Controllable Fabrication of Various Supramolecular Nanostructures Based on Nonamphiphilic Azobenzene Derivatives and Pillar[6]arene

Article abstract of DOI:10.1002/cjoc.201400508

Various novel types of supramolecular nanostructures formed by nonamphiphilic azobenzene derivatives, G1 or G2 have been successfully fabricated, where G2 is structurally similar with G1 but an extra phenoxy group is connected with the azobenzene motif. M

Full text of DOI:10.1002/cjoc.201400508

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DOI: 10.1002/cjoc.201400508
Controllable Fabrication of Various Supramolecular
Nanostructures Based on Nonamphiphilic Azobenzene
Derivatives and Pillar[6]arene
Shan Qin, Shuhan Xiong, Yang Han, Xiao-Yu Hu,* and Leyong Wang
Key Laboratory of Mesoscopic Chemistry of MOE, Center for Multimolecular Organic Chemistry,
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210093, China
Various novel types of supramolecular nanostructures formed by nonamphiphilic azobenzene derivatives, G1 or
G2 have been successfully fabricated, where G2 is structurally similar with G1 but an extra phenoxy group is con-
nected with the azobenzene motif . Micellar structures can be obtained from the self-assembly of G1, which further
transform to large-sized spindle structures; while nanorods can be initially formed by G2, which will gradually ag-
gregate to form layered structures with much larger size. Moreover, it is found that upon addition of WP6, which
can form inclusion-complex with G1 or G2, separately, the nonamphiphilic G1 and G2 thus converse to su-
pramolecular amphiphiles WP6⊃G1 and WP6⊃G2, respectively. Consequently, both of the above WP6⊃G1 and
WP6⊃G2 complexes can further assemble to form supramolecular binary vesicles, which will gradually transform
to nanotubes (WP6⊃G1) or well-ordered nanosheets (WP6⊃G2).
Keywords nanostructure, azobenzene derivatives, pillararene, self-assembly, UV-responsiveness
Introduction
various well-defined architectures and may find analo-
gous potential applications in encapsulation and con-
trolled release, has rarely been studied and is far from
satisfactory. Therefore, the exploration of nonamphi-
philic molecules that can spontaneously form
well-defined nanostructures such as micells, vesicles,
nanorods, and nanosheets in aqueous solution will in-
crease our understanding and broaden the applications
of these well-ordered nanoaggregates in the field of bio-
technology and biomedicine, particularly for drug deliv-
ery.
During our investigation about the construction of
stimuli-responsive supramolecular vesicles based on
supramolecular amphiphiles for controlled drug deliv-
ery,[10] we surprisingly found that a kind of nonamphi-
philic azobenzene derivatives bearing long alkyl chains
at one end could also spontaneously aggregate to form
well-defined nanoscale structures. Moreover, with the
addition of water-soluble pillar[6]arene (WP6), such
nonamphiphilic molecules could transform to su-
pramolecular amphiphiles via pillaraene-based host-
guest interaction,[11] which could further assemble to
form nanostructures with completely different mor-
phology compared with that of the nonamphiphilic ones.
This finding may help us to have a better understanding
on the self-assembly of nonamphiphilic molecules.
Herein, we report the successful fabrication of vari-
Molecular self-assembly is an attractive and power-
ful strategy for the fabrication of sophisticated and func-
tional supramolecular architectures.[1] Based on this
concept, various functional nanoscale structures which
can be used in the fields of drug delivery,[2] controlled
release,[3] nanoreactors,[4] supramolecular polymers,[5]
and detection[6] can be precisely constructed and ma-
nipulated by rational design and modification of the
self-assembling building blocks at the molecular level.
Over the past decade, supramolecular nanostructures
formed by the self-assembly of supramolecular amphi-
philes bearing both hydrophilic and hydrophobic seg-
ments have attracted much attention because they can
not only spontaneously aggregate to form different as-
semblies depending on the repelling and coordinating
forces between their hydrophilic and hydrophobic parts
of the component molecules and the surrounding me-
dium,[7] but also undergo structure transitions in re-
sponse to numerous stimuli, such as pH, temperature,
redox, and light.[2c,8] However, to the best of our
knowledge, although a variety of nanostructures with
various sizes and functions have been reported based on
supramolecular amphiphiles,[7,9] the investigation on
nanoscale structures formed by nonamphiphilic mole-
cules in aqueous solution, which can also assemble into
*
E-mail: huxy@nju.edu.cn; Tel.: 0086-025-83597090; Fax: 0086-025-83597090
Received July 25, 2014; accepted September 1, 2014; published online XXXX, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400508 or from the author.
