Acid-responsive organogel mediated by arene-perfluoroarene and hydrogen bonding interactions

Article abstract of DOI:10.1039/c2sm07281e

A phenyl and a 2,3,5,6-tetrafluorophenyl ring, each bearing a tris(n-dodecyloxy)benzylamine moiety via an amide bond, were tethered together through an imine linkage to give a non-covalent synthon (imine 1) with a strong capacity of supramolecular self-as

Full text of DOI:10.1039/c2sm07281e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Soft Matter
Cite this: Soft Matter, 2012, 8, 5486
www.rsc.org/softmatter
PAPER
Acid-responsive organogel mediated by arene–perfluoroarene and hydrogen
bonding interactions†
*
Huixian Wu, Ben-Bo Ni, Chong Wang, Feng Zhai and Yuguo Ma
Received 1st December 2011, Accepted 14th March 2012
DOI: 10.1039/c2sm07281e
A phenyl and a 2,3,5,6-tetrafluorophenyl ring, each bearing a tris(n-dodecyloxy)benzylamine moiety via
an amide bond, were tethered together through an imine linkage to give a non-covalent synthon (imine
1) with a strong capacity of supramolecular self-assembly. Gelation was observed in several organic
solutions, within which fibrous aggregate morphologies were visualized by SEM and AFM. Both
arene–perfluoroarene stacking and amide–amide hydrogen bonding interactions were responsible for
1
such self-assembly behaviours, as evidenced by H NMR studies. Hydrolysis of the imine linkage
catalyzed by acid strongly weakened the intermolecular interactions, resulting in dissociation of the low
molecular weight gelator and giving rise to an acid-mediated gel–sol transition.
(m.p. ¼ 25 ꢀC) than either of the components.19 In contrast to the
Introduction
edge-to-face molecular packing fashion adopted by pure benzene
or hexafluorobenzene in the solid state, an alternating face-to-
face stacking arrangement was observed for these two molecules
in their co-crystals.20 The face-to-face stacking motif is stabilized
by both electrostatic21 and van der Waals interactions between
the two different phenyl rings. Specifically, the former is
a quadrupolar interaction, which originated from opposite
charge distributions in the aromatic planar structure of benzene
and hexafluorobenzene.20 A similar alternating stacking motif
has also been observed for complexes of aromatic and per-
fluorinated aromatic compounds.22–33 Because of this interesting
molecular stacking fashion, extensive attention has been paid to
the application of arene–perfluoroarene interactions in the field
of crystal engineering in past decades to generate unique crystal
structures,22–26 for instance, with molecular arrangements
feasible for topochemical reactions (e.g. butadiyne polymeriza-
tions,27 olefin photodimerization and 1,3-diene photo-
polymerization,28 1,3-dipolar cycloaddition between azides and
alkynes,29 etc.). It has also been used to achieve specific liquid
crystalline (LC) behaviours of materials.30–33 However, when
stepping beyond the field of crystal engineering, for instance, in
a solution self-assembly system, examples that harness this
interaction to construct superstructures are relatively rare, and
only a few precedents of bio-related molecules are reported.34,35
One possible reason for why this interaction is rarely used to
modulate molecular assembly in the solution state is that its
strength is relatively low, which is usually not enough to
outweigh the counteracting entropic effect caused by solvation.36
In the limiting examples where arene–perfluoroarene interactions
were still effective in dilute solutions, aromatic and per-
fluorinated aromatic moieties were prearranged carefully into
peptides34 and oligonucleotides,35 with the assistance of back-
bone hydrogen bonding and/or dipole–dipole interactions, the
The development of low molecular weight gelators (LMWGs)
has been attracting great interest in the past 10 years.1–15 As an
example of ‘‘soft matter’’, an organogel based on molecular
assembly exhibits numerous appealing properties: well-defined
nanoscale one-dimensional molecular alignment, ease of fabri-
cation, thermo-reversibility, and external physical/chemical
stimuli-responsive ability, etc. These features promise organogels
with various potential applications in areas such as drug
delivery,1,2 sensors,3–12 light-harvesting systems,13 electronic
devices,14,15 molecular logic gates,16 and templates.17 Gelation of
small molecules is speculated to rely on the chemical and/or
physical cross-linking of supramolecular polymer chains into
three-dimensional networks, which can immobilize solvent
molecules.18 Normally, their aggregation into fibrous super-
structures is driven by multiple weak noncovalent interactions,
such as hydrogen bonding, van der Waals, electrostatic and
solvophobic interactions. Although various LMWGs have been
reported to date, to our best knowledge, no previous examples
have been reported where arene–perfluoroarene interactions
were incorporated for organogelator design.
Arene–perfluoroarene interaction has been extensively
studied, since Patrick and Prosser first reported a 1 : 1 mixture of
benzene (m.p. ¼ 5 ꢀC) and hexafluorobenzene (m.p. ¼ 5 ꢀC)
gave rise to a co-crystal with a much higher melting point
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Lab
of Polymer Chemistry & Physics of Ministry of Education, College of
Chemistry, Peking University, Beijing, 100871, China. E-mail: ygma@
pku.edu.cn; Fax: (+86) 10-6275-6660; Tel: (+86) 10-6275-6660
† Electronic
supplementary
information
(ESI)
available:
Corey–Pauling–Koltun (CPK) modeling of imine 1; pictures of the gel
in testing organic solvents; 1H NMR monitoring the result of the
hydrolysis reaction; SEM images of the 1 : 1 mixture of 3 and 4; and
NMR spectra of new compounds. See DOI: 10.1039/c2sm07281e
5486 | Soft Matter, 2012, 8, 5486–5492
This journal is ª The Royal Society of Chemistry 2012

View Article Online
arene–perfluoroarene interaction was able to modulate the
stability and the arrangement of these biologically important
structures.
accomplished by refluxing the reactants in toluene with
p-MeC6H4SO3H as the catalyst to give imine 1 as a glassy solid
with a yield of 30%.
