Conjugated polymers with repeated sequences of group 16 heterocycles synthesized through catalyst-transfer polycondensation

Article abstract of DOI:10.1021/jacs.6b01916

Periodic π-conjugated polymers of the group 16 heterocycles (furan, thiophene, and selenophene) were synthesized with controlled chain lengths and relatively low dispersities using catalyst-transfer polycondensation. The optical gap and redox potentials of these copolymers were fine-tuned by altering the heterocycle sequence, and atomic force microscopy revealed nanofibrillar morphologies for all the materials. Grazing incidence wide-angle X-ray scattering of the thiophene-selenophene copolymers indicated that the π-stacking distance increased with incorporation of the larger heteroatom (from ～3.7-4.0 ?), while the lamellar spacing decreased (from ～15.8-15.2 ?). The study also revealed that periodic sequences allow electronic properties to be tuned while retaining nanofibrillar morphologies similar to those observed for poly(3-hexylthiophene).

Full text of DOI:10.1021/jacs.6b01916

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Article
Conjugated Polymers with Repeated Sequences of Group 16
Heterocycles Synthesized through Catalyst-Transfer Polycondensation
Chia-Hua Tsai, Andria Fortney, Yunyan Qiu, Roberto R Gil,
David Yaron, Tomasz Kowalewski, and Kevin J.T. Noonan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01916 • Publication Date (Web): 22 Apr 2016
Downloaded from http://pubs.acs.org on April 23, 2016
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Journal of the American Chemical Society
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Conjugated Polymers with Repeated Sequences of Group 16
Heterocycles Synthesized through Catalyst-Transfer Polycon-
densation
ChiaꢀHua Tsai,‡ Andria Fortney,‡ Yunyan Qiu, Roberto R. Gil, David Yaron,* Tomasz Kowalewski,*
Kevin J. T. Noonan*
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Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania, 15213, United States
ABSTRACT: Periodic πꢀconjugated polymers of the group 16 heterocycles (furan, thiophene and selenophene) were synthesized
with controlled chain lengths and relatively low dispersities using catalystꢀtransfer polycondensation. The optical gap and redox
potentials of these copolymers were fineꢀtuned by altering the heterocycle sequence, and atomic force microscopy revealed nanoꢀ
fibrillar morphologies for all the materials. Grazing Incidence Wide Angle XꢀRay Scattering of the thiopheneꢀselenophene copolyꢀ
mers indicated that the πꢀstacking distance increased with incorporation of the larger heteroatom (from ~3.7 – 4.0 Å) while the laꢀ
mellar spacing decreased (from ~15.8 – 15.2 Å). The study also revealed that periodic sequences allow electronic properties to be
tuned while retaining nanofibrillar morphologies similar to those observed for poly(3ꢀhexylthiophene).
Introduction
mers of alkylthiophene and alkylselenophene have shown that
electronic properties vary smoothly with composition,5 they
also reveal a need to explore the impact of randomness and
sequence on solidꢀstate order.
Precise monomer insertion along a polymer backbone alꢀ
lows biological macromolecules to attain a wide range of
structural and functional properties. This continues to inspire
researchers to synthesize macromolecules while controlling
monomer sequence as a means to manipulate the properties of
manꢀmade materials.1 Their efforts are reflected by recent
numerous reports demonstrating increasing levels of control
over monomer insertion in stepꢀgrowth and chain growth
polymerization processes.1 Results of those studies add signifꢀ
icantly to understanding the impact of sequence on material
properties of polydisperse synthetic polymers.
