Ethynylene-linked oligomers based on benzodithiophene: Synthesis and photoelectric properties

Article abstract of DOI:10.1002/cjoc.201400079

Two conjugated ethynyl-linked oligomers, oligo(benzodithiophene-ethynylene- benzothiadiazole) (O1) and oligo(benzodithiophene-ethynylene-carbazole) (O2), were synthesized by Sonogashira coupling reaction. Their degrees of polymerization were 7 and 10, respectively. Their photophysical and electrochemical properties were investigated. O1 exhibitd two strong absorption bands at 404 nm and 483 nm, and O2 at 401 nm and 429 nm. The results of UV-Vis, cyclic voltammetry (CV) and theoretical calculations showed that O1 has a narrower band gap than O2. The conductivities of O1 and O2 were 1.05×10^-15 and 6.98×10^-16 S/cm, respectively, and would increase to 1.23×10^-10 and 1.05×10 ^-10 S/cm after doping with iodine. Two oligomers based on benzodithiophene with different structures and energy levels. Copyright

Full text of DOI:10.1002/cjoc.201400079

FULL PAPER
DOI: 10.1002/cjoc.201400079
Ethynylene-Linked Oligomers Based on Benzodithiophene:
Synthesis and Photoelectric Properties
Songyang Jiang, Yuwen Ma, Yingyi Wang, Chengyun Wang,* and Yongjia Shen
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai 200237, China
Two conjugated ethynyl-linked oligomers, oligo(benzodithiophene-ethynylene-benzothiadiazole) (O1) and
oligo(benzodithiophene-ethynylene-carbazole) (O2), were synthesized by Sonogashira coupling reaction. Their de-
grees of polymerization were 7 and 10, respectively. Their photophysical and electrochemical properties were in-
vestigated. O1 exhibitd two strong absorption bands at 404 nm and 483 nm, and O2 at 401 nm and 429 nm. The re-
sults of UV-Vis, cyclic voltammetry (CV) and theoretical calculations showed that O1 has a narrower band gap than
O2. The conductivities of O1 and O2 were 1.05×10−15 and 6.98×10−16 S/cm, respectively, and would increase to
1.23×10−10 and 1.05×10−10 S/cm after doping with iodine.
Keywords benzodithiophene, conjugated polymer, Sonogashira reaction, photoelectric property, conductivity
Introduction
in small molecules and polymers, mostly in OFETs and
in OPVs.[3-10]
Organic photoelectric materials have recently at-
tracted intense research interest because of their struc-
tural diversity. Organic semiconductors consisting of
these materials have better flexibility and lower produc-
tion cost. Moreover, the physical properties (optical be-
havior, charge carrier mobility, HOMO/LUMO energy
levels, and structural ordering) of these organic semi-
conductors can be tuned by various chemical function-
alizations. There are two kinds of charge transports in
polymer transistors, namely, interchain transport (π-π
stacking orientation) and intrachain transport. The speed
of intrachain transport is much faster than that of inter-
chain. Additionally, polymers usually possess good
flexibility and thermal stability, which render them the
most promising materials for flexible devices with large
area.[1] The rapid development of organic photoelectric
materials will definitely make contributions for the
global energy crisis.
Polythiophenes, including one-dimensional, two-
dimensional and three-dimensional conjugated systems,
are one of the largest families of organic semiconduc-
tors, which have been used in organic photovoltaic cells
(OPVs) due to their high charge-carrier mobility and
facile synthesis to tune their energy levels.[2] Benzo-
[1,2-b:4,5-b']dithiophene (BDT) has a symmetric and
planar conjugated structure, so BDT-based conjugated
polymers present a tight and regular stacking, which
was widely studied as a photoelectric functional mate-
rial. BDT unit has been already used as conjugated core
BDT was first used in the organic thin film transistor.
In 1997, Katz et al. reported a new type of p-type or-
ganic semiconductor based on benzodithiophene build-
ing blocks with expected thermal stability.[11] Subse-
quently. Ong et al. found that the polymer exhibited
excellent field-effect transistor properties with mobility
of 0.15 cm2•V−1•s−1 in thin-film transistor when BDT
unit was linked with other electron transport unit.
Moreover the preparation of thin-film semiconductor
could be processed without thermal annealing.[8]
In 2007, a polymer based on BDT-thiophene was
applied in OFETs with a high field-effect transistor mo-
bility of 0.25 cm2•V−1•s−1 and current on/off ratio of
105－106 when measured in ambient conditions.[9] In
recent years, the field-effect property was found to be
improved by introducing aromatic ethinyl to BDT
unit.[12-14]
Conjugated polymers based on BDT units were first
applied in BHJ solar cells by Hou et al. at 2008. In their
following work, they repeatedly used BDT units as a
component for the construction of conjugated polymers,
and succeeded in improving the PCE of OPVs beyond
7% in 2010.[15] And more BDT-containing photoelectric
materials were studied and reported,[16-22] indicating that
BDT plays a pivotal role in photovoltaic polymers and
other relevant applications.
