Pillar[6]arene-based photoresponsive host-guest complexation

Article abstract of DOI:10.1021/ja302998q

The trans form of an azobenzene-containing guest can complex with a pillar[6]arene, while it cannot complex with pillar[5]arenes due to the different cavity sizes of the pillar[6]arene and the pillar[5]arenes. The spontaneous aggregation of its host-guest complex with the pillar[6]arene can be reversibly photocontrolled by irradiation with UV and visible light, leading to a switch between irregular aggregates and vesicle-like aggregates. This new pillar[6]arene-based photoresponsive host-guest recognition motif can work in organic solvents and is a good supplement to the existing widely used cyclodextrin/azobenzene recognition motif.

Full text of DOI:10.1021/ja302998q

Article
pubs.acs.org/JACS
Pillar[6]arene-Based Photoresponsive Host−Guest Complexation
Guocan Yu,† Chengyou Han,† Zibin Zhang,† Jianzhuang Chen,† Xuzhou Yan,† Bo Zheng,†
Shiyong Liu,*,‡ and Feihe Huang*,†
†Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China
‡Hefei National Laboratory of Physical Sciences at the Microscale, CAS Key Laboratory of Soft Matter Chemistry, Department of
Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
S
* Supporting Information
ABSTRACT: The trans form of an azobenzene-containing guest
can complex with a pillar[6]arene, while it cannot complex with
pillar[5]arenes due to the different cavity sizes of the pillar[6]arene
and the pillar[5]arenes. The spontaneous aggregation of its host−
guest complex with the pillar[6]arene can be reversibly photo-
controlled by irradiation with UV and visible light, leading to a
switch between irregular aggregates and vesicle-like aggregates.
This new pillar[6]arene-based photoresponsive host−guest recog-
nition motif can work in organic solvents and is a good supplement
to the existing widely used cyclodextrin/azobenzene recognition
motif.
been rarely investigated.7b,d,8 Furthermore, pillar[6]arene-based
photoresponsive host−guest complexation has not been
reported yet. Herein, we investigate the complexation of an
azobenzene-containing guest 3 with pillar[5]arenes 1a, 1b, and
1c and pillar[6]arene 2. Because of the different cavity sizes of
the pillar[5]arenes and the pillar[6]arene, trans-3 cannot
complex with the pillar[5]arenes 1, while it can reside in the
cavity of the pillar[6]arene 2. A photoresponsive threading−
dethreading switch can be achieved upon UV and visible light
irradiation due to the trans−cis photoisomerization of 3,
accompanied by disassembly and assembly of the correspond-
ing aggregates.
1. INTRODUCTION
Stimuli-responsive host−guest systems have attracted much
interest due to their application in a broad range of fields, such
as memory storage, smart supramolecular polymers, drug
delivery systems, sensors, protein probes, and functional
nanodevices.1 Numerous external stimuli such as temperature-
change, pH-change, redox, and light have been utilized in these
systems.2 Among them, light is of special interest because it can
work rapidly, remotely, cleanly, and noninvasively.3 A variety of
photoresponsive groups have been utilized as switching units to
control the microscopic structures and relevant functions of
various supramolecular systems.4 Among them, azobenzene has
been proved to be especially advantageous for inducing
dramatic structural changes in supramolecular systems due to
its photoinduced E/Z isomerization, accompanied by large
molecular property changes.5
2. EXPERIMENTAL SECTION
Synthetic Procedures (See Scheme S1 in the Supporting
Information). Pillar[5]arenes 1a, 1b, and 1c and pillar[6]arene 2
were reported previously.6b,8
Pillararenes are a new type of macrocyclic hosts. Their
repeating units are connected by methylene bridges at the para-
positions, forming a unique rigid pillar architecture, which is
different from the basket-shaped structure of meta-bridged
calixarenes. The unique structure and easy functionalization of
pillararenes have afforded them outstanding ability to
selectively bind different kinds of guests and provided a useful
platform for the construction of various interesting supra-
molecular systems, including cyclic dimers, chemosensors,
supramolecular polymers, and transmembrane channels.6,7
They have been described as “fascinating cyclophanes with a
bright future”.6p Until now, two kinds of pillararenes,
pillar[5]arenes and pillar[6]arenes, containing five and six
units, respectively, have been reported.6−8 The host−guest
chemistry of pillar[5]arenes has been widely explored.
