Topochemical polymerization of phenylacetylene macrocycles: A new strategy for the preparation of organic nanorods

Article abstract of DOI:10.1021/ja3116422

Soluble organic nanorods were prepared from phenylacetylene macrocycles using the topochemical polymerization of butadiyne moieties placed both inside and outside the macrocycles' skeletons. Macrocycles containing amide groups were self-assembled in a columnar fashion through the formation of an organogel in ethyl acetate. Upon irradiation with UV light, the Raman signals associated with butadiyne units completely vanished, indicating the creation of covalently linked nanorods.

Full text of DOI:10.1021/ja3116422

Communication
pubs.acs.org/JACS
Topochemical Polymerization of Phenylacetylene Macrocycles: A
New Strategy for the Preparation of Organic Nanorods
Simon Rondeau-Gagne, Jules Romeo Neabo, Maude Desroches, Jeremie Larouche, Josee Brisson,
́
́
́
́
́
́
and Jean-Franco
̧
is Morin*
Dep
́
artement de Chimie and Centre de Recherche sur les Mater
́ ́ ́ ́
iaux Avances (CERMA), Universite Laval, 1045 Ave de la Medecine,
Pavillon Alexandre-Vachon, Queb
́
ec, QC, Canada G1V 0A6
S
* Supporting Information
ABSTRACT: Soluble organic nanorods were prepared
from phenylacetylene macrocycles using the topochemical
polymerization of butadiyne moieties placed both inside
and outside the macrocycles’ skeletons. Macrocycles
containing amide groups were self-assembled in a
columnar fashion through the formation of an organogel
in ethyl acetate. Upon irradiation with UV light, the
Raman signals associated with butadiyne units completely
vanished, indicating the creation of covalently linked
nanorods.
he supramolecular self-assembly of macrocycles is one of
Tthe most popular strategies for preparing organic
nanotubes with new properties for various applications,
including host−guest chemistry,1 scaffolding,2 and gas storage.3
One major drawback of these supramolecular assemblies is their
relative kinetic instability. Because they rely on weak
interactions such as hydrogen-bonding, π−π, and van der
Waals interactions, the conditions under which they can be
employed and characterized are very restricted, limiting their
use in devices. Thus, strategies to improve their stability have
been developed, and one of the most promising ones is
covalent attachment of the macrocycles once the supra-
molecular assembly is formed. This strategy has proven to be
successful with many types of macrocycles, such as cyclo-
dextrin,4 cyclic oligopeptides,5 and calixarene.6 Many attempts
to prepare nanotubes from phenylacetylene macrocycles
(PAMs) in the crystalline state using 1,3-butadiyne top-
ochemical polymerization7 have been reported, but most of
them failed8 or have led to rapid decomposition to produce
undefined, insoluble carbon residues.9,10
We recently attempted to use this strategy to make stable
organic nanotubes from PAM 1 (Figure 1)11 by cross-linking
the butadiyne moieties through photoinduced topochemical
reactions to yield polydiacetylene (PDA)-walled nanotubes.
However, we found that only very short PDA oligomers were
formed under UV irradiation and that the yield of polymer-
ization was very low, although PDA formation from non-
macrocyclic 1,4-diarylbutadiyne derivatives in the gel state
proved to be efficient.12 We hypothesized that the butadiyne
moieties were unreactive because of the high rigidity of the
macrocycle.13 One way to drive the topochemical polymer-
ization is to heat the butadiyne-containing materials in the solid
state. Shimizu14 and Lauher15 independently used this strategy
Figure 1. Phenylacetylene macrocycles 1 and 2.
very recently to prepare insoluble, nonprocessable nanotubes
from macrocycles with flexible backbones. However, heating
PAM 1 in the solid state, which was not crystalline but in a
dried gel state, destroyed the hydrogen-bonding network
holding the macrocycles in a face-to-face configuration.
Therefore, we have designed the new PAM 2 (Figure 1)
possessing at each end a cross-linkable butadiyne unit with an
alkyl fragment attached, providing a more flexible environment
that should enable the topochemical polymerization. The cross-
linking of those units at the exterior of the macrocycle should
lead to a covalently linked, stable organic nanotube that can be
further heated to allow topochemical polymerization of the
butadiyne moieties present within the PAM backbone.
Herein we report the synthesis, self-assembly, and top-
ochemical polymerization in the gel state of PAM 2 bearing
multiple cross-linkable butadiyne moieties, leading to soluble,
one-dimensional (1D) nanorods. It was not possible to confirm
whether or not the resulting architectures possess an inner
cavity. To the best of our knowledge, this is the first report of a
controlled topochemical polymerization involving a six-
membered PAM, although many over the years have pursued
this goal. The use of rigid, conjugated, carbon-rich macrocycles
represents a necessary step toward the preparation of shape-
Received: November 28, 2012
Published: December 18, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of PAM 2
persistent, nongraphitic nanotubes and nanorods with unique
semiconducting properties.
hexanes. In those solvents, translucent gels were obtained at
concentrations as low as 1.0 wt %. In ethyl acetate, however,
very stable but opaque gels were obtained at the same
concentration, leading to larger crystallites within the organogel
(see Figure S19 in the SI). It is worth noticing that further
characterization of the ethyl acetate organogel had to be
performed quickly since it turned blue very rapidly under
ambient conditions, indicating the formation of PDA within the
supramolecular assembly.
