Polymorphism of amino acid-based dendrons: From organogels to microcrystals

Article abstract of DOI:10.1002/asia.201000529

There is a delicate balance for a low-weight molecule to behave as a gelator or crystal. The synthesis of two novel amino acid-based naphthalene-dendrons, Nap-G1 and Nap-G2 is described. Both dendrons display polymorphic properties in organic solvents. Nap-G1 developed a fibrous network with β-sheet architecture in cyclohexane but exhibited a spherulitic network in mixed solvents (chloroform/petroleum ether 1:5, v/v). On the other hand, Nap-G2 acted as an efficient organogelator in chloroform but formed crystalline fibers in relatively high polarity solvents (such as acetone and methanol). Combinations of characterizations have been employed to study the polymorphism. In the balance: Two amino acid-based naphthalene-dendrons display different polymorphic properties in organic solvents. Nap-G1 developed a fibrous network with β-sheet architecture in cyclohexane but exhibited a spherulitic network in mixed solvents while Nap-G2 acts as an efficient organogelator in chloroform but formed crystalline fibers in relatively high polarity solvents (see picture).

Full text of DOI:10.1002/asia.201000529

DOI: 10.1002/asia.201000529
Polymorphism of Amino Acid-Based Dendrons: From Organogels to
Microcrystals
Gui-Chao Kuang,[a] Ming-Jun Teng,[a] Xin-Ru Jia,*[a] Er-Qiang Chen,*[a] and Yen Wei[b]
Abstract: There is a delicate balance
for a low-weight molecule to behave as
a gelator or crystal. The synthesis of
two novel amino acid-based naphtha-
lene-dendrons, Nap-G1 and Nap-G2 is
described. Both dendrons display poly-
morphic properties in organic solvents.
Nap-G1 developed a fibrous network
with b-sheet architecture in cyclohex-
ane but exhibited a spherulitic network
in mixed solvents (chloroform/petrole-
um ether 1:5, v/v). On the other hand,
Nap-G2 acted as an efficient organoge-
lator in chloroform but formed crystal-
line fibers in relatively high polarity
solvents (such as acetone and metha-
nol). Combinations of characterizations
have been employed to study the poly-
morphism.
Keywords: amino acids · b sheets ·
dendrons · polymorphism · self-
assembly
1. Introduction
and prolonging aging time,[9] have been developed to ascer-
tain the mechanism of gelation, however, it is still difficult
to manipulate a compound to act as a gelator or a crystal
clearly. Clarification of the mechanism for molecular pack-
ing in gel phase and bulk crystals of compounds also re-
mains a challenge.[10] A seductive strategy to understand the
polymorphism is to design molecular architectures that pos-
sess large aromatic groups, including anthraquinone,[11] an-
thracene,[9] dehydrobenzoannulenes,[12] naphthalene[13], etc.
Gelation arises from a delicate balance of molecules that
dissolve or self-assemble in selective solvents, depending on
the interactions between gelator–gelator and gelator–sol-
vent.[10] It has been previously seen that the solvent-mediat-
ed intermolecular interactions play a critical role in the for-
mation of gels or crystals. In fact, the solvent diversity may
contribute to the complexity of gelation by three means:
1) efficient gelation would occur by controlling gelator–sol-
vent interactions to a proper hydrophobic and hydrophilic
balance; 2) the formation of a 3D network requested for ge-
lation would be prohibited if crystal growth and branching
are unfavorable in a selected solvent; 3) complicated gel
morphologies would be observed owing to the internal
structures and polarity of solvents.[14] Very recently, studies
investigating the impact of introducing different solvents on
morphology have been reported.[6] For example, Liu and co-
workers described the transition from dipeptide-based gel
phase to flower-like microcrystal by adding ethanol as a co-
solvent to toluene.[6a] Wan and co-workers demonstrated the
sugar-appended gel to crystal transition by changing the
ratios of the mixed solvents of water/1,4-dioxane.[6b]
The past decades have witnessed a surge of research prog-
ress dedicated to dendritic gelators owing to their advantag-
es of tunable architectures and diverse functionalities.[1]
A
number of dendritic gelators were created by using polyam-
ide,[2] amino acids,[3] and poly(benzyl ether)[4] as building
blocks. The precisely controlled structures such as chirality,
size, and shape show direct impact on the macroscopic gela-
tion properties.[1] Although the kinetic properties of gelation
and the mechanism of low-molecular weight organogelators
have been studied extensively, the polymorphism of dendrit-
ic gelators remains an elusive goal. Until now, many helpful
ways, such as subtle change in structure,[5] introducing co-
solvents,[6] changing supersaturation,[7] adding additives,[8]
[a] Dr. G.-C. Kuang, M.-J. Teng, Prof. X.-R. Jia, Prof. Dr. E.-Q. Chen
Beijing National Laboratory for Molecular Sciences
and Key Laboratory of Polymer Chemistry
and Physics of the Ministry of Education
College of Chemistry and Molecular Engineering
Peking University
Beijing, 100871 (China)
Fax : (+86)10-6275-1708
E-mail: xrjia@pku.edu.cn
eqchen@pku.edu.cn
[b] Prof. Dr. Y. Wei
Department of Chemistry
Tsinghua University
Beijing, 100084 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000529.
