π-Bridge Substitution in DASAs: The Subtle Equilibrium between Photochemical Improvements and Thermal Control**

Article abstract of DOI:10.1002/chem.202004988

Donor–acceptor Stenhouse adducts (DASAs) are playing an outstanding role as innovative and versatile photoswitches. Until now, all the efforts have been spent on modifying the donor and acceptor moieties to modulate the absorption energy and improve the c

Full text of DOI:10.1002/chem.202004988

Accepted Article
Accepted Article
Title: π-Bridge Substitution in DASAs: the Subtle Equilibrium Between
Photochemical Improvements and Thermal Control
Authors: David Martínez-López, Eduardo Santamaría-Aranda, Marco
Marazzi, Cristina García-Iriepa, and Diego Sampedro
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1/2020

Chemistry - A European Journal
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π-Bridge Substitution in DASAs: the Subtle Equilibrium Between
Photochemical Improvements and Thermal Control
[a]
[a]
[a,b,c]
David Martínez-López, Eduardo Santamaría-Aranda, Marco Marazzi,*
Iriepa,*[b,c] and Diego Sampedro*[a]
Cristina García-
[
[
[
a]
b]
c]
Dr. D. Martínez-López, E. Santamaría-Aranda, Dr. M. Marazzi and Dr. D. Sampedro
Departamento de Química
Centro de Investigación en Síntesis Química (CISQ), University of La Rioja
Madre de Dios, 53, 26006 Logroño, Spain
E-mail: marco.marazzi@uah.es , diego.sampedro@unirioja.es
Dr. M. Marazzi and Dr. C. García-Iriepa
Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering
Universidad de Alcalá
Ctra. Madrid-Barcelona km 33,600, E-28871 Alcalꢀ de Henares (Madrid), Spain
E-mail: cristina.garciai@uah.es
Dr. M. Marazzi and Dr. C. García-Iriepa
Chemical Research Institute “Andrꢁs M. del Río” (IQAR)
Universidad de Alcalá
E-28871 Alcalꢀ de Henares (Madrid), Spain
Supporting information for this article is given via a link at the end of the document.
Abstract: Donor–Acceptor Stenhouse Adducts (DASAs) are playing
an outstanding role as innovative and versatile photoswitches. Until
now, all the efforts have been spent on modifying donor and acceptor
moieties, to modulate the absorption energy and improve the
cyclization and reversion kinetics. However, there is a strong
dependence on specific structural modifications and a lack of
predictive behavior, mostly due to the complex photoswitching
mechanism. Here, by means of a combined experimental and
theoretical study, we systematically explore the effect of chemical
modification of the π-bridge linking the donor and acceptor moieties,
finding significant impact on the absorption, photocyclization and
relative stability of the open form. Especially, a position along the π-
bridge is found to be the most suited to red-shift the absorption while
preserving cyclization. However, thermal back-reaction to the initial
isomer is blocked. These effects are explained in terms of an
increased acceptor capability offered by the π-bridge substituent that
can be modulated. This strategy opens the path toward derivatives
with infra-red absorption and a potential anchoring point for further
functionalization.
conversion between a linear triene-colored-hydrophobic and a
cyclic-colorless-hydrophilic form (Figure 1).[5]
Different experimental and computational studies have focused
[
6–12]
[13–15]
on distinct DASAs applications,
properties,
switching
mechanism[
16–20]
or reactivity
[21]
.
Introduction
Molecular photoswitches, that is, molecules that can be
interconverted between two states by light, have attracted the
attention of scientists in the last decades due to their versatility
and outstanding applications.[1–3] Nevertheless, new families of
photoswitches continue to arise aimed mainly at increasing the
range of properties and applications. A fascinating example is the
emerging Donor−Acceptor Stenhouse Adducts (DASAs), firstly
reported by Read de Alaniz in 2014.[4] DASAs are promising T-
type photoswitches characterized by an efficient reversible
Figure 1. Upper panel: DASAs' overall reactivity. Photoisomerization coupled
to cyclization generates the closed form while the extended form is thermally
recovered. A general DASA structure is also given including the π-bridge
(orange) connecting donor (green) and acceptor (red). Lower panels following
the color code: donors, acceptors and π-bridge substituents (in grey if studied
only theoretically) investigated in the present work. Theoretical models always
include the diethylamino moiety in D, instead of the synthesized diheptylamino
moiety.
1
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The photoswitch activation with visible and far-red/near-infrared
light is a desired and crucial property for biological and materials
applications, as it increases the tissue and surface penetration
capability, respectively.[2,22,23] So far, several DASAs derivatives
absorbing in the visible region have been reported.[5,24] Different
generations of DASAs have been prepared in order to enhance
the switchability of these molecules in polar and non-polar media
Hence, in order to increase the knowledge on these systems and
test the vability of π-bridge substitution, in this paper we present
a comprehensive computational and experimental work mainly
focused on the effect of substituting different positions of the π-
bridge on the DASAs properties (Figure 1). It was found that
DASAs with a modified π-bridge have different absorption
properties, photochemical isomerization mechanism and thermal
and to red-shift the absorption spectra. It should be remarked that, equilibria.
up to now, this tunability has been achieved solely by changing
two different DASA moieties: acceptor (A) and donor (D) (Figure
1
). For instance, it was reported in 2014 that replacing 1,3- Results and Discussion
disubstituted barbituric acid-based acceptor group (A1 in Figure
) by a 1,3-indanedione (A2 in Figure 1) results in a red-shift of
1
The paper is organized as follows. First, the design of several
DASAs substituted on the π-bridge is presented. Several
positions and functional groups were considered. Next, the
absorption of selected candidates was thoroughly analyzed. One
of the main aims for these compounds is the activation using far
red or even near infrared light. Then, the photoisomerization was
explored. This is the first step in the mechanistic path, and it is
related with the potential energy surface in the excited state.
