The Metastability of an Electrochemically Controlled Nanoscale Machine on Gold Surfaces

The advent of supramolecular chemistry has provided chemists with the wherewithal to construct molecule-level machines 3] in an efficient manner using the protocol of template-direction. Synthetically accessible, linear motor molecules come in the guise of bistable [2]rotaxanes in which the ring component can be induced to move relative to the dumbbell-shaped one by altering the redox characteristics of the molecules. Such precisely controllable nanoscale molecular machines and switches have attracted a lot of attention 3] because of their potential to meet the expectations of a visionary and to act as some of the smallest components for the engineering of nanoelectromechanical systems (NEMs) and the fabrication of nanoelectronic devices. Although the redox-switching properties of numerous bistable [2]rotaxanes have been demonstrated in solution, the lack of coherence of the switches in this phase makes it difficult to harness the potential envisaged by Feynman. It is essential that we establish how to self-assemble these tiny switches in an orderly manner at surfaces and to investigate their switching properties in conjunction with their introduction into solid-state devices that have been shown to function as two-dimensional molecular electronic circuits. The fabrication of such devices required the design and synthesis of bistable [2]rotaxanes that are amphiphilic, so that they can be transferred 14±16] as molecular monolayers using the Langmuir ± Blodgett (LB) technique into a device setting. A molecular switch tunnel junction (MSTJ) has been fabricated by sandwiching such self-organized LB monolayers between a bottom Si electrode and a top Ti/Al electrode in a crossbar device architecture. The switch-on (high conductance) and switch-off (low conductance) states of each junction can be addressed respectively upon applying a 2 V or a 2 V bias. The proposed electromechanical switching mechanism (Figure 1) suggests that the ground state, where the cyclobis(paraquat-p-phenylene) (CBPQT , blue) ring initially encircles the tetrathiafulvalene (TTF, green) unit, represents the switch-off state. When a 2 V bias is applied, the CBPQT ring moves mechanically to the 1,5-dioxynaphthalene (DNP, red) ring system as a result of oxidation of the TTF unit to its radical cation. Although, when the bias is removed, neutrality is restored to the TTF unit, the CBPQT ring continues to reside on the DNP ring system, forming the metastable state. The observation of a switch-on state can be attributed to this slow-decaying metastable state that can be erased by applying a 2 V bias for a fleeting moment during the switching cycle. Since the mechanical motion associated with this decay is an activated process, these devices exhibit a hysteretic current ± voltage response. Herein, we describe how we have utilized a custom-designed variable temperature (VT) electrochemical apparatus to investigate the redox-switching behavior of an Au surface-confined linear motor-molecule, that is, a disulfide-tethered bistable [2]rotaxane SSR ¥ 4PF6, together with the corresponding dumbbell-shaped control compound SSD. In both cases, the appended disulfide function is used to immobilize the redox-active [2]rotaxane and dumbbell control onto gold surfaces as selfassembled monolayers (SAMs). The [2]rotaxane SSR ¥ 4PF6 was obtained (Figure 2) by a template-directed protocol wherein a CBPQT ring was clipped around the TTF unit of the dumbbellshaped precursor SSD. Here, we report i) the results of a semiquantitative electrochemical investigation carried out on the surface-confined SSR and the control (SSD) at room temperature in MeCN, leading to the identification of a [28] W. L. Jorgensen, J. Tirado-Rives, J. Am. Chem. Soc. 1988, 110, 1657. [29] Gaussian 98 (Revision A.11.3), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc. , Pittsburgh, PA, 2002. [30] C. Breneman, K. Wiberg, J. Comput. Chem. 1990, 11, 361. [31] S. J. Weiner, P. A. Kollman, D. A. Case, U. C. Singh, C. Ghio, G. Alagona, S. Profeta, P. Weiner, J. Am. Chem. Soc. 1984, 106, 765. [32] D. Fincham, D. Heyes, Adv. Chem. Phys. 1985, 63, 493. [33] M. Parrinello, A. Rahman, J. App. Phys. 1981, 52, 7182. [34] T. Darden, D. York, L. Pedersen, J. Chem. Phys. 1993, 98, 10089. [35] C. Zannoni, in The Molecular Physics of Liquid Crystals (Eds. : G. R. Luckhurst, G. W. Gray), Academic Press, London 1979, pp. 51 ± 83. [36] I. Haller, Prog. Solid State Chem. 1975, 10, 103. [37] K. Toyne, in Thermotropic Liquid Crystals (Ed. : G. W. Gray), Wiley, London, 28 ±63. [38] M. Hird, in Physical Properties of Liquid Crystals, Vol. 1: Nematics, (Eds. : D. A. Dunmur, A. Fukuda, G. R. Luckhurst), EMIS, IEE, London 2000, pp. 3 ± 16. [39] A. Ferrarini, P. L. Nordio, J. Chem. Soc. Perkin Trans. 1998, 2, 456. [40] M. E. Tuckerman, B. Berne, G. Martyna, J. Chem. Phys. 1992, 97, 1990.