The preparation of 3,6-bis(3-hexylthien-2-yl)-s-tetrazine and its conjugated polymers

Article abstract of DOI:10.1002/pola.24774

We demonstrated, for the first time, that 3,6-bis(3-hexylthien-2-yl)-s- tetrazine (TTz) with hexyl group at the 3-position of thiophene rings can be prepared using a modified sulfur-assisted Pinner synthesis. Although the hexyl group creates large steric hindrance to the tetrazine ring formation reaction, and the reaction under a traditional condition only produces trace amount of the target product, the yield of this reaction under a modified reaction condition using anhydrous hydrazine at 68 °C can reach 65%. Two new copolymers of the resulting TTz and hexyl- or 2-ethylhexyl-substituted cyclepentadithiophene have been prepared. The polymers show a broader light absorption in film than in solution attributing to the large distribution of effective conjugation length of polymer chain due to the existence of both cis- and trans-orientations of the 3-hexylthiophene units in the planar polymer chain in solid state. Although the polymers show a narrow band gap and a deep HOMO level, which are desirable for generating an efficient light absorption and a larger open circuit voltage (V_oc) of the resulting solar cell devices, the device performance is not as good as expected. It is attributed to the random distribution of the cis- and trans-conformations along the polymer chain.

Full text of DOI:10.1002/pola.24774

The Preparation of 3,6-Bis(3-hexylthien-2-yl)-s-tetrazine and Its
Conjugated Polymers
JIANFU DING, ZHAO LI, ZHE CUI,* GILLES P. ROBERTSON, NAIHENG SONG, XIAOMEI DU, LUDMILA SCOLES
Institute for Chemical Process and Environmental Technology (ICPET), National Research Council of Canada (NRC),
1200 Montreal Road, Ottawa, Ontario, Canada K1A 0R6
Received 1 April 2011; accepted 11 May 2011
DOI: 10.1002/pola.24774
Published online 31 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: We demonstrated, for the first time, that 3,6-bis(3-
hexylthien-2-yl)-s-tetrazine (TTz) with hexyl group at the 3-posi-
tion of thiophene rings can be prepared using a modified sul-
fur-assisted Pinner synthesis. Although the hexyl group creates
large steric hindrance to the tetrazine ring formation reaction,
and the reaction under a traditional condition only produces
trace amount of the target product, the yield of this reaction
under a modified reaction condition using anhydrous hydra-
zine at 68 ꢀC can reach 65%. Two new copolymers of the
resulting TTz and hexyl- or 2-ethylhexyl-substituted cyclepenta-
mer chain due to the existence of both cis- and trans-orienta-
tions of the 3-hexylthiophene units in the planar polymer chain
in solid state. Although the polymers show a narrow band gap
and a deep HOMO level, which are desirable for generating an
efficient light absorption and a larger open circuit voltage (Voc
)
of the resulting solar cell devices, the device performance is
not as good as expected. It is attributed to the random distribu-
tion of the cis- and trans-conformations along the polymer
§
C
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chain. 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 49: 3374–3386, 2011
dithiophene have been prepared. The polymers show
a
broader light absorption in film than in solution attributing to
the large distribution of effective conjugation length of poly-
KEYWORDS: bulk heterojunction; conjugated polymer; narrow
band gap; s-tetrazine; steric hindrance; solar cell
INTRODUCTION s-Tetrazine (Tz) in conjugated molecules is
one of the most electron-deficient moiety with the electron-
pulling ability equivalent to tetranitrophenyl unit.1 Owing to
the successful work on developing sulfur-assisted Pinner syn-
thesis of Tz unit in conjugated molecules by Abdel-Rahman,2,3
and the recent extensive study on the synthesis of a wide
range of tetrazines by Audebert,4 the development of new con-
jugated Tz structures in conjugated small molecules1–4 and
polymers has achieved significant progresses.4(a),5 These
materials showed a great potential for the use in optical and
electronic devices such as nonlinear optical devices,6 sensors,7
and smart windows.8 The highly electron-deficient property of
Tz unit is extremely attractive for constructing push–pull
structures in pendent groups for nonlinear optical applica-
tions1(a),6 and inside polymer main chains for solar cell appli-
cations.9 Because of the extremely strong electron pulling
effect of Tz, the resulting polymers with this push–pull struc-
ture have a narrow band gap and deep HOMO level,9 which
enhance sun light absorption and promote open circuit voltage
(Voc) of solar cell devices. Although a few different approaches
can be used to prepare Tz, which are summarized in a recent
review article by Audebert,1(a) the sulfur-assisted Pinner
synthesis appears the most practicable way to produce Tz unit
in conjugated aromatic molecules such as dithienyl-s-tetra-
zine.1–4 Furthermore, to further adjust the properties of the
final product, such as solubility and processability, it is desired
to introduce an alkyl group to the 3- or 4-position of the
thiophene ring as shown in Scheme 1.
Recently, we have successfully prepared a TTz compound with
the hexyl group at the 4-position, 3,6-bis(4-hexylthien-2-yl)-s-
tetrazine) (TTz-6out) using the sulfur-assisted Pinner synthe-
sis.9 After bromination, this molecule was copolymerized with
2,6-bis(trimethylstannane) monomer of 4,4-dihexyl-4H-cycle-
penta[2,1-b:3,4-b0]dithiophene) (CPDT) by Stille coupling reac-
tion to yield PCPDTTTz-6:6out (Scheme 1), where the first 6 in
the suffix represents hexyl group on CPDT, and the second 6
with a subscript ‘‘out’’ represents hexyl group at the 4-position
of thiophen ring, which faces away from the Tz ring. It is used
to distinguish the hexyl group at the 3-position, which faces
toward Tz ring in poly[2,6-(4,4-dihexyl-4H-cyclepenta[2,1-
b:3,4-b0]dithiophene)-alt-5,50-(3,6-bis(3-hexylthien-2-yl)-s-tetra-
zine)] (PCPDTTTz-6:6in), which will be reported in this article.
