Photocontrolled reversible supramolecular assemblies of an azobenzene-containing surfactant with α-cyclodextrin

Article abstract of DOI:10.1002/anie.200604982

(Figure Presented) Inclusion and exclusion of an azobenzene-containing surfactant with α-cyclodextrin (α-CD) can be used to fabricate photostimulus-responsive vesicle-like aggregates that can undergo assembly and disassembly reversibly (see picture). The movement of α-CD is like that of a molecular shuttle.

Full text of DOI:10.1002/anie.200604982

Angewandte
Chemie
DOI: 10.1002/anie.200604982
Photoresponsive Vesicles
Photocontrolled Reversible Supramolecular Assemblies of an
Azobenzene-Containing Surfactant with a-Cyclodextrin**
Yapei Wang, Ning Ma, Zhiqiang Wang, and Xi Zhang*
Amphiphilic surfactants can self-organize into molecular
assemblies, such as micelles and vesicles, which have been
[
1]
widely used as capsules for drug delivery and microreactors
[
2]
for nanoparticle preparation. Covalently attached func-
tional groups on surfactants that are responsive to external
[
3]
[4]
[5]
stimuli—for example, pH, temperature, redoxstate,
[
6]
[7]
Figure 1. Illustration of the photocontrolled reversible assemblyand
disassemblyof AzoC10; red bar: azobenzene moiet y, blue spot:
pyridinium group.
light, and even carbon dioxide and argon —can control
the assembly and disassembly of these amphiphilic surfac-
tants. Different from the self-organization of surfactants
induced by a hydrophobic effect, most supramolecules are
formed on the basis of host–guest interaction, and some of
1
which was fully characterized by H NMR spectroscopy and
[
8]
them can also be fine-tuned by external stimuli. The
understanding and control of noncovalent interactions in
those supramolecular assemblies may open a way for the
electrospray ionization mass spectrometry (ESI-MS). As
expected, the azobenzene part at the tail of the hydrophobic
alkyl chain of AzoC10 can be photoisomerized. As shown in
Figure 2, upon irradiation with UV light at 365 nm the
[
9]
design of advanced materials and devices. As an example,
the two isomers of azobenzene, the trans and cis forms, can be
reversibly switched to each other upon photoirradiation.
Driven by hydrophobic and van der Waals interactions, trans-
azobenzene can be well-recognized by a-cyclodextrin (a-
[
10]
CD), a type of cyclodextrin with six glucose units. However,
when trans-azobenzene is transformed to cis-azobenzene, a-
CD cannot include the bulky cis form any more because of the
mismatch between the host and guest. Therefore, the host–
guest assembly and disassembly between azobenzene and a-
CD by external photostimuli can act as a driving force to build
[
11]
up molecular shuttles, motors, and machines.
Herein, we attempted to make use of photocontrolled
inclusion and exclusion reaction of an azobenzene-containing
surfactant with a-CD for fabricating a supramolecular system
that can undergo reversible assembly and disassembly (see
Figure 1). It is anticipated that this research will provide a
model system that combines photochemistry and host–guest
chemistry for a stimulus-responsive vesicle.
Figure 2. Reversible photoisomerization of AzoC10 upon irradiation
with UV and visible light, respectively. UV/Vis spectra before (c)
and after (a) irradiation of AzoC10 (c=7”10 m) with UV light of
À5
365 nm.
To realize the photocontrolled assembly and disassembly,
we designed and synthesized the surfactant 1-[10-(4-phenyl-
azophenoxy)decyl]pyridinium bromide (termed AzoC10),
absorption band at around 347 nm decreases remarkably, and
concomitantly the band at around 430 nm increases slightly.
The absorption bands at 347 and 430 nm are ascribed to p–p*
and n–p* transitions, respectively. The change of the absorp-
tion bands induced by UV irradiation is indicative of the
photoisomerization of AzoC10 from the trans to the cis state.
When irradiated by visible light at 450 nm, the p–p*
absorption increases again with a slight decrease in the n–
p* absorption, which indicates that the photoisomerization of
AzoC10 undergoes a change from the cis to the trans state.
We wondered whether AzoC10 could be associated with
a-CD and how strong the association was. To answer these
questions, we measured the association constant of AzoC10
with a-CD by detecting the UV absorption of AzoC10 at the
same concentration while increasing the concentration of a-
[
*] Y. P. Wang, N. Ma, Prof. Z. Q. Wang, Prof. X. Zhang
KeyLab of Organic Optoelectronics & Molecular Engineering
Department of Chemistry, Tsinghua University
Beijing 100084 (P.R. China)
Fax: (+86)10-6277-1149
E-mail: xi@mail.tsinghua.edu.cn
[
**] This work was financiallysupported bythe National Basic Research
Program (2007CB808000) and the National Natural Science
Foundation of China (20334010, 20473045, and 20574040).
