Multiple-Stimuli Responsive Luminescent Gels Based on Cholesterol Containing Benzothiadiazole Fluorophores

Article abstract of DOI:10.1002/cjoc.201500283

Two new fluorescent organogelators based on cholesterol containing benzothiadiazole group have been designed and synthesized. Three methods for gels preparation have been presented, such as heating-cooling process, ultrasonic treatment and mixed solvents

Full text of DOI:10.1002/cjoc.201500283

FULL PAPER
DOI: 10.1002/cjoc.201500283
Multiple-Stimuli Responsive Luminescent Gels Based on
Cholesterol Containing Benzothiadiazole Fluorophores
Huibin Sun,a,b Shujuan Liu,a Qiang Zhao,*,a and Wei Huang*,a,b
a Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM),
Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts &
Telecommunications, Nanjing 210023, Jiangsu, China
b Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National
Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing
211816, Jiangsu, China
Two new fluorescent organogelators based on cholesterol containing benzothiadiazole group have been de-
signed and synthesized. Three methods for gels preparation have been presented, such as heating-cooling process,
ultrasonic treatment and mixed solvents under room temperature. For both of the gels, their states and emission col-
＋
ors exhibit striking changes upon addition of Hg2 . The gelation properties, structural characteristics and fluores-
cence of the gels were studied by FT-IR, UV-Vis absorption and PL spectra. The underlying mechanism of gelation
property was studied by X-ray diffraction combined with theoretical calculation, and the important role of π-π in-
teractions in forming the gels has been proved.
Keywords organogel, nanofibers, nanotubes, fluorescence, sensor
Introduction
tion because of its value in gene therapy.[21] Since
Sijbesma[22,23] and Naota[24,25] investigated organogel by
sonication, some examples of sonication-induced mor-
phology changes of gels have been reported.[26] How-
ever, their structure-property relationships and mecha-
nisms are not clear and more extensive investigation is
neccesary. Therefore, the design, synthesis, and investi-
gation of novel sonication-responsive organogels are
very important.
Low-molecular-weight gelators (LMWGs) have
been arousing great interest during the past few years
due to their potential uses as soft material.[1-4] They can
form stable three dimensional (3D) nanoscale super-
structures by self-assembly stabilized through weak
noncovalent forces, such as π-π stacking, van der Waals
forces, hydrogen bonding and hydrophobic-hydrophobic
interactions. Recently, organogels that consist of nano-
tubes by self-assembly have also been investigated.[5,6]
As we know, nanotubes, which have hollow structure
with less than 100 nm inner diameters, have highly
potential applications in drug delivery and nano-
objects.[7] In addition, the helical nanostructures con-
structed from π-conjugated fluorescent LMWGs, such
as twisted ribbons and fibers, are also a topic of
increasing interest[8] due to their academic importance
and promising applications in chiral recognition, opto-
electronic devices and biomaterials.[9-12]
Stimuli-responsive organogels are a kind of fasci-
nating LMWGs in which the morphology of molecules
is regulated by external stimuli, including light,[13,14]
pH,[15,16] ions,[17,18] sound,[19,20] and so on. A fascinating
characteristic of stimuli-responsive organogels is their
potential applications in sensor in view of the changes
of morphology or other properties upon external stimuli.
Particularly, ultrasound has been received much atten-
As a class of well-known organogel factor, choles-
terol-based derivatives can form helical structure upon
ultrasound through hydrogen bonding, π-π interactions,
hydrophobic-hydrophobic interactions and chirality of
the molecule. In order to further understand the role of
hydrogen bonding and π-π interactions and study the
changes of fluorescent properties in the process of
gelation of cholesterol-based π-conjugated organolgels,
we designed and synthesized two cholesterol-based
mutiple-stimuli responsive luminescent LMWGs, 1a
and 1b (Scheme 1). Both of them contain π-conjugated
backbone based on benzothiadiazole, which is not only
a good fluorophore, but also a recognition group for
＋
＋
Hg2 due to the specific interaction between Hg2 and
sulfur atom.[27] Interestingly, 1b can form gels in dichlo-
romethane and show slightly better gelation ability than
1a even though there is no hydrogen bonding presented.
