Investigations on the electrochemical properties of new conjugated polymers containing benzo[c]cinnoline and oxadiazole moieties

Article abstract of DOI:10.1016/j.polymer.2011.10.059

A four-step route was designed to synthesize 3,8-benzo[c]cinnoline dicarboxylic acid (4). New conjugated polymers, POXD (I) and POXD (T), containing benzo[c]cinnoline and oxadiazole moieties, were obtained by thermal cyclodehydration of their soluble polyhydrazide precursors PHA (I) and PHA (T), respectively. Two reduction peaks were observed for these new conjugated polymers during CV cathodic scan. From the CV voltammograms combined with the results from molecular simulation, we concluded that the first reduction occurred at oxadiazole moiety and benzo[c]cinnoline moiety was responsible for the second reduction. It indicates that oxadiazole has stronger electron affinity than benzo[c]cinnoline. We proposed a mechanism to explain this two-stage reduction process. Due to the planar and electron-accepting ability of benzo[c]cinnoline and oxadiazole moieties, POXD (I) and POXD (T) exhibited very low LUMO (-3.42 and -3.45 eV) and HOMO (-6.23 and -6.27 eV) energy levels. They can be used as hole-blocking or electron-injection layers for OLED applications.

Full text of DOI:10.1016/j.polymer.2011.10.059

Polymer 52 (2011) 6011e6019
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Investigations on the electrochemical properties of new conjugated polymers
containing benzo[c]cinnoline and oxadiazole moieties
,
a
a
a
a
b
c
Jyh-Chien Chen a , Hsin-Chung Wu , Chi-Jui Chiang , Li-Chia Peng , Tuo Chen , Li Xing , Shun-Wei Liu
*
a Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan
b Pfizer Worldwide Research and Development, 200 Cambridge Park Drive, Cambridge, MA 02140, USA
c Department of Electronic Engineering, Mingchi University of Technology, No. 84, Gongzhuan Rd., New Taipei City 24301, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A four-step route was designed to synthesize 3,8-benzo[c]cinnoline dicarboxylic acid (4). New conju-
gated polymers, POXD (I) and POXD (T), containing benzo[c]cinnoline and oxadiazole moieties, were
obtained by thermal cyclodehydration of their soluble polyhydrazide precursors PHA (I) and PHA (T),
respectively. Two reduction peaks were observed for these new conjugated polymers during CV cathodic
scan. From the CV voltammograms combined with the results from molecular simulation, we concluded
that the first reduction occurred at oxadiazole moiety and benzo[c]cinnoline moiety was responsible for
the second reduction. It indicates that oxadiazole has stronger electron affinity than benzo[c]cinnoline.
We proposed a mechanism to explain this two-stage reduction process. Due to the planar and electron-
accepting ability of benzo[c]cinnoline and oxadiazole moieties, POXD (I) and POXD (T) exhibited very
low LUMO (ꢀ3.42 and ꢀ3.45 eV) and HOMO (ꢀ6.23 and ꢀ6.27 eV) energy levels. They can be used as
hole-blocking or electron-injection layers for OLED applications.
Received 26 August 2011
Received in revised form
28 October 2011
Accepted 31 October 2011
Available online 4 November 2011
Keywords:
Electrochemical properties
Oxadiazole
Benzo[c]cinnoline
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
conjugated polymers can effectively improve the electron mobility,
lower LUMO, HOMO energy levels and in some cases decrease the
Conjugated polymers have been attracting intensive research
activities because of their versatile applications for organic light-
emitting diodes (OLEDs) [1e4], photovoltaic cells [5e8] and thin
film transistors [9]. In comparison to their inorganic counterparts,
conjugated polymers can be solution (ink-jet) processable, and
manufactured at relatively low costs. Furthermore, their chemical
structures can be designed by organic reactions to meet the various
requirements, resulting in the abundance of new conjugated poly-
mers. However, most conjugated polymers such as poly(p-phenyl-
enevinylene)s (PPVs) [10], poly(p-phenylene)s (PPPs) [11],
poly(thiophene)s (PTs) [12], poly(fluorene)s (PFs) [13] and their
energy gaps once donor-acceptor pair is formed [18e20]. Several
structures with strong electron affinity (electron-accepting) have
been investigated, including 2,1,3-benzothiadiazole [7], pyridine
[21], quinoxaline [18], cyanovinylene [22],1,3,4-oxadiazole [23], 2,20-
bis(trifluoromethyl)biphenyl [24] and others. However, compared to
the numerous literatures focused on electron-rich conjugated poly-
mers, the reports on the new moieties with strong electron affinity
are relatively rare. PBD (2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole) was the first documented electron-transporting material
used in a double layer OLED, which showed lower turn-on voltage
and higher electroluminescence efficiency [25]. 1,3,4-Oxadiazole
moiety has also been introduced in main chains or side chains of
various conjugated polymers [26e28]. The resulted oxadiazole-
containing conjugated polymers improved device performance.
