Triggering a [2]Rotaxane Molecular Shuttle by a Photochemical Bond-Cleavage Strategy

Article abstract of DOI:10.1021/acs.orglett.7b00393

The successful triggering of ring-shuttling motion between two stations in a [2]rotaxane is demonstrated by employing a photochemical bond-cleavage strategy. A photolabile bulk barrier is covalently introduced into two identical stations of the thread to

Full text of DOI:10.1021/acs.orglett.7b00393

Letter
pubs.acs.org/OrgLett
Triggering a [2]Rotaxane Molecular Shuttle by a Photochemical
Bond-Cleavage Strategy
Chuan Gao, Zhou-Lin Luan, Qi Zhang, Shun Yang, Si-Jia Rao, Da-Hui Qu,* and He Tian
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China
University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
S
* Supporting Information
ABSTRACT: The successful triggering of ring-shuttling motion
between two stations in a [2]rotaxane is demonstrated by
employing a photochemical bond-cleavage strategy. A photolabile
bulk barrier is covalently introduced into two identical stations of
the thread to prevent dynamic shuttling of the macrocycle,
resulting in a “gated” state. Irradiation of UV light (λ = 365 nm)
results in the complete removal of the bulk barrier and the
balanced shuttling motion of the macrocycle, indicating an “open” state of the rotaxane. In addition, the process from the “open”
rotaxane to the “gated” rotaxane was executed by a chemical-rebonding method.
ynthetic chemists have created and developed various
enrich light-controlled rotaxane systems, as the photochemical
process would occur precisely.
S
artificial molecular machines,1 such as rotaxanes,1i,k
catenanes,1h,c and motors,1j anticipated to mimic the
machine-like behaviors in biological systems.2 Rotaxane, well-
known for its unique shuttling motion in interlocked nanoscale
systems, has proved to be versatile as a molecular switch and
molecular machine.2,3 These functional applications strongly
rely on the responsiveness of the stimuli of rotaxane systems.4
The reliable external stimuli modes include pH,4a,b redox,2a,4c
light,2b,4d−f chemical input,4g and microenvironmental
changes4h in which light has been considered the most
promising mode to trigger rotaxane-based molecular machines
and shuttles.1g,5 In most cases, light-controlled rotaxane
molecular shuttles have been incorporated into the backbones
of azobenzene, stilbene, or fumaramide moieties.6 The trans/cis
or E/Z isomerizations of the above-mentioned photoresponsive
functional units can induce macrocyclic ring shuttling between
two distinguishable recognition sites in effective modes.
Recently, other strategies for light-controlled rotaxanes have
been reported,4d,e,7 such as the photoacid strategy7a and the
photosensitizer strategy.4d,e,7b These intelligent methodologies
provide important potential driving modes to trigger a
molecular shuttling motion in bistable or multistable rotaxanes.
However, it is still desirable to develop new, reliable, and
efficient light-controlled modes for the development of
rotaxane-based molecular shuttles.
A phototrigger, also known as a type of photolabile or
photoremovable group, represents an inherently irreversible
photoresponsive mode.8 Although it has been widely developed
in biomaterials and photoresponsive supramolecular polymer-
ization,8c−e little effort has been made for their applications in
the control of motion in rotaxane molecular shuttle systems.
Indeed, the inherent irreversibility of a phototrigger increases
the difficulty in constructing a switchable rotaxane system;
however, on the other hand, we foresee that the successful
introduction of a phototrigger would provide a novel strategy to
To control the degenerate rotaxanes, many chemists have
introduced a photoresponsive azobenzene unit6c or other
functional units9 between two identical stations to mediate the
shuttling movement of rotaxanes. Herein, we report the
successful triggering of a subunit shuttling motion in a
degenerate [2]rotaxane via a photochemical bond-cleavage
strategy (Figure 1a). In this design, a photolabile bulk barrier is
introduced and covalently bonded into and between two
recognition sites of the thread to prevent dynamic shuttling of
the macrocycle between two identical stations, resulting in a
“gated” state that has absolute distribution of the macrocycle on
one of the two stations. Upon irradiation of UV light (λ = 365
nm), complete photolysis occurs in a controllable manner,
resulting in the removal of the bulk barrier and the subsequent
recovery of balanced shuttling motion of the macrocycle, which
is referred to as an “open” state that has the same distribution
of the macrocycle in the two stations with a fast shuttling
motion. The inverse process from the “open” state to “gated”
state can be realized via a chemical rebonding of the
corresponding phototrigger into the rotaxane.
