Unraveling the Photoswitching Mechanism in Donor-Acceptor Stenhouse Adducts

Article abstract of DOI:10.1021/jacs.6b01722

Molecular photoswitches have opened up a myriad of opportunities in applications ranging from responsive materials and control of biological function to molecular logics. Here, we show that the photoswitching mechanism of donor-acceptor Stenhouse adducts (DASA), a recently reported class of photoswitches, proceeds by photoinduced Z-E isomerization, followed by a thermal, conrotatory 4π-electrocyclization. The photogenerated intermediate is manifested by a bathochromically shifted band in the visible absorption spectrum of the DASA. The identification of the role of this intermediate reveals a key step in the photoswitching mechanism that is essential to the rational design of switching properties via structural modification.

Full text of DOI:10.1021/jacs.6b01722

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Communication
Unraveling the Photoswitching Mechanism
in Donor–Acceptor Stenhouse Adducts
Michael M. Lerch, Sander J. Wezenberg, Wiktor Szymanski, and Ben L. Feringa
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01722 • Publication Date (Web): 06 May 2016
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Unraveling the Photoswitching Mechanism in Donor–
Acceptor Stenhouse Adducts
Michael M. Lerch†, Sander J. Wezenberg†, Wiktor Szymanski†,‡, Ben L. Feringa*,†
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† Centre for Systems Chemistry, Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groꢀ
ningen, The Netherlands
‡Department of Radiology, University of Groningen, University Medical Center Groningen, Hanzeplein 1, 9713 GZ, Groꢀ
ningen, The Netherlands
Supporting Information Placeholder
demonstrated the remarkable control over micelle stability that
they can provide and the potential as phaseꢀtransfer tags.43–48Upon
ABSTRACT:Molecular photoswitches have opened up a myriad
of opportunities in applications ranging from responsive materiꢀ
alsand control of biological function to molecular logics. Here, we
irradiation with visible light (λ= 540 – 580 nm) in aromatic solꢀ
vents (e.g. toluene), the linear trieneꢀform Aof DASA cyclizes to
show thatthe photoswitching mechanism of donor–acceptor Stenꢀ
a zwitterꢀionic formB(Figure 1 and S1–S6). FormBis thermally
house adducts (DASA), a recently reported class of photoswitchꢀ
unstable and reverts to the trieneisomerAon the timeꢀscale of
es, proceeds by photoinducedZ–E isomerization,followed by a
seconds to minutes (Figure 1). The triene formA is strongly colꢀ
thermal,conrotatory 4πꢀelectrocyclization. The photogeneratedinꢀ
ored (ελmax ~105 M–1 cm–1), whereasBis colorless.43 Although
termediate is manifested bya bathochromically shiftedband in the
mechanistic studies have, to the best of our knowledge, not been
visible absorption spectra of the DASA. The identification of the
reported to date,46,49a 4πꢀelectrocyclizationis anticipated to be
role of this intermediate reveals a key step in the photoswitching
crucial, by analogy to the Piancatelli rearrangement.50–52
mechanism that is essential tothe rational design of switching
properties viastructural modification.
Molecular switches undergo reversible changes in their strucꢀ
ture in response to external stimuli, such aslight or changes in the
chemical environment.1 The use of light as an external stimulus is
often preferred over chemical stimuli due to its nonꢀinvasive
nature,2–6and orthogonality to many other processes. Furthermore,
light can be applied withprecise spatial and temporal control.3,4
The photoinducedchangesto molecular structure are manifested in
changes to molecular properties, such as dipole moment, conjugaꢀ
tion and charge, which can alterthe function of molecules in more
complex systems.7–11Photoswitchable control elements have been
Figure 1. Molecular structure and photoswitching of donor–
acceptor Stenhouse adducts (DASAs; 1ꢀ2) in toluene.
applied successfully in material sciences,5,12–16supramolecular
chemistry17–21and biological systems.22–27The range of wellꢀ
established photoswitches includes azobenzenes, diarylethenes
and spiropyrans.1,15,28–32The properties of photoswitches can often
be tuned easily to provide optimum performance for specific
applications, primarily because of the detailed understanding
available of the underlying electronic and steric factors that conꢀ
trol their photoswitching behavior.
