Chemistry Reference
In-Depth Information
The magnetic properties of structures based on the hexacarboxylic acid-substituted PTM radical have
also been reported. The study of its self-assembly in the solid state revealed that it is possible, through a
tuning of the crystallization conditions, to control the structural dimensionality going from 0- to 2-D solid
state structures in which the association of radicals through direct hydrogen bonds to form layers gives
rise to the presence of weak ferromagnetic intermolecular interactions between radicals.
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2.2.2 Materials with electronic properties
The enormous current interest in molecule-based electronics arises from the potential use of such systems
in molecular-scale devices; like nano-sized transistors, switches and wires.
65,66
The study of intramolecular
electron transfer (IET) processes has been of prime interest in this area because the understanding and
control of IET could lead to a better design and improvement of such molecule-sized electronic devices.
67
For such studies, the use of molecular wires with two electroactive units connected by a rigid bridge has
definite advantages over systems with two freely diffusing units.
68
There are two kinds of such systems
showing electronic properties (Scheme 2.3): one consisting of an electron donor (D) and an electron
acceptor (A) group as electroactive units, called D-A dyads; the other composed of the same unit at
both sides of the bridge but with different oxidation states, which are named as mixed-valence (M-V)
compounds. According to the degree of electronic coupling between the two electroactive units through
the bridge, the compounds showing IET can be classified in three classes. Class I contains those compounds
in which the two units are electronically independent of any significant electronic coupling, while Class III
comprises those systems in which there is a large electronic coupling with a complete delocalization of
the electron along the whole molecular system. By contrast, Class II compounds show an intermediate
situation, in which the electron is transferring between the two subunits.
M-V compounds and D-A dyads of Class II are excellent candidates for IET studies, since the rate and
efficiency of such phenomena can be easily followed and studied from the position, intensity and width
of the so-called intervalence transition (IVT) band, also named the charge transfer band, which usually
appears at the near-infrared (NIR) region. IET phenomena may take place either through thermally or
optically induced mechanisms, as shown schematically in Figure 2.18. To have a thermally-induced IET,
the electron must move from one site to another along the nuclear coordinate overcoming the thermal energy
barrier,
E
th
, which arises from the nuclear reorganization required for the passage of the electron from one
site to another. This nuclear reorganization is produced both within the molecule as in the surrounding
solvent molecules. On the contrary and according with the Frank - Condon principle, an optically-induced
IET takes place, either directly from the ground state [A - B
+
] to a vibrationally excited level of the first
e
−
h
n
,
∆
M
+
M
+
M
Bridge
Bridge
M
e
−
h
n
,
∆
D
Bridge
A
D
Bridge
A
Scheme 2.3
Schematic representation of D-A dyads (bottom) and M-V compounds (top)
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