Unprecedented parallel packing of unsymmetrical bolaamphiphiles driven by π-π Stacking of cinnamoyl groups

Article abstract of DOI:10.1039/c002541k

The self-assembly and molecular packing mode of a novel, unsymmetrical bolaamphiphile, sodium 4-(6-hydroxyhexyloxy) cinnamate (SHHC) was studied. Tube-like structures were obtained in SHHC aqueous solution at pH 9.2. Upon UV irradiation, the SHHC molecules in the system were found to undergo four-center-type photocyclodimerization and photoisomerization. The combination of IR, XRD and Zeta potential results suggested the formation of a tail-to-tail type bilayer membrane structure: the SHHC molecules stretched upright with the cinnamate headgroups toward the outside in each layer, whereas the tail OH groups of two layers remained inside and formed interlayer hydrogen bonds. This parallel packing mode of SHHC molecules in the membrane was also confirmed by the pH-induced self-assembly transition. With increasing pH from 9.2 to 12.0, the low-curvatured tubes transformed into spherical vesicles, which was in line with the increased outer leaflet head group area as increasing pH. Our results demonstrated that upon careful molecular design, the orientation of unsymmetrical bolaamphiphiles can be controlled; π-π interactions between cinnamoyl groups were found to be strong enough to drive the formation of well-oriented molecular assemblies. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002541k

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/softmatter | Soft Matter
Unprecedented parallel packing of unsymmetrical bolaamphiphiles driven by
p–p stacking of cinnamoyl groups†
*
Ying Zhao, Jing Zhao, Yun Yan, Zichen Li and Jianbin Huang
*
*
Received 5th February 2010, Accepted 30th March 2010
First published as an Advance Article on the web 25th May 2010
DOI: 10.1039/c002541k
The self-assembly and molecular packing mode of a novel, unsymmetrical bolaamphiphile, sodium
4-(6-hydroxyhexyloxy) cinnamate (SHHC) was studied. Tube-like structures were obtained in SHHC
aqueous solution at pH 9.2. Upon UV irradiation, the SHHC molecules in the system were found to
undergo four-center-type photocyclodimerization and photoisomerization. The combination of IR,
XRD and Zeta potential results suggested the formation of a tail-to-tail type bilayer membrane
structure: the SHHC molecules stretched upright with the cinnamate headgroups toward the outside in
each layer, whereas the tail OH groups of two layers remained inside and formed interlayer hydrogen
bonds. This parallel packing mode of SHHC molecules in the membrane was also confirmed by the pH-
induced self-assembly transition. With increasing pH from 9.2 to 12.0, the low-curvatured tubes
transformed into spherical vesicles, which was in line with the increased outer leaflet head group area as
increasing pH. Our results demonstrated that upon careful molecular design, the orientation of
unsymmetrical bolaamphiphiles can be controlled; p–p interactions between cinnamoyl groups were
found to be strong enough to drive the formation of well-oriented molecular assemblies.
bolaamphiphiles, we are interested to regulate the molecular
Introduction
packing mode via proper molecular design.
Over the past years, self-organized structures of bolaamphiphiles
have attracted increasing attention owing to their rich
morphologies.1–4 So far, a full spectrum of molecular assemblies
has been observed in the systems of bolaamphiphiles, including
micelles,5,6 vesicles,7,8 lamellae,9 ribbons,10 fibers11–13 and tubes.14–19
Among this work, the relationship between the morphology of
the self-assembled structures and the chemical structure of
unsymmetrical bolaamphiphiles is of special interest. Different
from those symmetrical analogues with two identical head
groups attached to each end of a hydrophobic chain, the
unsymmetrical bolaamphiphiles have two different head groups,
which are usually of different sizes and properties.20 Such
molecular structures are supposed to readily form unsymmetrical
monolayer lipid membranes (MLM) with different inner and
outer leaflets. A few successful examples have been achieved in
this aspect.7,12,15,21,22 However, most unsymmetrical bolaamphi-
philes are found to form symmetrical MLM due to the favorable
antiparallel packing of the molecules.9,10,23–25 So far, the delicate
manipulation of the molecular packing motif is still a challenge
that restricts the development of the study on the unsymmetrical
bolaamphiphiles. In order to have a better understanding on the
molecular structure that determines the packing mode of the
molecules in the membrane formed from unsymmetrical
Inspired by the commonly seen antiparallel packing of
unsymmetrical bolaamphiphiles, we realized that a strong
interaction between the homohead-groups must be introduced to
obtain a parallel packing of the unsymmetrical bolamphiphiles.
Moreover, in order to facilitate the examination of the symmetry
of the membrane formed by the designed molecules, the incor-
poration of an easy-to-check functional group in proximity to
one head of the unsymmetrical bolaamphiphiles is necessary.
