Self-assembly of discrete homochiral, helical, hydrogen-bonded nanocages: From vesicles to microspheres and tubules capable of gelating solvents

Article abstract of DOI:10.1002/chem.200902750

The chiral tris-monodentate imidazolinyl ligands la-c exhibit a strong tendency to form the discrete, helical [2+3] nanocages 3 ([l₂-2 ₃]) with tartaric acids 2. Circular dichroism (CD) spectra and theoretical calculations reveal that supramolecular handedness of capsulelike architectures is determined only by the chirality of the imidazolinyl ligands rather than tartaric acids. The chirality of imidazolinyl Ii-gands is transferred to the helicity of the complexes through the directed hydrogen bonds between the N3 atom of imidazoline rings and the carboxyl of tartaric acids. These hydrogen-bonded nanocages can spontaneously self-assemble into spherical vesicles, during which the hydrogen bonding that arises from the hydroxyl groups of tartaric acids plays a crucial issue. The vesicles formed by [{(S,S,S)-1a}₂(2^L)₃] (3a) may further evolve into microspheres that gelate organic solvents after being aged at -20°C for 24 h, and can also be unprecedentedly transformed to tubular assemblies capable of rigidifying the solvents when subjected to ultrasound irradiation.

Full text of DOI:10.1002/chem.200902750

DOI: 10.1002/chem.200902750
Self-Assembly of Discrete Homochiral, Helical,
Hydrogen-Bonded Nanocages: From Vesicles to Microspheres and
Tubules Capable of Gelating Solvents
Liwei Yan, Ying Xue, Ge Gao, Jingbo Lan, Fan Yang, Xiaoyu Su, and Jingsong You*[a]
Abstract: The chiral tris-monodentate
imidazolinyl ligands 1a–c exhibit
gands is transferred to the helicity of
the complexes through the directed hy-
drogen bonds between the N3 atom of
imidazoline rings and the carboxyl of
tartaric acids. These hydrogen-bonded
nanocages can spontaneously self-as-
semble into spherical vesicles, during
which the hydrogen bonding that arises
from the hydroxyl groups of tartaric
acids plays a crucial issue. The vesicles
formed by [{(S,S,S)-1a}2(2L)3] (3a) may
further evolve into microspheres that
gelate organic solvents after being aged
at À208C for 24 h, and can also be un-
precedentedly transformed to tubular
assemblies capable of rigidifying the
solvents when subjected to ultrasound
irradiation.
a
strong tendency to form the discrete,
helical [2+3] nanocages 3 ([12·23]) with
tartaric acids 2. Circular dichroism
(CD) spectra and theoretical calcula-
tions reveal that supramolecular hand-
edness of capsulelike architectures is
determined only by the chirality of the
imidazolinyl ligands rather than tartaric
acids. The chirality of imidazolinyl li-
Keywords: gels · nanocages · nano-
tubes · self-assembly · vesicles
Introduction
combinations thereof.[2] Liu et al. demonstrated for the first
time that the commercially available [Pd6L4]12+ (Pd=ethyl-
Three-dimensional cage/capsulelike supramolecular archi-
tectures formed by noncovalent interactions are currently
gaining intensive attention due to their aesthetically appeal-
ing topologies and potential applications in host–guest
chemistry and catalysis.[1] Although numerous reports con-
tinue to elucidate the details of preparation and characteri-
zation of nanometer-scale cages/capsules, considerably less
is known about further aggregation and supramolecular as-
sembly of these discrete entities in solutions. Recent studies
have shown that larger spherical, vesicular, and tubular ag-
gregates can be formed by self-assembled nanocapsules or
nanocages. Atwood et al. described the supermolecular self-
assembly of a near-spheroidal hexameric nanocapsule based
on pyrogallol[4]arene into submicron spheres and tubules or
ACHTUNGERTNeNUNG nediamine palladium(II), L=2,4,6-tris(4-pyridyl)triazine)
cationic metal–organic octahedral nanocages can self-assem-
ble into monodisperse hollow vesicles in solution.[3]
Various biological or chemical assemblies including vesi-
cles, tubules, fibrils, and viral helical coats perform numer-
ous biochemical actions in nature. In particular, vesicles
have achieved prominence due to their potential applica-
