Supramolecular architectures from bent-core dendritic molecules

Article abstract of DOI:10.1002/anie.201407705

Control of the self-assembly of small molecules to generate architectures with diverse shapes and dimensions is a challenging research field. We report unprecedented results on the ability of ionic, bent dendritic molecules to aggregate in water. A range of analytical techniques (TEM, SEM, SAED, and XRD) provide evidence of the formation of rods, spheres, fibers, helical ribbons, or tubules from achiral molecules. The compact packing of the bent-core structures, which promotes the bent-core mesophases, also occurs in the presence of a poor solvent to provide products ranging from single objects to supramolecular gels. The subtle balance of molecule/solvent interactions and appropriate molecular designs also allows the transfer of molecular conformational chirality to morphological chirality in the overall superstructure. Functional motifs and controlled morphologies can be combined, thereby opening up new prospects for the generation of nanostructured materials through a bottom-up strategy.

Full text of DOI:10.1002/anie.201407705

Angewandte
Chemie
DOI: 10.1002/anie.201407705
Chirality Transfer
Supramolecular Architectures from Bent-Core Dendritic Molecules**
Miguel Cano, Antoni Sꢀnchez-Ferrer, Josꢁ Luis Serrano, Nꢁlida Gimeno,* and M. Blanca Ros*
Abstract: Control of the self-assembly of small molecules to
generate architectures with diverse shapes and dimensions is
a challenging research field. We report unprecedented results
on the ability of ionic, bent dendritic molecules to aggregate in
water. A range of analytical techniques (TEM, SEM, SAED,
and XRD) provide evidence of the formation of rods, spheres,
fibers, helical ribbons, or tubules from achiral molecules. The
compact packing of the bent-core structures, which promotes
the bent-core mesophases, also occurs in the presence of a poor
solvent to provide products ranging from single objects to
supramolecular gels. The subtle balance of molecule/solvent
interactions and appropriate molecular designs also allows the
transfer of molecular conformational chirality to morpholog-
ical chirality in the overall superstructure. Functional motifs
and controlled morphologies can be combined, thereby open-
ing up new prospects for the generation of nanostructured
materials through a bottom-up strategy.
applications such as chiral resolution, nonlinear optics, and
photovoltaics are claimed.[4g–i,5] Thus, in addition to the stated
functional properties,[5] BCLCs offer high potential in both
supramolecular chemistry and materials science.
Interestingly, Ho, Hsu, and co-workers[6] reported the
ability of bent-core molecules to promote supramolecular
chirality by self-assembly in a solvent. Thus, an achiral bent-
core molecule formed hierarchical helical superstructures in
a THF/water solution.[6a] They proposed the molecular
conformational chirality as the origin of such superstructures.
Control over the self-assembly of small molecules to
generate architectures with diverse shapes and dimensions by
supramolecular chemistry is still a topical, challenging, and
dynamic research field.[7,8] Systems ranging from the nano-
and micrometer scales to macroscopic dimensions, along with
diverse morphologies with shapes and structures that can
influence the physical properties, are considered in this field,
with applications in nanotechnology to biotechnology.[7,8]
However, many such systems are still difficult to synthe-
size, especially in the case of aqueous systems, which could
give rise to breakthroughs in biological, medical, and
materials science. The prediction of structural features from
noncovalent interactions is still in its infancy and, despite
interest in new approaches, systematic studies on the self-
assembly of bent-core molecules in solution have not been
published to date, despite their mesomorphic abilities.
Herein, we report major advances in the potential of the
bent-core packing to generate controlled aggregates in water.
The choice of ionic bent-core/PPI-based (PPI = polypro-
pyleneimine molecules to achieve this target arose not only
because they form BCLCs,[9] but also because their inherent
amphiphilicity could favor segregation effects in solvents,
including water.[10] Thus, we explored how pathways to create
aggregates of bent-core molecules affected their final supra-
molecular structure by changing the molecular design. The
effect of changing the length of the flexible tails, either in an
inner (m: 4, 10) or an outer (n: 8, 14) position was evaluated
initially with first generation dendrimers PPI1-B1-m-
n (Scheme 1). We then extended our research to further
ionic dendrimers by incorporating intrinsic chirality into the
molecules (PPI1-B1-4-7* and PPI1-B1-4-8*) with the aim of
controlling the supramolecular chirality. Furthermore, an
isomeric rod-core dendrimer (PPI1-C1-4-8) was studied to
demonstrate the exclusivity of the bent-core design to induce
supramolecular chirality (Scheme 1).
