White emission from dinuclear cyclometalated platinum(II) complex in single-emitting layer PLEDs

Article abstract of DOI:10.1016/j.tet.2011.01.042

A dinuclear platinum(II) complex of (dfppy)₂Pt ₂(dipic) has been prepared, where dfppy is 2,4-difluorophenylpyridine and dipic is a biphenyl-bridged bi-picolinic acid derivative. Its physical and optoelectronic properties, as well as

Full text of DOI:10.1016/j.tet.2011.01.042

Tetrahedron 67 (2011) 2118e2124
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Tetrahedron
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White emission from dinuclear cyclometalated platinum(II) complex
in single-emitting layer PLEDs
,
a
a
a
a
Yafei Wang a, Xianping Deng a, Yu Liu a , Meijun Ni , Ming Liu , Hua Tan , Xiaoshuang Li ,
*
,
b,
*
Weiguo Zhu a , Yong Cao
*
a College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education, Xiangtan University, Xiangtan 411105, China
b Institute of Polymer Optoelectronic Material and Devices, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
dinuclear platinum(II) complex of (dfppy)2Pt2(dipic) has been prepared, where dfppy is 2,4-
Received 15 October 2010
Received in revised form 1 January 2011
Accepted 14 January 2011
Available online 20 January 2011
difluorophenylpyridine and dipic is a biphenyl-bridged bi-picolinic acid derivative. Its physical and op-
toelectronic properties, as well as molecular orbitals calculation have been investigated and compared
with those of its mono-nuclear (dfppy)Pt(pic) complex. Both platinum(II) complexes exhibited almost
identical photoluminescence (PL) spectra with deep blue emission in dilute dichloro-methane (10ꢀ5 M)
and different PL spectra with red emission in their neat films. Stable white emissions were obtained in
the (dfppy)2Pt2(dipic)-doped polymer light-emitting devices using a blend of poly(vinylcarbazole) and 2-
(4-biphenyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole as a host matrix at dopant concentrations from
1 wt % to 10 wt %. In contrast, the (dfppy)Pt(pic)-doped devices exhibited orange-red emissions in the
same device configuration. It indicates that dinuclear platinum(II) complex with a non-planar structure is
an effective way to control formation excimers of platinum(II) complex and get white-emitting PLEDs
with single dopant.
Keywords:
Electroluminescence
Dinuclear platinum(II) complex
Polymer light-emitting devices
White light
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, another kind of the SEL-based WOLEDs with single
dopant has been developed, in which white light emission results
White organic light-emitting devices (WOLEDs) have been
attracting much interest because of their great potential applica-
tions for flat-panel displays with low-cost, high efficiency, and
flexible property.1e4 Usually, white light emission can be obtained
from two kinds of device structures. One is multiple-emissive-layer
(MEL) structure,5e12 in which different layers emit different light in
visible spectral regions. The other is single-emissive-layer (SEL)
structure where multi-emitters are doped in the same emissive
layer.13e16 In the MEL-based WOLEDs, a charge-blocking layer is
usually inserted between emissive layer and cathode in order to
confine the charges and excitons within the MEL regions for
achieving high-efficiency emission. However, this approach often
causes expensive processing cost, high driving voltage, and low
efficiencies. In contrast, the SEL-based WOLEDs have simple pro-
cess and lower cost than the MEL-based WOLEDs. Nevertheless, it is
a crucial challenge to get pure white emission and color stability
from a mixing emission of high-lying monomer emission and low-
lying excimer emission. The reported excimer-emitting materials
are mostly blue-emitting fluorescence17,18 and phosphorescence
materials with planar molecular structure.19e23 Compared to fluo-
rescence materials, phosphorescence materials, such as platinum
(II) complexes have higher efficient luminescence as they can har-
vest single and triplet excitons leading to 100% internal quantum
efficiency in theory. To our best knowledge, most of the reported
phosphors used as single dopant in the SEL-based WOLEDs are
bidentate platinum(II) complex of (dfppy)Pt(acac)20 and N^C^N-
coordinated tridentate platinum(II) complex of PtL2Cl.21e23 For
example, Jabbour et al. made a (dfppy)Pt(acac)-doped WOLED us-
ing 2,6-bis(N-carbazolyl)-pyridine (26mCPy) as a host matrix,
which exhibited near-white emission with a power efficiency of
29.1 lm Wꢀ1 and a Commission Internationale de l’eclairage (CIE)
ꢀ
1931 chromaticity coordinates of (0.46, 0.47).20 Cocchi et al. used
PtL2Cl and its derivatives as dopant to fabricate another WOLED
with an external quantum efficiency (EQE) of 18.3%, a power effi-
ciency of 11.8 lm Wꢀ1, and a CIE 1931 chromaticity coordinates of
(0.33, 0.38).23 Nevertheless, there are few reports on the white
polymer light-emitting devices (WPLEDs) using dinuclear platinum
(II) complex as single dopant.
