Synthesis of photoresponsive cholesterol-based azobenzene organogels: Dependence on different spacer lengths

Article abstract of DOI:10.3762/bjoc.11.122

A series of azobenzene-cholesterol organogel compounds (M₀ -M₁₂ ) with different spacers were designed and synthesized. The molecular structures were confirmed by ¹H NMR and ¹³C NMR spectroscopy. The rapid and reversible photoresponsive properties of the compounds were investigated by UV-vis spectroscopy. Their thermal phase behaviors were studied by DSC. The length of the spacer plays a crucial role in the gelation. Compound M₆ is the only one that can gelate in ethanol, isopropanol and 1-butanol and the reversible gel-sol transitions are also investigated. To obtain visual insight into the microstructure of the gels, the typical structures of the xerogels were studied by SEM. Morphologies of the aggregates change from flower-like, network and rod with different sizes. By using IR and XRD characterization, it is found that intermolecular H-bonding, the solvents and van der Waals interaction are the main contributions to the specific superstructure.

Full text of DOI:10.3762/bjoc.11.122

Synthesis of photoresponsive cholesterol-based azobenzene
organogels: dependence on different spacer lengths
Yuchun Ren, Bin Wang* and Xiuqing Zhang
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1089–1095.
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan
Province of China, China West Normal University, Nanchong 637009,
China
doi:10.3762/bjoc.11.122
Received: 16 January 2015
Accepted: 08 June 2015
Published: 29 June 2015
Email:
Bin Wang* - cwnuwangb@126.com
Associate Editor: H. Ritter
*
Corresponding author
©
2015 Ren et al; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
azobenzene; cholesterol; organogel; photoresponsive
Abstract
A series of azobenzene–cholesterol organogel compounds (M0–M12) with different spacers were designed and synthesized. The
molecular structures were confirmed by 1H NMR and 13C NMR spectroscopy. The rapid and reversible photoresponsive properties
of the compounds were investigated by UV–vis spectroscopy. Their thermal phase behaviors were studied by DSC. The length of
the spacer plays a crucial role in the gelation. Compound M6 is the only one that can gelate in ethanol, isopropanol and 1-butanol
and the reversible gel–sol transitions are also investigated. To obtain visual insight into the microstructure of the gels, the typical
structures of the xerogels were studied by SEM. Morphologies of the aggregates change from flower-like, network and rod with
different sizes. By using IR and XRD characterization, it is found that intermolecular H-bonding, the solvents and van der Waals
interaction are the main contributions to the specific superstructure.
Introduction
In the past few decades, low molecular mass organic gelators self-association through weak van der Waals interactions
(
LMOGs) have attracted increasing attention not only for basic [16,17]. Up to now, numerous attempts have been made to
self-assembly behavior but also for their potential application in develop novel supramolecular architectures, which can bring
areas such as templates [1], light harvesting [2], fluorescent about new predictable gelation abilities for the construction of
scensing [3], etc. The driving force for the spontaneous forma- novel cholesterol-based gelators.
tion of gel could be relatively non-covalent interactions such as
π–π stacking [4-7], hydrogen bonding [7-9], dipole–dipole [10], Azobenzene is one of the smartest molecules among all known
and van der Waals interactions [11,12]. A series of cholesterol- photochromic compounds because of its E/Z isomerization [18].
based gelators were reported [13-15]. These new classes of Based on this, photomechanical soft materials containing
organogelator architectures have been systematically studied azobenzene have been utilized in supramolecular systems [19].
because of their reversible gel process and unique directional Examples of several cholesterol-linked azobenzenes with
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Beilstein J. Org. Chem. 2015, 11, 1089–1095.
gel–sol reversible changes also have been studied [20-23]. Results and Discussion
R. Zentel found that the photoinduced reversible gel–sol To investigate the photoresponsiveness of these compounds, the
changes between isotropic and anisotropic gels of a class of dilute THF solution of compounds were irradiated by UV and
semicarbazide–azobenzene-based gelators [24]. A new multi- visible light. Figure 1a shows an obvious increase in the peak at
stimuli photoresponsive organogel containing azobenzene 258 nm and a concurrent decrease in the strong peak at 358 nm.
