Organogels and liquid crystalline properties of amino acid-based dendrons: A systematic study on structure-property relationship

Article abstract of DOI:10.1021/cm201913p

Self-assembly behaviors of a series of amino acids-based dendrons, from the first generation (G1) to the third generation (G3) with various focal moieties or peripheral groups, were systematically studied. The supramolecular structures in organogels, thermotropic and lyotropic liquid crystals (LCs) were measured. The influence of the focal groups, dendritic branches, and generation numbers on the mesophase of organogels or LCs was studied by a combination of experimental techniques including transmission electronic spectrometry (TEM), atomic force microscopy (AFM), infrared (IR) spectra, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). It was found that the gelation ability of the dendrons in organic solvents was highly related to the generation; namely, none of the G1 dendrons could form organogels, G2 dendrons displayed good gelation ability, and G3 dendrons gelled the organic solvents with the lowest critical gelation concentration. Oscillatory shear measurements indicated that the gels behaved as viscoelastic materials with good tolerance to external shear force. In addition, all of G3 dendrons and some G2 dendrons were capable of self-organizing to afford the thermotropic and lyotropic LCs.

Full text of DOI:10.1021/cm201913p

Article
pubs.acs.org/cm
Organogels and Liquid Crystalline Properties of Amino Acid-Based
Dendrons: A Systematic Study on Structure−Property Relationship
Gui-Chao Kuang, Xin-Ru Jia,* Ming-Jun Teng, Er-Qiang Chen,* Wu-Song Li, and Yan Ji
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, China 100871
S
* Supporting Information
ABSTRACT: Self-assembly behaviors of a series of amino acids-
based dendrons, from the first generation (G1) to the third
generation (G3) with various focal moieties or peripheral groups,
were systematically studied. The supramolecular structures in
organogels, thermotropic and lyotropic liquid crystals (LCs)
were measured. The influence of the focal groups, dendritic
branches, and generation numbers on the mesophase of organogels
or LCs was studied by a combination of experimental techniques
including transmission electronic spectrometry (TEM), atomic
force microscopy (AFM), infrared (IR) spectra, wide-angle
X-ray diffraction (WAXD), and small-angle X-ray scattering
(SAXS). It was found that the gelation ability of the dendrons in organic solvents was highly related to the generation; namely, none
of the G1 dendrons could form organogels, G2 dendrons displayed good gelation ability, and G3 dendrons gelled the organic
solvents with the lowest critical gelation concentration. Oscillatory shear measurements indicated that the gels behaved as
viscoelastic materials with good tolerance to external shear force. In addition, all of G3 dendrons and some G2 dendrons were
capable of self-organizing to afford the thermotropic and lyotropic LCs.
KEYWORDS: amino acids, dendrons, self-assembly, organogel, thermotropic and lyotropic liquid crystal
peripheral moieties. A series of amphiphilic twin-dendritic com-
pounds behaving as thixotropic organogel in solution and ther-
motropic LC in bulk have been investigated.14 However, it is
not common for dendritic molecules to behave simultaneously
as organogels and lyotropic and thermotropic LCs, owing to
the subtle relationship between the intrinsic units and super-
molecular structures. Up to now, polypeptide-based dendrons
with both gel and LC properties have been rarely reported.
Although Kato et al. have revealed that a dendritic oligopeptide
containing the pyrene focal group displayed both gel and
LC properties, the hexagonal columnar LC mesophase was
observed by addition of 2,4,7-trinitrofluorenone (TNF), which
can increase intermolecular force by donor−acceptor inter-
action.15 It is anticipated that the elucidation of the principles of
the self-assembly process of gel and LC will allow the develop-
ment of a variety of biologically inspired systems.16
We initiated a research program to address the versatile self-
assembly properties of glycine/L-aspartic acid dendrons in
organic solvents in 2005.17 It is found that the third generation
(G3) dendron displays not only effective organogel but also
lyotropic LC in benzyl alcohol. On the basis of our under-
standing of hydrogen bonding and π−π stacking working in concert
to regulate the supramolecular assembly and solvent-dendron
INTRODUCTION
■
Considerable effort has been made to advance our knowledge
of the soft materials of polypeptide-based dendritic molecules,
such as organogels and liquid crystals (LCs), since the inspira-
tion from the pioneering works by Aida,1 Smith,2 and Percec.3
More and more synthetic dendritic compounds acting as organ-
ogelators have been reported, which enriches the soft materials
and enlarges the view boundary of researchers.4 Amino acids-
based dendritic gelators have aroused particularly increasing
interest due to their broad structural diversities and potential
biomedical applications. Dendritic gelators from various amino
acids, including lysine,5 L-glutamate acid,6 L-aspartic acid,7 and
ornithine,8 etc., have been documented. Intriguing findings are
convinced that the focal group,1 chiral center,9 and genera-
tions10 have pronounced effects on the self-assembly process.
