Trace mild acid-catalysedZ→Eisomerization of norbornene-fused stilbene derivatives: intelligent chiral molecular photoswitches with controllable self-recovery

Article abstract of DOI:10.1039/d0sc05213b

Stilbene derivatives have long been known to undergo “acid-catalyzed”Z→Eisomerization, where a strong mineral acid at high concentration is practically necessary. Such severe reaction conditions often cause undesired by-reactions and limit their potential application. Herein, we present a trace mild acid-catalyzedZ→Eisomerization found with stilbene derivatives fused with a norbornene moiety. By-reactions, such as the migration of the C-C double bond and electrophilic addition reactions, were completely inhibited because of the ring strain caused by the fused norbornene component. Direct photolysis of theEisomers at selected wavelengths led to theE→Zphotoisomerization of these stilbene derivatives and thus constituted a unique class of molecular switches orthogonally controllable by light and acid. The catalytic amount of acid could be readily removed, and theZ→Eisomerization could be controlled by turning on/off the irradiation of a photoacid, which allowed repeated isomerization in a non-invasive manner. Moreover, theZisomer produced by photoisomerization could spontaneously self-recover to theEisomer in the presence of a catalytic amount of acid. The kinetics ofZ→Eisomerization were adjustable by manipulating catalytic factors and, therefore, unprecedented molecular photoswitches with adjustable self-recovery were realized.

Full text of DOI:10.1039/d0sc05213b

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Trace mild acid-catalysed Z / E isomerization of
norbornene-fused stilbene derivatives: intelligent
chiral molecular photoswitches with controllable
self-recovery†
Cite this: Chem. Sci., 2021, 12, 2614
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
b
a
Taotao Hao,a Yongsheng Yang,a Wenting Liang,
Chunying Fan,a Xin Wang,
*
a
a
a
Wanhua Wu,
and Cheng Yang
Xiaochuan Chen,
*
Haiyan Fu,
Hua Chena
*
a
Stilbene derivatives have long been known to undergo “acid-catalyzed” Z / E isomerization, where
a strong mineral acid at high concentration is practically necessary. Such severe reaction conditions
often cause undesired by-reactions and limit their potential application. Herein, we present a trace mild
acid-catalyzed Z / E isomerization found with stilbene derivatives fused with a norbornene moiety. By-
reactions, such as the migration of the C]C double bond and electrophilic addition reactions, were
completely inhibited because of the ring strain caused by the fused norbornene component. Direct
photolysis of the E isomers at selected wavelengths led to the E / Z photoisomerization of these
stilbene derivatives and thus constituted a unique class of molecular switches orthogonally controllable
by light and acid. The catalytic amount of acid could be readily removed, and the Z / E isomerization
could be controlled by turning on/off the irradiation of
a photoacid, which allowed repeated
Received 21st September 2020
Accepted 28th December 2020
isomerization in a non-invasive manner. Moreover, the Z isomer produced by photoisomerization could
spontaneously self-recover to the E isomer in the presence of a catalytic amount of acid. The kinetics of
Z / E isomerization were adjustable by manipulating catalytic factors and, therefore, unprecedented
molecular photoswitches with adjustable self-recovery were realized.
DOI: 10.1039/d0sc05213b
rsc.li/chemical-science
On the other hand, protic acids have long been known to
promote the Z / E isomerization of stilbene derivatives via an
A-SE2 mechanism.11 However, such acid-catalysed isomeriza-
Introduction
The Z–E isomerization of stilbene derivatives has been known
for over one century,1 and it has now become one of the most
representative prototype reactions applied to molecular
machines,2 such as molecular motors,3 photoswitches,4 molec-
ular ratchets5 and half adders,6 mainly due to the signicant
geometrical changes that occur during reversible isomerization.
