Dithieno[a,e]pentalene based conjugated polymers: Synthesis and characterization

Article abstract of DOI:10.1002/cjoc.201300516

The concept of introducing π-conjugated but anti-aromatic repeating units into conjugated polymers has been explored. The thermal stability, optical and electrochemical properties of dithieno[a,e]pentalenes (DTP) based polymers have been studied. The effect of introducing electron withdrawing repeating units into DTP based polymer on the physical properties of polymer has also been investigated. The new polymers showed broad absorption in visible and near-infrared region. The concept of introducing π-conjugated but anti-aromatic repeating units into conjugated polymers has been explored. The thermal stability, optical and electrochemical properties of dithieno[a,e]pentalenes (DTP) based polymers have been characterized. The effect of introducing electron withdrawing repeating units into DTP based polymer on the physical properties of polymer has also been investigated. The new polymers showed broad absorption in visible and near-infrared region. Copyright

Full text of DOI:10.1002/cjoc.201300516

FULL PAPER
DOI: 10.1002/cjoc. 201300516
Dithieno[a,e]pentalene Based Conjugated Polymers:
Synthesis and Characterization
Chao Hu and Qing Zhang*
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
The concept of introducing π-conjugated but anti-aromatic repeating units into conjugated polymers has been
explored. The thermal stability, optical and electrochemical properties of dithieno[a,e]pentalenes (DTP) based
polymers have been studied. The effect of introducing electron withdrawing repeating units into DTP based poly-
mer on the physical properties of polymer has also been investigated. The new polymers showed broad absorption
in visible and near-infrared region.
Keywords pentalene, conjugated polymer, low-bandgap polymer, anti-aromatic
Introduction
Chemical Company, Alfa Aesar Chemical Company,
Sinopharm Chemical Reagent Co. Ltd. and Darui
Chemical Co. Ltd. Tetrahydrofuran (THF) and toluene
were freshly distilled over sodium wire under nitrogen
prior to use. Other materials were used without further
purification. All flash chromatography separations were
carried out on silica gel (200 － 300 mesh). The
2-bromo-3-iodothiophene,[33] 4-hexylphenylacetylene,[34]
and dibromo monomer 6[35,36] were synthesized accord-
ing to reported methods. Bis(trimethylstannyl) monomer
4 and 5 were reported in our previous work.[37,38]
Ring-fused structures with extended π-conjugation
have received considerable amount of attention as
building blocks for organic semiconducting materials,
recently.[1-16] Fused heteroaromatic units such as thieno-
[3,2-b]thiophene[7,17-22] and thieno[3,4-b]thiophene[23,24]
have been successfully incorporated into many high
performance conjugated polymers for field-effect tran-
sistor (OFET) and organic photovoltaic (OPV) applica-
tions. The five: five fused heterocyclic systems are iso-
electronic with the 10-π-electron pentalene dianion and
can be considered as hetero analogs of the dianion.[25]
The parent pentalene is an eight-π-electron system. It is
antiaromatic and highly reactive.[25-27] Some pentalene
derivatives such as dibenzopentalenes have been per-
ceived as useful building blocks for synthesis of conju-
gated polymers because of their rigid planar and conju-
gated structure.[28-32] Small molecules based on dibenzo-
pentalenes have been synthesized and applied in organic
electronic devices.[31] Substituted dithieno[a,e]penta-
lenes (DTPs) are stable compounds and the efficient
synthetic methods for them have been developed
lately.[28,29] The DTPs can be interesting building blocks
for conjugated polymers. However, the application of
DTP derivatives as building blocks for conjugated
polymer has never been reported. Herein, we reported
the first synthesis of conjugated polymers with DTP as
building blocks. The optical and electrochemical prop-
erties of polymers were also investigated.
Measurements and characterization
NMR spectra were recorded on a Mercury plus 400
MHz machine. GPC analysis was performed on a
Shimadzu LC-20 A coupled with refractive index de-
tector using THF as eluent with polystyrenes as stan-
dards. TGA was carried out on a TA instrument
QS000IR at a heating rate of 20 ℃•min−1 under nitro-
gen gas flow. UV-vis spectra were recorded on a Perkin
Elmer Lambda 20 UV-Vis spectrophotometer. Electro-
chemical measurements were conducted with a CHI 600
electrochemical analyzer under nitrogen atmosphere in a
deoxygenated anhydrous solution of tetra-n-butylam-
moniumhexafluorophosphate (0.1 mol/L) in acetonitrile.
