Diyne-functionalized fullerene self-assembly for thin film solid-state polymerization

Article abstract of DOI:10.1021/ma401677v

C₆₀ fullerene derivatives bearing aliphatic chains can self-assemble into versatile supramolecular structures. Cross-linking of such self-assembled morphologies is an attractive approach to enhance the structural stability of these self-organized structures. We describe the synthesis of a C₆₀ functionalized with a single alkyl chain bearing a diacetylene moiety. In a thin film, the molecule self-assembles into lamellar arrays. The character of the side chain attached to the fullerene is key to the observed packing ability. The stabilization proceeds through solid-state polymerization of the diacetylene moieties. By blending the fullerene derivative with a cyanine dye, various nanostructured fullerene morphologies are obtained that can be selectively stabilized by thermal polymerization. These films can serve as basis for nanostructured fullerene scaffolds that can find applications in optics and electronics.

Full text of DOI:10.1021/ma401677v

Article
pubs.acs.org/Macromolecules
Diyne-Functionalized Fullerene Self-Assembly for Thin Film Solid-
State Polymerization
Jean-Nicolas Tisserant,†,§ Roland Hany,† Eric Wimmer,† Antoni Sanchez-Ferrer,§ Jozef Adamcik,§
́
Gaetan Wicht,† Frank Nuesch,†,∥ Daniel Rentsch,† Andreas Borgschulte,‡ Raffaele Mezzenga,§
̈
̈
and Jakob Heier*,†
†
̈
Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Functional Polymers, Empa, Uberlandstrasse 129,
CH-8600 Dubendorf, Switzerland
̈
‡
̈
Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Hydrogen and Energy, Empa, Uberlandstrasse 129,
CH-8600 Dubendorf, Switzerland
̈
§Department of Health Sciences and Technology, Institute of Food, Nutrition and Health, ETH Zurich, CH-8092 Zurich, Switzerland
̈
̈
∥Institut des Mater
́
iaux, Ecole Polytechnique Fed
́ ́
erale de Lausanne, EPFL Station 12, CH-1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: C60 fullerene derivatives bearing aliphatic chains can
self-assemble into versatile supramolecular structures. Cross-linking of
such self-assembled morphologies is an attractive approach to
enhance the structural stability of these self-organized structures.
We describe the synthesis of a C60 functionalized with a single alkyl
chain bearing a diacetylene moiety. In a thin film, the molecule self-
assembles into lamellar arrays. The character of the side chain
attached to the fullerene is key to the observed packing ability. The
stabilization proceeds through solid-state polymerization of the
diacetylene moieties. By blending the fullerene derivative with a
cyanine dye, various nanostructured fullerene morphologies are
obtained that can be selectively stabilized by thermal polymerization. These films can serve as basis for nanostructured fullerene
scaffolds that can find applications in optics and electronics.
Self-organized structures as described above have proven to be
a very efficient way to facilitate solid-state polymerization,
which stabilizes a morphology without destroying it. Wang et
al. incorporated diacetylene functional groups into a fullerene
derivative known to self-assemble into a bilayer structure on the
molecular level.10 The cross-linked material was remarkably
resistive against organic solvents. For fullerene derivatives
several cross-linking routes have been explored, including
epoxydes,13 styrene derivatives,14 oxetanes,15 or polyacety-
lenes.16,17
Polymerization of diacetylenes proceeds through a 1,4
topochemical cycloaddition which Werner first described in
1969 as a diffusion-free, entirely lattice-controlled process.18
Schmidt later proposed a mechanism for such a solid-state
reaction based on 2 + 2 cycloaddition of olefins.19 Top-
ochemical reactions retain position and symmetry of the
monomer units; they require a stacked arrangement of
monomer units such that one unit can react with its two
neighbors.20−22 Conjugated diacetylenes polymerize in the
INTRODUCTION
■
Organic semiconducting molecules in thin films offer promising
alternatives to inorganic semiconductors in the fields of both
electronics and optics. Besides numerous other appealing
characteristics, fullerenes have attracted attention as they
belong to the class of small molecule n-type organic
semiconductors with excellent electronic properties.1,2 Full-
erenes and their derivates have been shown to self-assemble
into various mesoscopic shapes that can be controlled by the
chemical nature of their side group.3−6 A well-known route for
molecular self-assembly is based on amphiphilic molecules. For
instance, amphiphilic fullerenes containing cations7 or anions8
associate into bilayers that assemble into nanorods or vesicles.
A lot of attention has been paid to structures formed from
alkylated fullerenes. The nature of these molecules is not
strictly hydrophobic−hydrophilic, but it rather shows a
“hydrophobic amphiphilicity” due to π−π interacting C60 and
the van der Waals interacting alkyl chain.9,10 Nakanishi et al.
synthesized a fullerene derivative with three hexadecyl chains.
From a bilayer composed of C60 and interdigitated alkyl chains,
fibers, disks, spheres, and cones form, depending on the
solvent.11 The same system assembled epitaxially on highly
oriented pyrolytic graphite (HOPG), forming lamellar
structures resulting in a 1D alignment of the fullerenes.12
Received: August 9, 2013
Revised: December 20, 2013
© XXXX American Chemical Society
A
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was then extracted twice with CH2Cl2, the organic phase was collected
and dried over MgSO4, and the solvent was then removed under
reduced pressure. The crude product was dissolved in CH2Cl2 and
purified by column chromatography (Silica gel 60, Merck as stationary
phase, CH2Cl2 as eluent). After drying, 420 mg (87% yield) of PCB-ol
was obtained.
