Oxadiazole containing poly(p-phenylenevinylene)s: Synthesis and characterization

Article abstract of DOI:10.1039/c2nj40194k

A series of poly(p-phenylenevinylene) based polymers (MEH-OXD-PPVs) functionalized with Y-shaped double 1,3,4-oxadiazole-containing side chains were synthesized through a modified Gilch reaction. They are all soluble in common organic solvents such as chloroform, tetrahydronfuran and 1,1,2,2- tetrachloroethane. The chemical structures of MEH-OXD-PPVs were determined by ¹H NMR, GPC and elemental analysis. Thermogravimetric analysis shows that they have good thermal stability with the decomposition temperature ranging from 312°C to 326°C. The absorption and fluorescence emission maxima of the polymers exhibit an appreciable blue-shift with the increase of oxadiazole-containing moieties. Electrochemical investigation shows that the HOMO energy levels of the polymers vary regularly with the change of content of oxadiazole-containing moieties. Additionally, by introducing more OXD-PV unit during copolymerization, the PL quantum efficiencies of the polymers are significantly enhanced compared to that of MEH-PPV. The successful manipulation of optical and electronic properties of the MEH-OXD-PPVs indicates an effective concept for developing PPV-based functional materials with tunable optoelectronic properties by changing the amount of electron-withdrawing oxadiazole-containing moieties.

Full text of DOI:10.1039/c2nj40194k

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PAPER
Oxadiazole containing poly(p-phenylenevinylene)s: synthesis and
characterizationw
Pengfei Li,z Shanpeng Wen,z Weidong Cheng, Jibo Zhang, Shiyu Yao, Bin Xu and
Wenjing Tian*
Received (in Montpellier, France) 13th March 2012, Accepted 28th May 2012
DOI: 10.1039/c2nj40194k
A series of poly(p-phenylenevinylene) based polymers (MEH-OXD-PPVs) functionalized with
Y-shaped double 1,3,4-oxadiazole-containing side chains were synthesized through a modified
Gilch reaction. They are all soluble in common organic solvents such as chloroform,
tetrahydronfuran and 1,1,2,2-tetrachloroethane. The chemical structures of MEH-OXD-PPVs
1
were determined by H NMR, GPC and elemental analysis. Thermogravimetric analysis shows
that they have good thermal stability with the decomposition temperature ranging from 312 1C to
326 1C. The absorption and fluorescence emission maxima of the polymers exhibit an appreciable
blue-shift with the increase of oxadiazole-containing moieties. Electrochemical investigation shows
that the HOMO energy levels of the polymers vary regularly with the change of content of
oxadiazole-containing moieties. Additionally, by introducing more OXD-PV unit during
copolymerization, the PL quantum efficiencies of the polymers are significantly enhanced
compared to that of MEH-PPV. The successful manipulation of optical and electronic properties
of the MEH-OXD-PPVs indicates an effective concept for developing PPV-based functional
materials with tunable optoelectronic properties by changing the amount of electron-withdrawing
oxadiazole-containing moieties.
However, most of the PPVs have good hole-transport properties
Introduction
but poor electron-transport abilities with the hole mobility being
about one to three orders of magnitude higher than the electron
mobility,14 which limits their applications as active materials in
polymer LEDs and PVCs.15–21 To overcome this drawback, one
of the most widely employed methods is to enhance the electron
affinity of the conjugated polymers by introducing electron-
withdrawing groups, such as the 1,3,4-oxadiazole unit.22
Conjugated polymers have excellent optoelectronic properties
which are currently being employed in various applications,
such as light-emitting diodes (LEDs), photovoltaic cells
(PVCs), thin film transistors (TFTs), and solid-state lasers1–10
due to their feasible design of the molecular structures and
easy modification of their photophysical properties.11 Besides,
conjugated polymers also provide many merits over inorganic
materials such as low cost, easy processability, ease of forming
large areas, and mechanical flexibility.12
The 1,3,4-oxadiazole unit is a very popular chromophore in
organic small molecules and polymers for optoelectronic devices,
notably for organic LEDs and PVCs.12,23 The electron-deficient
nature of oxadiazole can help to improve the electron mobility
when incorporated into organic optoelectronic materials. In
addition, the oxadiazole moiety is known to provide enhanced
thermal stability, redox stability, and good film-forming proper-
ties.24,25 Therefore, enormous efforts have been devoted to
incorporate oxadiazole moieties into polymers either as polymer
backbones26–29 or as side chains14,22–24 in order to develop new
materials with improved optical and electronic properties. In
comparison with main chain (backbone) modifications, side chain
modifications have less impact on the hole-transport properties of
the backbones; namely the main chain PPV does retain its own high
hole mobility.32 On the other hand, many efforts have been devoted
to investigate the effect of oxadiazole concentration on the photo-
physical and electrochemical properties of polymers. Chen and
co-workers once reported the influence of backbone-incorporated
Among various conjugated polymers, poly(p-phenylene vinylene)
(PPV) and its derivatives have attracted tremendous attention due
to their excellent luminescent and mechanical properties. Their ease
of processing, long device lifetime as well as amenability to chemical
modifications offer additional reasons for further investigation
of this class of materials.13 For example, by proper modification
of the chemical structure, the fluorescence of PPVs can span
over the visible range and further into the near-infrared region.
