Engineering topochemical polymerizations using block copolymer templates

Article abstract of DOI:10.1021/ja507318u

With the aim to achieve rapid and efficient topochemical polymerizations in the solid state, via solution-based processing of thin films, we report the integration of a diphenyldiacetylene monomer and a poly(styrene-b-acrylic acid) block copolymer templat

Full text of DOI:10.1021/ja507318u

Article
pubs.acs.org/JACS
Engineering Topochemical Polymerizations Using Block Copolymer
Templates
Liangliang Zhu,† Helen Tran,† Frederick L. Beyer,‡ Scott D. Walck,‡ Xin Li,§ Hans Ågren,§
Kato L. Killops,*,∥ and Luis M. Campos*,†
†Department of Chemistry, Columbia University, New York, New York 10027, United States
‡Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States
§Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm,
Sweden
∥Edgewood Chemical Biological Center, Aberdeen Proving Ground, Maryland 21010, United States
S
* Supporting Information
ABSTRACT: With the aim to achieve rapid and efficient
topochemical polymerizations in the solid state, via solution-based
processing of thin films, we report the integration of a
diphenyldiacetylene monomer and a poly(styrene-b-acrylic acid)
block copolymer template for the generation of supramolecular
architectural photopolymerizable materials. This strategy takes
advantage of non-covalent interactions to template a topochemical
photopolymerization that yields a polydiphenyldiacetylene
(PDPDA) derivative. In thin films, it was found that hierarchical
self-assembly of the diacetylene monomers by microphase segregation of the block copolymer template enhances the
topochemical photopolymerization, which is complete within a 20 s exposure to UV light. Moreover, UV-active cross-linkable
groups were incorporated within the block copolymer template to create micropatterns of PDPDA by photolithography, in the
same step as the polymerization reaction. The materials design and processing may find potential uses in the microfabrication of
sensors and other important areas that benefit from solution-based processing of flexible conjugated materials.
growing interest for these materials to be exploited in excitonic
photovoltaic devices (polydiacetylene undergoes singlet
fission),23 sensors,24,25 bioelectronic materials,26 and other
optoelectronic devices,27 the past decade has witnessed
increased interest in controlling the polymerization of DAs.
The key strategies have been based on grafting DA monomers
on surfaces to form monolayers that are well-aligned17 and
introducing functional groups on the DA monomers to control
their aggregation and crystallization, followed by thermal or
photochemical polymerization.18,28 The reactivity of such
constructs is usually sluggish, where polymerization proceeds
within minutes to hours by heating or UV light.29,30 Recently,
Shimizu and co-workers have shown that DA-containing
macrocycles can crystallize with the appropriate arrangement
for polymerization, though the reaction takes 3 h with
heating.31,32 Exploiting biomimetic supramolecular interactions,
Tovar and co-workers found that the arrangement of
diphenyldiacetylene (DPDA) monomers using oligopeptides
can guide the supramolecular assembly, where photopolyme-
rization occurs within 30 min to yield polydiphenyldiacetylene
(PDPDA).33 In all these cases, the polymerizations are system-
specific and not amenable for thin-film processing. Solution-
INTRODUCTION
■
Chemical transformations in constrained media have enabled
grand technological advancements and a deep fundamental
understanding of reaction mechanisms.1−4 While unimolecular
processes have less stringent requirements to engineer reactivity
in the solid state,5−9 bimolecular and polymerization reactions
can be challenging.10−13 In the latter case, preorganization of
the reactive components is crucial, where bond-making and
bond-breaking must occur with minimal atom displacement. To
align the building blocks, these topochemical transformations
are generally carried out in highly ordered systems, such as
molecular cages,14 monolayers,15,16 nanoporous materials,17
and crystals.18−20 The ability to template topochemical
reactions in amorphous media (such as polymers), however,
has received little attention due to the lack of control of
intermolecular alignment of the reactive species.3 Interestingly,
block copolymers (BCPs) can lead to ordered phases where
appropriate functionalization yields close packing arrange-
ments.21 Thus, we sought to exploit the use of side-chain
supramolecular BCP templates to investigate chemical
reactivity in constrained media.