groups was designed and studied by D. Zhu and his co-workers The photostationary state was attained after 94 min of irradi-
[
25]. Q. Zhang synthesized a series of new symmetric dicholes- ation and the spectral changes are attributable to the trans-to-cis
terol-linked gelators [26]. Upon UV–vis irradiation of the gels, isomerization under UV light irradiation. Further visible light
a reversible gel–sol transition occurred by breaking and (450 nm) irradiation on the cis-solution led to an increase in the
reforming of van der Waals interactions. The cholesterol-substi- absorption intensity at 358 nm and a decrease around 258 nm,
tuted diacetylenic polymerized gels exhibit enhanced stability and the original spectrum recovered by the reverse cis-to-trans
upon thermo- or photo-stimuli [27]. Although similar com- isomerization (Figure 1b). The absorption band at 358 nm
pounds have been discussed, it is still not very clear about the corresponding to the π–π* transition of azobenzene increased,
relationship between the spacer length and the gelation ability respectively. It should be emphasized that such reversible spec-
of gelators. Herein we synthesized a series of compounds with tral transformation could be repeated several times. The trans-
different spacers between cholesteryl and azobenene units to-cis isomerization of the azobenzenze unit had been demon-
(
Scheme 1). In these compounds, M6 is the only one that can strated. Compounds M2, M5, M6, and M12 have the same
gelate in ethanol, isopropanol and 1-butanol. It is found that recoverable photoresponsive properties as M0. Their UV–vis
intermolecular H-bonding, the solvents and van der Waals inter- spectra were shown in Supporting Information File 1, Figures
action are the main contributions to the specific superstructure.
S1–S4. M3 and M10 have the photochromic properties.
Scheme 1: Synthesis route and chemical structure of the compounds.
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Beilstein J. Org. Chem. 2015, 11, 1089–1095.
Figure 1: Changes in the absorption over time in the UV–vis spectra of dilute THF solution of M0: (a) upon UV-light irradiation (λ = 365 nm) and
(b) upon visible-light irradiation (λ = 450 nm) of the solution obtained after irradiation of 365 nm.
Liquid crystal properties of the synthesized
Table 1: Phase transition temperatures of the compounds.
compounds
The structures of these compounds are shown in Scheme 1. The
thermal behaviors of the seven compounds were analyzed by
differential scanning calorimetry (DSC). The DSC curves of the
cholesterol-based supramolecular complexes are shown in
Figure 2 and phase transition temperatures are listed in Table 1.
The DSC thermogram of M0 presented three peaks at 158, 211
and 249 °C on heating, corresponding to the crystal to crystal
transition, crystal to mesophase transition and mesophase to
isotropic transition. On the cooling run, DSC curves presented
isotropic phase to mesophase, mesophase to crystal and crystal
to crystal at 190, 155 and 126 °C. Each curve of compounds
sample
heating (°C)
cooling (°C)
M0
M2
M3
M5
M6
K 158 K2 211 N 249 I
K 92 I
I 190 N 155 K1 126 K2
I 59 K
K 164 N 220 I
K 120 N 197 I
K 78 I
I 224 N 213 K
I 193 N 83 K
—
M10
M12
K 90 I
K 75 N 120 I
I 54 K
I 120 N 42 K
K: crystalline state, N: nematic phase, I: isotropic liquid.
M2, M6 and M10 displayed a single endothermic melting peak. the crystal to mesophase transitions, and the second peaks
The heating DSC curves of M3, M5 and M12 contained two displayed the mesophase to isotropic transitions. The DSC ther-
endothermic peaks. The peaks at lower temperature were due to mogram of M3, M5 and M12 were observed two exothermic
Figure 2: DSC thermogram of the compounds M0-n obtained on (a) heating and (b) cooling.