Taking these two considerations together, we designed a simple
unsymmetrical bolaamphiphile that contains a cinnamate group.
Cinnamate groups, which are photo responsive as well as pH
sensitive, are able to undergo four-center-type 2 + 2 photo-
cyclodimerizations when two such groups are in the range of
26–28
ꢀ
3.6–4.1 A.
This reaction is commonly observed with cinna-
mate acid derivatives in a crystalline state where the cinnamoyl
groups are parallel to each other and in close proximity. This is
also applicable when amphiphiles containing cinnamoyl groups
self-assemble into ordered molecular assemblies, such as vesicles
and LB films,29–32 since parallel packing of the molecules in these
structures is the only choice. In our design, sodium cinnamate is
used as one of the polar heads of the unsymmetrical bolaam-
phiphile, where the p–p stacking between the cinnamoyl groups
is expected to drive the parallel packing of the molecules. The
other small polar head we choose for the molecule is OH, to use
its weak polarity. These two head groups are connected via an
alkyl chain consisting of 6 carbons, to weaken the hydrophobic
interaction contributed by the alkyl chains so as to highlight the
strength of the p–p stacking interaction between those cinna-
moyl groups.
Beijing National Laboratory for Molecular Science (State Key Laboratory
for Structural Chemistry of Unstable and Stable Species), College of
Chemistry and Molecular Engineering, Peking University, Beijing,
100871, People’s Republic of China. E-mail: yunyan@pku.edu.cn;
jbhuang@pku.edu.cn; zcli@pku.edu.cn; Fax: +86-10-62751708; Tel:
+86-10-62765058
Therefore, the aim of this paper is to investigate the molecular
packing and self-assembling behaviors of the delicately designed
† Electronic supplementary information (ESI) available: Supplementary
figs S1–5. See DOI: 10.1039/c002541k
3282 | Soft Matter, 2010, 6, 3282–3288
This journal is ª The Royal Society of Chemistry 2010

View Article Online
unsymmetrical bolaamphiphile, sodium 4-(6-hydroxyhexyloxy)
cinnamate (SHHC). We are curious to find out whether the p–p
stacking of cinnamate groups is strong enough to drive a parallel
packing of the SHHC molecules in their self-assemblies. TEM,
XRD, UV, IR and pH variation experiments were used to
examine the system. It was pleasing to note that parallel packing
of SHHC molecules indeed took place. Our results demonstrate
that through proper molecular design, control of the packing
mode of unsymmetrical bolaamphiphiles can be easily achieved,
which paves the way for fabricating unsymmetrical molecular
assemblies on a large scale.
Transmission electron microscopy (TEM). One drop of the
sample dispersion was placed onto a formvar-coated copper grid
and the excess liquid was sucked away by filter paper. Then one
drop of staining agent (2% uranyl acetate aqueous solution) was
dropped onto the sample grid by the same process as above.
Negatively stained TEM micrographs were obtained with
a JEOL-100 CX II transmission electron microscopy.
UV–vis measurements. The SHHC solutions containing cin-
namoyl moieties in quartz test tubes were irradiated using a high-
pressure Hg lamp system of 500 W. After different irradiation
times, the resulting diluted solutions (0.1 mmol dmꢀ3) were
analyzed at 25 ꢁC by an UV-vis spectrometer equipped with
a thermostatic bath.
Experimental section
Infrared (IR) spectra. The SHHC solutions in test tubes were
put into the vacuum drying equipment for 48 h. Then micro-
scopic Fourier transform IR spectra were collected with a Nicolet
iN10 MX spectrometer (Thermo Scientific, America) operating
Materials
Synthesis of SHHC. 1-bromo-6-hydroxyhexane was prepared
from the corresponding diol and hydrobromic acid33 (Beijing
Chemical Co. A.R.). A mixture of 1-bromo-6-hydroxyhexane
(2.18 g, 0.012 mole) and 4-hydroxycinnamic acid (1.64 g, 0.01
mole) in 40 cm3 ethanol was added into 10 cm3 of 2 mol Lꢀ1
NaOH aqueous solution. This mixture was allowed to react for
24 h under magnetic stirring at 60 ꢁC. Then the reaction mixture
was cooled and filtered to collect solid, brownish products. The
crude product was purified by re-crystallization from ethanol–
water (volume ratio 4/1) 5 times to obtain a white solid (1.04 g).
Final yield: 36%. 1H NMR (D2O, 200MHz): d (ppm): 7.45 (d, 1
H), 7.2 (d, 2 H), 6.9 (d, 2 H), 6.3 (d, 1 H), 4.0 (t, 2 H), 3.5 (t, 2 H),
1.6–1.7 (m, 2 H), 1.3–1.5 (m, 6 H). Elemental analysis, calculated:
C: 62.93%, H: 6.64%; found: C: 62.75%, H: 6.37%.
in the range of 4000 to 650 cmꢀ1
.