The seminal work by Wegner on the topochemical
polymerization of diacetylene (DA) demonstrated that its
molecular orientation was essential to react by 1,4-addition to
yield polydiacetylene, a conjugated polymer.22 With the
Received: July 18, 2014
Published: September 11, 2014
© 2014 American Chemical Society
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Figure 1. Chemical structures of the block copolymer PS-b-PAA, the monomer IDA, and the corresponding polydiacetylene after the BCP-
templated photopolymerization of the supramolecular ensemble PS-b-(PAA-sg-IDA). Note: this is a simplified representation because it is expected
that the monomers are randomly distributed along the PAA backbone at IDA:AA ratios less than 1.
based processing by standard techniques with these systems
before and after polymerization is difficult due to the lack of
molecular preorganization required for the reaction to proceed
and the insolubility of the resulting polymers. Therefore, the
development of processable and modular solid-state templates
for topochemical polymerizations of DAs that do not rely on
unpredictable crystal engineering strategies, with fast reactivity
(within seconds), are highly desired.10
To tackle the various challenges associated with obtaining
solution-processable thin films of polydiacetylene by conven-
tional techniques, we employ a BCP34−37 template-based
approach that drastically enhances the topochemical photo-
polymerization.38 Additionally, strongly phase segregating
BCPs,39 such as polystyrene-b-poly(acrylic acid) (PS-b-PAA)
templates can be chemically modified to access micropatterns
by photolithography, which may be beneficial for the
fabrication of multiple devices on a single wafer. Figure 1
shows the supramolecular grafting strategy that exploits strong
Figure 2. Tuning of the packing interactions by varying the molar
ratio. Partial H NMR spectra (400 MHz, DMSO-d6, 298 K) of (A)
IDA only and PS-b-P(AA-sg-IDA) at R = (B) 0.1, (C) 0.2, (D) 0.5, (E)
0.75, (F) 1.0, and (G) 1.5. R = moles of the IDA monomer relative to
1
hydrogen-bonding interactions between the imidazolyl diphen-
yl-diacetylene (IDA) monomer40−42 and PS-b-PAA. It must be
moles of the acrylic acid units.
noted that the figure is a simplified representation for the
templating strategy. It is not expected that single BCP chains
(ranging from 0.1 to 1.5) was investigated by proton nuclear
will template single-chain polymerizations of IDA. The
magnetic resonance spectroscopy (1H NMR, Figure 2).
As expected, the imidazolyl proton signals broaden upon
hydrogen bonding and reach a maximal downfield shift at R =
0.2. By increasing R, the imidazolyl proton signals shift upfield.
supramolecular complex of these two systems is termed PS-b-
P(AA-sg-IDA), where sg stands for supramolecular graft of the
IDA monomers to the PAA block. This approach takes
advantage of the fact that (1) the DPDA moiety has a larger
molar extinction coefficient compared to a conventional DA,
(2) noncovalent interactions provide modularity to tune the
monomer loading ratios, (3) the complexes can be processed
from solution by spin-coating, and (4) microphase segregation
We attribute these changes to weakened hydrogen bonding
interactions between the IDA monomer and PAA, which may
be due to steric crowding at high loading.21 Thus, we focused
on loading ratios of R = 0.2 and 0.4 for optimal interactions
between the IDA monomers and PAA.
To gauge the importance of the BCP template, photo-
polymerization of the IDA monomer in solution was studied
of the side-chain functional BCPs induces local order that
enhances the topochemical polymerization.21,34,43
without PS-b-PAA. The IDA monomer was dissolved in
chloroform and irradiated with 254 nm for 20 min. Linear
RESULTS AND DISCUSSION
■
The supramolecular strategy allows for facile tuning of the
packing interactions of the IDA monomer by simply varying the
molar ratio (R) of the IDA monomer relative to the acrylic acid
units along the BCP backbone. The IDA monomer is
synthesized in only four steps, and PS-b-PAA, which is also
commercially available, can be prepared by established
procedures.21 To evaluate the appropriate loading ratio of the
monomer, the hydrogen bonding interaction between PS-b-
PAA and the imidazole group of IDA at various R values
absorption and photoluminescence spectra were collected for
solutions before and after irradiation (Figure 3A, D). The
absorption maxima at 321 and 344 nm, and the emission
maxima at 409 and 433 nm, originate from the IDA
chromophore. Prolonged irradiation to the solutions resulted
in minor spectral changes to both the absorption and
photoluminescence spectra, which is indicative that the
photopolymerization of the free IDA monomer does not
proceed appreciably in solution (Figure 3A,D).
