Chemistry Reference
In-Depth Information
is a synergism between them, in new physical phenomena and novel applications that are difficult
to achieve in conventional inorganic solids. The design of such multifunctional molecular systems
that are able to change their functions by an external stimuli - like temperature, light and electric
field - results in more appealing systems because of their potential applications as molecular-scale
switches in nanotechnology.
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A molecular-scale switch is a fundamental component of any true molecular
magnetic/electronic/photonic device. To achieve a real switching effect, the molecule must be stable in two
(or more) states exhibiting very different properties which may be interconverted reversibly in response
to an external stimuli, such as redox, temperature, pressure or irradiation. To date, several molecular
switches exhibiting changes in one property, such as color,
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luminescence,
31
optical nonlinearity,
32
or
magnetic properties,
33
have been reported. However, only very recently the number of useful properties
being simultaneously modulated on a bistable molecule-based system has been extended to three
properties (electrical, optical and magnetic), although in this case the changes in the properties have an
intermolecular origin.
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PTM radicals are nice examples of molecular building blocks for obtaining such multifunctional molec-
ular materials, since they permit to combine the intrinsic magnetic characteristics of these molecules with
optical, conducting and electrochemical properties.
2.2.1 Materials with magnetic properties
The design of molecular materials, especially of those of a purely organic nature, with relevant mag-
netic properties has been one of the major challenges of the last decades. Due to the light nature of
elements composing organic compounds, only the magnetic exchange interactions determine the magnetic
behavior at temperatures well above 0.1 K. Other kinds of magnetic interactions (such as hyperfine or
spin-orbit interactions) as well as sources of magnetic anisotropies can be considered as negligible above
this temperature in most open shell organic materials. Consequently, magnetic organic systems may be
described at zero applied magnetic fields by the effective spin Hamiltonian approach, which takes the form
of Equation 2.1:
H
=−
2
J
ij
S
i
·
S
j
(2.1)
where
J
ij
represents the effective exchange interaction parameter for the magnetic centres
i
and
j
-the
spin-containing building blocks - which have total quantum spin numbers
S
i
and
S
j
, respectively, and
the summation runs over all the adjacent pairs of centers. According to the formalism of Equation 2.1,
when
J
ij
is positive the two spins tend to be parallel to each other in the ground state and the magnetic
interaction (or coupling) is ferromagnetic (FM). On the contrary, if
J
ij
is negative the two spins align in
an antiparallel fashion in the ground state and the interaction is antiferromagnetic (AFM).
Most organic magnetic materials exhibit paramagnetic behaviors at high temperatures where the spins of
the material behave independently of each other. However, as the temperature is decreased, the exchange
interactions become comparable with the thermal energy of the system and the neighboring spins tend
to align in accordance with the signs of their
J
ij
parameters, appearing either in a short or a long range
ordering of the spins. Alignments of spins can be propagated along one, two or three dimensions of
the solid, but only if the alignment occurs in three dimensions can a real long range magnetic ordering
appear. In this case the solid exhibits a cooperative magnetic property below its critical temperature
(T
C
)
, behaving as a bulk magnet. Keeping in mind the characteristics previously described, the design
of organic molecular materials showing bulk magnetic properties involves three main aspects or steps
that must be taken in consideration carefully: (i) the stability of the spin-containing building blocks;
(ii) the coupling routes or magnetic interaction mechanisms between the neighboring spin-containing
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