A platinum electrode was used as a working electrode, a
platinum wire was used as an auxiliary electrode, and an
Ag/Ag+ electrode was used as a reference electrode.
Polymer thin films were coated on platinum electrode
and ferrocene was chosen as a reference. A potential
scan rate of 50 mV•s−1 was used for all experiments.
Experimental
Synthesis of monomer
Materials
2-Bromo-3-(4-hexylphenyl)ethynylthiophene (1)
Chemicals were purchased from Sigma-Aldrich
*
E-mail: qz14@sjtu.edu.cn; Tel.: 0086-021-34202726; Fax: 0086-021-54741297
Received July 3, 2013; accepted August 16, 2013; published online October 2, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300516 or from the author.
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Chin. J. Chem. 2013, 31, 1404—1408

Dithieno[a,e]pentalene Based Conjugated Polymers: Synthesis and Characterization
A mixture of 2-bromo-3-iodothiophene (5.07 g, 17.5
mmol), bis(triphenylphosphine)palladium(II) dichloride
(0.061 g, 0.087 mmol) and copper(І) iodide (0.033 g,
0.173 mmol) in triethylamine (150 mL) was degassed
for 10 min. 4-Hexylphenylacetylene (3.43 g, 18.4 mmol)
was added to the solution. After stirring at room tem-
perature for 12 h, the mixture was filtered through a
celite pad and the filtrate was collected. The solvent was
removed under reduced pressure and residue was puri-
fied by chromatography on silica gel with petroleum
ether (boiling range 60－90 ℃) as an eluent to give the
range 60－90 ℃) as an eluent to give the titled product
as a brown solid (0.35 g, 44%). H NMR (400 MHz,
1
CDCl3) δ: 7.53 (d, J＝8.0 Hz, 4H), 7.24 (d, J＝8.0 Hz,
4H), 6.80 (d, J＝4.8 Hz, 2H), 6.70 (d, J＝4.8 Hz, 2H),
2.64 (t, J＝8.0 Hz, 4H), 1.68－1.58 (m, 4H), 1.50－
1.19 (m, 12H), 0.90 (t, J＝6.8 Hz, 6H); 13C NMR (100
MHz, CDCl3) δ: 148.35, 145.97, 142.77, 139.26, 137.18,
130.79, 129.02, 127.85, 127.36, 121.46, 36.22, 31.96,
31.49, 29.29, 22.85, 14.36.
Compound 3
1
N-Bromosuccinimide (0.210 g, 1.18 mmol) was
added to the solution of 2 (0.30 g, 0.561 mmol) in chlo-
roform (45.0 mL) and acetic acid (45.0 mL) at 0 ℃.
The mixture was stirred for 1 h at 0 ℃ and then was
quenched with water. The mixture was extracted with
diethyl ether (100 mL) for three times. The combined
organic layer was dried with anhydrous sodium sulfate.
Solvent was removed under reduced pressure and the
residue was purified by recrystalization in acetone to
afford a dark orange solid (0.30 g, 76%). 1H NMR (400
MHz, CDCl3) δ: 7.31 (d, J＝8.0 Hz, 4H), 7.18 (d, J＝
8.0 Hz, 4H), 6.56 (s, 2H), 2.63 (t, J＝8.0 Hz, 4H), 1.68
－1.58 (m, 4H), 1.50－1.19 (m, 12H), 0.90 (t, J＝6.8
Hz, 6H); 13C NMR (100 MHz, CDCl3) δ: 147.10,
145.65, 142.06, 138.31, 138.28, 130.05, 129.22, 127.67,
124.42, 113.76, 36.21, 31.92, 31.43, 29.26, 22.82, 14.33.
