Small molecular glasses based on multiposition encapsulated phenyl benzimidazole iridium(III) complexes: Toward efficient solution-processable host-free electrophosphorescent diodes

Article abstract of DOI:10.1021/jp909297d

Three electrophosphorescent small molecular Ir3+ complexes, Ir(HexPhBI)₃1 (HexPhBI = 1-Hexyl-2-phenyl-1H-benzo[d]imidazole), Ir(CzPhBI)₃ 2 (CzPhBI = 9-(6-(2-phenyl-1H-benzo[d]imidazol-1-yl) hexyl)-9H-carbazole), and Ir(Cz₂

Full text of DOI:10.1021/jp909297d

J. Phys. Chem. B 2010, 114, 141–150
141
Small Molecular Glasses Based on Multiposition Encapsulated Phenyl Benzimidazole
Iridium(III) Complexes: Toward Efficient Solution-Processable Host-Free
Electrophosphorescent Diodes
Hui Xu,* Dong-Hui Yu, Le-Le Liu, Peng-Fei Yan, Li-Wei Jia, Guang-Ming Li, and
Zheng-Yu Yue*
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education and School of Chemistry
and Materials, Heilongjiang UniVersity, Harbin 150080, People’s Republic of China
ReceiVed: September 28, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009
Three electrophosphorescent small molecular Ir3+ complexes, Ir(HexPhBI)3 1 (HexPhBI ) 1-Hexyl-2-phenyl-
1H-benzo[d]imidazole), Ir(CzPhBI)3 2 (CzPhBI ) 9-(6-(2-phenyl-1H-benzo[d]imidazol-1-yl)hexyl)-9H-
carbazole), and Ir(Cz2PhBI)3 3 (Cz2PhBI ) 9-(6-(4-(1-(6-(9H-carbazol-9-yl)hexyl)-1H-benzo[d]imidazol-2-
yl)phenoxy)hexyl)-9H-carbazole), were synthesized in which 3 was designed with the structure of multiposition
encapsulation. Compared to the hexyl-substituted 1, 2 and 3 end-capped with the conjugated carbazole moieties
have improved thermal stability. X-ray diffraction analysis proved the amorphous state of 2 and 3. High-
photoluminescent efficiencies of 3 are achieved as 72% in solution and 61% in solid. It indicates that the
peripheral carbazoles not only facilitate the separation of triplet-emission cores and reduce the intermolecular
aggregation but also supply a routine for the intermolecular energy transfer. Electrochemical analysis showed
the more oxidation states of 3, which might be anticipated to make it superior to 1 and 2 in hole injection and
transporting. The important role of the peripheral carbazole moieties in carrier injection/transporting and the
optical properties of the complexes were further investigated by Gaussian simulation. A dramatic
electroluminescent (EL) performance, including external quantum efficiency of nearly 6%, low turn-on voltage
of 2.5 V, and high brightness over 6000 cd m-2, from the host-free spin-coated device of 3 was achieved.
The superiority of multiencapsulation in EL was proved by comparing the EL performance of 2 and 3. By
making comparison between the host-free and phosphor-doping devices, it indicated that the combined
modification of the aliphatic chains and functional groups in multipositions is a feasible approach to realize
the high-efficiency small molecular phosphorescent materials.
Introduction
free or heavy-blending devices.15-27 Because of their controllable
intermolecular interactions and reduced T-T quenching, they
The organic light-emitting diods (OLEDs) based on the
electrophosphorescent emitters have been paid much attention
with the great potential for full-color flat-panel displays and
light sources.1,2 Since the phosphorescent emitters can harvest
both singlet and triplet excitons, their theoretical internal
electroluminescent (EL) quantum efficiency can reach 100%.1
However, most of the phosporescent emitters present the worse
concentration quenching and triplet-triplet (T-T) annihilation
in their solid states.3 The external quantum efficiency (EQE) of
the devices using pure Ir(ppy)3 (ppy ) 2-phenylpyridine) as
the emitting layers was less than 0.8%.2 To reduce the
concentrationquenching,thecommonapproachisdoping/blending3,4
the emitters in suitable host systems. Hosts, such as polyvinyl-
carbazole (PVK), 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole (PBD), and poly(9,9-dioctylfluorene) (PFO), served
as the energy media and the matrixes dispersing the phospho-
rescent emitters. Many excellent performances were realized.4-17
However, these doping/blending systems were inferior in phase
stability, processability, and repeatability. One of the main
solutions is linking the hosts and guests through covalent bonds.
The electrophosphorescent dendrimers and polymers were
designed as the integrations of hosts and guests for the host-
are expected as the alternatives to the doping/blending systems.
Actually, dendrimers and polymers also have another important
advantage of solution processability. Although the solution-
processed electrophosphorescent devices exhibited worse per-
formances than the corresponding vacuum-deposited devices,
it is believed that the approaches of solution-process are much
more desirable for low-cost large-area commercial applications.10
The devices based on these pure dendrimers, as well as
phosphorescent polymers, exhibited good EL performance. Also
recently with the introduction of several functional dendrons
based on carbazole, triphenyl amine, fluorene moieties, and the
design of “double-dendron” configuration, the electrophospho-
rescent performance of the dendrimers has been improved
greatly.22,28,30 Relatively, the polymer and dendrimer systems
are complicated and have some difficulties in synthesis. How
to benefit from the structure design of the dendrimers and
achieve an excellent EL performance through simpler molecular
structures is an interesting and significant challenge.
Recently, the small molecular electrophosphorescent emitters
with reduced concentration quenching for the host-free OLEDs
became attractive. Several nondoped vacuum-deposition devices
of pure small molecules with high efficiencies were reported;6,31-35
however, there are still few reports about the similar host-free
devices through solution-process. Efficient small molecular
solution-processable amorphous phosphors would be superior
* To whom correspondence should be addressed. Tel: +86 451 81773032.
Fax: +86 451 86608042. E-mail: (H.X.) hxu@hlju.edu.cn and (Z.-Y.Y.)
yuezhengyu@hlju.edu.cn.
10.1021/jp909297d  2010 American Chemical Society
Published on Web 12/02/2009

142 J. Phys. Chem. B, Vol. 114, No. 1, 2010
Xu et al.
SCHEME 1: Synthesis Procedure and the Molecular Structures of the Ir3+ Complexes
in synthesis, processability, and stability and have the potential
as one of the most important alternative for host-free solution-
processed electrophosphorescent devices. It is well-known that
the separation of triplet excitons can efficiently decrease the
quenching effect.2,29 For small molecules, the flexible aliphatic
chains cannot only efficiently restrain the intermolecular interac-
tion and aggregation but also improve the dissolvability and
film-forming ability. However, according to the previous works
about dendrimers, only using aliphatic moieties as the wrapping
groups the electric performance of the materials would remark-
ably decrease. Recently, we reported a series of zinc(II)
complexes based on carbazol-9-yl hexyl modified (hydroxyl-
phenyl) benzimidazole.36 This rigid ring end-capped aliphatic
chain can remarkably decrease the aggregation, which induced
the hypsochromic emission of the complex. The peripheral
carbazole moieties also played an important role in the carrier
injection/transporting. However, for phosphorescent molecules,
only one position modification is not enough. Thanks to the
pioneer work reported by Burn et al.,25 multiposition encapsula-
tion seems to be essential for our molecular design.
peripheral carbazole moieties were introduced to inject/transport
hole and sensitize the Ir3+ complex cores. The flexible hexyls
can reduce the intermolecular interaction and facilitate the
solution-process of the complexes. Gaussian simulation indicated
that the electron-cloud densities of the frontier orbitals (the
highest occupied molecular orbital (HOMO), HOMO-1, HOMO-
2, the lowest unoccupied molecular orbital (LUMO), LUMO+1
and LUMO+2) of the complexes were entirely located on the
cores formed by Ir3+ and phenyl benzimidazole moieties.