€
because of phase separation and undesired Forest energy transfer
in the SEL-based WOLEDs.
* Corresponding authors. Tel.: þ86 731 58298280; fax: þ86 731 58292251 (W.Z.,
Y.L.); tel.: þ86 20 87114635 (Y.C.); e-mail addresses: liuyu03b@126.com (Y. Liu),
zhuwg18@126.com (W. Zhu), poycao@scut.edu.cn (Y. Cao).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.01.042

Y. Wang et al. / Tetrahedron 67 (2011) 2118e2124
2119
In order to achieve novel excimer-emitting platinum(II) com-
2.2. Photophysical properties
plex and its doped WPLEDs with stable white emission, here we
reported a dinuclear platinum(II) complex of (dfppy)2Pt2(dipic), in
which dfppy is 2,4-difluorophenylpyridine and dipic is a biphenyl-
bridged bi-picolinic acid derivative. For comparison, its mono-
nuclear platinum(II) complex of (dfppy)Pt(pic) was synthesized,
where pic is picolinic acid. The synthetic route of both platinum(II)
complexes is shown in Scheme 1. The photophysical, electro-
chemical, and electroluminescent properties, as well as molecular
orbitals calculated by density functional theory (DFT) for both of
platinum(II) complexes were studied. We found that the binuclear
platinum(II) complex of (dfppy)2Pt2(dipic) presented stable white
emission in the PLEDs using a blend of poly(N-vinylcarbazole)
(PVK) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD) as a host matrix with increasing dopant concentrations from
1 wt% to 10 wt% and tuning applied voltages from 7 V to 12 V.
However, the mononuclear (dfppy)Pt(pic) complex exhibited only
red emission in the PLEDs with the same configuration at different
concentrations from 1 wt% to 10 wt%.
The UV/vis absorption spectra of (dfppy)Pt(pic) and
(dfppy)2Pt2(dipic) in dilute dichloromethane (DCM,10ꢀ5 M) at 298 K
are shown in Fig. 1, and their pertinent UV/vis absorption data are
summarized inTable 1 for comparison. As a nearly linear and isolated
arrangement between the PtePt segments, dinuclear platinum(II)
complex exhibits a similar UV/vis spectral pattern to that of mono-
nuclear counterpart.25,26 Two moderate UV absorption bands are
observed at around 325 nm and 350 nm from the spin-allowed and
spin-forbidden metal-to-ligand charge transfer transitions (1MLCT
and 3MLCT), respectively. However, the dinuclear platinum(II) com-
plex exhibits an increased extinction coefficient of MLCT absorption
compared to the mononuclear platinum(II) complex. It is suggested
to be relative to the additional intramolecular acceptor(A)edonor
(D)eacceptor(A) interaction of (dfppy)2Pt2(dipic), where acceptor(A)
is a (dfppy)Pt(pic) chromophore and donor(D) is biphenyl unit.