Anal. calcd for C36H36Br2S2: C 62.43, H 5.24; found C
62.11, H 5.18.
product as a colorless liquid (4.31 g, 71%). H NMR
(400 MHz, CDCl3) δ: 7.49 (d, J＝8.0 Hz, 2H), 7.21 (d,
J＝2.4 Hz, 1H), 7.18 (d, J＝8.0 Hz, 2H), 7.04 (d, J＝
2.4 Hz, 1H), 2.63 (t, J＝8.0 Hz, 2H), 1.68－1.58 (m,
2H), 1.50－1.19 (m, 6H), 0.91 (t, J＝6.8 Hz, 3H); 13C
NMR (100 MHz, CDCl3) δ: 144.03, 131.82, 130.02,
128.74, 125.96, 124.97, 120.21, 116.82, 93.64, 82.69,
36.20, 31.96, 31.46, 29.18, 22.87, 14.37.
Scheme 1 Synthesis of monomers
C6H13
I
C6H13
Cs2CO3, Pd2(dba)3
CsF, P(t-Bu)3 BF4
benzene-1,4-diol
.
Br
S
1,4-dioxane
44%
Pd(PPh3)2Cl2,
CuI, Et3N
Br
S
71%
1
Synthesis of polymers
C6H13
C6H13
Polymer P1
Tris(dibenzylideneacetone)dipalla-
dium (0.0040 g, 0.0044 mmol) and tri-o-tolylphosphine
(0.0050 g, 0.0160 mmol) were added to a solution of 3
(0.140 g, 0.202 mmol) and 4 (0.251 g, 0.202 mmol) in
toluene (12.0 mL) under nitrogen. The solution was
subjected to three cycles of evacuation and admission of
nitrogen. The mixture was heated to 110 ℃ for 24 h.
After cooled to room temperature, the mixture was
poured into methanol (100 mL) and was stirred for 2 h.
The precipitate was collected by filtration. The product
was purified by washing with methanol and hexane in a
Soxhlet extractor for 24 h each time. It was extracted
with hot chloroform in a Soxhlet extractor for 24 h. Af-
ter removing solvent, a black solid was collected (0.25 g,
S
S
Br
NBS
Br
S
S
CHCl3, HOAc
76%
C6H13
C6H13
3
2
4,8-Di(4-hexylphenyl)-2,6-dithia-dicyclopenta[a,e]-
pentalene (2) Hydroquinone (0.634 g, 5.76 mmol),
cesium carbonate (1.88 g, 5.77 mmol), cesium fluoride
(0.962 g, 6.334 mmol), tris(dibenzylideneacetone)di-
palladium (0.040 g, 0.044 mmol), tri-tert-butylphospho-
niumtetrafluoroborate (0.050, 0.173 mmol) were added
into a reaction vessel under nitrogen. A degassed solu-
tion of 1 (1.0 g, 2.879 mmol) in 1,4-dioxane (20 mL)
was transferred to the vessel. The vessel was sealed with
a Teflon stopcock and was heated at 140 ℃ for 48 h.
After cooled to room temperature, the mixture was fil-
tered through a celite pad and was washed thoroughly
with chloroform. The filtrate was collected and evapo-
rated to dryness. The residue was purified by chroma-
tography on silica gel with petroleum ether (boiling
1
86%). H NMR (CDCl3, 400 MHz) δ: 6.2－7.9 (br,
14H), 2.4－3.2 (br, 12H), 0.4－2.1 (br, 98H). Anal.
calcd for C94H124S6: C 78.06, H 8.64; found C 78.32, H
8.66; GPC: Mn＝20.1 kDa; PDI＝4.52.