State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, Changchun 130012, P. R. China.
E-mail: wjtian@jlu.edu.cn; Fax: +86431 85193421;
Tel: +86431 85166368
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj40194k
z These authors contributed equally to this work.
c
1626 New J. Chem., 2012, 36, 1626–1633
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oxadiazole content on the photophysical and electrochemical
properties of the resulting copolymers.13 Being connected as
part of the side chains, the content of the oxadiazole groups
were also adjusted to tailor the chemical structures of the
target copolymers to optimize their optical and electronic
properties.22,23 The above efforts have achieved some results
in optimizing the polymer’s photophysical properties, but they
failed in tuning energy levels rationally; i.e., they cannot simulta-
neously regulate photophysical and electrochemical properties
by adjusting oxadiazole contents due to the linear pattern of
used oxadiazole-containing side chains. Each linear side chain
possesses only one oxadiazole unit, so it may not induce a
sufficient electron-withdrawing ability that is strong enough to
influence the energy levels of the polymer.
were purchased from Beijing Chemical Reagent. Tetrahydro-
furan (THF) was dried over sodium and benzophenone.
Diethyl 5-(3-bromopropoxy)-isophthalate, 1,4-bis(bromomethyl)-
2-methoxy-5-(2-ethyl-hexy-loxy)benzene (monomer 7) and poly(1-
methoxy-4-(2-ethylhexyloxy)-p-phenylene vinylene) MEH-PPV
were synthesized according to the literature.12,30,31
Measurements
1H NMR spectra were recorded on Bruker AVANCE 500 MHz
spectrometer with chloroform-d as solvent and tetramethylsilane
(TMS) as internal standard. The elemental analysis was carried
out with a Thermoquest CHNS-Ovelemental analyzer. The gel
permeation chromatographic (GPC) analysis was carried out
with a Waters 410 instrument with tetrahydrofuran as the eluent
(flow rate: 1 mL minꢀ1, at 35 1C) and polystyrene as the
standard. The thermogravimetric analysis (TGA) was performed
on a Perkine Elmer Pyris 1 analyzer under nitrogen atmosphere
(100 mL minꢀ1) at a heating rate of 10 1C minꢀ1. UV-visible
absorption spectra were measured using a Shimadzu UV-3100
spectrophotometer. Photoluminescence spectra were collected on
a Shimadzu RF-5301PC spectrophotometer and Maya 2000 Pro.
optical fiber spectrophotometer. The PL quantum efficiencies
were determined in an integrating sphere (C-701, Labsphere
Inc.), with a 405 nm Ocean Optics LLS-LED as the excitation
source, and the laser was introduced into the sphere through the
optical fiber. Electrochemical measurements of these derivatives
were performed with a Bioanalytical Systems BAS 100 B/W
electrochemical workstation.
In this work, we present the synthesis of a series of new soluble
PPV-based polymers with different mole ratios between two
repeating units, {[2-(3-{3,5-bis[5-(4-dodecyloxy)phenyl]-1,3,4,-
oxadiazole-2-yl}phenoxy)-hexyloxy-1,4-phenylenevinylene] (OXD-
PV) and [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyl enevinylene]
(MEH-PV). Instead of the commonly used linear side chain, a
two-dimensional side chain is adopted in the target polymers. Being
located at the end of flexible alkoxyl chain, phenyl ring possesses
two branches in the meta positions. Each branch comprises three
parts: a oxadiazole unit as the head, a phenylene spacer as the
body, and a long flexible alkoxyl end chain as the tail. The whole
two-dimensional side chain looks like a ‘‘Y’’, so it is named a
Y-shaped side chain. The structural design is based on the
consideration that the PPV backbone is a good hole-transporting
material and the flexible alkoxyl chains provide solubility.
