Two novel orange cationic iridium(III) complexes with multifunctional ancillary ligands used for polymer light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2012.09.035

Two novel orange cationic iridium complexes [(npy)₂Ir(o-phen)] PF₆ and [(npy)₂Ir(c-phen)]PF₆ were synthesized. Hnpy: 2-(naphthalen-1-yl)pyridine; o-phen: a 1,10-phenanthroline derivative containing an electron-transporting functional group of 2,5-diphenyl-1,3,4- oxadiazole and a crystallization-resistant tert-butyl functional group; c-phen: a 1,10-phenanthroline derivative containing a hole-transporting functional group of carbazole and a crystallization-resistant 2-ethylhexyl functional group. Both of them are amorphous and possess high thermal stability with 5% weight-reduction temperatures (ΔT_5%) of 386°C and 383°C, and glass-transition temperatures (T_g) of 267°C and 195°C respectively. They were used as phosphorescent dopants in polymer light-emitting diodes (PLEDs) fabricated by solution-processed technology with configuration of ITO/PEDOT:PSS/PVK:PBD:iridium complex/TPBI/CsF/Al. At the optimal doping concentration of 2.0 wt%, the corresponding PLEDs exhibited the maximum current efficiencies of 9.1 cd A^-1 and 10.0 cd A ^-1, the maximum external quantum efficiencies of 6.5% and 7.1%, and the maximum luminance of 2314 cd m^-2 and 3157 cd m^-2 respectively, with the same CIE coordinates of (0.57, 0.40). The results indicate that cationic iridium complexes are promising candidates for PLED applications when they are designed reasonably.

Full text of DOI:10.1016/j.orgel.2012.09.035

Organic Electronics 13 (2012) 3211–3219
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Two novel orange cationic iridium(III) complexes with multifunctional
ancillary ligands used for polymer light-emitting diodes
Huaijun Tang a,b, , Yanhu Li b, Baofeng Zhao b, Wei Yang b, Hongbin Wu b, Yong Cao b
⇑
a The Engineering Laboratory of Polylactic Acid-Based Functional Materials of Yunnan Province, School of Chemistry and Biotechnology,
Yunnan University of Nationalities, Kunming 650500, PR China
b Institute of Polymer Optoelectronic Materials and Devices, Key Lab of Specially Functional Materials of the Ministry of Education,
South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel orange cationic iridium complexes [(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-phen)]
PF6 were synthesized. Hnpy: 2-(naphthalen-1-yl)pyridine; o-phen: a 1,10-phenanthroline
derivative containing an electron-transporting functional group of 2,5-diphenyl-1,3,4-oxa-
diazole and a crystallization-resistant tert-butyl functional group; c-phen: a 1,10-phenan-
throline derivative containing a hole-transporting functional group of carbazole and a
crystallization-resistant 2-ethylhexyl functional group. Both of them are amorphous and
Received 11 August 2012
Received in revised form 13 September
2012
Accepted 15 September 2012
Available online 22 October 2012
possess high thermal stability with 5% weight-reduction temperatures (DT5%) of 386 °C
Keywords:
and 383 °C, and glass-transition temperatures (Tg) of 267 °C and 195 °C respectively. They
were used as phosphorescent dopants in polymer light-emitting diodes (PLEDs) fabricated
by solution-processed technology with configuration of ITO/PEDOT:PSS/PVK:PBD:iridium
complex/TPBI/CsF/Al. At the optimal doping concentration of 2.0 wt%, the corresponding
PLEDs exhibited the maximum current efficiencies of 9.1 cd AÀ1 and 10.0 cd AÀ1, the max-
imum external quantum efficiencies of 6.5% and 7.1%, and the maximum luminance of
2314 cd mÀ2 and 3157 cd mÀ2 respectively, with the same CIE coordinates of (0.57, 0.40).
The results indicate that cationic iridium complexes are promising candidates for PLED
applications when they are designed reasonably.
Cationic iridium complex
Polymer light-emitting diode
Oxadiazole
Carbazole
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
liquid crystal display backlights, and solid-state lighting
sources [2,16]. LECs usually require only a single emissive
In the recent years, cationic iridium(III) complexes have
attracted more and more interest because of their unique
photophysical properties and applications in light-emitting
electrochemical cells (LECs) [1–7], organic light-emitting
diodes (OLEDs, including both small-molecule and poly-
mer light-emitting diodes) [8–14] and bioimaging [15].
Both LECs and OLEDs are under intense investigation due
to potential applications in full-color flat-panel displays,
layer without extra ancillary layers and use ionic charges
to facilitate electronic charge injection from the electrodes
into the active layer [1–7]. In comparison with conven-
tional OLEDs, LECs offer several advantages such as very
simple device architectures, being independent of the
thickness of the active layer and being used with air-stable
electrodes [1–7]. However, some grievous drawbacks such
as slow response, severe excited-state quenching due to
LECs being composed of neat films of emissive materials
have hindered the practical applications of the LECs
[9–11]. On the contrary, such drawbacks can be signifi-
cantly suppressed in OLEDs by doping the phosphors in
host materials as emissive layer as well as using extra
functional layers. Thus some cationic iridium complexes
⇑
Corresponding author at: The Engineering Laboratory of Polylactic
Acid-Based Functional Materials of Yunnan Province, School of Chemistry
and Biotechnology, Yunnan University of Nationalities, Kunming 650500,
PR China.
