Charge transfer induces formation of stimuli-responsive, chiral, cohesive vesicles-on-a-string that eventually turn into a hydrogel

Article abstract of DOI:10.1002/asia.201403205

A charge transfer (CT) mediated two-component, multistimuli responsive supergelation involving a L-histidineappended pyrenyl derivative (PyHisOMe) as a donor and an asymmetric bolaamphiphilic naphthalene-diimide (Asym-NDI) derivative as an acceptor in a 2:1 mixture of H₂O/MeOH was investigated. Asym-NDI alone self-assembled into pH-responsive vesicular nanostructures in water. Excellent selectivity in CT gel formation was achieved in terms of choosing amino acid appended pyrenyl donor scaffolds. Circular dichroism and morphological studies suggested formation of chiral, interconnected vesicular assemblies resembling "pearls-on-a-string" from these CT mixed stacks. XRD studies revealed the formation of monolayer lipid membranes from these CT mixed stacks that eventually led to the formation of individual vesicles. Strong cohesive forces among the interconnected vesicles originate from the protrusion of the oxyethylene chains from the surfaces of the chiral vesicles.

Full text of DOI:10.1002/asia.201403205

DOI: 10.1002/asia.201403205
Full Paper
Gels
Charge Transfer Induces Formation of Stimuli-Responsive, Chiral,
Cohesive Vesicles-on-a-String that Eventually Turn into a Hydrogel
Subham Bhattacharjee[a] and Santanu Bhattacharya*[a, b]
Abstract: A charge transfer (CT) mediated two-component,
multistimuli responsive supergelation involving a l-histidine-
appended pyrenyl derivative (PyHisOMe) as a donor and an
asymmetric bolaamphiphilic naphthalene-diimide (Asym-
NDI) derivative as an acceptor in a 2:1 mixture of H2O/MeOH
was investigated. Asym-NDI alone self-assembled into pH-re-
sponsive vesicular nanostructures in water. Excellent selectiv-
ity in CT gel formation was achieved in terms of choosing
amino acid appended pyrenyl donor scaffolds. Circular di-
chroism and morphological studies suggested formation of
chiral, interconnected vesicular assemblies resembling
“pearls-on-a-string” from these CT mixed stacks. XRD studies
revealed the formation of monolayer lipid membranes from
these CT mixed stacks that eventually led to the formation
of individual vesicles. Strong cohesive forces among the in-
terconnected vesicles originate from the protrusion of the
oxyethylene chains from the surfaces of the chiral vesicles.
Introduction
with giant superstructures, by using intriguing spatiotemporal
programming language and elegant architectural plans. In-
spired by nature, various divergent reversible, noncovalent,
weak interactions including mostly H-bonding forces have
been utilized to fabricate a number of artificial assemblies with
preferential handedness.[4] In particular, the gelation-triggered
construction of helical architectures[5] from chiral or even achi-
ral molecular building blocks has drawn immense attention re-
cently.
Charge-transfer (CT) crystals comprising alternately/cofacially
arranged donor and acceptor aromatic molecular scaffolds are
interesting, as they have excellent conducting properties.[1a]
Metallic conductivity of CT crystals comprising tetrathiafulva-
lene and 7,7,8,8-tetracyanoquinodimethane was reported
nearly three decades ago.[1a] Thereafter, several other reports
emphasized the utilization of CT interaction in superconductivi-
ty, semiconductivity, and ferroelectricity.[1b,c] However, research
work to construct CT-mediated two-component assemblies,[2]
especially gels,[3] has been performed only relatively recently.
