Natural molecular machines, such as
ATP-synthase, myosin, kinesin, or dynein are complex and inspiring biological
assemblies whose structures and working mechanisms have been elucidated in a few
cases. These intriguing systems have prompted scientists to design and build
artificial molecular machines by mimicking their biological counterparts. In
this sense, phenomena that control the form and function of living systems, such
as self-assembly, molecular recognition, and multivalency have been employed in
supramolecular chemistry and template-directed synthesis in order to construct
functional molecular devices. Attempts to extend the concept of a machine to the
molecular level, by taking advantage of biomimicry, have yielded a plethora of
switches, tweezers, shuttles, and even molecular muscles, walkers and rotary
motors. Each of these molecular machines has been designed specifically to
perform particular functions upon application of an external energy input.
The development of synthetic molecular
machines has been greatly enhanced by pooling intellectual resources of
appropriate groups. In this regard, the collaboration between Stoddart (UCLA &
CNSI) and Balzani (Bologna) has resulted in an important contribution to the
field of Supramolecular Chemistry and Molecular Nanotechnology. Stoddart has
focused his activities on designing and building molecular-level devices and
machines in the frame of bottom-up approach to nanotechnology. Balzani has
investigated such devices and machines powered by chemical energy,
electrochemical energy, or light in order to understand the properties and
operational mechanisms of these kinds of artificial machines as a prelude to
optimizing their performance.
Inspired by the concept of multivalency
and using template-directed synthesis, two trivalent mechanically interlocked
molecular machines were conceived (Science 2004, 303,
1845−1849 and J. Am. Chem. Soc., In press) with two orthogonal
recognition sites for dibenzo[24]crown-8 (DB24C8), and 2,3- dinaphtho[24]crown-8
(DN24C8) – one a dialkylammonium ion (CH2NH2 +CH2) and the other a bipyridinium
dication (BIPY2+). Whereas at low pH, the CH2NH2 +CH2 sites bind the
DB24C8/DN24C8 macrocycles preferentially, at high pH, deprotonation occurs with
loss of hydrogen bonding, allowing the macrocycles to move to the BIPY2+ sites
where they can acquire some stabilizing π–π stacking interactions. 1H NMR
spectroscopy and cyclic voltammetry, aided and abetted by absorption
spectroscopy, have been employed to unravel the details of the mechanism by
which the rig and platform components move on the alternate addition of base and
acid. For each molecular elevator, the platform operates by taking three
distinct steps associated with each of the three deprotonation/reprotonation
processes. Thus, molecular elevators are more reminiscent of a legged animal
than they are of passenger on freight elevators. |
