Synthesis and testing of new end-functionalized oligomers for molecular electronics Austen K. Flatt, a Shawn M. Dirk, a Jay C. Henderson, a Dwanleen E. Shen, a Jie Su, b Mark A. Reed b, * and James M. Tour a, * a Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, 6100 Main Street, MS-222, Houston, TX 77005, USA b Departments of Electrical Engineering, Applied Physics, and Physics, Yale University, P.O. Box 208284, New Haven, CT 06520, USA Received 8 May 2003; revised 4 September 2003; accepted 4 September 2003 Abstract—Several new classes of oligomers have been synthesized with functionalities designed to aid in the understanding of molecular device behavior, specifically when molecules are interfaced between proximal electronic probes. The compounds synthesized are series of azobenzenes, bipyridines and oligo(phenylene vinylene)s that bear acetyl-protected thiols for ultimate attachment to metallic surfaces. Some initial electrochemical and solid-state test results are also reported. q 2003 Elsevier Ltd. All rights reserved. 1. Introduction Due to physical and economic constraints, silicon based semiconductor technology is rapidly nearing a production brick wall. 1 As the miniaturization of solid-state silicon circuitry continues in order to increase speed, capacity and computing power, a point will be reached at which processors can no longer be made smaller, faster and cheaper. It has been proposed that by incorporating organic molecules into functioning molecular circuits, one may overcome many of the challenges that complimentary metal-oxide semiconductor (CMOS) technology is facing. 2 Work performed in the molecular electronics field has demonstrated that single molecules exhibit reversible switching behavior, which may lead researchers to molecu- lar memory and logic devices. 3–5 Our research has centered around oligo(phenylene ethynylene)s (OPEs) which, with the redox active nitro group, have demonstrated negative differential resistance (NDR) at variable temperatures. 2 Several new classes of potential molecular electronics molecules have been synthesized in our laboratory in order to develop further understanding of the switching pro- cess. 6–9 In this paper, we will discuss our synthetic work on azobenzene derivatives, pyridine systems, and oligo- (phenylene vinylene)s (OPVs) that have been synthesized as possible device candidates and that all bear protected thiol end groups for self-assembled attachment to metallic probes. Due to the additional redox center of azobenzenes, the electron deficient nature of pyridyl oligomers, and the high electrical transport seen in OPVs, 10 these molecules are good candidates to study device behavior. In addition to these syntheses, we performed electro- chemical testing of selected compounds, a method found to be useful for qualitative comparisons of molecular electronic devices. 11 We also include some results obtained from planar test devices using the bipyridyl compounds which show a resetable on-off state and NDR behavior. 2. Azobenzenes It has been shown that OPEs containing a redox active aromatic nitro functionality exhibit NDR at various temperatures. 3 The proposed mechanism is that the redox center contributes to the switching behavior of the mononitro OPE. 12 However, other theories have recently been put forth for NDR behavior including molecule/metal- based contact variations that could result in NDR-like performance. 13,14 By incorporating an azo functionality into an OPE, an additional redox center is created where switching behavior is likely to be observed. In addition to the redox active site, azobenzenes are known to change between the E and Z configurations when irradiated with light, giving rise to other probable switching mechanisms, 15 although we are not exploiting that manifold here. The azobenzene derivatives synthesized are shown in 0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2003.09.004 Tetrahedron 59 (2003) 8555–8570 * Corresponding authors. Tel.: þ1-713-348-6246; fax: þ1-713-348-6250; e-mail: tour@rice.edu Keywords: oligomers; molecular electronics.