Photoinduced Release of a Chemical Fuel for Acid–Base-Operated Molecular Machines

Article abstract of DOI:10.1002/chem.201800474

Back and forth motions of the acid–base-operated molecular switch 1 are photo-controlled by irradiation of a solution, which also contains the photolabile pre-fuel 4. The photo-stimulated deprotection of the pre-fuel produces controlled amounts of acid 2,

Full text of DOI:10.1002/chem.201800474

A Journal of
Accepted Article
Title: Photoinduced Release of a Chemical Fuel for Acid-Base
Operated Molecular Machines
Authors: Stefano Di Stefano, Chiara Biagini, Osvaldo Lanzalunga, Luigi
Mandolini, and Flaminia Di Pietri
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To be cited as: Chem. Eur. J. 10.1002/chem.201800474
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Photoinduced Release of a Chemical Fuel for Acid-Base Operated
Molecular Machines
Chiara Biagini,[a] Flaminia Di Pietri,[a] Luigi Mandolini,[a] Osvaldo Lanzalunga[a] and
Stefano Di Stefano*[a]
Abstract: Back and forth motions of the acid-base operated
molecular switch 1 are photo-controlled by irradiation of a solution
which also contains the photolabile prefuel 4. The photo-stimulated
deprotection of the prefuel produces controlled amounts of acid 2
whose base-promoted decarboxylation fuels the back and forth
motions of the Sauvage type [2]-catenane based molecular switch.
Since switch 1 and prefuel 4 do not interact in the absence of
irradiation, an excess of the latter with respect to 1 can be added to
the solution from the beginning. In principle, a photo-control of the
back and forth motions of any molecular machine whose operation is
guided by protonation/deprotonation, could be attained by use of
prefuel 4, or of any other protected acid that undergoes deprotection
groups depending on whether the source of energy required for
the operation is light or a chemical fuel.
Light has been often chosen as a stimulus to obtain the control
of both rotary and translational motion, with the advantage of
producing no waste products.[3] For example, Feringa developed
a series of rotary motors based on overcrowded alkenes where
a rotor performs complete 360° rotation with respect to a stator
around a double bond.[4] The whole process is powered by the
repetition of a sequence of two reactions, consisting of a light
induced cis-trans isomerization of the double bond and a
spontaneous thermal relaxation step. Light powered linear
motion in rotaxane-based molecular shuttles has been achieved
by Leigh’s group, using a nanosecond laser pulse,[5] and by
Balzani’s group, using sunlight.[6]
by irradiation with light at
a proper wavelength, followed by
decarboxylation under conveniently mild conditions.
Nanoscale motion, both in covalent and supramolecular
structures, has been also triggered and controlled by sequences
of chemical reactions. Acid-base chemistry, and other reversible
interactions have been widely used in rotaxane and catenane
based molecular switches, albeit with many limitations
concerning the lack of net directionality and the impossibility to
perform autonomous motion.[2o,p] No doubt an important
contribution to the resolution of both these hurdles was recently
due to Leigh’s group. They designed and realized a [2]-catenane
in which one of the two macrocycles performed autonomous and
unidirectional cycles around the other.[7]
Introduction
In natural systems, complex molecular architectures assembled
from small and simple building blocks are able to convert energy
from various sources into molecular-level directional motion,
thus performing many of the useful tasks at the basis of
fundamental biological processes.[1] The extraordinary efficiency
demonstrated by nature in the design of such stimuli-responsive
systems has always been of inspiration for scientists in many
research fields, especially in chemistry. Controlling nanoscale
motion in artificial systems and interfacing them with other
structures on the same scale, as well as with the macroscopic
world, would potentially lead to revolutionary results influencing
all of the fields of science and technology. For these reasons,
although the exquisite level of complexity of natural molecular
machines still appears out of reach, the design of artificial
molecular machines represents an extremely attractive research
topic.[2]
In the frame of chemical fuelled molecular switches, we recently
reported that the Sauvage-type catenane 1 depicted in Scheme
1, undergoes back and forth motions from state A to states B (B’
and B”), and back again to state A, in the presence of 1 mol
equiv of 2-phenyl-2-cyanopropanoic acid 2 used as a chemical
fuel. [8] While in states B the catenane-based switch possesses a
well-defined co-conformation with the two phenanthroline
subunits tightly held in proximity by the shared proton, state A
cannot be associated to any precisely defined co-conformation.