solid state under the influence of pressure, heat, UV, or γ
radiation to yield conjugated structures.23
Phase separation and dewetting are well-established methods
to manufacture nanostructured thin films with new function-
alities. Cross-linking would be most attractive if applied to the
phase of a blend film such that insoluble meso- or macroporous
structured scaffolds can be manufactured.24 Fullerene blend
films with various electron donors are currently widely studied
for organic solar cell applications.25 Most studied are blends
with semiconducting polymers,26−28 but blends with small
organic molecules have gained great interest.29 Stabilizing
specific blend morphologies of fullerenes is appealing as it is
believed that coarsening of the nanomorphology impacts device
performance.13 In our previous work, we have shown that
liquid−liquid dewetting (LLD) is the dominant phase-
separation process in blends of polymers and cyanine dyes,
providing easy means of hierarchical structuring.30
10,12-Tricosadiynoic Acid [6,6]-Phenyl-C61-butyl Ester, C23 PCB-
Diyne. 150 mg of PCB-ol was placed in a dry round-bottom flask
under argon atmosphere, and then 125 mL of dry CH2Cl2 was added
with stirring at room temperature. A mixture of 294 mg of 10,12-
tricosadiynoic acid, 262 mg of DCC, and 16 mg of DMAP dissolved in
25 mL of dry CH2Cl2 was then added at room temperature in the dark.
The solution was stirred in the dark for 4 days. The solvent was
removed under reduced pressure, and the crude product was purified
by column chromatography (Silica gel 60, Merk as stationary phase,
and hexane/CH2Cl2, 2:1 as eluent). After that, the solid was washed
twice with CH3OH. After filtration and drying, 160 mg of C23 PCB-
diyne was obtained (78% yield). Chemical structures and purities were
1
verified by H NMR and mass spectrometry (MS) (see Supporting
Microcontact printing (μCP) of functional thiols on
surfaces31,32 is a well-described method to induce long-range
order in a dewetting33 or phase-separating34 system. We have
shown in a previous study that the pattern replication could be
controlled by the contact line pinning of hydrophobic droplets
of phenyl-C61-butyric acid methyl ester (PCBM) on hydro-
phobic self-assembled monolayer (SAM) lines.35 The ability to
stabilize such fullerene patterns and meso-structures is a new
concept and may prove useful for further processing of these
functional surfaces. We here demonstrate the synthesis of a C60
derivative carrying an alkyl chain with one single diyne moiety
to (i) enhance the self-assembly of the fullerene and (ii)
polymerize the mesoscopic structure obtained both in bulk
films and microstructured films achieved by a phase separation
process.
Information).
Synthesis of C11 Diynoic Acid. 1-Bromo-1-hexyne. In a three-
necked flask, 0.8 g of 1-hexyne, 1.96 g of N-bromosuccinimide (NBS),
0.17 g of AgNO3, and 50 mL of acetone were added. The mixture was
stirred for 3 h at room temperature, mixed with 100 mL of hexane, and
concentrated under reduced pressure. The solid phase was removed by
filtration, and the filtrate was purified by fractional distillation under
reduced pressure. 740 mg of yellow oil was obtained (45% yield). H
NMR (400.1 MHz, CDCl3): 2.16 (t, J = 7.0 Hz, 2H), 1.45 (m, 2H),
1.37 (m, 2H), 0.86 (t, J = 7.3 Hz, 3H) ppm.
4,6-Undecadiynoic Acid. In a three-necked flask, 17.8 mg of
copper(I) chloride and 20 mL of 30% butylamine aqueous solution
were added. A few crystals of hydroxylamine hydrochloride were
added to the solution until the blue color disappeared. 300 mg of 4-
pentynoic acid was added, and the mixture was cooled to 0 °C. 700 mg
of 1-bromo-1-hexyne in 10 mL of diethyl ether was added dropwise.
The solution was then stirred at room temperature for 2 h and was
stopped with 2 N HCl by adjusting the pH to 2. The organic layer was
extracted three times with ethyl acetate. The organic layers were
collected and dried, and the solvent was removed under reduced
pressure. The crude product was recrystallized in hexane. 390 mg of
1
EXPERIMENTAL SECTION
■
Synthesis of PCB-Diynes. The synthesis shown here for the first
time used only commercially available reactants and mild conditions.
Scheme 1 shows the two-step synthesis of the diacetylene-function-
1
4,6-undecadiynoic acid was obtained (71% yield). H NMR (400.1
MHz, CDCl3): 2.59 (m, 4H), 2.25 (t, J = 6.9 Hz, 2H), 1.49 (m, 2H),
1.41 (m, 2H), 0.90 (t, J = 7.3 Hz, 3H) ppm.
Scheme 1. Synthetic Route for Obtaining PCB-Diyne via
4,6-Undecadiynoic Acid [6,6]-Phenyl-C61-butyl Ester, C11 PCB-
Diyne. C11 PCB-diyne was synthesized by coupling PCB-ol with 4,6-
undecadiynoic acid, as described above for C23 PCB-diyne. Yield 20%.
1H NMR (400.1 MHz, CDCl3): 7.92 (m, 2H), 7.55 (m, 2H), 7.48 (m,
Reduction of PCBM with DIBAL and Steglich Esterification
with Diynoic Acid
a
1H), 4.18 (t, J = 6.3 Hz, 2H), 2.89 (m, 2H), 2.51 (m, 4H), 2.24 (t, J =
6.9 Hz, 2H) 1.8−2.0 (m, 4H), 1.48 (m, 2H), 1.40 (m, 2H), 0.89 (t, J =
7.2 Hz, 3H) ppm.
General Methods. PCBM (Solenne B.V., The Netherlands) and
10,12-tricosadiynoic acid (Sigma-Aldrich) were used as received; the
cyanine dye 1,1′-diethyl-3,3,3′,3′-tetramethylcarbocyanine perchlorate
(CyC) was synthesized in our laboratory.36 Films were spin-coated on
glass substrates at different spin speeds (250−4000 rpm) from a 1 wt
% solution of PCB-diyne, PCB-diyne/CyC, or PCBM/CyC in
chlorobenzene (CB). The composition of blends varied from 0.8:1
to 2:1 (mol/mol) between CyC and the fullerenes. Films were
temperature-annealed at 100 or 120 °C in the dark under vacuum
(10−3 mbar) for times ranging from 1 to 24 h. To check the
(in)solubility, films were immersed in a large volume (typically 50 mL
for a 1 cm2 film) of a good solvent (CB, CH2Cl2, or CHCl3) and were
sonicated for 10 min.