In this paper, BDT was selected as a donor unit due to
its excellent electron donating capability and electron
transmiting capability, and benzothiadiazole[23-29] or
*
E-mail: cywang@ecust.edu.cn (C. Y.Wang); Tel.: 0086-021-64252967; Fax: 0086-021-64252967
Received February 12, 2014; accepted March 28, 2014; published online April 22, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400079 or from the author.
298
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Ethynylene-Linked Oligomers Based on Benzodithiophene: Synthesis and Photoelectric Properties
carbazole[30-32] was linked with BDT via acetylene as a
bridge to form a D-linker-A conjugated system. By
comparison, carbazole showed relatively weaker elec-
tron-withdrawing ability than benzothiadizole. Fur-
thermore, due to the rigidity of acetylene bond, the new
oligomers exhibited more reasonable planarity and ri-
gidity, contributing a better π-π stacking effect which
would further enhance the electron transmission as a
whole. The two novel oligomers were discussed from
their optoelectronic property as well as conductivity.
MHz, CDCl3) δ: 7.48 (d, J＝5.6 Hz, 2H, thienyl H),
7.37 (d, J＝5.6 Hz, 2H, thienyl H), 4.27 (t, J＝6.8 Hz,
4H, OCH2), 1.87 (q, J＝6.8 Hz, 4H, CH2), 1.53－1.58
(m, 4H, CH2), 1.38－1.25 [m, 16H, (CH2)4], 0.88 (t, J＝
7.2 Hz, 6H, CH3).
Synthesis of 2,6-dibromo-4,8-dioctyloxybenzo-
[1,2-b:4,5-b']dithiophene (5) Compound 4 (1.02 g,
2.28 mmol) was dissolved into 30 mL of methylene
chloride in a 100 mL three-neck flask. Bromine (0.764 g,
4.78 mmol) was dissolved into 20 mL of methylene
chloride in a funnel and slowly dropped into the flask
under an ice-water bath, and then the mixture was
stirred for 7 h at room temperature. After removing sol-
vent, the crude product was purified by column chro-
matography (methylene chloride∶petroleum ether＝
1∶7), compound 5 (1.238 g, 2.048 mmol, yield 93.65%)
Experimental
Materials
All solvents and reagents, unless otherwise stated,
were CP or AR level. Solvents for chemical synthesis
such as THF, triethylamine were purified by dehydra-
tion and distillation with standard methods. Pd(PPh3)2-
Cl2 was synthesized according to the reference and
stored in dry container.[33]
1
was obtained as white solid. m.p. 58.3－59.3 ℃; H
NMR (400 MHz, CDCl3) δ: 7.42 (s, 2H, thienyl H),
4.19 (t, J＝6.8 Hz, 4H, OCH2), 1.83 (quintuple, J＝6.8
Hz, 4H, CH2), 1.49－1.55 (m, 4H, CH2), 1.37－1.30 [m,
16H, (CH2)4], 0.90 (t, J＝6.8 Hz, 6H, CH3); 13C NMR
(100 MHz, CDCl3) δ: 142.3, 131.2, 130.9, 123.2, 115.0,
74.2, 31.8, 30.4, 29.4, 29.3, 26.0, 22.7, 14.1; HRMS
(ESI) m/z: [M＋H]＋ calcd for C26H37Br2O2S2 603.0602,
found 603.0604.
Instruments and measurements
1H NMR and 13C NMR data were obtained from a
Bruker Avance III 400 (in CDCl3, TMS as the internal
standard). MS data were obtained by Micromass
LCTTM (HREI-TOF). FT-IR spectra were recorded on
a Nicolet Magna-IR 550 spectrometer. The UV-Vis
spectra were recorded by a Varian Cary 100 UV-Vis
Spectrometer (CHCl3 as solvent). Fluorescence spectra
were recorded by a Varian Cary Eclipse Fluorescence
Spectrometer (CHCl3 as solvent). Cyclic voltammo-
grams were performed with a Versastat II electro-
chemical workstation. Thermogravimetric analysis
(TGA) was carried on a Netzsch STA 409 PC/PG under
nitrogen atmosphere. Molecular weights of the polymer
were obtained on a Waters 1515 using a calibration of
curve of polystyrene standards, with tetrahydrofuran as
the eluent. The conductivity was measured by PC408
digital insulation resistor tester at room temperature.