Supporting information for this article (synthesis of AzoC10,
association constant for AzoC10–a-CD, measurement of the critical
aggregate concentration, photoisomerization rates, and ESI-MS
and XRD data) is available on the WWW under http://www.ange-
wandte.org or from the author.
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Communications
CD. According to the modified Benesi–Hildebrand equation,
the association constant Kazo/a-CD for the 1:1 inclusion complex
4
À1
of a-CD with trans-AzoC10 is 2.82 ” 10 m . As shown in
Figure 3, the absorption of trans-AzoC10 at 346 nm is
Figure 4. TEM images of a) AzoC10 and b) AzoC10 mixed with a-CD
1:1 molar ratio); c) after irradiation at 365 nm for 500 s; d) after
irradiation at 450 nm for 2500 s, then further irradiation at 365 nm for
(
Figure 3. a) Photoisomerization spectra of AzoC10 in the presence of
a-CD (1:1 molar ratio) before and after UV irradiation. b) Changes of
AzoC10–CD inclusion (absorbance at 350 nm) upon alternating irradi-
5
00 s; e) cis-rich AzoC10 mixed with a-CD (1:1 molar ratio). The
À5
concentration of AzoC10 is 7”10 m, which is larger than the critical
aggregate concentration (CAC) of trans-AzoC10 (3.5”10 m) and cis-
ation with UV and visible light. The concentration of AzoC10 is
À5
À5
7
”10 m.
À5
AzoC10 (3.9”10 m).
enhanced after association with a-CD. The molar extinction
coefficients of trans-AzoC10 in the absence and presence of
aqueous solution of AzoC10 to a molar ratio of 1:1, the
vesicle-like aggregates disassemble (see Figure 4b). One
plausible reason is that the inclusion of the azobenzene
group of AzoC10 by a-CD renders this amphiphilic AzoC10
hydrophilic. As a result, vesicle-like aggregates disassemble.
Third, when the above system is irradiated by UV light at
365 nm, we found interestingly that the vesicle-like aggre-
gates form again (see Figure 4c). As discussed before, a-CD
may leave the azobenzene group when AzoC10 is photo-
isomerized to the cis state, because a-CD cannot include the
cis-azobenzene of AzoC10. This means that AzoC10 in the
presence of a-CD becomes amphiphilic once again after UV
irradiation. Therefore, the self-organization of AzoC10 in the
cis form leads to the formation of a vesicle-like structure.
Moreover, the aggregates (Figure 4c) are disassembled by
further irradiation at 450 nm, and are largely recovered on
subsequent UV irradiation at 365 nm (Figure 4d).
Notably, there is some difference between Figure 4a and c
in terms of the size and shape of the vesicle-like aggregates.
There are several reasons that may cause the difference.
1) Figure 4c shows the vesicle-like aggregates formed by cis-
AzoC10 in the presence of a-CD, which should be different
from the aggregates formed by trans-AzoC10 (Figure 4a). To
clarify this issue, we used UV light to irradiate the solution of
trans-AzoC10 for a sufficient time and then added a-CD to
the solution. In this way, most of the AzoC10 was photo-
isomerized into the cis form, thus rendering it hardly able to
interact with a-CD. However, on comparing Figure 4e and a,
3
4
À1
À1
a-CD are 6.95 ” 10 and 1.07 ” 10 m cm , respectively.
Therefore, the enhanced absorption should be caused by the
enhanced molar extinction coefficient after association. In
other words, the enhanced absorption suggests that trans-
AzoC10 forms an inclusion complexwith a-CD.
The inclusion complexof AzoC10 with a-CD can undergo
trans–cis photoisomerization reversibly. As shown in Fig-
ure 3a, upon irradiation by UV light at 365 nm the absorption
of trans-AzoC10 decreases dramatically, and concomitantly
an absorption peak attributable to cis-AzoC10 appears.
Interestingly, the absorption of cis-AzoC10 in the presence
of a-CD is almost the same as that of pure cis-AzoC10, which
suggests that the cis-azobenzene moiety of AzoC10 hardly
interacts with a-CD. On alternating irradiation of the solution
with UV and visible light, this reversible photoisomerization
process can be recycled many times (see Figure 3b).
