Photocontrolled morphological conversion and chiral transfer of a snowflake-like supramolecular assembly based on azobenzene-bridged bis(dibenzo-24-crown-8) and a cholesterol derivative

Article abstract of DOI:10.1039/c9cc01874c

A snowflake-like supramolecular clockwise-helical assembly was fabricated via the host-guest interaction of trans-azobenzene-bridged bis(dibenzo-24-crown-8) (trans-1) and a cholesterol derivative, while a snowflake-like supramolecular non-helical assembly

Full text of DOI:10.1039/c9cc01874c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
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DOI: 10.1039/C9CC01874C
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Photocontrolled morphological conversion and chiral
transfer of a snowflake-like supramolecular assembly
based on azobenzene-bridged bis(dibenzo-24-crown-8)
and a cholesterol derivative
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
a
a
a
a,b
Hui-Juan Wang, Heng-Yi Zhang,* Huang Wu, Xian-Yin Dai, Pei-Yu Li and Yu Liu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A snowflake-like supramolecular clockwise-helical assembly lanthanide luminescent molecular11 and so on. Many cholesterol
was fabricated via the host–guest interaction of trans- linked crown ethers were reported due to their applications in
1
2
13
14
15
azobenzene-bridged bis(dibenzo-24-crown-8) (trans-1) and a catalyst, ion selection, organic gels and biology. Some
cholesterol derivative, while a snowflake-like supramolecular cholesterol-stoppered rotaxanes constructed by crown ether
non-helical assembly can be obtained upon complexation of represented gelation and switchable properties to pH, redox
1
6
cis-1 after UV-irradiation, accompanying the disappearance of change. All these cholesterol derivatives linked crown ether only
the circular dichroism signals in the azobenzene zone of 1.
showed the property of either cholesterol or crown ether, while the
host-guest interaction supramolecular chiral system based on
cholesterol derivatives was not reported, to the best of our
knowledge. Here we introduced a photoresponsive azobenzene-
bridged bis(dibenzo-24-crown-8) to a cholesterol derivative that
result in a macroscopic chirality (nanoscale chirality that could be
Chiral supramolecular assemblies have been attracting extensive
interest due to their controllable structural features, accessible
preparation, and potential applications in chiral recognition and
1
separation. Especially their chirality transfer is expected to apply for
the development of versatile chiral chiroptical switching, chiral
1
recognized by microscope) and chiral transfer from guest to host.
2
catalysis and sensing systems. Cholesterol as a chiral motif and it’s
Interestingly, the chirality in macroscopic level and chiral transfer
effect is photocontrolled. The illustration of the conversion of
chiral/archiral snowflake nanostructures under different light
irradiation is shown in Scheme 1.
derivatives are a sort of significant candidate for the construction of
the chiral supramolecular assemblies and has been widely used in
1
a,3
building blocks of chiral supramolecular nanostructures,
such as
8
4
5
6
7
nanofiber, nanotube, nanoroll, helic robin, fluffy globular and
nanoflower. Recently, Zhao et al. constructed a manipulated
8
property transfer nanosystem through the coassembly of two
4
a
cholesterol building blocks, and a precise control over chirality and
6
dimensions self-assembly by an azobenzene modified cholesterol.
On the other hand, the binding or encapsulation of a chiral guest in
an achiral cavity has been proven to be an effective method for
1
a
chirality transfer. Crown ether has a flexible cavity that can
construct various supramolecular assemblies with specific functions.
Recently, we constructed some stimuli-responsivity supramolecular
assemblies by crown ethers, such as luminescent lanthanide
supramolecular assembly tuned by the photoreaction of
9
anthracene, reversibly photoswitchable supramolecular assembly
applied as a photoerasable fluorescent ink,10 dual-stimuli reversible
a. Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, P. R. China.
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Scheme 1 Schematic representations of 1, 2 and 21.
b.
Brief synthesis routes of azobenzene-bridged bis(dibenzo-24-
crown-8) (1) and cholesterol modified secondary ammonium salt (2)
were shown in Scheme S1 (ESI†). Benefiting from the
Tianjin 300072, China.
