Alternating copolymers based on 2,1,3-benzothiadiazole and hexylthiophene: Positioning effect of hexyl chains on the photophysical and electrochemical properties

Article abstract of DOI:10.1002/ejoc.201100182

A series of donor-acceptor alternating π-conjugated copolymers based on 2,1,3-benzothiadiazole and hexylthiophene units has been synthesized by the palladium-catalyzed Stille cross-coupling method. Various precursory monomers possessing dibromo and bis(tributylstannyl) functionalities were readily prepared in high yields. Microwave-assisted polymerization proved efficient for the production of high-molecular-weight copolymers, ranging from 13550 to 52490 g mol^-1. All copolymers exhibited excellent solubilities in most common organic solvents. The thermal properties of these copolymers were investigated by thermogravimetric analysis and differential scanning calorimetry, and the polymers showed high thermal stabilities. Incorporation of the benzothiadiazole unit into polyhexylthiophene chains affected the photophysical and electrochemical properties. The thin-film absorption spectra of all polymers are significantly red-shifted relative to the corresponding absorption bands in solution and exhibit broader absorption bands. The optical band gaps were estimated to be in the range of 2.02-1.74 eV. The highest occupied and lowest unoccupied molecular orbital energy levels are in the ranges of -5.37 to -5.66 eV and -3.33 to -3.44 eV, respectively. In the X-ray diffraction analysis of the deposited film of the copolymer P3, strong diffraction peaks were observed at 2θ = 5.72° (15.43 A) and 23.12° (3.84 A). These values relate to the distances between chains with interdigitated hexyl chains and to π-π stacking between the conjugated chains.

Full text of DOI:10.1002/ejoc.201100182

FULL PAPER
DOI: 10.1002/ejoc.201100182
Alternating Copolymers Based on 2,1,3-Benzothiadiazole and Hexylthiophene:
Positioning Effect of Hexyl Chains on the Photophysical and Electrochemical
Properties
Ashraf A. El-Shehawy,*[a,b] Nabiha I. Abdo,[a,c] Ahmed A. El-Barbary,[c] and Jae-Suk Lee*[a]
Keywords: Copolymers / Conjugation / Palladium / Cross-coupling / X-ray diffraction
A series of donor–acceptor alternating π-conjugated copoly-
mers based on 2,1,3-benzothiadiazole and hexylthiophene
units has been synthesized by the palladium-catalyzed Stille
cross-coupling method. Various precursory monomers pos-
sessing dibromo and bis(tributylstannyl) functionalities were
readily prepared in high yields. Microwave-assisted polyme-
physical and electrochemical properties. The thin-film ab-
sorption spectra of all polymers are significantly red-shifted
relative to the corresponding absorption bands in solution
and exhibit broader absorption bands. The optical band gaps
were estimated to be in the range of 2.02–1.74 eV. The high-
est occupied and lowest unoccupied molecular orbital energy
rization proved efficient for the production of high-molecu- levels are in the ranges of –5.37 to –5.66 eV and –3.33 to
lar-weight copolymers, ranging from 13550 to 52490 gmol–1
–3.44 eV, respectively. In the X-ray diffraction analysis of the
All copolymers exhibited excellent solubilities in most com- deposited film of the copolymer P3, strong diffraction peaks
.
mon organic solvents. The thermal properties of these copol-
ymers were investigated by thermogravimetric analysis and
differential scanning calorimetry, and the polymers showed
high thermal stabilities. Incorporation of the benzothiadia-
zole unit into polyhexylthiophene chains affected the photo-
were observed at 2θ = 5.72° (15.43 Å) and 23.12° (3.84 Å).
These values relate to the distances between chains with
interdigitated hexyl chains and to π–π stacking between the
conjugated chains.
Introduction
neither centrosymmetric nor do they possess symmetry
axes; three possible isomeric diads can be formed from the
coupling of 3AT monomeric units. As a result, four types
of spectroscopically distinct nonequivalent triads are de-
tected in the NMR spectra.[3–6] Head-to-head couplings
mostly represent unfavorable configurations, due to steric
repulsion between the alkyl chains and the lone pairs of
electrons on adjacent sulfur atoms of the thiophene
rings.[4–6]
High-performance polymer BHJ solar cells made from
mixtures of poly(3-hexylthiophene) (P3HT) and [6,6]-
phenyl C61 methyl butyrate (PCBM) have been reported,
with maximum PCEs of ηe = 4–5%.[7] Although new ap-
proaches to improving the efficiencies of P3HT/PCBM cells
are still being reported, the relatively large band gap of
P3HT (ca. 1.90 eV) limits the fraction of the solar spectrum
that can be utilized. After P3HT, with absorption bands
covering only a part of the visible light range, had been
investigated, some narrow band gap polymers were synthe-
sized with the goal of improving the absorption properties
of the polymers further, for application in polymer solar
cells (PSCs).[8] The design of several classes of low-band-
gap conjugated polymers that combine electron-rich and
electron-deficient units has recently attracted considerable
attention.
There is currently a considerable level of interest in the
preparation of π-conjugated polymers, due to their promis-
ing optical and electronic properties.[1] Huge advances in
the development of novel photovoltaic materials and the
structures of their electronic devices have recently been
achieved. Power conversion efficiencies (PCEs) can now
reach 7.7%.[2] Over the past decade, research has focused
on regioregular poly(3-alkylthiophenes) (P3ATs) as the
standard electron-donating materials in polymer bulk het-
erojunction (BHJ) solar cells, while important progress has
been made in understanding the device science and the as-
sociated improvements in device efficiency.[1,2] The sub-
strates used in the preparation of oligo- and poly-3ATs are
[a] Department of Nanobio Materials and Electronics, School of
Materials Science and Engineering, and Research Institute for
Solar & Sustainable Energies (RISE), Gwangju Institute of
Science and Technology (GIST),
1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of South
Korea
Fax: +82-62-9702304
E-mail: jslee@gist.ac.kr
[b] Department of Chemistry, Faculty of Science,
Kafr El-Sheikh University,
Kafr El-Sheikh 33516, Egypt
Fax: +20-47-3215175
E-mail: elshehawy2@yahoo.com
[c] Department of Chemistry, Faculty of Science, Tanta University,
Tanta 31527, Egypt
Benzothiadiazole (BT) is one of the best moieties for in-
duction of a narrow band gap. Although the BT structure
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100182.
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tends to cause low solubility, copolymers of BT with several units and also of the positions of their hexyl groups on
electron-rich units (donors) that can improve the process- the photophysical and electrochemical properties has been
ability have been investigated,[8,9] showing high PCEs of up investigated.
to 6.1%.[10] However, the intramolecular charge transfer
(ICT) mechanism in the donor–acceptor (D–A) structure
allows extension of the conjugation length.[1a,2b,11] These
low-band-gap polymers have been used for photovoltaic
cells and other organic electronics.[2,12] Increased effective
conjugation length in D–A polymers also leads to higher
hole mobility.[1a,2b,11e,13]
Results and Discussion
Synthesis of Precursor Monomers
Several precursory monomers possessing dibromo and
bis(tributylstannyl) functionalities were readily prepared in
We have recently reported a simple and efficient protocol relatively high yields from the starting material 4,7-di-
for the synthesis of dihexyl-2,2Ј-bithiophenes, based on 2- bromo-2,1,3-benzothiadiazole (1, Scheme 1), prepared by
bromo-4-hexylthiophene instead of the commonly used brominating 2,1,3-benzothiadiazole.[8c,15] Palladium-cata-
monomer 2-bromo-3-hexylthiophene.[14] Kumada and Su- lyzed Suzuki cross-coupling reactions between 1 and 3-hex-
zuki cross-coupling reactions were conducted to synthesize ylthiophene-2-boronic acid pinacol ester (2) or 3-hexylthio-
the desired products as head-to-tail and tail-to-tail regioiso- phene-4-boronic acid pinacol ester (6) produced the corre-
mers with high yields and excellent selectivity.[14] Here we sponding oligomers 4,7-bis(3-hexylthiophen-2-yl)benzo-
wish to report the synthesis and characterization of several [c][2,1,3]thiadiazole (3) and 4,7-bis(4-hexylthiophen-2-yl)-
alternating π-conjugated copolymers based on 2,1,3-benzo- benzo[c][2,1,3]thiadiazole (7), respectively, in high yields.
thiadiazole and hexylthiophene (HT) units. The incorpora- The oligomer 7 could also be synthesized through a Suzuki
tion of the BT unit into a polyhexylthiophene (PHT) im- cross-coupling reaction between 2-bromo-4-hexylthiophene
proves the UV/Vis absorption, thermal stability, electro- (11), prepared by our previously reported method,[14] and
chemical properties and interchain interactions and thus en- 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester)
hances the charge transport properties of the resulting co- (10) in toluene in the presence of a catalytic amount of tet-
polymers. The impact of the number of hexylthiophene rakis(triphenylphosphane)palladium(0) [Pd(PPh3)4].
