Cooperative self-assembly of discoid dimers: Hierarchical formation of nanostructures with a pH switch

Article abstract of DOI:10.1021/ja4025148

Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1?1 with an aggregation constant of ca. >10¹⁰ M^-l in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH.

Full text of DOI:10.1021/ja4025148

Article
pubs.acs.org/JACS
Cooperative Self-Assembly of Discoid Dimers: Hierarchical Formation
of Nanostructures with a pH Switch
M. Tassilo Fenske,† Wolfgang Meyer-Zaika,‡ Hans-Gert Korth,† Henning Vieker,§ Andrey Turchanin,§
and Carsten Schmuck*,†
‡
†Institut fur Organische Chemie, and Institut fur Anorganische Chemie, Universitat Duisburg-Essen, Universitatsstraße 7, 45141
Essen, Germany;
̈
̈
̈
̈
§Faculty of Physics, University of Bielefeld, Universitatsstraße 25, 33615 Bielefeld, Germany.
̈
S
* Supporting Information
ABSTRACT: Derivatives of the self-complementary 2-guanidiniocar-
bonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to
dimerize to 1•1 with an aggregation constant of ca. >1010 M−l in
DMSO) aggregate in a diverse manner depending on, e.g., variation of
concentration or its protonation state. The mode of aggregation was
analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these
zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at
the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further
aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly
cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like
aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-
assisted π−π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by
the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional
sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization
of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic
amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions,
which reassemble into the original rod-like aggregates upon readjustment to neutral pH.
form classical nanostructures such as vesicles or tubes instead of
rods.
INTRODUCTION
■
Whereas the self-assembly of polymer-based materials is well
established, the controlled self-assembly of small, nonpolymeric
building blocks into well-defined larger nanostructures is still an
emerging area of research, especially for self-assembly in polar
solution. In recent years, progress has been made using, for
example, the stacking of aromatic amphiphiles,1,2 aggregation of
metallo-supramolecular structures,3,4 or electrostatically driven
self-assembly of organic ions.5,6 It is even more interesting
but also more challengingto find self-assembling systems
which allow for switching back and forth between different
types of nanostructures using an external stimulus7 such as
temperature,8,9 metal ions,10,11 light,12,13 pH,14−16 or sol-
vent.17,18 Obviously, in the latter case switching back and
forth between different nanostructures is not really possible
without further ado. We now report that the small self-
complementary zwitterions 2(b−e) and 3 (Scheme 1) can
hierarchically self-assemble in polar solution (DMSO) first into
planar dimers, which then further aggregate in a cooperative
manner into rigid nanofibers or flat sheets, depending on the
packing parameter of the monomers. These higher aggregates
result from π-stacking of the discoid dimers formed by the self-
complementary zwitterions. Therefore, the aggregation mode
can be controlled by changing the protonation state of the
zwitterions. Protonation leads to cationic amphiphiles which
Ten years ago we reported that 2-guanidiniocarbonyl pyrrole
3-carboxylate zwitterion 1 forms very stable ion-paired dimers
1•1 (Scheme 1) even in polar solution (K > 1010 M−1 in
DMSO; K > 102 M−1 in water, respectively).19,20 However, this
zwitterion did not form any specific aggregates larger than
dimers but precipitated even at low micromolar concentrations
(DMSO). It is well-known that aromatic systems can aggregate
into one-dimensional fibers when functionalized with amide
groups.21−25 These aggregates are often formed via intermo-
lecular π−π-stacking of aromatic cores and are further stabilized
by intermolecular hydrogen bonds along the periphery of such
stacks. Based on these observations we envisioned that a new
type of zwitterions 2(b−e) and 3, derived from 1 by
introducing a sec-amide group in the 3-position of the pyrrole,
will also form dimers, which could then further aggregate into
rod-like nanostructures in a comparable manner as reported for
planar aromatic discoid molecules. In this case, however, the
individual planar aromatic stacking units would be the result of
a self-assembly processes themselves. For characterization of
the resulting aggregates, and to elucidate the mechanism
Received: March 11, 2013
© XXXX American Chemical Society
A
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certain stiffness. Interestingly, the sec-amide 2a, which does not
have an amide NH in the peripheral amide group, did not form
any such aggregates (see Supporting Information). Also, the
parent zwitterion 1 did not form any ordered aggregates but
precipitates from solution at concentrations > ca. 10 mM. AFM
and SEM images of 2b and 2c (Figure 9A) revealed fibers
appearing almost identically. Aggregates of both compounds
are characterized by a thickness of about 40−50 nm, with some
small variation in their respective height. The fibers visible in
the AFM-image of butyl amide 2b (Figure 1A) have an average
height of ca. 1.1 0.2 nm, while fibers formed by 2-hexyl amide
Scheme 1. Molecular Structure of Self-Complementary
Zwitterions 1, 2(a−e), and 3 and Their Corresponding
Dimers
2c have a height of ca. 1.9
0.2 nm (c.f. Figure 8A and
Supporting Information for corresponding height profiles),
which is in agreement with the increased molecular size of 2c
compared to 2b. Indeed, the measured heights correspond
quite well to the dimensions of dimerized monomers 2b•2b
and 2c•2c, respectively, as estimated from molecular mechanics
calculations (Figure 2). These dimensions were also confirmed
underlying their formation, concentration-dependent UV/vis
measurements, atomic force microscopy (AFM), helium ion
microscopy (HIM), and scanning, as well as transmission
electron microscopy (SEM, TEM) were employed.