E-mail: hyzhang@nankai.edu.cn, yuliu@nankai.edu.cn
c.
†
Electronic Supplementary Information (ESI) available: Detailed synthesis,
characterization data, Job’s plots, association constants, absorption spectra and
transmission electron microscopy images. See DOI: 10.1039/x0xx00000x
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photoisomerism of azobenzene unit, 1 showed high conversion for 50 minutes, 1 comes to another photostationary state and a
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1
DOI: 10.1039/C9CC01874C
efficiency, excellent reversibility and repeatability. The trans-cis series of new signals appeared in the H NMR spectrum. As shown in
photoisomerization of 1 could be confirmed by the variation of the Figure S12 (ESI†), the protons (H , H and H ) of the azobenzene
ultraviolet (UV) absorption spectra upon alternating irradiation with moieties showed obvious upfield shifts (∆δ = 0.10-0.94 ppm). Also
a
b
C
1
7
d
UV and visible light. Generally, the trans-isomer of an azobenzene the aromatic protons (H ) of the dibenzo-24-crown-8 moieties
derivative can transform to the cis-isomer under the irradiation by showed appreciable upfield shifts (∆δ = 0.10 ppm). The integral of
UV light, and it can reverse back to the trans-isomer through either the proton signals indicated that the isomerization efficiency of
1
8
irradiation with visible light or heating. To quantify the rate of trans-1 to cis-1 upon irradiation at 365 nm light is 95%. Then after
conversion between trans-1 and cis-1, the irradiation time the irradiation with visible light (> 420 nm) for 30 minutes, 1 comes
dependence of the UV absorption spectra for 1 upon the irradiation to another photostationary state (trans-1). The isomerization
at 365 nm was detected (Fig. S11, ESI†). In dichloromethane, the efficiency of cis-1 to trans-1 upon irradiation with visible light (> 420
UV/vis spectrum of 1 shows a strong π-π* transition at 358 nm and a nm) is 89%, which was calculated by the integral of the proton
weak n-π* band near 440 nm, which is the typical absorbance of signals. Subsequently, after putted in dark for one day the resulting
1
trans-azobenzene. After the irradiation with UV light (365 nm) for 15 isomerized solution go completely back to the initial state. The H
seconds, the absorption at 358 nm sharply decreased, a stronger n- NMR results further confirm the high conversion efficiency, excellent
π* transition arose at 450 nm and weaker bands arose at 310 nm and reversibility and repeatability for 1. Further, direct morphological
2
77 nm which is the typical absorbance of the cis-azobenzene.19 As information of 1 was investigated by transmission electron
can be seen from Figure 1a, the colour of 1 changed from light yellow microscopy (TEM), scanning electron microscopy (SEM) and dynamic
to orange after irradiation with UV light (365 nm) for 15 second, light scattering (DLS). As shown in Figure 2 and Fig. S14 (ESI†), the
which was ascribed to the trans-isomer transformed to cis-isomer aggregation of both trans-1 and cis-1 formed nanoparticles in its TEM
accompanying with the new absorption peak arose at 450 nm. It is and SEM images. And, the DLS data showed trans-1 in CH Cl solution
2 2
also interesting to further investigate the repetitiveness of the form nanoparticles with an average hydrodynamic diameter of about
photoisomerism of 1. As shown in Figure 1b, the absorption of trans- 412 nm, while cis-1 is about 458 nm with the same morphology (Fig.
1
at 358 nm, the characteristic absorption of the π-π* transition of S14c, d, ESI†).
trans azobenzene, markedly decreased under the light irradiation at
65 nm for 15 seconds, indicating photo isomerization of 1 from
3
trans to cis. After irradiated with visible light (> 420 nm) for 2
seconds, the absorption at 358 nm was recovered, suggesting the
reverse photoisomerization from cis to trans. Significantly, this cycle
could be completely repeated ten times without any fatigue (Figure
1
b), indicating good reversibility and repetitiveness properties of 1.