Fig. 1 also depicts the photoluminescence (PL) spectra of these
two platinum(II) complexes in dilute DCM (10ꢀ5 M) and the neat
Scheme 1. Synthesis of cyclometalated platinum(II) complexes.
2. Results and discussion
2.1. Syntheses and characterization
The methyl bi-picolinate derivative (4) was synthesized via
a
Williamson etherforming reaction between 4,40-dihydrox-
ybiphenyl and methyl 3-(6-bromohexyloxy) picolinate (3) in the
presence of K2CO3 with a moderate yield of 38.5%. The bi-pico-
linic acid derivative of H2dipic (5) was obtained by hydrolysis of
methyl bi-picolinate derivative (4) under NaOH aqueous solution
with a high yield of 87%. The mononuclear platinum(II) complex
of (dfppy)Pt(pic) and binuclear platinum(II) complex of
(dfppy)2Pt2(dipic) were obtained according to our previous
work24 by a chloride cleavage of the (dfppy)2Pt2Cl2 dimmer with
picolinic acid and its di-picolinic acid derivative, respectively.
(See Supplementary data). The 1H NMR spectra (Fig. S1), ele-
mental analysis, and TOF-MS data confirm that both of the
platinum(II) complexes were successfully obtained.
Fig. 1. The UV/vis absorption and PL spectra in dilute DCM (10ꢀ5 M) and the PL spectra
in the neat films for (dfppy)2Pt2(dipic) and (dfppy)Pt(pic) at 298 K.

2120
Y. Wang et al. / Tetrahedron 67 (2011) 2118e2124
Table 1
Photophysical and electrochemical properties of (dfppy)2Pt2(dipic) and (dfppy)Pt(pic)
Compounds
UV/vis (nm)
Emissionb (nm)
Emissionc (nm)
HOMO (eV)
LUMO (eV)
Eg (eV)
(dfppy)Pt(pic)
(dfppy)2Pt2(dipic)
264, 325(0.652),a 350(0.652)a
418
425
601
438, 627
ꢀ6.63
ꢀ5.83
ꢀ3.46
ꢀ2.69
3.17
3.14
258, 322(2.03),a 353(1.54)a
a
Absorption extinction coefficient (104).
b
c
Measured in dilute DCM (10ꢀ5) at room temperature.
Measured in the neat film at room temperature.
films at room temperature. All of the PL data are also summarized
in Table 1. Both mono- and di-nuclear platinum(II) complexes ex-
hibit a similar PL profile in dilute DCM. The maximum emission
peaks at 425 nm and 418 nm are observed for the dinuclear plati-
num(II) complex and mononuclear platinum(II) complex, re-
spectively, which are assigned to the electronic transition from
3MLCT to ground state. Furthermore, the PL spectra exhibit varieties
with increasing (dfppy)2Pt2(dipic) concentrations from 10ꢀ5 M to
10ꢀ1 M in DCM, in which low-energy emission band exhibits an
increased intensity accompanying with another red-shift band
from high-energy emission (Fig. S2). The variety in PL profiles of
(dfppy)2Pt2(dipic) is attributed to its different aggregation state at
different concentrations in DCM.
peaked at 440 nm is stronger than that from the low-lying energy
emission peaked at 628 nm. In contrast, for the PL spectrum of
(dfppy)-Pt(pic), the intensity from the high-lying energy emission
peaked at 440 nm is much weaker than that from the low-lying en-
ergy emission peaked at 601 nm. It is obvious that the (dfppy)Pt(pic)
complex exhibits stronger excimer emission than the (dfppy)2Pt2(-
dipic) complex. Therefore, this AeDeA-based dinuclear platinum(II)
complex is available to display a decreased excimer emission due to
the distorted interaction of two (dfppy)Pt(pic) chromophores at
terminal.