Polymer P2 Tris(dibenzylideneacetone)dipalladium
(0.0040 g, 0.0044 mmol) and tri-o-tolylphosphine
(0.0050 g, 0.0160 mmol) were added to a solution of 3
(0.065 g, 0.094 mmol), 6 (0.075 g, 0.094 mmol), and 5
(0.191 g, 0.188 mmol) in toluene (12.0 mL) under ni-
trogen. The solution was subjected to three cycles of
evacuation and admission of nitrogen. The mixture was
heated to 110 ℃ for 8 h. After cooled to room tem-
perature, the mixture was poured into methanol (100
Chin. J. Chem. 2013, 31, 1404—1408
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Scheme 2 Synthesis of polymers
C6H13
C10H21
S
C10H21
C10H21
S
C10H21
Stille
cross-coupling
S
S
S
*
Sn
Sn
+
3
S
*
S
S
n
S
S
C10H21
C10H21
C10H21
C10H21
C6H13
4
P1
C6H13
S
C6H13
S
N
N
Stille
C12H25
S
Br
S
cross-coupling
Sn
Sn
+
+
3
S
S
C
12H25
Br
S
6
C6H13
C6H13
5
C6H13
C6H13 C6H13
S
C6H13 C6H13
S
S
N
N
C12H25
S
S
S
S
*
S
n
S
S
m
*
S
C12H25
S
S
P2
C6H13 C6H13
C6H13 C6H13
C6H13
mL) and was stirred for 2 h. The precipitate was col-
lected by filtration. The product was purified by wash-
ing with methanol and hexane in a Soxhlet extractor for
24 h each time. It was extracted with hot chloroform in
a Soxhlet extractor for 24 h. After removing solvent, a
black solid was collected (0.21 g, 88%). 1H NMR
(CDCl3, 400 MHz) δ: 6.8－8.1 (b, 22H), 2.5－3.0 (b,
24H), 1.0－2.0 (b, 120H), 0.6－1.0 (b, 36H). Anal.
calcd for C158H202N2S13: C 74.53, H 8.00, N 1.10; found
C 74.62, H 8.74, N 1.01; GPC: Mn＝14.5 kDa; PDI＝2.32.
pentalenes (2) was synthesized according to Tilley’s
method with slight modification. The Pd-catalyzed re-
ductive homocoupling of haloenynes was carried out
with tris(dibenzylideneacetone)dipalladiumas catalyst
and
tri-tert-butylphosphoniumtetrafluoroborate
as
ligand, hydroquinone as reducing agent, cesium carbon-
ate as base and cesium fluoride as additive. The bromi-
nation of 2 with NBS at 0 ℃ gave the monomer 3 in
77% yield. Benzodithiophene (BDT) was chosen as
co-monomer for P1. The BDT and 4,7-bis(4-dodecyl-
thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBTz) were
chosen as co-monomers for P2. The BDT is a rigid pla-
nar structure with extended π-conjugation. This struc-
ture facilitates π-electron delocalization within the
polymer chain and improves π-π interactions between
polymer chains in solid states.[24] The DTBTz was a
repeating unit for enhancing the donor/acceptor interac-
tion of polymers, therefore, reducing the bandgap of
polymer.[36]
Results and Discussion
Synthesis and characterization
The synthetic routes for monomer and polymers are
shown in Schemes 1 and 2. The precursor, 2-bromo-3-
(4-hexylphenyl)ethynylthiophene (1) was synthesized
through Sonogashira cross-coupling reaction between
2-bromo-3-iodothiophene and 4-hexylphenylacetylene
at room temperature in 71% yield. The dithieno[a,e]-
The polymer P1 was synthesized by Stille cross-
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Chin. J. Chem. 2013, 31, 1404—1408

Dithieno[a,e]pentalene Based Conjugated Polymers: Synthesis and Characterization
coupling reaction in the presence of tris(dibenzylidene-
100000
acetone)dipalladium as catalyst and tri-o-tolylphosphine
as ligand. The P1 was purified by washing with metha-
nol and hexane successively in a Soxhlet extractor for
24 h each time, then were extracted with hot chloroform
in a Soxhlet extractor for 24 h. The copolymer P2 con-
sisting of 2,4,7-bis(4-dodecylthiophen-2-yl)benzo[c]-
[1,2,5]thiadiazole (6), 3 and 5 at 1∶1∶2 ratio was also
synthesized by Stille cross-coupling reaction. The
polymer was purified by the similar method as that for
P1. Both of the two polymers are black solid and readily
soluble in common solvent such as chloroform, toluene
and chlolobenzene.
P1
P2
80000
60000
40000
20000
0
400
600
800
Wavelength/nm
Thermal stability
Thermogravimetric analyses of P1 and P2 are shown
in Figure 1. Both polymers showed good thermal sta-
bilities with decomposition temperatures over 250 ℃.
1.0
0.8
0.6
0.4
0.2
0.0
P1
P2
100
P1
P2
80
60
40
20
0
300
400
500
600
700
800
900
Wavelength/nm
Figure 2 UV-Vis absorption spectra of P1 and P2 in chloroform
solution (top) and as thin films (bottom).