A distinctive feature of such system is that no additional antifuel
is needed to restore the switch to its initial form since the
transformation of the reactant 2 to the product 3 (2-phenyl-2-
cyanopropane) is step by step coupled to the transition
A→B→A. A first proton transfer from 2 to 1 produces the proton
catenate 1H+, which is paired to a carboxylate ion (Scheme 1,
state B’). The subsequent decarboxylation step leads to state
B”, where 1H+ is paired to an anion in which the negative charge
was suggested to be strongly localized on the nitrogen atom of
Many efforts have been made towards this goal. Relatively
simple, prototypal examples of stimuli responsive systems,
composed either of single molecules or a limited number of
components, have been designed and studied, in order to
investigate how motions at the nanoscale level can be triggered
and controlled. Such systems can be divided into two main
[a]
C. Biagini, F. Di Pietri, Prof. L. Mandolini, Prof. O. Lanzalunga and
Dr. S. Di Stefano
the cyano group (Scheme 1, state B”).[9]
A final, rate
determining proton transfer from 1H+ to the anion, restores the
catenane to its neutral form 1, giving the waste product 3. This
system was of inspiration to Leigh and co-workers who recently
exploited the decarboxylation reaction of trichloroacetic acid for
the unidirectional operation of rotary and linear molecular
motors.[10]
Dipartimento di Chimica and Istituto CNR di Metodologie Chimiche-
IMC, Sezione Meccanismi di Reazione
Dipartimento di Chimica, Università degli Studi di Roma “La
Sapienza”, P.le A. Moro 5, 00185 Rome, Italy.
E-mail: stefano.distefano@uniroma1.it
Supporting information for this article is given via a link at the end of
the document.
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function. Such protecting group can be easily and rapidly
removed by irradiating the solution according to a mechanism
involving the singlet excited state of the protected carboxylic
acid.[12] Thus we envisaged that protection of acid 2 with DMB
would represent a good strategy to obtain a prefuel enabling a
photocontrolled release of the fuel through irradiation.
Prefuel 4 was prepared by condensation of DMB with acid 2 in
the presence of dicyclohexylcarbodiimide and catalytic amounts
of 4-dimethylaminopyridine.
Figure 1 shows the variations of the UV-Vis spectrum of a 0.050
mM prefuel 4 solution in dichloromethane (2 mL in 1 cm cuvette)
upon irradiation centered at 335 nm. Light irradiation was
performed on a spectrofluorimeter equipped with a 150 W xenon
lamp (10 nm spectral bandwidth). In accordance to what
observed by Corrie and Wan for other protected acids,[11]
production of fuel 2 and 1-phenyl-4,6-dimethoxybenzofuran
(DMBF) from prefuel 4 (eqn 1) is a clean process as witnessed
by the presence of two sharp isosbestic points (Figure 1).
1H NMR monitoring of a more concentrated solution of 4
confirms a clean deprotection process upon irradiation at 335
nm (see Figure S1 in the SI). Once verified that the light induced
production of 2 from 4 was well-working, we set the conditions
for the photo-controlled back and forth motions (A→B→A) of
catenane 1 which are outlined in Scheme 2.[13]
Scheme 1. Switching motions of catenane 1 fuelled by acid 2. Fast and
quantitative proton transfer (step A→B’) fixes the contact between the
interlocked macrocycles in the region of the phenanthroline nitrogen atoms.
Loss of CO2 (step B’→B”), followed by slower back proton transfer from 1H+
to its counteranion, restores the catenane to its original form (step B”→A).
