Polarized emission behavior of Pt complex-doped polymer films

Article abstract of DOI:10.1080/15421406.2012.688619

Organic light-emitting diodes (OLEDs) based on conjugated polymers exhibiting polarized emission have attracted much attention because of their potential as backlights for conventional liquid crystal (LC) displays and 3-D displays. We have reported a new

Full text of DOI:10.1080/15421406.2012.688619

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Polarized Emission Behavior of Pt
Complex-Doped Polymer Films
a
a
a
Motoi Kinoshita , Kenta Ibaraki , Yoshihiro Matsuura , Yunmi
a
a
a
Nam , Atsushi Shishido & Tomiki Ikeda
a
Chemical Resources Laboratory, Tokyo Institute of Technology ,
R1-12, 4259 Nagatsuta, Midori-ku , Yokohama , 226-8503 , Japan
Published online: 02 Aug 2012.
To cite this article: Motoi Kinoshita , Kenta Ibaraki , Yoshihiro Matsuura , Yunmi Nam , Atsushi
Shishido & Tomiki Ikeda (2012) Polarized Emission Behavior of Pt Complex-Doped Polymer Films,
Molecular Crystals and Liquid Crystals, 563:1, 83-91, DOI: 10.1080/15421406.2012.688619
To link to this article: http://dx.doi.org/10.1080/15421406.2012.688619
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Mol. Cryst. Liq. Cryst., Vol. 563: pp. 83–91, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.688619
Polarized Emission Behavior of Pt Complex-Doped
Polymer Films
∗
MOTOI KINOSHITA, KENTA IBARAKI,
YOSHIHIRO MATSUURA, YUNMI NAM,
ATSUSHI SHISHIDO, AND TOMIKI IKEDA
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-12,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Organic light-emitting diodes (OLEDs) based on conjugated polymers exhibiting po-
larized emission have attracted much attention because of their potential as backlights
for conventional liquid crystal (LC) displays and 3-D displays. We have reported a
new series of LC polymers based on donor-acceptor architecture containing an oxadi-
azole (OXD) and several amine moieties in the same side chain, which exhibit bipolar
character with a wide energy band gap.
In this study, to develop phosphorescent OLEDs with polarized emission, we synthe-
sized a platinum complex, Pt(4-decyloxy)salen, with emitting property, and fabricated
the device consisting of Pt complex as a guest and LC polymers as a host material.
The Pt complex exhibited high thermal stability, good solubility and an emission peak
at 530 nm in dichloromethane. To obtain electroluminescense from the Pt complex, we
fabricated ITO/rubbed PEDOT:PSS/Pt(4-decyloxy)salen:host polymer/MgAg devices.
The devices emitted a yellowish green light, which was dominated by the triplet emission
of the platinum complex. In addition, polarized electroluminescence was observed in
an aligned LC polymer film.
Keywords Electroluminescence; platinum complex; polarized emission
Introduction
Organic light-emitting diodes (OLEDs) have recently received a great deal of attention for
their application as full-color, flat-panel displays as well as from the standpoint of scientific
interest [1–4]. They are attractive because of low-voltage driving, high brightness, capability
of multi-color emission by the selection of emitting materials and easy fabrication of large-
area and thin-film devices. In addition, OLEDs exhibiting polarized emission have attracted
much attention because of their potential as backlights for conventional liquid crystal (LC)
displays and 3-D displays [5–10].
Recently, OLEDs based on phosphorescent materials such as Ir complexes and bipolar
host materials, have shown high efficiency [11–20]. To develop high performance OLEDs,
not only efficient emitting material but also bipolar host materials with wide energy band
gaps are required. The host materials used for OLEDs should meet the requirements of
∗
Address correspondence to Dr. Motoi Kinoshita, Chemical Resources Laboratory, Tokyo In-
stitute of Technology, R1-12,4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. Tel: (+81)
4
2-924-5241. Fax: (+81) 42-924-5275. E-mail: mkinoshita@res.titech.ac.jp
83

