Photoswitchable supramolecular catalysis by interparticle host-guest competitive binding

Article abstract of DOI:10.1002/chem.201202711

On and off: Ester hydrolysis catalyzed by a Zn^II-coordinated β-cyclodextrin dimer can be switched on and off using light in the presence of gold nanoparticles with azobenzene units attatched to their surfaces. Under visible light, the azobenzene units are trans and bind tightly to the dimer, thus leading to reduced catalysis. Under UV light, the azobenzene units are cis and bind loosely to the dimer, thus allowing substrates to bind and hydrolysis to occur. Copyright

Full text of DOI:10.1002/chem.201202711

COMMUNICATION
DOI: 10.1002/chem.201202711
Photoswitchable Supramolecular Catalysis by Interparticle Host–Guest
Competitive Binding
Liangliang Zhu,[a] Hong Yan,[a] Chung Yen Ang,[a] Kim Truc Nguyen,[a]
Menghuan Li,[b] and Yanli Zhao*[a, b]
Supramolecular catalysts are of great interest because of
their enzyme-like noncovalent interactions with substrates.[1]
For example, some cyclodextrin dimers (bisCDs) and metal-
lo-bisCDs have been used as supramolecular catalysts that
are hydrolase mimics.[2] Generally, supramolecular catalysts
are used homogeneously with respect to the substrates, and
these catalysts cannot be recovered easily for reuse. Thus,
effective and reversible immobilization methods[3] for the re-
covery of these catalysts are urgently needed. Although,
chemists have investigated supramolecular catalysts by using
in situ tuning of the reaction,[4] the development of switcha-
ble supramolecular catalysts that can be reversibly immobi-
lized still remains a challenge.
We wanted to design and prepare supramolecular cata-
lysts with the following attributes: 1) their activity can be
switched on and off in situ by tuning competitive binding
events, and 2) they contain an appropriate carrier that
would allow reversible immobilization. CD–azobenzene
dyads are the kernel of many light-driven artificial molecu-
lar machines, which have attracted a lot of attentions be-
cause of their intrinsic property to be controlled remotely
without generating byproducts.[5] By taking advantage of the
reversible assembly/disassembly of the CD host upon the
trans–cis photoisomerization of the azobenzene guest, this
dyad has been employed in the preparation of a variety of
functional materials and devices, such as molecular
switches,[6] molecular logic gates,[7] tunable soft materials,[8]
responsive polymers,[9] nanoelectronics,[10] and drug-delivery
systems.[11] Inspired by these developments, we propose to
employ this phototriggerable host–guest binding process as
the controllable inhibition process in supramolecular cataly-
sis.
We chose gold nanoparticles (AuNPs) as the catalytic sup-
port onto which the CD-based supramolecular catalysts
would be reversibly immobilized because (1) AuNPs can be
easily prepared and isolated to serve as an effective hetero-
geneous nanocarrier and (2) the surface plasmon resonance
(SPR) and the chromic property of AuNPs are sensitive to
interparticle events;[12] these properties can be used either to
indicate or monitor the switchable catalytic process. Thus, a
key part of the preparation of the catalytic system is to graft
azobenzene units onto the AuNP surface thus, placing the
CD-based supramolecular catalysts between AuNPs through
host–guest binding with the azobenzene units.
It has been well documented that the association constant
for the binding of a b-CD ring with the trans isomer of azo-
benzene is greater than 103 mÀ1, indicating strong binding af-
finity within the dyad.[6–11] As a result, the CD-based supra-
molecular catalysts will initially encapsulate the trans-azo-
benzene units on the AuNP surface instead of the substrates
for the catalytic reaction. Upon photoisomerization of the
azobenzene unit from the trans to the cis form under UV-
light irradiation, the binding affinity between the b-CD ring
and the azobenzene unit will be weakened substantially,
thus allowing the b-CD ring to encapsulate the reaction sub-
strates and catalyze the transformation. When illuminating
the system with visible light, the cis-to-trans photoisomeriza-
tion of the azobenzene unit will cause the b-CD ring to
form the inclusion complex with the trans-azobenzene unit,
thus terminating the catalytic reaction again (Figure 1).
Therefore, interparticle host–guest competitive binding was
established .