Scheme 1. Synthesis of precursory compounds containing dibromo and bis(tributylstannyl) functionalities. Reaction conditions:
(i) Pd(PPh3)4, toluene, 100 °C, 12 h; (ii) NBS, THF, 0 °C to room temp.; (iii) LDA, (C4H9)3SnCl, –80 °C, then room temp.; (iv) Zn dust,
EtOH–glacial AcOH, reflux, 5 h; (v) C6H13–MgBr, NidpppCl2, ether, 40 °C, 12 h.
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Alternating Copolymers Based on 2,1,3-Benzothiadiazole and Hexylthiophene
Dibromination of the oligomers 3 and 7 by addition of and synthesized as outlined in Scheme 2. All the copoly-
N-bromosuccinimide (NBS, 2.5 molequiv.) in tetrahydro- mers were prepared by the Stille cross-coupling methodol-
furan (THF) afforded the corresponding precursor oligo- ogy in the presence of a catalytic amount of Pd(PPh3)4 in
mers 4,7-bis(5-bromo-3-hexylthiophen-2-yl)benzo[c][2,1,3]- dry dimethylformamide (DMF). To investigate appropriate
thiadiazole (4) and 4,7-bis(5-bromo-4-hexylthiophen-2-yl)- polymerization conditions, the copolymers P2–P4 were first
benzo[c][2,1,3]thiadiazole (8), respectively. Lithiation of synthesized under either thermal or microwave polymeriza-
compounds 3 or 7 with lithium diisopropylamide (LDA) tion conditions. Stille cross-coupling of the monomers 4
at –80 °C, followed by quenching with tributyltin chloride, and 5 afforded the corresponding copolymer P2 with alter-
afforded the corresponding bis(tributylstannyl) derivatives nating BT and two HT moieties linked together in a tail-
5 and 9, respectively, in high yields after silica gel flash col- to-tail configuration. The copolymer P2 could be also pre-
umn chromatography.
pared by Stille cross-coupling of 4,7-dibromo-2,1,3-benzo-
In another synthetic pathway, the 5,5Ј-bis(tributylstan- thiadiazole (1) with the monomer 13. The copolymer P3
nyl)-2,2Ј-bithiophene derivatives 13 and 17 were readily was also prepared through a Stille cross-coupling of mono-
synthesized as shown in Scheme 1. The compound 4,4Ј-di- mers 4 and 9. However, the HT units in P3 are alternatively
hexyl-2,2Ј-bithiophene (12) was prepared in 95% yield linked in a head-to-tail and tail-to-head configuration, rela-
through Suzuki cross-coupling of 6 and 11 by our pre- tive to the central BT unit.
viously reported method.[14] Moreover, 3,3Ј-dihexyl-2,2Ј-bi-
Under the same thermal and/or microwave reaction con-
thiophene (16) was prepared in 94% yield by reduction of ditions that had been used for the synthesis of the copoly-
3,3Ј,5,5Ј-tetrabromo-2,2Ј-bithiophene (14)[16] with zinc dust mers P2 and P3, several trails for the synthesis of the copol-
in ethanol at reflux followed by a Grignard reaction with ymer P4 by copolymerization of monomers 8 and 9 proved
hexylmagnesium bromide in the presence of 1,3-bis(diphen- unsuccessful. Only oligomeric materials were observed
ylphosphanyl)propane nickel(II) chloride [Ni(dppp)Cl2]. Fi- when the reaction mixtures were poured into methanol,
nally, 4,4Ј-dihexyl-5,5Ј-bis(tributylstannyl)-2,2Ј-bithiophene possibly as a consequence of the steric repulsion resulting
(13) and 3,3Ј-dihexyl-5,5Ј-bis(tributylstannyl)-2,2Ј-bithio- from the hexyl–hexyl side chain interactions during the
phene (17) were readily synthesized in high yield by lithi- Stille cross-coupling reaction. Stille cross-coupling of the
ation of the corresponding monomers 12 and 16, respec- monomers 1 and 17, however, occurred readily, affording
tively, with LDA at –80 °C followed by quenching with tri- the desired copolymer P4 with alternating BT and two HT
butyltin chloride. All synthesized compounds were con- moieties linked together in a head-to-head configuration.
1
firmed by H and 13C NMR and elemental analyses. Their For comparison, the copolymer P1 was also synthesized by
spectroscopic data were found to be in agreement with the Stille cross-coupling of the monomer 1 with 9.
proposed structures (see the Supporting Information for the
The number of HT units and the positions of their hexyl
side chains can have an impact on the properties of the
resulting copolymers, so the Stille cross-coupling was ex-
tended to the synthesis of other copolymers, each contain-
ing units of four HT components. Under microwave reac-
1H and 13C NMR spectra).
Polymer Synthesis
With the precursory monomers bearing dibromo and tion conditions, Stille cross-couplings of 17 with 4 or with
bis(tributylstannyl) functionalities in hand (see Scheme 1), 8 afforded the desired copolymers P5 and P6 (Scheme 2),
several π-conjugated copolymers were carefully designed respectively, containing alternating BT and four HT moie-
Scheme 2. Synthesis of the π-conjugated copolymers P1–P6 by the Stille cross-coupling polymerization method.
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ties in their repeating units. The polymer backbones of P5 because the steric effects due to the hexyl–hexyl side chain
and P6 have similar compositions, except that the positions interactions in the Stille cross-coupling polymerization were
of their hexyl chains on the thiophene rings are different.
much less severe (Table 1).
After precipitation into methanol, the crude copolymers
were filtered and washed extensively with methanol, fol- firmed by elemental and H NMR analyses; detailed char-
lowed by successive Soxhlet extractions with methanol and acterization data are included in the Experimental Section
The structures of the resulting copolymers were con-
1
1
acetone to remove any byproducts and oligomers. Analysis (see the Supporting Information for the H NMR spectra).
of the resulting copolymers showed that the palladium cata- As anticipated, the copolymers P2 and P4 each displayed
lyst had been removed completely. All copolymers exhibited only one multiple peak at ca. 2.70 ppm, originating from
good solubilities (Ͼ5 mgmL–1) at room temperature in the α-methylene protons (thiophene-CH2) of the hexyl
most common organic solvents such as THF, chloroform, chains, indicating only one dominant orientation of the
toluene, xylene, and chlorobenzene. The molecular weights hexyl chains. Two signals with almost the same chemical
(Mn) and molecular weight distributions (PDI) of the re- shifts (but not with the same relative intensities), at ca. 2.5
sulting copolymers were determined by gel permeation and ca. 2.7 ppm and assigned to the thiophene-CH2 pro-
chromatography (GPC) in CHCl3 with use of polystyrene tons, were clearly observed in the 1H NMR spectra of each
standards and the results are summarized in Table 1. All of the polymers P1, P3, P5, and P6, indicating the presence
copolymers showed symmetrical unimodal molecular of regiorandom configurations of HT units in the polymer
weight distributions (see the Supporting Information for the backbones. Similar NMR observations have been reported
GPC profiles).
