Novel cyclometalated platinum (II) complex containing carrier-transporting groups: Synthesis, luminescence and application in single dopant white PLEDs

Article abstract of DOI:10.1016/j.dyepig.2012.11.006

In order to investigate the influence of carrier-transporting groups on the luminescent properties, especially on excimer/aggregation emission, and to obtain single emissive dopants used in white polymer light-emitting diodes with a single emissive-layer structure, two novel platinum complexes derived from a platinum(II) (2-(4′,6′-difluorophenyl)pyridinato-N,C ²′)(picolinate) unit containing carrier-transporting groups were synthesized and characterized, where a triphenylamino or an oxadiazole-triphenylamino functionalized unit was pending into the picolinate by non-conjugated linkage, respectively. Their optophysical, electrochemical and electroluminescent properties were investigated. Compared to their parent platinum complex, both functionalized platinum complexes exhibited a significantly red-shifted photoluminescent profile in neat film and higher photoluminescent quantum efficiency in dichloromethane at room temperature. Furthermore, the platinum complex with oxadiazole-triphenylamino ambipolar transporting group exhibited better photoluminescent properties and offered potential application as a single dopant in a single emissive-layer-based white polymer light-emitting diode. More stable white light emissions were observed in this platinum complex-doped devices at dopant concentrations of 4-8 wt%. The maximum luminous efficiency of 1.01 cd A^-1 and the maximum brightness of 2177 cd m^-2 were obtained in the polyfluorene-hosted devices.

Full text of DOI:10.1016/j.dyepig.2012.11.006

Accepted Manuscript
Novel Cyclometalated Platinum (II) Complex Containing Carrier- Transporting
Groups: Synthesis, Luminescence and Application in Single-dopant White PLEDs
Aihui Liang, Yanhu Li, Weiguo Zhu, Yafei Wang, Fei Huang, Hongbin Wu, Yong Cao
PII:
S0143-7208(12)00306-3
10.1016/j.dyepig.2012.11.006
DYPI 3765
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 August 2012
Revised Date: 4 November 2012
Accepted Date: 6 November 2012
Please cite this article as: Liang A, Li Y, Zhu W, Wang Y, Huang F, Wu H, Cao Y, Novel Cyclometalated
Platinum (II) Complex Containing Carrier- Transporting Groups: Synthesis, Luminescence
and Application in Single-dopant White PLEDs, Dyes and Pigments (2012), doi: 10.1016/
j.dyepig.2012.11.006.
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ACCEPTED MANUSCRIPT
Highlights
1. Two novel platinum complexes derived from 2(2,4-difluorophenyl)pyridine were
synthesized.
2. The photophysical and electroluminescent properties of the platinum complexes
were studied.
3. The platinum complex with an acceptor-donor-acceptor structure exhibited blue-
shifted excimer’s emission than that with a donor-acceptor structure in the neat film as
evident from their photoluminescence spectra.
4. Both platinum complexes displayed excellent photoluminescent quantum
efficiency.
5. The fabricated PLEDs exhibited stable white electroluminescent properties.

ACCEPTED MANUSCRIPT
Novel Cyclometalated Platinum (II) Complex Containing Carrier-
Transporting Groups: Synthesis, Luminescence and Application in
Single-dopant White PLEDs
Aihui Liang a,b, Yanhu Li b, Weiguo Zhu a*, Yafei Wang a, Fei Huang b,
Hongbin Wu b, Yong Cao b
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 Materials & Devices, State Key Laboratory of
Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, China
Corresponding author: Dr. Weiguo Zhu
Tel: +86-731-58298280
Fax: +86-731-58292251
E-mail addresses: zhuwg18@126.com
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ABSTRACT: In order to investigate the influence of carrier-transporting groups on the
luminescent properties, especially on excimer/aggregation emission, and to obtain single
emissive dopants used in white polymer light-emitting diodes with a single emissive-
layer structure, two novel platinum complexes derived from a platinum(II) (2-(4′,6′-di-
fluorophenyl)pyridinato-N,C2′)(picolinate) unit containing carrier-transporting groups
were synthesized and characterized, where a triphenylamino or an oxadiazole-triphenyl-
amino functionalized unit was pending into the picolinate by non-conjugated linkage,
respectively. Their opto-physical, electrochemical and electroluminescent properties were
investigated. Compared to their parent platinum complex, both functionalized platinum
complexes exhibited a significantly red-shifted photoluminescent profile in neat film and
higher photoluminescent quantum efficiency in dichloromethane at room temperature.
Furthermore, the platinum complex with oxadiazole-triphenylamino ambipolar transport-
ing group exhibited better photoluminescent properties and offered potential application
as a single dopant in a single-emissive-layer-based white polymer light-emitting diode.
More stable white light emissions were observed in this platinum complex-doped devices
at dopant concentrations of 4-8 wt%. The maximum luminous efficiency of 1.01 cd A-1
and the maximum brightness of 2177 cd m-2 were obtained in the polyfluorene-hosted
devices.
Keywords: Electrophosphorescence; Platinum (II) complex; White polymer light-
emitting devices; Synthesis
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1. Introduction
White organic and polymeric light-emitting devices (WOLEDs/WPLEDs) have
attracted a great deal of attention because of their potential applications in full color
displays, back lighting of flat-panel displays and solid-state lighting [1-4]. To date, white
light emission based on polymers and small molecules have been obtained by several
methods and technologies [5–14]. There were two device structures to build the small
molecule-based WOLEDs by sequential deposition; one is multiple-emissive-layer (MEL)
structure [15-21] and the other is single-emissive-layer (SEL) structure [22-24]. However,
these WOLEDs fabricated by deposition exhibited problems, such as complicated
processes, high cost and poor stability. In order to overcome such problems, WOLEDs
and WPLEDs fabricated by solution-processing technique were recently developed, in
which all small organic molecules and polymers used as emitters were dissolved in the
given solvents and spin-coated to form an emissive layer [25-31].