However, the host−guest chemistry of pillar[6]arenes has
For the synthesis of 3, a solution of 4-bromomethylazobenzene B
(5.40 g, 19.7 mmol) in ethanol (50.0 mL) and trimethylamine (30% in
ethanol, 50.0 mL) was allowed to react at 80 °C for 24 h. The solution
was concentrated under reduced pressure. The residue was diluted
with water (20.0 mL) and washed with dichloromethane (5 × 5 mL).
Excess NH4PF6 was added to the aqueous solution to produce an
orange precipitate, which was collected by filtration and washed with
water to give 3 as an orange solid (6.54 g, 86%). mp 216.5−217.8 °C.
The proton NMR spectrum of 3 is shown in Figure S1. 1H NMR (500
MHz, 10:1 chloroform-d/acetonitrile-d3, room temperature) δ (ppm):
7.99 (d, J = 8.0 Hz, 2H), 7.93 (d, J = 8.0 Hz, 2H), 7.63 (d, J = 8.0 Hz,
2H), 7.52 (s, 3H), 4.54 (s, 2H), 3.16 (s, 9H). The 13C NMR spectrum
of 3 is shown in Figure S2. 13C NMR (125 MHz, acetonitrile-d3, room
temperature) δ (ppm): 153.38, 152.16, 133.76, 131.67, 129.86, 129.17,
Received: March 28, 2012
Published: April 27, 2012
© 2012 American Chemical Society
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K, μ = 0.529 mm−1, 27 170 measured reflections, 13 739 independent
reflections, 823 parameters, 11 restraints, F(000) = 1344, R1 = 0.1758,
wR2 = 0.4244 (all data), R1 = 0.1400, wR2 = 0.3813 [I > 2σ(I)], max.
residual density 0.818 e·Å−3, and goodness-of-fit (F2) = 1.450. CCDC-
851276.
3. RESULTS AND DISCUSSION
Host−Guest Studies. The complexation of trans-3 with
pillar[5]arenes 1a, 1b, and 1c and pillar[6]arene 2 was studied
by 1H NMR spectroscopy first. As shown in Figure 1, when 1.0
122.74, 122.55, 117.04, 68.44, 54.93, 54.90, 54.86, 52.26, 52.23, 52.19,
51.36. LRESIMS is shown in Figure S3: m/z 253.9 [M − PF6]+
(100%). HRESIMS: m/z calcd for [M − PF6]+ C16H20N3, 254.1652;
found 254.1641; error −4.3 ppm.
Preparation of Scanning Electron Microscopy (SEM)
Samples. The SEM samples were prepared on clean Si substrates.
Each sample solution was deposited onto a Si substrate, placed in a
refrigerator for 30 min, and freeze-dried in a freeze-drying machine at
−20 °C under reduced pressure.
Transmission Electron Microscopy (TEM) and Dynamic Light
Scattering (DLS) Studies. The self-assembled structures of trans-3,
2⊃trans-3, and 2⊃cis-3 were revealed using TEM. An equimolar
solution of 5.00 × 10−4 M 2 and trans-3 was made first in a mixture of
CHCl3 and CH3CN (v:v, 10:1). The samples of 2⊃trans-3 were
prepared by drop-coating this solution onto a carbon-coated copper
grid, while the samples of 2⊃cis-3 were prepared by drop-coating this
solution on a carbon-coated copper grid under the irradiation of UV
light until the solvent evaporated. TEM experiments were performed
on a JEM-1200EX instrument. The solution of trans-3 (5.00 × 10−4
M) was left to stand overnight before being used for DLS tests, while
for a solution of 2 (5.00 × 10−4 M) and trans-3 (5.00 × 10−4 M), it
was left to stand overnight and irradiated by UV light for 10 min
before being used for DLS tests. The irregular aggregates in solution
were eliminated by using a microporous membrane. The diameters of
the assemblies were measured on a Nano-ZS ZEN3600 instrument.