84
M. Kinoshita et al.
energy level matching for charge carrier injection and acceptance of both holes and electrons
in a nano-layered film, and hence should desirably possess bipolar character to permit the
formation of both stable cation and anion radicals.
Previously, we synthesized LC polymers containing oxadiazole and amine moieties in
the same side chain [21–25]. They showed LC behavior, high quantum efficiency and po-
larized blue emission. Furthermore, LC polymers had a wide band-gap and bipolar charge-
transporting ability. However, these LC polymers emitted fluorescence, due to which at
most 25% of the excitation states generate light. In this study, we prepared a phosphores-
cent Pt complex-doped LC host polymer and investigated its photo- and electroluminescent
behavior.
Experimental
Synthesis
ꢀ
N,N -Bis(4-n-decyloxy-salicylaldehydo)ethylenediaminateplatinum(II)complex (Pt(4-dec-
yloxy)salen) was prepared starting from 2,4-dihydroxybenzaldehyde by the synthetic route
shown in Scheme 1. All reagents were purchased from Tokyo Kasei Co. Ltd., Kanto Kagaku
1
Co. Ltd. and Aldrich Co. Ltd.. The synthesized compounds were identified by H NMR
(JEOL, GX300 (300 MHz)), elemental analysis and mass spectra (JMS-700 spectrometer
with fast atom bombardment (FAB)).
Synthesis of 4-(decyloxy)salicylaldehyde (1). 2,4-Dihydroxybenzaldehyde (6.9 g, 50
mmol), 1-bromodecane (11.2 g, 51 mmol), KHCO3 (5.2 g, 52 mmol) and a trace amount of
◦
KI were added to DMF (70 ml). The resulting mixture was stirred at 135 C for 4 h. After
Scheme 1. Synthetic route of the Pt complex.

