Ultrafast Light-Driven Substrate Expulsion from the Active Site of a Photoswitchable Catalyst

Article abstract of DOI:10.1002/anie.201702861

The photoswitchable piperidine general base catalyst is a prototype structure for light control of catalysis. Its azobenzene moiety moves sterically shielding groups to either protect or expose the active site, thereby changing the basicity and hydrogen-bonding affinity of the compound. The reversible switching dynamics of the catalyst is probed in the infrared spectral range by monitoring hydrogen bond (HB) formation between its active site and methanol (MeOH) as HB donor. Steady-state infrared (IR) and ultrafast IR and UV/Vis spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within a few picoseconds. Thus, the force generated by the azobenzene moiety even in the final phase of its isomerization is sufficient to break a strong HB within 3 ps and to shut down access to the active site.

Full text of DOI:10.1002/anie.201702861

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Angewandte
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Accepted Article
Title: Ultrafast light-driven substrate expulsion from the active site of a
photoswitchable catalyst
Authors: Manuel Pescher, Luuk van Wilderen, Susanne Gruetzner,
Chavdar Slavov, Josef Wachtveitl, Stefan Hecht, and Jens
Bredenbeck
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201702861
Angew. Chem. 10.1002/ange.201702861
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10.1002/anie.201702861
Angewandte Chemie International Edition
COMMUNICATION
Ultrafast light-driven substrate expulsion from the active site of a
photoswitchable catalyst
Manuel Pescher,[+][a] Luuk van Wilderen,[+][a] Susanne Grützner,[b] Chavdar Slavov,[c] Josef Wachtveitl,[c]
Stefan Hecht[b] and Jens Bredenbeck*[a]
Abstract: The azobenzene-containing photoswitchable piperidine
general base catalyst is a prototype structure for light control of
catalysis. Its azobenzene moiety moves sterically-shielding groups
to either protect or expose the active site, thereby changing the
compound’s basicity and hydrogen-bonding affinity. The catalyst’s
reversible switching dynamics is probed in the infrared spectral
range by monitoring hydrogen bond (HB) formation between its
active site and methanol (MeOH) as HB donor. Steady-state infrared
(IR) and ultrafast IR and UV/Vis-spectroscopies are used to uncover
ultrafast expulsion of MeOH from the active site within a few pico-
seconds. Thus, the force generated by azobenzene even in the final
phase of its isomerisation is sufficient to break a strong HB within 3
ps and to shut down access to the active site.
catalysis (via uncaging or photoswitching) has to be distin-
guished from photo-catalysis,[11] where a photoreaction is
promoted by electronic excitation.
The previously developed catalyst under study (Figure 1A,
blue structures),[12,13] shortly called ”Azocat” (derived from
AB-based catalyst), is a prototype system for conformation-
al control of reactivity. This molecule catalyses the Henry
reaction, a base-catalysed addition of nitroalkanes to alde-
hydes or ketones, for which reversible active-site accessibil-
ity has already been shown.[12,13] Other types of active cen-
ters can in principle be controlled in a similar fashion.
Azocat consists of four main structural moieties: the active
centre, formed by a tert-butylpiperidine (t-BP); a linker
group (γ-butyrolactone; γ-BL) providing a stiff connection
between t-BP and AB due to the combination of a spiro-
center and sp2-hybridised atoms; the AB switch; and steric
shields (two 2,6-dimethylbenzene groups, labeled R’ in
Figure 1A) blocking Azocat’s active site in the closed state.
Illumination with 365 nm induces trans-cis isomerization in
the AB moiety leading to a large structural rearrangement of
the system. The cis-form represents the open or active form
of the catalyst, because the catalytic centre located at the t-
BP can be accessed by reaction partners. The reverse
