PH-Gated photoresponsive shuttling in a water-soluble pseudorotaxane

Article abstract of DOI:10.1039/c8cc00688a

A fluorescent bistable pseudorotaxane was devised and shown to display pH-gated photoresponsive ring shuttling. At pH > 7.5 shuttling does not take place after light stimulation while at pH ≈ 5 the macrocycle is quantitatively translocated.

Full text of DOI:10.1039/c8cc00688a

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pH-Gated Photoresponsive Shuttling in a Water-Soluble
Pseudorotaxane
A. Zubillaga, †a P. Ferreira, †a A. J. Parola,a S. Gago,* a and N. Basílio* a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A fluorescent bistable pseudorotaxane was devised and shown to overcome in order to improve performance, as recently
display pH-gated photoresponsive ring shuttling. At pH > 7.5 recognized by Leigh and co-workers.25 Herein, we demonstrate
shuttling does not take place after light stimulation while at pH ≈ 5 the great, although unexplored, potential of the
the macrocycle is quantitatively translocated.
chalcone/flavylium photochromic system to devise
photoresponsive pseudorotaxanes. The present system
Pseudorotaxanes, assembled from linear guest molecules displays excellent switching fidelity, positional integrity and
(axle) and macrocyclic receptors, are precursors of long-lived states that together with water solubility, pH-gated
mechanically interlocked molecules such as rotaxanes and photoresponsive ring-shuttling and fluorescence emission
catenanes that may be used as synthetic molecular machines properties make it potentially appealing for biological
and switches.1 3 Bistable [2]rotaxanes, consisting of a linear applications. Flavylium systems display highly attractive pH-
–
molecule and a macrocycle that can be switched by means of dependent photochemical properties that were previously
external stimuli between two recognition sites, constitute a explored to devise models for optical molecular memories and
prototypical example of a synthetic molecular machine with mimicking elementary properties of neurons.30–33 In this work,
potential applications in several fields such as molecular we have designed and synthesized trans-chalcone
1 (Scheme
electronics, switchable catalytic systems, drug delivery 1) in 3 steps from readily available starting materials (see
systems, molecular muscles, molecular pumps, etc.4 9 Different Supporting Information, SI). The corresponding flavylium
–
chemical and physical external stimuli can be used to switch dication
2
(Scheme 1) is quantitatively formed after UV-light
in acid media (pH = 1) as evidenced
appealing as it allows spatial and temporal control in a remote from H NMR and UV-Vis absorption spectroscopy (see SI, Fig.
manner without introducing new reagents or producing S8 and Fig. S9). Both and were envisaged to form inclusion
bistable rotaxanes, but probably light is one of the most irradiation (366 nm) of
1
1
1
2
chemical waste.10 13 In order to devise photoswitchable complexes with cucurbit[7]uril (CB7) in aqueous solution based
rotaxanes and pseudorotaxanes, light-responsive units such as on the well documented high binding affinity and selectivity of
azobenzenes, stillbenes, olefins, spiropyrans, hydrazones, etc these macrocyclic nanocontainers for organic cations.34–36
can be introduced in the structures of the thread or
–
macrocycle.14 26 Alternatively, redox or pH-responsive units
–
can also be used together with photosensitizers or photoacids,
respectively, present in solution or chemically attached to the
(pseudo)rotaxane components.27–29 Despite of the vast
amount of work reported on this topic, there are several
limitations, such as modest positional integrity movements,
short-lived states and poor switching fidelity, that must be
Scheme 1. Structures of trans-chalcone
studied in this work.
1, flavylium cation 2 and CB7 molecules
This article is dedicated to Prof. Fernando Pina on the occasion of his 70th birthday.