We are particularly interested in introducing arene–per-
fluoroarene interactions into solution self-assembly systems. To
this end, we attached a tris(n-dodecyloxy)benzylamine segment,
which is a promising gel-formation unit, to both ends of a Schiff
base consisting of phenyl and 2,3,5,6-tetrafluorophenyl rings,
through an amide group. Previously, we have shown that
molecules containing a similar Schiff base structure would pack
into columns where phenylene–tetrafluorophenylene alternating
face-to-face stacking was observed in the crystalline state.29 The
amide group is used here to further stabilize the face-to-face
packing of the Schiff base structure in solution by hydrogen
bonding,24 while the tetrafluorinated aniline based imine is highly
acid sensitive. Herein, we present self-assembly behaviours of
this carefully designed molecule, named imine 1, as well as its
gelation behaviours and gel–sol transition under catalytic
amounts of acid.
Self-association of imine 1 in solution
1
The H NMR spectroscopy not only helped confirm the molec-
ular structure, but also provided insight into the molecular self-
1
assembly in solution. A set of H NMR spectra of imine 1 in
CDCl3 was collected at varied concentrations, ranging from
2.0 ꢁ 10ꢂ3 mol Lꢂ1 to 2.7 ꢁ 10ꢂ2 mol Lꢂ1 (Fig. 1). The concen-
tration-dependence of the chemical shift revealed that this
molecule underwent self-association in solution. Specifically, the
increase of concentration caused an upfield shift for nuclei
resonances of the aromatic and imine protons (in the range of 7.7
to 8.6 ppm), suggesting face-to-face stacking occurred between
the Schiff base structures.39 Meanwhile, a downfield shift was
observed for the resonance of amide NH protons (with chemical
shifts of 6.2–6.9 ppm), serving as evidence for intermolecular
hydrogen bonding interactions between amide groups. Overall,
these data suggested that both the arene–perfluoroarene stacking
among the Schiff base moieties and the hydrogen bonding
interaction among amide groups were both contributing to the
self-assembly of imine 1 in solution.
Results and discussion
Synthesis
The synthesis of imine 1 is shown in Scheme 1. The preparation
3,4,5-tridodecyloxybenzylamine37
fluorobenzoic acid38 were according to literature procedures. A
Subsequently, the gelation capability of imine 1 was investi-
gated. This molecule was speculated to be an organogelator as it
has structural features very favourable for gelation: (a) arene–
perfluoroarene interaction is capable of inducing infinite
supramolecular polymerization, (b) 3,4,5-tris(n-dodecyloxy)
benzylamine units can further stabilize the fibrous aggregates by
amide–amide hydrogen bonding and enable the formation of
three-dimensional networks via van de Waals interaction
between the long dodecyloxy sidechains. As expected, thixo-
tropic gels were observed at room temperature in a variety of
organic solutions of imine 1, including n-alkanes, ethyl acetate,
1,4-dioxane, and some combinations of solvents such as ethyl
acetate–n-alkanes and 1,4-dioxane–n-alkanes. Gelation proper-
ties and respective minimum gelation concentrations are
summarized in Table 1. It was noted that in aromatic solvents,
such as toluene and benzene, gelation process were not observed.
Given that hydrogen bonding interaction should not be severely
affected by these aromatic solvents, we reasonably suggested that
of
and
4-azidotetra-
condensation reaction between amine 3 and aldehyde 4 was
Scheme 1 Synthesis of imine 1. (a) (COCl)2, CH2Cl2, r. t., 24 h. (b) Et3N,
CH2Cl2, 0 ꢀC, 20 min. (c) SnCl2$2H2O, CH2Cl2/MeOH (1 : 1, v : v), r. t.,
overnight. (d) p-MeC6H4SO3H, toluene, reflux, overnight.
Fig. 1 1H NMR spectra of imine 1 in CDCl3 at (a) 2.0 ꢁ 10ꢂ3 mol Lꢂ1
;
(b) 3.2 ꢁ 10ꢂ3 mol Lꢂ1; (c) 5.9 ꢁ 10ꢂ3 mol Lꢂ1; (d) 1.4 ꢁ 10ꢂ2 mol Lꢂ1; (e)
2.7 ꢁ 10ꢂ2 mol Lꢂ1
.
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 5486–5492 | 5487

View Article Online
Table 1 Results of gelation tests in different organic solvents
conditions for DTS test, which were carried out as a function
of time. Within the investigated regime, storage moduli G0
(ꢃ105 Pa) were found to be an order of magnitude larger than
respective loss moduli G00 (ꢃ104 Pa), and both G0 and G00
remained constant over 3000 s, implying that the gel was quite
rigid and stable.
Solvent
Imine 1
3 and 4 (1 : 1e)
n-Dodecane
n-Hexane
Ethyl acetate–n-hexane (1 : 3, v : v)
1,4-Dioxane–n-dodecane (1 : 3, v : v)
1,4-Dioxane
Ethyl acetate
Benzene
Toluene
TGa(1.8b)
TG(0.6)
TG(1.3)
OGc(3.7)
OG(5.5)
OG(2.5)
Sd
S
S
S
S
S
S
S
S
The morphology of the gel was examined by scanning electron
microscopy (SEM). Fig. 3 shows the representative SEM image
of the xerogel obtained after solvent evaporation. As expected,
a three-dimensional network with an extended fibrous structure
was observed. The width of individual fibers ranged from 40 nm
to 200 nm, suggesting that the individual fibers were likely to be
composed of bundles of stacking arrays of imine molecules.
Additionally, aggregates of imine 1 were visualized by atomic
force microscopy (AFM) in tapping mode (Fig. 4). A dilute
solution of imine 1 in n-hexane (1.4 ꢁ 10ꢂ5 mol Lꢂ1) was drop-
cast on to a fresh mica substrate and allowed to dry in air before
imaging. The observed fibrous structures indicate aggregation of
1 occurred upon solvent evaporation. The average height of the
fibers measured from the AFM images was 3.5 ꢄ 0.2 nm, which
was in accordance with the extended molecular width estimated
by the CPK model (Fig. S1†). Morphologies observed by SEM
and AFM revealed that imine 1 underwent anisotropic aggre-
gation in solution, as a result of both face-to-face arene–per-
fluoroarene interaction and hydrogen bonding interaction.