Compared with the linear side chain that has one electron-deficient
unit, each Y-shaped side chain contains two electron-deficient
1,3,4-oxadiazole units to provide a much enhanced electron-
withdrawing ability. Through such a design, the Y-shaped side
chain is expected to act more effectively in tuning the polymer’s
optical and electronic properties. Besides, the combination of
branched alkoxyl chains with heterocyclic rings causes large steric
hindrance, which would play an important role in influencing
intermolecular interactions, thus in tailoring the photophysical
properties of the target polymers. MEH-OXD-PPVs were
synthesized by a modified Gilch polymerization method12,23 and
Synthesis of monomer
2-(6-Bromohexyloxy)-1,4-dimethylbenzene (1). 2,5-Dimethyl-
phenol (6.10 g, 0.05 mol), 46 mL of 1,6-dibromo- hexane
(0.30 mol), 30 g of anhydrous potassium carbonate, 0.3 g of
KI, and 200 mL of dry acetone were refluxed for 24 h under
N2 atmosphere. The reaction mixture was cooled to room
temperature, then it was filtered to remove the salts and
concentrated under vacuum. The crude product was purified
by column chromatography (hexane/ethyl acetate = 20 : 1) to
1
give 11.16 g (90%) of a yellowy liquid. H NMR (500 MHz,
1
CDCl3) (ppm, from TMS): 7.00(d, 1H), 6.66(d, 1H), 6.63(s, 1H),
3.95(t, 2H), 3.43(t, 2H), 2.31(s, 3H), 2.17(s, 3H), 1.90–1.89(m, 2H),
1.82–1.80(m, 2H), 1.54–1.52(m, 4H).
characterized by H NMR, elemental analysis and GPC. Their
photophysical and electrochemical properties were systematically
described. As expected, the resulting polymers indeed exhibit good
solubility in chloroform, tetrahydronfuran and 1,1,2,2-tetrachloro-
ethane, as well as fine-tuned photophysical and electrochemical
properties by adjusting the amount of the OXD-PV unit.
Additionally, they possess high molecular weights (between
13 300 and 653 000) and good thermal stabilities. The successful
regulation of optoelectronic properties of the MEH-OXD-PPVs
leads to a clear understanding, at a molecular level, of the
structure–property relationships in this series of materials.
Diethyl 5-(6-(2,5-dimethylphenoxy)hexyloxy)isophthalate (2).
Compound 1 (5.68 g, 0.02 mol), 4.76 g of diethyl 5-hydroxy-
isophthalate (0.02 mol), 30 g of anhydrous potassium carbonate,
0.3 g of KI, and 200 mL of dry acetone were refluxed for 24 h
under N2 atmosphere. After being cooled to room temperature,
the reaction mixture was filtered to remove the salts. The
solvent was evaporated under vacuum and then the residue
was poured into 200 mL of methanol under stirring. A white
solid precipitated out, which was collected by filtration to give
1
Experimental section
7.15 g (80.8%) of compound 2. H NMR (500 MHz, CDCl3)
(ppm, from TMS): 8.27(s, 1H), 7.74(s, 2H), 7.00(d, 1H),
6.66–6.63(m, 2H), 4.42–4.37(m, 4H), 4.06(t, 2H), 3.96(t, 2H),
2.31(s, 3H), 2.17(s, 3H), 1.85–1.84(m, 4H), 1.60–1.57(m, 4H),
1.41(t, 6H). Anal. calcd for C26H34O6: C, 70.56; H, 7.74.
Found: C, 70.49; H, 7.79.
Materials
N-Bromosuccinimide, 5-hydroxyisophthalate, potassium tert-
butoxide (t-BuOK) were purchased from Acros. 1,3-Dibromo-
propane, p-hydroxyanisole, POCl3 and all the other solvents
c
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5-(6-(2,5-Dimethylphenoxy)hexyloxy)isophthalohydrazide (3).
Compound 2 (3.10 g, 0.007 mol), 30 mL (excess) of hydrazine
monohydrate, and 60 mL of methanol were heated under reflux
for 48 h. After being cooled to room temperature, the reaction
mixture was recrystallized from ethanol. A white solid product
was obtained with a total amount of 2.32 g (yield 80.1%).
1H NMR (500 MHz, CDCl3) (ppm, from TMS): 9.77(s, 2H),
7.85(s, 1H), 7.46(s, 2H), 6.98(d, 1H), 6.72(s, 1H), 6.62(d, 1H),
4.50(s, 4H), 4.06 (t, 2H), 3.95(t, 2H), 2.24(s, 3H), 2.08(s, 3H),
1.77–1.75(m, 4H), 1.51–1.49(m, 4H). Anal. calcd for
C22H30N4O4: C, 63.75; H, 7.30; N, 13.52. Found: C, 63.70; H,
7.29; N, 13.45.