process is triggered by wavelengths > 450 nm, bringing the
shield to a proximal position and inactivating the catalyst.
The back reaction occurs also thermally, but the lifetime of
the open form at room temperature is several hundred
hours.[13] The isomerization quantum yields of AB are about 0.4-
0.5 for cis-trans and 0.1-0.2 for trans-cis.[14] In the photo-
stationary state cis/trans and trans/cis ratios of 95:5 and 90:10
are achieved for Azocat.[13] Its high photostability allows for a
large number of switching cycles. The t-BP moiety exhibits a
nitrogen-atom with a free electron pair, making it an ideal
acceptor for HBs. The formation of a strong HB to Azocat’s
catalytic centre explains its catalytic activity.
Here, we study the kinetics and structural changes of Azo-
cat’s switching process in absence and presence of HB-
forming partners. The solvent is perchloroethylene (PCE)
which is apolar and aprotic. The switching process and its
interaction with a substrate (MeOH in our case, which do-
nates a strong HB to the active site) are investigated by
steady-state and time-resolved ultrafast IR and UV-Vis
spectroscopies (experimental details and methods[15] in the
SI) to understand the catalyst’s isomerisation process fol-
lowed by HB network reorganisation. The binding of MeOH
to the active site can be easily followed in the IR, because
the OH-stretching vibration changes upon HB formation.[16]
Steady-State FTIR spectroscopy. In order to understand the
ultrafast structural changes of the catalyst and its environ-
ment we first discuss the steady-state FTIR spectra of Azo-
cat, its active site mimic methylpiperidine (NMP) and of
MeOH at different concentrations.
Precise temporal and spatial control of (bio)chemical pro-
cesses opens up new possibilities for applications. In recent
years, a variety of molecular properties have successfully
been rendered photoswitchable, for instance secondary
structure formation in peptides,[1] protein function[2] or force
generation in molecular motors.[3]
Typically, photocontrol occurs via irreversible bond cleav-
age or via reversible switching.[4] The usage of light to irre-
versibly photodissociate and thereby activate a caged com-
pound allows the initiation of a reaction at a defined point in
time. Unfortunately the process induced by the initial cleav-
age cannot be reversed or shut down. This limitation is
overcome by making the activation reversible and repeata-
ble, utilizing geometric rearrangements of molecular struc-
ture. This approach has been used to photoswitch biologi-
cal activity of proteins[4,5] and DNA transcription.[6] Consid-
erable efforts are being made to reversibly photoregulate
catalysis, which is perhaps the most attractive function to
control from a chemist’s point of view.[7] A prominent exam-
ple is the previously developed azobenzene (AB) based
supramolecular catalyst,[8] which brings two metal centers in
proximity in the active state. Another system is a light-
responsive cavitand[9] which binds and thereby activates the
catalyst. Osorio-Planes et al.[10] recently reported regulation
of catalytic activity via photoswitchable intramolecular
HBing that inhibits the active site. Note that photoregulated
[a]
M. D. Pescher, Dr. L. J.G.W. van Wilderen, Prof. J. Bredenbeck
Institute for Biophysics, Johann-Wolfgang-Goethe Universität
Frankfurt am Main, Germany
E-mail: bredenbeck@biophysik.uni-frankfurt.de
S. Grützner, Prof. S. Hecht
Department of Chemistry, Humboldt-Universität zu Berlin
Berlin, Germany
[b]
[c]
Dr. C. Slavov, Prof. J. Wachtveitl
Institute of Physical and Theoretical Chemistry, Johann-Wolfgang-
Goethe Universität
Frankfurt am Main, Germany
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
Reversibility and isomerisation of the catalyst. Upon
illumination, Azocat isomerises reversibly, which is evident
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10.1002/anie.201702861
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COMMUNICATION
by the symmetric FTIR difference spectra (see Figure S1).
The band labeled