To further elucidate these aspects, in this report we focused
on conjugated copolymers with wellꢀdefined repeating seꢀ
quences comprised of 3ꢀhexylthiophene, 3ꢀhexylselenophene
and 3ꢀhexylfuran units. To assure the sequence is wellꢀ
defined, the copolymers were synthesized by linking short
oligomers through catalyst transfer polycondensation (CTP).6
Polymer chains assembled through polycondensation typically
grow through a stepꢀgrowth mechanism, which yields broad
molecular weight distributions. CTP on the other hand, proꢀ
ceeds in a chain growth process, yielding polymers with narꢀ
row molecular weight distributions and controlled molecular
weights.6 We demonstrate that, similar to previous reports on
random copolymers,5 the optical bandgaps and redox potenꢀ
tials of the wellꢀdefined periodic copolymers vary with comꢀ
position in a predictable, linear manner. In addition, the periꢀ
odic sequences exhibit wellꢀdefined morphologies and the
packing patterns mimic those of regioregular P3HT, one of the
most wellꢀstudied semiconducting polymers.
Sequence control may be especially effective in tuning the
electronic properties of conjugated polymers because their
electronic structure is highly delocalized.2 This is manifested
in the widely recognized ability to create low band gap polyꢀ
mers by linking electron rich and electron poor building
blocks to form donorꢀacceptor copolymers.3 However, when
these building blocks have substantially different shapes, sizes
and polarities, a sequence designed to target a particular optiꢀ
cal bandgap or redox potential may lead to unintended conseꢀ
quences on chain packing and material morphology. Such
unintended consequences may be minimized through the use
of units with similar structures, such as the group 16 heterocyꢀ
cles: thiophene, selenophene and furan. It is reasonable to
expect that the similar shapes of these building blocks will
make it possible to vary the sequence without substantially
impacting morphology. However, reports of distinct nanoscale
phase separation in block copolymers of 3ꢀalkylthiophene and
3ꢀalkylselenophene,4 indicate simple replacement of a heteroaꢀ
tom in the heterocycle may have a significant impact on solidꢀ
state packing. Furthermore, while studies of random copolyꢀ
Results and Discussion
The utility of CTP has already been demonstrated with the
design of more complex architectures bearing πꢀconjugated
segments including: graft,7 block,8 gradient,4d,9 star,10 and doꢀ
norꢀacceptor polymers.11 Alternating copolymers have been
explored using Kumada CTP,12 typically with thiophene diꢀ
mers bearing different sideꢀchain units, but extensions beyond
dimers are scarce.13 In this study, we initially focused on a
bithiophene monomer (1), to address potential concerns of
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competitive chainꢀtransfer and termination in CTP of thioꢀ
transfer and termination observed by Kiriy and coworkers in
phene dimers and trimers.13
their experiments with thiophene dimers and trimers may have
been a consequence of the catalyst (PhNi(PPh3)2Br) used to
initiate polymerization.13 They postulated that bidendate phosꢀ
phine nickel catalysts would likely result in improved control,
which is consistent with our observations.
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Monomer 1 was synthesized and activated using 2,2,6,6ꢀ
tetramethylpiperidinylmagnesium chloride lithium chloride
complex (TMPMgCl·LiCl) to selectively deprotonate the 5ꢀ
position,14 and polymerizations were initiated using
Ni(dppp)Cl2 (Scheme 1). After 15 min, all reaction mixtures
were quenched and precipitated with 6M HCl/MeOH solution,
and washed with methanol only. We obtained relatively good
yields of the final polymer in most cases due to the formation
of a single regioisomer as the monomer (Table 1). Gel perꢀ
meation chromatography (GPC) of the crude polymer samples
confirmed excellent control over the molecular weights with
relatively low dispersities (Table 1). Although we do observe
variation in the molecular weight distribution (in the range of
1.10 – 1.25) when experiments from Table 1 are repeated, the
targeted molecular weights are highly repeatable. The variance
in dispersity could be related to catalyst initiation, as McNeil
and coworkers have reported large changes in dispersity upon
changing the rate of precatalyst initiation.15
The aromatic signals of the T-Se polymer main chain apꢀ
pear at 6.92 ppm (HA, Figure 1) and 7.18 ppm (HB, Figure 1),
corresponding to the thiophene and selenophene ring, respecꢀ
tively. Analysis of this copolymer using 2D NMR spectroscoꢀ
py led to the assignment of a tailꢀtoꢀtail (TT) regiodefect at
7.00 ppm (HC, Figure 1), which results from precatalyst initiaꢀ
tion and is similar to the TT defect reported for P3HT.16 The
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Br terminated selenophene chain end produced one H NMR
signal for the ring proton (HD, Figure 1) and was assigned with
the aid of the starting material. The H terminated selenophene
end group (HE, Figure 1) appears at 7.52 ppm and is correlated
with a signal at 7.17 ppm (HF, Figure 1). HF is visible in the
COSY NMR spectrum as it overlaps with the major aromatic
resonance of the selenophene ring in the polymer main chain
(Figure S17, Supporting Information). Integration of the sigꢀ
nals for the end groups did not afford the expected 1:1 ratio.