Therefore, the flexible hexyls and carbazole moieties surround-
ing the cores actually form the peripheral shell to isolate the
excitons. According to the space-filling models, the double bulky
chains of 3 point to six different directions and almost fill in
all of the blank spaces in 2 (Scheme 1). The investigation
showed that the carbazole end-capped aliphatic chains facilitate
the formation of amorphous state of the complex more ef-
ficiently. Moreover, 3 with a very high PL quantum yield (PL
QY) of more than 60% in film exhibits much weaker quenching
effect than 1 and 2. Considering the strong wrapping effect of
the aliphatic chains, the biggest degree of encapsulation of the
core in 3 will make a remarkable contribution to its lower
exciton concentration and weaker intermolecular interaction.
Both electrochemical results and Gaussian simulation indicated
the important role of the peripheral carbazole moieties as the
intermolecular hole-transfer groups. The EL performance of the
host-free spin-coated OLED based on 3 was improved as much
as twice of that of 2.
In this work, a double-position modified phenyl benzimida-
zole ligand 9-(6-(4-(1-(6-(9H-carbazol-9-yl)hexyl)-1H-ben-
zo[d]imidazol-2-yl)phenoxy)hexyl)-9H-carbazole (Cz2PhBI) and
its six-position encapsulated complex Ir(Cz2PhBI)3 3 were
designed and conveniently synthesized, as well as Ir(HexPhBI)3
1 (HexPhBI ) 1-Hexyl-2-phenyl-1H-benzo[d]imidazole) and
Ir(CzPhBI)3 2 (CzPhBI ) 9-(6-(2-phenyl-1H-benzo[d]imidazol-
1-yl)hexyl)-9H-carbazole) for comparison (Scheme 1). The

Phenyl Benzimidazole Iridium(III) Complexes
J. Phys. Chem. B, Vol. 114, No. 1, 2010 143
Experimental Section
J1 ) 6.9 Hz, J2 ) 7.5 Hz, 2H); 1.84-1.65 (m, 4H); 1.36-1.12
(m, 4H); ESI-MS m/z (%): 443 (100) [M+]. Elemental anal calcd
(%) for C31H29N3: C, 83.94; H, 6.59; N, 9.47. Found: C, 83.85;
H, 6.57; N 9.58.
Materials and Instruments. All the reagents and solvents
used for the synthesis of the ligands and complexes were
purchased from Aldrich and Acros companies and used without
further purification.
9-(6-(4-(1-(6-(9H-Carbazol-9-yl)hexyl)-1H-benzo[d]imida-
zol-2-yl)phenoxy)hexyl)-9H-carbazole (Cz2PhBI). White crys-
Infrared spectra were recorded using a Bruker-EQUINOX55
1
tals with yield of 78%. mp 128-132 °C. H NMR (300 MHz,
1
FT-IR spectrometer. H NMR spectra were recorded using a
CDCl3, TMS): δ ) 8.11 (dd, J1 ) 7.5, J2 ) 6.9 Hz, 4H);
7.82-7.76 (m, 1H); 7.59 (d, J ) 8.7 Hz, 2H); 7.50-7.36 (m,
6H); 7.36-7.17 (m, 9H); 6.96 (d, J ) 8.7 Hz, 2H); 4.33 (t, J )
7.2 Hz, 2H); 4.23 (t, J ) 6.9 Hz, 2H); 4.17 (t, J ) 7.5 Hz, 2H);
3.96 (t, J ) 6.6 Hz, 2H); 1.99-1.85 (m, 2H); 1.84-1.58 (m,
8H); 1.30-1.18 (m, 6H). ESI-MS m/z (%): 708 (100) [M+].
Elemental anal calcd (%) for C49H48N4O: C, 83.02; H, 6.82; N,
7.90; O, 2.26. Found: C, 83.14; H, 6.79; N, 8.08; O, 2.11.
Varian Mercury plus 300NB spectrometer relative to tetram-
ethylsilane (TMS) as internal standard. Molecular masses were
determined by a FINNIGAN LCQ Electro-Spraying Ionization-
Mass Spectrometry (ESI-MS), or a MALDI-TOF-MS. Elemental
analyses were performed on a Vario EL III elemental analyzer.
Absorption and photoluminescence (PL) emission spectra of the
target compound were measured using a SHIMADZU UV-3150
spectrophotometer and a SHIMADZU RF-5301PC spectropho-
tometer, respectively. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were performed on
Shimadzu DSC-60A and DTG-60A thermal analyzers under
nitrogen atmosphere at a heating rate of 10 °C min-1. Cyclic
voltammetry (CV) was performed on an Eco Chemie’s Autolab.
2-Phenyl-benzimidazole. Benzene-1,2-diamine (27.0 g, 0.25
mol) and benzaldehyde (25.28 mL, 0.25 mol) were solved in
200 mL DMF. Then sodium metabisulfite (47.53 g, 0.25 mol)
in 50 mL water was added in portions. The mixture was heated
to 90 °C overnight and then cooled to room temperature and
poured into water. The precipitate was filtered and then
recrystallized by ethanol as white needle crystals (42.0 g, 82%).
General Procedure for Preparation of Ir3+ Complexes.
Under N2, iridium trichloride hydrate (1 mmol, 267 mg) was
added to a solution of the ligands (1 mmol) in 2-methoxyethanol
(30 mL) and water (10 mL). The mixture was heated to 120 °C
with constant stirring for one day and then cooled to room
temperature. The amorphous green precipitates were filtrated,
washed with water (3 × 30 mL) and methanol (3 × 30 mL),
dried and used as the crude dichloro-bridged iridium dimers.
The mixture of the dimers (0.2 mmol), the corresponding ligands
(0.44 mmol), and anhydrous K2CO3 (67 mg, 0.50 mmol) in
glycerol (10 mL) was heated to 210 °C for 2 days and then
cooled to room temperature. Excess water was added. The
solution was extracted with CH2Cl2 (3 × 30 mL). The organic
phase was dried with MgSO4. The solvent was removed in
vacuo. The residues were purified by the flash column chro-
matography using CH2Cl2 as eluent to afford the complexes.
