An ambipolar poly(meta-phenylene) copolymer with high triplet energy to host blue and green electrophosphorescence

Article abstract of DOI:10.1039/c1jm11212k

An ambipolar polymer host for electrophosphorescence is developed by incorporating a hole transporting carbazole unit and electron transporting oxadiazole unit into a poly(meta-phenylene) backbone with a high triplet energy level.

Full text of DOI:10.1039/c1jm11212k

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PAPER
An ambipolar poly(meta-phenylene) copolymer with high triplet energy to host
blue and green electrophosphorescence
a
a
a
Jun Liu,a Lu Li,ab Chaokun Gong, Zhibin Yu and Qibing Pei
*
Received 22nd March 2011, Accepted 11th April 2011
DOI: 10.1039/c1jm11212k
An ambipolar polymer host for electrophosphorescence is developed by incorporating a hole
transporting carbazole unit and electron transporting oxadiazole unit into a poly(meta-phenylene)
backbone with a high triplet energy level.
LUMO/HOMO energy levels and high conductivity.15–17
Introduction
However, PFs have an ET of only about 2.2 eV, which is
lower than those of green (2.4 eV) and blue (2.6 eV) phos-
phorescent dopants. Therefore, PFs cannot efficiently host
blue and green electrophosphorescence.18–20 Recently, several
high ET conjugated polymers (ET ¼ 2.5–2.6 eV) featuring
a meta-linkage in the backbone were reported as promising
host materials for green PPLEDs, such as poly(3,6-carba-
zole),13,14,21,22 poly(3,6-silafluorene)23–25 and poly(3,6-fluo-
rene).26,27 We reported that poly(meta-phenylene) had an ET as
high as 2.64 eV and could be used as a conjugated polymer
host for efficient blue electrophosphorescence.28,29 While the
meta-linkages in the backbone of these polymers increase the
ET, they also increase the bandgap of the polymers and lead
to larger barriers for charge carrier injections. The resulting
single-layer devices exhibit poor electroluminescence (EL)
performance. Although insertion of an additional charge
carrier injection layer boosts EL efficiency, the device fabri-
cation becomes more complicated.
Polymer light-emitting diodes (PLEDs) promise new flat-panel
display and solid-state lighting technologies fabricated by low-
cost solution processing.1 The materialization of this potential
largely hinges on phosphorescent PLEDs (PPLEDs), in which
phosphorescent dopants are dispersed in a polymer host and
used as the emissive layer. PPLEDs can harvest both singlet
and triplet excitons and achieve an internal electron-to-photon
quantum efficiency of 100%.2–11 Ideally, owing to solution
processing in fabrication, PPLED devices have a single-layer
configuration with the emissive layer sandwiched between the
cathode and anode. During device operation, electrons and
holes are injected from electrodes to the polymer host and
generate singlet and triplet excitons. The excitons are trans-
ferred to the triplet states of the phosphorescent dopants and
generate light. An ideal polymer host, on the one hand,
should possess proper LUMO/HOMO energy levels matching
with the cathode/anode to facilitate charge carrier injec-
tion.12,13 On the other hand, a polymer host should have
a high triplet energy level (ET) to prevent triplet energy back
transfer from the dopant to the host and the consequent
quenching of triplet excitons.14 Unfortunately, polymer host
materials with both high ET and proper LUMO/HOMO
energy levels are scarce.
Herein, we improve charge carrier injection/transporting
abilities of high-ET conjugated polymer hosts by incorpo-
rating both hole transporting unit and electron transporting
unit to the conjugated polymer backbone. Poly(meta-phenyl-
ene) has been proved by us to exhibit ET high enough to host
blue and green electrophosphorescence.28 Carbazole and
oxadiazole are well-known hole transporting materials and
electron transporting materials, respectively. They both have
been introduced to conjugated polymers to improve charge
carrier transporting.30–33 In this manuscript, we develop
a novel ambipolar high-ET conjugated polymer host (PmPCz-
Ox, see Scheme 1) with a carbazole unit and oxadiazole unit
PPLEDs typically use a non-conjugated polymer, poly
(vinylcarbazole) (PVK), as the host because of its high ET.
