Stabilization of a kinetically favored nanostructure: Surface ROMP of self-assembled conductive nanocoils from a norbornene-appended hexa-peri-hexabenzocoronene

Article abstract of DOI:10.1021/ja064461h

Newly designed norbornene-appended hexabenzocoronene 1 self-assembles, upon diffusion of an Et₂O vapor into its CH₂Cl₂ solution, to form either graphitic nanocoils or nanotubes, depending on the self-assembling conditions. The coiled assembly, selectively formed at 15 °C, is a kinetic intermediate for the tubular assembly and transforms into nanotubes on standing at 25 °C. However, post-ring-opening metathesis polymerization of the norbornene pendants of 1 enhances the thermal stability of the coiled assembly as well as the tubular one and disables a thermodynamic coil-to-tube transition. The polymerized nanocoils show an electroconductivity of 1 × 10^-4 S cm^-1 upon doping with I₂, while the nonpolymerized nanocoils are disrupted upon being doped.

Full text of DOI:10.1021/ja064461h

Published on Web 10/18/2006
Stabilization of a Kinetically Favored Nanostructure: Surface
ROMP of Self-Assembled Conductive Nanocoils from a
Norbornene-Appended Hexa-peri-hexabenzocoronene
Takuya Yamamoto,† Takanori Fukushima,*,†,‡ Yohei Yamamoto,† Atsuko Kosaka,†
Wusong Jin,† Noriyuki Ishii,§ and Takuzo Aida,*,†,‡
Contribution from the 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, 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, and 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
Received June 23, 2006; Revised Manuscript Received September 9, 2006; E-mail: fukushima@nanospace.miraikan.jst.go.jp;
aida@macro.t.u-tokyo.ac.jp
Abstract: Newly designed norbornene-appended hexabenzocoronene 1 self-assembles, upon diffusion
of an Et2O vapor into its CH2Cl2 solution, to form either graphitic nanocoils or nanotubes, depending on the
self-assembling conditions. The coiled assembly, selectively formed at 15 °C, is a kinetic intermediate for
the tubular assembly and transforms into nanotubes on standing at 25 °C. However, post-ring-opening
metathesis polymerization of the norbornene pendants of 1 enhances the thermal stability of the coiled
assembly as well as the tubular one and disables a thermodynamic coil-to-tube transition. The polymerized
nanocoils show an electroconductivity of 1 × 10-4 S cm-1 upon doping with I2, while the nonpolymerized
nanocoils are disrupted upon being doped.
Introduction
In general, self-assembly is a thermodynamically controlled
process1 where a variety of temporarily formed kinetic inter-
mediates eventually transform into a single organized structure
with higher symmetry. However, in some cases, kinetic inter-
mediates are more attractive than thermodynamic products. Here,
we report selective formation and stabilization of a kinetically
favored self-assembled conductive nanocoil,2 a potential can-
didate for the realization of nanoscale solenoids.
Recently, we reported self-assembled nanotubular objects of
hexabenzocoronene (HBC) amphiphiles (e.g., 3) (Figure 1),
which consist of helically rolled bilayer tapes composed of
bilaterally coupled columns of π-stacked HBC units.3 In the
course of this study we noticed that under certain conditions
coiled assemblies occasionally result together with the nano-
tubes. Although the nanocoil is highly attractive for its potential
Figure 1. Molecular structures of amphiphilic HBCs with polymerizable
(1 and 2) and nonpolymerizable (3) end groups and ROMP catalyst 4.
† ERATO-SORST Nanospace Project (JST).
of helical electroconduction, such a less symmetric assembly
likely exists only intermediately in the self-organization process.
How does one select and stabilize this kinetic intermediate? This
is a challenging issue in supramolecular chemistry.
‡ The University of Tokyo.
§ Biological Information Research Center (AIST).
(1) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes;
VCH: Weinheim, 1995.
(2) Examples of coiled assemblies, see: (a) Cornelissen, J. J. L. M.; Fischer,
M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 1998, 280, 1427-
1430. (b) Sone, E. D.; Zubarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed.
2002, 41, 1705-1709. (c) Jung, J. H.; John, G.; Yoshida, K.; Shimizu, T.
J. Am. Chem. Soc. 2002, 124, 10674-10675. (d) Shimizu, T.; Masuda,
M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401-1443. (e) Messmore,
B. W.; Sukerkar, P. A.; Stupp, S. I. J. Am. Chem. Soc. 2005, 127, 7992-
7993. (f) Yang, W.-Y.; Lee, E.; Lee, M. J. Am. Chem. Soc. 2006, 128,
3484-3485.
(3) (a) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura,
T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481-
1483. (b) Jin, W.; Fukushima, T.; Niki, M.; Kosaka, A.; Ishii, N.; Aida, T.
Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10801-10806. (c) Yamamoto,
Y.; Fukushima, T.; Jin, W.; Kosaka, A.; Hara, T.; Nakamura, T.; Saeki,
A.; Seki, S.; Tagawa, S.; Aida, T. AdV. Mater. 2006, 18, 1297-1300. (d)
Fukushima, T. Polym. J. 2006, 38, 743-756.
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A R T I C L E S
Yamamoto et al.
Figure 2. Schematic representation of the concept of post-ROMP stabilization of self-assembled nanocoils and nanotubes from 1.
Scheme 1. Synthesis of Norbornene-Appended HBC 1a
a (a) Triethylene glycol monochlorohydrin, K2CO3, KI, dry DMF, 80 °C, 72%; (b) Ac2O, Et3N, DMAP, dry CH2Cl2, 20 °C, 83%; (c) FeCl3, MeNO2, dry
CH2Cl2, 20 °C, 61%; (d) aqueous KOH, THF; (e) 5-norbornene-2-carboxylic acid, DCC, DMAP, dry CH2Cl2, reflux, 28%.
Here, we highlight our serendipitous finding that self-
assembly of norbornene-appended HBC 1 results in formation
of either a coiled or a tubular object depending on the self-
assembling conditions (Figure 2). Furthermore, a thermodynamic
coil-to-tube transition, occurring eventually, can be disabled by
subsequent ring-opening metathesis polymerization (ROMP)4
of the norbornene pendants of 1. Post-polymerization or cross-
linking of self-assembled molecular building blocks is often
utilized for enhancement of the structural robustness of non-
covalent supramolecular architectures.5 Prior to the present study
we reported covalent stabilization of HBC graphitic nanotubes
using reversible dimerization chemistry of coumarin and thiol
functionalities incorporated into HBC as pendants.6 To achieve
permanent stitching of nanotubularly assembled HBC, we
applied acyclic diene metathesis (ADMET)4,7 to preformed
nanotubes from allyl-group-appended HBC 2.8 However, post-
polymerization with ADMET did not proceed under conditions
suitable for controlled self-assembly. Alternatively, we later found
that ADMET of nonassembled 2 in CH2Cl2 triggers spontaneous
formation of surface-polymerized nanotubes with an enhanced
thermal stability.8 To accomplish post-polymerization that allows
for stabilization of predetermined nanostructures, HBC 1 having
more reactive norbornene functionalities was thus designed.
Results and Discussion
Compound 1 was synthesized over five steps from hexa-
phenylbenzene derivative 53b (Scheme 1).9 Triethylene glycol
monochlorohydrin was allowed to react with 5 in DMF at 80 °C
in the presence of K2CO3/KI, yielding 6 in 72%. The hydroxy
groups of 6 were protected with acetyl groups using Ac2O/Et3N/
DMAP in CH2Cl2 at 20 °C to afford 7 in 83% yield. Oxidative
(4) For reviews, see: (a) Buchmeiser, M. R. Chem. ReV. 2000, 100, 1565-
1604. (b) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29.
(c) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760-3765.
(5) (a) Regen, S. L.; Yamaguchi, K.; Samuel, N. K. P.; Singh, M. J. Am. Chem.
Soc. 1983, 105, 6354-6355. (b) Ringsdorf, H.; Schlarb, B.; Venzmer, J.
Angew. Chem., Int. Ed. Engl. 1988, 27, 113-158. (c) Zubarev, E. R.; Pralle,
M. U.; Li, L.: Stupp, S. I. Science 1999, 283, 523-526. (d) Won, Y.-Y.;
Davis, H. T.; Bates, F. S. Science 1999, 283, 960-963. (e) Clark, T. D.;
Kobayashi, K.; Ghadiri, M. R. Chem.sEur. J. 1999, 5, 782-792. (f) Meier,
W. Chem. Soc. ReV. 2000, 29, 295-303. (g) Gin, D. L.; Gu, W.; Pindzola,
B. A.; Zhou, W.-J. Acc. Chem. Res. 2001, 34, 973-980. (h) Hartgerink, J.
D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684-1688. (i) Mueller,
A.; O’Brien, D. F. Chem. ReV. 2002, 102, 727-757. (j) Kim, Y.; Mayer,
M. F.; Zimmerman, S. C. Angew. Chem., Int. Ed. 2003, 42, 1121-1126.
(k) Hecht, S.; Khan, A. Angew. Chem., Int. Ed. 2003, 42, 6021-6024.
(6) (a) Motoyanagi, J.; Fukushima, T.; Ishii, N.; Aida, T. J. Am. Chem. Soc.
2006, 128, 4220-4221. (b) Motoyanagi, J.; Fukushima, T.; Kosaka, A.;
Ishii, N.; Aida, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5120-
5127.
