A comparison of the properties of two structurally equivalent but regiochemically different mono-alkylated polybithiophenes prepared through AABB-type stille polycondensation

Article abstract of DOI:10.1002/pola.26448

Previous routes to polymers with mono-alkylated bithiophenes have proceeded through polymerization of monoalkyl-2,2′-bithiophene monomers through oxidative or AB-type cross-coupling polymerizations. The resulting polymer regiochemistry affects both the location and orientation of the polymer side-chains. In contrast, AABB-type cross-coupling polymerizations can control the location and in some cases the orientation of the side-chains. To study how this control can impact polymer properties, two poly(monodecyl-2,2′- bithiophene) polymers have been synthesized through Stille AABB-type polycondensations of 2,5-bis(trimethylstannyl)thiophene with different monomers. The alkyl side-chains are located on every other thiophene, but polymer 1 consists of both head-to-tail and head-to-head dyads, whereas polymer 2 is made up of only head-to-head dyads. ¹H NMR, ¹³C NMR, and heteronuclear single quantum correlation spectroscopy are used to confirm and contrast the polymer regiochemistries. The physical properties of the two polymers are analyzed using UV-vis spectroscopy, differential scanning calorimetry, and grazing-incidence X-ray diffraction. Polymer 2 is found to display significantly more aggregation in solution than 1, and it displays different thermal properties. The film properties of polymers 1 and 2, however, are very similar, with nearly identical UV-vis profiles and d-spacing values as determined by grazing incidence X-ray diffraction.

Full text of DOI:10.1002/pola.26448

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A Comparison of the Properties of Two Structurally Equivalent but
Regiochemically Different Mono-Alkylated Polybithiophenes Prepared
Through AABB-Type Stille Polycondensation
Pamela M. Lundin, Gaurav Giri, Zhenan Bao
Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305-5025
Correspondence to: Z. Bao (E-mail: zbao@stanford.edu)
Received 5 September 2012; accepted 21 October 2012; published online 4 December 2012
DOI: 10.1002/pola.26448
ABSTRACT: Previous routes to polymers with mono-alkylated
and heteronuclear single quantum correlation spectroscopy
are used to confirm and contrast the polymer regiochemis-
tries. The physical properties of the two polymers are ana-
lyzed using UV–vis spectroscopy, differential scanning
bithiophenes have proceeded through polymerization of
0
monoalkyl-2,2 -bithiophene monomers through oxidative or
AB-type cross-coupling polymerizations. The resulting polymer
regiochemistry affects both the location and orientation of the
polymer side-chains. In contrast, AABB-type cross-coupling
polymerizations can control the location and in some cases
the orientation of the side-chains. To study how this control
calorimetry, and grazing-incidence X-ray diffraction. Polymer 2
is found to display significantly more aggregation in solution
than 1, and it displays different thermal properties. The film
properties of polymers 1 and 2, however, are very similar,
with nearly identical UV–vis profiles and d-spacing values as
determined by grazing incidence X-ray diffraction. VC 2012
0
can impact polymer properties, two poly(monodecyl-2,2 -
bithiophene) polymers have been synthesized through Stille
AABB-type polycondensations of 2,5-bis(trimethylstannyl)thio-
phene with different monomers. The alkyl side-chains are
located on every other thiophene, but polymer 1 consists of
both head-to-tail and head-to-head dyads, whereas polymer 2
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2013, 51, 908–915
KEYWORDS: conjugated polymers; polythiophenes; structure-
1
13
is made up of only head-to-head dyads. H NMR, C NMR,
property relationships; NMR; X-ray diffraction
14
INTRODUCTION Polythiophenes have found use as semicon-
able. Most of these polymers have been prepared via elec-
trochemical or chemical oxidative polymerization of mono-
ductor materials in a number of device applications, includ-
1
–6
0
9–11,13
ing photovoltaics and transistors.