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Figure 3. BCP template-based photopolymerization in solution. (A) Absorption spectra of a solution of IDA monomer (dashed line) without
irradiation and (solid line) after irradiation at 254 nm for 20 min. (B) Absorption spectra of a solution of PS-b-P(AA-sg-IDAR=0.2) (dashed line)
without irradiation and (solid line) after irradiation at 254 nm for 20 min and (C) corresponding image of the solutions under ambient light. (D)
Emission spectra (λex = 365 nm) of the IDA monomer (dashed line) without irradiation and (solid line) after irradiation at 254 nm for 20 min. (E)
Emission spectra (λex = 365 nm) of a solution of PS-b-P(AA-sg-IDAR=0.2) (dashed line) without irradiation and (solid line) after irradiation at 254 nm
for 20 min and (F) corresponding image of the solutions under a UV light. Spectra were collected at 40 μM of IDA in chloroform at 298 K.
In contrast to the free IDA monomer studies, a reaction of
PS-b-P(AA-sg-IDAR=0.2) in solution was observed. This may be
analogous to other solution-polymerization studies of DAs,
where preassembly of liposomes is required for polymerization
to occur44 or simply from hydrogen-bonded IDA mesogens to
the BCPs. But given the observed color changes, we postulate
that the yellow color arises from a solvated polydiacetylene, as
it has been previously shown in other studies,45 in addition to
unreacted IDA and oligomers in the mixture. In the infrared
spectrum of PS-b-P(AA-sg-PDPDA), the 2208 cm−1 vibration
appeared due to increased conjugation length along the
alternating alkenes and alkynes after photopolymerization.
The band at 1630 cm−1 became broader by overlapping with
the band from the double bonds. (Figure S2, Supporting
Information).45,46 Also, the absorption bands of the IDA
monomer in PS-b-P(AA-sg-IDAR=0.2) showed noticeable
changes after 20 min of irradiation (Figure 3B)28,46 with an
apparent color change from colorless to pale yellow (Figure
3C). This spectral shift has been observed previously in
materials with similar DPDA moieties45,46 and results from
relatively low degrees of polymerization (or oligomerization)
coupled with effects from the substituents of DPDA.47 The
most notable change was observed in the photoluminescence
spectra, where the 420 nm peak corresponding to the IDA
monomer diminished, but did not disappear, after 20 min of
irradiation, concomitantly with the appearance of an emission
peak near 520 nm corresponding to contributions from
PDPDA (Figure 3E, and Figure S3, Supporting Information).
Similarly, a change in photoluminescence was observed with λex
= 365 nm (Figure 3F). Theoretical calculations confirm that the
optical transitions of PDPDA are lower in energy than the free
IDA monomer and correlate well with the experimentally
observed band shift in the simulated absorption spectra
(Figures S4 and S5; Tables S1 and S2, Supporting
Information). Both absorption and emission spectral variations
became saturated after 20 min (Figure S3, Supporting
Information). Again, the low photoluminescence peak intensity
of PDPDA at 520 nm can be attributed to low degrees of
polymerization, given that this system is disordered in solution.
Attempts to isolate PDPDA were unsuccessful because of its
similar solubility to that of BCP and unreacted IDA monomers.
Nonetheless, we postulate that the BCP template led to some
alignment for the topochemical reaction of the DA to proceed
in solution, though not sufficient for efficient polymerization.