Furthermore, the amino acid layer sequence shows huge
influence on the gelation properties.11
On the other hand, investigation of the LC properties of
dendritic compounds is also a current topic for their potential
applications in functional materials.12 As two kinds of LC,
thermotropic LCs (occurring by heating) and lyotropic LCs
(induced by isotropic solvents) show strict differentiation
depending on the molecular shape and structures. Most
reported materials exhibiting both thermotropic and lyotropic
LCs are amphiphilic compounds.13 As to dendritic molecules,
LC properties can be endowed with dedicated structural designs
of molecules at the focal point, in dendritic branches, and on
Received: July 5, 2011
Revised: November 18, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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interaction required for lyotropic LC property, we have enabled
significant advances in designing polypeptide dendritic mole-
cules to show organogel, lyotropic, and even thermotropic LC
properties.18 In implementing our previous studies, photo-
responsibility of dendrons focally modified with azo and cinnamate
groups also have been investigated.19
Herein, we describe systematically the self-assembly proper-
ties of dendrons with different amino acid combinations as
building blocks, including glycine and L-aspartic acid (gly−asp),
glycine and L-glutamic acids (gly−glu), and D-alanine and
L-aspartic acid (ala−asp). Both their gel and LC properties were
systematically investigated and compared in detail. Some
fundamental molecular parameters, for design principles, to
regulate the supramolecular structures also have been revealed
by changing the structure of the focal moieties, dendritic branches,
generation numbers, and peripheral groups.
group of the lower-generation dendron with trifluoroacetic acid
(TFA) and then coupling the resultant N-deprotected inter-
mediate to the C-deprotected G1 (Boc-G1-COOH), which
was prepared by hydrogenative debenzylation of the dipeptide.
The synthetic procedures of dendrons referred to this article
have been reported previously,17,18a,b,d with the exception of
Acrylic-G2ala−asp and Boc-G3ala−asp, which are described in the
Supporting Information.
Organogelation Behavior. To avoid evaporation of the
liquid components, gels were prepared in a sealed vial (i.d. ≈ 1 cm).
Scheme 2. Structures of G1, G2, and G3 Dendrons
RESULTS AND DISCUSSION
■
Molecular Design and Synthesis. In our studies, different
amino acids-based dendrons with generations 1−3 that consisted
of glycine/L-aspartic acid (gly−asp), glycine/L-glutamic acids
(gly−glu), and D-alanine/L-aspartic acid (ala−asp) were con-
vergently synthesized (Scheme 1) and further modified, either on
Scheme 1. Convergent Synthesis of the Different
Generations of Dendrons Boc-Gn (n = 1, 2, and 3)
a
a
Reagents and conditions: (a) p-toluenesulfonic acid, benzene, benzyl
alcohol, refluxing; (b) CH3OH, KOH; (c) Boc-D-alanine, DCC, −10 °C;
(d) TFA, CH2Cl2; (e) Pd−C, H2, ethanol; and (f) DCC, Boc-G1ala−asp
-
COOH, N-methylmorpholine, NHS, −10 °C. Boc = tert-butoxycarbonyl,
DCC = dicyclohexylcarbodiimide, TFA = trifluoroacetic acid, NHS =
N-hydroxy-succinimide. ala−asp: D-alanine and L-aspartic acid.
the periphery or at the focal point. Scheme 2 summarizes the total
of 23 dendritic molecules we synthesized. The detailed syn-
thetic procedures for partial dendrons have been previously
reported (Supporting Information).17,18a,b,d As a representative,
Scheme 1 depicts the general procedures involved in the syn-
thesis of D-alanine and L-aspartic acids-based dendrons. The
standard DCC coupling of Boc-D-alanine and benzyl protected
L-aspartic acid generated the first generation (G1) of the dendron.