It is also noteworthy that the morphology of 1a gels
*
E-mail: iamqzhao@njupt.edu.cn, wei-huang@njtech.edu.cn
Received April 6, 2015; accepted August 12, 2015; published online September 29, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500283 or from the author.
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Chin. J. Chem. 2015, 33, 1140—1144

Multiple-Stimuli Responsive Luminescent Gels Based on Cholesterol
3: 4-Iodophenol (2.2 g, 10.0 mmol), cholesteryl
chloroformate (5.39 g, 12.0 mmol) and K2CO3 (5.6 g,
40.58 mmol) in 60 mL acetone were stirred at 59 ℃
for 24 h under nitrogen atmosphere. The reaction mix-
ture was concentrated in vacuum, and then washed
twice with CH2Cl2. The filtrate was collected and con-
centrated under reduced pressure. The resulting crude
product was purified by column chromatography (SiO2,
CH2Cl2∶hexane＝1∶1) to yield 3 as a white solid
consisted of left-handed helical nanostructures, while
that of 1b was composed of nanotubes with the uniform
dimensions as shown in the photographs of SEM and
TEM. The multiple-stimuli responsive properties of 1a
and 1b gels were studied, and FT-IR, UV-Vis
absorption spectra and PL spectra were carried out to
investigate the driving force.
Experimental
1
(4.33 g, 70%). H NMR (400 MHz, CDCl3) δ: 7.61 (d,
Materials and instruments
J＝8.8 Hz, 2H), 6.89 (d, J＝8.8 Hz, 2H), 5.35 (d, J＝
5.0 Hz, 1H), 4.58－4.43 (m, 1H), 2.49－2.31 (m, 2H),
2.04－1.58 (m, 6H), 1.58－0.87 (m, 23H), 0.85 (d, J＝
6.5 Hz, 3H), 0.82－0.77 (m, 6H), 0.61 (s, 3H).
The reaction of cholesteryl chloroformate with
4-iodoaniline or 4-iodophenol gave compounds 2 and 3,
respectively. Target compounds 1a and 1b were sub-
sequently synthesized by Sonogashira coupling reaction
of 4,7-diethynyl-2,1,3-benzothiadiazole with 2 and 3,
respectively. 4,7-Dibromo-2,1,3-benzothiadiazole and
4,7-diethynyl-2,1,3-benzothiadiazole were synthesized
according to previous literatures.[28,29] The structures of
1a: Compound 2 (0.332 g, 0.526 mmol), 4 (0.045 g,
0.24 mmol), and CuI (0.005 g, 0.026 mmol) were added
to a 100 mL flask under a nitrogen atmosphere. In the
glovebox, Pd(PPh3)4 (0.030 g, 0.026 mmol) was added
to the flask. When a degassed mixed solution of anhy-
drous THF (20 mL) and anhydrous diisopropylamine
(20 mL) were added to the flask, the mixture was stirred
under reflux for 24 h. The reaction mixture was concen-
trated in vacuum, the residue was purified by column
chromatography (SiO2, CH2Cl2∶THF＝20∶1) to yield
red solid (0.209 g, 75%). 1H NMR (400 MHz, CD2Cl2) δ:
7.68 (s, 2H), 7.43 (d, J＝46.1 Hz, 8H), 6.66 (s, 2H),
5.34 (d, J＝5.1 Hz, 2H), 4.50－4.42 (m, 2H), 2.45－
2.10 (m, 4H), 2.05－1.68 (m, 12H), 1.65－0.89 (m,
46H), 0.85 (d, J＝6.5 Hz, 6H), 0.82－0.72 (m, 12H),
0.62 (s, 6H); 13C NMR (100 MHz, CDCl3) δ: 152.53,
151.54, 151.02, 139.03, 138.48, 132.46, 123.31, 123.28,
120.18, 117.13, 89.88, 79.18, 79.13, 56.69, 56.14, 49.99,
42.33, 39.71, 39.52, 37.93, 36.83, 36.56, 36.19, 35.80,
31.92, 31.85, 28.23, 28.02, 27.64, 24.29, 23.84, 22.83,
22.57, 21.06, 19.29, 18.72, 11.87.