To prepare the azobenzene-functionalized AuNPs, we first
synthesized stable azobenzene-containing disulfide com-
pound L1 (Figure 1 and Figure S1 in the Supporting Infor-
mation), the sulfide functionality being necessary for subse-
quent binding to the AuNPs. In its initial state, the azoben-
zene unit in compound L1 adopts a trans conformation as
[a] Dr. L. Zhu, Dr. H. Yan, C. Y. Ang, K. T. Nguyen, Prof. Dr. Y. Zhao
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
1
evidenced by the H NMR spectrum (see the Supporting In-
formation, Figure S2A). The trans-to-cis photoisomerization
of the azobenzene unit occurs when the compound is irradi-
ated by UV light at 365 nm. The presence of the cis isomer
was confirmed by the 1H NMR spectra, which shows a
group of resonances in the aromatic region that are upfield
shifted relative to those of the trans isomer (see the Sup-
porting Information, Figure S2B). The maximum trans-to-cis
photoisomerization efficiency is approximately 85% accord-
21 Nanyang Link, Singapore 637371
E-mail: zhaoyanli@ntu.edu.sg
[b] M. Li, Prof. Dr. Y. Zhao
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202711.
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formation, Figure S6). Specifi-
cally, two distinct decomposi-
tion steps of L1 were observed
showing that this ligand under-
goes substantial decomposition
between 2708C and 4508C.
The weight loss of Au@L1L2
before it reaches 2708C corre-
sponds to the decomposition of
L2, and the residue after
4508C contributes to the inor-
ganic Au core. Thus, we can
deduce the average molar ratio
between the attached azoben-
zene unit and the Au core to
be approximately 2400:1.
ZnII-coordinated
pyridyl-
bisCD (bCDZn), which is a
highly efficient supramolecular
catalyst for ester hydrolysis,[14]
was employed to investigate
Figure 1. Structures of the supramolecular catalyst bCDZn, the ligand components, L1 and L2, and a schematic
representation of the phototriggered interparticle host–guest competitive binding for switchable catalysis. The
stable catalytic states in this system are labeled as Au@L1L2ꢀbCDZn and p-Au@L1L2+bCDZn. The pro-
posed mechanism is also illustrated: the catalytic activity of bCDZn is passivated because the metallo-bisCDs
are prone to bind with the trans-azobenzene unit of L1 on the AuNP surface in preference to the substrates;
upon photoisomerization of the trans-azobenzene unit to its cis isomer, dissociation of bCDZn from L1 on the
AuNPs readily occurs and allows bCDZn to bind with the substrates, thus leading to the activation of the cata-
lytic reaction.
host–guest
binding
with
Au@L1L2. To ensure quantita-
tive binding, we prepared an
aqueous solution with a rela-
tively high concentration of
Au@L1L2 (0.19 mm), and con-
ducted all of the optical studies
in a cuvette with a path length
ing to the NMR-signal integration, an efficiency that is high
relative to those of most other azobenzene derivatives[6–10]
and ensures sufficiently efficient trans–cis interconversion
on the nanoparticle surface. This photoisomerization is re-
versible and the cis isomer can be converted back into the
trans isomer by irradiation using visible light (see the Sup-
porting Information, Figure S2C).
of 1 mm. Hybrid Au@L1L2 has a characteristic SPR band
at 524 nm (Figure 2A, curve a); the TEM image (Figure 2C)
indicates that the nanoparticles are mainly monodispersed.
