Synthesis and characterization of fully conjugated donor-acceptor-donor triblock copolymers

Article abstract of DOI:10.1021/ma200084z

A synthetic approach is established to provide a monofunctional telechelic poly(3-octylthiophene) (P3OT) bearing a single bromine-substituted end group that is of potential use in the preparation of well-defined block copolymers. Telechelic P3OT was prepared via a chain growth process by a catalyst-transfer condensation polymerization (CTCP) of 5-bromo-4-octyl-2-thienylmagnesium iodide initiated by a phenylnickel(II) initiator. Optimization of the conditions for quenching the reaction allowed for the installation an α-bromo functionality at the terminus of the polymer. We demonstrate the utility of this well-defined monofunctional polymer, Ph-P3OT-Br, by coupling it to a poly(quinoxaline) (PQ) bearing boronate ester end groups to provided a new class of donor-acceptor-donor (D-A-D) triblock copolymers. The formation of the triblock copolymers was confirmed by gel-permeation chromatography (GPC) and ¹H NMR spectroscopy. The optical properties of the polymers were investigated using UV-visible absorption and fluorescence spectroscopy. Efficient quenching of the fluorescence from the individual blocks of the triblock copolymers is consistent with the occurrence of electron transfer. AFM images illustrate a nanoscale phase separation of the electron-rich P3OT and electron-poor PQ blocks.

Full text of DOI:10.1021/ma200084z

ARTICLE
pubs.acs.org/Macromolecules
Synthesis and Characterization of Fully Conjugated
DonorꢀAcceptorꢀDonor Triblock Copolymers
Kathy B. Woody,† Benjamin J. Leever,‡ Michael F. Durstock,‡ and David M. Collard†,*
†School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
‡Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States
S Supporting Information
b
ABSTRACT: A synthetic approach is established to provide a
monofunctional telechelic poly(3-octylthiophene) (P3OT)
bearing a single bromine-substituted end group that is of
potential use in the preparation of well-defined block copoly-
mers. Telechelic P3OT was prepared via a chain growth process
by a catalyst-transfer condensation polymerization (CTCP) of
5-bromo-4-octyl-2-thienylmagnesium iodide initiated by a
phenylnickel(II) initiator. Optimization of the conditions for
quenching the reaction allowed for the installation an R-bromo
functionality at the terminus of the polymer. We demonstrate
the utility of this well-defined monofunctional polymer,
PhꢀP3OTꢀBr, by coupling it to a poly(quinoxaline) (PQ)
bearing boronate ester end groups to provided a new class of donorꢀacceptorꢀdonor (DꢀAꢀD) triblock copolymers. The
formation of the triblock copolymers was confirmed by gel-permeation chromatography (GPC) and 1H NMR spectroscopy. The
optical properties of the polymers were investigated using UVꢀvisible absorption and fluorescence spectroscopy. Efficient
quenching of the fluorescence from the individual blocks of the triblock copolymers is consistent with the occurrence of electron
transfer. AFM images illustrate a nanoscale phase separation of the electron-rich P3OT and electron-poor PQ blocks.
’ INTRODUCTION
extensively studied and provide abundant opportunities to prepare
materials with unique morphologies, however these polymers only
contain one electroactive segment.7ꢀ11 Donorꢀacceptor block
copolymers with a conjugated electron donating segment and an
aliphatic segment containing pendant electron acceptors have also
been prepared.12ꢀ14 However, the aliphatic segment in rodꢀcoil
block copolymers serves as an insulator. There are few examples of
fully conjugated block copolymers, which either have an aliphatic
spacer between the conjugated segments3,15 or are directly
linked.16 The paucity of fully conjugated DꢀA block copolymers
can largely be attributed to the synthetic challenges in preparing
such materials and the lack of electron accepting polymers, which
generally contain nitrogen-based heteroaromatic units in the
conjugated backbone (e.g., pyridine,17 quinoxaline,18 quinoline,19
or thienopyrazine20) or are substituted with electron-withdrawing
substituents (e.g., fluorine21 or cyano substituents22). Such block
copolymers could provide opportunities to control the scale and
morphology of phases in BHJ devices.
Conjugated polymers have been widely studied for use as
organic semiconductors in electronic devices such as solar cells.
These materials have the potential to provide low-cost, flexible,
and lightweight devices for which the cost of materials processing
is significantly lower than for inorganic semiconductors.1 In bulk-
heterojuction (BHJ) organic solar cells, donor (D) and acceptor
(A) materials are blended to provide the photoactive layer.
Absorption of photons leads to the formation of excitons, and
diffusion of these excitons to the DꢀA interface leads to
formation of charge carriers.2 For example, prototype photo-
voltaic cells incorporating a regioregular poly(3-alkylthiophene)
as the donor and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) as the acceptor display promising efficiencies.3 Initia-
tives to improve the performance of organic solar cells largely
focus on the design and synthesis of new low band gap
conjugated polymers4 and changing the morphology of the
blends.5 Thoroughly blending of the donor and acceptor materi-
als improves device performance by reducing the size of the
phase-separated domains such that they are on the order of the
exciton diffusion length (10ꢀ20 nm).6
The development of new synthetic methodologies is required
to provide access to donorꢀacceptor block polymers that make
use of the phase separation of the two blocks to impart new or
Block copolymers phase separate on the nanoscale and the size
of the phases formed by the two blocks is directly related to the
length of each block. Rodꢀcoil block copolymers containing a
semirigid conjugated segment and a flexible segment have been
Received: January 13, 2011
Revised:
May 5, 2011
Published: May 18, 2011
r
2011 American Chemical Society
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Figure 1. Preparation of a telechelic poly(3-octylthiophene) with a single R-bromothienyl end group, and coupling to a poly(5,8-quinoxaline) (PQ)
bearing two boronate ester end groups to afford a triblock copolymer.