The poor solubility of poly(1,3,4-oxadiazole)s, resulted from the
planar and rigid oxadiazole heterocycles, was generally circum-
vented by preparing their soluble polyhydrazide precursors first,
followed by thermal cyclodehydration.
derivatives [14,15] are p-electron excessive (electron-rich) in nature
and hence have better hole-injection and hole-transporting ability
[16,17]. As a result, the imbalanced hole and electron mobility could
lead to poor performance of the organic electronic devices. In addi-
tion, the adjustments of HOMO, LUMO energy levels and band gaps
are also critical for conjugated polymers to be applied in multi-
layered or bulk hetero-junction devices. It has been reported that
the incorporation of strong electron affinity (EA) moieties into
Benzo[c]cinnoline and its derivatives are planar, nitrogen-
containing heterocyclic moieties. They have been reported as
physiologically active molecules [29]. The nitrogen atoms at 5
and
6 positions are strongly electron-attracting that may
* Corresponding author. Tel.: þ886 2 27376526; fax: þ886 2 27376544.
E-mail address: jcchen@mail.ntust.edu.tw (J.-C. Chen).
dramatically affect the reactivity of the substituted functional
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.10.059

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J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
group on the ring. The HOMO and LUMO energy levels of benzo
[c]cinnoline molecule were reported to be lower than those of
their carbon-containing counterparts [30]. Therefore, it can be
expected that benzo[c]cinnoline can be a potential electron-
accepting moiety. In this study, as the first part of our serious
efforts on incorporation of benzo[c]cinnoline ring into various
conjugated polymers, poly(1,3,4-oxadiazole)s containing benzo
[c]cinnoline moiety were prepared via their polyhydrazide
precursors followed by thermal cyclodehydration. Their electro-
chemical properties were investigated by cyclic voltammetry
(CV). A mechanism to explain the observed two-stage reduction
(CV cathodic scan) was proposed according to the results from
molecular simulation. We concluded that the oxadiazole had
stronger electron affinity than benzo[c]cinnoline. The possible
applications for OLEDs were also discussed.
2.3. Synthesis of monomers
2.3.1. 4,40-Dibromo-2,20-dinitrobiphenyl (1)
To a 250-mL, three-necked, round-bottomed flask equipped
with a condenser were added 2,5-dibromonitrobenzene (11.43 g,
40.70 mmol), dimethylformamide DMF (77 mL) and activated
copper powder (8.22 g, 129.45 mmol). The mixture was heated at
120 ꢁC for 3 h under nitrogen atmosphere. After cooled to room
temperature, the reaction mixture was added toluene (150 mL) and
filtered. The clear filtrate was collected, washed with brine and
water, dried over anhydrous MgSO4. The solvent was evaporated.
The crude solid was recrystallized from ethanol/toluene (4:1) to
afford 5.73 g (70% yield) of light yellow crystals. mp 149e151 ꢁC
(lit [31]. mp 149 ꢁC). 1H NMR (500 MHz, DMSO-d6,
d, ppm): 8.44
(d, J ¼ 2.0 Hz, 2H, Ar-Ha), 8.08 (dd, J1 ¼ 8.2 Hz, J2 ¼ 2.0 Hz, 2H,
Ar-Hb), 7.49 (d, J ¼ 8.2 Hz, 2H, Ar-Hc).
2. Experimental
NO2
2.1. Materials
c
b
Br
Br
Tetrabutylammonium perchlorate (TBAP) used in cyclic vol-
tammetric measurements was recrystallized twice with ethyl
acetate and dried at 120 ꢁC under reduced pressure overnight. All of
the other reagents were purchased from commercial companies
and used as received. All of the solvents used in this study were
purified according to standard methods prior to use.
a
O2N
1
2.3.2. 3,8-Dibromobenzo[c]cinnoline (2)
2.2. Measurements
To a 500-mL, three-necked, round-bottomed flask equipped
with a condenser was added LiAlH4 (1.80 g, 47.49 mmol) in anhy-
drous ether (70 mL) under nitrogen atmosphere. 4,40-Dibromo-
2,20-dinitrobiphenyl (1) (2.40 g 6.00 mmol) in anhydrous ether
(100 mL) and benzene (100 mL) was added into the reaction
mixture. After stirred for 2 h at room temperature, the reaction
mixture was heated at 45 ꢁC for 15 min. Water (10 mL) was then
slowly added to the reaction mixture to decompose excess LiAlH4.
The reaction mixture was filtered and solvents were evaporated.
