J. Phys. IV France 114 (2004) 511-513 EDP Sciences, Les Ulis DOI: 10.1051/jp4:2004114119
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Artificial molecular devices based on tetrathiafulvalene J.O. Jeppesen1,2,∗, C. Patrick Collier2, J.R. Heath2, Y. Luo2, K.A. Nielsen1,2, J. Perkins2, J. Fraser Stoddart2 and E. Wong2 1
Department of Chemistry, University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark 2 Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1569, USA
Abstract. Over the past two decades, the relative motion of the components in mechanically interlRFNHGPROHFXOHV FDWHQDQHVDQGURWD[DQHV KDVEHHQH[WHQVLYHO\GHPRQVWUDWHGLQVROXWLRQ More recently, the area of molecular electronics has advanced considerably and over the past four years, catenanes and rotaxanes have been utilized as the active molecules in solid-state switchable tunnel junction devices. We present here new amphiphilic switchable [2]rotaxanes suitable for integration into nano-scale devices. Key words. Rotaxanes – tetrathiafulvalenes – molecular devices.
1. INTRODUCTION Today’s computer technology, which relies on silicon based chips, is rapidly approaching the upper limits of its physical capabilities. The hardware used in today’s computers is based on a largedownward (top-down) approach to the manufacture of integrated circuit architectures based on solidstate switches [1]. This particular approach to manufacturing computational hardware has been incredibly successful. It has produced an exponential growth in cost-efficient computational performance over the last 25 years. However, it is beginning to dawn upon electronic engineers that the top-down approach is going to run into serious difficulties, since this approach leads physicists and engineers to manipulate progressively smaller pieces of matter. As we approach the time when the limits of silicon-based technology will be reached, a huge effort is being made to find an alternative to this technology. A promising strategy is offered by the small-upward (bottom-up) approach. Indeed, scientists all over the world are turning toward molecules – and entering the nanoscale regime – with the idea of fabricating electronic devices from the bottom-up as a supplement to the present top-down approach. Over the past two decades, many catenanes and rotaxanes have been synthesized and characterized in solution. However, it is the integration of these molecules into device settings that has been receiving much attention [1,2] over the past three years. 1.1. Catenanes The switchable [2]catenane [2,3], 1•4PF6 shown in Figure 1a, consists of an electron rich macrocycle containing a tetrathiafulvalene (TTF) unit and a dioxynaphthalene (DNP) unit catenated with the electron-poor tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+). Under normal conditions, the TTF unit is encircled by the tetracationic cyclophane. However, upon electrochemical (or chemical) oxidation of the TTF unit to the corresponding dication, circumrotation of the macrocyle occurs – as a consequence of electrostatic repulsion between charges – and the TTF unit inside CBPQT4+ is replaced by the DNP unit. This process is completely reversible upon electrochemical (or chemical) reduction of the TTF dication, and hence provides the basis for a reversible molecular switch. In the solid state, the [2]catenane 1•4PF6 is arranged as continuous polar π-electron donor∗
Corresponding author: J.O. Jeppesen, e-mail:
[email protected]
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acceptor stacks. This arrangement of these largely hydrophilic molecules can be transformed into a monolayer on a Langmuir trough, using hydrophobic dimyristoylphosphatidic (DMPA–) counterions. This act of self-assembly provides the opportunity to prepare molecular junctions [3,4] where these electroactive molecules are sandwiched between two metal electrodes. The device fabricated from the [2]catenane 1•4PF6 is stable and it can be cycled over approximately a two-month period before there is any appreciable decrease in its performance. However, the junction resistance between its two states is only a factor of around 2. 1.2. Rotaxanes In order to improve the performance of the molecular device, [2]rotaxanes closely related to the [2]catenane 1•4PF6 have been prepared [5]. In the device fabricated from the [2]catenane 1•4PF6, the electroactive species is closed to one of the electrodes. Ideally, the electroactive units should span the molecular junction. Opening the [2]catenane loop to form a [2]pseudorotaxane provides an answer. An important consideration is that the pseudorotaxane must be amphiphilic in order to allow deposition of the molecules on a surface using Langmuir–Blodgett techniques, but another important consideration is the spacing of the molecules in a junction – a [2]rotaxane has a bulky stopper on both ends of the dumbbell, helping to separate the molecules and hence allowing the cyclophane to shuttle more freely between the recognition stes. These small changes in the design have given rise to much better device performance. However, synthetically it is not as easy at it looks. The effort which is required to obtain such compounds typical require 20–30 synthetic steps. Recently [5], amphiphilic bistable [2]rotaxanes, such as 2•4PF6 depicted in Figure 1b, have been designed and synthesized. This compound contain a hydrophobic tetraarylmethane and a hydrophilic, dendritic stopper and is
a)
b)
Figure 1. a) A switchable [2]catenane (left), its continuous donor-acceptor stack exhibited in the solid state (topright), and a molecular junction of the electroactive [2]catenane sandwiched between a polysilicon and a titanium electrode (bottom-right). b) An amphiphilic bistable [2]rotaxane (top) and its incorporation (bottom) into a molecular tunnel switch junction.
comprised of two π-electron rich stations—a monopyrrolo-TTF (MPTTF) unit and a DNP moiety— which can act as recognition sites for the tetracationic cyclophane, CBPQT4+, to reside around. The performance of the bistable [2]rotaxane 2•4PF6 and closely related molecules in a device setting have been intensively investigated [4] and will not be discussed further herein, except to highlight that solid-state switches fabricated from this kind of amphiphilic [2]rotaxanes are far superior to the switch [3] fabricated from the TTF based [2]catenane 1•4PF6. Furthermore, they have been incorporated into a two-dimensional circuit as an 8 × 8 crossbar device producing a rewritable 64-bit random access memory (RAM) or a XOR logic gate [4]. Acknowledgements This research was funded by the Carlsbergfondet and the Familien Hede Nielsens Fond in Denmark and by the Defense Advanced Research Projects Agency (DARPA) in the United States.
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