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Spectroelectrochemical experiments have been carried out proving that the PTM radical can be reversibly
reduced to the corresponding anion and, therefore, the functionalized substrates act as a surface chemical
switch in which the magnetic and optical properties are used as read-out mechanisms. Figure 2.36 shows the
UV-Vis spectrum corresponding to the reduction - oxidation process of electroactive PTM self-assembled
monolayers. Upon reduction to the PTM anion, the characteristic PTM radical absorption band at 384 nm
disappears, while a band centered at 525 nm is observed. This system behaves thus as a real bistable
chemical switch, since it is possible to interconvert the self-assembled monolayer between two states
which exhibit different properties: an OFF state associated with the PTM anion SAM (non-fluorescent and
diamagnetic) and an ON state corresponding to the PTM radical SAM (fluorescent and paramagnetic).
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2.4 Conclusions
In this chapter the chemical and physical properties of PTM radicals have been described, mainly from
the molecular materials point of view. The main focus is on multifunctionality, giving different insights to
the magnetic, electronic and optical properties of PTM-based materials with representative examples for
each of them.
Regarding the magnetic properties of PTMs, the magnetism of high-spin PTM radicals have been
described in detail as have those of extended systems. The strengths of intramolecular ferromagnetic
exchange couplings of PTM radicals through the
m
-phenylene unit are strong and, consequently, such
polyradicals are robust high-spin molecules showing outstanding chemical and thermal stabilities. Met-
allocenes were also shown to act as effective magnetic couplers that effectively transmit ferromagnetic
interactions between PTM radicals. Investigation of the transmission of magnetic interactions through
hydrogen bonds has also been carried out with PTM radicals with carboxylic acid groups. This fact,
together with the ability of hydrogen bonds to transmit magnetic interactions and link molecules with
hydrogen bonding acceptor and donor groups, made possible the obtaining of magnetic extended systems
based on PTM radicals functionalized with several carboxylate/carboxylic groups. These magnetic extended
systems resulted in two new families of open framework materials with either a metal - organic or purely
organic nature, named metal - organic radical open frameworks (MOROFs) or purely organic radical open
frameworks (POROFs), respectively, with porous structures and interesting magnetic properties.
Regarding the electronic properties of PTMs, the most relevant results were obtained in the study of
IET phenomena observed in D-A dyads, where the PTM radical unit acts as a strong electron acceptor unit
due to its low reduction potential. In addition, M-V compounds derived from PTM radicals in which one
of both PTM units is reduced to the anionic form enabling the generation of the mixed-valence species
have also been discussed. Moreover PTM derivatives are electroactive molecules susceptible to reduction
or oxidation to the anionic or cationic forms, allowing in this way the 'on - off' switching of their magnetic
character.
Going one step further, the possibility to functionalize surfaces with PTM molecules as spin-containing
units towards their use in molecular spintronics has also been described. PTM radicals show very weak
spin-orbit couplings and, consequently, they have been proposed as possible molecules to favor the spin
polarization conservation during the electronic or spin transport. In this line, PTM radicals have been
recently deposited on surfaces of a different nature, proving for all cases that the magnetic character of
the molecule persist on the surface.
Regarding the optical properties, the capability of PTM radicals to generate NLO optical responses, either
as octupolar materials or as component of a 'push - pull' system, has been presented and the corresponding
hyperpolarizability values have been provided.
Finally, the intrinsic redox properties of PTM radicals together with the increasing interest for molecular-
scale switches made interesting the exploration of switching systems based on PTM radicals. Specifically, it
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