The second (G2) and third generations (G3) were synthesized
by repeating two-reaction cycle, that is, by removing the Boc
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dendritic molecules and lead higher degree organization.22 Third,
Boc-G2ala−asp dendrons exhibit excellent gelation ability in
methanol and 1,2-dichloroethane (DCE), while both Boc-
G2gly−asp and Boc-G2gly−glu show good solubility in these solvents.
We are not sure at this point why Boc-G2ala−asp behaves as the best
gelator; we speculate that the stereogenic center of alanine may be
responsible for better gelation or the additional methyl groups may
provide additional hydrophobic interaction to increase the dendron’s
solubility.23 Finally, G2 dendrons with amine and acrylic as the focal
groups have stronger gelation tendency than other G2 dendrons
because of their smaller steric hindrance. For example, Acrylic-
Scheme 2. continued
G2gly−asp could gel more organic solvents at a very low concentration
18b
(e.g., 4 mg/mL in THF) as compared with Boc-G2gly−asp
.
Sol−Gel Phase Transition Temperature. To further
evaluate the dendritic effect and focal moiety on the thermal
stability of the gel, the gel−sol transition temperatures (Tgel) of
ala−asp
Boc-G2ala−asp, Acrylic-G2ala−asp, and Boc-G3
in 1,2-
Weighed quantities of dendritic gelator and liquid were heated
to obtain a homogeneous solution. Then, the vial was placed
under room temperature (25 °C) for 1 h. If the cooled samples
were not visually phase-separated and did not flow perceptibly
when the vials were inverted, they were considered to be gels.
Ultrasound, as another alternative for gel preparation, has
gained more attention.20 Here, the gelation of Boc-G2ala−asp
in 1,2-dichloroethane (DCE) was observed exclusively under
ultrasound, while only precipitate could be afforded without
sonication. The gelation ability of the dendrons is summarized
in Table 1. Through the systematic studies on the gelation of
the obtained compounds, some structural factors essential for
the fibrous assembly can be summarized. First, the dendrons
with higher generation show lower critical gelation concen-
tration (CGC). All of the G1 dendrons cannot gel any solvent
we used, while G2 dendrons form gel with similar CGC values
regardless of the amino acids branches, and the gels from G3
dendrons have the lowest CGC values, which is probably
attributed to more hydrogen-bonding sites from the amide
groups. Such dendritic effect is consistent with the results in
Smith’s reports.21 Second, all of Boc-G2-COOH dendrons,
without benzyl groups on the periphery, cannot form gel in
common solvents, indicating the indispensability of π−π
stacking between the phenyl moieties. Detailed mechanistic
and theoretical studies by Smith’s group demonstrated that
benzyl groups would enhance the enthalpic contribution between
dichloroethane were compared under different concentrations
and the results are shown in Figure 1. As expected, the Tgel
Figure 1. Plots of Tgel against the concentration of Boc-G2ala−asp
,
Acrylic-G2ala−asp, and Boc-G3ala−asp in 1,2-dichloroethane with the
heating rate 1 °C/min in oil bath.
values increased with the concentration increasing, and then
reached maximum and remained almost constant thereafter.
a
Table 1. Organogelation Abilities of the Dendrons
Boc-
Boc-
Boc-
Acrylic-
G2ala−asp
Acrylic-
H2N-
Boc-
Boc-
Boc-
Boc-G2-
COOH
solvents
CHCl3
G1
G2ala−asp
G2gly−glu
G2gly−asp
G2gly−asp
G2ala−asp
G3ala−asp
G3gly−glu
G3gly−asp
S
S
S
S
S
S
S
S
S
S
S
S
P
S
S
S
G(6)
G(5)
G(9)
G(7)
G(5)
G(6)
G(4)
G(5)
G(7)
PG
G(5)
G(6)
G(7)
G(10)
G(4)
G(8)
G(6)
G(8)
G(5)
PG
G(6)
G(7)
G(13)
G(17)
G(8)
G(4)
G(5)
PG
G(9)
G(11)
G(14)
G(5)
G(6)
G(2)
G(1)
G(2)
G(6)
P
G(22)
G(4)
G(16)
G(13)
G (15)
G(3)
G(2)
G(4)
G(8)
P
G(13)
G(7)
G(5)
G(7)
G(5)
G(6)
G(3)
G(4)
G(12)
P
P
S
S
S
S
P
P
S
P
P
S
S
S
CH3OH
EtOH
G(30)
G(27)
G(15)
S
S
S
G(18)
S
EtOAc
THF
G(50)
G(22)
S
S
benzene
toluene
acetone
DCE
G(7)
G(11)
PG
G(6)
G(6)
G(10)
PG
S
G(10)
S
S
P
S
S
P
G(10)
P
G(3)
PG
cyclohexane
DMSO
DMF
PG
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
H2O
P
P
P
P
P
P
P
P
a
: P, precipitate; PG, partial gel; G, gels, S, soluble (>100 mg mL−1). The values given in parentheses are the minimum concentration (mg mL−1) to
achieve gelation at 25 °C.
