Hierarchical preferences of hydroxylated oxanorbornane-based achiral amphiphiles

Article abstract of DOI:10.1021/la4034172

Achiral amphiphiles with hydroxylated oxanorbornane headgroups showed specific morphological characteristics and hierarchical preferences depending upon the nature of lipophilic units. Detailed scanning electron microscopic (SEM) studies showed that twist

Full text of DOI:10.1021/la4034172

Article
pubs.acs.org/Langmuir
Hierarchical Preferences of Hydroxylated Oxanorbornane-Based
Achiral Amphiphiles
D. Sirisha Janni and Muraleedharan K. Manheri*
Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600 036, India
S
* Supporting Information
ABSTRACT: Achiral amphiphiles with hydroxylated oxanorbornane head-
groups showed specific morphological characteristics and hierarchical
preferences depending upon the nature of lipophilic units. Detailed scanning
electron microscopic (SEM) studies showed that twisted ribbonlike
aggregates are characteristic of monoalkoxyaryl lipids with hydrocarbon
chain length in the range C10−C13; these systems also had a preference
toward lamellar arrangement. Asymmetric packing of these lipids is a unique
occurrence and shows that the presence of molecular chirality is not an
absolute requirement for curvature effects in their supramolecular
assemblies. Aryl units in these systems were found to be important for
the observed morphological preferences, which became evident from
comparative studies involving simple long chain esters without this moiety.
Single-crystal X-ray diffraction analysis of one of the lipids from the latter
group gave finer details of strong and weak secondary interactions, which operate during their assembly process. Introduction of
more than one alkyl chain on the aromatic ring caused a notable shift in the packing propensity toward columnar arrangement.
Most of these cone-shaped molecules were found to give doughnut-shaped aggregates from acetone solution through the
intermediary of fibrous structures, which was confirmed through SEM, transmission electron microscopic, and atomic force
microscopic studies.
we have synthesized a new series of hydroxylated oxanorbor-
nenyl imides connected to the long chain alkoxy aryl units
through an ethanolamine spacer. Favorable orientation of
hydroxyl groups for hydrogen bonded assembly, flexibility of
the linker for proper packing, and the possibility of varying the
number of alkyl chains on the aromatic ring are the attractive
features of this design. Systematic studies involving this class of
compounds have shown that chirality in an individual lipid is
not an absolute requirement for hierarchical preferences like
twisted ribbons during their self-assembly. This has significance
from both design and mechanistic standpoints and highlights
the influence of headgroup tilt and conformational bias in
dictating the assembly profile. Formation of doughnut-shaped
assemblies from acetone solutions of di- and trialkyl lipids and
an insight into their development are other important
highlights of this work.
INTRODUCTION
■
Formation of higher-order structures through molecular self-
organization and their development into new functional
materials is one of the hottest areas of research today.1−3
The nature and directionality of secondary interactions
involved and the orientation of interacting groups with respect
to each other play decisive roles in such processes. A predictive
power on these aspects would help us to design properties
through appropriate choice of component(s). For this, a
thorough understanding on the preference profiles in supra-
molecular architectures is necessary. Although networks based
solely on hydrogen bonds are abundant, a great deal of
attention is also being placed on using this in conjunction with
hydrophobic clustering and π-stacking possibilities to create
new organic materials of well-defined nano- and microscale
structures.4−6 Concerted efforts have given insight into the
organizational preferences of not only small organic molecules
but also more complex systems such as polymers, dendrimers,
and metallomesogens, which culminate in special properties
with applications in diverse areas such as optoelectronics,7,8
data storage,9 soft magnetic systems, stimuli-responsive
materials, and so forth.10−14
EXPERIMENTAL SECTION
■
Amphiphiles studied here were prepared in 4−6 synthetic steps,
which started with Diels−Alder reaction of furan with maleic
anhydride, treatment of the resulting adduct with ethanolamine
to get an imide intermediate, its esterification with appropriate
alkoxy benzoic acids (or long chain carboxylic acids), and a final
We have recently shown that polyhydroxylated oxanorbor-
nane scaffolds can be used as a “sugar surrogate” to create
structural mimics of glycolipids.15 Their self-assembly and
aggregation characteristics were comparable to that of natural
analogs reported in the literature.16,17 In continuation of this,
Received: September 4, 2013
Revised: November 1, 2013
Published: November 18, 2013
© 2013 American Chemical Society
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cis-dihydroxylation using osmium tetroxide (Supporting In-
formation Schemes S1 and S2). The alkoxy benzoic acids used
in esterifications were separately prepared by alkylation of
various hydroxy benzoates and hydrolysis of the ester group
using alcoholic KOH. Their coupling with the imide
intermediate was carried out under EDCI/HOBt/DIPEA
conditions. Long chain carboxylic acid esters used for
comparative studies were prepared by reaction of the
corresponding acid chloride with the imide intermediate. We
have presented the synthetic details of imide intermediate and
the alkoxy benzoates in the Supporting Information. Specific
information on the esterification and dihydroxylation steps are
presented below.
