A class of narrow-band high-spin organic polymers II. Polymers with indirect exchange interaction

Abstract

It is shown that indirect exchange magnetic interaction may be crucial for the ground state spin multiplicity of a class of organic polymers. Such polymer chains possess a system of extended π-conjugation and each elementary unit contains at least one radical substituent R . with a localized partially occupied molecular orbital (LPOMO): ▪ The occurrence of LPOMOs is caused either by (i) steric hindrance which inhibits the π-π interaction of LPOMOs and the π-system along the polymer chain by making the two planes of π-conjugation almost orthogonal or (ii) specific topology of the π-network of the radical substituent R . preserving its π-POMO even when linked to the polymer chain. The almost perfect localization of POMOs results in vanishing direct (Hund) exchange interaction between unpaired electrons. These uncompensated spins still induce spin polarization of the overall system of π-conjugation which may break the spin degeneracy. An approach similar to the s-d(s-f) model of indirect exchange is applied. The ground state spin multiplicity depends on the exchange constants of the corresponding effective spin Hamiltonian. The latter constants are calculated for several model polymers in order to establish the type (ferro- or antiferromagnetic) and magnitude of the indirect exchange interaction.