To our delight, the SPR band underwent a bathochromic
shift to 528 nm after the addition of 1 equivalent (relative to
that of the azobenzene unit) of bCDZn (Figure 2A, curve
b). Subsequently, by introducing an excess amount of
bCDZn, the band was further red-shifted to 530 nm (Fig-
ure 2A, curves d and e). The positive signal at 360 nm in the
induced circular dichroism (ICD) spectra confirms that the
host–guest binding occurs (Figure 2B), because the p–p*
electronic transition of the azobenzene unit aligned parallel
to the axis inside the CD cavity produces a positive Cotton
effect.[15] Nanoparticle aggregates can be observed in the
TEM image (Figure 2D) at this stage, featuring interparticle
host–guest binding. In addition, the solution color became
deeper after the addition of bCDZn (see the inserts in Fig-
ure 2C and 2D). For comparison purposes, control studies
were conducted by the addition of pyridyl bisCD to the
Au@L1L2 solution and analyzing the resulting mixture by
using UV/Vis and ICD studies. The positive ICD signal and
the fact that the SPR band does not undergo a red-shift (see
the Supporting Information, Figures S7 and S8) show that
intraparticle host–guest binding rather than interparticle
host–guest binding predominates. These data indicate that
the ZnII-coordinated rigid linkage between the two CD rings
in bCDZn promotes interparticle host–guest binding (see
Water-solubility is an essential prerequisite for the forma-
tion of CD–azobenzene complexes. Nevertheless, current
approaches to impart hydrophilic character to azobenzene-
containing disulfides are still relatively limited. Therefore, a
mixed self-assembled monolayer (mSAM) strategy involving
both L1 and a hydrophilic ligand L2 was employed to pre-
pare water-soluble hybrid AuNPs. L2 bears a polyethylene
glycol moiety that has been proven to be unable to complex
with the b-CD ring.[13] Upon optimization, an appropriate
molar ratio (L1/L2=1:7, see the Supporting Information,
Figure S3) facilitated the formation of an mSAM on the
AuNP surface. The FT-IR spectrum of the functionalized
AuNPs, Au@L1L2, shows a number of characteristic peaks
(1500–1600 cmÀ1) corresponding to the vibrations of aromat-
ic moieties of compound L1, thus confirming the successful
conjugation of L1 onto the AuNP surface (see the Support-
ing Information, Figure S4). As Au@L1L2 is well dispersed
in water, a moderate Raman signal of Au@L1L2 in aqueous
solution was observed (see the Supporting Information, Fig-
ure S5). The thermal-stability investigation provided more
detailed information on Au@L1L2 (see the Supporting In-
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Photoswitchable Supramolecular Catalysis
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tures.[2,14] Before studying the kinetics of the hydrolysis, we
needed to clarify the effect of the thermal isomerization[6a,16]
of the azobenzene unit on AuNPs. Even at room tempera-
ture, the cis-azobenzene unit gradually converts back into
the corresponding trans isomer in the dark. The apparent
rate constant (k=0.0019 minÀ1, first-order exponential
growth fitting) for the recovery process was calculated in ac-
cordance with the changes of the p–p* band (see the Sup-
porting Information, Figure S10). Fortunately, one of the
isoabsorptive points at 420 nm (see the Supporting Informa-
tion, Figure S10 A) overlaps with the absorption band of the
hydrolyzate, 4-nitrophenolate.[17] Therefore, the UV/Vis
signal at 420 nm (e=7130mÀ1 cmÀ1) was used to monitor the
catalyzed hydrolysis reactions. The absorption of the cata-
lysts at this wavelength was subtracted from the overall ab-
sorbance at this wavelength to compare the relative rates of
hydrolysis (Figure 3). In addition, we only used Au@L1L2
complexed with 1 equivalent of bCDZn in the catalytic
study, because free bCDZn may interfere with the switcha-
ble catalysis.
Figure 2. Formation of Au@L1L2ꢀbCDZn and the phototriggered disas-
sembly process in water at 298 K. (A) Absorption spectra (the SPR
band) of (a) a solution of Au@L1L2, (b) a solution of Au@L1L2 and
1 equiv of bCDZn (c) a solution of Au@L1L2 and 1 equiv of bCDZn
that has been photoirradiated using 365 nm UV light, (d) a solution of
Au@L1L2 and 10 equiv of bCDZn (e) a solution of Au@L1L2 and
20 equiv of bCDZn. (B) Induced circular dichroism (ICD) spectra of (a)
a solution of Au@L1L2, (b) a solution of Au@L1L2 and 20 equiv of
bCDZn. (C) TEM image of Au@L1L2. (D) TEM image of Au@L1L2
after the addition of 20 equiv of bCDZn. The inserts in (C) and (D)
shows the corresponding photographs of the solutions.
the schematic representation of Au@L1L2ꢀbCDZn in
Figure 1).
Figure 3. Switchable supramolecular catalysis for the hydrolysis of NA.