This out-polymer (PCPDTTTz-6:6out) was tested for polymer
solar cell applications and devices with power conversion effi-
ciency (PCE) up to 5.4% have been achieved,9(a) indicating a
high potential of the Tz polymers for solar cell applications.
Additional Supporting Information may be found in the online version of this article. Correspondence to: J. Ding (E-mail: jianfu.ding@nrc-cnrc.gc.ca)
*Present address: School of Chemical Engineering, Sichuan University, Chengdu 610065, China. E-mail: cuizhe@scu.edu.cn.
NRCC publication number of this article is 52824.
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3374–3386 (2011) 2011 Government of Canada. §Exclusive worldwide publication
C
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rights in the article have been transferred to Wiley Periodicals, Inc.
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SCHEME 1 Structures of TTz monomers and polymers, the suffix 6 with a subscript ‘‘out’’ or ‘‘in’’ in the sample name represents
hexyl group on the thiophene ring which faces away from or toward the Tz ring.
An advantage of the in-polymers (PCPDTTTz-6:6in) over the
out-counterpart is that the alkyl group will provide a protec-
tion to the Tz ring from undesired side reactions. It is well
known that Tz is a reactive aromatic unit and many high
nitrogen-containing tetrazines are widely used as energetic
materials.10 The Tz unit in conjugated molecules can take
part in Diels–Adler cycloaddition with a double bond to lose
a N2 molecule.11 This reaction even occurs with fullerene
derivatives at elevated temperatures.12 Although this reac-
tion is desired for the preparation of many special chemi-
cals,11 it becomes a concern when a Tz containing polymer
is used together with a fullerene derivative such as [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) for solar cell
applications. The solution of the mixture of a polymer with
PCBM is usually prepared and coated at a high temperature
to form a high quality film with a controlled morphological
structure.9 Our preliminary test showed this process could
induce cycloaddition of Tz ring with C60 when the tempera-
ture reached 110 ꢀC. The conversion of this reaction in
ortho-dichlorobenzene (o-DCB) can reach ꢁ20% in 1 h at
this temperature for the out-polymer. This reaction will de-
grade the structure and property of the polymer and must
be prevented or controlled in the device fabrication. Obvi-
ously, we can expect to reduce this undesired reaction for
the in-polymer (PCPDTTTz-6:6in), where the Tz ring is sur-
rounded by the hexyl groups to obtain a steric protection to
prevent this side reaction (Scheme 1). Our test indeed
showed that the reaction rate of PCPDTTTz-6:6in with C60 is
about 28-fold lower than the ‘‘out’’ analog under the same
condition.
Unfortunately, this steric effect also makes the preparation of
the corresponding monomer, 3,6-bis[3-hexylthien-2-yl]-s-tet-
razine (TTz-6in) extremely difficult. This synthesis has to go
through dihydrotetrazine for any possible approaches, with
two examples shown in Scheme 2 for the Pinner and Stolle
syntheses.2,13 In these reactions, a hydrazine molecule has to
approach the carbon atom of the cyano group or oxadiazole
group, respectively. The alkyl substituent at the 3-position of
SCHEME 2 Pinner and Stolle´ synthesis for the preparation of tetrazines.
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The obtained 2-cyano-3-hexylthiophene was then converted
to TTz-6in using the sulfur-assisted Pinner synthesis followed
by an aromatization. The sulfur-assisted Pinner synthesis has
been successfully used to produce TTz-6out in a high yield in
our previous work.9(a) However, the reaction to prepare TTz-
6in is much more difficult. Under a similar reaction condition
by using hydrazine monohydrate in anhydrous ethanol at 78
ꢀC in our initial tests, we only observed a trace amount of
the desired product in the reaction. We attributed it to the
high steric hindrance created by the adjacent hexyl group to
the reaction on the cyano group. It is interesting to find that
a reaction at 70 ꢀC by using anhydrous hydrazine gave a
much higher yield, and therefore, this reaction for the forma-
1
tion of dihydro-s-tetrazine was monitored by H NMR. In this
case, 2-cyano-3-hexylthiophene, sulfur, and anhydrous hydra-
zine at molar ratios of 1.0:0.6:4.0 were added into 0.6 mL of
methanol-d4 in a capped NMR tube. The reaction solution
was placed in a water bath with temperature controlled suꢀb-
ꢀ
ꢀ
SCHEME 3 Synthetic approach for the preparation of the TTz-
sequently at 22 C (for 1 h), 50 C (for 1 h) and then 70 C
with occasional shaking. NMR spectra of the reaction solu-
tion were collected at different times and compared with the
predicted spectrum of the two possible dihydrotetrazine
products in Figure 1.
Br2-6in monomer using the sulfur-assisted Pinner synthesis.
the thiophene ring will obviously make this process more
difficult than at the 4-position. Although the one with a
methyl substitution at the 3-position appeared possible for
Stolle´ synthesis,14 we do not find in the literature any syn-
thesis of similar structure with the substituent at this posi-
tion larger than a methyl group.1(a) Furthermore, a methyl
group attached to this position will make the nitrile inert in
the sulfur-assisted Pinner synthesis.3 Interestingly, we found
that TTz-6in with a hexyl group at 3-position can be pre-
pared in a reasonable yield by the sulfur-assisted Pinner
synthesis under a modified reaction condition. Therefore, in
this article, we will report our approach to optimize the syn-
thesis of TTz-6in, the preparation of the corresponding poly-
mers and their characterizations for solar cell applications.
1
The H NMR spectra showed that the molecular structure of
2-cyano-3-hexylthiophene did not have obvious change when
the reaction solution was kept at room temperature and
ꢀ
then at 50 C for 1 h. But the sulfur powder in the solution
ꢀ
was completely disappeared after 1 h reaction at 50 C, indi-
cating that most of the sulfur had reacted with hydrazine to
form the sulfur-hydrazine adduct as indicated in the reaction
mechanism proposed by Audebert (Scheme 4).4(b) Although
the formed adduct did not react with the nitrile at 50 ꢀC, it
ꢀ
readily reacted at 70 C as indicated by the appearance of a
pair of new peaks at 7.29 (50) and 6.91 (40) ppm. Their
intensities steadily increased with time over the next 17 h.