E-mail address: tanghuaijun@sohu.com (H. Tang).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.09.035

3212
H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
had been used as phosphorescent dopants in OLEDs for
obtaining high EL efficiency [8–14].
trix solvent. Elemental analyses (EA) were performed on a
Vario EL Elemental Analysis Instrument. Ultraviolet–visible
(UV–Vis) absorption spectra were recorded on a HP8453E
spectrophotometer. Photoluminescence (PL) spectra were
recorded on a Fluorolog–3 spectrophotometer. Differential
scanning calorimetry (DSC) curves were obtained on a Net-
zsch DSC200 analyzer at a heating rate 10 °C minÀ1 under
N2, in DSC measurements of the iridium complexes, both
of them underwent two heating cycles. Thermogravimetry
(TG) curves were obtained on a Netzsch TG209 thermal
analyzer at a heating rate 20 °C minÀ1 under N2. Cyclic vol-
tammetry (CV) was performed on a computer-controlled
CHI800C electrochemical analyzer with the cationic irid-
ium complexes dissolved in anhydrous and Ar-saturated
MeCN solutions containing 0.1 mol LÀ1 tetra-n-butylam-
monium hexafluorophosphate (TBAPF6) as the supporting
electrolyte at a scanning rate of 50 mV sÀ1. A glass carbon
electrode was used as working electrode, Ag/AgCl elec-
trode as reference electrode, and platinum wire as counter
electrode. Ferrocene (4.8 eV under vacuum) was used as
the internal standard.
In contrast to neutral iridium complexes, cationic irid-
ium complexes can be more easily synthesized and puri-
fied with very high yield even getting near to theoretical
values [3,5]. Nevertheless, few cationic iridium complexes
had been used as phosphorescent dopants in OLEDs,
mostly because they are special inorganiÀc salts which con-
tain inorganic acid ion such as PFÀ6 ; ClO4 ; BF4À for balanc-
ing positive charge [1–14]. As special inorganic salts,
most of them usually are crystalline and possess inferior
compatibility with host materials that results in phase seg-
regation in OLEDs [9]. Moreover, excluding strong polar
solvents, cationic iridium complexes usually are insoluble
or slightly soluble in many ordinary organic solvents such
as dichloromethane, toluene, chlorobenzene, which have
hampered their OLEDs being fabricated by conventional
solution-processed technology. At the same time, their
OLEDs also are difficult to be fabricated by vacuum-depos-
ited technology because of their poor sublimability [9].
The compounds for OLEDs containing dendritic or long
chain-like alkyls usually are beneficial to improving amor-
phous nature, miscibility with host materials and their sol-
ubility [17–19]. Some organic groups such as carbazole,
triphenylamine and oxadiazole having carrier-transporting
function are beneficial to improving the performance of EL
materials [16]. Orange EL materials are not only important
monochromatic materials [20–25], but also are important
components for preparing two-element complementary
white-light OLEDs with blue EL materials [20,21]. Up to
now, special study of orange cationic iridium complexes
used in OLEDs is not documented. In this work, two novel
orange cationic iridium complexes were synthesized by
using 2-(naphthalen-1-yl)pyridine (Hnpy) as main ligand
(anionic ligand) and 1,10-phenanthroline derivatives as
auxiliary ligand (neutral ligand). For the auxiliary ligand,
one 1,10-phenanthroline derivative contains a electron-
transporting functional group of 2,5-diphenyl-1,3,4-oxadi-
azole and a crystallization-resistant functional group of
tert-butyl, the other 1,10-phenanthroline derivative con-
tains a hole-transporting functional group of carbazole
and a crystallization-resistant functional group of 2-ethyl-
hexyl. The cationic iridium complexes are expected to pos-
sess good solubility, amorphous nature, miscibility with
host materials and carrier-transporting property, thus,
they are preferred for OLED application with good EL
performance.
2.2. Synthesis and characterization of the cationic iridium
complexes
The synthetic routes of the cationic iridium complexes
are shown in Scheme 1, experimental details and charac-
terization data are given in the following.
2.2.1. 2-(Naphthalen-1-yl)pyridine (1, npy)
2-(Naphthalen-1-yl)pyridine (npy) was synthesized
according to the published procedures [26]. 2-Bromopyri-
dine (2.90 g, 18.4 mmol), 1-naphthaleneboronic acid
(3.16 g, 18.4 mmol), and tetrakis-(triphenylphosphine)pal-
ladium (0.20 g, 0.17 mmol) were dissolved in toluene
(30.0 mL) and ethanol (10.0 mL), then an aqueous solution
of 2 mol LÀ1 Na2CO3 (11.0 mL) was added to the mixture.