Polymerization. All the polymerization reactions and mani-
pulations were carried out under nitrogen atmosphere. The
preparation of polymer 8a–8g was conducted using the
modified Gilch method.12,23 The reaction conditions for
the preparation of Polymer 8b–8g were kept the same as that
of polymer 8a.
Polymer 8a (OXD-MEH-PPV(100/0)). A solution of 6 mL
of potassium tert-butoxide (1.0 M THF solution, 6 mmol) was
slowly added over 1 h to a stirred solution of monomer 6
(0.56 g, 0.5 mmol) in 25 mL of dry THF that was cooled to
ꢀ5 1C. Under a N2 atmosphere, the mixture was stirred at
room temperature for 24 h. The polymerization solution was
poured into 500 mL of MeOH. A crude polymer precipitated
out, which was redissolved in chloroform and reprecipitated
into methanol. Then the residue was extracted with methanol
for 24 h in a Soxhlet apparatus to remove the impurities and
oligomers. The insoluble fraction was then collected via
extraction with CHCl3 for 24 h. The chloroform solution is
then concentrated to provide 0.34 g (71% yield) of orange
5,50-(5-(6-(2,5-Dimethylphenoxy)hexyloxy)-1,3-phenylene)-
bis(2-(4-(dodecyloxy)phenyl)-1,3,4-oxadiazole) (5). Compound
3 (2.90 g, 0.007 mol), 5 g (excess) of fresh 4-dodecyloxy-
benzoyl chloride, 1.08 g of pyridine and 40 mL of THF were
stirred at 0–5 1C for 4 h. The solvent was removed under
reduced pressure to give a white sticky matter (compound 4).
The obtained crude product was heated slowly for 30 min in
50 mL of POCl3 under N2 atmosphere. The mixture was
refluxed for 8 h. After about 30 mL of POCl3 was distilled
off, the reaction mixture was cooled to room temperature and
then poured into 300 mL of ice-water under stirring. After
being extracted with chloroform (3 ꢁ 100 mL), the combined
organic extracts were washed with water and NaHCO3 (aq),
dried over anhydrous MgSO4. After removal of the solvent,
the crude product was poured into a large amount of methanol.
A yellow precipitate was collected by filtration and then it was
recrystallized from methanol/chloroform. Two step yield 53%.
1H NMR (500 MHz, CDCl3) (ppm, from TMS): 8.40(s, 1H),
8.11(d, 4H), 7.81(s, 2H), 7.04(d, 4H), 7.00(d, 1H), 6.66–6.64(m,
2H), 4.17(t, 2H), 4.05(t, 4H), 3.98(t, 2H), 2.31(s, 3H), 2.18(s,
3H), 1.93–1.80(m, 10H), 1.63–1.61(m, 4H), 1.52–1.46(m, 4H),
1.38–1.27(m, 30H), 0.89(t, 6H). Anal. calcd for C68H82N4O6: C,
75.43; H, 8.65; N, 5.86. Found: C, 75.41; H, 8.70; N, 5.45.
1
precipitate. H NMR (500 MHz, CDCl3), (ppm, from TMS):
7.95–7.94(br, Ar–H), 7.63(br, Ar–H), 6.91(br, Ar–H and vinyl
proton), 5.96(m, Ar–H), 4.11–3.49(m, –OCH2– and –OCH3),
1.94–1.25(m, –CH2–), 1.01–0.86(m, –CH3). (C60H78N4O6)n
Elem. Anal. calcd: C, 75.75; H, 8.26; N, 5.89; Found: C,
75.66; H, 8.21; N, 5.80.
Polymer 8b (OXD-MEH-PPV(80/20)). The feed ratio
between monomer 6 and monomer 7 is 80 : 20. The resulting
copolymer was obtained as a brown powder with a yield of
65%. 1H NMR (500 MHz, CDCl3), (ppm, from TMS):
7.95–7.94(br, Ar–H), 7.63(br, Ar–H), 7.05–6.86(m, Ar–H and
vinyl proton), 5.96(m, Ar–H), 4.12–3.79(m, –OCH2– and
–OCH3), 0.97(m, –CH3). [(C60H78N4O6)0.80(C17H24O2)0.20]n
Elem. Anal. calcd: C, 75.33; H, 8.37; N, 4.56; Found: C,
75.31; H, 8.41; N, 4.51.
Polymer 8c (OXD-MEH-PPV(67/33)). The feed ratio
between monomer 6 and monomer 7 is 67 : 33. The resulting
copolymer was obtained as a red powder with a yield of 54%.