Ir(HexPhBI)3 1. Orange powder with yield of 25%. 1H NMR
(300 MHz, CDCl3, δ): 7.74 (t, J ) 7.8 Hz, 3H), 7.267-6.43
(m, 15H), 6.28 (t, J ) 7.8 Hz, 3H), 6.13 (d, J ) 8.4 Hz, 3H),
4.63 (tt, J1 ) 7.2 Hz, J2 ) 7.2 Hz, 6H), 2.06-1.78 (m, 6H),
1.41-1.12 (m, 18H), 0.91 ppm (t, J ) 6.9 Hz, 9H). MALDI-
TOF-MS (m/z (%)): 1026 (100) [M+]. Anal. Calcd (%) for
C57H65IrN6: C, 66.70; H, 6.38; N, 8.19. Found: C, 66.77; H,
6.46; N, 8.02.
1
mp 295-296 °C. H NMR (300 MHz, DMSO-d6, TMS): δ )
12.92 (s, N-H, 1H); 8.21 (d, J ) 6.9 Hz, 2H); 7.77-7.43 (m,
5H); 7.23 ppm (d, J ) 4.8 Hz, 2H). ESI-MS m/z (%): 194 (100)
[M+]. Elemental anal calcd (%) for C13H10N2: C, 80.39; H, 5.19;
N, 14.42. Found: C, 80.43; H, 5.28; N, 14.29.
4-(1H-Benzo[d]imidazol-2-yl)phenol. The synthetic proce-
dure was the same as 2-phenyl-benzimidazole, but 4-hydoxy-
benzaldehyde was used instead of benzaldehyde (45.0 g, 85%).
1
mp 281-282 °C. H NMR (300 MHz, DMSO-d6, TMS): δ )
12.64 (s, N-H, 1H); 9.96 (s, O-H, 1H); 8.03 (d, J ) 8.7 Hz,
2H); 7.73-7.36 (m, 2H); 7.25-7.09(m, 2H); 6.94 ppm(d, J )
8.4 Hz, 2H). ESI-MS m/z (%): 210 (100) [M+]. Elemental anal
calcd (%) for C13H10N2O: C, 74.27; H, 4.79; N, 13.33; O, 7.61.
Found: C, 74.38; H, 4.81; N, 13.54; O, 7.27.
Ir(CzPhBI)3 2. Yellow powder with yield of 35%. 1H NMR
(300 MHz, CDCl3, δ): 8.09 (d, J ) 7.8 Hz, 6H), 7.58 (d, J )
7.2 Hz, 3H), 7.44 (t, J ) 7.8 Hz, 6H), 7.28 (d, J ) 9.0 Hz,
6H), 7.23 (t, J ) 7.5, 6H), 7.14 (d, J ) 8.1 Hz, 3H), 6.85-6.69
(m, 6H), 6.58 (t, J ) 7.5, 3H), 6.07 (d, J ) 8.1 Hz, 3H),
4.65-4.31 (m, 6H), 4.23-4.04 (m, 6H), 1.95-1.68 (m, 12H),
1.42-1.23 ppm (m, 12H). MALDI-TOF-MS (m/z (%)): 1521
(100) [M+]. Anal. Calcd (%) for C93H86IrN9: C, 73.39; H, 5.70;
N, 8.28. Found: C, 73.41; H, 5.77; N, 8.15.
General Procedure for Preparation of the Ligands. Under
N2, the benzimidazole derivatives were dissolved in acetone.
The equivalent KOH was added and the mixture was stirred
for 30 min. Then the bromides were added in portions. The
mixture was heated to reflux for overnight and quenched by
water, and then the mixture was extracted by dichloromethane
(3 × 30 mL). The organic layer was dried with MgSO4. The
solvent was removed in vacuo. The residues were recrystallized
with petroleum ether/ethyl acetate (1:1).
1-Hexyl-2-phenyl-1H-benzo[d]imidazole (HexPhBI). White
crystals with yield of 55%. mp 40-42 °C. 1H NMR (300 MHz,
CDCl3, TMS): δ ) 7.87-7.79 (m, 1H); 7.28 (dd, J1 ) 7.8 Hz,
J2 ) 5.4 Hz, 2H); 7.57-7.48 (m, 3H); 7.46-7.37 (m, 1H);
7.35-7.27 (m, 2H); 4.25 (t, J ) 7.5 Hz, 2H); 1.89-1.73 (m,
2H); 1.29-1.15 (m, 6H); 0.85 ppm (t, J ) 6.9 Hz, 3H); ESI-
MS m/z (%): 278 (100) [M+]. Elemental anal calcd (%) for
C19H22N2: C, 81.97; H, 7.97; N, 10.06. Found: C, 81.78; H,
7.89; N, 10.33.
Ir(Cz2PhBI)3 3. Green-yellow powder with yield of 32%. 1H
NMR (300 MHz, CDCl3, δ): 8.07 (d, J1 ) 7.8 Hz, J2 ) 2.4 Hz,
12H), 7.47-7.29 (m, 24H), 7.25-7.13 (m, 18H), 7.07 (d, J )
8.1 Hz, 3H), 6.87-6.74 (br, 3H), 6.54 (t, J ) 7.5, 3H),
6.25-6.08 (br, 3H), 6.00-5.84 (br, 3H), 4.47-3.93 (m, 24H),
1.81-1.61 (m, 18H), 1.35-1.14 (m, 30H). MALDI-TOF-MS
(m/z (%)): 2318 (100) [M+]. Anal. Calcd (%) for
C147H143IrN12O3: C, 76.17; H, 6.22; N, 7.25; O, 2.07. Found:
C, 76.31; H, 6.19; N, 7.38; O, 2.36.
Theoretical Calculations. Computations on the electronic
ground state of the ligands and complexes were performed using
Becke’s three-parameter density functional in combination with
the nonlocal correlation functional of Lee, Yang, and Parr
(B3LYP).37 6-31G(d) basis sets were employed for the ligand
and the relativistic effective core potential of Los Alamos and
double-ꢀ basis sets were employed for the Ir (LANL2DZ).38
9-(6-(2-Phenyl-1H-benzo[d]imidazol-1-yl)hexyl)-9H-carba-
zole (CzPhBI). White crystals with yield of 90%. mp 114-116
1
°C. H NMR (300 MHz, CDCl3, TMS): δ ) 8.10 (d, J ) 7.8
Hz, 2H); 7.879-7.768 (m, 1H); 7.72-7.62 (m, 2H); 7.51-7.39
(m, 5H); 7.36-7.28 (m, 5H); 7.25 (t, J ) 7.8 Hz, 2H); 4.24 (tt,

144 J. Phys. Chem. B, Vol. 114, No. 1, 2010
Xu et al.
SCHEME 2: Synthesis Procedure of the Ligands
The ground-state geometries were fully optimized at the B3LYP
level. All computations were performed using the Gaussian 03
package.39
interaction would increase the intermolecular interaction. Thus,
if using aryl groups it is necessary to restrain the intermolecular
interaction through the steric effect by increasing the volume
of the aromatic substituents or the angle of torsion between them.