enhanced electronic properties. One approach used to prepare
donorꢀdonor (DꢀD) fully conjugated block copolymers is to
couple the termini of two separate prepolymers that bear com-
plementary functionality.16,23,24 More recently, with the advent of
catalyst-transfer condensation polymerizations (CTCP) of haloar-
ylmagnesium halides,25 similar DꢀD block copolymers architec-
tures have become accessible by chain extension of the active end
of quasi-living polymers with a second monomer.26ꢀ31 Here we
describe the use of CTCP to prepare a telechelic poly(3-
octylthiophene) (P3OT) bearing a single bromine-substituted
end group that is of potential use in the preparation of well-
defined block copolymers. We demonstrate the utility of this new
well-defined monofunctional polythiophene by coupling it under
Suzuki coupling conditions to a poly(quinoxaline) (PQ) that has
two boronate ester end groups. This provides a DꢀAꢀD triblock
copolymer, Figure 1. Poly(3-alkylthiophene)s are commonly
employed as a donor material in BHJ solar cells.32 For this study,
we chose poly(5,8-quinoxaline) as an electron-poor acceptor
block because it is susceptible to n-doping by reduction (Ered
=
ꢀ1.98 V versus Ag/Agþ) and the polymer is rendered soluble by
decoration of the backbone with flexible side chains.18 The
structural and optical properties of the polymers are described.
Figure 2. Origin of two types of end groups from propagating end in the
catalyst-transfer condensation polymerization (CTCP) of 5-bromo-4-
alkyl-2-thienylmagnesium bromides.
’ RESULTS AND DISCUSSION
Synthetic Approach. Our synthetic approach to well-defined
triblock copolymers involves three steps: (i) the synthesis of a
well-defined monofunctional telechelic P3OT donor block that
bears a single R-bromothienyl end group, (ii) the synthesis of a
PQ acceptor block bearing complementary boronate ester
functionality at both ends, and (iii) coupling of the two polymer
chains under Suzuki cross-coupling conditions, Figure 1.
Telechelic Bromine-Terminated Poly(3-alkylthiophene)
Donor Block. We explored several methods to attain well-defined
telechelic P3OT that bears a single R-bromothienyl end group that
subsequently could be coupled to a poly(5,8-quinoxaline) block. It
is well established that poly(3-alkylthiophene)s with low polydis-
persity can be prepared by nickel(II)-catalyzed polymerization
of 5-bromo-4-alkyl-2-thienylmagnesium iodides.25 Unlike most
condensation polymerizations of metalloaryl halides, this reaction
proceeds by a catalyst-transfer chain growth mechanism in which
the nickel remains associated with the polymer chain and the
propagating end interconverts between an R-bromothienyl and an
R-thienylnickel complex upon addition of each monomer,
Figure 2.33ꢀ35 The quasi-living nature of this polymerization
relies on the rate of propagation being substantially greater
than that of dissociative reductive elimination of nickel from
the propagating chain end. While significant details about this
process have been elucidated, quenching of the polymeriza-
tion, typically by pouring the reaction mixture into an acidic
aqueous solution, affords a mixture of R-bromothienyl
(ThꢀBr/ThꢀBr) and unfunctionalized thienyl end groups
(ThꢀBr/ThꢀH), Figure 2.36 In addition, polymer chains can
undergo metathesis to generate polythiophene with unfunc-
tionalized thienyl end groups on both chain ends (ThꢀH/
ThꢀH). Previously, McCullough and co-workers showed that
it was possible to end-functionalize polythiophene using a
Grignard reagent such as ethynylmagnesium bromine as an
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Figure 3. Initiation of the polymerization of 2-bromo-3-octyl-5-io-
dothiophene (6) with phenylnickel(II) initiator (5) leads to phenyl
initiated P3OT.
end-capping reagent.37 The end-functionalized polythio-
phene in these studies have been used to prepare rodꢀcoil
block copolymers.8,10
Figure 4. 1H NMR (300 MHz, CDCl3) of phenylnickel(II)-initiated
poly(3-octylthiophene), allowed to react for 1 h and then: (A) quenched
by pouring reaction mixture into MeOH; (B) maintained for 12 h
at ꢀ15 °C and poured into MeOH.
In an effort to attain further control over the preparation of
poly(3-alkylthiophene), several groups have explored the use of
preformed arylnickel(II) complexes as initiators, thereby result-
ing in the installation of the aryl group at the initiated end of the
chain.38ꢀ40 We chose to prepare poly(3-octylthiophene) by
employing phenylnickel(II) bromide as an initiator, and then
focus on the retention of the R-bromothienyl functionality at the
terminus derived from the propagating chain end. This was
guided, in part, by the proposed mechanistic origins of the
different combinations of end groups.40
1:1 ratio with the phenyl end groups. The thiophene at the other
end of the polymer chain bears either an R-thienyl bromide or
hydrogen atom (X = H or Br, Figure 4A). The relative amounts
of these two types of end groups can be determined by 1H NMR.
The β-proton on the terminal thiophene (proton f, Figure 4)
appears at 6.90 ppm if it bears an R-hydrogen atom, and at 6.83
ppm (proton f0) if the polymer is terminated with a bromine
atom.42 Initial batches of polymer prepared in this manner
contained a mixture of R-bromothienyl (56%) and unfunctiona-
lized thienyl (44%) end groups, Figure 4A. The sum of the
integral of protons f and f0 (i.e., from the propagating chain end)
is equal to that of the β-proton on the thiophene adjacent to the
phenyl end group (proton d of the initiating end), consistent
with efficient initiation of the polymerization by the
phenylnicklel(II) complex 6 and chain growth without termina-
tion or chain transfer prior to quenching..