B
ent molecules are the origin of bent-core liquid crystals
(BCLCs), which have properties different to those of classical
mesophases.[1] Their molecular geometry forces the molecules
to adopt a compact packing, which restricts their rotational
freedom and leads to novel and intriguing polar mesophases.
A distinctive feature of this class of mesogen is the possibility
of the formation of mesophases with spontaneous macro-
scopic chirality from achiral molecules.[1e,f,l,2] Their formation
mechanisms are still under debate,[1e,f,2a,b,d,3,4] but potential
[*] Dr. M. Cano, Dr. N. Gimeno, Prof. Dr. M. B. Ros
Instituto de Ciencia de Materiales de Aragꢀn
Departamento de Quꢁmica Orgꢂnica. Facultad de Ciencias
Universidad de Zaragoza-CSIC
Campus San Francisco. 50009-Zaragoza (Spain)
E-mail: bros@unizar.es
Dr. A. Sꢂnchez-Ferrer
ETH Zurich
Department of Health Sciences and Technology, IFNH
Schmelzbergstrasse 9, LFO, E29, 8092 Zurich (Switzerland)
Prof. Dr. J. L. Serrano
Instituto de Nanociencia de Aragꢀn
Departamento de Quꢁmica Orgꢂnica
Facultad de Ciencias, Universidad de Zaragoza
Campus San Francisco, 50009-Zaragoza (Spain)
[**] We greatly appreciate financial support from the Spanish Govern-
ment (MICINN-FEDER projects MAT2012-38538-C03-01 and
CTQ2012-35692) and Aragon’s Government (GA) (project E04).
Support from Prof. R. Mezzenga is greatly appreciated. We are also
grateful to Juan de la Cierva-MICINN, JAEDOC-CSIC (N.G.), and to
GA (M.C.) fellowship programs for support. Thanks are given also
to the nuclear magnetic resonance, mass spectra, and thermal
analysis services from the ICMA (Uni. Zaragoza-CSIC) and the LMA
from INA (Uni. Zaragoza) for SEM and TEM equipment.
It is demonstrated here that these ionic dendrimers are
suitable molecules not only to obtain different bent-core
mesophases, but that they also aggregate to form a variety of
useful architectures in water. However, the presence of
a bent-core structure alone is not sufficient to provide
either aggregates or similar morphologies, including helical
structures.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407705.
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Figure 1. Scanning electron microscopy (SEM) images of dialyzed
aqueous solutions of PPI1-B1-10-14 (A), PPI1-B1-10-8 (B), PPI1-B1-4-8
(C–F), PPI1-B1-4-8* (G), and PPI1-C1-4-8 (H).
Scheme 1. Chemical structure and nomenclature of the ionic bent-core
dendrimers.
spheres that aggregate into circular objects (diameters of 20–
60 mm; Figure 1B and see Figure S6 in the Supporting
Information).
The new PPI-based dendrimers were synthesized and
characterized according to previously published methods[9]
(see the Supporting Information) and they were used in
aggregation studies together with the dendrimer PPI1-B1-10-
14 previously reported by us.[9] Self-assembly of these
dendrimers in the bulk (see Table S1 in the Supporting
Information) was investigated by a range of analytical
techniques (POM, DSC, TGA, and XRD; see the Supporting
Information). The materials with a long terminal chain and
a flexible spacer (n: 14 or m: 10) form a lamellar mesophase,
assigned as an SmCP phase, and this is consistent with
previous results.[9] However, the dendrimer with short flexible
chains (n: 8 and m: 4) arranges into a Colr liquid-crystal phase,
as evident by X-ray diffraction (XRD) studies.[9]
Remarkable differences were observed in the case of
aggregates formed from dendrimers with a short inner spacer
(m: 4). PPI1-B1-4-14 and PPI1-B1-4-8 both form helical
aggregates, which are new chiral arrangements formed from
achiral units.[8d] Very long twisted tapes (several micrometers
in length and around 30 nm in width) with a pitch in the range
of 110 nm are generated by the self-assembly of PPI1-B1-4-14
(see TEM and FESEM images in Figure 2A–D and Figure S6
in the Supporting Information). Interestingly, the entangled
fibrillar nanostructure immobilizes THF, as evident by the
test-tube method (Figure 2E). This observation is reminiscent
of the rare recent reports of organogels based on a bent-core
organized gelator,[4g,12] and thus relates to the active field of
molecular gels in general.[13]
For PPI1-B1-4-8, with the shortest outer and inner flexible
tails (m: 4, n: 8), TEM and SEM studies indicate the
formation of long helical ribbons and empty nanotubes
(tubules) that are several micrometers in length (Figure 1C–
F and Figure 3) with polydisperse diameters (ca. 400–600 nm
diameter) as a result of multiple wrappings (Figure 1E);
tubules have attracted great interest in nanotechnology for
the preparation of functional materials.[14] However, gel
formation was not detected in this case.