of appropriate solvent, sonicated, and drop-casted on silica or
HOPG (highly ordered pyrolytic graphite) substrates. It was
allowed to dry at ambient temperature in a desiccator overnight
and then was used for imaging. To prepare the sample for
transmission electron microscopic (TEM) studies, a solution of
the compound in the desired solvent (1 mg/1.5 mL) was drop-
casted onto a carbon coated copper TEM grid and allowed to
dry in a desiccator as mentioned above. For studies involving
amino acid as a chiral additive (Figure 6), 1 equiv of amino acid
was dissolved in 0.1 mL of water and added to a methanolic
solution of the sample (1 mg/1.4 mL). After sonication, a drop
of the mixture was placed on silica substrate and allowed to dry
under the conditions mentioned above before imaging.
General Procedure for the Coupling of Imide
Intermediate with Alkoxy Benzoic Acids. To a stirred
solution containing a mixture of alkoxy benzoic acid (1 equiv)
and 1-hydroxybenzotriazole (HOBt, 1 equiv) in dry dichloro-
methane at 0 °C was added iPr2NEt (1.2 equiv) and 1-ethyl-3-
[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDCI,
1.1 equiv). The reaction mixture was stirred at 0 °C for 10 min,
to which the imide intermediate (5, Supporting Information)
was added, and the mixture was allowed to stir at 0 °C for 30
min and then at room temperature for 24 h. After completion
of the reaction, the mixture was diluted with dichloromethane
and washed with water (30 mL). The organic layer was dried
over anhydrous Na2SO4, filtered, the solvent was evaporated
under reduced pressure, and the residue was purified by column
chromatography on silica gel using 30−40% EtOAc/Hexanes to
get the esters in 60−94% yield. Spectroscopic details of the
resulting compounds are shown in the Supporting Information.
General Procedure for the Preparation of Aliphatic
Esters. To a stirred solution of the imide intermediate (5, 1.0
equiv) and Et3N (1.5 equiv) in dry dichloromethane was added
the appropriate acid chloride (1.1 equiv) at 0 °C under N2
atmosphere. The reaction mixture was warmed to room
temperature and allowed to stir for 2 h. After completion of
the reaction, the mixture was washed with water and extracted
with dichloromethane. The organic layer was dried using
anhydrous Na2SO4 and evaporated under reduced pressure.
The residue was purified by column chromatography on silica
gel using 20−30% EtOAc/Hexanes. Yields and spectroscopic
details of resulting compounds are shown in the Supporting
Information.
RESULTS AND DISCUSSION
Structures of amphiphiles with alkoxy aryl lipophilic units used
in the present study are shown in Figure 1. They can be placed
■
Figure 1. Hydroxylated oxanorbornane-based lipids used in the study.
into three groups based on the number of alkyl groups present
on the aromatic ring. Compounds 1a−g form one group with a
single alkyl chain at the 4-position but individually differ in the
number of methylene units in the chain. At the same time 2a−c
and 3a−c have two and three alkyl chains at 3,5 or 3,4,5-
positions, respectively. Differences in the hydrophilic−hydro-
phobic ratios were expected to affect their self-assembly profiles
and morphological characteristics. To understand this, scanning
electron microscopic (SEM) examination of samples prepared
by directly drop-casting their methanol solutions on silica
substrate was initially carried out.
