Supramolecular cross-linking of [60]fullerene-tagged polyphenylacetylene by the host-guest interaction of calix[5]arene and [60]fullerene

Article abstract of DOI:10.1002/anie.201004074

Hand in hand: A polyphenylacetylene with C₆₀ moieties can be cross-linked by a homoditopic tetrakiscalix[5]arene host by formation of a specific supramolecular complex (see picture). This noncovalent interaction leads to an increase of the mole

Full text of DOI:10.1002/anie.201004074

Angewandte
Chemie
DOI: 10.1002/anie.201004074
Supramolecular Cross-Linking
Supramolecular Cross-Linking of [60]Fullerene-Tagged Polyphenyl-
acetylene by the Host–Guest Interaction of Calix[5]arene and
[60]Fullerene**
Takeharu Haino,* Eri Hirai, Yoshihisa Fujiwara, and Kouki Kashihara
The cross-linking of polymeric chains is one of the most
important topics in polymer science. The cross-linking of
linear polymers creates three-dimensional networks and
reduces structural flexibility, leading to dramatic changes in
macroscopic morphologies and properties.[1] These are irre-
versible because the three-dimensional networks are con-
structed with covalent bonds. The introduction of a reversible
nature into polymer cross-linkage should generate a new class
of intelligent polymer materials; their macroscopic properties
might be turned “on” and “off” by external stimuli.
these studies, we found that covalently linked double
calix[5]arenes encapsulate fullerenes in their cavities to
form 1:1 host–guest complexes.[7] The molecular association
between a C60 molecule and a double-calix[5]arene is strong
enough to create a supramolecular polymeric array of
[60]fullerenes.[8] We set out to make use of this unique host–
guest motif as a new class of supramolecular cross-linkage
between polymer chains. Homoditopic host 2[8a] has two
double-calix[5]arene units, each of which can encapsulate a
C
60 moiety grafted onto an adjacent polymer chain to create a
The reversible nature of the cross-linkages can be realized
by employing noncovalent bonds. Grafting of supramolecular
entities onto conventional polymeric scaffolds can produce a
cross-linked three-dimensional polymer network with non-
covalent bonds. Complementary hydrogen bonding,[2] salt
bridges,[3] and metal–ligand interactions[4] produced stably
cross-linked polymers, which displayed unusual behaviors and
properties. Development of a new supramolecular cross-
linkage offers an alternative way to control the three-dimen-
sional structures of cross-linked polymer networks. The cross-
linkages driven by host-guest interactions are limited to
date.[5] Thus, much work remains to be done to develop a new
class of supramolecular cross-linkages with specific host–
guest interactions.
Fullerene and its analogues are an intriguing class of
molecules owing to their unique physical and chemical
properties. Fullerene-containing supramolecular polymers
that self-assemble through host–guest interactions have
been occasionally reported.[6] Our group has been developing
fullerene hosts based on a calix[5]arene. During the course of
stable noncovalent cross-linkage. Herein, we present the
supramolecular cross-linking of [60]fullerene-tagged poly-
phenylacetylene poly-1a, which is established by the host–
guest complexation of 2 in an interchain manner (Figure 1);
this process alters the molecular weight and solid-phase
morphology of the original polymer.
A rhodium catalyst, [{Rh(nbd)Cl}2] (nbd = norborna-
diene), is known to effectively polymerize phenylacetylenes
to give high-molecular weight, stereoregular polyphenylace-
tylenes.[9] The copolymerization of phenylacetylenes 3 and 4
at a feed ratio of [3]/([3] + [4]) = 0.9 proceeded smoothly
under rhodium catalysis in chlorobenzene to give poly-1a in
51% yield.[10] Poly-1a was composed of 3 and 4 in a ratio of
93:7, as determined by 1H NMR spectroscopy. Increasing the
fraction of 4 incorporated into the polymer caused a serious
decrease in product solubility and yield. The polymerization
of 3 was carried out under rhodium catalysis in toluene to give
poly-1b in 78% yield.
