From vesicles to solid spheres: Terminal functional group induced morphology modification

Article abstract of DOI:10.1039/b927579g

Two similar naphthalimide based organogelators were synthesized. These two compounds can gelate a variety of organic solvents and form interesting morphologies. Transmission and scanning electron microscopy of the xerogels of the two compounds showed vesicle and solid sphere morphologies, respectively, even though their molecular structures are very close. The mechanism of the self-assembly process was investigated by ¹H NMR, IR, 2D-NOESY spectra, wide-angle X-ray diffraction and rheological experiments. The study reveals that the cooperation and competition of multiple intra/inter-molecular interactions are the main determining factors for these compounds' self-assembly into vesicles and solid spheres. A single functional group able to determine the formation of vesicles in non-typical amphiphilic system is rarely encountered. Therefore, these results provide further insights into morphology control, especially the formation of vesicles in non-typical amphiphilic systems.

Full text of DOI:10.1039/b927579g

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PAPER
www.rsc.org/softmatter | Soft Matter
From vesicles to solid spheres: terminal functional group induced morphology
modification†
*
Qi Wang, Junchen Wu, Zuguang Gong, Ying Zou, Tao Yi and Chunhui Huang
Received 5th January 2010, Accepted 22nd March 2010
First published as an Advance Article on the web 5th May 2010
DOI: 10.1039/b927579g
Two similar naphthalimide based organogelators were synthesized. These two compounds can gelate
a variety of organic solvents and form interesting morphologies. Transmission and scanning electron
microscopy of the xerogels of the two compounds showed vesicle and solid sphere morphologies,
respectively, even though their molecular structures are very close. The mechanism of the self-assembly
1
process was investigated by H NMR, IR, 2D-NOESY spectra, wide-angle X-ray diffraction and
rheological experiments. The study reveals that the cooperation and competition of multiple intra/inter-
molecular interactions are the main determining factors for these compounds’ self-assembly into
vesicles and solid spheres. A single functional group able to determine the formation of vesicles in
non-typical amphiphilic system is rarely encountered. Therefore, these results provide further insights
into morphology control, especially the formation of vesicles in non-typical amphiphilic systems.
Introduction
Vesicles have been extensively studied over the past decades for
their potential applications in drug delivery,1 templates for pro-
cessing well-defined materials and food processing.2 As we
already know, numerous amphiphilic polymers, surfactants and
lipids can form vesicles through self-organization.3 Recently,
vesicles formed from low weight molecules through noncovalent
interactions have also been attracting attention.4 For example,
Shinkai and co-workers reported
a diazacrown-appended
Scheme 1 a) Molecular structures of 1a and 1b; b) proposed antiparallel
b-sheet model for compounds 1a and 1b in the gel.
cholesterol gelator that could assemble into vesicles and then, by
vesicle-vesicle interactions, yield an extended network.4a Li and
co-workers presented the first example of a non-typical amphi-
philic structure containing no hydrophilic segment which could
form vesicles in polar methanol and assemble into fibrillar gels in
apolar solvents.4b Taking into account the dynamic character of
those vesicles, they may show advantages in biomedical appli-
cations and smart materials. Despite a wide range of reported
smart materials capable of self-organizing with high efficiency
and selectivity,5 the design of nonamphiphilic structures which
can be controlled by changing the building blocks that sponta-
neously form vesicles and balls has rarely been reported.
chemical input.6 In this work, we attached pyridine as a terminal
group to compound 1a (Scheme 1a). This terminal pyridine
group may act as a proton acceptor to form hydrogen bonds with
the proton of amide in the linker, thus tuning the structure for
self-assembly. For comparison, compound 1b with a terminal
methylphenyl group replacing pyridine was also synthesized. It is
surprising that the self-assembly of the two compounds showed
quite different vesicle and solid sphere morphologies, respec-
tively, in spite of their molecular structures being very similar.
This may be the first example of a non-typical amphiphilic
naphthalimide derivative whose morphology can be controllably
altered from vesicles to solid spheres by modulation of the
building blocks.