S
a
c
b
Transparent gel. Minimum gelation concentration (mmol Lꢂ1).
Opaque gel. Solution. e Molar ratio.
d
the disruption of arene–perfluoroarene stacking interaction,
mainly by the competing aromatic interaction from solvent
molecules, was most likely to be the reason for the failure of
gelation.
The viscoelastic nature of a gel can be characterized by rheo-
logical measurements if the storage modulus G0 exceeds the loss
modulus G00 and if there is no dependence of G0 on the oscillatory
frequency.40,41 To assess the viscoelasticity of the gel rheologi-
cally, a gel of imine 1 was prepared at a concentration of 8.2 ꢁ
10ꢂ3 mol Lꢂ1 in 1,4-dioxane–n-dodecane (1 : 3, v : v) on a parallel
plate (diameter ¼ 25 mm) and was stood at 20 ꢀC for 24 h before
being subjected to testing. The rheological results, including
dynamic strain sweep (DSS), dynamic frequency sweep (DFS),
and dynamic time sweep (DTS) patterns, are shown in Fig. 2.
DSS was performed at a frequency of 6 rad sꢂ1 as a function of
strain, ranging from 0.01 to 100%, to examine the linear visco-
elastic strain regime. Both moduli, G0 and G00, remained roughly
constant below 0.2% strain, implying the upper limit of the linear
regime corresponds to a critical strain value of 0.2%. G00 was
observed to increase when subjected to a higher strain, indicating
local rearrangements taking place in the gel.42,43 Mechanically-
induced fracture resulted when the gel was subjected to a strain
above 0.6%, which was evidenced by a decrease in the absolute
values of both G0 and G00. The gel was then subjected to
a nondestructive DFS experiment performed at a constant strain
of 0.1% to obtain the linear viscoelastic frequency regime. Data
collected with frequencies ranging from 100 to 0.5 rad sꢂ1 showed
that G0 was constantly greater than respective G00, suggesting that
the gel was viscoelastic in the testing frequency range. Based on
the DSS and DFS results, a constant strain of 0.1% and
frequency of 6 rad sꢂ1 were chosen as the linear viscoelastic
Self-association behaviors of a 1 : 1 mixture of 3 and 4 in solution
Note that in the molecular design, we purposely fused the phenyl
ring to the 2,3,5,6-tetrafluorophenyl ring via an imine linkage to
facilitate the alternating face-to-face molecular stacking of imine
1. In order to justify that covalently fusing these two moieties
into one molecule is indispensible for molecular self-assembly to
occur, we investigated the self-assembly behaviors of a mixture of
imine 1’s synthetic precursors, compounds 3 and 4, in solution. A
1
set of H NMR spectra of a mixture of compounds 3 and 4 at
a molar ratio of 1 : 1 was collected in CDCl3 within a similar
concentration range as Fig. 1. In contrast to the concentration-
dependent 1H NMR spectra of imine 1, no evident chemical shift
changes of the aromatic resonances were detected in this mixture
system upon concentration variation (Fig. 5), indicating the
absence of face-to-face aromatic stacking interactions between 3
and 4 in solution. Moreover, upon increasing the concentration,
Fig. 3 (A) SEM image of a xerogel of imine 1 from ethyl acetate–n-
hexane (1 : 3, v : v) with a concentration of 8.2 ꢁ 10ꢂ3 mol Lꢂ1. Insert:
photo of the gel. (B) SEM image of the gel sample made from ethyl
acetate–n-hexane (1 : 3, v : v, 8.2 ꢁ 10ꢂ3 mol Lꢂ1) after undergoing gel–
sol transition triggered by adding a catalytic amount of acidic stimuli
(p-MeC6H4SO3H).
Fig. 2 DSS, DFS and DTS plots of imine 1 tested on a 8.2 ꢁ 10ꢂ3 mol
Lꢂ1 gel in 1,4-dioxane–n-dodecane (1 : 3, v : v) at 20 ꢀC. Storage modulus
G0: ; loss modulus G00: . DSS was tested at u ¼ 6 rad sꢂ1; DFS was
tested at strain ¼ 0.1%; DTS was tested at u ¼ 6 rad sꢂ1 and strain ¼
0.1%.
5488 | Soft Matter, 2012, 8, 5486–5492
This journal is ª The Royal Society of Chemistry 2012

View Article Online
then subjected to SEM imaging (Fig. S9†), showing that no
fibrous aggregate was formed in any case. All these results
provided clear evidence that self-assembly capacity of imine 1
was different from that of the 1 : 1 mixture of its synthetic
precursors, 3 and 4 (Table 1). Thus, the covalent imine linkage
between the phenyl and tetrafluorophenyl moieties employed in
our molecular design, indeed, played an essential role in facili-
tating intermolecular interactions as well as molecular self-
assembly.
Fig. 4 (A) AFM image of a film cast from n-hexane solution of imine 1
(1.4 ꢁ 10ꢂ5 mol Lꢂ1) on mica substrate. (B) The cross section analysis of
the white line in (A).
Acid-responsive gels
Acid-catalyzed hydrolysis of imine is known to be a highly effi-
cient reaction. Provided that a rather distinct gelation capability
was observed between imine 1 and its precursors, inducing a gel–
sol transition process by adding acid to the gel may be achievable
with the current system. It is very attractive to explore acid-
responsive low molecular weight organogels. To the best of our
knowledge, no precedent of such an example has been reported.