X-ray diffraction (XRD). Self-supported cast films for the
XRD studies were prepared by dispersing the sample solutions in
precleaned glass slides. Then the slides loaded with sample
solutions were dried over silica gel overnight to obtain a film of
the self-assembled structures. Reflection XRD studies were
carried out for the film with a model XKS-2000 X-ray diffrac-
tometer (Scintaginc). The X-ray beam was generated with a Cu
ꢀ
anode, and the wavelength of the Ka1 beam was 1.5406 A. The
X-ray beam was directed toward the film edge, and the scanning
was done up to a 2q value of 10ꢁ.
Zeta potential measurement. Zeta potentials were measured
using
a temperature-controlled ZetaPALS Zeta Potential
Methods
Analyzer (Brookhaven Instruments Corporation).
Preparation of samples. The solutions were obtained by dis-
solving the solid SHHC powder in 0.01 mol Lꢀ1 borax buffer
solution (pH 9.2) and ultrapure water (Milli-Q, 18.2 MU cm).
Upon addition of NaOH aqueous solution, the pH of SHHC in
ultrapure water was adjusted to 12.0 using a SevenMulti type pH
meter with InLab Semi-Micro electrodes (Mettler Toledo,
Switzerland). These samples were vortex mixed and allowed to
equilibrate for at least one week at 25 ꢁC. All analytical
Results and discussion
TEM observation
10 mM SHHC is insoluble in water at room temperature. Upon
heating, the majority of the solids can be solublized, but some
ꢁ
measurements were performed at 25 C.
Dynamic light scattering measurements. Light scattering
measurements were performed with a Brookheven light scat-
tering-apparatus, equipped with a 400 mW argon ion laser
operating at a wavelength of 532 nm. A refractive index matching
bath of filtered cis-decalin surrounded the cylindrical scattering
cell, and the temperature was controlled at 25 ꢂ 0.5 ꢁC using
a Haake C35 thermostat.
Fig. 1 The hydrodynamic radii distribution of the self-assembled
structures in 0.01 mol Lꢀ1 SHHC solution measured at pH 9.2, 25 ꢁC, in
the presence of 10 mM NaCl.
Scheme 1 The synthetic pathway to the asymmetrical bolaamphiphile
SHHC.
This journal is ª The Royal Society of Chemistry 2010
Soft Matter, 2010, 6, 3282–3288 | 3283

View Article Online
insoluble solids are still seen on the bottom of the test tube.
However, at pH above 9, all the solids disappear and the system
becomes a bluish, homogeneous phase at room temperature.
Dynamic light scattering experiments indicated the presence of
polydispersed large particles. As demonstrated in Fig. 1, the
average hydrodynamic radius increases from 64 nm to 485 nm as
the scattering angle changed from 90 to 45ꢁ, suggesting tube or
rod like structures were formed.
Scheme 2 The photoreactions of the cinnamoyl moieties.
The TEM micrographs revealed (Fig. 2a) formation of a large
amount of tubular structures in this 10 mM SHHC solution.
Their hollow nature can be clearly visulized in Fig. 2b. In fact,
some separate vesicles were observed along with these tubes (See
Fig. S1 in the ESI†). In addition, the wall thickness of the tubes,
which can be obtained from Fig. 2b, is 3.7 nm. This value is
about two times the stretched length of the SHHC molecule,
which indicates that the walls of the tubes are formed by a bilayer
of SHHC molecules. This will be discussed later, in combination
with XRD results. Such tubular structures can be well repeated
and observed easily in a full view of the microscope (See Fig. S1–
2 in the ESI†). The tubes run several micrometres in length and
50–100 nm in diameter. Most tubes are closed at both ends, but
in some cases open ends are observed as well. It seems that the
curvature of the tubes is not homogenous. It was found that in
some places the tubes are squeezed into vesicular structures. The
heterogeneous curvature endows the tubes with good flexibility.
These observations suggest that the unsymmetrical bolaamphi-
philic SHHC is able to self-assemble into membrane structures in
aqueous solution.
Fig. 3 UV absorption spectra with different irradiation times of
0.01 mol Lꢀ1 SHHC solution at pH 9.2.
ꢀ
reactive C]C bonds are within a distance of 3.6–4.1 A. The UV
Photosensitive self-assembly of SHHC
absorption spectra of the SHHC solution at various exposure
times are shown in Fig. 3. Before UV irradiation, the SHHC
solution exhibits an absorption maximum at 287 nm, which is
ascribed to the absorption of cinnamoyl groups in the form of
parallel packing.14 The maximum absorption of noninteracting
cinnamoyl groups in solution is usually observed above 310 nm;
it is the occurrence of p–p stacking in the parallel molecular
array that leads to the blue shift of the absorbance band.15 This
UV result suggests that SHHC molecules indeed adopt parallel
packing in the membrane. After irradiation, the intensity of the
maximum at 287 nm decreases, meanwhile, the position of the
maximum absorbance blue shifts to 270 nm. These changes can
Next, we were interested in examining the molecular packing
mode, namely, parallel or antiparallel orientation of the SHHC
molecules in the membrane. To achieve this, the solution with
tubular structures was exposed to UV irradiation. In principle,
two photoreaction pathways are possible for the cinnamoyl
units:34 (i) trans–cis photoisomerization, and (ii) photo-
cyclization (so-called photodimerization), as illustrated in
Scheme 2.
