RESEARCH ARTICLES
◥MECHANICAL BONDS
Active mechanisorption driven by pumping cassettes
Liang Feng^1 †, Yunyan Qiu^1 †, Qing-Hui Guo2,3, Zhijie Chen^1 , James S. W. Seale^1 , Kun He^4 , Huang Wu^1 ,
Yuanning Feng^1 , Omar K. Farha1,5, R. Dean Astumian^6 , J. Fraser Stoddart1,2,3,7*
Over the past century, adsorption has been investigated extensively in equilibrium systems,
with a focus on the van der Waals interactions associated with physisorption and electronic
interactions in the case of chemisorption. In this study, we demonstrate mechanisorption, which
results from nonequilibrium pumping to form mechanical bonds between the adsorbent and
the adsorbate. This active mode of adsorption has been realized on surfaces of metal-organic
frameworks grafted with arrays of molecular pumps. Adsorbates are transported from one well-
defined compartment, the bulk, to another well-defined compartment, the interface, thereby
creating large potential gradients in the form of chemical capacitors wherein energy is stored
in metastable states. Mechanisorption extends, in a fundamental manner, the scope and
potential of adsorption phenomena and offers a transformative approach to control chemistry
at surfaces and interfaces.
A
dsorption-based phenomena play an es-
sential role in present-day approaches to
catalysis, energy storage, and environ-
mental remediation. Langmuir ( 1 ) and
Lennard-Jones ( 2 ) observed (Table 1) in
the 1930s that adsorbates interact with sur-
faces through van der Waals interactions
(physisorption) and/or by electronic interac-
tions (chemisorption). Sorption is generally
thought of as a passive process in which ad-
sorbates move from regions of high to low
concentration such that the amount of ad-
sorbates at surfaces is always changing in
the direction determined by approach to
equilibrium. Although our understanding
of the equilibrium aspects, involving these
interactions and their practical applications,
has increased drastically ( 3 – 7 ), little effort
has been made to adsorb molecules away
from equilibrium. One approach to develop-
ing nonequilibrium adsorption ( 8 , 9 ) is to
immobilize artificial molecular machines
on surfaces.
Conceptually, molecules can be pumped
( 10 ) by modulation of energy barriers and
wells. A dynamic superstructure can be im-
plemented chemically in the form of a pseudo-
rotaxane ( 11 ) with a switchable electrostatic
stopper and a steric barrier surrounding an
addressable recognition site that presents
awellinwhichthebindingenergycanbe
modulated. Recently, we have referred to
this combination of components as a pump-
ing cassette ( 12 – 19 ). Artificial molecular pumps
(AMPs) use pumping cassettes to induce many
rings to reside away from equilibrium on col-
lecting chains. The essential modus operandi
of AMPs is (Fig. 1) to recruit rings from bulk
solution to the addressable recognition sites
within the pumping cassettes in the first
phase of an external modulation and force
them onto a collecting chain in the second
phase. Specific examples of AMPs that rely
on redox chemistry for their operation are
described in recent reviews ( 17 – 20 ). There is
great flexibility with regard to powering theseAMPs where energy can be introduced in
the form of chemical fuel ( 12 , 13 ) or elec-
tricity ( 14 – 16 ) to produce high-energy oligo-
rotaxanes with a precisely controllable number
of rings.
Although AMPs have been used to drive non-
equilibrium chemistry ( 9 , 21 – 26 ), they exhibit
limited organization in bulk solution on ac-
count of their random orientations. By orga-
nizing AMPs on surfaces ( 27 ), it is possible to
use the pumping cassette to do exactly what it
does in the bulk solution, but in a confined
configuration. The result is the transport of
rings from one well-defined compartment, the
bulk, to another well-defined compartment,
the interface, thereby creating a very large
chemical potential gradient commensurate
with storing energy in a metastable state.
Metal-organic frameworks (MOFs) ( 28 – 31 )
provide the ideal platforms for achieving
this goal because they contain ( 32 – 34 ) build-
ing blocks arranged precisely in a periodic
framework in which the AMPs can be im-
mobilized and their operations maintained.
Numerous examples of robust dynamics ( 27 )
have been observed in MOFs wherein a com-
ponent undergoes rotational ( 35 – 38 ) or lin-
ear ( 39 , 40 ) movements of a nondirectional
nature ( 35 – 42 ). In one instance, Danowskiet al.
( 43 )havedescribedtheunidirectionalro-
tary motion of motors in a MOF. None of these
examples, however, displays nonequilibrium
adsorption.
Here, we report the phenomenon of mech-
anisorption ( 44 ), which results from nonequili-
brium pumping (Fig. 2A) to form mechanical
bonds between the adsorbent and the ad-
sorbate. This phenomenon is associated with
molecules that are transported actively to a sur-
face compartment by using pumping cassettes
and are retained in a highly nonequilibriumRESEARCHSCIENCEscience.org 3 DECEMBER 2021•VOL 374 ISSUE 6572 1215
Table 1. A summary of the features of three sorption types happening on a solid surfaceÑ
physisorption, chemisorption, and mechanisorption.Sorption type Physisorption Chemisorption MechanisorptionRepresentation.....................................................................................................................................................................................................................
Thermodynamics.....................................................................................................................................................................................................................Equilibrium systems Equilibrium systems Nonequilibrium systems
Interactions.....................................................................................................................................................................................................................Van der Waals interactions Electronic interactions Mechanical bonds
Selectivity.....................................................................................................................................................................................................................Not selective Selective Highly selective
Adsorption.....................................................................................................................................................................................................................Spontaneous Spontaneous Energy required
Desorption.....................................................................................................................................................................................................................Energy required Energy required SpontaneousLocation
Langmuir monolayer
or BET multilayer
Monolayer Stack
.....................................................................................................................................................................................................................(^1) Department of Chemistry, Northwestern University,
Evanston, IL 60208, USA.^2 Stoddart Institute of Molecular
Science, Department of Chemistry, Zhejiang University,
Hangzhou 310021, China.^3 ZJU-Hangzhou Global Scientific
and Technological Innovation Center, Hangzhou 311215,
China.^4 Northwestern University Atomic and Nanoscale
Characterization Experimental Center (NUANCE),
Northwestern University, Evanston, IL 60208, USA.
(^5) Department of Chemical and Biological Engineering,
Northwestern University, Evanston, IL 60208, USA.
(^6) Department of Physics and Astronomy, University of Maine,
Orono, ME 04469, USA.^7 School of Chemistry, University of
New South Wales, Sydney, NSW 2052, Australia.
*Corresponding author. Email: [email protected]
These authors contributed equally to this work.