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Figure 2. Dynamic moduli, G′ and G″ vs angular frequency on double logarithmic scale for dendrons Boc-G2ala−asp and Boc-G3ala−asp gels at different
concentrations in 1,2-dichloroethane at 25 °C. Stress amplitude 0.1 Pa.
On the other hand, the Tgel values of Boc-G3ala−asp are higher
than those of Boc-G2ala−asp at the same concentration. The
same trend was also observed for the dendrons with the
glycine/L-glutamic acid (gly−glu) and glycine/L-aspartic acids
(gly−asp) as the building blocks. From the architectural point
It is common sense that the gels are highly sensitive to
external force; that is, gels will yield or flow under an applied
stress.24 In this work, the mechanical properties measurements
of Boc-G2ala−asp and Boc-G3ala−asp gels at different concen-
trations were performed (Figure 3). Both G′ and G″ showed a
wide plateau region until the strain amplitude reached one
critical stress value known as the yield value. After exceeding
this value, they started to drop dramatically. This should be due
to the partial collapse of the gels. This observation is consistent
with universally accepted results that a gel is prone to show a
viscous response under long time scales (low frequencies), and
it also agrees well with the studies that the gels show relaxation
processes at shorter time scales (high frequencies). Apparently,
comparison of the yield values of the gels demonstrates that the
G′ and G″ of Boc-G2ala−asp gel are obviously larger than those of
Boc-G3ala−asp at the same concentration. These results are well
consistent with the results of viscoelasticity tests, as mentioned
above.
Driving Force Analysis. The cooperation of hydrogen
bonding and π−π stacking are considered as the major driving
forces responsible for the fibrous aggregates. As demonstrated
in our previous report, the π−π stacking between the phenyl
rings can be proved by pyrene probe, whose emission intensity
ratio I1/I3 is an indicator for the polarity of microenviron-
ment.17,18 For example, it was found that the I1/I3 ratio of
pyrene decreased gradually with increasing the concentration of
Boc-G3gly−asp and Boc-G3gly−glu, which indicates the formation
of a nonpolar microenvironment by virtue of the π−π stacking
of the peripheral phenyl groups.
of view, the intermolecular interactions of the Boc-G3ala−asp
ala−asp
might be stronger than those of the Boc-G2
because of
more amide groups in the dendritic branches, which can lead to
more thermally stable supramolecular organogels. Moreover, it
was found that the Tgel values of Acrylic-G2ala−asp gels were
about 20−60 °C higher than those of Boc-G2ala−asp gels (con-
centration from 0.5 to 3.2 wt %), which might be attributed to
the focal acrylic group that can reduce the steric hindrance and
provide more π−π interaction, as compared to the focal Boc
moiety. This observation indicates that a minor structural
difference in the molecules can affect the property greatly.
Rheological Properties. To assess the dynamic nature
and relaxation modes of Boc-G2ala−asp and Boc-G3ala−asp gels
in 1,2-dichloroethane (DCE), the oscillatory shear measure-
ment was conducted (Figure 2). Data was collected by using a
stress-controlled rheometer with parallel plate-type geometry,
as a function of angular frequency from 70 to 0.1 rad/s, by
using a stress-controlled rheometer with parallel plate-type
geometry. Both the storage modulus (G′) and the loss modulus
(G″) of Boc-G2ala−asp and Boc-G3ala−asp exhibit slight frequency
dependence, with G′ an order of magnitude larger than G″,
implying that the gels are a typical elastic rather than viscous
soft matter. It is worth mentioning that the storage modulus of
Boc-G2ala−asp gel is obviously larger than that of Boc-G3ala−asp
,
indicating that the gel from Boc-G2ala−asp is more rigidity
associated with the elastic gel structure. However, the detailed
reason for the observed modulus increasing of Boc-G2ala−asp
organogel is not clear at present.