UV-Vis titration of 1 with CB7 afforded a binding constant of
K11 = 9.8x105 M-1 (in H2O at 25 °C, see SI, Fig. S10) comparable
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with the value reported for tetraethylammonium cation
(1.0x106 M-1 in D2O at 25 °C).37 The proximity of the K values
suggests
a
similar binding mode for both guests, i.e.,
inside the
encapsulation of the triethylammonium group of
1
cavity of CB7 which was confirmed by the observation of
1
upfield complexation-induced H NMR chemical shifts for the
triethylammonium and propyl spacer protons after addition of
1 equivalent of CB7 (see SI, Fig. S11). Noteworthy, the
1
aromatic H NMR signals, that hardly change with 1 equiv. of
Scheme 2. Illustration showing the reversible displacement of the CB7 wheel after
photoinduced interconversion of 1:CB7 into 2:CB7 complexes and thermal back
reaction.
CB7, undergo considerable broadening and shifts in the
presence of excess of host suggesting the inclusion of the
phenol group inside the cavity of a second CB7 molecule
(probably due to hydrophobic effects) to form 2:1 host-guest
complexes
(K21 = 2.0x102 M-1 in D2O at 25 °C) under these conditions (SI,
Fig. S12-S14).38
1
The H NMR spectra in the absence and in the presence of 1
equivalent of CB7 (see Supporting Information, Fig. S16) show
slight downfield complexation induced chemical shifts for the
signals of the ethyl ammonium protons (a and b) and
considerable upfield shifts for the signals of the phenyl group
The interaction of CB7 with flavylium species
2 (generated
(f and g) along with proton e indicating that in the case of 2 the
photochemically upon irradiation of 1) was also investigated
CB7 ring is located around the 2-phenyl group between the
two positive charges (see Scheme 2). In the presence of excess
of CB7 (10 equiv.) the signals of the triethylammonium group
are displaced upfield and the f and g signals of the phenyl
group return to lower field. These observations suggest that
the first CB7 molecule, sitting around the phenyl, is pushed to
the pyrylium moiety to allow the accommodation of the
second host on the ammonium group.
by UV-Vis (Supporting Information, Fig. S15) and fluorescence
(Fig. 1) titrations at pH = 2 (acidic conditions are required to
avoid the thermal back reaction to form
experiment). As can be observed in Fig. 1., the changes in the
emission spectrum of upon addition of CB7 can be used to
monitor pseudorotaxane formation. The emission intensity
increases from free to the 1:1 CB7 host-guest complex.
1 during the titration
2
2
2:
Upon further addition of CB7 the emission intensity decreases
again due to the formation of a 2:1 host-guest complex with
lower fluorescence emission quantum yield. Fitting the
variations observed for the emission intensity at 490 nm with
the CB7 concentration to a 2:1 binding model affords the
binding constants K11 = 1x107 M-1 and K21 = 1.2x104 M-1 (in H2O,
pH = 2, at 25 °C) in excellent agreement with UV-Vis
Focusing on the 1:1 host-guest complexes, which can be
obtained with equimolar concentrations of host and guest, the
results suggest that the position of the CB7 wheel can be
switched from the quaternary ammonium group in the case of
1
to the phenyl group for 2 upon irradiation with UV light
(Scheme 2). Previous works documented on the construction
of stimuli-responsive CBn-based pseudorotaxanes but the
majority relies on pH and redox responsive assemblies.43,44 The
absorption experiments. The higher CB7 affinity observed for
2
in comparison with 1 was expected on the basis of previous
fact that only
a small number of light-responsive CBn
studies regarding the complexation of flavylium multistate
systems with CB7.39–42 The determined fluorescence quantum
pseudorotaxanes/rotaxanes were reported is a consequence
of the challenges associated with such systems.39,45–53
yield for the free flavylium dye 2 (φ≈1%) and for the 1:1 host
guest complex CB7 =24%) provide quantitative basis for
Fig. 2. (a) shows the spectral variations observed after
2:
(φ
the observed variations on the fluorescence properties.
irradiation of
equivalent of CB7 at pH = 4.2. This experiment indicates that
CB7 complex (88% is formed under these conditions, see
mathematical model in the SI) is quantitatively converted into
CB7 (> 93% of complex) with a quantum yield of 10%.
