Efficient blue and bluish-green iridium phosphors: Fine-tuning emissions of FIrpic by halogen substitution on pyridine-containing ligands

Article abstract of DOI:10.1016/j.orgel.2013.09.026

Three new iridium compounds, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic, were designed and synthesized by introducing the F, Cl and Br atoms to the 4-position of pyridine ring in the frame of sky-blue emitter, FIrpic. Adding F atom stabilizes the HOMO level of FIrpic but keeps the LUMO level of FIrpic almost unchanged, which consequently broadens the HOMO-LUMO gap of FIrpic and finely tunes the emission to 465 nm of 4-F-FIrpic from 470 nm of FIrpic. In contrast, introducing of Cl and Br atoms simultaneously lowers the HOMO and LUMO levels of FIrpic, which brings about the squeeze of HOMO-LUMO gap in FIrpic and makes the emissions of 4-Cl-FIrpic and 4-Br-FIrpic red-shift to 475 and 479 nm, respectively. The phosphorescent organic light-emitting devices using the three iridium compounds as dopants were fabricated with the following configuration: ITO/MoO₃/TAPC/TCTA:dopants/Tm/LiF/Al. The device based on 4-F-FIrpic showed a blue emission with the Commission Internationale de L'Eclairage coordinate of (0.15, 0.28), and revealed rather high efficiencies, with maximum current efficiency of 29 cd A^-1, power efficiency of 29 lm W ^-1 and external quantum efficiency of 14.6%.

Full text of DOI:10.1016/j.orgel.2013.09.026

Organic Electronics 14 (2013) 3163–3171
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient blue and bluish-green iridium phosphors: Fine-tuning
emissions of FIrpic by halogen substitution on
pyridine-containing ligands
Cong Fan a, Liping Zhu b, Bei Jiang a, Cheng Zhong a, Dongge Ma b, , Jingui Qin a, Chuluo Yang a,
⇑
⇑
a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials,
Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s
Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new iridium compounds, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic, were designed and
synthesized by introducing the F, Cl and Br atoms to the 4-position of pyridine ring in
the frame of sky-blue emitter, FIrpic. Adding F atom stabilizes the HOMO level of FIrpic
but keeps the LUMO level of FIrpic almost unchanged, which consequently broadens the
HOMO–LUMO gap of FIrpic and finely tunes the emission to 465 nm of 4-F-FIrpic from
470 nm of FIrpic. In contrast, introducing of Cl and Br atoms simultaneously lowers the
HOMO and LUMO levels of FIrpic, which brings about the squeeze of HOMO–LUMO gap
in FIrpic and makes the emissions of 4-Cl-FIrpic and 4-Br-FIrpic red-shift to 475 and
479 nm, respectively. The phosphorescent organic light-emitting devices using the three
iridium compounds as dopants were fabricated with the following configuration: ITO/
MoO3/TAPC/TCTA:dopants/Tm/LiF/Al. The device based on 4-F-FIrpic showed a blue emis-
sion with the Commission Internationale de L’Eclairage coordinate of (0.15, 0.28), and
revealed rather high efficiencies, with maximum current efficiency of 29 cd Aꢀ1, power effi-
ciency of 29 lm Wꢀ1 and external quantum efficiency of 14.6%.
Received 26 July 2013
Received in revised form 12 September
2013
Accepted 14 September 2013
Available online 27 September 2013
Keywords:
Halogen atoms
Iridium compounds
Hammett parameters
Phosphorescent organic light-emitting
diode
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
cyclometalated iridium (III) compounds are proved to be
the most promising blue phosphor dyes due to their high
Phosphorescent organic light-emitting diodes (PhOL-
EDs) using organo-transition metal complexes as the phos-
phor dyes can harvest both electro-generated singlet and
triplet excitons, achieving 100 % internal quantum effi-
ciency [1]. Among the red-green-blue trichromatic emis-
sions, blue light plays the indispensable role for
achieving full-color displays and generating white light in
combination with a complementary color for illumination
[2]. However, the efficiencies and stabilities of blue PhOL-
EDs still need to be greatly improved [3]. To the present,
phosphorescent quantum yield, microsecond-range life-
time and thermal stability among the organo-transition
metal complexes [4].
Due to the spin–orbit coupling (SOC) and configuration
interaction (CI), iridium (III) compounds exhibit sophisti-
cated fine structures of electronic states [5,6]. The emissive
lowest excited triplet state (T1) is in fact nondegenerate
and splitted into three substrates without the external
magnetic field [7]. Each substrate possesses relatively indi-
vidual radiative and non-radiative kinetic characteristics,
depending on the degree of the contributions from the vir-
tual excited singlet states mixed in each substrate for the
CI effect [8]. These involved virtual excited singlet states
would break the spin-forbidden transition and provide
⇑
Corresponding authors. Tel.: +86 27 68756101 (C. Yang).
E-mail addresses: mdg1014@ciac.jl.cn (D. Ma), clyang@whu.edu.cn
(C. Yang).
1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.09.026

3164
C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
the allowednesses of optical transitions to the electronic
singlet ground state (S0). As a result, modification on
iridium compounds by introducing functional groups could
disturb the fine electronic states and tune their photolumi-
nance (PL) emissions.
some of which were expected to arise due to coupling to
19F, and these couplings had not been assigned. Elemental
analysis of carbon, hydrogen and nitrogen was performed
on Vario EL-III microanalyzer. EI-MS spectra were recorded
on VJ-ZAB-3F-Mass spectrometer. Thermogravimetric
analysis (TGA) was undertaken with a NETZSCH STA
449C instrument and the thermal stability of the samples
under nitrogen atmosphere was determined by measuring
their weight loss, heated at a rate of 10 °C minꢀ1 from 25 °C
to 600 °C.
