Systematic studies on structural parameters for nanotubular assembly of hexa-peri-hexabenzocoronenes

Article abstract of DOI:10.1021/ja801179e

Thirteen different hexa-peri-hexabenzocoronenes (HBCs) I-III were newly synthesized, and their self-assembling behaviors were investigated. Taking into account also the reported behaviors of amphiphilic HBCs, some structural parameters of HBC essential for the tubular assembly were revealed. Points to highlight include (1) the importance of two phenyl groups attached to one side of the HBC unit, (2) essential roles of long paraffinic side chains on the other side of the phenyl groups, and (3) no necessity of hydrophilic oligo(ethylene glycol) side chains. The hierarchical nanotubular structure, rendered by virtue of a synchrotron radiation technique, was virtually identical to our previous proposal, where the nanotubes are composed of helically coiled bilayer tapes with a tilting angle of ～45°. Each tape consists of π-stacked HBC units, where the inner and outer HBC layers are connected by interdigitation of paraffinic side chains. The coiled structure is most likely caused by a steric congestion of the phenyl groups attached to the HBC unit, whose tilting direction may determine the handedness of the helically chiral nanotube.

Full text of DOI:10.1021/ja801179e

Published on Web 06/25/2008
Systematic Studies on Structural Parameters for Nanotubular
Assembly of Hexa-peri-hexabenzocoronenes
Wusong Jin,*,† Yohei Yamamoto,† Takanori Fukushima,*,$,†,‡ Noriyuki Ishii,§
,|
Jungeun Kim,# Kenichi Kato, Masaki Takata,#, and Takuzo 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,
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,
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,
Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun,
Hyogo 679-5198, Japan, RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo
679-5148, Japan, and Department of AdVanced Materials Science, Graduate School of Frontier
Sciences, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
Received February 16, 2008; E-mail: jin@nanospace.miraikan.jst.go.jp; fukushima@riken.jp; aida@macro.t.u-tokyo.ac.jp
Abstract: Thirteen different hexa-peri-hexabenzocoronenes (HBCs) I-III were newly synthesized, and their
self-assembling behaviors were investigated. Taking into account also the reported behaviors of amphiphilic
HBCs, some structural parameters of HBC essential for the tubular assembly were revealed. Points to
highlight include (1) the importance of two phenyl groups attached to one side of the HBC unit, (2) essential
roles of long paraffinic side chains on the other side of the phenyl groups, and (3) no necessity of hydrophilic
oligo(ethylene glycol) side chains. The hierarchical nanotubular structure, rendered by virtue of a synchrotron
radiation technique, was virtually identical to our previous proposal, where the nanotubes are composed
of helically coiled bilayer tapes with a tilting angle of ∼45°. Each tape consists of π-stacked HBC units,
where the inner and outer HBC layers are connected by interdigitation of paraffinic side chains. The coiled
structure is most likely caused by a steric congestion of the phenyl groups attached to the HBC unit, whose
tilting direction may determine the handedness of the helically chiral nanotube.
Introduction
(TEG) chains on the other. Noteworthy is that these amphiphilic
HBC derivatives self-assemble in polar solvents such as THF to
Self-assembled nanostructures consisting of π-conjugated mol-
ecules have attracted increasing attention because of their potential
for organic and supramolecular electronics.1 Polycyclic aromatic
hydrocarbons are often utilized as building blocks for these
materials,2 where flexible side chains are usually incorporated into
a rigid aromatic core, so that incompatible segments are phase
separated from one another at a molecular level to enhance the
ordering of aromatic units responsible for charge-carrier transport.3
Hexa-peri-hexabenzocoronene (HBC) is the one with a highly
extended π-conjugated system. Pioneering works have been made
by Mu¨llen and co-workers on the synthesis and self-assembly of
HBC derivatives with paraffinic side chains, affording discotic
liquid crystals and nanofibers.4 Meanwhile, we developed a new
class of Gemini-shaped HBC derivatives such as 1a and 1b (Figure
1), which possess two hydrophobic dodecyl chains (C12H25) on
one side of the HBC core and two hydrophilic triethylene glycol
form several tens of micrometers long nanotubes with a uniform
diameter of 20 nm and a wall thickness of 3 nm. The proposed
structure of the nanotube consists of helically rolled-up bilayer tapes
composed of π-stacked HBC units, where the inner and outer HBC
(1) For recent reviews, see: (a) Lee, M.; Cho, B. K.; Zin, W.-C. Chem.