The aggregation of the ionic dendrimers in solution (THF
and water) was carried out according to published procedur-
es.[10g,11] These studies provided convincing evidence of the
formation of different aggregates (the results are summarized
in Table S1 in the Supporting Information).
These dendrimers, which incorporate four bent-core
structures, are suitable molecules for self-organization upon
the addition of water (poor solvent) to give different
supramolecular morphologies, depending on the chemical
structure of the ionic molecule (see Figure S6 in the Support-
ing Information). The presence of long hydrocarbon tails, as
in PPI1-B1-10-14, induces the formation of small rods
(< 1 mm in length and 100 nm in width) (Figure 1A and see
Figure S6 in the Supporting Information). However, this is
not the case for compounds with shorter terminal chains (n:
8). In this case, TEM results showed that PPI1-B1-10-8 forms
long fibrillar aggregates (several micrometers), which have
a width of around 30 nm. These aggregates coil to form small
The formation of tubules was studied by 1H NMR
spectroscopy and SEM. Starting from a [D8]THF solution,
the signals of the aromatic protons were shifted upfield in the
NMR spectra on increasing the amount of D2O, an observa-
tion that supports the existence of a solvent effect (see
Figure S4 in the Supporting Information). Cast samples for
SEM studies showed that small twisted ribbons are formed at
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Figure 3. TEM image of PPI1-B1-4-8 showing a lamellar arrangement
(insets: selected-area electron diffraction (SAED) and fast Fourier
transform (FFT) images) in the helical ribbons and tubes. Proposed
mechanism for the hierarchical self-assembly of the ionic dendrimers
in the presence of water (A–D).
Figure 2. Transmission electron microscopy (TEM; A–C), field-emis-
sion scanning electron microscopy (FE-SEM; D), and SEM (E) images
of PPI1-B1-4-14 from dialyzed aqueous solutions and formation of an
organogel (test-tube method, inset in (E)).
optical activity, despite the fact that it is formed from achiral
molecules.
low D2O proportions and these undergo fusion to form helical
nanoribbons that progressively close into nanotubes (tubules;
see Figure S5a,b in the Supporting Information). Heating the
solution up to 508C results in the tubular aggregates opening
and breaking to form ribbons (see Figure S5c in the Support-
ing Information). Cooling the mixture to room temperature
causes these ribbons to fuse together to give high-quality large
tubules through a sort of self-healing process (see Figure S5d
in the Supporting Information).
TEM data and SAED reflections from samples of PPI1-
B1-4-8 revealed an inner lamellar nanostructured organiza-
tion in the helical ribbons with a d-spacing of 4.4 nm. Based
on these data, the molecular size of PPI1-B1-4-8 (length of the
bent-core acid: 4.6 nm), and the ability of the bent core to
form tilted structures (as in the SmCP mesophase),[1f,2c] we
propose the packing model shown in Figure 3. Folded
dendrimers, which are partially exposed to the water, arrange
in layers with the aromatic cores compacted within and the
ionic parts facing upwards towards the water to form twisted
ribbons. Progressive lateral growth steps will guide this
arrangement to form helical ribbons, which eventually close
to give tubules.
Despite the complexity of the molecules, the self-assem-
bled structures described here show several similarities with
the topologies of other supramolecular aggregates based on
chiral surfactants or peptides,[7e,14,15] thus suggesting
a common mechanism for the formation of tubules by the
helical coiling of tapelike aggregates. Furthermore, the
stacking of twisted layers to form strands resembles models
proposed for the formation of helical nanofilaments through
a local saddle splay deformation, which generates the B4
helical nanofilament phase[4] reported in bent-core molecules.
This chiral mesophase has great potential[4g–i] and has strong
SAXS and WAXS experiments on dry powder samples of
different aggregates (see Figure S8 in the Supporting Infor-
mation) also supported the molecular lamellar disposition
when assembled in water. Sub-micrometer lamellar-like
structures are suggested in all cases, which evolve from
seeding filaments to different morphologies depending on the
chemical structure.
