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Linear Artificial Molecular Muscles for Mechanical Nanoactivation

Two switchable, palindromically-constituted bistable [3]rotaxanes have been designed and synthesized (Appl. Phys. Lett. 2004, 85, 5391- 5393) with a pair of mechanically-mobile rings encircling a single dumbbell. These designs are reminiscent of a “molecular muscle” for the purposes of amplifying and harnessing molecular mechanical motions. The location and switching of the two cyclobis(paraquat- pphenylene) (CBPQT4+) rings can be controlled to be on either tetrathiafulvalene (TTF) or naphthalene (NP) stations, either chemically (1H NMR spectroscopy) or electrochemically (cyclic voltammetry), such that switching of inter-ring distances from 4.2 to 1.4 nm mimics the contraction and extension of skeletal muscle, albeit on a shorter length scale ( J. Am. Chem. Soc. 2005, 127, 9745-9759).

The active form of the bistable [3]rotaxane bears disulfide tethers attached covalently to both of the CBPQT4+ ring components for the purpose of its self-assembly onto a gold surface. An array of flexible microcantilever beams, each 500 x 100 x 1 µm in size and coated on one side with a monolayer of 6 billion of the active bistable [3]rotaxane molecules, undergoes controllable and reversible bending up and down when it is exposed to the synchronous addition of aqueous chemical oxidants and reductants in the form of Fe(ClO4)3 and ascorbic acid, respectively. The beam bending is correlated with flexing of the surface-bound molecular muscles, whereas a monolayer of the dumbbell alone is inactive under the same conditions. Along with a simple calculation from a force balance diagram, these observations support the hypothesis that the cumulative nanoscale movements within surface-bound “molecular muscles” can be harnessed to perform micro-scale mechanical work.

The controlled actuation of cantliever beams by surface-bound nanoscale objects five orders of magnitude smaller in size demonstrates the potential of switchable, bistable interlocked molecules to function (Box) as nanoscale molecular machines. Numerous labs were involved in the preparation, fabrication, and investigation of these interesting functional molecules. The synthesis of the palindromic [3]rotaxane was carried out by synthetic chemists, mechanical as well as bioengineers carried out the fabrication of the nanoscale devices, and industrial partners were integral to the analysis of their function. This combination of synthesis, selfassembly, engineering, and analysis highlights the interdisciplinary nature of this work.

 

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