As reported, the aggregation of some surfactants, such as
cetyltrimethylammonium bromide, can be disrupted upon
addition of the cyclodextrin, due to formation of an inclusion
complexof hydrophobic alkyl chains with water-soluble
[
12]
cyclodextrin.
We wondered how the presence of a-CD
and photoirradiation could affect the aggregation of AzoC10.
To address this question, we performed transmission electron
microscopy (TEM) measurements. First, as indicated by
Figure 4a, the pure AzoC10 can form vesicle-like aggregates
in aqueous solution. Second, when a-CD is added to an
2
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Chemie
we find that after this treatment cis-AzoC10 forms similar
aggregates to those of trans-AzoC10, which indicates that the
photoisomerization of the azobenzene group has no signifi-
cant influence on the size and shape of the vesicle-like
aggregates of AzoC10 in the present situation. 2) Although a-
CD prefers to bind with the azobenzene moiety of AzoC10, it
can interact with the alkyl chain of AzoC10 as well. Upon
UV irradiation, the weak interaction between a-CD and cis-
azobenzene may drive some of the a-CD to slide onto the
alkyl chain, and thus the self-organization of AzoC10
complexed with a-CD could form different vesicle-like
aggregates.
The influence of a-CD on the aggregation behavior of
AzoC10 is supported by the different kinetic behavior of
photoisomerization under various conditions. Usually, the
photoisomerization of the azobenzene group proceeds in first
order in solution; the initial trans–cis photoisomerization rate
zene groups of AzoC10 in the presence of a-CD can still form
aggregates like cis-AzoC10 alone.
After UV irradiation of the inclusion complexof AzoC10
and a-CD, we mentioned that a-CD is likely to move from the
azobenzene group to the hydrophobic alkyl chain of AzoC10.
If this is true, then a-CD can also move back from the alkyl
chain to the azobenzene group under irradiation by visible
light. Therefore, the movement of a-CD is like that of a
molecular shuttle, which can influence the aggregation of
AzoC10 in aqueous solution. To confirm this speculation, we
[
13]
1
used H NMR spectroscopy to provide evidence about the
interaction of a-CD and AzoC10 under different conditions.
À4
The concentration of AzoC10 in D O chosen was 2.5 ” 10 m,
2
at which the NMR signals of AzoC10 can be collected
adequately. As shown in Figure 5, in the aryl region only
constants (k ) are shown in Table 1. On increasing the
t
Table 1: Photoisomerization rate constants of AzoC10 at different
concentrations in the absence or presence of a-CD (1:1).
À1
À1
c(AzoC10) [m]
kt [s
]
kc [s ]
À5
1
1
3
7
7
0
0.1723
0.1003
0.0237
0.0126
0.0055
0.0286
–
0.0130
0.0059
0.0052
À5
0
(CD)
À5
.5”10
”10 (CD)
”10
À5
1
À5
Figure 5. H NMR spectra of a) AzoC10, b) AzoC10 in the presence of
a-CD (1:1 molar ratio), and c) after irradiation at 365 nm for 500 s,
then d) irradiation at 450 nm for 2700 s. The concentration of AzoC10
À4
is 2.5”10 m. The solvent is D O; d(H O)=4.79 ppm.
2
2
concentration of AzoC10, the value of k decreases remark-
t
ably. Note that the isomerization rate of AzoC10, with a
concentration lower than the CAC, decreases upon addition
of a-CD (1:1), but increases when AzoC10 is mixed with a-
CD above the CAC. The most likely reason could be as
follows. At a concentration lower than the CAC, AzoC10 is
freely dispersed in the bulk solution, and hence there is
enough space for AzoC10 to undergo trans–cis conversion;
the added a-CD molecules will incorporate AzoC10 in their
cavities, thereby restricting the trans–cis isomerization of
AzoC10 to some extent. The situation is different for AzoC10
aggregating at a concentration above the CAC. In that case,
the azobenzene group does not have enough space for the
trans–cis conversion, which results in weakening of the
photoisomerization. On the other hand, the space for the
photoisomerization of azobenzene in the presence of a-CD
may be larger than that in the vesicle aggregates. Therefore,
we can draw the conclusion that the added a-CD increases the
initial isomerization rate of AzoC10 at a concentration above
the CAC, thus indicating that a-CD destroys the aggregates of
AzoC10, which also agrees well with the previous TEM
observations.
protons of the hydrophilic pyridine group of AzoC10 are
readily detected by NMR measurements, and no proton of the
azobenzene group is clearly observed. This fact suggests the
formation of AzoC10 aggregates on the basis of a hydro-
[
14]
phobic effect.