at about 3425 cm-1 is less intense and
As in AB, the

N=N band (1520 cm-1) serves as isomeriza-
has a reduced fwhm. The highest-wavenumber absorption
tion marker, as it is only IR active in the cis-form due to the
local point symmetry being broken upon isomerisation.
MeOH binds to the active center in equilibrium with
MeOH in solution. MeOH can only bind to Azocat’s active
site in the open cis-state. We monitored the changes in-
band (
Bands
/
) at 3643 cm-1 is the narrowest and smallest.

and

appear upon UV-irradiation while the

/-
band concomitantly disappears. Therefore, the
/

and
/

bands are interpreted as interconverting OH-species. The
difference between the integral over the band and that
of the bands is attributed to a significant increase of the
/
duced by HBing in the
induced difference spectra of Azocat in Figure 1A. In ab-
sence of MeOH only CH bands are visible in the region
below 3100 cm-1. Upon addition of MeOH three bands
OH region, as shown in the light-
/
transition dipole moment upon HB formation.

To check if the observed spectral changes originate from
MeOH binding to the active site, MeOH was titrated to the
active-site mimic NMP in PCE (Figure 1B). NMP itself does
not absorb in the
changes are due to the added MeOH. The absorption
above 3100 cm-1 appear. The OH-band labeled with
 is
centered around 3250 cm-1 and shows the highest induced
absorption and largest fwhm (full width at half maximum).
OH spectral region, so any observed
ξ
χ
α
trans, inactivated
cis, reactive
O
O
O
A
B
δ
O
0.3
0.0
R
R
N
N
H
O
N
N
N
'
365 nm
450 nm
O
R'
R'
N
2
+
H
H
-0.3
O
R'
R'
0.6
0.4
0.2
0.0
N
N
H
H
H
'
+
+
O
O
O
H
O
3000
3200
3400
3600
γ α/β
O
O
O
ξ'
ξ
δ
χ
O
O
H
R
R
C
D
N
N
O
H
2
0
H
N
N
N
365 nm
450 nm
'
O
R'
N
R'
O
+
O
H
-2
H
O
H
R'
R'
-4
+
H
O
O
O
1.6
H
H
H
O
1.2
0.8
0.4
0.0
O
O
H
H
O
O
O
O
3
+
H
H
H
H
O
H
3000
3200
3400
3600
wavenumber / cm-1
Figure 1. MeOH binds to the active site. Next to each FTIR spectrum, molecular structures are drawn with Greek labels corresponding to those at the vibrational
bands. The arrows in all spectra denote the direction of the spectral changes due to increasing MeOH concentration. Panels A and C present the light-induced
difference spectra of Azocat (with
R = tert-butyl and R’ = 2,6-dimethylbenzene in the structures) with low and high MeOH concentration, respectively. In the
spectra after 365 nm excitation (light blue) bands appearing upon formation of the reactive cis-form (light blue) have a positive sign. Correspondingly, in the
spectra after 450 nm excitation (dark blue) bands vanishing upon formation of the inactive trans-form (dark blue) have a negative sign. The light blue spectra have
the catalytically inactive trans-form subtracted, the dark blue that of the reactive cis-form. Panels B and D show the absorption spectra of increasing MeOH con-
centration in the presence and absence of NMP, respectively. Concentrations: A) Azocat (2 mM) is switched in the absence (gray) and presence of MeOH (49
mM; light blue, and 166 mM; blue). B) MeOH is added stepwise to NMP (1M) in PCE, mimicking the occupation of Azocat’s active site. MeOH concentrations are:
33 mM, 98 mM, 228 mM, 420 mM C) A higher MeOH concentration (685 mM) in presence of Azocat (69 mM) provides a pool of MeOH clusters that can supply
MeOH to form an HB to Azocat. D) MeOH is added to pure PCE. MeOH concentrations are: 66 mM, 131 mM, 163 mM, 196 mM, 228 mM, 260 mM.
This article is protected by copyright. All rights reserved.

10.1002/anie.201702861
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spectra show the same three spectral features as those
observed in Azocat in presence of MeOH. The titration
to 228 mM). The difference signal of Azocat at high MeOH
concentrations is thus attributed to a superposition of the
/,  and -bands. The MeOH molecules that bind to
shows that the
which a band at 3425 cm-1 appears (the
MeOH concentrations. In addition, a narrow band (the
band) at 3643 cm-1 increases. In agreement with the inter-
pretation in Glew and Rath’s work[17] we assign the
band to free MeOH molecules. The -band represents the

-band at 3250 cm-1 grows in first, after
-band) with higher
Azocat’s active site therefore originate from the different
MeOH cluster species as well as from monomers and di-
mers. Upon switching from cis to trans, these species are
repopulated by MeOH expelled from the active site.
In summary, at low MeOH concentrations MeOH targeting
the active site is mainly recruited from MeOH monomers. At
higher concentrations the Azocat-bound MeOH molecules
are also recruited from the MeOH clusters. Inactivation of
the catalyst due to cis-trans isomerization reverses the
process, and all binding-site bound MeOHs detach.
/-
/-

monomeric MeOH, while the-band is due to the OH serv-
ing as a HB acceptor. Due to the small spectral separation
the two bands cannot be distinguished here. The
that is formed first represents N-bound MeOH molecules.
When a second MeOH binds to the N-bound -MeOH, the
OH of the second MeOH results in the band (Figure 1B,
right panel).[17,18] Furthermore, a shoulder
red side of the band, due to former
HB donor as well as acceptor.
-band