The result mixture was stirred at 100 °C for 24 h. The sol-
vents were distilled off and the residue was dissolved in
dichloromethane (50.0 mL), then washed with water, and
dried (anhydrous Na2CO3). After the evaporation of sol-
vent, the pure product was obtained by silica gel column
chromatography, eluting with a mixture of CH2Cl2 and
n-hexane (volume ratio, 1:4). Yield 72.0% (2.70 g), white
solid. 1H NMR (300 MHz, CDCl3, 25 °C, ppm): d = 8.80 (d,
1H, 3J = 4.5 Hz, ArH), 8.10 (dd, 1H, 3J = 7.2 Hz, ArH), 7.92
(d, 2H, 3J = 6.9 Hz, ArH), 7.80–7.87 (m, 1H, ArH), 7.45–
7.63 (m, 5H, ArH), 7.32–7.36 (m, 1H, ArH). Anal. Calc. for
C15H11N (%): C, 87.77; H, 5.40; N, 6.82. Found (%): C,
2. Experimental
87.53; H, 5.48; N, 6.99.
2.1. General information
2.2.2. [(npy)2Ir(o-phen)]PF6
The chloro-bridged dimmer (npy)2Ir(l-Cl)2Ir(npy)2 (2)
Chemicals and reagents were purchased from commer-
cial sources and used without further purification unless
otherwise stated. 1H NMR spectra were recorded on a Bru-
ker AV300 spectrometer operating at 300 MHz, tetrameth-
ylsilane (TMS) was used as internal standard. Mass spectra
(MS) were obtained on a Bruker Esquire HCT PLUS liquid
chromatography mass spectrometer (LC-MS) with an elec-
trospray ionization (ESI) interface using acetonitrile as ma-
was prepared following a literature method [27]. The
auxiliary ligand 2-(4-tert-butylphenyl)-5-(4-(1-ethyl-1
H-imidazo
[4,5-f][1,10]phenanthrolin-2-yl)phenyl)-1,3,
4-oxadiazole (o-phen) was synthesized using the proce-
dures previously developed [28]. [(npy)2Ir(o-phen)]PF6
was synthesized by reacting (npy)2Ir(l-Cl)2Ir(npy)2
(0.51 g, 0.40 mmol) with o-phen (0.44 g, 0.84 mmol) in
glycol (30 mL) under argon at 150 °C for 16 h, followed

H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
3213
Scheme 1. Synthetic routes of the cationic iridium complexes.
by ion exchange reaction with a solution of ammonium
hexafluorophosphate in water (0.30 mol LÀ1, 12.0 mL) [3],
the resulting flocculent precipitate was filtered, washed
with water and then dried. The pure product was obtained
by silica gel column chromatography, eluting with a mix-
ture of CH2Cl2 and acetonitrile (volume ratio, 10:1). Yield
90.3% (0.92 g), red solid. 1H NMR (300 MHz, CDCl3, 25 °C,
ppm): d = 9.31 (s, 1H, ArH), 9.13 (d, 1H, 3J = 9.0 Hz, ArH),
8.56–8.65 (m, 4H, ArH), 8.37 (s, 2H, ArH), 8.12 (t, 4H,
3J = 8.4 Hz, ArH), 7.96 (s, 3H, ArH), 7.73–7.86 (m, 5H,
ArH), 7.33–7.62 (m, 10H, ArH), 7.04 (t, 1H, 3J = 6.0 Hz,
ArH), 6.88 (t, 1H, 3J = 6.6 Hz, ArH), 6.40 (dd, 2H,
3J = 5.7 Hz, ArH), 4.87 (q, 2H, 3J = 9.0 Hz, –CH2–), 1.39 (s,
9H, –C(CH3)3), 0.88 (t, 3H, 3J = 7.5 Hz, –CH3). ESI–MS (m/
Z): 1125.2 [M–PF6]+; Anal. Calc. for C63H48F6IrN8OP (%): C,
59.57; H, 3.81; N, 8.82. Found (%): C, 59.38; H, 3.85; N, 8.93.
with stirring. The mixture was extracted with CH2Cl2,
washed with water and dried (anhydrous Na2SO4). The sol-
vent was distilled off and the residue was chromato-
graphed over silica gel, eluting with
a mixture of
petroleum ether (b.p. 60–90 °C) and ethyl acetate (volume
ratio, 10:1). Yield 89.0% (9.96 g), yellow oil. 1H NMR
(300 MHz, CDCl3, 25 °C, ppm): d = 8.08 (d, 2H, 3J = 5.7 Hz,
ArH), 7.42–7.46 (m, 2H, ArH), 7.37 (d, 2H, 3J = 6.0 Hz,
ArH), 7.19–7.23 (m, 2H, ArH), 4.25 (t, 2H, 3J = 5.4 Hz, –N–
CH2–), 1.81–1.86 (m, 2H, –CH2–), 1.23–1.38 (m, 10H, al-
kyl-H), 0.85 (t, 3H, 3J = 5.4 Hz, –CH3). Anal. Calc. for
C20H25N (%): C, 85.97; H, 9.02; N, 5.01. Found (%): C,
86.07; H, 8.85; N, 5.08.