The Z–E isomerization of stilbene derivatives has been exten-
sively investigated and is currently known to be triggered by
physical (light or temperature) or chemical activators, such as
boron uoride-ether complexes,7 acids8 and bromine, in
conjunction with light.9 Photoisomerization is most commonly
applied in molecular machines due to its non-invasive and
convenient operating features.10
tion commonly requires the use of strong mineral acids at high
concentrations as the solvent, which signicantly limits the
application of this method due to low functional group toler-
ance and undesired side reactions caused by the harsh reaction
conditions.12 The use of a large excess of strong acids is
particularly unfavourable for molecular machines in terms of
non-accumulative repeating operations. For an ideal chemically
activated molecular machine, the chemical inductor should be
mild, work in a catalytic amount, and be readily removable/
deactivated.13 To our knowledge, such molecular machines
based on stilbene derivatives have not yet been documented.
Although stimulus-responsive molecular switches have been
widely constructed based on chemical and physical stimuli,
including light,14 electrochemistry,15 temperature,16 pH,17 solvent,18
and additives,19 and we have established rotating switches con-
sisting of molecular universal joints based on thermal20 and redox
stimuli,21 the molecule switches obtained by such sophisticated
design and synthesis are generally responsive to only one of the
above stimuli, and few molecular switches can be manipulated by
two independent effectors,2a,f,13 which would offer a higher level of
aKey Laboratory of Green Chemistry & Technology, College of Chemistry, Sichuan
University, 29 Wangjiang Road, Chengdu 610064, China. E-mail: yangchengyc@scu.
edu.cn
bInstitute of Environmental Science, Department of Chemistry, Shanxi University,
Taiyuan 030006, China
† Electronic supplementary information (ESI) available. CCDC 5Z: 1949916,
5E1/5E2: 1949886 and 6E: 1949917. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d0sc05213b
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control.22 Herein, we report Z / E isomerization of a novel kind of was synthesized by a one-pot stereoselective Pd-catalysed
stilbene derivative unprecedentedly catalysed by even a trace domino reaction (Scheme S6†).23 As illustrated in Fig. 2, a new
amount of a mild organic acid. Double control of the Z–E isom- set of proton signals separate from the original signals of 5Z
1
erization by the two independent external stimuli of light and acid, appeared during the H NMR spectral measurements of 5Z in
so-called orthotropic control,22a was realized (Fig. 1a), resulting in CDCl3 (Fig. 2a and b). This chemical conversion progressed
intriguing and advanced functions in these molecular photo- smoothly over time even under strict light-proof conditions,
switches. The unique self-recovery behavior has been widely pre- which is unexpected because both the Z and E forms of stilbene
sented in nature, e.g. Nymphaea tetragona, which self-recover derivatives are, in general, thermally stable. This observation
between OFF (night) and ON (day) with light and chemical stimuli prompted us to perform further NMR investigations in different
(Fig. 1c). Hence, the overall dynamic properties of the newly deuterated solvents. Indeed, 5Z has excellent thermal stability,
prepared stilbene derivative switches mimicked the behaviors of and no chemical conversion was observed in the 1H NMR
the macroscopic Nymphaea tetragona well.
spectra by heating 5Z in DMSO-d6 at 130 ꢀC for over 24 h (Fig. 2d
and e). Further NMR experiments revealed that spontaneous
chemical conversion could be observed only in newly opened
CDCl3 but not in other deuterated solvents. To examine the
possible effects of oxygen, we deoxygenated the solutions by
bubbling nitrogen into the CDCl3 solution, which inhibited the
reaction. Surprisingly, bubbling oxygen did not facilitate the
chemical conversion but instead inhibited it (Fig. S41m†);
therefore, we speculated that removal of volatile gas dissolved in
CDCl3 by bubbling either N2 or O2 should be responsible.
The newly formed compound was then isolated and char-
acterized as the E isomer 5E on the basis of HR mass and 2D
NMR spectroscopic studies (Fig. S15–S18†). We then considered
that the tiny amount of DCl due to the photodecomposition of
CDCl3 may play a role in the spontaneous isomerization of 5Z.
