Highly π-extended polymers based on phenanthro-pyrazine: Synthesis, characterization, theoretical calculation and photovoltaic properties

Article abstract of DOI:10.1016/j.polymer.2013.09.015

Two novel narrow bandgap conjugated polymers containing phenanthro-pyrazine unit have been successfully synthesized by the Stille coupling reaction. Comparing to the common polymers containing dithiophen-quinoxaline or dithiophen-thieno[3,4-b]pyrazine moi

Full text of DOI:10.1016/j.polymer.2013.09.015

Polymer 54 (2013) 6191e6199
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Highly
p
-extended polymers based on phenanthro-pyrazine:
Synthesis, characterization, theoretical calculation and photovoltaic
properties
a
,
b
a
b
a
a
a
Zhiming Wang , Zhao Gao , Ying Feng , Yulong Liu , Bing Yang , Dandan Liu ,
a
a,
a
*
Ying Lv , Ping Lu , Yuguang Ma
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China
School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang 111003, People’s Republic of China
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2013
Received in revised form
Two novel narrow bandgap conjugated polymers containing phenanthro-pyrazine unit have been suc-
cessfully synthesized by the Stille coupling reaction. Comparing to the common polymers containing
dithiophen-quinoxaline or dithiophen-thieno[3,4-b]pyrazine moiety, the conjugation degree of these
new polymers is extended through the direction perpendicular to the main chain by introducing
phenanthrene-9,10-dione to maintain a rigid conjugated bridge. The obtained polymers exhibit solution-
processing ability, high thermal stabilities, broad visible absorption bands and narrow optical bandgaps.
Theoretical studies disclose that the P2 exhibits wholly coplanar conformation in 1-D and 2-D direction,
and the PCE value is 6-folded higher than P1 under the same photovoltaic measurement condition.
Ó 2013 Elsevier Ltd. All rights reserved.
3
September 2013
Accepted 11 September 2013
Available online 19 September 2013
Keywords:
2-Dimentional conjugation
Pyrazine
Narrow bandgap
1. Introduction
have attracted tremendous interest in the design and synthesis of
organic narrow bandgap materials. By rational design of monomers
The study of narrow bandgap conjugated polymers is an ever-
growing research field because of their great promise in applica-
tion such as polymer solar cells through convenient solution-
processable method to solve the impending energy crisis [1e4].
Comparing to their inorganic counterparts, soluble polymer semi-
conductors are generally packed poorly and show low level of or-
der, which leads to the low mobility and power conversion
efficiency in photovoltaic devices [5]. Thus, the strategy that
generally be applied to design and synthesis conjugated polymers
with narrow bandgap includes creating highly planar structure,
strengthening the intermolecular interactions, enhancing their
molecular orbital overlapping, and using donor-acceptor effects
and choosing the suitable polymerization method, kinds of novel
polymers are prepared and perform high PCE on account of the
intermolecular pep stacking and intramolecular charge transfer in
D-A system [19,20]. In these works, the dithiophene group in the
polymer main chain can be applied as a bridge and the formation of
hydrogen bonds between the nitro atoms (in thieno[3,4-b]pyrazine
(NeH) or quinoxaline (NeH)) and b-hydrogen atoms (in one or two
neighboring thiophene rings (SeH)), can minimize the dihedral
angle and strengthen planarity along the main chain direction (1-D
conjugation), resulting in strong intramolecular interaction to
achieve planar structure (Scheme 1) [21].
In the expectation of broadening the absorption spectra and
extending the coplanarity to further promote the intermolecular
[6e9]. Among many of the building blocks for narrow bandgap
organic materials, heterocyclic aromatic units usually provide
tendency to form large orbital overlapping and strong intermo-
lecular interaction [10e12]. Recently, dithiophen-thieno[3,4-b]
pyrazine [13e15] and dithiophen-quinoxaline moieties [16e18]
p
e
p
stacking of these polymers to get narrow bandgap materials,
here, we attempted to extend the 2-dimensional conjugation de-
gree by attaching rigid -conjugated moieties perpendicular to
dithiophen-thieno[3,4-b]pyrazine or dithiophen-quinoxalin unit
22e25]. Phenanthrene-9,10-dione (PQ) was chosen as a basic
p
[
constructing unit, which could react with diamino monomer to
maintain a rigid conjugated bridge and develop the 2-dimensional
conjugated polymers [26,27]. The newly formed phenanthro-
pyrazine ring could ensure nearly all atoms be in one plane [28].
*
Corresponding author. Tel.: þ86 431 85167057; fax: þ86 431 85168480.
E-mail address: lup@jlu.edu.cn (P. Lu).
0
032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.09.015

6192
Z. Wang et al. / Polymer 54 (2013) 6191e6199
Scheme 1. The structures of dithiophen-thieno[3,4-b]pyrazine, dithiophen-quinoxaline and phenanthro-pyrazine.
ꢁ
1
In addition, the intermolecular interaction between the electron-
donating thiophene unit and electron-accepting phenanthro-pyr-
azine unit could shorten the distance between the polymer chains
to form a higher level of packing order, which would facilitate the
charge or carrier transport along the 2-dimensional region. What’s
more, conceiving the solution processability of the polymer, the
dihexyl-fluorene moiety was attached to the 3,6-position of PQ
unit, which showed the V-shape conformation and reduced the
polymerization steric hindrance to some extent [29,30], and tuned
the HOMO level of the polymers [31,32]. The dihexyl-fluorene
moiety could simultaneously broaden the absorption spectrum.
