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Fig. 3.35 Principles of
catenane and rotaxane
structures
organized a group of 15 people focusing exclusively on molecular switching
elements. Williams said in an interview:
“I would say let someone else working in this field, say 'Let's make diodes and
transistors'. Instead of re-inventing the silicon electronics at the level of individual
elements, we're going to attack at a higher level. What we want should look as a
fully integrated device.” Hewlett-Packard started to work in the field of molecular
components in close collaboration with chemists from the University of California,
Los Angeles, Fraser Stoddart and James Heath. At this time, they studied the
possibility of using molecular catenanes to create molecular switches. They chose
the molecule which was a combination of the cyclic cyclophane molecule
(Fig. 3.36 ) containing two bipyridine groups connected to a complex crown ether
molecule. Crown ethers are well-known nonplanar cyclic molecules, in the central
cavity of which other comparatively small atomic fragments can be built in. In
particular, crown ethers retain well metal ions, which is widely used in practice.
The crown ether used for the formation of the catenane molecule has a
tetrafulvalene group and dioxynaphthalene fragment built in on opposite sides of
the crown ring. Thus, the catenane molecule involved linked cyclic fragments, one
of which contained an electron donor, tetrathiafulvalene, and the second one—a
cyclophane ring with an electron acceptor (bipyridine). Electron transition from
donor to acceptor leads to Coulomb repulsion of the ring fragments, and the
molecule undergoes a conformational change resulting in increased distance
between the charged groups.
The process of conformational rearrangement is illustrated in Fig. 3.36 which
shows a three-dimensional structural model of the catenane molecule. Its compact
size and the flexibility of the crown ether ring should greatly facilitate this process.
Based on the catenane molecule considered above, an electric current switch was
created. The molecule was placed between a polycrystalline layer of n -type silicon
and a metal electrode. The switch opened when the potential of 2 V was applied, it
closed at 2 V, and its state could be read at 0.1 V (Fig. 3.37 ).
At the end of the 1990s, the group involving Hewlett-Packard and the University
of California set a goal to develop a working model of a molecular mass storage
device. The rotaxane R molecule (Fig. 3.38 ) was chosen as a molecular memory
element.
A typical rotaxane molecule is a polyester chain on which a cyclic molecular
fragment is mounted like a bead on a string. This fragment with embedded electron
acceptor group can move along the chain. In order for the cyclic fragment not to slip
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