Many electrophosphorescent dendrimers had the similar mul-
tiaryl substituted structures. With the aim of achieving a simple
small molecular phosphor used in host-free spin-coated devices,
alkyl moieties are more suitable as the encapsulating groups,
which not only reduce the intermolecular interaction efficiently
by steric effect and easily wrap the complex cores to separate
the phosphors, but also improve the solution-processability of
the complex. However, along with the reduction of interaction
between the complex cores, the electric inertia of the alkyls
also remarkably weakens the carrier injection/transporting in
the materials. Recently, it was reported by Samuel et al.41 that
although their Ir3+ cores were also separated, carbazole dendrons
made a great contribution to the improvement of the carrier
injection/transport in the dendrimers. Therefore, we believed
that the combinations of carrier-transporting moieties and
aliphatic chains (CT-AC), such as the carbazole end-capped
alkyls in this work, should be the suitable encapsulating groups
for the small molecular electrophosphorescent complexes. The
CN ligands we chose were 2-phenyl-benzimidazole (PBI)
derivatives. The N-H in benzimidazole rings supplied one
position for alkyl substitution. Through introducing a hydroxyl
at para- position of phenyl, a double-position substitutable PBI
derivative was obtained. The space-filling models simulated by
Gaussian calculation showed that after alkylation six bulky arms
were formed in 3, which point to six different directions and
almost fill in all of the blank spaces in three carbazole-9-yl hexyl
encapsulated 2 (Scheme 1). This strongly demonstrated our
molecular design.
Device Febrication and Testing. The OLEDs with configu-
rations of ITO/PEDOT:PSS (50 nm)/Ir3+ complexes (30 nm)/
TPBI(30 nm)/LiF (1 nm)/Al (100 nm) and ITO/PEDOT:PSS
(50 nm)/Ir3+ complexes-TCTA (20%, 30 nm)/TPBI (30 nm)/
LiF (1 nm)/Al (100 nm), where PEDPT:PSS is poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) as hole-injection/
transportinglayer,TPBIis1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-
2-yl)benzene as electron-transporting hole-blocking layer, and
TCTA is tris(4-(9H-carbazol-9-yl)phenyl)amine as host, were
fabricated as in the following procedure: PEDOT:PSS films were
first deposited on precleaned ITO-coated glass substrates and
then cured at 120 °C in air for 30 min. Then the film of the
emitting layer was spin-cast with chlorobenzene as the solvent
(concentration: 10 mg mL-1) at a spin rate of 1200 rpm,
followed by deposition of TPBI at a rate of 0.2 nm s-1 at a
pressure below 1 × 10-6 mbar. Then a layer of LiF with 1 nm
thickness was deposited at a rate of 0.1 nm s-1 to improve
electron injection. Finally, a 100 nm thick layer of Al was
deposited at a rate of 0.6 nm s-1 as the cathode. The EL spectra
were measured using a PR650 spectra colorimeter. The
current-density-voltage and brightness-voltage curves of the
devices were measured using a Keithley 2400/2000 source meter
and a calibrated silicon photodiode. All the experiments and
measurements were carried out at room temperature under
ambient conditions.
Results and Discussions
Design and Synthesis. To reduce the T-T quenching, one of
the most efficient approaches is separating the phosphorescent
centers. Although some simple aryl groups can be used to
encapsulate the complex cores, their strong π-π stacking
The PBI derivatives were synthesized by condensation of
benzaldehyde derivatives and o-phenylenediamine with the

Phenyl Benzimidazole Iridium(III) Complexes
J. Phys. Chem. B, Vol. 114, No. 1, 2010 145
Figure 1. TGA and DTG curves of 1-3.
Figure 2. WAXRD spectra of 1-3.
existence of a mild oxidant sodium metabisulfite. Then the
ligands were prepared conveniently by alkylation of the PBI
derivatives with different alkyl bromides under alkaline condi-
tions as depicted in Scheme 2. The Ir3+ complexes were prepared
according to the following well-established, two-step approach40
(Scheme 1): the chloride-bridged dimers were first formed by
treating IrCl3 ·nH2O with excessive ligands in a mixed solvent
of 2-ethoxyethanol and water. Then the crude dimers reacted
with excessive ligands to convert to the facial (fac) isomer at
210 °C in glycerol. The complexes were purified through flash
1
column chromatography and characterized by H NMR, el-
emental analysis, and MALDI-TOF mass spectrometry.
Thermal and Phase Properties. The temperatures of de-
composition (Td) of 1 is 330 °C. Td of 2 and 3 increases to
402 °C (Figure 1). It was found that 1 started decomposition at
273 °C, and at 398 °C the weight loss ratio was 24%, which
just corresponds to the loss of three hexyls in 1. However, for
2 and 3 the decomposition occurred from 360 °C. At 490 °C,
the weight loss ratios of 2 and 3 were 48 and 61%, respectively,
which are attributed to the loss of the carbazol-9-yl hexyls in 2
and 3. It means that the decomposition of the hexyls at lower
temperature can be efficiently prevented by end-capping with
carbazole moieties. The intra- and intermolecular π-π interac-
tions between different carbazole moieties may retard the
decomposition of hexyls, since the additional interactions make
the decomposition need more energy, which might be the main
reason for the improved thermal stability of 2 and 3.
Figure 3. The absorption and PL spectra of the ligands in solution.
The phase states of the complexes are what we were very
concerned about, because the amorphous or disordered states
often correspond to the weak intermolecular interaction, which
is very important for decreasing T-T quenching. Although the
complexes did not have any melting points according to their
DSC analysis, in order to clearly find out the phase status of
the spin-coated films, wide-angle X-ray diffraction (WAXRD)
analysis of 1-3 was performed (Figure 2). For 1 two sharp
peaks with 2θ angles at 8 and 20° proved the existence of
crystallization in 1. Obviously, only with the alkyl substitution
the aggregation of the Ir3+ cores cannot be avoided absolutely.
Although it was doubt if the planar structure of carbazole might
facilitate aggregation, only the broad and weak peaks were found
in the WAXRD spectra of 2 and 3. Moreover, the broad peak
around 8° in the spectrum of 3 was much weaker than that of
2 and hardly to be recognized. Both 2 and 3 are amorphous. It
means that the bulky carbazol-9-yl hexyl moieties in 2 and 3
efficiently prevent the aggregation, and 3 with more carbazol-
9-yl hexyls seems to be more disordered. It is clear that the
intermolecular interaction and aggregation of 3 is the weakest
among these three complexes, which might have strong effects
on the reduction of concentration quenching and T-T annihilation.
Figure 4. The absorption and PL spectra of 1-3 in solution (solid)
and film (dash) and the emission photos of the complexes (insert, for
3 because CCD camera was saturated the emission looks like white.)
Optical Properties. To examine the photophysical properties
of the complexes, UV/vis and PL spectra of the ligands and
1-3 were measured in their dilute CH2Cl2 solutions (1 × 10-5
mol L-1) (Figures 3 and 4). The absorption peak at 289 nm of
HexPhBI is attributed to the benzimidazole moieties, and the
other peak at 231 nm corresponds to its phenyl moiety. For
CzPhBI and Cz2PhBI, except for the absorption peaks originated
from phenyl benzimidazole, the peaks at 346, 332, 263, and
236 nm are attributed to their carbazole moieties. Notably, the
emissions of CzPhBI and Cz2PhBI are similar to that of

146 J. Phys. Chem. B, Vol. 114, No. 1, 2010
Xu et al.