Hydrolysis reaction of imine 1 with p-MeC6H4SO3H as
a catalyst was monitored by 1H NMR. The majority of the
reactant was decomposed into its two precursors within 30 min
(see Supporting Information†), suggesting cleavage of the imine
linkage under acidic conditions was facile and effective. The
sensitivity of the gels formed by imine 1 towards acid was
observed in subsequent study. When a catalytic amount of p-
MeC6H4SO3H was added onto the top of a gel, gel–sol transition
occurred within less than one hour. In another experiment shown
in Fig. 6, a colored aqueous droplet could stand steadily on the
top of an ethyl acetate–n-hexane (1 : 3, v : v) gel for several days.
Note that the density of the aqueous droplet was larger than that
of the ethyl acetate–n-hexane (1 : 3, v : v) gel system, this
phenomenon is in accordance with the rheological result that the
gel formed by imine 1 was stable and viscoelastic. However, after
a drop of HCl (10%) was added to the aqueous droplet, gel–sol
transition, resulting from the acid-induced irreversible fracture of
the gel, occurred. After this phase transition, the acidic droplet
sank to the bottom of the container due to gravitational attrac-
tion. Changes of aggregate morphology upon acid-induced gel–
sol transition were studied by SEM, which showed the fibrous
nanostructures of imine 1 disappeared after treatment with acid
(Fig. 3B). Overall, these results unambiguously showed that the
organogels formed by imine 1 are acid-responsive.
Fig. 5 1H NMR spectra of a 1 : 1 mixture of 3 and 4 in CDCl3 at (a) 1.4
ꢁ 10ꢂ3 mol Lꢂ1; (b) 2.8 ꢁ 10ꢂ3 mol Lꢂ1; (c) 5.8 ꢁ 10ꢂ3 mol Lꢂ1; (d) 1.4 ꢁ
10ꢂ2 mol Lꢂ1; (e) 2.7 ꢁ 10ꢂ2 mol Lꢂ1
.
the downfield shift of the amide NH protons in 3 and 4 was
smaller in magnitude, compared to that observed for imine 1.
These results suggested that intermolecular association was
greatly weakened when the covalent linkage between the phenyl
and tetrafluorophenyl moieties is broken.
The gelation capacity of the mixture of 3 and 4 in organic
solvents, shown in Table 1, was also examined. Test solutions of
compounds 3 and 4 mixed at a molar ratio of 1 : 1 were prepared
at various concentrations, followed by standing at room
temperature for a couple of hours to see if gelation occured.
However, gelation was not observed, even in solutions with
nearly saturated concentrations (ꢃ40 mM). These solutions were
Conclusion
In summary, a Schiff base molecule, imine 1, was designed and
synthesized to realize for the first time a catalytic acid-responsive
organogel system furnished by arene–perfluoroarene interac-
tions. Both arene–perfluoroarene interactions and amide–amide
hydrogen bonding were identified as the driving forces for the
self-assembly of imine 1. Additionally, imine 1 was found to be
a good gelator in ethyl acetate, 1,4-dioxane, and n-alkanes, as
well as in ethyl acetate–n-alkane and 1,4-dioxane–n-alkane
mixtures. Whereas in aromatic solvents, where arene–per-
fluoroarene interactions between the solute molecules were dis-
rupted by the competing aromatic interaction from solvent
molecules, gelation was not observed. Gelation did not occur in
a solution of a 1 : 1 mixture of Schiff base precursors 3 and 4
Fig. 6 Gel–sol transition of a gel of 1 in ethyl acetate–n-hexane (color-
less portion) triggered by adding a drop of HCl (10%) to the colored
aqueous droplet on the top of the gel (Rhodamine B was used as dye).
Different time intervals after adding the drop of acid are indicated in the
photos.
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 5486–5492 | 5489

View Article Online
either. On the basis of NMR spectra, no arene–perfluoroarene
stacking were observed in this mixture system, only weak
hydrogen bonding interactions, providing an explanation for the
absence of the gelation process. This interesting result was
exploited to realize the first acid-responsive organogel with a gel–
sol transition process induced by acid.
(M + 23)+. Anal. Calcd. for C50H80F4N4O4: C, 68.46; H, 9.19; N,
6.39. Found: C, 68.21; H, 9.17; N, 6.64.
4-Aminotetrafluoro-N-(3,4,5-tris(dodecyloxy)benzyl)
benza-
mide (3). Compound 2 (8.07g, 9.2 mmol) was dissolved in 50 mL
of mixture of CH2Cl2/MeOH (1 : 1, v : v), and SnCl2$2H2O
(2.7 g, 12 mmol, 1.3 equiv) was added to the solution. The reac-
tion mixture was stirred overnight at room temperature. After the
reaction, the solvents were evaporated under reduced pressure.
The solid residue was purified through column chromatography
over silica gel with ethyl acetate/petroleum ether (60–90 ꢀC) (1 : 5,
v : v) as eluant to afford 3 as a white powder: 5.80 g (74.4%). 1H
NMR (300 MHz, CDCl3, ppm): d 6.52 (s, 2H), 6.17–6.08 (b, 1H),
4.54 (d, J ¼ 5.4 Hz, 2H), 4.24–4.20 (b, 2H), 3.99–3.90 (m, 6H),
1.82–1.70 (m, 6H), 1.50–1.41 (m, 6H), 1.38–1.18 (m, 48H), 0.88 (t,
J ¼ 6.8 Hz, 9H). 13C NMR (100 MHz, CDCl3, ppm): d 159.0,
153.3, 144.5 (JC–F ¼ 247 Hz), 137.5, 136.1 (J ¼ 238 Hz), 132.4,
128.4, 105.9, 102.9, 73.4, 69.1, 44.3, 31.9, 30.3, 29.8, 29.7, 29.6,
29.4, 29.4, 26.1, 22.7, 14.1. 19F NMR (282 MHz, CDCl3, ppm):
d ꢂ65.4, ꢂ83.3. MS (ESI, m/z): Calcd: 850.6 (M+), Found: 873.6
(M + 23)+. Anal. Calcd. for C50H82F4N2O4: C, 70.55; H, 9.71; N,
3.29. Found: C, 70.46; H, 9.76; N, 3.38.
Experimental section
General
All solvents were obtained from commercial sources and purified
and/or dried according to standard procedures. Unless otherwise
indicated, all of the commercial reagents were used as received.
