steady state indefinitely, before being released
on demand into the bulk by a process that
involves the breaking of coordinative, me-
chanical, and noncovalent bonds. Unlike
physisorption or chemisorption, we show that,
when coupled with an energy source, the pas-
sive movement of adsorbates from regions of
high to low concentration is reversed, giving
rise to active adsorption by which adsorbate
concentrations are maintained with large dif-
ferences in chemical potential on surfaces and
in bulk solution. This nonequilibrium pump-
ing is achieved on MOF nanosheets and nano-
particles, which not only function as extended
stoppers but also make it possible to align and
organize arrays of AMPs. These AMP-grafted
MOFs are capable of undergoing repeated
and near-quantitative adsorption of rings—cyclobis(paraquat-p-phenylene) (CBPQT4+,
Fig. 1A)—from solution to form high-energy
oligorotaxanes under redox control. These
rings, which contain redox-switchable bipyr-
idinium (BIPY2+) units, are sequestered under
reducing conditions from solution and trans-
ferred on oxidation in a precise manner by
pumping cassettes using an energy-ratchet
mechanism onto the collecting chains of the
oligorotaxanes ( 9 , 45 ). This redox-controlled
active transport (Fig. 2B) transfers rings
from bulk solution to much higher local con-
centrations on surfaces, enabling repeat-
able mechanisorption to occur in a way that
drives an entire integrated system away from
equilibrium.Construction of AMP-grafted MOFs
MOFs have many features—well-defined com-
position, high stability, controllable stoichi-
ometry, and variable spacing of the surface
sites—that made them particularly suitable
for our initial investigation of mechanisorp-
tion. We selected a robust, ultrathin MOF
nanosheet composed of Zr-BTB ( 46 , 47 ),
where BTB stands for 4,4′,4′′-benzene-1,3,5-
triyl-tris(benzoic acid) (Fig. 3A), as an ideal
platform on which to graft AMPs. This two-
dimensional MOF is constructed from six‐
connected [Zr 6 (m 3 -O) 4 (m 3 -OH) 4 ] (Zr 6 ) clusters
and tritopic carboxylate linkers (BTB). The
involvement of water during the synthesis
hampers ( 46 ) the packing of the Zr‐BTB layers
substantially, leading to the formation of
nanosheets. Transmission electron micros-
copy (TEM) of Zr-BTB (fig. S89) reveals that the
nanosheets exhibit slightly wrinkled planes,
with the Zr 6 clusters appearing as dark spots.
These nanosheets can be easily dispersed into
solution, forming colloidal suspensions, ex-
hibiting a Tyndall effect (fig. S88) and offer-
ing a robust and dispersible platform for the
attachment of AMPs. The phase purity of Zr-
BTB has been verified by powder x‐ray dif-
fraction (PXRD) (fig. S77), and its monolayer
structure confirmed by the asymmetric dif-
fraction peaks ( 48 ). Proton nuclear magnetic
resonance (^1 H NMR) spectroscopy of di-
gested Zr‐BTB indicates (figs. S1 to S11) that
the Zr 6 clusters, in the form of as-synthesized
nanosheets, are capped mainly by benzoates
(BA−). After numerous attempts, we were able
to substitute the capping benzoates with
OH−/H 2 O ligands by treating the nanosheets
with either HCl/H 2 O or HCl/MeCN. These
exposed Zr−OH−/H 2 O sites are extremely
reactive toward carboxylic acids, even at room
temperature ( 49 , 50 ).
A carboxyl-terminated molecular pump,
MPCG13+, was designed (Fig. 1B). This pro-
totype consists of a Coulombic barrier and a
switchable recognition site at one end forming
a pseudo pumping cassette, in addition to a re-
versible loading and unloading carboxylate1216 3 DECEMBER 2021•VOL 374 ISSUE 6572 science.orgSCIENCE
Fig. 1. The structural formulas for the CBPQT4+ring and three generations of molecular pumps (MP)Ñ
MPCG13+, MPCG23+, and MPCG33+.(A) The structural formula for the CBPQT4+ring. Selected key
protons on CBPQT4+(Phen, CH 2 ,a, andb) are labeled to aid the interpretation of^1 H NMR spectra
reproduced in Fig. 3. (B) The prototype MPCG13+consists of a PY+Coulombic barrier and a BIPY2+
recognition site at one end (the pseudo pumping cassette), a reversible loading and unloading
carboxylate gate (CG) at the other end, and a ring-collecting oligomethylene (OM) chain located in
the middle. Selected key protons on MPCG13+(numbered 1 and 4 to 10) are labeled to aid the
interpretation of^1 H NMR spectra reproduced in Fig. 3. (C)ComparedwithMPCG13+, MPCG23+has an
additional steric barrier in the form of an isopropylphenylene (IPP) unit inserted between the BIPY2+
recognition site and the collecting chain (OM)—which is attached to IPP by a triazole (TRZ) ring
resulting from a click reaction between an azide and an alkyne—allowing for the repeated collection
of CBPQT4+rings from solution. (D)ComparedwithMPCG23+, MPCG33+consists of a collecting
polyethylene glycol (PEG) chain instead of an OM chain located between the steric barrier (IPP) and the
carboxylate gate (CG), allowing for the repeated collection of CBPQT4+rings from solution. MPCG33+is
prepared by a click reaction between polyAZCG—which has an azide (AZ) attached to the PEG chain
terminated by CG—and an alkyne functionalized with the pumping cassette. The PF 6 – counterions are
omitted for the sake of clarity.
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