2.2.4. 3-Formyl-9-(2-ethylhexyl)-9H-carbazole (4)
POCl3 (2.0 mL, 21.5 mmol) was added dropwise into
dried N,N-dimethylformamide (DMF, 25.0 mL) with stir-
ring in an ice bath, then 9-(2-ethylhexyl)-9H-carbazole
(5.60 g, 20.0 mmol) was added. The mixture was slowly
heated to 80 °C and kept for 6 h. The cooled reaction mix-
ture was slowly poured into ice-water (about 200 mL) with
stirring, and neutralized to pH ꢀ 7.0 with Na2CO3. The mix-
ture was extracted with CH2Cl2 (3 Â 50 mL), washed with
water and dried (anhydrous Na2SO4). After the evaporation
2.2.3. 9-(2-Ethylhexyl)-9H-carbazole (3)
After a mixture of KOH (14.00 g, 250.0 mmol) and car-
bazole (6.60 g, 40.0 mmol) in acetone (80.0 mL) was stirred
for 30 min, another mixture of 2-ethyl-1-bromohexane
(11.60 g, 60.0 mmol) in acetone (20.0 mL) was added drop-
wise in with stirring, and then the stirring was kept for
10 h. The reaction mixture was slowly poured into water

3214
H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
of solvent, the pure product was obtained by silica gel col-
umn chromatography, eluting with a mixture of petroleum
ether (b.p. 60–90 °C) and CH2Cl2 (volume ratio, 2:1). Yield
72.0% (4.44 g), brownish-yellow oil. 1H NMR (300 MHz,
CDCl3, 25 °C, ppm): d = 9.94 (s, 1H, –CHO), 8.42 (d, 1H,
4J = 1.2 Hz, ArH), 7.99 (d, 1H, 3J = 7.8 Hz, ArH), 7.85 (dd,
1H, 3J = 8.4 Hz, 4J = 1.5 Hz, ArH), 7.37–7.42 (m, 1H, ArH),
7.27–7.31 (q, 2H, ArH), 7.16–7.21 (t, 1H, ArH), 4.12 (t, 2H,
3J = 7.2 Hz, –N–CH2–), 1.67–1.76 (m, 2H, –CH2–), 1.11–
1.12 (m, 10H, alkyl-H), 0.75 (t, 3H, 3J = 6.6 Hz, –CH3). Anal.
Calc. for C21H25NO (%): C, 82.04; H, 8.20; N, 4.56. Found (%):
C, 81.95; H, 8.14; N, 4.64.
1H, 3J = 6.3 Hz, ArH), 7.29–7.64 (m, 12H, ArH), 7.04 (t, 1H,
3J = 6.0 Hz, ArH), 6.88 (t, 1H, 3J = 7.2 Hz, ArH), 6.41 (dd,
2H, 3J = 6.6 Hz, ArH), 4.90 (q, 2H, –N–CH2–), 4.38 (t, 2H,
3J = 6.9 Hz, –N–CH2–), 1.88–1.95 (m, 2H, –CH2–), 1.66 (t,
3H, 3J = 6.3 Hz, –CH3), 1.26–1.43 (m, 10H, alkyl-H), 0.86
(t, 3H, 3J = 6.6 Hz, –CH3). ESI–MS (m/Z): 1126.3 [M–PF6]+.
Anal. Calc. for C65H55F6IrN7P (%): C, 61.41; H, 4.36; N,
7.71. Found (%): C, 61.27; H, 4.48; N, 7.85.
2.3. Fabrication and measurements of EL devices
Cationic iridium complexes were dissolved in chloro-
benzene and filtered with a 0.45
dium tin oxide (ITO) coated glass substrates with a sheet
resistance of 15–20 /h were used as anodes. After a sub-
lm filter. Patterned in-
2.2.5. 2-(9-(2-Ethylhexyl)-9H-carbazol-3-yl)-1H-imidazo
[4,5-f][1,10]phenanthroline (5)
1,10-Phenanthroline-5,6-dione was synthesized as the
reported [29]. A mixture of 1,10-phenanthroline-5,6-dione
(2.10 g, 10.0 mmol), 3-formyl-9-(2-ethylhexyl)-9H-carba-
X
strate being sufficiently cleaned with acetone, detergent,
distilled water and 2-propanol in ultrasonic baths, and
then treated with oxygen plasma for 4 min, an anode buf-
fer layer of poly(3,4-ethylenedioxythiophene) doped with
poly(styrene sulfonate) (PEDOT:PSS) was spin-coated on
and dried by baking in a vacuum oven at 80 °C for 8 h.
The light-emitting layer consisted of poly(N-vinylcarbaz-
ole) (PVK, host materials), 2-(4-biphenylyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (PBD, electron-transporting
materials), and cationic iridium(III) complexes (phospho-
rescent dopants) at different concentrations was spin-
coated on the buffer layer in a glove box containing less
than 10 ppm oxygen and moisture, then baked at 100 °C
in inert atmosphere for 20 min to remove solvent residue.
The thickness of these spin-coated films was measured by
an Alfa Step 500 surface profilometer. A hole-blocking and
zole
(3.07 g,
10.0 mmol),
CH3COONH4
(15.40 g,
200.0 mmol) and glacial acetic acid (50.0 mL) was refluxed
for 2 h. Water (about 100 mL) was added in and the solu-
tion was neutralized to pH ꢀ 7.0 with concentrated ammo-
nia after the reaction mixture being cooled, the resulting
flocculent precipitate was filtered, washed with water
and ethanol, and then dried in a vacuum oven. The product
was used directly in the next reaction without further puri-
fication and characterization.