Ultrafast spectroscopy. In order to investigate the expulsion
process of molecules that are bound to the catalyst’s active
site in real time we performed ultrafast laser experiments.
Isomerization of the catalyst. The isomerization reaction

’ appears on the


species now being
The difference spectra of Azocat (see Figure 1A) basically
exhibit the same features as MeOH binding to NMP, which
we therefore interpret as MeOH binding to the active site
(evident by the disappearance of free MeOH and by the
of Azocat can be followed by tracking the

N=N vibration
(Figure S2).[20] We observe the disappearance of the
N=N
band (bright green trace in Figure S3) within the laser pulse,
indicating immediate isomerization of the AB moiety. The ultra-
fast dynamics were also probed by UV-Vis spectroscopy
(see Figure S4 and Figure S5). The UV-Vis difference spec-
tra essentially show the same dynamics of Azocat without
MeOH and with an excess of MeOH. This indicates that
MeOH is not modulating the isomerization process.
appearance of bound MeOH showing
The large redshift of the OH-stretching wavenumber of
about 400 cm-1 between free MeOH (
band) and MeOH
bound to the N-atom of the piperidine moiety ( band) cor-
, ’ and  bands).


responds to the formation of a strong HB (binding enthalpy
of about 20-25 kJ/mol), both in Azocat as well as in the
active site mimic NMP.[19]
MeOH expulsion. Figure 2 shows time-resolved IR spectra
of the
PCE. Upon cis-trans isomerization of the Azocat, a positive
signal appears at the monomer band, concomitant with
negative signals in the range of HBed MeOH. At 2 ps a
negative signal appears at the species (the black spec-
OH region of a solution of Azocat and MeOH in
At higher MeOH concentration MeOH clusters are
formed. When switching at higher concentrations of MeOH
(Figure 1C), clusters or chains of MeOH molecules are the pre-
dominant species. To compare the spectral signatures of these
species to the difference signals caused by switching of Azocat,
the spectra of a titration of pure MeOH to PCE are shown in
Figure 1D. In agreement with previous studies [17,18] the -band
at 3333 cm-1 is assigned to HB chain formation of MeOH mole-
cules, where MeOH accepts as well as donates HB’s. MeOH
chains truncate at the  and -groups (see right panel in Figure
1D for schematic molecular configurations). The -band appears
at 3527 cm-1, while the -band overlaps with the -band of the
MeOH monomers at 3643 cm-1. Due to HBs at the chain’s end
being weaker, formation of rings and long chains is preferred.[17]
Upon trans-cis isomerization (i.e. activation, see Figure 1C),
MeOH is recruited from these different species. Concomi-
/

trum), reporting the expulsion of MeOH from the active site.
Like in the FTIR spectra of Figure 1A and C, three main
bands (
is probably covered by the broad 3200 cm-1-3500 cm-1 fea-
ture, belonging to the -species. The -band, representing
/,  and ) are observed in Figure 2. The-band


MeOH clusters, has the opposite sign with respect to the
FTIR results (Figure 1C). This is due to the heating induced
by the pump pulse, which breaks up MeOH clusters after
the heat has been dissipated from Azocat into the solution.
In the steady-state switching experiments the
-band is
positive because the signal is dominated by MeOH mole-
cules that are released from the active site and join the
MeOH clusters. The observed signal size is in line with an
estimated[21] temperature rise of a few tenths of K (see SI).
To further investigate the dynamics we carried out the ex-
periment for different MeOH concentrations and plotted the
integral of the free OH band for each concentration (Figure
3A). The MeOH monomer band appears instantaneously
(within the pulse duration, see Figure 3B, blue curve) for all
MeOH concentrations investigated. Without MeOH the
integral remains essentially zero, thereby providing an in-
ternal validation for the used integration method. With
MeOH a plateau appears after 3 ps that lasts up to 100 ps,
with only a slight decrease after 10 ps. This slight decay
may be due to the integral borders not matching the 
band perfectly. In order to avoid overestimation of the inte-
gral the borders were set conservatively. The integrals’
tant with the disappearance of the
big negative signal is observed for the
(see also Fig. 1A, where at 166 mM MeOH the onset of the
negative -band can be seen), in addition to a smaller
negative signal of the -band at 3527 cm-1. This
band is not