1H NMR (500 MHz, CDCl3), (ppm, from TMS): 8.02–7.96(br,
Ar–H), 7.66–7.63(br, Ar–H), 6.93–6.92(br, Ar–H and vinyl
proton), 5.96(s, Ar–H), 4.13–3.92(m, –OCH2– and –OCH3),
1,3-Bis[2-(4-(dodecyloxy)phenyl)-1,3,4-oxidiazole-5-yl]-5-[3-(2,5-
dibromomethyl)phenoxyhexyloxy]-benzene (6) (monomer 6). To a
250 mL three-neck flask, compound 5 (11.07 g, 11.6 mmol),
N-bromosuccinimide (NBS) (4.6 g, 25.6 mmol), catalytic amounts
of benzoyl peroxide (BPO), and 100 mL of benzene were added.
The mixture was heated under reflux and stirred until succinimide
was on top of the solution. After the disappearance of the
compound 5, confirmed by TLC, the mixture was cooled to
room temperature. The succinimide was filtered off, and the
mother liquor was poured into excess water. The mixture was
extracted with dichloromethane, and the combined organic
layers were washed several times further with water, dried over
anhydrous MgSO4. After filtration, the solvent was removed by
evaporation under vacuum. The product was purified by column
chromatography on silica gel using hexane to give 5.15 g (yield
40%) of Monomer 6. 1H NMR (500 MHz, CDCl3) (ppm, from
TMS): 8.40(s, 1H), 8.11(d, 4H), 7.83(s, 2H), 7.281(s, 1H), 7.05
(d, 4H), 6.93–6.90(m, 2H), 4.54(s, 2H), 4.45(s, 2H), 4.19(t, 2H),
4.09(t, 2H), 4.05(t, 4H), 1.95–1.92(m, 4H), 1.86–1.80(m, 4H),
1.67–1.66(m, 6H), 1.52–1.46(m, 4H), 1.39–1.27(m, 34H), 0.89
(t, 6H). Anal. calcd for C60H80Br2N4O6: C, 64.74; H, 7.24; Br,
14.36; N, 5.03. Found: C, 64.70; H, 7.29; Br, 14.31; N, 5.09.
2.96–1.25(m, –CH2–), 1.02–0.87(m, –CH3). [(C60H78N4O6)0.67
-
(C17H24O2)0.33]n Elem. Anal. calcd: C, 75.33; H, 8.37; N, 4.56;
Found: C, 75.32; H, 8.39; N, 4.58.
Polymer 8d (OXD-MEH-PPV(50/50)). The feed ratio
between monomer 6 and monomer 7 is 50 : 50. The resulting
copolymer was obtained as a red powder with a yield of 51%.
1H NMR (500 MHz, CDCl3), (ppm, from TMS): 7.98–7.97(br,
Ar–H), 7.64–7.62(br, Ar–H), 7.05–7.02(br, Ar–H and vinyl proton),
6.93–6.92(br, Ar–H), 4.14–3.94(m, –OCH2– and –OCH3),
1.95–1.25(m, –CH2–), 1.01–0.87(m, –CH3). [(C60H78N4O6)0.50
(C17H24O2)0.50 n Elem. Anal. calcd: C, 75.33; H, 8.37; N, 4.56;
Found: C, 75.35; H, 8.37; N, 4.48.
-
]
Polymer 8e (OXD-MEH-PPV(33/67)). The feed ratio between
monomer 6 and monomer 7 is 33 : 67. The resulting copolymer
1
was obtained as a red powder with a yield of 52%. H NMR
(500 MHz, CDCl3), (ppm, from TMS): 8.02–7.69(br, Ar–H),
c
1628 New J. Chem., 2012, 36, 1626–1633
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7.47–7.44(br, Ar–H), 6.97–6.96(br, Ar–H and vinyl proton),
5.96(s, Ar–H), 4.15–3.94(m, –OCH2– and –OCH3), 1.95–1.23(m,
protons peaked at about 2.31 ppm and 2.18 ppm disappeared
after bromination and two new peaks appeared at about 4.54
and 4.45 ppm, which confirmed the success of bromination. The
purity of the monomers and the intermediate compounds were
–CH2–), 1.01–0.87(m, –CH3). [(C60H78N4O6)0.33(C17H24O2)0.67]n
Elem. Anal. calcd: C, 75.33; H, 8.37; N, 4.56; Found: C, 75.35; H,
8.31; N, 4.52.
1
confirmed by H NMR spectroscopy and elemental analysis.