To clarify the existence of hydrogen bonding, temperature
1
and concentration-dependent H NMR experiments were per-
formed. Taking Boc-G3gly−glu gel (13 mg/mL in CDCl3) as an
example, the signals at 8.49 and 5.72 ppm due to the protons of
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Figure 3. Storage modulus G′ and loss modulus G″ (1 rad/s) for dendrons Boc-G2ala−asp and Boc-G3ala−asp gels as a function of imposed stress in 1,2-
dichloroethane at 25 °C.
1
Figure 4. H NMR spectra of Boc-G2ala−asp in CDCl3 at 2, 10, 20, 50 mg mL−1 at 25 °C.
amide groups were gradually upfield shifted to 8.10 and 5.66 ppm
with increasing temperatures from 24 to 55 °C.18a Moreover,
the 1H NMR spectra of Boc-G2ala−asp were recorded at different
concentrations (Figure 4). By increasing the concentration
from 2 to 50 mg/mL, the signals at 5.24, 5.26, and 6.41 ppm
ascribed to the amide moieties are gradually downfield shifted
to 5.35, 5.37, and 6.63 ppm, respectively. These results strongly
demonstrate the progressive involvement of these groups in
hydrogen bonding.25
FTIR is another powerful tool for investigating organogels
because it can measure the sample either in the gel state or in
the sol state. To further confirm the hydrogen bonding interac-
tion, the FTIR spectra of Boc-G2 (Boc-G2ala−asp, Boc-G2gly−glu
and Boc-G2gly−asp) and Boc-G3 (Boc-G3ala−asp, Boc-G3gly−glu
,
,
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and Boc-G3gly−asp) both in solution and gel state were recorded
(Figure 5). It is found that the solution of Boc-G2gly−glu displays
Figure 6. AFM images of (a) Boc-G2ala−asp, (b) NH2-G2ala−asp, (c and
d) Boc-G3ala−asp with different magnifications in 1,2-dichloroethane.
series of bead-like fibers were observed, which might be owing
to the chiral center of D-alanine. However, it is difficult to make
certain whether they are helices or not.28 Of interest for the
case of Boc-G3gly−asp and Boc-G3ala−asp gels, the magnified
images further visualize thinner fibril structures in the fiber
bundles, indicating a hierarchical self-assembly process.
Figure 5. IR spectra of second and third generation dendrons in
solution and gelation states.
the signals at 3420, 1671, and 1520 cm−1 assigned to N−H
bending vibrations, amide I, and II bands, respectively,
evidencing no hydrogen bonding in its sol state.26 For Boc-
G3ala−asp dendron in sol state, the FTIR spectrum shows
different information from Boc-G2gly−glu; namely, the predom-
inating amide I and II bands shifted to 1625 and 1532 cm−1 and
a shoulder peak appeared around 3423 cm−1, which indicate the
intramolecular hydrogen bonding interaction and free NHs
when the molecules are in the solution.27 However, the FTIR
spectra of the corresponding gels present different characters;
namely, the signals ascribed to N−H bending vibrations shift to
lower wavenumbers with a 100 cm−1 frequency in 1,2-dichloro-
ethane; the amide I and II bands of Boc-G3ala−asp and Boc-
G3gly−glu gels shift to 1629, 1533 cm−1 and 1635, 1544 cm−1,
with the exception of the amide I band signal of Boc-G3gly−asp
gel, which appears at a higher wavenumber of 1658 cm−1,
indicating the existence of both the intra- and intermolecular
hydrogen bonding interactions.
Morphologies and Architectures of Organogels. To
gain better insight into the molecular organization, the xerogels
were examined by atomic force microscopy (AFM) and trans-
mission electron microscopy (TEM). As shown in Figures 6
and 7, all the xerogels exhibited highly developed and inter-
twined fibrous networks, suggesting a great extent of supra-
molecular aggregates. The fibers showed a large aspect ratio
(length over width), with several micrometers in length and
approximately 30 nm in width. TEM images show that the fiber
density of Boc-G2ala−asp gel is higher than that of Boc-G3ala−asp
gel, which is consistent with our rheological observation. It is
worth mentioning that, in the case of Boc-G2ala−asp xerogel, a
It has been well-developed by Smith et al. that the properties
and morphologies of two-component gels can be easily modu-
lated by changing their molar ratio.29 Inspired by their excellent
work, we expected to observe the deferent morphologies of two
components gels from Boc-G1ala−asp-COOH and NH2-G2ala−asp
.
