Magnetic Properties of Iminonitroxide and Nitronylnitroxide Based Diradicals

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Abstract
The properties of a series of stable diradicals based either on iminonitroxide (IN) or nitronylnitroxide (NN) are described. Both radical fragments are coupled by various phenylene and phenylethynylene (triple bond) bridges. They are characterized by EPR studies both in solution and in glassy state. Magnetic coupling is evidenced at a very large distance between both radical fragments (up to 36 Å). The intramolecular coupling is found to be antiferromagnetic in all cases and in particular for meta- or ortho- substituted phenylene. Semiempirical calculations using configuration interaction show that the linkers with triple bonds induce a stronger conjugated character than the corresponding ones involving a double bond, in agreement with the experimental optical spectra, and that the relevant geometrical parameters are the twist angles; the corresponding barriers of rotation are then evaluated. Calculations show that, for some values of the twist angle, an antiferromagnetic state may be stabilized.