Photoinitiated Polymerization of Columnar Stacks of Self-Assembled Trialkyl-1,3,5-benzenetricarboxamide Derivatives

Article abstract of DOI:10.1021/ja037927u

A disk-shaped molecule, N-(5-sorbyl-pentyl)-N′,N″ -di(n-octyl)benzene-1,3,5-tricarboxamide (1), has been synthesized and assembled into a columnar stack in cyclohexane. Using a photoinitiated process, we polymerized monomer 1 in its self-assembled state a

Full text of DOI:10.1021/ja037927u

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Photoinitiated Polymerization of Columnar Stacks of
Self-Assembled Trialkyl-1,3,5-benzenetricarboxamide
Derivatives
Mitsutoshi Masuda,† Pascal Jonkheijm, Rint P. Sijbesma,* and E. W. Meijer*
Contribution from the Laboratory of Macromolecular and Organic Chemistry,
EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB, EindhoVen, The Netherlands
Received August 14, 2003; E-mail: r.p.sijbesma@tue.nl; e.w.meijer@tue.nl
Abstract: A disk-shaped molecule, N-(5-sorbyl-pentyl)-N′,N′′-di(n-octyl)benzene-1,3,5-tricarboxamide (1),
has been synthesized and assembled into a columnar stack in cyclohexane. Using a photoinitiated process,
we polymerized monomer 1 in its self-assembled state and analyzed the resulting poly-1. On cooling a
boiling cyclohexane solution of 1, the molecules aggregate via amide hydrogen bonds, as supported by
the position of the N-H stretching band in FT-IR spectroscopy. Evidence of helical columnar stacking of
1 in the aggregate is provided by an induced CD effect upon blending 1 with a chiral side-chain homologue
3 in a so-called “sergeants and soldiers” experiment. The columnar assembly in cyclohexane was
polymerized by UV light (365 nm) irradiation in the presence of 2,2-dimethoxy-2-phenylacetophenone as
radical photoinitiator. The polymerization occurs selectively to give 1,4-polymer with isolated trans CdC
1
bonds, as shown by FT-IR, and H and 13C NMR spectroscopy. Lack of polymerization of methyl sorbate
(4) under identical conditions, and low incorporation of 4 in copolymerizations with 1 in cyclohexane, suggests
that the polymerization preferentially occurs within the columnar assembly. Size exclusion chromatography
shows that the degree of polymerization (Xn) based on number average molecular weight is approximately
65. Incorporation of small amounts of nonpolymerizable derivatives 2 and 3 into the columns has no effect
on Xn and conversion. Molecular models show that the polymerizable sidearm of 1 is long enough to span
the distance between the monomers in the chiral stack. Under the atomic force microscope (AFM), purified
samples of polymerized 1 displayed nanometer-sized fibrous morphologies with a high-axial-ratio (>150),
uniform width (60 nm), and a thickness (1.0 nm) which corresponds to the width of the benzene
tricarboxamide core of 1, whereas aggregates of nonpolymerized 1 yield a featureless image due to their
instability.
Introduction
chiral components to columnar assemblies of achiral 1,3,5-
benzenetricarboxamides was shown to bias their helicity6d in a
Self-assembled columnar structures of stacked aromatic
compounds are attractive building blocks for supramolecular
electronics,1-3 because they possess the necessary one-
dimensional nanometer-sized architecture to function in charge
or energy transport. 1,3,5-Benzenetricarboxamides are syntheti-
cally easy to access and self-assemble in the crystal,4 in the
liquid crystalline state,5 and in both organic and aqueous media
to form columnar structures via triple amide hydrogen bonding.6
In previous work, we have studied derivatives with chiral side
chains, which express supramolecular helicity within the column
due to helical strands of amide hydrogen bonds. Addition of
supramolecular form of the “sergeants and soldiers” principle.7
Covalent fixation of self-assembled objects is an important
issue in supramolecular chemistry,8,9 and polymerization of a
chiral or helical arrangement of molecules in liquid crystalline
phases has important technological applications.10 Polymeriza-
tion of columnar aggregates fixes and stabilizes objects with a
well-defined structure and enables further processing such as
modification and positioning. So far, many attempts have been
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† Current address: Nanoarchitectonics Research Center, National Institute
of Advanced Industrial Science and Technology (AIST), Tsukuba Central
5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
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J. AM. CHEM. SOC. 2003, 125, 15935-15940
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A R T I C L E S
Masuda et al.