Chin. J. Chem. 2014, XX, 1—5
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Qin et al.
COMMUNICATION
ous novel types of supramolecular nanostructures
formed by nonamphiphilic azobenzene derivatives, G1
or G2, where G2 is structurally similar with G1 but an
extra phenoxy group is connected with the azobenzene
motif (Scheme 1). It was found that micellar structures
could be obtained from the self-assembly of G1, which
would further transform to large-sized spindle structures;
whereas, nanorods could be initially formed by G2,
which would gradually aggregate to form layered struc-
tures with much larger sizes. Moreover, with the addi-
tion of WP6, supramolecular amphiphiles WP6⊃
G1 and WP6⊃G2 could be obtained via the pil-
lararene-based host-guest interaction, leading to the
formation of supramolecular binary vesicles, which
would gradually transform to nanotubes (WP6⊃G1) or
well-ordered nanosheets (WP6⊃G2) through further
self-assembly process.
reversible change from the cis- to the trans-state of G1
or G2. All the above results clearly demonstrated that
such azobenzene-based nonamphiphilic molecules G1
and G2 exhibited good UV-responsiveness.
Scheme 1 Cartoon illustration of the self-assembly of nonam-
phiphilic azobenzene derivatives
Figure 1 UV-vis absorption spectra of guests (G1 and G2) in
ethanol under UV irradiation at 365 nm for 10 min, and then after
further irradiation with visible light at 435 nm for 10 min. (a) G1
(6×10−5 mol/L); (b) G2 (6×10−5 mol/L).
Results and Discussion
Subsequently, an interesting phenomenon was found
that when we injected G1 or G2 (0.0025 mmol, dis-
solved in 0.5 mL of THF) into 49.5 mL of deionized (DI)
water, a white opalescent phenomenon was obviously
observed from the aqueous solution of G1 or G2, indi-
cating the formation of microaggregates. Then the sizes
and morphologies of these nanostructural aggregates
formed by nonamphiphilic molecule G1 or G2 in aque-
ous solution were investigated via transmission electron
microscopy (TEM) and dynamic light scattering (DLS)
experiments. As shown in Figure 2 and Figure 3, DLS
result showed that the freshly prepared G1 solution
formed aggregates with a narrow size distribution, giv-
ing an average diameter of 37 nm (Figure 3a), and TEM
images indicated the solid spherical morphology with a
diameter ranging from 20 to 50 nm, convincingly indi-
cating the formation of micellar structure. Surprisingly,
we found an interesting phenomenon that such micellar
structures would gradually transform to large-sized
spindle structures with increasing incubation time. Fig-
ure 2b showed the TEM image of the freshly prepared
micellar structures after an incubation time of one week,
in which a large number of beaded structure and some
Initially, two azobenzene-based nonamphiphilic
molecules G1 and G2 were designed, where G2 is
structurally similar with G1 but an extra phenoxy group
is connected with the azobenzene motif. As expected,
both of them exhibited good photoresponsive properties
as investigated by the UV-vis spectroscopy (Figure 1).