We also demonstrated that the duration of the A→B→A motion
of 1 can be varied from minutes to days by introducing
substituents in the para position of the phenyl group of 2, with
electron-withdrawing groups accelerating and electron-donating
groups retarding the motion.[9]
A major drawback related to the use of 2 and its derivatives as
fuels for the cyclic motions of 1 lies in the impossibility to use an
excess fuel with respect to the machine. Indeed, if an excess
fuel is used, a mechanistic shunt is opened that consumes all of
the excess acid, without the involvement of concomitant motions
of the switch: if undissociated acid 2 is still present when the ion
pair of state B” is completely formed, such acid will donate the
proton to the anion partner in B”, and 1H+ will remain protonated
until excess acid is present. Such disadvantage poses severe
limitations to an autonomous operation of the molecular switch,
which does not exploit the energy supplied by the excess fuel. A
possible way to overcome this problem would consist in the use
of a prefuel that can be activated to release the fuel in a
controlled manner. Here, the idea is to add to catenand 1 a
photosensitive prefuel that is able to generate fuel 2 by
irradiation with light at a properly selected wavelength.
Figure 1. Variations of UV-Vis spectrum of a 0.050 mM solution of prefuel 4 in
CH2Cl2 at 25 °C after repeated irradiations at λ = 335 nm. The inset shows that
the absorbance at 302 nm increases according to
dependence.
a first-order time
Results and Discussion
Figure 2 shows the UV-Vis spectra of 1, 1H+ (CF3CO2– salt), 2, 4,
and DMBF. In order to minimize undesirable photo-induced side
3’5’-Dimethoxy-benzoin (DMB) was demonstrated by Corrie and
Wan[11] to be an optimal protecting group for the carboxylic
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reactions, deprotection of prefuel 4 to fuel 2 was carried out by
irradiation of the solution at 335 nm. At this wavelength prefuel 4
has still a weak absorption tail (see insets in Figure 2b) and so
do compounds 1, 1H+ and DMBF whereas fuel 2 does not
absorb at all.
We previously showed[9] that the A→B→A motion can be
followed by UV-Vis spectroscopy since the anion in state B” has
a characteristic absorption band centered at 375 nm. This is a
fortunate situation, as all of the other species do not absorb at
this wavelength (Figure 2).
A reaction between 1 and 2 was initially carried out at lower
concentrations with respect to the kinetic runs previously
performed[8,9] with the aim to check whether the system still
works under conditions convenient for the photochemical control
of the catenane 1 motions. Figure 3 compares the absorbance
variation at 375 nm as a function of time related to the reaction
between 0.050 mM 1 and 0.050 mM 2 with that of the same
reaction carried out at higher concentrations,[9] 0.30 mM 1 and
0.30 mM 2. Reaction times are practically unchanged confirming
that the mechanism in Scheme 1 is still operating at lower
concentrations. At the same time, the maximum absorbance
values, reached at t ≈ 100 s in both cases, are in accordance
with a complete formation of intermediate B”, which then
disappears to give product 3 and catenane 1 in its neutral state.
Scheme 2. Photo-controlled motions of catenane 1. Irradition of prefuel 4
produces fuel 2, which in turn allows the switching of 1 from state A to states
B and back again to state A.
Figure 2. UV-Vis spectra of: (a) 1 (blue), 1H+CF3CO2 (red); (b) 4 (black), 2
(pink) and DMBF (green) (CH2Cl2, 25 °C). The inset shows the enlargement of
–
Figure 3. UV-Vis monitoring of the reaction between 0.050 mM (a) and 0.30
mM (b, data from ref 9) equimolar 1 and 2 (CH2Cl2, 25 °C). Curves are plots of
a standard first order equation with k = 2.4×10-4 s-1 in both cases.
the absorption tail of 4.
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Thus, a solution of 1 (0.050 mM) and a 10-fold excess of prefuel
4 (0.50 mM) was repeatedly subjected to irradiation periods at
335 nm.[14] The result of this experiment is reported in Figure 4.
In the pre-phase I, the solution is irradiated for 120 s in the
spectrofluorimeter and then the cuvette is transferred into the
spectrophotometer in order to follow the catenane motion.