1H NMR spectroscopy provides evidence for the supra-
molecular complexation of C60 by the double calix[5]arene
unit of 2. Calix[5]arenes are typically subject to a ring-flipping
process that occurs rapidly at room temperature, resulting in a
broad signal of the bridge methylene group. Upon complex-
ation of a C60 molecule to the calix[5]arene cavity, the signal of
the bridge methylene appears as an AB quartet owing to the
increased energetic cost of the ring-flipping process. This
result reflects the attractive van der Waals interactions that
arise from the effective face-to-face contacts of the calix[5]-
arene aromatic rings with the molecular surface of C60.[7b]
Figure 2 shows how the bridge methylene signals of 2 change
upon the addition of poly-1a. The methylene signal at
approximately d = 3.8 ppm gradually broadened, and even-
tually split the two broad signals at approximately d = 3.4 and
4.0 ppm, whereas the addition of poly-1b did not induce any
change to the spectrum. These observations suggest that the
two cavities of 2 selectively encapsulate the C60 moieties of
poly-1a to link the polymer chains.
[*] Prof. Dr. T. Haino, E. Hirai, K. Kashihara
Department of Chemistry
Graduate School of Science, Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526 (Japan)
Fax: (+81)82-424-0724
E-mail: haino@sci.hiroshima-u.ac.jp
Homepage: http://home.hiroshima-u.ac.jp/orgchem/
Prof. Dr. Y. Fujiwara
Department of Mathematical and Life Sciences
Graduate School of Science, Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526 (Japan)
[**] This work is supported by Grants-in-Aid for Scientific Research
(Nos. 18350065, 21350066) of the JSPS, a Grant-in-Aid for Science
Research (No. 19022024) in a Priority Area “Super-Hierarchical
Structures” from MEXT (Japan), and the Yamada Science Founda-
tion.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004074.
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7899

Communications
Figure 1. a) Representation of supramolecular cross-linking by a host–guest interaction between a C60 moiety (pink spheres) and the ditopic cross-
linker. b) Polyphenylacetylenes poly-1a, b, ditopic cross-linker 2, and phenylacetylenes 3 and 4.
Figure 3. Stern–Volmer plots of 2 (1.0ꢀ10À6 molLÀ1) a) in the presence
*
*
of poly-1a ( ) and poly-1b ( ) in toluene at 293 K, and b) in the
^
presence of poly-1a in toluene (+), in chloroform ( ), and in
o-dichlorobenzene (ꢀ).
Figure 2. 1H NMR spectra (300 MHz) of 2 (3.2ꢀ10À4 molLÀ1) in the
presence of poly-1a (a–e: 0.00, 2.53, 5.06, 7.59, and 10.12 mgmLÀ1) or
poly-1b (g–j: 1.23, 2.46, 3.69, 4.92 mgmLÀ1), and poly-1a (f) and poly-
1b (k) at 298 K in [D]chloroform.
and (3.4 Æ 6.9) ꢀ 105 LmolÀ1 for the fluorescence quenching of
poly-1a and poly-1b, respectively. The Ksv value of poly-1a is
about ten times larger than that of poly-1b, indicating that
host–guest complexation of the C60 moiety with 2 enhances
the quenching effect.
To obtain the association constants for the two binding
sites of host 2, nonlinear regression analysis was carried out.
Plotting the absolute intensities of the emission versus
concentrations of the C60 moiety in toluene gave a hyperbolic
curve. A 2:1 association model was applied to determine the
association constants K11 and K21 ((4.36 Æ 0.58) ꢀ 104 and
(7.50 Æ 1.70) ꢀ 104 LmolÀ1, respectively). These values closely
match the reference value obtained from tert-butyl full-
erenoacetate 5 and double calix[5]arene 6 (see the Supporting
Information).[12] Accordingly, the double-calix[5]arene units
Supramolecular complexation between the C60 moiety of
poly-1a and host 2 was also detected by using fluorescence
spectroscopy. Host 2 had an absorption band at about 420 nm
owing to its highly conjugated nature, which is probably
assignable to a p–p* transition. Excitation at the band
wavelength gave rise to a strong emission at around 460 nm
in toluene (see the Supporting Information). Fullerene is
known to be a good energy acceptor;[11] thus, this emission can
be quenched when a C60 moiety is encapsulated within the
cavities of host 2. The addition of poly-1a quenched the
emission well, whereas poly-1b did not (Figure 3a). Stern–
Volmer plots gave constants Ksv of (34.9 Æ 1.5) ꢀ 105 LmolÀ1
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of 2 potently bind the C60 moieties even though they are
connected to a large polymeric backbone.
between the C60 moiety and the double-calix[5]arene unit in
these three solvent systems. The more competitive solvation
of chloroform and o-dichlorobenzene weakens the host–guest
interaction, and host 2 was unable to create stable cross-
linkages between the polymer chains.