The yellow solid was collected and purified by column chroma-
tography using dichloromethane as the eluent to afford 1.60 g (79%
yield) of yellow needle-like crystals. mp 241e242 ꢁC (lit [32]. mp
All melting points were determined on a Mel-Temp capillary
melting point apparatus. Proton (1H NMR) and carbon (13C NMR)
nuclear magnetic resonance spectra were measured at 500 and
125 MHz on a Bruker Avance-500 spectrometer, respectively.
Infrared spectra were obtained with a Digilab-FTS1000 FTIR. Mass
spectroscopy was conducted on a Finnigan TSQ 700 mass spec-
trometer. Inherent viscosities were determined with a Cannon-
Ubbelohde No. 100 viscometer at 30.0 ꢂ 0.1 ꢁC. Molecular
weights were measured on a JASCO GPC system (PU-980) equipped
with an RI detector (RI-930), a Jordi Gel DVB Mixed Bed column
(250 mm ꢃ 10 mm), using dimethylacetamide (DMAc) as the eluent
and calibrated with polystyrene standards. Thermal gravimetric
analyses (TGA) were performed in nitrogen with a TA TGA Q500
242e243 ꢁC). 1H NMR (500 MHz, DMSO-d6,
d, ppm): 8.94
(d, J ¼ 2.0 Hz, 2H, Ar-Ha), 8.90 (d, J ¼ 8.8 Hz, 2H, Ar-Hc), 8.24
(dd, J1 ¼ 8.8 Hz, J2 ¼ 2.0 Hz, 2H, Ar-Hb). EIMS (m/z): calcd. for
C12H6Br2N2, 335.9; found, 335.9 [Mþ].
thermogravimetric analyzer using a heating rate of 10 ꢁC minꢀ1
.
Differential scanning calorimetry (DSC) measurements were
carried out under N2 atmosphere using a PerkineElmer DSC 4000
analyzer at a heating rate of 20 ꢁC/min. UVevis spectrometry was
N N
a
carried out on
a
Cary-100 UVevis spectrometer. Photo-
luminescence (PL) measurements were carried out on a Per-
kineElmer F4500 photoluminescence spectrometer. PL quantum
yield (FPL) of the polymer in concentrated sulfuric acid was
measured by using 10ꢀ5 M quinine sulfate in 1 N H2SO4 as reference
standard (FPL ¼ 0.546). Cyclic voltammetric (CV) measurements
were carried out on a CH Instrument 611C electrochemical analyzer
at room temperature in a three-electrode electrochemical cell with
a working electrode (polymer film coated on ITO glass), a reference
electrode (Ag/Agþ, referenced against ferrocene/ferrocium (Fc/Fcþ),
0.09 V), and a counter electrode (Pt gauze) at a scan rate of
100 mV sꢀ1. CV measurements for polymer films were performed in
an electrolyte solution of 0.1 M tetrabutylammonium perchlorate
(TBAP) in acetonitrile. The potential window at oxidative scan was
0e2.5 V and reductive scan was 0w ꢀ2.5 V, respectively. Wide-
angle X-ray diffraction (WXRD) measurements were performed
using polymer powder on a Rigaku TTRAX III with 18 kw rotation
anode Cu target.
Br
Br
c
b
2
2.3.3. 3,8-Dicyanobenzo[c]cinnoline (3)
To a 100-mL, three-necked, round-bottomed flask equipped
with a condenser were added 3,8-dibromobenzo[c]cinnoline (2)
(1.014 g, 3.00 mmol), K4[Fe(CN)6]$3H2O (0.557 g, 1.32 mmol),
Na2CO3 (0.636 g, 6.00 mmol), DMAc (20 mL), and Pd(OAc)2 (0.005 g,
0.02 mmol). The flask was vacuumed and backfilled with N2 several
times. The reaction mixture was heated at 120 ꢁC under nitrogen
atmosphere for 8 h. Ethyl acetate (50 mL) was then added to the
reaction mixture when it was cooled to room temperature. The
precipitate of the reaction mixture was collected by filtration and

J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
6013
Scheme 1. Synthetic route for 3,8-benzo[c]cinnoline dicarboxylic acid (4).
washed with water, methanol (50 mL), dichloromethane (50 mL)
and acetone (50 mL). The product was dried at 80 ꢁC under reduced
pressure for 12 h to afford 0.600 g (87% yield) of brown solid. The
crude product was used for next step without further purification.
mp >300 ꢁC (decompose). EIMS (m/z): calcd. for C14H6N4, 230.0;
found, 230.0 [Mþ].
The precipitate that formed was collected and washed with water
and hot methanol to afford 0.380 g (89% yield) of light green solid.
2.4.2. Polyhydrazide PHA (T)
Polyhydrazide PHA (T) was prepared from 3,8-benzo[c]cinno-
line dicarboxylic acid (4) and terephthalic dihydrazide using the
same procedures for PHA (I) in 84% yield.
N N
2.5. Film preparation and thermal cyclodehydration of
polyhydrazides
NC
CN
Polyhydrazides, PHA (I) and PHA (T) were dissolved in DMAc
(0.05%, w/v). The formed polymer solutions were filtered through
3
a syringe with a 0.45 mm filter. The polymer solutions were spin-
coated on a clean ITO glass which was then put in an air circu-
lated oven at 100 ꢁC for 1 h, 250 ꢁC for 2 h, followed by 300 ꢁC for
6 h under reduced pressure to afford polyoxadiazole films, POXD (I)
and POXD (T), respectively.