The understanding of the specific mechanism by which DASAs
undergo photoswitching is essential toenable the full potential of
these highly promising switches to be realized. It is furthermore
important for addressingthe pronounced solventꢀdependence of
the photoswitching and the ability to tune thermal halfꢀlife of B
and wavelength of excitation for A. Insightinto the mechanism is
expected to lead to the application of DASAs in more complex
functional systems, as was the case withspiropyrans after their
switching mechanism was understood well enough to use these
photochromic compounds in a highly diverse range of applicaꢀ
tions.15,30–32,53–55
An especially important tunable property is that of switching
withvisible light.26,33–36Indeed visible light controlled phoꢀ
toswitches are receivingincreasing attention mainly due to photoꢀ
damage and ꢀdegradation observed with short wavelength irradiaꢀ
tion in biological and material systems.26,37–39Photoswitches that
respond to light in the wavelength range650 nm to 900 nmare
especially important for biological applications,40,41as was recentꢀ
ly demonstrated by Woolley and coꢀworkers with the switching of
azobenzenes in whole blood.42
Herein, we show that the photoswitching mechanism of DAꢀ
SAs involves an actinic step which precedesa thermal 4πꢀ
electrocyclization. Weidentifya photoꢀgenerated intermediateA’
(proposed Z–Eisomerization),which then undergoes thermal 4πꢀ
electrocyclization to give the closed formB. Temperature dependꢀ
ent UV/Vis absorption spectroscopy and NMR measurements (in
addition to TDꢀDFT calculations)provide insight into the mechaꢀ
nism and reaction kinetics.
A new class of visible light photoswitches,the donor–acceptor
Stenhouse adducts (DASAs), has been recently introduced by
Read de Alaniz, Hawker and coꢀworkers(Figure 1).43–45 DASAs
are stable, synthetically accessible Tꢀtype photoswitchꢀ
es.1,43,44Initial reports explored the structural scope of DASAsand
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a)
Thermal
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R/A
π
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conrot.
H+-transfer
k2,obs.
OH
N
O
O
Irradiation
k1 (h
O
O
O
O
O
O
(E)
OH
O
O
O
O
ν
)
N
O
(Z)
k-1 (hν)
k-1,thermal
k-2,obs.
N
H
A
B
A'
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OH
N
O
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O
O
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E
Scheme 1.Definition of the various phases involved in the phoꢀ
toswitching of compound 1 at 293 K (a) and 253 K (b) on experiꢀ
mental data: Blue solid line = absorption maximum of A at
545 nm; and light blue dotted line = absorption maximum of A’ at
600 nm.
A'
irradiation (Scheme 1b; Figure S17). Upon cessation of irradiaꢀ
tion, the absorbance of the band decreases exponentially with an
isosbestic point maintained at 566 nm (Figure S17).
B
A
Figure 2.Photoswitching mechanism of DASAs: a) Proposed
photoswitching mechanism;b) Representation of the photoswitchꢀ
ing pathway in an energy level diagram.
These data indicatethat the formation of isomer A’, the absorpꢀ
tion of which is redꢀshifted compared to that of1 (SI section
5.2.6),is due to photoinduced Z–Eisomerization. Upon irradiation,
A’ is generated from A, with aphotostationary state (PSS) reached
rapidly (φ = 0.17; Scheme 1, phase ii, Figures S7–S10) and mainꢀ
tained under irradiation. k1(hν) and kꢀ1(hν)are both photochemical
reaction rates that are dependent on the photokinetic factors (Figꢀ
ure 2a). At low temperature (< 253 K) the absorption (Scheme 1b,
phaseiii) remains unchanged once the PSS is reached, indicating
equilibrium at k1,obs.[A] = kꢀ1,obs.[A’] (vide infrafor kinetic analꢀ
yses).At higher temperatures, A’ not only reverts to A (kꢀ1(hν) and
kꢀ1,thermal) but can also cyclize to B (k2). This leads to a decrease in
absorption during irradiation (Scheme 1a, phase iii, 293 K). As kꢀ
1,thermal> k2,obs., most of A’ is switched back to A (Scheme 1, phase
iv). However, B also can revert back to Athermally (Scheme 1,
phase v) viaA’ (kꢀ2,obs. and kꢀ1,thermal). A’ does not reach a signifiꢀ
cant steady state concentration under these conditions, and hence,
the transient absorption in backswitching from B to A is not obꢀ
served (kꢀ1,thermal>> kꢀ2,obs.; Figures S7–S19). Furthermore, A’
generated through irradiation at low temperature (<253 K),does
not react further to formB(FigureS17).Thesedata and the model
developed are summarized in the energy level diagram in Figure
2b. The photoswitching behavior of 2 under identical condiꢀ
tions(Figure S4–S6) is similar to the one of compound 1. Despite
the fact that the rate of thermal relaxation is higher for 2, a transiꢀ
ent absorption band is also observed.
Consideration of possible reaction mechanisms led us to proꢀ
pose initial Z–Eisomerization (to form A’, Figure 2) followed by
cyclization to B (Figure 2). Considering the stereochemistry of B
(confirmed by Xꢀray analysis by the groups of Hawker and Read
de Alaniz43,44), the cyclization is expected to be a conrotatory,
thermally allowed 4πꢀelectrocyclization.43,44Hence, we focused on
observing the intermediate A’. Studyof the photoswitching proꢀ
cess in toluene led to the observation of a transient band in the
UV/Vis absorption spectrum during photoswitching of 1(Figure 3,
Figure S11–S13). Upon irradiation, the main absorption band of
the linear triene (“open”) A at545 nm diminishes, while a new,
redꢀshifted absorption band at 600 nm transiently appears.