However, PVK is a good hole transporter but a poor electron
transporter. Therefore, PVK has to be admixed with electron
transporting materials for charge carrier balance, which raises
the probability of phase separation during long-term device
operation.2–8 Conjugated polymers, such as poly(2,7-fluorene)s
(PFs), are excellent host materials because of their suitable
incorporated to
a poly(meta-phenylene) backbone. This
ambipolar polymer host retains high ET (2.60 eV) of the poly
(meta-phenylene) backbone and possesses suitable LUMO
(ꢀ2.43 eV) and HOMO (ꢀ5.57 eV) energy levels owing to the
oxadiazole unit and the carbazole unit, respectively. There-
fore, PmPCz-Ox is expected to be a suitable host for blue
and green PPLEDs with high efficiencies and low driving
voltages.
aDepartment of Material Science and Engineering, University of
California, Los Angeles, California, 90095, USA. E-mail: qpei@seas.
ucla.edu
bState Key Laboratory of Electronic Thin Films and Integrated Devices,
Department of Optoelectronic, Information, University of Electronic
Science and Technology of China, Chengdu, 610054, P. R. China
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0.500 mmol), 2-(3,5-dibromophenyl)-5-phenyl-1,3,4-oxadiazole
(3) (0.1907 g, 0.500 mmol), Pd(PPh3)4 (0.0101 g, 0.0100 mmol)
and Aliquat 336 (1 drop) under argon atmosphere was added
K2CO3 (2.0 mL 2.0 M aqueous solution) and toluene (6 mL). The
mixture was stirred at 90 ꢁC for 48 h. After being cooled down to
room temperature, the mixture was poured into CHCl3 and
washed with water three times, followed by drying over anhy-
drous Na2SO4 and filtration. The filtrate was concentrated and
precipitated in methanol to give the crude product as a white
powder, which was further purified by re-precipitation twice in
CHCl3/CH3OH. The powder was collected and dried in vacuum
1
overnight. Yield: 0.17 g (51.5%). H NMR (400 MHz, CDCl3)
d (ppm): 8.14–7.93 (br, 7H), 7.55–7.27 (br, 8H), 7.10 (br, 4H),
4.15 (br, 2H), 3.91 (br, 2H), 1.80 (br, 4H), 1.30 (br, 4H). Anal.
Calcd. for C38H31N3O2: C, 81.26; H, 5.56; N, 7.48. Found: C,
80.33; H, 5.60; N, 7.21.
Characterization
1H and 13C NMR spectra were recorded on a Bruker arx-400
spectrometer. Elemental analysis was performed with a Perkin-
Elmer 2400 elemental analyzer. Molecular weights of the poly-
mer were measured with gel permeation chromatography (GPC)
method using mono-disperse polystyrene standards and tetra-
hydrofuran (THF) as the eluent. Thermal gravimetic analysis
(TGA) was carried out on a Perkin-Elmer TGA-7 system with
a heating rate of 20 ꢁC/min. Differential scanning calorimetry
(DSC) was performed on a Perkin Elmer DSC-6 system with
Scheme 1 Chemical structures and synthetic routes of PmPCz-Ox.
Experimental section
Materials
9-(6-(3,5-Dibromophenoxy)hexyl)-9H-carbazole (1)28 and 2-(3,5-
dibromophenyl)-5-phenyl-1,3,4-oxadiazole (3)13 were prepared
according to the reported procedures. All the reactions were
carried out under argon gas protection. The phosphorescent
ꢁ
a heating rate of 20 C/min. Cyclic voltammetry (CV) was con-
dopants,
fac-tris[2-(2-pyridyl-kN)-5-mehylphenyl]iridium(III)
ducted in a solution of Bu4NBF4 (0.1 M) in acetonitrile with Pt
wire, Pt plate and saturated calomel electrode (SCE) as the
working electrode, counter electrode and reference electrode,
respectively. The polymer film was dip-coated on the working
electrode from its solution in methylene chloride. Absorption
spectra were obtained from a Shimadzu UV-1700 UV/Vis spec-
trophotometer. Fluorescence spectra at room temperature and
phosphorescence spectra at 77 K were measured with a PTI
QuantaMaster 30 spectrofluorometer. Current–voltage and
brightness–voltage curves of PPLED devices were recorded by
a computer-controlled Keithley 2400/2002 source unit calibrated
with a Photoresearch PR-655 spectrophotometer. Electrolumi-
nescence spectra and CIE coordinates were recorded with the
Photoresearch PR-655 spectrophotometer.
