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 diradicals and one-dimensional polymers, having frontier non-overlapping non-bonding MOs (NBMOs) localized within the monoradical units have been studied theoretically. The non-overlap and localization of the NBMOs is determined by the topology of the pi -systems, and can be explained by means of the extended Coulson-Rushbrooke-Longuet-Higgins theorem. The localization and non-overlapping of the NBMOs results in vanishing of the direct (Hund) and the kinetic exchange interaction between the unpaired electrons. The indirect exchange of the unpaired electrons (spin polarization exchange) is the only contribution to the effective exchange integral and occurs via the delocalized pi -electrons in the filled energy bands. A comparison of our models is accomplished with the indirect exchange in rare earth compounds (f-s model) and with the superexchange of the Kramers-Anderson model. (C) 2001 Elsevier Science B.V. All rights reserved.