Meanwhile, another pair of new peaks appeared in 7 h at
7.40 (500) and 6.93 ppm (400) and slowly increased in inten-
sity. These two pairs of the peaks might be attributed to 1,4-
and 1,2-isomers of the dihydrotetrazine. However, to accu-
rately assign them is difficult. One possible way is to use 15N
NMR, where the peak of two tetrazine N in the 1,4-isomer
will be a doublet with a coupling constant of ꢁ6.5 Hz due to
the two-bond 15NANA1H coupling,16 while this coupling
does not exist in 1,2-isomer. But this measurement is impos-
sible in solution due to the extremely low abundance of 15N.
Therefore, we used the Advanced Chemistry Development
CþH NMR prediction software to predict the spectra of 1,2-
and 1,4-isomers.17 The predicted spectrum of a 1:1 mixture
of 1,2- and 1,4-isomers was shown in Figure 1(a). It indi-
cates that the chemical shifts of the thiophene protons in
1,2-isomer is slightly higher than 1,4-isomer, probably due to
the higher conjugation of 1,2-dihydro-s-tetrazine. As there is
very good concordance between the predicted and the exper-
imental spectra, we believe that we can reasonably assign
the first pair of peaks (50 and 40) to 1,4-dihydro-s-tetrazine
and second pair (500 and 400) to 1,2-dihydro-s-tetrazine. Fur-
thermore, with the appearance of the second pair, a new
RESULTS AND DISCUSSION
The Preparation of TTz-6in Monomer
Based on the reasons mentioned above, the sulfur-assisted
Pinner synthesis was chosen for the preparation of TTz-6in.
Therefore, a synthesis approach as shown in Scheme 3 was
designed. The synthesis started from 2-bromo-3-hexylthio-
phene, which was converted to 3-hexylthiophene-2-carboxal-
dehyde through a lithiation reaction.15 This product was fur-
ther converted to 2-cyano-3-hexylthiophene by a two-step
reaction with hydroxylamine and then acetic anhydride.
Although this process gave a high yield, the obtained product
always contained ꢁ7% of 2-cyano-4-hexylthiophene isomer,
which was produced during the lithiation reaction. This iso-
mer was hard to remove in the aldehyde form, but most of
this impurity can be removed in the nitrile form by a column
chromatographic purification with the content reduced to
less than 3%. It is important to remove this isomer as much
as possible, because the existence of this isomer will reduce
the regioregularity of the final conjugated polymer and ham-
per its chain stacking capability.
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FIGURE 1 (a) Predicted 1H NMR
spectrum of 1:1 mixture of bis(3-
hexylthiophen-2yl)-1,2-dihydro-s-
tetrazine and bis(3-hexylthio-
phen-2yl)-1,4-dihydro-s-tetrazine
by ACD software.17 (b) NMR
spectra of the reaction solution
taken at deferent reaction times
at indicated temperatures.
peak of the a-protons in the hexyl group also emerged at a
slightly higher frequency (2.85 ppm). This also suggests a
higher conjugation of the molecule, consistent with the 1,2-
dihydro-structure [see Fig. 1(a)].
equal to the increase of the 1,2-isomer, indicating 1,4-isomer
was being slowly converted to 1,2-isomer in this period. It
should be noted that peaks 500 and 400 completely disap-
peared when additional 3 equiv of anhydrous hydrazine was
added at 42 h. Meanwhile the conversion rate to 1,4-dihy-
dro-s-tetrazine dramatically increased from 40 to 66%. This
result indicates that the 1,2-isomer is completely reverted to
the 1,4-isomer under the assistance of excess hydrazine. It
also means the presence of high concentrated hydrazine in
the solution will depress the conversion of 1,4-isomer to 1,2-
isomer.
The conversion rate to these two dihydrotetrazines was esti-
mated by comparing the integral intensities of these two
pairs of the peaks (50, 40 and 500, 400) with that of the methyl
1
group at 0.89 ppm in the H NMR spectra (Fig. 1). The result
is plotted in Figure 2 along with the conversion rate of the
starting material calculated from peak 5 and 4. It shows that
conversion rate of the starting material steadily increases in
17 h and then levels off. The conversion to 1,4-dihydro-s-tet-
razine shows the same trend but with lower values and
reaches the maximum of 47% at 17 h. The difference
between these two curves is obviously due to the formation
of 1,2-isomer, as well as small amount of byproducts as indi-
cated by the small peaks in Figure 1(b). After 17 h, the
decrease of the 1,4-isomer peak intensities was more or less
Figure 2 also shows that the conversion of the starting mate-
rial was leveled off after 17 h, but it resumed when extra hy-
drazine was added into the reaction. This result indicates
that the cease of the reaction is due to the exhaustion of hy-
drazine. It should be noted that a large amount of gas
released from the reaction in the initial 7 h. The gas turned
wet pH paper blue, indicating the gas is mostly composed of
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SCHEME 4 Reaction mechanism
of the sulfur-assisted Pinner syn-
thesis proposed by Audebert.4(b)
ammonia. It means that the decomposition of hydrazine to
ammonia competed with the Tz ring formation in this pro-
cess. With the decrease of the hydrazine concentration, the
ring formation reaction reduced its speed and almost
stopped after 17 h. In this case, the addition of extra hydra-
zine into the reaction pushed the ring formation reaction for-
ward again.