Synthesis of 2,6-trimethylsilyl-4,8-dioctyloxy-
benzo[1,2-b:4,5-b']dithiophene (6)
Compound 5
(1.01 g, 1.67 mmol) was dissolved into 30 mL THF and
20 mL triethylamine in a 100 mL three-neck flask, de-
gassed with Ar over 30 min. Then, Pd(PPh3)2Cl2 (58.5
mg, 0.08 mmol) and CuI (31.8 mg, 0.167 mmol) were
added into the flask under Ar atmosphere. Subsequently,
trimethylsilylacetylene (1.31 g, 13.34 mmol) was added
dropwise under Ar atmosphere. The mixture was stirred
at room temperature for 7 h. After removing solvent, the
crude product was purified by column chromatography
(methylene chloride∶petroleum ether＝1∶15), com-
pound 6 (0.962 g, 1.505 mmol, yield 90.18%) was ob-
1
tained as yellow solid. m.p. 99.6－101.2 ℃; H NMR
(400 MHz, CDCl3) δ: 7.56 (s, 2H, thienyl H), 4.21 (t,
J＝6.8 Hz, 4H, OCH2), 1.84 (quintuple, J＝6.8 Hz, 4H,
CH2), 1.51－1.54 (m, 4H, CH2), 1.37－1.30 [m, 16H,
(CH2)4], 0.90 (t, J＝6.8 Hz, 6H, CH3), 0.28 (s, 18H,
SiCH3); 13C NMR (100 MHz, CDCl3) δ: 144.1, 132.0,
130.4, 126.2, 123.1, 101.9, 98.0, 74.4, 32.0, 30.7, 29.6,
29.5, 26.2, 22.9, 14.4, 0.22; HRMS (ESI) m/z: [M＋H]＋
calcd for C36H55O2S2Si2 639.3182, found 639.3177.
Synthesis of 2,6-bisacetenyl-4,8-dioctyloxybenzo-
[1,2-b:4,5-b']dithiophene (7) Compound 6 (0.855g,
1.338 mmol) was dissolved into 20 mL THF in a 50 mL
flask, then TBAF (1.267 g, 4 mmol) was added into the
flask. The mixture was stirred for 40 min. Subsequently,
the solvent was removed and the crude product was pu-
rified by column chromatography (methylene chlo-
ride∶petroleum ether＝1∶7), compound 7 (0.637 g,
1.288 mmol, yield 96.24%) was obtained as claybank
solid. m.p. 66.2－67.8 ℃; 1H NMR (400 MHz, CDCl3)
δ: 7.62 (s, 2H, thienyl H), 4.23 (t, J＝6.8 Hz, 4H,
Synthesis
Synthesis of 4,8-dioctyloxybenzo[1,2-b:4,5-b']-
dithiophene (4) Compound 3 (0.96 g, 4.36 mmol) and
zinc powder (0.682 g, 10.43 mmol) were suspended in
15 mL of water, then NaOH (2.6 g, 650 mmol) was
added into the mixture. The mixture was heated to re-
flux for 1 h. During the reaction, the color of the mix-
ture changed from yellow to red and then to orange.
Subsequently, 1-bromooctane (2.53 g, 13.12 mmol) and
TBAB (0.28 g, 0.87 mmol) were added into the flask.
After being refluxed for 12 h, the reactant was poured
into 70 mL cold water and extracted by 100 mL of me-
thylene chloride for three times. The organic layer was
dried by anhydrous Na2SO4. After removing solvent, the
crude product was purified by column chromatography
(methylene chloride∶petroleum ether＝1∶15), com-
pound 4 (1.45 g, 3.25 mmol, yield 74.3%) was obtained
1
as white crystal. m.p. 46.0－47.1 ℃; H NMR (400
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Jiang et al.
FULL PAPER
OCH2), 3.48 (s, 2H, alkynyl H), 1.85 (quintuple, J＝6.8
Hz, 4H, CH2), 1.50－1.55 (m, 4H, CH2), 1.36－1.30 [m,
16H, (CH2)4], 0.90 (t, J＝6.8 Hz, 6H, CH3); 13C NMR
(100 MHz, CDCl3) δ: 144.0, 131.7, 130.2, 126.6, 121.9,
100.6, 83.5, 74.2, 31.8, 30.5, 29.4, 29.2, 26.0, 22.7, 14.1;
HRMS (ESI) m/z: [M＋H]＋ calcd for C30H39O2S2
495.2391, found 495.2390.
room temperature, the cold reaction mixture was poured
into 100 mL water. Subsequently, the mixture was fil-
tered, and filter cake was washed by water. The crude
product was purified by recrystallization from n-hexane.
