346
NANOMACHINES AND NANODEVICES
313 nm
> 380 nm
____t
-.+------TRANS ISOMER
(a)
365 nm light -
N = N
CIS ISOMER
1
- 420 nm light
(b)
Figure 13.1 1. (a) Cis-to-trans UV-light-induced isomerization of azobenzene; (b) a molecular
machine based on light-induced isomeric changes of the azobenzene polymer, which contracts
when it is converted to the cis form, causing the cantilever beam to bend.
becoming more difficult and more expensive to make. This has motivated an effort
to synthesize molecules, which display switching behavior. This behavior 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 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 a 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.
Furthermore the two states must be distinguishable by some probe, and the
application of the probe R is called the read mode. Figure 13.12 is a schematic
s1 and sz= External Stimuli ]
A - B
1 A =state o B = state 1 I
SZ
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 S, returns the molecule to state 0.