General Procedure for cis-Dihydroxylation: Prepara-
tion of Amphiphiles 1a−j, 2a−c, and 3a−c. To a stirred
solution containing a mixture of the oxanorbornene derivative
(1.0 equiv), N-methyl morpholine N-oxide (2.4 equiv) and
pyridine (30 μL for 100 mg of the alkene) in t-BuOH/H2O
(3:1) was added osmium tetroxide (0.02 M solution in t-
BuOH, 0.01 equiv), and the solution was heated at 80 °C for
7−8 h. After completion of the reaction, the mixture was cooled
to room temperature, treated with 15% Na2SO3(aq) (1 mL),
and allowed to stir for 5−10 min. After removing the solvents
under reduced pressure, the crude mixture was diluted with
dichloromethane, dried using anhydrous Na2SO4, filtered,
evaporated under reduced pressure, and the residue was
purified by column chromatography on silica gel using
EtOAc−DCM−MeOH in a gradient mode to get the products
as colorless solids. Yields and spectroscopic details of the
resulting compounds are shown in the Supporting Information.
Preparation of Samples. To prepare samples for scanning
electron microscopic (SEM) and atomic force microscopic
(AFM) studies, the compound (1 mg) was dissolved in 1.5 mL
In general, they were found to form fibrous or ribbonlike
aggregates, which were discrete in the case of mono alkyl
systems, interlinked in dialkyl series, and highly cross-linked in
the trialkyl derivatives; the only exception was the globular
assembly in the case of 3a. SEM images of aggregates from 1a,
1b, 2a, and 2b are shown in Figure 2 and those from other
compounds are given in the Supporting Information (Figure
S1).
One of the most astonishing observations was the presence
of twisted ribbonlike aggregates in the sample of 1b having the
C12 alkyl chain. Because a change from fibrous to twisted
ribbonlike morphology happened on increasing the chain
length to C12, other homologues in this series with C10, C11,
C13, and C14 alkyl chains (1d−g) were also synthesized and
included in the study to understand the structural requirements
for the observed hierarchical assembly. Of these, lipids 1d−f
with C10, C11, and C13 alkyl chains also gave twisted ribbons
as that of 1b. However, compound 1g having a C14 alkyl chain
gave spherical aggregates of more or less uniform sizes under
this condition (Figure 3D), suggesting that there is an optimal
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Figure 2. SEM images of the samples of (A) 1a, (B) 2a, (C) 1b, and
(D) 2b prepared by directly drop-casting their methanol solutions (1
mg/1.5 mL) on silica substrate.
Figure 4. (A) SEM images of a sample of 1g (B, enlarged view)
prepared by drop-casting its MeOH solution (1 mg/1.5 mL) directly
on silica substrate. (C) DLS histogram of 1g in MeOH.
series, the lattice parameters of 1c with a 16 carbon chain (a =
100 Å, b = 42 Å) supported a rectangular columnar
arrangement.
Because there is no internal element to induce chirality, the
observed “handedness” is likely arising from curvature in
lamellar arrangement brought about by uniform tilting of
headgroups when molecules assemble. In the case of lipids 1b
and 1d−1f, this is probably happening over a long-range, which
gives rise to the twisted structure. A pictorial representation of
this using the bilayer formed through a head−head interaction
of the lipid 1b is presented in Figure 5.
Figure 3. SEM images of samples of (A) 1d, (B) 1e, (C) 1f, and (D)
1g prepared by directly drop-casting their methanol solutions (1 mg/
1.5 mL) on silica substrate.
chain-length requirement for the observed supramolecular
chirality. Widths of these twisted ribbons ranged between 135
and 230 nm. Their pitches, however, were not uniform.
Interestingly, they remained in either left-handed or right-
handed forms. Unlike most of the known glycolipids that
exhibit such morphologies, the amphiphiles studied here are
achiral in nature, which makes the chiral preference during
supramolecular organization significant.
