Preparation and properties of oxadiazole-containing polyacetylenes as electron transport materials

Article abstract of DOI:10.1002/pola.23904

A series of functional polyacetylenes (PAs) bearing diphenyl oxadlazole pendant groups (P1-P4) were prepared, and the resultant polymers are completely soluble In common organic solvents, Their structures and properties were characterized and evaluated by

Full text of DOI:10.1002/pola.23904

Preparation and Properties of Oxadiazole-Containing Polyacetylenes as
Electron Transport Materials
XIN WANG,1,2 SHANYI GUAN,1 HONGYAO XU,1,3 XINYAN SU,1 XUHUI ZHU,4 CHUN LI4
1College of Material Science and Engineering and State Key Laboratory of Chemical Fibers and Polymeric Materials, Donghua
University, Shanghai 201620, People’s Republic of China
2School of Chemistry, Chizhou University, Chizhou 247000, People’s Republic of China
3State Key laboratory of Crystal Material, Shandong University, Jinan 250100, People’s Republic of China
4Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory SPA-OXAcial Functional Materials, MOE, South China
University of Technology, Guangzhou 510640, People’s Republic of China
Received 19 November 2009; accepted 17 December 2009
DOI: 10.1002/pola.23904
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A series of functional polyacetylenes (PAs) bearing
diphenyl oxadiazole pendant groups (P1–P4) were prepared,
and the resultant polymers are completely soluble in common
organic solvents. Their structures and properties were charac-
terized and evaluated by DSC, TGA, UV, PL, CV, and EL analy-
ses. The results show that all the resulting polymers possesses
low LUMO energy level and high thermal stability, and the re-
sultant functional polyacetylenes without spacer group
between the polyacetylene conjugated main chain and oxadia-
zole pendant groups (P1–P3) show lower LUMO energy level
(ꢀꢁ3.87 eV) and higher thermal properties (Tg) than that (P4)
with a flexible spacer. The resultant polymer (P2) was applied
as an ETM in bilayer electroluminescent devices and effectively
enhances external quantum efficiency and the brightness of
C
V
device, and decreases turn-on voltages of devices.
2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
1406–1414, 2010
KEYWORDS: electron transport material; light-emitting diodes;
oxadiazole; polyacetylene; thermal properties
INTRODUCTION Organic and conjugated polymer light-emit-
ting diodes (OLEDs) have been extensively studied during
the past decade because of their potential applications in a
new generation flat display and lighting technologies.1–3 For
ideal high-performance electroluminescent devices, the injec-
tion and transport balance of electrons and holes is need-
ful.4–9 However, most electroluminescent (EL) organic mate-
rials often show more efficiently hole injection due to the
inherent richness of p-electrons in the EL materials, which
results in an unbalance of charge injection in emitting layer
and low device emissive efficiency. Therefore, many electron
injection/transporting materials were prepared and investi-
gated.10–12
crystallization under device operation. However, most of the
aromatic polyoxadiazoles exhibits poor solubility in organic
solvents.7 Nevertheless, the incorporation of oxadiazole moi-
eties as pendant groups into poly(methyl methacrylate) has
exhibited higher Tgs and better processiblity relative to small
oxadiazole molecules.16,17 Attachment of the oxadiazole unit
to the phenylene ring of poly(phenylenevinylene) also
showed well electron injection/transport properties.18
Polyacetylenes (PA) is a prototypical linear conjugated poly-
mer with alternating single and double bond, shows well
electrically conductive after doping.19,20 Proper structural
design may tune the backbone-pendant interplay into har-
mony and synergy, generating new substituted PAs with
novel functionalities. Recently, Xu and coworkers found that
azobenzene-containing substituted polyacetylenes possess
large third-order nonlinear optical susceptibility (up to
10ꢁ10 esu) and novel optical limiting properties.21–25 Many
other functional polyacetylenes with electro-optic activity,
photonic responsiveness, and biologic compatibility have
been investigated.23,24,26–31 However, to the best of our
knowledge, the functional polyacetylenes with electron trans-
port properties were less investigated.
Oxadiazoles turn out to be very well electron injection/trans-
porting materials and have attracted significant attention
owing to their prominent electron affinity, high photolumi-
nescence quantum yield.6,7,13 Unfortunately, the small oxadia-
zole molecules are easily crystallized over time due to joule
heating during device operation,6,14 which results in low effi-
ciency and poor stability.6,7,15 Compared to low-molecular-
weight oxadiazole compounds, oxadiazole-containing poly-
mers are expected to have higher Tg and less susceptible to
Correspondence to: H. Xu (E-mail: hongyaoxu@163.com)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1406–1414 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
was equipped with a three-electrode cell. Pt wire was used
as a counter electrode and an Ag/Agþ was used as a refer-
ence electrode in the CV measurement. The polymer thin
films were cast onto a carbon-glass of a working electrode.
