Supra-dendron Gelator Based on Azobenzene–Cyclodextrin Host–Guest Interactions: Photoswitched Optical and Chiroptical Reversibility

Article abstract of DOI:10.1002/chem.201603998

A novel amphiphilic dendron (AZOC₈GAc) with three l-glutamic acid units and an azobenzene moiety covalently linked by an alkyl spacer has been designed. The compound formed hydrogels with water at very low concentration and self-assembled into chiral-twist structures. The gel showed a reversible macroscopic volume phase transition in response to pH variations and photo-irradiation. During the photo-triggered changes, although the gel showed complete reversibility in its optical absorptions, only an incomplete chiroptical property change was achieved. On the other hand, the dendron could form a 1:1 inclusion complex through a host–guest interaction with α-cyclodextrin (α-CD), designated as supra-dendron gelator AZOC₈GAc/α-CD. The supra-dendron showed similar gelation behavior to that of AZOC₈GAc, but with enhanced photoisomerization-transition efficiency and chiroptical switching capacity, which was completely reversible in terms of both optical and chiroptical performances. The self-assembly of the supra-dendron is a hierarchical or multi-supramolecular self-assembling process. This work has clearly illustrated that the hierarchical and multi-supramolecular self-assembling system endows the supramolecular nanostructures or materials with superior reversible optical and chiroptical switching.

Full text of DOI:10.1002/chem.201603998

A Journal of
Accepted Article
Title: Supra-dendron gelator based on Azobenzene-cyclodextrin
Host-Guest interaction: Photoswitched Optical and Chiroptical
reversibility
Authors: Fan Xie, Long Qin, and Minghua Liu
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To be cited as: Chem. Eur. J. 10.1002/chem.201603998
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Supra-dendron Gelator Based on Azobenzene-cyclodextrin Host-
Guest interaction: Photoswitched Optical and Chiroptical
Reversibility
Fan Xie,a Guanghui Ouyanga , Long Qina and Minghua Liu ab*
Abstract: A novel amphiphilic dendron (AZOC8GAc) with three L-
glutamic acid units and an azobenzene moiety covalently linked by
an alkyl spacer was designed. The compound formed hydrogels
with water in very low concentration and self-assembled into chiral
twist structures. The gel showed a reversible macroscopic volume
phase transition responding to pH variations and photo-irradiation.
During the photo-triggered changes, although the gel showed a
complete reversibility in their optical absorptions, only an incomplete
chiroptical property was reached. On the other hand, the dendron
could form a 1:1 inclusion complex via a host-guest interaction with
gels.[18-20]Besides the conventional optical switch upon
alternative UV/Vis irradiation, the chiroptical switch, which is a
reversible change of the chirality in the system, has also
attracting great interest.[21-25] In order to realize the chiroptical
switch based on azobenzene, it is important to transfer the
chirality to the azobenzene moiety since the azobenzene is
achiral. During such chiral transfer, the self-assembly of the
component molecules is necessary. However, in many cases,
although the strong π-π interactions between the azobenzenes
can help the self-assembly, they often cause irreversibility of
trans-cis isomerization.[26-28]Thus, it is necessary to subtly
change the molecular design. An effective way is to incorporate
the azobenzene moiety into the polymer system. While an
alternative way could be through the host-guest inetractions
between the azobenzene and cyclodextrins. Since the pioneer
work of Bortolus and Monti[29] on the azobenzene-cyclodextrin
inclusion complexes, it has been well-established that trans-
azobenzene form host-guest inclusion complex with
cyclodextrins (α-CD, β-CD and γ-CD) in water and the bulky cis-
azobenzene would never fit in the cavity of α-CD due to the
steric repulsion. In this paper, we have designed a dendron
attached by the azobenzene with an alkyl spacer, as shown in
Figure 1.
-cyclodextrin (α-CD), designated as
a supra-dendron gelator
AZOC8GAc /α-CD. The supra-dendron showed a similar gelation
behavior to that of AZOC8GAc but with an enhanced
photoisomerization transition efficiency and chiroptical switching
capacity, which was completely reversible both in the optical and
chiroptical performances. The self-assembly of the supra-dendron is
a hierarchical or multi supramolecular self-assembling process. The
work clear illustrates that the hierarchical and multi-supramolecular
self-assembling systems endows the supramolecular nanostructures
or materials with superior reversible optical and chiroptical switching.