(Ir(mppy)3) and bis[(4,6-difluorophenyl)pyridinato-N,C2]-(pico-
linato)iridium(III) (FIrpic), were purchased from American Dye
Source.
9-(6-(3,5-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenoxy)hexyl)-9H-carbazole (2)
A mixture of 9-(6-(3,5-dibromophenoxy)hexyl)-9H-carbazole (1)
(0.54 g, 1.08 mmol), Pd(dppf)2Cl2 (0.048 g, 0.065 mmol),
potassium acetate (0.64 g, 6.53 mmol), bis(pinacolato)diboron
(0.61 g, 2.40 mmol) and DMSO (15 mL) was stirred at 80 ꢁC
overnight. After being cooled down, the mixture was poured into
brine and extracted with CH2Cl2. The organic layer was washed
with water three times, dried over Na2SO4, filtered and concen-
trated. The residual was purified by column chromatography
with CH2Cl2/hexane ¼ 1/1 as the eluent to give the title
Device fabrication
1
compound as a white solid. Yield: 0.47 g (73.0%). H NMR
Indium tin oxide (ITO) glass substrates were ultrasonically
cleaned for 30 min each sequentially with detergent, de-ionized
water, acetone and isopropanol. Then they were dried in a heat-
ing chamber at 70 ꢁC. PEDOT:PSS (Clevios VP Al 4083 from H.
C. Starck Inc.) was spin-coated on the ITO glass substrates at
3000 rpm for 60 s and then baked at 120 ꢁC for 15 min to give an
approximate thickness of 40 nm. The emissive layer (70 nm)
was then spin-coated from the solution of the polymer host
(20 mg mLꢀ1) and the dopants in chlorobenzene. Finally, a thin
layer of CsF (1.0 nm) followed by a layer of aluminum (100 nm)
were deposited in a vacuum thermal evaporator through
a shadow mask at a pressure of 10ꢀ6 Torr. The active area of the
devices is 0.126 cm2.
(400 MHz, CDCl3) d (ppm): 8.10 (d, 2H), 7.86 (s, 1H), 7.46 (m,
6H), 7.23 (td, 2H), 4.32 (t, 2H), 3.97 (t, 2H), 1.91 (m, 2H), 1.74
(m, 2H), 1.53 (m, 4H), 1.33 (s, 24H). 13C NMR (100 MHz,
CDCl3) d (ppm): 150.07, 140.42, 133.50, 125.62, 123.50, 122.82,
120.33, 118.71, 108.66, 83.77, 67.56, 42.97, 29.28, 28.92, 27.05,
25.94, 24.88. HR-MS (m/z (%)): 596.37 (100) [M + H]+. Anal.
Calcd. for C36H47B2NO5: C, 72.62; H, 7.96; N, 2.35. Found: C,
72.93; H, 8.05; N, 2.35.
PmPCz-Ox
To a mixture of 9-(6-(3,5-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)phenoxy)hexyl)-9H-carbazole (2) (0.2827 g,
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Results and discussion
Design and synthesis of PmPCz-Ox
The chemical structure of the polymer host PmPCz-Ox is shown
in Scheme 1. The hole-transporting carbazole unit is connected
to the poly(meta-phenylene) backbone through an alkyloxy
spacer and the electron-transporting oxadiazole unit is in
conjugation with the poly(meta-phenylene) backbone. As shown
in Scheme 1, The diboronate monomer 2 was synthesized from
carbazole-tethering dibromobenzene
1 and bis(pinacolato)
diboron under palladium catalysis. The 2,5-diphenyl-1,3,4-oxa-
diazole dibromide monomer 3 was obtained in a four-step
synthesis following the literature procedures.13 With the two
monomers in hand, we synthesized the alternating polymer
PmPCz-Ox by Suzuki polycondensation with palladium catal-
ysis.34 Its chemical structure is verified by 1H NMR and elemental
analysis. PmPCz-Ox is readily soluble in common organic
solvents, such as chloroform, toluene and chlorobenzene. The
weight average molecular weight (Mw) and polydispersity (PDI)
of PmPCz-Ox, as determined by GPC with polystyrene stan-
dards, are 5700 and 1.59, respectively.