The measurements were carried out at room temperature
and the electrolyte of 0.1 M TBAP in acetonitrile with nitro-
gen-saturated.
Absorption spectra were recorded on a Shimadzu UV-265
spectrometer and fluorescence spectra were obtained on a
Shimadzu RF-5301PC spectrofluorimeter.
The luminance properties of the devices were measured
with a photodiode calibrated by a Spectra Scan Spectropho-
tometer. External quantum efficiency (EQE) was verified by
measuring in the integrating sphere after encapsulation of
the devices with UV-cured epoxy and thin cover glass. Cur-
rent-brightness-voltage characteristics were made by using a
keithley source measurement unit (Keithley 2400 and 2000)
with a calibrated silicon photodiode. The EL spectra were
measured by JY SPEX CCD 3000 spectrometer. All the meas-
urements were carried out in ambient atmosphere at room
temperature.
Polymerization
All the polymerization reactions and manipulations were
performed under prepurified nitrogen using Schlenk techni-
ques either in vacuum-line system of an inert-atmosphere
glove box, except for the purification of the polymers, which
were done in open air. Typical procedure for the polymeriza-
tion is given below: Into a baked 20-mL Schlenk tube with a
side arm was added 1 mmol of the monomer. The tube was
evacuated under vacuum and then flushed with dry nitrogen
three times through the side arm. Dioxane (3 mL) was
injected into the tube to dissolve the monomer. The catalyst
solution was prepared in another tube by dissolving 4.6 mg
(0.01 mmol) [Rh(nbd)Cl]2 and 2.02 mg (0.02 mmol) Et3N in
2 mL of dioxane, which was transferred to the monomer so-
lution using a hypodermic syringe. The reaction mixture was
stirred at 60 ꢂC under nitrogen for 6 h. The mixture was
then diluted with 5 mL of dioxane and added dropwise to
200 mL of methanol under stirring. The precipitate was cen-
trifuged and redissolved in THF. The THF solution was added
dropwise into 200 mL of methanol to precipitate the poly-
mer. The dissolution–precipitation process was repeated
three times, and the finally isolated precipitant was dried
CHART 1 The structures of P1–P4.
In this work, a series of oxadiazole-containing PAs (Chart 1)
with
different
terminal
alkoxyl
group
([HC
¼
CAC6H4AC2N2OAC6H4AOCmH2mþ1]n, m ¼ 4, 8, 10 for P1,
P2, P3, respectively) or with a spacer group (OCH2) and a
terminal alkoxyl group at same time ([HC
¼
CAOCH2AC6H4AC2N2OAC6H4AOC8H17]n, P4) were prepared.
Their structure and properties are to be characterized and
evaluated. The application of P2 as electron injection materi-
als in bilayer electroluminescent devices will be examined.
EXPERIMENTAL
Materials
Monomer molecules (M1–M4) were synthesized according
to the Scheme 1 and refs. 27,30–32. The red emissive mate-
rial, 4,7-di (5-3,5-bis(naphthalene-1-yl)phenyl-4-hexylthio-
phen-2-yl)-2,1,3-benzothiadiazole (DB) was prepared based
on ref. 33. Poly(ethylenedioxythiophene): poly(styrene sulfo-
nate) (PEDOT:PSS) was used as hole transport layers and
also serve to planarise the anode.7 Acetonitrile, tetrabutylam-
monium perchlorate (n-Bu)4NClO4 (TBAP) (99%) (Fluka),
PEDOT (Bayer), and all the other reagents and solvents were
used as received.
ꢂ
under vacuum at 30 C to a constant weight.
2-(4-Butyloxyphenyl)-5-(4-ethynylphenyl)-1,3,4-
oxadiazole (M1)
FTIR (KBr, m cmꢁ1): 3280, 2952, 2879, 2100, 1612, 1495,
842. 1H-NMR (300 MHz, CDCl3, d, ppm): 0.87 (t, 3H, CH3),
1.48 (m, 2H, CH2CH3), 1.84 (m, 2H, OCH2CH2), 3.24 (s, 1H,
BCH), 4.05 (t, 2H, OCH2), 7.02 (d, 2H, J ¼ 8.8 Hz, Ar-H), 7.64
(d, 2H, J ¼ 8.3 Hz, Ar-H), 8.05 (d, 2H, J ¼ 8.9 Hz, Ar-H), 8.09
(d, 2H, J ¼ 8.4 Hz, Ar-H). 13C-NMR (100.61 MHz, CDCl3, d,
ppm): 164.5, 163.4 (Oxa-C), 157.4, 132.8, 127.1, 126.6,
125.8, 122.5, 117.6, 114.9 (Ar-C), 82.3 (CBCH), 79.9 (BCH),
68.5 (OCH2), 31.8 (OCH2CH2), 19.1 (CH2CH3), 14.1 (CH3).
