Novel iridium complexes as high-efficiency yellow and red phosphorescent light emitters for organic light-emitting diodes

Article abstract of DOI:10.1016/j.tet.2008.09.033

A series of novel biscyclometallated iridium complexes based on spirobifluorene ligands and acetyl acetonate (acac) ancillary ligands have been synthesized and characterized. Their electrochemical properties were investigated by cyclic voltammetry (CV). HOMO, LUMO, and energy band gaps of all the complexes were calculated by the combination of UV-vis absorption spectra and CV results. TGA and DSC results indicated their excellent thermal stability and amorphous structure. All the iridium complexes were fabricated into organic light-emitting devices with the device configuration of ITO/PEDOT:PSS (50 nm)/PVK (50 wt %):PBD (40 wt %):Ir complex (10 wt %) (45 nm)/TPBI (40 nm)/LiF (0.5 nm)/Ca (20 nm)/Ag (150 nm). Yellow to red light emission has been achieved from the iridium complexes guest materials. Complex C1 (yellow light emission) achieved an efficiency of 36.4 cd/A (10.1%) at 198 cd/m² and complex C4 (red light emission) reached external quantum efficiency of 4.6%. The slight decrease of external quantum efficiency at high current density revealed that the triplet-triplet (T₁-T₁) annihilation was effectively suppressed by the new developed complexes.

Full text of DOI:10.1016/j.tet.2008.09.033

Tetrahedron 64 (2008) 10814–10820
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Novel iridium complexes as high-efficiency yellow and red phosphorescent
light emitters for organic light-emitting diodes
,
,
Jun Hong Yao a b, Changgua Zhen a, Kian Ping Loh b, Zhi-Kuan Chen a
*
a Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore
b Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel biscyclometallated iridium complexes based on spirobifluorene ligands and acetyl
acetonate (acac) ancillary ligands have been synthesized and characterized. Their electrochemical
properties were investigated by cyclic voltammetry (CV). HOMO, LUMO, and energy band gaps of all the
complexes were calculated by the combination of UV–vis absorption spectra and CV results. TGA and DSC
results indicated their excellent thermal stability and amorphous structure. All the iridium complexes
were fabricated into organic light-emitting devices with the device configuration of ITO/PEDOT:PSS
(50 nm)/PVK (50 wt %):PBD (40 wt %):Ir complex (10 wt %) (45 nm)/TPBI (40 nm)/LiF (0.5 nm)/Ca
(20 nm)/Ag (150 nm). Yellow to red light emission has been achieved from the iridium complexes guest
materials. Complex C1 (yellow light emission) achieved an efficiency of 36.4 cd/A (10.1%) at 198 cd/m2
and complex C4 (red light emission) reached external quantum efficiency of 4.6%. The slight decrease of
external quantum efficiency at high current density revealed that the triplet–triplet (T1–T1) annihilation
was effectively suppressed by the new developed complexes.
Received 14 February 2008
Received in revised form 24 August 2008
Accepted 11 September 2008
Available online 20 September 2008
Keywords:
Iridium complex
Phosphorescent materials
Spirobifluorene
Organic light-emitting diodes (OLEDs)
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
decrease device efficiency and impair the device stability. At high
doping concentrations, the intermolecular interaction in thin film
Currently, organic electroluminescent materials are of interest
in academic and industrial fields due to their application in light-
weight, low-cost, flexible large-area optoelectronic devices.1–4
Since the first organic light-emitting diode (OLED) was fabricated,5
great progress has been achieved through exploration of novel
high-efficient materials and optimization of device structures.6,7
Thinner, lighter, self-emission, high efficiency, lower energy con-
sumption, easier fabrication, and high-resolution make OLEDs very
promising for full-color display.
Among the light-emitting materials for OLEDs, phosphorescent
materials, which can harvest both singlet and triplet excitons have
attracted great attention recently because of their higher efficiency
than fluorescent materials.8,9 Iridium complexes are the most
popular choice as electrophosphorescent guest materials due to
their higher efficiency and facile color tunablity comparing with
other metal–ligand complexes.10–14 However, there are some issues
that need to be addressed to enhance their efficiency further. For
small molecular materials, crystallization of their thin films may
lead to the formation of excimers and exciplexes, which will
will lead to self-quenching of luminescence. To address the above
issues, amorphous spiro-annulated structures15 are adopted as
ligands for iridium complexes. Their 3-D bulky and steric con-
figuration was expected to effectively suppress the close packing
among the molecules in solid state.16–18 In addition, their excel-
lent solubility allows for solution process, which will be an
advantage over other small molecules that need to be thermally
deposited into films. Their good film-forming ability will offer
a uniform thin film with enhanced morphological stability during
device fabrication and operation. Concurrently, color tuning can
be realized by modifying the conjugation length of the ligands.
Here we present the synthesis and characterization of a series of
spirobifluorene based ligands and the corresponding iridium
complexes. The chemical structures of the complexes are shown
in Figure 1. The thermal and electrochemical properties of the
target iridium complexes were investigated by thermogravimetric
analysis (TGA) and differential scanning calorimeter (DSC). The
light emission was tuned by modification of the cyclometalated
ligand structure.11,19 The synthesized iridium complexes
were employed as guest materials for OLEDs. Blended with
poly(vinyl carbazole) (PVK) and 2-(4-biphenylyl)-5-(4-tert-butyl-
phenyl)-1,3,4-oxadiazole (PBD) host materials, the device perfor-
mance was evaluated.
* Corresponding author.
E-mail address: zk-chen@imre.a-star.edu.sg (Z.-K. Chen).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.09.033

J.H. Yao et al. / Tetrahedron 64 (2008) 10814–10820
10815
dimers were converted to Ir complexes after reacting with acac
group under reflux either in dichloromethane or in 2-ethoxy-
ethanol in the presence of a base. The procedure is depicted in
Scheme 2.