(A) Plots of relative absorbance at 420 nm versus reaction time during
the hydrolysis of NA in aqueous solution at 298 K: (a) without catalyst;
(b) in the presence of bCDZn; (c) in the presence of Au@L1L2; (d) in
the presence of p-Au@L1L2; (e) in the presence of Au@L1L2ꢀbCDZn;
(f) in the presence of p-Au@L1L2+bCDZn. (B) Normalized absorption
changes at 420 nm during an ongoing hydrolysis process with photoirra-
diation for several cycles. Each cycle lasts 150 min. In each cycle, the sol-
ution is irradiated with visible light for 45 min and then its absorbance is
measured for 30 min, followed by irradiation with 365 nm UV light for
45 min and the absorption measurement for another 30 min.
The hybrid Au@L1L2 has excellent photochemical per-
formance. Normally, illumination of Au@L1L2 with a UV-
light source with a wavelength of 365 nm and an intensity of
15 W for approximately 45 minutes is necessary for
Au@L1L2 to reach its photostationary state, p-Au@L1L2,
where the azobenzene unit is in its cis form. Furthermore,
the trans–cis photoswitching process of the azobenzene unit
on Au@L1L2 is reversible and repeatable by alternating be-
tween photoirradiation with 365 nm UV light and visible
light (see the Supporting Information, Figure S9). Encour-
aged by the reversible photoswitching of the Au@L1L2
hybrid, we proceeded to study the photoswitchability of
Au@L1L2ꢀbCDZn. The SPR band of Au@L1L2ꢀbCDZn
was blue-shifted after sufficient irradiation at 365 nm (Fig-
ure 2A, curve c), thus indicating phototriggered disassembly
of the CD–azobenzene complex followed by the redisper-
sion of the AuNPs (see the schematic representation of p-
Au@L1L2+bCDZn in Figure 1). Therefore, the bCDZn
catalyst can be released from the nanoparticles by the trans-
to-cis photoisomerization of the azobenzene unit, a process
that is indicated by SPR band shifts.
The initial rate of spontaneous hydrolysis of NA (1.9 mm)
was relatively slow; the rate increased by approximately 40-
fold in the presence of bCDZn (0.19 mm of the b-CD ring,
0.1 equivalent with respect to the substrate, see curves a and
b in Figure 3A). This rate enhancement can be attributed to
the formation of a complex between the catalyst, bCDZn,
and the substrate, an interaction that facilitates hydroly-
sis.[2,14] The presence of either Au@L1L2 or its photostation-
ary state p-Au@L1L2 alone does not catalyze this reaction,
as evidenced by the lack of obvious hydrolysis-rate enhance-
ments (Figure 3A, curves c and d).
To determine whether the catalytic activity of the system
is photoswitchable, 4-nitrophenyl acetate (NA) and bis(4-ni-
trophenyl) carbonate (BNPC) were selected as catalytic-hy-
drolysis substrates because of their suitable structural fea-
Switchable catalysis was observed when NA was treated
with composite Au@L1L2ꢀbCDZn. There is only a small
enhancement in the hydrolysis rate in the presence of
Au@L1L2ꢀbCDZn (Figure 3A, curve e) because the b-CD
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Y. L. Zhao et al.
Table 1. The rate of hydrolysis of NA and BNPC as catalyzed by differ-
ent species.
rings of the catalyst bCDZn are occupied by the trans-azo-
benzene units on the AuNP surface. However, when the
composite was converted into its photostationary state, p-
Au@L1L2+bCDZn, upon its irradiation with UV light, an
approximate 30-fold enhancement in the hydrolysis rate was
observed (Figure 3A, curve f), which is close to enhance-
ment observed when NA is treated with bCDZn (Figure 3A,
curve b). These data suggest that a photoinduced disassem-
bly process leading to the release of bCDZn and p-
Au@L1L2 is operating, the former then being free to cata-
lyze the hydrolysis reaction. The switched-on catalytic activi-
ty is similar to that of a control experiment in which NA is
treated with a water-soluble homogeneous system consisting
of an azobenzene derivative and bCDZn (see the Support-
ing Information, Figure S11). Because the timescale of the
hydrolysis of NA is generally longer than that of the photoi-
somerization of Au@L1L2ꢀbCDZn, the progress of the re-
action can be either be impeded or promoted as required by
irradiation of the reaction mixture with visible and UV
light, respectively. This photoswitching of catalysis can be
observed from the increase and the decrease of the hydroly-
sis rate with irradiation alternating between UV and visible
light, respectively (Figure 3B).