1
the target compounds were characterized by H NMR
and 13C NMR spectroscopy.
Synthesis and characterization
2: 4-Iodoaniline (0.219 g, 1.0 mmol) in CH2Cl2 (10
mL) was stirred at room temperature for 12 h, while
cholesteryl chloroformate (0.539 g, 1.2 mmol) in
CH2Cl2 (10 mL) and triethylamine (0.4 mL) were added
dropwise. The reaction mixture was concentrated in
vacuum, and the residue was purified by column chro-
matography (SiO2, CHCl31∶MeOH＝20∶1) to yield
white solid (0.33 g, 54%). H NMR (400 MHz, CDCl3)
δ: 7.33 (d, J＝8.8 Hz, 2H), 7.21 (d, J＝9.9 Hz, 2H),
6.46 (s, 1H), 5.33 (d, J＝5.1 Hz, 1H), 4.61－4.46 (m,
1H), 2.41－2.21 (m, 2H), 2.03－1.69 (m, 6H), 1.63－
0.87 (m, 23H), 0.85 (d, J＝6.5 Hz, 3H), 0.82－0.77 (m,
6H), 0.61 (s, 3H).
Scheme 1 Chemical structures and synthetic routes of 1a and 1b
Chin. J. Chem. 2015, 33, 1140—1144
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Sun et al.
FULL PAPER
(5∶1) upon ultrasonic treatment or heating–cooling
process (Figure S2 in the Supporting Information). It
can be seen that 1b gels displayed green light upon 365
nm irradiation. As a further investigation, we took a
group of photos illustrating the dynamic process of 1b
gel formation in chloroform/acetonitrile (5∶1) (Figure
1). The results showed that hydrogen bonding is non-es-
sential in the gelation process in our system.
1b: Compound 3 (0.332 g, 0.526 mmol), 4 (0.045 g,
0.24 mmol), and CuI (0.005 g, 0.026 mmol) were added
to a 100 mL flask under a nitrogen atmosphere. In the
glovebox, Pd(PPh3)4 (0.030 g, 0.026 mmol) was added
to the flask. When a degassed mixed solution of anhy-
drous THF (20 mL) and anhydrous diisopropylamine
(20 mL) were added to the flask, the mixture was stirred
under reflux for 24 h. The reaction mixture was concen-
trated in vacuum, and the residue was purified by
column chromatography (SiO2, CH2Cl2∶THF＝20∶1)
1
to yield red solid (0.203 g, 73%). H NMR (400 MHz,
CDCl3) δ: 7.72 (s, 2H), 7.61 (d, J＝8.7 Hz, 4H), 7.18 (d,
J＝8.7 Hz, 4H), 5.36 (d, J＝5.1 Hz, 2H), 4.62－4.45 (m,
2H), 2.53－2.33 (m, 4H), 2.03－1.60 (m, 12H), 1.57－
0.87 (m, 46H), 0.85 (d, J＝6.5 Hz, 6H), 0.82－0.77 (m,
12H), 0.62 (s, 6H); 13C NMR (100 MHz, CDCl3) δ:
154.35, 152.50, 151.54, 139.07, 133.23, 132.46, 123.31,
121.32, 120.18, 117.13, 96.60, 85.46, 79.18, 56.69,
56.14, 49.99, 42.33, 39.71, 39.52, 37.93, 36.83, 36.56,
36.19, 35.80, 31.92, 31.85, 28.23, 28.02, 27.64, 24.29,
23.84, 22.83, 22.57, 21.06, 19.29, 18.72, 11.87.
Figure 1 The formation process of gel of compound 1b by
adding 0.1 mL of acetonitrile dropwise to the solution of 1b in
0.5 mL of chloroform.
Table 1 Gelation ability of 1a and 1b (20 mg•mL−1)
Solvent
1aa 1ab 1bc 1bd
Preparation of gels
Dichloromethane
Acetone
OG OG OG OG
Thermodynamic gels were acquired after the hot
solution (at about 75 ℃) has been cooled to room
temperature (15 ℃). Ultrasonic gels were obtained
when the hot solution was heated (at about 75 ℃) until
the solid was completely dissolved to form transparent
solutions and was treated immediately with ultrasound
for 30 s (0.33 W•cm2, 40 kHz, 15 ℃). Compounds that
could form gels in mixed solvents were obtained by
adding non-solvent dropwise to solution of compounds.
