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The Metastability of an Electrochemically Controlled [28]W. L. Jorgensen, J. Tirado-Rives, J. Am. Chem. Soc. 1988, 110, 1657. guise of bistable [2]rotaxanes in which the ring component can [29]Gaussian 98 (Revision A.11.3), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. be induced[5] to move relative to the dumbbell-shaped one by 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. altering the redox characteristics of the molecules. Such Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. precisely controllable nanoscale molecular machines and Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, switches have attracted a lot of attention[2, 3] because of their P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghava- potential to meet the expectations of a visionary[6] and to act as chari, 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. some of the smallest components for the engineering of Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. nanoelectromechanical systems (NEMs) and the fabrication of Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. nanoelectronic devices.[7] Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc., Although the redox-switching properties of numerous bista- Pittsburgh, PA, 2002. [8] [30]C. Breneman, K. Wiberg, J. Comput. Chem. 1990, 11, 361. ble [2]rotaxanes have been demonstrated in solution, the lack [31]S. J. Weiner, P. A. Kollman, D. A. Case, U. C. Singh, C. Ghio, G. Alagona, S. of coherence of the switches in this phase makes it difficult to Profeta, P. Weiner, J. Am. Chem. Soc. 1984, 106, 765. harness the potential envisaged by Feynman.[6] It is essential that [32]D. Fincham, D. Heyes, Adv. Chem. Phys. 1985, 63, 493. we establish how to self-assemble these tiny switches in an [33]M. Parrinello, A. Rahman, J. App. Phys. 1981, 52, 7182. orderly manner at surfaces[9±12] and to investigate their switching [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. properties in conjunction with their introduction into solid-state Luckhurst, G. W. Gray), Academic Press, London 1979, pp. 51 ± 83. devices that have been shown[7] to function as two-dimensional [36]I. Haller, Prog. Solid State Chem. 1975, 10, 103. molecular electronic circuits. The fabrication of such devices [37]K. Toyne, in Thermotropic Liquid Crystals (Ed.: G. W. Gray), Wiley, London, required the design and synthesis[13] of bistable [2]rotaxanes that 28 ± 63. [7, 14±16] [38]M. Hird, in Physical Properties of Liquid Crystals, Vol. 1: Nematics, (Eds.: D. A. are amphiphilic, so that they can be transferred as Dunmur, A. Fukuda, G. R. Luckhurst), EMIS, IEE, London 2000, pp. 3 ± 16. molecular monolayers using the Langmuir ± Blodgett (LB) tech- [39]A. Ferrarini, P. L. Nordio, J. Chem. Soc. Perkin Trans. 1998, 2, 456. nique into a device setting. A molecular switch tunnel junction [40]M. E. Tuckerman, B. Berne, G. Martyna, J. Chem. Phys. 1992, 97, 1990. (MSTJ) has been fabricated[7] by sandwiching such self-organized Received: July 15, 2003 [Z908] 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 2Vor a À 2 V bias. The proposed electromechanical switching mech- anism (Figure 1) suggests that the ground state, where the cyclobis(paraquat-p-phenylene) (CBPQT4, blue) ring initially The Metastability of an encircles the tetrathiafulvalene (TTF, green) unit, represents[17] the switch-off state. When a 2 V bias is applied, the CBPQT4 Electrochemically Controlled ring moves mechanically to the 1,5-dioxynaphthalene (DNP, red) ring system as a result of oxidation of the TTF unit to its radical Nanoscale Machine on GoldSurfaces cation. Although, when the bias is removed, neutrality is restored to the TTF unit, the CBPQT4 ring continues to reside on the DNP [a] [b] Hsian-Rong Tseng, Dongmin Wu, ring system, forming the metastable state. The observation of a [b] [b] Nicholas Xuanlai Fang, Xiang Zhang,* and switch-on state can be attributed to this slow-decaying meta- [a] J. Fraser Stoddart* stable state that can be erased by applying a À2 V bias for a fleeting moment during the switching cycle. Since the mechan- [1] The advent of supramolecular chemistry has provided chemists ical motion associated with this decay is an activated process, [2, 3] with the wherewithal to construct molecule-level machines in these devices exhibit a hysteretic current ± voltage response. [4] an efficient manner using the protocol of template-direction. Herein, we describe how we have utilized a custom-designed Synthetically accessible, linear motor molecules come in the variable temperature (VT) electrochemical apparatus to inves- tigate the redox-switching behavior of an Au surface-confined[18] [a] Dr. H.