Finally, the thermal equilibrium was computed. This final part of
the mechanism may be related with the dark equilibria found in
these species and responsible for the cyclization process. For all
these sections, complementary experimental results will be
presented.
[
25]
ca. 30 nm in toluene. Similarly, a second generation of DASAs
with an even more red-shifted absorption was achieved by
introducing novel aniline or indoline-based donors.[26,27] Recently,
a third generation has emerged whose absorption tunability rises
from novel structural changes of the carbon acid acceptor.[24]
However, further shifting the absorption wavelength in the
infrared region still constitutes a challenge.
Another key aspect of DASAs still poorly understood is the effect
that the relative energy of each thermal isomer has on the overall
photoswitching process. In contrast with other types of
photoswitches, the complete mechanism for DASAs implies a
photochemical step for the isomerization of a C=C double bond
and several thermal steps that allow for the formation of the final
structure, the closed form.[16,20,28] All these intermediates are in
thermal equilibrium and could strongly affect the overall
photoswitching process. In turn, the relative stability of these
isomers can be modified by several factors. For instance, the
Design and synthesis of π-bridge-substituted DASAs
Up to five different positions are, in principle, susceptible for
modification in DASAs’ π-bridge. The selection of the substitution
site among the different possibilities (e to i, Figure 1) was done
based on several mechanistic, synthetic and computational
criteria. First, the hydroxyl group in f has been shown to be crucial
to secure DASA functioning,[30] hence it was preserved. Second,
the typical synthetic route for DASAs, consisting of the opening
of a furan ring by the donor moiety, points out the central positions,
g and h, as preferred over the lateral ones, e and i (Scheme 1).
Thus, we initially considered both g- and h-substituted DASAs
from a computational point of view. However, experimentally we
aimed for the substitution of furan with Br and Ph to yield g-
substituted DASAs, as they can be prepared by accessible
building blocks and their effect could be relevant enough to
change the properties of DASAs. Moreover, both Br and Ph
groups could be further functionalized, if required for specific
applications.
effect of the solvent polarity has been deeply studied in these
compounds.[
5,15,17,18,24,27]
Due to the marked difference in polarity
of the open and closed forms, the change in the solvent polarity
for instance going from toluene to acetonitrile) greatly changes
(
the switching process. The effect on both switching process and
dark equilibrium could be changed not only by the type of solvent,
but also by small structural modifications or concentration of the
photoswitch.[13,16,20,27,29]
The basic design features of DASAs have been mainly
maintained through the three generations reported. Changes in
the
D and A moieties have contributed to improve the
performance in some cases, but also to the worsening of the
photoswitching process or even the complete inactivation of the
back reaction.[
5,13,24,30]
However, the π-bridge has been kept
mainly intact, although it could be considered a potentially
relevant modification point. Substitution on the bridge could not
only directly alter the relative stability of the different thermally
connected isomers, but it also could affect the optical properties
and the overall switching process. Even more, substitution at the
bridge could be also useful, in principle, as anchoring point for
subsequent functionalization or to bind these compounds in
different applications. Nevertheless, this could result in a
substantial modification of the steric and/or electronic properties
of the DASA, thus requiring investigations to clarify how and to
which extent the photoswitching behavior would be affected.
More in general, a study of the outcomes induced by chemical
modifications of the DASA backbone on the overall switching
mechanism, is of high interest as it could lead to better rationalize
the complex interplay between the D-A push-pull effect, the
photochemical reactivity, and the thermal intermediates until
cyclization is accomplished.
Regarding the acceptor and donor moieties under study, they
have been selected based on different motivations. First, we
aimed to evaluate the effect of the bridge substitution on both first
(
D1 derivatives) and second (D2 derivatives) generation DASAs.
D1 and D2 were chosen because DASAs derivatives including
these moieties present quite blue and red-shifted absorption,
respectively[5,26] and hence, evaluating the additive character of
bridge substitution effect on their maximum absorption
wavelength is worthwhile. Considering the acceptor moiety, A1
was selected as numerous data has been already reported for
DASAs derivatives including this moiety and so, comparison with
our results could be straightforward. In addition, A2 was chosen
as it is one of DASAs acceptors giving a significant red-shifted
absorption (together with other acceptor recently reported).[
Hence, the evaluation of derivatives including D2 and A2 moieties
could aid in testing the limit of DASAs’ red-shift absorption.
24]
2
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Preliminary calculations using time dependent-density functional
theory (TD-DFT, see Computational Details) were performed to
understand the main effects that π-bridge modification could
cause on the electronic distribution of these molecules. It is
remarkable the fact that, the nature of the X-substituent group is
crucial, since the π-bridge, when unsubstituted, is almost neutral,
i.e., it acts as a mere linker between D and A moieties.