Based on the model of sky blue-emissive bis [2-(40,60-
0
difluoro)phenylpyridinato-N,C2 ]iridium(III) picolinate (FIr-
pic) [9], some strategies are successfully adopted to broaden
the gaps between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) of iridium (III) compounds to obtain blue emission,
such as adding electron-donating groups (–OMe [10], –
NMe2 [11]) to the pyridine moiety to elevate the LUMO le-
vel, or adding electron-withdrawing groups (–F [12], –CF3
[13], –CN [14], –COOMe [15], –PO (–SO2) [16]) to the phenyl
moiety to pull down the HOMO level. Recently, Baranoff
et al. demonstrated that the Hammett parameters would
be useful to correlate the chemical structures of halogen-
substituted phenyl rings to the electronic properties of
FIrpic [17,18]. The HOMO and LUMO levels of FIrpic were
2.2. Photophysical characterization
UV–vis absorption spectra were recorded on Shimadzu
UV-2550 spectrophotometer with baseline corrected. Stea-
dy-state emission spectra were recorded on Hitachi F-4500
fluorescence spectrophotometer. Absolute photolumines-
cence quantum yields measured in CH2Cl2 were recorded
on FLS920 spectrometer with Xenon light source (450 W)
through the Edinburgh Instruments integrating sphere.
The integrating sphere is 150 mm in diameter and has its
inner surface coated with Barium Sulfate (BaSO4). Time-re-
solved measurements in CH2Cl2 solution were performed
on FLS920 spectrometer using the time-correlated single-
photon counting (TCSPC) option and the Edinburgh Instru-
ments picoseconds pulsed diode laser (Model: EPL-375) as
the light source. The excited state lifetime data were ana-
lyzed using F900 software by minimizing the reduced chi
differently affected by the inductive
(rm) effect and
resonance ( p) effect of halogen atoms. Interestingly, few
r
studies report the employment of electron-withdrawing
groups to the pyridine moiety, which are generally sup-
posed to stabilize the LUMO level and consequently narrow
the HOMO–LUMO gap of FIrpic.
In this article, we designed and synthesized three irid-
ium compounds, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic,
by introducing fluorine, chlorine and bromine atoms to
the 4-position of pyridine ring in the frame of FIrpic. The
4-F-FIrpic molecule exhibited blue-shift emission with
peak emission at 465 nm compared to the 470 nm of FIrpic,
whereas 4-Cl-FIrpic and 4-Br-FIrpic showed red-shift emis-
sions with peak emission at 475 and 479 nm, respectively
[19]. The analysis of cyclic voltammetry and quantum
chemistry calculations indicated that F atom decreased
the HOMO level of FIrpic while kept the LUMO level of
FIrpic almost undisturbed. The phosphorescent organic
light-emitting device based on 4-F-FIrpic exhibited a blue
emission with CIE coordinate of (0.15, 0.28), and revealed
rather high efficiencies, with maximum current efficiency
of 29 cd Aꢀ1, power efficiency of 29 lm Wꢀ1 and external
quantum efficiency of 14.6%.
squared function
( ) and visual inspection of the
v2
weighted residuals. After collecting the data, a lifetime va-
lue in the window of the F900 software was initially given,
then the F900 software will automatically get a analyzed
lifetime value and a corresponding value of the reduced
chi squared function (v
2). When the given value of v2 is
in the range of 1.0–1.2, the analyzed lifetime value of the
iridium complex is supposed to be well fitted. All the sam-
ples were fresh and carefully prepared. Deaerated samples
were prepared by purging argon for 60 min.
2.3. Cyclic voltammetry
Cyclic voltammetric measurements were carried out by
using a CHI voltammetric analyzer. Tetrabutylammonium
hexafluorophosphate (0.1 M) was used as the supporting
electrolyte. The conventional three-electrode cell with a
Pt work electrode of 2 mm diameter, a platinum-wire
counter electrode and a Ag/AgCl reference electrode was
employed. The scan rate is 0.1 V sꢀ1. At the end of each
experiment, the ferrocene/ferricenium (Fc/Fc+) couple
was used as the internal standard. The HOMO and LUMO
energy levels (eV) of the four compounds are calculated
according to the formula: ꢀ[4.8 eV+(E1/2(reversible)/Eonset(irre-
versible) ꢀ E1/2(Fc/Fc+))].
2. Experimental section
2.1. General information
All reactions were carried out using Schlenk tube in an
argon atmosphere. All reagents commercially available
were used as received unless otherwise stated. The sol-
vents (tetrahydrofuran, diethyl ether, dichloromethane)
were purified by conventional procedure and distilled un-
der argon before using. 1H NMR spectra were measured on
Varian Unity 300 MHz spectrometer using CDCl3 as solvent
and tetramethylsilane as an internal reference. 13C NMR
(75 MHz) spectra were measured on Varian Unity
300 MHz spectrometer using CDCl3 as solvent. The chemi-
cal shifts reported for the 13C NMR spectra reflected the
positions of the observed peaks at the frequency given,
2.4. Theoretical calculations
The geometrical and electronic properties of all the four
iridium compounds were carried out with the Gaussian 09
package [20]. All the geometries were optimized at B3LYP/
cc-pVDZ level with cc-pVDZ-pp pseudo potential on irid-
ium (III) metal. Then the time-dependent density

C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
3165
functional theory (TD-DFT) was used to calculate their trip-
2.5.2. Synthesis of 2-(20,40-difluorophenyl)-4-chloropyridine
(4-Cl-dfppy)
let state energies and a subsequent nature transition orbi-
tal (NTO) analysis was used to visualize the triplet state
distribution.
The procedure is same as 4-F-dfppy by using 2,4-dichlo-
ropyridine as the reagent. Yield: 70%. 1H NMR (300 MHz,
CDCl3) d [ppm]: 8.50 (d, J = 5.1 Hz, 1H), 7.97–7.89 (m,
1H), 7.69 (s, 1H), 7.19–7.17 (m, 1H), 6.95–6.80 (m, 2H).