After cooling to room temperature, the solution turned to
slurry, which was diluted with MeOH, filtered, and rinsed
with MeOH to obtain a white powder. The H NMR spectrum
1
of the crude product shows a rather pure product (Fig. 3),
indicating the selectivity of this reaction to the 1,4-isomer is
high under this reaction condition. This result confirms our
1
observation in the H NMR study that the conversion of 1,4-
1H NMR study of this reaction under different reaction con-
ditions, for example, at 78 ꢀC in methanol-d4, resulted in
complex NMR spectra, indicating side reactions predomi-
ꢀ
nated. Reactions using hydrazine monohydrate at 70 C only
yielded a conversion to the desired product lower than 10%
in 2 days. These results explain why our initial attempt for
making TTz-6in failed by reacting with hydrazine monohy-
ꢀ
drate at 78 C, which was successfully used to prepare TTz-
6out isomer.9(a) However, the reason behind this low conver-
sion by the use of hydrazine monohydrate is not fully under-
stood. NMR study showed the starting 2-cyano-3-hexylthio-
pene mostly remained in the reaction solution after 1 day at
70 ꢀC, while the sulfur powder was consumed at the early
stage of the reaction. This might indicate the presence of
water in the reaction reduces the reactivity of the sulfur-
hydrazine adduct based on the reaction mechanism shown
in Scheme 4.
Based on these observations, a new reaction was designed.
Therefore, anhydrous hydrazine was added into the ethanol
reaction solution in three portions (2, 1, and 1 equiv) at 0, 6,
and then 12 h at 68 ꢀC. The reaction was stopped at 20 h.
FIGURE 2 Conversion with the reaction time of the starting
materials, and to the formation of 1,4- and 1,2-isomers calcu-
lated from NMR spectra shown in Figure 1.
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FIGURE 3 NMR spectrum in C6D6
of the crude product from 2-
cyano-3-hexylthiophene by the
modified sulfur-assisted Pinner
reaction.
isomer to 1,2-isomer is depressed by the presence of hydra-
zine. Similar phenomenon was also reported for another ring
closure reaction leading to dihydro-s-tetrazine.12
silica gel in CH2Cl2 at room temperature to form the
dibromo monomer, 3,6-bis{5-bromo-3-hexylthien-2-yl]-s-tet-
razine (TTz-Br2-6in). It is worth to note that the aromatic
proton of TTz-Br2-6in has a much smaller chemical shift
(7.05 ppm) than its TTz-Br2-6out analogs (7.95 ppm).9(a) The
3-H of the thiophene ring in TTz-Br2-6out is strongly
deshielded by the Tz ring, hence, its peak shifted toward a
higher frequency, while this effect does not apply to the 4-H
in TTz-Br2-6in.
The obtained white powder was then dissolved in 20 mL of
CHCl3 and was aromatized by isoamyl nitrite at 40 ꢀC. The
reaction was completed in 8 h. Recrystallization of the crude
product in 2-propanol gave red shinning needle-like crystal
in a yield of 65%. TTz-6in was then brominated using N-bro-
mosuccinimide (NBS) in the presence of a small amount of
SCHEME 5 Reaction scheme for the preparation of the copolymers of TTz and CPDT by Stille coupling reaction.
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as shown in Scheme 5. The structure of the polymers was con-
firmed by 1H NMR. However, the NMR spectra of both the
polymers had one aromatic proton peak missing when the
spectrum was collected in o-DCB-d4. This is due to that the
missing peak is overlapped with one of the solvent peaks.
Both the proton peaks were observed in the spectrum col-
lected in toluene-d8 (see Supporting Information). Gel
permeation chromatography (GPC) study showed that both
polymers had a high-molecular weight with number–average
molecular weights of 15.7 and 48.0 kDa for PCPDTTTz-6:6in
and poly[2,6-(4,4-bis(2-ethylhexyl-4H-cyclepenta[2,1-b:3,4-
b0]dithiophene)-alt-5,50-(3,6-bis(3-hexylthienyl-2-yl)-1,2,4,5-
tetrazine)] (PCPDTTTz-2,6: 6in), respectively. The differential
scanning calorimetry (DSC) analysis and thermal gravimetric
analysis (TGA) under nitrogen [Fig. 4(a) andꢀ (b)] showed
both polymers started to decompose at ꢁ190 C. It resulted
ꢀ
in a broad exothermal peak at ꢁ260 C in DSC curves and a
stage with ꢁ5% weight loss from 200 to 300 ꢀC in TGA
curves. As discussed in our previous publications,9 this
decomposition is due to the breaking of the Tz ring. We can
see that the decomposition temperature of the in-polymer is
similar to its out-counterpart,9 indicating the hexyl substitu-
ent at the 4-position will not create additional protection to
the Tz ring in the polymers for thermal decomposition.
The Steric Effect on the Chain Conformation
of the Polymers
FIGURE 4 DSC and TGA curves of PCDPTTTz-6:6in and
The hexyl group at the 4- or 3-position of the thiophene ring
in PCPDTTTz-6:6out or PCPDTTTz-6:6in might have a signifi-
cant impact on the conformation of the polymer chains. As
indicated in Scheme 6, the methylene unit of the hexyl group
on the thiopene ring will overlap in the space with the
hydrogen atom on the adjacent CPDT unit to create a high
steric energy for the cis-orientation of the 4-hexylthiophene
unit in PCPDTTTz-6:6out. Therefore, only trans-conformation
PCDPTTTz-2,6:6in.