All of the reactions were carried out in oven-dried round-bottom
flasks. 1H, 19F and 13C NMR spectra were recorded on a Mercury
plus 300 MHz or Bruker 400 MHz using CDCl3 as solvent.
Chemical shifts are reported in parts per million (ppm) and
coupling constants are reported in hertz (Hz). 1H NMR chemical
shifts were referenced to TMS (0 ppm), 19F NMR chemical shifts
were referenced to CF3COOH (0 ppm) and 13C NMR chemical
shifts were referenced to CDCl3 (77.0 ppm). Mass spectra were
recorded on a VG ZAB-HS mass spectrometer. Elemental
analysis was performed using an Elementar VARIO EL
elemental analyzer. Electro-spray ionization (ESI) mass spec-
trometry was performed using a Bruker Apex IV FTMS
instrument.
4-Formyl-N-(3,4,5-tris(dodecyloxy)benzyl)benzamide (4). 4-
Formylbenzoic acid (3.42 g, 22.8 mmol) was dissolved in anhy-
drous CH2Cl2 (20 mL) and (COCl)2 (20 mL, 228 mmol, 10
equiv.) was injected into the solution. A drop of DMF was added
to induce the reaction and the reaction mixture was stirred under
anhydrous atmosphere for 24 h at room temperature. The
solvent and excessive (COCl)2 was removed under reduced
pressure to afford 4-formylbenzoyl chloride as yellow viscous
liquid. It was used in the following reaction without further
purification.
Procedures of synthesis
4-Azidotetrafluoro-N-(3,4,5-tris(dodecyloxy)benzyl) benzamide
(2). 4-Azidotetrafluorobenzoic acid (2.84 g, 12.1 mmol) was
dissolved in anhydrous CH2Cl2 (20 mL), and (COCl)2 (10.4 mL,
121 mmol, 10 equiv.) was injected into the solution. A drop of
N,N-dimethylformamide (DMF) was added to induce the reac-
tion, and the reaction mixture was stirred under anhydrous
atmosphere for 24 h at room temperature. The solvent and
excessive (COCl)2 was removed under reduced pressure to give 4-
azidotetrafluorobenzoyl chloride as a red viscous liquid. It was
used in the following reaction without further purification.
3,4,5-Tridodecyloxybenzylamine (8.0 g, 12.1 mmol) and Et3N
(5.5 ml, 36.4 mmol, 3 equiv) were dissolved iꢀn anhydrous CH2Cl2
(20 mL) and the solution was cooled to 0 C under anhydrous
atmosphere. 4-Azidotetrafluorobenzoyl chloride (12.1 mmol, 1
equiv.) was dissolved into anhydrous CH2Cl2 (5 mL) and the
solution was added dropwise to the reaction system. The reaction
mixture was stirred under anhydrous atmosphere for 20 min at
0 ꢀC. The solvent was removed under reduced pressure. The solid
residue was purified through column chromatography over silica
gel with ethyl acetate/petroleum ether (60–90 ꢀC) (1 : 8, v : v) as
eluant to afford compound 2 as a white powder: 8.07 g (76.4%).
1H NMR (300 MHz, CDCl3, ppm): d 6.51 (s, 2H), 6.10–6.19 (b,
1H), 4.54 (d, J ¼ 5.4 Hz, 2H), 3.99–3.89 (m, 6H), 1.85–1.68 (m,
6H), 1.52–1.40 (m, 6H), 1.40–1.16 (m, 48H), 0.88 (t, J ¼ 6.6 Hz,
9H). 13C NMR (100 MHz, CDCl3, ppm): d 157.6, 153.3, 144.0
(JC–F ¼ 251 Hz), 140.4 (JC–F ¼ 249 Hz), 137.4, 131.9, 121.9,
111.4, 105.8, 73.4, 69.0, 44.4, 31.9, 30.3, 29.7, 29.7, 29.6, 29.4,
29.4, 26.1, 22.7, 14.1. 19F NMR (282 MHz, CDCl3, ppm):
d ꢂ62.9, ꢂ72.7. MS (ESI, m/z): Calcd: 876.6 (M+), Found: 899.6
3,4,5-Tridodecyloxybenzylamine (15.0 g, 22.8 mmol) and Et3N
(15 mL, 68.3 mmol, 3 equiv.) were dissolved in anhydrous
CH2Cl2 (20 mL) and the solution was cooled to 0 ꢀC under
anhydrous atmosphere. 4-Formylbenzoyl chloride (22.8 mmol, 1
equiv.) was dissolved in anhydrous CH2Cl2 (5 mL) and the
solution was added dropwise to the reaction system. The reaction
mixture was stirred under anhydrous atmosphere for 20 min at
0 ꢀC. Then, the solvent was removed under reduced pressure and
the solid residue was purified through column chromatography
over silica gel with ethyl acetate/petroleum ether (60–90 ꢀC)
(1 : 5, v : v) as eluant to afford compound 4 as a yellow powder:
1
4.03 g (22.2%). H NMR (300 MHz, CDCl3, ppm): d 10.08 (s,
1H), 7.95 (m, 2H), 6.54 (s, 2H), 6.31–6.39 (b, 1H), 4.55 (d, J ¼
5.7 Hz, 2H), 3.99–3.90 (m, 6H), 1.84–1.71 (m, 6H), 1.53–1.40 (b,
6H), 1.40–1.16 (b, 48H), 0.88 (t, J ¼ 6.8 Hz). 13C NMR
(100 MHz, CDCl3, ppm): d 191.4, 166.1, 153.4, 139.5, 138.2,
137.8, 132.6, 129.8, 127.7, 106.6, 73.5, 69.2, 44.7, 31.9, 30.3, 29.7,
29.7, 29.6, 29.4, 29.4, 26.1, 22.7, 14.1. MS (ESI, m/z): Calcd:
791.6 (M+), Found: 814.6 (M + 23)+, 792.6 (M + 1)+. Anal. Calcd.
for C51H85NO5: C, 77.32; H, 10.81; N, 1.77. Found: C, 77.30; H,
10.98; N, 1.80.
