When stimulated by light, electricity or chemical reagents, a class of molecules called rotaxanes — composed of mutually-recognizable and intercommunicating ring and dumbbell-shaped components — are shown to experience relative mechanical intramolecular motions in solution, allowing them to behave as linear molecular shuttles (1-3). Created by a bottom-up approach based on self-assembly and self-organization, rotaxanes are a potential material for developing functional nanoelectromechanical systems (NEMS). Integrating a bottom-up approach, based on the self-organization of rotaxane motor-molecules, together with a top-down approach based on micro/nano fabrication, a new class of functioning nano- and meso-scale mechanical devices becomes possible. Artificial motor molecules, like rotaxanes, may be able to mimic essential biological processes where motion is involved (4). It is also conceivable that rotaxanes can be used in molecular valves, light switches, and nanomechanical amplifiers, based on their bistable characteristics.
Thus far, it has been demonstrated by numerous researchers (3,5,6) that rotaxanes can exhibit relative movements of their ring and dumbbell-shaped components in solution. However, in solution, the large number of randomly distributed molecules that are in constant Brownian motion cannot be addressed individually. In order to realize macroscopic actuation from rotaxane-based NEMS devices, the internal mechanical motions of these molecules need to be coherently and cooperatively orchestrated. For this purpose the molecules need to be self-organized at interfaces that are addressable on the nanoscale. Most importantly, however, at these interfaces the molecules must retain the switching behavior they exhibit clearly in the solution phase such that mechanical actuation remains a viable and realistic application.
In this work, surface pressure-molecular area (p-A) isotherms and Atomic Force Microscope (AFM) studies were used to indicate that redox-controllable rotaxanes are mechanically switchable in closely packed Langmuir films. These results, not only constitute a proof of principle, but they also provide the impetus to go on and develop rotaxane-based functional nanomechanical devices.
(1) V. Balzani, A. Credi, F.M. Raymo, J.F. Stoddart, Angew. Chem. Int. Ed. 39, 3348 (2000).
(2) J.F. Stoddart, H.-R. Tseng, Proc. Natl. Acad. Sci. USA 99, 4797 (2002).
(3) H.-R. Tseng, S.A. Vignon, J.F. Stoddart, Angew. Chemie 42, 1491 (2003).
(4) D.S. Goodsell, Our Molecular Nature: The Body's Motors, Machines, and Messages (Copernicus, New York, 1996).
(5) L. Raehm, J.M. Kern, J.-P. Sauvage, Chem. Eur. J. 5, 3310 (1999).
(6) A.M. Brouwer et al., Science 291, 2124 (2001).
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