N
O
O
N
O
O
Ir
Ir
For the Ir complexes, there are two kinds of isomers, facial (fac)
and meridional (mer) configuration. Meridional isomers are ki-
netically favored products. Their structures can be investigated by
X-ray diffraction and NMR spectroscopy.28,29 The 1H NMR spectra
suggest that all of the Ir complexes are formed exclusively as facial
isomers, where the two ligands surrounding the iridium atom are
chemically equivalent.25 The small ancillary ligands effectively re-
duced the steric hindrance brought by bulky main ligands. The
facial isomers are expected to have higher efficiency and better
device performance comparing with the meridional isomers.28
2
2
C1
C2
N
O
Ir
N
O
O
Ir
2.2. Optical analysis
O
The UV–vis absorption spectra of the series of Ir complexes C1–
C4 in anhydrous dichloromethane solution are shown in Figure 2.
All the complexes showed similar spectra, with three main
absorption bands for each material. As can be seen, there are two
low-intensity metal to ligand charge transfer (MLCT) transitions
from 370 nm to 550 nm. The band at longer wavelength is assigned
2
2
to triplet 3MLCT d–
p* excitations, while the band at shorter
C3
C4
wavelength is attributed to singlet state 1MLCT excitation. The
high-intensity peaks, which start from 330 nm to 380 nm belong to
Figure 1. Structures of bis-cyclometalated Ir complexes.
the
p/p
*
transitions localized on the phenylpyridine-based
ligands. The figures also showed that increasing the conjugation
length of the cyclometalating ligand caused a red shift both of the
2. Results and discussion
2.1. Synthesis
p/p
*
and d/
p
*
transition bands of the complex, which is
consistent with the reported results.11,25,26
The photoluminescence (PL) of the complexes was measured at
room temperature in anhydrous dichloromethane (DCM) solution
on Luminescence Spectrometer LS50B (Perkin Elmer). The emission
spectra of all the complexes are shown in Figure 3. The PL spectra
can be divided into two main groups. Group 1 included complexes
C1–C3, whose peak wavelengths fall into the range of 545–560 nm
and they all emitted yellow light. The peak wavelength of complex
C4 was 612.5 nm and the emission light is red. The color change in
light emission complies with the theory that the light emission of Ir
complexes can be tuned by changing the structures of the ligands.
The solution photoluminescence efficiencies (PLQE) were mea-
sured using quinoline bisulfate in 0.1 N H2SO4 as a standard. The
PLQEs for complexes C1, C2, C3, and C4 in degassed solution of
toluene are 52%, 54%, 55%, and 28%, respectively. The optical
properties of spirobifluorene based Ir complexes in solution are
summarized in Table 1.
General synthetic routes towards the ligands are outlined in
Scheme 1. First, 4,40-di-tert-butylbiphenyl was synthesized from
biphenyl, which reacted with tert-butyl chloride in dichloro-
methane in the presence of catalytic amount of anhydrous ferric
chloride in quantitative yield.20 2-Bromo-4,40-di-tert-butylbiphenyl
1 was obtained from 4,40-di-tert-butylbiphenyl through a bromina-
tion reaction catalyzed by anhydrous ferric chloride in chloroform,
proceeding in 50% yield. Compound 1 was then converted to
its lithium salt 4,40-di-tert-butylbiphenyl-2-lithium by reacting with
n-butyl lithium. 2-Bromo-9-fluorenone reacted with the lithium salt
of 1 in anhydrous THF solution at ꢀ78 ꢁC, followed by a cyclization
reaction, producing spirobifluorene 2 in a yield of 54%.21 Following
standard halogen–lithium reaction, 2-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-(20,70-di-tert-butyl)-9,90-spirobifluorene 3 was
synthesized in a yield of 64% from compound 2.22 Finally, ligands L3
and L4 were synthesized by following standard Suzuki coupling
reactions between compound 3 with 423 and 2-chloroquinoline,
respectively, with yields of 74% for L3 and 99% for L4.24
2.3. Thermal analysis (TGA and DSC)
Due to the poor reactivity of 2-bromopyridine, ligands L1 and L2
cannot be obtained from direct coupling reaction between spi-
robifluorene boronic ester and 2-bromopyridine. Thus, ligands L1
and L2 were synthesized by different routes with ligands L3 and L4.
First, 2-fluorenonepyridine 5 was synthesized by oxidizing 2-
fluorenepyridine in pyridine solution by saturated oxygen at room
temperature, which proceeded in 97% yield. Following the cycli-
zation procedure, compound 1 and 2-bromobiphenyl reacted with
compound 8 to form spiro compounds L1 and L2 in yields of 69%
and 48%, respectively.
A standard two-step procedure was used to synthesize the
final heteroleptic Ir complexes from the ligands.11,25–27 Firstly, the
ligands were reacted with iridium chloride trihydrate in a mix-
ture of 2-ethoxyethanol and water in the ratio of 3:1. After
refluxing overnight under nitrogen, the intermediate compounds
chloride-bridged dimers were obtained with medium yields. The
The thermal stability of the Ir complexes in nitrogen was eval-
uated by TGA using TGA Q500 instrument (heating rate of
10 ꢁC minꢀ1). Complex C1 exhibited excellent thermal stability. The
onset temperature of weight loss is 373 ꢁC, and the temperature for
5% weight loss is 460 ꢁC. The first as well as the only one weight loss
is due to the cleavage of ancillary ligand (acac group). Complex C3
showed a lower decomposition temperature than complex C1. The
onset decomposition temperature is 260 ꢁC. There were two stages
of decomposition, which corresponded to loss of ancillary ligand
(acac group) and the tert-butyl groups that covalently linked to the
cyclometalated ligands.