To conrm this hypothesis, we added a drop of aqueous DCl to
the CDCl3 solution of 5Z at room temperature and observed that
isomerization rapidly produced 5E in quantitative yield, as
Results and discussion
Z-to-E isomerization of norbornene-fused stilbene derivatives
catalysed by a trace amount of mild acid
The present study was essentially prompted by a casual obser-
vation of spontaneous chemical conversion of 5Z (Fig. 1), which
1
determined by monitoring the H NMR spectral changes.
Alkenes are known to undergo electrophilic addition with
mineral acids.24 Alternatively, (Z)-stilbene derivatives, mediated
by a strong mineral acid,8 isomerize to the E isomers, where
proton transfer to the C]C double bond is the rate-determining
step. The latter was dened as acid-catalysed isomerization,
even though a large excess (commonly >50 wt%) of the strong
mineral acids was generally required. The above observation is
Fig. 1 (a) Orthotropic control of the Z–E isomerization of norbor-
nene-fused stilbene derivatives by acid and light, (b) the chemical
structure of the stilbene derivatives, and (c) schematic representation
of the self-recovery process. The overall dynamic properties of the
norbornene-fused stilbene derivative switches mimic those of the
macroscopic Nymphaea tetragona, which self-recover between OFF
(night) and ON (day) under light and chemical stimuli.
Fig. 2 1H NMR spectra of 5Z (a) in CDCl3 sealed with K2CO3, (b) in
newly opened CDCl3 (which contains a tiny amount of acid), (c) with
addition of 0.5 eq. of TFA to CDCl3, (d) in DMSO-d6 measured soon
after preparing the solution, and (e) in DMSO-d6 after heating at 130 ꢀC
for 24 h. All measurements were performed at room temperature.
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thus unprecedented and prompted us to test the catalytic effect
of different kinds of acids. The isomerization of 5Z occurred
smoothly in the presence of 0.5 eq. of mineral acid, including
HCl, HBr and H2SO4. Surprisingly, when a milder, organic acid,
triuoroacetic acid (TFA), was used instead of DCl to CDCl3, 5Z
also underwent isomerization to give 5E within 30 min (Fig. 2c).
Catalytic isomerization could be accomplished in the presence
of 1% eq. of TFA by increasing the reaction time (Table S1†). The
isomerization also proceeded smoothly in the presence of
a weaker organic acid, including diuoroacetic acid and
dichloroacetic acid.
To understand this exceptional phenomenon, we synthe-
sized a series of stilbene derivatives 4–10 (Fig. 1b) through
a reported method,23 the possible absolute congurations of
which are shown in Scheme S6,† and we further examined their
proton-catalysed isomerization behaviour. The addition of an
equal amount of TFA to the CDCl3 solution of 1Z did not cause
any isomerization even aer standing at room temperature for 1
week (Fig. S37c†). The same was true with 2Z (Fig. S38c†).
However, the addition of TFA to the solution of 3Z not only
afforded Z / E isomerization product 3E but also led to
migration of the C]C double bond to give endocyclic alkene
3C. A 3E : 3C ratio of 37 : 63 was nally achieved when equi-
librium was reached aer four days (Fig. S39e†). In contrast, 4Z–
6Z gave the E isomers in quantitative yield under the catalysis of
TFA (Fig. S40–S44†), demonstrating a unique chemical reaction
behaviour due to the introduction of the norbornene moiety.
1
1
Fig. 3 (a) H NMR spectra of 5E in CDCl3, (b) H NMR spectra of the
solution after the addition of 2.0 eq. of DCl to the solution of 5Z in
CDCl3 for 2 h, and (c–f) 1H NMR spectra of the solution after the
addition of 20 eq. of DCl to the solution of 5Z in CDCl3 for (c) 2 h, (d)
16 h, (e) 24 h, and (f) 48 h.
freshly prepared 3Z spontaneously isomerized even in the
absence of an acid to give endocyclic alkene 3C and 3E, and
a 3C/3E ratio of 89 : 11 was achieved aer six days (Fig. S39c†).
