Journal of Fluorine Chemistry
Fluoride-responsive organogel containing azobenzyl and
cholesterol units
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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
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0022-1139/ß 2014 Elsevier B.V. All rights reserved.