Besides the possibility of achieving different nanostruc-
tured architectures by selecting the appropriate ionic bent-
core dendrimers, the exclusive possibility of inducing chiral
structures attracted our attention. Why do only some
dendrimers induce supramolecular chirality? And why are
two different chiral architectures built?
From consideration of the chiral aggregates of PPI1-B1-4-
14 and PPI1-B1-4-8, it appears that the small flexible spacer
(m: 4 versus m: 10) connecting the hydrophilic and bent-core
parts of the dendrimer controls the twisting ability of the
lamellar assembly. This causes adjacent ester groups to rotate
at different angles relative to each other, thereby resulting in
the formation of the optimum molecular geometry with
a molecular twist.[3d,6,16] Furthermore, this molecular confor-
mational chirality can successfully be transferred from the
molecular level into supramolecular chirality through self-
assembly, both in the mesophase[3d,16] and in the presence of
a solvent.[6,12] Interestingly, in the case of our aggregates in
water, this transfer only occurs when a short chain is present
to link the hydrophobic and hydrophilic structures.
Concerning the second question, the TEM images clearly
show that helical ribbons and tubules are only formed by
PPI1-B1-4-8. The mechanism and pathway for tubule for-
mation have motivated both experimental and theoretical
studies,[15] and the reason why some ribbons have a cylindrical
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curvature, which leads to tubes, whereas others have a saddle-
like shape, is still a matter of debate. The formation of twisted
versus helical ribbons is proposed to be related to the lateral
assembly width of the tapes, which is typically—but not
always—influenced by the length of the alkyl tails which form
the membranes that control the fluid or crystalline organ-
izations.[15c,d] Thus, the presence of longer chains favors
twisted ribbons.[15a,f] In our case, the terminal chains of the
dendrimer seem to exert this control (ca. 30 nm versus
ca. 300 nm for PPI1-B1-4-14 and PPI1-B1-4-8, respectively)
to afford the two chiral supramolecular structures.
TEM and SEM studies on PPI1-B1-4-14 and PPI1-B1-4-8
showed the formation of both left- and right-handed helical
strands (Figures 1F and 2C,D), although specific optical
activity was not detected by CD measurements. Nanostruc-
tures with precise chirality could offer many advantages,[17]
and attempts to direct the screw-sense of the helical
assemblies of our systems were investigated by the incorpo-
ration of intrinsic chirality. Regrettably, neither PPI1-B1-4-7*,
bearing a terminal tail that was successfully used by Ho, Hsu,
and co-workers,[6b] nor PPI1-B1-4-8* provided evidence of
pure twisted aggregates (Figure 1G, and see the Supporting
Information). The stereogenic centers are probably too far
from the origin of the scrolling collective process to have any
influence, but unsuitable molecular packing for conforma-
tional chirality transfer cannot be ruled out.
Finally, concerning the key role of rigid-core designs to
promote helical supramolecular structures, it can be stated
that PPI1-C1-4-8, a linear isomer of PPI1-B1-4-8, forms long
nontwisted fibers (Figure 1H and see the Supporting Infor-
mation). Thus, the molecular packing is of particular impor-
tance for the expression of the conformational chirality, which
is induced by the presence of a molecular kink in the bent-
core molecules, and to the morphological chirality in the
overall superstructure; but other structural requirements
should also be fixed in the molecule to achieve this goal.
In conclusion, ionic bent-core dendrimers are suitable
molecules both to obtain different bent-core mesophases and
to allow their assembly in water to provide useful materials.
Rods, spheres, nontwisted or twisted fibers, helical ribbons,
and tubules can be obtained by using these achiral molecules.
The compact packing that characterizes bent-core structures
and induces the attractive bent-core mesophases can be
achieved in the presence of water, thereby providing systems
ranging from specific single objects to supramolecular gel
structures. Nevertheless, a subtle balance between molecule/
solvent interactions, which can be controlled through suitable
molecular design, is a crucial parameter to control the
morphology of the aggregation.
of the B4-like phase,[3e,4] in this case also suitable for
processing from solution. Finally, our results raise new
questions and offer new prospects in terms of both theoretical
and experimental approaches for understanding the param-
eters that govern the induction of supramolecular chirality
from achiral molecules.
Received: July 29, 2014
Published online: October 5, 2014
Keywords: chirality transfer · ionic dendrimers · liquid crystals ·
.
self-assembly · supramolecular chemistry
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