The timeꢀdependent behavior of the visible absorption of 1at
293 Kis as follows (Figure 3b, Scheme 1a, and Figures S7, S11–
S13): a rapid initial increase in absorbance at λmax 600 nm is
observed upon irradiation, followed by a slower decay. Cessation
of irradiation is followed by a rapid decrease in absorbance
at600 nm. Notably, this absorption bandis not observed upon
thermal relaxation from state B to A. The maximum absorption
reached at 600 nm was directly dependent on the photon flux and
wavelength of irradiation (Figures S18 and S19). At 253 K, the
absorption band at 600 nm increases and is then stable during
The photoswitching of 1 and 2by photochemicalZ–Eisomerization
is followed by a conrotatory, thermal 4πꢀelectrocyclization. The
rate limiting step for the overall reaction from A to Bis k2,obs.. For
reversion of B to A, the rate limiting step is kꢀ2,obs.. Importantly,
the proposed mechanism (Figure 2) further involves a lateꢀstage
protonꢀtransfer. In the presence of water, these steps are expected
to be fast. However, some solvents may favor isomeric distribuꢀ
tions (similar to spiropyrans,31,56 diarylethenes57 and
azobenzenes58,59)and thus influence the kinetics and thereby the
possible observation of the absorption band.Halogenated solvents
(e.g. dichloromethane) favor the elongated triene structure A.44
Nevertheless, photoswitching to A’ is observed (FiguresS47–
S49). Polar protic solvents (e.g. methanol and water) result in
irreversible photoswitching fromA to B44 with no observed transiꢀ
ent absorption (Figure S43).No decomposition of B in water upon
prolonged irradiation was observed (Figures S44 and S45). Moreꢀ
over, under aqueous conditions A cyclizes slowly, but spontaneꢀ
ously to B in the dark (Figure S46).
Figure 3. Observation of a transient absorption band: a) Absorpꢀ
tion spectra at indicated timeꢀpoints during photoswitching of 1
(293 K, 4 µM in toluene) under broad band visible irradiation; b)
Timeꢀdependent change in absorption of A (at 545 nm) and A’ (at
600 nm) with enlarged irradiation regime.
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To further investigate the nature of intermediate A’, and to
studies and kinetic modelling. This material is available free of
charge via the Internet at http://pubs.acs.org.
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connect its structure to the observed bathochromic shift of the
transient absorption, TDꢀDFT calculations were performed (SI
section 12). The obtained results confirmed that Z–Eisomerization
can cause a bathochromic shift in the absorption spectrum. This
finding is further supported by reports on the bathochromic shift
of the absorption spectrum upon photoisomerization of analogous
cyanine dyes that cannot undergo cyclization.60,61Moreover, lowꢀ
AUTHOR INFORMATION
Corresponding Author
* b.l.feringa@rug.nl
1
temperature HꢀNMR spectroscopy measurements with inꢀNMR
ACKNOWLEDGMENT
irradiation show the photogeneration of a single unstable intermeꢀ
diate in deuterated dichloromethane that mainly affects chemical
shifts in the polyene region (SI section 11).
The Netherlands Organization for Scientific Research (NWOꢀ
CW, Top grant to B.L.F., VIDI grant no. 723.014.001 for W.S.
and Veni grant no. 722.014.006 to S.J.W.), the Royal Netherlands
Academy of Arts and Sciences Science (KNAW), the Ministry of
Education, Culture and Science (Gravitation program
024.001.035) and the European Research Council (Advanced
Investigator Grant, no. 227897 to B.L.F) are acknowledged for
financial support. The Swiss Study Foundation is acknowledged
for a fellowship to M.M.L. We thank P. van der Meulen for help
with the NMR studies and T. TiemersmaꢀWegman for ESIꢀMS
analyses andProf.W.R. Browne and J. Chen for technical assisꢀ
tance and discussion.
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Finally, the kinetics of photoswitching were studied in more
detail. Reaction rates of the different steps were measured directly
or indirectly (Table 1) and fitted to a kinetic model based on our
mechanistic hypothesis (Figure 2, SI section 13). The timeꢀ
dependence of the production and consumption of Bwas calculatꢀ
ed by making the assumption that A’ and Bshow negligible abꢀ
sorbance at 545 nm and A and Bshow negligible absorbance at
600 nm. Overall, the measured and modelled reaction rates agree
qualitatively and are in line the proposed energy level diagram
(Figure 2b).
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