Artificial Molecular Pump Operating in Response to Electricity and Light

Article abstract of DOI:10.1021/jacs.0c06663

The ability to control the relative motions of component parts in molecules is essential for the development of molecular nanotechnology. The advent of mechanically interlocked molecules (MIMs) has enhanced significantly the opportunities for chemists to harness such motions in artificial molecular machines (AMMs). Recently, we have developed artificial molecular pumps (AMPs) capable of producing highly energetic oligo- and polyrotaxanes with high precision. Here, we report the design, synthesis, and operation of an AMP incorporating a photocleavable stopper that allows for the use of orthogonal stimuli. Our approach employs a ratchet mechanism to pump a ring onto a collecting chain, forming an intermediate [2]rotaxane. At a subsequent time, application of light triggers the release of the ring back into the bulk solution with temporal control. This process is monitored by the quenching of the fluorescence of a naphthalene-based fluorophore. This design may find application in the fabrication of molecular transporting systems with on-demand functions.

Full text of DOI:10.1021/jacs.0c06663

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Communication
Artificial Molecular Pump Operating in Response to Electricity and Light
Qing-Hui Guo, Yunyan Qiu, Xinyi Kuang, Jiaqi Liang, Yuanning Feng, Long
Zhang, Yang Jiao, Dengke Shen, R. Dean Astumian, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c06663 • Publication Date (Web): 31 Jul 2020
Downloaded from pubs.acs.org on July 31, 2020
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Artificial Molecular Pump Operating in Response
to
Electricity and Light
†,
§
†,
†
†
†
§
Qing-Hui Guo, Yunyan Qiu, Xinyi Kuang, Jiaqi Liang, Yuanning Feng,
Long Zhang,† Yang Jiao,† Dengke Shen,† R. Dean Astumian,*,‡ and J. Fraser Stoddart*,†,¥, #
†Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208, United States
‡Department of Physics, University of Maine, 5709 Bennet Hall, Orono, Maine 04469,
United States
¥Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road,
Nankai District, Tianjin 300072, China
#School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
*E-mail: astumian@maine.edu stoddart@northwestern.edu
§These authors contributed equally to this work
MAIN TEXT
*Correspondence Address
Professor J Fraser Stoddart
Department of Chemistry
Northwestern University
2145 Sheridan Road
Evanston, IL 60208 (USA)
Email: stoddart@northwestern.edu
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ABSTRACT: The ability to control the relative motions of component parts in
molecules is essential for the development of molecular nanotechnology. The advent of
mechanically interlocked molecules (MIMs) has enhanced significantly the
opportunities for chemists to harness such motions in artificial molecular machines
(AMMs). Recently, we have developed artificial molecular pumps (AMPs) capable of
producing highly energetic oligo- and polyrotaxanes with high precision. Here, we
report the design, synthesis, and operation of an AMP incorporating a photocleavable
stopper that allows for the use of orthogonal stimuli. Our approach employs a ratchet
mechanism to pump a ring onto a collecting chain, forming an intermediate [2]rotaxane.
At a subsequent time, application of light triggers the release of the ring back into the
bulk solution with temporal control. This process is monitored by the quenching of the
fluorescence of a naphthalene-based fluorophore. This design may find application in
the fabrication of molecular transporting systems with on-demand functions.
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Biomolecular machines,1 such as transmembrane ion-pumps,2 use energy to control directional
transport of ions across membranes. Operation of these machines is necessary for the
maintenance of many other biological functions. Sophisticated and highly efficient
biomolecular machines1b have long been a source of motivation to physical scientists to
emulate their functions by designing and constructing artificial molecular counterparts.3 The
burgeoning of research on wholly synthetic molecular machines4—built from the bottom-up
on the nanometer scale—has been one of the results of this effort to emulate biology. The past
three decades have witnessed the synthesis of unnatural compounds5 that are capable of
executing controlled relative motions within their molecules. Mechanically interlocked
molecules6 (MIMs), particularly switchable rotaxanes7 and catenanes8, have become a focal