Thermally induced phase transition behavior of Ir complexes
was investigated by DSC under nitrogen atmosphere on a DSC Q100
instrument (scanning rate of 10 ꢁC minꢀ1). The DSC curve revealed
that the Ir complex is amorphous. On heating to 320 ꢁC, no crys-
tallization and melting were observed, which is matched with the

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J.H. Yao et al. / Tetrahedron 64 (2008) 10814–10820
i
Br
1
2
O
ii
OH
Br
O
O
iii
iv
B
Br
Br
3
4
v, vi
Br
NH2
Br
N
5
Br
N
N
vii
L3
+
O
O
B
Cl
N
N
L4
O
viii
ix
Br
+
MgBr
N
N
N
iii
6
R
R
R
R
6
OH
R
R
iii
Br
N
N
R = H,
R = t-butyl,
L1
L2
Scheme 1. Synthetic routes for the ligands. Reagents and conditions: (i) Br2, CHCl3, anhydrous FeCl3, 0 ꢁC; (ii) n-BuLi, THF, ꢀ78 ꢁC; (iii) HCl, acetic acid, reflux; (iv) n-BuLi, THF,
ꢀ78 ꢁC, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane; (v) HCl, H2O, NaNO2, 0 ꢁC; (vi) pyridine, 40 ꢁC, Na2CO3; (vii) Na2CO3, toluene, Pd(PPh3)4, reflux, 24 h; (viii) THF,
Pd(PPh3)4, reflux, 24 h; (ix) pyridine, O2, overnight.
prediction that the spirobifluorene structure is amorphous. Such
amorphous materials are particularly favored for OLEDs because
aggregation and crystallization will affect the device stability and
lifetime significantly.30 The amorphous materials also exhibit ex-
cellent processibility, homogeneity, and isotropic properties, which
are favorable for OLEDs.31
electrochemical data and energy levels of all the Ir complexes are
calculated and summarized in Table 2.
It is known that the HOMO of Ir complexes is determined by the
5d orbital of Ir with substantial mixing with the
p orbitals of
the ligand and LUMO is related to the
p
*
orbitals of ligand.25,32,33
The LUMO energy level is cyclometalating ligand dependent, which
is not affected by the nature of the ancillary ligands.34 Comparing
the energy levels of C1 and C2, it was found that their LUMOs are
the same and C2 showed higher HOMO, indicating that the in-
troduction of tert-butyl groups decreases the energy band gap of
the Ir complex due to their electron-donating abilities.35
2.4. Electrochemical properties
The electrochemical behavior of the materials was investigated
by cyclic voltammetry (CV). All the Ir complexes showed obvious
oxidation peaks and underwent a reversible one-electron oxidation
process, demonstrating excellent electrochemical stability of their
cations. However, no reduction peak was observed. Thus, their
energy band gaps were estimated from the long-wavelength ab-
sorption edge data collected from spectroscopic method. The
2.5. Electroluminescent (EL) properties
All the OLED devices were fabricated by a combination of high
vacuum thermal deposition and spin-coating on pre-cleaned ITO

J.H. Yao et al. / Tetrahedron 64 (2008) 10814–10820
10817
R1
R2
R2
250
200
150
100
50
C1
C2
C3
C4
N
Cl
Cl
i
Ir
Ir
IrCl3 3H2O
+
N
N
2
2
R1
R2
R1
chloride-bridged dimer
R1
N
O
Ir
ii or iii
O
2
R2
heteroleptic Ir complex
0
550
600
650
Wavelength (nm)
700
750
800
Scheme 2. Synthetic routes for the Ir(III) complexes. Reagents and conditions: (i) 2-
ethoxyethanol–H2O¼3:1, reflux, 24 h; (ii) DCM, acac, tetrabutylammonium hydroxide,
ethanol, reflux, 5 h; (iii) 2-ethoxyethanol, acac, Na2CO3, reflux, 24 h.
Figure 3. PL spectra of Ir complexes C1–C4 in anhydrous DCM.
glass substrates employing the synthesized Ir complexes as guest
materials and PVK as host material. The device configuration is ITO/
PEDOT:PSS (50 nm)/PVK (50%):PBD (40%):Ir complex (10%)
(45 nm)/TPBI (40 nm)/LiF (0.5 nm)/Ca (20 nm)/Ag (150 nm). The
emissive Ir complexes were doped into a host polymer matrix of
PVK and blended with the electron transport molecule, 2-(4-
biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). The
doping concentration of guest Ir complexes was fixed at 10 wt %. It
is important to note that doping concentrations of guest higher
than 5 wt % are typically required to efficiently quench the host
luminescence and achieve good carrier transport in phosphores-
cent OLEDs.
All the devices display intense yellow or red emission in the
range of 545–609 nm in the EL spectra, which resembled their PL
spectra of the Ir complexes in DCM solution. No emission from PVK
was observed, indicating that efficient energy transfer from the
host materials to guest materials occurred. Furthermore, the EL
emission of the device originated from the triplet excited states of
the phosphors. The peak emission wavelength of EL spectra and
their CIE coordinates are summarized in Table 3.
Table 1
Photophysical properties of Ir complexes C1–C4 in anhydrous DCM
Ir Complexes
UV–vis
PL, lem (nm)
lonset
lmax
1MLCT,
d–
3MLCT,
d–
(nm)
(nm),
p/p
*
p
*
p*
C1
C2
C3
C4
518.5
535.3
537.5
610
329.5
340.5
339.5
351.5
397.5
390.5
386.5
417.5
473.5
468.5
451.5
480
548.5 (591.5)
550 (592)
555.5 (592)
612.5
Note: the data in the parentheses are the wavelength of shoulders and sub-peaks.
a large barrier for injection of holes from PEDOT:PSS, which
translates into a high onset voltage.36 Lower turn-on voltage is
expected to be obtained by employing novel host materials.