However, above ca. pH = 5 the photoinduced conversion of
CB7 into CB7 decreases. The absorption spectra observed
1 with 365 nm light in the presence of 1
1:
1
1
2:
= 1x107 M-1
1:
2:
K1:1
K2:1
= 1.2x104 M-1
0
at the photostationary state (maximum possible
photoconversion) at different pH values (Fig. 2.(b)) allows the
determination of an apparent pKa = 6.1. The pH-coupled
0.1
1
10
100
[CB7]/[2] ratio
0
photochemical interconversion of 1:CB7 into 2:CB7 arises from
500
600
700
the well-known chemical network of reversible reactions
exhibited by flavylium systems.30 The primary photochemical
event is the trans to cis photoisomerization of the chalcone.
Once formed, the cis species equilibrates fast with the
hemiketal species through a ring-closure tautomerization.
Finally, depending on the pH, this species dehydrates to give
the flavylium cation. Conversely, at high pH values irradiation
of the trans-chalcone leads to the formation of a mixture of
/ nm
λ
Fig. 1. Emission spectra of flavylium 2 (2.2 ꢀM, λex = 445 nm) at pH = 2 (HCl = 0.01 M)
registered with increasing amounts of CB7. The black full lines correspond to the
spectra of free dye and 1:1 complex while the dotted line was obtained for 300 ꢀM of
CB7. For clarity, the spectra registered for intermediate concentrations of CB7 between
2.2 and 300 ꢀM are not shown. The inset shows the fluorescent intensity (490 nm)
plotted against the CB7 concentration (from 0 to 300 ꢀM of CB7).
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cis-chalcone and hemiketal while under more acidic conditions Complete recovery of
this mixture is rapidly converted into flavylium cation.
1
:
CB7 can be achieved at pD = 5.3 but at
the cost of reducing the conversion yield (88%) of the
photoproduct (see Supporting Information, Fig. S18).
2:CB7
1.5
1.5
1
(a)
(b)
λ
irr = 365 nm
1
φ = 10%
1
pKa = 6.1
0.5
0
0.5
0
0
3
4
5
pH
6
7
8
300
400
500
300
400
500
λ
/ nm
λ
/ nm
Fig. 2. (a) UV-Vis spectral variations of trans-chalcone
of 1 equivalent of CB7 at pH = 4.2 (10 mM of citrate buffer) after irradiation with
a 365 nm UV-light source. (b) Spectra taken at the photostationary state at
different pH values obtained after irradiation of (26 M) with 1 equivalent of
CB7 in 10 mM citrate buffer.
1 (26 ꢀM) in the presence
1
ꢀ
The net result is the observation of the photochemical
conversion of trans-chalcone into flavylium cation . Because
shuttling of the CB7 ring is only observed when the flavylium
1
2
cation is formed, the system behaves as an AND molecular Scheme 3. Illustration showing the pH-coupled photoresponsive shuttling
according to an AND molecular logic gate. At pH around
5 the system is
logic gate that works with H+ and light inputs. Moreover, if the reversible and returns to the trans-chalcone in the dark. However, the system
can be LOCKED in the flavylium state by acidification (ca pH = 2) after irradiation
pH is low enough (pH ≈ 2) the pseudorotaxane can be LOCKED and UNLOCKED by addition of base to pH > 5. Simplified representation of the
potential energy of the system at selected pH values.
in the aromatic station because the flavylium cation is the
thermodynamically stable species under highly acidic
conditions and does not return to the trans-chalcone state /
quaternary ammonium station in the dark. This process is
reversible and the pseudorotaxane can be UNLOCKED by
neutralization with NaOH. It is worth noting that when the
trans-chalcone solution is acidified in the dark the system will
be out of the equilibrium and can, in principle, evolve to the
flavylium cation. However, the kinetic barrier is high and this
process can take several months to reach the equilibrium at
room temperature (the t1/2 is ca. 100 days at pH =2 for a
similar flavylium derivative).51 The stimuli-responsive
operation of this system is summarized in Scheme 3.
