Formation of water-dispersible nanotubular graphitic assembly decorated with isothiouronium ion groups and its supramolecular functionalization

Article abstract of DOI:10.1021/ja0671518

A newly designed Gemini-shaped hexabenzocoronene amphiphile (1), carrying an isothiouronium ion-appended side chain, self-assembles in CH ₂Cl₂ to form a nanotubular object, whose graphitic wall is densely covered by a positively charged molecular layer of isothiouronium ion pendants. The graphitic nanotube can be dispersed uniformly in aqueous media owing to effective hydration as well as electrostatic repulsion. Post-supramolecular functionalization of the nanotube surface is possible, without disruption of the tubular morphology, by taking advantage of a specific interaction of the isothiouronium ion pendants with oxoanion guests. Mixing with sodium poly(4-styrenesulfonate) results in wrapping of the nanotube, while complexation with an electron-accepting oxoanion such as anthraquinone carboxylate allows photoinduced electron transfer from the graphitic wall to the bound guest molecules.

Full text of DOI:10.1021/ja0671518

Published on Web 12/23/2006
Formation of Water-Dispersible Nanotubular Graphitic
Assembly Decorated with Isothiouronium Ion Groups and Its
Supramolecular Functionalization
Guanxin Zhang,† Wusong Jin,† Takanori Fukushima,*,†,‡ Atsuko Kosaka,†
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 October 5, 2006; E-mail: fukushima@nanospace.miraikan.jst.go.jp; aida@macro.t.u-tokyo.ac.jp
Abstract: A newly designed Gemini-shaped hexabenzocoronene amphiphile (1), carrying an isothiouronium
ion-appended side chain, self-assembles in CH2Cl2 to form a nanotubular object, whose graphitic wall is
densely covered by a positively charged molecular layer of isothiouronium ion pendants. The graphitic
nanotube can be dispersed uniformly in aqueous media owing to effective hydration as well as electrostatic
repulsion. Post-supramolecular functionalization of the nanotube surface is possible, without disruption of
the tubular morphology, by taking advantage of a specific interaction of the isothiouronium ion pendants
with oxoanion guests. Mixing with sodium poly(4-styrenesulfonate) results in wrapping of the nanotube,
while complexation with an electron-accepting oxoanion such as anthraquinone carboxylate allows
photoinduced electron transfer from the graphitic wall to the bound guest molecules.
Introduction
Self-assembled π-electronic nano-objects1 with post-func-
tionalizable surface groups via noncovalent interactions are
expected to serve as potential scaffolds for the ultimate
molecular design of finely integrated electronic and optoelec-
tronic materials.2 To achieve this goal, the nano-objects must
be sufficiently robust to allow the accommodation of a guest
without structural disruption. Furthermore, proper choice of
surface groups that enable selective noncovalent interactions is
essential. However, such π-electronic molecular assemblies that
fulfill the above requisites are very rare.3 Here, we report
successful formation of a hexa-peri-hexabenzocoronene (HBC)
graphitic nanotube4 with isothiouronium ion surface pendants
(Figure 1) that can be functionalized without disruption by
oxoanion guests via a specific hydrogen-bonding interaction.5,6
HBC7 is one of the representatives of polycyclic aromatic
Figure 1. Schematic illustrations of (a) the graphitic nanotube from HBC
1 and (b) its bilayer wall (C12; dodecyl chain), and (c) a possible binding
mechanism for the complexation with AQ.
hydrocarbons that attracts considerable attention, since they
tend to form columnar assemblies important for electronic
and optoelectronic applications.8 Mu¨llen and co-workers are
(4) (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) Jin, W.;
Fukushima, T.; Kosaka, A.; Niki, M.; Ishii, N.; Aida, T. J. Am. Chem.
Soc. 2005, 127, 8284-8285. (d) Motoyanagi, J.; Fukushima, T.; Ishii, N.;
Aida, T. J. Am. Chem. Soc. 2006, 128, 4220-4221. (e) Motoyanagi, J.;
Fukushima, T.; Kosaka, A.; Ishii, N.; Aida, T. J. Polym. Sci., Part A:
Polym. Chem. 2006, 44, 5120-5127. (f) Yamamoto, T.; Fukushima, T.;
Yamamoto, Y.; Kosaka, A.; Jin, W.; Ishii, N.; Aida, T. J. Am. Chem. Soc.
2006, 128, 14337-14340. (g) 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.
† JST.
‡ The University of Tokyo.
§ AIST.
(1) For recent reviews, see: (a) Simpson, C. D.; Wu, J.; Watson, M. D.; Mu¨llen,
K. J. Mater. Chem. 2004, 14, 494-504. (b) Wu¨rthner, F. Chem. Commun.
2004, 1564-1579. (c) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.;
Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491-1546. (d) Elemans, J.