reported concerning polymerization of self-assemblies in organic
and aqueous media, but relatively few successful attempts have
been reported for polymerization of columnar structures5a,11 or
other one-dimensional assemblies such as nanoscale fibers12 or
giant wormlike micelles.13 Major issues that complicate the
covalent fixation of columnar aggregates by means of poly-
merization are the possibility of cross-linking between columns
and incompatibility of the repeat distance of the polymer chain
with the stacking distance of the monomers in the column.
elastomers,17 and for the polymerization of bilayer18 and
monolayer16c,19 membranes and hexagonal mesophases.11f,20
However, the strict steric requirements for the 1,4-polymeriza-
tion of diacetylenes limit the use of this functional group in
less ordered phases. Therefore, we have chosen the sorbate
moiety as a polymerizable unit in trialkyl-benzenecarboxamide
derivative 1. In this Article, the synthesis of the monomer 1,
its self-assembly properties, the polymerization within the
assemblies, the studies of the stereochemistry, and the morphol-
ogy of the polymers obtained are disclosed.
Molecular Design
To polymerize columnar self-assemblies based on C3-sym-
metrical benzene-tricarboxamides with minimal distortion of the
well-defined architecture, we focused on two additional require-
ments for the molecular design of the polymerizable group: (1)
avoiding cross-linking and (2) compatibility of the repeat
distance. Two completely different strategies to prevent inter-
columnar reaction can be envisaged. In one strategy, either a
step polymerization or a chain polymerization of molecules with
multiple reactive groups is used, and placing the reactive groups
away from the periphery prevents intercolumnar reaction.11e
A
strategy using multiple reactive groups has been shown to work
well in self-assembled polymer aggregates,13,14 or in condensed
phases, where the dynamics of the self-assembly processes are
slow. In solution, where the dynamics are much faster, an
alternative strategy to prevent cross-linking is proposed by using
a chain polymerization and a single polymerizable group per
molecule. When a chain polymerization is used to connect con-
secutive layers of columnar aggregates of benzene-tricarbox-
amides, the polymerizable group attached to the self-assembly
unit is required to give a polymer chain with a distance between
repeating units that is equal to, but not shorter than, the stacking
distance of the columnar assemblies, which is 0.36 nm.4 If the
distance is less, the length of the polymer chain does not match
the columnar length, the polymerization will terminate early,
or the columnar architecture may even be lost. Furthermore,
the angle between consecutive monomers (ca. 60° in the crystal
structure of a benzene-1,3,5-tricarboxamide derivative4) requires
additional length. This requirement precludes the use of efficient
1,2-polymerization processes such as the radical polymerization
of (meth)acrylates, which give a repeating distance of around
0.25 nm. 1,4-Chain polymerizations, on the other hand, give a
repeating unit distance around 0.48 nm,15 giving some freedom
in the offset of groups from disk to disk. Polymerizations of
diacetylenes and dienes have been used extensively in topo-
chemical reactions in the crystalline state,16 in thermoplastic
Results
Synthesis. Monomer 1 was synthesized by the hydrolysis of
one of the methyl ester groups of benzene-1,2,3-tricarboxylic
acid trimethylester using 1 equiv of sodium hydroxide in 69%
yield, followed by amidation of the two remaining ester groups
with n-octylamine in 91% yield and the condensation of the
carboxylic acid group with 6-aminopentyl sorbate in 90% yield
(Scheme 1). Monomer 1 was fully characterized by 1H and 13
NMR, FT-IR spectroscopy, and elemental analysis. The stereo-
chemistry of the sorbyl group in 1 was confirmed to be more
than 98% of the E,E-configuration based on H NMR spec-
troscopy. The synthesis of the nonpolymerizable derivatives,
achiral 2 and chiral 3, has been reported previously.6c,d
Neat monomer 1 forms an enantiotropic columnar mesophase
at elevated temperature (melting temperature (Tm) ) 158-161
C
1
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O’Brien, D. F. J. Am. Chem. Soc. 2001, 123, 3595-3596.
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31, 9403-9405. (b) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Hanabusa, K.;
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2000, 16, 7545-7457.
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805.