Nonetheless, appropriate substitutions in this part of the molecule
can result in important modifications of the electronic properties
of the bridge and, due to this, of the whole DASA. Careful
selection of substitution may lead the π-bridge to behave as a D-
or A-moiety, hence strikingly increasing the charge transfer
character of the S
propose a set of substituents to unveil the chemical nature of the
π-bridge, spanning from strong donors (-OMe, -NMe ) to strong
acceptors (-CN, -NO ), also including substituents with mixed
0 1
→ S vertical transition. Therefore, we may
2
2
inductive and resonance effects (-Br, -Ph and its derivatives -(p-
OMe)Ph, -(p-Cl)Ph, -(p-CN)Ph). In terms of absorption properties
and electronic densities, our TD-DFT results surprisingly suggest
that the substituted π-bridge acts as an acceptor in all cases.
Indeed, when analyzing the S → S absorption, mainly described
0 1
by a HOMO → LUMO transition, it is evident that the LUMO
orbital shows an increased electronic density on the π-bridge
2
group X, even for -NMe (Figure S2). Hence, the π-bridge
naturally acts as an acceptor, increasing the overall DASA
acceptor ability by coupling through conjugation with the ending
A moiety (A1/A2 in Figure 1).
Such electron density displacement toward the newly established
acceptor (π-bridge and A1/A2), typical of the S charge transfer
1
character, can be therefore improved by selecting X-substituents
with large acceptor strength. Nevertheless, such strength cannot
be increased indefinitely: in Figure 2 we compare the calculated
charge transfer character of three D2-X-A2 compounds with
0 1 2
Figure 2. S →S charge transfer character of D2-X-A2 (X = -Br, -CN, -NO ),
calculated by TD-DFT (B3LYP/6-31+G(d)) in toluene, applying natural bond
order analysis. Electron displacement among the moieties (donor, π-bridge and
acceptor) is shown by arrows from positive (blue) to negative (red) regions. The
thickness of the arrows is related to the amount of charge transfer.
Inspired by these prospective computational data and within the
limits imposed by the acceptor strength of the π-bridge X-
substituent, a set of novel DASAs has been synthetized through
a modification of the standard procedures[25,26,31] in order to
include π-bridge substituents, as shown in Scheme 1. This
implies the preparation of substituted furans that are
subsequently opened by the donor moiety to afford the final
compounds. A series of DASAs with the general notation D-X-A
were prepared in which the donor moiety (D), the acceptor moiety
2
increasing X-acceptor strength: -Br < -CN < -NO . The -Br
derivative already shows how the π-bridge and A form a unified
acceptor moiety; in the -CN derivative the acceptor character is
partially displaced toward the π-bridge, maintaining an overall D-
A electronic molecular structure; while the -NO derivative results
2
in a D-A-D molecule, since the conventional ending A moiety is
converted into a donor. Hence, concerning the photoreactivity,
the -NO derivative is expected to behave differently compared to
2
other π-bridge substituted DASAs (see Photoswitching
Mechanism section).
(
A) and the π-bridge substituent (X) were modified. Figure 1
includes the structure of the compounds prepared (D:
diheptylamine or N,N-diheptylisoindoilin-5-amine; A: barbituric
acid or 1,3-indanedione; X: H, Br or Ph). With this set of
compounds in hand, we could determine the effect of every part
of the DASA molecule on the different properties.
3
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Table 1. Absorption energy computed at the B3LYP/6-31+G(d) level of theory
for compounds D1-X-A1, considering both the g or h substitution sites (see
Figure 1). The solvent effect (toluene) is applied by polarizable continuum
model. Please note that these are qualitative values. Quantitative values are
given in Table 2, by applying the ADC2 level of theory.
Substituent (X)
H
Substitution site
Absorption energy eV (nm)
-
2.516 (493)
2.351 (527)
2.436 (509)
2.398 (517)
2.475 (501)
2.299 (539)
2.530 (490)
1.931 (642)
2.440 (508)
g
h
g
h
g
h
g
h
Br
Ph
CN
NO
2
Scheme 1. Synthetic path for π-bridge substituted DASAs.
Preparation of the unsubstituted DASAs (D-H-A) implies the
condensation of furfural with the corresponding acceptor followed
by opening of the furan by the donor moiety (see Supporting
Information). In contrast, π-bridge substituted compounds require
the preparation of the corresponding furan derivative prior to the
condensation and ring opening (Scheme 1). To do this, 3-
With the computational absorption spectra and the prepared
compounds in hand, we could focus on the effect of the π-bridge
X-substituent on DASA absorption properties. In Figure 3 and
Table 1 we show both theoretical and experimental results. It
should be noted that, to reduce the computational cost, the
diheptylamino group in D1/D2 of synthesized DASAs has been
replaced by the diethylamino group since, comparing their
3
bromofuran was reacted with POCl to prepare the bromofuran
carbaldehyde. This compound could be used to prepare the Br-
substituted compounds or further reacted under Suzuki
conditions to yield 3-phenylfuran-2-carbaldehyde. It is important
to note that this synthetic route allows only for the preparation of
g-substituted compounds. This is due to the regiochemistry in the
carbonylation step, in which the aldehyde group is placed in
position 2 of the furan while the Br is located in 3 (Scheme 1).
The synthesis of such type of g-substituted compounds was
already proposed, although with different donor and acceptor
moieties, and not attempting a photochromic neither mechanistic
study.[32]
simulated absorption spectra (Figure S1), only a 5 nm
bathochromic shift has been found, due to almost negligible
electron density differences.