2.3.4. 3,8-Benzo[c]cinnoline dicarboxylic acid (4)
To a 100-mL, three-necked, round-bottomed flask equipped
with a condenser were added crude 3,8-dicyanobenzo[c]cinnoline
(3) (1.00 g, 4.35 mmol), diethylene glycol (20.0 mL), KOH (1.10 g,
19.60 mmol), and water (0.5 mL). The reaction mixture was heated
at 220 ꢁC for 12 h under nitrogen atmosphere. When the reaction
mixture was cooled to room temperature, the precipitate that
formed was collected and dissolved in hot water (250 mL). After
filtration, the clear filtrate was acidified by concentrated HCl. The
precipitate was collected and washed with water to afford 0.60 g
(52% yield) of yellow solid. mp >300 ꢁC (lit [33]. mp 335 ꢁC). 1H
2.6. Fabrication of electron-only devices
To fabricate the electron-only device for the current density
investigation, a polyhydrazide solution (0.5%, w/v in m-cresol) was
spin-coated on top of Al/glass substrate then converted to the
corresponding polyoxadiazole by thermal cyclodehydration in
vacuum. Aluminum tris(8-hydroxyquinolinate) (Alq3) and Al elec-
trode were then sequentially deposited through a shadow mask,
giving an active device area of 0.2 cm2. The comparison device of
Alq3 single layer was prepared identically. The devices were
completed with an encapsulation in a glove box (O2 and H2O
concentration below 0.1 ppm). The evaporation rates of the organic
NMR (500 MHz, DMSO-d6, d, ppm): 13.71 (s, 2H, carboxylic acid-
Hd), 9.19 (d, J ¼ 1.6 Hz, 2H, Ar-Ha), 9.06 (d, J ¼ 8.5 Hz, 2H, Ar-Hc), 8.53
(dd, J1 ¼ 8.5 Hz, J2 ¼ 1.6 Hz, 2H, Ar-Hb). EIMS (m/z): calcd. for
C14H8N2O4, 268.0; found, 267.9 [Mþ].
N N
a
d
HOOC
COOH
c
b
4
2.4. Synthesis of polyhydrazide by direct polycondensation
2.4.1. Polyhydrazide PHA (I)
To a 50-mL, round-bottomed, three-necked flask equipped with
a mechanical stirrer, a condenser and a nitrogen inlet were added
3,8-benzo[c]cinnoline dicarboxylic acid (4) (0.268 g, 1.00 mmol),
NMP (12.0 mL), LiCl (1.540 g), triphenyl phosphite (1.1 mL) and
pyridine (2.9 mL). The reaction mixture was heated at 100 ꢁC for
30 min under nitrogen atmosphere. Isophthalic dihydrazide
(0.194 g, 1.00 mmol) was then added. The reaction mixture was
heated at 140 ꢁC for 6 h under nitrogen. After cooled to room
temperature, the reaction mixture was poured into water (300 mL).
Fig. 1. 1H NMR spectrum of 3,8-benzo[c]cinnoline dicarboxylic acid (4) in DMSO-d6.

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J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
Scheme 2. Direct bromination of benzo[c]cinnoline.
and metal layers were controlled to be 0.1 nm/s by quartz-crystal
monitor and were calibrated by the thickness measurements with
a surface profiler (150; Veeco Dektak). Subsequently, the electrical
characteristics of each device were measured by a programmable dc
power supply (2400; Keithley) in atmosphere at room temperature.
It was reported that direct bromination of benzo[c]cinnoline
would lead to an isomeric mixture containing bromides on 1, or 1,4
or 1,7 positions as shown in Scheme 2 [34]. Therefore, 3,8-
dibromobenzo[c]cinnoline (2) has to be prepared according to the
proposed synthetic route (Scheme 1).
The dibromo functional groups of compound (2) can be poly-
merized or coupled with other moieties by Suzuki, Heck and Stille
coupling reactions. If 2,5-dichloronitrobenzene is used as the
starting material in Scheme 1, the formed 3,8-dichlorobenzo[c]
cinnoline can under go aromatic nucleophilic substitution due to
the electron-withdrawing benzo[c]cinnoline structure. In this
study, we converted the bromides of compound (2) into cyano
groups, which were further hydrolyzed to form 3,8-benzo[c]cin-
noline dicarboxylic acid (4). Although it was reported that 3,8-
benzo[c]cinnoline dicarboxylic acid (4) can be prepared by photo-
chemical method or by acetophenone in the presence of alkoxide in
alcoholic solvent, the yield was extremely low (9%) with tedious
purification process and long reaction time (60 h) [33,35]. Our
proposed synthetic route provided higher yield with easier workup
procedures.
3. Results and discussion
3.1. Synthesis of monomers
The diacid, 3,8-benzo[c]cinnoline dicarboxylic acid (4), was
prepared by a four-step synthetic route as shown in Scheme 1. 2,5-
Dibromonitrobenzene was first coupled by Ullmann coupling
reaction to form 4,40-dibromo-2,20-dinitrobiphenyl (1). The dinitro
compound (1) was then reduced by LiAlH4 to form 3,8-
dibromobenzo[c]cinnoline (2). The bromide functional groups
were converted to cyano groups. The final product, 3,8-benzo[c]
cinnoline dicarboxylic acid (4) was obtained by the hydrolysis of
dicyano compound (3). Fig. 1 shows the 1H NMR spectrum of 3,8-
benzo[c]cinnoline dicarboxylic acid (4).
