Shedding Light on the Photoisomerization Pathway of Donor-Acceptor Stenhouse Adducts

Article abstract of DOI:10.1021/jacs.7b09081

Donor-acceptor Stenhouse adducts (DASAs) are negative photochromes that hold great promise for a variety of applications. Key to optimizing their switching properties is a detailed understanding of the photoswitching mechanism, which, as yet, is absent. Here we characterize the actinic step of DASA-photoswitching and its key intermediate, which was studied using a combination of ultrafast visible and IR pump-probe spectroscopies and TD-DFT calculations. Comparison of the time-resolved IR spectra with DFT computations allowed to unambiguously identify the structure of the intermediate, confirming that light absorption induces a sequential reaction path in which a Z-E photoisomerization of C₂-C₃ is followed by a rotation around C₃-C₄ and a subsequent thermal cyclization step. First and second-generation DASAs share a common photoisomerization mechanism in chlorinated solvents with notable differences in kinetics and lifetimes of the excited states. The photogenerated intermediate of the second-generation DASA was photo-accumulated at low temperature and probed with time-resolved spectroscopy, demonstrating the photoreversibility of the isomerization process. Taken together, these results provide a detailed picture of the DASA isomerization pathway on a molecular level.

Full text of DOI:10.1021/jacs.7b09081

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Communication
Shedding Light on the Photo-Isomerization
Pathway of Donor–Acceptor Stenhouse Adducts
Mariangela Di Donato, Michael M. Lerch, Andrea Lapini, Adèle D. Laurent,
Alessandro Iagatti, Laura Bussotti, Svante P. Ihrig, Miroslav Medved, Denis
Jacquemin, Wiktor Szymanski, Wybren Jan Buma, Paolo Foggi, and Ben L. Feringa
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09081 • Publication Date (Web): 17 Oct 2017
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Shedding Light on the PhotoꢀIsomerization Pathway of Donor–
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Acceptor Stenhouse Adducts
Mariangela Di Donato∥,&,§,1, Michael M. Lerch†,1, Andrea Lapini∥,§, Adèle D. Laurent2,
Alessandro Iagatti∥,&, Laura Bussotti∥, Svante P. Ihrig†, Miroslav Medveď3,4, Denis
Jacquemin2,5, Wiktor Szymański‡,†, Wybren Jan Buma6, Paolo Foggi∥,&,⊥, Ben L.
Feringa†,*
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†
‡
∥
&
§
Centre for Systems Chemistry, Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
Department of Radiology, University of Groningen, University Medical Center Groningen, Hanzeplein 1, 9713 GZ, Groningen, The Netherlands
LENS (European Laboratory for Non Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino, Italy
INO (Istituto Nazionale di Ottica), Largo Fermi 6, 50125 Firenze, Italy
Dipartimento di Chimica “Ugo Schiff”, Università di Firenze, via della Lastruccia 13, 50019 Sesto Fiorentino, Italy
⊥
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Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, 06100 Perugia, Italy
CEISAM, UMR CNRS 6230, BP 92208, 2 Rue de la Houssinière, 44322 Nantes, Cedex 3, France
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Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University in Olomouc, 17.
listopadu 1192/12, CZꢀ771 46 Olomouc, Czech Republic
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Department of Chemistry, Faculty of Natural Sciences, Matej Bel University, Tajovského 40, SKꢀ97400 Banská Bystrica, Slovak Republic
Institut Universitaire de France, 103 bd St Michael, 75005 Paris Cedex 5, France
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands.
Supporting Information Placeholder
ABSTRACT: Donorꢀacceptor Stenhouse adducts (DASAs) are
negative photochromes that hold great promise for a variety of
applications. Key to optimizing their switching properties is a detailed
understanding of the photoswitching mechanism, which, as yet, is
absent. Here we characterize the actinic step of DASAꢀ
photoswitching and its key intermediate, which was studied using a
combination of ultrafast visible and IR pumpꢀprobe spectroscopies
and TDꢀDFT calculations. Comparison of the timeꢀresolved IR
spectra with DFT computations allowed to unambiguously identify
the structure of the intermediate, confirming that light absorption
induces a sequential reaction path in which a ZꢀE photoisomerization
of C2ꢀC3 is followed by a rotation around C3ꢀC4 and subsequent
thermal cyclization step. First and second generation DASAs share a
common photoisomerization mechanism in chlorinated solvents with
notable differences in kinetics and lifeꢀtimes of the excited states. The
photogenerated intermediate of the second generation DASA was
photoꢀaccumulated at low temperature and probed with timeꢀresolved
spectroscopy, demonstrating the photoꢀreversibility of the
isomerization process. Taken together, these results provide a detailed
picture of the DASA isomerization pathway on a molecular level.