A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. AdV. Mater.
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J. AM. CHEM. SOC. 2007, 129, 719-722
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A R T I C L E S
Zhang et al.
Scheme 1. Synthesis of Isothiouronium Ion-Appended HBC 1a
a (a) 2-[2-(2-Chloroethoxy)ethoxy]ethyl p-toluenesulfonate, K2CO3, 18-crown-6, DMF, 60 °C, 90%; (b) PdCl2(PPh3)2, CuI, DBU, benzene, 60 °C, 42%;
(c) Ph2O, reflux, 22%; (d) NaI, acetone, reflux, 86%; (e) FeCl3, MeNO2, CH2Cl2, 25 °C, 46%; (f) 1,3-dimethylthiourea, THF, reflux, 77%.
Scheme 2. Synthesis of Isothiouronium Ion-Appended HBC 2a
a (a) FeCl3, MeNO2, CH2Cl2, 25 °C, 61%; (b) 1,3-dimethylthiourea, THF, reflux, 52%.
pioneers in columnar assemblies of HBC derivatives with
paraffinic side chains9 that display anisotropic carrier transport
properties.
Recently, we have reported the formation of graphitic
nanotubes by controlled self-assembly of HBC-based nonionic
amphiphiles, where a graphitic bilayer wall consisting of
π-stacked HBC is covered by densely packed triethylene glycol
(TEG) chains.4 On the basis of this approach, surface-modified
graphitic nanotubes are available by self-assembly of HBC
amphiphiles carrying end-functionalized TEG chains.4c-f
However, such a covalent molecular design strategy always
requires time-consuming optimization of self-assembling condi-
tions to ensure the formation of nanotubular structures. In this
sense, noncovalent, post-functionalization of preformed nano-
tubes is considered to be more advantageous for diversifying
(6) (a) Yeo, W.-S.; Hong, J.-I. Tetrahedron Lett. 1998, 39, 3769-3772. (b)
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Minagawa, M.; Morita, K.; Kato, Y.; Taniguchi, S.; Kato, R.; Teramae, N.
J. Chem. Soc., Perkin Trans. 2 2002, 866-870. (e) Kato, Y.; Nishizawa,
S.; Teramae, N. Org. Lett. 2002, 4, 4407-4410. (f) Kato, R.; Cui, Y.-Y.;
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(7) Clar, E.; Ironside, C. T.; Zander, M. J. Chem. Soc. 1959, 142-147.
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Kreouzis, T. Phys. ReV. B 1995, 52, 13274-13280. (b) van de Craats, A.
M.; Warman, J. M.; Fechtenko¨tter, A.; Brand, J. D.; Harbison, M. A.;
Mu¨llen, K. AdV. Mater. 1999, 11, 1469-1472. (c) Kreouzis, T.; Donovan,
K. J.; Boden, N.; Bushby, R. J.; Lozman, O. R.; Liu, Q. J. Chem. Phys.
2001, 114, 1797-1802. (d) van de Craats, A. M.; Stutzmann, N.; Bunk,
O.; Nielsen, M. M.; Watson, M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus,
H.; Friend, R. H. AdV. Mater. 2003, 15, 495-499. (e) O’Neill, M.; Kelly,
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(9) (a) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99, 1747-
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5592-5629.
the range of available functional groups. In the present work,
we designed isothiouronium ion-appended HBC amphiphiles 1
and 2, since the guanidinium ion family are known to bind a
variety of oxoanions.5,6 Fortunately, compound 1 formed
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720 J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007

Water-Dispersible Nanotubular Graphitic Assembly
A R T I C L E S
Compound 1, a new class of ionically charged amphiphilic
HBC, showed a much different solubility from previously
reported nonionic versions. After many trials, we succeeded in
obtaining a nanotubular object from 1 in CH2Cl2, which is a
good solvent for other amphiphilic HBCs.4 Thus, a CH2Cl2
suspension of 1 (1 mg/mL) was once heated to reflux, and the
resulting hot solution was allowed to cool to 25 °C, whereupon
a yellow suspension resulted (Figure 2a). Electronic absorption
spectroscopy of an air-dried suspension showed a main absorp-
tion band at 366 nm with red-shifted absorption bands at 427
and 459 nm (Figure 2b), characteristic of tubularly assembled
HBCs.4 SEM and TEM micrographs revealed the formation of
a nanotubular assembly with a uniform diameter of 18 nm and
a wall thickness of 3 nm (Figure 3a,b). The observed dimensions
of the nanotube are quite similar to those of previously reported
nanotubular objects from nonionic HBC amphiphiles.4 Infrared
spectroscopy of a cast film of the nanotubes displayed CH2
stretching vibration bands at 2917 (Vanti) and 2848 cm-1 (Vsym),10
indicating that the dodecyl chains of 1 are stretched to form a
crystalline domain by interdigitation. Moreover, a solid sample
of tubularly assembled 1 exhibited an X-ray diffraction peak at
2θ ) 25.8°,10 assignable to a regularly stacked HBC with a d
spacing of 3.45 Å.