Polymerization of monomer 6 was performed with an excess of
potassium tert-butoxide in dry THF under nitrogen atmosphere at
room temperature according to the standard Gilch method
(Scheme 2). To tune the electro-optical properties of OXD-PPV,
we copolymerized monomer 6, with various feed ratios of
monomer 7, in THF. During the polymerization, the viscosity of
the reaction mixture was increased without any precipitation, and
intense fluorescent light was observed. In this work, the resulting
polymers were completely soluble in common organic solvents,
such as chloroform, tetrahydronfuran, and 1,1,2,2-tetrachloro-
ethane. The good solubility of the OXD-MEH-PPVs is attributed
to the easy free rotation between the PPV backbone and oxygen
atom-linked bulky side chains.
Polymer 8f (OXD-MEH-PPV(20/80)). The feed ratio
between monomer 6 and monomer 7 is 20 : 80. The resulting
copolymer was obtained as a dark red powder with a yield of 53%.
1H NMR (500 MHz, CDCl3), (ppm, from TMS): 8.08–8.04(br,
Ar–H), 7.54–7.49(br, Ar–H), 7.20–6.98(m, Ar–H and vinyl proton),
5.96(s, Ar–H), 4.17–3.94(m, –OCH2– and –OCH3), 1.83–1.25(m,
–CH2–), 1.01–0.88(m, –CH3). [(C60H78N4O6)0.20(C17H24O2)0.80]n
Elem. Anal. calcd: C, 75.33; H, 8.37; N, 4.56; Found: C,
75.34; H, 8.32; N, 4.52.
Results and discussion
Synthesis and characterization
Molecular weights and thermal properties
The synthetic routes for the 1,3,4-oxadiazole containing
monomer 6, corresponding polymers as well as copolymers
with various ratios of OXD-PV and MEH-PV are shown in
Scheme 1. Monomer 6 was synthesized in the following five
steps. Diethyl 5-hydroxyisophthalate, 1, was synthesized from
2,5-dimethylphenol by the reacting with 1,6-dibromo-hexane
and potassium iodide in the presence of anhydrous potassium
carbonate in acetone. Followed by the reaction in the mixture
of 2,5-dimethyl-phenol, anhydrous potassium carbonate,
potassium iodide and dry acetone, compound 2 was synthesized.
After being treated with hydrazine monohydrate, the hydrazide,
compound 3 was achieved. Condensation of hydrazide, 3, with
4-dodecyloxy-benzoyl chloride yielded compound 4, which is
sufficiently pure for next step reaction. The result shows
this method was feasible and the yield of compound 5 (56%
for two steps) was considerable. After the bromination reaction
with NBS, the newly designed monomer, 6 was obtained. The
synthetic strategy of incorporating the oxadiazole-containing
moiety as an ether linkage is trying to improve the solubility of
the target polymer. In the co-monomer 7, the bis(bromomethyl)-
benzene backbone also contains two oxygen atom-linked side
chains (a 2-ethyl-hexoxyl group and a methoxyl group) to
provide a molecular similarity between the two monomers. In
1H NMR spectra of monomer 6, as shown in Fig. 1, benzyl
The molecular weights of the resulting copolymers are depicted
in the Table 1. Using THF as the eluent and polystyrene as the
standards, the GPC data exhibit a number-average (Mn) mole-
cular weight from 8500 to 136 000 and a weight-average (Mw)
molecular weight from 13 300 to 653000, respectively. The
polydispersity index (PDI) is from 1.55 to 4.86.
The thermal stability of polymer is important for device
longevity. Fig. 2 displays the TGA chart of polymers 8a–8g.
The TGA analysis reveals that, a relatively high thermal
stability with an initial weight loss (5%) temperature of 312 1C,
extending to 326 1C. It is interesting to observe that the thermal
stabilities of the polymers are progressively improved with the
increasing amount of oxadiazole-containing moieties.
Optical properties
1. Ultraviolet-visible (UV-vis) spectroscopies of polymers
8a–8g. The UV-vis absorption properties of polymers 8a–8g
Fig. 1 1H NMR spectrum and chemical structure of the monomer 6
Scheme 1 Synthesis of monomer 6.
in CDCl3 solution.
c
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Table 2 Optical data of polymers 8a–8g
In solutiona In filmb
lambasx/nm lmemax/nm lambasx/nm lemmax/nm Egopt/eV FPL
Opticalc
Polymer
8a (100/0) 453/305 516
8b (80/20) 463/304 522
8c (67/33) 470/305 532
8d (50/50) 478/403 538
8e (33/67) 479/304 547
8f (20/80) 486/304 550
8g (0/100) 499/332 554
452/313 535
468/306 548
489/309 564
489/303 564
497/315 573
505/314 574
2.31
2.28
2.21
2.19
2.17
2.16
2.12
0.26
0.18
0.21
0.21
0.22
0.12
0.10
510
585
a
b
1 ꢁ 10ꢀ5 M in anhydrous chloroform. Spin-coated from a 5 mg mLꢀ1
chloroform solution. The optical band gap (Eogpt) was obtained from
absorption edge.
c
Scheme 2 Synthesis of copolymers.