Cis–trans isomerization occurs in all cases, whereas photo-
dimerization only occurs in molecular assemblies where the two
Fig. 2 TEM images of the aggregates formed in 0.01 mol Lꢀ1 SHHC
solution at pH 9.2 before UV irradiation. (a) Represents a general view,
whereas (b) demonstrates the clearer tubular structure with a wall
thickness of 3.7 nm.
Fig. 4 IR spectra of 0.01 mol Lꢀ1 SHHC solution before and after UV
irradiation at pH 9.2.
3284 | Soft Matter, 2010, 6, 3282–3288
This journal is ª The Royal Society of Chemistry 2010

View Article Online
be attributed to a loss of conjugation in cinnamoyl chromo-
phores, which is due to the occurrence of photoreactions of the
cinnamoyl groups in the tubular membrane of SHHC.16 No
noticeable changes were observed upon further prolongation of
the irradiation time to more than 4 h, suggesting completion of
the photo reaction in the system within 4 h.
Molecular packing in the membrane
The occurrence of photodimerization suggests that the cinna-
myol moieties of SHHC molecules are in a parallel mode.
However, at this stage, we cannot tell exactly which molecular
packing mode is present from those illustrated in Scheme 3. In
order to find out the actual molecular arrangement in the
membrane of the SHHC molecules, XRD experiments, which
can obtain the membrane-stacking periodicity (d), are required.
It has been proved that the membrane structures can be well-
preserved in casting films.16,18,46 Therefore, the cast films of the
SHHC solution was prepared and measured by XRD. Fig. 5
shows the XRD patterns of tubes dried on glass. The small-angle
region shows the strong (001), (002), and (004) reflection peaks.
The (003) reflection is very weak so that it is only discernible at
high magnifications. These periodic diffraction peaks suggest the
existence of well-defined lamellar structures.16,18,46 According to
the Bragg equation, the layer spacing is 3.6 nm, which agrees very
well with the TEM result given in Fig. 2b. Since the stretched
molecular length of SHHC is 1.8 nm (modelled by Chem 3D),
this result indicates that double molecular layers are indeed
formed in the membrane, in which the SHHC molecules take an
upright, fully stretched conformation. In this regard, only two
packing modes, namely Scheme 3a and b, are possible. Other
modes in Scheme 3 give mono molecular layers.
The photodimerization was further confirmed by using IR
spectroscopy. As seen in Fig. 4, striking changes of the vibration
of carbonyl groups were observed. Before irradiation, the
conjugated carbonyl groups in the form of sodium salt
(–COONa) have
a strong stretching vibration band at
1551 cmꢀ1 35
.
Upon UV light irradiation, this band shifts from
1551 to 1578 cmꢀ1, indicating the loss of conjugation of the
carboxyl group. Moreover, the intensities of the vinylene C]C
stretching vibration band at 1636 cmꢀ1 is significantly lowered,
and the ]C–H stretching and deformation vibration bands at
3040 cmꢀ1 and 970 cmꢀ1 disappear after 4 h irradiation.36–42
These are powerful proofs for the loss of the vinylene C]C bond
due to photodimerization. Meanwhile, the appearance of the
stretching and bending vibration of saturated C–H of the
cyclobutane at 2899 and 1343 cmꢀ1 37
respectively, after UV
,
irradiation, unambiguously confirms the occurrence of the pho-
todimerization in the tubular membranes formed by SHHC
molecules.
It is noteworthy that a weak vinylene C]C stretching vibra-
tion at 1638 cmꢀ1 still exists after 4 h irradiation. This suggests
that the SHHC molecules are not completely dimerized. Photo-
dimerization can hardly be 100% in all of the cinnamate deriv-
ative membranes because the single molecules left among
dimerized ones have no chance to be dimerized any more. In our
case, the amphiphilic nature of the SHHC molecules allows
equilibrium between the separate SHHC molecules and those
self-assembled ones. Therefore, both the separated SHHC
molecules and those left in the membranes after photo-
dimerization should be responsible for the uncompleted dimer-
ization. Upon analysis of the integral area of the C]C vibration
band using the CH2 symmetrical stretching vibration band at
2863 cmꢀ1 as a reference (which doesn’t change upon UV irra-
diation), the degree of photodimerization is about 58%.36 This
dimerization yield is noticeably high compared with that of other
cinnamate acid derivatives in similar dilute solutions. For
instance, one of the highest photodimerization efficiencies
reported by Bassani and co-workers43 for cinnamates in dilute
solution is about 45%. The high efficiency of the photo-
dimerization indicates that the array of SHHC molecules in the
membrane is highly ordered.