RESULTS AND DISCUSSION
■
Microscopic Studies of Aggregates Based on 2(a−c).
The first concrete evidence for the formation of larger
aggregates than just dimers came from AFM-, SEM-, and
TEM-studies. Images obtained from 1 mM solutions of butyl
amide 2b and 2-hexylamide 2c clearly showed one-dimensional
aggregates (Figure 1) with a length of several hundred
nanometers. The fibers are rather straight, thus indicating a
Figure 2. Calculated structure of a one-dimensional rod obtained from
the stacking of discoid dimers 2b•2b (one shown in yellow). The
modeling suggests H-bonds between amide groups along the inner
core of the rod.
by DOSY experiments, which delivered a hydrodynamic radius
of ca. 1.4 nm (assuming spherical particles) for dimer 2b•2b
(10 mM solution in DMSO) in good agreement with the
results from microscopic images.26 However, as the lateral
thickness of these fibers is significantly bigger than the size of
the dimers, this suggests that the fibers are not individual rods
(formed by π−stacking of dimerized zwitterions), but rather
larger bundles resulting from further lateral aggregation of such
individual rods. This could indeed be confirmed by a SEM-
image of the (identical) surface generated for the AFM-
experiment. It is clearly visible that these fibers are aggregates of
the expected smaller rods (Figure 1B). The same was observed
Figure 1. AFM- (A), SEM- (B), and TEM-images (C) of 1.0 mM
solutions (DMSO) of zwitterions 2b (A, B) and 2c (C) (Scale bar: A
= 2 μm; B = 1 μm; C = 50 nm; z-scale A = 1.4 nm).
B
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in TEM-images of compound 2c: again, it can be clearly seen
that the fibers are formed from aggregates of smaller rods. The
dimensions of these individual rods (1.7 0.1 nm, see Figure
1C, zoom region) contained within the bigger bundles are in
excellent agreement with their height obtained from the AFM-
experiment and the molecular size expected for the discoid
dimers of these zwitterions (see Figure 2). Based on these
images, the bigger strands are likely to consist of 10−30
individual fibers.
high dimerization constant of the zwitterions (K > 1010 M−1 in
DMSO),19,20 the changes in the UV spectra, which start already
upon dilution below a concentration of 0.1 mM cannot reflect
the formation of the dimers from the zwitterions but must
represent the stacking of these dimers into larger aggregates.
The occurrence of a clear isosbestic point further suggests a
distinct and ordered structure for these larger aggregates, rather
than just unspecific aggregation (as in 1). The bathochromic
shift of ca. 20 nm is then a result of the electronic changes
accompanying the stacking of the discoid dimers into the rods.
For amide functionalized aromatics, it had already been
shown that the formation of related one-dimensional rods from
π−π-interactions of aromatic cores can be promoted by
hydrogen bonds along the stacks.21−25 As the amide group in
our zwitterions is also crucial for the formation of the one-
dimensional aggregates it is most likely that H-bonds between
the butyl amide groups also help stabilize the structure. Neither
the parent zwitterion 1 without any amide group nor zwitterion
2a with a dimethyl amide group, both lacking the potential to
form H-bonds, showed any evidence for the formation of
specific aggregates in AFM-, SEM-, and TEM-experiments.
