Photoresponsive switches at surfaces based on supramolecular functionalization with azobenzene-oligoglycerol conjugates

Article abstract of DOI:10.1002/anie.201403331

The synthesis, supramolecular complexation, and switching of new bifunctional azobenzene-oligoglycerol conjugates in different environments is reported. Through the formation of host-guest complexes with surface immobilized β-cyclodextrin receptors, the bifunctional switches were coupled to gold surfaces. The isomerization of the amphiphilic azobenzene derivatives was examined in solution, on gold nanoparticles, and on planar gold surfaces. The wettability of functionalized gold surfaces can be reversibly switched under light-illumination with two different wavelengths. Besides the photoisomerization processes and concomitant effects on functionality, the thermal cis to trans isomerization of the conjugates and their complexes was monitored. Thermal half-lives of the cis isomers were calculated for different environments. Surprisingly, the half-lives on gold nanoparticles were significantly smaller compared to planar gold surfaces. The thermal and light-induced photoisomerization of bifunctional azobenzene-oligoglycerol conjugates was examined in solution and on supramolecularly functionalized gold nanoparticles and planar gold surfaces. The wettability of the functionalized gold surfaces could be reversibly switched with light. Thermal half-lives of Z-isomers were determined for different environments to provide information about the stability of the functional switches for further applications.

Full text of DOI:10.1002/anie.201403331

Angewandte
Chemie
DOI: 10.1002/anie.201403331
Supramolecular Switches
Photoresponsive Switches at Surfaces Based on Supramolecular
Functionalization with Azobenzene–Oligoglycerol Conjugates**
Olaf Nachtigall, Christian Kçrdel, Leonhard H. Urner, and Rainer Haag*
Dedicated to Professor Armin de Meijere on the occasion of his 75th birthday
Abstract: The synthesis, supramolecular complexation, and
switching of new bifunctional azobenzene–oligoglycerol con-
jugates in different environments is reported. Through the
formation of host–guest complexes with surface immobilized
b-cyclodextrin receptors, the bifunctional switches were cou-
pled to gold surfaces. The isomerization of the amphiphilic
azobenzene derivatives was examined in solution, on gold
nanoparticles, and on planar gold surfaces. The wettability of
functionalized gold surfaces can be reversibly switched under
light-illumination with two different wavelengths. Besides the
photoisomerization processes and concomitant effects on
functionality, the thermal cis to trans isomerization of the
conjugates and their complexes was monitored. Thermal half-
lives of the cis isomers were calculated for different environ-
ments. Surprisingly, the half-lives on gold nanoparticles were
significantly smaller compared to planar gold surfaces.
states in different environments are important for possible
applications.
Herein, we examined the switching dynamics and thermal
half-life of amphiphilic azobenzene–glycerol conjugates as
well as their host–guest complex with a cyclodextrin deriva-
tive, in different polar solvents, which were immobilized on
gold surfaces. For this purpose, we synthesized three different
bifunctional azobenzene–glycerols, which contained an ada-
mantane and different generations of oligoglycerol dendrons
G1-AB-Ad, G2-AB-Ad, and G3-AB-Ad (Figure 1). The
oligoglycerol dendrons were attached to induce water sol-
ubility and for potential bioapplications. A thiol-functional-
ized b-cyclodextrin compound (b-CD-SH) and its protected
version prot. b-CD-SR were synthesized as a host molecule
(see the Supporting Information).[10] The formation of the
host–guest complex of G3-AB-Ad and prot. b-CD-SR was
carried out in an equimolar D2O solution for 24 h and was
confirmed by NMR spectroscopy (Figure 2a). The NMR
spectrum of the complex exhibited changes in the chemical
shifts compared to the spectra of the single compounds, which
indicated a significant host–guest interaction. Moreover,
NOESY experiments showed cross-peaks between the pro-
tons in the cavity of cyclodextrin and adamantane. The
nuclear Overhauser effect confirmed the successful formation
of the complex.[11] The absence of cross-peaks between the
azobenzene and cyclodextrin indicated that there was no
interaction between these molecular parts.
