Conductive one-handed nanocoils by coassembly of hexabenzocoronenes: Control of morphology and helical chirality

Article abstract of DOI:10.1002/anie.200704747

(Chemical Presented) One-handed military discipline: The hexabenzocoronene (HBC) derivative 1 can coassemble with the chiral HBCs (S)- or (R)-2 to yield graphitic nanocoils (see picture). Self-assembly of 2 alone gives noncoiled fibrous assemblies. A sergeants-and-soldiers effect leads to the formation of one-handed nanocoils. These can be covalently stabilized by post-surface ROMP of the pendant norbornene groups to give a uniform cast film. Upon doping with I₂, this film becomes electroconductive without any detectable morphological disruption.

Full text of DOI:10.1002/anie.200704747

Communications
DOI: 10.1002/anie.200704747
Helical Nanocoils
Conductive One-Handed Nanocoils by Coassembly of
Hexabenzocoronenes: Control of Morphology and Helical Chirality
Takuya Yamamoto, Takanori Fukushima,* Atsuko Kosaka, Wusong Jin, Yohei Yamamoto,
Noriyuki Ishii, and Takuzo Aida*
Electroconductive one-handed helical nanofibers are attrac-
tive in view of their potential for the realization of nanoscale
solenoids. While there are many reported examples of the
construction of helical nanofibers by self-assembly of p-
electronic molecules,[1,2] most of these involve twisted rib-
bons,[3] which do not provide the coiled pathways essential for
electromagnetic properties. Some coiled assemblies of aro-
matic molecules have been reported,[4] although they are still
very rare and their conducting properties have not been
investigated, mainly because of their insufficient morpholog-
ical robustness for electrochemical doping. Thus, the design of
nanostructures that satisfy the three requisites for electro-
magnetic properties—coiled pathways, one-handedness, and
electroconductivity—is still a highly challenging research
topic.
stacked HBC array that exhibits conductivity upon oxidative
doping.[6] However, the resultant nanocoils are a mixture of
right- and left-handed ones, which means that further
structural elaboration is needed to realize the one-handed
helical graphitic array that is essential for the exploration of
electromagnetic properties.
Herein we report the selective formation of covalently
stabilized conductive nanocoils with a one-handed helical
chirality by coassembly of norbornene-appended HBC deriv-
atives 1 and 4 (Figure 1). This achievement is the result of a
A few years ago we found that a Gemini-shaped
hexabenzocoronene (HBC) bearing triethylene glycol and
dodecyl chains self-assembles into a graphitic nanotube.[5]
More recently, we have also found that the incorporation of
pendant norbornene groups into the amphiphilic HBC (1)
gives rise to a nanocoiled assembly with uniform diameter and
helical pitch.[6] The metastable coiled structure exists for a
sufficiently long time, probably because of a steric effect of
the pendant norbornene groups, and therefore allows post
ring-opening metathesis polymerization (ROMP) of the
norbornene groups to covalently stabilize the kinetically
selected assembly against a thermodynamic coil-to-tube
transformation. The polymerized nanocoil consists of a p-
[*] Dr. T. Fukushima, Prof. Dr. T. Aida
Department of Chemistry and Biotechnology
School of Engineering and Center for NanoBio Integration
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
(Japan)
Fax : (+81)3-5841-7310
E-mail: aida@macro.t.u-tokyo.ac.jp
Homepage: http://macro.chem.t.u-tokyo.ac.jp/Home.html
Figure 1. Formation of one-handed nanocoils through a sergeants-and-
soldiers effect in the coassembly of 1 with 4.
Dr. T. Yamamoto, Dr. T. Fukushima, A. Kosaka, Dr. W. Jin,
Dr. Y. Yamamoto, Prof. Dr. T. Aida
ERATO-SORST Nanospace Project, Japan Science and Technology
Agency (JST)
National Museum of Emerging Science and Innovation
2-41 Aomi, Koto-ku, Tokyo 135-0064 (Japan)
E-mail: fukushima@nanospace.miraikan.jst.go.jp
long-term effort to optimize the molecular structure and self-
assembly conditions to suppress the concomitant formation of
undesired structures, such as noncoiled fibers and nanotubes.