Although both the packing modes in Scheme 3a and b are
parallel packing, it is noteworthy that they are with different
outmost groups. This allows us to differentiate these two packing
modes by examining their surface electrical properties. There-
fore, zeta potential measurements were carried out. The obtained
zeta potential for the SHHC tubes at pH 9.2 is ꢀ50 mV, sug-
gesting the outer leaflet of the tubes are composed of carboxyl
groups. On the basis of the above experimental results, we can
conclude that the SHHC molecules adopt mode Scheme 3a in the
membrane, i.e., parallel double molecular layers with the bulky
cinnamoyl group towards the outside. Of course, we cannot
exclude the possibility of a small fraction of SHHC molecules in
the mode of 3b coexisting with 3a, but, in general, the bulky
groups tend to be predominant in the outer leaflet.16
PH responsive self-assembly of SHHC
The well-orientated molecular arrangement in the membrane
allows for tailoring the self-assembled structures by pH
It should be pointed out that the photoreactions signifi-
cantly affected the conformation of the SHHC molecules. This
is evidenced by the variation of the stretching vibration band
of the hydrogen bonds. Before UV irradiation, the hydrogen
bonds exhibit a strong, broad band at 3346 cmꢀ1, indicating
the presence of hydrogen bonds with an average bond length
and angle; after UV irradiation, this band splits into three
groups,44,45 suggesting formation of three categories of OH
groups due to the rearrangement of the molecular orientation
after photoreaction. This change of the molecular conforma-
tion disfavors the self-assembly of SHHC molecules, which
results in the break of the tubular structures (see Fig. S3 in the
ESI†).
Scheme 3 Possible molecular packing mode of SHHC molecules in the
membrane. The large solid sphere (C) represents the carbonate group,
whereas the small one (ꢃ) represents the hydroxyl group.
This journal is ª The Royal Society of Chemistry 2010
Soft Matter, 2010, 6, 3282–3288 | 3285

View Article Online
Fig. 5 XRD patterns of 0.01 mol Lꢀ1 SHHC solution at pH ¼ 9.2.
Fig. 7 The hydrodynamic radii distribution of the self-assembled
structures in 0.01 mol Lꢀ1 SHHC solution measured at pH 12.0, 25 ꢁC.
variations. As the pH of the system is increased to 12.0,
predominant vesicular aggregates with diameters of 20–200 nm,
together with some remaining tubes, were observed (Fig. 6a).
Meanwhile, dynamic light scattering results (Fig. 7) revealed
the presence of smaller structures than those in the pH
9.2 systems. These structures are less angular dependent when
compared with those in the pH 9.2 system, in line with the
formation of predominant vesicles. The smaller angular depen-
dence in this case may reflect the polydispersity of the vesicles
and the remaining small fraction of tubular structures, as
revealed in Fig. 6a.
increases when P decreases. This means that as the a value in our
case increases with increasing pH, P is decreased, which favors
formation of high-curvatured self-assemblies. As we have dis-
cussed in previous work, vesicles have a smaller packing
parameter than tubes.48 That is why transition from tubes to
vesicles occurs with increasing pH in the solution of SHHC.
As the pH rose to 12, the degree of photodimerization of
SHHC was found to be 44%, lower than that at pH 9.2 (Fig. 8),
which is in line with the parallel packing fashion of SHHC
molecules. Because the repulsion between the head groups
increases with increasing pH, it is more difficult for SHHC
molecules to pack closely for the occurrence of dimerization.
Again, the dimerization disfavors the presence of vesicles, but the
lower efficiency of photodimerization allows some vesicles to
remain even after 4 h photoreaction (Fig. 6b).