2.3. DFT calculations
At the same time, the PL spectra in the (dfppy)2Pt2(dipic) and
(dfppy)Pt(pic) neat films exhibit two resolved emission bands with
a high-lying one at around 440 nm and a low-lying one at about
600 nm. However, distinct differences in the emission intensity and
location are present in the two PL spectra. For the PL spectrum of
(dfppy)2Pt2(dipic), the intensity from the high-lying energy emission
In order to investigate the effect of electronic transitions on PL
property, the molecular orbital calculations by density functional
theory (DFT: b3lyp/6-31g**: lanl2dz) were performed for both
platinum(II) complexes. The resulting high occupied molecular
orbital (HOMO) and low unoccupied molecular orbital (LUMO)
graphs are shown in Fig. 2 and their energy levels are listed in
Fig. 2. Molecular orbitals of (dfppy)2Pt2(dipic) and (dfppy)Pt(pic).

Y. Wang et al. / Tetrahedron 67 (2011) 2118e2124
2121
Table 2
Energies for the frontier orbitals of (dfppy)2Pt2(dipic) and (dfppy)Pt(pic) calculated
by DFT
Compounds
HUMO-1
(eV)
HUMO
(eV)
LUMO
(eV)
LUMOþ1
LUMOþ2
(eV)
(eV)
(dfppy)Pt(pic)
(dfppy)2Pt2(dipic)
ꢀ6.302
ꢀ5.864
ꢀ6.138
ꢀ5.864
ꢀ2.356
ꢀ2.247
ꢀ1.973
ꢀ2.247
ꢀ1.562
ꢀ1.754
Table 2. Just as most of cyclometalated complexes,27 the electronic
cloud is distributed among the Pt center and the cyclometalated
ligand of dfppy for HOMO, while the electronic cloud is localized
principally on the dfppy, the Pt center, and the pic ancillary ligand
for LUMO. The distribution of electronic cloud for the dinuclear
platinum(II) complex of (dfppy)2Pt2(dipic) is similar to that for the
mononuclear platinum(II) complex of (dfppy)Pt(pic). However, the
bridged biphenyl unit provides a little contribution to the HOMO
and LUMO. It should be related to that the conjugate degree be-
tween two (dfppy)Pt(pic) chromophores is destroyed by the ali-
phatic ether chain in (dfppy)2Pt2(dipic). As shown in Table 2, the
average energies of HOMO and LUMO are ꢀ5.86 and ꢀ2.25 eV for
(dfppy)2Pt2(dipic) and ꢀ6.22 and ꢀ2.16 eV for (dfppy)Pt(pic), re-
spectively, which is calculated by its near-degenerate pairs of
HOMO, HOMO-1 and LUMO, LUMOþ1. Therefore, the energy gap of
(dfppy)2Pt2(dipic) is narrower than that of (dfppy)Pt(pic). The
AeDeA-based dinuclear platinum(II) complex exhibits bath-
ochromic emission compared to its mononuclear platinum(II)
complex.
2.4. Thermal and dispersible properties
The thermal stability of platinum(II) complexes was evaluated
by thermogravimetric analysis (TGA) under N2 stream with
a scanning rate of 20 ꢁC/min and their TGA curves are shown in
Fig. S3. The decomposition temperature (Td) values are 236 ꢁC for
(dfppy)2Pt2(dipic) and 294 ꢁC for (dfppy)Pt(pic), which correspond
to a 5% weight loss. Both platinum(II) complexes exhibit high
thermal stability.
In order to make clear their dispersibility in polymer matrix, the
films of (dfppy)2Pt2(dipic) and (dfppy)Pt(pic) doped into a blend of
PVKþPBD at 10 wt% doping concentration were made. Their surface
morphologies were recorded by atomic force microscopy (AFM)
and are shown in Fig. 3. The (dfppy)2Pt2(dipic) and (dfppy)Pt(pic)-
Fig. 3. Atomic force microscope images for the platinum(II) complexes-doped
PVKePBD films at 10% doping concentration: (a) (dfppy)2Pt2(dipic); (b) (dfppy)Pt(pic).