Regarding the results, a quantitative agreement between the
experimental and predicted wavelength at maximum absorbance,
λ
max, was achieved by performing calculations with the second-
order algebraic diagrammatic construction (ADC(2)) single-
configuration reference method. Indeed, on one hand TD-DFT
can achieve only a qualitative agreement, with considerable shifts
from the experimental value, as shown by our results (Table S2)
[14]
and previous works on DASAs. On the other hand, the use of
more sophisticated multi-configurational methods is not needed
to predict DASA absorption, since it can be described by a single
HOMO → LUMO transition. Apart from λmax values, the spectral
shape is the other important aspect to reproduce accurately
absorption spectra, and this can be properly simulated at TD-DFT
level when including the vibrational resolution (Figures S4-
S6).[14,33–36] Hence, we have applied an energy shift to the
vibrationally-resolved TD-DFT spectra in order to match the
ADC(2) λmax values (Figure 3b). By applying this computational
strategy, a quite good agreement with experiment of both spectral
Absorption spectra
A critical feature of photoswitches is the absorption wavelength
needed to be activated. This is especially relevant for DASAs as
these compounds absorb in the visible region. Careful structural
modifications have been used to shift the absorption energy until
reaching the red window of the spectrum.[24,26] This is important
for many applications requiring low energy light like, for instance,
biological media. A general screening of relevant DASAs
including the prepared compounds and additional potential
candidates was done through prospective absorption energy
0 1
energetics and shape has been found, with a wide S →S
calculations. We used the TD-DFT framework, recognized as
absorption band characterized by a tail extending to shorter
wavelengths.
qualitatively correct in previous DASA studies[
14,15,17,18,20,30]
(see
Computational Details). Then, the effect of π-bridge substitution
was considered. First, we evaluate the effect of X-substitution in
both positions g and h. Our data show that the g position leads to
more consistent red-shifts compared to the h position, especially
when increasing the acceptor ability of the X-substituent (Table
Focusing on D1-X-A1 compounds, we observed that by
introducing a π-bridge substituent, the absorption spectrum is
red-shifted (Table 2 and Figure 3a). In particular, increasing the
X-acceptor strength (-Br < -CN < -NO
absorption red-shift.
2
) results in an amplified
1).
4
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The smallest red-shift (~0.1 eV) is induced by –Ph. Due to steric
constraints, it is placed almost orthogonal to the π-bridge (Figure
S6B), hence considerably reducing its resonant effect and mainly
acting through its limited inductive effect.
induced photoisomerization of the adequate C=C double bond.
To investigate this issue we have calculated, at the TD-DFT level,
the excited-state minimum-energy path of D2-Br-A2, D2-CN-A2
and D2-NO
2
-A2 in toluene, through the polarizable continuum
model (Figure 4).[ As explained in the previous section, the nitro
group seriously affects the electronic structure of the molecule.
The computed results confirm the expectations: on one hand, D2-
37]
Br-A2 and D2-CN-A2 reach a planar S
DASA photochemistry.[28] Indeed, a S
energy barrier along the
torsion coordinate is expected to connect this minimum with a
/S conical intersection corresponding to a 90 degrees twisted
structure around the C =C photoisomerizable bond, constituting
the first step toward cyclization. On the other hand, D2-NO -A2
reaches directly an unproductive S /S intersection region,
corresponding to pyramidalization of the -NO group, hence
1
minimum, typical of
1
S
1
0
g
f
2
1
0
2
conferring only photostability (i.e. internal conversion) properties
to the compound and hampering the photoisomerization (Figures
4
and S3).
Figure 3. a) Absorption spectra experimentally recorded in toluene, with color
code corresponding to λmax, indicated at the top of each spectrum. Two series
are shown, including (left) the effect of the π-bridge substitution in D1-X-A1 (X:
-
H, -Ph, -Br), and (right) the effect of changing both donor and acceptor (D2 and
A2), when X: -Br. b) ADC(2)//DFT spectrum simulation of D1-X-A1 (X: -H, -Ph,
Br, -CN).
-
This trend is confirmed for D2-X-A2, D1-X-A2 and D2-X-A1
compounds (Table 2). Hence, we demonstrate that by changing
the π-bridge X-substituent, irrespective of D and A moieties, a
significant red-shift of the absorption can be achieved, reaching
0.21-0.23 eV when considering the most effective -CN (Table S6).
As aforementioned, -NO has quite different electronic
2
a
Figure 4. S
Franck–Condon structure (reaction coordinate = 1) at the B3LYP/6-31+G(d)
level of theory for the D2-Br-A2, D2-CN-A2 and D2-NO -A2 derivatives. -Br
black) and -CN (red) derivatives reach a planar minimum in the excited state
, full symbols), whereas the -NO derivative reaches an intersection region
group.
1
minimum energy path computed in toluene starting from the
distribution that could affect the switching mechanism (vide infra).
In addition, the red-shift caused by each X-substituent is a
constant value, i.e. the same shift has been computed for D1-X-
A1 and D2-X-A2 families (0.13-0.14 eV for -Ph, 0.14-0.18 for -Br
and 0.21-0.23 for -CN). A similar additive effect has been
computed for the D and A moieties, reaching a 0.25-0.28 eV red-
shift when modifying D1-X-A1 into D2-X-A2, irrespective of the X-
substituent (Table S7).
2
(
(
S
1
2
with the ground state trough pyramidalization of the N atom of the -NO
2
Photoswitching mechanism
Once checked the effect of the π-bridge substituent on the optical
properties, the next step in the reaction mechanism is the light-
Table 2. Calculated (ADC(2) level, black) and experimentally determined (solvent: toluene, blue) λmax, given in eV (nm). The coupled effects of different donor D
and acceptor A groups, as well as various π-bridge X-substituents, are shown.