Considering that PS-b-P(AA-sg-IDAR=0.2) is solution-proc-
essable, and that microphase segregation of BCPs can enhance
molecular alignment through ordered domains,21,34,43 the
photopolymerization was studied in thin films. In addition to
organizing the IDA monomers through hydrogen bonding to
the PAA backbone, the PS-b-PAA template imparts hierarchical
order of the IDA monomers through BCP microphase
segregation in the solid-state thin films, which is induced by
solvent vapor annealing.47,48 An atomic force microscope
(AFM) image of an as-cast film of PS-b-P(AA-sg-IDAR=0.2
)
displays a featureless surface with roughness ca. 15 nm (Figure
4A). The solvent vapor annealing process was optimized and
exposure of the films to DMF for 36 h led to smoother films
(roughness ca. 2 nm, compare height scale bar), with a dot-like
nanopattern topography resulting from the microphase
segregation of PS-b-P(AA-sg-IDAR=0.2) (Figure 4B). Interest-
ingly, a higher loading ratio (R = 0.4) had a subtle enhancement
on the packing uniformity of the nanostructure after solvent
vapor annealing (Figure 4C,D). Microphase separation of the
PS and PAA blocks into roughly spherical domains was
observed via high-angle annular dark-field scanning trans-
mission electron microscopy (HAADF-STEM) in both the as-
cast materials and the annealed materials (Figure 4E, and
Figure S6, Supporting Information). Resolved regions of the
lamellar domains attributed to the PDPDA stacks are circled,
which are similar to previously reported systems (see GIXRD in
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Figure 5. BCP template-based photopolymerization in solid-state thin
films. (A) Emission spectra (λex = 365 nm) of a thin film of PS-b-
P(AA-sg-IDAR=0.2) (dashed line) without irradiation and (solid line)
after irradiation at 254 nm for 20 s. The solvent-annealed films
(orange) exhibit a higher emission intensity after irradiation when
compared to the as-cast films (blue). (B) Time-resolved plots of the
intensity at 520 nm (which corresponds to the PDPDA polymer)
relative to 420 nm (which corresponds to the IDA monomer) for as-
cast films (blue square) and solvent-annealed films (orange circle) of
PS-b-(PAA-sg-IDAR=0.2). (C) Emission spectra (λex = 365 nm) of a thin
film of IDA monomer only (dashed line) without irradiation and
(solid line) after irradiation at 254 nm for 20 s.
Figure 4. Thin-film self-assembly characterization: AFM topography
images of PS-b-P(AA-sg-IDAR=0.2) for an (A) as-cast and (B) solvent-
annealed film; AFM topography images of PS-b-P(AA-sg-IDAR=0.4) for
an (C) as-cast and (D) solvent-annealed film. (E) HAADF-STEM
image of a solvent-annealed film of PS-b-P(AA-sg-IDAR=0.4) after
irradiation, where the dark spherical regions correspond to the PS
domains. Examples of lamellar crystalline domains are circled. Scale
bars are 200 nm and (E) 50 nm.
Figure S6a,b, Supporting Information).21,49−51 The lamellar
crystallite features were only rarely observed in the as-cast
sample, indicating that improved organization of the side chains
upon annealing facilitated formation of PDPDA lamellar stacks
(see Figure S6c−f, Supporting Information).
The relative intensity of the 520 nm emission peak
corresponding to PDPDA is higher for the solvent-annealed
films than the as-cast films (Figure 5A, solid lines). We
postulate that BCP microphase segregation further aligns the
IDA monomers (Figure 4, and Figure S7, Supporting
Information) and enhances the topochemical polymerization,
leading to a greater extent of polymerization and resulting in an
increased PDPDA emission peak intensity. These differences
are more apparent when monitoring the intensity of PDPDA
peak relative to emission peak of the IDA monomer with
reaction time (I520/I420, Figure 5B, Figure S7, Supporting
Information). From this data, the relative photoconversion
efficiency can be roughly calculated to be 73% and 80% for the
as-cast film and a solvent-annealed film, respectively. As a
control experiment, the emission of a thin film of free, non-
templated IDA monomer was completely quenched and no
spectral changes were observed upon irradiation (Figure 5C).
These results indicate that the IDA monomer can be rapidly
polymerized to PDPDA using a BCP-templated self-assembled
morphology in the solid state, which induces the appropriate
alignment of IDA monomers.
Relative to templating in solution, significant enhancement of
the topochemical polymerization was expected in thin films,
where the hierarchical BCP microphase segregation can serve as
a handle to align the IDA monomers. In fact, the photo-
polymerization of PS-b-(PAA-sg-IDA) in both the as-cast and
solvent-annealed thin films was over one order of magnitude
faster than in solution. The reaction proceeded within 20 s of
UV irradiation, which is the fastest externally triggered reaction
time.28−30,52 Such fast time scales are particularly attractive for
high-throughput processing. As compared to the solution
reaction (Figure 3E), the photoluminescence spectra of both
the as-cast and solvent-annealed thin films display a 420 nm
peak corresponding to the IDA monomer that diminished
concomitantly with the appearance of a pronounced emission
peak near 520 nm, corresponding to PDPDA (Figure 5A).