This result is in contrast to a previous report in which
polymerizations were conducted at room temperature. At this
higher temperature a significant portion of the isolated polymers
lacked the aryl end group derived from the initiator,40 suggesting
the occurrence of chain transfer whereby nickel dissociates from
one propagating chain and reinitiates chain growth from another
monomer. While initiation of the polymerization with the
phenylnickel(II) complex installs a phenyl group at one end of
the P3OT chain, the mixture of termini derived from the
propagating chain end presents a significant hindrance to the
preparation of block copolymers since it is important that all of
the polymer chains have identical functionality. Polymers bearing
an unfunctionalized thienyl end group would not undergo a
subsequent coupling reaction to provide block copolymers,
thereby leading to homopolymer impurities. In our hands,
modifications of reaction time, temperature and quenching
conditions were unsuccessful in completely preventing dissocia-
tive reductive elimination, and the polymerization reactions
always gave a mixture of R-bromothienyl and thienyl termini
derived from the propagating chain end. Since efforts to prevent
this dissociation from occurring were unsuccessful, we decided
instead to simply allow time for the dissociative reductive elimination
Addition of bromobenzene to a solution of Ni(PPh3)4 in
toluene led to the precipitation of the bright yellow initiator 5
that was collected by filtration, Figure 3.41 Initiation of the
polymerization of 2-bromo-3-octyl-5-iodothiophene (6) with 5
at 0 °C resulted in the formation of the phenyl initiated poly(3-
alkylthiophene) with either bromine or hydrogen groups at
the terminus derived from the propagating chain end (i.e,
PhꢀP3OTꢀX) Figure 3. Kiriy has previously shown that if
low temperatures are maintained during this polymerization the
resulting polymers are exclusively propagated from the phenyl
initiator with formation of chains with Ph/ThꢀBr and Ph/
ThꢀH combinations of end groups. However, at room tempe-
rature there is evidence for chain transfer in which nickel
dissociates from one chain and initiates a new chain that lacks
the aryl end group derived from the initiator, resulting in chains
initiated from 5-bromo-4-alkyl-2-thienylnickel(II) and end-
capped with R-bromothienyl and unfunctionalized thienyl end
groups (e.g., ThꢀBr/ThꢀBr and ThꢀBr/ThꢀH combinations
of end-groups, plus some ThꢀH/ThꢀH as a byproduct).40 Even
in the absence of chain transfer, the R-bromothienyl end groups
derived from the propagating chain end form as a result of some
reductive elimination with dissociation of the nickel from the
polymer backbone which could occur during the polymerization
or after consumption of the monomer.
1
Analysis of the aromatic region of the H NMR allows for
characterization of the two termini of the phenylnickel(II)-
initiated polymers, Figure 4A. The protons of the phenyl end
groups derived from the initiator appear as a distinct set of
multiplets at 7.61 (d), 7.38 (t), and 7.27 (m) ppm, with integrals
of 2:2:1 respectively corresponding to the AM2X2 spin system of
the phenyl end group. The β-hydrogen of the thiophene unit
adjacent to the phenyl end group (Hd, Figure 4A) appears as a
singlet at 7.16 ppm: As expected, the integral of this peak reveals a
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Figure 5. Synthesis of bromine terminated poly(5,8-quinoxaline) (PQBr2).
of the nickel(II) from the polymer chain at the end of the
polymerization to provide R-bromothienyl end groups. Rather
than following the commonly used procedure of quenching the
polymerization by pouring the reaction mixture into methanol
after a relatively short reaction time (i.e., upon consumption of
the monomer), the reaction mixture was placed in a freezer at
ꢀ15 °C for 12 h prior to precipitation into methanol. Holding
the reaction at low temperatures for an extended period after
consumption of the monomer allowed for almost complete
reductive elimination and dissociation of the nickel from the
polymer backbone to provide P3OT terminated with R-bro-
mothienyl end groups (PhꢀP3OTꢀBr). Thus, while propaga-
tion is significantly faster than chain transfer during the
polymerization, which is important to the success of this proce-
dure, reductive elimination with dissociation of nickel from the
chain does take place, albeit slowly, once the monomer is
Figure 6. Synthesis of thiophene-quinoxline-thiophene trimer by Suzuki
cross-coupling.
1
consumed. The H NMR of the polymer reveals a 1:1 ratio of
the β-proton on the thiophene adjacent to the phenyl propagat-
ing group (proton d, Figure 4B) and the R-bromothienyl group
(proton f0, Figure 4B), consistent with a polymer containing
a high proportion of phenyl groups at the initiated end and
R-bromothienyl end groups derived from the propagating end
(i.e., the Ph/ThꢀBr combination of end groups). While there is a
small peak present at 6.90 ppm, the agreement in the integrals of
peaks d and f0 indicate that the presence of unfunctionalized
thienyl end groups is minimal. Chain transfer products which
would lead to polythiophene with ThꢀBr/ThꢀBr and ThꢀBr/
ThꢀH end groups are absent based on the appearance of the
NMR spectra (i.e., the absence of peaks corresponding to a
ThꢀBr initiating end, and equal integrals of peaks d and f0).
The precipitated solid was subjected to sequential extraction
in a Soxhlet extractor with methanol, acetone, hexanes and
chloroform. From this procedure, extracts into hexanes consisted
of a material with a lower degree of polymerization (DP = 14),
and the chloroform fraction provided polymer with higher
molecular weight (DP = 22), and low polydispersity indices
(1.2 and 1.3, respectively). In these polymerizations we
employed a 1:20 initiator to monomer ratio. The molecular
weights appear to be consistent with the control of the chain
length by the molar ratio of the initiator and monomer, albeit the
polymer has been fractionated.25
Figure 7. Synthesis of donorꢀacceptorꢀdonor triblock copolymers by
Suzuki cross-coupling.
condensation polymerizations of 5,8-dibromoquinoxalines 4a and
4b using Yamamoto coupling conditions,18 Figure 5. Monomer 4
was synthesized by published procedures with minor modific-
ations:43 4,7-Dibromobenzothiadiazole (1) was reduced with
sodium borohydride to afford diamine 2, which was used without
purification in a subsequent condensation with diketone 3. Nickel-
(0)-catalyzed polymerization of monomers 4a and 4b resulted in
the corresponding dibromo-terminated polymers PQ(C8)Br2 and
PQ(EH)Br2, respectively. The protons of the quinoxaline rings at
the termini of the chains appear as two distinct doublets in the 1H
NMR at 6.98 and 7.73 ppm (see Supporting Information for
spectra). This is consistent with unsymmetrically substituted
quinoxaline groups, and is in contrast to the signal for the symmetri-
cally substituted quinoxaline rings in the backbone of the polymer
that appear as a singlet at 8.33 ppm. The degree of polymerization of
the polymer was determined by comparing the relative integrals of
the protons of the end groups and those of the polymer backbone.