Introduction
Stimuli-responsive supramolecular gels, which undergo
reversible behaviors during the gel to sol or other macroscopic
volume phase transitions upon external stimuli such as
temperature, light, metal ions, electrical or magnetic field, have
attracted great interests.[1-5] A lot of soft smart materials have
been fabricated based on the responsive gels, which find many
applications in various fields such as biological substance
release,[6-8] mechanical shape memory,[9-10] environmental
remediation[11-14]and so forth. Light is one of the most important
stimuli since it is easily controlled and non-destructive. Thus,
various photoactive gels have been developed. Those
photochromic groups such as azobenzene, spiropyran or
dithienylethene were well incorporated into the gels through the
covalent bonds or via the supramolecular approach.[15-17] Among
various photoactive gelators, the azobenzene derivative is one
of the most investigated systems and many azobenzene
derivatives were successfully incorporated into the functional
Figure 1 Photographs of the gels and the illustration on the reversible phase
transitions of (a) conventional gelator AZOC8GAc and (b) Host-Guest
inclusion supra-dendron AZOC8GAc/α-CD. The AZOC8GAc gelator:0.41 wt%
in all cases, AZOC8GAc/α-CD mol ratio is 1/1.
[a] X. Fan, Dr. L. Qin, Prof. Dr. M. Liu
CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics,
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190,
People‟s Republic of China. E-mail: liumh@iccas.ac.cn.
[b] Prof. Dr. M. Liu
Collaborative Innovation Centre of Chemical Science and Engineering,
Tianjin, P. R. China.
With such a molecular design both the gelation ability and the
optical reversibility were greatly enhanced. However, the
chiroptical switching is still not satisfactory. Thus, we further
fabricate the inclusion complex with cyclodextrins (CDs) and
found that the dendron could from a 1:1 host-guest complex
Supporting information for this article is available on the WWW under
http://dx.doi.org/xxxx.
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presents sol-like, while the samples with the gelator concentration between
0.25 wt% and 0.69 wt% form hydrogel.
with α-cyclodextrin (α-CD) in Figure 1, which could be
designated as supra-dendron. Such a supra-gelator shows a
similar gelation behavior to that of the AZOC8GAc but with an
enhanced chiroptical switching capacity, which is completely
reversible both in the optical and chiroptical performances. The
self-assembly of the supra-dendron is a hierarchical or multi
supramolecular self-assembling process. The results indicated
that such a hierarchical self-assembly process endows the
supramolecular system with more complexity and superior
reversible optical as well as chiroptical switching.
Since the dendritic compound AZOC8GAc with four carboxylic
acids was pH sensitive, the pH titration curve at an AZOC8GAc
solution concentration of 1 mM was presented in Figure S1. The
relatively flat regions of the curve correspond to the protonation
constants pKa values, which were found to be 3.4±0.1 and
4.46±0.1, respectively. The corresponding compositions of
AZOC8GAc at various pH were calculated from the equation
pKa
= pH −
lg ([base]/ [acid],[33-34] the negative charge
distribution from 0 to 4 under different pH values were obtained.
Based on the above results, the assemblies formed at selected
pH values showed different phase behavior and micro properties.
It was found that the pH value of the original hydrogel was
located at 3.4, well-defined helical twists were obtained (Figure
3b), exhibited the widths ranged from 7 to 15 nm. TEM images
(Figure S2) revealed the ultrafine nanofibres with an average
Results and Discussion
Hydrogelation and pH-responsiveness of Azo-dendron
(AZOC8GAc)
The molecular structure of the hydrogelator AZOC8GAc was
shown in Figure 1. The solid compound was dispersed in water
(1ml) and slightly heated to 50 °C, a light yellow solution could
be obtained. After cooling down for some time to room
temperature, transparent hydrogel could be obtained, which was
confirmed by the inverted test tube method.[30] It was found that
the lowest critical gelation concentration (CGC) to form a
hydrogel was near to 0.25 wt% and the sample required four
hours to reach stability. Moreover, the formation of the hydrogel
with the gelator concentration increasing to 0.69 wt% only
needed 30 minutes. Figure 2a shows the absorption spectra of
the dendron in various concentrated solution or gels. The
maximum absorption peak showed a blue-shift with gelator
concentration increasing, indicating the π-π stacking of the
azobenzene moieties. Dynamic rheological measurements[31-32]
were performed to characterize the rheological behavior of the
sample with different concentration of 0.41wt% and 0.15 wt%.