The external quantum efficiency (EQEext, or hext) and luminance
efficiency (LE) of complex C1 are shown in Figure 4. Device with
10 wt % of C1 exhibited the highest efficiency with hlum¼36.4 cd/A
and EQEext¼10.1% at 0.5 mA/cm2 (brightness: 198 cd/m2). When
the current density increased from 0.1 mA/cm2 to 1.0 mA/cm2,
10.0 mA/cm2, and 100 mA/cm2, the external quantum efficiency of
the device varied from 8.6% to 9.6%, 8.6%, and 5.8%, respectively. In
a controlled experiment, when PPy2Ir(acac) was used as the dop-
ant, the corresponding external quantum efficiencies of the device
are 8.0%, 7.5%, 6.2%, and 4.2% at the four different levels of driving
current. The decrease of EQE with increasing current density is
attributed to increased triplet–triplet annihilation of phosphor-
bound excitons and field-induced quenching effects,37 which are
It can be seen from Figure 4 that the turn-on voltage (defined as
the voltage required to give a luminance of 1 cd/m2) of the devices
is 5.8 V. The relatively higher turn-on voltage is ascribed to a low-
lying HOMO level of PVK (at about ꢀ5.9 eV), and consequently,
0.35
C1
C2
C3
C4
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Table 2
Electrochemical properties of Ir complexes C1–C4 in anhydrous DCM
∗
π → π
Ir complexes Eonset (eV) Epa (eV) Epc (eV) HOMO (eV) LUMO (eV) Eg (eV)
C1
C2
C3
C4
0.76
0.69
0.81
0.74
0.86
0.83
0.90
0.84
0.79
0.73
0.84
0.77
ꢀ5.16
ꢀ5.09
ꢀ5.21
ꢀ5.14
ꢀ2.77
ꢀ2.77
ꢀ2.90
ꢀ3.11
2.39
2.32
2.31
2.03
1(MLCT)
Epa: anodic peak potential, Epc: cathodic peak potential.
3(MLCT)
Table 3
Summary of electroluminescence (EL)
Ir complexes
EL lmax (nm)
CIE coordinates (x, y)
C1
C2
C3
C4
554.5
552
560
0.469
0.463
0.470
0.644
0.528
300
350
400
450
Wavelength (nm)
500
550
600
0.532
0.522
0.356
609.5
Figure 2. UV–vis absorption spectra of Ir complexes in anhydrous DCM.

10818
J.H. Yao et al. / Tetrahedron 64 (2008) 10814–10820
a
b 100
1000
100
10
10
10000
1000
1
1
10
100
0.1
0.1
0.01
0.01
1E-3
1E-4
10
1
1
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
0
5000
10000 15000
Luminance (cd/m2)
20000
Figure 4. (a) V–I–L curves of device based on Ir complex C1 and (b) luminance efficiency and external quantum efficiency of the devices based on Ir complex C1.
1H NMR and 13C NMR spectra were measured in CDCl3 solution
Table 4
Device characteristics of Ir complexes C1–C4
on a Bruker DPX (400 MHz) NMR spectrometer with tetrame-
thylsilane (TMS) as the internal standard. Mass spectra (MS) were
recorded on a Bruker Autoflex TOF/TOF (MALDI-TOF) instrument
using dithranol as a matrix. UV–vis–NIR absorption spectroscopy
was measured by Shimadzu UV-3101 spectrometer at 25 ꢁC. Fluo-
rescence spectra were recorded on Luminescence Spectrometer
LS50B (Perkin Elmer) at room temperature in anhydrous
dichloromethane. The CV measurements were performed in an-
hydrous dichloromethane with 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as the supporting electrolyte at a scan
rate of 0.1 V/s at room temperature under the protection of nitro-
gen. A platinum plate was used as working electrode, a gold stick
was used as counting electrode, and Al/AgCl (3 M KCl solution) was
used as reference electrode.
Ir complexes hext (%) VTurn-on (V) lmax (nm) LEmax (cd/A) L (cd/m2) at 20 V
C1
C2
C3
C4
10.1
5.3
5.3
4.6
5.8
7.2
6.2
7.0
550
552
560.5
609
36.4
19.0
19.4
30,956
19,271
15,943
2702
7.44
key issues for phosphorescent devices. In our case, the slight drop
of quantum efficiency clearly demonstrated that the T–T annihila-
tion has been suppressed effectively by the introduction of bulky 3-
D ligands through chromophore isolation38 and preventing the
energy transfer from emissive dopant to the host.39 The device
characteristics based on all the Ir complexes are summarized in
Table 4.
4.2. 2-Bromo-4,40-di-tert-butylbiphenyl (1)40
3. Conclusion
To a solution of 4,40-di-tert-butylbiphenyl (3.99 g, 15 mmol) and
catalyst anhydrous ferric chloride (20 mg) in chloroform (30 mL) at
0 ꢁC was added bromine (2.4 g, 15 mmol) dissolved in chloroform
(10 mL) dropwise. The reaction mixture was stirred overnight. The
resultant reaction mixture was quenched with sodium carbonate
until the orange color disappeared. The organic layer was separated
and the water phase was washed with water and extracted with
hexane (50 mL) three times. The combined organic phase was
washed with saturated brine, dried over anhydrous MgSO4, fil-
trated, and concentrated in vacuo to get a mixture of product and
starting material. The conversion was estimated from proton NMR
spectrum to be about 50%. The product and the starting material
have the same polarity and cannot be separated by silica gel column
chromatography. The starting material will not affect the next step
of the reaction. Thus product mixture was not purified and was
used for next reaction directly.