MALDIꢀTOF experiments were attempted to probe this furꢀ
ther, but the higher molecular weights limited this analysis.
Further studies aimed at exploring the end group fidelity of
these TMPMgCl·LiCl activated polymerizations is necessary
to explore the end groups in more detail.
Scheme 1. Synthesis of Periodic Copolymers.
C6H13
C6H13
TMPMgCl—LiCl
S
S
Br
S
S
S
S
Ni(dppp)Cl2
THF
n
1
T-T
C6H13
C6H13
C6H13
C6H13
TMPMgCl—LiCl
Table 1. Polymerization Data for All Monomers.
Se
Se
Br
Ni(dppp)Cl2
THF
n
a
Polymer
M/Cat.
Mn
Yield (%)
Đ
C6H13
C6H13
2
T-Se
25
50
100
12700
27100
36100
1.14
1.11
1.16
82
85
74
T-T
C6H13
Se
C6H13
Se
S
S
S
Br
TMPMgCl—LiCl
S
S
25
50
100
15000
27000
39800
1.16
1.11
1.18
86
78
60
T-Se
n
Ni(dppp)Cl2
THF
C6H13
T-T-Se
C6H13
C6H13
C6H13
3
C6H13
Se
C6H13
Se
50
32100
1.23
77
T-T-Se
T-Se-Se
F-T-Se
S
Br
TMPMgCl—LiCl
Se
C6H13
Se
50
39000
1.16
83
n
Ni(dppp)Cl2
THF
C6H13
C6H13
C6H13
T-Se-Se
50
100
11700
23100
1.18
1.22
42
30
4
C6H13
Se
C6H13
Se
aGPC traces were recorded at 40 °C versus polystyrene standꢀ
ards using THF as the eluent.
O
O
Br
TMPMgCl—LiCl
S
S
n
Ni(dppp)Cl2
THF
Trimers of 3ꢀhexylthiophene and 3ꢀhexylselenophene were
synthesized by repeated coupling reactions to afford monoꢀ
mers 3 and 4 (Supporting Information). Polymerization of 3
and 4 produced periodic copolymers with either 33 or 66%
selenophene content (T-T-Se and T-Se-Se). The molecular
weight and dispersities were indicative of chainꢀgrowth, imꢀ
plying that ringꢀwalking16b across three heterocycles is still
effective. End group signals in all the thiophene selenophene
copolymers are similar and have been assigned using the T-Se
copolymer as a reference (Supporting Information). For all the
thiophene selenophene copolymers, the only defect that could
C6H13
C6H13
C6H13
C6H13
F-T-Se
5
To build on the success with 1, monomer 2 was synthesized
containing 3ꢀhexylthiophene and 3ꢀhexylselenophene with a
headꢀtoꢀtail (HT) arrangement of the alkyl chains. This perꢀ
fectly alternating copolymer cannot be obtained by combining
thiophene and selenophene comonomers in one polymerizaꢀ
tion, due to the reactivity ratios.4d The polymerization of 2
proceeded with excellent control over Mn and low Đ, followꢀ
ing an identical procedure to that for dimer 1 (T-Se, Table 1).
