COLL 293 |
| Yun Hee Jang, Yong-Hoon Kim, Seung Soon Jang, Sungu Hwang, and William A. Goddard III. Materials and Process Simulation Center, California Institute of Technology, Beckman Institute (139-74), Pasadena, CA 91125 |
| The central components of the programmable molecular switch recently demonstrated by Stoddart and Heath are [2]rotaxane and [2]catenane, which consist of a cyclobis-(paraquat-p-phenylene) shuttle (CBPQT4+)(PF6-)4 moving between two stations threaded together, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). First we carried out molecular dynamics simulations on various coverages of [2]rotaxane monolayers on Au(111). We find that (1) the ring adopts different orientations depending on the coverage (it lies down parallel to the surface at lower coverage when free space is available around it, but at higher coverage it tilts around each of its axis so as to accommodate the high surface pressure) and that (2) a π-π stack between the free station and the shuttle on the other station is preferably formed in all coverages indicating that both [2]rotaxane and [2]catenane might have similar configurations of their key components. Then we calculated the electronic structure of those key components using density functional theory. We find that a critical function of the (CBPQT4+)(PF6-)4 shuttle is to induce a downshift of the frontier orbital energy levels of the station where it is on. This downshift alters the relative positions of energy levels of TTF and DNP, which in turn alters the electron tunneling rate through them. Another potential role of the (CBPQT4+)(PF6-)4 shuttle is to provide low-lying LUMO levels. This reduced HOMO-LUMO gap would significantly facilitate the electron tunneling through the system. Finally, using the surface Green′s function formalism, we calculated the current-voltage (I-V) curve of those key components positioned between two gold electrodes in various configurations. |
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Nanoscience and Nanotechnology
6:00 PM-8:00 PM, Monday, March 29, 2004 Disneyland -- North Exhibit Hall, Poster
Division of Colloid and Surface Chemistry |