Greenish yellow organic light emitting devices based on novel iridium complexes containing 2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole

Article abstract of DOI:10.1016/j.dyepig.2013.08.011

2-Cyclohexenyl-1-phenyl-1H-benzo[d]imidazole (Hcyclopbi) and its heterocyclometalated iridium complexes bis[2-cyclohexenyl-1-phenyl-1H-benzo[d] imidazole](acetylacetonate)iridium(III) [(cyclopbi)₂Iracac] and bis[2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole][2-(3-(trifluoromethyl) -1H-pyrazol-5-yl)pyridinate]iridium(III) [(cyclopbi)₂IrCF ₃] were firstly designed and synthesized. Owing to the strong electron withdrawing ability of 2-(3-(trifluoromethyl)-1H-pyrazol) compared to that of acetylacetonate, blue shifted phosphorescence 542 nm was obtained for (cyclopbi)₂IrCF₃ compared to that of (cyclopbi) ₂Iracac 545 nm with photoluminescence quantum yields (φ) 0.22 and 0.25, respectively, as well the highest occupied molecular orbit (HOMO) energy level was estimated to be -5.10 eV for (cyclopbi)₂IrCF₃, 0.19 eV lower than that of -4.91 eV for (cyclopbi)₂Iracac. When (cyclopbi)₂Iracac was doped into 4,4′-bis(9H-carbazol-9-yl) biphenyl (CBP) as the emitting layer, high efficiency organic light emitting devices (OLEDs) were obtained with maximum current efficiency (η_c) 22.3 cd/A at 3.1 mA/cm² and maximum brightness 16,300 cd/m² at 171.6 mA/cm², which were comparable to those of bis(2-phenylpyridine)(acetylacetonate)iridium(III) [(ppy) ₂Iracac] with the same device structure. This results indicate that (cyclopbi)₂Iracac and (cyclopbi)₂IrCF₃ are potential phosphorescent dyes used for OLEDs.

Full text of DOI:10.1016/j.dyepig.2013.08.011

Dyes and Pigments 99 (2013) 1010e1015
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Greenish yellow organic light emitting devices based on novel iridium
complexes containing 2-cyclohexenyl-1-phenyl-1H-benzo[d]
imidazole
Liangliang Han a, Dongyu Zhang b, Yuanhang Zhou a, Ying Yang a, Han Young Woo c,
,
Fali Bai a, Renqiang Yang a
*
a Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
b Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
c Department of Cogno-Mechatronics Engineering (WCU), Pusan National University, Miryang 627-706, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
2-Cyclohexenyl-1-phenyl-1H-benzo[d]imidazole (Hcyclopbi) and its heterocyclometalated iridium
complexes bis[2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole](acetylacetonate)iridium(III) [(cyclo-
pbi)2Iracac] and bis[2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole][2-(3-(trifluoromethyl)-1H-pyr-
azol-5-yl)pyridinate]iridium(III) [(cyclopbi)2IrCF3] were firstly designed and synthesized. Owing to the
strong electron withdrawing ability of 2-(3-(trifluoromethyl)-1H-pyrazol) compared to that of acetyla-
cetonate, blue shifted phosphorescence 542 nm was obtained for (cyclopbi)2IrCF3 compared to that of
(cyclopbi)2Iracac 545 nm with photoluminescence quantum yields (4) 0.22 and 0.25, respectively, as well
the highest occupied molecular orbit (HOMO) energy level was estimated to be ꢀ5.10 eV for (cyclo-
pbi)2IrCF3, 0.19 eV lower than that of ꢀ4.91 eV for (cyclopbi)2Iracac. When (cyclopbi)2Iracac was doped
into 4,40-bis(9H-carbazol-9-yl)biphenyl (CBP) as the emitting layer, high efficiency organic light emitting
devices (OLEDs) were obtained with maximum current efficiency (hc) 22.3 cd/A at 3.1 mA/cm2 and
maximum brightness 16,300 cd/m2 at 171.6 mA/cm2, which were comparable to those of bis(2-
phenylpyridine)(acetylacetonate)iridium(III) [(ppy)2Iracac] with the same device structure. This results
indicate that (cyclopbi)2Iracac and (cyclopbi)2IrCF3 are potential phosphorescent dyes used for OLEDs.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 7 June 2013
Received in revised form
8 August 2013
Accepted 13 August 2013
Available online 26 August 2013
Keywords:
Organic light emitting devices
Greenish yellow phosphorescence
2-Cyclohexenyl-1-phenyl-1H-benzo[d]
imidazole
Heterocyclometalated iridium complexes
Photoluminescence
Electroluminescence
1. Introduction
styrylisoquinoline were firstly employed as cyclometalated ligands
and used for OLEDs application with maximum current efficiency