From the frequency sweep measurements ( Figure 2b) , both G′
and G″ of the hydrogel sample at 0.41 wt% increased with the
increasing frequency, and showing a storage modulus G′
greater than the loss modulus G″, which evidenced a true gel
compared to sol-like sample at 0.15 wt%. As shown in Figure S
1d, the strain sweep tests were conducted for the 0.41wt% gel
from 0.1% to 100% strain at 20 °C, with a constant frequency of
6.28 rad s−1. When the strain increased over 50%, the G″ value
was greater than G′, revealing the collapse of the gel networks
and the dominant fluid character of the hydrogel.
diameter around
5 nm and further deduced the helical
nanostructures may due to the helical entanglement of Nano
fibers. If the system was acidified to pH 2, stable hydrogel was
still obtained. In this case, thicker nanofibers with average
diameter of 8 nm were obtained, but without obvious helical
structures. Moreover, when increasing the sample pH to 4, the
increased ionization of carboxyl group resulted in a partial
hydrogel, in which helical nanostructures randomly distributed
and partly relaxed. At pH 5.5 or above, the hydrogel converted
to a sol and the morphologies changed completely from helixes
to non-continuous packed structures. UV-Vis and CD spectra
further revealed the pH responsiveness of the hydrogel (Figure
3a). For the UV-Vis spectra of the system pH fell in the range of
2-4, the maximum peak appeared at 326nm, which can be
ascribed to the π-π* transition of the trans-form azobenzene
moiety in an aggregated state. Upon regulating the sample to pH
5.5, the peak further shifted to 343 nm which is exactly
corresponding to the disassembly with pH increasing. In the CD
spectra,[35-36] the hydrogel at pH 3.4 presented
a strong
symmetric exciton couplet Cotton effect with a crossover at 328
nm, and positive peak and negative valley at 315 nm and 346
nm, respectively. This spectrum indicated that the chiral transfer
from chiral carbon atom to achiral azobenzene head group was
due to the self-assembly of the dendron. When pH changed, the
CD spectrum showed also a change. An intensive negative CD
splitting band was observed at pH 2 of the hydrogel, appearing a
minimum negative band at 365nm and a maximum positive band
at 301 nm. For the system pH at 5.5, negative CD band with
decreased intensity was located at 341nm. These spectral
changes reflected that the packing of the azobenzene moiety
could be subtly regulated by the pH variations of the gel.
Figure 2 (a) concentration-dependent UV/Vis spectra, and (b) evolution of G′
and G″ as a function of angular frequency AZOC8GAc assembly at different
concentration, in which the sample with gelator concentration at 0.15 wt%
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intensity upon visible light stimuli. Therefore, the CD intensity at
346 nm was steadily decreased after several UV/Vis irradiation
cycles which manifesting an irreversible supramolecular
chiroptical switching, as shown in Figure 4c. Furthermore, Figure
4d revealed that the photo-triggered isomerization process from
tans-azobenzene to cis-azobenzene resulted in a dramatic
morphology change, during which the helical twists
disassembled into lamellar fragments but the CD signal was not
completely disappeared. AFM section analyses (Figure 4e and
4f) provided more precise information about the helical assembly.
Most helical fibers was obtained with a height of approximately
7.8 nm and a helical pitch of around 90 nm, whereas few thinner
fibrils with a diameter of around 4.3 nm and thicker helixes with
height of 15nm were also observed. Depending on these
analysis, it could interpret that the helical nanostructures were
predominantly bundles of the nanofibers. The results discussed
above indicated that the segmentally conformational change of
the gelator may bring more vital effects on the stabilization of
helixes than the spectral changes with respect to the
supramolecular hydrogel.