Polarized Emission Behavior of Polymer Films
85
being cooled to room temperature, the mixture was poured into 1N HCl aq. and extracted
with chloroform, and the organic layer was evaporated to dryness. After evaporation of the
solvent, the residue was purified by column chromatography on silica gel (ethyl acetate :
1
hexane = 9 : 1) to yield colorless oil. Yield: 10 g, 72%; H NMR (300 MHz, CDCl3) δ:
0
6
3
.86 (t, 3H, J = 6.7 Hz), 1.18-1.55 (m, 16H), 1.78 (q, 2H, J = 7.0 Hz), 3.97 (t, 2H, J =
.6 Hz), 6.94 (d, 1H, J = 9.0 Hz), 7.39 (t, 1H, J = 3.1 Hz), 7.52 (dd, 2H, J = 9.1 Hz,
+
.1 Hz), 10.3 (s, 1H). Mass (FAB) found: m/z 279 [M] (278.19 calcd. for C17H26O3).
ꢀ
Synthesis of N,N -bis(4-decyloxysalicylidene)ethylenediamine (2). 2-Hydroxy-4-decy-
loxybenzaldehyde (9.8 g, 34 mmol) and ethylenediamine (1.0 g, 17 mmol) were added
to ethanol (50 ml), and the mixture was stirred and refluxed for 30 min under argon. The
Schiff-base ligand was separated as a yellow solid and was washed with H2O, methanol
1
and ethanol. Yield: 4.2 g, 43%; H NMR (300 MHz, CDCl3) δ: 0.86 (t, 6H, J = 7.0 Hz),
1
.15–1.48 (m, 32H), 1.74 (q, 4H, J = 6.7 Hz), 3.83 (s, 4H) 3.92 (t, 4H, J = 6.6 Hz), 6.22
(
(
dd, 2H, J = 9.0 Hz, 3.1Hz), 6.69 (d, 2H, J = 6.7 Hz), 7.08 (d 2H), 8.17 (s, 2H). Mass
+
FAB) found: m/z 580 [M] (580.42 calcd. for C36H56N2O4).
ꢀ
Synthesis of Pt(4-decyloxy)salen. K2[PtCl4] (3.0 g, 7.2 mmol) and N,N -bis(4-
decyloxysalicylidene)ethylene-diamine (8.3 g, 14 mmol) were added to 1M of KOH aq. and
◦
the mixture was stirred and heated at 40 C for 24 h under argon. The reaction mixture was
evaporated and purified by flash column chromatography (dichloromethane). The product
was evaporated to dryness. The yellow solid was recrystallized from dichloromethane, and
1
dried in vacuo to obtain orange powder. Yield: 0.6 g, 10%; H NMR (300 MHz, CDCl3) δ:
0
.86 (t, 6H, J = 7.0 Hz), 1.25-1.72 (m, 32H), 3.71 (s, 4H), 3.92 (t, 4H, J = 6.6 Hz), 6.22
(
dd, 2H, J = 6.3 Hz, 3.8 Hz), 6.69 (d, 2H, J = 2.4 Hz), 7.08 (d, 2H, J = 8.1 Hz), 7.82 (s,
+
2H). Mass (FAB) found: m/z 773 [M] (773.37 calcd. for C36H54N2O4Pt). Anal. calcd. for
C36H54N2O4Pt: C, 55.87; H, 7.03; N, 3.62; O, 8.27; Found: C, 55.64; H, 6.86; N, 3.44; O,
.44.
8
Molecular Properties of Pt(4-decyloxy)salen
Thermal properties of the compound were determined with an Olympus BH-2 polarizing
microscope equipped with Mettler hot-stage (FP-90 and FP-82), and a differential scanning
◦
calorimeter (Seiko I&E, SSC-5200 and DSC220C) at a heating/cooling rate of 3 C/min.
Electronic absorption spectra were measured using a JASCO U-550 spectrophotometer.
Photoluminescence (PL) and EL spectra were obtained using a Hitachi F-7000 lumines-
cence spectrometer. Fluorescence quantum yields of the polymers were determined by
using quinine sulfate as a standard compound.
Figure 1 shows DSC curves of Pt(4-decyloxy)salen. When a crystalline sample was
◦
◦
heated, two endothermic peaks were observed at 69 C and 114 C. Upon further heating,
◦
it melted at 233 C to yield an isotropic liquid. No mesophase was observed, even on
cooling. This was supported by XRD patterns (Figure 2). Figure 3 shows the electronic
absorption and PL spectra of Pt(4-decyloxy)salen in a dichloromethane solution. The
electronic absorption spectrum of the Pt complex shows bands peaking at 315, 330 and
∗
400 nm. The absorption peaks at 315 and 330 nm are due to typical ligand-centered ππ
transition, and another absorption band at 400 nm may be assigned probably to metal-to-
ligand charge transfer (MLCT). The Pt complex exhibited green phosphorescence with a
maximum peak at 534 nm (φem = 0.36) at room temperature.

86
M. Kinoshita et al.
Figure 1. DSC thermograms of the Pt complex.
◦
◦
◦
Figure 2. XRD patterns of the Pt complex at several temperatures; (a) 55 C, (b) 85 C, (c) 170 C,
◦
(d) 250 C.

Polarized Emission Behavior of Polymer Films
87
Figure 3. UV-vis absorption and photoluminescent spectra of the Pt complex in dichloromethane.
Polarized Emission Behavior
Polarized PL spectra of the Pt(4-decyloxy)salen doped in an LC host were measured
to explore anisotropic emission behavior of the Pt complex. The LC cell was prepared
as follows. S1114 and 0.1 mol% of Pt(4-decyloxy)salen were dissolved separately in
dichloromethane and the solutions were mixed together. After the solvent was removed
completely under vacuum, the LC mixture was sandwiched between two parallel transparent
glass substrates with a gap of 20 µm by silica particles as a spacer. The inner surfaces of
the two substrates were coated with rubbed PEDOT:PSS to obtain homogeneous alignment
of LCs. The monodomain structure of the LC cell was confirmed by polarizing optical
microscopy (POM), and homogeneous alignment was observed.
Figure 4 shows the polarized PL spectra of the LC cell. The emission intensity of the
aligned film turned out to be high along the rubbing direction and low in the direction
perpendicular to it, yielding dichroic ratio (RPL) of 1.7 at 550 nm (RPL = AꢁA ⊥ is a
Figure 4. Polarized photoluminescent spectra of the Pt complex doped S-1114 in the homogeneous
LC cell at a concentration of 0.10 mol% (λex = 400 nm). (//) Parallel to the rubbing direction; (⊥)
perpendicular to the rubbing direction.