tions in biomimetic models, drug or gene carriers, and nano-
structured materials, and so on.[4] Traditionally, vesicles con-
sist of a variety of amphiphilic components.[5] In contrast,
the design of vesicles with non-amphiphilic segments has
been a challenging and fascinating field of research.[3,6]
Multimonodentate nitrogen-containing heterocyclic rings
with arene cores have recently been of increasing interest in
the design and construction of a variety of cagelike topolo-
gies.[1,7] Recently, we disclosed that the chiral tris-monoden-
tate imidazolinyl ligand (S,S,S)-1a could preferentially form
homochiral, helical sandwich-shaped architectures (M)-[M3-
[a] L. Yan, Prof. Dr. Y. Xue, Prof. Dr. G. Gao, Dr. J. Lan, F. Yang,
Dr. X. Su, Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology
of Ministry of Education
AHCTUNGTRENNUNG
{(S,S,S)-1a}2] with a set of d3–d10 transition-metal ions.[8]
College of Chemistry, and State Key Laboratory of Biotherapy
West China Medical School, Sichuan University
29 Wangjiang Road, Chengdu 610064 (P.R. China)
Fax : (+86)28-85412203
Herein we wish to report an unprecedented finding that the
non-amphiphilic, homochiral, helical hydrogen-bonded
[2+3] nanocages 3 ([12·23]), which are formed by self-assem-
bly of the chiral C3-tris(imidazoline) ligands 1 and tartaric
acids 2 in a 2:3 ratio, may spontaneously aggregate into vesi-
cles, and further evolve into microspheres and tubules capa-
E-mail: jsyou@scu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902750.
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FULL PAPER
(S,S,S)-1a is insoluble in [D7]DMF and l-tartaric acid is in-
soluble in CDCl3, the NMR spectroscopic studies were car-
ried out in both of the deuterated solvents accordingly. The
1H NMR spectra of 3a in CDCl3 showed one set of new sig-
nals in a simple, highly symmetrical pattern, and the signals
for ligand (S,S,S)-1a disappeared completely, which clearly
indicated the quantitative formation of a single self-assem-
bled architecture (Figure 2a,b and Figure S22 in the Sup-
Figure 2. 1H NMR spectra for l-tartaric acid with (S,S,S)-1a at 293 K:
a) the ligand (S,S,S)-1a and b) (S,S,S)-1a/l-tartaric acid=2:3 in CDCl3
(6 mm). c) l-tartaric acid, and d) (S,S,S)-1a/l-tartaric acid=2:3 in
[D7]DMF (6 mm).
ble of gelating solvents when subjected to aging and an irra-
diation with ultrasound, respectively. This is a completely
new phenomenon observed for hydrogen-bonded nanocages.
Results and Discussion
porting Information). In [D7]DMF, the chiral proton of 2L
was strongly shifted upfield (Figure 2c–d). The self-assem-
bled complex 3a could be further confirmed by the electro-
spray ionization time-of-flight mass measurement (ESIMS
Synthesis and characterization of discrete hydrogen-bonded
nanocages [12·23] (3): With the chiral C3-tris(imidazoline) li-
gands in hand, the solution behavior of (S,S,S)-1a with l-tar-
taric acid (2L) was first investigated as the most studied ex-
ample. We found that (S,S,S)-1a could exhibit a strong ten-
dency to induce the generation of a discrete cage with 2L
(Figure 1). The self-assembled nanocapsule [{(S,S,S)-
1a}2(2L)3] (3a) was prepared by the reaction of (S,S,S)-1a
with 2L in a 2:3 ratio in methanol. Given that the ligand
1
TOF; Figure S5 in the Supporting Information). The H and
13C NMR spectroscopic studies in combination with ESIMS
(TOF) measurements suggested that the other C3-tris(imida-
zoline) ligands such as (S,S,S)-1b and (R,R,R)-1c could also
form the [2+3] nanocages [{(S,S,S)-1b}2(2L)3] and [{(R,R,R)-
1c}2(2L)3] (3b and 3c, respectively) with l-tartaric acid (Fig-
ures S1, S2, S6, S7, S23, and S24 in the Supporting Informa-
tion). However, when the R1 group of the imidazoline ring
was changed to the long chain, a diminished tendency to
form cagelike architectures was observed with (S,S,S)-1d
(Figure S27 in the Supporting Information). In addition, we
also investigated the other diacids such as m-, o-, and p-
phthalic acids, succinic acid, and oxalic acid, but found that
they only created such [2+3] assemblies with difficulty.