Using the solution processing technique, some high-performance white-emitting
devices were achieved by a spin-coating process. For example, Wu et al [32] obtained a
WPLED with a power efficiency (PE) of 20.3 lm W-1 and a luminescent efficiency (LE)
of 42.9 cd A-1, in which the blue-emitting small molecules of iridium complexes were
spin-coated to form SEL with a blend of poly(N-vinylcarbazole) (PVK) and 2,2'-(1,3-
phenylene)bis[5-(4-tert-butyl- phenyl)-1,3,4-oxadiazole] (OXD-7). Jen et al [33] made a
WOLED containing PVK, OXD-7, the blue-emitting FIrpic, the green-emitting Ir(ppy)3
and the red-emitting Os-R1 in SEL, which exhibited a maximum external quantum
efficiency (EQE) of 5.85 %, a PE of 6.15 lm W–1 and a LE of 10.9 cd A–1. Nevertheless,
this class of devices, blending various emitters in the SEL structure, could have poor
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stability and can barely obtain a pure white emission due to phase separation and
undesired Förster energy transfer from the high- energy emitter to the low-energy one. In
order to decrease the emitter types in the SEL WOLEDs/WPLEDs, a class of emitters
used as a single dopant was recently developed, which could present not only high-lying
monomer emission, but also low-lying excimer emission in the devices. Meanwhile, our
group reported some mononuclear platinum complexes and iridium complexes, as well as
dinuclear platinum complexes as single emissive dopants in the SEL-based WPLEDs
[34]. However, to our best knowledge, the platinum complex containing ambipolar
transport groups has not yet been reported in the spin-coated WPLEDs using a single
dopant [35].
As triphenylamine (TPA) and oxadiazole (OXD) are well-known hole- and electron-
transporting moieties, respectively, applied in the fields of OLEDs and organic solar cells
[36], they can form ambipolar transport groups and are suggested to be available to
improve the optoelectronic properties of their resulting platinum complexes while they
are simultaneously introduced into the platinum complex as substituent groups. In order
to investigate the influence of the ambipolar transport groups on the luminescent
properties, especially on excimer/aggregation emission of these platinum complexes and
further to obtain a class of single emissive dopants used in the SEL-based WPLEDs, a
novel platinum complex with ambipolar transporting groups of TPA and OXD, i.e.
FPt(OXD6TPA6Pic), was designed and synthesized, where FPt is a platinum(II)
(2-(4′,6′-difluorophenyl) pyridinato-N,C2′) unit, and OXD6TPA6Pic is an oxadiazole-
triphenylamino functionalized picolinate, respectively. For comparison, another platinum
complex with only a hole-transporting group of TPA, i.e. FPt(TPA6Pic), was also synthe-
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sized, where TPA6Pic is a triphenylamino functionalized picolinate. The photophysical,
electrochemical and thermal properties, as well as electroluminescent properties for both
platinum (II) complexes were studied. As expected, the pending carrier transporting
groups have exhibited a significant influence on the luminescent properties of their
corresponding platinum complexes. The FPt(OXD6TPA6Pic) with ambipolar transport
groups can be used as a single dopant to obtain WPLEDs containing a polymer matrix of
PVK or poly(9,9-dihexylfluorene) (PFO), respectively. The best device performance with
a maximum LE of 1.0 cd/A and brightness of 855 cd/m2 was obtained in the FPt(OXD6-
TPA6Pic)-doped PFO device at dopant concentrations of 8 wt% and a white emission
with a stable CIE coordinate of (0.30, 0.38) was observed at the same time.
2. Experimental Section
2.1. Materials and equipment
All solvents were carefully dried and distilled prior to use. Commercially available
reagents were used without further purification unless otherwise stated. 3-(6-(4-(Diphenyl
-amino)benzyloxy)hexyloxy)picolinic acid (TPA6PicH) and 3-(6-(4'-((4-((6-(4-(5-(4-
methyl)-1,3,4-oxadiazole-2-yl)phenoxy)hexyloxy)methyl)phenyl)(phenyl) amino) biphen
-yl-4-yloxy)hexyloxy)picolinic acid (OXD6TPA6PicH) were prepared according to the
reported process, respectively [37]. For further comparison, platinum (II) (4′,6′-(difluoro-
phenyl) pyridinato-N,C2′) (picolinate) [FPt(pic)] was also made based on our previous
work [34]. All reactions were performed under nitrogen atmosphere and were monitored
by thin-layer chromatography (TLC). Flash column chromatography and preparative
TLC were carried out using silica gel from Merck (200-300 mesh). All 1H NMR spectra
were acquired at a Bruker Dex-400NMR instrument using CDCl3 as a solvent. Elemental
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analysis was performed on a Harrios elemental analysis instrument. Time-of-flight mass
spectrometry (TOF-MS) was performed in the positive ion mode with a matrix of
dithranol at a Bruker-autoflex III smartbeam. The UV absorption and photoluminescence
(PL) spectra were measured with a Varian Cray50 and Perkin-Elmer LS50B Lumine-
scence Spectrometer, respectively. EL spectra and CIE coordinates were recorded with an
Insta-Spec IV CCD system (Oriel). Luminance and luminous efficiency-current density
(LE–J) data were measured with a Si photodiode and calibrated by using a PR-705
spectrascan spectrophotometer (Photo Research).
2.2 Synthesis of FPt(TPA6Pic)
To a mixture of K2PtCl4 (0.3 g, 0.84 mmol) and water (5 mL) was added 2-(2,4-
difluorophenyl)pyridine (0.4 g, 2.1 mmol) and 2-ethoxyethanol (15 mL). The mixture
o
was stirred vigorously at 100 C under inert gas atmosphere for 20 h. After cooling to
room temperature (RT), the colored precipitate was filtered off, washed with water and
hexane, respectively. A red solid of dimer of [FPtCl]2 was obtained (0.42 g, 78.0 %).