ReV. 2001, 101, 3869–3892. (b) Carroll, R. L.; Gorman, C. B. Angew.
Chem., Int. Ed. 2002, 41, 4378–4400. (c) Molecular Electronics:
Science and Technology; Aviram, A., Ratner, M., Eds.; New York
Academy of Sciences: New York, 1998. (d) Meijer, E. W.; Schenning,
A. P. H. J. Nature 2002, 419, 353–354. (e) vander Auweraer, M.; de
Schryver, F. C. Nat. Mater. 2004, 3, 507–508. (f) Hoeben, F. J. M.;
Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005,
105, 1491–1546. (g) Schenning, A. P. H. J.; Meijer, E. W. Chem.
Commun. 2005, 3245–3258. (h) Grimsdale, A. C.; Wu, J.; Mu¨llen, K.
Chem. Commun. 2005, 2197–2204. (i) Kumar, S. Chem. Soc. ReV.
2006, 35, 83–109. (j) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem.
Soc. ReV. 2007, 36, 1902–1929. (k) Wu, J.; Pisula, W.; Mu¨llen, K.
Chem. ReV. 2007, 107, 718–747. (l) Laschat, S.; Baro, A.; Steinke,
N.; Giesselmann, F.; Ha¨gele, C.; Scalia, G.; Judele, R.; Kapatsina,
E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Angew. Chem., Int. Ed.
2007, 46, 4832–4887. (m) de la Torre, G.; Claessens, C. G.; Torres,
T. Chem. Commun. 2007, 2000–2015. (n) Ajayaghosh, A.; Praveen,
V. K. Acc. Chem. Res. 2007, 40, 644–656. (o) Ryu, J.-H.; Hong, D.-
J.; Lee, M. Chem. Commun. 2008, 1043–1054. (p) Thompson, B. C.;
Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77. (q) Goodby,
J. W.; Saez, I. M.; Cowling, S. J.; Go¨rtz, V.; Draper, M.; Hall, A. W.;
Sia, S.; Cosquer, G.; Lee, S.-E.; Raynes, E. P. Angew. Chem., Int.
Ed. 2008, 47, 2754–2787.
† ERATO-SORST Nanospace Project, JST.
‡ Department of Chemistry and Biotechnology, The University of Tokyo.
§ National Institute of Advanced Industrial Science and Technology
(AIST).
# Japan Synchrotron Radiation Research Institute.
RIKEN SPring-8 Center.
| Department of Advanced Materials Science, The University of Tokyo.
$ Present address: Advanced Science Institute, RIKEN, 2-1 Hirosawa,
Wako, Saitama 351-0198, Japan.
9
9434 J. AM. CHEM. SOC. 2008, 130, 9434–9440
10.1021/ja801179e CCC: $40.75
2008 American Chemical Society

Nanotubular Assembly of Hexa-peri-hexabenzocoronenes
A R T I C L E S
renone pendants8 at one or both of the TEG termini. These
observations prompted us to synthesize 13 different HBC deriva-
tives (Figure 3) for rational understanding of the self-assembling
process and also analyze the hierarchical nanotubular structure by
means of wide-angle X-ray diffraction (WAXD) using a synchro-
tron radiation beam at SPring-8, BL02B2.