Although the formation of globular aggregates from 1g
having just an extra methylene unit compared to 1f was
surprising, detailed SEM analysis showed regions having
continuous linear array of these spherical assemblies (Figure
4A,B), which suggests that they probably originate from fibrous
or ribbonlike aggregates through a pearlinglike phenomena that
has been reported in the case of tubular vesicles from stearoyl-
oleoyl-phosphatidyl-choline in the presence of alkyl chain
modified dextran.18 A dynamic light scattering (DLS) study of
1g in MeOH showed the average diameter of these spherical
aggregates as 825 nm (Figure 4C).
To get an insight into the assembly, samples of 1a−f were
initially subjected to powder X-ray diffraction (PXRD) analysis.
Diffraction patterns of 1a−b and d−g were nearly identical with
d1, d2 and d3 in the ratio 1:1/2:1/3, which suggested lamellar
arrangements. The PXRD profile of 1b, a representative
example, is presented in Figure 9A and those of others are
included in the Supporting Information. The data for 1a, 1b,
1d, and 1e supported a bilayer arrangement with d1 being
approximately equal to twice the molecular length. A tendency
toward interdigitation was however observed in 1f and 1g
having longer chains; d1 in these cases were slightly less than 2l,
l being the molecular length, measured from structures
minimized using Chem3D. Unlike other compounds in this
Figure 5. Cartoon representation showing a possible mode of self-
assembly of 1b (A) that could result in supramolecular chirality (B).
As can be anticipated, flexibility associated with the linker
connecting the headgroup could allow the system to change the
curvature and hence the nature of the twisting. This is probably
responsible for the formation of both left- and right-handed
twisted ribbons during the aggregation of 1b and 1d−f. To
understand the effect of chiral additives, samples of 1b from
MeOH containing serine, tartaric acid, or arginine (1 equiv)
were prepared and used in SEM studies. These molecules were
expected to associate with the polar belt in between the alkyl
chains by interacting with the headgroups and exerting
additional influence on supramolecular chirality. Both D- and
L-isomers of serine and tartaric acids were used to understand
the difference, if any, in the nature of the aggregates. As evident
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from Figure 6, presence of L-serine and D-(−)-tartaric acid
promoted the formation of left-handed twisted ribbons
Figure 7. SEM images of samples of 1h (A), 1i (B), and 1j (C)
prepared by drop-casting their MeOH solution (1 mg/1.5 mL) on
silica substrate.
Single-crystal X-ray diffraction analysis showed the compound
(Figure 8A) as belonging to a monoclinic system with a P21/c
space group. The solved structure showed 2-fold positional
disorder for the alkyl side chain with relative occupancies of
58:42. The disordered components were located through
successive difference Fourior synthesis. The components were
partitioned and refined with the sum of their occupancies
restrained at 1. A detailed examination of the lattice showed the
involvement of an array of weak and strong hydrogen bonding
interactions in maintaining the assembly (bond length and
bond angle details are presented in the Table S3 of the
Supporting Information). Prominent among these are H2A···
O3 and H3A···O2 hydrogen bonding (H2A···O3, 2.044 Å;
H3A···O2, 2.032 Å; ∠O2−H2A···O3 = 151.43°, ∠O3−H3A···
O2 = 151.93°), which link molecules along the b-axis as shown
in Figure 8C. In addition to these, the bridge-head oxygen was
found to maintain weak CH···O hydrogen bonds with H1
and H2 of another one to give additional stabilization to the
stacking (Figure 8C). Laterally, there were weak CH···O
interactions between O3 and H3 of each molecule with the
Figure 6. Effect of additives on the aggregation of 1b from methanol;
images show morphologies of samples prepared in the presence of 1
equiv each of L-serine (a), L-arginine (b and c), D-serine (d and e), D-
(−)-tartaric acid (f), and L-(+)-tartaric acid (g); M and P in these
images refer to left-handed and right-handed twisted ribbons.
preponderantly. However, samples containing D-serine and L-
(+)-tartaric acid had both left- and right-handed twists. They
formed separate dendritic domains, suggesting that chiral
preference during nucleation has a direct effect on subsequent
supramolecular organization. Unlike L-serine, L-arginine facili-
tated the formation of both left- and right-handed twists and
the profile was comparable with that of the sample with D-
serine.
