Physics and Fabrication

Abstract

This paper describes the synthetic organic phase of a project directed toward the construction of molecular scale electronic devices. Outlined is a convergent synthetic route to orthogonally fused conjugated organic oligomers. The final systems are to have a potentially conducting chain fused perpendicularly to a second potentially conducting chain via a σ bonded network. One of the core segments synthesized is based on a spirobithiophene moiety with a central silicon atom. It is formed by a zirconium-promoted bis(bicyclization) of a tetrapropargylsilane. The second core is a 9,9′-spirobifluorene system. Terminal halide groups provide the linkage points for further extension of the chains via Pd-catalyzed or Pd/Cu-catalyzed cross coupling methods. All four branching arms are affixed to the core in a single operation, thus making the syntheses highly convergent. In the cases of the larger functionalized systems, alkyl substituents on the thiophenes afford soluble materials. In order to prepare the molecules with >50 Å lengths, an iterative divergent/convergent approach had to be utilized for the construction of oligo(thiophene-ethynylene) branching arms. Organopalladium-catalyzed procedures are used extensively for the syntheses of the orthogonally fused compounds. Since the time of the first room-filling computers, there has been a tremendous drive to compress the size of computing instruments. In order to bring this desire to its extreme, it was conceived that one may be able to construct single molecules that could each function as a self-contained electronic device, 1 specifically, molecular scale electronic devices. 2 The slow step in existing computational architectures is often the time it takes for an electron to travel between any two points. By moving to very small dimensions, for example, to the molecular scale, the transmit time would be minimized, hence the computational system could possibly operate at a far greater speed than is presently attainable from conventional patterned architectural arrays. There is another technical advantage that might be gained from molecular scale devices. Present computational systems utilize approximately 10 10 silicon-based devices. If devices were to be based upon single molecules, using routine chemical syntheses, one could prepare over 10 23 devices in a single reaction flask. Though the task of addressing large arrays of ordered molecular scale devices is presently unattainable, the potential is exciting. We recently demonstrated the transport of electrons through single linear conjugated molecules. 1cc,3 Linear molecules can be considered two-terminal systems or single molecular wires. The testing of actual molecular devices, for example transistors where three terminals are required, remains to be demonstrated due to the problem of addressing more than two terminals on a molecular-sized structure. However, we show here that the synthetic organic protocol exists for the preparation of the requisite molecular device architectures. The flexible syntheses of two spiro core systems and the X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) For some recent background work on molecular scale electronics, see: (a) Academic: New York, 1992. For some background references to molecular wires and devices, see: (g) Waldeck, D. H.; Beratan, D. N. Science 1993, 261, 576. (h) Grosshenny, V.; Harriman, A.; Ziessel R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1100. (i) Langler, L.; Stockman, L; Heremans, J. P.; Bayor, V.; Olk, C. H.; Van Haesendonck, C.; Bruynseraede, Y; Issi, J.-P. Synth. Met. 1995, 70, 1393. (j) Pascual, J. I.; Méndez, J.; Gómez-Herrero, J.; Baró, A. M.; Garcia, N.; Landman, U.; Luedtker, W. D.; Bogachek, E. N.; Cheng, H.-P. Flamigni, L; Balzani, V.; Collin, J.; Sauvage, J.; Sour, A.; Constable, E. C.; Thompson, A. M. W. C. Dupuis, M.; Clementi, E.; Aviram. A. J. Am. Chem. Soc. 1990, 112, 4206. (bb) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. S.; Burgin, T.; Whitesides, G. W.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Soc. 1995, 117, 9529-9534. (cc) Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Tour, J. M.; Whitesides, G. M.; Muller C. J.; Reed, M. A. Polym. Prepr. (Am. (2) One of the troublesome issues in the area of molecular electronics is simply the definition of "molecular electronics", since some authors refer to it as any molecular-based system such as a film or a liquid crystalline array. Other authors, including us, have preferred to reserve the term "molecular electronics" for single molecule tasks, such as single molecule-based transistors. Due to this confusion, we have chosen here to follow the Petty et al. (Introduction to Molecular Electronics, Petty, M. C.; Bryce, M. R.; Bloor, D. Eds.; Oxford Univ. Press: New York, 1995) terminology by using two subcategories, namely "molecular materials for electronics" for bulk applications and "molecular scale electronics" for single molecule applications. (3) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II, Allara, D. L.; Tour, J. M.; Weiss, P. S.