Compound 11 (2.12 g, 3.98 mmol, yield 80%) was ac-
quired as a earthy yellow powder. m.p. 97.0－98.1 ℃;
1H NMR (400 MHz, CDCl3) δ: 8.33 (s, 2H, ArH), 7.71
(d, J＝8.8 Hz, 2H, NH), 7.18 (dd, J1＝1.6 Hz, J2＝8.8
Hz, 2H, ArH), 4.22 (t, J＝7.2 Hz, 2H, NCH2), 1.83－
1.78 (m, 2H, CH2), 1.30－1.22 (m, 10H, CH2), 0.86 (t,
J＝6.8 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ:
139.5, 134.5, 129.4, 124.0, 110.9, 81.6, 43.3, 31.8, 29.3,
29.1, 28.8, 27.2, 22.6, 14.1; HRMS (ESI) m/z: [M＋H]＋
calcd for C20H24I2N 531.9998, found, 531.9996.
Synthesis of benzothiadiazole (8) O-Phenylenedi-
amine (3.25 g, 0.03 mol) was dissolved into 60 mL me-
thylene chloride and 18 mL triethylamine in a 250 mL
three-neck flask, then SOCl2 (7 mL, 0.096 mol) was
added dropwise in 1 h through a funnel. Kept the mix-
ture refluxing for 16 h, then 30 mL water and 80 mL 1
mol/L NaOH aqueous solution were successively added
when the reactant cooled to room temperature. The
mixture was extracted by 100 mL of methylene chloride
for three times. The organic layer was dried by anhy-
drous Na2SO4. After removing solvent, compound 8
(3.326 g, 24.43 mmol, yield 81.3%) was obtained as red
Synthesis of oligo(2,6-bisacetenyl-4,8-dioctyloxy-
benzo[1,2-b:4,5-b']dithiophene-alt-3,6-dibromobenzo-
thiadiazole) (O1) Compound 7 (123.7 mg, 0.25 mmol)
and compound 9 (73.5 mg, 0.25 mmol) were dissolved
in the mixture of tetrahydrofuran (8 mL) and Et3N (5
mL) in a 50 mL three-neck flask, then degassed with Ar
over 30 min. Then, Pd(PPh3)4 (30 mg) and CuI (10 mg)
were added into the flask under Ar atmosphere. The
mixture was stirred at room temperature for 12 h. Sub-
sequently, the mixture was filtered and the solvent was
removed. The residue was dissolved in 3 mL CHCl3,
and precipitated with adding 35 mL methanol. The pro-
cedure was repeated 3 times to remove unreacted
monomers and low molecular weight oligomers. After
this procedure, the precipitation was collected by vac-
uum filtration and dried in vacuum for 3 h at 60 ℃. The
oligomer O1 (100 mg, 50.76%) was obtained as red
1
brown crystal. H NMR (400 MHz, CDCl3) δ: 8.04－
8.00 (m, 2H, ArH), 7.62－7.57 (m, 2H, ArH).
Synthesis of 3,6-dibromobenzothiadiazole (9)
Compound 8 (1.36 g, 0.01 mol) was dissolved into 35
mL 40% HBr aqueous solution in a 250 mL three-neck
flask. Then 30 mL 40% HBr and 1.6 mL Br2 were
added dropwise from a funnel. Subsequently, another 20
mL 40% HBr was added again from funnel. Kept the
reactant refluxing for 12 h, and then cooled to room
temperature. The reaction mixture was poured into ex-
cess saturated NaHSO3 aqueous solution. Then the
mixture was filtrated, and the yellowish white solid was
washed by 30 mL of water thrice and 20 mL diethyl
ether thrice. The crude product was purified by recrys-
tallization from ethyl alcohol. Compound 9 (1.901 g,
6.47 mmol, yield 64.27%) was obtained as a yellowish
1
brown powder. λmax: 404, 483 nm; H NMR (400 MHz,
CDCl3) δ: 7.88－7.45 (m, 4H, ArH), 4.30－4.18 (m, 4H,
OCH2), 2.01－1.89 (m, 4H, CH2), 1.40－1.25 (m, 20H,
CH2), 0.90 (t, J＝6.8 Hz, 6H, CH3); IR (KBr) ν: 3410,
2147, 1637, 879 cm−1.
Synthesis of oligo(2,6-bisacetenyl-4,8-dioctyloxy-
benzo[1,2-b:4,5-b']dithiophene-alt-N-octyl-3,6-diio-
1
white powder. m.p. 187.2－187.3 ℃; H NMR (400
MHz, CDCl3) δ: 7.74 (s, 2H, ArH); 13C NMR (100 MHz,
CDCl3) δ: 132.4, 122.7, 113.9.