(7) Davidson, T.; Wagener, K. B. Acyclic Diene Metathesis (ADMET)
Polymerization. In Materials Science and Technology: Synthesis of
Polymers; Schluter, A. D., Ed.; Wiley-VCH: New York, 1999; Chapter 4.
(8) Jin, W.; Fukushima, T.; Kosaka, A.; Niki, M.; Ishii, N.; Aida, T. J. Am.
Chem. Soc. 2005, 127, 8284-8285.
(9) See Supporting Information.
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Stabilization of a Kinetically Favored Nanostructure
A R T I C L E S
Figure 3. SEM and TEM (inset) micrographs of air-dried suspensions of nanocoils and nanotubes. (a) Nanocoils and (b) nanotubes formed by Et2O vapor
diffusion into CH2Cl2 solutions of 1. Self-assembled nanocoils after being allowed to stand at 25 °C for (c) 3 and (d) 5 days. (e) Polymerized nanocoils and
(f) polymerized nanotubes by post-ROMP. (g) Polymerized nanocoils after heating at 75 °C for 12 h, and (h) polymerized nanotubes after heating at 100 °C
for 12 h.
cyclization of 7 by FeCl3/MeNO2 in CH2Cl2 at 20 °C resulted
in formation of HBC derivative 8 in 61% yield. The acetyl
groups were removed with aqueous KOH in THF to obtain
hydroxy-group-terminated HBC derivative 9. This substance was
subjected to a condensation reaction with 5-norbornene-2-
carboxylic acid using DCC/DMAP in CH2Cl2 under reflux,
giving target norbornene-appended amphiphilic HBC 1 in 28%
yield.
While HBC 2 self-assembles in THF to give nanotubes,8
norbornene-appended HBC 1 was highly soluble in THF and
hardly formed ordered assemblies. After many attempts, we
Figure 4. Electronic absorption spectra of (a) nanocoils and (b) nanotubes
from 1 in a range of 300-600 nm. Blue curves: before ROMP. Red
curves: after ROMP. Spectra are normalized at 427 nm.
noticed that, upon diffusion of an Et2O vapor into a CH2Cl2
solution of 1 (0.22 mM) at 25 °C,9 controlled self-assembly of
1 took place quantitatively to give nanotubes having a uniform
diameter of 20 nm and a wall thickness of 3 nm (Figure 3b).
Interestingly, when the concentration of 1 in CH2Cl2 was higher
(0.65 mM), vapor diffusion at 15 °C resulted in exclusive
formation of a coiled assembly in quantitative yield.9 As shown
in Figure 3a, the diameter of the nanocoils (30 nm) is obviously
greater than that of the nanotubes, while their wall thicknesses
are nearly identical to one another. Each nanocoil consists of a
helically rolled 20-nm-wide single bilayer tape with a pitch of
60 nm. Electronic absorption spectra of the nanocoils and
nanotubes were quite analogous to one another, displaying red-
shifted absorption bands, characteristic of π-stacked HBC, at
427 and 459 nm (Figure 4). A morphological similarity thus
observed between the coiled and tubular assemblies suggests
that the former is a kinetic precursor for the latter. In fact, when
a suspension of the nanocoils, obtained at 15 °C by vapor dif-
fusion, was warmed to 25 °C and allowed to stand, the nanocoils
were gradually transformed into nanotubes. According to SEM
microscopy, the self-assembled system after the initial 3 days
still contained the nanocoils, which disappeared after 5 days to
leave the nanotubes almost exclusively (Figure 3c and d). As
confirmed by TEM, the diameter and wall thickness of the
resulting nanotubes are identical to those formed at 25 °C by
vapor diffusion (Figure 3b). Considering that the nanocoils are
thus preferred only kinetically, their exclusive formation under
slightly modified assembling conditions is remarkable.
We found that post-ROMP of the norbornene pendants of 1
in the self-assembled nanocoils enables stabilization of their
thermodynamically labile form. Thus, an Et2O solution of
Grubbs catalyst 44 (10 mol %) was added to an Et2O suspension
of the nanocoils, and the mixture was stirred at 20 °C for 24 h.9
Filtration of the reaction mixture allowed quantitative isolation
of a yellow solid substance, which was completely insoluble in
CH2Cl2, a good solvent for monomer 1. In agreement with this
observation, no molecular-ion peak due to monomeric 1 was
detected by MALDI-TOF mass spectrometry of the filtrate
(Figure S1).9 Compared with the intact nanocoils before ROMP,
the isolated product in the infrared spectroscopy displayed a
large drop in intensity of a vibrational band at 1335 cm-1 (Figure
5a), characteristic of norbornene (dC-H out-of-phase in-plane
bending mode).10 The relative integral value of this band sug-
gested roughly 80% conversion of the cyclo-olefinic double
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Yamamoto et al.