Fig. 2 Absorption spectra and fluorescence spectra of PmPCz-Ox in
dilute solution and in solid film.
peak at 337 nm stemming from the carbazole unit. The absorp-
tion spectrum in the thin film is red-shifted by 10 nm compared to
that in solution because of the interaction between the polymer
chains in the solid state. The PL spectrum of PmPCz-Ox in
solution shows two vibronic peaks at 352 nm and 368 nm with
a broad band at 410 nm. The former two peaks are characteristic
of the carbazole unit while the latter broad band comes from the
exciplex between the carbazole unit and the oxadiazole unit.35 In
the thin film, the emission from the carbazole unit disappears and
the exciplex emission dominates the PL spectrum owing to the
close packing of the carbazole unit and the oxadiazole unit. The
exciplex between the carbazole unit and oxadiazole unit has
always been observed in blends of PVK and 2-(4-biphenylyl)-5-
(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD).2–8 Such blends are
frequently used as hosts for PPLEDs. The formation of the
exciplex has been shown not to affect EL performance of
PPLEDs. Fig. 3 shows the overlap of the fluorescent spectrum of
PmPCz-Ox along with the absorption spectra of FIrpic and Ir
The thermal properties of PmPCz-Ox are investigated using
TGA and DSC under nitrogen atmosphere. PmPCz-Ox exhibits
good thermal stability with 5% weight loss temperature (Td) at
ꢁ
404 C. The polymer does not show a glass transition, crystalli-
ꢁ
zation, or melting behavior in the temperature range 25 C–250
ꢁC, implying good morphology stability at normal device oper-
ation conditions.
The compatibility of PmPCz-Ox and phosphorescent dopants
is explored by atom force microscopy (AFM). Fig. 1 shows the
phase images of the film spin-coated from the blends of PmPCz-
Ox with FIrpic or Ir(mppy)3. The phase images exhibit a uniform
surface without pinholes, particles or phase separation, indi-
cating excellent compatibility of PmPCz-Ox with FIrpic or Ir
(mppy)3. This compatibility ensures good dispersion of the
phosphorescent dopants in the host and is necessary for excellent
device performance.
€
(mppy)3. The broad overlap is beneficial for efficient Forster
energy transfer from the polymer host to the dopants.
The phosphorescence spectrum of PmPCz-Ox dissolved in
THF was measured at 77 K (see Fig. 4). Deconvolution of the
phosphorescence spectrum gives three peaks at 474 nm, 501 nm
and 542 nm. Using peak energy at 474 nm, the ET of PmPCz-Ox
Photophysical properties
Fig. 2 shows the absorption and photoluminescence (PL) spectra
of PmPCz-Ox in dilute CH2Cl2 solution and in a thin film.
PmPCz-Ox in dilute solution has a broad absorption band
spanning from 308 nm to 255 nm, attributed to the oxadiazole-
incorporated poly(meta-phenylene) backbone. There is a small
Fig. 1 AFM phase images of the films spin-coated from the blends of
PmPCz-Ox (20 mg mLꢀ1 solution in chlorobenzene) with (a) FIrpic (10
wt%) or (b) Ir(mppy)3 (1 wt%).
Fig. 3 PL spectrum of PmPCz-Ox in thin film and absorption spectra of
FIrpic and Ir(mppy)3 in dilute solution.
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Fig. 4 Deconvolution of the phosphorescence spectrum of PmPCz-Ox
in THF at 77 K.
is calculated to be 2.60 eV. This value is slightly lower than that
of the blue phosphorescent dopant, FIrpic (2.62 eV), and higher
than that of green phosphorescent dopant, Ir(mppy)3 (2.41 eV).36
Therefore, when PmPCz-Ox is hosting green electro-
phosphorescence, triplet excitons can be confined in Ir(mppy)3
without triplet energy back transfer. However, for hosting blue
electrophosphorescence, the confinement of triplet excitons in
FIrpic is questionable. The ET of PmPCz-Ox is slightly lower
than that of the previously reported conjugated polymer host
based on poly(meta-phenylene) (2.64 eV),28,29 probably due to the
conjugation of the oxadiazole unit with the poly(meta-phenyl-
ene) backbone.
Fig. 5 PL spectra of PmPCz-Ox blends with FIrpic (a) or Ir(mppy)3 (b)
with specified phosphorescent dopant contents.