TABLE 1: The PL and Electrochemical Properties of the Compounds
d
compound
absorption peaks (nm)a
PL peaks (nm)a PL QY τPH (µs) Eox/Ered (V) HOMO (eV) LUMO (eV) Eg (eV)
HexPhBI 289, 231
350, 365(s)
352, 368
352, 368
1.65/-1.78
1.18/-1.73
1.00/-1.81
0.44/-1.74
0.43/-1.77
0.39/-1.84
-6.39
-5.92
-5.74
-5.18
-5.17
-5.13
-2.96
-3.01
-2.93
-3.0
-2.97
-2.9
3.43
2.91
2.81
2.18
2.20
2.23
CzPhBI
346, 332, 293, 263, 236, 221
Cz2PhBI 346, 332, 293, 263, 236, 221
1
2
3
445, 423, 388, 311, 298, 245, 227
445, 413, 376, 346, 312, 294, 262, 239 509, 541(s)
428, 407, 373, 349, 317, 295, 267, 237 491, 517(s)
514, 546(s)
0.20b/0.11c 0.39
0.34b/0.20c 0.34
0.72b/0.61c 0.20
a 10-6 M in CH2Cl2. b 10-6 M in degassed toluene using quinine sulfate in 0.5 M sulfuric acid as standard. c PL QY of the spin-coated films.
d Onset voltages in CV curves.
N-substituted carbazole derivatives, which proves the antenna
effectofthecarbazolemoietiesinthesealkyllinkeddonor-acceptor
systems.
Compared with the absorption spectra of the ligands, in the
absorption spectra of 1-3, the strong absorption bands observed
at 230-350 nm closely resemble those of the free ligands.
Especially for 2 and 3 the absorption peaks originated from the
carbazole moieties around 346 and 262 nm are recognized and
become stronger along with the increasing of the density of
carbazoles. These short-wavelength absorptions are assigned to
a ligand-centered 1π-π* transition. The broad absorption peaks
around 400 nm are metal-to-ligand charge-transfer (MLCT)
transitions. The next long tail absorptions from 450 to 500 nm
can reasonably be assigned to the mixed states containing
Figure 5. CV curves of the ligands.
3
3
spin-orbit coupling enhanced π-π* and MLCT transitions.
PL emission of 1 contains a main peak at 514 nm. The
emission of 2 is at 509 nm with a slight blue shift of 5 nm,
which should be induced by the influence of pendent carbazoles
on the energy of MLCT state through changing the coordinate
environment, local polarity and polarizability of the complex.
A much more remarkable hypsochromic shift of the emission
of 3 was observed as 20 nm (the main peak of 3 is at 491 nm).
With the exception that more pendent carbazoles induced
stronger changing of the energy of MLCT state, the effect of
the electron-donating alkoxy moieties on the frontier orbitals
of 3 should be the main factor for this considerable blue shift.
It is also noticeable that the PL spectrum of 3 in solution displays
much more discernible vibronic progressions than those of 1
and 2; in addition, in films from 1, 2, to 3 the intensity of their
shoulder peaks gradually increase. These features suggest the
successively increased molecular rigidity from 1, 2, to 3.
Furthermore, the emissions of these complexes in solutions and
spin-coated films are nearly the same, which also showed their
reduced aggregation in solid. PL QY of the complexes in
solution gradually increases from 1, 2, to 3 (Table 1). It is
noticed that compared with Ir(pbi)2acac (pbi ) 2-phenyl-N-
capped aliphatic chains can realize the simultaneous improve-
ments in sensitizing and separating Ir3+ cores. However, the
unusually steep increasing of PL QY from 2 to 3 implies that
there should be other important factors. It is believed that long
lifetime would increase the possibility of the interaction between
excited states, so the short lifetime facilitates the reduction of
T-T annihilation. The lifetime (τPH) of these complexes were
measured in air at room temperature and listed in Table 1. τPH
of 2 (0.34 µs) is slightly shorter than that of 1 (0.39 µs), but for
3 its lifetime steeply decreases to 0.2 µs, which successfully
fits to the trend of their PL QY Thus with the amorphous state
and shortest lifetime 3 exhibits much improved PL efficiencies.
With the stable PL emission and PL QY of 0.72 in solution
and 0.61 in solid, 3 is favorable among the green-emitting Ir3+
complexes. It proves that increasing the density of the sur-
rounding carbazole moieties assuredly improves the optical
performance of the complex.
Electrochemical Properties. To demonstrate the contribution
of the peripheral carbazole moieties on the carrier injection in
the complexes, circle voltammetry (CV) analysis of the ligands
and complexes was performed at room temperature in aceto-
nitrile measured against a Ag/Ag+ electrode (0.34 V vs saturated
calomel electrode (SCE)) with tetrabutylammonium hexafluo-
rophosphate (0.1 M in acetonitrile) as the supporting electrolyte
(Figures 5 and 6 and Table 1). All of the ligands have similar
irreversible reduction peaks attributed to the phenyl benzimi-
dazole moieties. Because of the electron-donating effect of the
alkoxyls, the reduction potential of Cz2PhBI is higher than those
phenyl benzimidazole, acac ) acetyl acetone, ΦPL ) 0.73)42
1
has much lower PL efficiency of only 0.2. The wrapping of the
light-emitting cores by optical inertia hexyls should be the main
reason for this remarkable decreasing. By the modification with
carbazoles, 2 has the double PL efficiency of 0.34. Along with
increasing of the density of carbazole moieties ΦPL of 3 greatly
increases to 0.72, which is already equal with that of
Ir(pbi)2acac. It indicates that as the energy-absorbing antennas
in the complexes, the peripheral carbazoles can remarkably
improve the efficiency of intra- and intermolecular energy
transfer. Significantly, from solution to film the PL QY of 1
and 2 remarkably decreased by 45 and 40%, respectively.
However, for 3 this reduction of efficiency mainly induced by
aggregation variation was only 15%. Obviously, the weakest
intermolecular interaction of 3 should be one of the most
important reasons for its smallest reduction of PL QY in solid.
It is also indicated that increasing the density of carbazole end-
of other two ligands. According to the equation reported by de
Leeuw et al.,43 EHOMO ) -(E
+ 4.4 eV) and ELUMO )
Oxy
onsetfSCE
Red
-(E
+ 4.4 eV), all of the ligands have LUMO levels
onsetfSCE
around -3.0 eV, which are comparable with the common
electron-transporting materials Alq3 and TPBI. HexPhBI only
has one irreversible oxidation peak with very high potential of
2.25 V, corresponding to HOMO level of -6.39 eV. Thus,
HexPhBI has weak hole-injection ability, even hole-blocking
ability. For CzPhBI, except for the oxidation peak originated
from the PBI moieties, the irreversible oxidation peak at 1.44

Phenyl Benzimidazole Iridium(III) Complexes
J. Phys. Chem. B, Vol. 114, No. 1, 2010 147
were conducted employing the Gaussian 03 package. Different
with HexPhBI whose electron clouds of HOMO and LUMO
levels are located on the phenyl benzimidazole group, CzPhBI
and Cz2PhBI are donor-acceptor system with flexible hexyls
as linker, in which HOMO levels and LUMO levels are
contributed by carbazole moieties and phenyl benzimidazole
moieties, respectively (Supporting Infomration SI1). Notably,
the electron clouds of HOMO and HOMO-1 levels of Cz2PhBI
are located on the two different carbazoles bonding to phenyl
and benzimidazole, respectively. This is just in agreement with
the oxidation curve of Cz2PhBI in which the shoulder oxidation
peak is attributed to the former carbazole and the main oxidation
peak is attributed to the latter. Furthermore, the energy gap
between HOMO and HOMO-1 levels of Cz2PhBI is 27 meV,
which is in accord with the potential gap of 22 mV between its
oxidation peaks. Obviously, the carbazole bonded with the
phenyl makes a remarkable contribution to improve the hole-
injection ability of the ligands.