As shown in Figure 1, upon irradiation of the G1 or G2
solution with UV light at 365 nm for 10 min, the ab-
sorption band at around 350 nm decreased remarkably,
meanwhile, the absorption band at around 445 nm in-
creased slightly. The absorption bands of the azoben-
zene group at 350 nm and 445 nm are ascribed to π-π*
and n-π* transitions, respectively.[8d] Such changes of
the absorption bands induced by UV irradiation sug-
gested the photoisomerization of the azobenzene motif
in G1 or G2 from the trans-state to the cis-state. On the
contrary, upon further irradiation of the above solution
with visible light at 435 nm for 10 min, the absorption
band at around 350 nm attributed to the trans-state of
G1 or G2 increased almost to its original intensity,
while the absorption peak at 445 nm attributed to the
corresponding cis-state decreased slightly, indicating the
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Controllable Fabrication of Various Supramolecular Nanostructures
larger micells could be seen. And after 2 weeks, almost
all of these structures had been transformed to large-
sized spindle structures (Figure 2c). Moreover, DLS
results also confirmed such gradually size-increasing
phenomenon of freshly prepared small micellar struc-
tures to spindle aggregates with much larger size (Figure
3b). With respect to the azobenzene derivative G2, it
was found that nanorods with an average diameter of
about 0.35 μm could be observed from its freshly pre-
pared solution based on the TEM and DLS results (Fig-
ures 2d, 2e, and Figure 3c). However, after two weeks,
the morphology of these nanorods transformed into
large-sized disordered layered structures with an aver-
age diameter of 0.85 μm (Figure 2f and Figure 3d). The
different morphologies of the nanostructures formed
from G1 or G2 may be associated with the presence of
an extra phenoxy group in G2, where the cooperative
intermolecular hydrogen bonding and π-π interactions
within the azobenzene and benzamido motifs of G2,
make it more easily to assemble into layer-like struc-
tures by π-π stacking.
G2 would be transformed to supramolecular amphi-
philes WP6⊃G1 and WP6⊃G2, respectively via pil-
laraene-based host-guest interaction, and the obtained
supramolecular amphiphiles would be possibly able to
form supramolecular nanoaggregates in water by
self-assembly. Subsequently, UV-vis absorption spec-
troscopy was used to study the host-guest complexation
between WP6 and azobenzene derivative G1 or G2. As
shown in Figure 4, upon the addition of WP6 into
guests G1 and G2, respectively, the absorption band
around 290 nm ascribed to the characteristic absorption
of WP6 increased dramatically compared with its origi-
nal absorption; meanwhile, the absorption peak around
340 nm assigned to the characteristic absorption of the
azobenzene guest (G1 or G2) also increased remarkably
together with a slightly hypochromatic shift (Figures 4a,
4b). The above observations clearly indicated the forma-
tion of supramolecular complexes between WP6 and
guests (G1, G2), mainly driven by the hydrophobic in-
teractions. With the presence of WP6, the trans-azo-
benzene motif of the guests G1 or G2 could thread into
the cavity of WP6 to form supramolecular amphiphiles
WP6⊃G1 and WP6⊃G2, resulting in the changes of
the absorbance intensity at 290 nm and 334 nm.
Figure 2 TEM images: (a) G1 aggregates in water (1% THF
was added to improve the solubility of G1); (b) G1 aggregates
after standing for one week; (c) G1 aggregates after standing for
two weeks; (d) G2 aggregates in water (1% THF was added to
improve the solubility of G2); (e) enlarged image of (d); (f) G1
aggregates after standing for two weeks. [G1]＝5×10−5 mol/L,
[G2]＝5×10−5 mol/L.
Figure 3 DLS data of the aggregates formed from G1 or G2: (a)
DLS results of Figure 2a; (b) DLS results of Figure 2c; (c) DLS
results of Figure 2d; (d) DLS results of Figure 2f.
Figure 4 UV-vis absorption spectra of WP6 with azobenzene
guests (G1 or G2): (a) WP6 (2.5×10−5 mol/L) in water (black
line), G1 (2.5×10−5 mol/L) in water (red line, 1% EtOH was
added to improve the solubility of G1), a mixture of WP6 and G1
(blue line, molar ratio, 1∶1); (b) WP6 (2.5×10−5 mol/L) in wa-
ter (black line), G2 (2.5×10−5 mol/L) in water (red line, 1%
EtOH was added to improve the solubility of G2), a mixture of
WP6 and G2 (blue line, molar ratio, 1∶1).
Since we know that water-soluble pillar[6]arenes
(WP6)[12] have been demonstrated to have a strong
binding affinity with azobenzene derivatives in water
driven by hydrophobic and electrostatic interac-
tions,[12a,13] we envision that such nonamphiphilic G1 or
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Qin et al.
COMMUNICATION
Encouraged by the above-mentioned results, we
speculated that the obtained supramolecular amphiphiles
WP6⊃G1 and WP6⊃G2 might exhibit different
self-assembly behaviors. Then, we investigated the
morphology and size of these aggregates formed by
WP6⊃G1 and WP6⊃G2 supramolecular amphiphiles,
respectively in aqueous solution via TEM and DLS
measurement. As shown in Figure 5, WP6⊃G1 formed
supramolecular binary vesicles (Figure 5a) with an av-
erage diameter of 82 nm (Figure 5e). Interestingly, after
two weeks, it was found that such vesicular structures
further transformed into large-sized nanotubes (Figure
5b) with an average diameter of 340 nm (Figure 5f).