Here, we report the synthesis and characterization of two new
solution-processable narrow bandgap materials, and their primary
photovoltaic application.
make 10 mg mL
solutions, followed by blending with PCBM
(purchased from Lumtec. Corp) in blend ratios (1:1, w/w). The active
layers were obtained by spin-coating the blend solutions and the
thickness of films were about 70 nm, as measured with the Ambios
Technology XP-2. Finally, the cathode of LiF (0.5 nm)/Al (100 nm) was
thermally deposited to finish the device fabrication. The active area
2
was about 5 mm . Currentevoltage (JeV) characteristics were
recorded using a Keithley 2400 Source Meter in the dark and under
ꢁ
2
100 mW cm simulated AM 1.5 G irradiation (Sciencetech SS-0.5 K
Solar Simulator). The spectral response was recorded by a SR830
lock-in amplifier under short circuit conditions when devices were
illuminated with a monochromatic light from a Xeon lamp. All
fabrication and characterizations were performed under ambient
atmosphere at room temperature.
2
. Experimental part
2
2
.3. Synthesis of monomers and polymers
.3.1. M1
2.1. Materials and measurement
3
,6-dibromophenanthrene-9,10-dione (366 mg, 1 mmol), 2-(9,9-
dihexyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxabo-rolane
1.15 g, 2.5 mmol) and 23.1 mg Pd(PPh (1% mol) were added in the
All the reagents and solvents used for the syntheses were pur-
chased from Aldrich or Acros companies and used without further
purification. The polymers were collected after Soxhlet extractions
with methanol, acetone and n-hexane for 12 h, respectively. All
reactions were performed under a dry nitrogen atmosphere.
(
3 4
)
mixture of 10 mL toluene and 2 mL 2.0 M K
2
CO
3
, and then the
ꢀ
mixture was stirred at 85 C under the N
2
atmosphere for 24 h. The
reaction was stopped by adding water, and the product was extracted
by CHCl three times. After dried over MgSO , the solvent was
removed by rotary evaporation. The product was purified by column
chromatography using mixture of CHCl and petroleum ether as the
eluent to give the white solid. Yield: 75%. H NMR (500 MHz, CDCl
1
The H NMR and spectra were recorded on AVANCZ 500 spec-
3
4
trometers at 298 K by utilizing deuterated chloroform (CDCl
dimethyl sulphoxide (DMSO) as solvent and tetramethylsilane
TMS) as standard. The elemental analysis was operated by Flash EA
112, CHNS-O elemental analysis instrument. The MALDI-TOF mass
3
) or
3
(
1
1
3
,
d
): 8.39 (s, 2H), 8.35e8.33 (d, 2H, J ¼ 8.24 Hz), 7.85e7.83 (d, 2H,
spectra were recorded using an AXIMA-CFRÔ plus instrument. Uve
vis absorption spectra were recorded on UV-3100 spectropho-
tometer. Fluorescence measurements were carried out with RF-
J ¼ 8.2 Hz), 7.79e7.76 (t, 4H, J ¼ 7.6, 6.7 Hz), 7.34e7.72 (d, 2H,
J ¼ 7.9 Hz), 7.66 (s, 2H), 7.40e7.36 (m, 6H), 2.08e2.03 (m, 8H), 1.46e
þ
1
.00 (m, 24H), 0.75e0.65 (m, 20H); MALDI-TOF-MS (m/z): [M þ H]
5
301PC. The differential scanning calorimeter (DSC) analysis was
calcd for C64
H
72
O
2
: 872.5; Found: 873.6; Anal. Calcd. for C64
72 2
H O : C,
ꢀ
determined using a NETZSCH (DSC-204) instrument at 10 C/min
under nitrogen flushing. Cyclic voltammetry (CV) were performed
with a BAS 100W Bioanalytical Systems, using a glass carbon disk
88.03; H, 8.31; O, 3.66; Found: C, 88.00; H, 8.27; N, 3.65.
2
.3.2. M2
,7-dibromobenzo[c][1,2,5]thiadiazole (1.32 g, 4.5 mmol), 2-
thiophen-2-yl)-1,3,2-dioxaborinane (1.68 g, 10 mmol) and 23.1 mg
(
F
¼ 3 mm) as working electrode, a platinum wire as auxiliary
4
þ
electrode and Ag/Ag as reference electrode. Cyclic voltammetric
(
4 4
studies were carried out containing 0.1 M [n-NBu ][BF ] as sup-
porting electrolyte. All solutions were purged with nitrogen stream
for 10 min before measurement.
Pd(PPh
mL 2.0 M K
the N
0 mL water, and the product was extracted by CHCl
After dried over MgSO , the solvent was removed by rotary evap-
3
)
4
(1% mol) were added in the mixture of 30 mL toluene and
ꢀ
6
2
CO
3
, and then the mixture was stirred at 85 C under
2
atmosphere for 48 h. The reaction was stopped by adding
three times.
2
3
2.2. OPV cells fabrication and measurements
4
oration. The product was preliminary purified by column chroma-
The BHJ solar cells were fabricated with the active layer consisting
tography using petroleum ether as the eluent to give the red solid.
1
of the Polymer:PCBM with blend ratios (1:1, w/w). ITO glass was
cleaned by detergent, acetone and boiled in H . A 50 nm layer of
poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate)
PEDOT:PSS) (Bayer PVPAl 4083) as a modified layer was spin-coated
Yield: 85%. H NMR (500 MHz, CDCl
7.89 (s, 2H), 7.47 (d, J ¼ 5.1,1.2 Hz, 2H), 7.22 (t, J ¼ 5.1, 3.6,1.2 Hz, 2H).