Subsequently, the complex was formed on gold nano-
particles (AuNPs). Therefore, functionalized AuNPs were
synthesized by the reduction of tetrachloroauric(III) acid
HAuCl4 with sodium borohydride in DMSO in the presence
of b-CD-SH.[12] Pure CD-coated nanoparticle solutions were
collected by centrifugation. Comparative NMR measure-
ments of the free b-CD and the b-CD functionalized AuNPs
ensured the successful coating of the nanoparticles. The host–
guest complex formed in aqueous solution except for G1-AB-
Ad, which was due to its poor solubility in water. For G1-AB-
Ad a mixture of DMSO/water (1:3) was used. Again, the
complexation step was monitored by NMR-spectroscopy for
the inclusion of G3-AB-Ad and b-CD functionalized particles
(Figure 2b). NOESY spectra showed cross-peaks between b-
CD-SH and G3-AB-Ad on AuNPs. Moreover, dynamic light
scattering (DLS) was used to determine the particle sizes of
the CD-coated AuNPs and the corresponding complexes with
the glycerol–azobenzene conjugates. For CD-coated AuNPs
the observed size of 2.7 nm was in good agreement with the
work of Kaifer and Huskens et al.[12,13] Upon the formation of
the host–guest complex with G1-AB-Ad, the size increased
T
he light-directed switching of molecules on surfaces has
great potential for interfacial changes, such as hydrophobicity
and bio-fouling behavior. The light-induced isomerization of
azobenzene is a paragon for a molecular switch.[1] The trans to
À
cis isomerization of the N N-double bond is triggered by UV
light, whereas back-switching can be realized either by heat or
visible light. Azobenzenes have been used in a variety of
applications and more recently were also investigated to
switch the function of genes or proteins.[2–5] Self-assembled
monolayers (SAM) on gold are an excellent method to study
functional surfaces and sense changes on the surface with
many experimental techniques.[6,7] However, molecular
switching of azobenzenes on planar gold surfaces is often
problematic owing to the excellent packing of alkanethiol
SAM on gold.[8] Therefore, a new approach to immobilize
functional switches on surfaces is required. Supramolecular
reversible switches are quite promising for establishing
a
switchable surface.[9] Additionally, characterizing the
switching mechanism as well as the stability of the switching
[*] O. Nachtigall, Dr. C. Kçrdel, L. H. Urner, Prof. Dr. R. Haag
Institut fꢀr Chemie und Biochemie, Freie Universitꢁt Berlin
Takustrasse 3, 14195 Berlin (Germany)
E-mail: haag@chemie.fu-berlin.de
Homepage: http://www.polytree.de
[**] This work was funded by the DFG within the SFB 658 and SFB 765.
O.N. thanks Dr. Matthew J. Webb for his kind support and the
Friedrich-Ebert-Stiftung for a scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403331.
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try on surfaces, a modu-
lar functional surface
was established for
reversible coating with
bifunctional molecular
switches.
Adsorption spectra
were recorded for all
three systems: inclusion
complex in solution,
grafted on gold nano-
particles, and on planar
gold surfaces (Figure 4).
Samples were illumi-
nated with 366 nm to
trigger the photo-iso-
merization of the azo-
benzene moiety. The
lamp we used was
a long-wave UV lamp.
The first spectrum was
assigned to the host–
guest complex formed
by G3-AB-Ad and
prot. b-CD-SH in an
aqueous solution. Upon
illumination
with
366 nm the adsorption
spectrum changed (blue
dotted line). The initial
absorbance band for the
Figure 1. Amphiphilic azobenzenes (guest) and cyclodextrins (host) for supramolecular architectures.
due to the additional contribution of the supramolecular
nanoparticle complex. Upon complexation of larger oligogly-
cerol dendrons, the particle size increased up to 8.7 nm for
G3-AB-Ad at b-CDAuNPs, and was clearly dependent on the
size of the guest.
p–p*-transition at 360 nm, 365 nm for gold nanoparticles of
the trans-azobenzene decreased upon illumination showing
the isomerization. The increase at 450 nm could be assigned
to the n–p*-transition of the formed cis isomer.