Molecular photoswitches1 allow reversible optical control in a
plethora of applications.2–7 Each photoswitch has its own
favorable and adverse properties that ultimately determine how
well it is suited for photoregulating a particular responsive
system. Highly desirable properties include switching with visible
light8,9 and negative photochromism.10
Donorꢀacceptor Stenhouse adducts (Figure 1a) are an emerging
class of photoswitches.11,12 They are particularly attractive due to
their modular nature and rapid synthesis and undergo a large
structural change upon photoswitching with visible light.
Theoretical studies have provided a first effort to rationalize their
photoswitching characteristics.13,14 Structural improvements of
these adducts have led to a second generation of DASAs.15,16
Successful applications of DASAs have already emerged for
smart materials,17–22 sensors,23,24 catalysis25 and drugꢀcarriers.11,26
Although such applications are promising and highlight the
potential of DASAs, the understanding of their photoswitching
behaviour is far from complete.
Figure 1. Photoswitching of DASAs: (a) Photoswitches used in
this study; (b) Refined mechanistic proposal; and (c) Findings of
this work. The red (blue) relative energy levels correspond to
DASA 1 (2) (see sections 6.1 and 6.3 in SI). In the electron
density differences (EDD) plot, the blue (red) regions correspond
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to decrease (increase) in electron density upon electronic
transition for DASA 1.
pumpꢀprobe delays, the appearance of a positive band (figure 2,
blue EADS), redꢀshifted compared to the bleaching signal,
identifies the formation of photogenerated intermediate
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Initial insights into the DASA photoswitching mechanism stem
corresponding to the transient absorption band previously
identified for 1.27 For sample 1, the A→A’ photoreaction happens
on a 2 ps timescale as indicated by the appearance of a positive
band peaking at 567 nm in the second EADS (red EADS in figure
2a) while both the ground state bleaching of A and its excited
state absorption band, peaked at 460 nm, significantly decrease.
On the following 10 ps timescale, the induced absorption band
associated with the photoproduct is subject to a spectral evolution
due to a vibrational cooling process in the ground state of the nonꢀ
isomerized form A (vide infra). In case of sample 2 (figure 2b) the
2 ps component is mostly associated with an excited state
relaxation of A, while the appearance of the intermediate band
(peaked at 649 nm) occurs on a longer 27 ps timescale (red to blue
curve evolution in figure 2b). The quantum yield for the
intermediate formation estimated from the residual bleaching
signal in transient absorption spectra constitutes ~17% for 1 and
~10% for 2.
In order to get detailed insight into the conformation of the
intermediate, visibleꢀpump/MidꢀIR probe spectra were measured
for both samples in the 1100ꢀ1750 cmꢀ1 spectral range. Figure 3
shows the EADS obtained by global analysis31 of the timeꢀ
resolved infrared (TRIR) spectra for sample 1 and 2, together with
their steadyꢀstate FTIR spectra, which identify the vibrational
bands of the elongated form. The time constants obtained from the
analysis of TRIR spectra match those obtained from the visible
pumpꢀprobe experiments except for the long timescale. This is
due to the larger error on the long time constant in the TRIR
measurements, for which a smaller acquisition time window was
used.
from the observation of an intermediate, presenting
a
bathochromically shifted transient absorption band in the UV/vis
spectrum with respect to the main absorption band upon light
irradiation.27 Timeꢀ and temperatureꢀdependent studies revealed a
strong dependence on temperature and light intensity, and
suggested that this intermediate results from a photoinduced Z−E
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isomerization followed by
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thermal conrotatory 4πꢀ
electrocyclization (figure 1b).27 This observation suggests the
separation of the mechanism into an actinic step and a thermal
step. Intermediate A’’ (figure 1b) is needed for successful
cyclization, but a simple ZꢀE isomerization leads only to
intermediate A’. Intermediate A’’ could result from
a
photoisomerization around C2ꢀC3 followed by a rotation around
C3ꢀC4 or from a concerted Hulaꢀtwist movement as commonly
observed in confined spaces such as protein cavities.28
Timeꢀresolved spectroscopy in both the UV/vis and IR region has
proven very successful in elucidating structural changes occurring
on the femto/picosecond timeꢀscale29,30 and in characterizing the
electronic properties of shortꢀlived reaction intermediates. Here
we report inꢀdepth studies on the photoswitching mechanism of
DASAs using ultrafast visible and MidꢀIR spectroscopy,
elucidating the timescale of the photoinduced isomerization
process and the structure of the intermediate resulting from the
actinic step. This intermediate was trapped and manipulated at
low temperature and could be confirmed structurally to be A’.