Figure 8 presents the AFM images of dried gels prepared from
methanol with different molar ratios of Boc-G1ala−asp-COOH
and NH2-G2ala−asp. In the case of the gel of NH2-G2ala−asp and
Boc-G1ala−asp-COOH with the molar ratio of 1:1, one-dimensional
fibers with helix-like structure are observed (Figure 8b). A
similar morphology of Boc-G2ala−asp xerogel has also been
observed in our previous observations. Although we cannot
ascertain that the fibers are really helical or not, we suppose that
the chiral center in the structure of D-alanine might endow the
assemblies with helix conformation. When the molar ratio
changed to 1:5, the morphology of the gel becomes a mess and
no regular fiber can be found. According to the report by Smith
et al.,29 in such a case, the fibers from two components con-
nected to each other in three dimensions, leading the gel phase
to be developed in a cubic way because more carboxyl groups
were involved. Additionally, the average height of the fibers
increased from 20 nm for NH2-G2ala−asp (5 mg/mL) alone to
50 nm (molar ratio 1:1) after adding Boc-G1ala−asp-COOH and
then to several hundred nanometers with the ratio of 1:5, which
is well consistent with the results of two-component lysine
dendritic gels (Figure 8d and e).30
The packing patterns of a series of xerogels made up of the
self-assembled fibers from the G2 and G3 dendrons with dif-
ferent amino acids as components were examined by wide-angle
X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS).
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Figure 7. TEM images of (a) Boc-G2ala−asp, (b) NH2-G2ala−asp, and (c) Boc-G3ala−asp xerogels made in 1,2-dichloroethane.
Figure 8. AFM images of xerogel of NH2-G2ala−asp and Boc-G1ala−asp-COOH mixture in methanol in a ratio of (a) 1:0, (b and d) 1:1, (c and e) 1:5.
It reconfirmed that small changes in the structures of the
gelators result in dramatic changes in the aggregation mode.
As shown in Figure 9a and b, both Boc-G2gly−asp and Boc-
G2gly−glu self-organized into lamellar architecture, with the layer
thickness of 3.20 and 3.55 nm, respectively. On the basis of
CPK molecular calculation, we can assume that each of the
lamella consist of two layers of partially interdigitated two
addition, when the focal point is modified with an acrylic group,
Acrylic-G2ala−asp xerogel displays a columnar hexagonal phase.
Liquid Crystalline Properties. The molecular shape with
a high aspect ratio is a crucial prerequisite to form LC
mesophase. Percec et al. have extended this idea to the
dendrimers/dendrons field.31 Our entry into the field of LCs
was rather serendipitous. Occasionally, we found dendron Boc-
G3gly−asp self-organized into lyotropic nematic LCs in benzyl
alcohol.17 Following this, we launched an extensive study on
the LC properties of natural amino acids-based dendrons,
from which the result was convincing that a small structural
change of the dendrons also had a huge impact on their LC
behaviors.18b We have now undertaken systematic studies on
the LC properties of related dendrons and made an effort to
formulate a structure−property correlation. Herein, we essen-
tially attempt to address the major structural issues by using a
combination of techniques including SAXS, WAXD, polarized
light microscopy (PLM), and differential scanning calorimetry
(DSC).
dendrons. In great contrast to Boc-G2gly−asp and Boc-G2gly−glu
,
when the glycine is replaced by D-alanine in the dendritic
branches, the Boc-G2ala−asp xerogel displays a hexagonal lattice,
with an intercolumnar distance of 5.70 nm. Contrary to the
lamellar architecture of Boc-G3gly−asp and Boc-G3gly−glu xerogels,
the scattering pattern of Boc-G3ala−asp is essentially different.
The SAXS patterns recorded numerous reflections, with the
first two peaks at 2.51 and 1.78 nm in the low angle region
(Figure 9f). These scattering peaks can be assigned to be Miller
indices (10), (11), respectively, of a columnar centered rectan-
gular lattice with a = 4.96 nm and b = 5.52 nm.
The focal group tremendously affected the packing mode of
the gel phase. For example, replacing the focal Boc by an amine
unit, the WAXD pattern of the corresponding NH2-G2ala−asp
xerogels showed two peaks appearing at 3.83 and 1.45 nm in
the low angle region, indicating the structural transition from a
hexagonal columnar arrangement to a rectangular lattice. In
The dendritic effect related with the LC development is
observed. The high generation dendrons facilitated to form
LC mesophase, but none of the G1 dendrons studied here
displayed thermotropic and lyotropic LCs. The G2 dendrons,
such as Boc-G2ala−asp, are capable of forming thermotropic LCs.
