table 2). The optimized doping concentrations of (DMBT) Ir(acac)
2
and (TBT) Ir(acac) were higher than that of (BT) Ir(acac), which suggested that the
2
2
introduction of relatively bulky dimethyl or trimethylsilyl groups on the phenyl ring
of main ligand effectively suppressed self-quenching, even at high concentrations.
The electroluminescent (EL) spectra of the optimized phosphorescent OLEDs
fabricated using (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) (Figure 4)
2
2
2
showed emission maxima at 563, 571, and 578 nm, respectively. The emission
maxima of the (DMBT) Ir(acac) and (TBT) Ir(acac) devices were changed by only 1
2
2
nm as the dopant concentration was increased from 3 to 11% while the (BT) Ir(acac)
2
device showed a red-shift of 2 nm under the same conditions. This indicated that the
(BT) Ir(acac) device had a higher degree of dopant aggregation and stronger
2
intermolecular interactions at a high dopant concentration than (DMBT) Ir(acac) and
2
(TBT) Ir(acac) devices. (Figure S7) The Commission Internationale de L’Eclairage
2
(CIE) coordinates of the (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac) devices
2
2
2
at the optimal doping concentrations were (0.49,0.50), (0.52,0.47), and (0.55,0.44),
2
respectively, at 1,000 cd/m .
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Figure 5(a) shows the current density-voltage-luminance (J-V-L) curves of the
phosphorescent OLEDs. The current densities of the OLEDs increased with
increasing concentration of (DMBT) Ir(acac) and (TBT) Ir(acac) dopants because of
2
2
improved charge hopping at high dopant concentrations [29]. The Vturn-on and Lmax
values of the Ir(III) complexes with bulky substituents on the cyclometalated ligands
2
2
were 4.5 V/4,532 cd/m for the (DMBT) Ir(acac) device and 4.5 V/6,837 cd/m for
2
the (TBT) Ir(acac) device. The highest Lmax of the (TBT) Ir(acac) device could be
2
2
attributed to its better recombination efficiency with higher ФPL as compared to the
(DMBT) Ir(acac) and (BT) Ir(acac)-based devices. The (DMBT) Ir(acac) and
2
2
2
(TBT) Ir(acac) devices exhibited higher maximum external quantum efficiencies
2
(EQEmax)/maximum current efficiencies (CEmax) of 7.8%/21.4 cd/A and 9.9%/25.3
cd/A, respectively, compared to that of (BT) Ir(acac) (4.4%/13.0 cd/A) (Figure 5(b)).
2
The low driving voltage and high EQE of the (TBT) Ir(acac) device improved the PE
2
value to 12.37 lm/W, which was the highest value among the three Ir(III)
complex-based devices.
These results showed that the (DMBT) Ir(acac) and (TBT) Ir(acac) complexes with
2
2
bulky substituents on the phenyl ring of the cyclometalated main ligands significantly
improved the device performances such as EQE, CE, and PE as compared to the
(BT) Ir(acac)-doped device. Introduction of bulky substituents on the phenyl ring of
2
(DMBT) Ir(acac) and (TBT) Ir(acac) complexes could effectively prevent the
2
2
concentration self-quenching at high doping concentrations and suppress the
triplet-triplet annihilation in the device [24,25]. In particular, a trimethylsilyl
substituent on the 4-position of the phenyl ring led to better device performances than
the dimethyl substituents on 3,4-position of the phenyl ring. The device characteristics
at other doping concentrations of (BT) Ir(acac), (DMBT) Ir(acac), and (TBT) Ir(acac)
2
2
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are shown in Figures S8, S9, and S10.
4. Conclusions
In this study, we successfully synthesized dimethyl- and trimethylsilyl–substituted
2-phenylbenzothiazole-based heteroleptic Ir(III) complexes, (DMBT) Ir(acac) and
2
(TBT) Ir(acac), for their application in solution-processed orange phosphorescent
2
OLEDs. Compared to (BT) Ir(acac), the synthesized (DMBT) Ir(acac) and
2
2
(TBT) Ir(acac) complexes exhibited improved thermal stability and emission quantum
2
efficiency. The introduction of bulky dimethyl and trimethylsilyl substituents on the
phenyl ring of the main ligand in these complexes led to reduced molecular
aggregation in the emissive layer of the device fabricated by solution processing.
Consequently, the triplet-triplet annihilation in the orange phosphorescent OLEDs was
effectively suppressed. The devices fabricated using (DMBT) Ir(acac) and
2
(TBT) Ir(acac) showed better performances than those fabricated using (BT) Ir(acac).
2
2
The device based on the (TBT) Ir(acac) complex, which had a trimethylsilyl
2
substituent on the phenyl ring, exhibited the best performance with the highest EQE,
Lmax, CE, and PEmax. In particular, among all the fabricated phosphorescent OLEDs,
the (TBT) Ir(acac) device exhibited the highest CE of 25.26 cd/A at a high doping
2
concentration of 7%.
Acknowledgements
This research was supported by the Ministry of Trade, Industry and Energy (MOTIE,
Korea) under the Industrial Technology Innovation Program. No. 10067715, and
National Research Foundation (NRF) grants, funded by the Korean government
[Grant No. NRF2017R1A2A2A05001345 and No. 2011-0030013 through GCRC
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Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
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A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V.
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G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
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5
5
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5
28 Table captions
29 Table 1. Optical and electrochemical properties of the synthesized iridium(III)
30 complexes.
31 Table 2. Performances of the orange phosphorescent OLEDs.
23