with regard to the molecular structures and regioregularity
of oligo- and PATs.[3–6,7e,3c,17]
Table 1. Polymerization results for the copolymers P1–P6 and their
thermal properties.[a]
Thermal Properties
Polymer Mn
(kgmol–1 [b]
PDI
Yield
Td
Tg
)
(Mw/Mn)[b] (%)[c]
(°C)[g,h]
(°C)[g,i]
The thermal properties of the copolymers P1–P6 were
investigated by thermogravimetric analysis (TGA) under ni-
trogen. The TGA thermograms are shown in Figure 1 and
the recorded TGA data are collected in Table 1. TGA deter-
mined that the thermolysis threshold could be defined by
a weight loss of approximately 5%. The thermolysis onset
temperatures of the copolymers were found to be above
350 °C and the decomposition temperatures (Td) were in
the 359.6–436.4 °C range, indicative of good thermal sta-
bilities. The negligible weight losses might be attributable to
the high thermal stability characteristics of the thiophene
and benzothiadiazole moieties. The high thermal stabilities
of the polymers prevent the deformation of the morpho-
logies of the copolymers and the degradation of the poly-
P1
18.63
2.10 87.0
365.8
72.4
76.1
––
66.1
82.3
43.6
46.2
P2[e]
P2[f]
P3
31.26 (16.74)[d] 1.53 (2.64)[d] 91.0 (73.0)[d] 423.3
28.55
1.62
89.0
––
27.72 (13.55)[d] 232 (2.89)[d] 91.0 (68.0)[d] 428.7
P4
P5
P6
33.79 (18.77)[d] 141 (2.09)[d] 98.5 (79.0)[d] 436.4
52.49
48.68
1.49
1.81
87.6
90.6
415.9
359.6
[a] All polymerizations were carried out in DMF in the presence
of Pd(PPh3)4 under microwave polymerization reaction conditions
unless stated otherwise. [b] Calculated from GPC (eluent CHCl3,
30 °C, polystyrene standards). [c] Based on the weight of pure poly-
mer obtained after Soxhlet extraction followed by drying under
vacuum for 24 h. [d] Values in parentheses are for those obtained
from Stille cross-coupling under thermal polymerization. [e] Based
on those data obtained from the polymerization of 4 and 5.
[f] Based on those data obtained from the polymerization of 1 and
13. [g] Measured on the polymer samples obtained by microwave meric active layer under applied electric fields, so the syn-
polymerization. [h] Determined by TGA under nitrogen at a heat-
thesized copolymers may be considered good candidates for
ing rate of 10 °Cmin–1. [i] Determined by DSC under nitrogen at a
optoelectronic device applications, especially for organic
heating rate of 10 °Cmin–1.
light-emitting diodes (OLEDs), organic field-effect transis-
tors (OFETs), and polymer solar cells (PSCs).
The GPC data revealed that microwave-assisted polymer-
ization had proved particularly efficient for obtaining the
desired copolymers with higher Mn and lower PDI values
(Table 1). Under the same reaction conditions, the copoly-
mers P5 and P6 were obtained with much higher molecular
weights (Mn = 52490 and 48680, respectively) than the co-
polymers P1–P4 (Table 1). Increasing the number of HT
units improved not only the solubilities but also the molecu-
lar weights of the resulting copolymers. The relatively low
Mn value of P3 relative to P2, irrespective of synthetic route
(Table 1), is probably due to the steric repulsion effect re-
sulting from the interactions of the hexyl chains at the 4-
positions of the thiophene rings in monomer 9. Copolymer-
ization of the monomers 4 and 5, with hexyl side chains at
the 3-positions of the thiophene rings, occurred smoothly Figure 1. TGA thermograms of the copolymers P1–P6.
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Alternating Copolymers Based on 2,1,3-Benzothiadiazole and Hexylthiophene
Differential scanning calorimetry (DSC) of the copoly-
mers P1–P6 was also investigated and the DSC curves are
shown in Figure 2. The polymer samples were heated to
300 °C under nitrogen and the DSC data were obtained
from the second heating cycles. As shown in Table 1, the Tg
values decreased with increasing numbers of HT units. The
copolymers P1–P4 exhibited Tg values at 72.4, 76.1, 66.1,
and 82.3 °C, respectively. On the other hand, the copoly-
mers P5 and P6 showed relatively lower Tg values at 43.6
and 46.2 °C, respectively. All copolymers failed to show
melting points within the temperature limits used in the
DSC measurement.
Figure 2. DSC thermograms of the copolymers P1–P6.
Figure 3. UV/Vis absorption spectra of the copolymers P1–P6:
a) in chlorobenzene solution, and b) as thin films spin-cast on glass
slides.
Optical Properties
The UV/Vis absorption spectra of the copolymers P1–P6
in solution and of spin-coated films prepared from their
As shown in Figure 3, the thin-film absorption spectra
chlorobenzene solutions on glass slides are shown in Fig- for all polymers are significantly red-shifted and exhibit
ure 3a and Figure 3b, respectively. The absorption maxima broader absorptions than their corresponding absorptions
and estimated optical band gaps (Egop) are summarized in in solution. From their onset absorption wavelengths, the
Table 2. Each polymer is characterized by two major ab- optical band gaps of all polymers in the solid state were
sorption peaks, a commonly observed feature for alternat- significantly reduced to lower values relative to their corre-
ing donor–acceptor copolymers. The low-wavelength ab- sponding values in solution. The film of copolymer P3
sorption peak (349–405 nm) is attributed to the π–π* transi- showed a maximum absorbance wavelength at 575.2 nm, a
tion of donor units whereas the high-wavelength peak (483– wavelength red-shifted by 40.8 nm relative to its value in
575 nm) is believed to be related to the intramolecular π– solution, whereas the absorption onset (710.4 nm) was red-
π* transition between the ground and the excited states of shifted by about 76.0 nm from its absorption onset wave-
the A–D charge transfer.
length (634.4 nm) in solution. Moreover, the optical band
Table 2. Optical and electrochemical properties of the copolymers P1–P6.[a]
UV/Vis absorption
Cyclic voltammetry[c,d]
Polymer
Solution
Film
λmax (nm) λonset (nm) Egop (eV)[b]
λmax (nm) λonset (nm) Egop (eV)[b]
HOMO (eV) LUMO (eV) Egec (eV)
P1
P2
P3
P4
P5
P6
513.5
503.5
534.4
526.2
483.5
525.3
593.2
606.8
634.4
613.5
574.4
628.1
2.09
2.04
1.95
2.02
2.16
1.97
531.7
546.5
575.2
555.6
515.2
564.1
644.2
648.3
710.4
656.7
613.3
682.4
1.92
1.91
1.75
1.89
2.02
1.82
–5.66
–5.46
–5.37
–5.53
–5.53
–5.38
–3.37
–3.35
–3.41
–3.44
–3.35
–3.33
2.29
2.11
1.96
2.09
2.18
2.05
[a] All data were obtained from polymers prepared under microwave reaction conditions. [b] Optical band gap (Egop) was calculated from
the intersection of the tangent on the low energetic edge of the absorption spectrum with the baseline. [c] The onset potentials were
obtained from the intersection of the two tangents down at the rising current and the baseline changing current of the CV curves. [d] The
thin films were prepared by drop-casting on ITO glass substrates.
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gap of P3 in the solid state was reduced to 1.75 eV relative rams of the synthesized copolymers, and the derived CV
to the corresponding value in solution (1.95 eV). These re- data [HOMO, LUMO, and electrochemical band gaps
sults suggest the presence of more extensive intermolecular (Egec)] are summarized in Table 2. The estimated EHOMO
interactions and ordering in the solid state (film), as in the and ELUMO values were found to be in the ranges of –5.37
case of P3HT.[17b,18]
to –5.66 eV and –3.33 to –3.44 eV, respectively. All HOMO
Both in solution and in the solid state, P5 revealed blue energy levels were found to be below the air oxidation
shift absorption relative to P6 (Table 2), possibly due to a threshold (ca. –5.27 eV),[22] so the polymers might show
less coplanar configuration of the adjacent thiophene rings good stabilities toward air and oxygen (a prerequisite in
induced by the different positions of hexyl side chains in considerations of device application). The EHOMO and
the polymer backbone of P5 relative to P6. A less coplanar ELUMO values for some polymers are either rather close to
configuration, however, significantly reduces the effective or lower than the reported value for P3HT. The estimated
conjugation length.