2.2.6. 1-Ethyl-2-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1H-
imidazo[4,5-f][1,10]phenanthroline (c-phen, 6)
NaH (0.6 g, 25.0 mmol) was added in a mixture of dried
compound 5 (2.49 g, 5.0 mmol) in 50.0 mL DMF (dried with
MgSO4 and distilled in vacuum at 76 °C) with stirring. After
being stirred at room temperature for 1 h and then C2H5Br
(2.18 g, 20.0 mmol) being added, the mixture was heated
to 110 °C and kept for 15 h in the presence of Ar. The
cooled reaction mixture was poured in water and extracted
with CHCl3 (3 Â 80 mL). The extract was washed with
water for several times and dried with anhydrous Na2SO4.
The solvent was distilled off to afford the crude product
after Na2SO4 was filtrated. The crude product was purified
by silica gel column chromatography, eluting with CH2Cl2.
Yield 55.1% (1.45 g), pale yellow powder. 1H NMR
(300 MHz, CDCl3, 25 °C, ppm): d = 9.12–9.20 (m, 3H, ArH),
8.67 (d, 1H, 3J = 8.1 Hz, ArH), 8.47 (s, 1H, ArH), 8.15 (d,
1H, 3J = 7.8 Hz, ArH), 7.82 (d, 1H, 3J = 8.4 Hz, ArH), 7.71–
7.75 (q, 2H, ArH), 7.46–7.59 (m, 3H, ArH), 7.30 (d, 1H,
3J = 7.5 Hz, ArH), 4.74 (q, 2H, –N–CH2–), 4.38 (t, 2H,
3J = 6.9 Hz, –N–CH2–), 1.88–1.95 (m, 2H, –CH2–), 1.64 (t,
3H, 3J = 7.2 Hz, –CH3), 1.26–1.37 (m, 10H, alkyl-H), 0.87
(t, 3H, 3J = 6.6 Hz, –CH3). Anal. Calc. for C35H35N5 (%): C,
79.97; H, 6.71; N, 13.32. Found (%): C, 80.15; H, 6.58; N,
13.27.
electron-transporting
layer
of
1,3,5-tris(N-phen-
ylbenzimidazol-2-yl)-benzene (TPBI) was thermally evap-
orated with a base pressure of 3 Â 10À4 Pa, then so do
the electron injection layer of CsF and subsequent alumi-
num cathode layer. The thickness of the thermal deposi-
tion layers was monitored by a quartz crystal thickness/
ratio monitor (Model: STM-100/MF, Sycon). The current
density–luminance–voltage (I–L–V) characteristics were
measured by a Keithley 236 sourcemeasurement unit and
a calibrated silicon photodiode. EL spectra and CIE coordi-
nate were recorded by a spectrophotometer (SpectraScan
PR-705, Photo Research).
3. Results and discussion
3.1. Thermal properties
The thermal properties of the complexes were mea-
sured by TG and DSC as shown in Fig. 1. The TG results
showed that both [(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-
phen)]PF6 have high thermal stability with 5% weight-
reduction temperatures
(DT5%) of 386 °C and 383 °C
2.2.7. [(npy)2Ir(c-phen)]PF6
respectively. Because the coordination bond is the weakest
bond in such complexes, the neutral auxiliary ligands usu-
ally lost firstly in thermal decomposition [30]. While in our
case, the high thermal stability of [(npy)2Ir(o-phen)]PF6
and [(npy)2Ir(c-phen)]PF6 was probably due to the loss of
the auxiliary ligands being hindered because of high inter-
molecular cross-linking caused by the dendritic tert-butyl
[(npy)2Ir(c-phen)]PF6 was prepared following the simi-
lar procedure as that of above [(npy)2Ir(o-phen)]PF6. Yield
88.3%, red solid. 1H NMR (300 MHz, CDCl3, 25 °C, ppm):
d = 9.33 (s, 1H, ArH), 9.16 (d, 1H, 3J = 7.8 Hz, ArH), 8.56–
8.64 (m, 4H, ArH), 8.48 (s, 1H, ArH), 8.11–8.20 (m, 3H,
ArH), 7.96 (s, 1H, ArH), 7.78–7.86 (m, 5H, ArH), 7.71 (t,

H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
3215
[(npy)2Ir(o-phen)]PF6
[(npy)2Ir(c-phen)]PF6
100
90
1st heating
2nd heating
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
[(npy)2Ir(o-phen)]PF6
80
[(npy)2Ir(c-phen)]PF6
70
50
100 150 200 250 300 350
60
Temperature (oC)
100
200
300
400
500
Temperature (oC)
Fig. 1. TG and DSC (inset) curves of the cationic iridium complexes.
and long chain-like 2-ethylhexyl, at the same time, big
bulk and irregularity of the neutral auxiliary ligands also
retard their loss.