3
,
d
): 8.13 (d, J ¼ 3.6, 1.2 Hz, 2H),
O
2 2
(
2.3.3. M3
ꢀ
onto the pre-cleaned ITO glass substrate and then dried at 120 C for
5 min on a hot plate. The polymer was dissolved in chlorobenzene to
M2 (300 mg,1.0 mmol) and Fe powder (1.3 g, 20 mmol) were put
into the two-neck bottle, and the 50 mL ice acetate was added. The
1

Z. Wang et al. / Polymer 54 (2013) 6191e6199
6193
mixture was refluxed for 30 min. When the color of the liquid was
changed from orange to white, the reaction was stopped. The
product does not need to be purified and used as the starting ma-
terial to synthesize M6.
7.98e7.66 (d, 2H, J ¼ 7.95 Hz), 7.86e7.84 (d, 2H, J ¼ 7.95 Hz), 7.80e
7.72 (m, 6H), 7.70e7.69 (d, 2H, J ¼ 4.0 Hz), 7.48e7.47 (d, 2H,
J ¼ 5.1 Hz), 7.42e7.34 (m, 6H), 7.19e7.17 (t, 2H, J ¼ 4.8, 1.0 Hz), 2.14e
2.04 (m, 8H), 1.15e1.05 (m, 24H), 0.77e0.72 (m, 20H); MALDI-TOF-
þ
MS (m/z): [M þ H] calcd for C76
H
78
N
2
S
3
: 1114.5; Found: 1115.6;
2.3.4. M4
Anal. Calcd. for C76
78
H N
2
S
3
: C, 81.82; H, 7.05; N, 2.51; S, 8.62;
Tributyl(thiophen-2-yl)stannane (3.73 g, 10 mmol), 2,5-dibromo-
Found: C, 81.00; H, 7.00; N, 2.20; S, 8.46.
3
3 4
,4-dinitrothiophene (1.1 g, 3.33 mmol) and 77 mg Pd(PPh ) (1%
mol) were put into the two-neck bottle, and the 100 mL toluene was
added. The mixture was refluxed for 24 h under N atmosphere. And
then the reaction was stopped and cooled to room temperature. The
mixture was first purified by column chromatography using THF as
the eluent to get orange liquid. And then it was concentrated and
2.3.9. M9
2
M8 (200 mg, 0.20 mmol) and NBS (90 mg, 0.5 mmol) were
dissolved in 25 mL THF and stirred for 2 h at room temperature.
After that, the mixture was poured into 200 mL methanol. A
blackish green was precipitated and filtrated. The product was
purified by column chromatography using petroleum ether as the
purified by column chromatography using petroleum ether as the
1
1
eluent to get orange solid. Yield: 80%. H NMR (500 MHz, CDCl
3
,
d):
eluent to give the gray-green solid. Yield: 90%. H NMR (500 MHz,
7
.61e7.60 (dd, 2H, J ¼ 5.1, 1.2 Hz), 7.55e7.54 (dd, 2H, J ¼ 3.6, 1.2 Hz),
3
CDCl , d): 8.53e8.51 (m, 4H), 7.84e7.83 (m, 4H), 7.79e7.52 (m, 4H),
þ
7
.19e7.17 (d, 2H, J ¼ 5.1, 3.6, 1.2 Hz); MALDI-TOF-MS (m/z): [M þ H]
calcd for C12 : 338.0; Found: 339.6; Anal. Calcd. for
: C, 42.59; H, 1.79; N, 8.28; S, 28.43; Found: C, 42.03; H,
.68; N, 8.10; S, 28.23.
7.72e7.70 (d, 2H, J ¼ 7.9 Hz), 7.46e7.37 (m, 8H), 6.65 (m, 2H), 2.20e
H N O S
6 2 4 3
2.01 (m, 8H), 1.22e1.01 (m, 24H), 0.88e0.72 (m, 20H); MALDI-TOF-
þ
C
1
H N O S
12 6 2 4 3
MS (m/z): [M þ H] calcd for C76
H
76Br
2
N
2
S
3
: 1270.3; Found: 1273.8;
Anal. Calcd. for C76
H76Br
2
N
2
S
3
: C, 71.68; H, 6.02; N, 2.20; S, 7.55;
Found: C, 70.98; H, 6.00; N, 2.18; S, 7.22.
2.3.5. M5
Under the N
2
atmosphere, M4 (670 g, 2.0 mmol), SnCl
2
(3.8 g,
2.3.10. P1
2
0 mmol) and HCl (3 mL) were dissolved in 10 mL acetate ether. The
M7 (540 mg, 0.43 mmol), Sn(C
4 9 6
H ) (250.0 mg, 0.43 mmol) and
mixture was refluxed for 24 h and then poured into water. The
product was extracted by acetate ether and 2% NaOH solution. The
solvent was removed by rotary evaporation and the concentrated
red oil was used directly in the next step to synthesize M8.