Results and Discussion
Synthesis of HBC Derivatives. The HBC derivatives in Figure
3 are classified into three categories. HBCs belonging to Group
I all carry phenyl groups and dodecyl side chains. HBCs IA
and IB possess at their phenyl groups oxyethylene side chains,
which however are shorter than those of 1a. While HBC IC
carries methoxy groups instead of the oligoether side chains,
ID possesses nonsubstituted phenyl groups. These compounds
are designed to investigate possible roles of the hydrophilic
oxyethylene side chains in the tubular assembly. HBCs in Group
II all carry TEG chains, while their hydrophobic part bears
paraffinic side chains of different lengths or nothing on an
extreme of this series. They are intended to investigate effects
of the paraffinic side chains on the tubular assembly. On the
other hand, HBC derivatives in Group III are miscellaneous.
As described in detail in Supporting Information, HBC
derivatives IA and IB were prepared by alkylation of 3 with
tosylated derivatives of the corresponding oligo(ethylene glycol)
monomethyl ethers, followed by oxidative cyclization of the
resulting hexaarylbenzenes with FeCl3 in CH2Cl2/MeNO2
(Scheme 1).9 A similar oxidation of 2 yielded IC.9 For the
synthesis of HBC derivatives IIA-C (Scheme 2), compound 5
with two bromophenyl groups was subjected to a PdII-catalyzed
coupling reaction with the corresponding alkylmagnesium
bromides to give hexaarylbenzenes 6a-6c.9 After the cleavage
of their methoxy groups with BBr3, the phenolic hydroxyl
groups of the resulting hexaarylbenzenes were alkylated with
2-[2-(2-methoxyethoxy)ethoxy]ethoxy p-tosylate, affording
hexaarylbenzenes 8a-8c. Then, they were converted into HBCs
IIA-C by oxidation with FeCl3. HBC derivatives IID, IIIA-C,
and IIIE were prepared from the corresponding diphenylacety-
lene and cyclopentadienone derivatives by a two-step synthetic
protocol involving the Diels-Alder reaction and subsequent
oxidation with FeCl3.9 For the synthesis of HBCs ID and IIID,
dibrominated IIIE was likewise prepared from 4,4′-dibromo-
diphenylacetylene and didodecylcyclopentadienone and sub-
jected to Pd0-catalyzed coupling reactions with phenylboronic
acid and TEG-appended acetylene, respectively.9 All the HBC
derivatives except ID were unambiguously characterized by
NMR spectroscopy and MALDI-TOF mass spectrometry (Fig-
ure S1).9 HBC ID showed an expected molecular ion peak in
MALDI-TOF mass spectrometry but was hardly characterized
by NMR spectroscopy because of its very poor solubility.
Self-Assembly of HBC Derivatives I. We investigated the self-
assembly of HBCs IA-C under conditions identical to those
optimized for 1a. Typically, a THF suspension of IA with
diethylene glycol side chains (1 mg/mL) was once heated at 50
°C, and the resulting yellow-colored solution was allowed to
cool to 25 °C. A yellow suspension formed within a few hours.
Electronic absorption spectroscopy of a cast film of the
Figure 1. Molecular structures of HBCs 1a-c.
Figure 2. Schematic illustrations of the hierarchical structures of a nanotube
formed from HBC 1a.
layers are connected by interdigitation of the dodecyl side chains,
while the TEG chains are located on both sides of the tubular wall
(Figure 2). Namely, the tubular assembly appears to take place by
an incompatibility between the hydrophobic and hydrophilic parts
of the molecule. However, in some cases, we noticed that large
hydrophobic groups incorporated into the TEG termini hardly
hamper the nanotube formation. Examples include Gemini-shaped
HBC amphiphiles with coumarin,6 norbornene,7 and trinitrofluo-
(2) For reviews, see: (a) Harvey, R. G. Polycyclic Aromatic Hydrocarbons;
Wiley-VCH: New York, 1997. (b) Randic, M. Chem. ReV. 2003, 103,
3449–3605. (c) Carbon-Rich Compounds; Haley, M. M.; Tykwinski,
R. R., Eds.; Wiley-VCH, Verlag GmbH and Co., KGaA: Weinheim,
2006. (d) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483.