Scheme 3. Preparation of polyhydrazides and polyoxadiazoles.

J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
6015
Table 1
Inherent viscosities and molecular weights of polyhydrazides and polyoxadiazoles.
Table 2
Thermal properties of polyhydrazides and polyoxadiazoles.
Polymer
Yield
Mn (g/mol)a
hinh (dL/g)
PDIa
Polymer
Td (ꢁC)a
Tg (ꢁC)b
Char yield (wt%)c
PHA (I)
89%
84%
ed
6500
6000
e
0.23b
0.20b
0.27c
0.15c
1.40
1.35
e
PHA (I)
284
292
459
469
190
210
230
250
55
60
58
59
PHA (T)
POXD (I)
POXD (T)
PHA (T)
POXD (I)
POXD (T)
ed
e
e
a
a
Obtained from GPC using DMAc as solvent and calibrated with polystyrene
Measured by TGA at a heating rate of 10 ꢁC/min in nitrogen.
Measured by DSC at a heating rate of 20 ꢁC/min in nitrogen.
Residual weight percentage at 800 ꢁC in nitrogen.
b
c
standards.
b
c
Measured in NMP at 30 ꢁC on 0.5 g dLꢀ1
.
Measured in conc. H2SO4 at 30 ꢁC on 0.5 g dLꢀ1
Quantitative yield in thermal cyclodehydration.
.
d
1H NMR spectrum of PHA (I). Protons of hydrazide linkages
appeared at 10.94 and 11.19 ppm. After thermal cyclodehydration,
the IR absorption peaks at 3235 (NeH stretching) and 1655 (C]O
stretching) cmꢀ1 disappeared and peaks attributed to the forma-
tion of oxadiazole ring, such as 1558 (C]N stretching) and 1077
(CeOeC stretching) cmꢀ1 appeared.
3.2. Synthesis of polymers
Polyhydrazides PHA (I) and PHA (T) were prepared by direct
polycondensation (Scheme 3) from 3,8-benzo[c]cinnoline dicar-
boxylic acid (4) with isophthalic dihydrazide and terephthalic
dihydrazide, respectively. Table 1 shows the inherent viscosities
and molecular weights of these polymers. The inherent viscosities
of PHA (I) and PHA (T) were 0.23 and 0.20 dL/g, measured in NMP
at 30 ꢁC with a concentration of 0.5 g/dL. The number average
molecular weights (Mn) were 6500 and 6000 g/mol measured by
GPC in DMAc solvent. Polyoxadiazoles, POXD (I) and POXD (T),
were prepared by thermal cyclodehydration of their polyhydrazide
precursors. After thermal cyclodehydration, the formed POXD (I)
and POXD (T) were insoluble in any organic solvents. Their
molecular weights can not be measured by GPC. The inherent
viscosities of POXD (I) and POXD (T), measured in concentrated
sulfuric acid, were 0.27 and 0.15 dL/g, respectively.
These values (molecular weights and inherent viscosities) are
not high when compared with those of other polyhydrazides and
polyoxadiazoles. However, all of these polymers can form contin-
uous, smooth, and pin-hole free films on ITO glass by spin coating
process. The low molecular weights might be partly resulted from
the low solid content (only 3%, w/v) we used during the direct
polycondensation. In conventional direct polycondensation, the
solid content was 7e8%, w/v [36]. For some monomers with ether
or bulky pendant groups, the solid contents can be as high as 30%,
w/v [37]. The higher solid content is beneficial to obtain polymers
with high molecular weights. Due to the poor solubility resulted
from the rigid and planar benzo[c]cinnoline structure, attempts to
prepare PHA (I) and PHA (T) with solid contents higher than 3%
would lead to precipitation during polymerization. Fig. 2 shows the
3.3. Thermal stability
The thermal stability was evaluated by the decomposition
temperatures at 5% weight loss (Td) and glass transition tempera-
tures (Tg) as listed in Table 2. Fig. 3 shows the TGA thermograms of
benzo[c]cinnoline-containing polyhydrazides and polyoxadiazoles.
Two stages of weight loss were observed. The first stage was
resulted from thermal cyclodehydration. The second stage was
attributed to the decomposition the formed polyoxadiazoles. The
Tds of PHA (I) and PHA (T) were 284 and 292 ꢁC, respectively. The
onset temperature of thermal cyclodehydration was around 280 ꢁC.
The Tds of POXD (I) and POXD (T) were 459 and 469 ꢁC, respec-
tively. The Tgs of PHA (I) and PHA (T) were 190 and 210 ꢁC. After
thermal cyclodehydration, the Tgs of POXD (I) and POXD (T) were
230 and 250 ꢁC. Polymers with p-catenation exhibited higher
thermal stability than their m-catenated analogs. These values are
comparable to those of conventional polyoxadiazoles. It indicates
the good thermal stability of the benzo[c]cinnoline moiety.
