Molecular compounds, comprised of mechanically interlocked components, can now be obtained[1-3] efficiently using template-directed protocols that rely upon supramolecular assistance to covalent synthesis. Since the weak noncovalent interactions that orchestrate the synthesis of such compounds — e.g., catenanes and rotaxanes — containing mechanical bonds live on between the components inside the molecules thereafter, they can be activated such that their components move with respect to each other in either a linear fashion (e.g., the ring component along the rod of the dumbbell component of a -rotaxane) or a rotary manner (e.g., one ring in a -catenane circumrotating through the other ring). Thus, -rotaxanes can be likened to linear motors and -catenanes to rotary motors. Moreover, these molecules can be activated[6,7] by switching the recognition elements on and off between the components chemically, electrically, and optically such that they perform motions e.g., — shuttling actions or muscle-like elongations and contractions — reminiscent of the moving parts in macroscopic machines. Such motor-molecules and molecular machines hold considerable promise for the fabrication of sensors, actuators, amplifiers and switches at the nanoscale level.
Beyond the verification[4-7] of solution-phase mechanical processes, we have demonstrated[8-11] recently that relative mechanical movements between the components in interlocked molecules can be stimulated (i) chemically in condensed phases (e.g., Langmuir monolayers and Langmuir-Blodgett films), (ii) electrochemically as self-assembled monolayers on gold, and (iii) electronically within the settings of solid-state devices. Not only has reversible, electronically-driven switching been observed in devices incorporating a bistable -catenane but a crosspoint random access memory circuit and a simple logic circuit have been fabricated recently using an amphiphilic, bistable -rotaxane. The experiments provide compelling evidence that switchable catenanes and rotaxanes perform mechanically in a soft-matter environment.
The lecture will highlight how the emergence of the mechanical bond in chemistry in chemistry during the last two decades has brought with it a real prospect of integrating a bottom-up approach, based on self-assembly and self-organization of motor-molecules, with a top-down approach, based on micro- and nanofabrication, to harness molecular machinery.[12,13] It all adds up to an integrated systems-oriented approach to nanotechnology that finds its inspiration in the transfer of concepts like molecular recognition from the life sciences into materials science.
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2. Self-Assembly in Natural and Unnatural Systems, Angew. Chem. Int. Ed. Engl. 1996, 35, 1155.
3. Interlocked Macromolecules, Chem. Rev. 1999, 99, 1643.
4. A Molecular Shuttle, J. Am. Chem. Soc. 1991, 113, 5131.
5. A Chemically and Electrochemically Switchable Molecular Shuttle, Nature 1994, 369, 133.
6. Artificial Molecular Machines, Angew. Chem. Int. Ed. 2000, 39, 3349.
7. A Photochemically Driven Molecular-Level Abacus, Chem. Eur. J. 2000, 6, 3558.
8. Operating Linear Motor-Molecules Mechanically in Condensed Phases, Nano Letters Submitted.
9. The Metastability of an Electrochemically Controlled Nanoscale Machine on Gold Surfaces, ChemPhysChem 2004, 5, 111.
10. A Catenane-Based Solid State Electronically Reconfigurable Switch, Science 2000, 289, 1172.
11. Two-Dimensional Molecular Electronics Circuits, ChemPhysChem 2002, 3, 519.
12. An Operational Supramolecular Nanovalve, J. Am. Chem. Soc. 2004, 126, 3370.
13. A Molecular Elevator, Science 2004, 303, 1845.