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5
32 Table 1.
Optical
Electrochemical
a
a/b
a
c
d
e
f
λ
abs
λ
em
FWHM
(nm)
Φ
PL
E
g
HOMO
LUMO
Dopant
(
nm)
13, 326, 356, 404,
40, 480
21, 331, 359, 404,
48, 483
19, 333, 364, 415,
53, 504
(nm)
(%)
(eV)
2.25
(eV)
(eV)
3
3
3
(
BT)
2
Ir(acac)
563/564
78
74
76
0.26
-5.35
-3.10
-3.03
-3.10
4
(
DMBT)
2
Ir(acac)
569/571
576/578
0.38
0.49
2.21
2.18
-5.24
-5.30
4
2
(TBT) Ir(acac)
4
5
5
5
5
5
5
5
5
33
34
35
36
37
38
39
40
a
–5
Maximum PL emission wavelength measured in 1 × 10 M CH
2
Cl
Maximum PL emission wavelength measured in the neat film state.
2 2
Values were measured using (BT) Ir(acac) (ΦPL = 0.26) as a reference in 1 × 10 M CH Cl solution.
2
solution.
b
c
d
e
f
–5
2
Energy gap calculated from the onset of the absorption edge.
Estimated from the cyclic voltammogram.
Estimated from the HOMO energy level and the band gap.
5
41
24

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5
42
Table 2.
b
c
λ
max, FWHM
V
turn
EQE
(%)
a
2
c
c
Dopant
x%
3
CIE (x,y)
L
max (cd/m )
CE (cd/A)
PE (lm/W)
(nm)
(V)
4.5
(
(
(
(
(
(
(
(
(
(
(
(
0.49,
561, 70
562, 72
562, 73
563, 75
570, 70
571, 72
571, 72
571, 72
578, 74
578, 74
578, 74
579, 76
2,711
4,948
2,443
3,678
2,535
3,881
4,532
3,567
3,551
4,958
6,837
4,351
2.74/2.52
7.03/6.45
3.51/2.90
7.29/6.79
4.90/3.35
5.57/4.06
5.63/4.67
7.44/5.21
0
.50)
0.49,
5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.38/4.38 12.96/12.96
0
.50)
2
(BT) Ir(acac)
0.50,
7
3.63/2.67
3.28/2.79
10.91/8.00
9.90/8.40
0
.49)
0.50,
11
0
.49)
0.52,
3
4.02/3.70 11.30/10.40
4.76/3.94 13.02/10.78
0
.47)
0.52,
5
7
0
.47)
2
(DMBT) Ir(acac)
0.52,
7.76/7.76 21.43/21.43 11.10/10.36
0
.47)
0.52,
11
6.67/5.81 18.26/16.18
10.00/7.26
5.08/3.62
8.38/5.67
0
.47)
0.55,
3
5
7
4.27/3.57
10.33/8.63
0
.44)
0.55,
5.17/4.48 12.49/10.83
0
.44)
2
(TBT) Ir(acac)
0.55,
9.94/9.94 25.26/25.26 12.37/12.21
0
.44)
0.55,
11
7.92/6.97 18.63/16.40
9.74/7.36
0
.44)
a
b
c
2
5
5
5
43
44
45
Values measured at a luminance of 1,000 cd/m .
Values measured at a luminance of 1 cd/m .
2
2
Values measured at maximum efficiency and luminance of 1,000 cd/m
.
25

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Figure captions
Fig. 1. (a) UV-Visible absorption and (b) PL spectra of (BT) Ir(acac),
2
(
DMBT) Ir(acac), and (TBT) Ir(acac).
2 2
Fig. 2. Frontier molecular orbitals HOMO and LUMO of (BT) Ir(acac),
2
(
DMBT) Ir(acac), and (TBT) Ir(acac).
2 2
Fig. 3. Cyclic voltammograms of (BT) Ir(acac), (DMBT) Ir(acac), and
2
2
(
TBT) Ir(acac).
2
Fig. 4. EL spectra of orange phosphorescent OLEDs fabricated using (BT) Ir(acac),
2
(
DMBT) Ir(acac), and (TBT) Ir(acac).
2 2
Fig. 5. (a) Current density-voltage-luminance (J-V-L) and (b) external quantum
efficiency–current density–current efficiency (EQE-J-CE) curves of (BT) Ir(acac),
2
(
DMBT) Ir(acac), and (TBT) Ir(acac)-based devices.
2 2
26

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Fig 1.
27

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Fig 2.
28