Accordingly, the changes in the UV spectrum of 2a (Figure 4)
Molecular Modeling Studies. Molecular mechanics
calculations were performed to visualize the one-dimensional
rods seen in the microscopic images of 2b and to probe their
molecular dimensions. A Monte Carlo conformational search
(Macromodel v 8.5, OPLS_2005 force field with GB/SA water
solvation, 50 000 steps) revealed that the planar discoid dimers,
formed from the self-assembly of zwitterion 2b, can further
stack into a one-dimensional rod (Figure 2). The dimerized
zwitterions form the inner core of these stacks which is coated
by the butyl amide groups. The estimated overall thickness of
the rod (ca. 1.5 nm) is in excellent agreement with the diameter
of the individual rods as extracted from the TEM images.
UV-Dilution Studies. Having thereby affirmed that sec-
amide substituted zwitterions 2b and 2c aggregate first into
dimers and then into one-dimensional rod-like structures, we
set out to investigate the concentration dependency of this
aggregation process using UV/vis dilution studies. It quickly
became obvious that the disassembly of aggregates based on
zwitterions like 2b only occurs at very low concentration (<0.3
mM). Consequently, NMR-dilution experiments, covering a
concentration range from 40 mM to 1.5 mM, were not
informative in these cases (see Supporting Information). UV-
spectroscopy, being more sensitive at lower concentration,
however, provided the means to monitor the disassembly
process. For example, UV-spectra of sec-amide 2b were
measured within the concentration range of 0.33 mM and 9
nM (Figure 3). Plotting the molar absorptivity ε against the
corresponding wavelengths at different concentrations reveals a
bathochromic shift of the absorption maxima of about 20 nm
below the concentration of 0.11 mM; with an isosbestic point
at 312 nm. At concentrations below 18 nM no further changes
of the molar extinction coefficients ε were observed. Due to the
Figure 4. Concentration dependent UV/vis absorption spectra of 2a
(0.33 mM to 4 nM in DMSO). Arrow indicates the direction of
change with decreasing concentration.
are completely different from those for 2b. Upon dilution from
0.16 mM to 4 nM a slight increase of the molar extinction
coefficients ε was observed, but no isosbestic point and only a
small bathochromic shift of about 8 nm were detected. In
contrast, all other zwitterions (2c−e, 3) which again have an
amide NH show similar UV changes as 2b (see Supporting
Information) and also formed ordered aggregates as seen in the
corresponding microscopic images (Figure 9). Therefore, one
can conclude that the formation of the one-dimensional rods
which are visible in the AFM-, SEM-, and TEM-experiments for
2b is favored by H-bonds along the side of the stack as
observed for other amide functionalized aromatics. However,
the important difference here is that the aggregating disc itself is
already the result of a self-assembly process, namely, the
dimerization of the zwitterion.
The molecular mechanics calculations also suggested such
stabilizing H-bonds between the amide groups of the stacked
dimers (Figure 2). For the formation of these H-bonds,
however, the amide group has to rotate out of the plane of the
Figure 3. Concentration dependent UV/vis absorption spectra of 2b
(0.33 mM to 9 nM in DMSO). Arrow indicates the direction of
change with decreasing concentration.
C
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dimers, so that the carbonyl-oxygen and the amide-NH point
toward the corresponding amide groups of an associated disc
above or below, respectively. This rotation changes the
electronic properties of the dimers and together with the π-
stacking is hence most likely responsible for the observed
hypsochromic shift in the UV spectra which occurs during rod-
formation. To test this, we simulated the consequences of such
a rotation by time dependent DFT calculations (PBE0/6-
31G(d) level, PCM method with DMSO as solvent) of the two
conformers A and B of model compound 4 (Figure 5).
Figure 6. Plotting the degree of aggregation α of 2b against the
concentration (black squares). The data can only be fitted to a strongly
cooperative aggregation mode (green) but clearly not to an isodesmic
growth model (red).
disassembled at this (and lower) concentrations. Hence the
molar absorptivity at this concentration corresponds to the
unstacked dimers (α = 0). The molar absorptivity of the fully
aggregated species (α = 1) can be obtained from an
extrapolation of the spectral changes at higher concentration
(see Supporting Information for more details). With these two
values for the molar absorptivities of the unstacked dimers and
the aggregated rods the degree of aggregation for each
concentration can be easily calculated. As shown in Figure 6,
the degree of aggregation α does not increase smoothly.