2.5. Device fabrication and measurement
13C NMR (75 MHz, CDCl3)
d [ppm]: 165.12, 164.97,
162.24, 162.07, 161.79, 161.62, 158.87, 158.72, 153.56,
150.20, 144.27, 132.08, 132.01, 124.19, 124.05, 122.46,
111.99, 111.71, 104.62, 104.29, 103.93. MS (EI, m/z): [M]+
calcd for C11H6ClF2N, 225.02; found, 225.01. Anal. calcd
for C11H6ClF2N (%): C 58.56, H 2.68, N 6.21; found: C
58.47, H 2.54, N 6.40.
The sky-blue FIrpic, hole-injection material MoO3,
hole-transporting
material
1,1-bis[(di-4-tolylamino)
phenyl]cyclohexane (TAPC), host material 4,40,400-tris
(carbazol-9-yl)-triphenylamine (TCTA) and electron-trans-
porting material 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene
(Tm) were commercially available. The three iridium com-
plexes were purified by crystallization in CH2Cl2/hexane
solution. Commercial indium tin oxide (ITO) coated glass
2.5.3. Synthesis of 2-(20,40-difluorophenyl)-4-bromopyridine
(4-Br-dfppy)
with sheet resistance of 10
X
/square was used as the
A mixture of 2,4-dibromopyridine (7.1 g, 30 mmol), 2,4-
difluorophenylboronic acid (4.7 g, 30 mmol), Pd(PPh3)4
(22 mg, 5% mmol), Na2CO3 (3.8 g, 36 mmol) in 20 ml of tet-
rahydrofuran (THF) and 5 ml of distilled water was re-
fluxed for 2 days under argon. The mixture was extracted
with CHCl3 and dried over anhydrous Na2SO4. The product
was purified by column chromatography on silica gel using
1: 10 (v:v) ethyl acetate/petroleum as eluent to give a
white solid. Yield: 30%. 1H NMR (300 MHz, CDCl3) d
[ppm]: 8.49 (d, J = 5.1 Hz, 1H), 8.02–7.91 (m, 2H), 7.41–
7.39 (m, 1H), 7.01–6.86 (m, 2H). 13C NMR (75 MHz, CDCl3)
d [ppm]: 165.13, 164.97, 162.20, 162.04, 161.79, 161.62,
158.69, 153.47, 150.10, 133.01, 132.11, 132.03, 127.19,
127.05, 125.48, 122.31, 122.16, 112.01, 111.73, 104.67,
104.33, 103.98. MS (EI, m/z): [M]+ calcd for C11H6BrF2N,
268.97; found, 268.98. Anal. calcd for C11H6BrF2N (%): C
48.92, H 2.24, N 5.19; found: C 49.01, H 2.04, N 5.22.
starting substrates. Before device fabrication, the ITO glass
substrates were pre-cleaned carefully and treated by UV/
O3 for 2 min. Then the sample was transferred to the depo-
sition system. 10 nm of MoO3 was firstly deposited to ITO
substrates, followed by 60 nm TAPC, 20 nm emissive layer
and 30 nm Tm. Finally, a cathode composed of 1 nm of lith-
ium fluoride and 120 nm of aluminum was sequentially
deposited onto the substrates in the vacuum of 10ꢀ6 Torr
to construct the devices. In the deposition of the emissive
layer, the host and guest were placed into different evapo-
rator sources. The deposition rates of both host and guest
were controlled with their correspondingly independent
quartz crystal oscillators. The evaporation rates were mon-
itored by a frequency counter and calibrated by a Dektak
6M profiler (Veeco). The I–V–B of all devices was measured
with a Keithey 2400 Source meter and a Keithey 2000
Source multimeter equipped with a calibrated silicon pho-
todiode. The electroluminance (EL) spectra were measured
by JY SPEX CCD3000 spectrometer. All measurements were
carried out at room temperature under ambient
conditions.
2.5.4. Synthesis of 4-F-FIrpic
A mixture of 2.5 equiv of 2-(20,40-difluorophenyl)-4-
fluoropyridine (4-F-dfppy, 0.53 g, 2.5 mmol) and 1 equiv
of IrCl3ꢁ3H2O (0.35 g, 1 mmol), 15 ml of 2-ethoxyethanol
and 5 ml of H2O was refluxed at 120 °C for 24 h under an
argon atmosphere. After back to room temperature, the
mixture was poured into water and the formed precipitate
was filtered, washed by water, ethanol and diethyl ether to
obtain the intermediate, assumed to be a chloro-bridged
dimer. Without further purification, a mixture of the inter-
mediate (0.39 g, 0.3 mmol), Na2CO3 (0.32 g, 3 mmol), and
picolinic acid (0.19 g, 1.5 mmol) was added into 20 ml 1,
2-dichloroethane solvent. The reaction was refluxed at
80 °C for 12 h under an argon atmosphere, then extracted
with CH2Cl2 and dried over anhydrous Na2SO4. The product
was obtained by column chromatography on silica gel
using 1: 10 (v:v) ethanol/CH2Cl2 as eluent to give a yellow
solid. Overall yields: 40%. 1H NMR (300 MHz, CDCl3) d
[ppm]: 8.70 (t, J = 6.9 Hz, 1H), 8.35 (d, J = 7.8 Hz, 1H),
8.03–7.94 (m, 3H), 7.77 (d, J = 4.8 Hz, 1H), 7.47 (t,
J = 6.3 Hz, 1H), 7.36 (t, J = 6.6 Hz, 1H), 7.02–6.96 (m, 1H),
6.80–6.75 (m, 1H), 6.55–6.40 (m, 2H), 5.86–5.83 (m, 1H),
5.60–5.56 (m, 1H). 13C NMR (75 MHz, CDCl3) d [ppm]:
172.75, 171.52, 168.66, 167.29, 165.24, 164.83, 162.92,
162.64, 162.51, 160.73, 153.08, 151.99, 150.90, 150.36,
148.58, 138.89, 128.92, 127.82, 127.69, 115.13, 114.85,
114.17, 111.57, 111.16, 110.89, 99.28, 99.15, 98.87, 98.60,
2.5.1. Synthesis of 2-(20,40-difluorophenyl)-4-fluoropyridine
(4-F-dfppy)
A
mixture of 2-chloro-4-fluoropyridine (0.66 g,
5 mmol), 2,4-difluorophenylboronic acid (0.79 g, 5 mmol),
Pd(OAc)2 (0.023 g, 0.1 mmol), PPh3 (0.105 g, 0.4 mmol)
and K2CO3 (1.1 g, 8 mmol) in 10 ml of 1,2-dimethoxyeth-
ane was refluxed at 80 °C for 12 h under argon atmosphere.