Donor D1
Donor D2
-Br
π-bridge X:
-H
-Ph
-Br
-CN
-NO
2
-H
-Ph
-CN
-NO
2
2
2
.20 (564)
.17 (572)
2.07 (599)
2.08 (597)
2.03 (611)
2.02 (612)
1.99 (623)
1.86 (667)
1.97 (629)
1.79 (693)
Acceptor A1
2.16 (574)
2.02 (614)
1.94 (639)
1.80 (689)
1.78 (697)
1.71 (725)
1.71 (725)
Acceptor A2
1.66 (745)
5
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To further explore the photochemical part of the mechanism, we
computed the key structures along the path for the available
experimental compounds (Figure 5). As can be seen, this part of
the mechanism is only slightly affected as the main features are
maintained for the substituted DASAs. Relevant changes are the
red-shift in the absorption spectra and the lowering or even the
vanishing of the energy barrier in the excited state which should
imply an even faster photoreaction for the π-bridge substituted
compounds.
spectroscopic window between reactants and products was
already found for some first-generation DASAs, although no
explanation was attempted.[27] To find a possible explanation, we
have hypothesized the presence of a complex ground-state
equilibrium between different E/Z isomers, as recently
suggested.[16,20] Especially, three different conformations were
considered (EEZ, EZZ and EEE, fixing the stereochemistry on D-
H-A compounds), as they were experimentally observed.[ While
confirming that the EEZ conformation is always the most stable,
this stability decreases for Ph- and even more for Br-derivatives,
therefore suggesting a more prominent role of EZZ and EEE
conformations when Br-substitution takes place (see Figure S10
and Table S8). Although their absorption does not lead to clear
differences in the spectrum, as expected for structures conserving
essentially the same conjugation length, we do expect that the
irradiation of EEZ, EZZ and EEE Br-derivatives could lead to
additional intermediates absorbing at 400nm, which should not be
observed when irradiating the only stable conformer, EEZ, of H-
and Ph-derivatives. The absorption of several intermediates in
this region does not rule out the possible decomposition of some
of the species in equilibrium. In fact, the increase of these bands
at longer times may suggest this alternative. However, even if
these bands are due to decomposition, this should be partial and
involve mainly the open form and /or the intermediates as the
closed forms were found to be stable.
20]
Figure 5. TD-DFT excited state stationary points along the photochemical
reaction path in toluene for the π-bridge (Ph, Br) substituted DASAs at the g
position, compared to unsubstituted DASA (H). Franck-Condon structure (Z
isomer), S
1 1 1 0
minimum and S transition state leading to the S /S conical
intersection (CI) are shown for D1-X-A1.
The cyclization was experimentally confirmed for all prepared
compounds. For instance, upon irradiation on the band maximum
of the open D1-Br-A1, this compound isomerizes to the closed
form (see Figure 6, a). However, it was found that a combined
effect of light and heat was operating in these compounds. That
is, although the cyclization process is considerably faster upon
irradiation, these compounds were found to isomerize also in the
dark. This isomerization process can be easily followed by UV as
the band corresponding to the open form (λmax = 610 nm)
disappears while the band for the closed transparent form (below
250 nm) grows. This agrees well with the previously studied
DASAs and with our theoretical results: absorption for the cyclized
form of D1-(g)Br-A2, at B3LYP/6-31G+(d) level in toluene, falls in
the window 254-263 nm. In order to isolate the effect of light and
heat in the cyclization, we followed the disappearance of the band
at 610 nm in a sample kept in the dark at 20ºC. After that, the
sample was irradiated at the same temperature (Figure 6, b). As
it can be seen, the cyclization takes place even in the dark,
although it is considerably slower (estimated half-life of ca. 18h).
However, once light is activated, the closing process takes place
more rapidly with a measured half-life for the open form of 1422 ±
1
6
s (i.e. approximately 0.4 hours). Thus, it is evident from Figure
b, that the isomerization and closing process for these
compounds takes place either by effect of light or temperature,
although it is clearly faster under irradiation in the band maximum.
From the practical point of view, the thermal cyclability of these
compounds may hamper their use in some applications. This will
be further studied in the next section.
Figure 6. a) Isomerization of D1-Br-A1 upon irradiation with 610 nm light in
During the photoisomerization, it was also noted the formation of
non-specific bands between 300 - 400 nm. While these bands
could be assigned to one or several intermediates along the
photoisomerization pathways, it may also be due to partial
decomposition. Indeed, an absorption peak lying in the
CH
2
Cl
2
.b) Monitoring of the disappearence of the open form in the dark at 20ºC
(
white background) and in the presence of 610 nm light (orange background).
.
6
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Chemistry - A European Journal
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Thermal equilibria
energy barrier for the cyclization is lowered, especially for h-
substituted compounds. This should imply a faster thermal
equilibrium in which the abundance of the different isomers will be
controlled by their relative energies, as it was experimentally
confirmed by this work. Most of the intermediates are stabilized,
especially the final closed forms. For both Br and Ph substituted
compounds, there is a clear stabilizing effect, higher for the h-
substituted compounds. This implies that once formed the closed
form, the reverse reaction to the open form will be much slower or
even not taking place for the π-substituted compounds, as
experimentally found here for the g-substituted Br and Ph DASAs.
As shown by our computational data, this result could be
extended to the h-substituted compounds, although their
synthesis was not performed.