Instruments
Thermal analyses were performed on a TA Iinstruments DSC
9000 for differential scanning calorimetry (DSC) under nitro-
gen at a scanꢂrate was 10 ꢂC/min within the temperature
range 30–300 C.
Cyclic voltammetry (CV) was determined on a LK98C elec-
trochemical analyzer at a scanning rate of 50 mV/s, which
OXADIAZOLE-CONTAINING POLYACETYLENES AS ETM, WANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 The synthesis routes of monomer molecules (M1–M4).
2-(4-Octoxyphenyl)-5-(4-ethynylphenyl)-1,3,4-
oxadiazole (M2)
BCH), 3.94 (t, 2H, OCH2CH2), 4.78 (s, 2H, CH2CB), 7.1 (m,
4H, J ¼ 7.62 Hz and J ¼ 8.86 Hz, Ar-H), 8.05 (m,4H, J ¼ 7.62
Hz and J ¼ 8.86 Hz, Ar-H). 13C-NMR (100.61 MHz, CDCl3, d,
ppm): 164.3, 163.9 (Oxa-C), 161.9, 160.0, 128.6, 117.6,
116.3, 115.4, 115.0 (Ar-C), 78.0 (CBCH), 75.2(BCH), 68.3
(OCH2), 55.9 (OCH2CB), 31.8 (CH2C2H5), 29.3–26.0
(OCH2C4H8), 22.7 (CH2CH3),14.1 (CH3).
FTIR (KBr, m cmꢁ1): 3272, 2932, 2851, 1611, 1456, 843. 1H-
NMR (300 MHz, CDCl3, d, ppm): 0.88 (t, 3H, CH3), 1.35 (m,
10H, (CH2)5CH3), 1.83 (m, 2H, OCH2CH2), 3.24 (s, 1H, BCH),
4.04 (t, 2H, OCH2CH2), 7.03 (d, 2H, J ¼ 8.9 Hz, Ar-H), 7.68
(d, 2H, J ¼ 8.4 Hz, Ar-H), 8.07 (m, 4H, Ar-H). 13C-NMR
(100.61 MHz, CDCl3, d, ppm): 164.8, 163.5 (Oxa-C), 162.1,
132.7, 128.7, 126.6, 125.3, 124.1, 116.0, 115.0(Ar-C), 82.8
(CBCH), 79.9 (BCH), 68.3(OCH2), 31.8(OCH2CH2), 29.1–29.3
(OC2H5C3H6C3H7), 26.0 (CH2C2H5), 22.7(CH2CH3), 14.1 (CH3).
Poly(2-(4-butyloxyphenyl)-5-(4-ethynylphenyl)-1,3,4-
oxadiazole) (P1)
Yellow solid, yield 92%, Mw ¼ 2.93 ꢃ 104, Mw/Mn ¼ 1.36
(polystyrene calibration). FTIR (KBr, m cmꢁ1): 3045, 2925,
2857, 1613, 1496, 837. 1HNMR (300 MHz, CDCl3, d, ppm):
0.96 (br, CH3), 1.45 (br, CH2CH3), 1.90 (br, OCH2CH2), 3.82
(br, OCH2), 5.92(br, cis ¼ C-H), 6.67 (br, Ar-H and trans ¼ C-
H), 6.91 (br, Ar-H), 7.67 (br, Ar-H). 13C-NMR (100.61 MHz,
CDCl3, d, ppm): 164.3–163.7 (Oxa-C), 134.9 (C¼¼CH), 128.8–
127.7(Ar-C), 123.9 (C¼¼CH), 115.0–114.9 (Ar-C), 68.5 (OCH2),
31.8 (OCH2CH2), 19.1 (CH2CH3), 14.1 (CH3).
2-(4-Decyloxyphenyl)-5-(4-ethynylphenyl)-1,3,4-
oxadiazole (M3)
FTIR (KBr, cmꢁ1): 3273, 2920, 2851, 2100, 1611, 1456, 843.
1H-NMR (300 MHz, CDCl3, d, ppm): 0.87 (t, 3H, CH3), 1.25–
1.48 (m, 14H, (CH2)7CH3), 1.83 (m, 2H, OCH2CH2), 3.25 (s,
1H, BCAH), 4.04 (t, 2H, OCH2), 7.02 (d, 2H, J ¼ 8.9 Hz, Ar-
H), 7.64 (d, 2H, J ¼ 8.5 Hz, Ar-H), 8.08 (m, 4H, Ar-H). 13C-
NMR (100.61 MHz, CDCl3, d, ppm): 164.5(Oxa-C), 157.4,
132.7, 128.3, 127.1, 125.6, 122.1, 117.5, 114.9(Ar-C), 82.3
(CBCH), 79.9 (BCH), 68.6(OCH2), 31.8(CH2C2H5), 29.3–26.0
(OCH2C6H12), 22.7(CH2CH3), 14.1 (CH3).