P
P
P
P
P
P
P
P
P
P
P
P
P
Methanol
Acetonitrile
p-Xylene
OG OG OG
Tetrahydrofuran
S
S
S
S
OG OG
Chloroform
S
P
P
S
P
P
Tetrahydrofuran/acetonitrile (5∶1)
Tetrahydrofuran/methanol (5∶1)
Chloroform/methanol (5∶1)
OG OG
OG OG
S
S
S
S
OG OG
OG OG
Results and Discussion
Chloroform/acetonitrile (5∶1)
a,c
Gelation properties of 1a and 1b
Thermodynamic gel (samples were heated to 75 ℃ to be
dissolved and then cooled to 15 ℃). b,d Ultrasound for 30 s (0.33
W•cm−2, 40 KHz); OG: opaque gel; P: heated to dissolve and
precipitated; S: solution.
The gelation properties of 1a and 1b were carried
out with various solvents by inverting the test tube
method. 1a and 1b can form stable organogels in several
organic solvents, including mixed solvents, under room
temperature (Table 1), and the gel state can be main-
tained for more than a month. Generally, in our system,
thermodynamic gels were obtained by cooling the hot
(about 75 ℃) and transparent solution to low tempera-
ture (about 15 ℃) and sonication gels were acquired
through the process of exposing the transparent solution
with ultrasound for about 30 s (0.33 W•cm−2, 40 kHz).
Interestingly, upon ultrasonic treatment or heating-
cooling process, 1a could form reversible gels not only
in dichloromethane and p-xylene, but also in mixed
solvents, such as THF/acetonitrile (5∶1) and THF/
methanol (5∶1) (Figure S1 in the Supporting Infor-
mation). The gels emitted bright yellow light upon 365
nm irradiation. It is surprising that 1b, without hydrogen
bonding, could form reversible gels in dichloromethane
and tetrahydrofuran, as well as in the mixed solvents of
chloroform/acetonitrile (5∶1) and chloroform/methanol
＋
Response to Hg2 of the gels
Interestingly, the gel state, solution and emission
color exhibited striking changes upon addition of 6.6
＋
equiv. Hg2 (0.276 mol•L−1 in acetonitrile, 0.2 mL) into
＋
the gel. The addition of Hg2 resulted in a transition
from yellow gel to black solution after 30 min and 2 d
for 1a and 1b, respectively, and the fluorescence was
quenched simultaneously (Figure 2). The much longer
time required for 1b changing from yellow gel to black
solution than that of 1a showed that the gelation
properties of 1b are obviously better than 1a. After
adding appropriate amount of ethylene diamine tetra-
acetic acid (EDTA) to the broken gels, the gels could
＋
not be recovered, indicating that Hg2 might destroy
the molecule structures of 1a and 1b via the specific
＋
interaction between Hg2 and sulfur atom.[31]
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Chin. J. Chem. 2015, 33, 1140—1144

Multiple-Stimuli Responsive Luminescent Gels Based on Cholesterol
Figure 2 Photograph of gels of 1a (left) and 1b (right) in
dichloromethane (10 mg/0.5 mL) under daylight (top) and UV
＋
light (bottom) before and after Hg2 addition (0.276 mol•L−1 in
acetonitrile, 0.2 mL).
Self-assembly process and mechanism study
The supramolecular structures formed by the xero-
gels (air-dried gels) of 1a and 1b were investigated by
scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) (Figure 3). The xerogels of
1a formed in dichloromethane (20 mg•mL−1) showed
network structures consisting of nanofibers with a width
of 30－100 nm (Figure 3a). From the TEM image as
shown in Figure 3b, it can be found that the xerogels of
1a showed left-handed helical fibers. The gels of 1a that
exhibit non-uniform nanofibers may be due to the
accumulation among nanofibers in different dimensions.
On the other hand, the morphologies of xerogels from
1b in dichloromethane (20 mg•mL−1) consisted of
hollow nanotubes by entrapping the solvent molecules.