Egec values for polymers are somewhat higher than the corre-
Previous reports on similarly conjugated polymer back- sponding values for Egop. The reason might lie in the inter-
bones indicated that attachment of the alkyl chains at the face barrier present between the polymer film and the elec-
4-positions of thiophene rings introduces the minimum trode surface.[20c,23]
amount of steric hindrance in the related D–A polymer.[19]
When the hexyl chains on thiophene rings directly linked to
the acceptor unit (BT) are attached at the 3-positions, se-
vere steric hindrance is introduced between the flanking thi-
enyl groups and the central benzothiadiazole unit. In such
a case the conjugated backbone is thereby twisted at the D–
A linkage, significantly affecting the effective conjugation
between the donor and the acceptor (BT). Relatively large
optical band gaps were observed for these copolymers (e.g.,
1.92 eV for P1, 1.91 eV for P2, and 2.02 eV for P5). Al-
though the two hexyl side chains of P4 are located at the
4-positions of thiophene rings relative to the central BT
unit, P4 shows an optical band gap very similar to that in
the case of P2 (see Table 2), possibly as a result of steric
repulsion between the hexyl chains and the lone pairs of
electrons on the adjacent sulfur atoms of the thiophene
rings in P4. The copolymer P3 showed the most strongly
red-shifted maximum absorbance wavelength and absorp-
tion onset in relation to P2 and P4. Although P3 has the
same number of 3HT units as P2 and P4, the positions of
the hexyl side chains in P3 seemed to provide high regioreg-
ularity in the polymer backbone. However, the HT units in
Figure 4. Cyclic voltammograms of the copolymers P1–P6.
P3 are alternatively linked in a head-to-tail and tail-to-head
configuration: the more favorable configuration of P3HT.
Probing this library of polymers with identical conju-
Furthermore, the enchainment of hexyl side chains in P3 gated backbones by cyclic voltammetry provides direct evi-
might also provide a larger range of hexyl–hexyl hydro- dence that differences in the positioning of the hexyl chains
phobic interactions as well as π–π interactions.
affect the energy levels of these related copolymers. The
LUMOs are essentially dominated by the common acceptor
unit (BT), which explains why all six copolymers show sim-
ilar reduction potentials and LUMO energy levels (Figure 4
and Table 2). As shown in Table 2, P1 showed the lowest
Electrochemical Properties
The electrochemical properties of the copolymers P1–P6 HOMO energy level of –5.66 eV. With incorporation of one
were investigated by cyclic voltammetry (CV) to determine more electron-rich HT in each repeating unit of the original
the positions of their highest occupied and lowest unoccu- copolymer P1, the HOMO energy levels show dramatic dif-
pied molecular orbitals (HOMOs and LUMOs, respec- ferences. The copolymer P3, with two HT units alterna-
tively). Several methods for evaluation of HOMO and tively linked in a head-to-tail and tail-to-head configuration
LUMO energy levels have been proposed in the litera- relative to the central BT unit, shows an increase in the
ture.[20] The HOMO and LUMO energy levels (EHOMO and electron density of the conjugated backbone, leading to an
ELUMO, respectively) of the synthesized copolymers were elevated EHOMO (–5.37 eV). The EHOMO values for P3 and
calculated by use of the following empirical formulas:[8b,21] P6 differ only slightly (–5.37 eV vs. –5.38 eV), indicating
EHOMO = –(Eox + 4.40) eV and ELUMO = –(Ere + 4.40) eV, that increasing the number of HT units has an almost negli-
where Eox and Ere are the onset oxidation and reduction gible influence on the extended conjugation of the D–A
potentials, respectively. Figure 4 shows cyclic voltammog- polymer. The copolymer P5 showed a lower EHOMO
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Alternating Copolymers Based on 2,1,3-Benzothiadiazole and Hexylthiophene
Figure 5. a) X-ray diffraction patterns of P3; b) proposed packing structure of P3.
(–5.53 eV) than P6 (–5.38 eV), possible due to the relatively XRD pattern. The interlayer distance suggests that the
higher severe steric hindrance introduced between the hexyl chains of P3 interdigitate as illustrated in Figure 5b.
flanking thienyl groups and the central benzothiadiazole However, the charge mobility is crucially dependent not
unit in P5 relative to P6, which significantly reduces the only on the π stacking distance but also on the intermo-
effective conjugation length.
lecular distance in the direction normal to the π stacking.[26]
The obtained results indicate that the HOMO is essen- Moreover, the π stacking is an important factor in de-
tially localized at the HT units (and partially on the BT termining the nature of the intermolecular interaction. The
unit), which also explains similar HOMO levels for P4 and small π stacking distance and the high degree of lateral
P5 (–5.53 eV). Similar observations relating to the impact chain packing could therefore explain why the copolymer
of the positions of alkyl chains on thiophene rings on the P3 showed the most strongly red-shifted maximum ab-
optoelectronic properties of various copolymers based on sorbance wavelength and absorption onset relative to other
alkylthiophenes and various acceptors including BT units polymers. Similar effects have been observed in alternating
have been reported in the literature.[19,24] Because the open- copolymers based on thiazole, 1,3,4-thiadiazole, benzobis-
circuit voltages (Voc values) of polymer solar cells shows thiazole, and thiazolothiazole moieties incorporated with
linear correlation with the differences between the HOMO hexylthiophene.[27,28]
level of the electron donor and the LUMO level of the elec-
tron acceptor, the lower HOMO levels for some of the syn-
thesized copolymers could result in higher Voc values for
the corresponding polymer solar cells.[12e]
Conclusions
X-ray Diffraction (XRD) Investigation
Simple and efficient methods based on the common Stille
To investigate the solid-state morphology and molecular cross-coupling copolymerization have been developed for
organization of P3 (the copolymer that showed the most the synthesis of six DA-type structurally similar polymers
strongly red-shifted absorbance), XRD analysis was per- with conjugated backbones made up of 2,1,3-benzothiadiaz-
formed, both on the deposited film of P3 and on its bulk ole and hexylthiophene moieties. Microwave-assisted poly-
powder sample (Figure 5a). Two well-defined diffraction merization proved efficient for providing high-molecular-
peaks are observed for the thin film drop-cast from the weight copolymers and low PDI values. The obtained re-
chlorobenzene solution of P3 onto a silica slide. The first sults showed that the side chains of the polymer backbone
peak, assigned at 2θ = 5.72° (15.43 Å), demonstrates the can have an impact on the photophysical and electrochemi-
semicrystalline nature of the material and is associated with cal properties of the polymer. Significant red shifts were
the distance between chains with interdigitated hexyl observed in the UV/Vis absorption spectra of all polymers
chains, whereas the second peak, assigned at 2θ = 23.12° in the solid state relative to their solutions in chlorobenzene.
(3.84 Å), is believed to be due to π–π stacking between the Hexyl side chains at the fourth positions in thiophene rings
conjugated chains in a direction normal to the former one. (relative to the central BT units) induce less steric hindrance
The π–π stacking distance in P3 was very similar to that in and thus more planar molecular structures. These planar
P3HT (3.8 Å).[3g,17b,25] However, the potential for this π–π molecular structures enhance π electron delocalization and
stacking induces planarization of the chains, thus enhanc- favor intermolecular interactions. Electrochemical charac-
ing the interdigitation.
terization revealed that these polymers have low HOMO
When XRD measurements were performed on a powder levels, indicating good resistance against oxidative doping.
sample of P3, two distinct similar peaks, assigned at 5.77° The potential of the synthesized copolymers for application
and 22.75° (see also Figure 5a), were observed in the in the field of OFETs and PSCs is now under investigation.