Considering the large amount of complicated noncovalent
intermolecular interactions such as H bonds, p–p interactions,
steric hindrance as well as hydrophobic/hydrophilic interactions
during the assembly of the molecules, it is really a challenge to
achieve controllable self-assembly through modulation of the
molecular structure. Our group previously reported a class of
naphthalimide derivative gels which formed various morphol-
ogies in response to outside stimuli, such as sonication and
Experimental
General methods
All starting materials were obtained from commercial suppliers
and used as received. 1H NMR and 13C NMR spectra were
recorded on a Mercuryplus, at 400 and 100 MHz, respectively.
Proton chemical shifts are reported in parts per million downfield
from tetramethylsilane. 2D spectra were recorded using a Bruker
DRX-500 spectrometer. Standard Bruker pulse sequences were
Department of Chemistry & Laboratory of Advanced Materials, Fudan
University, Shanghai, 200433, P. R. China. E-mail: yitao@fudan.edu.cn;
Fax: +86-21-55664621; Tel: +86-21-55664185
† Electronic supplementary information (ESI) available: Details of
synthesis and spectroscopic characterisation of the compounds. See
DOI: 10.1039/b927579g
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used for the experiments. Matrix assisted laser desorption ioni-
zation time-of-flight (MALDI-TOF) mass spectra were recorded
on an AXIMA-CFRPLVS mass spectrometer (Shimadzu). ESI-
MS data were recorded on a Waters Quattro Micro API LC-MS-
MS spectrometer (Waters, USA). Element analysis was carried
out on a VARIOEL3 apparatus (ELEMENTAR). Melting
points were determined on a hot-plate melting point apparatus
XT4-100A. All solutions were absolute anhydrous.
Gelation test for organic fluids
The gelators and solvents were put in a septum-capped test tube
ꢁ
and heated (>75 C) until the solid was dissolved. The sample
vial was then cooled to 25 ꢁC (room temperature). Qualitatively,
gelation was considered successful if no sample flow was
observed upon inversion of the container at room temperature
(the inverse flow method).
Wide-angle X-ray scattering
Results and discussion
Experiments were performed on a SAXSess high-flux small-angle
X-ray scattering instrument (Anton Paar) equipped with
a Kratky block-collimation system at room temperature. The X-
rays were generated by using a Philips PW3830 sealed-tube X-ray
generator (Cu target, l ¼ 0.1542 nm) with a power of 40 kV and
50 mA. Gels of 1a and 1b were measured in glass capillaries. The
incident X-ray beam was perpendicular to the long axis of the
capillary. The diffraction data were corrected for background
scattering.
Synthesis and characterization
Two naphthalimide derivatives 1a and 1b containing dipeptides
with different functional building blocks R (1a, pyridine; 1b,
methylphenyl) were designed for the purpose of studying the
morphology control of the molecules in the self-organizing
process. Details of synthesis and characterization of 1a and 1b
are described in the electronic supplementary information
(ESI).† The first step of synthesis was the preparation of two
different terminal groups connected by a long alkyl chain
(Scheme 2). Treatment of 11-bromoundecanoic acid in thionyl
chloride generated an acyl chloride compound which was reacted
with 4-amino-pyridine and p-methylaniline to afford two single
peptide compounds 2a and 2b, respectively.
Rheology
Experiments were performed on a MCR 301 Anton Paar
(Austria) rheometer, with a Couette cell and a temperature
control unit. The measurements were carried out on freshly
prepared gels by using a controlled-stress rheometer. Parallel-
plate geometry of 25 mm diameter and 1 mm gap were employed
throughout the dynamic oscillatory tests. The following tests
were performed: increasing amplitude of oscillation up to 100%
apparent strain shear (kept a frequency of 1 rad sꢀ1) and
The protection of L-tyrosine was begun by reacting it with
thionyl chloride in methanol for 3 h. The functional amino group
of compound 3 was further protected by reacting it with Boc2O in
the presence of triethylamine in distilled THF. Compounds 6a
and 6b were synthesized by the reaction of compounds 2a and 2b
with the corresponding phenolic hydroxyl group of compound 4
followed by regenerating the active carboxylic acid group of the
L-tyrosine moiety in lithium hydroxide solution.
frequency sweeps at 20 C (from 100–0.1 rad sꢀ1, 0.1% strain).