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Beilstein J. Org. Chem. 2015, 11, 1089–1095.
peaks on cooling process, which indicated the transitions from the change of spacer length in the molecular skeleton has a
isotropic phase to mesophase at higher temperature and significant effect on the gelation abilities of the compounds. To
mesophase to crystal transitions at lower temperature
investigate the photoresponsiveness of the three gels, we
performed the photoirradiation of gels using a UV lamp. The
formed gels at room temperature were in an opaque state.
Gelation behaviors
The gelation behaviors of the seven compounds were tested in Furthermore, the gels with both solvents are very stable for
2 solvents at room temperature and the results were listed in months of storage in a closed container, and no significant
2
Table 2. Among the compounds examined, M6 is the only one changes can be found. However, upon UV irradiation, the gel to
that forms a gel in ethanol, 1-butanol and isopropanol, while the sol phase transition became transparent as a result of trans-to-
other compounds cannot gelate in any solvent. Generally cis isomerization. Moreover, the solution has a tendency to
speaking, the gels were formed by a heating and cooling cycle reverse the sol to gel phase transition under visible irradiation.
process. Examination of the data shown in the table reveals that The corresponding visual images are shown in Figure 3.
Scanning electron microscopy studies
Table 2: Gelation performances of the seven cholesteryl derivatives at
room temperature.
To obtain a visual insight into the morphology of the aggrega-
tion mode, the typical structures of the xerogels prepared by the
freeze-drying of the three gels in different solvents were studied
by SEM (Figure 4). With the reference to the images, it can be
clearly seen that the structures of the three gelators are signifi-
cantly different from each other. The morphologies of the
aggregates change from flower-like to network and rod with
different sizes. Obviously, the gel from ethanol mainly shows a
similar flower-like morphology. When we focused on the
isopropanol xerogel, we could see that there are many regularly
three-dimensional network structures. In 1-butanol, the
organogelator molecules in the gel phase were self-assembled
into tightly staked rods. Different solvents caused a sharp differ-
ence between these morphologies of the xerogels.
Solvent
M0 M2 M3 M5 M6 M10 M12
Methanol
Ethanol
I
I
I
I
I
I
I
I
I
I
I
G
I
I
I
1
1
1
1
1
-Propanol
I
I
I
I
I
I
-Butanol
I
I
I
P
I
G
I
I
I
-Pentanol
-Octanol
I
I
I
I
I
I
P
I
I
I
I
I
I
,4-Butanediol
I
I
I
I
I
I
Isopropanol
Acetone
DMSO
I
I
I
I
G
S
I
I
I
I
P
I
I
P
I
I
P
I
I
I
I
DMF
I
P
S
P
S
S
S
S
P
S
S
S
I
I
I
S
S
I
I
I
EtOAc
I
I
P
I
P
P
S
S
S
S
P
S
S
S
I
P
S
S
S
S
S
P
S
S
S
I
Kerosene
Benzene
Toluene
Xylene
I
I
S
S
S
S
I
S
S
S
S
I
S
S
S
S
P
S
S
S
I
S
S
S
S
P
S
S
S
I
The FTIR spectroscopy
It should be emphasized that the organogels achieved through
strength and directionality of hydrogen bonding. Further infor-
mation about the intermolecular interactions of these gels was
obtained from the FTIR spectroscopy. Figure 5 shows the IR
spectra of M6 powder and its gels in ethanol, isopropanol and
THF
n-Hexane
Dichloromethane
Chloroform
S
S
S
S
S
I
1
3
-butanol. Gels show two peaks which located at 3740 and
Tetrachloromethane S
Distilled water
672 cm−1, characteristic of the hydrogen bonding O–H
I
stretching vibration. As for the powder, the absorption bands
cannot be found in the same vibrational area. The peak
G: turbid gel; S: solution; P: precipitate; I: insoluble.
Figure 3: Photographic images of the M6 sol-gel transition behavior after UV or visible light irradiation in different solution (a) ethanol, (b) isopropanol
and (c) 1-butanol.
