With the advent of nanotechnology, the fascination of achieving the
miniaturization of macroscale machines, with the prospect of matching-
if not eventually emulating- the performance of nanoscale biological
machines, has acted as an incentive to chemists and engineers to develop
artificial nanoscale machines at the molecular and
supramolecular levels. A machine can be defined as a device with
stationary and movable parts that modifies mechanical energy to do work
when fueled by an external force. Mechanically interlocked molecules and
supermolecules- bistable rotaxanes and catenanes as well as
pseudorotaxanes- are some of the most intriguing systems which can be
considered as nanoscale machines because of the stimuli-induced,
controlled mechanical movements of one part of the molecule- in this
case, one interlocked ring component- with respect to the other
have designed and synthesized a number of molecular machines based on
chemically, electrochemically, and photochemically switchable bistable
interlocked molecules for a variety of applications, for example,
pH-driven molecular elevator,3 molecular muscles that can
bend microcantilever beams,4 as well as molecular nanovalves5
for control release of molecules from nanopores. We have also developed
a light-harvesting molecular triad,6 which produces
photocurrents, an energy which we have utilized to power supramolecular
machines in the form of a pseudorotaxanes.
The molecular elevator3 (Box 1) comprises a tripodal rig-like
component bearing two competitive recognition sites on each of its three
legs, positioned at two different levels. The bistable tripod-shaped
component is triply interlocked with a platform component, which
contains three macrocyclic rings fused with a trigonal core, forming the
molecular elevator. The platform could be switched mechanically between
two recognition units on the triply bistable rig component by switching
the pH of the medium. In response to the chemical stimuli, the platform
component travels a distance of 0.7 nm from the upper to the lower level
of the rig, generating a force up to 200 pN, a number which is one order
of magnitude higher than that exerted by the natural motors, like myosin
and kinesin. Furthermore, the base-induced "power stroke" of the
platform from the upper to the lower level opens up a cavity (1.5 nm by
0.8 nm), which can potentially act as a host for a variety of guest
The concept of a bistable\]otaxane has been expanded4 into a
doubly bistable palindromic\]otaxane (Box 2) to develop a molecular
muscle. These molecular muscles, when self-assembled on microcantilever
beams (500x100x1 μm), are capable of bending and stretching the beams
when appropriate redox-reagents are injected into the device environment
in a microfluidic cell. The palindromic bistable\]otaxane is composed of
two cyclobisparaquat(p-phenylene) (CBPQT4+) rings- each
carrying a disulfide tether for gold-surface attachment- interlocked
onto a symmetrical dumbbell component which bears two π-electron rich
tetrathiafulvalene (TTF) units close to its ends and two
1,5-dioxynaphthalene (DNP) units bridged by a rigid di-alkyne spacer at
its center, and two 2,6-diisopropyl-phenol stoppers at both termini.
Chemical oxidation of both TTF units to TTF2+ dications
induces an electrostatic charge repulsion between the TTF2+ dications
and CBPQT4+ rings, which drives them towards the DNP sites
near the center, a process which bends the underlying microcantilever
beams onto which the\]otaxane molecules are attached covalently upward
by ca. 35 nm to their apparent saturation point. Reduction of the TTF
units back to their neutral states moves both CBPQT4+ rings
back to their original location, making the cantilevers straight.
reported the construction of redox-switchable and reversible molecular
nanovalves5 (Box 3), employing bistable\]otaxanes as the
redox-controllable gatekeepers and mesoporous silica nanoparticles as
nanoreservoirs. To estimate the overall performance of the nanopores
they were filled with different luminescent probe molecules. The
nanovalves can be closed by adding two equivalents of oxidant to oxidize
the TTF unit on the rotaxane backbone, a process which forces the CBPQT4+
ring to shuttle mechanically from the oxidized TTF unit to the DNP unit
on account of the charge repulsion between the CBPQT4+ ring
and the oxidized TTF2+ dicationic unit. The controlled
release of probe molecules can be demonstrated by adding reducing agents
to open the nanovalves. This process reduces the oxidized TTF units back
to their neutral state, so that the CBPQT4+ rings move away from the
openings of the nanopores and once again complex with the neutral TTF
unit, causing a subsequent release of the probe molecules.
molecular triad6 (Box 4)- composed of three unique electroactive
components, namely, (i) an electron-donating TTF unit, (ii) a
chromophoric porphyrin (P) unit, and (iii) an electron-accepting C60
unit- has been developed to harness light and convert it into electrical
energy. A disulfide-based anchoring group was tagged to the TTF end of
the triad in order to promote its self-assembly onto gold surfaces. When
irradiated near the absorption maximum (Soret band) of the chromophore P
at 413 nm, the triad undergoes a photo-induced electron transfer (PET)
from the singlet excited state of porphyrin (*P) to the
electron-accepting C60 unit, followed by a charge shift to
the better electron-donating TTF unit to generate the final
charge-separated state, TTF.+-P-C60.-.
This charge-separated state generates photocurrents in a closed circuit
in the form of a unidirectional electron flow from the working cathode
through (i) the photoactive triad, (ii) the electrolyte solution, to
(iii) the counter electrode, and through (iv) the outer circuit where
the current is measured (ΔI ~1 μA cm-2, Φphotocurrent
~ 1%). The triad has been utilized as a nanoscale power supply to drive
the dethreading of the BHEEN C CBPQT4+ pseudorotaxane in the
presence of 413 nm light at an applied potential (Vap = 0 V) that is
much lower than is required for direct electrochemical reduction (E1/2
= -300 mV) of the CBPQT4+ ring. In accordance with the PET
mechanism at Vap = 0 V, the charge-separated state of the
triad affords a C60 unit on the triad-functionalized
Au-working electrode, resulting in an effective terminal potential of
-550 mV which is the reduction potential of the C600/-.
unit. This potential is high enough to reduce the CBPQT4+
ring and induce its dissociation from the BHEEN stalk- a process which
has been monitored by detecting the BHEEN-based fluorescence intensity.
Based on the increase in the fluorescence intensity (320-370 nm), 6.7%
of the 0.37 mM\]seudorotaxane in MeCN underwent dissociation in the
presence of triad-excitation over 2900 s, an estimation which is
commensurate with the triad's ability to photoreduce 7% of the CBPQT4+
ring by generating 1.1 μA cm-2 photocurrent during the period
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