NATURE OF EXCHANGE INTERACTIONS IN STACKED RADICALS RADICAL-IONS OF CYCLIC POLYENES

From full configuration interaction calculations on stacked cyclic pol yene radicals/radical-ions, within the Hubbard and the Pariser-Parr-Po ple models, we show that the ground state is not always a triplet stat e as predicted by the McConnell mechanism for organic ferromagnetism. On the contrary, the (2n + 1) cyclic polyene radical stacks always pos sess a singlet ground state. In 2n cyclic polyene radical-ions, for sm all 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 ba sed on topological arguments.