Film characterization included scanning force microscopy (SFM),
optical microscopy, UV−vis spectroscopy, attenuated total reflectance
Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman
spectroscopy. SFM images were measured on a Nanosurf Mobile S in
tapping mode at a resonance frequency of 170 kHz with rectangular
silicon cantilevers (Mikromasch, Nanosensors TM) with a typical
force constant of ∼40 N m−1 and a tip radius of ∼10 nm; images were
a
10,12-Tricosadiynoic acid was commercially available; 4,6-undeca-
diynoic acid was synthesized via coupling of 1-bromohexyne and 4-
pentynoic acid.
alized fullerene derivatives. In a first reaction, PCBM was reduced to
the corresponding alcohol PCB-ol using diisobutylaluminium hydride
(DIBAL). PCB-ol was then esterified with diynoic acids in the
presence of N,N′-dicyclohexylcarbodiimide (DCC) and 4-
(dimethylamino)pyridine (DMAP) to yield C23 and C11 PCB-diyne.
4-([6,6]-Phenyl-C61)butanol, PCB-ol. PCBM (500 mg) was placed
in a dry 500 mL round-bottom flask under argon, and then 150 mL of
dry CH2Cl2 was added and cooled to 0 °C. 5 equiv of DIBAL (1 M in
CH2Cl2) was added dropwise under stirring. The reaction was run
overnight at room temperature. CH3OH (4 mL) was then added, and
the organic layer was washed two times with water. The aqueous layer
B
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analyzed with the WsXM scanning probe microscopy software.37
High-resolution SFM images were collected using a Nanoscope 8
multimode scanning force microscope (Bruker) operated in tapping
mode under ambient conditions. The microscope was covered with an
acoustic hood to minimize vibrational noise. The cantilevers with a
nominal tip radius of <10 nm (Bruker) were driven at oscillation
frequencies in the range of 150−200 kHz. Images were flattened using
the Nanoscope 8.1 software, and no further image processing was
carried out. Attenuance spectra taking scattering into account were
measured on a Varian Cary 50 UV−vis spectrophotometer. ATR-FTIR
spectra were measured on a Bio-Rad FTS 6000 spectrophotometer for
powders and a Bruker Hyperion (ATR coupled to an optical
microscope) for measurements on thin films. Raman spectra were
collected on a Senterra (Bruker) Raman microscope. To avoid long-
term exposure to the high intensity of the Raman laser, many short
measurements were taken at various positions of the sample. The data
was averaged over 50 spectra taken at λ = 532 nm with the lowest
possible laser power of 0.2 mW focused on an area of 5 μm2.
Differential scanning calorimetry (DSC) was measured on a
PerkinElmer DSC 8000 (under nitrogen flow, at heating/cooling
rates of 10 °C min−1 from 20 to 140 °C). Thermogravimetric analysis
(TGA) experiments were conducted on a Netzsch TG under an argon
atmosphere, with a heating rate of 20 °C min−1, from 30 to 900 °C.
Wide-angle X-ray scattering (WAXS) experiments were performed
using a Rigaku MicroMax-002+ microfocused beam (4 kW, 45 kV,
0.88 mA) with the λCu Kα = 0.154 18 nm. The scattering intensities
were collected in transmission mode by a Fujifilm BAS-MS 2025
imaging plate system (15.2 × 15.2 cm2, 50 μm resolution). An effective
scattering vector range of 0.5 nm−1 < q < 25 nm−1 was obtained, where
q is the scattering wave vector defined as q = 4π sin θ/λCu Kα with a
scattering angle of 2θ. Samples for WAXS were drop-cast from
chlorobenzene solutions (10 g L−1) on freshly cleaved mica substrates
(Pelco Mica Discs,Ted Pella, Inc.) and subsequently annealed as
described earlier.
Preparation of Patterned Substrates. Metals for thermal
deposition were purchased from Cerac Inc. 3 nm of chromium
thermally deposited onto glass served as an adhesion layer for the
deposition of 10−30 nm of gold. Both metals were deposited at a rate
of 0.1 Å s−1 and at a starting pressure of 6 × 10−6 mbar. The
poly(dimethylsiloxane) (PDMS) stamps were prepared by molding a
10:1 (wt) base:curing agent mixture (Sylgard 184-Dow Corning) on a
silicon wafer micropatterned by photolithography. After curing at
room temperature for 24 h, a drop of 0.5 mM ethanolic solution of
octadecanethiol (Sigma-Aldrich) was deposited on the PDMS stamp
for 5 s. The stamp was dried under nitrogen for 10 s and held in
contact with the gold surface for 10 s. The substrates were washed
with ethanol before coating.
Figure 1. (a) SFM image of a single crystalline platelet drop-cast from
10 mg mL−1 C23 PCB-diyne in CB. (b−d) SFM images on such a
platelet showing a periodic packing of the monomers with a periodicity
of ca. 8 nm. (e) WAXS data measured on a film as cast (blue), on a
film annealed for 24 h at 100 °C (green), on a film annealed for 24 h at
100 °C, then for 3h at 120 °C and measured at 120 °C (orange), and
for 8 h at 120 °C (red).