Aggregation only occurs above a critical concentration of ca. 15
μM. This behavior is typical for a cooperative nucleation
elongation mechanism. Aggregation first requires the initial
formation of a small nucleus. Once this nucleus (often a dimer,
i.e., in this case a tetramer formed of two discoid dimers of type
2•2) is formed, above the critical concentration (which
depends on the association constant K2 for the formation of
this nucleus), rapid aggregation occurs because the subsequent
elongation steps (which are characterized by an association
constant K) are energetically more favorable than the initial
formation of the nucleus (K > K2). The alternative to such a
cooperative aggregation would be the so-called isodesmic
growth, in which each aggregation step from monomer to
dimer to any larger aggregate is characterized by the same
association constant K. It is clearly evident that aggregation of
2b is not an isodesmic process (red line in Figure 6)23,28 but a
strongly cooperative one. The cooperative nucleation elonga-
tion model (also called K2/K-model) can be quantitatively
described with the following equation:28
Figure 5. TD-DFT computed UV-spectra of the two conformers A
and B of model compound 4. Depicted as lines are the oscillation
strengths (f) for both conformers. For the visualization of the UV-
spectra Gauss functions with a half width of Δλ/2 = 15 were used.
Conformers 4A and 4B differ only in the orientation of their
peripheral amide bond. While this functional group is oriented
90° perpendicular to the plane of the zwitterion within
structure 4A (as a model for its orientation in the aggregated
stacks) it is parallel in 4B (as in the nonstacked dimers). Using
time dependent DFT calculations the influence of these two
possible orientations of the amide group onto the electronic
structure of the zwitterion was estimated. A shift of the UV-
maxima of 17 nm was calculated if the amide bond is rotated
from the perpendicular (4A) to the parallel orientation (4B).
This shift is in excellent agreement with the bathochromic shift
observed upon dilution and hence deaggregation of the dimers
of 2b. Therefore, a similar change in the orientation of the
amide group seems to occur during rod-formation.
Next to the structural effects discussed above, the mechanism
of aggregation can be deduced from a quantitative analysis of
the UV-dilution experiments.27 For this analysis the degree of
aggregation α was plotted against the corresponding concen-
trations (Figure 6). As evident from the UV spectra of
compound 2b (Figure 3) the molar absorptivity does not
change anymore upon dilution below 4 nM. This indicates that
the aggregates responsible for the UV changes are completely
2Kc + σ − 4Kc + σ2
ε(c) =
(εf − ε ) + ε
a
a
2K2c2
ε denotes the measured molar absorptivity at each concen-
tration; εf and εa are the values for the free and the aggregated
species; K is the binding constant, σ is the degree of
cooperativity (K2/K), and c is the total compound concen-
tration in the sample. The data fitting (green line in Figure 6)
provides an association constant of K = 1.8 × 104 M−1 for the
stacking of the dimeric discs into the rods and a degree of
cooperativity of σ = 0.09, indicating that this is indeed a highly
cooperative process.
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Is the cooperative nucleation elongation mechanism
compatible with the formation of π-stacked rods from discoid
dimers as discussed so far? As deduced from the UV spectral
changes (Figure 4) aggregation requires the rotation of the
amide group out of planarity. At least for the two model
conformers 4A and 4B the energy difference between these two
orientations is ca. 15 kcal/mol. For the stacking of the first two
planar discoid dimers to form a nucleus four amide groups have
to be rotated out of planarity. Hence, this energy penalty has to
be paid four times. For any subsequent stacking of the next
discoid dimer onto an already existing nucleus or any larger
aggregate, only two amide groups have to be rotated.
Consequently, the formation of the nucleus is expected to be
less favorable than the subsequent elongation step. Already
based on this simple model, a cooperative aggregation is
expected and reasonable.
Table 1. Calculated Aggregation Constant K and Degree of
Cooperativity σ for the Aggregation of Dimers Based on
Zwitterions 2(b-e) and 3 into Rods
a
The aggregation mechanism of compound 2b can therefore
be described as shown in Figure 7. Due to the extraordinarily
a
The data are obtained from the corresponding UV dilution studies.
between the peripheral aromatic moieties. The aggregation
constants of butyl amide 2b and its homo congener 3 are again
very similar. A similar tendency is observed for the degrees of
cooperativity σ. The values for 2b−c and 3 are nearly identical,
whereas for 2d and 2e the degree of cooperativity increases
significantly.
Figure 7. Schematic self-assembly of the zwitterion 2b starting at
infinite diluted solution (A) via dimerization (B) and nucleation (C)
to formation of one-dimensional rod-like structures (D).
Microscopic Studies of Aggregates Based on 2(c−e)
and 3. To further determine the structural influence of the
peripheral amide substituent onto the mode of aggregation,
additional AFM-images of zwitterions 2(c-e) and 3 were taken.