0.04
0.02
[(npy)2Ir(o-phen)]PF
6
0.00
The complexes [(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-
phen)]PF6 showed no melting peaks or crystallization
peaks but only high glass-transition temperatures (Tg) at
267 °C and 195 °C respectively. The results suggested that
both of them are amorphous and difficult to crystallize
and consequently cause phase disengagement in EL de-
vices, which are desirable for high stability and efficiency
[17–19]. Compared with being easy to crystallize of the
archetype cationic iridium complex [Ir(ppy)2(phen)][PF6]
-0.02
-0.04
-2.5 -2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
[(npy)2Ir(c-phen)]PF
0.03
0.00
6
-0.03
-0.06
(Hppy = 2-phenylpyridine,
phen = 1,10-phenanthroline)
[31], the amorphous nature of them is presumably caused
by the dendritic tert-butyl and long chain-like 2-ethyl-
hexyl in the 1,10-phenanthroline derivatives. Moreover,
the additional groups of 2,5-diphenyl-1,3,4-oxadiazole
and carbazole are appended to the 1,10-phenanthroline
unit via a rotating single bond and are non-coplanar with
1,10-phenanthroline unit due to steric hindrance, which
also are barriers for crystallization.
-2.5 -2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
Potential (V) vs. Fc+/Fc
Fig. 2. Cyclic voltammograms of the cationic iridium complexes in CH3CN
solution of 1 Â 10À3 mol LÀ1 at the scan rate of 50 mV sÀ1. Potentials were
recorded versus Fc+/Fc.
ples of the complexes are both caused by IrIII/IrIV oxidation
and the reduction of the phen unit respectively, their cyclic
voltammograms are very similar, and their HOMO and
LUMO levels are also close to each other as listed in Table
1. The HOMO levels of the iridium complexes are À5.34 eV
and À5.38 eV, and the LUMO levels are À2.83 eV and
À2.84 eV respectively, these data lie between the HOMO
(À5.80 eV) and LUMO (À2.2 eV) levels of PVK which sug-
gest PVK is suitable host material for them [11].
3.2. Electrochemical properties and HOMO–LUMO energy
levels
The electrochemical behavior of the complexes was
studied by cyclic voltammetry (CV) using ferrocene as
the internal standard. The cyclic voltammograms are
shown in Fig. 2 and the results are summarized in Table
1. The complexes [(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-
phen)]PF6 exhibit very similar cyclic voltammograms, each
complex has a reversible one-electron oxidation couple at
positive potentials, the E1/2, ox are 0.54 V and 0.58 V respec-
tively. This can be attributed to metal-centered IrIII/IrIV oxi-
dation couple [9]. At negative potentials, each has a
3.3. UV–Vis absorption and PL spectra
The UV–Vis absorption and PL spectra of two cationic
iridium complexes in CH2Cl2 solution (a), in pristine com-
plex films (b) and doped in PVK films (c) are shown in
Fig. 3, and some optical data of the complexes in CH2Cl2
solution are listed in Table 2. The UV–Vis absorption spec-
tra of the complexes in dilute solution (in Fig. 3a) and in
reversible reduction couple, the E1/2,
are À1.97 V and
red
À1.96 V respectively. Such reversible reduction couple
can be assigned to orbitals centered mainly on the Ir-phen
fragment and presumably caused by the reduction of the
phen unit [32]. Because the oxidation and reduction cou-

3216
H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
Table 1
Table 2
Electrochemical data and energy levels of the cationic iridium complexes.
Some optical data of the complexes in CH2Cl2 solution.a
Complex
E1/2,
E1/2,
HOMO
(eV)b
LUMO
(eV)b
Eg
Complex
UV–vis absorption
PL
ox
red
(V)a
(V)a
(eV)b
kabs,max
(nm)
e
kex
(nm)
kem,max
(nm)
[(npy)2Ir(o-
phen)]PF6
[(npy)2Ir(c-
phen)]PF6
0.54
0.58
À1.97
À1.96
À5.34
À5.38
À2.83
À2.84
2.51
2.54
(L molÀ1 cmÀ1
)
[(npy)2Ir(o-
phen)]PF6
[(npy)2Ir(c-
phen)]PF6
295
286
34,932
45,096
370
370
584
586
Measured in Ar–saturated CH2Cl2, 0.1 mol LÀ1 TBAPF6, scan rate
a
50 mV sÀ1, versus Fc+/Fc couple.
1.0 Â 10À5 mol LÀ1, at room temperature.
a
b
HOMO = Àe(E1/2, ox) + (À4.8) eV, LUMO = Àe(E1/2, red) + (À4.8) eV and
Eg = LUMO À HOMO.
1MLCT is caused by the strong spin–orbit coupling induced
by the iridium heavy atom [9]. However, the UV–vis
absorption spectra of the complexes doped in PVK films
are completely different from that of the former two, and
the absorption is actually dominated by PVK instead of cat-
ionic iridium complexes for 2 wt% being a very low doping
[(npy)2Ir(o-phen)]PF
1.2
1.0
0.8
0.6
0.4
0.2
0.0
6
(a)
[(npy)2 Ir(c-phen)]PF
6
Absorption
concentration. The molar extinction coefficients (e) of
PL
[(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-phen)]PF6 are
34932 L molÀ1 cmÀ1 and 45096 L molÀ1 cmÀ1 respectively,
such high extinction coefficients probably indicate the en-
ergy can be efficiently transmitted to central Ir(III) through
the peripheric organic ligands.
