Switchable nanoporous sheets by the aqueous self-assembly of aromatic macrobicycles

Article abstract of DOI:10.1002/anie.201210373

Slim guests are welcome: Aromatic macrobicyclic amphiphiles underwent self-assembly through a face-to-face interaction to form dimeric micelles, which further associated laterally to form porous sheets with nanometer-sized pores. The resulting sheets efficiently intercalated planar aromatic guest molecules, whereupon the porous sheets were reversibly transformed into closed sheets (see picture). Copyright

Full text of DOI:10.1002/anie.201210373

Angewandte
Chemie
DOI: 10.1002/anie.201210373
Host–Guest Systems
Switchable Nanoporous Sheets by the Aqueous Self-Assembly of
Aromatic Macrobicycles**
Yongju Kim, Suyong Shin, Taehoon Kim, Dongseon Lee, Chaok Seok, and Myongsoo Lee*
The molecular self-assembly of aromatic building blocks is
a powerful approach to the construction of dynamic nano-
structures that are able to respond to external stimuli by
changing their shape and/or macroscopic properties.[1] In this
context, it is possible to modulate the interplay between
multiple noncovalent interactions through the rational design
of molecular modules. The directionality of these interactions
plays a critical role in controlling the shape of the self-
assembled structures.[2] Laterally grafted rod building blocks
lead to one-dimensional nanofibers through unidirectional
guiding of the elongated aromatic rods in the self-assembly
process.[3] Anisotropic micelles also undergo directional
growth to form low-dimensional supramolecular structures
in selected solvents.[4] For example, hydrophobic oblate
micelles with hydrophilic side faces stack on top of each
other to form 1D nanofibers.[5] On the other hand, hydro-
phobic aromatic rod bundles with hydrophilic up and down
grow through side-to-side interactions to form freely sus-
pended 2D structures in bulk solution.[6] A reduction in the
strength of the aromatic interactions leads the resulting
planar sheets to form 2D networks.
Figure 1. Molecular structure of aromatic macrobicycles with dendrons
attached to their basal plane and schematic representation of the 2D
growth of the flat micelles derived by pairwise stacking.
However, the elaborate construction of 2D structures
requires the rational design of molecular building blocks that
are able to grow in two dimensions. One possibility is the side-
by-side arrangement of 2D molecular building blocks, such as
flat disk-shaped aromatic molecules. Nonetheless, most flat
aromatic amphiphiles stack together through face-to-face
interactions to form nanofibers.[2a,7] To frustrate continuous
aromatic stacking in one dimension, one can introduce
flexible chains on the basal plane of the aromatic structures.
The basal-plane chains should enforce a lateral arrangement
of the flat aromatic segments and their two-dimensional
growth rather than growth through conventional 1D aromatic
stacking (Figure 1). With this idea in mind, we designed a flat
aromatic bicycle with a hydrophilic dendron at the center of
the basal plane. Another important issue regarding 2D planar
structures is the possibility of the integration of dynamic
response characteristics triggered by environmental changes.
The combination of principles of 2D planar structures with
responsive properties would generate a new class of intelli-
gent nanomaterials.
Herein we report the spontaneous formation of 2D porous
sheets in aqueous solution through the two-dimensional self-
assembly of an aromatic macrobicycle with a hydrophilic
dendron attached to its basal plane. Remarkably, unlike
conventional nanoporous materials, the resulting porous
sheets underwent dynamic motion between open and closed
states, as triggered by guest intercalation (Figure 4).[8]
The self-assembling molecules that form this aggregate
consist of a macrobicyclic aromatic segment and a hydrophilic
oligoether dendron grafted at the center of the basal plane
(Figure 1). The synthesis of the flat aromatic amphiphiles
began with the Sonogashira coupling of a dendron-substituted
diiodobenzene derivative with 2,6-dibromoethynylbenzene to
provide a tetrabromo building block (see the Supporting
Information). Suzuki coupling of the tetrabromo compound
with 3-((triisopropylsilyl)ethynyl)phenylboronic acid ester,
followed by silyl-group deprotection with tetra-n-butyl-
ammonium fluoride, then provided a precursor with four
terminal alkyne groups. The final aromatic amphiphiles were
synthesized efficiently by intramolecular Glaser-type cou-
pling of the terminal alkyne groups under dilute reaction
[*] Y. Kim, T. Kim, Prof. M. Lee
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry, Jilin University
Changchun 130012 (P. R. China)
E-mail: mslee@jlu.edu.cn
S. Shin, D. Lee, Prof. C. Seok
Department of Chemistry, Seoul National University
Seoul 151-747 (Korea)
[**] This research was supported by the National Science Foundation of
China (Grant No. 21221063), the Programme of Introducing Talents
of Discipline to Universities (111 Project of China, B06009), and the
Air Force Research Laboratory (FA2386-12-1-4078). We thank Prof.
T.-L. Choi at SNU for his helpful input into this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210373.