The Preparation and Characterization of the Polymers
TTz-Br2-6in was copolymerized with 2,6-bis(trimethylstan-nyl)-
4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene and 2,6-
bis(trimethylstannyl)-4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-
b:3,4-b0]dithiophene by Stille coupling reaction to yield two
polymers,9 PCPDTTz-6:6in and PCPDTTz-2,6:6in, respectively,
SCHEME 6 Illustration of the cis- and trans-conformations of a repeat unit of PCPDTTTz-6:6out and PCPDTTTz-6:6in. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 5 Models of the trans- and cis-conformations for the ‘‘out’’ and ‘‘in’’ isomers with minimized steric energy calculated by
MM2 simulation.18
will be predominately taken in this polymer chain, resulting
in a regioregular chain conformation. This feature is the
same as that of regioregular poly(3-hexylthiophene). Reloca-
tion of the hexyl group to the 3-position of the thiophene
ring in PCPDTTTz-6:6in will eliminate this overlap as
indicated in Scheme 6. Although the methylene unit might
create some interaction with the lone pair on the nitrogen
atom in Tz ring, the energy might be much smaller than
overlapping with a hydrogen atom due to the smaller size
of the lone pair. Therefore, this feature will offer the
Photovolatic Properties of the Polymers
The UV–vis spectra of the polymer solutions in chloro-
benzene, toluene, and the polymer films are displayed in
Figure 6. The spectra of PCPDTTz-6:6in show a maximum at
585 nm in chlorobenzene and 576 nm in toluene with a
shoulder around 640 nm. The peak of the film becomes
much broader with an increased intensity of the shoulder,
indicating chain stacking occurred both in solution and solid
state for this polymer. PCPDTTz-2,6:6in displays similar UV–
vis spectra, but the shoulder peak disappears for the
polymer
a low steric energy for both the cis- and
trans-oreintations, and a random distribution of cis- and
trans-conformations along the polymer chain will be taken
for PCPDTTTz-6:6in.
To verify this effect, the molecular mechanical MM2 method
has been used to calculate the steric energy of the cis- and
trans-conformations of a chain segment which includes the
interaction of the methylene unit with the hydrogen atom
in the out-polymer and with the lone pair in the in-poly-
mer.18 Therefore, a simplified molecular model in Figure 5
was used for this calculation, where the hexyl group was
represented by a methyl group to avoid unnecessary inter-
ference of the additional carbons in the side group. The
trans- and cis-conformations of the out- and in-isomer mod-
els (out-trans, out-cis, in-trans, and in-cis) were simulated
to obtain minimized steric energies, which are also listed in
Figure 5.18 The calculation showed that the cis-conforma-
tion of the out-isomer possesses an extra high steric energy
of 66.1 kcal/mol, while the three others have a similar low
energy around 64.2 kcal/mol. This calculation confirmed
that the low-energy trans-conformation will be predomi-
nated in the polymer chain of PCPDTTTz-6:6out, while both
the trans- and cis-conformations will be taken in
PCPDTTTz-6:6in.
FIGURE 6 UV–vis Spectra of the Tz copolymers in chloroben-
zene, in toluene and as a film. The spectra for PCPDTTz-6:6in
were shifted up by 0.7. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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nm, corresponding to an optical band gap of 1.70 and 1.73
eV, respectively. It is interesting that the film spectra of both
samples do not show red shift as usually observed for solar
cell polymers such as the PCPDTTz-6:6out conterpart.9 We
believe that this is due to the low steric energy for both the
cis- and trans-orientations of the thiophene ring in the main
chain. It leads to a highly random conformation and a
broader distribution of effective conjugation length of the
polymer chain in the solid state. This feature results in a
broad light absorption and could be beneficial for improving
the performance of solar cell devices. The shorter maximum
absorption wavelength of these two polymers than PCPDTTz-
6:6out also indicates that the polymer chains are less copla-
nar in solid state due to the random chain conformation.
The electrochemical properties of the polymers were studied
using cyclic voltammetry by coating a polymer thin film on
the platinum working electrode from its toluene solution.
The results are shown in Figure 7(a) and (b). Calculated
from the onset potential of the first oxidation and reduction
waves,19 the HOMO and LUMO energy levels are ꢂ5.45 and
ꢂ3.90 eV for PCPDTTz-6:6in and ꢂ5.50 and ꢂ3.87 eV for
PCPDTTz-2,6:6in. The HOMO energy levels are 0.05ꢁ0.1 eV
higher than its out-counterpart (see Table 1), indicating the
hexyl group at the 3- or 4-positions has very small effect on
the energy level. These values are also about 0.15 eV deeper
than the benzothiadiazole analogs (PCPDTBT).9a,20 Benzo-
thiadiazole is a widely used electron-deficient unit in solar
cell polymers. This property means a higher Voc can be
expected from these polymers.
Polymer solar cell devices were then fabricated from these
two polymers with a general structure of ITO/PEDOT:PSS/
polymer:PC71BM/LiF/Al. The weight ratio of polymer/
PC71BM was 1:2. Typical current density–voltage (J–V)
curves in dark and under illumination of AM 1.5G (100
mW/cm2) are shown in Figure 8. Device based on
PCPDTTTz-2,6:6in gives an open-circuit voltage (Voc) of 0.69
V, a short-circuit current density (Jsc) of 3.36 mA/cm2, and a
modest fill factor (FF) of 29.4%, leading to a PCE of 0.68%.
The device from PCPDTTTz-6:6in has a lower Voc of 0.57 V,
an improved Jsc of 6.23 mA/cm2 and a FF of 34%. The PCE
thus reaches 1.21%. The improvement of PCE arose mostly
from the increased Jsc. This can be explained by the structure
FIGURE 7 Cyclic voltammograms of the polymer films on a Pt
electrode referred to an Ag quasi-reference electrode in 0.1 M
Bu4NPF6/anhydrous CH3CN solution at a scan rate of 50 mV/
min.
solution samples, indicating much less chain stacking of this
polymer in solution. We believe this is attributed to the
increased solubility of the polymer due to the branched side
group. Both the polymer films show much broader absorp-
tion covering 400–700 nm with an onset at 730 and 715
TABLE 1 Characterization Data of the Polymers
a
1%b
c
c
d
d
e
e
f
Mn
Td
kSmax
kFmax
E0re
E00
ELUMO
EHOMO
Eg
ox
Polymer
(kDa)
PDIa
(ꢀC)
(nm)
(nm)
(V)
(V)
(eV)
(eV)
(eV)
PCPDTTTz-6:6in
PCPDTTTz -2,6:6in
PCPDTTTz-6:6out
PCPDTTTz-2,6:6out
a
15.7
48.0
20.0
29.4
2.24
1.82
1.41
1.51
236
231
234
242
583
592
587
562
575
571
600
564
e
ꢂ0.83
ꢂ0.86
ꢂ0.82
ꢂ0.82
0.72
0.77
0.76
0.87
ꢂ3.90
ꢂ3.87
ꢂ3.91
ꢂ3.91
ꢂ5.45
ꢂ5.50
ꢂ5.49
ꢂ5.60
1.55 (1.70)
1.63 (1.73)
1.58 (1.68)
1.69 (1.66)
Average number molecular weight and polydispersity index (GPC vs.
polystyrene standards in chlorobenzene).