point for the design and synthesis of artificial molecular machines9 (AMMs). The mechanical
bond10 in MIMs permit their component parts to translocate relatively long distances in a
controlled manner. The flexibility with which different units can be arranged in the MIMs
allows for the systematic design of machine-like functions.
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The fundamental principles governing9b,11 these controlled relative motions in MIMs rely on
switchable noncovalent bonding interactions12 and can be induced by a range of external
stimuli, e.g., chemicals,8d,8e,13 electricity,14 and light.15. Electricity14- and light15-driven
AMMs16 represent synthetic targets with the appealing advantages of ease of operation and
elimination of waste products. In addition, the use15 of light to trigger structural changes,
associated with the AMMs, offers16 spatial and temporal control. Here, we report a rotaxane-
based linear molecular pump that unidirectionally transports a ring, powered16 by an orthogonal
combination of electrical and optical stimuli. Previously, we have utilized14b,17 chemical and
electrochemical redox stimuli to power the operation of artificial molecular pumps (AMPs),
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creating a locally high concentration of cyclobis(paraquat-p-phenylene) (CBPQT4+) rings on
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a collecting chain.
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Herein, we describe the design, synthesis (Scheme 1) and operation (Figure 1) of a dumbbell-
shaped AMP with a pumping cassette18 on one end and terminated by a photocleavable stopper
at the other end. When stimulated sequentially by electricity and light, the CBPQT4+ ring
undergoes unidirectional transport onto, along the collecting chain and off from the other end
of the dumbbell, forming an intermediate [2]rotaxane R7+ along the way. The photocleavable
molecular pump PcMP3+ consists (Figure 1a) of (i) a redox-active bipyridinium (BIPY2+) unit
located between (ii) a 2,6-dimethylpyridinium (PY+) Coulombic barrier17b and (iii) an
isopropylphenylene (IPP) steric barrier17c, connected through (iv) an oligomethylene collecting
chain to (v) a photocleavable stopper (PcS) by (vi) a triazole (T) ring. The choice of the
photochemical cleavable stopper was based on an o-nitrobenzyl alcohol derivative 1 which has
been used19 widely as a photolabile protecting groups in organic19a,19b and polymer19c
chemistry, and also in biology19d. The phenyl group on the α position (Scheme 1) introduced
in 120 guarantees the stopper is large enough in the pump PcMP3+ to prevent the de-threading
of the ring from the collecting chain. The synthesis of PcMP•3PF6 was accomplished (Scheme
1) in three steps, starting from commercially available 3,4-dimethoxy-2-nitrobenzaldehyde.
See Section B in Supporting Information (SI) for further details relating to the synthesis of the
photocleavable molecular pump.
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The redox-driven pumping process uses electricity to recruit a ring from the bulk solution by
an energy ratchet mechanism11b,21. This mechanism relies17,18 on the manipulation of the
heights of kinetic barriers and the depths of thermodynamic wells resulting from interactions
between the ring and the units of which the pumping cassette is comprised. As the ring begins
the process of threading onto the pump, the Coulombic barrier presented by PY+ is very large
if the ring is in its oxidized state but is much smaller if the ring is in its reduced state.
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Furthermore, the binding interaction between the CBPQT2(+•) ring and BIPY+• is very strong22,
whereas the interaction between the CBPQT4+ ring and BIPY2+ is repulsive (Figure 1b, I). In
contrast, the barrier imposed by IPP is essentially independent of the redox state of the ring.
As a result of these redox-dependent interactions, reduction followed by oxidation recruits a
ring from the bulk solution and deposits it on the collecting chain, forming the [2]rotaxane R7+.
After the photocleavage of the stopper in R7+, the ring escapes into the bulk solution completing
the pumping action.
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The application of a constant reduction potential of −700 mV to a reaction mixture of PcMP3+
and CBPQT4+ in MeCN, inside the working half-cell of a custom-built electrolysis apparatus
(Section C, SI), reduces all of the BIPY2+ to BIPY+• units present in both the ring and on the
pump. The CBPQT2(+•) ring and BIPY+• unit enter into radical-pairing interactions, forming