After cooling the sample from isotropic fluid state, birefringent
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Figure 9. WAXD patterns of xerogel (a) Boc-G2gly−asp made from benzene,18b (b) Boc-G2gly−glu made from ethyl acetate,18a (c) Boc-G2ala−asp made
from 1,2-dichloroethane by ultrasound, (d) NH2-G2ala−asp made from chloroform, and (e) Acrylic-G2gly−asp made from THF. (f) SAXS of Boc-
G3ala−asp made from chloroform.
Figure 10. PLM images of (a) Boc-G2ala−asp at 150 °C, (b) Acrylic-G2gly−asp at 100 °C,18b and (c) Acrylic-G2ala−asp at 180 °C.
and thread textures were observed under PLM (Figure 10a).
Various temperature SAXS/WAXD measurements of Boc-
G2ala−asp at various temperatures were performed to identify the
mesophase structures. At 110 °C, the pattern shows diffraction
peaks at 4.41, 2.20, and 1.52 nm, with a sharp peak centered at
0.48 nm in the wide angle region (Figure S1 in the Supporting
Information). The q-ratio of 1:2:3 confirmed that the meso-
phase possessed an arrangement of a lamellar structure. In
addition, a shoulder peak at 1.44 nm and the sharp peak at
0.48 nm indicated the existence of some sort of highly ordered
molecular packing. All Boc-G3 dendrons (including Boc-
G3gly−asp, Boc-G3gly−glu, and Boc-G3ala−asp) not only performed
as efficient organogelators but also self-organized into lyotropic
LCs in benzyl alcohol with either spherulites or oily streak
textures (Figure 11d−f). DSC experiments revealed that only
one transition temperature appeared at low concentrations in
the cooling process, implying only one lyotropic LC phase
(Figure S4 in the Supporting Information). The tendency of
taking a lamellar phase for the dendrons Boc-G3ala−asp and Boc-
G3gly−glu on cooling is consistent with the results from amino
acid-based dendrons described previously (Figure 12).17
significantly depended on the building blocks. For example,
neither Boc-G2gly−asp nor Boc-G2gly−glu could self-organize into
mesophase architecture. (2) The nature of the LC behavior of
the dendrons is directly correlated with the focal groups. When
the Boc group was removed, none of NH2-G2 dendrons
(including NH2-G2ala−asp, NH2-G2gly−asp, and NH2-G2gly−glu
)
presented LC properties. However, both Acrylic-G2gly−asp and
Acrylic-G2ala−asp, with an acrylic group at focal point, formed
thermotropic LCs after cooling the samples from isotopic state.
For example, an exothermic peak of LC phase transition was
observed for Acrylic-G2ala−asp at 205 °C in the cooling traces
(Figure S3 in the Supporting Information). We presume that
the acrylic moieties favor π−π stacking and depress the steric
hindrance, which facilitates the formation of LCs.18b The focal
conic fan texture for both Acrylic-G2gly−asp and Acrylic-G2ala−asp
is observed by PLM, as shown in Figure 10b and c. The SAXS
diagram of Acrylic-G2gly−asp (recorded at 90 °C) revealed a
columnar hexagonal phase deduced from the diffractions with a
q-ratio of 1:√3. In contrast, when glycine was replaced by
D-alanine in the structure of Acrylic-G2ala−asp, the lamellar
packing pattern was observed in its LC phase (Figure S1b in
the Supporting Information). (3) The amino acid components
of dendrons are an important parameter in the LC phase
Regarding the LC properties, there are a few points worthy
of note. (1) The thermotropic LC properties of G2 dendrons
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Figure 11. Representative PLM textures of mesophase of (a and b) Acryl-G2gly−asp 30%, (c) Acryl-G2ala−asp 10%, (d) Boc-G3gly−asp 40%, (e) Boc-
G3gly−asp 8%, and (f) Boc-G3ala−asp 18% in benzyl alcohol.
interaction of phenyl groups; the dendrons with D-alanine as
building blocks, Boc-G2ala−asp, showed excellent properties both
in organogel and LCs. Efforts are ongoing to construct novel
peptide dendritic systems for addressing the ultrasound effect
and chiral issue.