Figure 3 (a) CD and UV-Vis spectra, (b) AFM images of the AZOC8GAc
hydrogel at different pH values. Gelator AZOC8GAc is 0.41wt%.The inset
image of Fig 3b showing the amplified defined helicity for the hydrogel at pH
3.4.
Light controlled reversible morphology and chirality
As is well known, diverse light responsive supramolecular
assemblies are designed as functional materials which is related
to the reversible E/Z-isomerization of azobenzenes.[37-38] Our
azobenzene attached dendron showed similar photo-
responsiveness to the light.
Figure
4
showed the
supramolecular chirality changes accompanied with the
morphological transformations for the conventional hydrogel
upon alternative UV/Vis irradiation. As the hydrogel proceeding
with reversible Gel-Sol transition stimulated by UV light (365 nm),
the assembly showed the characteristic changes in the
absorbance spectra (Figure 4a) with a decrease in the π-π*
absorption band of the trans-form azobenzene moieties at 326
nm and the appearance of the n-π* transition of cis-form
azobenzene groups at 431nm. Upon irradiation with visible light
(420 nm), the return efficiency from cis-form to trans-form
reached to 94%. Though the gel showed a complete reversible
optical switching, the transition efficiency of trans to cis
isomerization was only 65% based on the quantitative
calculation (as seen in Figure S3),[39-40] which may result from
the azobenzene chromophores having strong π-π* interactions
and being restricted heavily in the hydrogel. For the CD spectral
measurements (Figure 4b), significant Cotton effect was
observed in the gel state, and the CD intensity presented a slight
photo driven decrease but could not recover to the original
Figure 4 (a) UV-Vis spectra and (b) CD spectra of AZOC8GAc hydrogel under
alternative UV/Vis irradiation (AZOC8GAc hydrogel, Green line; the sample
under UV irradiation for 15min, Red line; the sample under Vis irradiation for
30min, Blue line). (c) CD intensity of AZOC8GAc hydrogel as a function of the
irradiation cycle at 346nm. (d) AFM images revealing the morphological
changes of AZOC8GAc hydrogel under UV irradiation. (e, f) AFM sectional
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analysis of AZOC8GAc hydrogel (height along the red arrows and helical pitch
labelled by blue triangles).UV light (30W at 365 nm) and visible light (40 W
compact fluorescent lamp, >400nm). Gelator AZOC8GAc is 0.41wt%, the
assembly pH is 3.4.
complex deviated from the lamellar. The largest d spacing of
1.73 nm is close to the single H-G molecule length with the alkyl
chain inclined.
Host-guest and light controlled reversibility of the
supramolecular chirality
It is well-known that trans-azobenzene can fit into the cavity of
cyclodextrin (CDs) and
spontaneously form supramolecular
complex.[41-42] To this end, isothermal titration calorimetry (ITC)[43]
was carried out to determine the stoichiometry and binding
constants of inclusion complexes. The titration curves of each
titration were found in Figure S6 and corresponding ITC results
of each titration were summarized in Table 1 of SI, revealing that
complexation stoichiometry between AZOC8GAc gelator and α-,
β- or γ-cyclodextrin was approaching to be 1/1, 5/4, or 2/1.
Moreover, the binding constants for the inclusion with the α-, β-
or γ- cyclodextrin were calculated to be 98.4 ×102, 26.6×102 and
29.1×102 M-1. Thus, for host−guest inclusion complex with
azobenzene, the α-CD had a higher association constant to
serve as a suitable host compared with β- or γ-CD. 1H NMR
spectroscopy was further carried out to investigate the binding
behavior of AZOC8GAC with CDs. As shown in Figure 5a, in
comparison with the spectrum of AZOC8GAC assembly, peaks
related to protons Ha-d on azobenzene part of AZOC8GAC
displayed pronounced downfield shifts after adding α- or γ-CD
on a stoichiometric ratio, whereas the initial signals of the
azobenzene moiety still partly retain in AZOC8GAc/β-CD
inclusion. However, Figure S8 showed that the signals related to
protons He, Hf on alkyl chain of AZOC8GAc displayed almost
negligible downfield shifts with an increasing amount of α-CD, in
which the △δ values were 0.22 for He, 0.08 for Hf,[44] and while
the peaks related to protons H3 and H5 located inside the cavity
of α-CD exhibited gradual upfield shifts. Furthermore, 2D nuclear
Figure 5 (a) 1H NMR spectra (azobenzene region) (b) CD / UV-Vis spectra of
AZOC8GAc hydrogel and AZOC8GAc/ Cyclodextrin complex. (c) Schematic
illustration of the „host-guest‟ inclusion mode with the stoichiometric ratio at 1:1
for α- , 5:4 for β- , and 2:1 for γ-CD, AZOC8GAc gelator: 0.41 wt %).