Organic light emitting devices (OLEDs) are currently an active
research area due to their potential applications in flat-panel dis-
plays and solid-state lightings [1e5]. Ir(III) complexes have attrac-
ted much attentions on improving OLEDs’ efficiency because the
(hc) 43.9 cd/A by Cheng and co-workers [10e12]. According to our
work [13], through changing the ancillary ligand, hetero-
cyclometalated iridium complexes containing syrylbenzothiazole
show saturated red phosphorescence with Commission Interna-
tional de l’Eclairage (CIE) coordinates (0.64, 0.36), which were also
reported by Xiao et al. [14]. As well as known iridium complexes
based on 1-phenyl-1H-benzo[d]imidazole were one of the most
efficient phosphores with high electron mobility which was firstly
reported by Lin and co-workers in 2004 [15e17]. Followed by the
styrylbenzoimidazole derivatives and their iridium complexes,
high efficiency OLEDs were obtained comparable to that of tris(2-
close-lying
pep* and metal to ligand charge transfer (MLCT)
states in these complexes together with the heavy atom effect
enhance the spin-orbit coupling, which result in utilization both
singlet and triplet excitons, causing nearly one unity of internal
quantum efficiency [5e9]. Alkenyl-heterocyclic ligands were easily
to obtain low energy phosphorescence in their cyclometalated
iridium complexes with high photoluminescence (PL) and elec-
troluminescence (EL) quantum efficiency which have been widely
studied during the past decades. Styrylpyridine, styrylquinoline,
phenylpyridine)iridium
[Ir(ppy)3]
[18].
However,
styryl-
heterocyclic derivatives always show lower triplet energy
compared to that of phenyl-heterocyclic derivatives because of the
increased conjugation length, which lead to orange or red phos-
phorescence for their iridium complexes. Kang et al. [19,20] have
employed yellowish green iridium complex tris[2-(1-cyclohexenyl)
* Corresponding author. Fax: þ86 532 80662778.
E-mail address: yangrq@qibebt.ac.cn (R. Yang).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.08.011

L. Han et al. / Dyes and Pigments 99 (2013) 1010e1015
1011
pyridine]iridium [Ir(chpy)3] as the phosphors which exhibit high EL
external quantum efficiency 18.7% higher than that of Ir(ppy)3
without influence the triplet energy significantly. The highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of Ir(chpy)3 were higher than that of the
host which result as more balanced hole and electron mobility in
the emitting layer (EML). Taking into account 1-phenyl-1H-benzo
[d]imidazole show higher electron mobility than that of pyridine
derivatives and better stability of cyclohexenyl-heterocyclic than
styryl-heterocyclic derivatives [21]. Thus the designing of
cyclohexenyl-benzoimidazole ligand and its iridium complex are
expected to obtain high efficiency phosphorescent organic light
emitting devices (PhOLEDs).
In this article we designed novel alkenyl-benzoimidazole deriva-
tive 2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole (Hcyclopbi),
which was simply synthesized by condensation of N-phenyl-o-phe-
nylenediamine and cyclohexene-1-carbaldehyde. Heterocyclo
metalated iridium complexes bis[2-cyclohexenyl-1-phenyl-1H-benzo
[d]imidazole](acetylacetonate)iridium(III) [(cyclopbi)2Iracac] and bis
[2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole][2-(3-(trifluorome
thyl)-1H-pyrazol-5-yl)pyridinate]iridium(III) [(cyclopbi)2IrCF3] were
also synthesized as greenish-yellow phosphorescent dyes. When
(cyclopbi)2Iracac was doped into 4,40-bis(9H-carbazol-9-yl)biphenyl
(CBP) as the emitting layer, maximum current efficiency (hc) 22.3 cd/A
was obtained at 3.1 mA/cm2 and maximum brightness 16,300 cd/m2
at 171.6 mA/cm2, which were comparable to those of bis(2-phe
nylpyridine)(acetylacetonate)iridium(III) [(ppy)2Iracac] with the
same device structure.
degreased with solvents and cleaned for 5 min by exposure to an
UV-ozone ambient, then it was immediately loaded into the
evaporation system. With a base pressure of w3 ꢁ 10ꢀ4 Pa, the
organic and metal cathode layers were grown successively by using
an in vacuo mask exchange mechanism without breaking vacuum.