Eur. J. Org. Chem. 2011, 4841–4852
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FULL PAPER
raphy (n-hexane/EtOAc 10:2) to generate 3 (1.41 g, 88.7% yield) as
a bright orange oil. 1H NMR (400 MHz, CDCl3): δ = 7.64 (s, 2 H,
2ϫCH, benzothiadiazole), 7.43–7.42 (d, J = 4.00 Hz, 2 H,
2ϫSCH), 7.10–7.09 (d, J = 4.00 Hz, 2 H, 2ϫSCH=CH), 2.68–264
(t, J = 16.00 Hz, 4 H, 2ϫArCH2), 1.60 (m, 4 H, 2ϫArCH2CH2),
1.26 [m, 12 H, 2ϫCH3(CH2)3], 0.82–0.79 (t, J = 12.00 Hz, 6 H,
2ϫCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 154.23, 141.65,
132.12, 129.86, 129.18, 127.41, 125.82, 31.52, 30.63, 29.30, 29.06,
22.49, 14.00 ppm. C26H32N2S3 (468.74): calcd. C 66.62, H 6.88, N
5.98, S 20.52; found C 66.50, H 6.75, N 6.01, S . 20.49.
Experimental Section
Materials: Unless otherwise stated, all manipulations and reactions
involving air-sensitive reagents were performed under dry, oxygen-
free nitrogen. All reagents and solvents were obtained from com-
mercial sources and dried by standard procedures before use. All
reactions were monitored by TLC to follow their completion. 4,7-
Dibromo-2,1,3-benzothiadiazole (1),[8c,29] 3,3Ј,5,5Ј-tetrabromo-
2,2Ј-bithiophene (14),[16] 3,3Ј-dibromo-2,2Ј-bithiophene (15),[16] 2-
(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(6),[14] 2-bromo-4-hexylthiophene (11),[14] and 4,4Ј-dihexyl-2,2Ј-bi-
thiophene (12)[14] were prepared by literature procedures.
4,7-Bis(5-bromo-3-hexylthiophen-2-yl)benzo[c][2,1,3]thiadiazole (4):
NBS (1.6 g, 9.01 mmol) was added in one portion to an ice-cold
solution of compound 3 (1.7 g, 3.63 mmol) in dry THF (65 mL).
The reaction mixture was stirred at 0 °C for 30 min, followed by
stirring overnight at room temperature. After quenching with satu-
rated aqueous NH4Cl solution, the reaction mixture was extracted
with ethyl acetate and the separated organic layers were collected,
washed with water, and finally dried with anhydrous Na2SO4. The
solvent was evaporated under vacuum and the resulting crude
product was purified by silica gel flash column chromatography (n-
hexane/EtOAc 10:2) to generate 4 (2.07 g, 91% yield) as a sticky
orange oil. 1H NMR (400 MHz, CDCl3): δ = 7.60 (s, 2 H, 2ϫCH,
benzothiadiazole), 7.05 (s, 2 H, 2ϫCH, thiophene), 2.62–258 (t, J
= 16.00 Hz, 4 H, 2ϫArCH2), 1.59 (m, 4 H, 2ϫArCH2CH2), 1.22
[m, 12 H, 2ϫCH3(CH2)3], and 0.82–0.79 (t, J = 12.00 Hz, 6 H,
2ϫCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 153.82, 142.35,
133.47, 131.90, 129.61, 126.50, 113.14, 31.45, 30.44, 29.34, 28.96,
22.45, and 13.99 ppm. C26H30Br2N2S3 (626.53): calcd. C 49.84, H
4.83, Br 25.51, N 4.47, S 15.35; found C 50.15, H 4.78, N 4.51, S
15.39.
Instrumentation and Methods: 1H and 13C NMR spectra were mea-
sured with a Varian spectrometer (400 MHz for H and 100 MHz
1
for 13C) in CDCl3 at 25 °C with TMS as the internal standard and
chemical shifts were recorded in ppm units. The coupling constants
(J) are given in Hz. Flash column chromatography was performed
with Merck silica gel 60 (particle size 230–400 mesh ASTM). Neu-
tralized silica gel was prepared by addition of triethylamine to the
silica gel in the same eluent as used for column chromatography.
Analytical thin layer chromatography (TLC) was conducted with
Merck 0.25 mm 60F silica gel precoated aluminum plates and UV-
254 fluorescent indicators. The UV/Vis absorption spectra were ob-
tained with a Varian Cary UV/Vis/NIR-5000 spectrophotometer
on the pure polymer samples. The thermal degradation tempera-
ture was measured by thermogravimetric analysis (TGA-TA Q-50
instrument) under nitrogen. Differential scanning calorimetry
(DSC) was performed with a TA instrument (DSC-TA Q-20) under
nitrogen at a heating rate of 10 °Cmin–1. Cyclic voltammetry (CV)
measurements were performed with a B-class solar simulator: Po-
tentiostate/Galvanostate (SP-150 OMA company); The supporting
4,7-Bis[3-hexyl-5-(tributylstannyl)thiophen-2-yl]benzo[c][2,1,3]-
thiadiazole (5): Lithium diisopropylamide (LDA, 2 m solution in
heptane/THF/ethylbenzene, 8.22 mL, 16.43 mmol) was added
dropwise over a period of 30 min to a solution of compound 3
(1.54 g, 3.26 mmol) in dry THF (70 mL) cooled to –80 °C, followed
by stirring at the same temperature for 1 h. The color of the reac-
tion mixture changed from orange to dark green. Tributyltin chlo-
ride (4.46 mL, 16.43 mmol) was then added dropwise over a period
of 30 min. After addition was complete, the reaction mixture was
allowed to return slowly to room temperature, followed by stirring
overnight. After quenching with saturated aqueous NH4Cl solu-
tion, the reaction mixture was extracted with ethyl acetate and the
organic layers were collected and washed repeatedly with water.
After drying (anhydrous Na2SO4), the organic solvent was evapo-
rated under vacuum and the resulting residue was purified by silica
gel flash column chromatography (n-hexane/Et3N 95:5) to afford
the desired product 5 (3.07 g, 90% yield) as an orange oil. 1H NMR
(400 MHz, CDCl3): δ = 7.61 (s, 2 H, 2ϫCH, benzothiadiazole),
7.12 (s, 2 H, 2ϫCH, thiophene), 2.70–2.66 (t, J = 16.00 Hz, 4 H,
2ϫArCH2), 1.65 (m, 16 H, 2ϫAr-CH2CH2, 6ϫSnCH2), 1.37–
1.16 [m, 36 H, 2ϫCH3(CH2)3, 6ϫSnCH2(CH2)2], 0.93–90 (m, 24
H, 2ϫCH3, hexyl, 2ϫCH3, butyl) ppm. 13C NMR (100 MHz,
CDCl3): δ = 154.18, 142.64, 137.97, 137.79, 137.59, 129.54, 127.59,
31.56, 30.77, 32.26, 29.18, 29.09, 27.29, 22.53, 14.02, 13.65,
10.88 ppm. C50H84N2S3Sn2 (1046.83): calcd. C 57.37, H 8.09, N
2.68, S 9.19, Sn 22.68; found C 57.28, H 8.02, N 2.51, S 9.08.
electrolyte
was
tetrabutylammonium
hexafluorophosphate
(TBAPF6) in acetonitrile (0.1 m) and the scan rate was 50 mVs–1.