In contrast, all norbornene-fused stilbenes 4Z–10Z were stable
and did not show spontaneous migration of the C]C double
bond or isomerization in solution. Even in pure TFA, 5Z led to
5E only, and no formation of 5C or other by-products was
observable (Fig. S41k†), which was attributed to the intense ring
strain25 in 5C due to the fused norbornene moiety. In contrast,
1Z and 3Z produced complicated unknown products in TFA
(Fig. S37e and S39f†).
The quantitative conversion of 5Z to 5E under proton-
catalysis conditions was slightly unexpected; in particular, the
much larger fused norbornene moiety was expected to cause
greater steric repulsion upon rotation of the benzene ring to the
norbornene side in the E isomer. To clarify this nding, we grew
single crystals of both 5Z and 5E and carefully analysed the
corresponding bond and angle parameters through X-ray
single-crystal studies (Fig. 4). The a angle (130.53ꢀ) of 5Z was
larger than the b angle (128.12ꢀ) of 5E, implying a smaller steric
repulsion caused by the norbornene side than the benzene side.
The dihedral angles q between the methoxybenzene plane
(blue) and the methylbenzene plane (red) were 56.15ꢀ for 5Z and
36.48ꢀ for 5E (Fig. 4b and e), which clearly indicated that the E
isomer had better coplanarity and therefore more effective
conjugation than the Z isomer.
The mechanism of proton-catalysed isomerization and
quantitative Z-to-E conversion
To gain insight into the mechanism of such unprecedented
proton-catalysed isomerization, we monitored the 1H-NMR
spectral change of 5Z in the presence of DCl. As illustrated in
Fig. 3b, the addition of DCl into the CDCl3 solution of 5Z
afforded 5E quantitatively within 2 h to give a relative integra-
tion of Ha0 of 0.35 (Fig. 3a and b), demonstrating that the proton
was partially deuterated. On the other hand, the addition of DCl
to the CDCl3 solution of 5E did not yield any 5Z, but the inte-
0
0
grated intensity of Ha also decreased with time (Fig. S41†),
indicating that the Ha proton of 5E was also exchanged by
deuterium. These results implied that both Z and E isomers
experienced a carbenium ion intermediate formed by proton
transfer. Interestingly, when excess DCl (20 eq.) was added to
0
5Z, the protons Hd and Hf0 were also replaced by deuterium at
approximately 82% aer 48 h (Fig. 3c–f). Based on these
observations, we propose that the proton-catalysed isomeriza-
tion of 5Z is accomplished through the process shown in
Scheme S10.† Thus, D+ transfers to the C]C double bond of 5Z
rst to give intermediate 5I. Carbenium ion 5I is then inverted
to the relatively stable species 5II. Elimination of Ha or D from
5II leads to the E isomer. Further deuterium experiments indi-
0
cated that the deuteration of Hd and Hf0 is a slow process and
0
that Hd and Hf0 can be completely replaced by D only in the
Moreover, detailed analyses of the single crystal of 5E
revealed that the norbornene moiety leans to one side of the
presence of a large excess of DCl.
ring. H18 deviated from the C]C plane (orange) at a distance of
A comparison between 5Z and 3Z enabled us to better
understand the role of the norbornene moiety. Surprisingly,
˚
1.613
A
(Fig. 4f). In contrast, the H4 proton on the
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could be ascribed to the stronger steric repulsion between the
methyl group and the methoxy benzene ring in the E isomer,
which makes the E-type carbocation much less stable.
Theoretical study of the proton-catalysed reaction
Styrene is known to undergo an addition reaction with TFA.26
However, no addition products could be detected with the
norbornene-fused stilbene derivatives (Fig. S40–48 and S96†).