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Previously, we have reported a series of dendrons based
on amino acids that not only acted as efficient organogela-
tors but also showed liquid-crystalline properties.[15] Motivat-
ed by the important role that underlying solvents play in
dendritic gelators, we prepared two novel dendrons Nap-G1
and Nap-G2 by using aspartic acid (Asp) and alanine (Ala)
as the building block and naphthalene as the focal group.
Though possessing quite similar structures, both display
quite different polymorphic architectures in the solvents
with different polarity. Nap-G1 self-assembled into a fibrous
network with b-sheet architecture in cyclohexane but exhib-
ited spherulitic-network morphology in the mixed solvents
(chloroform/petroleum ether 1:5, v/v). In contrast, Nap-G2
acted as an efficient organogelator in chloroform but
showed crystalline fibers in higher polarity solvents such as
acetone and alcohol. We demonstrate that in both dendrons
the naphthyl group plays a crucial role for additional driving
forces to form polymorphic architecture. Very recent obser-
vations, that introduce the bulk aromatic group at the N ter-
minal contribute to the complexation of self-assembly de-
scribed by Adam et al., are consistent with our postulation
on naphthyl producing polymorphism.[5]
vents were changed to polar solvents, such as chloroform
and ethyl acetate, Nap-G2 acts as an efficient gelator. How-
ever, only the viscous fluids were observed when Nap-G2
was dissolved in the solvents with higher polarity such as
acetone, ethanol, and methanol. Conversely, Nap-G1
showed good solubility in these polar solvents. This obvious
difference in gelation behavior indicates that Nap-G2 is a
more polar compound owing to more amide fragments in-
cluded in its branches. Meanwhile, the gel transition temper-
ature (Tgel) was determined when the stable gel, inverted in
a thermostated oil bath, changed to a fluid-like solution
(i.e., fell under the gravity influence). The results demon-
strate that solvents have a direct impact on the gel stability
(Table 1).
Nap-G1 can form an opaque gel with cyclohexane at a
concentration above 0.4 wt% (Figure 1a). Transmission
electron microscopy (TEM)[17] images reveal that most of
2. Gelation Properties
Nap-G1 and Nap-G2 gels were prepared by a sonication
method. In brief, a certain amount of the compound was dis-
solved in an organic solvent, and then the solution was sub-
jected to the ultrasound for a sufficient time. Whether the
gel could be obtained after the sonication was dependent on
the amount of gelator and the solvents used.[16] The behavior
of the dendrons in various solvents is listed in Table 1. Both
Nap-G1 and Nap-G2 display good solubility in dimethyl
sulfoxide (DMSO) and N,N-dimethyl formamide (DMF).
Nap-G1 demonstrates its gelation properties in nonpolar
solvents such as cyclohexane, decane, and petroleum ether,
whereas Nap-G2 precipitates in these liquids. When the sol-
Figure 1. a) Picture of a gel of Nap-G1/cyclohexane; b) TEM image of
xerogel Nap-G1/cyclohexane on a copper grid covered with Formvar
film; c) and d) TEM image of Nap-G1 xerogel obtained from the mixed
solvent of chloroform/petroleum ether (1:5, v/v) with different magnifica-
tions.