Figure 6. CV curves of 1-3.
V is attributed to the carbazole moieties. After introducing
carbazole moieties HOMO level of CzPhBI rises to -5.92 eV.
Cz2PhBI has a reversible oxidation peak at 1.47 V also attributed
to its carbazole moieties. The difference between CzPhBI and
Cz2PhBI is that for Cz2PhBI there is a distinct shoulder peak at
1.25 V, which means that the carbazole groups in two different
environments exhibit different oxidation behaviors. Compared
with the oxidation curve of CzPhBI and the Gaussian simulation
results in the following section, the shoulder peak should
correspond to the carbazole moieties bonded with the phenyls.
This results in a further raise of HOMO level of Cz2PhBI as
-5.74 eV.
All of the complexes have irreversible reduction peaks
attributed to their benzimidazole moieties, which are similar to
those of their ligands. The first reversible oxidation peaks of
1-3 around 0.5 V are originated from Ir3+. It was found that
for 1 the intensity of its first oxidation peak is much weaker
than those of 2 and 3. The main reason should be that in 1 the
Ir3+ cores are wrapped by electric inertia hexyls, which restrains
the intermolecular carrier transporting. Compared with the
ligands, the potentials of the irreversible oxidation peaks
attributed to the phenyl moieties of the complexes (around 1.75
V) remarkably decrease, which should be induced by the
formation of C-Ir bonds. Because of the electron-donating
effect of hexyloxy for 3, this peak shifts to the lower voltage
about 1.6 V. Both of 2 and 3 have the reversible oxidation peaks
originated from their carbazoles. Different with 2, the carbazole
moieties bonded with the phenyls and benzimidazoles in 3
revealed two different oxidation peaks, whose potential gap is
110 mV. It is believed that the intramolecular potential gaps
facilitate the intramolecular charge transfer, and high intramo-
lecular charge mobility might form the basis of the charge
transporting materials.44 Thus, its more oxidation states might
be anticipated to make 3 superior to 1 and 2 in hole injection
and transporting.
The contour plots of the six lowest unoccupied (LUMO to
LUMO+5) and the six highest occupied molecular orbitals
(HOMO to HOMO-5) for 1-3 are presented in Supporting
Infomration SI2. As shown in Table 2, HOMO of 1 and 2
involves the contributions of Ir 7d and 8d orbitals (52.4%) and
a π orbital localized on the phenyl (ca. 47%). Differently, the
oxygen atoms of 3 make an unnegligible contribution of 0.71%
to its HOMO, which induce a drop of 1% of the contribution
of Ir d orbitals to HOMO of 3. LUMOs of the complexes are
largely dominated by the π* orbitals of phenyl-benzimidazole
moieties with very small contributions of Ir d orbitals. The
closed HOMO and LUMO levels of the complexes are in
agreement with the electrochemical results. Moreover, the
energy gap of 3 is 0.1 eV larger than those of 1 and 2, which
may induce the emission blue shift of about 7 nm. This result
is also consistent with the optical analysis results. Furthermore,
it is showed that HOMO-3 to HOMO-5 of 2 are attributed to
its three carbazole moieties. The electron-cloud densities of
HOMO-5 to HOMO-7 of 3 are located on its three carbazole
moieties bonded with the phenyls, and its HOMO-8 to HOMO-
10 are mainly contributed by the other three carbazole moieties.
Although the carbazole moieties in 2 and 3 do not participate
in the formation of the HOMO levels, obviously, as the
peripheral groups of the cores they are the bridges between
different complex molecules and play very important roles in
hole-injection/transporting.
EL Properties. Inspired by the amorphous states and
excellent PL and electrochemical properties of 2 and 3, the host-
free device A and B with the configuration of ITO/PEDOT:
PSS (50 nm)/Ir3+ complexes (40 nm)/TPBI (30 nm)/LiF (1 nm)/
Al (100 nm) were fabricated by spin-coat. Device A and B were
based on 2 and 3 respectively. B had the much lower turn-on
voltage (2.5 V) than that of A (3.5 V) (Figure 7). At the same
voltages, the current densities (J) of B were much bigger than
those of A. Therefore, through increasing the density of pendant
Theoretical Calculation. To understand the nature of the
electrochemical and optical properties of 1-3, DFT calculations
TABLE 2: The Contribution of d Orbitals of Iridium to the Frontier Orbitals and the Frontier Energy Levels of the
Compounds
proportion (%)
compound
HOMO-2
HOMO-1
HOMO
LUMO
LUMO+1
LUMO+2
HOMO (eV)
LUMO (eV)
Eg (eV)
HexPhBI
CzPhBI
Cz2PhBI
1
2
3
-5.66
-5.36
-5.33
-4.44
-4.60
-4.54
-0.93
-1.01
-0.79
-0.82
-0.98
-0.82
4.73
4.35
4.54
3.62
3.62
3.72
54.86
48.91
45.98
47.47
47.13
44.99
52.43
52.45
51.40
2.09
1.79
1.95
0.55
0.84
0.07
2.97
3.02
2.31

148 J. Phys. Chem. B, Vol. 114, No. 1, 2010
Xu et al.
Figure 7. B-J-V curves and EQE-J curves of the spin-coating host-
free device A and B.
Figure 8. B-J-V curves of the spin-coating doping device C and D.
doping concentration of 20% remarkably influenced the carrier
injection and transporting in the devices. Compound 3 with the
better carrier injection/transporting ability supported the rapid
increasing of the current density during the voltage increasing.
However, for 2 to obtain the same current density with that of
D much higher voltage was required. In the doping devices,
the host mainly has the strong effect on dispersing light-emitting
centers and improving the energy transfer. Thus, for C and D,
TCTA could efficiently restrain the concentration quenching and
T-T annihilation. It is noticeable that C and D had similar
maximum brightness and efficiencies, which suggested that the
emission cores in 2 and 3 actually have the similar EL properties.
Moreover, compared with host-free A, both of the brightness
and efficiency of phosphor-doping C increased 5 times.
However, compared with B these data of D were only improved
for 2 and 1.5 times. Obviously, the dispersion ability of TCTA
played the crucial role in the performance improvement from
A to C. The remarkable differences between A and B must be
mainly originated from the much stronger encapsulation and
improved carrier injection/transporting ability of 3. Therefore,
to achieve a good EL performance at least two-position
encapsulation is necessary for the ligand structure design of
small-molecular phosphorescent emitters. It is also noticed that
although D had higher efficiencies at bigger current densities
and exhibited stronger efficiency stability, the maximum ef-
ficiency of C was more than that of D. In doping devices, at
the much low current densities, the excitons were first formed
on the host TCTA. Then the energy transported from the host
to the complexes through either the exciton migration or Fo¨rster
energy transfer, the more thorough encapsulation of the cores
in 3 makes TCTA more difficult to get close to the light-emitting
cores, which might limit the probability of the exciton migration.
