Nature of the magnetic interaction in organic radical crystals. 4. Magnetic interaction in mixed molecular radical crystals

The nature of the spin exchange interaction within the half-filled band of one-dimensional stacks of 1-D mixed molecular radical crystals (MMRC) with the general formula: ....PAH....R-.....PAH.....R-.......PAH....R-......PAH. ...R.(....)(R-., radical; PAH, diamagnetic polycyclic aromatic hydrocarbon) was investigated theoretically. The various contributions to the Heisenber g effective exchange integral, the direct (Coulomb), kinetic, and indirect spin exchange for the MMRCs, are evaluated quantitatively, using band theor y, and are compared with those of the corresponding parent molecular radica l crystals (MRC, without hydrocarbons). The nature and the magnitude of the exchange interaction in MMRCs are significantly different in comparison wi th the parent MRCs: the ferromagnetic interaction in MRC changes into antif erromagnetic in MMRCs and vice versa.