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3.2 Pioneering Ideas of Molecular Electronics
In 1974 Ari Aviram and Mark Ratner considered a hypothetical molecule, which
they called “molecular rectifier” (Fig.
3.7
). It contains two fragments, one being the
electron acceptor (tetracyanoquinodimethane, TCNQ) and the other the electronic
donor (tetrathiafulvalene, TTF). Electron acceptor is the molecule possessing an
electronic structure with the lowest molecular orbital unoccupied. BF
3
can serve as
the simplest example of such a molecule
.
The boron atom has the following
electronic structure: 1
s
2
2
s
2
2
p
x
2
p
y
2
p
z
. Upon formation of the BF
3
molecule this
electron distribution is changed to 1
s
2
2
s
1
2
p
x
2
p
y
2
p
z
. Three electrons form three
bonds with fluorine atoms, with one orbital remaining free. In the donor molecule
electrons are situated at the highest occupied molecular orbital. The electronic
structure of the nitrogen atom forming the NH
3
molecule is 1
s
2
2
s
2
2
p
x
2
p
y
2
p
z
.
Three electrons form three nitrogen-hydrogen bonds, but two more electrons are
still available in the outer electron shell. Such electronic structure of NH
3
and BF
3
leads to the formation of a stable molecular complex BF
3
NH
3
due to the so-called
Fig. 3.7 Aviram and
Ratner molecular rectifier:
chemical structure (a),
scheme of the electronic
levels (b), shift of the
electronic level under the
application of the direct (c)
and reverse (d) voltage