For zwitterion 2c (Figure 8A) identical rod-like aggregates as
for 2b (Figure 1) are observed which is hardly surprising given
the structural similarity of both zwitterions. The larger tert-octyl
amide 2d (Figure 8B), showed a much stronger tendency for
lateral aggregation of the rods into larger bundles. In the AFM-
image large areas covered with strongly aggregated, almost
layer-like structures with an average height of 1.8 0.2 nm are
seen. The 2,6-diisopropyl-phenyl amide 2e produced only
layer-like structures (Figure 8C). In this case most of the
surface was covered with a layer of an average thickness of 1.5
0.2 nm. Again, the height of this layer fits very well with the
estimated size of the discoid dimers 2e•2e.
high association constant of the zwitterionic binding motif 1 (K
> 1010 M−1 in DMSO)20 butyl amide 2b exists in its
monomeric state only at extreme dilution (A). Already at
nanomolar concentrations, mainly dimers 2b•2b are present
(B). At a critical concentration of about 15 μM these discoid
dimers start to aggregate into the first nuclei (C), which then
on further increase of the concentration readily grow in a
strongly cooperative manner into one-dimensional rods (D).
A similar cooperative growth was found for all other
zwitterions 2(c−e) and 3 with an amide NH group. The
corresponding aggregation constant K and the degree of
cooperativity σ as obtained from the respective UV-dilution
experiments (see Supporting Information) are summarized in
Table 1. It can be seen that the nature of the amide substituents
has an effect on the association constant and hence the stability
of the larger aggregates. The structural difference between 2b
and 2c is only small, which is reflected in their respective and
nearly identical aggregation constants. The larger tert-octyl side
chain of 2d, however, leads to a 30 times larger aggregation
constant. This increase most likely reflects enhanced van der
Waals interactions between the larger alkyl side chains along
the core of the π-stacked dimers. An even larger aggregation
constant is obtained for the diisopropylphenyl amide 2e. This
increased stability points toward additional π−π-interactions
The AFM-image of homo amide 3 (Figure 8D) exclusively
shows circular layer-like structures with a height of about 2.0
0.2 nm, which again correlates to the dimensions calculated for
the dimerized zwitterion 3•3. Such pronounced layer-like
aggregation was somehow unexpected with respect to its
structural similartity to 2b, which readily produced fiber-like
bundles of rods (see above). Somehow the rods formed by
homo amide 3 seem to be more prone to lateral aggregation
than in the case of 2b. To further investigate the aggregation of
3, SEM-, HIM-, and TEM-images were recorded (Figure 9).
E
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able to resolve these individual rods, the width of which (2.3
nm, see Figure 9C and zoom region) corresponds again quite
well to the size of the dimerized zwitterion 3·3. Hence, despite
the overall different appearance of the AFM images of 3, its
aggregation mode seems to be identical to that of the other
zwitterions. Obviously, the main difference is that the lateral
aggregation of the individual rods and the fiber-like bundles is
more pronounced. Hence, homoamide 3 has a stronger
inclination to form two-dimensional aggregates (upon evapo-
ration of the solvent on a mica surface) than 2b.
In conclusion, the different tendency of the zwitterions to
form two-dimensional aggregates, as seen in the AFM-images
(Figure 8A→D), depends on their possibility for lateral
interactions between the rods. The alkyl amides 2b, 2c, and
2d can interact laterally only by van der Waals forces and
therefore have only a weak tendency to form two-dimensional
aggregates. Accordingly, in the AFM images mainly linear
structures are seen. The 2,6-diisopropylphenyl amide 2e can
most likely form additional π−π-interactions to neighboring
strands, which increases the tendency toward two-dimensional
aggregation. Finally, homo amide 3, which might also form
lateral hydrogen bonds as well, appears only as a 2D-layer in the
AFM-image.
Figure 8. AFM-images of 1.0 mM solutions of the zwitterions 2c (A),
2d (B), 2e (C), and 3 (D). (Scale bar: 2 mm; z-scale A = 3.9 nm, B =
3.9 nm, C = 2.7 nm, D = 3.0 nm.)
Studies on Switching the Aggregation Mode by
Protonation. The ability to influence supramolecular
aggregation by varying an external factor (e.g., pH) in a
reversible manner is of pivotal interest. As recognized before, the
aggregation of the zwitterionic binding motif 1 can be directly
affected by changing its protonation state.19 Only the zwitterion
can form dimers; neither the protonated nor the deprotonated
forms obtained after addition of either acid or base are self-
complementary. Therefore, supramolecular structures formed
by the dimers of zwitterions of type 2 should be sensitive to
changes in their protonation state as well, and a drastic change
in their aggregation mode and the morphologies of the
resulting structures should occur. Attempts to investigate such a
morphology switch by deprotonation failed, since the addition
of base to a DMSO solution of zwitterions 2(b−e) only
resulted in complete precipitation. Obviously the anionic
compounds are not soluble enough to study their aggregation
in DMSO. However, addition of aqueous HCl (1 M, 10 equiv)
to the respective zwitterions in DMSO produced a homoge-
neous solution, in which complete protonation of the
1
zwitterions was confirmed by H NMR.