Synthesis of 3,6-diiodocarbazole (10) Carbazole
(3 g, 17.94 mmol), KI (3.9 g, 23.5 mmol) and KIO3 (5.1
g, 23.5 mmol) were dissolved into 75 mL HAc in a 250
mL three-neck flask, and the temperature of the mixture
was maintained at 80 ℃ for 6 h. After cooling to room
temperature, the mixture was filtrated, and the crude
product was washed successively by water, saturate
Na2CO3, saturate Na2S2O3 and water. The crude product
was purified by recrystallization from ethyl alcohol.
compound 10 (5.0 g, 11.93 mmol, yield 66.52%) was
docarbazole) (O2)
O2 (0.147 g, 70.91%) was
1
prepared similarly to O1. λmax: 401, 429 nm; H NMR
(400 MHz, CDCl3) δ: 8.40－7.16 (m, 8H, ArH), 4.30－
4.27 (m, 6H, OCH2 and NCH2), 1.89－1.87 (m, 6H,
CH2), 1.40－1.25 (m, 30H, CH2), 0.92－0.85 (m, 9H,
CH3); IR (KBr) ν: 3410, 2135, 1286, 1261, 1033 cm−1.
All the spectra of H NMR, 13C NMR and other
1
relevant data were list in Supporting Information.
Results and Discussion
1
obtained as a white solid. m.p. 204.5.5－204.7 ℃; H
Synthesis and characterization
NMR (400 MHz, CDCl3) δ: 8.33 (d, J＝1.6 Hz, 2H,
ArH), 8.07 (s, 1H, NH), 7.68 (dd, J1＝1.6 Hz, J2＝8.8
Hz, 2H, ArH), 7.22 (d, J＝8.8 Hz, 2H, ArH).
Synthesis of N-octyl-3,6-diiodocarbazole (11)
Compound 10 (2.1 g, 5 mmol) was dissolved into 20
mL DMF in a 100 mL three-neck flask, 1-bromooctane
(1.16 g, 6 mmol) and K2CO3 (3.5 g, 25 mmol) were
added in the flask. Then the mixture was maintained at
80 ℃ for 4 h under Ar atmosphere. After cooling to
Synthesis Scheme 1 illustrates the general syn-
thetic routes toward the BDT monomer 7. Compounds
1－3 were prepared according to the literature.[8,34]
Compound 4 was synthesized by a reduction reaction
and a substitution reaction. Compound 3 was firstly re-
duced by Zn powder in NaOH aqueous solution, and
then the substitution reaction occurred after 1-bromo-
dodecane was added into the mixture. Compound 4 was
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Chin. J. Chem. 2014, 32, 298—306

Ethynylene-Linked Oligomers Based on Benzodithiophene: Synthesis and Photoelectric Properties
brominated via Br2 to prepare compound 5 at low tem-
perature. Then compound 6 was synthesized by Sonoga-
shira coupling reaction in mixed solvents of THF and
TEA. This step occurred under Argon and dry condi-
tions, utilizing Pd(PPh3)Cl2/CuI as catalyst. The trimeth-
ylsilyl of compound 6 was removed by TBAF to obtain
monomer 7.
tion time was determined by TLC test. The yield is up to
93.65%, higher than that of literature.[9]
The synthetic methods of compound 7 were summa-
rized as follows. At first, we used KOH and K2CO3 as
base, MeOH/DCM or isopropanol as solvent,[36] but the
result was not satisfactory. Due to worse solubility of
base, the reaction was unsuccessful, especially when
MeOH and DCM were used as reaction solvents.
Therefore, we used tetrabutylammonium fluoride
(TBAF) acting as base and THF as solvent. Then the
base dissolved well and the reaction rate was fast. The
best reaction molar ratio of compound 6 vs. TBAF was
1∶3, and the reaction time was approximately 30 min.
This phenomenon may be ascribed to the strong com-
bining capacity of fluorine atom and silicon atom,
demonstrating that TBAF is a good choice to remove
trimethylsilyl.
Scheme 1 Synthesis of 2,6-bisacetenyl-4,8-dioctyloxybenzo-
[1,2-b:4,5-b']dithiophene
O
O
O
OH
Cl
N
S
S
S
1
2
O
C8H17-n
C8H17-n
Scheme 2 illustrates the general synthetic routes of
monomer 9, monomer 11, O1 and O2. Monomer 9 and
monomer 11 were synthesized according to previous
literature.[37,38]
O
O
S
S
S
Br
Br
S
S
S
O
3
O
O
n-C8H17
n-C8H17
4
Scheme 2 Synthesis of monomer 9, monomer 11, O1 and O2
5
Br
C8H17-n
NH2
N
N
N
N
O
O
S
I
S
S
NH2
Si
Si
8
Br
S
9
I
n-C8H17
I
I
6
N
N
N
H
H
C8H17-n
C8H17
11
O
10
S
C8H17
S
O
N
N
S
S
O
7
9
n-C8H17
n
S
7
O
C8H17
As shown in Scheme 1, Zn powder (about 1.2 equiv.)
was used excessively to ensure that the quinoid structure
was fully reduced for the preparation of compound 4.
