gate (CG) at the other end with a ring-collecting
chain located in the middle of a pseudo-
dumbbell. An acid-base reaction (Fig. 3C and
fig. S13) between Zr-BTB and MPCG13+in
MeCN at room temperature for 3 days gen-
erated Zr-BTB-MPCG1. Its PXRD pattern dem-
onstrates (fig. S77) that the structure of Zr-BTB
is maintained during the grafting of the mo-
lecular pump, whereas^1 H NMR spectroscopy,
carried out on digested (D 2 SO 4 /CD 3 SOCD 3 )
Zr-BTB-MPCG1, indicates the preservation of
MPCG13+during the grafting process (Fig. 3E
and fig. S24). The approximate composition
of Zr-BTB-MPCG1 was calculated (table S2)
to be Zr 6 (BTB)2.4(MPCG1)0.4,showingthateach
Zr 6 cluster is coordinated to 0.4 MPCG13+and
that the Zr-BTB-MPCG1 contains ~10^17 orga-
nized MPCG13+per milligram or, expressedanother way, 10^13 per square centimeter. The
average distance between pairs of molecular
pumps in Zr-BTB-MPCG1 is estimated to be
3.1 nm (fig. S48), a separation that is large
enough to ensure the independence of the dy-
namics associated with each MPCG13+, while
preserving the essential characteristics and
properties of the robust MOF.
The surface modification of Zr-BTB is real-
ized through the partial replacement of the
OH−/H 2 O ligands on the MOF surface by
MPCG13+carboxylates. The difference in the
local bonding state between Zr-BTB and Zr-
BTB-MPCG1 has been probed (figs. S73 and S74)
by infrared (IR) spectroscopy. Above 3500 cm−^1 ,
a sharp band at 3698 cm−^1 is observed for Zr-
BTB and is assigned to the stretching mode of
the OH−/H 2 O coordinated to Zr. The shift of
this band to 3648 cm−^1 indicates the changes
of Zr 6 coordinating environments and the
successful substitution of OH−/H 2 O ligands
on the Zr 6 clusters by the CG of MPCG13+.
Additionally, the signals of the coordinating
MPCG13+can also be identified (fig. S70) in the(^13) C magic-angle spinning (MAS) solid-state (SS)
NMR spectra of Zr-BTB-MPCG1: carbons from
methylene and methyl groups that resonate at
30 to 40 parts per million (ppm) and carbon
from BIPY2+/2,6-dimethylpyridinium (PY+)
groups that resonate at 139 ppm can both be
identified. Compared with the^1 H MAS SSNMR
spectrum of Zr-BTB, the Zr-BTB-MPCG1 spec-
trum shows (fig. S70) an increase in the peak-
area intensities for protons from the benzenoid
rings and a decrease in the peak-area intensities
of terminal OH−/H 2 O groups, demonstrating
the successful exchange of the OH−/H 2 O li-
gands by MPCG13+.
Redox-driven mechanisorption and
acid-triggered desorption
The redox-driven mechanisorption ( 9 , 45 ) of
CBPQT4+rings on Zr-BTB-MPCG1 operates by
an energy-ratchet mechanism (fig. S57). The
redox state of the system influences the height
of the electrostatic barriers for threading of
the rings. The barrier, arising from PY+, is much
larger for CBPQT4+than for its radical state
CBPQT2(•+). The redox state also influences the
binding energy strongly because of radical-
pairing interactions and Coulombic repul-
sion. Upon reduction, all the BIPY2+units are
reduced to BIPY•+radical cations, leading to
the threading (Fig. 3C) of CBPQT2(•+)rings
onto the pumping cassettes to form trisradical-
tricationic complexes ( 51 ) based on strong
radical-radical interactions. After oxidation,
Coulombic repulsion between the PY+/BIPY2+
units and the CBPQT4+ring in the pumping
cassette force the ring to move onto the col-
lecting chain of MPCG13+, resulting in the
formation of a two-dimensional array of
[2]rotaxanes with unreacted dumbbells inter-
spersed among them.
SCIENCEscience.org 3 DECEMBER 2021•VOL 374 ISSUE 6572 1217
Fig. 2. Mechanisorption on MOF nanosheets with organized molecular pumps.(A) A graphical
representation of a mechanisorption mechanism summarizing how redox and acid-base chemistry can
be used to adsorb and desorb precisely a fleet of rings onto and off the surface of an MOF. Step 1:
Grafting of polyAZCG onto the MOF surface followed by a click reaction with an alkyne containing
the pumping cassette. Step 2: Adsorption of rings by AMP-grafted MOF nanosheets after multiple redox
cycles. Step 3: Acid-driven desorption of rings and MPCG33+.(B) The cascade processes taking place
at each representative molecular pump, leading to quantitative ring adsorption on organized polymer chains
attached to the MOF surface and subsequent acid-triggered ring release: (i) grafting of MPCG33+on
the MOF surface; (ii) repeated redox-driven adsorption of rings; (iii) reduction-driven adsorption of rings from
solution via the pumping cassette and the formation of trisradical-tricationic complexes on the basis
of radical-radical interactions; (iv) oxidation-driven repulsion of rings from the pumping cassette onto the
collecting chain (PEG) and the formation of mechanically interlocked extended oligorotaxanes; (v) acid-
triggered desorption of rings associated with the cleavage of coordination bonds between MPCG33+and
surfaces as well as mechanical bonds between rings and surfaces. The conformations and arrangement of
MPCG33+are idealized for the sake of clarity.
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