Estimated from LUMO ¼ ꢂ(E0re þ 4.73) eV and HOMO ¼ ꢂ(E0ox þ 4.73)
eV.19
b
f
Decomposition temperature of 1% weight loss.
Energy gap: value in the bracket was calculated from the onset of the
thin film UV absorption.
c
Solution (in chlorobenz ene) and film absorption.
d
Onset potentials from CV measurements of thin films in
Bu4NPF6/CH3CN solution versus Ag.
a 0.1M
3382
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
fraction peak for the in-polymers, indicating a much loose p–
p stacking of these two in-polymers in the solid state.
EXPERIMENTAL
Materials
2-Bromo-3-hexylthiophene (97%), anhydrous hydrazine
(95%), anhydrous dimethylformamide (DMF), hydroxylamine
hydrochloride, isoamyl nitrite, NBS, and all the solvents were
purchased from Sigma-Aldrich and used as received. Anhy-
drous tetrahydrofuran (THF) was purified by running
through a Pure Solv-400-4-MD solvent purification system.
Tetrakis(triphenylphosphine)palladium(0) [(PPh3)4Pd(0)] was
prepared based on a reported method and stored in a glove
box.22
Characterization
FIGURE 8 J–V curve of PCPDTTTz-6:6in:PC71BM and PCPDTTTz-
2,6:6in:PC71BM (weight ratio 1:2) solar cell devices under illumi-
nation of AM 1.5G, 100 mW/cm2.
GPC, Waters Breeze HPLC system with 1525 Binary HPLC
Pump, and 2414 Differential Refractometer were used for
measuring the molecular weight and polydispersity index
(PDI). Chlorobenzene was used as eluent, and commercial
polystyrenes were used as standard. 1H and 13C NMR spec-
tra were recorded in CDCl3 for organic molecules and in o-
DCB-d4 at 140 ꢀC or in toluene-d8 at 100 ꢀC for polymers
using a 400-MHz Varian Unity Inova spectrometer. The NMR
solvent peaks were used as references (CDCl3, 7.25 ppm; o-
DCB-d4, 6.92, 7.20 ppm; toluene-d8, 2.09, 7.09 ppm). The
DSC analysis and TGA were performed at a heating rate of
difference between two polymers. PCPDTTTz-6:6in has a
small and linear alkyl side chain, which means better chain
packing and higher charge carrier mobility, and hence, the Jsc
increased from 3.36 to 6.23 mA/cm2. Comparatively,
PCPDTTTz-2,6:6in has relatively weak electronic coupling
with PCBM acceptor in the device due to the slightly bulky
ethylhexyl side groups. It will lead to less recombination of
dissociated charges.21 In addition with the slightly deeper
HOMO level (0.05 eV), this effect leads to a device with a Voc
about 0.12 eV higher than that from PCPDTTTz-6:6in.
ꢀ
10 C/min under a nitrogen atmosphere (50 mL/min) using
a TA Instruments DSC 2920 and TGA 2950, respectively. DSC
was calibrated with the melting transition of indium. Cyclic
voltammetry (CV) measurements were carried out under ar-
gon in a three-electrode cell using 0.1 M Bu4NPF6 in anhy-
drous CH3CN as the supporting electrolyte. The polymer was
coated on the platinum-working electrode. The CV curves
were recorded with reference to an Ag quasi-reference elec-
trode, which was calibrated using a ferrocene/ferrocenium
(Fc/Fcþ) redox couple (5.10 eV below the vacuum level) as
an external standard.19 The half wave potential of the Fc/
Fcþ redox couple was found to be 0.42 V versus the Ag
quasi-reference electrode. Therefore, the HOMO and LUMO
energy levels of the copolymers can be estimated using the
It is a big surprise to us that this type of polymers have
much poor photovoltaic performance then their out-counter-
part.9 This poor performance is partially attributed to the
lower Voc of the in-polymers related with their higher HOMO
level (See Tables 1 and 2). However, we believe the major
reason is related to the random distribution of the cis- and
trans-orientations of the thiophene rings along the polymer
chain as discussed above. This random conformation results
in a loose p–p stacking of the polymer in film, leading to a
poor device performance. This was confirmed by X-ray dif-
fraction study of the polymer films. As reported in Ref. 9c,
the out-polymers showed the [100] peak at 2h of 6.6ꢀ. A
[010] peak at 2h of 27ꢀ was also observed for PCPDTTTz-
6:6in. However, we do not observed any well-developed dif-
empirical equation EHOMO ¼ ꢂ (E0ox þ 4.68) eV and ELUMO
¼
ꢂ(E0re þ 4.68) eV, where Eo0 x and Er0e stand for the onset
potentials for the first oxidation and reduction pair relative
to the Ag quasi-reference electrode, respectively.
TABLE 2 Summary of the Device Fabrication Conditions and Photovoltaic Performance
Tempertureb
(ꢀC)
Jsc
(mA/cm2)
Voc
Polymera
(V)
FF (%)
PCE (%)
PCPDTTTz-6:6in
PCPDTTTz-2,6:6in
PCPDTTTz-6:6out
PCPDTTTz-2,6:6out
a
80
80
85
21
6.23
3.36
12.5
10.2
0.57
0.69
0.75
0.81
34.0
29.4
59.0
38.7
1.21
0.68
5.53
3.20
Devices were fabricated from a solution of polymer/PCBM71 (1/2, w/w) in 1,2-dichlorobenzene containing
3% of 1,8-diiodooctane.
b
Device fabrication temperature.