Naphthalene-diimide (NDI) derivatives, some of the most
promising n-type organic semiconductors, may be used as ac-
ceptors for suitable donor scaffolds to fabricate such CT-in-
duced assemblies.[3e,f] Recently, Ghosh and co-workers reported
CT-mediated assemblies by using NDI moiety as an acceptor
and pyrene as a donor.[2c,d]
On the other hand, most supramolecular gels with a diver-
gent class of molecular scaffolds are formed through the en-
tanglement of fibrillar networks; in contrast, a particularly intri-
guing class of gels, especially hydrogels, is vesicular gels.[6] The
individual vesicles that may be considered as “monomers” un-
dergo fusion or come in close contact with each other through
intervesicular cohesive forces to develop vesicle strings resem-
bling apparent polymerization of the sole vesicles that macro-
scopically eventually act as a viscoelastic gel. Vesicle gels are
also promising in the sense that they have potential applica-
tions in the field of medicinal chemistry, including drug deliv-
ery and tissue engineering.[7]
Supramolecular chirality can be regarded as an expression of
absolute molecular chirality in macromolecular form. Nature
has the amazing ability to construct helical assemblies that
span from nanoscopic to macroscopic dimensions, together
Two-component gels are fundamentally different from
single-component ones, at least in terms of the design strat-
egy. Two-component gels can be formed through metal-ion
coordination,[8] H-bonding or halogen-bonding interactions,[9]
donor–acceptor type interactions,[3] acid–base (salt-type) inter-
actions,[10] and so forth. The design of two-component gels re-
quires a more clever approach than that required for single-
component gels, because the structure, mechanical, optical,
and other properties of two-component gels can be conven-
iently tuned and considerably improved simply by changing
the molar ratio of the components.[10e] In the present work, we
demonstrate the remarkable role of CT, which assists in two-
[a] S. Bhattacharjee, Prof. Dr. S. Bhattacharya
Department of Organic Chemistry
Indian Institute of Science
Bangalore 560012, Karnataka (India)
Fax: (+91)80-23600529
E-mail: sb@orgchem.iisc.ernet.in
[b] Prof. Dr. S. Bhattacharya
Honorary Professor
Jawaharlal Nehru Centre for Advanced Scientific Research
Bangalore 560064, Jakkur (India)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201403205.
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component supramolecular gelation. The first phase of our in-
vestigation focused on the self-assembly properties of a novel
asymmetric bolaamphiphilic NDI derivative (Asym-NDI). Inter-
estingly, Asym-NDI self-assembles in water to generate pH-re-
sponsive vesicular assemblies, and this has been proven
through dye encapsulation along with various microscopic
studies. Then, we investigated CT gelation between a l-histi-
dine-appended pyrenyl derivative (PyHisOMe) as a donor and
Asym-NDI as an acceptor. Excellent selectivity in gel formation
was also achieved in terms of the choice of the amino acid ter-
mini in the pyrenyl donors. Detailed investigations clearly sug-
gest the formation of chiral interconnected vesicles resembling
“pearls-on-a-string” in the nanoscale dimension, whereas they
macroscopically form hydrogels through intertwining of the
vesicle strings. Intermolecular H-bonding interactions among
the hydrazide groups of Asym-NDI play a crucial role in assist-
ing the CT interactions. Moreover, the hydrogel also responds
to a variety of stimuli and undergo reversible sol–gel transition
upon removal of the stimuli. To the best of our knowledge,
this is the first report of CT-induced supramolecular gels with
“vesicles-on-a-string” morphology that show excellent selectivi-
ty in gelation, supramolecular chirality, and multistimuli re-
sponsiveness.
Self-assembly studies of Asym-NDI
Examination of bolaamphiphilic Asym-NDI (50 mm) in MeOH, in
which it molecularly dissolves, by UV/Vis spectroscopy revealed
well-resolved absorption bands at l=355 and 375 nm with vi-
bronic features (Figure 1a). In contrast, in water both absorp-
Figure 1. a) UV/Vis absorption spectra of Asym-NDI in MeOH and H2O,
[Asym-NDI]=50 mm. b) AFM image of Asym-NDI (50 mm) and c) height
image of a single vesicle. d) Energy-minimized structure of Asym-NDI. e) Ab-
sorption spectrum of the encapsulated dye in the vesicular compartment of
Asym-NDI. f,g) Photographs of a suspension of Asym-NDI (1 mgmLÀ1) in
water and precipitates of the vesicular aggregates along with encapsulated
dye.
Results and Discussion
Synthesis
The syntheses of PyHisOMe, PyPheOMe, PyTrpOMe, and PyPhe-
COOH were performed by using standard peptide coupling re-
1
agents, and these compounds were characterized by H NMR,
tion bands were redshifted to l=357 and 381 nm, respective-
ly, with concomitant decreases in the intensities of the bands.
In addition, the UV/Vis absorption spectrum in water displayed
significant scattering in the l=400–550 nm range, which sug-
gests aggregation-induced loss of transparency of the solution
in water even at a concentration of 50 mm (Figure 1 f). Concen-
tration-dependent UV/Vis studies of Asym-NDI were performed
in an attempt to determine the critical aggregation concentra-
tion in water (Figure S1a, Supporting Information). However,
the critical concentration leading to aggregation could not be
determined precisely from the concentration dependence of
the absorbance at l=380 nm.
13C NMR, and FTIR spectroscopy; ESI-MS; and elemental analy-
ses (Scheme 1 and the Supporting Information). The Asym-NDI
derivative was synthesized by heating an equimolar mixture of
1,4,5,8-naphthalenetetracarboxylic dianhydride, isoniazide, and
3,4,5-tris{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}benzohydrazide
in DMF at reflux under an inert atmosphere for 12 h. The crude
product was purified by silica gel column chromatography (3–
4% MeOH/CHCl3; see Scheme 1 and the Supporting Informa-
tion).