After cooled to room temperature, the mixture was poured
into water, extracted with CHCl3 and dried over anhydrous
Na2SO4. The product was purified by column chromatogra-
phy on silica gel using 1: 10 (v:v) ethyl acetate/petroleum
as eluent to give a white solid. Yield: 50%. 1H NMR
(300 MHz, CDCl3) d [ppm]: 8.68–8.63 (m, 1H), 8.10–8.02
(m, 1H), 7.51 (d, J = 10.2 Hz, 1H), 7.03–6.88 (m, 3H). 13C
NMR (75 MHz, CDCl3) d [ppm]: 170.34, 166.88, 165.05,
164.89, 162.25, 162.10, 161.71, 161.55, 158.89, 158.74,
154.87, 151.51, 151.43, 131.97, 131.90, 122.36, 111.75,
111.60, 111.50, 111.36, 110.02, 109.80, 104.46, 104.11,
103.76. MS (EI, m/z): [M]+ calcd for C11H6F3N, 209.05;
found, 209.03. Anal. calcd for C11H6F3N (%): C 63.16, H
2.89, N 6.70; found: C 63.35, H 3.01, N 6.95.

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C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
98.19. MS (EI, m/z): [M]+ calcd for C28H14F6IrN3O2, 731.06;
found, 731.01. Anal. calcd for C28H14F6IrN3O2 (%): C 46.03,
H 1.97, N 5.57; found: C 46.13, H 2.00, N 5.62.
3.2. Photophysical properties
The UV–vis absorption and photoluminescence spectra
of 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic in CH2Cl2 solution
were shown in Fig. 1. In general, the intense short wave-
length absorptions between 250 and 350 nm were due pri-
2.5.5. Synthesis of 4-Cl-FIrpic
The procedure is the same as the synthesis of 4-F-FIrpic
by using 2-(20,40-difluorophenyl)-4-chloropyridine (0.56 g,
2.5 mmol) as the ligand. Overall yields: 56%. 1H NMR
(300 MHz, CDCl3) d [ppm]: 8.61 (d, J = 6.6 Hz, 1H), 8.35–
8.22 (m, 3H), 7.98 (t, J = 7.5 Hz, 1H), 7.73 (d, J = 4.5 Hz,
1H), 7.46 (t, J = 6.9 Hz, 1H), 7.29–7.19 (m, 2H), 6.99–6.97
(m, 1H), 6.54–6.39 (m, 2H), 5.84 (d, J = 8.1 Hz, 1H), 5.69
(d, J = 8.4 Hz, 1H). 13C NMR (75 MHz, CDCl3) d [ppm]:
172.34, 166.52, 164.98, 163.14, 152.66, 151.11, 148.78,
148.06, 146.46, 138.55, 128.56, 127.01, 123.40, 123.11,
122.91, 122.71, 122.53, 114.71, 114.48, 114.24, 98.62,
98.28, 97.92, 97.57. MS (EI, m/z): [M]+ calcd for C28H14Cl2-
F4IrN3O2, 763.00; found, 763.08. Anal. calcd for C28H14Cl2-
F4IrN3O2 (%): C 44.04, H 1.85, N 5.50; found: C 44.12, H
1.88, N 5.42.
marily to the spin-allowed p–
p* transitions of the ligands,
while the weak long wavelength absorptions between 400
and 460 nm could be assigned to the admixture of 1MLCT
and 3MLCT transitions [6]. The three iridium compounds
showed intense photoluminescence emission in CH2Cl2
solution at room temperature. 4-F-FIrpic molecule exhib-
ited small blue-shift emission compared to FIrpic (470,
489 nm) with the maximum emissions at 465 and
487 nm [19], which was contrary to the common view that
the electron-withdrawing F atom attaching on pyridine
ring should bring down the LUMO level and subsequently
narrow the HOMO–LUMO energy gap. On the other hand,
4-Cl-FIrpic and 4-Br-FIrpic molecules manifested red-shift
photoluminescence spectra with the maximum emissions
at 475, 501 nm and 479, 501 nm, respectively. Their abso-
lute photoluminescence quantum yields (PLQYs) measured
in deaerated CH2Cl2 solution were 40% for 4-F-FIrpic, 67%
for 4-Cl-FIrpic and 59% for 4-Br-FIrpic. Under the same
conditions, the value was 50% for FIrpic. The photo-excited
decay curves of the three iridium compounds were shown
in Fig. 2. Their fitting lifetimes were calculated to be
2.5.6. Synthesis of 4-Br-FIrpic
The procedure is the same as the synthesis of 4-F-FIrpic
by using 2-(20, 40-difluorophenyl)-4-bromopyridine (0.67 g,
2.5 mmol) as the ligand. Overall yields: 70%. 1H NMR
(300 MHz, CDCl3) d [ppm]: 8.53 (d, J = 6.3 Hz, 1H), 8.44–
8.33 (m, 3H), 7.98 (t, J = 6.6 Hz, 1H), 7.73 (d, J = 4.8 Hz,
1H), 7.47 (t, J = 6.0 Hz, 1H), 7.34 (d, J = 5.1 Hz, 1H), 7.19
(d, J = 6.3 Hz, 1H), 7.13–7.12 (m, 1H), 6.55–6.40 (m, 2H),
5.86 (d, J = 6.9 Hz, 1H), 5.60 (d, J = 6.6 Hz, 1H). 13C NMR
(75 MHz, CDCl3) d [ppm]: 165.46, 152.98, 151.77, 149.08,
148.39, 148.27, 138.94, 135.56, 129.01, 126.93, 126.64,
126.40, 126.12, 125.93, 114.95, 114.71, 98.74, 98.38,
98.02. MS (EI, m/z): [M]+ calcd for C28H14Br2F4IrN3O2,
850.90; found, 850.68. Anal. calcd for C28H14Br2F4IrN3O2
(%): C 39.45, H 1.66, N 4.93; found: C 39.80, H 1.71, N 5.01.