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M.; Amstutz, A. Science 1997, 276, 384-389.
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15936 J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003

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Trialkyl-1,3,5-benzenetricarboxamide Derivatives
Scheme 1
A R T I C L E S
°C, clearing temperature (Tc) ) 171 °C) as concluded from the
flowerlike texture observed for a sample under polarized optical
microscopy and the miscibility with the known columnar
mesophase of 2, indicating that the monomers stack on top of
each other. Compound 1 has a higher melting point but a lower
clearing temperature than the saturated chain chiral analogue 2
(Tm ) 102 °C, Tc ) 204 °C) and achiral analogue 3 (Tm ) 119
°C, Tc ) 236 °C).
Self-Assembly in Cyclohexane. Cyclohexane was chosen as
solvent for the self-assembly and the polymerization of 1 for
its low polarity, its compatibility with amide hydrogen bond
formation, and its low reactivity toward growing radicals.21
Monomer 1 is soluble in boiling cyclohexane (approximately 1
× 10-2 mol/L), and solutions become slightly viscous on
cooling, indicating self-assembly formation similar to other
1,2,3-benzenetricarboxamide derivatives.6a,b At concentrations
below 1 × 10-1 M, gel formation is not observed, probably
due to the absence of physical cross-links between columns,
whereas longer chain homologues do form gels at similar
concentrations.6b In FT-IR spectra of cyclohexane solutions of
1 (Figure 1b-d), an amide N-H stretching band is observed
around 3241 cm-1 at 22 °C. The appearance of the NH stretch
region does not change upon dilution to a concentration of 1 ×
10-4 M, and N-H bands attributable to molecularly dissolved
species, which are observed in CHCl3 solution (Figure 1a),
remain absent. A non-H-bonded amide I band of 1 in CHCl3
appears at 1665 cm-1. In cyclohexane, this band appears at 1647
and 1630 cm-1. These results indicate that the monomers are
stacked on top of each other via amide hydrogen bonds as has
been observed for the columnar self-assemblies of 2, 3, and
their mixtures. The hydrogen-bonded stacks are in agreement
with an X-ray structure of a model benzene 1,3,5-tricarboxy
amide.4
Figure 1. Solution IR spectra of 1 at 22 °C. (a) 1.18 × 10-4 mol/L in
CHCl3, (b) 8.92 × 10-5 mol/L in cyclohexane, (c) 8.92 × 10-4 mol/L in
cyclohexane, and (d) 8.92 × 10-3 mol/L in cyclohexane.
solution of chiral 3, a negative Cotton effect was observed at
the absorption maxima of the phenyl and sorbyl chromophores.
A strong sergeant and soldiers effect7 was observed. At a
wavelength of 222 nm, the Cotton effect reaches saturation at
5% of 3 (Figure 2, inset), indicating that a small amount of 3
controls the helicity of columns of 1 in a highly cooperative
self-assembly process. The CD spectra also show a significant
Cotton effect at 250 nm, the absorption maximum (λmax) of the
sorbyl group, confirming that the molecules of 1 are incorporated
in the helical assemblies and that the sidearm as well as the
benzene tricarboxamide core are in a well-defined chiral
environment.
Variable-temperature (VT) CD spectra of a solution of a
mixture of 1 and 3 containing 6.75% 3, at a total concentration
of 6.7 × 10-5 M, indicate that the ellipticity is gradually lost
upon heating and reversibly disappears at 75 °C (Figure 3). The
loss of ellipticity upon heating in this experiment is presumably
the combined result of the loss of helicity of the columns and
The self-assembly in dilute solution was investigated using
CD spectroscopy on mixtures of 1 and 3 in cyclohexane (Figure
2). Obviously, a solution of achiral monomer 1, at a concentra-
tion of 3.35 × 10-5 mol/L, does not show a Cotton effect.
However, upon mixing this solution with a small amount of a
(21) Odian, G. Principles of Polymerization, 3rd ed.; A Wiley-Interscience
Publication: New York, 1991.
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A R T I C L E S
Masuda et al.
Figure 2. CD spectra of mixtures of 1 and 3 in cyclohexane at 22 °C at
a total concentration of 3.3 × 10-5 mol/L. Inset: ∆ꢀ value at 222 nm as a
function of the mole fraction of chiral 3.