Firstly,
a
10-nm-thick 4,40,400-tris(N-3-methylphenyl-N-phenyl-
amino)triphenylamine (m-MTDATA) hole injection layer (HIL) was
deposited, followed by a 20-nm-thick 4,40-bis[N-(1-naphthyl)-N-
phenyl-amino]biphenyl (NPB) hole transporting layer (HTL). Then a
30-nm-thick EML consisting of various weight ratio iridium com-
plexes doped into a 4,40-bis(9H-carbazol-9-yl)biphenyl (CBP) host
was prepared via thermal codeposition. Next, a 30-nm-thick 1,3,5-
tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) layer was
used to block hole and transport electron. A shadow mask with
rectangular 2 mm ꢁ 2 mm openings was used to define the cathode
consisting of a 1-nm-thick LiF layer, followed by a 100-nm-thick Al
layer. The EL spectra were recorded with a PR655 spectrophotom-
eter. The brightnessecurrentevoltage (BeIeV) characteristics of
OLEDs were measured with Keithley 2400. All the measurements
were carried out at room temperature under ambient conditions.
2.3. Synthesis
As shown in Scheme 1, the cyclometalated ligand Hcyclopbi was
synthesized by condensation of N-phenyl-o-phenylenediamine and
cyclohexene-1-carbaldehyde in 2-ethoxyethanol, after purified on
column chromatography using dichloromethane and ethyl acetic as
eluent, colorless solution and brown solid was obtained when the
solvent was evaporated with satisfied yield 68.5%, which was firstly
reported to our knowledge now and the structure was character-
ized by NMR, and HRMS. Hcyclopbi was used to react with
2. Experimental details
2.1. Reagents and measurements
IrCl3$3H2O to give cyclometalated Ir-m-chloro-bridged dimer,
which could react with acetylacetone or 2-(3-(trifluoromethyl)-1H-
pyrazol-5-yl) pyridine to afford heterocyclometalated (cyclo-
pbi)2Iracac or (cyclopbi)2IrCF3, respectively, in the presence of
Na2CO3. Both the complexes show strong phosphorescence on sil-
ica gel plates under the excitation of 365 nm. The synthesis
methods are described as below:
All reactions were performed under argon. Solvents were care-
fully dried and distilled from appropriate drying agents prior to use.
Commercially available reagents were used without further puri-
fication unless otherwise stated. All reactions were monitored by
using thin-layer chromatography (TLC) with Merck pre-coated
glass plates. Compounds were visualized with UV-light irradiation
at 254 and 365 nm. High resolution mass spectra (HRMS) were
obtained using a Bruker Maxis UHR-TOF, Ion Source: APCI system.
Nuclear magnetic resonance (NMR) spectra were measured in
appropriate deuterated solvents (CDCl3 or DMSO-d6) on a Bruker
AVANCE 600 MHz Fourier transform NMR spectrometer; chemical
shifts were quoted relative to the internal standard tetramethylsi-
lane for 1H NMR data. Absorption and photoluminescence spectra
were measured with a Hitachi U-4100 UVeViseNIR scanning
spectrophotometer and HORIBA JOBIN YVON FluoroMax-4 spec-
trophotometer, respectively. The electrochemistry characteristics of
the two complexes were investigated using a cyclic voltammetry
employing a diameter of 3 mm glass carbon electrode as working
electrode, a platinum wire as counter electrode and an Ag/AgCl
(3.0 M KCl) electrode as reference electrode. The cyclic voltam-
mograms were obtained from a one-compartment glass cell in
dichloromethane containing 10ꢀ3 M iridium complexes and 0.1 M
tetra(n-butyl)ammonium hexafluorophosphate (n-Bu4NPF6) as a
supporting electrolyte, where the scan rate was 100 mV sꢀ1, the
HOMO and LUMO energy level was calculated as the reference
method [18].