A notable increase in the number of right-handed twisted
ribbons in the presence of D-serine, L-tartaric, and L-arginine
suggests that the flexibility they impart to the lipid assembly is
different compared to that from other additives examined here.
Although a molecular level picture on this is currently lacking, it
is reasonable to assume that the ability of diastereomeric
complexes of 1b with these chiral systems to assume different
curvatures is contributing to the formation of left- and/or right-
handed twists.
Apart from being a point of attachment for alkyl chains, the
aromatic rings in 1a−g seem to play an important role in
directing their assembly toward fibrous/ribbonlike structures.
This became evident when the aggregation of lipids 1h−j
(Figure 7) was studied under identical conditions. SEM analysis
of their samples, drop-casted from methanol (1 mg/1.5 mL) on
silica substrate, showed continuous sheetlike morphologies as
shown in Figure 7A−C. Their PXRD spectra had peaks
corresponding to d1, d2, and d3 of 34, 17, and 8.8 Å (1h); 33,
17, and 8.7 Å (1i); 43, 22, and 14 Å (1j) in the ratio 1:1/2:1/4
(for 1h and 1i) and 1:1/2:1/3 (for 1j), indicating lamellar
arrangements.
2
corresponding atoms on the adjacent one to give a R2 (8)
motif. Apart from these, the imide CO acted as a trifurcated
donor to form weak hydrogen bonds with H1 and H7 of the
adjacent molecule in the same layer and H8 of another one
diagonally above it, which is presented in Figure 8B. The
overall arrangement of molecules in the lattice, stabilized by a
combination of hydrogen bonding interactions and hydro-
phobic clustering, is presented in Figure 8D.
An increase in the number of alkyl chains in 2a−c and 3a−c
was reflected in their PXRD pattern. Although the bis-hexyl
derivative 2a was lamellar with d1 = 41.2 Å corresponding to
bimolecular thickness, the C12 and C16 derivatives 2b and 2c
gave peaks in the low angle region with d1, d2, and d3 of 30.2,
19.6, and 13.2 Å and 31.3, 22.1, and 15.7 Å, respectively in the
ratio 1:0.707:0.5, indicative of tetragonal columnar arrange-
ment. Similarly, the trialkyl derivatives 3a and 3b had peaks
corresponding to d1, d2, and d3 of 34, 27.5, and 18 Å and 42.4,
36.5, and 22.4 Å, respectively.
The PXRD graphs of representative compounds (1b, 2b, and
3b) are presented in Figure 9 and those of others are shown in
the Supporting Information (Table S1 and Graphs S1−12).
The lattice parameters a = 68 Å, b = 32 Å for 3a and a = 84.8 Å,
b = 40.5 Å for 3b are suggestive of rectangular columnar
organization in these systems. Being nonionic with a fairly wide
lipophilic surface, the compounds 2b, 2c, 3a, and 3b are likely
forming inverted columns with alkyl groups dispersed radially
outward. Unlike these, the lipid 3c with three C16 alkyl chains
had peaks corresponding to d1, d2, and d3 of 58.1, 32.6, and 19.7
Gratifyingly, we could secure crystals of 1i by slow
evaporation of its solution in a MeOH-DCM mixture (1:4).
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Figure 8. (A) X-ray crystal structure of 1i. (B) Weak hydrogen bonding interactions at the imide carbonyl. (C) Hydrogen bonding network
involving head−head interactions. (D) Hydrogen bonding network involving head−head interactions and hydrophobic clustering in the crystal
system.
Å, respectively, in the ratio 1:1/2:1/3, indicating a lamellar
arrangement.
As with the aggregates from methanol, morphologies of
samples from other solvents such as ethanol, t-BuOH,
trifluoroethanol, tetrahydrofuran, and acetonitrile−water were
also found to originate from fibrous primary aggregates
(Supporting Information, Figure S2). A consistent and unique
preference was however observed in the case of samples of 2a,
2b, and 3a−c drop-casted from acetone. SEM and TEM images
of samples from 2b presented in Figure 10 clearly show that an
increase in the number of alkyl chains has resulted in a
markedly different morphology. The aggregates were dough-
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Figure 9. Powder XRD profiles of 1b (A), 2b (B), and 3b (C).