0.47 ls (4-F-FIrpic), 0.51 ls (4-Cl-FIrpic) and 0.44 ls (4-
Br-FIrpic), respectively, which were almost monoexponen-
tial and indicated the triplet nature of excited states [22].
All the thermal and photophysical data of 4-F-FIrpic, 4-
Cl-FIrpic and 4-Br-FIrpic were summarized in Table 1.
3.3. Electrochemical properties
The electrochemical behaviors of 4-F-FIrpic, 4-Cl-FIrpic
and 4-Br-FIrpic were examined by the cyclic voltammetry
(CV) using Fc/Fc+ couple as the internal standard. For com-
parison, the electrochemical behaviors of FIrpic were mea-
sured under the same conditions. As shown in Fig. 3, all
four iridium compounds exhibited reversible oxidation
process in CH2Cl2 solution. The half-wave oxidation poten-
tials (Eox versus Fc/Fc+) were 0.90 V (FIrpic), 1.02 V
(4-F-FIrpic), 1.03 V (4-Cl-FIrpic) and 1.02 V (4-Br-FIrpic),
respectively. Consequently, their HOMO energy levels
determined from the half-wave oxidation potentials were
ꢀ5.70 eV (FIrpic), ꢀ5.82 eV (4-F-FIrpic), ꢀ5.83 eV (4-Cl-FIr-
pic) and ꢀ5.82 eV (4-Br-FIrpic), respectively. For the
reduction behaviors in deaerated THF solution, FIrpic
and 4-F-FIrpic exhibited reversible behaviors, whereas
4-Cl-FIrpic and 4-Br-FIrpic exhibited irreversible behav-
iors. The reduction potentials (Ered versus Fc/Fc+) estimated
from the reduction onsets were ꢀ2.41 V (FIrpic), ꢀ2.40 V
(4-F-FIrpic), ꢀ2.25 V (4-Cl-FIrpic) and ꢀ2.12 V (4-Br-FIr-
pic), respectively. Accordingly, their LUMO energy levels
were ꢀ2.39 eV (FIrpic), ꢀ2.40 eV (4-F-FIrpic), ꢀ2.55 eV
(4-Cl-FIrpic) and ꢀ2.68 eV (4-Br-FIrpic). Noticeably, F, Cl
and Br atoms all lowered the HOMO level of FIrpic. How-
ever, adding F atom made LUMO level of FIrpic nearly
invariant, but Cl and Br atoms both stabilized the LUMO le-
vel. These data were listed in Table 2.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the ligands, 2-(20,40-difluorophenyl)-4-
fluoropyridine (4-F-dfppy), 4-chloro-2-(20,40-difluorophenyl)
pyridine (4-Cl-dfppy) and 4-bromo-2-(2’,4’-difluorophenyl)
pyridine (4-Br-dfppy) was depicted in Scheme 1. 4-F-dfppy
and 4-Cl-dfppy were obtained by a modified Suzuki cross
coupling reaction between 2-chloro-4-fluoropyridine/2,4-
dichloropyridine and 2,4-difluorophenyl boronic acid [10].
4-Br-dfppy was separated from the Suzuki cross coupling
reaction between 2,4-dibromopyridine and 2,4-difluoro-
phenyl boronic acid. The three iridium compounds were pre-
pared in a conventional two-step procedure, as shown in
Scheme 2 [21]. All the compounds were fully characterized
by 1H and 13C NMR spectroscopy, mass spectrum and ele-
mental analysis (see details in Section 2). The thermal
decomposition temperatures of the three iridium compounds
(Td, corresponding to 5% weight loss in the thermogravimet-
ric analysis) were in the range of 340–360 °C (Fig. S1), indi-
cating satisfactory thermal stabilities.

C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
3167
Scheme 1. Synthesis of the ligands.
Scheme 2. Synthesis of the iridium compounds.
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
10000
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
FIrpic
1.0
0.8
0.6
0.4
0.2
0.0
8000
PL
0.8
6000
0.6
4000
UV-Vis
0.4
2000
0.2
0
0
500
1000
1500
2000
0.0
250
300
350
400
450
500
550
600
Time (ns)
Wavelength (nm)
Fig. 2. Photo-excited decay curves of 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-
FIrpic in deaerated CH2Cl2 solution at room temperature.
Fig. 1. UV–vis absorption and PL spectra of 4-F-FIrpic, 4-Cl-FIrpic, 4-Br-
FIrpic and FIrpic in CH2Cl2 solution at room temperature.

3168
C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
Table 1
Thermal and photophysical properties of 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic.
ꢂ 103)
kPL,max
(nm)
U
(%)b
sobs (ls)
a
a
b
Compounds
Td (°C)
kabs max
,
(nm) (
e
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
362
357
344
259 (66.3), 383 (6.7)
262 (83.8), 386 (10.6)
263 (60.5), 385 (8.0)
465, 487
475, 501
479, 501
40
67
59
0.47
0.51
0.44
a
Measured in CH2Cl2 solution (extinction coefficient in parentheses).
b
U
= absolute photoluminescence quantum yields in deaerated CH2Cl2;
sobs = excited-state lifetimes.
