Fluoride-responsive organogel containing azobenzyl and cholesterol units

Article abstract of DOI:10.1016/j.jfluchem.2014.12.005

In this paper, two new cholesterol-based compounds O1 and O2 were designed and synthesized. The compound O2 could selectively gel in 1,4-dioxane with porous ribbon structure. The aggregation mode of O2 molecules were characterized by SEM, IR, UV-vis and X

Full text of DOI:10.1016/j.jfluchem.2014.12.005

Journal of Fluorine Chemistry 170 (2015) 24–28
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Fluoride-responsive organogel containing azobenzyl and
cholesterol units
*
Lijun Geng, Guoliang Feng, Shiguang Wang, Xudong Yu , Zhice Xu, Xiaoli Zhen, Tao Wang
College of Science and Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science & Technology, Shijiazhuang 050018,
PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
In this paper, two new cholesterol-based compounds O1 and O2 were designed and synthesized. The
compound O2 could selectively gel in 1,4-dioxane with porous ribbon structure. The aggregation mode
of O2 molecules were characterized by SEM, IR, UV–vis and XRD experiments. Interestingly, the gel of O2
was also fluoride-responsive. The addition of TBAF on the gel surface would trigger the gel–sol transition,
and the collapse time could be controlled by amount of fluoride ions. The response process was also
accompanied by dramatic color changes from orange to purple. From the NMR titration experiments, it
was deduced that the hydrogen bonding competition between the anion-receptor and self-assembly of
the receptor played an important role for the gel–sol transition.
Received 12 November 2014
Received in revised form 9 December 2014
Accepted 13 December 2014
Available online 27 December 2014
Keywords:
Organogel
Fluoride
ß 2014 Elsevier B.V. All rights reserved.
Assembly
Responsive
Gel–sol transition
Supramolecular
1. Introduction
are performed in solution, which restricted them from potential
application.
LMOGs (Low Molecular-mass Organic Gelators) have attracted
increasing attention in the material chemistry due to their merits
such as stimulus responsibility, reversibility, as well as self-healing
properties, which make them play important roles in the filed of
drug controlled release, molecular devices, biological issue, and
intelligent materials [1–8]. These organogels are constructed by
In the past decades, fluoride-ion-responsive LMOGs are
beginning to attract particular attention [33–35]. Although many
kinds of fluoride-ion-responsive LMOGs were designed and
studied, few of them was able to sense fluoride anions with high
selectivity among fluoride and other anions [36]. So far, design of
the anion receptors with high selectivity toward fluoride in
organogel issue is still a challenge. In this paper, two new
cholesterol-based compounds with azo units as the chromophore
and imidazole group as the anion acceptor were designed and
synthesized. Compound O2 was able to aggregate in 1,4-dioxane to
form stable and red color gel. Further study showed that
compound O2 could selectively recognize fluoride anion either
in solution or gel state, accompanied by dramatic color and phase
changes.
non-covalent interactions such as hydrogen bonding,
p–p
stacking, van der Waals interaction, which are very sensitive to
external stimuli such as ions, sonication, heat, pH, as well as
biological molecules [9–18]. In particular, anion-responsive gels
have drawn special attention in recent years [19,20]. The
introduction of anions could change the gel state in micro/
macrolevels depending on the interaction between anions and
gelator molecules [21–23].
The design and preparation of fluoride receptors have received
considerable attention due to their important roles in the field of
biology, chemistry, and environment [24,25]. It is well-known that
the NH units such as amides, (thio)ureas, ammonium, imidazole
units could be served as hydrogen bonding sites for anions in the
diluted solution [26–32]. However, most of the recognition events
2. Results and discussion
The synthesis and characterization of O1 and O2 could be seen
from Scheme 1 and supporting information. From Table 1, with the
concentration of 25 mg/mL, O2 could selectively gel in 1,4-dioxane
to form red and opaque organogel by classic heating-cooling
process. Without the –NO2 group, O1 displayed poor solubility in
the test organic solvents. By heating-cooling process, molecules of
O1 precipitated in 1,4-dioxane. SEM images revealed that they
*
Corresponding author. Tel.: +86 18231193228.
E-mail address: 081022009@fudan.edu.cn (X. Yu).
http://dx.doi.org/10.1016/j.jfluchem.2014.12.005
0022-1139/ß 2014 Elsevier B.V. All rights reserved.