Phase I (from 120 s to about 3h) nicely reveals the back proton
transfer from state B” to catenane 1 in its neutral form and
waste product 3. A second pre-phase follows in which the
cuvette is again transferred into the spectrofluorimeter and the
solution irradiated for additional 120 s, then the second back-
proton transfer (phase II) is followed in the spectrophotometer
up to about 6 h. The cycle is repeated two more times (phases
III and IV). The fraction of catenane 1 which is observed to
perform the (A→)B→A motion can be quoted by comparison of
the ΔA value at 375 nm recorded at each cycle, with the
corresponding value in Figure 3a. In cycles I and II about 42% of
catenane 1 molecules are observed to move from state B” to
state A, which implies the operation of the whole motion
A→B→A.[15] Such fraction somewhat decreases in the
subsequent cycles III and IV.
liberated a fraction of the catenane molecules underwent a
complete A→B→A cycle.
Our attempts to increase the fraction of the catenane molecules
undergoing the back and forth motions were frustrated by
unavoidable difficulties intrinsically dependent on the nature of
switch 1. As expected, when irradiation times were lowered from
120 to 60 s, the observed percentage of catenane 1 undergoing
back and forth motions between states A and B decreased from
about 42% to about 20-25% (see Figure S4). On the other hand,
when the irradiation time per cycle was increased to 180 s,
during the first two cycles almost 60% of catenane molecules
were observed to undergo the motions but, unfortunately, in the
subsequent cycles such fraction strongly decreased (Figure S5).
The decrease in efficiency in subsequent cycles was even more
marked when an irradiation time per cycle as high as 300 s was
applied (see Figure S6). In this case, during the first cycle
almost 70% of the catenane molecules were observed to
perform the back and forth motion while the percentage was
practically halved during the second cycle.
Figure 4. UV-Vis monitoring of 0.050 mM 1 in the presence of 0.50 mM
prefuel 4 after 120 s of irradiation of the solution at time 0, 10920, 21840 and
32760 s at 335 nm (CH2Cl2, 25 °C). The black horizontal bar corresponds to a
42% conversion of state A into state B”.
Production of the two waste products 3 and DMBF from photo-
deprotection of prefuel 4 in the presence of catenane 1 was also
1
verified by H NMR spectroscopy. A 2.0 mM equimolar solution
of 1 and 4 (2 mL in 1 cm cuvette) was irradiated at 335 nm for 1
h. Comparison of the 1H NMR spectrum of the crude product
with that of a typical catenane 1 promoted decarboxylation of 2
and of an authentic sample of DMBF (Figure S2) showed the
production of both waste products 3 and DMBF. Half of the initial
amount of prefuel 4 appears to have been consumed under
these conditions.
To sum up, the photo-controlled motion of catenane 1 has been
achieved. Initially, a 10-fold molar excess of prefuel was added
and the fuel release was gradually controlled by irradiation of the
solution at the proper wavelength. Each time the fuel was
–
Figure 5. UV-Vis spectra of (a) 0.010 mM 1 and (b) 0.010 mM 1H+CF3CO2
after 0 (black), 5 (red), 10 (green), 15 (blue), 20 (pink), and 25 (cyano) min
irradiation at 335 nm (CH2Cl2, 25 °C).
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In order to shed light on the origin of the loss of efficiency of the
back and forth motions of catenane 1 on increasing the number
of cycles, the behavior of 1 under irradiation conditions was
investigated. Figure 5a reports the variations of the absorbance
of a 0.010 mM 1 solution in dichloromethane upon 5 subsequent
irradiations for 5 minutes each at 335 nm. From comparison of
Figure 5a with Figure 2a and particularly from the presence of
the sharp isosbestic point at 275 nm in both of the figures, it
appears evident that irradiation causes transformation of 1 into
its protonated form 1H+. In contrast, the spectra related to
–
1H+CF3CO2 recorded under similar conditions (Figure 5b) do
not show significant variations, even after long irradiation times
(25 min). 1H NMR analysis of a 0.20 mM solution of 1 in
dichloromethane subjected to a prolonged irradiation (2 h) at
335 nm led to the same conclusion as shown in Figure 6.
