| Abstract
| - The central component of the programmable molecular switch recently demonstrated by Stoddartand Heath is [2]rotaxane, which consists of a cyclobis(paraquat-p-phenylene) shuttle (CBPQT4+)(PF6-)4(the ring) encircling a finger and moving between two stations, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). As a step toward understanding the mechanism of this switch, we report here itselectronic structure using two flavors of density functional theory (DFT): B3LYP/6-31G** and PBE/6-31G**.We find that the electronic structure of composite [2]rotaxane can be constructed reasonably well from itsparts by combining the states of separate stations (TTF and DNP) with or without the (CBPQT)(PF6)4 shuttlearound them. That is, the “CBPQT@TTF” state, (TTF)(CBPQT)(PF6)4−(DNP), is described well as acombination of the (TTF)(CBPQT)(PF6)4 complex and free DNP, and the “CBPQT@DNP” state, (TTF)−(DNP)(CBPQT)(PF6)4, is described well as a combination of free TTF and the (DNP)(CBPQT)(PF6)4 complex.This allows an aufbau or a “bottom-up” approach to predict the complicated [n]rotaxanes in terms of theircomponents. This should be useful in designing new components to lead to improved properties of theswitches. A critical function of the (CBPQT4+)(PF6-)4 shuttle in switching is that it induces a downshift ofthe frontier orbital energy levels of the station it is on (TTF or DNP). This occurs because of the net positiveelectrostatic potential exerted by the CBPQT4+ ring, which is located closer to the active station than thefour PF6-'s. This downshift alters the relative position of energy levels between TTF and DNP, which inturn alters the electron tunneling rate between them, even when the shuttle is not involved directly in theactual tunneling process. Based on this switching mechanism, the “CBPQT@TTF” state is expected to bea better conductor since it has better aligned levels between the two stations. A second potential role ofthe (CBPQT4+)(PF6-)4 shuttle in switching is to provide low-lying LUMO levels. If the shuttle is involved inthe actual tunneling process, the reduced HOMO−LUMO gap (from 3.6 eV for the isolated finger to 1.1 eVfor “CBPQT@TTF” or to 0.6 eV for “CBPQT@DNP” using B3LYP) would significantly facilitate the electrontunneling through the system. This might occur in a folded conformation where a direct contact betweenfree station and the shuttle on the other station is possible. When this becomes the main switchingmechanism, we expect the “CBPQT@DNP” state to become a better conductor because its HOMO−LUMO gap is smaller and because its HOMO and LUMO are localized at different stations (HOMOexclusively at TTF and LUMO at CBPQT encircling DNP) so that the HOMO-to-LUMO tunneling would bethrough the entire molecule of [2]rotaxane. Thus an essential element in designing these switches is todetermine the configuration of the molecules (e.g., through self-assembled monolayers or incorporation ofconformation stabilizing units).
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