L. Geng et al. / Journal of Fluorine Chemistry 170 (2015) 24–28
25
CH(CN)2
H
C(CN)2
N2+Cl-
N
N
NH2
R
N
N
NH2NH2
NaNO2
HCl
CH2(CN)2
C2H5ONa
R
R
R
N
a, R=H
a, R=H
1
2
b, R=NO2
b, R=NO2
O
Cl
H2N
N
O
H
H
NH
N
H2N
N
NH
H
N
N
O
N
NH2
HN
O
R
R
H
a, R=H
H
O1, R=H
O2, R=NO2
3
b, R=NO2
H
Scheme 1. Chemical structure and synthesis route of O1 and O2.
Table 1
From the UV–vis spectra (Fig. 2a), the absorption band of O2 in
the solution exhibited one peak at 421 nm with yellow color,
The gelation properties of O1 and O2 (25 mg/mL).
which was attributed to conjugated
p system of azobenzyl and
O1
O2
imidazole groups. In the gel state, the peak became broad with a
new shoulder at 532 nm, which displayed red shift, indicating the
THF
P
I
P
Methanol
Ethanol
I
enhanced
p conjugated system in gel state. FT-IR experiment
I
I
1,4-Dioxane
Xylene
P
I
OG
confirmed that the hydrogen bonding played an important role for
the gelation and molecular assembly. From Fig. 2b, the –NH
stretching bands of O2 in gel state from 1,4-dioxane were
positioned at 3477, 3435, 3366 cmÀ1, indicating the existence of
multiple hydrogen bondings; the bands at 1720, 1637, were
ascribed to the corresponding C55O vibrations, and the peak at
1565 cmÀ1 was ascribed to amino II band.
I
I
I
I
I
Ethyl acetate
n-Propanol
Cyclohexane
n-Hexane
I
I
I
I
I: insoluble; P: precipitate; OG: opaque gel.
To gain
a deeper insight into the molecular assembly
assembled to microsphere structure with diameters ranged from 5
to 10 m. From the magnified image (Fig. 1a–c), it was observed
that the spheres were with nanoporous surface. While ribbons
with porous surface were formed by O2 molecules in the gel issue
(Fig. 3d–f).
mechanism, XRD experiments were also performed. From Fig. 3,
the peak of the gel for O2 at 2.5 nm was ascribed to the d value of
single molecule. The peaks at 2.5, 1.25, 0.6 were with the ratio of
1:1/2:1/4, reflecting the lamer structure of the aggregation.
m
Fig. 1. SEM images of the precipitate of O1 (a–c) and the gel of O2 (d–f). (b) The magnified picture of (a), (c) the magnified picture of (b); (e) the magnified picture of (d), (f) the
magnified picture of (e); Scale bar: (a) 30 m; (b) 5 m; (c) 1 m; (d) 5 m; (e) 500 nm; (f) 500 nm.
m
m
m
m

26
L. Geng et al. / Journal of Fluorine Chemistry 170 (2015) 24–28
Fig. 2. (a) Absorption spectra of O2 in the solution of 1,4-dixane (10À4 M) and the gel (25 mg/mL); (b) IR spectrum of the gel of O2.
Fig. 3. XRD data of Xerogel of O2 in 1,4-dioxane.
Fig. 4. The gel phase changes after the addition of fluoride. (a) Gel; (b) treated with FÀ instantly; (c) after 70 min; (d) after 90 min; (e) after 120 min; (f) after 150 min; (g) after
170 min.
Generally speaking, hydrogen bonding plays an important role
in either gel assembly or anion recognition events. Therefore, the
addition of anions into gels would change the gel assembly in
molecular level due to the hydrogen bonding competition. Seen
from Fig. 4a, by classic heating-cooling process, the O2 formed
stable gel in 1,4-dioxane with red color. Upon addition of 20 equiv.
of fluoride anion (solid) on the gel surface, the interface became
purple instantly. The gradually collapse process could be seen from
Fig. 4c–f. After staying for 3 h, the gel corrupted and became purple
wholly (Fig. 4g). Also, the corruption rate could be controlled by the
equivalent of fluoride anions ranging from minutes to days
(Table 2).
(5 Â 10À5 M) in solution at 443 nm disappeared, while a new
strong peak at 563 nm appeared and increased, accompanied with
the obvious color changes from yellow to blue, indicating
formation of the complex O2 + FÀ. Other anions such as AcOÀ,
ClÀ, BrÀ, IÀ and HSO4 did not triggered any obvious color or
À
spectral changes (Fig. 5). The above results suggested the high
selectivity of the organogelator O2 toward tested anions. As
expected, addition of these anions event at 50 equiv. on the gel
surface, there was no obvious color or phase changes.
Table 2
The collapse time controlled by the amount of FÀ.
To further examine the color changes and the selectivity of O2
toward anions. The UV–vis experiments in the solution state were
carried out to study the host–guest interaction. From Fig. 5, upon
the addition of 20 equiv. of fluoride anions, the peak of O2
Equiv. of fluoride
Collapse time
2
5
20
50
100
1 day
18 h
4 h
1 h
10 min

L. Geng et al. / Journal of Fluorine Chemistry 170 (2015) 24–28
27
Fig. 5. UV–vis titrations of O2 (5 Â 10À5 M) with different anions (100 equiv.).
Fig. 6. UV–vis titrations of O2 solution (5 Â 10À5 M). (a) Upon the addition of fluoride anions; (b) upon the addition of AcOÀ anions.
Fig. 7. The NMR titrations of O2 with FÀ (30 equiv.).
3. Conclusion
By a precise study, addition of fluoride (0–80 equiv.) resulted in
remarkable and regular decrease of peak at 443 nm and increase of
the peak at 563 nm, indicating the strong and stable interaction
among O2 with FÀ via hydrogen bonding among –NHs as the anion
acceptor (Fig. 6a). While AcOÀ triggered very weaker changes
compared with that of fluoride (0–100 equiv.). NMR titrations
were also performed to confirm the bonding properties of O2 with
FÀ. From Fig. 7, the addition of fluoride anions triggered the
complete disappearance of the –NH, indicating a deprotonation
process. KOH as a kind of a strong base was added into the solution
of O2 in order to further check the binding mechanism. UV–vis
experiment revealed that KOH triggered very similar spectrum
changes of O2 just like that of FÀ (Fig. S1), which was in accordance
with the NMR titration experiment.
Two new kinds of cholesterol-based compounds were designed
and synthesized. The structural modification on the benzyl unit
had great impact on the gelaion properties. The gel was
characterized by SEM, XRD, IR as well as UV–vis measurements.
Interestingly, the gel was selectively fluoride-responsive among
test anions, expressing gel–sol transition and red-purple color
changes for easily nake-eye detection. Multiple interactions such
as hydrogen bonding, hydrophobic interaction, as well as
p–p
stacking were essential for the gel formation. It was deduced that
addition of fluoride anions destroyed the balance among organo-
gelator molecules, resulting in the gel–sol transition due to the
hydrogen bonding competition.

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L. Geng et al. / Journal of Fluorine Chemistry 170 (2015) 24–28
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