Figure 10. A−C respectively show SEM, TEM, and AFM images of samples of 2b prepared by drop-casting its acetone solution (1 mg/1.5 mL) on
silica substrates for SEM and AFM, and on a carbon coated copper grid for TEM.
nut-shaped with inner and outer diameters in ranges 1−320 nm
and 200 nm−3 μm, respectively. A sample of 2b for AFM
imaging was prepared by drop-casting its acetone solution (1
mg/1.5 mL) on silica substrate. The outcome from this study
was also in accordance with the results from SEM and TEM
experiments. Generally, central pores in these assemblies were
found to disappear over time to give the morphology shown in
Figure 10C.
In the SEM image of 2b, there were isolated regions
containing coiled fibers and partially formed “doughnuts”
(Figure 11), suggesting that they could be the precursors
Figure 11. Selected areas from the SEM images of sample from 2b (A
and B) showing the intermediary of coiled fibers during the
development of doughnut-shaped aggregates (C).
during the formation of doughnuts. Transition from fibrous to
Figure 12. (A and B) SEM and 3-D AFM images of the samples of 3b
from acetone (1 mg/1.5 mL); (C) coiled fibers en route to doughnut-
shaped assemblies. The cross-sectional height profile of aggregates of
3b along the line shown in panel D is displayed in panel E.
the final morphology appears to happen during solvent
evaporation and is likely facilitated by mixing of hydrophobic
domains.
As mentioned, the tendency to form doughnut-shaped
structures was also seen in trialkyl lipids. The SEM and AFM
images of samples from 3b are presented in Figure 12 and those
of 2a, 2c, 3a, and 3c are included in the Supporting Information
(Figure S20). Although the central pores in compound 3b
appear to be wider compared to those in 2b, no consistency
could be observed to correlate this with the number of alkyl
chains. AFM images of 3b were comparable to those of 2b
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(Figure 10). Here again, fibers seem to be the probable
precursors for the formation of doughnuts, which is evident
from the image shown in Figure 12C.
Because the SEM images presented above were taken on
silica substrate, it became important to see whether its
hydrophilic nature is contributing to the morphological
preference. To test this, experiments involving the lipids 1b,
1g, 2b, and 3b were repeated using hydrophobic HOPG
(highly ordered pyrolytic graphite) as the substrate, and the
results are presented in Figure 13. Twisted ribbonlike
phosphocholine (DNPC), which not only show new avenues in
material design but also highlight the remarkable ways in which
amphiphiles can pack to give three-dimensional order.24 Recent
theoretical studies on curvature formation in lamellar structures
have taken into account the free energy of the system with
contributions from the bending and surface tension.25
Conformational preferences of individual lipids, their tilt with
respect to the layer normal, solvation effects, and secondary
interactions are some of the specific factors that determine the
energy state of the assembly and hence the observed
hierarchical preference.26 In the absence of a chiral center,
the curvature could result if there is asymmetric packing over a
long-range with adequate stability. Most of the known reports
on supramolecular chirality from achiral systems are based on
compounds having tendency to stack one above the other in a
spiral fashion; initially formed fibrils in these cases develop in to
nano- or microscale assemblies through further aggregation.27
In comparison, the lipids studied here are achiral and also do
not have any unsaturation on the alkyl chain to control its
orientation. Preferential formation of left-handed twisted
ribbons in presence of L-serine and D-(−)-tartaric acid, and a
notable increase in right-handed twisted assemblies in the
presence of D-serine, L-tartaric, and L-arginine tend to suggest
that the stabilities and accessible curvatures of their
diastereomeric complexes with 1b are different.
Formation of doughnut-shaped assemblies from 2a, 2b, and
3a−c is another noteworthy observation from the present work.