ꢁ
Compounds 1a and 1b were synthesized by the reaction of the
naphthalic precursor as previously synthesized by our group with
6a and 6b in the presence of dicyclohexylcarbodiimide in
dichloromethane solution. Structural characterization of all the
Transmission and scanning electron microscopy
compounds was carried out by H, 13C NMR and mass spec-
1
A JEOL JEM-2100 F (UHR) mode field-emission transmission
electron microscope was used for TEM imaging, operating at
200 kV and fitted with an EDX analysis accessory. The samples
were prepared by coating the diluted wet gels onto a copper grid
covered with Formvar film at room temperature and the samples
were then transferred into a freeze drier (EYELA, FDU-1200)
for 24 h. A Shimadzu SUPERSCAN SSX-550-SEM was used for
SEM imaging with an accelerating voltage of 15 kV. The wet gel
samples were firstly diluted and applied to the moulds (40 mm ꢂ
40 mm square shaped, 0.5 mm thick mica plates) in four different
concentrations. Thereafter, the samples in the moulds were
transferred into a freeze drier (EYELA, FDU-1200) for 24 h and
coated with Au.
troscopy.
Fluorescent and IR spectra
The fluorescent spectra were recorded with a LFS920 (Edin-
burgh) spectrophotometer equipped with a 1000 W xenon lamp
as the excitation source. Fourier transform infrared (FTIR)
spectra were measured using an IRPRESTIGE-21 spectrometer
(Shimadzu) with KBr pellets.
Scheme 2 Synthesis route to compounds 1a and 1b.
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Gelation properties of the compounds
The organogels were obtained after the hotꢁsolution of 1a or 1b
had been cooled to room temperature (25 C) and aged for at
least 30 min. As expected, 1a and 1b can specifically gelate certain
organic solvents.7 However, the specific gelation abilities of 1a
and 1b are completely different even though they have similar
structures (Table 1). 1a could form transparent gels in alcohols
(ethanol, isopropanol, n-propanol) and 1,4-dioxane. In contrast,
1b was barely soluble in most alcohols, even methanol, but can
form gels in acetonitrile. The critical gelation concentration
(CGC) of 1b is rather low (5 mg mLꢀ1) compared with 1a
(25 mg mLꢀ1). Upon heating (>80 ꢁC), all gels of 1a and 1b could
easily be converted into the original sol and retransformed to gels
by cooling the sol to room temperature and aging for about one
hour. Comparison of the gelation ability of the two compounds
implies that the two different terminal groups significantly
influence gel formation. By changing the pyridine moiety (1a) to
methylphenyl (1b), the gelation solvents and the CGC are
dramatically changed. Furthermore; in polar solvents, 1a more
easily forms transparent gels while in non-polar solvents the
gelation behavior of 1b is stronger.
Morphology of the gels
To gain better insight into the molecular organization in gel
form, the morphologies of xerogels obtained from 1a and 1b were
investigated by scanning electron microscopy (SEM) (Fig. 1). In
acetonitrile and low molecular weight alcohols, 1a self-assembled
into spherical vesicles of differing diameters from 100–500 nm
(Fig. 1a–c). Vesicles formed in i-propanol were more uniform
(Fig. 1c). In high molecular weight alcohols such as n-propanol
a caky structure was observed (Fig. 1d). In 1-hexanol and S-
(+)-2-octanol, the morphology of the xerogels revealed fibrous
structure (Fig. 1e, f); while in 1,4-dioxane, burl-like superstruc-
ture of 2–3 mm size was observed (Fig. 1g). The sphere
morphology was also observed in the SEM image of the 1b
xerogel from acetonitrile with a diameter of 50–120 nm (Fig. 1h).