Finally, we should note that the substituents proposed here do not
imply relevant steric effects on the calculated backbone geometry
along the whole cyclization path, when compared to unsubstituted
DASAs. Nevertheless, the incorporation of different, more bulky,
substituents, could in principle add significant steric effects to the
noticed electronic ones, and should be therefore carefully
evaluated.
These newly prepared DASAs feature excellent optical properties.
Nevertheless, the complete switching mechanism implies not only
the formation of the closed form upon irradiation, but also the
reversible recovery of the open form under different stimulus,
usually thermal activation. This type of switches has been
reported to have a marked sensitivity to external factors such as
the solvent, but also to slight modifications in their structure.[5,24]
Thus, we then aimed to check the reversibility of these
compounds under different stimuli. For all π-bridge substituted
compounds D-X-A, the photoreaction from the open to the closed
form is performed swiftly under red light. However, these
compounds showed no appreciable reversibility back to the open
form even when different reaction conditions were tried. After
cyclization, the open form could not be recovered by the use of
apolar solvents (toluene, dichloromethane), heating (closed forms
were found to be thermally stable at reflux temperature of different
solvents) or light (using the wavelength of maximum absorbance,
ca. 350 nm). These results have two clear implications. First, as
these compounds could not be reverted to the initial state, their
use as switches is seriously hampered. Second, the thermal part
of the switching mechanism is modified somehow by the π-bridge
substitution to strongly stabilize the closed form.
In the application point of view, especially considering their
capacity to be covalently anchored and their solvent dependence,
π-bridge substituted DASAs could be proposed as fine-tuned
In order to further explore these effects, we computed all the
ground state stationary points along the complete path for the π-
bridge substituted Br and Ph DASAs in both the g- and h-
positions (Figure 7).
[21]
molecular probes or as novel substrate for DASA’s reactivity.
Also, depending on the type of closed form that is generated
(
zwitterionic or non-zwitterionic, see Figure 7), new applications
could be envisioned taking advantage of the infrared DASA
chemical transformation as a first step of a sequence. An
inspirational example could be retinal in visual rhodopsin, where
retinal undergoes cis-trans photoisomerization within the opsin
cavity as a first step, followed by charge migration from retinal to
opsin, and requiring replacement of a new retinal molecule within
opsin to start a new cycle.[38] In any case, the detailed mechanistic
information gathered here, should be useful in the design of new
species and their application in different systems.
Steric effects
Until now, we have mainly considered electronic effects, due to
the donor-acceptor intrinsic nature of this type of compounds.
Nevertheless, it should be noted that, especially when substituting
at g position, some steric constraints could arise, mainly due to
potential 1,3 allylic strain generated when converting
(
photochemically, Figure 5, or thermally, Figure 7) the Z isomer
into the E isomer. Therefore, we have analyzed both optimized
structures (i.e. the respective calculated energy minima) by
comparing D1-(g)H-A1, D1-(g)Br-A1 and D1-(g)Ph-A1. Driven
by previous studies on Z/E isomerization,[3,38] we have focused on
two key geometrical parameters: the C(f)=C(g) bond length and
the C(e)-C(f)=C(g)-C(h) dihedral angle (see Figure 1 for atom
labeling). We have found that an increase of the steric constraint
corresponds to a notable increase of the C(f)=C(g) bond length
for both Z (-H: 1.402 Å, -Br: 1.412 Å, -Ph: 1.419 Å) and E (-H:
Figure 7. DFT ground state stationary points (minima and connecting transition
states) along the mechanistic path for D1-X-A1, computed in toluene. a) Br atom
in positions g and h. b) Ph group in positions g and h. For comparison purposes,
the path computed for the unsubstituted DASA is also shown (black line).
1.406 Å, -Br: 1.416 Å, -Ph: 1.424 Å) isomers. At the same time,
the out-of-plane distortion also increases with the steric constraint,
being the C(e)-C(f)=C(g)-C(h) dihedral angle 179.5º (-H), 179.1º
As can be seen, several key aspects are preserved when
including substituents in the π-bridge. For instance, the high
energy barrier for Z / E isomerization (thus, requiring light
activation) is maintained, although it is reduced by 8 and 5
kcal/mol for the Br and Ph derivatives, respectively. Similarly, the
(
-Br) and 173.1º (-Ph) for the Z isomer, while 0.05º (-H), 0.64º (-
Br) and 12.1º (-Ph) for the E isomer.
The much higher influence of Ph can be associated with the fact
that the aromatic ring is not perpendicular to C(f)=C(g), but it
7
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Chemistry - A European Journal
10.1002/chem.202004988
FULL PAPER
forms an angle of 66 degrees (Z isomer) and 63 degrees (E Experimental Section
isomer). This affects more the E isomer due to the 1,3 allylic strain.
In terms of reactivity, on one hand these data support the
photochemical mechanism proposed in Figure 5, since a lower
C(f)=C(g) double bond character and a more pronounced C(e)-
C(f)=C(g)-C(h) pre-twist can suggest an explanation of the lower
excited state barrier found when substituting -H with -Br, and even
more the absence of such a barrier when X= -Ph (i.e. D1-(g)Ph-
A1 is expected to undergo the fastest photoisomerization).
On the other hand, we do not find a clear relationship relying steric
effects with thermal reactivity, since the E isomer, with respect to
the Z isomer, is more destabilized for the unsubstituted compound
Materials and methods.