In particular, poly-
alkyl-2,2 -bithiophene,
polycondensation of 5-metallo-5 -bromo-monoalkyl-2,2 -
bithiophene (Scheme 1).
or via AB-type cross-coupling
0
0
(
3-alkylthiophenes) (P3ATs) have been especially well-stud-
1
1–13
ied, with poly(3-hexylthiophene) (P3HT) in particular being
considered a benchmark polymer that is often used to evalu-
ate new polymers and novel device architectures. In thin-
film transistor devices fabricated from regioregular P3HT
A polymer produced by these
methods may contain both head-to-tail and head-to-head
dyads, the ratio of which depends on the polymerization con-
ditions. The dyad will affect both the location and orientation
of the alkyl side-chain. Successive head-to-tail dyads result in
alkylation on every other thiophene with all alkyl chains
pointing in the same direction. When head-to-head dyads are
created, the alkyl side-chains will be located on adjacent thi-
ophene units and will point in different directions. These
head-to-head dyads can result in torsion along the polymer
backbone and impact the polymer properties.
2
ꢀ1 ꢀ1
(
rr-P3HT), mobilities of up to 0.2 cm
V
s
have been
7
achieved. Bulk-heterojunction solar cells comprised of
rr-P3HT and [6,6]-phenyl-C61-butyric acid methyl ester have
produced power conversion efficiencies of over 4%.
8
Another class of polythiophenes that have been a subject
of interest is mono-alkylated poly(bithiophenes). These
0
polymers can be known as poly(3-alkyl-2,2 -bithiophenes)
(
P3ABTs) or poly(4-alkyl-2,2-bithiophenes) (P4ABTs),
Recently, we were interested in the synthesis of a polythio-
phene which could be modified with a dye molecule. Poly-
(3-(10-bromodecyl)-2,2 -bithiophene) was chosen as the
precursor because the primary alkyl bromide could easily be
modified with the dye by a substitution reaction, and the
location of the alkyl chain on every other thiophene would
depending on the starting monomer. Both types differ from
P3ATs in that the alkyl side-chain is incorporated onto every
other rather than all thiophene units, and they have been
explored as P3AT alternatives,
in which decreasing the number of alkyl side-chains is desir-
0
9
–13
especially for applications
V
C
2012 Wiley Periodicals, Inc.
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SCHEME 1 Synthetic approaches to P3ABT and P4ABT, and the resulting head-to-tail and head-to-head dyads.
better accommodate the bulky dye molecule. We chose to
synthesize this P3ABT polymer via a Stille polycondensation
of the 3-alkyl-2,5-dibromothiophene with 2,5-bis(trimethyl-
stannyl)thiophene rather than through the methods previ-
ously discussed because this AABB-type polymerization
method limits the location of the alkyl side-chain to being on
every other thiophene unit exclusively. The side-chain orien-
tation, however, is still not controlled, as both head-to-tail
and head-to-head dyads are possible with this synthetic
route (Fig. 1). Although this route has been used previously
Aldrich and recrystallized three times from hot ethanol prior
to use. Anhydrous chlorobenzene solvent was purchased in a
V
R
SureSeal bottle from Aldrich and used as received. Tris-
(dibenzylideneacetone)dipalladium(0) (Pd dba , Aldrich),
2
3
tri(ortho-tolyl)phosphine (TCI), and all other reagents were
purchased from commercial sources and used as received.
Polymerizations were performed using a CEM Discover
microwave reactor. Filtrations of precipitated polymers were
performed with Supelco Nylon 66 membranes (pore size
0.45 lm).