2.3.1. 2-Cyclohexenyl-1-phenyl-1H-benzo[d]imidazole (Hcyclopbi)
N-phenyl-o-phenylenediamine (1.84 g, 10 mmol) and cyclo-
hexene-1-carbaldehyde (1.10 g, 10 mmol) were charged into a two-
neck round bottomed flask and 2-ethoxyethanol (20 ml) was added
under nitrogen atmosphere. The mixture was heated at 110 ꢂC for
24 h. The solvent was removed under vacuum and the resulting
mixture was extracted into dichloromethane. The organic extract
was washed with brine solution, dried over anhydrous Na2SO4 and
filtered. After the solvent was evaporated, the crude product was
purified on a silica gel column using acetic ether and dichloro-
methane as eluent to give the desired product as a brown solid
(1.88 g, 68.5%). 1H NMR (DMSO-d6, 600 MHz):
J ¼ 7.86 Hz, 1 H), 7.6126 (t, J ¼ 7.44 Hz, 2 H), 7.5510-7.5261 (m, 1 H),
7.4525-7.4384 (m, H), 7.2543-7.1808 (m, H), 7.0781 (d,
d
¼ 7.6869 (d,
2
2
J ¼ 7.92 Hz, 1 H); HRMS (APCI): m/z calcd for C19H18N2: 274.1470;
found: 274.1515.
2.3.2. Bis[2-cyclohexenyl-1-phenyl-1H-benzo[d]
imidazole](acetylacetonate)iridium(III) [(cyclopbi)2Iracac]
Hcyclopbi (0.6 g, 2.2 mmol) was dissolved into a mixed solution
of 2-ethoxyethanol (15 ml) and water (5 ml) in a 50 ml round
bottomed flask, then IrCl3$3H2O (0.35 g, 1 mmol) was added. The
mixture was stirred under nitrogen at 100 ꢂC for 12 h. The mixture
was cooled to room temperature and the precipitate was collected
and washed with water, ethanol, and acetone, respectively, then
2.2. Device fabrication and measurement
OLEDs were grown on a glass substrate pre-coated with a w130-
nm-thick indium-tin-oxide (ITO) layer with a sheet resistance of
w20 U/,. Prior to organic layer deposition, the substrate was
dried in vacuum to give cyclometalated Ir-m-chloro-bridged dimer.

1012
L. Han et al. / Dyes and Pigments 99 (2013) 1010e1015
Scheme 1. Synthetic routes of cyclometalated ligand Hcyclopbi and bis-cyclometalated iridium complexes (cyclopbi)2Iracac and (cyclopbi)2IrCF3.
The dimer complex, 2.0 mmol acetylacetone, 4.0 mmol Na2CO3 and
15 ml 2-ethoxyethanol were charged into a round bottomed flask,
the mixture was then refluxed under an argon atmosphere for
another 12 h. After cooling to room temperature, the precipitate
was filtered off and washed with water, ethanol and ether,
respectively. The crude product was purified by silica column
chromatography with dichloromethane/petroleum ether (1:10, v/
v) as eluent to give 0.55 g (cyclopbi)2Iracac as an orange powder.
26.57, 23.82, 23.57, 23.47, 23.36; HRMS (APCI): m/z calcd for
47H39F3IrN7: 951.2848; found: 951.2901.
C
3. Results and discussion
3.1. Photophysics characteristics
The UVeVis and photoluminescence (PL) spectra of complexes
(cyclopbi)2Iracac and (cyclopbi)2IrCF3 in dichloromethane were
shown in Fig. 1. For complex (cyclopbi)2Iracac, the absorption
Yield: 65.6%. 1H NMR (CDCl3, 600 MHz):
d
¼ 7.5581e7.4978 (m, 10
H), 7.4395 (d, J ¼ 8.04 Hz, 2 H), 7.1961 (t, J ¼ 7.98 Hz, 2 H), 7.0955 (t,
J ¼ 7.98 Hz, 2 H), 6.9799 (d, J ¼ 8.04 Hz, 2 H), 5.1585 (s, 1 H), 1.8574
(s, 6 H), 1.9098e1.7802 (m, 4 H), 1.6122e1.5701 (m, 4 H), 1.4554e
1.4328 (m, 4 H),1.2735e1.2556 (m, 4 H); 13C NMR (CDCl3,150 MHz):
before 352 nm was attributed to the
pep* transition of ligand
Hcyclopbi, while from 352 to 524 nm was the characteristic of
metal to ligand charge transfer (MLCT) absorption of iridium
complexes similar to those reported previously [22,23]. For com-
d
¼ 184.39, 178.14, 167.91, 141.15, 136.37, 135.80, 129.27, 129.08,
128.86, 128.73, 128.21, 122.93, 120.45, 120.19, 114.33, 109.34, 101.10,
plex (cyclopbi)2IrCF3, the
pep* transition absorption was much
36.45, 28.56, 26.39, 23.58, 23.07; HRMS (APCI): m/z calcd for
stronger than that of (cyclopbi)2Iracac because of the blend
pep*
C
43H41IrN4O2: 838.2859; found: 838.2871.