All solvents were distilled prior to use. Some solvents such as
dichloromethane, diethyl ether, acetonitrile and tetrahydrofuran were
distilled with the solvent purification system Pure solvtm 4-MD. Thin Layer
chromatography (TLC) was performed using Polygram Sil G/UV254 F254
plates (0,2mm silica gel layer with fluorescence indicator on pre-coated
plastic sheets). Column chromatography was carried out with silica gel
(
230-240 mesh) as stationary phase. Mixtures of hexane/ethyl acetate
were used as mobile phase. H and 13C spectra were recorded on a Bruker
1
ARX-300 and/or a Bruker Avance 400 spectrometers. CDCl and CD OD
3
3
(
4.5 kcal/mol), followed by -Ph (4.4 kcal/mol) and -Br (1.6
have been used as the usual deuterated solvent with TMS as internal
standard. Chemical shifts are given in ppm and coupling constants in hertz.
Absorption molecular spectra were recorded on two distinct equipment:
HP-8453A UV-VIS-NIR diode array spectrophotometer (190-1100 nm) or
HP 8451A diode array spectrophotometer (190-820 nm). All the
experiments were carried out in quartz cuvettes (1 cm path length).
kcal/mol) substitutions.
In this case, we should therefore conclude that the driving force is
mainly of electronic nature, as analyzed in the previous sections.
Electrospray mass spectra were recorded on
a HP 5989B mass
spectrometer with an HP59987A interface in positive-ion mode. High
resolution mass spectrometry was performed in a HP Bruker Microtof-Q
with an Apollo II electrospray source in positive-ion mode.
Conclusion
We have studied a new generation of DASAs by modifying the π-
bridge through chemical substitution at the g and h positions.
Synthesis and Characterization of D1-Br-A1
Barbituric acid (3 mmol, 470 mg,
1 equiv.) and 3-bromofuran-2-
Especially,
h substituted DASAs were investigated only
carbaldehyde (3.3 mmol, 580 mg, 1.1 equiv.) were dissolved in water and
reacted at 70º C for 2 hours. After that, the reaction was cooled to rt and
the solid was isolated by filtration. The product did not need further
purification and could be used immediately in the next step. The adduct
computationally, while the most suitable g substituted DASAs
were studied both computationally and experimentally. This new
π-bridge substituted DASAs can be easily synthetized with high
yields and the synthetic path could be straightforwardly modified
to get novel analogues or to link these compounds to different
substrates. Regarding the optical properties, both the D-A and π-
bridge substitution effects were found to be additive to the spectral
energy shift, allowing to reach infra-red absorption. More in
general, methodology shown here allows to predict from scratch
the optical properties of these compounds and their
photochemical behavior. The acceptor electronic nature of these
π-substituted DASAs was established, including their limit to
preserve the photoswitching function.
(0.1 mmol, 31.3 mg, 1 equiv.) was dissolved in THF at 0ºC, then
diheptylamine (0.11 mmol, 23.43 mg, 1.1 equiv.) was added dropwise.
Promptly, the solution changed its color from yellow to blue-purple. The
reaction was stirred for additional 15 minutes. The final compound was
purified by HPLC as a bluish solid (60%), using a linear gradient 95-0%
water (containing 0.1% TFA) / acetonitrile over a period of 30 minutes. The
open form was found to be unstable in solution at rt and clear NMR data
could not be obtained. The following NMR data correspond to the closed
form. 1H NMR (300 MHz, Methanol-d
) δ 7.91 (d, J = 2.3 Hz, 1H), 4.81
dd, J = 3.6 Hz, J = 2.3 Hz, 1H), 3.94 (d, J = 3.6 Hz, 1H), 3.23 (m, 10H),
4
(
.66 (m, 4H), 1.29 (m, 16H), 0.90 (m, 6H). 13C NMR (75 MHz, MeOD) δ
1
Clearly, substitution in the π-bridge has profound implications for
the properties of DASAs and their switching mechanism. On one
hand, a significant and consistent red-shift in the absorption
maxima could be achieved. As this effect was found to be additive,
substitution in the bridge could be combined with other structural
modifications to allow for the use of long wavelengths with these
compounds. Nevertheless, experiments on g-substituted DASAs
reveal, that cyclization in the dark is relevant and may constitute
a significant reaction pathway.
(ppm) 198.6, 164.9, 154.6, 151.3, 132.8, 85.3, 67.8, 45.3, 32.7, 32.7, 29.9,
29.8, 28.1, 27.5, 27.4, 27.3, 23.6, 14.4. UV-VIS (CH Cl , open form): λ
Br [M+ H]+
2
2
-
1
-1
(
nm) 610 (ε = 15768 M cm ). EM-ES (+): calcd for C25
41 3 4
H N O
526.2280, found 526.2275.
Synthesis and Characterization of D1-Ph-A1
-Bromofuran-2-carbaldehyde (1.91 mmol, 335 mg,
dissolved in 1,2-dimethoxyethane (DME) and, to this solution was added
phenylboronic acid (7.64 mmol, 931 mg, equiv.),
3
1 equiv.) was
4
tetrakistriphenylphosphine palladium (0) (0,382 mmol, 446 mg, 0.2 equiv.)
and potassium carbonate (6.68 mmol, 474 mg, 3.5 equiv.). This mixture
was under reflux for 48 hours. The reaction was monitored by TLC and
when it was completed, the solvent was removed under reduced pressure.
On the other hand, this specific type of substitution has several
limitations that affect the switching process. As shown by the
computational results on the –NO
2
derivative, the
photoisomerization could be affected if the substituent is not
adequately chosen. Even if photoisomerization takes place with a
different set of substituents, thermal reversion is not possible in
these derivatives. Results shown herein allow for a detailed
description of the behavior of a new set of DASAs. However,
further efforts would be necessary to design and prepare different
π-bridge compounds that could maintain the excellent optical
properties while keeping reasonable switching ability.