Then a mixture of the dimer (0.2 g, 0.24 mmol), compound TPA6PicH (0.30 g, 0.60
mmol) and sodium carbonate (90 mg, 0.85 mmol) in 2-ethoxyethanol (15 mL) at about
80-90 oC was stirred vigorously under inert gas atmosphere for 24 h. After cooling to RT,
the mixture was extracted with dichloromethane (CH2Cl2) and the resulting organic layer
was dried over anhydrous magnesium sulfate. The solvent was removed and the residue
was purified by a dry flash silica gel column using CH2Cl2 as eluent to gain FPt(TPA6Pic)
(0.22 g, 53.0 %) as a brown red solid. 1H NMR (CDCl3, 400 MHz), δ (ppm): 9.14-9.15 (d,
J = 4.8 Hz, 1H), 8.63-8.64 (d, J = 5.2 Hz, 1H), 7.97-7.99 (d, J = 5.8 Hz, 1H), 7.86-7.89 (t,
1H), 7.65-7.67 (d, J = 5.4 Hz, 1H), 7.52-7.53 (t, 1H), 6.85-7.21 (m, 15H), 6.84-6.86 (d, J
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= 5.8 Hz, 1H), 6.58-6.59 (t, 1H), 4.42 (s, 2H), 4.16-4.18 (t, 2H), 3.49-3.52 (t, 2H),
13
1.96-2.03 (m, 2H), 1.90-1.92 (m, 2H), 1.63-1.65 (m, 2H), 1.47-1.50 (m, 2H). C NMR
(100 MHz, CDCl3) δppm: 170.11, 158.09, 149.89, 147.85, 147.26, 141.39, 139.54,
132.97, 129.20, 128.85, 128.14, 125.12, 124.16, 124.02, 122.70, 114.63, 114.45, 99.50,
70.39, 67.65, 29.16, 28.81, 25.76, 25.53. Anal. Calcd for C42H37F2N3O4Pt: C 57.27, H
4.23, N 4.77. Found: C 57.32, H 4.20, N 4.81. MALDI-TOF (m/z): calcd for [M]+ 880.24,
found [M]+ 880.20.
2.3 Synthesis of FPt(OXD6TPA6Pic)
This was prepared according to the synthetic procedure of FPt(TPA6Pic). A brown red
1
solid was obtained with a yield of 36.0 %. H NMR (CDCl3, 400 MHz), δ (ppm): 9.19-
9.20 (d, J = 5.2 Hz, 1H), 8.66-8.68 (d, J = 5.4 Hz, 1H), 8.00-8.06 (m, 5H), 7.85-7.87 (t,
1H), 7.78-7.76 (d, J =5.8 Hz, 1H), 7.52-7.58 (m, 1H), 7.46-7.48 (d, J = 8.5 Hz, 2H),
7.40-7.42 (d, J = 8.4 Hz, 2H), 7.32-7.34 (d, J = 8.0 Hz, 2H), 7.23-7,27 (t, 4H), 7.16-7.18
(t, 1H), 7.08-7.12 (m, 6H), 7.00-7.02(d, J = 8.8 Hz, 3H), 6.93-6.95 (d, J = 8.4 Hz, 2H),
6.76-6.77 (d, J = 6.4 Hz, 1H), 6.54-6.60 (t, 1H), 4.46 (s, 2H), 4.20-4.21 (t, 2H), 4.01-4.06
(t, 4H), 3.51-3.55 (t, 2H), 2.45 (s, 3H), 1.99-2.05 (m, 2H), 1.84-1.87 (m, 4H), 1.61-1.78
(m, 10H). 13C NMR (100 MHz, CDCl3) δppm: 170.21, 164.37, 164.27, 161.88, 158.41,
150.04, 147.74, 147.22, 146.47, 142.01, 141.33, 139.66, 135.16, 133.01, 132.90, 129.24,
128.89, 128.65, 127.64, 126.80, 124.23, 116.36, 114.98, 114.83, 99.68, 72.71, 70.36,
68.18, 67.84, 29.73, 29.18, 29.14, 26.05, 25.92, 25.49, 21.66. Anal. Calcd for
C69H63F2N5O7Pt: C 63.39, H 4.86, N 5.36. Found: C 63.42, H 4.87, N 5.32. MALDI-TOF
(m/z): calcd for [M]+ 1306.43, found [M]+ 1306.38.
2.4 Fabrication of PLEDs
The devices were made based on the following standard procedure. The ITO glass
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substrate was cleaned by ultrasonic bath sequentially in acetone, detergent, deionized
o
water and isopropanol, and baked at 80 C. A 40 nm-thick layer of poly(ethylendioxy-
thiophene):poly(styrene sulfonic acid) (PEDOT:PSS, Baytron P4083, Bayer AG) was
spin-coated onto the precleaned ITO-glass substrates after an O2 plasma treatment and
dried in vacuum box for over 8 h. In the PFO-hosted devices, a 40 nm thick layer of PVK
(10 mg/mL in dichlorobenzene) was spin-coated on the top of PEDOT and baked at 120
oC for 30 min. Then a mixture of the platinum complex with PVK+PBD (or PFO) was
respectively spin-coated onto the top of PPEDOT (or PVK) to form the emitting layer
from the solution in chlorobenzene (or in p-xylene) at a concentration of 20 mg/mL by
adapted rotate speed and baked at 120 oC for 20 minutes. The thickness of all spin-coated
films was measured by Profilometer (Tencor Alfa-Step 500). Finally, a thin layer of CsF
(1.5 nm) with about 120 nm thick Al capping layer was deposited through a shadow mask
in a vacuum chamber under a base pressure of ~10-4 Pa. The deposited thickness was
monitored by a crystal thickness monitor (Sycon). The devices were made in a controlled
atmosphere dry-box (Vacuum Atmosphere Co.) in N2 atmosphere except processing
PEDOT:PSS layer. The active area was 0.19 cm2 and five parallel measurements were
performed for each device.