RESULTS AND DISCUSSION
■
= 5.92 nm and a correlation length of ξ1 = 31.4 nm. This
lamellar periodicity indeed corresponds to the total length of
two interdigitated PCB-diyne molecules (see Figure S7,
Supporting Information). At high q-values, a sharp reflection
at qF = 6.28 nm−1 and a set of peaks at qT = 12.4, 13.6, and 14.0
nm−1 are visible and correspond to the fullerene−fullerene
distance of dF = 1.00 nm (ξF = 20.4 nm) and the crystalline
packing of the alkyl tails with an average distance around dT =
4.5 Å (ξT = 51.3 Å), respectively. We therefore suggest that the
lamellae shell is composed of C60, while the alkyl chains are
interdigitated and aligned with a periodicity of 0.45 nm. The
difference between the periodicity observed is WAXS and SFM
may arise from surface tilting. Some SFM scans suggest a
hexagonal packing of cylinders (see Figure S8, Supporting
Information); this morphology is not supported by WAXS and
could be assigned to a surface layer which forms in film regions
where lamellae are oriented parallel to the substrate, but the
thickness is irreconcilable with the lamellar period. Upon
thermal annealing of such films, WAXS data reveal that two
Self-Assembly of PCB-Diyne. We first discuss the self-
assembly of a C60 derivative where the fullerene is linked via an
ester to an alkyl chain (C23 or C11) bearing a conjugated
diacetylene moiety. The synthesis route for C23 and C11 PCB-
diyne is shown in Scheme 1 and described in the Experimental
Section. In films obtained by drop-casting of the C23 diyne
molecule from CB solution, micrometer-large polycrystalline
domains form (Figure 1a). High-resolution scanning force
microscopy (SFM) images reveal that the molecules pack in a
1D periodic lattice fashion with a typical periodicity of ca. 8 nm
(Figures 1b−d). We can identify regions where the lamellae are
oriented perpendicularly to the substrate (Figures 1b,c). A grain
boundary between two crystalline plates is shown in Figure 1d.
The lamellar packing of the molecules was revealed by wide-
angle X-ray scattering (WAXS) experiments as shown in Figure
1e. Two sharp reflections are observed at low q-values at q1 =
1.06 nm−1 and at q2 = 2q1, which correspond to a lamellar
packing of the PCB-diyne molecules with a layer distance of d1
C
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different morphologies develop depending on the annealing
temperature during the polymerization process. Annealing the
sample at 100 °C keeps the lamellar structure of the samples
and the crystallinity of the alkyl domains as observed by the two
sharp reflections at q1 = 1.02 nm−1 and q2 = 2.06 nm−1 and the
set of peaks at qT = 12.4, 13.6, and 14.1 nm−1, respectively. The
corresponding distances to these two features are the layer
distance d1 = 6.18 nm with a correlation length of ξ1 = 38.3 nm
and the average distance between alkyl tails of dT = 4.5 Å (ξT =
48.6 Å). Moreover, a sharp peak at qF = 6.28 nm−1 is also
present which indicates that the fullerene moieties are well
packed into their domain with an interfullerene distance of dF =
1.00 nm (ξF = 19.4 nm). Temperature-dependent WAXS
experiments revealed that this lamellar structure is stable
against further annealing at 120 °C, and only the alkyl chains
melt, which is shown by a broadening of the peak at qT while
the peaks at q1, q2, and qF are maintained. This morphological
stability will be discussed further when considering the
topochemical polymerization of C23-PCB-diyne. Annealing
the sample directly at 120 °C induces a transition to the
amorphous state as observed by the presence of a single broad
peak at qT = 13.7 nm−1 which corresponds to the tail-to-tail
distance of dT = 4.6 Å. Moreover, the fullerene moieties are
dispersed into the polymerized alkyl tail matrix as shown by the
two broad peaks at q1 and q2 which indicate a random
dispersion of such bulky motives into the continuous aliphatic
phase. A model for the packing based on WAXS and SPM
experiments is sketched in Figure 2 where the lamellae distance,
Information). From here one would expect a packing of the
molecules into micelles.40 We attribute the different observed
packing behavior to three factors. Both the rigid diacetylene
moiety in the middle of the alkyl chain, the use of an organic
solvent which may change the effective chain excluded volume
compared to a flexible alkyl chain in water,41 and, last but not
least, surface effects which may shift the observed conformation
from micelles to lamellas. This observation is supported by a
study by Nakanishi et al. where it was shown that fullerenes
bearing long alkyl chains self-assembled into interdigitated
bilayer structures independently on the CPP.42
Polymerization of C23 PCB-Diyne. Thermal polymer-
ization was chosen among other possible polymerization
methods to differentiate the topochemical reaction from the
photopolymerization of the C60 alone.43 Topochemical
reactions occur over a wide temperature range with typical
activation energies of 22 kcal mol−1.44 According to the WAXS
data, annealing the films at 100 °C for 24 h sustained the
micromorphology of the film; additionally, the film was left
fully insoluble in the coating solvent. The above-discussed
packing leads to a high degree of order in a film cast from CB
that supports the solid-state polymerization mechanism. The
theoretical polymer structure resulting from the topochemical
reaction between multiple stacked diacetylenes is shown in
Figure 2 for C23 PCB-diyne.23 We sketch the transformation of
a monomer-made lamella to an insoluble polymer-made
lamella. Here the self-assembly of the C23 PCB-diyne provides
the favored stacking of the diacetylene moieties.20−22 The
evolution of the morphology of the drop-casted films was
explored by SPM in parallel to the WAXS experiment.
According to the scattering data, no amorphous regions exist
in the film; the SFM scans therefore show large polycrystalline
domains embedded in a microcrystalline matrix (Figure 3a).
These structures are stable against annealing at 100 °C (Figure
3b).