Figure 2. (a) Photograph of a CH2Cl2 suspension (left) and a water
dispersion (right) of tubularly assembled 1. (b) Electronic absorption spectra
at 25 °C of an air-dried CH2Cl2 suspension (broken curve) and a water
dispersion (2.5 × 10-5 M; solid curve) of tubularly assembled 1. The spectra
were normalized at 427 nm.
nanotubes that were highly dispersible in water, necessary for
the uniform surface functionalization.
Results and Discussion
Isothiouronium ion-appended HBC amphiphile 1 was syn-
thesized over six steps according to Scheme 1.10 2-[2-(2-
Chloroethoxy)ethoxy]ethyl p-toluenesulfonate was allowed to
react with 3 in DMF in the presence of K2CO3/18-crown-6-
ether, yielding 4 in 90%. Sonogashira coupling reaction of 4
with biphenylacetylene 5 using PdCl2(PPh3)2/CuI/DBU afforded
6 in 42% yield. The resultant unsymmetrical alkyne was
subjected to Diels-Alder reaction with cyclopentadienone
derivative 7,4b giving hexaphenylbenzene 8 in 22% yield.
Compound 8 was converted by the reaction with NaI into the
corresponding iodide 9 in 86% yield. Oxidative cyclization of
9 by FeCl3/MeNO2 in CH2Cl2 resulted in the formation of HBC
derivative 10 in 46% yield. This substance was allowed to react
with 1,3-dimethylthiourea in THF, affording target compound
1 in 77% yield. Likewise, HBC amphiphile 2 was prepared over
two steps from hexaphenylbenzene derivative 114e (Scheme 2).10
In contrast with 1, HBC amphiphile 2 possessing two
isothiouronium groups has an extremely low solubility. It was
hardly soluble in CH2Cl2 even upon heating. Exceptionally, a
hot water/THF mixture (1:1 v/v) was able to solubilize 2.
However, cooling of this hot solution resulted in the formation
of nontubular, ill-defined fibrous aggregates (Figure 3c,d).
Of interest, tubularly assembled 1 was finely dispersed in
water at 25 °C, where component 1, in contrast, was insoluble
even under reflux. Thus, a CH2Cl2 suspension of the nanotubes
(1 mg/mL, 1 mL) was centrifuged at 25 °C, and the resulting
solid substance was subjected to immersion in MeCN (2 mL)
followed by centrifugation. After the repetition of this solvent-
Figure 3. (a) SEM and (b) TEM micrographs of an air-dried CH2Cl2 suspension of tubularly assembled 1. (c) SEM and (d) TEM micrographs of an
air-dried suspension, formed upon cooling a hot water/THF (1:1 v/v) solution of 2 from 60 to 25 °C. (e) SEM and (f) TEM micrographs of an air-dried water
dispersion of tubularly assembled 1 obtained by solvent exchange of CH2Cl2 with water. (g) TEM micrograph of an air-dried suspension, formed by mixing
tubularly assembled 1 and PSS at a ratio [1]/[SO3-] of 1:2 in water. (h) TEM micrograph of an air-dried sample of a water/MeCN (1:1 v/v) dispersion of
tubularly assembled 1 containing AQ at a ratio [1]/[AQ] of 1:6.
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A R T I C L E S
Zhang et al.
no longer soluble in common organic solvents including CHCl3
and THF, where unstitched nanotubes are readily disrupted by
dissociation. The successful noncovalent stitching is due to a
multivalent interaction of the isothiouronium ion-appended
nanotube with PSS.