(The film thickness was w70 nm.).
dopant concentrations from 1 wt % to 10 wt % at 10 V are shown in
Fig. 4. Three distinct peaks at about 430 nm, 484 nm, and 546 nm
are observed in these EL spectra, respectively. Their CIE coordinates
vary from (0.25, 0.28) to (0.31, 0.41) with increasing dopant con-
centrations from 1 wt % to 10 wt % at 10 V. White emission from
doped films exhibit
a
roughness with Ra¼0.603 nm and
Ra¼0.537 nm, respectively. It indicates that both platinum(II)
complexes have a good dispersibility in the PVKePBD matrix.
2.5. Electrochemical property
The electrochemical property of the (dfppy)2Pt2(dipic) complex
was examined by cyclic voltammetry. The electrochemical curve is
shown in Fig. S4 and its corresponding data are listed in Table 1. The
(dfppy)2Pt2(dipic) complex presents an irreversible oxidation po-
tential (Eox) at about 1.49 V and a reversible reduction potential
(Ered) at about ꢀ1.75 V. The HOMO and LUMO energy levels are
calculated to be ꢀ5.46 eV and ꢀ2.89 eV according to an empirical
formula: ELUMO¼ꢀ(Eredþ4.34) and EHOMO¼ꢀ(Eoxþ4.34), re-
spectively.28 Compared to our reported (dfppy)Pt(pic) (Table 1),29
(dfppy)2Pt2(dipic) has a narrow energy gap leading to a minor
red-shift emission, which is consistent with the theoretical
calculation.
2.6. Electroluminescent property
The electroluminescent (EL) spectra and their CIE chromaticity
diagrams of the (dfppy)2Pt2(dipic)-doped PVKePBD devices in the
Fig. 4. The EL spectra and CIE 1931 chromaticity diagram of the (dfppy)2Pt2(dipic)-
doped PVKePBD PLEDs with different concentrations from 1 wt % to 10 wt %.

2122
Y. Wang et al. / Tetrahedron 67 (2011) 2118e2124
these devices are exhibited at the given dopant concentrations.
Fig. 5 shows the EL spectra and the CIE 1931 chromaticity of the
PLED at 2 wt % dopant concentration under different applied volt-
ages from 7 V to 12 V. Obviously, the CIE coordinates at white-re-
gion have minor change from (0.30, 0.38) to (0.32, 0.37). For
comparison, the EL spectra and their corresponding CIE coordinates
of the other PLEDs at 1 wt%, 4 wt%, and 10 wt% dopant concentra-
tions with increasing applied voltages are shown in Fig. S5. Minor
change for these EL spectra and their corresponding CIE co-
ordinates are also observed at the given dopant concentrations and
applied voltages. Therefore, the dinuclear platinum(II) complex is
a promising class of single dopant to construct stable white-emit-
ting SEL-based PLEDs. In these fabricated devices, the device
exhibited pure white emission with a maximum brightness of
277 cd/cm2 and a maximum current efficiency 0.1 cd/A at 4 wt%
dopant concentration.
observed at the dopant concentrations from 1 wt % to 4 wt % and
gradually decreases with increasing the dopant concentrations.
This weak emission peak does not disappear until the dopant
concentration increases to 8 wt %. The corresponding CIE co-
ordinates vary from (0.39, 0.47) to (0.52, 0.44) with increasing
dopant concentrations from 1 wt % to 10 wt % at 10 V. The best
performance is obtained in the device with a maximum brightness
of 560 cd/cm2 and a maximum current efficiency of 0.30 cd/A at 4%
dopant concentration.
Therefore, constructing an AeDeA-based dinuclear platinum(II)
complex is available to control its formation excimers and get white-
emitting PLEDs with single dopant. Mononuclear platinum(II)
complex presents greater tendency to form excimers than dinuclear
platinum(II) complex. As a result, the SEL-based WPLEDs are more
available to obtain using the dinuclear platinum(II) complex rather
than mononuclear platinum(II) complex as single dopant.