Poly(2-(4-octoxyphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole)
(P2)
Yellow black solid, yield 95%, Mw ¼ 3.19 ꢃ 104, Mw/Mn
¼
1.42 (polystyrene calibration). FTIR (KBr, m cmꢁ1): 2925, 2854
(CH2, CH3), 1612, 1496 (C¼¼C), 837 (p-Ar). 1H-NMR (300 MHz,
CDCl3, d, ppm): 0.88 (br, CH3), 1.28 (br, (CH2)5CH3), 1.75 (br,
OCH2CH2), 3.90 (br, OCH2), 5.98 (br, cis ¼ C-H), 6.72 (br, Ar-H
and trans ¼ C-H), 6.89 (br, Ar-H), 7.67 (br, Ar-H). 13C-NMR
(100.61 MHz, CDCl3, d, ppm): 164.3–163.7 (Oxa-C), 134.9
2-(40-Octoxyphenyl)-5-(40-propynyloxyphenyl) -1,3,4-
oxadiazole (M4)
FTIR (KBr, cmꢁ1): 3270, 2922, 2853, 2127, 1612, 1497, 837.
1H-NMR (300 MHz, CDCl3, d, ppm): 0.88 (t, 3H, CH3); 1.23
(m, 10H, (CH2)5CH3), 1.71 (m, 2H, OCH2CH2), 2.54 (s, 1H,
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ARTICLE
RESULTS AND DISCUSSION
(C¼¼CH), 128.8–127.7(Ar-C), 123.9 (C¼¼CH),115.0–114.9 (Ar-
C), 68.3 (OCH2), 31.8 (CH2C2H5), 29.6–26.0 (OCH2C4H8), 22.7
(CH2CH3), 14.1 (CH3).
Syntheses and Solubility of the Polymers
All polymerization reactions were carried out under nitrogen
atmosphere using a standard Schlenk vacuum-line system.
We first synthesized functional polyacetylene (PA) containing
oxadiazole group without spacer and alkoxyl tail and found
that the resultant PA is insoluble in common organic solvent.
However, the functional PAs bearing oxadiazole with alkoxyl
tail (AOCmH2mþ1, m ¼ 4, 8, 10 for P1, P2, P3, respectively)
are soluble in common organic solvent, hinting that introduc-
tion of flexible terminal group obviously improve the solubil-
ity of resulting polymers. Similar phenomena were also
observed in our previous work on azobenzene-containing
PAs.24 Furthermore, it is found that the solubility of resultant
PAs is affected by the length of the flexible terminal chain,
and increases with the flexible terminal alkoxyl chain length.
For example, polyacetylene P1 with short terminal alkoxyl
chain (m ¼ 4) is partial soluble in common organic solvent
such as THF, CHCl3, toluene and dioxane, and P2–P4 with
long flexible terminal alkoxyl group (m ¼ 8 or 10) com-
pletely soluble in above common organic solvents and also
display well film-forming properties. Therefore, the struc-
tures of resultant polymers can be characterized by solution
spectral methods.30,31
Poly(2-(4-decyloxyphenyl)-5-(4-ethynylphenyl)-1,3,4-
oxadiazole) (P3)
Yellow black solid, yield 98%, Mw ¼ 2.54 ꢃ 104, Mw/Mn
¼
1.71 (polystyrene calibration). FTIR (KBr, m cmꢁ1): 2926,
1
2855(CH2, CH3), 1611, 1496(C¼¼C), 838(p-Ar). H-NMR (300
MHz, CDCl3, d, ppm): 0.87 (br, CH3), 1.26 (br, (CH2)7CH3),
1.70 (br, OCH2CH2), 3.83 (br, OCH2), 5.92 (br, cis ¼ C-H),
6.68 (br, Ar-H and trans ¼ C-H), 6.93 (br, Ar-H), 7.69 (br, Ar-
H). 13C-NMR (100.61 MHz, CDCl3, d, ppm): 164.3–163.7
(Oxa-C), 134.9 (C¼¼CH), 128.8–127.7(Ar-C), 123.9 (C¼¼CH),
115.0–114.9 (Ar-C), 68.8 (OCH2), 31.8 (CH2C2H5), 29.7–25.9
(OCH2C6H12), 22.7 (CH2CH3), 14.1 (CH3).
