several hours released the mechanisorbed
rings, MPCG13+and Zr-BTB, for subsequent
use ( 53 ). After desorption, the solid was di-
gested and analyzed by^1 H NMR spectroscopy
(fig. S31), confirming the complete release
of MPCG13+and CBPQT4+rings. The mech-
anisorbed rings are desorbed by lowering the
barrier associated with the gate as a result
of acid-triggered cleavage of coordination
and mechanical bonds. Rings escape spon-
taneously from a high-concentration compart-
ment into the bulk compartment with a much
lower concentration.
After the successful alignment of AMPs on
MOF nanosheets, we extended the construction
of AMP-grafted MOFs to three dimensions
(figs. S67 to S69). The surfaces of UiO-66 nano-
particles ( 54 ) can also be decorated with an
abundance of MPCG13+. Accordingly, CBPQT4+
rings can be adsorbed subsequently onto the
surface of the UiO-66-MPCG1 by redox cy-
cling and desorbed by treatment with acid.
The approach is generalizable to other types
of surfaces.
Repeated and near-quantitative
mechanisorption
Taking cues from previous investigations
( 12 – 15 ), we designed a system to accumulate
a couple of rings by incorporating a steric bar-
rier between a BIPY2+unit and a collecting
chain to form a full pumping cassette. The
steric barrier prevents adsorbed rings from
interfering with the subsequent recruiting
of rings from solution during the repeated
redox cycles. As a proof-of-concept, an iso-
propylphenylene (IPP) unit was introduced
(Fig. 1C) as a steric barrier into the backbone of
MPCG23+by a copper-catalyzed azide–alkyne
(click) cycloaddition ( 55 ). We synthesized Zr-
BTB-MPCG2 through sequential acid-base and
click reactions performed on these nanosheets
(figs. S32 and S33). The success of the click re-
action was verified by IR spectroscopy (figs.
S75 and S76), where the intensity of an azide
stretching band at 2095 cm−^1 decreased after
the click reaction. The^1 H NMR spectrum of
the digested Zr-BTB-MPCG2 indicates (fig.
S41) that 68% of 12-azidododecanoic acid
had reacted and that the density of MPCG23+
on the Zr-BTB-MPCG2 surface is close to that
of MPCG13+on Zr-BTB-MPCG1 (fig. S48). Once
again, by an energy-ratchet mechanism (Fig. 4),
CBPQT4+rings bind and traverse synchro-
nously the pumping cassettes of the dumbbell
arrays after a single redox cycle and become
localized on the collecting chains. A second
redox cycle results in the installation of a
second set of rings on the collecting chains of
most of the [2]rotaxanes, which thereby be-
come [3]rotaxanes grafted to Zr-BTB-MPCG2-
CBPQT(+)-II. Because the amount of BTB in the
solid material remains constant throughout
the redox cycling, BTB is used as an inter-
nal standard for the determination of the
CBPQT4+ring-loading efficiency.^1 H NMR
spectroscopic analysis (fig. S47) of digested
(D 2 SO 4 /CD 3 SOCD 3 ) materials indicates a 96
and 92% threading efficiency after the first
and second redox cycles, respectively, in keep-
ing with the efficiency observed in solution
( 13 ). Notably, these highly energetic and me-
chanically interlocked arrays maintain (figs.
S44 and S45) their chemical stability in MeCN
foratleast2weekswithoutlosinganyofthe
mechanisorbed rings from Zr-BTB-MPCG2.
The material also demonstrates high thermal
stability, as indicated by thermogravimetric
analysis (TGA) coupled with mass spectrom-
etry (figs. S85 to S87). These results suggest
that Zr-BTB-MPCG2 acts on CBPQT4+rings to
create (Fig. 4) a gradient in their local solid
and solution concentrations, moving increas-
ingly away from equilibrium, from 55.4 to
111 mmol/liter after the second redox cycle
(fig. S49). The cycled mechanical adsorption ofrings on Zr-BTB-MPCG2 was confirmed (figs.
S71 and S76) by IR spectroscopy and^13 C and(^1) H MAS SSNMR spectra, all of which demon-
strates the stepwise away-from-equilibrium
mechanisorption of the CBPQT4+rings toward
ever higher local concentrations on the Zr-
BTB surface.
Encouraged by these results, we designed
and synthesized a high-capacity mechani-
sorption system—namely, Zr-BTB-MPCG3—
constructed (Fig. 1D) from the MPCG33+with
a polyethylene glycol (PEG) collecting chain
located between the IPP steric barrier and the
CG gate. The PEG chain (the number average
molecular weight,Mn= 2000) allows ( 15 ) for
multiple repeated and near-quantitative redox-
driven mechanical adsorptions of CBPQT4+
rings on Zr-BTB-MPCG3. We produced Zr-
BTB-MPCG3-CBPQT(+)-I, -II, -III, -IV, and -V
(Fig. 5) by subjecting Zr-BTB-MPCG3 to mul-
tiple redox cycles. The cyclical redox-driven
processes deliver rings consecutively onto the
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Fig. 4. Flashing energy-ratchet mechanism driving rings away from equilibrium toward a higher local
concentration on the Zr-BTB-MPCG2 surface.CBPQT4+concentration analysis in the solid and the
solution phases before and after the second pumping cycle of Zr-BTB-MPCG2-CBPQT(+)-I. The pump arrays
act on the CBPQT4+rings to create a gradient in their local solid and solution concentrations, moving
increasingly away from equilibrium, from 55.4 and 7.67 mmol/liter to 111 and 5.75 mmol/liter after the
second redox cycle, respectively. Molar concentration of CBPQT4+rings on surfaces is estimated by dividing
the amount of CBPQT4+rings on MOF surfaces (accessed by solution-state NMR) by the estimated volume of
the MOF surfaces (the MOF surface area multiplied by the height of collecting chains). Bottom portion of the
figure shows energy profiles representing the free energies of the integrated system, as the rings are pumped
collectively from the solution phase by pumping cassettes onto the solid surfaces. The mechanisorbed
high-concentration rings can be released into the bulk spontaneously after lowering the barrier associated
with the gate. The green curved arrows on the energy profiles represent the kinetically favored reaction
pathways during the adsorption process. The energy diagram is illustrated with purple as the reduced state
and blue as the oxidized state.
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