The transition from tubes to vesicles with increasing pH is due
to the increased repulsion between the cinnamate groups, which
brought up the average head group area of the surfactants. Since
the pKb of SHHC is 5.4 (see Fig. S4 in the ESI†), the amount of
cinnamate in the pH 12.0 system is about 1.20 times that in the
pH 9.2 solution. That is, at pH 9.2, about 15% of the SHHC
molecules are in the form of cinnamic acid, which is much more
hydrophobic than those in the cinnamate form. Because the
SHHC molecules in the acid form will be greatly enriched in
the membranes, the fraction of SHHC in the cinnamate form in
the membrane is less than 85%. Therefore, the fraction of SHHC
in the cinnamate form in the pH 9.2 system is much smaller than
that in the pH 12.0 system. This means that there will be
considerable extra repulsion as the pH is increased from 9.2 to
12.0. According to Israelachvili,47 the critical packing parameter
(P) is defined as P ¼ v/al, where v is the volume of the hydro-
phobic tail of the surfactant molecule, a is the head group area at
the aggregate/water interface, and l is the length of the hydro-
phobic chain. The curvature of the molecular assemblies
Discussion
The driving force behind the parallel packing of SHHC mole-
cules is p–p stacking rather than hydrophobic interactions. The
IR spectra suggest that the CH2 groups are not densely packed,
because the nas and ns appear at 2934 and 2863 cmꢀ1 (Table 1),
respectively, before UV irradiation, which are in contrast with
typical all-trans zig-zag conformations of methylene chains
where the two vibrations occur at 2918 and 2847 cmꢀ1 49–51
.
Therefore, p–p stacking is the dominant driving force for the
formation of the membrane of SHHC. The strength of the p–p
stacking interaction between cinnamoyl groups hasn’t been
clearly elucidated in previous work, although it is well-known
Fig. 6 TEM images of the aggregates formed in 0.01 mol Lꢀ1 SHHC
Fig. 8 IR spectra of 0.01 mol Lꢀ1 SHHC solution before and after UV
solution at pH 12.0 before (a) and after (b) 4 h UV irradiation.
irradiation at different pH. (a) pH 9.2; (b) pH 12.0.
3286 | Soft Matter, 2010, 6, 3282–3288
This journal is ª The Royal Society of Chemistry 2010

View Article Online
Table 1 Characteristic infrared bands of the photosensitive SHHC
molecules before and after UV irradiation
hydrophobic interactions. Therefore, our results demonstrate
that the interaction between the cinnamoyl groups is so strong
that it even overcomes the electrostatic repulsion between the
COOꢀ groups and drives the formation of well-oriented molec-
ular assembly (Scheme 4). So, the occurrence of the photo-
dimerization of cinnamoyl groups in any cinnamate acid
derivatives is not only a regional confinement effect, but rather
a kind of self-attraction behavior. To our knowledge, this is the
first time a strong interaction between cinnamoyl groups has
been found.
Frequency/cmꢀ1
Before
After
Assignment and description
3040
2934
disappearance
2940
]CH– stretching vibration
CH2 asymmetric stretching
vibration
2863
None
2863
2899
CH2 symmetric stretching vibration
saturated C–H of the cyclobutane
stretching vibration
As a result of parallel packing of the SHHC molecules, the OH
groups of the molecules are allowed to locate on the other side of
the mono-molecular membrane. However, intramembrane
hydrogen bonding is not favored due to the loose packing of the
alkyl chain. As a result, intermembrane hydrogen bonding
occurs. Therefore, a double layer membrane is formed (Scheme
4). Such hydrogen bonding is greatly affected by the photo-
dimerization because of the change of the bond angle. This
inspires us to investigate whether, if groups other than OH are
used as the smaller head of a bolaamphiphile (when use sodium
cinnamate as the bulky head), asymmetrical membranes with
parallel molecular arrangements can be easily fabricated. Work
on this aspect is ongoing in our lab.
1636
1609
1638
1607
vinylene C]C stretching vibration
C]C stretching vibration in
benzene ring
nonconjugated C]O in –COONa
stretching vibration
conjugated C]O in –COONa
stretching vibration
C]O stretching vibration in
benzene ring
CH2 bending vibration
saturated C–H of the cyclobutane
bending vibration
C–O–C asymmetric stretching
vibration
C–O–C symmetric stretching
vibration
trans-vinylene C–H out-of-plane
deformation
None
1551
1515
1578
disappearance
1513
1475
None
1473
1343
1262
1024
970
1259
1029
disappearance
Conclusion
We have realized the unprecedented parallel packing of a simple
symmetrical bolaamphiphile that contains a cinnamoyl group
through delicate molecular design. The p–p stacking interaction
between cinnamoyl groups were found to be very strong and
drove the parallel packing of the SHHC molecules. The p–p
stacking interaction is so strong that it can even overcome
unfavorable electrostatic repulsion between the COOꢀ groups.
Our results demonstrate that by introduction of strong interac-
tions in the vicinity of one head of bolaamphiphiles, unsym-
metrical membranes can be readily fabricated.
that these compounds undergo photodimerization in crystalline
states and in ordered molecular assemblies. Previous reports all
attribute the occurrence of photodimerization to the parallel
packing of the cinnamoyl groups in the ordered molecular
assembles. In those works, the cinnamate acid derivatives are
usually ordinary amphiphiles with long alkyl chains (n > 10),
where the strong hydrophobic interaction between the alkyl
chains automatically drives the formation of molecular assem-
blies. Therefore, they can only adopt parallel packing of the
cinnamoyl groups in the self-assembled structures. In those cases,
no information about the ‘self-assembly’ of the cinnamoyl
groups is noticed, and the occurrence of the photodimerization is
considered as a regional confinement effect.31,52,53 However, the
SHHC molecule was designed in such a way that two orienta-
tions of the molecules in a membrane are possible; the alkyl chain
between the bulky cinnamate group and the small hydroxyl
group is short so that it does not generate appreciable
Acknowledgements
This research was supported by National Natural Science
Foundation of China (20903005, 20873001, 20633010 and
50821061), and the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry
(SRF for ROCS, SEM).