Scheme 2 Schematic representations of assembly based on the photo-responsive
interconversion of 2trans-1 and 2cis-1.
The properties of 2 were investigated by UV/Vis, circular dichroism
(CD), TEM and SEM. As shown in Figure 3c, the UV spectrum of 2
showed a sharp peak at 238 nm which is assigned to cholesterol
moiety. As expected, an intense Cotton effect appeared at the
absorption region of 2. It is ascribed to the intrinsic molecular
chirality of cholesterol units. From the TEM and SEM images of the
self-assembly of 2 (Fig. 2c, d), the morphological information was a
shuttle-like nanosheet. As illustrated in Scheme S2 (ESI†), the model
of molecular arrangement was estimated based on the hydrophobic
cholesterol tail and hydrophilic secondary ammonium salt head.
Combining the properties of 2, we can reasonably deduce that the
formation of nanosheet is ascribed to the intermolecular hydrogen
binding and π-π stacking.
Fig. 1 a) Chemical structures and changes of the photographic images of 1 upon
alternating UV and visible light irradiation, b) UV/Vis spectra of 1 and (inset) absorption
changes at 358 nm under irradiation with UV light (365 nm) and visible light (> 420 nm).
(
[1] = 3 × 10 M), c) UV/Vis spectra of 2, trans-1, cis-1, 2trans-1 and 2cis-1. ([1] = [2]
5
It is well documented that the secondary alkylammonium ion
noncovalently associates with dibenzo-24-crown-8 through the
3 × 105 M), d) UV/Vis spectra of 21 assembly and (inset) absorption changes at 359
=
nm under irradiation with UV light (365 nm) and visible light (> 420 nm). ([1] = 3 × 10
5
20
cooperative N-H…O and C-H…O hydrogen-bonding interactions.
2 2
M, 1:2 = 1:2). All samples were prepared in CH Cl at 298 K.
The assembly behaviors of 1 and 2 were investigated by UV/Vis, TEM,
SEM and CD spectrum. Upon the addition of different volume of 2 to
the solution of 1, the absorption peak of 1 at 277 nm and 358 nm
gradually increased (Figure S13). The binding constant was calculated
1H NMR experiments were performed in CDCl
isomerization efficiency of 1. After the irradiation with 365 nm light
to investigate the
3
2
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as 13400 ± 700 M1 for the association of 1 and 2. To investigate the
In CD studies, both trans-1 and cis-1 showed a silentViCewotAtroticnleeOffnelincet
photo responsiveness about the assembly of 1 and 2, the UV/vis at its UV-absorption region, and 2 presentDeOdI:a10n.1o0b39v/ioCu9sCCp0o1s8it7i4vCe
spectra of free 1, 2 and 21 assembly irradiated with alternative Cotton effect at its UV-vis absorption region (228-275 nm) (Fig. 3a,
lights were detected. Compared to that of free trans-1 and cis-1, the c). Interestingly, upon the assembly of trans-1 and 2, an obvious
absorption of both 2trans-1 and 2cis-1 showed a little blue shift Cotton effect appeared at the region of 300-400 nm assigned to the
from 280 nm to 276 nm (Fig. 1c). As expected, the assembly also absorption of trans-1 (Fig. 3b, d). After the irradiation with UV light
showed good photocontrolled reversibility and
(365 nm), the CD spectrum of 2cis-1 showed no obvious Cotton
effect in the wavelength of 300-400 nm. Further, we did several
cycles and all of them showed the similar phenomenon. The CD
results indicate that the chirality transferred from 2 to 1, and which
could be controlled reversibly and repeatedly by light. More
interestingly, the 21 supramolecular assembly exhibited
significantly different assembling behaviour from free trans-1 or free
2
. The photocontrolled chiral transfer for 2trans-1 was also
confirmed by the observation from TEM and SEM (Fig. 2e, 2f). From
the TEM and SEM images of 2trans-1, a number of clockwise rotary
snowflake-like nanostructures was shown (Fig. S15, ESI†). Magnified
TEM images displayed clear clockwise rotary snowflake-like
nanostructures, indicating their helical property. However, after
irradiation with UV light the configuration transferred from 2trans-
1