The PL spectra of two cationic iridium complexes in
CH2Cl2 solution (in Fig. 3a) all are mainly located at orange
light band from 540 nm to 710 nm, the maximum emission
wavelength of [(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-
phen)]PF6 are 584 nm and 586 nm respectively. The PL
spectra of the complexes in pristine solid complex films
(in Fig. 3b) also are very similar to those in solution. Nev-
ertheless, the PL spectra of the complexes in pristine solid
complex films have small red shifts relative to those in
solution, due to stronger interactions between the iridium
200
300
400
500
Ir(o-phen)]PF
600
700
800
1.2
1.0
0.8
0.6
0.4
0.2
0.0
[(npy)2
[(npy)2
6
(b)
Ir(c-phen)]PF
6
PL
Absorption
400
500
[(npy)2Ir(o-phen)]PF
600
700
PL
800
200
300
complexes with a greater degree of
p overlap [33]. The
1.2
1.0
0.8
0.6
0.4
0.2
0.0
maximum emission wavelength of [(npy)2Ir(o-phen)]PF6
and [(npy)2Ir(c-phen)]PF6 in pristine solid complex films
are 591 nm and 595 nm respectively. The PL spectra of
the complexes doped in PVK at 2 wt% have two emission
peaks. The peaks at the left with maximum emission wave-
lengths of 407 nm can be assigned to PVK, because the low
doping concentrations are not high enough to consume the
excitons in PVK. The peaks at the right with maximum
emission wavelengths of 584 nm are almost completely
identical with the PL spectra of the complexes in solution,
obviously, such emission peaks originate from the cationic
iridium complexes.
6
(c)
[(npy)2Ir(c-phen)]PF
6
Absorption
400
500
600
700
800
200
300
Wavelength (nm)
Fig. 3. UV–Vis absorption and PL spectra of cationic iridium complexes:
(a) in CH2Cl2 solution, 1.0 Â 10À5 mol LÀ1
, kex = 370 nm; (b) pristine
complex films (on quartz plates), kex = 370 nm. (c) Doped in PVK films
(2 wt%, on quartz plates), kex = 320 nm.
3.4. EL properties
pristine solid complex films (in Fig. 3b) are substantially
similar to each other, all of which can be assigned to the
cationic iridium complexes. The strong absorption bands
(about 200–380 nm) of the absorption spectra can be as-
The cationic iridium complexes were used as phospho-
rescent dopants at three different doping concentrations in
multilayer polymer light-emitting diodes (PLEDs). The con-
figuration of the devices is ITO/PEDOT:PSS (40 nm)/emit-
signed to the spin-allowed 1p pà transition in the ligands.
–
ting
layer
(70 nm)/TPBI
(30 nm)/CsF
(1.5 nm)/Al
The relatively weak absorption bands on the right of the
strong absorption bands in the long wavelength region
(>380 nm) can be assigned to an admixture of 1MLCT (me-
(120 nm), the emitting layer is PVK (70 wt%):PBD
(30 wt%):iridium complex (x wt%), x = 1.0, 2.0 and 4.0.
The EL spectra of the devices are shown in Fig. 4. When
two complexes were doped at same concentration, EL de-
vices exhibit almost identical EL spectra. All EL devices
tal–ligand charge transfer), and 3MLCT and 3p
–
pà states
[9]. The admixture of 3MLCT and 3p
–
pà with higher-lying

H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
3217
(2)
1.0%
2.0%
4.0%
1.0%
2.0%
4.0%
(1)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 4. EL spectra of the complexes [(npy)2Ir(o-phen)]PF6 (1) and [(npy)2Ir(c-phen)]PF6 (2) in PLEDs, device configuration: ITO/PEDOT:PSS/PVK:PBD:com-
plex (70 wt%:30 wt%:x wt%)/TPBI/CsF/Al, x = 1.0, 2.0 and 4.0.
have strong emission peaks at 540–750 nm which are pri-
marily consistent with the PL spectra of the complexes,
however, the shape of these EL peaks incompletely resem-
ble their PL spectra, the low and subtle shoulders on the
peaks on the right side of the PL spectra have become
stronger in the EL spectra, this disparity was possibly
caused by slight optical microcavity effect [34,35]. Besides
the strong emission peaks at the right, weak emission at
380–480 nm originated from the PVK-PBD exciplexes can
be observed when the doping concentrations are 1.0 wt%
because the low doping concentration is not enough to
consume the excitons of the PVK [36], however, such weak
emission peaks had not emerged at 2.0 wt% and 4.0 wt%.
The performances of the EL devices using two cationic
iridium complexes at different doping concentrations are
summarized in Table 3. It can be seen that 2.0 wt% is an
optimal doping concentration to achieve the highest effi-
ciency. The luminance and current density versus voltage
characteristics of the EL devices at 2.0 wt% doping concen-
tration are showed in Fig. 5. At 2.0 wt% doping concentra-
tion, the turn-on voltages of the EL devices using
[(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-phen)]PF6 are 8.6 V
and 6.8 V respectively, and the maximum luminances are
2314 cd mÀ2 and 3157 cd mÀ2 respectively. It is very obvi-
ous that the luminance are related to current density, since
both of them increase with the increasing of the voltages
and decline after the peak value. The current efficiency ver-
sus current density characteristics of the EL devices at
2.0 wt% doping concentration are showed in Fig. 6. The
maximum luminance (current) efficiencies (LEmax) of the
devices using [(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-
phen)]PF6 are 9.1 cd AÀ1 and 10.0 cd AÀ1, and the maxi-
mum external quantum efficiencies (QEmax) are 6.5% and
7.1% respectively. The EL efficiencies decrease with
increasing current density because of triplet–triplet anni-
hilation [37] and field-induced quenching effects [38]
which are common phenomena for most phosphorescent
OLEDs. Although the efficiencies declines sharply at very
low current density (<5 mA cmÀ2), the efficiency decreases
slowly and linearly with increasing current density after
5 mA cmÀ2, which is indicative of good device stability.