3. Results and discussion
3.1. Synthesis
As shown in Scheme 1, the platinum (II) complexes of FPt(TPA6Pic) and FPt(OXD6-
TPA6Pic) were synthesized by two-step procedures, which commenced from the reaction
of K2PtCl4 with 2,4-difluoropyphenylpyridine and a chloride cleavage of the resulting
chloro-bridged dimer with either TPA6PicH or OXD6TPA6Pic under moderate
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conditions based on the reported procedures [38-40], respectively. Since the ancillary
ligand OXD6TPA6PicH offers larger steric hindrance than TPA6PicH, the resulting
platinum complex of FPt(OXD6TPA6Pic) is more difficult to obtain and has a lower
1
yield than FPt(TPA6Pic). Both platinum complexes were confirmed by H NMR, 13C
NMR, elemental analysis and TOF-MS.
3.2. Optical Analysis
Figure 1 shows the UV-vis absorption spectra of FPt(OXD6TPA6Pic), FPt(TPA6Pic)
and FPt(pic) [platinum (II) (4′,6′-(difluorophenyl) pyridinato-N,C2′) (picolinate)] in
CH2Cl2 at 298 K, and their UV absorption data are summarized in Table 1. An intense
high-energy absorption band at about 304 nm and a moderate low-energy absorption
band at 350 nm were observed for these functionalized platinum complexes with
carrier-transporting groups, in which the former band is assigned to the intraligand-based
π-π* transitions and the latter band is assigned to the mixing transitions between the
spin-allowed singlet metal-to-ligand charge transfer (1MLCT) and the spin- forbidden
triplet metal-to-ligand charge transfer (3MLCT) [34]. Compared to the parent platinum
complex of FPt(pic), both functionalized platinum complexes of FPt(TPA6Pic) and
FPt(OXD6TPA6Pic) exhibited an additional strong high-lying absorption bands at about
305 nm, which is attributed to the absorption from the fluorescent chromophores of TPA
and/or OXD [41,42].
The PL spectra of these platinum complexes are shown in Figure 2a, and their
emission data are summarized in Table 1. Nearly identical PL profiles containing a
maximum emission peak at 462 nm with a shoulder at 506 nm are exhibited for both
FPt(TPA6Pic) and FPt(OXD6TPA6Pic) in CH2Cl2 solution. Compared to FPt(Pic), both
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functionalized platinum complexes displayed a wider PL profile due to the intrinsic
emission’s influence of the pending donor unit of triphenylamino and acceptor unit of
oxadiazole in the picolinate. Furthermore, FPt(OXD6TPA6Pic) exhibited wider PL
spectrum than FPt(TPA6Pic), which should be attributed to the additional effect of the
oxadiazole’ emission in dilute CH2Cl2 solution.
Figure 2b shows the PL spectra of these three platinum complexes in their neat films.
An obviously red-shifted low-energy emission peak over 600 nm is observed, which is
attributed to the planar molecular configuration of platinum complexes leading to
excimer formation [43]. The maximum emission peaks of the proposed eximers are
located at 681 nm for FPt(TPA6Pic) and 650 nm for FPt(OXD6TPA6Pic), respectively.
Both functionalized platinum complexes displayed a significantly red-shifted PL spectra
compared to FPt(Pic) in the neat films. The significantly red-shifted excimer emissions
for FPt(TPA6Pic) and FPt(OXD6TPA6Pic) should result from their additional inter-
molecular interaction through the space between donor and acceptor units. Interestingly,
FPt(OXD6TPA6Pic) exhibited a 31 nm blue-shifted PL spectrum than FPt(TPA6Pic) in
its neat film due to the electron-withdrawing effect of OXD under this state. The
diference of PL profile between in solution and in film indicates that intermolecular
interaction through the space between donor and acceptor units mainly exists in the film.
The salvation effect can efficiently restrain the intermolecular donor-acceptor interaction.
To further investigate the effect of carrier-transporting groups in ancillary ligands on
the PL performances of their platinum complexes, their fluorescence quantum yields (Фf)
were measured using 9,10-diphenylanthrancene as the standard (Φf = 0.9) in degassed
CH2Cl2 under photo-excitation at 380 nm [44]. As shown in Table 1, the Φf of 0.74 for
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FPt(TPA6Pic), 0.81 for FPt(OXD6TPA6Pic) and 0.58 for FPt(pic) were obtained in
CH2Cl2, respectively. It is obvious that both functionalized platinum complexes displayed
a higher Φf than the parent FPt(pic). Therefore, introducing carrier-transporting groups
into the ancillary ligand is a means to improve the luminescence efficiency of its resulting
platinum (II) complexes in CH2Cl2. Interestingly, for these functionalized platinum
complexes, the FPt(OXD6TPA6Pic) with ambipolar transporting groups of TPA and
OXD has the higher Φf than the FPt(TPA6Pic) with only hole transporting group.
3.3 Thermal Property
The thermal stability of platinum complexes was evaluated by thermogravimetric
analysis (TGA) under N2 stream with a scanning rate of 20 oC/min, and their degradation
temperatures (Td) for 5% weight loss are listed in Table 2. The recorded Td level is 253
oC for FPt(TPA6Pic), 264 oC for FPt(OXD6TPA6Pic) and 294 oC for FPt(Pic), respective
-ly. It indicates that these functionalized platinum complexes possess good thermal
property. Compared to FPt(Pic), FPt(TPA6Pic) and FPt(OXD6TPA6Pic) showed a lower
decomposition temperature because of a non-conjugating linkage between the parent
picolinic acid and the pending carrier-transporting groups.