2.3.3. Bis[2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole][2-(3-
(trifluoromethyl)-1H-pyrazol-5-yl)pyridinate]iridium(III)
[(cyclopbi)2IrCF3]
The synthetic procedure for (cyclopbi)2IrCF3 was similar to that
of (cyclopbi)2Iracac, except that 2.0 mmol 2-(3-(trifluoromethyl)-
1H-pyrazol-5-yl)pyridine was used instead of acetylacetone. The
crude product was purified by silica column chromatography with
dichloromethane/petroleum ether (1:10, v/v) as eluent to give
0.52 g (cyclopbi)2IrCF3 as a yellow powder. Yield: 55%. 1H NMR
(CDCl3, 600 MHz):
d
¼ 8.3850 (d, J ¼ 5.22 Hz, 1 H), 7.7336e7.4262
(m,11 H), 7.3395 (m, 1 H), 7.1294 (t, J ¼ 6.06 Hz,1 H), 6.9674e6.8943
(m, 4 H), 6.8601 (d, J ¼ 7.86 Hz,1 H), 6.8005e6.7478 (m, 2 H), 5.8529
(d, J ¼ 7.92 Hz, 1 H), 5.6751 (d, J ¼ 8.1 Hz, 1 H), 1.9276e1.2584 (m, 16
H); 13C NMR (CDCl3, 150 MHz):
d
¼ 183.58, 181.12, 167.99, 167.33,
156.75, 150.99, 150.27, 140.59, 139.74, 137.04, 136.12, 136.08, 135.94,
135.55, 129.46, 129.34, 129.21, 129.00, 128.96, 128.92, 128.68,
128.30, 128.06, 123.50, 122.53, 121.93, 121.75, 121.00, 120.40, 119.65,
119.19, 114.81, 112.27, 109.89, 109.27, 102.29, 35.88, 34.95, 26.82,
Fig. 1. UVeVis and PL spectra of complexes (cyclopbi)2Iracac and (cyclopbi)2IrCF3 in
dichloromethane solution.

L. Han et al. / Dyes and Pigments 99 (2013) 1010e1015
1013
Table 2
Electroluminescence characteristics of complexes (cyclopbi)2Iracac and (cyclo-
pbi)2IrCF3 with different doping concentration, (ppy)2Iracac was used as reference.
Brightnessc CIE (x, y)d
a
b
Complex
Doping
EL
Uon hc
concentration (nm) (V) (cd/A) (cd/m2)
(wt%)
(cyclopbi)2Iracac
8
537, 5.4 22.3
561
540, 5.3 22.1
565
16,300
14,100
0.42, 0.56
0.42, 0.55
12
(cyclopbi)2IrCF3
(ppy)2Iracac
8
12
8
550 6.5 12.9
4200
4400
29,000
20,700
0.40, 0.56
0.41, 0.56
0.31, 0.63
0.31, 0.63
552 6.6
9.2
520 5.3 16.9
520 5.8 26.4
12
a
Turn on voltage.
b
Maximum current efficiency.
Maximum brightness.
CIE at 8 V.
c
d
5-yl) pyridine compared to acetylacetone. The optical band gaps
(Eg) of (cyclopbi)2Iracac and (cyclopbi)2IrCF3 were calculated about
2.28 and 2.20 eV, as a result the LUMO were calculated from HOMO
and Eg, estimated to be ꢀ2.63 and ꢀ2.90 eV, respectively. For
Fig. 2. Cyclic voltammetry curves of (cyclopbi)2Iracac and (cyclopbi)2IrCF3 in
dichloromethane solution.