3.4. DFT calculations
(rp = 0.06) exhibited similar resonance effect on FIrpic,
which indicated that substitutes from H to F would exert
The calculated HOMO and LUMO orbital distributions of
FIrpic, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic were shown
in Fig. 4. The HOMO orbitals of 4-F-FIrpic, 4-Cl-FIrpic and
little influence on the electron density of iridium (III) ion.
On the other hand, the Cl (rp = 0.23) and Br (rp = 0.23) still
exhibited strong resonance effect to the FIrpic molecule,
which consequently could bring in the stabilization of
LUMO level in FIrpic.
4-Br-FIrpic involved the participation of
p-orbitals from
the pyridine rings, which indicated the F, Cl and Br atoms
would affect the HOMO energy levels. The calculated val-
ues of HOMO/LUMO levels, energy gaps and triplet ener-
gies for the four iridium compounds were summarized in
Tab. 2. Compared to the HOMO of FIrpic (ca. ꢀ5.57 eV),
the replacement of H atom by F, Cl and Br atoms signifi-
cantly lowered the HOMO level to ca. ꢀ5.72 eV (F),
ꢀ5.76 eV (Cl) and ꢀ5.75 eV (Br). Meanwhile, the F, Cl and
Br atoms lowered the LUMO level of FIrpic (ca. ꢀ1.93 eV)
as well, but Cl (ca. ꢀ2.19 eV) and Br (ca. ꢀ2.20 eV) atoms
stabilized the LUMO more than the F atom (ca. ꢀ2.02 eV).
Consequently, the energy gap of 4-F-FIrpic (ca. 3.70 eV)
was wider than FIrpic (ca. 3.64 eV), but 4-Cl-FIrpic (ca.
3.57 eV) and 4-Br-FIrpic (ca. 3.55 eV) displayed narrowed
energy gaps. The calculated triplet energy of 4-F-FIrpic
(ca. ET 2.75 eV) was the highest among the four iridium
compounds, while the triplet energies of 4-Cl-FIrpic (ca.
ET 2.68 eV) and 4-Br-FIrpic (ca. ET 2.67 eV) were lower than
the triplet energy of FIrpic (ca. ET 2.70 eV).
3.5. Phosphorescent OLEDs
To evaluate the three new iridium compounds as the
emissive dyes in phosphorescent organic light-emitting
devices, we designed the following device structure: ITO/
MoO3 (10 nm)/TAPC (60 nm)/TCTA: 4-F-FIrpic (device A),
4-Cl-FIrpic (device B), 4-Br-FIrpic (device C) (20 nm)/Tm
(30 nm)/LiF (1 nm)/Al (120 nm). For comparison, we also
fabricated device D with FIrpic as the dopant. The device
configuration was shown in Fig. 5. In these devices, MoO3
and LiF served as hole- and electron-injecting materials;
1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and
1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm) were used as
hole- and electron-transporting materials, respectively;
4,40,400-tris(carbazol-9-yl)-triphenylamine (TCTA) was em-
ployed as host material; the four iridium compounds were
used as the guest emitters with optimized doping level at
10%. The current density–voltage–luminance (J–V–L) char-
acteristics and current efficiency–power efficiency-EQE
versus current density of all these devices were shown in
Fig. 6. All the data were collected in Table 3.
The calculation results were in good accordance with
the experimental results and can be interpreted by the
inductive (rm) and resonance (rp) effects of halogen atom,
which were quantitatively determined as the Hammett
parameters [17]. As the Hammett parameters in Table 2
showed, the F, Cl and Br atoms were in para-position to
the iridium (III) ion but meta-position to the 2,4-difluoro-
As Fig. 6 revealed, the device B based on 4-Cl-FIrpic dis-
played the best performance among the four iridium com-
pounds, with a turn-on voltage of 2.9 V, a maximum
luminance of 13,482 cd mꢀ2, a maximum current efficiency
of 39 cd Aꢀ1, a maximum power efficiency of 41 lm Wꢀ1, a
maximum EQE of 16% and a CIE coordinate of (0.18, 0.40).
phenyl ring. Due to the inductive (
rm) effect to the 2,4-di-
fluoro-phenyl ring, F, Cl and Br atoms would all stabilize
the HOMO level of FIrpic. However, the H (
rp = 0) and F
Reduction Behaviours
Oxidation Behaviours
4-F-FIrpic
(b)
(a)
4-F-FIrpic
4-Cl-FIrpic
FIrpic
4-Br-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
FIrpic
-2.5
-2.0
-1.5
-1.0
0.0 0.2 0.4
0.6 0.8 1.0 1.2 1.4
Potential (V) VS. Fc/Fc+
Potential (V) VS. Fc/Fc+
Fig. 3. Cyclic voltammogram of FIrpic, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic versus Fc/Fc+ couple: (a) oxidation behaviors in freshly distilled CH2Cl2
solution; (b) reduction behaviors in freshly distilled THF solution with oxygen removal.

C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
3169
Fig. 4. Calculated HOMO/LUMO distributions of FIrpic, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic, and their triple states in nto-HOMO/nto-LUMO distribution.
Table 2
Calculated HOMO/LUMO, energy-gap and triplet-energy values of FIrpic, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic.
HOMOa (eV)
LUMOa (eV)
rm (meta to
rp
Computed
HOMO (eV)
Computed
LUMO (eV)
Computed energy
gap (eV)
Computed triplet
energy (eV)
b
phenyl ring)b
(para to Ir)
FIrpic
ꢀ5.70
ꢀ5.82
ꢀ5.83
ꢀ5.82
ꢀ2.39
ꢀ2.40
ꢀ2.55
ꢀ2.68
0
0
ꢀ5.57
ꢀ5.72
ꢀ5.76
ꢀ5.75
ꢀ1.93
ꢀ2.02
ꢀ2.19
ꢀ2.20
3.64
3.70
3.57
3.55
2.70
2.75
2.68
2.67
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
0.34
0.37
0.39
0.06
0.23
0.23
a
HOMO/LUMO energy levels determined from the oxidation/reduction potentials in the CV.