Critical Aggregation Concentration (CAC) Determination of
trans-3. Some parameters such as the conductivity, osmotic pressure,
fluorescence intensity, and surface tension of the solution change
sharply around the critical aggregation concentration. The dependence
of the solution conductivity on the solution concentration is used to
determine the critical aggregation concentration. Typically, the slope
of the change in conductivity versus the concentration below CAC is
steeper than the slope above the CAC. Therefore, the junction of the
conductivity−concentration plot represents the CAC value. To
measure the CAC of trans-3, the conductivities of the solutions at
different concentrations (from 0 to 0.63 mM) were determined, and
by plotting the changes of the conductivity versus the concentration,
we could get the critical aggregation concentration (CAC) of trans-3.
Crystallography. X-ray Crystal Data of P5. Crystallographic data:
1
Figure 1. Partial H NMR spectra (500 MHz, 10:1 chloroform-d/
acetonitrile-d3, room temperature) of 2.00 mM trans-3 (a), 2.00 mM
trans-3 and 1a (b), 2.00 mM trans-3 and 1b (c), 2.00 mM trans-3 and
1c (d), 2.00 mM trans-3 and 2 (e), and 2.00 mM 2 (f).
equiv of 1a, 1b, or 1c was added to a solution of trans-3, no
chemical shift changes were observed for protons of trans-3.
However, upon addition of 1.0 equiv of pillar[6]arene 2,
chemical shift changes of some protons on trans-3 appeared.
Protons Hd shifted upfield from 7.98 to 7.71 ppm (Δδ = −0.27
ppm). The resonance peaks related to Ha, Hb, and Hc
disappeared after complexation (spectrum e in Figure 1). The
reason is that these protons are located within the cavity of 2
and shielded by the electron-rich cyclic structure upon forming
a threaded structure between 2 and trans-3. Extensive
broadening effect occurred due to complexation dynamics.6d
In addition, no chemical shift changes happened for protons He
and slight chemical shift changes occurred for protons Hf and
Hg, indicating that they are not involved in the noncovalent
interactions between 2 and trans-3 because trans-3 is too long
to be wrapped by host 2 completely. On the other hand, the
protons on pillar[6]arene 2 also exhibited slight chemical shift
changes due to the interactions between trans-3 and 2. The
peak related to H1 on the benzene rings shifted downfield
slightly. The signals corresponding to H2 and H5 shifted upfield
−0.07 and −0.04 ppm, respectively. 2D NOESY NMR
spectrum is a useful tool to study the relative positions of the
components in host−guest inclusion complexes, so it was
further used to investigate the complexation between 2 and
trans-3. NOE correlation signals were observed between
protons Hd on the azobenzene unit of trans-3 and protons
H1 on pillar[6]arene 2 as well as the bridging methylene
protons H5 (A and B in Figure S11). On the other hand, no
colorless, C H O , FW 1031.37, triclinic, space group P1, a =
̅
65 90 10
12.1787(4), b = 13.9544(5), c = 20.7259(7) Å, α = 80.666(3)°, β =
83.051(3)°, γ = 81.950(3)°, V = 3424.0(2) Å3, Z = 2, Dc = 1.000 g
cm−3, T = 100 K, μ = 0.522 mm−1, 46 719 measured reflections, 12081
independent reflections, 691 parameters, 0 restraints, F(000) = 1120,
R1 = 0.0932, wR2 = 0.2714 (all data), R1 = 0.0815, wR2 = 0.2547 [I >
2σ(I)], max. residual density 0.759 e·Å−3, and goodness-of-fit (F2) =
1.054. CCDC-851275.
X-ray Crystal Data of P6. Crystallographic data: colorless,
C78H108O , FW 1237.64, triclinic, space group P1, a = 13.2244(10),
̅
12
b = 13.7845(10), c = 23.3051(12) Å, α = 89.203(5)°, β = 79.553(5)°,
γ = 76.373(6)°, V = 4058.4(5) Å3, Z = 2, Dc = 1.013 g cm−3, T = 293
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NOE correlation signals were observed between protons He−g
on the guest and the protons on the pillar[6]arene 2.