In contrast to the hydrolysis of NA, the hydrolysis of
BNPC (0.95 mm) involves a two-step process.[18] For the first
step, which is indicated by a decrease in the intensity of the
UV/Vis signal at 280 nm (where BNPC absorbs), switchable
catalysis was not observed (see the Supporting Information,
Figure S12), simply because the binding affinity of BNPC to
the b-CD dimers is relatively strong[19] and the azobenzene
unit cannot effectively replace BNPC from the b-CD cavity.
In contrast, this binding process does not occur for sub-
strates containing only one phenyl group. The switching
effect could be observed when monitoring the absorption of
4-nitrophenolate at 420 nm, the species that is formed
during the second step of the hydrolysis reaction (see the
Supporting Information, Figure S13). The apparent rates of
hydrolysis of NA and BNPC in the presence of the different
species were calculated (Table 1). In particular, the rates of
the hydrolysis reactions catalyzed by Au@L1L2ꢀbCDZn
and p-Au@L1L2+bCDZn reveal significant differences in
the catalytic activity of the composite when the azobenzene
unit is in its trans- and cis-configured form. By comparing
the apparent rate constants (see the Supporting Informa-
tion), we concluded that the differences in the hydrolysis
rates originate mainly from the amount of free bCDZn in
solution. There is more bCDZn available for catalysis when
the azobenzene unit undergoes the trans-to-cis photoisome-
rization on the AuNP surface.
Entry
Substrate
Catalyst
Rate (ꢁ10À8) [MminÀ1
]
1
2
3
4
5
6
7
8
NA
NA
NA
NA
NA
NA
BNPC
BNPC
BNPC
BNPC
BNPC
BNPC
none
bCDZn
Au@L1L2
p-Au@L1L2
Au@L1L2ꢀbCDZn
p-Au@L1L2+bCDZn
none
bCDZn
Au@L1L2
4.52Æ0.32
178Æ4.2
12.2Æ0.45
7.65Æ0.87
54.1Æ1.9
132Æ4.6
null[a]
334Æ12.9
9
null
10
11
12
p-Au@L1L2
Au@L1L2ꢀbCDZn
p-Au@L1L2+bCDZn
null
190Æ11.4
250Æ16.2
[a] null means that the reaction is too slow to be measured accurately.
toisomerization of the azobenzene unit on the gold nanopar-
ticle surface under UV irradiation, the azobenzene units are
unable to bind with the b-cyclodextrin rings as tightly and
thus the catalyst becomes available for catalyzing the hy-
drolysis of the substrates. We have also demonstrated that
the catalytic process can be switched on and off repeatedly
by alternating between UV- and visible-light irradiation. Be-
cause the host–guest competitive binding described here is
an interparticle process, the switchable supramolecular cata-
lyst can be reversibly immobilized on the gold nanoparticle
surface, a process that can be followed by observing the
changes to the SPR band of the gold nanoparticles. The re-
sults herein are valuable for the design and preparation of
the next generation of catalytic materials with facile control
over immobilization and catalytic activity.
Experimental Section
Synthesis of the compound L1: [4-[(6-Bromohexyl)oxy]phenyl]phenyldia-
zene (A)[20] (0.87 g, 2.4 mmol) and hexamethyldisilathiane (0.52 g,
0.61 mL, 2.9 mmol) were dissolved in dry THF (20 mL). The solution was
cooled down to between 0 and 58C and a solution of TBAF (1m) in THF
(2.66 mL, 2.66 mmol) was then added. After the solution was warmed up
to room temperature over 1.5 h, aqueous saturated ammonium chloride
(50 mL) was added. The solid precipitates were collected by filtration
and purified using silica gel chromatography (petroleum ether/ethyl ace-
tate=9:2) to obtain compound L1 (282 mg, 37.4%). mp. 106–1098C.
1H NMR (400 MHz, CDCl3, 298 K) d=7.90 (m, 8H), 7.49 (m, 6H), 7.02
(d, J=9.0 Hz, 4H), 4.06 (t, J=6.4 Hz, 4H), 2.74 (t, J=6.8 Hz, 4H), 1.84
(m, 4H), 1.74 (m, 4H), 1.55 (m, 8H). 13C NMR (100 MHz, CDCl3,
298 K) d=161.70, 152.84, 146.91, 130.33, 129.04, 124.77, 122.55, 114.70,
68.16, 38.97, 29.10, 28.69, 28.23, 25.75. HRMS (ESI) m/z: [M+H]+ calcd
for C36H43N4O2S2, 627.2827; found m/z, 627.2830.