Notably, the intensity of the 520 nm peak is more intense for
the thin films, relative to solution, which suggests that the solid-
state topochemical photopolymerization of the IDA monomers
proceeded to higher degrees of polymerization.
Given the fast and efficient photopolymerization observed
with this BCP-templated system, we sought to investigate
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photopatterning strategies to generate hierarchical shapes at the
micrometer length scale. Thin films of PS-b-P(AA-sg-IDA) are
soluble in common solvents. Hence, 4-bromostyrene (BrS) was
incorporated, yielding P(S-co-BrS)-b-PAA, which simultane-
ously cross-links during photopolymerization at 254 nm
(Figure 6A,B).53,54 Solvent-annealed thin films of P(S-co-
Exploiting such rapid reactivity, we fabricated micropatterns
by photolithography through the simultaneous cross-linking of
4-bromostyrene-functional BCP thin films during photo-
polymerization. Future studies will be geared toward exploiting
this templated supramolecular grafting strategy, which could be
useful in other topochemical reactions that require highly
uniform molecular alignment.
EXPERIMENTAL SECTION
■
1
General. H NMR and 13C NMR spectra were measured on a
Bruker 400 MHz spectrometer. The fast atom bombardment (FAB)
mass spectra and high-resolution mass spectrometry (HR-MS) were
recorded on a JMS-HX110 HF mass spectrometer (ionization mode:
FAB+). Absorption spectra were recorded on a Shimadzu 1800
spectrophotometer, and the photoluminescence emission spectra were
collected with a Jobin Yvon Fluorolog-3 spectrofluorometer (Model
FL-TAU3). The photoirradiation was carried on PL Series compact
UV lamp (4 W) with an irradiation wavelength of 254 nm. The
distance between the lamp and the sample was kept within 3−5 cm.
Argon bubbling was applied upon the irradiation of those solution
samples. Infrared spectra were obtained using a PerkinElmer
Spectrum400 FTIR spectrometer using a PIKE ATR attachment.
The atomic force microscopy images were collected on a Park System
PSI XE100 atomic force microscope. Thermal gravimetric analysis was
conducted with a TA Instruments Q50. The thickness of thin films was
determined with a Rudolph EL III ellipsometer. Transmission electron
microscopy was performed on a JEOL JEM-2100F field emission TEM
at 200 kV. High-angle annular dark-field (HAADF) STEM images
were collected using a Gatan Model 806 HAADF-STEM detector with
a 40 μm condenser aperture, 2 cm camera length, and 0.5 nm spot size.
Data analysis was performed using Gatan Digital Micrograph 3
software. The fluorescence images were captured by a LEICA TCS
SP5 confocal microscope.
Figure 6. Photopatterning experiments. (A) Solvent-annealed thin
films of P(S-co-BrS)-b-P(AA-sg-IDA) are aligned with a shadow mask
and cross-linked with 254 nm. (B) After a rinse and reannealing step,
hierarchical patterns are achieved, (C) as observed by confocal
microscopy in the 510−530 nm channel. Scale bar is 500 μm.
BrS)-b-P(AA-sg-IDA) exhibit comparable microphase-segre-
gated nanostructures as PS-b-P(AA-sg-IDA) (Figure S8,
Supporting Information). Through standard shadow-mask
photolithography, micropatterns of the hierarchical solvent-
annealed thin films of P(S-co-BrS)-b-(PAA-sg-PDPDA) were
fabricated and visualized using confocal microscopy in the
510−530 nm channel, which corresponds to emission from
PDPDA (Figure 6C, and Figure S9, Supporting Information).
The faint background fluorescence in Figure 6C may arise from
light leakage through the shadow mask, because this process
was carried out on a benchtop setup without the traditional
automated mask aligner. Moreover, confocal microscopy
imaging in the 460−480 nm channel shows that the IDA
monomer is readily consumed, and the faint signal can be
attributed to contributions from the PDPDA and the IDA
monomer. In this system, the combination of cross-linkable
templates and self-assembly has enabled us to fabricate thin
films of micropatterned PDPDA, a strategy that could be useful
for the fabrication of microarrays of conjugated photo-
luminescent materials.