Fig. 3. 1H NMR spectra of 1:CB7 in D2O (0.4 mM) at pD = 4.96 (acetate buffer 10 mM) in
the presence of 1 equivalent of CB7. Before (bottom), after irradiation with 365 nm
light (middle) and upon standing overnight in dark at 60 ºC. The signals marked with *
are assigned to ca. 10% of 2:CB7 that equilibrates with 1:CB7 at this pD value.
The pH-coupled photoresponsive shuttling was also tested by
1H NMR experiments. At pD = 7.4, irradiation of
1:CB7 with a
365 nm light source leads to the disappearance of the signals
corresponding to this species and the appearance of broad,
poorly defined signals that can be assigned to an equilibrium
mixture of complexed trans-chalcone, cis-chalcone, hemiketal
In summary, we have devised
a water-soluble bistable
pseudorotaxane that shows pH-gated photoresponsive ring
shuttling. The CB7 ring is only translocated after the
application of pH and light stimuli (an AND molecular logic
behavior). Noteworthy, the operating pH values fall within
those that can be employed for biological systems. At neutral
or slightly basic conditions no translocation occurs while at pH
values around 5 near quantitative ring translocation can be
observed. The system is reversible in the dark but can be made
irreversible (locked) at more acidic pH values and unlocked by
neutralization. In addition to these special stimuli responsive
features, the present system also displays good fluorescent
properties which further increase his potential for molecular
machine-based applications.
and ca. 10% of 2:CB7 in agreement with an apparent pKa = 6.1
(see SI, Fig. S17). The system is reversible and heating the
sample for 2 hours in the dark at 80 °C leads to the recovery of
the spectrum corresponding to
when the same experiment is conducted at pD = 4.96 (Fig. 3),
it can be clearly observed that CB7 is completely converted
into CB7 upon excitation with 365 nm light. As already
demonstrated, CB7 into CB7 interconversion leads to
1:CB7. On the other hand,
1
2:
1:
2:
displacement of the CB7 ring from the quaternary ammonium
to the aromatic station. Again, heating the sample in the dark
leads to the recovery of the spectrum corresponding to 1:CB7
This work was supported by the Associated Laboratory for
Sustainable Chemistry-Clean Processes and Technologies-
LAQV, which is financed by the Portuguese FCT/MEC
(UID/QUI/50006/2013) and co-financed by the ERDF under the
PT2020 Partnership Agreement (POCI-01-0145-FEDER-
007265). The FCT/MEC is also thanked for supporting the
confirming the reversibility of the system. However, at 25 °C
the thermal recovery is much slower (t1/2 = 151 hours at pH =
5, see Fig. S19 in the SI) which account for long-lived
photostationary state. It is worth noting that at this pD value
ca. 10% of 2:CB7 (estimated from the integration of the signals
corresponding to protons a) was not reconverted into 1:CB7.
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National Portuguese
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NMR
Network
(RECI/BBB-
17564–17567.
BQB/0230/2012), for project RECI/BBB-BEP/0124/2012 and for 25
D. A. Leigh, V. Marcos, T. Nalbantoglu, I. J. Vitorica-Yrezabal, F. T.
Yasar and X. Zhu, J. Am. Chem. Soc., 2017, 139, 7104–7109.
T.-G. Zhan, M.-Y. Yun, J.-L. Lin, X.-Y. Yu and K.-D. Zhang, Chem.
Commun., 2016, 52, 14085–14088.
the award of a postdoctoral grant (SFRH/BPD/84805/2012) to
N.B.
26
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S. Silvi, A. Arduini, A. Pochini, A. Secchi, M. Tomasulo, F. M. Raymo,
M. Baroncini and A. Credi, J. Am. Chem. Soc., 2007, 129, 13378–
13379.
Conflicts of interest
There are no conflicts to declare.
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Phototriggered ring shuttling in a water-soluble fluorescent pseudorotaxane can
enabled and disabled at different pH values.