Pd(PPh 2 mg, 2% was added into the three-neck bottle, and then
3 4
)
1.0 mL toluene, 1.0 mL DMF was added. The mixture was heated to
ꢀ
110 C and stirred for 48 h under N
2
atmosphere. The reaction was
quenched by adding 20 mL HCl (10%). The product was extracted by
CHCl and EDTA solution. The organic layer was collected and dried
over MgSO . The solvent was removed by rotary evaporation. The
concentrated oil was precipitated in methanol to form solid. The
solid was filtrated and purified by Al column to get rid of Pd
catalyst. And then the polymer was finally extracted by soxhlet
3
2.3.6. M6
4
M1 (440 mg, 0.5 mmol) and M3 were added to the solution of
ꢀ
acetate and refluxed for 12 h at 80 C. The mixture was then put into
the 500 mL methanol, the solid was occurred and filtrated. The
product was purified by the column chromatography using petro-
2 3
O
1
apparatus. Yield: 55%. H NMR (CDCl
3
, 500 MHz): d 9.50e7.50 (m,
1
leum ether as the eluent to give the orange solid. Yield: 95%. H NMR
10H), 7.49e7.26 (m, 10H), 7.25e6.50 (m, 6H), 2.30e1.60 (m, 8H),
1
3
(
500 MHz, CDCl
3
,
d): 9.62 (s, 2H), 8.91 (s, 2H), 8.24 (s, 2H), 8.13e8.11
1.30e0.40 (m, 44H). C NMR (500 MHz, CDCl
3
):
d
¼ 151.3, 148.9,
(
7
2
d, 2H, J ¼ 8.24 Hz), 7.94 (s, 2H), 7.88e7.85 (m, 4H), 7.81e7.78 (m, 4H),
147.5, 146.6, 145.8, 143.9, 142.3, 140.2, 139.9, 136.9, 133.3, 132.8,
127.6, 126.3, 125.6, 123.8, 120.8, 119.0, 118.5, 54.3, 39.4, 30.3, 28.9,
.68e7.67 (d, 2H, J ¼ 7.9 Hz), 7.62e7.34 (m, 6H), 7.30e7.29 (m, 2H),
3
3
.14e2.04 (m, 8H), 1.26e1.23 (m, 4H), 1.15e1.05 (m, 24H), 0.77e0.72
22.8, 21.6, 12.3. M
calibration). Anal. Calcd. for (C78
2.52; S, 5.78; Found: C, 80.90, H, 6.89, N, 2.09, S, 5.04.
n
: 22.0*10 , M
w
: 35.2*10 , M
w n
/M : 1.6 (GPC, PS
þ
(
1
m, 20H); MALDI-TOF-MS (m/z): [M þ H] calcd for C79
82 2 2
H N S :
78 2 2 n
H N S ) (%): C, 84.43; H, 7.27; N,
82 2 2
123.6; Found: 1124.4; Anal. Calcd. for C79H N S : C, 84.44; H, 7.36;
N, 2.49; S, 5.71; Found: C, 83.24; H, 7.06; N, 2.31; S, 5.55.
2.3.11. P2
2
.3.7. M7
P2 was synthesized similarly as P1 except that using M9 as the
1
M6 (480 mg, 0.43 mmol) and NBS (180 mg, 1.0 mmol) were dis-
starting material instead of M7. Yield: 50%. H NMR (CDCl
3
,
solved in 40 mL THF, and the mixture was refluxed for 2 h. The
mixture was stopped by pouring into 500 mL methanol. And the
deep red solid occurred and filtrated. The product was purified by the
500 MHz):
6H), 2.30e1.60 (m, 8H), 1.00e0.80 (m, 24H), 0.70e0.30 (m, 20H).
NMR (125 MHz, CDCl ):
¼ 150.7, 149.8, 141.4, 140.2, 139.4, 138.5,
135.6, 134.1, 131.7, 126.3, 125.6, 121.9, 120.8, 119.2, 118.7, 54.3, 39.5,
d
¼ 9.50e7.50 (m, 10H), 7.49e7.26 (m, 8H), 7.25e6.50 (m,
13
C
3
d
column chromatography using petroleum ether as the eluent to give
1
3
3
the deep red solid. Yield: 90%. H NMR (500 MHz, CDCl
3
,
d): 9.52e
30.5, 28.7, 22.6, 21.6, 12.9. M : 25.2*10 , M
n
: 21.0*10 , M
w
w n
/M : 1.2
9
.50 (d, 2H, J ¼ 8.24 Hz), 8.90 (s, 2H), 8.14e8.12 (d, 2H, J ¼ 8.24), 8.08
(GPC, PS calibration). Anal. Calcd. for (C76H N S ) (%): C, 81.82; H,
76 2 3 n
(
(
(
s, 2H), 7.89e7.84 (m, 8H), 7.58 (s, 2H), 7.42e7.35 (m, 6H), 7.20e7.19
7.05; N, 2.51; S, 8.62; Found: C, 79.40, H, 6.69, N, 2.13, S, 8.11.
3. Results and discussion
d, 2H, J ¼ 4.9 Hz), 2.17e2.06 (m, 8H), 1.18e1.06 (m, 24H), 0.81e0.72
þ
m, 20H); MALDI-TOF-MS (m/z): [M þ H] calcd for C78
2 2 2
H78Br N S :
1264.4; Found: 1267.7; Anal. Calcd. for C78
2 2 2
H78Br N S : C, 73.92; H,
6
2
0
.20; N, 2.21; S, 5.06; Found: C, 73.62; H, 6.18; N, 2.18; S, 5.00.
3.1. Synthesis
.3.8. M8
The general synthetic strategy towards the polymers is outlined
in Scheme 2. In the present work, the required dibenzo[a,c]phen-
azine precursor M1 was prepared by Suzuki coupling reaction be-
tween 3,6-dibromophenanthrene-9,10-quinone [33] and 2-(9,9-
dihexyl-9H-fluorene-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane in good yield of 75%. In this way, the soluble alkyl
chain and the two active carbonyl units were introduced. M2 was
also achieved by Suzuki coupling reaction using 4,7-dibromobenzo
M5 was added into the acetate solution of M1 (440 mg,
ꢀ
.5 mmol) and the mixture was stirred at 80 C for 2 h. After that,
the reaction was stopped by pouring them in 500 mL methanol. A
blackish green was precipitated and filtrated. The product was
purified by column chromatography using petroleum ether as the
1
eluent to give the blackish green solid. Yield: 80%. H NMR
(
500 MHz, CDCl
3
,
d): 9.29e9.28 (d, 2H, J ¼ 7.96 Hz), 8.73 (s, 2H),

6194
Z. Wang et al. / Polymer 54 (2013) 6191e6199
Scheme 2. The synthesis route to the monomers and polymers.