Annealing the film at 120 °C fully destroys the crystalline
character of the film. This is confirmed by SFM scan (Figure
3c). The shape of large domains remains intact (Figures 3g,h),
suggesting that the melt resulting from annealing does not flow.
Interestingly, the polymerization reaction is also taking place in
this film, rendering it insoluble in the casting solvent after 8 h of
annealing. We must conclude that the observed tail-to-tail
distance of 4.6 Å between the alkyl chains still allows for the
topochemical polymerization.
For applications in optoelectronics, organic materials are
typically deposited onto a substrate in a thin film. In the next
section we discuss the behavior of PCB-diyne and PCB-diyne
blend films spin-coated onto a substrate. Cross-linking was also
observed for films of C23 PCB-diyne spin-coated from CB
solution (see Figure S9, Supporting Information). With
annealing time the PCB-diyne film partially dewetted and
formed polycrystalline domains very similar to those obtained
by drop-casting. We assume that the fast process of spin-coating
does not allow the formation of an equilibrium morphology,
and the annealing of the films induces a further chain
rearrangement of the molecules until a configuration allowing
for the topochemical reaction is adopted. The annealed
polycrystalline films were insoluble in good solvents of the
monomer such as CB, dichlorobenzene, dichloromethane, and
chloroform, indicating a successful solid-state polymerization
reaction. Similar to the drop-casted films, crystallization is not a
necessary condition for polymerization. We recall here that
Figure 2. Schematics of the proposed self-assembly of C23 PCB-diyne
and its solid-state polymerization reaction.
the fullerene−fullerene and the tail-to-tail distances are shown.
A similar packing behavior in lamellae was observed by
Chikamatsu and co-workers for a monoalkylated fullerene
derivative.38 We believe that the driving force for the lamellar
arrangement is the immiscibility of the alkyl chains with the
fullerenes on one hand and the tendency of the alkyl chains to
crystallize on the other hand. The critical packing parameter
(CPP) is commonly used to predict the packing ability of
surfactant molecules in water from the curvature of the water−
lipid interface. The CCP is obtained by comparing the volume
of the lipid tail to the product of the cross-sectional area of the
head and the length of the lipid chain.39 For C23 PCB-diyne, a
CPP of 0.13 was found. The conformation was calculated using
the density functional theory (DFT) (see Figure S7, Supporting
D
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120 °C. The characteristic FTIR absorption bands of the
monomer at 2330, 2193, and 2077 cm−1, which correspond to
the stretching of the diacetylene moiety (CC−CC st),
disappeared upon annealing. The alkene (CC st) vibrations
at ∼1600 cm−1 was detected after annealing by Raman
spectroscopy (see Figure S15, Supporting Information).
These observations are in line with the reaction shown in
Figure 2. In the DSC thermograms an endothermic melting
point appeared at Tm = 122 °C during the first heating cycle.
This observation is in agreement with the WAXS experiments.
In the second heating cycle, however, the melting peak
disappeared. The resulting thermoset does not recrystallize
during cooling; neither does it melt in the second heating cycle
due to a fixed conformation of the aliphatic chains. The
polymerization reaction itself is predicted to be slightly
exothermic.45 We explain the absence of a signature for this
process by the rearrangement of the alkyl chains over a broad
range of temperatures prior to polymerization. We quantified
the insolubility of the cross-linked films by immersing the
samples in the solvent used for spin-coating and monitoring the
absorbance changes by UV−vis spectroscopy. Figure 4c shows
the absorbance spectra of as-cast films. The spectrum of the
nonannealed film is almost identical to the spectrum of a pure
PCBM film. The UV−vis spectrum of the annealed film differs
from the nonannealed film; we observed a broadening of the
characteristic absorption peak at 330 nm as well as a general
increase in absorption at larger wavelengths. These changes
reflect the crystallization and roughening of the film which was
shown above. The polymerization of self-assembled diacetylene
does not lead to new absorption bands in the UV−vis region
due to polydiacetylenes of various length and conformation.10
For a film annealed at 120 °C for 8 h or at 100 °C for 24 h, the
absorption remained almost unaltered after solvent wash
(Figure 4c and Figure S10), whereas the nonannealed film
fully dissolved. Annealing of spin-coated films is also suitable to
study the kinetics of PCB-diyne rearrangement and polymer-
ization. In Figure 4d, we show the fraction of the film remaining
on the substrate as determined by UV−vis as a function of
annealing time. The stabilization time was almost the same for
films of different thicknesses; full insolubility was in all cases
reached after 5 h of annealing at 120 °C. Such a slow
polymerization also supports the hypothesis that chain
rearrangement has to occur prior to polymerization. The rate-
limiting step is in this case the local ordering of molecules
which allows consecutive rapid polymerization. TGA experi-
ments were performed to exclude any degradation or weight
loss in the range of annealing temperature (see Figure S11,
Supporting Information).
Figure 3. Stabilization of pure C23 PCB-diyne films: (a) SFM image
of a drop-cast film before annealing; (b) after annealing at 100 °C; (c)
after annealing at 120 °C; (d−f) corresponding phase images; (g)
single crystals obtained by drop-casting a solution of 10 mg mL−1 C23
PCB-diyne in CB; (h) the same domains are insoluble after annealing
7 h at 120 °C, the film has been washed with CB.
only the local chain arrangement determines whether the
reaction takes place.