Hammett parameters cited from Ref. [17].
b
At practical brightness of 100 cd mꢀ2 (0.40 mA cmꢀ2
3.7 V), device B appeared current efficiency of 37 cd Aꢀ1
,
,
(0.26, 0.47). This also implicated that the exciton-combina-
tion zone in device C may be close to the metal cathode,
and thus the excitons could be easily quenched by the trip-
let-polaron annihilation, consequently resulting in the low
efficiency of device C [24].
The device A based on 4-F-FIrpic represented blue emis-
sion with CIE coordinate of (0.15, 0.28). Moreover, device A
achieved satisfactory efficiencies with a maximum current
efficiency of 29 cd Aꢀ1, a maximum power efficiency of
29 lm Wꢀ1 and a maximum EQE of 14.6%. At practical
brightness of 100 cd mꢀ2 (0.41 mA cmꢀ2, 3.7 V), device A
appeared current efficiency of 28 cd Aꢀ1, power efficiency
of 23 lm Wꢀ1 and EQE of 14%. At high brightness of
1000 cd mꢀ2 (4.57 mA cmꢀ2, 4.9 V), device A still remained
high current efficiency of 21 cd Aꢀ1, power efficiency of
14 lm Wꢀ1 and EQE of 11%. The efficiencies of device
power efficiency of 31 lm Wꢀ1 and EQE of 15.3%. At high
brightness of 1000 cd mꢀ2 (3.17 mA cmꢀ2, 4.7 V), device B
still maintained high current efficiency of 30 cd Aꢀ1, power
efficiency of 20 lm Wꢀ1 and EQE of 12.5%. These efficien-
cies were better than the bluish-green iridium analogous
reported in the literature [23]. The device C based on 4-
Br-FIrpic exhibited fair performance, with a turn-on volt-
age of 3.1 V, a maximum current efficiency of 13 cd Aꢀ1, a
maximum power efficiency of 13 lm Wꢀ1 and external
quantum efficiency (EQE) less than 5%. Compared to device
B, the electroluminance (EL) spectrum of device C dis-
played more strengthened shoulder emission around
501 nm than that of 4-Cl-FIrpic (see Supporting Informa-
tion), which resulted in its red-shift CIE coordinate of

3170
C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
Fig. 5. The device configuration and chemical structures of used materials.
50
45
40
35
30
25
20
15
10
4-F-FIrpic
600
500
400
300
200
100
0
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
FIrpic
4-Cl-FIrpic
(a)
(b)
104
103
102
101
10 11
4-Br-FIrpic
FIrpic
5
0
0.01
0.1
1
10
100
2
3
4
5
6
7
8
9
Voltage (V)
Current Density (mA/cm2)
50
45
40
35
30
25
20
15
10
5
21
18
15
12
9
4-F-FIrpic
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
FIrpic
(c)
4-Cl-FIrpic
4-Br-FIrpic
FIrpic
(d)
6
3
0
0
0.01
0.1
1
10
100
0.01
0.1
1
10
100
Current Density (mA/cm2)
Current Density (mA/cm2)
Fig. 6. (a) J–V–L characteristics of device A–D; (b) current efficiency versus current density of device A–D; (c) power efficiency versus current density of
device A–D; (d) EQE versus current density of device A–D.
Table 3
Performance summary of device A–D.
Dopant
Device
V turn-on (V)a
L max (cd/m2)b
EQE (%)c
LE (cd/A)c
PE (lm/W)c
CIE (x, y)d
4-F-FIrpic
4-Cl-FIrpic
4-Br-FIrpic
FIrpic
A
B
C
D
2.9
2.9
3.1
2.9
18,773
13,482
3745
14.6/14/11
16/15.6/12.5
4.6/3.8/1
29/28/21
39/38/30
13/10/2
29/23/14
41/34/20
13/8/2
(0.15, 0.28)
(0.18, 0.40)
(0.26, 0.47)
(0.15, 0.30)
21,318
15.7/15.3/11.3
32/31/23
32/28/13
a
b
c
Turn-on voltages at 1 cd mꢀ2
Maximum luminance.
.
Order of measured efficiency values: maximum, then values at 100, 1000 cd mꢀ2 for device A–D.
Commission International de I’Eclairage (CIE) coordinates measured at 5 V.
d
A based on 4-F-FIrpic were comparable with the device D
based on FIrpic, which displayed a maximum current effi-
4. Conclusion
ciency of 32 cd Aꢀ1
, a maximum power efficiency of
In summary, three new iridium compounds have been
synthesized and investigated by introduction the F, Cl
and Br atoms to the 4-position of pyridine ring in FIrpic.
The introduction of F atom on the pyridine ring enlarged
32 lm Wꢀ1 and a maximum EQE of 15.7%. Noticeably, the
device A showed superior blue color purity than device D
with the CIE coordinate of (0.15, 0.30).

C. Fan et al. / Organic Electronics 14 (2013) 3163–3171
3171
(c) C.-H. Yang, M. Mauro, F. Polo, S. Watanabe, I. Muenster, R.
Fröhlich, L. De Cola, Chem. Mater. 24 (2012) 3684.
[4] (a) Y. Chi, P.-T. Chou, Chem. Soc. Rev. 39 (2010) 638;
(b) L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater.