We subsequently investigated the circular dichroism (CD)
spectra of the self-assembled cages [1a2·23] formed by
(S,S,S)-1a or (R,R,R)-1a and three tartaric acids (2) includ-
ing l-tartaric acid (2L), d-tartaric acid (2D), and meso-tarta-
ric acid (2meso). More surprisingly, the signs of the CD signals
were dependent only on the absolute configuration of li-
gands rather than tartaric acids, which was consistent with
our previously reported helical metal complexes.[8] All the
Figure 1. A schematic representation of the self-assembly of nanocapsules
3 ([12·23]).
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J. You et al.
assemblies formed by (S,S,S)-1a showed a negative Cotton
effect similar to that of ligand (S,S,S)-1a (and vice versa;
Figure 3).
Figure 3. CD spectra of the assemblies [1a2·23] derived from (S,S,S)-1a or
(R,R,R)-1a and 2L, 2D, or 2meso in DMF (0.15 mm).
Molecular modeling: Theoretical calculations were per-
formed to better understand the supramolecular organiza-
tion in these structures. All calculations were carried out by
using the Gaussian 03 program, and full geometry optimiza-
tions of the complexes [1a2·23] were performed at the HF/
STO-3G level of theory.[9] The stationary point was con-
firmed through the harmonic frequency analysis in the same
basis set as a minimum with all positive frequencies. The op-
timized geometries of the complexes generated by (S,S,S)-
1a with l-, d-, and meso-tartaric acids (denoted as [{(S,S,S)-
2ACTHNUTRGNEUNG
1a}2(2L)3], [{(S,S,S)-1a}2(2D)3], and [{(S,S,S)-1a} (2meso)3], re-
spectively) are shown in Figure 4a–c (left). From Figure 4a–
c (left), one can see that these complexes have similar left-
handed structures, (M)-[{(S,S,S)-1a}223], which are held to-
gether with twelve directed hydrogen bonds to form the cap-
Figure 4. a) HF/STO-3G-calculated structures of enantiomers of (M)-
[{(S,S,S)-1a}2(2L)3] (left) and (P)-[{(R,R,R)-1a}2(2D)3] (right). Hydrogen-
bond lengths: O···H=2.260, N···H=1.701 ꢁ. b) HF/STO-3G-calculated
structures of enantiomers of (M)-[{(S,S,S)-1a}2(2D)3] (left) and (P)-
[{(R,R,R)-1a}2(2L)3] (right). Hydrogen-bond lengths: O···H=2.040,
N···H=1.671 ꢁ. c) HF/STO-3G-calculated structures of enantiomers of
À
sulelike assembly. The N···H distances in O H···N hydrogen
À
bonds between the OH of the carboxyl groups of tartaric
acids and the nitrogen atoms in the 3-position of the imida-
zoline rings are in the range from 1.671 to 1.721 ꢁ. The
(M)-[{(S,S,S)-1a}2ACHTNUGTRNENUG 2ACHUTNGTRENNUGN
(2meso)3] (left) and (P)-[{(R,R,R)-1a} (2meso)3] (right). Hy-
À
O···H distances in C H···O hydrogen bonds between the
drogen-bond lengths: O···H=2.205, N···H=1.721 ꢁ. All hydrogen atoms
except those at selected 1= interacting sites are omitted for clarity. Hy-
carbonyl groups (C=O) of tartaric acids and the aromatic
hydrogen atoms of the central benzene rings are in the
range from 2.040 to 2.260 ꢁ. The two central benzene rings
of (S,S,S)-1a lie nearly parallel to each other with interplane
distances of 5.78, 5.82, and 5.69 ꢁ for [{(S,S,S)-1a}2(2L)3],
3
drogen bonds viewed from the side are shown as dotted lines (for color
figure, see Figure S28 in the Supporting Information).