Morphologies of the films prepared by casting solutions of
poly-1a in the presence and absence of 2 were investigated by
field emission scanning electron microscopy (FESEM). Poly-
1a formed agglomerated nanostructures consisting of par-
ticle-like aggregates in the range of 10–30 nm (Figure 5a,b). It
It is accepted that the stability of the host–guest complex
between a calix[5]arene and fullerene is solvent dependent.[13]
To evaluate the effect of various solvents on the supramolec-
ular association of poly-1a and host 2, Stern–Volmer con-
stants (Ksv) were determined to be (38.0 Æ 0.7) ꢀ 104, (9.60 Æ
0.02) ꢀ 104, and (5.60 Æ 0.09) ꢀ 104 LmolÀ1 in toluene, chloro-
form, and o-dichlorobenzene, respectively, on the basis of
concentrations of the C60 moiety (Figure 3b). The Ksv values
are obviously solvent-dependent and proportional to the
association constants between 5 and 6 in these solvents.
Accordingly, the values indicate that the binding affinities of
host 2 and the C60 moieties of poly-1a are diminished in
solvents in the order toluene, chloroform, and o-dichloro-
benzene.
Size-exclusion chromatography (SEC) is a powerful
technique for determining the molecular weight distribution
of polymeric materials. SEC of poly-1a in toluene gave an
elution peak and an estimation of its average molecular
weight (Mn = 13000, Mw = 18000, PDI = 1.38). Upon addition
of 2 to a solution of poly-1a, the elution peak maximum
shifted, and free host 2 did not elute until a molar ratio of the
binding site of 2 and the C60 moiety reached 1:1 (Figure 4a).
Figure 5. Field-emission scanning electron microscope images of thin
films on glass plates prepared by casting benzene solutions of
a,b) poly-1a (6.93ꢀ10À6 molLÀ1), c,d) poly-1a (6.93ꢀ10À6 molLÀ1
)
with 2 (9.7ꢀ10À6 molLÀ1). Scale bars: a) 5 mm, b) 500 nm, c) 40 mm,
d) 10 mm.
has been noted that p–p interactions and the immiscible
nature of fullerenes often induce their self-assembly, giving
rise to spherical particles.[14] The polymer has long alkyl chains
as well as C60 moieties; the immiscible nature of the C60
moiety can account for their microphase separation. This
presumably leads to further aggregation to form the particle-
like nanostructures. By contrast, the addition of ditopic host 2
to poly-1a brought about a dramatic morphological change in
which the particle-like nanostructures completely disap-
peared, and a wide-spread fibrous network formed (Fig-
ure 5c,d).
Atomic force microscopic (AFM) observation of the cast
films of poly-1a in the absence and presence of 2 prepared on
HOPG gave more detailed insights into the assembled state
of poly-1a on the nanoscale (Figure 6). In the absence of 2,
nanoparticle-like agglomerates were observed in the film
prepared from a solution of poly-1a as seen in the FESEM
observation (Figure 6a). In contrast, supramolecular cross-
linking of poly-1a by the complexation of 2 led to a dramatic
change in morphology. Figure 6b shows a phase image of the
well-oriented fibrils. This indicates that the highly ordered
nanostructures are built from the self-assembly of these
fibrils. The peak-to-peak profile of the phase gave a uniform
pitch of (19.6 Æ 1.6) nm, suggesting that the supramolecular
assembly of poly-1a with 2 produced fibrils possessing a
Figure 4. Size-exclusion chromatograms of poly-1a in the presence of
2 in a) toluene and b) o-dichlorobenzene. The molar ratios of [2]/[C60
unit] are: 0.0 (black), 0.25 (blue), 0.5 (red), and 1.0 (green).