1
5,16
to synthesize P3ABTs,
to the best of our knowledge the
1
13
H and C NMR spectra were recorded using Varian Inova
resulting regiochemistry and its impact on polymer proper-
ties have not been studied. This article therefore attempts to
learn more about the impacts different regiochemical orien-
tations can have on polymer properties by comparing two
mono-alkylated bithiophenes that differ only in the ratio of
head-to-tail versus head-to-head dyads along the polymer
chain. Polymer 1 is prepared by Stille polycondensation of
the asymmetric monomer 2,5-dibromo-3-decylthiophene
with 2,5-bis(trimethylstannyl)thiophene; the resulting poly-
mer consists of a mixture of head-to-tail and head-to-head
dyads (Scheme 2). Polymer 2 is synthesized under similar
3
2
00 or 500 MHz instruments in deuterated chloroform at
93 K unless otherwise noted. Heteronuclear single quantum
correlation (HSQC) spectra were recorded using a Varian
0
0
conditions with the exception that the symmetric 5,5 -
0
0
0
0
00
dibromo-3,3 -didecyl-2,2 :5 ,2 -terthiophene monomer is
used with the 2,5-bis(trimethylstannyl)thiophene monomer
(
Scheme 3). Because both its monomers are symmetrical, the
backbone of polymer 2 is comprised entirely of head-to-head
dyads.
EXPERIMENTAL
General Information
1
7
2
,5-Dibromo-3-decylthiophene
and 2-bromo-3-decylthio-
FIGURE 1 Head-to-tail and head-to-head dyads in poly(mo-
noalkylatedbithiophene) polymers from AABB-type Stille
polycondensation.
1
8
phene were prepared according to literature procedures.
2,5-Bis(trimethylstannyl)thiophene was purchased from
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SCHEME 2 Synthesis of polymer 1.
Inova 600 MHz instrument in deuterated tetrahydrofuran at
93 K. Size exclusion chromatography was performed in THF
Monomer Synthesis
3,3 -Didecyl-2,2 :5 ,2 -Terthiophene (3)
0
0
0
0
00
2
solution and the molecular weights were calculated using a
calibration curve based on polystyrene standards. UV–vis
spectra were obtained using a Carey 6000i spectrometer.
Differential scanning calorimetry (DSC) measurements were
This compound was prepared by a literature procedure with
20
modifications. To a 35-mL microwave reactor vial was
added 2-bromo-3-decylthiophene (1.36 g, 4.48 mmol, 2.50
equiv.), 2,5-bis(trimethylstannyl)thiophene (735 mg, 1.79
ꢁ
performed on a TA Instruments Q2000 from 40 to 250 C at
2 3
mmol), Pd dba (33.0 mg, 0.0360 mmol, 4.02 mol% palla-
ꢁ
ꢀ1
a
scanning rate of 10
C
min
under
a
nitrogen
dium), and tri(ortho-tolyl)phosphine (43.7 mg, 0.144 mmol,
8.04 mol%), and toluene (15 mL). Argon was bubbled
through the solution for 20 min to degas it, and then the
vial was sealed with a snap cap and placed in the microwave
atmosphere.
Grazing incidence X-ray diffraction (GIXD) was performed at
the Stanford Synchrotron Radiation Lightsource on beamline
ꢁ
reactor for 1 h, cycling between 120 and 170 C. The reac-
11-3. A photon energy of 12.73 keV was used, and the scat-
tion was quenched with water (10 mL) and extracted with
dichloromethane (3 ꢂ 15 mL). The combined organic
extracts were washed with brine, dried over sodium sulfate,
and concentrated. Flash chromatography on silica with 1%
dichloromethane in hexanes afforded 838 mg of the product
tering intensity was recorded on a 2D image plate (MAR-
3
45) with a pixel size of 150 lm (2300 ꢂ 2300 pixels),
located at a distance of 400 mm from the sample center. The
distance between the sample and detector was calibrated
using a LaB6 polycrystalline standard. The incidence angle
1
as a yellow oil. Yield: 89%. Purity ( H NMR): >95%.