[{(S,S,S)-1a}2(2D)3], and [{(S,S,S)-1a}
N
accordance with the CD studies. It should be noted, howev-
er, that Hong et al. reported that the achiral tris-monoden-
tate imidazolinyl ligand 1e formed the helical [2+3] nano-
cages with d- or l-tartaric acid in aqueous solvents, and the
handedness of the cages was induced by the chiral tartaric
acids through charged hydrogen bonds.[10]
The imidazoline acceptors are directed toward the out-of-
plane of the C3-symmetric facial ligand, and the tilting angle
of about 358 between the central benzene and the imidazo-
line ring gives rise to the curvature needed for the formation
of a helical structure. It seems that the chirality of ligand 1a
is transferred to the helicity of the complex through the di-
rected hydrogen bonds between 1a and 2. Therefore, it can
be concluded that the M helicity is induced by (S,S,S)-1a
rather than tartaric acids to afford (M)-[{(S,S,S)-1a}223],
whereas the P helicity is induced by (R,R,R)-1a to afford
(P)-[{(R,R,R)-1a}223] (Figure 4a–c (right)), which is in good
For the M-type helical supramolecular self-assembled cap-
sules (M)-[{(S,S,S)-1a}223], the approximation HF/STO-3G
method yielded the relative electronic energies of [{(S,S,S)-
2ACTHNUTRGNEUNG
1a}2(2D)3] (0.0 kcalmolÀ1), [{(S,S,S)-1a} (2meso)3] (7.3 kcal
molÀ1), and [{(S,S,S)-1a}2(2L)3] (15.4 kcalmolÀ1). For the P-
type helical structures, the relative electronic energies of
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Hydrogen-Bonded Nanocages
FULL PAPER
[{(R,R,R)-1a}2(2L)3], [{(R,R,R)-1a}
(2meso)3], and [{(R,R,R)-
distribution, which was indicative of the formation of well-
equilibrated structures. Controlled experiments with either
2L or ligands 1 in DMF ruled out any artifacts, which re-
vealed that the presence of both components was essential
for the formation of spherical particles.
More interestingly, the hollow vesicular feature of the
self-assembled spheres of 3a in DMF was observed in the
transmission electron microscopy (TEM) images (Figure 6a).
The average diameter of spherical vesicles was in accord-
ance with the AFM and DLS studies. The TEM images
without staining revealed the contrast difference between
the periphery and the inner part of the spheres, which is ex-
pected from the 2D projection of 3D vesicular structures
2A
responding enantiomers of [{(S,S,S)-1a}2(2L)3], [{(S,S,S)-
2A
1a}2(2D)3], and [{(S,S,S)-1a} (2meso)3], respectively.
CTHUNGTRENNUNG
Vesicular aggregates: We further aimed at exploration of
the aggregation behavior of these discrete hydrogen-bonded
capsules 3a–c and 3e ([1e2·(2L)3]) in solution. Atomic force
microscopy (AFM) was first conducted to investigate the
self-assembly behavior. The AFM images clearly revealed
that 3a–c and 3e spontaneously
self-assembled into spherical
particles with uniform shape
and size in DMF (Figure 5a–d).
The average diameters of the
particles of 3a–c and 3e esti-
mated from the fitted histo-
grams of the size distribution
after subtracting the tip-broad-
ening parameters were 29.3,
370, 122, and 190 nm, respec-
tively (Figure S15 in the Sup-
porting Information).[11] Taking
3a as an example, the histo-
gram obtained from the individ-
ual diameters of 300 particles
shows a Lorentzian distribution
(R2 =0.9658) with an average
size of 29.3 nm and full width at
half-maximum
(fwhm)
of
8.8 nm (Figure 5e). The ratios
of the diameter and height of
aggregates formed by 3a–c and
3e were estimated to be aorund
7, 57, 11, and 39, respectively,
which indicated a considerable
flattening of the spheres, pre-
sumably as a result of either the
removal of the solvent after the
transfer of the nanosized as-
semblies from solution to
a
mica surface or the high local
force exerted by the AFM tip
(Figures S11–S14 in the Sup-
porting Information).[5c,6,12] Fur-
ther dynamic light scattering
(DLS) experiments with 3a
confirmed the formation of su-
permolecular aggregates with
an average diameter of 28.5 nm
(Figure 5f), which was in agree-
ment with the AFM study.
Moreover, the spherical assem-
Figure 5. Tapping-mode AFM height images of aggregates formed from a) 3a in DMF (5 mm), b) 3b in DMF
(5 mm), c) 3c in DMF (5 mm), and d) 3e in H2O (5 mm) on a freshly cleaved mica surface after the solvent was
evaporated (scale bar: a) 200 nm, b) 2 mm, c) 1 mm, d) 1 mm), and e) the corresponding histogram (Lorentzian
fit) of 3a. f) The intensity-weighted distribution of the aggregates obtained from the DLS measurement of the
blies displayed a narrow size sample of 3a (5 mm) in DMF at 258C.