Table 1. Gelation behavior of Nap-G1 and Nap-G2 in the common sol-
vents at 258C.
the interconnected straight fibers are hundreds of nanome-
ters long and approximately 40 nm wide (Figure 1b). In
comparison, to form fibrous assemblies in cyclohexane,
Nap-G1 gel displays a spherulitic-network morphology in
the mixed solvent of chloroform/petroleum ether (v/v=1:5)
with the needle-like fibers growing from the typical “perma-
nent junction zones” (Figure 1c, d). Such spherulitic-mor-
phology development may involve the following three steps:
1) the fiber branches initiated from a nucleation center;
2) highly branched fibers radially grew from the central nu-
cleation site; and 3) ultimately the branches impinged on or
entangled each other to shape the 3D networks.[18] The net-
work formation process can probably be considered as a
continuous fibers growth-branching process. The final evolu-
tion diagram is demonstrated in Figure 1. If the fiber growth
overwhelms the branching, a fiber network with a large
Solvents
Nap-G1
CGC[a]
Nap-G2
CGC[a]
[b]
[b]
Tgel
Tgel
Decane
G
G
G
U
988C
658C
838C
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
P
P
P
n.d.
n.d.
n.d.
908C
598C
n.d.
n.d.
n.d.
n.d.
n.d.
Petroleum ether
Cyclohexane
Ethyl acetate
Chloroform
Acetone
Methanol
Ethanol
DMF
S
S
S
S
S
S
S
G
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
VF
VF
VF
S
DMSO
S
[a] G, P, VF, and S denote gelation, precipitation, viscous fluid, and solu-
tion, respectively. The critical gelation concentration (CGC, wt%) is
shown in parentheses and
S represents no gelation formed below
100 mgmLÀ1. [b] The diameter of the vial is 10 mm and the heating rate
is 18CminÀ1 in an oil bath. n.d.=not determined.
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Dendrons of Amino Acids
aspect ratio (the length over the width) will form (Fig-
ure 1b). In the opposite case, a spherulitic-morphology net-
work will develop (Figure 1c, 1d).
3. Polymorphic Structure Analysis
The superstructure of Nap-G1 in cyclohexane was investi-
1
Nap-G2 gel could be prepared immediately in chloroform
under sonication, and the gel became more and more
opaque after aging for several hours (see Figure S4 in the
Supporting Information). The fast gel formation may be at-
tributed to the ultrasound triggering, which probably accel-
erated the fibrous aggregation of the gelators. The more
opaque gel appearance could be the result of either further
development of the fibers or the transition from gel phase
to crystal after extended aging time. TEM images of Nap-
G2 xerogel reveal the intertwined nanobelts with a large
aspect ratio. The average width of the nanobelts is around
100 nm (Figure 2a and b), much wider than that of the
gated by FTIR, Circular dichroism (CD), H NMR spectros-
copy, 2D nuclear Overhauser effect spectroscopy (NOESY),
and wide-angel X-ray diffraction (WAXD). We found that
the Nap-G1/cyclohexane gel was highly related to the b-
sheet structure. Xu and co-workers[20] and Ulijn and co-
workers[21] have studied some aromatic short-peptide deriva-
tives that are capable of forming hydrogels. They demon-
strate that the b-sheet packing is a key structure for the gel-
phase assemblies. For Nap-G1 in chloroform, the solution
shows the characteristic FTIR absorption bands at 3438
(nN-H), 1660 (amide I, nC O), and 1521 cmÀ1 (amide II, dN-H
;
=
Figure 3a). However, these bands shift to 3227, 1634, and
1535 cmÀ1 in the gel phase of Nap-G1/cyclohexane, thus in-
Figure 2. a) and b) TEM images of Nap-G2/chloroform xerogel on
copper grid covered with Formvar film at different magnifications; c) and
d) TEM images of Nap-G2 crystals from acetone on a copper grid with
different magnifications; e) gel picture of Nap-G2/chloroform.
fibers of dendrons (glycine and aspartic acid as building
blocks) reported in our previous papers.[15] This might be
mainly due to the different focal functional groups. Al-
though the dendrons based on amino acids bear the similar
dendritic portion, the stronger aromatic stacking between
the naphthyl groups may enhance the extent of interdigita-
tion, thereby leading to widening of the nanobelts.[19] Fig-
ure 2c and 2d show the TEM images of microbelts obtained
from the viscous fluid of Nap-G2/acetone. The magnified
image (Figure 2d) depicts the microbelts, which are shown
to be much thinner compared to that shown in Figure 2a.
The different aggregate morphologies of Nap-G2 in chloro-
form and acetone reflect the influence of the polarity of sol-
vents. It is clear that the microbelt aggregates from acetone
showed almost no branch off, which leads to the viscous
fluid but not the gel.
Figure 3. a) FTIR spectra of Nap-G1 in a chloroform solution and in the
gel of cyclohexane; b) CD spectrum of Nap-G1/cyclohexane gel.