With respect to the WP6⊃G2 supramolecular amphi-
phile, it could also assemble into small-sized su-
pramolecular vesicles (Figure 5c) with an average di-
ameter of 117 nm (Figure 5g). However, after two
weeks, the obtained WP6⊃G2 vesicles further aggre-
gated to form well-ordered nanosheets (Figure 5d) with
much larger size (ca. 9949 nm, Figure 5h).
upon irradiation of the WP6⊃G1 or WP6⊃G2
solution with UV light at 365 nm for 0.5 h, the absorp-
tion bands at around 290 and 340 nm almost had no
change; meanwhile, upon further irradiation with visible
light at 435 nm for 0.5 h, nearly no change could be ob-
served. For such phenomenon, we speculated that since
the two azobenzene guests were nonamphiphilic, the
strong hydrophobic effect in aqueous solution may
make the trans-azobenzene derivative G1 or G2 which
fits well with the WP6 cavity hardly to dethread from
the cavity of WP6, resulting in the non-photoresponsive
property of such azobenzene-based supramolecular am-
phiphiles.
Figure 6 UV-vis absorption spectra of the WP6⊃G1 and
WP6⊃G2 complexes in water (1% EtOH was added to improve
the solubility of the guests) under UV irradiation at 365 nm for 30
min, and then after further irradiation with visible light at 435 nm
for 30 min: (a) WP6⊃G1 complex (WP6＝3×10−5 mol/L, G1
＝3×10−5 mol/L); (b) WP6⊃G2 complex (WP6＝3×10−5
mol/L, G2＝3×10−5 mol/L).
Figure 5 (A) TEM images: (a) freshly prepared WP6⊃G1
aggregates; (b) WP6⊃G1 aggregates after standing for two
weeks; (c) freshly prepared WP6⊃G2 aggregates; (d) WP6⊃G2
aggregates after standing for two weeks. (B) DLS data of the
aggregates formed from WP6 with G1 or G2: (e) DLS results of
Figure 5a; (f) DLS results of Figure 5b; (g) DLS results of Figure
5c; (h) DLS results of Figure 5d.
Conclusions
For some supramolecular amphiphilic complex[14]
based on the host-guest recognition of pillar[6]arene
host and azobenzene-containing guest, it has been re-
ported that the photo-controllable threading-dethreading
switch can be obtained due to the photoisomeration of
the azobenzene motif.[13,15] However, such reversible
switch could not be achieved for our amphiphilic supra-
molecular self-assembly system. As shown in Figure 6,
In summary, the self-assembly behaviors of nonam-
phiphilic azobenzene derivatives G1 and G2 were in-
vestigated in aqueous solution, and various novel types
of supramolecular nanostructures could be obtained
based on their self-assembly process. G1 itself initially
self-assembled into small spherical micells, which
would gradually convert to large-sized spindle structures
with increasing incubation time. Compared with G1,
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Controllable Fabrication of Various Supramolecular Nanostructures
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azobenzene derivative G2 that has an extra phenoxy
group connected with the azobenzene motif could form
nanorod-like aggregates, which would then transform
into disordered layered structures probably due to the
presence of cooperative intermolecular hydrogen bond-
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zamido motifs of G2. Significantly, it was found that
upon addition of WP6, which could form inclu-
sion-complex with G1 or G2, separately, the nonam-
phiphilic G1 and G2 thus converse to supramolecular
amphiphiles WP6⊃G1 and WP6⊃G2, respectively.
Consequently, both of the above WP6⊃G1 and
WP6⊃G2 complexes could assemble to form supra-
molecular binary vesicles, which will gradually trans-
form to nanotubes (WP6⊃G1) or well-ordered nano-
sheets (WP6⊃G2). The present study provides a novel
strategy for the controllable fabrication of well-defined
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Acknowledgement
This work was supported by the National Natural
Science Foundation of China (No. 21202083), and
Jiangsu Provincial Natural Science Foundation of
China (No. BK20140595).
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