With regard to the exactly complete inclusion of azobenzene
with α-CD, we focused on investigations of 1:1 AZOC8GAc/α-
CD complex hydrogel. Firstly, for AZOC8GAc/α-CD complex
after irradiating with 365nm UV light, 1H NMR spectroscopy
(Figure S7 c) showed the resonance peaks corresponding to the
protons Ha‟-d‟ of cis-azobenzene part arose with no obvious peak
shifts which was similar to the free cis-azobenzene of
AZOC8GAc assembly upon photoirradiation (Figure S7 d ).
These phenomena provided evidence that AZOC8GAc/α-CD
complex exhibited excellent light responsive isomerization
Overhauser
enhancement
spectroscopy
(NOESY)
of
AZOC8GAc/α-CD complex showed the clear NOE correlations
between the protons H3 and H5 on inner α-CD cavity and protons
on azobenzene part of AZOC8GAc (Figure S9). All these results
demonstrated that the azobenzene moiety of AZOC8GAc was
threaded into the cavity of α-CD, but the alkyl chain moiety of
AZOC8GAc just have extremely weak interaction with α-CD.[45]
The FT-IR spectra (Figure S10 a) demonstrated that in case of
AZOC8GAc/α-CD complex, the N-H stretching frequencies at
3306.5 cm-1 were disappearing with the stretching frequencies of
primary and secondary alcoholic groups of CDs appeared
around at 3375 cm−1,[46] while amide I at 1655cm-1 and amide II
at 1545cm-1 significantly decreased associated with weakened
hydrogen bonding. Therefore, NMR spectra or the FT-IR
spectral study gave the supporting evidence for the formation of
more stable inclusion complex between AZOC8GAc and α-CD.
As seen in Figure 5c, azobenzene moiety was exactly located
inside the cavity of α-CD to form 1:1 AZOC8GAc/α-CD inclusion
while the azobenzene was not easily accessible to β-CD though
it had larger cavity diameter due to lower binding constant. The
X-ray diffraction patterns of four xerogels were also investigated,
as shown in Figure S10 b. For the AZOC8GAc assembly, there
were four diffraction peaks with the d spacing ratio of roughly
1:1/2:1/3 and 1:4, corresponding to the d spacing of 3.8 nm in a
lamellar structure. Since the length of 3.8 nm was larger than the
extended molecular length of 2.37nm but smaller than twice the
length, indicating the interdigitated bilayer structure of the
hydrogel.[47] However, for the complex hydrogel of
AZOC8GAc/α-CD system, three diffraction peaks corresponding
to the d spacings of 1.73 ,1.34,and 0.92 nm in the low angle
region were observed. This means that the structure of the H-G
behavior
and the cis-form azobenzene was mostly located
outside the cavity of α-CD. Furthermore, it was interesting to
investigate this complex assembly featuring light-controlled
complete reversible optical and chiroptcial properties
accompanied with morphological transformations. Figure 6a
revealed the absorption peak of the complex hydrogel has red-
shift to 333 nm due to the host-guest inclusion between
azobenzene group and cyclodextrin cavity. The exciton couplet
Cotton effect from AZOC8GAc/α-CD hydrogel observed was
similar to the AZOC8GAc hydrogel which may ascribe to the
chirality transfer from the chiral center to azobenzene part
through short alkyl spacer was more significant than from α-CD
with inherent chirality. In addition, the host-guest interactions
decreased π-π* packing of azobenzene groups and then
improved the photo transition efficiency from trans to cis-form.