1
be detected using H NMR spectroscopy was a TT coupling,
Quenching studies with monomer 2 confirmed deprotonaꢀ
tion at the expected position on the thiophene ring and are
included in the Supporting Information (Figure S3). The
polymerization seemed to be unaffected by the use of a dimerꢀ
ic unit as the monomer, even when the dimeric unit contained
two different heterocycles. This suggests the increased chainꢀ
which arises precatalyst initiation. In each polymerization, the
deprotonation with TMPMgCl·LiCl produces only one active
regioisomer as the starting monomer and affords primarily HT
coupling, similar to the report on P3HT from Mori and
coworkers.14c,17
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To broaden the scope of sequence control to include a periꢀ
almost identical to the end group discussed above for the T-
Se polymer. The proton for the Br terminated selenophene
chain end produced one signal at 6.99 ppm. Some small sigꢀ
nals appear near the major aromatic resonance of the thioꢀ
phene and we speculate these correspond to the thiophene
units near the TT defect and at the end of the polymer chain. It
is difficult to completely rule out whether these result from
minor defects present along the polymer chain.
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odic copolymer with 3 different heterocycles, furan was incorꢀ
porated into a trimer (monomer 5 – Scheme 1). We recently
reported CTP of poly(3ꢀhexylfuran) and demonstrated that
furan is amenable to controlled polycondensation when comꢀ
bined in a dimer with thiophene.18 The periodic copolymer F-
T-Se was prepared in a similar manner to the others from
monomer 5, but lower yields of the material were obtained
(Table 1). The monomer purity for this trimer was more diffiꢀ
cult to control and a small amount of the opposite isomer (Br
on the furan and H on the selenophene) was present during the
The presented results indicate that it is possible to obtain
polymers of F-T-Se with controlled molecular weights and
narrow dispersities. It is of note however, that polymerizations
involving furan containing units, proceeded with lower yields
and resulted in lower than expected molecular weights.18 Altꢀ
hough the largest oligomer explored by Kiriy and coworkers
was terthiophene, they noted that this might not be the limit in
controlled polycondensation.13 To explore this, we also preꢀ
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polymerization reaction. The H NMR spectrum of F-T-Se
showed little evidence of defects due to the presence of this
isomer during polymerization. Deprotonation of a proton adjaꢀ
cent to an alkyl chain will be difficult using TMPMgCl·LiCl
due to steric effects, and the selenophene proton of the minor
isomer is adjacent to the hexyl substituent. Mori and coworkꢀ
ers have made a similar observation with 3ꢀhexylthiophene
previously.19
pared
a thiophene tetramer, and polymerized it using
Ni(dppp)Cl2. The resultant P3HT (Mn = 25400, Đ = 1.14) is
indicative of a wellꢀcontrolled polymerization, implying that
ring walking can extend to four rings. The oligomer length
suitable for CTP is clearly dependent on many factors (monoꢀ
mer structure, catalyst structure, additives) and further studies
are necessary to probe this behavior in more detail. The
polymerization of the tetramer using CTP means larger monꢀ
omer sequences can potentially be targeted in the controlled
synthesis of semiconducting polymers.
Figure 2. UVꢀVis absorption spectra recorded in solution (CHCl3)
and in the solid state for all the synthesized polymers.
The optical properties of all the polymers were probed both
in solution and the solidꢀstate (Figure 2). The thiopheneꢀ
selenophene copolymers display a clear trend, whereby inꢀ
creasing selenophene content results in a red shifted λmax. The
λmax in CHCl3 shifts from 455 nm for T-T, to 480 nm for the
T-Se-Se periodic copolymer. While not synthesized here,
poly(3ꢀhexylselenophene) continues this trend by exhibiting
the most redꢀshifted absorption of the series (λmax = 499 nm,
solution).20 In the solidꢀstate, the thiopheneꢀselenophene sysꢀ
tems exhibit the same trend as observed in solution. These
results point to a wellꢀcontrolled bandgap by precise control
over the selenophene content in the polymer backbone similar
to that observed in the statistical copolymers.4b,4d,4e,5 In addiꢀ
tion, the vibronic structure in the solidꢀstate UVꢀVis spectrum
is wellꢀresolved and indicative of an ordered arrangement.