Notes and references
1 J. Sirieix, N. Lauth-de Viguerie, M. Riviere and A. Lattes, New
J. Chem., 2000, 24, 1043–1048.
2 Y. Yan, J. B. Huang, Z. C. Li, F. Han and J. M. Ma, Langmuir, 2003,
19, 972–974.
3 T. Lu, F. Han, Z. C. Li, J. B. Huang and H. L. Fu, Langmuir, 2006,
22, 2045–2049.
4 M. Roussel, V. Lognone, D. Plusquellec and T. Benvegnu, Chem.
Commun., 2006, 3622–3624.
5 Y. Yan, J. B. Huang, Z. C. Li, X. L. Zhao, B. Y. Zhu and J. M. Ma,
Colloids Surf., A, 2003, 215, 263–275.
6 B. Song, Z. Q. Wang, S. L. Chen, X. Zhang, Y. Fu, M. Smet and
W. Dehaen, Angew. Chem., Int. Ed., 2005, 44, 4731–4735.
7 J. H. Fuhrhop, H. Hungerbuhler and U. Siggel, Langmuir, 1990, 6,
1295–1300.
8 J. Guilbot, T. Benvegnu, N. Legros, D. Plusquellec, J. C. Dedieu and
A. Gulik, Langmuir, 2001, 17, 613–618.
Scheme 4 An illustration of the self-assembly and molecular packing
mode of SHHC molecules in the membrane.
This journal is ª The Royal Society of Chemistry 2010
Soft Matter, 2010, 6, 3282–3288 | 3287

View Article Online
9 M. Berchel, C. Meriadec, L. Lemiegre, F. Artzner, J. Jeftic and
T. Benvegnu, J. Phys. Chem. B, 2009, 113, 15433–15444.
10 S. Q. Zhou, C. Xu, J. Wang, W. Gao, R. Akhverdiyeva, V. Shah and
R. Gross, Langmuir, 2004, 20, 7926–7932.
32 L. P. Xu, C. J. Yan, L. J. Wan, S. G. Jiang and M. H. Liu, J. Phys.
Chem. B, 2005, 109, 14773–14778.
33 J. M. Chong, M. A. Heuft and P. Rabbat, J. Org. Chem., 2000, 65,
5837–5838.
11 R. C. Claussen, B. M. Rabatic and S. I. Stupp, J. Am. Chem. Soc.,
2003, 125, 12680–12681.
34 S. Karthikeyan and V. Ramamurthy, J. Org. Chem., 2007, 72, 452–
458.
12 K. Kohler, G. Forster, A. Hauser, B. Dobner, U. F. Heiser, F. Ziethe,
W. Richter, F. Steiniger, M. Drechsler, H. Stettin and A. Blume,
J. Am. Chem. Soc., 2004, 126, 16804–16813.
13 R. Iwaura and T. Shimizu, Angew. Chem., Int. Ed., 2006, 45, 4601–
4604.
35 S. Ida, T. Tsubota, S. Tanii, M. Nagata and Y. Matsumoto,
Langmuir, 2003, 19, 9693–9698.
36 S. Perny, P. Le Barny, J. Delaire, T. Buffeteau and C. Sourisseau, Liq.
Cryst., 2000, 27, 341–348.
37 S. D. M. Allen, M. J. Almond, J. L. Bruneel, A. Gilbert, P. Hollins
and J. Mascetti, Spectrochim. Acta, Part A, 2000, 56, 2423–2430.
38 B. Chae, S. W. Lee, M. Ree, Y. M. Jung and S. B. Kim, Langmuir,
2003, 19, 687–695.
39 S. W. Lee, S. I. Kim, B. Lee, W. Y. Choi, B. Chae, S. B. Kim and
M. Ree, Macromolecules, 2003, 36, 6527–6536.
14 J. H. Fuhrhop, D. Spiroski and C. Boettcher, J. Am. Chem. Soc.,
1993, 115, 1600–1601.
15 M. Masuda and T. Shimizu, Langmuir, 2004, 20, 5969–5977.
16 N. Kameta, M. Masuda, H. Minamikawa, N. V. Goutev, J. A. Rim,
J. H. Jung and T. Shimizu, Adv. Mater., 2005, 17, 2732.
17 M. Ambrosi, E. Fratini, V. Alfredsson, B. W. Ninham, R. Giorgi, P. Lo
Nostro and P. Baglioni, J. Am. Chem. Soc., 2006, 128, 7209–7214.
