Supramolecular Peapods Composed of a Metalloporphyrin Nanotube and Fullerenes

Article abstract of DOI:10.1021/ja038178j

An acyclic dimer of a dendritic zinc porphyrin bearing six carboxylic acid functionalities (1_acid) interacts with fullerenes, such as C₆₀ and C₇₀, to form supramolecular peapods , composed of a hydrogen-bonded zinc porphyrin nanotube and fullerenes (3). According to TEM, the peapods are very long (>1 μm) and have a uniform diameter of 15 nm. Without fullerenes, the zinc porphyrin dimer forms only a heavily entangled, irregular assembly. In contrast with 1_acid, an ester version of the acyclic dimer without hydrogen-bonding capability (1_ester) hardly interacts with fullerenes. Copyright

Full text of DOI:10.1021/ja038178j

Published on Web 10/28/2003
Supramolecular Peapods Composed of a Metalloporphyrin Nanotube and
Fullerenes
Tatsuya Yamaguchi,‡ Noriyuki Ishii,† Kentaro Tashiro,‡ and Takuzo Aida*,‡
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, and Biological Information Research Center, AIST, Tsukuba Central 6,
1-1-1 Higashi Tsukuba, Ibaraki 305-8566, Japan
Received August 28, 2003; E-mail: aida@macro.t.u-tokyo.ac.jp
Recently, carbon nanomaterials such as fullerenes and carbon
nanotubes have attracted considerable attention as potential com-
ponents for the next-generation electronic nanodevices. From this
point of view, an important challenge is to fabricate one-dimensional
clusters of fullerenes. The first success was demonstrated in 1998
by the inclusion of C60 into a single-walled carbon nanotube.1 Such
“peapod”-like carbon nanocomposites2 have also become of great
interest, since electronic interactions between guest fullerenes and
nanotube pods may generate unique physical properties.
Here we report the first example of “supramolecular peapods”
composed of a linear zinc porphyrin nanotube and fullerenes such
as C60 and C70 (Scheme 1). This achievement was demonstrated
Figure 1. TEM micrographs of (a,b) 1acid with C60 and (c,d) 1acid alone.
Samples were cast from TCE after being heated once at 120 °C, followed
by incubation at 40 °C for 4 days, and stained with RuO4.
Scheme 1. One-Dimensional Supramolecular Polymerization of
Acyclic Zinc Porphyrin Dimer 1acid Directed by a π-Electronic
Interaction with Fullerenes
by the fullerene-triggered unidirectional supramolecular polymer-
ization of an acyclic zinc porphyrin dimer having six carboxylic
acid (CO2H) functionalities (1acid).3 1acid also bears large [G4]-poly-
(benzyl ether) dendritic wedges, which are intended to overcome
the solubility problem of carboxylic acid-appended metalloporphy-
inclusion complexes (e.g., Kassoc ) 107-108 M-1 with RhIII; 105-
rins and allow easier visualization by transmission electron
107 M-1 with Zn) in solution.4,5 On the other hand, the correspond-
microscopy (TEM).
ing monomers do not interact with fullerenes under identical
conditions. We also found that an acyclic zinc porphyrin dimer
with methyl ester functionalities (1ester) hardly shows spectral
We recently found that cyclic dimers of metalloporphyrins
display very high affinities toward fullerenes and form stable
changes associated with the metalloporphyrin-fullerene interaction.
In sharp contrast, the corresponding carboxylic acid version (1acid),
‡ University of Tokyo.
† AIST.
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10.1021/ja038178j CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
though acyclic, exhibited notable spectral changes, similar to those
observed for the inclusion complexation. Typically, a 1,1,2,2-
tetrachloroethane (TCE) solution of a mixture of 1acid (1.2 × 10-5
M) and C60 (2.4 × 10-5 M) was heated once at 120 °C and then
allowed to stand at 40 °C for 4 days. TEM observation of the
resulting mixture showed the presence of very long (>1 µm) fibers
with a uniform diameter of 15 nm (Figure 1a,b). A virtually identical
TEM micrograph was observed when C70 was used in place of C60.3
On the other hand, 1acid alone (1.2 × 10-5 M) without C60, under
conditions otherwise identical to those described above, gave a
heavily entangled, irregular assembly (Figure 1c,d).
A TCE solution of 1acid (1.2 × 10-6 M) showed exciton-coupled
split Soret absorption bands at 427.8 and 435.6 nm,3 indicating a
twisted or tilted geometry of the two zinc porphyrin chromophores.6
On the other hand, upon titration with C70, the exciton coupling
gradually disappeared to give a less intensified Soret absorption
band at 428.2 nm.3 This spectral change, together with the TEM
Figure 2. 13C NMR spectrum of a 1:2 mixture of 1acid and 13C-enriched