Then n-C8H17Br (1.5 equiv.) was added into the mixture.
The reaction needed to be sustained for 12 h, otherwise
the substitution reaction could not proceed completely.
This factor will affect the yield, and the yield of this
step is up to 74.3%, which is higher than those in simi-
lar literatures.[18,35]
O1
C8H17
O
O
S
7
11
S
n
N
C8H17
C8H17
O2
In the transformation from compound 4 to com-
pound 5, 4,8-subsituted BDT (4) has two pairs of pro-
tons, and the chemical behaviors of these protons are
similar as the protons on a thiophene unit. The protons
on 2 and 6 positions exhibit lower pKa value than the
protons on 3 and 7 positions,[3] so the protons on 2 and 6
positions are more reactive. However, we must strictly
control the amount of liquid bromine (1 equiv.), and the
temperature should be lower than room temperature. We
conduct this reaction under ice-water bath, and the reac-
O1 and O2 were synthesized by a Palladium-cata-
lyzed Sonogashira coupling reaction. The reaction con-
dition and purification process were almost under the
same condition besides the reaction time of O2 was 3 d,
O1 Just 12 h. All of them were proceeded under argon
and dry environment in mixed solvents of THF and
Et3N, catalyzed by Pd(PPh3)Cl2 and CuI. The oligomers
were precipitated from CH3OH/CHCl3 for three times in
order to remove small molecules.
Characterization The oligomers were character-
Chin. J. Chem. 2014, 32, 298—306
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Jiang et al.
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1
ized by H NMR (Figure 1 and Figure 2) and FT-IR
100
80
60
40
20
O1
O2
spectroscopy. 1H NMR spectrum indicated the chemical
structure of each monomer unit of the alternating oli-
gomers, and the results confirmed their chemical struc-
tures. Characteristic proton resonances for O1 were as-
signed as follows: resonance at δ 7.88－7.45 for Ar-H,
resonance at δ 4.30－4.18 for methylene H located near
oxygen atom, and resonance at δ 1.40－1.25 for methyl
=
C
C
C
=
N
1
H. O2 can be characterized by H NMR in the same
way. (see the Experimental Section for details).
N-C
=
C-H
3500 3000 2500 2000 1500 1000 500
Wavenumber/cm-1
Figure 3 FT-IR spectra of O1 and O2.
Table 1 Molecular weights and thermal properties of O1 and
O2
Oligomer
Mn/Da
Mw/Da
PDI
1.13
n
Td/℃
O1
O2
4502
7369
5079
7658
7
285
326
1.04 10
degrees of polymerization (n) are 7 and 10, and their
PDI are 1.13 and 1.04, respectively. The low value of n
may be attributed to steric hindrance and solubility of
polymers. O1 has a worse solubility in organic solvent
than O2, because O2 has more octyl groups. More alkyl
chains were introduced into the system, which can en-
hance their solubility. However, more octyl groups will
hinder the rotation of conjugated structure. These fac-
tors will hamper the polymerization.
Figure 1 1H NMR spectrum of O1.
UV-Vis absorption and fluorescence emission spectra
The UV-Vis absorption spectra and fluorescence
emission spectra of O1 and O2 in CHCl3 solutions are
shown in Figure 4 and Table 2, respectively. The ab-
sorption spectrum of O1 showed two major peaks lo-
cated at 404 and 483 nm, and O2 also showed two ma-
jor peaks located at 401 and 429 nm.
500
0.6
Figure 2 1H NMR spectrum of O2.
400
The FT-IR spectra of O1 and O2 are shown in Fig-
ure 3. For O1, we can find a ≡C－H stretching vibra-
tion at 3409 cm−1 and C≡C stretching vibration at 2147
cm−1, C＝N stretching vibration at 1637 cm−1, C－Br
stretching vibration at 879.42 cm−1. For O2, its FT-IR
spectrum is similar to that of O1, N－C stretching vi-
bration at 1286.25 cm−1, ＝C－O－C stretching vibra-
tion at 1261.28 and 1033.71 cm−1.
