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

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they can be reversibly (magnetically, optically or electrochemically) switched between two (or more) stable

states when grafted on the surfaces. In addition, it is required that such states exhibit different responses

in order to be able to read the status of the switch. A strategy to scale down to molecular level memory

devices is focused on the fabrication of charge storage devices by substrate immobilization and patterning

of molecules that can be reversibly oxidized and reduced. 130 A simple and versatile technique to address all

these molecular building blocks on surfaces is the preparation of self-assembled monolayers (SAMs), 131,132

which allows the functionalization of surfaces with a layer of molecules, two-dimensionally organized, that

gives to the substrate new properties governed by the inherent characteristics of the molecules grafted on it.

There are extremely few examples of self-assembled monolayers based on organic radicals in the

literature. 133 As described in Section 2.2.2.3, silicon oxide and gold surfaces have been grafted with

PTM radicals making use of either covalent or non-covalent interactions. 85 The magnetic properties of

the functionalized surfaces 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. The opti-

cal characterization corroborated the chemical nature of the monolayer, exhibiting an absorption band at

382 nm and fluorescent emission band at 690 nm, which are characteristic of the radical character of the

grafted PTM molecules. 134 The EPR spectrum was also recorded to demonstrate the radical character of

the PTM functionalized surfaces. The EPR showed a signal at g

0024, with a line width of 5.2 Gauss,

which is close to that observed for many other PTM radicals. Electrochemical experiments were also

carried out proving that the grafted PTM radicals can be reversibly converted to the corresponding anion

and, therefore, the functionalized surfaces act as chemical switches in which the magnetic and optical

properties can be used as read-out mechanisms (Figure 2.34).

More interesting with respect to potential device applications is the possibility to locally address the

PTM radical molecules on the surfaces in order to fabricate multifunctional switchable patterned surfaces.

Thus, both the covalently and the electrostatic PTM bonded surfaces have been patterned by microcontact

printing (Figure 2.35) and visualized by laser scanning confocal microscopy or fluorescence microscopy

due to the fluorescent nature of the PTM molecules.

−

+ e −

− e −

− 2

− 4

−

600

−

400

−

200

Volta ge (mV)

Figure2.34 Left: Scheme of the redox process on a gold surface with covalently bonded PTM radicals. Right:

Cyclic voltammograms in dichloromethane, with 0.1M n-Bu 4 NPF 6 (vs Ag/AgCl) at different scan rates: a) 50,

Society.)

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