3.4. Organo-solubility and crystallinity
The solubility was investigated at a concentration of 0.5%, w/v in
various solvents. The results are summarized in Table 3. PHA (I) was
soluble in all tested solvents except THF at room temperature, while
p-catenated PHA (T) was only soluble in the same solvents at 60 ꢁC.
Fig. 2. 1H NMR spectrum of PHA (I) in DMSO-d6.
Fig. 3. TGA thermograms of polyhydrazides and polyoxadiazoles.

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J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
Table 3
Solubility of polyhydrazides.a
Polymer
NMP
DMAc
DMF
DMSO
THF
PHA (I)
PHA (T)
þþ
þ
þþ
þ
þþ
þ
þþ
þ
ꢀ
ꢀ
Abbreviations: THF, tetrahydrofuran; DMF, dimethylformamide; DMAc, dimethy-
lacetamide; NMP, N-methyl-2-pyrrolidinone; DMSO, dimethyl sulfoxide.
a
Solubility was determined using 0.005 g of polymer in 1 mL of solvent. þþ
(soluble at room temperature); þ (soluble on heating at 60 ꢁC); ꢂ (partially soluble
on heating at 60 ꢁC; ꢀ (insoluble on heating at 60 ꢁC).
After thermal cyclodehydration, the formed POXD (I) and POXD (T)
were only soluble in concentrated sulfuric acid. This is resulted
from the planar and rigid structures of benzo[c]cinnoline and
oxadiazole as well.
Crystallinity was investigated by wide angle X-ray diffraction.
Fig. 4 shows the diffraction patterns of these polymers. Poly-
hydrazides and polyoxadiazoles synthesized in this study all
showed crystalline peaks. Previous studies have reported the
amorphous character of polyhydrazides, which would become
crystalline polyoxadiazoles after thermal cyclodehydration [37,38].
It was attributed to the formation of planar and rigid oxadiazole
ring. In this study, the crystalline character of both polyhydrazides
and polyoxadiazoles might be attributed to the presence of planar
oxadiazole and benzo[c]cinnoline as well.
Fig. 5. UVevis and PL spectra of polyhydrazides and polyoxadiazoles.
The similar red shifts of the emission peaks were observed after
thermal cyclodehydration. The PL quantum yields of these new
polymers ranged from 0.18 to 0.31%. These values are exceptionally
low when compared with those of polyoxadiazole and its
derivatives.
3.5. Optical properties
3.6. Electrochemical properties
The UVevis and photoluminescence (PL) spectra of these poly-
mers (10ꢀ4e10ꢀ5 M in concentrated sulfuric acid) are shown in
Fig. 5. Photoluminescence quantum yield was estimated using
quinine sulfate as the standard. The data are summarized in Table 4.
The maximum absorption wavelengths (lmax) of PHA (I) and PHA
(T) were at 265 and 269 nm, respectively. These wavelengths were
red-shifted to 325 and 345 nm after the formation of POXD (I) and
POXD (T). The red shift could be attributed to the longer conjugated
length after the formation of oxadiazole ring. This effect is more
pronounced for p-catenated POXD (T) (76 nm) than for m-cate-
nated POXD (I) (60 nm).
The electrochemical redox behavior was characterized by cyclic
voltammetry. Fig. 6 shows cyclic voltammograms (cathodic scan) of
PHA (I), PHA (T), POXD (I) and POXD (T). Polymer films remained
stable during repeating scans and showed essentially the same
cyclic voltammograms. No oxidation peak below 2.50 V was
observed for PHA (I) and PHA (T). The oxidation onset potentials of
POXD (I) and POXD (T) were found to be 1.52 and 1.56 V, respec-
tively. One irreversible reduction peak was observed for PHA (I) and
red
PHA (T). The reduction onset potentials (E
) were at ꢀ1.53
onset
and ꢀ1.64 V, respectively. This peak was attributed to the reduction
of benzo[c]cinnoline moiety. After the formation of oxadiazole by
thermal cyclodehydration, two irreversible reduction peaks were
observed for POXD (I) and POXD (T), respectively. For example, two
reduction peaks with onset potentials of ꢀ1.26 and ꢀ1.55 V were
found for POXD (T) as shown in Fig. 6. Comparing the cyclic vol-
tammograms of polyhydrazides and polyoxadiazoles, we can
reasonably assume that the first peak was attributed to the
reduction of oxadiazole moiety and the second peak was the
resulted from the reduction of benzo[c]cinnoline moiety. This
implies that oxadiazole moiety had stronger electron affinity than
benzo[c]cinnoline moiety.
The photoluminescence (PL) spectra of PHA (I) and PHA (T)
showed emission peaks at 453 and 456 nm, respectively. The
emission peaks of POXD (I) and POXD (T) were at 459 and 472 nm.
To provide further evidence for the observed reduction behavior
of these polymers, theoretical calculations were carried out with
the Maestro graphical interface of Schrodinger molecular modeling
Table 4
Optical properties of polyhydrazides and polyoxadiazoles.