A mole ratio plot (Figure S13) based on the proton NMR
data of Hd on 2 demonstrated that the complex between 2 and
trans-3 has 1:1 stoichiometry in 10:1 (v/v) chloroform/
acetonitrile. The ESI-MS experiments confirmed the formation
of a 1:1 complex between 2 and trans-3. The relevant peak was
found at m/z 1493.1 (Figure S9), corresponding to [2⊃trans-
3−PF6]+. Peaks related to other stoichiometries were not
Single crystals of pillar[5]arene 1b and pillar[6]arene 2 were
obtained by slow evaporation of their respective solutions in a
mixture of dichloromethane and acetonitrile (1:2, v/v).
Although pillar[5]arene 1b and pillar[6]arene 2 have the
same repeating unit and are cyclic molecules, their X-ray crystal
structures (Figure 2) have an obvious difference. Pillar[5]arenes
1
found. H NMR titration experiments were carried out in 10:1
chloroform-d/acetonitrile-d3 to investigate the binding abilities
between 2 and tran-3 (Figure S12). The association constant
(Ka) of 2⊃trans-3 was determined to be (2.22 0.34) × 103
M−1 by fitting the chemical shift changes of Hd on the guest as a
function of the initial concentration of 2 (Figure S14).
As expected, the azobenzene-containing guest 3 exhibited
good photoresponsive properties as investigated by UV−vis
spectroscopy (Figures 3 and S4). Upon irradiation with UV
light at 365 nm, the absorption band at around 318 nm
decreased remarkably, and concomitantly the band at around
435 nm increased slightly. The absorption bands of the
azobenzene unit at 318 and 435 nm are ascribed to π−π* and
Figure 2. Ball-stick views of the crystal structures of 1b (a) and 2 (b).
Hydrogens and the solvent molecules were omitted for clarity. Carbon
atoms are red, and oxygen atoms are blue.
1b has a pentagon-like cyclic structure, while pillar[6]arene 2
has a hexagon-like cyclic structure. By treating pillar[5]arenes
and pillar[6]arene as a regular pentagonal pillar and a regular
hexagonal pillar, respectively, and ignoring the influence of the
substituents on the oxygen atoms of the benzene rings, we
calculated the structural parameters of pillar[5]arenes and
pillar[6]arene (Figure S18). In terms of the cavity size, the
diameter of the internal cavity of pillar[5]arenes is ∼4.7 Å,
equal to that of α-cyclodextrin (∼4.7 Å).9 The diameter of the
internal cavity of pillar[6]arene is ∼6.7 Å, close to that of β-
cyclodextrin (∼6.0 Å).9 These phenomena mentioned above
suggested that due to the difference in cavity sizes between
pillar[5]arenes 1 and pillar[6]arene 2 (Figures 2 and S18), the
azobenzene-containing guest trans-3 could not complex with
pillar[5]arenes 1, while it could thread into the cavity of
pillar[6]arene 2 to form a [2]pseudorotaxane 2⊃trans-3 (its
cartoon representation is shown in Figure 4) with its positive
trimethylammonium group close to the oxygen atoms on
pillar[6]arene 2, the benzene ring containing Hc and Hd located
in the cavity of pillar[6]arene 2, and the other benzene ring
outside of the cavity. The interactions between pillar[6]arene 2
and trans-3 might include hydrogen bonds and π−π stacking
interactions. It should be noted that, although pillar[5]arenes 1
and α-cyclodextrin have similar internal cavity size, pillar[5]-
arenes 1 cannot complex trans-3, while α-cyclodextrin can bind
azobenzene derivatives in water.9 A possible reason is that
pillar[5]arenes 1 have a rigid pillar structure, while α-
cyclodextrin has a flexible truncated conic structure.
Figure 3. (a) UV−vis spectra (10:1 chloroform/acetonitrile, v/v) of
5.00 × 10−4 M trans-3 (initial and after irradiation with UV light at 365
nm for 3 min) and an equimolar solution of 5.00 × 10−4 M trans-3 and
2 (initial, after irradiation with UV light at 365 nm for 3 min, and then
after irradiation with visible light at 435 nm for 10 min) and (b)
changes of the absorbance at 300 nm of an equimolar solution of 2 and
trans-3 upon alternating irradiation with UV and visible light for 10
min.