ꢁ
was chosen to be 0.10–0.14 to maximize signal-to-noise
2
1
ratio. The beam size was 50 ꢂ 150 lm , which resulted in a
H NMR (300 MHz, CDCl , d): 7.17 (d, J ¼ 5.2 Hz, 2H), 7.05
3
beam exposure on the sample 150 lm wide over the entire
length of the 25 mm sample. Typical exposure time was 300
s. The data was distortion corrected (theta-dependent image
distortion introduced by planar detector surface) before per-
forming quantitative analysis on the images. Numerical inte-
gration of the diffraction peak areas was performed with the
(s, 2H), 6.94 (d, J ¼ 5.2 Hz, 2H), 2.78 (t, J ¼ 7.8 Hz, 4H),
1.65 (m, 4H), 1.43–1.22 (m, 28 H), 0.87 (t, J ¼ 6.6 Hz, 6H).
00
00
0
0
00
5
,5 -Dibromo-3,3 -didecyl-2,2 :5 ,2 -Terthiophene (4)
This compound was synthesized as reported in a literature
20
00 0 0 00
from 3,3 -didecyl-2,2 :5 ,2 -terthiophene (817
procedure
mg, 1.55 mmol), NBS (617 mg, 3.47 mmol, 2.24 equiv.), and
1
9
software WxDiff.
SCHEME 3 Synthesis of polymer 2.
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DMF (8 mL), and was purified by flash chromatography on
silica with 1% dichloromethane in hexanes to afford 173 mg
of the product as a yellow oil. Yield: 16%. Purity ( H NMR):
times in 10-mL portions of chlorobenzene. The chloroben-
zene fractions were combined and concentrated, and the
polymer was precipitated in acetone and dried under vac-
uum to give 8.2 mg of the product as a shiny gold solid.
Yield: 14%. The polymer had a bimodal molecular weight
1
95%.
1
H NMR (300 MHz, CDCl , d): 6.98 (s, 2H), 6.89 (s, 2H), 2.69
3
distribution: Mw ¼ 175,000 and 8840 g/mol, M ¼ 104,000
n
(
0
t, J ¼ 7.8 Hz, 4H), 1.63–1.56 (m, 4H), 1.37–1.20 (m, 28 H),
and 5500 g/mol, and PDI ¼ 1.68 and 1.61.
.87 (t, J ¼ 6.7 Hz).
1
ꢁ
H NMR (500 MHz, chlorobenzene-d , 70 C, d): 7.07 (br,
5
General Procedure for Polymer Synthesis. To a 10-mL
microwave reactor flask with a stir bar, equimolar amounts
of 2,5-dibromo-3-alkylthiophene and 2,5-bis(trimethylstan-
nyl)thiophene were added. Chlorobenzene was added to that
the concentration of each monomer was 0.10 M, and the so-
lution was degassed by bubbling argon through the solution
for 20 min. During this time period, Pd dba (4.0 mol% Pd)
6
1
H), 2.82 (br, 4H), 1.73 (br m, 4H, 1.43 (br m, 4H), 1.41–
24 2
.22 (br m, 28 H), 0.89 (br m, 6H); Anal. calcd for C18H S :
C 71.00, H 7.94; found C 64.89, H 7.73.
Preparation of Polymer Films for UV–vis Spectroscopy
Polymer solutions with concentrations of 2 mg/mL were pre-
pared and dropcast onto glass slides. The films were allowed
to dry overnight in a covered petri dish prior to analysis.
2
3
and tri(ortho-tolyl)phosphine (8.0 mol%) were added. At the
end of the 20 min, the argon stream was removed, and the
vessel was capped with a snap cap. The reaction was placed
Preparation of Polymer Films for GIXD
Polymer solutions with concentrations of 10 mg/mL were
ꢁ
in a microwave reactor and heated to 200 C under 300 W
ꢁ
prepared, heated at 80 C for 5 min, and filtered. The filtered
for 30 min. The solution was then precipitated in 500 mL
methanol and filtered.