To understand this interesting phenomenon, two possible
reaction routes were compared based on density functional
theory (DFT) calculations. The computational simulation sug-
gested that 4Z and TFA formed an activated complex com_4Z,
which led to the transition state TS-4Z-TFA and then the inter-
mediates 4Z-INT. The bond rotation of 4Z-INT gave 4E-INT-1
and 4E-INT-2, which pass through the transition states TS-4E-
TFA and TS-4C-TFA, respectively, to nally give 4C and 4E
(Fig. 5). The calculations demonstrated that the Gibbs free
energy of 4Z is higher by 2.7 kcal molꢁ1 than that of 4E but is
lower by 9.0 kcal molꢁ1 than that of 4C, which is entirely
consistent with the results of quantitative conversion to 4E
under acid catalysis.
In particular, TS-4C-TFA showed an extremely high Gibbs
energy of 30.5 kcal molꢁ1, further supporting the complete
inhibition of the migration of the C]C double bond pathway.
On the other hand, 3C showed a Gibbs energy lower than that of
3Z by ꢁ5.0 kcal molꢁ1 and lower than that of 3E by
ꢁ3.1 kcal molꢁ1 (Fig. S97†), agreeing well with the high ratio of
the endocyclic alkene product obtained with 3Z. Thus, the
presence of a fused norbornene signicantly increased the
energy of both 4C and the intermediates affording 4C and hence
completely impeded the migration of the C]C double bond
route. On the other hand, the Gibbs energies of the rate-
determining step to the transition state (TS-1Z-TFA vs. TS-3Z-
Fig. 4 Single-crystal structures of 5Z and 5E (thermal ellipsoid plots at
the 50% probability level). The structural parameters for 5Z (a–c) and
5E (d–f).
methoxybenzene ring lies almost completely in the plane, which
unavoidably causes strong steric interaction with the methyl-
benzene ring with cis conguration (Fig. 4c). These results
accounted for the extremely high Z / E conversion in these
norbornene-fused stilbenes. Similar differences could be found
by comparison of the corresponding parameters obtained from
the single-crystal structures of 6Z and 6E (Fig. S49, Table S2†).
On the other hand, 7Z and 8Z, in which the alkylene proton
was replaced by a methyl group, also underwent isomerization
in the presence of TFA but gave the E isomers in 52% and 10%
yields, respectively, when equilibrium was reached (Fig. S45 and
S46†). The lower E/Z ratio for the isomerization of 7Z and 8Z
Fig. 5 Calculated energy profiles of 4Z to 4E and 4C.
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TFA) in the 1Z and 3Z pathways were 41.5 kcal molꢁ1 and different, and the isomerization could be conventionally
26.5 kcal molꢁ1 (Fig. S97†), respectively. The more conjugation detected from UV-vis spectral changes. As illustrated in Fig. 6a,
of the benzene ring with vinyl in 3Z1d as well as the tension25 5Z exhibited one broad absorption band that peaked at lmax
¼
resulting from the fusion of ve-membered rings should be 298 nm. The addition of TFA to the solution of 5Z in DCM led to
responsible for the much lower Gibbs energies, and this an apparent increase in absorption to nally give three clear
accounted for the lack of observation of isomerization of 1Z vibrational bands with peaks at 307, 321, and 337 nm, with two
under mild acid conditions.
isosbestic points (Fig. S64†). It should be noted that, as a very
diluted substrate 5Z is required for UV-vis detection, we used
superstoichiometric TFA to accelerate the isomerization.