3.4. Electrochemical Property
The electrochemical behavior of the three cyclometalated platinum complexes was
measured by cyclic voltammetry using ferrocene as the internal standard and their data
are listed in Table 2. Irreversible oxidation peaks presented from −1.61V to −2.29 V for
these platinum complexes. On the basis of these onset oxidation potentials (Eox) and the
formula of EHOMO = − (Eox + 4.34) eV, the highest occupied molecular orbital energy
levels (EHOMO) were calculated to be − 5.95 eV for FPt(TPA6Pic) and − 6.03 eV for
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FPt(OXD6TPA6Pic), respectively. As the reduction peak was not displayed, the lowest
unoccupied molecular orbital energy levels (ELUMO) had to be calculated based on the
equation of ELUMO = EHOMO + Eg, in which energy band gap (Eg) was estimated from the
UV-vis absorption threshold [45]. As a result, the ELUMO was − 3.04 eV for FPt(TPA6Pic)
and − 3.09 eV for FPt(OXD6TPA6Pic) were obtained. Apparently, the pending carrier-
transporting groups have an important influence on the HOMO and LUMO energy levels
of their platinum complexes. Both functionalized platinum complexes exhibited an
increasing ELUMO and ELUMO value compared to the parent FPt(Pic) due to the effect of
the carrier-transporting groups. Furthermore, FPt(OXD6TPA6Pic) displayed a decreased
ELUMO and ELUMO value compared to FPt(TPA6Pic) due to the effect of the OXD acceptor
unit.
3.5. Electroluminescent Property
To investigate the influence of ambipolar transporting group on electroluminescence
(EL) properties of its platinum complex, FPt(OXD6TPA6Pic) was chosen as the single
dopant in the devices. The SEL-based devices were fabricated with a structure of ITO/
PEDOT:PSS (40 nm)/dopant + PVK-PBD (70 nm)/CsF(1.5 nm)/Al (120 nm) (devices A)
and ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/dopant + PFO (70 nm)/CsF(1.5 nm)/Al
(120 nm) (devices B). The emitting layer consist of the FPt(OXD6TPA6Pic) dopant and
polymeric host matrix of PFO or PVK-PBD blend, in which the weight concentration of
the dopant is 4 and 8 wt %, PBD weight ratio is 30 wt % in the PVK-PBD blend,
respectively. The energy bands of FPt(OXD6TPA6Pic), PVK, PFO and PBD are shown
in the supplementary information accompanying this work (Figure S2).
Figure 3 showed the EL spectra of the PVK-hosted devices A and the PFO-hosted
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devices B at different dopant concentrations of 4 wt % and 8 wt % at 12 V. All of these
EL spectra exhibited a broad emission band from 400 to 750 nm. For devices A, three
emission peaks at 438, 484, 516 nm and a shoulder at 544 nm were observed. For devices
B, four emission peaks at 421, 439, 481, 511 nm and a shoulder at 540 nm were observed.
In these emission peaks, the high-lying emission peaks below 450 nm are assigned to the
host matrix, the low-lying emission peaks in a range of 450-520 nm are attributed to
intrinsic emission of FPt(OXD6TPA6Pic). Additionally, the shoulder in the low-lying
region, according to the reports by Chen et al [46], is attributed to the electromers
between host matrix and dopant. Compared to the PL spectra in the neat film, however,
little excimer emission at about 650 nm can be observed in the EL spectra for the
PFO-hosted devices B at 8 wt% dopant concentration. This implies that the excimer
emission from the platinum complexes can be efficiently tuned by the pendant ambipolar
transporting groups in their ancillary ligands under an electric field.
It was noted that the contribution between the host matrix and the FPt(OXD6TPA6Pic)
guest on the EL spectra is significantly different in the devices A and devices B. The
emission intensity from host matrix for PFO is much stronger than that for PVK. This
feature suggests that the energy transfer is more incomplete from the PFO instead of
PVK-PBD matrix to FPt(OXD6-TPA6Pic). The incomplete energy transfer can be used to
obtain a white emission in the FPt(OXD6TPA6Pic)-doped PFO devices. Figure 3 inset
shows the Commission Internationale de L’Eclairage (CIE) coordinates for these devices.
The devices B displayed more efficient white light emission than the devices A. In order
to investigate the EL stability of the devices, the EL spectra of the devices B at the dopant
concentration of 8 wt % was measured under different applied voltages, as shown in
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Figure S3. A minor change for the EL CIE coordinates from (0.29, 0.39) to (0.30, 0.38)
is observed with the increasing driving voltages from 8 V to 13 V. Therefore, the devices
B exhibited stable white emission under different driving voltages.
Figure 4 shows the luminous efficiency-current density-luminance (LE-J-L) charac-
teristics of the devices A and devices B. Table 3 summarizes the performance of these
devices at 4 wt% and 8 wt% dopant concentrations. The near-white emissions with the
CIE coordinates (0.28 ± 0.04, 0.40 ± 0.08) were observed for these FPt(OXD6TPA6
-Pic)-doped devices at a applied voltage of 12V. The maximum LE of 1.01 cd A-1 at a
current density of 3.20 mA cm-2 and the maximum brightness of 2177 cd m-2 were
obtained in the PFO-hosted devices B, respectively. For the recorded maximum LE and
brightness, the devices B exhibited higher levels than the devices A. At this time, the CIE
coordinates located at the near-white, even white region range from (0.24, 0.32) to (0.30,
0.38) at the dopant concentrations of 4 wt% ~ 8 wt% for the devices B at 12V. This
indicates that PFO is a better host matrix than PVK-PBD for the FPt(OXD6TPA6Pic)-
doped devices to obtain high-performance WPLEDs.
4. Conclusion
In summary, two novel cyclometalated platinum complexes, FPt(OXD6TPA6Pic) and
FPt(TPA6Pic), containing carrier-transporting groups were obtained. Both functiona-
lized platinum complexes exhibited significantly red-shifted excimer emission in neat
film and increased PL quantum efficiency in CH2Cl2 solution compared to the parent
FPt(pic). Furthermore, the FPt(OXD6TPA6Pic) containing ambipolar transporting groups
exhibited better PL property and potential application used as a single dopant in the SEL
WPLEDs. Stable white light emissions were observed in the FPt(OXD6TPA6Pic)-doped
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PFO devices at the dopant concentrations of 4 wt% ~ 8 wt% and different driving
voltages of 8V~13V. The maximum LE of 1.01 cd A-1 and the maximum brightness of
2177 cd m-2 were obtained, respectively. The results demonstrated that introducing the
ambipolar transporting groups of TPA and OXD into the ancillary ligand can lead to
beneficial properties for the constructing platinum complex in the SEL-based WPLEDs.