0.4
300
200
0.2
100
0.0
350 400 450 500 550 600 650 700
The molecular weight was measured by GPC using
polystyrene as the standard, with THF as the eluent, and
corresponding data were summarized in Table 1. The
number-average molecular weight (Mn) is 4502 Da for
O1 and 7369 Da for O2, respectively. Corresponding
Wavelengh/nm
Figure 4 UV-Vis absorption spectra of O1 (blue line) and O2
(red line) and fluorescence emission spectra of O1 (violet) and
O2 (green) in CHCl3 solutions.
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Ethynylene-Linked Oligomers Based on Benzodithiophene: Synthesis and Photoelectric Properties
Both the oligomers exhibited an absorption band at
approximately 400 nm to 500 nm arising from π-π*
transitions of the large conjugated system. However, O1
showed a strong absorption with λmax at 483 nm, which
is 54 nm red shifted from those of O2 (λmax at 429 nm).
It reveals that the light with lower energy or larger
wavelength could be absorbed by O1. This phenomenon
may be ascribed to the difference of their structures.
Benzothiadiazole is probably a group with stronger
electron-withdrawing ability than carbazole. In general,
D-π-A system has a lower band gap than normal system,
because the intramolecular charge transfer effect exists
in the D-π-A system, which is helpful for the transporta-
tion of electron.
The fluorescence emission spectra showed a broad
emission band over 450－680 nm, and the maximum
emission peak of O1 is located at 550 nm, approximate
to the peak of O2 at 545 nm. The two oligomer’s stocks
shift is approximate to 145 nm.
Thermal properties
The thermal properties of the target oligomers were
obtained by thermogravimetric analysis (TGA) under
nitrogen. Table 1 summarizes the results and their TGA
thermograms are given in Figure 6. The TGA curves
showed that the oligomers were thermally stable with an
initial decomposition temperature in a N2 atmosphere at
128 ℃ for O1 and 252 ℃ for O2, and 5% weight loss
(Td) of 285 ℃ for O1 and 326 ℃ for O2, respectively.
When weight loss was over 30%, O2 was more ther-
mally stable than O1 due to the better stability of car-
bazole than BDT unit.
Table 2 Photophysical properties of O1 and O2
Absorption
Excitation
Eg/eV wavlengh/
nm
Maximum
emission
λmax/nm
Oligomer
peak λmax
/
nm
1.1
O1
O2
404, 483 1.92
401, 429 2.27
404
401
550
545
1.0
O1
O2
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
The optical band gaps estimated from the onsets of
their absorption spectra were according to the litera-
ture,[39] and the hν-(αhν)2 dependences of O1 and O2
were shown in Figure 5. The results showed that their
optical band gaps were 1.92 and 2.27 eV, respectively.
For the construction of small band gap polymers, many
examples[40,41] have demonstrated that utilizing alterna-
tive electron deficient and electron rich units is an effec-
tive method. The difference of optical band gap is par-
ticularly owed to the different molecular structures and
the number of electron donating functional groups.[42,43]
Both of them have relatively small band gap, so the
electron transition is easy to occur. It is suitable for the
applications in photoelectric materials, especially for
O1. The ideal band gap of photoelectric materials is
below 2 eV.[44]
0.0
100 200 300 400 500 600 700 800
Temperature/oC
Figure 6 TGA curves of O1 and O2 at a heating rate of 10
℃/min under N2 atmosphere.
Cyclic voltammetry (CV)
The electrochemical behavior of the oligomers was
investigated by cyclic voltammetry (CV), as shown in
Figure 7. The oligomer powder was dissolved in an
electrolyte solution of 0.1 mol/L tetrabutylammonium
hexafluorophosphate (TBAPF6) in CHCl3. In the meas-
urement, we used Pt as working electrode, saturated
calomel electrode (SCE) as reference electrode and
glassy carbon electrode as counter electrode at room
temperature. Cyclic voltammetries (CVs) performed in
the whole potential range between 0 and 1.5 V showed
several redox processes.
The highest occupied molecular orbital (HOMO)
levels and the lowest unoccupied molecular orbital
(LUMO) levels were calculated by CV curve and λonset
.
The result was shown in Table 3. The HOMO levels of
O1 and O2 are −5.64 and −5.66 eV, and the LUMO of
O1 and O2 are −3.72 and −3.39 eV, respectively. Both
of them have a relatively deep HOMO energy level
which implies that these oligomers have good stability
in the air. These oligomers maybe have potential
Figure 5 The calculated curve of optical band gap of O1 and
O2 (h＝6.626×10−34 J•s; ν＝1/λ; α=absorbance).
The fluorescence emission spectra of O1 and O2 in
CHCl3 at room temperature are also shown in Figure 4.
Chin. J. Chem. 2014, 32, 298—306
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Jiang et al.