[
(
c][1,2,5]thiadiazole as the starting material, which coupled with 2-
thiophen-2-yl)-1,3,2-dioxaborinane to obtain 4,7-di(thiophen-2-
2
compound under SnCl and HCl in solution of ethyl acetate to afford
M5 [35]. And then, M9 could be achieved in the similar process as
M7 using M1 and M5 as the reactants. After M7 and M9 were
successfully prepared, we then tried to polymerize them by
yl)benzo[c]-[1,2,5]thiadiazole (M2). And then, the following
reduction process was performed in the presence of Fe dust and
acetate for 30 min to get the monomer M3. With M1 and M3 in
hand, the quinoxaline monomer M6 could be obtained as a yellow
Pd(PPh
3
)
4
, a commonly used catalyst for Still coupling reaction.
4 9 4
and Sn (C H )
Polymerization of M7 in the presence of Pd(PPh
3
)
4
2
ꢀ
powder with a nearly 100% yield by stirring them together at 80 C
in refluxed DMF/toluene gave a high molecular weight polymer P1
in a reasonable yield [36]. Under the same condition, P2 was also
successfully synthesized. The resulting copolymers were charac-
terized, spectroscopically and corresponded well to their expected
molecular structures. They were readily soluble in common organic
for 12 h. M7, the precursor for P1, was then synthesized by
bromination of M6 in the presence of NBS in high yield of 90%. For
monomer M4, the Stille coupling reaction was first performed to
get the yellow crystals of M4 with two nitro reactive group [34].
And it was then reduced to corresponding ortho-diamine
3
solvent such as toluene, THF and CHCl due to the introduction of

Z. Wang et al. / Polymer 54 (2013) 6191e6199
6195
Fig. 1. The 1H NMR spectra of P2 and M9.
long alkyl chain on fluorene group. The weight-average molecular
weights (M ) of P1 and P2 determined by gel permeation chro-
Fig. 2. TGA spectra of P1 and P2.
w
matography (GPC) using THF as the eluent and polystyrene as
standard is 35,200 and 25,500, respectively, with a polydispersity
index of 1.6 and 1.2.
3
.4. Photophysical properties
The absorption spectra of P1 and P2 in film state were shown in
Fig. 3. The absorption of monomer M9 and P3HT are also measured
for comparison purpose in Fig. S7. Absorption of P1 in film was
dominated by a series of peaks. The peak around 310 nm came from
the dihexylfluorene moiety, while the absorption peak around
3
.2. NMR characterization
The structure of the polymers was confirmed by NMR, FTIR and
1
elemental analyses. Take P2 as an example, Fig. 1 showed the H
NMR spectra of P2 together with its monomer M9. In the spectrum
of M9, the absorptions of the CH protons on the long alkyl chain
2
linked with fluorene unit were observed at 2.09, 1.13 and 0.75 ppm,
respectively, and P2 showed the similar proton resonances at this
area but they became broad in P2. In low field region
400 nm was attributed to the characteristics of pep* transition of
the main chain, and the absorption band around 650 nm was due to
the intramolecular charge transfer (ICT) because of the donor-
acceptor effects. Compared to P3HT, which exhibited main ab-
sorption peak at 520 nm, the absorption of P1 was obviously
broadened ranging from 400 to 800 nm, and the absorption in the
(d > 8.50 ppm), the absorptions of the H protons of phenanthrene
ring of M9 were observed at 9.29 and 8.73 ppm.
The absorptions of the H protons of thiophene ring were
observed at 6.65 and 7.40 ppm indicating that the monomer was
successfully formed. P2 also exhibited these proton resonances in
this area similar to M9. All this proved that the building block of M9
was inserted to the polymer 2. From 7.10 to 7.90 ppm, the absorp-
tions of the aromatic fluorene hydrogen, aromatic phenanthrene
hydrogen and thiophene hydrogen of P2 were hard to distinguish,
presumably due to their strong overlapping. No unexpected peaks
were found and all the peaks could be readily assigned. Thus, the
polymeric product was indeed P2 with a molecular structure as
shown in Scheme 2. Similar observations were found in the spec-
trum of P1 as shown in Supporting Information.
3.3. Thermal properties
Temperature for 5 wt% loss has often been used as a degradation
temperature (T ) to estimate thermal stability of a polymer. Both
d
the polymers were thermally stable and lost little of their weights
ꢀ
at a temperature as high as 300 C (Fig. 2). The T
d
was found to be at
ꢀ
ꢀ
3
85 C and 395 C for P1 and P2, respectively. When pyrolyzed at
00 C, P1 and P2 still kept 63.9% and 62.7% of their original
weights, respectively. In DSC heating cycles up to 350 C, whereas
P1 and P2 did not show any thermal transition (as shown in Fig. S6),
which was due to their 2-dimensional rigid structure.
ꢀ
8
ꢀ
Fig. 3. Absorption spectra of P1 and P2 in film state.