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J. You et al.
hydrogen bonding, van der Waals, and p–p stacking interac-
tions.
Figure 6. a) TEM images (unstained) of 3a in DMF (5 mm) on a carbon-
coated copper grid, and b) an enlarged image of the area marked in (a).
c) TEM image of 3a in DMF (5 mm) on a carbon-coated copper grid
stained with phosphotungstic acid aqueous solution (10 gdmÀ3).
To get insight into the noncovalent interactions that ac-
tually influence the aggregation, the two hydroxyl groups of
2L were blocked to form the ether derivative 2-OMeL and
the ketal derivative 2-OPrL. Similar to 2L, the NMR spectro-
scopic and ESIMS (TOF) studies revealed that both of the
two l-tartaric acid derivatives could quantitatively generate
the [2+3] nanocages with (S,S,S)-1a (Figures S3, S4, S8, S9,
S25, and S26 in the Supporting Information). However,
these capsulelike supramolecular architectures were not ca-
pable of evolving into any clear and uniform vesicular as-
semblies (Figure S16 in the Supporting Information). These
observations distinctly demonstrated that hydrogen bonding
from the hydroxyl groups of tartaric acids played a key role
in the formation of vesicular aggregates.
typically observed by TEM. The structure is clear from a
magnified image of a vesicle (Figure 6b). Notably, the
images taken from samples cast from solutions in DMF re-
mained intact even after 45 days.
To further shed light on whether the vesicular aggregates
were composed of the discrete hydrogen-bonded capsule
3a,[2,10] metal-coordinated nanocage [{(S,S,S)-1a}2Cu3Cl6]
was investigated using TEM because its structure has
proven incapable of disassembly in the solvent systems em-
ployed.[8] As depicted in Figure 7, the spherical aggregates
with an average diameter of 20 nm were observed, although
the images were obviously darker, presumably due to the
high content of heavy metal ions in the resultant superstruc-
tures. Therefore, in combination with the ESIMS (TOF) re-
sults, it is reasonable to assume that the vesicular objects il-
lustrated in Figure 6 consist of a number of discrete capsules
3a.
Gelation of nanocages: Subsequently, the size of the aggre-
gates of capsules was observed to be dependent upon the
concentration of the solution. Higher concentrations could
result in larger spherical aggregates. Taking 3a as the repre-
sentative example, TEM images indicated an increase in the
size of the vesicular aggregates from approximately 30 to
200 nm as the concentration of 3a in DMF increased from 5
to 29 mm (Figures 6 and 8a). As illustrated in Figure 8a, the
Figure 7. a) TEM image (unstained) of nanocages [{(S,S,S)-1a}2Cu3Cl6]
(5 mm) in DMF. b) The crystal structure of [{(S,S,S)-1a}2Cu3Cl6]. Atom
color code: C (light gray), N (black), Cu (dark gray), and Cl (gray); H is
omitted.
We further investigated in detail the influence of the ratio
of the two components on the aggregation by 1H NMR spec-
troscopic and microscopy studies. We found that both cages
and vesicles started to become less prevalent as the ratio of
the tris-monodentate imidazolinyl ligand and tartaric acid
changed away from 2:3, which also clearly indicated that the
cage structure is important to the formation of vesicles. Al-
though the specific interactions between these capsules are
not clear at this stage, we rationalized that the spherical
vesicles might be stabilized by numerous subtle noncovalent
interactions such as neighboring cooperative intermolecular
Figure 8. a) TEM image of 3a (29 mm) in DMF on a carbon-coated
copper grid, stained with phosphotungstic acid aqueous solution
(10 gdmÀ3). Inset: TEM image of unstained 3a (scale bar: 100 nm).
b) SEM image of the sample of 3a in DMF (29 mm, aged at À208C for
24 h) on a mica surface after the solvent was evaporated. c) TEM image
(unstained) of 3a (29 mm) in DMF after sonication. d) Photographs of a
solution of 3a at room temperature (left), a transparent gel of 3a after
being aged at À208C for 24 h (right top), and an opaque gel of 3a after
sonication (right bottom) in DMF (29 mm).