A three-electrode cell was used; a Pt wire and silver/silver chloride
[Ag in KCl (0.1 m)] were used as the counter and reference elec-
trodes, respectively. The polymer films for electrochemical measure-
ments were spin-coated from a polymer/chlorobenzene solution on
ITO glass slides (ca. 10 mgmL–1). XRD experiments were per-
formed with a Bruker D8 advanced model diffractometer and with
Cu-Kα radiation (λ = 1.542 Å) at a generator voltage of 40 kV and
a current of 40 mA. The gel permeation chromatographic (GPC)
analysis was carried out with a Shimadzu (LC-20A Prominence
Series) instrument; chloroform was used as a carrier solvent (flow
rate: 1 mLmin–1, 30 °C) and calibration curves were produced with
the aid of standard polystyrene samples. Microwave-assisted poly-
merizations were performed with a focused microwave synthesis
systemCEM (Discover S-Class System). The low-temperature reac-
tions were essentially performed in
a low-temperature bath
(PSL1810-SHANGHA EYELA). A syringe pump (KD Scientific,
KDS-100) was used for delivery of accurate and precise amounts
of reagents during the dropwise addition processes.
4,7-Bis(3-hexylthiophen-2-yl)benzo[c][2,1,3]thiadiazole (3): A mix-
ture of 4,7-dibromo-2,1,3-benzothiadiazole (1, 1.0 g, 3.4 mmol), 2-
(3-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2,
2.5 g, 8.5 mmol), and Pd(PPh3)4 (0.5 g, 43.5 mol-%) was dissolved
in toluene (35 mL) and degassed with nitrogen for 20 min, which
was followed by addition of an aqueous solution of K2CO3 (2 m,
30 mL). The reaction mixture was stirred vigorously and heated
under reflux at 100 °C for 12 h. After cooling, the reaction mixture
was extracted with toluene and the organic layers were collected,
washed with water, and finally dried with anhydrous Na2SO4. The
solvent was evaporated under reduced pressure and the obtained
crude product was purified by silica gel flash column chromatog-
4,7-Bis(4-hexylthiophen-2-yl)benzo[c][2,1,3]thiadiazole (7)[8c,29]
Method A: A mixture of compound 1 (1.0 g, 3.4 mmol), 2-(4-hexyl-
thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6, 2.5 g,
8.5 mmol),[14] and Pd(PPh3)4 (0.5 g, 43.5 mol-%) was dissolved in
toluene (35 mL) and degassed with nitrogen for 20 min, which was
followed by addition of an aqueous solution of K2CO3 (2 m,
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Alternating Copolymers Based on 2,1,3-Benzothiadiazole and Hexylthiophene
30 mL). The reaction mixture was vigorously stirred and heated
under reflux at 100 °C for 12 h. After having been allowed to cool
to room temperature, the reaction mixture was extracted with tolu-
ene and the organic layers were separated, washed thoroughly with
water, and finally dried with anhydrous Na2SO4. The solvent was
evaporated under vacuum and the resulting crude product was
purified by silica gel flash column chromatography (n-hexane/
EtOAc 10:2) to generate 7 (1.35 g, 85% yield) as a bright orange
solid.
stirred overnight. After having been quenched with saturated aque-
ous NH4Cl, the reaction mixture was extracted with ethyl acetate.
The organic layers were collected, washed thoroughly with water,
and finally dried with anhydrous Na2SO4. The solvent was removed
under reduced pressure and the obtained residue was purified by
silica gel flash column chromatography (n-hexane/Et3N 95:5%) to
afford 9 (3.9 g, 92.4% yield) as a dark red oil. 1H NMR (400 MHz,
CDCl3): δ = 8.15 (s, 2 H, 2ϫCH, benzothiadiazole), 7.82 (s, 2 H,
2ϫCH, thiophene), 2.66–2.63 (t, J = 12.00 Hz, 4 H, 2ϫArCH2),
1.70 (m, 16 H, 2ϫArCH2CH2, 6ϫSnCH2), 1.20–1.43 [m, 36 H,
2ϫCH3(CH3)3, 6ϫSnCH2(CH2)2], 0.91 (m, 24 H, 2ϫCH3, hexyl,
2ϫCH3, butyl) ppm. 13C NMR (100 MHz, CDCl3): δ = 152.62,
144.77, 134.03, 129.85, 125.73, 125.62, 125.50, 32.19, 31.85, 30.30,
29.52, 29.18, 27.32, 22.66, 14.10, 13.65, 11.02 ppm. C50H84N2S3Sn2
(1046.83): calcd. C 57.37, H 8.09, N 2.68, S 9.19, Sn 22.68; found
C 57.30, H 8.14, N 2.71, S 9.14.
Method B: A mixture of 2,1,3-benzothiadiazole-4,7-bis(boronic
acid pinacol ester) (10, 1 g, 2.48 mmol), 2-bromo-4-hexylthiophene
(11, 1.5 g, 6.2 mmol),[14] and Pd(PPh3)4 (0.5 g, 43.5 mol-%) was dis-
solved in toluene (35 mL) and degassed with nitrogen for 20 min,
which was followed by addition of an aqueous solution of K2CO3
(2 m, 30 mL). The reaction mixture was vigorously stirred and
heated under reflux at 90 °C for 12 h. After having been allowed
to cool to room temperature, the reaction mixture was extracted
with toluene and the organic layers were separated, washed thor-
oughly with water, and finally dried with anhydrous Na2SO4. The
solvent was evaporated under vacuum and the crude product was
purified by silica gel flash column chromatography (n-hexane/
EtOAc 10:2) to generate 7 (1.08 g, 93% yield) as a bright orange
4,4Ј-Dihexyl-5,5Ј-bis(tributylstannyl)-2,2Ј-bithiophene
(13):[17b,18]
LDA (6.24 mL, 12.5 mmol, 2.5 mol equiv.) was added dropwise
over a period of 1 h to a solution of 4,4Ј-dihexyl-2,2Ј-bithiophene
(12, 1.67 g, 5 mmol)[14] in dry THF (75 mL), cooled to –80 °C. The
reaction mixture was further stirred for 1 h at –80 °C, which was
followed by addition of a solution of tributyltin chloride (3.37 mL,
solid. 1H NMR (400 MHz, CDCl3): δ = 7.97 (s, 2 H, 2ϫCH, 12.50 mmol, 2.5 mol equiv.) in dry THF (5 mL) over a period of
benzothiadiazole), 7.82 (s, 2 H, 2ϫSCH=CH), 7.04 (s, 2 H,
2ϫSCH), 2.71–2.67 (t, J = 16.00 Hz, 4 H, 2ϫArCH2), 1.70 (m, 4
H, 2ϫArCH2CH2), 1.34 [m, 12 H, 2ϫCH3(CH2)3], 0.90–0.88 (t,
15 min. After addition was complete, the reaction mixture was al-
lowed to warm slowly to room temperature and stirred for an ad-
ditional 4 h. Water (30 mL) and ethyl acetate (30 mL) were then
J = 8.00 Hz, 6 H, 2ϫCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = added to the reaction mixture and the organic layer was separated
152.64, 144.36, 138.99, 128.99, 126.02, 125.53, 121.52, 31.70, 30.65,
30.47, 29.04, 22.62, 14.10 ppm. C26H32N2S3 (468.74): calcd. C
66.62, H 6.88, N 5.98, S 20.52; found C 66.76, H 6.94, N 6.07, S
20.55.
and washed twice with water (30 mL), followed by drying over an-
hydrous Na2SO4. The solvent was evaporated under reduced pres-
sure and the obtained oil residue was purified by flash column
chromatography on neutralized silica gel (n-hexane) to afford the
desired product 13 as a light yellow oil (4.15 g, 91% yield). 1H
NMR (400 MHz, CDCl3): δ = 6.73 (s, 2 H, 2ϫCH, thiophene),
4,7-Bis(5-bromo-4-hexylthiophen-2-yl)benzo[c][2,1,3]thiadiazole (8):
NBS (1.6 g, 9.01 mmol) was added in one portion to an ice-cold
solution of compound 7 (1.7 g, 3.63 mmol) in dry THF (65 mL).