Boronate Ester Functionalized Poly(5,8-quinoxaline). The
difunctional electron-accepting block in this study, poly(2,3-(4-
octyloxyphenyl)quinoxaline-5,8-diyl), PQ(C8), and poly(2,3-(2-
ethylhexyl)quinoxaline-5,8-diyl), PQ(EH), were prepared by
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The PQ(C8)Br2 sample prepared in this study had a DP of eight,
and the PQ(EH)Br2 analogue had a DP of 14.
singlet at 7.0 ppm and the poly(quinoxaline) gives a singlet at 8.0
ppm (see Supporting Information).
Coupling of Donor and Acceptor Blocks. We chose to
convert the end groups of PQBr2 into boronate esters that could
be coupled to the R-bromine terminated poly(3-octylthiophene),
PhꢀP3OTꢀBr, by a Suzuki cross-coupling reaction to couple
the two blocks together. In order to test this approach, 5,8-
dibromoquinoxaline (4a) was treated with an excess of bis-
(pinacolato)diboron. This led to rapid and quantitative conver-
sion to bisborolane 7, as confirmed by 1H NMR (the singlet for
the aromatic hydrogen of 4a at 7.83 ppm disappeared, and a new
singlet appeared at 7.93 ppm for the bisborolane-quinoxaline),
Figure 6. The palladium catalyzed Suzuki cross-coupling reaction
between bisborolane 7 and 2-bromo-3-octylthiophene afforded
bisthienylquinoxaline 8 with an isolated yield of 92% and serves
as a model for the coupling of the donor and acceptor blocks.
GPC profiles of PhꢀP3OTꢀBr, PQ(C8)Br2 and the result-
ing DꢀAꢀD triblock copolymer are shown in Figure 8. The
elution curve of the hexanes-soluble fraction of PhꢀP3OTꢀBr
(dotted line) corresponds to an Mn of 2.4 kDa. The
dibromopoly(quinoxaline), PQ(C8)Br2 (dashed line), has an
Mn of 5.8 kDa. After Suzuki coupling of the donor and acceptor
polymers and subsequent extractions, the chloroform fraction
gives an elution curve corresponding to an Mn of 10.3 kDa (solid
1
line). Taken together the H NMR spectra and GPC data are
consistent with formation of the DꢀAꢀD triblock copolymer,
P3OTꢀPQ(C8)ꢀP3OT.
Treatment of PQBr2 with
a large excess of bis-
(pinicolatediboron) afforded the boronate ester-terminated
poly(quinoxaline), PQꢀB(OR0)2 as a yellow powder, Figure 7.
1
While the H NMR signals of the aromatic end groups of the
boronate ester terminated polyquinoxalines are coincident with
those of the dibromo-substituted analogue, the presence of the
boronate end groups was confirmed by the appearance of a CꢀO
stretching band at 1248 cmꢀ1 in the IR spectrum (see Supporting
Information).
The DꢀAꢀD triblock copolymers were prepared by combin-
ing the boronate ester terminated poly(quinoxaline)s, PQꢀB-
(OR0)2, with 4 equiv of the hexanes-soluble PhꢀP3OTꢀBr
under Suzuki cross-coupling conditions, Figure 7. The lower
molecular weight hexanes-soluble fraction of the poly(3-al-
kylthiophene) donor block was used in order to simplify the
purification process after the coupling reaction whereby any
unreacted polymer could be washed out of the mixture by
extraction. After coupling the resulting block copolymers were
precipitated by addition of the reaction mixture to a large volume
of cold methanol, and the resulting solid was purified by
successive extractions in a Soxhlet extractor with methanol,
acetone, hexanes and chloroform. Any unreacted PhꢀP3OTꢀ
Br, which was used in excess, was extracted into hexanes, and the
block copolymer was extracted into the chloroform fraction.
Polymer Characterization. The number (Mn) and weight
(Mw) average molecular weights and polydispersity (PDI) of the
homopolymers and triblock copolymers were determined by
GPC and 1H NMR spectroscopy, Table 1. The relative segment
lengths of the block copolymers were determined by integration
of the 1H NMR signals of the protons of the aromatic rings: The
βꢀhydrogen atoms of the thiophene backbone of P3OT gives a
Figure 8. GPC profiles of PhꢀP3OTꢀBr (dotted line), PQ(C8)Br2
(dashed line), and P3OT-PQ(C8)ꢀP3OT (solid line).
Figure 9. DSC heating scans of PhꢀP3OTꢀBr, PQ(EH)Br2, and
P3OT-PQ(EH)ꢀP3OT (10 °C minꢀ1).
Table 1. Properties of PhꢀP3OTꢀBr (D), PQBr2 (A) and DꢀAꢀD triblock copolymers
polymer
Mwa,b (kg molꢀ1
)
Mnb,c (kg molꢀ1
)
PDIb,d
DPe,f
Tmg (°C)
Tch (°C)
PhꢀP3OTꢀBr hexanes extract
PhꢀP3OTꢀBr CHCl3 extract
PQ(EH)Br2
3.1
5.3
2.7
4.2
1.2
1.3
2.1
1.8
2.6
2.8
12
20
14
8
176
185
157
159
12.0
8.3
5.7
110
-
-
PQ(C8)Br2
4.6
72 (br)
75 115
-
P3OT-PQ(EH)ꢀP3OT
P3OT-PQ(C8)ꢀP3OT
26.0
10.0
42
92
28.8
10.3
36
-
a Weight-average molecular weight (Mw). b Determined by gel permeation using polystyrene standards. Number-average molecular weight (Mn).
c
d Polydispersity index (PDI). e Degree of polymerization (aryl repeat units). f Determined by end group analysis. g Melting temperatures determined by a
DSC scan rate of 10 °C.minꢀ1. h Recrystallization temperatures determined by a DSC scan rate of 10 °C.minꢀ1
.