Poly(2-(40-octoxyphenyl)-5-(40-propynyloxyphenyl)-1,3,4-
oxadiazole) (P4)
Brown black solid. Yield: 25%. Mw ¼ 2.67 ꢃ 104, Mw/Mn
¼
1.53 (polystyrene calibration). FTIR (KBr, m cmꢁ1): 2923,
2854 (CH2, CH3), 1611, 1497 (C¼¼C), 839 (p-Ar). 1H-NMR
(300 MHz, CDCl3, d, ppm): 0.89 (br, CH3), 1.31 (br,
(CH2)5CH3), 1.76 (br, OCH2CH2), 3.96 (br, OCH2), 5.2 (br,
OCH2CH¼¼), 6.85 (br, Ar-H and trans ¼ C-H), 7.36 (br, Ar-H).
13C-NMR (100.61 MHz, CDCl3, d, ppm): 164.3, 163.2 (Oxa-C),
161.6, 144.3(Ar-C), 139.2 (CACH), 128.4, 126.4 (Ar-C), 123.0
(CACH), 115.7 (Ar-C), 68.1(C,OCH2C7H15), 66.3 (CH2 C¼¼),
31.8 (CH2C2H5), 29.7–26.0 (OCH2C4H8), 22.6 (CH2CH3), 14.1
(CH3).
Structure Characterization
The structures of P1–P4 were characterized by spectroscopic
1
techniques, such as FTIR and H-NMR, and they all gave sat-
isfactory spectra data corresponding to their expected molec-
ular structures. For example, the characteristic BCAH and
CBC absorption bands of the monomers completely disap-
pear in the IR spectra of their polymers, proving that the
CBC bond of monomers have transformed to C¼¼C unit of
resulting polymers. All the characteristic proton resonance
absorption bands in the 1H-NMR spectra of resulting poly-
mers (P1–P4) significantly change broad and the new broad
resonance absorption band located at about d 5.9 or 6.8 cor-
responding to the cis or trans olefin absorption of the PAs
appears in the spectra of resultant polymers compared with
the spectra of the monomers, which also further confirms
that the CBC unit has transferred to C¼¼C bond. According
to Percec and coworkers method,34–40 we quantified the cis
content of the polymers based on the following equation:
Device Fabrication
The bilayer blended devices [ITO/PEDOT:PSS (50 nm)/
PVK(40 nm)/DB:P2 (80 nm)/Ba(4 nm)/Al(200 nm)] using a
poly(vinylcarbazole) (PVK) as a hole transport material layer,
and a mixture of DB and P2 as the emitting and electron
transfer film layer were prepared. The fabrication process of
the diodes followed a standard procedure below. The pat-
terned indium-tin oxide (ITO, 15 X/square) coated glass sub-
strates were cleaned with acetone, detergent, distilled water
and 2-propanol, successively in an ultrasonic bath. After
treatment with oxygen plasma, PEDOT: PSS was spin-coated
ꢂ
onto the ITO substrate and dried in a vacuum oven at 80 C
for 8 h. PVK was coated on top of a dried PEDOT:PSS layer.
Subsequently, a thin film of the emitting layer was coated
from a mixed toluene solution (35 mg/mL) at different ratio
of DB:P2, 100:0, 90:10, and 80:20 (wt/wt), inside a glove
box under vacuum, respectively. As PVK is barely soluble in
toluene, it remains unperturbed upon deposition of the top
emitting layer. Then, a thin layer of barium as an electron
injection cathode, and aluminum protection layer were ther-
mally deposited by vacuum evaporation through a mask
under 10ꢁ4 Pa. The deposition speed and thickness of the
barium and aluminum layers were monitored with a thick-
ness/rate meter. The cathode area is defined as the active
area of the device, typically, 0.17 cm2 in this study. All steps
except PEDOT:PSS layer were performed in N2-filled dry
boxes.
cis content ð%Þ ¼ ½A5:92=ððA þ A5:92Þ=9Þꢄ ꢃ 100
(1)
where A ¼ A7.69 þ A6.93 þ A6.68, corresponding to the peak
areas of the eight aryl protons of the polymers and the peak
areas of trans olefinic proton, and A5.92 corresponds to the
peak areas of cis olefinic proton. Then, the cis olefin contents
can be estimated based on integration areas of the peak, i.e.,
the cis contents of P1, P2, and P3 are 93.1, 96.4, and 81.8%,
respectively. Similar to P1–P3, the cis content of P4 is
43.3%. The low cis content of P4 may result from the cis-
trans isomerization during the polymerization due to the
small stereo-hinder of the flexible ether group as spacer.41
The characteristic acetylene and propargyl carbons of the
monomer resonances were not found, and two new peaks
OXADIAZOLE-CONTAINING POLYACETYLENES AS ETM, WANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The TGAs of P1–P4 are measured and the data are also
listed in Table 1. From Table 1, these polymers display very
high thermal stability in nitrogen and their Tds are up to
ꢂ
more than 330 C (Td, 5% loss weight). To prove the thermal
stability, we made TGA-FTIR experimental and investigated
the UV spectra of the pristine P1-P4 film annealed under
ꢂ
ambient atmosphere at 200 C for 30 min. The results show
no obvious IR absorption from decomposed product and the
UV absorption pattern and profile of polymer films don’t
ꢂ
appear any obvious changes before 200 C, implying that no
significant molecular decomposition and isomerization hap-
pens at the condition. The high thermal stability may be due
to the reason that the backbones of P1–P4 are surrounded
by a rigid diaryl oxadiazole groups with high thermal prop-
erties to shield the polymer main chains from the thermal
attack.27,28,30,32,44 Similar phenomena was found by Percec
and coworkers.41,45 When the polymers were heated to more
than 200 ꢂC, some weak aromatic FTIR spectral absorption
were observed, which may be originated from the decompo-
sition induced by backbone isomerization of polyacety-
lene.38,41,45 The detailed thermal decomposition mechanism
is in progress, which will be reported in our future publica-
tion. Simultaneously, it is found that the PAs directly bearing
oxadiazole groups (P1–P3) also exhibit higher Td than that
with a spacer between PA main chain and oxadiazole pend-
ant (P4). This is consistent with the results of the DSC
measurement.