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conditions (CuCl/CuCl2 in pyridine).[9] The resulting mole-
small, discrete micelles. The diameters of the micelles and the
pores were measured to be approximately 3.5 nm and 3–5 nm,
respectively. The micellar diameter is about twice the length
of the molecule and thus indicates the presence of dimeric
micelles with a face-to-face stacking of the aromatic basal
planes. Further apparent evidence for the formation of in-
plane dimeric micelles was provided by topochemical poly-
merization of the sheets. Polymerization of the diacetylene
groups by irradiation of the solution with UV light for 6 h
yielded only dimers (see Figure S1 in the Supporting Infor-
mation). This result demonstrates that the primary structure
of the 2D sheets consists of dimeric micelles in which the two
aromatic planes within the micellar core face one another in
a slipped p–p stack for efficient dimerization of the diacety-
lene groups.[10]
1
cules were characterized by H and 13C NMR spectroscopy
and MALDI-TOF mass spectrometry and were shown to be
in full agreement with the structures presented.
Molecule 1 self-assembles into nanoporous sheets in
dilute aqueous solutions. Cryogenic transmission electron
microscopy (cryo-TEM) showed sheetlike objects with
curved edges against the vitrified solution background (Fig-
ure 2a); this observation is indicative of the formation of
flexible sheets in bulk solution. Notably, a high-resolution
To further confirm the primary structure of the 2D
aggregates, we conducted vapor pressure osmometry (VPO)
measurements in 1,4-dioxane in the concentration range of
6.1 to 17.7 gkgÀ1 (sample/solvent). The molecular weight of
the primary aggregate was measured to be 3617.3 Da, which is
twice that of the single molecule (1772.2 Da; see Figure S2)
and indicative of the formation of a dimeric aggregate. To
gain insight into the packing arrangement of the dimeric
micelles, we performed molecular-modeling experiments
through quantum-mechanical calculations with the
GAMESS software (Figure 2d). Energy minimization
showed that the aromatic plane adopts a boat conformation
in which the central benzene ring protrudes from the opposite
side of the basal plane to that occupied by the dendrimer. The
paired aromatic segments are “slipped” with respect to one
another to reduce steric hindrance between the protruding
benzene rings. The slipped stacking of the aromatic segments
is reflected in a red-shifted absorption maximum in aqueous
solution relative to that in chloroform (Figure 2c)[11] and
efficient topochemical polymerization of the dimeric micelles
as described above.
All of these observations indicate that 1 self-assembles
through a face-to-face stacking arrangement of the flat
aromatic segments into dimeric micelles with a flat aromatic
core. These dimeric micelles in turn grow laterally through
side-to-side hydrophobic interactions to form 2D sheets.
However, the lateral association of the dimeric micelles leads
to in-plane defects as a result of the weak lateral interactions
between the very thin aromatic cores owing to the slipped
packing arrangement. Thus, porous sheets are formed. This
weak lateral association of the micelles is also reflected in the
formation of flexible sheets.
Figure 2. a) Cryo-TEM image of a solution of 1 (200 mm; scale bar:
200 nm). b) Negatively stained TEM image of nanoporous sheets from
a 100 mm aqueous solution of 1 (scale bar: 100 nm). The insets in (a)
and (b) show magnified images (scale bars: 20 nm) and the line scan
profile along the yellow line. c) Absorption and emission spectra of
1 (100 mm) in CHCl3 (red line) and aqueous solution (black line);
lex =318 nm. d) Molecular modeling of the dimeric micelle without
coronene and the dimeric micelle containing coronene.
To investigate the role of the dimeric stacking of the
aromatic planes in the formation of the 2D porous sheets, we
modified the molecular structure. With the aim of interfering
with the dimeric stacking of the aromatic planes, we replaced
the monosubstituted aromatic segment with a basal plane
disubstituted at the two opposite faces to form 2 (Figure 1). In
great contrast to 1, 2 formed only irregular aggregates in
solution (see Figure S3). This behavior implies that the
dimeric stacking of the aromatic planes is essential for the
formation of a well-defined 2D porous structure.
image revealed that the sheets contained in-plane nanopores
with an average diameter of approximately 4 nm (Figure 2a,
inset). To obtain more information on these sheets, we also
performed TEM experiments with the samples cast onto
a TEM grid (and negatively stained with uranyl acetate). A
low-magnification image showed planar sheets with rugged
surfaces against a dark background. A higher-magnification
image showed that the rugged surfaces consisted of uniform
micelles and nanopores (Figure 2b, inset) and thus suggested
that the sheets had formed through a lateral association of
The formation of 2D sheets based on pairs of face-to-face-
stacked aromatic molecules stimulated us to investigate
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whether the planar nanostructure would encapsulate flat
aromatic guest molecules, such as coronene, through p–p
stacking interactions in aqueous solution (Figure 2d). Indeed,
the 2D sheets readily solubilized coronene in aqueous
solution with preservation of their 2D structure. Upon the
addition of coronene to an aqueous solution of 1, the
fluorescence intensity at 530 nm associated with the coronene
emission increased until 0.5 equivalents of coronene had been
added (see Figure S4).[12] Upon the further addition of
coronene to the solution, the fluorescence intensity did not
change, and precipitation was observed. Therefore, it can be
concluded that the maximum coronene loading per amphi-
philic molecule is 0.5 equivalents. This result implies that the
flat conjugated aromatic guest is sandwiched between the two
aromatic basal planes of the dimeric micelles through hydro-
phobic and p–p stacking interactions. Atomic force micros-
copy (AFM) images showed that the layer thickness increased
from 2.1 to 2.5 nm upon the addition of coronene (Fig-
ure 3a,b). Thus, it appears that the coronene molecules, which
have a flat conjugated surface, are effectively intercalated
between the aromatic planes of the dimeric micelles.