As shown in Figure 1b, the methyl-6-nitroveratryloxycarbon-
yl group, known as a typical phototrigger that responds to UV
light,8b,d,e,10 was selected as the bulky photolabile group. A well-
known Leigh-type11a,d−f hydrogen-bonding-assembled [2]-
rotaxane was chosen to investigate the phototriggered shuttling
motion. The dumbbell-shaped thread 4, which contains two
succinamide recognition sites, was treated with phototrigger 5
to yield compound 3, and the subsequent five-component
clipping reaction involving p-xylylenediamine, isophthaloyl
Received: February 13, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.7b00393
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1
Figure 2. H NMR spectra (400 MHz, DMSO-d6, and 298 K) of (a)
dumbbell-shaped thread 3 and (b) [2]rotaxane 1. See Figure 1b for
proton assignments.
due to the introduction of the macrocycle. Meanwhile, similar
shielding signals from protons H6−8 and H12 to protons H6′−8′
and H12′ are shown in Figure 2b, while the 0.11 ppm downfield
shift of H13′, the protons of the phenyl stoppers, was attributed
to the deshielding effects of the aromatic rings of the
macrocycle. In principle, rotaxane 1 exists as four stereoisomers,
as it contains two centers of point chirality: one (C1) arising
from the presence of the mechanically interlocked ring and the
other (CA) due to four different covalently linked substituents.
However, the signals of the stereoisomers were not observed
1
according to H NMR (Figure 2b), which is probably because
the macrocycle is located far away from the ester moiety,
making the mechanical point chirality poorly exhibited.
Therefore, all these observed results verified the interlocked
structure of the target [2]rotaxane 1, as shown in Figure 1.
The photocleavage reaction of [2]rotaxane 1 was detected by
1H NMR spectroscopy, as shown in Figure 3. A hand-held UV
Figure 1. (a) Schematic representation of the two modes of the
[2]rotaxane before and after irradiation with UV light; (b) synthetic
routes of the target [2]rotaxane 1, the dumbbell-shaped thread
compound 3, and 4, the photolysis reaction8f of [2]rotaxane 1 to yield
degenerate [2]rotaxane 2, and the chemical rebonding reaction from
[2]rotaxane 2 to [2]rotaxane 1.
chloride, and thread 3 in the presence of triethylamine11
resulted in the formation of [2]rotaxane 1.
The interlocked structure of the target [2]rotaxane 1 was
1
confirmed with H NMR and 13C NMR spectroscopies and
high-resolution electrospray ionization (HR-ESI) mass spec-
trometry. To confirm the interlocked structure of [2]rotaxane
1, we compared the H NMR spectra of dumbbell-shaped
1
Figure 3. H NMR spectra (400 MHz, DMSO-d6, 298 K, and 1.24 ×
1
10−2 M) of the photocleavage process of [2]rotaxane 1 to yield an
“open” [2]rotaxane 2.
thread 3 and [2]rotaxane 1, as shown in Figure 2a,b. The
protons of [2]rotaxane 1 (Figure 1b) were properly assigned,
and the chemical shifts at δ = 8.02, 7.62, 6.94, 8.31, and 4.17
ppm are attributed to the protons of the macrocycle (Ha−g).
Significantly, signals of the methylene protons (H9, H10) on the
succinamide station of dumbbell-shaped thread 3 were
observed as a single signal in Figure 2a because of the
symmetrical structure of thread 3. However, in Figure 2b, part
of these protons (H9′, H10′) were shielded (Δδ = −1.22 ppm)
lamp was used as the source of 365 nm UV light with an
intensity of 8W/cm2. The DMSO-d6 solution of [2]rotaxane 1
was sealed in a quartz NMR tube and irradiated continuously
by the UV light, which was monitored by 1H NMR
spectroscopy every 10 min. Obviously, as shown in Figure 3,
the continuous irradiation of UV light led to the disappearance
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DOI: 10.1021/acs.orglett.7b00393
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of the signals (δ = 6.14 ppm) of the protons HA and the signals
(δ = 1.62 ppm) of the protons HB as well as the appearance and
gradual intensity enhancement of the signals (δ = 2.78 ppm) of
the protons HB′ on the photocleavage reaction byproducts and
the signals (δ = 5.38 ppm) of the protons Hx on the hydroxyl of
[2]rotaxane 2. Slight changes observed in the signals of HE and
HF are also related to the minor change of the corresponding
chemical environment. Significantly, the signals (δ = 5.19 ppm)
of the protons H1 on the thread moiety of [2]rotaxane 1
disappeared completely, and the simultaneous appearance of a
new signal (δ = 4.07 ppm) can be explained as a change in
bonding from the carbonic ester in [2]rotaxane 1 to the
hydroxyl in [2]rotaxane 2. These observations directly show the
efficient and complete photocleavage reaction of the methyl-6-
nitroveratryloxycarbonyl group, leaving a hydroxyl group in the
obtained [2]rotaxane 2, in an expected photocontrolled mode,
as shown in Figure 1b. Meanwhile, the rate constant k = 0.0686
min−1 for the photocleavage process, which was calculated
according to the chemical kinetic eq (Figure S1).
