Molecular Elements of Computers - Molecular Computing: Origins and Promises - page 65

phthalocyanine derivatives (ring C). Potential barriers between the wells are due to
intermediate fluorine atoms. As input signal the chain (SN) n is utilized along which
the electron comes to neutralize the charge of the C¼N + group. This leads to a
change in the shape of the potential well and cessation of electron tunneling along
the stack of molecules. Rings of type D at the element edges ensure signal exit
through the (SN) n chain and connection to the voltage difference across the Ni-S
link.
The basis of these two examples is the concept of control groups, i.e., molecular
fragments with the electronic structure which is rearranged under the influence of
some factor, thereby interrupting electron motion in the main chain. Other types of
control groups also exist.
Thus, in addition to the mechanism of charge neutralization caused by electron
transfer along the molecular chain at the nitrogen atom, as discussed above, Carter
proposed to use the electronic structure of control groups through:

￿ Tautomeric rearrangement driven by the change in the external electric field
(Fig. 3.15a )

￿ Intramolecular charge photo-transfer (Fig. 3.15b )

Figure 3.15d shows an example of a more complex molecular control group,
which can be manipulated both by an external electric field and light radiation.

Fig. 3.15 Molecular control groups proposed by Carter