Again, AFM and TEM were employed to investigate the
supramolecular structures formed by the protonated com-
pounds 2b•H+−2e•H+. To our delight a drastic change in
morphology was indeed observed after protonation. For
example, as discussed above, AFM-images recorded from
butyl amide 2b (Figure 10A, also see Figure 1) revealed rods
formed by linear aggregation of dimers 2b•2b. In contrast, the
protonated form 2b•H+ produced solvent filled vesicles (Figure
10B). These vesicles have a height of ca. 3−4 nm and a size of
ca. 50 nm. Given a certain softness of these structures in the
AFM experiment, this height corresponds to a classical
bimolecular layer structure of the vesicles. As 2b•H+ resembles
classical cationic amphiphiles the formation of such vesicular
structures is reasonable. After readjustment of the original
protonation state (2b•H+→2b) by addition of an equal
amount of base (aq. NaOH), the original rods of stacked
discoid dimers 2b•2b are formed again (Figure 10 C). All other
zwitterions behave in a similar way. Figure 11 shows AFM- and
SEM-images for 2c•H+−2e•H+ (for 3•H+ see Supporting
Figure 9. SEM- (A), HIM- (B), and TEM-images (C) of 1 mM
solutions of zwitterion 3. (Scale bar: A = 1 μm; B = 50 nm; C = 100
nm.)
The existence of a layer-like structure was confirmed in all three
cases. SEM-images showed a foil-like material that appeared to
be shrunken due to sample preparation, as coating the sample
with carbon required heating. HIM- and TEM-images,
however, also show the expected linear aggregates besides
layer structures. In both cases, the linear aggregates seemed to
be embedded into the sheet-like structure. The dimensions of
around 8 nm estimated from the HIM-image (Figure 9B)
suggested that these linear aggregates are bundles resulting
from lateral aggregation of individual rods. TEM-images were
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Figure 10. Switching the morphology of aggregates of 2b by reversible
protonation and deprotonation (1.0 mm, DMSO): (A) AFM image of
zwitterion 2b; (B) Protonated version 2b•H+ after the addition of 10
equiv of HCl; (C) Reformation of the zwitterion 2b after the addition
of the same amount of NaOH to the solution of B. Precipitated NaCl
can be seen between the supramolecular rods. Scale bar = 2 μm; z-
scale: A = 1.4 nm, B = 6.6 nm, C = 3.6 nm.
Information). While only small differences in the aggregation of
dimers formed by butyl amide 2b and 2-hexyl amide 2c were
detected (Figures 1A and 8A), more pronounced differences
were visible in the aggregates formed by their protonated
congeners 2b•H+ and 2c•H+. Only vesicular and therefore
spherical particles are formed from a 10 mM solution of
protonated butyl amide 2b•H+ in DMSO (Figure 11A). From
an equivalent solution of protonate 2-hexyl amide 2c•H+, tube-
like structures emerged in addition to vesicles with an average
Figure 11. AFM (A, B) and TEM-images (C) of a 10 mM solution of
the protonated versions of 2b•H+ (A), 2c•H+ (B, C), and AFM (D,E)
images of a 1 mM solution of 2d•H+ (D) and 2e•H+ (E). (Scale bar:
A, B, D, E = 2 mm; C = 100 nm; z-scale A = 6.8 nm, B = 6.5 nm, C =
2.7 nm, D = 2.6 nm, E = 7.4 nm.)
surface forming flat monolayers.31,32 Therefore, the data shown
here clearly demonstrates that self-complementary zwitterions
of type 2 allow for reversible morphology switching of the
resulting aggregates. Simply by reversible protonation and
deprotonation fiber-like structures can be transformed into
vesicles and tubes and vice versa.
height of 3.3
0.2 nm (Figure 11B). As these tube-like
structures have the same diameter as the vesicles, it could not
be excluded from the AFM image that these structure are
simply clusters of individual vesicles. However, TEM-images of
2c•H+, like the one depicted in Figure 11C, clearly showed that
the described structures are indeed hollow tubes. As shown in
the zoom-region, even fused multilayer substructures are visible.
Therefore, it appears that these nanotubes originate from the
fusion of individual vesicles.