(Figure 1b, II) a trisradical tricationic complex22. Subsequent oxidation, by employing an
oxidation potential of +700 mV restores the three positive charges to the pump, bringing the
total charge up to +7. As a result of Coulombic repulsion, the ring passes over the IPP barrier
(not over PY+) onto the collecting chain in a thermally activated process, affording (Figure 1b,
III) a kinetically trapped out-of-equilibrium [2]rotaxane R7+. The structure of the isolated
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rotaxane has been confirmed by H (Figures 2b and S2), DOSY (Figure S3) and NOESY
(Figure S6) NMR spectroscopy. Comparisons of the 1H NMR spectra of PcMP3+ (Figure 2a)
and R7+ (Figure 2b) reveal a large upfield movement in the chemical shifts of the probe-
resonances23 for H17′ through H25′, arising from the shielding influence of the CBPQT4+ ring
trapped on the collecting chain.
Continuous UV irradiation (365 nm) triggers the photocleavage of the stopper PcS in R7+,
resulting in the release (Figure 1b, IV) of the threaded ring into the bulk solution, leaving a
pseudodumbbell PDB3+ containing the pumping cassette18 with a terminal carboxylic acid and
the cleaved stopper PcS*. The reaction proceeds to completion as shown (Figure 2c) by the
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return of the chemical shifts21 for Hα and Hβ on CBPQT4+ ring and the collecting chain (H17*
to H25*) to their original values (Figures 2a, S2, and S5) on account of similar chemical
environments experienced by PcMP3+ and PDB3+. This observation shows the absence of
noncovalent bonding interactions between the collecting chain and the ring, thus demonstrating
the complete release of the ring from the [2]rotaxane.
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The release of the ring upon UV irradiation (Figure 3a) was monitored (Section D, SI) by 1H
NMR (Figures S7−S12) spectroscopy as follows―a solution of R•7PF6 in CD3CN (c0 = 1 mM)
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was subjected to UV irradiation for 25 min, during which time a series of H NMR spectra
were recorded. The molar fractions of the resulting R7+, PDB3+, PcS*, and CBPQT4+ were
obtained (Figure 3b) from integration of the proton resonances (Figures S10−S12) for H18',
H18*, H30*, and Hα, respectively. The rate constants for both the photocleavage and ring release
process―with respect to the decreased intensities of proton resonances H30' for the stopper PcS,
and Hα' for the threaded CBPQT4+ ring, respectively―were found (Figure 3c) to be (0.216 ±
0.003) and (0.218 ± 0.003) min–1. The identical first-order rate constants reveal that the
photolysis and ring release processes occur synchronously.
The efficient and controlled release of the ring from the collecting chain was also demonstrated
(Figure 4a) by fluorescence quenching experiments. The addition of 3 equiv of R•7PF6 to the
MeCN solution of the 1,5-dioxynaphthalene (DNP) fluorophore, quenched the fluorescence on
account of the formation (Figure 4a) of a donor-acceptor inclusion complex24 between DNP
and the CBPQT4+ ring released during the photolysis of the [2]rotaxane. The quenching was
monitored (Figure 4b) by recording the fluorescence spectra over time. The normalized
fluorescence intensities at 327 and 342 nm decreased (Figure 4b) and reached the steady state
after a sum total of 18 min UV irradiation. Temporal control of the ring release process was
demonstrated (Figure 4c) by switching the UV light on and off alternately.
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In summary, we have demonstrated the operation of an artificial molecular pump by applying
orthogonal electrical and optical stimuli sequentially. The controlled capture of the ring from
the bulk solution energetically uphill using electricity and subsequent release of the ring
energetically downhill using light results in the unidirectional transport of the ring onto, along,
and off the dumbbell employing an energy ratchet mechanism. The fast release of the ring from
the intermediate [2]rotaxane occurs synchronously with the photolysis. The current choice of
the irreversible photocleavable stopper, in principles, grants access to drug delivery or cargo
release platforms in which rapid and controlled-release kinetics are essential. This pump design
provides increased richness for control of the relative motions of component parts in artificial
molecular machines. The use of orthogonal stimuli, which introduces this higher level of
control, may find applications in the integration of multiple molecular machines working in