Figure 1. 1H NMR Spectrum of the T-Se copolymer (CHCl3 at 22
°C). Top Panel – expansion of the aromatic region denoted by the
dotted square in bottom panel. Aromatic signals for the polymer
main chain, TT defect and end groups are labeled. The star symꢀ
bols (*) indicate 13C satellite signals for the solvent and polymer.
The F-T-Se periodic copolymer has the most redꢀshifted
λmax in solution, and the most blueꢀshifted λmax in the solid
state. This is consistent with our previous observations that the
smaller furan moiety leads to enhanced planarity in solution.18
This effect is not present in solidꢀstate, where packing forces
result in planar πꢀstacked structures for all the synthesized
polymers.
1
Analysis of monomer 5 and the F-T-Se polymer using H
coupled HSQC experiments aided in the assignment of the end
group signals in the 1H NMR spectrum. A small signal appears
at 6.62 ppm, which corresponds to a bifuran TT defect from
precatalyst initiation (Figure S24, Supporting Information).
The H terminated selenophene end group appears at 7.51 ppm
and is again correlated with a small signal at 7.17 ppm. This is
The HOMOꢀLUMO gap as determined from cyclic voltꢀ
ammetry for the polymer series decreased with increasing
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molar fraction of selenophene and matched the changes obꢀ averages over the heterocycles, causing the excitation energy
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served for the optical band gap. DFT calculations were perꢀ
formed on oligomers of different sequences and lengths of up
to 12 heterocycles. The results are consistent with past work
on thiophene and selenophene oligomers, which found twistꢀ
ing of the structures to occur with low energy, both with and
without substitutents.21 Here, we consider planar structures
since, in the solid state, packing forces from πꢀstacking are
expected to favor planar configurations. Figure 3 illustrates
that both the experimentally observed excitation energy, and
that computed using TDDFT on oligomers of length 12, deꢀ
pend linearly on the proportion of selenophene rings. A small
disagreement between theory and experiment regarding λmax of
the homopolymers leads to somewhat different slopes, but
both theory and experiment find a linear dependence on comꢀ
position.
to depend linearly on the proportion of selenophene versus
thiophene units in Figure 3.
AFM images of ultraꢀthin films were prepared by dropꢀ
casting from dilute chloroform solutions directly onto silicon
substrates (Figures S36 – S40) with solvent vapor annealing.
In all cases, nanofibrillar structures were observed with widths
ranging from 24 – 35 nm, reminiscent of those reported for
regioregular P3ATs.23 In the course of AFM imaging, the surꢀ
faces of F-T-Se films visibly deteriorated upon rescanning.
Optical microscopy of the rescanned regions showed changes
that were consistent with photobleaching as has been observed
previously with furanꢀcontaining polymers.18
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The atomicꢀscale packing was studied using Grazing Inciꢀ
dence Wide Angle XꢀRay Scattering (GIWAXS) recorded at
the Synchrotron Radiation XꢀRay Source (CHESS). The gross
appearance of 2D GIWAXS patterns (Figure 4) was again
similar to the one widely reported for rrꢀP3HT. These studies
revealed the presence of outꢀofꢀplane reflections (020) origiꢀ
nating from πꢀstacking of polymer backbones along the subꢀ
strate, and inꢀplane reflections (100) resulting from layering of
those πꢀstacked aggregates parallel to it, with the interꢀlayer
spacing dictated by the length of alkyl chains. In all, they
showed a clear dependence on the overall content of selenoꢀ
phene rings in the sequence. The average πꢀstacking and laꢀ
mellar distances were obtained from the position of Bragg
peaks in outꢀofꢀplane and inꢀplane “slices” of 2D GIWAXS
patterns, with azimuthal averaging over, respectively, 5 – 15°
and 80 – 90° (Figure 4). As shown in Figure 4F, the πꢀ
stacking distance increased monotonically from 3.7 – 4 Å as
the proportion of selenophene rings increased. This can be
rationalized in terms of the larger heteroatom, Se versus S,
increasing the interchain πꢀstacking distance. Conversely, the
lamellar spacing decreased from 15.8 – 15.2 Å as selenophene
content was increased (Figure 4G). In the rrꢀP3AT family this
spacing is dictated by the length of the alkyl chains. The deꢀ
crease with selenophene content may be rationalized in terms
of the increase in πꢀstacking distance, which makes more volꢀ
ume available for side chains between the πꢀstacks. This inꢀ
creased volume enhances interdigitation of the side chains and
contracts the lamellar spacing. Seferos and coworkers have
used the alkyl chain length as a tool to match lamellar spacing
between 3ꢀalkylthiophenes and 3ꢀalkylselenophenes in random
copolymers previously.24
Figure 3. Excitation energies versus Se composition of thioꢀ
pheneꢀselenophene copolymers from TDDFT (blue), λmax in soluꢀ
tion (red), λmax in the solidꢀstate (green), and cyclic voltammetry
(orange range).