PREPARATION OF 3,6-BIS(3-HEXYLTHIEN-2-YL)-s-TETRAZINE, DING ET AL.
3383

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
3-Hexylthiophene-2-carboxaldehyde15
netic stir bar and was then sealed with a rubber septum.
The reaction system was connected to a thick-wall rubber
balloon for regulating the pressure inside the flask during
the reaction. Anhydrous hydrazine (95%, 2.0 g, 40.8 mmol)
was added into the solution using a syringe at room temper-
ature. The temperature was then raised to 50 ꢀC. The solu-
tion turned to brown with gas evolved. The sulfur in the so-
lution was completely dissolved within 5 min, and then the
temperature was increased to 68 ꢀC. The solution was
stirred at 68 ꢀC with the pressure regulated by the thick-
wall rubber balloon. Second and third portions of anhydrous
hydrazine (95%, 1.0 g, 20.4 mmol each) were added in 6 h
interval. The reaction was stopped in 20 h by cooling to
room temperature, where the solution turned to slurry. It
was mixed with 15 mL of MeOH and then filtered to collect
the solid, which was rinsed with MeOH and dried in air for
30 min to give a pale white powder.
2-Bromo-3-hexylthiophene (19.8 g, 80.0 mmol) was added
into a 500-mL round-bottomed flask with a stir bar, which
was then purged with argon under vacuum, and was added
with 300 mL of anhydrous THF. The solution was cooled to
ꢂ78 ꢀC using an acetone/dry ice bath. n-Butyl lithium (2.5
M) in hexane (32.0 mL, 80.0 mmol) was added dropwise
into the stirring solution over a period of 30 min and then
ꢀ
warmed to ꢂ40 C and stayed for 30 min. The solution was
cooled down to ꢂ78 ꢀC again, and anhydrous DMF (7.02 g,
96.0 mmol) was added in one shot. The solution was
allowed to warm up to room temperature and stirred over-
night. A total of 150 mL of water was added into the solu-
tion. The organic layer was separated, and the aqueous layer
was extracted with 200 mL of hexanes. The organic phases
were combined and washed with distilled water (100 mL)
twice, dried over anhydrous magnesium sulfate, and rotary
evaporated to remove the solvent. The liquid residue was
subjected to a silica-gel column chromatography (CHCl3/hex-
anes ¼ 3/7, Rf ¼ 0.2) to yield a light brown liquid (14.7 g,
yield 93.6%).
1H NMR (400 MHz, C6D6): d 6.84 (s, 2H), 6.64 (d, J ¼ 5.0 Hz,
2H); 6.55 (d, J ¼ 5.0 Hz, 2H); 2.71 (t, J ¼ 7.8 Hz, 4H); 1.51
(m, 4H); 1.14–1.26 (m, 12H) 0.85 (t, J ¼ 7.2 Hz, 6H).
The resulting powder was then dissolved in 20 mL of CHCl3
and isoamyl nitrite (5.5 g, 47 mmol) was added. The result-
ing solution was stirred at 40 ꢀC for 8 h. The solvent was
removed by a rotary evaporation, and the resulting red solid
was washed with methanol twice, and then subjected to
recrystallization in 2-propanol to yield red needle-like crystal
(2.7 g, 65% yield).
1H NMR (400 MHz, CDCl3): d 10.03 (s, 1H); 7.62 (d, J ¼ 5.0
Hz, 1H); 6.99 (d, J ¼ 5.0 Hz, 1H); 2.66 (t, J ¼ 7.8 Hz, 2H);
1.66 (m, 2H); 1.22–1.40 (m, 6H); 0.88 (m, 3H)).13C NMR
(100 MHz, CDCl3): d182.2, 152.9, 137.6, 134.4, 130.6, 31.5,
31.3, 28.9, 28.4, 22.5, 14.0.
2-Cyano-3-hexylthiophene
1H NMR (400 MHz, CDCl3): d 7.54 (d, J ¼ 5.0 Hz, 2H); 7.09
(d, J ¼ 5.0 Hz, 2H); 3.23 (t, J ¼ 7.6 Hz, 4H); 1.69 (m, 4H);
1.41 (m, 4H); 1.26–1.34 (m, 8H) 0.87 (t, J ¼ 7.2 Hz, 6H). 13C
NMR (100 MHz, CDCl3): d 161.8, 149.3, 132.0, 130.7, 129.3,
31.7, 30.5, 30.3, 29.1, 22.6, 14.1.
A mixture of 3-hexylthiophene-2-carboxaldehyde (9.82 g,
50.0 mmol) and hydroxylamine hydrochloride (5.2 g, 75
mmol) in pyridine/ethanol (60 mL, 1/1, v/v) was stirred
and refluxed overnight. The solution was rotary evaporated
to remove the solvent. The residue was extracted with chlo-
roform (100 mL), and the resulting solution was washed
with distilled water (2 ꢃ 50 mL) and then dried over anhy-
drous magnesium sulfate. After the solvent was removed
using a rotary evaporator. The viscous liquid residue was
then dissolved in acetic anhydride (30 mL) containing 0.3 g
of potassium acetate and was refluxed at 140 ꢀC for 3 h.
Then the solution was added with 50 mL of water and was
extracted with hexanes (50 mL) twice. The combined organic
extracts were washed subsequently with 50 mL of 5% aque-
ous sodium hydroxide solution and 50 mL of water twice,
dried over anhydrous magnesium sulfate, and rotary evapo-
rated to remove the solvent. The yellow liquid residue was
subjected to a silica-gel column chromatography (8% EtOAc/
hexanes, v/v, Rf ¼ 0.4) to yield a light green liquid (8.3 g,
86% yield).
