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Published March 12, 2009 | Supplemental Material
Journal Article Open

Free energy barrier for molecular motions in bistable [2]rotaxane molecular electronic devices


Donor−acceptor binding of the π-electron-poor cyclophane cyclobis(paraquat-p-phenylene) (CBPQT^(4+)) with the π-electron-rich tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) stations provides the basis for electrochemically switchable, bistable [2]rotaxanes, which have been incorporated and operated within solid-state devices to form ultradense memory circuits (ChemPhysChem 2002, 3, 519−525; Nature 2007, 445, 414−417) and nanoelectromechanical systems. The rate of CBPQT^(4+) shuttling at each oxidation state of the [2]rotaxane dictates critical write-and-retention time parameters within the devices, which can be tuned through chemical synthesis. To validate how well computational chemistry methods can estimate these rates for use in designing new devices, we used molecular dynamics simulations to calculate the free energy barrier for the shuttling of the CBPQT^4+ ring between the TTF and the DNP. The approach used here was to calculate the potential of mean force along the switching pathway, from which we calculated free energy barriers. These calculations find a turn-on time after the rotaxane is doubly oxidized of ~10^9−7) s (suggesting that the much longer experimental turn-on time is determined by the time scale of oxidization). The return barrier from the DNP to the TTF leads to a predicted lifetime of 2.1 s, which is compatible with experiments.

Additional Information

© 2009 American Chemical Society. Received: October 17, 2008; Revised Manuscript Received: January 12, 2009. Publication Date (Web): February 18, 2009. The computational work was initiated with support by the National Science Foundation (NIRT, W.A.G.). The collaboration was supported by the Microelectronics Advanced Research Corporation (MARCO; W.A.G. and J.F.S.) and its Focus Centers on Functional Engineered NanoArchitectonics (FENA) and Materials Structures and Devices, the Molectronics Program of the Defense Advanced Research Projects Agency (DARPA; J.F.S. and J.R.H.), the Center for Nanoscale Innovation for Defense (CNID; J.F.S.), and the MARCO Materials Structures and Devices Focus Center (J.R.H.). In addition, the facilities of the MSC (W.A.G.) were supported by ONR-DURIP and ARO-DURIP.

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