We investigated the influence of coronene intercalation
on the 2D structure by cryo-TEM analysis of a solution of 1 in
the presence of coronene (0.5 equiv; Figure 3c). The resulting
image shows large planar sheets with straight edges and thus
indicates that the flexible sheets become stiff upon the
intercalation of coronene and that the 2D structure is
preserved. Notably, a TEM image of a cast film negatively
stained with uranyl acetate shows large sheets with smoothly
embossed surfaces without any noticeable nanopores (Fig-
ure 3d). This image shows that the lateral pores are closed
upon the addition of coronene guest molecules. The magni-
fied image shows that the diameter of the in-plane micelles
decreases from 3.5 to 2.5 nm upon the addition of coronene
(Figure 2b, inset and Figure 3d, inset; see also Figure S5) and
thus indicates that the intercalation of the aromatic guests
causes the in-plane micelles to become laterally more closely
packed.
To corroborate the conformational change of the aromatic
basal planes upon intercalation of the coronene guest, we
performed molecular-modeling experiments. Energy minimi-
zation showed that the aromatic plane adopts an inverted
boat conformation in which the central benzene ring pro-
trudes from the face occupied by the dendrimer when
coronene is encapsulated (Figure 2d). This inversion of the
boat conformation provides an appropriate internal cavity to
contain a coronene molecule. The nonslipped arrangement of
the paired aromatic macrobicycles in the direction perpen-
dicular to the basal plane is consistent with the decrease in the
micellar diameter from 3.5 to 2.5 nm. These results indicate
that the intercalation of the flat aromatic guest molecules
increases the hydrophobicity of the aromatic segments of the
dimeric micelles through both the nonslipped arrangement of
the aromatic basal planes and the increased thickness of the
aromatic cores. Consequently, the strengthened side-to-side
hydrophobic interactions cause the primary micelles to be
more closely laterally packed without in-plane porous defects
and thus the formation of closed sheets (Figure 4). The
enhanced lateral interactions upon guest intercalation are
manifested by the change in the flexibility of the sheets, which
become highly stiff.
Full recovery of the original porous sheets was observed
by TEM upon removal of the coronene guest molecules by
extraction with toluene. Thus, the in-plane nanopores
undergo a reversible open/closed gating motion in response
to external guests without affecting the 2D structure. This
interesting behavior of the self-assembled porous sheets
originates from the face-to-face stacking motif of the rigid
flat aromatic segments of the amphiphilic molecules. The
face-to-face stacking of flat aromatic units in pairs enables the
Figure 3. a,b) AFM images of 1 (a) and 1–coronene (b; 50 mm aqueous
solution, 700ꢀ700 nm2) and cross-sectional analysis of the images.
c) Cryo-TEM image of a 100 mm aqueous solution of 1–coronene (scale
bar: 100 nm). d) Negatively stained TEM image of closed sheets from
a 100 mm aqueous solution of 1–coronene (scale bar: 100 nm). The
inset shows a magnified image (scale bar: 20 nm) and the line profile
along the yellow line.
Figure 4. Schematic representation of the switch between a nano-
porous sheet and a closed sheet, as triggered by the intercalation of
coronene.
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by the 2D structures. In this way, it is possible to switch
between open and closed states with preservation of the 2D
shape.
In conclusion, we have demonstrated that a rationally
designed macrobicyclic amphiphile consisting of a hydrophilic
dendron attached to the center of an aromatic plane under-
goes self-assembly into a 2D structure with nanosized lateral
pores through the lateral association of amphiphile dimers
with a uniform diameter of 3.5 nm. The porous sheets
efficiently intercalate flat aromatic molecules, such as coro-
nene, through the conformational inversion of the basal
planes of the dimeric micelles. Notably, the intercalation of
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Received: December 31, 2012
Revised: April 12, 2013
Published online: && &&, &&&&
Keywords: host–guest systems · macrobicycles ·
.
nanoporous sheets · self-assembly · supramolecular chemistry
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4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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Chemie
Communications
Host–Guest Systems
Slim guests are welcome: Aromatic
macrobicyclic amphiphiles underwent
self-assembly through a face-to-face
interaction to form dimeric micelles,
which further associated laterally to form
porous sheets with nanometer-sized
pores. The resulting sheets efficiently
intercalated planar aromatic guest mole-
cules, whereupon the porous sheets were
reversibly transformed into closed sheets
(see picture).
Y. Kim, S. Shin, T. Kim, D. Lee, C. Seok,
M. Lee*
&&&& — &&&&
Switchable Nanoporous Sheets by the
Aqueous Self-Assembly of Aromatic
Macrobicycles
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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