See Supporting Information for the scheme pertaining to the
compound labels below.
Synthesis of Compound 1. Compound 1 was prepared according
to a similar procedure described in the literature.55
Synthesis of Compound 2. A solution of hexyl bromide (1.05 g,
6.36 mmol) in anhydrous acetone (5 mL) was added dropwise into a
mixture of compound 1 (1.5 g, 6.41 mmol) and potassium carbonate
(0.8 g, 5.8 mmol) in anhydrous acetone (15 mL) at 60 °C. The
mixture was stirred for 12 h at 60 °C under Ar. The solvent was
removed in vacuo, and the residue was purified by silica gel
chromatography (hexane/ethyl acetate = 6:1) to afford gray
1
compound 2 (1.02 g, 50.4%). H NMR (400 MHz, CDCl3, 298 K):
δ = 7.45 (d, J = 9.2 Hz, 2H), 7.42 (d, J = 8.8 Hz, 2H), 6.84 (d, J = 8.8
Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H), 3.97 (t, J = 6.4 Hz, 2H), 1.79 (m,
2H), 1.46 (m, 2H), 1.35 (m, 4H), 0.91 (t, J = 7.2 Hz, 3H). 13C NMR
(100 MHz, CDCl3, 298 K): δ = 159.91, 156.32, 134.29, 134.06,
115.66, 114.68, 114.36, 113.62, 81,43, 81.00, 73.01, 72.82, 68.25,
31.57, 29.18, 25.76, 22.60, 14.04. MS (FAB+): calcd for [M]+ m/z =
318.2, found m/z: 318.3; HR-MS (FAB+): calcd for C22H22O2 [M]+
m/z = 318.1620, found m/z: 318.1619.
Synthesis of Compound 3. Compound 2 (0.6 g, 1.89 mmol) was
added to 1,6-dibromohexane (4.5 g, 18.4 mmol) in acetone (5 mL).
Potassium carbonate (520 mg, 3.77 mmol) was added and the mixture
refluxed for 6 h under Ar protection. The solution was filtered, and the
filtrate was precipitated in a 4-fold volume of hexane. The solid was
filtered, washed with petroleum (30 mL) and deionized water (20
mL), and dried under vacuum to obtain gray compound 3 (737 mg,
81.1%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 7.44 (d, J = 8.0 Hz,
4H), 6.83 (d, J = 7.6 Hz, 4H), 3.97 (m, 4H), 3.42 (t, J = 6.8 Hz, 2H),
1.90 (m, 2H), 1.81 (m, 4H), 1.50 (m, 6H), 1.34 (m, 4H), 0.91 (t, J =
6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3, 298 K): δ = 159.89,
159.74, 134.04, 134.04, 114.66, 114.64, 113.85, 113.67, 81,38, 81.27,
72.97, 72.89, 68.17, 67.86, 33.79, 32.66, 31.56, 29.13, 28.99, 27.91,
25.69, 25.28, 22.61, 14.03. MS (FAB+): calcd for [M]+ m/z = 480.2
(79Br), found m/z: 480.2 (79Br); HR-MS (FAB+): calcd for
C28H33O279Br [M]+ m/z = 480.1664, found m/z: 480.1668.
CONCLUSION
■
The BCP-templated strategy described here has enabled us to
efficiently drive the topochemical polymerization of a diphenyl
diacetylene derivative (IDA) in solution and thin films. It is
important to note that common strategies rely on crystal
engineering and/or stringent supramolecular recognition
design, which can be challenging to control in a predictable
fashion. The viability of our approach is rooted not only in
processability and modularity but also on the facile synthesis of
the materials. Hydrogen-bonding interactions between imida-
zole on the IDA monomer and the acrylic acid on the BCP
template provided effective molecular alignment that led to
rapid photochemical polymerization to synthesize a supra-
molecularly grafted PDPDA. Moreover, the microphase-
segregated structure arising from the BCP further improves
the organization of the IDA monomers, which led to the solid-
state, BCP-templated polymerization. Notably, the topochem-
ical reaction can be completed within tens of seconds.
