Nature of exchange interactions in stacked radicals/radical-ions of cyclic polyenes

Abstract

From full configuration interaction calculations on stacked cyclic polyene radicals/radical-ions, within the Hubbard and the Pariser—Parr—Pople models, we show that the ground state is not always a triplet state as predicted by the McConnell mechanism for organic ferromagnetism. On the contrary, the (2n + 1) cyclic polyene radical stacks always possess a singlet ground state. In 2n cyclic polyene radical-ions, for small n (n < 3), the ground state is a singlet. For larger n values, the ground state appears to be a triplet. We rationalize these results based on topological arguments.