Acknowledgements
Financial support from the National Natural Science Foundation of China (50973093,
51273168 and 21202139), the Innovation Group in Hunan Natural Science Foundation
(12JJ7002), the Specialized Research Fund and the New Teachers Fund for the Doctoral
Program of Higher Education of China (20094301110004), the Scientific Research Fund
of Hunan Provincial Education Department (10A119, 11CY023, 10B112), Natural
Science Foundation of Hunan (12JJ4019), and the Scientific Research Fund of Xiangtan
University (11QDZ18, 2011XZX08), Postgraduate Science Foundation for Innovation in
Hunan Province (CX2011B263).
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References
[1] Che CM, Chan SC, Xiang HF, Chan MCW, Liu Y, Wang Y. Tetradentate schiff base
platinum(II) complexes as new class of phosphorescent materials for high-efficiency
and white-light electroluminescent devices. Chem Commun 2004; 1484-6.
[2] Kamtekar KT, Monkman AP, Bryce MR. Recent advances in white organic light-
emitting materials and devices. Adv Mater 2010; 22: 572-82.
[3] Park YS, Kang JW, Kang DM, Park JW, Kim YH, Kwon SK, Kim JJ. Efficient,
color stable white organic light-emitting diode based on high energy level yellowish-
green dopants. Adv Mater 2008; 20: 1957 - 61.
[4] Holder E, Langeveld BMW, Schubert US. New trends in the use of transition metal-
ligand complexes for applications in electroluminescent devices. Adv Mater 2005; 17:
1109 -21.
[5] Zhao ZJ, Zhao YX, Lu P, Tian WJ. Fluorene-centered, ethynylene-linked carbazole
oligomers: synthesis, photoluminescence, and electroluminescence. J Phys Chem C
2007; 111: 6883-8.
[6] Mazzeo M, Vitale V, Della Sala F, Anni M, Barbarella G, Favaretto L, Sotgiu G,
Cingolani R, Gigli G. Bright white organic light-emitting devices from a single active
molecular material. Adv Mater 2005; 17: 34-9.
[7] a) Niu QL, Shao YX, Xu W, Wang L, Han SH, Liu NL, Peng JB, Cao Y, Wang J. Full
color and monochrome passive-matrix polymer light-emitting diodes flat panel
displays made with solution processes. Org Electron 2008; 9: 95-100. b) D’Andrade
BW, Forrest SR. white organic light-emitting devices for solid-state lighting. Adv
Mater 2004; 16: 1585-95. c) Kalinowski J, Fattori V, Cocchi M, Williams JAG.
Light-emitting devices based on organometallic platinum complexes as emitters.
Coord Chem Rev 2011; 255: 2401-5. d) Duggal R, Shiang JJ, Heller CM, Foust DF.
Local electric-field-driven repoling reflected in the ferroelectric polarization of
Ce-doped Sr0.61Ba0.39Nb2O6. Appl Phys Lett 2002; 80: 470-2. e) Paik KL, Baek NS,
Kim HK, Lee JH, Lee Y. White light-emitting diodes from novel silicon-based
copolymers containing both electron-transport oxadiazole and hole-transport carba-
zole moieties in the main chain. Macromolecules 2002; 35: 6782-91. f) Li SF, Zhong
GY, Zhu WH, Li FY, Pan JF, Huang W, Tian H. Dendritic europium complex as a
16

ACCEPTED MANUSCRIPT
single dopant for white-light electroluminescent devices. J Mater Chem 2005; 15:
3221-8.
[8] Sun Y, Giebink NC, Kanno H, Ma B, Thompson ME, Forrest SR. Management of
singlet and triplet excitons for efficient white organic light-emitting devices. Nature
2006; 440: 908-12.
[9] D’Andrade BW, Brooks J, Adamovich V, Thompson ME, Forrest SR. White light
emission using triplet excimers in electrophosphorescent organic light-dmitting
devices. Adv Mater 2002; 14: 1032-6.
[10]Zhang Y, Huang F, Jen AKY, Chi Y. High-efficiency and solution processible
multilayer white polymer light-emitting diodes using neutral conjugated surfactant as
an electron injection layer. Appl Phys Lett 2008; 92: 063303 - 5.
[11]D’Andrade BW, Thompson ME, Forrest SR. Controlling exciton diffusion in
multilayer white phosphorescent organic light emitting devices. Adv Mater 2002; 14:
147-51.
[12]Chen L, Li PC, Cheng YX, Xie ZY, Wang LX, Jing XB, Wang FS. White
electroluminescence from star-like single polymer systems: 2,1,3-benzothiadiazole
derivatives dopant as orange cores and polyfluorene host as six blue arms. Adv
Mater 2011; 23: 2986-90.
[13]Wu HB, Ying L, Yang W, Cao Y. Progress and perspective of polymer white light-
emitting devices and materials. Chem Soc Rev 2009; 38: 3391-400.
[14]Thompson J, Blyth RIR, Mazzeo M, Anni M, Gigli G, Cingolani R. White light
emission from blends of blue-emitting organic molecules: A general route to the
white organic light-emitting diode. Appl Phys Lett 2001; 79: 560-2.
[15]Yu XM, Kwok HS, Wong WY, Zhou GJ. High-efficiency white organic light-
emitting devices based on a highly amorphous iridium(III) orange phosphor. Chem
Mater 2006; 18: 5097-103.
[16]Yu XM, Zhou GJ, Lam CS, Wong WY, Zhu XL, Sun JX, Wong M, Kwok HS. A
yellow-emitting iridium complex for use in phosphorescent multiple-emissive-layer
white organic light-emitting diodes with high color quality and efficiency. J
Organomet Chem 2008; 693: 1518-27.