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X. Jia, E. Chen et al.
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dicating the presence of an antiparallel b-sheet (instead of
parallel) arrangement in the aggregates (Figure 3a). In the
CD spectrum of Nap-G1/cyclohexane gel (Figure 3b), two
positive Cotton effects near 215 and 285 nm, which can be
attributed to p–p* transition of the phenyl and naphthyl aro-
matics, respectively, are observed. Moreover, a negative
Cotton effect at 230 nm is ascribed to p–p transition of the
phenyl group. These results are consistent with the b-sheet
conformation of the amide bond.[22] The concentration-de-
cient in chloroform. As shown in Figure 4b, the additional
cross-peak generated by the intermolecular interaction be-
À
tween N H and the methine proton H* was confirmed
(Scheme 1). Therefore, the shifted proton should be only as-
cribed to the amide located near the naphthyl group. In this
case, Nap-G1 cannot form an ordered structure (e.g.,
b sheet) in chloroform solution through self-assembly.
The WAXD[23] results further supported the b-sheet archi-
tecture of Nap-G1 in cyclohexane. As shown in Figure 5a,
the xerogel of Nap-G1/cyclohexane can render the diffrac-
tions up to the seventh order, with their scattering vector
ratio of 1:2:3:5:6:7. This indicates that the xerogel possesses
a lamellar structure with the first-order diffraction corre-
sponding to a layer period of 32.6 ꢁ. Furthermore, we can
observe an additional diffraction at 4.3 ꢁ in the wide-angle
region, which can be ascribed to the spacing between pep-
tides within the b-sheet structure.[24] According to calcula-
tions from the CPK model, the size (from the focal point to
the periphery) of Nap-G1 is estimated to be about 18.8 ꢁ
with the assumption that Nap-G1 is in a fully extended
1
pendent H NMR spectroscopic experiments of Nap-G1 in
deuterated chloroform (CDCl3) were performed to further
understand the intermolecular hydrogen-bonding interaction
of Nap-G1 (Figure 4a, b). With the concentration up to
50 mgmLÀ1, Nap-G1/CDCl3 remained as a clear solution.
The proton signal at 6.47 ppm (3 mgmLÀ1) can be assigned
to the amide group of Nap-G1, which gradually shifts down-
field with increasing concentration, and this reflects forma-
tion of the hydrogen bonds. However, the 2D NOESY spec-
trum indicates the development of intermolecular hydrogen
bonds between the two amide groups of Nap-G1 is not suffi-
Figure 4. a) 1H NMR spectra of Nap-G1 in chloroform at different concentrations and recorded at 258C; b) portions of the NOESY spectrum of
50 mgmLÀ1 in chloroform at 258C.
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Dendrons of Amino Acids
this case, we presume that the
molecules are trapped in an
amorphous state, which might
be ascribed to the fast evapora-
tion of chloroform.
The solvents also played a
critical role in the self-assembly
of Nap-G2. The high solution
viscosity of Nap-G2 in acetone
and alcohols shall arise from
the long microbelts that inter-
cross each other. We found that
slow evaporation of the solvent
could lead the microbelts to
settle at the bottom of the cuv-
ette, thus forming a piece of dry
white sheet with the long axis
of microbelts parallel to the
sheet surface. The sheet could
be employed to run the
2D WAXD test in two orthogo-
nal directions (see the insets of
Figure 7a and b, wherein the
x direction is parallel to the
sheet surface and the z direc-
tion is the normal surface). Fig-
Scheme 1. Structures of Nap-G1 and Nap-G2. a) SOCl2, reflux, 2 h; b) CF3COO-G1, triethylamine, CHCl3, RT,
24 h; c) CF3COO-G2, triethylamine, CHCl3, RT, 24 h. For synthetic details, see the Supporting Information.
state. Hence, each lamella might consist of two layers of
Nap-G1, in which the naphthyl groups are partially interdi-
gitated with the benzyl groups. This packing fashion may
provide an additional driving force for the self-assembly of
Nap-G1.[20] Figure 5b depicts a schematic model of the mo-
lecular packing. As the lamellar period is smaller than twice
the extended molecular length of Nap-G1, interdigitated bi-
layers are easily formed through intermolecular hydrogen
bonding and p–p stacking interactions.