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large vesicular aggregates interconnected to form “pearls on
a string.” More interestingly, after the samples of 3a were
subjected to aging in DMF or N,N-dimethylacetamide
(DMA) at À208C for 24 h, transparent gels were seen with
a critical gelation concentration (CGC) of 2 wt% (Fig-
ure 8d). The scanning electron microscopy (SEM) image of
3a (29 mm) revealed that aging of the solution resulted in
the formation of uniform spherical aggregates with a size of
around 10 mm, many of which were observed to be adhered
together (Figure 8b). We speculated that the large particles
should be attributed to the spontaneous fusion of the small-
er ones after aging.[6a,13] Subsequently, these strings of micro-
spheres created a continuous interconnected globular net-
work that rigidified the solvent DMF (Figure 9). This class
To our surprise, transmission electron microscopy (TEM)
clearly revealed a dramatic morphological transformation of
the sample 3a after sonication. As shown in Figure 8c, the
TEM image obtained from the gel in DMF indicated the
presence of well-grown distinct cylindrical assemblies with
diameters of approximately 25 nm, which are responsible for
gelation (Figure 9). The clear contrast between the interior
and periphery of the unstained cylindrical structures is char-
acteristic of the projection images of hollow tubular aggre-
gates. In spite of a few reports on morphological transforma-
tion between the vesicular and tubular structures,[11b,16] it is
notable that there are no examples that describe such an ul-
trasound-induced switching of morphologies of supramolec-
ular aggregates of hydrogen-bonded nanocages. To further
investigate the fundamental building blocks of the macro-
scopic gel-phase network, ESIMS (TOF) spectra of gels ex-
pedited by aging or sonication were recorded accordingly.
Despite the different morphologies, their mass spectra
showed a main peak at m/z 1722.7 that was assigned to the
species [{(S,S,S)-1a}2(2L)3], clearly revealing that the cage-
like supramolecular structures were still present (Figure S10
in the Supporting Information).
Conventionally, supramolecular gels are prepared by heat-
ing a gelator in an appropriate solvent and subsequently
cooling the resulting isotropic supersaturated solution to
room temperature.[17] Since Naota et al. observed ultra-
sound-triggered gelation for the first time,[18] the employ-
ment of sonication as an unexpected but effective stimulus
to metal coordination, hydrogen bonding, p–p stacking, and/
or van der Waals interaction-dependent molecular gels has
received unprecedented attention.[19] We recently also dem-
onstrated a new gelation mechanism based on a coordina-
tion polymer, in which ultrasound changes the morphology
of the material from sheetlike microparticles into nanofib-
ers.[20] To our knowledge, that was the first example that de-
scribed the ultrasound-induced gelation through the forma-
tion of tubular architectures composed of the discrete supra-
molecular nanocages, although several multicomponent ge-
lators containing tartaric acids that show fibrillar, tubular,
and ribbonlike morphologies have been reported.[21]
Figure 9. A schematic representation of the self-assembly of vesicles 3a.
of gelation is unusual, as most previously reported examples
are based on self-aggregation of gelator molecules into long,
entangled fibrous networks.[14] As far as we know, there are
no reports on discrete hydrogen-bonded nanocages that can
develop into organogels through microspheres. In addition,
further investigation demonstrated that the capsulelike as-
semblies formed by 2-OMeL or 2-OPrL that lacked free hy-
droxyl groups were not only incapable of generating vesicu-
lar aggregates described above, but also could not evolve
into gelators.
Alternatively, as the self-assembled spheres of 3a were
submitted to sonication (0.45 WcmÀ2, 40 KHz) in DMF for
a period of 1–3 min, a complete and homogeneous liquid ge-
lation was also observed. This opaque white gel formed
could be stable for several months at room temperature
without visible changes (Figure 8d). The resulting gel was
thermoreversible, as the gel feature could be lost upon heat-
ing to more than 458C. In the case of DMF, 1 g of 3a was
capable of immobilizing 50 g of solvent (CGC=2 wt%).
Similarly, the gelation of 3a was also observed in N,N-di-
methyl acetamide (DMA). However, 3b–e containing only
alkyl-, aryl-, or lacking substituted groups on the imidazo-
line rings did not undergo gelation, thereby indicating that
both of the van der Waals and p–p stacking interactions that
arise from the alkyl and aryl segments are indispensable to
establish the gel.[15]
Conclusion
In conclusion, we have described that chiral tris-monoden-
tate imidazolinyl ligands together with tartaric acids self-as-
sembled into discrete, helical, hydrogen-bonded [2+3] nano-
cages, the handedness of which was determined only by the
chirality of the imidazolinyl ligands. These non-amphiphilic
cagelike supramolecular architectures then spontaneously
aggregated into vesicles, and further evolved into micro-
spheres that could gelate organic solvents after aging at
À208C. These vesicles could also be transformed to tubular
assemblies that resulted in the immobilization of solvents
when subjected to irradiation with ultrasound. Efforts are
now in progress to investigate the full scope of the transfor-
mation and to apply this strategy to other noncovalent sys-
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J. You et al.