concert.
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■ ASSOCIATED CONTENT
Supporting Information
Detailed information regarding the experimental methods, procedures, supportive figures and
schemes. This material is available free of charge via the Internet at http://pubs.acs.org.
■ ACKNOWLEDGEMENTS
The authors thank Northwestern University for financial support. This work made use of the
Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern
University, which has received support from the Soft and Hybrid Nanotechnology
Experimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois, and the
International Institute for Nanotechnology (IIN).
■ AUTHOR INFORMATION
Corresponding Authors
astumian@maine.edu
stoddart@northwestern.edu
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Notes
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Guo, Q.-H.; Qiu, Y. and Stoddart, J. F. have a patent application through Northwestern
University based on this work: A molecular pump powered sequentially by electricity and light.
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Captions to Scheme and Figures
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Scheme 1. Synthesis of the photocleavable molecular pump PcMP•3PF6
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Figure 1. The design and operation of PcMP3+ powered sequentially by electricity and light.
(a) Graphical representations and structural formulas of CBPQT4+, PcMP3+, and the photo-
cleaved units, PDB3+ and PcS*. The key protons on the PcMP3+ (1−34), CBPQT4+ (CH2, α, β,
and Xyl), PDB3+ (17*−25*), and PcS* (28*−34*) are labeled in order to aid the interpretation
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1
of the H NMR spectra (Figure 2) recorded in CD3CN. (b) Graphical representations of the
unidirectional transport of a ring onto, along, and off the dumbbell and the associated energy
profiles in which the green and red curved arrows indicate kinetically favored and disfavored
transitions, respectively. In Stage I, the positively charged ring cannot thread onto the dumbbell
because of strong Coulombic repulsions with PY+ and steric interactions with the PcS. In Stage
II, reduction by a negative potential of −700 mV lowers the Coulombic barrier and favors the
formation of a stable trisradical tricationic complex. In Stage III, oxidation by a positive
potential of +700 mV destabilizes the interaction between the ring and the recognition site
while simultaneously restoring the Coulombic barrier arising from PY+, preventing the escape
of the ring into the bulk solution. The ring must thus pass over the steric barrier IPP and thread
on the collecting chain. In Stage IV, photocleavage of the stopper PcS upon UV irradiation
allows the ring to return to the bulk solution.
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Figure 2. H NMR Spectra (500 MHz, CD3CN, 298 K) of (a) PcMP•3PF6, (b) the isolated
[2]rotaxane R•7PF6, and (c) a physical mixture of PDB•3PF6, CBPQT•4PF6, and PcS*
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resulting from UV irradiation of R7+ for 25 min. For full assignments, see Figures S1 and S4
in the Supporting Information.
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Figure 3. Kinetic analysis of ring release as monitored by 1H NMR spectroscopy. (a) Graphical
illustration of the ring release following UV irradiation. (b) Plots of molar fractions of R7+,
PDB3+, PcS*, and CBPQT4+ with time during photocleavage. (c) Plots of the ln(c) versus time
where c is the concentration of either CBPQT4+ (green squares) or PcS (blue circles).
Figure 4. Kinetic analysis of ring release as monitored by fluorescence quenching. (a)
Graphical illustration of ring release in the presence of a DNP fluorophore. (b) A stack of
fluorescence spectra taken at different times. Inset: The normalized fluorescence intensities at
327 and 342 nm as a function of the irradiation time. (c) Illustration of temporal control where
the light is turned on and off as indicated by the gray bars.
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Scheme 1
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Figure 1
ACS Paragon Plus Environment

Journal of the American Chemical Society
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Figure 2
ACS Paragon Plus Environment

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Figure 3
ACS Paragon Plus Environment

Journal of the American Chemical Society
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Figure 4
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ACS Paragon Plus Environment