FTIR spectroscopy, Raman spectroscopy, and DSC (Figures
4a,b) were used to monitor chemical changes upon annealing at
We conducted a control experiment with a PCB-diyne with a
much shorter alkyl-chain (C11). In that case, no insoluble
phase was formed after annealing (see Figure S12). C11 PCB-
diyne may not arrange into a structure that facilitates stacking
of the diacetylene moieties necessary for polymerization. This
finding is similar to recent results obtained for the polymer-
ization of alkyldiacetylene in thin Langmuir−Blodgett films,
where it was found that long, odd-numbered alkyl spacers
yielded considerably better polymerization due to a better
Figure 4. (a) FTIR spectra of as-coated C23 PCB-diyne film (top) and
after annealing at 120 °C for 8 h (bottom). (b) DSC thermograms
showing a thermoset behavior for C23 PCB-diyne. (c) Stabilization of
pure C23 PCB-diyne films monitored with UV−vis spectroscopy (red:
PCB-diyne film as-casted; black: after annealing; blue: annealed film
after washing with CB; green: nonannealed film after washing with
CB). (d) Kinetics of polymerization for C23 PCB-diyne films with
thicknesses of 17 nm (square), 35 nm (triangle), and 100 nm (cross)
upon annealing (120 °C). Films were washed with CB after indicated
times, and the remaining absorbance at 330 nm was measured. All
experiments conducted at 120 °C suggest that insolubility is not
correlated to a high degree of polymerization. This renders chemical
and structural analysis of the product challenging.20
accommodation to the restrictive single crystal structure.46−48
A
second control experiment was carried out to demonstrate that
the conjugated diacetylene units do not attack the fullerene
moiety of PCB-diyne, leading to different cross-linked products.
Blend films of PCBM and 10,12-tricosadiynoic acid were
annealed for 8 h and were then washed with CB. PCBM could
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completely be removed with CB after annealing and the UV−
vis spectrum of the washing solution coincides with the
spectrum of the starting solution (see Figure S13). Thus, the
major part of the fullerenes had not reacted during polymer-
ization and remained intact.49 This result is also supported by
the Raman spectra shown in Figure S15.
C23 PCB-Diyne/CyC Blend Film Morphology. Fullerene
derivatives are most widely used as model electron acceptors in
blend organic photovoltaics. In this field, the ability to
thermoset blend thin films may be crucial for stabilizing
nanoscale phase-separated morphologies against aging or
further processing. As an example to illustrate the general
ability of PCB-diyne to stabilize blend morphologies, we study
the blending of PCB-diyne and a cyanine dye, CyC. The cross-
linking reaction also proceeds in the fullerene phase of a film of
PCB-diyne blended with the cyanine dye CyC. The molecular
structure of CyC is shown in Figure 5a. In Figure 5b, we show
in Figure 5 is one example of PCB-diyne morphologies which
can be stabilized and forms the basis of fullerene scaffolds.
Depending on mixing ratios and film thicknesses, a large variety
of PCB-diyne morphologies can be manufactured (see Figure
S14).
C23 PCB-Diyne/CyC Blend Films on Patterned
Substrates. Figure 6a shows SFM scans of the morphology
Figure 5. Blend morphology stabilization: (a) structure of the cyanine
dye and sketch of vertically phase separated C23 PCB-diyne/CyC film;
(b) SFM image of a 10:1 C23 PCB-diyne/CyC molar blend film
coated at 500 rpm from CB; (c) SFM image of the insoluble
mesostructured C23 PCB-diyne film after thermal polymerization and
dissolution of CyC.
Figure 6. 1D template formation and stabilization: (a) SFM image of a
C23 PCB-diyne/CyC blend film on a SAM stripe pattern as indicated
below the figure (1:1 molar blend 1 wt % in CB, spin-coated at 500
rpm); (b) C23 PCB-diyne selectively removed with hexane; (c) CyC
selectively removed with CB after thermal polymerization of PCB-
diyne; (d) increasing the dye volume fraction in the film leads to an
optimized pattern transfer (1:2 C23 PCB-diyne/CyC molar blend 1 wt
% in CB, 500 rpm); (e) C23 PCB-diyne selectively removed with
hexane; (f) CyC selectively removed with CB after thermal
polymerization of C23 PCB-diyne; (g) experimental cross section of
the patterned morphology; (h) sketch of the process that leads to an
undercut structure.
SFM scans of a C23-PCB-diyne/dye blend film with a molar
ratio of 10:1. We found monodispersed droplets embedded in a
matrix film. Selectively dissolving the dye leaves sinkholes in the
matrix film as shown in the SFM scans in Figure 5c. The film
morphology thus comprises droplets of CyC in a matrix of
PCB-diyne. A sketch of the film morphology is shown in Figure
5a. We can explain the morphologies obtained from the phase
separation of the C23 PCB-diyne/CyC system by liquid−liquid
dewetting similarly to the PCBM/CyC blend system which we
investigated earlier.30 In the case of PCBM, a transient bilayer
forms with PCBM in contact with air and CyC in contact with
the substrate. Because of the higher surface energy of C23
PCB-diyne compared to PCBM (60 and 18 mN m−1,
respectively), the order of the initial transient bilayer is
reversed, with CyC (34.3 mN m−1) and C23 PCB-diyne now
forming wetting layers at the air and substrate interface,
respectively. For low dye fractions, the top dye layer dewets
from the PCB-diyne layer, forming the pattern of mono-
dispersed dye droplets on top of a PCB-diyne matrix film.
C23 PCB-diyne in blend films can also be thermally
polymerized. After washing in the casting solvent, this method
conserves the same morphology as dissolving the dye in a
selective solvent without polymerization (Figure 5c). This
experiment and the UV−vis spectra of annealed and non-
annealed films shown in Figure S12 prove that the C23 PCB-
diyne phase can be selectively rendered insoluble in blends.
Differently from the pure C23 PCB-diyne films, no large
polycrystalline domains formed during annealing as shown in
Figure 5c. This indicates that the dispersed solid CyC droplet
phase seems to hinder the growth of large polycrystals. The
polymerization still proceeds in blends. The morphology shown
of a C23 PCB-diyne/CyC blend film coated from a solution
with a 1:1 molar ratio onto a SAM stripe pattern. Figures 6b
and 6c show corresponding scans after PCB-diyne and CyC has
been selectively dissolved, respectively. We find cyanine
droplets embedded in a PCB-diyne matrix, whereby the
cyanine only forms a phase above the SAM stripes.