Back-switching was induced by illumination with 450 nm
using a high-pressure mercury lamp (100 W) with an optical
band-pass filter, which triggered the isomerization from the
cis to the trans form. The absorption spectrum changed, which
was caused by the switch. The p–p* absorption band
increased again but only to a certain point. This photo-
isomerization reached its photo-stationary state at the given
wavelength.
Similar observations were made for the complex on gold
nanoparticles. For trans-G3-AB-Ad complexed with CD
functionalized AuNPs, a spectrum was received with the
typical absorption of the p–p* transition at 360 nm of the
azobenzene and a broad absorption at 520 nm, which is
normal for the surface plasmon resonance of gold nano-
particles.[12] These solutions were illuminated with UV light to
induce the isomerization process. The pp*-transition band
decreased again, whereas the n–p* transition band at 450 nm
increased. As a result, the azobenzene was able to undergo
the isomerization immobilized on gold nanoparticles.
To ensure that the switching process worked on planar
gold surfaces, the host–guest system was transferred to semi-
transparent gold chips. Therefore, the chips were coated with
b-CD-SH before the inclusion complex was formed in a 1 mm
solution of the corresponding guests. For the UV/Vis meas-
The functionalization of planar gold surfaces was per-
formed on three different types of surfaces. The first were
semi-transparent surfaces with a 20 nm gold layer on glass for
UV/Vis spectroscopy; the second were gold chip sensors for
quartz crystal microbalance (QCM) for the online monitoring
of the host–guest complex formation. QCM measurements
were widely used in the detection of thin film formation.[14,15]
The third were gold (111) surfaces on glass with a 200 nm
metal layer for contact angle measurements. At first, the gold
sensor slides for QCM measurements were coated with the
thiol-functionalized b-CD in a 1 mm solution in DMSO in the
presence of sodium borohydride to prevent disulfide forma-
tion. Subsequently, the guest molecule G3-AB-Ad was rinsed
with a flow rate of 50 mLminÀ1 over the receptor-functional-
ized surfaces. During this process the frequency response was
recorded (Figure 3). The frequency decreased, which showed
the formation of the inclusion complex at the surface until
a saturation was achieved (I–II). In region III the surface was
equilibrated to Millipore water, which caused a slow but
steady increase in frequency. In region IV the host–guest-
functionalized surface was rinsed with sodium dodecylsulfate
(SDS) to induce decomplexation.[16] After this treatment the
initial frequency was observed. With this host–guest chemis-
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Figure 3. QCM sensogram of the online complexation of G3-AB-Ad on
planar gold surfaces coated with b-CD-SH.
isomerization from trans to cis, the hydrophilic glycerol head
groups tilted sideward and the hydrophobic azobenzene was
presented to the interface. Hence, an increase of the contact
angle could be expected by switching from trans to cis,
whereas a decrease of the contact angle should happen upon
back-isomerization. We measured an advancing contact angle
of 608 for the trans-G2-AB-Ad monolayer, which widened to
728 in the switched cis state (Figure 5). No significant change
in contact angle was observed for the G3 bearing azobenzene
owing to the large glycerol dendron. When the G3-oligogly-
cerol dendron was tilted to the side, the steric demand did not
alter the hydrophobicity in the switched state.