Dynamic light scattering (DLS) experiments were performed
to unambiguously confirm the existence of aggregates in solu-
tion ([Asym-NDI]=50 mm), and these experiments revealed an
average hydrodynamic diameter (Dh) of the aggregates of
(76Æ1) nm (Figure S1b). To investigate the nature of the
supramolecular assemblies, atomic force microscopy (AFM)
studies were performed by using a suspension of Asym-NDI
(50 mm). The AFM images in Figure 1b show the presence of
dense spherical disc-like aggregates with an average diameter
of approximately 25–35 nm and a height of approximately
4.5 nm (Figures 1b,c and S2a,b). The smaller size of the aggre-
Scheme 1. a,b) Molecular structures of the acceptor (Asym-NDI) and various
amino acid appended pyrenyl donors used in this investigation.
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gates observed under AFM relative to that observed by DLS
may be attributed to shrinkage of the aggregates during sol-
vent evaporation.[11] The diameter of Asym-NDI along the long
molecular axis, as determined by energy minimization of the
molecule by using B3LYP/6-31G* level computations, was ap-
proximately 3.2 nm (Figure 1d). Thus, the size of a single
Asym-NDI molecule is considerably smaller than the diameter
of the aggregates observed under AFM and DLS; this indicates
the vesicle-like nature of the aggregates.[11] Similar spherical
aggregates resembling vesicles were also observed under
transmission electron microscopy (TEM) investigations of a sus-
pension of Asym-NDI (50 mm; Figure S2c,d). XRD studies of
cast aggregates of Asym-NDI showed the presence of three
well-resolved Bragg reflections at 2q=2.59 (3.4 nm), 5.2
(1.7 nm) and 7.91 (1.12 nm) in the low-angle region with
a spacing ratio of 1:1/2:1/3 (Figure S2c). Thus, the presence of
regularly sharp XRD peaks suggest lamellar arrangement of
Asym-NDI with a periodicity of approximately 3.4 nm, which is
slightly larger than the molecular diameter (3.2 nm); this is in-
dicative of the formation of monolayer lipid membranes
(MLMs).[12] These MLMs, in turn, develop vesicular nanoassem-
blies through rolling-up, as evidenced from their turbid texture
as an aqueous suspension (Figure 1 f). Interestingly, the turbid
suspension transformed into a clear solution at pH 2 owing to
protonation of the pyridyl end group of Asym-NDI, which is in-
dicative of dissolution of the vesicular aggregates (Figure S3a).
Sedimentation experiments were performed to gain further
evidence of vesicle formation. In particular, the encapsulation
ability of the vesicular aqueous solution of Asym-NDI (Fig-
ure 1 f) was checked by using a hydrophilic dye, that is, meth-
ylene blue (MB).[11] A mixture of Asym-NDI (1 mg) and MB
(0.1 mg) in water (1 mL) was sonicated for approximately
15 min, and the resultant suspension was centrifuged, which
allowed precipitation of the encapsulated vesicular aggregates.
The supernatant was carefully removed, and the precipitate
was redispersed in water (1 mL); the same procedure was re-
peated twice to remove the un-encapsulated dye, if any. Inter-
estingly, the precipitate thus obtained retained its significantly
dark-blue color, which suggests encapsulation of the dye
inside the vesicular aggregates (Figure 1g). The precipitate
thus obtained was resuspended in water (1 mL), and the UV/
Vis spectrum of this suspension showed an absorption band at
l=664 nm (Figure 1e). This band is due to the entrapped MB
dye and further confirms its encapsulation with an encapsula-
tion efficiency of 2.2% (Figure S3b); it also confirms the exis-
tence of an inner aqueous compartment within the vesicular
aggregates. Poor encapsulation efficiency may be attributed to
the smaller size of the vesicles and hence the vesicular core.
against the above-mentioned NDI derivative. Interestingly,
Asym-NDI was able to form CT complexes with all of the above
pyrenyl derivatives in MeOH. The CT interaction was initially
evident from naked-eye detection of the deep violet color (re-
sulting from the formation of a CT complex) of the MeOH solu-
tion.
