Tubular stacking of water-soluble toroids triggered by guest encapsulation

Article abstract of DOI:10.1021/ja909279b

(Figure Presented) Coassembly of laterally grafted rod amphiphiles gives rise to the formation of water-soluble toroids with a hydrophobic interior. The toroids stack on top of each other to form a tubular container upon triggering by fullerene encapsulat

Full text of DOI:10.1021/ja909279b

Published on Web 12/03/2009
Tubular Stacking of Water-Soluble Toroids Triggered by Guest Encapsulation
Eunji Lee, Jung-Keun Kim, and Myongsoo Lee*
Center for Supramolecular Nano-Assembly and Department of Chemistry, Seoul National UniVersity,
599 Gwanak-ro, Seoul 151-742, Republic of Korea
Received November 1, 2009; E-mail: myongslee@snu.ac.kr
Construction of one-dimensional (1D) tubular structures by self-
assembly of designed molecules has attracted considerable attention
because of the potential applications and practical implications of
such structures.1 Supramolecular tubules can be constructed by
rolling-up of elementary ribbons,2 scrolling of 2D structures,3 and
stacking of flat macrocyclic molecules.4 It can be speculated that
1D stacking of supramolecular rings through solvophobic interac-
tions would provide a novel strategy for fabricating tubular
structures.5
Herein we present the formation of water-soluble toroids with a
hydrophobic cavity by coassembly of laterally grafted rod am-
Figure 2. (a) TEM image and (b) model and (inset) cryo-TEM image of
the ring structure of 1 containing 3 (90 mol % relative to 1) in aqueous
phiphiles and their tubular stacking triggered by guest (C60)
encapsulation (Figure 1). The synthesis of laterally grafted am-
phiphilic molecules 1 and 2 was performed according to procedures
described previously.3 Cryogenic transmission electron microscopy
(cryo-TEM) revealed that 1 based on a bis(ethylene oxide) chain
self-organizes into planar sheets, whereas 2 with a longer tris(eth-
ylene oxide) chain forms a ribbonlike assembly (Figure S1 in the
Supporting Information).
solution.
containing different amounts of 3 (Figure S4). Interestingly, at a 3
content of 40 mol %, the sheets were shown to split into a ribbonlike
structure (Figure S5). Eventually, the ribbons with a parallel
arrangement were gradually isolated into cylindrical fibers with a
uniform diameter of 8 nm at a 3 content of >70 mol % (Figure
S5). This dimension is in reasonable agreement with twice the fully
extended molecular length (∼4.1 nm by CPK modeling), thus
indicating bilayer packing with the alkyl chains inside.
More importantly, a further increase in the 3 content forced the
ribbon structure to transform into discrete nanostructures. The TEM
image in Figure 2a reveals the formation of highly curved discrete
rings at a 3 content of 80 mol %. The cross-sectional diameter of
the ring is highly uniform (∼4 nm). The diameter of the ring exterior
and the internal pore were measured to be ∼10 and 1.5-2 nm,
respectively. The cryo-TEM image in Figure 2b shows dark ringlike
objects against the vitrified solution background, demonstrating that
the rings are not formed during the solvent evaporation process
(0.005 wt % in aqueous solution). These dimensions together with
the extended molecular length (see Figure S6) indicate that the rings
are composed of a single layer of the molecules in which the rod
segments are oriented perpendicular to the plane of the rings and
arranged into highly curved barrel-like toroids.9 On the basis of
these results, the mixed solution above a certain 3 content can be
considered to coassemble into toroids that are composed of a
hydrophobic interior with a diameter of ∼2 nm and hydrophilic
exterior with a diameter of ∼10 nm (Figure 2b). Similar to 1, the
ribbon structure of 2 transforms into discrete toroids with a uniform
size upon addition of 70 mol % 3 (Figures S4 and S7).
This structural transformation from planar sheets to elongated
cylinders to discrete toroids upon addition of 3 can be explained
by the increasing volume fraction of hydrophilic segments through
coassembly. With increasing content of 3, which contains only a
hydrophilic flexible chain, the 2D sheets dissociate into smaller
aggregates with larger volume, allowing the increased number of
oligoether chains to be less confined without sacrificing the
anisotropic packing of the rod segments.10 Since in most cases of
ring structures the internal cavities have the same surface as the
exterior,11 it is remarkable that our rings consist of a hydrophobic
interior and hydrophilic exterior.
Figure 1. Chemical structures of 1-3 and schematic illustration of the
stacking of toroids in a 1D manner upon addition of C60 guest molecules.
This structural behavior stimulated us to investigate whether
increasing the volume fraction of hydrophilic chains would induce
highly curved nanostructures to relieve steric crowding at the flat
rod-coil interfaces of the sheets and the ribbons. With this in mind,
1 or 2 was mixed with 3 consisting of an aromatic rod and a
hydrophilic branched chain for coassembly.6 Notably, the average
hydrodynamic radii (RH) of both mixture solutions decreased with
increasing content of 3 (Figure S3), as confirmed by dynamic light
scattering (DLS) experiments.7,8 These observations reflect a
decrease in the size of the aggregates upon addition of 3.
To corroborate the structural change upon addition of 3, TEM
experiments were carried out on 0.005 wt % solutions of 1
9
18242 J. AM. CHEM. SOC. 2009, 131, 18242–18243
10.1021/ja909279b  2009 American Chemical Society