3. Conclusion
A dinuclear platinum(II) complex of (dfppy)2Pt2(dipic) and
a mononuclearplatinum(II)complexof(dfppy)Pt(pic)wereobtained.
Distinct difference in PL and EL properties were observed for both
platinum(II) complexes in their neat films and PLEDs. Stable white
emissions from the (dfppy)2Pt2(dipic)-doped devices were exhibited,
while the (dfppy)Pt(pic)-doped devices gave orange-red emissions.
Therefore, the AeDeA-based dinuclear platinum(II) complex is
a promising class of single dopant to construct stable white-emitting
SEL-based PLEDs. In order to obtain high-efficiency SEL-based
WPLEDs, optimization of the (dfppy)2Pt2(dipic)-doped devices need
be further carried out.
4. Experimental section
4.1. General
All solvents were carefully dried and distilled prior to use.
Commercially available reagents were used without further purifi-
cation unless otherwise stated. All reactions were performed under
nitrogen atmosphere and were monitored by thin-layer chroma-
tography (TLC). Flash column chromatography and preparative TLC
were carried out using silica gel from Merck (200e300 mesh). All 1H
NMR spectra were acquired at a Bruker Dex-400NMR instrument
using CDCl3 as a solvent. Mass spectra (MS) were recorded on
a Bruker Autoflex TOF (MALDI-TOF) instrument using dithranol as
a matrix. The UV absorption and photoluminescence spectra were
measured with a Varian Cray50 and PerkineElmer LS50B Lumi-
nescence Spectrometer, respectively. Electrochemical measure-
ments were made using a CHI660A electrochemical work station
with a scan rate of 100 mV sꢀ1 at room temperature under N2 at-
mosphere. A conventional three-electrode configuration consisting
of a Pt working electrode, a Pt-wire counter electrode, and a calomel
electrode reference electrode was used. The solvent in all mea-
surements was CHCN and the supporting electrolyte was 0.1 M tetra
(n-butyl)ammonium hexafluorophosphate. EL spectra were recor-
ded with an Insta-Spec IV CCD system (Oriel). Luminance was
measured with a Si photodiode and calibrated by using a PR-705
spectrascan spectrophotometer (Photo Research).
Fig. 5. The EL spectra and CIE 1931 chromaticity diagram of (dfppy)2Pt2(dipic)-doped
PVKePBD PLEDs with dopant concentration 2 wt % under different applied voltages.
For comparison, the EL spectra and CIE chromaticity diagrams of
the (dfppy)Pt(pic)-doped PVKePBD devices in the dopant con-
centrations from 1 wt % to 10 wt % at 10 V are shown in Fig. 6. The
intense orange-red emission rather than white emission is ob-
served and their maximum emission peaks vary from 558 nm to
612 nm with increasing dopant concentrations from 1 wt % to
10 wt %. In addition, a weak emission peak at about 430 nm is also
The single-emissive-layer devices were fabricated with a struc-
ture of ITO/PEDOT/PSS (50 nm)/dopantþPVKePBD (70 nm)/Ca
(10 nm)/Al (150 nm), in which ITO is used as an anode, PEDOT/PSS
is used as a hole-injection layer, and Ca/Al is employed as a cathode.
The emitting layer consists of the dopants of platinum(II) com-
plexes and host matrix of the PVKePBD blend. Doping weight
concentrations of platinum(II) complexes are 1 wt %, 2 wt %, 4 wt %,
8 wt %, and 10 wt %, respectively. PBD weight ratio is 30 wt % in the
PVKePBD blend.
Fig. 6. The EL spectra and CIE 1931 chromaticity diagram of the (dfppy)Pt(pic)-doped
PVKePBD PLEDs with different concentrations from 1 wt % to 10 wt %.