solutions were spin-cast onto native oxide wafters at a spin
rate of 3000 rpm for 40 s. The films were then annealed at
ꢁ
0
80 C for 30 min.
Poly(3-(10-decyl)-2,2 -bithiophene) (1)
2
,5-Dibromo-3-decylthiophene (76.2 mg, 0.199 mmol), 2,5-
bis(trimethylstannyl)thiophene (81.7 mg, 0.199 mmol),
Pd dba (3.7 mg, 0.0040 mmol, 4.1 mol% Pd), tri(ortho-tol-
RESULTS AND DISCUSSION
0
Poly(3-decyl-2,2 -bithiophene) 1 was synthesized via Stille
2
3
condensation of 2,5-dibromo-3-decylthiophene with 2,5-bis(-
trimethylstannyl)thiophene using a catalyst formed in situ
yl)phosphine (5.0 mg, 0.016 mmol, 8.2 mol%), and chloro-
benzene (2.0 mL) were used according to the general poly-
merization procedure. After filtration, the polymer was re-
dissolved in 80 mL chloroform and refluxed with 80 mL
from Pd dba and tri(ortho-tolyl)phosphine in chlorobenzene
2
3
21
under microwave conditions (Scheme 2). After work-up
and washing via Soxhlet extraction with acetone and hex-
anes, it was extracted into chloroform, and was found to be
soluble in toluene, tetrahydrofuran, and chlorobenzene. The
NH OH for 3 h. The layers were separated and the chloro-
4
form layer was stirred over 300 mg ethylenediaminetetraa-
cetate for 3 h, followed by addition of 80 mL deionized
water and stirring for 1 h. The layers were then separated
and the chloroform layer was concentrated and precipitated
in 500 mL methanol. Soxhlet extraction was performed on
the precipitated polymer with acetone, hexanes and chloro-
form until the extracts were colorless. The chloroform
extracts were concentrated, precipitated in 500 mL metha-
nol, filtered, and dried under vacuum to furnish the polymer
product. A mass of 18 mg of the product was obtained as a
shiny gold solid from the chloroform fraction of the Soxhlet
extraction. Yield: 30%. The polymer had a bimodal molecular
0
0
0
0
00 00 000
all-head-to-head polymer poly(3,3 -didecyl-2,2 :5 ,2 :5 ,2 -
quaterthiophene) (2) was synthesized from the symmetric
monomer 4 (Scheme 3). Polymer 2 was much less soluble
than 1, which precluded its work-up by standard procedures;
instead a processible sample was obtained by refluxing the
polymer solids in chlorobenzene, followed by precipitation in
acetone. Only a small amount of processible polymer was
obtained, preventing feasible further purification. Upon heat-
ing, small amounts were reasonably soluble in toluene, tetra-
hydrofuran, and chlorobenzene, which allowed the analyses
detailed in this article; however, polymer 2 may still contain
impurities that influence the observed properties detailed in
this manuscript. The molecular weights as measured by GPC
using THF as the eluent showed similar behavior. Both poly-
mers showed quite high (ꢃ200,000 g/mol) bimodal weight
distributions, which could be due to aggregation behavior, to
be described later. GPC analysis was also attempted with
chloroform and chlorobenzene solvents, respectively, but the
apparent aggregation was even worse. Although we assume
that both polymers have similar molecular weights based on
these GPC results, the aggregation behavior may mask differ-
ences in molecular weights which in turn would manifest in
different physical properties.
weight distribution: Mw ¼ 215,000 and 7460 g/mol, Mn
¼
9
3,400 and 5040 g/mol, and PDI ¼ 2.30 and 1.48.
1
ꢁ
H NMR (500 MHz, chlorobenzene-d , 70 C, d): 7.06 (br,
5
3H), 2.80 (br m, 2H), 1.71 (br m, 2H,), 1.43 (br m, 2H), 1.26
(
br m, 14 H), 0.88 (br m, 3H); Anal. calcd for C H S : C
1
8 24 2
71.00, H 7.94; found C 69.98, H 8.16.