CONCLUSION
■
We were able to show here that zwitterions of type 2 aggregate
hierarchically, first into discoid dimers, which then further
assemble into rods based on π−π-stacking. The formation of
rods by π-stacking of aromatic amides is well-known; in this
case, however, the stacking unit itself is already the result of a
self-assembly process, namely, the dimerization of the
zwitterions. Recently, two examples of similar hierarchical rod
formation, based on dimeric, and indeed trimeric, discoid
aggregates based on H-bonding, have been reported by
Meijer33 and Hud,34 respectively. However, as the structures
described within this paper are based on ionic interactions
within the discoid dimers, strongly cooperative π-stacking of
these dimers into rods following a nucleation elongation
mechanism was now also achieved in polar solvents (DMSO).
Depending on the size and nature of the amide side chains, the
tendency of the rods to form bigger bundles increases from
Whereas 2b•H+ and 2c•H+ formed vesicles or tubes, 2d•H+
and 2e•H+ produced flat layered aggregates in a concentration
dependent manner (c.f. Supporting Information). AFM images
of 2d•H+ (Figure 11D) and 2e•H+ (Figure 11E) obtained
from 1 mM solutions show structures with an average height of
1.5
0.2 nm and 1.4
0.2 nm, respectively. The surface
density of these layer-like structures is much higher in the case
of diisopropylphenyl amide 2e•H+, with some additional
precipitated material also present. The height of these flat
structures corresponds to the molecular dimension of ca. 1.3 to
1.6 nm for 2d•H+ and 2e•H+ (as suggested by molecular
modeling). Obviously 2d•H+ and 2e•H+, which have a more
cylindrical shape than 2b•H+ and 2c•H+ due to the larger size
of their corresponding side chains, not only form bigger
vesicles,29,30 but these vesicles then tend to collapse on the
G
dx.doi.org/10.1021/ja4025148 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(18) Fernan
6567−6569.
́
dez, G.; García, F.; San
́
chez, L. Chem. Commun. 2008,
molecule 2b to 2e. As the discoid dimers are essential for rod
formation, the self-assembly mode can be completely changed
by protonation of the zwitterions. Hence, zwitterions 2 are
converted into classical cationic amphiphiles 2•H+ which form
vesicles, tubes, and flat sheets, instead of linear rods. Such
switchable nanomaterials might be of interest for further
development of functional materials.
(19) Schmuck, C. Eur. J. Org. Chem. 1999, 2397−2403.
(20) Schmuck, C.; Wienand, W. J. Am. Chem. Soc. 2003, 452−459.
(21) Nguyen, T. Q.; Martel, R.; Avouris, P.; Bushey, M. L.; Brus, L.;
Nuckolls, C. J. Am. Chem. Soc. 2004, 126, 5234−5242.
(22) Nguyen, T.-Q.; Martel, R.; Bushey, M.; Avouris, P.; Carlsen, A.;
Nuckolls, C.; Brus, L. Phys. Chem. Chem. Phys. 2007, 9, 1515−1532.
(23) Smulders, M. M. J.; Nieuwenhuizen, M. M. L.; de Greef, T. F.
A.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer, E. W. Chem.
Eur. J. 2010, 16, 362−367.
(24) Aparicio, F.; Vicente, F.; San
8356−8358.
(25) Nieuwenhuizen, M. M. L.; de Greef, T. F. A.; van der Bruggen,
R. L. J.; Paulusse, J. M. J.; Appel, W. P. J.; Smulders, M. M. J.; Sijbesma,
R. P.; Meijer, E. W. Chem.Eur. J. 2010, 16, 1601−1612.
(26) Note that DOSY experiments do not only measure the radius of
the molecule/aggregate, but rather the (larger) radius of the solvation
sphere. Hence dimensions measured by DOSY are normally a bit
larger than the size of the molecule itself.
(27) De Greef, T. F. A.; Smulders, M. M. J.; Wolffs, M.; Schenning,
A. P. H. J.; Sijbesma, R. P.; Meijer, E. W. Chem. Rev. 2009, 109, 5687−
5754.
ASSOCIATED CONTENT
* Supporting Information
■
S
́
chez, L. Chem. Commun. 2010, 46,
Detailed synthetic procedures and spectroscopic character-
ization for all new compounds, as well as detailed description of
molecular modeling calculations, UV/vis titration, and micro-
scopic imaging experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: carsten.schmuck@uni-due.de.
■
Notes
(28) Wurthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.
̈
The authors declare no competing financial interest.
Eur. J. 2001, 7, 2245−2253.