Interestingly, compared to 1a, the diameter of the spheres is
much smaller (60 nm on average). However, evaporated from
Fig. 1 SEM images of the xerogels at room temperature (25 ꢁC). 1a in a)
acetonitrile, b) ethanol, c) isopropanol, d) n-propanol, e) 1-hexanol, f) S-
(+)-2-octanol, g) 1,4-dioxane (20 mg mLꢀ1) and h) 1b in acetonitrile
(5 mg mLꢀ1). The concentrations of the gels are 25 mg mLꢀ1 except where
otherwise mentioned. Scale bar: a) 1 mm; b) 2 mm; c) 2 mm; d) 10 mm; e)
5 mm; f) 2 mm; g) 10 mm; h) 1 mm.
solvents that do not form gels, the powder of 1a from ethyl
acetate and acetone shows a burl-like structure of 5 to 10 mm size,
whereas 1b shows needle structures in ethanol and acetone
(ESI†).
Table 1 Gelation ability of 1a and 1ba
TEM images also reveal important information about the
superstructures. The hollowness of 1a vesicles was confirmed by
TEM images (Fig. 2a,b). TEM images clearly show the 1a
xerogel in acetonitrile forming hollow spheres with the diameter
of about 200 nm. The wall thickness of the vesicles was estimated
to be 20–25 nm (Fig. 2b). Similar results were also observed for
1a in ethanol and i-propanol. Different from the hollow vesicle
structure of 1a, TEM images of the 1b xerogel from acetonitrile
indicate the formation of solid balls with a diameter of about 60–
90 nm (Fig. 2c). Each ball is in turn composed of smaller domains
with a diameter of less than 10 nm (Fig. 2d). It is evident that by
changing the pyridine (1a) to the methylphenyl (1b) of the
aliphatic chains the morphology changed from big vesicles to
a certain kind of smaller solid spheres. The different structures
reveal that the self-assembly process of the molecules strongly
depends on the functional moiety of the appended alkyl chains,
as well as on the gelation solvent.
Solvent
1ab
1bb
Methanol
Ethanol
S
G (30)
G (25)
G (25)
S
S
PG
S
S
PG
G (25)
I
I
I
I
I
I
S
S
i-Propanol
n-Propanol
Dichloromenthane
Chloroform
Acetone
P
PG
I
G (5)
I
PG
I
Toluene
Tetrahydrofuran
Acetonitrile
1,4-Dioxane
Xylene
Benzene
a
G: gel; PG: partial gel; P: precipitation; S: solution; I: insoluble. The
critical gelation concentrations of the gelators are given in parentheses
b
(mg mLꢀ1). Heated to dissolve and cooled to room temperature then
aged for 30 min (25 ꢁC).
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Spectral study
1
To gather information at the molecular level, liquid state H
NMR experiments were performed. A 2D-NOESY spectrum of
a d6-DMSO solution of compound 1a at 273 K revealed a unique
conformation of the chiral carbon in the L-tyrosine moiety. NOE
cross-peaks are in line with an anti conformation representing the
two amide functions (connected to the L-tyrosine) in opposite
directions (Fig. S2, ESI†).10 Such a conformation is favorable for
the organization of compound 1a into b-sheet structures via
intermolecular H-bonding (Scheme 1b). Temperature-dependent
1H NMR shows that the signals for Ha and Hb in the pyridine
unit of 1a were shifted upfield by 0.03 and 0.02 ppm, respectively,
from 30 to 70 ꢁC (Fig. 4). The chemical shift of protons in
pyridine with the change of temperature indicates that pyridine
forms an intramolecular H-bond with the amide group. The
increasing temperature weakens the intramolecular H-bond thus
causing a chemical shift of protons in pyridine. In contrast,
similar phenomena didn’t appear in compound 1b. The small
peaks around 7.6 ppm correspond to the amide groups of 1a and
1b both shifted upfield by 0.25 ppm. Concentration-dependent
1H spectra (6.5–39 mM) of 1a show a chemical shift of about
0.03 ppm for the naphthalic group due to p–p stacking; while no
chemical shift in the pyridine moiety was observed (Fig.S3,
ESI†). The 1H NMR spectra indicate that p–p interactions
became stronger as the concentration increased. On the other
hand, temperature can also strongly affect the intra- and inter-
molecular H-bonding during the gelation process.