(3) (a) 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.
(b) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122. (c)
Bre´das, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV.
2004, 104, 4971–5003. (d) Xiao, S.; Tang, J.; Beetz, T.; Guo, X.;
Tremblay, N.; Siegrist, T.; Zhu, Y.; Steigerwald, M.; Nuckolls, C.
J. Am. Chem. Soc. 2006, 128, 10700–10701.
(4) (a) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99,
1747–1785. (b) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem.
ReV. 2001, 101, 1267–1300. (c) Rohr, U.; Kohl, C.; Mu¨llen, K.; van
de Craats, A. M.; Warman, J. J. Mater. Chem. 2001, 11, 1789–1799.
(d) Simpson, C. D.; Wu, J.; Watson, M. D.; Mu¨llen, K. J. Mater.
Chem. 2004, 14, 494–504. (e) Wu, J.; Watson, M. D.; Zhang, L.;
Wang, Z.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 177–186. (f)
Palermo, V.; Morelli, S.; Simpson, C.; Mu¨llen, K.; Samori, P. J. Mater.
Chem. 2006, 16, 266–271.
(5) (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.
(8) (a) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki,
S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 314,
1761–1764. (b) Yamamoto, Y.; Fukushima, T.; N.; Saeki, A.; Seki,
S.; Tagawa, S.; Ishii, N.; Aida, T. J. Am. Chem. Soc. 2007, 129, 9276–
9277.
(6) Motoyanagi, J.; Fukushima, T.; Ishii, N.; Aida, T. J. Am. Chem. Soc.
2006, 128, 4220–4221.
(7) Yamamoto, T.; Fukushima, T.; Yamamoto, Y.; Kosaka, A.; Jin, W.;
Ishii, N.; Aida, T. J. Am. Chem. Soc. 2006, 128, 14337–14340.
(9) See Supporting Information.
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A R T I C L E S
Jin et al.
Figure 3. Newly designed HBC derivatives I-III.
suspension displayed a main broadband at 366 nm with red-
shifted absorption bands at 426 and 460 nm (Figure S2),9
characteristic of tubularly assembled HBC units.5–8 Scanning
electron microscopy (SEM) of an air-dried suspension allowed
for the visualization of cylindrical nano-objects with a hollow
channel (Figure 4a). Transmission electron microscopy (TEM)
showed that the nanotubes have a uniform diameter of 20 nm
with a wall thickness of 3 nm (Figures 4b and 4c). Infrared
spectroscopy of the nanotubes cast on a KBr plate showed CH2
stretching vibrations at 2918 (νanti) and 2849 (νsym) cm-1
,
characteristic of paraffinic chains with a stretched conformation.
Thus, it is most likely that the dodecyl chains interdigitate with
one another to support a bilayer structure of the nanotube wall.
All the above observations are analogous to those for the tubular
assembly of 1a. Moreover, a water droplet on a cast film of the
nanotubes of IA showed a contact angle of 52.7 °, which is
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Nanotubular Assembly of Hexa-peri-hexabenzocoronenes
A R T I C L E S
Scheme 1. Synthesis of Gemini-Shaped HBC Derivatives IA-Ca
a Reagents and conditions: (a) BBr3, CH2Cl2, 25 °C; (b) TsO(CH2CH2O)2Me or TsOCH2CH2OMe, KOH, THF, reflux; (c) FeCl3, MeNO2, CH2Cl2, 25 °C.
