Chemistry Reference
In-Depth Information
building blocks; and (iii) the propagation of the magnetic interactions along the three dimensions of
the material.
Substituted PTM radicals are stable species and, accordingly, they can be used as the constituent
building blocks of molecular magnetic materials. Regarding the coupling routes, a ferromagnetic inter-
action may result only if an orthogonal arrangement (zero overall overlap integral) between the two
singly-occupied molecular orbitals (SOMOs) of the two interacting subunits is produced. On the other
hand, if such an integral is non-zero, the resulting magnetic interaction would be antiferromagnetic for
non-degenerate orbitals. Magnetic interactions can take place either between neighboring molecules or,
in the case that there are two (or more) spin-containing units in the same molecule, intramolecularly.
Intermolecular interactions among organic radicals are mainly determined by the isotropic exchange inter-
actions between the unpaired electrons located on the SOMOs of the nearest neighbouring molecules.
The construction of magnetic materials requires that the structural subunits exhibit non-covalent interac-
tions suitable to be controlled in a predictable manner which are able to promote right isotropic exchange
interactions between the nearest neighboring open shell molecules. Up to now, the different types of
non-covalent intermolecular interactions that have been used for the assembly of such molecular subunits
are hydrogen bonding, transition metal ligation, stack-type alignment and bridging of ion radicals by their
counterions. In the case that the assembly of spin-containing molecules produces the proper magnetic inter-
actions and they are propagated along the three dimensions, the material will exhibit a bulk cooperative
magnetic property.
Intramolecular magnetic interactions play a major role in high-spin organic molecules.
35
Thus, when
a molecule has two high-lying orthogonal orbitals, close or equal in energy, with two less electrons
than necessary for a closed shell structure, its ground state becomes a triplet, as dictated by Hund's
rule. However, the actual nature of the interaction (ferromagnetic or antiferromagnetic) depends upon
the symmetry and topology of the degenerate or nearly degenerate SOMOs' orbitals. One of the general
approaches to obtain high-spin organic compounds is based on conjugated polyradicals with topologically
polarized
spins. These compounds are designed using appropriate ferromagnetic coupling units able to
align in parallel the spins of a pair (or more) of radical centers connected through such a unit.
m
-Phenylene
has become the most widely used ferromagnetic organic coupler because it is highly dependable.
In this section attention is focused on the developments and studies of magnetic materials derived from
PTM radicals. The magnetic properties of high-spin PTM radicals is described in detail as well as those of
extended systems based on PTM radicals. The possibility to functionalize surfaces with PTM molecules
as spin-containing units towards their use in molecular spintronics is also described.
α
2.2.1.1 High-spin PTM radicals
m-Phenylene Units as Intramolecular Ferromagnetic Couplers
With this aim,
m
-phenylene was used
as a ferromagnetic organic coupler of PTM radical centers in line with the efforts to increase the number
of persistent and stable high-spin radicals. Indeed, since the Schlenk hydrocarbon, first prepared in 1915,
high-spin alignments in ground states have been successfully demonstrated for several 1,3-phenylene
connected carbenes
36
and radicals based on triarylmethyl,
37
nitrogen-centred
38
and aminoxyl units.
39
The
work done in this direction included the synthesis, characterization and study of physicochemical properties
of molecules with two, three, or more triarylmethyl subunits, that are called bis-, tris-, or poly-triarylic
systems or, more generically, extended triarylmethyl systems. The first PTM polyradicals reported were
the diradical
2
and triradical
3
; both have a high-spin ground states with low-spin excited states, which
are inaccessible even at room temperature.
18b
The synthesis of larger polyradicals like
4
and
5
with
hyperbranched dendritic nature was also attempted. However, the numerous experimental problems found
during the synthesis and purification of pentadecaradical
5
, due to the presence of abundant magnetic defects
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