FULL PAPER
10
8
O1
O2
6
4
2
0
-2
-4
-6
Figure 8 Frontier molecular orbital profiles of O1 and O2.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential/V
Table 4 The calculated energy levels of oligomers by B3LYP/
6-31G
Figure 7 CV of O1 and O2 in 0.1 mol/L TBAPF6•CHCl3 solu-
tion at 20 mV/s.
Oligomer
HOMO/eV
LUMO/eV
Eg/eV
applications in organic photoelectric and photovoltaic
materials due to the relatively low HOMO energy
level.[45]
O1
O2
−5.3
−5.0
−2.6
−1.5
2.7
3.5
of O2 is −5.0 eV and the LUMO level is −1.5 eV. Ob-
viously, O1 has a narrow band gap than O2, it is due to
the different electron-withdrawing abilities of ben-
zothiadiazole and carbazole.
The calculated band gaps are obviously greater than
those estimated by absorption onset values. The results
are mainly caused by the difference of dimer models
used in the DFT calculations and the realistic oligomers.
The dimer models are little molecules, while O1 and O2
have degrees of polymerization for 7 and 10, respec-
tively.
Table 3 Electrochemical properties and band gap of O1 and O2
c
Oligomer Eonset-oxa/V HOMOb/eV Eg /eV LUMOd/eV
O1
O2
0.94
0.96
−5.64
−5.66
1.92
2.27
−3.72
−3.39
a Eonset-ox＝onset oxidation potential measured by cyclic voltam-
b
c
metry. HOMO＝−e(Eonset-ox＋4.7) (eV). Eg estimated from the
UV-Vis absorption spectra. d LUMO＝Eg＋HOMO.
Density functional theoretical calculations
Density functional theory (DFT) calculations were
performed at B3LYP/6-31G* level using dimer models,
Electrical conductivity
benzodithiophene-ethynylene-benzothiadiazole
and
Materials preparation: The solutions of O1 and O2
in CHCl3 were treated with iodine by various propor-
tions, and the CHCl3 was volatilized at room tempera-
ture. Then the residue was dried at 30 ℃ for 5 h. The
conductivity was measured by PC408 digital insulation
resistor tester at room temperature. The thin film used
for testing is prepared through tablet press under a
pressure of 20 MPa, which had a diameter about 12 mm.
Electrical conductivity: The results were shown in
Table 5. Pure O1 occupied a poor conductivity ap-
proximate to 1.0×10−15 S•cm−1, and the conductivity of
O1 increased to 4.37×10−12 and 1.23×10−10 with the
ratio of doped iodine at 10% and 30%, respectively.
Pure O2 was close to 7×10−16 S•cm−1, and the conduc-
tivity of O2 increased to 1.33×10−12 and 1.05×10−10
with the ratio of doped iodine at 10% and 30%, respec-
tively. The conductivity of oligomers was increased by
5－6 order of magnitudes. O1 has a larger conductivity
than O2 because of the difference in their oligomers
molecular structure. The benzothiadiazole has a stronger
electron-withdrawing ability than carbazole unit. The
electron transmission in O1 is better than O2, due to the
better ICT (intramolecular charge transfer) effect and
better π-π* stacking effect in O1 than O2 which has
benzodithiophene-ethynylene-carbazole. The optimized
geometries and electron distributions of O1 and O2
were calculated by DFT method with B3LYP/6-31G (d)
basis sets using Gaussian 03 code. In order to calculate
conveniently, we used oligomers’ dimer models, benzo-
dithiophene-ethynylene-benzothiadiazole and benzodi-
thiophene-ethynylene-carbazole, to replace O1 and O2.
The results were shown in Figure 8 and Table 4. The
HOMO of O1 was mainly delocalized through BDT
unit and acetylene bond, and the LUMO of O1 was
populated over benzothiadiazole and acetylene bond.
This phenomenon is an ideal condition for photoelectric
materials. The benzothiadiazole group plays an acceptor
role to BDT unit, which is beneficial to intramolecular
electron migration. It can be concluded that O1 will
have a good conductivity. The calculated HOMO level
of O1 is −5.3 eV and the LUMO level is −2.6 eV.
The HOMO of O2 is distributed almost overall con-
jugated backbone, and the LUMO of O2 is also popu-
lated over entire conjugated backbone. This phenome-
non can be attributed to the similar electron-with-
drawing ability of BDT and Carbazole. Compared with
HOMO state, the electron clouds in LUMO state
slightly flow to BDT unit. The calculated HOMO level
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Chin. J. Chem. 2014, 32, 298—306

Ethynylene-Linked Oligomers Based on Benzodithiophene: Synthesis and Photoelectric Properties
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