Not only such a polymeric compound with multiple oxoanion
groups but also guest molecules with a single oxoanion
functionality such as sodium anthraquinone-2-carboxylate (AQ;
Figure 1c) are able to bind to the nanotube surface. As observed
by TEM microscopy (Figure 3h), the nanotubular morphology
remained unchanged upon complexation with AQ. Excitation
of the nanotubes (1.1 × 10-5 M) in water/MeCN (1:1 v/v)12 at
427 nm resulted in a fluorescence emission at 526 nm with a
shoulder at 562 nm. On the other hand, upon titration with AQ,
the fluorescence intensity of the nanotubes decreased monotoni-
cally at the initial stage and leveled off later (Figure 4a). This
observation indicates the occurrence of a static quenching as
the consequence of a long-range photoinduced electron transfer
from the HBC units in the graphitic tubular wall to surface-
bound AQ molecules. The Stern-Volmer constant (KSV), as
determined from the initial fluorescence intensity change at 526
nm, was 3.3 × 106 M-1 (Figure 4b). As expected, the
fluorescence quenching by AQ was significantly less explicit
(KSV ) 3.7 × 103 M-1) in the presence of a large excess of
sodium benzoate (20 equiv) as a competitive oxoanion without
electron-accepting capability (Figure 4). While the guanidinium
ion family in aqueous media hardly forms stable 1:1 complexes
with oxoanions, those aligned on vesicular surfaces or liquid-
liquid interfaces have been reported to capture oxoanions
strongly.13 Thus, the notably large KSV value in the absence of
sodium benzoate justifies our expectation that AQ molecules
can be tightly trapped on the nanotube surface by the densely
aligned isothiouronium ion groups.
Figure 4. Fluorescence quenching of tubularly assembled 1 (11 µM) upon
titration with AQ in water/MeCN (1:1 v/v) at 25 °C. (a) Fluorescence
spectral changes and (b) their Stern-Volmer plots upon excitation at 427
nm in the absence ([AQ] ) 0, 0.02, 0.03, 0.07, 0.10, 0.17, 0.21, 0.35, 0.49,
0.63, 0.97, 1.5, 1.9, 4.4, 9.7, 19, 29, 39 µM) and presence ([AQ] ) 0, 10,
20, 30, 40, 50 µM) of sodium benzoate (220 µM).
exchange treatment, deionized water (2 mL) was added to the
residue, whereupon a yellow-colored, transparent solution
resulted (Figure 2a). Electronic absorption spectroscopy of this
solution showed red-shifted absorption bands due to π-stacked
HBCs (Figure 2b), whose maxima are the same as those
observed for the nanotube formed in CH2Cl2. By means of TEM
microscopy (Figure 3f), we found that the dimensions of the
nanotube in water are nearly identical to those in CH2Cl2. SEM
microscopy of an air-dried water dispersion clearly displayed
that the individual nanotubes are well separated from one another
(Figure 3e). Considering the fact that all the precedent nanotubes
from nonionic HBC amphiphiles tend to form thick bundles,4
this observation is remarkable and indicates that effective
hydration of the positively charged surface pendants as well as
their electrostatic repulsion should play an essential role in the
dispersion of the nanotubes in water.
Conclusions
In conclusion, incorporation of an isothiouronium ion end
group into one of the TEG chains of the amphiphilic HBC did
not hamper its nanotubular self-assembly. The resultant nanotube
was highly dispersible in water and displayed a sufficient
structural robustness despite possible electrostatic repulsions
among the surface pendants. Therefore, this new graphitic
nanotube may serve as a promising electroactive nanoscaffold
via post-functionalization with a wide variety of oxoanion
guests.
By taking advantage of a specific interaction of the isothiou-
ronium ion pendants with oxoanions,5,6 post surface function-
alization of the uniformly dispersed nanotubes is possible. For
example, by using a polymer having multiple oxoanion groups
such as sodium poly(4-styrenesulfonate) (PSS), we demonstrated
noncovalent surface stitching4c-f of tubularly assembled 1. Thus,
an aqueous dispersion of the nanotube (2.5 × 10-5 M, 80 mL)
was mixed with an aqueous solution of PSS (5.4 × 10-3 M,
0.74 mL), whereupon a yellow precipitate formed. TEM
microscopy of an air-dried suspension revealed that the included
nanotubes preserve their morphology without disruption (Figure
3g). Even though the precipitate, collected by filtration, was
washed thoroughly with water, a good solvent for PSS, the
resulting solid still contained PSS. Infrared spectroscopy10
showed vibration bands characteristic of phenyl sulfonate at
Supporting Information Available: Details of synthesis and
characterization of 1 and 2, infrared spectra, and X-ray diffrac-
tion pattern. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0671518
(11) Weiss, R. A.; Sen, A.; Willis, C. L.; Pottick, L. A. Polymer 1991, 32, 1867-
1874.
1038 and 1008 cm-1 11
along with those due to tubularly
,
assembled 1. The PSS/nanotube composite, thus formed, was
(12) To avoid solubility problems of AQ and the resulting composite, water/
MeCN (1:1 v/v) was employed in place of water.
(13) (a) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371-378. (b) Sakai,
(10) See Supporting Information.
N.; Matile, S. J. Am. Chem. Soc. 2003, 125, 14348-14356.
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