The question of whether the overall shape originates
spontaneously or from other simpler structures is pertinent
here. The results from SEM and AFM analysis of 2b and 3b
presented in Figures 11 and 12 rule out the likelihood of the
former mechanism and point toward a pathway involving
fibrous aggregates as intermediates. Nanometer-sized torroidal
structures have been reported from block copolymers,28
peptides,29−31 dendrimers,32 and also from melamine-oligo(p-
phenylenevinylene) supramolecular complexes.27 The mode of
assembly that results in such superstructures could vary from
one system to another and depend on the nature and shape of
amphiphiles. For example, torroidal aggregates from dumbbell-
shaped amphiphilic system reported by Kim et al. is believed to
develop through coalescence of initially formed spherical
micelles having hydrophobic cores and hydrophilic periph-
eries.33 A similar structure has also been obtained from cone-
shaped amphiphilic peptide Ac-GAVILRR-NH2, and the
initially formed spherical micellar aggregates in this case seem
to transform to nanopipelike intermediates. Bending of these
nanopipes and fusion of ends then result in final morpholo-
gies.31 Results from our studies involving 1a−g, 2a−c, and 3a−
c suggest that the molecular structure, hydrophilicity−hydro-
phobicity ratio, and the overall molecular shape have profound
influence on final morphologies. Notably, aggregates from
cone-shaped systems 2a, 2b, and 3a−c, though fibrous initially,
show a tendency to transform to doughnut-shaped structures
through coiling and domain mixing during curing stages.
Figure 13. SEM images of samples of (A) 1b, (B) 1g, (C) 2b, and (D)
3b prepared by directly drop-casting their solutions (1b and 1g from
methanol and 2b and 3b from acetone; 1 mg/1.5 mL) on HOPG
substrate.
aggregates from the methanol solution of 1b, formation of
spherical aggregates and pearlinglike phenomena from the
methanol solution of 1g (Supporting Information, Figure S30),
and doughnut-shaped aggregates from acetone solutions of 2b
and 3b were observed under this condition also, which suggests
that the solid support does not significantly influence their
assembly profile.
Aggregation profiles of lipids presented here, when viewed in
light of current developments in this area, prompts us to
present two important points for detailed discussion. First one
is on the origin of “handedness” during the self-assembly of 1b
and 1d−f. Such supramolecular chirality has in fact been
observed with a number of ionic and nonionic lipids, but these
systems invariably possess a chiral center either as part of the
main lipid or as part of the counterion. Although packing of
chiral headgroups in principle could induce curvature in
lamellar structures, the nature and length of a lipid chain also
have been found detrimental in getting stable twists.19 Control
of supramolecular chirality by using chiral counterions in
gemini surfactants reported by Oda et al.,20,21 time-dependent
development of twisted ribbons, helical ribbons, and nanotubes
from lysine-based amphiphile reported by Ziserman and co-
workers,22 and generation of helical aggregates from sugar
based amphiphiles23 are some interesting examples. Such
hierarchical preferences have also been found with mixtures
of lipids like 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phos-
phocholine (DC8,9PC) and 1,2-bis(dinonanoyl)-sn-glycero-3-
CONCLUSIONS
■
The aggregation behavior of a new class of oxanorbornane-
based amphiphiles and morphological characteristics of their
samples were systematically studied using a combination of
SEM, TEM, AFM, single-crystal XRD, and PXRD techniques.
Although formation of fibrous or ribbonlike structures from
alcoholic solvents is seen as a common characteristic, some of
the cone-shaped lipids with two and three alkyl chains were
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AUTHOR INFORMATION
Corresponding Author
*M. K. Manheri. E-mail: mkm@iitm.ac.in. Fax: +91 44 2257
4202. Tel: +91 44 2257 4233.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support of this work by the Department of Science
and Technology, INDIA (Grants SR/NM/NS-1034/2012) is
gratefully acknowledged. We also thank SAIF (IITM) for
HRSEM facility, Dr. Edamana Prasad (Department of
chemistry, IITM) for DLS facility, Dr. Babu Varghese (SAIF,
IITM) and Mr. V. Ramkumar (Department of chemistry,
IITM) for single-crystal XRD analyses, and Ms. Srividya
(Department of chemistry, IITM) for PXRD data. D.S.J. thanks
CSIR for fellowship.
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