Y. Wang et al. / Tetrahedron 67 (2011) 2118e2124
2123
4.1.1. Methyl 3-hydroxypicolinate (2). To
a
stirred mixture of
stirred in 2-ethoxyethanol (15 mL) under N2 atmosphere at 100 ꢁC
for 24 h. After cooled to rt, the mixture was extracted with DCM and
the mixed organic layer was dried over anhydrous MgSO4 and fil-
tered. The filtrate was evaporated to remove the solvent and the
residue was passed through a flash silica gel column using PE/DCM
(1:2) as eluent to gain (dfppy)2Pt2(dipic) as a yellow solid (0.32 g,
3-thydroxypicolinic acid (10 g, 71.9 mmol) in 350 mL CH3OH and
5 mL H2SO4 was added several drops benzene as dehydrate; the
resulting mixture was refluxed for another 24 h under the pro-
tection of nitrogen. After cooled to room temperature (rt), the
mixture was poured into brine and extracted with ethyl acetate
(EA). The organic layer was washed with brine for three times and
then dried over anhydrous Na2SO4. After removal of the solvent, the
intermediate (2) was gained as a white solid (11.0 g, 93.1%). 1H NMR
41.6%). 1H NMR (400 MHz, CDCl3, TMS),
d(ppm): 9.23e9.22 (d,
J¼5.3 Hz, 2H), 8.69e8.67 (d, J¼5.2 Hz, 2H), 8.05e8.03 (d, J¼8.0 Hz,
2H), 7.93e7.91 (t, 2H), 7.72e7.70 (d, J¼8.6 Hz, 2H), 7.58e7.54 (m,
2H), 7.46e7.44 (d, J¼8.4 Hz, 6H), 7.21e7.17 (t, 2H), 6.94e6.92 (d,
J¼8.6 Hz, 4H), 6.68e6.66 (t, 2H), 4.25e4.22 (t, J¼12.8 Hz, 4H),
4.04e4.01 (t, J¼12.5 Hz, 4H), 2.02e2.0 (t, J¼13.9 Hz, 4H), 1.89e1.85
(t, J¼13.3 Hz, 4H), 1.67e1.58 (m, 4H), 0.90e0.86 (t, J¼16.8 Hz, 4H).
TOF-MS: 1419.3. Anal. Calcd for C59H53F4N4O8Pt2: C 50.18, H 3.78, N
3.97. Found: C 50.24, H 3.91, N 4.08%.
(CDCl3, 400 MHz),
d (ppm): 7.68e7.62 (m, 2H), 7.52e7.50 (d,
J¼7.92 Hz, 2H), 7.45e7.43 (d, J¼7.92 Hz, 2H), 4.28e4.25 (t,
J¼12.4 Hz, 2H), 3.94 (s, 3H), 1.94e1.89 (m, 4H), 1.62e1.57 (m, 2H),
1.25e1.04 (m, 14H), 0.84e0.81 (t, J¼11.8 Hz, 3H), 0.59e0.58 (m, 4H).