6196
Z. Wang et al. / Polymer 54 (2013) 6191e6199
Fig. 4. The model structure of P1 or P2 and simulation of front molecular orbitals through DFT calculation at a B3LYP/6-31G(d) level.
blue region was improved. P2 also showed the similar absorption
peaks at 310, 400 nm, and the absorption of ICT was remarkably
red-shifted to 820 nm demonstrating longer conjugation wave-
length and stronger CT interaction in the polymer backbone [37].
The onsets of the absorption edge were at 845 nm and 1100 nm,
thus the bandgap of P1 and P2 were calculated to be 1.49 and
excitation would trend to shift the electron density distribution
from the central main chain to the pyrazine moieties on side chain
to some extent. For extrapolating of HOMOeLUMO gaps for poly-
mers, the linear curve of the HOMO, LUMO and bandgap against the
reciprocal of the number of monomer units (1/n) was employed
[38]. Alone with the repeat unit (n) increased, the gap was dropped
obviously. The modified extrapolation of 1/n afforded HOMOe
LUMO gap values of 1.71 and 1.13 eV for P1 and P2 (Table S1),
respectively, and this result was very close to the corresponding
experimental values obtained from optical absorption edge.
Meanwhile, the oligomer for P2 showed narrower bandgap than
that for P1, and it was not only due to a stronger CT interaction
between donor and acceptor unit in polymer, but also extended
conjugation from planar configuration.
1.21 eV, respectively. Comparing to the bandgap of P3HT of 1.94 eV,
the bandgap of P1 and P2 was obviously decreased, which proved
that the narrow bandgap polymers were achieved by our molecular
design and endowed their potential application in solar cells.
3.5. Theoretical calculation studies
To gain insight into the electronic structure of two highly
p-
extended polymers, density function theory (DFT) calculations
were performed at a B3LYP/6-31G(d) level for the geometry opti-
mization [16,25]. To simplify the calculation, only one to three
repeating units of each polymer were subject to the calculation,
with alkyl chains removed. The simulated electron density distri-
butions and the calculated HOMO and LUMO levels along with the
optimized geometries are shown in Fig. 4 and Fig. S8. The V-shape
conformations of the constructing units were observed for P1 and
P2, which could reduce the polymerization steric hindrance as
expected. And the HOMOs in both polymers were mainly populated
over the 1-D main chain, whereas the LUMO had sizable contri-
bution from perpendicular pyrazine fragments. Examination of the
frontier orbits of molecules suggests that the HOMOeLUMO
In P1, it is also observed that phenanthro-pyrazine unit exhibi-
ted the planarity geometry, which was commitment with the
design strategy. And the dihedral angel between plane of thiophene
ꢀ
and quinoxaline on main chain was calculated to be 25.6 (Dihedral
Angel-1). While in P2, the benzene unit in P1 was replaced by
thiophene here, the thiophene unit showed alternate arrangement
due to the strong hydrogen interactions between two neighboring
thiophene rings (SeH), which strengthened coplanarity along the
main chain direction (1-D conjugation) [16e18]. The dihedral angel
ꢀ
between thiophenes on main chain was decreased to 0.2 , which
resulted in the extended conjugation length and further narrower
bandgap compared to P1. This was agreed with our observation in
absorption spectra. Thus, a higher level of packing order could be

Z. Wang et al. / Polymer 54 (2013) 6191e6199
6197
g
formed in this way which would facilitate the charge carrier
transport along the two dimensional region [16e21,26,27].
Table 1
Photovoltaic parameters of devices based on Polymer:PCBM (1:1).
Polymer Thicknessa
V
oc (V)
b
J
c
FFd he (%)
V
oc (V)
f
V
oc
sc
2
(nm)
(mA/cm ) (%)
(V)
3.6. Electrochemical measurement
P1
P2
95
100
0.70
0.66
1.0
3.6
32
53
0.20
1.26
1.63
1.69
0.84
0.64.
The electrochemical behaviors of the polymers were investi-
a
gated by cyclic voltammogram (Fig. 5). P1 and P2 exhibited quasi-
reversible reduction wave. According to the onset potentials
of ꢁ1.07 V and ꢁ0.98 V, the LUMO level of P1 and P2 were calcu-
lated to be ꢁ3.65 and ꢁ3.73 eV by comparison to ferrocene (4.8 eV
vs. vacuum). According to the simulation results of front molecular
orbitals, the LUMO energy levels were all mainly originated from
the phenanthro-pyrazine unit. Comparing to the benzenepyrazine
The thickness of active layer.
The open-circuit voltage.
The short-circuit current density.
The fill factor.
The power conversion efficiency.
Theoretical Voc from the electrochemistry HOMOs according to Brabec theory (V).
b
c
d
e
f
g
Voc measured from the optical HOMOs.
[
13e15] and thiophenepyrazine, [16e18] the onset potentials were
and a device configuration of indium tin oxide (ITO)/PEDOT:PSS
(40 nm)/active layer (z100 nm)/LiF(1 nm)/Al (100 nm) was utilized
to evaluate the photovoltaic performances of OPVs. The polymer
and PCBM blend films were prepared from chlorobenzene solution,
while the LiF interlayer and Al was thermally deposited to finish the
device fabrication. The photovoltaic performance characteristics of
the OPVs were summarized in Table 1.