(nm)a
l
(nm)a
PL
max
FPL (%)b
UV
Polymer
l
max
PHA (I)
265
269
325
345
453
456
459
474
0.20
0.18
0.31
0.21
PHA (T)
POXD (I)
POXD (T)
a
Measured in conc. H2SO4 with a concentration of 10ꢀ4e10ꢀ5 M.
b
PL quantum yields were estimated using quinine sulfate (dissolved in 1 N
H2SO4(aq), assuming FPL of 0.546) as a standard.
Fig. 4. Diffraction patterns of polyhydrazides and polyoxadiazoles.

J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
6017
Fig. 7. Simulated LUMOs of (a) PHA (T), (b) POXD (T), and (c) POXD (T) radical anion.
electrochemical measuring system [39]. The results are summa-
rized in Table 5. The calculated LUMO energy levels were ꢀ3.18
and ꢀ3.07 eV for PHA (I) and PHA (T), respectively. The calculated
LUMO and HOMO energy levels were ꢀ3.42 and ꢀ6.23 eV for POXD
(I), ꢀ3.45 and ꢀ6.27 eV for POXD (T). The energy band gaps
calculated from electrochemical measurement were 2.81 eV for
POXD (I) and 2.82 eV for POXD (T). The energy levels for various
Fig. 6. Cyclic voltammograms (cathodic scan) of polyhydrazides and polyoxadiazoles.
suite (Maestro 9.1; Schrodinger, LLC: New York, 2010). The
repeating units of PHA (T) and POXD (T) were employed in the
molecular simulation. The details were described in the
Supplementary content. The simulated LUMOs of PHA (T) and
POXD (T) are shown in Fig. 7(a) and (b), respectively. For PHA (T),
the LUMO is concentrated on the benzo[c]cinnoline moiety. In
comparison, the LUMO of POXD (T) is distributed over the oxa-
diazoles and their conjugated benzene ring. Therefore the simula-
tion suggests that the first electron addition (reduction) is most
likely received by the oxadiazole ring. The LUMO was also calcu-
lated for the POXD (T) radical anion (charge
¼ ꢀ1, spin
multiplicity ¼ 2) to predict the location of the second electron
addition. Based on the results shown in Fig. 7(c), the second elec-
tron is added to the benzo[c]cinnoline moiety of POXD (T) radical
anion.
From the simulation results and the CV voltammograms of PHA
(T) and POXD (T), we concluded that during CV cathodic scan the
first reduction peak was from oxadiazole moiety and the second
reduction peak was from benzo[c]cinnoline moiety. Fig. 8 is our
proposed mechanism to explain the reduction behavior of POXD
(T) under CV cathodic scan.
The LUMO (electron affinity EA) and HOMO (ionization potential
IP) energy levels were calculated from the onset potentials of
reduction and oxidation, respectively. The absolute energy level of
ferrocene/ferrocenium (Fc/Fcþ) is assumed to be 4.8 eV below
vacuum level. The external Fc/Fcþ redox standard E1/2 was deter-
mined to be 0.09
V
versus Ag/Agþ in acetonitrile by our
Fig. 8. Mechanism of POXD (T) reduction under CV cathodic scan.

6018
J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
Table 5
Electrochemical properties of polyhydrazides and polyoxadiazoles.
(V)a
E
(V)a
EHOMO
(eV)
ELUMO
Eg, ec
Eg, opt
ox
red
Polymer
E
onset
onset
(eV)d
(eV)e
(eV)f
1st
2nd
PHA (I)
ꢀ1.53
ꢀ1.29
ꢀ1.64
ꢀ1.26
ꢀ5.58b
ꢀ6.23c
ꢀ5.77b
ꢀ6.27c
ꢀ3.18
ꢀ3.42
ꢀ3.07
ꢀ3.45
2.67
2.58
2.70
2.64
POXD (I)
PHA (T)
POXD (T)
1.52
1.56
ꢀ1.55
ꢀ1.55
2.81
2.82
a
Measured by cyclic voltammetry.
b
c
d
e
f
EHOMO ¼ ELUMO ꢀ Eg, opt
.
ox
EHOMO ¼ ꢀ(E
þ 4.80 ꢀ 0.09) (eV).
þ 4.80 ꢀ 0.09) (eV).
) ꢀ EA(E
onset
red
ELUMO ¼ ꢀ(E
onset
ox
red
Eg, ec ¼ IP(E
) (eV).
onset
onset
Calculated by the equation:1240/lonset
.
organic materials including poly{5-methoxy-2-[(20-ethylhexy)-
oxy]-p-phenylenevinylene} (MEH-PPV) [40], aluminum tris(8-
hydroxyquinolinate) (Alq3) [41], cyano-substituted poly(p-phenyl-
enevinylene) (CN-PPV) [40], 2-(4-biphenyl)-5-(4-tert-butyl-
phenyl)-1,3,4-oxadiazole (PBD) [42], poly(p-phenylene-1,3,4-
oxadiazole) (p-POD) [43], poly(pyridine-2,5-diyl) (PPy) [44], and
poly(quinoxaline-5,8-diyl) (PQX) [45] are shown in Fig. 9. The
benzo[c]cinnoline-containing polyoxadiazoles, POXD (I) and POXD
(T), reported in this study exhibited exceptionally low ELUMOs,
EHOMOs and large energy gaps. The LUMO energy levels are related
to the conjugation length and the electron-accepting ability of
polymers. For some electron-rich conjugated polymers, reduction
peak was hardly observed by electrochemical measurement.