Protonation of 1 is indeed well evidenced by the insurgence of
the typical signals in the high field region (1-0 ppm), of the triplet
at 2.4 ppm, and of the signals related to the protonated
phenanthroline core at low fields. As a matter of fact such
signals, although to a minor extent, were already evident in the
1H NMR spectrum of the crude product reported in Figure S2
(bottom trace). As regards to the proton source, we stress that
all of the dichloromethane samples used throughout this work
were flushed through basic alumina just before use, and that
1H NMR spectra of catenane 1, which has been previously
demonstrated to be an exceedingly strong base (pKa > 16[8])
because of topological reasons,[8,16] never revealed the presence
of even minute amounts of 1H+. Thus, whatever the nature of the
proton source, it should not be acidic enough to protonate the
strongly basic 1. Balzani, Sauvage and coworkers[17] reported
that the basicities of the singlet excited states of phenanthroline
and its 2,9-diphenyl and 2,9-dianisyl derivatives are much higher
than those of the corresponding ground states (ΔpKa of about 5,
8 and 14 in the given order). If a similar effect also operates with
catenand 1, the basicity of its excited state could be enhanced to
such an extent that proton abstraction from the dichloromethane
solvent becomes feasible. However, an alternative possibility is
that protonation of the excited state of 1 is due to trace amounts
of an undetected impurity cannot be ruled out. Thus,
independent of the precise nature of the proton source, the
photoinduced transformation of 1 to 1H+ causes a loss in
efficiency on increasing the number of cycles. However,
because inspection of Figure 5 indicates that this effect is
relatively small, the build-up of products that absorb the
irradiation light could be a concomitant cause of the reduced
photoconversion on successive cycles.
Figure 6. 1H NMR (CD2Cl2) spectrum of 1 (bottom), 1 subjected to 2 h
irradiation centered at 335 nm (middle), 1H+CF3CO2– (top). Signals from 8.6 to
7.2 ppm are due to aromatic protons, signals from 5.8 to 5.2 ppm are due to
double bond protons, signals from 3.3 to 0 ppm are due to methylene chain
protons (see ref 18a for details).
Conclusions
In this paper we have shown that it is possible to
photochemically control the chemically fuelled back and forth
motions of a molecular switch. In our system a light pulse
converts a first molecular species (prefuel) into a second one
(fuel) which, in turn, causes the large amplitude motions of a
molecular switch, as a results of a cascade effect typical of
biochemical systems. The main actor of such photocontrol is not
the acid-base operated molecular machine 1 but prefuel 4,
which can be transformed into the active fuel 2 at will by
irradiation of the solution at the proper wavelength. Since switch
1 and prefuel 4 do not interact in the absence of irradiation, an
excess of prefuel with respect to switch can be added to the
system from the beginning. The drawbacks related to the loss of
efficiency on increasing the number of motion cycles are due to
intrinsic properties of the molecular switch at hand and do not
spoil the generality of the principle. Control of the motion of any
molecular switch or motor that moves between two (or more)
states under the influence of protonation/deprotonation, could be
attained by use of prefuel 4, or any other protected acid that
undergoes deprotection by irradiation with light at a proper
wavelength, followed by decarboxylation under conveniently
mild conditions.
Experimental Section
Instruments and Methods
1H NMR spectra were recorded on a 300 MHz spectrometer. The spectra
were internally referenced to the residual proton signal of the solvent at
5.30 ppm for spectra recorded in CD2Cl2 and 7.26 ppm for spectra
recorded in CDCl3. Spectrophotometric measurements were carried out
on a diode array spectrophotometer equipped with a thermostatted cell
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10.1002/chem.201800474
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compartment. Irradiations were carried out on a spectrofluorimeter
equipped with a 150 W xenon lamp, setting the λexc at 335 nm and a
spectral bandwidth of 10 nm and using 2 mL quartz cuvettes with optical
path of 1 cm.
Metallo-Rotaxanes and Catenanes as Redox. Switches: Towards
Molecular Machines and Motors. In Molecular Switches, 1st Edition
(Ed.: B. L. Feringa), Wiley-VCH, Weinheim, 2001, pp. 249-307. d) V.