(cyclopbi)2Iracac, the LUMO was only contributed by the
p orbital
transition of Hcyclopbi and 2-(3-(trifluoromethyl)-1H-pyrazol-5-
yl)pyridine ligand. The blue shifted singlet MLCT (1MLCT) absorp-
tion band of complex (cyclopbi)2IrCF3 compared to (cyclo-
pbi)2Iracac was mainly because of the stronger electron
withdrawing property of 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)
pyridine than acetylacetone, which led to higher energy MLCT state
and blue-shifted photoluminescence [23e26]. However, the triplet
MLCT (3MLCT) absorption of the two complexes was same at
518.2 nm. Both the complexes show strong greenish yellow phos-
phorescence at room temperature in dichloromethane solution
with peak wavelength 545 and 542 nm, respectively. The PL
quantum yields (4) of (cyclopbi)2Iracac and (cyclopbi)2IrCF3 were
calculated to be 0.22 and 0.25, respectively, with fac-Ir(ppy)3 as the
standard.
of the cyclometalated ligand cyclopbi and independent of the
ancillary ligand, however, the 2-(3-(trifluoromethyl)-1H-pyrazol-5-
3.2. Electrochemistry characteristics
Fig. 2 depicts the cyclic voltammograms of the two complexes.
The onset oxidation (Eox) of (cyclopbi)2Iracac and (cyclopbi)2IrCF3
were 0.49 and 0.68 V, respectively, which were usually considered
as the metal-centered couple i.e. IrIII/IrIV of the complexes [27,28].
The oxidation and reduction onset of ferrocene were 0.47 and
0.29 V, respectively, then the HOMO of (cyclopbi)2Iracac and
(cyclopbi)2IrCF3 were estimated to be ꢀ4.91 and ꢀ5.10 eV,
respectively, according to the previously reported method [29]. In
cyclometalated iridium complexes, the HOMO energy level nor-
mally localized on the metal and cyclometalated ligand, and
because of the stronger electron donating ability of cyclohexenyl
than styryl, the HOMO of (cyclopbi)2Iracac was higher
than ꢀ5.11 eV of reported (psbi)2Iracac [18]. (cyclopbi)2IrCF3 exhibit
low HOMO compared to (cyclopbi)2Iracac was the result of strong
electron withdrawing ability of 2-(3-(trifluoromethyl)-1H-pyrazol-
Table 1
Photophysics and electrochemistry characteristics of complexes (cyclopbi)2Iracac
and (cyclopbi)2IrCF3 in dichloromethane solution.
Complex
Absorption (nm) PL
4
Eox
(V)
Eg
HOMO LUMO
(eV)
(nm)
(eV) (eV)
(cyclopbi)2Iracac 279.7, 378.3,
437.9, 518.2
(cyclopbi)2IrCF3 306.1, 364.1,
411.6, 518.2
545
0.22 0.49 2.28 ꢀ4.91 ꢀ2.63
0.25 0.68 2.20 ꢀ5.10 ꢀ2.90
Fig. 3. EL spectra of devices based on (cyclopbi)2Iracac and (cyclopbi)2IrCF3. a) With
doping concentration 3 wt% and 5 wt% at 8 V; b) With doping concentration 8 wt% at
8 V.
542

1014
L. Han et al. / Dyes and Pigments 99 (2013) 1010e1015
yl) pyridine ligand decreased the HOMO and LUMO of (cyclo-
pbi)2IrCF3 significantly. The photophysics and electrochemistry
characteristics of complexes (cyclopbi)2Iracac and (cyclopbi)2IrCF3
were summarized in Table 1.
3.3. Electroluminescence characteristics
In order to explore the EL characteristics of (cyclopbi)2Iracac and
(cyclopbi)2IrCF3, multi-layer devices were fabricated as ITO/m-
MTDATA (10 nm)/NPB (20 nm)/CBP:iridium complexes (30 nm)/
TPBi (30 nm)/LiF/Al. NPB was used as HTL, CBP was selected as host
because of the good overlap of the absorption of iridium complexes
and the PL of CBP (380 nm) [30], TPBi was used as electron trans-
porting and exciton blocking layer followed by LiF and Al as cath-
ode. As a comparison, (ppy)2Iracac was employed with doping
concentration 8 and 12 wt%.
The doping concentration was optimized from 3 wt%, 5 wt% to
8 wt% and 12 wt%, among which the device characteristics con-
taining 8 and 12 wt% emitters in the EML were summarized in
Table 2. Fig. 3a shows the EL spectra of (cyclopbi)2Iracac and
Fig. 5. Luminance versus current density relationship of (cyclopbi)2Iracac and
(cyclopbi)2IrCF3 with doping concentration 8 wt% and 12 wt%, respectively.