C O M M U N I C A T I O N S
of C60 within the cavities would force the rings to be more
hydrophobic up and down. To reduce the exposure of the
hydrophobic parts of the rings to a water environment, the rings
self-associate in a 1D manner to form elongated tubules with C60
inside. As a result, the fullerene array can be attained within the
1D spatial confinement of the hollow tubules (Figure 1).15
In conclusion, the results described here demonstrate that the
coassembly of laterally grafted amphiphilic analogues leads to the
formation of water-soluble toroids with a hydrophobic interior.
Notably, the toroids can efficiently encapsulate fullerene within their
internal cavities, which forces the toroids to stack on top of each
other to form a tubular container. This unique assembly could be
successfully utilized to spatially order the fullerene, which might
further broaden the application scope of fullerenes.
Acknowledgment. This work was supported by the National
Creative Research Initiative Program of the National Research
Foundation. E.L. and J.-K.K. acknowledge a fellowship from the
BK21 program of the Ministry of Education and Human Resources
Development.
Figure 3. (a) Fluorescence spectra (λex ) 316 nm) and (b) size distribution
graphs of aqueous solutions (0.005 wt %) of 1 and 3 (1/3 molar ratio )
1:9) without C60 and with 5-30 mol % C60. (c) TEM and (d) cryo-TEM
images of the cylinders of 1 and 3 after mixing with 30 mol % C60.
Supporting Information Available: Synthetic and other experi-
mental details. This material is available free of charge via the Internet
at http://pubs.acs.org.
The formation of barrel-like toroids with a hydrophobic cavity
led us to investigate whether the supramolecular nanostructures
would solubilize fullerenes (C60) in aqueous solution through
hydrophobic interactions. We therefore investigated the possibility
of encapsulating C60 into a coassembled solution of 1 and 3 (1/3
molar ratio ) 1:9). The emission spectrum of the coassembled
solution in the absence of C60 displayed a strong fluorescence with
a maximum at 415 nm (Figure 3a). In sharp contrast, upon addition
of C60 to the solution,12 the fluorescence intensity was significantly
suppressed, indicating that C60 was effectively encapsulated within
the hydrophobic interior of the rings.13 The fluorescence intensity
decreased with an increase in the fullerene content to a certain point
(30%) beyond which the fluorescence did not change with further
increment of fullerene (Figure 3a). Therefore, the maximum mole
percent of C60 loading per molecule can be considered to be ∼30%.
Remarkably, RH of the mixed solution of 1 and 3 increased upon
addition of C60 up to 30 mol % (Figure 3b). These results indicate
that the size of the aggregates increased with increasing C60
content.7,14 When a sample was cast from an aqueous solution
(0.005 wt %) and then negatively stained with uranyl acetate, the
TEM image of the mixed solution containing 30 mol % C60 revealed