(20 mL) was stirred at reflux for 10 h, and volatiles were then removed
under reduced pressure to afford compound 6. CH2Cl2 (60 mL), Et3N
(14.0 mL, 100 mmol), and amine (35.5 mmol) were added to the residue
at 08C. The resulting mixture was allowed to warm to room temperature
and stirred for 24 h. The solution was then washed with NaOH (10%,
50 mL) and the aqueous layer was extracted with CH2Cl2 (3ꢂ60 mL).
The combined organic layers were dried over MgSO4, and the solvent
was removed in vacuo to give a yellow solid, which could be purified by
column chromatography on silica gel.
tems. The work may open the door for the design of a new
generation of vesicles from non-amphiphilic architectures.
Experimental Section
1
General remarks: H NMR spectra were obtained using a Bruker AV-400
(400 MHz), a Varian Inova-400 (400 MHz), or a Varian INOVA-600
(600 MHz) instrument, whereas 13C NMR spectra were recorded using a
Bruker AV-400 (100 MHz), a Varian Inova-400 (100 MHz), or a Bruker
AV-600 (150 MHz) instrument. The 1H and 13C NMR spectroscopic
chemical shifts were measured relative to CDCl3, (CD3)2CO, CD3OD, or
[D7]DMF as the internal references. High-resolution (HR) MS were re-
corded using a Waters-TOF Premier mass spectrometer by positive ESI-
Q-TOF. The optical rotations were determined using a WZZ-2B polarim-
eter or Rudolph Autopol V polarimeter. Elemental analyses were per-
formed using a CARLO ERBA1106 instrument or a Heraeus CHN-O-
RAPID instrument. Melting points were determined and are uncorrect-
ed. DLS experiments were recorded using a HORIBA LB-550 instru-
ment. SEM images were obtained using a JSM-5900LV instrument at
20 kV. TEM studies were carried out using a JEM-100CXII (Figure S16
in the Supporting Information) or a HITACHI H-600 (others) instru-
ment, operating at 100 kV. Tapping mode AFM imaging was performed
under ambient conditions using a SEIKO SPA400 instrument by using
BS-Tap 300Al levers (Budget Sensors, silicone cantilevers). Ultrasound
irradiation was performed using a KQ3200 ultrasound bath (40 kHz,
0.45 WcmÀ2). Unless otherwise noted, all reagents were obtained from
commercial suppliers and used without further purification. Unless other-
wise indicated, all syntheses and manipulations were carried out under a
dry N2 atmosphere. Anhydrous solvents were dried by standard proce-
dures. 1,3,5-Tris(4,5-dihydro-1H-imidazol-2-yl)benzene (1e) and the com-
plex 3e were prepared by literature procedures.[10a,22] The chiral C3-tris-
Compound (S,S,S)-1b: Starting materials were (S)-valinol and isopropyl-
AHCTUNGTREGaNNUN mine. Compound (S,S,S)-1b was obtained in an 88% yield as a pale
yellow semisolid after purification by column chromatography on silica
gel with elution with ethyl acetate/methanol (6:1). [a]2D5 =67.8 (c=0.5 in
MeOH); 1H NMR (400 MHz, CDCl3): d=0.92 (d, J=6.8 Hz, 9H), 0.99
(d, J=6.4 Hz, 18H), 1.10 (d, J=6.8 Hz, 9H), 1.84–1.87 (m, 3H), 3.14 (t,
J=9.2 Hz, 3H), 3.44 (t, J=5.2 Hz, 3H), 3.74–3.77 (m, 3H), 3.87–3.94 (m,
3H), 7.65 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=17.8, 18.6, 19.3,
20.6, 33.2, 45.1, 46.6, 69.3, 128.9, 132.2, 164.0 ppm; ESIMS (TOF): m/z:
534.4 [M]+; elemental analysis calcd (%) for C33H54N6: C 74.11, H 10.18,
N 15.71; found: C 74.07, H 10.28, N 15.58.