FIGURE 1 DSC curve (measured under N2 at a heating rate of
20 ꢂC/min) of P1–P4.
associated with the resonances of the olefin carbon atoms of
the PA backbone were, however, observed in the 13C-NMR
spectra of these polymers, further substantiating the molecu-
lar structure of these polymers. In addition, the GPC mea-
surement results confirm that all the polymers show high
average molecular weights, further indicating that the poly-
mers are obtained30,31 and the detailed data have been given
in experimental section.
Optical Properties
The UV-vis absorption spectra of all polymers in film are
shown in Figure 2 and the data are summarized in Table 1.
Taken the absorption spectrum of P2 as an example, P2 in
neat film shows maximal absorption peak at 322 nm, corre-
sponding to the side chain aromatic oxadiazole group.43 The
absorption edge is measured to be 391 nm, from which the
p–p* band gap energy is estimated to be 3.17 eV. It is seen
from Table 1 that the band gap energies decrease with
increasing alkoxyl chain length. Moreover, it is found that the
PAs directly bearing oxadiazole groups (P1–P3) display
lower band gap energy than that with a spacer group (P4)
owing to larger p-electron conjugation effect in the former
than the latter.46,47
Thermal Stability
Thermal stability of polymer is very important for device
longevity. The glass transition temperatures (Tgs) of resultant
polymers were determined by a differential scanning calo-
rimꢂeter (DSC) in nitrogen atmosphere at a heating rate of
20 C/min. Figure 1 displays the DSC curve of the polymers
and their Tgs are summarized in Table 1. As shown in Figure
1, the Tgs of P1, P2, P3, and P4 are shown at 194.6, 193.3,
ꢂ
187.1, and 104.8 C, respectively. P1–P3 exhibited the higher
Tg than P4, which may be attributed to stronger electron
interaction between main chain and substituted inside
group.42,43 From Table 1, it is also found that the Tgs of the
resultant polymers (P1–P3) increase with decrease of
alkoxyl chain length.
Figure 2 also shows the PL spectra of P1–P4 in the film on
quartz. The photoluminescence spectra were obtained after
excitation at the absorption maxima of the oxadiazole moiety
TABLE 1 The Properties of P1–P4
kabs (nm)a
Eogpt (eV)b
k
(nm)a
Eonset (V)c
ELUMO (eV)d
EHOMO (eV)d
Td (ꢂC)e
Tg (ꢂC)f
PL
max
Sample
P1
P2
P3
P4
a
325
322
318
301
3.25
3.17
3.16
3.51
426
424
418
433
ꢁ0.37
ꢁ0.43
ꢁ0.53
ꢁ1.12
d
ꢁ4.03
ꢁ3.97
ꢁ3.87
ꢁ3.28
ꢁ7.28
ꢁ7.14
ꢁ7.03
ꢁ6.79
onset
346
379
353
337
194.6
193.3
187.1
104.8
The maximum absorption or emission wavelength.
Estimated from the absorption edge of the polymers.
Estimated from ELUMO ¼ ꢁ(E
þ 4.4) eV and EHOMO ¼ (ELUMO ꢁ
red
b
c
Egopt) eV).
e
Eonset is onset potential of polymer, measured in 0.1 M TBPA/CH3CN,
T
d measured under nitrogen at a heating rate of 10 ꢂC/min (5% loss weight).
Tg measured under nitrogen at a heating rate of 20 ꢂC/min.
f
scan rate 50 mV/s.
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ARTICLE
FIGURE 2 The UV-vis absorption and PL spectra of P1–P4 in
the film on quartz.