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Beilstein J. Org. Chem. 2015, 11, 1089–1095.
Figure 4: SEM images of xerogels formed with M6 from solution: (a) ethanol, (b) isopropanol and (c) 1-butanol.
observed at 3437cm−1 can be assigned to the intermolecular 3.70 nm, respectively. The d-spacing almost in ratio of 7:6:5:4,
association hydrogen stretching vibration. However, the band of may due to the molecular arrangement with order or existence
ethanol, isopropanol and 1-butanol shifted to 3438, 3439 and layers of the stacking between molecular. The last peak may
3
440 cm−1, respectively. The comparison of the IR spectra of relate to the periodic intermolecular distances at different direc-
powder and gels indicated the hydrogen-bond interaction in the tion and intermolecular separations. Two diffraction peaks at
gels. The bands of the asymmetric and symmetric CH2 17.32° and 25.40° were detected for the xerogels of another two
stretching vibration of M6 powder emerged at 2935 and solvents, corresponding to d values of 5.10 and 3.50 nm, res-
2
2
2
852 cm−1, while, in the ethanol gel, they shifted to 2934 and pectively. The different self-assembly modes of gelator in
858 cm−1, in isopropanol gel, they shifted to 2934 and various solvents cause the values changing. The XRD results
862 cm−1, in 1-butanol , they shifted to 2933 and 2859 cm−1. described above also demonstrated that the molecular inter-
The above mentioned FTIR spectral changes demonstrated action and the solvents had a significant impact on the gel for-
sufficiently that van der Waals interaction between the alkyl mation modes.
chains plays an important role in the self-assembly process of
organogels.
Figure 5: FTIR spectra of gels: (a) powder, (b–d) ethanol, isopropanol
Figure 6: XRD patterns of xerogels. (a) ethanol, (b) isopropanol and
and 1-butanol xerogels, respectively.
(c) 1-butanol.
Experimental
The XRD studies
To reveal the detailed changes in gel molecular of M6, XRD Materials
analyses were conducted and the results were displayed in The starting materials, thylene glycol, 1,3-propanediol, 1,5-
Figure 6. The pattern of the xerogel obtained from the 1-butanol pentadiol, 1,6-hexanediol, 1,10-decanediol, 1,12-dodecanediol,
gel exhibited five peaks at 12.84, 15.38, 17.22, and 20.44°, cholesterol, 4-toluene sulfonyl chloride, phenol, 4-aminoben-
corresponding to d-spacing of 6.90, 5.80, 5.14, 4.30, and zoic acid, iodomethane, sodium nitrite, hydrochloric acid,
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Beilstein J. Org. Chem. 2015, 11, 1089–1095.
sodium hydroxide, N,N'-dicyclohexylcarbodiimide (DCC), stirred at room temperature for 24 h. The solvent was evapo-
4
-dimethylaminopyridine (DMAP), all were purchased from rated under vacuum. The product was purified by column chro-
Sigma-Aldrich Chemicals (Shanghai, China). Other used matography over silica gel eluting with pure CH2Cl2, yield
reagents were commercially available and were distilled before 70%. Related compounds (M2, M3, M5, M6, M10, M12) were
used. The synthesis of the 4'-carboxy-4-methoxyazobenzene synthesized by a similar method.
ultilized in this study was described elsewhere [28].
Gelation test
General measurements
The potential gelator (25 mg) in solvent (1 mL) was placed in a
NMR spectra were recorded with a Bruker-400 NMR spectrom- sealed test tube and the solution was heated in a water bath until
eter. Powder X-ray diffraction (XRD) measurements were the solid was dissolved completely. The solution was cooled to
carried out on a Rigaku D/max 2500PC (Japan)-type single- room temperature. If the gelator formed a gel at this stage, it
crystal diffractometer using Cu Kα radiation to determine the was classified as “G”. When the solution remained all the time,
crystal structure. Infrared spectroscopy (IR) was measured on a they were referred to as solutions (S). The hot clear solution
Nicolet 6700 spectrometer from 4000 to 500 cm−1 at room systems, which precipitated when they are cooled down to room
temperature. The samples were prepared as KBr pellets. The temperature, are donated as P (precipitation), respectively.
