Alternating stacks of neutral and ion-radical hydrocarbons. A feasible path to ferromagnetic materials?

Models of mixed crystal stacks composed of alternating ion- radicals and neutral molecules have been studied computationally in order to gain more insight into the effective magnetic exchange interaction. The B3LYP/6311G level has been used for DFT calculations and compared to AM1-CI results for sandwich-type monoradical dimers and biradical trimers. The isolated cation-radicals have also been characterised with respect to spin-density distribution. Effective ferromagnetic exchange through the spin-polarized spacer requires strong charge localisation on the radical units. If the parent neutral molecules of the ion-radical and the spacer have close values of the ionisation potential, the frontier molecular orbitals (MOs) of charged and neutral molecules mix. This mixing causes delocalisation of the unpaired electrons over the spacer, thus stabilising the singlet