Synthesis and opto-electronic properties of a red-emitting heteroleptic platinum complex using pyrazol-based diketone derivative as ancillary ligand

Article abstract of DOI:10.1002/cjoc.201180356

A red-emitting heteroleptic cyclometalated platinum(II) complex containing an ancillary ligand of pyrazol-based diketone derivative was synthesized. Its optophysical and electroluminescent properties were studied. Compared to the reported (piq)Pt(acac) complex, this platinum(II) complex exhibited a blue-shifted UV absorption band at 300-450 nm, a low LUMO energy level and improved electroluminescent property. Using this platinum(II) complex as a single doping emitter and a blend of ploy(9,9-dioctylfluorene) and 2-tert-butylphenyl-5-phenyl- 1,3,4-oxadiazole as a host matrix, the fabricated polymer light-emitting devices displayed saturated red emission with a peak at 648 nm and a shoulder at 601 nm. Furthermore, the emission quenching of the platinum(II) complex was significantly suppressed in these devices at high current density due to an introduction of the non-planar pyrazol group into the ancillary ligand.

Full text of DOI:10.1002/cjoc.201180356

FULL PAPER
Synthesis and Opto-electronic Properties of a Red-Emitting
Heteroleptic Platinum Complex Using Pyrazol-based Diketone
Derivative as Ancillary Ligand
Deng, Jiyonga,b(邓继勇)
Wang, Yafeia(王亚飞)
Li, Xiaoshuanga(李小双)
Ni, Meijuna(倪美君)
Liu, Minga(刘明)
Liu, Yu*,a(刘煜)
Lei, Gangtiea(雷钢铁)
Zhu, Meixianga(朱美香)
Zhu, Weiguo*,a(朱卫国)
a College of Chemistry, Key Laboratory of Envirornment-Friendly Chemistry and Application of Ministry of Educa-
tion, Xiangtan University, Xiangtan, Hunan 411105, China
b Department of Chemistry and Engineering, Hunan Institute of Engineering, Xiangtan, Hunan 411104, China
A red-emitting heteroleptic cyclometalated platinum(II) complex containing an ancillary ligand of pyra-
zol-based diketone derivative was synthesized. Its optophysical and electroluminescent properties were studied.
Compared to the reported (piq)Pt(acac) complex, this platinum(II) complex exhibited a blue-shifted UV absorption
band at 300— 450 nm, a low LUMO energy level and improved electroluminescent property. Using this platinum(II)
complex as a single doping emitter and a blend of ploy(9,9-dioctylfluorene) and 2-tert-butylphenyl-5-phenyl-
1,3,4-oxadiazole as a host matrix, the fabricated polymer light-emitting devices displayed saturated red emission
with a peak at 648 nm and a shoulder at 601 nm. Furthermore, the emission quenching of the platinum(II) complex
was significantly suppressed in these devices at high current density due to an introduction of the non-planar pyra-
zol group into the ancillary ligand.
Keywords electroluminescence, platinum(II) complex, synthesis design, molecular device, optoelectronic proper-
ties
Introduction
diketone derivative is 3-methyl-1-phenyl-4-(3,4,5-tri-
fluorobenzoyl)-1H-pyrazol-5(4H)-one (Hmptfbp) and its
cyclometalated platinum(II) complex is (piq)Pt(mptfbp),
in which piq is 1-phenylisoquinoline. The molecular
structure and synthetic route of (piq)Pt(mptfbp) are
shown in Scheme 1. Its thermal, electrochemical, opto-
physical and EL properties were investigated. Employ-
ing (piq)Pt(mptfbp) as a single doping emitter and a
blend of ploy(9,9-dioctyl fluorene) (PFO) and
2-tert-butylphenyl-5-phenyl-1,3,4-oxadiazole (PBD) as
a host matrix, the single-emissive-layer (SEL) PLEDs
presented a saturated red emission peaked at 648 nm at
different dopant concentrations from 1 wt% to 8 wt%.
The maximum external quantum efficiency (QEext) of
0.7% was obtained at current density of 34.8 mA/cm2 in
the device at 4 wt% dopant concentration. Significantly,
the QEext still remained a level of 0.62% while the cur-
rent density increased to 100 mA/cm2. As expected, this
cyclometalated platinum(II) complex with a bulky an-
cillary ligand exhibited a restrainable emission quench-
ing in PLEDs at high current density.