Figure 4. FT-IR spectra of 1 and poly-1 as cast from cyclohexane.
1
poly-1 was characterized by FT-IR, H NMR, and 13C NMR
spectroscopy.16c,18b,22 The FT-IR spectrum of 1 displays a
conjugated CdC wagging mode of the sorbyl group at 1000
cm-1. The intensity of this band decreases upon irradiation
(Figure 4a and b). Simultaneously, a new band appears at 972
cm-1 which is attributed to an isolated trans CdC bond,
indicating 1,4-polymerization of 1.16c In addition, the FT-IR
spectrum of the product shows a band for a nonconjugated ester
carbonyl group at 1732 cm-1. The hydrogen-bonded N-H
stretching band is observed at the same frequency (3241 cm-1
)
before and after irradiation, indicating that polymerization of
the sorbyl ester groups does not significantly disturb the
columnar structure of the self-assembled aggregates.
1H NMR spectra of 1 and the purified poly-1 (Figure 5a and
b) indicate that irradiation results in 1,4-polymerization. The
vinyl proton peaks (7.21, 6.13-6.66, and 5.75 ppm) of the
sorbate monomer have disappeared, and a new single broad peak
(5.36 ppm), assigned to isolated trans double bonds of 1,4-
polymer, appears for polymerized 1 (Figure 5a and b). The
formation of 3,4-polymer is excluded because no vinyl proton
signals around 6.9-7.0 and 5.7-5.8 ppm are observed.18b,23 The
spectral regions around 0.9 and 1.6 ppm contain the methyl
signals of 1,4-polymerization product (a in Figure 5b) and 1,2-
polymerization product, respectively. Although these signals
overlap with the methyl group of the octyl chain (n) and with
a number of methylene protons (g, i, and l), respectively, the
integral ratio of these areas shows that 1,4-addition accounts
for more than 95% of the polymerization.
The 13C NMR spectrum of poly-1 provides more specific
information on the stereochemistry of the double bond. The
chemical shifts of two sets of tertiary carbons adjacent to the
double bond (carbon atom b in the structure of Figure 5b
resonating at 39.6 ppm, and carbon atom e in the structure of
Figure 5b resonating at 56.4 and 55.0 ppm24) are in good
agreement with the reported25 peak positions of poly(methyl
Figure 3. Temperature dependence of the Cotton effect at 222 nm of a
solution of 1 mixed with 6.75 mol % of 3 in cyclohexane.
a decreasing average column length. The latter decreases both
the cooperativity and the strength of the sergeant and soldiers
effect. To polymerize the aggregates in the helical state,
polymerizations were performed at room temperature.
Polymerization. Polymerization of self-assembled 1 in cyclo-
hexane was performed using Irgacure 651 (2,2-dimethoxy-2-
phenylacetophenone) as a radical photoinitiator and UV-light
(λmax ) 365 nm) to generate radicals. Direct photoactivation of
1, which has been shown to give low degrees of polymerization18b
and mainly dimerization,19 was prevented by filtering the light
through glass (cutoff wavelength 325 nm). Upon UV-irradiation
at 25 °C, the cyclohexane solution of 1 became slightly turbid
and more viscous. Progression of the polymerization was
monitored by FT-IR spectroscopy, using the changes in intensity
of the monomer band at 1000 cm-1 and the polymer band at
972 cm-1 (see below). After 5 h, the ratio of these bands did
not change significantly, and irradiation was stopped. This time
corresponds to approximately 4 times the half-life (70 min) of
the photoinitiator under the prevailing reaction conditions.
Poly-1 was isolated by removal of the solvent and subsequent
washing with diethyl ether to remove unreacted monomer.
Finally, the polymer was purified by Soxhlet extraction with
diethyl ether.
(22) Tsuchida, E.; Hasegawa, E.; Kimura, N.; Hatashita, M.; Makino, C.
Macromolecules 1992, 25, 207-212.
(23) Naf, F.; Degen, P. HelV. Chim. Acta 1971, 54, 1939-1949.
(24) See Supporting Information for spectra.
Spectroscopic Characterization. Polysorbates can be formed
via 1,2-, 1,4-, or 3,4-polymerization. The microstructure of
(25) Hertler, W. R.; Ovenall, D. W.; Reddy, G. S.; Sogah, D. Y. J. Am. Chem.
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