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n−π* transitions, respectively. The changes of the absorption
bands induced by UV irradiation indicated the photoisomeriza-
tion from the trans-3 state to the cis-3 state. On the contrary,
upon irradiation with visible light at 435 nm, the absorption
peak at 435 nm attributable to cis-3 decreased, while the
absorption band at 318 nm corresponding to trans-3 increased,
indicating a change from the cis-3 form to the trans-3 form.
Usually, the photoisomerization of the azobenzene group
proceeds with first-order kinetics in solution,4e,5b so the
interaction between pillar[6]arene 2 and trans-3 can be further
validated by the photoisomerization process. By plotting the
changes of absorbance versus time, the initial trans−cis
photoisomerization rate constant (kt) of 3 could be obtained
derivatives prefer the trans form in solution (spectrum b in
Figure 4). When a solution of trans-3 was irradiated by UV light
as 0.090
considerably to 0.067
0.009 s−1, while the value of kt decreased
0.006, 0.061 0.004, and 0.020
0.002 s−1, respectively (Figure S8), in the presence of 0.5, 0.8,
and 1.0 equiv of pillar[6]arene 2. Accompanied by the increase
of 2, the trans−cis photoisomerization rate constants (kt) of 3
decreased. The Ka value of 2⊃cis-3 was determined to be (2.64
0.29) × 102 M−1 by employing 1H NMR titration
experiments (Figure S17), which was lower than that of
2⊃trans-3. The reason that the photoisomerization rate
constant decreased upon addition of 2 could be as follows: in
the absence of 2, trans-3 molecules freely disperse in solution so
they can undergo the trans−cis conversion easily. When host 2
is added into the solution of trans-3, an inclusion complex of
pillar[6]arene 2 and trans-3 forms in which the macrocyclic
host encircled the axle. Subsequently, UV light irradiation can
convert the conformation of uncomplexed trans-3 molecules
from the trans form to the cis form, but the photoisomerization
of complexed trans-3 molecules is geometrically restricted. It is
this restructuring of the energy landscape for the system that
resulted in the reduction of the photoisomerization rate
constant.5b
1
Figure 4. Partial H NMR spectra (500 MHz, 10:1 chloroform-d/
Comparing the sizes of 3 and pillar[6]arene 2 (Figure S18),
we know that cis-3 is larger than the cavity of pillar[6]arene 2.
Therefore, upon irradiation with UV light, the conformation of
some trans-3 molecules changed from the trans state to the cis
state, and these 3 molecules could not thread into the cavity of
2 anymore. As shown in Figure 3a, the absorption band at 300
nm of an equimolar solution of 2 and trans-3 decreased
obviously after irradiation by UV light at 365 nm for 3 min. The
decreased absorption was caused by the trans−cis isomerization
of trans-3 molecules. The absorption intensity at 300 nm could
be recovered to the initial state after irradiation by visible light
at 435 nm for 10 min. Upon alternately exposing an equimolar
solution of 2 and trans-3 to light at 365 and 435 nm, this
reversible photoisomerization process could be recycled many
times (Figure 3b). It indicated that this threading/dethreading
transition could be controlled reversibly by irradiation with UV
and visible light due to the trans−cis photoisomerization of 3.
UV spectroscopic titrations were also performed to verify the
interactions between trans-3 and 2 in solution. Upon addition
of 2 to a solution of 5.00 × 10−4 M trans-3, a blue shift of the
maximum absorption peak from 318 to 294 nm was observed
(Figure S25a). On the contrary, a red shift occurred upon
addition of trans-3 to a solution of 5.00 × 10−4 M 2 (Figure
S25b). These shifts confirmed the host−guest complexation
between 2 and trans-3.