In the field of optoelectronic materials, cyclometa-
lated complexes have attracted significant attention be-
cause they have strong spin-orbital coupling and can
make full use of both singlet and triplet excitons to
achieve a 100% internal quantum efficiency in the-
ory.1-14 Among these cyclometalated complexes, plati-
num(II) complexes have a square-planar d8 electronic
configuration, which could cause molecular aggregation
leading to emission quenching. Therefore, platinum(II)
complexes generally display lower luminous efficiency
than the non-planar iridium complexes in organic/
polymer light-emitting devices (OLEDs/ PLEDs).15-20
In order to improve electroluminescence (EL) prop-
erties of platinum(II) complexes in these devices, an
effective method was reported to suppress molecular
aggregation and emission quenching by incorporating
some bulky groups into cyclometalated ligands.19,20 In
this paper, we present another method to get the same
aim via introducing a prazol group into ancillary ligand.
The resulting ancillary ligand of the pyrazol-based
*
E-mail: zhuwg18@126.com; liuyu03b@126.com; Tel.: 0086-0731-58298280; Fax: 0086-0731-58292251
Received December 16, 2010; revised May 5, 2011; accepted June 1, 2011.
Project supported by the National Natural Science Foundation of China (Nos. 50973093, 20772101, 20872124), Research Fund and the New Teacher
Fund for the Doctoral Program of Higher Education of China (Nos. 20094301110004, 200805301013), Science Foundation of Hunan Province (No.
2009FJ2002), Scientific Research Fund of Hunan Provincial Education Department (No. 10A119, 11CY023) and the Postgraduate Science Founda-
tion for Innovation in Hunan Province (No. CX2009B124).
Chin. J. Chem. 2011, 29, 2057— 2062
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Deng et al.
FULL PAPER
Scheme 1 Synthetic route of the (piq)Pt(mptfbp) complex
K2PtCl4
N
Cl
Cl
N
N
2-ethoxyethanol/water
Pt
Pt
1
F
F
F
F
F
F
COCl
O
O
O
N
N
ethanol
NHNH2
+
N2, reflux
O
O
N
1,4-dioxane
O
N
2
3
F
F
Hmptfbp
F
N
O
Na2CO3
2-ethoxyethanol
Pt
+
1
3
O
N
N
4
(Piq)Pt(mptfbp)
Results and discussion
Thermal property
The thermal stability of the (piq)Pt(mptfbp) and
(piq)Pt(acac) complexes is evaluated by thermogravim-
etric analysis (TGA) under N2 stream, and the data are
summarized in Table 1. The onset decomposition tem-
perature (Td) for 5% weight loss is 256 ℃ for
(piq)Pt(mptfbp) and 315 ℃ for (piq)Pt(acac), respec-
tively. Therefore, this platinum(II) complex containing a
pyrazol-based ancillary ligand still presents a high ther-
mal stability.
Photophysical properties
The UV-vis absorption and photoluminescent (PL)
spectra of the (piq)Pt(mptfbp) and (piq)Pt(acac) com-
plexes in dichloromethane (DCM) at room temperature
are shown in Figure 1 and their data are listed in Table 1.
For the (piq)Pt(mptfbp) complex, two typical absorption
bands are observed at about 306 and 412 nm, in which
the former band is attributed to the ligand center-based
π-π* electronic transition and the latter one is assigned
to the metal-ligand charge transfer (MLCT) electronic
transition. Compared to the reported (piq)Pt(acac), this
(piq)Pt(mptfbp) presented a bule-shifted UV absorption
band at 300—500 nm, which results from a steric effect
of the ancillary ligand by introducing a pyrazole group.
However, the mptfbp ancillary ligand has a minor effect
on the PL profiles. Intense red emissions for these two
platinum complexes are observed with a peak at 596 nm
and a shoulder at 640 nm under photo-excitation at
Figure
1
UV-vis absorption and PL spectra of the
(piq)Pt(mptfbp) and (piq)Pt(acac) complexes in dichloromethane
at 298 K.
room temperature. In order to understand the effect of
ancillary ligand on PL efficiency, the PL quantum yields
(Φ ) of both platinum complexes were measured in de-
f
gassed DCM at room temperature using (piq)2Ir(acac)
as the standard (Φ ＝0.2).24 As a result, (piq)Pt(mptfbp)
f
and (piq)Pt(acac) gave a PL quantum yield of 0.8 and
0.9, respectively. Obviously, attaching a pyrazole group
into diketone ancillary ligand has a minor influence on
PL efficiency of its platinum(II) complex.