We reported the synthesis of PAM 1 previously,11 and the
synthesis of PAM 2 is shown in Scheme 1. Starting from 5-
hexyn-1-ol, a bromination reaction on the terminal alkyne was
conducted with bromine to yield the corresponding bromoal-
kyne, which was immediately subjected to a Cadiot−
Chodkiewicz16 coupling reaction with 1-tetradecyne to yield
compound 2 in 96% yield over two steps. Next, the hydroxyl
group was replaced by an azido group using a tosylate
intermediate to give compound 3. The azido group was then
reduced into an amine using tin chloride, and the resulting
compound 4 was coupled to 3,5-diiodobenzoic acid11 using
standard Steglich conditions to give compound 5 in 84% yield.
Sonogashira coupling was used to install trimethylsilylacetylene
(TMSA) at the 3 and 5 positions, and the alkyne groups were
then deprotected. The resulting compound was coupled again
through Sonogashira coupling to 3,5-diiodo-1-octylbenzene11
in moderate yield. Another Sonogashira coupling was
performed at the iodo-bearing positions with triisopropylsily-
lacetylene (TIPSA), and the resulting compound 9 was
deprotected using tetrabutylammonium fluoride (TBAF) to
provide the half-macrocycle. Finally, a ring closure reaction
using Eglinton conditions was performed in dilute solution to
give PAM 2 in moderate 38% yield. PAM 2 was purified by
standard column chromatography and showed moderate to
good solubility in common organic solvents.
To gain better insight into the nanoscale morphology of the
ethyl acetate organogel, scanning electron microscopy (SEM)
analysis was performed (Figure 2). As expected, the organogel
Figure 2. SEM images of a dried ethyl acetate organogel of PAM 2 (10
mg/mL). Scale bars are (a) 2 μm and (b) 0.2 μm.
of PAM 2 was made of micrometers-long fibers with diameters
of ca. 200 nm. As we observed previously for PAM 1, the fibers
stacked to form few-micrometers-thick bundles, suggesting
strong interfiber interactions.8,17
Powder X-ray diffraction (PXRD) analysis on the same dried
gel was also performed to obtain information about the
intermolecular interactions and the distances within the
The gelation properties of PAM 2 were studied in common
organic solvents, and the detailed results are presented in Table
S1 in the Supporting Information (SI). PAM 2 gelified in
several solvents, including benzene, acetone, cyclohexane, and
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supramolecular assembly. The PXRD pattern is rather complex
and shows peaks in the small-angle region at 2θ = 7.0 and 4.0°,
corresponding to a ratio of 1:1/√3, which can be indexed as
the (100) and (110) reflections of a hexagonal lattice (Colh)
(Figure S20).11,18 The other peaks in the small-angle region
cannot yet be assigned with certainty but are likely to be due to
a columnar phase with different symmetry. The broad peak
centered at 2θ = 20.2° (4.39 Å) can be ascribed to the
intracolumnar liquidlike order between the alkyl chains. These
results suggest that the macrocycles within the organogel phase
are in the proper orientation to react with each other through
topochemical reactions to give a nanotubular structure.
To connect the macrocycles in order to fix the supra-
molecular assembly, topochemical polymerization of the
external butadiyne units was attempted by irradiating the
organogel with UV light (254 nm). Organogels obtained from
different solvents, all at a concentration of 10 mg/mL, were
tested. Interestingly, only the organogel obtained from ethyl
acetate turned blue under UV irradiation for 24 h (Figure S19).
After the reaction, the ethyl acetate was removed and
chloroform was added, providing an intense blue solution
with slight turbidity (Figure S19). This solution was analyzed
by size-exclusion chromatography (SEC) after it was filtered on
a 0.45 μm filter. It is worth mentioning that an appreciable
amount of insoluble blue material was left on the filter (30−50
wt % of the irradiated material). As shown in Figure S23, high-
molecular-weight materials were formed upon irradiation,
although PAM 2 was still present in rather high concentration.
Hence, a semipreparative SEC experiment was performed to
isolate the higher-molecular-weight fraction between 6 and 7.5
min. About 3 mg of intense blue material was thus obtained
from 20 mg of PAM 2, resulting in a yield of ca. 15% of soluble,
cross-linked material. It is worth mentioning that the nanorods
stayed soluble as long as they were not dried, in which case they
could be dispersed but not resolubilized. The other fractions
(retention time >8 min) were also collected, but no further
analysis was performed on them since they appeared as bluish-
green materials, suggesting incomplete topochemical polymer-
ization.