Compound L2 was prepared using a procedure described in the litera-
In summary, photoswitchable supramolecular catalysis of
ester hydrolysis has been achieved using host–guest compet-
itive binding of a ZnII-coordinated b-cyclodextrin dimer cat-
alyst with azobenzene units on the surfaces of gold nanopar-
ticles in the presence of reaction substrates. When the b-cy-
clodextrin rings are occupied by the trans-azobenzene units,
the catalyst no longer recognizes the substrate and therefore
its catalytic activity is passivated. After the trans-to-cis pho-
ture.[21]
Synthesis of bCDZn: ZnACTHNUTRGNEUNG(NO3)2·6H2O (40 mg, 0.134 mmol) was added to
a stirred solution of pyridyl bisCD (bCD)[14] (202 mg, 0.085 mmol) in
water (5 mL) at 458C. The resulting solution was stirred for another 6 h,
and then the mixture was concentrated under reduced pressure. Ethanol
(5 mL) was added to form precipitates, which were collected by filtration
and washed with ethanol. The sample was dried in vacuo to give bCDZn
(175 mg, 70.8%) as a white solid. mp. 234–2358C (decomp.). 1H NMR
(400 MHz, D2O, 298 K) d=7.95 (t, J=7.6 Hz, 1H), 7.39 (d, J=8.0 Hz,
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Photoswitchable Supramolecular Catalysis
COMMUNICATION
2H), 5.08–4.96 (m, 14H), 4.14 (m, 4H), 4.01–3.76 (m, 56H), 3.64–3.38
(m, 28H). 13C NMR (100 MHz, D2O, 298 K, d): 153.44, 141.72, 122.30,
101.80, 101.40, 83.90, 81.12, 73.10, 72.03, 71.82, 68.15, 60.35, 50.17, 48.82.
MS (ESI) m/z: [MÀ2H2O]2+ calcd 1216.87; found m/z, 1216.92. m/z:
[M+2H2O]2+ calcd 1252.90; found m/z, 1253.25.
2483–2489; c) H. Li, A. C. Fahrenbach, A. Coskun, Z. Zhu, G.
Barin, Y. L. Zhao, Y. Y. Botros, J.-P. Sauvage, J. F. Stoddart, Angew.
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Biomol. Chem. 2011, 9, 4226–4233.
Citrate-stabilized AuNPs with an average diameter of 13 nm were pre-
pared using a procedure described in the literature.[22]
Preparation of Au@L1L2: The hybrid gold nanoparticles were prepared
using a ligand-exchange reaction. Typically, 4 drops of a DMF solution of
L2 (0.29m) were added into stirred citrate-stabilized AuNP aqueous solu-
tion (6 mL, 8.5 nm of gold spheres) in an 8-mL cleaned vial. The solution
gradually turns colorless after approximately 30 mins. Then 4 drops of a
DMF solution of L1 (0.042m) were added to the solution and stirring was
continued for another 16 h, during which the solution turns red. Excess
L1 was removed by centrifugation (2500 rps, 23 min). The hybrid AuNPs
were collected by centrifugation (6000 rps, 60 min) from the resulting sol-
ution and washed with deionized H2O twice. Normally, the hybrid
Au@L1L2 was stored in aqueous solution, which was concentrated for
studies.
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Keywords: azo compounds
·
cyclodextrins
·
gold
nanoparticles
chemistry
·
photoisomerization supramolecular
·
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Received: July 28, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!

Y. L. Zhao et al.
Photoswitchable Catalysis
L. Zhu, H. Yan, C. Y. Ang,
K. T. Nguyen, M. Li,
Y. Zhao* ....................... &&&& — &&&&
Photoswitchable Supramolecular
Catalysis by Interparticle Host–Guest
Competitive Binding
On and off: Ester hydrolysis catalyzed
by a ZnII-coordinated b-cyclodextrin
dimer can be switched on and off using
light in the presence of gold nanoparti-
cles with azobenzene units displayed
on their surfaces. Under visible light,
the azobenzene units are trans and
bind tightly to the dimer, thus leading
to reduced catalysis. Under UV light,
the azobenzene units are cis and bind
loosely to the dimer, thus allowing sub-
strates to bind and hydrolysis to occur.
&
6
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!