Increasing the volume fraction of dye in the film leads to a
situation where the dye fully covers the SAM stripes (Figures
6d−f). This structure forms because CyC wetting droplets are
pinned to the SAM−gold interface. We achieved an almost
perfect transfer of the chemical substrate pattern into a phase
pattern. The corresponding imprinted and structured PCB-
diyne phase could also be thermally stabilized against solvents
(Figure 6f). With this contact-line pinning method we were
able to fabricate undercut structures (Figures 6g,h) which are
challenging to achieve with top-down methods such as
photolithography. The angle of the undercut structure depends
on the dye volume fraction.35 This is one very specific example
on how we can design photonic structures with the PCB-diyne/
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dye blend system. Films with topographical or compositional
pattern scatter ligh coherently.50 The materials and processes
can be utilized more generally for photonic applications: the
self-assembled fullerene itselves can possess useful optical
properties; here we specifically think of materials with high
dielectric constants.51 Polymer templates are frequently used as
base material for photonic materials52 and can act as matrix for
optically active materials.53
National Science Foundation (SNF) under grant number
200021_132502.
REFERENCES
■
(1) Makarova, T. L. In Semiconductors; American Institute of Physics:
Melville, NY, 2001; Vol. 35, pp 243−278.
(2) Dresselhaus, M. S.; Dresselhaus, G. Annu. Rev. Mater. Sci. 1995,
25, 487−523.
(3) Prassides, K.; Keshavarz, M.; Beer, E.; Bellavia, C.; Gonzalez, R.;
Murata, Y.; Wudl, F.; Cheetham, A. K.; Zhang, J. P. Chem. Mater.
1996, 8, 2405−2408.
CONCLUSION
■
We synthesized a fullerene derivative which self-assembles into
lamellar structures. The self-assembly in these nanostructures
leads to an arrangement of diacetylene moieties such that a
topochemical solid-state polymerization takes place upon
annealing at 100 °C, stabilizing the self-assembled morphology
and rendering the film insoluble in common solvents. The
associated chemistry is more straightforward than the chemistry
following a pathway based on the common belief that effective
cross-linking requires at least two diyne moieties.4 Annealing
the sample at 120 °C above the order−disorder transition of
the material also leads to a cross-linked insoluble film that is
now mainly amorphous. That is surprising as a certain chain
arrangement is prerequisite for the solid-state polymerization.
We conclude that to reach insolubility only a low number of
ordered cross-linking sites are necessary. The cross-linking
reaction also proceeds in spin-coated films but is accompanied
by large scale dewetting and crystallization. Most interestingly,
the reaction occurs in the C23 PCB-diyne phase of blend films,
opening a way to fabricate complex 3D insoluble objects. In this
case the large-scale crystallization is hindered by the presence of
dispersed dye domains. Selectively removing the dye after
polymerization of the C23 PCB-diyne leaves structured
fullerene scaffolds. The C23 PCB-diyne templates show
mesoporous structures in the submicrometer range; we are
currently investigating the feasibility of this process for
manufacturing nanoporous structures.
(4) Sawamura, M.; Kawai, K.; Matsuo, Y.; Kanie, K.; Kato, T.;
Nakamura, E. Nature 2002, 419, 702−705.
(5) Georgakilas, V.; Pellarini, F.; Prato, M.; Guldi, D. M.; Melle-
Franco, M.; Zerbetto, F. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 5075−
5080.
(6) Babu, S. S.; Mohwald, H.; Nakanishi, T. Chem. Soc. Rev. 2010, 39,
4021−4035.
(7) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem.,Int. Ed.
1999, 38, 2403−2405.
(8) Zhou, S. Q.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.;
Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001,
291, 1944−1947.
(9) Nakanishi, T. Chem. Commun. 2010, 46, 3425−3436.
(10) Wang, J. B.; Shen, Y. F.; Kessel, S.; Fernandes, P.; Yoshida, K.;
Yagai, S.; Kurth, D. C.; Mohwald, H.; Nakanishi, T. Angew. Chem., Int.
Ed. 2009, 48, 2166−2170.
(11) Nakanishi, T.; Schmitt, W.; Michinobu, T.; Kurth, D. G.; Ariga,
K. Chem. Commun. 2005, 48, 5982−5984.
(12) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.;
Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128,
6328−6329.
(13) Drees, M.; Hoppe, H.; Winder, C.; Neugebauer, H.; Sariciftci,
N. S.; Schwinger, W.; Schaffler, F.; Topf, C.; Scharber, M. C.; Zhu, Z.
G.; Gaudiana, R. J. Mater. Chem. 2005, 15, 5158−5163.
(14) Cheng, Y. J.; Hsieh, C. H.; Li, P. J.; Hsu, C. S. Adv. Funct. Mater.
2011, 21, 1723−1732.
(15) Cheng, Y. J.; Cao, F. Y.; Lin, W. C.; Chen, C. H.; Hsieh, C. H.
Chem. Mater. 2011, 23, 1512−1518.
(16) Markov, D. E.; Amsterdam, E.; Blom, P. W. M.; Sieval, A. B.;
Hummelen, J. C. J. Phys. Chem. A 2005, 109, 5266−5274.
(17) Nierengarten, J. F.; Setayesh, S. New J. Chem. 2006, 30, 313−
316.
(18) Wegner, G. Z. Naturforsch., Part B 1969, B24, 824.
(19) Schmidt, G. M. T. In Solid State Photochemistry; Verlag Chemie:
Weinheim, 1976.