(E)-2,3,5,6-Tetrafluoro-N-(3,4,5-tris(dodecyloxy)benzyl)-4-(4-
(3,4,5-tris(dodecyloxy)benzyl-carbamoyl)benzylideneamino) ben-
zamide (1). A solution of 3 (0.85 g, 1 mmol), 4 (0.791 g, 1 mmol, 1
equiv.) and p-MePhSO3H$H2O (1.9 mg, 0.01 mmol) in toluene
5490 | Soft Matter, 2012, 8, 5486–5492
This journal is ª The Royal Society of Chemistry 2012

View Article Online
(10 mL) was heated to reflux overnight. The solvent was removed
under reduced pressure at room temperature and the residue was
purified through column chromatography over silica gel with
Et3N/CH2Cl2/petroleum ether (60–90 ꢀC) (0.2 : 1 : 3, v : v : v) as
References
1 (a) Y. Nagai, L. D. Unsworth, S. Koutsopoulos and S. Zhang, J.
Controlled Release, 2006, 115, 18; (b) C. Li, J. Madsen, S. P. Armes
and A. L. Lewis, Angew. Chem., Int. Ed., 2006, 45, 3510; (c)
G. P. Andrews, L. Donnelly, D. S. Jones, R. M. Curran,
R. J. Morrow, A. D. Woolfson and R. K. Malcolm,
Biomacromolecules, 2009, 10, 2427.
2 (a) B. Xu, Langmuir, 2009, 25, 8375; (b) G. Liang, Z. Yang, R. Zhang,
L. Li, Y. Fan, Y. Kuang, Y. Gao, T. Wang, W. W. Lu and B. Xu,
Langmuir, 2009, 25, 8419.
3 (a) T. Ishi-I and S. Shinkai, Top. Curr. Chem., 2005, 258, 119; (b)
T. Kato, N. Mizoshita, M. Moriyama and T. Kitamura, Top. Curr.
Chem., 2005, 256, 219.
4 (a) K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata,
T. Komori, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc.,
1994, 116, 6664; (b) S. Yagai, T. Nakajima, K. Kishikawa,
S. Kohmoto, T. Karatsu and A. Kitamura, J. Am. Chem. Soc.,
2005, 127, 11134; (c) T. Suzuki, S. Shinkai and K. Sada, Adv.
Mater., 2006, 18, 1043.
1
eluant to afford 1 as a yellow glassy solid: 0.487 g (30%). H
NMR (300 MHz, CDCl3, ppm): d 8.52 (s, 1H), 7.96 (d, J ¼ 8.4
Hz, 2H), 7.88 (d, J ¼ 8.7 Hz, 2H), 6.85 (t, J ¼ 5.6 Hz, 1H), 6.61 (t,
J ¼ 5.4 Hz, 1H), 6.54 (s, 2H), 6.52 (s, 2H), 4.57 (d, J ¼ 5.7 Hz,
2H), 4.51 (d, J ¼ 4.5 Hz, 2H), 4.03–3.84 (m, 12H), 1.90–1.62 (m,
12H), 1.58–1.10 (m, 96H), 0.880 (t, J ¼ 7.2 Hz, 9H), 0.875 (t, J ¼
6.9 Hz, 9H). 13C NMR (100 MHz, CDCl3, ppm): 168.0, 166.4,
158.3, 153.3, 153.2, 143.7 (JC–F ¼ 245 Hz), 139.1 (JC–F ¼ 250
Hz), 138.3, 137.3, 137.1, 132.7, 132.3, 131.7, 129.7, 129.3, 127.7,
127.5, 112.3, 106.2, 105.7, 73.5, 73.4, 69.0, 44.7, 44.2, 31.9, 30.3,
30.3, 29.7, 29.7, 29.6, 29.4, 29.4, 29.3, 26.1, 26.1, 22.7, 14.1. 19F
NMR (282 MHz, CDCl3, ppm): d ꢂ64.3, ꢂ74.4. MS (ESI, m/z):
Calcd: 1625.3 (M+), Found: 1699.3 (M + 74) +. Anal. Calcd. for
ꢀ
5 (a) J. Eastoe, M. Sanchez-Dominguez, P. Wyatt and R. K. Heenan,
Chem. Commun., 2004, 2608; (b) Y. Ji, G.-C. Kuang, X.-R. Jia,
E.-Q. Chen, B.-B. Wang, W.-S. Li, Y. Wei and J. Lei, Chem.
Commun., 2007, 4233; (c) J. H. Kim, M. Seo, Y. J. Kim and
S. Y. Kim, Langmuir, 2009, 25, 1761.
C101H165F4N3O8: C, 74.63; H, 10.23; N, 2.59. Found: C, 74.30;
H, 10.32; N, 2.70.
6 (a) C. Geiger, M. Stanescu, L. Chen and D. G. Whitten, Langmuir,
1999, 15, 2241; (b) R. Wang, C. Geiger, L. Chen, B. Swanson and
D. G. Whitten, J. Am. Chem. Soc., 2000, 122, 2399.
Scanning electron microscopy (SEM)
7 (a) J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, J. H. van Esch and
B. L. Feringa, Science, 2004, 304, 278; (b) C. Wang, D. Zhang,
J. Xiang and D. Zhu, Langmuir, 2007, 23, 9195; (c) H. Yang, T. Yi,
Z. Zhou, Y. Zhou, J. Wu, M. Xu, F. Li and C. Huang, Langmuir,
2007, 23, 8224; (d) L. Zhu, X. Ma, F. Ji, Q. Wang and H. Tian,
Chem.–Eur. J., 2007, 13, 9216; (e) A. Ajayaghosh, V. K. Praveen,
S. Srinivasan and R. Varghese, Adv. Mater., 2007, 19, 411.
SEM measurements were carried out by a field emission scanning
electron microscope (FEEM, LEO 1530 VP) operated at an
accelerating voltage of 1.0 kV. All samples were applied to
a stainless silica wafer and allowed to dry under reduced
pressure.