The PL spectra of the (piq)Pt(mptfbp)-doped PFO-
PBD films at dopant concentrations from 1 wt% to 8
wt% are shown in Figure 2. Three main emission peaks
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Chin. J. Chem. 2011, 29, 2057— 2062

Synthesis and Opto-electronic Properties of a Red-Emitting Heteroleptic Platinum Complex
Table 1 Optophysical and electrochemical properties of the (piq)Pt(mptfbp) and (piq)Pt(acac) complexes
Compound
(piq)Pt(mptfbp)
(piq)Pt(acac)
UV/nm
325, 412
290, 476
PL/nm
596, 640
591, 643
Eox/V
0.7
EHOMO/eV
－6.25
ELUMO/eV
－3.74
Eg/eV
2.50
Td/℃
256
—
－5.71
－3.23
2.48
315
Electroluminescent property
The EL spectra of the (piq)Pt(mptfbp)-doped PFO-
PBD devices at the dopant concentrations from 1 wt%
to 8 wt% are shown in Figure 3. Similar EL spectra are
observed in these doped devices. The maximum EL
emission peak locates at 648 nm with a shoulder at 601
nm. Like the PL profiles of the (piq)Pt(mptfbp)-doped
PFO-PBD films, the high-energy emission peak at 420
nm is also displayed in these EL profiles. However, this
high-energy EL peak is very weak and obviously de-
creases with increasing dopant concentrations. While
the dopant concentration increases to 4%, it almost dis-
appears and a saturated red emission with a CIE coor-
dinate (0.61, 0.38) is observed. Therefore, EL emissions
are almost dominated by the (piq)Pt(mptfbp) dopant
rather than the PFO-PBD host in the devices at the
given dopant concentrations. While the dopant concen-
tration is over 4 wt%, the emission of the PFO-PBD
blend is almost quenched by the emission of
(piq)Pt(mptfbp) under electricity in the devices.
Figure 2 PL spectra of the (piq)Pt(mptfbp)-doped PFO-PBD
(20%) films at different dopant concentrations from 1 wt% to 8
wt%.
at 420, 596 and 640 nm are observed, in which the
high-energy peak at 420 nm results from emission of
PFO-PBD host and other two low-energy peaks come
from emission of the platinum(II) complex under
photo-excitation. With increasing dopant concentrations,
the emission from the platinum(II) complex is enhanced,
but the emission from the host matrix is decreased. This
implies that the energy transfer from the PFO-PBD host
to the platinum(II) complex is incomplete under
photo-excitation in the doped films at the dopant con-
centrations from 1 wt% to 8 wt%.
Electrochemical property
The electrochemical behavior of this (piq)Pt(mptfbp)
complex was studied by cyclic voltammetry using
ferrocene as an internal standard, and the data are listed
in Table 1. A reversible oxidation wave (Eox) at 0.7 V vs.
a saturated calomel electrode (SCE) is observed, but the
reduction wave (Ered) disappeared. Thus, the Ered was
obtained based on energy band gap (Eg) and Eox, in
which Eg was estimated by the UV-vis absorption
edge.13 The highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO)
energy levels (EHOMO, ELUMO) of this platinum(II) com-
plex were then calculated based on the following for-
mula, ELUMO＝－(Ered＋4.34) and EHOMO＝－(Eox＋
4.34).13 As a result, for this platinum(II) complex,
EHOMO and ELUMO are －6.25 and －3.74 eV, respec-
tively. Compared to (piq)Pt(acac), this complex exhib-
ited low LUMO and HOMO energy levels.24
Figure 3 EL spectra of the (piq)Pt(mptfbp)-doped PFO-PBD
(20%) devices at different dopant concentrations from 1 wt% to 8
wt%.
Figure 4 exhibits the current density-voltage and
brightness-voltage characteristics of the (piq)Pt-
(mptfbp)-doped PFO-PBD devices at the dopant con-
centrations from 1 wt% to 8 wt%. With increasing
dopant concentrations, the turn-on voltage of the de-
vices increases from 8.75 to 14.5 V. This means that EL
process in the (piq)Pt(mptfbp)-doped devices is domi-
nated by charge trapping process besides Förster and/or
Dexter transfer processes.24
Chin. J. Chem. 2011, 29, 2057— 2062
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Deng et al.
FULL PAPER
more, with increasing current density from 34 to 100
mA/cm2, this device still exhibited a QEext as high as
0.62%. This indicates that emission quenching of this
platinum(II) complex in the doped device at high cur-
rent density was significantly suppressed. Introducing a
non-planar pyrazol group into the diketone derivative is
available to suppress emission quenching of its plati-
num(II) complex.
Conclusion
In summary, a red-emitting cyclometalated plati-
num(II) complex containing a pyrazol-based diketone
derivative was obtained. The relation-ship between
structure of platinum(II) complex and its optophysical
property was primarily investigated. The emission
quenching of this platinum(II) complex in its doped
PLEDs was found to be efficiently suppressed although
these devices have not exhibited high emission effi-
ciency. In order to obtain higher emission efficiency,
optimization of the platinum(II) complex-doped devices
should be carried out.
