| STUDY |
So what do computations have to offer? Authors, has solved a puzzle in the rotaxane switch. The switch consists of a linear molecule that contains tetrathiafulvalene (TTF) and naphthyl groups, separated from each other along the strand. A ring of dibenzo-[24-crown-8] surrounds the molecule, like a wedding band on a finger. When voltage is applied to the molecule, the ring moves between positions, settling over either the TTF or the naphthyl group. Experimentally, these are distinguishable because one has high resistance (low current), while the other has a resistance 20 times lower. But the experiment didn't indicate which position--ring on TTF or ring on naphthyl--was on or off. Authors performed calculations of a model of the system, drawing on numerous techniques, including electron tunneling theories developed by Northwestern University chemistry professor Mark A. Ratner. The answer came from the pictures of the system's highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO). When the ring is over the naphthyl unit, authors found that the energy states of the HOMOs and LUMOs are basically degenerate--that is, nearly the same--making it easy for an electron or hole to tunnel across the molecule. But when the ring is over the TTF, the HOMOs and LUMOs are localized and therefore separated, making it less likely that electrons will conduct. Therefore, when the ring is over the naphthyl group, the switch is on. When the ring is over the TTF group, the switch is off. |
| UPDATE | 04.03 |
| AUTHOR | This data is not available for free |
| LITERATURE REF. | This data is not available for free |
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