Further addition of 2 brought about only an additional peak
assignable to free host 2. The Mn value and PDI (polydis-
persity index) of poly-1a in the presence of 0.25 and
0.5 equivalents of 2 were determined to be Mn = 25000 and
32000, and PDI = 2.80 and 2.81, respectively. The complex-
ation of 2 increased its molecular weight to over twice that of
free poly-1a. Surprisingly, excess 2 did not break previously
formed cross-linkages in toluene, even though the host–guest
complexation of a calix[5]arene and C60 is driven by non-
covalent interactions. From these results, it is evident that
homoditopic host 2 finds its complements on the polymer
chains to create remarkably stable cross-linkages by host–
guest interactions in toluene (Figure 4a). However, chroma-
tograms obtained in chloroform and in o-dichlorobenzene
contrasted sharply to that in toluene. The peak maximum of
poly-1a did not show a significant shift in the presence of host
2, and poly-1a and free host 2 eluted separately. These results
are quite consistent with the trend of binding affinities
Angew. Chem. Int. Ed. 2010, 49, 7899 –7903
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Communications
7.2 Hz), 1.20–17.0 (m, CH2, 17.86H), 0.88 ppm (t, CH3, 2.79H, J =
6.6 Hz).
Poly-1b: Monomer 3 (62.9 mg, 200 mmol) and triethylamine
(28.0 mL, 202 mmol) were dissolved in toluene (2 mL). A solution of
[{Rh(nbd)Cl}2] (1.0 mg, 2.2 mmol) in toluene (2.0 mL) was added to
this solution. After stirring at 308C for 24 h, the resulting mixture was
diluted with methanol. The resulting precipitates were filtered off and
dried under reduced pressure to give poly-1b (49 mg, 78%). Weights
were determined by SEC (Mn = 4.1 ꢀ 104 and Mw = 9.7 ꢀ 104). The
content of the cis–transoidal structure was estimated to be about
97 mol% based on the 1H NMR spectrum. 1H NMR (300 MHz,
CDCl3, 508C): d = 6.85 (br, aromatic, 2H), 6.54 (br, aromatic, 2H),
Figure 6. AFM images of assembled poly-1a on HOPG: a) without 2,
b) with 2 (left: height contrast; right: phase contrast). Scale bars:
100 nm.
=
5.73 (s, C C-H, 1H), 4.85 (br, CH2, 2H), 2.26 (m, C(O)CH2, 2H),
1.20–1.40 (m, CH2, 18H), and 0.86 ppm (t, CH3, 3H, J = 6.8 Hz).
Received: July 3, 2010
Revised: August 7, 2010
Published online: September 17, 2010
uniform diameter. The addition of double calix[5]arene 6
instead of 2 gave rise to the randomly aligned fibrils, which are
different from the morphologies of poly-1a observed in the
absence and presence of 2; these results imply that the
supramolecular cross-linking plays a key role for the forma-
tion of the highly ordered nanostructures (see the Supporting
Information).
In summary, we have demonstrated that homoditopic host
2 selectively encapsulates C60 moieties grafted onto a polymer
chain. Furthermore, this process creates a remarkably stable
cross-linkage, leading to an increase in molecular weight and
unique morphological changes to the polymer in its solid
state. The stability of the supramolecular cross-linkage
directly depends upon the solvent; thus, the supramolecular
cross-linkage of the polymer can be regulated by the solvent
system. The macroscopic solid-state morphologies of poly-1a
are highly influenced by the supramolecular cross-linking.
Nanoparticle-like morphologies are probably favored by the
immiscible nature of the C60 moiety, but the supramolecular
cross-linking makes the polymer bundle together to form
uniform fibrils. These, in turn, align into a well-oriented two-
dimensional array on a HOPG surface.
Keywords: calixarenes · cross-linking · fullerenes ·
.
polyphenylacetylene · supramolecular chemistry
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Experimental Section
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CDCl3, 508C): d = 6.30–7.40 (m, aromatic, 4H), 5.75 (s, C C-H, 1H),
4.60–5.20 (br, CH2, 2H), 4.78 (s, CH, 0.07H), 4.46 (t, O-CH2, 0.14H,
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