FIGURE 3 Cyclic voltammograms of M2, P2, and P4 recorded
in film (0.1 M TBPA/CH3CN, scan rate 50 mV/s).
(322 nm). The maximal emission wavelengths of P1–P4 are
listed in Table 1. It can be seen from Table 1 that, similar
with UV spectra, maximal photoluminescence peak wave-
length also decreases with increasing alkoxyl chain length.
energy level of P2 (ꢁ3.93 eV) is significantly lower than that
of P4 (ꢁ3.28 eV), hinting that the former has larger p-elec-
tron conjugation than the latter. It is also consistent with the
result of UV spectra.8,48 Similar results are observed in P1
and P3, that is, the directly incorporation of PA into oxadi-
zaole (Chart 2) not only provides a continuous conduction
path through the p-orbital overlapping along the polymer
backbone but also facilitate the generation of charge carriers
by either partial oxidation (i.e., p-doping) or partial reduction
(i.e., n-doping).8,43,48,49 Moreover, the HOMO energy levels
are also evaluated based on optical band gap from UV-vis
spectra and the LUMO energy level and the results are sum-
marized in Table 1. From Table 1, the HOMO energy levels of
all polymers are very low. It is favor to blocking the hole
transport.43
Electrochemical Properties
To order to match the energy band structural Scheme 2 of
PLED device, it is necessary to determine the energy levels
of the lowest unoccupied molecular orbital (LUMO) and the
highest occupied molecular orbital (HOMO) of each compo-
nent. Cyclic voltammetry was employed to investigate the re-
dox behavior of the polymer P1–P4 and estimate the HOMO
and LUMO energy levels of the polymers. As a comparison,
the redox behavior of M2 (the corresponding monomer of
P2) was investigated under the same conditions. The cyclic
voltammograms of P2, P4, and M2 are depicted in Figure 3
and energy level data of all polymers are summarized in Ta-
ble 1. During the cathodical scan, M2 exhibits two reduction
peaks at ꢁ1.33 and ꢁ2.28 V, respectively, and the onset
potential is at ꢁ1.10 V. However, P2 shows two irreversible
reduction peaks at ꢁ0.83 and ꢁ1.62 V, respectively, and
onset potential is at ꢁ0.43 V. The onset potentials of the n-
doping processes in a conjugated polymer can be utilized as
Application of P2 as ETM in a Bilayer
Red-Light-Emitting Device
Owing to the well solubility, higher thermal stability and
excellent electron injection ability or low LUMO energy level,
the bilayer OLED devices with a configuration of ITO/
PEDOT: PSS/PVK/DB:P2 (x:y wt/wt)/Ba/Al] using P2 with
directly linkage between the mainchain and the pendents as
ETM were prepared. To decrease the operating voltage and
smooth the surface roughness of the indium tin oxide (ITO)
a surrogate to estimate the LUMO energy levels5 according
to the empirical equation,6 ELUMO ¼ ꢁ(E
þ 4.4) eV,
onset
red
onset
red
where E
is the onset potentials for the reduction proc-
esses of a polymer. The LUMO energy level of P2 is thus esti-
mated to be ꢁ3.97 eV, which is lower by ꢁ0.75 eV than that
of the monomer M2 (ꢁ3.23 eV), hinting that P2 has well
electron injection property. The value is almost similar to
that of PPV-OXD (ꢁ3.93 eV) and its derivate (ꢁ3.98 eV).18
During the cathodical scan, however, the CV curve of P4
shows onset potential is at ꢁ1.12 V, the corresponding
LUMO energy level is at ꢁ3.28 eV. In addition, the LUMO
CHART 2
SCHEME 2 The synthesis routes of P1–P4.
OXADIAZOLE-CONTAINING POLYACETYLENES AS ETM, WANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 EQE as a function of current characteristics of ITO/
PEDOT:PSS/X/Ba/Al diodes (LED1: X ¼ PVK/DB, LED2: X ¼
PVK/DB:P2(90:10), LED3: X ¼ PVK/DB:P2(80:20)).
PEDOT: PSS/PVK/DB:P2 (90:10 wt/wt)/Ba/Al] (LED2) and
[ITO/PEDOT:PSS/PVK/DB:P2 (80:20 wt/wt)/Ba/Al] (LED3).
The film thickness of DB and P2 were controlled to be
80 nm. In the P2/ DB mixture, no phase separation was
observed up to the P2 fraction of 20 wt %.
The current density-voltage (a) and luminescence-voltage (b)
characteristics of LED1-LED3 are shown in Figure 4 and the
performances of devices are tabulated in Table 2. As can be
seen in Figure 4, the turn-on voltages of LED2 and LED3 are
about 4.1 and 4.5 V, respectively, which are significantly
lower than that of LED1 without P2 as electron-transporting
materials. The luminescence intensities of LED2 and LED3
also increase significantly with an increase in voltage [Fig.