Balzani, A. Credi, S. Silvi M. Venturi, Chem. Soc. Rev. 2006, 35, 1135-
1149. e) G. Podoprygorina, V. Böhmer, Tetra-Urea Calix[4]arenes -
From Dimeric Capsules to Novel Catenanes and Rotaxanes. In Modern
Supramolecular Chemistry: Strategies for Macrocycle Synthesis, 1st
Edition (Eds.: F. Diederich, P. J. Stang, R. R. Tykwinski), Wiley-VCH,
Weinheim, 2008, pp. 143-184. f) J. A. Wisner, B. A. Blight, Rotaxane
and Catenane Synthesis. In Modern Supramolecular Chemistry:
Strategies for Macrocycle Synthesis, 1st Edition (Eds: F. Diederich, P. J.
Stang, R. R. Tykwinski), Wiley-VCH, Weinheim, 2008, pp. 349-391. g)
V. Balzani, A. Credi, M. Venturi, Molecular Devices and Machines:
Concepts and Perspectives for the Nanoworld, 2nd Edition, Wiley-VCH,
Weinheim, 2008. h) J. F. Stoddart, Chem Soc. Rev. 2009, 38, 1802-
1820. i) J. E. Beves, B. A. Blight, C. J. Cambell, D. A. Leigh, R. T.
McBurney, Angew. Chem. Int. Ed. 2011, 50, 9260-9327. j) J.-C.
Chambron, J.-P. Sauvage, New J. Chem. 2013, 37, 49-57. k) X. Liu, C.-
H. Lu, I. Willner, Acc. Chem. Res. 2014, 47, 1673-1680. l) S. Erbaş-
Çakmak, D. A. Leigh, C. T. McTernan, A. L. Nussbaumer, Chem. Rev.
2015, 115, 10081-10206. m) M. A. Watson, S. L. Cockroft, Chem. Soc.
Rev. 2016, 45, 6118-6129. n) B. L. Feringa, Angew. Chem. Int. Ed.
2017, 56, 11060 –11078. o) J.-P. Sauvage, Angew. Chem. Int. Ed.
2017, 56, 11080–11093. p) J. F. Stoddart, Angew. Chem. Int. Ed. 2017,
56, 11094 –11125.
Materials
All reagents and solvents were purchased at the highest commercial
quality and were used without further purification, unless otherwise stated.
CH2Cl2 and CD2Cl2 were flushed through basic alumina immediately prior
to use. Catenane 1 and acid 2 (mixture of EE, EZ and ZZ geometrical
isomers) were available from a previous investigation.[8,18] DMB and
DMBF was prepared as reported in the literature (see SI).[11,19]
1-(3',5'-Dimethoxyphenyl)-2-oxo-2-phenylethyl-2-phenyl-2-
cyanopropanoate (4).
4-Dimethylaminopyridine
(DMAP)
(34
mg,
0.28
mmol),
dicyclohexylcarbodiimide (DCC) (137 mg, 0.66 mmol), acid 2 (100 mg,
0.57 mmol) and DMB (466 mg, 1.7 mmol) were dissolved in CH2Cl2 (15.0
mL) in the given order at 0 °C. The solution was held at 0 °C under
stirring for 2.5 h, then the dicyclohexylurea developed during the reaction
was removed by filtration. The solution was washed with aqueous HCl
(0.6 M) and subsequently with saturated NaHCO3. The resulting organic
phase was then dried, evaporated and the crude wax was subjected to
chromatography (SiO2, hexane / dichloromethane 1:6). Prefuel 4 (104 mg,
0.24 mol) was obtained as a pale yellow oil (42% yield).
[3]
[4]
W. R. Browne, B. L. Feringa, Nat. Nanotechnol. 2006, 1, 25-35.
A. Cnossen, W. R. Browne, B. L. Feringa, Top. Curr. Chem. 2014, 354,
139−162.
[5]
[6]
[7]
[8]
[9]
A. M. Brouwer, C. Frochot, F. G. Gatti, D. A. Leigh, L. Mottier, F.
Paolucci, S. Roffia, G. W. H. Wurpel, Science 2001, 291, 2124-2128.
V. Balzani, M. Clemente-Leon, A. Credi, B. Ferrer, M. Venturi, A. H.
Flood, J. F. Stoddart, Proc. Natl. Acad. Sci. 2006, 103, 1178−1183.
