Radially diblock nanotube: Site-selective functionalization of a tubularly assembled hexabenzocoronene

Article abstract of DOI:10.1021/ja075822b

An amphiphilic Gemini-shaped hexa-peri-hexabenzocoronene (1) with azide functionalities self-assembles to form graphitic nanotubes whose interior and exterior surfaces can be post-functionalized by "click chemistry". A click reaction of the nanotubes with

Full text of DOI:10.1021/ja075822b

Published on Web 01/11/2008
Radially Diblock Nanotube: Site-Selective Functionalization of a Tubularly
Assembled Hexabenzocoronene
Justin L. Mynar,† Takuya Yamamoto,† Atsuko Kosaka,† Takanori Fukushima,*,†,‡ Noriyuki Ishii,§ 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, 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 August 3, 2007; E-mail: fukushima@nanospace.miraikan.jst.go.jp; aida@macro.t.u-tokyo.ac.jp
Nanoscale tubular objects with functional surface groups have a
variety of potential applications.1-3 If the interior and exterior
functionalities are individually designed, the expediency of the
nanotubes could be greatly enhanced. However, such radially
diblock nanotubes are very rare.4 Recently, we have reported that
Gemini-shaped amphiphilic hexa-peri-hexabenzocoronenes (HBCs)
bearing triethylene glycol (TEG) chains self-assemble into graphitic
nanotubes, which consist of helically rolled bilayer tapes composed
of bilaterally coupled columns of π-stacked HBC units (Figure 1).5
Figure 1. Schematic structures of (a) Gemini-shaped amphiphilic HBC,
(b) self-assembled bilayer tape, and (c) graphitic nanotube.
By self-assembly of HBCs with certain functionalities attached to
the TEG termini, surface-functionalized graphitic nanotubes with
applications including electronics, optoelectronics, and chirotech-
nology have been fabricated.6,7 However, in principle, this design
Chart 1. Schematic Structures of Dendrons (G2-OH, G2-alkyne)
and Hexabenzocoronenes (1-3), and Illustration of Two-Stage
Site-Selective Functionalization of Tubularly Assembled 1 to Form
a Radially Diblock Nanotube
strategy never gives radially diblock nanotubes with different
interior and exterior functionalities. Here we report the successful
formation of such a new type of graphitic nanotube by site-selective
post modification of a nanotubular precursor bearing reactive surface
groups. Assuming that the interior reactive groups of the nanotube
have a lower accessibility than the exterior ones, we expected that
large molecules such as dendrons should graft on selectively to
the exterior surface.8
We have chosen to use the [3 + 2] cycloaddition of azides and
alkynes as a means of chemically modifying the graphitic nanotubes.
This reaction has been touted as a high-yielding reaction that not
only is insensitive to oxygen and water but also is tolerant to many
functional groups.9 Most importantly for self-assembled materials,10
the reaction can proceed under mild conditions, allowing for the
functionalization of the nanotubes without destroying their structural
integrity. Thus, a second generation Fre´chet-type dendron with an
alkyne focal core (G2-alkyne) was synthesized by carbodiimide
coupling of G2-OH with pentynoic acid (Chart 1).11 Meanwhile,
the HBC amphiphile was modified by reacting its tosylated TEG-
appended precursor with NaN3 to yield diazido-HBC (1, Chart 1).11
increase in diameter.11 Since the dendron is roughly 2 nm in size,
When a hexane vapor was allowed to diffuse at 25 °C into a THF
this observation indicates that the exterior surface of the nanotube
has been fully modified. Meanwhile, attempted self-assembly of a
dendronized-HBC (3), synthesized from 1 and G2-alkyne,11 resulted
in the formation of an irregular aggregate.
solution of 1, a yellow precipitate formed. Scanning electron
microscopy (SEM) and transmission electron microscopy (TEM)
of an air-dried sample showed the presence of bundled nanotubes
(Figure 2a), which were roughly 18 nm in diameter, as confirmed
by atomic force microscopy (AFM).11
Click chemistry was performed at 25 °C by adding CuSO4/
sodium ascorbate to a THF/MeOH/hexane suspension of a mixture
of G2-alkyne and tubularly assembled 1 ([G2-alkyne]/[1] ) 2).11
After 10 h without stirring, visualization by SEM and TEM
microscopy confirmed that the tubular objects have survived under