Therefore, the spectral characterizations (Figure 6b) showed
that the CD signal almost disappeared under UV irradiation, and
reversely recovered by the visible light. Therefore, for
AZOC8GAc/α-CD complex, repeated photoirradiation cycles
resulted in virtually the same chiroptical activity which indicated
good reversibility of this chiroptical switching (Figure 6c). The
AFM images described the reversible photo modulation on
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Figure 6 (a) UV-Vis spectra and (b) CD spectra of AZOC8GAc/α-CD hydrogel
under alternative UV/Vis irradiation (AZOC8GAc hydrogel, Light green line; the
sample under UV irradiation for 15min, Red line; the sample under Vis
irradiation for 30min, Blue line). (c) CD intensity of AZOC8GAc/α-CD hydrogel
as a function of the irradiation cycle at 346nm. (d) AFM images revealing the
reversible morphological changes of AZOC8GAc/α-CD hydrogel under
alternative UV/Vis irradiation. UV light (30W at 365 nm) and visible light (40 W
compact fluorescent lamp,>400 nm). (AZOC8GAc gelator: 0.41 wt%, the molar
ratio of AZOC8GAc/ α-CD is 1/1, the co-assembly pH is 3.2).
micro-level morphologies of AZOC8GAc/α-CD complex. As
shown in Figure 6d, the fragmental irregular nanoaggregates
regulated by host-guest inclusion finally transformed into more
amorphous particles upon UV irradiation. However, the same
nanostructures were also obtained for the recovered sample
irradiated by visible light.
Figure
7 Schematic illustration on the AZOC8GAC assembly: (a) For
AZOC8GAC conventional hydrogel, the amphiphilic gelator formed
interdigitated bilayer in which azobenzene groups packed face to face (π-π
stacking), and then light induced the transition of azo groups from trans-form
to cis-form and AZOC8GAC hydrogel collapsed. (b) For the AZOC8GAc/α-CD
supra-hydrogel, “host-guest” interaction between azobenzene and cyclodextrin
caused the loose packing and formed the nanoaggregate. Photo irradiation
further caused the isomerization of Azo accompanying with the disassembly of
AZOC8GAc/α-CD complex, while partial of cis-AZOC8GAc/α-CD complex still
formed the aggregate since there are close to the cyclodextrin.
original bilayer transformed into more relaxed stacking of the
supra-dendron, which formed irregular nanoaggregates. Upon
irradiating the complex sample with UV light, the decreased π-π*
packing of azobenzene groups due to “host-guest” interactions
caused complete photo isomerization of azobenzene moieties
within the AZOC8GAc/α-CD complex. Therefore, the
azobenzene group came out from the cavity and led to the
disassembly of the nanoaggregates. Since the cis-azobenzene
is close to the cyclodextrin, some of them can still form the
smaller nanoaggregate. Interestingly, the photo driven
morphological and chiroptical switching was fully reversible
under alternative UV/Vis irradiation with regard to the
AZOC8GAc/α-CD hydrogel.
Discussion
Based on the above results, a possible mechanism concerning
the novel AZOC8GAc assembly with multi-responsiveness to
chiral helixes and chiroptical properties could be proposed. The
AFM or TEM images have revealed that the dendron self-
assembled into helical fibrils. The FT-IR, UV/Vis spectra and X-
ray diffractions further confirmed that the gelator molecules
formed interdigitated bilayer in which strong π-π stacking of
azobenzenes groups as well as the hydrogen bond between
carbonyl groups played important roles in the formation of stable
hydrogel and thus produced split Cotton effect for azobenzene
parts. When AZOC8GAc hydrogel was subjected to UV
irradiation, the helical structures disassembled into lamellar
fragments with hydrogel collapsing. However, strong
interchromophores
interaction
confined
the
complete
Conclusion
isomerization of azobenzene groups and thus resulted in the
corresponding CD signal incompletely disappearing. The visible
light subsequently induced the cis-form recovering to its original
packing. Meanwhile, the photo reversible CD change between
trans-form and cis-form hydrogel was able to repeat several
times under alternative UV / visible light irradiation. In the case
of AZOC8GAc/α-CD complex, stable hydrogel was obtained with
similar exciton couplet. The strong “host-guest” interaction
between azobenzene group and cyclodextrin cavity caused the
In summary, we demonstrated that an amphiphilic dendron
AZOC8GAc with azobenzene moiety could form responsive
hydrogels to pH and light. The chirality localized at the glutamic
acid can be transferred to azobenzene moiety through a long
alkyl spacer and generated twisted helical nanostructures and
strong split Cotton effect. The hydrogel showed a reversible
optical switch upon alternative UV/Vis irradiation but with
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with a METTLER TOLEDO/FE20 PH meter at a room temperature
around 20 °C . The electrode was calibrated with a standard buffer
solution before being used. Three titrations were performed, and the
mean pKa values were calculated.