100
200
300
400
500
Table 1 Optical properties of P1 and P2
Temperature/oC
Solution λabs a/nm Film λoanbsset /nm
Eg /eV
b
Polymer
Figure 1 The TGA plots of P1 and P2.
onset
P1
P2
812
707
832
773
1.49
1.60
Optical properties
The UV-vis absorption spectra of P1 and P2 in chlo-
roform solution and the spectra of thin films are shown
in Figure 2. The optical properties of polymers are sum-
marized in Table 1. The absorption peak positions were
at 376, 513 and 691 nm for P1 and 376, 418 and 589 nm
for P2 in chloroform soltion, respectively; and the ab-
sorption peak positions were at 376, 513 and 693 nm for
P1 and 383, 441 and 589 nm for P2 in thin film, respec-
tively. In chloroform solution, the absorption onsets
were 812 and 707 nm for P1 and P2; in thin flim, the
absorption onsets were 832 and 773 nm for P1 and P2.
The optical bandgaps were calculated to be 1.49 and
1.60 eV for P1 and P2 (Table 1). The thin-film absorp-
tion onsets of polymers were red-shifted by about 20 nm
for P1 and 66 nm for P2 compared to their solution ab-
sorption onsets. The thin-film absorption spectrum of
P2 became broad in the 300 to 700 nm region. The
absorption of P2 covered the whole visible region and it
might be used as effective material for harvesting solar
energy.
a Chloroform solution (2×10−5 and 1×10−5 mol/L for P1 and P2,
respectively); b Eg=1240/ λabs
.
onset
P2 are displayed in Figure 3, and the electrochemical
properties are summarized in Table 2. Both P1 and P2
showed irreversible oxidizing cycles and quasi-reversi-
ble reducing cycles. The HOMO levels were estimated
from the onset oxidation. The HOMO energy levels of
P1 and P2 were calculatd to be −5.19 and −5.40 eV,
respectively (Table 2). The LUMO energy levels of P1
and P2 were estimated from the onset reduction. The
LUMO energy levels of P1 and P2 were calculatd to be
−3.39 and −3.76 eV, respectively. The electrochemical
bandgaps were calculated to be 1.80 and 1.64 eV for P1
and P2, respectively. After addition of benzothiadiazole
repeating units, the HOMO and LUMO energy levels of
copolymer P2 were both reduced. However, the benzo-
thiadiazole units which were electron-accepting units
had larger effect on the LUMO level than on the HOMO
level of the copolymer. This resulted in overall decreas-
ing of electrochemical bandgap of P2.
Electrochemichal properties
The cyclic voltammograms of the polymers P1 and
Chin. J. Chem. 2013, 31, 1404—1408
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Hu & Zhang
FULL PAPER
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P2
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Potential/V(vs. Ag/Ag+)
Figure 3 Cyclic voltammograms for P1 and P2 polymer films.
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LUMOa HOMOb
Egelectrochem c/V
Eorendset /V Eoonxset /V
Polymer
/eV
/eV
0.48
0.69
1.80
P1
−1.32
−0.95
−3.39
−5.19
−5.40
1.64
P2
−3.76
red
onset
ox
onset
a
b
LUMO ＝ −(4.71 ＋ E
);
HOMO ＝ −(4.71 ＋ E
);
c Egelectrochem ＝LUMO−HOMO.
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Conclusions
Two new polymers with dithieno[a,e]pentalenes as
repeating units have been synthesized. The conjugated
but anti-aromatic repeating unit can be used as building
block for low bandgap polymer. They can provide
π-orbital overlap in the conjugated main-chain. The
fused heterocyclic structures provide stability for the
polymers. The thermal stability, optical and electro-
chemical properties of dithieno[a,e]pentalenes based
polymer were characterized for the first time. The new
polymers showed broad absorption in visible and
near-infrared region and they might be used as material
for harvesting solar energy.
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Chem. 2013, 4, 3225.
This work was supported by National Natural Sci-
ence Foundation of China (NSFC Grant Nos. 21174084,
21274087) and by Doctoral Fund of Ministry of Educa-
tion of China (Grant No. 20120073110032).
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