Nobel Centennial Symposia
"Frontiers of Molecular Science"

December 4-7, 2001
Friiberghs Manor, Örsundsbro and Stockholm University

The Nature of the Mechanical Bond

by J. Fraser Stoddart
Department of Chemistry and Biochemistry and Exotic Materials Institute University of California at Los Angeles 405 Hilgard Avenue, Los Angeles California 90095, USA stoddart@chem.ucla.edu

Molecular compounds, comprised of mechanically interlocked components, can now be obtained^1-4 efficiently using template-directed protocols that reply 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 [2]rotaxane as in a molecular shuttle⁵) or a rotary manner (e.g., one ring in a [2]catenane circumrotating through the other ring as in a bistable switch⁶). Thus, [2]rotaxanes can be likened to linear motors and [2]catenanes to rotary motors. Moreover, these molecules can be activated^7-9 by switching the recognition elements on and off between the components chemically, electrically and optically such that the 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 promise¹⁰ for the fabrication of sensors, actuators, amplifiers and switches at the nanoscale level.

Although the vast majority of our research on switchable catenanes and rotaxanes has been carried out in the context^5-9 of solution-phase mechanical processes, we have demon- strated^10-14 recently that relative mechanical movements between the components in interlocked molecules can be stimulated electrically within the setting of a solid-state device. Not only has reversible, electronically-driven switching been observed¹³ in devices incorporating a bistable [2]catenane but a crosspoint random access memory circuit and a simple logic circuit have been fabricated¹⁴ recently using an amphiphilic, bistable [2]rotaxane. These experiments can be considered as positive evidence that switchable catenanes and rotaxanes perform mechanically in a soft-matter environment and can withstand simple device-processing steps.

The lecture will highlight how the emergence of the mechanical bond 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 create nanomechanical systems in order to harness, manipulate and transfer energy on the nanoscale level. It is an approach to nanoscience and nanotechnology that relies fundamentally upon concept transfer from the life sciences into materials science.

In the future, we can anticipate (1) the development of new (supra)molecular motors, (2) the designing of methods to induce them to operate coherently and controllably on surfaces and within frameworks as machines¹⁵ and functioning devices, (3) the elaboration of integrated power supplies to drive the machines and devices, (4) an integration of bottom-up and top-down procedures for the nano- and microfabrication of molecularly-driven sensors, actuators, amplifiers and switches, (5) an increased understanding and appreciation of the science and engineering that lies behind nanoscale processes, and (6) the emergence of an elite cadre of highly trained scientists and technologists with both broad perspectives and individual expertise in the fields of nanoscience and molecular nanotechnology. All this and more is in the nature of the mechanical bond as it impacts upon chemistry and beyond.

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