However, aggregation of the CT complex was not observed
in MeOH alone. To introduce solvophobic interactions among
the CT complexes, water was gradually added to the pre-
formed MeOH solution of the CT complex. More specifically,
addition of water to a MeOH solution (in a proportion of 2:1 v/
v H2O/MeOH) of the PyHisOMe/Asym-NDI=1:1 CT complex at
a PyHisOMe concentration of 2.4 mm caused immediate aggre-
gation, which eventually produced a translucent gel after
ageing for approximately 10–12 min (Figure 2). Interestingly,
Figure 2. The gel–sol transitions of the PyHisOMe/Asym-NDI=1:1 CT hydro-
gel triggered by a variety of stimuli (heat/cold, acid/base, Fe+3/EDTA), [PyHi-
sOMe]=2.4 mm in a 2:1 mixture of H2O/MeOH.
the 1:1 CT complex formed a gel at a concentration as low as
0.84 mm, and this demonstrates an encapsulation of more
than 44000 water molecules and almost 10000 methanol mol-
ecules by a single CT unit. Thus, the present hydrogel belongs
to the category of supergelators.[14] Surprisingly, neither the
mixture of PyPheOMe (or PyTrpOMe)/Asym-NDI=1:1 produced
a gel if water was added to the MeOH solution (H2O/MeOH=
1:2) of the mixture; instead, only precipitation was observed.
The gelation propensity of PyPheCOOH with Asym-NDI (1:1
molar ratio) was also checked by slowly adding water to the
MeOH solution of the mixture. However, in this instance gela-
tion was also not observed, although PyPheCOOH has one free
ÀCOOH group that may participate in intermolecular H-bond-
ing. Thus, remarkable selectivity in the formation of the gel in
terms of the choice of the amino acid termini in the pyrenyl
donor was observed. Moreover, the selective gelation of the
PyHisOMe/Asym-NDI=1:1 CT complex in a 2:1 mixture of H2O/
MeOH also suggests that the l-histidine unit has a significant
role in forming intermolecular H-bonds in addition to the inter-
molecular H-bonding mediated by the amide units.
Charge-transfer-interaction triggered hydrogelation
Wilson et al. examined the CT interactions among nine combi-
nations of electron-rich donors and electron-poor acceptors by
using various spectroscopic techniques. These studies revealed
high association constants between pyrene and the NDI deriv-
atives.[13] In the present report, various amino acid appended
pyrenyl derivatives were prepared as possible donor scaffolds
Supramolecular gels often respond to a variety of external
stimuli such as heat, light, metal ions, pH, salts, ultrasound,
and agitation, and they often revert back to the gel state with
concomitant removal of the stimuli.[15] With this in mind, we
also investigated the possible consequences of exposing the
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present 1:1 CT gel to various stimuli. The CT gel showed ther-
moreversibility (Figure 2a). Recently, Liu and co-workers dem-
onstrated the hydrogelation of l-histidine-based bolaamphi-
philes by using the metal-ion coordination ability of the imida-
zole N atom of the l-histidine unit.[12] To examine the impact
of the present gel system in the presence of metal ions, we
added Fe3+ to the preformed gel (PyHisOMe/Fe3+ =1:1). This
caused immediate dissolution of the gel state to produce an
intense greenish-colored sol (Figure 2b). The intense greenish
color of the sol indicates that the CT interactions between the
donor and acceptor pairs remain unaffected by metal-ion coor-
dination of the l-histidine unit of the gelators. Reformation of
the gel state was realized just by adding a small excess
amount of ethylenediaminetetraacetic acid disodium salt
(EDTA; Fe3+/EDTA=1:1.5); EDTA binds preferentially to the
metal ion, which frees the gelators. It is also well known that
the basic nitrogen atom of the l-histidine unit can be proton-
ated easily. The pyridyl N atom of Asym-NDI is also pH sensi-
tive. Thus, the preformed gel was treated with a few drops of
2n HCl solution. As a result, the 1:1 CT gel immediately turned
into a sol (pH 2) but still kept the CT interactions intact, as re-
vealed by retention of its violet color (Figure 2c). However, ge-
lation was realized if the added acid was neutralized by 2n
NaOH. Similarly, the addition of a few drops of 2n NaOH onto
the top of a preformed gel also led to the disruption of the gel
network along with abolition of the CT interactions (Figure 2d).