After having been stirred for 30 min at 0 °C, the reaction mixture
was stirred overnight at room temperature, followed by quenching
with saturated aqueous NH4Cl. The reaction mixture was extracted
with ethyl acetate, followed by washing of the separated organic
layers with water. The collected organic layers were dried with an-
hydrous Na2SO4 and the organic solvent was removed under vac-
uum. The crude product obtained was purified by silica gel flash
column chromatography (n-hexane/EtOAc 10:2) to generate 8
(2.25 g, 99% yield) as a red solid.[29] 1H NMR (400 MHz, CDCl3):
δ = 7.76 (s, 2 H, 2ϫCH, benzothiadiazole), 7.72 (s, 2 H, 2ϫCH,
thiophene), 2.65–2.61 (t, J = 16.00 Hz, 4 H, 2ϫArCH2), 1.66 (m,
4 H, 2ϫArCH2CH2), 1.34 [m, 12 H, 2ϫCH3(CH2)3], 0.90–0.88 (t,
2.55 (t,
4 H, 2ϫArCH2), 1.62 (m, 16 H, 2ϫArCH2CH2,
6ϫSnCH2), 1.33–1.29 [m, 36 H, 2ϫCH3(CH2)3, hexyl,
6ϫCH3(CH2)2, butyl], 0.96–0.88 (m, 24 H, 2ϫCH3, hexyl,
6ϫCH3, butyl) ppm. 13C NMR (100 MHz, CDCl3): δ = 138.00,
137.40, 126.11, 121.07, 33.84, 31.91, 31.80, 28.90, 28.70, 27.24,
22.73, 14.10, 13.80, 10.90 ppm. C44H82S2Sn2 (912.67): calcd. C
57.90, H 9.06, S 7.03, Sn 26.01; found C 58.11, H 9.09, S 6.98.
3,3Ј-Dihexy1-2,2Ј-bithiophene (16): Hexylmagnesium bromide
(7.51 mL, 15 mmol, 2 m ether solution) was added dropwise over a
period of 30 min, under a stream of nitrogen, to an ice-cooled solu-
tion of 3,3Ј-dibromo-2,2Ј-bithiophene (15, 1.62 g, 5.0 mmol) in ul-
tra dry ether (55 mL) containing NidpppCl2 (0.11 g, 0.21 mmol).
The reaction mixture was warmed to 40 °C and stirred under re-
J = 8.00 Hz, 6 H, 2ϫCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = flux. After 12 h, the reaction mixture was then ice-cooled and
152.23, 143.07, 138.49, 128.07, 125.31, 124.85, 111.60, 31.63, 29.73, treated with saturated aqueous NH4Cl solution. The organic layer
29.68, 28.95, 22.61, 14.10 ppm. C26H30Br2N2S3 (626.53): calcd. C was separated and washed thoroughly with water, followed by dry-
49.84, H 4.83, Br 25.51, N 4.47, S 15.35; found C 49.93, H 4.85,
N 4.56, S 15.40.
ing over anhydrous Na2SO4. The solvent was evaporated and the
crude product was purified by silica gel flash column chromatog-
raphy (n-hexane) to afford the desired product 16 as a light yellow
oil (1.57 g, 94% yield).[30] 1H NMR (400 MHz, CDCl3): δ = 7.28–
7.26 (d, J = 8.00 Hz, 2 H, 2ϫSCH), 6.96–6.94 (d, J = 8.00 Hz, 2
H, 2ϫhexyl-CCH), 2.50–2.47 (t, J = 12.00 Hz, 4 H, 2ϫArCH2),
1.54–1.22 [m, 16 H, 2ϫCH3(CH2)4], 0.84–0.82 (t, J = 8.00 Hz, 6 H,
2ϫCH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 142.32, 128.70,
128.50, 125.19, 31.62, 30.69, 29.09, 28.77, 22.56, 14.05 ppm.
C20H30S2 (334.58): calcd. C 71.80, H 9.04, S 19.17; found C 71.71,
H 9.14, S 19.20.
4,7-Bis[4-hexyl-5-(tributylstannyl)thiophen-2-yl]benzo[c][2,1,3]-
thiadiazole (9): LDA (7.52 mL, 15 mmol) was added over a period
of 45 min to a stirred solution of compound 7 (1.4 g, 3 mmol) in
dry THF (100 mL), cooled to –80 °C. The color of the reaction
mixture changed from dark orange to dark green. The reaction
mixture was further stirred for 1 h at –80 °C and for 2 h at room
temperature, after which the color of the reaction mixture had
turned to a deep violet. After the mixture had again been cooled
to –80 °C, tributyltin chloride (4.1 mL, 15 mmol) was then added
dropwise over a period of 30 min. The reaction mixture was then
allowed to warm slowly to room temperature, after which it was
3,3Ј-Dihexyl-5,5Ј-bis(tributylstannyl)-2,2Ј-bithiophene (17): LDA
(5.41 mL, 10.83 mmol, 2.5 mol equiv.) was added dropwise over a
Eur. J. Org. Chem. 2011, 4841–4852
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A. A. El-Shehawy, N. I. Abdo, A. A. El-Barbary, J.-S. Lee
FULL PAPER
period of 1 h to a solution of 3,3Ј-dihexyl-2,2Ј-bithiophene (16,
1.45 g, 4.33 mmol) in dry THF (75 mL), cooled to –80 °C. After
addition was complete, the reaction mixture was stirred for 1 h at
–80 °C, followed by addition of a solution of tributyltin chloride
(2.92 mL, 10.83 mmol, 2.5 mol equiv.) in dry THF (5 mL) over a
period of 15 min. The reaction mixture was then allowed to warm
slowly to room temperature and stirred for a further 4 h. Water
(30 mL) and ethyl acetate (30 mL) were then added and the organic
layer was separated and washed twice with water (30 mL), followed
by drying over anhydrous Na2SO4. The organic solvent was re-
moved under vacuum to yield the crude product, which was puri-
fied by column chromatography on neutralized silica gel (n-hexane)
to afford 17 as a light yellow oil (3.83 g, 97% yield).[17b,18] 1H NMR
(400 MHz, CDCl3): δ = 6.97 (s, 2 H, 2ϫArCH), 2.55–2.51 (t, J =
16.00 Hz, 4 H, 2ϫArCH2), 1.60 (m, 16 H, 2ϫArCH2CH2,
6ϫSnCH2), 1.35–1.07 [m, 36 H, 2ϫCH3(CH2)3, hexyl,
6ϫCH3(CH2)2, butyl], 0.91–0.88 (t, J = 12.00 Hz, 24 H, 2ϫCH3,
hexyl, 6ϫCH3, butyl) ppm. 13C NMR (100 MHz, CDCl3): δ =
142.56, 137.09, 136.10, 135.17, 31.64, 30.91, 29.17, 28.97, 28.73,
27.24, 22.58, 14.05, 13.66, 10.77 ppm. C44H82S2Sn2 (912.67): calcd.
C 57.90, H 9.06, S 7.03, Sn 26.01; found C 57.98, H 9.13, S 7.05.
In all cases, the final polymers were dried under vacuum for at least
24 h and then subjected for the required analysis. The chemical
yields, molecular weights (Mn), and molecular weight distributions
(PDI; Mw/Mn) of the resulting copolymers are summarized in
Table 1.
Poly{(benzo-2,1,3-thiadiazol-4,7-diyl)-5,5-[4,7-bis(4-hexylthiophen-
2-yl)benzo[c](2,1,3)thiadiazole]} (P1): 1H NMR (400 MHz, CDCl3):
δ = 8.17–7.79 (m, 4 H, 4ϫCH, benzothiadiazole), 7.05 (brs, 2 H,
2ϫCH, thiophene), 2.79–2.70 (brd, 4 H, 2ϫArCH2), 1.75–1.71
(m, 4 H, 2ϫ ArCH2CH2), 1.34–1.26 [m, 12 H, 2 ϫCH3(CH2)3],
0.90–0.85 (brt, 6 H, 2ϫCH3) ppm. (C32H32N4S4)n (600.88)n: calcd.
C 63.96, H 5.37, N 9.32 S, 21.35; found C 64.17, H 5.32, N 9.40,
S 21.30.