Scheme 2. Synthesis of Gemini-Shaped HBC Derivatives IIA-Ca
a (a) C16H33MgBr, C13H27MgBr, or C8H17MgBr, PdCl2(dppf), THF, reflux; (b) BBr3, CH2Cl2, 25 °C; (c) TsO(CH2CH2O)3Me, KOH, THF, reflux; (d)
FeCl3, MeNO2, CH2Cl2, 25 °C.
comparable to that in the case of tubularly assembled 1a
(45.5 °) reported previously.5a
the tubularly assembled HBCs adopt a stretched conformation
and interdigitate with one another to form a crystalline domain
that supports the bilayer structure of the tubular wall. The
essential role of the paraffinic side chains in the tubular assembly
has also been suggested by the failure of the formation of
nanotubes from 1c (Figure 1) carrying branched paraffinic side
chains incapable of crystallization.5b Thus, the octyl side chains
of IIC are not long enough to form a crystalline domain
important for the tubular assembly. Accordingly, HBC IID with
no paraffinic side chains did not form nanotubes (Figure 5d).
We then investigated the assembly of a HBC derivative with
longer paraffinic side chains such as IIA in CH2Cl2 and found
that it self-assembles in a controlled manner to give nanotubes
with a high structural integrity (Figure 5a). In this case, the
choice of the solvent for self-assembly was rather critical,
because an attempted assembly in THF under conditions
optimized for tubular assembly of 1a5a resulted in only a mixture
of nanotubes and ill-defined aggregates (Figure S4). Noteworthy
is that the nanotubes formed in CH2Cl2, as observed by TEM
microscopy (Figure 5a), have the largest wall thickness (3.5
nm) and diameter (25 nm) among those obtained from HBC
derivatives examined. The different size regimes, thus observed
for the nanotubular assembly of IIA, seem likely, considering
that the nanotube wall is supported by interdigitation of the
paraffinic side chains. Namely, longer side chains upon inter-
digitation could result in a thicker bilayer wall. When such a
thicker tape rolls up, a smaller curvature, i.e., a larger tube
diameter, may be preferred.
Similar to 1a and IA, less hydrophilic IB carrying short
ethylene glycol chains forms a nanotubular assembly (Figures
4d-f). However, to our surprise, even in the absence of ethylene
glycol chains (IC), controlled self-assembly to give nanotubes
took place (Figures 4g-i). Hence, one may conclude that the
amphiphilic molecular design is not essential for the nanotubular
assembly. Nevertheless, we noticed that the nanotubes of IB
and IC (Figures 4d and 4g, respectively) are much shorter than
that of IA (Figure 4a). Since IB and IC are less soluble than
IA in THF, the corresponding nanotubular assemblies tend to
precipitate out of the solution in a relatively early stage and
lose the opportunity to grow further. Controlled self-assembly
of IA-C also took place in CH2Cl2, a better solvent than THF,
where the resulting nanotubes appeared to be longer than those
formed in THF (Figure S3).9 Unfortunately, self-assembling
properties of ID were hard to investigate due to its extremely
poor solubility.
Self-Assembly of HBC Derivatives II. In contrast with the
HBC derivatives in Group I, those in Group II showed much
different self-assembling behaviors from one another. For
example, when a hot THF solution of IIC with octyl side chains
(1 mg/mL) was allowed to cool to 25 °C, a yellow suspension
resulted. However, TEM microscopy of an air-dried suspension
displayed ill-defined aggregates (Figure 5c), with only a small
amount of tubular and coiled assemblies. We also noticed that
the electronic absorption bands above 400 nm, informative of
the π-stacking manner of HBC, are quite obscure (Figure S2).9
Furthermore, infrared spectroscopy showed CH2 stretching
Just for curiosity, we synthesized HBC IIB carrying tridecyl
side chains with a view to investigate odd-even effects of the
paraffinic side chains on the assembly. Under conditions similar
to those for 1a,5a compound IIB formed nanotubes whose size
regimes (Figure 5b) and spectral profiles (Figure S2)9 were
essentially identical to those of the nanotubular assembly of 1a.
vibrations at a higher wavenumber region (2923, 2852 cm-1
)
than that observed for the dodecyl side chains in the tubular
assembly of 1a. As described in the above section and also
reported in our previous papers,5–8 the dodecyl side chains in
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A R T I C L E S
Jin et al.