Given that the hydrazide group of Asym-NDI is significantly
acidic, the addition of a strong
the concentration of the gelators (Figure 3b). The association
constant (K) of the 1:1 CT complex was determined to be ap-
proximately 0.9ꢁ103 mÀ1 (Figure S4).[3e] Temperature-dependent
UV/Vis studies of the 1:1 CT complex (PyHisOMe=1.7 mm)
were also performed. These revealed that the CT band was still
present in the heated sol and that the intensity of the CT band
gradually increased by decreasing the temperature to 258C,
which suggests aggregation-triggered enhancement in the CT
interactions (Figure 3c). UV/Vis studies were also performed
with the 1:1 CT complex at various compositions of H2O/MeOH
to determine the “critical” composition of the solvents necessa-
ry for aggregation (Figure 3d). Interestingly, the 1:1 CT complex
(PyHisOMe=0.25 mm) showed an almost negligible CT band
in the absorption spectrum in MeOH alone. The CT band cen-
tered at l=575 nm gradually appeared with a concomitant in-
crease in the proportion of water and suddenly increased up
to a significant extent in a 4:1 mixture of H2O/MeOH, which
suggests manifestation of the aggregation-induced enhanced
CT interactions (Figure 3d,e). Notably, although the CT band at
[PyHisOMe]=0.25 mm was negligible in the 2:1 mixture of
H2O/MeOH (optimum solvent composition for gel formation),
a significant enhancement in the CT intensity was observed in
the 4:1 mixture of H2O/MeOH, which suggests that the CT in-
teractions were dependent on both the concentration of the
gelator and the solvent composition.
The concentration-dependent fluorescence spectra of the
1:1 CT complex (Figure S5a) displayed emission in the range of
base presumably leads to depro-
tonation of the hydrazide group,
which plays a significant role in
H-bonding-assisted enhanced CT
interactions among the donor–
acceptor pairs. Thus, the depro-
tonation of the hydrazide group
resulted in weakening of the CT
interactions.
Titration of PyHisOMe (1 mm)
in
MeOH
with
increasing
amounts of Asym-NDI was per-
formed under UV/Vis spectrosco-
py (Figure 3a). Notably, the CT
band remained very weak even
at a 1:1 molar ratio of the CT
complex. This suggests that the
CT interactions among the
donor–acceptor pairs are pre-
sumably negligible in MeOH,
which results in failure to form
an effective gel in MeOH alone.
Concentration-dependent
ab-
sorption spectra of the 1:1 CT
complex in a 2:1 mixture of H2O/
MeOH, however, showed the
presence of a strong CT band,
which gradually disappeared
Figure 3. a) Titration of PyHisOMe in MeOH with increasing amounts of Asym-NDI under UV/Vis spectroscopy.
b) Concentration-dependent UV/Vis spectra of the 1:1 CT complex in a 2:1 mixture of H2O/MeOH. c) Temperature-
dependent UV/Vis spectroscopy of the 1:1 CT complex in a 2:1 mixture of H2O/MeOH; [PyHisOMe]=1.7 mm.
d) UV/Vis spectroscopy of the 1:1 CT complex at various compositions of H2O/MeOH and e) photographs of the
with a concomitant decrease in corresponding suspensions under normal light, [PyHisOMe]=0.25 mm.
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l=350 to 450 nm and slowly increased with a concomitant
decrease in the gelator concentration and showed a maximum
emission at a PyHisOMe concentration of 16 mm. The emission
intensity further diminished with a decrease in the concentra-
tion. However, the excimer emission of PyHisOMe, characteris-
tic for an aggregated PyHisOMe species, was not observed for
any of the concentrations, and this indicates formation of
a mixed stack donor–acceptor complex.[3a]
After that, the CD intensity gradually increased to a significant
extent with further aging of the solution, and the process start-
ed to saturate after approximately 30 min, which thus demon-
strated gradual evolution of the left-handed helical (M-helical)
aggregates from the isotropic state.[16] The intense bisignate
negative Cotton signal that evolved over 30 min intersected
the q=08 line with an isodichroic point at l=409 nm. The bi-
signate Cotton signal displayed a maximum negative band at
l=555 nm; this coincides with the absorption position of the
CT band in the UV/Vis spectra and a maximum positive band
at l=355 nm, which happens to be close to the lmax of the
pyrenyl unit of the 1:1 CT complex in the UV/Vis spectrum. A
strong negative Cotton effect in the region of the CT band
also suggests the induction of molecular chirality of PyHisOMe
to the 1:1 CT complex at the supramolecular level. A plot of
the CD intensity at l=354 nm versus time afforded a sigmoidal
curve, which indicates the rate of helical supramolecular poly-
merization increased significantly after approximately 8 min
and reached saturation after approximately 30 min (Figure 4a,
inset).