4.1.2. Methyl 3-(6-bromohexyloxy)picolinate (3). A mixture of
methyl 3-hydroxypicolinate (4.0 g, 26.1 mmol), 1,6-dibromohexane
(19.0 g, 78.3 mmol), K2CO3 (18.0 g, 130.5 mmol), and DMF (60 mL)
was stirred at 80 ꢁC for 24 h under nitrogen atmosphere. The
resulting mixture was cooled to rt, poured into water (200 mL), and
then extracted with DCM (3ꢂ30 mL). The combined organic layer
was dried over anhydrous MgSO4 and filtered. The filtrate was
evaporated to remove the solvent and the residue was passed
through a flash silica gel column using DCM as eluent to give the
compound 3 as a reddish brown oil (6.3 g, 76.5%). 1H NMR (CDCl3,
4.1.6. Complexes (dfppy)Pt(pic). It was prepared according to the
synthetic procedure of complex (dfppy)2Pt2(dipic) except the ratio
between the [(dfppy)PtCl]2 dimmer and picolinic acid, which is
1:2.5 according to the reported procedure.24 1H NMR (400 MHz,
CDCl3, TMS),
d
(ppm): 9.23e9.22 (d, J¼5.5 Hz, 1H), 9.10e9.09 (d,
J¼5.5 Hz, 1H), 8.24 (s, 2H), 8.08e8.06 (d, J¼8.1 Hz, 1H), 7.95e7.91 (t,
J¼15.1 Hz, 1H), 7.74e7.72 (t, J¼10.6 Hz, 1H), 7.23e7.20 (t, J¼12.6 Hz,
1H), 6.95e6.93 (d, J¼7.6 Hz, 1H), 6.71e6.66 (t, J¼19.1 Hz, 1H). TOF-
MS: 508.12. Anal. Calcd for C17H10F2N2O2Pt: C 40.24, H 1.99, N 5.52.
Found: C 40.41, H 1.91, N 5.38%.
400 MHz),
d
(ppm): 8.28e8.27(d, J¼3.8 Hz, 1H), 7.43e7.40 (t,
J¼11.8 Hz, 1H), 7.37e7.35 (d, J¼8.4 Hz, 1H), 4.09e4.06 (t, J¼11.9 Hz,
2H), 3.97 (s, 3H), 3.45e3.42 (t, J¼13.2 Hz, 2H), 1.92e1.87 (m, 8H).
Acknowledgements
4.1.3. Compound 4. A mixture of compound 3 (2.0 g, 6.3 mmol),
4,40-dihydroxybiphenyl (0.5 g, 2.7 mmol), potassium acetate (2.8 g,
20.8 mmol), and DMF (60 mL) was stirred at 80 ꢁC for 24 h under
nitrogen atmosphere. The resulting mixture was cooled to rt,
poured into water (100 mL), and then extracted with DCM
(3ꢂ30 mL). The combined organic layer was dried over anhydrous
MgSO4 and filtered. The filtrate was evaporated to remove the
solvent and the residue was passed through a flash silica gel col-
umn using PE/EA (10:1) as the eluent to give the compound 4 as
Financial support from the National Natural Science Foundation
of China (20772101, 50973093, and 20872124), the Science Foun-
dation of Hunan Province (2009FJ2002), the Specialized Research
Fund for the Doctoral Program of Higher Education (2009
4301110004), the New Teachers Fund for the Doctoral Program of
Higher Education of China (200805301013), and the Postgraduate
Science Foundation for Innovation in Hunan Province (CX2009B124
and S2008 yjscx09).
a white solid (1.3 g, 38.6%). 1H NMR (400 MHz, CDCl3, TMS),
d(ppm):
8.31 (s, 2H), 7.46e7.39 (m, 8H), 6.95e6.93 (d, J¼7.7 Hz, 4H),
4.11e4.08 (t, J¼12.4 Hz, 4H), 4.03e4.00 (t, J¼12.6 Hz, 4H), 3.97 (s,
6H), 1.90e1.84 (m, 16H). Anal. Calcd for C38H44N2O8: C 69.49, H
6.75, N 4.27. Found: C 69.35, H 6.64, N 4.32%.
Supplementary data
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.tet.2011.01.042.
4.1.4. Compound 5. A mixture of compound 4 (1.0 g, 1.5 mmol),
NaOH (50%, 5 mL), anhydrous ethanol (5 mL), and THF (10 mL) was
stirred vigorously for 2 h at 60 ꢁC, then for another 24 h at room
temperature. The mixture was poured into water and extracted
with DCM (3ꢂ30 mL). The combined organic layer was dried over
anhydrous magnesium sulfate, distilled, and then the solvent was
removed by rotary evaporation to gain compound 6 (0.82 g, 87.0%)
References and notes
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d (ppm): 13.1 (s, 2H),
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