To gain insight into the origins of this linear dependence, an
exciton model was developed in which the excited state is
envisioned as containing an exciton that is coherently delocalꢀ
ized along the polymer (Supporting Information).22 The Hamꢀ
iltonian matrix for the exciton on an oligomer with N heteroꢀ
cycles is:
ꢄꢅ
ꢆ
ꢆ
ꢄꢇ
⋱
⋱
⋱
ꢆ
ꢃ
ꢂ
ꢊ
ꢀ ꢁ
ꢆ
ꢄꢈꢉ
where αi is the energy of the exciton when it resides on site i,
with values ⍺Se, ⍺S, or ⍺O for selenophene, thiophene or furan,
respectively. These parameters establish the oneꢀdimensional
effective energy landscape along which the exciton is coherꢀ
ently delocalized. The coupling between adjacent rings, β, sets
the effective mass of the exciton or, equivalently, the energy
stabilization due to coherent delocalization of the exciton. The
same value, β, is used for all couplings except that between
Analysis of radial (Figure 4H) and azimuthal (Figure 4I)
widths of Bragg peaks in GIWAXS patterns also revealed that
the T-Se sequence exhibited a considerably higher level of πꢀ
stacking and lamellar disorder in comparison with all other
sequences. The most likely origin of this disordered nature of
T-Se is the curvature of the polymer backbone imparted by the
alternation of selenophene and thiophene rings. The curvature
can be traced to the pattern of bond angles along the conjugatꢀ
ed backbone (Figure 5). Each of the monomers bends the polꢀ
ymer backbone a slightly different amount according to the
internal angle determined by the heteroatom bridge (88.3° for
Se and 92.4° for S). For the homopolymers, the alternating
up/down direction of the monomer ensures the polymer reꢀ
mains overall linear. The same is true for all sequences except
the alternating T-Se copolymer (Figure 5), which exhibits a
curved backbone. Considering that the T-Se structure is the
only sequence that is expected to have a curved backbone, and
is the sequence with the largest disorder in the GIWAXS patꢀ
adjacent furans, which is assigned the value βOO
.
Such a model reproduces the TDDFT excitation energy
computed for a range of oligomers comprised of between 2
and 12 heterocycles to an accuracy of 0.05 eV. The resulting
parameters therefore provide a concise summary of the
TDDFT results (αSe: 5.615 eV, αS: 5.777 eV, αO: 6.356 eV, β:
ꢀ1.715 eV, βOO: ꢀ1.819 eV). Since the coupling between rings,
β, is large compared to the energy differences between heteroꢀ
cycles, the ripples in the energy landscape arising from variaꢀ
tions in the sequence have a negligible effect on the wavefuncꢀ
tion describing the exciton delocalization. The exciton thus
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terns, this strongly implicates the backbone curvature as the
source of the disorder in T-Se.