As shown in Figure 1, Nap-G1 xerogel obtained from the
mixed solvent of chloroform/petroleum ether (1:5, v/v) pres-
ents the spherulitic network, which is drastically different
from that of Nap-G1/cyclohexane. This implies that the mo-
lecular packing would also be different. The diffraction pat-
tern of Nap-G1/mixed-solvent xerogel exhibits a set of well-
resolved diffraction peaks (Figure 6), thereby indicating a
crystalline structure with an order higher than the lamellar
packing of Nap-G1 in cyclohexane. The first-order diffrac-
tion centered at 2q=3.48 corresponds to a d spacing of
2.6 nm, which is smaller than the layer period of the interdi-
gitated b-sheet structure but is still much larger than the
size of the Nap-G1 with an extended conformation. As the
WAXD powder pattern lacks dimensionality, the crystalline
structure cannot be solved at this moment. It is also intrigu-
ing to observe that the solvent with relatively high polarity
can completely inhibit the ordered packing of Nap-G1. In
Figure 6, the WAXD pattern from a powder sample ob-
tained after drying the Nap-G1/CHCl3 solution only shows
an amorphous halo in the low-angle region, the center (at
around 2q=5.88) of which corresponds to a dimension of
1.5 nm, which is close to the molecular size of Nap-G1. In
ure 7a and b depict the 2D WAXD patterns of a sheet
sample obtained from Nap-G2/acetone with the X-ray inci-
dent beam perpendicular and parallel to the z direction, re-
spectively. The ring pattern of Figure 7b indicates that the
ordered domains within the sheet are rotationally disor-
dered around the z direction. On the other hand, Figure 7a
reveals the orientation of the sample. On the basis of the
diffraction geometry, we assume that the diffraction arcs
along the z direction can be attributed to (hk0), whereas
those along the x direction are (00L). The detailed indices
of the integral-intensity profile of the 2D WAXD patterns
are shown in Figure 7c (see also Table S1 in the Supporting
Information). This indexing corresponds to an orthorhombic
cell with a=3.05 nm, b=2.66 nm, and c=0.83 nm. In this
case, it is very possible that the Nap-G2 molecules form col-
umns that pack parallel to form a 2D centered rectangular
structure. Previously,[15c] we have shown that a dendritic ge-
lator with an azo group at the focal point formed a similar
type of architecture, which has also been described by Jang
and Aida[25] and Percec[26] in their study on dendritic mole-
cules. The density of the Nap-G2 sample was measured to
be 1.30 gcmÀ3. With the value of the c axis of 0.83 nm, a cal-
culation based on the method by Percec et al.,[26] we deter-
mined that the number of the molecules located in each
column in the rectangular unit cell is 2.4. Figure 7d shows
the WAXD powder pattern of the Nap-G2/chloroform xero-
gel. It can be seen that some strong diffractions of the xero-
gel peak at similar positions appear in Figure 7c; however,
some diffractions (particularly those in the high-angle
region) are much broader, indicating that the packing of the
molecules are less perfect. Moreover, we can observe two
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Figure 5. a) WAXD pattern of Nap-G1 xerogel made from cyclohexane; b) the proposed bimolecular-layer model for gelation.
new diffractions at 2q lower than 5 degrees. This infers that
the molecular packing in xerogel is different from the ortho-
rhombic structure of Nap-G2 in viscous fluid, or the xerogel
of Nap-G2/chloroform may contain a mixed structure.
4. Conclusions
We have investigated polymorphic properties of two naph-
thyl-dendrons Nap-G1 and Nap-G2 in selective solvents.
Our results demonstrate that Nap-G1 formed a gel in cyclo-
hexane with b-sheet architecture but exhibited spherulitic-
network morphology in the mixed solvents such as chloro-
form/petroleum ether (1:5, v/v). Nap-G2 in chloroform self-
organized into intertwined nanobelts, acting as an efficient
organogelator. On the other hand, in high-polarity solvents
of acetone and alcohols, it formed crystal microbelts with an
orthorhombic structure. To the best of our knowledge, this
Figure 6. WAXD patterns of Nap-G1 xerogel from the mixed solvent of
chloroform/petroleum ether (1:5, v/v) and amorphous powder obtained
from drying the chloroform solution.
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Dendrons of Amino Acids
on Brucker 100 MHz spectrometers.
Matrix-assisted laser desorption/ioni-
zation time-of-flight (MALDI-TOF)
mass-spectra were acquired on
a
BIFLE XIII time-of-flight MALDI
mass spectrometer with a-cyano-4-hy-
droxycinnamic acid (CCA) as the
matrix, which gives the best resolution.