Figure 3 (spectrum of the photochemical mixture). Obviously,
kinetic changes of the proton signals (H8−10 and H12) can be
observed in Figure 3, which is completely consistent with the
observation in Figure 4, indicating the successful realization of
photocontrolled activation of rotaxane 1 via a phototriggered
strategy; however, this strategy could result in the formation of
byproducts.
Interestingly, the observed shuttling motion of [2]rotaxane 2
in DMSO is not consistent with the results reported by Leigh11f
in which the macrocycle was localized over the alkyl linker
because of a solvophobic effect in DMSO. In this case, the
linker between the two recognition sites of [2]rotaxane 2 was
composed of a hydroxyl-substituted chain and benzene rings.
The shuttling motion between the two recognition sites of
[2]rotaxane 2 could be observed in DMSO instead of locating
in the middle of the linker, which is probably due to the less
hydrophobic hydroxyl-substituted chain and the steric effect of
the benzene rings.
For further evidence and investigation of the shuttling
motion of [2]rotaxane 2, variable-temperature 1H NMR
measurements were conducted in DMSO-d6 to elucidate the
kinetic parameters (Figure S2). Both the shuttling rate (ks) and
the free energy of activation (ΔG⧧) are similar to those in the
previous report (Figure S3).11f Finally, the inverse process,
converting from the “open” rotaxane to the “gated” rotaxane,
was implemented by a chemical rebonding method (Figure 1b
and Figure S4), indicating that the whole process can be
reversibly driven. Although a chemical “re-stoppering” can be
performed, the process is not fully efficient (79% yield in Figure
1b), and it may be difficult to perform a second switching cycle
in situ, which limits the reversibility of the approach.
To further confirm the final product of the photocleavage
process, we purified the “open” [2]rotaxane 2 for further
1
investigation. The H NMR spectra of [2]rotaxane 1, 2, and
dumbbell-shaped thread 4 are shown in Figure 4. Significantly,
In conclusion, a novel strategy for the construction of a
photocontrolled molecular shuttling rotaxane was successfully
proposed and demonstrated via the introduction of a
phototrigger as a removable bulk barrier. Shuttling motion in
a typical [2]rotaxane was activated by irradiation with UV light,
and the inverse process was driven by an efficient and facile
chemical-rebonding method. We envision that this novel
phototriggered strategy could serve as an efficient method for
the construction of photocontrolled rotaxanes and enrich the
driving forces for the operation modes of molecular machines,
thus advancing the study of artificial molecular machines.
1
Figure 4. H NMR spectra (400 MHz, DMSO-d6, and 298 K) of (a)
[2]rotaxane 1, (b) [2]rotaxane 2, and (c) dumbbell-shaped thread 4.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00393.
as shown in Figure 4a,b, the key signals at 2.48, 2.39, 1.34, and
1.18 ppm, i.e., the chemical shifts of H9, H9′, H10, and H10′ in
[2]rotaxane 1, respectively, merged into two single peaks (H9, δ
= 1.96 ppm; H10, δ = 1.79 ppm) in [2]rotaxane 2, suggesting
the two succinamide stations are in identical chemical
environments in [2]rotaxane 2. Meanwhile, compared with
the signals of the protons H9 and H10 on reference thread 4
without an interlocked macrocycle, a shielding effect still
existed (Δδ = −0.59 ppm), proving the macrocycle still
included the two succinamide stations in [2]rotaxane 2.
Therefore, it can be concluded that the macrocycle included
two identical succinamide stations in [2]rotaxane 2 and that the
macrocycle was characterized by fast shuttling between the two
stations along the thread in [2]rotaxane 2. The corresponding
change of the observed deshielding effect of proton H11 and the
merger of the peaks of protons H5−7 and H12 also prove the
active shuttling. To consider whether a similar shuttling motion
of [2]rotaxane 2 occurs in a photocontrolled mode and
whether this mode has a mixed nature, further comparison is
needed between Figure 4b (spectrum of the pure sample) and
■
S
Details of experimental procedures and characterization
data, investigation of shuttling dynamics, and calculation
of the rate constant for photocleavage process (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dahui_qu@ecust.edu.cn.
ORCID
Da-Hui Qu: 0000-0002-2039-3564
He Tian: 0000-0003-3547-7485
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.7b00393
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ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21672060, 21421004), the Fundamental
Research Funds for the Central Universities, and the
Programme of Introducing Talents of Discipline to Universities
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