The restraining of host-guest energy transfer in 3 due to the
higher encapsulation density might be the main reason for its
lower maximum efficiencies at low current density. At the high
current densities, the stronger encapsulation of the cores and
the enhanced carrier injection/transporting ability of 3 supported
considerably stable efficiencies and facile carrier-trapping, which
made the efficiency of D gradually exceed that of C.
functional groups, the carrier-injection and transport can be
efficiently improved, which is in agreement with the electro-
chemical results. The maximum brightness of B was 6150 cd
m-2, which was more than twice of that of A (2808 cd m-2).
Moreover, the ratio of the brightness of B and A was nearly
fixed at the different current densities. It is more inspiring that
EQE of B at 100 cd m-2 was as high as 5.9%, which is equal
with that of a device using a blend of 8% (t-butyl) phenyl
benzimidazole Ir3+ complex and 30% PBD in PFO15 as the
emitting layer, higher than those devices based on the green-
emitting electrophosporescent Ir3+ polymers21,22 and comparable
with those of host-free spin-coat devices based on the similar
carbazole dendrons substituted phenyl benzimidazole Ir3+
complexes.27,30 At 1000 cd m-2, the efficiency of B remained
as 4.5%. It is shown that B had much higher efficiencies than
A, whose EQE were 2.5% at 100 cd m-2 and 1.7% at 1000 cd
m-2. In addition, the ratios of EQE of B to A were 2.36, 2.65,
and 3.02 at 100, 1000, and 2000 cd m-2, respectively. Since
high luminance generally corresponds to high exciton-concen-
tration, it indicates that 3 has much stronger ability to reduce
the concentration quenching and T-T annihilation effect than
2, and at higher exciton concentrations the predominance of 3
became more distinct, which is precisely consistent with our
molecular design.
Another question is whether the much better EL performance
of 3 is originated from its molecular structure design, which
would induce the better carrier injection/transporting, more
efficient separation of triplet excitons and weaker intermolecular
interaction in 3, or is attributed to the better luminescent
properties of its alkoxy substituted complex core. In order to
find out the origin of the highly improved EL performance of
3, the phosphor-doping devices C and D with the configuration
of ITO/PEDOT:PSS (50 nm)/Ir3+ complexes: TCTA (20%, 40
nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm) were also fabricated
by spin-coat, corresponding to 2 and 3 respectively. Herein,
TCTA was hole-transporting host. The turn-on voltages of C
and D were 3.5 V (Figure 8). C had the maximum brightness
of 9812 cd m-2 at 13 V with current density of 220 mA cm-2
.
The maximum EQE of C reached 11.4% at 5 V with very low
current density of 0.22 mA cm-2. The maximum brightness of
D was 10 521 cd m-2 at 11 V with current density of 204 mA
cm-2. Its maximum efficiency of 9.2% was achieved at 7.5 V
with current density of 10.6 mA cm-2. Since at the low voltages
the carrier injection/transporting was mainly determined by the
host TCTA, C and D had the same turn-on voltage. However,
along with the voltage increasing, at high current density the
carrier injection/transporting ability of the guests with the high
EL emissions of A-D were similar with the PL emissions
in solid of the corresponding complexes (Figure 9). It is
noticeable that compared with C the shoulder peaks at 550 nm
in the EL spectra of A were much stronger. Only at 12 V the
intensity of this peak in the spectrum of C is comparable with
that of A. Moreover, the emissions of A were slightly wider
than those of C. Although the shoulder peaks at 520 nm in the
spectra of B were also enhanced during the voltage increasing,
the spectra of B and D were almost the same at the same

Phenyl Benzimidazole Iridium(III) Complexes
J. Phys. Chem. B, Vol. 114, No. 1, 2010 149
Figure 9. EL spectra of A-D at different voltages and schemes of energy levels of A-D.
voltages. The enhancement of the shoulder peak might be
induced by the intermolecular interaction at high exciton
concentrations. Even at the lower exciton concentrations, 2
exhibited considerably strong interaction, which might be the
reason for the differences between the emissions of A and C.
However, 3 only exhibited relatively strong interaction at much
higher exciton concentrations. The same EL spectra of B and
D also indicated the weakened intermolecular interaction in 3.
was achieved. The device also had very low turn-on voltage of
2.5 V and high brightness over 6000 cd m-2. Compared with
2, 3 exhibited much better EL performances, such as bigger
brightness and higher efficiency, which are mainly attributed
to its reduced intermolecular interaction, improved carrier
injection/transporting ability, and limited emission quenching.
Acknowledgment. This work was financially supported by
NSFC under Grants 50903028 and 20972043, Natural Science
Foundation of Heilongjiang province under Grant QC08C10,
Education Bureau of Heilongjiang Province under Grants
11521206 and 11521216, Key Laboratory of Optical Electronics
and Energy/Environmental Materials of Heilongjiang Province,
and Heilongjiang University Starting Foundation of Young
Teachers. The authors want to thank Dr. Qiang Li, Dr. Chun-
Gui Tian, and Dr. Ju-Wen Zhang for their assistance in the
thermal, XRD, and electrochemical analysis.
Conclusion
In this work, we have synthesized a simple phenyl benzimi-
dazole Ir3+ complex 3 with the six-arm encapsulation. Our
investigation showed that the combined modification of the
aliphatic chains and functional groups in multipositions is a
feasible approach to realize the high-efficiency small molecular
phosphorescent materials in which the wrapping effect of the
aliphatic chains and the encapsulation effect of the functional
groups are utilized to reduce the concentration quenching and
T-T annihilation effect. Compounds 3 and 2 end-capped with
the conjugated carbazole moieties have improved thermal
stability than the hexyl-substituted 1. XRD analysis proved that
the carbazole end-capped aliphatic groups have stronger effect
on restrain the aggregation. Both 2 and 3 are amorphous.
Compound 3 exhibits much better PL properties in solution and
solid than 1 and 2. The peripheral carbazoles not only facilitate
the separation of triplet-emission cores, but also supply a routine
for the intermolecular energy transfer. Significantly, for 3 the
reduction of PL efficiency from solution to solid mainly induced
by aggregation variation was only 15%, which is only a half of
those of 1 and 2. Both electrochemical analysis and Gaussian
simulation suggested the important role of the peripheral
carbazoles in carrier injection/transporting. Furthermore, the
more oxidation states of 3 might be anticipated to make it
superior to 1 and 2 in hole injection and transporting. A dramatic
EQE of nearly 6% from the host-free spin-coated device of 3
Supporting Information Available: The contours of frontier
molecular oribitals of the ligands and complexes simulated with
Gaussian 03. This material is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151.
(2) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.;
Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4.
(3) Forrest, S. R. Nature 2004, 428, 911.
(4) Tsutsui, T.; Yang, M. J.; Yahiro, M.; Nakamura, K.; Watanabe,
T.; Tsuji, T.; Fukuda, Y.; Wakimoto, T.; Miyaguchi, S. Jpn. J. Appl. Phys.,
Part 2 1999, 38, L1502.
(5) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048.