However, a catalytic amount of TFA could indeed lead to
quantitative isomerization of 5Z at high concentrations as can
be seen in Fig. 2c, S41, S92 and Table S1.† The isomerization
rate increased with the amount of acid, and the extent of Z / E
conversion could be nely manipulated by controlling the
amount of acid and the reaction time. In the presence of a large
amount of TFA, isomerization was completed within a few
seconds (Fig. 6b). The acid-catalysed isomerization proceeded
smoothly even at temperatures as low as ꢁ40 ꢀC (Fig. S69†). The
isomerization rate could be nely tuned by varying the
temperature, and increasing the temperature led to an increase
in the reaction rate (Fig. S68†). By manipulating the above
Factors inuencing the proton-catalysed isomerization and
non-accumulative cycles
The rate of the proton-catalysed isomerization process was
clearly inuenced by the solvents used. For example, the
isomerization was completed within 2 h in DCM and CHCl3 in
the presence of 0.5 eq. of TFA (Table S1, Fig. S67†) but required
a longer time in n-hexane and toluene and several days in
CH3CN. However, when THF, MeOH, EtOH, or 1,4-dioxane was
used as the solvent, no isomerization could be observed, even
aer 72 h, which was attributed to the stronger solvation of
protons in these polar solvents.27
The UV-vis spectra of the Z and E isomers of these
norbornene-fused stilbene derivatives are distinctively
Fig. 6 (a) Changes in the UV-vis spectra of 5E (5 ꢂ 10ꢁ5 M in DCM) upon 337 nm UV irradiation for different time periods, (b) changes in time for
the absorbance at 337 nm after the addition of different amounts of TFA to the solution of 5Z (1 ꢂ 10ꢁ4 M in DCM), and (c) control of the
isomerization rate of 5Z by the addition of TFA and Et3N. Conditions: stage I: addition of TFA (VTFA : VDCM ¼ 0.0033); stage II: addition of Et3N;
stage III: addition of TFA (VTFA : VDCM ¼ 0.0066); stage IV: addition of Et3N; stage V: addition of TFA (VTFA : VDCM ¼ 0.0099). (d) Changes in the
proportion of 5E with eosin Y as the photoacid catalyst (0.1 mmol 5Z and 0.008 mmol eosin Y in 3.0 ml DCM) and the proportion of 5E
determined by HPLC analysis.
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factors, the isomerization rate could be exibly tuned from of any base, the on/off switching cycle could be repeated more
seconds to days. than 25 times (Fig. S70–S72†). Notably, certain Z forms of
Seminal pH-activated Z–E switches based on the isomeriza- molecular switches, such as azobenzene derivatives, can ther-
tion of hydrazone28 or enamine29 have recently been explored by mally isomerize to the E form, but the thermal isomerization
using a protic acid as the hydrogen bonding regulator, in which kinetics primarily depend on the inherent molecular properties
the acid acted as a reactant and the reverse process required the and are difficult to manipulate.33 In contrast, both the Z / E
addition of a base to neutralize the acid, which unavoidably led and E / Z isomerization kinetics of the present molecular
to the generation of waste and side products that eventually switch system are nely controllable. As exemplied in Fig. 7, by
hindered the cycle process.30 In the present system, isomeriza- changing the concentration of TFA, the recovery time could be
tion could be accomplished in the presence of only a catalytic conveniently manipulated in a wide time range and therefore
amount of protic acid, which allowed stopping the Z / E should signicantly improve the application scenarios of self-
isomerization by adding a small amount of alkali. As illustrated recovery.
in Fig. 6c, the Z / E isomerization of 5Z can be switched on/off
The potential functional group tolerance was further inves-
by adding TFA and Et3N sequentially. On the other hand, the tigated considering the mild catalysis conditions of the present
TFA or HCl could be easily removed from the solution simply by Z / E isomerization system. For example, compound 10Z,
bubbling with nitrogen (Fig. S94 and S95†), thus avoiding the which bears an electron-donating diphenylamino group and an
accumulative contamination commonly suffered in chemically electron-withdrawing cyano group, underwent isomerization
induced reversible processes.
smoothly under acid-catalysis conditions. The electronic push–
Moreover, catalytic isomerization by mild acids allowed pull effect extends the absorption of 10Z to the visible range,
another alternative for non-invasive control by the use of showing an absorption peak at 400 nm, which is a 102 nm
a photoacid. The Z / E isomerization of 5Z was examined in bathochromic shi compared with the parent molecule 4Z
the presence of 8% eq. of a commercial photoacid, eosin Y, (Fig. S59†). A blue uorescence emission was observed for 10Z
which releases protons upon photolysis but returns to a neutral upon excitation at 380 nm. Upon the addition of TFA to 10Z in
species in the dark.31 Indeed, 5Z underwent Z / E isomeriza- DCM, the UV/vis and PL (photoluminescence) spectra showed
tion when the system was photoirradiated with a 455 nm OLED, obvious changes to nally give 10E exclusively (Fig. S87 and
and the isomerization could be stopped by simply turning off S88†). The interconversion between 10E and 10Z could be
the light (Fig. 6d and S91–S93†).