We have reported previously that HBC derivative (S)-2,
which possesses a chiral handle in the hydrophilic chain, self-
assembles into a nanotube containing a one-handed helical
array of p-stacked HBC units.[3,7] This observation prompted
us to investigate whether the self-assembly of (S)-3 might lead
to the formation of a one-handed nanocoil. However, only
non-coiled fibrous assemblies and nanotubes were obtained[8]
Dr. N. Ishii
Biological Information Research Center
National Institute of Advanced Industrial Science and Technology
(AIST)
Tsukuba Central-6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566 (Japan)
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 2. SEM and TEM images of air-dried suspensions of polymerized coassemblies of 1 and 4. a–c) SEM images of the coassemblies formed
from 1 with 1 mol% of (S)-4 (a), 20 mol% of (S)-4 (b), or 20 mol% of (R)-4 (c). d) TEM image of the sample prepared with 20 mol% of (S)-4. e–
h) SEM images of the coassemblies formed with 50 (e), 60 (f), 80 (g; inset: TEM), and 95 mol% (h; inset: TEM) of (S)-4. The TEM images are
not informative of the handedness.
despite thorough investigations under a variety of self-
assembly conditions, including those optimized for the
formation of the nanocoil from 1 and the one-handed
nanotube from (S)-2. We then synthesized (S)-4, which has
a shorter spacer between the norbornene and HBC moieties,
with the expectation that a pronounced steric effect from the
norbornene groups might facilitate nanocoil formation.
However, as in the case of (S)-3, only nanotubular or fibrous
assemblies were obtained from (S)-4.[8] These failures imply
that the formation of nanocoils from Gemini-shaped HBC
derivatives relies on a very delicate balance between the
kinetic and thermodynamic parameters in the self-assembly
process.
We found that the enantiomers of 4 can drive the
stereoselective formation of nanocoils by coassembly with 1.
In a typical reaction, a 5-mL vial containing a mixture of 1
(0.52 mmol, 80 mol%) and (S)-4 (0.13 mmol, 20 mol%) in
CH2Cl2 (1 mL) was placed in a 50-mL vial containing 10 mL
of Et2O for vapor diffusion. The setup was kept at 158C in an
incubator for 24 h to give a yellow precipitate quantitatively.
Only left-handed nanocoils with a diameter of 30 nm and a
pitch of 60 nm were observed by SEM microscopy. To
enhance the morphological robustness, the coassembled
nanocoils were surface-polymerized by ROMP according to
a reported method.[6] Thus, the resulting vapor-diffusion
mixture was diluted with Et2O to a total volume of 20 mL
(Et2O/CH2Cl2 100:1 v/v) and the second-generation Grubbs
catalyst 5 (0.065 mmol) was added to the resulting suspension.
The mixture was stirred slowly at 208C for 24 h and the
reaction was then quenched with a few drops of ethyl vinyl
ether. The resultant solid was found to be insoluble in CH2Cl2,
which is a good solvent for both 1 and (S)-4. The coiled
structure of the coassembly was preserved during the
polymerization, as confirmed by SEM and TEM micrographs
(Figures 2b and 2d). The polymerized nanocoils were
approximately 30 nm in diameter, 60 nm in pitch, and 20 nm
in tape width. These dimensions are identical to those of the
self-assembled nanocoils formed from 1 alone before and
after ROMP of its pendant norbornene groups.[6] As
expected, the use of (R)-4 instead of (S)-4 for coassembly
with 1, followed by surface ROMP under identical conditions
to those described above, afforded nanocoils with the
opposite helical sense (Figure 2c).
Analogous coiled nanofibers were obtained by the
coassembly of 1 with (S)-4 over a wide composition range
(mol fraction of (S)-4: 1, 5, 10, 20, 30, 40, and 50%). However,
undesired products began to form when the mol fraction of
(S)-4 exceeded 50% (60, 70, 80, 90, 95, and 99%). For
example, both nanocoils and nanotubes were obtained at a
mol fraction of (S)-4 of 60 and 70% (Figure 2 f), while only
nanotubes with a diameter of 16 nm formed at 80 and 90%
mol fractions of (S)-4 (Figure 2g). A further increase of the
mol fraction of (S)-4 (95 and 99%) resulted in the formation
of fibrous assemblies (Figure 2 h). It should be noted that
simultaneous morphology and handedness control requires
much finer tuning of the self-assembly composition. As
already described, the coassembly of 1 with 20 mol% of
(S)- and (R)-4 allows selective formation of the left- and right-
handed nanocoils, respectively (Figures 2b and 2c). When the
mol fraction of 4 was lower than 20% (10, 5, and 1%),
however, the disfavored helical handedness was also obtained
for the resulting nanocoils (Figure 2a), whereas when the mol
fraction of 4 was higher than 50% (Figure 2e), the nanocoil
remained one-handed despite the undesired formation of
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Communications
other assemblies such as nanotubes and fibers (see above and
Figure 2 f).