morphology of the gel samples were examined by scanning Insoluble systems in which the potential gelator could not be
electron microscopy (SEM, S5200, FEI Company) under an dissolved even at the boiling point of the solvent were desig-
acceleration voltage of 30 kV. UV–vis absorption spectra were nated as insoluble (I).
recorded using a spectrophotometer (UV-3600, Shimadzu,
Japan). Differential scanning calorimetry (DSC) was carried out Conclusion
on a thermal analysis (TA) DSC-Q30 in nitrogen at a heating In summary, we have synthesized seven cholesterol-based com-
rate of 10 K/min.
pounds with azobenzene groups. The compounds with different
flexible alkyl spacers containing two, three, five, six, ten, or
twelve carbon atoms are denoted as M2–M12, respectively.
Synthetic procedures
Synthesis of m2–m12. 3β-[(2-hydroxyethyl)oxy]cholest-5-ene Upon UV–vis irradiation, these compounds can undergo a
(
m2) was synthesized similar to a previous report [29]. Choles- typical reversible trans–cis and cis–trans change due to the
terol 15.00 g (38 mmol) in dry dichloromethane (225 mL) and photo-isomerization of the azobenzene units. They possess
dry triethylamine (16.50 mL), p-tosyl chloride 10.35 g recoverable photoresponsive properties. Their thermal phase
(
54 mmol) was added. A catalytic amount of DMAP was also behaviors have been investigated by DSC, indicating good
added. The reaction mixture was then allowed to stir at 48 °C liquid crystal properties. The dramatic change in the gelation
for 12 h. The reaction mixture was washed sequentially with behavior was produced by the different spacer lengths between
diluted HCl aqueous solution, saturated brine solution, and cholesteryl and azobenzene. Compound M6 is the only good
water. The organic phase was dried over anhydrous Na2SO4. gelator that can gelate in ethanol, isopropanol and 1-butanol.
The solvents were removed by vacuum evaporation and re- Based on the results of SEM, IR and XRD, it demonstrated that
crystallized using chloroform and methanol. The residue (7.0 g, the solvents, intermolecular H-bonding, and van der Waals
1
3 mmol) was dissolved in anhydrous dioxane (60 mL) and dry interaction affected the aggregation mode and morphology of
ethylene glycol (20 g, 0.32 mol) was added. The mixture was gels. These results can be applied in fields of design and devel-
refluxed at 110 °C for 4 h. The solution was cooled and solvent opment of novel organogelators and soft matter.
was removed under vacuum. The white residue was dissolved in
chloroform. The organic layer washed with NaHCO3, water,
Supporting Information
and brine; and dried over anhydrous Na2SO4. Finally, the pro-
duct was purified by column chromatography over silica gel
Supporting Information File 1
eluting with petroleum ether/ethyl acetate to yield 90% of m2.
The procedure used for the preparation of m3, m5, m6, m10,
and m12 are similar to that for m2.
Spectroscopical and analytical data.
http://www.beilstein-journals.org/bjoc/content/
[
supplementary/1860-5397-11-122-S1.pdf]
4-Methoxy-4’-((cholesteryloxy)carbonyl)azo-
benzene (M0)
Acknowledgements
A mixture of 4'-carboxy-4-methoxyazobenzene (0.25 g, The work was financially supported by the Program for Scien-
1
1
0 mmol), cholesterol (0.38 g, 1 mmol), and DCC (0.21 g, tific and Technological Innovative Team in Sichuan Provincial
mmol), DMAP (0.06 g, 0.5 mmol) in 25 mL CH2Cl2 was Universities of China (2010008), Key Project of Natural
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Beilstein J. Org. Chem. 2015, 11, 1089–1095.
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