conducted by acid and visible light for multiple cycles without
any loss in performance. The tolerance of additional functional
groups, such as 6Z and 10Z, makes it possible to extend the
application of this series of compounds through further deriv-
Molecular switch orthogonally controlled by light and acid
The much stronger absorption of 5E in the long-wavelength atization or functionalization.
range is consistent with the better conjugation properties of
5E, and E / Z photoisomerization could be performed by
selective excitation at the absorption peak of 5E. As shown in
Chiral molecular switch
Norbornene-fused stilbene derivatives are inherently chiral,
bearing four stereogenic centers (C7, C8, C11, and C12) in the
Fig. 6a, when 5E was irradiated with 337 nm UV light in DCM,
spectral changes opposite to those of the acid-catalysed isom-
erization (Fig. S64†) were observed. No further absorption
change could be seen aer irradiation for 30 min, and a pho-
tostationary state (PSS) ratio of 5Z : 5E ¼ 88 : 12 was determined
by HPLC analysis. These results revealed that the Z / E and E
/ Z isomerization of these norbornene-fused stilbenes could
be independently manipulated by acid and light, which makes
them highly unique molecular switches capable of orthogonal
control.22
This orthogonal control makes these molecular switches
promising for accomplishing complex functions. For example,
they can be used as intelligent molecular switches with
controllable self-recovery. Self-recovery switches have wide
applications from everyday illumination to microcirculation
and are critical in computers and accessory protection.32
A
crucial issue for self-recovery switches lies in the precise control
of the recovery timescale. The E-to-Z photoisomerization
kinetics of the present system can be conveniently controlled by
the irradiation light power. Even in the presence of a certain
amount of TFA, the E / Z photoisomerization of 5E proceeded
smoothly under intensive irradiation, and turning off the light
Fig. 7 Photoisomerization and recovery cycles of 5Z (5 ꢂ 10ꢁ5 M in
DCM) in the presence of TFA (VTFA : VDCM) (a) 0.0016, (b) 0.0033 and (c)
0.0066 at room temperature. The red and grey areas represent pho-
toisomerization (with 337 nm UV irradiation) and self-recovery
led to spontaneous Z / E isomerization. Without the addition processes, respectively.
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hexane and DCM (Vn-hexane : VDCM ¼ 80 : 20). The rst eluted
fraction 5E1 and the second eluted fraction 5E2 showed perfect
mirror-image circular dichroism (CD) spectra, demonstrating
an enantiomeric relationship (Fig. 8a). Single crystals of both
5E1 and 5E2 were successfully grown, and the X-ray diffraction
analyses demonstrated that the absolute congurations of 5E1
and 5E2 were (7S,8R,11S,12R) and (7R,8S,11R,12S) (Fig. 8c),
respectively. The CD spectrum of 5E1 showed clear negative
peaks at 236 nm and 300 nm and a positive peak at 252 nm.
Irradiation of enantiopure 5E1 at 337 nm led to apparent CD
spectral changes (Fig. 8a).
In particular, in the short wavelength region of 230–245 nm,
signal inversion from positive to negative was observed. The
original negative peak at 236 nm was sharply inverted to
a positive peak. This makes it possible to detect E/Z or on/off
switching on the basis of the CD sign rather than the spectral
intensity of common molecular switches. As illustrated in
Fig. 8b, alternately turning on and off the photoirradiation led
to switching of the CD signal at 236 nm between ꢁ4 and +6,
offering an unambiguous on/off signal read-out of the molec-
ular switch. On the other hand, optical rotation measurements
indicated that (7R,8S,11R,12S)-5E2 has a specic rotation of 119
deg cm3 dmꢁ1 gꢁ1 at 20 ꢀC with the sodium D line (589 nm) that
drastically changed to 226 deg cm3 dmꢁ1 gꢁ1 in the PSS under
photoirradiation at 337 nm but completely recovered aer
adding TFA. Therefore, non-destructive read-out of the switch-
ing state of 5 could thus be realized by detecting the specic
rotation changes.