to 2cis-1 and its TEM images showed straight stretched non-
helical snowflake-like nanostructures bigger than that of 2trans-1
in size. The system can be switched back with visible light (> 420 nm)
and the cycle is repeatable. The chirality of 21 observed by TEM
and SEM is consistent with the CD results. These phenomena jointly
indicate the good reversibility of photocontrolled chirality transfer
and helical/non-helical nanostructure morphological conversion. The
strong noncovalent interactions between 1 and 2 endow effective
chiral transfer from 2 to trans-1 through the host-guest interaction
which is different to the classic majority rule. And the chirality
transfer was detected on two levels (supramolecular and
nanoscopic). Combining these observations, we can reasonably
deduce that the snowflake-like morphology is due to the secondary
Fig. 2 TEM images and schematic representations (inset) of a) trans-1, b) cis-1 (made
-
4
by trans-1 upon the irradiation with UV light (365 nm) for 50 min ( [1] = 3.3 × 10 M), c)
-
4
TEM image of 2, d) SEM image of 2, ( [2] = 6.6 × 10 M), TEM images of e) 2trans-1, f)
2
2
cis-1 (made by 2trans-1 upon the irradiation with UV light (365 nm) for 50 min (1:
= 1:2, [1] = 3.3 × 10 M), All samples were prepared in CH Cl at 298 K.
2 2
-
4
repetitiveness properties. As shown in Figure 1d, after irradiated
with UV light (365 nm), the absorption of 21 at 358 nm and 276 nm
-4
Fig. 3 Circular dichroism spectra of a) 2 ([2] = 10 M). b) 2trans-1 and 2cis-1 (1: 2 =
decreased obviously, and a new peak appeared at 310 nm. 1:2, [1] = 2×10-3 M). UV/Vis spectra of c) 2 ([2] = 10-4 M) d) 2trans-1 and 2cis-1 (1: 2 =
1
:2, [1] = 10-4 M) in CH
Cl at 298 K.
2 2
Subsequently the sample was irradiated with visible light (> 420 nm),
and the decreased intensity at 358 nm was recovered to its original
level. It could be ascribed to the assembly 2cis-1 revert to 2trans-
assembly of 21 and the hydrophobic alkyl chains of 2 is exposed to
the outer surface. As illustrated in Scheme 2, mainly through the
noncovalent interaction and hydrophobic interaction between 1 and
, the assembly of 21 presented a snowflake-like conformation.
The assembly of 2trans-1 showed a clockwise rotary snowflake-like
nanostructure which mainly due to the twisting of 2 mediated by
1
(Scheme 1). This experiment was redone ten times without any
fatigue. In other words, the absorption curves of 21 upon
irradiation with alternative UV/vis light was completely overlapped
with each other (Fig. 1d).
2
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and S. Shinkai, J. Am. Chem. Soc., 2000, 122, 8648-8653.
trans-azobenzene. However, once the assembly of 21 was
irradiated with UV light, it isomerized to 2cis-1, presented a non-
helical snowflake-like nanostructure which can be ascribed to the
opposite twisting behavior of the two cholesterol units mediated by
cis-azobenzene.
In summary, a snowflake-like supramolecular clockwise-helical
assembly has been constructed through the intermolecular
complexation of trans-1 with the cholesterol derivative 2. After
trans-1 in this supramolecular assembly was changed into cis-1 via
UV-irradiation, its helix was lost accompanying the disappearance of
the CD signals in the azobenzene zone of 1. The photocontrolled
conversion of morphology, transferred chirality and facile
preparation properties will give the supramolecular assembly
potential application value in materials, photo driven chiral switches
and information storage.
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1
1
5. A. Sewbalas, R. U. Islam, W. A. L. van Otterlo, C. B. de
DOI: 10.1039/C9CC01874C
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We thank the National Natural Science Foundation of China (Nos.
2
1772100 & 21432004), and the 973 Program (2015CB856500) for
financial support.
Conflicts of interest
There are no conflicts to declare.
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