C60 in TCE-d2 at -40 °C.
due to free C60 (δ 143.0 ppm).4a As estimated from the half-width
values of the signals, relaxation time T2 of the signal due to included
C60 (0.018 s) was only 18% of that of free C60 (0.097 s), indicating
a highly constrained motion of C60 in the tubular space. Upon
heating, the signal due to included C60 gradually shifted to a lower
magnetic field and appeared to coalesce at 60 °C with the signal
due to free C60.3 Such a high coalescence temperature has never
been observed for C60 complexed with nonpolymerizable, linear
cyclic dimers of porphyrins.4b,7 Therefore, the peapod architecture,
once formed, considerably lowers the complexation/decomplexation
dynamics of guest fullerenes.
In conclusion, we have demonstrated the first example of
“supramolecular peapods” consisting of a hydrogen-bonded zinc
porphyrin nanotube and fullerenes, by the fullerene-directed one-
dimensional supramolecular polymerization of an acyclic zinc
porphyrin dimer bearing six carboxylic acid functionalities (1acid).
The peapods are thermally stable and characterized by their
untangled, discrete architectures with very high aspect ratios. These
new π-electronic nanocomposite materials may serve as potential
components for photoelectronic devices.
micrographs (Figure 1), suggests that zinc porphyrin dimer 1acid
,
upon interaction with fullerenes, adopts a parallel and eclipsed
conformation, to form hydrogen-bonded [2 + 2] inclusion com-
plexes having four CO2H functionalities on each side ([2 +
2]-1acid⊃fullerene) (Scheme 1). The inclusion complexes can then
be polymerized unidirectionally via the dimerization of the CO2H
side groups, to afford fullerene-included “peapods” 3 with a zinc
porphyrin nanotube. The diameter of 3, as estimated from a
molecular model with the dendritic envelope, is 12 nm, which agrees
well with that observed by TEM (Figure 1b).
Infrared spectroscopy of a mixture of 1acid and C60, cast from a
TCE solution on a CaF2 plate, showed a CdO stretching vibration
at 1688 cm-1 assignable to a dimeric form of CO2H, without any
Acknowledgment. We thank Prof. T. Akasaka of Tsukuba
University for his generous supply of 13C-enriched C60. T.Y. thanks
the JSPS Young Scientist Fellowship.
1
shoulder at 1725 cm-1 due to monomeric CO2H.3 Although H
NMR spectroscopy of the mixtures of 1acid and fullerenes was less
informative due to significant spectral broadening, an acyclic dimer
with two CO2H groups (2acid), in combination with C70,3 gave
reasonable support for the fullerene-directed conformational change
of 1acid. A TCE-d2 solution of 2acid (1.5 × 10-3 M) at 20 °C showed
zinc porphyrin meso-H and bridging aromatic o-H/p-H (Ar-o-H/
Ar-p-H) at δ 10.16 and 7.14/7.23 ppm. Upon mixing 5 equiv of
Supporting Information Available: Synthetic procedures and
spectral data of 1 and 2; TEM of 1acid/C60 and 1acid/C70; selected UV-
vis, IR, 1H NMR spectra, and DLS profiles of 1 and 2 with fullerenes
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
C70 with 2acid, the meso-H and Ar-p-H signals shifted upfield to δ
References
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interaction with C70.7 Thus, 2acid, upon interaction with C70,
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1]-2acid⊃C70) as a temporary product, which is then trapped by
dimerization to give thermodynamically favored [2 + 2]-2acid⊃C70
(Scheme 1). Inclusion complex [2 + 2]-2acid⊃C70, thus formed,
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As expected, such a fullerene-induced spectral change was only
very small for the methyl ester version of 2acid (2ester),3 due to the
absence of any subsequent events to stabilize temporarily formed
[1 + 1]-2ester⊃C70.
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(6) Probably due to intra- and/or intermolecular H-bonding interactions, since
1ester did not show any exciton coupling (ref 3).
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Sakamoto, S.; Yamaguchi, K. Macromolecules 2000, 33, 9182.
(8) An example of self-assembly of dendritic macromolecules by dimerization
of carboxylic acid functionalities: Zimmerman, S. C.; Zeng, F.; Rei, D.
E. C.; Kolotuchin, S. V. Science 1996, 271, 1095.
The inclusion of fullerenes within the supramolecular zinc
porphyrin nanotube was also supported by 13C NMR spectroscopy
using 13C-enriched C60.3 At -40 °C, a TCE-d2 solution of a 1:2
mixture of 1acid and C60 showed a broad signal at δ 139.6 ppm,
assignable to included C60 (Figure 2), in addition to a sharp signal
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