Fig.
2 TEM images of the xerogels: (a, b) 1a in acetonitrile
(25 mg mLꢀ1); (c, d) 1b in acetonitrile (5 mg mLꢀ1). Scale bar: a) 0.5 mm;
b) 200 nm; c) 0.5 mm; d) 50 nm.
Rheology
The mechanical properties of 1a and 1b gels were studied by
dynamic oscillatory measurements (Fig. 3). The linear visco-
elastic regions (LVR) of gels 1a and 1b were determined by strain
amplitudes ranging from 0.01 to 100% at 6.28 rad sꢀ1. Both the
storage modulus (G0) and loss modulus (G00) remain constant up
to approximately 0.5% strain (G0 > G00). Interestingly, beyond
this level of deformation, 1a shows a weak strain overshoot8 so
that the G0 of 1a decreases, while the G00 increases to form a small
peak and then decreases (Fig. 3a). Dynamic oscillatory data
show that, when a small strain was imposed, the G0 and G00 of the
1a and 1b gels remain constant (linear region). The small peak
appearing at 0.5% strain in the 1a gel may be attributed to the
breakage of the spherical structure into layers, which causes an
increase in the loss modulus (G00).9 In sharp contrast to 1a, such
a kind of overshoot does not appear in the 1b gel. The 1b gel
shows a catastrophic disruption which is indicated by a step
decrease in the values of both moduli and the reversal of the
viscoelastic signal (G00 > G0; Fig. 3a). This result indicates that 1b
forms an intercrossed morphology rather than the hollow
spheres. The implementation of a frequency sweep between 0.1
and 100 rad sꢀ1 shows G0 > G00 which confirms that gels 1a and 1b
have predominantly elastic character (Fig. 3b). Thus, these
results further establish the difference of morphologies and self-
assemblies between 1a and 1b.
Fluorescent spectra show concentration-dependent emission
in both 1a and 1b (Fig. S4, ESI†). Compounds 1a and 1b exhibit
similar emission spectra in solution. In particular, the maximum
emission wavelength of 1a and 1b shifted from 518 to 532 nm and
514 to 524 nm, respectively, when the concentration increased
from 0.1 to 20 mM. This suggests that 1a and 1b have strong p–p
interactions in the more concentrated state as compared with
a more diluted one.
In order to check the contribution of hydrogen bonds in the
amide and carbamate groups to the gelation process, the infrared
Fig. 3 Dynamic oscillatory data for 1a and 1b gel at 20 ꢁC. a) Strain
sweep of gels at a frequency of 6.28 rad sꢀ1. b) Frequency sweep of gels at
a strain of 0.1%. Gels: 1a, 25 mg mLꢀ1 in i-propanol; 1b, 10 mg mLꢀ1 in
acetonitrile.
Fig. 4 1H NMR spectra (500 MHz) of 1a (2 mM) and 1b (2 mM) at 30,
50 and 70 ꢁC in DMSO. The signals marked a and b correspond to the
protons of 1a and the signals marked a0 and b0 correspond to the protons
of 1b which are mentioned in Scheme 1.