incomplete chiroptical reversibility. On the other hand, through
the host-guest interaction, the dendron can be derived into a
supra-dendron, in which azobenzene moiety formed a 1:1
complex with α-cyclodextrin. Such AZOC8GAc/α-CD supra-
dendron could form the hydrogel also. Besides the reversible
optical absorption upon alternative UV/Vis irradiation, the supra-
dendron gel showed also a complete chiroptical reversibility.
This offers great potential to develop new functional
The calculation of stoichiometry and binding constant ‘Ka’ of the
AZOC8GAc-cyclodextrin inclusion complexes: Isothermal titration
calorimetric measurements (ITC) were performed on a MicroCal Auto-
iTC200 instrument, which was composed of a reference cell and a
sample cell of 400ul. Stock solutions of host (1 mM, 1 mL) and guest (10
mM, 0.5 mL) in water were prepared using volumetric glassware. Before
each titration, all the solutions were degassed and kept constant
temperature. In a typical run, a 200μL syringe was full of guest (10 mM)
and the cell was loaded with host (1 mM, 400ul). The titration of the host
with the guest was carried out at 298 K, 20 injections of 2 μL with a
equilibration time interval of 120s each injection. Each injection of ligand
resulted in a heat pulse that is integrated with respect to time and
normalized for concentration to generate a titration curve of kcal/mol vs
molar ratio (guest/host). The resulting isotherm was fitted to a binding
model to generate the affinity (KD), stoichiometry (n) and enthalpy of
interaction (ΔH). All the data were analyzed with Microcal Origin 7.0
software provided by the manufacturer.
supramolecular
materials
with
the
bridge
between
supramolecular chemistry and chiral nanostructures. Nowadays,
the supramolecular self-assembly is towards more complex
system to realize the new functions.[48] The present result clear
indicates that the hierarchical or multi supramolecular self-
assembling endows the supramolecular system with superior
responsive behaviors.
Experimental Section
Chemicals and reagents: All starting materials used in synthesis of
AZOC8GAc were obtained from commercial suppliers and used without
any additional purification, in which the reactant Boc-GE was synthesized
according to our previous work.[49] All the organic solvents were purified
and dried according to standard methods. Milli-Q water (18.2 MW cm)
was used in all experiments.
Apparatus and measurements: The UV/Vis and CD spectra were
detected using JASCO UV-550 and JASCO J-810 spectrophotometers,
respectively. Fourier transform infrared (FT-IR) spectra were recorded on
a
JASCO FT/IR-660 plus spectrophotometer with a wavenumber
resolution of 4 cm-1at room temperature. X-ray diffraction (XRD) was
achieved on a Rigaku D/Max-2500 X-ray diffractometer (Japan) with
CuKa radiation (λ=1.5406Å), which was operated at 45 kV, 100 mA. AFM
was performed by using Fastscan mode (Nanoscope IIIa, Digital
Instruments) with a pyramidal Si3N4 tip. The HR-TEM images were
obtained on a Tecnai G2 F20 U-TWIN operating at accelerating voltages
of 200 kV. PH value was obtained on the METTLER TOLEDO/FE20 PH
at a room temperature around 20 °C. Rheological measurements were
conducted on Discovery DHR-1 Rheometer (TA Instruments) using a
20mm parallel plate geometry in a Peltier plate. The frequency-sweep
spectra were measured in a constant strain (0.1%) mode over the
frequency range of 0.1–100 rad s−1 at 20°C. The strain-sweep spectra
were measured in a constant frequency of 6.28 rad s−1 over the strain
range of 0.1–100% at 20 °C.TLC was performed on silica gel HF254
flake and column chromatography was carried out with 230–400 mesh
silica gel. Isothermal titration calorimetric measurement (ITC) were
performed on a MicroCal Auto-iTC200 instrument. 1H NMR spectra
and13C NMR spectra were recorded with a Bruker Fourier 300 (300 MHz)
spectrometer in CDCl3 or DMSO-d6 by using Me4Si as an internal
standard.1H NMR spectra or 2D-NOESY spectra of assembly were
recorded with a Bruker Avance DMX 400 spectrophotometer or a Bruker
Avance DMX 600 spectrophotometer in D2O. MS spectra were
determined with BEFLEX III for the ESI mass spectrometer. Elemental
analyses were performed on a Carlo–Erba-1106 instrument. All the
photographs of the gel were taken by using a Canon EOS60D.
