S 1 A B

Sf and S2- External Stimuli

Figure 13.12. Schematic representation of the elements of a molecular switch. An external stimulus Si changes a molecule from state 0 to state 1, and returns the molecule to state 0.

representation of the basic elements of a molecular switch, in which stimulus S, brings about a conversion from state 0 to state 1, and stimulus S2 induces the reverse conversion. There are a number of different kinds of molecular switches.

An example of a molecular switch is provided by the azobenzene molecule, which has the two isomeric forms sketched in Fig. 13.11a. Unfortunately the cis form of azobenzene is not thermally stable, and a slight warming causes it to return to die trans form, so optical methods of switching are not of practical use for applications in computing. Employing electrochemical oxidation and reduction can overcome this thermal instability of azobenzene. Figure 13.13 shows how the cis isomer is reduced to hydrazobenzene by the addition of hydrogen atoms at a more anodic (negative) potential, and then converted back to the trans isomer by oxidation, which removes tire hydrogen atoms.

A chiroptical molecular switch, such as the one sketched in Fig. 13.14, uses circularly polarized light (CPL) to bring about changes between isomers. The application of left circularly polarized light (—)-CPL to the molecular conformation M on the left side of die figure causes a rotation of die four-ring group on the top from a right-handed helical structure to a left-handed helical arrangement P, as shown. Right circularly polarized light (+)-CPL brings about the reverse transformation. Linearly polarized light (LPL) can be used to read the switch by monitoring the change in the axis of the light polarizer. The system can be erased using unpolarized light (UPL).

Conformational changes involving rearrangements of the bonding in a molecule can also be the basis of molecular switching. When the colorless spiropyran, shown on the left in Fig. 13.15, is subjected to UV light, hv,, the carbon-oxygen bond opens, forming merocyanine, shown on the right in Fig. 13.15. When the mero-cyanine is subjected to visible (red) light, hv2, or heat (A), the spiropyran reforms.

A catenane molecule has been used to make a molecular switch that can be turned on and off with the application of a voltage. A catenane is a molecule with a


Figure 13.13. Schematic of controlling the azobenzene switching process using both photo-isomerization (top of figure), and electrochemistry (bottom of figure), making this a dual-mode switch. (With permission from M. Gomez-Lopez and F. J. Stoddart, in Handbook of Nano-stmctured Materials and Nanotechnotogy, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 5, Chapter 3, p. 230.)



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