(cyclopbi)2IrCF3 at doping concentration
respectively. Being as expected CBP emission was obtained for 3 wt
% doped (cyclopbi)2Iracac and 3 wt%, 5 wt% doped (cyclopbi)2IrCF3
3 wt% and 5 wt%,
because of the incompletely Dexter energy transfer from host to
dyes caused by the low doping concentration. The EL spectra
around 410 nm were stronger for (cyclopbi)2IrCF3 compared to
(cyclopbi)2Iracac at corresponding doping concentration was due
to the delocalized HOMO of (cyclopbi)2IrCF3 which make a large
barrier for hole injection from NPB to the emitter. When the doping
concentration increased to 8 and 12 wt%, pure greenish yellow
phosphorescence was obtained for both the devices (Fig. 3b). For
(cyclopbi)2Iracac, the EL spectra show vibration structure with two
peak wavelength 537 and 561 nm at 8 wt% doping concentration,
slightly red shift was obtained when the doping concentration
improved to 12 wt%, the CIE changed from (0.42, 0.56) to (0.42,
0.55) which was measured at 8 V respectively as summarized in
Table 2. For (cyclopbi)2IrCF3, the peak wavelength was 550 nm with
CIE (0.40, 0.56) at doping concentration 8 wt% under applied
voltage 8 V.
Fig. 4a shows the hc and current density relationship of (cyclo-
pbi)2Iracac and (cyclopbi)2IrCF3 with doping concentration 8 and
12 wt% respectively, the maximum hc of 8 wt% doped (cyclo-
pbi)2Iracac device was 22.3 cd/A at current density 3.1 mA/cm2,
which was comparable to that of (ppy)2Iracac (Fig. 4b). When the
doping concentration increased to 12 wt%, maximum hc 22.1 cd/A
was still maintained indicating nearly no concentration quenching.
However, efficiency dropped significantly because of the strong
concentration quenching [31e33]. As described in Table 2, the turn
on voltage of device employed 8 wt% (cyclopbi)2Iracac was much
lower than that of (cyclopbi)2IrCF3 as the result of the high HOMO
energy of (cyclopbi)2Iracac which can balance the hole and electron
in the EML. The maximum brightness of 8 wt% (cyclopbi)2Iracac
doped device was 16,300 cd/m2, 14,100 cd/m2 was obtained when
the doping concentration increased to 12 wt% as described in Fig. 5.
The brightness of (cyclopbi)2IrCF3 based devices was relatively
lower than that of (cyclopbi)2Iracac may be resulted from the high
drive voltage caused by the unbalanced hole and electron mobility
in the EML.
4. Conclusions
In conclusion, 2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole
and its iridium complexes (cyclopbi)2Iracac and (cyclopbi)2IrCF3
were designed and synthesized for the first time. Photophysics,
electrochemistry and electroluminescence characteristics were
investigated and high efficiency greenish yellow OLEDs with
maximum hc 22.3 cd/A and maximum brightness 16,300 cd/m2 was
Fig. 4. Current efficiency versus current density of devices based on (cyclopbi)2Iracac,
(cyclopbi)2IrCF3 and (ppy)2Iracac with doping concentration
8 wt% and 12 wt%,
respectively. a) (cyclopbi)2Iracac and (cyclopbi)2IrCF3; b) (ppy)2Iracac.

L. Han et al. / Dyes and Pigments 99 (2013) 1010e1015
[8] Wang Q, Chen YH, Chen JS, Ma DG. Appl Phys Lett 2012;101:133302.
1015
obtained employing (cyclopbi)2Iracac as the emitter, which was
comparable to that of traditional phosphorescent dye (ppy)2Iracac.
We thought the higher energy level of cyclohexenyl compared to
that of phenyl and the excellent electron transporting ability of
benzoimidazole lead to easily hole injecting and balanced electron
and hole mobility in the emitting layer, which may helpful for
designing efficiency phosphores. On the other hand, it’s important
to develop greenish yellow phosphorescence dyes for white OLEDs
designing and explore their potential application in solid-state
lighting and display.
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Acknowledgment
This work was supported by Outstanding Young Scientists
Research Award Fund of Shandong Province (BS2010NJ021),
Shandong Provincial Natural Science Foundation (ZR2011BZ007),
Qingdao Municipal Science and Technology Program (11-2-4-22-
hz), and State Key Laboratory of Luminescent Materials and Devices
(South China University of Technology). We thank Dr. Xiao Li and
Haijun Chi for the NMR analysis in University of Science and
Technology Liaoning.
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