(6) Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.;
Thompson, M. E. Appl. Phys. Lett. 2003, 83, 3818.
(7) Yeh, S. J.; Wu, M. F.; Chen, C. T.; Song, Y. H.; Chi, Y.; Ho,
M. H.; Hsu, S. F.; Chen, C. H. AdV. Mater. 2005, 17, 285.

150 J. Phys. Chem. B, Vol. 114, No. 1, 2010
Xu et al.
(8) Yang, M. J.; Tsutsui, T. Jpn. J. Appl. Phys., Part 2 2000, 39, L828.
(9) Gong, X.; Robinson, M. R.; Ostrowski, J. C.; Moses, D.; Bazan,
G. C.; Heeger, A. J. AdV. Mater. 2002, 14, 581.
(30) Ding, J. Q.; Gao, J.; Cheng, Y. X.; Xie, Z. Y.; Wang, L. X.; Ma,
D. G.; Jing, X. B.; Wang, F. S. AdV. Funct. Mater. 2006, 16, 575.
(31) Wang, Y.; Herron, N.; Grushin, V. V.; LeCloux, D.; Petrov, V.
Appl. Phys. Lett. 2001, 79, 449.
(32) Song, Y. H.; Yeh, S. J.; Chen, C. T.; Chi, Y.; Liu, C. S.; Yu, J. K.;
Hu, Y. H.; Chou, P. T.; Peng, S. M.; Lee, G. H. AdV. Funct. Mater. 2004,
14, 1221.
(33) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal,
R. A., Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712.
(34) Liu, Z. W.; Guan, M.; Bian, Z. Q.; Nie, D. B.; Gong, Z. L.; Li,
Z. B.; Huang, C. H. AdV. Funct. Mater. 2006, 16, 1441.
(35) Liu, Y.; Ye, K. Q.; Fan, Y.; Song, W. F.; Wang, Y.; Hou, Z. M.
Chem. Commun. 2009, 3699.
(10) Takizawa, S.; Montes, V. A.; Anzenbacher, P., Jr. Chem. Mater.
2009, 21, 2452.
(11) Tao, Y. T.; Wang, Q.; Yang, C. L.; Zhang, K.; Wang, Q.; Zou,
T. T.; Qin, J. G.; Ma, D. G. J. Mater. Chem. 2008, 18, 4091.
(12) Zhang, K.; Chen, Z.; Yang, C. L.; Zhang, X. W.; Tao, Y. T.; Duan,
L.; Chen, L.; Zhu, L. N.; Qin, J. G.; Cao, Y. J. Mater. Chem. 2007, 17,
3451.
(13) Tao, Y. T.; Wang, Q.; Yang, C. L.; Wang, Q.; Zhang, Z. Q.; Zou,
T. T.; Qin, J. G.; Ma, D. G. Angew. Chem., Int. Ed. 2008, 47, 8104.
(14) Shen, F.-Z.; Xia, H.; Zhang, C.-B.; Lin, D.; He, L.; Ma, Y. G. J.
Phys. Chem. B 2004, 108, 1014–1019.
(36) Xu, H.; Xu, Z.-F.; Yue, Z.-Y.; Yan, P.-F.; Wang, B.; Jia, L.-W.;
Li, G.-M.; Sun, W.-B.; Zhang, J.-W. J. Phys. Chem. C 2008, 112, 15517.
(37) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(15) Wei, X. Q.; Peng, J. B.; Cheng, J. B.; Xie, M. G.; Lu, Z. Y.; Li,
C.; Cao, Y. AdV. Funct. Mater. 2007, 17, 3319.
(16) Mi, B. X.; Wang, P. F.; Gao, Z. Q.; Lee, C. S.; Lee, S. T.; Hong,
H. L.; Chen, X. M.; Wong, M. S.; Xia, P. F.; Cheah, K. W.; Chen, C. H.;
Huang, W. AdV. Mater. 2008, 20, 1.
(38) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634–1641.
(39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreyen, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Octhterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; L.Martin, R.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, rev.
C.02; Gaussian, Inc.: Pittsburgh, PA, 2004.
(17) Xiao, L. X.; Su, S.-J.; Agata, Y.; Lan, H.; Kido, J. AdV. Mater.
2009, 21, 1271.
(18) Chen, X.; Liao, J.-L.; Liang, Y.; Ahmed, M. O.; Tseng, H.-E.; Chen,
S.-A. J. Am. Chem. Soc. 2003, 125, 636.
(19) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby,
C. E.; Ko¨hler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004,
126, 7041.
(20) Liu, S.-J.; Zhao, Q.; Deng, Y.; Xia, Y.-J.; Lin, J.; Fan, Q.-L.; Wang,
L.-H.; Huang, W. J. Phys. Chem. C 2007, 111, 1166.
(21) Zhen, H. Y.; Luo, J.; Yang, W.; Chen, Q. L.; Ying, L.; Zou, J. H.;
Wu, H. B.; Cao, Y. J. Mater. Chem. 2007, 17, 2824.
(22) Zhen, H. Y.; Jiang, C. Y.; Yang, W.; Jiang, J. X.; Huang, F.; Cao,
Y. Chem.sEur. J. 2005, 11, 5007.
(23) Halim, M.; Pillow, J. N. G.; Samuel, I. D. W.; Burn, P. L. AdV.
Mater. 1999, 11, 371.
(24) Freeman, A. W.; Koene, S. C.; Malenfant, P. R. L.; Thompson,
M. E.; Frechet, J. M. J. J. Am. Chem. Soc. 2000, 122, 12385.
(25) Lo, S. C.; Anthopoulos, T. D.; Namdas, E. B.; Burn, P. L.; Samuel,
I. D. W. AdV. Mater. 2005, 17, 1945.
(26) Bolink, H. J.; Santamaria, S. G.; Sudhakar, S.; Zhen, C. G.;
Sellinger, A. Chem. Commun. 2008, 618.
(27) Burn, P. L.; Lo, S.-C.; Samuel, I. D. W. AdV. Mater. 2007, 19,
1675.
(28) Zhang, K.; Chen, Z.; Zou, Y.; Gong, S. L.; Yang, C. L.; Qin, J. G.;
Cao, Y. Chem. Mater. 2009, 21, 3306.
(29) (a) Zhou, G. J.; Wong, W.-Y.; Yao, B.; Xie, Z. Y.; Wang, L. X.
Angew. Chem., Int. Ed. 2007, 46, 1149. (b) Zhou, G. J.; Wong, W.-Y.;
Yao, B.; Xie, Z. Y.; Wang, L. X. Angew. Chem. 2007, 119, 1167.
(40) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.;
Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003,
125, 7377.
(41) Gambino, S.; Stevenson, S. G.; Knights, K. A.; Burn, P. L.; Samuel,
I. D. W. AdV. Funct. Mater. 2009, 19, 317.
(42) Huang, W.-S.; Lin, J. T.; Chien, C.-H.; Tao, Y.-T.; Sun, S.-S.; Wen,
Y.-S. Chem. Mater. 2004, 16, 2480.
(43) de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand,
R. E. F. Synth. Met. 1997, 87, 53.
(44) Plater, M. J.; Jackson, T. Tetrahedron 2003, 59, 4673.
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