88
M. Kinoshita et al.
dichroic ratio with Aꢁ and A⊥ being the values of the polarized absorption parallel and
perpendicular to the rubbing direction). Thus, the Pt complex is expected to function as a
polarized emitter with a suitable electron conducting host polymer for OLEDs.
OLED
Figure 5 shows the PL spectra and molecular structures of PVK as an amorphous host poly-
mer and PA6CB as an LC host polymer. To fabricate the OLED, Pt(4-decyloxy)salen doped
host polymer films were obtained by spin-coating on an ITO coated glass substrate covered
with rubbed PEDOT:PSS at a rate of 2500 rpm for 30 s with 1,1,2,2-tetrachloroethane
polymer solutions at several concentrations of Pt complex. Film thicknesses were deter-
mined by a Dektak surface profiler. An alloy of magnesium and silver (volume ratio: ca.
10:1) was vacuum deposited onto the polymer layer by simultaneous evaporation from two
separate sources. The EL behavior was evaluated with a DC voltage current source monitor
(Advantest R6243) and a fluorescence spectrometer (Hitachi F7000).
Figure 6 shows the EL spectra of the fabricated devices using Pt(4-decyloxy)salene-
doped PVK and PA6CB as host polymers. The devices emitted yellowish-green light
peaking at around 580 nm, which mainly originates from Pt(4-decyloxy)salen. A very
weak emission with λmax at approximately 600 nm was observed, which may result from
exciplex formed by the Pt complex and host polymers, respectively.
Figure 7 shows the polarized EL spectrum of the device, ITO/rubbed PEDOT:PSS/Pt
complex doped LC Polymer/MgAg. The EL intensity of the aligned film turned out to be
Figure 5. Chemical structures and photoluminescent spectra of films for PVK and PA6CB.

Polarized Emission Behavior of Polymer Films
89
Figure 6. Electroluminescent spectra of the Pt complex-doped PVK and PA6CB devices at various
concentrations.
higher along the rubbing direction. The dichroic ratio calculated from the polarized EL
spectra was REL = Iꢁ/I⊥ = 2.4 where REL is a dichroic ratio with Iꢁ and I⊥ being the values
of polarized emission parallel and perpendicular to the rubbing direction. When the device
was fabricated using PVK as a host polymer, the EL spectrum was polarization-independent
Figure 7. Polarized EL spectra of the Pt complex-doped PA6CB device at the concentration of
4
.0 mol%. (//) Parallel to the rubbing direction; (⊥) perpendicular to the rubbing direction.

90
M. Kinoshita et al.
because the Pt complex is not aligned in the amorphous host polymer. Thus, the Pt complex
was found to function as a polarized phosphorescent material for polarized OLEDs with
the combination of the aligned host polymer.
Conclusions
In this study, we synthesized Pt complex as a phosphorescent material and fabricated
the device consisting of Pt complex doped in an LC polymer to develop a polarized
phosphorescent OLED. The synthesized Pt complex showed high thermal stability, good
solubility and green emission in solution. The fabricated device using Pt complex doped in
host polymers emitted yellowish-green light, which is dominated by the triplet emission of
the platinum complex. In addition, the polarized EL spectrum was observed in an aligned
LC polymer film. Thus, as polarized OLEDs are of great interest due to their potential
use in efficient light sources for liquid crystal displays and three-dimensional displays, this
study suggests a way to achieve such polarized phosphorescence using an aligned metal
complex.
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