Comparison between the experimental transient infrared and
DFTꢀcomputed spectra supports a sequential photoswitching
mechanism (figure 1b and c), showing that in the analyzed
solvents (dichloromethane/chloroform) isomerization between the
elongated triene form A and the twisted intermediate form A’’
does not occur through a hulaꢀtwist mechanism. Furthermore, we
provide evidence that – even though the same intermediate is
formed for both first and second generation DASAs – the second
generation isomerizes at least ten times slower.
Figure 3. EADS obtained from global analysis31 of the TRIR
spectra of 1 (a) and 2 (b) in chloroform. The black spectrum on
top of each panel is the FTIR spectrum of the linear A form.
The longꢀliving component (blue line in figure 3a and b) allows to
identify the structural changes that occur during the formation of
the intermediate. For both samples, major spectral changes occur
in the 1150 and 1450 cmꢀ1 region, mainly associated with triene
chain CꢀC/C=C stretching and CꢀH rocking/scissoring vibrations
according to our DFT calculations. In particular, in the longꢀlived
spectral component (blue lines in figure 3a and b)
negative/positive bands are present at 1154/1188 cmꢀ1 for 1 and at
1143/1170 cmꢀ1 for 2. A differential band pattern in the same
regions has been previously observed in different molecules
undergoing cis/trans photoisomerization such as rhodopsin32,33
and the Photoactive Yellow Protein (PYP).34 This pattern strongly
suggests that a similar process occurs in the present compounds.
We have compared the long living experimental TRIR spectral
component with computed IR difference spectra obtained at the
Figure 2. EvolutionꢀAssociated Difference Spectra (EADS)
obtained from global analysis31 of time resolved visible data
recorded for 1 (a) and 2 (b) in chloroform.
Figure 2 reports the EADS (EvolutionꢀAssociated Difference
Spectra) obtained by global analysis31 of visible pumpꢀprobe data
recorded for DASA 1 and 2 in chloroform (SI section 4). DASA 1
does not cyclize in chlorinated solvents, while 2 undergoes a
reversible cyclization reaction to form a neutral cyclic form
reminiscent of B (SI section 3).15,16 Similar spectra with
comparable EADS lifeꢀtimes are obtained for 1 in toluene, where
reversible cyclization occurs (figure S4.1).
Upon photoexcitation, bleaching of the ground state absorption
(figure 2, black EADS) is observed as a negative signal peaking at
537 nm for 1 and 610 nm for 2. Importantly, this short living
component exhibits a blueꢀshifted excited state absorption,
peaking at 460 and 482 nm for 1 and 2, respectively. At long
B3LYP/6ꢀ31++G(d,p)/SMD level35 38 (SI section 6) for
–
isomerizations leading to both A’ and A’’. For both samples the
best match between computed and experimental spectra is
obtained for the A→A’ transition (figure 4). We thus conclude
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that for both DASAs, photoexcitation leads to the formation of the
A’ photoproduct, which, in turn, excludes the Hulaꢀtwist
mechanism.
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The TRIR spectra confirm the timescale of photoisomerization:
the intermediate bands in the 1100ꢀ1200 cmꢀ1 fingerprint region
appear on a 2 ps timescale for 1 and on a 24 ps timescale for 2. In
case of 1 additional positive bands appear in the second EADS, as
for instance at 1139 and 1331 cmꢀ1. These signals decay in the
following 10 ps evolution, suggesting that they are not attributable
to A’ but possibly to the hot ground state of the A species. Target
analysis31 of both the visible and IR transient data of 1 assuming a
branched decay of A* in both A’ and the hot ground state of A
successfully disentangles the individual spectra of these two
species (SI section 4.4). In contrast, the initial (vibrational)
relaxation of 2 occurs on the A* excited state surface. Notably, in
the first spectral component of both samples, assigned to the A*ꢀ
A difference spectrum, only a few low intense positive bands are
noticed, indicating much weaker IR absorption in A*. The most
prominent band shifts in the initial EADS are in the 1600 cmꢀ1
region, attributable to carbonyl stretch vibrations according to the
DFT calculations.