Novel 3-D spirobifluorene ligands for Ir complexes have been
designed, which suppress T–T annihilation and concentration
quenching and improve the film quality in order to enhance ef-
ficiency of OLEDs and stability. Yellow to red light emissions were
achieved for the Ir complexes. The thermal properties of all the Ir
complexes were evaluated by TGA and DSC, indicating their good
thermal stability and amorphous state. Devices were fabricated
by doping the complexes at the concentration of 10 wt % into
polymeric host materials PVK and PBD, showing satisfactory
device performance. Complex C1 achieved efficiency of 36.4 cd/A
(10.1%) at 198 cd/m2. The external quantum efficiency did not
decrease obviously with the increasing current density, confirm-
ing that the T1–T1 annihilation was largely suppressed by the
new designed complexes. The efficiencies of the device could be
further enhanced by optimizing device structure and changing
host materials.
4. Experimental
4.3. 2-Bromo-(20,70-di-tert-butyl)-9,90-spirobifluorene (2)21
4.1. General information
To a solution of the mixture of 4,40-di-tert-butylbiphenyl and
2-bromo-4,40-di-tert-butylbiphenyl (3.06 g, 5 mmol) in anhydrous
THF (50 mL) was added dropwise n-BuLi (5 mL, 6 mmol) in hexane
at ꢀ78 ꢁC, After stirring for 1 h, the mixture was transferred to
a solution of 2-bromofluorenone (1.3 g, 5 mmol) in THF (20 mL) at
ꢀ78 ꢁC and stirred overnight. Then the reaction was quenched with
All the starting materials were purchased from commercial
suppliers and used directly without further purification. Tetrahy-
drofuran (THF) was distilled from sodium–benzophenone imme-
diately prior to use.

J.H. Yao et al. / Tetrahedron 64 (2008) 10814–10820
10819
water and extracted with ethyl acetate (3ꢂ50 mL). The organic
layers were combined, washed with saturated brine, dried over
anhydrous MgSO4, filtrated, and concentrated in vacuo.
Then H2SO4 was added into the resulting suspension to eliminate
the volatile pyridine. The precipitate was filtered and purified by
column chromatography (silica gel, ethyl acetate–hexane¼1:5);
0.41 g of yellow powder product (97%) was obtained. 1H NMR
The residue was dissolved in glacial acetic acid (15 mL) and one
drop of concentrated HCl was added. The mixture was heated to
gentle reflux for 1 h. After the mixture was cooled down to room
temperature, the precipitate was filtered and washed with water.
The mixture of 2-bromo-(20,70-di-tert-butyl)-9,90-spirobifluorene
and 4,40-di-tert-butylbiphenyl was separated by silica gel column
chromatography eluted with hexane (Rf¼0.2) to provide white solid
(CDCl3, 400 MHz):
d [ppm] 8.71–8.70 (d, 1H), 8.27–8.26 (m, 2H),
7.79–7.78 (m, 2H), 7.71–7.69 (d, 1H), 7.65–7.63 (d, 1H), 7.59–7.57 (d,
1H), 7.54–7.50 (t,1H), 7.35–7.26 (m, 2H). 13C NMR (CDCl3,100 MHz):
d
[ppm] 156.5, 150.2, 145.2, 144.5, 140.9, 137.3, 135.2, 133.8, 129.7,
124.8, 123.0, 121.1, 121.0, 120.7. MS (MALDI): 258.10 (m/z), calcd for
C18H11NO: 258.09.
product, 1.37 g (54%). 1H NMR (CDCl3, 400 MHz):
d [ppm] 7.84 (d,
1H), 7.74 (d, 3H), 7.50 (d, 1H), 7.42 (m, 3H), 7.14 (t, 1H), 6.87 (s, 1H),
4.7. General synthetic procedure for L1 and L2 using L1
as an example
6.75 (d, 1H), 6.65 (s, 2H), 1.18 (s, 18H). 13C NMR (CDCl3, 100 MHz):
d
[ppm] 152.1, 151.3, 149.8, 148.5, 141.1, 141.0, 139.6, 131.0, 128.5,
128.0, 127.7, 125.4, 124.7, 121.5, 120.9, 120.2, 119.6, 35.2, 31.8.
To a solution of 2-bromobiphenyl (0.58 g, 2.5 mmol) in anhy-
drous THF (6 mL) was added dropwise n-BuLi (2.8 mL, 2.3 mmol) in
hexane at ꢀ78 ꢁC. After stirring for 1 h, the mixture was transferred
to a stirred solution of 2-(fluorenon-2-yl)pyridine (0.5 g, 2 mmol)
in anhydrous THF (6 mL) at ꢀ78 ꢁC. The reaction mixture was kept
stirring overnight and warmed to room temperature. Water was
added to terminate the reaction. The organic layer was separated
and water phase was extracted with ethyl acetate (3ꢂ20 mL). The
organic layer was combined, washed with saturated brine, dried
over anhydrous MgSO4, filtrated, and concentrated in vacuo. The
residue was dissolved in glacial acetic acid (6 mL) with one drop of
concentrated HCl and the solution was heated to reflux for 1 h.