23 (2011) 926;
the HOMO–LUMO gap of FIrpic by decreasing the HOMO
level while keeping the LUMO of FIrpic almost undis-
turbed, which consequently resulted in the blue-shift
emission of FIrpic. This observation broke the widely rec-
ognition that electron-withdrawing F atom is supposed
to lower the LUMO energy level. On the other hand, the
addition of Cl and Br atoms narrowed the HOMO–LUMO
gap of FIrpic and caused the emission red-shift by simulta-
neously stabilizing both the HOMO and LUMO levels of FIr-
pic. These iridium compounds exhibited good performance
in PhOLEDs. The 4-Cl-FIrpic-based device achieved a max-
imum current efficiency of 39 cd Aꢀ1, a maximum power
efficiency of 41 lm Wꢀ1 and a maximum EQE of 16% with
a bluish-green CIE coordinate of (0.18, 0.40). Remarkably,
the 4-F-FIrpic-based device achieved a maximum current
efficiency of 29 cd Aꢀ1, a maximum power efficiency of
29 lm Wꢀ1 and a maximum EQE of 14.6% with a satisfac-
tory blue CIE coordinate of (0.15, 0.28). We believe that
the device efficiencies would get improved with suitable
bipolar host materials.
(c) W.-Y. Wong, C.-L. Ho, J. Mater. Chem. 19 (2009) 4457;
(d) G. Zhou, W.-Y. Wong, X. Yang, Chem. Asian J. 6 (2011) 1706;
(e) Y. Liu, K. Ye, Y. Fan, W. Song, Y. Wang, Z. Hou, Chem. Commun.
(2009) 3699.
[5] L. Piela, Ideas of Quantum Chemistry, Elsevier, 2007.
[6] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S.
Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K.
Ueno, J. Am. Chem. Soc. 125 (2003) 12971.
[7] T. Hofbeck, H. Yersin, Inorg. Chem. 49 (2010) 9290.
[8] H. Yersin, Highly Efficient OLEDs with Phosphorescent Materials,
Wiley-VCH, Berlin, 2007.
[9] C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E.
Thompson, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 2082.
[10] L.-L. Wu, C.-H. Yang, I.-W. Sun, S.-Y. Chu, P.-C. Kao, H.-H. Huang,
Organometallics 26 (2007) 2017.
[11] D. Di Censo, S. Fantacci, F. De Angelis, C. Klein, N. Evans, K.
Kalyanasundaram, H.J. Bolink, M. Grätzel, M.K. Nazeeruddin, Inorg.
Chem. 47 (2008) 980.
[12] R. Ragni, E.A. Plummer, K. Brunner, J.W. Hofstraat, F. Babudri, G.M.
Farinola, F. Naso, L. De Cola, J. Mater. Chem. 16 (2006) 1161.
[13] S.-Y. Takizawa, H. Echizen, J.-I. Nishida, T. Tsuzuki, S. Tokito, Y.
Yamashita, Chem. Lett. 35 (2006) 748.
[14] S.H. Kim, J. Jang, S.J. Lee, J.Y. Lee, Thin Solid Films 51 (2008) 722.
[15] M. Marín-Suárez, B.F.E. Curchod, I. Tavernelli, U. Rothlisberger, R.
Scopelliti, I. Jung, D.D. Censo, M. Grätzel, J.F. Fernández-Sánchez, A.
Fernández-Gutiérrez, M.K. Nazeeruddin, E. Baranoff, Chem. Mater.
24 (2012) 2330.
[16] C. Fan, Y. Li, C. Yang, H. Wu, J. Qin, Y. Cao, Chem. Mater. 24 (2012)
4581.
[17] C. Hansch, A. Leo, R.W. Taft, Chem. Rev. 91 (1991) 165.
[18] E. Baranoff, B.F. Curchod, F. Monti, F. Steimer, G. Accorsi, I. Tavernelli,
U. Rothlisberger, R. Scopelliti, M. Grätzel, M.K. Nazeeruddin, Inorg.
Chem. 51 (2012) 799.
Acknowledgments
We are grateful to the National Basic Research Program
of China (973 Program 2013CB834805, 2009CB623602 and
2009CB930603), the National Science Fund for Distin-
guished Young Scholars of China (No. 51125013), and the
Fundamental Research Funds for the Central Universities
of China.
[19] S.J. Yeh, M.F. Wu, C.T. Chen, Y.H. Song, Y. Chi, M.H. Ho, S.F. Hsu, C.H.
Chen, Adv. Mater. 17 (2005) 285.
Appendix A. Supplementary material
[20] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H.
Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino,
G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J.
Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin,
K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J.
Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V.
Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.02, Gaussian
Inc., Wallingford, CT, 2009.
[21] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C.
Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc.
123 (2001) 4304.
[22] A.B. Tamayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N.
Ho, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377.
[23] Y.-C. Zhu, L. Zhou, H.-Y. Li, Q.-L. Xu, M.-Y. Teng, Y.-X. Zheng, J.-L. Zuo,
H.-J. Zhang, X.-Z. You, Adv. Mater. 23 (2011) 4041.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2013.09.026.
References
[1] (a) M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, M.E. Thompson,
S.R. Forrest, Nature 395 (1998) 151;
(b) Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin, D. Ma,
Angew. Chem. Int. Ed. 47 (2008) 8104.
[2] H. Sasabe, J.-I. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast,
N. Langer, O. Molt, E. Fuchs, C. Lennartz, J. Kido, Adv. Mater. 22
(2010) 5003;
(b) R. Wang, D. Liu, H. Ren, T. Zhang, H. Yin, G. Liu, J. Li, Adv. Mater.
23 (2011) 2823;
(c) L. Chen, C. Yang, J. Qin, J. Gao, H. You, D. Ma, J. Organomet. Chem.
691 (2006) 3519;
(d) C.-L. Ho, W.-Y. Wong, New J. Chem. 37 (2013) 1665.
[3] (a) C. Fan, F. Zhao, P. Gan, S. Yang, T. Liu, C. Zhong, D. Ma, J. Qin, C.
Yang, Chem.-Eur. J. 18 (2012) 5510;
[24] (a) Z. Wu, L. Wang, G. Lei, Y. Qiu, J. App. Phys. 97 (2005) 103105;
(b) S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328.
(b) L. Chen, H. You, C. Yang, D. Ma, J. Qin, Chem. Commun. (2007)
1352;