The lithographic methods used to make silicon chips for computers are approaching their limits in reducing the sizes of circuitry on chips. Nanosize architecture is becoming more cumbersome and more costly to make. This has motivated an effort to synthesize molecules, which display switching behaviour. This behaviour might form the basis for information storage and logic circuitry in computers using binary systems.
A molecule A that can exist in two different states such as two different conformations A and B, and which can be converted reversibly between the two states by external stimuli such as light or a voltage, can be used to store information. In order for the molecule to be used as zero or one digital state, necessary for binary logic, the change between the states must be fast and reversible by external stimuli. The two states must be thermally stable and be able to switch back and forth many times. Moreover, the two states must be distinguishable by some probe and the application of the probe R is called the read mode.
Figure 1.0 is the representation of the basic elements of a molecular switch, in which stimulus S1 brings about a conversion from state 0 to state 1, and stimulus S2 induces the reverse conversion. We have different kinds of molecular switches.
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A case in point is a chiroptical molecular switch illustrated in the figure below that 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 the figure causes a rotation of the four-ring group on the top from a right-handed helical structure to a left-handed helical arrangement P as illustrated. 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 by using unpolarised light.
Conformational changes involving rearrangements of the bonding in a molecule can also be the basis of molecular switching. When the colourless spiropyran shown below is subjected to UV light, hv1, the carbon-dioxide bond opens forming merocyanine; when the merocyanine is subjected to visible (red) light hv2 or heat (Δ), the spiropyran re-forms.
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