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Figure 4. 2DꢀGIWAXS scattering profiles with scale bar for log of normalized intensity (Left), and the azimuthally averaged
“slices” of 2DꢀGIWAXS scattering profiles of in plane (100) (Blue) and out of plane (020) (Red) Bragg peak reflections (Right)
for T-T (A, Mn = 36,100), T-T-Se (B, Mn = 32,100),T-Se (C, Mn = 39,800), T-Se-Se (D, Mn = 39,000) and F-T-Se (E, Mn =
23,100). πꢀstacking (F) and lamellar (G) spacing determined from out of plane (020) and in plane (100) Bragg peaks from GIWAXS
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scattering profiles. The radial (H) and azimuthal (I) full widths at half maxima (FWHM) provide a measure of disorder in the samples that
is also reflected in the melting temperatures (J), depicted in (J/mol per ring).
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Complete descriptions of experimental procedures, NMR spectra,
GPC traces, CV data, DSC data, optical data and exciton model
discussion are included in the supporting information (PDF file).
The optimized molecular geometries and predicted excitation
energies are included in a separate text file. The Supporting Inꢀ
formation is available free of charge on the ACS Publications
website.
The disordered nature of the T-Se copolymer is also evident
when the differential scanning calorimetry (DSC) traces of all
polymers (Figures S41 and S42, Supporting Information) are
compared. A distinct lowering of melting temperature (Figure
4J) and heat of fusion (Figure S43) are observed for the alterꢀ
nating T-Se copolymer. Furthermore, thermogravimetric analꢀ
ysis (TGA) (Figure S44) points to good thermal stability of all
polymers, with an initial decomposition temperature above
339 °C. Notably, the decomposition residue at 600 °C was
highest for the P3HT homopolymer, which points to some
dependence of carbonization on heteroatom content.
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AUTHOR INFORMATION
Corresponding Author
yaron@andrew.cmu.edu
tomek@andrew.cmu.edu
noonan@andrew.cmu.edu
Author Contributions
‡These authors contributed equally.
ACKNOWLEDGMENT
K.J.T.N. is grateful to the ARO (63038ꢀCHꢀYIP and W911NFꢀ
16ꢀ1ꢀ0053) and the NSF for a Career Award (CHEꢀ1455136).
T.K. gratefully acknowledges financial support from Heinz Enꢀ
dowments (grant #E1041). NMR Instrumentation at Carnegie
Mellon was partially supported by the NSF (CHEꢀ0130903 and
CHEꢀ1039870). Xꢀray scattering results are based partly on reꢀ
search conducted at Cornell High Energy Synchrotron Source
(CHESS) which is supported by the National Science Foundation
and the National Institutes of Health/National Institute of General
Medical Sciences under NSF awards DMRꢀ0936384 and DMRꢀ
1332208.
Figure 5. Top – Calculated bond lengths and angles from doꢀ
decamer calculations for thiophene and selenophene in the oligoꢀ
mer chain (end groups were excluded). Bottom – Energy miniꢀ
mized T-T-Se and T-Se dodecamer structures.
Conclusion
In summary, we have designed and synthesized 5 different
sequenced monomers and applied catalystꢀtransfer polyconꢀ
densation to afford periodic conjugated polymers with conꢀ
trolled molecular weights and relatively low dispersities. The
sequences have a predictable impact on the electronic properꢀ
ties of the materials as determined by UVꢀVis, CV and comꢀ
putation. Solidꢀstate order of the copolymers was probed using
Xꢀray scattering, DSC and TGA. These studies indicate that
changes in bond lengths from the chalcogen to the diene may
introduce curvature in the polymer backbone that can perturb
organization. The sequences that mostly closely retain the
morphology of P3HT are those for which the repeating seꢀ
quence retains a linear polymer backbone. Variations in the πꢀ
stacking distance were also noted due to changes in heteroaꢀ
tom size, though these do not seem to strongly perturb chain
packing. Overall, the incorporation of sequenced monomers
offers a means to fineꢀtune electronic structure while retaining
a nanofibrillar morphology. The potential to precisely tune
band gaps by controlling the sequence may be valuable when
incorporating these systems into organic electronic devices.
Further studies will elucidate the impact of randomness on the
solidꢀstate organization of group 16 copolymers.
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