Compound (R,R,R)-1c: Starting materials were (R)-2-amino-2-phenyl-
AHCTUNGTRENNUNG
=
À159.0 (c=0.5 in MeOH); 1H NMR (400 MHz, CDCl3): d=3.79 (t, J=
8.4 Hz, 3H), 4.37 (t, J=10.0 Hz,3H), 5.25 (2d, J=8.0 Hz, 8.0 Hz, 3H),
6.70 (d, J=8.0 Hz, 6H), 7.06–7.14 (m, 24H), 7.82 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=61.8, 67.8, 123.3, 124.0, 126.7, 127.3, 128.7, 128.8,
130.8, 131.6, 142.7, 143.7, 161.3 ppm; ESIMS (TOF): m/z: 739.4 [M+H]+;
elemental analysis calcd (%) for C51H42N6: C 82.90, H 5.73, N 11.37;
found: C 82.84, H 5.88, N 11.27.
Compound (S,S,S)-1d: Starting materials were (S)-valinol and dodecyl-
AHCTUNGTREGaNNUN mine. Compound (S,S,S)-1d was obtained in a 50% yield as a pale
yellow oil after purification by column chromatography on silica gel with
elution with ethyl acetate/methanol (10:1). [a]2D5 =47.6 (c=0.5 in
MeOH); 1H NMR (400 MHz, CDCl3): d=0.88 (t, J=7.2 Hz, 9H), 0.93
(d, J=6.8 Hz, 9H), 1.02 (d, J=6.8 Hz, 9H), 1.26–1.34 (m, 54H), 1.47–
1.50 (m, 6H), 1.84–1.89 (m, 3H), 2.80–2.83 (m, 3H), 2.98 (t, J=9.2 Hz,
3H), 3.06–3.13 (m, 3H), 3.56 (t, J=10.0 Hz, 3H), 3.88–3.95 (m, 3H),
7.72 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=14.1, 18.1, 19.0, 22.7,
26.8, 29.1, 29.4, 29.5, 29.59, 29.61, 29.63, 29.67, 31.9, 33.3, 49.4, 53.4, 70.5,
129.3, 132.4, 165.4 ppm; ESI-HRMS (TOF): m/z: calcd for C60H109N6
[M+H]+: 913.8714; found: 913.8710.
ACHTUNGTRENNUNG(imidazoline) ligands (S,S,S)-1a and (R,R,R)-1a were synthesized accord-
ing to our previously reported methods.[8] (2R,3R)-2,3-Dimethoxysuccinic
acid (2-OMeL) and (4R,5R)-2,2-dimethyl-1,3-dioxolane-4,5-dicarboxylic
acid (2-OPrL) were synthesized by following literature procedures.[23]
Preparation of samples for AFM: Samples of 3a–c (5 mm) in DMF and
3e (5 mm) in H2O were cast onto a freshly cleaved mica surface under
ambient conditions, and then dried at 558C for 0.5 h before making AFM
images.
Preparation of samples for TEM: TEM specimens were prepared by
gently placing a carbon-coated copper grid on a surface of the sample.
The TEM grid was removed, dried for 0.5 h at room temperature, and
then subjected to observation.
Preparation of samples for SEM: Samples of 3a in DMF were placed on
a freshly cleaved mica surface, left open to the atmosphere for drying,
shielded with gold, and then examined.
Acknowledgements
Gelation tests: A capped vial was charged with complex 3a (10.0 mg) dis-
persed in DMF (0.5 mL). The resulting mixture was aged at À208C for
24 h. The sample was simply confirmed by the “stable-to-inversion of a
test tube” method. Similarly, the gelation of 3a was observed in N,N-di-
methylacetamide (DMA).
This work was supported by grants from the National Natural Science
Foundation of China (nos. 20702035 and 20602027). We thank the Centre
of Testing and Analysis, Sichuan University for AFM, TEM, SEM, and
NMR spectroscopic measurements.
A capped vial was charged with complex 3a (10.0 mg) dispersed in DMF
(0.5 mL), and the mixture was then introduced into an ultrasonic cleaner
(0.45 WcmÀ2, 40 kHz) and submitted to sonication at 298 K for around
1–3 min. The sample was simply confirmed by the “stable-to-inversion of
a test tube” method. Similarly, the gelation of 3a was observed in DMA.
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Chem. Eur. J. 2010, 16, 2250 – 2257

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Received: October 6, 2009
Published online: January 11, 2010
Chem. Eur. J. 2010, 16, 2250 – 2257
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2257