2 2 2
Then, the crude was extracted three times with (CH Cl :H O) and the
organic fractions were collected. The solvent was removed and the pure
compound was purified by column chromatography on silica gel using as
eluent a mixture of hexane and ethyl acetate (4:1). In the next step, the
condensation of 3-phenylfuran-2-carbaldehyde (0.87 mmol, 150 mg, 1.1
equiv.) and barbituric acid (0.79 mmol, 101 mg, 1 equiv.) was performed.
Both compounds were dissolved in water, and the reaction was heated at
70ºC for 2 hours. Then, the reaction was cooled to RT, and the resulting
solid was filtrated. This compound did not need further purification. Finally,
this adduct (0.1 mmol, 0,31 mg, 1 equiv.) was dissolved in THF at 0ºC,
then diheptylamine (0.11 mmol, 23.43 mg, 1.1 equiv.) was added dropwise.
Promptly, the solution changed its color from yellow to blue-purple. The
reaction was stirred for additional 15 minutes. The final compound was
purified by HPLC as a bluish solid (65%), using a linear gradient 95-0%
8
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Chemistry - A European Journal
10.1002/chem.202004988
FULL PAPER
water (containing 0.1% TFA) / acetonitrile over a period of 30 minutes. The
open form was found to be unstable in solution at rt and clear NMR data
λ
max values between TD-DFT and ADC(2) methods, although maintaining
a correct description of the electronic nature.
could not be obtained. The following NMR data correspond to the closed
1
form. H NMR (300 MHz, Methanol-d
1
(
4
) δ 7.84 (d, J = 7.7 Hz, 2H), 7.77 (s,
Solvent effects were taken into account by the Polarizable Continuum
H), 7.42 (d, J = 6.9 Hz, 3H), 4.93 (s, 1H), 3.99 (d, J = 3.9 Hz, 1H), 3.27
s, 10H), 1.70 (s, 4H), 1.28 (s, 16H), 0.89 (s, 6H). 13C NMR (75 MHz,
Methanol-d ) δ 204.2, 165.2, 154.7, 148.5, 146.8, 131.9, 130.6, 129.5,
28.9, 86.0, 66.7, 52.7, 49.9, 47.9, 32.7, 29.8, 28.4, 27.5, 25.8, 23.6, 14.38.
UV-VIS (CH
calcd for C31
Model using the Integral Equation Formalism variant (IEF-PCM).[43,44]
4
All DFT and TD-DFT calculations were performed with the Gaussian16[45]
1
suite of programs, while ADC(2) calculations were carried out with the
Turbomole 7.3[46] program.
-
1
-1
2 2
Cl , open form): λ (nm) 590 (ε = 18040 M cm ). EM-ES (+):
+
46 3 4
H N O [M+ H] 524.3488, found 524.3482.
Synthesis and Characterization of D2-Br-A2
,3-indandione (1 mmol, 132 mg, 1 equiv.) was dissolved in water. To this
Acknowledgements
1
solution, 3-bromofuran-2-carbaldehyde (1.1 mmol, 175 mg, 1.1 equiv.)
was added dropwise and then, the mixture was heated at 70ºC for 2 hours.
After that, the reaction was cooled to rt, and the resulting precipitate was
filtered and used without further purification. This adduct (0.1 mmol, 30 mg,
This research was supported by the MINECO/FEDER (CTQ2017-
87372-P). D. M.-L. and E. S.-A are grateful to Universidad de La
Rioja for their doctoral fellowships, and M. M. for a postdoctoral
fellowship.
1
equiv.) was dissolved in THF at 0ºC, then N,N-diheptylindolin-5-amine
(0.11 mmol, 36 mg, 1.1 equiv.) was added dropwise. Promptly, the solution
changed its colour from yellow to blue-purple. The reaction was stirred for
additional 15 minutes. The final compound was purified by HPLC as a
bluish solid (40%), using a linear gradient 95-0% water (containing 0.1%
TFA) / acetonitrile over a period of 30 minutes. The open form was found
to be unstable in solution at rt and clear NMR data could not be obtained.
Keywords: DASA • photochemistry • thermal equilibrium •
photoswitches
1
The following NMR data correspond to the closed form. H NMR (300 MHz,
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Methanol-d ) δ 8.07 (d, J = 7.9 Hz, 1H), 8.01 (dd, J = 6.7, 1.8 Hz, 1H), 7.97
(s, 1H), 7.92 (s, 2H), 7.25 (s, 1H), 6.96 (s, 1H), 6.47 (d, J = 8.6 Hz, 1H),
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frequency calculation to ensure that the optimized structure corresponds
0
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1
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with the experiment. Concerning the TD-DFT calculations, a benchmark
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used also for excited state calculations (Table S1). The effect of the basis
set on the absorption energy was also benchmarked, finally selecting the
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g)X-A2 compounds, during irradiation, was analyzed by Natural Bond
[
42]
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Chemistry - A European Journal
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A new set of Donor-Acceptor Stenhouse adducts was computationally studied and prepared through the substitution on the -bridge.
This allows for the modification of absorption, photocyclization and relative stability of the isomers. This change provides a consistent
red-shift in the absorption maximum and a structural entry point for subsequent derivatization, although the reversibility is blocked.
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1
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