the mild click conditions (Figure 2b).11 Noteworthy, images
provided by AFM of the dendronized nanotubes showed a 4 nm
MALDI-TOF mass spectrometry of the reaction mixture showed
the presence of single- and double-click products 2 and 3,
respectively, along with unreacted 1.11 We hypothesized that the
steric bulk of the G2 dendron would prevent dendronization of the
HBC molecules located on the inner layer of the graphitic bilayer
wall. To determine the amount of the dendron grafted on, we
utilized analytical size exclusion chromatography (SEC) monitored
by absorption of the HBC moiety at 360 nm. In conformity with
the result of MALDI-TOF mass spectrometry, the SEC trace of
the reaction mixture in, for example, 6 h (Figure 3a) indicated the
presence of unreacted 1 and double-click 3 as major fractions with
† ERATO-SORST Nanospace Project (JST).
‡ The University of Tokyo.
§ Biological Information Research Center (AIST).
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10.1021/ja075822b CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. SEM and TEM (inset) micrographs of air-dried suspensions of self-assembled nanotubes (a) formed by vapor diffusion of hexane into a THF
solution of 1, (b) after the first-click reaction with G2-alkyne, (c) after the second-click reaction with 5-hexynenitrile, and (d) after the second-click reaction
with 5-hexyn-1-ol.
and second fractions were found to contain 3 and 5 (Figure 3b),
respectively, with a mole ratio 3/5 of 1.3.12 On the other hand,
diazido-HBC 1 and cross-double-click product 4 (Figure 3b) were
hardly detected.
In conclusion, we have developed an innovative method for the
preparation of radially diblock graphitic nanotubes, whose func-
tionalities on the interior and exterior surfaces differ from one
another. The dendritic grafts on the exterior surface will make it
possible to accommodate a greater number of functional groups.
The basic rationale of this concept, in principle, is not only limited
to the graphitic nanotubes but also can be elaborated into a general
method for site-selective and shape-persistent functionalization of
potentially labile self-assembled nano-objects.
Supporting Information Available: Details of synthesis, charac-
terization, experimental procedures for self-assembly of 1 and click
reactions, AFM and SEM micrographs, MALDI-TOF mass and
electronic absorption spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 3. (a) Analytical SEC traces (CHCl3, 360 nm) of the reaction
mixtures in 2, 6, 8, and 10 h after starting the click reaction of tubularly
assembled 1 with G2-alkyne. The traces were normalized with respect to
the total peak area. Asterisk indicates dimeric byproducts.11 (b) A recycling
preparative SEC trace (CHCl3, 360 nm) of a reaction mixture in 12 h after
starting the second-click reaction with 5-hexynenitrile.
References
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to continue for 10 h, the peak due to double-click 3 was enhanced
at the expense of the peak due to unreacted 1, while single-click 2
was hardly detected. A nearly identical SEC profile was observed
in 12 h. From the derived areas of the SEC peaks, roughly a half
amount of 1 remained unreacted. Considering a possible steric
congestion of the surface azide groups, a random click reaction on
the interior and exterior nanotube surfaces would result in the
preferential formation of single-click product 2. However, the results
with SEC suggest the more accessible HBC molecules, located on
the outer layer of the graphitic bilayer wall, are fully clicked
(dendronized), while the azide groups on the inner layer are mostly
intact. In the observed kinetics of the functionalization, the
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example, in a prolonged reaction, clicking of the interior azide
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the interior of the nanotubes was successfully functionalized, with
retention of the tubular structure, by the second-click reaction with
small alkynes such as 5-hexynenitrile and 5-hexyn-1-ol (Figure 2
parts c and d).11 Recycling preparative SEC of a 12-h reaction
mixture with 5-hexynenitrile gave two well-separated fractions
(Figure 3b). By means of MALDI-TOF mass spectrometry, the first
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