Organic polymers with indirect magnetic interaction caused by the molecular topology of the elementary units

The magnetic properties of two classes of open-shell pi -systems, namely di radicals and one-dimensional polymers, having frontier non-overlapping non- bonding MOs (NBMOs) localized within the monoradical units have been studie d theoretically. The non-overlap and localization of the NBMOs is determine d by the topology of the pi -systems, and can be explained by means of the extended Coulson-Rushbrooke-Longuet-Higgins theorem. The localization and n on-overlapping of the NBMOs results in vanishing of the direct (Hund) and t he kinetic exchange interaction between the unpaired electrons. The indirec t exchange of the unpaired electrons (spin polarization exchange) is the on ly contribution to the effective exchange integral and occurs via the deloc alized pi -electrons in the filled energy bands. A comparison of our models is accomplished with the indirect exchange in rare earth compounds (f-s mo del) and with the superexchange of the Kramers-Anderson model. (C) 2001 Els evier Science B.V. All rights reserved.