M. R. Wilson, J. Solà, A. Carlone, S. M. Goldup, N. Lebrasseur, D. A.
Leigh, Nature 2016, 534, 235-240.
1H NMR (300 MHz, 25°C, CDCl3): attention, prefuel 4 was obtained as a
1:1 mixture of diasteroisomers, each present as a couple of enantiomers.
δ=7.92 (m, 1H), 7.83 (m, 1H), 7.60-7.32 (m, 7H), 6.70 (s, 1H), 6.68 (s,
1H), 6.55 (m, 1H), 6.52 (m, 1H), 6.39 (bs, 1H), 3.72 (m, 6H), 2.04 ppm (m,
3H). 13C NMR (75 MHz, 25°C, CD2Cl2) δ: 192.3, 192.1, 167.6, 167.3,
161.0, 135.1, 134.9, 134.3, 134.2, 134.0, 133.6, 133.5, 129.01, 128.96,
128.84, 128.80, 128.73, 128.68, 128.62, 128.5, 126.0, 119.07, 118.92,
106.0, 105.7, 101.25, 101.20, 79.8, 79.6, 55.3, 48.3, 48.2, 25.0, 24.7
ppm. UV-Vis (CH2Cl2): λmax (ε) = 241 nm (8900), 282 nm (1552 mol-
1 dm3 cm-1). MS (EI) m/z: 429 [M+], 324 [M-105]+, 256 [M-173]+, 165 [M-
264]+, 130 [M-199]+, base peak 105, 77.
J. A. Berrocal, C. Biagini, L. Mandolini, S. Di Stefano, Angew. Chem. Int.
Ed. 2016, 55, 6997-7001.
C. Biagini, S. Albano, R. Caruso, L. Mandolini, J. A. Berrocal, S. Di
Stefano, Chem Sci. 2018, 9, 181–188.
[10] S. Erbas-Cakmak, S. D. P. Fielden, U. Karaca, D. A. Leigh, C. T.
McTernan, D. J. Tetlow, M. R. Wilson, Science 2017, 358, 340–343.
[11] Y. Shi, J. E. T. Corrie, P. Wan, J. Org. Chem. 1997, 62, 8278-8279.
[12] The two methoxy groups in the skeleton of DMB are essential for a
productive deprotection process in our conditions. In the presence of
catenand 1, the light induced deprotection of the analog of 4 devoid of
the two methoxy groups does not proceed at all.
Acknowledgements
Thanks are due to the Ministero dell’Istruzione, dell'Università e
della Ricerca (MIUR, PRIN 2010CX2TLM). This work was also
partially supported by Università di Roma La Sapienza (Progetti
di Ricerca 2015). The authors thank Prof. Maurizio Fagnoni for
his suggestions concerning photolabile protecting groups for the
carboxylic function.
[13] The mechanism of photodeprotection proposed by Corrie and Wan (ref
11) involves
a heterolytic cleavage of the C–O bond from an
intramolecular exciplex characterized by a charge transfer interaction
between the methoxybenzene ring and the electron-deficient oxygen of
the n,π* singlet excited acetophenone, leading to the formation of a
carboxylate and a cation precursor of DMBF. Fast deprotonation of the
cation by the carboxylate produces DMBF and the carboxylic acid.
Thus protonation of catenand 1 might involve either fuel 2 or the cation
precursor of DMBF. In both cases the same states B of Scheme 1 are
formed.
Keywords: Chemical Fuels • Molecular Motions •
Photochemical Control • Catenane • Molecular Machines
[14] When non-irradiated a solution of 0.050 mM 1 and 0.50 mM 4 in CH2Cl2
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Entry for the Table of Contents
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Photocontrol of the back and
forth motions of a molecular
switch is attained by use of an
inactive prefuel, which is at will
transformed into the active form
by irradiation with light at the
proper wavelength.
C. Biagini, F. Di Pietri, L. Mandolini,
O. Lanzalunga, and S. Di Stefano*
Page No. – Page No.
Photoinduced Release of a
Chemical Fuel for Acid-Base
Operated Molecular Machines
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