decreased nearly 0.3e0.4 eV, indicating the easier injection of
electrons. P1 and P2 showed irreversible oxidation wave. The onset
potentials were at 1.22 V and 1.27 V, respectively, the HOMO level of
P1 and P2 were thus calculated to be ꢁ5.93 and ꢁ5.99 eV by
comparison to ferrocene. The bandgap estimated from the CV were
2
.28 eV for P1 and 2.25 eV for P2, which were different from the
values calculated from the absorption edge. It was because that the
onset potentials in two polymers were not originated from the 1-D
main chain as the result DFT calculated, and the oxidation process
happened firstly on the peripheral fluorene unit, which was more
accessible to the electrode and the redox reaction according
diffusion theory in CV measurement [39]. The value of HOMO level
was exactly the same as that of the fluorene unit as reported in the
literature [40,41]. The values of HOMO levels of both polymers had
big difference compared to the results from DFT, and it was because
that the fluorene unit was as a substituted group in calculated
process, and did not participate in conjugated system and the for-
mation of the frontier molecular orbitals. So, a more reasonable
approach from the values of LUMO (ꢁ3.65 and ꢁ3.73 eV) in CV
measurement and the onsets of the absorption edge (1.49 and
Fig. 6 depicted the current densityevoltage (JeV) curves of the
OPVs measured under the illumination of simulated AM 1.5G
ꢁ2
conditions (100 W m ) in P2 device. As shown in the atomic force
microscopy (AFM) images (Fig. 7) of the blend films, the mor-
phologies of the blend films were moderately homogeneous and
there was no large phase separation, indicating that both of the two
polymers possessed good miscibility with PCBM. Polymer P1
2
exhibited device performance of Voc ¼ 0.70 V, Jsc ¼ 1.0 mA/cm and
FF ¼ 0.32, resulting in PCE ¼ 0.20%. Polymer P2 exhibited an
2
improved performance of Voc ¼ 0.66 V, Jsc ¼ 3.6 mA/cm and
FF ¼ 0.53, resulting in PCE ¼ 1.26%. The value is comparable to those
for some low-bandgap NIR-absorbing organic polymer diodes re-
ported [42,43].
Though P1 exhibited a higher Voc, its PCE was just about sixth of
P2. The better performance of P2 originated from two aspects.
Firstly, P2 showed broader absorption and covering the solar
spectrum better than P1. As shown in Fig. 8, the IPCE curves of the
OPV based on P2 exhibited a broad response covering 380e700 nm,
and the EQE was near 20% in the spectral region from 380 to
1.21 eV) were applied to estimate the values of HOMOs, and they
were thus calculated to be 5.14 eV and 4.94 eV for P1 and P2,
respectively, which were very close to the DFT predicted values.
3.7. Photovoltaic properties
6
00 nm, higher and broader than that of P1 [44]. Secondly, the
ꢀ
dihedral angel between thiophenes on P2 main chain was only 0.2
To demonstrate the potential application of the two conjugated
polymers P1 and P2 in OPVs, PCBM was used as electron acceptor,
Fig. 6. Current densityevoltage (JeV) curves of the polymer solar cell with P2:PCBM
Fig. 5. Cyclic voltammogram of P1 and P2.
(1:1) active layers under simulated AM 1.5 solar irradiation.

6198
Z. Wang et al. / Polymer 54 (2013) 6191e6199
2
Fig. 7. AFM images (10*10
m
m ) of the active layers from the polymers (polymer:PCBM).
The optical bandgap is calculated to be 1.49 and 1.21 eV for P1 and
P2 from the onsets of the absorption spectra. P2 exhibits wholly
coplanarity conformation in 1-D and 2-D direction as revealed by
the theoretical studies, and a higher PCE value of 1.26% of P2/PCBM
device than P1 is finally achieved under simulated AM 1.5 condi-
tions. The 6-folded improved PCE value in P2/PCBM device may be
originated from tuning the dihedral angel between dithiophen and
ꢀ
ꢀ
phenanthro-pyrazine from 25.6 for P1 to 0.2 for P2.
Acknowledgments
This work is financially supported by the National Basic
Research Program of China (973 Program, 2013CB834701), National
Science Foundation of China (Grant No. 21174050, 51203091) and
PCSIRT.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.09.015.
References
[
[
1] Yu G, Hummelen J, Wudl F, Heeger AJ. Science 1995;270:1789e91.
2] Halls JJM, Walsh CA, Greenham NC, Marseglia EA, Friend RH, Moratti SC, et al.
Nature 1995;376:498e500.
Fig. 8. EQE curves of the photovoltaic cells with polymer:PCBM system illuminated by
monochromatic light.
[3] Qin R, Li W, Li C, Du C, Veit C, Schleiermacher H, et al. J Am Chem Soc
2
009;131:14612e3.
and P2 exhibited wholly coplanar conformation as revealed by DFT,
which showed higher level of packing order and benefited for the
transport of carriers, resulting improved FF and nearly 6-folded
higher PCE than P1.
[4] Wong W-Y, Ho C-L. Acc Chem Res 2010;43:1246e56.
5] Halls JJM, Cornil J, dos Santos DA, Silbey R, Hwang DH, Holmes AB, et al. Phys
Rev B 1999;60:5721e7.
[
[
6] Cheng Y, Yang S, Hsu C. Chem Rev 2009;109:5868e923.
[7] Sendur1 M, Balan A, Baran D, Toppare L. J Polym Sci Part A: Polym Chem
011;49:4065e70.
8] Peet J, Kim J, Coates N, Ma W, Moses D, Heeger AJ, et al. Nat Mater 2007;6:
97e500.
2
Additionally, there was large difference between the obtained
[
Voc in OPV device and the Voc from the electrochemistry HOMOs
4
according to Brabec theory. And the difference was little from that
achieved by optical method [45]. A reasonable explanation was that
the fluorene unit only had a substituted effect and did not partic-
ipated in the formation of the frontier molecular orbital as
described in DFT.
