Biomedical Engineering Reference
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Fig. 12 Chemical formula and operation scheme of a molecular elevator
The transformation of the contracted 13 4+ species - whose length was estimated to
be 6.5 nm - to the extended 12 2+ one could be obtained by addition of Cu(I) ions in
acetonitrile/dichloromethane solution.
A Molecular Elevator
The structural and functional features of the acid base-controllable molecular shut-
tle described in Sect. 3.2.1 were employed to design and construct a two-component
molecular device, 14 9+ (Fig. 12 ), that behaves like a nanometer-scale elevator (Badjic
et al. 2004 ). This nanomachine, which is ca. 2.5 nm in height and has a diameter of
ca. 3.5 nm, consists of a trifurcated rig-like component containing two different
notches - one AMH and one BPM - at different levels in each of its three legs. The
latter are interlocked by a molecule made up of three crown ether macrocycles fused
trigonally to a central aromatic fl oor; such a platform can be made to stop at the two
different levels. The three legs of the rig carry bulky feet that prevent the loss of the
platform. Initially, the platform resides exclusively on the “upper” level, i.e., with
the three macrocycles surrounding the ammonium centers (Fig. 12a , state 0). This
preference results from strong N + -H···O hydrogen bonding and weak stabilizing
p - p stacking forces between the aromatic cores of the platform and rig components.
Upon addition of a strong, non-nucleophilic phosphazene base to an acetonitrile
solution of 14 9+ , deprotonation of the ammonium center occurs and, as a result, the
platform moves to the “lower” level, that is, with the three macrocycles surrounding
the BPM (Fig. 12b , state 1). This structure is stabilized by electron donor-acceptor
interactions between the electron-rich aromatic units of the platform and the elec-
tron-defi cient BPM of the rig component. Subsequent addition of acid restores the
AMH, and the platform moves back to the upper level. The elevator motion, which
can be followed by NMR spectroscopy, electrochemistry, and absorption and fl uo-
rescence spectroscopy, is quantitative and can be repeated many times on the same
solution. The distance traveled by the platform is about 0.7 nm, and from thermody-
namic considerations it can be estimated that the elevator movement from the upper
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