EXPERIMENTAL SECTION
■
General. All reagents and common solvents were obtained from
commercial sources and used as received, except for special statements.
1H NMR spectra were recorded on Bruker 400 MHz spectrometers at
room temperature using d6-dimethyl sulfoxide and CDCl3 as solvents
and tetramethylsilane as an internal standard. 13C NMR spectra were
recorded on Bruker 100 MHz spectrometers. Matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass-spectra were
Figure 12. 1D WAXD patterns of Boc-G3ala−asp 18% and Boc-G3gly−glu
30% in benzyl alcohol.
acquired on a BIFLE XIII time-of-flight MALDI mass spectrometer
with α-cyano-4-hydroxycinnamic acid (CCA) as the matrix, which can
give the best resolution. FTIR spectra were obtained using a Bruker
VECTOR22 IR spectrometer. Tapping-mode atomic force microscopy
(AFM) measurement was performed using a SPA-400 multimode AFM
and SPI3800N probe station. For the preparation of AFM samples, the
loose gel was dropped on a freshly cleaved mica surface and air-dried at
room temperature before imaging. TEM was performed using a JEM-
100 CXII microscope, operating at an acceleration voltage of 100 kV.
Powder wide-angle X-ray diffraction (WAXD) patterns of xerogels were
obtained using a Bruker D8 Discover diffractometer with GADDS as a
2D detector. Air scattering was subtracted from the sample patterns.
Diffraction patterns were recorded in transmission mode at room
temperature employing Cu Kα radiation. SAXS measurements were
performed on equipment with a SAXSess camera (Anton-Paar, Graz
Austria), which is connected with an X-ray generator (Philips) operating
at 40 kV and 50 Ma employing Cu Kα radiation (λ = 0.154 nm). The
1D scattering function (log I(q)) was obtained by integrating the 2D
scattering pattern, which was recorded on an imaging-plate detector
(Perkin-Elmer) using SAXSQuant software (Anton-Paar, Graz Austria).
Gel was naturally dried for two days at room temperature before the
WAXD experiment. DSC measurements were carried out on a thermal
analysis Q100 DSC system. Polarized light microscopy (PLM) was used to
observe the LC textures of the samples on a Leitz Laborlux 12 microscope.
The rheology was measured with a ThermoHaake RS300 rheometer.
self-assembly. Although both Acrylic-G2gly−asp and Acrylic-
G2ala−asp dendrons were capable of forming lyotropic LCs in benzyl
alcohol, the former displayed strong birefringence at concentrations
exceeding 30 wt % (in benzyl alcohol) (Figure 11a and b), while
the latter exhibited the lyotropic LC phase at concentrations as low
as 10 wt % (Figure 11c).
CONCLUSION
■
In summary, we have conducted systematic studies on the
structural changes of the dendritic branches, focal moieties,
generation, and peripheral groups for their organogel and LC
properties. The following conclusions can be draw: (1) Both
organogel and LC properties of the amino acid-based dendrons
subject to the dendritic effect. None of G1 dendrons could
form mesophase structure in bulk or solvent; G2 dendrons
showed versatile self-assembly ability depending on the build-
ing blocks in the dendritic branches and the focal group; G3
dendrons gelled organic solvents with the lowest critical
gelation concentration (CGC) and self-organized into lyotropic
LCs in benzyl alcohol. (2) Minor changes in the structures
significantly affect their self-assembly behavior. Only the
dendrons with a focal acrylic group, Acrylic-G2gly−asp and
Acrylic-G2ala−asp, could form both thermotropic and lyotropic
LCs; the dendrons with carboxyl ends, Boc-G2-COOH, lost
their self-assembly ability for being without the π−π stacking
ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization of Acrylic-G2ala−asp and Boc-G3ala−asp, SAXS
results of Boc-G2ala−asp and Acrylic-G2ala−asp, DSC diagrams of
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liquid crystalline. This material is available free of charge via
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: xrjia@pku.edu.cn; eqchen@pku.edu.cn.
■
ACKNOWLEDGMENTS
■
This work is financially supported by the National Natural Science
Foundation of China (20974004) and partially supported by the
National Basic Research Program of China (973 Program, No.
2011CB933300) grant to X.R.J. We acknowledge Dr. Hai-Ming
Fan and Professor De-Hai Liang at Peking University for their
help with rheological studies.
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