Besides the analysis of the IR spectra, our calculations of relative
energies, electron density differences (EDD) and geometrical
changes upon excitation provide additional information
supporting the proposed mechanism. The EDD plots (figure 1c
and S6.9) show that for both samples the electron density changes
upon excitation are localized on the πꢀconjugated linker. In the
excited state electron density flows towards the carbon atom
functionalized by an OH group, resulting in decreased electron
density on “double” bonds. Upon excitation, the structure remains
planar (figure S6.10), but the bond lengths increase by ca. 0.01ꢀ
0.02 Å along the chain, except for the central C3ꢀC4 bond, in line
with the EDD analysis. The experimental and theoretical red shift
of 2 vs. 1 in the excitation energy for A→A* (figure 2 and table
S6.2 in SI) is related to an extension of the πꢀconjugation to the
indoline moiety. The energy level diagram also reveals that the
A’→A’’ barrier is ca. 2 kcal/mol smaller for 2 suggesting faster
kinetics for the C3ꢀC4 bond rotation step, which is related to a less
distorted TS structure (difference of ca. 5°) for this derivative.
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Figure 4. A comparison of the experimental and DFTꢀcomputed
longꢀlasting IR spectral component of 1 (a) and 2 (b) in
chloroform. The conversion of A→A’ (blue) and A→A’’ (hula
twist, red) are depicted.
Finally, we photoꢀaccumulated intermediate A’ of compound 2 at
233 K under continuous illumination at 520 nm in deuterated
dichloromethane (SI section 5). Irradiation at 660 nm resulted in
backꢀswitching to the elongated A species as assessed by TRIR
spectra (figure S5.2), suggesting that reversible isomerization can
be induced by selectively pumping A or A’, while ringꢀclosure to
form B is only possible via thermal pathway.
The electrocyclization is several orders of magnitude longer than
the actinic step, thus being the rateꢀdetermining step as tentatively
concluded before.27 Our study identifies key factors for improving
switching characteristics, for instance, increasing the
photochemical quantum yield by optimizing A’ properties and
disfavor the reverse isomerization process. It also suggests that
the reason why firstꢀgeneration DASAs do not cyclize in
chloroform or dichloromethane, as opposed to second generation
DASAs, is solely a question of energy levels and barriers involved
in the thermally induced 4πꢀelectrocyclization, and is not due to
the actinic step. The presented results elucidate the timescale of
the actinic step and bode well for implementation of photoswitch
improvement.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and characterization of compounds,
additional UV/vis absorption spectra and photoswitching studies,
ultrafast visible and MidꢀIR spectroscopy; EADS (Evolution
Associated Difference Spectra) obtained by global analysis; SAS
(Species Associated Spectra) obtained by target analysis and DFT
computations (PDF).
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AUTHOR INFORMATION
Corresponding Author
*b.l.feringa@rug.nl
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Author Contributions
1 These authors contributed equally.
ACKNOWLEDGMENT
The authors gratefully acknowledge financial support from
LaserlabꢀEurope (LENS002289), the Ministry of Education,
Culture and Science (Gravitation program 024.001.035), The
Netherlands Organization for Scientific Research (NWO–CW,
Top grant to B.L.F., VIDI grant no. 723.014.001 for W.S.), the
European Research Council (Advanced Investigator Grant, no.
227897 to B.L.F.) and the Royal Netherlands Academy of Arts
and Sciences Science (KNAW). A.D.L., D.J. and M.M. are
thankful to the Campus France and the Slovak Research and
Development Agency for supporting their longꢀstanding
collaboration in the framework of Stefanik PHC program
(BridgET project, No. 35646SE and SKꢀFRꢀ2015ꢀ0003,
respectively), to the Czech Science Foundation (project no. 16ꢀ
01618S) and the Ministry of Education, Youth and Sports of the
Czech Republic (grant LO1305). This research used
computational resources of 1) the GENCIꢀCINES/IDRIS, 2)
CCIPL (Centre de Calcul Intensif des Pays de Loire), 3) a local
Troy cluster, and 4) the HPCC of the Matej Bel University in
Banska Bystrica by using the infrastructure acquired in projects
ITMS 26230120002 and 26210120002 supported by the Research
and Development Operational Programme funded by the ERDF.
The Swiss Study Foundation is acknowledged for a fellowship to
M.M.L.
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