0
0
0
0
00 00 000
Poly(3,3 -didecyl-2,2 :5 ,2 :5 ,2 -quaterthiophene) (2)
0
0
00
0
0
00
5
0
0
,5 -dibromo-3,3 -didecyl-2,2 :5 ,2 -Terthiophene (65.0 mg,
.0947 mmol), 2,5-bis(trimethylstannyl)thiophene (38.3 mg,
.0947 mmol), Pd dba (1.8 mg, 0.0020 mmol, 4.1 mol%
2
3
Pd), tri(ortho-tolyl)phosphine (2.4 mg, 0.0079 mmol, 8.3
mol%), and chlorobenzene (0.95 mL) were used according
to the general polymerization procedure. Because of insolu-
bility, after filtration, the solid polymer was refluxed six
NMR spectroscopy was used to analyze the regiochemical
make-up of polymers 1 and 2. Due to solubility reasons,
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1
3
FIGURE 3 C NMR spectra of the alkyl region of polymers 1
and 2.
and 2 was too low to obtain useful data, even after >11,000
scans with an extended T1 relaxation time of 2 s. However,
the alkyl region of the spectra of polymers 1 and 2 did gen-
1
erate adequate signal for analysis (Fig. 3). As in the H NMR
13
spectra, the C NMR spectrum of 1 is more complicated
1
FIGURE 2 Expanded aromatic and a-CH
2
regions of the
H
ꢁ
NMR spectra of polymers 1 and 2 chlorobenzene-d
5
at 70 C.
chlorobenzene-d5 and THF-d8 were used as the NMR sol-
vents, and the chlorobenzene-d samples were heated to 80
5
ꢁ
C to improve resolution. In chlorobenzene-d , the aromatic
5
1
region of the H NMR spectrum is somewhat obscured by
signals corresponding to protio-solvent, but it is still obvious
that polymer 1 shows a more complicated signal pattern in
this region than polymer 2, which implies that polymer 1
has significant amounts of different regiochemical dyads
along its polymer chain, as is expected (Fig. 2). In the alkyl
region corresponding to the a-CH resonances, the chemical
2
shift of 1 and 2 are virtually identical at d 2.81 and d 2.82,
respectively. Therefore, any difference between two alkyl
side-chains pointing toward each other versus away from
each other is indistinguishable in this polymer scaffold. The
1
H NMR spectra of polymers 1 and 2 in THF-d8 confirmed
these observations, although due to the lower acquisition
temperature, the peaks were not as well resolved in these
experiments.
1
3
Polymers 1 and 2 were also examined by C NMR spectros-
copy. Because of the number of potentially overlapping
resonances in the aryl region of chlorobenzene-d , THF-d8
5
was the solvent of choice in these experiments. Unfortu-
nately, the signal to noise in the aromatic region of both 1
FIGURE 4 HSQC spectra of polymers 1 and 2.
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FIGURE 5 Four different environments a central thiophene could occupy in a poly(monoalkylbithiophene).
than that of 2, indicative of more diverse regiochemistry
along the polymer backbone. To attempt to assign the
resonances in these spectra and obtain a more precise under-
standing of relative degrees of head-to-tail and head-to-head
dyads in polymer 1, further analysis of this region using
HSQC experiments was performed (Fig. 4). It was found that
coupling at the 5-position versus the 2-postion of the 2,5-
dibromo-3-decylthiophene monomer, as determined by react-
ing this monomer with a series of substoichiometric amounts
of 2-trimethylstannylthiophene (Scheme 4). Such a preference
would influence the ratio of head-to-tail to head-to-head cou-
plings that occur in the polymerization.
1
3
the C NMR resonances corresponding to the a-CH position
2
Having established that the regiochemical structures of poly-
mers 1 and 2 are indeed different, it was desirable to under-
stand how these differences manifest themselves as physical
occur within a region populated by other alkyl signals, thus
precluding precise assignment of the head-to-tail and head-
to-head resonances. However, this region of polymer 1 seems
to consist of several overlapping peaks leading to an elon-
gated cross-peak pattern whereas the same area of spectrum
of polymer 2 contains a simpler, more symmetrical pattern.
