MAGNETIC-PROPERTIES OF IMINONITROXIDE AND NITRONYLNITROXIDE BASED DIRADICALS

The properties of a series of stable diradicals based either on iminon itroxide (IN) or nitronylnitroxide (NN) are described. Both radical fr agments 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 di stance between both radical fragments (up to 36 Angstrom). The intramo lecular coupling is found to be antiferromagnetic in all cases and in particular for meta- or ortho-substituted phenylene. Semiempirical cal culations using configuration interaction show that the linkers with t riple bonds induce a stronger conjugated character than the correspond ing ones involving a double bond, in agreement with the experimental o ptical spectra, and that the relevant geometrical parameters are the t wist angles; the corresponding barriers of rotation are then evaluated . Calculations show that, for some values of the twist angle, an antif erromagnetic state may be stabilized.