After the reaction mixture was cooled to room temperature, the
precipitate was filtered and washed with water. To the solution was
added sodium hydroxide solution to tune to pH¼7. Then, the mixed
solution was extracted with ethyl acetate (3ꢂ20 mL). The organic
layer was combined, washed with saturated brine, dried over an-
hydrous MgSO4, and concentrated in vacuo. The crude product was
purified by column chromatography (silica gel, hexane first, then
EA–hexane¼1:10) to provide light yellow solid product L1, 0.54 g
4.4. 2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-(20,70-
di-tert-butyl)-9,90-spirobifluorene (3)22
To
a
solution of 2-bromo-(20,70-di-tert-butyl)-9,90-spirobi-
fluorene (1.44 g, 2.8 mmol) in anhydrous THF (14 mL) was added
dropwise n-BuLi (4.3 mL, 5.1 mmol) at ꢀ78 ꢁC. After stirring for 1 h,
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1 mL,
4.8 mmol) was added. The mixture was stirred overnight. Then the
reaction was quenched with water and extracted with dichloro-
methane (3ꢂ30 mL). The organic layer was washed with saturated
brine, dried over MgSO4, filtrated, and concentrated in vacuo. The
residue was purified by column chromatography (silica gel, ethyl
acetate–hexane¼1:10) to give white solid product, 0.99 g (64%). 1H
NMR (CDCl3, 400 MHz):
d [ppm] 7.86 (d, 3H), 7.71 (d, 2H), 7.37 (m,
3H), 7.19 (s, 1H), 7.08 (t, 1H), 6.67 (d, 1H), 6.62 (s, 2H).
4.5. 2-(40-Bromophenyl)pyridine (4)23
To a solution of 4-bromoaniline (2 g, 12 mmol) in concentrated
HCl (4 mL) was added slowly a solution of NaNO2 (1.66 g, 24 mmol)
in H2O (3 mL) at 0 ꢁC. The mixture was stirred at 0 ꢁC for 1 h and
pyridine (50 mL) was added. The mixture was further stirred at
40 ꢁC for 4 h and then sodium carbonate (20 g) was added and the
slurry was stirred overnight. After cooling to room temperature,
water was added and the water phase was extracted with ethyl
acetate. The organic layer was combined, washed with saturated
brine, dried over anhydrous MgSO4, filtrated, and concentrated in
vacuo. Pyridine was distilled off and the residue was purified by
column chromatography (silica gel, ethyl acetate–hexane¼1:10) to
give white solid product, 1.06 g (38%). 1H NMR (CDCl3, 400 MHz):
(69%). 1H NMR (CDCl3, 400 MHz):
d [ppm] 8.57–8.56 (d, 1H), 8.08–
8.06 (d, J¼8 Hz, 1H), 7.96–7.94 (d, J¼8 Hz, 1H), 7.89–7.85 (t, 3H),
7.63–7.53 (m, 2H), 7.40–7.35 (m, 4H), 7.13–7.09 (m, 4H), 6.78–6.71
(m, 3H). 13C NMR (CDCl3, 100 MHz):
d [ppm] 150.0, 149.1, 142.4,
141.7,139.7,137.0,128.6,128.4,128.2,127.4,124.8,124.5,123.1,122.3,
121.1, 120.7, 120.5. MS (MALDI): 393.138 (m/z), calcd for C30H19N:
393.151.
Compound L2 was synthesized by the same method as L1 with
a yield of 48% as yellow powder. 1H NMR (CDCl3, 400 MHz):
d [ppm]
8.58 (d, 1H), 8.12–8.10 (d, 1H), 7.97–7.88 (dd, 2H), 7.73–7.71 (d, 2H),
7.63–7.52 (m, 2H), 7.39–7.36 (m, 3H), 7.31–7.26 (m, 1H), 7.14–7.09
(m, 1H), 6.72–6.68 (m, 2H), 1.14 (s, 18H). 13C NMR (CDCl3, 100 MHz):
d
[ppm] 8.69–8.68 (d, 1H), 7.89–7.87 (d, J¼8.4 Hz, 2H), 7.8–7.74 (m,
d [ppm] 151.2, 150.8, 150.4, 149.7, 149.1, 143.3, 141.5, 139.7, 139.2,
2H), 7.71–7.69 (d, 2H), 7.61–7.59 (d, J¼8.4 Hz, 2H).
137.0, 128.4, 127.8, 127.2, 125.3, 124.6, 123.0, 122.2, 121.1, 120.6,
120.5, 119.5, 35.2, 31.8. MS (MALDI): 505.260 (m/z), calcd for
C38H35N: 505.276.
4.6. 2-(Fluorenon-2-yl)pyridine (5)
A solution of 2-fluorenylmagnesium bromide (20 mmol) in an-
hydrous THF (60 mL) was added dropwise to a stirred mixture of 2-
bromopyridine (2.39 g, 15 mmol) and Pd(PPh3)4 (0.17 g, 0.15 mmol)
in anhydrous THF (60 mL) at room temperature and stirred for 1 h.
Then the mixture was heated to gentle reflux under nitrogen and
stirred overnight. After cooling to room temperature, the mixture
was washed with water and extracted with dichloromethane
(3ꢂ50 mL). The organic layer was then washed with saturated
brine, dried over MgSO4, and concentrated in vacuo. After purified
by column chromatography (silica gel, ethyl acetate–hexane¼1:20)
0.70 g (19.2%) of 2-fluorenylpyridine was obtained.
4.8. General procedure for the synthesis of L3 and L4, using L3
as an example
A mixture of 2-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-(20,70-di-tert-butyl)-9,90-spirobifluorene (4) (0.554 g, 1 mmol),
2-(40-bromophenyl)pyridine (5) (1 mmol), Pd(PPh3)4 (0.05 g,
0.043 mmol), aqueous sodium carbonate (2 M, 0.71 mL), and tolu-
ene (2.9 mL) was deoxygenated and then heated to reflux under
nitrogen with stirring overnight. After the reaction mixture was
cooled down to room temperature, the mixture was washed with
water and extracted with ethyl acetate (3ꢂ20 mL). The organic
layers were then washed with saturated brine, dried over MgSO4,
and concentrated in vacuo. The crude products were purified by
column chromatography (silica gel, ethyl acetate–hexane¼1:10) to
give white solid product with the yield of 100%. 1H NMR (CDCl3,
To a solution of 2-fluorenepyridine (0.4 g, 1.6 mmol) in pyridine
(10 mL), tetramethylammonium hydroxide (1 mL) was added at
room temperature. Then air was filled into the system and the re-
action mixture was kept stirring overnight at room temperature.