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Figure 10. UVꢀvisible absorption spectra in chloroform (5 mg/
100 mL) for PhꢀP3OTꢀBr (dotted line), PQ(C8)Br2 (gray dashed
line), PQ(EH)Br2 (black dashed line), and DꢀAꢀD triblock copolymers
P3OTꢀPQ(C8)ꢀP3OT (gray solid line), and P3OTꢀPQ(EH)ꢀ
P3OT (black solid line).
The thermal transition temperatures of the polymers prepared
in this study were measured by differential scanning calorimetry
(DSC) in a nitrogen atmosphere, Table 1. DSC heating scans of
PhꢀP3OTꢀBr and PQ(EH)Br2 and corresponding triblock
copolymer are shown in Figure 9. The hexanes soluble fraction of
PhꢀP3OTꢀBr has a single endothermic peak on heating at
176 °C and a supercooled crystallization transition at 157 °C
upon cooling. The melting transition of PQ(EH)Br2 occurs at
110 °C. The corresponding triblock copolymer, P3OT-PQ-
(EH)ꢀP3OT has two melting transitions at 75 and 115 °C,
and one crystallization peak upon cooling at 92 °C. Depression of
melting points of the separate components in other conjugated
block copolymers has been previously observed.44,45 Accord-
ingly, the endothermic transitions of the triblock copolymer may
correspond to melting of the PQ(EH) and P3OT segments,
respectively. The triblock copolymer P3OT-PQ(C8)ꢀP3OT
does not show any thermal transitions, indicating that this
polymer is largely amorphous.
The solution UVꢀvisible absorption spectra of PQBr2,
PhꢀP3OTꢀBr, and DꢀAꢀD triblock copolymers P3OTꢀ
PQ(C8)ꢀP3OT and P3OTꢀPQ(EH)ꢀP3OT are shown in
Figure 10. The solution spectra were recorded for solution of the
polymer in chloroform (5 mg/100 mL). The precursors for the
polyquinioxaline blocks, PQ(C8)Br2 and PQ(EH)Br2 absorb at
392 and 376 nm, respectively. The telechelic poly(3-
octylthiophene) bearing phenyl and R-bromothienyl termini
absorbs at 450 nm. These absorptions are similar to previously
reported values for related homopolymers.18,43 The spectra of
the DꢀAꢀD triblock copolymers display absorption bands
that are similar to those of the constituent blocks. For
example, the P3OTꢀPQ(EH)ꢀP3OT triblock copolymer
has absorption bands at 378 and 450 nm. The height of the
absorbance band at 450 nm of P3OTꢀPQ(EH)ꢀP3OT is
smaller than that of the PhꢀP3OTꢀBr homopolymer be-
cause there is less of the P3OT segment present in the triblock
copolymer. As the block copolymer spectrum is, for the most
part, a superposition of the homopolymer spectrum, there is
no apparent ground state interaction between the donor and
acceptor segments.
Figure 11. Fluorescence spectra of solutions (top; 5 mg/100 mL) and
solid state (bottom; films cast from 15 mg/mL solutions in 1,4-
dichlorobenzene) of PQ(C8)Br2 (dashed line), PhꢀP3OTꢀBr
(dotted line) and DꢀAꢀD triblock copolymers, P3OTꢀPQꢀP3OT.
The solid state absorptions are broader and red-shifted for all
of the materials (see Supporting Information). The absorption
maxima for the homopolymers occur at 501 nm for
PhꢀP3OTꢀBr, at 396 nm for PQ(C8)Br2 and at 366 nm for
PQ(EH)Br2. The spectrum of the solid state DꢀAꢀD triblock
P3OTꢀPQ(C8)ꢀP3OT has two absorption transitions, at 490
and 407 nm, that may be ascribed to the donor and acceptor
segments, respectively. Similarly, P3OTꢀPQ(EH)ꢀP3OT has
absorption transitions at 500 and 366 nm.
While the absorption spectra of the block copolymers have
transitions that are similar to those of the constituent homopoly-
mers, the fluorescence spectra have much more noticeable
differences. The solution and thin film fluorescence spectra are
shown in Figure 11. The emission of a solution of the precursor
poly(3-octylthiophene) block occurs at 560 nm, and the emis-
sion of the poly(5,8-quinoxaline) precursors occur at 455 nm
(C8 analogue) and 473 nm (EH), consistent with previously
reported results for related homopolymers.18,43 However, the
emission spectra of the block copolymer shows almost complete
quenching of the fluorescence, consistent with electron transfer
from the donor to the acceptor segment. The solid state
fluorescence spectra were measured on films prepared by casting
15 mg/mL of the polymers in 1,4-dichlorobenzene onto quartz
slides. The emission maximum of poly(3-octylthiophene) occurs at
770 nm and that of the poly(5,8-quinoxaline)s occur at 507 nm
(C8) and 443 (EH). As with the solution spectra, films
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ARTICLE
prepare a new class of DꢀAꢀD triblock copolymers with poly(3-
octylthiophene) donor and poly(5,8-quinoxaline) acceptor
blocks. The synthetic method established in this study to afford
poly(3-octylthiophene) with the Ph/ThꢀBr combination of end
groups might be applicable to the preparation of a broad array
of new materials. Characterization of the DꢀAꢀD triblock
copolymers in this study reveals efficient fluorescence quenching
of the polymers in the solid state, supporting the occurrence of
electron transfer.
’ EXPERIMENTAL SECTION
General Methods. All starting materials were purchased from
commercial sources and used without further purification unless other-
wise stated. THF and Et2O were dried over sodium benzophenone ketyl
prior to distillation under argon. Column chromatography was per-
formed on flash grade silica (32ꢀ60 Å, Sorbent Technologies, Atlanta, GA).
NMR analysis was performed on a Bruker DSX 300 instrument using
CDCl3 as the solvent. Chemical shifts are referenced to internal
tetramethylsilane. IR analyses were performed on a Nicolet 4700 FTIR
with an ATIR attachment from Smart-Orbit Thermoelectronic Cor-
poration. Ultravioletꢀvisible analysis was performed on a Shimadzu
UV-2401PC spectrometer, and fluorescence spectroscopy was per-
formed on a Shimadzu RF-5301PC spectrofluorophotometer. AFM
scans were conducted using a Veeco Dimension V AFM. Thin film
samples were prepared from dichlorobenzene and chloroform solutions
(20 mg/mL and analyzed in tapping mode with a Nanosensors silicon
AFM probe (model PPP-NCHR). Elemental analyses were performed
by Atlantic Microlab, Inc. (Norcross, GA). 4,7-Dibromobenzo[c]-
[1,2,5]thiadiazole (1),47 Ni(PPh3)4,48 and 2-bromo-5-iodo-3-
octylthiophene25 were prepared using previously reported methods.
Synthesis of analogues a (R = ꢀC6H4ꢀOC8H17) are described below.
Homologues b (R = 2-ethylhexyl) were synthesized using similar
procedures unless otherwise stated. Spectral characterization of com-
pounds 7 and 8 and homologue b is provided in Supporting Information.
1,2-Bis(4-(octyloxy)phenyl)ethane-1,2-dione, 3a. Potas-
sium carbonate (15.2 g, 110 mmol) and tetrabutylammonium bromide
(8.0 g, 25 mmol) were added to a solution of 1-bromooctane (15 mL,
88 mmol) and 1,2-bis(4-hydroxyphenyl)ethane-1,2-dione (10.6 g,
41.4 mmol) in anhydrous DMF (150 mL) under argon. The mixture
was heated at 80 °C for 24 h, cooled and poured into H2O (700 mL).
The resulting mixture was filtered and the filtrate was recrystallized from
ethanol to afford 3a as a colorless solid (10.6 g, 52%): mp = 63ꢀ64 °C.
1H NMR (300 MHz, CDCl3): δ 7.92 (d, 3JHH = 8.7 Hz, 4H, Ar C2ꢀH),
Figure 12. Tapping-mode atomic force microscopy (AFM) images of
spin-coated thin films cast from chloroform onto ITO. P3OTꢀPQ-
(EH)ꢀP3OT: A, height; B, phase. P3OTꢀPQ(C8)ꢀP3OT: C,
height; D, phase.
of the block copolymer show almost complete quenching of the
fluorescence.
To investigate the morphology of the DꢀAꢀD triblock
copolymers, thin films of P3OTꢀPQ(EH)ꢀP3OT were cast
from chloroform and dichlorobenzene. Atomic force microscopy
height and phase images on the films are presented in Figure 12.
The nanostructure apparent in these images may be attributed to
phase separation of the poly(3-octylthiophene) segments and
the poly(5,8-quinoxaline) segments. While the nanoscale phase
separation of certain conjugated block copolymers may be
ascribed to the presence of crystalline and amorphous blocks,18,46
an alternate explanation in terms of π-stacking interactions has
also been posited.25 Accordingly, as with other block copolymers,
the thermodynamically driven phase separation is a function of
FloryꢀHuggins interaction parameters and does not strictly rely
on the presence of crystalline and amorphous blocks. Films of
homopolymer cast under the same conditions did not show such
textures.
3
3
6.93 (d, JHH = 8.7 Hz, 4H, Ar C3ꢀH), 4.02 (t, JHH = 6.6 Hz, 4H,
ꢀOCH2ꢀ), 1.74ꢀ1.84 (m, 4H), 1.28ꢀ1.47 (m, 20H), 0.88 (t, 3JHH
=
’ CONCLUSIONS
6.6 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 193.52 (CdO), 164.43
(Ar C4), 132.31 (Ar C2), 126.00 (Ar C1), 114.65 (Ar C3), 68.42
(CꢀO), 31.74, 29.25, 29.16, 28.96, 25.89, 22.61, 14.07. IR (ATIR): 2916
(ArCꢀH str.), 2844, 1664, 1597, 1573, 1508, 1463, 1421, 1251 (CꢀOstr.),
1162, 1058, 1014, 956, 889, 842, 759, 649, 617 cmꢀ1. HRMS: calcd for
C30H42O4 = 466.3083; obsd = 466.3091; Δ = 1.7 ppm. Anal. Calcd: C,
77.21; H, 9.07. Found: C, 76.38; H, 8.90.
In conclusion, we have established new synthetic routes to a
monofunctional, telechelic poly(3-octylthiophene) where the
single bromine end group can be used as a functional handle in
subsequent coupling reactions. This was achieved by CTCP of
2-bromo-3-octylthienylmagnesium iodide from a phenylnickel-
(II) initiator at 0 °C, and delaying quenching of the polymeri-
zation rather than precipitation as soon as the monomer has been
consumed. Working at low temperature slows down the dis-
sociative reductive elimination of nickel from the polymer during
propagation, ensuring placement of phenyl groups at the initiated
terminus of the chains in favor of competing chain transfer
processes. The delay in quenching of the polymerization after
consumption of the monomer allows time for reductive elimina-
tion of nickel and installation of the R-bromothienyl terminal
functionality. The telechelic PhꢀP3OTꢀBr block was coupled
to a poly(5,8-quinoxaline) bearing boronic esters at each end to
5,8-Dibromo-2,3-bis(4-(octyloxy)phenyl)quinoxaline, 4a.
NaBH4 (9 g, 238 mmol) was added in four equal portions 20 min apart
to a solution of 4,7-dibromobenzo[c][1,2,5]thiadiazole (1) (6.1 g, 21
mmol) and CoCl2 6H2O (0.1 g, 0.4 mmol) in EtOH (100 mL) under
3
argon. The solution was stirred for an additional 30 min and the solvent
was removed under reduced pressure. The residue was taken up in H2O
(100 mL) and the mixture was neutralized with 10% HCl (50 mL) and
extracted with CH2Cl2 (2 ꢁ 200 mL). The organic extracts were
combined and the solvent was removed under reduced pressure to
afford 2 as a colorless solid (3.5 g). 1H NMR (300 MHz, CDCl3): δ 6.16
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ARTICLE
(s, 2H), 3.37 (br s, 4H). The crude solid is unstable in light and air and
was used immediately without further purification.
quinoxaline CꢀH), 2.79 (b, 4H, ArꢀCH2ꢀ), 1.92 (b, 2H, ꢀCH2CH-
(CH2)2ꢀ), 1.07ꢀ1.36 (m, 16H) 0.67ꢀ0.87 (m, 12H). IR (ATIR):
2954, 2921, 2854, 1580, 1460, 1377, 1308, 1205, 1172, 1094, 923, 827,
767, 726, 653, 567, 445 cmꢀ1. GPC (THF, UVꢀvis detector): 5.7 kDa;
PDI = 2.1.
