Magnetic Ordering in Organic-Radical Assemblies

Ferromagnetic interactions in three-dimensionally stacked organic-radical assemblies were deduced under periodic boundary conditions. Although the topological-linkage patterns apparently contained pentavalent carbon atomic sites due to the three dimensionalities, it was shown that the degenerate non-bonding crystal orbitals (NBCOs) were subject to orbital mixing to form Wannier-transformed NBCOs, analogous to non-Kekulé polymers. In the ferromagnetic π–π stacking modes, the NBCOs were spread over more than two adjacent unit cells. On the other hand, in the π–π antiferromagnetic stacking modes, the NBCOs were localized only at one unit cell. High-spin preferences in the ferromagnetic stacking modes were attributed to anti-parallel-spin instabilities in effective two-electron wave functions consisting of products of NBCOs (PNBCOs). Our analysis was confirmed by theoretical calculations.