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spectra of 1a and 1b were measured in acetonitrile. The N–H
stretching bands were examined. The IR spectra of gel samples of
1a and 1b showed a strong NH stretching band at 3312 cmꢀ1
which indicates that strong intermolecular H-bonding exists in
both 1a and 1b gels (Fig. S5, ESI†).4b
Relation between morphologies and gel mechanical properties
The present results reveal that it is a thermodynamic balance
among the hydrogen bonding, p–p interaction and solvophilic
effect which makes the molecules self-assemble into the different
morphologies. Thus, we can propose that the vesicles of 1a were
composed of multi-layers of self-assembled molecules as indi-
cated by the space-filling model and TEM image.12 These layers
were generated by two-dimensional packing of the dimers
through p–p interactions of the naphthalic moieties. During the
formation of the vesicles, the pyridine functional moiety with
the appended alkyl chains entangled with each other by intra-
molecular hydrogen bonding, which interlocked the dimers and
formed the vesicles (Fig. 6). In contrast, the formation of the
solid balls of 1b may mainly be due to the effect of the terminal
methylphenyl group. During the gelation process, the appended
flexible alkyl chains try to extend to the exterior of the assembly
framework by solvophilic effects instead of being entangled with
each other by H-bonding interactions. This would inhibit 1b
from self-assembling into a bigger network. As a result,
compound 1b can only form small solid balls. The small balls
further aggregate into bigger balls by van der Waals interactions
and p–p stacking. The result proves that the two different
terminal groups are the key determinants for the formation of
the two different morphologies from vesicles to solid balls. It
should also be mentioned that the solvent may also play an
important role in determining the formation of self-assembly
structures.
Wide-angle X-ray scattering
The molecular packing of 1a and 1b in the xerogel was further
investigated by powder X-ray diffraction (Fig. 5). The scattering
patterns of the 1a xerogel in acetonitrile and i-propanol show
a sharp peak in the range of 4.5–4.7 nm. Molecular modeling
suggested that the self-assembly of the 1a dimer had a spacing
distance of approximately 4.5 nm. Considering the contribution
of the intermolecular H-bonding, this result supports the view
that the ordered structure was generated through intermolecular
H-bonding and p–p stacking (Fig. 6). The 1b xerogel of aceto-
nitrile displays a completely different X-ray diffraction profile,
with two peaks corresponding to d spacings of 5.6 and 0.97 nm.
These two peaks correspond to the framework of self-assembly
molecular layers and the average distance between the stacked
foldamers, respectively. Interestingly, the aggregation of b-sheet
monolayers of 1a and 1b in the xerogel phase was also clearly
identified by their XRD patterns. All the xerogel samples feature
a peak at about 0.46 nm, which is a typical value for the inter-
strand distance of b sheets.11
Conclusions
In this work, we have successfully synthesized two naph-
thalimide based organogels which could self-assemble to form
vesicles and spheres. Intermolecular hydrogen bonding, p–p
stacking and van der Waals interactions stabilize the rigid
conformations of compound 1a to form vesicles. When the
pyridine moiety of 1a was changed to methylphenyl (1b), both
the gelation properties and morphology were changed dramat-
ically. The different morphologies formed by 1a and 1b illus-
trate that the self-assembly of the peptides can be easily changed
by modulating the appended functional groups. These results
provide a deeper understanding of the gel formation process of
non-typical amphiphilic compounds. Moreover, it may open the
way to studying the effect of appending moieties on the gel
formation process. The well defined hydrophilic hollow vesicles
could be used in various applications, such as biomimetic
studies, drug and gene delivery systems and responsive mate-
rials. We intend to explore some of these possibilities in our
future research.
Fig. 5 X-Ray diffraction patterns of 1a and 1b xerogels in acetonitrile at
room temperature (1a, solid line; 1b, dash dot line).
Acknowledgements
This work was supported by the National Science Foundation of
China (grant no. 20771027), the National Basic Research
Program of China (grant no. 2009CB930400), Specialized
Research Fund for the Doctoral Program of Higher Education
(grant no. 200802460007), Shanghai Sci. Tech. Comm. (grant no.
08JC1402400), and the Shanghai Academic discipline project
(no. B108).
Fig. 6 Tentative model for the self-assembly of vesicles and balls from
compounds 1a and 1b.
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2684 | Soft Matter, 2010, 6, 2679–2684
This journal is ª The Royal Society of Chemistry 2010