The SEM images from sectional view and TEM images
indicated that the nanotubes had the same dimensions,
inner diameter with 16 nm and a wall thickness of 13
nm (Figure 3c and 3d). We found that the morphologies
of xerogels from 1a and 1b formed by thermodynamic
process were similar to those formed by ultrasonic
treatment or in mixed solvents.
In order to investigate the driving force leading to
the formation of the organogels, FT-IR, UV-Vis absorp-
tion and PL spectrum investigations were performed. As
shown in FT-IR spectrum, the gel samples of 1a in
dichloromethane (20 mg•mL−1) showed NH stretching
band at 3324 cm−1, corresponding to that in the solution
state (3334 cm−1), which means the obvious hydrogen
bonding interaction in the gel formed from 1a (Figure
S3 in the Supporting Information). The UV-Vis absorp-
tion spectrum of 1a solution (2×10−5 mol•L−1) in dichlo-
romethane showed the characteristic absorption peaks
appearing at 248, 315, 325 and 434 nm, corresponding
to those at 251, 322, 341 and 474 nm in gel state formed
in dichloromethane (20 mg•mL−1), displaying obvious
redshift phenomena from solution to gel state (Figure S4
in the Supporting Information). The same redshift phe-
nomena occurred between 1b gels and solution (Figure
S5 in the Supporting Information). The UV-Vis absorp-
tion spectra of 1a and 1b with an obvious redshift were
attributed to the formation of a J-type aggregate of the
Figure 3 SEM images of xerogels of 1a (a) and 1b (c) from
dichloromethane solution (20 mg•mL−1), TEM images of
xerogels of 1a (b) and 1b (d) from dichloromethane solution (20
mg•mL−1).
gelators, and π-π interactions of benzothiadiazole cores
may be one of the main driving force for the aggrega-
tion.[3 ] The PL spectrum of 1a displayed a 21 nm red-
0
shift from the solution (2×10−5 mol•L−1, λ＝535 nm) to
the gel (20 mg•mL−1, λ＝555 nm) (Figure S6 in the
Supporting Information). The emissions of 1b also
showed a redshift of 20 nm from the solution (2×10−5
mol•L−1, λ＝518 nm) to the gel (20 mg•mL−1, λ＝538
nm) (Figure S7 in the Supporting Information). The
redshift of UV-Vis absorption and PL spectra of the ge-
lators implies the remarkable π-π interactions between
benzothiadiazole cores in the gel state.
To gain a deeper insight into the assembly mecha-
nism, 1a and 1b in the xerogels state from dichloro-
methane were further investigated by powder X-ray
diffraction. We also used semiempirical AM1 based on
RB3LYP/3-21G to carry out ground-state optimized
molecular-geometry calculations (Figure S8 in the Sup-
porting Information). Compound 1a in gels state
showed one sharp peak at 2θ of 1.84° (Figure 4), corre-
sponding to d spacing of 4.81 nm, measured for the xe-
rogels formed in dichloromethane. This distance of 1a
indicates the molecules aggregate by linear type,[31,32]
which could be due to π-π interactions and hydrogen
bonding interactions. Especially, hydrogen bonding
interactions make molecule linear between benzothiadi-
azole group and cholesterol. However, 1b (the xerogel
formed in dichloromethane) displays a peak at 2.84°,
corresponding to d value of 3.10 nm (Figure 4), which is
shorter than the length of the bent molecular distance of
1b. The d value suggests the molecules assembled by
tight bilayers, in which π-π interactions play an impor-
tant part and the molecule tends to be bent between
benzothiadiazole group and cholesterol. As a result, We
supposed possible accumulation modes of 1a and 1b in
the gels state as shown in Figure 5.
Chin. J. Chem. 2015, 33, 1140—1144
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1143

Sun et al.
FULL PAPER
China (No. 2012CB933301), National Natural Science
Foundation of China (Nos. 21171098 and 51473078),
Program for New Century Excellent Talents in Univer-
sity (No. NCET-12-0740), and Priority Academic Pro-
gram Development of Jiangsu Higher Education Institu-
tions (No. YX03001) for financial support.
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＋
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Chin. J. Chem. 2015, 33, 1140—1144