(29) Dorfler, H.-D. Grenzflachen- und Kolloidchemie; 1. Aufl.; VCH:
̈
̈
ACKNOWLEDGMENTS
■
Weinheim, 1994.
Financial support of this work by the Deutsche Forschungsge-
meinschaft (DFG) is gratefully acknowledged. We thank Dr.
(30) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B.
̈
Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley &
Christoph Hirschhauser for helpful discussions during the
preparation of the manuscript.
̈
Sons: Chichester, U.K, 2003.
(31) Ravoo, B. J.; Darcy, R. Angew. Chem., Int. Ed. 2000, 39, 4324−
4326.
(32) Voskuhl, J.; Fenske, T.; Stuart, M. C. A.; Wibbeling, B.;
Schmuck, C.; Ravoo, B. J. Chem.Eur. J. 2010, 16, 8300−8306.
̌
́
(33) George, S. J.; de Bruijn, R.; Tomovic, Z.; Van Averbeke, B.;
REFERENCES
■
(1) Bae, J.; Choi, J. H.; Yoo, Y. S.; Oh, N. K.; Kim, B. S.; Lee, M. J.
Am. Chem. Soc. 2005, 127, 9668−9669.
Beljonne, D.; Lazzaroni, R.; Schenning, A.; Meijer, E. W. J. Am. Chem.
Soc. 2012, 134, 17789−17796.
́ ́
(2) Fernandez, G.; García, F.; Aparicio, F.; Matesanz, E.; Sanchez, L.
Chem. Commun. 2009, 7155−7157.
́
(34) Cafferty, B. J.; Gallego, I.; Chen, M. C.; Farley, K. I.; Eritja, R.;
Hud, N. V. J. Am. Chem. Soc. 2013, 135, 2447−2450.
(3) Lutzen, A.; Hapke, M.; Griep-Raming, J.; Haase, D.; Saak, W.
̈
Angew. Chem., Int. Ed. 2002, 41, 2086−2089.
(4) Frischmann, P. D.; Guieu, S.; Tabeshi, R.; MacLachlan, M. J. J.
Am. Chem. Soc. 2010, 132, 7668−7675.
(5) Tanaka, Y.; Katagiri, H.; Furusho, Y.; Yashima, E. Angew. Chem.,
Int. Ed. 2005, 117, 3935−3938.
(6) Schmuck, C.; Rehm, T.; Klein, K.; Grohn, F. Angew. Chem., Int.
̈
Ed. 2007, 46, 1693−1697.
(7) Fenske, T.; Korth, H. G.; Mohr, A.; Schmuck, C. Chem.Eur. J.
2012, 18, 738−755.
(8) García, F.; Aparicio, F.; Marenchino, M.; Campos-Olivas, R.;
́
Sanchez, L. Org. Lett. 2010, 12, 4264−4267.
(9) Nieuwenhuizen, M. M. L.; de Greef, T. F. A.; van der Bruggen, R.
L. J.; Paulusse, J. M. J.; Appel, W. P. J.; Smulders, M. M. J.; Sijbesma, R.
P.; Meijer, E. W. Chem.Eur. J. 2010, 16, 1601−1612.
(10) Hui, J. K.-H.; MacLachlan, M. J. Dalton Trans. 2010, 39, 7310−
7319.
(11) Golubkov, G.; Weissman, H.; Shirman, E.; Wolf, S. G.; Pinkas,
I.; Rybtchinski, B. Angew. Chem., Int. Ed. 2009, 48, 926−930.
(12) Kanaya, A.; Takashima, Y.; Harada, A. J. Org. Chem. 2011, 76,
492−499.
(13) Wang, Y.; Han, P.; Xu, H.; Wang, Z.; Zhang, X.; Kabanov, A. V.
Langmuir 2010, 26, 709−715.
(14) Dong, S.; Luo, Y.; Yan, X.; Zheng, B.; Ding, X.; Yu, Y.; Ma, Z.;
Zhao, Q.; Huang, F. Angew. Chem., Int. Ed. 2011, 50, 1905−1909.
(15) Rodler, F.; Linders, J.; Fenske, T.; Rehm, T.; Mayer, C.;
Schmuck, C. Angew. Chem., Int. Ed. 2010, 49, 8747−8750.
(16) Voskuhl, J.; Fenske, T.; Stuart, M. C. A.; Wibbeling, B.;
Schmuck, C.; Ravoo, B. J. Chem.Eur. J. 2010, 16, 8300−8306.
(17) 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.
H
dx.doi.org/10.1021/ja4025148 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX