Morphology control of selenophene-thiophene block copolymers through side chain engineering

Article abstract of DOI:10.1021/ma501231d

We report the synthesis of three fully π-conjugated diblock copolymers containing selenophene- and thiophene-based repeating units. All of these diblock copolymers undergo phase separation, and by systematically changing the compatibility of the two blocks through side chain modification, we are able to access different thin film morphologies. Introducing a bulky 2-ethylhexyl side chain increases solubility while retaining crystallinity of the selenophene block. While poly(3-hexylselenophene)-b-poly(3-hexylthiophene) and poly(3-(2-ethylhexyl)selenophene-b-poly(3-(2-ethylhexyl)thiophene) form more disordered fibrillar structures, poly(3-hexylthiophene)-b-poly(3-(2-ethylhexyl) selenophene) forms long (1-2 μm) solid state fibrillar structures that are reminiscent of the lamellae that are formed by nonconjugated block copolymers. We further use electron energy loss spectroscopy to visualize thiophene- and selenophene-rich domains at the nanometer scale in each of these examples. By studying new polymer compositions and relating them to solid state structure, we further our understanding of heterocycle induced phase separation and phase separation in general.

Full text of DOI:10.1021/ma501231d

Article
pubs.acs.org/Macromolecules
Morphology Control of Selenophene−Thiophene Block Copolymers
through Side Chain Engineering
Jon Hollinger and Dwight S. Seferos*
Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S
3H6, Canada
S
* Supporting Information
ABSTRACT: We report the synthesis of three fully π-conjugated
diblock copolymers containing selenophene- and thiophene-based
repeating units. All of these diblock copolymers undergo phase
separation, and by systematically changing the compatibility of the
two blocks through side chain modification, we are able to access
different thin film morphologies. Introducing a bulky 2-ethylhexyl
side chain increases solubility while retaining crystallinity of the
selenophene block. While poly(3-hexylselenophene)-b-poly(3-hex-
ylthiophene) and poly(3-(2-ethylhexyl)selenophene-b-poly(3-(2-
ethylhexyl)thiophene) form more disordered fibrillar structures,
poly(3-hexylthiophene)-b-poly(3-(2-ethylhexyl)selenophene) forms
long (1−2 μm) solid state fibrillar structures that are reminiscent
of the lamellae that are formed by nonconjugated block copolymers. We further use electron energy loss spectroscopy to visualize
thiophene- and selenophene-rich domains at the nanometer scale in each of these examples. By studying new polymer
compositions and relating them to solid state structure, we further our understanding of heterocycle induced phase separation
and phase separation in general.
Block copolymers of P3HT and fullerenes10,11 are successful as
INTRODUCTION
■
compatibilizing agents, for reducing interfacial tension, and
The synthesis of block copolymers is an effective strategy for
the preparation of nanopatterned materials. Experimental and
theoretical investigations on “classic” block copolymers (such as
increasing phase stability.12,13 P3HT has also been copoly-
merized with a perylene diimide-containing block by an elegant
one-pot sequential polymerization that allows for controlled
polystyrene, polymethyl acrylate, polyisoprene, etc.) have
molecular weights and narrow polymer dispersity.14 This
shown that a small number of variables determine the types
of ordered structures that will result from a given copolymer.1
polymer undergoes phase separation, and varying the volume
fraction of the two blocks results in different phase-separated
These parameters include the average degree of polymerization
of the block (Na), the volume fraction of the block in the
copolymer (fa), and the Flory interaction parameter (χ) of the
two blocks. By modifying these parameters, it is possible to
morphologies including lamellae and hexagonally packed
cylinders.15
Another approach focuses on using one conjugated and one
nonconjugated block (a rod−coil design). A number of P3HT-
access phase-separated domains on the nanometer scale that
based rod−coil copolymers have been investigated.16−18 The
vary in size (determined by N) and symmetry (determined by
the phase diagram and is dependent on χN and fa).2 Common
domain symmetries include lamellae, hexagonally packed
cylinders, body-centered cubic, and gyroids.3,4 Block copoly-
mers are useful in many applications including nanolithog-
raphy,5 ultrafiltration,6 and secondary alignment of small
molecules or nanoparticles.7,8
rod−coil design often improves the processability and
mechanical properties of the conjugated material.19 While
these materials form phase-separated domains, strong rod−rod
interactions in P3HT-containing block copolymers often result
in a phase diagram that consists predominantly of fibrillar or
lamellar structures. This is because P3HT has a short, linear,
regioregular alkyl side chain, resulting in a lamellar crystalline
packing structure, which is a strong driving force for nanofibril
formation. The strength of this interaction is quantified by the
Considering the success and utility of block copolymers,
there has been a recent emphasis to extend this strategy to π-
conjugated polymers. The prospect of controlling 10−20 nm
domains is very appealing, as these length scales are of great
importance to electronic events in organic electronic devices.9
There are a number of potential block architectures that are
particularly attractive. Because of the control afforded by the
Kumada catalyst transfer polymerization (KCTP) of poly(3-
hexylthiophene) (P3HT), almost all examples include P3HT.
́
Maier−Saupe parameter (μ) for rod−rod interactions. Many
groups have modified μ by altering the side chain to either a
Received: June 13, 2014
Revised: July 15, 2014
Published: July 24, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of Block Copolymers
branched or longer linear alkyl group.20 Atactic poly(3-(2-
ethylhexyl)thiophene) (P3EHT) incorporates a racemic
branching point in the side chain which decreases rod−rod
interactions, allowing for a larger phase space in P3EHT
copolymers containing a nonconjugated block.21,22 Several
examples of poly(3-dodecylthiophene)-block-poly(methyl
methacrylate) copolymers exhibit classical phase behavior.23−26
By fine-tuning the block copolymer structure and composition,
classical phase diagrams have been attained using P3HT blocks
as well.27
Despite the increase in processability and interesting phase
behavior, rod−coil copolymers possess an insulating block that
hinders performance in organic electronic devices.16 All-
conjugated rod−rod copolymers have been prepared to address
this issue. Rod−rod copolymers are largely limited to all-
thiophene backbones due to synthetic limitations as few
monomer types are compatible with KCTP. Some research
groups have circumvented this limitation by alternate syntheses,
typically involving end-group control and postpolymerization
techniques.28,29 Some other monomer units have been shown
to be amenable to block copolymerization directly through
pseudoliving conditions,30 and catalyst optimization will lead to
the controlled polymerization of an even wider selection of
conjugated monomers of varying structure and electronics.31,32
Largely, however, polymer chemists are limited to polythio-
phene motifs in all-conjugated block copolymers, and thus
alteration of side chain functional groups is a convenient route
to two distinct blocks. Such alterations include variation of
length33 or bulkiness34 of alkyl side chains or differences in the
chemical nature of the side chains such as polarity,35
functionality,36 or optical purity.37 This results in distinct
structural characteristics in each block and often leads to phase
separation in the solid state.
occurs adjacent to, as opposed to within, the π-conjugated main
chain. Our group has investigated the alteration of the main
chain through heteroatom substitution and the synthesis of
poly(3-alkylselenophene)-block-poly(3-alkylthiophene) (P3AS-
b-P3AT) copolymers. P3AS is the selenium containing
analogue of P3AT and differs from it in important ways.
P3AS has a narrower optical band gap than P3AT due to a
stabilization of the lowest unoccupied molecular orbital
(LUMO), which is advantageous for electronic applications.
The selenium atom is also larger and more polarizable, which
results in different crystallization behavior.38 Interactions
between selenium atoms in the solid state could also facilitate
charge transfer.39 Importantly, poly(3-hexylselenophene)-block-
poly(3-hexylthiophene) (P3HS-b-P3HT) undergoes phase
separation in the solid state.40 Gradient and statistical
copolymers have been investigated as well.41,42 This is a new
strategy for creating phase separating block copolymers,
namely, a copolymer consisting of two blocks of 5-member
heterocycles with identical side chains will undergo phase
separation if the heteroatoms are not identical. Here, we
present a study that considers the effect of both heteroatom
and side chain on the morphology of all-conjugated diblock
copolymers. We hypothesize that selecting appropriate side
chain combinations will allow us to access thin film structures
different from those achieved with identical side chains. For the
first time we will specifically address the effect of introducing a
branched side chain in P3AS-b-P3AT copolymers.
RESULTS AND DISCUSSION
■
Synthesis. Polymers were synthesized by the chain
extension of a separately formed macroinitiator using KCTP
(Scheme 1). We focus on a series of three block copolymers:
P3HS-b-P3HT, poly(3-hexylthiophene)-block-poly(3-(2-
ethylhexyl)selenophene) (P3HT-b-P3EHS), and poly(3-(2-
ethylhexyl)selenophene)-block-(3-(2-ethylhexyl)thiophene)
While the numbers of available thiophene side chains and
combinations thereof are nearly limitless, side chain alteration
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Table 1. Summary of Block Copolymer Characterization
a
a
b
c
d
polymer
Se (mol %)
S
(mol %)
GPC (NMR ) Mn (kDa)
Đ
annealed (pristine) film EG (eV)
P3HS(53)-b-P3HT(47)
P3HT(50)-b-P3EHS(50)
P3HT(40)-b-P3EHS(60)
P3EHS(53)-b-P3EHT(47)
53
50
60
47
50
40
47
20.2
1.34
1.18
1.14
1.18
1.67 (1.68)
1.77 (1.78)
1.77 (1.78)
15.7
25.9
53
32.2 (18.4)
1.75 (1.80)
a
b
c
Determined by 1H NMR spectroscopy. 1,2,4-Trichlorobenzene at 140 °C, measured against polystyrene standards. Determined by comparison of
d
o-tolyl-CH3 peak to combined α-CH2 peaks. Determined by onset of absorption in thin films.
Figure 1. Thin film absorption profiles of pristine (a) and annealed (b) films.
(P3EHS-b-P3EHT). All of these polymers were prepared with
a feed ratio that was expected to yield a 50:50 monomer ratio.
Minor discrepancies in the feed vs incorporation ratio are
unavoidable; we therefore use the notation where the values in
brackets indicate the relative molar incorporation ratios of each
monomer confirmed by integration of the characteristic
resonances of the aromatic protons on the heterocyclic
backbone units (see Supporting Information). Synthesis of
P3HS(53)-b-P3HT(47) was accomplished using Ni(dppp)Cl2
catalyst; however, when similar conditions were used for
P3EHS-b-P3EHT, additional resonances were present in the
NMR spectra in the aromatic region (see Supporting
Information). These resonances are identical to the ones
observed for statistical copolymers of 3-alkylthiophenes and 3-
alkylselenophenes. It is likely that the branch in the side chain
increases the monomer bulkiness and slows the kinetics of the
chain propagating transmetalation step.43 Thus, the solution
contained a mixture of macroinitiator and unconsumed
monomer upon the addition of the second monomer. Thus,
the second block contained a statistical distribution of the first
and second monomers. We therefore turned to a more active
catalyst, Ni(dppe)Cl2, and found that we could prepare block
copolymers without any evidence of a statistical region while
maintaining molecular weight control and narrow dispersity.
The Ni(dppe)Cl2 catalyst was used for any polymerizations
using a monomer with the 2-ethylhexyl side chain. Because the
branched side chains of P3EHS-b-P3EHT were expected to
affect the hydrodynamic radius of the resulting polymers (and
thus the calculated gel permeation chromatography (GPC)
molecular weight), we synthesized these copolymers using a
published external initiation procedure that installs an o-tolyl
group at one terminus, allowing for the determination of Mn
through NMR end-group analysis.44 We chose to use the less
soluble block as the macroinitiator in each case to avoid any
potential solubility complications upon chain extension that
could broaden the dispersity of the resultant polymers. The
initial three polymers used in this study are P3HS(53)-b-
P3HT(47), P3HT(50)-b-P3EHS(50), and P3EHS(53)-b-
P3EHT(47) (Table 1).
Polymer molecular weights were determined with GPC that
was calibrated using narrow dispersity polystyrene standards.
Dispersities are relatively low for all polymers (1.1−1.3), and
polystyrene equivalent molecular weights are very close to
expected values based on a theoretical degree of polymerization
of 100 and typical GPC overestimation of 1.2−2.3.45 End-group
analysis on P3EHS(53)-b-P3EHT(47) allowed for quantifica-
tion of the overestimation of GPC Mn by NMR. GPC
overestimation was 1.75 times the NMR molecular weight,
which is slightly larger than what is typically reported for P3HT
homopolymers with degree of polymerization of less than
100.46 This is consistent with the larger hydrodynamic radius of
the monomers with larger side chains.
Optoelectronic Properties. All of the block copolymers
are soluble in chlorobenzene, and polymer thin films were cast
from solution (8 mg/mL) onto glass substrates that had been
previously coated with poly(sodium 4-styrenesulfonate) (to
facilitate delamination, vide infra) by spin-coating (1000 rpm).
We note a considerable increase in solubility of any copolymer
which incorporates the 2-ethylhexyl side chain. This greatly
facilitates the characterization of the copolymers and aids
especially in the study of selenophene-containing copolymers
which are less soluble than their thiophene analogues. The
onset of film optical absorption was used to estimate the optical
band gap in the thin films and taken as the point at 10%
absorption of the peak maximum. The effect of thermal
annealing was examined by measuring optical absorption before
and after a 16 h annealing step in a nitrogen-filled glovebox
(Figure 1). Annealing temperatures were varied with the
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copolymer to the highest temperature that did not result in loss
of film integrity (observed as a significant blue-shift in
absorption or film disintegration upon delamination) and
noted in the text as these conditions depend on the structure of
the copolymers. P3HS(53)-b-P3HT(47) has the most narrow
optical band gap (1.68 eV), and while the annealing step (200
°C) increases the vibronic structure in the absorption profile,
the optical band gap remains relatively unchanged (1.67 eV).
The optical band gap of P3HT(50)-b-P3EHS(50) (1.78 eV) is
slightly larger than P3HS(53)-b-P3HT(47), likely due to the
backbone twisting induced by the bulkier 2-ethylhexyl side
chain. It is also relatively unchanged after annealing (200 °C).
P3EHS(53)-b-P3EHT(47) has the widest pristine optical band
gap (1.80 eV); however, upon annealing (100 °C) the optical
band gap narrows the most significantly (1.75 eV). This is
attributed to relatively large disorder of the bulky side chains
that exists immediately after spin-casting and relatively large
increase in order brought on by annealing. All three polymers
display increased vibronic fine structure after the annealing
step, suggesting that there is an increase in backbone rigidity
and interchain order. The increased vibronic fine structure
observed in the absorption profiles indicates that the block
copolymers undergo phase separation into regions rich in either
selenophene or thiophene.47
considering both the optical absorption and XRD data, it
appears that the EH side chain increases backbone twisting, and
this allows the polymer chains to pack more tightly in the edge-
on plane. The diffraction pattern for P3HT(50)-b-P3EHS(50)
contains two diffraction peaks that correspond to interlayer
distances of 15.8 and 14.1 Å. These diffraction peaks are
consistent with crystalline P3HT and P3EHS, respectively.
Interestingly, when a P3HT-b-P3EHT block copolymer was
examined in a previous study, only one diffraction peak is
observed.48 It should be noted that the relative amount of
P3EHT in the previous study was relatively low (17%) and is
the likely explanation for the lack of the expected higher angle
peak.
Differential Scanning Calorimetry. DSC was carried out
to determine thermal properties. Each polymer was subjected
to a heat−cool−heat cycle to examine both the as-cast and
post-melt behavior. While literature data exist for P3HT (Tm =
240 °C), P3HS (Tm = 110 and 250 °C),49 and P3EHT (Tm =
71 and 83 °C),20 P3EHS homopolymer was previously
unknown and was examined first. P3EHS has a melting
transition of 107 °C (see Supporting Information). P3HS(53)-
b-P3HT(47) exhibits two melting transitions in its first heating
cycle (120 and 230 °C), as we have previously reported (Figure
3).40 The transition at 120 °C corresponds to the type 2
Wide-Angle X-ray Diffraction. To probe the crystalline
nature of the samples, we turned to X-ray diffraction
experiments. Films were drop-cast from chlorobenzene solution
onto silicon substrates, annealed as described above, and 2-
dimensional diffraction patterns were collected. All polymers
have a strong diffraction peak which is strongly aligned in the
out-of-plane direction at 5°−7° (Figure 2). This is assigned as
Figure 3. DSC profiles of copolymers. First heating and cooling cycle
(top) displaced from second heating cycle (bottom).
Figure 2. Annealed thin film XRD pattern for P3HS(53)-b-P3HT(47)
and P3HT(50)-b-P3EHS(50) and P3EHS(53)-b-P3EHT(47). Inset is
representative 2D area detector image of P3HS(53)-b-P3HT(47).
polymorph of P3HS and is not present in the second heating
cycle, as this polymorph is readily converted to the type 1
polymorph upon heating. The transition at 230 °C corresponds
to the melting transition of both P3HT and P3HS (both the
type 1 polymorphs). While these two blocks have nominally
different melting transitions, these copolymers often exhibit
only a single melting peak due the proximity of the two
transitions. The thermal profile of P3HT(50)-b-P3EHS(50)
exhibits transitions at approximately 220 and 230 °C in both
heating cycles that likely both correspond to the P3HT block.
Multiple melting transitions have been reported previously for
P3HT and are attributed to a “melting−recrystallization−
melting” process.50 The absence of the P3EHS transition is
likely due to the low crystallinity of this block, in agreement
with XRD data and melting enthalpy. The melting enthalpy of
P3EHS (6.3 J/g) is notably less than that of P3HT (23.7 J/g51).
the lamellar spacing dictated by the length of the alkyl side
chains in an edge-on orientation. P3HS(53)-b-P3HT(47) has a
single sharp diffraction peak which corresponds to an interlayer
spacing (d-spacing) of 15.8 Å. This is in agreement with
previous reports.40 Only one diffraction peak is present in this
case because the d-spacings of both the P3HT and P3HS are
not sufficiently different to produce two resolved peaks.
P3EHS(53)-b-P3EHT(47) also has a single diffraction peak
which corresponds to an interlayer distance of 14.1 Å, notably
smaller than the copolymers with linear hexyl side chains. A
smaller d-spacing for the 2-ethylhexyl side chain compared to
the hexyl side chain has been reported previously, along with a
concurrent increase in π−π stacking distance.20,48 When
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To further probe this observation, we examined a more
P3EHS-rich version of this polymer [P3HT(40)-b-P3EHS-
(60)], and in this case a low temperature melting transition
(100 °C) that is assigned to the P3EHS block is observed. The
profile of P3EHS(53)-b-P3EHT(47) contains a single melting
transition at approximately 110 °C in both heating cycles which
corresponds roughly to the expected transition of the P3EHS
block. The expected transition of the P3EHT block is absent;
however, this transition is not always observed.48 Both
P3HS(53)-b-P3HT(47) and P3HT(50)-b-P3EHS(50) exhibit
a recrystallization transition during their cooling cycles;
however, no such exotherm is observed for P3EHS(53)-b-
P3EHT(47). For P3EHS(53)-b-P3EHT(47), a recrystallization
exotherm appears in the second heating cycle at 75 °C.
Electron Microscopy. To visualize the structures formed
by these polymers, we utilized transmission electron micros-
copy (TEM). For this purpose, we delaminated the films used
for absorption measurements and collected the samples onto
TEM grids. In order to facilitate delamination, we first coated
the glass substrates with a thin film of poly(sodium 4-
styrenesulfonate). Films were imaged after annealing. The
structure of P3HS(53)-b-P3HT(47) consists of nanofibrillar
crystallites that are typical of P3HT-type polymers and mostly
disorganized at larger (micrometer) scales (Figure 4).
P3EHS(53)-b-P3EHT(47) forms more continuous films of
fibrillar type structures that are broader. These fibrillar
structures are still quite disordered on larger scales.
Interestingly, P3HT(50)-b-P3EHS(50) forms long fibrillar
structures that are highly aligned on the scale of the TEM
images and reminiscent of the lamellar phase of a classical block
copolymer system. Domains persist for a minimum of 1−3 μm
and are ordered on these scales to a much greater degree than
P3HS(53)-b-P3HT(47) or P3EHS(53)-b-P3EHT(47).
This interesting difference in thin film morphology led us to
investigate other compositions of P3EHS-b-P3EHT and P3HT-
b-P3EHS. P3EHS(88)-b-P3EHT(12) forms very similar
morphologies as P3EHS(53)-b-P3EHT(47) (see Supporting
Information). P3HT(40)-b-P3EHS(60), which possesses a
slight excess of selenophene, results in material that forms
long lamellar domains similar to P3HT(50)-b-P3EHS(50) (see
Supporting Information).
We next turned to electron energy loss spectroscopy (EELS)
to examine the elemental composition of the structures
observed in TEM. Although selenium does not produce a
strong signal using EELS, sulfur does, and combining a dark
field TEM image with EELS images of sulfur is informative
(Figure 5). In the dark field image, regions rich in heavier
Figure 5. Dark field TEM (top row), EELS-sulfur image (middle row),
and composite image (bottom row) of P3HS(53)-b-P3HT(47) (left
column), P3HT(40)-b-P3EHS(60) (center column), and P3EHS(53)-
b-P3EHT(47) (right column). Bright regions in the EELS image are
sulfur-rich and dark regions are sulfur-deficient. The red regions of the
composite image are the bright regions from the dark field image, and
the blue regions are bright regions in the EELS image. Scale bars: 200
nm.
elements (selenium) appear brighter and regions with lighter
elements (sulfur) appear darker. In the case of P3HS(53)-b-
P3HT(47), the disordered fibrils that appear dark in the bright
field image appear bright in the dark field image. This suggests
the heavier selenium atom is present in this region but could
also correspond to a more crystalline P3HT region. Turning to
the EELS sulfur-selective image, we observe that the bright
regions in the dark field image correspond with dark features in
the EELS image. This confirms that these regions are sulfur-
Figure 4. Bright field TEM images of copolymers. Scale bar: 500 nm.
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Wide-angle X-ray diffraction experiments were carried out using a
Bruker D8 Discover diffractometer with a cobalt sealed tube source.
Data were collected using a Vantec 500 area detector. Differential
scanning calorimetry was performed on a TA Instruments Q100 DSC
(block copolymers) or a TA Instruments SDT Q600 TGA/DSC
(P3EHS). Bright field TEM was performed on a Hitachi H-7000
transmission electron microscope at 100 kV accelerating voltage. Dark
field TEM and EELS imaging were carried out on an FEI Tecnai 20
transmission electron microscope at 200 kV accelerating voltage.
Thin Film Preparation. Glass slides 2 × 2 cm were placed in
piranha solution for 15 min then cleaned with distilled water and dried
with a stream of nitrogen. A solution of poly(sodium 4-
styrenesulfonate) (10 mg/mL in a 1:1 vol:vol mixture of water and
ethanol) was spin-coated onto the slide (500 rpm 1.5 s, 4000 rpm 60
s) and then annealed at 130 °C for 15 min in air. A solution of
polymer (8 mg/mL in chlorobenzene) was then coated onto the slide
(500 rpm 1.5 s, 1000 rpm 60 s) and annealed in a nitrogen-filled
glovebox at the described temperature for 16 h. These films were then
used for absorption measurements. To prepare TEM samples, the
films were scored then the slide was immersed in water to delaminate
the films. The films were collected on a copper TEM grid.
Synthesis. 3-(2-Ethylhexyl)selenophene. A three-neck flask was
charged with selenium powder (3.9 g, 50.1 mmol), then evacuated,
and backfilled with nitrogen (flask 1). Ethanol (160 mL) was degassed
and added to the selenium. Sodium borohydride (3.03 g, 80.2 mmol)
was added in portions. After the solution turned clear and colorless,
the mixture was cooled in an ice bath. A solution of 3-chloromethyl-5-
ethyl-1-nonyn-3-ol (5.91 g, 31.3 mmol) in ethanol (15 mL) was
degassed and added to flask 1. The reaction was allowed to warm to
room temperature and stirred for 1.5 h. A solution of KOH (2.81 g,
50.1 mmol) was dissolved in 28 mL of ethanol with 1 mL of water.
The mixture in flask 1 was heated to reflux, and the KOH solution was
added and allowed to reflux for an additional 2 h. The reaction was
cooled in an ice bath, and 5% HCl was added. The reaction mixture
was diluted with ethyl ether, and the mixture was filtered. The liquor
was washed with acid, water, and brine, and then the solvent was
removed under reduced pressure. The resulting yellow oil was
subjected to a short silica column eluting with hexanes to yield the
final product (4.13 g, 54%). 1H NMR (CDCl3, 400 MHz): δ [ppm] =
7.90 (dd, J1 = 5.4, J2 = 2.4 Hz, 1H); 7.51 (m, 1H); 7.18 (dd, J1 = 5.2,
J2 = 1.2 Hz, 1H); 2.54 (d, J = 7.2, 2H); 1.55 (m, 1H); 1.32−1.23 (m,
8H); 0.90−0.85 (m, 6H). 13C NMR (CDCl3, 100 MHz): δ [ppm] =
144.0, 131.9, 129.4, 124.7, 40.3, 36.1, 32.6, 29.0, 25.7, 23.1, 14.2, 10.9.
HRMS-DART: calcd 245.08085; found 245.08071; Δ = 0.56 ppm.