Table 1 Polymerization results and thermal properties of polymers
8a–8g
Feed ratio Mn (104) Mw (104) PDI Tdb (1C)
a
a
Polymer
8a (100/0) 100 : 0
8b (80/20) 80 : 20
8c (67/33) 67 : 33
8d (50/50) 50 : 50
8e (33/67) 33 : 67
13.6
0.85
2.39
12.6
12.5
7.5
60.1
1.33
10.6
61.4
37.4
12.3
65.3
4.41 326
1.55 319
4.44 318
4.86 317
2.99 315
1.63 312
3.30 303
8f (20/80) 20 : 80
8g (0/100) 0 : 100
19.7
a
Calculated from GPC (eluent : THF; polystyrene standards). b Tempe-
rature at 5% weight loss by a heating rate of 10 1C minꢀ1 under nitrogen.
are presented in Table 2. Fig. 3 shows the normalized UV-vis
absorption of the copolymers in CHCl3 solution and film spin-
coated from CHCl3 solution.
The absorption spectra of polymers 8a–8f in the solutions of
CHCl3 display two absorption bands. The long wavelength
peak in the region of 400–550 nm originates from the p–p*
absorption of the conjugated polymer main chain, whereas the
short wavelength peak around 304 nm is attributed to the
aromatic side chain.12,32 It is interesting to observe that, with
the increasing amount of 1,3,4-oxadiazole moieties, the inten-
sity of the absorption at 453–486 nm decreased gradually while
that at around 304 nm increased gradually. Compared with
the absorption peak of MEH-PPV at 499 nm, the polymers
exhibit blue-shifted peaks, and the degree of the blue-shift
increases with the increasing amount of 1,3,4-oxadiazole
moieties. We believe the blue-shift arises from the reduced
Fig. 3 UV-vis absorption spectra of polymers 8a–8g in (a) CHCl3
solution and (b) films.
content of electron-donating oxygen atom attached to the
phenylene ring in the main chain and also due to the reduced
coplanarity of the polymer main chain.23,24 The bulky substi-
tuent in polymers 8a–8f causes the steric hindrance and will
make the phenylene ring or vinylene unit twist; therefore, the
p-conjugation will be interrupted.24 The absorbance spectra of
the copolymers in solid thin films are similar to those of the
copolymers in CHCl3 solutions, except for the red-shift of the
longer wavelength peaks, which suggests more efficient p-stacking
and stronger intermolecular interactions in the film.12 The distin-
guishable absorption features of polymers 8a–8g demonstrate that
OXD-PV unit was successfully incorporated into the PPV-based
polymer backbone with a pre-defined feed ratio.
Fig. 2 TGA thermogram of polymers 8a–8g.
c
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LUMO energy levels and the band gap of a polymer.34 Fig. 5
2. Photoluminescent (PL) spectroscopies of polymers 8a–8g.
Fig. 4 outlines the photoluminescence spectra of polymers
8a–8g in both CHCl3 solution and film spin-coated from CHCl3
solution. The characteristic values are listed in Table 2.
shows the cyclic voltammograms of the polymers 8a–8g, which
were performed at room temperature in anhydrous acetonitrile,
using a platinum disk as the working electrode, a platinum wire as
the counter electrode, an Ag/AgNO3 as the reference electrode, and
The emission spectra are quite similar for the solution and film
samples with one main emission peak. Interestingly, as OXD-PV
content is increased, the wavelength of the maximum emission
peak of polymers 8a–8f exhibits a gradually blue-shifted trend in
both solution and film, which is attributed to the reduced amount
of electron-donating oxygen atom attached to the backbone and
the increased amount of bulky substituents.23,24
tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 mol Lꢀ1
)
as the supporting electrolyte. Ferrocene/ ferrocenim (Fc/Fc+)
was used as the internal reference and showed a peak
at +0.05 V vs. Ag/Ag+.