ꢀ3
properties. In THF solutions with concentrations of 4 ꢂ 10
mg/mL at room temperature, the absorbance maxima of
polymers 1 and 2 are 544 and 538 nm, respectively, by UV–
vis spectroscopy [Fig. 6(a)]. When these solutions are heated
Analysis of the local maxima in the a-CH region of the HSQC
2
ꢁ
for 5 min at 50 C and then cooled back to room temperature
spectrum of polymer 1 reveals four irresolvable cross-peaks
for polymer 1 in comparison to the single cross-peak in the
spectrum of polymer 2. It is plausible that these four
resonances correspond to four different environments a 3-
alkyl thiophene could occupy in a polymer synthesized in an
AABB fashion when one looks beyond dyads to the potential
triads that would result (Fig. 5). Although the relative ratios
between these four peaks cannot be unequivocally deter-
mined due to the significant degree of overlap, the relative
heights of the two largest peaks is on the order of 2:1. This
ratio is consistent with the degree of preference for initial
and analyzed by UV–vis spectroscopy, the absorbance maxi-
mum of 1 remains similar at 539 nm, whereas that of 2
shows a hypsochromic shift to 467 nm [Fig. 6(b)]. The tem-
perature-dependent nature of the absorption maxima of 2
indicates that it experiences significant aggregation in solu-
tion, which is broken apart by heating. This comparatively
larger degree of aggregation may also explain why the opti-
cal density of 2 is lower than that of 1. UV–vis spectroscopy
was also used to analyze these polymers in the film state. As
films, the absorbance maxima of 1 and 2 are 542 and 546
nm respectively, and the profiles are quite similar [Fig. 6(c)].
SCHEME 4 Ratio of reaction at the 5-position versus the 2-position of 2,5-dibromo-3-decylthiophene with substoichiometric
amounts of 2-trimethylstannylthiophene.
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FIGURE 7 DSC curves of polymers 1 (black) and 2 (red).
ꢁ
ing, polymer 1 crystallized at 217 C, whereas polymer 2
ꢁ
showed two such transitions at 235 and 225 C. The differ-
ent behavior of these polymers upon cooling indicates that
the regiochemical structure of the polymers does impact the
thermal properties. During heating, polymers 1 and 2 show
ꢁ
endothermic transitions at 234 and 242 C, respectively, that
perhaps correspond to the melt temperature, although they
are not very pronounced (Fig. 7). Although the temperature
difference in this transition may be attributable to the regio-
chemical differences of the two polymers, the possibility that
different molecular weights or purity levels may also influ-
ence this result cannot be ruled out.
GIXD was used to further obtain information regarding the
film structure of polymers 1 and 2 (Fig. 8). The p–p spacing
distance, which has been shown to be side chain independ-
5
ent in P3ATs, is ꢃ3.81 Å for both polymers, although the
diffraction intensity of this in-lane peak is low. The low
intensity of the p–p spacing distance diffraction peak, while
still leading to high charge carrier mobilities, has been previ-
ously observed for the poly(3,4-dialkylterthiophene) (P34AT)
FIGURE 6 Normalized absorbance spectra of polymers
1
(
black) and 2 (red) in (a) THF at room temperature, (b) THF af-
22
ꢁ
systems. This spacing is slightly larger than those typically
observed for P3ATs (ꢃ3.7 Å), which is likely due to the more
disordered nature of the thin films and broadening of the
ter heating to 50 C for 5 min and then cooling back to room
temperature, and (c) film.
23–24
diffraction profile.