cylindrical aggregates with a diameter of 10 nm (Figure S9d). Closer
examination of the samples showed that the cylinders are composed
of lateral stripes with a regular spacing of 3.3 nm along the cylinder
axis (Figure 3c). The formation of cylindrical objects in bulk
solution was also confirmed by cryo-TEM (Figure 3d). The diameter
and lateral spacing of the cylinders are in good agreement with the
dimensional features of the individual toroids. This finding dem-
onstrates that fullerene drives the supramolecular rings to stack on
top of one another to form a tubular structure in which the fullerenes
are encapsulated within the internal cavity (Figure 1). Encapsulation
References
(1) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem.,
Int. Ed. 2001, 40, 988–1011.
(2) (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) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005,
105, 1401–1443. (c) Yang, W.-Y.; Lee, E.; Lee, M. J. Am. Chem. Soc.
2006, 128, 3484–3485.
(3) Lee, E.; Kim, J.-K.; Lee, M. Angew. Chem., Int. Ed. 2009, 48, 3657–3660.
(4) Rosselli, S.; Ramminger, A.-D.; Wagner, T.; Silier, B.; Wiegand, S.;
Ha¨ussler, W.; Lieser, G.; Scheumann, V.; Ho¨ger, S. Angew. Chem., Int.
Ed. 2001, 40, 3137–3141.
(5) Schappacher, M.; Deffieux, A. Science 2008, 319, 1512–1515.
(6) The diblock rod amphiphile 3 was characterized by 1H and 13C NMR
spectroscopies, elemental analysis, and MALDI-TOF mass spectrometry
(see the Supporting Information).
(7) Lee, E.; Kim, J.-K.; Lee, M. Angew. Chem., Int. Ed. 2008, 47, 6375–6378.
(8) It should be noted that 3 does not show any apparent aggregation behavior
in aqueous solution within this range of concentrations.
(9) This arrangement of the rod segments is also supported by the height of
the toroids, which corresponds to the rod length (∼3 nm). See: Sakai, N.;
Mareda, J.; Matile, S. Acc. Chem. Res. 2008, 41, 1354–1365.
(10) The formation of rings with internal cavities might be attributed to the
steric constraint of the bulky hydrophobic alkyl chains.
(11) (a) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L.
Science 2004, 306, 94–97. (b) Kim, J.-K.; Lee, E.; Huang, Z.; Lee, M.
J. Am. Chem. Soc. 2006, 128, 14022–14023. (c) Yagai, S.; Mahesh, S.;
Kikkawa, Y.; Unoike, K.; Karatsu, T.; Kitamura, A.; Ajayaghosh, A. Angew.
Chem., Int. Ed. 2008, 47, 4691–4694. (d) Huang, H.; Chung, B.; Jung, J.;
Park, H.-W.; Chang, T. Angew. Chem., Int. Ed. 2009, 48, 4594–4597.
(12) See the Supporting Information for the experimental details.
(13) (a) Wolffs, M.; Hoeben, F. J. M.; Beckers, E. H. A.; Schenning, A. P. H. J.;
Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 13484–13485. (b) Yamaguchi,
T.; Ishii, N.; Tashiro, K.; Aida, T. J. Am. Chem. Soc. 2003, 125, 13934–
13935.
(14) This trend was also confirmed by TEM experiments (Figure S9).
(15) Kawauchi, T.; Kumaki, J.; Kitaura, A.; Okoshi, K.; Kusanagi, H.;
Kobayashi, K.; Sugai, T.; Shinohara, H.; Yashima, E. Angew. Chem., Int.
Ed. 2008, 47, 515–519.
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