The Synthetic procedures of gelator AZOC8GAc:The syntheses of
AZOC8OH, AZOC8GAc illustrated in Scheme S1. More details of
syntheses and characterization of these compounds were provided in the
ESI.
Preparation and characterize of hydrogel: The typical hydrogel was
spontaneously formed by adding AZOC8GAc gelator (1.5, 2.5,4.1, 5.5,
6.9 mg) to pure water (1 mL) respectively, which was slightly heated to a
light yellow solution at 50 °C and then rest at room temperature for 30
minutes. The stability of hydrogel increased with concentration. Dynamic
rheological measurements were performed to demonstrate viscoelastic
property of AZOC8GAc hydrogel (0.41 wt%, the total volume was 1ml).
The hydrogels at various pH values was prepared by mixing AZOC8GAc
with different pH aqueous solutions, which were adjusted by NaOH and
HCl solutions. For AZOC8GAc/cyclodextrins (CDs) complex, The gelator
(4.1mg) was dissolved in water to form transparent aqueous solution.
Then, an aqueous solution of α-CD, β-CD or γ-CD was mixed into the
above solution, respectively. Subsequently, the mixed solution was
heated and cooled down, resulting in the rapid formation of co-
assembled hydrogel. The mol ratio of α-CD, β-CD or γ-CD to AZOC8GAc
was kept at 1, 0.8, 0.5, respectively and the total volume was kept at 1ml.
Photo-regulated experiment was carried out by UV light (30 UVA lamp,
λ=365 nm, 20cm away from the sample) at cool condition and further
converted the sample from Gel to Sol. The hydrogel obtained (0.41 wt%,
the total volume was 1ml) by the above methods were firstly diluted with
water, cast on a freshly cleaved mica surface and then dried in a vacuum
for AFM measurements. For the TEM measurements, a small amount of
diluted hydrogel was placed onto carbon coated copper grid and dried in
a vacuum or exposed to UV light. As for FT-IR spectra and XRD
measurements, the samples were cast onto silicon slices and dried as
mentioned above. In the process of measuring the UV and CD spectra of
hydrogels, a quartz cuvette with 0.1 mm width was used. All the samples
for the characterizations were irradiated by UV (30 W UV lamp with main
light at 365 nm) and visible light (40 W compact fluorescent lamp, >400
nm visible light) alternatively to see their changes.
Acknowledgements
We acknowledge the financial support from the National Natural
Science Foundation of China (No. 21473219, 91427302, and
2132106), the Basic Research Development Program
(2013CB834504) and the Fund of the Chinese Academy of
Sciences (No. XDB12020200).
Keywords: supra-dendron • chiroptical switching • chiral twist •
Potentiometric pH Titration: AZOC8GAc was first dissolved in water at
a concentration of 1.0 mM, and then the sample pH was adjusted to 11
with 0.1 M NaOH aqueous solution. Then 0.1 M HCl was gradually added
into this solution in small portions. The titration process was monitored
supramolecular hydrogel • photoresponsiveness
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Fan Xie, Long Qin and Minghua Liu *
Page No. – Page No.
Supra-dendron gelator based on
Azobenzene-cyclodextrin Host-Guest
interaction: Photoswitched Optical
and Chiroptical reversibility
The constructed dendron hydrogel showed a reversible optical switching but an
incomplete chiroptical switching. While the supra-dendron composed of 1:1
inclusion complex with α-CD displayed both complete reversible optical and
chiroptical switching.
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