High thermal stability is important for applications of
these systems. Therefore, we determined the thermal half-
lives of the cis isomers for all the synthesized azobenzenes in
different solvents, for the G3-AB-Ad cyclodextrin inclusion
complex in solution, on gold nanoparticles, and on planar gold
surfaces (Table 1). A large thermal half-life ensures stable
switching states. All values were calculated from the corre-
sponding UV/Vis spectra. A half-life of six minutes in
acetonitrile was measured for the simple cis-azobenzene–
adamantyl conjugate. In a protic polar solvent such as
methanol, the half-life was much shorter and was smaller
than one minute. Here the solvent effect provoked a reduced
energy barrier for the cis to trans isomerization. Such solvent
dependency illustrates a dipolar transition state and thus
a rotational mechanism of the isomerization moiety.[17,18]
The half-life increased dramatically by attaching the
glycerol dendrons. G1-AB-Ad had the highest half-life of
1773 min in methanol. Upon increasing glycerol dendron
generation, the half-lives of the respective cis isomers were
lower: G2-AB-Ad with 1223 min and G3-AB-Ad 702 min.
The increasing steric demand of the growing generations is
responsible for the lower half-life because the complexed cis
isomer is less favorable compared to smaller generations.
After changing to water as solvent, the half-life of G3-AB-Ad
was doubled to 1523 min. This opposite solvent effect implied
a stabilization of the cis isomer by water. The thermal half-life
of the host–guest-complex of G3-AB-Ad and b-CD was
significantly smaller with 478 min than for the free azoben-
Figure 2. a) 1H NMR spectra of the host cyclodextrin and guest G3-AB-
Ad and a 2D-NMR (NOESY) of the corresponding inclusion complex;
b) NMR spectra of b-CD, b-CD on AuNPs, G3-AB-Ad, and the 2D-NMR
(NOESY) of the corresponding inclusion complex on gold nanoparti-
cles. NOE: circled in red.
urements the gold chips were put upright in a cuvette and the
spectrum was recorded in transmission (Figure 4). We could
detect the transition at 350 nm as a small shoulder, which
decreased upon illumination. The overall absorption was
weak because the measurement was carried out in trans-
mission of the monolayer. To clarify the isomerization process
the polynomial fits of the adsorption difference are shown in
the spectrum as green lines. The continuous green line
represents the trans isomer at the surface, whereas the
dotted line refers to the switched cis isomer. Back-switching
is shown by the dashed green line, which did not reach the
initial intensity because of the photo-stationary state at this
wavelength. The G1/2/3-AB-Ad was able to switch reversibly
on semi-transparent surfaces. Additionally, we confirmed the
switching process by contact angle measurements. Upon
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zene. The solvent effect was re-
duced by the inclusion complex.
The solvent molecules did not
have the space for interacting
with the azobenzene moiety.
Therefore, the solvent effect is
reduced. However, the other oli-
goglycerol azobenzene deriva-
tives were not water soluble and
therefore the half-lives could not
be measured.
The situation changed dra-
matically on gold nanoparticles.
The cis-azobenzenes complexed
on CD-coated gold nanoparticles
had significant lower half-lives. A
half-life of 20 min was measured
for G1-AB-Ad on nanoparticles,
which increased only slightly for
the higher generation dendrons.
The cis-G2-AB-Ad switched in
Figure 4. UV/Vis spectra of the switching process of complexed b-CD/G3-AB-Ad of a) in solution, b)
on nanoparticles, and c) a planar surface
26 min back to the trans isomer, whereas cis-G3-AB-Ad
needed 33 min. This surprising trend was not observed for
light-induced back-switching, where the process on nano-
particles was as fast as in solution. Two different explanations
for this phenomenon are possible. On the one hand,
plasmonic effects of the gold nanoparticles could lead to
higher isomerization rates. The plasmon resonance of the
nanoparticles interacted with excitons and enhanced the
dissipation of exciton–hole pairs. On the other hand, the
isomerization catalyzed by gold nanoparticles by an electron-
transfer mechanism, which was discussed by Scaiano et al.[19]
The half-lives of the AB-complex on planar gold surfaces
were in good agreement with the observations made in free
solution. This would imply the dark back-switching inde-
pendent from solvent and is governed only by the interaction
of the host–guest. The half-life here decreased on planar
surfaces with the size of the dendrons, which was also
observed in solution (Table 1).