10820
J.H. Yao et al. / Tetrahedron 64 (2008) 10814–10820
400 MHz):
d
[ppm] 8.70 (s, 1H), 7.97 (t, 3H), 7.92 (d, 1H), 7.76 (m,
2H), 7.02 (d, 2H), 6.72 (m, 4H), 6.35 (d, 2H), 4.90 (s, 1H), 1.34 (s, 6H),
1.22 (d, 36H). MS (MALDI): 1398.966 (m/z), calcd for C89H79N2IrO2:
1398.562.
5H), 7.59 (d, 2H), 7.42 (d, 3H), 7.28 (d, 1H), 7.13 (t, 1H), 7.04 (s, 1H),
6.72 (d, 3H), 1.17 (s, 18H). 13C NMR (CDCl3, 100 MHz):
d
[ppm] 157.3,
151.1, 150.6, 150.3, 149.9, 149.1, 141.9, 141.6, 141.5, 140.3, 139.5, 138.3,
136.8, 128.0, 127.6, 127.5, 127.3, 126.8, 125.0, 124.4, 122.9, 122.1,
120.9, 120.5, 120.3, 35.0, 31.6, 29.9. MS (MALDI): 581.299 (m/z),
calcd for C44H39N: 581.308.
References and notes
1. Shirota, Y. J. Mater. Chem. 2000, 10, 1.
2. Borchardt, J. K. Mater. Today 2004, September 42.
Compound L4 was synthesized with a yield of 74% as white
3. Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471.
4. Hung, L. S.; Chen, C. H. Mater. Sci. Eng. R. 2002, 39, 143.
5. Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
6. He, G.; Pfeiffer, M.; Leo, K.; Hofmann, M.; Birnstock, J.; Pudzich, R.; Sadbeck,
J. Appl. Phys. Lett. 2004, 85, 3911.
7. Shih, P.-I.; Chien, C.-H.; Chuang, C.-Y.; Shu, C.-F.; Yang, C.-H.; Chen, J.-H.; Chi, Y.
J. Mater. Chem. 2007, 17, 1692.
8. Holder, E.; Langeveld, B. M. W.; Schubert, U. S. Adv. Mater. 2005, 17, 1109.
9. Kohler, A.; Wilson, J. S.; Friend, R. H. Adv. Mater. 2002, 14, 701.
10. Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl.
Phys. Lett. 1999, 75, 4.
needle crystal. 1H NMR (CDCl3, 400 MHz):
d [ppm] 8.35–8.33 (d,
J¼7.8 Hz, 1H), 8.10–8.02 (m, 3H), 7.93–7.91 (d, J¼7.8 Hz, 1H), 7.76–
7.74 (d, 3H), 7.69–7.64 (m, 2H), 7.48–7.38 (m, 5H), 7.14–7.11 (t, 1H),
6.74–6.71 (m, 3H), 1.14 (s, 18H). 13C NMR (CDCl3, 100 MHz):
d [ppm]
157.5, 151.0, 150.7, 150.2, 148.9, 148.3, 143.4, 141.3, 139.5, 139.5,
136.5, 129.8, 129.7, 128.3, 127.7, 127.7, 127.5, 127.2, 126.2, 125.1,124.4,
123.4, 120.9, 120.5, 120.4, 119.4, 35.0, 31.6. MS (MALDI): 555.295 (m/
z), calcd for C42H37N: 555.292.
11. Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.;
Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304.
12. Lamansky, S.; Kwong, R. C.; Nugent, M.; Djurovich, P. I.; Thompson, M. E. Org.
Electron. 2001, 2, 53.
4.9. General procedure for the synthesis of the Ir(III)
complexes
13. Tsuzuki, T.; Shirasawa, N.; Suzuki, T.; Tokito, S. Adv. Mater. 2003, 15, 1455.
14. Beeby, A.; Bettington, S.; Samuel, I. D. W.; Wang, Z. J. Mater. Chem. 2003, 13, 80.
15. Steuber, F.; Staudigel, J.; Stossel, M.; Simmerer, J.; Winnacker, A.; Speritzer, H.;
Weissortel, F.; Salbeck, J. Adv. Mater. 1999, 12, 130.
16. Geng, Y.; Datsis, D.; Gulligan, S. W.; Ou, J. J.; Chen, S. H.; Rothberg, L. J. Chem.
Mater. 2002, 14, 463.
A mixture of a ligand (1.4 mmol), iridium chloride trihydrate
(0.25 g, 0.7 mmol), water (7.5 mL), and 2-ethoxyethanol (22.5 mL)
was deoxygenated and then heated to reflux under nitrogen for 24 h.
After cooling to room temperature, the mixture was filtrated and
washed with ethanol to give product chloride-bridged dimer. Then,
a mixture of the chloride-bridged dimer (0.56 g, 0.2 mmol), acetyl
acetone (50 mg, 0.5 mmol), ethanol (0.3 mL), dichloromethane
(15.6 mL), and tetrabutylammonium hydroxide (129 mg) was de-
oxygenatedand heatedtoreflux under nitrogen for 2 h. Aftercooling
to room temperature, the mixture was evaporated in vacuo. After
column chromatographic purification (silica gel, dichloromethane),
the final Ir complex products were obtained with yields ranging
from 32% to 43%.