The Commission Internationale de L’Eclairage (CIE) color
coordinates of the EL devices at 2.0 wt% doping concentra-
tion are (0.57, 0.40) which corresponds to the orange re-
gion in the CIE chromaticity diagram. In contrast with
already reported orange organic EL materials, the EL per-
formance of the PLEDs using [(npy)2Ir(o-phen)]PF6 and
[(npy)2Ir(c-phen)]PF6 is still far lower than that of the EL
devices using the state-of-the-art orange materials, espe-
cially, some neutral iridium complexes [20,39], neverthe-
less, the EL performance of them already better than or
comparable with that of many previously reported orange
Table 3
Performance of the EL devices.
Complex
Doping concentration (wt%)
Von (V)
Lmax (cd mÀ2
)
LEmax (cd AÀ1
)
QEmax (%)
kmax (nm)
CIE (at 12 mA mÀ2
)
[(npy)2Ir(o-phen)]PF6
1.0
2.0
4.0
7.2
8.6
10.2
2424
2314
3252
9.0
9.1
7.3
6.4
6.5
5.2
618
620
620
(0.55, 0.39)
(0.57, 0.40)
(0.58, 0.40)
[(npy)2Ir(c-phen)]PF6
1.0
2.0
4.0
6.6
6.8
8.6
2538
3157
2580
9.4
10.0
7.9
6.7
7.1
5.7
618
618
620
(0.57, 0.40)
(0.57, 0.40)
(0.59, 0.40)

3218
H. Tang et al. / Organic Electronics 13 (2012) 3211–3219
3500
3000
2500
2000
1500
1000
500
180
[(npy)2 Ir(o-phen)]PF6
[(npy)2 Ir(c-phen)]PF6
150
120
90
60
30
0
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
[(npy)2Ir(o-phen)]PF
[(npy)2Ir(c-phen)]PF
6
6
0
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
Fig. 5. Luminance and current density (inset) versus voltage characteristics of the devices ITO/PEDOT:PSS/PVK (70 wt%):PBD (30 wt%):complex (2 wt%)/
TPBI/CsF/Al.
coordination with Ir(III) ion, both 2,5-diphenyl-1,3,4-oxa-
diazole and carbazole are beneficial to improving carrier-
transporting property.
15
10
5
4. Conclusions
0
Two novel orange cationic iridium complexes using
2-(naphthalen-1-yl)pyridine as main ligand and 1,10-
phenanthroline derivatives as auxiliary ligand containing
carrier-transporting and crystallization-resistant func-
tional groups were synthesized. Both of them have high
thermal stability, good amorphous nature, good solubility
and miscibility with host materials, and were used as phos-
phorescent dopants in PLEDs fabricated by conventional
solution-processed technology. Their PLEDs with configu-
ration of ITO/PEDOT:PSS/PVK:PBD:iridium complex/TPBI/
CsF/Al exhibit good EL performance with the maximum
luminance efficiencies of 9.1 cd AÀ1 and 10.0 cd AÀ1, and
the maximum external quantum efficiencies of 6.5% and
7.1% respectively. The results indicate that cationic iridium
complexes are promising candidates to PLEDs when they
are designed reasonably.
-5
-10
-15
-20
-25
[(npy)
[(npy)
2 Ir(o-phen)]PF
6
2 Ir(c-phen)]PF
6
100
120
140
0
20
40
60
80
Current density (mA⋅cm-2)
Fig. 6. Current efficiency versus current density characteristics of the EL
devices ITO/PEDOT:PSS/PVK (70 wt%):PBD (30 wt%):complex (2 wt%)/
TPBI/CsF/Al.
materials including some iridium complexes [23–
25,40,41]. Compared with vast amounts of neutral iridium
complexes already being designed, synthesized and used
in OLEDs, very few cationic iridium complexes have been
studied and used in OLEDs, so it is reasonable to believe
that the EL performance of OLEDs using cationic iridium
complexes would be improved rapidly if more investiga-
tion is given to them.
The above-mentioned experimental results show
[(npy)2Ir(o-phen)]PF6 and [(npy)2Ir(c-phen)]PF6 possess
nearly the same photophysical, electrochemical and EL
properties, presumably due to their similar chemical struc-
ture. Both of them are cationic iridium complexes using
the same main ligand of 2-(naphthalen-1-yl)pyridine and
the same anion of PFÀ6 . Moreover, although 2,5-diphenyl-
1,3,4-oxadiazole and carbazole are electron-transporting
and hole-transporting functional groups respectively, they
are both 1,10-phenanthroline derivatives and have same
Acknowledgements
This work was supported by China Postdoctoral Science
Foundation (No. 20090450858), Key Scientific Research
Fund Supported by Education Department of Yunnan Prov-
ince (No. 2011Z003) and Youth Fund of Yunnan Universi-
ties of Nationalities (No. 11QN05).
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