To investigate the potential of PmPCz-Ox as a phosphores-
cence host, we blended PmPCz-Ox with FIrpic or Ir(mppy)3 and
measured their PL spectra in thin films. The results are shown in
Fig. 5. When PmPCz-Ox is doped with FIrpic, the PL spectra
show emission from both PmPCz-Ox and FIrpic. With
increasing FIrpic content, the relative intensity of FIrpic emis-
sion increases while the relative intensity of PmPCz-Ox emission
decreases. These results indicate no serious triplet energy back
transfer from FIrpic to PmPCz-Ox and are consistent with the
similar ET of PmPCz-Ox and FIrpic. For the blends of the
polymer host with Ir(mppy)3, the same trend is observed. When
the Ir(mppy)3 content is higher than 5 wt%, the emission from
PmPCz-Ox is almost completely quenched. The energy transfer
from host to phosphorescent dopant is more complete in the case
of Ir(mppy)3 compared to that of FIrpic. Two possible factors
may be responsible for the difference. One is the more efficient
1.17 V and 1.50 V, respectively. The former is attributed to the
carbazole unit, while the latter the poly(meta-phenylene)
backbone. According to the empirical formula (EHOMO ¼ ꢀe
(Eox + 4.4) [eV], ELUMO ¼ ꢀe (Ered + 4.4) [eV]),37 the HOMO and
LUMO energy levels of PmPCz-Ox are calculated to be
ꢀ5.57 eV and ꢀ2.43 eV, respectively. The EHOMO matches well
with the work functions of the anode (ITO/PEDOT:PSS:
ꢀ5.2 eV), and the ELUMO with the cathode (CsF/Al: ꢀ2.2 eV).
These results indicate that the incorporation of carbazole unit
and oxadiazole unit would facilitate both hole injection and
electron injection into PmPCz-Ox.
€
Forster energy transfer from the host to Ir(mppy)3 than that
Electroluminescent properties
from the host to FIrpic. This would be consistent with the
observed larger spectral overlap in the case of Ir(mppy)3 (see
Fig. 3). The other one is the possible slight triplet energy back
transfer from FIrpic to PmPCz-Ox due to their ET alignment.
Polymers with high ET generally have large bandgaps and suffer
from difficult charge injections due to misalignment of their
HOMO/LUMO with Fermi energy levels of electrodes. There-
fore, their single-layer devices always exhibit very low EL effi-
ciencies.13,24,26 A charge carrier transporting material has to be
blended in the emissive layer3,4,6 or inserted as an additional
charge injection layer21,25,39–42 to improve charge carrier balance
for high EL efficiency. The proper LUMO/HOMO energy levels
of PmPCz-Ox are expected to facilitate charge carrier injections
and enable fabrication of single-layer devices. The single (emis-
sive) layer would employ a blend of PmPCz-Ox with 10 wt% of
FIrpic or 1 wt% of Ir(mppy)3. Accordingly, single-layer devices
Electrochemical properties
Cyclic voltammetry of PmPCz-Ox film was employed to inves-
tigate the redox behavior and to estimate the LUMO/HOMO
energy levels of the polymer host. The cyclic voltammogram
exhibits one irreversible reduction wave with an onset potential
at ꢀ1.97 V, due to the electron-deficient oxadiazole unit. There
are two irreversible oxidation waves with the onset potentials at
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Table 1 EL performance of the blue and green PPLED devices
were fabricated with a sandwich structure of ITO/PEDOT:PSS
(40 nm)/emissive layer (70 nm)/CsF (1 nm)/Al (100 nm).