ASSOCIATED CONTENT
* Supporting Information
■
S
Mass spectrometry (MS) of C23 PCB-diyne, NMR of
intermediates and products, UV−vis spectra of C23 and C11
PCB-diyne/CyC films, UV−vis spectra of PCBM/10,12-
tricosadiynoic acid blends, TGA of C23 PCB-diyne, optimal
molecular conformation calculated by DFT, SFM of bulk and
blend spin-coated morphologies, Raman spectroscopy of
pristine and annealed C23 PCB-diyne. This material is available
free of charge via the Internet at http://pubs.acs.org.
(20) Enkelmann, V. In Polydiacetylenes; Cantow, H.-J., Ed.; Springer:
Berlin, 1984; Vol. 63, p 91.
(21) Baughman, R. H. J. Polym. Sci., Part B: Polym. Phys. 1974, 12,
1511−1535.
(22) Enkelmann, V. Colloid Polym. Sci. 1978, 256, 893−903.
(23) Khandelwal, P. K.; Talwar, S. S. J. Polym. Sci., Part A: Polym.
Chem. 1983, 21, 3073−3082.
(24) Stein, A. Adv. Mater. 2003, 15, 763−775.
(25) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D.
E. Adv. Mater. 2007, 19, 1551−1566.
(26) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45−61.
(27) Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; Leveque, P.;
Audinot, J. N.; Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R.
Adv. Mater. 2010, 22, 763−768.
(28) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Chem. Rev.
2013, 113, 3734−3765.
(29) Mishra, A.; Bauerle, P. Angew. Chem., Int. Ed. 2012, 51, 2020−
2067.
AUTHOR INFORMATION
Corresponding Author
*E-mail jakob.heier@empa.ch (J.H.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Beatrice Fischer (Empa) for DSC and TGA
measurements, Ylenia Maniglio (Empa) for help in the
synthesis, M. Nagel (Empa) for helpful discussions, and A.
Gerecke (Empa) for MS measurements. We thank T. Geiger
(Empa) for the molecular conformation simulations. This work
was supported by the Swiss Competence Center for Energy and
Mobility, CCEM-CH, Dursol project, and by the Swiss
(30) Heier, J.; Groenewold, J.; Huber, S.; Nuesch, F.; Hany, R.
Langmuir 2008, 24, 7316−7322.
(31) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002−
2004.
G
dx.doi.org/10.1021/ma401677v | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(32) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.;
Whitesides, G. M. Chem. Rev. 2005, 105, 1103−1169.
(33) Xue, L. J.; Han, Y. C. Prog. Polym. Sci. 2011, 36, 269−293.
(34) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U.
Nature 1998, 391, 877−879.
(35) Tisserant, J. N.; Hany, R.; Partel, S.; Bona, G. L.; Mezzenga, R.;
Heier, J. Soft Matter 2012, 8, 5804−5810.
(36) Ernst, L. A.; Gupta, R. K.; Mujumdar, R. B.; Waggoner, A. S.
Cytometry 1989, 10, 3−10.
(37) Horcas, I.; Fernan
́ ́
dez, R.; Gomez-Rodríguez, J. M.; Colchero, J.;
Gom
́
ez-Herrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705.
(38) Chikamatsu, M.; Nagamatsu, S.; Yoshida, Y.; Saito, K.; Yase, K.;
Kikuchi, K. Appl. Phys. Lett. 2005, 87, 203504.
(39) Israelachvili, J. N. In Intermolecular and Surface Forces, 2nd ed.;
Academic: New York, 1991.
(40) Mezzenga, R.; Schurtenberger, P.; Burbidge, A.; Michel, M. Nat.
Mater. 2005, 4, 729−740.
(41) Birshtein, T. M.; Skvortsov, A. M.; Sariban, A. A. Macromolecules
1976, 9, 892−895.
(42) Nakanishi, T.; Shen, Y. F.; Wang, J. B.; Li, H.; Fernandes, P.;
Yoshida, K.; Yagai, S.; Takeuchi, M.; Ariga, K.; Kurth, D. G.; Mohwald,
̈
H. J. Mater. Chem. 2010, 20, 1253−1260.
(43) Rao, A. M.; Zhou, P.; Wang, K. A.; Hager, G. T.; Holden, J. M.;
Wang, Y.; Lee, W. T.; Bi, X. X.; Eklund, P. C.; Cornett, D. S.; Duncan,
M. A.; Amster, I. J. Science 1993, 259, 955−957.
(44) Chance, R. C.; Patel, G. N. J. Polym. Sci., Polym. Phys. Ed. 1978,
16, 859−881.
(45) Rubner, M. F. Macromolecules 1986, 19, 2114−2128.
(46) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. J. Phys. Chem.
B 1998, 102, 9550−9556.
(47) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E.
Macromolecules 1999, 32, 4343−4350.
(48) Menzel, H.; Horstmann, S.; Mowery, M. D.; Cai, M.; Evans, C.
E. Polymer 2000, 41, 8113−8119.
(49) Xu, S. L.; Kang, S. Z.; Gan, L. B.; Zhang, L.; Wang, C.; Wan, L.
J.; Bai, C. L. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 757−760.
(50) Xia, Y. N.; Gates, B.; Li, Z. Y. Adv. Mater. 2001, 13, 409−413.
(51) Koster, L. J. A.; Shaheen, S. E.; Hummelen, J. C. Adv. Energy
Mater. 2012, 2, 1246−1253.
(52) Braun, P. V.; Rinne, S. A.; Garcia-Santamaria, F. Adv. Mater.
2006, 18, 2665−2678.
(53) Lindsay, G. A.; Ashley, P. R.; Davis, M. C.; Guenthner, A. J.;
Sanghadasa, M.; Wright, M. E. Mater. Sci. Eng., B 2006, 132, 8−15.
H
dx.doi.org/10.1021/ma401677v | Macromolecules XXXX, XXX, XXX−XXX