A solution of 3,6-dibromobenzene-1,2-diamine (2) (3.5 g, 13 mmol)
and 3a (5.0 g, 11 mmol) in acetic acid (200 mL) was heated to reflux for
24 h. The solution was cooled to room temperature and poured into
H2O (200 mL). The mixture was filtered and the filtrate was purified by
column chromatography (30:70 v/v CH2Cl2:hexanes) to afford 4a as a
Boronate-Ester-Modified Poly(2,3-(4-octyloxyphenyl)qui-
noxaline-5,8-diyl), PQ(C8)ꢀB(OR)2. In a dry Schlenk flask under
argon, bis(pinacolato)diboron (0.4 g, 1.6 mmol), potassium acetate (0.18 g,
1.8 mmol) and Pd(dppf)Cl2 (0.06 g, 0.08 mmol) were added to a solution
of PQ(C8)Br2 (0.67 g, 0.02 mols Br end groups) in anhydrous 1,4-dioxane
(30 mL). The solution was stirred at 50 °C for 48 h, and then poured into
MeOH (100 mL). The mixture was filtered and the residue was washed
with MeOH (200 mL) to afford PQ(C8)ꢀB(OR)2 as a yellow solid (0.60 g,
90%). 1H NMR (300 MHz, CDCl3): δ 8.33 (b, 2H, quinoxaline CꢀH),
6.60 (b, 4H, phenyl CꢀH), 3.7 (b, 4H, ꢀOCH2ꢀ), 1.54ꢀ1.87 (m, 4H),
1.15ꢀ1.42 (m, 20H), 0.87 (b, 6H). IR (ATIR): 2924, 2852, 1600, 1512,
1
yellow solid (4.4 g, 56%), mp = 81ꢀ83 °C . H NMR (300 MHz,
CDCl3): δ 7.83 (s, 2H, quinioxaline CꢀH), 7.65 (d, 3JHH = 8.7 Hz, 4H,
3
phenyl C2ꢀH), 6.87 (d, JHH = 8.7 Hz, 4H, phenyl C3ꢀH), 3.99
3
(t, JHH = 6.6 Hz, 4H, ꢀOCH2ꢀ), 1.78ꢀ1.82 (m, 4H), 1.28ꢀ1.49
(m, 20H), 0.89 (t, 3JHH = 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl3):
δ 160.41 (phenyl C4), 153.53 (quinoxaline C2 and C3), 138.95
(quinoxaline C1 and C4), 132.44 (quinoxaline C6 and C7), 131.63
(phenyl C2), 130.26 (phenyl C1), 123.38 (quinoxaline C5 and C8),
114.31 (phenyl C3), 68.05 (CꢀO), 31.79, 29.33, 29.21, 29.16, 26.00,
22.63, 14.09. IR (ATIR): 2912 (Ar CꢀH str.), 2846, 1600, 1512, 1468,
1377, 1242, 1173 (CꢀO str.), 985, 821, 717, 656, 540 cmꢀ1. HRMS:
calcd for C36H44Br2N2O2 = 694.1770; obsd = 694.1767; Δ = 0.4 ppm.
Anal. Calcd: C, 62.07; H, 6.37; N, 4.02. Found: C, 61.81; H, 6.31;
N, 4.21.
1471, 1382, 1342, 1294, 1249, 1170, 1027, 979, 831, 665, 592, 538 cmꢀ1
.
Boronate-Ester-Modified Poly(2,3-(2-ethylhexyl)quinoxa-
line-5,8-diyl), PQ(EH)ꢀB(OR)2. Modification of PQ(EH)Br2
(0.52 g 0.014 mols of Br end group) was carried out according to the
procedure provided above to afford PQ(EH)ꢀB(OR)2 as a yellow solid
(0.47 g, 94%). 1H NMR (300 MHz, CDCl3): 8.14 (b, 2H, quinoxaline
CꢀH), 2.79 (b, 4H, ArꢀCH2ꢀ), 1.92 (b, 2H, ꢀC2 CH), 1.07ꢀ1.36
(m, 16H) 0.67ꢀ0.87 (m, 12H). IR (ATIR): 2954, 2921, 2854, 1580,
1460, 1377, 1308, 1248 (CꢀO str.), 1205, 1172, 1094, 923, 827, 767,
Monofunctional Telechelic Polythiophene, PhꢀP3OTꢀBr. In
a argon-filled glovebox, bromobenzene (1 mL, 1.5 g, 9.5 mmol) was added
to a solution of Ni(PPh3)4 (1.3 g, 1.17 mmol) and anhydrous toluene
(8 mL). The mixture was stirred overnight, and the solution turned from red
to yellow. The mixture was filtered and the solid was washed with toluene
(30 mL) to afford the arylnickel(II) initiator (5) as a yellow solid (0.5 g,
57%). In a separate oven-dried Schlenk flask, iPrMgCl (1 M in THF,
3.5 mL, 7.0 mmol) was added to a solution of 2-bromo-5-iodo-3-octylthio-
phene (2.8 g, 7.0 mmol) in THF (30 mL) at 0 °C. After 1 h, a solution of the
phenylnickel(II) initiator (5) (0.10 g, 0.14 mmol) in THF (2 mL) was
added to the reaction mixture. The mixture was stirred for 1 h at 0 °C and
transferredtoafreezer (ꢀ20 °C) for 12 h. Hexanes (20 mL) was added and
the mixture was poured into MeOH (100 mL). The resulting precipitate
was sequentially extracted in a Soxhlet extractor with acetone, hexanes and
chloroform to afford PhꢀP3OTꢀBr as a purple solid: the hexanes
extracted fraction (350 mg, 26%); the chloroform extracted fraction (637
726, 653, 567, 445 cmꢀ1
.