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Synthesis of IDA. To a mixture of compound 3 (650 mg, 1.35
mmol) and potassium hydroxide (151 mg, 2.78 mmol) in acetonitrile
(6 mL) was added imidazole (645 mg, 9.48 mmol). The reaction
mixture was refluxed for 8 h and then cooled to room temperature.
After flash column chromatography (ethyl acetate/methanol = 25:2),
the crude product was washed with deionized water (15 mL) and
dried under vacuum to obtain pure white compound IDA (255 mg,
the Army Research Office under award number W911NF-12-1-
0252. The self-assembly characterization was funded by an NSF
CAREER grant (DMR-1351293). H.T. thanks the Department
of Defense (DOD) for the National Defense Science &
Engineering Graduate (NDSEG) Fellowship. K.L.K. thanks the
Army Basic Research Program for funding. We thank Dr. J. Xia
and Dr. S. Wei for helpful discussions and D. Hanifi for the
assistance with the GISAXS studies.
1
40.4%). H NMR (400 MHz, CDCl3, 298 K): δ = 7.46 (s, 1H), 7.44
(d, J = 8.4 Hz, 4H), 7.06 (s, 1H), 6.91 (s, 1H), 6.84 (d, J = 8.8 Hz,
2H), 6.81 (d, J = 8.4 Hz, 2H), 3.96 (m, 6H), 1.78 (m, 6H), 1.47 (m,
4H), 1.34 (m, 6H), 0.91 (t, J = 6.8 Hz, 3H). 13C NMR (100 MHz,
CDCl3, 298 K): δ = 159.90, 159.67, 137.10, 134.04, 134.04, 129.50,
118.77, 114.67, 114.62, 113.91, 113.65, 81,42, 81.21, 73.01, 72.87,
68.16, 67.72, 46.94, 31.56, 31.03, 29.12, 28.97, 26.34, 25.69, 25.62,
22.60, 14.03. MS (FAB+): calcd for [M + H]+ m/z = 469.3, found m/
z: 469.4; HR-MS (FAB+): calcd for C31H37O2N2 [M + H]+ m/z =
469.2855, found m/z: 469.2860.
Synthesis of PS-b-PAA and P(S-co-BrS)-b-PAA. PS(7K)-b-PAA-
(8.1K) was synthesized by reversible addition−fragmentation chain
transfer (RAFT) according to previously reported literature.21
Likewise, P[S(5.4K)-co-BrS(0.7K)]-b-PAA(8.1K) was synthesized by
RAFT with 10 mol % incorporation of BrS.
Computational Details. Theoretical calculations were carried out
using the Gaussian 09 program package.56 The geometries of the
monomer model and the diacetylene model were optimized by density
functional theory (DFT) calculations, using the hybrid B3LYP
functional57 and the 6-31G* basis set.58 At the optimized geometries,
time-dependent (TD) DFT calculations were performed using the
range-separated hybrid CAM-B3LYP functional,59 with solvent effects
of chloroform taken into account by the polarizable continuum model
(PCM).60
Thin-Film Preparation. Solutions of PS-b-P(AA-sg-IDA) (6 mM in
dioxane) were spin-coated (2500 rpm, 45 s) on silicon chips. The
thicknesses of the films were 60−70 nm. The chips were inserted into
the cuvette holder with an angle about ∼45° to the incident light for
the optical tests.
Solvent Annealing. Solvent vapor annealing was optimized by
varying the solvent (toluene, benzene, chloroform, acetone, THF,
dioxane, and DMF) and annealing time (0.5 h, 1 h, 2 h, 4 h, 12 h, 36 h,
and 72 h). Thin films were analyzed by AFM. We found that the
optimal solvent vapor annealing conditions for our system involves
annealing in a 200 mL closed jar with DMF vapor for 36 h.
Photopatterning. A quartz mask with micron-sized chromium
patterns was aligned to the thin films in a nitrogen atmosphere.
Irradiation at 254 nm for 20 s afforded the cross-linked thin films and
polymerized the IDA monomers. Then the films were washed with
dioxane 3× and dried with a nitrogen stream. The micropatterns were
imaged with confocal microscopy, using different wavelength channels,
where the intensity of each channel was read under the same gain.
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AUTHOR INFORMATION
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Corresponding Authors
kathryn.l.killops.civ@mail.mil
lcampos@columbia.edu
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This research was funded by the Department of the Army Basic
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