Stoddart Mechanostereochemistry Group

 

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Operating Nanoelevators

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 ππ 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.

 

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