-R. Tseng, Prof. J. F. Stoddart linear motor-molecule, that is, a disulfide-tethered bistable The California NanoSystems Institute and [2]rotaxane SSR ¥ 4PF6 , together with the corresponding dumb- Department of Chemistry and Biochemistry bell-shaped control compound SSD. In both cases, the append- University of California, Los Angeles (USA) ed disulfide function is used to immobilize the redox-active Fax: (1) 310-206-1843 E-mail: [email protected] [2]rotaxane and dumbbell control onto gold surfaces as self- [b] D. Wu, N. X. Fang, Prof. X. Zhang assembled monolayers (SAMs). The [2]rotaxane SSR ¥ 4PF6 was The California NanoSystems Institute and obtained (Figure 2) by a template-directed protocol[4] wherein a Mechanical and Aerospace Engineering Department CBPQT4 ring was clipped around the TTF unit of the dumbbell- University of California, Los Angeles (USA) shaped precursor SSD. Here, we report i) the results of a Fax: (1) 310-206-2302 [19] E-mail: [email protected] semiquantitative electrochemical investigation carried out on 4 Supporting information for this article is available on the WWW under http:// the surface-confined SSR and the control (SSD) at room www.chemphyschem.org or from the author. temperature in MeCN, leading to the identification[20] of a ChemPhysChem 2004,5,111±116 DOI: 10.1002/cphc.200300992 ¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 111 bipyridinium units in the CBPQT4 ring are reduced to radical cations. SAMs of SSR4 and SSD were prepared by immersing a coiled gold wire (99.9%/length, 105 mm/diameter, 0.5 mm) into their solutions (0.1 mM)inCH2Cl2 for seven days. After washing the substrate with CH2Cl2 and drying under an air-flow, the SAM-modified gold wire was placed into the custom-built electrochemical cell for cyclic voltammetry (CV) measurements, which were carried out in an electrolyte solution of LiClO4 (0.05 M) in MeCN over a range of temper- atures (263 to 303 K) under an Ar atmosphere. Figure 1. Proposed electromechanical mechanism which accounts for the hysteretic current-voltage All potentials are referenced to an Ag/AgCl response by an amphiphilic, bistable [2]rotaxane monolayer in an MSTJ device, where the bottom electrode is Si and the top electrode is Ti/Al. The devices are described in ref. [7] and the rotaxane reference electrode. In the investigations of the molecules consist of i) an amphiphilic dumbbell component containing two recognition sites–a TTF-centered oxidation processes, namely tetrathiafulvalene (TTF, green) unit and a 1,5-dioxynaphthalene (DNP, red) ring system, and two TTF !TTF. and then TTF. !TTF2, three oxi- stoppers, one (upper) hydrophilic (black) and the other (lower) hydrophilic (blue) and ii) a ring dation peaks were observed (Figure 3a) for component, the tetracationic cyclophane (blue), cyclobis(paraquat-p-phenylene). See ref. [7] for the structures of rotaxanes that give hysteretic current ± voltage responses in the Figure 2. Template-directed synthesis of the disulfide-tethered, bistable [2]ro- taxane SSR ¥ 4PF6 from its dumbbell-shaped precursor SSD and their self- organization on gold as SAMs. Reagents and conditions: a) a,a'-[1,4-phenyl- enebis(methylene)]bis(4,4'-bipyridium) bis(hexafluorophosphate), 1,4-bis(bromo- methyl)benzene,N,N-dimethylformamide, RT, 10 days; b) chromatography on SiO2 :Me2CO/NH4PF6, followed by addition of H2O to the eluent. metastable state, translationally isomeric with its ground state, ii) the kinetics associated with the first-order decay of the metastable state back to its ground state, in SAMs of SSR4 at 288 K, and iii) a variable-temperature analysis of the kinetics over the range from 278 to 303 K, leading to an estimate of the 4 activation energy for the relaxation of the metastable state of Figure 3. a) The CV (green) of the surface-confined, bistable [2]rotaxane SSR . b) The CV (red) of the surface-confined dumbbell compound SSD. The associated 4 the surface-confined SSR back to its ground state. Finally, we redox and/or mechanical changes for SSR4 are shown inside the green box. The demonstrate how iv) the metastable state is quelled when the associated redox changes for SSD are shown inside the red box. 112 ¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org ChemPhysChem 2004,5,111±116 SSR4 at 310, 490, and 630 mV in the ratio of 1:9:10, associated with each peak, the surface coverage by the respectively.
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