Poly[(benzo-2,1,3-thiadiazol-4,7-diyl)-(4,4Ј-dihexyl-2,2Ј-dithiophen-
1
5,5Ј-diyl)] (P2): H NMR (400 MHz, CDCl3): δ = 7.68 (brd, 2 H,
2ϫCH, benzothiadiazole), 7.27 (s, 2 H, 2ϫCH, thiophene), 2.70
(brt, 4 H, 2ϫArCH2), 1.69 (m, 4 H, 2ϫArCH2CH2), 1.31–1.34
[m, 12 H, 2 ϫ CH3(CH2)3], 0.86–0.83 (brt, 6 H, 2ϫ CH3) ppm.
(C26H30N2S3)n (466.72)n: calcd. C 66.91, H 6.48, N 6.00, S 20.61;
found C 66.75, H 6.30, N 5.89, S 19.69.
General Procedure for Microwave-Assisted Stille Cross-Coupling Po-
lymerization: Equimolar amounts (0.5 mmol) of the desired di-
bromo and bis(tributylstannyl) derivatives were dissolved in dry
DMF in a very well dried screw-capped glass tube and degassed
with nitrogen for at least 30 min. A catalytic amount of Pd(PPh)4
(5 mol-% relative to Br) was then added to the reaction mixture,
which was degassed again with nitrogen for an additional 30 min.
The screw-capped glass tube was then microwave-irradiated for
5 min at 100 °C, 5 min at 120 °C, and 30 min at 150 °C.
Poly[(benzo-2,1,3-thiadiazol-4,7-diyl)-5Ј,5Ј-[4,7-bis(3Ј,4-dihexyl-
2,2Ј-bithiophen-5-yl)benzo[c][2,1,3]thiadiazole] (P3): 1H NMR
(400 MHz, CDCl3): δ = 8.02–7.67 (m, 4 H, 4ϫCH, benzothiadia-
zole), 7.27 (brs, 2 H, 2ϫCH, thiophene), 7.04 (brs, 2 H, 2ϫCH,
thiophene), 2.90–2.70 (m, 8 H, 4ϫArCH2), 1.79–1.70 (brm, 8 H,
4ϫArCH2CH2), 1.50–1.25 [m, 24 H, 4ϫCH3(CH2)3], 0.91–0.84
(brt, 12 H, 4ϫCH3) ppm. (C52H60N4S6)n (933.45)n: calcd. C 66.91,
H 6.48, N 6.00, S 20.61; found C 66.99, H 6.55, N 6.13, S 20.65.
Poly[(benzo-2,1,3-thiadiazol-4,7-diyl)-(3,3Ј-dihexyl-2,2Ј-dithiophen-
1
The end-capping process was performed in two separate steps. A
solution of phenylboronic acid pinacol ester (5 mol-%) in DMF
(0.5 mL) was added first, followed by microwave irradiation of the
reaction mixture for 2 min at 100 °C, 2 min at 120 °C, and 5 min
at 150 °C. The same process was repeated by addition of a solution
of bromobenzene (5 mol-%) in DMF (0.5 mL). After the second
end-capping process, the screw-capped glass tube was allowed to
return to room temperature and the reaction mixture was poured
into methanol. The crude polymer was collected by filtration and
washed repeatedly with methanol. The residual solid was loaded
into an extraction thimble and washed with methanol (24 h) fol-
lowed by acetone (24 h).
5,5Ј-diyl)] (P4): H NMR (400 MHz, CDCl3): δ = 8.10 (brs, 2 H,
2ϫCH, benzothiadiazole), 7.86 (brs, 2 H, 2ϫCH, thiophene), 2.70
(brt, 4 H, 2ϫArCH2), 1.69 (brs, 4 H, 2ϫArCH2CH2), 1.37–1.20
[m, 12 H, 2 ϫ CH3(CH2)3], 0.88–0.70 (brt, 6 H, 2ϫ CH3) ppm.
(C26H30N2S3)n (466.72)n: calcd. C 66.91, H 6.48, N 6.00, S 20.61;
found C 66.82, H 6.64, N 6.15, S 20.49.
Poly[(benzo-2,1,3-thiadiazol-4,7-diyl)-(4,4Ј,3ЈЈ,4ЈЈЈ-tetrahexyl-
2,2Ј;5Ј,2ЈЈ;5ЈЈ,2ЈЈЈ-quarterthiophen-5,5Ј-yl)] (P5): 1H NMR
(400 MHz, CDCl3): δ = 7.69 (brd, 2 H, CH, benzothiadiazole),
7.19 (brs, 2 H, 2ϫCH, thiophene), 7.06 (brs, 2 H, 2ϫCH, thio-
phene), 2.69–2.56 (br d, 8 H, 4 ϫ ArCH2), 1.72–1.57 (brd, 8 H,
4ϫArCH2CH2), 1.29–1.20 [m, 24 H, 4ϫCH3(CH2)3], 0.89–0.80
(brt, 12 H, 4ϫCH3) ppm. (C46H58N2S5)n (799.29)n: calcd. C 69.12,
H 7.31, N 3.50, S 20.06; found C 69.23, H 7.49, N 3.39, S 19.79.
General Procedure for Thermal Stille Cross-Coupling Polymeriza-
tion: A 25 mL round-bottomed flask fitted with a three-way stop-
cock was heated under reduced pressure and then allowed to cool
to room temperature under nitrogen. In this flask, equimolar
amounts (0.5 mmol) of the desired dibromo and bis(tributylstan-
nyl) derivatives were dissolved in dry DMF (20 mL) and degassed
(nitrogen) for at least 30 min. A catalytic amount of Pd(PPh)4
(5 mol-% relative to Br) was then added to the reaction mixture
and degassing with nitrogen was again performed for an additional
30 min. The reaction mixture was stirred vigorously and heated un-
der reflux at 100 °C for 72 h. End-capping was performed in two
steps. A solution of phenylboronic acid pinacol ester (5 mol-%) in
toluene (0.5 mL) was added, followed by heating of the reaction
mixture under reflux for at least 6 h. The same process was repeated
by addition of a solution of bromobenzene (5 mol-%) in toluene
(0.5 mL). After the second end-capping process, the reaction mix-
ture was allowed to return to room temperature and poured into
methanol. The crude polymer was collected by filtration and
washed repeatedly with methanol. The residual solid was loaded
into an extraction thimble and washed with methanol (24 h) fol-
lowed by acetone (24 h).
Poly[(benzo-2,1,3-thiadiazol-4,7-diyl)-(3,4Ј,3ЈЈ,3ЈЈЈ-tetrahexyl-
2,2Ј;5Ј,2ЈЈ;5ЈЈ,2ЈЈЈ-quarter thiophen-5,5Ј-yl)] (P6): 1H NMR
(400 MHz, CDCl3): δ = 7.93–7.92 (brd, 2 H, CH, benzothiadia-
zole), 7.77 (brs, 2 H, 2ϫCH, thiophene), 7.09 (brs, 2 H, 2ϫCH,
thiophene), 2.82–2.54 (brd, 8 H, 4ϫArCH2), 1.68–1.57 (brd, 8 H,
4ϫArCH2CH2), 1.27–1.18 [m, 24 H, 4ϫCH3(CH2)3], 0.81–0.76
(brt, 12 H, 4ϫCH3) ppm. (C46H58N2S5)n (799.29)n: calcd. C 69.12;
H, 7.31; N, 3.50; S, 20.06; found C 69.18, H 7.37, N 3.42, S 19.92.
Supporting Information (see footnote on the first page of this arti-
cle): NMR spectra of compounds 3–5, 7–9, 16, 17 and the copoly-
mers P1–P6, and GPC profiles of the copolymers P1–P6.
Acknowledgments
This work was supported by the Program for Integrated Molecular
Systems (PIMS) and the World Class University (WCU) program,
Korea (Project No. R31-20008–000–10026–0)
4850
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Eur. J. Org. Chem. 2011, 4841–4852

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Received: February 8, 2011
Published Online: July 5, 2011
4852
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 4841–4852