Because the CT gel is optically translucent in nature (Fig-
ure S5b), it also provides an opportunity to investigate the ag-
gregation properties in solution through time-dependent cir-
cular dichroism (CD) spectroscopy near or above the critical
gelator concentration. For this experiment, an appropriate
amount of water was added to the preformed solution of the
1:1 CT complex in MeOH ([PyHisOMe]=2 mm, 2:1 H2O/MeOH),
and the resulting solution was immediately mixed thoroughly
by mechanical agitation. The resultant solution was poured
into a 1 mm cuvette as fast as possible and the CD signals
were recorded with progressive aging of the solution. Interest-
ingly, the solution did not display any CD signal after 1 min,
which indicates that 1 min is not long enough to induce effi-
cient helical supramolecular polymerization (Figure 4a). A very
weak bisignate negative CD signal started to evolve only after
approximately 4 min, and this is indicative of detectable nucle-
ation of chiral supramolecular polymerization at this time.
To verify the existence of supramolecular chirality originating
from the molecular chirality of PyHisOMe, the temperature-de-
pendent CD spectra of the 1:1 CT complex were recorded (Fig-
ure S5c). A solution of the 1:1 CT complex ([PyHisOMe]=
2 mm) exhibited an intense bisignate negative Cotton signal at
258C. The intensive bisignate negative Cotton signal, however,
slowly lost intensity with a gradual increase in the temperature
and finally became CD silent at a temperature of approximate-
ly 508C, which is indicative of an apparent breakdown in the
left-handed helical aggregates.
The CD spectra of the 1:1 CT complex were also recorded at
variable compositions of MeOH and H2O at a PyHisOMe con-
centration of 0.25 mm to affirm the growth of the left-handed
assemblies beyond the “critical” solvent composition (Fig-
ure 4b). Interestingly, in MeOH alone, the CD signal was almost
silent, which indicates that the aggregation of the CT mixed
stacks in MeOH alone is negligible. The CD signal, however, ap-
peared with an increasing amount of water in MeOH in the
proportion of H2O/MeOH=4:1. A relatively stronger bisignate
negative Cotton effect was observed in a 9:1 mixture of H2O/
MeOH; this indicates an aggregation-induced enhancement in
the CD signal and the formation of left-handed helical assem-
blies by the CT mixed stacks in this solvent composition. Nota-
bly, the critical solvent composition for the aggregation of the
gelators at a PyHisOMe concentration of 0.25 mm was H2O/
MeOH=4:1, although the gelator at a relatively high concen-
tration (PyHisOMe=2 mm) self-assembled in a 2:1 mixture of
H2O/MeOH to a “stable” gel state. Thus, the self-assembly of
the gelators was not only dependent on the concentration but
also on the composition of the solvents and temperature.
XRD studies of the xerogels of the 1:1 CT complex from
a 2:1 mixture of H2O/MeOH displayed one strong Bragg reflec-
tion in the low-angle regime at 2q=2.59 that corresponds to
a repetitive distance of 3.4 nm of the layered structure (Fig-
ure 5a). Given that the diameter of the layered structure was
almost comparable to the extended length of the 1:1 CT com-
plex (ꢀ3.2 nm; Figure 5b), it is believed that the CT mixed
Figure 4. a) Evolution of the CD intensity upon aging a mixture of the 1:1 CT
complex in a 2:1 mixture of H2O/MeOH, [PyHisOMe]=2 mm. Inset shows the
plot of the CD intensity at l=354 nm versus time. b) CD spectra of the 1:1
CT complex at different compositions of H2O and MeOH, [PyHisO-
Me]=0.25 mm.
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a smaller area of the mica surface revealed the formation of
1D fiber-like structures resulting from closely packed individual
vesicles, and this led to the generation of the “pearls-on-a-
string” type nanostructured pattern (Figures 6d and S6a,b).
The formation of the fiber-like network through the closely
packed linear arrangement of the vesicles unambiguously
demonstrates that the vesicles are strongly cohesive in nature.
Moreover, cluster-type aggregates were observed from
a freeze-dried 1:1 CT gel above the critical gelator concentra-
tion (PyHisOMe=2.4 mm; 2:1 H2O/MeOH) under SEM, analysis
of which indicated the formation of the former through fusion
of the vesicular aggregates (Figure S6c). This result also ruled
out the possibility of evaporation-induced formation of vesicle
strings. Menger et al. reported similar “vesicles-on-a-string”
gels based on a series of gemini surfactants.[6a] Raghavan and
co-workers very recently demonstrated the conversion of
densely packed vesicle gels into helical microtubules as a func-
tion of time.[6d] Such equivalent morphological development
such as micelle chaining resulting from kinetic self-assembly
was also demonstrated by several groups.[17] In an attempt to
determine the existence of the free vesicles, an AFM study was
performed at a much lower concentration of the gelator ([Py-
HisOMe]=0.05 mm). This showed the formation of free vesicu-
lar nanostructures with an average diameter of 90–200 nm and
a height of 5–20 nm (Figure 6e,f). Similar concentration-depen-
dent morphological patterns were also observed by AFM in
water (Figure S7).