acetonitrile-d3, room temperature) of 2.00 mM trans-3 and 2 (a), 2.00
mM trans-3 (b), 2.00 mM trans-3 after irradiation at 365 nm for 10
min (c), 2.00 mM trans-3 and 2 after irradiation at 365 nm for 10 min
(d), and then after further irradiation at 450 nm for 10 min (e).
for 10 min, the ratio of the trans to cis form changed to about
50:50 (spectrum c in Figure 4). When the solution comprised
of equimolar amounts of trans-3 and 2 was irradiated by UV
light for 10 min, the chemical shifts of the protons Hc* on the
π-electron rich benzene ring of cis-3 shifted upfield dramatically
from 7.33 to 6.61 ppm, and the peak related to protons Hd*
shifted upfield from 6.87 to 6.85 ppm (spectra c and d in Figure
4). On the other hand, the peaks related to Ha* and Hb* shifted
upfield slightly (Δδ = −0.11 and −0.10 ppm, respectively) and
exhibited broadening effect (Figure S20). On the contrary, the
chemical shifts of protons He*, Hf*, and Hg* on the benzene
rings did not change (spectra c and d in Figure 4). These
phenomena indicated that the trimethylammonium group on
the cis-3 was bound by a rim of pillar[6]arene 2, while the rest
of guest cis-3 was outside the cavity of 2 as shown by a cartoon
representation of 2⊃cis-3 in Figure 4. Upon irradiation with
light at 450 nm for sufficient time, cis-3 was converted back to
trans-3, and the proton signals of spectrum e in Figure 4 were
the same as those of spectrum a in Figure 4.
Photocontrolled Threading−Dethreading Switch. To
Microstructures of the Self-Assembled Aggregates.
We further explored how the presence of pillar[6]arene 2 and
photoirradiation affects the aggregation of 3. The critical
aggregation concentration of the trans-3 was measured to be
1
further confirm this hypothesis, H NMR characterization was
conducted to provide evidence about the interaction of trans-3
and cis-3 with pillar[6]arene 2. Normally, azobenzene
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Figure 5. TEM images of trans-3 (a, the scale bar is 0.5 μm) with an inset showing an enlarged image (the scale bar is 100 nm), an equimolar
mixture of trans-3 and pillar[6]arene 2 (b, the scale bar is 0.5 μm), and an equimolar mixture of trans-3 and pillar[6]arene 2 after irradiation at 365
nm for 10 min (c, the scale bar is 5 μm), the fusion process from small-size vesicle-like aggregates to large-size vesicle-like aggregates (d, the scale bar
is 2 μm), DLS results of trans-3 solution (e) and the aggregates of 2⊃cis-3 (f), and the illustration of the formation of the aggregates (g). The
concentration of trans-3 was 5.00 × 10−4 M, which is larger than the critical aggregation concentration, 1.68 × 10−4 M of trans-3.
1.68 × 10−4 M (Figure S21). For some amphiphiles, such as
cetyltrimethylammonium bromide, it has been reported that
their aggregration can be reversibly controlled due to the
controlled formation of inclusion complexes with cyclodextrins
in water.10 As shown by a TEM image in Figure 4a, trans-3
formed solid spherical aggregates with an average diameter
about 90 nm, which was further verified by SEM experiments
(Figure S22a). When an equimolar amount of pillar[6]arene 2
was added, trans-3 threaded into the cavity of 2 to form
inclusion complex 2⊃trans-3, resulting in the disappearance of
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the solid spherical aggregates of trans-3 (Figure 4b).