[17]Sasabe H, Takamatsu JI, Motoyama T, Watanabe S, Wagenblast G, Langer N, Molt
17

ACCEPTED MANUSCRIPT
O, Fuchs E, Lennartz C, Kido J. High-efficiency blue and white organic light-
emitting devices incorporating a blue iridium carbene complex. Adv Mater 2010; 22:
5003-7.
[18]Rehmann N, Ulbricht C, Köhnen A, Zacharias P, Gather MC, Hertel D, Holder E,
Meerholz K, Schubert US. Advanced device architecture for highly efficient organic
light-emitting diodes with an orange-emitting crosslinkable iridium(III) complex.
Adv Mater 2008; 20: 129-33.
[19]Tian N, Thiessen, A, Schiewek R, Schmitz OJ, Hertel D, Meerholz K, Holder E.
Efficient synthesis of carbazolyl- and thienyl-substituted beta-diketonates and
properties of their red and green light-emitting Ir(III) complexes. J Org Chem 2009;
74: 2718-25.
[20]Ulbricht C, Rehmann N, Holder E, Hertel D, Meerholz K, Schubert US. Synthesis
and characterization of oxetane-functionalized phosphorescent Ir(III) complexes.
Macromol Chem Phys 2009; 210: 531-41.
[21]Tian N, Lenkeit D, Pelz S, Kourkoulos D, Hertel D, Meerholz K, Holder E.
Screening structure-property-correlations and device performance of Ir(III)
complexes in multi-layer PhOLEDs. Dalton Trans 2011; 40: 11629-35.
[22]a) D'Andrade BW, Holmes RJ, Forrest SR. Efficient organic electrophosphorescent
white-light-emitting device with a triple doped emissive layer. Adv Mater 2004; 16:
624 - 8. b) Cheng G, Li F, Duan Y, Feng J, Liu SY, Qiu S, Lin D, Ma YG, Lee ST.
White organic light-emitting devices using a phosphorescent sensitizer. Appl Phys
Lett 2003; 82: 4224-6.
[23]D’Andrade BW, Baldo MA, Adachi C, Brooks J, Thompson ME, Forrest SR. High-
efficiency yellow double-doped organic light-emitting devices based on phosphor-
sensitized fluorescence. Appl Phys Lett 2001; 79: 1045-7.
[24]Tian N, Aulin YV, Lenkeit D, Pelz S, Mikhnenko OV, Blom PWM, Loi M A,
Holder E. Cyclometalated red iridium(III) complexes containing carbazolyl-acetyl-
acetonate ligands: efficiency enhancement in polymer LED devices. Dalton Trans
2010; 39: 8613-5.
[25]Misra A, Kumar P, Kamalasanan MN, Chandra S. White organic LEDs and their
recent advancements. Semicond Sci Technol 2006; 21: R35-47.
18

ACCEPTED MANUSCRIPT
[26]Kido J, Hongawa K, Okuyama K, Nagai K, White light-emitting organic electro-
luminescent devices using the poly(N-vinylcarbazole) emitter layer doped with three
fluorescent dyes. Appl Phys Lett 1994; 64: 815-7
[27]Tekin E, Holder E, Marin V, de Gans BJ, Schubert US, Inkjet printing of
luminescent ruthenium- and iridium-containing polymers for applications in light-
emitting devices. Macromol Rapid Commun 2005; 26: 293-7.
[28]Marin V, Holder E, Wienk MM, Tekin E, Kozodaev D, Schubert US. Inkjet printing
of electron donor/acceptor blends: towards bulk heterojunction solar cells. Macromol
Rapid Commun 2005; 26: 319-24.
[29]Marin V, Holder E, Hoogenboom R, Tekin E, Schubert US. Light-emitting
iridium(III) and ruthenium(II) polypyridyl complexes containing quadruple hydrogen
-bonding moieties. Dalton Trans 2006; 13: 1636-44.
[30]Tekin E, Wijlaars H, Holder E, Egbe DAM, Schubert US. Film thickness
dependency of the emission colors of PPE-PPVs in the inkjet printed libraries. J
Mater Chem 2006; 16: 4294-8.
[31]Tekin E, Holder E, Kozodaev D, Schubert US. Controlled pattern formation of
MEH-PPV by inkjet printing. Adv Funct Mater 2007; 17: 277-84.
[32]Wu HB, Zhou GJ, Zou JH, Ho CL, Wong WY, Yang W, Peng JB, Cao Y. Efficient
polymer white-light-emitting devices for solid-state lighting. Adv Mater 2009; 21:
4181-4.
[33]Niu YH, Liu MS, Ka JW, Bardeker J, Zin MT, Schofield R, Chi Y, Jen AKY.
Crosslinkable hole-transport layer on conducting polymer for high-efficiency white
polymer light-emitting diodes. Adv Mater 2007; 19: 300-4.
[34]a) Wang YF, Liu Y, Li XS, Qi HR, Zhu MX, Wang L, Lei GT, Wei QL, Zhu WG,
Peng JB, Cao Y. Novel cyclometalated platinum (II) complex containing alkyl-
trifluorene picolinic acid as emitter for single-layer white PLEDs. Org Electron 2010;
11: 1954 - 9. b) Li XS, Wang YF, Liu M, Ni MJ, Zhang ZY, Liu Y, Luo J, Shi DY,
Chen Q, Lei GT, Zhu WG. Synthesis and opto-electronic properties of a dinuclear
Pt(II)-Ir(III) complex bridged with bipicolinic acid derivative. Dalton Trans 2012; 41:
2972-8.
[35]Furuta PT, Deng L, Garon S, Thompson ME, Fre´chet JMJ. Platinum-functionalized
19

ACCEPTED MANUSCRIPT
random copolymers for use in solution-processible, efficient, near-white organic
light-emitting diodes. J Am Chem Soc 2004; 126: 15388-9.
[36]a) Shu CF, Dodda R, Wu FI, Liu MS, Jen AKY. Highly efficient blue-light-emitting
diodes from polyfluorene containing bipolar pendant groups. Macromolecules 2003;
36: 6698-703. b) Xu ZW, Li Y, Ma XM, Gao XD, Tian H. Synthesis and properties
of iridium complexes based 1,3,4-oxadiazoles derivatives. Tetrahedron. 2008; 64:
1860-7.