ABA Triblock Copolymer, P3OTꢀPQ(C8)ꢀP3OT. K2CO3
(2 M, aq, 5 mL) was added to a solution of PQ(C8)ꢀB(OR)2 (0.55 g)
and hexanes-soluble PhꢀP3OTꢀBr (0.61 g) in THF (20 mL), and the
mixture was degassed by freezeꢀpumpꢀthaw (2 ꢁ 15 min cycles).
Pd(PPh3)4 (0.10 g, 0.09 mmol) was added and the solution was heated
to 60 °C for 3 d. The mixture was poured into MeOH (100 mL) and
filtered. The resulting solid was sequentially extracted in a Soxhlet
extractor with MeOH, acetone, hexanes and CHCl3. The solvent from
the CHCl3 fraction was removed under reduced pressure to afford
P3OTꢀPQ(C8)ꢀP3OT as a purple solid (0.62 g, 52%). 1H NMR (300
MHz, CDCl3): δ 8.56 (br s, quinoxaline CꢀH), 7.24 (br s, thiophene
CꢀH), 6.85 (m, 4H, phenyl CꢀH), 4.03 (m, ꢀOCH2ꢀ), 3.06
(m, ThꢀCH2ꢀ), 1.61ꢀ1.76 (m), 1.10ꢀ1.41 (m), 0.73ꢀ0.89 (m). IR
(ATIR): 2924, 2854, 1604, 1509, 1467, 1384, 1342, 1294, 1240, 1170,
1112, 1025, 979, 831, 723, 663, 632, 590, 536 cmꢀ1. GPC (THF,
UVꢀvis detector): 10.3 kDa; PDI = 2.8.
1
mg, 47%). H NMR (300 MHz, CDCl3) δ 7.01 (br s, 1H), 2.74ꢀ2.89
(m, 2H), 1.71ꢀ1.82 (m, 2H), 1.32ꢀ1.44 (m, 10H), 0.89ꢀ0.91 (m, 3H).
IR (ATIR): 2922, 2850, 1662, 1563, 1509, 1456, 1377, 1055, 823, 754, 723,
661 cmꢀ1. GPC(THF, UVꢀvis detector): hexanes fraction, 2.4 kDa, PDI =
1.3; chloroform fraction, 2.9 kDa, PDI = 1.0.
ABA Triblock Copolymer, P3OTꢀPQ(EH)ꢀP3OT. Coupling
of hexanes soluble PhꢀP3OTꢀBr (0.38 g) and PQ(EH)ꢀB(OR)2
(0.46 g) was carried out according to the procedure provided above to
prepare P3OTꢀPQ(EH)ꢀP3OT and the product was isolated as a purple
solid (0.55 g, 67%). 1H NMR (300 MHz, CDCl3): δ 8.14 (bs, quinoxaline
CꢀH), 6.98 (s, thiophene CꢀH), 2.80 (m, ThꢀCH2ꢀ), 1.83ꢀ1.95 (m),
1.56ꢀ1.69 (m), 1.06ꢀ1.44 (m), 0.66ꢀ0.93 (m). IR (ATIR): 2956, 2927,
2856, 1495, 1379, 1261, 1095, 1018, 923, 800, 725, 696, 673, 622, 541,
443 cmꢀ1. GPC (THF, UVꢀvis detector): 10.0 kDa; PDI = 2.6.
Poly(2,3-(4-octyloxyphenyl)quinoxaline-5,8-diyl), PQ-
(C8)Br2. In an argon-filled glovebox, Ni(COD)2 (0.71 g, 2.6 mmol)
was added to a Shlenk flask containing a solution of dibromide 4a
(1.5 g, 2.2 mmol), 2,20-bipyridine (0.44 g, 2.8 mmol) 1,5-cycloocta-
diene (1 mL, 8 mmol) in anhydrous N,N-dimethyformamide
(25 mL). The mixture was stirred at 60 °C for 48 h, and then poured
into MeOH (100 mL). The solution was filtered and the resulting
gray precipitate was dissolved in CHCl3 (50 mL) and the solution
was stirred with 10% aqueous HCl (20 mL) for 30 min. The organic layer
was separated and then stirred with 10% KOH (20 mL) for 30 min. The
organic layer was separated and the polymer was precipitated by pouring the
solution into MeOH (200 mL). The solution was filtered and PQ(C8)Br2
was obtained as a yellow solid. 1H NMR (300 MHz, CDCl3): δ 8.33 (br s,
2H, quinoxaline CꢀH), 6.60 (b, 4H, phenyl CꢀH), 3.7 (b, 4H,
ꢀOCH2ꢀ), 1.54ꢀ1.87 (m, 4H), 1.15ꢀ1.42 (m, 20H), 0.87 (b, 6H). IR
(ATIR): 2927, 2856, 1604, 1511, 1467, 1384, 1342, 1294, 1243, 1172, 1027,
977, 831, 736, 665, 592, 540 cmꢀ1. GPC (THF, UVꢀvis detector):
4.28 kDa; PDI = 1.93.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
1
spectroscopic data, H NMR of block copolymers, IR spectra,
solid state UVꢀvisible absorption spectra, and wide-angle X-ray
diffractograms. This material is available free of charge via the
Internet at http://pubs.acs.org.
Poly(2,3-(2-ethylhexyl)quinoxaline-5,8-diyl), PQ(EH)Br2.
Polymerization of monomer 4b (2.0 g, 3.9 mmol) was carried out
according to the procedure provided above to afford PQ(EH)Br2 as a
yellow solid (1.1 g, 73%). 1H NMR (300 MHz, CDCl3): δ 8.14 (b, 2H,
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: david.collard@chemistry.gatech.edu.
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ARTICLE
’ ACKNOWLEDGMENT
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We thank the Air Force Office of Scientific Research ꢀ
Georgia Tech BIONIC program, the Georgia Tech Center for
Organic Photonics and Electronics COPE), and the ACS Divi-
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