However, the signals of the azobenzene
group of AzoC10 appear in the presence of a-CD (1:1 molar
ratio), which indicates that a-CD can associate the azoben-
zene group of AzoC10 and induce the disassembly of the
aggregates. Further irradiation at 365 nm for sufficient time
ensures that azobenzene converts into the cis form in a
photostationary state. As indicated by Figure 5c, a-CD has
almost no interaction with the azobenzene group in this case.
Upon UV irradiation, if all a-CD molecules migrate back into
the bulk after disassembly of a-CD with AzoC10, then
AzoC10 may aggregate again and in that case the NMR
spectrum should be similar to that shown in Figure 5a.
However, the proton signals of azobenzene (see Figure 5c)
are not screened again, which suggests that upon UV
irradiation, some of the a-CD moves onto the alkyl chain of
AzoC10. Upon irradiation at 450 nm for a sufficient time, the
proton signals of Figure 5d are similar to those of Figure 5b,
which indicates that a-CD can move back and interact with
the azobenzene group of AzoC10 once again, thus behaving
like a molecular shuttle.
Interestingly, a-CD has little effect on the cis–trans
isomerization of AzoC10 when above its CAC. The cis–
trans photoisomerization rate constant (k ) of AzoC10
c
associated with a-CD after UV irradiation stays almost the
same as that of AzoC10 itself. This result is different from the
The above discussion is further supported by ESI-MS
data. Before UV irradiation, there is a peak with m/z 1387.79,
which corresponds to the 1:1 inclusion complexof AzoC10
trans–cis behavior of AzoC10, whose k value increases upon
t
association with a-CD, which suggests that the cis-azoben-
Angew. Chem. Int. Ed. 2007, 46, 2823 –2826
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2825

Communications
(
3
minus the bromine anion) and a-CD. After irradiation at
65 nm, a peak with m/z 1387.82 still exists, which suggests
[6] a) H. Ringsdorf, B. Schlarb, J. Venzmer, Angew. Chem. 1988,
00, 117; Angew. Chem. Int. Ed. Engl. 1988, 27, 113; b) Y.
1
Orihara, A. Matsumura, Y. Saito, N. Ogawa, T. Saji, A.
Yamaguchi, H. Sakai, M. Abe, Langmuir 2001, 17, 6072.
7] Y. X. Liu, P. J. Jessop, M. Cunningham, C. A. Eckert, C. L.
Liotta, Science 2006, 313, 958.
that cis-AzoC10 can also form a 1:1 inclusion complexwith a-
CD. As a-CD does not interact with the cis-azobenzene
group, it must form an inclusion complexwith the alkyl chain
of AzoC10 under these conditions. In other words, ESI-MS
data provide further evidence to support the speculation that
some a-CD shifts from the azobenzene group to the alkyl
chain of AzoC10 upon UV irradiation.
We have confirmed the formation of vesicle-like aggre-
gates that can be reversibly controlled by external photo-
stimuli. However, there is one unclear point as to whether the
vesicle aggregates have a bilayer structure. To answer this
question, we employed X-ray diffraction (XRD) to provide
information about the layered structure. To do so, we
fabricated a cast film from a solution of AzoC10 with a
[
[
8] a) R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. B. Folmer, J. H.
Ky Hirschberg, R. F. M. Lange, J. K. L. Lowe, E. W. Meijer,
Science 1997, 278, 1601; b) S. Kiyonaka, K. Sugiyasu, S. Shinkai,
I. Hamachi, J. Am. Chem. Soc. 2002, 124, 10954; c) R. Castro, I.
Cuadrado, B. Alonso, C. M. Casado, M. Moran, A. E. Kaifer, J.
Am. Chem. Soc. 1997, 119, 5760; d) E. R. Gillies, T. B. Jonsson,
J. M. J. FrØchet, J. Am. Chem. Soc. 2004, 126, 11936; e) Y. J. Ma,
W. F. Dong, M. A. Hempenius, H. Möhwald, G. J. Vancso, Nat.
Mater. 2006, 5, 724; f) D. Y. Chen, M. Jiang, Acc. Chem. Res.
2005, 38, 494.
[
9] a) M. Seiler, H. Dürr, I. Willner, E. Joselevich, A. Doron, J. F.
Stoddart, J. Am. Chem. Soc. 1994, 116, 3399; b) D. Philp, J. F.
Stoddart, Angew. Chem. 1996, 108, 1242; Angew. Chem. Int. Ed.