[37]Karakus C, Fischer LH, Schmeding S, Hummel J, Risch N, Schäferling M, Holder E.
Oxygen and temperature sensitivity of blue to green to yellow light-emitting Pt(II)
complexes. Dalton Trans 2012; 41: 9623-32.
[38]Fischer LH, Karakus C, Meier RJ, Risch N, Wolfbeis OS, Holder E, Schäferling M.
Referenced dual pressure and temperature sensitive paint for color camera read out.
Chem Eur J 2012; in press.
[39]Liang AH, Wang YF, Liu Y, Tan H, Cao YB, Li L, Li XS, Ma WJ, Zhu MX, Zhu
WG. Synthesis, photophysical and electrochemical characterization of the hetero-
leptic iridium complexes with modified ancillary ligand by carrier-transporting
groups. Chin J Chem 2010; 28: 2455-62.
[40] Wang YF, Deng XP, Liu Y, Ni MJ, Liu M, Tan H, Li XS, Zhu WG, Cao Y. White
emission from dinuclear cyclometalated platinum(II) complex in single-emitting
layer PLEDs. Tetrahedron 2011; 67: 2118-24.
[41]Pu YJ, Kurata T, Soma M, Kido J, Nishide H. Triphenylamine- and oxadiazole-
substituted poly(1,4-phenylenevinylene)s: synthesis, photo-, and electroluminescent
properties. Synth Met 2004; 143: 207-14.
[42]Wu G L, Yang Y, He C, Chen XM, Li YF. A new triphenylamine-based hyper-
branched polyfluorene with oxadiazole units on its side chains. Eur Polym J 2008; 44:
4047-53.
[43]Kozhevnikov VN, Donnio B, Bruce DW. Phosphorescent, terdentate, liquid-
crystalline complexes of platinum(II): stimulus-dependent emission. Angew Chem
Int Ed 2008; 47: 6286-9.
[44]Peng Q, Xu J, Li MJ, Zheng WX. Blue emitting polyfluorenes containing dendro-
nized carbazole and oxadiazole pendants: synthesis, optical properties, and electro-
20

ACCEPTED MANUSCRIPT
luminescent properties. Macromolecules 2009; 42: 5478- 85.
[45]Si ZJ, Li J, Li B, Hong ZR, Liu SY, Li WL. Electroluminescence from singlet
excited-state of the exciplex between (2,3-dicarbonitrilopyrazino[2,3-f][1,10] phenan
-throline) Re(CO)Cl and CBP. J Phys Chem C 2008; 112: 3920-5.
[46]Chen XW, Liao JL, Liang YM, Ahmed MO, Tseng HE, Chen SA. High-efficiency
red-light emission from polyfluorenes grafted with cyclometalated iridium
complexes and charge transport moiety. J Am Chem Soc 2003; 125: 636 – 7.
.
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Table 1
The optophysical data of FPt(Pic), FPt(TPA6Pic) and FPt(OXD6TPA6Pic)
[a]
Compounds
FPt(pic)
FPt(TPA6Pic)
UV-vis (nm) Emission[a] (nm) Emission[b] (nm) Фem (%)
325, 350, 404
304, 351, 404
475, 502
462, 506
462, 506
601
681
650
0.58
0.74
0.81
FPt(OXD6TPA6Pic) 310, 348, 404
Measured in DCM at room temperature;
[b] Measured in the neat film at room temperature.
Table 2
Electrochemical and thermal data of FPt(Pic), FPt(TPA6Pic) and FPt(OXD6TPA6Pic)
Compounds
FPt(pic)
FPt(TPA6Pic)
HOMO (eV) LUMO (eV) Eg (eV)[a] Td(oC)
-6.63
-5.95
-6.03
-3.46
-3.04
-3.09
3.17
2.91
2.94
294
253
264
FPt(OXD6TPA6Pic)
[a] Estimated according to the UV-vis absorption spectrum.
Table 3
The performances of device A and B at different dopant concentrations of 4 wt % and 8 wt %
J(mA/cm2)[b]
Devices LEmax(cd/A) V(V)[a]
Lmax(cd/m2) Vturn-on(V) CIE(x,y)[c]
A-4 wt%
A-8 wt%
B-4 wt%
B-8 wt%
0.80
0.93
0.82
1.01
8.0
7.4
7.4
7.8
1.1
3.2
3.9
1.8
318
314
2117
855
6.4
6.0
6.5
6.4
0.30, 0.43
0.32, 0.48
0.24, 0.32
0.30, 0.38
[a] Voltage at the maximum LE; [b] Current density at the maximum LE;
[c] Commission Internationale de L’Eclairage coordinates at 12 V.
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Figure captions
Figure 1. Normalized UV-vis absorption spectra of FPt(OXD6TPA6Pic), FPt(TPA6Pic)
and FPt(Pic) in CH2Cl2 at room temperature
Figure 2. Normalized PL spectra of FPt(OXD6TPA6Pic), FPt(TPA6Pic) and FPt(Pic) in
CH2Cl2 (a), and their neat films at room temperature (b)
Figure 3. EL spectra of the devices A and B at different dopant concentrations of 4 wt %
and 8 wt % at 12 V.
Figure 4. The luminous efficiency-current density-luminance (LE-J-L) characteristics of
the devices A and devices B
Scheme 1. Synthetic route of FPt(TPA6Pic) and FPt(OXD6TPA6Pic)
Figure S1. Normalized UV-vis absorption spectra of FPt(OXD6TPA6Pic) in CH2Cl2
and PL spectra of PVK and PFO film at room temperature
Figure S2. Energy band diagram of FPt(OXD6TPA6Pic), PVK, PFO and PBD
Figure S3. EL spectra of devices B under different applied voltages at the dopant
concentration of 8 wt%
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Scheme 1
28