TTz-Br2-6in
To a mixture of 3,6-bis[2-hexylthien-2-yl]-s-tetrazine (0.525
g, 1.266 mmol) and SiO2 (0.2 g) in CH2Cl2 (10 mL) was
added NBS (0.50 g, 2.80 mmol) and was stirred at room
temperature for 2 days in dark. The solution was filtered to
remove SiO2 and was then mixed with 20 mL of distilled
water and then extracted with 30 mL of CH2Cl2. The water
phase was separated and extracted with 30 mL of CH2Cl2
again. The combined organic phases were washed with 20
mL of distilled water twice, dried over anhydrous magne-
sium sulfate, and rotary evaporated to remove the solvent.
The resulting red solid residue was run through a silica-gel
column (20% CHCl3/hexanes, v/v, Rf ¼ 0.5) to yield a red
powder (0.48 g, 66% yield).
1H NMR (400 MHz, CDCl3): d 7.05 (s 2H); 3.16 (t, J ¼ 7.8 Hz,
4H); 1.65 (m, 4H); 1.39 (m, 4H); 1.26–1.33 (m, 8H) 0.88 (t, J
¼ 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3): d 161.0, 150.0,
134.8, 130.7, 119.5, 31.6, 30.5, 30.1, 29.0, 22.5, 14.1.
1H NMR (400 MHz, CDCl3): d 7.46 (d, J ¼ 5.0 Hz, 1H); 6.95
(d, J ¼ 5.0 Hz, 1H); 2.78 (t, J ¼ 7.8 Hz, 2H); 1.64 (m, 2H);
1.24–1.36 (m, 6H); 0.88 (m, 3H).13C NMR (100 MHz, CDCl3):
d 154.8, 131.6, 128.5, 114.3, 105.4, 31.4, 30.1, 29.8, 28.7,
22.5, 14.0.
PCPDTTTz-6:6in
TTz-6in
TTz-Br2-6in (0.2019 g, 0.353 mmol) and 4,4-dihexyl-2,6-bis
(trimethylstannanyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene
(0.2414 g, 0.359 mmol) were added to a mixture of 8 mL of
anhydrous toluene and 0.8 mL of DMF in a 25-mL flask. The
2-Cyano-3-hexylthiophene (3.95 g, 20.4 mmol), sulfur (0.39
g, 12.3 mmol), and ethanol (15 mL) were added into a 50
mL round-bottomed flask, which was equipped with a mag-
3384
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
CONCLUSIONS
system was purged with argon under vacuum and was
moved to a glove box to add (PPh3)4Pd(0) (8 mg, 0.07
mmol). The solution was stirred and refluxed under argon
for 5 h. A total of 5 mL of degassed anhydrous toluene was
added, and the solution was refluxed overnight. One drop of
trimethylstannanylbenzene followed by two drops of bromo-
benzene was added in 3 h intervals under refluxing. The so-
lution was cooled to room temperature in 5 h and poured
into methanol to precipitate the polymer. The resulting poly-
mer was further purified by Soxhlet extraction with acetone
and then hexanes. The polymer was recovered with CHCl3
and dried under vacuum for 16 h to get PCPDTTTz-6,6in as a
shining dark solid (190 mg, yield: 72%), Mn ¼ 15,700 Da,
PDI, 2.24).
Dithienyl-s-tetrazine with a hexyl group at the 3-position of
the thiophene ring (TTz-6in) is interesting for polymer solar
cell applications. However, the preparation of this type of
molecules is challenging. Using the sulfur-assisted Pinner
synthesis under a traditional condition only gives trace of
1
this product. Monitoring the reaction by H NMR shows that
reaction using anhydrous hydrazine at a low temperature
(68 ꢀC) can efficiently promote the conversion and depress
side reactions. We find that the decomposition of hydrazine
competes with this reaction; using excess hydrazine by add-
ing successively in several portions will improve the yield
and reduce side reactions. The yield of this reaction reached
65% by adding anhydrous hydrazine (4 equiv) in three
ꢀ
1H NMR (400 MHz, o-DCB-d4): d 7.36 (2H); {7.29 (2H); 7.14
(2H) in toluene-d8} 3.26 (4H); 2.03 (4H); 1.79 (4H); 1.48
(4H); 1.28–1.42 (8H); 1.11–1.27 (16H); 0.89 (6H); 0.80 (6H).
potions over 12 h at 68 C. From the TTz-6in monomer, two
new copolymers of TTz-6in with two cyclepentadithiophene
units (CPDT-6, and CPDT-2,6) have been prepared. The poly-
mers show broad light absorption, attributing to the wider
distribution of effective conjugating length of polymer chain
in the film due to the low steric energy of both the cis- and
trans-orientations of the thiophene unit in the chain. The
polymers also show a narrow band gap with a deep HOMO
level. Polymer solar cell devices are fabricated and tested
based on the blends of these polymers with PC71BM.
PCPDTTTz-2,6:6in
TTz-Br2-6in (0.1145 g, 0.200 mmol) and 4,4-bis(ethylhexyl)-2,6-
bis(trimethylstannanyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene
(0.1493g, 0.205 mmol) were added into 5 mL of anhydrous
toluene in a 25-mL flask. The system was purged with argon
under vacuum and was moved to a glove box to add
(PPh3)4Pd(0) (6 mg, 0.005 mmol). The solution was refluxed
overnight under argon. One drop of trimethylstannanylbenzene
followed by two drops of bromobenzene was added in
3 h intervals under refluxing. The solution was cooled to room
temperature in 5 h and poured into methanol to precipitate the
polymer, which was further purified by Soxhlet extraction with
acetone and then hexanes. The polymer was recovered by
The authors acknowledge the financial support from NRC-CSIC,
NRC-NSERC-BDC, and NRC-NANO programs.
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Patterned ITO glass substrates were cleaned with detergent
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