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Published December 17, 2004 | public
Journal Article

The Role of Physical Environment on Molecular Electromechanical Switching


The influences of different physical environments on the thermodynamics associated with one key step in the switching mechanism for a pair of bistable catenanes and a pair of bistable rotaxanes have been investigated systematically. The two bistable catenanes are comprised of a cyclobis(paraquat‐p‐phenylene) (CBPQT^(4+)) ring, or its diazapyrenium‐containing analogue, that are interlocked with a macrocyclic polyether component that incorporates the strong tetrathiafulvalene (TTF) donor unit and the weaker 1,5‐dioxynaphthalene (DNP) donor unit. The two bistable rotaxanes are comprised of a CBPQT^(4+) ring, interlocked with a dumbbell component in which one incorporates TTF and DNP units, whereas the other incorporates a monopyrrolotetrathiafulvalene (MPTTF) donor and a DNP unit. Two consecutive cycles of a variable scan rate cyclic voltammogram (10–1500 mV s^(−1)) performed on all of the bistable switches (∼1 mM) in MeCN electrolyte solutions (0.1 M tetrabutylammonium hexafluorophosphate) across a range of temperatures (258–303 K) were recorded in a temperature‐controlled electrochemical cell. The second cycle showed different intensities of the two features that were observed in the first cycle when the cyclic voltammetry was recorded at fast scan rates and low temperatures. The first oxidation peak increases in intensity, concomitant with a decrease in the intensity of the second oxidation peak. This variation changed systematically with scan rate and temperature and has been assigned to the molecular mechanical movements within the catenanes and rotaxanes of the CBPQT^(4+) ring from the DNP to the TTF unit. The intensities of each peak were assigned to the populations of each co‐conformation, and the scan‐rate variation of each population was analyzed to obtain kinetic and thermodynamic data for the movement of the CBPQT^(4+) ring. The Gibbs free energy of activation at 298 K for the thermally activated movement was calculated to be 16.2 kcal mol^(−1) for the rotaxane, and 16.7 and 19.2 kcal mol^(−1) for the bipyridinium‐ and diazapyrenium‐based bistable catenanes, respectively. These values differ from those obtained for the shuttling and circumrotational motions of degenerate rotaxanes and catenanes, respectively, indicating that the detailed chemical structure influences the rates of movement. In all cases, when the same bistable compounds were characterized in an electrolyte gel, the molecular mechanical motion slowed down significantly, concomitant with an increase in the activation barriers by more than 2 kcal mol^(−1). Irrespective of the environment—solution, self‐assembled monolayer or solid‐state polymer gel—and of the molecular structure—rotaxane or catenane—a single and generic switching mechanism is observed for all bistable molecules.

Additional Information

© 2004 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. Received: October 15, 2004; Published online: November 24, 2004. This research was funded by the Office of Naval Research (ONR), the National Science Foundation (NSF), the Moletronics Program of the Defense Advanced Research Projects Agency (DARPA), the Microelectronics Advanced Research Corporation (MARCO) and its Focus Centers on Functional Engineered NanoArchitectonics (FENA) and Materials Structures and Devices, and the Center for Nanoscale Innovation for Defense (CNID).

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