Figure 5. a) XRD studies of the 1:1 CT dried gel. b) An energy-minimized
structure of the 1:1 CT complex.
stacks form MLMs through alternate self-assembly of the
donor–acceptor pairs.[12] DLS studies of the solution of the 1:1
CT complex ([PyHisOMe]=0.25 mm, H2O/MeOH=4:1) revealed
the formation of aggregates with a Dh of (266Æ19) nm, which
suggests the existence of vesicular aggregates in solution (Fig-
ure S5d).
Scanning electron microscopy (SEM) of the 1:1 CT gel at a Py-
HisOMe concentration of 0.25 mm from a 4:1 mixture of H2O/
MeOH confirmed the formation of vesicular aggregates with
a diameter in the range of 80 to 230 nm, which are arranged
in a “pearls-on-a-string” type fashion (Figure 6a). Moreover, in
some cases the individual vesicles remained “fused” together
to form short fiber-like organizations in the nanoscale dimen-
sion (Figure 6b). To verify the “pearls-on-a-string” type mor-
phological features, AFM studies were also performed by using
a solution of the gelators ([PyHisOMe]=0.25 mm) from a 4:1
mixture of H2O/MeOH. AFM analysis of a relatively large area
of the mica surface evidenced the formation of entangled
fiber-like networks (Figure 6c). However, careful scanning of
The various rheological parameters of a soft material are ex-
tremely important in terms of the utility of the material in
practical applications. Rheologically, gels are viscoelastic solid-
like materials.[18] Oscillatory strain amplitude sweep experi-
ments were performed with the
1:1 CT gels at three different
concentrations (Figure 7a). The
G’ value, that is, the viscoelastic
character of the CT gel, progres-
sively increased upon increasing
the concentration of the gelator.
Oscillatory frequency sweep ex-
periments were also performed
for the CT gel at various concen-
trations. The frequency sweep
experiment showed that the G’
value was always greater than
the corresponding loss modulus,
G’’, value under an applied strain
of 0.01% in the angular frequen-
cy range of 1 to 100 radsÀ1 and
was also invariant to angular fre-
quency. This suggests the viscoe-
lastic nature of the CT gels over
the entire angular frequency
range (Figure S8).
Progressive evolution of the
1:1 CT gel from the molecularly
Figure 6. a,b) SEM and c,d) AFM images of the 1:1 CT system at different positions and magnifications, [PyHisO-
Me]=0.25 mm. e,f) AFM image of the 1:1 CT system at [PyHisOMe]=0.05 mm and the corresponding height
image.
dissolved isotropic state was also
realized from time-dependent
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Figure 7. a) Amplitude–sweep rheology data of 1:1 CT gels in a 2:1 mixture
of H2O/MeOH at three different concentrations. b) Evolution of the viscoelas-
tic gels from an apparently isotropic solution as a function of time, [PyHisO-
Me]=4 mm.
Figure 8. a,b) A cartoon representation of Asym-NDI and the 1:1 CT com-
plex. c)Demonstration of the formation of vesicles of Asym-NDI in water
through self-assembly of the MLMs; only the cross-section of the vesicle is
shown for clarity. d) Demonstration of the formation of chiral vesicles from
the left-handed helical arrangements of the mixed CT stacks in the MLMs;
only the cross-section of the unilamellar vesicle is shown for clarity. e) For-
mation of “vesicles-on-a-string” through interdigitation of the oligo-oxyethy-
lene chains.
measurement of the G’ and G’’ values at an angular frequency
of 6.28 radsÀ1 and a strain of 0.01% (Figure 7b). Thus, to a solu-
tion of the 1:1 CT complex in MeOH, an appropriate amount of
water was added at a time (H2O/MeOH=2:1). Subsequently,
the resultant solution was mixed properly and placed on the
rheometer as fast as possible. Initially, the G’ value was less
than the respective G’’ value, which was indicative of the solu-
tion state of the CT complex. Notably, the storage modulus G’
started to increase relative to the corresponding loss modulus
G’’, which suggested progressive evolution of the viscoelastic
character upon aging of the solution. Thenceforth, the me-
chanical strength progressively increased and was almost satu-
rated after 40 min; this confirmed robust gel formation from
the isotropic solution state.
vironment, and the relatively less polar pyridyl ends face the
inner core of the individual vesicles. Thus, the surfaces of the
vesicles are rich with polar oxyethylene chains, which may be
attributed to the cohesive nature of the vesicles. The individual
vesicles form a “pearls-on-a-string” or “fiber-like” assembly
through interdigitation of the oxyethylene chains (Figure 8e).