Figure 5. Switching process of the inclusion complexes G1/2/3-AB-Ad
monitored by contact angle measurements on gold surfaces for three
cycles.
We investigated whether the guest molecule decomplexed
by switching from trans to cis using DOSY NMR and QCM
experiments. DOSY NMR measurements of the host–guest
complex in D2O for the trans isomer and after illumination
with UV-light (cis isomer) showed no change of the diffusion
coefficient (Supporting Information, Figure S12), which indi-
cates there is no free guest. QCM measurements with
illumination were performed similar to the above mentioned
experiments. The complexation was confirmed by a decrease
in frequency. Over the period of the whole measurement the
functionalized chip was constantly rinsed with Milli-Q water.
After addition of the trans-azobenzene guest and equilibra-
tion, the sensor was illuminated with 366 nm for 30 min to
induce the cis isomerization. During this procedure the
frequency slightly increased for 2 Hz. A control experiment
with a blank sensor chip was performed the same way. After
starting the lamp und during the illumination with 366 nm the
frequency increased for 2 Hz. This identical small frequency
change indicates that no desorption occurs during switching
from trans to cis. The small frequency increase can be
Table 1: Half-lives of different azobenzenes and their complexes with b-
CD in various solvents and substrates.
compound
environment
t1/2 [min]
at 208C
Z-AB-Ad
Z-AB-Ad
Z-G1-AB-Ad
Z-G2-AB-Ad
Z-G3-AB-Ad
Z-G3-AB-Ad
Z-G3-AB-Ad @ prot. b-CD-SH
Z-G1-AB-Ad @ b-CD-S-AuNP
Z-G2-AB-Ad @ b-CD-S-AuNP
Z-G3-AB-Ad @ b-CD-S-AuNP
Z-G1-AB-Ad @ b-CD-S-Au
Z-G2-AB-Ad @ b-CD-S-Au
Z-G3-AB-Ad @ b-CD-S-Au
acetonitrile
methanol
methanol
methanol
methanol
water
water
water
water
water
6
<1
1773
1223
702
1523
478
20
26
33
613
517
465
air
air
air
4
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In summary, we have introduced a new supramolecular
amphiphilic switch that can readily be applied in solution as
well as on surfaces. The switching behavior of the host–guest
complex was carefully analyzed by different methods and
revealed an efficient process with a strong environment
dependence upon both solvent as well as the kind of surface
(curved vs. planar). This is in contrast to self-assembled
monolayers of densely packed azoswitches that cannot be
switched readily on planar gold surfaces. Furthermore, we
could demonstrate a clear switching behavior of the contact
angle and hence the hydrophobicity of the surface. Partic-
ularly the contact angles around 658 are discussed as
transitions between fouling and non-fouling surfaces.[20]
Therefore, the G2-switch on gold is a potential candidate
for light-controlled bioadhesion.
Received: March 14, 2014
Revised: April 30, 2014
Published online: && &&, &&&&
Keywords: azobenzene · half-life · host–guest chemistry ·
.
molecular switches · surface chemistry
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Communications
Communications
Supramolecular Switches
The thermal and light-induced photoiso-
merization of bifunctional azobenzene–
oligoglycerol conjugates was examined in
solution and on supramolecularly func-
tionalized gold nanoparticles and planar
gold surfaces. The wettability of the
functionalized gold surfaces could be
reversibly switched with light. Thermal
half-lives of Z-isomers were determined
for different environments to provide
information about the stability of the
functional switches for further applica-
tions.
O. Nachtigall, C. Kçrdel, L. H. Urner,
R. Haag*
&&&& — &&&&
Photoresponsive Switches at Surfaces
Based on Supramolecular
Functionalization with Azobenzene–
Oligoglycerol Conjugates
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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