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Fig. 7. The Fe-Ru mixed valence complex: (A) structure and (B) state encoding.
Fig. 8. The mixed-valence zwitterion: (A) structure and (B) state encoding.
Another real molecule proposed in literature is a mixed-valence complex
based on iron atoms (Fe) [ 26 ]. This molecule is called meta-Fe2: the active dots
are represented by two metal-ligand units, each composed of a cyclopentane
(Cp) and in iron atom (Fe), while an ethynyl linker connects them to a central
benzene ring. Scanning Tunneling Microscope (STM) imaging has revealed that
the presence of perturbations in the molecular environment leads to a charge
confinement inside the molecule in cationic form.
All the molecules already discussed need to be oxidized or reduced in order
to employ their nonzero net charge to realize different charge configurations
encoding binary information. As a consequence, particular attention has to be
payed to control redox processes. The zwitterion, a molecule which incorporates
a donor or acceptor site, permits to overlook such diculties. This special site
releases or attracts an electron, generating a free mobile charge in the molecule
while preserving the overall neutrality.
A practical example of mixed-valence zwitterion based on boron clusters has
been proposed as QCA device [ 16 ]. The octahedral cage of a closo-hexaborate
dianion (sketched in Fig. 8 (A)) is used as central linker group for four redox
centers, while other two axial linker could be used to bind the molecule on
a substrate. The boron cage attracts two electrons from two antipodal active
dots, thus becoming doubly negatively charged; two choices of opposite dots are
possible, subsequently two binary states can be encoded, as shown in Fig. 8 (B).
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