Figure 4. SEM and TEM micrographs of air-dried suspensions of IA (a-c), IB (d-f), and IC (g-i) assembled in THF.
Figure 5. TEM micrographs of air-dried suspensions of (a) IIA assembled in CH2Cl2, and (b) IIB, (c) IIC, and (d) IID assembled in THF.
Self-Assembly of HBC Derivatives III. Groups III involves
five different HBCs with much diverse structural variations.
HBC IIIA carries dodecyl groups instead of the TEG chains in
1a, while HBC IIIB involves a phenylene spacer for connecting
the dodecyl side chains to the HBC unit. HBC IIIC is a
dibrominated version of 1a at the periphery of the HBC core.
HBCs IIID and IIIE are essentially different from other HBC
derivatives in that they do not bear phenyl groups. The former
makes use of an alkyne functionality for anchoring the TEG
chains to the HBC core, while the latter consists only of a HBC
core and dodecyl side chains but does not possess TEG chains.
In THF under the self-assembling conditions optimized for 1a,
none of these HBC derivatives formed nanotubes. Use of
halogenated solvents such as CH2Cl2 and CHCl3 instead of THF
for the assembly again failed to form nanotubes. Moreover, we
attempted vapor diffusion5–8 using hexane or MeOH as a poor
solvent in conjunction with THF or CH2Cl2, but no tubular
objects resulted from HBC derivatives III. Figures 6a-e show
typical TEM or SEM micrographs of the resultant aggregates.
These nontubular assemblies did not exhibit red-shifted absorp-
tion bands (Figure S2).9
The most interesting result in the above experiments is
certainly the unsuccessful nanotubular assembly of HBC IIID
(Figure 6d), since it differs from 1a only in the structure of the
anchoring part for the TEG chains. In relation to this observa-
tion, Mu¨llen and co-workers have reported the formation of
nontubular filaments from amphiphilic HBC 9 (Chart 1) carrying
imidazolium ion pendants.10 Considering also the observation
that IIIE did not form nanotubes but nontubular filaments
(Figure 6e), we can conclude a crucial role of the phenyl groups
in the controlled self-assembly to form a tubular morphology.
Nevertheless, no nanotube formation from IIIB (Figure 6b)
suggests that the phenyl groups should not be located on both
sides of the HBC core. Thus, as can be seen in the HBCs
belonging to Groups I and II, an ideal structural motif for the
tubular assembly may involve two phenyl groups on the other
(10) Hamaoui, B. E.; Zhi, L.; Pisula, W.; Kolb, U.; Wu, J.; Mu¨llen, K.
Chem. Commun. 2007, 2384–2386.
(11) Similar X-ray diffractions due to periodically aligned phenyl groups
have been reported for a highly ordered one-dimensional assembly of
ˇ
a hexaaryl-HBC derivative. Pisula, W.; Tomovicˇ, Z.; Watson, M. D.;
Mu¨llen, K.; Kussmann, J.; Ochsenfeld, C.; Metzroth, T.; Gauss, J. J.
Phys. Chem. B 2007, 111, 7481–7487.
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A R T I C L E S
Figure 6. TEM micrographs of air-dried suspensions of (a) IIIA and (b) IIIB assembled in THF, (c) IIIC assembled in THF/MeOH (1:1 v/v), and (e) IIIE
assembled in THF. (d) SEM micrograph of an air-dried suspension of IIID assembled in THF by MeOH vapor diffusion.
Chart 1. Molecular Structure of HBC Derivative 910
characteristic of J-aggregates of π-stacked HBC units (Figure S2).