4(b)]. Compared with LED1, the brightness increase drasti-
cally from 334 cd/cm2 to 827 cd/cm2 (LED2) and 1,036 cd/
m2 (LED3), respectively. Simultaneously, the higher EQE (Fig.
5) and lower turn-on voltages in LED2 and LED 3 were
FIGURE 4 Voltage-current (a) and voltage-luminance (b) curves
of ITO/PEDOT:PSS/X/Ba/Al diodes (LED1: X ¼ PVK/DB, LED2:
X ¼ PVK/DB:P2(90:10), LED3: X ¼ PVK/DB:P2(80:20)).
electrode, the hole injection-transport layer, poly(3,4-ethyle-
nedioxythiophene) doped with poly(styrenesulfonate)
(PEDOT/PSS), was spin-coated onto the surface-treatꢂed ITO
substrate and dried on a hot plate for 30 min at 110 C. The
emissive material DB is an organic red emitters33 and PVK is
a hole transporting/electron blocking layer. For comparison,
we firstly fabricated a bilayer LED1 without P2, ITO/
PEDOT:PSS/PVK/DB/Ba/Al, the operating voltage of EL
emission was observed over 5.7 V with the EQE of 0.77%
and the brightness of 334 cd/m2 due to unbalanced charge
injection and transport.50,51 For improving the emissive
properties of device, we used P2 as an mixed electron-trans-
porting materials to fabricate two bilayer devices, ITO/
TABLE 2 LEDs Properties of P2 Electron Transport Layer
Devices
kEmLax(nm)
Von(V)
Lmax (cd/m2)
EQE (%)
LED1
LED2
LED3
643
649
649
5.7
4.1
4.5
334
827
0.77
1.42
1.65
FIGURE 6 Normalized EL spectra of ITO/PEDOT:PSS/X/Ba/Al
diodes (LED1: X ¼ PVK/DB, LED2: X ¼ PVK/DB:P2(90:10), LED3:
X ¼ PVK/DB:P2(80:20)).
1,036
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ARTICLE
8 Chen, S. H.; Chen, Y. Macromolecules 2005, 38, 53–60.
observed, hinting that doping P2 effectively improve the per-
formance of devices.
9 Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765–768.
Figure 6 shows the EL spectra of all LEDs at wavelength
from 643 to 649 nm and all devices show deep red emission,
which correspond to the intrinsic electroluminescence of DB,
indicating that these ELs are mainly originated from the DB
emission. That is, P2 mainly plays as a role to improve elec-
tron injection and transport in the device.
10 Lee, T.; Landis, C. A.; Dhar, B. M.; Jung, B. J.; Sun, J.; Sar-
jeant, A.; Lee, H.-J.; Katz, H. E. J Am Chem Soc 2009, 131,
1692–1705.
11 Zhang, X.-B.; Tang, B.-C.; Zhang, P.; Li, M.; Tian, W.-J.
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CONCLUSIONS
13 Lee, R.-H.; Hsu, H.-F.; Chan, L.-H.; Chen, C.-T. Polymer 2006,
A
series of oxadiazole-containing polyacetylene were
47, 7001–7012.
designed and characterized, and their properties were inves-
tigated. The incorporation of oxadiazole into PA with conju-
gated main chain has endowed resulting polymer with novel
electron affinity (or low LUMO energy level) and high ther-
14 Cha, S. W.; Choi, S.-H.; Kim, K.; Jin, J.-I. J Mater Chem
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15 Sun, Y.-M. Polymer 2001, 43, 9495–9504.
ꢂ
mal properties (Td >330 C, 5% loss weight). Their thermal
16 Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Roth-
properties and electron affinity increases with the flexible
terminal alkoxyl chain length. The functional PAs directly
bearing oxadiazole (P2, ꢁ3.97 eV) possess better thermal
stability and lower LUMO energy level than that with a
spacer group (P4, ꢁ3.28 eV). P2 as ETM effectively improves
the performance of bilayer LEDs, ITO/PEDOT:PSS/PVK/
DB:P2/Ba/Al, which the external quantum efficiency (EQE)
and the maximum brightness of devices are enhanced, and
turn-on voltages decreases. The work may pave the way for
designing new ETM with high thermal stability and well
electron affinity.
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Financial support from the National Natural Science Fund of
China (Grant Nos. 90606011, 20974018 and 50472038), Ph.D.
Program Foundation of Ministry of Education of China (No.
20070255012), Shanghai Leading Academic Discipline Project
(No. B603) and Open Project of The State Key Laboratory of
Crystal Materials (KF0809), the Program of Introducing Talents
of Discipline to Universities (No. 111-2-04), Anhui Provincial
Natural Science Foundation (No. 090414190), and Anhui Pro-
vincial University Natural Science Foundation (No.
KJ2009A122).
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