Figure
4
Current density-voltage characteristics of the
(piq)Pt(mptfbp)-doped PFO-PBD (20%) devices at different
dopant concentrations from 1 wt% to 8 wt%.
The external quantum efficiency-current density
characteristics of the (piq)Pt(mptfbp)-doped PFO-PBD
devices at the dopant concentrations from 1 wt% to 8
wt% are displayed in Figure 5. For comparison, these
device performances are listed in Table 2. The maxi-
mum QEext of 0.7% at 34 mA/cm2 and the highest
brightness of 637 cd/m2 at 247 mA/cm2 were obtained
in the device at 4 wt% dopant concentration. Further
Experimental
Physical measurements
1
All H NMR spectra were acquired at a Bruker
Dex-400NMR instrument using CDCl3 as a solvent.
Thermogravimetric analysis (TGA) was carried out with
a NETZSCH STA449 instrument from 25 to 700 ℃ at
a heating rate of 20 ℃/min under nitrogen atmosphere.
UV-vis absorption spectra were recorded on a Perkin-
Elmer Lambda 25 UV-vis absorption spectrophotometer.
Photoluminescence (PL) spectra were recorded with an
Insta-Spec IV CCD system (Oriel) under excitation of
325 nm line of a He-Cd laser (OmniChrome Co.).
Thickness of the spin-casted film was measured with
profilometer (Tencor Alfa-Step 500). Layer thickness of
thermal deposition was monitored by a crystal thickness
monitor (Sycon). Current density (J)-voltage (V) data
were collected using a Keithley 236 source measure-
ment unit. QEext were obtained by an integrating sphere
(IS-080, Labsphere) measuring the total light output in
all directions. Luminance was measured by a Si photo-
diode and calibrated by a PR-705 Spectrascan spectro-
photometer (Photo Research). EL spectra were recorded
using a CCD spectrophotometer (Instaspec 4, Oriel).
Figure 5 External quantum efficiency-current density charac-
teristics of the (piq)Pt(mptfbp)-doped PFO-PBD (20%) devices at
different dopant concentrations from 1 wt% to 8 wt%.
Table 2 Device performances of the (piq)Pt(mptfbp)-doped PFO-PBD (20%) devices at different dopant concentrations from 1 wt% to
8wt%
－
2
Dopants concentration Turn-on voltage/V Maximum brightness/(cd•m )
Maximum external quantum efficiency/%
0.44 (33.6 mA/cm2)
CIE
1%
2%
4%
8%
8.8
10
878 (19.8 V)
356 (25.2 V)
637 (18.5 V)
450 (25.5 V)
0.56, 0.35
0.59, 0.34
0.61, 0.38
0.61, 0.38
0.65 (20.0 mA/cm2)
0.70 (34.8 mA/cm2)
0.40 (6.3 mA/cm2)
13.3
14.5
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Synthesis and Opto-electronic Properties of a Red-Emitting Heteroleptic Platinum Complex
Device fabrication
provide red solid. The red solid was recrystallized in
dichloromethane/hexane to afford red crystal of com-
pound 4 (70 mg, 30.5%). 1H NMR (CDCl3, 400 MHz) δ:
8.72 (d, J＝6.4 Hz, 1H), 8.59 (d, J＝8.0 Hz, 1H), 8.06
(d, J＝6.8 Hz, 1H), 7.94 (d, J＝7.2 Hz, 2H), 7.68 (d,
J＝6.8 Hz, 2H), 7.50 (d, J＝6.8 Hz, 4H), 7.32 (d, J＝
6.8 Hz, 2H), 7.06—7.76 (m, 2H), 6.28—6.03 (m, 2H),
1.97 (s, 3H). Anal. calcd for C32H20F3N3O2Pt: C 52.61,
H 2.76, N 5.75; found C 51.98, H 2.52, N 5.78.
The phosphorescent devices using (piq)Pt(mptfbp)
as dopant and a blend of PFO and PBD as a host matrix
were fabricated on a clean ITO glass substrate with a
sheet resistance of 15 Ω/□ (Nanbo, Shengzhen). A
hole-injection layer of poly-3,4-ethylene dioxythio-
phene (PEDOT, 50 nm), a hole-transporting layer of
poly(vinyl-carbazole) (PVK, 50 nm) and an emitting
layer of platinum(II) complex doped into a PFO-PBD
blend (80 nm) were spin-coated on the top of the ITO
substrate successively. Subsequently, barium (4 nm) was
deposited on the top of the emitting layer under vacuum
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Chin. J. Chem. 2011, 29, 2057— 2062