PmPCz-Ox:10
wt% FIrpic
PmPCz-Ox:1
wt% Ir(mppy)3
As shown in Fig. 6, the EL spectra of the two devices exhibit
emission exclusively from FIrpic (lmax ¼ 472 nm) or Ir(mppy)3
(lmax ¼ 512 nm). No contribution from PmPCz-Ox is observed
in the EL spectra. Note the presence of emission from the
polymer in the PL spectra of the blends (see Fig. 5). The absence
of the host emission in the EL spectra can be attributed to charge
trapping effect by FIrpic and Ir(mppy)3.38
Emissive layer
Turn-on voltage (V)
7.6
160
0.50
0.14
472
(0.16, 0.32)
6.5
Maximum brightness (cd mꢀ2
)
7650
20.9
6.4
512
(0.24, 0.65)
Luminance efficiency (cd Aꢀ1
Power efficiency (lm/W)
EL maximum (nm)
)
CIE coordinates (x,y)
The primary PPLED device performance data are listed in
Table 1. Fig. 7 shows the luminance efficiencies as a function of
current densities. A typical blue electrophosphorescent device
has a turn-on voltage of 7.6 V, luminance efficiency of 0.50 cd
Aꢀ1 and maximum brightness of 160 cd mꢀ12. This performance is
somewhat worse than that of poly(meta-phenylene) tethered with
carbazole.28 The reason is unknown. Possibly, PmPCz-Ox
requires optimized device structure for good EL performance.
The green electrophosphorescent device has a turn-on voltage of
6.5 V, luminance efficiency of 20.9 cd Aꢀ1 and maximum
brightness of 6050 cd mꢀ12. This performance is among the
highest reported for green PPLEDs based on conjugated poly-
mer host, indicating that PmPCz-Ox is an excellent host for
green electrophosphorescence. For comparison, we use carba-
zole-tethering poly(meta-phenylene) without an oxadiazole
unit28 as the host to fabricate control green PPLED. The control
device exhibits a luminance efficiency of 12.8 cd Aꢀ1, which is
lower than of PmPCz-Ox. Therefore, the excellence of PmPCz-
Fig. 7 Dependence of luminance efficiency on current density of the blue
and green PPLEDs.
Ox is attributed to the high triplet energy level due to the poly
(meta-phenylene) backbone, and the proper LUMO/HOMO
energy levels owing to the oxadiazole/carbazole units. This result
demonstrates the molecular design strategy of ambipolar poly-
mer host materials with electron–hole transporting units incor-
porated into a high-ET conjugated polymer backbone.
Conclusion
In summary, we have developed an ambipolar conjugated
polymer host material PmPCz-Ox with both a hole transporting
carbazole unit and electron transporting oxadiazole unit incor-
porated into a poly(meta-phenylene) backbone. The poly(meta-
phenylene) backbone ensures high ET (2.60 eV) of PmPCz-Ox.
The carbazole unit raises the polymer’s HOMO energy level to
enhance hole injection, while the oxadiazole unit lowers the
LUMO energy level, to boost electron injection. The PPLED
devices with PmPCz-Ox doped with green or blue phosphores-
cent dopants as the active layer emit exclusive phosphorescent
light from the dopants. The preliminary devices show a lumi-
nance efficiency of 20.9 cd Aꢀ1 for green electrophosphorescence
and 0.5 cd Aꢀ1 for blue emission. The excellent EL performance
of the green PPLED demonstrates our molecular design of
ambipolar polymer hosts, in which the electron transporting
moiety and hole transporting moiety are incorporated into
a polymer backbone with high ET. We believe that through
further molecular design, ambipolar conjugated polymer hosts
can be developed with sufficiently high ET to host blue phos-
phorescent dopants.
Fig. 6 EL spectra of single layer devices, ITO/PEDOT:PSS/emissive
layer/CsF/Al, wherein the emissive layer is a blend of PmPCz-Ox with
10 wt% FIrpic (a) or 1 wt% Ir(mppy)3 (b).
9776 | J. Mater. Chem., 2011, 21, 9772–9777
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20 N. R. Evans, L. S. Devi, S. S. K. Mak, S. E. Watkins, S. I. Pascu,
Acknowledgements
€
A. Kohler, R. H. Friend, C. K. Williams and A. B. Holmes, J. Am.
Chem. Soc., 2006, 128, 6647.
The work reported here was supported by the United States
Department of Energy Solid State Lighting Program (Grant No.
DE-FC26-08NT01575).
21 Y.-C. Chen, G.-S. Huang, C.-C. Hsiao and S.-A. Chen, J. Am. Chem.
Soc., 2006, 128, 8549.
22 K. Zhang, Y. T. Tao, C. L. Yang, H. You, Y. Zou, J. G. Qijn and
D. G. Ma, Chem. Mater., 2008, 20, 7324.
23 Y. Q. Mo, R. Y. Tian, W. Shi and Y. Cao, Chem. Commun., 2005,
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