17. Kim, J. Pure Appl. Chem. 2002, 74, 2031.
18. Wong, K.-T.; Liao, Y.-L.; Su, H.-C.; Wu, C.-C. Org. Lett. 2005, 7, 5131.
19. You, Y.; Park, S. Y. J. Am. Chem. Soc. 2005, 127, 12438.
20. Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071.
21. Yu, W.-L.; Pei, J.; Huang, W.; Heeger, A. J. Adv. Mater. 2000, 12, 828.
22. Lo, S.-C.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. Macromolecules 2003, 36, 9721.
23. Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby, C. E.; Kohler, A.;
Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004, 126, 7041.
24. Zhang, K.; Chen, Z.; Yang, C. L.; Qin, J. G.; Cao, Y. Organometallics 2007, 26, 3699.
25. 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.
26. Chen, X.; Liao, J.-L.; Liang, Y.; Ahmed, M. O.; Tseng, H.-E.; Chen, S.-A. J. Am.
Chem. Soc. 2003, 125, 636.
27. Duan, J.-P.; Sun, P.-P.; Cheng, C.-H. Adv. Mater. 2003, 15, 224.
28. Ragni, R.; Plummer, E. A.; Brunner, K.; Hofstraat, J. W.; Babudri, F.; Farinola,
G. M.; Naso, F.; Cola, L. D. J. Mater. Chem. 2006, 16, 1161.
29. Mak, C. S. K.; Hayer, A.; Pascu, S. I.; Watkins, S. E.; Holmes, A. B.; Kohler, A.;
Friend, R. H. Chem. Commun. 2005, 4708.
Compound C1 was synthesized from L1 with a yield of 32% as
yellow solid. 1H NMR (CDCl3, 400 MHz):
d [ppm] 8.59 (d, 2H), 7.84
(d, 4H), 7.63 (m, 4H), 7.34 (m, 6H), 7.14 (m, 8H), 7.08 (m, 4H), 6.97 (d,
2H), 6.77 (d, 4H), 6.70 (d, 2H), 5.23 (s, 1H), 1.81 (s, 6H). MS (MALDI):
1075.309 (m/z), calcd for C65H43N2IrO2: 1075.286.
30. Sun, Y.-H.; Zhu, X.-H.; Chen, Z.; Zhang, Y.; Cao, Y. J. Org. Chem. 2006, 71, 6281.
31. Jung, S.; Kang, Y.; Kim, H. S.; Kim, Y. H.; Lee, C. L.; Kim, J. J.; Lee, S. K.; Kwon, S. K.
Eur. J. Inorg. Chem. 2004, 17, 3415.
Compound C2 was synthesized from L2 with a yield of 43% as
a yellow solid. 1H NMR (CDCl3, 400 MHz):
d [ppm] 8.58 (d, 2H), 7.68
32. Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.
(m, 8H), 7.31 (m, 6H), 7.10 (m, 4H), 6.99 (s, 2H), 6.91 (t, 4H), 6.70 (d,
2H), 6.58 (d, 2H), 6.48 (s, 2H), 5.24 (s, 1H), 1.79 (s, 6H), 1.15 (s, 18H),
0.89 (s, 18H). MS (MALDI): 1300.437 (m/z), calcd for C81H75N2IrO2:
1300.546.
33. Ding, J.; Gao, J.; Fu, Q.; Cheng, Y.; Ma, D.; Wang, L. Synth. Met. 2005, 155, 539.
34. Chen, L.; Yang, C.; Qin, J.; Gao, J.; You, H.; Ma, D. J. Organomet. Chem. 2006, 691,
3519.
35. Ono, K.; Joho, M.; Saito, K.; Tomura, M.; Matsushita, Y.; Naka, S.; Okada, H.;
Onnagawa, H. Eur. J. Inorg. Chem. 2006, 3676.
36. van, A.; DijkenBastiannsen, J. J. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W.;
Rothe, C.; Monkman, A.; Bach, I.; Stossel, P.; Brunner, K. J. Am. Chem. Soc. 2004,
126, 7718.
37. Gong, X.; Lim, S. H.; Ostrowski, J. C.; Moses, D.; Bazan, G. C.; Heeger, A. J. J. Appl.
Phys. 2004, 95, 948.
Compound C3 was synthesized from L3 with a yield of 37% as
yellow solid. 1H NMR (CDCl3, 400 MHz):
d [ppm] 8.37 (d, 2H), 7.73
(m, 6H), 7.57 (d, 2H), 7.34 (m, 8H), 7.03 (t, 2H), 6.88 (m, 4H), 6.58 (m,
6H), 6.17 (s, 2H), 5.15 (s, 1H), 1.15–1.13 (d, 36H). MS (MALDI):
1452.729 (m/z), calcd for C93H83N2IrO2: 1452.609.
38. King, S. M.; Al-Attar, H. A.; Evans, R. J.; Congreve, A.; Beeby, A.; Monkman, A. P.
Adv. Funct. Mater. 2006, 16, 1043.
39. Velusamy, M.; Thomas, K. R. J.; Chen, C.-H.; Lin, J. T.; Wen, Y. S.; Hsieh, W.-T.; Lai,
C.-H.; Chou, P.-T. Dalton Trans. 2007, 3025.
Compound C4 was synthesized from L4 with a yield of 32% as
red solid. 1H NMR (CDCl3, 400 MHz):
d
[ppm] 8.69 (d, 2H), 8.44 (m,
4H), 7.91 (m, 6H), 7.72 (m, 6H), 7.54 (d, 2H), 7.39 (m, 6H), 7.14 (m,
40. Kim, Y.-H.; Kim, H.-S.; Kwon, S.-K. Macromolecules 2005, 38, 7950.