ꢁ
8 (a) J. J. D. de Jong, P. R. Hania, A. Pugzlys, L. N. Lucas, M. de Loos,
R. M. Kellogg, B. L. Feringa, K. Duppen and J. H. van Esch, Angew.
Chem., Int. Ed., 2005, 44, 2373; (b) Y.-L. Zhao and J. F. Stoddart,
Langmuir, 2009, 25, 8442.
Atomic force microscopy (AFM)
AFM measurements were conducted in tapping mode on freshly
stripped mica surfaces by DI Nanoscope IIIa in air. Tap300Al
tips (Budget Sensors) with a force constant of 40 N mꢂ1 and
a resonance frequency of 300 kHz were used.
9 (a) S. Wang, W. Shen, Y. Feng and H. Tian, Chem. Commun., 2006,
1497; (b) C. Wang, Q. Chen, F. Sun, D. Zhang, G. Zhang, Y. Huang,
R. Zhao and D. Zhu, J. Am. Chem. Soc., 2010, 132, 3092.
10 (a) C. Wang, D. Zhang and D. Zhu, Langmuir, 2007, 23, 1478; (b)
A. Ghosh and J. Dey, Langmuir, 2009, 25, 8466; (c) M. B. Dowling,
J.-H. Lee and S. R. Raghavan, Langmuir, 2009, 25, 8519; (d)
T. Kar, S. Debnath, D. Das, A. Shome and P. K. Das, Langmuir,
2009, 25, 8639.
Rheological measurements
11 (a) T. Naota and H. Koori, J. Am. Chem. Soc., 2005, 127, 9324; (b)
J. Wu, T. Yi, T. Shu, M. Yu, Z. Zhou, M. Xu, Y. Zhou, H. Zhang,
J. Han, F. Li and C. Huang, Angew. Chem., Int. Ed., 2008, 47, 1063.
12 (a) C. Wang, D. Zhang and D. Zhu, J. Am. Chem. Soc., 2005, 127,
16372; (b) S.-I. Kawano, N. Fujita and S. Shinkai, J. Am. Chem.
Soc., 2004, 126, 8592; (c) S.-I. Kawano, N. Fujita and S. Shinkai,
Chem.–Eur. J., 2005, 11, 4735; (d) Y.-L. Zhao, I. Aprahamian,
A. Trabolsi, N. Erina and J. F. Stoddart, J. Am. Chem. Soc., 2008,
130, 6348.
Rheological measurement were carried out by using Physica
MCR 301 instrument with a parallel plate (diameter ¼ 25 mm) at
20 ꢀC.
Gelation test
A typical procedure for qualitative gelation testing was as
following: in a test tube (diameter ¼ 1 cm), imine 1 was mixed
with appropriate amounts of solvent and the mixture was heated
until tꢀhe solid dissolved. The resulting clear solution was cooled
to 20 C and annealed for about 1 h at this temperature. When
the test tube could be inverted without change of shape of its
content, it was indentified as a gel.
13 K. Sugiyasu, N. Fujita and S. Shinkai, Angew. Chem., Int. Ed., 2004,
43, 1229.
14 (a) T. Akutagawa, K. Kakiuchi, T. Hasegawa, S.-I. Noro,
T. Nakamura, H. Hasegawa, S. Mashiko and J. Becher, Angew.
Chem., Int. Ed., 2005, 44, 7283; (b) T. Kitahara, M. Shirakawa,
S.-I. Kawano, U. Beginn, N. Fujita and S. Shinkai, J. Am. Chem.
Soc., 2005, 127, 14980; (c) T. Kitamura, S. Nakaso, N. Mizoshita,
Y. Tochigi, T. Shimomura, M. Moriyama, K. Ito and T. Kato, J. Am.
ꢀ
Chem. Soc., 2005, 127, 14769; (d) J. Puigmartı-Luis, V. Laukhin,
A. P. del Pino, J. Vidal-Gancedo, C. Rovira, E. Laukhina and
ꢀ
Acknowledgements
D. B. Amabilino, Angew. Chem., Int. Ed., 2007, 46, 238.
15 (a) F. S. Schoonbeek, J. H. van Esch, B. Wegewijs, D. B. A. Rep,
M. P. de Haas, T. M. Klapwijk, R. M. Kellogg and B. L. Feringa,
Angew. Chem., Int. Ed., 1999, 38, 1393; (b) B. W. Messmore,
J. F. Hulvat, E. D. Sone and S. I. Stupp, J. Am. Chem. Soc., 2004,
126, 14452; (c) S.-I. Kawano, N. Fujita and S. Shinkai, Chem.–Eur.
J., 2005, 11, 4735.
This research was financially supported by the National Basic
Research Program (2007CB808000) from the Ministry of
Science and Technology and National Natural Science Foun-
dation of China (No. 21074004 & 50873002). The authors are
grateful to Prof. Dehai Liang for his help on rheological
measurements.
16 H. Komatsu, S. Matsumoto, S.-I. Tamaru, K. Kaneko, M. Ikeda and
I. Hamachi, J. Am. Chem. Soc., 2009, 131, 5580.
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 5486–5492 | 5491

View Article Online
17 (a) J. H. Jung, Y. Ono and S. Shinkai, Angew. Chem., Int. Ed., 2000,
39, 1862; (b) M. Shirakawa, N. Fujita and S. Shinkai, J. Am. Chem.
Soc., 2005, 127, 4164; (c) P. Gao, C. Zhan and M. Liu, Langmuir,
2006, 22, 775; (d) X. Peng, S. Karan and I. Ichinose, Langmuir,
2009, 25, 8514.