Furthermore, infrared spectral characteristics include antisymmetric
and symmetric stretching vibrations of the long alkyl side chains
at 2917 (νanti) and 2848 (νsym) cm-1, characteristic of a stretched
conformation of paraffins on interdigitation. Furthermore, powder-
pattern WAXD profiles of all the nanotubular assemblies are
roughly identical to one another (Figures S5a),9 except for the
case of HBC IIA having longer hexadecyl side chains, where
the d-spacing observed for the resulting larger-diameter nanotubes
(Vide ante) was slightly greater than those of the other cases.
Table 1. Structural and Spectral Characteristics of Tubularly
Apparently, the diffraction patterns of nontubular assemblies of
Assembled HBC Derivatives
IIC, IID, and IIIE (Figures S5b) were quite different from those
of the nanotubular assemblies.
diameter
(nm)a
d-spacing
(nm)b
absorption maxima
(nm)
C-H stretching
(cm-1) νanti
HBC
νsym
Having these features in mind, we conducted WAXD analysis
of the nanotubular assembly of 1b using a synchrotron radiation
X-ray beam.9 The nanotubes of 1b have a great advantage, since
they can be aligned unidirectionally into a macroscopic fiber,5c
thereby making the diffraction pattern informative of the orientation
of the π-stacked HBC units. Thus, a hot 2-MeTHF solution (50
°C) of (S)-1b (1.5 mg/mL) was allowed to cool to 25 °C, and the
resultant suspension was aged for three weeks. Then, a glass hook
was dipped into this suspension to collect the nanotube bundles
and pulled up to stretch the captured aggregate to form a
macroscopic fiber. By way of this simple macroscopic treatment,
most of the nanotubes are aligned unidirectionally along the longer
axis of the fiber.5c As shown in Figure 7a, we successfully obtained
a two-dimensional (2D) WAXD image with a qualified diffraction
pattern. At a ꢀ-angle (azimuthal angle from the longer-axis direction
of the fiber) of 0°, a strong diffraction arc with a d-spacing of 0.347
nm was observed (Figure 7), indicating that the HBC units are
π-stacked on top of each other and oriented perpendicular to the
longer axis of the macroscopic fiber. Other characteristic diffraction
arcs were observed at ꢀ-angles of approximately ( 45 and ( 27°,
where the d-spacings for the former were ∼0.48 and 1.03 nm, while
that for the latter was 0.416 nm. Because of the longitudinal
orientation of the nanotubes in the fiber, the diffraction feature of
the fiber allows for elucidating the packing diagram of the
assembled HBC molecules. As illustrated in Figure 8, the π-stacked
columnar arrays of the HBC molecules are helically stranded
(Figure 8b), where the central axes of these helical HBC columns
are tilted by 45° relative to the longer axis of the tube (Figure 8c).
1ac
1bd
IA
20
20
20
0.345
0.347
0.344
361, 429, 459
365, 398, 421
366, 426, 460
(363, 426, 459)e
364, 428, 458
(363, 429, 458)e
367, 425, 462
(363, 431, 461)e
358, 428, 460e
359, 430, 460
2918
2917
2918
2849
2849
2849
IB
IC
20
21
0.345
0.345
2918
2917
2847
2848
IIA
IIB
25
20
0.352
0.347
2917
2919
2848
2849
a Determined by TEM microscopy. b Assigned to the cofacial
HBC-HBC distance. c From ref 5a. d From ref 5b. e For the nanotubes
formed in CH2Cl2.
side of the paraffinic side chains. However, the unsuccessful
nanotube formation from IIIA (Figure 6a) suggests that the
paraffinic side chains should be located only on one side of
the HBC core. This configuration is certainly important for the
selective formation of a bilayer tape without forming multila-
mellar structures. Finally, the failure of the nanotubular assembly
of IIIC (Figure 6c), a dibrominated version of 1a, is not
surprising. As described in the following section, the nanotubular
morphology is constructed from bilaterally connected columnar
arrays of π-stacked HBC units, and such a bilateral connection
can be hampered by bulky side groups of the HBC core.