Figure 5. Infrared spectra (KBr) of (a) nanocoils and (b) nanotubes from
1 in a range of 1800-1300 cm-1. Blue curves: before ROMP. Red
curves: after ROMP. Spectra are normalized at 1731 cm-1
.
Figure 6. I-V profiles of polymerized nanocoils from 1 before (black
circles) and after (red circles) being doped with I2.
bond into an acyclic one.11 The absorption spectral profile of
the assembly, characteristic of the π-stacked HBC units, hardly
changed in this polymerization (Figure 4a), most likely due to
the spatial separation of the norbornene and HBC units by the
long oligoether chain. In relation to this observation, electron
micrographs (Figure 3e) and AFM images (Figure S2)9 showed
that the nanocoils after ROMP preserve the helical structure
and size regime. We also found that the polymerized nanocoils
are thermally stable. For example, upon heating at 75 °C in a
sealed ampule, the coiled structure, as observed by SEM (Figure
3g), survived even after 12 h in Et2O/CH2Cl2 (100/1 v/v)
whereas the nonpolymerized coil was disrupted completely.
Likewise, catalyst 4 was capable of metathesizing the
norbornene pendants of the tubularly assembled 1 at 20 °C,
affording thermally stable graphitic nanotubes, insoluble in CH2-
Cl2.9 MALDI-TOF mass spectrometry of the polymerization
mixture hardly detected monomeric 1 (Figure S1).9 By means
of infrared spectroscopy (Figure 5b), the conversion was
determined as approximately 80%.11 Since the norbornene
pendants are located on both inner and outer surfaces of the
tubular assembly, the rather high metathesis conversion suggests
that 4 is able to go inside the nanotubes. As observed by electron
microscopy (Figure 3f) and absorption spectroscopy (Figure 4b),
the size regime of the nanotubes and the geometry of their
π-stacked HBC units hardly changed in ROMP. The polymer-
ized nanotubes thus obtained perfectly survived when a sealed
ampule containing their suspension in Et2O/CH2Cl2 (100/1 v/v)
was heated at 100 °C for 12 h (Figure 3h). In contrast, the
nonpolymerized version under similar conditions started to lose
its morphological integrity even upon heating at 45 °C.
We investigated electroconductive properties of the graphitic
objects using 3-µm-gap two-probe electrodes. Both nonpolym-
erized and polymerized nanocoils, on casting from their suspen-
sions, gave uniform films. Upon exposure to I2, the former was
readily disrupted with a loss of the coiled structure (Figure S3).9
In contrast, the latter was successfully doped with retention of
the coiled morphology (Figure S3)9 and displayed a linear I-V
profile (Figure 6), which allowed determination of the conduc-
tivity as 1 × 10-4 S cm-1. On the other hand, the nonpolymer-
ized and polymerized nanotubes were heavily agglomerated and
did not give films qualified for the measurement.
Conclusions
For the covalent stabilization of HBC-based graphitic assem-
blies, we already reported an ADMET-triggered approach with
allyl-group-appended HBC amphiphile 2,8 which however is
hardly applicable to preassembled systems. In contrast, the new
methodology based on ROMP of a norbornene analogue (1)
may be referred to the ‘predetermined’ approach in which either
a coiled or a tubular object fabricated beforehand can be post-
polymerized (Figure 2). The polymerized graphitic nanocoils
from 1, thus obtained by taking advantage of this approach,
are conductive upon being doped with I2 and attractive for
realization of nanoscale solenoids.
Acknowledgment. We thank Prof. Tomoji Kawai and Dr.
Masateru Taniguchi (Osaka University) for providing microgap
electrodes and Akinori Kogure (Shimadzu Corp.) for AFM
measurements.
Supporting Information Available: Details of synthesis and
characterization of 1, experimental procedures for self-assembly
of 1 and post-ROMP, MALDI-TOF mass spectra, AFM images,
and SEM micrographs (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(10) (a) Shaw, R. A.; Castro, C.; Dutler, R.; Rauk, A.; Wieser, H. J. Chem.
Phys. 1988, 89, 716-731. (b) Kawai, N. T.; Butler, I. S.; Gilson, D. F. R.
J. Phys. Chem. 1991, 95, 634-639. (c) Parker, S. F.; Williams, K. P. J.;
Steele, D.; Herman, H. Phys. Chem. Chem. Phys. 2003, 5, 1508-1514.
(11) By reference to the intensity of the CdO stretching band at 1731 cm-1
,
the extent of conversion of norbornene pendants was determined on the
basis of the relative integral intensity of bands at 1335 cm-1 before and
after ROMP.
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