Interestingly, when the above system was irradiated by UV
light at 365 nm, vesicle-like aggregates with a diameter of about
5 μm (Figure 4c) appeared. These vesicle-like aggregates
presumably came from the self-assembly of unthreaded host−
guest complex 2⊃cis-3. It should be noted that irregular
aggregates from the inclusion complex 2⊃trans-3 can be still
observed as black dots in Figure 4c because not all of the trans-
3 molecules were converted to the cis-3 molecules by the UV
irradiation. On the other hand, the fusion process from small-
size vesicle-like aggregates to large-size vesicle-like aggregates
was also found in TEM images (Figure 4d). As shown by DLS,
the main diameter distribution of the solid spherical aggregates
formed from trans-3 is around 78 nm (Figure 5e), while the
main diameter distribution for the vesicle-like aggregates
formed from the unthreaded inclusion complexes between cis-
3 and 2 is about 4.8 μm (Figure 5f). The results determined by
DLS were in good agreement with the TEM images in Figure
4a and c. Small-angle X-ray scattering (SAXS) was used to
clarify the packing of the cis-3 and pillar[6]arene 2 complex in
the vesicle-like aggregates (Figure S24). The thickness of the
bilayer of the vesicle-like aggregates was calculated to be 1.7 nm
by SAXS and around 2.0 nm from the TEM experiments
(Figure S22b), in accordance with the length of two 2⊃cis-3
inclusion complexes with antiparallel packing. Moreover, the
vesicle-like aggregates disassembled upon irradiation with
visible light at 450 nm, and reformed on subsequent UV
irradiation at 365 nm. These irregular aggregates and vesicle-
like aggregates can be obtained by UV and visible light
irradiation reversibly, which results from the threading and
dethreading of the cavity of pillar[6]arene 2 by the photo-
responsive guest 3. A possible mechanism for this aggregation
control is shown in Figure 4g. In the mixture of chloroform and
acetonitrile (10/1, v/v), the azobenzene part of trans-3 is
solvophilic, while the trimethyl ammonium moiety of trans-3 is
solvophobic. Furthermore, hydrogen bonding between the
hydrogen atoms on the trimethyl ammonium group and the
nitrogen atoms can provide an additional driving force for the
aggregation of trans-3 (Figure S23). Therefore, it can self-
assemble to form solid spherical aggregates shown in Figure 4a.
When solvophilic pillar[6]arene 2 is added, a threaded
inclusion complex 2⊃trans-3 forms with the solvophobic
trimethyl ammonium moiety included in the cavity of 2 so
the regular aggregates of amphiphilic trans-3 in Figure 4a are
destroyed to form irregular aggregates in Figure 4b. When this
solution is irradiated with UV light, some trans-3 molecules are
converted into cis-3, so two host−guest complexes, threaded
2⊃trans-3 and unthreaded 2⊃cis-3, exist in solution. Threaded
2⊃trans-3 is solvophilic. However, the two ends of unthreaded
2⊃cis-3 are solvophilic, while its middle part is solvophobic
because the solvophobic trimethyl ammonium moiety is not
included in the cavity of pillar[6]arene 2. Therefore, both
vesicle-like aggregates from self-assembly of unthreaded 2⊃cis-3
and irregular aggregates from self-assembly of threaded
2⊃trans-3 can be observed in Figure 4c. It should be noted
that the fusion process shown in Figure 4d is a typical
intermediate state, supporting that this mechanism is
plausible.3h,5d
UV light, the conformation of 3 changes from the trans form to
the cis form, and the threading of 3 through the cavity of
pillar[6]arene 2 is prohibited due to the size mismatch, but it
can rethread through the cavity again upon visible light
irradiation. This photocontrolled switch of threading results in
microstructural changes of the self-assembled aggregates
formed from the host−guest inclusion complex between 2
and 3. These phenomena have been demonstrated by various
characterization techniques. Therefore, this new efficient
pillar[6]arene-based host−guest recognition motif exhibited a
photoresponsive threading−dethreading switch in organic
solvents. Acting as a good supplement to the existing widely
used cyclodextrin/azobenzene recognition motif in water, it will
have a broad application in the future fabrication of more
sophisticated photoresponsive supramolecular systems.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, NMR spectra, SEM images, and X-ray
crystallographic files (CIF) for 1b and 2 and other materials.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
sliu@ustc.edu.cn; fhuang@zju.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The National Natural Science Foundation of China (20834004,
91027006, 21125417), the Fundamental Research Funds for
the Central Universities (2012QNA3013), and the Zhejiang
Provincial Natural Science Foundation of China (R4100009)
are greatly acknowledged for their generous financial support.
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The cavity size of pillar[6]arene 2 is suitable for the
azobenzene-containing guest trans-3, while the cavity size of
pillar[5]arenes 1 is too small for trans-3. Upon irradiation with
8716
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