Conclusions
In summary, we demonstrated an unprecedented catalytic Z-to-
E isomerization of the C]C bond in a novel kind of
norbornene-fused stilbene derivative triggered by a trace
amount of mild organic acids, which is in sharp contrast to the
concentrated strong mineral acids required in conventional
acid-catalysed isomerization of stilbenes. Such mild catalysis of
Z-to-E isomerization occurred quantitatively without any elec-
trophilic addition or migration of the C]C double bond by-
reaction. The unique properties of these stilbene derivatives
make them promising as a novel kind of molecular switch that
has the following advantages: the kinetics of Z / E isomeri-
zation could be controlled well by the acidity and concentration
of acids, temperature and solvents. Photoirradiation at
Fig. 8 (a) CD spectral changes of 5E1 and 5E2 (5 ꢂ 10ꢁ5 M) in n-
hexane upon irradiation with 337 nm UV light. (b) CD intensity changes
at 236 nm for 5E1 (5 ꢂ 10ꢁ5 M in n-hexane) in the presence of TFA
(VTFA : Vn-hexane ¼ 0.0066) after irradiation with 337 nm and then
turning off the light until recovery for five cycles. (c) Single-crystal a selected wavelength led to E / Z isomerization in excellent
structures of 5E1 and 5E2 (thermal ellipsoid plots at the 50% probability
level).
conversion to realize the rst molecular switches that are
orthogonally manipulated by acid and light, respectively. The Z
/ E isomerization could be stopped without contamination by
using photoacids or removing the acid by bubbling, and
therefore, the acid-catalysed Z / E and photo-induced E / Z
norbornene component. Our previous studies demonstrated
that photochirogenesis is a powerful tool for reading and
isomerization switching process could proceed through
sensing the supramolecular arrangement of photosubstrates.34
multiple cycles in a non-accumulative fashion. Molecular
Compared to conventional photochromic systems, chiral
switches with controllable self-recovery were thus realized
molecular switches provide the unique advantage of non-
destructive read-out by detecting optical rotation.35 The enan-
tiomeric resolution of 5E was successfully achieved by chiral
based on these excellent properties. Theoretical studies indicate
that the ring strain of the norbornene-fused ve-membered ring
formed upon stilbene conjugation reduces the activation energy
HPLC (IA column, Fig. S91b†) using a mixed solvent system of n-
of proton transfer and prevents C]C migration, which is
2620 | Chem. Sci., 2021, 12, 2614–2622
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responsible for the unprecedented catalysis of isomerization.
We believe that similar properties should be achievable through
reasonable chemical modication of the fused ring of stilbene,
thus opening a new avenue for stilbene derivatives with highly
complex functions.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
10 (a) X. Guo, B. Shao, S. Zhou, I. Aprahamian and Z. Chen,
Chem. Sci., 2020, 11, 3016; (b) C. L. Fleming, S. Li,
We acknowledge the support of this work by the National
Natural Science Foundation of China (No. 21871194, 92056116,
21971169, 21572142 and 22072099), the National Key Research
and Development Program of China (No. 2017YFA0505903), the
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Science
& Technology Department of Sichuan Province
(2019YJ0160, 2019YJ0090, and 2017SZ0021), and the Funda-
mental Research Funds for the Central Universities
(20826041D4117). Compound characterization was performed
with the support of the Comprehensive Training Platform of
Specialized Laboratory, College of Chemistry and Professor
Peng Wu of the Analytical & Testing Center, Sichuan University.
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