18 N. Kameta, M. Masuda, H. Minamikawa and T. Shimizu, Langmuir,
2007, 23, 4634–4641.
40 Y. Yan, S. Guo, W. Xiong, J. B. Huang, Z. C. Li and J. M. Ma, Acta
Phys.-Chim. Sin., 2005, 21, 550–555.
41 F. F. Wang, F. He, Z. Q. Xie, Y. P. Li, M. Hanif, M. Li and Y. G. Ma,
Macromol. Chem. Phys., 2008, 209, 1381–1388.
19 N. Kameta, K. Yoshida, M. Masuda and T. Shimizu, Chem. Mater.,
2009, 21, 5892–5898.
42 H. Kihara and N. Tamaoki, Macromol. Chem. Phys., 2008, 209,
1424–1431.
20 A. H. Fuhrhop and T. Y. Wang, Chem. Rev., 2004, 104, 2901–2937.
21 J. H. Fuhrhop and D. Fritsch, Acc. Chem. Res., 1986, 19, 130–137.
22 K. N. Liang and Y. Z. Hui, J. Am. Chem. Soc., 1992, 114, 6588–6590.
23 J. Schneider, C. Messerschmidt, A. Schulz, M. Gnade, B. Schade,
P. Luger, P. Bombicz, V. Hubert and J. H. Fuhrhop, Langmuir,
2000, 16, 8575–8584.
24 J. Song, Q. Cheng, S. Kopta and R. C. Stevens, J. Am. Chem. Soc.,
2001, 123, 3205–3213.
25 S. J. George, T. F. A. de Greef, R. Bovee, J. L. J. van Dongen,
A. P. H. J. Schenning and E. W. Meijer, Chem.–Asian J., 2009, 4,
910–917.
43 V. Darcos, K. Griffith, X. Sallenave, J. P. Desvergne, C. Guyard-
Duhayon, B. Hasenknopf and D. M. Bassani, Photochem.
Photobiol. Sci., 2003, 2, 1152–1161.
44 M. J. Liu, P. Y. Wu, Y. F. Ding, G. Chen and S. J. Li,
Macromolecules, 2002, 35, 5500–5507.
45 L. Rintoul, E. Wentrup-Byrne, S. Suzuki and L. Grondahl, J. Mater.
Sci.: Mater. Med., 2007, 18, 1701–1709.
46 H. Yui, H. Minamikawa, R. Danev, K. Nagayama, S. Kamiya and
T. Shimizu, Langmuir, 2008, 24, 709–713.
47 J. N. Israelachvili, D. J. Mitchell and B. W. Ninhem, J. Chem. Soc.,
Faraday Trans. 2, 1976, 72, 1525–1538.
26 M. D. Cohen, G. M. J. Schmidt and F. I. Sonntag, J. Chem. Soc.,
1964, 2000.
48 Y. Yan, W. Xiong, X. S. Li, T. Lu, J. B. Huang, Z. C. Li and H. L. Fu,
J. Phys. Chem. B, 2007, 111, 2225–2230.
27 J. Bolt, F. H. Quina and D. G. Whitten, Tetrahedron Lett., 1976, 17,
2595–2598.
49 Y. Yan, J. B. Huang, Z. C. Li, J. M. Ma, H. L. Fu and J. P. Ye,
J. Phys. Chem. B, 2003, 107, 1479–1482.
28 G. Wegner, Pure Appl. Chem., 1977, 49, 443–454.
29 M. Yamamoto, N. Furuyama and K. Itoh, J. Phys. Chem., 1996, 100,
18483–18490.
30 T. Nakamura, K. Takagi, M. Itoh, K. Fujita, H. Katsu, T. Imae and
Y. Sawaki, J. Chem. Soc., Perkin Trans. 2, 1997, 2751–2755.
31 Q. Xia, X. S. Feng, J. Mu and K. Z. Yang, Langmuir, 1998, 14, 3333–
3338.
50 T. P. Ang, T. S. A. Wee and W. S. Chin, J. Phys. Chem. B, 2004, 108,
11001–11010.
51 P. Gao, C. L. Zhan and M. H. Liu, Langmuir, 2006, 22, 775–779.
52 M. Sugimoto, K. Shibahara, K. Kuroda, T. Hirao, H. Kurosawa and
I. Ikeda, Langmuir, 1996, 12, 2785–2790.
53 H. M. Fan, X. M. Zhu, L. N. Gao, Z. C. Li and J. B. Huang, J. Phys.
Chem. B, 2008, 112, 10165–10170.
3288 | Soft Matter, 2010, 6, 3282–3288
This journal is ª The Royal Society of Chemistry 2010