2,5-Dibromo-3-(2-ethylhexyl)selenophene. A degassed solution of
NBS (2.30 g, 12.9 mmol) in DMF (50 mL) was added dropwise to a
degassed solution of 3-(2-ethylhexyl)selenophene (1.26 g, 5.18 mmol)
in DMF (50 mL) in an ice bath in the dark. This reaction was allowed
to warm to room temperature and stirred for 16 h. The reaction was
quenched with saturated sodium thiosulfate solution and diluted with
Et2O. The phases were separated, and the organic phase was washed
three times with water and twice with brine and dried over MgSO4.
The solvent was removed under reduced pressure to yield an orange
oil. Column chromatography (hexanes) afforded the product as a pale
yellow oil (1.5 g, 72%). 1H NMR (CDCl3, 400 MHz): δ [ppm] = 6.95
(s, 1H), 2.41 (d, J = 7.2 Hz, 1H), 1.59−1.51 (m, 1H), 1.32−1.21 (m,
8H), 0.91−0.86 (m, 6H). 13C NMR (CDCl3, 100 MHz): δ [ppm] =
144.3, 134.7, 113.5, 111.7, 40.0, 35.0, 32.5, 28.8, 25.7, 23.1, 14.2, 10.9.
HRMS-DART: calcd 400.90187; found 400.90146; Δ = 1.02 ppm.
o-Tolylmagnesium Bromide. Magnesium turnings (0.63 g, 25.9
mmol) were added to a Schlenk flask with an argon atmosphere. Dry
THF (20 mL) was added to the flask, and the mixture was cooled in
an ice bath. 2-Bromotoluene (2.2 mL, 3.15 g, 18.4 mmol) was added
slowly, and the mixture was allowed to reflux for 45 min and then
cooled to room temperature. The solution was allowed to settle, and
the solution was transferred to a sealed flask via a cannula fitted with a
filter to remove excess magnesium. The concentration of the solution
(0.8 M) was determined by titration with salicylaldehyde phenyl-
hydrazone.54
deficient and selenium-rich. Sulfur appears to be diffusely
present in the sample, but more concentrated immediately
adjacent to the dark regions, as one would expect from these
phase-separated block copolymer structures. A similar observa-
tion is made when examining both P3EHS(53)-b-P3EHT(47)
and P3HT(40)-b-P3EHS(60). The alternating bright and dark
lamellar regions present in the dark field image invert when
compared to the EELS sulfur image, confirming that the
lamellae consist of alternating selenium and sulfur rich domains.
It is notable that although all of these copolymers form similar
crystalline structures (that is, they are all similarly ordered on
the 1−2 nm scale), they do not possess the same order on the
1−2 μm scale. The origin of these differences is still not clear
but could lie in the decreased rod−rod interactions and change
in χ that is achieved in the specific case of P3HT-b-P3EHS.
We also find that the domain sizes of the features observed in
the diblock copolymers differ depending on the nature of the
side chains. The fibrils in P3EHS(53)-b-P3EHT(47) are
qualitatively wider than the domains formed in either
P3HS(53)-b-P3HT(47) or P3HT(40)-b-P3EHS(60). P3AT-
type polymers are known to align along this axis (normal to the
long axis of the fibril),52 and so higher molecular weight
polymers (as is the case with P3EHS(53)-b-P3EHT(47))
should account for these observations.
CONCLUSIONS
■
In this study, we synthesized a series of thiophene−
selenophene diblock copolymers. Modification of the side
chains of each block results in notable changes in processability,
optical properties, and most importantly thin film morphology.
Selenophene−thiophene block copolymers with identical side
chains produce thin films that are ordered on the 1−2 nm scale,
as demonstrated by WAXD peaks, but relatively disordered on
the 1−2 μm scale. Compositional differences were examined;
however, no changes were observed in morphology within the
compositions studied. By using different side chains on each
block, the 1−2 nm scale order is maintained, and a much more
ordered lamellar structure is observed on the 1−2 μm scale.
The unstained images of these films are striking and elemental
mapping experiments allow us to concretely determine where
each domain is present in the film. Overall these polymers are
important for advancing our understanding of phase separation
and by further changing the composition we expect to access
additional structures with these systems. Experiments are
underway to thoroughly explore the complete phase diagram
of these copolymers across a broader compositional range and
confirm the long-range order observed in TEM imaging
through more advanced X-ray scattering experiments.