Engl. 1996, 35, 1154; c) L. J. Prins, D. N. Reinhoudt, P. Timmer-
man, Angew. Chem. 2001, 113, 2446; Angew. Chem. Int. Ed.
À5
concentration of 7 ” 10 m. XRD shows a Bragg peak at 2q
ꢀ
38, which corresponds to a d spacing of 29.7 Š. Considering
that AzoC10 is about 26 Š in length, we may draw the
conclusion that AzoC10 molecules are well interdigitated in
the vesicle-like aggregates. It is hoped that such a line of
research may provide a way to fabricate smart responsive and
reversible materials.
2001, 40, 2382; d) J. D. Badjic, A. Nelson, S. J. Cantrill, W. B.
Turnbull, J. F. Stoddart, Acc. Chem. Res. 2005, 38, 723; e) A. B.
Descalzo, R. Martíez-Mµæez, F. Sancenón, K. Hoffmann, K.
Rurack, Angew. Chem. 2006, 118, 6068; Angew. Chem. Int. Ed.
2006, 45, 5924; f) T. Auletta, M. R. de Jong, A. Mulder,
F. C. J. M. van Veggel, J. Huskens, D. N. Reinhoudt, S. Zou, S.
Zapotoczny, H. Schönherr, G. J. Vancso, L. Kuipers, J. Am.
Chem. Soc. 2004, 126, 1577.
Received: December 8, 2006
Published online: March 9, 2007
[
10] a) R. Breslow, S. D. Dong, Chem. Rev. 1998, 98, 1997; b) C.
Dugave, L. Demange, Chem. Rev. 2003, 103, 2475.
Keywords: isomerization · self-assembly·
[
11] a) S. A. Nepogodiev, J. F. Stoddart, Chem. Rev. 1998, 98, 1959;
b) A. Harada, Acc. Chem. Res. 2001, 34, 456; c) G. Wenz, B. H.
Han, A. Müller, Chem. Rev. 2006, 106, 782; d) H. Murakami, A.
Kawabuchi, K. Kotoo, M. Kunitake, N. Nakashima, J. Am.
Chem. Soc. 1997, 119, 7605; e) D. H. Qu, Q. C. Wang, J. Ren, H.
Tian, Org. Lett. 2004, 6, 2085; f) I. Tomatsu, A. Hashidzume, A.
Harada, J. Am. Chem. Soc. 2006, 128, 2226; g) I. A. Banerjee, L.
Yu, H. Matsui, J. Am. Chem. Soc. 2003, 125, 9542.
.
supramolecular chemistry· surfactants · vesicles
[
1] a) M. J. Ostro, P. R. Cullis, Am. J. Hosp. Pharm. 1989, 46, 1576;
b) P. G. Westmoreland, Jr., R. A. Day, A. L. Underwood, Anal.
Chem. 1972, 44, 737.
[
2] a) T. Dwars, E. Paetzold, G. Oehme, Angew. Chem. 2005, 117,
7
338; Angew. Chem. Int. Ed. 2005, 44, 7174; b) D. M. Vriezema,
[12] a) A. Diaz, P. A. Quintela, J. M. Schuette, A. E. Kaifer, J. Phys.
Chem. 1988, 92, 3537; b) U. R. Dharmawardana, S. D. Christian,
E. E. Tucker, R. W. Taylor, J. F. Scamehorn, Langmuir 1993, 9,
2258.
[13] I. Tomatsu, A. Hashidzume, A. Harada, Macromolecules 2005,
38, 5223.
[14] B. Song, Z. Q. Wang, S. L. Chen, X. Zhang, Y. Fu, M. Smet, W.
Dehaen, Angew. Chem. 2005, 117, 4809; Angew. Chem. Int. Ed.
2005, 44, 4731.
M. C. AragonØs, J. A.-A. W. Elemans, J. J. L. M. Cornelissen,
A. E. Rowan, R. J. M. Nolte, Chem. Rev. 2005, 105, 1445.
3] R. M. Izatt, J. D. Lamb, R. T. Hawkins, P. R. Brown, S. R. Izatt,
J. J. Christsen, J. Am. Chem. Soc. 1983, 105, 1782.
4] S. Shinkai, S. Nakamura, S. Tachiki, T. Kajiyama, O. Manabe, J.
Am. Chem. Soc. 1985, 107, 3363.
[
[
[
5] a) K. Hoshino, T. Saji, J. Am. Chem. Soc. 1987, 109, 5881;
b) C. A. Rosslee, N. L. Abbott, Anal. Chem. 2001, 73, 4808.
2
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