[9] Zhang M, Guo X, Li YF. Adv Energy Mater 2011;1:557e60.
[10] Nelson TL, Young TM, Liu JY, Mishra SP, Belot JA, Balliet CL, et al. Adv Mater
2010;22:4617e21.
[
[
11] Li YN, Singh SP, Sonar P. Adv Mater 2010;22:4862e6.
12] Baek MJ, Park H, Dutta P, Lee WH, Kang IN, Lee1 SH. J Polym Sci Part A: Polym
Chem 2013;51:1843e51.
13] Karsten BP, Janssen RAJ. Org Lett 2008;10:3513e6.
14] Wu PT, Bull T, Kim FS, Luscombe CK, Jenekhe SA. Macromolecules 2009;42:
[
[
6
71e81.
4
. Conclusions
[
[
[
15] Li J, Deng X, Zhang Z, Wang Y, Liu Y, He K, et al. J Polym Sci Part A: Polym
Chem 2012;50:4686e94.
16] Huo L, Tan Z, Zhou Y, Zhou E, Han M, Li Y. Macromol Chem Phys 2007;208:
In summary, two 2-dimensional narrow bandgap conjugated
1294e300.
polymers based on the dithiophen and phenanthro-pyrazine units
have been successfully designed and synthesized by the Still
coupling reaction. The obtained polymers show good solubilities in
common organic solvent, high thermal stabilities, and broader
visible absorption bands and narrower optical bandgap than P3HT.
17] Kroon R, Lundin A, Lindqvist C, Henriksson P, Steckler T, Andersson MR.
Polymer 2013;54:1285e8.
18] Song HJ, Lee TH, Han MH, Lee JY, Moon DK. Polymer 2013;54:1072e9.
19] Zou Y, Wan M, Sang G, Ye M, Li Y. Adv Funct Mater 2008;18:2724e32.
20] Sista P, Kularatne RS, Mulholland ME, Wilson M, Holmes N, Zhou X, et al.
J Polym Sci Part A: Polym Chem 2013;51:2622e30.
[
[
[

Z. Wang et al. / Polymer 54 (2013) 6191e6199
6199
[
21] Wang JY, Hau SK, Yip HL, Davies JA, Chen KS, Zhang Y, et al. Chem Mater
[33] Wu Z, Xiong Y, Zou J, Wang L, Liu J, Chen Q, et al. Adv Mater 2008;20:2359e64.
[34] Kenning DD, Mitchell KA, Calhoun TR, Funfar MR, Sattler DJ, Rasmussen SC.
J Org Chem 2002;67:9073e6.
[35] Kitamura C, Tanaka S, Yamashita Y. Chem Mater 1996;8:570e8.
[36] Marshall JA. Chem Rev 2000;100:3163e85.
2011;23:765e7.
[
[
[
22] Schwartz BJ. Annu Rev Phys Chem 2003;54:141e72.
23] Averbeke BV, Beljonne D, Hennebicq E. Adv Funct Mater 2008;18:492e8.
24] Wang M, Hu X, Liu P, Li W, Gong X, Huang F, et al. J Am Chem Soc 2011;133:
9
638e41.
25] Huo L, Zhang S, Guo X, Xu F, Li Y, Hou J. Angew Chem Int Ed 2011;50:
697e702.
26] Qin C, Fu Y, Chui C, Kan C, Xie Z, Wang L, et al. Macromol Rapid Commun
011;32:1472e7.
27] Li S, He Z, Yu J, Chen S, Zhong A, Tang R, et al. J Mater Chem 2012;22:
2523e31.
28] Foster EJ, Jones RB, Lavigueur C, Williams VE. J Am Chem Soc 2006;128:
569e74.
[37] Hancock JM, Gifford AP, Zhu Y, Lou Y, Jenekhe SA. Chem Mater 2006;18:
4924e32.
[38] Zade SS, Bendikov M. Org Lett 2006;23:5243e6.
[39] Wang Z, Lu P, Chen S, Gao Z, Shen F, Zhang W, et al. J Mater Chem 2011;21:
5451e6.
[40] Chi C, Wegner G. Macromol Rapid Commun 2005;26:1532e7.
[41] Zou Y, Zou J, Ye T, Li H, Yang C, Wu H, et al. Adv Funct Mater 2013;23:1781e8.
[42] Xia Y, Deng X, Wang L, Li X, Zhu X, Cao Y. Macromol Rapid Commun 2006;27:
1260e4.
[
[
[
[
9
2
1
8
[
[
29] Wu P, Ren G, Jenekhe SA. Macromolecules 2010;43:3306e13.
30] Yuan J, Zhai Z, Dong H, Li J, Jiang Z, Li Y, et al. Adv Funct Mater 2013;23:
[43] Petersen MH, Hagemann O, Nielsen KT, Jorgensen M, Krebs FC. Sol Energy
Mater Sol Cells 2007;91:996e1009.
8
85e92.
31] Chen H, Hou J, Zhang S, Liang Y, Yang G, Yang Y, et al. Nat Photonics 2009;3:
49e53.
32] Zhang M, Guo X, Yang Y, Zhang J, Zhang Z, Li Y. Polym Chem 2011;2:2900e6.
[44] Dimitrov SD, Nielsen CB, Shoaee S, Tuladhar PS, Du J, McCulloch I, et al. J Phys
Chem.Lett 2012;3:140e4.
[45] Scharbe MC, Mühlbacher D, Koppe M, Denk P, Waldauf C, Heeger AJ, et al. Adv
Mater 2006;18:789e92.
[
[
6