The electronic absorption spectra of the coassembled
nanocoils display red-shifted absorption bands at 426 and
459 nm due to the p-stacked HBC units (Figures 3a and 3d).
morphological robustness. Thus, an Et2O suspension of the
polymerized one-handed nanocoils obtained from 1 and (S)-
or (R)-4 (20 mol%) was cast onto the electrodes to give a
uniform film. While the film before doping showed only a
negligible electrical current, the doped film prepared upon
exposure to an I2 vapor
was conductive and
exhibited a linear I–V
profile.[8] The average
conductivity of the
doped film composed
of the polymerized
nanocoils with a left-
handed helical chirality
was found to be 1.4 ”
10À4 ScmÀ1, which is
comparable to those of
the right-handed ana-
logue and a racemic
reference
from
prepared
1
alone.[6,8] In
sharp contrast with the
nonpolymerized nano-
coils,[6] the polymerized
nanocoils retain their
morphological integ-
rity throughout the oxi-
dative doping with I2.[8]
Figure 3. Electronic absorption and CD spectra of polymerized nanocoils obtained from 1 and (S)-4 (a–c) or (R)-4
(d–f) in Et2O/CH2Cl2 (100:1 v/v; 30 mm) at 208C, in a quartz cell with a path length of 10 mm.
Although the wavelengths of these absorption bands are
unaffected by the mixing ratio of 1 and 4, their intensities
decrease notably with increasing mol fraction of 4. Character-
istic circular dichroism (CD) bands with positive and negative
signs for (S)-4 (Figures 3b and 3c) and ( R)-4 (Figures 3e and
3f), respectively, in accordance with the morphological
inversion of the nanocoils in the SEM observations, were
also observed in the region of these absorption bands. It
should also be noted that the intensities of these CD bands
change as a function of the mol fraction of 4. Considering the
hypochromic effect of 4 on the absorption spectral profile of
the coassembly, the CD intensities at 431 and 457 nm were
Figure 4. Plots of CD intensity/absorbance ratios at 431 (circles) and
457 nm (squares) versus the mol fraction of 4. Blue: coassembly of 1
with (S)-4. Red: coassembly of 1 with (R)-4.
divided by the corresponding absorbances to approximate the
net contribution of the p-stacked HBC array to the observed
chiroptical activity. When plotted against the mol fraction of
4, the ratio q/A shows a saturation tendency with a monotonic
increase up to 10% (Figure 4), which suggests that the
enhanced handedness of the coiled nanofibers formed by the
coassembly of 1 with (S)- or (R)-4 can be accounted for by the
successful operation of the sergeants-and-soldiers effect
(Figure 1).[9,10]
We also attempted the coassembly of 1 with (S)-3 (1, 5, 10,
and 50 mol%) but obtained a mixture of ill-defined structures
and nanocoils with no particular helical selection.[8] This result
suggests that it is difficult to coassemble 1 and (S)-3 into a
single object.
In conclusion, we have fabricated conductive nanocoils
with a defined handedness by taking advantage of the
sergeants-and-soldiers effect. One of the main difficulties
encountered in this research was that the coiled assembly is a
kinetically favored product and is therefore subject to trans-
form into a tubular assembly thermodynamically. This
situation meant that a coiled assembly with a sufficiently
long lifetime to enable its covalent stabilization had to be
designed. The other major difficulty arose from the fact that
the morphological preference of the “sergeant” (4; fiber) is
different from that of the “soldier” (1; coil). By overcoming
these synthetic difficulties, we were able to fabricate the first
The electroconductive properties of the polymerized
nanocoils were investigated by a two-probe method using
5 mm-gap electrodes, thereby taking advantage of their
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Received: October 13, 2007
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Published online: January 24, 2008
Keywords: chirality · conducting materials · polymerization ·
.
self-assembly · supramolecular chemistry
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