/

-band at 3643 cm-1, a
-band at 3333 cm-1



observed upon switching at lower MeOH concentrations
(Figure 1A). The negative bands are partially compensated
by the
 and -bands that are formed due to HBing of
MeOH to the active site of Azocat. Note that the absorption
cross-section for MeOH in clusters is larger than that for
MeOH involved in N-coordination (compare Figures 1B and
1D): 33 mM of MeOH with an excess of NMP results in 0.08
mOD at the -band (Figure 1B), while adding the same to a
solution containing already a large amount of MeOH leads to an
increase of 0.38 of the -band (Figure 1D, change from 196 mM
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10.1002/anie.201702861
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COMMUNICATION
plateau height depends on the concentration of Azocat and
MeOH. It approximately doubles in size upon doubling the
MeOH concentration (going from 0.04 OD•cm-1 at 4 ps for
41 mM MeOH to 0.08 OD•cm-1 at 82 mM MeOH), while
keeping the Azocat concentration at 35 mM. Further in-
creasing the MeOH concentration to 120 mM has no effect
on the plateau’s height. While the uncertainty in the ampli-
tude precludes a detailed quantitative analysis, this implies
that saturation is reached in the range of about 120 mM.
Therefore, no further MeOH molecules can bind to the ac-
tive site in the catalyst's cis-configuration and get expelled
upon isomerisation to the trans-form. This hypothesis is
confirmed by the fact that an increase of the Azocat con-
centration from 35 mM to 54 mM with co-increasing the
MeOH concentration to 200 mM leads to considerable in-
crease of the signal again. As now more Azocat molecules
are available that can form HBs to MeOH molecules in the
cis- conformation, more MeOH is expelled from the binding
site upon switching. A contribution to the free MeOH signal
could arise from MeOH clusters in the vicinity of Azocat,
which are dissociated by Azocat's motion. However, a sim-
ple molecular volume estimate for 54 mM Azocat (diameter
about 1.2 nm) suggests only about 0.06 MeOH molecules
being in the vicinity of one Azocat molecule (in addition to
the one MeOH bound to the active site), even at 200 mM
MeOH. The probability of a MeOH cluster being near Azo-
cat is thus small. We conclude that destruction of clusters
by the motion of Azocat does not play a role.
redshift of the OH-stretching vibration upon HBing is 500
cm-1 as compared to 400 cm-1 in MeOH). The most signifi-
cant difference is that the signal rise after 100 ps already
sets in at lower concentrations for TFE then for MeOH,
consistent with the fact that TFE starts to form clusters at
lower concentrations then MeOH. The time traces of the
bands
(see Figure 3B) show dynamics that nicely fit to that of the
 band of free MeOH. The -trace, which is due to MeOH
at the active site, depletes already during the IRF and then
stays constant. The -trace, (mainly representing MeOH
 and /which represent HBed MeOH molecules