The optical properties of the SEC-purified blue material were
studied by UV−vis spectroscopy in chloroform solution. A
broad absorption band with λmax = 654 nm appeared upon
irradiation, which is characteristic of the PDA backbone (Figure
S22). As an empirical proof of the formation of organic
nanorods, the thermochromic properties of this material were
studied in both solution and the solid state. As expected, no
sign of thermochromism in either solution or the solid state was
detected under the numerous conditions tested, which indicates
that the PDAs were formed in a very rigid environment without
the possibility of chain torsion. This result eliminates at some
point the possibility of having a covalent polymer chain in
which the macrocycles are attached at only one end to each
other. In this particular case, one can assume that the resulting
PDA would exhibit chromism properties due to the steric
hindrance between the macrocycles, which would force the
polymer chain to twist, especially when a polar solvent such as
dimethyl sulfoxide or N,N-dimethylformamide (DMF) is used
to break the intramolecular hydrogen-bonding network.
To assess whether all of the external diyne moieties were
consumed upon irradiation to form PDA, the purified blue
material was analyzed by Raman spectroscopy (Figure 3). The
powder spectrum of PAM 2 exhibits a strong band at 2222
cm−1 associated with the stretching mode of the diyne moieties.
Figure 3. Raman spectra of PAM 2 (red) and the SEC-purified blue
material (black).
Unexpectedly, this band disappeared completely upon irradi-
ation for 24 h, suggesting that all of the diyne moieties, even
those within the macrocycle skeleton, reacted to give PDA
derivatives (Figure S24). This was confirmed by the appearance
of new bands at 1473 and 2096 cm−1 attributed to the
stretching vibration modes of newly formed alkene and alkyne
groups, respectively. Although the driving force for the
topochemical polymerization of the diyne units inside the
macrocycle is not yet understood, we hypothesize that
topochemical polymerization reactions occur on the outer
part of the macrocycle first, placing the diyne moieties inside
the macrocycle into the proper position to undergo the second
topochemical polymerization. The lack of diyne signals in the
final material eliminates the possibility of having a single-point
attachment between two macrocycles. This result is in perfect
agreement with the absence of thermochromic properties of the
blue material obtained after UV irradiation.
PXRD analysis was performed on the nanorods obtained
after SEC purification, and the diffractogram is presented in
Figure S21. Interestingly, the broad peak at 2θ = 20.2° (4.39 Å)
was still present, indicating that the intramolecular alkyl chain
interactions were preserved. Also, a less intense and broad peak
appeared at 2θ = 3.6° (24.5 Å), corresponding to the diameter
of the nanorods.
High-resolution transmission electron microscopy
(HRTEM) measurements were recorded on the purified
nanorods (Figure 4). The nanorods formed were several tens
Figure 4. HRTEM images of the nanorods. Scale bars are (a) 200 nm
and (b) 20 nm. White arrows indicate the position of individualized
nanorods.
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of nanometers long and 2−3 nm wide, in perfect agreement
with the structural parameters calculated by molecular
modeling and the distance measured by PXRD. The length
of the nanorods also suggests that topochemical polymerization
can be accomplished over a long range (several tens of
macrocycles). Although it was not possible to visualize an
internal void in the nanorods by HRTEM, the presence of
columnar phases and the π−π stacking between the macro-
cycles within the organogel phase of PAM 2, as proven by
PXRD, serve as a strong indication that the resulting material is
made of covalently linked 1D stacks of macrocycles, probably
forming tubular architectures with a calculated internal
diameter of ca. 8 Å (Figure 24). In fact, intercolumnar
topochemical reactions are unlikely because the formation of
PDA is very dependent on the distance between monomers
(∼4.9 Å) and the angle between the reacting butadiyne units
(45°).7 For the same reason, cross-linking reactions occurring
in amorphous phases (if present) are highly improbable, leaving
the formation of 1D nanorods as the only plausible product, as
shown in Figure 5.
ACKNOWLEDGMENTS
■
This work was supported by NSERC through a Discovery
Grant. We thank Richard Janvier (U. Laval) for his help with
SEM and TEM experiments, Rodica Plesu (U. Laval) and Jean-
Franco̧ is Rioux (U. Laval) for their help in polymer
characterization, and Philippe Dufour (U. Laval) for HRMS
experiments and synthesis. S.R.-G. thanks the NSERC for a
Ph.D. scholarship.
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́ ́
S.; Neabo, J. R.; Daigle, M.; Morin,
In conclusion, new soluble organic nanorods were prepared
from phenylacetylene macrocycles through topochemical
reactions of butadiyne units properly located within the
macrocycle structure. This finding opens the way for the
formation of nongraphitic semiconducting nanorods and
nanotubes for electronic applications. Our efforts are now
being directed toward the preparation of new macrocycles as
building blocks that will lead to nanorods with different
diameters. We also plan to investigate the porous properties of
these new architectures for gas storage and separation. This
study will allow us to assess whether or not the nanorods have a
formal internal cavity.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data for all new
compounds, PXRD patterns, UV−vis spectra, and gelation
properties. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jean-francois.morin@chm.ulaval.ca
■
Notes
The authors declare no competing financial interest.
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