Orientation of the p–p stacking has
To understand the effect the differing regiochemical struc-
tures have on the thermal properties of polymers 1 and 2,
DSC was used. The polymers were heated beyond the melt
been shown to be very dependent on the processing condi-
25
tions chosen, and it is therefore difficult to draw any con-
clusion regarding the degree of crystallinity of the polymer
films. The out-of-plane lamellar packing is found to have a
ꢁ
temperature, cooled to 40 C, and heated again. Upon cool-
FIGURE 8 GIXD diffraction patterns of films cast from polymers 1 and 2.
914
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ARTICLE
spacing of ꢃ25.2 Å for both polymers 1 and 2. The film
properties of both polymers therefore seem to be quite simi-
lar, which is in agreement with the similarities found in film
UV–vis spectroscopy experiments discussed above. Moreover,
these GIXD findings are interesting in how close the values
obtained for these polymers are to those obtained in meas-
4 C. H. Woo, B. C. Thompson, B. J. Kim, M. F. Toney, J. M. J.
Fr eꢀ chet, J. Am. Chem. Soc. 2008, 130, 16324–16329.
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jugated Polymers; M. Leclerc, J.-F. Morin, Eds.; Wiley-VCH Ver-
lag GmbH & Co. KGaA: Weinheim, 2010; Chapter 3, pp 91–145.
6
I. Osaka, R. D. McCullough, In Conjugated Polymer Synthe-
5
,26
sis; Y. Chujo, Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Wein-
heim, 2010; Chapter 3, pp 59–90.
urements on P3AT systems.
7
2
G. Wang, J. Swensen, D. Moses, A. J. Heeger, J. Appl. Phys.
003, 93, 6137–6141.
CONCLUSIONS
Two new polymers 1 and 2, belonging to the P3ABT family
structure, have been synthesized through AABB-type
polymerizations, which is in contrast to previously reported
polymers of this type. As a consequence, the regiochemical
structure of polymers 1 and 2 differ from those P3ABT poly-
mers previously reported. Using NMR spectroscopy, polymer
8
G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009,
21, 1323–1338.
9
H. Masuda, D. K. Asano, K. Kaeriyama, Synth. Met. 2001,
119, 167–168.
10 P. Miao, C.-Y. Zhang, H. S. O. Chan, S.-C. Ng, Macromol.
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1
is found to consist of a mixture of head-to-tail and head-
11 J.-P. L e` re-Porte, J. J. E. Moreau, C. Torreilles, Eur. J. Org.
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to-head dyads in approximately a 2:1 ratio; conversely, poly-
mer 2 consists solely of head-to-head-couplings. Analysis of
the physical properties of these polymers confirms that these
differences in regiochemical structure manifest themselves in
different behavior. Polymer 2 is much less processible than
polymer 1, and by UV–vis spectroscopy it displays much
more aggregation. Furthermore, DSC shows that upon cool-
ing polymer 2 undergoes two different exothermic transi-
tions, possibly corresponding to crystallization, in contrast to
the single transition displayed by 1. The films cast from
polymers 1 and 2 were both crystalline. The thin film UV–vis
absorption profiles of both are comparable, with 2 showing
a slight red-shift as compared to 1. From GIXD analysis, it
was revealed that both polymers have p–p stacking distances
of ꢃ3.81 Å and interlayer lamellar spacing distances of
ꢃ25.2 Å.
12 D. R. Greve, J. J. Apperloo, R. A. J. Janssen, Eur. J. Org.
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Janssen, Colloids Surf. A 2002, 198–200, 323–330.
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8 Naraso, F. Wudl, Macromolecules 2008, 41, 3169–3174.
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ACKNOWLEDGMENTS
brand, R. C. Chiechi, J. C. Hummelen, J. Mater. Chem. 2011,
2
1, 1582–1592.
Funding for this research was provided by the DARPA QuBE
program (grant N66001-10-1-4060). G.G. was supported by the
National Science Foundation Division of Materials Research
Solid State Chemistry (DMR-0705687-002). The authors dedi-
cate this paper to the memory of our colleague Seong-Ho Choi.
2
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