/
clusters, in addition to contributions of N-bound MeOH) also
shows a similar fast decrease, but it exhibits another strong
decrease after 100 ps. This kinetics occurs in parallel with
the slow appearance of additional free MeOH, confirming
that it originates from the clusters. QM/MM studies of AB in
the condensed phase have revealed that the initial sub-
picosecond cis-trans isomerisation event involving the cen-
tral CN=NC moiety is only weakly affected by the solvent,[22]
while the subsequent orientational relaxation of the rings
and the approach to the trans-minimum are considerably
slowed down by the solvent with respect to the gas phase.
In Azocat, the steric shields, which have to be moved by the
AB moiety, are expected to decelerate the process further.
Our experiment reveals that the switching process never-
theless rapidly creates sufficient strain to break the strong
HB between the alcohol and the piperidine base and to
push MeOH and TFE from the binding site within the first 3
ps as reported by the fast rise of the  band and the de-
On longer time scales (between 100 ps and 3 ns) the integral of
the /-band stays constant for MeOH concentrations up to 82
mM (with the Azocat concentration remaining constant at 35
mM). However, higher MeOH concentrations of 120 mM and
200 mM lead to a continuous growth of the integral from 100 ps
onwards. The higher the MeOH concentration, the higher is the
increase of the integral at later time points . As shown in Figure
1D, this is the concentration range where the concentration
pletion of
 and  (Figure 3). At higher alcohol concentra-
tions, where the alcohol starts to form aggregates, an addi-
tional slower (> 100 ps) generation of free alcohol can be
[Azocat] [MeOH]
A
0
41
82
120
35
54
200
0.4
0.2
0.0
0.0
of MeOH clusters ( band) starts to rise.
Ultrafast release
of MeOH
Control experiments using the alcohol 2,2,2-trifluoroethanol
(TFE) instead of MeOH show very similar kinetics and con-
centration dependences (Figure S8), showing that Azocat is
able to break the HB of an even stronger HBing partner (the
Solvent
heating
ξ
δ
χ
α/β
0.6
0.3
B
0.0
2
4.8
-0.3
-0.6
237
1000
3162
IRF
ξ-trace
χ/δ-trace
-0.5
-3 0 3 6 10
100
1000
3150
3300
3450
3600
3750
time / ps
wavenumber / cm-1
Figure 3. Band integral of the /MeOH monomer band (panel A) as a func-
tion of time after switching Azocat to the cis state (closed). The experiment is
carried out in the absence and presence of different concentrations of MeOH
in PCE. The /band appears during the pump pulse. Panel B shows the time
traces of the highlighted  and  wavenumbers in Figure 2 (as dotted and
dashed lines, respectively), although the latter reflects both /species. In
blue, the IRF of the laser experiment is shown. The time scale is linear up to
7.5 ps, and logarithmic thereafter.
Figure 2. Selection of ultrafast pump-probe difference spectra upon cis-trans
switching in the νOH region of a mixture of 200 mM MeOH and 54 mM Azocat.
The small difference in signal size in the two separately measured spectral
windows is caused by different spatial overlap. The delay times are given in ps.
The integral of the /band (light blue box) is shown in Figure 3A. The time
courses at 3205 cm-1 (dotted) and 3376 cm-1 (dashed) are shown in Figure 3B.
The grey line at 3333 cm-1 depicts the position of the cluster species .
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[2]
[3]
a) A. A. Beharry, G. A. Woolley, Chem. Soc. Rev. 2011, 40, 4422; b) A.
Rullo, A. Reiner, A. Reiter, D. Trauner, E. Y. Isacoff, G. A. Woolley,
Chem. Comm. 2014, 50, 14613.
observed that we attribute to release of alcohol from the
aggregates due to the temperature increase of the sample
upon VIS excitation, shifting the equilibrium from aggre-
gates to free alcohol. This is supported by the depletion of
the band at later times and the synchronous increase of
the  band. A similar time scale for the rise of tempera-
ture and homogeneous distribution of heat in a solution
heated via laser excitation of a dye has been previously
reported.[23] It is noted that only OH stretch vibrations con-
tribute to this wavenumber range, i.e. signals of potential
slow conformational equilibration dynamics of other struc-
tural moieties can be excluded, as they would appear in the
transient for 0 mM MeOH in Figure 3A.
Using ultrafast and steady-state IR spectroscopy we have
characterised the photoswitching and substrate binding
behavior of Azocat. The ultrafast switching process makes
the photoswitchable base an interesting tool for the study of
chemical reactions which are difficult to time control other-
wise. The binding of MeOH (via an HB) to its active site is
used as a probe of Azocat’s active site accessibility and
thus its availability for catalysis. FTIR difference spectros-
copy shows the release of MeOH upon Azocat isomerisa-
tion. In time-resolved IR measurements we observe an
ultrafast release of active site-bound MeOH molecules. This
shows that the force generated by the azobenzene is suffi-
cient to break a strong HB within 3 ps, even during the final
structural rearrangements of the isomerisation when the
azobenzene geometry approaches the trans minimum of
the energy surface and shuts down access to the binding
site. These findings on the dynamics will aid and inspire the
design of future ultrafast photoswitchable systems for appli-
cations in catalysis, mechanochemistry, and pharmacology.
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10.1002/anie.201702861
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Manuel Pescher, Luuk van Wilderen,
Susanne Gruetzner, Chavdar Slavov,
Josef Wachtveitl, Stefan Hecht and Jens
Bredenbeck*
Page No. – Page No.
Ultrafast light-driven substrate expul-
sion from the active site of a
photoswitchable catalyst
Switching of the azobenzene moiety moves the steric shielding groups on an ul-
trafast time scale, expelling a strongly hydrogen bonded binding partner from the
active site. The catalyst is deactivated by blocking access to the binding site.
This article is protected by copyright. All rights reserved.