Structural Aspects of Self-Assembled Nanotubes. Table 1
summarizes the spectral data of the self-assembled nanotubes
obtained in this and previous studies.5a,b As already described,
typical absorption spectral features include two red-shifted bands
9
J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008 9439

A R T I C L E S
Jin et al.
Figure 7. (a) 2D-WAXD image of a macroscopic fiber (4 mm in length and 0.2 mm in diameter) composed of the nanotubes of 1b oriented unidirectionally
along the longer axis of the fiber. (b) X-ray diffraction profiles of the fiber at ꢀ-angles of 0 (red), 27 (green), and 45° (blue).
Figure 8. Schematic representations of the hierarchical self-assembled structures of HBC derivative 1b. (a) π-Stacked columnar array, (b) bilayer wall, and (c)
nanotube.
The d-spacing of 1.03 nm at ꢀ ) (45° corresponds to the
intercolumnar distance (Figure 8a). In each helical column,
the HBC molecules stack up one another with an offset geometry.
The d-spacings of 0.347 nm at ꢀ ) 0° and ∼0.48 nm at ꢀ ) (45°
are assigned to the cofacial and center-to-center distances between
the π-stacked HBC units (Figure 8a). Apparently, the offset helical
geometry of the π-stacked HBC units allows the attached phenyl
groups to be released from their possible steric congestion. For an
energetic reason, the phenyl groups under such chiral environments
might better tilt in either clockwise or counterclockwise direction
relative to the HBC disk. In this context, the diffraction peak with
a d-spacing of 0.416 nm at ꢀ ) (27° is noteworthy. We assume
that the phenyl groups may adopt a cofacial orientation with a tilting
angle of 27° relative to the longer axis of the nanotube (Figure
8a).11 Thus, the direction of this orientation may determine the
handedness of the helical chirality of the nanotube.5b
first and nearly perfect structural elucidation of the HBC
nanotube, using a synchrotron radiation technique. The diffrac-
tion pattern of the nanotube of 1b successfully rendered the
hierarchical structure composed of a number of bilayers laterally
coupled and grown helically (Figure 8). Through these studies,
we now recognized that our adventitious molecular design for
the prototype HBC amphiphile includes a variety of tricks that
lead to the nanotubular assembly. Most important may be the
presence of two phenyl groups on one side of the HBC unit,
which allows the molecule to π-stack on top of each other in
an offset, helical geometry. Although triethylene glycol (TEG)
chains in the prototype molecular design are not essential, they
help the resultant nanotubes stay as long as possible in solution
and grow up to a macroscopic dimension. Furthermore, the
flexible TEG chains can make the nanotube formation virtually
intact to their terminal functionalities, usable for surface-
functionalization of the nanotubes.
Conclusions
Through the systematic investigation of the self-assembling
behaviors of 13 new Gemini-shaped hexa-peri-hexabenzocoro-
nene (HBC) derivatives I-III, some structural parameters
essential for the nanotubular assembly were revealed. Although
we have often emphasized the importance of amphiphilic
molecular design for the nanotube formation,5–8 the real essence
of this design turned out to be for making this discotic
π-conjugated molecule less symmetrical by incorporating two
phenyl groups and paraffinic side chains on opposite sides. This
is quite encouraging, since one may take more advantage of
such a design flexibility for structural elaboration and function-
alization of the nanotubular assembly. Also noteworthy is the
Acknowledgment. The synchrotron radiation experiments were
carried out on the BL02B2 of SPring-8 under the Priority Nano-
technology Support Program administered by the Japan Synchrotron
Radiation Research Institute (JASRI) (Proposal No. 2008A1644).
Supporting Information Available: Details of synthesis and
characterization of HBC derivatives I-III, MALDI-TOF mass
and electronic absorption spectra, TEM micrographs, and X-ray
diffraction patterns. This material is available free of charge
via the Internet at http://pubs.acs.org.
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9440 J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008