onset
From the onset oxidation potentials (E
ox
) and the onset
onset
red
reduction potentials (E
) of the polymers, HOMO and
In Table 2, we listed the PL quantum efficiencies (FPL) of the
solid film of the polymers, which were measured under a 405 nm
Ocean Optics LLS-LED in the integrating sphere. The PL quantum
efficiencies of the OXD-MEH-PPVs range from 12% to 26%,
which are higher than that of MEH-PPV (10%), indicating that the
OXD-PV unit can greatly enhance the absolute PL quantum
efficiencies of the copolymers. With the OXD-PV content of
100%, the highest FPL value of 26% is achieved for polymer 8a
(OXD-PPV). With the incorporation of an OXD-PV moiety, the
PL quantum efficiencies of the polymers were increased, indicating
that the long, flexible side chains can dilute the main chains like a
solvent, causing a reduction in the aggregation of conjugational
segments, and thus an increase in PL quantum efficiency.14,33
LUMO energy levels as well as the energy gap were calculated
according to the following equations:35–38
onset
ox
HOMO (eV) = ꢀe(E
LUMO (eV) = ꢀe(E
+ 4.75)
onset
red
+ 4.75)
onset
Eegc (eV) = e(E
ox
onset
– E
red
)
onset
ox
Ag/Ag+.
onset
red
where E
and E
are the measured potentials relative to
The electrochemical properties of the copolymers are listed
in Table 3 and their band diagram is shown in Fig. 6.
Obviously, as the 1,3,4-oxadiazole units in the polymers
increased, their HOMO energy levels gradually decreased.
Similarly, their LUMO energy levels showed a decreasing
tendency after introducing more electron-withdrawing
pendant groups. Regarding that the threshold HOMO level
for air stable conjugated polymers is estimated to be ꢀ5.2 eV,39 it
is concluded that attaching 1,3,4-oxadiazole units to polymers can
effectively lower their HOMO levels, thus enhancing their chemical
stability in ambient conditions. The optical band gaps estimated
from absorption onset of the copolymers are significantly less than
the electrochemical data.
Electrochemical properties
Cyclic voltammogram (CV) is very helpful in measuring
electrochemical behaviors and evaluating the relative HOMO,
Mobility
To investigate the influence of Y-shaped side chain on the
PPVs’ charge-transport abilities, we estimated the hole mobility
and electron mobility of polymer 8a (OXD-MEH-PPV (100: 0))
by the space-charge-limited-current (SCLC) method.40–42
The J–V curves of the ITO/polymer 8a/LiF/Al device and the
Fig. 5 Cyclic voltammograms of the films of polymers 8a–8g cast on
a platinum electrode in 0.1 M TBAPF6 in acetonitrile at a 50 mV sꢀ1
scan rate.
Fig. 4 Normalized photoluminescence spectra of polymers 8a–8g in
(a) CHCl3 solution and (b) films.
c
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New J. Chem., 2012, 36, 1626–1633 1631

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Table 3 Electrochemical data of polymers 8a–8g
OXD-PV units, OXD-MEH-PPVs also exhibit enhanced PL
quantum efficiencies, compared with that of MEH-PPV.
Moreover, as demonstrated from the investigation of electro-
chemical properties, the HOMO and LUMO levels of the
polymers exhibit a decreasing tendency after incorporating
more oxadiazole-containing moieties. These results prove that
the design of a Y-shaped, double 1,3,4-oxadiazole-containing,
bulky side chain is very successful in tailoring the photophysical
and electrochemical properties of PPV-based polymers, which
provides a better understanding on how the chemical structures
affect polymer’s optical and electronic properties. High molecular
weights along with good thermal stability, good air-stability,
adjustable spectral properties and energy levels indicate that
OXD-MEH-PPVs are promising materials for optoelectronic
applications.
onset
onset
Polymer
E
ox
/eV
E
/eV HOMO/eV LUMO/eV Eegc/eV
red
8a (100/0) 0.65
8b (80/20) 0.44
8c (67/33) 0.41
8d (50/50) 0.40
8e (33/67) 0.39
8f (20/80) 0.37
8g (0/100) 0.36
ꢀ1.97
ꢀ2.05
ꢀ2.07
ꢀ2.07
ꢀ2.08
ꢀ2.06
ꢀ2.10
ꢀ5.40
ꢀ5.19
ꢀ5.16
ꢀ5.16
ꢀ5.14
ꢀ5.12
ꢀ5.11
ꢀ2.78
ꢀ2.70
ꢀ2.68
ꢀ2.68
ꢀ2.67
ꢀ2.69
ꢀ2.65
2.62
2.49
2.48
2.47
2.47
2.43
2.46
Acknowledgements
This work was supported by the State Key Development Program
for Basic Research of China (grant no. 2009CB623605), the
National Natural Science Foundation of China (grant no.
20874035), the 111 Project (grant no. B06009), and the Project
of Jilin Province (20080305). The authors thank Yajun Gao for
useful suggestions and discussions.
Fig. 6 Band diagram of polymers 8a–8g.
ITO/PEDOT:PSS/polymer 8a/Au device in the dark are
shown in the Supplementary Information.w
The electron mobility and the hole mobility of polymer 8a
Notes and references
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