The entanglement of the fiber-like structures creates 3D pock-
ets, which are eventually filled up by the solvent molecules.
Thus, the solvent molecules occupy the inner core of the vesi-
cles as well as the 3D hydrophilic pockets that, in turn, cause
gel formation. Notably, Asym-NDI did not form a coordination
complex with any of the metal ions, including Ag+, Cu2+, and
Zn2+. It also remained in the unprotonated form even at pH 3.
These observations suggest that the basicity of the pyridyl N
atom of Asym-NDI is rather low, presumably as a result of the
presence of the electron-withdrawing carbonyl group in the
para position of the pyridine ring. Therefore, we did not con-
sider H-bonding possibilities between the l-histidine unit of
PyHisOMe and the pyridine group of Asym-NDI during the CT-
induced gelation process.
The larger size (diameter ꢀ90–200 nm) and height (ꢀ5–
20 nm) of the individual vesicles relative to the extended
length of the 1:1 CT pairs indicates aggregation of the MLMs
into multilayer structures in the vesicular wall. On the other
hand, the supramolecular chirality was not reflected directly in
the morphological features of the gelators. Thus, the combina-
tion of CD, XRD, and morphological studies clearly suggests
that a left-handed helical arrangement of the mixed CT stacks
in the vesicular walls leads to the formation of chiral vesicular
aggregates (Figure 8d). However, a closer look at the single 1:1
CT complex suggests that either side of the CT unit is consider-
ably polar. The relative polarity of the two ends is significantly
different owing to the asymmetric bolaamphiphilic nature of
the CT unit. Given that the oligo-oxyethylene chains are more
polar, they probably experience the hydrophilic H2O/MeOH en-
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In conclusion, we explored the self-organization of novel asym-
metric bolaamphiphilic naphthalene-diimide (Asym-NDI) into
pH-responsive vesicles in water as the first phase of our inves-
tigations. However, in the second part, we demonstrated re-
markable CT-mediated gelation of PyHisOMe/Asym-NDI=1:1
charge-transfer (CT) mixed stacks in a 2:1 mixture of H2O/
MeOH. Aggregation of the CT stacks was not realized in MeOH
alone; however, the addition of water to the methanol solution
in a H2O/MeOH ratio of 2:1 caused immediate aggregation and
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the donor–acceptor pairs in MeOH increases significantly in
a 2:1 mixture of H2O/MeOH owing to hydrophobic effects. The
CT interactions are further assisted through intermolecular H-
bonding interactions among the hydrazide units of Asym-NDI
and are dependent on the concentration, temperature, and
solvent composition. Unlike the PyHisOMe/Asym-NDI=1:1 CT
complex, the failure of PyPheOMe, PyTrpOMe, and PyPheCOOH
to form gels with the acceptor, Asym-NDI, is noteworthy. This
further suggests that the l-histidine unit of PyHisOMe has
a crucial role in preventing precipitation of the aggregates
from solution. This occurs through several H-bonding interac-
tions of the histidine unit with the gelators as well as with the
solvent molecules, and thereby selective gel formation occurs
in a 2:1 mixture of H2O/MeOH. The presence of supramolecular
left-handed chiral assemblies was further evidenced from circu-
lar dichroism studies, although it was not directly apparent
from the morphological features of the gels. Rather, intercon-
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fashion as a result of intervesicular cohesive forces were de-
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ethylene chains from the surface of the vesicles. The “vesicles-
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mixture. XRD studies suggest the CT mixed stacks self-assem-
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membranes, and this eventually leads to the fabrication of the
chiral vesicular architectures. In addition, multistimuli respon-
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this is the first report of a supramolecular CT gel that possesses
all of these features, including “vesicles-on-a-string” morpholo-
gy. Accordingly, the present system may be useful in stimuli-re-
sponsive drug delivery or in materials-release applications.
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Keywords: charge transfer · helical structures · selectivity in
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Received: October 22, 2014
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FULL PAPER
Gels
Stringing it along: A charge transfer
(CT) mediated two-component supergel
shows excellent selectivity in its forma-
tion and has a stimuli-responsive
nature. The left-handed helical arrange-
ment of the mixed 1:1 CT stacks in the
monolayer lipid membranes leads to
the formation of chiral interconnected
vesicular aggregates with morphological
features that resemble “vesicles-on-a-
string”.
Subham Bhattacharjee,
Santanu Bhattacharya*
&& – &&
Charge Transfer Induces Formation of
Stimuli-Responsive, Chiral, Cohesive
Vesicles-on-a-String that Eventually
Turn into a Hydrogel
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