Polychlorotriphenylmethyl Radicals: Towards Multifunctional Molecular Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 65

preserve their radical character on the surface and confirming, in addition, the small spin-orbit couplings
under these conditions. These samples were also characterized by means of electron spin noise scanning
tunneling microscopy (ESN-STM). 103 This technique combines the spatial resolution of the STM with
the spectral resolution of EPR, and has recently been used to study small clusters of organic radicals on
surfaces. 104 Analysis of the data collected using this technique leads to the conclusion that it is possible
to detect the magnetic character of a decorated surface with 27 radicals.
Regarding the chemisorption of PTM radical derivatives on surfaces, different substrates have been inves-
tigated by preparing self-assembled monolayers (SAMs). 100 - 102 For this purpose, two different approaches
were followed: (i) direct anchoring of the PTM radical on the surface, and (ii) growth of a prefunctionalized
SAM which then reacts with the PTM radical derivative by the formation of either a covalent bond, a coor-
dination bond or via electrostatic interactions (Figure 2.25). The magnetic properties were characterized by
EPR in all cases, by cyclic voltammetry (CV) to study the electrochemical properties and by UV-Vis and
fluorescent spectra for the optical properties. Spectroelectrochemical experiments have also been carried
out proving that the PTM radical can be reversibly converted to the corresponding anion and, therefore,
the functionalized surfaces can act as chemical switches in which the magnetic and optical properties can
be used as read-out mechanisms (Section 2.3.2.2). Recently, it has been demonstrated that a gold surface
grafted with a PTM radical derivative linked with a conjugated bond to the gold shows electron transport
rates that are one order of magnitude larger than the surface grafted with the close shell counterpart of the
PTM derivative. This important result offers promising perspectives for fabricating devices for molecular
spintronics. 105

2.2.3 Materials with optical properties
In the last few years, the interest to develop novel second order nonlinear optical (NLO) materials has
considerably increased due to their potential application in emerging optoelectronic technologies. Tradi-
tionally, materials exhibiting second order NLO behaviors were inorganic crystals, such as lithium niobate
(LiNbO3) and potassium dihydrogenphosphate (KDP). However, organic materials, such as organic crys-
tals and polymers, have been shown to offer better nonlinear optical and physical properties, such as
ultrafast response times, lower dielectric constants, better processability and a remarkable resistance to
optical damage, as compared to inorganic materials. 106 Most of the efforts to discover new molecular
chromophores having large NLO properties have been focused on closed shell electronic organic species.
However, more recently, a large interest has been devoted to the investigation of materials having open
shell electronic structures. As it has been recently pointed out by Marks et al. 107 , species having open shell
electronic states, such as organic radicals 108 or paramagnetic transition metal complexes, 109

can exhibit

very large first order hyperpolarizabilities ( ) in comparison with analogous closed shell systems, thanks

to the presence of accessible low-lying charge transfer electronic states. In spite of this interest, only a few
examples of organic open shell species showing second order hyperpolarizabilities have been described up
to now, mostly due to the low stability of these species. 110
Systems presenting NLO properties can be grouped into two categories: (i) 'push - pull' systems and
(ii) octupolar systems. Donor-acceptor systems linked through a

β

backbone are one of the most devel-
oped 'push - pull' structures in the search for new compounds with efficient NLO responses. 111,112 The
requirements for molecules exhibiting interesting NLO responses are best met by highly polarizable donor-
acceptor (D-A) dyads, showing intramolecular electron transfer between the electron donating an electron
withdrawing groups. Since the pioneering work of Green et al. , 113 who reported obtaining a ferrocene
derivative with excellent NLO responses, there has also been considerable effort in using metallocenes as
donor groups in NLO molecular materials. 114 In addition to 'push - pull' dipolar molecules, more recently
there have appeared octupolar molecules exhibiting NLO responses. 115 These are nondipolar species whose
second order NLO response is related to multidirectional charge transfer excitations, rather than to dipolar

π