Conjugated polymers with longer conjugation length and strong
electron-accepting ability exhibit lower LUMO energy levels.
POXD (I) and POXD (T) had lower LUMO energy levels than
commonly used electron-transporting materials such as Alq3
(ELUMO ¼ ꢀ3.30 eV) [41], PBD (ELUMO ¼ ꢀ2.4 eV) [42], p-POD
(ELUMO ¼ ꢀ2.8 eV) [43], PPy (ELUMO ¼ ꢀ2.9 eV) [44], and PQX
(ELUMO ¼ ꢀ3.16 eV) [45]. It indicates that, in addition to oxadiazole,
the electron-accepting and planar benzo[c]cinnoline structure also
helps to lower the LUMO energy level. The HOMO energy levels are
also associated with the electron-accepting ability. Conjugated
polymers with stronger electron-accepting ability have lower
HOMO energy levels. POXD (I) and POXD (T) also had lower HOMO
Fig. 10. Current density vs. voltage characteristics of electron-only devices.
transported from emitter such as MEH-PPV. In addition, the large
band gap should also confine the luminescence in the emitting
layers [43].
In order to elucidate the electron-injection and electron-
transporting abilities of POXD (I) and POXD (T), electron-only
devices were fabricated with the structures of Al/POXD
(I)(150 nm)/Alq3(50 nm)/Al, Al/POXD (T)(150 nm)/Alq3(50 nm)/Al
and Al/Alq3(200 nm)/Al. Fig. 10 shows the IeV curves of these
electron-only devices. It is obvious that the turn-on voltage can be
significantly reduced by inserting a layer of POXD (I) or POXD (T)
between the Al and Alq3. At a current density of 10 mA/cm2, the
turn-on voltage of the electron-only devices with a layer of POXD (I)
or POXD (T) was determined to be 7.8 V. No detectable turn-on
voltage was observed for the electron-only device without POXD
(I) or POXD (T) layer, indicating the large electron injection barrier
from aluminum (4.20 eV) to the LUMO of Alq3 (3.30 eV). The
significant reduction in turn-on voltage can be explained by the
lower barriers for electron injection and fast electron-transporting
abilities resulted from the electron-accepting benzo[c]cinnoline
and oxadiazole moieties [46]. Combined with the good thermal
stability, benzo[c]cinnoline-containing polyoxadiazole POXD (I)
and POXD (T) can be promising candidates for electron-transporting
and electron-blocking materials for OLED applications.
energy levels than Alq3 (EHOMO
¼ ꢀ5.70 eV) [41], CN-PPV
(EHOMO ¼ ꢀ5.63 eV) [40], PPy (EHOMO ¼ ꢀ5.70 eV) [44], and PQX
(EHOMO ¼ ꢀ5.74 eV) [45]. The low ELUMOs and EHOMOs of POXD (I)
and POXD (T) were attributed to the combined effects of benzo[c]
cinnoline and oxadiazole moieties. These two planar moieties
exhibited electron-accepting ability. As show in Fig. 9, the low
ELUMOs would allow the easier electron injection from air-stable
aluminum cathode, and the low EHOMOs would block the hole
4. Conclusions
A four-step synthetic route have been successfully developed to
prepare 3,8-benzo[c]cinnoline dicarboxylic acid. Polyoxadiazoles
POXD (I) and POXD (T) were prepared by direct polycondensation
of this diacid and dihydrazides to form their polyhydrazide
precursors PHA (I) and PHA (T), followed by thermal cyclo-
dehydration. PHA (I) and PHA (T) were soluble in polar solvents,
while POXD (I) and POXD (T) polyoxadiazoles were only soluble in
concentrated sulfuric acid. These polymers all exhibited good
thermal stability. From CV measurements, two reduction peaks
were observed for POXD (I) and POXD (T), respectively. The results
of molecular simulation and CV measurements suggested that the
first reduction peaks was resulted from oxadiazole and the second
peak was from benzo[c]cinnoline. We concluded that oxadiazole
moiety had stronger electron affinity than benzo[c]cinnoline
moiety. POXD (I) and POXD (T) also exhibited low ELUMOs and low
EHOMOs due to the electron-accepting ability of planar oxadiazole
and benzo[c]cinnoline as well. These polymers can be promising
candidates as the hole-blocking or electron-injection layer for OLED
Fig. 9. Energy gap diagram.

J.-C. Chen et al. / Polymer 52 (2011) 6011e6019
6019
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