EXPERIMENTAL SECTION
■
General Considerations. All reagents were used as received
unless otherwise specified. 2,5-Dibromo-3-hexylselenophene,42 2,5-
dibromo-3(2-ethylhexyl)thiophene,20 and 3-chloromethyl-5-ethyl-1-
nonyn-3-ol53 were synthesized as previously reported. 2,5-Dibromo-
3-hexylthiophene, isopropylmagnesium chloride (2.0 M solution in
THF), Ni(dppp)Cl2, Ni(dppe)Cl2, 2-bromotoluene, and N-bromo-
succinimide (NBS) were purchased from Sigma-Aldrich. Solvents were
dried using an Innovative Technology solvent purification system.
Instrumentation. 1H NMR was performed using a Varian
Mercury 400 (400 MHz) spectrometer. GPC measurements were
carried out using a Malvern 350 HT-GPC system at 140 °C with 1,2,4-
trichlorobenzene stabilized with butylated hydroxytoluene and
calibrated with narrow dispersity polystyrene standards. Absorption
spectra were recorded using a Varian Cary 5000 spectrophotometer.
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Article
Polymer Synthesis. P3HS(53)-b-P3HT(47). 2,5-Dibromo-3-hex-
ylselenophene (178 mg, 0.477 mmol) was loaded into a Schlenk flask
placed under vacuum for 20 min. Dry THF (3.4 mL) was added
followed by i-PrMgCl (0.23 mL, 0.46 mmol) solution at room
temperature. In a separate flask, 2,5-dibromo-3-hexylthiophene (153
mg, 0.469 mmol) was prepared in the same manner. After 1 h, the 2,5-
dibromo-3-hexylselenophene/i-PrMgCl solution was added to a
separate flask containing Ni(dppp)Cl2 (5.1 mg, 0.0094 mmol) and
heated to 40 °C. The solution turned bright orange, and the reaction
was allowed to continue for 15 min, at which time the thiophene
solution (which had also reacted with the i-PrMgCl for 1 h) was added
slowly to the polymerization flask and allowed to react for an
additional 15 min. The reaction was quenched with 5% HCl and
precipitated in methanol, filtered through a Soxhlet thimble, and
extracted with methanol, hexanes, and chloroform. The chloroform
fraction was collected and concentrated and then passed through a
short silica column using chloroform. The solvent was removed under
reduced pressure to yield the polymer as a purple solid (113 mg, 63%).
1H NMR (CDCl3, 400 MHz): δ [ppm] = 7.12 (s, 0.53 H), 6.98 (s,
0.47 H), 2.80 (t, J = 8 Hz, 1H), 2.73 (t, J = 8 Hz, 1H), 1.75−1.65 (m,
2H), 1.47−1.39 (m, 2H), 1.39−1.30 (m, 4H), 0.95−0.88 (m, 3H).
GPC (1,2,4-trichlorobenzene, 140 °C): Mn = 20.2 kDa, Mw = 27.1
kDa, Đ = 1.34.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra, GPC elution profiles, additional absorption
profiles, two-dimensional XRD patterns, additional DSC
profiles, and TEM images. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail dseferos@chem.utoronto.ca (D.S.S.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSERC, the CFI, The Ontario
Research Fund, DuPont, and the Alfred P. Sloan Foundation
(for a Research Fellowship in Chemistry). The authors also
thank Dr. Jim Britten and Victoria Jarvis for assistance with
WAXD measurements, Nancy Dou and Christine Allen for
assistance with DSC measurements, and Doug Holmyard for
assistance with EELS imaging.
P3HT(50)-b-P3EHS(50). This polymer was prepared and isolated in
an analogous manner as P3HS(53)-b-P3HT(47), using Ni(dppe)Cl2
(1 mol %), to yield a purple solid (99 mg, 48%). 1H NMR (CDCl3): δ
[ppm] = 7.09 (s, 0.5H), 6.98 (s, 0.5H), 2.81 (t, J = 8 Hz, 1H), 2.66 (d,
J = 7.2 Hz, 1H), 1.75−1.66 (m, 1.5H), 1.48−1.25 (m, 7.5H), 0.95−
0.86 (m, 4.5H). GPC (1,2,4-trichlorobenzene, 140 °C): Mn = 15.7
kDa, Mw = 18.5 kDa, Đ = 1.18.
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