FTIR spectra were obtained by using
a Bruker VECTOR22 IR spectrome-
ter. CD spectra of the gels and solu-
tions were recorded on a Jasco J-810
spectropolarimeter equipped with
a
Julabo F25 thermostatic apparatus.
The samples were dropped into or pre-
pared in a quartz cuvette with a path
length of approximately 1 mm. The
fluorescence emission measurements
were carried out at room temperature
in chloroform by using a Hitachi F-
4500 fluorescence spectrophotometer.
Transmission electron microscopy
(TEM) experiments were performed
by using a JEM-100 CXII microscope
with an acceleration voltage of 100 kV.
Wide-angle X-ray diffraction (WAXD)
patterns of xerogels were obtained by
using a Bruker D8 Discover diffrac-
tometer with GADDS as a 2D detec-
tor. The diffraction patterns were re-
corded in transmission mode at room
temperature by employing CuKa radia-
tion. The background scattering was
recorded and then subtracted. The gel
samples were naturally dried for two
days at room temperature before
WAXD experiments.
Figure 7. a) and b) 2D WAXD patterns of the sheet sample of Nap-G2 obtained after drying the acetone solu-
tion slowly in air. The insets show the sample orientation with respect to the X-ray incident beam. x and z di-
rections are parallel and perpendicular to the sheet surface, respectively; c) integral-intensity profile of the
WAXD pattern shown in (a). The peaks are indexed with an assumed orthorhombic structure of Nap-G2 crys-
tal; d) WAXD powder pattern of Nap-G2/chloroform xerogel.
is the first report referring to the polymorphism of the self-
assembly of dendrons in selective solvents. These results
might provide a broad perspective for the design of novel
self-assembly architectures.
Acknowledgements
This work is supported by the National Natural Science Foundation of
China (NSFC No. 20774003 and 20974004 to X. R. Jia and No. 20774006
to E. Q. Chen).
Experimental Section
5. Gel Preparation
[1] D. K. Smith, Adv. Mater. 2006, 18, 2773.
[2] C. Kim, K. T. Kim, Y. Chang, J. Am. Chem. Soc. 2001, 123, 5586.
[3] a) H. F. Chow, J. Zhang, Chem. Eur. J. 2005, 11, 5817; b) H. F. Chow,
J. Zhang, Tetrahedron 2005, 61, 11279.
[4] a) M. Seo, J. H. Kim, J. Kim, N. Park, J. Park, S. Y. Kim, Chem. Eur.
J. 2010, 16, 2427; b) W. D. Jang, D. L. Jiang, T. Aida, J. Am. Chem.
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J. G. Rudick, P. A. Heiney, Chem. Eur. J. 2008, 14, 909; d) M. Yoshi-
da, Z. M. Fresco, S. Ohnishi, J. M. J. Frꢂchet, Macromolecules 2005,
38, 334; e) Y. Feng, Z. T. Liu, J. Liu, Y. M. He, Q. Y. Zheng, Q. H.
Fan, J. Am. Chem. Soc. 2009, 131, 7950.
Samples for gelation studies were prepared by ultrasound (200 W) in
10 mm (i.d.) vials sealed at one end. For example, Nap-G1 (3 mg) and cy-
clohexane (1 mL) were added to the vial sequentially. The sealed vial
was sonicated at room temperature. Gelation was determined by the ab-
sence of flow of the solvent when the tube was inverted. Critical gel con-
centration was determined by weighing up a minimum amount of gelator
needed for the stable gel formation.
6. Materials and Measurements
All reagents and organic solvents were obtained from commercial sour-
ces and were used as received except when specified otherwise. The syn-
thesis and characterization of the dendrons are described in the Support-
ing Information.
1H NMR spectra were recorded on Brucker 400 MHz spectrometers at
room temperature using [D6]DMSO and/or CDCl3 as the solvent and tet-
ramethylsilane as an internal standard. 13C NMR spectra were recorded
[5] D. J. Adams, K. Morris, L. Chen, L. C. Serpell, J. Bacsa, G. M. Day,
Soft Matter 2010, 6, 4144.
[6] a) P. L. Zhu, X. H. Yan, Y. Su, Y. Yang, J. B. Li, Chem. Eur. J. 2010,
16, 3176; b) J. X. Cui, Z. H. Shen, X. H. Wan, Langmuir 2010, 26,
97.
[7] R. Y. Wang, X. Y. Liu, J. Y. Xiong, J. L. Li, J. Phys. Chem. B 2006,
110, 7275.
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FULL PAPERS
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Received: August 2, 2010
Published online: February 24, 2011
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