30 M. Weck, A. R. Dunn, K. Matsumoto, G. W. Coates, E. B. Lobkovsky
and R. H. Grubbs, Angew. Chem., Int. Ed., 1999, 38, 2741.
31 S. W. Watt, C. Dai, A. J. Scott, J. M. Burke, R. L. Thomas,
J. C. Collings, C. Viney, W. Clegg and T. B. Marder, Angew.
Chem., Int. Ed., 2004, 43, 3061.
18 (a) R. G. Weiss, P. Terech, Molecular gels: materials with self-
assembled fibrillar networks,Springer, Amsterdam, 2006; (b)
X. Y. Liu, Top. Curr. Chem., 2005, 256, 1.
19 C. R. Patrick and G. S. Prosser, Nature, 1960, 187, 1021.
20 J. H. Williams, Acc. Chem. Res., 1993, 26, 593.
21 C. A. Hunter, Angew. Chem., Int. Ed. Engl., 1993, 32, 1584.
22 F. Ponzini, R. Zagha, K. Hardcastle and J. S. Siegel, Angew. Chem.,
Int. Ed., 2000, 39, 2323.
32 K. Kishikawa, K. Oda, S. Aikyo and S. Kohmoto, Angew. Chem., Int.
Ed., 2007, 46, 764.
33 C. Dai, P. Nguyen, T. B. Marder, A. J. Scott, W. Clegg and C. Viney,
Chem. Commun., 1999, 2493.
34 S. M. Butterfield, P. R. Patel and M. L. Waters, J. Am. Chem. Soc.,
2002, 124, 9751.
35 (a) K. A. Frey and S. A. Woski, Chem. Commun., 2002, 2206; (b)
G. Mathis and J. Hunziker, Angew. Chem., Int. Ed., 2002, 41, 3203.
36 R. Laatikainen, J. Ratilainen, R. Sebastian and H. Santa, J. Am.
Chem. Soc., 1995, 117, 11006.
€
23 (a) W. J. Feast, P. W. Lovenich, H. Puschmann and C. Taliani, Chem.
Commun., 2001, 505; (b) V. R. Vangala, A. Nangia and V. M. Lynch,
Chem. Commun., 2002, 1304; (c) C. E. Smith, P. S. Smith,
R. L. Thomas, E. G. Robins, J. C. Collings, C. Dai, A. J. Scott,
S. Borwick, A. S. Batsanov, S. W. Watt, S. J. Clark, C. Viney,
J. A. K. Howard, W. Clegg and T. B. Marder, J. Mater. Chem.,
2004, 14, 413.
37 J. van Herrikhuyzen, A. Syamakumari, A. P. H. J. Schenning and
E. W. Meijer, J. Am. Chem. Soc., 2004, 126, 10021.
38 J. F. W. Keana and S. X. Cai, J. Org. Chem., 1990, 55, 3640.
€
39 (a) M. Kastler, W. Pisula, D. Wasserfallen, T. Pakula and K. Mullen,
€
J. Am. Chem. Soc., 2005, 127, 4286; (b) J. Wu, A. Fechtenkotter,
€
J. Gauss, M. D. Watson, M. Kastler, C. Fechtenkotter, M. Wagner
~
24 (a) M. Gdaniec, W. Jankowski, M. J. Milewska and T. Po1onski,
€
and K. Mullen, J. Am. Chem. Soc., 2004, 126, 11311; (c)
R. B. Martin, Chem. Rev., 1996, 96, 3043.
Angew. Chem., Int. Ed., 2003, 42, 3903; (b) L. S. Reddy,
P. M. Bhatt, R. Banerjee, A. Nangia and G. J. Kruger, Chem.–
Asian J., 2007, 2, 505.
40 (a) J. D. Ferry, Viscoelastic properties of polymers, Wiley, New York,
1980; (b) K. Te Nijenhuis, Thermoreversible network: viscoelastic
properties and structure of gels, Springer, Berlin, 1997.
41 (a) I. Hisaki, H. Shigemitsu, Y. Sakamoto, Y. Hasegawa, Y. Okajima,
K. Nakano, N. Tohnai and M. Miyata, Angew. Chem., Int. Ed., 2009,
48, 5465; (b) S. Yao, U. Beginn, T. Gress, M. Lysetska and
€
25 (a) A. Sundararaman, L. N. Zakharov, A. L. Rheingold and F. Jakle,
Chem. Commun., 2005, 1708; (b) A. Hori, A. Shinohe, M. Yamasaki,
E. Nishibori, S. Aoyagi and M. Sakata, Angew. Chem., Int. Ed., 2007,
46, 7617.
26 S. Zhu, S. Zhu, G. Jin and Z. Li, Tetrahedron Lett., 2005, 46, 2713.
27 (a) G. W. Coates, A. R. Dunn, L. M. Henling, D. A. Dougherty and
R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 1997, 36, 248; (b) R. Xu,
V. Gramlich and H. Frauenrath, J. Am. Chem. Soc., 2006, 128, 5541.
28 G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller,
E. B. Lobkovsky and R. H. Grubbs, J. Am. Chem. Soc., 1998, 120,
3641.
€
ꢀ
F. Wurthner, J. Am. Chem. Soc., 2004, 126, 8336; (c) D. D. Dıaz,
K. Rajagopal, E. Strable, J. Schneider and M. G. Finn, J. Am.
Chem. Soc., 2006, 128, 6056.
42 (a) H. M. Princen, J. Colloid Interface Sci., 1983, 91, 160–175; (b)
T. G. Mason, J. Bibette and D. A. Weitz, J. Colloid Interface Sci.,
1996, 179, 439.
29 B.-B. Ni, C. Wang, H. Wu, J. Pei and Y. Ma, Chem. Commun., 2010,
46, 782.
43 M. Lescanne, A. Colin, O. Mondain-Monval, F. Fages and
J.-L. Pozzo, Langmuir, 2003, 19, 2013.
5492 | Soft Matter, 2012, 8, 5486–5492
This journal is ª The Royal Society of Chemistry 2012